I document four post-Jurassic tectonic events recorded in the
geology of the Chortis block of northern Central America
within the context of the regional evolution of the Caribbean
plate and the southernmost North American Cordillera. The earliest event is Aptian-early
Cenomanian, intra-arc rifting followed by late Cretaceous inversion of the rift
basin in the Frey Pedro range of east-central Honduras. A 3.5-km-thick stratigraphic section of
clastic, carbonate, volcaniclastic and volcanic rocks were deposited in the
intra-arc rift and on its rift shoulders.
The geochemistry of rift-related volcanic rocks shows magmatic arc
affinity.
North trending rifting of the
western Chortis block and NNW-SSE transtensional extension of northern Honduras
and the offshore Honduran borderlands reveal Miocene to Recent divergence
between the Caribbean and North America
plates. Observed boundary-normal
extension occurs where the angle between the plate boundary fault and the Caribbean
motion vector is greater than 10º and boundary-parallel transtension where the
angle is between 5 and 10º.
Correlation of regional aeromagnetic data with outcrop exposures
allows subdivision of the Chortis block into five terranes: 1) Central Chortis with
continental Paleozoic basement; 2) Eastern Chortis with Jurassic
metasedimentary basement; 3) Southern Chortis of low magnetic intensity and
covered by Miocene volcanic strata; 4) Siuna with oceanic island arc basement;
and 5) Northern Chortis where early Tertiary magmatism overprints the Central
and Eastern Chortis terranes. Common
geologic and geophysical characteristics of the Chortis terranes and Mexico
terranes allow improved reconstructions of the region prior to its Tertiary
fragmentation.
I spent three years in 1989-1992 in Honduras as a field geologist
for the Peace Corps mapping three geologic quadrangles in central and eastern
Honduras at a scale of 1:50,000 (Rogers and O’Conner, 1993;
Rogers, 1995b, and
in press). During my mapping and travels
in all parts of the country and to neighboring countries, I was able to examine
the entire stratigraphic section of northern Central America
ranging from 1.0 Ga Grenville age basement to Quaternary volcanic rocks and
alluvium (Figure
0.2). I also visited
exposures of structures ranging from Cretaceous-age folds to Quaternary age faults along with a variety of landscapes including high
plateaus, basin and range topography, volcanic terranes, and alluvial
plains.
Figure 0.2. Composite stratigraphic
columns for rocks exposed in Guerrero state, Mexico, central Honduras, and the
Nicaraguan Rise
How all of these diverse aspects of regional geology evolved
through time remained enigmatic to me and helped motivated me to spend five
years as a Ph.D. candidate at the University
of Texas at Austin. This time and new tools learned as a graduate
student have allowed me to gain better regional insights into the geology of Honduras.
A grant from the Petroleum Research Fund of the American Chemical Society
provided me three years of graduate student support and funded an additional 3
months of mapping in eastern Honduras.
This dissertation is an attempt to put all the field observations made
while in the Peace Corps and here at the University
of Texas into a single, coherent
tectonic and stratigraphic framework. It
is my hope that this framework, although incipient, will make the geology of Honduras
and northern Central America more accessible to a new
generation of geoscientists.
The goal of this dissertation is to understand all aspects of the
tectonic and stratigraphic history of the Chortis block of northern Central
America (Figure 0.1). Five
studies were carried out; each addresses a specific aspect of the Chortis block
and each forms one chapter of this dissertation.
Figure 0.1 is a regional map showing the
spatial coverage of each chapter; on Figure 0.2
are three cross sections
showing the age range of stratigraphy studied for each chapter.
All chapters were written as manuscripts to be submitted to
journals. This approach means that some
introductory material is repeated in more than one chapter. The chapters are
arranged according to the time period they address. The chapters dealing with the youngest tectonic events affecting the Chortis
block are placed first, and the oldest events are placed last. This order also reflects the order in which
work on each chapter was done. This approach was taken in order to be able to
“remove” the overprinting effects of younger events before starting a study of
an older tectonic event.
Chapter 1 is entitled
“Epeirogenic uplift above a detached slab in northern Central
America”. The study was a
collaborative effort with H. Karasson and R. van der Hilst (Massachusetts
Institute of Technology) to use P-wave tomographic images of the mantle beneath
the Chortis block to image the subducted slab of the Cocos plate. A box labeled Chapter 1 shows the region from
which tomographic and geomorphologic data was examined. Earlier Ph.D. work by
van der Hilst (1990)
had shown that the subducted Cocos slab appeared to be detached beneath Central
America. In this study, I
attempted to relate the detached slab to mantle upwelling and the formation of
a large plateau formed after 10 Ma. The
timing of the formation of the plateau and the slab break-off event suggested a
genetic relation between the two events.
The chapter also addresses the evolution of the Middle
America volcanic arc in Honduras
including the formation of a large ignimbrite province also of Miocene
age. The paper was presented in poster
form at two national meetings, as a poster for an NSF-Margins conference in San
Jose, Costa Rica,
and was published in article form in the November, 2002, issue of Geology (Rogers et al., 2002).
Chapter 2 is entitled:
“Plate tectonic controls on two styles of active, transtensional deformation
along the North American-Caribbean plate boundary zone (northern Central
America and offshore Honduran borderlands)”. This study focused on the Miocene to recent
tectonic development of the transtensional zone between the North American and Caribbean
plates that has shaped the geomorphology and geology of northern and western Honduras
(Figures 0.1 and
0.2). The conceptual
model for this paper was inspired by recent GPS results constraining the motion
of the Caribbean plate relative to the North
America plate (DeMets et al., 2000). Unlike Mexico,
which hosts a more complete record of post-Cretaceous strata, most of the
post-Cretaceous stratigraphic units of the Chortis block have been removed by
erosion (Figure 0.2). For this reason,
this study relied heavily on marine geophysical data collected by the University
of Texas Institute for Geophysics
in 1979 and 1989 that images the structure and stratigraphy of basins formed in
the Honduran borderlands at the northern edge of the Chortis block.
Chapter 3 is called “Late
Cretaceous amalgamation of the western Caribbean plate
by collision between the continental Chortis block and intraoceanic Caribbean
arc and oceanic plateau”. This study focused
on defining the deformational history of the southeastern margin of the Chortis
block by combining geologic map data collected by me in the Colon Mountains of
eastern Honduras in 1992 (Rogers, in press) with the geologic and geochemical
results of an unpublished 1994 Ph.D. dissertation at the University of Arizona
by Margaret Venable on the Siuna oceanic terrane of northern Nicaragua. In addition, I used on-land seismic
reflection data tied to exploration wells from the area to the northeast of the
Colon Mountains that was published by Mills and Barton (1996) along with
offshore seismic and well data from the eastern Nicaraguan Rise (Rockwell,
1985). The objective of the chapter was to show a 350-km-long belt of
continuous fold-thrust deformation named here the “Colon
belt” that marks the suture zone between the continental terranes of the Chortis
block and the oceanic Siuna terrane.
Chapter 4 is called “Cretaceous
intra-arc rifting, sedimentation and basin inversion in east-central Honduras”
describes the geology of a well exposed but previously unmapped section of
Paleozoic-early Cenomanian metamorphic, sedimentary, and igneous rocks in the
Frey Pedro study area of the Agalta Range
of eastern Honduras
(Figures 0.1 and
0.2). The objective of
the study is to use these new structural, stratigraphic, biostratigraphic, and
geochemical field data to better constrain the geologic and tectonic history of
this part of the Chortis block (Central Chortis terrane) during the period of
time from Aptian to early Cenomanian (Figure 0.2). The study revealed that the topographic
Agalta range exposes a thick stratigraphic section (3.5 km) deposited in an
Albian-Aptian intra-arc rift and on its rift shoulders. This rift feature, named here the Agua Blanca
rift, presently trends northwest and is parallel to three other belts of
deformed Cretaceous rocks in Honduras
(Comayagua, Minas de Oro and Montaña de la Flor belts) that also may correspond
to Cretaceous intra-arc rifts produced during the same phase of intra-arc
extension. In order to better understand
the Aptian-early Cenomanian tectonic setting for intra-arc rifting and
subsequent rift inversion on the Chortis block, I reconstructed the Chortis
block relative to the terranes studied by previous workers in southern Mexico. Five geologic and tectonic features were
selected to best realign the two areas: 1) areas of Precambrian basement
outcrops; 2) areas of similar Mesozoic stratigraphy; 3) areas of Mesozoic
volcanic arc rocks exhibiting a similar arc geochemistry; 4) areas exhibiting
parallel trends of late Cretaceous folds and thrusts; and 5) areas of similar
magnetic signature. Using this
reconstruction, I propose four main regional tectonic events, three of which
are expressed in the structure and stratigraphy of the Frey Pedro study
area.
Chapter 5 is called “Tectonic terranes of the continental Chortis block (Honduras and Nicaragua) inferred from integration of regional aeromagnetic
and geologic data” and uses a new aeromagnetic map of Honduras
made available to me by the Direccion General de Minas e Hidrocarburos in Honduras. This map reveals prominent magnetic domains which
are compared to the regional geology discussed in previous papers. I have also incorporated aeromagnetic data
compiled by USGS for southern Mexico
in order to compare the magnetic expression of once continuous basement
terranes.
P-wave tomographic images reveal that the northern Central
American highlands east of the modern volcanic arc overlie a detached slab.
Hypsometric analysis of the highlands in Honduras
demonstrates that the region is a dissected plateau that is disrupted by normal
faults near the North American–Caribbean plate margin. The dissected Central
American plateau contains a network of superimposed rivers with meanders cut
into bedrock; such a geomorphic character indicates that the regional uplift
occurred in the absence of tilting. I propose that the epeirogenic uplift of
northern Central America is the buoyant upper-plate
response to the influx of mantle asthenosphere following the break-off and
sinking of the slab.
The origin of anomalous topographic relief distal from plate
margins remains an unresolved geomorphic and tectonic issue in many regions.
Theories for relief development—including removal of mantle lithosphere (Bird,
1979; Platt and England, 1994) and subduction-related lithosphere deflection
(Mitrovica et al., 1989)—are debated in part because it is difficult to
directly observe mantle-related relief-building processes. Slab detachment,
however, can be imaged by seismic tomography, providing a means of observing
the mantle underlying areas of anomalous relief. Uplift attributed to asthenospheric upwelling is an effect of slab detachment in the
Mediterranean-Carpathian region and is supported by thermomechanical modeling
(Wortel and Spakman, 2000;
van de Zedde and Wortel, 2001).
Behind the volcanic arc in northern Central America
(Guatemala, Honduras,
El Salvador,
and Nicaragua)
mountainous topography with elevations of >1 km extends 400 km from the
plate margins. To investigate this anomalous topography, I combine analyses of
surficial landscape data using digital elevation model (DEM) and satellite
imagery (LANDSAT) to quantify the magnitude and extent of the uplift with
P-wave tomographic images of the mantle underlying the region. Within the
context of the regional geologic and tectonic setting, I propose that the
northern Central American plateau east of the modern arc developed in response
to mantle upwelling following slab detachment.
Subduction of the Cocos plate and its predecessor, the Farallon
plate, beneath the North American and Caribbean plates
has produced the Middle America Trench, the
modern Central American arc, and earthquakes along the plates interface (Figs.
1A and 1B). Seafloor magnetic-anomaly data from the Cocos plate indicate that Neogene rates of convergence at the Middle America Trench have varied.
Spreading on the southernmost Pacific-Cocos segment of the East Pacific Rise
achieved full spreading rates of 180–210 mm/yr between 10 and 19 Ma (Wilson,
1996) following breakup of the Farallon plate and the initiation of Cocos-Nazca
spreading at 23 Ma (Barckhausen et al., 2001;
Wilson and Hey, 1995). The
identified anomalies and symmetrical, but unidentified, post–10 Ma East
Pacific Rise anomalies support the interpretation of a fixed position of the
southernmost Pacific-Cocos segment of the East Pacific Rise. With the
Caribbean plate
immobile relative to the hotspot reference frame since 38 Ma (Müller et al.,
1999), the position of the Middle America Trench has likewise been stable.
Therefore, variable Cocos-Pacific spreading rates require that convergence at
the Middle America Trench increased between 10 and 19 Ma.
Figure 1.1. Setting of northern Central
America showing plates and topography, bathymetry
and tomography
Northern Central America straddles the
North American–Caribbean strike-slip plate margin, a tectonic scenario that
produces intraplate deformation (Figure 1.1A). South of the plate margin,
rotation about the arcuate Motagua-Polochic fault system in Guatemala has led
to the formation of about a dozen small rifts of late Miocene to Holocene age
(Guzman-Speziale, 2001;
Burkart and Self, 1985;
Figure 1.2), whereas north of
the plate margin, the displacement is taken up by post–middle Miocene
shortening (Guzman-Speziale and Meneses-Rocha, 2000).
Figure 1.2.
Northern
Central America
map of uplift related features
New P-wave tomographic images of the Middle America Trench margin
and beneath northern Central America to the depth of
1000 km reveal a 300-km-wide gap in the subducted slab directly beneath the
uplifted highlands (Figure 1.1B). The gap parallels the Middle America Trench
from the Tehuantepec Ridge to the Cocos-Nazca-Pacific triple junction trace (Figure
1.1A). Tomographic images in cross section of the mantle beneath northern Central
America show a complex, three-part geometry to the subducted Cocos
slab (Figure 1.1B). The upper part of the slab, which is poorly defined in the
images, represents lithosphere subducted since 3.8 Ma—if the present-day convergence
rate (7.5 cm/yr) and slab geometry are assumed—and corresponds to the slab
known from study of Wadati-Benioff zone earthquakes along the Middle America
Trench (Burbach et al., 1984) (Figure 1.1B). Beneath the high-velocity,
seismically active slab is an area of mantle velocity interpreted as a
300-km-long gap in the currently subducting slab. A slab (“lower slab,”
Figure
1.1B) dipping eastward is present at depths of 500–1000 km in the mantle.
If the lower part of the slab broke off near the trench
(50-km-downdip slab distance) and if slab geometry and convergence rate were
unaffected by the break, the minimum possible age for slab detachment is
somewhat younger than 3.8 Ma. However, slab detachment along ~50% of the
convergent margin of the Cocos plate would presumably lessen slab-pull forces
and slow convergence at the Middle America Trench; thus the lower slab broke
off prior to 3.8 Ma.
Directly overlying the detached slab (Figure 1.1B) is the Central
American plateau that extends from east of the modern volcanic arc to the Caribbean
Sea and includes the region of deformation of the North
American–Caribbean plate margin. To illustrate its plateau-like character, I
examined the hypsometry within catchments contained within three subregions of
the plateau in Honduras at increasing distances from the plate margin by using
data from the global 30 arc second GTOPO30 DEM (Figure 1.3A). These subregions
are (1) the active fault-bounded horst and graben province of western Honduras,
(2) a central tectonically stable province containing erosion surfaces between
800 and 1000 m and bounded to the north by an active zone of strike-slip faulting,
and (3) the heavily dissected mountains of eastern Honduras
with few erosion surfaces. The western province (subregion 1) contains Miocene
ignimbrite strata, Cretaceous carbonate and clastic strata and a basement of
schist and gneiss (Weyl, 1981). The central province (subregion 2) contains
mostly folded Cretaceous strata and metamorphic rocks, while metamorphic rocks
are prevalent in the eastern province (subregion 3). Precipitation is at a minimum in the central
province (1–1.5 m/yr), but increases to 2 m/yr in the west and to >3 m/yr in
the east (IGN, 1996). The choice of Honduras
is due to the availability of (1) topographic maps (1:50,000 scale) from which
the subregions of the plateau were delineated and (2) field observations of the
extent of Neogene deformation.
Figure 1.3. Comparative hypsometry of
Honduras
I calculated the differential hypsometry (i.e., the histogram of
elevation) from the count of elevation values at 30 m intervals (the vertical
resolution of the GTOPO30 DEM in Central America) with 1
km horizontal resolution for the three subregions (Figure 1.3B). Cumulative
hypsometry is the progressive summation of the differential hypsometry. The
cumulative hypsometry of the central province (Figure 1.3B) reveals that 50% of
the region is concentrated between 800 and 1100 m. The central province
represents the core of a moderately dissected plateau. Lower precipitation in
this landlocked area contributes to the preservation of the plateau. In the
western, rifted province, the elevation is more variable although the mean
elevation is approximately the same as in the central province (Figure 1.3B). I attribute the variability of elevation
values in the west to disruption of the plateau by rifting proximal to the
plate margin. In the eastern province, the mean elevation is lower, and the
differential hypsometry is skewed to the lower elevations. Higher precipitation
has resulted in greater denudation of the plateau in the eastern area.
The type and extent of the uplift are recorded in the deeply
entrenched rivers of the Central American plateau. These rivers contain
low-gradient reaches with high sinuosity as shown by the Rio Patuca in eastern Honduras
(Figure 1.4). I interpret these reaches as superimposed fluvial meanders
entrenched into bedrock during uplift. Entrenched meanders develop where
meandering alluvial rivers are superimposed into bedrock during uplift in the
absence of significant tilting (Gardner, 1975;
Schumm et al., 1987). Therefore, the presence of entrenched
meanders reflects near-vertical, epeirogenic uplift, and the distribution of
entrenched meanders documents the extent of the uplift.
Figure 1.4. Bedrock meanders of Rio Patuca, Honduras
The distribution of entrenched meanders was mapped in northern Central
America from composite, 1:100,000-scale LANDSAT images and
1:50,000-scale topographic maps (Figure 1.2). Entrenched meanders are found at
elevations of >300 m in all lithologies across northern Central
America. The continuity of entrenched meanders increases with distance
from the plate margins, achieving greatest density and continuity in the
relatively resistant metamorphic rocks of eastern Honduras
and northern Nicaragua
(Figure 1.2). Entrenched meanders decrease in abundance near the active
volcanic arc of El Salvador
and Nicaragua
and near the Motagua-Polochic faults and the actively deforming north coast of Honduras.
Although abundant in western Honduras,
entrenched meanders are disrupted into shorter segments by the normal faults of
the north-trending rift basins. I
interpret the distribution of entrenched meanders as resulting primarily from
the epeirogenic uplift of northern Central America but
also affected by faulting and volcanic overprinting near plate margins.
The uplift is no older than the youngest rocks cut by the rivers’
entrenched meanders. The youngest rocks that host entrenched meanders were
deposited during the middle Miocene silicic ignimbrite flare-up event that
blanketed the western third of northern Central America
with up to 2 km of strata (Williams and McBirney, 1969;
Weyl, 1980; Figure 1.2). Ar/Ar geochronology of ash recovered from drilling in the Caribbean Sea dates
the end of the 10-m.y.-long Central American ignimbrite flare-up at 10 Ma (Sigurdsson
et al., 2000), which agrees with published ages from the Coyol Group strata in
Nicaragua (Ehrenborg, 1996). Williams and McBirney (1969) recognized that the
thick ignimbrite strata blanketed low-relief terrain prior to uplift and that
large-scale pyroclastic eruptions buried the earlier drainage network and
allowed development of new meandering rivers that were subsequently entrenched.
The rifting that disrupts the entrenched meanders in western Honduras
commenced after 10.5 Ma (Gordon and Muehlberger, 1994). Significant topography
had developed by the latest Miocene, as shown by the valley-fill deposition of
the 300-m-thick Gracias Formation in the rift valleys. These deposits contain
abundant mammalian fauna of early Hemphillian age (9–6.7 Ma) (Olson and McGrew,
1941; Webb and Perrigo, 1984).
I relate the uplift of the now-dissected plateau and the
associated development of the network of entrenched meanders to a gap in the
Cocos slab directly beneath the plateau (Figure 1.1B). Following slab
detachment, hot asthenospheric mantle flows in to replace the space vacated by
the cold, lower slab as it sinks into the mantle (Wortel and Spakman, 2000). Thermomechanical modeling of the slab-detachment process demonstrates large
(>500 °C) transitory heating of
the base of the upper plate for several million years following the slab
detachment due to upwelling mantle (van de Zedde and Wortel, 2001). Asthenospheric upwelling can produce
decompression-induced volcanism, and the geochemistry of the basaltic lavas
from behind the volcanic front in Honduras
and Guatemala
is consistent with mantle upwelling (Walker et al., 2000). Slab detachment and
the uplift of the Central American plateau occurred between the end of the subduction-related ignimbrite flare-up at 10 Ma and prior to 3.8 Ma, the time
at which the tip of the Cocos slab was subducted (beginning of “modern
subduction,” Figure 1.5).
Figure 1.5.
Timing of
Neogene events affecting northern
Central
America
In collisional settings, slab detachment occurs as the force of
slab-pull near the trench is resisted by the attempted subduction of buoyant
lithosphere that produces a tear in the downgoing slab (Wortel and Spakman,
2000). I suggest that this mechanism also occurred along the noncollisional
Middle America Trench margin as a result of the decreasing age and increasing
buoyancy of the incoming Cocos oceanic plate during the 19–10 Ma interval of superfast spreading along the
southernmost Cocos-Pacific segment of the East Pacific Rise (Figure 1.1).
Although steady-state subduction at the Middle America Trench since 2.5 Ma has
been inferred from the presence of cosmogenic isotopes in the modern arc lavas
of Central America (Morris et al., 2002), my observations require highly
variable subduction during the Neogene.
Previous workers have proposed that along-strike variability of active,
interplate deformation along the North American – Caribbean
plate margin is closely controlled by the angle between the GPS-derived Caribbean
plate motion vector and the azimuth of plate-bounding transform faults. In this chapter, I present geologic,
earthquake, marine geophysical, and remote sensing data from on- and offshore
areas to show that Neogene-Recent transtensional deformation produced by the
North America-Caribbean plate boundary in northern Central America exhibits two
distinct structural styles: plate boundary-normal
(east-west) extension in the western study area (onshore 375-km-long plate
boundary segment of basin and range morphology in western Honduras and northern
Guatemala); and plate boundary-parallel
(NNW- SSE) transtension in the
eastern study area (onshore and offshore 600-km-long plate boundary segment of
margin-parallel ridges and basins in the Nombre de Dios range and Aguan Valley
of northern Honduras and offshore Honduran borderlands region). Detailed comparison of the strikes of mapped
onland faults in western Honduras and southern Guatemala and the trend of the
GPS-derived Caribbean plate vector shows that plate boundary-normal (east-west)
extensional rifting occurs in the western area when the angle of divergence
between the main plate boundary fault (Motagua fault zone) and the GPS-derived
Caribbean plate vector is equal to or greater than 10º. Marine geophysical mapping of offshore faults
and onland faults in Honduras
and the offshore Honduran borderlands shows that this style of plate
boundary-parallel (NNW-SSE) transtension occurs when the angle of divergence
between the main plate boundary fault (Swan
Islands fault) and the Caribbean
motion vector is between 5 and 10º. A
narrow, 35-km-wide tectonic transition area in north-central Honduras
separates the plate boundary-normal rifts of western Honduras
from the plate boundary-parallel rifts in northeastern Honduras
and the offshore Honduran borderlands.
Faults of the offshore Honduran morphologic and structural borderlands
extend onshore into the Nombre de Dios range and Aguan
Valley of northern Honduras
where subaerial styles of transtensional deformation are similar to those
documented in this chapter in the submarine Honduran borderlands. Tectonic geomorphology studies show pervasive
oblique-slip faulting with evidence for late Quaternary left-lateral offsets
and active uplift of stream networks.
Seismic data tied to wells in the Honduran borderlands shows that
plate-boundary related submarine faults in this region are active,
transtensional features that initiated in the middle Miocene with filling of asymmetric
half-grabens and continued through the Pliocene-Pleistocene. Detailed plate reconstructions show that the
north trending faults of western Honduras rifts developed in response to
increased interplate divergence (>10º) as the western margin of the
Caribbean plate shifted from the left-lateral Polochic fault to the left-lateral
Motagua fault about 8 Myr.
Plate-boundary parallel rifts of Mio-Pliocene age also formed during a
period of increased plate divergence which has become increasingly strike-slip with
less extension today.
Strain partitioning along active and ancient plate margins is an
increasingly recognized process that produces complex deformation along and
adjacent to the margin. Most previous
studies of strain partitioning have come from transpressive regions where
convergent plate motion is transformed into components of strike-slip and
thrust faulting in either ancient (cf. Teyssier et al., 1995;
Jones and Tanner,
1995; Claypool et al., 2002) or active settings (Calais et al., 2002;
Mann et
al., 2002). In contrast, there are few
examples documented from active transtensional settings where obliquely
divergent plate motion is partitioned into coexisting strike-slip and normal
components of deformation (Ben-Avraham and Zoback, 1992;
Ben-Avraham, 1992). A variety of techniques are available to
document the components of deformation in transtensional settings. Global position system (GPS)-based geodetic
studies can quantify the precise rates of motion along plate boundaries which
can be resolved or “partitioned” into plate boundary-normal and plate boundary-parallel
components of active deformation (e.g., DeMets et al., 2000;
Calais et al.,
2002) (Figure 2.1). These plate
boundary-normal and plate-boundary parallel components can also be identified
and studied in the field using standard geologic mapping of faults and fault
related geomorphic features.
Figure 2.1. Northern Caribbean
margin tectonic map
Key questions for understanding strain partitioning in
transtensional settings include: 1) what are the relative amounts of
strike-slip faulting vs. normal faulting along any one particular plate
boundary segment?; 2) what are the large-scale tectonic controls on different
styles of coexisting strike-slip and normal faulting along the strike of the
plate boundary?; 3) where are the critical transition points from one style of
transtensional strain partitioning to another and what are their tectonic
controls?; and 4) how are successive styles of transtension superimposed on
older styles as the tectonic controls change through time?
I present the northwestern margin of the Caribbean
plate as an example of strain partitioning in a transtensional setting and
examine in detail the resulting deformational patterns as observed both onland
in Honduras and
Guatemala and
beneath the adjacent Caribbean
Sea (Honduran borderlands) (Figure 2.2). This region is ideal for such a study for
several reasons:
1) GPS-based geodesy from
DeMets et al. (2000) places firm constraints on the variation of interplate
motion and deformational styles across a 3100-km-long segment of the North
America-Caribbean plate boundary (Figure 2.1). Using a more robust model for present-day
plate motion between the North America and Caribbean plates supported by
observations from stable areas within the Caribbean plate (Figure 2.1),
DeMets
et al. (2000) propose a strong correlation between the direction of plate
motion and the degree of transtension with a divergence angle between fault and
plate vector of about 5º marking the threshold between predominantly
strike-slip structures and predominately transtensional structures.
2) Plate boundary-related
normal faults of a variety of orientations and ages are present in northern Central America and in adjacent areas of the Honduran
borderlands (Figure 2.2). To the
west is an area of plate boundary-normal east-west extension (i.e., normal
faults bound rifts at right angles to the trend of the highly arcuate,
strike-slip plate boundary, the Motagua fault zone). These north trending normal faults have been
interpreted by previous workers as: 1) intraplate deformation and block
rotations about a highly arcuate, convex-southward left-lateral strike-slip fault
system (Plafker, 1976; Burkart and Self, 1985); and as 2) fault termination
structures related to the termination of left-lateral slip of the Motagua fault
zone (Langer and Bollinger, 1979;
Guzman-Speziale, 2001). To the east in the Honduran borderlands and
Nombre de Dios range of northern Honduras, normal faults bound elongate rifts
and intervening basement blocks that are subparallel to the main left-lateral
strike-slip plate boundary fault (Swan Islands fault zone) (Figure 2.2). Because these offshore borderlands structures
extend onshore into the Nombre de Dios range and Aguan
Valley of northern Honduras,
I document the onshore
record of deformation and uplift through studies of geology and tectonic
geomorphology.
3) Finally, plate margin
faults have known variations in position and orientation that can be tied to
quantitative plate reconstructions based on the opening history of the Cayman
trough and surrounding plate pairs.
Along with the fault information shown on
Figure 2.2, I have compiled
magnetic anomaly information from ocean basins surrounding Central America and
the Caribbean plate and used these data to constrain the transtensional opening
history back to Early Miocene (20 Ma). These tectonic data can constrain the
paleopositions of the north trending rifts in the western study area of western
Honduras and Guatemala
and the plate boundary-parallel rifts in the eastern study area of the Nombre
de Dios range, Aguan Valley,
and Honduran borderlands (Figure 2.2).
Figure 2.2. Neotectonic map of northern Honduras
The objectives of this chapter are to: 1) compare the orientation
of faulting to the GPS-derived plate vector for the western 1000 km of the
Caribbean-North American margin in northern Central America and the Honduran borderlands
and to calculate the predicted along-strike components of boundary-parallel
(east-west) and boundary-normal (NNW-SSE) extension; 2) document the variation
in structural style of rifts in both regions using compilations of previous
studies, tectonic geomorphology studies of onland areas in the Nombre de Dios
range and Aguan valley of northern Honduras and marine geophysical studies of
offshore areas in the Honduran borderlands; 3) relate the two basic styles of
rifting (north-south trending rifts in west, east-northeast-trending rifts in
east) to variations in the angle of plate divergence; 4) use quantitative plate
reconstructions to constrain a model for how both areas of rifting evolved from
the Early Miocene (~20 Ma) to the present.
Original data presented in this chapter includes the following: 1) analysis of topographic data, remotely
imaged mapping of faults using LANDSAT satellite imagery, and analysis of
stream channel data from the Nombre de Dios range and Aguan Valley of
northern Honduras; digital topographic data shown in
Figure 2.2 and LANDSAT
imagery (band 3) was made available to me in CD format by the Hurricane Mitch
study of the U.S. Geological Survey; 2) interpretation of previously unpublished
marine geophysical data from the offshore Honduran borderlands that was
made available to me by the University of Texas Institute for Geophysics
(UTIG); these data include multi-channel seismic reflection data collected by
UTIG during the CT-1 Caribbean cruise in 1981 and SeaMARC II sidescan imagery
and single-channel seismic data collected during the MW-9809 cruise in 1989;
and 3) plate reconstructions made using
the PLATES reconstruction software at UTIG; Lisa Gahagan at UTIG assisted
me in the preparation of the reconstructions used in this chapter. No original field-based studies in northern Honduras
were done for this study.
GPS-determined motion of the Caribbean plate relative to North
America predicts significant along-strike variations in the style of
deformation along the 3100 km long plate boundary (DeMets et al., 2000) (Figure
2.1). The plate boundary extends from
the Motagua-Polochic Valley of Guatemala to the Lesser Antilles
arc in the northeastern Caribbean and has a total
GPS-derived rate of mainly left-lateral displacement of 21 mm/yr.
DeMets et al. (2000) have noted that
GPS-based interplate velocities are consistent with the along-strike transition
in structural styles from: 1)
transtension (combination of left-lateral strike-slip and normal faulting)
in the northwestern corner of the plate in northwestern Central America and the
western Cayman trough; on this segment, the divergence between the Caribbean
plate vector and the trend of the plate boundary faults predicts oblique
opening over a wide area; 2) pure
strike-slip faulting in the central Cayman trough; on this segment the
Caribbean plate vector and the trend of the plate boundary faults is almost
exactly parallel and predicts concentrated shear on linear, strike-slip faults
bounding the southern edge of the Cayman trough (Rosencrantz and Mann, 1991); 3)
transpression (combination of left-lateral strike-slip faulting and
convergence) in the eastern Cayman trough and southern Cuba; on this
segment, divergence between the plate vector and faults south of Cuba predicts
transpression and oblique underthrusting (Calais and Mercier de Lépinay, 1993); 4)
even greater amounts of transpression in the Hispaniola region, particularly
in the zone of contact between the Caribbean
plate and Bahaman Platform (Mann et al., 2002); and 5) oblique subduction of oceanic crust beneath the northeastern
edge of the Caribbean plate in Puerto Rico and the Virgin Islands in the
northeastern corner of the plate (Jansma et al., 2000) (Figure 2.1).
Earthquake epicenters with M > 4.0 from the National
Earthquake Information Center (NEIC) database and focal mechanisms for
earthquakes from the Harvard University catalog of Centroid Moment Tensors with
4.0 < M < 7.1 are compiled on a topographic basemap in
Figure 2.2A. The greatest concentration of epicenters
aligns in a belt parallel to the Swan Islands-Motagua fault zone, a continuous
zone of subaerial and submarine active, left-lateral faulting that accommodates
much of the present-day North America-Caribbean plate motion (Molnar and Sykes,
1969; Rosencrantz and Mann, 1991;
Deng and Sykes, 1995). Earthquake focal mechanisms along this fault
are dominantly left-lateral (Deng and Sykes, 1995;
Van Dusen and Doser, 2000; Caceres-Calix, 2003) (Figure 2.2A). A 230-km-long,
left-lateral surface rupture averaging 1.08 m of horizontal and 0.3 meter of
vertical displacement occurred on February
4, 1976, along the Motagua fault of Guatemala
(Plafker, 1976; Kanamori and Stewart, 1978).
This M7.5 event also splayed to the south and activated ruptures along
north-south oriented rift faults in Guatemala
where focal mechanisms indicated dominantly normal displacements (Langer and
Bollinger, 1979).
Earthquake epicenters with focal mechanisms that are indicative
of east-west extension are also spatially associated with faults bounding north
trending rifts of western Honduras
and Guatemala. Here, 12 rifts are characterized by steep,
fault-bounded, intermontane valleys filled by late Miocene to Quaternary age sediments
and lakes (Manton, 1987; Gordon and Muehlberger, 1994; (Figure 2.2A).
Guzman-Speziale (2001) used earthquake focal
mechanisms in the rift area to calculate an east-west rate of extension of 8
mm/yr across all the rifts. He proposes
that the rifts terminate strike-slip displacement along the Motagua-Polochic
fault zone. Earthquake events in
offshore areas of the Honduran borderlands and Nicaraguan Rise are widely
scattered and infrequent indicating low levels of activity in these more
intra-plate areas and/or poor seismograph coverage (Figure 2.2A).
Active faults of the region are compiled on
Figure 2.2A and
consist of three groups: 1) linear faults mapped along the Motagua-Swan Islands strike-slip fault zone (Plafker, 1976;
Rosencrantz and Mann, 1991); 2) more discontinuous,
north trending normal faults associated with rift structures of western Honduras and Guatemala; and 3) a broad, 125-150-km-wide zone of submarine
faults occupying the offshore region of shelfal to abyssal depths (200-2000
m) known as the Honduran borderlands (Pinet, 1971;
Case and Holcombe, 1980). The westward extension of the latter group of
submarine faults extends onshore into the Nombre de Dios range and Aguan
Valley of northern Honduras. The westward extensions of these faults
disappear at the longitude of the north-trending Yojoa-Sula rift basin at 88º
west (Figure 2.2A). For the first and
second groups of faults, I have compiled previously mapped segments of the
fault along with topographic lineaments interpreted from LANDSAT imagery on
Figure
2.2A. The main criteria for including
topographic lineaments in the active fault compilation were their sharp morphologic
expression in the tropical landscape.
Motagua, Polochic, and
Jocotan fault zones. The Motagua
fault offsets a staircase of Quaternary river terraces in the Motagua Valley of
Guatemala that yield long-term, left-lateral slip rates of 0.45 to 1.88 cm/yr
(Schwartz, et al., 1979). There is no
quantitative estimate of the cumulative lateral offset on the Motagua fault
since alluvium of the Motagua Valley
obscures many of the adjoining and presumably offset rock units. The Motagua fault is directly along strike
of the offshore Swan Islands fault to the east and is flanked by the active,
left-lateral Polochic fault zone to the north (Burkart, 1983;
Burkart, 1994) and the inactive Jocotan-Chamelecon fault zone to the south (Ritchie,
1976). The Jocotan-Chamelecon fault is
inferred to be inactive because its trace has become fragmented by east-west
opening of more recent, north oriented rifts (Ritchie, 1976;
Plafker,
1976). Burkart (1983) documents 130 km
of left-lateral displacement on drainages along the Polochic fault which
occurred between 10 and 3 Ma.
Burkart (1994) propose a chronology of plate boundary faulting that started along
the Jocotan fault between 20 and 10 Ma, switched to the Polochic fault from 10
to 3 Ma and switched again to the Motagua fault from 3 Ma to the present.
Swan Islands fault zone. This active
submarine fault forms a prominent semi-continuous fault scarp on the seafloor
(Rosencrantz and Mann, 1991). Its
location is consistent with the high level of earthquake activity shown by the
left-lateral earthquake focal mechanisms in the area (Figure 2.2A). The fault juxtaposes oceanic crust of the
Cayman trough generated at the Mid-Cayman spreading center over the past 49 Ma
with continental and oceanic crust of the Honduran borderlands to the south (Figure
2.2B). The Swan Islands fault steps
right at the Swan Islands restraining bend, a major right-step in the
left-lateral fault trace associated with topographic uplift of the Swan Islands
of Honduras and active convergent deformation of the seafloor observed on
sidescan and seismic reflection profiles (Mann et al., 1991).
Mann et al. (1991) and Leroy et al. (2000)
propose that the right-step and restraining bend in the Swan Islands fault zone
formed when the Mid-Cayman spreading center lengthened in a southward direction
by 25 km between 19.5 and 25.9 Ma (Figure 2.2B). The Swan Islands fault zone extends eastward
where part of the Caribbean-North America interplate motion is transformed into
opening along the 100-km-long Mid-Cayman spreading center (Leroy et al., 2000)
and the other part of the interplate motion continues along strike-slip faults
extending along the southern edge of the Cayman trough to the island of Jamaica
(Rosencrantz and Mann, 1991).
Plate-boundary normal
faults and rift structures. These
normal faults and associated rifts are described by Plafker (1976),
Mann and
Burke (1984), Manton (1987),
Gordon and Muehlberger (1994) and
Guzman-Speziale
(2001) and are shown as the fault-bounded Quaternary basins in
Figure 2.2A
(note that white areas represents mapped Quaternary alluvium modified from
Kozuch, 1991). Rifts maintaining a trend
almost perpendicular to the arcuate Motagua fault zone extend from the Guatemala
City graben in the west to western Honduras
(Figure 2.2A). These rifts are
half-grabens with their western sides’ downthrown. Two rifts occur in the 30-km-wide zone
bounded by the Motagua and Jocotan faults and three rifts occur in the
triangular wedge defined by the Middle America volcanic
arc to the west and the Jocotan fault and its western extension to the north
(Plafker, 1976).
Further east, the rifts assume a more north orientation, are
wider with larger Quaternary-filled floors, are full grabens, and form the
discontinuous “Honduras
depression” described by Muehlberger (1976) and
Gordon and Muehlberger
(1994). Quaternary basaltic volcanism
occurs in the rift near Lake Yojoa
(Wadge and Wooden, 1981). These volcanic
rocks have been dated using K-Ar as 0.5 Ma (Italian Hydrothermal report, 1987).
None of the rifts has been studied in detail, but are mainly
known through 1:50,000 scale quadrangle mapping by the Honduran government
(Kozuch, 1991). I have
incorporated this previous mapping into the compilation maps shown in
Figure 2.2
and 2.4A. These previous quadrangle
geologic mapping efforts, including King (1972;
1973), Markey (1995), and
Rogers
(1995b), have not succeeded in finding any evidence for Quaternary faulting at
the faults bounding the easternmost rifts (Figure 2.2B).
Gordon (1994) carried out fault striation
studies on older, Tertiary rocks in western Honduras
and found a complex set of extension axes that are likely related to the pre-10.5
Ma. opening history of the rifts.
Because all are sharply defined topographically and appear to be
currently subsiding, older, pre-Quaternary rocks characterizing their
development are not exposed. A vertical
motion on the faults bounding the rifts is significant: the Comayagua rift has
subsided relative to the adjacent mountains by as much as 2 km (Everett,
1970) and appears to have been an incipient feature during the deposition of
middle Miocene ignimbrites (Dupre, 1970).
Gordon and Muehlberger (1994) conclude that the rifting in Honduras
began after 10.5 Ma. Webb and Perrigo
(1984) found Hemphillian-age (9.0-6.7 Ma) mammalian fauna in the rift
valley-fill deposits.
Offshore Honduran
borderlands and plate boundary-parallel rift structures. “Borderlands” is a physiographic term used to
describe a distinctive irregular morphology commonly occurring along
strike-slip plate margins that is characterized by a sub-parallel series of
elongate islands, basins and troughs with a variety of depths and
sedimentologic patterns (cf. Gorsline and Teng, 1989, for a structural and sedimentologic description of the strike-slip fault-controlled California
continental borderland province). The upper structure and late Neogene
sedimentation of the Honduran borderlands is described in papers by
Kornicker
and Bryant (1969) and Pinet (1971;
1972; 1975) using single-channel seismic
profiling and shallow coring. In
Figure 2.2B
and in this chapter, I combine UTIG deeper penetration, multi-channel seismic
reflection profiles, sidescan images of the seafloor, and offshore exploration
wells to better define the distribution and structure of the elongate basins
and ridges underlying the Honduran borderlands.
A total of 21 different basins are mapped on
Figure 2.2B. More densely spaced seismic profiling may
reveal that some of the basins may form more continuous basins than as shown on
Figure 2.2B. The basins form a belt that
narrows towards the Mid-Cayman spreading center then widens towards the
west. There are two parallel basin axes:
the Bonacca and Patuca basins separated by a basement high. These basins narrow to the west to form a
single, offshore basin: the Tela basin.
These basins are presently filling with deepwater, terrigenous
turbidites derived from modern river deltas along the north coast of Honduras
(major rivers are shown in Figure 2.2A) (Pinet, 1972;
1975).
2.4.1 Caribbean plate vector and predicted partitioning of
slip in the study area
Variation in Caribbean plate vector relative to trends of faults. In
Figure 2.3A, I use the GPS-based plate
motion data from DeMets et al. (2000) to plot three plate vectors for the Caribbean-North American plate boundary
over an 1100 km distance along the plate boundary. The objective is to graphically show the east
to west increase in the component of boundary-normal slip (NNW-SSE extension)
predicted by the increasing angle between the plate vector and the trend of
interplate faults. DeMets et al. (2000)
present a similar analysis but use a much less detailed fault trace (Figure
2.1) and a much wider spacing between comparison points than I show on
Figure 2.3A.
Figure 2.3. Motion to boundary fault map, graphs
The first vector is located 100 km east of the Mid-Cayman
spreading center on the Oriente fault zone (Figure 2.3A). In this area the total plate vector is 18.4
mm/yr in a west-southwest direction with a negligible component of extensional
opening (0.2 mm/yr). The second vector
is located on the Swan Islands fault zone at longitude 85ºW adjacent to the
Honduran borderland rift province shown in Figure 2.2B and shows 18.5 mm/yr of
plate motion composed of 18.4 mm/yr of strike-slip and a geologically significant
component of NNW-SSE extension (2.2 mm/yr).
The third vector is located at 89ºW near the mouth of the Motagua Valley
of Guatemala (i.e., junction of the Swan
Islands submarine fault and the
subaerial Motagua fault in the Motagua
Valley. This vector shows 18.5 mm/yr of strike-slip
consistent with the other two sites but a large increase in the NNW-SSE
extensional component to 4.8 mm/yr.
Plot of angular variation
along the plate boundary. The large
increase in the extensional component of plate motion from east to west is
related to the increase in the angle between the plate vector and the trend of
the Motagua-Swan Islands
fault zones. The along-strike variation
in angle is shown graphically in Figure 2.3B.
The angular difference remains relatively constant and < 5º in the
eastern area of the Mid-Cayman spreading center that is characterized by
strike-slip faulting on well defined faults (Rosencrantz and Mann, 1991). The value increases to amounts > 5º in the
central area adjacent to the offshore Honduran borderlands and increases up to
20º in the area of the Motagua Valley.
Strike-slip and
extensional components compared to locations of rift features. In
Figure 2.3C, I graphically show the
variation in the Caribbean-North America velocity components that are parallel
(diamonds) and perpendicular (squares) to the trend of the plate boundary
faults. These points are derived by
rotating the predicted plate velocities using the DeMets et al. (2000)
pole
information onto local fault trends at the locations separated by about 50 km (0.5 degree of longitude). Values of strike-slip remain relatively
constant around 18 mm/yr except for a decrease to 17 mm/yr in the area of
maximum predicted extension near the eastern end of the Motagua
Valley (Figure 2.3C). Values of extension are variable but
generally cluster around the median 0 degree line in pure strike-slip areas of
the Oriente fault and Mid-Cayman spreading center. Rates of extension increase in a westward
direction with the exception of the localized area of deformation near the Swan
Islands restraining bend near
longitude 84ºW. The maximum rate of
extensional opening is 6 mm/yr in the eastern Motagua
Valley near longitude 88.5ºW.
The area of plate boundary-parallel NNW-SSE extension in the
Honduran borderlands and northern coast of Honduras
(Nombre de Dios range and Aguan Valley)
is characterized by predicted rates of opening of less than 5 mm/yr. The area of plate boundary-normal, east-west
extension in western Honduras and Guatemala is characterized by rates of
opening that are greater than 5 mm/yr.
These rates rapidly decrease to the west of longitude 88.5ºW as the
Motagua fault abruptly curves to the west and northwest in the western part of Guatemala
(Figure 2.3C).
Summary of plate tectonic
predictions. These predicted areas
and rates of opening provide a useful model to compare detailed observations of
plate boundary deformation in onland areas of Honduras
and in the offshore Honduran borderlands.
I will begin by describing deformation of the area of north trending rifts,
then describe the plate-boundary parallel rifts and faults of northern Honduras
(Nombre de Dios range) and conclude with a description of deformation in the
offshore Honduran borderlands using marine geophysical data.
2.5 USE OF
DIGITAL ELEVATION MODEL (DEM) OF HONDURAS TO DEFINE ZONES OF ACTIVE AND INACTIVE TRANSTENSIONAL
RIFTING IN WESTERN AND CENTRAL
HONDURAS
Objectives. In this section, I analyze of regional
geomorphology using GTOPO30, 30 arc-second Digital Elevation Model (DEM) of
Honduras, combined with the interpretation of fault scarps from previous
geologic mapping and interpretation of LANDSAT imagery shown in
Figure 2.2, to
better define the exact area of plate boundary-normal rifting in central and
western Honduras and its boundaries with adjacent, more tectonically stable
morphologic provinces. The objective of
this section is to gain a better regional understanding of the relative levels
of tectonic activity of rifting across Honduras. Previous workers have assumed that either
rifts are confined to the western part of the country and that the eastern part
of the country is part of the stable Caribbean plate (e.g., Plafker, 1976;
Burkart and Self, 1985) while other workers like
Manton (1987),
Gordon and Muehlberger (1994) and
Ave Lallemant and Gordon (1999) have proposed that
active deformation of Honduras includes the central, northern, and even eastern
parts of the country.
Data used. All of topographic and LANDSAT imagery were
compiled by the U.S. Geological Survey on a two CD set called “Digital atlas of
Central America prepared in response to Hurricane Mitch”
(January, 1999). In Chapter 1 of this
dissertation and in Rogers et al. (2002), I used these same topographic data to
define the Central American plateau directly overlying a detached slab of the
Cocos plate.
Regional hypsometry. I calculated the differential hypsometry
(i.e., the histogram of elevation) from the count of elevation values at 30 m
intervals (the vertical resolution of the GTOPO30 DEM of Central America shown
in Figure 2.4A) with 1 km resolution for four sub-regions of tectonic interest
shown in Figure 2.4B. Cumulative
hypsometry is the progressive summation of the differential hypsometry. The analysis presented in this chapter
differs from the similar analysis done in Chapter 1 because for this chapter I subdivide
the plateau province to distinguish an additional, fourth zone, the inactive
rift province of central Honduras
(Figure 2.4B). Due to the 1 km
relsoution of the DEM, hypsometric analyses were limited to watersheds greater
than 100 km2 in order to provide comparable results. This precluded applying hypsometric analyses
to the small drainages of the North Coast of Honduras and the alluvial plains
of eastern Honduras.
Figure 2.4. Regional hypsometry, map and river profile charts
Longitudinal river and
stream profiles. In addition to hypsometry
of the four zones, I compile longitudinal profiles of major rivers, at 20 meter
vertical intervals, to compare the relative tectonic activity of the four zones
(Figure 2.4C). Perennial rivers tend to
develop concave forms (Richards, 1981) except where the influence of geology,
including tectonic activity, results in non-concave profiles. Higher gradient rivers commonly reflect
steeper, tectonically-related relief, such as that produced along
mountain-front fault scarps (Hovius, 2000).
Ongoing GPS study of
deformation in Honduras. GPS benchmarks have been
installed at five sites in central and eastern Honduras by C. DeMets
(University of Wisconsin, Madison) and myself in 2000, but have only been
observed once (Figure 2.4A).
Reoccupation of these GPS sites and integration of these data with other
areas of the Caribbean will be useful for
better establishing the stable versus faulting areas of Honduras.
Hypsometry indicates three main morphologic provinces of Honduras
that are outlined in white and numbered 1-3 on
Figure 2.4A. The three zones are the western rifts (zone
1), the plateau (zones 2) and the eastern province (zone 3). The plateau province is subdivided into a
region of inactive rifts (zone 2a) and the core plateau (zone 2b). The undeformed core of the plateau (zone 2b)
is described first in order to compare with adjacent zones. The north coast (unlabeled in
Figure 2.4) forms
a forth morphologic province which is discussed below in section 2.6.
Morphologic zone 2b, core
of Central American plateau. The
basement rocks underlying this morphologic zone consist of pre-Jurassic
metamorphic basement rocks and folded Cretaceous rocks (Chapter 4 and
Plate 1). This zone represents the core of the
moderately dissected Central American plateau defined by Rogers et al.
(2002). High-level erosion surfaces
characterize the relatively smooth upper surface of the plateau (Helbig, 1959;
Manton, 1987; and Rogers et al., 2002).
Cumulative hypsometry reveals that 50% of zone 2b is concentrated
between 700 and 1000 m ASL (Figure 2.4B).
The plateau is preserved probably because: 1) it is located in an inland
area removed from the Caribbean Sea and Pacific Ocean and subjected to lower
rainfall amounts (1-1.5 m/yr) than surrounding, more coastal areas to the west
(>2 m/yr) and east (>3 m/yr); and 2) it is a tectonically stable area
located on the Caribbean plate and therefore not subjected to active
faulting.
The longitudinal profiles of six rivers and streams from plateau
areas in zone 2b are shown in Figure 2.4C (river locations identified as a-f in
Figure 2.4A). Rivers in this zone have
straight profiles along their entire lengths reflecting the tectonically
undisturbed uniform gradients. Rogers et
al. (2002, Chapter 1) proposes that these rivers entrenched into bedrock
following the uplift of the Central American plateau after 10.5 Ma.
Morphologic zone 1, area
of active plate boundary-normal rifting.
This area exhibits the basin and range topography as seen on
Figure 2.4A
and is underlain by pre-Jurassic basement rocks, folded Cretaceous rocks and
overlying Miocene ignimbrite deposits in the western area (Rogers et al., 2002;
Chapter 1; Plate 1). Cumulative
hypsometry differs significantly from zone 3 and reveals a variable elevation
in this region that reflects the extreme topographic relief related to the
formation of half-grabens and elevated rift shoulders.
Rogers et al. (2002) propose that the Central
American plateau originally extended westward to include the area of zone 1 but
the plateau area was disrupted by the formation of north- trending rifts after
10.5 Ma (Gordon and Muehlberger, 1994).
The longitudinal profiles of five rivers from rifted areas in
zone 1 are shown in Figure 2.4C (river locations identified as a-e in
Figure 2.4A).
Rivers in this zone have low gradient
reaches on rift valley floors but abruptly steepen when they cross onto the
uplifted rift shoulders. These concave
and stepped profiles differ significantly from the straight profiles seen in
the plateau area of zone 2b.
Morphologic zone 2a, area
of inactive rifts. Geologic
quadrangle mapping by King (1972;
1973), Markey (1995) and
Rogers and O’Conner
(1993) in this area has revealed the presence of north-striking normal faults
cutting across a terrain of pre-Jurassic basement rocks, folded Cretaceous
sedimentary rocks, and Miocene volcanic deposits (Figure 2.4A and
Plate 1). Cumulative hypsometry from this area shows a
similar hypsometric pattern to that observed in zone 2b, the stable core of the
Central American plateau. This
observation along with geologic evidence from the above previous studies
suggests that this area of rifting has become inactive.
The longitudinal profiles of two rivers from rifted areas in zone
2a are shown in Figure 2.4C (river locations identified as a-b in
Figure 2.4A). Rivers in this zone have dominantly straight
profiles along their entire lengths reflecting the tectonically undisturbed
uniform gradients. Rogers et al. (2002,
Chapter 1) proposes that these rivers entrenched into bedrock following the
uplift of the Central American plateau after 10.5 Ma.
Morphologic zone 3, eastern
Honduras. The basement rocks underlying
this morphologic zone consist of pre-Mesozoic metamorphic basement rocks and
folded Cretaceous rocks (Chapter 3 and Plate 1). This zone represents the deeply dissected
eastern edge of the Central American plateau defined by Rogers et al.
(2002).
The longitudinal profiles of three rivers and streams from
plateau areas in zone 3 are shown in Figure 2.4C (river locations identified as
a-c in Figure 2.4A). Rivers in this zone
display the classic concave profile of graded rivers in tectonically stable regions,
adjusted to this higher rainfall area adjacent to the Caribbean Sea. The stepped profile of the Rio Wampu (Figure 2.4C,
profile a) is interpreted as a special case related to the breaching of its
divide (the stepped area) and piracy of tributaries west of the divide.
A map summarizing the locations of all normal faults and rifts in
Honduras and
the area of inferred inactive rifts and normal faults is shown in
Figure 2.3A. The eastern area of the country is considered
a tectonically stable block lacking any evidence for active faulting and is not
discussed further. In the next section I
show data supporting active faults of the Nombre de Dios range and Aguan
Valley and make the case that this
region is the onshore extension of plate boundary-parallel normal faults of the
offshore Honduran borderlands (Figure 2.3A).
In this section, I present an analysis of the regional
geomorphology of the Sierra Nombre de Dios and adjacent Aguan Valley that is
one of the most seismically active (Figure 2.2A) and highest relief (maximum
elevation: 2643 m ASL) areas of Honduras.
In addition to these suggestions of tectonic activity, LANDSAT imagery
compiled in Figure 2.5A indicates plentiful evidence for late Quaternary
strike-slip and normal faulting crosscutting the region that has been described
by previous authors including Manton (1987),
Manton and Manton (1999), and
Ave Lallemant and Gordon (1999). Key
objectives of this section are to: 1) determine the nature of fault
displacements along the prominent topographic lineaments in the area; 2) to
understand the relation of observed faulting to the predicted model for
tectonic transtension shown in Figure 2.3A; 3) to better define the boundary
between the active plate boundary zone deformation and the stable Caribbean
plate in eastern Honduras (morphologic zones 2, 3, and 4 summarized on
Figure
2.4A and discussed in the previous section); and 4) to correlate major faults
and structural styles onland to those observed offshore in the Honduran
borderlands (Figure 2.2A).
Figure 2.5. North coast LANDSAT image and geology
LANDSAT and topographic data used in this study were compiled by
the U.S. Geological Survey on a two CD set called “Digital atlas of Central
America prepared in response to Hurricane Mitch” (January,
1999). RADARSAT data was also used and
was obtained at the NASA web site as a free download. Geologic map data was compiled from the
previous compilation by Kozuch (1991) with minor modifications based on the more recent field studies by
Manton and Manton (1999) and
Ave Lallemant and Gordon (1999).
Methods. Methods included interpretation of 1:100,000
scale LANDSAT imagery with particular emphasis on establishing the continuity
and existence of lineaments and correlating them in a GIS environment with
previously mapped faults by Manton and Manton (1999) and by using river and
stream profiles to document areas of recent uplift along the trace of the
lineament. Because much of the area
remains unmapped or mapped only at a reconnaissance scale, I was able to
discover previously unrecognized faults and show continuity and interactions
between known faults.
The geologic setting of the Nombre de Dios range is summarized in
the map on Figure 2.5B and has been described by
Williams and McBirney (1969),
Manton (1987), Kozuch (1991),
Manton and Manton (1999) and
Ave Lallemant and
Gordon (1999). The range forms a heavily
faulted dome with its core underlain by sheared high-grade gneiss and schist,
felsic intrusions, and locally mafic volcanic strata of pre-Cenozoic age.
Manton (1987) notes that the dominant
east-northeast trend of recent faults is parallel to the trend of faults and
folds in the older rocks. For a domal
uplift of this size and elevation, younger, flanking sedimentary rocks are
limited. Thus leading to the interpretation
that most of the clastic, erosional products of the uplift have been
transported offshore into basins of the Honduran borderlands.
Manton and Manton (1999) describe highly
deformed, marine turbidites of Miocene age overlain by a south-dipping section
of coarse valley-fill conglomerate south of the city of Trujillo. These authors speculate that the present-day
area of the Nombre de Dios range and adjacent Aguan valley to the south may
have once formed part of the submarine Honduran borderland province in
pre-Miocene time and was uplifted by strike-slip tectonics. I have identified a possible Quaternary
marine terrace on the Caribbean coast at the eastern end
of the Aguan Valley
(Figure 2.5A, B). The terrace is on the
footwall of the La Esperanza fault and is cut by lineaments suggesting recent
subaerial emergence of the terrace and continued activity of the La Esperanza
fault.
The cloud-free LANDSAT image in
Figure 2.5A and the geologic map
in Figure 2.5B summarize the five major faults of the Nombre de Dios range that
form prominent topographic lineaments and are discussed in this section. All faults are linear, have east-northeast
strikes, and are parallel to offshore faults known from marine geophysical
mapping of the offshore Honduran borderlands and Swan
Islands fault zone and compiled on
the map in Figure 2.5B. Plate tectonic
predictions summarized on Figure 2.3A suggest that the faults could help
accommodate the total plate motion of about 18.5 mm/yr (strike-slip) and 2-3
mm/yr (NNW-SSE extension) predicted at this longitude. The Sierra Nombre de Dios is about 100 km
south of the main plate boundary fault zone (Swan
Island fault zone) (Figure 2.3A).
La Ceiba fault zone (fault
number 1 in Figure 2.5A).
Muehlberger (1976) named this fault forming the prominent, slightly
curvilinear mountain front of the Nombre de Dios range and
Gordon and Muehlberger (1994) show a SEASAT radar image of the scarp. The fault forms a steep and sharp boundary
between pre-Cenozoic basement rocks in the range and Quaternary fan and
pediment deposits along the front of the range.
North-flowing streams and rivers cross the La Ceiba fault mountain front
and enter the Caribbean Sea directly along the eastern
two-thirds of the range while crossing a broad plain of amalgamated alluvial
fans along the western one-third of the range.
To my knowledge, there has been no detailed study of this fault in the
field. East of the city of La
Ceiba, the trend of the fault turns abruptly to the
southeast and nearly merges with the Rio Viejo fault. To the west, the La Ceiba fault appears to
terminate on the normal fault forming the eastern boundary of the Sula rift (Honduras
depression). The La Ceiba fault aligns
with the inactive trace of the Jocotan-Chamelecon faults to the west (Figure
2.2)
Rio Viejo fault zone (fault number 3 in
Figure
2.5A). Manton
(1987) named this
fault that forms the southern boundary of the topographically highest part of
the Nombre de Dios range and form the southern edge of pre-Cenozoic basement
rocks exposed in the core of the range (Figure 2.5B). The strip of topographically uplifted
basement rocks is remarkably narrow (about 20 km) for its elevation, suggestive
of active strike-slip tectonics. The Rio
Viejo fault extends eastward almost to the coastal city of Trujillo. To the west it appears to terminate near the
western limit of high topography of the range and does not extend as far to the
west as the sub-parallel La Ceiba fault zone.
The close association of area of highest topography to the step-over
area between the La Ceiba and Rio Viejo fault zones suggests that the two
faults form an active restraining bend on left-lateral strike-slip faults. This interpretation is also supported by the
abrupt curve in the La Ceiba fault to align with the Rio Viejo fault (Figure 2.5A,
B). To my knowledge, there has been no
detailed study of this fault in the field.
Aguan fault zone (fault
number 2 in Figure 2.5A). Elvir
(1974) named this fault that forms the topographic boundary between the
elevated area of basement rocks of the western Nombre de Dios range and the
lower lying and younger rocks south of the range. This fault is the continuation of a lineament
that can be traced across the Quaternary floor of the western Aguan
Valley (shown as a dotted line in
Figure 2.5B). Documentation of a
Quaternary scarp is difficult due to the high impact of agriculture and development
on the Quaternary plains of the valley.
Lepaca fault zone (fault
number 4 in Figure 2.5A). This fault
forms the topographic boundary between the elevated area of basement rocks of
the central and eastern Nombre de Dios range and the lower lying and younger
rocks south of the range. Documentation
of a Quaternary scarp is difficult due to the high impact of agriculture and development
on the Quaternary plains of the valley.
I name this fault zone here for the first time.
La Esperanza fault zone
(fault number 5 in Figure 2.5A).
Manton
(1987) named this fault that forms the topographic boundary defining the
southern edge of the Aguan Valley. The fault juxtaposes Quaternary alluvium of
the valley floor with a mixture of pre-Cenozoic igneous, metamorphic, and
sedimentary rocks south of the fault.
The existence of a possible Quaternary marine terrace near the seaward
projection of this fault supports the idea of recent uplift of the area south
of the fault.
Normal faults defining the eastern edge of the Sula rift of the Honduras
depression abruptly truncate the east-northeast striking La Ceiba fault zone
(Figure 2.5A, B). North of the La Ceiba
fault, faults of the eastern boundary of the Sula rift change to more northeast
trends. The El Negrito and Morazon
half-grabens form outliers to the east of the Sula rift but exhibit
orientations more to the northeast than the more north- trending Sula
rift. The RADARSAT image shown in
Figure
2.6 documents the transition from the more northerly trends of normal faults
that disrupt the Quaternary (0.5 Ma) volcanic field in the Sula rift to the
more northeasterly trends of the El Negrito and Morazon rifts (average trends
of rift-bounding normal faults indicated by white arrows in
Figure 2.6). The Lean rift occurs as a full-graben
structure to the northeast of the El Negrito and Morazon rifts and to the north
of the La Ceiba fault (Figure 2.5B).
Avé
Lallemand and Gordon (1999) have previously related the origin of the Lean graben to left-slip motion on the La Ceiba fault.
Figure 2.6. Yojoa Rift RADARSAT image
I propose that the northeast trend of the northern Sula, El
Negrito, Morazon, and Lean rifts forms a 35-km-wide rift province of
intermediate orientation between plate boundary-normal, or north- trending
rifts of western Honduras
and plate boundary parallel, or east-northeast-trending rifts in the Nombre de
Dios range, Aguan Valley,
and Honduran borderlands. The position
of the proposed transition zone between the two much larger rift provinces is
plotted on Figure 2.3C.
Objectives. I compiled geomorphic features and drainages
of this region using 1:50,000 scale topographic maps combined with
interpretations of LANDSAT satellite imagery shown in
Figure 2.5A. The objective of this study is to use the
longitudinal profiles of these rivers and streams to assess the activity of the
five major faults described above along with previously unrecognized lineaments
identified on LANDSAT imagery. The
elevated domal uplift of the Nombre de Dios range with its quasi-radiating drainages
and steep mountain fronts (Figure 2.7A) is ideal for examining the effects of
recent tectonic activity related to east-northeast faults shown in
Figure 2.7B
that intersect these rivers at high angles.
Figure
2.7. North
Coast Honduras
morphology
Controls on geomorphology. The Nombre de Dios range is highly asymmetric
with its drainage divide south of the midline of the range (Figure 2.7A). This asymmetry is probably related to
increased precipitation on the seaward flank of the range and a rain shadow
developed on the landward (Aguan Valley)
side of the range in addition to the tectonic uplift of the range.
Of the many rivers and streams draining the north flank of the
range compiled in Figure 2.7A, the larger Río Cangrajal and Río Papaloteca have
penetrated farthest into the range and have captured tributaries draining the
south flank of the range. This southward penetration is evidenced by a stranded
paleo-divide north of and parallel with the modern divide (Figure 2.7A). The modern divide is characterized by a
low-relief surface interpreted as a high-level erosion surface. Similar, more extensive and possibly
correlative erosion surfaces have been described in central Honduras south of
the Nombre de Dios range and Aguan Valley (Helbig, 1959;
Manton, 1987; Rogers
et al., 2002; Chapter 1) (morphologic zones 2 and 3 in
Figure 2.4A).
Method. All lineaments interpreted from LANDSAT
imagery of the Nombre de Dios range are shown as thin, dotted lines on the map
in Figure 2.7B. In order to test whether
these lineaments represent previously unrecognized faults or whether they are
inactive faults or some other form of layering (e.g., foliations, bedding
planes, sills and dikes in basement units, joints), I compiled twelve
longitudinal and representative river and stream profiles across the range (Figure
2.8). River reach length and crossings
of the river channels at 20 m contour intervals were derived from 1:50,000
topographic maps. The resulting twelve
profiles of north- and south-flowing streams display elevation and gradient
from the modern drainage divide to the mountain front.
Figure 2.8. Stream profiles - Nombre de Dio Range
Results. North-flowing streams have upwardly convex
longitudinal profiles indicative of tectonic uplift of the Nombre de Dios range
that is outpacing the rate of fluvial downcutting (cf. Hovius, 2000, for a
comprehensive summary of tectonic effects on longitudinal river profiles) (Figure
2.8). On the plots in
Figure 2.8, the
thick line represents the elevation of the river or stream channel (vertical
scale in kms to left) crossing the 20 m contour interval (small squares)
compiled from 1:50,000 topographic maps.
Elevation and reach length data are used to calculate the gradient for
each segment of the river or stream
by taking the first derivative of the profile curve:
s = -dH/dL
where s is slope, H is height and L is
distance (Hack,1957). The slope or gradient for each stream reach is
plotted as the thinner, dashed line in Figure 2.8. The vertical scale of gradient in
meters/meters is shown to the right.
Parts of the stream or river with local areas of steep gradient are
shown as the spikes or “knickpoints” in the thin, dashed line. The cataracts common in bedrock rivers with
step-pool morphology are smaller than the 20 m contour interval of the
topographic maps and are not apparent on the plots.
All knickpoints are plotted in map view as small black dots on
the tectonic map of the Nombre de Dios range in
Figure 2.7B in order to compare
the locations of the knickpoints with major, mapped faults (heavy lines in
Figure
2.7B) and with topographic lineaments interpreted from the LANDSAT imagery
(thin lines in Figure 2.7B).
Correlation between the locations of faults and lineaments with
knickpoints are indicated by the “L” on the gradient plots in
Figure 2.8. The “S” on the plots in
Figure 2.8 indicates
locations where the streams or rivers cross the upper level, low relief erosion
surfaces shown in gray on the map in Figure 2.7A. The alignment on many of the profiles between
lineaments and knickpoints suggests that many of the lineaments may be active
faults that are uplifting the stream or river channel. Moreover, faults/lineaments commonly occur
downstream from upwardly convex profiles indicating reaches of the river
undergoing active uplift (cf. Figure 2.8, profiles 3, 5, 6, 7, 8, 9, 10, and 11). This suggests a range bounding fault in the
knickpoint area is contributing to the upstream uplift of the upthrown block of
the range. The coincidence of
knickpoints, upwardly convex river profiles, lineaments, and known faults makes
it unlikely that the knickpoints are the result of resistant lithologies in the
river channels (Hovius, 2000). Annual
rainfall in the Nombre de Dios range exceeds 3 meters per year (periodically in
the form of intense tropical storms and hurricanes, such as Mitch in 1998) so
it is likely that the erosive power of the rivers and streams exceeds rock
resistance.
The parallel trend of the lineaments with known, active faults
like the range-bounding La Ceiba and Rio Viejo fault zones suggests that the
Nombre de Dios range is being pervasively and internally sheared by
left-lateral faulting (Figure 2.7B). The
pervasiveness of the shearing may reflect the fact that the trend of active
faults is roughly parallel to the foliations and strike of older basement
structures (Manton and Manton, 1999) (Figure 2.5B).
The river profile method can be used to constrain the active
vertical uplift of the river. In
combination with this method, I also mapped apparent lateral offsets of river
channels in order to provide constraints on the horizontal slip on areas
exhibiting vertical uplift. Four
examples of apparent left-lateral offsets ranging from 1.5 to 2.4 km are compiled
from LANDSAT imagery and shown on Figure 2.9A-D. Bedrock-confined channels of the Río
Papaloteca and Río Lis Lis are deflected left-laterally where the rivers cross
the Río Viejo fault (2.4 km – Figure 2.9A) and the La Ceiba fault (1.5 km –
Figure
2.9B) respectively along the steep, northern mountain front of the Sierra Nombre de Dios. The bedrock-confined
channels of the Río Lepaca and Río Pimineta are deflected left-laterally 1.7 km
and 2.1 km, respectively, where they cross the Lepaca fault along the southern
mountain front of the range (Figure 2.9C, D).
These data suggest that pervasive left-lateral shearing is active and
accompanies the active vertical uplift constrained by the river profiles shown
in Figure 2.8.
Figure 2.9. Example of fault offsets of rivers
Method. I constrain the active tilting of multiple
fault blocks comprising the actively uplifting Sierra Nombre de Dios by
analyzing drainage basin asymmetry, a geomorphologic method to determine the
direction that a drainage basin has been tilted (Gardner et al., 1987). Drainage basin asymmetry is calculated as an
asymmetry factor (AF) by the equation:
AF=100(Ar/At)
where Ar is the drainage area on the
downstream right-side of the trunk stream and At is the total drainage area.
Drainage basin asymmetry was calculated for 21 north-draining watersheds
and 23 south-draining watersheds of the Sierra Nombre de Dios that are
summarized in Figure 2.10.
Results. Watersheds draining to the Caribbean of the
western Sierra Nombre de Dios are tilted westward while in the central section
of the range watersheds are tilted to eastward (Figure 2.10). The watersheds of the eastern part of range
near Trujillo draining to the Caribbean
Sea are tilted westward with the exception of the easternmost
watersheds.
Figure 2.10. Tilt analysis of Nombre de Dios range
Asymmetry of watersheds draining to the Aguan valley in the
western part of the range is complicated by the southward migration of the
drainage divide (Figure 2.7A). The
central watersheds draining to the Aguan valley originate in or cross a
topographic uplift centered on the Lepaca fault zone (Figure 2.10). Drainage basin asymmetry reflects an
eastward-plunging anticlinal uplift centered along the left-lateral Lepaca
fault. Half of the eastern watersheds
draining to the Aguan valley display a tilt to the west.
Tilting of the Aguan Valley. Meandering alluvial rivers
respond to tilting perpendicular to the channel by migrating in the direction
of tilt (Leeder and Alexander, 1987). The Río
Aguan has migrated south, toward the La Esperanza fault, in the eastern part of
its valley indicating that the fault is mainly downthrown to the northeast and
is active (Figure 2.10). In the central
part of the Aguan valley, the Río Aguan flows against the northern fault-bounded
valley wall just south of the Lepaca uplift indicating that the fault bounding
the northern edge of the valley is downthrown to the southeast. In the western alluvial Aguan valley, the
river displays no obvious migration toward known faults.
I propose several fault blocks coincident with and underlying the
regions of topographic uplifts named on Figure 2.10:
1) The Pico Bonito
restraining bend topographic uplift.
The uplift of this feature is inferred to be a convergent restraining
bend formed at a right-step between the La Ceiba and Rio Viejo left-lateral
fault zones. Active uplift is supported
by the river convex-upward river profiles shown in
Figure 2.8 and left-lateral
displacement is inferred from left-lateral river channel offsets compiled in
Figure
2.9A-B. Asymmetric watersheds shown in
Figure
2.10 indicate that the bend area mainly plunges westward.
2) La Ceiba topographic
uplift. The uplift of this feature
appears to be a peripheral effect related to the smaller, but fault-controlled
Pico Bonito restraining bend (Figure 2.10).
Tilt directions are variable but generally trend about a north
axis.
3) Lepaca topographic
uplift. The uplift of this feature
appears related to left-lateral motion along the Lepaca fault and the formation
of a large anticline parallel to the fault trace. The fault may accommodate motion along the
largely buried fault along the northern edge of the Aguan
Valley. Tilt directions vary across
the structure.
4) Trujillo topographic uplift. The uplift of this feature appears related to
left-lateral oblique motion along the east-northeast extension of the Rio Viejo
fault zone.
The regional geomorphology of this onland area as summarized on
Figure
2.10 is consistent with transtension in an NNW-SSE direction at right-angles to
the overall trend of the plate boundary faults (Figure 2.3A). The area lies within the region where
Caribbean plate motion diverges between 5 and 10 degrees from the plate
bounding faults (Figure 2.3B), so strain partitioned faults with major
strike-slip and minor normal displacement are predicted. Inclusion of the north coast region of Honduras
in the offshore Honduran borderlands province allows the borderlands to
maintain a width of approximately 150 km for its 600 km length (Figure 2.3A). Faults, topography, and river profiles show
that the Sierra Nombre de Dios is a fault-bounded horst while the Aguan valley
is a half-graben dominated by the La Esperanza normal fault (Figure 2.10). The Sierra Nombre de Dios horst is composed
of blocks with morphology consistent with localized uplift along restraining
bends of left-lateral strike-slip faults.
The La Ceiba, Aguan, and La Esperanza faults appear to be both normal
and left-slip faults, while the Río Viejo and Lepaca faults display only
left-slip indicators. The pattern of
late Quaternary river meanders in the Aguan
Valley indicates that down-to-the-northeast
throw on the La Esperanza fault is the main active control on the subsidence of
the rift.
Regional gravity field. GEOSAT marine gravity data compiled in
Figure
2.11A from Smith and Sandwell (1997) shows the large-scale basement structure
of the Honduran borderlands. To the
north of the borderlands, the Cayman trough shows a large gravity low, associated
with thin oceanic crust produced at the Mid-Cayman spreading center between 49.3 and 0 Ma (ages of magnetic
anomalies from Leroy et al. (2000) shown in
Figure 2.2B). A gravity high centered on the active
spreading center reflects active deformation of the adjacent crust, thicker
oceanic crust, or possible mantle upwelling related to the short, 100-km-long
spreading center. The Nicaraguan
carbonate platform, located south of the Honduran borderlands, is characterized
a smooth gravity high.
Figure 2.11 Bathymetry and topography of the Honduran Borderland
In comparison to both the Cayman trough and Nicaraguan Rise, the
Honduran borderlands exhibits a more disrupted gravity field produced by the
existence of elongate, fault-bounded, margin-parallel basins and ridges
characteristic of continental borderland provinces (Gorsline and Teng,
1989). Twin gravity highs/basement
ridges (outlined by yellow dashed lines in Figure 2.11A) extend along the
southern, faulted margin of the Cayman trough while a gravity low showing a
basin extends along the edge of the Nicaraguan Rise. A gravity/basement high, the Patuca Ridge, is
present within this basinal structure and projects into the Nombre de Dios
range of northern Honduras. The southern part of the Patuca
Basin projects into the Aguan
Valley (both gravity features are
shown by yellow dashed lines in Figure 2.11A).
Bathymetry. Predicted bathymetry (Smith and Sandwell,
1997) of the region derived from the Geosat gravity data shown in
Figure 2.11A
is compiled along with the GTOPO30 onland topography. Early workers like
Kornicker and Bryant (1969)
had considered the basement and bathymetric high along the southern edge of the
Cayman trough a single ridge called the Bonacca Ridge. Because regional gravity and bathymetry data
show two parallel ridges, I name the parallel features the North and South
Bonacca Ridges (locations shown on Figure 2.11B).
Kornicker and Bryant (1969) also considered
the Tela Basin
north of the Nombre de Dios range of northern Honduras
to extend as a continuous feature to the northeast of Honduras. I have renamed the basins in the eastern area
to more accurately reflect the complex borderlands basins seen on
Figure 2.11A
and B. The Bonacca
Basin is used for the basin
separating the North and South Bonacca Ridges and the Patuca
Basin is used for the basin between
the South Bonacca Ridge and the Nicaraguan Rise (locations shown on
Figure 2.11B). The Patuca and Tela
Basins are roughly colinear but are
separated by a bathymetric and structural saddle northeast of Honduras.
Along-strike correlation
of submarine features.
Figure 2.12
shows three bathymetric profiles across the Honduran borderlands using the map
data in Figure 2.11B that demonstrates how the newly named basement and
bathymetric features described above are correlated across the length of the
borderlands province. The locations of active
faults identified from marine seismic profiles discussed below are shown by
arrows.
The twin North and South Bonacca Ridges can be identified on
profiles A and B but merge on profile C to form the single, emergent Bay
Islands of northern Honduras
(Ave Lallemant and Gordon, 1999). The Swan
Islands restraining bend, an active
localized restraining bend structure on the Swan
Islands fault zone (Mann et al.,
1991) (Figure 2.2A), appears on profiles A and B but disappears along-strike on
profile C. The Patuca basin forms a deep
basin on profile A, is interrupted by the appearance of the Patuca basement
high on profile B. The northern half of
the Patuca basin projects westward into the Tela basin, the Patuca high
projects into the Nombre de Dios range, and the southern half of the Patuca
basin projects into the Aguan Valley. The oerall effect of the parallel ridges and
valleys of the Honduran borderlands is to widen from east to west (Figure 2.12).
Figure 2.12. Bathymetric Profiles of the Honduran Borderland
The flat area of the Nicaraguan carbonate platform on profiles A
and B projects into an area of exposed basement and Mesozoic rocks of northern Honduras. This region has been previously interpreted as an eroding plateau
(morphologic zones 3 and 4 in Figure 2.4A).
The subaerial topographic peaks shown profile C south of the Aguan
Valley (AV) include the 1600-m-high Botaderos range (B) and the 2304-m-high
Agalta range (Chapter 4), both of which are proposed to be the remnants of the
eroded Central American plateau (Rogers et al., 2002; Chapter 1). High-level erosional surfaces of the
northern flank of the Botaderos range south of the Aguan valley are mapped from
LANDSAT imagery on Figure 2.7B.
Objectives and marine geophysical
data used. The objectives of this
section are to use previously unpublished MCS seismic profiles across the
Honduran borderlands collected during the UTIG CT1 survey in 1979 combined with
SeaMARC II sidescan sonar images and single-channel seismic lines of the
seafloor and shallow subsurface collected in 1989 (Rosencrantz and Mann, 1991;
Mann et al., 1991) to document: 1) the locations of active faults related to
Caribbean-North American plate motion; 2) how these active faults control the
observed bathymetry and structure of the borderlands area seen on three
profiles on Figure 2.13; 3) to determine the type of displacement on those
active faults and whether they partition present-day plate motion in the manner
predicted on Figure 2.3; and 4) to correlate well logs with dated stratigraphy
to these lines in order to understand the initiation and history of the
offshore basins. The locations of the MCS and SCS seismic lines used in the
study are shown on Figure 2.2B; the footprint of the sidescan image is shown on
Figure 2.2A.
Figure 2.13. Multichannel seismic lines of the Honduran Borderland
For this study, I consider any fault breaking young seafloor sediments
to be “active”. The sidescan imagery
data was used in conjunction with the MCS profiles to insure that the fault
extended to the seafloor. The sidescan
imagery is also useful for seafloor mapping because it has three-dimensional
coverage of the seafloor and allows one to trace commonly curving and anastomosing
fault zones between widely separated two-dimensional seismic profiles.
MCS profile CT1-8a and 8b. This line trends north-northwest and provides
an excellent cross section to a depth of about 4 seconds two-way travel time of
the eastern part of the Honduran borderlands in the area southeast of the Swan
Islands restraining bend (Figure
2.2B). Seven active faults are recognized:
the two active faults to the north are parallel strands of the Swan Islands
fault zone discussed in detail by Rosencrantz and Mann (1991)
and the five active
faults to the south bound the Bonacca and Patuca basins and separate them from
the intervening basement highs, the North and South Bonacca ridges (Figure 2.13A). Correlation of line CT1-8 with the sidescan
image in Figure 2.14B shows that a linear fault running down the crest of the South
Bonacca ridge forms a major scarp on the seafloor and therefore is
presumed to be an active feature. At
depth, this fault forms one edge of a large basement ridge with normal faults
dipping away from its center (Figure 2.14C).
Figure 2.14: Multi-channel
seismic line - CT 8
Both basinal areas are symmetrical full grabens with bounding,
apparently normal faults dipping inward beneath the basin center. Both grabens contain about 0.5 second of well
stratified sedimentary fill. Although it
acts as a horst separating the Bonacca and Patuca basins, the South
Bonacca ridge is underlain by a thick section of basinal rocks and
therefore appears to be uplifted graben that has become uplifted and isolated
from the effects of modern sedimentation.
Three distinct seismic sequences are identified on this and the
other two seismic profiles and are numbered 1, 2, and 3 on
Figure 2.13. The lowest sequence 1 consists of tabular,
tilted reflectors up to 1.0 seconds TWT in thickness. The base of this sequence merges with the acoustic basement due to loss of energy
with depth and therefore the nature of the contact between sequence 1 and
basement is not clear. Seismic facies
associated with sequence 1 include a semi-continuous high amplitude facies consisting
of subparallel, semi-continuous high amplitude reflections with high amplitude
reflections dominant suggesting sand-rich channel-lobe complexes.
Sequence 2 consists of a
wedge-shaped, isolated, fault-bounded package of reflectors indicative of long-term
vertical motion on the adjacent fault scarps.
Seismic facies associated with sequence 2 include a semi- continuous high amplitude alternating with
low amplitude facies containing concave-up high amplitude reflections
and wedge-shaped external configuration suggesting partially channelized
turbidite/lobe complexes. For example,
on Figure 2.14C, the wedge-shape of sequence 2 is imaged adjacent to the
presently active basement block (South Bonacca Ridge) and linear fault shown in
Figure 2.14B. On
Figure 2.14D crossing
the Patuca basin, sequence 2 forms two similar wedges against normal faults
dipping southeast. Sequence 3 is the
uppermost unit and provides a tabular fill of the basinal areas of the Bonacca
and Patuca basins. A
progradational/shingled facies displays basinward progradation geometry of reflectors
that changes downdip into semi-continuous high amplitude facies.
Interpretation of seismic
sequences. Based on its geometry
alone, I interpret tabular sequence 1 as a pre-rift unit deposited prior to the
formation of the full and half-grabens seen on the line. The parallel, horizontal reflectors indicate
that this unit was deposited as a flat layer above a continental or island arc
crust at the northern edge of the Nicaraguan Rise. I interpret wedge-shaped sequence 2 as a
syn-rift unit that accompanied the main phase of rifting and fault scarp
formation. Sequence 3 is interpreted as
a post-rift unit. Fault control on
tabular sequence 3 is less strong than on the wedge-shaped units of sequence
2. Nevertheless, some fault scarps
penetrate unit 3 to the seafloor and indicate that faulting continues to the
present-day although generally not forming rifts with clastic wedge units (cf.
Figure
2.14C, D). The source of sedimentation
of sequence 3 is either peri-platform carbonate ooze derived from carbonate
production on the Nicaraguan Rise (cf. Figure 2.14D) or terrigenous clastic
material derived from major river systems of northern and eastern Honduras
(Figure 2.11B).
This proposed rift interpretation is similar to that proposed by
Pinet (1975) based on his study of single-channel seismic reflection study in
the area of the Tela basin (Figure 2.2A).
Pinet (1975) documents two stratigraphic sequences above an angular unconformity
disrupted by normal faults that form horsts and grabens filled by turbidites
(Figure. 2.2A). Noting that these sequences
are uniform in thickness, Pinet (1975) inferred that the block faulting
postdates the deposition and proposed a Late Miocene-Pliocene age of deposition
and Pliocene normal faulting event based on correlation to onshore geologic
history.
MCS profile CT1-6a and 6b. This line trends north and provides an
excellent cross section to a depth of about 5 seconds two-way travel time of the
central part of the Honduran borderlands in the area southwest of the Swan
Islands restraining bend (Figure 2.2B). Five active faults are recognized: the three
active faults to the north are parallel strands of the Swan Islands fault zone
discussed in detail by Rosencrantz and Mann (1991) and the two active faults to
the south bound the South Bonacca ridge (Figure 2.13A). Active faults of the Swan
Islands fault zone are highlighted
by white arrows in Figure 2.15B.
Correlation of line CT1-6 with the sidescan image in
Figure 2.15B shows
linear fault scarps running down the en
echelon crest of the North Bonacca ridge. At depth, this fault forms one edge of a
large half-graben (Bonacca basin) with normal faults dipping to the southeast
and the asymmetric wedge-shaped seismic unit 2 (Figure 2.15C). Normal faults bounding the more symmetrical Patuca graben
on the south end of the seismic line also locally penetrate the seafloor (Figure
2.15D).
Figure 2.15: Multi-channel seismic line - CT 6
Interpretation of seismic
sequences. Both the Bonacca and
Patuca rifts are much wider on line CT1-6ab than on line CT1-8ab to the west
supporting the basic observation of the north-south widening of the Honduran
borderlands from east to west seen on the regional bathymetric profiles in
Figure
2.12. As on line CT1-6ab, seismic
sequences 1-3 are observed and support the previous interpretation of a
pre-rift, rift, and post-rift history.
On line CT1-8ab, the drape character of post-rift sequence 3 into low
areas along the axes of the two rifts appears less disturbed by faulting than
on line CT1-6ab.
MSF profile CT1-3b. This is the westernmost MCS line and trends
in a northward direction across the borderlands (Figure 2.2A). Major contrasts with the previous two lines
include: a steepening of the scarp of the Swan
Islands fault zone at the edge of
the Cayman trough; a series of six half-grabens formed along normal faults
dipping southward; and tilting and faulting of sequence 3 suggesting that the
rift process is ongoing (Figure 2.13C).
Active faulting along the Swan
Islands fault zone is very
pronounced and is indicated by the white arrows in
Figure 2.16B.
Figure 2.16. Multi-channel seismic line - CT 3
Single-channel profile
showing active faults in the Tela basin.
SCS line 71 collected in 1989 along with the sidescan data trends east
to west across the Tela basin (Figure 2.17A).
Line 71 images two active fault scarps seen as faint lineaments on the
sidescan image (Figure 2.17B). In the
subsurface these faults control small east-northeast-trending rifts within the Tela
basin (Figure 2.17C).
Figure 2.17. Single channel seismic lines 171, Tela basin
Objectives and methods. Four oil exploration wells (DGE unpublished
report, 1987) were drilled at shelfal to slope water depths (66-337 m BSL)
along the southern edge of the Honduran borderlands adjacent to the Patuca rift
basin (Figure 2.2B). The locations of the
wells are shown on Figure 2.18 and allow correlation between the dated major
units in the wells and seismic units 1-3 observed on CT-1 lines 8ab, 6ab, and
3ab. Sonic log data available from Punta
Patuca-1 (DGS unpublished report, 1987) was used to converted the well log to
two-way travel time and this time section was then correlated to the south end
of MSC profile CT1-6ab crossing the Patuca basin (Figure 2.15D).
Figure 2.18. Well logs of the Honduran borderlands
The well log correlation made in this manner is problematic
because the Punta Patuca-1 well was drilled at outer shelf water depths of 170
m in an area outside of the main zone of rifting in the deeper water Patuca
basin. For that reason it is difficult
to clearly relate the well logs to the pre-, syn-, and post-rift sequences seen
on the deeper water parts of the seismic lines (Figure 2.15C). Despite this problem, all four wells show a
similar history suggesting that the general timing and lithologies of the wells
provide a reliable insight into the rift history of the Honduran
borderlands.
Major lithologic units
observed in wells. The four wells
were drilled to depths of 2397 to 3790 m and penetrated basement lithologies
consisting of late Cretaceous black slate and quartzite (Castilla-1), late
Cretaceous mudstone and volcanic rocks (Castana-1), late Eocene redbeds (Punta
Patuca-1), and early Cretaceous sandstone and shale (Gracias a Dios-1). The precise correlation of these rocks to
outcropping units in either the Bay Islands
or the Gracias a Dios range is unclear.
One possibility is that these rocks are metamorphosed Valle de Angeles Formation,
of late Cretaceous age (Chapter 4). This unit
commonly contains a diverse mix of clastic and volcanic rocks types and is generally
metamorphosed in northern Honduras
and the Bay Islands.
Ages of basal unconformity,
pre-rift, syn-rift, and post-rift sedimentation. All four wells penetrate a basal unconformity
overlying a variety of rock types that are dated biostratigraphically in the
well report and range from early Cretaceous through late Eocene (Figure 2.18). Clastic rocks of Middle Miocene age overlie
the unconformity and range in water depth from subaerial (?) redbeds to
coastal/shelf. I interpret this contact
as middle to early late Miocene transition from the acoustic basement to the
overlying tabular pre-rift seismic unit 1 (Figure 2.13).
Syn-rift seismic unit 2 correlates to a late Miocene section of
sandstone/shale with minor limestone in the Punta Patuca well and this time is
taken as the age for the wedge-shaped rift units seen on the MCS lines in
Figure
2.13. The environment based on well
biostratigraphy (Figure 2.18) remains coastal/shelf; so despite the rifting
event the basin environment remained shallow.
Post-rift seismic sequence 3 correlates to a Plio-Pleistocene
section of mudstone deposited at bathyal depths. The upper 300 m of the well was not logged (Figure
2.18). The Castaña-1 drilled near the
mouth of the Río Aguan and the bathymetric high that separates the Tela and
Bonacca basins, penetrated 400 m of limestone suggestive of localized clastic
bypass and carbonate deposition on the bathymetric high. Numerous bentonite horizons along with
volcanic fragments found in the middle Miocene clastic shelf strata of Punta
Patuca 1 and Gracias A Dios 1 represent airfall and fluvial linkage to the
ignimbrite province of the Miocene arc of Central America
(Sigurdsson et al., 2000).
In this chapter, I have presented geologic, earthquake, marine
geophysical, and remote sensing data from on- and offshore areas to show that
Neogene to Recent transtensional deformation produced at the North America-Caribbean
plate boundary in northern Central America exhibits two distinct structural
styles: plate boundary-normal
(east-west) extension in the western study area (onshore 375-km-long plate
boundary segment of basin and range morphology in western Honduras and southern
Guatemala); and plate boundary-parallel
(NNW-SSE) transtension in the eastern study area (the 600-km-long plate
boundary segment of margin-parallel ridges and basins onshore in northern
Honduras and in the offshore Honduran borderlands region).
In the western area of
transtension, these north trending normal faults have been interpreted by
previous workers as: 1) intraplate
deformation and block rotations about a highly arcuate, convex-southward
left-lateral strike-slip fault system (Plafker, 1976;
Burkart and Self, 1985);
and as 2) fault termination structures
related to the termination of left-lateral slip of the Motagua fault zone
(Langer and Bollinger, 1979; Guzman-Speziale, 2001). To the east in the Honduran borderlands and
Nombre de Dios range and Aguan Valley
of northern Honduras,
previous work has not been presented for regional transtensional
deformation. In the chapter, I have
shown that east-northeast trending faults in both the Nombre de Dios range and
the offshore borderlands are oblique-slip and extend in a zone about 150 km
south of the Swan Islands fault, the main plate boundary strike-slip
fault.
Detailed comparison of the strikes of mapped onland faults in
western Honduras and northern Guatemala and the trend of the GPS-derived
Caribbean plate vector shows that plate boundary-normal (east-west) extensional
rifting occurs in the western area when the angle of divergence between the
main plate boundary fault (Motagua fault zone) and the GPS-derived Caribbean
plate vector is equal to or greater than 10º (Figure 2.3). Marine geophysical mapping of offshore faults
and onland faults in Honduras
and the offshore Honduran borderlands shows that this style of plate
boundary-parallel (NNW-SSE) transtension occurs when the angle of divergence
between the main plate boundary fault (Swan
Islands fault) and the Caribbean
motion vector is between 5 and 10º. A
narrow, 35-km-wide tectonic transition area in north-central Honduras
separates the plate boundary-normal rifts of western Honduras
and from the plate boundary-parallel rifts in northeastern Honduras
and the offshore Honduran borderlands (Figure
2.3).
Objective and method. A fundamental observation is that the
individual rifts of the Honduran borderlands widen from east to west (Figure
2.12). This observation could be
explained by an east to west increase in the rate and amount of extension as
predicted by the plate tectonic model shown in
Figure 2.3. In order to verify an east to west increase
in extension as predicted by the model in Figure 2.3, I used the vertical shear
fault restoration method of Rowan and Kligfield (1989). This method assumes that hanging wall
material drops down vertically to fill the void produced by extension. The top of seismic sequence 1 (Figure 2.13),
the pre-rift sequence known from the four Honduran exploration wells to be a
clastic section of Middle Miocene age (Figure 2.18) forms the datum for the
reconstruction.
Each of the three MCS lines was first converted from time to
depth using the sonic log data from the Punta Patuca-1 well that was described
above (Figure 2.18). The restoration did
not include the vertical throw on the large scarp of the Swan Islands fault
zone that forms the main plate boundary strike-slip plate boundary (Figure 2.13),
because earthquake focal mechanisms compiled on
Figure 2.2A indicate that this
fault currently behaves as a vertical, left-lateral strike-slip fault. Sources of error in these fault restorations
include: 1) the fact that the trend of the three seismic lines is not exactly
perpendicular to the trend of the mapped normal faults as seen in
Figure 2.2B;
and 2) only one of the three lines, CT1-8ab, completely crosses all of the faults
of the borderlands belt (Figure 2.2B).
Results for the Honduran
borderlands. Rifts along the
easternmost line CT1-8ab record 11.0 km or 11.8% extension; rifts along central
line CT1-6ab record 17.2 km or 16.7% extension, and rifts along the westernmost
line CT1-3ab record 9.3 km or 21.3% extension (Figure 2.19). The results (percent extension) support the
tectonic model in Figure 2.3 predicting a progressive east to west increase in
extension.
Figure 2.19: Restored cross section of the Honduran borderlands
The GPS-based Caribbean plate vector of
Figure 2.3 operating over
the time interval of active rifting inferred from the exploration wells in
Figure
2.18 (Late Miocene to recent – 12 Ma) would predict the following amount of
extension at longitude 85ºW (roughly the location of line CT1-3ab and CT1-6ab)
assuming that the plate boundary motion is completely partitioned: 29.7 km of
plate boundary-normal opening or 24.7% extension over the 150-km-wide Honduran
borderlands (Figure 2.19). Using
variable rates and directions of Caribbean plate motion
based on marine magnetic anomalies and as discussed in the next section, a
value of 30.1 km of plate boundary-normal opening is derived. This is equivalent to 25.1% extension over a
distance of 150 km the past 12 Ma. Extension
values derived from the seismic lines, from the extrapolated GPS plate
velocity, and the plate restoration are compared graphically on
Figure
2.19. The smaller values (11.8 to 21.3
percent) obtained by the seismic lines might be explained by several factors:
1) motion is not completely partitioned as predicted by the model in
Figure 2.3 (i.e., more motion
is taken up by strike-slip faulting on the Swan Islands fault zone or on
strike-slip faults within the rifts and basement ridges than predicted by the
model); 2) two of the lines, CT1-3 and CT1-8, do not completely cross all of
the normal faults of the Honduran borderlands belt; and 3) the trend of the
three seismic lines is not exactly perpendicular to the trend of the mapped normal
faults as seen in Figure 2.2B.
The uncertainties of extension estimates for the Honduras
borderlands derive from the conversion of time to depth and in estimates of
fault dips at depths. The average
velocity for the syn-rift sequence 2 is 3.0 km/sec with a range of 2.5 to 3.2
km/sec and the late syn-rift sequence 3 is 2.2 km/sec with range of 2.0 to 2.4
km/sec (DGS, 1987). To address this
issue, end member velocity estimates were applied to several faults to
determine the range of fault dips and these ranges were extrapolated to the
entire profile to provide an error estimate of the extension derived from
seismic profiles. The slower velocities
yielded a shallower depth to the pre-rift sequence 1 of approximately 17
percent and faults dip decreased between 15 to 20 percent depending on
thickness of the syn-rift sequence. The
velocity adjustments applied to the thin post-rift sequence have negligible
effect on fault geometry. This change in
fault geometry decreased the total amount of extension by 15 percent or to 9.4
km (9 percent extension) for line CT1-8, 14.6 km (14.2 percent) for line CT1-6,
and 7.9 km (18.1 percent) for CT1-3.
Applying the higher velocity estimate resulted in a slight increase in the
depth of the pre-rift sequence. Higher
velocity estimates yield less than a five percent increase in fault dip, an
amount that increases the extension estimates by less than one percent.
Comparison of model to previous fault striation results from the Bay
Islands.
The Bay Islands
forms the emergent continuation of the North Bonacca Ridge and is therefore a
critical area for constraints on the timing and nature of extension across the
Honduran borderlands (Figure 2.2A).
Paleostress analysis on striated fault planes by Avé Lallemant and
Gordon (1999) from outcrops on Roatan
Island, the largest of the Bay
Islands, documents five phases of
brittle deformation since 36 Ma. Following
east-west shortening (F1), north-south Miocene extension is
represented by conjugate north-northwest-trending left-lateral and
east-northeast-trending right-lateral strike slip faults (F2), and
west-northwest to east-northeast trending conjugate normal faults (F3). Subsequent east-west extension occurred along
conjugate northeast to east and northwest to west trending left-lateral
strike-slip faults F4 and north to north-northwest-trending
conjugate normal faults F5.
The north striking F5 normal faults are the island’s most
pervasive set of faults as shown by aerial photography delineation of north-trending
lineaments that cut the uplifted Quaternary reefs on the NW shore of the island
(Figure 2.5b of Avé Lallemant and Gordon, 1999).
Miocene age north-south extension of F2 and F3
agrees with my marine structural and stratigraphic data from rifts in the Honduras
borderlands. However, later east-west extension of their F4 and F5
events is not obvious on the structures mapped using the seismic lines and
sidescan images. One possibility is that
the recent east-west extension of Roatan may be the result of local east-west
flexure of the island in the otherwise north-south extending borderlands.
Results for the north-trending
rifts of Honduras and Guatemala. Application of the current GPS-derived plate
vector to this 340-km-wide zone of rifting (centered on longitude 89ºW) yields
an extensional amount of 45.0 km of plate boundary-normal slip for the last 12
Ma (age of deformed offshore strata), or 15.3% extension. Application of the changing plate
reconstruction vectors to this 340-km-wide zone of rifting (centered on
longitude 89ºW) yields an extensional amount of 47.8 km of plate
boundary-normal slip for the past 12 Ma, or 16.3% extension.
Objective and methods. An important question is: how have the
transtensional features observed onland in Honduras
and offshore in the Honduran borderlands evolved through time. In
Figure 2.20, I present six quantitative
plate reconstructions made using the PLATES reconstruction software to
illustrate the complex evolution of this transtensional plate margin segment
for the past 20 Ma (early Miocene). The North
America plate is held fixed in the reconstructions. The present-day coastline of Central American
is shown in all the reconstructions as a frame of reference. Two vectors are shown on each reconstruction:
1) the current GPS-derived Caribbean plate motion vector
(DeMets et al., 2000), and 2) the plate vector derived from magnetic anomalies
in the surrounding oceanic basins including the Cayman trough (Rosencrantz, 1994). The reconstruction methodology results in a
20 m.y. average Caribbean plate vector of 21.4 mm/yr at
azimuth N78E relative to North America. That is slightly greater than the 18.5 mm/yr
at azimuth N77E GPS measured rate at 15oN, 85 oW. A small inset map at the bottom of each
reconstruction summarizes the major faults that are known from previous
geologic studies to be active during that time along with the angle between
these faults and the magnetic anomaly-derived plate vector inferred for that
time period.
Figure 2.20. Tectonic model for the Honduran borderlands
Present (0 Ma). At the present, the main left-lateral faults
of the plate boundary zone are the Motagua (Plafker, 1976) and Swan
Islands fault zone (Rosencrantz and
Mann, 1991) (Figure 2.20A). The Polochic
fault in Guatemala,
although active, is not treated in this analysis. The angle between the plate vector and the
faults increases from east to west. In
the Honduras
borderlands area, the angle is between 5 and 10º and in the area of north-trending
rifts of Honduras
the angle is greater than or equal to 10º.
Rifting of the Honduran borderlands is decreasing at this time
(seismic sequence 3 in Figure 2.13).
This decrease in rifting may relate to this area moving into an area of
less plate divergence. A zone of
inactive, north-trending rifts is described from an area of east-central Honduras
(IR in Figure 2.20A). These rifts were
likely to have been generated in an area of greater plate vector divergence in
the west and rafted into their present position as the plate moved
eastward.
Early Pliocene (4 Ma). At this time the basic fault geometry of the
present-day remains the same (Figure 2.20B).
In the Honduran borderlands, well data indicates that the rift phase
that had begun in the late Miocene is still occurring. According to
Burkart (1983), motion had
shifted from Polochic fault to the Motagua fault by this time. The orientation of the highly arcuate Motagua
fault would increase the angle of divergence and amount of strain partitioning
in the area of north-south rifting.
Late Miocene (8 Ma). During this period,
Burkart (1983) suggests
that the main plate boundary fault in Central America
shifted from the Polochic fault to the Motagua fault. About 130 km of offset has been documented on
the Polochic yet only a “few tens of kilometers” has been proposed for the
Motagua (Burkart, 1983). The north-south
trending rifting in western Honduras
commenced prior to this time, after 10.5 Ma (Gordon and Muehlberger, 1994),
resulting in rift valley-fill deposits containing early Hemphilian-age (9-6.7
Ma) mammalian fauna (Webb and Perrigo, 1984).
According to the offshore wells, late Miocene is the time that the rift
basins of the Honduran borderland initiated (Figure 2.13). It is possible that this shift of plate
motion to the more arcuate Motagua fault led to a rapid phase of transtensional
divergence manifested both in the formation of the north-south rifts and the
plate boundary-parallel rifts of the Honduran borderlands.
Middle Miocene (12 Ma). According to
Ritchie (1976), the Jocotan
fault was the main plate boundary fault at this time (Figure 2.20D). The reconstruction suggests that the Jocotan
fault would form a much straighter continuation of the Swan
Islands fault, than either the
Polochic or Motagua. The Middle Miocene
is the time of a major unconformity in the Honduran borderland between deformed
basement to Eocene rocks and overlying Middle Miocene, shallow-water clastic
sedimentary rocks (Figure 2.18). The
alignment of the Jocotan-Chamelecon faults with the La Ceiba fault suggests
that these faults formed the plate boundary.
Late Early Miocene (16 Ma)
and Early Miocene (20 Ma). The fault
geometry at this time is similar to that at 12 Ma (Figure 2.20D). The Jocotan fault was the main plate boundary
fault at this time (Figure 2.20E, F) and formed a much straighter continuation
of the Swan Islands
fault. Early Miocene deep marine
turbidites have been exposed on the Swan
Islands by restraining bend
tectonics (Mann et al., 1991).
The main conclusions of this chapter are as follows:
1) Geologic, earthquake, marine geophysical, and remote sensing
data from on- and offshore areas show that Neogene to Recent transtensional
deformation produced along the North America-Caribbean plate boundary in
northern Central America exhibits two distinct structural styles: plate boundary-normal (east-west) extension
in the western study area (onshore 375-km-long plate boundary segment of basin
and range morphology in western Honduras and northern Guatemala); and plate boundary-parallel (NNW-SSE)
transtension in the eastern study area (onshore and offshore 600-km-long
plate boundary segment of margin-parallel ridges and basins in the Nombre de
Dios range and Aguan Valley of northern Honduras and offshore Honduran
borderlands region) (Figure 2.2).
2) Detailed comparison of
the strikes of mapped onland faults in western Honduras and northern Guatemala
and the trend of the GPS-derived Caribbean plate vector shows that plate
boundary-normal (east-west) extension occurs in the western area when the angle
of divergence between the main plate boundary fault (Motagua fault zone) and
the GPS-derived Caribbean plate vector is equal to or greater than 10º (Figure
2.3). This oblique opening model differs
from previous interpretations like Burkart and Self (1985) that invoke block
rotations about the highly arcuate Motagua fault zone. GPS results from Central America
indicate coupling across the North America – Caribbean
margin in Guatemala
as the westernmost Chortis block is moving with North America
(C. DeMets, personal communication, 2003).
3) Marine geophysical mapping of offshore faults and onland
faults in Honduras
and the offshore Honduran borderlands shows that this style of plate
boundary-parallel (NNW-SSE) transtension occurs when the angle of divergence
between the main plate boundary fault (Swan
Islands fault) and the Caribbean
motion vector is between 5 and 10º (Figure 2.3). The amount of observed extension derived from
three seismic lines across the Honduran borderlands increases from west to east
in accord with the model predictions shown on
Figure 2.3.
4) A narrow, 35-km-wide
tectonic transition area in north-central Honduras
separates the plate boundary-normal north-trending rifts of western Honduras
and from the plate boundary-parallel rifts in northeastern Honduras
and the offshore Honduran borderlands (Figure 2.2).
5) Faults of the offshore Honduran morphologic and structural borderlands
extend onshore into the Nombre de Dios range and Aguan
Valley of northern Honduras
where subaerial styles of transtensional deformation are similar to those
documented in this chapter in the submarine Honduran borderlands (Figure 2.7). Tectonic geomorphology studies show pervasive
oblique-slip faulting with evidence for late Quaternary left-lateral offsets (Figure
2.9) and active uplift of stream networks (Figure 2.8).
6) Seismic data tied to wells in the Honduran borderlands (Figure
2.18) shows that plate-boundary related submarine faults in this region are
active, transtensional features that initiated in the middle Miocene with
filling of asymmetric half-grabens and continued through the
Pliocene-Pleistocene.
7) Quantitative plate reconstructions show that the
north-trending rifts of western Honduran rifts developed in response to
increased interplate divergence (>10º) as the western margin of the
Caribbean plate shifted from the left-lateral Polochic fault to the left-lateral
Motagua fault prior to 8 Ma.
Plate-boundary parallel rifts of Mio-Pliocene age also formed during a
period of increased plate divergence which has become increasingly strike-slip
and less extensional today.
Chapter 3: Late Cretaceous amalgamation of the
western Caribbean plate by collision between the continental Chortis
block and intraoceanic Caribbean arc and oceanic plateau
The northwest-verging Colon
fold-thrust belt of eastern Honduras
and Nicaraguan Rise and adjacent Siuna belt of northern Nicaragua
document a late Cretaceous collisional event between a south-facing passive
continental margin of the Chortis block of northern Central America
and an eastward and northeastward-moving, early to late Cretaceous Caribbean arc
system. This previously unrecognized,
thin-skinned arc-continental collisional zone, termed here the Colon
fold-thrust belt, can be traced for an along-strike distance of 350 km across
northern Central American and the Nicaraguan Rise. The deformed belt is described in this
chapter in three, colinear segments: 1) a 150 km-long belt of remote but
well-exposed Jurassic-Late Cretaceous rock outcrops in the Colon Mountains in eastern Honduras described from original
geologic mapping presented in this chapter; 2) a 75-km-long subsurface belt of
Jurassic-Late Cretaceous rocks beneath the Mosquitia
coastal plain of Honduras that has been mapped using onland seismic
reflection studies and exploration drilling for oil; 3) a 75-km-long subsurface
belt of Late Cretaceous to Eocene rocks on the eastern Nicaraguan Rise known from offshore seismic reflection
studies and exploration drilling for oil.
The belt of folding extends 50 km to the southeast into northern Nicaragua.
In addition to the along-strike continuity of the deformation
front and associated folds, all three deformed belts share similar timing of fold-thrust deformation (Colon Mountains: post-Campanian (Tabacon
Formation); Mosquitia plain: post-late Cretaceous (Valle de Angeles Formation);
Nicaraguan Rise: post-Cretaceous and possible extending to the end of Eocene);
and fold-thrust structural style
(southeastward-dipping thrusts and related northeastward-verging folds that
structurally elevate Cretaceous rocks).
The structural position of the Siuna belt of oceanic island arc origin
to the south of the Colon fold-thrust belt; its association with calc-alkaline
volcanic rocks of the Caribbean arc, and its Campanian (75 Ma) emplacement age
all suggest that the Siuna belt was overthrust to the northeast on the hanging
wall of the Colon fold-thrust belt. I
propose that the Siuna belt formed the leading edge of the east- and
northeast-facing Caribbean arc system that entered the
proto-Caribbean Sea between North and South America
during late Cretaceous time. Following
its collision with the passive margin of northern Central America (Chortis
block), the Caribbean arc and Caribbean-North American plate boundary migrated
northeastward with the northeast-migrating Caribbean plate. Due to subsequent Cenozoic rotation of the
amalgamated Chortis and Caribbean arc terranes,
deformational strike of the Colon
belt and Siuna belt are orthogonal to the Middle America
convergent margin and volcanic arc.
The Chortis block is a Precambrian-Paleozoic continental block
that presently occupies the northwestern corner of the Caribbean
plate (Gordon, 1990) (Figure 3.1). The Chortis
block is bounded to the north by left-lateral strike-slip faults of the
present-day North American-Caribbean plate boundary (Chapter 2) and to the
southwest by the Middle America trench and volcanic arc
of the present-day Cocos-Caribbean plate boundary (Chapter 1). The southern and eastern edge of the Chortis
continental block is not well defined because it is located in remote areas of
eastern Honduras
and northern Nicaragua,
is covered by lowlands of eastern Central America, or is
blanketed by waters of the Caribbean Sea and the
Cenozoic carbonate platform of the Nicaraguan Rise (Figure 3.1). GPS studies by
DeMets et al. (2000) show that
the southeastern edge of the Nicaraguan Rise and probably the eastern part of the Chortis
block are moving as part of the stable Caribbean plate.
Figure 3.1. Topographic and tectonic
setting of the late Cretaceous Colon fold-thrust
In Chapter 5 of this dissertation, the Chortis block is divided
into four new terranes: the North, Central, Southern, and Eastern Chortis
terranes (Figure 3.1). In addition, the
Siuna terrane is an oceanic terrane that was defined by Venable (1994)
in northern
Nicaragua.
A basic observation from the map shown in
Figure 3.1 is that the
overall structural strike of pre-Cenozoic rocks and its related topography on
the Eastern Chortis continental terrane and the Siuna
oceanic terrane is at right angles to trend of the Cenozoic Middle America
trench and volcanic arc (Figure 3.1).
Moreover, the trends of the Eastern Chortis and
Siuna terranes are at a high angle to the Central and Northern Chortis
terranes. Understanding the tectonic
history of these terranes and how their plate tectonic development led to the
juxtaposition of these diverse trends provides the motivation for this study in
the area shown by the box in Figure 3.1 and in detail in
Figure 3.2.
Figure 3.2. Pre-Tertiary geology of the continental Chortis block of eastern Honduras and
northeastern Nicaragua
Most previous work on the Chortis block has focused on the
northern edge of the block where it is juxtaposed with the Maya block of
southern Guatemala across the Motagua and Polochic left-lateral strike-slip
faults of the North American-Caribbean plate boundary (Figure 3.1).
Donnelly et al. (1990) and Burkart (1994)
have documented a late Cretaceous collisional orogeny that emplaced ophiolites
along the northern edge of the Motagua fault valley and produced north-south
shortening of pre-Cenozoic strata in eastern Guatemala
and Belize. Reconstructing the two sides of the late
Cretaceous collision across the Motagua suture zone is complex because as much
as 1100 km of late Eocene to Recent left-lateral motion accompanying the
opening of the Cayman trough has been superimposed onto the suture zone (Rosencrantz
et al., 1988; Leroy et al., 2000; Sisson et al., 2003) (Figure 3.1). This large-scale eastward migration of the Chortis
block along these strike-slip faults is supported by detailed fault kinematic
and radiometric dating of igneous rocks in the zone of southernmost Mexico
affected by the lateral shear (Riller et al., 1992;
Schaaf et al., 1995). Pre-Tertiary reconstructions of the Chortis
block place it about 1100 km further to the west along the southern margin of
Mexico (e.g., Azema et al., 1985; Dengo, 1985;
Pindell and Barrett, 1990;
Tardy
et al., 1994).
There have been few previous efforts to better constrain the
location of the southern and eastern margins of the continental Chortis block
and its tectonic relationships with oceanic and island arc terranes of southern
Central America.
Dengo (1985) placed the southern boundary of the Chortis block near the
Honduras-Nicaragua political boundary and the offshore boundary along the Hess
escarpment. Pindell and Barrett (1990)
and Tardy et al. (1994) have shown reconstructions for a Chortis block suturing
against arc and oceanic plateau terranes in southern Central America
but these reconstructions have remained largely conjectural since there are few
field constraints from this area. Case
et al. (1990) compiled crustal refraction lines that allowed them to infer a
boundary between continental rocks of the Chortis terrane and arc rocks of the
eastern Nicaraguan Rise and southern Central America.
Venable (1994) has been the only field-based
effort in northern Nicaragua
to focus on defining the suture zone between the Chortis block and an oceanic
terrane to the south which she named the Siuna terrane, based on her field area
in the Siuna mining district of northern Nicaragua
(Figure 3.1).
The specific objectives of this chapter include:
1) to present new geologic
field observations from the northeast-trending Colon Mountains of the eastern Chortis
block; these data have been published previously in the form of a Honduran
1:50,000 scale geologic map (Rogers, in press), three abstracts (Rogers, 1994;
1995b; Rogers et al., 2003), and one Spanish language article (Rogers, 1996); I
have also incorporated the paleontological results of
Scott and Finch (1999)
which are based on Cretaceous sedimentary rock samples collected in the Colon
study area. The only previous work in
eastern Honduras
was reconnaissance in nature along major rivers (Mills and Hugh, 1974).
2) to integrate
unpublished geologic field mapping, isotopic, and radiometric results from the
unpublished Ph.D. dissertation of Venable (1994) from the Siuna terrane of Nicaragua; these are the only modern geologic
data from this area of Nicaragua;
previous studies rely only on reconnaissance traverses along major rivers
(Zoppis Bracci and del Giudice, 1958;
Paz-Rivera, 1963).
3) to integrate published
seismic reflection and well data by Mills and Barton (1996) from the
along-strike continuation of the Colon Mountains beneath the Mosquitia coastal plain of easternmost Honduras; I have correlated the stratigraphy
encountered in these wells and seen on the seismic reflection lines into the
stratigraphy I mapped in the Colon Mountains.
4) to integrate
unpublished subsurface seismic reflection and well data by Rockwell (1985)
from
the along-strike continuation of the Colon Mountains beneath the submarine area of the eastern Nicaraguan Rise. These data are correlated both to the
Mosquitia plains subsurface study of Mills and Barton (1996) and to the
geologic study of the Colon Mountains.
Together these studies are used to constrain the stratigraphic
and tectonic history of a continuous, northeast-trending belt of deformation,
named here the Colon belt (Figure
3.1). This belt records the suturing
between the oceanic Siuna terrane to the south with the continental Eastern,
Central, and Northern Chortis terranes to the north.
A geologic map summarizing the geologic setting of the Colon
Mountains study area is shown in
Figure 3.2. This map, showing all
pre-Tertiary geologic units of eastern Honduras
and northeastern Nicaragua,
was compiled from Kozuch (1991) and
INETER (1995) supplemented with surface and
subsurface information from Rockwell (1985),
Mills and Barton (1996), and this study. Eastern Honduras is an ideal area to examine
pre-Tertiary tectonic history because the area has remained tectonically stable
in the Cenozoic and therefore has not been overprinted by tectonic events
affecting either the North American-Caribbean strike-slip boundary in the
Honduran borderlands (Chapter 2) or by tectonic events associated with the
subduction of the oceanic Cocos plate beneath the Caribbean plate (Chapter
1). One disadvantage of geologic studies
in eastern Honduras
is that there are few units of Tertiary age.
For this reason, correlations must be made into the subsurface of the
Mosquitia plain or of the Nicaraguan Rise in order to establish the age of the
Cretaceous deformation seen in the Colon
Mountains.
The Colon belt of
fold-thrust deformation is described in this chapter in three along-strike
segments shown on Figure 3.2: Colon Mountains
of eastern Honduras;
Mosquitia Plains of eastern Honduras;
and Nicaraguan Rise (Caribbean
Sea). The Siuna belt of
northeastern Nicaragua
is also described using data from Venable (1994) and is inferred to represent
the leading edge of the collided Caribbean arc
system.
Geologic mapping and radiometric studies have shown that the
Northern and Central Chortis terranes (areas north of
the Guayape fault system in Figure 3.2) have a Grenville to Paleozoic-age
basement composed of gneiss and schist (Case et al., 1990;
Donnelly et al.,
1990; Manton 1996;
Manton and Manton, 1999; and
Nelson, et al., 1997) (Figure
3.1). Seismic refraction studies
compiled by Case et al. (1990) show that terranes are underlain by continental
crust about 45 km in thickness.
Thinned, 30-35-km-thick continental crust consisting of Jurassic
sedimentary and metasedimentary rocks makes up the basement of the Eastern
Chortis terrane southeast of the Guayape fault system (Case et
al., 1990; Gordon, 1993a;
Rogers, 1995b: Viland, et al., 1996). Mapping in the Valle de Jamastran at the SW
termination of the Guayape fault, Rogers
(1995b) observed the transition from the sandstones and shales of the
Bathonian-age Agua Fria Formation (Gordon and Young, 1993) to greenschist
facies phyllites and quartzite. These
metasedimentary rocks, with metamorphic grade increasing to the east, were
followed east of the Guayape fault along the Rio Patuca into the metamorphic
rocks previously assumed by Kozuch (1991) to be the Paleozoic Cacaguapa Group.
The Siuna terrane is an oceanic terrane first defined and named
in northern Nicaragua
(Venable, 1994).
Case et al. (1990)
compiled seismic refraction data from the area of the Siuna terrane and showed
a 20-25-km island arc crust built on oceanic crust. A fundamental observation from the map shown
in Figure 3.1 is that the overall structural strike and related topography of
the Eastern Chortis continental terrane and the Siuna
oceanic terrane is at right angles to trend of the Middle America
trench and volcanic arc (Figure 3.1). Moreover, the trends of the Eastern
Chortis and Siuna terranes are at a high angle to the Central and Northern
Chortis terranes.
The linear and topographically prominent Guayape fault system
(GFS) (Finch and Ritchie, 1991; Gordon and Muehlberger, 1994) forms a major
terrane boundary between the northeasterly-striking rocks in the Eastern
Chortis terrane (basement composed of Jurassic metasedimentary
rocks) and more east-striking rocks in the Central and Northern Chortis
terranes (basement composed of Precambrian and Paleozoic crystalline
rocks). Gordon and Muehlberger (1994)
documented several kilometers of right-lateral strike-slip motion along the
fault during Neogene time.
Finch and
Ritchie (1991) proposed about 50 km of left-lateral motion based in part on the
apparent lateral offset of the Agua Fria Formation on either side of the fault. The map compilation shown in
Figure 3.2
indicates that the apparent left-lateral offset is closer to 60 km (length of
line indicated by “1” in Figure 3.2).
The sense of bending of the parallel ranges northwest of the fault in
the Central Honduras terrane also supports the
left-lateral interpretation by Finch and Ritchie (1991).
Despite the lack of crystalline basement east of the Guayape
fault and evidence of an apparent 60 km of left-lateral offset, a very similar
Mesozoic stratigraphy occurs on both the Central and Eastern Chortis
terranes on both sides of the fault.
Figure 3.3 compares the names, thicknesses, and ages of the main
formations found on both sides of the fault and shown in map view on
Figure
3.2.
Figure 3.3. Comparison of
Mesozoic stratigraphic units, nomenclature and thicknesses of the Chortis
terranes
Agua Fria Formation. The middle Jurassic Agua Fria Formation forms
the oldest basal clastic unit and least studied formation on both
terranes. The formation is at least 1700
meters thick and consists of coastal plain fluvial deposits, minor
shallow-marine carbonate, and rhythmically-bedded siliciclastic sedimentary
rocks that have been previously interpreted as marine turbidites by
Ritchie and
Finch (1985), Gordon (1990) and
Rogers (1995b).
The basal contact of this formation on older rocks has never been
observed. Underlying metamorphic
basement was inferred by Gordon in the Catacamas
Valley area of the Central
Chortis terrane based on the presence of recycled, metamorphic
clasts in conglomerate of the Agua Fria Formation (Gordon, 1993b).
Viland et al. (1996) document regional
deformation in the late Jurassic that metamorphosed parts of the Agua Fria
Formation prior to deposition of Cretaceous strata. A major unconformity is indicated between the
Agua Fria Formation and metamorphic basement from the much lower degree of
deformation in the Agua Fria than observed in
surrounding basement outcrops.
Yojoa Group. Early Cretaceous-Cenomanian shallow-marine
limestone of the Yojoa Group overlies the basal clastic rocks of the Agua Fria
Formation on both sides of the Guayape fault (Mills et al., 1967;
Scott and
Finch, 1999; Chapter 4) (Figure 3.3).
The Yojoa Group is divided into the upper and lower Atima Formations and
the intervening Mochito shale. Shallow
water shelf limestone of the Atima Formation is locally up to 1400 m in
thickness. Volcanic rocks including andesite
and dacite flows and pyroclastic rocks occur within the Atima Formation on the
Central and Eastern Chortis terranes (Carpenter, 1954;
Gordon, 1990; Chapter 4).
Valle de Angeles Formation. Overlying the Yojoa Group is a thick sequence
of coarse-grained, continental redbeds of the Valle de Angeles Formation
(Chapter 4) (Figure 3.3). Clastic rocks
of the formation include clay-rich, lithic conglomerate (both matrix- and
clast-supported) and sandstone and shale deposited as submarine and subaerial debris
flows (Rogers and O’Conner, 1993).
Conglomerates contain clasts of all underlying lithologies including the
Yojoa Group and even redbed clasts from the formation itself indicating
syndepositional reworking of previously deposited redbeds (Mills et al., 1967;
Finch, 1981; Rogers, 1994;
1995a).
Thickness of the lower Valle de Angeles Formation can range from several
hundred up to 1000 meters. Where
exposed, the contact between the Valle de Angeles redbeds and the limestone of
the underlying Yojoa Group has been described as unconformable, with karst
developed at the contact in at least one location (Rogers, 1995a;
Scott and
Finch, 1999).
Discontinuous shallow marine carbonate strata of Campanian age
(Esquias/Jaitique Formation) is recognized in central Honduras and was used as
a datum to separate the Valle de Angeles Formation into a lower and upper
sequence (Finch, 1981;
Scott and Finch, 1999) (Figure 3.3). Limestone associated with minor gypsum
deposits occurs locally in this unit and suggests an isolated marine
depositional basin (Horne et al., 1974;
Finch, 1981). Limestone of this unit varies from a few tens
of meters to several hundred meters in thickness.
The upper redbeds of the upper Valle de Angeles Formation are
generally finer-grained than the lower redbeds of the same formation. Redbeds are interbedded with minor mafic
volcanic rocks that thicken to more than 300 meters in eastern Honduras
(Weiland et al., 1992; Rogers, in press).
Scott and Finch (1999) proposed that the upper Valle de Angeles redbeds
rapidly blanketed the intra-redbed carbonates without producing an erosional
unconformity. The grain size transition
from the lower, coarse-grained to upper, fine-grain redbeds of the Valle de Angeles
Formation is gradational, with decreases in grain size and bedding thickness is
spread over several hundred meters of section (Rogers and O’Conner, 1993). Like the lower redbeds of the formation, the
upper redbeds have been interpreted as debris flows in the Tegucigalpa
region (Rogers and O’Conner, 1993).
Intrusive rocks. Felsic plutons ranging in age from Cretaceous
to early Tertiary intrude the stratigraphic units described above and are shown
on the compilation map in Figure 3.2 (Southernwood, 1986;
Kozuch, 1991; INETER, 1995). Ages of the plutons are known
mainly from field relations and from radiometric dating (cf. Chapter 5 for a
compilation of radiometric ages from Honduras).
3.6 THE COLON FOLD-THRUST BELT IN EASTERN HONDURAS
The Colon fold-thrust
belt is best expressed in folded, massive limestone lithologies of the
Cretaceous Atima Formation (Figure 3.3B) in the Colon
Mountains between the Rio Coco and
Rio Patuca (Figure 3.4). Maximum
elevation of the range is 880 m ASL; average elevation of the area of the surrounding
range the varies from 100 m in the lowlands along the Patuca and Coco Rivers to
an average of 300-400 m ASL in the Patuca Mountains northwest of the Colon
Mountains. A broad zone of open folds
affecting all Cretaceous units shown on the column in
Figure 3.3B parallels the
Colon belt and extends at least 20
km to the northwest of the belt into the Patuca
Mountains (Figure 3.4).
Figure 3.4. LANDSAT image of Colon Mountains
As seen on the LANDSAT image in
Figure 3.4, the Rio Patuca
closely follows the structural grain of the Colon
fold-thrust belt including the frontal reverse fault on the northeast side of
the Colon Mountains
(Figure 3.5). Both the Rio Patuca and
Rio Coco south of the Colon Mountains
are entrenched bedrock rivers showing no signs of deflection or offset by
Quaternary faults. Moreover, the
regional geomorphology of eastern Honduras
supports the interpretation that the area is a tectonically stable part of the Caribbean plate (Rogers et
al., 2002; Chapters 1, 2). The northeast
flow directions of the Rio Patuca and Coco reflects their origins as
northeast-flowing alluvial rivers prior to the late Neogene epeirogenic uplift
that incised their channels into bedrock canyons (Rogers et al., 2002).
Figure 3.5. Geologic map of Colon Mountains
The parallelism between the northeast-trending Guayape
strike-slip fault system and the northeast-trending Colon
Mountains (Figure 3.2) have led
previous workers to also interpret the deformation of the Colon
Mountains in terms of a large,
topographically-elevated “flower structure” produced by lateral shearing and
transpression on vertical, strike-slip fault planes (Mills and Hugh, 1974;
Gordon and Muehlberger, 1994;
Mills and Barton, 1996). This interpretation was mainly supported by
interpretation of satellite imagery (Figure 3.4) and aerial photographs and not
by detailed structural observations in the field.
Riverbank and stream exposures in the area of the confluence of
the Rio Wampu and Rio Patuca provide excellent cross sectional exposures of the
structure and stratigraphic units of the Colon
Mountains (Figure 3.5). Mapping and paleontological dates from
sedimentary units sampled in the Colon
Mountains (Scott and Finch, 1999) reveal
a pre-Cenomanian south-facing continental margin setting. Shallow-water, Cretaceous carbonate rocks and
clastic strata, totaling 4 km in thickness, were deformed by thin-skinned
style, northwestward-directed thrusting following Campanian time.
Basement and overlying
Yojoa Group. Low-grade
metasedimentary basement of the area is composed of phyllites and quartzites of
presumed Jurassic age (metamorphosed Agua Fria Formation). This metasedimentary unit crops out in the
north-central part of the study area (Figure 3.5) and extends to the northeast
to the Guayape fault (Gordon, 1993a) (Figure 3.2).
An estimated 1500 meters of massive shallow water limestone of
the Atima Formation of the Yojoa Group overlies the Agua Fria Formation, but I
was never able to observe this contact in the field. The limestone contains the upper Albian foraminifera
Cuneolina walteri, Pseudocyclammina hedbergi, and Praeglobotruncana delrioensis, as well
as Albian to lower Cenomanian foraminifera Pseudonummoloculina
(Nummoloculina) heimi indicating a stratigraphic position equivalent to the
Upper Atima Formation elsewhere in Honduras
(Scott and Finch, 1999).
Based on carbonate petrography,
Scott and Finch (1999) infer an
upward-shoaling carbonate succession from a basal peloid-foram-spicule
wackestone/packstone to an upper peloid-foram-bivalve wackestone/grainstone. Biofacies indicates that this succession
occurred as the result of marine transgression from middle to inner shelf
paleodepths suggesting a “keep-up” carbonate platform during relative sealevel
rise.
Krausirpe Formation. Calcareous marine shales containing organic
detritus of the Krausirpe Formation defined by Rogers
(1994) conformably overlie the carbonate rocks of the Upper Atima Formation
(Figure 3.3B). The Krausirpe Formation
is estimated to be 500 meters thick and, its biogenic component includes late
Albian to early Cenomanian palynomorphs as well as the foraminifera Globiegerinoides cushmani and Heterohelix globulosa (Scott and Finch,
1999). Biofacies indicate that these
beds represent a transition from carbonate shelf (Atima lithology) to
terrigenous shelf (Krausirpe lithology) with paleowater depths of up to 50
meters as indicated by the presence of planktic foraminifers and
dinoflagellates.
Late Cretaceous Valle de
Angeles Formation. Above the Yojoa
Group marine strata, the Valle de Angeles Formation contains medium to
coarse-grained, immature sandstone and conglomerate with subrounded clasts of
metamorphic, volcanic, carbonate, and redbed lithologies. Total thickness of the Valle de Angeles
redbeds is estimated to be 1500 meters in the eastern part of the study area
and decreasing to several hundred meters in the western part of the study area
below the volcanic flows of the Wampu Formation (Figure 3.5). Carbonate clasts were derived from limestone
of the Atima Formation and contain the foraminifera Orbitolina (Mesorbitolina) subconcava (late Aptian to late Albian)
and calcareous Dasyclad alga Dissocladella
undulate (Cenomanian) and the rudist Mexicaprina
sp. (late Albian) (Scott and Finch, 1999).
Limestone-clast conglomerate in the Valle de Angeles Formation is
much more abundant proximal to the limestone massif of the Colon
Mountains in the lower part of the
Valle de Angeles Formation redbeds (Figure 3.3B). In areas to the northwest of the Colon
Mountains, metamorphic clasts predominate
and limestone clasts are absent to rare.
Subangular pebble clasts of red sandstone appear throughout the section
becoming more prevalent toward the top of the redbeds.
Wampu volcanic unit. Flows of basaltic andesite within clastic
strata of the Valle de Angeles Formation are exposed along the Rio Patuca
upstream of its confluence with the Rio Wampu (Figure 3.5). These volcanic exposures define a
northeast-trending outcrop belt that is an outlier of the basaltic andesite
flows of the main Wampu volcanic field to the northwest that was studied by
Weiland et al. (1992) (Figure 3.5).
Weiland et al. (1992) report K/Ar ages of 70.4 (+/- 3.4) Ma and 80.7
(+/-4.3) Ma from volcanic flows in this northern area. In the upper 500 meter of the Valle de
Angeles Formation, Wampu volcanic flows are interbedded with the clastic strata
of the Valle de Angeles Formation.
Tabacon Formation. The Tabacon Formation is a 500-meter-thick,
massive bedded, subangular cobble breccia, composed of volcanic clasts derived
from the flows of the Wampu Formation and reworked clasts from the redbed
strata (Rogers, 1994). Its age is taken
as Late Cretaceous (< 70 Ma) based on its stratigraphic position above the
radiometrically dated Wampu volcanic rocks (70-80 Ma) (Weiland et al.,
1992). It is the youngest stratigraphic
unit present in the study area and is involved in the folding and thrusting.
Cretaceous erosion and
tilting event inferred from stratigraphic relationships. An angular
unconformity of 10 to 15 degrees separates the Krausirpe Formation from the
overlying clastic strata of the Valle de Angles Formation. A zone of epikarst is developed where the
Valle de Angeles Formation unconformably blankets the massive limestone of the
Atima Formation. Epikarst at the contact
and the complete erosion of the underlying Krausirpe strata records a period of
subaerial exposure of the limestone prior to the beginning of the late
Cretaceous Valle de Angeles deposition.
North of the Rio Patuca, limestone of the Atima Formation is absent, and
the Valle de Angeles redbeds were observed in direct unconformable contact with
the metasedimentary strata of the Agua Fria Formation. Patchy deposition of the Atima Formation
limestone has been commonly reported across the Chortis block (cf.
Finch, 1981; Donnelly et al., 1990; Chapter 4) and its complete absence north of
the Rio Patuca (Figure 3.5) may signify its non-deposition rather than its
deposition and subsequent erosion.
However, the angular unconformity between the Krausirpe and Valle de
Angeles Formation, the Atima clasts within the Valle de Angeles redbeds, and
the karstified erosion surface at the Valle de Angeles – Atima contact are all
evidence for a Cretaceous tectonic event that preceded the main late
Cretaceous-early Tertiary shortening event that formed the Colon fold-thrust
belt. In Chapter 4, I propose that the
unconformity observed at the base of the Valle de Angeles Formation is related
to intra-arc rifting and extensional deformation.
Late Cretaceous-early Tertiary
folding event inferred from stratigraphic and structural relationships. Because no obvious angular unconformity is
observed in the late Cretaceous section, I infer that the main shortening event
that formed the Colon fold-thrust
belt occurred after the deposition of the Tabacon Formation about 70 Ma (Figure
3.3B). Because the contact between the
Valle de Angeles redbeds and the Tabacon breccia is gradational with upward
coarsening, and increase in angularity and volcanic clast content, it is possible
that the Tabacon Formation is an early syn-tectonic deposit related to the
early phase of the shortening event. To
the northeast and southwest, away from the Wampu volcanic field, the breccia
clasts in the Tabacon Formation contain a greater abundance of metamorphic rock
fragments suggesting a regional uplift and source from the northwest in latest
Campanian to Maastrichtian time. Because
the Tabacon breccia blanketed the latest Cretaceous landscape and rests unconformably
on the Agua Fria phyllite and schist, I interpret the
Tabacon Formation as a synorogenic clastic wedge shed from subaerially-exposed
highlands formed during Late Cretaceous-early Tertiary shortening.
Weathering characteristics
of rock units. In the tropical
rainforest of eastern Honduras,
Colon belt stratigraphic units
weather to produce distinctive landforms easily recognizable on satellite
images and aerial photography (Figure 3.4).
This distinction aided in the development of the regional geologic map (Figure
3.2). The resistant limestone of the
Atima Formation and volcanic breccias of the Tabacon Formation are prominent
ridge formers. The extensive karst
topography developed on limestone of the Atima Formation distinguishes it from
the Tabacon Formation. Agua Fria
Formation metasedimentary rocks are moderately resistant, and coupled with
their diverse bedding plane foliations; produce a rugged upland topography with
a dendritic drainage. Wampu Formation
volcanic flows are somewhat less resistant than the Agua Fria
metasedimentary rocks and develop a rectangular drainage. Rocks of the Valle de Angeles Formation are
not resistant and form the non-alluvial lowlands of the region.
Comparison to previous
interpretations of stratigraphy. Mills and Hugh (1974) map the late Jurassic Todos Santos Formation and Atima Formation along the Rio Wampu and west of the
Rio Patuca in an area where I mapped late Cretaceous breccia of the Tabacon
Formation (Figure 3.5). I suggest that
the resistant beds of the Tabacon breccia were incorrectly identified by
Mills
and Hugh (1974) because the Tabacon unit forms strike ridges remarkably similar
in appearance to the ridge-forming Atima limestone when viewed from a distance
or on aerial photographs. Because Tabacon
breccia overlies metasedimentary Agua Fria Formation along the Rio Wampu,
Mills
and Hugh (1974) may have mistakenly assumed that the next higher clastic unit
would be the Todos Santos Formation.
Overall structure. The Colon belt is a fold-thrust belt of
imbricate, southeast-dipping reverse faults that place Cretaceous carbonate
strata of the Atima Formation over post-Cenomanian Valle de Angeles Formation
(Figure 3.6A and B).
Broad open folds are found 20 km northwest of the thrust front indicating a
broad, foreland zone of convergent deformation (Figures 3.4 and
3.5). The northeast-striking frontal thrust
parallels the Rio Patuca on the west flank of the Colon
Mountains (Figure 3.4). Reverse faults and complexly,
northeastward-plunging folds of the Colon Mountains coincide with the outcrop
area of shale of the Krausirpe Formation which forms an incompetent horizon on
which northwest directed shortening is facilitated (Figures
3.4 and
3.5). West of the Rio Patuca where the Krausirpe
Formation is absent, large folds with northeastward-trending axial surfaces deform Jurassic
metasedimentary rocks and overlying Cretaceous strata.
Figure 3.6:
Structural cross sections and multi-channel seismic profiles across
Colon fold-thrust belt
Main thrust faults and related folding. At least four thrust sheets comprise the Colon
Mountains as displayed on the
structure profile (Figure 3.6 A and B).
The location of the faults is revealed by the repetition of the
distinctive Atima – Krausirpe stratigraphy visible in the strike valleys of the
Colon Mountains. In the Patuca
Mountains, west of the Rio Patuca,
the ridges composed of volcanic breccia of the Tabacon Formation forms two open
synclines separated by a metamorphic rock-cored anticline flanked by Valle de
Angeles strata dipping off the anticline (Figure 3.6A). Although shortening is thin-skinned in the Colon
Mountains, involvement of the
Jurassic metasedimentary basement in the convergent deformation is manifested
by this major anticline in Jurassic metasedimentary rocks (Figure 3.5).
Folding and age of main
shortening event. Plots of poles to
bedding for each of the Cretaceous stratigraphic units are very similar and
cluster in the northwest quadrant reflecting the dominant southeast dips of the
region indicating a single episode of deformation (Figure 3.7A). These plots show that all formations
experienced an equivalent amount of shortening that post-dated the age (70-80
Ma) of the youngest unit (Tabacon Formation) (Figure 3.3B). A histogram of all dips measured in
Figure
3.7F supports this interpretation.
Figure 3.7. Summary of
structural measurements made in Colon
map area
Best fit great circles display upright folds trending about N45E
to N30E (Figure 3.7A). The larger
scatter of the poles to bedding planes in the Krausirpe and Valle de Angeles
Formations and increase in dips in the Krausirpe Formation (Figure 3.7F)
reflects the greater amount of intra-formational deformation of these
thinly-bedded, clastic units as compared to the more massively bedded Atima, Tabacon and Wampu units. Minor folds
(Figure 3.7E) are present in the thinly bedded shale of the Krausirpe Formation and in thinly bedded, fine-grained strata of
the Valle de Angeles Formation. In
contrast, minor folds are virtually absent from the massive-bedded Atima and
Tabacon Formations as observed in other areas of Honduras
(Chapter 4).
Kinematic data derived from striated fault measurements using the
method of Marrett and Allmendinger (1990) are consistent with oblique faulting
with reverse and right lateral motion.
The direction of reverse motion is consistent with the mapped NW
directed thrust faults. Right-lateral
strike-slip faults are manifested as linear offsets of the Colon
thrust front and are prominent along the southern margin of the range (Figure 3.4).
The mining district of Siuna, Nicaragua
is located 70 km southeast of the Colon
fold-thrust belt of Honduras
(Figure 3.2). In the Siuna mining area,
Cretaceous volcanic and sedimentary strata host economically important Zn-Cu-Au
volcanogenic massive sulfide and Cu-Au skarn deposits (Venable, 1994) (Figure 3.8). Volcanic rocks include flows of
calc-alkaline hornblende or pyroxene andesite that locally include pillow
basalts and pyroclastic material.
Sedimentary strata interbedded with the andesite contain calcareous
shale, sandstone, and tuff with thin limestone and chert containing shallow
marine fauna (Venable, 1994).
Figure 3.8. Geology of Siuna mining district
Serpentinite bodies, ultramafic cumulates and podiform chromite
are thrust imbricated with Cretaceous strata; tectonic transport to the north
and east (Venable, 1994) (Figure 3.8).
One outcrop of wehrlite is associated with the serpentinites. X-ray diffraction analysis shows the
serpentinite bodies composed of lizardite and chrysotile with minor magnate and
chromite (Venable, 1994). In thin
section the serpentinite contains nickel and chromium disseminated in chromite. Chromite boulders observed across the serpentinite outcrop indicate a weathered
podiform chromite body (Venable, 1994).
Undeformed diorite and granodiorite plutons intrude strata and
volcanic rocks and postdate the shortening deformation.
Venable (1994) reports an Ar39/Ar40 age of
59.89 (+/- 0.47) Ma for a biotite separate from a diorite pluton. Whole rock analysis of the diorite yielded a
Sm147/Nd144 ratio of 0.135624 and a present day Nd143/Nd144 of 0.512985 with a
present day epsilon Nd value of +6.8 corresponding to an initial epsilon Nd
value of +7.2 at 60 Ma indicating lack of contamination from continental crust. Small, undeformed hornblende andesite dikes
intrude the faulted contacts between the serpentinites and sedimentary
strata. Ar34/Ar40 dating of a hornblende
separate from the andesite dikes yield an age of 75.62 (+/- 1.33) Ma.
Venable (1994) defined the Siuna terrane as an oceanic island arc
active from the early to middle late Cretaceous and deformed in the late
Cretaceous. The arc rocks developed on
an oceanic basement with an unevolved isotopic signature.
Venable (1994) proposed that the Siuna arc
accreted to the southern margin of the continental Chortis block in the latest
Cretaceous.
I propose that the Siuna terrane formed the leading edge of the
Guerrero-Caribbean island arc that accreted to the margin of southern Mexico
and the Chortis block in late Cretaceous times.
This arc system formed the leading edge of the Caribbean
oceanic plateau province which formed in late Cretaceous time (95-88 Ma)
(Hoernle et al., 2002).
3.8 THE COLON FOLD-THRUST BELT BENEATH THE MOSQUITIA COASTAL PLAIN
OF EASTERN HONDURAS
The subsurface seismic
reflection grid and well study of
Mills and Barton (1996) is shown on
Figure 3.2 and covers a large area of the Mosquitia alluvial plain of
Honduras centered on the village of
Awas. The area is
underlain by a large, Quaternary alluvial plain related to fluvial deposition
from the combined outflow of the Rio Patuca and Cocos (Figure
3.2). The overall elevation of the plain is a
few tens of meters above sea level with the highest points confined to bedrock
hills (Mills and Hugh, 1974).
A small part of the Mosquitia alluvial plain is apparent on the southeast corner
of the LANDSAT image shown in
Figure 3.4.
Regional seismic lines correlated to the two wells drilled along
Line T-08 in Mills and Barton (1996) (Figure 3.6C) identified several of the
same lithologic formations described above from outcrops in the Colon Mountains
and summarized on the stratigraphic column in
Figure 3.3B. My interpretation of the units encountered in
the two wells differs significantly from the stratigraphic interpretation by
Mills and Barton (1996). Their approach
was to adopt new formation names for units that I consider to be correlatable
to the stratigraphic section of the Colon
Mountains. Part of the correlation problem with both
wells was related to the fact that no paleontological age determinations were
made so none of the units described by Mills and Barton (1996) were dated. My correlations below rely solely on
lithologic correlation. I visited the
wellsites in February of 1992 and examined cuttings from the wells.
The main units described in the two wells included the following
units shown in detail on Figure 13 of
Mills and Barton (1996) and schematically
on Figure 3.6C:
Metasedimentary rocks.
Mills and Barton (1996) designate the dark grey phyllite encountered during drilling as Paleozoic (?) while noting that the phyllite
may be metamorphosed Agua Fria Formation. I adopt the later interpretation
based on the similar appearance of the phyllite to metasedimentary strata of
the Agua Fria Formation in the Colon Mountains and elsewhere in Honduras (Rogers,
1995b; Viland et al., 1996). In the Colon
Mountains, these rocks consisted of
dark-colored phyllite with remnant bedding and local stringers of met
conglomerate. Mills and Barton (1996)
interpreted this unit as Paleozoic (?) schist whereas I interpret the unit as
Jurassic metasedimentary Agua Fria Formation, similar to that occurring in
outcrops in the Colon Mountains
(Figure 3.3B).
Dark-colored, tightly
cemented sandstone and shale.
Mills
and Barton (1996) interpreted this unit as unmetamorphosed Agua Fria Formation. I agree with their interpretation.
Red, fine-grained,
argillaceous, hard sandstone, shale and siltstone.
Mills and Barton (1996) found this to be a
unique unit not correlatable to other units in eastern Honduras
and proposed a new stratigraphic name, the Rio Riba Formation. In contrast, I correlate this unit with the
late Cretaceous Valle de Angeles Formation in the Colon
Mountains (Figure 3.3B).
Soft to hard, medium to
fine-grained, reddish sandstone, siltstone, and conglomerate.
Mills and Barton (1996) introduced the Ahuas
beds stratigraphic name for this unit while noting the possible correlation to
the Tabacon Formation. I make the correlation between the Tabacon Formation and
the Ahuas beds based on the volcaniclastic composition of both units.
Units not present in
subsurface of Mosquitia plains.
Units not present in the subsurface of the Mosquitia plains but observed
in outcrop of the Colon Mountains
include the Atima and Krausirpe Formations (Figure 3.3B).
Timing of folding and
thrusting in the subsurface of the Mosquitia Plains. Adopting my stratigraphic correlations, the
Embarcadero well tied to line T-108 shows Jurassic Agua Fria Formation thrust
over Late Cretaceous Valle de Angeles Formation which in turn is thrust over
Jurassic Agua Fria Formation (Figure 3.6C).
The northern part of the line and well show repetitions in the unit I
interpret as the Late Cretaceous (post-80 Ma) Tabacon Formation. These observations constrain the age of
thrusting in eastern Honduras
as at least post-dating the deposition of the Tabacon Formation, or latest
Cretaceous-Tertiary (post-80 Ma).
Structural style. The structural style on the seismic line
consists of imbricate, south-dipping reverse faults. The hanging wall of these faults forms a
structural culmination centered near the Embarcadero well seen in
Figure 3.6C. This elevated hanging wall in the subsurface
is directly along-strike from the topographically-elevated hanging wall of the Colon
Mountains (Figure 3.2).
Correlation of frontal
thrust between the Mosquitia Plain and Colon Mountains. Aeromagnetic data from
Direccion General de Minas e Hidrocarburos (Honduras)
that is discussed in detail in Chapter 5 is used to correlate the frontal
thrust along-strike in the subsurface of the Mosquitia Plains (Figure
3.9). Superimposing the thrust front
known from mapping in the Colon Mountains
and the Mosquitia subsurface study onto the aeromagnetic map reveals a good
correlation between the deeper basement structure expressed on the aeromagnetic
map and the shallow structure observed on the seismic data and by field
mapping. This correlation implies
involvement of the underlying magnetic basement in the shortening either as an inversion
of the rifted Jurassic margin (Chapter 5) or as a backstop preventing further
advancement of the thin-skinned thrusting.
Figure 3.9. Aeromagnetic map of eastern Honduras
3.9 THE COLON FOLD-THRUST BELT BENEATH THE NICARAGUAN RISE, CARIBBEAN SEA
The Nicaraguan Rise is a shallow-water (<100 m), tectonically
stable, horizontal Cenozoic carbonate platform overlying continental and arc
crust (Arden, 1975; Case et al., 1990). Nearshore areas are dominated by
terrigenous sedimentation related to major deltas of the Patuca and Coco
Rivers (Figure 3.2).
Rockwell (1985)
reprocessed about 850 km of seismic reflection airgun data (48 fold) and 315 km of original Texaco data collected by surveys
from 1977-1980 (offshore survey area shown in
Figure 3.2). This area of the Nicaraguan Rise is known as
the Gracias a Dios platform. The
objective was to re-evaluate petroleum prospects in the area and perhaps plan
further, non-exclusive seismic surveys in the area. Specific objectives included the enhancement
of weak, but somewhat coherent, pre-Tertiary reflectors and clarification of
shallow structural and stratigraphic relationships. Reprocessing substantially improved data down
to the base of the Tertiary and marginally improved pre-Tertiary data.
Three horizons were mapped: top of Eocene, top of Cretaceous, and
top of Atima (mid-Cretaceous) (Figure 3.6D, E).
A well tie to the Main Cape-1 well shown on
Figure 3.2 on an adjacent
line confirms the age of the first two horizons but no well tie was able to
substantiate the top of Atima Formation.
Other wells on the Nicaraguan Rise compiled by Robertson Research (1984)
support the idea that direct correlation is possible between the Cretaceous
stratigraphy in eastern Honduras
and the Cretaceous stratigraphy underlying the Eocene-Recent carbonate
platform. For example, the Caribe-1,
Caribe-2, and Caribe-3 wells encountered mid- to early Albian limestone below
late Cretaceous sandstone. This
succession suggests a direct correlation to Albian Atima Formation overlain by
late Cretaceous Valle de Angeles Formation.
The Diamante-1 well penetrated Cretaceous limestone, no older than late
Albian, overlain by dark shale with minor sandstone. This succession suggests Krausirpe Formation
overlying Atima Formation (Figure 3.3B).
The top of Eocene horizon represents an unconformity between
Eocene and Oligocene carbonate lithologies and shows that Eocene rocks pinch
out in a westward and northwestward direction on the platform by erosion
(Figure 3.6E). The wedge of Eocene rocks
is extended by a series of down-to-the-southeast normal faults. The relation of the normal faults to the
shortening of the Colon belt is not
clear as precise age constraints are lacking.
The normal faults may have formed in response to deeper seated thrusting
and uplift along the thrust front and hanging wall of the Colon
belt shown on Figure 3.2. Alternatively,
the normal faulting may be post-collisional and reflect the accommodation of
the eastern Chortis block during its eastward translation from Mexico
(Chapter 5).
The top of Cretaceous unit is also tilted east-southeast and away
from an east-southeast-dipping frontal thrust striking east-northeast and
colinear with the subsurface thrust mapped beneath the Mosquitia Plain (Figure
3.2; 3.6D). The structure of this
frontal thrust is similar to those previously discussed in the Colon Mountains
and Mosquitia plain: an anticlinal hanging wall block developed in
mid-Cretaceous Atima limestone overthrusts clastic sedimentary units correlated
to late Cretaceous Valle de Angeles Formation (Figure 3.6D).
Offshore observations constrain the age of thrusting on the
Nicaraguan Rise as at least post-dating the deposition of the Valle de Angeles
Formation, or latest Cretaceous-Tertiary (post-80 Ma). Tilting and normal faulting of the Eocene
unit indicates that thrusting could have continued into the Eocene but ended by
the beginning of the Oligocene.
Restoring the Chortis block to its pre-translation (pre-Eocene)
position adjacent to the truncated margin of southwestern Mexico re-aligns the
Colon fold-thrust belt of the Eastern Chortis terrane with the fold-thrust
belts and ophiolites north of the Motagua suture in Guatemala (Burkart,
1994; Donnelly et al., 1990) (Figure 3.10A).
The present day configuration of these elements is shown in
Figure 3.10B. This reconstruction also provides a best fit
of the following elements common to the Chortis block and SW Mexico
that are discussed at length in Chapters 4 and 5:
1) Influx of late
Cretaceous terrigenous sandstone and shale over early Cretaceous shallow
marine platform limestone of both southern Mexico
and Chortis;
2) Grenville-age basement
common to both areas;
3) Late Cretaceous
shortening structures common to both areas; and
4) Mid-Cretaceous arc
volcanism that is geochemically similar in both areas (Chapter 4).
These common features of the two regions are used to reconstruct
the late Cretaceous position of the Chortis block against southwestern Mexico
that is shown in Figure 3.10A.
Figure 3.10. Plate
reconstructions
3.10.2 Caribbean arc collision in the late Cretaceous
Three independent lines of evidence support my proposed
interpretation shown in Figure 3.10A that the eastern Chortis block records the
collision of the Caribbean arc with the southern margin
of North America in the late Cretaceous. The first is the 350 km-long Colon
fold belt with Campanian-age, northwest-directed shortening described in this
chapter. The second is the spatial
association and inferred accretion of the intra-oceanic Siuna island arc
complex on the southern margin of the Chortis block in the late Cretaceous
(Venable, 1994). The third is the
Pacific origin of the Caribbean arc and its position at
the leading edge of the Caribbean large igneous province
(Pindell and Barrett, 1990;
Sinton et al., 1997). The entry of this
arc-plateau feature into the area of the proto-Caribbean Sea shown in
Figure
3.10A led to the subduction of proto-Caribbean oceanic crust and partial
accretion of the Caribbean arc and the oceanic plateau at the “gateways” to the
Caribbean in Colombia in South America (Kerr et al., 1998) and in southern
Central America where large areas of the crust appear to have been built on
oceanic plateau material (Sinton et al., 1997).
In the reconstruction shown in
Figure 3.10A, the Guayape fault
system of eastern Honduras
manifests collisional deformation by left-lateral strike-slip faulting and
oroclinal bending of the inverted rift basins adjacent to the fault (Chapter
4). Later right-lateral motion of the
Guayape produces the pull-apart basin and the normal faults that partially
truncate the oroclinal bending (Gordon and Muehlberger, 1994) (Figure
3.11).
Figure 3.11. LANDSAT image of Guayape fault system
Kesler et al. (1990) shows that the continental blocks of Central
America and Mexico
display distinct clustering of lead isotopic ratios which provide a useful
basis to distinguish among the complexly amalgamated terranes of
the region. I follow their approach by
compiling lead data from Venable (1994) from the Siuna terrane (Table 1). As previously discussed, the crust of the Chortis
block can be subdivided into: 1) continental crust of the central and northern Chortis
terranes with Precambrian-Paleozoic crystalline basement; 2) crust of the
Eastern Chortis terrane consisting of thinned continental crust corresponding
to exposed Jurassic metasedimentary basement (Agua Fria Formation), and 3)
accreted oceanic island arc crust of the Siuna terrane (Figure 3.12A).
Table 1: Lead isotopic composition of the Siuna terrane,
Nicaragua (sample locations shown in
Figure 3.8) (data
from Venable, 1994)
|
|
sample
|
mineral
|
Host rock
|
206Pb/204Pb
|
207Pb/204Pb
|
208Pb/204Pb
|
|
Pb-1
|
417-231
|
galena
|
Cretaceous
volcanic
|
18.613
|
15.591
|
38.399
|
|
Pb-2
|
403-196
|
galena
|
serpentinite
|
18.598
|
15.559
|
38.315
|
|
Pb-3
|
371-031
|
galena
|
Cretaceous
volcanic
|
18.550
|
15.606
|
38.415
|
|
Pb-4
|
417-290
|
sphalerite
|
Cretaceous
volcanic
|
18.583
|
15.575
|
38.341
|
Analysis: A. Baadsgard, University
of Alberta
The plot of 207/206 Pb verses. 206/204 Pb data from volcanic host
rock displays distinctive clustering of common lead isotopes grouped by terrane
(Figure 3.12 B). Also displayed on the
plot are lead isotopic ratios from the Caribbean large
igneous province (Sinton et al., 1997; Hauff et al., 2000;
Hoernle et al., 2002), the Maya block of the North
American plate (Cumming et al., 1976;
1981; Sunblad et al., 1991), and the
Miocene volcanic cover of western Nicaragua
(Cumming et al., 1981). The lead ratios of
the Siuna terrane cluster outside of the Caribbean large
igneous province cluster indicating that the Siuna arc is not underlain by the Caribbean
plateau material. Instead, like in other parts of the Caribbean
arc system, the Siuna arc probably developed at the leading edge of the Caribbean
oceanic plateau province.
Figure 3.12: Map of Maya and Chortis blocks showing lead isotope data
The isotopic ratios from the Miocene volcanic cover of western Nicaragua
plot close to the Siuna cluster, suggesting that the volcanic pyroclastic
deposits may bury Siuna arc terrane to the southwest where Garayer and
Viramonte (1973) describes peridotite-bearing ultramafic rocks locally exposed
beneath the Nicaraguan volcanic cover.
Walther
et al.’s (2000) observation of high velocity mantle material in an upper crustal position beneath western Nicaragua
may also represent a part of the oceanic Siuna terrane.
1) The northeast-trending, 350-km-long Colon
fold-thrust belt is oriented at right angles to the modern convergent margin of
the Middle American trench and is roughly perpendicular to the trends of other
terranes in central and northern Honduras. The style of deformation, age of deformation,
and deformed lithologic units are similar along the length of the belt in the Colon
Mountains, on the Mosquitia coastal
plain, and on the Nicaraguan Rise.
2) Comprised of southeast-dipping imbricate thrusts and folds,
the Colon belt of eastern Honduras
and the Nicaraguan Rise records a northwest-southeast tectonic shortening event
(present geographic coordinates).
3) The Siuna oceanic island arc complex of northern Nicaragua
consists of late Cretaceous, calc-alkaline volcanic strata, serpentinite and
ultramafic cumulates. The Siuna terrane
is inferred to represent the accreted leading edge of the Caribbean
arc system onto the southern edge of the Chortis block. Coeval, late Cretaceous deformation of the Colon
belt and the accretion of the Siuna complex are interpreted as the product of
collision between the Caribbean arc and the Chortis
block.
4) This collision is approximately coeval, possibly a few million
years earlier, with the collision of the Caribbean arc
in Guatemala
that emplaced the northern subduction complex of the Motagua valley and the shortening
deformation in Guatemala
to the north of the Motagua valley.
5) The Cenozoic translation of the Chortis block into the Caribbean
realm offsets the original geometry of the co-linear collision. Part of the collisional belt (Colon)
has been rotated and translated as the Chortis block; the other part of the
collisional belt remained on the North American plate as the Motagua Valley of
Guatemala.
This chapter describes the geology of a well exposed but
previously unmapped section of Paleozoic-early Cenomanian metamorphic,
sedimentary, and igneous rocks in the Frey Pedro study area of the Agalta
Range of eastern Honduras. The objective of the study is to use these
new structural, stratigraphic, biostratigraphic, and geochemical field data to
better constrain the geologic and tectonic history of this part of the Chortis
block (Eastern Chortis terrane) during the period of
time from Aptian to early Cenomanian.
The study revealed that the topographic Agalta range exposes a thick
stratigraphic section (3.5 km) deposited in an Albian-Aptian intra-arc rift and
on the rift shoulders. This rift
feature, named here the Agua Blanca rift, presently trends northwest and is
parallel to three other belts of deformed Cretaceous rocks in Honduras
(Comayagua, Minas de Oro and Montaña de la Flor belts) that also may correspond
to Cretaceous intra-arc rifts produced during the same phase of intra-arc extension. These other three deformed belts are west of
the Agalta range and also form topographically-elevated mountain ranges.
Rift- and arc-related units include calc-alkaline volcanic rocks
and pyroclastic flows of the Manto Formation and volcaniclastic rocks of the
Tayaco Formation. These rift- and
arc-related units occupy the stratigraphic position between two major,
extensive, shallow-water carbonate units, the Lower and Upper Atima Formation,
also of middle Cretaceous age. Observed
thickening of volcaniclastic rocks of the Tayaco Formation strata in the Agua
Blanca rift accompanied erosion of the adjacent rift shoulders and eruption of
Manto calc-alkaline volcanic rocks both within and adjacent to the Agua Blanca
rift. During the late Cretaceous, the
Agua Blanca intra-arc rift was inverted by a regional shortening event. Rocks within the rift were intensely
shortened while rocks on the rift shoulders that are underlain by metamorphic
basement rocks were shortened less.
In order to better understand the Aptian-early Cenomanian
tectonic setting for intra-arc rifting and subsequent rift inversion on the Chortis
block, I reconstructed the Chortis block relative to the terranes studied by
previous workers in southern Mexico. Five geologic and tectonic features were
selected to use in realigning the two areas: 1) areas of Precambrian basement
outcrops; 2) areas of similar Mesozoic stratigraphy; 3) areas of Mesozoic
volcanic arc rocks exhibiting a similar arc geochemistry; 4) areas exhibiting
parallel trends of late Cretaceous folds and thrusts; and 5) areas of similar
magnetic signature. Using this
reconstruction, I propose four main regional tectonic phases, three of which
are expressed in structure and stratigraphy of
the Frey Pedro study area: Phase
1: Neocomian rifting of Chortis from Mexico (deposition of Tepemechin
Formation); Phase 2: Late Albian-Aptian
intra-arc rifting of the Chortis block (origin of Agua Blanca rift); and Phase 3: Late Cretaceous shortening related
to collision of the Guerrero-Caribbean arc system with reconstructed area of
Chortis-southern Mexico (inversion of Agua Blanca rift). The fourth and final phase, Paleogene carbonate sedimentation is
recorded in other areas that expose younger rocks of this period.
The Chortis block of northern Central America
forms the only continental part of the present-day Caribbean
plate and is therefore an important constraint on the origin and
Cretaceous-Cenozoic displacement history of the Caribbean
plate (Figure 4.1A). Closure of the
1100-km-long, Eocene-Recent strip of oceanic crust in the Cayman trough pull-apart basin (Rosencrantz et al., 1988;
Leroy et al.,
2000) places the Chortis block adjacent to an area of mainly continental terranes in southern Mexico
(Pindell and Barrett, 1990;
Sedlock et al., 1993; Dickinson and Lawton, 2001). A problem with previous reconstructions of
Chortis-southern Mexico
is that the geology of the Chortis area in Honduras
is much less studied and understood than correlative terranes in Mexico. Previous reconstructions of the Chortis block
and southern Mexico that have relied almost entirely on data from southern
Mexico include: Azema et al. (1985),
Pindell and Barrett (1990),
Riller et al.
(1992), Herrman et al. (1994), Tardy et al. (1994), and
Schaaf et al.
(1995).
Figure 4.1. Present day plate structures and topographic map of southern Mexico, Central
America and adjacent ocean basins
Improved understanding of the geology of the Chortis block over
the past 10 years has led to my subdivision of the Chortis block into four
terranes: the Eastern, Central, Southern and Northern terranes that I show on
Figure
4.1A and discussed at length in Chapter 5.
A basic observation from the map shown in
Figure 4.1B is that the
overall structural strike of pre-Cenozoic rocks and its related topography on
the Eastern Chortis continental terrane and the Siuna
oceanic terrane is at right angles to trend of the Cenozoic Middle America
trench and volcanic arc. Moreover, the
trends of the Eastern Chortis and Siuna terranes are at
high angle to the Central and Northern Chortis terranes. Understanding the tectonic history of these
terranes and how their plate tectonic development led to the juxtaposition of
these diverse trends provides the motivation for this field study in the Eastern
Chortis terrane. The Frey
Pedro study area on this terrane is located by the box in
Figure 4.1B.
The objective of this chapter is to present new structural,
stratigraphic, biostratigraphic, and geochemical field data from the East
Chortis terrane for the time period when it was joined to southern
Mexico
(Aptian-early Cenomanian). I then use
these new data to compare to previously published results from southern Mexico
to identify features on Chortis and southern Mexico
that best constrain the restoration of the two areas.
This field study describes the geology from a previously
unmapped, 2,000 km2 region in interior of east-central Honduras
(Frey Pedro area of Agalta Range). This region was selected for detailed study
because: 1) it has not been covered by Tertiary volcanic strata as found in
western Honduras (Chapter 1); and 2) it has not been subjected to Cenozoic
plate boundary deformation related to the North American-Caribbean plate
boundary zone deformation (DeMets et al., 2000) (Chapter 2). For these reasons, the area provides an
unobscured record of Cretaceous structural and stratigraphic events.
Methods included standard geologic mapping on a basemap of
1:50,000 scale topographic sheets, measured sections, and measurements and
analysis of structural information. Eight
weeks were spent in the field in February through April of 2000 and three weeks
were spent in January of 2001. Prior to
this study, I spent several weeks in 1990 and 1991 in this region of Honduras
with Mark Gordon and Mike Kozuch and became familiar with the major lithologic
units and tectonic problems.
Sedimentary samples were collected for micropaleontological
dating and were sent to Dr. Robert Scott of Precision Stratigraphy Associates
in Cleveland, Oklahoma. All results of his foraminifera picks and age
and environmental interpretations are presented in this chapter. Samples of volcanic rocks were collected for
geochemical analyses and were sent to Dr. Thomas Vogel (Michigan
State University)
for major element chemistry by X-Ray Defraction (XRF) and to Dr. Lina Patino (Michigan
State University)
for trace element chemistry by inductively-coupled plasma mass spectrometer (ICP-MS). All major and trace element data is reported
in this chapter. I prepared biotite
separates from two samples from the Cretaceous Chindona batholith and one
plagioclase separate from a volcanic rock of the Cretaceous Manto Formation for
Ar-Ar dating by Dr. Peter Copeland (University
of Houston). These samples have been irradiated but at
this time I have not received these Ar-Ar age results.
The Chortis block is a polycomponent block or “superterrane”
because it includes five distinct terranes whose boundaries are defined
principally by their contrasting geologic and magnetic basements (Figure 4.1B)
(Chapter 5). The basement of the Central
Chortis terrane is Paleozoic schist and gneiss of the Cacaguapa Group (Fackundiny,
1970; Horne et al., 1976a;
Metal Mining Agency of Japan (MMAJ), 1980;
Kozuch,
1991) and Grenville-age orthogneiss (Manton, 1996) (Figure 4.2). Basement of the eastern Chortis terrane
consists of Jurassic unmetamorphosed Agua Fria Formation strata overlying
metamorphosed Agua Fria strata (Gordon, 1993a;
Rogers,
1994; Chapter 3). Basement of the Southern
Chortis terrane, which is largely covered by up to 2 km of Neogene
volcanic rocks, is defined by its distinctive magnetic field that shows sharp
boundaries with adjacent terranes (Chapter 5).
The Siuna terrane consists of an assemblage of early Cretaceous oceanic
island arc rocks (including ultramafics) that developed on oceanic basement and
accreted to the eastern Chortis terrane in the late Cretaceous (Venable, 1994;
Chapter 3). Basement of the Northern
Chortis terrane consists of highly sheared high-grade gneiss,
felsic batholiths of late Cretaceous and early Tertiary age and Cretaceous age
metasedimentary strata (Horne et al., 1976b; Manton, 1987;
Manton and Manton,
1999; Ave Lallemant and Gordon, 1999).
Figure 4.2. Geologic
map of Honduras
Mesozoic strata, up to 4 km thick, overlie basement of the
central and eastern Chortis terranes (Figure 4.2). The middle Jurassic Agua Fria Formation and
its metamorphosed equivalent are exposed adjacent to and southeast of the Guayape
fault system in eastern Honduras
(Gordon and Muehlberger, 1994). The Agua
Fria Formation consists of marine sandstone and shale, shallow-water limestone,
terrestrial sandstone and shale and thin lignite beds known from both
reconnaissance and quadrangle mapping (Ritchie and Finch, 1985;
Gordon 1993b,
Rogers, 1995b). The Agua Fria Formation
was deformed and partially metamorphosed prior to deposition of Cretaceous
strata (Rogers, 1995b; Viland et al., 1996) although the age and tectonic
significance of this event is unknown. The
Tepemechin Formation is a thin conglomerate that unconformably overlies the Agua
Fria Formation and forms the base of the overlying Cretaceous carbonate
stratigraphy of the Yojoa Group (Gordon, 1993a) (Figure 4.2). Within the Yojoa Group, two thick units of
platform carbonates are recognized: the Upper and Lower Atima Formation locally
separated by the Mochito shale (Scott and Finch, 1999). The age of the Upper and Lower Atima
Formations is Albian-Aptian.
The late Cretaceous Valle de Angeles Formation records a shift
from carbonate to clastic deposition (Finch, 1981) (Figure 4.2). The Valle de Angeles Formation contains a
Lower, generally coarser interval and an Upper finer interval locally separated
by shallow marine carbonate strata of the Cenomanian-age Jaitique and Esquias
Formation (Horne et al., 1974;
Finch, 1981; Scott and Finch, 1999). Clastic rocks of the Valle de Angeles
Formation contain redbeds deposited both as debris flows in submarine settings
and as fluvial and debris flows in subaerial settings (Horne et al., 1974;
Wilson, 1974; Rogers and O’Connor, 1993).
The Upper Cretaceous Valle de Angeles Formation is the most widely
distributed Mesozoic unit on the Chortis block (Figure 4.2). The redbeds of Valle de Angeles Formation of
Carpenter (1954) was elevated to Group status by
Mills et al. (1967) with the
inclusion of the Illama Formation.
Subsequent workers have added local carbonate units as formation and
members to the Valle de Angeles Group (e.g. Finch, 1981). The grouping of all Mesozoic stratigraphic
elements above the middle Cretaceous Atima Formation into the Valle de Angeles
Group occurred absent detailed study of any Mesozoic stratigraphic element and
left unnamed and undefined the clastic redbeds for which the name Valle de
Angeles was originally intended. For
these reasons, I discontinue the use of the Valle de Angeles Group as a
catchall for the Mesozoic post-Atima stratigraphy of Honduras. Instead, individual formation names are used
where known. The upper and lower redbeds
are distinguished as the Upper Valle de Angeles and Lower Valle de Angeles
Formations respectively. In the absence
of intervening carbonate units to distinguish upper and lower redbeds, the term
Valle de Angeles Formation is used.
A series of four northwest-trending structural belts and
topographic mountain ranges record regional, late Cretaceous shortening of the
central Chortis terrane (Figure 4.2).
The westernmost Comayagua and the Minas de Oro belts are partially
buried by Miocene age pyroclastic deposits (Chapter 1) and overprinted by
active north trending rifts of western Honduras
(Chapter 2). The northern Montaña de
Flor and Frey Pedro belts are in the stable interior of the Chortis block, east
of the rifts and south of the actively deforming Honduras
borderlands (Chapter 2) (Figure 4.2).
The northwest trending belts are nearly perpendicular to the
northeast-trending Guayape fault system, active in the Neogene as a
right-lateral strike-slip fault (Gordon and Muehlberger, 1994) (Figure 4.2). The series of normal-fault bounded valleys
along the Guayape fault system including the one centered at Catacamas (Figure
4.2) formed in response to this phase of dextral motion on the Guayape
fault. The easternmost part of the
Montana de la Flor belt is truncated by one of these normal fault splays
related to shear on the Guayape fault system.
The easternmost part of the Frey Pedro belt is oroclinally bent in a
counter-clockwise sense. I infer that
this bending occurred as a result of an earlier period of left-lateral
strike-slip motion on the Guayape fault that accompanied formation of the late
Cretaceous Colon fold-thrust belt (Chapter 3).
The study area is centered on the eastern part of the Frey Pedro
structural belt and Agalta range north of the Catacamas valley (Figure 4.3). The study area is in a dry climatic zone
within the dissected, Central American plateau of Honduras (Rogers et al.,
2002; Chapter 1). Deep plateau
dissection and the tectonic stability of the area results in the preservation
and excellent exposure of a thick Cretaceous section in the Agalta range. Car access to this region of Honduras
is via the Juticalpa – Trujillo
unpaved highway. The main access
crossing the study area is the 24-km-long gravel road constructed in 1988 that
crosses the western Agalta range and Montana Frey Pedro, Montana Agua Blanca,
and Montana Jacaleapa at a right angle and connects the towns of San Francisco
de la Paz and Gualaco. Outcrops along
the road provide near-continuous exposure of the structure and stratigraphy of
the Frey Pedro belt (Figure 4.3). This
is the only major road in Honduras
that crosses one of the four northwest-trending structural belts shown on
Figure
4.2 at a high angle. Car and foot access
within the study area shown in Figure 4.3 is along a network of numerous,
unimproved logging roads and trails.
Figure 4.3. Location map of Frey Pedro study area
Previous geologic studies of the region were limited to mapping
in an area to the west of the study area by the Metal Mapping Agency of Japan (MMAJ,
1980a) and quadrangle mapping to the south along the Guayape fault system
(Kozuch, 1990; Gordon, 1993b, Gordon and Muehlberger, 1994).
Southernwood (1986) examined exposures of
limestone clast conglomerate of the Lower Valle de Angeles Formation along part
of the San Francisco de la Paz – Gualaco road prior to its improvement and
straightening in 1988.
The Agalta range splits into three parallel and smaller ranges or
“mountains” in the study area: the Montaña Frey Pedro is an antiformal block
cored by schist and gneiss as is the Jacaleapa range to its north (Figure 4.4).
