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CRYPTIC STRIKE-SLIP FAULTS OF THE CHORTIS BLOCK

GORDON, Mark B.; AVE LALLEMANT, Hans G., Dept. of Geology and Geophysics MS 126, Rice University, 6100 Main St., Houston, TX 77005-1892.

Since the Eocene (50 Ma) sinistral displacements of at least 1100 km occurred along the Cayman Trough, the boundary between the Caribbean and North American plates. Similar displacements should be expected along this boundary in Guatemala (Polochic fault, PF and Motagua fault, MF), but they have not been recognized.

Offsets of 130 km have been documented on the 1 km wide PF, consistent with u=100t ± one order of magnitude (u=displacement; t=thickness of fault zone) (Scholz, 1987). Although a Quaternary slip rate has been determined for the MF, the total amount of slip is currently unknown. Using Scholz' hypothesis and estimating the width of the Motagua Valley (5 km) as t for the post Eocene fault zone, u on the Motagua fault could be 500 km. Even so, these two faults do not account for the total strike-slip observed along the Cayman Trough.

Two other faults that have been suggested as candidates for accommodating some of this motion are the Guayape fault (GF) and the Jocotán-Chamelecón fault (JChF). Previous work shows that the GF may contribute on the order of 70 km of early sinistral slip (prior to neotectonic dextral slip). New mapping and structural analysis along the JChF indicates that the slip is sinistral and may still be active, particularly along the western portion of the fault. The fault is approximately 250 m thick suggesting u=25 km.

Even allowing for the large u (725 km) estimates above, and fault termination structures such as normal faults south of the principal plate boundary and thrust faults north of the boundary accommodating 100 km of slip (which is probably unreasonably large), the total amount of displacement along the Cayman Trough is not accounted for. We propose that distributed faulting across the entire width of the Chortís block on cryptic strike-slip faults accounts for the remaining u. We have recognized sinistral faults from the Island of Roatán in the north to the Gulf of Fonseca in the south. On the basis of this study and of published maps, we estimate that the missing sinistral displacement of at least 275 km can be accounted for by applying Scholz' equation to cryptic faults. The faults with broad gouge zones are more commonly exposed in the Cretaceous and older rocks than the Tertiary volcanic rocks, suggesting that the major slip occurred before 30 Ma.

In: Geological Society of America Abstracts with Programs, v. 27, p. 227-228, 1995

EVOLUTION OF NEOGENE MICROTECTONIC PHASES ON THE CHORTIS BLOCK (NORTHERN CENTRAL AMERICA)

GORDON, Mark B., Dept. of Geology, Rice University, Houston, TX 77251-1892.

The Chortís block has been moving eastward relative to North America since at least the Eocene. Deformation associated with the plate boundary has affected most of the Chortís block. At least five phases of faulting affected volcanic rocks of 19-10 Ma age.

(1) Tilted fault blocks were formed during a NE tension. The faults are now very low angle. The structural data were restored to paleo horizontal (bedding) prior to applying the fault slip inversion technique. The relationship between the bedding and fault tilt indicates an extension of at least 125% occurred locally.

(2) In central Honduras, NW-striking, high-angle normal faults formed during NE tension. These faults formed at approximately 14 Ma based on K-Ar dating. This phase could be a continuation of phase (1). This phase does not appear to be active in central Honduras whereas similar faults are active in east-central Honduras.

(3) SE compression and NE tension caused the formation of EW dextral faults and NS sinistral faults. This event could be contemporaneous with event (2). In central Honduras, this phase occurred before phase (5) (below).

(4) EW tension is associated with NS-striking grabens that are actively forming. Some faulting of this orientation was contemporaneous with phase (2) although the majority of these faults probably occurred during the formation of the major grabens after 10.5 Ma and continuing to the present.

(5) NS compression and EW tension were responsible for NW-striking dextral faults and NE-striking sinistral faults. NW-striking dextral faults have been proposed to link the various segments of the Honduras depression. Although no evidence was found for major NW-striking dextral faults, this phase may represent more diffuse faulting that links the basins. This phase is compatible with phase (4).

Some of the phases that are separated by the fault slip analysis are contemporaneous and may represent rapid permutations of the s2 and s3 axes. Displacement partitioning may be occurring in which strike-slip faults may occur during an overall extension. This deformation style shows that, although the principal Neogene deformation of the Chortís block has been EW extension, the deformation is more complex. The complexity may be related to counterclockwise rotation of the Chortís block as it is translated past the Yucatán Peninsula.

In: Geological Society of America Abstracts with Programs, v. 26, no. 7, p. A-207, 1994.

Active Faults of the Chortís Block

M B Gordon (Dept. Geology and Geophysics MS 126, Rice University, 6100 South Main Street, Houston, TX 77005-1892; tel. 713-527-4880; e-mail: markg@geophysics.rice.edu)

Active fault zones of the Chortís block have been identified by: (a) surface breaks from earthquakes; (b) earthquake hypocenters; (c) geologic mapping; (d) air photo interpretation and field reconnais-sance, and (e) Landsat, Seasat radar, and Shuttle Imaging Radar.

Faulting on the margins of the Chortís block occurs along the sinistral North America/Caribbean plate boundary and on the Middle America Trench, the Caribbean/Cocos plate boundary. Major earthquakes (M>7) have occurred along the North America/ Caribbean plate boundary faults (the Polochic, Motagua and Swan Islands faults) in 1785, 1816, 1856 and 1976. Active fault traces have been mapped onshore by geologic studies and offshore by SEAMARCII sonar imaging. A NE dipping Wadati-Benioff zone characterizes the Caribbean/Cocos plate boundary. Numerous small to intermediate earthquakes have occurred in this zone as well as rare great earthquakes.

Many intraplate faults occur within the Chortís block indicating that it is part of broad plate boundary zones between the North America, Caribbean and Cocos plates. Earthquakes occurring along the volcanic arc are typically shallow focus strike-slip events as determined from focal mechanism solutions and surface faulting. WNW striking faults (parallel to the arc) have dextral slip, and ENE faults have sinistral slip. Earthquakes and surface fault-ing occur along north-striking grabens south of the Motagua fault. Aftershocks of the 1976 Guatemala Motagua fault earthquake occurred on the Mixco fault system of the Guatemala City graben. Surface faulting accompanied these events. Earthquakes occurred along the Ipala graben in 1733 and 1765 and along the Comayagua graben in 1610, 1774, 1809 and 1982. Faults have been mapped in Quaternary age deposits. Other faults within the Chortís block are largely unconstrained by earthquake data. However, very fresh scarps appear on remote imagery data and aerial photographs. The La Ceiba, Río Viejo and Aguán faults have previously been identi-fied as probable active, sinistral faults parallel to the Swan Islands fault. Interpretation of aerial photographs and field reconnais-sance shows that the Chamelecón fault to the west is also active. Other possibly active faults include WNW-striking normal faults E of the Honduras depression and the dextral Guayape fault.

Published in: Eos (Transactions, American Geophysical Union), v. 75, p. 611., 1994

Multiple Phases of Neogene East-West and North-South Extension on the Chortís Block, Central America

M B Gordon (Dept. Geology and Geophysics, Rice University, Houston, TX 77251-1892; 713-527-4880; e-mail: markg@geophysics.rice.edu)

A wide zone of deformation is associated with the North America/Caribbean strike-slip plate boundary in Central America. E-W extension associated with active N-trending grabens dominates the Chortís block between the Guatemala City graben and the Honduras depression. On the basis of fault slip analysis, the style and direction of extension is constrained along the Honduras depression. This information is necessary to analyze the deformation associated with the plate boundary. The presence of Miocene volcanic rocks and an extensive road network makes the area ideal for study.

Previous field mapping has shown that faulting started before all of the volcanic rocks were deposited (Dupré, 1970). Evidence for pre 13.2±0.6Ma faulting includes a buried N-trending graben and a buried NW trending normal fault both on the eastern flank of the modern Comayagua graben. Because of the age bracketing, the two sets of faults probably are contemporaneous. WNW-striking microfaults in the older volcanic rocks are consistent with N-S extension (the faults have oblique slip). Previous workers have considered these faults to be dextral and to have formed as a recent reactivation of an earlier structural belt. Although the faults are probably a reactivation, little evidence for dextral shearing is present. Activity along the WNW normal faults appears to have ceased in the immediate vicinity of the Comayagua graben, but is present in a zone 50-200km to the E.

Inversion of fault slip data shows E-W extension is indeed causing the recent opening of grabens along the Honduras depression. More importantly, the extension occurs well to the north of the Chamelecón fault where previous workers had stated that normal faults do not exist.

Burkart and Self (1985) suggested that the Chortís block was broken into smaller blocks separated by grabens and rotating around Yucatán independently. The data from the Honduras depression supports their model except that it must be modified to account for episodic N-trending extension in the eastern part of the Chortís block.

In: Eos (Trans. Am. Geophys. Union), v. 74, no 43, p. 614, 1993

Revised Jurassic and Early Cretaceous (pre-Yojoa Group) stratigraphy of the Chortís block: Palaeogeographic and tectonic implications.

GORDON, MARK B., Department of Geology and Geophysics, Rice University, Houston, TX 77251-1892

Jurassic continental rocks, which in some locales include a marine component, have been mapped in a broad arc around the Caribbean Sea. These rocks have a key role in defining the early paleogeographic and tectonic evolution of the Caribbean region. Tectonic displacements of 1000km or greater that occurred in the Caribbean region during the Cenozoic have obscured the earlier tectonic history making accurate stratigraphic data essential to understand the tectonic history.

Geologic mapping on the Chortís block has defined two major Mesozoic units which are older than the well-known, lower Cretaceous Yojoa Group. The two units have been considered part of the same stratigraphic element, the Honduras Group. The Agua Fría Formation, a dominantly clastic mixed continental-marine unit, has been dated as Middle Jurassic based on recent discoveries of ammonites and plant fossils. Dates as old as Rhaetian for similar rocks have been reported in the literature, but have not been confirmed. No nonfaulted contact between the Agua Fría Formation and the Yojoa Group has been found. The Yojoa Group is commonly underlain by a conglomeratic unit, the unnamed siliciclastic member of the Honduras Group, that is lithologically distinct from the Agua Fría Formation. Available palynological data indicate that the unnamed siliciclastic member is restricted to the lower Cretaceous. The Honduras Group as currently defined is an inappropriate stratigraphic term. The two units of the group are not contiguous and have significant differences in lithology, and especially, in age. Inasmuch as its usage is causing stratigraphic confusion, I recommend abandoning the term Honduras Group. The Agua Fría Formation is left as an independent stratigraphic element. The unnamed siliciclastic member is herein named "the Tepemechín Formation" after a river south of Lake Yojoa.

Although thicknesses as great as 1500m have been reported for the Agua Fría Formation (e.g., Roberts and Irving, 1957), it is limited to rather narrow basins probably of strike-slip origin (Gordon, 1990). By contrast, the Tepemechín Formation is widespread yet thinner and was deposited immediately prior to the deposition of the Yojoa Group limestones. Thus, rifting probably occurred in the earliest Cretaceous and was followed by subsidence represented by the limestone. Thus, the Chortís block experienced strike-slip faulting in the Middle Jurassic followed by rifting throughout the block in the Early Cretaceous whereas neighboring regions experienced "pure" rifting from the Middle Jurassic to the Cretaceous.

In: Mesozoic and Early Cenozoic Development of the Gulf of Mexico and Caribbean region, Thirteenth Annual Research Conference, Gulf Coast Section, Society of Economic Paleontologists and Mineralogists, Programs and Abstracts, p. 27-28, 1992.

EARLY TERTIARY SINISTRAL MOTION ON THE GUAYAPE FAULT OF HONDURAS

GORDON, Mark B., Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78713-7909.

The Guayape fault (GF) is considered a major tectonic feature of the Chortís block (CB) by many authors, yet its tectonic evolution has only recently been studied by geologic mapping and reconnaissance studies. This work shows that the fault is currently a dextral fault which reactivates an earlier sinistral fault. Documentation for early sinistral faulting is given below (numbers refer to figure).

(1) A major structural contact has a large (>70 km) left-lateral separation across the fault. South of the Valle de Catacamas, basement rocks have been thrust over the Jurassic Agua Fría Formation west of the GF (Kozuch, 1989). This contact is offset on the eastern side of the GF.

(2) An SIR image reveals that Mesozoic structures have been deflected in sinistral shear close to the GF. This interpretation is confirmed by field data.

(3) North of the Valle de Catacamas, Mesozoic sedimentary rocks are deformed by folding and reverse faulting. The trends of these features (070) are in a proper orientation to have formed as subsidiary features during sinistral shear on the 030 GF.

(4) An early, sinistral tectonic phase is recorded by fault slip data. Inversion of the fault slip data indicates that s1 was 345° and s3 was 075. These values for tectonic stress are consistent with early sinistral shear along the GF. However, the majority of observed fault planes were formed during the later, dextral shear.

Geologic features along the fault only constrain the sinistral shear as occurring after the deposition of Cretaceous sedimentary rocks. The sinistral faulting ended prior to the deposition of ignimbrite sheets in the Miocene which have apparently only been affected by the dextral faulting. I propose that the sinistral faulting occurred during the early opening history of the Cayman Trough (Eocene). The fault later became inactive because the CB rotated into a position such that the fault was not in an ideal position to accommodate the slip along the plate boundary (PB). The fault has subsequently been reactivated as a dextral fault (Gordon, Eos, 68, 423, 1987).

In: Gordon, M.B., 1991, Early Tertiary sinistral motion on the Guayape fault of Honduras: Geological Society of America Abstracts with Programs, v. 23, p. A138.

Mesozoic Igneous Rocks on the Chortis Block: Implications for Caribbean Reconstructions

M B Gordon, (Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78713-7909)

The Mesozoic stratigraphy of the Chortis block is dominated by sedimentary rocks, but volcanic rocks have been reported in the literature from the Mesozoic section. Reports of a significant volcanogenic component in sandstones or conglomerates are less common. During recent field studies, volcanic rocks have been recognized directly east of the Guayape fault, in central Honduras, where they are interbedded with Jurassic Agua Fría sandstones and shales. These rocks have been mapped from Río Patuca to Río Wampú, and the volcanic section is at least 500 m thick. In the Catacamas Valley, several hundred meters of volcaniclastic rocks and lavas underlie the Aptian Atima limestone. Even though more volcanic rocks have been recognized in this study, Mesozoic volcanic rocks (mostly andesites and dacites) of the Chortis block are limited in extent. Except for the Jurassic rocks described above, they can only be mapped within about 150 km2 areas, and they are volumetrically insignificant.

Mesozoic intrusions are mostly small except for the 140 Ma Dipilto batholith (>1000 km2). Published dates of Mesozoic plutonic rocks range from 150 Ma to end of the Cretaceous. Most dates are Late Cretaceous. Although a broader range of dates will probably be found as more dates become available, few large intrusions of probable Mesozoic age have been mapped.

Most reconstructions place Chortis alongside Mexico throughout the Jurassic and Cretaceous and show a volcanic arc through Honduras. However, little evidence for a widespread volcanic arc exists because Mesozoic volcanics and intrusives are too small to demonstrate the presence of a continuous, active volcanic arc. Thus, there is a direct conflict between these reconstructions and available geologic and paleomagnetic data.

In: Eos (Trans. Am. Geophys. Un.), v. 70, p. 1342, 1989.

THE CHORTIS BLOCK: A RAFT OF MESOZOIC SEDIMENTS AND CENOZOIC VOLCANICS ON A SOLID FOUNDATION

GORDON, Mark B., and GOSE, Wulf A., Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78713-7909

Dengo (1969) introduced the name 'Chortis Block' for northern Central America south of the Motagua fault. Dengo established that the basement terranes north and south of the Motagua fault are distinctly different though each terrane is continental crust. North of the Motagua fault the Chuacús Series are high grade garnet amphibolites. South of the fault the basement is largely low-grade phyllites. Furthermore, no Late Paleozoic sediments such as the Santa Rosa Group have been discovered south of the Motagua fault. The basement terrane south of the fault, the Chortis Block, is also distinctly different from the Cretaceous oceanic basement of Costa Rica (Dengo and Bohnenberger 1969). Thus, the distinct nature of the basement relative to neighboring regions suggests that the Chortis Block is an independent terrane.

Mills et. al (1967) identify the Atima Limestone as the fundamental stratigraphic datum of Honduras. Although it is approximately age equivalent to limestones north of the Motagua fault, Clemons (1966) shows that limestones formed in distinct depositional environments. The first dated sedimentary rocks are the Middle Jurassic Agua Fría (formerly El Plan) clastics. Another clastic unit (unnamed siliciclastic member of the Honduras Group, formerly Todos Santos Formation) is stratigraphically below the Atima. Due to a lack of fossils, the relationship between the two parts of the Honduras Group is not well established. The Atima is also locally underlain by the Cantarranas Formation, an argillaceous limestone which is strongly deformed. It has been dated by ammonites as Neocomian. The Valle de Angeles Group, which is largely redbeds, overlies the Atima. The group includes at least two Late Cretaceous limestones which have been dated as Cenomanian and Turonian. Paleomagnetic data indicate that the youngest redbeds are probably Campanian. Thus, Valle de Angeles Group is clearly older than the Maastrichtian Sepur Formation north of the Motagua fault. No rocks similar to the Atima Formation and the Valle de Angeles Group exist in southern Central America. Inasmuch as the Chortis Block has a distinctive stratigraphy and is bounded by faults, it fits the Jones et al. (1981) criteria for a terrane.

Paleomagnetic data indicate that the Chortis Block, relative to neighboring provinces experienced a complex rotational history during the Cretaceous. These data further establish that the Chortis Block has a unique geologic history.

In: Geol. Soc. Am. Abstr. Progs., v. 21, p. 12, 1989.

Evidence from the Valle de Catacamas supports a right-lateral, neotectonic sense of slip for the Guayape Fault of Honduras

GORDON, MARK B., and MUEHLBERGER, WILLIAM R.,* Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78713-7909

The Guayape fault extends from the Caribbean coast southwest to the Pacific Coast at the Gulf of Fonseca. The Valle de Catacamas is a major physiographic basin centrally located along the W side of the Guayape fault. Increasingly deeper structural levels are encountered from W to E in the footwall block of the north range-bounding normal fault. At the W end of the valley Tertiary (?) mafic volcanics crop out in the footwall block. To the E the mafic volcanics are underlain by the Upper Cretaceous Valle de Angeles molasse. About 10 km farther E, the relief north of the frontal fault is much greater and quartz pebble sandstone (Honduras Group?) is overlain by limestone. The limestone is penetratively cleaved near the frontal fault with the cleavage essentially parallel to the fault. Thus, the normal faulting may be following the trend of a major preexisting weakness defined by the cleavage. At the NE end of the valley, the sedimentary rocks are underlain by metamorphic basement. Thus, the greater topographic relief and deeper structural levels in the footwall near the Guayape fault indicate that the normal faults have a greater displacement near the Guayape fault and this displacement decreases to the W away from the fault. Evidence for normal faulting along this range front includes mesoscale structures on fault planes and large scale faults which place younger rocks on older rocks.

In the SE corner of the valley, splays of the Guayape fault place metamorphic basement rocks against Tertiary volcanics exposed within the valley.

Preliminary analysis of fault slip data (slickenside planes and striae) suggests that s3 is approximately 310°, consistent with formation of the valley by right-lateral shear along the Guayape fault.

Fresh fault scarps, normal faulting on the N side, and the lower topographic relief on the S side suggests that the Valle de Catacamas is a young, asymmetric fault wedge basin formed by dextral shear along the Guayape fault.

In: Am. Assoc. Petrol. Geol. Bull., v. 72, p. 190, 1988.

EARLY TERTIARY SINISTRAL MOTION ON THE GUAYAPE FAULT OF HONDURAS

GORDON, Mark B., Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78713-7909.

(2) An SIR image reveals that Mesozoic structures have been deflected in sinistral shear close to the GF. This interpretation is confirmed by field data.

In: Gordon, M.B., 1991, Early Tertiary sinistral motion on the Guayape fault of Honduras: Geological Society of America Abstracts with Programs, v. 23, p. A138.

The Guayape Fault of Honduras: A Major Right-lateral Fault Cutting the Chortis Block

MARK B. GORDON, (Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78713-7909)

The Guayape Fault of Honduras was recognized as a major regional tectonic element by Elvir (1974) and Muehlberger (1976). The fault has been followed for 290 km from the Caribbean coast to the Valle de Jamastran region of Honduras (Ritchie and Finch, 1984). These authors suggest that the fault may connect with the 110 km long Choluteca linear from the Valle de Jamastran region to the Gulf of Fonseca. The linear appears to continue offshore in the Gulf of Fonseca as a disturbed zone in industry seismic data. Therefore, the Guayape fault probably cuts the entire isthmus and is at least 400 km long.

In connecting the Choluteca linear to the main segment of the Guayape Fault, a right-step must be made in the fault at the Valle de Jamastran. Because this region is experiencing extension, the right-step at this valley indicates that the fault is currently a right-lateral fault. Inversion of fault slip data from the Valle de Catacamas, 130 km further N, indicates a least principal stress direction consistent with the formation of the Valle de Catacamas as a fault wedge basin along a right-lateral fault. Just S of the Caribbean coast, the Sico Valley appears to be forming as a dextral pull-apart basin. Thus, the morphology of actively forming extensional basins indicates that the Guayape Fault is a major right-lateral fault.

Much of the slickenside striation data for both the Valle de Jamastran (Ritchie and Finch, pers. comm.) and the Valle de Catacamas may show a two stage history. I propose that an earlier left-lateral phase could have preceded the neotectonic right-lateral phase. As the Chortis Block is rotated back to its Early Tertiary position (Gose, 1985), the Guayape Fault is rotated to a position parallel to the current plate boundary (Motagua, Polochic faults). Therefore, in the Early Tertiary, the Guayape Fault could have been accommodating at least part of the left-lateral plate motion. As the Chortis Block rotated counterclockwise, the fault gradually became subparallel to the plate boundary in an orientation unsuitable to accommodate plate motion. It has since become reactivated as a major, right-lateral conjugate shear to the left-lateral plate motion.

In: EOS (Trans. Am. Geophys. Un.), v. 68, p. 423, 1987.

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