Robert D. Rogers
334 Williams Avenue North, Renton, Washington 98055 USA . Telephone: (206) 228-1592 email: email@example.com
The citation for this report is: Rogers, R.D., 1992, Geology of the Valle de Jamastrán Quadrangle, Honduras, Open File Report, Instituto Geográfico Nacional, Tegucigalpa, Honduras 56 p. A 1:50,000 scale geologic map of Valle de Jamastrán, Honduras accompanies this report: Rogers, R.D., 1995, Mapa Geológico de Honduras: Hoja de Valle de Jamastrán, Instituto Geográfico Nacional, Tegucigalpa, Honduras, escala 1:50,000.
Analysis of Valle de Jamastrán faults with motion indicators show that most features are related to the early sinistral and later dextral motion of the GFS and conform to a strain ellipse model of shearing. A N60oW dextral continuation of the Montaña de Comayagua Structural Belt trend is also apparent from this analysis.fig. 1). Field mapping in the Valle de Jamastrán and the surrounding region was carried out discontinuously between January and September of 1991 and augmented by air photo interpretation (1:20,000 and 1:40,000 scale) as part of the effort by the Honduran government to inventory its geological resources. Since the initiation of the National Geological Mapping Program of Honduras in the late 1960's, the project has published maps of 24 quadrangles (1:50,000 sale) with eight more currently in production (fig. 1 and table 1) and two editions of a national map (1:500,000 scale). This effort has been carried out by graduate students at the University of Texas at Austin, Wesleyan University and SUNY-Binhgamton; faculty at Tennessee Technological University and the College of Charleston; Dirección General de Minas e Hidrocarburos personnel; and Peace Corps volunteers. These maps along with reports or theses form the basic geological knowledge of Honduras used by mining and petroleum interests, academic studies and development projects. The Valle de Jamastrán mapping project was initiated to study the postulated relation between the Jamastrán valley and the Guayape Fault System (GFS) which is the dominate structural feature in the region. fig. 2). The Chortis block with its lithologically and temporally distinct basement is separated from the Maya block of the North American (NOAM) plate to the north by the Chixoy-Polochíc-Motagua transform fault and from the Cocos plate to the west by the Middle American Trench (MAT). The southern margin of the Chortis block with the Chorotega block (Costa Rica and Panama) may be the Santa Elana-Hess escarpment suture, while the eastern Chortis block margin is undefined (Dengo, 1985). The Chortis block has experienced rotation, shearing and stretching following Late Cretaceous-Early Tertiary collision with the Maya block to the north. This tectonic positioning of Honduras gives the country its rugged and fractured mountainous topography interspersed with structural basins.
Honduras is cut by northeast and northwest trending shear zones (fig. 2) presumably active since Chortis collision (suturing) with the Maya block before the Early Tertiary (Emmet, 1983). Based upon paleomagnetic evidence, Gose (1985) concludes that Honduras has experienced 50o counterclockwise rotation relative to North America since the Late Cretaceous. Dextral northwest trending shears, such as the Montaña de Comayagua structural belt (MCSB) (Emmet, 1983), along with extension related to the opening of the Cayman Trough (Gordon and Muehlberger, 1988) produced the discontinuous series of north-south trending structural basins known as the Honduras Depression (fig. 2). The northeast trending shears were first thought to be sinistral, forming the conjugate pairs to the northwest trends (Emmet, 1983 and Manton, 1987), and may have accommodated significant plate motion as part of a diffuse margin between the CARIB and NOAM plates (Gordon, 1990). Gordon (1990) finds evidence of dextral motion for the extensive northeast trending Guayape Fault System (GFS) in eastern Honduras based on geometric analysis of the Catacamas valley. Finch and Ritchie (1991), while defining the GFS, corroborate Gordon's interpretation and the resultant picture suggests large sinistral motion for northeast trends followed by a reversal to lesser dextral motion of at least the GFS. Gordon proposes counterclockwise rotation of the Chortis block around the NOAM-CARIB plate margin to account for the reversal of the earlier sinistral motion.
Many subsidiary structural features, the compressive deformation of the Cretaceous sequence and the extension of the entire stratigraphy, appear related to the northwest and northeast trending shears. The MAT arc subduction, pushing the Chortis block from the southwest, is related to the thick Tertiary volcanic sequence. Lack of compressional deformation in the Tertiary sequence points to a decrease in compression following Tertiary deposition as the Chortis block became locked into place following collision with the NOAM plate. The extensional faulting of Tertiary units and incision into bedrock of meandering river courses evidence regional uplift and may reflect the influence of the MAT in the general uplift of Honduras. Interaction of MAT and intra-Chortis shears are not reported but may be significant as they are coeval.fig. 2). Quadrangle mapping and reconnaissance geologic investigation of the GFS identifies this as a zone of initially sinistral displacement followed by a reversal to dextral motion (Finch and Ritchie, 1991; Gordon, 1990; Kozuch, 1991; Kozuch and Rogers, 1991). Five major river drainages form the trace of the shear zone (the Ríos Guayambre, Guayape, Pataste, Tinto, and Paulaya). Structural basins form along the GFS from near the town of El Paraíso to the Caribbean (Valle de Jamastrán, Azucualpa, Catacamas, Culmí, and Sico basins). The Sico basin is interpreted as a strike-slip basin induced by dextral GFS motion (Finch and Ritchie, 1991). The Catacamas and Azucualpa basins form along N50oE normal faults splays related to the dextral GFS motion (Gordon, 1990). The Culmí area may have experienced complex basin formation; down dropped and receiving sediments during the sinistral motion of GFS and then uplifted to its current position as a divide between the Tinto and Paulaya drainages during dextral GFS motion. Based on analysis of basin geometry Gordon (1990) cites the Valle de Jamastrán as a pull-apart basin formed by dextral GFS motion. However, Gordon acknowledges that only some of the structural elements conform to a dextral induced opening. Valle de Jamastrán basin development will be elaborated upon in a following section. Gordon and Muehlberger (1988) indicate that the Choluteca linear, which takes a right step or bend relative to the GFS near El Paraíso, is the southern continuation of the GFS. However, Finch and Ritchie (1991) contend that the GFS terminates near El Paraíso. Field data is enigmatic and considering the reversal of motion both interpretations may be correct. fig. 3) covers Paleozoic metamorphic basement through Quaternary volcanics. Gneiss, schists, phyllites, marbles and meta-intrusives form the basement Cacaguapa Schist. This sequence has undergone at least three deformation episodes with the oldest being at least 305 Ma (Horne et al., 1976).
The Jurassic-Cretaceous Honduras Group clastics composed of conglomerates, sandstones, shales, coal, carbonates, and minor volcanic beds were deposited on the basement and have experienced minor metamorphism in places (Gordon, 1990; Finch and Ritchie, 1985). The Honduras Group sediments may be quite thick and extensive in eastern Honduras as only the edges of this depositional basin have been explored. Honduras Group depositional environment is varied, including fluvial, coastal plain and marine deposition. Contrary to recent review (Donnelly et al., 1990), the El Plan Formation of Carpenter (1954) seems to be a member of the extensive Jurassic Agua Fría Formation of the Honduras Group (Finch, personal comm.; Gordon, 1990; Kozuch, 1990; and personal observation) and is currently being mapped as such by field workers.
Deposition of a thick sequence of shallow marine carbonates, the Yojoa Group, occurred in Early Cretaceous Albian to Aptian time in central Honduras (Donnelly et al., 1990; Kozuch, 1991). This marine transgressive sequence is the thickest limestone unit exposed in Honduras.
Predominantly terrestrial red bed deposition of the Valle de Angeles Group commenced in Late Cretaceous times interspersed by transgressive marine carbonates (Jiatique and Esquías Formations of Cenomanian age) in central Honduras (Finch, 1981). Quartz pebble conglomerates dominate the lower Valle de Angeles Group while fining to shales and sandstones in the upper Valle de Angeles Group. Based on the sedimentary structure of the Valle de Angeles Group in the Tegucigalpa area, Rogers and O'Conner (in press) interpret this group as proximal and distal tropical alluvial fans dominated by hyper-concentrated fluid flows. Little evidence exists for continuing Valle de Angeles Group deposition past the Late Cretaceous in central Honduras.
The Tertiary volcanic sequence rests unconformably on the Mesozoic strata. Matagalpa Formation mafic flows (Oligocene) have spotty exposure and may interfinger with the thick sequence of predominantly ignimbrite tuffs of the Miocene Padre Miguel Group. Padre Miguel Group deposition extends over most of Honduras but becomes thicker and younger to the south nearing the influence of the MAT subduction zone (R. Harwood, personal comm.). Minor fluvial and lacustrine deposits are present and increase in frequency and thickness in the upper Padre Miguel Group apparently grading into the fluvial volcanoclastic deposits of the Gracias Formation. Late Tertiary-Quaternary mafic flows cap the older volcanic sequence in places and appear related to structural trends in central Honduras. Late Tertiary and Quaternary alluvium appears as a thin veneer in many of the structural basins in Honduras. Intrusives of varying ages and compositions punctuate the stratigraphy.figs. 4 and 5). This section presents descriptions, the nature of contacts, the thickness, age and lithologic constraints, the morphologic expression, and interpretation of depositional environments for each mapped unit. fig. 5 ). Loma Copatillo (hill in grid 68;59) and the low hills flanking the mountains along the southwest margin of the Valle de Jamastrán near El Obraje expose the Cacaguapa Schist. Quartz mica schists and gneisses with quartz and calcite veins, displaying red iron staining on weathered surfaces while light to dark grey on fresh surfaces, make up this unit. Buried contacts with the overlying stratigraphy are probably faulted noting the close proximity of Cacaguapa strata to the Valle de Angeles red beds while the intervening Honduras Group is missing. The only observed contact is unconformable with overlying volcanic beds in the southwest section of the quadrangle.
Minimum age constraints on the basement sequence are limited to the Middle Jurassic age of the overlying Agua Fría Formation of the Honduras Group and radiometric ages of 140 Ma for the nonmetamorphosed Dipilto batholith to the southeast (by Horne and Clark, unpl. data in Donnelly et al., 1990). The basement may be as old as 305 Ma as reported by Horne et al. (1976) and has undergone at least three metamorphic events with the most recent being before 140 Ma. Morphologic expression of the basement is generally subdued in the quadrangle forming low, rounded, moderately dissected hills in the valley or along its side.
Metasedimentary rocks are exposed in the eastern section of the quadrangle south of the GFS, along the trend of the GFS to the northeast, and north of the GFS in a band oriented oblique to the shear zone (fig. 5). Unit thickness may be 700 meters or greater. The metasedimentary sequence is intruded by quartz monzonite in fractures and bedding foliations along and to the south of the GFS trend. The thickness of the metasedimentary sequence could be exaggerated if a large intrusive is present under the southeastern mountains. The contact between the Cacaguapa basement and Honduras Group metamorphic rock is buried and may be faulted. Finch and Ritchie (1990) on the Danli quadrangle show the basement-Honduras Group contact as tectonically deformed, meaning that the contact was folded and faulted in a post-Honduras Group deformation event. This event may have slightly metamorphosed the lower part of the Honduras Group section. This interpretation is a possibility in the Valle de Jamastrán.
The phyllites of the Honduras Group are of a lower grade than the basement phyllites exposed in central Honduras. Higher grade metamorphism is encountered in the east of the quadrangle. It is noted that the last regional basement deformation event produced the minor deformation of the Honduras Group and deformation is not related to later Tertiary intrusions or GFS. Evidence for this includes the lack of any metamorphic units younger than the Honduras Group, including units exposed near intrusions and the GFS; nonmetamorphosed Honduras Group exposed near the intrusion on the El Paraíso quadrangle; and wide spread metamorphosed Honduras Group south of the GFS on the Cifuentes, El Maguelar and Río de Apali (Jutiapa) quadrangles. The massive Jurassic (140 Ma) Dipilto batholith exposed on the Honduras-Nicaragua border to the southeast may be related to the slight metamorphism of the Honduras Group in this region as the metasedimentary strata are principally exposed in the deeper, older(?) sequence of post-basement rocks to the east and south of the GFS. Yasushi Watanabe (personal comm.) of the Japanese Geological Survey recently obtained K-AR dates of 186 and 243 Ma from micas of Honduras Group phyllites in the Danli and Valle de Jamastrán quadrangles. These dates are believed to represent deformation age. This information suggests that deposition of Honduras Group sediments and metamorphism are nearly coeval.
The upper contact of the metamorphic Honduras Group grades with decreasing metamorphism into the nonmetamorphosed Honduras Group sequence or is in fault contact with the Valle de Angeles Group. The metasedimentary unit expresses itself in the rugged, well dissected hills and mountains southeast of the GFS and the more rolling hills along and north of the GFS.fig. 5). Gordon (1990 and 1992) mapped a Honduras Group carbonate unit on the Santa Maria del Real quadrangle to the northeast, while Finch and Ritchie (1990) show fault emplaced limestone blocks within Honduras Group strata on the neighboring Danli sheet. While faulting is pervasive within the highly deformed Honduras Group, no evidence for fault emplacement of the calcareous strata was seen in the Valle de Jamastrán.
Honduras Group calcareous strata are comprised of massive blue-grey limestone with recrystalized calcite and a low grade marble. Exposed near Cuchilla de Cal is a sequence that grades upward from sandstone to calcareous phyllite to extremely indurated marble with chert nodules. The carbonates were also observed as weathered, crumbly white and pink limestone with a chalky appearance. Strata are a few tens of meters thick and thinner units (<10 meters) were mapped elsewhere in the Honduras Group sequence associated with calcareous phyllites. Typically, the carbonates outcrop along ridges where exposed by erosion, therefore the upper contact has been stripped away. However, at 71.6;48.5 a roadcut exposes a calcareous (marble?) unit within Honduras Group calcareous phyllites confirming the existence of the carbonates within the Honduras Group sequence. No paleontologic dates exist for the carbonates of the Honduras Group in the Valle de Jamastrán or elsewhere.fig. 5). Composed primarily of grey to dark green shales and mudstones, and tan to white, fine grained, rounded, well-sorted sandstones and quartz arenites, these sediments are mapped as the Honduras Group. Minor grey to tan-orange sandstone and reddish shales also exist. Matrix supported quartz pebbles appear in some sandstone beds. Minor white, well sorted, rounded, quartz pebble-cobble conglomerates (56.0;55.0) are interbedded within a shale-sandstone sequence that contains coal and fossil wood fragments. Bedding is predominantly planar in the Honduras Group, though some trough cross-bedding (up to 0.3 meters) exists in thick arenite strata. The arenites, where exposed, generally appear in thick (1-5 meters) to massive beds. Some sandstone units form lenses within the shaley sequence. The mudstones and shales are generally laminated or thinly bedded. Occasional down cutting of lensoidal sandstone into shale beds occur. Symmetrical ripples exist in a cutting at 69.0;65.5.
Thickness for the nonmetamorphosed Honduras Group sequence is at least 900 meters and may be much more as the upper contact is not exposed except by faulting with the Valle de Angeles Group and the lower contact is gradational with the metasedimentary strata of the Honduras Group. Aggregate Honduras Group thickness (meta and nonmetamorphosed) in the Valle de Jamastrán is 1600 meters as the units appear to represent an upper and lower division and not side by side. Total Honduras Group thickness in eastern Honduras may be much greater considering the extensive exposure in the quadrangles to the northeast (Villa Santa and El Maguelar) and southeast (Río de Apali and Cifuentes) and based on reconnaissance trips east of the GFS that found the Honduras Group extending at least to the Ríos Patuca and Cuyamel (fig. 6). Honduras Group sandstones and shales form the high rugged relief of the northern Valle de Jamastrán quadrangle. Streams are highly incised, forming steep valleys. In at least one area (57.2;55.2) a northeast trending sandstone ridge forms an impressive vertical hogback that exhibits structural control.
Age of all Honduras Group strata is constrained by fossil plant fragments and ammonites dated as Bajocian to Early Bathonian of the Middle Jurassic (Ritchie and Finch, 1985). The upper age limit for the Honduras Group sediments is the Albian to Aptian (lower Cretaceous) age of the Yojoa Group carbonates exposed elsewhere in Honduras. Gordon (1990) adopts a Necomian (Lower Cretaceous) age for the carbonates of the Honduras Group by suggesting that the Cantarranas Formation should be included within the Honduras Group. An Early Cretaceous age is reported for Honduras Group equivalent strata based on palynology (Gose and Finch, 1987).
The depositional environment of the Honduras Group sequence represents coastal plain fluvial sedimentation punctuated by transgressive marine inundations and braided rivers, possibly tectonically induced. Coal, shale and mudstone with sandstone lenses indicate floodplain environments with small sandy streams occasionally dissecting the finer sediments. The alternating sandstone-shale sequences (or sandstone-phyllite where metamorphosed) are fluvial floodplain deposits. The clean arenites with trough crossbeds demonstrate upper flow regime conditions, thus indicating large river channels within the floodplain deposits. The calcareous shale, sandstone, phyllite, and limestone deposits indicate shallow marine transgressions. Finch and Ritchie (1990) report shallow marine ammonite fossils in the Danli quadrangle, while fossil wood and leaf impression appear near Danli, in the Valle de Jamastrán, and in other Honduras Group localities to the east. Fossil plants identified as Calamites and Sphenopsid ferns by Robin Morris (Missouri Botanical Gardens, personal comm.) indicate an alluvial lowland environment. The symmetrical ripples suggest a tidal influence, possibly lagoonal deposition. Clast supported coarse conglomerates interbedded with sandstone, shale and coal beds in the upper(?) Honduras Group indicate a change in flow regime and transporting capacity of the paleo-rivers. Whether this is evidence of tectonism or changing hydraulic conditions is uncertain, but considering the conglomerates position near the Honduras Group-Yojoa Group unconformity elsewhere in Honduras the tectonic interpretation is preferred.
In summary, the Honduras Group reflects initial coastal plain deposition of coal, floodplain fines, and fluvial sands with intermittent marine incursions of carbonates, calcareous sands and silt. This grades upward through a thick sequence of coastal plain fluvial deposits and terminates in coarse clastic fluvial deposits indicating renewed tectonic activity. The inclusion of mildly metamorphosed sediments to the Honduras Group and the presence of these sediments throughout eastern Honduras, based upon a reconnaissance trip along the Río Patuca, suggests a much larger Honduras Group depositional basin than previously thought (fig. 6). It must be emphasized that the presence of metamorphic rock does not necessarily indicate Cacaguapa basement in Honduras. Low grade phyllites, quartzites and marbles may be part of Honduras Group strata and should be mapped with caution until their stratigraphic position can be determined for certain.fig. 5). With the exception of the ridge forming Cerro Los Jobos, all Valle de Angeles red beds are found on the basin floor or down faulted along the valley margins. Sedimentary structures are best exposed along the road between Danli and San Diego and also near Rancho Jamastrán, across the road from the mine field to the south. Mostly planer beds are apparent and conglomerates are generally matrix supported. Occasional clast supported conglomeratic lenses appear as well as minor cross-bedding and fining upward sequences, but no cut and fill structures appear. Relief is subdued in red bed terrain with conglomerates forming minor ridges. All red beds weather to reddish soils that contain quartz pebbles where the conglomerates erode.
Valle de Angeles and Honduras Group conglomerates differ in several aspects. Valle de Angeles conglomerates are generally red, intermediately rounded and sorted, matrix supported pebble clasts of quartz, limestone and basement metamorphics. In contrast, the white Honduras Group conglomerates contain clast supported, well sorted and rounded, of always quartz pebble and cobble clasts. Some Quaternary alluvium of reworked red beds strongly resemble Valle de Angeles strata but are poorly consolidated to unconsolidated and lack folding and faulting.
The lower contact of the Valle de Angeles beds is faulted against the Honduras Group where exposed. The only ridge of red beds, at Cerro Los Jobos, rests on metasedimentary strata of the Honduras Group and was thus presumably faulted with the intervening nonmetamorphosed strata missing. Various Tertiary and Quaternary volcanic units unconformably cover the red beds at 67.8;52.6 and near grid 54;51. The exposed Valle de Angeles Group thickness is at least 100 meters but probably no more than 300 meters. Age of the strata is constrained between the lower Cretaceous Yojoa Group and the Tertiary volcanics in central Honduras (Finch, 1981). Intra-Valle de Angeles limestones (Jaitique and Esquias Formations) can separate the lower coarse clastics from the upper finer sandstones and siltstones. No limestones of the Valle de Angeles Group appear in the Valle de Jamastrán. Rogers and O'Conner (in press) separate the Valle de Angeles Group in Tegucigalpa into the lower coarse clastic Villa Nueva Formation and the upper fine grained Río Chiquito Formation based upon lithologic and sedimentary differences and the observed gradational contact between the two formations. This distinction is not developed in the Valle de Jamastrán as the group is much thinner, poorly exposed and probably faulted, thus obscuring the distinction.
Depositional environment for the Valle de Angeles Group was interpreted by Rogers and O'Conner (in press) as humid tropical alluvial fans. This interpretation is supported by the red bed stratigraphy in the Valle de Jamastrán. Matrix supported conglomerates, planar bed forms and the absence of scour suggest deposition by hyper-concentrated and debris flows (Pierson and Costa, 1987 and Smith, 1986). Lenses of clast-supported conglomerates with minor crossbeds and fining upward sequences indicate normal fluvial deposition occurring during waning stages of hyper-concentrated flows and between debris flow events. Gypsum and evaporites noted elsewhere in the Valle de Angeles Group of Honduras do not necessary indicate an arid climate, as restricted circulation in a basin or a monsoonal climate can produce evaporite deposition in a humid region where evaporation periodically exceeds precipitation. Paleomagnetic studies by Gose (1987) places the Chortis block near its present longitude, in the tropics, during the Late Cretaceous deposition of the Valle de Angeles Group, thus precluding an extensive arid climate for the Chortis block during red bed deposition. Clays produced by tropical weathering and necessary for the initiation of high-viscosity and debris flows would have been abundant in the tropical climate that existed on the Chortis block during the Late Cretaceous. The presence of minor breccia units indicate short transport distances and deposition near a highland source.
The Moroceli quadrangle to the west of Danli contains thick Valle de Angeles beds of clast supported conglomerates with trough cross stratification. This represents a major fluvial channel within the fan deposit. Gradual fining upward of the group in the Tegucigalpa area represents waning of the tectonic uplift that served as the source of the sediment or a modification of the basin hydraulics. The quartz clasts of the group are similar to, and generally smaller, than the clasts of the Honduras Group and thus may be reworked from eroded Honduras Group strata. If the clasts are reworked, then rounding does not indicate distance of transport from the source area. The clasts originated from the abundant quartz veins within the basement. Weathering of basement schist, gneiss and phyllite supplies the clays necessary for the non-Newtonian flows deposited in the red beds and also may provide the iron from weathering of basement pyrite that gives the Valle de Angeles Group its distinctive red coloration (R. Finch, personal comm.).
Limestone clasts in the Valle de Angeles conglomerates are of unknown origin, but two sources are likely; the carbonates of the Honduras Group or the missing Yojoa Group strata. The limestone clasts are extremely indurated suggesting a Honduras Group origin, and at present there is no nearby source of the Yojoa carbonates. It is not known if the Yojoa Group was stripped off between the Honduras Group and Valle de Angeles Group deposition or if it was not deposited at all. If deposited, the Yojoa Group must have been thin in order to have been completely eroded off. With either option, the lack of the Yojoa Group in southern Honduras (none in the mapped Valle de Jamastrán, Danli and Tegucigalpa quadrangles) indicates a limit of the Yojoa Group basin to the south. Paleomagnetic data indicating a 50o counterclockwise rotation of the Chortis block since the Late Cretaceous (Gose, 1985), paleogeographic reconstructions that show the Chortis block moving into place from the west (Pindell and Barrett, 1990, plate 12), and field mapping suggests a highland (possibly early MAT subduction) to the southwest in Cretaceous time limiting Yojoa Group carbonate deposition and providing the source for the later Valle de Angeles fan sediments.
In summary, the Valle de Angeles Group was deposited by hyper-concentrated, debris, and fluvial flows of a tropical alluvial fan sequence probably from a southwestern tectonic highland in the Late Cretaceous.fig. 5). The intrusive is highly fractured, weathers to grussy soils and occasionally contains calcite veins and rounded quartz grains. The dikes intrude along fractures and upturned bedding foliations of the Honduras Group metasedimentary sequence. This intrusion is compositionally similar to the Danli intrusive (Tdi of Finch and Ritchie, 1990) and is believed to be a continuation of this unit. The intrusion continues to the El Paraíso sheet where it intrudes "classic" Padre Miguel ignimbrites leaving roof pendants (34.7;45.0, 34.1;44.8, 39.1;43.6). This precludes the intrusive from being genetically related to the pre-Cretaceous granitic Dipilto batholith (140 Ma) to the south. Paul Pushkar, as reported in Williams and McBirney (1969), obtained an enigmatic K-AR age of 11 Ma for an intrusion believed to be the Dipilto at the time. This age, however, agrees well with a post-Padre Miguel intrusion and is adopted here as a tentative age for the Danli and Valle de Jamastrán intrusions. The fractured nature of the Valle de Jamastrán intrusive, the dike swarm and emplacement along GFS trend in the Tertiary suggest a causal relation of the intrusion with the GFS. The intrusion is covered by younger mafic volcanics on the Danli and El Paraíso sheets. The 300 meters of quartz monzonite exposed in the Valle de Jamastrán is a minimum thickness as the intrusive probably extends much deeper. fig. 5). These dark maroon to purple, sandy to boulder-size blocky aggregates of andesite display steep, rugged topography with slopes often littered with house-size boulders containing car-size clasts. With thickness of 400-500 meters, these flows cover Padre Miguel volcanics and the Tertiary intrusion on the Danli and El Paraíso sheets. These flows were originally described as the Matagalpa Formation by Williams and McBirney (1969) and later as Tv2 mafic flows by Finch and Ritchie (1990). A ridge of basement strata blocks these flows from extending into the Valle de Jamastrán to the east, while on the El Paraíso sheet they extend from more than 10 kilometers west of the volcanic center conveniently located at the intersection of four topographic quadrangles (Valle de Jamastrán, Danli, El Paraíso, Río de Apali). Richard Harwood (Peace Corps volcanologist) confirmed the nature of these flows and probable eruptive center. He further interprets the mafic flows as phreatomagmatic deposits that occurred when mafic magma intersected groundwater resulting in violent eruptions of blocky "lahar" appearing material. Several zones of coarse mafic flows were found encompassing fine grained sandy deposits of mafic volcanic detrius. The mafic flows are fractured and faulted in the northeast GFS orientation and a near perpendicular northwest trend. These trends form the highest topographic ridges of the eruptive center. The upper mafic flow contact is in near concordance with the overlying Late Tertiary-Quaternary basalts. fig. 5). The basalts can appear nearly white on weathered surfaces and occasionally exhibit an exfoliation fracture. These basalts form a relatively thin (100 meters) cap on the volcanic sequence. Basalts display the same fracture and depositional trends as the underlying mafic flows. Topographic alignment of the ridges of the volcanic center identify the eruptive fissures for the volcanics, and their alignment with the predominate structural trends suggests these fissures are structurally controlled. Several small basalt flows that moved up along faults appear within the valley itself. A notable occurrence of this is at 59.5;55.6 where a small, dark reddish-maroon basalt flow seeped out along the normal bounding fault of the northwest valley wall. Small, dark, medium grained mafic dikes containing feldspar and biotite cut the Tertiary intrusive and Honduras Group strata at various locations within the Valle de Jamastrán quadrangle and may be feeder dikes for the mafic flows. Late Tertiary-Quaternary basalts in the Valle de Jamastrán have not been dated radiometrically, but elsewhere in Honduras K-AR ages of 2.0 to 0.5 Ma have been obtained for similar flows (ENEE,1987). fig. 5). The older, large alluvial fans consist of angular, unsorted, matrix-supported clasts of lithic fragments weathered from the bedrock highlands. The most prominent fan appears in the northwest corner of the valley composed primarily of detritus of Valle de Angeles Group, Honduras Group, and Padre Miguel volcanic derived from drainages to the west. It is highly dissected and distinguishable from Valle de Angeles red beds by its lack of consolidation and the inclusion of Tertiary volcanic fragments. Another major fan appears to the northwest of the intrusive ridge along the GFS on the southeast valley margin. This finer grained fan is composed of detritus from the Tertiary intrusive. The older fans are inactive and have been dissected, reworked into terraces and some have younger active fans depositing on their upper reaches. fig. 5). These terraces consist of sub-angular to rounded, cobble- to sand-size, clast supported material of the various lithologies present in the Valle de Jamastrán, with an observable change across the valley from mostly Padre Miguel material in the west to Honduras Group material in the east. Older higher terraces are dissected forming a hummocky topography, while younger surfaces are planar. Small limited Quaternary terraces appear along the present river courses. Elevation between the terraces vary between a few to ten meters with at least six surfaces present. The terraces are most pronounced in the eastern part of the valley along the drainages of the Río del Hato, Río San Francisco, Río Los Almendros, and the Río Guayambre. A resistivity and magnetic survey (Carruthers, 1983) indicates a deepening of the valley alluvium to the east (fig. 7), an observation confirmed by field investigation and aerial photography interpretation. Terraces and alluvial fans southeast of GFS along the Río San Francisco are elevated or hanging, indicating substantial lowering of base level in excess of other areas in the Valle de Jamastrán. No unequivocal examples of faulted alluvium appear in the valley. Base level appears to have been lowered in the valley following terrace deposition; however aggradation is occurring on terrace surfaces along the Río Las Laras and Río San Antonio. Complex fluvial response or recent tectonic activity explains this departure from the overall trend of degradation.
Alluvium currently exists as sand, pebble and cobbles along the major river drainages and represents the mobile phase actively being transported out of the Valle de Jamastrán watershed.Fig. 8). As mentioned earlier, previous investigators (Gordon, 1990; Finch and Ritchie, 1991) define the GFS as a major strike-slip feature with an early sinistral phase followed by dextral motion. Field mapping in the Valle de Jamastrán allows for further definition of the GFS and subsidiary structures.
The principle shear of the GFS at present is a broad (approximately 4 km wide) N45oE trending brecciated zone extending from grid 69;48 northeast to grid 81;61 in the Valle de Jamastrán (fig. 8). Rocks within this zone are tectonically brecciated with fractures generally parallel to the GFS trend. Along the central shear in the Valle de Jamastrán the Honduras Group metasediment bedding foliations are steep and oriented along GFS trend. The Tertiary intrusive was injected along these foliations in places. The intrusive is unroofed in the southern end of the Valle de Jamastrán shear. Valle de Angeles strata make their only appearance as a ridge-former at Cerro Los Jobos where it is faulted onto the Honduras Group metasedimentary strata. The brecciated zone, with tight isoclinal folding of Honduras Group metasediments and streams parallel to the shear zone, extends to the northeast through El Maguelar quadrangle. Brecciated Honduras Group and the Río Apali drainage are oriented along GFS trend to the southwest. The principle shear of the GFS forms the southeast margin of the topographic Valle de Jamastrán.
A N60oE normal fault forms the northwest valley wall and drops the stratigraphically higher Valle de Angeles red beds into Valle de Jamastrán while uplifting the Honduras Group to the northwest (fig. 8). The contact between these units defines the fault trace. Northeast topographic alignment of Honduras Group terrain to the northwest of the normal fault and northeast aligned topographic highs of red beds within the valley suggest a series of northeast trending normal faults down to the southeast. A basalt flow at 59.4;56.6 also defines the valley -bounding fault trace.
A second N60oE trending normal fault drops Valle de Angeles red beds onto Honduras Group metasedimentary strata to the northwest of Santa Maria (fig. 8). The red beds of Cerro Los Jobos were originally down-faulted as part of this block but have since undergone at least 200 meters of uplift. The morphologic expression of these N60oE trending faults is sharp and these faults are believed to be relatively young.
Metamorphic basement strata exposed at Loma Copatillo (68;59) and on the eastern flank of the Late Tertiary-Quaternary volcanic center in the southwest valley are extremely close to Valle de Angeles strata with the intervening Honduras Group missing. These buried contacts must be faulted and are interpreted as N20oW normal faults down into the valley (fig. 8). These faults are not exposed and could also be interpreted as thrusts verging into the valley. The Loma Copatilla fault may explain the occurrence of structurally deeper levels of Honduras Group metasediments to the east. The faulted basement block on the eastern flank of the Late Tertiary-Quaternary volcanic center served to block the mafic flows into the Valle de Jamastrán and therefore must have been exposed by faulting prior to the eruptive event. The N20oW trending faults are buried; therefore they are older than the exposed N60oE trending faults.
The Río San Antonio displays a rectangular drainage pattern in the northwest section of the quadrangle that reflects structural control (fig. 8). Fracture and fault plane orientations obtained along the river bed include northwest trending dextral and northeast trending normal faults that support this interpretation.
Faulting was observed in all units and significant intra-formational deformation (faulting and folding in the Honduras Group and faulting in the Valle de Angeles Group) is apparent. Folding is most evident in the Honduras Group metasedimentary strata near the GFS, but this may be related to poor exposure preventing recognition at other localities. North trending quartz filled gash fractures (up to one meter in length) appear at 71.8;63.8 and a few other localities within the quadrangle.
Figure 9a shows northwest to southeast cross section A-A' that crosses the principle shear of the GFS and northwest boundary fault. Tertiary quartz monzonite intrudes along the principle shear and the increased hydrostatic pressure of the injection of fluids into the fault zone should accentuate the faulting as the stress-strain envelope is pushed into tensional failure, creating a positive feedback between the intrusive and the shear zone. This relationship eases as the intrusion cools and solidifies. The time of the intrusion (post-Padre Miguel Group (11 Ma?)) should have been a period of accelerated motion on the GFS. The intrusion has been uplifted and unroofed since emplacement.
The southwest to northeast profile B-B' (fig. 9b) crosses the southwest and northeast bounding faults and displays the relation of the Late Tertiary-Quaternary volcanics to the valley. Also shown is the deepening of the alluvium cover to the east.Figure 10 displays these orientations broken into dextral, sinistral, normal and reverse slip orientations. These data are compared to oriented strain ellipse models showing secondary faults in order to determine the motion of the GFS and other structural trends that affect the Valle de Jamastrán deformation (fig. 11).
A sinistral strain ellipse model with the principle displacement zone (PDZ) oriented N45oE (GFS trend) should result in synthetic sinistral Riedel (R) and P shears of N30oE and N60oE respectively, antithetic dextral Riedel (R') shears of N20oW, normal faults oriented N10oW and reverse faults of N70oW (fig. 11a). The strain ellipse also shows the principle axes of extension and compression.
A dextral strain ellipse model with the PDZ oriented N45oE (GFS trend) should result in P shears of N30oE, R shears at N60oE, R' shears of N20oW, normal faults oriented N70oE and reverse faults of N10oW (fig. 11b). Compressional and extensional axes are near perpendicular to those of the sinistral N45oE strain ellipse.
Emmet (1983) compared oriented slip indicators to a strain ellipse model to aid in determining the N60oE dextral motion of the Montaña de Comayagua structural belt (MCSB) that extends from Montaña de Santa Barbara to at least San Juancito in central Honduras. Northwest fracture trends cut the Late Tertiary-Quaternary volcanic flows of the Valle de Jamastrán and form strong topographic and stream lineaments in the regions to the north and south of the Valle de Jamastrán quadrangle. As topographic lineaments suggest that the MCSB trend continues to the Valle de Jamastrán region, a N60oW dextral strain ellipse model is also compared to the Valle de Jamastrán slip data set (fig. 11c).
Total slip data (fig. 10) shows a wide scatter of orientations that is not unexpected in a zone experiencing multiple shearing events, but when separated into sinistral and dextral GFS trends and into the dextral N60oW MCSB trend, the data conform well to the expected fault orientations. No single deformation event can explain the scatter of the data.
Sinistral GFS oriented slip data (fig. 12) shows the dextral orientations associated with antithetic R' shears and the synthetic sinistral slip associated with the PDZ, R and P shears including the numerically minor P shears. Normal faults show a scatter about the predicted northern trend and include the orientations of the N20oW basin boundary faults between the metamorphic basement and the Valle de Angeles strata. Purely dextral GFS motion precludes these fault orientations. One reverse fault falls along the predicted orientation.
The dextral GFS trend (fig. 13) displays the synthetic PDZ, R and P shear data that match the predicted orientations. R' antithetic shears show a wide scatter around the expected value. Normal fault orientations match closely with the predicted values and include the N60oE valley boundary faults. Again, purely sinistral GFS motion precludes these normal fault orientations. Two reverse faults match the predicted orientations.
To explain the remaining data and the northwest topographic lineaments of the Valle de Jamastrán region, extension of the dextral MCSB trend was included in the strain ellipse model analysis (fig. 14). The N60oW dextral model when compared to the data shows a close match to the synthetic PDZ, R, and P shears. Antithetic R' shears group fairly close about the predicted value while normal and reverse fault orientations display a greater scatter. Most of the slip orientations corresponding to the N60oE dextral trend were obtained to the east of the actual Valle de Jamastrán.
It should be noted that N60oW dextral and N45oE sinistral motion results in similar strain ellipse orientations (fig. 11). This is manifested as an overlapping of the observed fault orientations for these trends (figs. 12 and 14).
The resultant picture suggests that the Valle de Jamastrán basin has been affected by both early sinistral and later dextral GFS motion. Basin-bounding normal fault orientations support this interpretation as both the N20oW and N60oE faults could not have formed during a single simple shearing episode. The sequence of GFS shearing is first left then right lateral as morphologic expression of the N20oW faults are buried and subsequently older than the N60oE faults related to the dextral motion on the GFS.
Coeval northwest dextral and northeast sinistral shear would appear as conjugate fractures of pure shear (fig 15a). Later switching to dextral motion of the GFS dissolves this relation and may permit a counterclockwise rotation to be taken up along the principle northwest and northeast shears while developing a connecting dextral shear (fig. 15b), possibly the E-W trending shear on the Danli geologic map (Finch and Ritchie, 1990).
Some pre-GFS data and older faults reactivated by the GFS that are not in the predicted orientations are probably incorporated into the analysis. It is assumed, however, that the most recent deformation will show the predominant trends.
An alternative explanation for the scatter of fault orientations was developed that assumes the strain ellipse model has a self-similar nature (fig. 16). This alters the strain ellipse model to include secondary strain ellipses oriented along the secondary shears (R, R', and P). The secondary shears overprint a smaller, possibly localized, strain ellipse on themselves distinct from the primary strain ellipse induced by and associated with the PDZ. In the modified ellipse model, the secondary shears ( R, R', and P) act as the PDZ to produce a third order set of shears (r, r', and p) in the predicted orientations. Third order ellipse producing fourth order shears may be possible theoretically, but material competency is a limiting factor preventing fracturing as rock strength exceeds the shear stress produced by the higher order shears.
The second order strain ellipses with associated third order shears can result in a scattering of fault orientations about the predicted values of the primary strain ellipse model. Unfortunately, this idea could not be tested fully in the Valle de Jamastrán region due to the complicated histories and motions of the primary shears. It has been brought to the author's attention that Moody and Hill (1957) presented an almost identical mechanical model of shearing and interested readers should consult this paper.
As mentioned previously, a geophysical study of the Valle de Jamastrán shows an increasing depth to resistant bedrock in the eastern valley (fig. 7) (Carruthers, 1983). A zone of non-resistant material at least 500 meters in depth parallels the GFS northwest of Cerro Helado. I reinterpret this material as sheared weathered bedrock, probably the Tertiary intrusive, and not a deep alluvium trough as previously proposed (Carruthers, 1983).
Stream courses along the N60oE trending fault marking the northwest valley boundary are generally deflected to the left as they enter the valley indicating tectonic control (fig. 17a). While this could be viewed as evidence of left lateral motion on this fault, four things must be considered. The first is that mapping shows this fault is strongly normal; second, valley alluvium thickens to the east; third, most faults in the Valle de Jamastrán region are oblique, showing dip-slip and strike-slip motion; and forth, streams flow down gradient regardless of the motion of any fault they may cross. The last seemingly obvious point has important implications. A stream crossing an east trending left lateral fault, for example, that also has normal motion hinging down to the west would show stream deflection to the right instead of to the left as the stream flows down gradient (fig. 17b). In the case of the stream deflections along the N60oE fault, oblique (normal and dextral) motion hinging the fault down to the east best explains the stream deflections and the thickening of alluviam to the east (fig. 17c).
Along the eastern side of the Río San Francisco, extending south onto the Río de Apali sheet, alluvial fans are hanging three to ten meters above younger terrace surfaces. Similar, but less spectacular, hanging fans exist south of the GFS along minor tributaries such as the Quebrada del Aguila. These fans were left hanging following a base level reduction probably due to tectonic uplift. Northeast of the Valle de Jamastrán on El Maguelar sheet and along GFS trend, vertically faulted alluvium was discovered during a reconnaissance trip with M. Kozuch (University of Colorado). At least two meters of vertically displaced terrace deposits are exposed along new road cuts. Soil samples are being dated with thermoluminosity methods by Kozuch to determine the age of the displacements.
Another geomorphologically interesting area in the Valle de Jamastrán is along the GFS near El Ojo de Agua where at least 50 meters of alluvial terrace deposits exist in a valley without any major surface drainage. This valley must have been carved out and filled with sediment by a large river, probably the Río San Francisco, that has since changed its course from the northeast to the northwest. An old, partially consolidated, thick alluvial terrace sequence near Chichicaste and Santa Maria appears too extensive to have been deposited by the present drainage. This suggests that the Río Guayambre at one time flowed to the northeast of the GFS near the present road between Chichicaste and Santa Maria instead of through and to the southeast of the GFS. Subsequently, the Río Guayambre was diverted to its present southeast course through the narrow water gap in the GFS near Chichicaste. The presence of bedrock hills in the old Río Guayambre course suggests a tectonic cause for the river diversion.
The alignment along GFS trend of the vertically faulted alluvium, the hanging valleys, the unroofed Tertiary intrusive, the extensive, elevated old alluvium of Chichicaste and Santa Maria and the probable diversion of the Río Guayambre suggest a Late Tertiary uplift of the hills along the GFS. This uplift is termed the Chichicaste push-up. The push-up modified the tectonic Valle de Jamastrán extensively in the northeast.
No Padre Miguel deposits large enough to account for the amount of Padre Miguel material found in the valley's terrace deposits are exposed in the watershed that currently drains into the Valle de Jamastrán. It was noted that Padre Miguel material in the terraces increases in amount in the western part of the topographic valley and that extensive Padre Miguel deposits exist to the west near El Paraíso. These Padre Miguel volcanics are now separated from the Valle de Jamastrán by a Late Tertiary-Quaternary volcanic center. I propose that the original tectonic Valle de Jamastrán extended much further to the southwest and received sediments from the Padre Miguel outcrops to the west before Late Tertiary-Quaternary volcanism modified the valley by blocking the drainage. This ended deposition of terraces composed of Padre Miguel material.
While it is likely that tectonism has modified the Valle de Jamastrán since its development, rock outcrops within the valley do not appear to have been faulted through the alluvial cover. The general impression is that the outcrops have been buried by and have even contributed debris to the alluvial cover.
The Yojoa Group carbonates were not deposited in southern Honduras or, alternatively, were thin and stripped off before Valle de Angeles red bed deposition.
Valle de Angeles Group sediments were laid down on a major unconformity evident by the lack of Yojoa carbonate strata on top of Honduras Group sediments. The missing Yojoa Group, lack of intra-Valle de Angeles shallow marine carbonates, paleomagnetic studies (Gose, 1985), and depositional facies analysis (Rogers and O'Conner, in press) suggest that the Valle de Angeles conglomerates, sands and fines originated from southern highlands of basement and Honduras Group. Deposited as debris flows, hyper-concentrated flows and fluvial flows that generally grade upward, the Valle de Angeles Group represents tropical alluvial fans that spread to the north and received some carbonates during marine transgression into central Honduras during the Late Cretaceous. Highland uplift may be related to early MAT subduction as the red beds contain tuff layers and volcanic detrius in southern central Honduras. The Valle de Angeles strata were compressionally faulted and folded during Late Cretaceous-Early Tertiary collision of the Chortis block with the Maya Block along the Motagua suture zone in Guatemala (Donnelly, et al. 1990).fig. 9b). The northeast extent of the valley is uncertain; a later uplift modified the basin geometry. During sinistral GFS motion the Tertiary intrusives that cut the Tertiary Padre Miguel Group on the adjoining El Paraíso sheet were intruded as massive stocks and as dike swarms injected along upturned bedding and vertical fractures within the Honduras Group metasedimentary strata. This felsic intrusion is dissimilar to the younger mafic volcanics. The intrusive may have locally enhanced GFS motion by the induction of tensile failure.
Tertiary northwest-trending dextral shearing imposes a similar stress regime as the sinistral GFS, possibly enhancing basin development, but its influence on the Valle de Jamastrán basin is minor due to the proximity of the GFS.
Late Tertiary-Quaternary dextral GFS shearing produced N60oE oblique dextral transtensional faults as the northwest boundary fault of the basin. This faults Valle de Angeles red beds into the valley against Honduras Group mountains. Somewhat thicker alluvium in the eastern part of the valley and left stepping streams suggest oblique hinged motion down to the east on the normal fault. The valley-filling terraces were being deposited by rivers draining Padre Miguel terrain to the southwest near El Paraíso.
N45oE trending low hills were uplifted creating the Chichicaste push-up. A brecciated zone defines the uplift. The Tertiary intrusive was unroofed at this time. The previously down-dropped block of Cerro Los Jobos was uplifted to its current ridge forming position. The Río Guayambre was diverted from the northwest of the uplift to its present course through the push-up near Chichicaste while the Río San Francisco was diverted from El Ojo de Agua wind gap to its present position leaving 50 meters of alluvium in this gap. The hanging alluvial fans and elevated terraces of the Río San Francisco were tectonically uplifted. The push-up certainly modified the Valle de Jamastrán tectonic basin. Uplifted alluvium in and to the northeast of Santa Maria may have been part of the Valle de Jamastrán basin and if so, gives the valley a more elongated form than previously thought.
Continued northwest-trending dextral shearing following cessation of sinistral GFS motion may have kept northwest trending normal faults active during dextral GFS motion while reactivating older faults with northwest trends.
At two periods during GFS motion igneous activity is associated with the fault. The Tertiary intrusive, emplaced along the GFS, is the first and may have accelerated fault motion. Later mafic volcanics used the GFS and probably the MCSB fractures as vents.fig. 15). Gordon (1990), Emmet (1983), Finch and Ritchie (1991), Manton (1987) and now this report credit intra-Chortis block structural basins to motion along strike-slip faults. Manton (1987) roughly defines a north-trending zone in central Honduras that marks the intersection of these trends, but he does not speculate on the characteristics of this zone. Northwest dextral and northeast sinistral shears can be viewed as conjugate sets to pure shear with a northerly compression axis (fig. 15a). The intersection of the GFS and the MCSB is near the Zamorano and Moroceli valleys. While these areas were not studied in detail, fault scarps forming valley boundaries have orientations that are expected from a pure shear intersection (fig. 15a). Richard Markey (Peace Corps) is mapping the Moroceli quadrangle and should shed light on this relation.
Dextral MCSB and dextralGFS trends may combine to induce a counterclockwise rotation of small blocks within the Chortis block. Gordon (1990) favored a counterclockwise rotation of the Chortis block around the NOAM plate as the mechanism inducing the dextral motion on the GFS. If the northwest and northeast shears are discrete but related structural features, then rotation would have to be taken up along an east-west trending dextral shear connecting the earlier shears (fig. 15b). The large shear developed across the southern margin of the Danli quadrangle fits this description, although Finch and Ritchie (1990) do not display motion on this fault. They consider it part of a "horse tail" splay that terminates the dextral GFS (Finch and Ritchie, 1991). Related dextral MCSB and GFS motions may result in a tight rotation centered within the Chortis block and not to rotation of the Chortis block about the NOAM plate.
Mentioning of a GFS termination leads into the question of a link between the GFS and the Choluteca linear to the southwest. Muehlberger and Gordon (1988) and Gordon (1990) link the two structural features, while Finch and Ritchie, (1991) and Ritchie and Finch (1985, 1989) believe the structures are separate, with the GFS dying out near El Paraíso. Considering the diverse motion history of the GFS, both of these camps may be right. Rotation of, or within, the Chortis block along with reversal of GFS motion provides a solution; the GFS and Choluteca linear may have been continuous during early sinistral motion, while becoming discrete during later dextral motion as rotation moved the GFS and Choluteca linear out of alignment. This rotation also explains the kink in the N55oE Choluteca linear and N35oE GFS trends as well as the apparent splaying out of the GFS to the west near El Paraíso and Danli.
An alternative explanation to the cause of reversal of motion along the GFS exists that does not depend upon Chortis block rotations about the NOAM plate. Motion of the CARIB plate past the NOAM plate is about 1cm/y, while subduction of the COCOS plate under the CARIB plate at the MAT is about 8 cm/y (Mann et al., 1990). It seems reasonable that the greater motion on the MAT influences the GFS, which is oriented perpendicular to the MAT and is in a position to serve as an intraplate shear and is not a curvilinear feature such as the Motagua Fault System. Subduction at the MAT is variable along the length of the subduction zone and this may initiate variable motion on the GFS. If, for example, the rate of subduction along the MAT to the northwestof the GFS were 1% faster than that to the south, 8 kilometers of dextral displacement would have occurred over the last 10 million years. This would be enough motion to create the basins produced by dextral motion observed along the GFS. This postulated relation does not preclude intra-Chortis counterclockwise rotation but could be the cause of rotation that does not depend on CARIB-NOAM plate boundary interaction. Further refinement of MAT subduction rates are needed to address this possibility.
The GFS is the largest of several northeast trending shears in eastern Honduras and Nicaragua. Lineaments of the Río Patuca, the Río Coco and the Cifuentes valley just east of the GFS define a broad complex zone of intraplate deformation that probably has a history similar to the GFS. The features are discontinuous and probably were intermittently active since the Early Tertiary. Their discontinuous nature, especially the Cifuentes valley in Nicaragua, suggests that they may have taken up some GFS motion as one fault became stuck and inactive while another took up the deformation. Obviously, much more study is needed to understand the tectonics and structure of the eastern Chortis block.
A final note on the GFS concerns the unusual Culmí valley located along the north central GFS in Olancho (fig. 2). This valley is filled with Tertiary volcanics and alluvium but serves as a drainage divide of the Río Tinto to the south and the Río Paulaya to the north with both rivers actively pirating tributaries of the Río Wampú that drains the Culmí basin. Based upon a reconnaissance of the basin with M. Kozuch and study of topographic sheets, it is proposed that the Culmí basin was formed during sinistral GFS motion that has been uplifted by later dextral motion on the GFS. Reversal of several water courses (Tinto, Pataste, possibly Guayape) from northern drainage through the Wampú drainage to their current southern drainage is probable if this interpretation is correct.
The tectonic significance of the Honduras Group is speculative, but several observations warrant discussion. Deposition of coastal plain to shallow marine sediments occurred on an active continental margin. Low grade metamorphism shortly after burial suggests rapid deposition in an actively subsiding basin. A scenario involving active tectonics is also supported by the presence of felsic volcanic deposits within the Honduras Group in eastern Honduras. The metamorphism of the Honduras Group was noted to increase in grade to the southeast and I have not encountered any large masses of Cacaguapa Schist continental basement to the southeast. The basement of Nicaragua appears to be a continuation of the low grade metasedimentary sequence found in eastern Honduras and contains possible oceanic crust in the form of pillow basalts (Donnelly et al., 1990). A tectonic solution that accounts for the above features is the presence of a fore-arc basin (Honduras Group depositional basin) of a subduction zone. This would place the oceanic crust (?) of Nicaragua against the continental crust of Honduras near the northeast trending Honduras-Nicaragua border.
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