Caribbean-South American Plate Boundary Research Articles

Strain calculations of active tectonic blocks in northeastern Venezuela from GNSS analysis

We study translation, rotation, and strain of active tectonic blocks in Northeastern Venezuela from the Global Navigation Satellite System (GNSS) observations. Since the installation of the geodetic network in 2003, one of the goals was to place at least three observation sites at each tectonic block to study the deformation of each one. Based on this premise, we define at least seven blocks: Bergantín and Caripe blocks south of the El Pilar Fault (EPF), and Cariaco Gulf, Land bridge, Paria, North Peninsula, and Margarita Island blocks north of the EPF. Our preferred block modeling shows angular rotations from 0.02 to 0.29° Ma−1. It is known that the EPF concentrates the active deformation in this region of the Caribbean-South American plate boundary. However, the existent rotation could accommodate part of the motion. The strain rate tensors (SRT) indicate NW-SE compression and NE-SW extension for the western blocks. To the east, the 4-Land bridge block keeps the NW-SE compression but shows a decrease in the extensional component. The 5-Paria blocks show a complete inversion in the sense of semi-axis. Additionally, we evaluate the possibility of different motions in Margarita block calculating translational vector, rotational velocity, and strain from three GNSS sites at each side thereof. Our results show remarkable similarities for the Macanao Peninsula and Eastern Margarita Island, pointing to both belonging to a single block.

Tectonic inversion in the Caribbean-South American plate boundary: GPS geodesy, seismology, and tectonics of theMw6.7 22 April 1997 Tobago earthquake

On 22 April 1997 the largest earthquake recorded in the Trinidad-Tobago segment of the Caribbean-South American plate boundary zone (Mw 6.7) ruptured a shallow (~9 km), ENE striking (~250° azimuth), shallowly dipping (~28°) dextral-normal fault ~10 km south of Tobago. In this study, we describe this earthquake and related foreshock and aftershock seismicity, derive coseismic offsets using GPS data, and model the fault plane and magnitude of slip for this earthquake. Coseismic slip estimated at our episodic GPS sites indicates movement of Tobago 135 ± 6 to 68 ± 6 mm NNE and subsidence of 7 ± 9 to 0 mm. This earthquake was anomalous and is of interest because (1) its large component of normal slip and ENE strike are unexpected given the active E-W dextral shearing across the Caribbean-South American plate boundary zone, (2) it ruptured a normal fault plane with a low (~28°) dip angle, and (3) it reactivated and inverted the preexisting Tobago terrrane-South America ocean-continent (thrust) boundary that formed during early Tertiary oblique plate convergence.

U–Pb detrital zircon age patterns of Cenozoic clastic sedimentary rocks in Trinidad and its implications

The Cenozoic evolution of northern South America can be simplified as a diachronous, west-to-east change from north-facing passive margin, to active convergence and transcurrent plate margin. As the current eastern end of the Caribbean-South America plate boundary, the Trinidad area records the most recent tectonic regime transition. Documenting the provenance of Cenozoic clastic rocks in Trinidad provides insights into the spatial and temporal relationships between mountain range uplifting, sediment dispersal, and drainage system development along the eastern end of the Caribbean-South American plate boundary. Four Eocene to Pliocene age outcropping sandstone samples were collected from Trinidad and 545 detrital zircon grains were analyzed using the U–Pb LA-ICP-MS method. A total of 404 analyses with less than 10% discordance were used for the final interpretation. Results show that the age distribution of the Eocene to the Early Oligocene samples is very restricted and dominated by Precambrian age grains with age peaks at ~1400Ma, ~1800Ma, and ~2000Ma, typically derived from the Guyana Shield. In contrast, the Late Oligocene and younger samples show much broader and mixed age distribution that includes Mesozoic, Paleozoic, and Precambrian peaks. This age shift was interpreted as arriving of the Great Caribbean Arc and oblique collision between the South America and Caribbean plates. Continuing collision uplifted the Andes belts to the west. The Eastern Cordillera of Colombia, the Merida Andes, and the northern Venezuela coastal ranges then became the second primary sources. Offshore sedimentation switched from the passive margin with multiple small drainage systems to the active convergent and transcurrent plate margin with a single large river-dominated delivery system.

Gravity inversion using seismically derived crustal density models and genetic algorithms: an application to the Caribbean-South American Plate boundary

SUMMARY Crustal density models derived from seismic velocity models by means of velocity–density conversions typically reproduce the main features of the observed gravity anomaly over the area but often show significant misfits. Given the uncertainty in the relationship between velocity and density, seismically derived density models should be regarded as an initial estimate of the true subsurface density structure. In this paper, we present a method for estimating the adjustments necessary to a seismically derived density model to improve the fit to gravity data. The method combines the Genetic Algorithm paradigm with linear inversion as a way to approach the non-linear and linear aspects of the problem. The models are divided into three layers representing the sedimentary column, the crystalline crust and the lithospheric mantle; the depths of these layers are determined from the seismic velocity model. Each of the layers is divided into a number of provinces and a density adjustment (Δρ) value is found for each province so that the residual gravity (difference between the observed gravity anomaly and the anomaly calculated for the seismically derived model) is minimized while keeping Δρ between predefined bounds. The preferred position of the province boundaries is found through the artificial evolution of a population of solutions. Given the stochastic nature of the algorithm and the non-uniqueness of the problem, different realizations can yield different solutions. By performing multiple realizations we can analyse a set of solutions by taking their mean and standard deviation, providing not only an estimate of the Δρ distribution in the subsurface but also an estimate of the associated uncertainty. Synthetic tests prove the ability of the algorithm to accurately recover the location of province boundaries and the Δρ values for a known model when using noise-free synthetic data. When noise is added to the data, the algorithm broadly recovers the features that define the known model despite greater standard deviations of the solutions and the occurrence of artefacts in the mean solutions. The algorithm was applied to four profiles across the Caribbean–South America Plate boundary. Some general patterns in the distribution of Δρ were observed consistently in the profiles and are correlated with the interpretations of the velocity models. Positive Δρ values in the sedimentary layer, negative Δρ values for island arc and extended island arc crust with an abrupt change to positive values in South American crust, and positive values in the mantle under the continent and island arc with a transition to negative values under the Caribbean oceanic crust.

Receiver function imaging in strongly laterally heterogeneous crust: Synthetic modeling of BOLIVAR data

We resolve a large (∼20 km) discrepancy in Moho depth determined from PdS receiver functions (RFs) and from active source seismic profiling in the complex Caribbean-South American plate boundary zone in eastern Venezuela. As part of the BOLIVAR experiment 20 broadband stations were deployed along an active source profile to record teleseisms. Using the extremely heterogeneous crustal model obtained from active source data, we generated 2D finite-difference elastic wave synthetics and from them calculated receiver functions and CCP stacks. We compare the observations with synthetic sections that have been spatially sampled at 0.25 km to 40 km. The densely sampled synthetics show that several events in the field data that were originally interpreted as the Moho are multiple reflections within sedimentary basins. Where the Moho has the steepest dip under the plate boundary the CCP stacks fail to image the Moho well, regardless of the density of spatial sampling. A suitable spatial sampling criterion for clearly imaging the lower crust and Moho is to overlap Fresnel zones by 50% at Moho depth, which for the 1 Hz receiver functions examined here, requires an instrument spacing of 15–20 km, with the actual field data density ranging from 20 km to 100 km.

A 3-D lithospheric model of the Caribbean-South American plate boundary

A 3-D structural model of the Caribbean-South American plate boundary was constructed by gravity modeling. The model was constrained by four wide-angle seismic refraction sections, Moho depth estimations from receiver functions, and additionally seismological hypocenters, surface geology, and geodynamic information. Density values were calculated from empirical velocity-density functions, and mineralogical-chemical composition considering specific P/T conditions. We tested different structural models for Western and Eastern Venezuela. In the final model, the fit of the measured and modeled gravity fields for a long Caribbean slab in Western Venezuela was better than the fit obtained for a short one. This interpretation is consistent with the constraining data. The slab is interpreted to extend further to the south beneath Northern Colombia and culminates in the area of the seismic cluster of the Bucaramanga nest. The modeling estimates a slab dip angle under Maracaibo and Merida Andes of 15°, which increases to 32° below 100 km depth. The dip direction of approx. N150°E ± 5 increases lightly eastward. In Eastern Venezuela, considering its short wavelength, lineaments analyzed from gravity data (by curvature methods and Euler deconvolution) seem to be related to shallow structures and density contrast in the Serrania del Interior and not from a deep detached slab beneath the continental crust. It is deduced from modeling results that this slab configuration has a very small influence on the gravity field. The slab was modeled according to the subduction-transform propagation model with purely westward subduction and a slab break off along a vertical dip-slip tear through the lithosphere.

Prehistoric earthquakes on the Caribbean–South American plate boundary, Central Range fault, Trinidad

Research Article| August 01, 2010 Prehistoric earthquakes on the Caribbean–South American plate boundary, Central Range fault, Trinidad Carol S. Prentice; Carol S. Prentice 1U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar John C. Weber; John C. Weber 2Department of Geology, Grand Valley State University, Allendale, Michigan 49401, USA Search for other works by this author on: GSW Google Scholar Christopher J. Crosby; Christopher J. Crosby 1U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025, USA *Current address: San Diego Supercomputer Center, University of California–San Diego, MC 0505, 9500 Gilman Drive, La Jolla, California 92093-0505, USA Search for other works by this author on: GSW Google Scholar Daniel Ragona Daniel Ragona 3Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California–San Diego, La Jolla, California 92121, USA †Current address: BP America, 501 Westlake Park Blvd., Houston, Texas 77079, USA Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (8): 675–678. https://doi.org/10.1130/G30927.1 Article history received: 02 Dec 2009 rev-recd: 19 Feb 2010 accepted: 21 Feb 2010 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Carol S. Prentice, John C. Weber, Christopher J. Crosby, Daniel Ragona; Prehistoric earthquakes on the Caribbean–South American plate boundary, Central Range fault, Trinidad. Geology 2010;; 38 (8): 675–678. doi: https://doi.org/10.1130/G30927.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Recent geodetic studies suggest that the Central Range fault is the principal plate-boundary structure accommodating strike-slip motion between the Caribbean and South American plates. Our study shows that the fault forms a topographically prominent lineament in central Trinidad. Results from a paleoseismic investigation at a site where Holocene sediments have been deposited across the Central Range fault indicate that it ruptured the ground surface most recently between 2710 and 550 yr B.P. If the geodetic slip rate of 9–15 mm/yr is representative of Holocene slip rates, our paleoseismic data suggest that at least 4.9 m of potential slip may have accumulated on the fault and could be released during a future large earthquake (M > 7). You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Caribbean–South American plate boundary at 65°W: Results from wide‐angle seismic data

We present the results of the analysis of new wide‐angle seismic data across the Caribbean–South American plate boundary in eastern Venezuela at about 65°W. The ∼500 km long profile crosses the boundary in one of the few regions dominated by extensional structures, as most of the southeastern Caribbean margin is characterized by the presence of fold and thrust belts. A combination of first‐arrival traveltime inversion and simultaneous inversion of PmP and Pn arrivals was used to develop a P wave velocity model of the crust and the uppermost mantle. At the main strike‐slip fault system, we image the Cariaco Trough, a major pull‐apart basin along the plate boundary. The crust under the Southern Caribbean Deformed Belt exhibits a thickness of ∼15 km, suggesting that the Caribbean Large Igneous Province extends to this part of the Caribbean plate. The velocity structures of basement highs and offshore sedimentary basins imaged by the profile are comparable to those of features found in other parts of the margin, suggesting similarities in their tectonic history. We do not image an abrupt change in Moho depth or velocity structure across the main strike‐slip system, as has been observed elsewhere along the margin. It is possible that a terrane of Caribbean island arc origin was accreted to South America at this site and was subsequently bisected by the strike‐slip fault system. The crust under the continental portion of the profile is thinner than observed elsewhere along the margin, possibly as a result of thinning during Jurassic rifting.

Estimating the Age of Near-Shore Carbonate Slides Using Coral Reefs and Erosional Markers: A Case Study from Curacao, Netherlands Antilles

Carbonate mass failures are widespread, particularly along steep tropical shorelines, however, the age, frequency, and cause of these failures are often poorly constrained. As shorelines undergo intensive human development, understanding when carbonate slopes failure occur is becoming increasingly important. Using both land and marine-based observations, we present methods for making valuable first-order age estimates of near-shore slope failures. As a case study, we analyze the partially submerged lower carbonate terrace along the southern shoreline of Curacao, Netherlands Antilles. This terrace sits adjacent to a well-documented carbonate slope failure. Our combined land-sea study constrains the slide contact, the amount of post-slide coral reef growth, and the location of erosional shoreline notches. From these observations, we place limits on the timing of slope failure, and conclude that the Caracas Bay slide did not occur recently (within the last ~3000 years) but more likely at least ~116,000 years ago, near the end of the last interglacial sea level high stand. Slope failure likely occurred along the unconformable contact separating the base of the coastal limestone with underlying Cretaceous basalt, and may have been triggered by a large earthquake along the Caribbean-South American plate boundary.

Tectonics, basin subsidence mechanisms, and paleogeography of the Caribbean-South American plate boundary zone

Using a mega-regional dataset that includes over 20,000 km of on- and offshore 2D seismic lines and 12 wells, we illustrate three different stages of fault formation and basin evolution in the Caribbean arc-South American continent collisional zone. Transpressional deformation associated with oblique collision of the Caribbean arc migrates diachronously over a distance of ∼1500 km from western Venezuela in Paleogene time (∼57 Ma) to a zone of active deformation in the eastern offshore Trinidad area. Each diachronous stage of pre-, syn-, and post-collisional basin formation is accompanied by distinct patterns of fault families. We use subsidence histories from wells to link patterns of long-term basinal subsidence to periods of activity of the fault families. Stage one of arc-continent collision Initial collision is characterized by overthrusting of the south- and southeastward-facing Caribbean arc and forearc terranes onto the northward-subducting Mesozoic passive margin of northern South America. Northward flexure of the South American craton produces a foreland basin between the thrust front and the downward-flexed continental crust that is initially filled by clastic sediments shed both from the colliding arc and cratonic areas to the south. As the collision extends eastward towards Trinidad, this same process continues with progressively younger foreland basins formed to the east. On the overthrusting Caribbean arc and forearc terranes, north-south rifting adjacent to the collision zone initiates and is controlled by forward momentum of southward-thrusting arc terranes combined with slab pull of the underlying and subducting, north-dipping South American slab. Uplift of fold-thrust belts arc-continent suture induces rerouting of large continental drainages parallel to the collisional zone and to the axis of the foreland basins. Stage two This late stage of arc-continent collision is characterized by termination of deformation in one segment of the fold-thrust belt as convergent deformation shifts eastward. Rebound of the collisional belt is produced as the north-dipping subducted oceanic crust breaks off from the passive margin, inducing inversion of preexisting normal faults as arc-continent convergence reaches a maximum. Strain partitioning also begins to play an important role as oblique convergence continues, accommodating deformation by the formation of parallel, strike-slip fault zones and backthrusting (southward subduction of the Caribbean plate beneath the South Caribbean deformed belt). As subsidence slows in the foreland basins, sedimentation transitions from a marine underfilled basin to an overfilled continental basin. Offshore, sedimentation is mostly marine, sourced by the collided Caribbean terranes, localized islands and carbonate deposition. Stage three This final stage of arc-continent collision is characterized by: 1) complete slab breakoff of the northward-dipping South American slab; 2) east-west extension of the Caribbean arc as it elongates parallel to its strike forming oblique normal faults that produce deep rift and half-grabens; 3) continued strain partitioning (strike-slip faulting and folding). The subsidence pattern in the Caribbean basins is more complex than interpreted before, showing a succession of extensional and inversion events. The three tectonic stages closely control the structural styles and traps, source rock distribution, and stratigraphic traps for the abundant hydrocarbon resources of the on- and offshore areas of Venezuela and Trinidad.

Mantle transition zone beneath the Caribbean-South American plate boundary and its tectonic implications

We analyzed receiver-function data recorded by a temporary broadband array deployed as part of the BOLIVAR project and the permanent seismic network of Venezuela to study the mantle transition zone structure beneath the Caribbean-South American plate boundary and Venezuela. Significant topography on both the 410-km and the 660-km discontinuities was clearly imaged in the CCP (common-conversion-point) stacked images. Beneath the southeastern Caribbean, the 410-km is featured by a narrow (∼ 200 km EW) ∼ 25-km uplift extending in the NS direction around 63° west, while the 660-km is depressed by ∼ 20 km in a narrow region slightly west to the uplift, a scenario that is more consistent with westward descent of the oceanic South American plate rather than a break-off of NNW dipping proto-Caribbean oceanic lithosphere along the El Pilar Fault. We also found a thick transition zone beneath the Falcon region in northwestern Venezuela, possibly associated with the subducted Nazca plate. A flat 410-km was observed beneath the Guayana shield, suggesting that the shield has a stable and moderately deep keel, which has little effect on the underlying transition zone structure.

Constraints on mantle flow at the Caribbean–South American plate boundary inferred from shear wave splitting

We measured shear wave splitting from SKS and SKKS data recorded by temporary stations deployed as part of the Broadband Onshore‐Offshore Lithospheric Investigation of Venezuela and the Antilles Arc Region project and the national seismic network of Venezuela. Approximately 3000 station‐event pairs yielded ∼300 with visible SKS and/or SKKS phases. We obtained 63 measurements at 39 of the 82 stations in the network using the method of Silver and Chan (1991) and conventional quality criteria. We combined our results with previous measurements made by Russo et al. (1996). The most prominent feature in the data is an area of large (>2.0 s) lag times with roughly east–west fast axes in northeastern Venezuela. Mineral physics models show split times this large are difficult to explain with horizontal foliation, but are more feasible with anisotropy characterized by a coherent vertical foliation and an east–west fast axis extending over most of the upper 250 km of the mantle. We interpret the large split times in northeastern Venezuela as a consequence of eastward translation of the Atlantic slab, which has left a strong vertical foliation in its wake parallel to the plate boundary. The peak split times correspond closely with the point the slab intersects the base of the anisotropic asthenosphere at 250 km. Away from this area of large split times the measured times fall to more standard values, but an east–west fast axis still predominates. We suggest this is linked to the rapidly varying strain field at the southern edge of the Atlantic which quickly disrupts the coherent strain field that causes the very large split times in northeastern Venezuela.

Negligible convergence and lithospheric tearing along the Caribbean–South American plate boundary at 64°W

Prior studies of the Caribbean–South American plate boundary have suffered from poor constraint on the structure of the crust and uppermost mantle. We use a recent wide‐angle velocity model from the Broadband Ocean‐Land Investigation of Venezuela and the Antilles arc Region project to constrain new seismic reflection data and previously published line drawing interpretations of the Caribbean–South American plate boundary at 64°W. Though commonly characterized as obliquely convergent, we determine that convergence is negligible in our study area. Previous estimates of Miocene to present north‐south shortening onshore eastern Venezuela have commonly been 115 km or higher, but we constrain shortening to ∼35 km onshore, with an additional ∼30 km offshore. With such minor convergence, we conclude that uplift and basin subsidence in eastern Venezuela does not derive from typical collisional orogeny. Instead, the largely vertical tectonics likely result from mantle dynamics associated with an eastward propagating, near‐vertical tear in the lithosphere along the former passive margin.

Characterizing the Caribbean–South American plate boundary at 64°W using wide‐angle seismic data

We present wide‐angle velocity modeling results from profile 64°W of the Broadband Ocean‐Land Investigation of Venezuela and the Antilles arc Region (BOLIVAR) project. Line 64W is a 460‐km‐long, approximately north‐south, onshore‐offshore reflection/refraction transect located approximately at 64°W longitude. The profile extends across the transform plate boundary between the southeastern Caribbean (CAR) and South American (SA) plates. East of the profile the plate boundary bends to the north, and SA subducts beneath CAR. We utilize first‐arrival tomography to resolve a smooth velocity field for the sediments and upper/middle crust and then use a layered approach to resolve a sharp velocity contrast for the Moho, simultaneously inverting refracted Pn and reflected PmP arrivals. We image crustal and upper mantle structure across the plate boundary zone. We interpret that the strike‐slip system that accommodates relative motion between CAR and SA extends near vertically through the entire crust and offsets the Moho. We see no evidence supporting a major component of convergence, and rather than a wide boundary zone of overlapping lithospheric plates, we interpret the plate boundary to be confined to the 33‐km‐wide, near‐vertical strike‐slip system. Previously interpreted thrust faults flanking the strike‐slip system appear to be confined to the upper/middle crust and may be related to the detachment of subducting South American lithosphere at the southern terminus of the Lesser Antilles subduction zone east of 64°W.

Chronology of Cenozoic tectonic events in western Venezuela and the Leeward Antilles based on integration of offshore seismic reflection data and on-land geology

Newly acquired, deep-penetration Broadband Onshore-Offshore Lithospheric Investigation of Venezuela and the Antilles Arc Region seismic reflection data from offshore western Venezuela (Bonaire Basin) and around the Leeward Antilles are combined with existing geologic and geophysical data sets to examine the chronology of Late Cretaceous–Cenozoic tectonic events in this part of the Caribbean–South American plate boundary zone. These tectonic events have controlled the maturation and structural trapping of known hydrocarbons in the offshore Bonaire Basin and the adjacent onland Falcn Basin. We infer three tectonic phases that are constrained using these combined data sets. (1) The late Eocene–early Oligocene, north-south opening of the 3–6-km (1.8–3.7-mi)-thick Falcn-Bonaire Basin occurred along east-west–striking normal fault systems that have locally been inverted by later tectonic phases. These Paleogene normal faults rifted the Upper Cretaceous arc crust and Paleogene marine depositional sequences within the offshore Bonaire Basin. (2) Northwest-striking normal faults crosscut the older normal faults of the Bonaire Basin and Leeward Antilles and form deep, submarine rifts that contain up to 4 km (2.5 mi) of sedimentary fill and form deep-water channels between islands of the Leeward Antilles. Offshore well data and age of onshore sediments in the Falcn Basin indicate that this second phase of rifting occurred mainly during the late Oligocene to early Miocene and remains active to the present. (3) Inversion of the subaerial Falcn Basin commenced during the middle Miocene. This inversion phase is reflected in the present-day pattern of an east-northeast–trending fold-thrust belt that can be traced over 200 km (124 mi) along strike in the Falcn Basin. A second offshore fold-thrust belt (La Vela) can be traced over a distance of 175 km (108 mi) along strike and parallel to the northeast-trending Falcn Basin coast. Restoration of imbricate thrusts seen on seismic lines perpendicular to the La Vela fold-thrust belt indicates a minimum of 7 km (4.3 mi) of northeast-southwest–directed, thin-skinned shortening. Geochemical work indicates that source rocks for scattered occurrences of hydrocarbons in the Falcn Basin and its coastal zone are Paleogene and Miocene marine shale. Reservoir rocks are Tertiary marine sandstone and shale deposited in Paleogene rifts formed during the first tectonic phase in the late Eocene to early Oligocene. Structural traps were formed by thrusting during the second tectonic phase in the late Oligocene to early Miocene.

Oblique collision and accretion of the Netherlands Leeward Antilles to South America

Accretion of the Netherlands Leeward Antilles volcanic island arc to northern South America (offshore Venezuela) was accompanied by deformation and clockwise block rotation. Structural analysis of Aruba, Curaçao, and Bonaire indicates that they were deformed during at least one ductile event (D1) and three brittle events (F1 − F3). The first deformation phase (D1 and F1) began in the Early Cretaceous, at approximately 110 Ma. Deformation features are consistent with displacement partitioning at the obliquely convergent Caribbean‐Atlantic (proto‐Caribbean) plate boundary. This deformation was accompanied by an approximate 90° clockwise rotation of the forearc terrane. The onset of the second phase of deformation (F2) occurred in the Eocene (∼55 Ma), also accompanied by a small amount of rotation, and structures are consistent with wrench tectonics in a pull‐apart regime. This coincided with the collision of the Caribbean plate with the Great Bahama Bank and the initiation of convergence between North and South America. The youngest deformation structures and rotation (F3) are also associated with wrench faulting. Likely, motion along the Oca fault and the Caribbean plate transform fault initiated a major restraining bend in the late Paleogene. Beginning ∼12 Ma, the northeastward escape of the Maracaibo block further increased the complexity of tectonism at the Caribbean–South American plate boundary. Deformation structures are consistent with a total of 135° clockwise rotation of the Leeward Antilles islands and regional diachronous deformation from west to east since the Early Cretaceous.

Mud volcanoes of the Orinoco Delta, Eastern Venezuela

Mud volcanoes along the northwest margin of the Orinoco Delta are part of a regional belt of soft sediment deformation and diapirism that formed in response to rapid foredeep sedimentation and subsequent tectonic compression along the Caribbean–South American plate boundary. Field studies of five mud volcanoes show that such structures consist of a central mound covered by active and inactive vents. Inactive vents and mud flows are densely vegetated, whereas active vents are sparsely vegetated. Four out of the five mud volcanoes studied are currently active. Orinoco mud flows consist of mud and clayey silt matrix surrounding lithic clasts of varying composition. Preliminary analysis suggests that the mud volcano sediment is derived from underlying Miocene and Pliocene strata. Hydrocarbon seeps are associated with several of the active mud volcanoes. Orinoco mud volcanoes overlie the crest of a mud-diapir-cored anticline located along the axis of the Eastern Venezuelan Basin. Faulting along the flank of the Pedernales mud volcano suggests that fluidized sediment and hydrocarbons migrate to the surface along faults produced by tensional stresses along the crest of the anticline. Orinoco mud volcanoes highlight the proximity of this major delta to an active plate margin and the importance of tectonic influences on its development. Evaluation of the Orinoco Delta mud volcanoes and those elsewhere indicates that these features are important indicators of compressional tectonism along deformation fronts of plate margins.

Calcite and quartz microstructural geothermometry of low-grade metasedimentary rocks, Northern Range, Trinidad

The Northern Range of Trinidad is a mountainous exposure of deformed metasedimentary rocks with Mesozoic depositional ages and Tertiary metamorphic ages located in the Caribbean–South American plate boundary zone. These rocks lack precise mineralogical indicators of metamorphic grade. Using temperature-sensitive quartz and calcite microstructures, and fission track data, we identify, describe, quantify, and map a nearly continuous spectrum of relatively low (>l50°C) to relatively high (250–400°C) deformation temperatures from east to west across the range. Average estimated rock exhumation rates also increase systematically along this trend. The analysis illuminates a sharp, east–west-trending, major thermal discontinuity (i.e. a fault) along the southern boundary of the range. Kinematic analysis of faults and shear bands adjacent to this boundary and modeling indicate that the fault probably dips southward at 80–85° and accommodated normal dip-slip at rates that decreased from west to east. Earlier experimental studies at high strain rates illustrated a strong strain rate dependency for quartz microstructural transitions. Our work indicates a possible strong temperature control for quartz microstructural transitions at natural strain rates.

A preliminary paleomagnetic pole for mid-Cretaceous rocks from Tobago: further evidence for large clockwise rotations in the Caribbean-South American plate boundary zone

The island of Tobago occupies the eastern end of the central (igneous) belt of the Caribbean-South American plate boundary zone. Volcaniclastic sediment of the Albian (∼ 100 Ma) Tobago Volcanic Group and dikes of similar age within it were sampled in two homoclinal sections with different attitudes. The mean of virtual geomagnetic poles for 12 sites (25.4°N, 24.1°E, A95 = 4.2°) is well defined, pre-tilting and apparently reliable, yet is far removed from a similar age reference pole for South America. Five other paleomagnetic studies of Cretaceous rocks from widely separated localities farther west in the plate boundary zone yield pole positions that are remarkably similar to the Tobago pole. Poles obtained from the Guajira Peninsula of Colombia, the islands of Aruba and Bonaire, and the Caribbean Mountains of Venezuela are among those that agree with the Tobago result. The paleolatitudes for study areas within the plate boundary zone are consistent with an origin on or near South America, yet the poles throughout the zone are rotated roughly 90°. Dextral relative motion between the Caribbean and South American plates was probably responsible for the rotation.

Thin-Skinned Deformation of the Cretaceous and Tertiary Sedimentary Section of Eastern Offshore Trinidad: ABSTRACT

The major oil fields of the Monagas Fold-Thrust Belt of Eastern Venezuela are anticlinal structures related to thin-skinned thrusting that transported Tertiary clastic sediments up a series of ramps. The ramps are thought to arise from a flat-detachment surface located within Lower Cretaceous or Pre-Cretaceous sediments. The ramps merge upward into a flat-detachment located in soft plastic shales of the Miocene foredeep sequence. In Eastern Offshore Trinidad, similar deformation occurs within the easternmost extension of this fold-thrust belt along the Caribbean-South American Plate Boundary. Thrusting in Eastern Offshore Trinidad has produced a high-relief duplex structure that formed by interconnection of a basal detachment, ramp-thrusts, and a roof-thrust. Excellent 3-D seismic data show the geometry of this duplex structure, and well data show that the basal-detachment is located within the Late Albian-Cenomanian Gautier Formation. The Gautier is a very plastic organic-rich shale that forms an excellent surface for detachment. A series of ramps cut upward from this basal detachment through hard, competent siliceous rocks of the Turonian through Maastrichtian Naparima Hill Formation. The thrusts abruptly flatten into an upper detachment at the lithologic contact with soft ductile shales of the Oligocene Lower Cipero Formation. The geometry of the thin-skinned thrusting ismore » similar to that observed in the Monagas Thrust Belt, but differs in that the lower flat-detachment surface lies within Cretaceous sediments equivalent to the Querecual Formation, and not within earlier Cretaceous or Pre-Cretaceous Venezuelan equivalents.« less