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Quaternary fault kinematics and stress tensors along the southern Caribbean from fault-slip data and focal mechanism solutions

The M! erida Andes is a mountain chain that trends SW–NE in western Venezuela, extending for some 350 km from the Colombian–Venezuelan border to the city of Barquisimeto. Its highest peaks reach 5000 m in elevation in the central portion, near M! This chain appears to be the northeastward topographic prolongation of the Eastern Cordillera of the Colombian Andes. The latter belongs to the main Andes chain that extends along the Pacific coast of South America. However, there is no direct genetic relationship between the M! Andes and Eastern Cordillera. The M! Andes uplift is not produced by convergence across a conventional type-B subduction, as most of the South American Andes are. In addition, the M! Andes and Eastern Cordillera are separated by the southern termination of the left-lateral strike-slip Santa Marta-Bucaramanga fault, and the NW–SE trending Santander Massif. The present M! erida Andes chain build-up results from Pliocene–Quaternary transpression due to oblique convergence between two continental blocks: South America and Maracaibo Triangular Block. The present geodynamic setting is responsible for ongoing strain partitioning along the M! erida Andes where the foothills and the mountain belt have been shortened transversely in a NW–SE direction whereas the Bocon ! o fault, roughly located in the core and along the M! Andes axis, accommodates dextral slip. Tectonic inheritance in the M! erida Andes plays a major role, since the chain growth partly results from inversion of Jurassic (half-) grabens, exposing Precambrian and Paleozoic rocks of the South American continental crust along the chain core. Evidence of very different kinds attests to the ongoing growth of the chain. From the geomorphic viewpoint, the axial valleys display well-preserved Quaternary staircase terrace systems with more than 500 m of vertical drop between the oldest terrace and present river beds. Rivers cutting across the structural grain of the chain show very distinct transverse ‘‘wine cup’’ profiles, attesting to the rapid uplift of the chain. In some cases, rivers have downcut over a 1000 m of relief, where the lower 200–300 m of these profiles exhibit subvertical walls. Other features that support the significant uplift of the chain and its youth are synorogenic mollasic deposits along both flanks of the chain, deposited in flexural basins, whose thicknesses reach 8 and 3 km on the northwest and southeast of the M! erida Andes, respectively. These continental deposits of essentially Plio–Quaternary age are arranged in up-dip convergence growth wedges, which are being deformed or destroyed by basin-vergent intracutaneous wedges, triangular zones and/or flat-and-ramp structures rooted under the M! Andes. The M! Andes core comprises igneous and metamorphic rocks formed at depths of 8–10 km, that are cropping out at its highest summits at about 5000 m elevation, implying a total uplift of the order of 12–15 km in the last 3–5 Ma (average uplift rate of 2–5 mm/a). Activation age and amount of uplift match rather well with those derived from fission-track thermochronology. The entire mountain belt exhibits widespread seismicity, essentially restricted to upper crustal levels (seismogenic layer). As well, focal mechanism solutions confirm the strain partitioning taking place in the M! Andes at present: right-lateral strike-slip along the sub-axial Bocon ! o fault and basinward thrust solutions on both flanks. Underfed/under-destruction pull-apart basins (Las Gonz! alez and Apartaderos) along the Bocon ! o fault appear to be complementary evidence of rapid uplift of the M! Andes.

Geomorphic and geologic evidence of ongoing uplift and deformation in the M! erida Andes, Venezuela

Abstract A lithospheric-scale, geodynamic model of the Caribbean southern boundary takes into account tectonic and stratigraphic elements previously unconsidered or not well constrained. The Falcón, Bonaire, Blanquilla and Grenada basins are parts of a former single back-arc basin associated with the migrating Mesozoic Caribbean Arc in Late Eocene–Oligocene times. Spreading of the crescent-shaped basin was triggered by declining Caribbean eastward motion in response to increased convergence between the two Americas at around 38–33 Ma. Dextral wrenching along the southern boundary started in western Venezuela at 17–15 Ma and progressed eastward to the El Pilar Fault by 12 Ma. Eastward motion of the Caribbean relative to South America was earlier accommodated by east-progressing, non-partitioned oblique subduction and collision, as illustrated today in eastern Venezuela around the Los Bajos–El Soldado fault system. Extrusion of the Maracaibo and Bonaire blocks towards the NNE started around 5 Ma, thus the dextral Boconó fault is a young feature. Total plate boundary Cenozoic dextral wrenching recorded onshore Venezuela does not exceed 100 km and is more likely to be in the order of 55 km.

Key issues on the post-Mesozoic Southern Caribbean Plate boundary

The present-day surface velocity field of the South American continent reflects the recent geodynamic features. It varies from quasi rigid body motion in the eastern part to mountain range deformation in the western Andes. Space geodetic observations provide velocities at discrete points only. To represent the continuous velocity field, an adequate deformation model has to be developed. Two different models are applied, the least squares collocation approach (LSC) and the finite element method (FEM). The input data are given by 329 velocities derived from continuously observing GPS stations and several GPS geodynamics projects. The different data sets are transformed to a common kinematic datum by deriving the rotation vector of the South American plate from station motions of the IGS Regional Network (RNAAC-SIR) in the rigid eastern part and reducing these plate motions from all the data sets. The resulting residual motions define the boundary conditions in the FEM and the input signals in the LSC. For the FEM a network of approximately 75000 linear elements is generated. The model theology is a homogeneous elastic material (Young’s modulus 70 GPa and Poisson ratio 0,3). For the LSC empirical covariance functions are derived from the observed velocity vectors. The comparison of both methods shows an agreement in the mm/a level. The result is a continuous surface velocity model for the South American continent which may be used for interpolation of point motions in geodetic networks and reference frames. A detailed geodynamic interpretation has not yet been done due to the simplicity of the model (homogeneous material, rough fault structures, no sophisticated dynamics).

Deformation of the South American Crust Estimated from Finite Element and Collocation Methods

Spatial and temporal relationships between compression, strike-slip and extension in the Central Venezuelan Andes: Clues for Plio-Quaternary tectonic escape

Position changes due to recent crustal deformations along the Caribbean- South American plate boundary derived from the CASA GPS project

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Position changes due to recent crustal deformations along the Caribbean- South American plate boundary derived from the CASA GPS project