Geomorphic and geologic evidence of ongoing uplift and deformation in the Merida Andes, Venezuela

The Merida 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 Wrida. 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 Wrida Andes and Eastern Cordillera. The Wrida Andes uplift is not produced by convergence across a conventional type-B subduction, as most of the South American Andes are. In addition, the Merida 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 Wrida 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 Wrida Andes where the foothills and the mountain belt have been shortened transversely in a NW-SE direction whereas the Bocono fault, roughly located in the core and along the Wrida Andes axis, accommodates dextral slip. Tectonic inheritance in the Wrida 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 1000m of relief, where the lower 200-300m 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 kin on the northwest and southeast of the Wrida 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 Wrida Andes. The Merida Andes core comprises igneous and metamorphic rocks formed at depths of 8-10km, 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 Wrida Andes at present: right-lateral strike-slip along the sub-axial Bocono fault and basinward thrust solutions on both flanks. Underfed/under-destruction pull-apart basins (Las Gonzalez and Apartaderos) along the Bocono fault appear to be complementary evidence of rapid uplift of the Wrida Andes. (C) 2002 Elsevier Science Ltd and INQUA. All rights reserved.