Models for the structure of the Northwestern Mountain front of the Venezuelan Andes: Neogene inversion of Jurassic Grabens

The present Venezuelan Andes match the location of a system of grabens active from Jurassic to Neocomian. Neogene oblique convergence between the Maracaibo block and South America, turned the old extensional system into a southeasterly dipping A-type subduction. Right-lateral slip along the Bocono fault zone assimilated the tangential component of the displacement. The perpendicular component inverted the buried normal-faults as reverse high angle structures, developing a mountain front syncline in the overlaying sedimentary sequences. The steep southeastern limb of the syncline is truncated by northwesterly vergent high angle reverse faults involving basement. In some sections a triangle zone is recognized below the syncline. The zone is bounded by thrusts detached within the Capacho-Colon package, southeasterly vergent backthrusts and high angle reverse faults that cut through the syncline. In some other sections, however, the space below the syncline is filled by a wedge of basement rocks dissected by a fan of blind or buried reverse faults. It is suggested that the structure evolved from early fault-bend folding. The transition was probably associated with the development and propagation of an imbricated fan of faults in the hanging wall of a southeasterly dipping basement ramp.