SEISMIC EVIDENCE FOR BLIND THRUSTING OF THE NORTHWESTERN FLANK OF THEVENEZUELAN ANDES

Surface geology and seismic and well data from the northwestern flank of the Venezuelan Andes indicate overthrusting of Andean basement rock s toward the adjacent Maracaibo Basin along a blind thrust fault. The frontal monocline is interpreted as the forelimb of a northwestward ve rging fault-related fold deformed over a crustal-scale ramp. The Andea n block has been thrust 20 km to the northwest and uplifted 10 km on a ramp that dips about 20-degrees-30-degrees southeastward. The thrust fault ramps up through crystalline basement rocks to a decollement hor izon within the shaly units of the Cretaceous Colon-Mito Juan formatio ns. Backthrusts in the monocline produce a wedge geometry and reduce t he amount of blind slip required on the decollement northwest of the A ndes. The rigid Andean uplift was caused by northwest-southeast compre ssive tectonic forces related to the convergence of the Caribbean plat e, the Panama volcanic arc, and northwestern South America. The thick (up to 6 km) molasse deposits accumulated in the foredeep basin indica te that the Venezuelan Andes started to rise as early as the early Mio cene. However, a late Miocene intramolasse unconformity marks the begi nning of the formation of the monocline and the greatest uplift. The c rustal-scale fault-related fold model may explain structural features seen in other areas of basement-involved foreland deformation.