Sediment-thickness and upper-lower crustal boundary of the Maracaibo block, Venezuela, from gravity study

The Maracaibo Block is a triangularly shaped fraction of independent continental lithosphere located in north-western Venezuela, controlled by a complex system of strike -slip and thrusts faults; the Oca-Ancon fault (north), the Santa Marta -Bucaramanga fault (southwest) and Bocono fault (east). In the present research we study two interfaces associated with the upper crust, the basement -sediment, and the upper -lower crustal interface, as well the interaction with the Caribbean plate. These interfaces were defined using gravimetric inversion methods in conjunction with well information, refraction and reflection seismic and seismological data. Applying the inversion algorithm of Cordell and Henderson to the gravimetric data, 10 km of sedimentary column are estimated in the deepest part of the Maracaibo Lake basin and 5 km in the Apure Barinas basin, both zones coincide with the minimum gravimetric anomaly of each basin. Using spectral analysis, a discontinuity with depths between 17 and 24 km is observed; located between the upper and lower crustal boundary. Four gravimetric domains are defined; to the north related to the displacement of the Oca-Ancon fault and its influence on the Bonaire block, to the west the displacement of the Santa Marta Bucaramanga fault, to the east the Apure fault and the central domain, relacionated with the Maracaibo block. There are 60 km of distance between the gravity minimums belonging to the Apure Barinas and Maracaibo Lake basins; this displacement is associated with the right lateral strike slip offsets of the Bocono fault. The displacement of Bocono fault, and the shortening in a northeast -southeast direction, is consistent with ongoing strain partitioning in the Merida Andes. A displacement of 110 km is proposed for the Santa Marta Bucaramanga fault and 100 km for the Oca Ancon fault, respectively. The first pulses of Oligocene uplift of the Merida Andes are due to the interaction of the Maracaibo block with the Caribbean plate. The uplift of the Merida Andes was dominated mainly by the northwest -southeast compression until 4 Ma ago; in the Pleistocene, the displacement of the Bocono fault and the rotation of the Trujillo block began to control the current structural configuration of the northeastern Merida Andes. This interaction gives rise to a pattern of gravimetric lows and highs associated with subbasins and basement highs, located between Oca Ancon and Bocono faults.