Gravity Anomalies, Geodynamic Modelling and the Eastern Venezuela Basin Evolution.

Abstract

This work examines the tectonic evolution of the Eastern Venezuela foreland basin by analysis of gravity anomalies and geodynamical modelling. Gravity data (8ºN-12ºN and 60ºW-66ºW) were processed to produce gravity anomaly maps, the most prominent feature being the minimum of -200 mGal isostatic, Bouguer and free-air anomalies associated with the basin. Positive gravity values characterize the northern terranes. Backstripping analysis of the sedimentary successions of four boreholes penetrating the Eastern Venezuela basin was applied to evaluate the history of subsidence. This demonstrated that an early passive margin phase (Cretaceous to early Oligocene) was followed by Oligocene to Recent tectonic subsidence of the foreland basin linked to the northern coastal compressional tectonic belt. Geohistorical analysis shows a major contribution to subsidence from early Oligocene, renewed during mid-Miocene times, in response to the loading of the South American plate on its northern margin. This tectonic loading pattern is younger from north to south. The observed gravity anomaly paired in eastern Venezuela is adequately reproduced by crustal models along profiles OO´ and II´ showing the isostatic negative anomaly over the foreland basin, primarily caused by the 10-13 km of sediments, the downwarping of dense lower crust and Moho down to 48 km depth. The positive gravity anomalies to the North are associated with southward thrusting of metamorphic and magmatic terranes, and dense Caribbean lithosphere. Dense subducting mantle may also be contributing to the northern positive anomaly belt as deep seismicity suggests.
Two mechanisms were applied to explain the formation of the Eastern Venezuela foreland basin during a collisional regime. First, the “hidden load” approach explains the deflection but includes an intracrustal load whose magnitude is around a third of the supracrustal loads computed by an iterative process. This gives rise to a major misfit between the gravity anomaly computed for the structural model which takes into account all the geological and geophysical constraints when the contribution of the “hidden load” is included in the calculations. Second, since the hidden load hypothesis fails to explain the gravity profiles, a remaining viable explanation is release of compressional strain energy involving N-S crustal shortening involving faulting. I therefore explored a fault-based hypothesis which does not depend on hidden gravitational loads, but takes into account the clear relationship between the subsidence and the complementary uplift. In conclusion the prime cause of the evolution of the system depends on the forces on fault planes as they move, where the tractions on a thrust fault develop when a frictionless fault occurs in response to horizontal deviatoric compression of an elastic layer. As compression acts at the edges of the plate, the stresses are re-orientated producing the movement of the two plates along dipping fault planes (including the El Pilar fault). This led to the formation of the Eastern Venezuela foreland basin and linked uplift of the Eastern Serranía del Interior by spasmodic release of elastic strain energy.