BASIN DEVELOPMENT ALONG THE PACIFIC MARGIN OF SOUTH AMERICA. A PETROLEUM EXPLORATION PERSPECTIVE

The basins of the Pacific margin of South America formed by periodic reactivation and modification of existing structures. Until the mid-Mesozoic, subsidence was influenced by Gondwana tectonism. Cretaceous dispersal resulted in a new style of basin formation that was, nevertheless, still rooted in the earlier basement fabrics, but dominated by subduction processes and strike-slip faults that dismembered the basement. Reactivation of basement structures is also expressed in structural traps and hydrocarbon charge. Hydrodynamically, the flow patterns in structurally-compartmentalized extensional basins and uniform flexural basins are unique.

Oblique subduction and its angle of descent are important factors that cause the leading-edge of the overriding continental lithosphere to be dissected by strike-slip faults, which are dominated by vertical and translational movements. In South America, this behavior is expressed in two families of NNE- to NE-directed, right-lateral, strike-slip faults, one in Chile and NW Argentina (e.g. Aconquija and Las Breñas-Transbrasiliano), the other in northern Peru and Ecuador through the Gulf of Guayaquil. This sub-parallel, left-stepping strike-slip system is linked by NW-striking, sinistral shear zones in Peru. Thin-skinned shortening is most pronounced in the Ene-Madre de Dios of Peru and along the margin of the Chaco basin in Bolivia. The different styles of foreland basin subsidence match this structural framework: narrow, discontinuous foreland basins in front of basement-involved transpressional orogens, and broad foreland basins where thin-skinned thrust-belt loading was pronounced. Locally, strike-slip processes have resulted in domino-style rotation of basement blocks, counter clockwise in coastal Peru, and clockwise in NW Argentina. The dilated trailing edges of these rotated crustal blocks are believed to explain the trondhjemites intruded to higher crustal levels in Peru (Cordillera Blanca), mineralization, and even small trailing-edge basins. Finally, although flat-slab processes are generally dated to the Neogene, the principal phase of Andean deformation and uplift, there is also evidence in Argentina for an earlier, Maastrichtian, episode of flat-slab subduction that resulted in regional down-warping and marine flooding.