Structural styles of the Andean foothills, Putumayo Basin, Colombia

Interpretation of seismic profiles, earthquake fault-plane solutions, radar images, and geometry of structures suggests that two different structural styles are viable alternatives for the Putumayo basin in Colombia. An eastern domain, varying in width from 4 to 13 km, might be characterized by strike-slip faulting parallel to the Andes because it exhibits similar structures to those formed in restraining bend settings, an example is the Orito fold, the largest known oil field in the basin. Correlation of seismic reflections with wells into the Orito fold and foreland indicates a post-Miocene age for this structure. Previous interpretations of contractional dip-slip movement on Andes-parallel structures, as proposed by Portilla (1991) with faults involving basement, are also viable. A 15 km-width western domain is interpreted as a region of foreland-dipping rocks uplifted above their regional level by wedging of pre-Cretaceous (?) rocks beneath known Jurassic rocks. Above the Jurassic rocks thin-skinned deformation occurs inside of the Mesozoic and Cenozoic sedimentary cover, also in the form of wedging. Mesozoic and Paleozoic (?) rocks were injected into of a late Cretaceous-early Paleocene unit composed of shale. The western domain is truncated to the west by a major reverse fault that places Paleozoic rocks over Mesozoic and Cenozoic rocks.