Abstract

Several authors have interpreted the Colombian Pacific margin as a succession of oceanic terrane accretions, with generation of forearc basins under a compressive-transpressive tectonic regime during the Late Cretaceous -Miocene. New data acquired in the last decade, consisting of reprocessed seismic lines, geological crosssections, stratigraphic data from wells, and new stratigraphic columns, are analyzed in this study. We suggest that the proposed models of successive terrane accretions are not supported by the new data; rather, the onshore basins were likely controlled by a marine platform from the Late Cretaceous to the Middle -Late Eocene, with reef growth to the end of this period. In the Late Cretaceous, contemporary to the sedimentation stage, effusive magmatism, mainly basaltic, also occurred. Since Oligocene time, the basins were affected by three different tectonic processes. First, during the Oligocene -Early Miocene, an extensional-transtensional event occurred in the Tumaco Basin and San Juan Sub -basin, which produced horst and graben structures and domino faults, along with the emplacement of igneous bodies in the eastern flank of the basins. The Atrato Sub -basin was affected by local reverse faulting towards its eastern edge. Second, during the Mid -Late Miocene, the western side of the Tumaco Basin was uplifted, and transpressive deformation occurred in the San Juan Sub -basin as evidenced by the San Juan and Garrapatas fault systems. Finally, in the Atrato Sub -basin, a transpressional regime is evidenced by the Baud & oacute; anticline. Our results suggest there is no evidence of oceanic terrane accretions in the abovementioned basins during the Late Cretaceous -Middle Eocene. Furthermore, we do not see evidence of a subduction system during that period. We conclude that subduction in western Colombia could have begun during the Early Oligocene instead.