Tectonostratigraphic evolution of a marginal basin during the transition from arc collision to subduction; the case of the northern Pacific forearc of Colombia

The forearc of the northern Pacific region of Colombia (Atrato Basin) initially formed in an intra-oceanic setting related to the Central American arc, and its sedimentary record provides a unique opportunity to study the evolution of source-to-sink systems during the transition from arc collision to subduction. Intracrustal weaknesses, and the highly oblique approach to the continental South American paleomargin, favored an initial soft collision of the island arc and associated marginal basins during the Early Miocene. This is suggested by the lack of widespread deformation in the collisional front and the accumulation of hemipelagic rocks with little terrestrial input, sourced by the colliding arc and the continental paleomargin. A regional unconformity associated with a shift toward terrigenous deposition is attributed to a harder collision and subsequent buoyancy-driven uplift of the oceanic terrane ca. 15-13 Ma. The final docking of the oceanic arc and related basins to the continent and coeval surface uplift near the suture zone ca. 12-7 Ma marked the transition from collision to subduction and the establishment of the Atrato Basin as a continental forearc. This was accompanied by postcollisional arc magmatism, shallowing of accumulation depths to nearshore conditions, and a dramatic decrease in the sediment contribution of the continental paleomargin. Subsequent shallow subduction of the Coiba microplate caused the Late Miocene uplift of the coastal Baudo Range and the fragmentation of the Atrato Basin into inner and outer (coastal) segments. This episode drove a shift from predominantly transverse to longitudinal drainage systems and the final transition from marine to terrestrial settings in northwestern Colombia.