Late cretaceous to miocene stratigraphy and provenance of the coastal forearc and Western Cordillera of Ecuador: Evidence for accretion of a single oceanic plateau fragment

The Western Cordillera and coastal forearc along the western margin of Ecuador share an allochthonous origin involving the accretion of mafic rocks of an oceanic plateau and intraoceanic volcanic arc. Oceanic plateau mafic rocks of the Pallatanga Formation (Western Cordillera) and Piñon Formation (coastal forearc) form the basement of the Pallatanga and Piñon blocks (terranes), respectively. The basement rocks of both regions are likely derived from the Caribbean Plateau that formed from ~100 to 87Ma. Provenance analyses indicate that detrital material derived from the South American continental margin first reached the Piñon and Pallatanga blocks during Paleocene sedimentation. Specifically, Paleocene-Eocene successions in the coastal forearc and Western Cordillera received detrital input from continental basement rocks of the Eastern Cordillera, and input of igneous material likely derived from the Tandapi volcanic arc and its continuation, the Macuchi submarine arc. In addition, detrital zircon U-Pb ages for uppermost Cretaceous to Miocene clastic deposits of the coastal forearc (including the broader Progreso basin region) and Western Cordillera recorded continuous input from syndepositional volcanic sources, consistent with sustained arc magmatism and close proximity to the Andean magmatic arc. This suggests that the continental magmatic arc in Ecuador has been well established since at least Maastrichtian time. The provenance analyses together with thermochronological, paleomagnetic, and stratigraphic data suggest that the Piñon and Pallatanga blocks were accreted to the South American continental margin at ~73Ma. Therefore, a previously proposed late Eocene age of Piñon accretion is unlikely. Further, a proposed allochthonous origin for the Macuchi arc cannot be reconciled with the fact that this arc supplied detrital material to both the Pallatanga and Piñon blocks. These points lead to a broader recognition that deformation episodes in Ecuador, such as a proposed Eocene increase in shortening, need not be associated with collisional events.