The Chota basin and its significance for the inception and tectonic setting of the inter-Andean depression in Ecuador

The inter-Andean structure is defined as an approximately north–south-trending, linear, topographic depression in Ecuador located between the Cordillera Real and the Cordillera Occidental. The depression swings westward toward the Gulf of Guayaquil in southern Ecuador, dissecting the topography of the Cordillera Occidental. The structural limits of the depression are reactivated faults, which formed during accretionary events after 140 Ma along the Ecuadorian continental margin. Several distinct basins progressively formed along the depression. A new, radiometric-based, chronostratigraphic framework for the sedimentary series of the Chota basin has been combined with data from other subbasins in the inter-Andean depression to reevaluate the timing and formation of the larger scale tectonic structures. Inception of the Chota basin commenced at ∼6–5 Ma (latest Miocene), and the progressively younger Quito-San Antonio-Guayllabamba, Ambato-Latacunga, and Riobamba-Alausí basins formed along a southward trend. Each basin was filled with alluvial fan, fluvial, lacustrine, and contemporaneous volcanic deposits. Synsedimentary transpressive deformation was dominant during basin development, though minor, synsedimentary, normal faulting is assumed to have occurred during short periods of weakening compression. The inter-Andean depression of Ecuador formed in the vicinity of a major restraining bend, which accommodates the northward displacement of the north Andean block with respect to the South American plate. A ramp basin model is proposed to explain the tectonosedimentary development of the inter-Andean depression