A PLIOCENE QUATERNARY COMPRESSIONAL BASIN IN THE INTERANDEAN DEPRESSION, CENTRAL ECUADOR

The segment of the Interandean Depression of Ecuador between Ambato an d Quite is characterized by an uppermost Pliocene-Quaternary basin, wh ich is located between two N-S trending reverse basement faults: the V ictoria Fault to the west, and the Pisayambo Fault to the east. The cl ear evidence of E-W shortening for the early Pleistocene (between 1.85 and 1.21 Ma) favours a compressional basin interpretation. The morpho logy (river deviations, landslides, folded and flexure structures) dem onstrates continuous shortening during the late Quaternary. The late P liocene-Quaternary shortening reached 3400+/-600 m with a rate of 1.4/-0.3 mm yr(-1). The E-W shortening is kinematically consistent with t he current right-lateral reverse motion along the NE-SW trending Palla tanga Fault. The Quito-Ambato zone appears to act as a N-S restraining bend in a system of large right-lateral strike-slip faults. The compr essive deformation which affects the Interandean Depression during the Pliocene is apparently coeval to the beginning subduction of very you ng oceanic lithosphere north of the Gulf of Guayaquil. The relatively buoyant new crust may have significantly increased the mechanical coup ling in the subduction zone from Pliocene to Present.