Allochthonous terranes in northwestern Ecuador

Abstract Formations and lithostratigraphic units of the Ecuadorian Northwestern Cordillera can be divided into three major groups. The first group comprises Lower Cretaceous E-MORB basalts, conformably overlain by tholeiitic to boninitic components of an ensimatic island arc. The second group consists mainly of tholeiitic to calc-alkaline volcanic and volcano-sedimentary island-arc deposits, unconformably on top of the first group, and of Eocene to Early Oligocene age. The third group is represented by calc-alkaline continental volcanics, ranging from Late Oligocene to Recent. Two main tectonic events occur during Early Eocene and Middle Oligocene times. The first event can be interpreted as an accretion of a slab of oceanic basalt, with its overlying arc material. The suture zone is marked by slices of MORB-type material, delineating the southward extension of the Colombian Cauca-Patia Fault Zone. The second event is related to a major rearrangement of plate-tectonic regimes in the Eastern Pacific during Oligocene times. Its nature is most probably transpressional, accompanied by thrusting.