MODE AND TIMING OF TERRANE ACCRETION IN THE FOREARC OF ECUADOR

We present new, and compile previous geochronological, thermochronological, geochemical and isotopic data acquired from magmatic and sedimentary rocks of the Ecuadorian Andean forearc, which relate to the collision of the Late Cretaceous Caribbean plateau and Great Arc of the Caribbean with the NW South American Plate. The volcanic basement (Piñon, Pallatanga and San Juan units) comprises ultramafic and mafic rocks that yield oceanic plateau geochemical affinities and U/Pb (SHRIMP) and ⁴⁰Ar/³⁹Ar crystallization ages of 88–85 Ma, supporting the hypothesis that they are derived from the Caribbean plateau. Ocean island arc sequences (San Lorenzo, Rio Cala) overlie the plateau and yield crystallization ages that range between ~86–65 Ma. Along-strike trends of thermochronological data, sedimentary data, and the age of termination of the arc sequence, indicates that the collision was oblique, commencing in the south at ~73 Ma and continuing northwards until ~65 Ma within Ecuador. Paleomagnetic analyses of volcanic rocks, which comprise the Piñon and San Lorenzo units of the southern forearc, indicate their pre-collisional extrusion at equatorial and shallow southern latitudes, strengthening the possibility that they extruded from the Galapagos Plume. Paleomagnetic declination data from the same basement and sedimentary cover rocks (Cayo Fm.) constrain 40–60° of clockwise rotation prior to collision, during the Campanian. The termination of rotation was probably synchronous with their collision with the South American plate. The accretion of the Pallatanga terrane , located in the western region of the present arc, is tightly constrained by an unconformable contact between the allochthonous basement sequence and overlying continent-derived Paleocene turbiditic rocks (Saquisilí Fm.). Magmatism associated with the Campanian–earliest Paleocene Rio Cala arc, which erupted through the Pallatanga terrane in the north, ceased during the Danian and was subsequently buried beneath Late Paleocene marine sedimentary rocks (Saguangal Fm.), which reworked continental clasts. On a regional scale, multi-phase ⁴⁰Ar/³⁹Ar and fission-track thermochronological data document exhumation related to terrane accretion occurring between ~73–65 Ma.