Lower plate retreat and opening of an Andean backarc basin

<p>The northern segment of the Andean Mountain System comprises at its western border a Cretaceous plateau-like province intruded by island arc sequences, which bear stratigraphic and geochemical affinities to the Caribbean Large Igneous Province. For these combined provinces a Pacific-derived origin has been postulated, supposing the accretion and partial subduction of an allochthonous terrane. This scenario may be challenged for the Andean part, supposing the opening of a backarc basin floored by oceanic crust, when integrating craton-related age spectra of zircon xenocrysts retrieved from subvolcanic rocks, the stratigraphic relations between pre-magmatic siliciclastic and overlying magmatic-volcanic arc sequences and the evidence of substantial extensional tectonics. In this view, an accelerated retreat of the lower plate, as expected to occur at the free end of a subduction system, would have prompted the opening of a wedge-like space comparable to the Lau basin of the western Pacific. In this contribution we present structural evidence for crustal deformations owing to plate convergence and deformations supposedly imposed by a N-S directed asthenospheric backflow on the inboard of the subducting slab.</p>