Extension and basin formation in the southern Andes caused by increased convergence rate: A mid-Cenozoic trigger for the Andes

The southern Andes between 33 degrees and 45 degreesS latitude are characte rized by a series of complex basins that spanned the contemporaneous active continental margin, which itself was characterized by volcanic activity. T he basins are filled with thick (up to 3000 m) accumulations of interbedded sedimentary and volcanic strata of late Oligocene-early Miocene age. We in terpret that these basins developed during a phase of moderate extension wi thin the plate margin system, triggered by an increased rate of convergence of the Farallon (Nazca) and South American plates between 28 and 26 Ma. Th is history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compr essional coupling. Although extensional basins of this age are only well-de scribed in the southern Andes, the convergence history and volcanic chronol ogy are similar farther north in the central Andes (18 degrees -33 degreesS ), leading to the speculation that extension may have characterized the lat e Oligocene-early Miocene interval regionally. We hypothesize that this ext ension was a necessary condition to subsequent building of the modern Andes Mountains.