The southern Andes between 33¿ and 45 ¿S latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin, which itself was characterized by volcanic activity. The basins are filled with thick (up to 3000 m) accumulations of interbedded sedimentary and volcanic strata of late Oligocene-early Miocene age. We interpret that these basins developed during a phase of moderate extension within the plate margin system, triggered by an increased rate of convergence of the Farallon (Nazca) and South American plates between 28 and 26 Ma. This history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling. Although extensional basins of this age are only well-described in the southern Andes, the convergence history and volcanic chronology are similar farther north in the central Andes (18¿--33 ¿S), leading to the speculation that extension may have characterized the late Oligocene-early Miocene interval regionally. We hypothesize that this extension was a necessary condition to subsequent building of the modern Andes Mountains. ¿ 2001 American Geophysical Union