6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 592-595 Structural and magmatic responses to steepening of a flat subduction, southern Mendoza, Argentina Victor A. Ramas & Andrés Folguera Laboratorio de Tectonica Andina, FCEyN, Universidad de Buenos Aires, Ciudad Universitaria, Pabellon 2, (1428) Buenos Aires, Argentina; andes@gl.fcen.uba.ar The reconnaissance of typical volcanic arc products in the Chachahuén area of the southem Mendoza foothills ( 35". 37"30'5) lB far ,,:< 500 km frum the coruincrual !cd to (he prupm"l uf a periuJ uf /1,,( :<ubduction in Late Miocene times (Kay 2002). Further studies on the structure and magmatic distribution in time and space of the volcanic rocks of southern Mendoza led to the confirmation of this hypothesis and to improve the comprehension of the geologie history of this segment of the Andes. Field work and observations made along the Principal Cordillera of Argentina and Chile and the adjacent foothills show that this segment of the Andes has a distinctive geologie evolution when compared to the northern and southern adjacent segments. Main structural units The Andes between 35° and 37°30'S are characterized by the following structural units that have been defined since the early XX century by Gerth (1931). From west to east they are: Active Magmatic Arc: The magmatic arc at these latitudes is developed in the western slope of the cordillera and has been characterized by numerous petrologic and geologie studies (Lopez Escobar 1984, Hildreth et al. 1999). Recent analyses have demonstrated a complex history of crustal erosion and consequent migration of the magmatic arc towards the foreland between 7 and 4 Ma (Kay et al. 2005). Las Loicas Trough: A Plio-Quaternary basin has been recognized east of the present magmatic arc, associated with large silicic calderas controlled by extensional north-northwest trending faults such as the Planchon - Azufre and Calabozos calderas, Puelche volcanic field, Mary, Varvarco, Domuyito and Domuyo calderas (see location in Folguera et al. 2005, this volume). This depression coincides with the north-south course of the Rio Grande valley, and has been filled by thick sequences of ignimbrites, lavas, and ash faU deposits of acidic composition. Principal Cordillera: Miocene granitoids have been emplaced by Mesozoic deformed rocks and are unconformably covered by thick sequences of Late Miocene volcanic rocks (Gerth 1931, Ramos and Nullo 1993). As a result of important orogenie shortening and uplift, synorogenic conglomerates and sandstones have been deposited in the foreland. The age of the older deposits is constrained between of 15.1 Ma at the base and 6.7 Ma at the top of the sequence by K/Ar ages. The angu lar unconforrnity between this sequence and the next synorogenic deposits is bracketed between 6.7 and 5.04 Ma, age obtained at the base of these deposits (Kraemer 2000). This sequence is unconforrnably covered by Late Pliocene-Quatemary deposits. Pliocene lavas of andesitic and basaltic composition are not deformed in this segment. Rio Grande basin: This foreland basin accumulated more than 2,000 m of synorogenic deposits in two depocenters, north and south of the Rîo Atuel (Yrigoyen 1993). The two previous Tertiary sequences are covered 592