PLIO-PLEISTOCENE EASTWARD MIGRATION OF THE ARC FRONT IN SOUTHERN ANDES (33-35ºS): TECTONIC REGIMES AND VOLCANISM SOUTH OF THE PAMPEAN FLAT-SLAB SEGMENT

First-order arquitectural changes in volcanic arcs are related to dramatic but not instantaneous changes of subduction systems and tectonic regimes of the upper crust beneath the volcanoes. Plio-Pleistocene evolution of Southern Andes records episodes of arc front migration and arc broadening and narrowing as a consequence of changes in subduction angle, convergence velocity and ridge collision. These major changes were accompanied by changes in the tectonic regimes. While a fixed arc front and narrowing of the arc would have characterized the segment 38-42ºS since the Pliocene, which was also coeval with the pass from east-west compression to north-east dextral transpression along the arc, the northernmost segment of the Southern Andes, just south of the present Pampean flat slab segment, show a different evolution. In fact, the arc front migrated to the east at the end of Pliocene and the tectonic regime changed from east-west compression to north-south compression in the present forearc and western Andean foothills. Geochemical features and morphostructural evolution of volcanic centers also record these changes. Here we analyze structural data from the western Andean foothills and Pliocene porphyry copper deposits together with data of Pleistocene active stratovolcanoes located ca. 50 km east to understand the relationship between first-order changes of the volcanic arc and tectonic regimes of the upper plate. While Miocene-Pliocene magmatic events were controlled by a bulk east-west compression, last episodes of volcanism and hydrothermal alteration at El Teniente copper mine are postdated by a NS compression, which is younger than ca. 2 Ma and suggests a causative link between this new regime and the coeval arc migration. Previous hypothesis have suggested passive slab flattening or tectonic erosion of the forearc region for explaining the Plio-Pleistocene eastward arc migration. Whatever the cause, we argue that this process was accompanied by an important change of the tectonic regime, which also imprinted both different geochemical signatures and contrasting volcano morphologies and eruptive styles along the active magmatic belts.