Crustal collapse in the Southern Andes after shallow subduction (34°30-40°30S)

ANDRÉS FOLGUERA; VANESA D. LITVAK; EZEQUIEL GARCÍA MORABITO; VICTOR A. RAMOS; GERMÁN BOTTESI; TOMÀS ZAPATA

Mendoza, Argentina

Simposio; Backbone of the Americas; 2006

Asociación Geológica Argentina y Geological Society of America

A series of NW extensional basins (34º30´-40º30´S) were developed during the last 4 Ma in response to the steepening of a Late Miocene shallow subduction zone: from north to south, Las Loicas (LL, 34º30´- 38º30´S), Sierra de Reyes (SR, 36º-37º30xS), Loncopué (36º15´-39º30´S), La Laja (LLa, 36º45´-37º45´S) and Bío Bío-Aluminé troughs (BBA, 38º30´-40º30´S). Common features are shared among them from a) extensional reactivation of basement structures in areas of thick-skinned structure, to b) tectonic control of volcanism. However, important differences rise as well: The southern depocenters (BBA and Loncopué troughs) are superimposed to Middle Cretaceous compressive structure, mildly reactivated into the Late Miocene, while as the northern ones are overprinting basically Late Miocene contractional topography (LL, SR and LLa troughs). On the other hand, Las Loicas trough is linked to bimodal volcanism from crustal melts, forming dome and caldera complexes, to poorly evolved mantle-derived products, while the other basins were basically filled by the lasts. These discrepancies are related from north to south to the contrasting intensity that the Late Miocene Andean uplift had. While to the north, the shallow subduction zone (34º30´-38º30´S) established around 12-7 Ma, produced important crustal thickening, denudating and erasing most of the Cretaceous fold and thrust belt, to the south the Cretaceous contractional deformation was barely reactivated. Las Loicas trough, whose amplitude tightly coincides with the Late Miocene shallow subduction zone, was the result of partial collapse of a relatively high orogen supported by deep crustal roots that partially melted when extensional induced-mantle liquids raised. On the contrary, Loncopué and BBA troughs formed in response to steepening of the oceanic slab in association with marginal drag of the shallow subduction zone developed to the north, and consequently over thin crust, which allowed rapid rising of mantle-derived products inhibiting anatectic processes.