FROM DUCTILE SHEAR ZONES TO CRETACEOUS OROGENESIS AT SOUTHERN PATAGONIAN ANDES

A fundamental problem in the evolution of the southernmost Andes, namely the origin of the ophiolitic remnants of a Mesozoic oceanic basin called the Rocas Verdes, is their evolution into a plate tectonic setting involving the possible subduction and/or underthrusting of a oceanic basin beneath a continent fragment. The Patagonian Andes (50-52ºS) provide a sequence of uplifted oceanic basin floor that is as well preserved as the Sarmiento Complex, a Late Jurassic ophiolitoid complex with chracateristic granitic basement of Jurassic granophyres. It is delimited to the east by a steep mylonite belt, consisting of a strongly foliated succession of silicic tuffs and siltstones of the Jurassic-Cretaceous Tobífera and Zapata Formations, that also formed part of the ocean basin floor. A backarc character of the Rocas Verdes basin was attained during Cretaceous times. Provenance studies in foreland strata, revealed that part of the Tobífera Formation (foliated and not) was exhumed during the latest Cretaceous, contemporaneous with the deposition of conglomerates of the Cerro Toro Formation. Syntectonic metamorphic mineral assemblage in sheared silicic tuffs (stilpnomelane, phengite, chlorite) and geothermobarometry indicate that dynamic-metamorphism ocurred at pressure-temperature conditions around 7-8 kbar and 350-450°C. The pressure-temperature estimates require significant crustal thicknening and would mean that part of the ocean basin floor was thrusted under a rifted continent fragment and consequently subduction/accretion within the deepest part of orogenic wedge is suggested. The zone of thickened crust probably expanded the width of the orogen and established an orographic barrier between the magmatic arc and the foreland basin. The hypothesis of subduction/underthrusting within an intracontinental setting, tenuous postulated for the Fuegian Andes and now extended to the southern Patagonian Andes, change the view about the mean of an Andean-type orogenesis. A critical question is about the role of Jurassic and earliest Cretaceous extension, bimodal magmatism and thermal weakening of the continental crust in establishing the thermo-mechanical conditions for the subsequent Cretaceous compressive orogenic phase.