THE INITIATION OF OROGENESIS IN THE PATAGONIAN ANDES BY THE COMPRESSIONAL INVERSION OF THE EXTENSIONAL ROCAS VERDES BASIN

The Patagonian Andes record a period of Cretaceous-Neogene orogenesis that began with the compressional inversion of a Jurassic rift basin called the Rocas Verdes. This rift is unusual because it was the only one of a series of Jurassic back arc and marginal basins south of Ecuador that was floored by oceanic crust. This region also includes high grade, kyanite-bearing metamorphic rocks of Cordillera Darwin and the northern margin of an obducted ophiolite sequence. New U-Pb and ⁴⁰Ar-³⁹Ar ages and the results of a structural study of basin inversion along the Beagle channel (55°S) show that basin closure involved two distinctive stages of shortening and uplift that are separated by tens of millions of years. ⁴⁰Ar-³⁹Ar ages on metamorphic muscovite that grew during the folding of Rocas Verdes rocks indicate that basin inversion began by ~98 Ma and formed a narrow (~60 km) wedge of imbricated thrusts. U-Pb crystallization ages on zircon and cross cutting relationships with granitic plutons indicate that this period of shortening, which resulted in the obduction of the oceanic floor of the Rocas Verdes basin on top of adjacent continental crust, ceased prior to ~86 Ma. During this same interval, continental crust was underthrust to depths of ~35 km beneath the early Andean arc. Another period of thrust faulting, culminating in the Paleogene, thickened the internal part of the orogen, resulting in the uplift and exhumation of the deeply buried continental crust composed of basement schists and Upper Jurassic volcanic rock. This latter period marked the beginning of a rapid expansion of the fold-thrust belt into the Magallanes foreland basin. The timing and kinematics of thrust faulting in the hinterland of this orogen allow us to predict the spatial extent and types of rocks that were uplifted by faults and influenced the depositional history of this foreland basin. The orogen also provides an important example of how the antecedent geology of a rift basin can influence inversion mechanisms, including processes that lead to ophiolite obduction and continental underthrusting in a back arc setting.