The Argentine Precordillera: A foreland thrust belt proximal to the subducted plate

Abstract

The Precordillera thrust belt of western Argentina is anomalously close, both horizontally and vertically, to the coeval subduction zone of the Nazca plate. The thin-skinned part of the belt has an unusually deep decollement that is well defined by industry seismic reflection and recent broadband experiments. New area and line-length balanced cross sections show that the central Precordillera has accrued ∼90 ± 21 km of shortening since 13 Ma; much of that shortening occurred between 12 and 9 Ma. Fault-slip data generally show shortening approximately west-northwest–east-southeast, orthogonal to the traces of the thrust and folds in the Precordillera and oblique to the mean vector of local global positioning system (GPS) data. The GPS strain rate is –63 ± 9 × 10 –9 /yr, whereas strain rate in the thrust belt, averaged over 13 m.y., is –56 ± 4 × 10 –9 /yr. Although the decollement of the Precordillera cannot cut into Paleozoic Cuyania(?) terrane basement east of the crest of the high Andes, broadband receiver function data show that significant crustal thickening must occur beneath and even east of the thrust belt. We suggest that top-to-the-west shear and thickening of the lower crust due to flat subduction explains the distribution of crustal thickening.