Abstract

We analyze the west vergent thrust system (WTS) along the western flank of the Altiplano in northern Chile (18°S-21°S). In our study area (19°20?S-19°50?S), the WTS consists of three thrust propagation monocline folds (flexures) developing growth strata. The relative uplift accommodated by the flexures is rapid between 26 and 8 Ma (0.1 mm/yr), diminishing to 0.02 mm/yr after 8 Ma. Approximately 2000 m of relative surface uplift was accommodated by the flexures since the late Oligocene. Sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 Ma, coeval with the shifting of deformation from the Altiplano to the sub-Andean zone, where the underthrusting of the Brazilian Craton would have resulted in crustal thickening, surface uplift in the orogen, and westward ductile subcrustal flow. Forearc tilting is accommodated by east vergent thrusts (ETS) issued from the Benioff zone beneath the Central Depression emerging into the Western Cordillera, contributing 500-1400 m of surface uplift. The WTS connects the ETS in the brittle-ductile crustal transition (?25 km depth), continuing farther east as the Altiplano low-velocity zone, configuring the western Altiplano as a crustal-scale fault bend fold. Forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ETS. Meanwhile, between 19°S and 21°S, the WTS accommodates dextral strike slip, and ?3 km of N-S shortening occurred in the Coastal Cordillera. Transcurrence and strain partitioning are probably the result of slight plate convergence obliquity, strong coupling within the interplate zone, westward continental concavity, and high elevation opposing horizontal contraction. Copyright 2005 by the American Geophysical Union.