Insight into the Neogene tectonic history of the northern Bolivian Orocline from new paleomagnetic and geochronologic data

[1] A paleomagnetic study of 36 sites ( 315 samples) of Neogene strata from the Peruvian Altiplano and adjacent sub-Andean belt ( Pilcopata area), combined with (40)Ar/(39)Ar dating of 13 lava flows, provide new constraints on the Cenozoic deformation history of the northern Bolivian Orocline. In the Peruvian Altiplano, (40)Ar/(39)Ar dating brackets the age of the strata sampled for paleomagnetism between 26.5 and 12.1 Ma for the Huaccochullo basin, 18.6 and 12.1 Ma for the Descanso-Yauri basin, and 16.8 and 15.5 Ma for the Ayaviri basin. Sub-Andean sediments are paleontologically constrained as Miocene, yet pre-9 Ma. Prefolding magnetizations isolated in the Huaccochullo, Descanso-Yauri, and Pilcopata localities indicate significant counterclockwise vertical axis block rotations of 11.3 degrees +/- 5.4 degrees, 31.0 degrees +/- 10.2 degrees and 7.8 degrees +/- 4.8 degrees, respectively. The pattern of rotations together with mapped structures suggest that deformation of the northern Altiplano was partitioned into large regions experiencing relatively minor rotation versus smaller, isolated basins exhibiting high-amplitude counterclockwise rotations that lie in a major left-lateral shear zone. Our new results imply significant transpressional deformation occurred throughout the Peruvian Altiplano since circa 12 Ma and are integrated in a model detailing the tectonic evolution of the northern Bolivian Orocline since 25 Ma.