Insight Into The Neogene Kinematic Development Of The Northern Altiplano From Paleomagnetic And Geochronologic Data

Paleomagnetic data associated with age constraints remain a unique source of information in investigating the geometric and kinematic development in major orogenies trough time. A paleomagnetic study of 36 sites (315 samples) of Neogene strata from the Peruvian Altiplano and adjacent sub-Andean fold and thrust belt (Pilcopata area), combined with 40Ar/39Ar dating of 13 lava flows, provide constraints on the Cenozoic deformation history of the northern Bolivian Orocline in a region previously almost void of paleomagnetic data. In the Peruvian Altiplano, 40Ar/39Ar 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° ± 5.4°, 31.0° ±10.2° and 7.8° ± 4.8°, 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.