Counterclockwise rotation of late Eocene–Oligocene fore‐arc deposits in southern Peru and its significance for oroclinal bending in the central Andes

Abstract

The results of a paleomagnetic study along the fore arc of southern Peru (15–18°S) and northern Chile (18–19°S) are reported from middle to late Miocene ignimbrites (7 sites), late Oligocene to early Miocene ignimbrites (72 sites), Paleogene sediments (20 sites), and Mesozoic and Paleocene volcanics and intrusions (31 sites). Comparison of locality‐mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5.2 ± 11.3° clockwise to 55.6 ± 7.0° counterclockwise. Spatially, the magnitude of counterclockwise rotations increases northward from ∼0° within the Chilean fore arc south of 18°30′S to >45° north of 16°30′S. In southern Peru, paleomagnetic rotations recorded in Paleogene red beds decrease from late Eocene to late Oligocene, whereas Miocene ignimbrites display no evidence of rotation. These new results confirm that the rotations recorded in the fore arc of southern Peru were acquired at least before ∼15 Ma, and probably before 25 Ma, and thus prior to the late Neogene shortening of the sub‐Andes. The onset of major Andean shortening in the Eastern Cordillera during the latest Eocene–earliest Oligocene is interpreted to have triggered the bending of the Peruvian fore arc. The region of the Peruvian fore arc with the largest rotations appears to be the fore‐arc counterpart of the Abancay deflection, a remarkable NE‐SW offset in the axis of the Eastern Cordillera induced by a major regional preorogenic structure. We underline that the Abancay deflection should be seen as the northwestern boundary, and therefore as a key element, of the Bolivian Orocline.