Paleomagnetism and tectonics of the southern Atacama Desert (25–28°S), northern Chile

Abstract

We report paleomagnetic results for 131 sites from the modern forearc of northern Chile (25°S and 28°S). Remanent magnetization in volcanic and intrusive rocks is mostly primary, while a secondary magnetization is observed in sedimentary rocks. Comparison of locality‐mean directions with expected paleomagnetic directions indicates vertical axis rotations from −7.3° ± 21.6° counterclockwise to 52.7° ± 17° clockwise. Jurassic to Early Cretaceous rocks from the Coastal Cordillera and Cretaceous to Paleocene rocks from the Central Depression show similar magnitude (>30°) clockwise rotations, while more variable rotations occur in Mesozoic to Eocene rocks of the Precordillera. Clockwise rotations in Mesozoic and Paleogene rocks occur in the Chilean Frontal Cordillera south of 27°30′S. Paleomagnetic results in three large Miocene ignimbrite sheets overlying rotated and nonrotated older rocks in the Precordillera and Pre‐Andean Depression which show no relative rotation between sites indicate that most rotations within the study area occurred prior to 18 Ma (early Miocene) and likely during and after the “Incaic” tectonic event, which affected large tracts of the central Andes. The postulated onset of rotations in the north Chilean forearc was contemporaneous with the beginning of horizontal shortening and uplift of the Eastern Cordillera in Bolivia and northwestern Argentina. Rotation of the Chilean forearc, enhancement of the curvature of the central Andes, and the formation of the Bolivian Orocline seem to be, for the most part, closely linked to the evolution of the Eocene‐Oligocene tectonics of the Eastern Cordillera.