Paleomagnetic evidence for rotation in the Precordillera of northern Chile: structural constraints and implications for the evolution of the Andean forearc

Abstract

Palaeomagnetic results are reported for two late Mesozoic to early Cenozoic continental sedimentary sequences from the Precordillera (Cordillera de Domeyko) of northern Chile. Comparison of isolated components of magnetisation from the Late Cretaceous-Palaeocene Purilactis Formation and the unconformably overlying Oligo-Miocene Paciencia Group with reference poles for South America indicate that significant post-Palaeocene and post-Lower Miocene clockwise rotation has taken place. A structural study of the area has revealed the presence of a number of thrust sheets that were periodically active throughout the Cenozoic. Stratigraphie, structural and palaeomagnetic data indicate that thrust sheet movement took place following deposition of both the Purilactis Formation (?lower Eocene Incaic Orogeny) and the Paciencia Group (?lowest Miocene Peheunche Orogeny) and resulted in the folding, uplift and clockwise rotation of the two sequences. Differences in the amount of rotation between the two units reflect incremental movement of the thrust sheets, 12° of rotation took place prior to deposition of the Paciencia Group followed by 17° of post-Lower Miocene clockwise rotation. This study demonstrates that palaeomagnetically detected rotation in the Precordillera of northern Chile, 250 km inland of the Peru-Chile trench, is related to the periodic movement of an easterly propagating thin-skinned thrust front. Between 22°30′ and 23°30′S the north Chilean forearc contains areas of non-rotation, thin-skinned compressional related rotations and large, block fault rotations indicating that the simplistic models previously presented to account for forearc rotation in the Central Andes should be modified. In particular, evidence implies that rotation could be related to a number of tectonic events and may not simply be due to one deformation event.