New and revised paleomagnetic data from the southern central andes: Testing tectonic rotations

Abstract

A paleomagnetic study performed in Plio-Pleistocene volcanic rocks of two localities in the Southern Central Andes is presented. The region corresponds to the transition zone between the Central and the Patagonian Andes. In the latter, deformation is almost exclusively controlled by the dextral-strike slip Liquiñe-Ofqui Fault Zone, that extends from ∼47 °S to ∼38 °S and accommodates the deformation generated by the oblique subduction of the Nazca Plate below South America. To the north of ∼36 °S, deformation is mainly contractional. Our study was carried out in the transition zone between both tectonic regimes. Sampling consisted of 31 paleomagnetic sites distributed in two localities: the Loncopué (∼38.0 °S, 70.7 °W) and Andacollo (∼37.2 °S, 70.8 °W) areas. We sampled Pliocene and Pleistocene volcanic rocks with ages younger than 5 Ma. After stepwise demagnetization and principal component analysis, site mean remanence directions were computed. Both localities show non-significant clockwise rotations of 1.7° ± 14.4° (Loncopué) and 8.1° ± 8.4° (Andacollo). Although the presently available database does not support significant tectonic rotation of the Andacollo lavas, preliminary results obtained from upper Miocene volcanics in a nearby area indicating ∼20° of clockwise rotation, encourage further studies to improve the resolution of the paleomagnetic data to determine if crustal block rotations have been, or still are, part of the Andean deformational processes. A revision of previously published paleomagnetic data in the northern Patagonian Andes suggests that, against original claims, no significant tectonic rotations associated to the Liquiñe-Ofqui Fault Zone, between ∼39 and 37.5 °S, can be unambiguously demonstrated.