Understanding kinematics of intra-arc transcurrent deformation: Paleomagnetic evidence from the Liquiñe-Ofqui fault zone (Chile, 38–41°S)

Abstract

The Liquiñe-Ofqui fault zone (LOFZ) is a major ~1000 km long dextral shear zone of southern Chile, likely related to strain partitioning of Nazca Plate oblique convergence with South America. To understand block rotation pattern along the LOFZ, we paleomagnetically sampled 55 sites (553 samples) between 38°S and 41°S. We gathered Oligocene to Pleistocene volcanics and Miocene granites at a maximum distance of 20 km from the LOFZ, and at both sides of it. Rotations with respect to South America, evaluated for 36 successful sites, show that crust around the LOFZ is fragmented in small blocks, ~1 to 10 km in size. While some blocks (at both fault edges) undergo very large 150°–170° rotations, others do not rotate, even adjacent to fault walls. We infer that rotations affected equidimensional blocks, while elongated crust slivers were translated subparallel to the LOFZ, without rotating. Rotation pattern across the LOFZ is markedly asymmetric. East of the fault and adjacent to it, rotations are up to 150°–170° clockwise, and fade out ~10 km east of fault. These data support a quasi-continuous crust kinematics, characterized by small rigid blocks drag by the underlying ductile crust flow, and imply 120 km of total fault offset. Conversely, crust west of the LOFZ is cut by seismically active NW-SE sinistral antithetic faults, and yields counterclockwise rotations up to 170° at 8–10 km from LOFZ, besides the unrotated blocks. Further data from the Chile fore arc are needed to understand block rotation kinematics and plate dynamics west of the LOFZ.