Exploring slip partitioning in the Southern Andes: New insights from fault slip data and crustal seismicity

Abstract

Oblique convergence between the Nazca and South American plates along the central and southern Chilean Andes (33°-47°S) is partitioned between the megathrust and the upper plate, This segment features two major along-strike bends—the Maipo Orocline (~34°S) and the Arauco Peninsula (~38°S)—and the intra-arc Liquiñe-Ofqui Fault System (LOFS) south of 38°S. We examine long- and short-term deformation along and across strike using (i) a harmonized catalog of roughly 2,300 post-Miocene fault-slip measurements from the forearc and arc regions, from which we derive P-T axes from kinematic inversions, and (ii) an integrated seismological database that combines GCMT and ANSS Mw≥5 upper-plate events (1976-2025) with published focal mechanisms from temporary local networks (Mw<4), categorized by forearc/arc/back-arc regions based on slab geometry. Results indicate that slip partitioning along the margin between 33°S and 47°S is more complex than previously recognized. We identify four consecutive tectonic segments along the continental margin based on the spatial distribution of P and T axes over both long- and short-term periods. (1) 33°-34°S, characterized by weak obliquity causing overall EW shortening and local NS shortening in both the forearc and arc domains, suggesting radial shortening; (2) 34°-37°S, dominated by mostly blind margin-parallel dextral faults as shown by crustal seismicity plus active NW and NE transverse structures; (3) 37°-41°S, where forearc margin orthogonal shortening in the submerged forearc coexists with nearly margin-parallel shortening in the emerged forearc. This is coeval with intra-arc dextral slip along the LOFS and EW shortening accommodated by transverse NE dextral and NW sinistral strike-slip faults; (4) 41°-47°S, a region controlled by the geometry and kinematics of the LOFS strike-slip duplex. Mesoscopic kinematic indicators (e.g., abundant slickenfibers, and no recognized pseudotachylite to date) suggest a significant aseismic component to upper-plate slip, implying that morphotectonic and paleoseismic slip rates may overestimate seismic hazard. Shallow crustal sources can locally influence expected long-return-period ground motion intensities; thus, integrating aseismic slip and utilizing improved GNSS/InSAR data are vital for future seismic hazard analyses in the Southern Andes.