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

Abstract

Active slip partitioning between the subduction megathrust and the upper plate is investigated in the oblique-convergence setting of the Nazca-SouthAmerican plate boundary between 33 degrees and 47 degrees S. This segment has two major along-strike bends: the Maipo Orocline (similar to 34 degrees S) and theArauco Peninsula (similar to 38 degrees S), whereas south of 38 degrees S, lies the intra-arc Liqui & ntilde;e-Ofqui Fault System (LOFS). Here we examine long-and short-term upper-plate deformation by combining a harmonized catalog of about 2,300 fault-slip measurements in the forearc and arc regions, from which we derive P-T axes using kinematic inversions, along with an integrated seismological database for upper-plate events (1976-2025), including global and local networks. These data are categorized by forearc, arc, and back-arc regions. We identify four distinct tectonic segments based on the spatial distribution of P and T axes in the long-and short-term: (1) 33 degrees-34 degrees S, showing both similar to E-W and similar to N-S subhorizontal shortening in the forearc and arc areas, suggesting active radial shortening; (2) 34 degrees-37 degrees S, dominated by mostly blind, seismogenic, margin-parallel dextral faults along with NW-and NE-trending structures running at a high angle with respect to the plate margin (called transverse faults here); (3) 37 degrees-41 degrees S, where margin-orthogonal subhorizontal shortening in the submerged forearc coexists with nearly margin-parallel shortening in the emerged forearc, with contemporaneous dextral slip along the LOFS and similar to E-W shortening accommodated by transverse NE-trending dextral and NW-trending sinistral seismically active faults; and (4) 41 degrees-47 degrees S, a region governed mainly by the geometry and kinematics of the LOFS strike-slip duplex. Kinematic indicators on mesoscopic faults (mostly slickenfibers) and a lack of pseudotachylytes suggest a considerable aseismic component to upper-plate fault slip, implying that morphotectonic slip rates may overestimate seismic hazard. Future detailed geodetic data may help better constrain the relative contributions of aseismic and seismic slip on the upper-plate faults in this Andean segment.