Lithospheric Relaxation During Slab Window Formation: Evidence From Earthquake Focal Mechanism Analysis in Southern Patagonia

Abstract

This work aims to analyze the crustal seismicity of Southern Patagonia (45°–48°S, 69°–75°W), where the South Chile Oceanic Seismic Ridge subducts, creating an asthenospheric window (AW). This region provides a unique natural laboratory for investigating the geodynamics of slab windows and their impact on lithospheric deformation and stress distribution. We seek to understand the stress field and deformation on an active AW by characterizing seismogenic structures. Previous studies identified this zone as a seismic gap due to slab removal, indicated by the absence of Pliocene to present‐day magmatism and minimal crustal deformation. Despite limited seismological data, we analyzed earthquake records from three temporary networks active in Southern Patagonia between 2004–2006 and 2018–2021 and focal mechanisms (FM) from literature and catalogs. Seismic records were processed using SEISAN software and two 1‐D velocity models. We determined 47 FM, primarily normal faulting and strike‐slip, with occasional reverse faulting events. Events are mostly tectonics, and influenced by local and regional structures, with some shallow earthquakes likely linked to anthropogenic activities. Tectonic FM allowed us to differentiate events occurring between three major structural boundaries: the trench, the Liquiñe‐Ofqui fault zone (LOFZ), and the eastern Andean thrust front near the foreland transition zone. These major boundaries allowed us to define three distinct blocks: Chiloe, Cachet, and an aseismic foreland. In the Chiloe block and over the AW area, FM are predominantly strike‐slip, linked to the LOFZ kinematics. The predominance of normal‐faulting FM and extension in the Cachet block, situated at the core of the AW, aligns with normal faults displacing Upper Miocene strata and Pliocene basalts. These findings are consistent with the interpretation of regional lithospheric relaxation following the arrival of the AW around 6 Ma.