ALBERT

Description: We examine the intra‐arc crustal seismicity of the Southern Andes Volcanic Zone (SVZ). Our aim is to resolve inter‐seismic deformation in an active magmatic arc dominated by both margin‐parallel (Liquiñe‐Ofqui fault system, (LOFS)) and Andean transverse faults (ATF). Crustal seismicity provides information about the schizosphere tectonic state, delineating the geometry and kinematics of high strain domains driven by oblique‐subduction. Here, we present local seismicity based on 16‐months data collected from 34 seismometers monitoring a ~200 km long section of the Southern Volcanic Zone, including the Lonquimay and Villarrica volcanoes. We located 356 crustal events with magnitudes between Mw 0.6 and Mw 3.6. Local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain, suggesting a convex shape of the crustal seismogenic layer bottom. Focal mechanisms indicate strike‐slip faulting consistent with ENE‐WSW shortening in line with the long‐term deformation history revealed by structural geology studies. However, we find regional to local‐scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity, within three distinctive latitudinal domains. In the northernmost domain, seismicity is consistent with splay faulting at the northern termination of the LOFS; in the central domain, seismicity distributes along ENE‐ and WNW‐striking discrete faults, spatially associated with, hitherto seismic ATF. The southernmost domain, in turn, is characterized by activity focused along a N15°E striking master branch of the LOFS. These observations indicate a complex strain compartmentalization pattern within the intra‐arc crust, where variable strike‐slip faulting dominates over dip‐slip movements.

Description: We examine the intra‐arc crustal seismicity of the Andean Southern Volcanic Zone. Our aim is to resolve interseismic deformation in an active magmatic arc dominated by both margin‐parallel (Liquiñe‐Ofqui fault system, LOFS) and Andean transverse faults. Crustal seismicity provides information about the schizosphere tectonic state, delineating the geometry and kinematics of high strain domains driven by oblique‐subduction. Here, we present local seismicity based on 16‐month data collected from 34 seismometers monitoring a ~200‐km‐long section of the Southern Volcanic Zone, including the Lonquimay and Villarrica volcanoes. We located 356 crustal events with magnitudes between Mw 0.6 and Mw 3.6. Local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain, suggesting a convex shape of the crustal seismogenic layer bottom. Focal mechanisms indicate strike‐slip faulting consistent with ENE‐WSW shortening in line with the long‐term deformation history revealed by structural geology studies. However, we find regional to local‐scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity, within three distinctive latitudinal domains. In the northernmost domain, seismicity is consistent with splay faulting at the northern termination of the LOFS; in the central domain, seismicity distributes along ENE‐ and WNW‐striking discrete faults, spatially associated with, hitherto seismic Andean transverse faults. The southernmost domain, in turn, is characterized by activity focused along a N15°E striking master branch of the LOFS. These observations indicate a complex strain compartmentalization pattern within the intra‐arc crust, where variable strike‐slip faulting dominates over dip‐slip movements.

Description: Oblique-slip tectonics in the intra-arc region of the Southern Andes accommodates heterogeneous deformation derived from plate convergence during the Pliocene and Quaternary. Long-term mechanical interaction between Andean transverse faults (i.e. NW-striking sinistral faults) and margin-parallel faults (i.e. NNE-striking faults) results in linked transtensional fault damage zones that facilitate structural conditions for the migration and emplacement of geofluids in the upper crust. We investigated the architecture of pre-eruptive units and the nature of faulting at the Tatara–San-Pedro–Pellado volcanic complex in the Southern Andes. Here, oblique-slip faulting crosscuts Miocene folded strata and granitoids. Our main results suggest that Quaternary volcanism and an associated geothermal systems developed on top of an ENE-oriented structural anisotropy defined by hundreds of faults and dikes interacting in a ca. 9 km long and 4 km wide rock volume, named the Tatara Damage Zone. Deformation in this domain is characterized by ENE- to WNW-striking transtensional oblique-slip faults flanked by (1) the seismically active NS-striking (dextral) Melado Fault to the west, (2) discrete NS- to ENE-striking dextral splay faults to the east and (3) the sinistral NW-striking Los Cóndores Fault to the north-east, which is reported for the first time in this work. The latter fault represents an excellent example of a long-lived Andean Transverse Fault. Furthermore, we suggest that the Los Cóndores Fault accommodates a margin-oblique slip component of bulk transpressional deformation. We demonstrate that Pliocene–Quaternary intra-arc oblique faulting developed after pre-Late Miocene compressional tectonics, and that this oblique faulting constrains the geometry of permeable pathways for the flow, emplacement, and eruption of quaternary geofluids. Furthermore, we evaluated the stress field for a discrete volcanic complex and provided key elements to better understand the role of Andean Transverse Faults in the spatial organization of Quaternary arc volcanism and geothermal systems in the Southern Andes.

Description: The Liquiñe-Ofqui fault system (LOFS) in south-central Chile provides a natural laboratory to assess the interplay between magma/hydrothermal fluid flow and crustal deformation. Understanding these processes is of paramount importance for geothermal energy exploration and seismic hazard assessment. We deployed a dense seismic network (Sielfeld et al., 2019) at the northern termination of the LOFS in south-central Chile (~38°S) between 2014 March and 2015 June. The main aim was to better understand the significance and implications of seismic activity in relation to geological information such as the complex fault-fracture network, volcanoes, and the stress field estimated from geological data. As a result, the network was designed to monitor the northern segment of the LOFS on a more regional scale rather than concentration on the activity of one volcano. The network covered a ~200‐km‐long section of the Southern Volcanic Zone, including several Holocene stratovolcanoes (Callaqui, Copahue, Caviahue Caldera, Tolhuaca, Lonquimay, Llaima, Sierra Nevada, Sollipulli, Villarrica, Quetrupillán, Lanín (La), and Mocho‐Choshuenco). Waveform data are open and available under network code 3H from the GEOFON data centre under license CC BY 4.0.