The Caburgua micro-seismic sequence: crustal intra-arc faulting in Southern Andes.

Abstract

Current strain compartmentalization in Southern Andes intra-arc region results in the activation of NW faulting. Left-lateralcompressive seismic faulting taking place at the Caburgua Lake originates at depth of 9 km, extending horizontally for 20 kmlong up to the surface with very small magnitudes < 0.6 Mw. Several morphological and bathymetric features are consistent withthe fault direction, as well as Holocene monogenic volcanism. Controversially, geologically mapped faults in this segment ofthe Andes, only report NS oriented faults associated with the right-lateral strike-slip faults, such the Liquiñe-Ofqui Fault Systemand Reigolil-Pirihueico Fault. The N55˚W oriented Caburgua oblique-slip fault can be traced by linear clustering of micro- crustalearthquakes in South-Central Chile Cordillera. In transpressional arc settings, crustal seismicity preferentially occurs within majorarc-parallel fault zones, accommodating a major portion of the strike-slip component of strain partitioning (McCaffrey 1996,Weller et al., 2010). Recently published works have recognized the influence of the activation of long-term oblique-to-the-arcfaults, decoupling and compartmentalizing the bulk strain into conjugate transpressional to transtensional domains (e.g. Langeet al., 2008; Pérez-Flores et al, 2016). Here we present results of an unexpected micro-seismic swarm occurring in the hithertoundescribed NW striking Caburgua Fault, cross-cutting the Caburgua Lake in the Araucania Region, Southcentral Chile. TheCaburgua seismic swarm was recorded by a temporal seismic network, covering the overall intra-arc upper crustal seismicity inthis region. This seismic sequence occurs few km norths of the well-recognized regional volcano-tectonic Villarrica-Quetrupillan-Lanin alignment (Lara et al., 2006, Cembrano and Lara, 2009, Sielfeld et al., 2018 under review). Complementary morphostructuralanalysis such topographic scarps and lineaments inline with Pleistocene to Holocene eruptive products, and bathymetric features,such, subaqueous domes and steep slopes, suggest that neotectonic activity has undergone along the Caburgua Fault for at least300 ka., based ok the minimum age of the reported volcanic products (Moreno and Lara, 2008). The Caburgua micro-seismicswarm depicts a unique opportunity to understand the tectonic role of Andean transverse faulting (ATF) in the Andean orogenyhistory and fluid migration within an active volcanic chain.