Transient tectonic regimes imposed by megathrust earthquakes and the growth of NW-trending volcanic systems in the Southern Andes

Abstract

Geophysical data show that megathrust earthquakes can promote transient tectonic regimes lasting up to several decades. However, such variations are not trivial to be recognises in the geological records of volcanic arcs. To better understand the occurrence of transient tectonic regimes we study the inter- and post-seismic deformation in the upper crust of the Andean volcanic arc affected by the M8.8 2010 Maule earthquake. We focus on the region around the Nevados de Chillán Volcanic Complex, Southern Central Chile, where we compare geological and earthquakes data. This NW-oriented Andean-transverse volcanic complex faces one of the regions that slipped the most during the 2010 M8.8 Maule earthquake. Field fault-slip data from lithostratigraphic units forming the basement of Nevados de Chillan Volcanic Complex point out three tectonic regimes. The principal is driven by a sub-horizontal and NE-oriented σ1 (transpressional kinematics). This regime is compatible with today's GPS data recorded during inter-seismic periods. The second and third tectonic regimes are less abundant and indicate transpression (σ1 sub-horizontal and NW-oriented) and normal (σ1 sub-vertical and E-W-oriented) faulting, respectively. The geological record was also compared with seismic data acquired from 2010 to 2015 in the investigated region. The moderate-magnitude seismic activity occurred beneath the volcanic arc after the Maule earthquake points out an enhanced crustal deformation if compared to inter-seismic periods. The inversion of focal mechanisms suggests that intra-arc regions may experience a short-lived post-seismic transtensional regime with an E-W oriented σ1 with a dip of about 45°. Our work suggests that megathrust earthquakes impose transient stress variations in the volcanic arc, possibly reactivating Oligo-Miocene NW-striking inherited crustal structures, upon which volcanic edifices develop. This may help explaining the occurrence of Andean-transverse volcanic complexes oriented sub-perpendicular to the NE-oriented σ1 driving convergence in the Southern Andes.