Double-vergent orogenesis in north Patagonia: different mechanisms of Andean deformation

Abstract

The Andes constitute an ideal tectonic setting for investigating different modes of subduction orogeny,given its segmented anatomy and the potential drivers of the upper/lower plates over the episodicperiods of mountain building (e.g., Mpodozis and Ramos, 1989). In this sense, integrated analyses oforogenic sections that contemplate from the fore to retroarc sections and their potential coupling are stilllacking. Therefore, we studied a forearc segment in northern Patagonia (~41-44ºS) in order to analyzeits crustal architecture and the structural control over the thick-infilling oceanic forearc basins andNeogene strata in the western Andean slope and Coastal Cordillera. Here, Neogene strata weredeposited during the tectonic switching from an accentuated late Oligocene-early Miocene extensionalregime to a mid-late Miocene contractional period and the breakup of the Farallon into the Nazca plate(e.g., Jordan et al., 2001). We performed a structural analysis through field work along with acquisitionof several seismic data (2D multichannel and refraction seismic data and drillholes from ENAP-Chile) inorder to identify the time of activity of the main forearc structures and their relation, in a crustal scale,with the east-vergence faults that form the retroarc fold-thrust belt. Our data reveal the presence of amajor west-vergent thrust in the western slope of the Andean cordillera and a trenchward directed splayin the Coastal Cordillera during mid-late Miocene time, contemporaneously with retroarc fold-thrust beltgrowth (e.g., Folguera et al., 2018). This suggests a double-vergent orogenesis for this Andeansegment, and proposes a discussion on the general modes of Andean deformation like in othersegments of the Central Andes (e.g., Riesner et al., 2018).