A high-resolution seismic investigation of active faulting in lakes of Patagonia

DONALDO M. BRAN; JORGE GABRIEL LOZANO; FERNANDO ALMARAZ; DIEGO WINOCUR; FLORENCIA B. RESTELLI; STEFANÍA BUNICONTRO; EMANUELLE LODOLO; LUCA BARADELLO; YASMIN S. GUTIERREZ; ALEJANDRO A. TASSONE

S.C. BARILOCHE

Congreso; IAL - IPA 2022; 2022

CONICET

Subaqueous lacustrine deposits are a valuable tool for the characterization of tectonicdeformation since they offer decisive cross-sections with thick and usually well-preserved sedimentary records where to look for markers. This fact plays a key role inplaces where direct morphological evidence is lacking due to high erosion and/or slowdeformation rates or is concealed by dense forests or extensive water bodies, as is thecase of Patagonian Andes. This work presents the results of high-resolution seismicresearch in lacustrine subaquous deposits that led to the identification andcharacterization of active structures in two lakes encompassed within different tectonicsettings. Lago Fagnano, located in the Fuegian Patagonia, runs along the activeMagallanes – Fagnano transform fault system. The area is characterized by shallowlow-magnitude seismic activity with large historical earthquakes, such as the Mw 7.51949. This event generated coseismic ruptures identified onshore, although their fullextension remains uncertain. Seismic reflection data in the lake revealed a series offault ruptures affecting glaciolacustrine and lacustrine deposits, several of which reachthe lake-floor suggesting these could represent the offshore continuation of the 1949rupture. In addition, seismic data shows how deformation is distributed along differentfault segments, representing along-strike variations of deformation. On the other hand,Lago Argentino is a large proglacial lake located in the Austral Patagonian Andesforeland. Despite the seismic activity recorded in the area, no studies regarding theneotectonic activity have been carried out so far. We have identified subvertical faultswithin the post-glacial lacustrine sediments of Rico and Sur arms of the lake thatconstitute the first evidence of active tectonics in the area. Subaquous faults could becorrelated with onshore geomorphic markers that affect basement rocks. Theidentification and characterization of active potentially seismogenic faults is a key stepto a thorough hazard assessment in Patagonia, where these types of studies are yetlimited. Not only to evaluate future fault ruptures but also to investigate the occurrenceof associated geohazards such as soil liquefaction, landslides and associated tsunami.