Is glacial isostatic adjustment controlling crustal deformation beneath the Southern Patagonian Icefield (48-52°S) ? Insight from the characterization of small magnitude earthquakes.

The Patagonian ice-field located in the Southern Patagonian Andes (Chile-Argentina border, 46-52°S) is the second largest ice cap remaining in the southern hemisphere. Voluminous ice-loss due to constant global temperature rise since the last glacial maximum (~20 ka) coupled to an abnormally low-viscosity of the underlying mantle are causing a ∼4 cm/yr uplift of the upper-plate. Because this region is very remote and underpopulated, and despite the occurrence of several Mw > 5 earthquakes, it always lacked dense seismological observations. In this work we analyzed 2 years of continuous broad-band data recorded by 27 seismometers, deployed around the Southern Patagonian Ice-field (SPI), between JAN 2019 and DEC 2020. We found that most of the seismicity occurs at crustal level and coincides in space with regional compressive structures that characterize the Magallanes Fold and thrust belt. This major feature of the Argentine retroarc accommodated the crustal shortening of the southern Chilean margin in response to the subduction of the Antarctic plate beneath the South-American Plate. However, focal focal mechanism solutions obtained in this work, reveal that the seismicity located right beneath the SPI accommodate EW extensional stress. The EW transition from extensional to compressive crustal deformation seems to coincide with the strain variations derived from modeled response to rapid ice loss. This important result suggests that the current crustal deformation of the Patagonian Andes at 46-52°S, could be mostly controlled by crustal relaxation induced by glacial isostatic adjustment.