Paleo-lacustrine terraces of Lago Argentino - Witnesses of crustal deformation at the Patagonian Icefields since Late Pleistocene?

E. MARDERWALD; P. BUSCH; J. M. ARAGÓN PAZ; L. MENDOZA; J. STRELIN; G. CONNON; J. L. HORMAECHEA; M. HORWATH; M. SCHEINERT; R. DIETRICH; A. RICHTER

Rostock

Conferencia; 27 th International Polar Conference "Polar Systems under Pressure"; 2018

University of Rostock

Lago Argentino is situated at the eastern edge of the Southern Patagonian Icefield, the largest temperate ice mass within the Southern Hemisphere. Intensive mass loss of the Patagonian icefields since the Little Ice Age (LIA) has induced exceptionally rapid crustal uplift due toglacial-isostatic adjustment (GIA) clearly evidenced by geodetic Global Navigation Satellite Systems (GNSS) observations (Richter et al. 2016, Lange et al. 2014, Dietrich et al. 2010). The glacier retreat after the LIA maximum has been preceded by much more dramatic ice-mass changes since the Last Glacial Maximum (LGM). According to current GIA models (Lange et al. 2014) the solid earth relaxation is very fast in this region due to the peculiar tectonic-reological setting imposed by the subduction of an active ocean ridge at the Chile (Ayzen) Triple Junctionand the opening of the Patagonian slab window underneath the icefields. The geodetically determined present-day visco-elastic deformation does therefore not provide information on these earlier ice-mass signals. However, between the LGM and the lateglacial Puerto Bandera readvance (climax about 13,000 cal yrs BP, Strelin et al. 2011), the maximum lake level in the Lago Argentino basin was several tens of meters higher than at present (185 m asl, IGN 2015) and has formed paleo-lacustrine terraces which are well preserved along the shores ofthe eastern and central parts of the lake. The paleo-lake level at the time of terrace formation with respect to the present-day lake level documents the cumulative differential vertical crustal deformation. Intense post-LGM glacial-isostatic uplift centered west of the lake is expected to result in a tilt between the late-Pleistocene and present lake levels with the oldest westernmost paleo-terraces raised higher (55 m) above the present lake level than further east (25 m). Tectonic deformation related to the plate collision to the west might have  increased the GIA-induced gradient in relative to present paleo-terrace height. We present first results of a geodetic determination of the geometry and relative height of ten paleo-terrace profiles and one paleo-delta at Lago Argentino based on kinematic GNSS profiling. GNSS buoy observations were combined with the operation of two tide gauges in the lake to derive the present-day mean lake level as a reference surface for the paleo-terraces. Additional data sources such as a high-resolution regional digital elevation model (IGN 2015) and historic aerophotogrammetric material are employed for the geomorphological identification and interpretation of the paleo-terraces.