Geophysical Methods Applied to the Study of Lakes and PaleoLakes in Tierra del Fuego

Abstract

During the last years, geophysical surveys have been carried out in order to study the possible tectonic origin of Udaeta Lake and the extension and thickness of glacio-lacustrine deposits cropping out in the Río Valdez mouth. In both cases, geophysical methods proved to be a powerful tool for the investigation of Quaternary tectonic and glacial activity in a zone of Tierra del Fuego, which is covered by a very dense forest that hinders the identification of rocky outcrops and natural exposures. The geometry of Udaeta Lake, located in the central section of the Magellan-Fagnano Fault System (MFFS), previously envisaged by other authors as a possible pull-apart basin, was investigated. Magnetic, gravimetric and detailed topographic profiles and vertical electrical soundings were surveyed. Geophysical results were integrated with fieldwork data allowing the identification of a transtensional zone with two E?W main sinistral strike-slip faults with a normal component that control the North and South coasts of the Lake. The central depression occupied by the Lake was also affected by inferred NE subparallel faults. The obtained results support a genesis of Udaeta Lake consistent with a pull-apart basin model and highlight the influence of MFFS on the formation of some water bodies located along the fault zone. Moreover, the constructed models contribute to a better understanding of earthquake rupture parameters and pattern of surface and in-depth deformation, improving the knowledge of the seismological behavior of the MFFS. In the Río Valdez mouth, located in the southeastern coast of Fagnano Lake, an outcrop of 7 m thick rhythmic glacio-lacustrine sediments was identified. It was interpreted that such outcrop was formed in a frontal morainic complex during the Pleistocene. Ground magnetic and resistivity surveys were carried out, in order to determine the thickness and areal extension of these lacustrine sediments. A maximum E-W extension of ~220 m, a minimum NNW-SSE one of ~180 m, and a minimum thickness of ~20 m were estimated. Considering this thickness and an assumed average sedimentation rate, the natural dam that promoted the paleoLake and their related lacustrine deposits could have existed prior to MIS 2, when the Last Glacial Maximum took place in the region.