Thorkelson, Derek

This thesis advances understanding of late Cenozoic landscape evolution and glaciation in southernmost South America using continental sedimentary deposits and landforms in the Lago Cardiel region in the foothills of the southern Patagonian Andes and along the Atlantic north and south of the Strait of Magellan. The evolution of the landscape in these two areas was determined through landform mapping and relative chronologic landform correlations. Paleomagnetic characteristics of late Cenozoic sediments and basalt flows and the stratigraphy and sedimentology of Pleistocene glacial sediments in sea cliffs and anthropogenic exposures provide a chronology and evidence of depositional environments during Pleistocene glaciations. The landscape in the Lago Cardiel area changed significantly following the last major period of tectonic uplift at the end of the Miocene. Large west-trending valleys that incise Miocene-aged basalt were abandoned by their formative rivers about 4.4 Ma. The closed basin that contains Lago Cardiel began to form on the relict plain surface before 4.0 Ma and grew in size throughout the Pliocene and Pleistocene by a combination of erosion by small streams, deflation, colluviation, and possibly tectonic collapse. Drainage reorganizations occurred at about 4.0 Ma and 3.6 Ma, most likely initiated by increased aggradation or isostasy during Pliocene glaciations. Eolian, fluvial, and mass-movement processes continued to alter the landscape throughout the Pleistocene with higher rates during glacial periods. Evidence of at least three glaciations is recorded in the stratigraphic exposures at the Atlantic Coast and the shores of the Strait of Magellan. At Cabo Vírgenes and Bahía Posesión, two glacial drift units were deposited in a grounding-line environment. These sediments are normally magnetized and date to the Brunhes Chron (<0.78 Ma). The Tres de Enero highway cut exposes three subglacial tills deposited during the Great Patagonian Glaciation (GPG) – two normally magnetized tills that I assign to the early Brunhes Chron and a lower reversely magnetized till deposited during the Matuyama Chron (2.581-0.78 Ma). The reversely magnetized till and other reversely magnetized GPG sediments indicate that the earliest Pleistocene glaciations occurred before 0.78 Ma. In the Río Gallegos Valley, a 0.86 Ma basalt flow caps a thick unit of normally magnetized glaciofluvial gravel, which was probably deposited during the Jaramillo Subchron (1.075-0.991 Ma). This thesis provides a timeline for the evolution of the landscape of the Lago Cardiel region from the Miocene to the present. It also contributes to our understanding of the age and depositional environments of GPG and post-GPG 1 glacial events in the Strait of Magellan region by documenting the magnetic polarity of glacial sediments throughout the region.

Peak metamorphism of the Wolverine metamorphic complex (WMC) occurred at conditions of 770°-830°C and 7.2-10.4 kbars, and was accompanied by partial melting and the development of tight to isoclinal northeast-vergent folds that are axial planar to a transposition foliation. The WMC is modelled as a diffuse northeast-vergent shear zone that formed beneath a southwest-vergent panel of rocks as the orogenic wedge detached and translated northeastward during Mesozoic contraction. Juxtaposition of Middle Jurassic greenschist-facies upper crustal rocks against upper amphibolite-facies rocks of the WMC, which contain Eocene 40Ar/39Ar cooling ages, suggests that the WMC remained at deep crustal levels until it was rapidly exhumed in the Eocene along the normal Wolverine fault. Mineral reaction and disequilibrium textures indicate a near- isothermal decompression path from 7.2-10.4 kbars to below 4 kbars, corresponding to a minimum of 11 km of exhumation prior to cooling below ~650°C.