Tephrochronology Applied to Late Cenozoic Paleoclimate and Geomorphic Evolution of the Central Transantarctic Mountains

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This award supports a program to study the geomorphic evolution of the Dry Valleys region of the Transantarctic Mountains. The isotopic ages and areal distribution of volcanic ash deposits will be used to constrain the chronology and rates of landform development and to help quantify the age of geomorphic surfaces. The depositional setting and degree of weathering of the ash will also help to constrain regional paleoclimatic conditions during and after deposition of the volcanic ash. This work will focus on the Quartermain Mountains, Asgaard Range, Olympus Range, and McKelvey Valley because these regions are known to contain some volcanic ash deposits and because they contain a variety of geomorphic settings over a substantial elevation range. This project will attempt to test assumptions that underlie the hypothesis that the Dry Valleys region represents a relict, semi- arid landscape that formed prior to build up of the continental ice sheets during middle Miocene time and that subsequent slope modification has been limited to minor glacial scouring concentrated at valley heads, glacier confluences, and deep- valley troughs. Among these assumptions are 1) that present geomorphic processes are ineffective denudation agents, 2) that widespread land surfaces within tectonically uniform blocks reflect base-level changes at the Transantarctic Mountains front, and 3) that geomorphic landforms of the Dry Valleys that superficially resemble box canyons, escarpments, mesas, and buttes of desert regions in the American southwest formed by denudation processes now operating in the Colorado Plateau. The isotopic age information generated by this project will provide constraints for models of geomorphic evolution in cold- desert regions and will have implications for models of landscape development and uplift history of the Transantarctic Mountains. If the hypothesis of ancient semi-arid erosion followed by minor glacial incision proves correct for the Dry Valleys regi on, then changes in base level, as defined by isotopically dated land surfaces in the Valleys, will provide a chronology of late Cenozoic mountain evolution and uplift independent of thermochronological studies that have been completed. This is important because the thermochronology studies, although well suited for determining long-term uplift rates, are not well suited to distinguishing early from late Cenozoic uplift. The volcanic ash data base developed in the course of this study will also be useful for studies of the McMurdo volcanic province.