LANDSCAPE EVOLUTION OF THE DRY VALLEYS, TRANSANTARCTIC MOUNTAINS - TECTONIC IMPLICATIONS

There are different views about the amount and timing of surface uplif t in the Transantarctic Mountains and the geophysical mechanisms invol ved. Our new interpretation of the landscape evolution and tectonic hi story of the Dry Valleys area of the Transantarctic Mountains is based on geomorphic mapping of an area of 10,000 km(2). The landforms are d ated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution. Following a lowering of base level about 55 m.y. ago, there was a phase of rapid denudation associated with planation and escarpment retreat, probably under semiarid conditions. Eventually , downcutting by rivers, aided in places by glaciers, graded valleys t o near present sea level. The main valleys were flooded by the sea in the Miocene during a phase of subsidence before experiencing a final s tage of modest upwarping near the coast. There has been remarkably lit tle landform change under the stable, cold, polar conditions of the la st 15 m.y. It is difficult to explain the Sirius Group deposits, which occur at high elevations in the area, if they are Pliocene in age. Ov erall, denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland, whi ch is in good agreement with the results of existing apatite fission t rack analyses. It is suggested that denudation reflects the difference s in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy. Most crustal uplift (2-4 km) is inferred to have occurred i n the early Cenozoic with 400 m of subsidence in the Miocene followed by 300 m of uplift in the Pliocene.