| Summary: | Separate regions within the Transantarctic Mountains, the uplifted flank of the West Antarctic rift system, appear to have distinct Neogene histories of glaciation and valley downcutting. Incision of deep glacial outlet valleys occurred at different times throughout central and northern Victoria Land. This is corroborated by measurements of cosmogenic nuclides 21Ne, robe and 26Al of glacial erosion surfaces and high-elevated moraines. 21Ne ages of two summit plateaus, at elevations of 1650 m in central Victoria Land and ~ 2800 m in northern Victoria Land, range from 3.84 to 11.2 Ma, respectively. The latter date indicates that these glacial erosion surfaces are the oldest known exposure dated surfaces on Earth. Glacial erosion terraces, remnants of early phases of valley downcutting, have 21Ne ages of 1.27 and 6.45 Ma for central and northern Victoria Land, respectively. Therefore, deglaciation of summit plateaus, valley downcutting and topographic uplift occurred during the Mid-Miocene in northern Victoria Land and not earlier than the Mid-Pliocene in central Victoria Land. In northern Victoria Land, ice flow directions changed markedly from the time a regional ice sheet occupied the level of the highest summits to the present condition with summits rising up to 800 above the valley glaciers. In central Victoria Land, the oldest documented ice flow direction occupying the summit erosion surface prior to incision was SW-NE, draining the East Antarctic Ice Sheet along an outlet glacier at least 10 times as wide as the present E-W-flowing David Glacier. This great variation in denudation histories probably results from differential tectonic uplift of various regions within the presently active rift flank. Three tectonic processes contribute to Late Neogene uplift: (1) ongoing extension in adjacent Ross Sea rift basins; (2) regional dextral transtension following SE-trending Precambrian and Palaeozoic structural trends which offsets the ~ N-S-trending grain of the rift and reactivates earlier faults; and (3) isostatic response to valley downcutting and related denudation. (C) 1999 Elsevier Science B.V.
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