Contrasting Neogene denudation histories of different structural regions in the Transantarctic Mountains rift flank constrained by cosmogenic isotopemeasurements

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 valle ys occurred at different times throughout central and northern Victoria Lan d. This is corroborated by measurements of cosmogenic nuclides Ne-21, Be-10 and Al-26 of glacial erosion surfaces and high-elevated moraines. Ne-21 ag es of two summit plateaus, at elevations of 1650 m in central Victoria Land and similar to 2800 m in northern Victoria Land, range from 3.84 to 11.2 M a, respectively. The latter date indicates that these glacial erosion surfa ces are the oldest known exposure dated surfaces on Earth. Glacial erosion terraces, remnants of early phases of valley downcutting, have Ne-21 ages o f 1.27 and 6.45 Ma for central and northern Victoria Land, respectively. Th erefore, deglaciation of summit plateaus, valley downcutting and topographi c uplift occurred during the Mid-Miocene in northern Victoria Land and not earlier than the Mid-Pliocene in central Victoria Land. In northern Victori a Land, ice flow directions changed markedly from the time a regional ice s heet occupied the level of the highest summits to the present condition wit h summits rising up to 800 above the valley glaciers. In central Victoria L and, 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-flowin g David Glacier. This great variation in denudation histories probably results from differen tial tectonic uplift of various regions within the presently active rift fl ank. Three tectonic processes contribute to Late Neogene uplift: (1) ongoin g extension in adjacent Ross Sea rift basins; (2) regional dextral transten sion following SE-trending Precambrian and Palaeozoic structural trends whi ch offsets the similar to N-S-trending grain of the rift and reactivates ea rlier faults; and (3) isostatic response to valley downcutting and related denudation, (C) 1999 Elsevier Science B.V. All rights reserved.