Southern Alaska as an example of the long-term consequences of mountain building under the influence of glaciers

Southern Alaska is a continent-scale region of ongoing crustal deformation within the Pacific-North American plate boundary zone. Glaciers and glacial erosion have dictated patterns of denudation in the orogen over the last s imilar to 5 Myr. The orogen comprises three discrete topographic domains fr om south to north, respectively: (1) the Chugach/St. Elias Range; (2) the W rangell Mountains; and (3) the eastern Alaska Range. Although present defor mation is distributed across the orogen, much of the shortening and uplift are concentrated in the Chugach/St. Elias Range. A systematic increase in t opographic wavelength of the range from east to west reflects east-to-west increases in the width of a shallowly dipping segment of the plate interfac e, separation of major upper plate structures, and a decrease in the obliqu ity of plate motion relative to the plate boundary. Mean elevation decays e xponentially from similar to 2500 to similar to 1100 m from east to west, r espectively. Topographic control on the present and past distribution of gl aciers is indicated by close correspondence along the range between mean el evation and the modern equilibrium line altitude of glaciers (ELA) and diff erences in the modern ELA, mean annual precipitation and temperature across the range between the windward, southern and leeward, northern flanks. Net , range-scale erosion is the sum of (1) primary bedrock erosion by glaciers and (2) erosion in areas of the landscape that are ice-marginal and are de glaciated at glacial minima. Oscillations between glacial and interglacial climates controls ice height and distribution, which, in turn, modulates th e locus and mode of erosion in the landscape. Mean topography and the mean position of the ELA are coupled because of the competition between rock upl ift, which tends to raise the ELA, and enhanced orographic precipitation ac companying mountain building, which tends to lower the ELA. Mean topography is controlled both by the 60 degrees latitude and maritime setting of acti ve deformation and by the feedback between shortening and uplift, glacial e rosion, and orographic effects on climate accompanying mountain building. ( C) 2000 Elsevier Science Ltd. All rights reserved.