THE INFLUENCE OF GLACIAL EROSION ON ACTIVE MOUNTAIN BUILDING, BASED ON EXHUMATION OF THE CHUGACH/ST. ELIAS RANGE, SOUTHEAST ALASKA

Studies of active mountain building in recent years have increasingly attributed significance to the control that erosional process and climate exert on the topography of orogenic belts. While linkages between denudation, deformation, and topography have been demonstrated in fluvially-dominated mountain belts, it is uncertain what role such coupling may have in glacially-dominated orogens. The Chugach/St. Elias Range in southeast Alaska is a young orogenic belt that contains key elements for exploring the role of glaciers in the evolution of mountain belts. This range has been built over the past 6 Myr due to rapid collision of the Yakutat microplate into North America. Given the distribution of primary structures and along-strike changes in deformation style, patterns of structurally-controlled bedrock uplift can be loosely predicted. Imposed upon this are major climatic variations, however, that predict a contrasting patterns of exhumation. A strong orographically-controlled precipitation gradient occurs inland across the axis of the range. The distribution of glaciers also varies from the heavily glaciated high core of the range to the margins that experience cyclic transition between fluvial and glacial systems. These predict that maximum denudation should occur along the coast and margins, between the upper and lower limits of cyclically fluctuating equilibrium line altitude. Although exhumation has been measured at several locations, the pattern of unroofing across the bulk of the range is unknown. We are using apatite radiogenic helium (U-Th)/He dating to constrain the low-temperature cooling and recent exhumation history of the range, to explore the role of glacial erosion. Sixty samples of variable composition were collected in 2001 from the western and central portion of the range and are currently being analyzed. These should provide first-order constraint with adequate spatial coverage to begin testing how climate and tectonics control denudation patterns and, in turn, how erosion influences the partitioning of deformation.