The Impact of Long-Term Glacial Erosion on the Active Chugach-St. Elias Mountains, southern Alaska
Buscher, Jamie Todd
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The influence of erosion on uplifting orogens has been demonstrated to be a primary force in landscape development. An understanding of fluvial erosion in mountain belts is fairly well documented, but the impact of glacial erosion is yet to be fully recognized. The uplift of the Chugach-St. Elias Mountains over the last 5-6 Ma under the influence of intense glaciation provides a unique setting to study the impact of glacial erosion on landscape development. The range has been built by rapid convergence (~5 cm/yr) of the Yakutat terrane with North America. Climatic forcing of northward-driven storms has created a disproportionate glacier distribution across strike, where extensive piedmont glaciers (low equilibrium line altitudes) cover the windward side of the range and small isolated glaciers (high equilibrium line altitudes) occupy the leeward side. If glacial erosion is greatest at the equilibrium line altitude, then glaciers will act as "buzzsaws" there to limit topographic development. Exhumation would therefore be expected to increase towards the coast. If glacial erosion is not dominant, exhumation would be expected to increase away from the coast towards the core of the range, where fault dip angles are high and deep crustal rocks are exposed. To determine the impact of long-term glacial erosion on exhumation of the Chugach-St. Elias Mountains, samples were collected along and across the strike of the range and analyzed by the apatite radiogenic helium (AHE) technique. Samples previously dated using the apatite fission track (AFT) method and located adjacent to our field area were also included in the analyses. The low-temperature sensitivity of these thermochronometers allows exhumation rates to be determined for shallow crustal depths. Both glacial and tectonic processes have influenced exhumation of the range. Exhumation rates increase to the south and east towards the collision zone, but coastal rates (0.36-2.5 mm/yr) are significantly higher than inland samples (0.038-0.24 mm/yr). These rates indicate that coastal glaciation plays a dominant role in landscape development and suggest that short-term erosion rates inferred from sediment yields are exaggerated. Although the exhumation rates are lower than expected, the correlation of exhumation patterns, glacier distribution, and equilibrium line altitude supports the "glacial buzzsaw hypothesis".
- Masters Theses