Glacier erosion at convergent margins: a numerical and field study in the Chugach-St. Elias Mountains of South Alaska
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Glacier erosion at convergent margins: a numerical and field study in the Chugach-St. Elias Mountains of South Alaska The Saint Elias range of South Alaska is known for its rapid erosion and uplift (order of 1 cm/yr), extreme relief at Mt. St. Elias reaching ~5800 m within 15 km of Pacific waters, and long history of tidewater glaciation extending to the Pliocene. The region is a natural laboratory to examine the coupling between tectonics, climate and topography, and the evolution of mountain ranges under the influence of glacial ice. To better understand the spatial and temporal distribution of glacial erosion and the parameters that control the landscapes they generate, I have built a physically based 2-dimensional model of glacial erosion forced by climate on glacial cycle time scale and applied it to the Seward-Malaspina glacier system. This numerical model should also prove useful in interpreting the growing thermochronological record of exhumation within the range, the sedimentary record in the Northeast Pacific and the climatic signature contained in the spatial and temporal distribution of sediment depocenters. The model integrates the seasonal evolution of glacial mass balance and basal hydrology, as well as accounts for ice dynamics, thermal regime, bedrock erosion by glacier quarrying and abrasion, sediment transfer, and tidewater processes. Field measurements of basin-wide erosion rates, glacier mass balance, ice velocity and glacier geometry conducted during the course of this study and as well as glaciological data available in the scientific literature are used to constrain model parameters. In addition, seismic profiling of Vitus Lake in front of the massive Bering-Bagley glacier system allowed computing a basin wide erosion rate of almost 6mm/yr averaged over 30 years of sediment accumulation for arguably the largest temperate glacier in the world. Model results indicate that ice flux per unit glacier width and sliding velocity, controlled by mass balance and valley width, exert an important influence over the distribution of glacier erosion. Yet, basal effective pressure as dictated by subglacial hydraulics and sediments protecting the substrate often overcome all other parameters in dictating erosional patterns, thereby suggesting that accounting for the degree of decoupling between ice and the bed and the presence of sediment is necessary to capture the essence of the distribution of glacial erosion in numerical models. Integration of these factors over glacial cycles allows establishing the spatial distribution of erosion for Seward-Malaspina Glacier that is consistent with long-term denudation revealed by the thermochronological data available for the region.