Examination of the interplay between glacial processes and exhumation in the Saint Elias Mountains, Alaska

Abstract

The combination of large, temperate glaciers and rapid crustal convergence in the Saint Elias Mountains (southeastern Alaska, USA, and Yukon Territory and British Colombia, Canada) provides an exceptional opportunity to study the interactions between the tectonic and surface processes that have shaped most active orogens on Earth during much of the Quaternary. This research fi rst provides a review of thermochronometric data sets recording exhumation under two major glacier systems of the Saint Elias Mountains, the Bagley-Bering and the Seward-Malaspina systems. These data sets are integrated over the single glacier systems and used in conjunction with glaciological data to investigate the interactions of glacial erosion and tectonics. Despite their proximity, the glaciological processes and geological settings of these two glacial systems differ signifi cantly. On the east side of the orogen, sediments eroded from bedrock underneath the Malaspina Glacier refl ect regions of rapid erosion under the slowly moving Seward Ice Field. Because the Seward Ice Field overlies a localized zone of major faulting and rapid exhumation, the strained and fractured bedrock is primed for erosion. On the west side, the Bering Glacier is the primary outlet for the Bagley Ice Field, which covers half of the crest of the orogen; however, few if any of the sediments at its terminus originate from under the Bagley Ice Field. Sediment transport is likely hindered by subglacial freeze-on processes that reduce the sediment-carrying capacity of subglacial rivers, though glacial surges are likely exceptions that deposit sediment far beyond the active margin of the glacier. Our study concludes that the widely invoked concepts of glacial erosion should be used with caution, as oversimplifi cation can fail to account for important site-specifi c differences in geologic and glacial conditions.

The role of the cryosphere in source-to-sink systems

“…3) (Shuster et al, 2005;Berger and Spotila, 2008;Thomson et al, 2010;Herman et al, 2013). One notable drawback to this method is that bedrock cooling ages can only be established where samples can be collected, which precludes sampling under modern ice streams that may be the locus of erosion over LGM time scales (Berger et al, 2008b;Enkelmann et al, 2010;Headley et al, 2013). One alternative is to use detrital thermochronometery on grains collected from glacial outwash.…”

Evaluation of detrital thermochronology for quantification of glacial catchment denudation and sediment mixing

“…Melt water transports sediment through and underneath the glacier facilitating bedrock abrasion and glacial flow. Freezing can cause blockage of the fluvial system and pressure can build up underneath the ice that can be released suddenly in outburst floods of large amounts of sediment-laden water (Raymond, 1987;Paterson, 1994;Lingle and Fatland, 1998;Headley et al, 2013). All these transport processes in glaciers raise the question of how representative are sediment samples collected at the glacier terminus to record the source of sediment of the entire catchment?…”

The Active Yakutat (Kula?) Plate and Its Southcentral Alaska Megathrust and Intraplate Earthquakes

“…But, a significant amount of the yak must be absorbed by subduction somewhere given the mass balance of the Saint Elias Range with its high rates of internal deformation and exhumation [16,[48][49][50][51][52][53][54], which does not even come close to the estimated amount of N transport of yak; i.e., nearly 100 km of yak within 2 Ma [33]. Then, where has the missing yak and even YAK to the NW gone?…”