DETRITAL THERMOCHRONOLOGY REVEALS EFFICIENCY OF GLACIAL EROSION IN THE ST. ELIAS RANGE, ALASKA

The massive ice fields and their associated outlet glaciers of the St.Elias orogen provide a unique setting to connect the patterns of modern glacial dynamics and erosion to tectonics. Combining information from the glaciology and thermochronology of this orogen provides new insights into the interaction between active tectonic processes, glacial erosion and subglacial transport. The cooling history of the ice-covered rocks is recorded in glacial detritus found in modern deposits near the glacial terminus. Combined with glacial velocity and surface measurements, these detrital samples can be used as tracers for glacial erosion processes and material transport. The detrital samples not only show a variance in the cooling histories along the orogenic strike but also large variations in the efficiency of erosion and evacuation of material out of the system. To the west of the orogen, the main outlet of the Bagley Ice Field, the Bering Glacier, appears to be disconnected hydraulically from the high elevation Ice Field, preventing material from being transported through the entire length of the glacier during quiescent glacier phases. We propose processes that could act to maintain this pattern of sediment deposition, including super-cooling of subglacial water with hydraulic shutdown during normal phases of the glacier and an active hydraulic system with sediment deposition far outboard of the current glacial terminus during surging phases. Further east, the Malaspina Glacier sediments reveal a region of extremely rapid erosion that is focused underneath the Seward Ice field, not in the deep and narrow Seward Throat where most the rapid surface velocity suggests high ice flux and intense erosion. However, these zones of highest exhumation and thus high rates of vertical advection do correlate with active faulting and localized strain at the orogen’s indenter corner.