HR: 14:40h
AN: H53E-05    [Abstracts]
TI: Long-Term Glacial Erosion Rates and Pre-Glacial Topography in Southwest British Columbia
AU: * Ehlers, T A
EM: tehlers@umich.edu
AF: Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109 United States
AU: Farley, K A
EM: farley@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Rusmore, M E
EM: rusmore@oxy.edu
AF: Department of Geology, Occidental College, Los Angeles, CA 90041 United States
AU: Woodsworth, G J
EM: gjw@paralynx.com
AF: Geologic Survey of Canada, 101-605 Robson Street, Vancouver, BC V6B5J3 Canada
AB: The hypothesis that Late Cenozoic climate change increased the topographic relief of mountain ranges relies on the assumption that alpine glaciers are more efficient at eroding valley bottoms than ridge crests. Although theoretical and field studies have made advances in quantifying glacial erosion processes, rigorous tests of this hypothesis have been limited by uncertainties in long-term glacial erosion rates and pre-glacial topographic relief. Here we interpret long-term ($>$10$^{6}$ yr) glacial erosion rates and pre-glacial topographic relief in the southern Coast Mountains, British Columbia, using apatite (U-Th)/He and apatite fission track cooling ages and a thermal-kinematic numerical model. Twenty-six new apatite (U-Th)/He samples were collected along two 60 km long transects that cross the glacially sculpted topography of Mount Waddington. Samples were collected between elevations of 0 and 4000 m, with a subset of samples along each transect collected at a constant elevation of 1600 m. Apatite (U-Th)/He ages range between 1.5 and 14.1 Myr. Two patterns are present in the spatial distribution of ages. First, ages generally increase in elevation with some key exceptions due to glacial erosion discussed later. Second, samples collected on the 1600 m contour on each transect vary systematically between ~1.5-8 Myr BP and are youngest near the high topography but are noticeably shifted to the west of the high topography of Mount Waddington. We interpreted the apatite (U-Th)/He ages using a coupled 3D thermal-kinematic model. The model was used to: (1) predict apatite He sample ages across the present-day topography for variations in model parameters, (2) identify the best-fit model simulations with a statistical comparison, (3) calculate the difference in ages (age anomaly) between model predicted and measured ages, and (4) calculate the change in topography that could produce the observed age anomaly. In regions like the Coast Mountains where glacial erosion is pervasive and sample ages are younger than the onset of glaciation then the calculated change in topography (step 4) is a proxy for the magnitude of glacial erosion. Calculated glacial erosion rates across two 60 km long transects range between ~0.4 and 0.8 mm/yr. Calculated preglacial topographic relief suggests massive valley widening resulted in a 16 km migration of the peak elevation. Topographic relief increased by +1600 to +2200 m along one transect and -200 to +900 m along the other. We find that glacial erosion magnitudes are variable over 60 km distance and that long-term glacial erosion rates are an order of magnitude lower than short-term ($<$10$^{4}$ yr) rates.
DE: 8107 Continental neotectonics
DE: 8130 Heat generation and transport
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting