HR: 0800h
AN: H51A-1100    [Abstracts]
TI: Quantifying Glacial Valley Widening and Deepening With Apatite (U-Th)/He Thermochronology: Coast Mountains, B.C.
AU: * Densmore, M S
EM: mdens@umich.edu
AF: Geological Sciences, University of Michigan, Ann Arbor, 48109
AU: Ehlers, T A
EM: tehlers@umich.edu
AF: Geological Sciences, University of Michigan, Ann Arbor, 48109
AU: Farley, K A
EM: farley@gps.caltech.edu
AF: Geological and Planetary Sciences, California Institute of Technology, Pasadena, 91125
AU: Woodsworth, G J
EM: glennw@nrcan.gc.ca
AF: Geological Survey of Canada, 101-605 Robson, Vancouver, V6B 5J3
AB: Climate change and glaciation significantly influence the erosion history and topography of orogens. Glacial modification of landscapes typically occurs by valley widening and deepening. However, few studies have been able to quantify the rates at which widening and deepening occur. We integrate apatite (U-Th)/He (AHe) thermochronometry and thermo-kinematic modeling to quantify the range of glacial erosion rates in the southern Coast Mountains, British Columbia. The Coast Mountains are an ideal setting for quantifying glacial erosion rates as they have experienced substantial topographic modification by glaciers from late Miocene to present. Forty-one AHe samples were collected in a 20x20 km region covering one large and multiple smaller glaciated valleys to quantify the erosion history of this region. Samples were collected on a grid at 4 km intervals and along a vertical profile in the largest valley. Currently available ages from 22 of these samples range from 1.7-11.7 Ma and generally increase with increasing elevation. Age versus elevation along the vertical transect range from 2.4 Ma at 43 m to 11.1 Ma at 2382 m. A distinct break in slope occurs in the age-elevation plot around 1700 m corresponding to a sample age of 5 Ma. Sample ages collected in tributary valleys range in age from 4.6-11.7 Ma and are consistently older than samples at the same elevation in the main valley, which range from 1.7-11.1 Ma. We interpret the difference in AHe ages in samples at similar elevations to be the result of variable amounts of glacial valley deepening and widening. The occurrence of older ages in smaller valleys than in the main valley at the same elevation suggests significantly more glacial widening and deepening in the main valley over the last 2-5 Ma. The break in slope seen in the age versus elevation plot indicates the maximum extent of topographic modification occurs between 1700-2300 m. Preliminary estimates of long-term glacial erosion rates in this area range between 0.4-0.8 mm/yr. Work in progress involves analyzing spatial variations in cooling ages with a 3D thermo-kinematic model to quantify the preglacial topography in 3D.
DE: 8130 Heat generation and transport
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1827 Glaciology (1863)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting