INFLUENCE OF GLACIAL VALLEY WIDENING AND DEEPENING ON THERMOCHRONOMETER AGE-ELEVATION PROFILES

Glaciation can significantly influence the erosion history and topography of orogens 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), apatite fission track (AFT), and zircon (U-Th)/He (ZHe) thermochronometry with thermo-kinematic modeling to quantify the erosion history of the heavily glaciated southern Coast Mountains, British Columbia. We collected a vertical profile spanning over 2300 m near Mt. Waddington, at intervals of ~250 m. Nine AHe ages range from 2.3 – 12.0 Ma, with a distinct break in slope in the age-elevation relationship between 1700 – 2000 m, corresponding to a sample age of 5 – 7 Ma. Additionally, 4 AFT and 3 ZHe cooling ages from subsets of these samples range from 6.9 – 39.2 Ma, and 10.5 – 21.7 Ma, respectively. Both the AFT and ZHe data show a break in slope occurring at 2400 m and 1000 – 1400 m, corresponding to samples ages of 12 – 20 Ma in both systems. The break in slope in the AFT and ZHe data is interpreted as associated with the onset time of exhumation in the Coast Mountains. However, we interpret the break in slope of the AHe data as an acceleration of erosion in the past 5 – 7 Myr associated with alpine glaciation.

We use a 3-D thermo-kinematic numerical model that tracks sample cooling histories and predicts cooling ages, to constrain the pre- and synglacial erosion rates. Free parameters in the model include basal heat flow, internal heat production, thermal conductivity, pre- and synglacial erosion rates, and onset time of glaciation. Hundreds of simulations were conducted to find the range of model parameters that minimize the difference between predicted and observed AHe, AFT, and ZHe ages. Preliminary results indicate a pre-glacial erosion rate of ~0.4 mm/yr that accelerated by a factor of 2 – 3 within the last 7 Myr. Work in progress is refining the onset time of accelerated glacial erosion as well as the average pre- and synglacial erosion rates. Although we cannot distinguish between glacial valley widening or widening and deepening as mechanisms for accelerated erosion rates, we suggest that one of these processes is responsible for the observed nonlinear AHe age-elevation profile.