Spatial and Temporal Variations in Topography and Glacial Erosion From Apatite (U-Th)/He Thermochronometry, Southern British Columbia

We integrate 40 new and 26 published apatite (U-Th)/He (AHe) cooling ages from the heavily glaciated Mount Waddington region of the Coast Mountains, British Columbia. Samples are used to characterize the history and spatial variations in glacial erosion of the region. The samples cover a ~50x50 km area and span nearly 4 km of elevation, with ages ranging from 1.5-15.0 Ma. The high sampling density over this region permits a 3D analysis of spatial variations in erosion across the Klinaklini River valley. AHe ages increase considerably above 2 km elevation, with average ages of 4.5 Ma below this level and 8.4 Ma above. Furthermore, the sample ages on the eastern flank of the valley are generally younger than samples on the west side at comparable elevations, with mean ages of 5.1 and 6.9 Ma, respectively. Normalizing samples to a common elevation using age-elevation relationships reveals more subtle spatial variations in the data. Elevation-corrected ages from samples on the western flank of the valley increase with distance from the valley axis by 6 Myr over 15 km. Conversely, samples on the eastern flank typically decrease by 2-3 Myr over 15 km. 3D thermal-kinematic modeling of the study area indicates that topographic perturbation of isotherms can be only partially responsible for the observed spatial variations in AHe ages. Rather, differences in cooling ages across the valley suggest both spatial and temporal variations in erosion and topography. More specifically, the abrupt increase in ages above 2 km indicates slower erosion rates and amounts at higher (>2 km) elevations in some parts of the study area. The variation in ages at equal elevations suggests slower erosion rates to the west of the Klinaklini River and enhanced glacial valley widening to the east. Model results suggest Plio-Pleistocene erosion rates range between 0.6-1.0 mm/yr below 2 km elevation. Work in progress involves more detailed quantification of the spatial and temporal variations in cooling ages with a 3D thermo- kinematic model.