LONG-TERM GLACIAL EROSION RATES AND MAGNITUDES: INSIGHTS FROM SOUTHWEST BRITISH COLUMBIA AND IMPLICATIONS FOR THE PATAGONIAN ANDES

Alpine glaciers are efficient agents of erosion and capable of significantly modifying topography. Furthermore, the flux of glacially eroded material can alter body forces within orogens and influence long-term deformation. Despite recent advances in theoretical and field studies that quantify glacial erosion processes, few studies have documented glacial erosion rates over long temporal (>10⁶ yr) or large spatial (>10's of km) scales. This study addresses the long-term rates and magnitudes of glacial erosion and regional deformation in the southern Coast Mountains, British Columbia, and highlights correlations with the southern (Patagonian) Andes. We use 26 new apatite (U-Th)/He (AHe) and previously published apatite fission track (AFT) cooling ages to address the post-late Miocene erosion history from two 60 km long transects of the heavily glaciated southern Coast Mountains, British Columbia. Observed AHe cooling ages from equal elevation samples range between 1.5-8 Ma and suggest massive valley widening resulted in a 16 km shift of the highest point in the topography in the last ~1.5 Ma. We evaluate temporal and spatial variations in erosion rates using a 3D thermo-kinematic model that predicts AHe and AFT ages at the surface for different glacial erosion histories. Comparison of model-predicted and observed cooling ages suggest up to a 300% increase in erosion rates, coincident with the onset of glaciation of this range. An additional 25 AHe data spanning the 150 km width of the orogen display a gradual decrease in cooling ages from the flanks towards the glaciated core of the range. Except for the western flank of the range, no evidence for large magnitude faulting was observed. These results suggest glacial erosion and isostasy are largely responsible for the topographic development of this range. The apparent lack of large offset faults across the orogen suggests that distributed strain across numerous small scale structures may have accommodated glacial unroofing of the range. The Coast Mountains and Patagonian Andes have similar climate regimes, topographic relief and morphology, plate, boundaries, and long-term deformation histories. We suggest that glacial erosion may also be the driving mechanism for topographic development of the southernmost Andes.