Long-Term Glacial Erosion in the Coast Mountains, British Columbia, Canada from Low- Temperature Thermochronology

We integrate a dense suite of low-temperature thermochronometry samples with numerical models to constrain long-term (>106 yr) denudation within the heavily glaciated Coast Mountains, British Columbia, Canada. We synthesize 84 new and published apatite (U-Th)/He (AHe), 13 apatite fission track (AFT), 9 zircon (U-Th)/He (ZHe) and 6 zircon fission track (ZFT) cooling ages that range from 1.4 - 15.4 Ma (AHe), 5.2 - 34.5 Ma (AFT) 2.4 - 26.6 Ma (ZHe) and 18.6 - 55.2 Ma (ZFT). This span of ages allows us to quantify both the pre- and post-glacial history of the region, as well as detailed spatial variations in erosion. Samples were collected over a ~2500 km2 region, spanning 4 km of relief across glaciated valleys and ridges. Cooling ages generally increase in age with increasing sample elevation throughout the region. However, distinctly different linear age-elevation relationships exist above and below ~2000 m. As a consequence of this, ages above 2000 m average 7.5, 18.0, 25.0 and 45.4 Ma for AHe, AFT, ZHe and ZFT, respectively, while below 2000 m the averages decrease to 4.2, 10.7 15.7 and 39.8 Ma. We use the spatial distribution of cooling ages and erosion rates calculated from a 1-D thermo-kinematic numerical model to quantify the effect of glaciation on long-term erosion rates. Calculated rates range from 0.2 - 2.2 mm/yr over the last 55 myr. Relatively young cooling ages and higher erosion rates occur preferentially at low elevations indicating increased glacial intensity in the lower flanks of the broad U- shaped valleys. Best-fit regression lines and inverse modeling of samples along vertical transects suggest most rapid cooling is constrained within the last ~6 Ma, coincident with the onset of regional alpine glaciation. Our results are consistent with denudation of a former paleotopographic high offset ~16 km to the SW from its present position. Current work in progress is developing and applying a 3-D numerical model to better constrain the spatial and temporal variations in erosion rates.