Plio-Quaternary glacial incision of a thermal structure inherited from Eocene core complex exhumation, Thor-Odin dome, British Columbia

The contribution of exogenic processes on the denudation history of mountain belts has been documented in many regions but the long-term and large scale effects of glacial erosion on denudation rates, relief development and sediment flux into basins are much debated. We use multiple low-temperature chronometers, apatite fission track (AFT) and apatite and zircon (U-Th)/He, along with thermal modeling to better constrain exhumation history of the Thor-Odin Dome in the Monashee Mountains, British Columbia. The present day topographic relief in the region reaches up to 2.5 km, with deeply incised valleys and ubiquitous glacial features such as hanging valleys, glacial lakes. In order to investigate the timing and rates of exhumation we collected samples from various altitudes across the region and along a vertical profile from Mt. Symons. AFT ages vary between 44-14 Ma, and show a strong correlation with elevation. Combined with track length distribution, AFT data indicates that the exposed crustal section experienced two episodes of cooling: a Middle Eocene cooling event related with dome exhumation by detachment tectonics, and a recent, post-Middle Miocene (< 14 Ma) event. We applied apatite (AHe) and zircon (ZHe) (U-Th)/He chronometry to a subset of these samples, primarily collected at valley bottoms (~500-600 m). Our results show that zircon crystals record the Middle Eocene cooling event (37-45 Ma), whereas apatite grains reveal late Miocene (6-12 Ma) ages. Thermal history reconstructions of low-elevation samples using inverse modeling strongly suggest a pulse of rapid exhumation at ~3 Ma, which coincides with the onset of glaciation in Northern Hemisphere. We employed best possible T-t paths from inverse modeling into forward models of AFT-AHe age pairs. The result indicates that these samples was residing at temperatures ~50-80 °C prior to the onset of rapid cooling at ~3 Ma, which fits well with the expected paleotemperatures from observed fission track length measurements. Our results also require ~2.5 km of erosion since 3 Ma, which can be correlated with off-shore sedimentation in the Astoria Fan. Further investigations on plausible denudation and relief scenarios using a three-dimensional thermo-kinematic numerical model are under way to quantify denudation rates, relief amplification, and paleotopography of the Monashee Mountains.