RECENT ADVANCES IN QUANTIFYING LONG-TERM RATES OF GLACIAL LANDSCAPE EVOLUTION WITH THERMOCHRONOLOGY

Alpine glaciers are efficient agents of erosion and capable of significantly modifying topography. 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. In this study, recent results from two complementary low-temperature thermochronology techniques are presented that quantify temporal and spatial variations in long-term erosion rates and paleotopography in the heavily glaciated Mount Waddington region of the Coast Mountains, British Columbia. The techniques presented include apatite (U-Th)/He (AHe) and ⁴He/³He thermochronometry. These thermochronometer systems are sensitive to rock cooling histories in the upper 2-3 km of the crust and are therefore well suited for quantifying the most recent stage of exhumation in orogens. 

Specific results from these approaches are as follows. First, variation in erosion over large spatial scales were determined using AHe and apatite fission track (AFT) cooling ages from two 60 km long transects. Observed AHe cooling ages from equal elevation samples range between 1.5 and 8 Ma and suggest thick alpine glaciers resulted in a 16 km shift of the highest point in the topography in the last 1.5-4.0 Myr. Temporal and spatial variations in erosion rates were evaluated using a 3D thermo-kinematic model that predicted AHe and AFT ages at the surface for different erosion histories. Comparison of model predicted and observed cooling ages suggests > 3 times increase in erosion rates over the last 1.5–7 Ma, coincident with the onset of glaciation of this range.  Second, results from apatite ⁴He/³He thermochronometry tightly constrain the timing and rate of erosion across a single valley in the same study area. Cooling histories determined from 5 proximal samples require accelerated denudation of the valley initiating at ~1.8 +/- 0.2 Ma. At least 2 km of overlying rock was removed from the valley at > 5 mm/yr, indicating that glacial valley deepening proceeded > 6 times faster than erosion rates prior to 1.8 Ma.

Taken together, these two approaches demonstrate that over million-year time scales glaciation (1) can significantly modify the position of range crests within an orogen, and (2) produces pulses of high erosion.