Post-glacial and litho-structural controls on the fluvial erosion of Southern Canadian Rocky Mountains

The songs of the majestic landscapes are composed by the symphony of tectonic, climatic, and lithologic processes. For tectonically quiescent landscapes, landscape dynamics can be complicated by the tension between fluvial and glacial erosion and composite lithologic erodibility contrasts. We investigate these complications by focusing our study on the Southern Canadian Rocky Mountains, a late-stage tectonic fold and thrust belt with spatially uniform climate. These ranges comprise deformed carbonate and mixed carbonate-siliciclastic rock sequences. They have also experienced extensive glacial sculpting evidenced by steepened river valley walls and U-shaped channels. To address these complications, we produce the first basin average erosion rates, derived from 36Cl-cosmogenic isotopes, for 22 catchments across the Rockies. We then compare these erosion rates with climate, topographic, and litho-structural factors using bivariate and multivariate Bayesian regression modelling to infer the dominant controls of the landscape evolution of the Rockies. We begin our analysis with the common factors used in landscape evolution studies such as mean annual temperature (MAT), mean annual precipitation (MAP), normalised difference vegetation index (NDVI) for climate, relief, gradient, channel steepness index (k_sn) for topography, extracting summary statistics (minimum, 10^th, 25^th, 50^th, 75^th, 90^th, maximum, 90^th –10^th, 75t^h –25^th, mean, and median) for each catchment. Because of the stepped nature of the topography in the Rockies resulting from lithologic strength contrasts and glacial modifications, we also compute standard deviation in k_sn and terrain ruggedness index (TRI). Finally, constraining lithologic erodibility is especially challenged by the high variability in the depositional architecture of mixed carbonate-siliciclastic rocks and facies structure of carbonate rocks in passive margins. Therefore, we measure and employ direct intact rock strength measurements using a Schmidt hammer and fine-scale geologic mapping for all the geologic units in each catchment. We partially account for the impact of fault-related damage zones on lithologic erodibility by calculating fault density derived from mapped structures. Additionally, we develop a new method that accounts for landscape stability based on the TOBIA index that accounts for the relationship between the bedding orientation (primary plane of weakness) and hillslope angle. Our findings reveal that the standard deviation of k_sn is a much more important regressor for topography of transitional landscapes than k_sn. Interestingly, we find that climatic factors have significant influence despite their limited variability in the region. Although fully accounting for lithologic erodibility remains beyond the scope of our field, our TOBIA index-based method is a significant step in constraining litho-structural controls on landscape evolution.