VALLEY ASYMMETRY MAPS FOR THE AMERICAN CORDILLERA REVEAL PATTERNS SUGGESTIVE OF TECTONIC, LITHOLOGIC, CLIMATIC AND FLUVIAL DRIVERS

Valleys become asymmetric when tectonic, lithologic, climatic, ecologic or hydrologic drivers cause hillslopes on one side of the valley to weather and erode differently than those opposite. We investigate how this local phenomenon affects broader landscape evolution by systematically mapping valley asymmetry throughout the American Cordillera, and at higher resolutions in the western U.S., using easily-accessible digital elevation models. Our maps reveal that valley asymmetry is present in a majority of mountainous environments, but that the orientation of this asymmetry is highly variable, producing distinct non-stochastic spatial patterns and provinces within which hillslopes of one aspect orientation are steeper on average. We have identified asymmetry trends that correlate with latitude, elevation and mountain-range-scale topography. Latitudinal trends at global scales are most likely related to influences that vary at this scale, such as plate tectonics, atmospheric circulation, or the magnitude and incidence angle of insolation. Mountain-range scale asymmetry patterns could be associated with the orientation of faulting, jointing, tilting, lithology, sedimentary bedding, river incision dynamics on mountain flanks, or range-scale topoclimate. In the granodioritic Idaho Batholith, the orientation of N-S oriented valley asymmetry reverses at roughly 2000 m elevation, which is likely related to changes in precipitation, temperature, vegetation cover and the dominance of glacial vs. fluvial processes. Spatial patterns of valley asymmetry appear to reflect the driving processes. Although asymmetry maps alone cannot definitively identify these processes, they are a useful tool for constraining possible influences.