The interaction between tectonics, topography, and climate in the San Juan Mountains, Southwestern Colorado
McKeon, Ryan Edward.
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Alpine glaciers have been referred to as "buzzsaws" on the grounds that they control the topographic development of actively deforming mountain ranges; however, the nature of the linkage between glacial erosion and topography in different tectonic and climatic settings remains unclear. In the San Juan Mountains of southwestern Colorado, an intracontinental mountain range with dramatically lower annual precipitation than previously studied ranges, distinct spatial variations in morphology resulting from Quaternary glaciation coincide with different exhumation histories that were derived using apatite (U-Th)/He thermochronology. The northwestern region had cooling ages of 3-10 Ma over an elevation range of 1300 m, moderate correlation between mean elevation and glacial thresholds, and regionally high values for relief and slope above cirque floors. The southern region, by contrast, had cooling ages of 19-32 Ma over an elevation range of 800 m, no correlation between mean elevation and glacial thresholds, and low values for relief and slope above cirque floors. The average magnitude of incision into a reconstructed maximum topography surface is nearly equal for the two study regions suggesting that the effects of glacial erosion are localized to high topography. The northwestern and southern regions show little variation in climate and fluvial and hillslope erosive potential, which implies that erosionally induced isostatic rebound is an unlikely source for the difference in cooling ages. Instead, I infer that active tectonism (possibly related to the Aspen anomaly) is responsible for different cooling ages and drove the greater degree of glacial modification of the northwestern region. As a result of the spatial variability in epeirogenic uplift, the San Juan Mountains appear to be both a mountain range that was just high enough to be glaciated, the southern region, and a mountain range where glacial erosion controls the elevation of high topography, the northwestern region; and thus are a microcosm for the diverse mountain ranges of the western United States.