The Interaction Between Climate, Tectonics, and Topography in the San Juan Mountains, Southwestern Colorado

Alpine glaciers have been found to produce rapid denudation rates that are capable of keeping pace with orogenic rock uplift, leading researchers to characterize glaciers as “buzzsaws” effectively limiting the altitude of developing orogens that intersect the glacial threshold. Research has concentrated on regions of maritime climate and convergent tectonics, leaving the impact of glaciation on intracontinental mountain ranges poorly understood. In the San Juan Mountains of southwestern Colorado, we examined the impact of extensive Quaternary glaciation on topography using swath profiles and morphologic characteristics of relief and slope in relation to glacial thresholds; in addition, apatite (U-Th)/He thermochronology was used to investigate the spatial variation of exhumation throughout the range. The northwestern region had cooling ages of 3-10 Ma, 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 19-39 Ma, no correlation between mean elevation and glacial thresholds and low values for relief and slope above cirque floors. Lithologic constrains on total denudation imply that active tectonism, not erosionally induced isostatic rebound, was responsible for the difference in cooling ages. In addition, modern climate patterns and late Quaternary glacial extent are similar for the northwest and southern regions and, therefore, climatic variations between the regions and fluvial erosion models cannot account for the differences observed. In the San Juan Mountains it appears that the presence of glacial ice alone does not result in the control of topographic development by glacial thresholds. The data implies that more recent tectonic activity in the northwestern region (related to the Aspen anomaly?) resulted in greater glacial modification of the landscape and suggests that topographic controls imposed by glacial thresholds are dependent on tectonic processes.