QUANTITATIVE TOPOGRAPHIC DIFFERENCES BETWEEN EROSIONALLY EXHUMED AND TECTONICALLY ACTIVE MOUNTAIN FRONTS: IMPLICATIONS FOR LATE CENOZOIC EVOLUTION OF THE SOUTHERN ROCKY MOUNTAINS

A model based on erosional exhumation and aseismic epeirogenic uplift rather than post-Laramide fault offset is proposed to explain the high relief, linear mountain front of Sierra Nacimiento in north-central New Mexico. Our model reconstructs the post-Laramide tectonic and geomorphic history of Sierra Nacimiento and is broadly applicable across the southern Rocky Mountains. Mapping of terraces in the Jemez River valley on the eastern flank of Sierra Nacimiento reveals terrace long profiles that diverge downstream, consistent with documented regional base level fall. Similarly, late Pliocene to Holocene fluvial deposits on the western Sierra Nacimiento piedmont imply these sediments were derived from westward elongating tributary streams related to an axial stream drainage system that has migrated away from the range and incised. This migration is best explained by epeirogenic warping of the Rio Grande rift flank in which Sierra Nacimiento and the eastern margin of the Colorado Plateau are embedded. Incision rates from the Jemez River and western Sierra Nacimiento piedmont suggest that exhumation of the range is proceeding like a wave, propagating from south to north. Analyses of geomorphic metrics show that a quantifiable difference exists between this erosionally exhumed mountain front and mountain fronts that are being uplifted along a truly active fault, like the Taos range in northeast New Mexico. This quantifiable difference appears in correlations between the metrics of drainage basin elongation, valley floor width to valley height ratios, and a newly developed metric, the drainage basin volume to drainage basin area ratio. These metrics show statistically significant correlations along active mountain fronts whereas no statistically significant correlation can be found between the metrics on mountain fronts that have been erosionally exhumed. These results underscore the importance of base level fall and epeirogenic uplift as the driving forces behind late Cenozoic exhumation of the southern Rocky Mountains.