Large-scale geomorphology and fission-track thermochronology in topographic and exhumation reconstructions of the Southern Rocky Mountains

Abstract

Long-term landscape evolution is the integrative sum of constructive rock-uplift processes and destructive erosional processes. For the Southern Rocky Mountains, it has been proposed that Phanerozoic rock uplift and erosion have been influenced by crustal structure that was inherited from the time of assembly of the continent in the Proterozoic. This paper compares large-scale geomorphology and long-term rock exhumation histories in three distinct crustal terranes to assess possible differences in Laramide and post-Laramide deformation of the Southern Rocky Mountains. We analyze modern topographic data and fission-track thermochronology for several portions of the Southern Rockies with distinctly different post-Laramide geologic histories and estimate the amount of Laramide and post-Laramide rock uplift. The areas of investigation include: (1) the Colorado Front Range, an area of regional elevated heat flow in the Yavapai province; (2) the New Mexico Taos Range, a region of localized high heat flow through-out the late Tertiary and Quaternary along the Rio Grande rift in the Mazatzal province; and (3) the New Mexico Sierra Nacimiento, a region of localized high heat flow in the Quaternary associated with the Jemez Mountains, also in the Mazatzal province. Both the Taos Range and Sierra Nacimiento lie in proximity to the Jemez lineament, which is thought to be a significant Proterozoic structural feature that has influenced Quaternary volcanism along its trend. We utilize and test a dimensionless topographic index called the ZR ratio, which is the ratio between mean elevation and mean relief, as a quantitative measure of the differences in orogen-scale topography. Digital topography (DEM) and GIS software (ARC/INFO) allow for the rapid determination of this ratio at various length scales. Our results show that the Taos Range has the lowest ZR ratio (most rugged topography), and the Sierra Nacimiento has the highest ratio (least rugged topography). Fission-track thermochronology results show that a partial annealing zone (PAZ) is preserved in the Front Range and the northern portion of Sierra Nacimiento (the region incidentally not on the high-heat flow Jemez lineament); both the Taos Range and the southern portion of Sierra Nacimiento have had the PAZ removed by erosion. Reconstructed amounts and timing of rock denudation based on the fission-track data are consistent with the ZR ratios in that the most rugged topography exhibits the greatest and most recent denudation, whereas the least rugged topography experienced far less denudation, most of which occurred immediately after the Laramide orogeny. Fission-track thermochronology and geomorphic results support an overall model of significant crustal thickening during the Laramide orogeny, followed by relatively low and differential rates of rock uplift, erosional exhumation, and isostatic rebound. Specific regions of greater post-Laramide rock exhumation and rugged topography are highly correlated with regions of known localized high heat flow and late Cenozoic volcanism, a finding consistent with the hypothesis that at least locally, the high mean elevation of the Southern Rockies is supported by a buoyant mantle. Both the style of Laramide deformation and subsequent magmatic input from a low-velocity, buoyant mantle may have been influenced by older crustal structure. All three factors, Proterozoic crustal structure, Laramide deformation, and post-Laramide up-lift, appear to play an important role in the ability of streams to integrate through the Rocky Mountain foreland. As rates of erosion are strongly tied to local relief of well-drained landscapes, stream integration at the large scale may explain the correlation between long-term denudation rates and modern topographic ruggedness ( ZR ratio), and be the limiting factor in the processes driving rock uplift in the post-Laramide Southern Rockies.