SPATIAL VARIABILITY OF EXHUMATION ALONG THE EASTERN MARGIN OF THE RIO GRANDE RIFT

The actively spreading Rio Grande rift influenced uplift and erosion and created many geologic features in Southwest North America, however the timing and progression of these processes is not well constrained in some of the smaller mountain ranges of the region. Two mountain ranges along the eastern margin of the rift in southern New Mexico were studied to improve our understanding of landscape evolution related rifting. Erosion patterns and bedrock ages were measured in the Guadalupe and Sacramento Mountains to evaluate the potential progression of rifting from south to north. Both mountain ranges are regionally close in location, however, the Sacramento Mountains sit at an elevation of approximately 3000 m, experience more precipitation (approximately 36.46 in average rainfall) and are therefore more heavily vegetated whereas the Guadalupe Mountains sit at an elevation of approximately 2600 m, do not receive as much rain (approximately 21.06 in average rainfall) and are less vegetated. These patterns of precipitation, vegetation, and elevation suggest that the Sacramento Mountains were uplifted more and contain more developed canyons than the Guadalupe Mountains. The primary erosional mechanism is stream incision, which has cut channels draining from the mountain into the adjacent subsiding basins. We generated longitudinal profiles of stream channels in ArcGIS 10 to determine if either range displayed evidence of uplift or differential erosion from knickpoints. Three of the five catchments studied showed evidence of knickpoints in both mountain ranges. The Sacramento catchment displayed one clear knickpoint while knickpoints in the Guadalupe catchments ranged from clear to relatively small and difficult to distinguish. Preliminary results for the Sacramento catchment suggest that the knickpoint was formed due to more resistive material whereas Guadalupe catchment knickpoints suggest they are associated with active uplift and less erosion. The difference in exhumation between the two ranges will be further evaluated with low temperature (U-Th)/He apatite ages collected from bedrock samples at the top and bottom of both mountain ranges.