Date of Award
Master of Science (MS)
Department of Geography-Geology: Hydrogeology
The actively spreading Rio Grande Rift influenced uplift and erosion in Southwest North America, however the timing and progression of these processes is not well constrained in some smaller mountain ranges of the region. The Guadalupe and Sacramento Mountains located along the eastern margin of the rift in southern New Mexico were studied to improve our understanding of landscape evolution related to rifting. To address landscape evolution and the influence of tectonics I analyzed spatial distributions of erosion for each mountains range. I identified knickpoints along five longitudinal stream profiles from five canyons (4 in the Guadalupe Mountains and 1 in the Sacramento Mountains). I then coupled knickpoint elevations with histogram data to understand where majority of erosion occurs throughout each canyon. Histogram data joined with lithology data allowed for understanding why knickpoints exist. I used apatite (U-Th)/He low temperature thermochronology (AHe) to understand the cooling history of each range by dating apatite gains picked from bedrock and stream sediment samples. I used ages determined from bedrock samples to calculate the distribution of ages throughout a canyon. Using random sample points generated in ArcGIS to represent a sediment sample collected at the mouth of a stream, I calculate the probability density function to assess the probability of ages found within a stream sediment sample. Results from knickpoint analysis revealed that all five canyons contained knickpoints. Histogram results showed that larger areas were prominent at higher elevations, with which the knickpoint locations did not correlate. Lithology distribution revealed that knickpoints occur within resistant rock units (e.g. limestone). Bedrock grain analysis showed that apatite ages were reset and displayed ages between 24.9 - 28.3 Ma Â± 5 Ma which is similar to when rift propagation began ~30 Ma ago. Similarities in ages were also seen when comparing bedrock ages from this study to previous studies. For each sample location, it was found that ~1.6 km was exhumed. Probability density function analysis revealed that majority of ages found within a stream sediment sample would be found at higher elevations because of the distribution of carbonates. The quality of apatite was also addressed due to low helium concentrations in sediment samples. It was found that in order to have a high enough helium concentration, to assure accurate mineral identification, and to reduce uncertainty an apatite grain picked must have a width greater than 70 um, must display a distinct crystal shape, and should not be rounded. Tying everything together, cooling histories determined from AHe analysis revealed that over the course of 30 Ma approximately 1.6 km of sediment has been exhumed across the eastern margin. This coupled with knickpoint and histogram analyses proves that evolution of the landscape, especially along the Guadalupe and Sacramento Mountains, is influenced by regional rifting.
Hoffman, Lauren, "Spatial Variability of Erosion Patterns along the Eastern Margin of the Rio Grande Rift" (2014). Theses and Dissertations. 185.