60th Annual Report on Research 2015

Conditions in the deep crust and mantle must drive surface processes, however we need to improve our understanding of the rates and timescales at which the deep earth and surface systems interact. The scientific objective of this project is to investigate if surface processes record the progression of extension and mantle processes at rift zone margins. The Rio Grande Rift is an ideal region to investigate these interactions due to results of deep earth observations. The Guadalupe Mountains in western Texas and southeastern New Mexico are positioned on the eastern margin of the Rio Grande Rift. Although the rocks of the Guadalupe Mountains have been well studied for their relationship to petroleum sources and reservoirs, recent faulting and sediment production via erosion is less well understood. This study helps constrain the timing and spatial distribution of topography in the Guadalupe Mountains to investigate the spatial variability of erosional, tectonic and mantle influences. First, we are studying the spatial patterns of landscape evolution in relation to faulting with geomorphic analyses of the landscape completed with digital elevation models in a geographic information system. The modeled results from digital elevation models will be compared to field estimates of erosion rates quantified with catchment averaged cosmogenic nuclide analyses. Second, we will constrain the timing and rates of faulting and surface processes with cosmogenic nuclide exposure ages along the western escarpment and individual faults.

Research Questions

The first problem we focused on the spatial distribution of landscape evolution in the Guadalupe Mountains influenced by faulting. Our specific hypotheses are: 1. Spatial patterns of erosion will be faster at the center of the range where the greatest displacement occurs, with decreasing erosion rates toward the northern and southern tips. 2. Variable uplift and erosion is caused by regional propagation of extension from south to north.

Methods and Results

To investigate the first problem, we collected sediment samples at the mouth of catchments to estimate the catchment averaged erosion rates with cosmogenic radionuclides. We received preliminary results from the Purdue PRIME Lab this week, but have not had time to complete calculations to include in this report. We also analyzed elevation datasets to determine the geomorphic patterns of landscape evolution related to north-south position in the mountain range and position along fault segments. Two types of geomorphic analyses were completed. The first investigated the volume of material that was eroded from stream catchments throughout the range. A digital elevation model was used to delineate stream channels and watersheds throughout the Guadalupe Mountains. Initial, pre-erosional surfaces were created above the catchments and the difference between the surfaces was used to find the volume of material eroded. An erosion rate was calculated by dividing the total volume by an estimate of exhumation initiation at 35 Ma determined from apatite U-Th/He cooling ages measured in three bedrock samples from the range. Across the entire range, the greatest volume of sediment was eroded from the southern end of the range. Larger catchments are found near more complex arrays of faults (Figure 1). A comparison between east and west shows that average slopes are greater in east draining catchments than west draining catchments.   

The second geomorphic analysis looked at the complexity of the stream channels. The streams were traced in Google Earth to determine where most active flows were likely to occur using different available time images. Channels were then assigned a Shreve stream order number. The streams were brought into ArcGIS to determine the lengths of stream segments. The lengths and order numbers were compared from north to south along the western escarpment of the range and along individual fault segments. Stream order numbers were higher along the fault that created the western escarpment than the faults within the Brokeoff Mountains to the west. This result suggests that evolution of the escarpment has been active more recently than the other faults. 

Continuing Work

The remaining work will be to analyze and summarize quantitative measurements of exhumation and uplift and refine geomorphic analyses. The spatial patterns observed in the geomorphic analyses above will be compared to catchment averaged erosion rates and bedrock exposure ages along faults and the escarpment. We will also investigate if fault displacement is similar to the timing of alunite mineral formation in cave deposits observed in the Guadalupe Mountains based on cosmogenic exposure ages. We are waiting for sample results from Purdue PRIME lab and have prepared a second batch of samples to be sent out for analysis in October. Fieldwork in the spring will allow us an opportunity to collect additional samples for analysis over the summer. 

Research Team and Impact

This grant provided some financial support to four undergraduate students. Three of the students, Tyler Rothschild, Dakota Csanda, and Kacey Garber, helped process samples at Illinois State University to prepare them for cosmogenic analyses at Purdue PRIME Lab. One student, Kirsten Schaefer, completed the geomorphic analysis of stream catchments with GIS. This grant also allowed me to take four students, Kacey Garber, Christine Salinas, Dakota Csanda, and Tyler Rothschild to the field to investigate field sites and collect samples. Christine Salinas completed an analysis of stream networks in relation to fault positions with combined GIS and Google Earth work as part of an independent study. During Spring and Summer 2015, these students working with me met on a weekly or biweekly schedule to discuss research papers, progress with samples and the geologic background of the field site. Christine Salinas, Dakota Csanda and Kacey Garber will be presenting research posters at the Geological Society of America Annual Meeting in Baltimore in November. Kirsten Schaefer received an extended internship with the National Park Service, where she has been asked to apply her GIS skills.