LONG-TERM THERMAL HISTORIES OF ROCKS IN THE RIO GRANDE RIFT BASIN AND RANGE TRANSITION ZONE USING ZIRCON (U-TH)/HE THERMOCHRONOLOGIC DATA

Multiple thermochronologic methods have been used to place important constraints on time periods of extension in the Rio Grande rift and Basin and Range Province. In particular, the relatively low temperature sensitivities of apatite and zircon (U-Th)/He (AHe and ZHe, respectively) make them valuable for capturing recent exhumation due to normal faulting. However, differential radiation damage and helium retention suggests that these systems can be highly sensitive to long-term thermal histories that rocks have experienced since their formation in the Proterozoic. We analyzed samples on a transect across the Rio Grande rift – Basin and Range transition in southern New Mexico as a natural laboratory to investigate the Proterozoic-Cenozoic thermal histories of rocks exposed in fault block uplifted mountain ranges. Main deformation events that have affected this region include the Ancestral Rocky Mountains, the Laramide Orogeny, and extension during the development of the Basin and Range Province and Rio Grande rift. For each sample location, forward models were constructed using HeFTy software that include periods of exhumation and/or burial due to each tectonic event, and each forward model predicts specific age-eU relationships. Inverse modeling of AHe and ZHe data will be done using HeFTy software to compare to forward models and to test whether AHe and ZHe data record long and complex cooling histories. Preliminary data for some of these ranges yield ZHe ages that range from 19-649 Ma and show a negative correlation with eU, suggesting that ZHe data obtained from this region are an important record of pre-Cenozoic tectonic exhumation. Results from study will constrain long-term timing, magnitude and rates of cooling experienced in these fault blocks across the Rio Grande rift – Basin and Range transition zone, and may yield important insight into the timing and duration of deformation related to the Ancestral Rocky Mountains, Laramide Orogeny, and Cenozoic extension.