(U-TH)/HE THERMOCHRONOLOGIC INVESTIGATION OF THE LONG-TERM THERMAL HISTORY OF PROTEROZOIC ROCKS IN WEST TEXAS AND SOUTHERN NEW MEXICO

Multiple thermochronologic methods (apatite fission-track and (U-Th)/He) have been used to place important time constraints on periods of Cenozoic extension in the Rio Grande rift. However, Proterozoic rocks in this region have experienced a much longer thermal history than is typically recorded by these data. To investigate long-term (>1 Ga) thermal histories, we combine GIS analysis with zircon (U-Th)/He (ZHe) thermochronology to document periods of pre-Cenozoic burial and exhumation. Advancements in the understanding of differential radiation damage and helium retention in zircon suggest that long-term thermal histories may be extracted from the rocks. We analyzed samples from the southern Rio Grande rift in west Texas and southern New Mexico. This region serves as natural laboratory to investigate the Proterozoic-Cenozoic thermal histories of rocks exposed in fault block uplifted mountain ranges. Prior to Cenozoic extension, the southern Rio Grande rift had been affected by multiple deformation events, including the Grenville Orogeny, Ancestral Rocky Mountains, Jurassic rifting, and the Laramide Orogeny, each of which may be recorded in the thermal history of Proterozoic rocks. For each sample location, forward models and predictive age-eU plots were constructed using HeFTy software that analyze periods of exhumation and/or burial from each tectonic event. Inverse modeling of ZHe data using HeFTy software will be used to evaluate forward models and predicted age-eU plots, to test whether ZHe data record long and complex cooling histories. Preliminary data from some of these areas 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 this study will constrain the long-term timing, magnitude, and rates of cooling experienced in these fault blocks across the southern Rio Grande rift and may yield important insight into the timing and duration of deformation related to pre-Cenozoic deformational events. In addition, ZHe data and results combined with GIS will be used to produce time-series animations to better understand and visualize the extensive thermal history of this region.