GIS Analysis and (U-Th)/He Thermochronologic Investigation of Proterozoic Rocks in Southern New Mexico

Abstract

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 and Basin and Range Province. However, Proterozoic-aged rocks in this region have experienced a much longer thermal history that is not typically recorded by these data. To investigate long-term (>1 Ga) thermal histories, we combine GIS analysis of past deformational events with zircon (U-Th)/He (ZHe) thermochronology to document periods of pre-Cenozoic burial and exhumation. Recent advancements in the understanding of differential radiation damage and helium retention in zircon suggest that long-term thermal histories experienced since their formation in the Proterozoic, may be extracted from the rocks. We analyzed samples along a transect across the Rio Grande rift -Basin and Range transition zone in west Texas, southern New Mexico and south-eastern Arizona. This region serves as natural laboratory to investigate the Proterozoic-Cenozoic thermal histories of rocks exposed in fault block uplifted mountain ranges. Prior to recent exhumation in the Cenozoic, Rio Grande rift and Basin and Range extension, the Rio Grande rift -Basin and Range transition zone has been affected by multiple deformation events, including the Ancestral Rocky Mountains, Jurassic rifting and the Laramide Orogeny. GIS was used to combine published deformational maps of New Mexico to construct a deformational sequence map of southern New Mexico, illustrating areas that experienced uplift and/or burial during the Ancestral Rocky Mountains, Laramide Orogeny and Rio Grande rift. For each sample location, forward models and predictive age-eU plots were constructed using HeFTy software that include periods of exhumation and/or burial due to each tectonic event. Inverse modeling of ZHe data will be done using HeFTy software to compare to forward models and predicted age-eU plots, to test whether ZHe data record long and complex cooling histories. Preliminary data from 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, Jurassic rifting, Laramide Orogeny, and Cenozoic extension.