Understanding brittle deformation patterns and their sequence of events in southern New Mexico is vital for the discovery and extraction of natural resources. The purpose of this project is to understand the scarcely studied nature and kinematics of faults located within the southern Rio Grande Rift. The region of interest preserves multiple fault populations, most notably NS-trending faults and NW-SE-trending faults. The main hypothesis to test is that, while both fault sets have contributed to extension in the southern Rio Grande rift, the NW-SE-trending faults also preserve evidence for multiple deformation events dating back possibly to the Precambrian era. In order to correlate the existing faults and their kinematics to previously studied tectonic events, a paleostress analysis was performed to determine the orientations of the maximum (σ 1 ) and minimum (σ 3 ) principal stresses of fault populations.

The region in and around the southern Rio Grande rift has experienced a long and complex tectonic history since the Precambrian era. In addition to recording extension directions due to the opening of the Rio Grande rift, faults can also possibly record contractional deformation related to the Laramide orogeny, extension along the boundary of the Mesozoic Chihuahua Trough, and possibly strike-slip movement since the Precambrian related to the Texas Lineament. The northern and central segments of the Rio Grande rift preserve mostly N-S-trending faults, whereas the southern segment preserves NW-SE-trending faults. The main hypothesis to test is that although both fault sets were active during extension of the rift, the NW-SE trending faults may preserve evidence for underlying reactivated older faults, possibly dating back to the Precambrian. Using exposed faults in the southern rift, a paleostrain analysis was performed to determine maximum extension (S₁) and maximum shortening (S₃) directions. Fault kinematic data was collected from six mountain ranges in southern New Mexico and western Texas. Results support a model where the entire Rio Grande rift evolved within a general EWdirected extensional stress field. This resulted in extension along NS-trending dip-slip faults in the northern and central segments of the rift. In contrast, in the southern rift EW-directed extension may have been accomplished through reactivation of much older underlying structures in the crust, resulting in NW-trending dip-slip and oblique-slip faults. This observation could help explain the geometric "bend" in the Rio Grande rift as it continues south into Texas and northern Mexico. Further investigation of the kinematics is underway and is critical to understanding the importance of reactivation during continued extension within the southern Rio Grande rift. vi

No abstract