FAULT KINEMATICS OF THE SOUTHERN RIO GRANDE RIFT: EXPLORING THE POSSIBILITY OF FAULT REACTIVATION

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. N – S trending faults are primarily dip slip based on the orientation of slickenlines and are interpreted to be normal based on shear sense criteria. In contrast, the NW – SE trending faults preserve a wide range of fault plane orientations and slickenlines vary from strike – slip to dip- slip.

Results support a model where the entire Rio Grande rift evolved within a general EW-directed 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.