FAULT KINEMATICS AND PALEOSTRAIN ANALYSIS OF THE SOUTHERN RIO GRANDE RIFT: A COMPARATIVE STUDY TO THE CENTRAL RIFT

The purpose of this project is to understand fault kinematics that record extension of the southern Rio Grande rift in southern New Mexico. The area of interest preserves multiple faults sets including N – S-trending and 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 era. Using exposed faults in southern New Mexico, a paleostrain analysis was performed to determine maximum extension (S₁) and maximum shortening (S₃) directions. 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.

Fault kinematic data was collected from five mountain ranges in southern New Mexico and western Texas. N – S trending faults are 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 plane orientations and slickenlines vary from strike – slip to dip- slip. Future investigation will help determine sense of shear, but preliminary conclusion suggest some faults may record reverse sense of slip.

Preliminary results support a model where the Rio Grande rift evolved within a general EW-directed extensional stress field. This resulted in extension along NS-trending 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. This observation could help explain the large 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.