FAULT KINEMATICS AND PALEOSTRAIN ANALYSIS OF THE SOUTHERN RIO GRANDE RIFT

Few paleostress analyses have been performed in the southern portion of the Rio Grande rift (RGR). The purpose of this project is to further understand the kinematics of faults produced by extension of the southern RGR. The area of interest preserves multiple groups of faults, most notably N – S-trending faults and NW – SE-trending faults. The main hypothesis to test is that, while both fault sets were formed during the extension in the southern RGR, the NW – SE-trending faults may preserve evidence for pre-existing deformation events dating back possibly to the Precambrian era. In addition to extension during development of the RGR, faults in southern New Mexico can also possibly record contractional deformation related to the Laramide orogeny, extension along the border of the Mesozoic Chihuahua Trough, and possibly strike-slip movement since the Precambrian along the Texas Lineament.

Initial fault kinematic data were obtained from four mountain ranges in the southern rift where N – S -trending faults and NW – SE-trending faults are both preserved. The N – S-trending faults preserve slickenlines with an array of rake measurements, but are generally dip slip. These faults are interpreted to be normal faults based on younger rocks displaced on top of older rocks and inspection of shear sense criteria along the fault plane. In contrast, NW – SE-trending faults preserve a wide range of fault plane orientations, and slickenlines vary from strike-slip to dip slip. Although future careful investigation of these faults will help to determine the sense of slip, preliminary findings suggest that some of these faults may record reverse-sense slip.

Our preliminary findings based on fault analysis in the southern RGR suggest that, while E – W–oriented stresses in the northern and central segments of the rift resulted in extension along N – S-trending faults, in southern New Mexico this stress field may have caused reactivation of much older underlying structures in the crust. This observation would help to explain the large bend in the rift as it continues south into Texas and northern Mexico. More careful investigation of the kinematics recorded along these faults is currently underway, and is critical in understanding the importance of reactivation during continued extension within the southern RGR.