New Mexico Geological Society Annual Spring Meeting — Abstracts

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 (σ₁) and minimum (σ₃) principal stresses of fault populations.

Initial results are presented from the Cookes range, southern New Mexico. NS-trending faults preserve slickenlines with steep rakes. These faults are interpreted to be normal faults based on younger rocks on top of older rocks and inspection of shear sense criteria along the fault plane (Fossen, 2010). In contrast, minor faults along NW-SE-trending faults have a range of 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.

In addition to Rio Grande rift extension, NW-SE-trending faults in southern New Mexico have also been attributed to contraction during the Laramide orogeny (Chapin C. and Cather S.,1981), extension along the border of the Mesozoic Chihuahua Trough (Haenggi W. T., 2002), and may even be related to strike-slip movement during the Precambrian (Muehlberger W., 1965). Future research will focus on relating fault populations to these previously recognized periods of deformation in southern New Mexico. To do this, additional localities in the southern Rio Grande rift will be investigated where NS-trending faults and NW-SE-trending faults are both preserved. We will specifically look for the relative ages of these two fault populations to determine in NS-trending faults consistently cross-cut NW-SE-trending faults. Our preliminary findings based on fault analysis in the Cookes Range suggest that, while EW extension in the northern and central segments of the rift produced new NS-trending faults, in southern New Mexico this stress field may have resulted in the reactivation of much older faults and fractures in the crust, producing the large bend in the rift as it continues into Texas and northern Mexico.

References:

1. Chapin C. E. and Cather S. M. (1981) Relations of tectonic to ore deposits in the southern Cordillera. Arizona Geological Survey Digest., 173 - 198.
2. Fossen, H. (2010) Structural Geology (2^nd ed.), 222–223.
3. Haenggi W. T. (2002) Tectonic history of the Chihuahua trough, Mexico and adjacent USA, Part II, 55, Boletin de la Sociedad Geologica Mexicana, 38 -94.
4. Muehlberger W. R. (1965) Late Paleozoic movement along the Texas lineament, 27, Transactions of the New York Academy of Sciences, 385 – 392.