STRUCTURAL ANALYSES OF THE TASCOTAL MESA FAULT, CHRISTMAS MTS., WEST TEXAS

The Big Bend region of west Texas hosts a complex structural history. This is where the Ouachita fold belt meets the southern Rocky Mountains and associated structures are cut by N and NW-trending normal faults related to Rio Grande rifting and/or Basin and Range extension. The Christmas Mountains, north of Big Bend National Park, were chosen to test ideas on the southward propagation of faults associated with Rio Grande rifting. The Christmas Mountains are the remnants of elliptical laccocalderas that formed ~42 Ma and tilted the predominantly Cretaceous sedimentary rocks and Tertiary intrusive, extrusive, and volcaniclastic rocks. Remote sensing interpretations of lineament patterns were compiled in a geodatabase to support field mapping of major structural features of the Trans-Pecos region. This supported a model of the regional stress field to distinguish compressional versus extensional strain. We hypothesize that there are step-over segments along a N25°W trend consistent with an extensional transfer zone. We observe lineament trends of N°10-20°W in the northern corner of the study area within a series of weathered volcanic units. In the south, the trends are N45°W to N50°W on field mapped fractures and kilometer scale lineaments interpreted from remote sensing. The structural and slope analyses and field data show a correlation between the fracture systems and the topographically highest areas near the southeast and gentle slopes toward the northwest. Our data also show an approximately orthogonal rotation of the maximum compression direction based on lineament trends from remote sensing and from the fracture and shear zone data collected in the field. Many have contested that the Rio Grande Rift does not continue south into Mexico. We have documented fracture and lineament data via remote sensing of Landsat, ASTER, SPOT imagery and field mapping of small scale faults, shear zones and fractures to test the hypothesis that the Tascotal Mesa Fault served as a controlling structure to transfer slip to the south. We propose that the Rio Grande Rift continues south into northern Mexico as others have proposed via a transtensional accommodation zone which is influenced by inherited Precambrian structure.