STRUCTURAL GEOLOGY AND TECTONICS OF THE FRANKLIN MOUNTAINS, WEST TEXAS

The Franklin Mountains in southern New Mexico and west Texas are a N-S trending range in the Rio Grande Rift. The structure of the mountain range is the result of two deformation events, the Laramide Orogeny and Rio Grande Rift extension. We describe the kinematics and geometry of faults within the range to constrain the tectonic evolution of the region. The western Franklin Mountains expose lower Paleozoic to Cretaceous rocks that were folded and faulted during Laramide contraction. Folds trend NW-SE with shallow hinges. Folds are bounded by shallowly SW dipping thrust faults developed in evaporites within Permian strata. These structures are cut by steeply east dipping reverse faults that place Paleozoic over Cretaceous strata. Kinematic P and T axis calculated from both sets of faults suggests that the reverse faults are back thrusts develop during the same progressive northeast directed shortening event that produced the low angle faults. The reverse faults produced a narrow triangle zone along the west side of the range with steep to overturned strata. This triangle zone appears to have been a major zone of weakness and controlled the location of subsequent rift related normal faults along the western side of the range. The unimodal shortening direction recorded in the Franklins contrasts with the complex rotation of shortening directions observed in the Laramide orogen of central New Mexico. Extension during Rio Grande Rifting has caused uplift and west tilting of the range. Normal faults strike 345-350 along much of the west side of the range and have steep to vertical dips. Slickenside lineations along the fault planes are nearly down dip. P and T kinematic axis calculated from the faults indicates east-west extension and subvertical shortening. The nearly east-west extension on faults with north-northwest strikes suggests a small right-handed component to rift related strain. A complex set of range crossing normal faults accommodates differential extension and tilting within range, with the most deeply exhumed strata occurring in the central part of the range. Our data show that Laramide contractional structures strongly influenced the location of subsequent rift related faults demonstrating the importance of crustal anisotropy in controlling extensional structure.