CONTRACTIONAL STRUCTURES IN THE WESTERN FRANKLIN MOUNTAINS SOUTHERN NEW MEXICO AND WEST TEXAS

One of the most unusual features of the structural geology of the Franklin Mountains near El Paso Texas are the presence of folds and thrust faults in Permian and younger rocks and the lack of these structures in older rocks. Here I present results of new mapping along the western flank of the Franklin Mountains and a possible explanation of this geometry. Folds attributed to Late Cretaceous through Eocene contraction in the Franklin Mountains are found within the Panther Seep Formation and in younger rocks. Folds trend to the northwest and have sub-horizontal axis. The folds are generally open and have angular hinges. Folds are best exposed near Cerro de Cristo Rey near downtown El Paso, and in the northern Franklin Mountains near Anthony Gap. Shortening across the folds is estimated between 10-25% based on the fold geometry. The Panther Seep Formation consists of thinly bedded limestone, dolomite, shale and two prominent beds of gypsum near the top of the formation. These thinly bedded units and the gypsum beds in particular are intensely deformed and contain outcrop scale folds and low angle thrust faults. The faults dip gently to the southwest. Kinematic indicators such as slickensides, fold asymmetry, asymmetric vein arrays, and C/S fabrics indicate top to the northeast transport. In a number of places, the thrust faults are associated with strike slip faults. The strike-slip faults and the geometry of tensile fractures suggest a significant wrench component to the overall strain field during thrusting suggesting the structures formed during right-handed transpression. I suggest that the relatively weak Panther Seep formation served as a de’collement surface during northeast directed thrusting within the El Paso area. This relationship explains why older Paleozoic strata in the Franklin Mountains were not affected by contractional deformation since they formed the footwall for the thrust system.