LOW ANGLE NORMAL FAULTING IN THE FRANKLIN MOUNTAINS OF SOUTHERN NEW MEXICO AND WEST TEXAS

The Franklin Mountains are a north south trending range in the southern Rio Grande Rift bound to the west by the Mesilla Basin and to the east by the Hueco Basin. These deep basins cover much of the structure responsible for the large scale extension within the southern Rio Grande rift. However, faults exhumed from the deep parts of the basin are exposed in the Franklin Mountains. The most dramatic structural feature within the Franklins are a family of north-northwest trending normal faults with moderate to gentle east dips. Displacement on the faults is smallest at the north and south ends of the range and increases towards the central part of the mountain range. Stratigraphic separation is 10’s of meters at the northern end of the range, increases to 1400 m at the New Mexico state line, and is nearly 2000 m near north Mt. Franklin, in the central part of the range. Displacement decreases rapidly towards the south where the range terminates in central El Paso, Texas. The occurrence of faults with the largest displacements at the highest elevations in the mountain range suggests that low angle normal faults are a critical ingredient in the development of the topography of the Franklin Mountains.