STRUCTURAL GEOLOGY AND GEOPHYSICS OF THE PIPELINE COMPLEX, NORTHERN FRANKLIN MOUNTAINS, EL PASO, TEXAS

The Pipeline Complex is a series of hills on the eastern flank of the Franklin Mountains, El Paso, Texas. Their origin has been attributed to recent landsliding (Lovejoy, 1976a-e); or to Eastern Boundary Fault Zone (EBFZ) fault plates rotated to low angle (Seager, 1981). Mapping of the Complex indentified east dipping fault plates with simple internal structures. The plates are stacked with each plate being younger than and cutting the lower. The main boundary fault (MBF) is a N-S trending (25-35$\sp\circ$E dip) curvilinear scissors fault with zero displacement at the north and 10000 feet (3050m) in the south. Internal structures were caused by differential movement of the plates as a result of MBF scissors motion. Bedding dips 20-25$\sp\circ$W, while the same units in the Franklins dip 50-60$\sp\circ$W. This relationship and the internal structures are difficult to explain with previous theories. The Pipeline Complex probably developed as a toppling failure into a bench identified on seismic line 6-B. The failure was associated with, but not structurally related to, EBFZ development. A magnetic map (along with susceptibility measurements) was made of the Complex. Talwani-type models of gravity (20 miles (36km) long, 1000 stations) and magnetic profiles across the Franklins suggest the existence of a dike (3.00 gm/cm$\sp3$) beneath the Franklins. In all models because of surface geology constraints the upper EBFZ was modeled with a high-angle east dip. At depth the dip was uncertain. Modeling suggested that the Western Boundary Fault Zone (WBFZ) was located west of the Anthony synclinorium. The magnetic profile exhibited a large (250 gamma) monoclinal anomaly which did not match the gravity profile. The EBFZ was evident on a seismic line (6-B) across the Franklins and was interpreted as a high-angle, non-listric, east dipping normal fault. Between the EBFZ and the ridge of the Franklins is a structurally complex bench (2000 feet (600m) depth). Neither the WBFZ or the Franklin Mountain dike are visible. Seismic lines in the Mesilla Bolson showed that basin fill averages 2500 feet (760m) thick