SEISMIC STUDIES OF THE DEPTH EXTENT OF THE NORTH SISTER ANDESITE INTRUSION, WEST EL PASO, TEXAS

Multiple Eocene intrusive bodies crop out on the western side of the Franklin Mountains, in El Paso, Texas, and in the southern Mesilla Basin of Texas and New Mexico. Previous studies of these andesitic intrusions concentrated on the petrology and petrogenesis of the intrusions. However, the purpose of this study is to better understand the geometry of the intrusions by conducting seismic refraction surveys near the edges of one intrusion, known locally as the Three Sisters site. We collected seismic data along five lines on the eastern side of the north hill; four of the lines were oriented perpendicular to the outcrop edge and one was oriented parallel to the outcrop edge. Of the four perpendicular lines, two lines overlap one another and create a line about thirty-five meters longer than the other three lines. Data were analyzed using Pickwin and modeled with Plotrefa. Analysis shows maximum P-wave velocities at depths of 20 meters are less than 3000 meters per second; this is not consistent with other bodies of andesite in the region. The eastern side of the outcrop may not consist of andesite, and the small exposure observed on the southeast side of the hill is either debris from the top of the hill or a very small, localized intrusion.