Quaternary geologic and geomorphic framework for neotectonic analysis of the northeastern Franklin Mountains, El Paso, Texas

Abstract

The Quaternary geology and geomorphology of a 45 km2 area along the northeastern Franklin Mountains near El Paso, Texas was characterized as part of a paleoseismic evaluation of the East Franklin Mountains fault. The East Franklin Mountains fault is a normal fault that extends from the northern end of the Franklin Mountains near Anthony Gap, southward for about 40 km along the east side of the Franklin Mountains to the International Boundary at El Paso, Texas. Five Quaternary stratigraphic units were defined on the basis of mapping and correlation with the work documented by the Desert Project in southeastern New Mexico. The elements of the geomorphic framework consist of Paleozoic bedrock, Dona Ana alluvium (> 400 ka), and Jornada I alluvium (400 ka to 250 ka) on the foot wall block, and Jornada H alluvium (1 50 ka to 25 ka), Isaack's Ranch alluvium (1 5 ka to 8 ka), and Organ alluvium (8 ka to present) on both footwall and hanging wall blocks. Topography was used as the initial criteria for delineating the Dona Ana and Organ alluvium. Jornada I, Jornada II, and Isaack's Ranch alluvium were distinguished using soil development, drainage pattern analyses, induration, degree of weathering, microtopography, rock varnish, and desert pavement. Geologic and geomorphic mapping suggests that the youngest faulted alluvium is Jornada II in age and that the oldest unfaulted alluvium is Isaack's Ranch in age. Trenching across the East Franklin Mountains fault suggests four surface rupturing events since the beginning of Jornada II time about 150 ka, with a total 8.8 m of offset. The average displacement for each event is about 2.4 m. A total of 61 m of offset has occurred since the stabilization of the Done Ana surface in the Hueco bolson about 610 ka ago. Therefore, the above offsets yield slip rates of about 0. 1 0 mm/yr for the past 6 1 0 ka and 0.06 mm/yr for the past 150 ka. Recurrence intervals are on the order of every 40 ka with a maximum surface-wave magnitude (Ms) of 7.1 and a moment magnitude (Mw) of 7.0.