Una Falla Críptica: the East Franklin Mountains Fault, El Paso, Texas and Its Late Quaternary Behavior

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems
Type: Oral
Date: 4/28/2020
Time: 03:30 PM
Room: 240
Description: 

The East Franklin Mountain Fault (EFMF) is a north-south striking Quaternary normal fault that runs through El Paso, Texas and Juarez, Mexico (combined population ~2.5 million). Three paleoseismic studies were funded by NEHRP since 1995. The first two involve three trenches on the far northern part of the EFMF, 3 km south of the Texas-New Mexico border. The main trench cut across an 8 m-high scarp on a Jornada IIa alluvial fan (age 95-150 ka) and exposed 4 to 5 displacement events, based on colluvial wedges and fault terminations. The earlier trench study (before luminescence dating) correlated a strong caliche paleosol on the footwall and hanging wall as the same, yielding cumulative vertical displacement of ~7 m. Later re-study of this trench used luminescence dating and concluded that the strong caliche soil on the hanging wall was developed on younger alluvium deposited against the toe of the scarp. This implies a minimum vertical displacement of ~10 m since the age of the Jornada Ia footwall. The other two trenches demonstrated that middle-to-late Holocene Organ Alluvium is not faulted, and that the cumulative vertical displacement since deposition of the Camp Rice Fm. (Plio-Pleist.) is greater than 50 m. In 2015 we trenched a second site 17 km farther south, in a 1 km-stepover at McKelligan Canyon (Beaumont Medical Center). This trench yielded a similar number of displacement events in the same time period, but displacements-per-event were ~half as large as farther north because the trench was located near the termination of the scarp. Smaller displacements per event equated to thinner colluvial wedges, so inter-event caliche paleosols developed through each wedge and into the next underlying one, obscuring wedge boundaries. The overlap of event ages at the two sites permits a single rupture segment >25 km long from McKelligan Canyon northward. Additional trenching is required to locate the southern end of this segment, but most of the fault in El Paso and Juarez is heavily urbanized, so locating viable trench sites will be difficult.

Presenting Author: James P. McCalpin

Authors