Geophysical constraints on the Hueco and Mesilla Bolsons: Structure and geometry
The Hueco and Mesilla Bolsons are part of the intramountain basins of the Rio Grande Rift system. These bolsons are the primary source of groundwater for the El Paso-Ciudad Juarez metropolitan area and contain faults that show evidence of repeated earthquakes during the Quaternary. The region is also associated with has low-level (M<4) seismicity. The collection and analysis of precision gravity data, coupled with information from water wells, multichannel analysis of surface waves (MASW) studies and previously published seismic reflection lines, have been used to examine the structure and faulting within these bolson. This study reveals that the Hueco and Mesilla Bolsons are very different structurally. The southern Mesilla Bolson contains about 500 m of sediment. Faults are difficult to trace and have less than 50-100 m of displacement across them. The southernmost bolson contains numerous Tertiary intrusions and the thickness of Cretaceous bedrock appears to decrease from south to north, possibly delineating the edge of Laramide age deformation within the bolson. The northern Hueco Bolson contains 1800 to 2500 m of basin fill. Displacement along the East Franklin Mountains fault (EFMF), a fault with evidence for repeated earthquakes within the past 64,000 years, is about 1500 m, and displacement on intrabasin faults is 200-300 m. Several intrabasin faults appear to control the saline to freshwater contact within the bolson. The EFMF may extend over 30 km south of the end of its mapped trace at the end of the Franklin Mountains and a number of intrabasin faults also extend south into the urbanized regions of the study area. The EFMF and other basin structures appear to be offset or disrupted at the speculated edge of Laramide deformation that lies beneath the bolson. Horizontal Gradient Methods (HGM) were applied to the gravity data and were successful for tracing faults and older Laramide features within the Hueco Bolson beneath the urbanized regions of the cities. HGM were not as successful at tracing faults within the Mesilla Bolson, however they were helpful for tracing the subsurface extent of igneous intrusions including the Mt. Cristo Rey, River, Three Sisters, and the Westerner outcrops. Some of these features appear linked at depth by a series of dikes and faults. MASW data were used to determine the average shear wave velocity in the upper 30m (Vs 30) at ∼70 sites within the Hueco Bolson. These observations were combined with similar data collected previously in Juarez to produce regional velocity and site classification maps. The results show low velocities are found close to the river within fluvial deposits with higher velocities close to the Franklin Mountains where bedrock is close to the surface and higher velocities in upland regions of northeast El Paso were soils appear to be more highly cemented. These data will be used in conjunction with information on bolson geometries to model the expected effects of strong ground motion from earthquakes in the El Paso-Ciudad Juarez region.^
Avila, Victor Manuel, "Geophysical constraints on the Hueco and Mesilla Bolsons: Structure and geometry" (2016). ETD Collection for University of Texas, El Paso. AAI10133864.