New Mexico Geological Society Annual Spring Meeting — Abstracts

The crustal structure of the Rio Grande Rift is an important link to the understanding of mantle, crustal and surface processes in continental rift environments. The 2003 Potrillo Volcanic Field (PVF) experiment was designed as a detailed seismic investigation of the structure and composition of the Southern Rio Grande Rift (SRGR) at the PVF, a young and well-known xenolith locality. Our results provide new insights into the structure of the SRGR and PVF. Along the 205-km-long profile, the velocity structure of the upper 3-5 km reflects the basins and ranges of this recently extended area. Basin fill ranges in velocity from 2.5 to 4.5 km/s. In the ranges, velocities are 4.7 to 5.3 km/s and reflect uplifted Paleozoic sedimentary rock. A middle crust interface that marks the transition from upper to middle crust steps up from ~15 to ~11 km below the PVF leads to thickening of the middle crust in this region. Velocities increase from approximately 6.15 to 6.4 km/s above this interface to 6.7 to 7.1 km/s at the base of the crust. Whereas near-vertical incidence records exhibit laminar reflectivity at the Moho, velocity modeling does not show a pronounced lower crust transitional layer. Crustal thickness varies from 35 km near Hachita, NM to as little as ~30 km beneath El Paso, TX. Upper mantle velocities decrease west to east from 7.9 to 7.75 km/s, consistent with a warm upper mantle and high heat flow values of 75 to 125 mW/m2 . We interpret the west to east changes in the middle crust, crustal thickness and upper mantle velocity as the manifestation of the transition from southern Basin and Range province to the Rio Grande Rift proper.