Crustal Structure of the Southern Rio Grande Rift (SRGR): Modeling Results for the Potrillo Volcanic Field (PVF) Seismic Experiment

The crustal structure of the Rio Grande Rift is an important link to the understanding of mantle, crustal and surface processes associated with classic continental rift environments. Magmatic and extensional structures associated with the rift have profoundly modified the lithospheric structure of the southern Rocky Mountains. Previous geophysical and geological studies have illuminated distinct structural characteristics between the northern extension of the RGR, the Colorado Plateau to the West and the Great Plains to the East. Modern active source seismic studies, gravity modeling as well as xenolith suites recovered from two Quaternary maars (Kilbourne Hole and Potrillo Maar) and Eocene intrusions in the Potrillo Volcanic Field (PVF), provide much insight into the structure and crustal evolution of the southern extent of the Rio Grande rift. The Potrillo Volcanic Field (PVF) experiment carried out in May of 2003 was designed to improve the resolution of previous studies and to gain a better understanding of the crustal properties, structure and evolution of the southern Rio Grande Rift and relate it to xenoliths from within PVF. This experiment was comprised of 8 shots of 1000-2000 lbs., 793 seismic recorders (TEXANS) deployed at variable spacing of 100 m, 200m and 600m over 205 km. Earlier seismic results from Sinno et al. indicate distinct thinning of the crust by 4-6 km into the SRGR from both the northern Rio Grande rift and the adjacent Basin and Range province, and a regional Pn velocity of approximately 7.7 km/s. A broad gravity high of 20-30 mGal as well as high heat flow values in the range of 75 to 125 mWm-2 are also associated with the SRGR. Data recorded during the PVF experiment show energy propagation across the entire length of each shot gather as well as distinct Pg, Pn, PmP, and numerous intracrustal reflection phases. Complex reflectivity associated with PmP at 11 s as well as a deeper phase suggesting P to S conversions off the Moho are clearly indicated on the near vertical reflection record sections. Tomographic modeling clearly delineates velocity anomalies associated with Basin and Range as well as possible earlier Laramide deformation. A change in mid-crustal velocities from 5.5 - 6 km/s west of the PVF to 6 - 6.5 km/s to its east indicates modification of the crustal structure related to Potrillo magmatism. Large- scale modification of the lithosphere is indicated by overall thinning of crust from West to East out of the southern Basin and Range. Crustal thickness may be as great as 35 km and likely dips to the east. Here we present a crustal model for the SRGR synthesizing results from previous results, current tomographic, ray-based and gravity modeling as well as xenolith data analyzed from the Potrillo volcanic field.