CRUSTAL STRUCTURE OF THE SOUTHERN RIO GRANDE RIFT DETERMINED FROM SEISMIC REFRACTION, SURFACE WAVE DISPERSION, AND GRAVITY DATA

A series of seismic refraction profiles have been recorded in south-central New Mexico with the goal of determining the crustal structure in the southern Rio Grande rift. These lines consist of three regional profiles: a reversed E-W line across the rift, an unreversed N-S axial line, and an unreversed NW-SE line. The reversed E-W line shows no significant dip along the Moho ((TURN)32 km thick crust) and 7.7 km/sec Pn velocity. Results from the N-S axial line and the NW-SE line indicate an apparent Pn velocity of 7.95 km/sec and significant dip along the Moho in the south and southeast directions, respectively. When interpreted together, these data indicate a crustal thinning in the southern rift of 4 to 6 km with respect to the northern rift and the adjacent Basin and Range province. The regional Pn velocity is approximately 7.7 km/sec. An analysis of regional Bouguer gravity anomalies in the area is consistent with these seismic results and indicate the crust is thinnest ((TURN)26 km) within an oval N-S trending area near El Paso, Texas. Rayleigh wave dispersion measurements show the regional Sn velocity and Poisson's ratio of the upper mantle to be 4.15 km/sec and 0/3, respectively.