Integrated geophysical studies of continental rifts in West Texas and New Mexico

An integration of multiple geophysical methods combined with geologic information has been used to study the structure, geology and tectonic setting of the present day Rio Grande rift and the late Middle Proterozoic Delaware aulacogen. The Rio Grande rift study focused on the structure of the rift in south-central New Mexico. Seismic data and well drilling information were used to constrain gravity models of the crust. These models suggest that the Engle and Palomas basins are half graben structures bounded by normal faults. The Tularosa basin with a full graben structure is the best developed basin in this part of the rift. An analysis of the deeper structure based on these models suggests that the crust is 28-33 km thick, which is thin compared to the unextended crust surrounding the rift. A series of four studies of the Precambrian basement geology of the Permian basin region make up remainder of the dissertation. These studies are focused on understanding the geology, structure and tectonic setting of the Abilene gravity minimum and the Pecos mafic intrusive suite. The Abilene gravity minimum is a 600 km long gravity low extending from the Texas-Oklahoma border to the Diablo platform in west Texas. The Abilene gravity minimum may represent a continental margin arc batholith equivalent in scale to the Sierra-Nevada batholith in California. It may be a result of subduction along the southern margin of the continent during the formation of the southern Granite-Rhyolite province. I propose that the location of the Grenville deformation front is farther south and alined along a series of east-west oriented gravity highs extending between the Llano and Van Horn uplifts. The Pecos mafic intrusive suite is represented by a series of layered mafic intrusions in the basement under the Central basin platform and Roosevelt uplift in the Permian basin. The Pecos mafic intrusive suite contains the igneous rocks associated with formation of the Delaware aulacogen. Paired gravity and magnetic maxima indicate that the intrusion may be composed of four large intrusive centers which extend north under the rest of the Central basin platform and Roosevelt uplift covering a minimum area of 9200 km$\sp2$. Seismic reflection data over this intrusive suite are characterized by sub-horizontal layered reflectivity which is tied to compositional variation in a layered mafic intrusion through well cuttings and a synthetic seismogram for the Nellie well. Integration of the seismic interpretation with gravity models shows that the Pecos mafic intrusive suite is 3-10 km thick and 50-100 km wide. The overall structure of the suite is that of a sill with a dike-like keel. In places the intrusion forms a sill, and the base of the sill is imaged in the reflection data. This rift event is contemporaneous with formation of the Midcontinent rift system, Pikes Peak Batholith and the intrusion of diabase sills in California, Nevada and Arizona. (Abstract shortened by UMI.