Through an integrated interpretation of gravity, magnetic, seismic reflection, and well data we are able to define the geometry, lateral extent, and tectonic setting of late Middle Proterozoic mafic rocks (the Pecos mafic intrusive suite) in the basement of the Permian basin of west Texas and eastern New Mexico. In this region, seismic reflection data are characterized by subhorizontal layered reflectivity within basement that can be tied to compositional variation in a layered mafic intrusion through well cuttings and a synthetic seismogram. In places the intrusion forms a sill, and the base of the sill is imaged in the reflection data. 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 locally dikelike keel. North--south trending gravity and magnetic anomalies associated with the Pecos mafic intrusive suite suggest that it is composed of four large intrusive centers which cover a minimum area of 9200 km2, making it one-seventh the size of the Bushveld Complex and one of the largest layered mafic intrusions in the world. We interpret this massive intrusion to be associated with collisional rifting in the foreland of the Grenville orogeny. ¿ American Geophysical Union 1995