Bouguer gravity anomaly, low-pass, and band-pass maps of southeastern California, western Arizona, and southern Nevada are analyzed to determine regional crustal structure. Based on 100--250 km band-pass wavelength filtering of the Bouguer anomaly map, a low-amplitude gravity maximum is associated with the highly extended terrane along the Colorado River region of California, Arizona, and Nevada. Two crustal models, constructed using Bouguer gravity anomaly data and constrained by geological mapping, aeromagnetic data, and seismic refraction data, are consistent with the dome-shaped, midcrustal lens of probable dioritic composition centered under the Whipple Mountains proposed by McCarthy et al. (1989), although other models are not precluded. This midcrustal lens from the western edge of the Colorado River extensional corridor to east of the Buckskin and Hualapai mountains in Arizona. Dioritic material is exposed in various mountain ranges in the Colorado River extensional corridor and may be part of the source of the gravity maximum that is associated with each range. ¿American Geophysical Union 1991