The southern Basin and Range physiographic province extends southward from the Colorado Plateau and Great Basin to the United States-Mexico boundary, and from the lower Colorado River valley eastward to the Pecos River valley (Fig. 1). This region is divided into six parts: lower Colorado River valley, Sonoran Desert, Mexican Highland section, Transition Zone-Datil-Mogollon section, Rio Grande rift zone, and Sacramento section.
The principal physiographic characteristic of the southern Basin and Range province is a series of discontinuous mountain ranges alternating with subparallel intermontane depositional basins. Most of the mountain ranges trend west-northwest to north, but a few are northeasterly. Their higher summits range in altitude from 900 to rarely above 2500 m in western and eastern parts of this region, and from 1600 to 3265 m in its central part. Intermontane-basin floors range from close to sea level near Yuma to above 1500 m altitude in the transition zone below the Colorado Plateau. The mountain ranges have diverse structures, ages, and rock types, even between nearby ranges. Exposed rocks range from Precambrian to Quaternary and include virtually all rock types: sedimentary, volcanic, plutonic, and metamorphic. Some ranges are structurally and lithologically simple, but other mountain ranges have complex structures, commonly involving many episodes of low-angle and/or high-angle faulting, folding, and/or igneous intrusion, and similarly complex stratigraphy and lithology.