Wide-ranging reconnaissance mapping in northern Mexico has given new insight into the basement of Mexico which is mostly hidden by Mesozoic supracrustal rocks. Ouachita rocks under the central highlands of Mexico, separate Grenville rocks in central Chihuahua from Grenville rocks of the San Luis Potisi area. A reinterpretation of regional gravity data combined with the limits of the Parral-Proaņo overthrust belt (north of Parral to the San Luis Potisi area) refine the definition of Ouachita-Grenville basement boundaries. The proposed correlation between Ouachita crust and the Cretaceous overthrust is believed to reflect a less buoyant younger crust.

During the opening of the Gulf of Mexico, numerous internal basins developed on the crustal platform. Laramide compression resulted in tight folding of supracrustal rocks within the Mesozoic basins while gentle monoclinal folding developed on the platforms.

The most complex portion of the Mexican basement is the Mesozoic marginal basin extending along the western margin of Mexico. The pre-Mesozoic craton's western margin corresponds closely with the Cananea-Inde structure. Crustal blocks such as the Precambrian Caborca Terrane and the Santa Maria del Oro (SMO) Terrane are believed to be large exotic basement fragments included in the basin. The SMO is a poorly exposed highly metamorphosed belt of unknown age along the western margin of the craton. The Cananea-Inde structure separates the more intense metamorphic NW oriented SMO fabrics from the NE trending Ouachita fabrics.

Probable major left-lateral transpressional motion on the Altar Megashear emplaced the Caborca Terrane into its present position in the Mid-Cretaceous. Major Mid-Cretaceous deformation follows the trace of this structural boundary. To the south, the Altar Megashear divides the Guerrero Terrane into two adjacent basinal sequences brought together during this same Mid-Cretaceous event. Mapping in Sonora gave little support to the concept of the Mojave-Sonora Megashear (MSM). The only mapped structure possibly corresponding to the previously proposed MSM appears much less significant than the Altar Megashear and terminates against it.