The area between the Colorado Plateau on the north, New Mexico and Chihuahua to the east, and southern and Baja California to the southwest has undergone a complicated thermal and tectonic history climaxing in medial Cretaceous time and manifested in a sequence of events which continue to the present time, independent of plate boundary transform faulting. This sustained sequence of events was initiated by subduction‐related magmatic emplacement, largely between 120 and 50 Ma, beginning in the west and sweeping eastward across the region. The emplacement of these rocks generated a large isostatic welt which began to elevate in late medial Cretaceous time in the west central portion of the area and spread outward until in Oligocene time the west coast of southern California was emergent almost to the continental escarpment. Both erosional and tectonic unloading accompanied elevation, with the axis of the welt achieving its maximum elevation in earliest Cenozoic time when north central Sonora may have had elevations of the order of 5000 m. Body forces caused the welt to relax laterally, producing thrust and nappe structures which were westward vergent in the west and eastward vergent in the east. To the east these structures are included in what is called the Larimide Orogeny; to the west they include such features as the Santa Rosa mylonite belt and related structures. As with the magmatic emplacement, the gravitational spreading began in the west. During the Eocene, additional rebound in response to deep erosion and tectonic unroofing began to expose midcrustal rocks within the core area of the welt, and subhorizontal detachment began along the brittle‐ductile boundary. During Oligocene and early Miocene time, renewed isostatic uplift resulted from rising magma, and continued erosional and tectonic denudation spawned both brittle and ductile extension, hydrothermal alteration, and mineral deposition. From mid‐Miocene to the present time the central portion of the area has subsided, while the marginal mountain ranges have elevated.