Plutonic and volcanic rocks of Middle Jurassic to Miocene age are widely exposed in southern California (U.S.A.), the peninsula of Baja California, and the states of Sonora, Sinaloa, and Jalisco (Mexico).
The Jurassic emplacement occurred in two belts, one in the extreme western margin of the continent, the other 500 kilometers inland in central Sonora. During early Cretaceous to Oligocene time, the axis of magmatism migrated from the peninsular ranges of southern and Baja California eastward into central Mexico. In the Miocene, magmatism moved westward to the area soon to become the Gulf of California.
In the continental margin island arc-type plutonic and volcanic magma erupted into Triassic and Jurassic ophiolite basement. In the Peninsular Ranges, the granitic rocks intruded the apron of Triassic-Jurassic clastic strata derived from the Precambrian craton to the east. Farther inland, Late Cretaceous and Early Tertiary plutons intruded Precambrian, early Paleozoic, and Cretaceous cratonic strata.
In the continental borderland and mainland Mexico, the observable plutons are shallowly emplaced into low-grade metamorphic rocks. In the Peninsular Ranges, uplift and deep erosion have exposed diapir-shaped bodies emplaced within upper amphibolite grade schists and gneisses.
In the continental margin, gabbro and low-potassium tonalite predominate; in the western Peninsular Ranges, gabbro is associated with a variety of rocks, ranging from diorite to granite. In the eastern Peninsular Ranges, leuco-tonalite is predominant, with granite becoming progressively important eastward into mainland Mexico.
The distribution of Jurassic arcs suggests that an oceanic arc was sutured onto North America in Early Cretaceous time. The tremendous volume of added sialic magma dilated the western edge of the continent by almost 300 kilometers, perhaps causing the continent to override the trench at the end of the Oligocene.