Spatial, temporal, and chemical patterns of igneous rocks in southern California, southern Arizona, and southwestern New Mexico strongly support low-angle subduction beneath southwestern North America in Late Cretaceous through middle Tertiary time. The data suggest constant dip-variable depth subduction from 130 to 80 m.y. B.P. and variable dip-variable depth subduction from 80 to 20 m.y. B.P. Flattening of the subducting slab from 80 to 50 m.y. B.P. and steepening of the slab from 40 to 20 m.y. B.P. are indicated by the data.