Four maps are presented here that show the location and extent of magmatic fields between eastern Alaska and northern Mexico during the successive time intervals of 55--40, 40--25, 25--10, and 10--0 Ma, and four others show the distribution of metamorphic core complexes during the same Cenozoic time intervals. The maps are based on U.S. Geological Survey and Canadian Cordilleran data bases containing about 600 isotopic dates and extensive literature review. For nearly 60 Ma the development of metamorphic core complexes has coincided with the locus of areally extensive and voluminous intermediate-composition magmatic fields. The association is suggestive of a close link between magmatism and core complex formation, namely that magma directly and indirectly lowers the strength of the crust. Magmatism thus controls the location and timing of core complex formation. The stresses responsible may be inherited from Mesozoic crustal thickening, locally created by uplift and magmatic thickening of the crust, and imposed by the global pattern of plate motions and driving forces. Since the Miocene, rates of magmatism, extension, and core complex formation have declined. The modern Basin and Range province is not a suitable model for the situation that existed during major magmatic culminations. The singular event of early Miocene time, the merging of two large magmatic fields, extinguishing the Laramide magmatic gap, explains several disconnected observations: the hyperextension episode of the Colorado River corridor, rapid reorientation of stress patterns across much of western North America, and subsequent rapid tectonic movements in California. Magma-triggered breakup of western North America lithosphere coincided with development of the San Andreas transform system. Thermal destruction of the Laramide magmatic gap created a California ''microplate'' about 22 Ma ago that moved rapidly away from North America. Thus two plate tectonic processes, thermal destruction of the lithosphere ''bridge'' and northward growth of a transform system, interacted to produce Miocene and later tectonic patterns and events. ¿ American Geophysical Union 1991