A PRELIMINARY METAMORPHIC MAP OF CALIFORNIA

California has fostered the development of many ideas about the relationship between plate tectonics and metamorphism. Ernst (1983) compiled a map that was the first to illustrate the relationship between tectonics and metamorphism in northern California. Our new metamorphic map (1:2,000,000) illustrates primarily the effects of Mesozoic subduction, accretion, and igneous intrusion, but Precambrian and Paleozoic metamorphic rocks also are exposed in eastern California. Blueschists in the Coast Ranges are related to the same subduction cycle as the Cretaceous Sierra Nevada batholith, but the metamorphic effects of the batholith are limited to small areas adjacent to or underlain by elements of the batholith. Low-grade metamorphism in the western Sierra Nevada is mostly older than the batholith and, therefore, does not represent a low P/T metamorphic belt paired with the high P/T blueschists in the Coast Ranges. No orogen-scale, high-grade event has overprinted the pre-batholithic metamorphism in the Sierra-Klamath region. Consequently, the orogen preserves multiple episodes and styles of Phanerozoic metamorphism that reflect processes prior to, during, and after accretionary events: (1) amphibolite facies metamorphism of Paleozoic ultramafic rocks; (2) hydrothermal metamorphism by intrusions in volcanic arcs and ophiolites; (2) sub-greenschist facies burial metamorphism of volcanic arcs; (3) rare blueschists; (4) greenschist facies metamorphism in regional shear zones; (5) multiple episodes of contact metamorphism by Mesozoic igneous intrusions. Regional-scale faults commonly are metamorphic discontinuities, and are intruded and overprinted by latest Jurassic plutons. The present level of exposure in the Sierra Nevada-Klamath region apparently has not suffered a major dynamothermal metamorphism since its final amalgamation during late Jurassic time. The preservation of multiple episodes and styles of metamorphism may be typical of orogens formed by accretion and subduction.