The Yolla Bolly terrane of the Franciscan assemblage in northern California is a typical subduction complex that has undergone penetrative deformation and metamorphism to the high-pressure/low-temperature blueschist facies. Detailed mapping combined with sedimentologic analysis has enabled us to: (1) reconstruct a probable paleosedimentary environment; (2) analyze the interaction between deformation and metamorphism during and after subduction; and (3) speculate on subsequent deformational history including tectonic accretion to North America.

The Yolla Bolly terrane consists of several thrust-bound lithologic units: a lower unit of disrupted mudstone and thin-bedded sandstone (broken formation) containing scarce volcanic flows and beds of radiolarian chert; a middle unit of predominantly thick-bedded to massive sandstone (metagraywacke) that includes several zones of radiolarian chert; and an upper unit of mudstone and thin-bedded sandstone (broken formation) containing numerous intrusive and extrusive volcanic rocks as well as rare radiolarian chert. Coeval ages (Tithonian to Valanginian) on radiolarians from all three units, together with the sedimentologic data, suggest that the rocks represent a continent-derived submarine fan, deposited in a complex transform graben possibly similar to the present-day Gulf of Californ a or basins in the California continental borderland.

Radiometric ages of approximately 100 m.y. on metagraywacke containing lawsonite and aragonite indicate that these rocks were subducted and metamorphosed to a high-pressure/low-temperature mineral assemblage about 30 m.y. after they were formed. Closely following subduction, the rocks were probably involved in a collision that imbricated and tectonically returned the subduction complex to the surface.

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