Sedimentation, metamorphism and tectonic accretion of the Franciscan assemblage of northern California

Abstract

Summary The Yolla Bolly terrane of the Franciscan assemblage in northern California is seemingly a typical subduction complex, having undergone penetrative deformation and metamorphism to the high pressure-low temperature blueschist facies. Detailed mapping combined with sedimentological analysis has enabled us to: (1) reconstruct a probable palaeosedimentary environment, (2) analyse the interaction during and after subduction between deformation and metamorphism, and (3) speculate on subsequent deformational history including tectonic accretion of the terrane to North America. Rocks of the Yolla Bolly terrane consist of three thrust-fault-bounded lithological units: a lower unit of disrupted mudstone and thin-bedded sandstone (broken formation) containing scarce volcanic and radiolarian chert horizons, a middle unit predominantly of thick-bedded to massive sandstone (metagreywacke) that includes several horizons of radiolarian chert, and an upper unit of mudstone and thin-bedded sandstone (broken formation) with numerous intrusive and extrusive volcanic rocks plus rare radiolarian chert. Radiolarians from all three units are of the same age (Tithonian to Valanginian) and together with the sedimentological 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 California or basins of the California Continental Borderland, rather than a trench setting. Metagreywacke containing lawsonite and aragonite yields radiometric ages of approximately 110 Myr and indicate that these rocks were subducted to depths of 20–30 km about 30 or 40 Myr after they were deposited. Shortly after subduction, the rocks were probably involved in a collision that imbricated and tectonically returned the subduction complex to the surface.