Blueschist metamorphism of the Eastern Franciscan belt, northern California

Rocks of the Eastern Franciscan belt, northern California, are divided into two tectonostratigraphic terranes metamorphosed to the blueschist facies, both with a distinct lithologic association and deformational history. The easternmost terrane, the Pickett Peak terrane of Early Cretaceous isotopic age, consists of crenulated mica schist and gneissic to schistose metagraywacke, with lesser alkalic mafic metaigneous rocks and scarce metachert. The Pickett Peak terrane retains evidence of three periods of penetrative deformation, the first of which is characterized by segregation layering, and the second and third by crenulation cleavages. Blueschist-facies conditions persisted during the first two deformations.

The Yolla Body terrane of Late Jurassic and Early Cretaceous paleontologic age lies structurally below and to the west of the Pickett Peak terrane. It is characterized by voluminous metagraywacke and lesser argillite, coherent interbedded radiolarian chert, and alkalic gabbroic dikes and sills. The Yolla Bolly terrane retains evidence for two phases of penetrative deformation that were coaxial with the second and third phases of deformation in the Pickett Peak terrane. The first phase of deformation (parallel to the second phase in the Pickett Peak terrane) was also accompanied by blueschist-facies metamorphism.