Structural Cross Section of the South Fork Mountain Schist, Franciscan Subduction Complex, Northern California Coast Ranges

Abstract:

Much of the Franciscan subduction complex is made up of mélanges, is poorly exposed, or both, making it difficult to extract structural data and information on its response to subduction and exhumation. The Eastern Belt of the Franciscan, however, contains coherent thrust sheets that contain a detailed record of the regional deformation history. The South Fork Mountain Schist is a 6km thick body of blueschist facies pelitic and mafic schist that forms the easternmost of the coherent thrust sheets, lying adjacent to the Coast Range Fault, which separates the Franciscan from the Great Valley sequence. As such, it has potentially preserved the history of motion along the Coast Range Fault, both when it was a megathrust plate boundary and when it was reactivated as a normal fault.

Thomes Creek, east of Paskenta in the northern California Coast Ranges, runs roughly perpendicular to the strike of the Coast Range Fault and the South Fork Mountain Schist. The excellent exposure in the creek bed is ideal for constructing a structural cross section. At least three episodes of deformation can be identified, which produced overturned and asymmetric folds. The earliest fabric discernible in the field is a strongly differentiated crenulation cleavage, produced mainly by dissolution-precipitation creep, containing syn-kinematic lawsonite and sodic amphibole. This fabric is likely to reflect very high strain associated with the subduction zone megathrust. Foliation dips vary from south east to north east, with east dipping foliation being the most common. The vergence of asymmetric minor folds affecting this foliation is opposite on opposite limbs of larger scale folds, allowing the identification of fold structures on the scale of hundreds of meters amplitude. These phases of folding are likely to represent the history of underplating and exhumation of the schist body.