SUBDUCTION ZONE PALEOSTRESS AND DEFORMATION, FRANCSICAN COMPLEX, NORTHERN CALIFORNIA COAST RANGES

The South Fork Mountain Schist Formation in the Eastern Belt of the Franciscan primarily consists of ocean sediments which were subducted and then accreted to the overriding plate at depths of ~35 km and subsequently metamorphosed at ~340°C, close to the limit of quartz plasticity and the lower limits of the temperature conditions which produce slow earthquakes today. The mechanics of deformation at these depths and conditions, where rocks are deformed by both brittle and ductile deformation simultaneously, are not yet entirely understood, but the ductile deformation has been seen to involve both pressure solution and dislocation creep.

This study utilizes electron backscatter diffraction to measure the paleostress recorded by recrystallized quartz grains close to the upper and lower bounding faults of the South Fork Mountain Schist as well as within the main body. Stress results are then combined with previously derived flow laws for both pressure solution and dislocation creep to calculate strain rates attributable to both deformation mechanisms. Preliminary results find that two samples of metasediments which were deformed close to the upper bounding fault experienced differential stresses of 54.5 MPa and 51.8 MPa, while stress was 65.1 MPa close to the lower bounding fault. Deformation within the main body occurred under a stress of 35.5 MPa. Previous work has suggested that while underplated sediments experience distributed deformation throughout their entire thickness, deformation is concentrated along the upper and lower bounding faults. Our results broadly support this model, though the upper bounding fault may have largely ceased activity after activation of the lower fault.