DEFORMATION MECHANISM EVOLUTION DURING SUBDUCTION RELATED DEFORMATION, EASTERN BELT FRANCISCAN COMPLEX, NORTHERN CALIFORNIA

The South Fork Mountain Schist member of the Eastern Belt of the Franciscan Complex primarily consists of marine sediments which were subducted and accreted to the overriding plate, deforming at ~35km depth and ~340°C. Textural relationships between blueschist facies minerals and deformational fabrics are used to relate episodes of deformation to different stages of subduction and accretion. Evidence is preserved both of pressure solution and dislocation creep driven deformation. A model for pressure solution driven differentiation within a simple shear regime is presented and is used to estimate a minimum strain rate while a combination of the plate convergence rate and shear zone thickness is used to estimate a maximum strain rate. These constraints are then used to evaluate the suitability of competing pressure solution flow laws, finding that a thin film flow law best predicts the observed strain rates. Previous workers have demonstrated that pressure solution rates are greatly enhanced in the presence of micas. Deformation mechanism maps were made both with a modification to account for increased deformation rates in the presence of micas and without the modification; maps with the modification describe deformation of weakly or undifferentiated rock while maps without the modification describe deformation of strongly differentiated rock which has fewer quartz-mica boundaries. The deformation mechanism maps demonstrate that progressive differentiation lowered the strain rate and promoted a switch from pressure solution to dislocation creep, explaining the preserved composite microstructures. These results both describe the history of deformation in this location and point to a means by which the rheology of a unit can change without requiring a change in the boundary conditions driving deformation.