KINEMATIC ANALYSIS OF SYNSUBDUCTION SHEAR FABRICS IN FRANCISCAN COMPLEX MÉLANGE NEAR SAN SIMEON, CALIFORNIA

Asymmetric fabrics in Franciscan Complex shale matrix mélange near San Simeon, central California, record synsubduction shear that has implications for the Late Cretaceous to early Tertiary tectonic evolution of the region. Outcrop-scale structural data including S-, C-, and C’-planes, slickenlines, block long axes, and offset blocks indicate that most asymmetric fabrics developed in a NE-dipping sinistral shear zone following Late Cretaceous chaotic mixing of the mélange. Approximately 70% of C-planes strike within ~45° of the major NW-striking structures in the region – the Neogene San Gregorio-Hosgri fault and the Late Cretaceous-early Tertiary Nacimiento fault zone. Slip vectors on these NW-striking C-planes consistently record sinistral or oblique sinistral shear. This dominant shear regime accommodated E-W to NE-SW shortening and N-S to NW-SE extension. Angles between S- and C-planes are typically ~25-35° and range up to ~45°, suggesting dominantly simple shear. Sinistral shear in the mélange is kinematically incompatible with Neogene to recent dextral faulting, indicating that these fabrics predate the early Miocene development of the Pacific-North America transform. We propose that sinistral shear in the mélange is related to Late Cretaceous to early Tertiary slip on the adjacent Nacimiento fault zone. These kinematic data are compatible with previous studies that have suggested the juxtaposition of the Salinian block against the Nacimiento block was accommodated by hundreds of kilometers of sinistral slip on the Nacimiento fault zone. This sinistral slip may have been driven by subduction of an aseismic ridge and/or oblique convergence between the Farallon and North America plates during Late Cretaceous to early Tertiary flat-slab subduction.