Displacement along tectonic plate boundaries is largely accommodated within narrow zones of intensely deformed and metamorphosed rock. Historically, these plate boundary fault zones have been categorized as either steadily creeping or seismogenic and capable of rapid and destructive slip. Recent advances in geophysical monitoring and an increase in the number of catalogued and characterized earthquakes reveal that fault zone deformation encompasses a spectrum of slip behaviors between these end-members. I will present experimental deformation studies and structural observations of serpentinites and clay-rich gouge from the San Andreas Fault that show phyllosilicates impart an important control on fault zone mechanics over the full spectrum of slip behavior. Convergent and some strike-slip plate boundaries are dominated by phyllosilicate-rich lithologies, yet the structure and mechanics of these rocks are not well constrained. I will discuss how deformation processes in these rocks deviate from our standard models of fault zones, and the implications of these differences for seismic hazards.