The Punchbowl fault is an ancient segment of the San Andreas System that was active between 1 and 4 Ma and exhumed from a depth of 2--4 km, thereby providing the opportunity to study processes along the top of the seismogenic zone of today's San Andreas fault. X-ray diffraction, X-ray texture goniometry, and scanning and transmission electron microscopy data characterize the detailed microstructural and chemical changes that occurred along the Punchbowl fault and address the importance of mineral transformations, rock fabric, and fluid activity on the hydromechanical behavior of the fault zone. On the basis of bulk and clay fraction X-ray analyses, the fault rocks have mineral assemblages that are distinct from the protolith, indicating that mineral reactions have occurred. However, microstructural observations show that many of the changes in mineral assemblages occurred after the cessation of faulting. Moreover, measures of phyllosilicate fabrics across the Punchbowl fault from X-ray goniometry are uniformly weak, and so phyllosilicate-induced permeability anisotropy in the zone was small. This absence of fabrics limits the effectiveness of fluid focusing along the fault, although some anisotropy may have been imparted by compartmentalization of the fault zone by occasional clay shears. Together these observations indicate that neither clay growth nor fabric-imposed fluid focusing appeared to have played a significant role during strike-slip faulting and cataclasite formation, which suggests that fault-weakening mechanisms that rely on clay growth and elevated fluid pressure cannot be applied to the Punchbowl fault.