To understand the frictional behavior of natural faults at seismic slip rates, high-speed rotary shear experiments were conducted on disaggregated ultracataclasite from the Punchbowl fault. The experimental gouge layers were sheared at normal stresses of 0.2–1.3 MPa and velocities of 0.1–1.3 m/s to total displacements of 1.3–84 m. We employ thermomechanical FEM models and microstructural observations to consider spatial and temporal variation of normal stress and temperature in the samples and understand microprocesses. Four distinct gouge units form during shear. A slightly sheared starting material (Unit 1) and a strongly sheared and foliated gouge (Unit 2) are produced when frictional heating is insignificant and the coefficient of sliding friction is 0.4–0.6. A random fabric gouge with rounded prophyroclasts (Unit 3) and an extremely fine, microfoliated layer (Unit 4) develop when significant frictional heating occurs at greater velocity and normal stress, and the coefficient of sliding friction drops to approximately 0.2. Unit 3 forms at the critical temperature for vaporization of water and is associated with localization of slip to Unit 4 and elevation of temperature. The critical displacement for dynamic weakening in the rotary configuration can be understood as a consequence of the progressive inward migration of the friction-generated thermal front and the weaker localized slip surface and associated fluidized zone.