Sierra Mazatán in northwest Mexico is the southernmost metamorphic core complex in the North American Cordillera. New geologic, structural, and 40Ar/39Ar thermochronologic data demonstrate that the core complex detachment fault accommodated 15–35 km of slip at a rate of 3.3–7.7 mm/yr between 20.5 and 16 Ma. These data also suggest that the lower plate was tilted eastward 20°–50° since 20 Ma, indicating that the detachment fault initially dipped 30°–60°. Rapid slip on the fault occurred concurrently with subduction in Sonora and thus was not related to the ca. 12 Ma plate boundary change at this latitude or the late Miocene opening of the Gulf of California. Rapid extension at Sierra Mazatán was synchronous with core complexes along the length of the Cordillera, suggesting a distinct 20–15 Ma core complex event, the fundamental causes of which remain a mystery.