Tectonic implications of early Miocene extensional unroofing of the Sierra Mazatán metamorphic core complex, Sonora, Mexico

Sierra Mazatan in northwest Mexico is the southernmost metamorphic core complex in the North American Cordillera. New geologic, structural, and 4 0 Ar/ 3 9 Ar 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 Mazatan 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.