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Basement outcrops are rare in northeastern Mexico, but the effect of older basement structures on the current structural plan is clear. Two large structures, the San Marcos fault and the northern edge of the Sierra Madre Oriental, are oriented roughly east-west, and have been postulated as sites of the Mojave-Sonora megashear. Both structures were probably active in the late Jurassic. The San Marcos fault continued to be active into the Cretaceous, while the northern edge of the Sierra Madre Oriental became inactive. Both structures were strongly reactivated during the Laramide orogeny. Potential fields data integrated with regional geologic mapping suggests a series of horsts and grabens were created during Middle-Late Jurassic rifting of the basement, but that relative displacements are far less than those proposed for the Mojave-Sonora megashear system. The northern edge of the Sierra Madre Oriental, with its large left-step along the Monterrey salient, is kinematically more compatible with the postulated left-lateral offset along the Mojave-Sonora megashear, while the San Marcos fault has numerous right-stepping offsets, which is kinematically incom-patible with the Mojave-Sonora megashear hypothesis. In general, however, these data do not support the concept of a large-offset left-lateral megashear in northeastern Mexico.

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