It has generally been assumed that the last major compressive deformation in the Sierra Madre Oriental (Mexico) took place during the Laramide orogenesis (Upper Cretaceous – Early Eocene). We have studied the N120° Rio Bravo fault zone probably inherited from the Jurassic opening of the gulf of Mexico. This fault zone is located along the international boundary between United States and Mexico. We demonstrate that it was active mainly during the Oligocene. In the Ojinaga area (Chihuahua), the Sierra Madre Occidental, Oligocene volcanic sequences overlying conformably the sedimentary Upper Cretaceous sequence, are both tightly folded before 30 Ma. We think this folding is associated with motion of a major left-lateral fault, the Rio Bravo left lateral fault. These left-lateral fault system affects also the Sabinas fold-belt and extends below the Burgos bassin. This deformation is also imaged by gravimetric data and the offsets the Palaeocene-Eocene oil fields that are displaced left laterally. We propose that during the Oligocene, this ~1000 km long left-lateral shear zone that might be called the Rio Bravo fault was active during the Oligocene with a total offset of 40–60 km.

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