THE RIO GRANDE FAULT, A MAJOR MID-TERTIARY LEFT-LATERAL SHEAR ZONE

It is generally assumed that the last major compressive deformation in the Sierra Madre Oriental took place during the Laramide orogenesis (Upper Cretaceous – Early Eocene). We have studied the N120° Babia lineament inherited from the Jurassic opening of the Gulf of Mexico and located at the boundary between United States and Mexico. We demonstrate that it was active during Oligocene. In the Ojinaga area (Chihuahua), Oligocene volcanic sequences overlying the Upper Cretaceous are tightly folded, parallel to the N120° trend of the Rio Grande river. Thus the valley is underlined by a major sinistral fault which separates morphologically the Oligo-Miocene Big Bend Volcanic province of Texas from the tightly folded calcareous Mesozoic limestones of Chihuahua (Mexico). To the east, the sinistral fault-system extends below the Burgos basin. It offsets the Paleocene-Eocene oil-fields of roughly 40-60km. We propose that during the Oligocene, the shear controlled the distribution of the Burgos normal faults as demonstrated by their pattern that is compatible with sinistral wrench. Indeed, the Oligocene depocenters are also offset by ~40km. We conclude that a ~900km long shear zone that might be called the Rio Grande Fault was active during mid-Tertiary with a total offset of 40-60km. Its activity should obviously affect the Tertiary depocenters in southwestern Texas and within the Burgos basin. Based on offshore industrial multichannel seismic reflection data, we have evidenced NE-SW grabens offsetted by ~50km on both side of the Rio Grande Delta. This suggests a possible offshore extension of the Rio Grande sinistral fault-system.