ARE THE BORDERLANDS RIFT SYSTEM AND THE GULF OF MEXICO RELATED? GEOPHYSICAL EVIDENCE

The opening of the Gulf of Mexico (GoM) in Late Jurassic time involved development of a volcanic rifted margin (VRM) in Texas and a transform margin (T-COB) in Mexico. The junction between the T-COB and the VRM is also the terminus of an aulacogen (the Borderland Rift system, BRS), and these three elements meet near the present mouth of the Rio Grande at ~120°. The BRS can be traced ~2000 km WNW along the US-Mexico border as far as the Independence Dike Swarm of east-central California. The BRS is best known in the Sabinas basin and farther west, where salt diapirs and surface exposures permit geological scrutiny. Radiometric and stratigraphic ages from these regions indicate that BRS rift-related igneous activity occurred 145-150 Ma. Possible connection of the BRS with the GoM is more difficult because the triple junction is deeply buried beneath sediments, requiring geophysical investigations. The most readily available geophysical data are regional gravity and magnetics. However the gravity and magnetic anomalies within the BRS reflect numerous tectonic events (e.g., Mesoproterozoic crust, Ouachita foldbelt, Permo-Triassic volcanism, Mesozoic transforms and Cenozoic extensional tectonics and magmatism) that have occurred within the region. An isostatic residual gravity anomaly map indicates a series of gravity minima along the BRS in Chihuahua and Coahuila however in Nuevo Leon and Tamaulipas there is a large amplitude gravity maximum that partially overlies the Tamaulipas Arch. However, this anomaly which may be partially due to a Permian-Triassic volcanic arc is mainly northeast of the Tamaulipas Arch within the BRS and may be partially caused by BRS igneous rocks. Magnetic data are more complicated to interpret due mainly to the numerous volcanic rocks of various ages within the BRS. The known basins (Chihuahua Trough, Sabinas basin) are associated with linear magnetic lows and highs that parallel the above basins and extend to the junction of the T-COB and VRM. The geophysical data will need additional analysis including 2-D modeling before any definite conclusions can be made its usefulness in supporting the idea that the BRS developed as a narrow rift associated with the GOM opening.