THE GULF OF MEXICO: A LATE JURASSIC BACK-ARC BASIN

The origin of the Gulf of Mexico (GoM) is not well understood. Most investigators infer that it formed as the westernmost arm of Tethys. We propose an alternate interpretation, that the GoM opened during Late Jurassic time as a short-lived back-arc basin (BAB) associated with east dipping subduction of Pacific seafloor beneath Mexico. This interpretation explains two GoM enigmas: 1) Why was the GoM opening pole (79-84°W, 23-30°N) so different from that of the Central Atlantic (15-18°W, 65-67°N)? and 2) Why was GoM opening so short-lived (~165-145 Ma), when there was no collision or other obvious reason for seafloor spreading to stop? BAB opening is generally around a local pole of rotation and continues for a few 10's of Ma. GoM BAB opening followed the Permo-Triassic (287-232 Ma) East Mexico Arc and accompanied activity of the Early Mesozoic (225-155 Ma) Nazas Arc. The GoM BAB hypothesis also sheds light on the relationship between the GoM and a narrower rift system to the west, the Borderland Rift System (BRS). The BRS, which can be traced along the US-Mexico border into the 150-145 Ma Independence Dike Swarm of eastern California, is clearly associated with the Jurassic-Cretaceous Cordilleran arc and widens eastwards to merge with the GoM. Late Jurassic rifting in the BRS was succeeded by thermotectonic subsidence through Early Cretaceous time. In addition, the segmentation of the transitional crust beneath the northern GoM into a magmatically robust segment beneath the Texas coast and a stretched margin beneath Louisiana is also consistent with the behavior of a BAB: igneous activity is most prolific nearest the arc and diminishes with distance from the trench. A BAB interpretation for the GoM is compatible with faunal evidence documenting transgression into the GoM from the Pacific rather than the Atlantic realm. A possible objection is that the spreading ridge was oriented at high angles to the Nazas arc trend, whereas modern BAB spreading ridges generally parallel the associated arc. There are some exceptions: spreading ridges associated with the Miocene Sea of Japan and Andaman Sea BABs trend perpendicular to the associated arc. Such geometries reflect the presence of extensional stresses that are not orthogonal to the subduction zone, a situation that also existed in the GoM region during Late Jurassic time.