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Any basin with seafloor spreading that forms over an active subduction zone is a backarc basin (BAB). The Gulf of Mexico (GoM) opened behind the 232-150 Ma Nazas arc over an east-dipping subduction zone in Late Jurassic time, beginning ca. 165 Ma, and thus is a BAB. The hypothesis that the Gulf of Mexico formed as a backarc basin explains two 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 the GoM opening so short-lived (ca. 165-142 Ma), when there was no collision or other obvious reason for seafloor spreading to stop? The GoM BAB hypothesis also illuminates the relationship between the GoM and the Border rift system, which can be traced from the GoM near the mouth of the Rio Grande >2000 km along the U.S.-Mexico border into the Independence Dike Swarm of eastern California. Late Jurassic rifting in the Border rift system 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 BAB behavior: igneous activity is most prolifi c nearest the arc and diminishes with distance from the trench. A possible objection to the GoM BAB hypothesis is that the spreading ridge was oriented at high angles to the Nazas arc trend, whereas modern oceanic BAB spreading ridges generally parallel the associated arc. Continental BABs like the GoM develop spreading ridge orientations that are often at high angles to the associated convergent margin; for example, 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. [ABSTRACT FROM AUTHOR]