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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]

Potential field data along the Texas portion of the Gulf of Mexico indicate a large-amplitude coast-parallel magnetic maximum and a smaller Bouguer gravity high. Models constrained by seismic-refraction data indicate that these maxima manifest a deeply buried volcanic rifted passive margin or other magnetic high in the outer transitional crust. Buried 12-15 km, the source is 220 km wide, similar to the Vøring Plateau in Norway and the U.S. East Coast. This margin, which formed during the opening of the Gulf of Mexico, differs in origin from the transform boundary of the northeast Mexico margin (Tehuantepec transform), and we infer a Jurassic triple junction related to the Borderland rift system, which is traceable as far as southeast California. [ABSTRACT FROM AUTHOR]

We use U-Pb ages for 2655 individual detrital zircon grains in 30 samples of Upper Triassic (Carnian-Norian) sandstones of the southwestern USA (our data) and northern Mexico (other data) to infer regional Late Triassic provenance relations and tectonic features, which included a prerift uplift in Texas precursory to Jurassic opening of the Gulf of Mexico. Detrital zircons in sandstones from central paleorivers of the Chinle-Dockum fluvial system on the High Plains (USA) and Colorado Plateau, and from the marine terminus of the fl uvial system along the eastern flank of the Auld Lang Syne backarc basin in the Great Basin, reflect regional dispersal of sediment from the Ouachita orogen and adjacent Mesoproterozoic basement exposed on the northern flank of the rift uplift. The composite grain population of Ouachita-derived sands displays dominant U-Pb age peaks at 1100-1050 Ma (Grenvillian) and subordinate Neoproterozoic (630-550 Ma) and Paleozoic (475-400 Ma) age peaks inferred to document recycling of sand grains from the Ouachita system. The Ouachita detritus was supplemented by contributions from Mesoproterozoic basement rocks of southwest Laurentia (compound 1805-1655 Ma and unitary 1440 Ma age peaks) and from Permian-Triassic arc assemblages of northeastern Mexico (composite 285-215 Ma age peak). Master Chinle-Dockum paleodrainages extended east-southeast-west-northwest for 2000 km from Texas to Nevada, fl owing along the axis of a backarc trough formed by dynamic subsidence behind the Cordilleran magmatic arc, from headwaters in the Texas prerift uplift to the sediment trap of the backarc marine basin where the Auld Lang Syne Group accumulated. Sandstones from peripheries of the linked fluvial and marine depositional systems contain contrasting populations of detrital zircons derived either from the relict Amarillo-Wichita uplift, which fed abundant Cambrian zircons derived from an uplifted aulacogen-floor igneous assemblage, to selected basal units of the fluvial system, or from the Cordilleran arc assemblage to the south and west. The youngest Dockum sample from the Ouachita foreland on the High Plains contains abundant Devonian zircons probably derived from the Ouachita metamorphic core zone now present only in the Texas subsurface. Sandstones along the southern flank of the Chinle backarc basin on the southern Colorado Plateau contain populations of detrital zircons similar to those in sandstones of the Antimonio- Barranca forearc basin (Sonora) that was farther south beyond the arc axis. Sediment transported longitudinally from the Texas uplift to the El Alamar paleoriver and Potosí subsea fan of northeastern Mexico contains populations of detrital zircons similar but not identical to those in Chinle-Dockum fluvial strata of the High Plains shed transversely from the rift uplift. [ABSTRACT FROM AUTHOR]