NEW UNDERSTANDINGS OF THE MESOZOIC EVOLUTION OF THE GULF OF MEXICO

Recent studies reveal the following 4 important geotectonic constraints about of the Gulf of Mexico (GoM). 1) Opening was preceded by Late Triassic uplift and erosion. U-Pb ages for 2655 detrital zircons from Upper Triassic (Carnian–Norian) sandstones of the SW USA Chinle-Dockum fluvial system reflect sediment shed from the Ouachita orogen and Mesoproterozoic basement, probably from central Texas (Dickinson et al. 2010). Chinle-Dockum paleodrainages flowed west for 2000 km from Texas to Nevada. Texas prerift uplift had an areal extent of 175 × 10³ km² (300 × 600 km), implying average erosion of 4 km to yield the estimated sediment volume of 700 × 10³ km³. 2) The Late Triassic uplift was adjacent to what we interpret as a volcanic rifted margin (Mickus et al., 2009) potential field data along the Texas portion of the GoM indicate a large-amplitude coast-parallel magnetic high and a smaller Bouguer gravity high, consistent with a deeply buried volcanic rifted passive margin in the outer transitional crust. Buried beneath 12–15 km of sediment, the source is 220 km wide, similar to the Vøring Plateau in Norway and the U.S. East Coast. 3) Three salt diapirs from S. Louisiana exhume alkalic igneous rocks; these domes overlie an isolated magnetic high, which may mark a buried volcanic province. Magmatic Ti-rich biotite and kaersutite yield ⁴⁰Ar/³⁹Ar ages of 158.6 ± 0.2 and 160.1 ± 0.7 Ma (Stern et al. 2011). These ages correlate with stratigraphic evidence for GoM opening. The samples are strongly enriched in incompatible trace elements, indicating the igneous rocks are low-degree melts of metasomatized upper mantle. This information supports the idea that crust beneath S Louisiana formed as a magma-starved rifted margin on the N GoM flank ~160 Ma. 4) GoM opened behind the 232–150 Ma Nazas arc over an east-dipping subduction zone beginning ca. 165 Ma, and thus formed as a backarc basin (Stern and Dickinson, 2010). The GoM=BAB explanation illuminates the relationship between the GoM and Border rift system, which can be traced from the GoM >2000 km along the U.S.–Mexico border into E California. This geometry explains the segmentation of the northern GoM into a magmatically robust segment beneath the Texas coast and a stretched margin beneath Louisiana and is consistent with BAB behavior: igneous activity is most prolific nearest the arc and diminishes with distance from the trench.