WHY HASN'T THE TEXAS OR LOUISIANA PASSIVE MARGIN TURNED INTO A SUBDUCTION ZONE?

Formation of new subduction zones is a fundamental tectonic process but we are only beginning to understand how this happens. Important in this regard is the Wilson Cycle, whereby 1) rifting of a continent leads to 2) seafloor spreading and development of a passive continental margin, which then 3) collapses to form a new subduction zone that closes the ocean, culminating in 4) continent-continent collision. Passive margin (PM) collapse – the third stage - is key to understanding the Wilson Cycle. Why hasn’t PM collapse happened yet beneath the NW Gulf of Mexico (GoM) PM, which comprises old oceanic lithosphere that has also been pushed down by some of the thickest sediments (10-15 km) on Earth? If this does happen in the future, where is it likely to get started? Continental crust here adjoins Jurassic (~160 Ma) GoM oceanic crust. Magnetic and gravity modeling indicate that different types of transitional crust underlie Texas and Louisiana GoM margins. Transitional crust beneath coastal Texas is a Volcanic Rifted Margin (VRM) whereas that beneath coastal Louisiana comprises thin, serpentine-rich amagmatic transitional crust like that of the Galicia margin. Mickus et al. (2009) infer that Moho beneath mafic lower crust (lower crust with Vp= 7.1–7.7 km/s) lies as deep as 40 km. In contrast to the Texas margin, the Louisiana margin appears to be an amagmatic PM, with10 km or more of overlying sediments overlying perhaps 10 km of serpentine-rich crust. Both sectors of the NW GoM PM are deeply buried beneath sediments; the Texas VRM beneath ~15 km of sediment, making it (along with the Bengal PM) the thickest accumulation of PM sediments anywhere in the world. Thicker Texas crust is hotter (500°C Moho) compared to Louisiana (Moho T <400°C), and thus is weaker. Texas is more favorable than Louisiana for PM collapse because thick VRM crust here could be converted to denser eclogite at slightly higher P and T, but this is less likely to occur beneath Louisiana PM. Reverse faulting of continent over GoM crust should signal the first stage of PM collapse, but no signs of reverse faulting (earthquakes, thrust faults) have appeared. Regional compression might trigger PM collapse, but this occurred in Paleogene time as a result of Laramide in the west and collision of Cuba with Florida in the east without any affect in the NW GoM PM.