PENNSYLVANIAN COLLISION AND ITS CONTROL ON MESOZOIC RIFTING AROUND THE NORTHERN GULF OF MEXICO

Rifting of Laurentia from Greater Gondwana in Cambrian time was associated with a magmatic triple junction centered beneath modern Dallas, one arm of which is preserved as the S. Oklahoma Aulacogen. The position of this hotspot and the trend of its two successful arms (which opened to form the Iapetus/Rheic ocean in Early Paleozoic time) carved an irregular southern margin of Laurentia, which has since controlled the tectonic evolution of the region. This re-entrant margin was modified by the southward subduction of Laurentia beneath N. Gondwana, juxtaposing strong Laurentian lithosphere craton in the north with weak (hydrated and partially molten) arc lithosphere of N. Gondwana to the south. This asymmetry in lithospheric strength persisted for ~100 m.y. and controlled Gulf of Mexico rifting beginning in Triassic and continuing through Jurassic time. The Ouachita-Marathon foldbelt marks the northern limit of rifting, which was restricted to the hot, wet, and weak Gondwanan arc and forearc and did not affect cold, strong Laurentia. The only expression of Mesozoic rifting was Triassic uplift in Texas, which shed large volumes of clastic sediments westwards, now preserved as Late Triassic Dockum and Chinle Groups. Even Pennsylvanian foreland basins and Ancestral Rockies faults intersecting the Ouachita-Marathon orogen were not rejuvenated by Triassic-Jurassic extension. Extension in weak Gondwanan lithosphere resulted in a broad rift zone that now buried beneath Mesozoic and younger sediments. Buried fragments of thicker continental crust – the Sabine and Monroe uplifts, the Wiggins Arch, and Florida – are abandoned fragments of Gondwanan arc and forearc lithosphere. Because they are buried, we know little about these "Gondwana orphans" and also the deeper basins associated with the buried broad region of distributed extension. It will require joint efforts by academia, industry, and government to probe and better understand this region.