Mid-jurassic tectonothermal event superposed on a Paleozoic geological record in the Acatlan Complex of southern Mexico: Hotspot activity during the breakup of Pangea

摘要

U-Pb isotopic analyses of zircon from the lowest structural units of the Acatlan Complex of southern Mexico indicate that Paleozoic tectonothermal events are overprinted by mid-Jurassic (175+/-3 to 171+/-1 Ma), low pressure migmatization (5-6 kb), polyphase deformation, and intrusion of felsic and mafic magmas. Ensuing rapid cooling recorded by, Ar-40/Ar-39 muscovite, biotite and K-feldspar ages is estimated to have taken place at 21+/-3degreesC/my at exhumation rates of 0.6 mm/yr. Such rapid exhumation requires a combination of erosion and tectonic unroofing that is recorded by top-to-the-west kinematic data. Synchronous tectonic unroofing is also recorded 100 km to the east in the adjacent Oaxaca terrane, where top-to-the-north, extensional shear zones occur in Paleozoic strata.This pattern of extension suggests tectonic unroofing in response to domal uplift (radius > 100 km) like that associated with core complexes, slab windows, and hotspots. Most tectonic analyses for the Jurassic place the Acatlan Complex in the forearc region of an arc in Colombia lying 600-800 km inboard of the subduction zone, presumably in response to flat-slab subduction. Modern analogues suggest that flat-slab subduction reflects subduction of young buoyant oceanic lithosphere adjacent to either a mid-oceanic ridge, or a plume. Since core complexes are typical of arc-backarc regions, and slab windows generally produce metamorphic belts, the forearc setting and associated domal uplift suggest a plume to be the most likely cause of this Jurassic tectonothermal pulse in southern Mexico. This plume activity is synchronous with the opening of the Gulf of Mexico during the breakup of Pangea, to which it may have contributed.