RECORD OF THE CLOSURE OF THE RHEIC OCEAN IN THE ACATLÁN COMPLEX OF SOUTHERN MEXICO

The Paleozoic Acatlán Complex of southern Mexico comprises a low-grade polydeformed siliciclastic sequence (Petlalcingo Group), interpreted as either a passive margin or trench and forearc deposit, part of which contains detrital zircons as young as 375-355 Ma. These rocks are tectonically juxtaposed against high-grade metasediments, locally eclogitic mafic-ultramafic rocks and granitoids (Piaxtla Group) that are inferred to represent obducted oceanic and/or continental lithosphere, and which have yielded 480-345 Ma ages (new U-Pb zircon data). Both units were deformed in the Carboniferous prior to being unconformably overlain by clastic metasediments, limestones and volcanics (coeval with subvolvanic arc-related plutons) of the Early Permian Tecomate Formation. Subsequent, late Early Permian deformation involving north-south dextral transpression and south-vergent thrusting was synchronous with tectonic juxtapositioning of the complex against Grenville-age basement (Oaxacan complex), the platformal cover of which contains Tremadocian fossils of Gondwanan affinity and Carboniferous fauna of Midcontinent affinity. The Leonardian Matzitzi Formation that unconformably overlies the complex postdates its penetrative deformation. One school of thought has considered the Acatlan Complex to be a vestige of the Iapetus Ocean. However, new age data constrain deformation of the complex to the Late Paleozoic and suggest it instead record the closure of the younger Rheic Ocean during the collision of Gondwana and southern Laurentia (Ouachita orogeny). This is consistent with the available kinematics in which northward motion of Mexico is required for the final assembly of Pangea.