THE ACATLÁN COMPLEX OF SOUTHERN MEXICO: ALLEGHANIAN OVERPRINT OF RHEIC OCEAN VESTIGES

The Acatlán Complex of southern Mexico has traditionally been considered a southerly extension of the Appalachian Orogen. The complex comprises an assemblage of multiply deformed meta-sedimentary units, locally eclogitic mafic-ultramafic suites, and K-feldspar megacrystic granitoid bodies. The Acatlán Complex is tectonically juxtaposed to the east against ca. 1 Ga granulite-facies gneisses of the Oaxaca Complex, the Early Paleozoic cover of which contains Gondwanan fauna. Previous geochronology suggested an Early Paleozoic history for much of the Acatlán Complex with Ordovician-Silurian (Acatecan), Devonian (Mixtecan), and Permian deformation and plutonic events – similar to the Taconian-Acadian-Alleghanian, Iapetus-related, history of the southern Appalachian orogen.

Our new geochronologic investigations have challenged this view. At least part of the Cosoltepec Formation is now constrained to the Carboniferous rather than pre-Late Ordovician. The depositional age of the Tecomate Formation has been shown to be Early Permian rather than Siluro-Devonian. High-grade metamorphism of the Piaxtla Group is now known to be Carboniferous rather than Late Ordovician-Early Silurian. The crystallization of both the K-feldspar megacrystic granites (including those previously thought to be Late Devonian) and the type mafic unit (Xayacatlán Formation) of the Piaxtla Group have been shown to be Late Ordovician. The development of the Magdalena Migmatite, and the crystallization and metamorphism of mafic-ultramafic lenses in the Chazumba Formation record a Jurassic thermal pulse related to the opening of the Gulf of Mexico.

These new data support the proposition that the Acatlán Complex is not a direct relative of the southern Appalachians, which mainly records the history of the Iapetus Ocean, but instead its geology records the history of the Rheic Ocean and the final amalgamation of Pangea.