DEFORMATIONAL HISTORY OF THE PIAXTLA GROUP WITHIN THE ACATLÁN COMPLEX

The Acatlan Complex lies on the western side of the ~1 Ga Oaxacan Complex and has been interpreted as the vestiges of either the Rheic or Iapetus ocean. It comprises two major interleaved structural sequences: (i) the Petlalcingo Group, a thick metasedimentary sequence; and (ii) the Piaxtla Group, high-grade metasediments, migmatites, granite gneiss and eclogitic mafic boudins, both of which are unconformably overlain by the Late Devonian-Middle Permian Tecomate Formation. The San Francisco de Asis (SFA) slice of the Piaxtla Group records a complex structural history including: (a) eclogite facies metamorphism preserved in tectonic lenses, which has yielded concordant U-Pb zircon age of ~346 Ma; (b) three phases of structures (foliation, folds and stretching lineations) and SW-vergent thrusting contemporaneous with migmatization with leucosomes yielding condordant SHRIMP ages on zircon of ~340 Ma; (c) two crenulation cleavages (orientated south west and east) overprinted by decussate muscovite that has yielded a K-Ar age of 288 ± 14 Ma; and (c) NNW-trending upright folds that produce the map pattern. These data suggest that the SFA slice was subducted followed by obduction dehydration melting, during the Mississippian followed by and transpressional interleaving with the Petlalcingo Group and Tecomate Formation exhumation during the Pennsylvanian-Early Permian.