POLYPHASE DEFORMATION IN THE NORTHERN ACATLAN COMPLEX, SOUTHERN MEXICO: A RECORD OF CLOSURE OF THE RHEIC OCEAN FOLLOWED BY CONVERGENCE ON THE PACIFIC MARGIN OF PANGEA

Two major tectonothermal events have been identified in basement rocks of the Paleozoic Acatlán Complex of southern Mexico, which are believed to record: (i) closure of the Rheic Ocean in the Mississippian (F₁ and F₂) that produced an accretionary prism, and (ii) convergence along the paleo-Pacific margin of Pangea in the Early Permian (F₃), which produced N-S steeply dipping shear zones. In the northern Acatlán Complex blueschist and amphibolite facies rocks of continental rift and oceanic affinity (Piaxtla Suite) are in tectonic contact with greenschist facies siliciclastics (Petlalcingo Lithodeme) interpreted as continental rise deposits. Both units record three sets of penetrative structures: (i) bedding-parallel foliation (S₁) accompanied by blueschist/amphibolite facies (Piaxtla Suite) and greenschist facies (Petlalcingo Lithodeme); (ii) west-dipping, spaced crenulation cleavage (S₂) axial planar to north-south folds in S₁ under similar metamorphic conditions; and (iii) greenschist facies structures consisting of south and east-dipping crenulation cleavage (S₃) axial planar to south-vergent sheath folds (F₃) in the Piaxtla Suite, and a NW-dipping crenulation/solution cleavage axial planar to north-south, east-vergent, upright folds in the Petlalcingo Group. These structures are deformed by kink bands (F₄) that plunge NE and SW in units of the Piaxtla Suite of unknown age.