Abstract: Structure of the San Miguel ultramafic body and its host rock, the Acatlan Complex, southern Mexico: a fragment of the lapetus or Rheic oceans

The San Miguel ultramafic body appears to represent a slice of oceanic lithosphere that was thrust onto a thick psammitic-pelitic sequence (Chazumba Fonnation) during the Paleozoic. These units fonn part of the Acatlan Complex that lies west of the 1 Ga Oaxacan Complex that . is inferred to represent part of the Amazon craton. The structural sequence consists of the following: ( 1) a foliation, SE. defined by mica which appears to be associated with the thrust at the base of the ultramafic body; (2) a micaceous, crenulation cleavage axial planar to the main folds, FM, which are generally isoclinal and recumbent with curvilinear fold axes that appear to represent segments of sheath . folds with long limbs generally trending NNW-SSE SM is strongly developed at the base of the ultramafic body suggesting further thrust movement during this phase of defonnation; (3) late, upright, open-tight, gently N-plunging, N-S (and some conjugate) folds, FL, which vary from cylindrical to conical; (4) moderately NW-dipping, NE-trending kink bands; and (5) variably oriented faults. The outcrop pattern is a mushroom-shaped interference pattern produced by superimposition of FL on FM· The general NNW-SSE orientation of the FM fold axes is probably parallel to the direction of thrusting. The FM and FL structures are cut by granitic sheets that have been dated at) 72 ± I Ma (Sm-Nd garnet-whole rock age) and 175 ± 3 Ma (Rb-Sr muscovitewhole rock age). Several kilometres to the north, structures geometrically correlated with FM show top-to-the-south kinematics during which the 287 ± 2 Ma Totoltepec pluton was thrust over the (?)Devonian Tecomate Formation. The Totoltepec pluton is unconfonnably overlain by Middle Jurassic rocks. This appears to bracket the FM and FL defonnation in the Penno-Triassic, however, elsewhere Mississippian rocks are reported to unconfonnably overlie NS trending folds, suggesting that FM and FL fold are preCarboniferous, post-Devonian(?). Thus, the main and late structures may relate to the Pennian amalgamation of Pangea and Triassic opening of the Gulf of Mexico. Alternatively, they may record the Devonian transpressive collision between Laurentia and Gondwana during the closure of Iapetus, or convergent tectonic events on the margin of Gondwana adjacent to the Rheic ocean. Correlatives of the FE folds farther north do not affect the (?)Devonian Tecomate Fonnation and may be related to the Late Ordovician-Early Silurian Acatecan Orogeny, which is inferred to record either the collision between Laurentia and Gondwana or active tectonics on the southern, Amazonian margin of the Rheic Ocean.