THE ~1 GA OAXACAN COMPLEX OF SOUTHERN MEXICO: AN ARC TERRANE ON THE AMAZONIAN MARGIN

The Oaxacan Complex of southern Mexico consists of paragneisses and within-plate, rift-related orthogneisses (anorthosite, charnockite, metagabbro, and metagranite) intruded at >1140 Ma and 1012±12 Ma that were involved in two polyphase tectonothermal events: the ~1100 Ma Olmecan event and the 1004±3 to 979±3 Ma Zapotecan event. The Olmecan event produced migmatites during polyphase deformation and is synchronous with the intrusion of ~1117 Ma, rift-related granite. The Zapotecan event involved polyphase deformation under granulite facies metamorphic conditions at 700°-825°C and 7.2-8.2 kb under restricted PH2O conditions, which are inferred to be the result of underthrusting of the northern Oaxacan Complex. Between 979 Ma and 945 Ma, the northern Oaxacan Complex cooled through ~400°C at a rate of ~8°C/my, by which time it had risen through a depth of ~10 km. At 917 ± 6 Ma, the northern Oaxacan Complex was intruded by a shallow-level, calc-alkaline granitoid pluton that has been related to subduction along the Amazonia-Oaxaquia margin. After 945 Ma, the second stage of cooling took place at the rate of ~2°C/my: extrapolating this cooling rate to 0°C indicates that the rocks reached the surface at ~750 Ma. Of the various mechanisms that might explain the exhumation of the Oaxacan Complex within a convergent tectonic setting, tectonic switching between shallow and steep subduction appears to be consistent with the observations: (i) 1012 ± 12 Ma intrusion of a rift-related, AMCG (anorthosite-mangerite-charnockite-granite) suite at shallow levels in the backarc region related to steep subduction; (ii) ~1004-978 Ma Zapotecan orogenesis during which Oaxaquia was thrust beneath an arc or continent to 27-30 km depth during flat slab subduction; and (iii) rapid exhumation to 15 km depth and intrusion of the Etla pluton during backarc rifting related to steep subduction. The second cooling stage could then be related to thermal relaxation of the lithosphere terminating during the breakup of Rodinia. Following exhumation Tremadocian sedimentary rocks that contain Gondwanan shallow-water fauna were deposited unconformably upon the Oaxacan Complex.