990 and 1100 Ma Grenvillian tectonothermal events in the northern Oaxacan Complex, southern Mexico:: roots of an orogen

Inliers of similar to 1.0-1.3 Ga rocks occur throughout Mexico and form the basement of the Oaxaquia microcontinent. In the northern part of the largest inlier in southern Mexico, rocks of the Oaxacan Complex consist of the following structural sequence of units (from bottom to top), which protolith ages are: (1) Huitzo unit: a 1012 +/- 12 Ma anorthosite-mangerite-charnockite-granite (AMCG) suite; (2) El Catrin unit: greater than or equal to 1350 Ma orthogneiss migmatized at 1106 +/- 6 Ma; and (3) El Marquez unit: greater than or equal to1140 Ma para- and orthogneisses. These rocks were affected by two major tectonothermal events that are dated using U-Pb isotopic analyses of zircon: (a) the 1106 +/- 6 Ma Olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds; and (b) the 1004-978 +/- 3 Ma Zapotecan event produced at least two sets of structures: (ZI) recumbent, isoclinal, Class 1C/3 folds with gently NW-plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism; and (Z2) tight, upright, subhorizontal VVNW- to NNE-trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions. Cooling through 500 degreesC at 977 +/- 12 Ma is documented by 40Ar/39Ar analyses of hornblende. Fold mechanisms operating in the northern Oaxacan Complex under Zapotecan granulite facies metamorphism include flexural and tangential-longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes. Subsequent Phanerozoic deformation includes thrusting and upright folding under lower-grade metamorphic conditions. The Zapotecan event is widespread throughout Oaxaquia, and took crustal rocks to a depth of similar to 25-30 km by orogenic crustal thickening, and is here designated as Zapotecan Orogeny. Modem analogues for Zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens. Contemporaneous tectonothermal events took place throughout Oaxaquia, and in various parts of the Genvillian orogen in Laurentia and Amazonia. (C) 2003 Elsevier Science B.V. All rights reserved.