Ductile shear zone of the Late Paleozoic at eastern México: The Teziutlán Metamorphic Complex as a Middle America piece for reconstructing Pangea

Luis Javier Gutiérrez Trejo, Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México
Mario Alfredo Ramos Arias, Instituto de Geología, Universidad Nacional Autónoma de México
Edgar Ángeles Moreno, Centro de Geociencias, Universidad Nacional Autónoma de México

The Teziutlán Metamorphic Complex (TMC) in eastern-central México crops out as isolated tectonic windows adjacent to eastern crystalline basement of Oaxaquia. Consist of an assemblage of metasedimentary and metaigneous rocks of the Late Paleozoic, in which it's observed polyphase of deformation in low to medium metamorphic grade that preserves gneissic, schistose and mylonitic textures. The age of deformation is constrained between the minimum age of deposition at Mississippian-Permian in protoliths as well the tonalitic suite intrusions and the overlayed slightly deformed red beds with detritus derived from a Permo-Triassic arc.

The TMC structurally consist of a pile up by mylonitic shearing, in which is possible to distinguish four lithodemic units: i) La Soledad Lithodeme, represent the lowermost unit composed by mylonitc quartz feldspar gneisses, metagabbros interfingering with mica schist; ii) Chicuaco Lithodeme, constituted by meta rhyolite, meta granitic-tonalitic gneisses and mica chlorite schist; iii) Cozolexco Lithodeme is a mafic association composed by meta basalts intercalated with meta gabbros and tonalites; and iv) the uppermost unit, El Mirador Lithodeme, which lithologic association consist of white mica schist interlayered with meta volcanosediments.

Foliation S1 is observed like lepidoblastic and nematoblastic texture with Chl+Msc+Act, also quartszfeldspatic granoblastic domains in gneissic bands. S1 foliation is closed recumbent refolded to F2 and transposed in axial planar foliation to S2/1, both with vergence at 220° SW /40°. Altogether with kinematic indicators given by feldspar porphyroclast and pyrite shadow pressures with σ and δ figures, mica fish and s-c´ fabric which are produced by a subhorizontal shear zone with top to the SW.

On this study, K-Ar and Ar-Ar ages were extracted from white mica and amphibole immerse in the foliated and mylonitic fabric, which indicate that cooling ages occurred between 310±4 Ma and 270±3 Ma. Such range of time reflects a tectonothermal event in the Permian, which is in good agreement with the diachronic and progressive approaching between peri-Gondwanan and southeastern Laurentia terranes at the last stages of Pangea assemblage; regarding that TMC represent a critical piercing point for understanding assembly of Pangea in the Middle America segment.


Development of tectonic microstructures