PERMO-CARBONIFEROUS DEFORMATIONAL EVENTS IN THE XAYACATLÁN TYPE AREA, SOUTHERN MEXICO ASSOCIATED WITH MISSISSIPPIAN EXTRUSION AND MIDDLE PERMIAN TRANSTENSIONAL ARC DEVELOPMENT ON THE WESTERN MARGIN OF PANGEA

The Xayacatlán type area, eastern Mixteca terrane, southern Mexico, consists of four N-S fault blocks (from west to east): (i) Tizac block composed of the Middle Ordovician Huerta Unit: greenschist facies, interbedded psammites and pelites; (ii) Bravo block consisting of the Neoproterozoic-Upper Ordovician Amate Unit greenschist-amphibolite facies arkoses and psammites (Amate Unit: intruded by the Xayacatlán dike swarm (442±1 Ma) with fault slices of the Permian Tecomate Formation; (iii) Estaca block made up of the Mississippian Salada Unit over which the Permian Tecomate Formation has been thrust: the Salada unit is composed of greenschist facies psammites, pelites and thin tholeiitic mafic slices, the Permian Tecomate Formation consists of low grade clastic rocks and marbles; and (iv) the Tonahuixtla block containing the Tecomate formation and Totoltepec intrusion. These rocks record two penetrative tectonothermal events. The first event produced a penetrative cleavage under greenschist facies conditions that affected the Neoproterozoic-Amate, Huerta, and Salada units. Muscovite defining this cleavage yielded a ⁴⁰Ar/³⁹Ar plateau age of 324 ± 2 Ma. This, together with the 352 ± 3 Ma age of the youngest detrital zircon in the Salada Unit brackets the time of deformation within the Middle Mississippian. This cleavage is contemporaneous with deformation elsewhere in the Acatlán Complex that has been related to extrusion of high-pressure rocks. The second event produced two sets of Middle Permian fabrics (N-trending, subhorizontal sheath folds associated with a phyllitic cleavage, S-vergent thrusts, N-S, vertical, dextral shear zones, folded by upright folds) under a dextral transtensional regime. ⁴⁰Ar/³⁹Ar analysis of a sericitic whole-rock phyllite yielded a plateau age of 263 ± 3 Ma, which is inferred to date the cleavage development. The Middle Permian age of the dextral transtensional deformation in the Tecomate formation is broadly synchronous with the development of a Permian magmatic arc that extends along the backbone of Mexico and suggests that the arc formed in a dextral transtensional environment that may have been due to oblique convergence between the paleo-Pacific and western Pangea.