DEFORMATIONAL HISTORY OF THE COSOLTOPEC AND TECOMATE FORMATIONS, ACATLÁN COMPLEX, SOUTHERN MEXICO

The Cosoltepec and Tecomate formations form part of the Paleozoic Acatlan complex of southern Mexico, a polydeformed sequence of metasedimentary units, granitoids and eclogites, tectonically juxtaposed against Grenville-age basement and overlain by Lower Paleozoic rocks containing fauna of Gondwanan affinity. The older Cosoltepec Fm is a thick siliciclastic unit with a provenance of Grenville age like the neighboring Oaxaca Complex. Overriding the formation is an eclogitic mafic-ultramafic unit (Xayacatlán Fm) interpreted as a westward-obducted ophiolite whose emplacement coincided with intrusion of the mylonitic Esperanza granitoids at ca. 440 Ma. Both formations are interpreted as parts of an accretionary complex. They are overlain by, and infolded with, deformed Devonian volcano-sedimentary rocks (Tecomate Fm), and cut by deformed granitoids dated at ca. 371 Ma (La Noría gt) and ca. 287 Ma (Totoltepec gt). Three deformtional phases affect the Cosoltepec Fm: (1) Inferred tight-isoclinal folds (F_C1) with unknown kinematics that deform bedding and possess an axial planar cleavage (S_C1). (2) Tight-isoclinal, NW- to NE-trending curvilinear (sheath) folds (F_C2) that deform S_C1 and S_C1-parallel quartz veins and possess an axial planar slaty cleavage (S_C2). An associated mineral lineation (L_C2) records N-S dextral shear and N to S thrusting that placed the Totoltepec gt onto the Cosoltepec Fm. (3) NW- to NE-trending inclined-upright open folds (F_C3) with a strong axial planar crenulation cleavage (S_C3). Megascopic F_C3 are responsible for the regional structure. Two phases affect the Tecomate Fm: (1) Tight-isoclinal folds (F_T1) that deform bedding and are associated with a strong mineral lineation (L_T1) recording N to S thrusting. (2) NW- to NE-trending upright open folds (F_T2) that re-fold F_T1 and possess a strong axial planar cleavage (S_T2). These correlate with F_C2 and F_C3, respectively. D₁ is pre-Tecomate and may record obduction at ca. 440 Ma during the collision (Acatecan orogeny) of E. Laurentia and Gondwana. D₂ likely records the collision of Amazonia with S. Laurentia at ca. 287 Ma. D₃ is of uncertain significance but predates the San Miguel gt. dated at ca. 173 Ma.