Phanerozoic Structures in the Grenvillian Northern Oaxacan Complex, Southern Mexico: Result of Thick-Skinned Tectonics

Abstract

Shear zones and upright folds affecting the northern Oaxacan Complex are documented as follows: (1) SE-directed shearing dated at 479 ± 4 Ma (40Ar/39Ar biotite laser total fusion age); (2) E-directed thrusting dated at 247 ± 3 Ma (40Ar/39Ar biotite plateau age); (3) NW to NNW-trending, steeply inclined folds of mid-Triassic-Jurassic age bracketed between events (2) and (4); and (4) NNW-trending vertical shearing dated at 141 ± 9 Ma (40Ar/39Ar biotite laser total fusion age). Inasmuch as the northern Oaxacan Complex is inferred to have been exposed to conditions well above the ~300°C closure temperature for argon in biotite since it was exhumed at ~710-760 Ma, it is inferred that these ages record thermal re-equilibration associated with hot fluids flowing along active shear zones. These structural events correspond respectively with: (1) the onset of Paleozoic deposition in the Tremadocian, presumably associated with SE-directed, listric normal faulting/shearing; (2) E-vergent thrusting associated with the development of an arc along the length of Mexico; (3) folding in sympathy with the dextral shear during opening of the Gulf of Mexico; and (4) Jurassic-Cretaceous boundary, normal fault readjustments. These data indicate that the northern Oaxacan Complex was involved in thick-skinned tectonics during most Phanerozoic episodes of deformation.