Paleostress-field analysis by en-echelon veins in the Atokura Formation, Kanto Mountains, central Japan

En-echelon veins hosted by sandstone and mudstone of the Atokura Formation were examined to clarify the paleostress field in that formation which constitutes the Atokura Nappe in a northern part of the Kanto Mountains. The veins are morphologically classified into two groups:1) a similar fold type with smoothly curved limbs, the vein tips pinch out and are not linked to any other fractures (type A) and, 2) a chevron fold type, the vein tips linked to joints formed before the vein opening (type B). The two types show a different texture and assemblage of infilling minerals. The type A consists mostly of fibrous to polygonal quartz and chlorite with lesser amounts of calcite and albite, whereas the type B of polygonal to prismatic calcite and quartz with lesser amounts of plagioclase and chlorite. Regardless of the vein type, morphology of the en-echelon sigmoidal veins is represented by a similar fold, but that of bridges is close to a parallel fold. The presence of undulatory extinction and subgrains in the fibrous quartz and pressure-solution cleavage in the bridges, which are considered to have simultaneously formed, indicates that the en-echelon veins were formed in a brittle-ductile shear zone. According to the paleostress analysis with conjugate sets of type-A vein arrays, the maximum (σ1) principal stress axis is inferred to have been oriented in the NW-SE to NNE-SSW direction and horizontal, intermediate (σ2) principal stress axis vertical, and minimum (σ3) principal stress axis in the NE-SW to WNW-ESE direction and horizontal. The veining could have occurred during the formation of E-W trending open upright folds of the Upper Cretaceous Atokura Formation.