Strain geometries in the Sanbagawa Metamorphic Belt inferred from deformation structures in metabasite

Abstract

Abstract Ductile deformation in the Sanbagawa Metamorphic Belt has been considered to be characterized by uniaxial elongation parallel to the east–west‐trending lineation, based on strain analysis of radiolarian fossils in metachert. However, only limited data were available to test this idea in strongly recrystallized schists. Strain measurements on deformed pillow structures in central Shikoku show uniaxial flattening, whereas pressure shadows around pyrite in the Kanto Mountains indicate constrictional strain. In addition, in the low‐grade part of the Sanbagawa Metamorphic Belt belonging to the Northern Chichibu Belt in the Kanto Mountains, deformed amygdules in pillow lava show flattening strain, which is consistent with the results of strain measurements using radiolarian fossils in the same area. These observations, together with the results of other strain analyses reported so far, indicate that the strain field is not uniform and that the east–west‐trending shearing and stretching were not pervasive in the Sanbagawa Metamorphic Belt.