The origin of strain patterns resulting from contemporaneous deformation and metamorphism in the Sambagawa metamorphic belt

Abstract

Abstract The magnitudes of plastic strains of 104 metacherts were determined from the deformed shape of initially spherical radiolarians in the Sambagawa high‐P type metamorphic belt of Western Shikoku, Japan. The strain magnitude increases with increasing metamorphic temperature from several per cent to 250%. The a2/a3 ratio of strain ellipsoids in the higher metamorphic grades decreases with increasing metamorphic grade while the a1/a2 ratio increases rapidly. The long axis of the strain ellipsoid for every grade is nearly parallel to the length of the metamorphic belt, suggesting that the flow direction of the synmetamorphic deformation was uniform along the belt. A map of strain zones within the Sambagawa high‐P type metamorphic belt reveals that the metamorphic belt underwent a progressive bulk inhomogeneous shear deformation and that the high‐grade zones represent a deep‐seated boundary shear zone on the accretionary wedge between a subducting oceanic plate and the immobile rigid continental plate.