Oblique Diapirism of the Yakushima Granite in the Ryukyu Arc, Japan

Abstract

Field observations and structural analyses of the orientations of ductile flow fabrics and brittle fractures are used to infer diapiric rise of the Yakushima pluton into epizonal Paleogene sediments of an accretionary prism in Miocene times. Inside the pluton, preferred orientations of orthoclase megacrysts define a weak primary flow fabric. Shape and intensity of fabric ellipsoids derived from patterns in the alignment of the megacrysts reveal a circulation cell within the domal structure of the Yakushima granite. The asymmetry of this circulation pattern suggests that the pluton rose obliquely upward to the southeast, toward the Ryukyu trench. Fold axes in the ductile strain aureole of the pluton are distorted into conformity with the pluton’s shape in the direction of magma ascent Late aphte sheets, whose orientations record the local paleostress pattern, were emplaced into a concentric fracture system in and around the pluton. The aphtes indicate the lifting, and tangential spreading with concentrically disposed intermediate stress axes, of a brittle roof above a buoyant ellipsoidal body of residual magma. Comparable with strain patterns in theoretical models of a viscous sphere rising in ductile surroundings, the deformation pattern in and around the pluton is attributed to the final upsurge of oblique diapiric emplacement.