Flow patterns during exhumation of the Sambagawa metamorphic rocks, SW Japan, caused by brittle-ductile, arc-parallel extension

Abstract

Abstract Mesoscopic and microscopic structural analyses of the high-pressure/temperature Sambagawa metamorphic rocks (accretion complexes), SW Japan, have been carried out. Deformation characterized by extreme layer-normal thinning and nearly arc-parallel stretching occurred during exhumation in the Late Cretaceous. Asymmetric quartz c -axis fabrics and orientation of shear band cleavages reveal a pervasive top-to-the-west sense of shear in the Sambagawa metamorphic rocks during the exhumation stage. The 3D strain geometries, inferred from quartz c -axis fabric patterns, vary from plane strain to flattening across the metamorphic belt. We hypothesize that the data are most reasonably explained by a model of counter-flow in the subduction channel. The counter-flow was induced by a left-lateral oblique subduction of the oceanic (Izanagi) plate, which was strongly coupled with the subducting sediments. The 3D strain geometries suggest that the counter-flow (i.e. simple shear in the model) must have been accompanied by some arc-normal ‘press’ component. The mode of deformation changed from ductile to brittle arc-parallel extension, when the rocks were elevated and cooled below the temperature condition for the brittle-ductile transition of quartz ( c. /it 300 °C). The normal faulting (i.e. brittle extension) at subgreenschist conditions was often accompanied by the precipitation of actinolite. This change in deformation mechanism with decreasing temperature is recorded by a conjugate set of normal faults found in the oligoclase-biotite zone in the study area, for which the palaeostress directions conform to the ductile strain geometries.