Role of nappe boundaries in subduction-related regional deformation: spatial variation of meso- and microstructures in the Seba eclogite unit, the Sambagawa belt, SW Japan

The Seba eclogite unit of the Sambagawa high-P/T metamorphic belt has a kilometre-scale synformal structure and its base is the boundary between an overlying eclogite nappe and an underlying non-eclogitic nappe. The formation of the nappe boundary is related to a phase of strong penetrative ductile deformation (DA). Spatial variations of meso- and microstructures of schistose eclogite in the Seba unit reveal that there is (i) a decreasing intensity of DA, and (ii) a time-dependent widening of the DA shear zone, away from the nappe boundary. These features indicate that the nappe boundary behaved as a core zone during the DA phase and, furthermore, that development of the regional deformation, DA, is strongly controlled by the presence of the nappe boundary. A new model is proposed where each phase of regional deformation in subduction-type metamorphic belts is related to ancient activity of a particular nappe boundary. In this model a single phase of deformation may be related to exhumation in one nappe and subduction in an adjacent one.