Deformation facies analysis and tectonic evolution of low-grade schists in the Joetsu region, central Japan
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概要
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The present study was performed to examine the detailed exhumation process of deepsheeted materials in accretionary complexes by analysis of deformed minorstructures recorded in a low-grade schist zone, reconstructed by tracing sporadically distributed lowgrade schist bodies with similar constituent rocks, metamorphism, and deformation history in the Joetsu Belt and the Western Zone of the Ashio Belt belonging to the Paleozoic to Mesozoic systems of the Inner Zone of Southwest Japan. Geometric and kinematic analyses of deformed minor- and microstructures divided the whole deformation process into two deformation phases, i.e., the earlier deformation phase (phase 1 deformation) showing a sinistral shear regime with the deformation mechanism of crystal plasticity and diffusive mass transfer, and the later deformation phase (phase 2 deformation) showing a dextral shear regime with the deformation mechanism of fracture, slip, and cataclasis. Deformation facies analysis showed the temporal and spatial sequences (deformation zoning) of the deformed minorstructures. The former showed that deformed minorstructures formed under conditions of decreasing mean ductility with time in response to rock body exhumation, and the latter showed that mean ductility represented by the assemblage of deformed minorstructures decreased with distance from the axial part of a major isoclinal syncline in the western limb. The two inverted shear regimes were caused by alternate movement of oceanic and continental plates, i.e., NE-directed oblique subduction of the oceanic plate and NE-ENE-directed oblique upthrusting being reactivation of the Hida Nappe belonging to the continental plate. The mutual movements of the two plates played an important role in the development of the accretionary complex of the Japanese Islands.
- 新潟大学の論文
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