Origin of the Median Tectonic Line

The southern maginal region of the HT/LP Ryoke metamorphic belt is characterized by a mylonite zone and nappe structures. The tectono-metamorphic history in the southern margin of the Ryoke belt, and the initial stage of formation of the Median Tectonic Line (MTL) were studied at the middle-southern region along the MTL in the Chubu district. Four phases of deformation can be recognized in this region. The first deformation (D1) formed the S1 foliation. The structural features of D1 can not be clearly identified. The metamorphic conditions during D1 were assumed to be medium pressure in the Mikawa Plateau. Extensive growth of porphyroblasts occurred after D1 and before D2 (inter-D1-D2) under non-deformational conditions. The metamorphic conditions changed with a remarkable decrease in pressure from D1 to inter-D1-D2. The D2 deformation was penetrative and involved pressure-solution, resulting in a distinct foliation (S2), which is typically observed in the Ryoke metamorphic rocks. The metamorphic conditions changed with a distinct increase in temperature from inter-D1-D2 to D2. The oldest plutonic mass of the Older Ryoke granites (the Kamihara tonalite) was deformed during D2. The D3 deformation formed a large-scale recumbent fold during the earlier stage of D3. During the later stage of D3, the deformation was concentrated in the lower structurally portions, resulting in the formation of the large-scale mylonite zones and nappe structures. The mylonite zones are developed horizontally and have a top to the west shear sense. The high temperature portion of the Ryoke belt was emplaced onto the low temperature portion. The second older plutonic mass of the Older Ryoke granites (the Tenryukyo granite) were emplaced and deformed during D3. The D3 deformation occurred with distinct decrease in temperature and pressure. The intrusion of the Younger Ryoke granites took place after D3. The D4 deformation is characterized by the formation of the nappe complex and crush melange. The nappe structures of D4 characteristically contain the rocks formed on the surface and rocks derived from other terrane. During the later stage of D4, the Ryoke nappe complex was thrust over the Sambagawa rocks. After the coupling of the Ryoke belt and the Sambagawa belt, the high-angle MTL was formed with a sinistral strike-slip component, resulting in the formation of upright folds.
D1, inter-D1-D2, and D2 are typical of the Ryoke belt, and are the result of regional metamorphic processes. Whereas D3 and D4 are characteristic of the southern margin of the Ryoke belt and represent a distinct decrease in temperature and pressure. The initial stage of the MTL is correlated with the D3 and D4 tectonism, and occurred with the uplift of the Ryoke belt from depth and the transportation of the Ryoke belt toward the south. The western portion of the Ryoke belt was uplifted at about 90 Ma and was followed by the uplift of the eastern portion at 60 Ma. The high-angle MTL and the upright folds postdate the tectono-metamorphic processes of the Ryoke belt and the formation of the initial MTL as a horizontal shear zone.