Reconstruction of the paleostress field on the basis of 3-D analyzes of healed miocrocracks in granitic rocks

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To estimate the influence of the bending of the Median Tectonic Line(MTL) by the late Miocene collision of the Izu-Ogasawara arc, we examined the 3-D orientations of healed microcracks in the Ryoke granitic rocks and the Tanzawa Tonalite and determined the paleostress field. The results are as follows :1. δHmax directions recorded as the orientation of healed microcracks from the Nojima Granodiorite(Awaji Island), Misugi Tonalite(Mie Pref.) and Shinshiro Tonalite(Aichi Pref.)are nearly parallel to the current strike of the MTL. These results suggest that the microcrack-bearing granitic bodies along the MTL are rotated together with the MTL during the arc-arc collision. However, those from the Takato Granite(Nagano Pref.) are almost normal to that of the MTL. It is not able to be explained by the bending, and open to research more.2. δHmax directions recorded as the orientation of both healed and sealed microcracks from the Tanzawa Tonalite which was collided at c. 5Ma are fairly constant throughout the body(NNE-SSW). Geothermometry on fluid inclusions constituting healed microcracks suggest that the formation of them was occurred at the condition of 300〜400℃. This trend was explained by the divergent δHmax trajectories at the northeastern side of colliding center which has the movement direction of NW.