土岐花崗岩中の石英に発達するマイクロクラックの三次元方位分布による古応力場の復元と生成環境

Estimation of the paleostress field from the 3-D orientation distribution of microcracks and their geothermal conditions in the Toki Granite, central Japan

高木 秀雄*; 三輪 成徳*; 横溝 佳侑*; 西嶋 圭*; 円城寺 守*; 水野 崇   ; 天野 健治  

Takagi, Hideo*; Miwa, Shigenori*; Yokomizo, Yoshiyuki*; Nishijima, Kei*; Enjoji, Mamoru*; Mizuno, Takashi; Amano, Kenji

花崗岩の古応力場を復元するための手法開発を目的として、土岐花崗岩中を対象としてヒールドマイクロクラックを3次元的な測定及び流体包有物を対象とした解析を行った。ヒールドマイクロクラックの3次元方位測定の結果、ヒールドマイクロクラック形成時の主応力軸は南北方向であり、流体包有物の解析から形成時期は7060Maであることが示された。この結果は、日本海拡大による西南日本の回転を考慮すると当時のプレート運動方向と調和的であり、本研究で開発した手法が古応力場を復元するための手法として有効であることを示した。

Based on the 3-D orientations of intracrystalline healed, sealed and open extension microcracks in quartz grains in the Late Cretaceous Toki Granite, we discuss the paleostress field and the history of microcracking combining the microthermometry of fluid inclusions in healed microcracks and sealing material identification in sealed microcracks. Twenty one oriented samples are collected mainly from the DH-15 core (240-1000 mabh) drilled by Japan Atomic Energy Agency (JAEA) and additionally from five outcrops in the Tono region. 3-D orientations of healed microcracks indicate the orientation of N-S to NW-SE in almost all sites, whereas those of sealed and open microcracks indicate the dominant orientation of E-W. Two or three orthogonal sets of microcracks are common in both healed and sealed microcracks. The formation of these sets can be attributed to the switch of principal stress axis due to stress release just after the crack formation. Healed microcracks probably reflect more regional paleostress field because of consistency of the orientations than the case of sealed and open microcracks that show rather scattering orientations. N-S to NW-SE trending healed microcracks are formed around 60 Ma on the basis of K-Ar biotite ages of the Toki Granite and formation temperature (c. 300-400C) of fluid inclusions estimated from microthermometry in the case of intrusion depth (3.5 km=100 MPa) of the Toki Granite. Thus the orientation trended NW-SE after the restoration of clockwise rotation of SW Japan at c. 15 Ma. Following the healed microcrack formation, E-W trending high-angle sealed microcracks filled with carbonate mineral are formed. Open microcracks presumably formed at near-surface at the last stage of sealed microcrack formation after c. 20 Ma before the Mizunami Group deposited unconformably on the granite.