飛騨地体の構造とテクトニクス  [in Japanese] Structure and tectonics of the Hida massif, central Japan  [in Japanese]

The Hida massif including Unazuki crystalline schists thrusts as a nappe over the inner terranes of Southwest Honshu. High-grade gneisses in the massif were produced from late Middle Paleozoic sediments which were depositedon the Sino-Korea continental margin, or within a rift zone formed on the continental margin. The gneisses underwent two times the ductile deformations in a deep-sheeted state in the early stage, and were intruded by older Funatsu granite during early Traiassic time. The granite-gneiss complex suffered shearing to form two sets of mylonite zone (Funatsu shear zone) during the time of exhumation of the Hida massif. The structural analyses of granitic mylonite zones shed a new light on the exhumation process of the massif. Syndeformational granites intruded along the de-collement from which the Hida gneiss formations were detached, and along the thrusts of stacked gneiss sheets. The granite was subjected mylonitization first affected by subhorizontal displacement to form Unazuki-stage mylonites and secondary by dextral-reverse movement of the Hida massif consisting of stacked gneisses, resulted in northward tilting of the massif. The Funatsu-stage mylonites were formed through the secondary movement. The age of the first and secondary movements is roughly constrained by a Pb-U age of sphene (240 Ma) of a mylonitized granite and K-Ar ages of hornblende (215 Ma) of mylonitized mafic rocks in the mylonite zone. The younger granites (Funatsu granites) were intruded after the second movement, cutting the mylonite zones. The Funatsu shear zone was a northeastward extension of the Honam shear zone in South Korea. Using the Honam-Funatsu shear zone as a reference frame, the paleo-geographical setting (pre-Sea of Japan) of the Hida massif in the eastern margin of Eurasian continent is reconstructed; the massif was situated around Zal. Pyotra Velikogo (Bay of Pyotr the Great) and was probably the extension of Variscan origenic belt formed along the periphery of the Sino-Korean continental mass.