Metamorphic Thermal Structure of the Outer Zone of Southwest Japan in the Koshibu-gawa Section, Nagano Prefecture, Central Japan

Raman spectral analyses were conducted of carbonaceous materials (CM) in meta-sedimentary rocks to estimate the metamorphic thermal structure of the Outer Zone of Southwest Japan in the Akaishi Mountains area, central Japan. Twenty samples of pelitic and psammitic rocks were collected along the Koshibu River and by traversing the Sanbagawa, Chichibu, and Shimanto belts. The datasets of the full width at half maximum of the D1-band (FWHMD1) -which is a sensitive thermal parameter of the Raman spectra of CM-imply that each rock records a single metamorphic thermal event. The peak metamorphic temperatures estimated with Raman CM geothermometry show 286 339 degrees C in the Sanbagawa Belt, 284-350 degrees C in the Chichibu Belt, and 328-347 degrees C in the Shimanto Belt with a continuous spatial change including at the boundaries between belts. From northwest to southeast, a clear temperature decrease is observed in the Sanbagawa Belt, but an increase in the Chichibu Belt with a constant high-T condition-similar to those recognized in southeast part of the Chichibu Belt-in the Shimanto Belt. A comparison of geological structures suggests that the present thermal structures in the Chichibu and Shimanto belts were formed with the formation of a "twist-bend" structure related to the collision of the Izu-Ogasawara (Izu-Bonin) Arc with respect to the Honshu Arc at middle Miocene. In contrast, the thermal structure in the Sanbagawa Belt was mainly formed before about 20 Ma but the above middle Miocene tectonics may involve the formation of the apparent steep geothermal gradient.