11:00 AM - 11:15 AM
[SGL24-02] Evaluation of the thermal effect of intrusion of the Kaikomagatake granitic body by metamorphic thermal analysis of basement rocks of the Outer Zone of SW Japan and reconstruction of the Miocene tectonics in the Akaishi Mountain range
Keywords:Akaishi Mountain range, Outer Zone of Southwest Japan, Kaikomagatake granitic body, Raman carbonaceous material geothermometry, intrusion thermal modeling, numerical analysis
[Introduction] The opening of the Japan Sea and the collision of the Izu-Ogasawara Arc with the Honshu Arc were major tectonic events at the East Asian Continental Margin during the Miocene. The effects of these events are well recognized in central Japan, where the geological structure of the Outer Zone of Southwest Japan in the Akaishi Mountain range was regionally modified with the formation of a twist-bend structure. In the Koshibugawa area, which is located on the axis of the twist-bend structure, metamorphic thermal analysis shows that the thermal structure was clearly modified by the twist-bend formation. In the northern part of the Akaishi Mountain range, further north than the Koshibugawa area, the geological modification appears to have formed a more severe twist-bend structure. However, the Kaikomagatake granitic (KG) body intrudes into this area, and the thermal effect of this intrusion needs to be considered for the evaluation of the metamorphic thermal structure. we therefore conducted metamorphic thermal analysis and thermal modeling on the basement rocks along the Kurokawa and Todaigawa rivers (Ina city, Nagano prefecture), in the northern part of the Akaishi Mountain range, to evaluate the thermal effect of the intrusion.
[Study flow] We first estimated the peak metamorphic temperatures around the intrusion by Raman carbonaceous material geothermometry (RCMT). We then calculated the peak thermal structure around the intrusion by numerical thermal modeling and constrained the initial temperatures of magma (Tmagma) and basement rocks (Tbase) at the time of intrusion by fitting to the temperatures obtained from RCMT. In addition, we also estimated Tmagma by thermodynamic calculations using the bulk chemical composition of the KG body independently of the metamorphic analysis. By comparing both Tmagma values, we eventually verified whether the metamorphic thermal structure recognized in nature can be explained by the thermal effect of the intrusion heat source.
[Results] In the study area, the Sanbagawa, Chichibu, and Shimanto belts are aligned from west to east, and the Todai Formation of shallow marine origin is narrowly distributed along the boundary between the Sanbagawa and Chichibu belts. The KG body intruded into the Shimanto belt. The temperatures estimated with RCMT were ~330–420 °C in the Sanbagawa belt, ~280 °C in the Todai Formation, 280–320 °C in the Chichibu belt, and 320–490 °C in the Shimanto belt. From west to east towards the intrusion boundary, the thermal structure shows a temperature decrease in the Sanbagawa belt, while a temperature increase is recognized from the Todai Formation to the Shimanto belt with a continuous spatial change, including at the geological boundaries. The fitting results by thermal modeling of the latter thermal structure were Tmagma = ~850 °C and Tbase = ~180 °C. In contrast, the results of thermodynamic calculation show the Tmagma range as ~750–1100 °C.
[Discussion] The consistency between the metamorphic analysis and thermodynamic calculation suggests that the temperature increase from the Todai Formation to the Shimanto belt was caused by the thermal effect of the KG body intrusion. This implies that major geological modifications did not occur after the intrusion; in other words, the formation of the twist-bend structure was complete before the intrusion. The geothermal gradient at the time of intrusion is constrained as ~20 °C/km based on the Tbase value obtained from this study and the emplacement depth estimated by a previous study, suggesting that the intrusion occurred under a geothermal gradient relatively lower than those typically estimated for common geological settings at volcanic arcs.
[Study flow] We first estimated the peak metamorphic temperatures around the intrusion by Raman carbonaceous material geothermometry (RCMT). We then calculated the peak thermal structure around the intrusion by numerical thermal modeling and constrained the initial temperatures of magma (Tmagma) and basement rocks (Tbase) at the time of intrusion by fitting to the temperatures obtained from RCMT. In addition, we also estimated Tmagma by thermodynamic calculations using the bulk chemical composition of the KG body independently of the metamorphic analysis. By comparing both Tmagma values, we eventually verified whether the metamorphic thermal structure recognized in nature can be explained by the thermal effect of the intrusion heat source.
[Results] In the study area, the Sanbagawa, Chichibu, and Shimanto belts are aligned from west to east, and the Todai Formation of shallow marine origin is narrowly distributed along the boundary between the Sanbagawa and Chichibu belts. The KG body intruded into the Shimanto belt. The temperatures estimated with RCMT were ~330–420 °C in the Sanbagawa belt, ~280 °C in the Todai Formation, 280–320 °C in the Chichibu belt, and 320–490 °C in the Shimanto belt. From west to east towards the intrusion boundary, the thermal structure shows a temperature decrease in the Sanbagawa belt, while a temperature increase is recognized from the Todai Formation to the Shimanto belt with a continuous spatial change, including at the geological boundaries. The fitting results by thermal modeling of the latter thermal structure were Tmagma = ~850 °C and Tbase = ~180 °C. In contrast, the results of thermodynamic calculation show the Tmagma range as ~750–1100 °C.
[Discussion] The consistency between the metamorphic analysis and thermodynamic calculation suggests that the temperature increase from the Todai Formation to the Shimanto belt was caused by the thermal effect of the KG body intrusion. This implies that major geological modifications did not occur after the intrusion; in other words, the formation of the twist-bend structure was complete before the intrusion. The geothermal gradient at the time of intrusion is constrained as ~20 °C/km based on the Tbase value obtained from this study and the emplacement depth estimated by a previous study, suggesting that the intrusion occurred under a geothermal gradient relatively lower than those typically estimated for common geological settings at volcanic arcs.