10:45 AM - 11:15 AM
[AHW24-05] The discharge rate of the Arima-type deep brine discharge in Ooshika-mura, Nagano Prefecture
★Invited Papers
Keywords:slab-derived fluid, discharge rate, Median Tectonic Line, geologic structure, Kashio
Arima-type hot spring water is thought to originate from the dehydration from the hydrous minerals in a subducting slab. In the present research, this dehydrated saline water is called “ADB (Arima-type Deep Brine)”. It is important to estimate the amount of the discharge of ADB in order to understand the water circulation in subduction zones. Few studies, however, have been conducted on such estimation because the chemical and isotopic compositions of the ADB is unknown.Ooshika-Mura is located at the southern part of Nagano prefecture, central Japan. It is thought that the ADB is diluted by local meteoric water, gushes out at several points (e.g. the Kashio hot spring, famous as the Arima-type hot spring), and flows into the 4 rivers in the region. In the present research, the flow rate and Cl- concentration of river water is measured at several points along each river. The flow rate of the ADB is estimated at each observation point, based on the results of above measurements and the previous study on the chemical and isotopic composition of the ADB in the researched region. These estimated flow rate values are regarded as the discharge rate of the ADB discharge inside the upstream watershed areas. The ADB discharge rate over the whole area of Ooshika-Mura is estimated approximately 0.8 L/s, by summing up each flow rate at the most downstream point of each river. The estimation is consistent with previous calculation of the dehydration rate from subducting slabs. More than 50% of the discharged ADB in the whole area concentrates within a few kilometers of the confluence of Kashio River and Shiokawa River. The fault plane of the MTL intersects with the Shiokawa River at the area, which is consistent with the ADB discharging model in Kii peninsula, shown by the previous study.