Deep-seated fluid around the Median Tectonic Line, Kii Peninsula and its relationship to geologic structure

Abstract

Through a geological survey, this study examines the geochemical properties of groundwater and river water distributed in the environs of the Median Tectonic Line MTL on Kii Peninsula, Japan, and their upwelling paths in the upper crust. Carbon dioxide-bearing water spouting in the environs of the MTL is assumed to be contaminated by Arima-type fluid derived from the subducted Philippine Sea slab, based on the result of water quality, oxygen-hydrogen isotope ratio, and helium isotope ratio. Arima-type fluid is presumed to ascend along the MTL fracture zones and related faults parallel to the MTL. NaCl-type and Ca HCO 3 2-type groundwa-ters with high concentrations of Cl discharge in the northern part of the study area at the related faults, and CaSO 4-type and Ca HCO 3 2-type groundwaters including some high concentrations of Cl spout out on the MTL in the southern part of the study area. Arima-type fluid is diluted by the local groundwater system and erupts at the intersection of faults and rivers. Cataclasites and microfractures accompanied in the faults are filled with calcite veins, showing that the fluid penetrates into them, where calcite then precipitates. As a result, the discharge path to the ground surface is sealed and eruption points then migrate to fresh fractures.