Relationships between stream water chemistry and catchment geology in a mountainous region in Nagano Prefecture, Japan II: mid to southern part along the Median Tectonic Line

Abstract

Major ion concentrations were determined for a large number of stream water samples collected from the mountainous area around the Median Tectonic Line (MTL) in the mid to southern part of Nagano Prefecture, Japan. The MTL, a major fault in the accretionary sedimentary complex to Japan Arc, extending linearly in the east–west direction in southwest Japan, bends in this area to south–north along the Akaishi Mountain chain. To the east of the MTL are, in order, Jurassic metamorphic rocks, including black/green schists and ultra-basic serpentinite (Sambagawa-Mikabu Belt); Jurassic mudstone with voluminous chert and calcite (Chichibu Belt); and Cretaceous sandstone (Shimanto Belt). On the other hand, on the west side of the MTL, Cretaceous granitic rocks (including tonalite and quartzdiorite) intruded and significantly metamorphosed the Jurassic sedimentary systems (Ryoke-Belt). In this area, more than 200 water samples were collected from small streams. Ion concentrations were analyzed by ion chromatography, whereby higher ion concentrations relatively rich in Mg²⁺ and Ca²⁺ and lower concentrations relatively rich in Na⁺ were obvious for the streams in Sambagawa-Mikabu, Chichibu, and Ryoke Belts, respectively. These tendencies could be explained by whole rock analysis on XRF (X-ray fluorescence spectrometry) as well as by a preliminary leaching experiment of the powdered rock samples with water. The results strongly suggested that the analytical data of the major ions in the stream waters might be compiled into a hydrogeochemical map, corresponding to the geological map.