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Geochemical Journal, Vol. 45, pp. 309 to 321, 2011 Hydrochemistry of the groundwaters in the Izu collision zone and its adjacent eastern area, central Japan YOICHI MURAMATSU,1* YUTA NAKAMURA,2 JITSURO SASAKI3 and AMANE WASEDA4 1Department of Liberal Arts, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 2Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan 3Department of Pure and Industrial Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 4Japan Petroleum Exploration Co., Ltd., Research Center, 1-2-1 Hamada, Mihama-ku, Chiba 261-0025, Japan (Received April 8, 2010; Accepted March 6, 2011) Chemical and stable isotopic (δD, δ18O, δ34S) compositions of rivers and groundwaters, mineral constituents of rock samples from wells, and δ34S values of anhydrite in the Izu collision zone and its adjacent eastern area, southern Kanto Plain, central Japan, were analyzed to constrain the water-rock reactions and flow systems of the groundwaters. Inside the accreted Izu–Bonin–Mariana (IBM) basin, a two-dimensional map of the geothermal gradient calculated roughly using the discharge groundwater temperatures and the borehole temperature logging data confirms that the aqui- fer is recharged by the local meteoric water (LMW) and the high density seawater in the area. The oxygen and hydrogen isotopic compositions reveal that the Ca·Na–SO4 groundwaters in the Tanzawa Mts. and the high Na–Cl groundwaters in the coastal area are of meteoric water and weakly altered fossil seawater origins, respectively. Sulfur in the SO4 rich groundwaters is derived from anhydrite and gypsum based on the sulfur isotopic compositions. The sulfate-type groundwaters were produced by the following process: the LMW infiltrated downward with dissolution of the sulfate 2+ minerals from hydrothermal veins in the Tanzawa Group, produced the Ca–SO4 groundwater as a result of Ca exchange partly on Na–smectite layer of mixed-layer chlorite–smectite in the sedimentary rocks of the Tanzawa Group. The Ca–Cl groundwaters in the eastern margin of the Tanzawa Mts. were produced by mixing of LMW with fossil seawater recharged from the surface of the coastal area, and Ca2+ exchange of the mixed-layer mineral in pyroclastic rocks of the Tanzawa Group. Outside the accreted IBM basin, the Na–HCO3 groundwaters in the shallow aquifer were formed by dissolution of authigenic calcite with LMW, and Na+ exchange in the Kazusa Group. The moderate Na–Cl groundwaters in the deep aquifer were formed by mixing of the deep seated fossil seawater with the Na–HCO3 waters in permeable sandstone and conglomerate of the Kazusa Group. Keywords: Izu collision zone, groundwater, hydrochemistry, formation mechanism, recharge, anhydrite, cation exchange tectonic difference between the two areas since the INTRODUCTION Miocene. Within non-volcanic area of the Kanagawa Prefecture, Drilling of thermal wells for hot-spring bathing pur- central Japan, the northern tip of the oceanic Philippine poses since the 1980’s was performed extensively on a Sea Plate is generally thought to have been colliding with deep thermal aquifer at the depths more than 1000 m in the continental Eurasian Plate at the northern margin of the non-volcanic area of the Kanagawa Prefecture. As a the Izu Peninsula. Intense Quaternary crustal movements, result, geological structure and hydrochemistry of the area such as active faulting with high slip-rates and rapid up- have been reported by many investigators (e.g., Imahashi lift or subsidence, occur along this inland plate boundary et al., 1996; Seki et al., 2001; Oyama et al., 1995; Ozawa (Yamazaki, 1992). The flow system of groundwaters in and Eto, 2005), while details of the water-rock reactions the accretion area of the Philippine Sea Plate may differ and flow systems of the groundwater in the area have not from those in the eastern non-volcanic area based on the been clearly described previously. In this paper, we present the results of chemical and stable isotopic (δD, δ18O, δ34S) compositions of rivers and groundwaters from *Corresponding author (e-mail: [email protected]) the wells in the non-volcanic area of the Kanagawa and Copyright © 2011 by The Geochemical Society of Japan. southeastern Yamanashi Prefectures, to constrain the flow 309 Kanto Mts N N Sea of Japan Study area Tono Akiyamakawa fault ki m 500 –Aika Makime w fault a fault Doshigawa fault Pacific Ocean – Tanzawa Mts. Susugaya 1500 m 1000 m 3500 m 2000 m 2500 m Kannawa fault Tokyo Bay Kozu fault – Mat 1000 m suda Subductio 1500 m Quaternary system 2000 m Volcanic rocks n in 15 Ma Neogene system Sag Sagami Bay Sedimentary rocks ami trough Volcanic rocks Granitic rocks Accreted IBM basin Palaeogene system 0 50 km Pre–Tertiary system Fig. 1. Geological map of the southern Kanto Plain (after Hayashi et al., 2006). Thin-broken lines indicate counters of depth of upper boundaries of the pre-Neogene systems from sea level. system of the groundwaters around the accretion area of Eto, 2005; Hayashi et al., 2006; Takahashi et al., 2006). the Philippine Sea Plate. The Kobotoke and Sagamiko Groups are correlated with the Shimanto Group. The Cretaceous Kobotoke Group composed of shale and sandstone is widely dis- OVERVIEW OF GEOLOGY tributed through the northwestern Tanzawa Mountains A simplified geological map of the southern Kanto (Mts.) to the Kanto Mts. The Paleaogene Sagamiko Group Plain located southwest of Tokyo, including the Kanagawa composed of sandstone, conglomerate, and mudstone and southeastern Yamanashi Prefectures, is shown in Fig. crops out in the southern side of the Kobotoke Group 1. The western part of the study area is topographically (Sakai, 1987) and occurs deeper than 1005 m below the situated at the Tanzawa Mountain land (maximum alti- surface at Ebina City (location 15 in Fig. 2; Ozawa and tude of 1673 m above sea level), and the eastern part at Eto, 2005). There are normal NE-SW trending faults, and the Sagamigawa low land (10 m above sea level), normal and reverse NW-SE trending faults in the Tanzawa Sagamihara platform (50 m above sea level), Tama hill Mts., and the Tonoki–Aikawa reverse fault runs along the (70 to 90 m above sea level), and Shimosueyoshi plat- boundary between the Sagamiko and Tanzawa Groups. form (40 to 60 m above sea level) in eastwardly order, The Miocene Tanzawa Group, which is equivalent to the while the Miura Peninsula is located in the southeastern Green Tuff formation, is mainly composed of dacitic part of the study area at the Miura hill (maximum alti- lapilli tuff, tuff-breccia, andesitic lapilli tuff and tude of 241 m above sea level). The geology of the study tuffaceous mudstone (Oka et al., 1979). It has been meta- area has been reported by many investigators, based on morphosed under conditions from zeolite to amphibolite geological and logging surveys obtained during drilling facies, which were produced by the intrusion of the of many deep thermal and seismic survey wells and seis- Tanzawa plutonic complex (K–Ar dating of 4.3 to 7.6 Ma; mic reflection surveys (e.g., Ishii, 1962; Omori et al., Kawano and Ueda, 1966) composed mainly of tonalite 1986; Suzuki, 2002; Kanagawa Pref., 2003; Ozawa and with minor gabbro (Seki et al., 1969). According to Zhang 310 Y. Muramatsu et al. N Tokyo Yamanashi Otsuki RTR FTR Sagamihara 2 1 3 29 4 6 30 24 5 28 Yamanaka Ebina Yokohama 22 16 Lake 31 711 25 15 19 23 26 20 17 Shizuoka 21 Tokyo Bay 12 13 Hiratsuka Yokosuka Odawara Ca Na–SO4 18 Miura Na–SO4 Sagami Bay Peninsula Na–Cl Ca–Cl 27 14 TAF Na–HCO3 0 20 km Ca–HCO3 Fig. 2. Location of the water samples in the study area. and Yoshimura (1997) and Zhang et al. (1997), the low Group is composed of tuffaceous sandstone and mudstone. grade metamorphic rocks of the southern Tanzawa Mts. The Miocene to early Pliocene Miura Group, composed can be divided into stilbite, laumontite, prehnite– of tuffaceous sandstone and mudstone, unconformably pumpellyite, epidote, epidote–amphibole and amphibole overlies the Hayama Group, and is exposed on the north- zones in increasing order of metamorphic grade. The ern, central and southern parts of the peninsula. There Miocene Aikawa Group equivalent to the Green Tuff for- are E-W trending normal faults in the peninsula (Omori mation, which is bordered on the Tanzawa Group by the et al., 1986). The Late Pliocene to Early Pleistocene Makime–Susugaya tectonic line, is mainly composed of Kazusa Group, composed of tuffaceous sandstone and andesitic lapilli tuff, mudstone and tuffaceous sandstone. sandy mudstone, unconformably lies on the Miura group. The Kannawa reverse fault runs along the boundary The Sagami Group consists of neritic, lacustrine and flu- between the Tanzawa and Ashigara Groups at the south- vial deposits. ern margin of the Tanzawa Mts. The Izu–Bonin–Mariana (IBM) arc of the Philippine Sea Plate is thought to be SAMPLING AND ANALYTICAL PROCEDURES bordered on the Honshu arc of the continental Eurasian Plate by the Kannawa and Kozu–Matsuda faults Cutting samples were collected at 5-m intervals from (Sugimura, 1972). The Pliocene–Pleistocene Ashigara the two wells (locations 6 and 16 in Fig. 2) for X-ray Group is mainly composed of sandstone, mudstone and powder diffraction analysis. We collected thirty-one sam- conglomerate. The accreted IBM basin (Soh et al., 1998) ples of river water and groundwater from wells drilled between the Tonoki–Aikawa fault and the Kannawa, for hot-spring bathing and domestic purposes in 2006– Kozu–Matsuda faults formed by accretion of the IBM arc 2007 (Fig.

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