, Geology, geologic structure, and hot springs in southern Kyushu, Japan

/1,**1 11, Geology, geologic structure, and hot springs in southern Kyushu, Japan +, +3 0 +- +3 0 +. + 23***0/ +,+-/, 23*2/00 +*+ Tetsuo KOBAYASHI + and Toru YANO, + Earth and environmental Sciences, Faculty of Science, Kagoshima University +,+-/, korimoto, Kagoshima 23***0/, Japan, Kagoshima Prefectural Police +*+, Kamoike-shinmachi, Kagoshima 23*2/00, Japan Abstract The geology, geologic structure, and hot springs in southern Kyushu are discussed in this paper. The basement rocks of this area are composed mainly of the sedimentary complexes of the Shimanto Terrane which is divided into two groups : Cretaceous Northern Shimanto Terrane and Tertiary Southern Shimanto Terrane. The graben structures in southern Kyushu have been formed in relation to the opening of the Okinawa Trough since ca. +* Ma, and followed by contemporaneous volcanism. Volcanism was most active in between. Ma and, Ma which formed the fundamental graben structure such as Hokusatsu Triangular Graben, Hitoyoshi basin, and Kagoshima Graben. Since, Ma, the volcanism within the Kagoshima Graben became most active involving at least three calderas : Ata, Aira, and Kakuto calderas from south to north. Active volcanoes also exist in this graben, indicating an active volcano-tectonic depression. Most hot springs in southern Kyushu are distributed in these grabens, suggesting an intimate genetic relation between volcanism and hot springs. The heat supply of the hot springs from the basement rocks is sourced not only from the present magma, but also from Tertiary granites. Key words : Shimanto Terrane, Kagoshima Graben, Hokusatsu Triangular Graben, Okinawa Trough, hot spring +* Ma. Ma, Ma, Ma

12 : + +331 +33,, +,*** km Fig. + +30/ Fig. + Geologic and structural map of the Ryukyu arc, Okinawa Trough, Taiwan and vicinity (modified from Kimura, +32/). Legend + : central graben,, : basin occupied by Pleistocene igneous intrusions, - : major fault and fault scarp,. : buried major fault and fault scarp, / : eastern boundary of the Ryukyu Ridge, 0 : trench, 1 : active volcanoes, 2 : submarine intrusions or volcanoes since Late Pleistocene time, TG : Tokara Gap, KG : Kerama Gap.

/1,**1 13 Fig., +303 +30,,*-* km +* - -+ +/ Ma Fig., Geologic sketch map of southern Kyushu indicating the distribution of Kagoshima Graben (modified from Tsuyuki, +303). B : Basement sedimentary complexes of the Shimanto Terrane and the Chichibu Terrane of Triassic to Tertiary ages, G : Tertiary granitic rocks, V : volcanic rocks, Kr : Kirishima volcano, S : Sakurajima volcano, Km : Kaimondake volcano,

14 +32/ +321 +/ Ma +. Ka +312 +-.0 Ma : +300 +33- +* Ma +33+ 0 Ma -* Fig. - Kodama et al. +33/, Ma Kamata and Kodama +333 0 Ma, Ma,, Ma +33- +33-0 Ma Fig. - Distribution of crustal extension of Okinawa Trough and associated paleomagnetism (after Kamata and Kodama, +33-). Open arrow with PHS shows the present subduction direction of the Philippine Sea plate.

/1,**1 15 -, +331a : Arita, +3/. Fig., +331a /,* +*2 + Fig..., Fig.. Contour line map of the basement rocks in southern Kyushu based on drilling data for hot springs. Figures in parentheses from Kubota (+320) and Agency for Natural Resources and Energy (+322). H : Hokusatu Triangular Graben, K : Kagoshima Graben.

16 +, 20* m Fig. /,**0,*** +331a +320 Fig. 0 A B : +32, Fig. 0 A Fig. 0 B +303 +32. Fig. / Fig. / Distribution of the Kagoshima graben, caldera structure, and Curie point depths in southern Kyushu (modified from Kubota,,**0). + : active volcano,, : graben, - : fault,. : caldera and estimated caldera, / : Cretaceous to Tertiary basement rocks, 0 : Curie point depth (contour interval is + km).

/1,**1 17 Fig. 0 Distribution of graben in southern Kyushu (A) and the E-W cross section of the Kagoshima Graben (B : Hayasaka, +32,). HT : Hokusatsu Triangular Graben, KG : Kagoshima Graben, Kt : Kakuto caldera, Ar : Aira caldera, At : Ata caldera..,**0 +331a Koto +3+0 p. +2

18,.1 Ma,.0 Ma +333 - Ma - Ma + Ma. Fig. 1 Table + +** Fig. 1 Geologic map of southern Kyushu (modified from Yamamoto et al.,,**-).

/1,**1 19 Table + Summary of the geology of southern Kyushu. Ma +41*41. -4.+41 /4--4. +*4//4-,.+*4/ +.+, 0/,. +.00/,./+.0.+ Fig. 2,**. -* km +32. +. Ma,**1 +31. +30.

20 Fig. 2 Generalized geologic map of the Shimanto Terrane in southern Kyushu (modified from Teraoka,,**.). F is an estimated left-lateral fault. +. Ma +312 +, Ma Shibata and Nozawa, +302 +31, Yagi et al., +31/ +2 Ma +331 /* Ma +300 ; Shibata and Nozawa, +300

/1,**1 21.,,**+ 2 Ma 1 Ma 0 Ma Fig. 3 +331b / Ma +331b / Ma..- Ma -.. Ma+.3 Ma +.1 Ma+.* Ma 0.- Ma,*** - Ma Kaneoka et al, +32.,.- Ma+.3 Ma,**+. Ma, Ma,***. Ma, Ma +321 - Ma,./ Ma,,**1 Fig. 3 Distribution of Late Quaternary volcanic rocks in southern Kyushu (modified from Nagao,,**+). Solid lines indicate the depth of subducting slab. V.P. : volcanic province.

22 +.1 Ma Ueno et al., +333 +31+ Fig. 0B - Ma +312 *.2 Ma +*.1 Ma +33* *.2 Ma +331a *.2 Ma : Kaneoka and Suzuki, +31* - Ma Kaneoka et al, +32. +32* + Ma, Ma, Ma+ Ma *./2 Ma*.-. Ma,*** +, +333,3 ka +302 *.2 Ma +, - - +331a :,**-, Ma +331b / +,*** m +33, +33, /+ Fig. +* +33, +303

/1,**1 23 Fig. +* Distribution of natural hot springs in southern Kyushu (modified from Tsuyuki, +33,) +33, 0 +, +* - +**. / 0 +-.0 0 /, 2* Fig. ++

24 Fig. ++ Genetic model of hot springs at the foot of Sakurajima volcano (after Tsuyuki, +33,) Fig. +, Flow model chart of hot springs in the southwestern area of Kirishima volcano (after Tsuyuki, +33, and Fujita et al.,,***) +* Fig. +, /** m0** m 0* +,

/1,**1 25 +331 + Ma 0* *.-/ Ma*.- Ma +323 ; +323 Ma -** m+,*** m Fig. +- +31+ +33, Fig. 0B 0/* m 1* +31, +,*** m,-2 -+* +330,** m,** +312

26 Fig. +- Configuration map of basement rocks underneath Kagoshima city (A : modified from Hayasaka and Oki, +31+) and the E-W cross section of this area showing the horizon of hot water in the deposits (B : Tsuyuki, +33,)

/1,**1 27 0 +* Ma 0 Ma. Ma, Ma + Ma +. Ma +302 :.0 +-,/+-.-. Arita, T. (+3/.) : The geological structure of Kagoshima Prefecture, Kyushu district, Japan. Sci. Rep. Kanazawa Univ., no.,, +,+-+. +330 : JAMSTEC no. +,,++,+3.,*** : /* +++. +30, : no. 2.10,. +32, : A no. + 1012. +31+ : no.. +/,3. +31. : 2*.,3.-*. +312 : -2 p. +33- : - no..+ +,3+.2. Kamata, H. and Kodama, K. (+333) : Volcanic history and tectonics of the Southwest Japan Arc. The Island Arc, 2, -3-.*-. Kaneoka, I. and Suzuki, M. (+31*) : K-Ar and fission-track ages of some obsidians from Japan.

28 J. Geol. Soc. Japan, 10, -*3-+-. Kaneoka, I., Aramaki, S., Kobayashi, T. and Oki, K. (+32.) : Pliocene and Pleistocene volcanism in southern Kyushu : K-Ar ages of Fumoto and Isaku pyroclastic flows and related rocks. Bull. Volcanol. Soc. Japan,,3, /30,. +30. : +/ -.+-.-. +300 : K-Ar dating V /0 +3+,++. Kimura, M. (+32/) : Back-arc rifting in the Okinawa Trough. Marine and Petroleum Geology,,,,,,,.*. Kodama, K., Tashiro, H. and Takeuchi, T. (+33/) : Quaternary counterclockwise rotation of south Kyushu, southwest Japan. Geology,,-, 2,-2,0. +30/ : 1+.-1./1. Koto, B. (+3+0) : The great eruption of Sakura-jima in +3+.. J. Coll. Sci. Imp. Univ. Tokyo, -2, +,-1. +320 : -0./3.1..,**0 : 0*,,3,-3. +331 :.+,,2 p.,**- : --0 p, +323 : K-Ar 2. -32.*,. +33+ : no. -,+,,+0. +32. :,3.//0.,**+ : ++ +, C, 3+ p. +331 : K-Ar K-Ar : 3,./..0.. +333 : no. -, 2+21. +33* : +,/+--.,*** : no. -- 0+02.,**1 : ++-. +32* : no. +- -/10. +323 : K-Ar +323

/1,**1 29 1. +312 :,3 //+//.. Shibata, K. and Nozawa, T. (+300) : K-Ar ages of granites from Amami-oshima, Ryukyu islands, Japan. Bull. Geol. Surv. Japan, +1,.-*.-/. Shibata, K. and Nozawa, T. (+302) : K-Ar ages of acid rocks of Noma-misaki and Hioki mountains, Kyushu, Japan. Bull. Geol. Surv. Japan, +3,,--,-0. +312 : K-Ar 2. //+//-. +322 : 0, 03 p. +31, :, 2231.,**. : no. /33.*.2. +32/ : //.1/,. +333 : K-Ar +*/ /2//22.,*** : K-Ar./ +-++.2. +303 : / no., 2/+*+. +33, : ++*/. +31, : -- p.,**1 : ++- 3/++,. Ueno, M., Takeda, T., Otsuka, H. and Shimada, N. (+333) : Paleomagnetic evidence for the timing of the gold mineralization in Kagoshima, Japan. Mineral Deposits : Process to Processing (Stankey C. J. et al., eds), +,.++,.. +331a : 3S-. 233/. +331b :,* + +321 : K-Ar -1,*/,+-. Yagi, K., Ishikawa, H. and Kojima, M. (+31/) : Petrology of a lamprophyre sheet in Tane-gashima island, Kagoshima prefecture, Japan. J. Japan. Assoc. Mineral. Petrol. And Economic Geologists, 1*,,+-,,.. Yamamoto, M., Kawano, Y., Imai, A. and Nishimura, K. (,**-) : Miocene granites and the Hishikari gold deposits in Kyushu. Hutton Symposium V, Field Guidebook, Geol. Surv. Japan, Interim Report, no.,2, 0+2*. +321 : 3-22+23/.