01R942. 

 ῌ
Geothermal Island-Arc Genesis at the Plate Margins
On the Similarity of the two Geothermal Island-Arc Genesis
in Southern Part of Kyushu, Japan and Sumatra, Indonesia  +3  .  +3  !"  +3  0  +.  #$Susumu NISHIMURA (NPO Think-tank Kyoto Institute of Natural History)

Abstract
Geothermal island-arc genesis are found in graben and caldera along the big-fault
systems of Southern part of Kyushu, Japan and Sumatra, Indonesia. The magma of ignimbrite and welded tu# (‘Shirasu’) might be originated from the remelt of oceanic plate
and/or crustal materials, when the front of Philippine and Indian Sea plates is subducted
to nearly ++* km in depth. These oceanic plates are subducted obliquely to trench axis.
The volcanism of these ignimbrite and Shirasu is so violent as to make a big caldera, of
which Bouguer anomalies are about /* mgal lower than those of the surrounding area,
and the big caldera has the diameter of +/%,* km which is disturbed the big-fault systems.
After these activities, the dacite and andesite volcanisms occur, which make the central cone of volcanoes. The magma of these volcanoes is quite di#erent from that of ignimbrite and welded tu# in the genesis. The magma must be originated from the partialmelt of the upper mantle under the volcanic belt of island arc. There are hot-springs
within +/ km distance from crater of volcanoes.
Acording to a plausible model for the migration of H,O and the generation of basalt
magma in subduction zones, the hydrated peridotite is formed by addition of slab-derived
H,O beneath the forearc region and should be dragged downward on the slab by the subduction of oceanic lithosphere. Amphibolite and chlorite in the dragged hydrated peridotite layer decomposes to release H,O at around ++* km just beneath a volcanic front.
When the front of migration of H,O from the hydrated peridotite reaches the region withsolidus temperature of hydrous peridotite. Partial melting takes place to produce initialmagma. Successive addition of H,O to the region expands the partial molten zone upwards in which the degree of partial melting increases upwards. From the top of thepartially molten zone a mantle diapir starts rising to make a volcanism.
The di#erent two types of volcanic activities with geothermal activities are found at
the volcanic belts in the dragged downward slabs of the around +/* and ,/* km depths.
Key words : Island-arc, geothermal area, Hot Spring, Southern part of Kyusyu, Sumatra῍ῌ&'()*+,-./0123435/672896:;<=>?@A BCDE" FGH(2IJK/LM./0123435/67)NO@A &'(2*+,-.ELMPQ ,/ /1 ¼ ',**1-Im3C3 (GRGOPQ43  !"#$%& '()* +,-  ". /0& 0&123'7+,45 ' /* mgal- 162
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TMqtug. +31/ ; hij3klmT\. +33/- n? +31/ PJ4l+ 3C3o1 ] '(). Nishimura ed., +32*-
N'OPQ (GRG)H 3¡O1¢pX]6. +32* P£J?z ,Y89:. E 3C3qtu1*". *r¤s¥¦ 4§W. ¨,Y 3 © 1 W. [tuPttªX] '%&. +32*- «¬=ukl. Eῌ3C3 N'OPQ (GRG)H­®1 W. * uv
1wJ1=X]

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D± ',***- U * z{ῌ²³q)H4 ++* km ??zV '| +-. OPQ
(GRGn´ Nµ}n´4~¶< '. ,**+- 1. b3µqNµ· ;< =3µq4¸ X"= " ¸ X".
89:? 3z SI

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4 +/»,/ km x ¸ <. ¸ < =y89:. 3C3q '| /- 89:

tu

44

Fig. + Distribution of earthquake epicenters indicated the depth of upper part of
subduction slab in Kyushu area (modiﬁed from Takahashi, ,***). Solid lines
indicate the depth of epicenters. Arrow is shown the direction of subduction
and its ﬁgure is subduction rate (modiﬁed from Taira, ,**+).
 + 
 !" ,***# $%&'()*+, -.# /0 1" ,**+# 2345678

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+ :67#$89$;<+ =>?@A!BC0 DEFGHIEJK!

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R["I

45

Distribution of ‘natural’ hot springs with calderas in Southern Kyushu (modiﬁed from Tsuyuki, +33,).
 ,   +33/     ! "#$% &'()*+,-./ 01 '2

()3#4567()*89 :; &' ῌ <=>?@! AB ,C AKobayashi, +322 ;DE +33,CF
GHIJKLMNB / JKOMNPQRS?9TUVWX? YZO
 R[R\ ]'^&'&?_`F ab GHIMNRc?#defghXFijXT*kl(<mF #deMNnopaR[ῌ R qrsk`tVsuv?wxyX ADE +33, ; z ,**0CF {|}k`H~H" "?X{"?yX AB 2CF
GHI#oT GHIJKLH JKYZ"{G ASuwohC ?<g ?R[ ?@! AB 3 ; Sidarto et al., +33.CF Gk` ,/ km GH
IJKX(na"{ ALiwaC ?<? {MN{S

BC

46

Fig. -Generalized geological map and Bouguer anomaly map of Southern Kyushu (Kobayashi, +322).
 -  Kobayashi, +322 1 -  +33.  ,  +0   +33/ !"#$%&' ()*+,*+-./01234 ,***5 67897:;<= +33/ >?@89A' BC SO.D, E ClD E FGF<HI J < K',- LMN<>FOP7FGFQRSTUVWVX7YZ<FGF7R[ST7\]^ Tatsumi (+3235 _ (+33/5`a= 6bc=defgh=' (Nishimura, +3315 i +* 67j<k LMNl>FOPm
nopqO7F*O^r7UVWV7Js+/* km tu

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47Fig. . Distribution of earthquake epicenters indicated the depth of upper part ofsubduction slab in Sumatra (modiﬁed from Hamilton, +31.).
.
  !"#$Hamilton, +31. %&'

 +,-**       !"#$%&'()*+&', -# ./01&'234567 89:;<=>?@ABC$- ῌ

D*E FGHIJK L/0M1 JNO P,QJKRST .$C&'U V&'UJK@ WXYZR[4 \T&']@,.=^_`_"a$JKEbcCMd$C$efgMhi jklE&'23@mn[JK@.oJKM W &'23@mn[JKMpTkl qrQTstuQv +* E wxyMz_ji{|} ~89 @&'234 @rC$QR
[ FC

P 9( } ,T@ 7x/0M 01* km F z_i{|}E.j T ~Fukao et al., +33. j$ MTQ&'
23 {|}@mn @rQv u# ~v ++ ; Nishimura, +331
j$C@r@ ῌ wx&'()Ez__Fa4 ++* km  MC$ ῍wxME&'()eC |&';  |&';  ]&'; - 4C$ ῎ &'()ME&'23EC$1T4 J°c

48Fig. / Volcanic and geothermal areas along Sumatra fault zone (Nishimura, ,***).
Arrow is shown the direction of subduction and its ﬁgure is subduction rate.
 / ῌ  ! "#$ %Nishimura, ,*** &'(  ,***a ; ,***b ; ,**+

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-.

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 ,***a ; ,***b ; ,**+

0Y&'4 +**A+/* km |}!wxyHzQZ ++* km |}B&.6GHzQZL' > YC¡< +,-**¢_YC£¤<¥ YC<¦*£>GY#Y<§¨>   >Y#Y©<E Y#Y©ª«Y#Y¡K¦ ¬!­® ¯p /1 q m,**1noaCXY"#$%&

49Fig. 0 The FῌAῌM diagrams of volcanic rocks from (a) southern part of Kyushu and(b) Sumatra (Nishimura, +32*).
 0 a  FFeOFe,O-ῌAK,ONa,OῌMMgO b  FῌAῌM  !

"#$%& ,** km '()*"#+,-./ +/* km 0(1234
56789:

;<;=>? @?AB.CῌDEFG!
-** km AH0,IJK"I+,$%&/ LMNO89:G - PQ

RS1TUV3W;<;

XYZ.[BCῌ\9! ]^_`ab4cdefAHghiaIjk-l  mNishimura, +331n!

ῌ῏῍῎50Fig. 1 The K,OῌMgO diagrams of volcanic rocks from (a) southern part of Kyushu and (b) Sumatra(Nishimura, +32*).
 1 a 
 K,OῌMgO  b 
 K,OῌMgO Fig. 2

The model of geothermal genesis, in Kirishima area (Fujita, ,**0).
 2 ῌ  ,**0 /1  ,**1

Fig. 3 Geological sketch map around Suwoh and Liwa (modiﬁed from Sidarto et al., +33.).
 3 
   +33.
   Sidarto et al., +33. !"#$%&'(Fig. +* A plausible model for migration of H,O and the generation of basalt magams insubduction zones (modiﬁed from Tatsumi, +323).
 +* )*+*,-./ ( 0123456789 :;<=< 9 >=:;<=< 9 =?@A B%/Tatsumi, +323 !"#CD'(51ª52

Fig. ++ A model of subduction system across Tohoku area, Japan (modiﬁed from Nishimura, +331),
using Plume Tectonics conducted by Fukao et al. (+33.).
 ++ Fukao et al. +33.
 Pῌ ! "#$%&'()*+ Nishimura, +331 .  ῌ !"#% &'()*+,-./01$2 +/* km 34/5 678%"&'($),-. +/* km 9:;<=>?@A/5BC=DE ; Nishimura, +331F GHIJKLMN$&'('O 6Oblique subduction zoneF /5PQ=DE/%RSTUVWXYZ[A \/%Z]X=DE

 ]X=DE^_E` abcde 6fF ghijklh 6ignimbriteF "mn=>o pqrs"tuvwx"yA R z{e"|}$~ "y RῌnO/DE  6,**+F : $$K   
Fukao, Y., Maruyama, S., Obayashi, M. and Inoue, H. (+33.) : Geologic implication of the wholemantle P-wave tomography, Jour. Geological Society, Jap., +**, .ῌ,-.

6,**0F : `¡`¢/£¤' Y¥¦§¨ ©ª «¬­® ./ῌ1- ¯vj

f°n± ²Hamilton, W. (+31.) : Earthquake Map of the Indonesian Region, + : /,***,***, USGS, Map I-21/-C.
Kobayashi, T. (+322) : Tectonic Setting and Major Pyroclastic Eruption in Southern Kyushu,

« /1 ° ,**1$05 ­®¯

53

A guide book for Sakurajima Volcano, Organizing Committee Kagoshima International
Conference on Volcanoes, +ῌ0, July +3ῌ,-, +322, Kagoshima.
Nishimura, S. (+331) : Report of the Review of Topical Report on Volcanology of the Siting
Studies at Muria Peninsula, Indonesia, -* p, International Atomic Energy Agency, Project
INS/3/*+,, April ,,, +331. Venna.
Nishimura, S. ed. (+32*) : Physical Geology of Indonesian Island Arcs., ,-3 p, Kyoto Univ., Kyoto.

  +32* :    ,- B-+ +ῌ+3.

  ,***a :  !"#$%&' () ()*+ .3 ,*1ῌ,+0.
  ,***b : ,-./0123 () ()*+ /* ++-ῌ++3.
  ,**+ : () 456 ()*+ /+ 32ῌ+*1.
  ,**. :  ()7789:;  < =()*+>#?@ +,1ῌ+-2 A
BCDEFG HIJK:LMNO +31/ : JK:LMP QR .3  +, STQR /*  - S
22 U VWISidarto, T.C.A., Santosa, S. and Gunawan,W. (+33.) : Geology of the Kotaagung Quadarangle,Sumatera, + : ,/*,*** Explanatory note and geological map, Geological Research and Development Centre.

DXYZXYH[\]N^ ,*** : _% ++ `abcde0fabghi%hPj=kXlmDnopqῌ !"ῌ0r0stuvwLM 0stxyz{"@ _% +,  - S +/. U HI_ |} ,**+ : 0f~kAY  01+ + j I ,*** : $ῌ  ῌY CY  FG I
Tatsumi, Y. (+323) : Migration of ﬂuid phases and genesis of basalt magmas, J. Geophys., Res.,3., .031ῌ.1*1.

  +33/ :   + y X ~kAY ! +20 U FGI +33, :  !"() 01VW()   ¡¢£¤ ++/ U VWIy-¥¦§¨:©ª +33/ : « 2 ¬J§¨:©ª _% -  +* ST_% . - S +.* U VWI