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Tectonic Movements in the Central Kyushu, JapanKeiji TAKEMURABeppu Geothermal Res. Lab., Kyoto University

AbstractThe central part of Kyushu, Japan is a locus of active arc-volcanism associated withnorthwest subduction of the Philippine Sea plate, and there the regional graben-likestructure (”Beppu-Shimabara graben”) from Beppu Bay to the Shimabara peninsulaextends. Subduction of the Philippine Sea plate has played an important role in volcanic,geothermal and tectonic activity in central Kyushu.
There are two main graben-like areas in central Kyushu such as Hohi-volcanic zonein the eastern part and Unzen graben in the west. The tectonic movements in theHohi-volcanic zone are a#ected by the activity of Median Tectonic Line and KokuraTagawa Line related with the movement of Philippine Sea Plate subduction. Initialnorthward subduction in late Miocene formed the half graben structure of initial Hohivolcanic zone. Relative convergence direction of the Philippine Sea plate shifted counterclockwise at about +./ Ma, and the same time west-northwestward subduction enhancedright slip on the Median Tectonic Line. Geologic, gravimetric and seismic data indicatethat the Median Tectonic Line has shifted its active trace northward in central Kyushu.
As a result, the depocenter adjacent to the transcurrent fault migrated northeastward inthe Hohi volcanic zone, from Shonai basin (early Quaternary) to Beppu Bay basin (lateQuaternary). The latest depocenter of the Beppu Bay is surrounded by active faults thatdelineate a rhomboidal basin on the Median Tectonic Line. Eastern Central Kyushu(Hohi-volcanic zone) shows the basin forming history and tectonic evolution at thetermination of a large transcurrent fault system of Median Tectonic Line.
Combining fault motions, which were modeled as dislocation planes embedded in anelastic isotropic half space, restored the subsurface structure in Hohi-volcanic zone.

143»g¼²144

The tectonic movements in Unzen graben must be related to the activity of back arcspreading of Okinawa Trough or volcanic activity around Northwest Kyushu hot region.
Key words : Central Kyushu, Beppu-Shimabara Graben, Median Tectonic Line, SeismicReﬂection Survey, Gravity, Tectonic Development      :   - ῌ +        !"#$%&'(&)*+,- ./ 01 23ῌ4 56 +3137 89:1-;< 5= +7/ >?@AB 5Kamata, +323 7 CD

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145

Fig. + Distribution of pre-Tertiary basement rocks and active faults around Beppu Bay (Takemura et al.,+33.). Information on land is mainly by Hoshizumi et al. (+322), and information in Beppu Bay is mainlyby Yusa et al. (+33,). Line A, G, H : multichannel seismic reﬂection survey line, a : Neogene sedimentsand volcanics, b : Ryoke Metamorphic rocks and granite, c : Sanbagawa metamorphic rocks, Thicksolid and broken lines on land show faults ; ticks point down dip. Open triangles are Quaternaryvolcanoes. A : Listric fault region, which is developed at the upper part of the anticlinal structure. B :Basin structure region buried by thick sediments. C : Trough-like structure region. Contours in bayarea show depth (m) to the pre-Tertiary basements. BBF : Beppu Bay Cross Fault separating BeppuBay into B and C regions, RS : Margin of Ryoke Belt basement rocks beneath the sediments, AS :Asamigawa fault, BK : Beppu Kita fault, Yu : Yufudake, Tu : Tsurumidake, Ga : Garandake, Ta :TakasakiyamaInset : The platetectonic environment of the Beppu Bay region. MTL ; Median Tectonic Line, ISTL :Itoigawa Shizuoka Tectonic Line, OT : Okinawa Trough, RT : Ryukyu Trench, NT : Nankai Trough, K :Kyushu, S : Shikoku, H : Honshu.
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Fig. , Bouguer anomaly map and nomenclature of depression (Itoh et al., +332). The anomaly map iscompiled from Komazawa and Kamata (+32/) and Takemura et al. (+33-). The shaded zone is agravity slope coincident with the western margin of the Hohi Volcanic Zone. BB : Beppu Bay, SN :Shonai, KJ : Kuju, SY : Shin-Yabakei, SS : Shishimuta, AS : Aso. ,  ! Itoh et al., +332KL¦§ῌ¨© +32/ ª« +33- $¬< ­iAbn(®I(¯°±E<BB :  SN : \ KJ : ²³ SY : , ´µ SS : ¶!© AS : ·¸-   /  +

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PHSsubduction

MiddlePleistoceneHolocene

WNW

Strain in Hohivolcanic zone

 ;Itoh et al., +332=5Volcanism in Hohivolcanic zone

western part :contractioneastern part :extension

Episode - (*.1 Ma) : Shift of active trace of MTLEarlyPleistocene

WNW

centraleruption 0**less voluminousStage , : Migration

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higher K

 +tholeiitic

Table +147

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LateMiocenePliocene

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EarlyPleistocene

calc-alkalic

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KTL§(L)MTL*§(R)

KJ, SN, BBasymmetricKTL§§(L)MTL*§(R)

Episode + (0 Ma) : Initiation of Hohi volcanic zoneSee Fig. , for abbreviations of basins. §§ : high fault activity, § : low fault activity, (R) :right-lateral motion, (L) : left-lateral motion¨©ªn , «¬5 §§ : ­LH>?$ § : ®LH>?$ R : JK#S?$ L : JK#S?5  FS, !"#$ %#$ &'()*+,-./-/01 2345%#6789: ;77

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K#S?>?TUVWMXYZE +* km [0)\R M!"#56]^_ $ 0 Ma `\abcῌdc1 ;efghi= jk?lmn /o + Zpqr4\5efghist$ uvwxyz{UV F;| 0 Ma= }~ Zm\ !"# ;Itoh et al., +332 -=5 %9:)~ῌMJK#S? 4>?m 4\5 %#%#67]dῌstm -\5 RR efghist @4\ !"# ;o +  stage +=5 ῌ)+, D#efghi2345)JK#S?¡-¢G)HI]cmksD£ ;PQ$+33,=5 4\M$ MTL ¤~¥u ;trace * trace += 7¦R\148:;<=Fig. - Active fault distribution around the Hohi volcanic zone (Itoh et al., +332) after Research Group forActive Tectonics in Kyushu (+323), Research Group for Active Faults (+33+) and Takemura et al. (+33.).
The thin broken line and thick dotted line are the oldest trace of the MTL and the reﬂection seismic lineH in Fig. . respectively. See Fig. , for abbreviations of basins.
 -   Itoh et al., +332+323 +33+ Takemura et al. +33.  H :   +33, !"#$% MTL &'()*Fig. . Reinterpreted seismic proﬁle across the mouth of Beppu Bay (Itoh et al., +332)FS+ and FS, represent locations of conspicuous ﬂower structures. See Fig. - for line location.
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hi o(149Fig. / A tectonic synthesis of the Hohi volcanic zone (Itoh et al., +332).
Bouguer anomaly contour is drawn on base maps.
 /   Itoh et al., +332    KJ$ %& SN!"#'(')*+, %& SN - ./0 123 4567  8 9( :;<=>?@ !ABAC DEF$*GH+I$J  KLM(NOP+, %& SN J Q +rhomboidal basin RST, J

 9( trace + U trace ,W H *-XY,T,  *Z%& SN, [Z-\],X^-_/4V!"-'(`a rhomboidal basin NRS

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ῌItoh, Y., Takemura, K. and Kamata, H. (+332) : History of basin formation and tectonicevolution at the termination of a large transcurrent fault system : deformation mode ofcentral Kyushu, Japan. Tectonophysics, ,2., +-/῍+/*.
Kamata, H. (+323) : Volcanic and structural history of the Hohi volcanic zone, central Kyushu,Japan. Bull. Volcanol., /+, .+῍/*.

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Kusumoto, S., Fukuda, Y. and Takemura, K. (+333b) : A distinction technique between volcanic and tectonic depression structures based on the restoration modeling of gravityanomaly : a case study of the Hohi volcanic zone, central Kyushu, Japan. Jour. Volcanol.
Geothermal Res., 3*, +2-῍+23.
Kusumoto, S., Takemura, K., Fukuda, Y. and Takemoto, S. (+333a) : Restoration of depressionstructure at the eastern part of central Kyushu, Japan, by means of dislocation modeling.
Tectonophysics, -*,, ,21῍,30.

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Takemura, K., Yusa, Y. and Itoh, Y. (+33.) : Quaternary tectonic movements around BeppuBay at the western end of the Median Tectonic Line, southwest Japan. Spec. Pub. Jour.
Geod. Soc. Japan (Proc. CRCM ‘3-, Kobe), .*+῍.*/.

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