Volcanic history and tectonics of the Southwest Japan Arc

Research ArticleVolcanic history and tectonics of the Southwest Japan Arc

HIROKI KAMATA1 AND KAZUTO KODAMA2

1School of Earth Sciences, Faculty of Integrated Human Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501,Japan (E-mail: [email protected]), and 2Department of Geology, Kochi University, Akebono-cho,

Kochi 780-8072, Japan (E-mail: [email protected])

Abstract Remarkable changes in volcanism and tectonism have occurred in a synchronousmanner since 1.5–2 Ma at the junction of the Southwest Japan Arc and the Ryukyu Arc.Although extensive volcanism occurred in Kyushu before 2 Ma, the subduction-related volcanism started at ca 1.5 Ma, forming a NE–SW trend volcanic front, preceded bysignificant changes in whole-rock chemistry and mode of eruptions at ca 2 Ma. The MedianTectonic Line has intensified dextral motion since 2 Ma, with a northward shift of its activetrace of as much as 10 km, accompanied by the formation of rhomboidal basins in CentralKyushu. Crustal rotation and incipient rifting has also occurred in South Kyushu and the northern Okinawa Trough over the past 2 million years. We emphasize that the commencement age of these events coincides with that of the transition to the westwardconvergence of the Philippine Sea plate, which we interpret as a primary cause of these synchronous episodes. We assume that the shift in subduction direction led to anincrease of fluid component contamination from subducted oceanic slab, which then produced island-arc type volcanism along the volcanic front. Accelerated trench retreatalong the Ryukyu Trench may have caused rifting and crustal rotation in the northernRyukyu Arc.

Key words: Hohi volcanic zone, Kagoshima graben, Kyushu, Median Tectonic Line, Philippine Sea plate, Ryukyu Arc, Southwest Japan Arc, subduction, tectonics, volcanism.

the earliest Quaternary period. Several major volcanic events were reported to have occurred at ca 2 Ma (Kamata et al. 1988a,b; Nakada &Kamata 1991), and crustal-scale deformation has taken place in Kyushu since 2 Ma (Kamata1989b; Kodama et al. 1995; Itoh et al. 1998). ThePHS plate changed its subduction direction in theLate Pliocene–Early Pleistocene (Matsuda 1980;Kaizuka 1984; Seno 1985; Okamura 1988). In thispaper we focus on the synchronous nature of theseevents and propose a tectonic model in which allthese events were caused by a change in the con-vergence direction of the PHS plate.

VOLCANIC EVENTS

Volcanic events are divided into three categories:(i) alignment of volcanoes, (ii) changes in the chem-istry of volcanic rocks, and (iii) changes in mode oferuptions.

INTRODUCTION

A variety of styles of volcanism and crustal de-formation can be observed at the junction of the Southwest Japan Arc and the Ryukyu Arc,under which the Philippine Sea (PHS) plate hasbeen subducting northwestward. The PHS plateresumed subduction at ca 6 Ma after a halt or slow-down of subduction during the period 10–6 Ma(Kamata & Kodama 1994; Itoh & Nagasaki 1996).The difference in arc orientation led to obliquesubduction of the PHS plate beneath the South-west Japan Arc through the Nankai Trough andnormal subduction beneath the Ryukyu Arcthrough the Ryukyu Trench (Fig. 1).

Recent geologic studies in the Kyushu andChugoku Districts have revealed that the majorgeologic events were initiated synchronously in

Accepted for publication 15 February 1999.©1999 Blackwell Science Asia Pty Ltd.

The Island Arc (1999) 8, 393–403

394 H. Kamata and K. Kodama

A volcanic front is defined as a trenchward limitof an alignment of volcanoes lying roughly paral-lel to a trench axis with a certain space interval in the subduction zone (Sugimura 1960; Sugimura& Uyeda 1973). In most subduction zones the volcanic front overlies a contour, of approximately110 km depth, on the dipping seismic plane(Tatsumi 1986). The volcanic front in Kyushu and Chugoku Districts is parallel to the depth contours of the deep seismic plane and is per-pendicular to the present subduction direction ofthe PHS plate (Fig. 2). The volume of volcanicrocks younger than 1.5 Ma is greater in the moreactive, present seismic zone, suggesting that thevolcanism is related to the seismicity occuring nearthe 100–120 km depth interval. Kamata et al.(1988b) compiled radiometric ages for the volcanicrocks along the volcanic front in Central Kyushuand Chugoku Districts and concluded that thelinear alignment of volcanoes emerged at ca 1.5 Ma. Likewise, Sakaguchi (1988) reportedthat the volcanic rocks around Kagoshima grabenin South Kyushu (Fig. 2) are younger than 1.5 Ma,which means that the volcanic front on the north-

ern Ryukyu Arc also formed at ca 1.5 Ma (Kamata1998).

Volcanic rocks in Central Kyushu consist of calc-alkalic and high-alkali tholeiite series in composi-tion. The potassium–silica diagram indicates thatthe volcanic rocks younger than 1.6 Ma have ahigher potassium content than older rocks (Fig. 3).In contrast, Nakada and Kamata (1991) suggestedthat the percentage of calc-alkalic to tholeiitic rockdecreases with time. The increase in potassiumcontent and tholeiitic ratio might result fromsmaller degrees of melting of the mantle source inthe Early Quaternary period (Nakada & Kamata1991).

Basaltic rocks in Central and North Kyushu aredivided into three types: the Northwest Kyushualkalic basalts (NWKB, Fig. 4), calc-alkalic mag-nesian basaltic andesite of the Yabakei-type (YbB)and high-almina basalt of the Kuju-type (KjB)(Nakada & Kamata 1991). Spider diagrams ofincompatible elements indicate that the island-arcbasalts with a niobium anomaly have erupted since2 Ma in Kyushu (Fig. 4). This suggests that thefluid components from subducted slab have conta-

Fig. 1 Index map showing tectonicframework around the Japanese Islands.[HVZ, Hohi volcanic zone; MTL, MedianTectonic Line.]

SW Japan and Ryukyu Arcs since 2Ma 395

minated the mantle wedge more progressively andefficiently since 2 Ma to produce island-arc typevolcanism along the volcanic front.

In Central Kyushu, Plio-Pleistocene volcanicrocks are widely distributed in a 70 by 40 km area.This area forms a volcano-tectonic depression(Hohi volcanic zone; Figs 1, 2) filled with more than5000 km3 of andesitic materials (Kamata 1989b).Evidence indicates that the Hohi volcanic zone wasformed by north–south extension since 6–5 Ma.Geologic evidence includes the east–west trend ofnormal faults that developed and was preserved inthe Quaternary period (Ikeda 1979; Chida & Ikeda1991). The north–south extension rate in the Hohi volcanic zone has decreased monotonicallywith time since 5 Ma to the present (Fig. 5). ThePliocene volcanism is characterized by lava-floweffusion that formed huge lava plateaus (Table 1).

The voluminous lava flows were extensively buriedin a large graben that formed at ca 5 Ma (Kamata1989b). The volcanism with fissure eruption and graben formation was initiated by a large-scale crustal melting as the result of a strongnorth–south extension that was caused by subduc-tion of the PHS plate, which restarted at ca 6 Ma(Kamata & Kodama 1994).

In contrast, the Pleistocene volcanism inCentral Kyushu is characterized by eruptions oflarge-scale pyroclastic flows (ignimbrites) of morethan 10 km3 in magma volume, forming calderas ofca 10 km in diameter (Fig. 5). Using K–Ar andfission-track methods, these ignimbrites weredated at 1.3 Ma (Shikido ignimbrite; Takemura et al. 1988), 1.0 Ma (Yabakei; Danhara et al. 1997), 0.9 Ma (Imaichi; Kamata et al. 1994) and 0.6 Ma(Yufugawa; Hoshizumi & Kamata 1991).

Fig. 2 Distribution of volcanic rocks,plate motions and the depth contour of thedeep-seismic plane at the junction of theSouthwest Japan and Ryukyu Arcs. Darkareas represent the distribution of volcanicrocks younger than 1.5 Ma (after Kamata1998). Stippled areas represent the Hohivolcanic zone. The solid arrows show themotion of the Philippine Sea plate (afterSeno 1977). The open arrows show pastmotion of the Philippine Sea plate before1.5 Ma (according to Seno & Maruyama1984; Seno 1985). Dashed lines show thedepth of the seismic plane (Shiono 1974;Yoshii & Kobayashi 1981). [HVZ, Hohivolcanic zone; PHS, Philippine Sea plate.A, Aso volcano; K, Kuju volcano; Y, Yufu-Tsurumi volcano; a, Futagoyama volcano;b, Himeshima volcano; c, Takeyamavolcano; d, Shikuma volcano; e, Sengokuvolcano; f, Tokuyama-Kimpo volcano; g,Chojagahara volcano; h, Nosakayamavolcano; i, Kumoimine volcano, j, Aoyasanvolcano; k, Chikura volcano; l, Nabeyamavolcano; p, Omine volcano; q, Akaivolcano; r, Kumamoto-Kimbo volcano; s,Unzen volcano.]


The ignimbrite eruption requires the develop-ment of a large-volume (> 10 km3) magma chamberstable (in repose) for > 0.1 million years betweeneruptions. The storage of voluminous felsic magmaat a relatively shallow depth in the crust is causednot by high magma production but by continuousaccumulation of magma without significant tap-ping during a long repose time (Takahashi 1995).The occurrence of ignimbrites in Central Kyushuemplaced exclusively after 1.5 Ma (Fig. 5) suggeststhat the crust peripheral to the magma chamberreached a condition favorable for efficient magmaaccumulation after the extension rate in the Hohivolcanic zone had decreased (Takahashi 1995;Kamata 1996). Since then the magma chamberssustained and evolved on the balance of contrac-tion caused by continuous magma supply fromdepth and crustal extension due to the graben for-mation. Other evidence for the transition of vol-canism around 2 Ma includes a monotonic decreasein eruption volume of each eruptive unit and achange in eruption style from fissure eruption tocentral eruption (Table 1; Fig. 5).

STRUCTURAL GEOLOGIC EVENTS

Major structural geologic events on the SouthwestJapan and the Ryukyu Arcs include (i) accelerationof dextral-fault movement along the Median Tectonic Line (MTL) (ii) progressive formation ofrhomboidal basins in Central Kyushu (iii) crustalrotation of intra-arc blocks at South Kyushu and(iv) incipient rifting at the northern OkinawaTrough and the Kagoshima graben.

Fig. 3 K2O-SiO2 diagram in wt%. (d)Volcanic rocks older than 1.6 Ma and (s)volcanic rocks younger than 1.6 Ma inHohi volcanic zone (HVZ) except ejecta ofAso volcano; (triangle) ejecta of Asovolcano. Chemical compositions recalcu-lated to be H2O-free. (Data from wet chem-ical analyses of Kamata 1987, 1989b.)

Fig. 4 Spider diagrams of incompatible elements for the NorthwestKyushu basalts (NWKB), Yabakei-type basalts (YbB) and Kuju-typebasalts (KjB) (after Nakada & Kamata 1991). [IAB, island-arc basalt; OIB,ocean-island alkalic basalt.]


The MTL is one of the most prominent geologicbreaks in Southwest Japan (Fig. 1). It extends 900km in an east–west direction and comprisesseveral parallel or an echelon fault systems forming a 10-km-wide shear zone. Kamata (1992)inferred that the MTL connects to the west to theOita-Kumamoto Tectonic Line in Kyushu (Fig. 6).Sinistral faulting was dominant along the MTLduring the Cretaceous to Early Paleogene periods,although thrust movement in the Late Paleogeneis detectable in several areas (Ichikawa 1980). Bycontrast, dextral faulting has dominated since the earliest Quaternary period to the present time(Kaneko 1966; Sangawa 1986). The oldest recordof the dextral displacement was dated ca 2 Ma andwas reported from the eastern margin of the MTLon the basis of the analysis of faults that juxta-

posed confluent fans and rivers (Sangawa 1980;Tsukuda 1990).

The dextral motion on the MTL has activatedthe formation of rhomboidal basins in CentralKyushu since 2 Ma. Geologic, gravimetric andseismic data indicate that the MTL has shifted itsactive trace northward as much as 10 km (trace 1in Fig. 7; Itoh et al. 1998). As a result, the depocen-ter basins adjacent to the MTL migrated andformed progressively northeastward in the Hohivolcanic zone: the Kuju basin (KJ in Fig. 7) formedfirst, after which the Shonai basin (SN) formed,followed by the Beppu Bay basin (BB). The KujuBasin was formed at ca 6 Ma during initiation ofsubduction, whereas the Shonai Basin formed inthe Early Quaternary period (2–1.5 Ma) as dextralmotion along the MTL accelerated.

Fig. 5 Rate of extension (10-3 cm/year)versus time (in million years) for the Hohivolcanic zone (HVZ) (Kamata 1989b; Taka-hashi 1995). Large circles represent eruptivevolume in 102 km3, the small circle in 101

km3, for pyroclastic-flow deposits (afterKamata 1989a; Kamata et al. 1994). Note thateruption of lava flows was predominant in5–2 Ma whereas the large-volume pyroclas-tic-flow volcanism started at ca 2–1 Ma.

Table 1 Transition of tectonic and volcanic activities of the Hohi volcanic zone in Central Kyushu

Geologic period Tertiary (Pliocene) Quaternary (Pleistocene)

Age 5–1.5 Ma 1.5–0.7 Ma After 0.7 MaEruption volume Large Medium SmallVolcanic edifice Lava plateau Small-scale lava plateau Lava dome and stratovolcano

Alignment of volcanoes East–west Northeast–southwest Northeast–southwestEruption style Fissure eruption Central eruption Central eruptionStress field Strongly extensional Extensional Extensional and locally compressional

Ignimbrite Unknown Shikido, Yabakei, Imaichi Yufugawa, Aso

K2O (wt%) Low High High


Fig. 6 Dextral faults (the Oita-Kumamoto Tectonic Line and the Median Tectonic Line) and the Hohi volcanic zone (solid dots) in Kyushu and ShikokuIslands (Kamata 1992). Open arrows show the present subduction direction of the Philippine Sea plate (Seno 1977). Small solid arrows show the activestrike–slip faults. [HVZ, Hohi volcanic zone; KT, Kokura-Tagawa Fault zone; MTL, Median Tectonic Line.]

Fig. 7 Northward shift of the MedianTectonic Line (MTL) and progressive for-mation of associated rhomboidal basins inthe Hohi volcanic zone (HVZ) in the earlyPleistocene–Holocene periods (after Itoh etal. 1998). Bouguer anomaly contours in 2.5mgal intervals (Komazawa & Kamata 1985)are shown.


Paleomagnetic data from the Miocene–Pliocenesedimentary rocks in South Kyushu show that alarge part of South Kyushu has been rotated by about 30 degrees counter-clockwise during the past 2 million years. This rotation started synchronously with rifting of the continental crust beneath South Kyushu and the northernOkinawa Trough. This commenced around thePliocene–Pleistocene boundary and continues tothe present. Kodama et al. (1995) proposed a tectonic model to explain the counter-clockwiserotation of South Kyushu in terms of the rotationof three intra-arc blocks (Fig. 8). The westernboundary of these blocks is defined as the north–south trending Kagoshima graben, a volcano-tectonic depression developed since the latestPliocene (Aramaki 1984; Uto et al. 1997). Accord-ing to marine geophysical surveys (Sibuet et al.1987), the northern Okinawa Trough is in therifting stage of back-arc basin evolution. Kodamaet al. (1995) emphasized that the rotation of SouthKyushu took place synchronously with the crustalextension and volcanism occuring since 2 Ma.

TECTONIC IMPLICATIONS

In summary, the volcanic and structural geologicevents of significance occurred in the Early Pleis-tocene. We conclude that the synchronous nature

Fig. 8 Tectonic model for counter-clock-wise rotation of South Kyushu separated intothree blocks of A, B and C, from north tosouth (modified from Kodama et al. 1995). Ascrustal stretching in Kagoshima graben andthe northern Okinawa Trough progressesnorthward, the blocks are pushed eastward,accompanied by counter-clockwise rotation.

of these events was caused by a change in the PHSplate motion at ca 2 Ma. Matsuda (1980) andKaizuka (1984) proposed that the PHS plate wassubducting northward at ca 3 Ma (based on thestructural deformation in the northern part of theIzu Peninsula and the southern Kanto District inCentral Japan) and that afterward the subductionchanged to a more westward direction. Okamura(1988) proposed that the westward change in sub-duction direction took place at the Late Plioceneto the Early Pleistocene, because of the shift inpaleoposition of the Kyushu-Palau Ridge in theshelf slope along the Nankai Trough off southKyushu. Seno (1985) stressed that the change inthe PHS plate motion from north-northwest towest-northwest occurred ca 1.5 Ma, according tothe analysis of submarine morphology in the triplejunction off Boso Peninsula in Central Japan. Thisevidence suggest that the on-land volcanic and tectonic events in Kyushu synchronize with thewestward shift in the direction of the PHS platemotion at ca 2 Ma.

It has been argued that the Japan Sea startedto contract at ca 2 Ma, resulting from the onset ofeastward migration of the Amurian plate, the east-ernmost portion of the Eurasia plate, and north-west–southeast extension of the Baikal rift zone(Zonenshain & Savostin 1981; Tamaki & Honza1985; Tokuyama et al. 1992; Ishibashi 1995; Wei &Seno 1998). The mechanical coupling between the


synchronism of volcanic events can be betterexplained by the oceanic slab subduction, whichcaused changes in the chemical and physical con-dition of magma at ca 2 Ma.

The island-arc type volcanic rocks on the volcanic front in Kyushu were produced by the progressive contamination of mantle wedge by components derived from the subducting slab (Nakada & Kamata 1991). Partial melting of crustal material is a possible explanation of the 5–3 Ma calc-alkaline andesite magmas in theHohi volcanic zone (Fig. 9a). The volcanic rocksyounger than 2 Ma are characterized by a smallerdegree of partial melting and more progressivecontamination by slab-derived components (Fig.9b). Kamata (1998) interpreted these island-arc type volcanic materials along the volcanic frontto be produced by more efficient contamination,which was caused by subduction of the PHS plateafter 1.5 Ma. Although the relationship betweenvolatile contamination and the change in the PHS subduction direction is not well understood,increase in degree of coupling between theEurasia and the PHS plates may have caused pro-gressive contamination in the mantle above thedowngoing slab. It is also probable that the degreeof frictional/shear heating on the upper surface ofthe subducting slab is higher in active seismicareas (Acharya 1981), resulting in an enlargementof the partially molten zone above the dragged

Fig. 9 Schematic diagram showing convection and contamination ofthe mantle wedge and igneous activity in Central and North Kyushu(modified from Nakada & Kamata 1991). [CA, calc-alcalic; PHS, Philip-pine Sea plate; TH, tholeiitic.]

Amurian plate and the PHS plate might also havecaused tectonic effects such as an acceleratedmigration of Nankai sliver along the MTL.However, we believe that the change in the PHSplate motion played a more significant role thanthe Amurian eastward motion, because the

Fig. 10 Tectonic development of the Southwest Japan Arc and the Ryukyu Arc before 6 Ma, from 6 to 2 Ma and after 2 Ma. Open arrow shows the convergence direction of the Philippine Sea plate. [CCW, counter-clockwise rotation; CW, clockwise rotation. HVZ, Hohi volcanic zone; MTL, Median Tec-tonic Line; PHS, Philippine Sea plate.]


hydrated peridotite layer (Tatsumi & Eggins1995).

The oblique subduction of the PHS platebrought about an acceleration of dextral slip on theMTL (Nakamura et al. 1984; Nakamura et al. 1987(Figs 1, 10), which led to the northward shift of thewestern end of the MTL accompanied by the pro-gressive formation of rhomboidal basins in CentralKyushu (Fig. 7).

Paleomagnetic results from South Kyushu indi-cate about 30 degrees of counter-clockwise tec-tonic rotation relative to the Eurasian continent(Fig. 10). The retreating trench model of Viallon et al. (1986) predicts a pair of clockwise rotationsfor the southern Ryukyu Arc and a counter-clock-wise rotation for the northern Ryukyu Arc.According to Miki et al. (1990) and Kamata andKodama (1994), the southern Ryukyu Arc experi-enced clockwise rotation during 6–2 Ma (Fig. 10).In contrast, the northern Ryukyu Arc has experi-enced counter-clockwise rotation since 2 Ma(Kodama et al. 1995), during which the northernOkinawa Trough has been in an incipient spread-ing stage (Sibuet et al. 1987). We propose that therifting and crustal rotation in South Kyushu havebeen caused by the onset of arc-normal subductionof the PHS plate after 2 Ma, coupled with a morerapid retreat of the Ryukyu Trench.

ACKNOWLEDGEMENTS

The valuable comments of Kozo Uto, Tetsuzo Seno,Xavier Le Pichon, Loren Kroenke, YoshiyukiTatsumi, Asahiko Taira, Keiji Takemura, YasutoItoh, Masaki Takahashi and Keiko Suzuki-Kamataare greatly appreciated.

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Volcanic history and tectonics of the Southwest Japan Arc