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Tectonics of an arc-arc junction: an example from Kyushu Island at the junction of the Southwest Japan Arc and the Ryukyu Arc
TECTOHOPHYSICS Te~tonophysi~ 233(1994)69-H
Tectonics of an arc-arc junction: an example from Kyushu Island at the junction of the Southwest Japan Arc and the Ryukyu Arc Hiroki Kamata a, Kazuto Kodama b n @g&a
office, Geological Survey of Japan, Govemment Bldg. No. 2,Bekkan, 4-I-67, Otemae, Chum-ku,Osaka54% Japan, b Llepwtmentof Geology, Kochi ~nive~~~, 2-5-1, Akdono-cho, Kochi 780, Japan
(Received April 6, 1993; revised version accepted January 7, 1994)
Abstract Three distinctive geologic events occurred synchronously during the last 6 m.y. on Kyushu Island, lying at the junction of the Southwest Japan Are and the Ryukyu Arc. These events included: (1) dextral-fault displacement along a 100 km section of the Median Tectonic Line; (2) formation of a rectangular volcano-tectonic depression 70 km long and 40 km wide; and (3) approximately 30” of counterclockwise crustal rotation. We propose an integrated tectonic model that explains these three tectonic features in terms of subduction of the Philippine Sea plate, which resumed at around 6 Ma after a halt of more than 5 m.y. Oblique subduction of the Philippine Sea plate beneath the Southwest Japan Arc detached a fore-arc sliver that was displaced along a dextral fault, the Median Tectonic Line, north of the Nankai Trough, A volcano-tectonic depression, the Hohi volcanic zone, was formed by the pull-apart structure on the western margin of the Median Tectonic Line. By contrast, normal subduction of the Philippine Sea plate under the Ryukyu Arc caused back-arc spreading, observed as crustal extension in the northern Okinawa Trough. The crustal extension caused the counterclockwise rotation for the northernmost Ryukyu Arc including southern Kyushu.
1. Introduction Arc-arc junctions are not only geomo~hologitally peculiar but also exhibit a variety of tectonic features such as strike-slip faults, ridges and troughs (Fitch, 1972; Jarrard, 1986; Seno et al., 1986). The development of these features has been attributed to local stress concentration in the end of intra-arc transcurrent faults (e.g., Fitch, 1972). These interpretations, however, have generally been based on a lack of detailed, on-land geologic and geophysical documentation. We will review recent studies of Kyushu Island, located at HO-1951~94/$07.~
0 1994
SXU 0040-1951(94)00008-w
the junction of the Southwest Japan and the Ryukyu arcs (Fig. 11, and highlight that Kyushu experienced three distinctive geologic events: intra-arc strike-slip fault displacement; formation of a volcano-tectonic depression; and counterclockwise tectonic rotation. All of these events took place during the last 6 m.y. and since the Philippine Sea plate began to subduct beneath Kyushu. We will propose a tectonic model that ascribes these three events to the inception of the subduction of the Philippine Sea plate and will put emphasis on the difference in subduction direction relative to the two island arcs.
Etsevier Science B.V. All rights reserved
H. Kamata, K. Kodama / Tectortophysics 233 (1994) 69-81
70
2. Strike-slip faults The Median Tectonic Line (MTL) is a striking geologic structure, which extends for 900 km in an E-W direction in southwestern Japan (Fig. 21 and separates a fore-arc sliver from the arc crust (Fig. 3). Sinistral faulting had been dominant along the MTL during Cretaceous to early Paleogene times, while in the late Paleogene thrust movement has been demonstrated in several of its portions (e.g., Ichikawa, 1980). By contrast, dextral faulting has dominated since at least the early Quaternary to the present (Kaneko, 1966; Sangawa, 1986). The previous oldest record of the dextral displacement was dated at about 2 Ma, which was reported from the eastern margin of the MTL, on the basis of the analysis of faults 6-E
12bE I
that juxtaposed confluent fans and rivers (Sangawa, 1980; Tsukuda, 1990). Recently, Kamata (1992) re-estimated the initiation of dextral displacement on the MTL and suggested an age of 5 Ma, based on structural analyses of the western extension of the MTL, or the Oita-Kumamoto Tectonic Line, along with its branches as the Hinagu, the Futagawa and the Imahata faults (Fig. 2; Watanabe, 1984; Okada and Chida, 1991; Chida and Ikeda, 1991). Kamata (1992) noted that an ENE-WSW-trending negative gravity anomaly, running along the OitaKumamoto Tectonic Line, is bent clockwise at two locations (P and R in Fig. 4). He interpreted the negative anomaly pattern as rhomboidal pull-apart tectonic voids (e.g., Glazner, 1991) caused by dextral displacement along the Oital&Et
i
,loookm
Fig. 1. Index map of the Japanese Islands and associated plates and arcs.
H. Kamata, K K&m
71
/ Tec~o~phy~~c~ 233 (19941 69-81
Kumamoto Tectonic Line. He then dated the commencement of the dextral movement to be not later than 5 Ma using the average dextral displacement rate of the MTL (Kamata, 1992). This estimation is insistent with the timing of formation of several fore-arc basins, which is regarded to be contemporaneous with the MTL activity (Sugiyama, 1991).
3. Volcano-t~~nic d~p~ssion On central Kyushu, Plio-Pleistocene volcanic rocks are widely distributed in a 70 by 40 km area. This area forms a volcano-tectonic depression (HVZ in Fig. 2) filled with more than 5000
km3 of andesitic material (Kamata, 1989b). Much evidence indicates that the HVZ was formed by N-S extension in Quaternary times. Geologic evidence includes the E-W trend of normal faults that developed in the early to late Quaterna~ (Fig. 2; Ikeda, 1979; Chida and Ikeda, 1991). Geophysical evidence includes the results of geodetic surveys (Tada, 1985), earthquake focal mechanisms (Hatanaka and Shimazaki, 19881 and geothermal investigations (Yuhara and Ehara, 1981; Yano et al., 19871. The HVZ forms a boxshaped graben surrounded by steep gravity gradients (Fig. 4). Gravity analysis (Kamata, 1993), drilled core description (Tamanyu, 1985; Kamata, 1989b) and seismic refraction exploration (Kubotera et al., 1982) all indicate that pre-Tertiary
s
strike-slip fault ,
normal fault
Fig. 2. Dextral faults (the Median Tectonic Line and the Oita-Kum~oto Tectonic Line) and the Hohi vofcanic zone (dots are oultine of HVZ) on Kyushu and Shikoku islands. Open arrows with PHS show the present subduction direction of the Philippine Sea plate, after Seno (1977). Small solid arrows are active strike-slip faults. KT = Kokura-Tagawa Fault Zone. Map location shown in Fig. 1.
H. Kamata, X Kodanta/ Tectonophysics 233 &W4~ 69-81
72
basement rocks occur up to 3 km below the surface. Radiometric ages of volcanic rocks in the HVZ show a systematic zonation (Kamata, 1989b) younging inwards from the margins, with outer ages between 6 and 5 Ma and inner ages of 0.2 Ma (Fig. 5). A similar zonal pattern is also observed in the Bouguer gravity anomalies. No radiametric ages older than 2 Ma were reported from within the -5 mGa1 contour line, while those younger than 1 Ma were found only within the - 15 mGa1 contour line. Moreover, those rocks younger than 0.5 Ma occur in the subcircuIar region with an anomaly of more than -30 mGal (Ss in Figs. 4 and 5, Shishimuta caldera; Kamata 1989a). These features imply close spatial and temporal links between the Plio-Pleistocene volcanism and the graben formation, Calcafkalic high-Mg basaltic andesites were erupted in the early stage of the HVZ activity (5-3 Ma), while high-alumina basalts typical of island arcs were erupted in the later stage (2-O Ma) (Kamata et al., 1988b; Nakada and Kamata, 1991). The eruption rate of the HVZ decreased with time, as is quantitatively indicated by changes of both area and erupted volume for each I m.y. interval, which shows m~mum activity produc-
NW
Median
(Oita-Kumamoto
mantle
ing about 3000 km3/m.y. interval 5-4 Ma (Fig. 6).
early on during the
4. In&a-are rotation Paleomagnetism of the Late Miocene rocks on southern Kyushu indicates westward declinations. The paleomagnetic directions of the Uchiumigawa Group on southeastern Kyushu (3 in Fig. 7; Kodama and Nakayama, 1993) and the Kukinaga Group in Tanegashima Island (1 in Fig. 7; Kodama et al., 1991) provide a common mean direction that deviates from the present dipole field direction by 26.8 k 5.6” in declination and 0.1 rt 3.9” in inclination. Several westerly deflected magnetization directions were also obtained from the Middle Miocene granite on southern Kyushu (2 in Fig. 7) by Ishikawa and Torii (1986) who estimated a 45 _t 24” westward shift. Because the youngest age of these rotated strata is the Late Miocene, the counterclockwise rotation took place after about 6 Ma. The Eurasian mean paleopole has been in agreement with the geographic pole from the Late Miocene onward (Irving and Irving, 19821, during which no relative motion has been recognized between Eurasia and southwest-
Tectonic Line Tectonic Line)
SE Pacific
Ocean
wedge
I
I/
+(aseismic) I
Fig. 3. Profile of the Southwest Japan Arc and subducting Philippine Sea plate. Configuration of plate and crust is after Ishihara and Yoshida (1992) and Isozaki et al. (1990). Location of profile line shown in Fig. 1.
em Honshu (e.g*$ Otofuji et al., 199ll* These ~al~~magnetic data led Kodama and Nakayama Cl9931to con&de that soutbem ICyusku underwent about W of ~~~t~~~I~~is~ tectonic ~&ation relative t~3the Eurasian continent after the latest Miocene. They i~t~~~t~d that this cmmtercloekwise rotation was caused by widespread
stretchiny of contixxmtal crust as exempIified by a number of submarine normal faults behind soutbem Ky~sbu (Fig. 8; Sibuet et at., 1987). Xncontrast, Miki et d. (1990) suaest &&wise ~~t~ti~~ for the s~~tb~rn Ry&y~ Arc on the basis of ~aIe~rnag~~tisrn of Ish~g~~jima in thr: Ryuky~ Arc (Fig. 8). They claim that this &ck-
Fig. 4. Bouguer nnamaiy contours in mCal (p = 2.3 g/cm3 after Komazawa and Kamata, 1985). H = high; L = low; OKC = OitaKumamoto Tectonic Line: As = Asa volcano; & ~1Kuju volcano; Yf -; Y&u-Tsurumi volcano; 8’~ = Futago voIcano; SS = ~~~~~rn~t~~Dots show the outline of the H&i volcartic zone. Map &cation &own in Fig. 1.
74
H. Kumata, K. Kodamu / Tectonophysics 233 U994) 69-81
wise rotation was caused by a wedge-shaped spreading of the back-arc basin east of Taiwan (Fig. 8; e.g., Otofuji and Matsuda, 1987).
5. Subduction of the Philippine Sea plate The Philippine Sea plate is currently subducting northwestward under the Southwest Japan Arc and the Ryukyu Arc (Figs. 2 and 8; Seno, 1977). However, subduction history of the Philippine Sea plate in the past is controversial. Seno and Maruyama (1984) reported that the Philippine Sea plate subducted northwestward at 17 Ma. Several workers suggest that the Philippine Sea plate ceased to have subducted in some pe-
riod between the present and Miocene times (Uto, 1989; Kamata, 1992). Our compilation, however, suggests that the Philippine Sea plate has been subducting no~hwes~ard since about 6 Ma. Evidence for this includes the following: (I) Matsubara and Seno (1980) pointed out that the Philippine Sea plate has had a northwestward direction of subduction from at least 5 Ma on the basis of its average convergence rate (3.5-4.5 cm/yr) and the length of the subducted slab (150-200 km). (2) The WVZ recorded its rn~irn~ eruption rate at about 5 Ma, which has been followed by a monotonous decrease in eruption rate to the present (Fig. 6). Temporal variations of the chemical composition of the volcanic rocks on central LEGEND
Suou-nada
Sea
~Jouatemary aLava
voJearlks and ahrvium
dames youtger than 0.1 ?&
Pliocene volcanic8 Miocene vokanks
and intruskes
Pre-Tertiary basements
’ RADIOMETRIC
AGE
0 0.2Ma9ge
4.OkkwwI
Fig. 5. Distribution of volcanic rocks and their radiometric ages superimpased on the Bouguer anomaly map of the Hohi volcanic zone WVZ). Radiometric ages after Kamata (1989b). Bouguer anomaly contours in mGal after Komazawa and Kamata (1985). Solid lines show the distribution of volcanic rocks divided by 1 m.y. intervals. Mi = Miyanoharu; Ss = Shishimuta. Map location shown in Fig. 1.
H. timata, K. Kodama / Tect~~~hysi~s233 (1994’)69-81
Kyushu show (a) typical chemical characteristics related to the subduction of the oceanic slab from 6-5 Ma to the present, and (b) progressive contamination of the mantle wedge by this oceanic slab over time (Nakada and Kamata, 1991). These results suggest that the Philippine Sea plate has been continuously subducting since 6 Ma, giving rise to calcalkaline volcanic activity at the earliest stage. (3) Niitsuma and Akiba (1985) pointed out that the Late Miocene to Pliocene sed~ents commenced deposition at 7-6 Ma in fore-arc basins along Nankai Trough, and that these sediments were of turbidite origin derived from the collision of the Izu-Bonin Arc with the Japanese Arc. Thus, they suggested that the Philippine Sea plate subducted from 7-6 Ma to at Ieast 3 Ma. (4) Sugiyama (1991, 1992) demonstrated that the northwestward subduction of the Philippine Sea plate resumed at about 5 Ma, because some bends in the geologic structures in the outer zone of the Southwest Japan were formed by dextral movement of the MTL. There is other evidence implying a 10-6 Ma halt to subduction of the Philippine Sea plate.
Fig. 6. Temporal variations of volume and area for volcanic rocks in the Hohi volcanic zone. Data plotted in center of each million-year interval used after Kamata (1989b).
75
(1) Tatsumi and Maruyama (1989) demonstrated that the Setouchi high-Mg andesites were produced by subduction of young, hot oceanic lithosphere of the Philippine Sea plate. The age of the Setouchi volcanism ranges from 17 to 10 Ma (Takahashi, 1981; Tatsumi, 1983). (2) According to DSDP Leg 31 in the northern Shikoku Basin (Ingle and Karig, 1975) terrigenous turbidite occurs in very limited core sections dated between 5 and 4 Ma. This suggests that the trough was not deep enough to trap terrigenous deposits at around 5 Ma. In other words, the Philippine Sea plate did not start subducting in the current setting until about 5 Ma (Seno and Maruyama, 1984). (3) Calc-alkaline volcanism was absent but mantle-zenoliths bearing basanites erupted during the period 9.5-6.5 Ma in the western part of the Southwest Japan Arc, suggesting that the oceanic plate did not subduct during the period 10-6 Ma (Uto, 1989).
6. The tectonic model We interpret the three geologic events on Kyushu in terms of resumed subduction of the Philippine Sea plate beneath the arc-arc junction. The major tectonic changes are detectable after 6 Ma (Fig. 9). Our model emphasises the difference in orientation of the subduction direction relative to the Southwest Japan Arc and the Ryukyu Arc (Fig. 10). We propose that after 6 Ma oblique subduction under the Southwest Japan Arc yielded a strong coupling between the fore-arc sliver and the subducting slab (e.g., Kimura, 1985), which resulted in dextral displacement on the MTL and its western extension, the Oita-Kumamoto Tectonic Line (Fig. 9). The dextral-fault movement along the Oita-Kumamoto Tectonic Line, coupled with its pull-apart geometry, formed a graben of 70 by 40 km (HVZ) that has extended in the N-S direction. The graben is filled with voluminous volcanic materials erupted since about 6 Ma, because it lies on the volcanic front which is presently deliniated as lava domes younger than 0.1 Ma (Fig. 5) (Kamata et al., 1988a).
H. Ka~ta, K. Kodama/ Tectoiqo~hysics233 (1994) 69-81
76
By contrast, the subduction beneath the Ryukyu Arc after 6 Ma gave rise to back-arc spreading in the Okinawa Trough (Fig. 9). The retreating trench hypothesis explains a pair of countercl~~ise and ciockwise crustal rotations in the two ends of a back-arc basin (e.g., Viallon et al., 1986). Crustal extension in the initial rifting
stage in the northern Okinawa Trough caused an approximately 30” of counterclockwise rotation for southern Kyushu (Fig. 9; Kodama and Nakayama, 1993). We propose that this rotation required crustal flexure on southern Kyushu, which one can observe as a structural bend of the preTertiary strata, known as the Hokusatsu Bend
Tsushima Island
Goi;S Islands
J”
f?5&
Sanbagawa Seit Chichibu Belt Northern Shimanto Belt Southern Shimanto Belt Neogene deposits Miocene granite Paleomagnetic declination
Fig. 7. General geology south of the Median Tectonic Line (MTL) on Kyushu and Shikoku islands. Paleomagnetic declinations: 1 = Late Miocene deposits at Tanegashima Island (Kodama et al., 1991); 2 = Middle Miocene granitic rocks at Osumi Peninsula (ishikawa and Torii, 1986); 3 = Late Miocene deposits at Miyazaki (Kodama and Nakayama, 1993); 4-5 = Miocene igneous rocks at Goto and Tsushima islands, (Ishikawa and Tagami, 1991); 6 = Oligocene deposits at Kitakyushu tishikawa, 1990); 7 = Cretaceous granitic rocks at Fukuoka (Ito and Tokieda, 1986); 8 = Miocene granitic rocks at Okueyama (Torii and Ishikawa, 1986); 9 = Middie Miocene volcanic rocks at Sanin (Otofuji et al., 1991); IQ = Cretaceous Izumi Group (Kodama, 1989). HKB = Hokusatsu Bend (Murata, 1987b); HTB = Hitoyoshi Bend, (Murata, 1987a); NIB = Nojiri Bend (Teraoka et al., 1981); OkYL= Oita-Kumamoto Tectonic Line; HT/z = Hohi volcanic zone. Map location shown in Fig. 1.
H. Kumaia,
1%Kkiama / ~ecfo~p~ysics 233(1994) 69-B
(HKB in Fig. 7). Our model opposes the previous interpretation that the Hokusatsu Bend formed solely at about 15 Ma, contemporaneous with the opening of the Japan Sea (Murata, 1987b; Kano et al., 19901, and asserts that the Hokusatsu Bend may have been used as a pivot of crustal rotation of southern Kyushu since 6 Ma (Fig. 8).
77
It has already been argued that oblique subduction of the Philippine Sea plate re-activated the MTL as a dextral detachment fault (Fitch, 1972; Matsuda, 1973). Similar examples have been reported from Burma and Sumatra (Fitch, 19721, southern Kuril (Kimura, 1986; DeMets, 1992), western Aleutian, southern Mariana, southern
paleomagnetic declination 0
pivot of crustai rotation
.*Yj crustal extension
24’N
Fig. 8. Distribution of crustal extension and associated paleomagnetism. Normal faults after Sibuet et al. (1987). Open arrow with PHS is the same as Fig. 2. UKL = Ojta-Kumamoto Tectonic Line. Map Iocation shown in Fig. 1.
H. KWata, K Kodmna/ Tectmwphysics233 (1994) 69-81
78
Yappu and Palau (Seno et al., 19861, and from western Ryukyu (Kuramoto and Konishi, 1989). Generally, when a plate subducts beneath an arc-arc junction, there is a difference in the subduction orientation relative to each arc orientation. For example, the Pacific plate subducts normal to the Northeast Japan Arc, while it subducts obliquely to the Kuril Arc (Fig. 10). Fore-arc slivers detached from overriding plates by strike-slip faults form, in some cases, collision zones at arc-arc junctions. For example, the Kuril fore-arc sliver collides with the northernmost Japanese Arc (Fig. 10; Kimura, 1986). Likewise, oblique subduction of the Philippine Sea plate to the southern Ryukyu Arc transferred the fore-arc sliver westward to cause collision in Taiwan (Fig. 10; Kuramoto and Konishi, 1989).
iefore 6 Ma
.-*:i
.-
4
crustalextension . paleomegnetii declination
On central Kyushu, however, a large graben was formed in the arc-arc junction (HVZ in Fig. 10). Crustal strain concentration on the western margin of the dextral fault seems to have been relieved by the fo~ation of en echelon grabens (e.g., HVZ) and normal-fault zones in the Unzen and Yatsushiro Bay areas (Fig. 2). Our model opposes previous models, which regard central Kyushu as a convergent area where either collision or subduction of fore-arc microplates has taken place (Kobayashi, 198.5; Isozaki, 1989). In short, the late Cenozoic tectonics of Kyushu can be explained by the subduction of the Philippine Sea plate, which restarted at about 6 Ma, and by the different subduction orientations of the downgoing slabs relative to the two connecting arcs.
.
.-•
6 Ma - present
F
c
l
.
.
. .
v
.R
Fig. 9. Tectonic development of the Southwest Japan Arc and the Ryukyu Arc before and after 6 Ma. Open arrow with PHS is the same as Fig. 2. HVZ = Hohi volcanic zone; MTL = Median Tectonic Line.
79
fore-am sliver
Fig. 10. Tectonic framework around the Japanese Islands and associated plates. Hvz = Hohi volcanic zone; MTL = Median Tectonic Line.
Acknowledgements Thoughtful comments by Tetsuzo Seno and Akihiro Murata are deeply appreciated. The manuscript was improved by reviews by Peter J.J. Kamp and Jean-Claude Sibuet.
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canic region in the central part of Oita prefecture, Kyushu district, southwest Japan. J. Geogr. Sot. Jpn., 52 lo-29 (in Japanese with English abstract). Ingle, J.C. and Karig, D.E., 1975. Site reports: 8, Site 297. Init. Rep. DSDP, 31: 275-316. Irving, E. and Irving, G.A., 1982. Apparent polar wander paths, Carboniferous through Cenozoic and the assembly of Gondwana. Geophys. Surv., 5: 141-188. Ishihara, K. and Yoshida, A., 1992. Configuration of the Philippine Sea slab and seismic activity an and around Kyushu. J. Seismol. Sot. Jpn., 45: 45-51 (in Japanese with English abstract). Ishikawa, N., 1990. Differential rotation of the western part of southwest Japan: paleomagnetic study in the northern part of the Kyushu Island. Rock Magn. Paleogeophys., 17: 73-79. Ishikawa, N. and Tagami, T., 1991. Paleomagnetism and fission-track geochronology on the Goto and Tsushima Islands in the Tsushima strait area: implications for the opening mode of the Japan Sea. J. Geomagn. Geoelectr., 43: 229-253. Ishikawa, N. and Torii, M., 1986. Westerly deflected remanent direction of the Middle Miocene granite from the Osumi Peninsula. Rock Magn. Paleogeoph~,, 13: 8-11. Isozaki, Y., 1989. Convergent processes of microplates-application of major plate convergence to micro plate tectonics. Rep. Inst. Kuroshio Sphere, Kochi Univ., 4: 59-75. Isozaki, Y., Maruyama, S. and Furuoka, F., 1990. Accreted oceanic materials in Japan, Tectonophysics, 181: 179-205. Ito, H. and Tokieda, K., 1986. Tilting movements of the Japanese Islands inferred from Cretaceous and Early Tertiary paleomagnetic data. J. Geomagn. Geoelectr., 38: 361-386. Jarrard, R.D., 1986. Terrane motion by strike-slip faulting of fore-arc slivers. Geology, 14: 780-783. Kamata, H., 1989a. Shishimuta caldera, the buried source of the Yabakei pyroclastic flow in the Hohi volcanic zone, Japan. Bull. Volcanol., 51: 41-50. Kamata, H., 1989b. Volcanic and structural history of the Hohi volcanic zone, central Kyushu, Japan, Bull. Volcanol., 51: 315-332. Kamata, H., 1992. Right-lateral movement of the OitaKumamoto Tectonic Line as a western extension of the Median Tectonic Line, originated from rightward, oblique subduction of the Philippine Sea plate. Mem. Geol. Sot. Jpn., 40: 53-63 (in Japanese with English abstract). Kamata, H., 1993. Subsurface geologic structure and its genesis of Beppu Bay and adjacent area in central Kynshu, Japan. J. Geol. Sot. Jpn., 99: 39-46 (in Japanese with English abstract). Kamata, H., Hoshizumi, H. and Koyaguchi, T., 1988a. Formation age of the volcanic front in central Kyushu-western Sanin districts. Earth Mon., 10: 568-574 (in Japanese). Kamata, H., Uto, K. and Uchiumi, S., 198% Geochronology and evolution of the post-Shishimuta caldera activity around the Waitasan area in the Hohi volcanic zone, Kyushu, Japan. Bull. Volcanol. Sot. Jpn., Ser. 2, 33: 305320.
Kaneko, S., 1966. Transcurrent displacement along the Median Line, south-western Japan. N.Z.J. Geol. Geophys., 9: 45-59. Kano, K., Kosaka, K., Murata, A., and Yanai, S., 1990. Intra-arc deformations with vertical rotation axes: the case of the pre-Middle Miocene terranes of Southwest Japan. Tectonophysics, 176: 33-354. Kimura, G., 1985. Cretaceous subduction mode in Hokkaido. Kagaku, 55: 24-31 (in Japanese). Kimura, G., 1986. Oblique subduction and collision: Fore-arc tectonics of the Kurile Arc. Geology, 14: 404-407. Kobayashi, Y., 1985. A hypothesis on formation of the Beppu-Shimabara Graben. Abstr. Seismol. Sot. Jpn., 1985-2: 38 (in Japanese). Kodama, K., 1989. Paleomagnetic study of the Upper Cretaceous Izumi strike-slip basin along the Median Tectonic Line in Southwest Japan. In: J. Hillhouse (Editor), Deep Structure and Past Kinematics of Accreted Terranes. Geophys. Monogr., AGU, 50: 239-248. Kodama, K. and Nakayama, K., 1993. Paleomagnetic evidence for post-Late Miocene intra-arc rotation of South Kyushu, Japan. Tectonics, 12: 35-47. Kodama, K., Ozawa, T., Inoue, K., Maeda, Y. and Takeuchi, T., 1991. Paleomagnetism and ~st-Middle Miocene punter-cl~~ise rotation of Tanegashima Island off south Kyushu, Japan. J. Geomagn. Geoelectr., 43: 721-740. Komazawa, M. and Kamata, H., 1985. The basement structure of the Hohi Geothermal Area obtained by gravimetric analysis in central-north Kyushu, Japan. Rep. Geol. Sure. Jpn., 264: 305-333 (in Japanese with English abstract). Kubotera, A., Ito, K., Murakami, H. and Mitsunami, T., 1982. Crustal structure of Kuju Volcano Group as revealed by explosion seismology. Bull Volcanol. Sot. Jpn., 27: 81-95 fin Japanese with Engiish abstract). Kuramoto, S. and Konishi, K., 1989. The Southwest Ryukyu Arc is a migrating microplate (forearc sliver). Tectonophysics, 163: 75-91. Matsubara, Y. and Seno, T., 1980. Paleogeographic reconstruction of the Philippine Sea at 5 m.y. B.P. Earth Planet. Sci. Lett., 51: 406-414. Matsuda, T., 1973. The Median Tectonic Line as an active strike-slip fault system. In: Median Tectonic Line. Tokai Univ. Press, pp. 239-251 (in Japanese with English abstract). Miki, M., Matsuda, T. and Otofuji, Y., 1990. Opening mode of the Okinawa Trough: paleomagnetic evidence from the South Ryukyn Arc. Tectonophysics, 175: 335-347. Murata, A., 1987a. Conical folds in the Hitoyoshi Bending, South Kyushu, formed by the clockwise rotation of the Southwest Japan Arc. J. Geol. Sot. Jpn., 93: 91-105. Murata, A., 198x1. Hokusatsu Bend and clockwise rotation of the Southwest Japan Arc. J. Fat. Sci. Univ. Tokyo, Sect. II, 21: 33-349. Nakada, S. and Kamata, H., 1991. Temporal change in chemistry of magma source under Central Kyushu, Southwest Japan: Progressive contamination of mantle wedge. Bull. Volcanol., 53: 182-194. Niitsuma, N. and Akiba, F., 1985. Neogene tectonic evolution
H. Kamaia, K. Kodama / Tectonophysics 233 (1994) 69-81 and plate subduction in the Japanese Island Arcs. In: N. Nasu et al. (Editors), Formation of Active Ocean Margins. Terra Sci. Publ., Tokyo, pp. 75-108. Okada, A. and Chida, N., 1991. No. 106, Yatsushiro. In: Res. Group Active Faults (Editors), Active faults in Japan, Rev. Ed. Univ. Tokyo Press, pp. 370-373 (in Japanese). Otofuji, Y. and Matsuda, T., 1987. Amount of clockwise rotation of Southwest Japan: fan shape opening of the southwestern part of the Japan Sea. Earth Planet. Sci. Lett., 85: 289-301. Otofuji, Y., Itaya, T. and Matsuda, T., 1991. Rapid rotation of southwest Japan-palaeomagnetism and K-Ar age of Miocene volcanic rocks of southwest Japan. Geophys. J. Int., 105: 397-405. Sangawa, A., 1980. Quatema~ crustal movement and landform development in middle part of Southwest Japan. Prof. K. Nishimura Retire. Memorial Vol., pp. 60-65 (in Japanese). Sangawa, A., 1986. The history of fault movement since late Pliocene in the central part of Southwest Japan. R. Sot. N.Z. Bull., 24: 75-85. Seno, T., 1977. The instantaneous rotation vector of the Philippine Sea plate relative to the Eurasian plate. Te~onophysi~, 42: 209-226. Seno, T. and Maruyama, S., 1984. Paleogeographic reconstruction and origin of the Philippine Sea. Tectonophysics, 102: 53-84. Seno, T., Kimura, G., and Kobayashi, Y., 1986. Is an arc-arc junction a triple junction? Abstr. Int. Kaiko Conference Subduction Zones, Tokyo-Shim&u, pp. 134-135. Sibuet, J.-C., Letouzey, J., Barrier, F., Charvet, J., Foucher, J.-P., Hilde, T.W.C., Kimura, M., Chiao, L.-Y., Marsset, B., Muller, C. and Stephan, J.-F., 1987. Back arc extension in the Okinawa Trough. J. Geophys. Res., 92: 14,0414,063. Sugiyama, Y., 1991. Right-lateral strike-slip basins in the Second Paleo-Seto Inland Sea-A model of basin development associated with the migration of active domain of a large-scale strike-slip fault. Struct. Geol., 36: 99-108 (in Japanese with English abstract). Sugiyama, Y., 1992. The Cenozoic tectonic history of the forearc region of southwest Japan, based mainly on the data obtained from the Shixuoka district. Bull. Geol. Surv. Jpn., 43: 91-112 (in Japanese with English abstract). Tada, T., 1985. Spreading of the Okinawa Trough and its relation to the crustal deformation in the Kyushu (2). J.
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Seismol. Sot. Jpn., 38: 1-12 (in Japanese with English abstract). Takahashi, M., 1981. Anomalous island-arc volcanic zonemagmatism of Middle Miocene in Outer Zone and Setouchi Province of Southwest Japan. Earth Mon., 3: 382388 (in Japanese). Tamanyu, S., 1985. Stratigraphy and geologic structures of the Hohi geothermal area, based mainly on the bore hole data. Rep. Geol. Surv. Jpn., 264: 115-142 (in Japanese with English abstract). Tatsumi, Y, 1983. High magnesian andesites in the Setouchi volcanic belt, southwest Japan and their possible relation to the evolutionary history of the Shikoku Inter-arc Basin. In: Geodynamics of the Western Pacific-Indonesian region. Geodyn. Ser., Am. Geophys. Union, 11: 305-314. Tatsumi, Y. and Maruyama, S., 1989. Boninites and high-Mg andesites: Tectonics and petrogenesis. In: A.J. Crawford (Editor), Boninites. Unwin Hyman, London, pp. 50-71. Teraoka, Y., Imai, I. and Okumura, K., 1981. The bending structure of the Outer Zone of Kynshu. Stud. Late Mesozoic Tectonism Jpn., 3: 87-98 (in Japanese). Torii, M. and Ishikawa, N., 1986. Paleomagnetic directions from Middle Miocene igneous rocks in central, eastern Kyushu Island, Rock Magn. Paleogeophys., 13: 1-5. Tsukuda, E., 1990. Active tectonics of the Median Tectonic Line. Bull. Geol. Surv. Jpn., 41: 405-406 (in Japanese). Uto, K., 1989. Neogene volcanism of Southwest Japan: Its time and space based on K-Ar dating. Ph.D. Thesis, Univ. Tokyo, 184 pp. Viallon, C., Huchon, P. and Barrier, E., 1986. Opening of the Okinawa basin and collision in Taiwan: a retreating trench model with lateral anchoring. Earth Planet. Sci. Lett., 80: 145-155. Watanabe, K., 1984. Active faults and their significance in the region to the west of Aso caldera, Kumamoto Prefecture. Mem. Fat. Educ. Kumamoto Univ., 33: 35-47 (in Japanese with English abstract). Yano, Y., Kamata, H. and Ogawa, K., 1987. On the conditions for the formation of a hot water convection system above the slope of a caldera basement: modeling of Shishimuta caldera, central Kyushu. J. Jpn. Assoc. Pet. Technol,, 52: 399-412 (in Japanese with English abstract). Yuhara, K. and Ehara, S., 1981. Geothermal fields of arc volcanism in Japan. Bull. Volcanol. Sac. Jpn. 26: 185-203 (in Japanese with English abstract).
Tectonics of an arc-arc junction: an example from Kyushu Island at the junction of the Southwest Japan Arc and the Ryukyu Arc Hiroki Kamata a, Kazuto Kodama b n @g&a
office, Geological Survey of Japan, Govemment Bldg. No. 2,Bekkan, 4-I-67, Otemae, Chum-ku,Osaka54% Japan, b Llepwtmentof Geology, Kochi ~nive~~~, 2-5-1, Akdono-cho, Kochi 780, Japan
(Received April 6, 1993; revised version accepted January 7, 1994)
Abstract Three distinctive geologic events occurred synchronously during the last 6 m.y. on Kyushu Island, lying at the junction of the Southwest Japan Are and the Ryukyu Arc. These events included: (1) dextral-fault displacement along a 100 km section of the Median Tectonic Line; (2) formation of a rectangular volcano-tectonic depression 70 km long and 40 km wide; and (3) approximately 30” of counterclockwise crustal rotation. We propose an integrated tectonic model that explains these three tectonic features in terms of subduction of the Philippine Sea plate, which resumed at around 6 Ma after a halt of more than 5 m.y. Oblique subduction of the Philippine Sea plate beneath the Southwest Japan Arc detached a fore-arc sliver that was displaced along a dextral fault, the Median Tectonic Line, north of the Nankai Trough, A volcano-tectonic depression, the Hohi volcanic zone, was formed by the pull-apart structure on the western margin of the Median Tectonic Line. By contrast, normal subduction of the Philippine Sea plate under the Ryukyu Arc caused back-arc spreading, observed as crustal extension in the northern Okinawa Trough. The crustal extension caused the counterclockwise rotation for the northernmost Ryukyu Arc including southern Kyushu.
1. Introduction Arc-arc junctions are not only geomo~hologitally peculiar but also exhibit a variety of tectonic features such as strike-slip faults, ridges and troughs (Fitch, 1972; Jarrard, 1986; Seno et al., 1986). The development of these features has been attributed to local stress concentration in the end of intra-arc transcurrent faults (e.g., Fitch, 1972). These interpretations, however, have generally been based on a lack of detailed, on-land geologic and geophysical documentation. We will review recent studies of Kyushu Island, located at HO-1951~94/$07.~
0 1994
SXU 0040-1951(94)00008-w
the junction of the Southwest Japan and the Ryukyu arcs (Fig. 11, and highlight that Kyushu experienced three distinctive geologic events: intra-arc strike-slip fault displacement; formation of a volcano-tectonic depression; and counterclockwise tectonic rotation. All of these events took place during the last 6 m.y. and since the Philippine Sea plate began to subduct beneath Kyushu. We will propose a tectonic model that ascribes these three events to the inception of the subduction of the Philippine Sea plate and will put emphasis on the difference in subduction direction relative to the two island arcs.
Etsevier Science B.V. All rights reserved
H. Kamata, K. Kodama / Tectortophysics 233 (1994) 69-81
70
2. Strike-slip faults The Median Tectonic Line (MTL) is a striking geologic structure, which extends for 900 km in an E-W direction in southwestern Japan (Fig. 21 and separates a fore-arc sliver from the arc crust (Fig. 3). Sinistral faulting had been dominant along the MTL during Cretaceous to early Paleogene times, while in the late Paleogene thrust movement has been demonstrated in several of its portions (e.g., Ichikawa, 1980). By contrast, dextral faulting has dominated since at least the early Quaternary to the present (Kaneko, 1966; Sangawa, 1986). The previous oldest record of the dextral displacement was dated at about 2 Ma, which was reported from the eastern margin of the MTL, on the basis of the analysis of faults 6-E
12bE I
that juxtaposed confluent fans and rivers (Sangawa, 1980; Tsukuda, 1990). Recently, Kamata (1992) re-estimated the initiation of dextral displacement on the MTL and suggested an age of 5 Ma, based on structural analyses of the western extension of the MTL, or the Oita-Kumamoto Tectonic Line, along with its branches as the Hinagu, the Futagawa and the Imahata faults (Fig. 2; Watanabe, 1984; Okada and Chida, 1991; Chida and Ikeda, 1991). Kamata (1992) noted that an ENE-WSW-trending negative gravity anomaly, running along the OitaKumamoto Tectonic Line, is bent clockwise at two locations (P and R in Fig. 4). He interpreted the negative anomaly pattern as rhomboidal pull-apart tectonic voids (e.g., Glazner, 1991) caused by dextral displacement along the Oital&Et
i
,loookm
Fig. 1. Index map of the Japanese Islands and associated plates and arcs.
H. Kamata, K K&m
71
/ Tec~o~phy~~c~ 233 (19941 69-81
Kumamoto Tectonic Line. He then dated the commencement of the dextral movement to be not later than 5 Ma using the average dextral displacement rate of the MTL (Kamata, 1992). This estimation is insistent with the timing of formation of several fore-arc basins, which is regarded to be contemporaneous with the MTL activity (Sugiyama, 1991).
3. Volcano-t~~nic d~p~ssion On central Kyushu, Plio-Pleistocene volcanic rocks are widely distributed in a 70 by 40 km area. This area forms a volcano-tectonic depression (HVZ in Fig. 2) filled with more than 5000
km3 of andesitic material (Kamata, 1989b). Much evidence indicates that the HVZ was formed by N-S extension in Quaternary times. Geologic evidence includes the E-W trend of normal faults that developed in the early to late Quaterna~ (Fig. 2; Ikeda, 1979; Chida and Ikeda, 1991). Geophysical evidence includes the results of geodetic surveys (Tada, 1985), earthquake focal mechanisms (Hatanaka and Shimazaki, 19881 and geothermal investigations (Yuhara and Ehara, 1981; Yano et al., 19871. The HVZ forms a boxshaped graben surrounded by steep gravity gradients (Fig. 4). Gravity analysis (Kamata, 1993), drilled core description (Tamanyu, 1985; Kamata, 1989b) and seismic refraction exploration (Kubotera et al., 1982) all indicate that pre-Tertiary
s
strike-slip fault ,
normal fault
Fig. 2. Dextral faults (the Median Tectonic Line and the Oita-Kum~oto Tectonic Line) and the Hohi vofcanic zone (dots are oultine of HVZ) on Kyushu and Shikoku islands. Open arrows with PHS show the present subduction direction of the Philippine Sea plate, after Seno (1977). Small solid arrows are active strike-slip faults. KT = Kokura-Tagawa Fault Zone. Map location shown in Fig. 1.
H. Kamata, X Kodanta/ Tectonophysics 233 &W4~ 69-81
72
basement rocks occur up to 3 km below the surface. Radiometric ages of volcanic rocks in the HVZ show a systematic zonation (Kamata, 1989b) younging inwards from the margins, with outer ages between 6 and 5 Ma and inner ages of 0.2 Ma (Fig. 5). A similar zonal pattern is also observed in the Bouguer gravity anomalies. No radiametric ages older than 2 Ma were reported from within the -5 mGa1 contour line, while those younger than 1 Ma were found only within the - 15 mGa1 contour line. Moreover, those rocks younger than 0.5 Ma occur in the subcircuIar region with an anomaly of more than -30 mGal (Ss in Figs. 4 and 5, Shishimuta caldera; Kamata 1989a). These features imply close spatial and temporal links between the Plio-Pleistocene volcanism and the graben formation, Calcafkalic high-Mg basaltic andesites were erupted in the early stage of the HVZ activity (5-3 Ma), while high-alumina basalts typical of island arcs were erupted in the later stage (2-O Ma) (Kamata et al., 1988b; Nakada and Kamata, 1991). The eruption rate of the HVZ decreased with time, as is quantitatively indicated by changes of both area and erupted volume for each I m.y. interval, which shows m~mum activity produc-
NW
Median
(Oita-Kumamoto
mantle
ing about 3000 km3/m.y. interval 5-4 Ma (Fig. 6).
early on during the
4. In&a-are rotation Paleomagnetism of the Late Miocene rocks on southern Kyushu indicates westward declinations. The paleomagnetic directions of the Uchiumigawa Group on southeastern Kyushu (3 in Fig. 7; Kodama and Nakayama, 1993) and the Kukinaga Group in Tanegashima Island (1 in Fig. 7; Kodama et al., 1991) provide a common mean direction that deviates from the present dipole field direction by 26.8 k 5.6” in declination and 0.1 rt 3.9” in inclination. Several westerly deflected magnetization directions were also obtained from the Middle Miocene granite on southern Kyushu (2 in Fig. 7) by Ishikawa and Torii (1986) who estimated a 45 _t 24” westward shift. Because the youngest age of these rotated strata is the Late Miocene, the counterclockwise rotation took place after about 6 Ma. The Eurasian mean paleopole has been in agreement with the geographic pole from the Late Miocene onward (Irving and Irving, 19821, during which no relative motion has been recognized between Eurasia and southwest-
Tectonic Line Tectonic Line)
SE Pacific
Ocean
wedge
I
I/
+(aseismic) I
Fig. 3. Profile of the Southwest Japan Arc and subducting Philippine Sea plate. Configuration of plate and crust is after Ishihara and Yoshida (1992) and Isozaki et al. (1990). Location of profile line shown in Fig. 1.
em Honshu (e.g*$ Otofuji et al., 199ll* These ~al~~magnetic data led Kodama and Nakayama Cl9931to con&de that soutbem ICyusku underwent about W of ~~~t~~~I~~is~ tectonic ~&ation relative t~3the Eurasian continent after the latest Miocene. They i~t~~~t~d that this cmmtercloekwise rotation was caused by widespread
stretchiny of contixxmtal crust as exempIified by a number of submarine normal faults behind soutbem Ky~sbu (Fig. 8; Sibuet et at., 1987). Xncontrast, Miki et d. (1990) suaest &&wise ~~t~ti~~ for the s~~tb~rn Ry&y~ Arc on the basis of ~aIe~rnag~~tisrn of Ish~g~~jima in thr: Ryuky~ Arc (Fig. 8). They claim that this &ck-
Fig. 4. Bouguer nnamaiy contours in mCal (p = 2.3 g/cm3 after Komazawa and Kamata, 1985). H = high; L = low; OKC = OitaKumamoto Tectonic Line: As = Asa volcano; & ~1Kuju volcano; Yf -; Y&u-Tsurumi volcano; 8’~ = Futago voIcano; SS = ~~~~~rn~t~~Dots show the outline of the H&i volcartic zone. Map &cation &own in Fig. 1.
74
H. Kumata, K. Kodamu / Tectonophysics 233 U994) 69-81
wise rotation was caused by a wedge-shaped spreading of the back-arc basin east of Taiwan (Fig. 8; e.g., Otofuji and Matsuda, 1987).
5. Subduction of the Philippine Sea plate The Philippine Sea plate is currently subducting northwestward under the Southwest Japan Arc and the Ryukyu Arc (Figs. 2 and 8; Seno, 1977). However, subduction history of the Philippine Sea plate in the past is controversial. Seno and Maruyama (1984) reported that the Philippine Sea plate subducted northwestward at 17 Ma. Several workers suggest that the Philippine Sea plate ceased to have subducted in some pe-
riod between the present and Miocene times (Uto, 1989; Kamata, 1992). Our compilation, however, suggests that the Philippine Sea plate has been subducting no~hwes~ard since about 6 Ma. Evidence for this includes the following: (I) Matsubara and Seno (1980) pointed out that the Philippine Sea plate has had a northwestward direction of subduction from at least 5 Ma on the basis of its average convergence rate (3.5-4.5 cm/yr) and the length of the subducted slab (150-200 km). (2) The WVZ recorded its rn~irn~ eruption rate at about 5 Ma, which has been followed by a monotonous decrease in eruption rate to the present (Fig. 6). Temporal variations of the chemical composition of the volcanic rocks on central LEGEND
Suou-nada
Sea
~Jouatemary aLava
voJearlks and ahrvium
dames youtger than 0.1 ?&
Pliocene volcanic8 Miocene vokanks
and intruskes
Pre-Tertiary basements
’ RADIOMETRIC
AGE
0 0.2Ma9ge
4.OkkwwI
Fig. 5. Distribution of volcanic rocks and their radiometric ages superimpased on the Bouguer anomaly map of the Hohi volcanic zone WVZ). Radiometric ages after Kamata (1989b). Bouguer anomaly contours in mGal after Komazawa and Kamata (1985). Solid lines show the distribution of volcanic rocks divided by 1 m.y. intervals. Mi = Miyanoharu; Ss = Shishimuta. Map location shown in Fig. 1.
H. timata, K. Kodama / Tect~~~hysi~s233 (1994’)69-81
Kyushu show (a) typical chemical characteristics related to the subduction of the oceanic slab from 6-5 Ma to the present, and (b) progressive contamination of the mantle wedge by this oceanic slab over time (Nakada and Kamata, 1991). These results suggest that the Philippine Sea plate has been continuously subducting since 6 Ma, giving rise to calcalkaline volcanic activity at the earliest stage. (3) Niitsuma and Akiba (1985) pointed out that the Late Miocene to Pliocene sed~ents commenced deposition at 7-6 Ma in fore-arc basins along Nankai Trough, and that these sediments were of turbidite origin derived from the collision of the Izu-Bonin Arc with the Japanese Arc. Thus, they suggested that the Philippine Sea plate subducted from 7-6 Ma to at Ieast 3 Ma. (4) Sugiyama (1991, 1992) demonstrated that the northwestward subduction of the Philippine Sea plate resumed at about 5 Ma, because some bends in the geologic structures in the outer zone of the Southwest Japan were formed by dextral movement of the MTL. There is other evidence implying a 10-6 Ma halt to subduction of the Philippine Sea plate.
Fig. 6. Temporal variations of volume and area for volcanic rocks in the Hohi volcanic zone. Data plotted in center of each million-year interval used after Kamata (1989b).
75
(1) Tatsumi and Maruyama (1989) demonstrated that the Setouchi high-Mg andesites were produced by subduction of young, hot oceanic lithosphere of the Philippine Sea plate. The age of the Setouchi volcanism ranges from 17 to 10 Ma (Takahashi, 1981; Tatsumi, 1983). (2) According to DSDP Leg 31 in the northern Shikoku Basin (Ingle and Karig, 1975) terrigenous turbidite occurs in very limited core sections dated between 5 and 4 Ma. This suggests that the trough was not deep enough to trap terrigenous deposits at around 5 Ma. In other words, the Philippine Sea plate did not start subducting in the current setting until about 5 Ma (Seno and Maruyama, 1984). (3) Calc-alkaline volcanism was absent but mantle-zenoliths bearing basanites erupted during the period 9.5-6.5 Ma in the western part of the Southwest Japan Arc, suggesting that the oceanic plate did not subduct during the period 10-6 Ma (Uto, 1989).
6. The tectonic model We interpret the three geologic events on Kyushu in terms of resumed subduction of the Philippine Sea plate beneath the arc-arc junction. The major tectonic changes are detectable after 6 Ma (Fig. 9). Our model emphasises the difference in orientation of the subduction direction relative to the Southwest Japan Arc and the Ryukyu Arc (Fig. 10). We propose that after 6 Ma oblique subduction under the Southwest Japan Arc yielded a strong coupling between the fore-arc sliver and the subducting slab (e.g., Kimura, 1985), which resulted in dextral displacement on the MTL and its western extension, the Oita-Kumamoto Tectonic Line (Fig. 9). The dextral-fault movement along the Oita-Kumamoto Tectonic Line, coupled with its pull-apart geometry, formed a graben of 70 by 40 km (HVZ) that has extended in the N-S direction. The graben is filled with voluminous volcanic materials erupted since about 6 Ma, because it lies on the volcanic front which is presently deliniated as lava domes younger than 0.1 Ma (Fig. 5) (Kamata et al., 1988a).
H. Ka~ta, K. Kodama/ Tectoiqo~hysics233 (1994) 69-81
76
By contrast, the subduction beneath the Ryukyu Arc after 6 Ma gave rise to back-arc spreading in the Okinawa Trough (Fig. 9). The retreating trench hypothesis explains a pair of countercl~~ise and ciockwise crustal rotations in the two ends of a back-arc basin (e.g., Viallon et al., 1986). Crustal extension in the initial rifting
stage in the northern Okinawa Trough caused an approximately 30” of counterclockwise rotation for southern Kyushu (Fig. 9; Kodama and Nakayama, 1993). We propose that this rotation required crustal flexure on southern Kyushu, which one can observe as a structural bend of the preTertiary strata, known as the Hokusatsu Bend
Tsushima Island
Goi;S Islands
J”
f?5&
Sanbagawa Seit Chichibu Belt Northern Shimanto Belt Southern Shimanto Belt Neogene deposits Miocene granite Paleomagnetic declination
Fig. 7. General geology south of the Median Tectonic Line (MTL) on Kyushu and Shikoku islands. Paleomagnetic declinations: 1 = Late Miocene deposits at Tanegashima Island (Kodama et al., 1991); 2 = Middle Miocene granitic rocks at Osumi Peninsula (ishikawa and Torii, 1986); 3 = Late Miocene deposits at Miyazaki (Kodama and Nakayama, 1993); 4-5 = Miocene igneous rocks at Goto and Tsushima islands, (Ishikawa and Tagami, 1991); 6 = Oligocene deposits at Kitakyushu tishikawa, 1990); 7 = Cretaceous granitic rocks at Fukuoka (Ito and Tokieda, 1986); 8 = Miocene granitic rocks at Okueyama (Torii and Ishikawa, 1986); 9 = Middie Miocene volcanic rocks at Sanin (Otofuji et al., 1991); IQ = Cretaceous Izumi Group (Kodama, 1989). HKB = Hokusatsu Bend (Murata, 1987b); HTB = Hitoyoshi Bend, (Murata, 1987a); NIB = Nojiri Bend (Teraoka et al., 1981); OkYL= Oita-Kumamoto Tectonic Line; HT/z = Hohi volcanic zone. Map location shown in Fig. 1.
H. Kumaia,
1%Kkiama / ~ecfo~p~ysics 233(1994) 69-B
(HKB in Fig. 7). Our model opposes the previous interpretation that the Hokusatsu Bend formed solely at about 15 Ma, contemporaneous with the opening of the Japan Sea (Murata, 1987b; Kano et al., 19901, and asserts that the Hokusatsu Bend may have been used as a pivot of crustal rotation of southern Kyushu since 6 Ma (Fig. 8).
77
It has already been argued that oblique subduction of the Philippine Sea plate re-activated the MTL as a dextral detachment fault (Fitch, 1972; Matsuda, 1973). Similar examples have been reported from Burma and Sumatra (Fitch, 19721, southern Kuril (Kimura, 1986; DeMets, 1992), western Aleutian, southern Mariana, southern
paleomagnetic declination 0
pivot of crustai rotation
.*Yj crustal extension
24’N
Fig. 8. Distribution of crustal extension and associated paleomagnetism. Normal faults after Sibuet et al. (1987). Open arrow with PHS is the same as Fig. 2. UKL = Ojta-Kumamoto Tectonic Line. Map Iocation shown in Fig. 1.
H. KWata, K Kodmna/ Tectmwphysics233 (1994) 69-81
78
Yappu and Palau (Seno et al., 19861, and from western Ryukyu (Kuramoto and Konishi, 1989). Generally, when a plate subducts beneath an arc-arc junction, there is a difference in the subduction orientation relative to each arc orientation. For example, the Pacific plate subducts normal to the Northeast Japan Arc, while it subducts obliquely to the Kuril Arc (Fig. 10). Fore-arc slivers detached from overriding plates by strike-slip faults form, in some cases, collision zones at arc-arc junctions. For example, the Kuril fore-arc sliver collides with the northernmost Japanese Arc (Fig. 10; Kimura, 1986). Likewise, oblique subduction of the Philippine Sea plate to the southern Ryukyu Arc transferred the fore-arc sliver westward to cause collision in Taiwan (Fig. 10; Kuramoto and Konishi, 1989).
iefore 6 Ma
.-*:i
.-
4
crustalextension . paleomegnetii declination
On central Kyushu, however, a large graben was formed in the arc-arc junction (HVZ in Fig. 10). Crustal strain concentration on the western margin of the dextral fault seems to have been relieved by the fo~ation of en echelon grabens (e.g., HVZ) and normal-fault zones in the Unzen and Yatsushiro Bay areas (Fig. 2). Our model opposes previous models, which regard central Kyushu as a convergent area where either collision or subduction of fore-arc microplates has taken place (Kobayashi, 198.5; Isozaki, 1989). In short, the late Cenozoic tectonics of Kyushu can be explained by the subduction of the Philippine Sea plate, which restarted at about 6 Ma, and by the different subduction orientations of the downgoing slabs relative to the two connecting arcs.
.
.-•
6 Ma - present
F
c
l
.
.
. .
v
.R
Fig. 9. Tectonic development of the Southwest Japan Arc and the Ryukyu Arc before and after 6 Ma. Open arrow with PHS is the same as Fig. 2. HVZ = Hohi volcanic zone; MTL = Median Tectonic Line.
79
fore-am sliver
Fig. 10. Tectonic framework around the Japanese Islands and associated plates. Hvz = Hohi volcanic zone; MTL = Median Tectonic Line.
Acknowledgements Thoughtful comments by Tetsuzo Seno and Akihiro Murata are deeply appreciated. The manuscript was improved by reviews by Peter J.J. Kamp and Jean-Claude Sibuet.
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