Geological Society of America Bulletin

Downloaded from gsabulletin.gsapubs.org on February 11, 2014 Geological Society of America Bulletin

Volcano-tectonic interactions during rapid plate-boundary evolution in the Kyushu region, SW Japan

S.H. Mahony, L.M. Wallace, M. Miyoshi, P. Villamor, R.S.J. Sparks and T. Hasenaka

Geological Society of America Bulletin 2011;123, no. 11-12;2201-2223 doi: 10.1130/B30408.1

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Volcano-tectonic interactions during rapid plate-boundary evolution in the Kyushu region, SW Japan

S.H. Mahony1,†, L.M. Wallace2, M. Miyoshi3, P. Villamor2, R.S.J. Sparks1, and T. Hasenaka4 1Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, UK 2GNS Science, 1 Fairway Drive, Avalon, Lower Hutt 5010, New Zealand 3Beppu Geothermal Research Laboratory, Institute for Geothermal Sciences, Kyoto University, Noguchibaru, Beppu, Oita 874-0903, Japan 4Department of Earth and Environmental Sciences, Kumamoto University, Kumamoto 860-8555, Japan

ABSTRACT INTRODUCTION led to a complex volcanic history; since 15 Ma there have been “fl ood” basalt and andesite Evolution of the local plate tectonic and At subduction margins, volcanic and tectonic lavas, backarc monogenetic volcanoes, and arc volcanic system relationship at Kyushu processes are intrinsically linked. The distri- volcanoes varying in behavior from lava domes Island is defi ned by major changes in tec- bution and style of volcanism depends on the to calderas. tonics and volcanic style at ca. 15, 10, 6, regional tectonic framework, factors includ- Abundant geophysical and geological infor- and 2 Ma. Plate reconstructions presented ing: subducting slab dynamics, morphology mation for Kyushu allows an in-depth study here suggest that prior to 15 Ma, the Pacifi c and composition, mantle-wedge geodynam- of how volcanism over the past 15 Ma has plate subduction dominated Kyushu tectonics, upper-plate structure and composition, and responded to changes in plate convergence ics. From 15 to 6 Ma, the evolving relative regional tectonic deformation. Likewise, tecton- directions and subducting slab characteristics. In plate motions shifted the triple junction ics can be strongly infl uenced by the location of Kyushu the volcanic-tectonic system is young, between the Pacifi c plate, Philippine Sea active volcanism; for example, the upper plate complex, and rapidly changing. Geochemical plate, and southwest Japan northwards, can be thermally weakened by magmatism, and data are integrated into our analysis by using so that the Philippine Sea plate was sub- thereby magmatism can lead to the localization patterns in distribution of fl uid mobile elements, ducted beneath Kyushu. We suggest that a of deformation. Volcanic processes can perturb giving further insight into the volcanic and tec- lack of subduction-related volcanism from regional stress fi elds and infl uence the style of tonic evolution. We suggest that the volcano- 10 to 6 Ma is due to shallow subduction of faulting. Some plate boundaries undergo rapid tectonic evolution of the Kyushu region is con- the young Shikoku Basin lithosphere. By temporal and spatial changes, particularly sistent with the history of Philippine Sea plate 6–5 Ma, changes in the Philippine Sea plate where there is strong along-strike variation in motion as constrained by paleomagnetic and motion led to more rapid, nearly trench- the age of the subducting plate, or where a triple seafl oor spreading studies (Hall et al., 1995a, normal, subduction of the Eocene west Phil- junction is present. 1995b; Sdrolias et al., 2004). Our hypothesis ippine Basin crust beneath Kyushu. This Kyushu is located in southwest Japan for the volcano-tectonic evolution of Kyushu model is supported by an increase in arc- (Fig. 1), where the Philippine Sea plate is sub- is at odds with most published interpretations like geochemistry of lavas since ca. 6.5 Ma. ducted beneath the Amurian plate, a subplate by suggesting a mid-Miocene onset of the Izu Subduction of fl uid-rich features such as the of the Eurasian plate (e.g., Petit and Fournier, arc collision with central Japan (e.g., Seno and Kyushu-Palau ridge introduced large vol- 2005; DeMets et al., 2010). Philippine Sea plate Maruyama, 1984; Watanabe, 2005; Saito et al., umes of fl uids into the Kyushu arc system, subduction has led to a well-developed volcanic 2007). This highlights the need to resolve the leading to voluminous volcanism across arc in Kyushu. However, over the past 15 Ma, fundamental disconnect between current under- Kyushu, focused particularly in areas where there have been periods of apparent slow sub- standing of past Philippine Sea plate motion and the ridge subduction occurs in tandem with duction, changes in subduction direction, sub- the timing of Izu arc collision. local extensional tectonics. Key issues, such duction of a ridge, shallow plate subduction, as the timing of Izu arc collision with central clockwise and anticlockwise vertical axis rota- CENOZOIC TECTONIC SETTING IN Japan and the history of motion of the Phil- tion of the forearc region between the volcanic THE REGION OF SOUTHWEST JAPAN ippine Sea plate, are reassessed here, result- front and the trench, rollback of the subducting ing in a model that favors Izu arc–central slab and backarc and intra-arc rifting, as well as Throughout the Cenozoic, the long-term evo- Japan collision at ca. 8–6 Ma, rather than changes in relative motion of the adjacent tec- lution of the Pacifi c margin of Southeast Asia the more widely accepted date of ca. 15 Ma. tonic plates. These major Cenozoic changes in provided the overriding driving force behind tectonic setting (e.g., Cambray and Cadet, 1994; more localized margin changes in Japan. Prior Kamata and Kodama, 1994; Yamaji, 2003) have to late Cenozoic time, the entire length of the

†E-mail: [email protected]

GSA Bulletin; November/December 2011; v. 123; no. 11/12; p. 2201–2223; doi: 10.1130/B30408.1; 14 ﬁ gures; 4 tables; Data Repository item 2011289.

Japanese islands was underthrust by the Pacifi c a 12–30 Ma timing (Otofuji and Matsuda, 1983; lithosphere formed due to rifting and spreading plate (Taira, 2001; Hall, 2002). Since that time, Tamaki et al., 1992), while other data sets sug- in the eastern part of the Philippine Sea plate paleomagnetic data and spreading histories of gest a more short-lived opening history from between 30 and 15 Ma (Watts and Weissel, 1975; marginal basins have been interpreted to sug- 14 to 16 Ma (Otofuji et al., 1991). To accom- Kobayashi and Nakada, 1979; Okino et al., gest three major periods of regional tectonic modate the Sea of Japan opening, the Japanese 1999; Hall, 2002; Sdrolias et al., 2004) (Fig. 1). development in the adjacent Southeast Asia and islands parted from the Asian mainland (Tamaki The Kyushu-Palau ridge divided the young Shi- the southwest Pacifi c regions, at 45 Ma, 25 Ma, et al., 1992; Jolivet et al., 1994; Maruyama et koku Basin (eastern Philippine Sea plate) from and 5 Ma (Hall et al., 1995a, 1995b; Hall, 1998). al., 1997; Lee et al., 1999), with ~45° clockwise the 40–60 Ma West Philippine Basin (western Although the exact history and timing is con- rotation of Kyushu and southwest Honshu and Philippine Sea plate) lithosphere. The Kyushu- troversial, the Sea of Japan opened up west of ~45° anticlockwise rotation of northeast Hon- Palau ridge is a remnant Eocene– Oligocene Japan sometime between the mid-Oligocene shu and Hokkaido (Jolivet and Tamaki, 1992; arc that split away from the Izu-Bonin-Mariana and the mid-Miocene; some data sets indicate Otofuji et al., 1991, 1994). The Shikoku Basin arc during Shikoku Basin rifting. These events strongly infl uenced the position and nature of the triple junction (Fig. 1) between the Pacifi c 124° 128° 132° 136° 140° 144° 148° plate, Philippine Sea plate, and Eurasian plate (or Amurian subplate), which has evolved since Okhotsk Eurasian 15 Ma, impacting the tectonic history of central plate plate HOKKAIDO and southwest Japan. (Amurian 44° subplate) Present Day

Kuril Trench Japan is currently located astride two subduction margins: the Paciﬁ c plate is being subducted 40° beneath northern Japan, while the Philippine Sea plate is being subducted beneath southwest Sea of Japan

10 mm/yr Japan (Fig. 1). Much of the upper plate in south- HONSHU west Japan is part of the Amurian plate (Wei and Seno, 1998). The contemporary Philippine Sea 90 mm/yr plate adjacent to southwest Japan can be divided Japan Trench 36° spatially into two parts due to lithospheric age SHIKOKU differences. Young buoyant Shikoku Basin lithosphere (ca. 26–15 Ma) is subducted beneath southwest Honshu at the Nankai Trough, while

Izu-Bonin-Mar the older (40–60 Ma) West Philippine Basin lithosphere is subducted beneath central and 32° southern Kyushu at the Ryukyu Trench. A vol- Izu-Bonin-Marianas Arc KYUSHU 72 mm/yr canic arc has not yet developed in southwest Nankai Trough Honshu where the Shikoku Basin is subducted ianas Trench Pacific at a shallow angle and the leading edge has only Shikoku Basin plate reached 70 km depth (Fig. 2); in contrast the (26–15 Ma) 130 Ma West Philippine Basin portion of the Philippine KPR 28° Sea plate is subducted seismically to depths of Okinawa Trough 150–200 km beneath Kyushu (Fig. 2), and a well-developed volcanic arc is located above West Philippine the 105 km depth contour (England et al., 2004; Basin (60–40 Ma) Parece Vela 0 150 km Nakajima and Hasegawa, 2007). 79 mm/yr Basin N Ryukyu Trench Philippine Sea plate 24° Major Active Tectonic Features in Kyushu

Figure 1. Overview of plate tectonics of the Japanese arc system, indicating the interaction Major tectonic features in Kyushu are two between the three main plates—the Eurasian plate (Amurian subplate), the Philippine Sea extensional grabens and two strike-slip domains. plate, and the Paciﬁ c plate. White outlined areas indicate land. Convergence rates at each In central Kyushu, the Beppu-Shimabara graben respective plate boundary are labeled in mm/yr. The gray lines with solid triangles mark (Fig. 3) exhibits ongoing north-south exten- subduction zones. The gray dashed line cutting across Honshu and up the western side of sional faulting (e.g., Kamata, 1992; Kamata and Honshu and Hokkaido marks a zone of convergent tectonics, which is not a subduction Kodama, 1999), which coincides with abundant zone. The main subduction zones are marked—the Ryukyu Trench, Nankai Trough, Izu- magmatic activity. The extensional faulting con- Bonin-Mariana Trench, Japan Trench, and Kuril Trench. The Okinawa Trough is a recent tinues southwest to backarc rifting in the Oki- backarc basin (since 6 Ma). The Sea of Japan, West Philippine Basin, Shikoku Basin, and nawa Trough (e.g., Lee et al., 1980; Sibuet et Parece Vela Basin are mature basins. The Kyushu-Palau ridge (KPR) is a remnant arc al., 1987). The Kagoshima graben in southern currently being subducted beneath Kyushu. Bathymetry image © 2009 Terrametrics. Kyushu (Fig. 3) hosts recent active east-west

2202 Geological Society of America Bulletin, November/December 2011 Downloaded from gsabulletin.gsapubs.org on February 11, 2014 Volcano-tectonic interactions during rapid plate-boundary evolution in the Kyushu region, SW Japan

130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140°E 37°

36°

35°

34°

33°

32°

N 0 100 km 31°N

Figure 2. Conﬁ guration of the subducted Philippine Sea plate modiﬁ ed from Nakajima and Hasegawa (2007). Triangles mark the locations of active volcanoes. The thick gray dashed line marks the position of the leading edge of the subducted Philippine Sea plate. The gray contour lines mark the depth contours of the subducted Philippine Sea plate slab. The tight contours over Kyushu represent the steeply dipping West Philippine basin lithosphere, compared to the shallower dip of the Shikoku Basin lithosphere under southwest Honshu.

extension and active normal faulting (e.g., Ara- of volcanism, geochemical characteristics of mag- Key geochemical signatures can be used to maki, 1984; Kodama et al., 1995; AIST, 2008) mas, and the presence of volcanic gaps. Detailed infer the source conditions for magma gen- at rates up to 7–8 mm/yr (Wallace et al., 2009a), volcanic histories are presented in Tables 1–4, esis beneath volcanic regions, through well- which may be related to the recent trenchward which describe the general changes within each established global correlations between tectonic migration of the active volcanic arc due to slab time division. Unless otherwise stated in the processes and magma chemistry (e.g., Pearce rollback (Yamaji, 2003). The southern bound- appropriate table, all information regarding vol- and Cann, 1971, 1973; Pearce and Norry, 1979; ary of Beppu-Shimabara graben extension in cano locations and products comes from the online Pearce and Peate, 1995). Here we plot major Kyushu is the Oita-Kumamoto line, a system AIST database (AIST, 2008). This information is and trace element data of Kyushu volcanic of dextral strike-slip faults that are thought supplemented by observations from Ocean Drill- rocks on diagnostic diagrams (SiO2 versus to be the westward continuation of Shikoku ing Program (ODP) cores on ash-layer frequency alkalis, alkalis-FeO*-MgO (AFM), SiO2 versus

Island’s Median Tectonic Line (Okamura et (Cambray and Cadet, 1994). K2O, MnO-TiO2-P2O5, Zr-Nb-Y, Y versus Sr/Y al., 1995; Fig. 3). An east-west–trending dis- diagrams, and normal mid-oceanic-ridge basalt continuity in global positioning system (GPS) Geochemistry of Volcanic Rocks (N-MORB)–normalized trace element pattern; velocities across southern Kyushu, just north see Figures DR1–DR21 [footnote 1]). These of the Kagoshima graben, indicates active left- In order to examine the relationship between key geochemical diagrams for Kyushu volca- lateral shear cutting across southern Kyushu tectonics and magmatism in Kyushu, we inves- nic rocks were used to classify the rock types (Nishimura and Hashimoto, 2006; Wallace et tigated the temporal and spatial variations in and constrain the magmatic source and the tec- al., 2009a). Wallace et al. (2009a) have sug- geochemical characteristics of volcanic prod- tonic background of magmatism. In particular, gested that this shear occurs in response to sub- ucts by compiling reported data. The details we focus on the following ﬁ ve geochemically duction of the Kyushu-Palau ridge. are provided in the GSA Data Repository Fig- distinctive rock types observed in Kyushu, ures DR1–DR21.1 which indicate tectonic setting as reﬂ ected VOLCANIC HISTORY OF KYUSHU in the nomenclature: (1) ocean island basalt (OIB); (2) island arc basalt (IAB); (3) interme- This paper discusses volcanism from 15 to 1GSA Data Repository item 2011289, Geochemi- diate type between OIB and IAB (OIB/IAB); 10 Ma, 10–6 Ma, 6–2 Ma, and Quaternary vol- cal discrimination diagrams of the volcanic rocks (4) high-magnesian andesite; and (5) adakitic in Kyushu, is available at http://www.geosociety canism from 2 to 0 Ma. These time divisions are .org/pubs/ft2011.htm or by request to editing@ andesite, dacite, and rhyolite, hereafter referred based on major changes in the style and intensity geosociety.org. to as adakite.

Geological Society of America Bulletin, November/December 2011 2203 Downloaded from gsabulletin.gsapubs.org on February 11, 2014 Mahony et al. Additional notes Additional notes O, B/Nb and other fl uid-mobile B/Nb and other fl O, elements with time 2 (Nagao et al., 1995) (Kakubuchi et al., 1994) (Kakubuchi change from OIB/IAB to OIB Additional notes off NW Kyushu (Shinjo et al., 2000) off NW Kyushu Associated with mid-Miocene subduction IAB to OIB, Kakubuchi et al., 1994 Kakubuchi IAB to OIB, 2005 (if not AIST, 2008) (if not AIST, (if not AIST, 2008) (if not AIST, et al., 2000; Miyoshi et al., 2008b Miyoshi et al., 2000; (if not AIST, 2008) (if not AIST, N.D. Kimura et al., and Nakamura, 1994; Ishikawa N.D. IAB/HMA Nagao et al., 1999 ood andesites” “fl 2.5–2 Ma S-type felsic Alkalic basalt OIB Uto et al., 2004 1995; Sano, Alkalic basalt OIB Uto et al., 2004 1995; Sano, TABLE 3. VOLCANIC ACTIVITY FROM 6 TO 2 MA TO 6 FROM ACTIVITY VOLCANIC 3. TABLE TABLE 2. VOLCANIC ACTIVITY FROM 10 TO 6 MA TO 10 FROM ACTIVITY VOLCANIC 2. TABLE TABLE 1. VOLCANIC ACTIVITY FROM 15 TO 10 MA TO 15 FROM ACTIVITY VOLCANIC 1. TABLE N.D. N.D. HMA Low-boron Shiraki et al., 2000 N.D. N.D. HMA IAB/ADK/Low-boron N.D. Small scale scoria cones and lava fl ows fl scoria cones and lava scoria cones and lava fl ows fl scoria cones and lava Plateau lavas (2.5–2 Ma) Plateau lavas erupted into a tectonic depression. Location type Volcanism Composition Geochemistry Reference Iki Island mainly Monogenetic volcanoes, Location type Volcanism Composition Geochemistry Reference Iki Island mainly Monogenetic volcanoes, Kitamatsuura plateaus Lava Tholeiitic basalt OIB to OIB/IAB 2007 NUMO, change from OIB/ Temporal plateaus. lava 8, 100-m-thick and Shimoshima Unzen Location type Volcanism Composition Geochemistry Reference Fukuoka Monogenetic volcanoes Basalt OIB 2003 Hoang and Uto, Scattered centers Kitamatsuura and (Ma) 3.5–0.6 (Ma) 10 to 6 3.5–0.6 2.49 (Ma) Figure 6 shows the locations of the volcanic centers in this table. OIB/IAB—intermediate type between OIB and island arc basalt; N.D.—no details were used in this study; IAB—island arc basalt; HMA— IAB—island arc basalt; details were used in this study; N.D.—no OIB/IAB—intermediate type between OIB and island arc basalt; centers in this table. the locations of volcanic Figure 6 shows Figure 5 shows the locations of the volcanic centers in this table. N.D.—no details were used in this study; IAB—island arc basalt; SI—Shikoku Island; HMA—high-magnesian andesite. Island; SI—Shikoku IAB—island arc basalt; details were used in this study; N.D.—no centers in this table. the locations of volcanic Note: Figure 5 shows activity. age of volcanic *Approximate Figure 8 shows the locations of the volcanic centers in this table. N.D.—no details were used in this study; OIB/IAB—intermediate type between OIB and island arc basalt; IAB—island arc basalt; ADK— IAB—island arc basalt; OIB/IAB—intermediate type between OIB and island arc basalt; details were used in this study; N.D.—no centers in this table. the locations of volcanic Figure 8 shows Note: *Approximate age of volcanic activity. age of volcanic *Approximate Note: *Approximate age of volcanic activity. age of volcanic *Approximate Region Date* SWNWNW 17–12 15, 9–6 15–4.3, 4.3–3.5, Southern Kyushu Hirado Island Plutons N.D. I-type and N.D. IAB to OIB type Uto et al., 2004 Due to an asthenospheric heat source Region Date* NWSI and NE 14 14 Setouchi Shimoshima N.D. N.D. N.D. N.D. HMA (High Boron) HMA 2006 Tatsumi, Nagao et al., 1992 NWNW 4.5– NWNW 4.3–3.5 3 10–6 Iki Island Higashimatsuura Kitamatsuura plateau Lava plateaus Lava Lavas Basalt Basalt Alkalic basalt OIB to OIB/IAB OIB N.D. 2007 NUMO, Uto et al., 2004 1995; Sano, 2007 NUMO, Temporal plateaus. lava 8, 100-m-thick Voluminous adakite; HMA—high-magnesian andesite; OIB—ocean island basalt. adakite; NWNW 2.7–2.2 N.D.NWNWSW 2.5–0.5 Arita Between 4 6–1.2SW Pre-UnzenSW Southern pre-UnzenSW 4.8–1.6C Hisatsu 2.5–1 6.4–5.9C HokusatsuC 6 Monogenetic volcanoes From Stratovolcano Initially Island arc andesite tuff breccias ZoneVolcanic Hohi Nansatsu Sendai 2.8–2.1 3.8–2.2 N.D. cones and pyroclastic Lavas Pre-Aso caldera Pre-Kuju Basalt Andesite and dacite plateaus cones Lava and pyroclastic Lavas Rhyolite N.D. IAB Basalt N.D. plateau Lava N.D. OIB Andesite N.D. Uto and Uchiumi, 1997 N.D. Some HMA Andesite N.D. OIB N.D. 2007 NUMO, Shiraki Matsumoto et al., 1992; N.D. HMA (3.8 Ma) IAB (2.2 Island arc-type N.D. IAB N.D. 2008 Miyoshi, N.D. Increased K Hedenquist et al., 1994 N.D. NWNW Episodes from NW 15–4.3, 4.3–3.5, NW 9–6 7–2 Between Kitamatsuura Hirado Island N.D. N.D. IAB to OIB type Uto et al., 2004 NWCSW 7 6.4–5.9 8 Shimoshima Nansatsu Yabakei N.D. N.D. N.D. Basalt N.D. N.D. OIB IAB OIB/IAB 1990 and Matsumoto, Kakubuchi Nagao et al., 1992 Hedenquist et al., 1994 the onset of subduction-related volcanism marked Likely HMA erupted at Shimoshima 7 Myr previously Region Date* high-magnesian andesite; OIB—ocean island basalt. OIB—ocean island basalt. high-magnesian andesite;

2204 Geological Society of America Bulletin, November/December 2011 Downloaded from gsabulletin.gsapubs.org on February 11, 2014 Volcano-tectonic interactions during rapid plate-boundary evolution in the Kyushu region, SW Japan ) ( Continued rhyolitic Ito pyroclastic fl ow deposit ow fl Ito pyroclastic rhyolitic Small volume 3 the Aira caldera Additional notes trending SW of Aso trending SW of Aso Post–Aso caldera volcanism Post–Aso Marked change at 0.7 Ma from N-S extension to N-S change at 0.7 Ma from N-S extension Marked compression, coincides with change in volcanism style compression, coincides with change in volcanism Produced the 110 km N.D. 1989 1989 al., 2006 al., 2006 et al., 2000 Hunter, 1998 Hunter, Aramaki, 1990 (if not AIST, 2008) (if not AIST, Nagao et al., 1999 Uto and Uchiumi, 1997; Sudo Uto and Uchiumi, 1997; Kobayashi, 1984; Sugimoto et 1984; Kobayashi, Sugimoto et 1984; Kobayashi, IAB Ma) N.D.Tsukui and Aramaki, 1984; N.D. Kamata, Kamata et al., 1988; N.D. Kamata, Kamata et al., 1988; N.D. 1985; Watanabe, Ono and N.D.N.D. Matsumoto et al., 1991 line along a fault Aso 4, formed directly before Active Matsumoto et al., 1991 line along a fault formed between Aso 1 and 2, Active OIB/IAB OIB/IAB Ohta et al., 1992 ADK/IAB ADK/IAB ADK/IAB Kaneoka and Ozima, 1970 N.D. N.D. OIB/IAB Sugimoto et al., 2005 N.D. N.D. N.D. Basalt OIB N.D. Dacite Dacite Rhyolite Andesite Alkalic basalt OIB Uto et al., 2004 1995; Sano, Dacite, rhyoliteDacite, ADK Itoh, 1990 Rhyolite, dacite Rhyolite, Andesite, basaltAndesite, IAB/OIB(1 Basalt, andesite N.D. et al., 2004 Miyabuchi small volume lavas small volume TABLE 4. VOLCANIC ACTIVITY FROM 2 TO 0 MA TO 2 FROM ACTIVITY VOLCANIC 4. TABLE caldera forming; 0.7–0.3, caldera forming; N.D. N.D. fl o w s Maar Basalt N.D. N.D. volcano O increase in lavas. Lava fl ows fl Lava 2 stratocones scoria cones (0.5–0.3 Ma) Stratovolcano biotite rhyolites Two lava domes lava Two Andesite N.D. N.D. pyroclastic cones pyroclastic Postcaldera central Postcaldera formed fl ank volcano fl formed pyroclastic fl ow deposits ow fl pyroclastic Monogenetic volcanoes, Monogenetic volcanoes, lava fl ows, cinder cones, cinder cones, ows, fl lava formed fl ank monogenetic fl formed pyroclastic cones, lava fl ows, ows, fl lava cones, pyroclastic 1.6 Ma K Aso eruptions, Caldera-forming Aira eruption Caldera-forming Akai Intracaldera (Aso) volcanism zone Kimpo Omine Intracaldera (Aso) volcanism Hisatsu Location type Volcanism Composition Geochemistry Reference Akadaki, Onidake, Onidake, Hinodake) Maruyama (Ojikajima, Kyonotake, Kyonotake, Aira caldera Sumiyoshi-ike Hokusatsu/pre– Kishuku, Fukue, Kishuku, Fukue, Take-dake (Aso) Take-dake 0.6 Kuju complex Andesite volcano N.D. ADK/IAB N.D. 1.1 0.04 Tsurumi dome stratovolcanoes Lava dacite Andesite, OIB/IAB / 0.45 Pre-Aso caldera ows, fl Andesite pyroclastic 0.09 0.15 (Ma) 0.025 6–1.2 Aso 4 until 0.025 From 0.022From Naka-dake, 0.022–0.006 Oninomi 2; 0.12, Aso 3; 0.09, 0.12, Aso 3; 2; Region Date* CCCC 0.6–0.4 N.D. N.D. 0.7 1.6, Waita-yama domes lava Stratovolcano, Hane-yama Hohi volcanic Pre-Kuju Andesite N.D. N.D. N.D. Kamata, 1998 N.D. Rhyolite N.D. N.D. Daishi, 2006 Daishi, 2006 SWSWSW 0.1 0.007 1.6, 1.4 Yonemaru- Pre-Kirishima Aojiki shield Lava Maar Andesite N.D. N.D. N.D. N.D. N.D. CCC 2 Approx. 0.7–0.3 1–0.7, Pre-Tsurumi N.D. Shishimuta FT/TM/OA/NH plateau Lava Caldera N.D. Andesite 0.7, plateau; lava 1–0.7, N.D. N.D. N.D. N.D. N.D. CCCCC 0.35–0.02 1.5–1.1 0.015 0.4–0.3, Himeshima 1.1 domes and pyroclastic Lava 0.04 Hiji Futago Kanagoeows fl domes and lava Lava Yufu dacite Andesite, ows fl Lava Stratovolcano dome stratovolcanoes Lava ADK dacite Andesite, dacite (0.4–0.3) Andesite, IAB OIB/IAB / Andesite Nakada, 1986 Kamata, 1987, 1989 N.D. N.D. C C 0.14, Aso Aso 1; 0.3, SW SWSWSWSW 0.022–0 0.016 0.5–0.35 Sakurajima 0.5 Wakamiko stratovolcano Postcaldera Imuta dacite Andesite, Satsuma- Caldera N.D. dome Lava Rhyolite N.D. Dacite N.D. N.D. on the southern Formed rim of Aira caldera Shimomura, 1960 6 × 3 km submarine caldera in the north eastern corner of N.D. NWNW 1.7–1.4, 1–0.6 1.07–0.66 Iki IslandNW Goto Islands Monogenetic volcanoes, NWNWNW 1.3 From 1.3–1, 0.8–0.4 0.5–0.3 Formed 1.1 Taradake Ukujima Unzen domes lava Stratovolcano, stratovolcano Polygenetic Kurose basalt, dacite Andesite, and domes ows fl Lava rhyolite Basalt, andesite, OIB/ OIB/IAB (remains) neck Volcanic OIB N.D. Sudo et al., 1998 Basalt OIB N.D. C C C C SW SW 3–1, episodic activity

Geological Society of America Bulletin, November/December 2011 2205 Downloaded from gsabulletin.gsapubs.org on February 11, 2014 Mahony et al.

Ocean island basalt–type rocks are characteristic of an enriched asthenosphere source (e.g., Saal et al., 1998; Parman et al., 2005; Jackson et al., 2008). These rocks are plotted on the ﬁ elds of ocean island tholeiite and/or ocean

island alkaline in MnO-TiO2-P2O5 diagram, and are plotted on the fi elds of within-plate tholeiite and/or within-plate alkaline, and/or plume (P) -MORB fi elds in Zr-Nb-Y diagram. Island arc basalts are enriched in fl uid mobile Kagoshima bay Additional notes eruption in Japan large ion lithophile elements (e.g., K, Rb, and Ba) and light rare earth elements, and are

Located on the Kikai caldera rimLocated on the Kikai caldera depleted in fl uid immobile high fi eld strength elements (e.g., Nb, Zr) and heavy rare earth elements. Island arc basalts are plotted on the fi elds 20 × 17 km diameter caldera, 40 km SW of Kyushu. 20 × 17 km diameter caldera, 40 SW of Kyushu.

The 0.006 Ma VEI 7 eruption the largest Holocene was The 0.006 Ma of island arc tholeiite and calc-alkaline basalt in

MnO-TiO2-P2O5 diagram, and are plotted on the ﬁ eld of volcanic arc basalt in Zr-Nb-Y diagram. We deﬁ ned OIB- and IAB-type as the rocks that plot around the boundary area between OIB and island arc basalt in the MnO-TiO -P O and

) 2 2 5 Zr-Nb-Y diagrams, implying contributions from both OIB and island arc basalt sources. We also (if not AIST, 2008) (if not AIST, describe rocks as OIB/IAB if they are deﬁ ned as Continued al., 1984; Machida, 1999 al., 1984;

Torrence and Grattan, 2002 Torrence OIB in one diagram and as island arc basalt in another diagram. Boron (B) is a useful element for identify- ing subducted oceanic-slab infl uence on the subarc mantle compositions, because B is enriched in altered oceanic crust and sea-fl oor sediment (e.g., Ishikawa and Nakamura, 1993; Smith et al., 1995), and is selectively parti- tioned into fl uid phase during fl uid-fl ux melting at the base of mantle wedge (e.g., Moran N.D. N.D. et Walker Ono et al., 1982;

Andesite IAB N.D. et al., 1992; Bebout et al., 1999). In contrast,

Andesite, rhyoliteAndesite, N.D. N.D. OIB and MORB have low B contents (Sun and McDonough, 1989; Ryan and Langmuir, 1993; Chaussidon and Jambon, 1994; Chaussidon and

TABLE 4. VOLCANIC ACTIVITY FROM 2 TO 0 MA ( TO 2 FROM ACTIVITY VOLCANIC 4. TABLE Marty, 1995; Ryan et al., 1996; Leeman and Sis- son, 1996) reﬂ ecting an absence of a subducting slab component. B/Nb (Fig. 4) is a sensi- tive indicator of slab involvement. Boron and

calderas Nb have similar solid and/or melt distribution Lava domes, domes, Lava since 0.1 Ma Stratovolcano Andesite N.D.coefﬁ N.D. cients so that B/Nb are not signiﬁ cantly

Stratovolcanoes, small Stratovolcanoes, affected by partial melting and crystal fraction- postcaldera stratovolcano postcaldera stratovolcano producing large ignimbrites Pre–Kikai caldera volcanism N.D. N.D. 1982; and Self, Newhall ation. However, B and Nb have entirely different chemical behavior in ﬂ uid-related processes with B having a much higher mobility than Nb. Boron/Nb variation also does not reﬂ ect crustal centers Location type Volcanism Composition Geochemistry Reference assimilation, because the ratio in crust is vanish- Takeshima

Suwanosejima ingly low (Ishikawa and Nakamura, 1994; Lee- man and Sisson, 1996; Ishikawa and Tera, 1997; Ishikawa et al., 2001; Sano et al., 2001; Tonarini et al., 2004).

(Ma) Here we classify rocks containing more

than 53 wt% of SiO2 and 6–8 wt% of MgO as high-magnesian andesite by following Shiraki (1993). Setouchi high-magnesian andesite magmas, distributed in southwest Japan, are inter- Figure 9 shows the locations of the volcanic centers in this table. N.D.—no details were used in this study; OIB/IAB—intermediate type between OIB and island arc basalt; ADK—adakite; IAB—island arc ADK—adakite; OIB/IAB—intermediate type between OIB and island arc basalt; details were used in this study; N.D.—no centers in this table. the locations of volcanic Figure 9 shows Note: *Approximate age of volcanic activity. age of volcanic *Approximate Region Date* SWSW 0.3–0.15, 0.1SWSWSW Kirishima 0.5–0.4SWSWMa), (0.3–0.15 volcano Shield 0.3 0.11SW 0.006 Kobayashi 0.095, 0.0063 0.004 Kakuto Kikai Ata Ikeda-ko Kaimondake 0.7 Caldera dome lava Stratovolcano, maar dome, Caldera, lava eruptions Caldera-forming andesite dacite, Rhyolite, Yahazu-dake, Caldera Basalt, andesite N.D. Caldera rhyolite Dacite, IAB rhyolite Dacite, N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. Lies directly west of Ata caldera Matsumoto and Ui, 1997 25 km (E-W) ×12 submarine caldera in the mouth of Lies directly west of Ata caldera Associated with Kirishima volcano Associated with Kirishima volcano basalt. Volcanoes FT/TM/OA/NH: FT—Fukuman-yama and Tateishi-yama; TM—Takahira-yama and Mizuguchi-yama; OA—Ojika-yama and Amagoi-dake; NH—Noine-dake and Hanamure-yama. NH—Noine-dake and Amagoi-dake; OA—Ojika-yama and Mizuguchi-yama; TM—Takahira-yama Tateishi-yama; and FT—Fukuman-yama FT/TM/OA/NH: Volcanoes basalt. SWSWSW 1–0.9 0.7 0.01–0 Kuroshima 16 new Approx. Tairajima, Stratovolcanopreted Andesite as either due N.D. to hydrous partial N.D. melting

2206 Geological Society of America Bulletin, November/December 2011 Downloaded from gsabulletin.gsapubs.org on February 11, 2014 Volcano-tectonic interactions during rapid plate-boundary evolution in the Kyushu region, SW Japan of the mantle or due to the interaction between and coexistence of high-magnesian andesites ing of the maﬁ c lower crust under garnet and/or the mantle and silicic slab melt in subduc- and OIB-type rocks (Kakubuchi et al., 1995; amphibole stable conditions (e.g., Atherton and tion zones (e.g., Tatsumi, 2003). The origin of Shiraki et al., 2000; Miyoshi et al., 2008). We Petford, 1993). In the case of Kyushu, high Sr/Y Setouchi high-magnesian andesite magmas are contend that the exact origin and plate tectonic adakites were distributed in the volcanic front often ascribed to the subduction of a young and context of both the Setouchi and Kyushu high- and backarc area. The volcanic front adakites hot oceanic plate (Shikoku Basin) and backarc magnesian andesites are not well understood. in Yufu at 0.04 Ma were interpreted by Sugi- opening (Tatsumi, 2003, 2006). However, the Adakitic silicic rocks are characterized by moto et al. (2006) to be generated by mixing origin of the Kyushu high-magnesian andesite is high Sr/Y ratios (see Fig. 4). Hypotheses for between MORB mantle and partial melt of the still controversial. Previous work suggests that the generation of adakites remain controversial subducted terrigenous sediments on the basis of Kyushu high-magnesian andesite was generated (e.g., Castillo, 2006). Defant and Drummond calculation using trace elements and isotopic independently from subduction, by interaction (1990) suggested that adakites are formed by data. Shiraki et al. (2000) argued the Pliocene between injected asthenospheric mantle and direct melting of the young (<25 Myr) hot sub- backarc adakites are generated by interaction preexisting metasomatized lithospheric mantle, ducting slab or sediments. Alternatively adakite between injected asthenospheric mantle and on the basis of their mature stage of volcanism magma could be generated by the partial melt- preexisting metasomatized lithospheric mantle,

35° Southwest Honshu AMURIAN Tsushima PLATE Islands

34° Fukuoka MTL Northwestern Volcanic FG Beppu Region HVZ Shikoku HM BSG Oita AR Central Island Goto Volcanic KJ 33° Islands U Region Aso Nagasaki

Shimabara OKTLNon- Peninsula Volcanic Back-arc Region Kyushu Kumamoto Southern KS 32° Volcanic Region AI Forearc SJ Nankai Trough

N 72 mm/yr IK Kagoshima AT Bay AMUR/PSP 31° Okinawa Trough KG PHILIPPINE SEA −5000 0 0 50 km −5000meters0 KK PLATE

128° 129° 130° 131° 132° 133° 134° 135° Figure 3. Detailed active tectonic setting of Kyushu. Several key cities, volcanoes, and tectonic features are highlighted on this map. Black text indicates a city or other geographical location. Red triangles mark key volcano locations. Tectonic features have blue text; the two grabens are bounded by dotted blue lines. Green text indicates the three main volcanic regions and the nonvolcanic region. The forearc region is deﬁ ned as the area between the volcanic front and the trench, the volcanic front marked here by the volcanoes FG-KJ-Aso-KS-AI-SJ-IK-AT-KK. The backarc region is that behind the volcanic front. Abbreviations: FG—Futago volcano; HVZ—Hohi volcanic zone, marked by red line; HM—Higashimatsuura volcanics; AR—Arita volcanic rocks; U—Unzen volcano; KJ—Kuju volcano; Aso—Aso volcano; KS—Kirishima volcano; AI—Aira caldera; SJ—Sakurajima volcano; IK—Ikeda-ko volcano; AT—Ata caldera; KK—Kikai caldera; BSG—Beppu-Shimabara graben; MTL—Median Tec- tonic Line; OKTL—Oita-Kumamoto tectonic line; KG—Kagoshima graben.

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34° on the basis of the coexistence of adakites and Himeshima OIB-type rocks. Futago Oninomi Yufu Active Volcanic Regions Kuju 33° Here we have divided Kyushu into three Aso regions of active volcanism: the northwestern, central, and southern (Fig. 3). We deﬁ ned these regions as areas with similar volcanic history. A common major change of volcanism in these Latitude 32° three volcanic regions is the widespread lava plateaus that preceded other eruption styles. The Kirishima northwestern volcanic region is dominated by episodic backarc volcanism, with many monogenetic volcanoes and lava plateaus (we deﬁ ne Aira Mantle lava plateaus as a series of dominantly long, 31° values Imuta runout lava eruptions from distributed sources). The central volcanic region is a long-lived area 0246 0.1 0.2 0.3 20 40 60 of volcanism (including the recent onset of arc B/Nb B/Zr Sr/Y volcanism), changing in style through time. Types of volcanism in the central volcanic Figure 4. Along-arc variations of B/Nb, B/Zr, and Sr/Y ratios of volcanic products from region have included voluminous lava plateaus, around the recent volcanic front in Kyushu. Black circles in B/Nb and B/Zr diagrams show lava domes, cinder cones, and calderas (such as data for the basaltic rocks. White circles in Sr/Y diagram show data for the hornblende- the 18 km × 25 km Aso caldera, Fig. 3). The bearing silicic products. southern volcanic region experienced widespread lava plateau volcanism throughout the 130° 132° Pliocene, but since 0.3 Ma there has been volcanic front caldera formation and postcaldera stratovolcanoes (e.g., Aira caldera 18 km × 20 km, Iki Island Sakurajima postcaldera stratovolcano, Fig. 3). 15–4.3 Ma Setouchi 34° 14 Ma There is a fourth region of Kyushu, the “nonvolcanic” region located partially in the northern forearc in Figure 3. This region has no record of volcanism, although there are Miocene I-type (ca. 12–15 Ma) and S-type (14–17 Ma) granitic Hirado plutons (e.g., Kimura et al., 2005). There is also 15 Ma, 9–6 Ma a marked peak in frequency of voluminous vol- I canic ash layers in ODP sites 296, 442, 443, and 445 in the Shikoku Basin (Cambray and Cadet, 1994), suggesting substantial volcanism I in southwest Japan in the Miocene. S Volcanic History 15–10 Ma Possible location Felsic of heat source, 11 Ma Shimoshima I The I- and S-type plutons (Fig. 5) form part 14 Ma plutons of more widespread igneous activity (Table 1) in southwest Japan between 17 and 13 Ma (Kano I et al., 1991; Kimura et al., 2005). Furukawa N and Tatsumi (1999) proposed that the Setou- chi high-magnesian andesite volcanism formed 0 50 km due to subduction of the young, hot Philippine Sea plate between 17 and 10 Ma (Takahashi, S 31° 1981; Tatsumi, 1983). Tatsumi (2006; Tatsumi et al., 2001; Tatsumi, 1982) suggested that the Figure 5. Volcanism in Kyushu from 15 to 10 Ma. Shaded areas of varying size represent Setouchi high-magnesian andesite volcanism is lava plateaus, or in some cases represent volcanism of an unknown nature. Triangles rep- associated with the opening of Japan Sea and resent stratovolcanoes, or monogenetic volcanoes. There is a band of I- and S-type felsic subduction of hot Shikoku Basin lithosphere. plutons (represented by black dots with an I and an S, respectively) trending northeast- High-magnesian andesite volcanism also southwest across central Kyushu. There is minor volcanism in the northwestern and cen- occurred in Kyushu almost simultaneously with tral volcanic regions. The locations of the I- and S-type felsic plutons are after Kimura et the Setouchi high-magnesian andesites at Shi- al. (2005). moshima (Nagao et al., 1992; Fig. 5).

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Volcanic History 10–6 Ma Voluminous andesitic lavas characterized the Miyoshi, 2008). High-magnesian andesites have The 10–6 Ma period (Fig. 6; Table 2) is early stage of the Hohi volcanic zone (6–2 Ma) low B/Nb ratios that are similar to MORB or marked by a lack of obvious subduction-related (Kamata, 1989). The early stages erupted OIB (0.05–0.5; Ryan et al., 1996). On the other volcanism (Fig. 7), which has led to the sug- high-magnesian andesite at the Hohi volcanic hand, the island arc basalts show signifi cantly gestion that there was a hiatus in subduction zone tectonic edges, followed later (2–0 Ma) higher B/Nb than high-magnesian andesites. (Kamata and Kodama, 1994). Low rates of by island arc basalts (Kamata, 1987; Nakada Therefore the elevated B/Nb suggests that the explosive volcanism are indicated by the low and Kamata, 1991), which focused toward the addition of slab-derived fl uid to the source man- abundance of volcanic ash layers in ODP cores center of the Hohi volcanic zone (Kamata and tle beneath the Aso area occurred between 3.8 from the Shikoku Basin (Cambray and Cadet, Kodama, 1994). These lavas became compo- and 2.2 Ma (Miyoshi, 2008). In this western part 1994). Effusive volcanism mostly occurred in sitionally more K enriched with time and after of the central volcanic region, the adakite and the northwestern region, with minor late volca- peaking in activity at 5 Ma, the Hohi volcanic island arc basalt magmas coexisted with high- nism in the southern region (Fig. 6). zone decreased in activity throughout the Plio- magnesian andesites. Pre-caldera volcanism in cene to the present (Kamata and Kodama, 1994). Aso was dominated by andesite lava plateaus Volcanic History 6–2 Ma Around the same time, volcanism increased in (Ono and Watanabe, 1985). From 6 to 2 Ma, volcanic activity in Kyushu the Shimabara peninsular area (Yokose et al., In the southern volcanic region, andesite lava increased relative to the 10–6 Ma period (Fig. 8; 1999), the eventual location of Unzen volcano. plateaus developed in the Nansatsu, Hokusatsu, Table 3), with notable formation of many andes- On the western edge of the Hohi volcanic zone, and Hisatsu areas, parallel to and westward of itic lava plateaus in areas of future caldera volca- Aso volcano erupted high-magnesian andesites the Kagoshima graben (Nagao et al., 1999) nism. The northwest volcanic region continued at 3.8 Ma and island arc basalts at 2.2 Ma, which (Fig. 8; Table 3). Ocean Drilling Program sites forming basaltic lava plateaus and monogenetic showed an increase in K2O, B/Nb, and other 296, 442, and 443 adjacent to Kyushu in the Shi- volcanoes, as well as new rhyolitic volcanism. fl uid-mobile elements content with time (Fig. 4; koku Basin show a high frequency of volcanic ash layers from 6 Ma to the present (Cambray and Cadet, 1994). 130° 132° Volcanic History 2–0 Ma The most recent episode of arc volcanism in Iki Island Ya b a ke i Kyushu started at the beginning of the Quater- 8 Ma 34° nary (e.g., Watanabe, 2005). Signifi cant changes in the mode of eruption and whole-rock chemistry occurred throughout Kyushu (Kamata and Kitamatsuura Hirado Kodama, 1994; see Fig. 7 for a summary). An basalts, 10–6 Ma 9–6 Ma active volcanic arc related to Philippine Sea plate subduction formed in the central and southern regions of Kyushu, extending from Aso and trending northeast-southwest (Fig. 3). Volcanic Arc volcanism has been prevalent in Kyushu for activity the past 2 Myr, with a marked pulse of large- 7–2 Ma magnitude explosive eruptions with associated caldera formation from 0.3 Ma (e.g., Aso, Aira, Shimoshima Kakuto, Ata, and Kikai; Fig. 9). The large calde- basalts, 7 Ma ras in the arc were mostly preceded by Pliocene to early Pleistocene voluminous andesitic lava fl ows. Ocean Drilling Program data on Shikoku Basin cores also show a marked increase in ash layer frequency after 2 Ma, especially in site 442 (Cambray and Cadet, 1994). N The northwestern volcanic region showed Nansatsu continued backarc volcanism, typically with 6.4–5.9 Ma new monogenetic volcano centers as well as 0 50 km Unzen volcano, which lies in a transitional zone between the arc and backarc regions. 31° There is a nonvolcanic gap in the volcanic front to the southeast of Unzen. Taken together, the Figure 6. Volcanism in Kyushu from 10 to 6 Ma. Shaded areas of varying size represent location of Unzen and the gap are enigmatic. lava plateaus, or in some cases represent volcanism of an unknown nature. Triangles The central volcanic region was the location represent stratovolcanoes, or monogenetic volcanoes. Dashed gray lines in the central of the fi rst Quaternary caldera volcanism in region mark the Beppu-Shimabara graben. Episodes of effusive volcanism occurred Kyushu with Shishimuta caldera, followed sporadically through time, mainly located along the very western edge of Kyushu (i.e., by multiple caldera-forming eruptions at Aso Nansatsu, Kitamatsuura). The location of the Nansatsu volcanic rocks is taken from volcano (Fig. 9; Table 4). The Hohi volcanic Shinjo et al. (2000). zone saw a marked increase in K2O content in

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15–10 Ma 10–6 Ma 130°E 130°E

A Kitamatsuura Boron) 10–6 Ma Setouchi HMAHM (High-Boron)(High- Yabakei 8 Ma 33°N 14 Ma 33°N

Leading I Hirado edge? 7–2 Ma 15–7 Ma Low-Boron HMA I Leading Shimoshima Shimoshima edge? 14 Ma S 7 Ma 32°N 32°N 32°N I –12 Ma 17 OIB Felsic plutons OIB/IAB I IAB HMA Nansatsu ADK 130°E S

6–2 Ma 2–0 Ma

130ºE130°E Iki Himeshima <4 Ma Iki Fukuoka Kurose 4–1 Ma Futago 5–3 Ma Ojikajima Yufu-Tsurumi Low-Boron HMA 33°N Taradake Kuju Aso 2.2 Ma Unzen Aso 3.8 Ma Fukue Low-BoronLow-Boron HMA HMA 4 Ma

Hisatsu

32°N HisatsuHisat 11 MaMa Kirishima su N HokusatsuHoku s HokusatsuHokusatsu atsu

0 100 km Nansatsu Kaimon

Figure 7. Concise summary of the geochemistry of volcanoes in Kyushu through the past 15 Ma. The dots represent analyzed samples from named volcanoes and/or areas of volcanism that have been classiﬁ ed by type (different shad- ing). Solid black dots are ocean island basalt (OIB); dark gray dots with white spots are hybrids between OIB and island arc basalt (IAB). Plain gray dots are IAB; pale-gray dots with dark-gray spots are high-magnesian andesite (HMA); and white with dark-edge dots are adakites (ADK). Black circles with an I or S in them, respectively, represent the locations of I- and S-type felsic plutons (from Kimura et al., 2005). Where the color is not in a small circle (e.g., large Fukuoka area on the 6–2 Ma map), the volcanism is distributed throughout this area, but not necessarily spatially continuous.

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lavas by 1.6 Ma, which preceded the marked cano is a postcaldera volcano associated with TECTONIC CONFIGURATION OF change from north-south extension to north- Kakuto and Kobayashi calderas, comprising a THE KYUSHU REGION FROM 15 MA south compression in the Hohi volcanic zone series of cones on its fl anks (International Vol- TO PRESENT at 0.7 Ma (Kamata et al., 1988), coinciding canological Association, 1962), all formed in with the change in volcanism style from cal- an elliptical volcano zone (~30 km × 20 km) In order to understand the changing char- dera formation (Shishimuta caldera, 8 km trending northwest-southeast. Interestingly, acter of volcanism in Kyushu, the evolution wide, >3 km deep; Kamata, 1989) to small- the location and northwest-southeast orienta- of the plate-boundary confi guration must be volume lavas and domes. The southern volca- tion of Kirishima and its associated vents coin- reconstructed. Philippine Sea plate motion has nic region has seen an eastward migration of cide and align with the left-lateral shear zone profoundly impacted the tectonic and volcanic the volcanic front since 6 Ma. The Nansatsu, identifi ed from GPS observations (Wallace et evolution of southwest Japan. According to Hokusatsu, and Hisatsu volcanic areas, char- al., 2009a) that crosscuts southern Kyushu. paleomagnetic data, the Philippine Sea plate acterized by island arc basalts, represent the Farther south, offshore Kyushu in the Ryukyu has undergone large northward translation and location of the early volcanic front (Watanabe arc (Fig. 9), the Kikai caldera had multiple clockwise rotation (~90° relative to the sur- et al., 1992). The Kagoshima graben represents caldera- forming eruptions, including a VEI 7 rounding plates) since 50 Ma (Hall et al., 1995a, the current location (Figs. 7 and 9). Southern eruption at 0.063 Ma (Ono et al., 1982; Walker 1995b, and references therein), although there Kyushu has had several Quaternary caldera- et al., 1984; Machida, 1999). Many volcanic is controversy regarding the exact timing of forming eruptions, at Kakuto, Kobayashi, Aira, centers in the Ryukyu arc became active dur- the northward movement of the Philippine Sea Wakamiko, and Ata calderas. Kirishima vol- ing the Quaternary (Fig. 9; Table 4). plate (Yamazaki et al., 2010). The Philippine Sea plate has also undergone notable changes in its kinematics, likely due to major regional tec- 130° 132° tonic events, including collisions on its bound- Fukuoka aries (Eguchi, 1984; Hall et al., 1995a, 1995b; Iki Island 4.5–2.49 Ma Sdrolias et al., 2004). Hall et al. (1995a, 1995b) 4.3–3.5 Ma HVZ andesite suggested that between 40 and 50 Ma the Philip- 34° lava plateau pine Sea plate underwent ~50° clockwise rota- Higashimatsuura basalts, 3 Ma tion, negligible rotation from 25 to 40 Ma, ~34° HVZ formation clockwise rotation from 5 to 25 Ma, and ~5.5° Kitamatsuura 6 Ma clockwise rotation from 5 Ma to present. Cur- basalts rently, geodetic data indicate ~1°/Myr clockwise Arita rhyolites rotation rate for the Philippine Sea plate (Sella 2.7–2.2 Ma et al., 2002), generally consistent with the 5 Ma Pre-Kuju Volcanic to present paleomagnetic estimates of vertical 2.8–2.1 Ma activity axis rotation rate. At 2 Ma, a shift in Philippine Pre-Unzen Sea plate motion occurred, causing Philippine 2.5 Ma Volcanic Sea plate/Amurian plate relative plate motion to activity, 4 Ma Aso, 3.8–2.2 Ma become more oblique in southwest Japan (in a dextral sense), leading to Pliocene reactivation Hisatsu of the Median Tectonic Line as a right-lateral 6–1.2 Ma strike-slip fault (Kamata and Kodama, 1994; Hokusatsu Itoh et al., 1998; Kamata, 1998). Studies of 4.8–1.6 Ma Philippine Sea plate kinematics suggest that a N Okinawa Sendai major northward shift in the position of the Phil- Trough 2.5–1 Ma ippine Sea plate/Eurasia relative pole of rotation opening 0 50 km from 15°N to 48°N occurred at ca. 5 Ma (e.g., Nansatsu Hall et al., 1995a), which would have caused a 6.4–5.9 Ma 31° major change in relative motion at the southwest Japan–Philippine Sea plate boundary around Figure 8. Volcanism in Kyushu from 6 to 2 Ma. Shaded areas of varying size represent lava 5 Ma. plateaus, or in some cases represent volcanism of an unknown nature. Triangles represent Alternative models for Philippine Sea plate stratovolcanoes, or monogenetic volcanoes. Reinitiation of subduction at ca. 6 Ma is closely kinematics have recently been developed from followed by the formation of the Hohi volcanic zone (HVZ; marked by dotted gray line). the interpretation of paleomagnetic data from Volcanism in the HVZ initially formed at the edges of the zone and focused into the center sites on the northern portion of the Philippine with time. Extensive andesite lava plateaus also formed in the HVZ. Widespread effusive Sea plate (Yamazaki et al., 2010). These new volcanism continued in the northwestern and southern regions. Dashed gray lines in the data are interpreted to suggest that the Philip- central region mark the Beppu-Shimabara graben; dashed and solid lines in the southern pine Sea plate underwent most of its north- region mark the Kagoshima graben. The sporadic lavas located in the Fukuoka area are ward movement prior to 15 Ma (suggesting from scattered, small monogenetic volcanoes. The exact locations of these volcanoes are that between 50 and 15 Ma, the Philippine Sea unknown; so the locations of the dated samples are plotted here instead of volcano locations. plate rotated 90° clockwise about a pole near The locations of the Hisatsu, Hokusatsu, and Nansatsu volcanic rocks are taken from Shinjo 23°N/162°E, and that northward movement of et al. (2000). the Philippine Sea plate after 15 Ma is negligible

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Kanagoe Hane-yama Iki island Futago Himeshima Kimpo 34°

Kurose Hiji Yufu Ukujima Tsurumi Ojikajima FT TM OA Akadaki NH Kishuku (Fukue) Shishimuta CA Waita-yama Kyonotake Tara Kuju -dake Aso Hinodake Omine Onidake Unzen Akai

Kobayashi CA Hisatsu Kakuto CA Kirishima Hokusatsu Imuta Aira CA Wakamiko CA Satsuma-Maruyama Sakurajima Aojiki Yonemaru-Sumiyoshi-ike Ata CA Ikeda-ko CA 31° Kaimondake

Kuroshima Kikai CA Satsuma-Io Jima 2–1 Ma 1–0.3 Ma Okinawa Trough 0.3–0.01 Ma 0.001–0 Ma Kuchinoshima Kuchinoerabu-jima Nakano-shima N Kogaja-Jima and Suwanose-jima Oki and Gaja-Jima Suwanose-jima Tairajima Akesuki-Jima 0 65 km

Figure 9. Volcanism in Kyushu from 2 to 0 Ma. Shaded areas of varying size represent lava plateaus, or in some cases represent volcanism of an unknown nature. Triangles represent stratovolcanoes, or monogenetic volcanoes; stars represent calderas. The Quaternary is shaded into four sections: 2–1 Ma (white with dark outline), 1–0.3 Ma (pale gray), 0.3–0.01 Ma (dark gray), and 0.001–0 Ma (black). Many lava shield volcanoes and lava plateaus formed at ca. 2 Ma (nontriangle or star “areas” of color here), in locations of future caldera volcanism. From 1 to 0.3 Ma, the major change in volcanism is the start of caldera formation in Kyushu, at Shishimuta and Kobayashi calderas (represented by larger stars). The Hohi volcanic zone (HVZ) gains an inﬂ ux of stratovolcanoes from 0.6 Ma in the Kuju volcano complex; FT—Fukuman-yama and Tateishi-yama; TM—Takahira-yama and Mizuguchi-yama; OA—Ojika-yama and Amagoi-dake; NH—Noine-dake and Hanamure-yama. From 0.3 Ma, volcanism in Kyushu is dominated by large calderas, namely Aso, Kakuto, Aira, Ata, and Kikai (calderas are represented by large stars). These calderas all form within grabens, the Beppu-Shimabara graben in the central region (outlined by dashed/dot and dashed lines) and the Kagoshima graben in the southern region (outlined by solid black lines). Volcanism in the southern volcanic region generally youngs southward. Holocene volcanism has seen the volcanic front extend down in the Ryukyu arc, with the largest Holocene eruption occurring at Kikai volcano. Through the Holocene, the long-lived centers of Fukue (Hinodake), Unzen, Aso, Kuju, Yufu/Tsurumi, Kirishima, and Sakurajima continue to have active stratovolcanoes. The Holocene has had no widespread lava plateau eruptions. The locations of the Hisatsu and Hokusatsu volcanic rocks are taken from Shinjo et al. (2000).

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(Yamazaki et al., 2010). However, the Yamazaki to Shikoku Island and Kyushu. Note that the west Japan plate boundary adjacent to the Phil- et al. (2010) data set does not include samples Shikoku Basin (30–15 Ma; Okino et al., 1999; ippine Sea plate was dominated by left- lateral between 25 and 10 Ma, so it is diffi cult to assess Sdrolias et al., 2004) and the Sea of Japan (ages transpression (Fig. 10). The past kinematics of the timing of post–25 Ma changes in Philippine for the Sea of Japan opening vary from 30 to Philippine Sea plate motion from Hall et al. Sea plate motion from these data reliably. We 12 Ma: Otofuji and Matsuda, 1983; Tamaki et (1995a) and Sdrolias et al. (2004) requires rapid suggest that these data (and their associated al., 1992; and Lee et al., 1999; to 16–14 Ma: northeastward migration of the triple junction uncertainties) can be fi t equally well by 0.4°– Otofuji et al., 1991) had largely fi nished open- between southwest Japan, the Philippine Sea 0.5°/Myr (in latitude) of northward movement ing by 15 Ma, so the development of these plate, and the Pacifi c plates from 15 to 5 Ma. of the Philippine Sea plate from 29 Ma to pres- basins does not impact our reconstructions. At 15 Ma, the triple junction was located near ent, comparable to the ~0.7°/Myr northward Shikoku Island, and by 10 Ma it had migrated movement of the Philippine Sea plate from 5 to Tectonic Reconstruction for 15–10 Ma northeast ~300 km to a location somewhere 15 Ma that we expect in the reconstructions pre- Between 15 and 10 Ma, the kinematics of the between the Kii Peninsula and Boso Peninsula sented here using rotation poles from previous Philippine Sea plate relative to Eurasia in the region. East of this triple junction, subduction of studies of Philippine Sea plate kinematics. southwest Japan region were markedly different the Pacifi c plate is occurring. Thus, if our recon- from today (according to paleomagnetic studies struction is correct, prior to 15 Ma, subduction Methods of Hall et al., 1995a). The portion of the south- of the Pacifi c plate likely occurred beneath most

To reconstruct the plate-boundary conﬁ guration for southwest Japan over the past 15 Ma, Eurasian/Amurian plate published ﬁ nite rotation poles for the Philip- pine Sea plate relative to Eurasia from Gaina 36°N and Müller (2007) are used for the 2–15 Ma reconstructions. Note that the Gaina and Mül- ler (2007) Philippine Sea plate–Eurasia poles Subduction are largely derived from kinematic studies by 34° Seno et al. (1993), Hall et al. (1995a, 1995b), and Sdrolias et al. (2004). For 2 Ma to present, Subduction Pacific poles of rotation for the Philippine Sea plate, 32° plate the Amurian plate, and other tectonic plates and blocks in the region are derived from inversion IBM (10 Ma) of earthquake slip vectors and contemporary GPS data (Wallace et al., 2009a). Paleomag- 30° ~~ 8 8cm/a cm/a netic data from the Kyushu forearc (Kodama SBSC and Nakayama, 1993; Kodama et al., 1995) are SBSC (15 Ma, has just finished rifting) used to constrain the anticlockwise rotation of (10 Ma) 28° the Kyushu forearc from ca. 6 Ma to present. IBM (15 There remains considerable uncertainty about when Kyushu forearc rotation began: Kamata Left-lateral transpression ~8 cm/a Ma) and Kodama (1999) proposed that it started in KPR (10 Ma) 26° the Quaternary (<2.5 Ma), suggesting the rotation was very rapid (~15°/Myr). Other stud- KPR (15 Ma) ies suggest that the rotation has been ongoing since 6 Ma (Kato et al., 1998; Yamaji, 2003), 24° Philippine as this is consistent with evidence for exten- Sea sional deformation in southern Kyushu for the plate past 5 Ma, and slab rollback of the Philippine Sea plate since at least 5 Ma (Yamaji, 2003). 22° N 0 200 km For the purposes of this study, we assume that anticlockwise rotation of the Kyushu forearc 128°E 130° 132° 134° 136° 138° 140° 142° 144° has been ongoing since 6 Ma, at a constant rate (~5°/Myr). In our reconstructions, the positions Figure 10. Southwest Japan tectonics from 15 to 10 Ma, from a reconstruction using poles of of major physiographic features are tracked, rotation of the Philippine Sea plate relative to Eurasia from Gaina and Müller (2007). The such as the Kyushu-Palau ridge, the Izu-Bonin- gray shaded features are the positions of the Izu-Bonin-Mariana arc (IBM), and Kyushu- Mariana arc, and the Shikoku Basin spreading Palau ridge (KPR) at 15 Ma, and the solid black line is the position of the Shikoku Basin center. We assume that the position of the Izu- spreading center (SBSC) at 15 Ma. The dashed-outlined portions of the 15 Ma KPR, SBSC, Bonin Trench remains in the same location that and IBM approximately show the portions of those features that have been subducted since it is today relative to the Izu-Bonin-Mariana arc 15 Ma. The light-gray dashed lines outline the 10 Ma positions of the IBM, SBSC, and KPR. throughout the time period of this reconstruc- The black arrows (labeled in cm/yr) show approximate Philippine Sea plate–Amurian plate tion. A similar assumption is made for the posi- relative rates of motion from 15 to 10 Ma. The present-day east coast Kyushu and Shikoku tion of the Ryukyu and Nankai Troughs relative Island coast line is shown in light gray, while the 15 Ma position of the coast line is in black.

Geological Society of America Bulletin, November/December 2011 2213 Downloaded from gsabulletin.gsapubs.org on February 11, 2014 Mahony et al. of southwest Japan. At 15 Ma, the Shikoku Watanabe, 2005; among others; see further dis- occurred beneath northern Kyushu throughout Basin spreading center and the Kyushu-Palau cussion of this issue later in the paper). this period, and the Kyushu-Palau ridge col- ridge were intersecting the subduction margin lision with the subduction margin may have somewhere offshore the Ryukyu Islands, south- Tectonic Reconstruction for 5–2 Ma been the cause of anticlockwise rotation of the west of Japan. Around 5 Ma, Philippine Sea plate motion Kyushu forearc (Wallace et al., 2009a), which Sdrolias et al. (2004) suggested that most changed and began converging in a north- started ca. 2–5 Ma (Kodama et al., 1995; of the 25–5 Ma clockwise rotation of the westerly direction relative to southwest Japan Kodama and Nakayama, 1993). Backarc rift- Philippine Sea plate (34° from paleomagnetic (Seno and Maruyama, 1984; Seno, 1989; ing in the Okinawa Trough probably began ca. studies) occurred between 15 and 5 Ma. If Hall et al., 1995a). Compared to the 15–5 Ma 5 Ma (Sibuet et al., 1987; Wu et al., 2007), so this interpretation is correct, the along-strike period, from 5 to 2 Ma there is minimal along- in our reconstruction, the Okinawa Trough has migration of the Philippine Sea plate–Pacifi c strike migration of the triple junction and not yet opened prior to 5 Ma. At 5 Ma, at the plate–southwest Japan triple junction would the impingement points of the Kyushu-Palau beginning of rapid anticlockwise rotation of have been even more rapid than shown here. ridge, Izu-Bonin-Mariana arc, and Shikoku Kyushu, the coastline of Kyushu was also co- For example, if 30° of clockwise rotation of Basin spreading center (Fig. 12). Notably, the linear with the rest of southwest Japan accord- the Philippine Sea plate occurred between 15 subduction point of the Kyushu-Palau ridge ing to this reconstruction. and 5 Ma (at a rate of ~3°/Myr), the Philip- pine Sea plate–Pacifi c plate–southwest Japan triple junction would be located well south of Kyushu at ca. 10 Ma. Thus, depending on the Eurasian/Amurian Plate details of past Philippine Sea plate rotational 36°N kinematics (e.g., Sdrolias et al., 2004), subduction of the Pacifi c plate (and start of Philippine Sea plate subduction) beneath Kyushu may Pacific have initiated as recently as 10 Ma. 34° Plate

Tectonic Reconstruction for 10–5 Ma IBM (5 Ma) Tectonic reconstruction here is for the period oblique subduction 32° 10–5 Ma rather than the similar 10–6 Ma period ~ 7 cm/a SBSC (5 Ma) studied for the volcanism, simply as the ﬁ nite ~ 7-8 cm/a rotation poles used in this reconstruction (Gaina SBSC (10 M and Müller, 2007) are listed for 5 Ma, not 6 Ma. 30° The timing of changes in Philippine Sea plate ~ 8 cm/a IBM (10 Ma) KPR (5 Ma) Izu Bonin Mariana kinematics are not well constrained, so for the Arc (10 Ma) a) purposes of this study the tectonics discussed in this 10–5 Ma period are considered to be com- 28° parable to the 10–6 Ma volcanism. left-lateral transpression The large left-lateral component of relative plate motion at the southwest Japan plate KPR (10 Ma) 26° boundary between 15 and 5 Ma continued to cause rapid northeast migration of the position of the Philippine Sea plate–Paciﬁ c plate– Philippine Sea Plate southwest Japan triple junction, the Kyushu- 24° Palau ridge, Shikoku Basin spreading center, and the Izu-Bonin-Mariana arc relative to the southwest Japan plate boundary (Fig. 11). By 22° N 10 Ma, the Kyushu-Palau ridge was inter- 0 200 km secting the plate boundary offshore southern Kyushu, while the recently extinct Shikoku 128°E 130° 132° 134° 136° 138° 140° 142° 144° Basin spreading center intersection point was positioned adjacent to southwestern Shikoku. Figure 11. Southwest Japan tectonics from 10 to 5 Ma, based on a reconstruction using poles According to our reconstruction, the Izu- of rotation for the Philippine Sea plate relative to Eurasia for this time period from Gaina Bonin-Mariana arc point of intersection with and Müller (2007). The gray shaded features are the position of the Izu-Bonin-Mariana the margin at 10 Ma was just east of the Kii arc (IBM), Kyushu-Palau ridge (KPR) at 10 Ma, and the solid black line is the position of Peninsula, and by 8–6 Ma it had migrated close the Shikoku Basin spreading center (SBSC) at 10 Ma. The dashed-outlined portions of the to its current position, adjacent to the Boso 10 Ma KPR, SBSC, and IBM approximately show the portion of those features that has Peninsula. However, such a result is at odds been subducted since 10 Ma. The light-gray dashed lines outline the 5 Ma positions of the with most published literature, which indicates IBM, SBSC, and KPR. The black arrows (labeled in cm/yr) show approximate Philippine a ca. 15 Ma age for the initiation of Izu-Bonin- Sea plate–Amurian plate relative rates of motion from 10 to 5 Ma. The present-day east Mariana arc collision in its current location coast Kyushu and Shikoku Island coast line is shown in light gray, while the 10 Ma position in central Japan (Seno and Maruyama, 1984; of the coast line is in black.

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Tectonic Reconstruction for 2 Ma–Present Comparison with Previous Reconstructions the west of the Kyushu-Palau ridge) began sub- At ca. 2 Ma, the Philippine Sea plate shifted of Southwest Japan Tectonics ducting beneath Kyushu. Around 5–6 Ma, the its subduction direction from northwest to kinematics of the Philippine Sea plate changed west-northwest (Nakamura et al., 1984). The Our tectonic reconstructions suggest that, prior signifi cantly (Hall et al., 1995a), causing the Phil- increase in the westward component of subduc- to 15 Ma, subduction of the Pacifi c plate occurred ippine Sea plate–southwest Japan plate boundary tion caused the onset of dextral motion on the beneath most of southwest Japan and Kyushu. in the Kyushu region to change from strike-slip Median Tectonic Line (Kamata and Kodama, Starting ca. 15 Ma (depending on the details of dominated to nearly pure convergence, coincid- 1994; Itoh et al., 1998; Kamata, 1998). This Philippine Sea plate kinematics), the Kyushu ing with evidence for reinitiation of subduction- change in subduction direction also caused the region was straddling the Philippine Sea plate– related volcanism in Kyushu ca. 5–6 Ma (Seno, subduction point of the Kyushu-Palau ridge southwest Japan boundary zone, which was 1989; Kamata and Kodama, 1999; Fig. 12). to start migrating southwards (opposite to dominated by left-lateral transpression (Fig. 10). Previously published tectonic reconstruc- its previous northwards migration direction), From 15 Ma until ca. 5–6 Ma, buoyant Shikoku tions of southwest Japan give widely varying from its location offshore northern Kyushu at Basin crust was subducting beneath Kyushu results. Variation in reconstructions prior to ca. 2 Ma, to its present position ~80 km to the (Figs. 10 and 11); at ca. 5–6 Ma, the older, more 1995 can be reasonably explained by the lack of south (Fig. 13). deeply subducted west Philippine Basin slab (to paleomagnetic data, which have become available for later studies (e.g., Hall et al., 1995a). Most reconstructions of the southwest Japan region assume that the Izu-Bonin-Mariana arc, Kyushu-Palau ridge, and Shikoku Basin spread- Eurasian/Amurian plate ing center have been generally at the same loca- 36°N Pacific tion as they are today for the past 15 Myr (e.g., plate Seno and Maruyama, 1984; Hibbard and Karig, 1990; Jolivet et al., 1994; Taira, 2001; Kimura et al., 2005), or that these features were located 34° even farther to the northeast ca. 15 Ma than they are today (Otsuki, 1990). However, these reconstructions are not consistent with the kinematics of the Philippine Sea plate, as constrained by SBSC (5 Ma) 32° ~5 cm/a paleomagnetic and seafl oor spreading studies (Hall et al., 1995a, 1995b; Hall, 2002; Sdrolias SBSC (2 Ma) IBM (2 Ma) et al., 2004) and seismic tomography studies KyushuKyush IzuIz BonBoninin MarMariana (Miller et al., 2006). As shown in Figures 10–13, 30° Arc (5 Ma) u

c u Bonin Mariana if the estimates of past Philippine Sea plate ~6 cm/a Palau RidgeR (5 Ma) ( 5 (PSP rel Ma) motion used to constrain the reconstructions Eurasia) shown here are correct, they require the conﬁ g- idge 28° uration of the southwest Japan plate boundary (5 M i ana to have evolved dramatically since 15–20 Ma, a) including rapid northeastward migration of the Philippine Sea plate–Paciﬁ c plate–southwest 26° Japan triple junction.

KPR (2 Ma) Our reconstructions of the southwest Japan region are consistent with the more regional 24° Philippine Sea plate southeast Asia reconstructions presented by Hall (2002), Sdrolias et al. (2004), and Gaina and Müller (2007), which is expected given that their pre–2 Ma estimates of Philippine Sea 22° N 0 200 km plate–Eurasian plate kinematics were used to derive these reconstructions. For example, at 128°E 130° 132° 134° 136° 138° 140° 142° 144° 15 Ma, Sdrolias et al. (2004) position the Shi- koku Basin spreading center point of intersec- Figure 12. Southwest Japan tectonics from 2 to 5 Ma, based on a reconstruction using poles tion just south of Kyushu, similar to the 15 Ma of rotation of the Philippine Sea plate relative to Eurasia from Seno et al. (1993). The gray scenario presented here. An earlier reconstruc- shaded features are the position of the Izu-Bonin-Mariana arc (IBM) and Kyushu-Palau tion by Lee et al. (1999) is among the only ridge (KPR) at 5 Ma, and the solid black line is the position of the Shikoku Basin spread- studies focused on the southwest Japan region ing center (SBSC) at 5 Ma. The dashed-outlined portions of the 5 Ma KPR, SBSC, and that accounted for the evolving motion of the IBM show the portion of those features that has been subducted since 5 Ma. The light-gray Philippine Sea plate in a similar way; they also dashed lines outline the 2 Ma positions of the IBM, SBSC, and KPR. The black arrows suggested that subduction of the Paciﬁ c plate (labeled in cm/yr) show approximate Philippine Sea plate–Amurian plate relative rates of beneath all of southwest Japan prior to 15 Ma motion from 5 to 2 Ma. The present-day east coast Kyushu and Shikoku Island coast line is and that, for some period after 15 Ma, the shown in light gray, while the 5 Ma position of the coast line is in black. southwest Japan–Philippine Sea plate bound-

Geological Society of America Bulletin, November/December 2011 2215 Downloaded from gsabulletin.gsapubs.org on February 11, 2014 Mahony et al. ary was strike-slip dominated. Our study IMPLICATIONS OF THESE magnesian andesites are interpreted to indicate supports Lee et al.’s (1999) reconstruction, RECONSTRUCTIONS FOR VOLCANO- subduction of the young hot Shikoku Basin lith- which places the 15 Ma Izu-Bonin-Mariana TECTONIC EVOLUTION OF osphere (Kano et al., 1991; Yamaji and Yoshida, arc impingement point somewhere between SOUTHWEST JAPAN 1998; Tatsumi and Hanyu, 2003). Shimoda et Kyushu and Shikoku Island; they suggest this al. (1998) argued on the basis of their isotopic reconstruction is also consistent with evidence Volcanic-Tectonic Evolution from 15 to 10 Ma data that the Setouchi high-magnesian andesites for widespread shortening between Kyushu were generated by interaction of MORB mantle and Korea at this time. Our study also supports Igneous activity is represented by I- and with partial melt of subducted Shikoku Basin Miller et al.’s (2006) reconstruction, which S-type plutons trending northeast-southwest sediment at an anomalously high temperature. suggested that the Pacifi c plate– Philippine through central Kyushu (Fig. 14A) and small Both these scenarios are reasonably consis- Sea plate–southwest Japan triple junction was volcanic remnants. Volcanic centers in south- tent with our tectonic reconstruction (Fig. 10). located near the Kii Peninsula at ca. 10 Ma, west Honshu, Shikoku, and on Kyushu erupted By 14 Ma, young Shikoku Basin lithosphere based on evidence from the subducted Pacifi c the Setouchi high-magnesian andesites from was being subducted beneath Kyushu and Shi- plate slab morphology from seismic tomo- 12 to 15 Ma (Tatsumi, 1983; Tatsumi et al., koku Island, and also beneath the Kii Penin- graphic images. 2001). High Sr/Y ratios in the Setouchi high- sula region by 13 Ma (Fig. 14A). Moreover, if the already subducted Izu-Bonin-Mariana arc had a more northeasterly trend than currently observed (assuming that the subducted Izu- Eurasian/Amurian plate Bonin-Mariana arc had the same orientation as 36°N Pacific plate Figure 14. Volcanic and tectonic evolution 34° ~5 cm/a of southwest Japan from 15 Ma to present. Volcanic centers are denoted by black triangles with black fi ll (polygenetic volcanism), no fi ll (monogenetic volcanism), and gray 32°

SBSC (2 Ma) ﬁ ll (areas of volcanism, e.g., lava-dominated

I A

zu Bonin Mariana

~7 cm/a rc (2 Ma) eruptions). Plutons are marked by solid

IBM (present) black circles with either a white I (I-type 30° pluton) or S (S-type pluton), with pluton locations from Kimura et al. (2005). Inset KPR (present) enlarged diagrams of Kyushu volcanic centers in the bottom right of each ﬁ gure rep-

28° SBSC (present) resent the dotted region on the main map. Figures 14A (15–10 Ma), 14B (10–5 Ma), 14C (5–2 Ma), and 14D (2–0 Ma) show

Kyushu Palau Ridge (2 Ma) southwest Japan tectonics from 15 to 0 Ma, 26° from a reconstruction using poles of rotation of the Philippine Sea plate relative to Eurasia from Gaina and Müller (2007). The gray shaded features are the positions of the 24° Philippine Sea plate Izu-Bonin-Mariana arc (IBM) and Kyushu- Palau ridge (KPR) at the oldest end of the age range for each ﬁ gure, and the solid black 22° N 0 200 km line is the position of the Shikoku Basin spreading center (SBSC) at the oldest end of 128°E 130° 132° 134° 136° 138° 140° 142° 144° the age range. The dashed-outlined portions of the KPR, SBSC, and IBM approximately Figure 13. Southwest Japan tectonics from 2 Ma to present, based on a reconstruction using show the portions of those features that have poles of rotation of tectonic blocks and plates (including Philippine Sea plate and the Amu- been subducted. The light-gray dashed lines rian plate) from Wallace et al. (2009a). The gray shaded features are the positions of the outline the position of the IBM, SBSC, and Izu-Bonin-Mariana arc (IBM), and Kyushu-Palau ridge (KPR) at 2 Ma, and the solid black KPR at the youngest end of the age range. line is the position of the Shikoku Basin spreading center (SBSC) at 2 Ma. The dashed- The black arrows (labeled in cm/yr) show outlined portions of the 2 Ma KPR, SBSC, and IBM show the portion of those features that approximate Philippine Sea plate–Amurian has been subducted since 2 Ma. The light-gray dashed lines outline the present-day position plate relative rates of motion for each age of the IBM, SBSC, and KPR. The black arrows (labeled in mm/yr) show Philippine Sea range. The present-day east coast Kyushu plate–Amurian plate relative rates of motion from 0 to 2 Ma. The present-day east coast and Shikoku Island coast line is shown in Kyushu and Shikoku Island coast line is shown in light gray, while the 2 Ma position of the light gray, while the position of the coast line coast line is in black. at the oldest end of the age range is in black.

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Eurasian/Amurian plate Eurasian/Amurian plate 36°N A 36°N B

Subduction Pacific 34° 34° plate

IBM (5 Ma) I I I Subduction Pacific I S plate Oblique subduction 32° 32° IBM (1 S ~7 cm/a S IBM (10 Ma) IBM (15 Ma) ~7-8 cm/a BSC (5 Ma) KPR (5 Ma) SBSC (10 Ma) 0 Ma) SBSC 30° 30° ~8 cm/a SBSC~8 (15 cm/a Ma) IBM (10 Ma) (10 Ma)

28° 28° KP Left-lateral transpression Left-lateral transpressionR KPR (10 Ma) (10 Ma) ~8 cm/a 26° 26°

KPR (15 Ma) I I 24° Philippine I 24° Philippine Sea plate Sea I S plate 22° N 22° N 0 200 km 0 200 km S 15–10 Ma 10–5 Ma 128°E 130° 132° 134° 136° 138° 140° 142° 144° 128°E 130° 132° 134° 136° 138° 140° 142° 144°

Eurasia/Amurian plate Eurasian/Amurian plate X C X D 36°N Pacific 36°N plate Pacific plate ~5 cm/a 34° 34° IBM (2 Ma)

IBM (present) IBM (5 Ma) SBSC SBSC (present) (2 Ma) SBSC (5 Ma) 32° IBM (2 Ma) 32° KPR (5 ~5Ma) cm/a SBSC (2 Ma) KPR (2 Ma) ~7 cm/a KPR (2 Ma) 30° 30° KPR (present) ~6 cm/a (PSP rel Eurasia) 28° 28°

26° 26°

Philippine Sea plate 24° 24° Philippine Sea plate

N N 22° 0 200 km 22° 0 200 km 5–2 Ma 2–0 Ma 128°E 130° 132° 134° 136° 138° 140° 142° 144° 128°E 130° 132° 134° 136° 138° 140° 142° 144° Volcanics key: Polygenetic volcano Monogenetic volcano Areas of lava-dominated eruptions Pluton

Geological Society of America Bulletin, November/December 2011 2217 Downloaded from gsabulletin.gsapubs.org on February 11, 2014 Mahony et al. observed today), the Shikoku Basin lithosphere by Kamata and Kodama (1994) and mainly mantle plume on the basis of their petrological could have begun being subducted beneath the based on interpretation of the geochemistry data. However, when the volcanic-tectonic sys- entire southwest Japan region even earlier. Our of rather sparse volcanic rocks in this period, tem is reconstructed as a whole, we suggest that reconstructed age of onset of Shikoku Basin– which lack a slab fl uid signature. Between ca. the Nansatsu and Kitamatsuura volcanic areas lithosphere subduction beneath southwest Japan 15 and 6 Ma, the relative plate motion offshore were part of the same volcanic system. The calc- (ca. 14 Ma) is similar to the age of the erupted Kyushu on the Nankai Trough–Ryukyu Trench alkaline Kitamatsuura volcanism was followed a Setouchi high-magnesian andesites (13.7 Ma), is dominated by strike slip with some slow con- few million years later by the Arita rhyolite vol- suggesting the subduction could be a cause of vergence (Fig. 14B), suggesting that subduction canoes, representing the next stage in the volca- the volcanism. The OIB-type volcanism, which was much slower from 10 to 6 Ma (and perhaps nic evolution of the region, with focusing of the occurred at 15 Ma in the northwestern volcanic even longer, from ca. 14 to 6 Ma, similar to the volcanism into more mature volcanic centers. region in Kyushu (Shinjo et al., 2000; Uto et al., suggestion of Taira, 2001) than it is today. 2004), suggests that asthenospheric injection Our reconstructions highlight an additional Volcanic-Tectonic Evolution from 6 to 2 Ma occurred beneath this region. explanation for the apparent cessation of Our reconstructions suggest that the intersec- subduction-related volcanism from 10 to 6 Ma: Subduction of the Philippine Sea plate in a tion between the Izu-Bonin-Mariana arc and the the presence of the young, buoyant Shi- north-northwest direction, which began at 6.5 Ma, southwest Japan plate boundary was located koku Basin lithosphere that was probably had a major impact on the volcanic and tectonic somewhere between Kyushu Island and Shi- being subducted obliquely at a shallow angle events that followed. By 6 Ma, the Kyushu- koku Island at ca. 15 Ma. Paleomagnetic studies beneath Kyushu during this time (Fig. 14B). Palau ridge had migrated to northeast Kyushu, document 45º of clockwise rotation of southwest In modern-day southwest Honshu, the Shikoku and remained there until 2 Ma (Fig. 14C). We Honshu in the early to middle Miocene (Otofuji Basin lithosphere is being subducted at a very propose that the Kyushu-Palau ridge subduc- and Matsuda, 1983; Celaya and McCabe, 1987; shallow angle, with the leading edge of the plate tion adjacent to northeast Kyushu was the cause Otofuji et al., 1991). Note that this time win- being currently at 70 km depth (Fig. 2), and an of the 30º anticlockwise rotation of the Kyushu dow for the rotation of southwest Honshu is just absence of active subduction-related volcanism. forearc documented from paleomagnetic stud- prior to the time frame (15 Ma) of our recon- The modern setting in southwest Honshu thus ies during this time (Kodama et al., 1995). This struction, and so for simplicity it is not included provides an analog for Kyushu at 10–6 Ma, rotation was initiated by an along-strike change in our reconstructions. Lee et al. (1999) pro- where the recently formed Shikoku Basin litho- from subduction of the shallowly dipping Shi- posed that the Izu-Bonin-Mariana arc collision sphere was subducted in a similar fashion. koku Basin lithosphere and Kyushu-Palau ridge, with Kyushu acted as a pivot point for the rapid In spite of this proposed halt in subduction, to subduction of the older denser West Philippine clockwise rotation of southwest Honshu during low-intensity volcanism continued throughout basin lithosphere, which was subducted steeply the Sea of Japan opening, consistent with our this period in the western parts of Kyushu at Iki, and likely rolled back farther south. Together reconstruction. Rapid arc rotation about a pivot Kitamatsuura, Shimoshima, and Nansatsu (see these competing forces created a torque that gen- point at a subduction/collision transition is a Fig. 6 for locations). Volcanism at Iki Island is erated rotation of the Kyushu forearc. Rollback common process worldwide (e.g., Wallace et al., consistently characterized by OIB-type geo- of the West Philippine Basin slab adjacent to the 2005, 2009b), and the along-strike change from chemistry through time, refl ecting its backarc Ryukyu Trench and Kyushu, and rotation of the indentation of the Izu-Bonin-Mariana arc at the setting (Shinjo et al., 2000). The episodic Kita- Kyushu forearc, has infl uenced the kinematics southwest Japan plate margin (near Kyushu) to matsuura basalt volcanism has OIB and alkaline of north-south–directed extension in the Beppu- subduction of the Pacifi c plate farther northeast characteristics, but the youngest unit at 6 Ma is Shimabara graben (including the Hohi volcanic is a plausible mechanism to trigger rapid Mio- calc-alkaline (Sakuyama et al., 2009). The physi- zone) of central Kyushu. cene clockwise rotation of southwest Honshu. cal shape of Kyushu at 6 Ma was very different Coincident with the onset of extension, the to now. Notably, prior to the opening of the Oki- Hohi volcanic zone was the site of major erup- Volcanic-Tectonic Evolution from 10 to 6 Ma nawa Trough, the Kitamatsuura and Nansatsu tive outpourings from 6 to 2 Ma, with a peak regions would have been geographically closer in production at 5 Ma. During this time period, Many workers have noted an absence of (e.g., Fig. 11 shows the reconstructed geography the Kyushu-Palau ridge subduction point under- subduction-related volcanism in Kyushu of Kyushu at 6 Ma). The calc-alkaline nature of went minimal along-strike migration due to the between 10 and 6 Ma (Fig. 14B), interpreting the 6 Ma Kitamatsuura basalts and the island arc nearly orthogonal convergence between the this as a cessation in subduction of the Philip- basalt volcanism in the Nansatsu area of south- Philippine Sea plate and Eurasia (Fig. 14C). pine Sea plate offshore Kyushu (e.g., Uto, 1989; ern Kyushu from 6.4 to 5.9 Ma (Fig. 6), together We suggest that the voluminous volcanism in Kamata, 1992). Confl icting views are held by with the ODP observations, indicate onset of the Hohi volcanic zone during this period is other authors who either suggest there was no subduction-related volcanism at ca. 6.5 Ma. This related to a combination of upper-plate exten- late Miocene halt in subduction of the Philip- suggests that the older, more steeply dipping sion and Kyushu-Palau ridge subduction. The pine Sea plate (e.g., Maruyama et al., 1997; West Philippine Basin portion of the Philippine subduction of the Kyushu-Palau ridge directly Kimura et al., 2003), who suggest there was Sea plate had migrated to the Kyushu region beneath northeast Kyushu at 5 Ma introduced subduction since 7 Ma (Niitsuma, 1988), or who (consistent with our reconstructions; Fig. 14B) a large amount of fl uids into the melt hot zone propose an alternative time period for a “stag- and had been subducted deeply enough to initiate feeding the Hohi volcanic zone, which trig- nant” phase in subduction (e.g., Taira [2001] the generation of island arc–type magmas in the gered arc magmatism. Volcanic responses to the suggests 14–8 Ma). Integrated Ocean Drilling Nansatsu and Kitamatsuura regions. Sakuyama reinitiation of subduction are confi rmed by an Program data support low levels of volcanism et al. (2009) suggested that the change from observed increase in K content in Hohi volcanic in this period (Cambray and Cadet, 1994) with OIB-type alkali to calc-alkaline basalts in the zone magmas from 6 Ma onwards (Nakada and a marked increase starting between 6 and 7 Ma. Kitamatsuura region was due to the shallow- Kamata, 1991), confi rming an increased fl uid The proposed halt in subduction is summarized ing of the melt-extraction depth in an upwelling signature and subducted slab infl uence.

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Voluminous lava plateaus were produced in volcanic front. The two major areas of caldera ducted Kyushu-Palau ridge with upper-plate areas corresponding to present-day Aso and formation in Kyushu appear to coincide with extension is seen to dramatically increase the Aira calderas. One characteristic of Kyushu vol- areas of extensional normal faulting, namely the volume of magma produced. For example, the canism appears to be voluminous andesite lavas Kagoshima graben and the Beppu-Shimabara backarc extension termination point coincides in an area prior to caldera-forming eruptions graben (Fig. 3). The fi rst of these calderas to with the Kyushu-Palau ridge subduction point involving more silicic magmas (part of the cal- form was the Shishimuta caldera from 1 to near Aso volcano. Shishimuta caldera, or even dera cycle). It is suggested that the magma intru- 0.9 Ma (Fig. 9). Shishimuta caldera demon- the whole Hohi volcanic zone located slightly sion into the lower crust associated with these strates how subtle changes in the tectonics of a northeast of Aso, may represent an older ver- precursory lavas serve to thermally weaken an region may affect the related volcanism. From 1 sion of this extensional and fl uid-rich subduc- area (Annen et al., 2006), and lead to conditions to 0.7 Ma, a north-south extensional stress fi eld tion environment. An example of this scenario is conducive to caldera formation. was present in the Hohi volcanic zone (reported seen in Kamchatka at the Kluchevskoy volcano During the 6–2 Ma period, the northwestern by Kamata et al., 1988), during which time the complex, which is one of the most productive region volcanoes erupted OIB-type basalt, unin- large Shishimuta caldera formed (Kamata et al., volcanoes in the world. Kluchevskoy volcano fl uenced by subduction components. The initia- 1988). At 0.7 Ma, a local, weak compressional complex sits in a 200-km-wide graben structure tion of subduction and arc processes perhaps led stress regime initiated in the Hohi volcanic zone located above where the Emperor Seamount to the pulse of backarc volcanism; OIB-type (Kamata et al., 1988; Itoh et al., 1998), and since chain (on the Pacifi c plate) subducts beneath volcanism occurred around Fukuoka from 4 to that time only lava dome volcanoes have formed the Eurasian plate (Dorendorf et al., 2000). The 1 Ma (Hoang and Uto, 2003; Fig. 8). in the Hohi volcanic zone. This indicates a tec- high magma production rate and enriched 18O tonic relationship with the style of volcanism, isotopes were suggested by Dorendorf et al. Volcanic-Tectonic Evolution from 2 Ma and suggests that other large calderas (i.e., Aso, (2000) to be a consequence of the combination to Present Aira, Ata, Kakuto, Kobayashi, and Kikai) may of high fl uid input over time due to seamount have formed in periods of local extension within chain subduction, and intra-arc extension where The subduction direction of the Philippine Sea their respective grabens. The alternative possi- the seamount chain is subducted. The examples plate beneath southwest Japan at 2 Ma changed bility in the volcano-tectonic “chicken-and-egg” from Kyushu and Kamchatka demonstrate that from north-northwest to northwest (Yamaji, conundrum is that intense magmatism associ- volcanism is strongly infl uenced by the age and 2003; Fig. 14D), causing signifi cant volcanic ated with calderas and underplating weakened composition of subducted lithosphere. and tectonic consequences such as initiation of the upper plate to induce extension. This sug- Aso caldera has had four major caldera- dextral strike slip on the Median Tectonic Line gests that grabens preferentially form in areas of forming episodes since 0.3 Ma, demonstrating (Itoh et al., 1998). This change in plate motion high magma fl ux, such as the extensive lava pla- common cyclic behavior for a large caldera also caused the migration direction of the teaus that occurred in Kyushu prior to calderas. system. Cyclic volcanism at calderas is usually Kyushu-Palau ridge subduction point along the The location of calderas could be explained attributed to magma fl uxing, magma differentia- Nankai Trough–Ryukyu Trench to change from by the positions of physical features (such as tion, and episodic generation of silicic magma northeast to southwest (the Kyushu–Palau ridge the Kyushu-Palau ridge) on the subducting chambers (e.g., Rytuba, 1994; Riciputi et al., subduction point currently migrates southwest at Philippine Sea plate. The subduction of sea- 1995; Troll et al., 2002), so no specifi c tectonic 40 km/Myr; Wallace et al., 2009b). We contend mounts and ridges introduces a large amount controls are needed to explain the episodic that southwestward migration of the buoyant, of fl uid-rich minerals into the subduction zone caldera behavior of Aso volcano. However, an shallowly subducting Shikoku Basin lithosphere as well as tectonically eroded sediments (Bangs alternative idea for Aso relates to the fact that into the northern Kyushu region since 2 Ma has et al., 2006). Once the slab reaches a depth of the Kyushu-Palau ridge appears to subduct been the cause of lithospheric shortening in the ~100 km, these fl uid-rich sediments induce the directly beneath it (Fig. 1). Bathymetric images Hohi volcanic zone since 0.7 Ma and cessa- formation of hydrous island arc–type magmas, of the nonsubducted part of the Kyushu-Palau tion of Hohi volcanic zone graben development leading to explosive volcanism (England et ridge (e.g., Fig. 1) show that the Kyushu-Palau (Kamata et al., 1988). al., 2004). Recent Aso and Kirishima basalts ridge is not a smooth continuous feature but The rate of volcanism in the Shimabara gra- both show signifi cantly higher B/Nb and B/Zr lumpy with changes in width and height along ben (near Unzen volcano) has increased since (Fig. 4) than the northern volcanic front basalts its length. Subduction of this “lumpy” remnant 6 Ma (Yokose et al., 1999), supporting the idea (Oninomi, Yufu, and Kuju). In addition, the arc would cause an uneven fl uid fl ux into the that increasing extension in the region related hornblende-bearing silicic products from the mantle, leading to periods of highly fl uid-rich, to rollback of the Philippine Sea slab offshore northern volcanic front show high Sr/Y ratios voluminous volcanism, as seen at Aso. We have Kyushu (Okinawa Trough) has facilitated a and adakite composition (Fig. 4). The adakitic discussed many instances of volcanic-tectonic greater rate of volcanism. Rollback of the Philip- magmatism in the northern volcanic front prob- evolution and interactions in Kyushu; however, pine Sea plate has also impacted the location of ably corresponds to the subduction of the young there are still aspects of Kyushu’s volcanic set- volcanism in southern Kyushu (Fig. 14D). From (and hot) Shikoku Basin. The source mantle ting that are poorly understood, for example, the ca. 6 to 1 Ma, the volcanic front was located near of Aso and Kirishima basalts may be metaso- nonvolcanic southeastern region between Aso the Nansatsu, Hokusatsu, and Hisatsu volcanic matized by the modern slab-derived fl uid from volcano and Kirishima volcano. centers; however, increased Philippine Sea plate the subducted old West Philippine basin and/ slab rollback shifted the volcanic front eastward or Kyushu-Palau ridge. Higher ratios of B/Nb IMPLICATIONS FOR TIMING OF to its current location in the Kagoshima graben and B/Zr are noted at Kirishima volcano than IZU ARC COLLISION WITH (Yamaji, 2003; Figs. 8 and 9). at Aso, supporting the idea of the Kyushu-Palau CENTRAL JAPAN In the past one million years, a number of ridge migrating southwestwards along the large caldera volcanoes (e.g., Aira, Aso, etc.; Nankai Trough–Ryukyu Trench since 5–6 Ma. Most published tectonic histories of Japan see Fig. 9) have formed in Kyushu along the The combination of the position of the sub- assume a mid-Miocene onset of collision of

Geological Society of America Bulletin, November/December 2011 2219 Downloaded from gsabulletin.gsapubs.org on February 11, 2014 Mahony et al. the Izu Peninsula with central Japan (Seno and reconstructions suggest that highly oblique sub- the subduction margin acts as a pivot point for Maruyama, 1984; Hibbard and Karig, 1990; duction of the newly formed, shallowly subduct- the rotation of the arc (Wallace et al., 2005). Taira, 2001; Kimura et al., 2005). Such a model ing Shikoku Basin provides a likely explanation The kinematics of rotation of southwest Honshu requires the Izu collision to have been ongo- for this. Under this model, Kyushu from 10 to and the Sea of Japan opening are very similar ing in its current location since ca. 15 Ma, and 6 Ma (Fig. 11) is analogous to modern-day south- to these modern-day examples, and it is likely assumes that the confi guration of tectonics in west Honshu where the buoyant Shikoku Basin that a similar scenario has occurred in southwest southwest and central Japan has changed little is currently being subducted at a low angle and Japan (e.g., rapid arc rotation about a collisional since that time. The primary evidence support- the leading edge of the Philippine Sea plate is at pivot point). Lee et al. (1999) suggested that a ing a mid-Miocene timing for the Izu collision is ~70 km depth (Fig. 2), and there is currently no wide zone of early to middle Miocene shorten- the intrusion of the 15.7–7.4 Ma Kofu Granitic active arc volcanism (Fig. 14B). If the Shikoku ing between the Korean Peninsula and south- Complex (which is thought to be derived from Basin was being subducted beneath Kyushu from west Japan, near Tsushima Island, provides evi- the Izu arc) into the Cretaceous–Paleogene Shi- 10 to 6 Ma, as our reconstruction and the lack dence for a collisional pivot point during the Sea manto belt (Kawano and Ueda, 1966; Shibata of subduction related volcanism suggests, the Izu of Japan opening. et al., 1984; Saito and Kato, 1996; Saito et al., arc would have been intersecting the plate bound- The history of migration of the southwest 1997). The Shimanto Belt is an ancient accre- ary ~100–200 km southwest of its current loca- Japan–Philippine Sea plate–Pacifi c triple junc- tionary complex and one of the primary bedrock tion for much of this time period. tion along the plate boundary is central to under- terranes in central Japan in the region of the Izu (2) Middle Miocene (16–14 Ma; Otofuji et al., standing the evolution of Japanese tectonics and collision. The other piece of evidence cited in 1991) estimates for the most rapid opening of the volcanism since the Miocene. The fundamental support of a middle Miocene Izu collision is the Sea of Japan are incompatible with the onset of disagreement between the timing of Izu arc mid-Miocene age of trough-fi ll sediments adja- the Izu collision ca. 15 Ma. If the Izu collision collision with central Japan and the history of cent to the Koma-Kushigatayama block of the began in its current location in central Japan at motion of the Philippine Sea plate clearly needs Izu collision zone (Aoike, 1999). The assump- 15 Ma, the newly formed Shikoku Basin crust to be resolved, as this has major implications tion that the Izu collision began in the middle and the buoyant Izu-Bonin arc would have been for our understanding of the history of triple Miocene has dominated the majority of pub- subducted beneath southwest Japan adjacent to junction migration along the plate boundary. It lished Miocene to present tectonic interpreta- the area of most rapid Sea of Japan opening dur- is obvious that the data constraining the history tions of Japan. ing rifting. For backrifting to occur in the Sea of of Philippine Sea plate motion, the evidence for However, if the past kinematics of the Philip- Japan, the slab being subducted beneath south- timing of Izu arc collision, and timing of back- pine Sea plate, as determined from paleomag- west Japan must have been able to roll back. It arc rifting in the Sea of Japan require serious netic data and seafl oor spreading studies (Hall is highly unlikely that the Shikoku Basin crust reevaluation. However, we contend that recon- et al., 1995a, 1995b; Hall, 2002; Sdrolias et al., and the Izu arc were converging on southwest structions accounting for current knowledge of 2004), are correct, the Izu arc collision with and central Japan at the time of the opening of the past motion of the Philippine Sea plate (e.g., central Japan should have occurred much more the Sea of Japan, because these buoyant features Hall et al., 1995a, 1995b; Sdrolias et al., 2004) recently, probably ca. 6–8 Ma, as suggested would have retarded convergence at the trench are more consistent with the tectonic and volca- by the reconstructions of Lee et al. (1999) and and resisted slab rollback and rapid backarc nic evolution of Kyushu and southwest Japan. Toda et al. (2008). Our age for onset of the Izu opening in the Sea of Japan. For mid-Miocene collision is consistent with the inferred age of rifting in the Sea of Japan to occur, it is more CONCLUSIONS the Tanzawa block (part of the Izu arc) col- likely that the older, denser Pacifi c plate was sub- lision at 6.8 Ma (Yamamoto and Kawakami, ducting beneath most of southwest Japan at that The diverse volcanism that has occurred in 2005). These reconstructions differ markedly time (and that the Philippine Sea plate–southwest Kyushu since the early Miocene can largely from most published reconstructions of Japa- Japan–Pacifi c plate triple junction was located be explained as a consequence of evolution of nese tectonics (e.g., Seno and Maruyama, 1984; near Kyushu at ca. 15 Ma), consistent with our the plate-boundary confi guration in the south- Hibbard and Karig, 1990; Jolivet et al., 1994; reconstructions shown in this paper (Fig. 10). west Japan region. In the plate tectonic recon- Taira, 2001; Kimura et al., 2005; Yamazaki et (3) Approximately 45° of clockwise tec- structions shown, the subduction of the Pacifi c al., 2010, among others), many of which were tonic rotation of southwest Honshu occurred plate occurred beneath most of southwest Japan developed prior to the collection of paleomag- in the middle Miocene, simultaneous with the prior to 15 Ma, and volcanism before this time netic and other data sets documenting the past Sea of Japan opening. The kinematics of rota- is likely related to subduction of the Pacifi c history of the Philippine Sea plate. As discussed tion of southwest Honshu and the simultane- plate. From 15 to 5 Ma, relative plate motion in the previous section, our reconstructions ous opening of the Sea of Japan are consistent at the Philippine Sea plate–southwest Japan explain many features of the volcano-tectonic with rotation of southwest Honshu about a pivot boundary is highly oblique (dominated by left- evolution of the Kyushu region that are diffi cult point located near Kyushu. Collision of the Izu- lateral strike slip), and we suggest that a lack of to understand in the context of most published Bonin-Mariana arc with Kyushu is expected just subduction-related volcanism from 10 to 6 Ma reconstructions of southwest and central Japan. prior to 15 Ma in our reconstructions, and we can be explained by (1) a smaller convergent We suggest that the tectonic and volcanic his- suggest that the collision formed a pivot point component of relative plate motion and (2) low- tory of Kyushu and southwest Japan supports for the clockwise rotation of southwest Honshu. angle subduction of young, buoyant Shikoku the plate reconstructions presented here, for sev- All modern-day examples of rapid tectonic rota- Basin lithosphere, similar to what is occurring eral different reasons. tion of arcs are associated with an along-strike in southwest Honshu today. (1) Prior to now, there has been no explanation transition from collision to subduction (Wal- A change in Philippine Sea plate motions ca. for the hiatus in subduction-related volcanism lace et al., 2005, 2009b). In many cases these 5 Ma led to more rapid, nearly trench-normal in Kyushu between 10 and 6 Ma (Cambray and rotating arcs are also associated with backarc convergence, and subduction of the Eocene– Cadet, 1994; Kamata and Kodama, 1999). Our rifting, and collision of a buoyant feature with Oligocene west Philippine Basin beneath

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Science Letters, v. 121, p. 277–291, doi:10.1016/0012 Ishikawa, T., and Nakamura, E., 1994, Origin of the slab Kyushu. This helps explain the increasingly -821X(94)90073-6. component in arc lavas from across-arc variation arc-like signature of volcanism in Kyushu since Chaussidon, M., and Marty, B., 1995, Primitive boron of B and Pb isotopes: Nature, v. 370, p. 205–208, 6 Ma. The voluminous recent volcanism in the isotope composition of the mantle: Science, v. 269, doi:10.1038/370205a0. p. 383–386, doi:10.1126/science.269.5222.383. 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England, P., Engdahl, R., and Thatcher, W., 2004, Systematic ally elevated titanium, tantalum and niobium (TITAN) variation in the depths of slabs beneath arc volcanoes: in ocean island basalts with high 3He/4He: Geo- ACKNOWLEDGMENTS Geophysical Journal International, v. 156, p. 377–408, chemistry Geophysics Geosystems, v. 9, p. Q04027, doi:10.1111/j.1365-246X.2003.02132.x. doi:10.1029/2007GC001876. Furukawa, Y., and Tatsumi, Y., 1999, Melting of a subduct- Jolivet, L., and Tamaki, K., 1992, Neogene kinematics in the We express our utmost thanks to Mark Cloos and ing slab and production of high-Mg andesite mag- Japan Sea region and the volcanic activity of the north- Chuck Connor for their valuable input and comments, mas: Unusual magmatism in SW Japan at 13–15 Ma: east Japan arc, in Tamaki, K., Suyehiro, K., Allan, J., the Nuclear Waste Management Organization of Japan Geophysical Research Letters, v. 26, p. 2271–2274, and McWilliams, M., eds., Proceedings of the Ocean (NUMO) for funding much of this work, Neil Chap- doi:10.1029/1999GL900512. Drilling Program, Scientifi c Results, v. 127–128, man for encouraging our ideas, and Nicholas Barnard Gaina, C., and Müller, R.D., 2007, Cenozoic tectonic and p. 1311–1331. depth/age evolution of the Indonesian gateway and Jolivet, L., Tamaki, K., and Foumier, M., 1994, Japan Sea, for his comments. 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MANUSCRIPT ACCEPTED 31 MARCH 2011 lavas from Kajiki and Kokubu areas in northern part Uto, K., 1989, Neogene volcanism of southwest Japan: Its and Ushine area in southern part of caldera: Bulletin time and space based on K-Ar dating [Ph.D. thesis]: of the Volcanological Society of Japan, v. 45, p. 1–12. University of Tokyo, 184 p. Printed in the USA

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