Detailed volcanic morphology of Daisan-Miyako Knoll in the southern Ryukyu Arc

Marine Geology 404 (2018) 97–110

Contents lists available at ScienceDirect

Marine Geology journal homepage: www.elsevier.com/locate/margo

Detailed volcanic morphology of Daisan-Miyako Knoll in the southern Ryukyu Arc

T

⁎

Hiroki Minamia, , Yasuhiko Oharaa,b a b

Hydrographic and Oceanographic Department of Japan, Tokyo 100-8932, Japan Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan

A R T I C LE I N FO

A B S T R A C T

Editor: Michele Rebesco

High-resolution bathymetric mapping using an autonomous underwater vehicle and a survey vessel was conducted at Daisan–Miyako Knoll in the southern Ryukyu Arc. This paper presents for the first time the detailed volcanic morphology of a submarine edifice that previously was poorly explored, and discusses its formation process and relation to arc volcanism and tectonics. Bathymetry shows morphological evidence that Daisan–Miyako Knoll is a stratovolcano with three caldron-like depressions (> 1 km in diameter), a central cone, and multiple lava flows at the summit. The main edifice is conical with a basal diameter of 8–10 km and relief of 1000–1100 m, with a summit depth of 779 m. The central cone that stands inside the depressions has a circular crater. Numerous blocks up to 10 m wide are distributed around the crater, indicating a previous explosive eruption. Lava flows erupted from the central cone have a maximum thickness of 46 m at the flow margins and a maximum flow length of 1.9 km. The total flow area is 1.5 km2 and the estimated volume is 0.03 km3. The relatively thick lava flows (up to 46 m) with steep-sided margins (30–40°) and curtain-folded surface textures suggest that silicic magmas erupted, and their fresh appearance indicates that lava-flow eruptions (the most recent volcanic activity) occurred during the Quaternary. The volcanic features at Daisan–Miyako Knoll are aligned mainly in the NW–SE direction, suggesting that volcanism here may be controlled primarily by arcparallel extension. No volcanic edifices have been confirmed on the trench side of Daisan–Miyako Knoll, therefore this edifice defines the location of the arc volcanic front in this region. From the larger rhyolitic edifices (> 10 km wide) around Daisan–Miyako Knoll along the front, it is inferred that silicic arc volcanism is extending from the central to southern submarine Ryukyu Arc.

Keywords: Arc volcanism Submarine volcano Ryukyu Arc High-resolution mapping

1. Introduction On Earth, submarine volcanism is a fundamental geological process for magma genesis and crustal construction (Rubin et al., 2012). While the mid-ocean ridge system where new oceanic crust is created is the primary area for submarine volcanism worldwide, subduction zone also plays an important role in generating arc magmas and forming continental crust (Stern, 2002). The spatial distribution and characteristics of submarine arc volcanoes in the subduction zone provide an important constraint on arc magmatic processes. Arc volcanism in the southern part of the Ryukyu subduction zone is poorly understood because it is completely underwater. No subaerial volcanoes exist in the southern Ryukyu Arc in contrast to the northern part of the arc where active island volcanoes clearly define the Quaternary volcanic front. The identification of a chain of submarine edifices in the southern arc in the 1970s (Hamamoto et al., 1979; Kato

⁎

et al., 1982) coupled with dipole magnetic anomalies over the edifices and dredged volcanic samples implies that the volcanic front extends underwater from the northern to the southern arc (Ueda, 1986; Oshida et al., 1992; Watanabe et al., 1995; Shinjo et al., 1998). However, in addition to its submarine nature, the deeper water (> 500 m) in the southern arc compared with the north (Fig. 1a) has hindered our understanding of individual edifices. Even if edifices are mapped using the latest high-frequency (12–100 kHz) ship-based sonars with a 1° beam width, the deeper water prevents us from collecting high-resolution (sub-meter) bathymetry. Because low-resolution (greater than tens of meters) bathymetry is insufficient for resolving fine-scale volcanic features, high-resolution bathymetry is needed to understand the products and processes of submarine volcanism (Clague et al., 2011; Caress et al., 2012; Carey et al., 2018). Only two hydrothermally active edifices in the southern Ryukyu Arc, namely, the Tarama Knoll and Irabu Knoll (Fig. 1b), have been mapped with autonomous underwater

Corresponding author. E-mail address: [email protected] (H. Minami).

https://doi.org/10.1016/j.margeo.2018.07.008 Received 29 March 2018; Received in revised form 17 July 2018; Accepted 22 July 2018 Available online 24 July 2018 0025-3227/ © 2018 Elsevier B.V. All rights reserved.

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

Fig. 1. (a) Tectonic map of Ryukyu subduction zone. The red triangles mark the active subaerial volcanoes (Japan Meteorological Agency, 2013; Geological Survey of Japan, 2013). The red dashed line marks the Quaternary volcanic front. The black triangle marks the location of Daisan–Miyako Knoll. The open triangle marks the inferred location of a submarine eruption off Iriomote-jima in 1924. The thick black dashed lines mark the boundaries between the northern, central, and southern Ryukyu Arc. The black dotted lines indicate the slab depths from Nakamura and Kaneshiro (2000). Relative plate motions between the Philippine Sea Plate and Eurasian Plate are shown by arrows (Seno et al., 1993). Abbreviations for island names are as follows: Kume-jima (Km), Miyako-jima (My), Ishigaki-jima (Is), Iriomote-jima (Ir). The box indicates the location shown in (b). (b) Tectonic map of the southern Ryukyu Arc. Bathymetry data were compiled from the J-EGG500 (JODC Expert Grid Data for Geography) bathymetry and from multiple cruises conducted by the Japan Coast Guard (~100 m grid cell size). The faults are from Research Group for Active Faults in Japan (1991). The blue open triangles mark the hydrothermally active edifices. The blue dashed line marks the location of the volcanic front discussed in this paper. The divergent black dashed arrows indicate the direction of arc-perpendicular and arc-parallel extensions deduced from Kubo and Fukuyama (2003). The box indicates the area shown in Fig. 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

et al., 1995). Seismic refraction profiles show the existence of continental crust beneath the sedimentary layer of the entire Okinawa Trough, implying that the trough is still in a rifting stage prior to seafloor spreading (Sibuet et al., 1995). The crustal thickness becomes thinner toward the south, varying from 23–25 km in the north to 10–16 km in the south (Iwasaki et al., 1990; Hirata et al., 1991; Nakahigashi et al., 2004; Klingelhoefer et al., 2009). The southern trough is considered to be the most rifted because of its deeper depth and well-developed rift grabens such as the Miyako Rift and Yaeyama Rift (Fig. 1b). In the southern arc, two types of extension exist, namely, arc-perpendicular extension in the back-arc region and arc-parallel extension in the fore-arc and arc regions. Arc-perpendicular extension is caused by active rifting with extension axes of N150°E for the Pleistocene time-period and N170°E for the late Pleistocene–Holocene timeperiod (Sibuet et al., 1998). Arc-parallel extension has been deduced in a variety of geological and geophysical investigations. On-land fault analysis at Miyako-jima Island, located southwest of Daisan–Miyako Knoll, showed that normal faults strike at around N40°W and reflect post-Pleistocene displacements (Research Group for Active Faults of Japan, 1991; Kuramoto and Konishi, 1989; Fabbri and Fournier, 1999). Seafloor bathymetric surveys off Miyako-jima Island and its adjacent islands identified NW–SE and NNW–SSE surface faults or lineaments oriented perpendicular to the arc (Hamamoto et al., 1979; Arai et al., 2014). Large across-arc faults form graben-like depressions such as the Kerama Gap and Miyako Saddle (Fig. 1b). Seismic reflection profiles demonstrate that these faults reach the seafloor (Nishizawa et al., 2017), and submersible observations of one of the faults suggest that recent movements and the timing of faulting occurred after or during the Pleistocene age (Matsumoto et al., 2009). The present-day arcparallel extension is confirmed by focal mechanism solutions of shallow

vehicle (AUV) high-resolution bathymetry (Okino et al., 2015). Because of our poor understanding of many other edifices, especially those between Irabu Knoll and Daisan–Kume Knoll over a distance of ~200 km, the characteristics of arc volcanism there are completely unknown (Fig. 1b). The Daisan–Miyako Knoll is one such poorly-explored edifice in this region. In this paper, we present the detailed volcanic morphology of Daisan–Miyako Knoll obtained with an AUV and a surface vessel. Based on this morphology, we reconstruct the recent formation process and relative chronology of volcanic activity at the knoll. Furthermore, we examine the characteristics of arc volcanism around the knoll and discuss the interaction with regional tectonics. Our study provides new insights into the interaction of magmatic and tectonic process in the Ryukyu Arc and in other arc systems. 2. Geological setting The Ryukyu subduction zone is a convergent plate margin in the northwestern Pacific Ocean formed by subduction of the Philippine Sea Plate beneath the Eurasian Plate (Fig. 1a). The 1200-km-long Ryukyu subduction zone consists of the Ryukyu Trench, the Ryukyu Arc, and the Okinawa Trough (Fig. 1a), and is divided into three parts morphologically and geologically, namely, the northern, central (middle), and southern parts separated by the Tokara Strait and the Kerama Gap, respectively (Kizaki, 1986). The Okinawa Trough is an active back-arc basin behind the Ryukyu Arc. The water depth in the trough varies from ~200 m in the north to ~2300 m in the south (Sibuet et al., 1998). The trough is marked by normal faults trending NE–SW or E–W associated with the extensional rifting, and these faults form a series of en echelon rift grabens (Sibuet 98

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

into CARIS, whose swath editor tool was used to check visually for any water-column anomalies (bubble plumes) for each ping.

earthquakes (Kubo and Fukuyama, 2003) and crustal deformations estimated from GPS observations (Nakamura, 2004). The arc-parallel extension occurred after the Pleistocene age and is ongoing (Kuramoto and Konishi, 1989; Fabbri and Fournier, 1999; Otsubo and Hayashi, 2003; Nishizawa et al., 2017). A possible mechanism producing the arcparallel extension is considered to be arc-stretching by the increasing curvature of the oceanward migrating arc as the back-arc basin opens (Fabbri and Fournier, 1999; Fournier et al., 2001; Kubo and Fukuyama, 2003). The arc volcanism becomes more ambiguous from north to south. In the northern and central arc, active volcanic islands clearly define the volcanic front, which is located above where the subducted slab reaches depths of ~90–100 km (Fig. 1a) (Nakamura and Kaneshiro, 2000). By contrast, the southern arc has no subaerial volcanoes. A number of submarine edifices have been identified along the southwestern extension of the subaerial volcanic front (Hamamoto et al., 1979; Kato et al., 1982), and some are associated with distinct dipole magnetic anomalies (Ueda, 1986; Oshida et al., 1992). Volcanic rocks from these edifices essentially show a bimodal composition from basalt to rhyolite through minor andesite and have island-arc affinity (Chung et al., 2000; Watanabe, 2000; Matsumoto et al., 2001; Yamanaka et al., 2015). In 1924, an unknown submarine volcano erupted off Iriomote-jima Island (Fig. 1a) and produced huge floating pumices (Kato, 1991); this is the only known historical eruption in the southern Ryukyu Arc. Although the exact eruption location has not been identified, submarine edifices off the islands of Iriomote-jima and Ishigaki-jima have been surveyed intensively with surface vessels and submersibles since the eruption. Some of the edifices, such as the Iriomote Knoll (Fig. 1b), were revealed to be volcanic in origin, indicating part of the volcanic front is located off the islands (Watanabe et al., 1995; Watanabe, 2000). Daisan–Miyako Knoll is located about 120 km northeast off Miyakojima in the southern Ryukyu Arc (Fig. 1b), and the depth of the subducted slab beneath the knoll is 80–90 km (Nakamura and Kaneshiro, 2000). In 1976, the knoll was first mapped by the Hydrographic and Oceanographic Department of Japan, Japan Coast Guard (JCG) as a conical edifice with a depth of 943 m at its shallowest point (Kato et al., 1982). However, because of insufficient sonar resolution, the 1976 survey did not identify any volcanic features such as a caldera or a central cone. Later, a dipole-type magnetic anomaly with an amplitude of 490 nT and a wavelength of 3 km was observed over the knoll (Ueda, 1986). Dredging recovered a dacite rock from the summit region in 1986 (Oshima et al., 1988) and pumice blocks from the southern flank of the edifice in 1997 (Shinjo et al., 1998), but their geochemical compositions were not documented. Although what little evidence is available points to the knoll being volcanic in origin, to date there have been no geophysical or geological surveys focusing on Daisan–Miyako Knoll, and the character of its volcanism remains completely unknown.

3.2. AUV bathymetry and side-scan data The high-resolution bathymetric mapping was conducted using JCG's AUV Gondou, which is a cruising-type vehicle designed and constructed by International Submarine Engineering. The detailed specifications of the Gondou are given in Minami and Ohara (2016). The Gondou carries an R2Sonic SONIC2022 multibeam echo sounder with variable 200–400 kHz frequency and 1° beam width both along-track and across-track. In total, seven dives were performed in 2015 and 2016. The Gondou was set to maintain a constant altitude of 70 or 100 m above the bottom with a cruising speed of 3 knots. The line spacing was set to either 120 m or 40 m at the central cone to prevent data gaps. After the standard horizontal and vertical corrections for raw sounding data, the navigation drifts between each survey line during each dive were adjusted. The absolute positions of our AUV bathymetry were co-registered with the Global Positioning System (GPS)-navigated shipboard EM122 bathymetry collected by S/V Takuyo. The final positional uncertainty of the AUV bathymetry was 30 m, which is the resolution of EM122 reference bathymetry. The fully navigation-corrected soundings were gridded at a cell size of 1 m using the swathangle weighting algorithm in the CARIS software. ESRI ArcMap software was used to measure the area of the lava flows detected by the AUV bathymetry. Side-scan sonar imagery was used to highlight seafloor textures and structures and to improve morphological interpretation. The side-scan data were acquired with an EdgeTech 2200-M sonar installed on the Gondou. The frequency was 120 kHz and two or three pings were transmitted every 1.5 s with a cruising speed of 3 knots. The range was set to 200 m and 240 m for the altitudes of 70 m and 100 m, respectively. The corrected navigation data were merged with the side-scan data, and a slant range correction was made. 4. Results 4.1. General morphology of Daisan–Miyako Knoll The shipboard bathymetry shows that Daisan–Miyako Knoll is a conical edifice that is slightly elongated in the NW–SE direction (Fig. 2). It has a basal diameter of 8–10 km and rises to 1000–1100 m above the surrounding seafloor (Fig. 3). The shallowest depth is 779 m at the summit, where three depressions are nested together. The slope is relatively gentle (0–10°) on the lower flanks and becomes steeper (10–30°) on the upper flanks (Fig. 2b). There are volcanic cones around the main edifice, mainly on its northeastern and southwestern sides (Fig. 2). Each cone is ~500–1000 m in diameter and ~100–200 m in relief with slopes of 16–30°. Some cones have crater-like depressions in their summits. The cones on the northeastern side are principally aligned in the NW–SE direction (~N50°W). There are extensively developed lineaments around Daisan–Miyako Knoll (Fig. 2). The lineaments strike ENE–WSW or NE–SW and have vertical offsets of a few meters to a few tens of meters. They are interpreted as surface faults formed by the rifting in the Okinawa Trough (Sibuet et al., 1995). A group of lineaments west to northwest of the Knoll form a distinctive rift graben called the Miyako Rift (Figs. 1b and 2c). No lineaments have been confirmed on Daisan–Miyako Knoll itself.

3. Methods 3.1. Ship bathymetry and acoustic backscatter intensity The bathymetry and acoustic backscatter intensity were acquired concurrently in 2015 using a Kongsberg EM122 multibeam sonar system (12 kHz) mounted on JCG's S/V Takuyo. The sonar system generates 288 beams per ping with a beam width of 2° × 2°. The data were processed using the CARIS HIPS and SIPS software. Standard depth corrections for surface sound speed, sound speed profile, attitude, sensor offset, and sonar bias were applied to the raw soundings during the data acquisition and processing. The corrected soundings were gridded at a cell size of 30 m using a swath-angle weighting algorithm that gives higher weight for gridding to the soundings of the inner (nadir) beams than to those of the outer beams. The backscatter data were extracted in CARIS. Backscatter mosaics were generated using the Geocoder tool in CARIS with a cell size of 30 m, and a full-blend mosaicking algorithm was used. The water-column data were imported

4.2. Detailed morphology of Daisan–Miyako Knoll The primary target of the AUV dives was the summit region where higher backscatter was observed from the shipboard EM122 sonar (Fig. 4). 99

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

Fig. 2. (a) Bathymetric map of Daisan–Miyako Knoll and its surroundings. Bathymetry data are from the S/V Takuyo EM122 surveys conducted in 2015. The grid cell size is 30 m. The map is illuminated from the south. The map projection is UTM51N. (b) Slope map with depth contours of same area as in (a). The contour interval is 50 m. (c) Morphological interpretation map of same area as in (a). The black triangle marks the summit (depth 779 m) of Daisan–Miyako Knoll. The closed black square marks the location (125°48.6′E, 25°48.6′N, 1130 m deep) where dacite was dredged by Oshima et al. (1988). The open black square marks the location (125°47.87′E, 25°46.91′N, 1650 m deep) where pumice blocks were dredged by Shinjo et al. (1998). Lines A–A′ and B–B′ indicate the locations of the depth cross-section shown in Fig. 3. The box indicates the area shown in Fig. 4.

summit. The central-outer depression has a diameter of 1.5–2 km and has an elliptical shape. Its rim is less sharp than those of the other two depressions in the AUV bathymetry. Its inner-wall slopes at < 15° with a smooth surface, and the relief from the rim to the floor is 60–70 m. The

4.2.1. Depressions The AUV bathymetry demonstrates the presence of three caldronlike depressions (Fig. 5). The three depressions are referred to here as the central-outer depression, the central-inner depression, and the northwestern (NW) depression based on their relative locations at the 100

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

A 800

NW

1000

depth (m)

A’ SE

central cone

1200

central-inner depression

1400

NW depression

1600 1800 2000 0

2500

5000

7500

10000

12500

distance (m)

B 800

SW

1000

depth (m)

B’ NE

central centralcone cone

1200 central-outer depression

1400 1600 1800 2000 0

2500

5000

7500

10000

12500

distance (m) Fig. 3. Depth cross-sections of Daisan–Miyako Knoll. Profiles A–A′ and B–B′ correspond to the lines in Fig. 2c.

On the upper northwestern flank of the cone, there is a crater (crater C1) that is ~70 m across, and there is another crater (crater C2) on the lower western flank (Fig. 5). There are numerous blocks around the summit crater of the central cone (Fig. 5), varying in size from several meters to ~10 m. Sharp acoustic shadows are seen behind the blocks in the side-scan imagery (Fig. 8b), indicating that the blocks are steep-sided. The blocks are distributed on the southern side of the cone (at least ~1 km from the summit crater) but cannot be identified on the northern side in our bathymetry where the surface is covered by lava flows (Fig. 5). There may be blocks on the upper flank of the cone, but due to the rugged seafloor in this area it is difficult to say definitively (Fig. 8a).

western part of the central-outer depression is cut by both the NW depression and central-inner depression. The central-inner depression has a diameter of ~1.2 km and has a circular shape. It sits inside the central-outer depression. The rim is sharper than that of the central outer depression and as sharp as that of the NW depression. The inner-wall slopes at 20–30° and has radial gully-like features on its upper part (Fig. 5). The relief from the rim to the floor is ~300 m. There is a steep bathymetric high (high C) mid-way up the inner-wall (Fig. 5). The NW depression has a diameter of ~1.4 km and a horseshoe shape that opens to the northwest. It merges with the central-inner depression at its southeastern side and shares a rectilinear rim with the central-inner one. The rim is sharper than that of the central-outer depression. The inner-wall slopes at 20–40° and has radial gully-like features on its upper part. The relief from the rim to the floor is up to ~500 m. There are two prominent bathymetric highs (highs A and B) mid-way up the inner-wall (Fig. 5). High A sits on the northeastern side and is at least 60 m across and 60 m tall with slopes in excess of 70° in places. High B sits on the southern side of the depression. Because of the steepness of the local terrain, the AUV bathymetry is incomplete over highs A and B. There are two small crater-like depressions (craters C3 and C4) on the depression floor (Fig. 5). Crater C3 is 120–140 m across and ~20 m deep, while crater C4 is 70–90 m across and 15–20 m deep. On the northwestern flank of the main edifice of Daisan–Miyako Knoll, NW–SE-orientated (~N55°W) flow-like features were identified in both the shipboard (Fig. 4a) and AUV (Fig. 6) bathymetry. The flowlike features extend downslope ~4 km from the depth of ~1300 m at the NW depression to ~2000 m. These features are associated with higher backscatter (Fig. 4b), smooth surfaces, and gentle slopes (Figs. 6 and 7). The morphology is concave (~50–100 m deep) on the upper flank and convex (~50 m high) on the lower flank (Fig. 7).

4.2.3. Lava flows Lava flows emanating from the central cone are imaged in the AUV bathymetry (Fig. 5). The flows are clearly distinguished from the surrounding seafloor by sharp margins with steep slopes of 30–40° and ropey and curtain-folded surface textures (Fig. 8c). The surface textures of the flows are not uniform, and flows north of the central cone have blocky textures whose relief and width are up to several meters (Fig. 8d). The main flows erupted from the summit crater of the central cone, cascaded down the northeastern flank (Fig. 5), and branched northeastward (flow A-n) and southeastward (flow A-s). Flows A-n and A-s are 1.9 km and 1.4 km long, respectively, and ~10–40 m up to 46 m thick at their margins (Fig. 9). Another flow (flow A-w) erupted from the summit crater, advanced northwestward, and followed the rim of the central-inner depression, changing its flow direction to the west and southwest as it flowed down to the depression floor. Flow A-w is 0.8 km long and ~10–20 m thick. A minor flow (flow B) overlapping flow A-w seems to have erupted from crater C1; flow B is ~370 m long and ~10–20 m thick. The lava flows cover an area of 1.5 km2. To estimate the volume, we measured the flow thickness at 64 locations at the flow margins and simply averaged them. By multiplying this average thickness (~20 m) by the area (1.5 km2), the estimated volume is 0.03 km3. Although our estimate contains uncertainties due to a lack of pre-eruption bathymetry and not knowing the thickness accurately everywhere, this is the first time that the full extent of lava flows in the Ryukyu Arc have been mapped and the erupted lava volume has been estimated quantitatively.

4.2.2. Central cone The central cone stands on the eastern rim of the central-inner depression and is also situated inside the central-outer depression (Fig. 5). The cone has a circular summit crater that is 70–80 m deep and 250 m wide. The crater rim appears rugged in the side-scan imagery (Fig. 8a). The cone has a basal diameter of 900–1000 m and rises 200 m from the base (at a depth of 1060 m) to the summit (at a depth of 860 m) (Fig. 9). Its flanks slope at a near constant angle of approximately around 25°. 101

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

125°46'E

125°48'E

25°50'N

a

depth (m) 700

1100

Fig.6 1500

1900

25°48'N

Fig.5 2300

0

1

2

4 km 125°46'E

125°48'E

b

25°50'N

artifacts?

backscatter intensity high

25°48'N

Fig.6

Fig.5 low

Fig. 4. (a) Tracklines for AUV Gondou dives. The background bathymetry is from shipboard EM122 sonar. The boxes indicate the areas shown in Figs. 5 and 6. (b) Backscatter intensity map of same area as in (a). The backscatter is from EM122 sonar. Black and white indicate high and low intensity, respectively. It is noted that the N–S-trending high intensities east of 125°48′E are probably artifacts caused by the strong reflections at the nadir of the ship track.

5. Discussion

4.2.4. Ridges At least three ridge-like structures (ridges R1, R2, and R3) are identifiable where the central-inner depression and NW depression merge (Fig. 5). These ridges lie predominantly in the NW–SE (~N50°W) direction. Ridge R1 is the most distinguished of the three, and is ~400 m long, ~80 m wide, and up to ~70 m tall with steep slopes in excess of 60°. Because of this steepness, the Gondou collided with ridge R1 during one of the mapping dives and aborted the mission, resulting in data gaps. Ridge R2 is composed of at least three minor ridges. One of the minor ridges is ~50 m wide and ~30 m tall. Ridge R3 is ~160 m long, ~60 m wide and ~30 m tall.

5.1. Formation process of Daisan–Miyako Knoll Here, we interpret the observed volcanic morphology and discuss the formation process of Daisan–Miyako Knoll based on the morphology. Of the three depressions, the central-outer depression is the oldest for the following reasons. First, the central-outer depression must have formed before the central-inner one. Second, the NW depression cuts the western part of the central-outer depression and appears to cut the western part of the central-inner depression (Fig. 5). Third, the rim 102

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

Fig. 5. (a) Detailed bathymetric map of summit of the Daisan–Miyako Knoll. The AUV bathymetry (1 m grid) overlies the shipboard EM122 bathymetry (30 m grid). The map is illuminated from the north. The map projection is UTM51N. (b) Morphological interpretation of map in (a). The box indicates the location shown in Fig. 8. Lines E–E′, F–F′, G–G′, and H–H′ mark the locations of the depth cross-sections shown in Fig. 9. (c) Perspective view of Daisan–Miyako Knoll. The view is from the southeast and is illuminated from the north with a vertical exaggeration of two. The white dotted lines mark the caldera rims.

formation of central-inner depression. Ridges R1–3 probably formed before the two depressions, but this is uncertain based solely on morphology. The ridges and highs are probably remnants of intruded dikes. The central-outer and -inner depressions can be interpreted as calderas because of their circular and elliptical shapes and sizes (> 1 km in diameter). At Daisan-Miyako, the central cone with lava flow eruptions stands inside the central-inner depression. This structure resembles that of Akita-Komagatake volcano in the NE Japan Arc where a pyroclastic cone with lava flow eruptions occurs inside the southern caldera (Wachi et al., 1997; Fujinawa et al., 2004). The flow-like features on the northwestern flank (Figs. 4 and 6), which are concave on the upper

of the central-outer depression lacks sharpness and the surface is much smoother than those of the other two depressions (Fig. 5), indicating erosion including sedimentation or deposition after its formation. These morphological features all point to the central-outer depression being older than the central-inner depression and NW depression. It is hard to identify the sequence in which the central-inner depression and NW depression formed. They are merged and share a common rim that appears quite rectilinear and uniformly sharp in our AUV bathymetry (Fig. 5). The NW depression might have affected the central-inner depression that appears with a rectilinear shape on its western side, suggesting the NW depression was formed after the 103

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

c debris avalanche deposits? ridge R1

NW depression

ridge R2 high B

high A lava flow B

central-inner depression

summit crater central-outer depression high C lava flow A-n lava flow A-w central cone

lava flow A-s

2700

2300

1900

1500

1100

depth (m) 700

Fig. 5. (continued)

the lava fields (Fig. 4). Acoustic water-column anomalies in sonar data are sometimes observed at active volcanoes from hydrothermal venting or emission of volcanic gases (Chadwick et al., 2014; Minami and Ohara, 2016, 2017). However, Daisan–Miyako Knoll appears to be dormant at present, considering that our shipboard multibeam sonar and AUV side-scan sonar did not detect any acoustic water-column anomalies. Determining the absolute eruption ages to confirm the formation sequence will require further work involving radiometric dating of rock samples and direct visual observations.

flank and convex on the lower flank (Fig. 7), are interpreted as debris avalanche deposits from the NW depression created by sector collapse, as observed at young active submarine arc volcanoes such as Monowai in the Kermadec Arc (Chadwick et al., 2008; Wright et al., 2008; Watts et al., 2012) and NW Rota-1 in the Mariana Arc (Chadwick et al., 2012; Schnur et al., 2017). We interpret that the sector collapse created the NW depression and removed material from the summit and upper northwestern flank, then that material extended downslope and deposited on the lower flank. The central cone was built after the two central depressions formed. The cone is similar to pyroclastic cones observed on subaerial volcanoes (Moriya, 1986) with nearly constant slopes (~25° here) (Fig. 9). The presence of the summit crater demonstrates that explosive eruptive activity occurred. The numerous blocks around the summit crater (Figs. 5 and 8) are interpreted as ejecta (cf. the giant 1–9-m-wide pumice clasts seen at the Havre volcano on the Kermadec Arc) produced by a deep (> 500 m below the sea surface) submarine eruption (Carey et al., 2018). The most recent volcanic activity at Daisan–Miyako Knoll is the eruption of lava flows from the central cone (Fig. 5). Flows A-n, A-s, and A-w seem to have been produced simultaneously. Flow B is the most recent flow because it overlaps flow A-w. This overlapping feature indicates that there have been at least two lava-flow eruptions at the central cone. The relatively thick (up to 46 m) lava flows with talus-like steep margins (30–40°) (Fig. 9), and ropey and curtain-folded surface textures (Figs. 5 and 8) are similar to silicic lava flows on subaerial volcanoes such as the Rocche Rosse (Bullock, 2015; Bullock et al., 2018), the north lobe of Glass Mountain (Fink, 1980; Fink and Anderson, 2017) and Cordón-Caulle (Tuffen et al., 2013; Farquharson et al., 2015), suggesting that silicic magmas erupted at Daisan–Miyako. The clearness of the ropey and curtain-folded lava morphology in the AUV bathymetry indicates that there is little or no sediment cover on the flows, making the eruptions relatively recent. The relative lack of sedimentation is also inferred from the high acoustic backscatter from

5.2. Implications of tectonic control of arc volcanism by arc-parallel extension Here, we discuss possible tectonic control of the volcanism of Daisan–Miyako Knoll based on the observed morphology and regional tectonics in the southern Ryukyu Arc. The bathymetry shows that the trend of the volcanic features of Daisan–Miyako Knoll is principally NW–SE: the main edifice of the Knoll is elongated slightly in the NW–SE direction (Fig. 2), linear ridges R1–3 at the summit run in the NW–SE direction, the depressions appear to be aligned in the NW–SE direction (Fig. 5), and the cones around the main edifice are aligned in the NW–SE direction (Fig. 2). This systematic orientation of surface volcanic features implies that magma intruded preferentially along the NW–SE direction. If we assume that the direction of minimum principal stress is normal to the trend of the volcanic features, a NE–SW extensional stress field is inferred. In the southern Ryukyu Arc, recent extension has been recognized as arc-parallel, which refers to tensional stress axes parallel to the arc (in the NE–SW direction) (Fig. 1b) as described in section 2. Similar arcparallel (along-arc) extension has been observed in the Mariana Arc (Wessel et al., 1994; Stern and Smoot, 1998; Martínez et al., 2000; Kato et al., 2003; Bird, 2003; Heeszel et al., 2008) and other arc systems (McCaffrey, 1996; Feuillet et al., 2002). Regarding the southern 104

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

125°45'E

125°46'E

125°47'E

125°48'E

25°51'N

a

25°50'N

D’

D

C’

C 0

1

2 km

2100

25°51'N

125°45'E

125°46'E

1700 125°47'E

1300

depth (m) 900 125°48'E

b

25°50'N

D’

D

C’

C Fig. 6. (a) Detailed bathymetric map of northwestern flank of Daisan–Miyako Knoll. The AUV bathymetry (1 m grid) overlies the shipboard EM122 bathymetry (30 m grid). The map is illuminated from the north. The map projection is UTM51N. Lines C–C′ and D–D′ marks the location of the depth cross-sections shown in Fig. 7. (b) Side-scan mosaic of northwestern flank of Daisan–Miyako Knoll. Bright (white) colors indicate high reflectivity and dark (black) colors indicate low reflectivity or acoustic shadows.

between arc-parallel and arc-perpendicular extensions around the volcanic chain in which Daisan–Miyako Knoll sits; therefore, we cannot rule out the possibility that arc-perpendicular extension may affect the volcanism at the Knoll. In fact, our ship-based bathymetry confirmed that the E–W- and ENE–WSW-trending surface faults are mainly on the back-arc side of Daisan–Miyako Knoll (Fig. 2), which appears to reflect arc-perpendicular extension. However, no distinct E–W- and ENE–WSW-trending volcanic features and lineaments were identified at the Knoll from either the ship-based (Fig. 2) or AUV bathymetry (Fig. 5). Although it is possible that volcanic deposits from recent eruptions may have masked pre-existing features, arc-perpendicular

Mariana Arc, it has been suggested that rapid changes in direction and magnitude of the back-arc opening can cause arc-parallel extension (Martínez et al., 2000). Heeszel et al. (2008) interpreted that the increasing curvature of the Mariana Arc system with time or oblique subduction may induce arc-parallel extension, and the occurrence of cross-arc volcanism may be controlled by the arc-parallel extension in the upper plate and associated passive mantle upwelling. Another potential control of the volcanism at Daisan–Miyako Knoll is arc-perpendicular (across-arc) extension, which refers to tensional stress axes perpendicular to the arc (Fig. 1b). Kubo and Fukuyama (2003) showed that there is a possible boundary of the two stress fields 105

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

C

C’

depth (m)

1200 1250 1300 1350 1400 0

200

400

600

800

1000

D

1200

D’

depth (m)

1900

1950

2000 0

200

400

600

800

1000

1200

distance (m) Fig. 7. Depth cross-sections of northwestern flank. Profiles C–C′ and D–D′ correspond to the lines in Fig. 6.

occur inside the depressions. Lava flows are thick (up to ~46 m), and have ropey surface textures, steep margins (30–40°) and relatively short flow lengths (< 2 km) (Fig. 5). One dacite rock was dredged from the previously mapped flow margin (Oshima et al., 1988) and pumice blocks were dredged from the southern flank of the main edifice (Shinjo et al., 1998) (Fig. 2c). The samples are very limited but nevertheless indicate silicic volcanism. Recent surveys nearby revealed the presence of a dacitic-rhyolitic caldera volcano, namely, Daisan–Kume Knoll (~10–16 km wide) northeast of Daisan–Miyako Knoll (Fig. 1b) (Harigane, 2015; Minami and Ohara, 2017), where hydrothermal venting has been confirmed. To the southwest of Daisan–Miyako Knoll, there is another rhyolitic volcanic edifice (~10 km wide) named Iriomote Knoll, where pumice and volcanic rocks were sampled with a SiO2 content of ~70–72 wt% (Fig. 1b) (Watanabe, 2000). At the Iriomote Knoll, indicators of present or past hydrothermal activity were observed such as hydrogenous manganese oxides, hydrothermally altered pumice, and dead mussels similar to ones living at other hydrothermal vents in the Okinawa Trough (Watanabe et al., 1995). A historical submarine eruption in 1924 off Iriomote-jima Island (Fig. 1a) produced rhyolitic pumices (SiO2 ~73 wt%) (Kato, 1991) and this likely represents the silicic volcanism along the front, although its exact eruption point is yet to be located. Therefore, Daisan–Miyako Knoll and surrounding larger edifices (Daisan–Kume Knoll and Iriomote Knoll) demonstrate that silicic volcanism is predominant along the southern Ryukyu volcanic front. The presence of silicic submarine volcanism since the midPleistocene age has been previously documented in the central Ryukyu Arc (Yokose et al., 2010). Caldera-like structures, rhyolitic samples, and indicators of hydrothermal activities have been confirmed at volcanic edifices (e.g., Amami Calderas, Daiichi–Amami Knoll, Io–torishima Bank) along the volcanic front or rear-arc chain in the central arc (Yokose et al., 2010; Ishizuka et al., 2014; Minami and Ohara, 2016). Our results for Daisan–Miyako Knoll coupled with previous results for the surrounding edifices suggest that the silicic volcanism in the central arc could extend to the southern arc along the submarine volcanic front. The genesis of silicic magma in the Ryukyu Arc is not well understood. One possible mechanism producing silicic magma may come from studies at the Izu–Bonin Arc. The modern volcanism at the northern Izu-Bonin Arc is bimodal in composition and is characterized by basalt-dominant island volcanoes and rhyolite-dominant submarine

extension does not seem to be evident from recent volcanic activity. Therefore, we interpret that tectonic control on arc volcanism by arc-parallel extension is more important in the southern Ryukyu Arc. Located roughly 50 km southwest of Daisan–Miyako Knoll, Daiichi–Miyako Knoll is another unexplored edifice in the southern Ryukyu Arc (Fig. 1b). If the volcanism there is controlled by rifting activity (arc-perpendicular extension), the expected elongation of the edifice would be ENE–WSW or E–W as inferred from the trend of the nearby Miyako Rift. However, Daiichi–Miyako Knoll is elongated linearly in the NNW–SSE direction (Fig. 1b), suggesting that arc-parallel extension also seems to control the volcanism at Daiichi–Miyako Knoll. The slight difference in elongation between Daiichi–Miyako Knoll (NNW–SSW) and Daisan–Miyako Knoll (NW–SE) may reflect a local difference of the extension direction due to the curved shape of the southern Ryukyu Arc (Fig. 1a). We reason that crustal stretching and magma injection are enhanced locally in response to the arc-parallel extension, and that the linearly elongated edifice (Daiichi–Miyako Knoll) and the depression (Miyako Saddle) are formed as volcanic and tectonic surficial expressions of the extension (Fig. 1b).

5.3. Location and characteristics of volcanic front in southern Ryukyu Arc Bathymetry has provided morphological evidence suggesting that Daisan–Miyako Knoll is a submarine volcano that experienced lava-flow eruptions possibly during the Quaternary. On the trench-side of the Knoll, broad and flat shallow banks (e.g., Miyako–Sone; “Sone” means bank in Japanese) (Fig. 1b) with water depths less than ~500 m are distributed where well-indurated carbonate rocks corresponding to the Quaternary Ryukyu Limestone are observed (Arai et al., 2014) and no volcanic edifices have been identified to date. Therefore, Daisan–Miyako Knoll is the most trench-ward volcano that defines the location of the Quaternary volcanic front in this region. The morphology of Daisan–Miyako Knoll and surrounding larger (> 10 km wide) volcanic edifices suggest that submarine volcanism along the front is predominantly silicic. At Daisan–Miyako Knoll, past explosive eruptions are strongly inferred by the formation of calderalike depressions, a central cone with a summit crater, and the numerous surrounding blocks (Fig. 5). Furthermore, the morphological characteristics suggest that highly viscous silicic magmas were associated with the recent volcanic activity. The steep ridges R1–3 and highs A–C 106

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

125°47.8'E

125°47.8'E

25°48.7'N

25°48.7'N

a

0

50

reflectivity high

100 m

low (shadow)

125°47.7'E

125°47.7'E

25°48.7'N

25°48.7'N

b

0

50

100 m

125°48.3'E

125°48.3'E

25°48.7'N

25°48.7'N

c

0

50

100 m

125°48'E

125°48'E

d

0

50

100 m

(caption on next page) 107

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

Fig. 8. Selected close-up views of Fig. 5. All maps are scaled the same. The panels on the left show the bathymetry. The color scale is the same as in Fig. 5a. The panels on the right show the side-scan imagery. Bright (white) colors indicate high reflectivity and dark (black) colors indicate low reflectivity or acoustic shadows. The white arrow indicates the ensonification direction of the side-scan sonar.

6. Conclusions

calderas (Tamura and Tatsumi, 2002). Geochemical studies of the Quaternary Izu-Bonin Arc volcanoes suggest that the rhyolitic magmas may have been produced by partial melting of calc-alkaline andesite in the upper-to-middle crusts (Tamura and Tatsumi, 2002). The Izu–Bonin Arc is characterized by the presence of a thick layer of middle crust with a P-wave velocity (Vp) of 6.0–6.5 km/s (Suyehiro et al., 1996; Kodaira et al., 2007), and similarly a middle crust with Vp = 6.1–6.5 km/s is suggested by a recent seismic refraction study in the southern Ryukyu Arc (Nishizawa et al., 2017), thus the partial melting of the middle crust may form abundant silicic magmas in the southern Ryukyu Arc. Future geochemical and petrological studies of individual submarine edifices in the central-to-southern Ryukyu Arc will provide important constraints on the arc magmatic processes.

The following conclusions are derived from our results and interpretations. 1. For the first time, the volcanic morphology of Daisan–Miyako Knoll has been fully mapped from the combined bathymetry of the AUV and surface vessel. The results have confirmed that Daisan–Miyako Knoll is a stratovolcano with three caldron-like depressions (> ~1 km in diameter), a central cone with a summit crater, lava flows, crater-like depressions (< ~150 m in diameter), and ridges on its summit region. 2. The formation process of Daisan–Miyako Knoll is interpreted based on the detailed volcanic morphology and is summarized as follows: (1) Development of the main edifice, (2) Formation of the central-

depth (m)

E

E’

980 990 1000 1010 1020 1030 1040 1050 1060

thickness ~46m

0

100

200

300

400

500

distance (m)

F

F’

depth (m)

1140 1150

flow A-n

1160 1170 1180 0

100

200

300

400

500

distance (m)

G

G’

depth (m)

1140 1150

flow A-s

1160 1170 1180 0

100

200

300

400

500

distance (m)

H

H’

850

depth (m)

900 950 1000

central cone

1050 1100 0

200

400

600

800

1000

1200

1400

distance (m) Fig. 9. Depth cross-sections of volcanic structures of Daisan–Miyako Knoll. Profiles E–E′, F–F′, G–G′, and H–H′ correspond to the lines in Fig. 5b. 108

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara

outer depression (possibly caldera), (3) Formation of the centralinner depression (possibly caldera), (4) Formation of the NW depression and linear flow-like features on the NW flank (possibly sector collapse with debris flows), (5) Formation of the central cone, (6) Eruption of lava flows A-n, A-s and A-w from the summit crater of the central cone, (7) Eruption of lava flow B from the flank crater of the central cone. The fresh appearance of the lava-flow morphology indicates that Daisan–Miyako Knoll was active during the Quaternary. 3. The volcanic features of Daisan–Miyako Knoll principally trend NW–SE, which is normal to the extension axis (NE–SW) of the regional stress field; this is known as arc-parallel extension. It is suggested that the volcanism at the Knoll is controlled primarily by arcparallel extension in the southern Ryukyu Arc. 4. Daisan–Miyako Knoll is located on the volcanic front and the silicic characteristics of the surrounding large (> 10 km wide) volcanic edifices and Daisan–Miyako Knoll indicates that silicic arc volcanism continues underwater from the northern to southern Ryukyu Arc.

1029/1999TC900001. Farquharson, J.I., James, M.R., Tuffen, H., 2015. Examining rhyolite lava flow dynamics through photo-based 3D reconstructions of the 2011–2012 lava flow field at CordónCaulle, Chile. J. Volcanol. Geotherm. Res. 304, 336–348. https://doi.org/10.1016/j. jvolgeores.2015.09.004. Feuillet, N., Manighetti, I., Tapponnier, P., Jacques, E., 2002. Arc parallel extension and localization of volcanic complexes in Guadeloupe, Lesser Antilles. J. Geophys. Res. 107 (B12), 2331. https://doi.org/10.1029/2001JB000308. Fink, J.H., 1980. Surface folding and viscosity of rhyolite flows. Geology 8, 250–254. Fink, J.H., Anderson, S.W., 2017. Emplacement of Holocene Silicic Lava Flows and Domes at Newberry, South Sister, and Medicine Lake volcanoes, California and Oregon: U.S. Geological Survey Scientific Investigations Report 2017–5022–I. https://doi.org/10. 3133/sir20175022I. (41 p.). Fournier, M., Fabbri, O., Angelier, J., Cadet, J.-P., 2001. Regional seismicity and on-land deformation in the Ryukyu arc: implications for the kinematics of opening of the Okinawa Trough. J. Geophys. Res. 106 (B7), 13751–13768. https://doi.org/10.1029/ 2001JB900010. Fujinawa, A., Iwasaki, M., Honda, K., Nagao, A., Wachi, T., Hayashi, S., 2004. Eruption history in the post-caldera stage of Akita-Komagatake Volcano, Northeastern Japan Arc: correlation between eruptives constituting volcanic edifices and air-fall tephra layers. Bull. Volc. Soc. Japan 49, 333–354 (in Japanese with English abstract). Geological Survey of Japan, 2013. Volcanoes of Japan 1:2,000,000 map series no. 11, third ed. Geological Survey of Japan (AIST), Tsukuba. Hamamoto, F., Sakurai, M., Nagano, M., 1979. Submarine geology off the Miyako and Yaeyama Islands. Rep. Hydro. Ocean. Res. 14, 1–38 (in Japanese with English abstract). Harigane, Y., 2015. Structural development of volcanic caldera in Kume-jima western offshore. In: Cruise Report R/V Natsushima Cruise NT14-22, p24, . http://www. godac.jamstec.go.jp/catalog/data/doc_catalog/media/NT14-22_all.pdf. Heeszel, D.S., Wiens, D.A., Shore, P.J., Shiobara, H., Sugioka, H., 2008. Earthquake evidence for along-arc extension in the Mariana Islands. Geochem. Geophys. Geosyst. 9, Q12X03. https://doi.org/10.1029/2008GC002186. Hirata, N., Kinoshita, H., Katao, H., Baba, H., Kaiho, Y., Koresawa, S., Ono, Y., Hayashi, K., 1991. Report on DELP 1988 cruise in the southern Okinawa trough, part 3: crustal structure of the southern Okinawa trough. Bull. Earthq. Res. Inst., Univ. Tokyo 66, 37–70. Ishizuka, O., Shimoda, G., Harigane, Y., Inoue, T., Arai, K., Sato, T., Katayama, H., Minami, H., Ohara, Y., 2014. Active submarine reararc volcanoes west of Iotorishima, Ryukyu arc. In: Programme and Abstracts of The Volcanological Society of Japan 2014 Fall Meeting A1-35, (in Japanese). Iwasaki, T., Hirata, N., Kanazawa, T., Melles, J., Suyehiro, K., Urabe, T., Möller, L., Makris, J., Shimamura, H., 1990. Crustal and upper mantle structure in the Ryukyu Island Arc deduced from deep seismic sounding. Geophys. J. Int. 102, 631–651. https://doi.org/10.1111/j.1365-246X.1990.tb04587.x. Japan Meteorological Agency (Ed.), 2013. National Catalogue of the Active Volcanoes in Japan, fourth ed. Japan Meteorological Agency. Kato, Y., 1991. Submarine volcanic eruption at Iriomote in 1924. Gekkan Chikyu 13, 644–649 (in Japanese). Kato, S., Katsura, T., Hirano, K., 1982. Submarine geology off Okinawa Island. Rep. Hydrogr. Res. 17, 31–70 (in Japanese with English abstract). Kato, T., Beavan, J., Matsushima, T., Kotake, Y., Camacho, J.T., Nakao, S., 2003. Geodetic evidence of back-arc spreading in the Mariana Trough. Geophys. Res. Lett. 30, 1625. https://doi.org/10.1029/2002GL016757. Kizaki, K., 1986. Geology and tectonics of the Ryukyu Islands. Tectonophysics 125, 193–207. https://doi.org/10.1016/0040-1951(86)90014-4. Klingelhoefer, F., Lee, C.-S., Lin, J.-Y., Sibuet, J.-C., 2009. Structure of the southernmost Okinawa Trough from reflection and wide-angle seismic data. Tectonophysics 466, 281–288. https://doi.org/10.1016/j.tecto.2007.11.031. Kodaira, S., Sato, T., Takahashi, N., Ito, A., Tamura, Y., Tatsumi, Y., Kaneda, Y., 2007. Seimological evidence for variable growth of crust along the Izu intraoceanic arc. J. Geophys. Res. 112, B05104. https://doi.org/10.1029/2006JB004593. Kubo, A., Fukuyama, E., 2003. Stress field along the Ryukyu Arc and the Okinawa Trough inferred from moment tensors of shallow earthquakes. Earth Planet. Sci. Lett. 210 (1–2), 305–316. https://doi.org/10.1016/S0012-821X(03)00132-8. Kuramoto, S., Konishi, S., 1989. The southwest Ryukyu Arc is a migrating microplate (forearc sliver). Tectonophysics 163, 75–91. https://doi.org/10.1016/0040-1951(89) 90119-4. Martínez, F., Fryer, P., Becker, N., 2000. Geophysical characteristics of the southern Mariana Trough, 11°50′N–13°40′N. J. Geophys. Res. 105 (B7), 16591–16607. https://doi.org/10.1029/2000JB900117. Matsumoto, T., Kinoshita, M., Nakamura, M., Sibuet, J.-C., Lee, C.-S., Hsu, S.-K., Oomori, T., Shinjo, R., Hashimoto, Y., Hosoya, S., Imamura, M., Ito, M., Tukuda, K., Yagi, H., Tatekawa, K., Kagaya, I., Hokakubo, S., Okada, T., Kimura, M., 2001. Volcanic and hydrothermal activities and possible “segmentation” of the axial rifting in the westernmost part of the Okinawa Trough -preliminary results from the YOKOSUKA/ SHINKAI6500 Lequios Cruise-JAMSTEC. J. Deep Sea Res. 19, 95–107 (in Japanese with English abstract). Matsumoto, T., Shinjo, R., Nakamura, M., Kimura, M., Ono, T., 2009. Submarine active normal faults completely crossing the southwest Ryukyu Arc. Tectonophysics 466, 289–299. https://doi.org/10.1016/j.tecto.2007.11.032. McCaffrey, R., 1996. Estimates of modern arc parallel strain rates in fore-arcs. Geology 24 (1), 27–30. https://doi.org/10.1130/0091-7613(1996)024<0027:EOMAPS>2.3. CO;2. Minami, H., Ohara, Y., 2016. Detailed morphology and bubble plumes of Daiichi-Amami Knoll in the central Ryukyu Arc. Mar. Geol. 373, 55–63. https://doi.org/10.1016/j. margeo.2016.01.008.

Acknowledgments We would like to thank the crew of S/V Takuyo and the operation team of AUV Gondou who helped with the data acquisition. We thank I.S.E., Ltd. for providing technical assistance for our AUV operation and maintenance. We thank JCG officer Noritsune Seo and Arc Geo Support Co., Ltd. for their assistance in processing the AUV data. We also thank Matthew J. Wilson for improving the final manuscript. Constructive comments from William W. Chadwick Jr. and an anonymous reviewer, as well as the editorial handling of M. Rebesco, greatly improved the manuscript. References Arai, K., Machiyama, H., Chiyonobu, S., Matsuda, H., Sasaki, K., Humblet, M., Iryu, Y., 2014. Subsidence of the Miyako-Sone submarine carbonate platform, east of Miyakojima Island, northwestern Pacific Ocean. Island Arc 23, 1–15. https://doi.org/10. 1111/iar.12051. Bird, P., 2003. An updated digital model of plate boundaries. Geochem. Geophys. Geosyst. 4, 1027. https://doi.org/10.1029/2001GC000252. Bullock, L.A., 2015. Structure, Emplacement and Textural Evolution of Young Obsidian Lavas in the Aeolian Islands, Italy. PhD thesis. Keele University, UK. Bullock, L.A., Gertisser, R., O'Driscoll, B., 2018. Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands). Bull. Volcanol. 80 (5), 48. https://doi. org/10.1007/s00445-018-1222-4. Caress, D.W., Clague, D.A., Paduan, J.B., Martin, J.F., Dreyer, B.M., Chadwick Jr., W.W., Denny, A., Kelley, D.S., 2012. Repeat bathymetric surveys at 1-metre resolution of lava flows erupted at Axial Seamount in April 2011. Nat. Geosci. 5, 483–488. https:// doi.org/10.1038/ngeo1496. Carey, R., Soule, S.A., Manga, M., White, J., McPhie, J., Wysoczanski, R., Jutzeler, M., Tani, K., Yoerger, D., Fornari, D., Caratori-Tontini, F., Houghton, B., Mitchell, S., Ikegami, F., Conway, C., Murch, A., Fauria, K., Jones, M., Cahalan, R., McKenzie, W., 2018. The largest deep ocean silicic volcanic eruption of the past century. Sci. Adv. 4, e1701121. https://doi.org/10.1126/sciadv.1701121. Chadwick Jr., W.W., Wright, I.C., Schwarz-Schampera, U., Hyvernaud, O., Reymond, D., de Ronde, C.E.J., 2008. Cyclic eruptions and sector collapses at Monowai submarine volcano, Kermadec arc: 1998–2007. Geochem. Geophys. Geosyst. 9, Q10014. https:// doi.org/10.1029/2008GC002113. Chadwick Jr., W.W., Dziak, R.P., Haxel, J.H., Embley, R.W., Matsumoto, H., 2012. Submarine landslide triggered by volcanic eruption recorded by in-situ hydrophone. Geology 40 (1), 51–54. https://doi.org/10.1130/G32495.1. Chadwick Jr., W.W., Merle, S.G., Buck, N.J., Lavelle, J.W., Resing, J.A., Ferrini, V., 2014. Imaging of CO2 bubble plumes above an erupting submarine volcano, NW Rota-1, Mariana Arc. Geochem. Geophys. Geosyst. 15, 4325–4342. https://doi.org/10.1002/ 2014GC005543. Chung, S.-L., Wang, S.-L., Shinjo, R., Lee, C.-S., Chen, C.-H., 2000. Initiation of arc magmatism in an embryonic continental rifting zone of the southernmost part of Okinawa Trough. Terra Nova 12, 225–230. https://doi.org/10.1046/j.1365-3121. 2000.00298.x. Clague, D.A., Paduan, J.B., Caress, D.W., Thomas, H., Chadwick Jr., W.W., Merle, S.G., 2011. Volcanic morphology of West Mata Volcano, NE Lau Basin, based on highresolution bathymetry and depth changes. Geochem. Geophys. Geosyst. 12, QOAF03. https://doi.org/10.1029/2011GC003791. Fabbri, O., Fournier, M., 1999. Extension in the southern Ryukyu arc (Japan): link with oblique subduction and back arc rifting. Tectonics 18, 486–497. https://doi.org/10.

109

Marine Geology 404 (2018) 97–110

H. Minami, Y. Ohara Minami, H., Ohara, Y., 2017. The Gondou hydrothermal field in the Ryukyu Arc: a huge hydrothermal system on the flank of a caldera volcano. Geochem. Geophys. Geosyst. 18, 3489–3516. https://doi.org/10.1002/2017GC006868. Moriya, I., 1986. Morphometry of pyroclastic cones in Japan. In: The Geographical Reports of Kanazawa University. 3. pp. 58–76. Nakahigashi, K., Shinohara, M., Suzuki, S., Hino, R., Shiobara, H., Takenaka, H., Nishino, M., Sato, T., Yoneshima, S., Kanazawa, T., 2004. Seismic structure of the crust and uppermost mantle in the incipient stage of back arc rifting - northernmost Okinawa trough. Geophys. Res. Lett. 31, L02614. https://doi.org/10.1029/2003GL018928. Nakamura, M., 2004. Crustal deformation in the central and southern Ryukyu Arc estimated from GPS data. Earth Planet. Sci. Lett. 217, 389–398. https://doi.org/10. 1016/S0012-821X(03)00604-6. Nakamura, M., Kaneshiro, S., 2000. Determination of subducted Philippine Sea Plate in the Nansei islands deduced from hypocenter data. University of the Ryukyus 70, 73–82 (in Japanese with English abstract). Nishizawa, A., Kaneda, K., Oikawa, M., Horiuchi, D., Fujioka, Y., Okada, C., 2017. Variations in seismic velocity distribution along the Ryukyu (Nansei-Shoto) Trench subduction zone at the northwestern end of the Philippine Sea plate. Earth Planets Space 69, 86. https://doi.org/10.1186/s40623-017-0674-7. Okino, K., Asada, M., Noguchi, T., Komaki, K., Masakazu, F., Tara, K., Manaka, T., Koide, S., Tomita, D., 2015. Preliminary report of AUV URASHIMA dives at Tarama and Irabu hydrothermal fields. In: Japan Geoscience Union Meeting 2015, abstract SCG64-09. Oshida, A., Tamaki, K., Kimura, M., 1992. Origin of the magnetic anomalies in the southern Okinawa Trough. J. Geomagn. Geoelectr. 44, 345–359. https://doi.org/10. 5636/jgg.44.345. Oshima, S., Takanashi, M., Kato, S., Uchida, M., Okazaki, I., Kasuga, S., Kawashiri, C., Kaneko, Y., Ogawa, M., Kawai, K., Seta, H., Kato, Y., 1988. Geological and geophysical survey in the Okinawa trough and the adjoining seas of Nansei Syoto. Rep. Hydrogr. Res. 24, 19–43 (in Japanese with English abstract). Otsubo, M., Hayashi, D., 2003. Neotectonics in Southern Ryukyu arc by means of paleostress analysis. Bull. Fac. Sci. Univ. Ryukyus 76, 1–73. Research Group for Active Faults in Japan, 1991. Active Faults in Japan: Sheet Maps and Inventories. Tokyo Univ. Press, Tokyo (437 pp). Rubin, K.H., Soule, S.A., Chadwick Jr., W.W., Fornari, D.J., Clague, D.A., Embley, R.W., Baker, E.T., Perfit, M.R., Caress, D.W., Dziak, R.P., 2012. Volcanic eruptions in the deep sea. Oceanography 25 (1), 142–157. https://doi.org/10.5670/oceanog. 2012.12. Schnur, S.R., Chadwick Jr., W.W., Embley, R.W., Ferrini, V.L., de Ronde, C.E.J., Cashman, K.V., Deardorff, N.D., Merle, S.G., Dziak, R.P., Haxel, J.H., Matsumoto, H., 2017. A decade of volcanic construction and destruction at the summit of NW Rota-1 seamount: 2004–2014. J. Geophys. Res. 122, 1558–1584. https://doi.org/10.1002/ 2016JB013742. Seno, T., Stein, S., Gripp, A.E., 1993. A model for the motion of the Philippine Sea plate consistent with NUVEL-1 and geological data. J. Geophys. Res. 98 (B10), 17941–17948. https://doi.org/10.1029/93JB00782. Shinjo, R., Motoyama, I., Nakamura, M., Takaki, Y., Nishida, H., Morii, Y., Tanaka, H., 1998. Report on RN97 cruise by T/S Nagasaki Maru in the southern Okinawa Trough. Bull. Coll. Sci. Univ. Ryukyu 65, 39–51. Sibuet, J.-C., Hsu, S.-K., Shyu, C.-T., Liu, C.-S., 1995. Structural and kinematic evolutions of the Okinawa Trough backarc basin. In: Taylor, B. (Ed.), Backarc Basins. Springer US, New York, pp. 343–379.

Sibuet, J.-C., Deffontaines, B., Hsu, S.-K., Thareau, N., Le Formal, J.-P., Liu, C.-S., 1998. Okinawa trough backarc basin: early tectonic and magmatic evolution. J. Geophys. Res. 103 (B12), 30245–30267. https://doi.org/10.1029/98JB01823. Stern, R.J., 2002. Subduction zones. Rev. Geophys. 40 (4), 1012. https://doi.org/10. 1029/2001RG000108. Stern, R.J., Smoot, N.C., 1998. A bathymetric overview of the Mariana forearc. Island Arc 7 (3), 525–540. https://doi.org/10.1111/j.1440-1738.1998.00208.x. Suyehiro, K., Takahashi, N., Ariie, Y., Yokoi, Y., Hino, R., Shinohara, M., Kanazawa, T., Hirata, N., Tokuyama, H., Taira, A., 1996. Continental crust, crustal underplating and low-Q upper mantle beneath an oceanic island arc. Science 272, 390–392. https:// doi.org/10.1126/science.272.5260.390. Tamura, Y., Tatsumi, Y., 2002. Remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs: an example from the Izu-Bonin arc. J. Petrol. 43, 1029–1047. https://doi.org/10.1093/petrology/43.6.1029. Tuffen, H., James, M., Castro, J.M., Schipper, I., 2013. Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile. Nat. Commun. 4, 2709. https://doi.org/10.1038/ncomms3709. Ueda, Y., 1986. Geomagnetic anomalies around the Nansei Syoto (Ryukyu Islands) and their tectonic implications. Bull. Volc. Soc. Japan 31, 177–192 (in Japanese with English abstract). Wachi, T., Doi, N., Koshiya, S., 1997. Tephra stratigraphy and eruptive activities of the Akita-Komagatake Volcano. Bull. Volc. Soc. Japan 42, 17–34 (in Japanese with English abstract). Watanabe, K., 2000. Dive surveys on knolls off the north-east of the Iriomote Island. –the Iriomote Knoll and the Daiichi and Daini Kohama Knolls-JAMSTEC. J. Deep Sea Res. 16, 19–28. Watanabe, K., Shibata, A., Furukawa, H., Kajimura, T., 1995. Topography of submarine volcanoes off the North-Northeast Coast of Iriomote Island, the Ryukyu Islands. Bull. Volc. Soc. Japan 40, 91–97 (in Japanese with English Abstract). Watts, A.B., Peirce, C., Grevemeyer, I., Pauletto, M., Stratford, W., Bassett, D., Hunter, J.A., Kalnins, L.M., de Ronde, C.E.J., 2012. Rapid rates of growth and collapse of Monowai submarine volcano in the Kermadec Arc. Nat. Geosci. 5 (7), 510–515. https://doi.org/10.1038/ngeo1473. Wessel, J.K., Fryer, P., Wessel, P., Taylor, B., 1994. Extension in the northern Mariana inner forearc. J. Geophys. Res. 99 (B8), 15181–15203. https://doi.org/10.1029/ 94JB00692. Wright, I.C., Chadwick Jr., W.W., de Ronde, C.E.J., Reymond, D., Hyvernaud, O., Gennerich, H.-H., Stoffers, P., Mackay, K., Dunkin, M., Bannister, S.C., 2008. Collapse and reconstruction of Monowai submarine volcano, Kermadec arc, 1998–2004. J. Geophys. Res. 113, B08S03. https://doi.org/10.1029/2007JB005138. Yamanaka, T., Nagashio, H., Nishio, R., Kondo, K., Noguchi, T., Okamura, K., Nunoura, T., Makita, H., Nakamura, K., Watanabe, H., Inoue, K., Toki, T., Iguchi, K., Tsunogai, U., Nakada, R., Ohshima, S., Toyoda, S., Kawai, J., Yoshida, N., Ijiri, A., Sunamura, M., 2015. The Tarama Knoll: geochemical and biological profiles of hydrothermal activity. In: Ishibashi, J., Okino, K., Sunamura, M. (Eds.), Subseafloor Biosphere Linked to Hydrothermal Systems. Springer, Tokyo, pp. 497–504. https://doi.org/10. 1007/978-4-431-54865-2_40. Yokose, H., Sato, H., Fujimoto, Y., Mirabueno, M.H.T., Kobayashi, T., Akimoto, K., Yoshimura, H., Morii, Y., Yamawaki, N., Ishii, T., Honza, E., 2010. Mid-Pleistocene submarine acidic volcanism of the Tokara Islands, Japan. J. Geogr. 119, 46–68 (in Japanese with English abstract).

110