Crust and uppermost mantle structure of the Kyushu-Palau Ridge, remnant arc on the Philippine Sea plate

Abstract

We acquired 27 wide-angle seismic profiles to investigate variation in crustal structure along the Kyushu-Palau Ridge (KPR), a 2600-km-long remnant island arc in the center of the Philippine Sea plate; 26 lines were shot across the strike of the KPR at 13°-31°N, and one was shot along the northernmost KPR. The derived P-wave velocity (Vp) models show that the KPR has a crustal thickness of 8-23 km, which is thicker than the neighboring backarc basin oceanic crusts of the West Philippine Basin to the west and the Shikoku and Parece Vela Basins to the east. While the KPR crust consists mainly of lower crusts with a Vp of 6.8-7.2 km/s, the thicker crust contains a thick middle crust with Vp of 6.0-6.8 km/s. In general, the KPR crust is thicker in the north than in the south. The uppermost mantle velocities just below the KPR bathymetric highs are lower than 8.0 km/s and are commonly associated with a slightly high Vp of 7.2 km/s at the base of the crust. Large amplitude reflection signals are sometimes observed at far offsets on several lines suggesting the existence of several reflectors at depths of 23-40 km in the mantle beneath the KPR. The characteristics of these reflections are similar to these observed beneath the Izu-Ogasawara (Bonin) island arc, the tectonically conjugate arc of the KPR before backarc basin spreading. Very thin crust and high Pn velocities characterize the transition between the KPR and the eastern basins, which is probably a relic of the initial stage of the rifting. West of the KPR, the crust varies in structure from north to south as a result of the different tectonic settings in which it evolved.

Nishizawa et al. Earth, Planets and Space (2016) 68:30 DOI 10.1186/s40623-016-0407-3

Crust anduppermost mantle structure ofthe Kyushu-Palau Ridge, remnant arc onthe Philippine Sea plate

Azusa Nishizawa*, Kentaro Kaneda and Mitsuhiro Oikawa

Background

The Kyushu-Palau Ridge (KPR) is a 2600-km-long bathymetric high extending northsouth at the center of the Philippine Sea plate in the northwestern Pacic (Fig.1). The KPR is regarded as a remnant of the proto IzuOgasawara (Bonin)Mariana (IBM) island arc that was separated by backarc spreading of the Shikoku and Parece Vela Basins in the late Eocene (e.g., Mrozowski and Hayes 1979; Seno and Maruyama 1984; Okino etal. 1994). Topographic characteristics of the KPR are as follows: large height and width of the KPR in the north than in the south, with the volcanic edice decreasing in size from north to south; a steep approximately linear slope on the eastern side of the ridge, which was formed by

prior to the backarc spreading of the Shikoku and Parece Vela Basins (e.g., Tokuyama 2007); spar like structures extending westward from the central part of the ridge, which has similar orientations to the seamount chains to the west of the northern IzuOgasawara Ridge.

Figure2 left shows the Bouguer gravity anomaly map. A low anomaly indicating thicker crust correlates with the KPR bathymetric highs. Ishihara and Koda (2007) deduced the crustal thickness of the KPR by three-dimensional gravity modeling and showed that the KPR crust has a continuous crustal root along the ridge and that the crustal thickness exceeds 15 km in most of its northern part and 10 km in most of its southern part. They detected a belt of thin crust with thickness of approximately 5 km along the transition zone between the Shikoku Basin and KPR. The magnetic anomaly map (Fig.2 right) shows many dipolar anomalies are distributed along the KPR, and their intensity is larger in the

*Correspondence: [email protected] and Oceanographic Department, Japan Coast Guard, Tokyo 135-0064, Japan

2016 Nishizawa et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

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north than in the south. Yamazaki and Yuasa (1998) identied three northsouth rows of long-wavelength magnetic anomalies over the KPR, IzuOgasawara volcanic front, and Nishi-Shichito Ridge (rear arc of the IzuOgasawara arc). They interpreted that these magnetic anomalies reected magnetization of the deeper crust along the arcs and that the similarity of the three rows of anomalies was due to their separation from a single paleoIzuOgasawara island arc. Ishizuka et al. (2011) investigated in detail the ages and geochemical characteristics of the volcanic rocks sampled from almost the entire length of the KPR. They showed that the KPR was active between 25 and 48Ma, but most of the ages

of the volcanic rocks from the KPR lie within the narrow range of 2528 Ma, and no systematic variation in age can be recognized in along-arc and across-arc directions. However, Eocene ages have only been obtained from the northern half of the KPR, to the north of 23N, and a lack of ages older than 32.5 Ma in the southern half could imply that the KPR was established on Cretaceous terranes in the northern region, but on the West Philippine Basin oceanic crust with an age of around 3646Ma in the south (Ishizuka etal. 2011; Sasaki etal. 2014).

Many 2-D seismic refraction proles using ocean bottom seismographs (OBSs) as receivers have been carried out across the IzuOgasawara island arc and the KPR.

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The rst good quality seismic experiment across the IzuOgasawara island arc was acquired at 3215 N in the northern part of the arc in 1995, and a P-wave velocity (Vp) model down to the Moho was obtained (Suyehiro etal. 1996). A Vp model of the arc at around 3030 N was also presented by Nishizawa etal. (2006), showing that there is no signicant variation in the arc crustal models between 3030 and 3215 N, with both models characterized by a middle crust with a Vp of 6 km/s, a lower crust of 6.87.3 km/s, and a total thickness of around 20 km. However, extensive seismic refraction and reection surveys conducted under Japanese Continental Shelf Survey (CSS) Project in 20042008 showed large variations in crustal models over large distances along the IzuOgasawara arc from north to south. For instance, Kodaira et al. (2007b) presented a >1000-km-long wide-angle seismic prole along the volcanic arc,

revealing two scales of variation in arc crust beneath the volcanic front; at one scale the Izu arc north of 30N is much thicker than the Ogasawara arc in the south; at the intervolcano scale of around 50km, the crust is thicker below each volcano. A seismic study along the rear arc of the Izu arc (Kodaira etal. 2008) also shows marked variations in crustal thickness that are attributed mainly to thickness variations of the middle crustal layer with Vp of 6.06.8km/s. Kodaira etal. (2008) proposed that the rear-arc crust is composed of a remnant-arc crust that was separated from the volcanic front, probably in the Oligocene, and that most of the rear-arc crust was created before separation from the volcanic front.

Tonalitic rocks dredged from the Komahashi-Daini Seamount (on line KPr4 in Fig. 5), the shallowest seamount in the northern KPR, which are dated at 3738 Ma, were formed during early stage of the

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IzuOgasawaraMariana (IBM) arc volcanism (Haraguchi et al. 2003). The tonalite complex seems to correspond to this middle crust of the intra-oceanic island arcs with Vp of around 6km/s. Distribution of the middle crust along the ridge axis could provide a fundamental insight into an evolution of the proto-IBM arc.

As mentioned before, the Kyushu-Palau and Izu Ogasawara Ridges were originally a single intra-oceanic arc produced by subduction of the Pacic plate beneath the Philippine Sea plate. However, the crustal structure of the KPR appears to be dierent from that of IzuOgasawara island arc, since the KPR was less aected by subduction-related volcanism after initiation of the backarc spreading. Therefore, comparison of the crustal structure of the KPR and IzuOgasawara arc can help constrain the early evolution of the IzuOgasawara arc system.

Recently, the Daito Ridges, other signicant bathymetric highs characterizing the Philippine Sea plate, were comprehensively investigated geologically and geophysically under the CSS Project in Japan (e.g., Ishizuka etal. 2013; Nishizawa et al. 2014). Three large bathymetric highs, the Amami Plateau, the Daito Ridge, and the Oki-Daito Ridge, from north to south (Fig.1) are also originated from paleo-island arcs. Nishizawa et al. (2014) showed that these bathymetric highs usually have a middle crust with a Vp of 6.36.8km/s, a lower crust with Vp=6.87.2km/s, a Pn velocity of 7.67.8km/s, and a crustal thickness of 1525km. These structures are similar to those of the IBM island arc, which are immature paleo-island arcs.

Several seismic refraction experiments were previously carried out over the KPR (e.g., Murauchi et al. 1968; Shinohara et al. 1999; Arisaka et al. 2003). However, these velocity models are too sparsely distributed to understand all the characteristics of the KPR crust along the entire length. In 20042008, the CSS Project also designed a massive seismic survey of the KPR region. We conducted 27 seismic reection and refraction lines across the KPR between 13 and 31N and one line along the ridge in the northernmost part. Using these survey data, Nishizawa et al. (2007) presented four Vp models for the KPR crusts at 1521N and showed that thicker crust exists beneath the KPR than the adjacent back-arc basin oceanic crusts of the West Philippine Basin and Parece Vela Basin. Kaneda et al. (2015) presented an overview on most of the seismic refraction results obtained by Japan Coast Guard under the CSS Project. References including basic information of the surveys and data were provided in Kaneda etal. (2015) by citation of the open cruise reports. Thier scientic results from the KPR region, however, were only briey summarized. The purpose of this paper is to compile all the KPR seismic structural models including the above results and to

characterize in detail the variation of the KPR Vp models along the ridge axis, which will provide key information for the construction of evolution model of the Philippine Sea plate involving the KPR.

Methods

The seismic survey consisted of 27 lines which were located to sample well the variations in seaoor topography of the KPR from 13 to 31N. Because the row of the KPR bathymetric highs is curved and the ridge width is thin compared with the IzuOgasawara arc, it is difcult to design an along-ridge seismic line. Therefore, 26 of the 27 seismic lines cross the KPR strike almost perpendicularly and only one line was shot along the ridge at the northern end of the KPR. The line lengths range from 105km for KPr31 to 610km for SPr10 (Fig.1).

We deployed OBSs as a receiver at an average interval of 5km along each line. Each OBS is equipped with a three-component 4.5Hz geophone and a hydrophone. A tuned airgun array of 36 airguns with a total volume of 8040 cubic inches (132 L) or a non-tuned airgun array of 4 airguns with a total volume of 6000 cubic inches (98 L) was shot at an interval of 200m (90s) for the wide-angle seismic proles. Multichannel seismic (MCS) reection data using a 480 ch. (6000m long) or 240 ch. (3000m) hydrophone streamer were also collected on the coincident lines. The airgun array was shot at a 50-m interval for each MCS line. Navigation was provided by the ships Global Positioning System, and each OBS instrument was relocated using the direct water wave arrivals (Oshida etal. 2008).

The procedures of the OBS data processing and velocity analysis are the same as presented by Nishizawa etal. (2014). That is, the OBS record sections from three-component geophone and hydrophone outputs were produced through frequency ltering, deconvolution, and a slant stack to increase the signal-to-noise ratio. Travel times of the reection and refraction signals were picked from these record sections and used to construct a P-wave velocity model. We introduced the thickness of the top most sedimentary layer constrained by MCS data into the initial velocity model for each line. Then, we obtained velocity models using the tomo2d tomographic inversion coded of Korenaga etal. (2000), forward modeling with two-dimensional ray tracing (Fujie etal. 2000; Kubota etal. 2009) and comparison with synthetic seismograms calculated by a nite dierence method, E3D (Larsen and Schultz 1995). The horizontal grid size of the tomographic inversion is 0.5km in all the velocity models, and the vertical grid size gradually increased with depth according to the relation 0.05 + (0.01 depth (km))1/2 km. Almost all velocity models have tomo-graphic inversion mists less than 50ms. We examined

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the resolving power of the results using conventional checkerboard resolution tests and checked ray coverage for each model. Although the forward modeling was time-consuming, the method was very useful to interpret weak and distant refraction signals and intermittent reection signals. We estimated the deeper structure at depths greater than 15km mainly by forward modeling, since the ray coverage in the deeper part was usually very low.

Results

Figures 3 and 4 show all the P-wave velocity models across the KPR and demonstrate the variation along the ridge axis from north to south. We split the KPR into ve regions shown in Fig.1 and describe in detail representative proles for each region. We loosely divide our crustal models into three parts (upper, middle, and lower crust) based on their P-wave velocities, velocity gradients, and thickness proportions following Calvert (2011) and Nishizawa etal. (2014) and will describe their characteristics below.

Region 1

We show the Vp model for KPr4 (Fig.5), which is representative of the northernmost region of the KPR where the bathymetric highs are very shallow and wide. The shallowest portion along KPr4 corresponds to Komahashi-Daini Seamount.

Tomographic inversion mist is less than 50 ms, and the checkerboard test result and ray diagram (Fig. 5 bottom) show the model shallower than 15 km is reliable. The Vp model of Komahashi-Daini Seamount has a thick middle crust with a thickness of around 7km and Vp of 66.5 km/s. Since the region below 15 km depth is less resolvable, and the crustal thickness beneath the seamount is 18km, the Vp at the bottom of lower crust of 7.1km/s and Pn velocity of 7.6km/s were estimated by two-dimensional ray tracing. The transition region between the Shikoku Basin and the KPR is characterized by thin crust less than 5km thick excluding the topmost sediment layer and a high Pn (uppermost mantle) velocity of 8.18.2km/s (Fig.5). On the other hand, a small bathymetric high exists to the west of Komahashi-Daini Seamount at a distance of 200km, and it has a slightly thicker curst with no thick middle crust layer. A thick sediment layer with Vp smaller than 3.5km/s and with a maximum thickness of around 3 km exists between Komahashi-Daini Seamount and the small bathymetric high. Deeper Moho at the southwestern end of the Vp model indicates a subducting bathymetric high beneath the forearc of the Nansei-Shoto island arc.

The total thicknesses of the KPR crusts are appreciably greater than those of the Shikoku Basin for all the lines of

KPr4, SPr10, KPr6, KPr8, and SPr7 in Region 1 (Fig.3). Particularly thick middle crust was found also for SPr10 and KPr6, and slightly thin middle crusts are estimated for KPr8 and SPr7 where the widths of the ridge topography are narrower than those of the northern lines.

It is notable that thin crust occurs at the boundary of the KPR and Shikoku Basin on all lines and is less than 3km thick on line KPr8 (Fig.3). Pn velocities in this area are higher than 8.0km/s, which are signicantly higher than those of the Shikoku Basin. To the west of the KPR, the upper sedimentary layer is relatively thick.

Region 2

Four lines DAr4, KPr11, KPr12, and KPr31 were located to cross the region where the KPR lies close to several seamounts and areas of shallow seaoor the west of the KPR.

We show the result for KPr11 in Fig.6 as a representative in this region. KPr11 was positioned around 10km south of DAr4 which connects to the Daito Ridge to the west. The Vp model beneath the KPR for KPr11 shows a total crustal thickness of around 13 km, a middle crust thickness of about 5km, Vp at the bottom of lower crust of 7.2 km/s, and Pn velocity of 7.6 km/s. Slightly shallow Moho at a distance of 140km is required to explain observed PmP arrivals. The eastern transition zone from the KPR to the Shikoku Basin is characterized by abrupt crustal thinning to less than 5km. At the western transition, around 20-km-thick KPR crust thins to 10 km beneath the Minami-Daito Basin. We detected a very fast Pn velocity of 8.3km/s at this transition.

Similar Vp models were obtained for DAr4, KPr12, and KPr31 (Fig.3). The boundary between the KPR and the Shikoku Basin is again characterized by very thin crust of 3 to 5km and Pn velocities of 8.08.1km, which are dierent from values found generally in the Shikoku Basin. On the other hand, the Minami-Daito Basin has thick crust of around 10km and high Pn velocity of 8.18.3 km/s, which is notably dierent from a typical oceanic crust. The crustal thickness in the western transition to the Minami-Daito Basin is not thinner than that of the Minami-Daito Basin, in general.

Region 3

Region 3 corresponds the area where the eastern edge of the KPR bounds the deepest part of the Shikoku Basin and the KPR topography is slightly deeper and narrower than in other regions. We shot ve lines including KPr13, SPr11, KPr14, KPr32, and KPr15 in this region and compiled each Vp model in Fig.3.

We show the Vp model for KPr13 in Fig.7. The crustal thickness beneath the KPR is about 10km with a middle crust of 2km. Vp at the bottom of the lower crust and

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the Pn velocity are 7.2 and 8.2 km/s, respectively. The Pn velocity beneath the KPR is very high, over 8.2km/s, much dierent from other lines. Since this line has a thin crust of less than 10km as a whole, the checkerboard test result in Fig. 7 bottom shows good recovery. We could conrm the high Pn velocity and thin crust not only by the tomographic inversion but also by 2-D forward ray-tracing method.

Common characteristics among the ve lines are follows: Crustal thickness below the KPR is more than 10 km, but less than 5 km beneath the eastern transition into the Shikoku Basin. To the west of the KPR, the crustal thicknesses of the Minami-Daito Basin on lines KPr13, SPr11, and KPr14 and of the Mangetsu Basin on lines KPr32 and KPr15 are larger than in the Shikoku Basin. The crust of the Minami-Daito Basin is slightly thinner in Region 3 than in Region 2. Uppermost mantle velocities for the lines other than KPr13 beneath the KPR

were estimated to be 7.67.8 km/s, while Pn velocities beneath the Minami-Daito and Mangetsu Basins are 8.2 and 8.0km/s, respectively.

Region 4

Region 4 corresponds to be the central part of the KPR where the KPR is basically bounded by the Parece Vela Basin (PVB) to the east and by the West Philippine Basin to the west. There are 11 seismic lines across the KPR from 22N to 15N, and the Vp models are shown in Fig.4. Since the previous paper by Nishizawa etal. (2007) has already reported velocity models for KPr19, Kpr20, Spr5, and KPr26 (white lines in Fig.1 right), we will highlight the result for KPr24 in Region 4 (Fig.8). The seaoor topography to the west of the KPR is at, 5000m deep, but a rather rugged seaoor fabric with N-S strike characterizes PVB to the east of the KPR. As shown in the checkerboard test result (Fig.8 bottom), it is difficult

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to determine deeper structure beneath the KPR from the tomographic inversion. Thus, we inferred velocities in the lower crust and uppermost mantle by trial-and-error tting of observed distant travel times using the forward modeling. Vp model for KPr24 has a middle crust with thickness of 4.5km and a total crustal thickness of 14km. The P-wave velocities at the base of the crust and in the uppermost mantle are estimated to be 7.2 and 7.7km/s, respectively.

We observed large amplitude reection signals at far osets in several OBS records on KPr24. Figure9 shows an example of the record section obtained at OBS 43 in the PVB. The signals in red ellipses in Fig. 9a were mapped to a black curve with white arrows at depth of 25km in Fig.9c using the travel time mapping method by Fujie etal. (2006). The other black curves correspond to the mapping results from reection signals recorded by

other OBSs. Although similar signals with large amplitudes at far osets were also observed on lines KPr21 and KPr22, their appearances, that is, their travel times and osets from the KPR are dierent. Furthermore, such signals were not observed on line KPr25.

KPr17 is located from the southeastward extension of the Oki-Daito Ridge to the PVB in the east (Fig.1). The Vp model (Fig. 10) is distinctive compared with other KPR models; that is, the total crustal thickness beneath the KPR is only around 8 km including the thick top sedimentary layer with Vp less than 4km/s and with a thickness of about 3km. The thin total crustal thickness is due to an anomalously thin lower curst. However, the KPR Pn velocity of 7.8km/s is well constrained as shown in the checkerboard result (Fig. 10 bottom), which is similar to other KPR models. Large amplitude reection signals were recorded on many record sections over

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osets of around 80 km along KPr17. Assuming a Vp below the Moho of roughly 8.0km/s, the travel times of these signals can be explained by reectors at 1014km below the Moho under the Oki-Daito Ridge and KPR at distances of 40160 km and of 200250 km. These reectors are much deeper compared with those below KPr24.

The KPR Vp models in Region 4 (Fig. 4) dier from line to line, but there are some common features: thicker crust and especially middle crust than in the adjacent basins, Pn velocities less than 8.0 km/s, thin crust and high Pn velocities over 8.0km/s at the eastern boundary with the PVB.

Region 5

Lines KPr40 and KPr41 are situated to the south of the intersection between the CBF Rise and KPR, and the southernmost proles in this study. On line KPr40, the Vp model beneath KPR reveals that the middle crust is about 5km thick and the Vp at the bottom of the lower crust is 7.2 km/s. The total crustal thickness is 15 km, and Pn velocity is 7.8km/s (Fig.11). The model for KPr41

shows slightly thinner crust and slower Vp at the bottom of the crust compared with the KPr40 (Figs.4, 11).

Discussion

Pwave velocity structure ofthe KPR

Although the velocity variation is large, we can derive some common characteristics of the KPR by comparing the Vp models along the KPR. The Vp models show the KPR has thicker curst with thicknesses of 8km (KPr 17 and KPr20) to 23km (KPr19), compared with standard oceanic crusts of 7.1 km + 0.8 km (e.g., White et al.

1992), and also lower Pn velocities less than 8.0 km/s. When we compare the KPR models with the neighboring backarc basin oceanic crust of the West Philippine Basin (Arisaka etal. 2003) to the west and of the Shikoku and Parece Vela Basins to the east (e.g., Nishizawa etal. 2007, 2011), the KPR crust is always thicker (Figs.3, 4). The thick KPR crust is mainly composed of lower crust with Vp of 6.87.2km/s (Figs.3, 4). Crust thicker than 18km also has over 5km of middle crust with Vp of 6.0 to 6.8km/s. For example, the middle crust beneath KPr4, SPr10, and KPr19 is thicker than 6 km. Pn velocities

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just below the KPR are less than 8.0km/s for almost all seismic lines, often accompanying a slightly high Vp of 7.2km/s at the base of the crust. The lower Pn and higher crust bottom velocities were also found beneath the paleo-island arcs at the Daito Ridge region (Nishizawa et al. 2014) and IzuOgasawara intra-oceanic arc (e.g., Suyehiro etal. 1996).

An exception to the thicker KPR crust is found on line KPr17 as shown in Fig.10. Usually, we can estimate crustal thicknesses from Bouguer gravity anomaly distribution. Smaller values of Bouguer gravity anomaly along the KPR in Fig.2 left roughly coincide the regions with thick crusts. However, the low gravity anomaly below the KPR along KPr17 is mainly caused by thick low velocity materials with Vp<4km/s and thickness of around 3km. Although the origin of the thick materials is unknown, the lower Pn velocity of 7.8km/s beneath the ridge seems to be similar to the KPR in general.

When we could record S-wave signals occasionally converted from P-wave at the base of the top sedimentary layer, we tried 2-D forward modeling to estimate

preliminary values of Vp/Vs. The results show that Vp/ Vs for the uppermost mantle beneath the KPR is around 1.73, and not larger than 1.8. Therefore, we infer that the mantle serpentinization is probably not responsible for the low uppermost mantle velocities.

The crustal structure of the IzuOgasawara intraoceanic arc, the tectonically conjugate island arc of the KPR, was well investigated also by the CSS project of Japan (e.g., Kodaira et al. 2007b, 2008). In the rear arc (Nishi-Shichito Ridge) area of the IzuOgasawara arc, Kodaira etal. (2008) showed that the thick middle crust contributes to the total crustal thicknesses. Kodaira etal. (2007a) also focused on the distribution of the middle crust in the Vp model along the volcanic front in the Izu arc, where is characterized by bimodal volcanism. They found that basaltic volcanoes have thick middle crust and rhyolitic volcanoes thinner middle crust and deduced that continental crust grows predominantly below the basaltic volcanoes in the Izu arc. Furthermore, they revealed that the thicker middle crust below the volcanic front and rear arc correspond to the high

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magnetic anomalies as suggested by Yamazaki and Yuasa (1998). Similar high magnetic anomalies are also distributed along the northern KPR (Yamazaki and Yuasa 1998), and KPr4 and SPr10 lines cross these anomalies which match the distinctive dipolar magnetic anomalies in Fig.2 right. Our Vp models show that thick middle crust exists below both of the lines. A clear dipolar anomaly on the KPr19 (Fig.2) also indicates thicker middle crust beneath Okinotori-shima Island.

Alongridge variation inVp models ofthe KPR

Figures 3 and 4 show that the KPR crust is roughly thicker than approximately 20km in the north (Regions 1 and 2) with exceptions of KPr8, KPr7, and KPr31, than in the south (Regions 3, 4, and 5), although the maximum crustal thickness beneath the KPR varies with each prole. The crustal structure of the IzuOgasawara arc revealed a similar but clearer variation from north to south along the volcanic front (Kodaira etal. 2007b).

They showed large changes in the crustal thickness from 26 to 35km beneath northern Izu arc to 922km in the Ogasawara arc, assuming the Moho corresponds to the 7.6km/s velocity contour.

Only line KPr1 was acquired parallel to the ridge

axis at the western part of the north KPR (Fig. 1). The Vp model (Fig. 3 top right) shows several bathymetric highs with thicker crust, distributed at an interval of 50100 km, which are subducting beneath the land-ward plate as reported by Nishizawa etal. (2009). Both the forward modeling and tomographic inversion support a large variation in Pn velocity along KPr1. Higher Pn velocities are obtained below the Kikai Basin and Amami-Sankaku Basin. The higher Pn velocities below the Amami-Sankaku Basin are also detected at KPr6 and KPr8 in Fig.3. While the Pn velocities at the Kikai Basin varies from 7.7km/s at KPr4 to 8.1km/s at SPr10, such inhomogeneity may be related to the unknown tectonic evolution of these basins.

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Transition betweenthe KPR andthe backarc basins onthe east

The seaoor topography at the boundary between the KPR and the Shikoku or Parece Vela backarc basins is characterized by sharp and steep scarps supposed to be formed by the initial rifting, breakup, and early separation of the proto-arc. The sediment ponds identied by the multi-channel seismic proles (e.g., KRr4 and KPr11 in Fig. 12) are also related to the rifting. The Vp models below the transition area are characterized by thinner crust with a thickness less than 4km (Figs.3, 4) and often slightly higher Pn velocity of >8.0km/s compared with those of the Shikoku and Parece Vela Basins. This thinner crust and higher Pn velocity are considered as a result from the initial extension of the paleo-arc crust and upward migration of deeper mantle materials with higher Vp, respectively.

Although serpentinized mantle is sometimes formed near the base of the rifted crust along non-volcanic passive margins in the North Atlantic (e.g., Whitmarsh etal.

1996; Reid 1994), the Pn velocity of >8.0 km/s in our result does not indicate existence of serpentinized materials. Moreover, our preliminary analysis of S-wave travel times showed Vp/Vs in the crust and uppermost mantle in the transition area was around 1.73, signicantly less than 1.8, which is dierent from Vp/Vs of serpentinized rocks. On the other hand, similar thin crust and high Pn velocities at the arc-basin transition are also found along western edge of the IzuOgasawara arc (Takahashi etal. 2009) and Mariana arc (Takahashi etal. 2007), the conjugate to the KPR before the separation of the proto-island arc. These features indicate that there was not water supply into the uppermost mantle at the rifting stage.

The seismic structure at the northern end of the eastern KPR transition is very important for locating the western limit of the future Nankai megathrust earthquake with a magnitude of around 9 (Cabinet Office of Japan 2012). There is an intrinsic dierence in lithosphere thicknesses between the Shikoku Basin with an age less than 30Ma (e.g., Okino etal. 1999) and the Daito Ridges

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of Cretaceous to Eocene age (Ishizuka and Yuasa 2007). Moreover, this study revealed the large discontinuity in crustal thicknesses between the thick crusts below the KPR and signicantly thin crusts beneath the transition on the subducting Philippine Sea plate, which suggests the northernmost KPR transition could be a possible segment boundary in a future large earthquake at the Nankai Trough.

Transition betweenthe KPR andthe western backarc basins or other structures

Sedimentary aprons with thicknesses of around 12km are commonly observed along the western slope of the KPR (Fig.12), which indicates the sediments were deposited after KPR formation. However, in the deeper part, the crustal models of west of the KPR show large variations among the seismic lines as shown in Figs.3 and 4. This is because the tectonic settings of the western side vary from north to south along the KPR, such as the Kikai and Amami-Sankaku Basins in Region 1, the Daito Ridge as a paleo-island arc and Minami-Daito Basins as intra-arc basins in Region 2, the Minami-Daito Basins and Mangetsu Basin in Region 3, the Oki-Daito Ridge as an island arc, the West Philippine Basin (WPB) as a back-arc basin and the CBF Rift as the spreading center of the WPB in Region 4, and WPB in Region 5. Vp models for the Daito Ridges were already compiled by Nishizawa etal. (2014), and those for the several other basins will be published in another paper.

Deep reectors belowthe KPR

We sometimes observed large amplitude reection signals at far osets in OBS record sections as shown in Fig. 9. These reection signals are mapped to several reectors at depths of 2240km below the seaoor of the KPR. The appearance of these reection signals varies on each line, and it is very difficult to constrain the spatial distribution or continuity of these deep reectors. These deep reectors might be distributed discontinuously across the region.

Similar several deep reectors are found below the southern IzuOgasawara arc at depths of around 2535km (Takahashi etal. 2009). Moreover, Takahashi et al. (2008) also observed upper mantle reections at depths of 20 and 30km beneath the western side of the West Mariana Ridge and from 30 to 35 km below the Mariana arc. In a petrological model for arc crust evolution of the IBM arc proposed by Tatsumi etal. (2008, 2015), a middle crust was produced from the basaltic initial arc crust, and then the restite with more mac composition transfers to the mantle. Therefore, the origin of the upper mantle reectors might be related to the transformation of the mac/dense crustal materials due to

repeated crustal growth, which is consistent with slower mantle Vp below the arc. Sato etal. (2009) inferred the distribution of a seismic reector at depths of 2040km in the upper mantle below the volcanic front along the northern IzuOgasawara arc denes the base of the low Vp uppermost mantle composed of the restite and olivine cumulates.

Conclusions

We acquired 27 seismic lines to determine the P-wave velocity structure along the KPR. Our results show the following:

1. Although the crustal thickness below the KPR bathy-metric high varies along the ridge axis, the northern KPR is generally thicker than the southern KPR. The KPR crust is signicantly thicker than the adjacent backarc basin oceanic crusts to both the east and west. The thick crust is mainly due to a thickened lower crust, but especially thick crust>20km also has a thick middle crust. The uppermost mantle velocities beneath the KPR bathymetric high are less than 8.0km/s and sometimes accompanied by a slightly higher Vp of around 7.2km/s at the base of the crust. Deeper reectors beneath the KPR Moho were often observed and might be distributed discontinuously and regionally. These above characteristics are also found in the IzuOgasawara intra-oceanic island arc, the conjugate arc to the KPR.

2. The eastern side of the KPR corresponds to the transition to the Shikoku and Parece Vela Basins. The Vp models show that the transition zone is characterized by a slightly higher Pn velocity and thinner crust elsewhere in the backarc basins, which may be a feature produced during the initial rifting stage of the proto-IBM arc.

3. Thick sedimentary aprons are usually observed at the shallower part of the western slope of the KPR. The Vp models to the west of the KPR, however, vary depending on their regional tectonics.

In this paper, we have presented many seismic proles using the large amount of the seismic data obtained from the Japanese CSS Project. However, some aspects of these data have not yet been analyzed, such as other reections from crust and uppermost mantle, S-wave signals, etc. Further study including other geophysical and geological investigation is still required to construct and revise the Philippine Sea plate tectonic evolution models.

Authors contributions

AN, KK, and MO contributed to the analysis and interpretation of the seismic data. AN compiled the regional tectonic information and drafted the manuscript. All authors read and approved the nal manuscript.

Nishizawa et al. Earth, Planets and Space (2016) 68:30

Page 17 of 18

Acknowledgements

The authors gratefully acknowledge Prof. Emeritus J. Kasahara, and ex-members of Continental Shelf Surveys Co. Ltd., for data processing and analyses. Y. Katagiri and N. Watanabe and the members of the Continental Shelf Surveys Office, Hydrographic and Oceanographic Department, JCG, are thanked for the management of the seismic surveys. The manuscript beneted from constructive reviews and suggestions by anonymous reviewers and the editor Prof. T. Yamazaki. Careful English editing by the reviewer was very helpful. Most of the gures in this paper were produced using the GMT graphic package of Wessel and Smith (1998).

Competing interests

The authors declare that they have no competing interests.

Received: 10 August 2015 Accepted: 7 January 2016

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