Structural variation of the Bonin ridge revealed by modeling of seismic and gravity data
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A previous study of a longitudinal profile along the Bonin ridge concluded that remarkably thin forearc crust (< 10 km thick) along the northern half of the ridge indicates that the crust there was formed by forearc spreading during the initial stage of subduction along the Izu-Bonin intra-oceanic arc. However, a profile across the Bonin ridge shows a thicker crust. In this study, we construct a model that takes into account seismic and gravity data from both profiles. Re-modeling of the seismic data showed a north-south aligned area of thin crust (∼10 km thick) at the center of the Bonin ridge; this structure was confirmed by gravity data. The seismic data at the eastern end of the across-arc profile suggests that the crust thickens beneath the trenchward slope of the Bonin ridge. However, a petrological model suggests a trenchward extension of forearc oceanic crust that formed during the initial stage of subduction. Although further detailed investigation is required, we suggest that this contradiction can be explained either by the subduction of buoyant crust immediately beneath the forearc oceanic crust, or by the presence of a serpentinized mantle wedge beneath the forearc oceanic crust.
Key wordsSeismic imaging gravity forearc Bonin ridge subduction zone
As discussed in previous studies (e.g., Stern and Bloomer, 1992; Pearce et al., 1992; Ishizuka et al., 2006), the magmatism and structure of the forearc of the IBM system provide fundamental information about the initiation of subduction as well as magmatic processes in an infant arc. The most prominent topographic feature along the forearc in the Izu-Bonin region is the Bonin ridge (Fig. 1), a north-south trending massif about 400 km long and bounded by the Ogasawara trough to the west and the Izu-Bonin trench to the east. The oldest known volcanic activity in the IBM arc system occurred on the Bonin ridge at Chichi-jima and Muko-jima (Fig. 1). 40Ar/39Ar dates imply that there were brief periods of Eocene (46–48 Ma) boninitic volcanism on both islands, which were caused by the melting of depleted mantle at shallow levels, aided by the introduction of hydrous fluids from a sinking oceanic plate (Ishizuka et al., 2006). On the other hand, normal arc volcanism (i.e., tholeiitic-calcalkaline andesitic volcanism) occurred at Haha-jima, 40 km south of Chichi-jima, and boninite has not been reported there (e.g., Taylor and Nesbitt, 1995). 40Ar/39Ar dates show that the tholeiiticcalcalkaline andesitic volcanism at Haha-jima occurred at about 41–45 Ma, that is, 2–3 Ma after the boninitic magmatism at Chichi-jima and Muko-jima. According to Ishizuka et al. (2006), the petrological data indicate that the process of crustal formation in the Izu-Bonin forearc changed at around 45 Ma in response to a change of the composition and melting conditions of the sub-arc mantle.
2. Previous Seismic Studies
The importance of seismic data for understanding the crustal composition of island arcs has been recognized since a layer of P -wave velocity 6 km/s was identified under the northern Izu arc by Suyehiro et al. (1996). Since then, a series of high-resolution deep seismic studies in the IBM arc have presented new insights into the formation of arc crust (e.g., Kodaira et al., 2007a, b, 2008; Takahashi et al., 2007, 2008a; Calvert et al., 2008; Tatsumi et al., 2008; Sato et al., 2009). A seismic image along the volcanic front of the Izu arc (Kodaira et al., 2007a) shows a layer of arc crust (more than 30 km thick) for which the P-wave velocity (6.0–6.8 km/s) is greater beneath basaltic volcanoes than beneath rhyolitic volcanoes. This observation implies that this layer, interpreted to be of felsic to intermediate composition, has formed predominantly under the basaltic volcanoes. A seismic study along the rear-arc of the Izu arc (Kodaira et al., 2008) shows marked variations of crustal thickness that are attributed mainly to thickness variations of the middle crustal layer of P-wave velocity 6.0–6.8 km/s. Kodaira et al. (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.
Takahashi et al. (2009) provided an image from a seismic reflection/refraction profile across the central Izu-Bonin arc, which crossed three arcs of different ages: the Eocene forearc (Bonin ridge), the present arc (volcanic front), and the Oligocene rear-arc. For the Eocene arc crust, they observed seismic velocities of 6.4–6.6 and 6.8–7.4 km/s in the middle and lower crust, respectively, whereas those of the current volcanic front were slower (5.7–6.5 km/s for the middle crust and 6.7–7.1 km/s for the lower crust). According to known petrologic studies, these structural differences reflect different mechanisms of crustal growth in areas of basaltic and andesitic magmas.
A recent seismic study along the Bonin ridge (Kodaira et al., 2010) revealed striking structural variation along the Bonin ridge. The seismic data show a remarkably thin crust (<10 km thick) along the northern half of the Bonin ridge (beneath Chichi-jima and Muko-jima) and abrupt crustal thickening (to ∼20 km) toward the south of the ridge (beneath Haha-jima). Velocity-depth profiles of the thin forearc crust of the Bonin ridge were seismologically identical to those of typical oceanic crust. This observation strongly supports the view that forearc oceanic crust along the Izu-Bonin intra-oceanic arc was formed by forearc spreading during the initial stage of subduction (Kodaira et al., 2010).
However, Kodaira et al. (2010) pointed out that the seismic structure modeled to the north of Chichi-jima was not entirely consistent with the structure across the Bonin ridge reported by Takahashi et al. (2009). The ridge-parallel profile of Kodaira et al. (2010) and the cross-ridge profile of Takahashi et al. (2009) intersect between Chichi-jima and Muko-jima, where the ridge-parallel profile shows a crust of about 10 km thickness, and the cross-ridge profile shows a crust of about 18 km thickness. Kodaira et al. (2010) gave three possible explanations for this inconsistency: differences in the initial models used, the effect of smoothing during inversion, or edge effects in the modeling. However, they did not further investigate the differences of the structures derived from the two seismic profiles. In this study, we have re-modeled the seismic data from both the cross-ridge (SPr2) and ridge-parallel (OGr1) profiles to further examine the three-dimensional structure, and further constrained the resultant model with gravity modeling.
3. Re-modeling of Seismic Data
The two sets of seismic data (SPr2 and OGr1; Fig. 1) used in this study were acquired with the same seismic system (Takahashi et al., 2009; Kodaira et al., 2010). Eight 1500-in3 airguns (total volume 197 L) were fired at 200-m intervals along both profiles. The ocean bottom seismographs (OBSs) for both lines were 4.5-Hz three-component gimbal-mounted geophones and hydrophones, and 16-bit digital data were continuously recorded to a maximum frequency of 100 Hz (Shinohara et al., 1993; Kanazawa and Shiobara, 1994). OBSs were deployed at approximately 5-km intervals along both lines, 105 OBSs on line SPr2 and 110 OBSs on line OGr1. Detailed descriptions of data acquisition and processing are provided by Takahashi et al. (2008b, 2009) and Kodaira et al. (2010).
To assess the resolution of the model for line SPr2, we applied a checkerboard test (Fig. 7(b)) using the same procedure as used previously for lines OGr1 and SPr2 (Takahashi et al., 2009; Kodaira et al., 2010), but, here, we used a finer checkerboard pattern (i.e., a 10 km horizontal by 5 km vertical pattern to 15 km depth). The checkerboard test showed good recovery beneath the Bonin ridge for the final modification of the model for line SPr2, but the eastern edge of the model (520–570 km) was not resolved at depths greater than 10 km.
4. Gravity Modeling
To confirm the structures shown by seismic data around the Bonin ridge, we calculated gravity anomalies using density data derived from the seismic data and compared them to gravity data recorded over the Izu-Bonin arc by the Hydrographic Department of Japan and the Geological Survey of Japan. The shipboard gravity data are available via the NOAA National Geophysical Data Center. The observed free-air gravity anomaly data were merged with gravity anomaly data derived from satellite altimetry (Sandwell and Smith, 1997). During the gravity modeling in this study, we aimed to fit the long-wavelength gravity anomaly.
Free-air gravity data primarily reflect density contrasts caused by variations of seafloor topography, but sub-seafloor density structures can also produce gravity anomalies. We calculated the theoretical gravity anomaly that would represent the crustal structure which we determined from the seismic data. Two-dimensional density structures were created for each profile, including seafloor topography, as a set of rectangular prisms, each 0.5 km long × 0.5 km wide and horizontally infinite in the direction normal to the profile. The P-wave velocities were converted to densities using the velocity-density relationship of Ludwig et al. (1970) and the water density was set at 1030 kg/m3. The gravity anomaly was calculated using the formula of Blakely (1996). The density structures at both ends of the two profiles were extended by sufficiently long distances (300 km) to avoid artificial edge effects. It should be noted that the densities at the intersection are not exactly identical between the two profiles. This is because the original seismic velocity models at the intersection are not exactly the same. This difference of the seismic velocity may reflect several factors, such as local inhomogeneity and anisotropy which cannot be resolved by the observed seismic data.
The effect of the long-wavelength gravity anomaly produced by the subducting Pacific plate on the gravity model was estimated using a three-dimensional subducting slab model. The thickness of the slab was set at 90 km, and the density contrast between the slab and the surrounding mantle was assumed to be +65 kg/m3 in accordance with a previous study by Furuse and Kono (2003). A crustal layer of 2800–3000 kg/m3 density was placed above the dense slab. The subducting plate was modeled as a set of prisms of 10 km × 10 km widths. The depth of the slab was estimated from a deep seismic plane down to ∼500 km depth (International Seismological Centre, 2001) and a plate interface imaged from seismic data by Takahashi et al. (2009). Along the north-south profile (line OGr1), the position of the subducting plate at depths of ∼30–35 km was interpreted from seismic reflections (Kodaira et al., 2010). The density model for line OGr1 also incorporated the subducted plate at depths of ∼30–35 km.
5. Discussion and Conclusions
From our analyses of gravity data and seismic profiles along and across the Bonin ridge, we conclude that there is a north-south aligned zone of thin crust (∼10 km thick) in the center of the Bonin ridge, and that the width of the zone of thin crust is as little as ∼20 km. Although our checkerboard test indicated that our modeling did not well-resolve the eastern end of the SPr2 profile, it did suggest that the crust thickens beneath the trenchward slope of the Bonin ridge.
Forearc seafloor spreading has been proposed as the process most likely to have created the forearc crust along the IBM arc (e.g., Stern and Bloomer, 1992; Shervais, 2001; Stern, 2004; Ishizuka et al., 2006). According to Stern and Bloomer (1992), the forearc spreading stage occurred at 48–45 Ma during which the abrupt sinking of the Pacific plate along a fracture zone caused an adiabatic upwelling of the asthenosphere, which, in turn, produced the forearc oceanic crust. During this stage, melting of the depleted mantle at shallow levels aided by hydrous fluid from the sinking oceanic plate caused boninitic volcanism. This was followed by a transitional stage (45–41 Ma), wherein a transition to true subduction with a down-dip motion of the Pacific plate caused a reorganization of asthenospheric circulation that resulted in a cooling of the forearc and forced the magmatic axis to retreat because of mantle melting at deeper levels. This process caused tholeiitic-calcalkaline andesitic volcanism. During the subsequent stable subduction stage (41 Ma to present), the down-dip motion of the Pacific plate continued.
To explain the formation of ophiolite, Stern (2004) proposed a model involving subduction of a buoyant oceanic plateau beneath forearc oceanic crust. According to his model (see figure 3 of Stern, 2004), emplacement of an ophiolite above a buoyant crust apparently creates a thick crust on the trenchward side of the forearc. If the buoyant crust underlies forearc oceanic crust only on the trenchward side of the forearc (as shown in figure 3 of Stern, 2004), the crust would thicken trenchward. This model may also explain the strong gravity anomaly in the Bonin ridge area, because subduction of the buoyant crust would force uplift of the forearc. Another possible interpretation is that the model represents serpentinization of a forearc mantle wedge. A recent seismic tomography study by Nakajima et al. (2009) showed a significant decrease of seismic velocity at the tip of the mantle wedge at the northern end of the Izu arc. Although Nakajima et al. (2009) resolved the mantle wedge structure only at the northern edge of the Philippine Sea plate, they also found a region of low P-wave velocity and high Poisson’s ratio (V p /V s )atthe trenchward tip of the mantle wedge beneath the Kanto region, central Japan. They interpreted this structure to represent serpentinized mantle wedge. If the development of the serpentinized mantle wedge continues to the south along the Izu arc, the crust of the forearc may appear to be thick because the P-wave velocity of the serpentinized mantle is comparable to the P-wave velocity of the lower crust (e.g., Hyndman and Peacock, 2003). The Poisson’s ratio structure at the forearc of the Bonin arc is fundamental to identifying the most appropriate model. To obtain the V p /V s structure, future studies should include arc-parallel active-source profiles as well as passive seismic studies.
In conclusion, our re-modeling of both gravity data and cross-ridge and ridge-parallel seismic data along the Bonin ridge showed the existence of a 20-km-wide, north-south aligned zone of thin crust (∼10 km thick) along the axis of the Bonin ridge. This structure is consistent with a model whereby forearc oceanic crust was created by forearc spreading during the initial stage of arc crust formation. Although a more detailed investigation is needed, the trench-ward thickening of forearc crust revealed by our modeling is interpreted as either a result of the subduction of the buoyant crust beneath forearc oceanic crust, or the existence of a serpentinized mantle wedge for which the seismic velocity gives the impression of thick crust.
This study was funded by the Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, and partially supported by Grant-in-Aid for Creative Scientific Research (19GS0211) and Grant-in-Aid for Scientific Research (B) (20340122). We gratefully acknowledge the help of the captain, crew, and technical staff on board R/V Kaiyo during the acquisition of the seismic data.
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