Serpentinized Mantle Wedge Research Articles

Investigating Segmentation in Cascadia: Anisotropic Crustal Structure and Mantle Wedge Serpentinization from Receiver Functions

AbstractThe Cascadia subduction zone exhibits along‐strike segmentation in structure, processes, and seismogenic behavior. While characterization of seismic anisotropy can constrain deformation processes at depth, the character of seismic anisotropy in Cascadia remains poorly understood. This is primarily due to a lack of seismicity in the subducting Juan de Fuca slab, which limits shear wave splitting and other seismological analyses that interrogate the fine‐scale anisotropic structure of the crust and mantle wedge. We investigate lower crustal anisotropy and mantle wedge structure by computing P‐to‐S receiver functions at 12 broadband seismic stations along the Cascadia subduction zone. We observe P‐to‐SV converted energy consistent with previously estimated Moho depths. Several stations exhibit evidence of an “inverted Moho” (i.e., a downward velocity decrease across the crust‐mantle boundary), indicative of a serpentinized mantle wedge. Stations with an underlying hydrated mantle wedge appear prevalent from northern Washington to central Oregon, but sparse in southern Oregon and northern California. Transverse component receiver functions are complex, suggesting anisotropic and/or dipping crustal structure. To constrain the orientation of crustal anisotropy we compute synthetic receiver functions using manual forward modeling. We determine that the lower crust shows variable orientations of anisotropy along‐strike, with highly complex anisotropy in northern Cascadia, and generally NW‐SE and NE‐SW orientations of slow‐axis anisotropy in central and southern Cascadia, respectively. The orientations of anisotropy from this work generally agree with those inferred from shear wave splitting of tremor studies at similar locations, lending confidence to this relatively new method of inferring seismic anisotropy from slow earthquakes.

Mantle wedge serpentinization effects on slab dips

Mantle wedge serpentinization effects on slab dips

Imaging the nonvolcanic tremor zone beneath the San Andreas fault at Cholame, California using station-pair double-difference tomography

Nonvolcanic tremors have been detected deep beneath the San Andreas fault (SAF) near Cholame, California. The tremors are modulated by small stress changes induced by Earth tides and regional and teleseismic earthquakes, implying elevated pore fluid pressure in the tremor zone. In this area, tremors lie beneath the seismogenic zone, limiting the usefulness of local earthquakes for resolving seismic velocity properties in the tremor zone. Station-pair double-difference tomography that uses differential tremor arrival times is used, for the first time, to provide new detailed information about the complexity of the lower-crust and upper-mantle transition and the occurrence of deep tremors in zones of localized low shear wave velocity anomalies. The anomalies may represent zones containing high-pore pressure fluids that migrate into the tremor zone episodically from the southwest and that are ultimately derived from dehydration of deep serpentinized mantle wedge material. These results improve our understanding of the tremor process, why tremors are concentrated in specific zones along the SAF, and possible relationships between tremors and large historic earthquakes.

SIMS chlorine isotope analyses in melt inclusions from arc settings

The relative contribution of volatiles from altered oceanic crust, serpentinized mantle, and subducted sediments on the composition of primary arc magmas is poorly constrained. We measured the chlorine stable isotope composition in olivine-hosted melt inclusions in order to provide a first order constraint on the δ37Cl values of primary magmas, since melt inclusions are not or only little affected by near surface processes. Chlorine isotope analyses were obtained with a CAMECA IMS 1280-HR at the University of Lausanne. A series of six Cl-bearing glass standards with δ37Cl values of −1.1 to +1.7‰, SiO2 between 50 and 76wt% and Cl concentration between 0.15 and 3.25wt% were used for calibration. We determined that SiO2, Al2O3 and K2O in glasses are primarily responsible for the observed variations in instrumental mass fractionation.We obtained a δ37Cl reproducibility on the standard glasses typically better than 0.3‰ (2SD) and a total uncertainty, corresponding to the propagated error of SIMS and Pyro-IRMS uncertainty, of 0.4‰ (2SD). Melt inclusions from Lesser Antilles, Vanuatu and Aeolian arcs display ranges of δ37Cl values from −1.9 to +0.6‰, −1.7 to +0.4‰ and −3.4 to −1.4‰ respectively. Combined with Cl/K2O ratios, these data suggest that Cl addition mainly originates from serpentinites (lithospheric serpentinites or serpentinized mantle wedge) in the Lesser Antilles and Vanuatu arcs. In contrast, Cl added to the mantle wedge beneath the Aeolian Islands may record a higher proportion of fluids derived from the subducting sediments. The observed differences of δ37Cl values in melt inclusions from the Lesser Antilles and Vanuatu arcs and the Aeolian Islands may directly reflect the different subduction geometries present.

Seismic evidence for a cold serpentinized mantle wedge beneath Mount St Helens

Mount St Helens is the most active volcano within the Cascade arc; however, its location is unusual because it lies 50 km west of the main axis of arc volcanism. Subduction zone thermal models indicate that the down-going slab is decoupled from the overriding mantle wedge beneath the forearc, resulting in a cold mantle wedge that is unlikely to generate melt. Consequently, the forearc location of Mount St Helens raises questions regarding the extent of the cold mantle wedge and the source region of melts that are responsible for volcanism. Here using, high-resolution active-source seismic data, we show that Mount St Helens sits atop a sharp lateral boundary in Moho reflectivity. Weak-to-absent PmP reflections to the west are attributed to serpentinite in the mantle-wedge, which requires a cold hydrated mantle wedge beneath Mount St Helens (<∼700 °C). These results suggest that the melt source region lies east towards Mount Adams.

Solid-state flow and fluid infiltration in serpentinized mantle wedge

Solid-state flow and fluid infiltration in serpentinized mantle wedge

Brucite as an important phase of the shallow mantle wedge: Evidence from the Shiraga unit of the Sanbagawa subduction zone, SW Japan

Large parts of the shallow mantle wedge are thought to be hydrated due to release of fluids from the subducting slab and serpentinization of the overlying mantle rocks. If serpentinization proceeds under low SiO2 activity, brucite can be a major phase in the low-temperature (<450°C) part of the serpentinized mantle wedge, but only very few natural examples have been documented. A combined petrological, geochemical, and geological study shows that brucite is widely distributed in the wedge mantle-derived Shiraga metaserpentinite body in the Sanbagawa metamorphic belt of SW Japan. Thermodynamic modeling combined with bulk rock composition and point counting indicates that the original fully hydrated shallow parts of the Sanbagawa mantle wedge contained ~10–15vol.% brucite before the onset of exhumation of the Shiraga body and before peak metamorphic conditions. A distinct zone of brucite-free essentially monomineralic antigorite serpentinite occurs limited to a 100-m-thick marginal zone of the body. This indicates a limited degree of Si-metasomatism by slab-derived fluids in the shallow mantle wedge. The presence of brucite may strongly affect the H2O budget and mechanical properties of serpentinite; these should be taken into consideration when examining the behavior of the shallow mantle wedge.

Carbonation of subduction-zone serpentinite (high-pressure ophicarbonate; Ligurian Western Alps) and implications for the deep carbon cycling

Abstract Much of the long-term carbon cycle in solid earth occurs in subduction zones, where processes of devolatilization, partial melting of carbonated rocks, and dissolution of carbonate minerals lead to the return of CO2 to the atmosphere via volcanic degassing. Release of COH fluids from hydrous and carbonate minerals influences C recycling and magmatism at subduction zones. Contradictory interpretations exist regarding the retention/storage of C in subducting plates and in the forearc to subarc mantle. Several lines of evidence indicate mobility of C, of uncertain magnitude, in forearcs. A poorly constrained fraction of the 40–115 Mt/yr of C initially subducted is released into fluids (by decarbonation and/or carbonate dissolution) and 18–43 Mt/yr is returned at arc volcanoes. Current estimates suggest the amount of C released into subduction fluids is greater than that degassed at arc volcanoes: the imbalance could reflect C subduction into the deeper mantle, beyond subarc regions, or storage of C in forearc/subarc reservoirs. We examine the fate of C in plate-interface ultramafic rocks, and by analogy serpentinized mantle wedge, via study of fluid–rock evolution of marble and variably carbonated serpentinite in the Ligurian Alps. Based on petrography, major and trace element concentrations, and carbonate C and O isotope compositions, we demonstrate that serpentinite dehydration at 2–2.5 GPa, 550 °C released aqueous fluids triggering breakdown of dolomite in nearby marbles, thus releasing C into fluids. Carbonate + olivine veins document flow of COH fluids and that the interaction of these COH fluids with serpentinite led to the formation of high-P carbonated ultramafic-rock domains (high-P ophicarbonates). We estimate that this could result in the retention of ∼0.5–2.0 Mt C/yr in such rocks along subduction interfaces. As another means of C storage, 1 to 3 km-thick layers of serpentinized forearc mantle wedge containing 50 modal % dolomite could sequester 1.62 to 4.85 Mt C/yr. We stress that lithologically complex interfaces could contain sites of both C release and C addition, further confounding estimates of net C loss at forearc and subarc depths. Sites of C retention, also including carbonate veins and graphite as reduced carbonate, could influence the transfer of slab C to at least the depths beneath volcanic fronts.

Receiver function images of the Hellenic subduction zone and comparison to microseismicity

Abstract. New combined P receiver functions and seismicity data obtained from the EGELADOS network employing 65 seismological stations within the Aegean constrained new information on the geometry of the Hellenic subduction zone. The dense network and large data set enabled us to estimate the Moho depth of the continental Aegean plate across the whole area. Presence of a negative contrast at the Moho boundary indicating the serpentinized mantle wedge above the subducting African plate was seen along the entire forearc. Furthermore, low seismicity was observed within the serpentinized mantle wedge. We found a relatively thick continental crust (30–43 km) with a maximum thickness of about 48 km beneath the Peloponnese Peninsula, whereas a thinner crust of about 27–30 km was observed beneath western Turkey. The crust of the overriding plate is thinning beneath the southern and central Aegean and reaches 23–27 km. Unusual low Vp / Vs ratios were estimated beneath the central Aegean, which most likely represent indications on the pronounced felsic character of the extended continental Aegean crust. Moreover, P receiver functions imaged the subducted African Moho as a strong converted phase down to a depth of about 100 km. However, the converted Moho phase appears to be weak for the deeper parts of the African plate suggesting nearly complete phase transitions of crustal material into denser phases. We show the subducting African crust along eight profiles covering the whole southern and central Aegean. Seismicity of the western Hellenic subduction zone was taken from the relocated EHB-ISC catalogue, whereas for the eastern Hellenic subduction zone, we used the catalogues of manually picked hypocentre locations of temporary networks within the Aegean. Accurate hypocentre locations reveal a significant change in the dip angle of the Wadati–Benioff zone (WBZ) from west (~ 25°) to the eastern part (~ 35°) of the Hellenic subduction zone. Furthermore, a zone of high deformation can be characterized by a vertical offset of about 40 km of the WBZ beneath the eastern Cretan Sea. This deformation zone may separate a shallower N-ward dipping slab in the west from a steeper NW-ward dipping slab in the east. In contrast to hypocentre locations, we found very weak evidence for the presence of the slab at larger depths in the P receiver functions, which may result from the strong appearance of the Moho multiples as well as eclogitization of the oceanic crust. The presence of the top of a strong low-velocity zone at about 60 km depth in the central Aegean may be related to the asthenosphere below the Aegean continental lithosphere and above the subducting slab. Thus, the Aegean mantle lithosphere seems to be 30–40 km thick, which means that its thickness increased again since the removal of the mantle lithosphere about 15 to 35 Ma ago.

Crustal large-scale serpentinized mantle peridotite body in the Sulu ultrahigh-pressure metamorphic belt, eastern China: Evidence from gravity and magnetic anomalies

From analysis of the geological and geophysical data (gravity, magnetic, seismic and petrophysics), we propose that geophysical anomalies are produced by a serpentinized mantle peridotite body (SMPB) situated in the middle to lower crust in the Sulu Belt. The SMPB was formed by crustal emplacement of mantle peridotites accompanied by ultrahigh-pressure (UHP) metamorphism. Our finding suggests an emplacement mechanism for the serpentinized mantle wedge (SMW), early in the subduction process. This is different from the classic view, which holds that the serpentinized forearc mantle is formed by in situ hydration processes (Blakely et al., 2005). The petrophysical properties of the SMPB are similar to those of the serpentinized forearc mantle or SMW in modern subduction-zones worldwide, but the formation mechanisms for SMPB and SMW are different. This observation is important for understanding the geodynamic processes that operated in the large UHP metamorphic belt in the Dabie-Sulu area, eastern China.

Non-volcanic seismic swarm and fluid transportation driven by subduction of the Philippine Sea slab beneath the Kii Peninsula, Japan

To understand the mechanism of an intensive non-volcanic seismic swarm in the Kii Peninsula, Japan, we used a dense seismic linear array to measure fine-scale variations of seismic velocities and converted teleseismic waves. A low-velocity anomaly confined to just beneath the seismic swarm area is clearly imaged, which correlates spatially with an uplifted surface area and a highly conductive and strong attenuative body. These results suggest that fluids such as partial melt or water are present beneath this non-volcanic seismic swarm area. It is notable that the island arc Moho below the seismic swarm area is at a depth of approximately 32 km in the northern part of the seismic swarm area and shallows to approximately 20 km towards the south, due to the raised structure of the serpentinized mantle wedge. In addition, we show that the hydrated oceanic crust of the subducting Philippine Sea slab is characterized by low velocities with a high Poisson's ratio at depths of less than 40 km. In contrast, dehydration conversion from oceanic basalt to eclogite takes place at depths greater than 50 km. Water released from the subducting oceanic crust could cause serpentinization of the mantle wedge and infiltration into the forearc base of the overlying plate. The interaction between dehydration of the subducting oceanic crust and hydration of the mantle wedge and overlying plate exerts an important role in driving the non-volcanic seismic swarm activity in the Kii Peninsula.

Thermal structure of subduction zones: effects of hydrolytic weakening and serpentinization of mantle wedge

Thermal structure of subduction zones: effects of hydrolytic weakening and serpentinization of mantle wedge

Downdip distributions of slow slipand serpentinization of mantle wedge in the Japanese subduction zones

Downdip distributions of slow slipand serpentinization of mantle wedge in the Japanese subduction zones

Deep long‐period earthquakes west of the volcanic arc in Oregon: Evidence of serpentine dehydration in the fore‐arc mantle wedge

AbstractHere we report on deep long‐period earthquakes (DLPs) newly observed in four places in western Oregon. The DLPs are noteworthy for their location within the subduction fore arc: 40–80 km west of the volcanic arc, well above the slab, and near the Moho. These “offset DLPs” occur near the top of the inferred stagnant mantle wedge, which is likely to be serpentinized and cold. The lack of fore‐arc DLPs elsewhere along the arc suggests that localized heating may be dehydrating the serpentinized mantle wedge at these latitudes and causing DLPs by dehydration embrittlement. Higher heat flow in this region could be introduced by anomalously hot mantle, associated with the western migration of volcanism across the High Lava Plains of eastern Oregon, entrained in the corner flow proximal to the mantle wedge. Alternatively, fluids rising from the subducting slab through the mantle wedge may be the source of offset DLPs. As far as we know, these are among the first DLPs to be observed in the fore arc of a subduction‐zone system.

Overriding plate structure of the Nicaragua convergent margin: Relationship to the seismogenic zone of the 1992 tsunami earthquake

We present 2‐D seismic velocity models and coincident multichannel seismic reflection images of the overriding plate and the inter‐plate boundary of the Nicaragua convergent margin along two wide‐angle seismic profiles parallel and normal to the trench acquired in the rupture area of the 1992 tsunami earthquake. The trench‐perpendicular profile runs over a seamount subducting under the margin slope, at the location where seismological observations predict large coseismic slip. Along this profile, the igneous basement shows increasing velocity both with depth and away from the trench, reflecting a progressive decrease in upper‐plate rock degree of fracturing. Upper mantle‐like velocities are obtained at ∼10 km depth beneath the fore‐arc Sandino basin, indicating a shallow mantle wedge. A mismatch of the inter‐plate reflector in the velocity models and along coincident multichannel seismic profiles under the slope is best explained by ∼15% velocity anisotropy, probably caused by subvertical open fractures that may be related to fluid paths feeding known seafloor seepage sites. The presence of a shallow, partially serpentinized mantle wedge, and the fracture‐related anisotropy are supported by gravity analysis of velocity‐derived density models. The downdip limit of inter‐plate seismicity occurs near the tip of the inferred mantle wedge, suggesting that seismicity could be controlled by the presence of serpentinite group minerals at the fault gouge. Near the trench, the inferred local increase of normal stress produced by the subducting seamount in the plate boundary may have made this fault segment unstable during earthquake rupture, which could explain its tsunamigenic character.

Oxygen isotope zoning in garnets from Franciscan eclogite blocks: evidence for rock–buffered fluid interaction in the mantle wedge

The oxygen isotope compositions of eclogite and amphibolite garnets from Franciscan Complex high-grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories. SIMS oxygen isotope analysis of single garnets from five eclogite blocks from three localities (Ring Mountain, Mount Hamilton, and Jenner Beach) shows an abrupt decrease in the δ18O value by ~1–3 ‰ from core to rim at a distance of ~120 ± 50 μm from the rim in nine out of the 12 garnets analyzed. In contrast, amphibolite garnets from one block (Ring Mountain) analyzed show a gradual increase in δ18O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ18O in eclogite garnet cores range from 5.7 to 11.6 ‰, preserving the composition of the eclogite protolith. The abrupt decrease in the δ18O values of the garnet rims to values ranging from 3.2 to 11.2 ‰ suggests interaction with a lower δ18O fluid during the final stages of growth during eclogite facies metamorphism (450–600 °C). We hypothesize that this fluid is sourced from the serpentinized mantle wedge. High Mg, Ni, and Cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock. Oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185–240 °C at Ring Mountain and Mount Hamilton. Rind formation temperatures together with the lower δ18O garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low-temperature retrogression. We suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge. As subduction continues, metasomatic exchange between high-grade blocks and surrounding ultramafic rock is recorded in low δ18O garnet rims, and later as temperatures decrease, with rind formation.

Behavior of fluid-mobile elements in serpentines from abyssal to subduction environments: Examples from Cuba and Dominican Republic

Serpentinites from subduction environments represent an important sink for fluid-mobile elements. In order to constrain geochemical behavior of fluid-mobile elements hosted by serpentine phases during subduction processes, we carried out a geochemical study (trace elements and Pb isotopes) of a series of serpentinites and cumulates from the accretionary wedge of Greater Caribbean (Cuba and Dominican Republic). The trace element compositions of the primary and alteration-related phases were analyzed in situ using LA–HR-ICP-MS techniques. The studied samples represent parts of the subducted proto-Atlantic oceanic lithosphere, which has experienced low to high grade metamorphism (greenschist to eclogite facies), before being exhumed; a subset of these samples were derived from the mantle wedge. This sampling provides the opportunity to trace the chemical mobility of fluid-mobile elements during prograde metamorphism along a cold geotherm in an oceanic subduction setting.Serpentinites display strong enrichment in fluid-mobile elements indicating extensive fluid–rock interaction. In situ analyses allow distinction of three types of serpentines related to the nature of primary minerals (olivine, ortho- or clinopyroxene). Compositions of subducted samples, especially in fluid-mobile elements, are relatively close to those of abyssal peridotites without noticeable evidence of mobility for trace elements during subduction-related prograde metamorphism, with the exception of B. This confirms that the observed enrichment results from seawater/peridotite interactions during residence time in the ocean. It also suggests that most mobile elements stored in serpentine minerals are immobile during subduction processes. A major consequence of this observation is that serpentine minerals are a good sink for mobile elements in subduction zones, until their dehydration. Additionally, Pb isotopes and over-enrichment in As–Sb in high-grade subducted serpentines (antigorite) suggest the contribution of a sedimentary component during a secondary hydration taking place at the lizardite/antigorite transition. We propose that this new serpentinization event, taking place at greater depth, results from mixing between sediments and serpentinites in the subduction channel.Mantle wedge serpentinites present imprints of hydrothermal fluids: they are B-rich but without strong enrichment in As and Sb, and show evidence for moderate contributions of a radiogenic Pb-component. This suggests that the fluids that produced the mantle wedge serpentinites derived from the dehydration of the oceanic crust, with moderate to no contribution of sediments. We posit that mantle wedge serpentinization took place around 20–25km depth: at such depth and temperature conditions (T>200°C), the subducted sediments still released their B-rich pore fluids while their structural water incorporated in hydrous minerals (phengite, lawsonite) remained stable. The existence of various potential reservoirs for fluid-mobile elements in subduction zone environments (subducted serpentinites, mantle wedge serpentinites, as well as subducted sediments and altered oceanic crust) that potentially release their fluids at different depths has strong implications for arc lava formation.

Petrogenetic relationships between jadeitite and associated high-pressure and low-temperature metamorphic rocks in worldwide jadeitite localities: a review

Jadeitite-bearing serpentinite-matrix melange is distributed in the Caribbean (Guatemala, Cuba, and Dominican Republic), circum-Pacific (Japan, Western USA, and Papua New Guinea), Alpine-Himalayan (Italy, Iran, Greece, and Myanmar), and Caledonian (Russia and Kazakhstan) orogenic belts, and always contains high-pressure, low-temperature (H P -L T ) metamorphic rocks. There are also jadeitite xenoliths in kimberlitic pipes in the Colorado Plateau (USA). The oldest occurrences of jadeitite are Early Paleozoic in Japan, Russia, and Kazakhstan, suggesting subduction-zone thermal structures evolved the necessary high pressure/temperature conditions for jadeitite formation since Early Paleozoic; the youngest occurrence is a xenolith from the Colorado Plateau. Major occurrences consist principally of fluid precipitates (P-type) that infiltrated the mantle wedge; fewer occurrences document metasomatic replacement (R-type) of plagiogranite, metagabbro and eclogite, and both types may be possible in the same occurrence or system. The P-T conditions for jadeitite formation can be extended beyond the previously argued limits of blueschist-facies conditions. Some jadeitite formed at epidote amphibolite and others at eclogite facies conditions. Available geochronological data of both jadeitite and associated H P -L T rock show temporal discrepancies between jadeitite formation and H P -L T metamorphism at some localities. The close association between older jadeitite and younger H P -L T rock in a single melange complex implies different histories for the subduction channel and jadeitite-bearing melange. Jadeitite-bearing serpentinite melange can stay at the mantle wedge for a considerable time and, as a result, experience multiple fluid-infiltration events. The subduction channel can occasionally incorporate overlying serpentinized mantle wedge material due to tectonic erosion. With time, the disrupted mantle wedge containing jadeitite veins is mixed with younger blueschists, exhumed eclogites and various fragments of suprasubduction-zone lithologies. Consequently, recrystallization and re-precipitation of jadeitite are reactivated along a slab–mantle wedge interface. All these possible scenarios and their combinations yield a complicated petrological record in jadeitite. With further investigation, the rock association of jadeitite–H P -L T metamorphic rocks–serpentinite has the potential to yield a greater understanding of subduction channels and overlying mantle wedge.

Thermal control of the seismogenic zone of southern central Chile

We developed thermal models for the Chile subduction zone along two profiles at 38.2°S and 42°S within the rupture area of the 1960 M = 9.5 Valdivia earthquake and south of the 2010 M = 8.8 Maule earthquake. The age difference of the subducting Nazca Plate has a major impact on the thermal regime, being much younger and hotter in the south. Seafloor heat flow observations confirm this difference but also indicate that in the southern area, heat advection at the outer rise cools the incoming plate. Heat flow values derived from the depth of gas hydrate bottom-simulating reflectors are in general agreement with probe and borehole measurements. The positions where the plate interface reaches temperatures of 100–150°C and 350–450°C differ between the two profiles. If these temperatures control the updip and downdip limits of the interplate seismogenic zone, the seismogenic zone widens and shifts landward to greater depths from south to north. Observed microseismicity, however, seems to fade at temperatures much lower than 350–450°C. This discrepancy can be explained in three alternative ways: (1) deformation in a thick subduction channel controls the seismic/aseismic transition; (2) microseismicity recorded over a limited time period does not represent the rupture depth of large interface earthquakes; or (3) the serpentinized mantle wedge controls the downdip limit.

Structural variation of the Bonin ridge revealed by modeling of seismic and gravity data

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.