Gondwana Research. Pressure temperature conditions of ongoing regional metamorphism beneath the Japanese Islands

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1 Gondwana Research 16 (2009) Contents lists available at ScienceDirect Gondwana Research journal homepage: Pressure temperature conditions of ongoing regional metamorphism beneath the Japanese Islands S. Omori a,, S. Kita b, S. Maruyama a, M. Santosh c a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo , Japan b Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Sendai, Miyagi, , Japan c Department of Natural Environmental Science, Faculty of Science, Kochi University, Kochi , Japan article info abstract Article history: Received 6 March 2009 Received in revised form 3 July 2009 Accepted 5 July 2009 Available online 14 July 2009 Keywords: Subduction zone Tectonics Ongoing regional metamorphism Dehydration-induced earthquake hypothesis Japanese Islands We evaluate the pressure temperature (P T) conditions of ongoing regional metamorphism at the top of the oceanic crust of the subducted Pacific and Philippine Sea plates through a combination of phase diagrams and hypocenter distribution and based on the dehydration-induced earthquake hypothesis. The brute-force method was employed to find the best match thermal structure to link the hypocenter distribution and dehydration. The estimated thermal structure varies far from the values obtained from numerical simulation. Our estimates are consistent with the qualitative physical prediction for the variation of temperature in different subduction zones and provide quantitative constraints for the models. In northeastern Japan, the P T path for the top of the oceanic crust turns to the high-t side at a depth of around 90 km. The depth corresponds to the location of the volcanic front and an active convection of the wedge mantle below this depth is suggested. Our computations also reveal the effect of an exceptional scenario beneath the Kanto region. The temperature in the Kanto region, where the cold lid of the Philippine Sea plate prevents heating by the return-flow of mantle wedge above, is much lower than that of northeastern Japan. The subduction of younger Philippine Sea plate leads to a higher-temperature in the oceanic crust. In the central Shikoku region, the thermal structure exhibits high-t/p nature. Heating by shear deformation can explain the high-t/p path in the depth range from 20 to 35 km. The Kyushu area shows moderate type T/P path reaching up to eclogite facies conditions. In the Kii and central Shikoku region, the thermal structure exhibits high-t/p nature. However, the absolute values for the areas seem to have problem in physical context. Our results has risen the significance of sediment subduction in the southwest Japan and requirement for further improvements in this technique including the aspect of variation of the bulk composition of the subducted material International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction Metamorphic rocks exhumed to the surface of the Earth preserve important information on the depths related to the physicochemical conditions and processes associated with crust mantle tectonics (e.g., Spear, 1993; Masago et al., 2009; Zhang et al., 2009) as well as the plate tectonic architecture of orogenic belts (e.g., Miyashiro, 1994; Goscombe and Gray, 2009; Santosh et al., 2009). Although, metamorphic rocks suffer variable degrees of retrogression during their exhumation and thus loose much of the critical information on the processes at depth, a careful examination can lead to potential information on the tectonic history. However, it is also suggested that the exhumation of the regional metamorphic rocks, particularly in subduction zones, occurs within a limited tectonic environment (e.g. Maruyama et al., Corresponding author. address: omori@geo.titech.ac.jp (S. Omori). 1996; Aoki et al., 2008), and therefore the tectonic record decoded from the metamorphic signature might also be limited. Regional metamorphism is an ongoing process in the active subduction zones, and is closely related to the processes operating in the subduction factory (Tatsumi and Kogiso, 2003) such as arc-back arc magmatism, continent crust formation, and material cycle. The P T conditions of ongoing metamorphism are therefore critical factors to decipher the subduction process. It is expected that the thermal architecture of a subduction zone would vary according to its properties such as geometry, age of the plate, and history of the subduction zone. In previous studies, thermomechanical simulations of the subduction zone have been employed to estimate the thermal constitution of various subduction zones (e.g. Peacock, 1996; Iwamori, 1998; Gerya and Yuen, 2003). In this paper, we present an alternative method to estimate the temperature of ongoing regional metamorphism in active subduction zone, by the application of the inversion method from the distribution of earthquake hypocenters and on the basis of the dehydration-induced earthquake hypothesis (e.g. Omori et al., 2002, 2004). We focus our study on the ongoing regional metamorphism beneath the Japanese X/$ see front matter 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi: /j.gr

2 S. Omori et al. / Gondwana Research 16 (2009) Fig. 1. Quaternary geology and tectonic environment of the Japanese Islands. The locations of the cross-sections given in Figs. 3 5 are also shown. The Japanese Islands are underlain by the subducting Pacific and Philippine Sea Plates. Contour depths of the subducting oceanic slab are also shown. The iso-depth contours of the Pacific slab (Hasegawa et al., 1994; Nakajima and Hasegawa, 2006) are shown by the green curves with an interval of 50 km, and those of the compromised model of the Philippine Sea slab (Kwang et al., 2004; Nakajima et al., 2009) are shown by blue curves with an interval of 10 km. The area of slab slab contact where the bottom of the Philippine Sea slab is in contact with the upper surface of the Pacific slab (Nakajima et al., 2009) is shaded by light green. Islands, where the Pacific and Philippine Sea plates are subducted. The metamorphic process that we consider in this work is at a depth range from about 30 km to 150 km, a region which has been defined as the metamorphic factory by Maruyama et al. (2009). Dehydration embrittlement experiments of mantle material have shown that the dehydration of serpentinite could induce brittle deformation in a rock, and therefore the dehydration in the subducted slab should have a genetic relation to the intraslab earthquakes (Raleigh and Paterson, 1965; Meade and Jeanloz, 1991; Dobson et al., 2002; Jung et al., 2004). Following the experimental works, several studies have been conducted on the relation between the origin of the double seismic zone and position of the dehydration reactions in the subducting slab peridotite (e.g., Nishiyama, 1992; Seno and Yamanaka, 1996; Peacock, 2001; Omori et al., 2002; Yamasaki and Seno, 2003). It was also shown that the deep earthquakes in the mantle transition zone have a close link to the dehydration of the dense hydrous Mgsilicates in the subducted slab (Omori et al., 2004). The concept of the dehydration-induced earthquake was extended to the seismicities in the mid oceanic ridge basalt (MORB) part of the subducted oceanic crust (Hacker et al., 2003; Kita et al., 2006). The temperature of subducted slab is a critical factor in the study of dehydration in the subducted slab, because most of the dehydration Table 1 Geologic and geometric properties of the subducted plate. Area Age (Ma) Subduction angle (degree) Subduction speed (cm/y) Notes References NE Japan (A) , 2 Kanto (B) PHS plate 1, 2 overridden Kanto (C) PHS plate 1, 2 overridden Kii (D) ca , 4 Central Shikoku (E) , 4 Kyushu (F) N , 4 1: Gorbatov and Kostoglodov (1997), 2: Hasegawa et al. 2009, 3: Seno and Maruyama (1984); 4: Hirose et al. (2008).

3 460 S. Omori et al. / Gondwana Research 16 (2009) reactions are temperature dependent. In order to estimate the temperature of the subducted slab, most of the previous studies employed numerical simulation and attempted to estimate the thermal structure in specific subduction zones such as those of northeast Japan, southwest Japan, Kyushu and Cascadia (Peacock, 2001; Hacker et al., 2003; Yamasaki and Seno, 2003; Abers et al., 2006; Iwamori, 2007). Consequently, several attempts to correlate the double seismic zones and dehydration of serpentinite in the slab using the thermal structures obtained by the thermomechanical simulations. However, in most previous cases, the location of the dehydration reactions was not consistent with the hypocenter distribution in the slab; the discrepancy seems to be too large when the uncertainty of hypocenter location and error bar of P T estimate were taken into account. Omori et al. (2002) pointed out this problem and considered that the discrepancy between the location of dehydration and seismicities in the slab arose from incompleteness in the thermomechanical model of subduction zone to estimate the thermal structure of the slab. Subsequently, they proposed a logical inversion to estimate temperature in the subducted slab by making a link between hypocenter distributions of the double seismic zones and temperature of dehydration obtained from phase diagrams, on the basis of the dehydration-induced earthquake hypothesis. Alternate attempts were also made to evaluate the link between the hypocenter distribution and dehydration reactions in the subducted slab by Hacker et al. (2003). These workers empirically corrected a phase Fig. 2. Cross sections of study areas: a) NE Japan, b) Kanto-1, c) Kanto-2, d) Kii, e) central Shikoku, and f) Kyushu. Plate boundaries are drawn based on seismic tomographies by Nakajima et al. (2009) for a c, and by Hirose et al. (2008) for e f. Black and red cross denotes hypocenters. Red triangle corresponds to active volcano. Locations of the areas are shown in Fig. 1. In the Kanto area, a lid of Philippine Sea Plate occupies space above the Pacific plate.

4 S. Omori et al. / Gondwana Research 16 (2009) Fig. 2 (continued). diagram for MORB+H 2 O to provide the link between dehydration and earthquakes in the oceanic crust beneath the thermal structure computed from numerical simulation. In this contribution, we estimate the temperature in the subducted MORB crust under the zone of ongoing metamorphism beneath the Japanese Islands using an inverse logic in linkage among hypocenter distribution, location of dehydration, and temperature. We employ computed phase diagrams and high-precision hypocenter data around the Japanese Islands to estimate the best-fit P T path in the subducting MORB crust by inferring a close link between the metamorphic dehydration and earthquakes. 2. Study area, tectonic environments, and earthquakes The regions examined in the present work are NE Japan, Kanto, Kii, central Shikoku, and Kyushu (Fig. 1). Geometric and geologic properties of the subducting plate in each area vary according to the history of development of the Pacific plate and the Philippine Sea plate (Table 1). In NE Japan and Kanto regions, the Pacific plate with an age of about 130 Ma is subducting at moderate angle of The most distinct signature of the Kanto area compared with that of NE Japan is that the Philippine Sea plate is overriding onto the Pacific plate under the Kanto Mountain. Such geometry has resulted in a complex threedimensional topography of the Philippine Sea plate beneath this area, and it has been suggested that the Philippine Sea plate might have markedly influenced the thermal framework of the Pacific plate (Hasegawa et al., 2007). The Philippine Sea plate is being subducted in Kii, central Shikoku, and Kyushu areas. The plate has much complex history of development as compared to the Pacific plate. Seno and Maruyama (1984) described the geologic history of the Philippine Sea plate as follows. The western Philippine basin is separated by Kyushu Palau arc, which

5 462 S. Omori et al. / Gondwana Research 16 (2009) and S-wave tomography (Nakajima et al., 2009). The major difference with the NE Japan domain is the presence of Philippine Sea plate under the forearc region between arc crust and the underlying subducting Pacific slab. Earthquake clusters in the Pacific oceanic crust shift 20 km deeper as compared to the case discussed above for NE Japan. The region where the earthquake nest occurs in this case is at about km depth. In addition, the volcanic front shifts from 270 km in the earlier case to 330 km, around 60 km westward. The different modes of subduction of the Philippine Sea plate and its hypocenter distributions are shown in the cross-sections in Fig. 2d, e and f. In the cross-section along Kii Peninsula (Fig. 2d), the Median Fig. 3. H 2 O content maps for MORB+H 2 O model bulk composition up to 5 GPa, 900 C. Bulk composition was fixed to Na 2 O:CaO:MgO:FeO:Al 2 O 3 :SiO 2 =2.6:11.1:7.6:10.6:15.7: 50.6 in weight and excess H 2 O. Dashed curve is the dehydration solidus after Schmidt and Poli (1998). Gray dashed curves correspond to boundary among representative metamorphic facies; Ec: eclogite, AmEc: amphibole eclogite, BS: blueschist, LwBS: lawsonite blueschist, GS: greenschist, EA: epidote amphibolite, and Am: amphibolite. was a transform fault that turned into an active subduction zone in the Tertiary to generate the Kyushu Palau island arc, which demised at 30 Ma. To the west of this arc, Philippine Sea plate is older than approximately 40 Ma. The oldest portion dates back to at least 100 Ma. The youngest portion lies in the middle of the western Philippine Sea, and is about 40 Ma old. The eastern Philippine Sea is divided into the Central Shikoku Palace Vera Basin and the central portion is as young as 8 Ma, which was a spreading center that died at 8 Ma. Symmetric magnetic stripes indicate that the spreading started approximately 30 Ma ago, and therefore the eastern portion should be older than this age. Thus the Philippine Sea plate has a complex history, not only in the different age segments, but also in constitution. For example, back-arc basin basalts, composed of andesites and granites, are subducting under SW Japan. The mode of subduction is also remarkably different according to the different ages and different lithologies. Under Kyushu, steep subduction occurs similar to the case of NE Japan, although the constituent lithology is different due to dominant granites and arc basement. In the major part of SW Japan, young and buoyant Philippine Sea plate is subducting, but locally with moderate angle of subduction as in Kii Peninsula (about 45 dip; Kwang et al., 2004). To the east underneath Kanto, again the older slab is subducting, but with buoyant Izu Mariana acidic crust. Under the Kanto Mountain, a complex three-dimensional topography is present (Nakajima et al., 2009). Fig. 2a shows the cross-section for the NE Japan domain (section A) illustrating the hypocenter distribution of intraslab earthquakes and S-wave tomography as studied by Kita et al. (2006). The double seismic plane, which was first documented in this area (Umino and Hasegawa, 1975; Hasegawa et al., 1978; Hasegawa et al., 2009), is distinct in the slab peridotite. Earthquakes in the MORB layer exhibit clustered distribution; a notable cluster is observed at a depth range of 40 to 60 km, and the maximum depth of the seismicity in the oceanic crust is limited to 120 km, in the observation range. The E W cross-section in the Kanto region (section B and C) is shown in Fig. 2b and c with hypocenter locations of the intraslab earthquakes Fig. 4. Cross sections of the subduction zones showing depth distributions of hypocenters in the MORB crust; a) NE Japan, b) Kanto-1, c) Kanto-2, d) Kii, e) central Shikoku, and f) Kyushu. Locations of the sections are shown in Fig. 1. Red curve denote 2 km layer from slab surface and assumed to be the MORB crust. Dots and hatched area represent single seismic event and hypocenter cluster, respectively. Depth distribution of the earthquakes in the MORB crust is shown by pink stripes. Thick black line and red triangle denote land area on the surface and active volcano, respectively.

6 S. Omori et al. / Gondwana Research 16 (2009) linking dehydration reactions, the locations of which are calculated using internally consistent thermodynamic dataset of rock forming minerals and fluids. A brute-force algorithm identifies the best-fit thermal structure, which links the hypocenter distribution and dehydration reactions. It is assumed that the dehydration reactions proceed at nearly equilibrium P T conditions. This assumption is justified empirically in that progressive regional metamorphism corresponds to the progress of dehydration reactions (e.g., Spear, 1993; Miyashiro, 1994) Phase diagram The phase equilibrium calculation was done using perple X program by Connolly (2005) with the internally consistent thermodynamic dataset of Holland and Powell (1998 and updates) and builtin solid solution models in perple X. Phases included in the calculation are taken from the default set. A model system for MORB+H 2 Owas assumed for the Na 2 O CaO MgO FeO Al 2 O 3 SiO 2 H 2 O phase diagram. The MORB composition was taken from Dick et al. (2000) and adopted to the model system as Na 2 O:CaO:MgO:FeO:Al 2 O 3 : Fig. 4 (continued). Tectonic Line is located 200 km away from the Nankai trough (trench). The Daisen active volcano stands 370 km away from the trench. About 40 km thick Philippine Sea plate, which is as young as 10 Ma is subducting below the Kii peninsula. Seismogenic zone starts 100 km away from the trench and ends near the Median Tectonic Line (MTL). A very narrow and thin mantle wedge is present in the whole Shikoku region and the Seto Inland Sea. The age of subducted Philippine Sea plate is approximately 20 Ma; it is young and buoyant with a shallow dip of about 10 to 20 (Fig. 2e). At the western end of the land area near by the Daisen volcano, the top of the underlying Philippine Sea plate is at about 60 km depth and frequent occurrence of seismicity is restricted underneath the Median Tectonic Line, starting from the location underneath Shikoku about 100 km far from the active trench (Nankai trough) (Zhao et al., 2004). The profile across the Kyushu Island (Fig. 2f) shows that the Philippine Sea plate here is as thick as the case in NE Japan, around 80 km, aged about 90 Ma, and is subducting at an angle over 45. Frequent seismicity occurs in the depth range of 50 to 80 km on the subducted MORB crust and seismicities in the slab peridotite are confirmed down to ca. 200 km. The Sakurajima active volcano is located 220 km from the trench, which may correspond to the socalled volcanic front formed by subduction. 3. Dataset and method To estimate the temperature of the ongoing regional metamorphism, we take into account the concept of the extended dehydrationinduced earthquake hypothesis within the subducting slab (Omori et al., 2002, 2004) that takes into account the dehydration in the slab induce earthquake. In the present study, we have estimated the thermal structure by an inversion from the hypocenter observation Fig. 5. a) Diagram showing the method for geometric generation of the model P T paths. A model path is defined by five nodes (N 0 4 ). Node N 0 is defined by initial P T condition (P 0, T 0 ). Location of the next node to N i is defined by an angle (a i ) and distance (r i ) from the node Ni. The last node N 4 is located on the maximum limit of the P T range of interest. P 0, T 0, a 1 4, and d 1 3 are changed independently to generate model P T paths. b) An example of the model paths plotted onto P T space. The paths are 3888 excerpts from paths used for the brute-force search.

7 464 S. Omori et al. / Gondwana Research 16 (2009) SiO 2 = 2.6:11.1:7.6:10.6:15.7:50.6 in weight proportion. H 2 O was assumed to be in excess amount during the calculation. At first, stable mineral assemblages are calculated in P T space up to 5 GPa and T from 300 to 900 C. Then total H 2 O contents in solid phases are calculated from the mineral assemblages, and plotted on to a H 2 O contents map (Fig. 4). In the diagram, a change in H 2 O content represents metamorphic reaction, both discontinuous and continuous reactions, associated with the H 2 O fluid, and dehydration occurs when the P T path of the subducting rock passes the H 2 O contour from the high-h 2 O to the low-h 2 O side. Dehydration does not occur in the reverse case Hypocenter data The double difference relocated hypocenter data were used for the present study: data prepared for the present work following the method of Kita et al. (2006) for the Tohoku section, Nakajima et al. (2009) for the Kanto section, and Hirose et al. (2008) for the sections in SW Japan were adopted. The hypocenters in a belt within 2 km from the surface, which is assumed to be the MORB crust, of the subducted slab were picked out, and seismic zones were identified and traced on each cross-section given in the original reports cited above and the depth distribution of the earthquakes in the MORB crust were thus defined (Fig. 4). The depth distribution of the earthquakes in each section shows repeated seismic and aseismic pattern; this pattern is examined by the brute-force search for the thermal condition of the subducted MORB crust Search for the best-fit thermal structure According to the hypothesis of dehydration-induced intraslab earthquakes, the location of dehydration and distribution of earthquakes are linked under a proper thermal structure in the subducted slab. We examined this concept by brute-force algorithmic search for Fig. 6. a) Model P T path superimposed onto the H 2 O content map and a depth range of dehydration along the path. b) An example of the match scoring between the depth range of an observed seismicities and the depth range of dehydration along a model P T path. Percentage of matching is calculated for each seismic and aseismic zone, then average of the zones is taken for the total matching score of the model P T path.

8 S. Omori et al. / Gondwana Research 16 (2009) the P T path of the MORB crust. Candidate P T paths are generated in a simple geometric model in the P T space to collect all possible paths in the P T range of interest generated by a geometric algorithm (Fig. 5). Subsequently, the depth range of dehydration along the paths is calculated using the H 2 O content values by phase equilibrium calculation (Fig. 6a). Finally, each model depth distribution of dehydration is compared to that of earthquakes in a scoring rule, which counts the match between seismicity and dehydration, as well as the match between aseismicity and lack of dehydration (Fig. 6b). The best-fit candidate P T paths are chosen from the high-scores. It should be noted that the phase equilibrium calculation was done in an assumption of isochemical (closed) system with excess fluid. However, the natural system should be an open system at least for the fluid phase and the rock does not coexist with excess fluid in the depth range of our interest. In the strict sense therefore, a phase diagram for H 2 O excess system cannot be used for H 2 O under-saturated system, because the mineral assemblage differs depending on the amount of water in the system. However, the discrepancy between the model system and the natural one can be solved by making a limitation at the calculation of dehydration depth along the model paths, if we only consider the existence or lack of dehydration along the P T paths. The limitations are that H 2 O content in solid phase never increases along the path, and dehydration occurs when the rock passes the H 2 O contour to lesser content of H 2 O than the previous value. These limitations are thermodynamically consistent when we discuss the depth distributions along the model P T paths. 4. Results From the brute-force searching, model P T paths were generated and were examined in each area under evaluation. Fig. 7 shows the histograms of the matching score in each area. In most cases, the highest-score group consists of a few model paths, and we chose the paths in the group as the best-fit thermal condition of the MORB crust in each area. Fig. 8 shows the selected P T paths of the MORB crusts in each area superimposed onto the H 2 O content maps. All results are limited in the stability field of the hydrous minerals because of the principle of our method, which essentially requires dehydration reaction. In the NE Japan section (A), the highest-score group consists of three model paths and they form two groups based on shape and location (Fig. 8a). The two paths with clockwise shape are the higherscore paths and the anticlockwise path is the third one. In the two sections of Kanto area (B and C), three and two paths are chosen from the high-score groups, respectively (Fig. 8b and c). They exhibit clockwise shape and lower-t location compared with that of NE Japan. The distinctive distribution of the hypocenters in the Kii area (D) shows that most of the earthquakes occurred in the oceanic peridotite and those in the MORB crust are rare and limited to the shallower depth in the trench side (Fig. 2d). Two contrasting groups of the paths are obtained for this area: very steep low-t type and almost flat high-t type (Fig. 8d). In the central Shikoku area (E), two model paths are selected, which exhibit very high-t/p slope (Fig. 8e). An evaluation of Fig. 7. Histograms of the scores by the brute-force search. A F represent study areas in Fig. 1. Red rectangles denote the paths selected in each area and plotted in Fig. 8. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

9 466 S. Omori et al. / Gondwana Research 16 (2009) Fig. 8. Estimated P T paths of the surface of the subducted slab superimposed onto the H 2 O content maps of MORB +H 2 O(Fig. 6). A: NE Japan, B:Kanto, C: Kii, D: central Shikoku, and E: Kyushu. Depth distributions of the hypocenter in the top of the MORB crust corresponding those in Fig. 5 are also shown. Blue bars represent the depth range of seismicity. Possible dehydration temperatures are superimposed on the H 2 O map. Black allows are the estimated P T path of the surface of subducted slab in each location.

10 S. Omori et al. / Gondwana Research 16 (2009) the estimated paths in Kii and central Shikoku are made in the following section. In the Kyushu section (F), three model paths are selected; the low-p/t path similar to central Shikoku bends toward high-p side at 1 GPa. 5. Discussion The brute-force searching generated model in a geometric rule rather than a thermomechanical model, and therefore a physical interpretation of the selected paths is required. Previous studies on subduction zone thermal structure through numerical simulation have shown that the age of the subducted slab, subduction angle and speed, and degree of shear heating are the key parameters that control the temperature of the slab (e.g. Peacock, 1996). Recently advanced models brought out the significance of the coupling between the subducting slab and wedge mantle just above the slab and the convection in the mantle wedge (Arcay et al., 2005, 2007). We evaluate the brute-force results by considering the physical interpretations and by comparison with previous predictions from numerical simulations (Fig. 9). In NE Japan, two contrasting paths are obtained (Fig. 8a). The anticlockwise path is qualitatively consistent with the previous thermal modelling (e.g. Hacker et al., 2003; Iwamori, 2007), although the temperature obtained in our study is C higher than that in the above works. It is possible that the subducting cold slab cools down the contact with mantle wedge, and lesser shear heating in the mantle wedge leads to low-t/p path for the MORB crust in deeper than the Moho. This is one of the explanations for the anticlockwise shape of the subduction path. On the other hand, the clockwise path requires increasing heating effect from the mantle wedge in the deeper domain. Although such an effect has not been considered in the previous thermal model, such a scenario is plausible if a strong convection in the mantle wedge compensates the cooling induced by the subducted cold slab. Hasegawa et al. (2005) presented the nature of seismic tomography in the mantle wedge which shows that high-t material is upwelling in the opposite directions of subduction toward the volcanic front in NE Japan. This scenario is consistent with the convection model with the curve moderately bending around 100 km depth (Fig. 8a). The thermomechanical model of Arcay et al. (2005) presented a similar clockwise Fig. 9. Comparison of the P T path of the MORB crust among the areas in NE Japan, Kanto and Kyushu area. Numerical estimates for the ongoing metamorphism and petrological estimates for regional metamorphism are also shown. Blue: NE Japan, Purple: Kanto, Orange: Kyushu, SMB: Sambagawa metamorphic rocks (Ota et al., 2004; Aoki et al., in press), Dotted curves, H03: numerical simulation for NE Japan by Hacker et al., 2003, IW07: numerical simulation for NE Japan (blue), Kanto (purple) and Kyushu (orange) area following Iwamori (2007). turn of the P T path. This shape would be caused by an effect of corner flow in their model. This prediction is consistent with our interpretation from the combination of brute-force estimate and tomographic observation. Therefore, we prefer the result of clockwise path for the NE Japan in the present case. The depth at which the path swings to the higher-t side must have a significant implication for the wedge mantle convection and consequently the formation of the volcanic front. A new magmagenesis model in such a higher-t subduction zone thermal structure is presented in elsewhere (Kogiso et al., 2009). In this context, the reason why the former thermomechanical models (e.g. Hacker et al., 2003; Iwamori, 2007) have yielded cooler thermal structure is evident; the models might have considered a weaker effect of the corner flow convection and ablation of the slab surface from the mantle wedge. For further constraints, it is necessary to examine the whole NE Japan area to determine which path really corresponds to the thermal structure of the subducting MORB. The two sections in Kanto area (B and C) also yielded clockwise type paths. The paths are located in higher-p and lower-t area than that of the paths for NE Japan. The subduction angle, speed and age of the plate are almost similar in Kanto area compared with that of NE Japan (Table 1) and therefore the difference should be caused by the other factors. The most distinct feature of the Kanto area is the Philippine Sea Plate overriding the Pacific plate(fig. 2b and c). The effect of the Philippine Sea plate is suggested to make the temperature of the subducted Pacific plate lower by disturbing the convection in the hanging wall mantle (Hasegawa et al., 2007). The present result of a colder MORB in Kanto than that of NE Japan is basically consistent with their suggestion. Moreover, the clockwise shape of the path in the section B is also explained by the thermal shielding of the Philippine Sea plate. The path turns toward high-t at about km depths, and the depth range corresponds to the depth of the beginning the mantle wedge beneath the Philippine Sea plate. As mentioned above, the convection in mantle wedge could lead to the heating of the slab surface. In addition, the bottom part of the subducting Philippine Sea plate is also a heat source for the Pacific plate. The plate is being continuously supplied to the depth which has the same effect as that of convection, and therefore a steady state temperature at the contact between high-t bottom of the Philippine Sea plate and low-t surface of the Pacific plate would become higher than that of the shallower part where the surface of the Philippine Sea plate contacts with the Pacific plate. In the section C, the Philippine Sea plate covers the whole of the Pacificplatein the depth range of interest (Fig. 2c). The reason why the paths in the section C (Fig. 8c) show a clockwise trend without the appearance of mantle wedge can be explained by this mechanism. The observation in the Kanto area also implies the significance of the corner flow in the mantle wedge to control the temperature of the slab surface. Subduction of the younger PHS plate in Kii and central Shikoku is expected to cause higher-t regional metamorphism than that in the case of the Pacific plate (e.g. Iwamori, 1998; Peacock and Wang, 1999). The brute-force result in Kii shows that the two contrasting groups of P T paths could explain the depth range of dehydration. The low-t result should be discarded because the thermal structure in the central Shiokoku, where the 20 Ma slab is subducted, and exhibits a higher-t nature. However, these higher-t type paths both in Kii and central Shikoku seem to have too high-t/p slope when considering the subduction geometry of these areas (Fig. 2d and e). If the temperature rise from 300 to 800 C within the depth from 20 to 30 km which corresponds to about 20 km distance on the slab surface, then very high temperature gradient along the slab surface (25 C/km) must exist. On the other hand, the S-wave tomographic image in this area shows the existence of a very low-velocity material in the subducted crust (Fig. 2d). Such low-velocity region is considered to represent a fluid-rich zone by Hirose et al. (2008). If the fluid already exists in the area, the dehydration-induced earthquake model may not work because the concept of dehydration induction of earthquake is that the earthquake occurs when fluid is released into a dry rock. Another

11 468 S. Omori et al. / Gondwana Research 16 (2009) explanation for the low-velocity zone is the presence of a sediment layer. Sediment subduction by tectonic erosion process has recently has been considered as a significant process in the subduction zone (Komabayashi et al., 2009; Yamamoto et al., 2009). If large amount of sediments are present in the subducted crust, then it is necessary to employ a different phase diagram to discuss the depth distribution of dehydration mechanism. These factors perhaps explain the reasons why the results obtained from the brute-force method in this study display some physical problems in these regions and warrant further studies to obtain more realistic temperature estimates. However, our brute-force approach underlines the importance of the subduction of crustal materials in the Kii Shikoku area. The Kyushu area shows a moderate type T/P path reaching to the eclogite facies conditions. The younger age of the subducted plate in this area is possibly caused higher-t subduction than the subduction of the Pacific plate. The anticlockwise path is qualitatively consistent with the previous thermal modelling by Iwamori (2007), although the temperature is C higher than their model calculation (Fig. 9). The higher-t tendency of the brute-force result is similar to the NE Japan and Kanto area. 6. Conclusions The P Tconditions of the ongoing regional metamorphism beneath Japanese Islands are estimated on the basis of the dehydration-induced earthquake hypothesis. In the NE Japan, Kanto, and Kyushu area, reasonable estimates were obtained which highlight the significance of corner flow convection for the thermal structure of the slab surface. The estimates in Kii and central Shikoku area suggest the possibility of subduction of large amounts of crustal material in this area. 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