Insight from Seismic Tomography and Mineral Physics ⁎ Dapeng Zhao A, Shigenori Maruyama B, Soichi Omori C

Home , Northeastern Japan Arc, Seismic tomography

Gondwana Research 11 (2007) 120–131 www.elsevier.com/locate/gr

Mantle dynamics of Western Pacific and East Asia: Insight from seismic tomography and mineral physics ⁎ Dapeng Zhao a, Shigenori Maruyama b, Soichi Omori c,

a Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan b Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan c Research Center for Evolving Earth, Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan Received 17 February 2006; received in revised form 12 June 2006; accepted 13 June 2006 Available online 17 August 2006

Abstract

Recent results of high-resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of Western Pacific and East Asia. The most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation (corner flow) processes in the mantle wedge. The combination of the two processes may have caused the back-arc spreading in the Lau basin, affected the morphology of the subducting Philippine Sea slab and its seismicity under southwest Japan, and contributed to the formation of the continental rift system and intraplate volcanism in Northeast Asia, which are clearly visible in our tomographic images. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent (a) small mantle plumes, (b) upwellings associated with the slab collapsing down to the lower mantle, or (c) sub-slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab. Combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite+water, we proposed a petrologic model for arc volcanism. Arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water-rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge. A large amount of fluids can be released from hydrated MORB at depths shallower than 55 km, which move upwards to hydrate the wedge corner under the fore-arc, and never drag down to the deeper mantle along the slab surface. Slab dehydration reactions at 120 km depth are the antigorite-related 5 reactions which supply water-rich fluids for forming the volcanic front. Phase A and Mg-surssasite breakdown reactionsat 200 and 300 km depths below 700 °C cause the second and third arcs, respectively. Moreover, the dehydration reactions of super-hydrous phase B, phases D and E at 500–660 km depths cause the fluid transportation to the mantle boundary layer (MBL) (410–660 km depth). The stagnant slabs extend from Japan to Beijing, China for over 1000 km long, indicating that the arc–trench system covers the entire region from the Japan trench to East Asia. We propose a big mantle wedge (BMW) model herein, where hydrous plumes originating from 410 km depth cause a series of intra-continental hot regions. Fluids derived from MBL accumulated by the double-sided subduction zones, rather than the India–Asia collision and the subsequent indentation into Asia, are the major cause for the active tectonics and mantle dynamics in this broad region. © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Keywords: Seismic tomography; Dehydration; Water in the mantle; Western Pacific; Volcanism; Intraplate tectonics

1. Introduction most active seismic and volcanic activities in the broad region from Western Pacific to the Eastern Asia continent (Fig. 1). Clarification The Western Pacific region is tectonically one of the most active of the three-dimensional (3-D) structure of the crust and mantle and complicated regions on Earth. The westward subduction of the under this region can greatly improve our understanding of Pacific plate and northward subduction of the Indo-Australia plate seismotectonics, volcanism and deep Earth dynamics. are distinctive in this region. Such a double-sided subduction has So far we have conducted extensive tomographic studies of caused the most significant trench–arc–backarc systems and the local, regional and global scales to determine high-resolution 3- D seismic velocity structure of the mantle under the Western ⁎ Corresponding author. Pacific and East Asia region. In this paper we summarize these E-mail addresses: [email protected] (D. Zhao), multi-scale tomographic results and explore their implications [email protected] (S. Maruyama), [email protected] (S. Omori). for arc magmatism, back-arc spreading, intraplate volcanism,

1342-937X/$ - see front matter © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2006.06.006 D. Zhao et al. / Gondwana Research 11 (2007) 120–131 121

Fig. 1. Location of the western Pacific region. East Asia is the location of double-sided subduction zone (red shadow in the map), where the old Pacific plate subducts from the east, and the Indo-Australia plate subducts from the south. seismotectonics and mantle dynamics. To support our seismo- 2. Dehydration reactions in slab peridotite in the MASH system logical observations, we also refer to the recent results of high- pressure and high-temperature mineral physics studies on the Recent ultrahigh-pressure experiments and relevant thermo- role of water in the mantle. Abbreviations of the minerals and dynamic calculations have enabled us to determine the stability mantle phases are shown in Table 1. field of dense hydrous magnesium silicates (DHMS) down to the

Table 1 Abbreviations and compositions of the phases in the model hydrous-peridotite Abbreviation Phase Chemical formula

A phase A Mg7Si2O8(OH)6 Aki Akimotoite Mg2Si2O6 Atg Antigorite Mg48Si34O85(OH)62 Ath Anthophyllite Mg7Si8O22(OH)2 Br Brucite Mg(OH)2 Chl Clinochlore Mg5Al2Si3O10(OH)8 cHum Clinohumite Mg9Si4O16(OH)2 D phase D MgSi2O4(OH)2 E phase E Mg2.3Si1.28O3.65 (OH)2.42 En Enstatite (ortho, high-P clino, low-P clino) Mg2Si2O6 Fld fluid H2O (+silicate components) Fo Forsterite Mg2SiO4 hyRn Hydrous ringwoodite Mg1.89Si0.98O3.7(OH)0.3 hyWd Hydrous wadsleyite Mg1.89Si0.98O3.7(OH)0.3 Mpv Mg-silicate perovskite Mg2Si2O6 Msur Mg-sursassite Mg5Al5Si6O21(OH)7 Mts Mg-tschermak pyroxene MgAl2SiO6 Pe Periclase MgO

Prp Pyrope Mg3Al2Si3O12 Rin Ringwoodite Mg2SiO4 shB superhydrous phase B Mg10Si3O14(OH)4 Sp Spinel MgAl2O4 St Stishovite SiO2 Tc Talc Mg3Si4O10(OH)2 Wad Wadsleyite Mg2SiO4 122 D. Zhao et al. / Gondwana Research 11 (2007) 120–131 uppermost lower mantle. These studies show that water in the The stability of the hydrous phases depends on the thermal subducting slab peridotite would be transported to the bottom of structure of descending slab at a fixed bulk composition. The the upper mantle by several DHMS (e.g., phase A, phase D, phase expected P–T space of dehydration reactions is analyzed using E and super-hydrous phase B) and hydrous polymorphs of model thermal paths of the subduction and stagnation of the slab olivine, via solid–solid reactions after antigorite decomposition (Fig. 3). There are three important features in the temperature– (Komabayashi et al., 2004). The DHMS would dehydrate to dehydration-range diagrams: liberate free water at the upper mantle-lower mantle boundary or a deeper level (Komabayashi, in press). (1) Deep dehydration reactions occur due to the decomposition Fig. 2 shows the dehydration reactions in the hydrous peri- of phase D which only occurs within a low-Tslab (reactions dotite down to the mantle boundary layer. The diagram was (17) and (18) in Fig. 2), the compressional decomposition constructed after Omori et al. (2004) with an update of high-P of phase E (reactions (14) and (16)), and other dehydration portion by Komabayashi (in press). Bulk compositions assumed reactions including superhydrous phase B (reactions (1)– for the diagram are also shown in Fig. 2. The low-P portion (12) and (15)). If isobaric heating during stagnation is not (Pb10 GPa) and high-P portion (PN10 GPa) of the diagram were considered, then the maximum depth of dehydration is constructed by assuming the MgO–Al2O3–SiO2–H2Osystem limited to 520 km, except in the case of a very low-T slab and the MgO–SiO2–H2O system, respectively. (Fig. 3).

Fig. 2. P–T diagram showing dehydration reactions in the subducting peridotite in the MgO–Al2O3–SiO2–H2O system and compositional triangle in the MgO–SiO2– H2O system. Solid curves show dehydration reaction curves, and numbers on the curves correspond to the reactions shown in Table 2. Location of the reaction (18) is not constrained well. Reaction (30) is a continuous reaction which occurs continuously in the area bounded by the reactions (28), (29), (32) and (33). Arrows show the direction of dehydration. Dashed curves denote major phase transformation boundaries of olivine. Stability fields of the hydrous phases are shaded by gray color. The bulk composition of the low-P portion (Pb10 GPa) was assumed to be on the thick line between brucite and talc in the ternary composition diagram plus 4 wt.% of

Al2O3. The high-P portion (PN10 GPa) of the diagram was constructed assuming the antigorite composition. D. Zhao et al. / Gondwana Research 11 (2007) 120–131 123

Table 2 assumption that deep earthquakes are a result of dehydration (e.g., Dehydration reactions in Fig. 2 Seno and Yamanaka, 1996; Omori et al., 2002). No. Reaction The specific subduction zones should be investigated case by 1 hyRin=Mpv+Pe+Fld case. In the following we combine the mineral physics results with 2 shB+Mpv=hyRin+Fld seismic tomography to discuss dehydration reactions in the 3 shB+St=Mpv+Fld Tonga-Kermadec, SW Japan and NE Japan subduction zones as 4 shB+St=Aki+Fld well as East Asia above the stagnant slab. 5 hyRin+St=Aki+Fld 6 shB+St=hyRin+Fld 7 hyWad+St=Aki+Fld 3. Tonga slab subduction and Lau back-arc spreading 8 hyRin+St+Fld=hyWad 9 D=shB+St+Fld The Tonga-Fiji region is a unique subduction zone on Earth. It 10 shB+St=hyWad+Fld contains two-thirds of all deep earthquakes in the world and has 11 E=hyWad+St+Fld 12 E=hyWad+En+Fld the associated Lau back-arc spreading center which has the largest 13 shB+St+Fld=E divergent rate among the back-arc basins in the world. The 14 shB+D+Fld=E installation of 12 broadband stations in the Tonga and Fiji islands 15 shB+Fld=A+D and 25 ocean bottom seismographs (OBS) in the Lau back arc and 16 A+D+Fld=E the Tonga forearc (Wiens et al., 1995) provided a unique oppor- 17 D+Br=shB+Fld 18 D+Br=A+Fld tunity to determine the 3-D structure of this region and to clarify 19 E=Fo+En+Fld the formation of the back-arc spreading and its relation to the 20 E=cHum+En+Fld subduction process. Zhao et al. (1997a) used the data recorded by 21 A+En+Fld=E those land seismic stations and OBS stations to determine a 22 cHum+En=Fo+Fld detailed tomographic image down to 700 km depth beneath the 23 A+En=cHum+Fld 24 A+En=Fo+Fld Tonga arc and the Lau back arc (Fig. 4). The subducting Tonga 25 A+En+Msur=Prp+Fld slab is imaged as a 100-km-thick zone with a P-wave velocity 4– 26 Fo+En+Msur=Prp+Fld 6% higher than the surrounding mantle. Beneath the Tonga arc 27 Chl+Atg=Msur+A+Fld and the Lau back arc, low-velocity (low-V) anomalies of up to 6% 28 En+Chl=Fo+Prp+Fld are visible. The slow-velocity anomaly beneath the Tonga arc 29 Chl=En+Fo+Sp+Fld – 30 Chl+En=Mts+Fo+Fld represents a dipping zone about 30 50 km above the slab, ex- 31 Atg=A+En+Fld tending from the surface to about 140 km depth. This feature is 32 Atg=Fo+En+Fld similar to the low-V zones found beneath the Japan, Alaska and 33 Ath+Fo=En+Fld Cascadia volcanic fronts (Zhao et al., 1992, 1995, 2001). Beneath 34 Tc+Fo=Ath+Fld 100 km depth, the amplitude of the back-arc anomalies is reduced, 35 Atg+Tc=En+Fld − − 36 Atg+Br=A+Fld but moderately slow anomalies ( 2% to 4%) are visible down to 37 A+Atg=Fo+Fld 400 km or even to 500 km depth. This deep extent of the mantle 38 Atg+Br=Fo+Fld wedge slow anomalies has been confirmed by detailed resolution 39 iceVII=Fld analyses (Zhao et al., 1997a), seismic attenuation tomography Reaction coefficients are not included. (Roth et al., 1999, 2000) and waveform modeling studies (Xu and Wiens, 1997). These results indicate that geodynamic systems (2) There is a concaved area that lacks dehydration in the depth associated with the back-arc spreading are not limited to the near- range of 200–500 km. Reactions involving Mg-sursassite surface areas, but are related to deep processes. (reaction (25) in Fig. 2) play an important role for dehydra- The slow velocity anomalies at depths of 300–500 km in the tion events in this depth range. In the low-T slabs (Fig. 3), mantle wedge (Fig. 4) could be caused either by the upwelling dehydration events occur continuously in this depth range. flow in the mantle wedge or by volatiles resulting from the deep However, in the moderate-T slabs, dehydration events ter- dehydration reactions occurring in the subducting slab (Nolet, minate when the coldest part of the slab intersects reaction 1995; Komabayashi et al., 2004; Komabayashi, in press). (25) toward the high-T side. Volatiles would have the effect of lowering the melting tempe- (3) The hottest slab does not experience deep dehydration rature and the seismic velocity, and may produce small amounts of reactions. In such high-T slabs (Fig. 3) water is liberated partial melt (Collier and Sinha, 1992). Temperatures in fast sub- and the slab is totally dehydrated by the decomposition of ducting slabs like Tonga are low enough for water to reach the clinochlore (reactions (28) and (30)), thus no dehydration stability depths of dense hydrous magnesian silicate phases (Par- can be expected at depths exceeding this reaction curve. son and Wright, 1996; Taylor et al., 1996; Omori et al., 2004), which may allow water penetration down to depths of 660 km. These results clearly show that the down-going slab never The thermal structure of the Tonga subduction zone is of the dehydrates continuously, and a vacant field of dehydration exists lowest-T type on Earth, thus the dehydration could occur at any at an intermediate depth (Fig. 3). It has long been known that there depth in the slab peridotite (Fig. 3). The phase diagram (Figs. 2 is a bimodal depth distribution of deep earthquakes. Such a and 3) and the tomographic image (Fig. 4) suggest that the distribution is consistent with the phase diagram and justifies the dehydration reactions which occur below the low-V zones are 124 D. Zhao et al. / Gondwana Research 11 (2007) 120–131

Fig. 3. Model P–T paths for the subducting slabs and the corresponding depth range of dehydration reactions in each thermal condition of the slab. The reaction curves in the left P–T diagram correspond to those in Fig. 2.

Nos. 27, 31, 35, and 36 at depths shallower than 300 km, and Partial melting of the mantle wedge by volatiles from the deep Nos. 17 and 18 around 400 km depth to the mantle boundary slab is important in localizing low seismic velocities; the slow layer. anomalies we observe at depths of 300–500 km (Fig. 4)may

Fig. 4. East–west vertical cross section of P-wave velocity image from 0 to 700 km depth beneath the Tonga arc and the Lau back-arc region. Red and blue colors denote slow and fast velocities, respectively. Solid triangles denote active volcanoes. Earthquakes within a 40-km width from the cross section are shown in open circles. The velocity perturbation scale is shown at the bottom (Zhao et al., 1997a). D. Zhao et al. / Gondwana Research 11 (2007) 120–131 125 represent this process. The slow velocity areas beneath the Tonga Sea slab is also imaged clearly. Intermediate-depth earthquakes arc and the Lau back arc seem to be separated at the shallow occur within the Philippine Sea slab down to a depth of 80 km. levels, but merge at depths greater than 100 km (Fig. 4). This The aseismic portion of the Philippine Sea slab extends down to suggests that although the arc and back-arc magma systems are 200 km depth with a dipping angle of about 45°. The sudden separated at shallow levels, where most of the magma is gene- change of the slab geometry happens right beneath the Daisen rated, there may be some interchanges between the magma sys- volcano that is located on the coast area of the Japan Sea. A tems at depths greater than 100 km. Interchange with slab-derived prominent low-V anomaly is visible at depths of 20–50 km be- volatiles at depths greater than 100 km may help to explain some neath the Daisen volcano and right above the subducting Phi- of the unique features in the petrology of back-arc magmas lippine Sea slab, which may represent the arc magma under the relative to typical mid-ocean ridge basalts, including excess volcano associated with the dehydration of the Philippine Sea slab volatiles and large ion lithophile enrichment (Faul et al., 1994). (Zhao et al., 2004a). On 6 October 2000, the western Tottori earthquake with 4. Pacific and Philippine sea slabs under Japan magnitude 7.3 occurred about 20 km southwest of the Daisen volcano with a focal depth of 12 km (Zhao et al., 2004a). The A high-resolution (25–45 km) 3-D P-wave velocity model is hypocenter was located right beside the low-V zone in the crust determined by using a large amount of arrival times from local, (Fig. 5). In the hypocentral area of the western Tottori earthquake, regional and distant earthquakes recorded by the dense seismic significant low-velocity and high Poisson's ratio anomalies were network on the Japan Islands (Zhao, 2004). Fig. 5 shows a vertical detected, which suggests the existence of fluids in the crust and cross-section of the tomographic image down to 500 km depth uppermost mantle under the volcano (Zhao et al., 2004a). In under western Japan. In this region the subducting Pacific slab is addition, in the focal area a number of low-frequency micro- located at depths greater than 450 km. The subducting Philippine earthquakes were detected at depths of around 30 km both before and after the 2000 Tottori mainshock (Ohmi and Obara, 2002). Low-frequency microearthquakes in the deep crust have been found earlier in other volcanic areas of the Japan Islands and were attributed to the activity of arc magma and fluids in the lower crust and uppermost mantle around the Moho discontinuity (Ukawa and Obara, 1993; Hasegawa and Yamamoto, 1994). Moreover, a detailed magnetotelluric imaging detected high electric conductivity anomalies down to a depth of 30 km beneath the western Tottori area (Oshiman, 2002). All these pieces of evidence indicate a close relationship among the slab dehydration, magma chamber under the Daisen volcano and the occurrence of the large crustal earthquake (Zhao et al., 2004a). The subducting Philippine Sea plate beneath SW Japan has an age of 20–50 Ma, thus the temperature of the slab is relatively high. It is estimated that the dehydration beneath the Daisen corresponds to the reactions (28) and (30) in the phase diagram (Fig. 2) at which clinochlore dehydrates. After the decomposition of the clinochlore, no hydrous mineral is stable in such a high-T slab, therefore the slab beneath SW Japan loses water at this point. Note that intraslab earthquakes beneath Kyushu Island occur down to 200 km depth in the subducting Philippine Sea slab. Such a discrepancy can be explained by the steep slab subduction and the older slab age under Kyushu than those under Shikoku and SW Honshu. Both the steep subduction and old plate make the slab colder, so that the stability of hydrous minerals and seismicity extend to the deeper areas. At depths of 150–450 km significant low-V anomalies are visible between the subducting Philippine Sea slab and the subducting Pacific slab (Fig. 5). The low-V zones are connected Fig. 5. Vertical cross section of P-wave velocity image from 0 to 500 km depth with the Pacific slab. We consider that the low-V anomalies are beneath Southwest Japan (Zhao et al., 2004a). Location of the cross-section is caused by fluids from the deep dehydration of the Pacific slab as shown in the insert map. Red and blue colors denote slow and fast velocities, well as the convective circulation process of the mantle wedge, respectively. The open triangle and the thick line on the top denote the Daisen similar to the deep low-V zones in the Fiji-Tonga subduction zone volcano and the land area, respectively. Earthquakes within a 30-km width from the cross section are shown in white circles. The velocity perturbation scale is (Fig. 4). shown at the bottom. The white star symbol shows the hypocenter location of The Pacific slab has a large subduction rate of 7–8 cm/year and the western Tottori earthquake (M 7.3) that occurred on 6 October 2000. an age of about 130 Ma around Japan, thus the slab may have a 126 D. Zhao et al. / Gondwana Research 11 (2007) 120–131

Fig. 6. East–west vertical cross section of P-wave velocity image along a profile passing through central Japan. Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scale is shown at the bottom. White dots denote earthquakes that occurred within a 40-km width from the profile. The thick bar and solid triangles denote the land area and active arc volcanoes, respectively. The three curved lines show the Conrad and the Moho discontinuities and the upper boundary of the subducting Pacific slab (Zhao, 2004). lower temperature, which allows deep slab dehydration at 400 km by the dense seismic networks on the Japan Islands (Zhao, 2004). depth, similar to the Tonga slab. Fig. 6 shows an E–Wvertical The tomographic image has a spatial resolution of 25–35 km for cross-section under Eastern Japan down to 500 km depth esti- the crust and mantle wedge and 40–50 km for the subducting slab mated from a joint inversion of local and teleseismic data recorded and the mantle under the slab down to a depth of 500 km. The

Fig. 7. Vertical cross sections of P-wave velocity images determined by a global tomographic inversion (Zhao, 2004). Locations of the cross sections are shown in the insert map. Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scale is shown below the cross sections. Black triangles denote volcanoes. The reversed triangles show the location of the Japan Trench. White dots denote earthquakes that occurred within 150 km of the profiles. The two solid lines denote the 410 and 660 km discontinuities. D. Zhao et al. / Gondwana Research 11 (2007) 120–131 127 subducting Pacific slab and arc-magma related slow anomalies in down to the deeper portion of the mantle wedge above the Pacific the mantle wedge are imaged clearly. As the Pacific slab has a low- slab. The high-V anomalies could be the droplets from the leading T, the stability of DHMS in the slab could reach to the MBL, and edge of the Philippine Sea slab, but it is also possible that they are the dehydrated water by the reactions (14)–(17) is imaged as low- blocks from the interior of the slab. If they are the droplets from the V anomalies just above the Pacific slab (450 km depth) in Fig. 5. slab leading edge, their current positions as shown in the tomo- The deep low-V zones at depths of 200-450 km should have a graphic image can be explained by the corner flow in the mantle higher temperature due to the corner flow in the mantle wedge, wedge considering the subduction factors of the Pacific and which can heat the Philippine Sea slab above them. Thus the Philippine Sea slabs under this region, though quantitative assess- Philippine Sea slab could lose its brittleness at a shallower depth ment has to be made with 3-D numerical simulations. and so intermediate-depth earthquakes do not occur within the We have made detailed resolution analyses and confirmed slab deeper than 80 km. The heating by the hot low-V zones from that all the high-V and low-V anomalies as mentioned above are below may also reduce the stiffness of the Philippine Sea slab so reliable features. For details, see Ochi et al. (2001), Zhao (2004) that the slab suddenly bends where the slab seismicity stops. The and Zhao et al. (2004a). A more recent tomographic study has arc magma under the Daisen volcano and the dehydration of the further confirmed these features (Abdelwahed and Zhao, 2005). Philippine Sea slab may also contribute to the changes in mecha- nical properties and density of the slab because the bending occurs 5. Stagnant slab and intraplate volcanism in Northeast Asia right beneath the Daisen volcano (Fig. 5). Two pieces of high-velocity (high-V) anomalies are visible at Several active intraplate volcanoes, e.g., Wudalianchi and depths of 330–470 km under the deep low-V zones (Fig. 5). The Changbai, exist in NE Asia (Figs. 7 and 8). The Wudalianchi high-Vanomalies may be pieces of the Philippine Sea slab districted volcano erupted in AD 1719 and 1721. The Changbai volcano by the hot anomalies above the Pacific slab, which are collapsing erupted in AD 1050, 1120, 1193 and 1410. The origin of these

Fig. 8. North–south (a) and east–west (b) vertical cross sections of P-wave velocity images under the Changbai intraplate volcano in NE China (Zhao et al., 2004b). Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scale is shown below the cross sections. Black triangles in (a) and (b) denote the intraplate volcanoes. White dots denote earthquakes that occurred within 100 km of the profiles. The two dashed lines denote the 410 and 660 km discontinuities. (c) Locations of the cross sections in (a) and (b). Black and red triangles denote seismic stations and volcanoes, respectively. The contour lines show the depths of the Wadati-Benioff deep seismic zone. 128 D. Zhao et al. / Gondwana Research 11 (2007) 120–131 active intraplate volcanoes is still unclear. Some researchers con- mantle transition zone, and deep earthquakes occur at depths of sidered them to be hotspots (e.g., Turcotte and Schubert, 1982), 500–600 km under the region, suggesting that the subducting while others invoked the asthenospheric injection to explain them Pacific slab is stagnant in the transition zone. (Tatsumi et al., 1990). Fig. 7 shows two vertical cross sections of velocity images 3-D seismic images of the mantle down to 800 km depth are under the Changbai and Wudalianchi volcanoes from a global determined beneath the Changbai volcano by applying teleseis- tomography model (Zhao, 2001, 2004). In addition to the first P mic tomography to relative travel time residuals recorded by a wave data, later phase data of pP, PP, PcP and Pdiff waves were portable seismic network composed of 22 seismic stations (Zhao also used in the inversion, and depth variations of the Moho, 410 et al., 2004b; Lei and Zhao, 2005)(Fig. 8). The results show a and 660 km discontinuities were taken into account. In the upper columnar low-V anomaly extending to 430 km depth under the mantle, the subducting Pacific slab is imaged clearly and earth- Changbai volcano. High-velocity anomalies are visible in the quakes occurred down to about 600 km depth within the slab

Fig. 9. (a) Tectonic features on the surface in Northwest Pacific and Northeast Asia. Black patches denote the Cenozoic basalts. A, Baikal rift; B, Shanxi graben; C, Tancheng-Lujiang fault zone; D, Okinawa trough. (b) A big mantle wedge (BMW) model showing the upper mantle structure and processes beneath NE Asia. The subducting Pacific slab becomes stagnant in the mantle transition zone. The deep dehydration process of the slab and convective circulation process in the mantle wedge cause upwelling of high-temperature asthenospheric materials, leading to the formation of the continental rift system as well as intraplate volcanoes in NE Asia (modified from Tatsumi et al., 1990). D. Zhao et al. / Gondwana Research 11 (2007) 120–131 129

(Fig. 7). Under NE China, the slab becomes stagnant in the dynamic process in the BMW such as deep subduction of the transition zone. In the lower mantle, pieces of fast anomalies are Pacific slab, upwelling of hot asthenospheric materials, and litho- visible under the stagnant slab in the transition zone. Similar spheric fractures. features were also found in other global tomographic models (e.g., An unexpected feature in Fig. 7 is that prominent slow Bijwaard et al., 1998; Fukao et al., 2001). These results suggest anomalies appear beneath the subducting Pacific slab and extend that the subducting slab meets strong resistance when it en- down to the lower mantle. This feature was also imaged by Fukao counters the 660 km discontinuity. The slab bends horizontally, et al. (2001). Prominent slow anomalies are visible in the depth and accumulates there for a long time (ca. 100–140 m.y.), and range of 260–500 km under the subducting Pacific slab (Fig. 6). then finally collapses to fall down as blobs onto the core–mantle The sub-slab slow anomalies have a lateral extent of 70–160 km, boundary as a result of very large gravitational instability from exceeding the resolution scale there. Reconstruction tests and phase transitions (Maruyama, 1994). resolution analyses confirmed that it is a reliable feature. We will A few researchers used receiver function methods to analyze discuss this feature in the next section. the waveform data from the portable seismic network (Fig. 8c) and obtained similar results (e.g., Ai et al., 2003). Fig. 7 also 6. Discussion and conclusions shows that very slow anomalies exist in the upper mantle right beneath the Wudalianchi and Changbai volcanoes, right above the Subducting slabs carry water stored in hydrous minerals into stagnant Pacific slab in the mantle transition zone. This result is the mantle transition zone and lower mantle. Mineral physics quite similar to the images under the Fiji-Tonga region where the studies show that the water storage capacity of the upper mantle back-arc volcanoes in Fiji and the Lau spreading center are and lower mantle is less than 0.2 wt.%, while the transition zone located above very slow anomalies in the mantle wedge right has a storage capacity of approximately 0.5–1wt.%duetoawater above the subducting Tonga slab (Fig. 4). solubility of about 1–3 wt.% in wadsleyite and ringwoodite, As mentioned above, slow velocity anomalies in the back-arc which are the major constituents of the transition zone (Ohtani region are generally associated with the back-arc magmatism and et al., 2004; Ohtani, 2005). Thus, the transition zone may be a volcanism caused by the deep dehydration process of the major water reservoir in the Earth's interior. The water transport subducting slab and the convective circulation process of the capacity of different slabs may vary, depending on their age and mantle wedge. Because the very old (hence very cold) Pacific plate thermal state. Hot and young slabs generally carry little water into is subducting beneath Eastern Asia at a rapid rate (7–10 cm/year), the deep mantle, while old and cold slabs with a lower thermal the dehydration reactions may not fully complete at the shallow gradient may transport significant amounts of water into the deep depth (100–200 km) of the mantle. Hydrous Mg-Si minerals in the mantle, though their water transport capacity is still a matter of subducting Pacific slab may continue to release fluids through debate (Poli and Schmidt, 2002). dehydration reactions at the depths of mantle transition zone Dehydration of a subducting slab could occur in four different (Omori et al., 2004; Ohtani et al., 2004; Komabayashi et al., 2004). regions depending on the temperature profiles of the slabs: (1) the Such a deep dehydration is caused by isobaric heating in the uppermost mantle by the decomposition of serpentine, chlorite, stagnant slab (reactions (1)–(12) in Fig. 2). The degree of heating phengite, and lawsonite, which could trigger island arc volcanism; depends on the history of the subduction zone and the degree of (2) the transition zone by decomposition of phase E and the cooling of the MBL by the subducted slab. A well cooled MBL decrease in the solubility of water in wadsleite with increasing would not cause dehydration, however the slab would collapse depth or isobaric heating in stagnant slab; (3) the upper part of the down to the lower mantle as mentioned above, then the thermal lower mantle by decomposition of hydrous ringwoodite and/or potential for the dehydration in the stagnant slab would be the superhydrous phase B; and (4) the deep lower mantle (about recovered. These processes may have led to the large-scale 1200 km) by the decomposition of phase D in the slab (Shieh et upwelling of the hot asthenospheric materials under NE China and al., 1998; Ohtani et al., 2004). In addition, the base of the upper caused the intraplate volcanism and continental rift systems in the mantle could be a region of dehydration melting of rising hot and region (Zhao et al., 2004b, 2006). wet plumes (Ohtani et al., 2004). Based on these results, we believe that a big mantle wedge There should be lateral heterogeneity in water content in the (BMW) is formed above the long stagnant slab under East Asia transition zone even if it has a high water storage capacity, i.e., the and the Changbai volcano is a kind of back-arc intraplate volcano transition zone adjacent to the subducting slabs may be enriched whose formation is closely related to the deep subduction and in water relative to the normal transition zone (Ohtani et al., 2004). stagnancy of the Pacific slab in the transition zone as well as its The stratification of the mantle in terms of the water content, i.e., deep dehydration processes in the BMW (Fig. 9). Tatsumi et al. relatively lower water content in the upper and lower mantle and (1990) first invoked the asthenospheric injection to explain the high water content in the transition zone, may be altered by formation of the Wudalianchi and Changbai volcanoes, but they various processes such as convective circulation, interaction of did not consider the stagnant Pacific slab under the region because the subducting slabs and ascending plumes with the transition such a slab structure was unknown at that time. Here we modify zone, and mobility of hydrogen and permeable flow of fluids in their model to emphasize the role of the stagnant Pacific slab and the transition zone. the BMW in the formation of the intraplate volcanism in East Asia Significant low-velocity anomalies are visible in the upper (Fig. 9). The extensional rift systems and faults widely existing in mantle and transition zone beneath the subducting Pacific slab East Asia (Fig. 9a) may be the surface manifestation of a deep (Figs. 6 and 7). Our extensive synthetic tests and resolution 130 D. Zhao et al. / Gondwana Research 11 (2007) 120–131

Fig. 10. A schematic diagram illustrating the formation of low-velocity anomalies in the mantle beneath the subducting slab. The sub-slab slow anomalies can be caused by (a) small mantle plumes, (b) upwellings associated with the slab subduction, or (c) sub-slab dehydration associated with deep earthquakes caused by the reactivation of large faults in the slab which are generated by the large normal-faulting earthquakes in the oceanic plate near the trench axis or transform faults and are preserved during the slab subduction (see text for details). analyses show that these sub-slab low-V zones are reliable fea- earthquakes on these faults near the surface. The seawater can be tures (Zhao, 2004; Abdelwahed and Zhao, 2005). However, it is preserved within the slab and be carried down to the deep part of difficult to interpret these slow anomalies under the slab. There the upper mantle and the transition zone. When some of the large may be three possibilities. The first is that they represent a small faults in the slab reactivate to cause the large deep earthquakes, the mantle plume rising from the lower mantle. Malamud and preserved water can be released to the mantle below the slab if the Turcotte (1999) suggested that more than 5000 plumes exist in faults have an adequate dipping angle relative to the dipping slab the mantle and the large number of seamounts represent the (Fig. 10). That is, the end of the fault at the lower slab boundary surface evidence for small plumes. If this conjecture is correct, it should be shallower than that at the upper slab boundary so that is not surprising that small plumes appear under the subducting the free water can move up and get out of the slab due to the slab, as detected by our tomographic imagings (Figs. 6 and 7). positive buoyancy force in the deep lithostatic condition. Studies The second is that they represent a hot upwelling portion of a of focal mechanisms of deep earthquakes show that such faults as local-scale convection associated with the subduction of the shown in Fig. 10 do exist in the subducting Pacific slab (Jiao et al., Pacific slab. The third possibility is that the sub-slab slow 2000; Tibi et al., 2003). The occurrence of deep earthquakes anomalies are caused by the slab dehydration. This is hard to indicates that the interior portion of the slab is still brittle and the understand because it is generally considered that fluids from the temperature there should be still quite low. Thus free water can slab dehydration are released to the upper mantle wedge to cause exist within the faults in the slab. The free water may keep the arc and back-arc magma as shown in Fig. 4. Here we propose a faults lubricated and so the faults within the slab may be reacti- model for the slab dehydration at the lower boundary of the slab vated frequently to generate deep earthquakes. (Fig. 10). Transform faults and fracture zones in the oceanic crust are Acknowledgements possible water drainage into the oceanic plate down to 50 km depth. The transform faults and fractures would also be mecha- This work was partially supported by Grant-in-aid for Scientific nical weak zones in the subducting slab. Large normal-faulting Research (Kiban-B 11440134 and Kiban-A 17204037) from earthquakes frequently occur within the oceanic plate right before Ministry of Education and Science, Japan and a special COE grant its subduction into the mantle close to the oceanic trench (Fig. 10). from Ehime University to D. Zhao. We thank the constructive The normal faultings associated with these large outer-rise earth- reviews by T. Nishiyama and an anonymous referee. We appreciate quakes can cut through the oceanic plate and left many faults and the helpful discussion and collaboration with A. Yamada. cracks within the subducting slab (Kanamori, 1971a,b). These faults within the slab may be reactivated to cause intermediate- References depth and deep earthquakes (Silver et al., 1995; Zhao et al., 1997b; Jiao et al., 2000), though several other models have been Abdelwahed, M., Zhao, D., 2005. Deep structure of the Japan Islands estimated proposed to explain the occurrence of intermediate-depth and from the joint inversion of local and teleseismic data. Program and Abstracts, deep earthquakes (see Tibi et al., 2003 for a recent review). We 2005 Fall Meeting, Seismol. Soc. Japan, p. 149. Ai, Y., Zheng, T., Xu, W., He, Y., Dong, D., 2003. A complex 660 km discontinuity consider that sea water can infiltrate into the interior of the oceanic beneath northeast China. Earth Planet. Sci. Lett. 212, 63–71. plate through the transform faults, fractures and the large normal Bijwaard, H., Spakman, W., Engdahl, E., 1998. Closing the gap between regional faults near the trench at least soon after the occurrence of large and global travel time tomography. J. Geophys. Res. 103, 30055–30078. D. Zhao et al. / Gondwana Research 11 (2007) 120–131 131

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