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Age of the subducting Pacific slab beneath East Asia and its geodynamic implications
Earth and Planetary Science Letters 464 (2017) 166–174
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Earth and Planetary Science Letters www.elsevier.com/locate/epsl
Age of the subducting Pacific slab beneath East Asia and its geodynamic implications Xin Liu a,b,∗ , Dapeng Zhao b,∗ , Sanzhong Li a , Wei Wei c a b c
Key Lab of Submarine Geosciences and Prospecting Techniques, MOE, and College of Marine Geosciences, Ocean University of China, Qingdao, 266100, China Department of Geophysics, Tohoku University, Sendai 980-8578, Japan Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake Administration, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 15 November 2016 Received in revised form 17 January 2017 Accepted 12 February 2017 Available online xxxx Editor: A. Yin Keywords: the Pacific slab the Izanagi plate mantle transition zone lithosphere age slab subduction East Asia
a b s t r a c t We study the age of the subducting Pacific slab beneath East Asia using a high-resolution model of Pwave tomography and paleo-age data of ancient seafloor. Our results show that the lithosphere age of the subducting slab becomes younger from the Japan Trench (∼130 Ma) to the slab’s western edge (∼90 Ma) beneath East China, and the flat (stagnant) slab in the mantle transition zone (MTZ) is the subducted Pacific plate rather than the proposed Izanagi plate which should have already collapsed into the lower mantle. The flat Pacific slab has been in the MTZ for no more than ∼10–20 million years, considerably less than the age of the big mantle wedge beneath East Asia (>110 million years). Hence, the present flat Pacific slab in the MTZ has contributed to the Cenozoic destruction of the East Asian continental lithosphere with extensive intraplate volcanism and back-arc spreading, whereas the destruction of the North China Craton during the Early Cretaceous (∼140–110 Ma) was caused by the subduction of the Izanagi (or the Paleo-Pacific) plate. © 2017 Elsevier B.V. All rights reserved.
1. Introduction In the Northwest Pacific and East Asian region, the Pacific plate is subducting northwestward beneath the Eurasian plate, the Philippine Sea (PHS) plate and the Okhotsk plate (Fig. 1). Earthquakes associated with the subducting Pacific slab form a clear Wadati–Benioff deep seismic zone extending down to the mantle transition zone (MTZ). In some regions (e.g., Mariana) the subducted slab has penetrated into the lower mantle, whereas in other regions (e.g., NE Asia) the slab has remained in the MTZ due to the resistance resulting from a positive buoyancy effect rendered by mineral phase changes and a viscosity jump at the 660-km discontinuity (e.g., Bijwaard et al., 1998; Helffrich, 2000; Zhao, 2004; Huang and Zhao, 2006; Fukao and Obayashi, 2013; Wei et al., 2012; King et al., 2015; Chen et al., 2017). The western edge of the flat slab in the MTZ beneath the East Asian continent is more than 2300 km away from the Japan Trench where the Pacific plate has been subducting (Fig. 1). Seismic tomography and electrical conductivity data favor a hydrated upper mantle existing above the flat slab, forming a big mantle wedge (BMW) (Zhao, 2004;
*
Corresponding authors. The author Xin Liu at: Key Lab of Submarine Geosciences and Prospecting Techniques, MOE, and College of Marine Geosciences, Ocean University of China, Qingdao, 266100, China. E-mail addresses: [email protected] (X. Liu), [email protected] (D. Zhao). http://dx.doi.org/10.1016/j.epsl.2017.02.024 0012-821X/© 2017 Elsevier B.V. All rights reserved.
Ichiki et al., 2006; Chen et al., 2017), because of the subductiondriven corner flow and fluids from deep slab dehydration and/or fluids brought down from the shallow mantle wedge by convection (Zhao et al., 2011). The hot and wet upwelling in the BMW is considered to be responsible for the extensive Late Mesozoic–Cenozoic tectono-magmatism in East Asia, causing significant intraplate volcanoes and earthquakes, continental lithosphere destruction, and a boundary in the surface topography and gravity anomaly in East Asia (Zhao et al., 2011; Figs. 1 and 2). Although it is well known that plate subduction plays a crucial role in mantle dynamics and the tectonic evolution of East Asia, the nature of the flat slab in the MTZ is still not well understood, for example, the slab’s lithosphere age and the duration of its subduction from the trench (i.e., the slab’s subduction age), which are critical information for linking seismic tomography to plate reconstruction (e.g., Spakman and Hall, 2010; Sigloch and Mihalynuk, 2013; Zahirovic et al., 2014; Hall and Spakman, 2015; Wu et al., 2016) and time-dependent geodynamic modeling (e.g., Billen, 2008; Goes et al., 2008; Liu et al., 2008; Bower et al., 2015; Seton et al., 2015). Global plate reconstructions suggest that the Izanagi plate had subducted beneath East Asia prior to the Pacific plate (e.g., Maruyama et al., 1997; Müller et al., 2008a; Seton et al., 2012). The poorly constrained ages of the subducting oceanic lithosphere lead to some key questions, such as: Is the present flat slab beneath East Asia the subducted Izanagi slab or
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Fig. 1. Tectonic settings of the study region (after Liu and Zhao, 2016). The red triangles denote active volcanoes. The pink triangles denote locations of the Cenozoic basalts (Xu et al., 2012). The thin blue lines denote depth contours of the present upper boundary of the subducting Pacific slab estimated from seismicity. The thick red line shows a boundary of gravity anomaly in East Asia (Fig. 2). The thick blue line shows the western edge of the flat slab in the MTZ beneath East Asia estimated from the tomographic model (Wei et al., 2012) used in this study. The eight black lines denote locations of the vertical cross-sections shown in Fig. 6. The solid sawtooth lines and the black dashed line denote the plate boundaries. The seafloor ages are from a global age model (Müller et al., 2008b). SLB, the Songliao Basin; NCC, the North China Craton; CCO, the Central China Orogen; SCC, the South China Craton; ECS, the East China Sea; PHS, the Philippine Sea. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the Pacific slab? Which slab was associated with the destruction of the North China Craton? How long has the present flat slab stayed in the MTZ? To resolve these issues, here we attempt to map distributions of the slab’s lithosphere age and its subduction age in the upper mantle and the MTZ beneath East Asia, by reconciling a high-resolution model of regional P-wave velocity (Vp) tomography (Wei et al., 2012) and the paleo-age data of ancient seafloor (Müller et al., 2008a; Seton et al., 2012). We mean the slab’s lithosphere age to be the period from the birth of the oceanic lithosphere at the midocean ridge to the present, whereas the slab’s subduction age is the time period from the plate subduction at the trench to the present. Our results shed new light on the evolution of the flat slab in the MTZ, as well as the East Asian tectonics during the Late Mesozoic to the Cenozoic.
MTZ beneath East Asia. The western edge of the flat slab in the MTZ derived from this tomographic model roughly coincides with a boundary in the surface topography and gravity anomaly in East Asia (Figs. 1 and 2). The ancient seafloor age data used in this study were obtained from a series of global plate reconstruction models since ∼140 Ma (Müller et al., 2008a; Seton et al., 2012). The reconstruction of the Panthalassa Ocean in these models was based on the premise that the Pacific plate formed at ∼170 Ma as a triangle, originating from a triple junction between the Farallon, Phoenix, and Izanagi plates (Engebretson et al., 1985; Boschman and van Hinsbergen, 2016). The mean error of the reconstructed seafloor ages is ∼10 million years (Myr). The reconstruction results show that the Pacific plate subduction beneath East Asia initiated at ∼60 Ma following the subducted Izanagi plate.
2. Data
3. Method
The Vp tomographic model used in this work was determined by inverting 1,401,797 high-quality arrival-time data recorded at ∼2000 local seismic stations deployed in the East Asia region (Wei et al., 2012). The spatial resolution of this Vp model is ∼1◦ in the lateral direction and ∼50–100 km in depth. This model shows the subducting Pacific slab clearly as a high-velocity zone in the
To determine the lithosphere age of the oceanic slab beneath East Asia, we adopt the reconstruction models of the Pacific Ocean at 0 Ma and 60 Ma (Müller et al., 2008a) (Figs. 3a, b), because the reconstruction results show that the mid-ocean ridge between the Izanagi and Pacific plates subducted at ∼60 Ma (Fig. 3b), which means that the Pacific plate has ceased growing at ∼60 Ma in
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Fig. 2. Distribution of the Bouguer gravity anomalies in the study region. The scale is shown at the upper-left corner. The gravity data are derived from Balmino et al. (2012). The thick red line shows a boundary of gravity anomaly in East Asia. The thick blue line shows the western edge of the flat slab in the MTZ beneath East Asia estimated from the tomographic model (Wei et al., 2012) used in this study. The other labeling is the same as that in Fig. 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the Western Pacific region. At first, we move the ancient seafloor isochrons of the Pacific plate at 60 Ma, and make them coincide with the seafloor isochrons of the Pacific plate at 0 Ma in the Western Pacific region (Fig. 3c). As shown in Figs. 3a, b, we set three reference points (A, B and C) on the Pacific plate at 60 Ma, then we simply find their corresponding points (A , B and C ) on the Pacific plate at 0 Ma. Assuming that the plate has not been internally deformed, for any point K on the ancient seafloor isochrons of the Pacific plate at 60 Ma, we calculate its corresponding point K on the moved isochrons by minimizing the distance r =
(d1 − d1 )2 + (d2 − d2 )2 + (d3 − d3 )2 with a
grid search approach, where d1 , d2 , d3 , d1 , d2 , d3 are distances between points K–A, K–B, K–C, K –A , K –B , K –C , respectively (Figs. 3d, e). Then we bend the moved ancient seafloor isochrons along the present upper boundary of the subducting Pacific slab beneath East Asia, keeping the distances between the adjacent points of the moved ancient seafloor isochrons (Figs. 3f and S1a). This bending approach resembles the reverse of the 3-D balance method for unfolding a deformed surface (Gratier et al., 1991) or subducted slabs in the upper mantle (Chatelain et al., 1993; Wu et al., 2016). We thus obtain the age distribution of the subducting oceanic lithosphere in the upper mantle and the MTZ beneath East Asia (Fig. 4). The present upper boundary of the subducting Pacific slab beneath the study region (Fig. 1) is derived from previous seismological studies (Zhao et al., 2012; Hayes et al., 2012). The extent of the flat slab in the MTZ is derived from the high-resolution regional tomography (Wei et al., 2012). To determine the slab’s subduction age distribution, we adopt the reconstruction models of the Pacific Ocean from 60 Ma to present (Müller et al., 2008a; Seton et al., 2012). At first, we move
the Pacific plate boundary (trench) in the Western Pacific region at i Ma, by minimizing the distance r with a grid search approach similar to the above-mentioned method (Figs. 3g, h). Then we bend the moved trench along the present upper boundary of the subducting Pacific slab beneath East Asia (Figs. 3f and S1b), thus we obtain the slab’s subduction age of i Ma. We repeat this process to all the reconstruction models from 60 Ma to present with 1 Ma interval. We thus obtain the slab’s subduction age distribution in the upper mantle and the MTZ beneath East Asia (Fig. 5). In addition, we further determine the lithosphere age of the oceanic slab at the time when the slab started to subduct at the trench (T trench ) with a relation of T trench = T lith − T subduct , where T lith is the lithosphere age with respect to the present and T subduct is the slab’s subduction age (Figs. S2–S4). Note that our results depend on the global plate reconstruction model adopted. In addition, the Izanagi plate is an inference rather than an observation, and so the mid-ocean ridge between the Izanagi and Pacific plates is hard to determine precisely (e.g., Rowley, 2008). 4. Results Figs. 4 and 6 display the obtained results of the subducting oceanic lithosphere age (SOLA) distribution in the upper mantle and the MTZ beneath East Asia, which show that the lithosphere age of the subducting slab ranges from ∼130 Ma at the present-day trench to ∼90 Ma at the western tip of the slab in the MTZ. The SOLA is ∼130–125 Ma beneath the NE Japan Arc, ∼125–110 Ma beneath the Japan Sea, ∼105–95 Ma beneath the Yellow Sea, ∼110–100 Ma beneath the East China Sea, and
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Fig. 3. The seafloor ages (in Ma) in and around the Western Pacific region (a) at present (0 Ma) and (b) at 60 Ma (after Müller et al., 2008a). Note that the seafloor ages in (a, b) are with respect to the present rather than to 60 Ma. The age scale is the same as that in Fig. 1. (c) The red lines show the isochrons of seafloor at 60 Ma, which are moved to coincide with the seafloor isochrons of the Pacific plate at 0 Ma (the blue lines) in the Western Pacific region. The thin black lines in (a, b, c) denote the present coastlines. (d, e) Schematic diagrams showing the procedure of moving the isochrons. (f) A schematic diagram showing the procedure of bending the moved ancient seafloor along the present upper boundary of the subducting Pacific slab (UBPS). (g, h) Schematic diagrams showing the procedure of moving the Pacific plate boundary at i Ma in the Western Pacific region. See the text for details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
∼110 Ma beneath the Ryukyu Arc. The SOLA beneath the Songliao Basin is in a range of ∼105–95 Ma, whereas it is ∼95–90 Ma beneath the North China Craton (NCC), and ∼100–90 Ma beneath the South China Craton. The Wudalianchi, Changbai, Ulleung, and Jeju active intraplate volcanoes are located above the flat slab in the MTZ, whereas the Datong active volcano exists beyond the western edge of the flat slab. The SOLA is ∼100 Ma beneath Wudalianchi, ∼110 Ma beneath Changbai, and ∼105 Ma beneath Jeju (Fig. 4). Beneath the Ulleung volcano, the SOLA is estimated to be ∼115–105 Ma, where a fracture zone exists in the subducting slab. This fracture zone is oriented normal to the ancient seafloor isochrons and extends southeastward to a fracture zone in the Pacific Ocean near the Japan Trench. Although we cannot reconstruct precisely the position of the fracture zone due to the limited resolution of our results, hypocenters of two large deep earthquakes (M ≥ 7.0) are located close to the estimated position of the fracture zone beneath SW Japan (Figs. 4 and 6e). This result suggests that the faults in the slab and/or the SOLA differences may cause seismic velocity heterogeneities in the subducting slab, which are often visible in tomographic images of subduction zones (e.g., Zhao, 2015). In addition, the boundary in the surface topography and the gravity anomaly distribution in East Asia (the red thick line in Fig. 4) is roughly sub-parallel to the isochrons of the flat slab, and a sharp bend of this boundary coincides with the fracture zone in the flat slab (Fig. 4).
Figs. 5 and 6 display the obtained results of the slab’s subduction age (T subduct ) distribution in the upper mantle and the MTZ beneath East Asia, which show that the subduction age of the imaged slab ranges from 0 Ma at the present-day trench to ∼30 Ma at the western tip of the slab in the MTZ. The subduction age is ∼5–0 Ma beneath the NE Japan Arc, ∼15–5 Ma beneath the Japan Sea, ∼20–15 Ma beneath the Yellow Sea, ∼15 Ma beneath the East China Sea, and ∼15–10 Ma beneath the Ryukyu Arc. The subduction age beneath the Songliao Basin is in a range of ∼25–20 Ma, whereas it is ∼30–20 Ma beneath the NCC, and ∼25–15 Ma beneath the South China Craton. A potential problem in our present approach is that the flat slab in the MTZ may be stretched, compressively thickened or even buckled (Ribe et al., 2007). Beneath the NE Japan Arc, our results show that the lithosphere age of the Pacific slab when it started to subduct at the trench (T trench ) is ∼130–120 Ma (Figs. 7a, b), and the thickness of the Pacific slab is ∼80–100 km as revealed by high-resolution local tomography and receiverfunctions results (e.g., Kawakatsu et al., 2009; Zhao et al., 2012; Liu and Zhao, 2016). Our results also show that T trench of the slab’s western edge beneath East Asia is ∼60 Ma (Figs. 7a, b). The younger flat Pacific slab in the MTZ revealed by this study should be thinner than the older Pacific slab beneath NE Japan, because the thickness of the oceanic plate is age-dependent (Kawakatsu et al., 2009). However, the flat slab is estimated to be ∼140 km thick beneath NE China and the northern NCC from waveform model-
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Fig. 4. A map view showing the lithosphere age distribution (the white lines) and P-wave velocity (Vp) tomography (the colors) of the subducting Pacific slab. The Vp tomographic image (Wei et al., 2012) is along the present upper boundary of the subducting Pacific slab (the thin red contour lines) at depths <500 km, and we use the Vp image at 500 km depth representing the flat slab in the MTZ (i.e., in the region west of the 500-km depth contour of the slab). The magenta stars denote epicenters of large earthquakes (M ≥7.0) during 1975–2016 derived from the Bulletin of the International Seismological Center. The hypocenters of these large earthquakes are shown in Fig. 6. The other labeling is the same as that in Fig. 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. The same as Fig. 4 but the white contour lines show the subduction age distribution of the Pacific slab. Along the Kuril–Japan–Izu–Bonin–Mariana trench axis, the slab’s subduction age is zero.
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Fig. 6. Vertical cross-sections of P-wave tomography along the eight profiles shown in Fig. 1. The red and blue colors denote low and high velocity perturbations, respectively, whose scale (in %) is shown at the bottom-middle. The color bar with blue numbers above each cross-section shows the subducting Pacific lithosphere ages from the west (East China) to the east (near the trench axis), whose scale (in Ma) is shown at the bottom-left. The surface topography along each profile is shown above the lithosphere age bar. The color bar with red numbers below each cross-section shows the subduction ages of the Pacific slab, whose scale (in Ma) is shown at the bottom-right. The red and pink triangles atop each cross-section denote locations of active volcanoes and Cenozoic basalts, respectively, within a 1◦ width of each profile. The background seismicity and large earthquakes (M ≥7.0) that occurred within a 1◦ width of each profile are shown in white circles and red stars, respectively. The two black dashed lines denote the 410 and 660 km discontinuities. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ing (Li et al., 2013). Numerical simulations considering the slab buckling in the MTZ suggest that the western edge of the flat slab beneath East China is the former mid-ocean ridge between the Izanagi and Pacific plates subducted at ∼60 Ma (Honda, 2016). Unfortunately, although undulations exist in the upper and lower boundaries of the flat slab (Fig. 6), as well as the 410-km and 660-km discontinuities beneath East Asia (e.g., Gu et al., 2012; Tian et al., 2016), we cannot quantitatively identify potential slab buckling using our tomographic model or other seismic results due to the limited spatial resolution of the seismic images. Considering the resolution scale of the tomographic model, the slab morphology and the paleo-age data used in this study, we estimate the errors of T lith , T subduct and T trench to be ±5–10 Ma for the older slab (∼130 < Ttrench < 90 Ma), and ±10–15 Ma for the younger slab (∼90 < Ttrench < 60 Ma) beneath East China where slab buckling may occur.
5. Discussion Our results (Figs. 4 and 6) show that the western edge of the flat slab in the MTZ is ∼90 Ma old, while the Pacific plate near the Japan Trench formed at ∼135 Ma. This result suggests that the flat slab in the MTZ beneath East Asia is the subducted Pacific plate rather than the Izanagi plate. The subducted Izanagi plate may have already sunk into the lower mantle (Seton et al., 2015). Many previous studies have attempted to link deep mantle tomography with the plate reconstructions, and estimated the slab ages at depth (e.g., van der Voo et al., 1999; van der Meer et al., 2010; Domeier et al., 2016). However, because the global tomographic models generally have a low resolution, most of the previous studies focused on the slab in the lower mantle by assuming near-vertical slab sinking and ignoring the slab deformation and stagnancy in the MTZ or the uppermost lower mantle (Zhao, 2004; Fukao and Obayashi, 2013; Marquardt and Miyagi, 2015). Episodic slab sinking may be more realistic, at least at depths <∼1000 km,
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Fig. 7. (a) A map view showing the distribution of the Pacific slab’s sinking rate beneath East Asia. (b) A map view showing the distribution of the Pacific slab’s advancing rate beneath East Asia. The dark blue contour lines in (a) and the white contour lines in (b) show the distribution of the oceanic lithosphere age at the time when the Pacific plate started to subduct at the trench (T trench ). The other labeling in (a, b) is the same as that in Fig. 1. (c) The green area shows the relation between the Pacific slab depth and its corresponding subduction age obtained by this study. (d) The green area shows the relation between the Pacific slab’s advancing distance and its corresponding subduction age obtained by this study. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
though a linear sinking rate is statistically significant (Domeier et al., 2016). With the slab’s subduction ages obtained by this study (Fig. 5), we estimate the vertical sinking rate (V sink ) distribution of the dipping Pacific slab in the upper mantle and the MTZ, using a relation V sink = S dep / T subduct , where S dep is slab depth. The obtained results (Figs. 7a, c) show that the slab sinking rate is in a range of 3.1–6.9 cm/yr. Our result also shows that the flat Pacific slab has remained in the MTZ beneath East Asia no longer than ∼10–20 Myr (Fig. 7c). In addition, we also estimate the advancing rate (V adv ) of the Pacific slab in the upper mantle and the MTZ, using a relation V adv = S dis / T subduct , where S dis is a distance between a position on the Earth’s surface directly above a point j on the imaged slab and the corresponding position where the point j entered the subduction zone (Fig. S5). The obtained results (Figs. 7b, d) show that the slab advancing rate ranges from 6.1 to 7.7 cm/yr. These results may provide a useful constraint on the time-dependent geodynamic modeling.
Extensive intraplate volcanism and back-arc spreading mark the Cenozoic destruction of the East Asian continental lithosphere (Xu et al., 2012; Li et al., 2012a; Fig. 1), which is considered to be associated with upwelling of hot and wet asthenospheric materials in the BMW above the flat Pacific slab in the MTZ (e.g., Zhao et al., 2011; Fig. 6). In addition, the NCC destruction during the Early Cretaceous with cratonic lithospheric removal and/or replacement has been attributed to the deep subduction of the Paleo-Pacific (or the Izanagi) plate beneath East Asia (Zhu et al., 2012; Liu et al., 2013). Our present results suggest that the present flat Pacific slab has stayed in the MTZ for no more than ∼10–20 Myr. Thus, the present flat Pacific slab may have been only associated with the Cenozoic tectono-magmatism in East Asia, and it has nothing to do with the Early Cretaceous tectono-magmatism in the region. We think that an initial BMW may have formed during that time (∼140–110 Ma) and resulted in the NCC destruction. Continuous slab subduc-
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tion beneath East Asia since the Early Mesozoic (before ∼250 Ma; Li et al., 2012b) may have brought a great amount of water into the upper mantle beneath East Asia, feeding the BMW and the intraplate volcanoes. Hence, the BMW may have existed much longer than any flat slabs in the MTZ beneath East Asia. 6. Conclusions The age distribution of the subducting Pacific plate beneath East Asia is investigated using a high-resolution tomographic model and paleo-age data of ancient seafloor. Major results of this work are summarized as follows. (1) The imaged subducting oceanic lithosphere becomes younger from the Japan Trench (∼130 Ma) to the slab’s western tip (∼90 Ma) beneath East Asia. Such a feature indicates that the flat slab now in the MTZ beneath East Asia is the subducted Pacific slab rather than the Izanagi slab which should have already sunk into the lower mantle. (2) The slab’s subduction age ranges from 0 Ma at the present-day trench to ∼30 Ma at the western tip of the flat slab in the MTZ beneath East Asia. The stagnant duration of the flat Pacific slab in the MTZ is no more than ∼10–20 million years, much shorter than the age of the big mantle wedge beneath East Asia (>110 million years). (3) It is the present Pacific slab that has contributed to the Cenozoic lithosphere destruction, extensive intraplate volcanism, and back-arc spreading in East Asia, whereas the North China Craton destruction during the Early Cretaceous (∼140–110 Ma) was caused by the subduction of the Izanagi (or the Paleo-Pacific) plate. Acknowledgements We thank Prof. R. Müller for sharing their age data of ancient seafloor. We appreciate the very helpful suggestions from Profs. R. Hall and W. Spakman, and the discussion with Dr. Y. Wang. We are very grateful to Prof. An Yin (the Editor), Prof. R. Hall and an anonymous reviewer who provided thoughtful review comments and suggestions which have improved this paper. The free software GMT (Wessel and Smith, 1998) is used for making the figures. This work was supported by grants from the JSPS (Kiban-S 23224012) and the MEXT (26106005) to D. Zhao, and grants from the Chinese NSFC (41602207, 41190072 and 41325009) to S. Li and X. Liu. Appendix A. Supplementary material Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2017.02.024. References Balmino, G., Vales, N., Bonvalot, S., Briais, A., 2012. Spherical harmonic modelling to ultra-high degree of Bouguer and isostatic anomalies. J. Geod. 86, 499–520. Bijwaard, H., Spakman, W., Engdahl, E., 1998. Closing the gap between regional and global travel time tomography. J. Geophys. Res. 103, 30055–30078. Billen, M., 2008. Modeling the dynamics of subducting slabs. Annu. Rev. Earth Planet. Sci. 36, 325–356. Boschman, L., van Hinsbergen, D., 2016. On the enigmatic birth of the Pacific Plate within the Panthalassa Ocean. Sci. Adv. 2, e1600022. http://dx.doi.org/10.1126/ sciadv.1600022. Bower, D., Gurnis, M., Flament, N., 2015. Assimilating lithosphere and slab history in 4-D Earth models. Phys. Earth Planet. Inter. 238, 8–22. Chatelain, J., Guillier, B., Gratier, J., 1993. Unfolding the subducting plate in the central New Hebrides Island ARC: geometrical argument for detachment of part of the downgoing slab. Geophys. Res. Lett. 20, 655–658. Chen, C., Zhao, D., Tian, Y., Wu, S., Hasegawa, A., Lei, J., Park, J., Kang, I., 2017. Mantle transition zone, stagnant slab and intraplate volcanism in Northeast Asia. Geophys. J. Int. 209, 68–85. http://dx.doi.org/10.1093/gji/ggw491.
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Earth and Planetary Science Letters www.elsevier.com/locate/epsl
Age of the subducting Pacific slab beneath East Asia and its geodynamic implications Xin Liu a,b,∗ , Dapeng Zhao b,∗ , Sanzhong Li a , Wei Wei c a b c
Key Lab of Submarine Geosciences and Prospecting Techniques, MOE, and College of Marine Geosciences, Ocean University of China, Qingdao, 266100, China Department of Geophysics, Tohoku University, Sendai 980-8578, Japan Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake Administration, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 15 November 2016 Received in revised form 17 January 2017 Accepted 12 February 2017 Available online xxxx Editor: A. Yin Keywords: the Pacific slab the Izanagi plate mantle transition zone lithosphere age slab subduction East Asia
a b s t r a c t We study the age of the subducting Pacific slab beneath East Asia using a high-resolution model of Pwave tomography and paleo-age data of ancient seafloor. Our results show that the lithosphere age of the subducting slab becomes younger from the Japan Trench (∼130 Ma) to the slab’s western edge (∼90 Ma) beneath East China, and the flat (stagnant) slab in the mantle transition zone (MTZ) is the subducted Pacific plate rather than the proposed Izanagi plate which should have already collapsed into the lower mantle. The flat Pacific slab has been in the MTZ for no more than ∼10–20 million years, considerably less than the age of the big mantle wedge beneath East Asia (>110 million years). Hence, the present flat Pacific slab in the MTZ has contributed to the Cenozoic destruction of the East Asian continental lithosphere with extensive intraplate volcanism and back-arc spreading, whereas the destruction of the North China Craton during the Early Cretaceous (∼140–110 Ma) was caused by the subduction of the Izanagi (or the Paleo-Pacific) plate. © 2017 Elsevier B.V. All rights reserved.
1. Introduction In the Northwest Pacific and East Asian region, the Pacific plate is subducting northwestward beneath the Eurasian plate, the Philippine Sea (PHS) plate and the Okhotsk plate (Fig. 1). Earthquakes associated with the subducting Pacific slab form a clear Wadati–Benioff deep seismic zone extending down to the mantle transition zone (MTZ). In some regions (e.g., Mariana) the subducted slab has penetrated into the lower mantle, whereas in other regions (e.g., NE Asia) the slab has remained in the MTZ due to the resistance resulting from a positive buoyancy effect rendered by mineral phase changes and a viscosity jump at the 660-km discontinuity (e.g., Bijwaard et al., 1998; Helffrich, 2000; Zhao, 2004; Huang and Zhao, 2006; Fukao and Obayashi, 2013; Wei et al., 2012; King et al., 2015; Chen et al., 2017). The western edge of the flat slab in the MTZ beneath the East Asian continent is more than 2300 km away from the Japan Trench where the Pacific plate has been subducting (Fig. 1). Seismic tomography and electrical conductivity data favor a hydrated upper mantle existing above the flat slab, forming a big mantle wedge (BMW) (Zhao, 2004;
*
Corresponding authors. The author Xin Liu at: Key Lab of Submarine Geosciences and Prospecting Techniques, MOE, and College of Marine Geosciences, Ocean University of China, Qingdao, 266100, China. E-mail addresses: [email protected] (X. Liu), [email protected] (D. Zhao). http://dx.doi.org/10.1016/j.epsl.2017.02.024 0012-821X/© 2017 Elsevier B.V. All rights reserved.
Ichiki et al., 2006; Chen et al., 2017), because of the subductiondriven corner flow and fluids from deep slab dehydration and/or fluids brought down from the shallow mantle wedge by convection (Zhao et al., 2011). The hot and wet upwelling in the BMW is considered to be responsible for the extensive Late Mesozoic–Cenozoic tectono-magmatism in East Asia, causing significant intraplate volcanoes and earthquakes, continental lithosphere destruction, and a boundary in the surface topography and gravity anomaly in East Asia (Zhao et al., 2011; Figs. 1 and 2). Although it is well known that plate subduction plays a crucial role in mantle dynamics and the tectonic evolution of East Asia, the nature of the flat slab in the MTZ is still not well understood, for example, the slab’s lithosphere age and the duration of its subduction from the trench (i.e., the slab’s subduction age), which are critical information for linking seismic tomography to plate reconstruction (e.g., Spakman and Hall, 2010; Sigloch and Mihalynuk, 2013; Zahirovic et al., 2014; Hall and Spakman, 2015; Wu et al., 2016) and time-dependent geodynamic modeling (e.g., Billen, 2008; Goes et al., 2008; Liu et al., 2008; Bower et al., 2015; Seton et al., 2015). Global plate reconstructions suggest that the Izanagi plate had subducted beneath East Asia prior to the Pacific plate (e.g., Maruyama et al., 1997; Müller et al., 2008a; Seton et al., 2012). The poorly constrained ages of the subducting oceanic lithosphere lead to some key questions, such as: Is the present flat slab beneath East Asia the subducted Izanagi slab or
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Fig. 1. Tectonic settings of the study region (after Liu and Zhao, 2016). The red triangles denote active volcanoes. The pink triangles denote locations of the Cenozoic basalts (Xu et al., 2012). The thin blue lines denote depth contours of the present upper boundary of the subducting Pacific slab estimated from seismicity. The thick red line shows a boundary of gravity anomaly in East Asia (Fig. 2). The thick blue line shows the western edge of the flat slab in the MTZ beneath East Asia estimated from the tomographic model (Wei et al., 2012) used in this study. The eight black lines denote locations of the vertical cross-sections shown in Fig. 6. The solid sawtooth lines and the black dashed line denote the plate boundaries. The seafloor ages are from a global age model (Müller et al., 2008b). SLB, the Songliao Basin; NCC, the North China Craton; CCO, the Central China Orogen; SCC, the South China Craton; ECS, the East China Sea; PHS, the Philippine Sea. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the Pacific slab? Which slab was associated with the destruction of the North China Craton? How long has the present flat slab stayed in the MTZ? To resolve these issues, here we attempt to map distributions of the slab’s lithosphere age and its subduction age in the upper mantle and the MTZ beneath East Asia, by reconciling a high-resolution model of regional P-wave velocity (Vp) tomography (Wei et al., 2012) and the paleo-age data of ancient seafloor (Müller et al., 2008a; Seton et al., 2012). We mean the slab’s lithosphere age to be the period from the birth of the oceanic lithosphere at the midocean ridge to the present, whereas the slab’s subduction age is the time period from the plate subduction at the trench to the present. Our results shed new light on the evolution of the flat slab in the MTZ, as well as the East Asian tectonics during the Late Mesozoic to the Cenozoic.
MTZ beneath East Asia. The western edge of the flat slab in the MTZ derived from this tomographic model roughly coincides with a boundary in the surface topography and gravity anomaly in East Asia (Figs. 1 and 2). The ancient seafloor age data used in this study were obtained from a series of global plate reconstruction models since ∼140 Ma (Müller et al., 2008a; Seton et al., 2012). The reconstruction of the Panthalassa Ocean in these models was based on the premise that the Pacific plate formed at ∼170 Ma as a triangle, originating from a triple junction between the Farallon, Phoenix, and Izanagi plates (Engebretson et al., 1985; Boschman and van Hinsbergen, 2016). The mean error of the reconstructed seafloor ages is ∼10 million years (Myr). The reconstruction results show that the Pacific plate subduction beneath East Asia initiated at ∼60 Ma following the subducted Izanagi plate.
2. Data
3. Method
The Vp tomographic model used in this work was determined by inverting 1,401,797 high-quality arrival-time data recorded at ∼2000 local seismic stations deployed in the East Asia region (Wei et al., 2012). The spatial resolution of this Vp model is ∼1◦ in the lateral direction and ∼50–100 km in depth. This model shows the subducting Pacific slab clearly as a high-velocity zone in the
To determine the lithosphere age of the oceanic slab beneath East Asia, we adopt the reconstruction models of the Pacific Ocean at 0 Ma and 60 Ma (Müller et al., 2008a) (Figs. 3a, b), because the reconstruction results show that the mid-ocean ridge between the Izanagi and Pacific plates subducted at ∼60 Ma (Fig. 3b), which means that the Pacific plate has ceased growing at ∼60 Ma in
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Fig. 2. Distribution of the Bouguer gravity anomalies in the study region. The scale is shown at the upper-left corner. The gravity data are derived from Balmino et al. (2012). The thick red line shows a boundary of gravity anomaly in East Asia. The thick blue line shows the western edge of the flat slab in the MTZ beneath East Asia estimated from the tomographic model (Wei et al., 2012) used in this study. The other labeling is the same as that in Fig. 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the Western Pacific region. At first, we move the ancient seafloor isochrons of the Pacific plate at 60 Ma, and make them coincide with the seafloor isochrons of the Pacific plate at 0 Ma in the Western Pacific region (Fig. 3c). As shown in Figs. 3a, b, we set three reference points (A, B and C) on the Pacific plate at 60 Ma, then we simply find their corresponding points (A , B and C ) on the Pacific plate at 0 Ma. Assuming that the plate has not been internally deformed, for any point K on the ancient seafloor isochrons of the Pacific plate at 60 Ma, we calculate its corresponding point K on the moved isochrons by minimizing the distance r =
(d1 − d1 )2 + (d2 − d2 )2 + (d3 − d3 )2 with a
grid search approach, where d1 , d2 , d3 , d1 , d2 , d3 are distances between points K–A, K–B, K–C, K –A , K –B , K –C , respectively (Figs. 3d, e). Then we bend the moved ancient seafloor isochrons along the present upper boundary of the subducting Pacific slab beneath East Asia, keeping the distances between the adjacent points of the moved ancient seafloor isochrons (Figs. 3f and S1a). This bending approach resembles the reverse of the 3-D balance method for unfolding a deformed surface (Gratier et al., 1991) or subducted slabs in the upper mantle (Chatelain et al., 1993; Wu et al., 2016). We thus obtain the age distribution of the subducting oceanic lithosphere in the upper mantle and the MTZ beneath East Asia (Fig. 4). The present upper boundary of the subducting Pacific slab beneath the study region (Fig. 1) is derived from previous seismological studies (Zhao et al., 2012; Hayes et al., 2012). The extent of the flat slab in the MTZ is derived from the high-resolution regional tomography (Wei et al., 2012). To determine the slab’s subduction age distribution, we adopt the reconstruction models of the Pacific Ocean from 60 Ma to present (Müller et al., 2008a; Seton et al., 2012). At first, we move
the Pacific plate boundary (trench) in the Western Pacific region at i Ma, by minimizing the distance r with a grid search approach similar to the above-mentioned method (Figs. 3g, h). Then we bend the moved trench along the present upper boundary of the subducting Pacific slab beneath East Asia (Figs. 3f and S1b), thus we obtain the slab’s subduction age of i Ma. We repeat this process to all the reconstruction models from 60 Ma to present with 1 Ma interval. We thus obtain the slab’s subduction age distribution in the upper mantle and the MTZ beneath East Asia (Fig. 5). In addition, we further determine the lithosphere age of the oceanic slab at the time when the slab started to subduct at the trench (T trench ) with a relation of T trench = T lith − T subduct , where T lith is the lithosphere age with respect to the present and T subduct is the slab’s subduction age (Figs. S2–S4). Note that our results depend on the global plate reconstruction model adopted. In addition, the Izanagi plate is an inference rather than an observation, and so the mid-ocean ridge between the Izanagi and Pacific plates is hard to determine precisely (e.g., Rowley, 2008). 4. Results Figs. 4 and 6 display the obtained results of the subducting oceanic lithosphere age (SOLA) distribution in the upper mantle and the MTZ beneath East Asia, which show that the lithosphere age of the subducting slab ranges from ∼130 Ma at the present-day trench to ∼90 Ma at the western tip of the slab in the MTZ. The SOLA is ∼130–125 Ma beneath the NE Japan Arc, ∼125–110 Ma beneath the Japan Sea, ∼105–95 Ma beneath the Yellow Sea, ∼110–100 Ma beneath the East China Sea, and
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Fig. 3. The seafloor ages (in Ma) in and around the Western Pacific region (a) at present (0 Ma) and (b) at 60 Ma (after Müller et al., 2008a). Note that the seafloor ages in (a, b) are with respect to the present rather than to 60 Ma. The age scale is the same as that in Fig. 1. (c) The red lines show the isochrons of seafloor at 60 Ma, which are moved to coincide with the seafloor isochrons of the Pacific plate at 0 Ma (the blue lines) in the Western Pacific region. The thin black lines in (a, b, c) denote the present coastlines. (d, e) Schematic diagrams showing the procedure of moving the isochrons. (f) A schematic diagram showing the procedure of bending the moved ancient seafloor along the present upper boundary of the subducting Pacific slab (UBPS). (g, h) Schematic diagrams showing the procedure of moving the Pacific plate boundary at i Ma in the Western Pacific region. See the text for details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
∼110 Ma beneath the Ryukyu Arc. The SOLA beneath the Songliao Basin is in a range of ∼105–95 Ma, whereas it is ∼95–90 Ma beneath the North China Craton (NCC), and ∼100–90 Ma beneath the South China Craton. The Wudalianchi, Changbai, Ulleung, and Jeju active intraplate volcanoes are located above the flat slab in the MTZ, whereas the Datong active volcano exists beyond the western edge of the flat slab. The SOLA is ∼100 Ma beneath Wudalianchi, ∼110 Ma beneath Changbai, and ∼105 Ma beneath Jeju (Fig. 4). Beneath the Ulleung volcano, the SOLA is estimated to be ∼115–105 Ma, where a fracture zone exists in the subducting slab. This fracture zone is oriented normal to the ancient seafloor isochrons and extends southeastward to a fracture zone in the Pacific Ocean near the Japan Trench. Although we cannot reconstruct precisely the position of the fracture zone due to the limited resolution of our results, hypocenters of two large deep earthquakes (M ≥ 7.0) are located close to the estimated position of the fracture zone beneath SW Japan (Figs. 4 and 6e). This result suggests that the faults in the slab and/or the SOLA differences may cause seismic velocity heterogeneities in the subducting slab, which are often visible in tomographic images of subduction zones (e.g., Zhao, 2015). In addition, the boundary in the surface topography and the gravity anomaly distribution in East Asia (the red thick line in Fig. 4) is roughly sub-parallel to the isochrons of the flat slab, and a sharp bend of this boundary coincides with the fracture zone in the flat slab (Fig. 4).
Figs. 5 and 6 display the obtained results of the slab’s subduction age (T subduct ) distribution in the upper mantle and the MTZ beneath East Asia, which show that the subduction age of the imaged slab ranges from 0 Ma at the present-day trench to ∼30 Ma at the western tip of the slab in the MTZ. The subduction age is ∼5–0 Ma beneath the NE Japan Arc, ∼15–5 Ma beneath the Japan Sea, ∼20–15 Ma beneath the Yellow Sea, ∼15 Ma beneath the East China Sea, and ∼15–10 Ma beneath the Ryukyu Arc. The subduction age beneath the Songliao Basin is in a range of ∼25–20 Ma, whereas it is ∼30–20 Ma beneath the NCC, and ∼25–15 Ma beneath the South China Craton. A potential problem in our present approach is that the flat slab in the MTZ may be stretched, compressively thickened or even buckled (Ribe et al., 2007). Beneath the NE Japan Arc, our results show that the lithosphere age of the Pacific slab when it started to subduct at the trench (T trench ) is ∼130–120 Ma (Figs. 7a, b), and the thickness of the Pacific slab is ∼80–100 km as revealed by high-resolution local tomography and receiverfunctions results (e.g., Kawakatsu et al., 2009; Zhao et al., 2012; Liu and Zhao, 2016). Our results also show that T trench of the slab’s western edge beneath East Asia is ∼60 Ma (Figs. 7a, b). The younger flat Pacific slab in the MTZ revealed by this study should be thinner than the older Pacific slab beneath NE Japan, because the thickness of the oceanic plate is age-dependent (Kawakatsu et al., 2009). However, the flat slab is estimated to be ∼140 km thick beneath NE China and the northern NCC from waveform model-
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Fig. 4. A map view showing the lithosphere age distribution (the white lines) and P-wave velocity (Vp) tomography (the colors) of the subducting Pacific slab. The Vp tomographic image (Wei et al., 2012) is along the present upper boundary of the subducting Pacific slab (the thin red contour lines) at depths <500 km, and we use the Vp image at 500 km depth representing the flat slab in the MTZ (i.e., in the region west of the 500-km depth contour of the slab). The magenta stars denote epicenters of large earthquakes (M ≥7.0) during 1975–2016 derived from the Bulletin of the International Seismological Center. The hypocenters of these large earthquakes are shown in Fig. 6. The other labeling is the same as that in Fig. 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. The same as Fig. 4 but the white contour lines show the subduction age distribution of the Pacific slab. Along the Kuril–Japan–Izu–Bonin–Mariana trench axis, the slab’s subduction age is zero.
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Fig. 6. Vertical cross-sections of P-wave tomography along the eight profiles shown in Fig. 1. The red and blue colors denote low and high velocity perturbations, respectively, whose scale (in %) is shown at the bottom-middle. The color bar with blue numbers above each cross-section shows the subducting Pacific lithosphere ages from the west (East China) to the east (near the trench axis), whose scale (in Ma) is shown at the bottom-left. The surface topography along each profile is shown above the lithosphere age bar. The color bar with red numbers below each cross-section shows the subduction ages of the Pacific slab, whose scale (in Ma) is shown at the bottom-right. The red and pink triangles atop each cross-section denote locations of active volcanoes and Cenozoic basalts, respectively, within a 1◦ width of each profile. The background seismicity and large earthquakes (M ≥7.0) that occurred within a 1◦ width of each profile are shown in white circles and red stars, respectively. The two black dashed lines denote the 410 and 660 km discontinuities. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ing (Li et al., 2013). Numerical simulations considering the slab buckling in the MTZ suggest that the western edge of the flat slab beneath East China is the former mid-ocean ridge between the Izanagi and Pacific plates subducted at ∼60 Ma (Honda, 2016). Unfortunately, although undulations exist in the upper and lower boundaries of the flat slab (Fig. 6), as well as the 410-km and 660-km discontinuities beneath East Asia (e.g., Gu et al., 2012; Tian et al., 2016), we cannot quantitatively identify potential slab buckling using our tomographic model or other seismic results due to the limited spatial resolution of the seismic images. Considering the resolution scale of the tomographic model, the slab morphology and the paleo-age data used in this study, we estimate the errors of T lith , T subduct and T trench to be ±5–10 Ma for the older slab (∼130 < Ttrench < 90 Ma), and ±10–15 Ma for the younger slab (∼90 < Ttrench < 60 Ma) beneath East China where slab buckling may occur.
5. Discussion Our results (Figs. 4 and 6) show that the western edge of the flat slab in the MTZ is ∼90 Ma old, while the Pacific plate near the Japan Trench formed at ∼135 Ma. This result suggests that the flat slab in the MTZ beneath East Asia is the subducted Pacific plate rather than the Izanagi plate. The subducted Izanagi plate may have already sunk into the lower mantle (Seton et al., 2015). Many previous studies have attempted to link deep mantle tomography with the plate reconstructions, and estimated the slab ages at depth (e.g., van der Voo et al., 1999; van der Meer et al., 2010; Domeier et al., 2016). However, because the global tomographic models generally have a low resolution, most of the previous studies focused on the slab in the lower mantle by assuming near-vertical slab sinking and ignoring the slab deformation and stagnancy in the MTZ or the uppermost lower mantle (Zhao, 2004; Fukao and Obayashi, 2013; Marquardt and Miyagi, 2015). Episodic slab sinking may be more realistic, at least at depths <∼1000 km,
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Fig. 7. (a) A map view showing the distribution of the Pacific slab’s sinking rate beneath East Asia. (b) A map view showing the distribution of the Pacific slab’s advancing rate beneath East Asia. The dark blue contour lines in (a) and the white contour lines in (b) show the distribution of the oceanic lithosphere age at the time when the Pacific plate started to subduct at the trench (T trench ). The other labeling in (a, b) is the same as that in Fig. 1. (c) The green area shows the relation between the Pacific slab depth and its corresponding subduction age obtained by this study. (d) The green area shows the relation between the Pacific slab’s advancing distance and its corresponding subduction age obtained by this study. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
though a linear sinking rate is statistically significant (Domeier et al., 2016). With the slab’s subduction ages obtained by this study (Fig. 5), we estimate the vertical sinking rate (V sink ) distribution of the dipping Pacific slab in the upper mantle and the MTZ, using a relation V sink = S dep / T subduct , where S dep is slab depth. The obtained results (Figs. 7a, c) show that the slab sinking rate is in a range of 3.1–6.9 cm/yr. Our result also shows that the flat Pacific slab has remained in the MTZ beneath East Asia no longer than ∼10–20 Myr (Fig. 7c). In addition, we also estimate the advancing rate (V adv ) of the Pacific slab in the upper mantle and the MTZ, using a relation V adv = S dis / T subduct , where S dis is a distance between a position on the Earth’s surface directly above a point j on the imaged slab and the corresponding position where the point j entered the subduction zone (Fig. S5). The obtained results (Figs. 7b, d) show that the slab advancing rate ranges from 6.1 to 7.7 cm/yr. These results may provide a useful constraint on the time-dependent geodynamic modeling.
Extensive intraplate volcanism and back-arc spreading mark the Cenozoic destruction of the East Asian continental lithosphere (Xu et al., 2012; Li et al., 2012a; Fig. 1), which is considered to be associated with upwelling of hot and wet asthenospheric materials in the BMW above the flat Pacific slab in the MTZ (e.g., Zhao et al., 2011; Fig. 6). In addition, the NCC destruction during the Early Cretaceous with cratonic lithospheric removal and/or replacement has been attributed to the deep subduction of the Paleo-Pacific (or the Izanagi) plate beneath East Asia (Zhu et al., 2012; Liu et al., 2013). Our present results suggest that the present flat Pacific slab has stayed in the MTZ for no more than ∼10–20 Myr. Thus, the present flat Pacific slab may have been only associated with the Cenozoic tectono-magmatism in East Asia, and it has nothing to do with the Early Cretaceous tectono-magmatism in the region. We think that an initial BMW may have formed during that time (∼140–110 Ma) and resulted in the NCC destruction. Continuous slab subduc-
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tion beneath East Asia since the Early Mesozoic (before ∼250 Ma; Li et al., 2012b) may have brought a great amount of water into the upper mantle beneath East Asia, feeding the BMW and the intraplate volcanoes. Hence, the BMW may have existed much longer than any flat slabs in the MTZ beneath East Asia. 6. Conclusions The age distribution of the subducting Pacific plate beneath East Asia is investigated using a high-resolution tomographic model and paleo-age data of ancient seafloor. Major results of this work are summarized as follows. (1) The imaged subducting oceanic lithosphere becomes younger from the Japan Trench (∼130 Ma) to the slab’s western tip (∼90 Ma) beneath East Asia. Such a feature indicates that the flat slab now in the MTZ beneath East Asia is the subducted Pacific slab rather than the Izanagi slab which should have already sunk into the lower mantle. (2) The slab’s subduction age ranges from 0 Ma at the present-day trench to ∼30 Ma at the western tip of the flat slab in the MTZ beneath East Asia. The stagnant duration of the flat Pacific slab in the MTZ is no more than ∼10–20 million years, much shorter than the age of the big mantle wedge beneath East Asia (>110 million years). (3) It is the present Pacific slab that has contributed to the Cenozoic lithosphere destruction, extensive intraplate volcanism, and back-arc spreading in East Asia, whereas the North China Craton destruction during the Early Cretaceous (∼140–110 Ma) was caused by the subduction of the Izanagi (or the Paleo-Pacific) plate. Acknowledgements We thank Prof. R. Müller for sharing their age data of ancient seafloor. We appreciate the very helpful suggestions from Profs. R. Hall and W. Spakman, and the discussion with Dr. Y. Wang. We are very grateful to Prof. An Yin (the Editor), Prof. R. Hall and an anonymous reviewer who provided thoughtful review comments and suggestions which have improved this paper. The free software GMT (Wessel and Smith, 1998) is used for making the figures. This work was supported by grants from the JSPS (Kiban-S 23224012) and the MEXT (26106005) to D. Zhao, and grants from the Chinese NSFC (41602207, 41190072 and 41325009) to S. Li and X. Liu. Appendix A. Supplementary material Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2017.02.024. References Balmino, G., Vales, N., Bonvalot, S., Briais, A., 2012. Spherical harmonic modelling to ultra-high degree of Bouguer and isostatic anomalies. J. Geod. 86, 499–520. Bijwaard, H., Spakman, W., Engdahl, E., 1998. Closing the gap between regional and global travel time tomography. J. Geophys. Res. 103, 30055–30078. Billen, M., 2008. Modeling the dynamics of subducting slabs. Annu. Rev. Earth Planet. Sci. 36, 325–356. Boschman, L., van Hinsbergen, D., 2016. On the enigmatic birth of the Pacific Plate within the Panthalassa Ocean. Sci. Adv. 2, e1600022. http://dx.doi.org/10.1126/ sciadv.1600022. Bower, D., Gurnis, M., Flament, N., 2015. Assimilating lithosphere and slab history in 4-D Earth models. Phys. Earth Planet. Inter. 238, 8–22. Chatelain, J., Guillier, B., Gratier, J., 1993. Unfolding the subducting plate in the central New Hebrides Island ARC: geometrical argument for detachment of part of the downgoing slab. Geophys. Res. Lett. 20, 655–658. Chen, C., Zhao, D., Tian, Y., Wu, S., Hasegawa, A., Lei, J., Park, J., Kang, I., 2017. Mantle transition zone, stagnant slab and intraplate volcanism in Northeast Asia. Geophys. J. Int. 209, 68–85. http://dx.doi.org/10.1093/gji/ggw491.
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