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Mid-Mantle Heterogeneities Associated with Izanagi Plate: Implications for Regional Mantle Viscosity ∗ Juan Li A, , David A

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Mid-Mantle Heterogeneities Associated with Izanagi Plate: Implications for Regional Mantle Viscosity ∗ Juan Li A, , David A Earth and Planetary Science Letters 385 (2014) 137–144 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Mid-mantle heterogeneities associated with Izanagi plate: Implications for regional mantle viscosity ∗ Juan Li a, , David A. Yuen b,c a Key Laboratory of the Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029 Beijing, China b School of Environment Sciences, China University of Geosciences, 430074 Wuhan, China c Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, USA article info abstract Article history: We employed S-to-P converted waveforms to detect mid-mantle scattering beneath northeastern China Received 20 July 2013 and the adjacent Japan Sea. Broadband and short-period waveforms recorded by seismic arrays for eight Received in revised form 16 October 2013 moderate-sized deep earthquakes were analyzed using a non-linear array stacking technique, and mid- Accepted 23 October 2013 mantle scatterers within the depth range 930–1120 km were clearly revealed. The heterogeneities have Available online 12 November 2013 an overall lateral extent of ∼800 km, and mostly occur within a region with a high velocity anomaly. Editor: P. Shearer The accumulation of MORB-like slab materials at mid-mantle depths might cause a different chemical Keywords: composition than that of the surrounding peridotitic mantle. The spatial isolation of the heterogeneities mid-mantle discontinuity from the stagnant Pacific slab suggests an origin related to the subduction of ancient Izanagi plate. subduction In combination with the reconstruction history of plate motions, we estimate the viscosity of the topmost Izanagi plate lower mantle to vary from 1.0 × 1022 to 1.6 × 1023 Pa s, which can be used as an independent constraint viscosity on the rheology of the lower mantle on a regional scale. © 2013 Elsevier B.V. All rights reserved. 1. Introduction depth of ∼920 km beneath the Tonga subduction zone (Kawakatsu and Niu, 1994), and scatterers around depths of 1000–1200 km Mapping the mantle heterogeneities in the lower mantle can and 1600 km have been mapped beneath Izu–Bonin–Mariana and assist our understanding of the distribution of geochemical reser- other subduction zones (Niu and Kawakatsu, 1997; Kaneshima and voirs and the scale of mantle circulation (e.g., Kawakatsu and Helffrich, 1999; Castle and Creager, 1999; Vinnik et al., 2001; Niu, 1994; Kaneshima and Helffrich, 1999; Helffrich and Wood, Vanacore et al., 2006; Niu, 2013). A velocity discontinuity at 2001; Castle and van der Hilst, 2003). The sources and loca- ∼1050 km has been revealed beneath mantle upwelling regions, tions of the distinct chemical reservoirs that provide the differ- e.g., Iceland and the Hawaiian Islands, arguing against a globally ent signatures of ocean island basalts (OIB) and mid-mantle ridge continuous mineralogical phase change near the depth of the mid- basalts (MORB) has been actively debated (e.g. Hofmann, 1997; mantle (Shen et al., 2003). Kellogg et al., 1999). Models with reservoir boundaries at different Seismic tomography has revealed a prominent stagnant slab depths in the mantle and chemically distinct blobs embedded in lying horizontally in the upper mantle transition zone (MTZ) un- the lower mantle have both been invoked to explain the geochem- der the Japanese subduction zone and extending 1000–2000 km ical, heat flow and seismic observations (Wen and Anderson, 1997; to the west (Fukao et al., 2001; Huang and Zhao, 2006), which Kellogg et al., 1999; Tackley, 2000). makes this location an ideal place to investigate geodynamical Compared with the heterogeneous upper mantle, the lower problems (Fig. 1). We undertook a systematic and thorough search mantle seems to be generally homogeneous, except for the lower- for deep mantle heterogeneities beneath northeastern China and most several hundred kilometers above the core–mantle boundary. the adjacent Japan Sea. Array stacking techniques were applied A smoothly increasing wave speed and density profile through to detect the weak signature of the scattered waves indicative the lower mantle has been inferred from various global mod- of the lower mantle structure. Broad mid-mantle scatterers with els (Dziewonski and Anderson, 1981; Kennett and Engdahl, 1991). depths ranging from 930 to 1120 km were revealed to the east Studies using scattered seismic waves, however, have detected seis- of the trapped stagnant slab. We compared our results to deep mic discontinuities and/or reflectors in the mid- to lower mantle. images obtained from seismic investigations and plate reconstruc- A local discontinuity with little topography has been imaged at a tion studies. We argue that subduction of the ancient Izanagi plate has caused the mid-mantle scatterers. From the spatial distribu- * Corresponding author. tion of this heterogeneity, the local rheology of the top portion E-mail address: [email protected] (J. Li). of the lower mantle can be constrained quite well, which in turn 0012-821X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.10.042 138 J. Li, D.A. Yuen / Earth and Planetary Science Letters 385 (2014) 137–144 Fig. 1. Location of the eight moderate-sized events investigated and the station distributions. (a) The large beachballs show the source mechanisms of the seven events with the S1000P phase observed; the pink dot indicates event #8 with no S1000P identified. The red point clusters are the conversion points calculated at their estimated depth for the S1000P phase, and the adjacent numbers indicate the depths of the observed scatterers. The P velocity model (Fukao et al., 2001) for the depth range 900 to 1000 km is shown in the background. Line AB indicates the position of the cross-section shown in Fig. 5. Yellow points indicate the conversion location of SdP phase of the Mw 6.9 earthquake analyzed by Niu (2013). (b) Distribution of seismic arrays; the triangles indicate the stations used. Insert is a schematic plot showing the ray paths of the P and SdP phases. provides an additional constraint for unraveling of the dynamical 2.1. Preprocessing processes and tectonic history under the western Pacific up to at least 50 Myr. We processed the raw data using the following basic steps. First, we examined every vertical-component seismogram and ex- cluded bad traces and those with SNR < 5. When necessary, we re- 2. Data and method versed the polarity of the recorded seismograms. We applied a band-pass filter between 1 and 5 s to the retained seismograms, and handpicked the first peak of the P waves. We then aligned We used source-sided S-to-P conversion waves to detect mid- these to zero seconds, which was then used as a reference time mantle scatterers. This kind of wave, named SdP, starts as a down- for further stacking. We normalized the seismograms according to going S wave and is subsequently converted to a P wave at the their maximum amplitude in the time window −10 to 120 s rel- mantle discontinuity or reflector (Fig. 1). Compared to the direct ative to the direct P waves (Kawakatsu and Niu, 1994; Castle and P wave, the mid-mantle S-to-P converted wave has a lower slow- Creager, 1999). A shorter time window was selected for two earlier ness, and is thus received at a steeper angle of incidence. Because seismic records because of the limitations of the record time. of the small amplitude of the SdP phase, a large-scale regional The waveforms recorded by UW for events #2 and #7 are seismic array stacking technique (e.g., Kawakatsu and Niu, 1994; shown in Fig. 2(a)–(b). As well as the direct P and surface con- Li et al., 2008) was used to enhance the coherent later arrivals, verted pP phases, we found another clear phase ∼40–50 s after the which helped to identify the mid-mantle discontinuities. direct P wave in most of the individual seismograms. No seismic We studied a total of eight moderate-sized deep earthquakes waves would be expected to arrive in this time window according (5.3 mb 6.0) that occurred after the year 1980 beneath the to the 1D global reference model. The measured particle motion, Russian–Chinese border and the Japan Sea (Fig. 1(a)). For earth- the incident angle, the approaching direction of this arrival, and quakes that occurred before 2008, we used the EHB catalog with later detailed analysis suggest that this is an S to P conversion the source parameters relocated (Engdahl et al., 1998); for later wave at a mid-mantle depth of ∼1000 km (named as S1000P here- events, the location information was taken either from the NECI after). No S to P converted phase is visible in any individual records catalog or previous work, if available (e.g., Li et al., 2013). All these in a similar time window for the other six events. events have a simple source-time function, which significantly re- duces contamination of the complex source rupture process to the 2.2. Nth-root slant stacking weak later arrivals. The seismic arrays used in this study include the Pacific Northwest Regional Seismic Network (UW), the Caltech We applied Nth-root stacking to enhance the low amplitude of Regional Seismic Network (CI), the German Regional Seismic Net- S to P conversions from mantle discontinuities. Details of the stack- work (GRSN) and the Alaska Regional Network (AK) (Fig. 1(b)). ing procedure are given by Kawakatsu and Niu (1994) and Li et al. Both the UW and CI networks have ∼100–200 short-period sta- (2008). We varied the slowness within the range ±1s/deg (rela- tions, while the GRSN and AK arrays consist of ∼30 and ∼20 tive to P) with an increment of 0.02 s/deg.
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