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Abstract: Oceanic plates subducted at trenches penetrate into the deep mantle, and encounter a structural boundary at a depth of 410km where olivine, the dominant element of mantle rocks, transforms into a higher density form wadsleyite. This transformation may be delayed within the coldest core of subducting plates (slabs) due to kinetic effects, and it has been suggested that metastable olivine may persist deeper than 410km. Using high density seismic array data in Japan, we show the direct image of the structure corresponding to this metastable olivine wedge (MOW) beneath southwest Japan. Numerical simulation of a subducting slab, including the kinetic effect of water (H2O) on the olivine–wadsleyite transformation, indicates that the presence of the imaged MOW requires an insignificant amount of water (less than 100wt. ppm) be present in the slab mantle, thus a deep dry cold slab. We infer that the transportation of water into the deep mantle occurs along the top surface of the subducting slab, but no significant amount within the slab itself. We also demonstrate that a numerical simulation including the kinetics of 660-km phase transformation can reconcile the observed deep depression of the 660-km discontinuity with a gentle Clapeyron slope. [Copyright &y& Elsevier]

In order to better understand the deep subduction geometry of the Pacific plate and genesis of intraplate volcanism in northeast China (NE China), we computed a total of 45,505 receiver functions from 788 teleseismic events recorded by 255 stations (NECESSArray temporal and permanent stations) in NE China. We used a common-conversion-point stacking (CCP) method to generate a 3D reflectivity volume beneath the study area. To position the P-to-S conversions to the correct depths, we employed 3D crustal and mantle models as references to make time to depth conversion. The 3D reflectivity volume was generated in an area between 115°–135°E and 40°–49°N, in the depth range of 300 to 800 km. We found significant topographic relief on the 660-km discontinuity across the study area. In particular, in a westward Pacific plate subduction section between 40°N and ∼45.5°N, the 660-km discontinuity is depressed by as much as ∼30–40 km along the western extension of the deep seismicity. The depression is elongated along the strike of the deep seismicity and is confined to a 200–300 km region in the E–W direction of subduction. To the west of this depression the 660-km discontinuity is uplifted by 5–10 km in a rectangular area of ∼100 km by 200 km centered at about 125°E and 43°N. In the north, the 660-km discontinuity is moderately depressed (∼20 km) in a broad area that extends further west. The high and low regions in the 660-km topographic map correlate, respectively, with low- and high-velocity anomalies in the P- and S-wave tomographic velocity images at the same depth. Our results suggest that slab stagnation might not be occurring in the southern part of the NE China, where the Changbaishan volcanic complex is located, thus the magmatism is unlikely caused by dehydration of the flat-lying Pacific slab in the transition zone. The low velocity mantle upwelling arising from a gap of stagnant slabs is a likely source that feeds the volcanic complex in NE China. [ABSTRACT FROM AUTHOR]

Abstract: We analyzed tens of thousands of receiver-function data recorded by 185 broadband seismic stations to study the crustal structure beneath the northeast China. Moho depth and average crustal ratio were measured at each station using the H–κ grid searching technique. For stations located above unconsolidated sediments, we applied the H–β method to remove strong shallow reverberations and generate subsurface receiver functions that allow for effective H–κ analysis. The measurements show that the Songliao basin has a relatively thin crust (∼31 km), and the Moho depth increases significantly from southeast (∼27 km) to northwest (∼35 km). The northwestward tilting of the Songliao basin may suggest that it was initiated by lithospheric flexure due to the load of the Great Xing'an range in the Jurassic before the large-scale extension in the Cretaceous. Moho depth varies from 26.7 km to 42.3 km across the study area, with the shallowest and deepest Moho being located at the eastern flank of the Songliao basin and the Great Xing'an range, respectively. The Moho depth correlates well with the surface topography in the western part, but not the central and eastern parts of the study area. The residual topography computed based on the Airy's isostasy model indicates that the high topography at the eastern flank of the Songliao basin, the Changbaishan volcanic center, and the southern end of the Great Xing'an range is likely dynamically supported by the upper mantle. [Copyright &y& Elsevier]