P-wave arrival times of both regional and teleseismic earthquakes were inverted to obtain mantle structures of East Asia. No fast （slab） velocity anomalies was not find beneath the 660-kin discontinuity through tomo...P-wave arrival times of both regional and teleseismic earthquakes were inverted to obtain mantle structures of East Asia. No fast （slab） velocity anomalies was not find beneath the 660-kin discontinuity through tomography besides a stagnant slab within the transition zone. Slow P-wave velocity anomalies are present at depths of 100-250 km below the active volcanic arc and East Asia. The western end of the flat stagnant slab is about 1 500 km west to active trench and may also be correlated with prominent surface topographic break in eastern China. We suggested that active mantle convection might be operating within this horizontally expanded ＂mantle wedge＂ above both the active subducting slabs and the stag- nant flat slabs beneath much of the North China plain. Both the widespread Cenozoic volcanism and associated extensional basins in East Asia could be the manifestation of this vigorous upper mantle convection. Cold or thermal alaomalies associated with the stagnant slabs above the 660-km discontinuity have not only caused a broad depression of the boundary due to its negative Clapeyron slope but also effectively shielded the asthenosphere and continental lithosphere above from any possible influence of mantle plumes in the lower mantle.展开更多
This is the first of two papers that describes a regional tomography investigation, which combines P-wave arrival times of both regional and teleseismic earthquakes to obtain 3D mantle structures of East Asia up to 1 ...This is the first of two papers that describes a regional tomography investigation, which combines P-wave arrival times of both regional and teleseismic earthquakes to obtain 3D mantle structures of East Asia up to 1 000 km depth. The most important findings of this tomography study are reported in this paper as follows. （1） No fast P-wave velocity anomalies can be related to subducted oceanic slabs beneath the 660 km discontinuity; instead the subducted oceanic slabs become flattened and stagnant within the transition zone. （2） The high velocity anomalies in the transition zone extend up to 1 500 km to the westward of the active trenches, which is a unique feature in the worldwide subduetion systems. （3） Slow P-wave velocity anomalies are visible up to -250 km underneath most of the East Asia on the east of 115°E, similar to the area of the stagnant slabs. These observations have important implications for the geodynamic process at depths beneath the East Asia, which might in turn control the widespread Cenozoic volcanism and associated extensional tectonics seen at the Earth＇s surface.展开更多
Seismic observations have shown structural variation near the base of the mantle transition zone （MTZ） where subducted cold slabs, as visualized with high seismic speed anomalies （HSSAs）, flatten to form stagnant ...Seismic observations have shown structural variation near the base of the mantle transition zone （MTZ） where subducted cold slabs, as visualized with high seismic speed anomalies （HSSAs）, flatten to form stagnant slabs or sink further into the lower mantle. The different slab behaviors were also accompanied by variation of the ＂660 kin＂ discontinuity depths and low viscosity layers （LVLs） beneath the MTZ that are suggested by geoid inversion studies. We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics. A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ. We found that the viscosity reduction of subducted crustal material leads to a sepa- ration of crustal material from the slab main body and its transient stagnation in the MTZ. The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20 30 My from the start of the plate subduction. The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model. Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals, dehydration should occur at the phase transformation depth, ~660 kin. The variation of the disconti- nuity depths and highly localized low seismic speed anomaly （LSSA） zones observed from seismic P waveforms in a relatively high frequency band （~ 1 Hz） support the hypothesis of dehydration from hydrous slabs at the phase boundary. The LSSAs which correspond to dehydration induced fluids are likely to be very local, given very small hydrogen （H＋） diffusivity associated with subducted slabs. The image of such local LSSA zones embedded in HSSAs may not be necessarily captured in tomography studies. The high electrical conductivity in the MTZ beneath the northwestern Pacific subduction zone does not necessarily require a broad range of high water content homogeneously.展开更多
基金grants(B-11440134,S-12002006)to Dapeng Zhao from the Japan Society for the Promotion of ScienceSupport for Shunping Pei came from a postdoct grant of Peking University+1 种基金supported by the National Natural Science Foundation of China(Nos.40125011,90814002 and 41074041)the Chinese Academy of Sciences(No.KZCX2-EW-QN102)
文摘P-wave arrival times of both regional and teleseismic earthquakes were inverted to obtain mantle structures of East Asia. No fast （slab） velocity anomalies was not find beneath the 660-kin discontinuity through tomography besides a stagnant slab within the transition zone. Slow P-wave velocity anomalies are present at depths of 100-250 km below the active volcanic arc and East Asia. The western end of the flat stagnant slab is about 1 500 km west to active trench and may also be correlated with prominent surface topographic break in eastern China. We suggested that active mantle convection might be operating within this horizontally expanded ＂mantle wedge＂ above both the active subducting slabs and the stag- nant flat slabs beneath much of the North China plain. Both the widespread Cenozoic volcanism and associated extensional basins in East Asia could be the manifestation of this vigorous upper mantle convection. Cold or thermal alaomalies associated with the stagnant slabs above the 660-km discontinuity have not only caused a broad depression of the boundary due to its negative Clapeyron slope but also effectively shielded the asthenosphere and continental lithosphere above from any possible influence of mantle plumes in the lower mantle.
基金grants(B-11440134,S-12002006)to Dapeng Zhao from the Japan Society for the Promotion of ScienceSupport for Shunping Pei came from a postdoct grant of Peking University+1 种基金supported by the Chinese Academy of Sciences(No.KZCX2-EW-QN102)the National Natural Science Foundation of China(Nos.41074041,90814002 and 40125011)
文摘This is the first of two papers that describes a regional tomography investigation, which combines P-wave arrival times of both regional and teleseismic earthquakes to obtain 3D mantle structures of East Asia up to 1 000 km depth. The most important findings of this tomography study are reported in this paper as follows. （1） No fast P-wave velocity anomalies can be related to subducted oceanic slabs beneath the 660 km discontinuity; instead the subducted oceanic slabs become flattened and stagnant within the transition zone. （2） The high velocity anomalies in the transition zone extend up to 1 500 km to the westward of the active trenches, which is a unique feature in the worldwide subduetion systems. （3） Slow P-wave velocity anomalies are visible up to -250 km underneath most of the East Asia on the east of 115°E, similar to the area of the stagnant slabs. These observations have important implications for the geodynamic process at depths beneath the East Asia, which might in turn control the widespread Cenozoic volcanism and associated extensional tectonics seen at the Earth＇s surface.
基金supported partly by a Grant-in-Aid for Scientific Research(B)(Grant Number 23340132) from the Ministry of Education,Culture,Sports,Science and Technology(MEXT),Japan
文摘Seismic observations have shown structural variation near the base of the mantle transition zone （MTZ） where subducted cold slabs, as visualized with high seismic speed anomalies （HSSAs）, flatten to form stagnant slabs or sink further into the lower mantle. The different slab behaviors were also accompanied by variation of the ＂660 kin＂ discontinuity depths and low viscosity layers （LVLs） beneath the MTZ that are suggested by geoid inversion studies. We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics. A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ. We found that the viscosity reduction of subducted crustal material leads to a sepa- ration of crustal material from the slab main body and its transient stagnation in the MTZ. The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20 30 My from the start of the plate subduction. The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model. Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals, dehydration should occur at the phase transformation depth, ~660 kin. The variation of the disconti- nuity depths and highly localized low seismic speed anomaly （LSSA） zones observed from seismic P waveforms in a relatively high frequency band （~ 1 Hz） support the hypothesis of dehydration from hydrous slabs at the phase boundary. The LSSAs which correspond to dehydration induced fluids are likely to be very local, given very small hydrogen （H＋） diffusivity associated with subducted slabs. The image of such local LSSA zones embedded in HSSAs may not be necessarily captured in tomography studies. The high electrical conductivity in the MTZ beneath the northwestern Pacific subduction zone does not necessarily require a broad range of high water content homogeneously.
