The Izu-Bonin subduction zone in the Northwest Pacific is an ideal location for understanding mantle dynamics such as cold lithosphere subduction. The slab produces a lateral thermal anomaly, inducing local topographi...The Izu-Bonin subduction zone in the Northwest Pacific is an ideal location for understanding mantle dynamics such as cold lithosphere subduction. The slab produces a lateral thermal anomaly, inducing local topographic changes at the boundary of a post-spinel phase transformation, considered to be the origin of the ‘660-km discontinuity.’ In this study, the short-period(1–2 Hz) S-to-P conversion phase S660P was used to obtain the fine-scale structure of the discontinuity. More than 100 earthquakes that occurred from the 1980s to the 2020s and were recorded by high-quality seismic arrays in the United States and Europe were analyzed. A discontinuity in the ambient mantle with an average depth of ~670 km was found beneath the 300–400-km event zone in the northern Bonin region near 33°N. Meanwhile, the ‘660-km discontinuity’ has been pushed upward, away from the slab, possibly because of a hot upwelling mantle plume. In the central part of the subduction zone, the 660-km discontinuity is depressed to an average depth of(690 ± 5) km within the slab at approximately 150 km below the coldest slab core, indicating a(300 ± 100) ℃ cold anomaly estimated using a post-spinel transformation Clapeyron slope of(-2.0 ± 1.0) MPa/K. In southern Bonin near 28°N, the discontinuity was found to be further depressed at an average depth of(695 ± 5) km below the deepest event and with a focal depth of ~550 km. The discontinuity is located where the slab bends abruptly to become sub-horizontal toward the west-southwest. Near the zone of the isolated Bonin Super Deep Earthquake, which occurred at ~680 km on May 30,2015, the discontinuity is depressed to ~700 km, suggesting a near-vertical penetrating slab and an S-to-P conversion in the coldest slab core, where a large low-temperature anomaly should exist.展开更多
The 660-km discontinuity that separates the Earth's upper and lower mantle has primarily been attributed to phase changes in olivine and other minerals.Resolving the sharpness is essential for predicting the compo...The 660-km discontinuity that separates the Earth's upper and lower mantle has primarily been attributed to phase changes in olivine and other minerals.Resolving the sharpness is essential for predicting the composition of the mantle and for understanding its dynamic effects.In this study,we used S-to-P conversions from the 660-km interface,termed S660P,arriving in the P-wave coda from one earthquake in the Izu–Bonin subduction zone recorded by stations in Alaska.The S660P signals were of high quality,providing us an unprecedented opportunity to resolve the sharpness of the discontinuity.Our study demonstrated,based on the impedance contrast given by the IASP91 model,that the discontinuity has a transitional thickness of^5 km.In addition,we observed a prominent arrival right after the S660P,which was best explained by S-to-P conversions from a deeper discontinuity at a depth of^720 km with a transitional thickness of^20 km,termed S720P.The 720-km discontinuity is most likely the result of a phase transition from majoritic garnet to perovskite in the segregated oceanic crust(mainly the mid-oceanic ridge basalt composition)at the uppermost lower mantle beneath this area.The inferred phase changes are also consistent with predictions from mineral physics experiments.展开更多
Digital waveform data recorded by the vertical component short period stations at the American networks of SCSN, NCSN and PNSN and three components broadband stations at the Germany and Swiss networks and arrays of GR...Digital waveform data recorded by the vertical component short period stations at the American networks of SCSN, NCSN and PNSN and three components broadband stations at the Germany and Swiss networks and arrays of GRFN, GRSN and SDSNet for the events between 1981 and 2000 under Izu-Bonin are used as data sets. The N-th root slant stack method was used to pick up the SdP phase converted at the velocity interface beneath source and the regionalized difference of the 660 km discontinuity beneath Izu-Bonin is studied. It is found that while the dip angles of the subducting slab and the maximal depths of sources increase gradually from 35N to 26N, the 660 km discontinuity appears regionalized differences. The discontinuity exists at 660 km while there is no effect from subducting slab, but it is depressed to the depth of 720 km while there are obvious effects. The dispersion of converted points is still an unsolved problem which maybe result from the complex structure of the discontinuity, converted phases which were misjudged, or the assumption of one dimensional spherical earth model.展开更多
The Izu-Bonin arc system is sediment-poor(~400 m thick with no accretionary prism)and,therefore,the influence of the altered oceanic crust(AOC)is most likely the source of the documented along-arc lava compositional v...The Izu-Bonin arc system is sediment-poor(~400 m thick with no accretionary prism)and,therefore,the influence of the altered oceanic crust(AOC)is most likely the source of the documented along-arc lava compositional variations,especially in Pb isotopes.Izu-Bonin arc lava geochemistry suggests an influx of subduction component from an Indian-type AOC.However,samples drilled from the western Pacific geochemical reference site at Integrated Ocean Drilling Program Site 1149 implies subduction of a Pacific-type AOC.To solve the apparent discrepancy of slab input versus arc output in this arc system,samples of the AOC were dredged from vertical fault scarps of the subducting Pacific Plate along a transect from 27.5°N to 34.50 N.Samples range from tholeiitic to mildly alkalic mid-ocean ridge basalts as well as trachybasalts,basaltic trachyandesites,tephrites,and phono-tephrites.Isotope ratios also exhibit a range of values(87Sr/86Sr=0.70282-0.70673,143 Nd/144 Nd=0.512552-0.513174,206Pb/204 Pb=18.43-20.00,207 Pb/204 Pb=15.40-15.67,208Pb/204Pb=37.75-39.55).Our results suggest that there is a geochemical variation in the AOC that is neither completely due to seawater or hydrothermal alteration,nor to petrogenetic processes.Rather,this variation is the result of the Pacific-Izanagi Ridge system tapping into a heterogeneous,plume-polluted mantle source during the Mid-Cretaceous volcanic event.The observed Pacific-type AOC is not responsible for the Indian-type Pb isotopic signature of Izu-Bonin arc lavas.This leads us to propose an alternative scenario where the Izu-Bonin arc lava Indian-type Pb isotopic signature originates from slab-derived fluids interacting and adsorbing Pb from an Indian-type mantle wedge through zone-refining.展开更多
The SdP, pdP and sdP phases are picked up with the Nth root slant stack method from the digital waveform data recorded by the networks and arrays in USA, Germany and Switzerland for the earthquakes occurring beneath I...The SdP, pdP and sdP phases are picked up with the Nth root slant stack method from the digital waveform data recorded by the networks and arrays in USA, Germany and Switzerland for the earthquakes occurring beneath Izu-Bonin and Japan Sea. The mantle discontinuities and the effects of subducting slab on the 660 km and 410 km discontinuities are studied. It is found that there are mantle discontinuities existing at the depths of 170, 220, 300, 410, 660, 850 and 1150 km. Beneath Izu-Bonin, the 410 km discontinuity is elevated, while the 660 km discontinuity is depressed; for both discontinuities, there are regionalized differences. Beneath Japan Sea, however, there is no depth variation of the 410 km discontinuity, and the 660 km discontinuity is depressed without obvious effect of the subducting slab.展开更多
文摘The Izu-Bonin subduction zone in the Northwest Pacific is an ideal location for understanding mantle dynamics such as cold lithosphere subduction. The slab produces a lateral thermal anomaly, inducing local topographic changes at the boundary of a post-spinel phase transformation, considered to be the origin of the ‘660-km discontinuity.’ In this study, the short-period(1–2 Hz) S-to-P conversion phase S660P was used to obtain the fine-scale structure of the discontinuity. More than 100 earthquakes that occurred from the 1980s to the 2020s and were recorded by high-quality seismic arrays in the United States and Europe were analyzed. A discontinuity in the ambient mantle with an average depth of ~670 km was found beneath the 300–400-km event zone in the northern Bonin region near 33°N. Meanwhile, the ‘660-km discontinuity’ has been pushed upward, away from the slab, possibly because of a hot upwelling mantle plume. In the central part of the subduction zone, the 660-km discontinuity is depressed to an average depth of(690 ± 5) km within the slab at approximately 150 km below the coldest slab core, indicating a(300 ± 100) ℃ cold anomaly estimated using a post-spinel transformation Clapeyron slope of(-2.0 ± 1.0) MPa/K. In southern Bonin near 28°N, the discontinuity was found to be further depressed at an average depth of(695 ± 5) km below the deepest event and with a focal depth of ~550 km. The discontinuity is located where the slab bends abruptly to become sub-horizontal toward the west-southwest. Near the zone of the isolated Bonin Super Deep Earthquake, which occurred at ~680 km on May 30,2015, the discontinuity is depressed to ~700 km, suggesting a near-vertical penetrating slab and an S-to-P conversion in the coldest slab core, where a large low-temperature anomaly should exist.
基金We are grateful for the thoughtful and constructive comments provided by two anonymous reviewers and the editor(Dr.Wei Leng).We also thank Jinfeng Hu for his contributions to this work at an early stage.Seismic data from the USArray network were accessed via the Data Management Center(DMC)of the Incorporated Research Institutions for Seismology(IRIS).Some figures were prepared using Generic Mapping Tools(GMT,Wessel and Smith,1999)GNUPLOT.This work was funded by the National Natural Science Foundation of China(grant no.91858205).
文摘The 660-km discontinuity that separates the Earth's upper and lower mantle has primarily been attributed to phase changes in olivine and other minerals.Resolving the sharpness is essential for predicting the composition of the mantle and for understanding its dynamic effects.In this study,we used S-to-P conversions from the 660-km interface,termed S660P,arriving in the P-wave coda from one earthquake in the Izu–Bonin subduction zone recorded by stations in Alaska.The S660P signals were of high quality,providing us an unprecedented opportunity to resolve the sharpness of the discontinuity.Our study demonstrated,based on the impedance contrast given by the IASP91 model,that the discontinuity has a transitional thickness of^5 km.In addition,we observed a prominent arrival right after the S660P,which was best explained by S-to-P conversions from a deeper discontinuity at a depth of^720 km with a transitional thickness of^20 km,termed S720P.The 720-km discontinuity is most likely the result of a phase transition from majoritic garnet to perovskite in the segregated oceanic crust(mainly the mid-oceanic ridge basalt composition)at the uppermost lower mantle beneath this area.The inferred phase changes are also consistent with predictions from mineral physics experiments.
基金State Natural Science Foundation of China (49874020) and the Special Funds for Major State Basic Research of China (95-13-04-06).
文摘Digital waveform data recorded by the vertical component short period stations at the American networks of SCSN, NCSN and PNSN and three components broadband stations at the Germany and Swiss networks and arrays of GRFN, GRSN and SDSNet for the events between 1981 and 2000 under Izu-Bonin are used as data sets. The N-th root slant stack method was used to pick up the SdP phase converted at the velocity interface beneath source and the regionalized difference of the 660 km discontinuity beneath Izu-Bonin is studied. It is found that while the dip angles of the subducting slab and the maximal depths of sources increase gradually from 35N to 26N, the 660 km discontinuity appears regionalized differences. The discontinuity exists at 660 km while there is no effect from subducting slab, but it is depressed to the depth of 720 km while there are obvious effects. The dispersion of converted points is still an unsolved problem which maybe result from the complex structure of the discontinuity, converted phases which were misjudged, or the assumption of one dimensional spherical earth model.
基金funded by the United States National Science Foundation grants(NSF-1333698)to P.Castillo(NSF-1333235)to S.Straub。
文摘The Izu-Bonin arc system is sediment-poor(~400 m thick with no accretionary prism)and,therefore,the influence of the altered oceanic crust(AOC)is most likely the source of the documented along-arc lava compositional variations,especially in Pb isotopes.Izu-Bonin arc lava geochemistry suggests an influx of subduction component from an Indian-type AOC.However,samples drilled from the western Pacific geochemical reference site at Integrated Ocean Drilling Program Site 1149 implies subduction of a Pacific-type AOC.To solve the apparent discrepancy of slab input versus arc output in this arc system,samples of the AOC were dredged from vertical fault scarps of the subducting Pacific Plate along a transect from 27.5°N to 34.50 N.Samples range from tholeiitic to mildly alkalic mid-ocean ridge basalts as well as trachybasalts,basaltic trachyandesites,tephrites,and phono-tephrites.Isotope ratios also exhibit a range of values(87Sr/86Sr=0.70282-0.70673,143 Nd/144 Nd=0.512552-0.513174,206Pb/204 Pb=18.43-20.00,207 Pb/204 Pb=15.40-15.67,208Pb/204Pb=37.75-39.55).Our results suggest that there is a geochemical variation in the AOC that is neither completely due to seawater or hydrothermal alteration,nor to petrogenetic processes.Rather,this variation is the result of the Pacific-Izanagi Ridge system tapping into a heterogeneous,plume-polluted mantle source during the Mid-Cretaceous volcanic event.The observed Pacific-type AOC is not responsible for the Indian-type Pb isotopic signature of Izu-Bonin arc lavas.This leads us to propose an alternative scenario where the Izu-Bonin arc lava Indian-type Pb isotopic signature originates from slab-derived fluids interacting and adsorbing Pb from an Indian-type mantle wedge through zone-refining.
基金the National Natural Science Foundation of China(Grant No.49874020) the Ph D Programs Foundation of Ministry of Education of China(Grant No.99000108).
文摘The SdP, pdP and sdP phases are picked up with the Nth root slant stack method from the digital waveform data recorded by the networks and arrays in USA, Germany and Switzerland for the earthquakes occurring beneath Izu-Bonin and Japan Sea. The mantle discontinuities and the effects of subducting slab on the 660 km and 410 km discontinuities are studied. It is found that there are mantle discontinuities existing at the depths of 170, 220, 300, 410, 660, 850 and 1150 km. Beneath Izu-Bonin, the 410 km discontinuity is elevated, while the 660 km discontinuity is depressed; for both discontinuities, there are regionalized differences. Beneath Japan Sea, however, there is no depth variation of the 410 km discontinuity, and the 660 km discontinuity is depressed without obvious effect of the subducting slab.
