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Herath P, Stern TA, Savage MK, Bassett D, Henrys S. Wide-angle seismic reflections reveal a lithosphere-asthenosphere boundary zone in the subducting Pacific Plate, New Zealand. SCIENCE ADVANCES 2022; 8:eabn5697. [PMID: 36149954 PMCID: PMC9506715 DOI: 10.1126/sciadv.abn5697] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/10/2021] [Accepted: 08/08/2022] [Indexed: 06/16/2023]
Abstract
New wide-angle seismic reflection data from offshore New Zealand show that the lithosphere-asthenosphere boundary (LAB) is more structured than previously thought. Three distinct layers are interpreted within a 10- to 12-km-thick LAB zone beginning at a depth of ≈70 km: a 3 (±1)-km-thick layer at the bottom of the lithosphere with a P-wave (VP) azimuthal anisotropy of 14 to 17% and fast azimuth subparallel to the direction of absolute plate motion and a 9 (±2)-km-thick, low VP channel with a P-wave-to-S-wave velocity ratio (VP/VS) of >2.8 in the upper 7 km of the channel and 1.8 to 2.6 in the lower 2 km of the channel. The high VP/VS ratios indicate that this channel may contain 3 to 20% partial melt that facilitates decoupling of the lithosphere from the asthenosphere and reduces resistance for plate motion. Furthermore, the strong azimuthal anisotropy above the low-velocity layer suggests localization of strain due to melt accumulation.
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Affiliation(s)
- Pasan Herath
- Institute of Geophysics, Victoria University of Wellington, Wellington, New Zealand
| | - Tim A. Stern
- Institute of Geophysics, Victoria University of Wellington, Wellington, New Zealand
| | - Martha K. Savage
- Institute of Geophysics, Victoria University of Wellington, Wellington, New Zealand
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2
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Genetic Analysis of Geothermal Resources and Geothermal Geological Characteristics in Datong Basin, Northern China. ENERGIES 2020. [DOI: 10.3390/en13071792] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Datong Basin is a Cenozoic fault basin located in the central part of the North China Block with strong tectonic activity. The unique geological environment of Datong Basin is believed to have good conditions for the formation of geothermal resources. Based on the research of the classification, genesis and geothermal geological characteristics of geothermal resources, the geological conditions, seismic activity, volcanic activity, geophysical exploration results, terrestrial heat flow and hot springs in Datong Basin are analyzed. The possibility of the occurrence of geothermal resources in Datong Basin is determined, and the genesis and occurrence mechanisms of geothermal resources in Datong Basin are judged. The results show that Datong Basin satisfies the geological geothermal conditions of the formation of geothermal resources and is of great research value. The formation of geothermal resources in the Datong Basin is affected by the uplift of the Qinghai–Tibet Plateau and the destruction of the North China Craton. The geothermal resources in Datong Basin are formed by the combination of modern volcanic activity and strong inner-plate tectonic activities. The geothermal system is a combination of convective hydrothermal systems and partial melt systems. At the same time, it is concluded that the key research areas for the occurrence of geothermal resources are mainly in the northeastern part of the basin. It is recommended to carry out detailed and comprehensive exploration of the northeastern part of Datong Basin.
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Liu J, Hirano N, Machida S, Xia Q, Tao C, Liao S, Liang J, Li W, Yang W, Zhang G, Ding T. Melting of recycled ancient crust responsible for the Gutenberg discontinuity. Nat Commun 2020; 11:172. [PMID: 31924776 PMCID: PMC6954225 DOI: 10.1038/s41467-019-13958-w] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2019] [Accepted: 12/10/2019] [Indexed: 11/09/2022] Open
Abstract
A discontinuity in the seismic velocity associated with the lithosphere-asthenosphere interface, known as the Gutenberg discontinuity, is enigmatic in its origin. While partial mantle melts are frequently suggested to explain this discontinuity, it is not well known which factors critically regulate the melt production. Here, we report geochemical evidence showing that the melt fractions in the lithosphere-asthenosphere boundary were enhanced not only by accumulation of compacted carbonated melts related to recycled ancient marine sediments, but also by partial melting of a pyroxene-rich mantle domain related to the recycled oceanic eclogite/pyroxenites. This conclusion is derived from the first set of Mg isotope data for a suite of young petit-spot basalts erupted on the northwest Pacific plate, where a clearly defined Gutenberg discontinuity exists. Our results reveal a specific linkage between the Gutenberg discontinuity beneath the normal oceanic regions and the recycling of ancient subducted crust and carbonate through the deep Earth.
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Affiliation(s)
- Jia Liu
- Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, 310012, Hangzhou, China.
| | - Naoto Hirano
- Center for Northeast Asian Studies, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai, 980-8576, Japan
| | - Shiki Machida
- Chiba Institute of Technology, Ocean Resources Research Center for Next Generation, Chiba, 275-0016, Japan
| | - Qunke Xia
- School of Earth Sciences, Zhejiang University, 310027, Hangzhou, China
| | - Chunhui Tao
- Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, 310012, Hangzhou, China.,School of Oceanography, Shanghai Jiao Tong University, Shanghai, 200240, China
| | - Shili Liao
- Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, 310012, Hangzhou, China
| | - Jin Liang
- Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, 310012, Hangzhou, China
| | - Wei Li
- Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, 310012, Hangzhou, China
| | - Weifang Yang
- Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, 310012, Hangzhou, China
| | - Guoying Zhang
- Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, 310012, Hangzhou, China
| | - Teng Ding
- Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, 310012, Hangzhou, China.,School of Oceanography, Hohai University, Nanjing, China
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Harmon N, Rychert C, Agius M, Tharimena S, Le Bas T, Kendall JM, Constable S. Marine Geophysical Investigation of the Chain Fracture Zone in the Equatorial Atlantic From the PI-LAB Experiment. JOURNAL OF GEOPHYSICAL RESEARCH. SOLID EARTH 2018; 123:11016-11030. [PMID: 31007998 PMCID: PMC6472653 DOI: 10.1029/2018jb015982] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 04/19/2018] [Revised: 11/08/2018] [Accepted: 11/10/2018] [Indexed: 06/09/2023]
Abstract
The Chain Fracture Zone is a 300-km-long transform fault that offsets the Mid-Atlantic Ridge. We analyzed new multibeam bathymetry, backscatter, gravity, and magnetic data with 100% multibeam bathymetric data over the active transform valley and adjacent spreading segments as part of the Passive Imaging of the Lithosphere Asthenosphere Boundary (PI-LAB) Experiment. Analyses of these data sets allow us to determine the history and mode of crustal formation and the tectonic evolution of the transform system and adjacent ridges over the past 20 Myr. We model the total field magnetic anomaly to determine the age of the crust along the northern ridge segment to better establish the timing of the variations in the seafloor fabric and the tectonic-magmatic history of the region. Within the active transform fault zone, we observe four distinct positive flower structures with several en échelon fault scarps visible in the backscatter data. We find up to -10 mGal residual Mantle Bouguer Anomaly in the region of the largest positive flower structure within the transform zone suggesting crustal thickening relative to the crustal thinning typically observed in fracture zones in the Atlantic. The extensional/compressional features observed in the Chain Transform are less pronounced than those observed further north in the Vema, St. Paul, and Romanche and may be due to local ridge segment adjustments.
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Affiliation(s)
- Nicholas Harmon
- Ocean and Earth ScienceUniversity of Southampton, Waterfront CampusSouthamptonUK
| | - Catherine Rychert
- Ocean and Earth ScienceUniversity of Southampton, Waterfront CampusSouthamptonUK
| | - Matthew Agius
- Ocean and Earth ScienceUniversity of Southampton, Waterfront CampusSouthamptonUK
| | - Saikiran Tharimena
- Ocean and Earth ScienceUniversity of Southampton, Waterfront CampusSouthamptonUK
- Now at the Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadenaCAUSA
| | - Tim Le Bas
- National Oceanography CentreSouthamptonUK
| | | | - Steven Constable
- Scripps Institution of OceanographyUniversity of California, San DiegoSan DiegoCAUSA
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5
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Lavayssière A, Rychert C, Harmon N, Keir D, Hammond JOS, Kendall J, Doubre C, Leroy S. Imaging Lithospheric Discontinuities Beneath the Northern East African Rift Using S-to- P Receiver Functions. GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS : G(3) 2018; 19:4048-4062. [PMID: 30774560 PMCID: PMC6360955 DOI: 10.1029/2018gc007463] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 02/27/2018] [Revised: 08/21/2018] [Accepted: 08/26/2018] [Indexed: 06/09/2023]
Abstract
Imaging the lithosphere is key to understand mechanisms of extension as rifting progresses. Continental rifting results in a combination of mechanical stretching and thinning of the lithosphere, decompression upwelling, heating, sometimes partial melting of the asthenosphere, and potentially partial melting of the mantle lithosphere. The northern East African Rift system is an ideal locale to study these processes as it exposes the transition from tectonically active continental rifting to incipient seafloor spreading. Here we use S-to-P receiver functions to image the lithospheric structure beneath the northernmost East African Rift system where it forms a triple junction between the Main Ethiopian rift, the Red Sea rift, and the Gulf of Aden rift. We image the Moho at 31 ± 6 km beneath the Ethiopian plateau. The crust is 28 ± 3 km thick beneath the Main Ethiopian rift and thins to 23 ± 2 km in northern Afar. We identify a negative phase, a velocity decrease with depth, at 67 ± 3 km depth beneath the Ethiopian plateau, likely associated with the lithosphere-asthenosphere boundary (LAB), and a lack of a LAB phase beneath the rift. Using observations and waveform modeling, we show that the LAB phase beneath the plateau is likely defined by a small amount of partial melt. The lack of a LAB phase beneath the rift suggests melt percolation through the base of the lithosphere beneath the northernmost East African Rift system.
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Affiliation(s)
- Aude Lavayssière
- National Oceanography CentreUniversity of SouthamptonSouthamptonUK
| | | | - Nicholas Harmon
- National Oceanography CentreUniversity of SouthamptonSouthamptonUK
| | - Derek Keir
- National Oceanography CentreUniversity of SouthamptonSouthamptonUK
- Dipartimento di Scienze della TerraUniversità degli Studi di FirenzeFirenzeItaly
| | - James O. S. Hammond
- Department of Earth and Planetary Sciences, BirkbeckUniversity of LondonLondonUK
| | | | - Cécile Doubre
- Institut de Physique du Globe de Strasbourg, UMR 7516Université de Strasbourg/EOST, CNRSStrasbourgFrance
| | - Sylvie Leroy
- CNRS, UMR 7193, Institut des Sciences de la Terre de ParisSorbonne UniversitéParisFrance
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6
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Chichester B, Rychert C, Harmon N, van der Lee S, Frederiksen A, Zhang H. Seismic Imaging of the North American Midcontinent Rift Using S-to- P Receiver Functions. JOURNAL OF GEOPHYSICAL RESEARCH. SOLID EARTH 2018; 123:7791-7805. [PMID: 31032165 PMCID: PMC6473666 DOI: 10.1029/2018jb015771] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 03/15/2018] [Revised: 08/03/2018] [Accepted: 08/17/2018] [Indexed: 06/09/2023]
Abstract
North America's ~1.1-Ga failed Midcontinent Rift (MCR) is a striking feature of gravity and magnetic anomaly maps across the continent. However, how the rift affected the underlying lithosphere is not well understood. With data from the Superior Province Rifting Earthscope Experiment and the USArray Transportable Array, we constrain three-dimensional seismic velocity discontinuity structure in the lithosphere beneath the southwestward arm of the MCR using S-to-P receiver functions. We image a velocity increase with depth associated with the Moho at depths of 33-40 ± 4 km, generally deepening toward the east. The Moho amplitude decreases beneath the rift axis in Minnesota and Wisconsin, where the velocity gradient is more gradual, possibly due to crustal underplating. We see hints of a deeper velocity increase at 61 ± 4-km depth that may be the base of underplating. Beneath the rift axis further south in Iowa, we image two distinct positive phases at 34-39 ± 4 and 62-65 ± 4 km likely related to an altered Moho and an underplated layer. We image velocity decreases with depth at depths of 90-190 ± 7 km in some locations that do not geographically correlate with the rift. These include a discontinuity at depths of 90-120 ± 7 km with a northerly dip in the south that abruptly deepens to 150-190 ± 7 km across the Spirit Lake Tectonic Zone provincial suture. The negative phases may represent a patchy, frozen-in midlithosphere discontinuity feature that likely predates the MCR and/or be related to lithospheric thickness.
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Affiliation(s)
- Ben Chichester
- National Oceanography Centre Southampton, Ocean and Earth SciencesUniversity of SouthamptonSouthamptonUK
| | - Catherine Rychert
- National Oceanography Centre Southampton, Ocean and Earth SciencesUniversity of SouthamptonSouthamptonUK
| | - Nicholas Harmon
- National Oceanography Centre Southampton, Ocean and Earth SciencesUniversity of SouthamptonSouthamptonUK
| | - Suzan van der Lee
- Department of Earth and Planetary SciencesNorthwestern UniversityEvanstonILUSA
| | - Andrew Frederiksen
- Department of Geological SciencesUniversity of ManitobaWinnipegManitobaCanada
| | - Hao Zhang
- Department of Geology and GeophysicsUniversity of UtahSalt Lake CityUTUSA
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7
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Rychert CA, Harmon N. Predictions and Observations for the Oceanic Lithosphere From S-to- P Receiver Functions and SS Precursors. GEOPHYSICAL RESEARCH LETTERS 2018; 45:5398-5406. [PMID: 30034045 PMCID: PMC6049891 DOI: 10.1029/2018gl077675] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 02/23/2018] [Revised: 05/02/2018] [Accepted: 05/09/2018] [Indexed: 06/08/2023]
Abstract
The ocean lithosphere is classically described by the thermal half-space cooling (HSC) or the plate models, both characterized by a gradual transition to the asthenosphere beneath. Scattered waves find sharp seismic discontinuities beneath the oceans, possibly from the base of the plate. Active source studies suggest sharp discontinuities from a melt channel. We calculate synthetic S-to-P receiver functions and SS precursors for the HSC and plate models and also for channels. We find that the HSC and plate model velocity gradients are too gradual to create interpretable scattered waves from the base of the plate. Subtle phases are predicted to follow a similar trend as observations, flattening at older ages. Therefore, the seismic discontinuities are probably caused by a thermally controlled process that can also explain their amplitude, such as melting. Melt may coalesce in channels, although channels >10 km thick should be resolvable by scattered wave imaging.
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Affiliation(s)
| | - Nick Harmon
- Ocean and Earth ScienceUniversity of SouthamptonSouthamptonUK
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Rychert CA, Harmon N, Armitage JJ. Seismic Imaging of Thickened Lithosphere Resulting From Plume Pulsing Beneath Iceland. GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS : G(3) 2018; 19:1789-1799. [PMID: 30166946 PMCID: PMC6108382 DOI: 10.1029/2018gc007501] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 02/20/2018] [Accepted: 05/04/2018] [Indexed: 06/08/2023]
Abstract
Ocean plates conductively cool and subside with seafloor age. Plate thickening with age is also predicted, and hot spots may cause thinning. However, both are debated and depend on the way the plate is defined. Determining the thickness of the plates along with the process that governs it has proven challenging. We use S-to-P (Sp) receiver functions to image a strong, persistent LAB beneath Iceland where the mid-Atlantic Ridge interacts with a plume with hypothesized pulsating thermal anomaly. The plate is thickest, up to 84 ± 6 km, beneath lithosphere formed during times of hypothesized hotter plume temperatures and as thin as 61 ± 6 km beneath regions formed during colder intervals. We performed geodynamic modeling to show that these plate thicknesses are inconsistent with a thermal lithosphere. Instead, periods of increased plume temperatures likely increased the melting depth, causing deeper depletion and dehydration, and creating a thicker plate. This suggests plate thickness is dictated by the conditions of plate formation.
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Affiliation(s)
- Catherine A. Rychert
- National Oceanography Centre Southampton, Ocean and Earth SciencesUniversity of SouthamptonSouthamptonUK
| | - Nicholas Harmon
- National Oceanography Centre Southampton, Ocean and Earth SciencesUniversity of SouthamptonSouthamptonUK
| | - John J. Armitage
- Dynamique des Fluides Géologiques, Institut de Physique du Globe de ParisParisFrance
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Rychert CA, Harmon N, Tharimena S. Scattered wave imaging of the oceanic plate in Cascadia. SCIENCE ADVANCES 2018; 4:eaao1908. [PMID: 29457132 PMCID: PMC5812736 DOI: 10.1126/sciadv.aao1908] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/23/2017] [Accepted: 01/11/2018] [Indexed: 06/08/2023]
Abstract
Fifty years after plate tectonic theory was developed, the defining mechanism of the plate is still widely debated. The relatively short, simple history of young ocean lithosphere makes it an ideal place to determine the property that defines a plate, yet the remoteness and harshness of the seafloor have made precise imaging challenging. We use S-to-P receiver functions to image discontinuities beneath newly formed lithosphere at the Juan de Fuca and Gorda Ridges. We image a strong negative discontinuity at the base of the plate increasing from 20 to 45 km depth beneath the 0- to 10-million-year-old seafloor and a positive discontinuity at the onset of melting at 90 to 130 km depth. Comparison with geodynamic models and experimental constraints indicates that the observed discontinuities cannot easily be reconciled with subsolidus mechanisms. Instead, partial melt may be required, which would decrease mantle viscosity and define the young oceanic plate.
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Tharimena S, Rychert C, Harmon N. A unified continental thickness from seismology and diamonds suggests a melt-defined plate. Science 2017; 357:580-583. [PMID: 28798127 DOI: 10.1126/science.aan0741] [Citation(s) in RCA: 35] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2017] [Accepted: 06/21/2017] [Indexed: 11/02/2022]
Affiliation(s)
- Saikiran Tharimena
- Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK.
| | - Catherine Rychert
- Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK
| | - Nicholas Harmon
- Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK
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Rychert CA, Harmon N. Constraints on the anisotropic contributions to velocity discontinuities at ∼60 km depth beneath the Pacific. GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS : G(3) 2017; 18:2855-2871. [PMID: 29097907 PMCID: PMC5652234 DOI: 10.1002/2017gc006850] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 02/03/2017] [Accepted: 06/27/2017] [Indexed: 06/07/2023]
Abstract
Strong, sharp, negative seismic discontinuities, velocity decreases with depth, are observed beneath the Pacific seafloor at ∼60 km depth. It has been suggested that these are caused by an increase in radial anisotropy with depth, which occurs in global surface wave models. Here we test this hypothesis in two ways. We evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests. We do this by fitting an example surface wave data set near the East Pacific Rise. We also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in S-to-P and P-to-S receiver functions and SS precursors using synthetic seismograms. We test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering. The result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0-10 Ma seafloor, which would also have a radial anisotropy signature. An increase in radial anisotropy with depth at 60 km depth is not well-resolved in surface wave models, and could be artificially observed. Shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops (<6%) in SS precursor imaging, but not receiver functions. The effect is diminished if strong anisotropy also exists at 0-60 km depth as suggested by surface waves. Overall, an increase in radial anisotropy with depth may not exist at 60 km beneath the oceans and does not explain the scattered wave observations.
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13
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Abstract
The lithosphere-asthenosphere boundary (LAB) beneath ocean basins separates the upper thermal boundary layer of rigid, conductively cooling plates from the underlying ductile, convecting mantle. The origin of a seismic discontinuity associated with this interface, known as the Gutenberg discontinuity (G), remains enigmatic. High-frequency SS precursors sampling below the Pacific plate intermittently detect the G as a sharp, negative velocity contrast at 40- to 75-kilometer depth. These observations lie near the depth of the LAB in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base. I propose that the G reflectivity is regionally enhanced by dynamical processes that produce melt, including hot mantle upwellings, small-scale convection, and fluid release during subduction.
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Affiliation(s)
- Nicholas Schmerr
- Department of Terrestrial Magnetism, 5241 Broad Branch Road, NW, Washington, DC 20015, USA.
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14
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Lithospheric layering in the North American craton. Nature 2010; 466:1063-8. [PMID: 20740006 DOI: 10.1038/nature09332] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2009] [Accepted: 06/25/2010] [Indexed: 11/09/2022]
Abstract
How cratons-extremely stable continental areas of the Earth's crust-formed and remained largely unchanged for more than 2,500 million years is much debated. Recent studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at depths too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. Here we show that changes in the direction of azimuthal anisotropy with depth reveal the presence of two distinct lithospheric layers throughout the stable part of the North American continent. The top layer is thick ( approximately 150 km) under the Archaean core and tapers out on the surrounding Palaeozoic borders. Its thickness variations follow those of a highly depleted layer inferred from thermo-barometric analysis of xenoliths. The lithosphere-asthenosphere boundary is relatively flat (ranging from 180 to 240 km in depth), in agreement with the presence of a thermal conductive root that subsequently formed around the depleted chemical layer. Our findings tie together seismological, geochemical and geodynamical studies of the cratonic lithosphere in North America. They also suggest that the horizon detected in receiver function studies probably corresponds to the sharp mid-lithospheric boundary rather than to the more gradual lithosphere-asthenosphere boundary.
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Kawakatsu H, Kumar P, Takei Y, Shinohara M, Kanazawa T, Araki E, Suyehiro K. Seismic evidence for sharp lithosphere-asthenosphere boundaries of oceanic plates. Science 2009; 324:499-502. [PMID: 19390042 DOI: 10.1126/science.1169499] [Citation(s) in RCA: 411] [Impact Index Per Article: 25.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the lithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions.
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Affiliation(s)
- Hitoshi Kawakatsu
- Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
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16
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Affiliation(s)
- Catherine A. Rychert
- Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, M/C 0225, La Jolla, CA 92093, USA
| | - Peter M. Shearer
- Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, M/C 0225, La Jolla, CA 92093, USA
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17
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Mierdel K, Keppler H, Smyth JR, Langenhorst F. Water Solubility in Aluminous Orthopyroxene and the Origin of Earth's Asthenosphere. Science 2007; 315:364-8. [PMID: 17234945 DOI: 10.1126/science.1135422] [Citation(s) in RCA: 192] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Plate tectonics is based on the concept of rigid lithosphere plates sliding on a mechanically weak asthenosphere. Many models assume that the weakness of the asthenosphere is related to the presence of small amounts of hydrous melts. However, the mechanism that may cause melting in the asthenosphere is not well understood. We show that the asthenosphere coincides with a zone where the water solubility in mantle minerals has a pronounced minimum. The minimum is due to a sharp decrease of water solubility in aluminous orthopyroxene with depth, whereas the water solubility in olivine continuously increases with pressure. Melting in the asthenosphere may therefore be related not to volatile enrichment but to a minimum in water solubility, which causes excess water to form a hydrous silicate melt.
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Affiliation(s)
- Katrin Mierdel
- Institut für Geowissenschaften, Universität Tübingen, Wilhelmstr. 56, 72074 Tübingen, Germany
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18
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Chen L, Zheng T, Xu W. A thinned lithospheric image of the Tanlu Fault Zone, eastern China: Constructed from wave equation based receiver function migration. ACTA ACUST UNITED AC 2006. [DOI: 10.1029/2005jb003974] [Citation(s) in RCA: 152] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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19
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Yang Y, Forsyth DW. Rayleigh wave phase velocities, small-scale convection, and azimuthal anisotropy beneath southern California. ACTA ACUST UNITED AC 2006. [DOI: 10.1029/2005jb004180] [Citation(s) in RCA: 120] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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