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Sample records for deep upper mantle

  1. Seismic evidence for water deep in Earth's upper mantle.

    PubMed

    van der Meijde, Mark; Marone, Federica; Giardini, Domenico; van der Lee, Suzan

    2003-06-06

    Water in the deep upper mantle can influence the properties of seismic discontinuities in the mantle transition zone. Observations of converted seismic waves provide evidence of a 20- to 35-kilometer-thick discontinuity near a depth of 410 kilometers, most likely explained by as much as 700 parts per million of water by weight.

  2. Iron-titanium oxyhydroxides which transport water into the deep upper mantle and mantle transition zone

    NASA Astrophysics Data System (ADS)

    Matsukage, K. N.; Nishihara, Y.

    2015-12-01

    We experimentally discovered a new hydrous phase in the system FeOOH-TiO2 at pressures of 10-16 GPa and temperatures of 1000-1600°C which corresponds to conditions of the deep upper mantle and the Earth's mantle transition zone. Seven different compositions in the FeOOH-TiO2 system having molar ratios of x = Ti/(Fe + Ti) = 0, 0.125, 0.25, 0.375, 0.5, 0.75 that were prepared by mixing reagent grade a-FeOOH (goethite) and TiO2 (anatase) powders were used as starting materials. High-pressure and high-temperature experiments were carried out using Kawai-type multi-anvil apparatus (Orange-1000 at Ehime University and SPI-1000 at Tokyo Institute of Technology). In this system, we identified two stable iron-titanium oxyhydroxide phases whose estimated composition is expressed by (FeH)1 - xTixO2 . One is the Fe-rich solid solution (x < 0.23) with e-FeOOH type crystal structure (e-phase, orthorhombic, P21nm) that was described by the previous studies (e.g., Suzuki 2010), and the other is the more Ti-rich solid solution (x > 0.35) with a-PbO2 type structure (a-phase, orthorhombic, Pbcn). The a-phase is stable up to 1500ºC for a composition of x = 0.5 and at least to 1600ºC for x = 0.75. Our result means that this phase is stable at average mantle temperature in the Earth's mantle transition zone. The Iron-titanium-rich hydrous phases was possible to stable in basalt + H2O system (e.g., Hashimoto and Matsukage 2013). Therefore our findings suggest that water transport in the Earth's deep interior is probably much more efficient than had been previously thought.

  3. Melting in the Earth's deep upper mantle caused by carbon dioxide.

    PubMed

    Dasgupta, Rajdeep; Hirschmann, Marc M

    2006-03-30

    The onset of partial melting beneath mid-ocean ridges governs the cycling of highly incompatible elements from the mantle to the crust, the flux of key volatiles (such as CO2, He and Ar) and the rheological properties of the upper mantle. Geophysical observations indicate that melting beneath ridges begins at depths approaching 300 km, but the cause of this melting has remained unclear. Here we determine the solidus of carbonated peridotite from 3 to 10 GPa and demonstrate that melting beneath ridges may occur at depths up to 330 km, producing 0.03-0.3% carbonatite liquid. We argue that these melts promote recrystallization and realignment of the mineral matrix, which may explain the geophysical observations. Extraction of incipient carbonatite melts from deep within the oceanic mantle produces an abundant source of metasomatic fluids and a vast mantle residue depleted in highly incompatible elements and fractionated in key parent-daughter elements. We infer that carbon, helium, argon and highly incompatible heat-producing elements (such as uranium, thorium and potassium) are efficiently scavenged from depths of approximately 200-330 km in the upper mantle.

  4. Seismic characteristics of central Brazil crust and upper mantle: A deep seismic refraction study

    USGS Publications Warehouse

    Soares, J.E.; Berrocal, J.; Fuck, R.A.; Mooney, W.D.; Ventura, D.B.R.

    2006-01-01

    A two-dimensional model of the Brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the Porangatu and Cavalcante lines, each approximately 300 km long. When the lines were deployed, they overlapped by 50 km, forming an E-W transect approximately 530 km long across the Tocantins Province and western Sa??o Francisco Craton. The Tocantins Province formed during the Neoproterozoic when the Sa??o Francisco, the Paranapanema, and the Amazon cratons collided, following the subduction of the former Goia??s ocean basin. Average crustal VP and VP/VS ratios, Moho topography, and lateral discontinuities within crustal layers suggest that the crust beneath central Brazil can be associated with major geological domains recognized at the surface. The Moho is an irregular interface, between 36 and 44 km deep, that shows evidences of first-order tectonic structures. The 8.05 and 8.23 km s-1 P wave velocities identify the upper mantle beneath the Porangatu and Cavalcante lines, respectively. The observed seismic features allow for the identification of (1) the crust has largely felsic composition in the studied region, (2) the absence of the mafic-ultramafic root beneath the Goia??s magmatic arc, and (3) block tectonics in the foreland fold-and-thrust belt of the northern Brasi??lia Belt during the Neoproterozoic. Seismic data also suggested that the Bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle. Finally, the Goia??s-Tocantins seismic belt can be interpreted as a natural seismic alignment related to the Neoproterozoic mantle domain. Copyright 2006 by the American Geophysical Union.

  5. Upper mantle and crustal structure of southwestern Scandinavia: Results of the TopoScandiaDeep project

    NASA Astrophysics Data System (ADS)

    Köhler, A.; Balling, N.; Ebbing, J.; England, R.; Frassetto, A.; Gradmann, S.; Jacobsen, B. H.; Kvarven, T.; Maupin, V.; Medhus, A. Bondo; Mjelde, R.; Ritter, J.; Schweizer, J.; Stratford, W.; Thybo, H.; Wawerzinek, B.; Weidle, C.

    2012-04-01

    The origin of the Scandinavian mountains, located far away from any presently active plate margin, is still not well understood. In particular, it is not clear if the mountains are sustained isostatically either by crustal thickening or by light upper mantle material. Within the TopoScandiaDeep project (a collaborative research project within the ESF TOPO-EUROPE programme), we therefore analyse recently collected passive seismological and active seismic data in the southern Scandes and surrounding regions. We infer crustal and upper mantle (velocity) structures and relate them to results of gravity and temperature-composition modelling. The Moho under the high topography of southern Norway appears from controlled source seismic refraction and Receiver Functions as relatively shallow (<= 45 km) compared to the deeper conversion (>55 km) imaged beneath the low topography in Sweden and elsewhere in the Baltic Shield area outside Norway. The Receiver Function modeling as well as the active seismic results suggest that the differences in the observed Moho response may represent the transition between tectonically reworked Moho under southern Norway and an intact, cratonic crust-mantle boundary beneath the Baltic Shield. Furthermore, the 410km-discontinuity and the LAB is imaged, the latter one suggesting a lithospheric thickening in NE direction. Upper mantle P-wave and S-wave velocities in southern Sweden and southern Norway east of the Oslo Graben are correspondingly relatively high while lower velocities are observed in the southwestern part of Norway and northern Denmark. The lateral velocity gradient, interpreted as the southwestern boundary of thick Baltic Shield lithosphere, is remarkably sharp. Differences in upper mantle velocities are found at depths of 100-400 km and amount to ± 2-3%. S-to-P wave conversions, interpreted to originate from the lithosphere-asthenosphere boundary, are preliminary estimated to 90-120 km depth. Inversion of Rayleigh and Love

  6. Mantle transition zone structure and upper mantle S velocity variations beneath Ethiopia: Evidence for a broad, deep-seated thermal anomaly

    NASA Astrophysics Data System (ADS)

    Benoit, Margaret H.; Nyblade, Andrew A.; Owens, Thomas J.; Stuart, Graham

    2006-11-01

    Ethiopia has been subjected to widespread Cenozoic volcanism, rifting, and uplift associated with the Afar hot spot. The hot spot tectonism has been attributed to one or more thermal upwellings in the mantle, for example, starting thermal plumes and superplumes. We investigate the origin of the hot spot by imaging the S wave velocity structure of the upper mantle beneath Ethiopia using travel time tomography and by examining relief on transition zone discontinuities using receiver function stacks. The tomographic images reveal an elongated low-velocity region that is wide (>500 km) and extends deep into the upper mantle (>400 km). The anomaly is aligned with the Afar Depression and Main Ethiopian Rift in the uppermost mantle, but its center shifts westward with depth. The 410 km discontinuity is not well imaged, but the 660 km discontinuity is shallower than normal by ˜20-30 km beneath most of Ethiopia, but it is at a normal depth beneath Djibouti and the northwestern edge of the Ethiopian Plateau. The tomographic results combined with a shallow 660 km discontinuity indicate that upper mantle temperatures are elevated by ˜300 K and that the thermal anomaly is broad (>500 km wide) and extends to depths ≥660 km. The dimensions of the thermal anomaly are not consistent with a starting thermal plume but are consistent with a flux of excess heat coming from the lower mantle. Such a broad thermal upwelling could be part of the African Superplume found in the lower mantle beneath southern Africa.

  7. Rheology of fine-grained forsterite aggregate under deep upper mantle conditions

    NASA Astrophysics Data System (ADS)

    Nishihara, Y.; Ohuchi, T.; Kawazoe, T.; Spengler, D.; Tasaka, M.; Hiraga, T.; Kikegawa, T.; Suzuki, A.; Ohtani, E.

    2010-12-01

    Under the conditions of the Earth’s mantle, both diffusion creep and dislocation creep can be the dominant deformation mechanism depending on physical and chemical environments. These two mechanisms are quite different in terms of stress dependence of viscosity and development of lattice-preferred orientation. Thus it is important to understand the dominant deformation mechanism in the mantle. Previous studies on rheology of olivine under high-pressure (>3 GPa) mostly focused on dislocation creep (e.g. Kawazoe et al., 2009; Durham et al., 2009). Knowledge of diffusion creep of olivine under deep upper mantle condition (>100 km) has been quite limited. In order to clarify the dominant deformation mechanism in the upper mantle, we have conducted deformation experiments at high-pressure and high-temperature using fine-grained forsterite aggregate. Experiments were carried out using a D-DIA apparatus “D-CAP (deformation cubic-anvil press)” installed at NE7 beamline, PF-AR, High Energy Accelerator Research Institute, Tsukuba, Japan. The samples are sintered aggregate of 90%forsterite + 10%enstatite with average grain size of ~1 μm. High-pressure was generated by MA6-6 assembly (e.g. Kawazoe et al., 2010) using cubic (Mg,Co)O pressure medium and WC anvils with 5 mm truncation edge length. High-temperature was generated using graphite furnace and was monitored by WRe thermocouple. Deformation experiments were conducted at pressure of 3-5.5 GPa, temperature of 1573 K, and uniaxial strain rate of 7 x 10-6-2 x 10-4 s-1. Sample stress was measured by two-dimensional X-ray diffraction using monochromatized synchrotron X-ray (50 keV) and imaging plate detector (e.g. Nishihara et al., 2009). Sample strain was measured by X-ray radiography. H2O concentration in starting material and recovered samples was determined based on FTIR analyses (Paterson, 1982). Steady state flow stress was determined at each deformation condition. The stress-strain rate data taken at

  8. Density of hydrous silicate melt at the conditions of Earth's deep upper mantle.

    PubMed

    Matsukage, Kyoko N; Jing, Zhicheng; Karato, Shun-ichiro

    2005-11-24

    The chemical evolution of the Earth and the terrestrial planets is largely controlled by the density of silicate melts. If melt density is higher than that of the surrounding solid, incompatible elements dissolved in the melt will be sequestered in the deep mantle. Previous studies on dry (water-free) melts showed that the density of silicate melts can be higher than that of surrounding solids under deep mantle conditions. However, melts formed under deep mantle conditions are also likely to contain some water, which will reduce the melt density. Here we present data constraining the density of hydrous silicate melt at the conditions of approximately 410 km depth. We show that the water in the silicate melt is more compressible than the other components, and therefore the effect of water in reducing melt density is markedly diminished under high-pressure conditions. Our study indicates that there is a range of conditions under which a (hydrous) melt could be trapped at the 410-km boundary and hence incompatible elements could be sequestered in the deep mantle, although these conditions are sensitive to melt composition as well as the composition of the surrounding mantle.

  9. Rheology of the Deep Upper Mantle and its Implications for the Preservation of the Continental Roots: A Review

    SciTech Connect

    Karato, S.

    2010-01-01

    The longevity of deep continental roots depends critically on the rheological properties of upper mantle minerals under deep upper mantle conditions. Geodynamic studies suggest that the rheological contrast between the deep continental and oceanic upper mantle is a key factor that controls the longevity of the continental roots. Current understanding of rheological properties of deep upper mantle is reviewed to examine how a large enough rheological contrast between the continental and oceanic upper mantle develops that leads to the longevity of the deep continental roots. Based on the microstructures of naturally deformed deep continental rocks as well as on the observations of seismic anisotropy, it is concluded that power-law dislocation creep dominates in most of the deep upper mantle. Deformation by power-law creep is sensitive to water content and therefore the removal of water by partial melting to form depleted continental roots is a likely mechanism to establish a large rheological contrast. The results of experimental studies on the influence of temperature, pressure and water content on plastic flow by power-law dislocation creep are reviewed. The degree of rheological contrast depends critically on the dependence of effective viscosity on water content under 'wet' (water-rich) conditions but it is also sensitive to the effective viscosity under 'dry' (water-free) conditions that depends critically on the influence of pressure on deformation. Based on the analysis of thermodynamics of defects and high-temperature creep, it is shown that a robust estimate of the influence of water and pressure can be made only by the combination of low-pressure (< 0.5 GPa) and high-pressure (> 5 GPa) studies. A wide range of flow laws has been reported, leading to nearly 10 orders of magnitude differences in estimated viscosities under the deep upper mantle conditions. However, based on the examination of several criteria, it is concluded that relatively robust

  10. Density of Hydrous Ultramafic Silicate Melt under the Earth's Deep Upper Mantle Conditions

    NASA Astrophysics Data System (ADS)

    Jing, Z.; Matsukage, K. N.; Karato, S.

    2005-12-01

    Density of silicate melts is a critical material property in our understanding of geochemical evolution of the Earth. Previous studies (e.g., Agee & Walker 1993; Suzuki et al., 1995) showed that the density of dry silicate melts can be higher than that of surrounding solids under deep upper mantle conditions. However, melts formed under such conditions likely contain some water (Bercovici & Karato, 2003), which will reduce the melt density. In this study, we performed sink/float experiments between 10 and 14GPa and at 2173K to determine the density of hydrous ultramafic silicate melts, using a Kawai-type multianvil apparatus. We choose a target melt composition based on the experimental study by Litasov & Ohtani (2002). With this chemical composition, olivine reacts with the melt above the liquidus, so we used diamond as the density marker. However, diamond is much denser than a melt with a typical mantle like Fe/Mg ratio. Therefore in this study we determined the density of melts with high Fe contents, and from the relation between Fe content and melt density, we inferred the melt density with Earth-like Fe/Mg. Four Fe-rich compositions with 5wt% water and different iron content were chosen as starting materials. Density crossovers between melts and diamond were observed for all compositions. The densities of four melts at 14GPa and 2173K were calculated using the Birch-Murnaghan equation of state. The pressure derivative of isothermal bulk modulus (Kt') of the melts was estimated to be around 4. The density of mantle melt with mantle value of content and 5wt% water at 14GPa, 2173K was extrapolated to be ~3.42±0.4g/cm3. We compared our density results for hydrous melts with previous results on dry melts and found that water is more compressible than other components in melt. The estimated partial molar volume of water at 14GPa and 2173K is ~8±2cm3/mol, which is significantly lower than the value at low pressures. The conditions under which the density crossover

  11. Evidence for deep melting in the European upper mantle from seismology

    NASA Astrophysics Data System (ADS)

    Cobden, L. J.; Trampert, J.; Fichtner, A.

    2016-12-01

    The recent development of full waveform seismic tomography on continental scales has provided new insights into the seismic structure of the lithosphere and asthenosphere. In particular, we can map shorter wavelength, high-amplitude velocity anomalies which would traditionally be damped and spatially smeared using classical methods. Quantitative interpretation of these anomalies - expressed as absolute rather than relative velocities- may open up the possibility of identifying important dynamic processes such as melting, that would otherwise go undetected or unconstrained. In this study we focus on the S-wave speed (Vs) structure beneath Europe, as obtained from full waveform inversion. The European upper mantle is characterised by seismic wave speeds that are slower than the global average. However, especially low velocities (< 4.0 km/s) are seen beneath Iceland and other parts of the mid-Atlantic ridge, as well as beneath Iberia, reaching a minimum between 120-130 km. Using Vs alone, in the absence of any other observable (e.g. Vp, density), it is difficult to constrain the chemical composition beneath Europe. This is because chemistry (C) and temperature (T) have sensitivities to Vs which trade off with each other. However, even considering the full range of possible chemical compositions, taking into account mineral intrinsic anelasticity, and allowing for presence of water, it is very difficult to create sufficiently low velocities to fit the slowest regions of the tomography model, using simple variations in T or C. Doing so requires either extremely high temperatures or unrealistically high attenuation. However, the slowest velocities can readily be modelled by including c. 1-2% melt. We discuss whether melt provides the most likely explanation for the slow regions, considering also the effect of more advanced anelasticity models such as "elastically accommodated grain boundary sliding", recently suggested by Karato et al. (2015). The possibility of deep

  12. Slab dehydration and fluid migration at the base of the upper mantle: implications for deep earthquake mechanisms

    NASA Astrophysics Data System (ADS)

    Richard, Guillaume; Monnereau, Marc; Rabinowicz, Michel

    2007-03-01

    Water enters the Earth's mantle via subduction of oceanic lithosphere and sediments. A lot of this water immediately returns to the atmosphere through arc volcanism, but part, retained in Dense Hydrous Magnesium Silicates (DHMSs) and Nominally Anhydrous Minerals (NAMs) like olivine, is expected to be subducted as deep as the bottom of the upper mantle (660 km depth). Then, due to its low solubility in lower mantle minerals, water is likely to be released as a hydrated fluid during the spinel-post-spinel phase change. The dynamics of this fluid phase is investigated through a 1-D model of compaction, in which a source term has been introduced to take the fluid precipitation into account. The competition between the advective transport by the descending slab and the buoyant rise of the fluid results in three distinct situations, depending on the properties of the solid and the fluid phases. Low matrix permeability and high fluid viscosity inhibit compaction and favour the entrainment of fluid towards the deep mantle. In this case, the entire slab water content would enter the lower mantle and would be mixed at large scale. However, realistic values of the fluid viscosity and matrix permeability make this possibility unlikely. When effective, compaction results in an accumulation of fluid at and below the phase boundary. Then, depending on the value of the matrix viscosity, the situation evolves differently. Above 1020 Pa s, accumulation of fluid extends below the phase boundary and the pressure difference between the fluid and the matrix increases continuously, exceeding the yield strength of rocks. As a result, cracks would form and evolve towards the formation of dykes. In case of very low mantle viscosity, possibly due to strong grain size reduction during phase change, compaction becomes very efficient and the fluid remains confined within the phase change horizon, without increasing pressure. In the long term, this last situation appears unstable and would also

  13. Mission Moho: Rationale for drilling deep through the ocean crust into the upper mantle

    NASA Astrophysics Data System (ADS)

    Ildefonse, B.; Abe, N.; Kelemen, P. B.; Kumagai, H.; Teagle, D. A. H.; Wilson, D. S.; Moho Proponents, Mission

    2009-04-01

    Sampling a complete section of the ocean crust to the Moho was the original inspiration for scientific ocean drilling, and remains the main goal of the 21st Century Mohole Initiative in the IODP Science Plan. Fundamental questions about the composition, structure, and geophysical characteristics of the ocean lithosphere, and about the magnitude of chemical exchanges between the mantle, crust and oceans remain unresolved due to the absence of in-situ samples and measurements. The geological nature of the Mohorovičić discontinuity itself remains poorly constrained. "Mission Moho" is a proposal that was submitted to IODP in April 2007, with the ambition to drill completely through intact oceanic crust formed at a fast spreading rate, across the Moho and into the uppermost mantle. Although, eventually, no long-term mission was approved by IODP, the scientific objectives related to deep drilling in the ocean crust remain essential to our understanding of the Earth. These objectives are to : - Determine the geological meaning of the Moho in different oceanic settings, determine the in situ composition, structure and physical properties of the uppermost mantle, and understand mantle melt migration, - Determine the bulk composition of the oceanic crust to establish the chemical links between erupted lavas and primary mantle melts, understand the extent and intensity of seawater hydrothermal exchange with the lithosphere, and estimate the chemical fluxes returned to the mantle by subduction, - Test competing hypotheses of the ocean crust accretion at fast spreading mid-ocean ridges, and quantify the linkages and feedbacks between magma intrusion, hydrothermal circulation and tectonic activity, - Calibrate regional seismic measurements against recovered cores and borehole measurements, and understand the origin of marine magnetic anomalies, - Establish the limits of life in the ocean lithosphere. The "MoHole" was planned as the final stage of Mission Moho, which requires

  14. Deep global cycling of carbon constrained by the solidus of anhydrous, carbonated eclogite under upper mantle conditions

    NASA Astrophysics Data System (ADS)

    Dasgupta, Rajdeep; Hirschmann, Marc M.; Withers, Anthony C.

    2004-10-01

    eclogite solidus is likely to intersect the oceanic geotherm at a depth close to 400 km. Carbonated eclogite bodies entering the convecting upper mantle will thus release carbonate melt near the top of the mantle transition zone and may account for anomalously slow seismic velocities at depths of 280-400 km. Upon release, this small volume, highly reactive melt could be an effective agent of deep mantle metasomatism. Comparison of the carbonated eclogite solidus with that of peridotite-CO 2 shows a shallower solidus-geotherm intersection for the latter. This implies that carbonated peridotite is a more likely proximal source of magmatic carbon in oceanic provinces. However, carbonated eclogite is a potential source of continental carbonatites, as its solidus crosses the continental shield geotherm at ca. 4 GPa. Transfer of eclogite-derived carbonate melt to peridotite may account for the geochemical characteristics of some oceanic island basalts (OIBs) and their association with high CaO and CO 2.

  15. Deep seismic structure and tectonics of northern Alaska: Crustal-scale duplexing with deformation extending into the upper mantle

    USGS Publications Warehouse

    Fuis, G.S.; Murphy, J.M.; Lutter, W.J.; Moore, T.E.; Bird, K.J.; Christensen, N.I.

    1997-01-01

    Seismic reflection and refraction and laboratory velocity data collected along a transect of northern Alaska (including the east edge of the Koyukuk basin, the Brooks Range, and the North Slope) yield a composite picture of the crustal and upper mantle structure of this Mesozoic and Cenozoic compressional orogen. The following observations are made: (1) Northern Alaska is underlain by nested tectonic wedges, most with northward vergence (i.e., with their tips pointed north). (2) High reflectivity throughout the crust above a basal decollement, which deepens southward from about 10 km depth beneath the northern front of the Brooks Range to about 30 km depth beneath the southern Brooks Range, is interpreted as structural complexity due to the presence of these tectonic wedges, or duplexes. (3) Low reflectivity throughout the crust below the decollement is interpreted as minimal deformation, which appears to involve chiefly bending of a relatively rigid plate consisting of the parautochthonous North Slope crust and a 10- to 15-km-thick section of mantle material. (4) This plate is interpreted as a southward verging tectonic wedge, with its tip in the lower crust or at the Moho beneath the southern Brooks Range. In this interpretation the middle and upper crust, or all of the crust, is detached in the southern Brooks Range by the tectonic wedge, or indentor: as a result, crust is uplifted and deformed above the wedge, and mantle is depressed and underthrust beneath this wedge. (5) Underthrusting has juxtaposed mantle of two different origins (and seismic velocities), giving rise to a prominent sub-Moho reflector. Copyright 1997 by the American Geophysical Union.

  16. Transition region of the earth's upper mantle

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.; Bass, J. D.

    1986-01-01

    The chemistry of the earth's mantle is discussed using data from cosmochemistry, geochemistry, petrology, seismology, and mineral physics. The chondritic earth, the upper mantle and the 400-km discontinuity, the transition region, lower mantle mineralogy, and surface wave tomography are examined. Three main issues are addressed: (1) whether the mantle is homogeneous in composition or chemically stratified, (2) whether the major element chemistry of the mantle is more similar to upper mantle peridotites or to chondrites, and (3) the nature of the composition of the source region of basalts erupted at midocean ridges.

  17. Transition region of the earth's upper mantle

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.; Bass, J. D.

    1986-01-01

    The chemistry of the earth's mantle is discussed using data from cosmochemistry, geochemistry, petrology, seismology, and mineral physics. The chondritic earth, the upper mantle and the 400-km discontinuity, the transition region, lower mantle mineralogy, and surface wave tomography are examined. Three main issues are addressed: (1) whether the mantle is homogeneous in composition or chemically stratified, (2) whether the major element chemistry of the mantle is more similar to upper mantle peridotites or to chondrites, and (3) the nature of the composition of the source region of basalts erupted at midocean ridges.

  18. Deep Europe today: Geophysical synthesis of the upper mantle structure and lithospheric processes over 3.5 Ga

    USGS Publications Warehouse

    Artemieva, I.M.; Thybo, H.; Kaban, M.K.; ,

    2006-01-01

    We present a summary of geophysical models of the subcrustal lithosphere of Europe. This includes the results from seismic (reflection and refraction profiles, P- and S-wave tomography, mantle anisotropy), gravity, thermal, electromagnetic, elastic and petrological studies of the lithospheric mantle. We discuss major tectonic processes as reflected in the lithospheric structure of Europe, from Precambrian terrane accretion and subduction to Phanerozoic rifting, volcanism, subduction and continent-continent collision. The differences in the lithospheric structure of Precambrian and Phanerozoic Europe, as illustrated by a comparative analysis of different geophysical data, are shown to have both a compositional and a thermal origin. We propose an integrated model of physical properties of the European subcrustal lithosphere, with emphasis on the depth intervals around 150 and 250 km. At these depths, seismic velocity models, constrained by body-and surface-wave continent-scale tomography, are compared with mantle temperatures and mantle gravity anomalies. This comparison provides a framework for discussion of the physical or chemical origin of the major lithospheric anomalies and their relation to large-scale tectonic processes, which have formed the present lithosphere of Europe. ?? The Geological Society of London 2006.

  19. Upper mantle diapers, lower crustal magmatic underplating, and lithospheric dismemberment of the Great Basin and Colorado Plateau regions, Nevada and Utah; implications from deep MT resistivity surveying

    NASA Astrophysics Data System (ADS)

    Wannamaker, P. E.; Doerner, W. M.; Hasterok, D. P.

    2005-12-01

    In the rifted Basin and Range province of the southwestern U.S., a common faulting model for extensional basins based e.g. on reflection seismology data shows dominant displacement along master faults roughly coincident with the main topographic scarp. On the other hand, complementary data such as drilling, earthquake focal mechanisms, volcanic occurrences, and trace indicators such as helium isotopes suggest that there are alternative geometries of crustal scale faulting and material transport from the deep crust and upper mantle in this province. Recent magnetotelluric (MT) profiling results reveal families of structures commonly dominated by high-angle conductors interpreted to reflect crustal scale fault zones. Based mainly on cross cutting relationships, these faults appear to be late Cenozoic in age and are of low resistivity due to fluids or alteration (including possible graphitization). In the Ruby Mtns area of north-central Nevada, high angle faults along the margins of the core complex connect from near surface to a regional lower crustal conductor interpreted to contain high-temperature fluids and perhaps melts. Such faults may exemplify the high angle normal faults upon which the major earthquakes of the Great Basin appear to nucleate. A larger-scale transect centered on Dixie Valley shows major conductive crustal-scale structures connecting to conductive lower crust below Dixie Valley, the Black Rock desert in NW Nevada, and in east-central Nevada in the Monitor-Diamond Valley area. In the Great Basin-Colorado Plateau transition of Utah, the main structures revealed are a series of nested low-angle detachment structures underlying the incipient development of several rift grabens. All these major fault zones appear to overlie regions of particularly conductive lower crust interpreted to be caused by recent basaltic underplating. In the GB-CP transition, long period data show two, low-resistivity upper mantle diapirs underlying the concentrated

  20. Dihedral angle of carbonatite melts in mantle residue near the upper mantle and transition zone

    NASA Astrophysics Data System (ADS)

    Ghosh, S. K.; Rohrbach, A.; Schmidt, M. W.

    2015-12-01

    Carbonate melts are thought to be ideal metasomatic agents in the deep upper mantle (Green & Wallace, 1988) and these melts are low in viscosities (10-1-10-3 Pa·s) compared to primitive basalt (101-102 Pa·s), furthermore the ability to form an interconnected grain-edge melt network at low melt fractions (< 1%) make carbonate melts extremely mobile. They are molten at relatively low temperatures and have solidus temperatures hundreds of degrees lower than silicate melts at >3 GPa (Dasgupta et al. 2006, Ghosh et al., 2009), dissolve a number of geochemically incompatible elements much better than silicate melts (Blundy and Dalton, 2000). Previous studies of carbonate melt dihedral angles in olivine-dominated matrices yielded 25-30oat 1-3 GPa, relatively independent of melt composition (Watson et al., 1990) and temperature (Hunter and McKenzie, 1989). Dihedral angles of carbonate melts in contact with deep mantle silicate phases (e.g. garnet, wadsleyite, and ringwoodite) which constitute more than 70 % of the deep upper mantle and transition zone have not been studied yet. We have performed multi-anvil experiments on carbonate-bearing peridotites with 5.0 wt% CO2 from 13.5 to 20 GPa 1550 oC to investigate the dihedral angle of magnesio-carbonatite melts in equilibrium with garnet, olivine (and its high-pressure polymorphs), and clinoenstatite. The dihedral angle of carbonate melts in the deep upper mantle and transition zone is ~30° for majorite garnet and olivine (and its polymorphs) dominated matrices. It does not change with increasing pressure in the range 13.5-20 GPa. Our results suggest that very low melt fractions of carbonatite melt forming in the deep upper mantle and transition zone are interconnected at melt fractions less than 0.01. Consistent with geophysical observations, this could possibly explain low velocity regions in the deep mantle and transition zone.

  1. Seismic evidence for Earth's crusty deep mantle

    NASA Astrophysics Data System (ADS)

    Frost, Daniel A.; Rost, Sebastian; Garnero, Edward J.; Li, Mingming

    2017-07-01

    Seismic tomography resolves anomalies interpreted as oceanic lithosphere subducted deep into Earth's lower mantle. However, the fate of the compositionally distinct oceanic crust that is part of the lithosphere is poorly constrained but provides important constraints on mixing processes and the recycling process in the deep Earth. We present high-resolution seismic array analyses of anomalous P-waves sampling the deep mantle, and deterministically locate heterogeneities in the lowermost 300 km of the mantle. Spectral analysis indicates that the dominant scale length of the heterogeneity is 4 to 7 km. The heterogeneity distribution varies laterally and radially and heterogeneities are more abundant near the margins of the lowermost mantle Large Low Velocity Provinces (LLVPs), consistent with mantle convection simulations that show elevated accumulations of deeply advected crustal material near the boundaries of thermo-chemical piles. The size and distribution of the observed heterogeneities is consistent with that expected for subducted oceanic crust. These results thus suggest the deep mantle contains an imprint of continued subduction of oceanic crust, stirred by mantle convection and modulated by long lasting thermo-chemical structures. The preferred location of the heterogeneity in the lowermost mantle is consistent with a thermo-chemical origin of the LLVPs. Our observations relate to the mixing behaviour of small length-scale heterogeneity in the deep Earth and indicate that compositional heterogeneities from the subduction process can survive for extended times in the lowermost mantle.

  2. Water Recycling, Lower Mantle Slab Subduction, and Viscous Layering of the Deep Mantle

    NASA Astrophysics Data System (ADS)

    Williams, Q.; McNamara, A.; Garnero, E.

    2005-12-01

    We explore the causes and possible consequences of a water/hydrogen-depleted layer in the lowermost ~1000 km of Earth`s mantle. At least three distinct, non-exclusive mechanisms exist that could generate such a layer: (1) descending melts could extract water from the deep mantle, and possibly sequester it within D``; (2) hydrogen could be stripped from deep mantle material during core formation, through formation of iron hydrides; and (3) the accreting planet could have nearly completely degassed, with the terrestrial water budget being accreted in a late hydrous veneer. In the latter two instances, the water budget of the mantle, and particularly the deep mantle, must entirely be generated by injection of water into the interior from the near surface. Our hypothesis is thus that the lower portion of Earth`s mantle might be (or have been) essentially dry, in contrast to the possible presence of 10's to 100's of ppm water in the overlying material. The principal geophysical effect of a water-depleted zone likely involves a marked increase in viscosity: for reference, such a decrease in water content produces about a 2-order of magnitude increase in the viscosity of upper mantle material. Fluid dynamic simulations show that a layer with a 2-order of magnitude viscosity increase in the bottom 1000 km of Earth`s mantle produces a substantial impediment to subduction, with subducted material laterally spreading out above this viscous layer. This behavior is compatible with tomographic images showing a lack of slab continuity into the deepest mantle, and the viscosity contrast thus produces a barrier to water ingress into the deep viscous layer, allowing it to remain anhydrous for extended time periods. Notably, the boundary between the viscous layer and overlying mantle and slab material undergoes substantial deflections, and because of the chemical similarity of the layers, should be seismically undetectable. Our results provide a straightforward mechanism through

  3. Upper-mantle origin of the Yellowstone hotspot

    USGS Publications Warehouse

    Christiansen, R.L.; Foulger, G.R.; Evans, J.R.

    2002-01-01

    Fundamental features of the geology and tectonic setting of the northeast-propagating Yellowstone hotspot are not explained by a simple deep-mantle plume hypothesis and, within that framework, must be attributed to coincidence or be explained by auxiliary hypotheses. These features include the persistence of basaltic magmatism along the hotspot track, the origin of the hotspot during a regional middle Miocene tectonic reorganization, a similar and coeval zone of northwestward magmatic propagation, the occurrence of both zones of magmatic propagation along a first-order tectonic boundary, and control of the hotspot track by preexisting structures. Seismic imaging provides no evidence for, and several contraindications of, a vertically extensive plume-like structure beneath Yellowstone or a broad trailing plume head beneath the eastern Snake River Plain. The high helium isotope ratios observed at Yellowstone and other hotspots are commonly assumed to arise from the lower mantle, but upper-mantle processes can explain the observations. The available evidence thus renders an upper-mantle origin for the Yellowstone system the preferred model; there is no evidence that the system extends deeper than ???200 km, and some evidence that it does not. A model whereby the Yellowstone system reflects feedback between upper-mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep-mantle plume hypothesis.

  4. The upper mantle transition region - Eclogite

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.

    1979-01-01

    The upper mantle transition region is usually considered to be peridotite which undergoes a series of phase changes involving spinel and post-spinel assemblages. There are difficulties associated with attempts to explain the 220, 400 and 670 km discontinuities in terms of phase changes in a peridotitic mantle. Moreover, in a differentiated earth there should be large quantities of eclogite in the upper mantle. Eclogite is denser than Al2O3-poor mantle to depths of 670 km, but it stays in the garnet stability field to pressures in excess of those required to transform depleted mantle to denser phases such as ilmenite and perovskite. Eclogite, therefore, remains above 670 km. The seismic properties of the transition region are more consistent with eclogite than peridotite. Most of the mantle's inventory of incompatible trace elements may be in this layer, which is a potential source region for some basalt magmas. The radioactivity in this layer is the main source of mantle heat flow, 0.7 microcalorie/sq cm-sec, and drives upper mantle convection.

  5. The upper mantle transition region - Eclogite

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.

    1979-01-01

    The upper mantle transition region is usually considered to be peridotite which undergoes a series of phase changes involving spinel and post-spinel assemblages. There are difficulties associated with attempts to explain the 220, 400 and 670 km discontinuities in terms of phase changes in a peridotitic mantle. Moreover, in a differentiated earth there should be large quantities of eclogite in the upper mantle. Eclogite is denser than Al2O3-poor mantle to depths of 670 km, but it stays in the garnet stability field to pressures in excess of those required to transform depleted mantle to denser phases such as ilmenite and perovskite. Eclogite, therefore, remains above 670 km. The seismic properties of the transition region are more consistent with eclogite than peridotite. Most of the mantle's inventory of incompatible trace elements may be in this layer, which is a potential source region for some basalt magmas. The radioactivity in this layer is the main source of mantle heat flow, 0.7 microcalorie/sq cm-sec, and drives upper mantle convection.

  6. Deep mantle: Enriched carbon source detected

    NASA Astrophysics Data System (ADS)

    Barry, Peter H.

    2017-09-01

    Estimates of carbon in the deep mantle vary by more than an order of magnitude. Coupled volcanic CO2 emission data and magma supply rates reveal a carbon-rich mantle plume source region beneath Hawai'i with 40% more carbon than previous estimates.

  7. Upper mantle anisotropy structure beneath eastern Tibet

    NASA Astrophysics Data System (ADS)

    Wang, Y.; Wang, W.; Wen, L.; Chen, X.

    2013-12-01

    Continental collision between the Indian and the Eurasian plates resulted in uplift of the Tibetan plateau and the thickening of the crust. A lot of work has been done on the crust structures beneath Tibet, and several tectonic models are proposed to explain the mechanism of the uplift and thickening. But due to the absence of the upper mantle structures, those models are still under debate. Fine upper mantle velocity and anisotropy structures can help us understand the dynamic process of the Tibetan plateau. Previous studies used shear wave splitting and surface wave analysis to study anisotropy structures beneath the Tibetan plateau. But those two methods provide a poor vertical resolution in upper mantle. Waveform modeling of upper mantle triplication phases can provide a good vertical resolution, but present methods for calculating synthetic seismograms cannot process anisotropic media. We develop a method based on the generalized reflection and transmission method (GRTM) to calculate synthetic seismograms for wave propagating in stratified VTI media, so we can waveform model upper mantle triplications propagating in anisotropic media. Recently, the increasing number of permanent and temporary seismic stations near Tibet provides us a good opportunity to study fine upper mantle structures beneath the Tibetan plateau. In this study, we waveform model the tangential and radial seismic triplication data recorded in Chinese digital seismic stations at a epicentral distance of 10-30 degree for several events occurring in middle Tibet to constrain fine upper mantle velocity and anisotropy structures beneath eastern Tibet. We also use mineral physics modeling method to explore thermal and compositional models that would explain the inferred seismic structures.

  8. Density heterogeneities of the European upper mantle

    NASA Astrophysics Data System (ADS)

    Yegorova, T.

    2003-04-01

    DENSITY HETEROGENEITIES OF THE EUROPEAN UPPER MANTLE T.Yegorova Institute of Geophysics, National Academy of Sciences of Ukraine egorova@igph.kiev.ua Large-scale 3D gravity modelling in one degree averaging has been carried out for the European crust and Northern Atlantic; the model comprises two heterogeneous regional layers sediments and crystalline crust, confined by reliable seismic horizons the “seismic” basement and the Moho interface (Geophys. J. Int., 2002, 151: 11-31). As a result, residual gravity (crust free) anomalies, caused by density heterogeneities in the upper mantle and reaching some hundred mGal in amplitude, were obtained. The present study has been undertaken to substantiate mantle origin of the crust-free anomalies on the basis of their correlation with the upper mantle heterogeneities revealed by other methods (first of all seismology and geothermal data). Existence of fundamental relations between subsurface geotectonic units and upper mantle structure is substantiated by revealed correlation of gravity mantle anomalies (MA) with: thickness of both crust and lithosphere, heat flow data, age of last tectono-thermal activation. Temperature regime of the upper mantle has been proven to be the main factor determining the density distribution below the crustal base. On the basis of revealed correlation between MA and lithosphere thickness (LT) a scheme of rough estimates of the lithosphere thickness has been obtained for the whole European continent and Northern Atlantic. The main features of the obtained pattern of the LT are generally in good agreement with known parameters of the LT. Density distribution in the upper mantle have been estimated by converting the MA into density values of the subcrustal layer, confined by the Moho and the depth level in the upper mantle (adopted for two versions at the depth of 100 and 200 km). On the other hand, revealed correlation between mantle gravity anomalies and velocity heterogeneities (which is

  9. Does MORB reflect upper mantle diversity?

    NASA Astrophysics Data System (ADS)

    Murton, B. J.; Smith, H.; Fitton, G.

    2013-12-01

    It is often stated that MORB provides a window into the composition of the earth's upper mantle. Although MORB displays a range of compositions, its spatial scale tends to be much longer than that of oceanic hotspots or mantle plumes, which also display greater compositional heterogeneity. Hence it is tempting to conclude that oceanic upper mantle is more homogeneous than mantle involved in hotspots and plumes. Observations from the interaction between Iceland and the adjacent Reykjanes Ridge offer a chance to test this view. A remarkable feature of this interaction is the rapid diminution of heterogeneity in basaltic lavas from onshore Iceland to off-shore along the adjacent mid-ocean spreading ridge (the Reykjanes Ridge). Young volcanic rocks on Iceland exhibit a wide range of trace-element and isotopic compositions reflecting a diversity of composition within the plume mantle beneath Iceland. The neovolcanic axis of the adjacent spreading ridge is also affected by the Iceland plume: the presence of a large diameter bathymetric swell, V-shaped ridges migrating out from Iceland, and associated enriched geochemical compositions all point to a ~1000-km diameter regional plume influence. Despite this, the diversity of lava composition decreases rapidly along the Reykanes Ridge away from Iceland. This decrease is unlikely to be an artefact of sampling as the Reykjanes Ridge has a very high density of samples acquired from dredge stations located every 2-3 km, each of which recovered a number of individual lavas. Collapsing the diversity of young lava compositions on Iceland produces a mean composition that lies on a mixing line between lavas from the northern Reykjanes Ridge and the highly enriched peripheral Icelandic volcano of Snaefjelsness. We argue that this decrease in heterogeneity is real and is evidence for either a mantle process that homogenises the outflowing Icelandic plume as it flows away from Iceland beneath the Reykjanes Ridge, or alternatively a

  10. Preface: Deep Slab and Mantle Dynamics

    NASA Astrophysics Data System (ADS)

    Suetsugu, Daisuke; Bina, Craig R.; Inoue, Toru; Wiens, Douglas A.

    2010-11-01

    We are pleased to publish this special issue of the journal Physics of the Earth and Planetary Interiors entitled "Deep Slab and Mantle Dynamics". This issue is an outgrowth of the international symposium "Deep Slab and Mantle Dynamics", which was held on February 25-27, 2009, in Kyoto, Japan. This symposium was organized by the "Stagnant Slab Project" (SSP) research group to present the results of the 5-year project and to facilitate intensive discussion with well-known international researchers in related fields. The SSP and the symposium were supported by a Grant-in-Aid for Scientific Research (16075101) from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. In the symposium, key issues discussed by participants included: transportation of water into the deep mantle and its role in slab-related dynamics; observational and experimental constraints on deep slab properties and the slab environment; modeling of slab stagnation to constrain its mechanisms in comparison with observational and experimental data; observational, experimental and modeling constraints on the fate of stagnant slabs; eventual accumulation of stagnant slabs on the core-mantle boundary and its geodynamic implications. This special issue is a collection of papers presented in the symposium and other papers related to the subject of the symposium. The collected papers provide an overview of the wide range of multidisciplinary studies of mantle dynamics, particularly in the context of subduction, stagnation, and the fate of deep slabs.

  11. Deep Mantle Fluids Bottled Up in Diamonds

    NASA Astrophysics Data System (ADS)

    Weiss, Y.; Pearson, D. G.

    2015-12-01

    Many mantle xenoliths and mineral inclusions in diamonds reflect refertilisation and enrichment by mantle metasomatism, a key mechanism for controlling abrupt changes in the chemical and physical properties of the continental lithospheric mantle (CLM) globally. However, the nature of the fluids involved can normally only be constrained indirectly from geochemical proxies or calculated using mineral/melt partition coefficients. Direct samples of mantle metasomatic fluids, shielded from any late stage alteration, are encased as microinclusions in fast-growing diamonds - "fibrous diamonds". These trapped high-density fluids (HDFs) provide a unique chemical and physical record for tracing the sources of deep mantle fluids and constraining the processes that shape their nature.Diamond HDFs vary between four major compositional types: saline, silicic and high-Mg plus low-Mg carbonatitic. A strong connection has been established between high-Mg carbonatitic HDFs and a carbonated peridotite source. In addition, the silicic and low-Mg carbonatitic HDFs have been related to hydrous eclogite (±carbonate). However, the compositionally extreme saline fluid endmember remained enigmatic and its source in the deep lithosphere has remained ambiguous. Our new data on fluid-rich diamonds show the geochemical fingerprints of a subducting slab as the source of deep mantle fluids of saline composition. In addition, for the first time, we show that these deep saline fluids are parental, via fluid rock interaction, to in-situ forming carbonatitic and silicic melts in the lithosphere. This model provides a strong platform for resolving the effects of the compositional spectrum of mantle fluids, which alter the deep lithosphere globally and play key roles in diamond formation.

  12. Isotopic Evidence For Chaotic Imprint In The Upper Mantle Heterogeneity

    NASA Astrophysics Data System (ADS)

    Armienti, P.; Gasperini, D.

    2006-12-01

    Heterogeneities of the asthenospheric mantle along mid-ocean ridges have been documented as the ultimate effect of complex processes dominated by temperature, pressure and composition of the shallow mantle, in a convective regime that involves mass transfer from the deep mantle, occasionally disturbed by the occurrence of hot spots (e.g. Graham et al., 2001; Agranier et al., 2005; Debaille et al., 2006). Alternatively, upper mantle heterogeneity is seen as the natural result of basically athermal processes that are intrinsic to plate tectonics, such as delamination and recycling of continental crust and of subducted aseismic ridges (Meibom and Anderson, 2003; Anderson, 2006). Here we discuss whether the theory of chaotic dynamical systems applied to isotopic space series along the Mid-Atlantic Ridge (MAR) and the East Pacific Rise (EPR) can delimit the length-scale of upper mantle heterogeneities, then if the model of marble-cake mantle (Allègre and Turcotte, 1986) is consistent with a fractal distribution of such heterogeneity. The correlations between the isotopic (Sr, Nd, Hf, Pb) composition of MORB were parameterized as a function of the ridge length. We found that the distribution of isotopic heterogenity along both the MAR and EPR is self- similar in the range of 7000-9000 km. Self-similarity is the imprint of chaotic mantle processes. The existence of strange attractors in the distribution of isotopic composition of the asthenosphere sampled at ridge crests reveals recursion of the same mantle process(es), endured over long periods of time, up to a stationary state. The occurrence of the same fractal dimension for both the MAR and EPR implies independency of contingent events, suggesting common mantle processes, on a planetary scale. We envisage the cyclic route of "melting, melt extraction and recycling" as the main mantle process which could be able to induce scale invariance. It should have happened for a significant number of times over the Earth

  13. Methane-derived hydrocarbons produced under upper-mantle conditions

    SciTech Connect

    Kolesnikov, Anton; Kutcherov, Vladimir G.; Goncharov, Alexander F.

    2009-08-13

    There is widespread evidence that petroleum originates from biological processes. Whether hydrocarbons can also be produced from abiogenic precursor molecules under the high-pressure, high-temperature conditions characteristic of the upper mantle remains an open question. It has been proposed that hydrocarbons generated in the upper mantle could be transported through deep faults to shallower regions in the Earth's crust, and contribute to petroleum reserves. Here we use in situ Raman spectroscopy in laser-heated diamond anvil cells to monitor the chemical reactivity of methane and ethane under upper-mantle conditions. We show that when methane is exposed to pressures higher than 2 GPa, and to temperatures in the range of 1,000-1,500 K, it partially reacts to form saturated hydrocarbons containing 2-4 carbons (ethane, propane and butane) and molecular hydrogen and graphite. Conversely, exposure of ethane to similar conditions results in the production of methane, suggesting that the synthesis of saturated hydrocarbons is reversible. Our results support the suggestion that hydrocarbons heavier than methane can be produced by abiogenic processes in the upper mantle.

  14. Seismic imaging of crust and upper mantle beneath northeast China

    NASA Astrophysics Data System (ADS)

    Huang, Jinli

    2017-04-01

    Northeast China is located in the composite part of Paleo Asia ocean and Pacific ocean Domain, it has undergone multi-stage tectonism. The geological structure is complicated and volcanic activity is strong. In this region, two major geologic and geophysical boundaries are distinct, the NNE-trending North South Gravity Lineament (NSGL) and Tanlu fault. With respect to North China Craton (NCC), Northeast China is more closely adjacent to the subduction zone of Pacific slab, along the eastern boundary of Northeast China the subducting Pacific plate approaches depths of 600 km, many deep earthquakes occurred here. This region becomes an ideal place to investigate deep structure related to deep subduction, deep earthquakes as well as intraplate volcanism. In this study, we determined high-resolution three dimensional P- and S-wave velocity models of the crust and upper mantle to 800 km depth by jointly inverting arrival times from local events and relative residuals from teleseismic events. Our results show that main velocity anomalies exhibited block feature and are generally oriented in NE to NNE direction, which is consistent with regional tectonic direction. The NSGL is characterized by a high-velocity (high-V) anomaly belt with a width of approximately 100 km, and the high-V anomaly extents to the bottom of upper mantle or mantle transition zone. The songliao basin, which is located between NSGL and Tanlu fault tectonic boundaries, obvious low-velocity anomaly extends to about depth of 200 km. Under the Great Xing'an Range on the west side of NSGL, the low velocity extend to the lithosphere. Our results also revealed that most of deep earthquakes all occurred in deep subduction zone with high-velocity anomaly. Further, we also observed that extensive low velocity exists above deep-earthquakes zones, this result suggests that deep subduction of the Pacific slab maybe affect overlying lithosphere, resulting in the state of molten, semi-molten or high water.

  15. Seismic Q of the lunar upper mantle

    NASA Technical Reports Server (NTRS)

    Nakamura, Y.; Koyama, J.

    1982-01-01

    Shallow moonquake data are used to determine the frequency dependence of Q values for both compressional and shear waves in the upper mantle of the moon at frequencies between 3 and 8 Hz. The seismic P wave Q is estimated to be at least 4000 and is nearly independent of frequency or decreases slightly with increasing frequency, while the S wave Q increases from at least 4000 at 3 Hz to at least 7000 at 8 Hz. The rate of increase of Q(S) is approximately proportional to the 0.7 + or - 0.1 power of the frequency above 5 Hz. With the absence of other dissipation mechanisms, compressional heat loss may be a dominant factor in the lunar interior. Uncertainty remains, however, in the absolute values of Q's owing to the largely unknown detailed structure of the lunar upper mantle.

  16. Attenuation tomography of the upper mantle

    NASA Astrophysics Data System (ADS)

    Adenis, Alice; Debayle, Eric; Ricard, Yanick

    2017-08-01

    We present QsADR17, a global shear wave attenuation model of the upper mantle. Synthetic tests confirm that large-scale shear attenuation anomalies are resolved in the whole upper mantle with limited vertical smearing (≤50 km). QsADR17 shows strong correlation with surface tectonics down to 200 km depth, with low attenuation beneath continents and high attenuation beneath oceans. The attenuation signal near 250 km depth is dominated by a high-quality factor along subduction zones. Attenuating anomalies are found beneath mid-ocean ridges down to 150 km and under most Pacific hot spots from the lithosphere down to the transition zone. The presence of broad attenuating anomalies at 150 km depth in the Pacific Ocean suggests that several thermal plumes pond in the asthenosphere. Evidence for compositional heterogeneities is found in the lithosphere at the base of cratons and in a number of active regions.

  17. Seismic Q of the lunar upper mantle

    NASA Astrophysics Data System (ADS)

    Nakamura, Y.; Koyama, J.

    1982-06-01

    Shallow moonquake data are used to determine the frequency dependence of Q values for both compressional and shear waves in the upper mantle of the moon at frequencies between 3 and 8 Hz. The seismic P wave Q is estimated to be at least 4000 and is nearly independent of frequency or decreases slightly with increasing frequency, while the S wave Q increases from at least 4000 at 3 Hz to at least 7000 at 8 Hz. The rate of increase of Q(S) is approximately proportional to the 0.7 + or - 0.1 power of the frequency above 5 Hz. With the absence of other dissipation mechanisms, compressional heat loss may be a dominant factor in the lunar interior. Uncertainty remains, however, in the absolute values of Q's owing to the largely unknown detailed structure of the lunar upper mantle.

  18. Physical state of the western U.S. upper mantle

    NASA Technical Reports Server (NTRS)

    Humphreys, Eugene D.; Dueker, Kenneth G.

    1994-01-01

    Using observed P wave images of the western U.S. upper mantle, which show lateral variations of up to 8%, and existing scaling relations, we infer that the low-velocity mantle is hot and partially molten to depths of 100-200 km, and that the high-velocity upper mantle is subsolidus. Most the high-velocity upper mantle within a few hundred kilometers of the coastline appears to be relatively dense, suggesting that it is relatively cool (i.e., a thermal lithosphere). This is expected for features associated with the subducting Juan de Fuca and Gorda slabs, and the high velocity upper mantle beneath the Transverse Ranges has been attributed to the sinking of negatively buoyant mantle lithosphere. Other high-velocity mantle structures near the continental margin are consistent with this interpretation. In contrast, the generally high elevations of the continental interior imply a buoyant upper mantle there, an inference that holds for both the high- and the low-velocity upper mantle. The only resonable way to produce the high-velocity low-density upper mantle is through basalt depletion, thereby creating mantle of increased solidus temperature and decreased density. We distinguish a marginal domain, within approximately 250 km of the Pacific coast, from an interior domain. This is based on the inferred upper mantle compositional difference and regional associations: beneath the marginal domain, upper mantle structures trend parallel to the surface physiography and young tectonic structures, whereas upper mantle structures beneath the continental interior trend northeasterly. This northeast orientation is discordant with the young tectonic structures, but aligns with young volcanic activity. The high lateral gradients in observed upper mantle seismic structure found throughout the western United States imply high lateral gradients in the associated temperature or partial melt fields. Because these fields diffuse on time scales of less than a few tens of millions of

  19. Physical state of the western U.S. upper mantle

    NASA Astrophysics Data System (ADS)

    Humphreys, Eugene D.; Dueker, Kenneth G.

    1994-05-01

    Using observed P wave images of the western U.S. upper mantle, which show lateral variations of up to 8%, and existing scaling relations, we infer that the low-velocity mantle is hot and partially molten to depths of 100-200 km, and that the high-velocity upper mantle is subsolidus. Most the high-velocity upper mantle within a few hundred kilometers of the coastline appears to be relatively dense, suggesting that it is relatively cool (i.e., a thermal lithosphere). This is expected for features associated with the subducting Juan de Fuca and Gorda slabs, and the high velocity upper mantle beneath the Transverse Ranges has been attributed to the sinking of negatively buoyant mantle lithosphere. Other high-velocity mantle structures near the continental margin are consistent with this interpretation. In contrast, the generally high elevations of the continental interior imply a buoyant upper mantle there, an inference that holds for both the high- and the low-velocity upper mantle. The only resonable way to produce the high-velocity low-density upper mantle is through basalt depletion, thereby creating mantle of increased solidus temperature and decreased density. We distinguish a marginal domain, within approximately 250 km of the Pacific coast, from an interior domain. This is based on the inferred upper mantle compositional difference and regional associations: beneath the marginal domain, upper mantle structures trend parallel to the surface physiography and young tectonic structures, whereas upper mantle structures beneath the continental interior trend northeasterly. This northeast orientation is discordant with the young tectonic structures, but aligns with young volcanic activity. The high lateral gradients in observed upper mantle seismic structure found throughout the western United States imply high lateral gradients in the associated temperature or partial melt fields. Because these fields diffuse on time scales of less than a few tens of millions of

  20. Surface wave tomography applied to the North American upper mantle

    NASA Astrophysics Data System (ADS)

    van der Lee, Suzan; Frederiksen, Andrew

    Tomographic techniques that invert seismic surface waves for 3-D Earth structure differ in their definitions of data and the forward problem as well as in the parameterization of the tomographic model. However, all such techniques have in common that the tomographic inverse problem involves solving a large and mixed-determined set of linear equations. Consequently these inverse problems have multiple solutions and inherently undefinable accuracy. Smoother and rougher tomographic models are found with rougher (confined to great circle path) and smoother (finite-width) sensitivity kernels, respectively. A powerful, well-tested method of surface wave tomography (Partitioned Waveform Inversion) is based on inverting the waveforms of wave trains comprising regional S and surface waves from at least hundreds of seismograms for 3-D variations in S wave velocity. We apply this method to nearly 1400 seismograms recorded by digital broadband seismic stations in North America. The new 3-D S-velocity model, NA04, is consistent with previous findings that are based on separate, overlapping data sets. The merging of US and Canadian data sets, adding Canadian recordings of Mexican earthquakes, and combining fundamental-mode with higher-mode waveforms provides superior resolution, in particular in the US-Canada border region and the deep upper mantle. NA04 shows that 1) the Atlantic upper mantle is seismically faster than the Pacific upper mantle, 2) the uppermost mantle beneath Precambrian North America could be one and a half times as rigid as the upper mantle beneath Meso- and Cenozoic North America, with the upper mantle beneath Paleozoic North America being intermediate in seismic rigidity, 3) upper-mantle structure varies laterally within these geologic-age domains, and 4) the distribution of high-velocity anomalies in the deep upper mantle aligns with lower mantle images of the subducted Farallon and Kula plates and indicate that trailing fragments of these subducted

  1. Seismic velocity variations beneath central Mongolia: Evidence for upper mantle plumes?

    NASA Astrophysics Data System (ADS)

    Zhang, Fengxue; Wu, Qingju; Grand, Stephen P.; Li, Yonghua; Gao, Mengtan; Demberel, Sodnomsambuu; Ulziibat, Munkhuu; Sukhbaatar, Usnikh

    2017-02-01

    Central Mongolia is marked by wide spread recent volcanism as well as significant topographic relief even though it is far from any plate tectonic boundaries. The cause of the recent magmatism and topography remains uncertain partially because little is known of the underlying mantle seismic structure due to the lack of seismic instrumentation in the region. From August 2011 through August 2013, 69 broadband seismic stations were deployed in central Mongolia. Teleseismic traveltime residuals were measured using waveform correlation and were inverted to image upper mantle P and S velocity variations. Significant lateral variations in seismic velocity are imaged in the deep upper mantle (100 to 800 km depth). Most significant are two continuous slow anomalies from the deep upper mantle to near the surface. One slow feature has been imaged previously and may be a zone of deep upwelling bringing warm mantle to beneath the Hangay Dome resulting in uplift and magmatism including the active Khanuy Gol and Middle Gobi volcanoes. The second, deep low velocity anomaly is seen in the east from 800 to 150 km depth. The anomaly ends beneath the Gobi Desert that is found to have fast shallow mantle indicating a relatively thick lithosphere. We interpret the second deep slow anomaly as a mantle upwelling that is deflected by the thick Gobi Desert lithosphere to surrounding regions such as the Hentay Mountains to the north. The upwellings are a means of feeding warmer than normal asthenospheric mantle over a widely distributed region beneath Mongolia resulting in distributed volcanic activity and uplift. There is no indication that the upwellings are rooted in the deep lower mantle i.e. classic plumes. We speculate the upwellings may be related to deep subduction of the Pacific and Indian plates and are thus plumes anchored in the upper mantle.

  2. Upper mantle material in the Brazilian shield

    NASA Astrophysics Data System (ADS)

    Berbert, C. O.; Svisero, D. P.; Sial, A. N.; Meyer, H. O. A.

    1981-04-01

    Information on the nature of the upper mantle can be obtained from nodules in kimberlites and basalt and from mantle-derived magmas, mineral inclusions in diamonds, as well as from the fields of geodesy, seismology, geothermy, geomagnetism and petrological models for the upper mantle. In Brazil studies of these kinds are still in the stage of data gathering. This article intends to present some of this data related to the alpine peridotites, nodules in basalts, mineral inclusions in diamonds, and kimberlites, without any pretension of deeper-going interpretation. Alpine peridotites are found all over Brazil and are grouped in three main classes: the serpentinized dunites-peridotites of small and medium size; the gabbro-pyroxenite-peridotite association in large complexes, the latter described only in the central part of Brazil; and the pyroxenite-gabbroic gneisses of the Goianira-Trindade type. Kimberlites have been described in Minas Gerais and Piaui states, but they also exist in Mato Grosso and possibly in Rondonia, Goiás, Roraima and Bahia. Inclusions in diamonds studied from Minas Gerais, Piauí, Mato Grosso, Paraná, Sa˜o Paulo and Goiás include olivine, pyroxene, garnet, chromite, sulphides, ilmenite, zircon and rutile. Ultramafic nodules in basalts and basanites from Rio de Janeiro, Rio Grande do Norte, Paraíba states and Fernando de Noronha Island are essentially Iherzolites, like the ones described from Paraguay.

  3. Dehydrogenation of goethite in Earth’s deep lower mantle

    PubMed Central

    Hu, Qingyang; Kim, Duck Young; Liu, Jin; Meng, Yue; Yang, Liuxiang; Zhang, Dongzhou; Mao, Wendy L.; Mao, Ho-kwang

    2017-01-01

    The cycling of hydrogen influences the structure, composition, and stratification of Earth’s interior. Our recent discovery of pyrite-structured iron peroxide (designated as the P phase) and the formation of the P phase from dehydrogenation of goethite FeO2H implies the separation of the oxygen and hydrogen cycles in the deep lower mantle beneath 1,800 km. Here we further characterize the residual hydrogen, x, in the P-phase FeO2Hx. Using a combination of theoretical simulations and high-pressure–temperature experiments, we calibrated the x dependence of molar volume of the P phase. Within the current range of experimental conditions, we observed a compositional range of P phase of 0.39 < x < 0.81, corresponding to 19–61% dehydrogenation. Increasing temperature and heating time will help release hydrogen and lower x, suggesting that dehydrogenation could be approaching completion at the high-temperature conditions of the lower mantle over extended geological time. Our observations indicate a fundamental change in the mode of hydrogen release from dehydration in the upper mantle to dehydrogenation in the deep lower mantle, thus differentiating the deep hydrogen and hydrous cycles. PMID:28143928

  4. Dehydrogenation of goethite in Earth's deep lower mantle

    NASA Astrophysics Data System (ADS)

    Hu, Qingyang; Kim, Duck Young; Liu, Jin; Meng, Yue; Yang, Liuxiang; Zhang, Dongzhou; Mao, Wendy L.; Mao, Ho-kwang

    2017-02-01

    The cycling of hydrogen influences the structure, composition, and stratification of Earth’s interior. Our recent discovery of pyrite-structured iron peroxide (designated as the P phase) and the formation of the P phase from dehydrogenation of goethite FeO2H implies the separation of the oxygen and hydrogen cycles in the deep lower mantle beneath 1,800 km. Here we further characterize the residual hydrogen, x, in the P-phase FeO2Hx. Using a combination of theoretical simulations and high-pressure–temperature experiments, we calibrated the x dependence of molar volume of the P phase. Within the current range of experimental conditions, we observed a compositional range of P phase of 0.39 < x < 0.81, corresponding to 19–61% dehydrogenation. Increasing temperature and heating time will help release hydrogen and lower x, suggesting that dehydrogenation could be approaching completion at the high-temperature conditions of the lower mantle over extended geological time. Our observations indicate a fundamental change in the mode of hydrogen release from dehydration in the upper mantle to dehydrogenation in the deep lower mantle, thus differentiating the deep hydrogen and hydrous cycles.

  5. Processes of deep terrestrial mantles and cores

    NASA Technical Reports Server (NTRS)

    Jeanloz, Raymond

    1991-01-01

    Ultrahigh pressure experiments are currently focused on revealing processes occurring deep inside planets. This is in addition to the traditional emphasis on the constitution of planetary interiors, such as the identification of the high pressure perovskite phase of (Mg,Fe)SiO3 as the predominant mineral inside the Earth, and probably Venus. For example, experiments show that the mechanism of geochemical differentiation, separation of partial melts, differs fundamentally in the lower mantles of Earth and Venus than at near surface conditions. In addition to structural transformations, changes in chemical bonding caused by pressure can also be significant for planetary interiors. Measurements of AC and DC electrical conductivity can be obtained at ultrahigh pressures and temperatures, to greater than 80 GPa and 3000 K simultaneously, using the laser heated diamond cell. Anhydrous lower mantle assemblages (perovskite + or - oxide phases) exhibit an electrical conductivity that depends strongly on Fe content. Contrary to traditional assumptions, temperature affects the conductivity of lower mantle assemblages relatively little. The Earth's deep focus seismicity can be explained by the recycling of water into the mantle.

  6. Processes of deep terrestrial mantles and cores

    NASA Technical Reports Server (NTRS)

    Jeanloz, Raymond

    1991-01-01

    Ultrahigh pressure experiments are currently focused on revealing processes occurring deep inside planets. This is in addition to the traditional emphasis on the constitution of planetary interiors, such as the identification of the high pressure perovskite phase of (Mg,Fe)SiO3 as the predominant mineral inside the Earth, and probably Venus. For example, experiments show that the mechanism of geochemical differentiation, separation of partial melts, differs fundamentally in the lower mantles of Earth and Venus than at near surface conditions. In addition to structural transformations, changes in chemical bonding caused by pressure can also be significant for planetary interiors. Measurements of AC and DC electrical conductivity can be obtained at ultrahigh pressures and temperatures, to greater than 80 GPa and 3000 K simultaneously, using the laser heated diamond cell. Anhydrous lower mantle assemblages (perovskite + or - oxide phases) exhibit an electrical conductivity that depends strongly on Fe content. Contrary to traditional assumptions, temperature affects the conductivity of lower mantle assemblages relatively little. The Earth's deep focus seismicity can be explained by the recycling of water into the mantle.

  7. Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions.

    PubMed

    Walter, M J; Kohn, S C; Araujo, D; Bulanova, G P; Smith, C B; Gaillou, E; Wang, J; Steele, A; Shirey, S B

    2011-10-07

    A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle.

  8. Deep Mantle Cycling of Oceanic Crust: Evidence from Diamonds and Their Mineral Inclusions

    NASA Astrophysics Data System (ADS)

    Walter, M. J.; Kohn, S. C.; Araujo, D.; Bulanova, G. P.; Smith, C. B.; Gaillou, E.; Wang, J.; Steele, A.; Shirey, S. B.

    2011-10-01

    A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle.

  9. Attenuation Tomography of the Upper Mantle

    NASA Astrophysics Data System (ADS)

    Adenis, A.; Debayle, E.; Ricard, Y. R.

    2014-12-01

    We present a 3-D model of surface wave attenuation in the upper mantle. The model is constrained by a large data set of fundamental and higher Rayleigh mode observations. This data set consists of about 1,800,000 attenuation curves measured in the period range 50-300s by Debayle and Ricard (2012). A careful selection allows us to reject data for which measurements are likely biased by the poor knowledge of the scalar seismic moment or by a ray propagation too close to a node of the source radiation pattern. For each epicenter-station path, elastic focusing effects due to seismic heterogeneities are corrected using DR2012 and the data are turned into log(1/Q). The selected data are then combined in a tomographic inversion using the non-linear least square formalism of Tarantola and Valette (1982). The obtained attenuation maps are in agreement with the surface tectonic for periods and modes sensitive to the top 200km of the upper mantle. Low attenuation regions correlate with continental shields while high attenuation regions are located beneath young oceanic regions. The attenuation pattern becomes more homogeneous at depths greater than 200 km and the maps are dominated by a high quality factor signature beneath slabs. We will discuss the similarities and differences between the tomographies of seismic velocities and of attenuations.

  10. Robustness of Global Radial Anisotropy Models of the Upper Mantle

    NASA Astrophysics Data System (ADS)

    Xing, Z.; Beghein, C.; Yuan, K.

    2014-12-01

    discrepancies, inferences on the depth of continental roots (~200-250km) based on either the extent of the dlnVS>0 anomalies or the depth at which ξ changes sign remain independent of the crustal model employed. We also note that VSV>VSH dominates the deep upper mantle except in central Pacific, which is characterized by VSH>VSV down to the transition zone.

  11. Towards a Global Upper Mantle Attenuation Model

    NASA Astrophysics Data System (ADS)

    Karaoglu, Haydar; Romanowicz, Barbara

    2015-04-01

    Global anelastic tomography is crucial for addressing the nature of heterogeneity in the Earth's interior. The intrinsic attenuation manifests itself through dispersion and amplitude decay. These are contaminated by elastic effects such as (de)focusing and scattering. Therefore, mapping anelasticity accurately requires separation of elastic effects from the anelastic ones. To achieve this, a possible approach is to try and first predict elastic effects through the computation of seismic waveforms in a high resolution 3D elastic model, which can now be achieved accurately using numerical wavefield computations. Building upon the recent construction of such a whole mantle elastic and radially anisotropic shear velocity model (SEMUCB_WM1, French and Romanowicz, 2014), which will be used as starting model, our goal is to develop a higher resolution 3D attenuation model of the upper mantle based on full waveform inversion. As in the development of SEMUCB_WM1, forward modeling will be performed using the spectral element method, while the inverse problem will be treated approximately, using normal mode asymptotics. Both fundamental and overtone time domain long period waveforms (T>60s) will be used from a dataset of over 200 events observed at several hundred stations globally. Here we present preliminary results of synthetic tests, exploring different iterative inversion strategies.

  12. Chemical equilibration of the Earth's core and upper mantle

    USGS Publications Warehouse

    Brett, R.

    1984-01-01

    The oxygen fugacity (fO2) of the Earth's upper mantle appears to lie somewhat above that of the iron-wu??stite buffer, its fO2 is assumed to have been similar to the present value at the time of core formation. In the upper mantle, the Fe-rich liquid protocore that would form under such conditions of fO2 at elevated temperatures would lie predominantly in the system Fe-S-O. Distribution coefficients for Co, Cu, Ni, Ir, Au, Ir, W, Re, Mo, Ag and Ga between such liquids and basalt are known and minimum values are known for Ge. From these coefficients, upper mantle abundances for the above elements can be calculated by assuming cosmic abundances for the whole Earth and equilibrium between the Fe-S-O protocore and upper mantle. These calculated abundances are surprisingly close to presently known upper mantle abundances; agreements are within a factor of 5, except for Cu, W, and Mo. Therefore, siderophile element abundances in the upper mantle based on known distribution coefficients do not demand a late-stage meteoritic bombardment, and a protocore formed from the upper mantle containing S and O seems likely. As upper mantle abundances fit a local equilibrium model, then either the upper mantle has not been mixed with the rest of the mantle since core formation, or else partition coefficients between protocore and mantle were similar for the whole mantle regardless of P, T, and fO2. The latter possibility seems unlikely over such a P-T range. ?? 1984.

  13. High electrical conductivity in upper mantle

    NASA Astrophysics Data System (ADS)

    Bell, Peter M.

    In a joint study by geophysicists at two national laboratories, Los Alamos Scientific Laboratory and Lawrence Livermore National Laboratory, it was concluded that upper-mantle rocks may have relatively high electrical conductivity, presumably because of small amounts of carbon situated along grain boundaries. This impurity conductivity is similar to values determined in the laboratory for low-grade oil shale during pyrolysis. The residual char has electrical conductivity of 10-1 to 10-2 S/m, which is as much as 106 greater than the conductivity of unpyrolized low-grade oil shale (T. Shankland and A. Duba, Carbon-enhanced electrical conductivity in rocks (abstract), Eos, 63, 438, 1982). This much is known; the implications follow.

  14. Upper mantle anisotropy structure beneath eastern Tibet and its exploration

    NASA Astrophysics Data System (ADS)

    Wang, Y.; Wen, L.

    2014-12-01

    Continental collision between the Indian and the Eurasian plates resulted in uplift of the Tibetan plateau and the thickening of the crust. A lot of work has been done on the crust structures beneath Tibet, and several tectonic models are proposed to explain the mechanism of the uplift and thickening. But due to the absence of the upper mantle structures, those models are still under debate. Fine upper mantle velocity and anisotropy structures can help us understand the dynamic process of the Tibetan plateau. Waveform modeling of upper mantle triplication phases can provide a good vertical resolution of upper mantle velocity structures, but present methods for calculating synthetic seismograms cannot process anisotropic media. We develop a method based on the generalized reflection and transmission method (GRTM) to calculate synthetic seismograms for wave propagating in stratified VTI media, so we can waveform model upper mantle triplications propagating in anisotropic media. In this study, we waveform model the tangential and radial seismic triplication data recorded in Chinese digital seismic stations at a epicentral distance of 10-30 degree for one events occurring in middle Tibet to constrain fine upper mantle velocity and anisotropy structures beneath eastern Tibet. The result shows that horizontal S wave velocity is larger than vertical S wave velocity in the upper mantle beneath eastern Tibet. We also build a mineral physics modeling method, which can calculate upper mantle anisotropy structures based on mantle temperatures, compositions and directions of mantle flow, and use this method to explore compositional and dynamic models that would explain the inferred seismic structures. The results suggest that in our sampling region, the water content is lower than 0.4 wt%, and there is vertical mantle flow beneath the lithospheric lid.

  15. Global Upper Mantle Azimuthal Anisotropy From Probabilistic Tomography

    NASA Astrophysics Data System (ADS)

    Beghein, C.; Yuan, K.

    2014-12-01

    The new model of Yuan and Beghein (2013), hereafter YBaniSV13, is the first global model to constrain 3-D azimuthal anisotropy in the deep upper mantle. It is compatible with previous models in the uppermost 200km of the mantle, but also displays 1% anisotropy above, inside, and below the Mantle Transition Zone (MTZ). Another interesting characteristic of this model is the change in fast seismic direction detected, on average, at ~250km depth and at the MTZ boundaries. These results have important consequences for our understanding of mantle deformation and convection patterns in the mantle. It is therefore important to assess the robustness if these features. We already tested that the model does not strongly depend on the reference 1-D mantle model, on the presence of discontinuities in this reference model, or on the crustal model and Moho depth used to calculate the laterally varying partial derivatives. In this work, we apply a model space approach, the Neighborhood Algorithm (NA) of Sambridge (1999), to determine quantitative model uncertainties and parameter trade-offs. First, the NA generates an ensemble of models with a sampling density that increases toward the best fitting regions of the model space, and then performs a Bayesian appraisal of the models obtained that allows us to determine the likelihood of azimuthal anisotropy in different region of Earth's interior. Such approaches have the advantage of sampling the model null-space, and therefore provide more reliable model uncertainties than traditional inverse techniques. We use YBaniSV13 as initial model, and search the model space around it, allowing for large enough deviations to test the robustness of the anisotropy amplitude. We compare results from a model space search based on the chi-square misfit and from a model space search based on the variance reduction, which is another useful measure of data fit that is independent of data uncertainties. Preliminary results for the chi-square driven

  16. European upper mantle tomography: adaptively parameterized models

    NASA Astrophysics Data System (ADS)

    Schäfer, J.; Boschi, L.

    2009-04-01

    We have devised a new algorithm for upper-mantle surface-wave tomography based on adaptive parameterization: i.e. the size of each parameterization pixel depends on the local density of seismic data coverage. The advantage in using this kind of parameterization is that a high resolution can be achieved in regions with dense data coverage while a lower (and cheaper) resolution is kept in regions with low coverage. This way, parameterization is everywhere optimal, both in terms of its computational cost, and of model resolution. This is especially important for data sets with inhomogenous data coverage, as it is usually the case for global seismic databases. The data set we use has an especially good coverage around Switzerland and over central Europe. We focus on periods from 35s to 150s. The final goal of the project is to determine a new model of seismic velocities for the upper mantle underlying Europe and the Mediterranean Basin, of resolution higher than what is currently found in the literature. Our inversions involve regularization via norm and roughness minimization, and this in turn requires that discrete norm and roughness operators associated with our adaptive grid be precisely defined. The discretization of the roughness damping operator in the case of adaptive parameterizations is not as trivial as it is for the uniform ones; important complications arise from the significant lateral variations in the size of pixels. We chose to first define the roughness operator in a spherical harmonic framework, and subsequently translate it to discrete pixels via a linear transformation. Since the smallest pixels we allow in our parameterization have a size of 0.625 °, the spherical-harmonic roughness operator has to be defined up to harmonic degree 899, corresponding to 810.000 harmonic coefficients. This results in considerable computational costs: we conduct the harmonic-pixel transformations on a small Beowulf cluster. We validate our implementation of adaptive

  17. Upper mantle anisotropy in the New Zealand region

    SciTech Connect

    Klosko, E.K.

    1999-02-01

    Shear-wave splitting parameters of fast polarization direction (F) and delay time (dt) are determined using data from the Southern Alps Passive Seismic Experiment (SAPSE), on the South Island of New Zealand and in the surrounding region. Our results clearly show that F are subparallel to trends of the Alpine and Marlborough Faults, and to the Pacific-Australian plate boundary. The dt values range from 0.6-2.2 s with an average value of 1.6 s; the largest values are from the central South Island. The main source of the observed shear-wave splitting is an anisotropic region between 40-400 km. The width of the zone is approximately 200 km. We attribute the coincidence of surface structural trends with the measured F, and the large dt values, to significant shear deformation in a 200 km thick zone along the plate boundary extending from the surface to deep within the upper mantle.

  18. Topographic Expression of Deep Crustal and Mantle Processes

    NASA Astrophysics Data System (ADS)

    Whipple, K. X.; Ouimet, W. B.; Baldwin, J. A.

    2006-12-01

    The topography of mountain ranges records much information about the history of tectonic events and climatic conditions. Although this record is not easily read, as there are many imperfectly understood factors that influence landscape morphology, there is much encouraging evidence that the river steepness index (a cousin of the Hack gradient index) exhibits a robust correlation with rock uplift rate. Published, and soon to be published, examples span a wide range of field conditions from sites around the globe (e.g., Western US, Bolivia, Nepal, China, Taiwan). Differences in substrate rock properties can have a pronounced influence on channel steepness for a given rock uplift rate, however, and analyses are best limited to regional studies where climatic conditions and lithology are largely uniform. In such settings, careful analysis of river profiles and the relief structure of the landscape can reveal important information about the history of rock uplift, recording temporal changes in uplift/river incision rates. This potential adds considerably to the value of topographic analyses, especially where uplift is driven by deep crustal or mantle processes and not clearly expressed in upper crustal structures and geodetic shortening data. However, this potential comes at the price of complicating the interpretation of the relationship between topography and the modern snapshot of deep crustal/mantle conditions provided by geophysical imaging techniques. Over what timescales does topography retain a record of past events in earth's interior? Important questions remain unanswered about the age of topography in many landscapes where deep crustal or mantle dynamics have been invoked. We discuss the factors that control landscape response time, both to an increase in rock uplift rate and to the cessation of tectonic activity, and present constraints on the antiquity of topography in several field settings.

  19. Redox state of earth's upper mantle from kimberlitic ilmenites

    NASA Technical Reports Server (NTRS)

    Haggerty, S. E.; Tompkins, L. A.

    1983-01-01

    Temperatures and oxygen fugacities are reported on discrete ilmenite nodules in kimberlites from West Africa which demonstrate that the source region in the upper mantle is moderately oxidized, consistent with other nodule suites in kimberlites from southern Africa and the United States. A model is presented for a variety of tectonic settings, proposing that the upper mantle is profiled in redox potential, oxidized in the fertile asthenosphere but reduced in the depleted lithosphere.

  20. An in situ experimental study of Zr4+ transport capacity of water-rich fluids in the temperature and pressure range of the deep crust and upper mantle

    NASA Astrophysics Data System (ADS)

    Mysen, Bjorn

    2015-12-01

    Throughout the Earth's history, mass transport involved fluids. In order to address the circumstances under which Zr4+ may have been transported in this manner, its solubility behavior in aqueous fluid with and without NaOH and SiO2 in equilibrium with crystalline ZrO2 was determined from 550 to 950 °C and 60 to 1200 MPa. The measurements were carried out in situ while the samples were at the temperatures and pressures of interest. In ZrO2-H2O and ZrO2-SiO2-H2O fluids, the Zr4+ concentration ranges from ≤10 to ~70 ppm with increasing temperature and pressure. Addition of SiO2 to the ZrO2-H2O system does not affect these values appreciably. In these two environments, Zr4+ forms simple oxide complexes in the H2O fluid with ∆H ~ 40 kJ/mol for the solution equilibrium, ZrO2(solid) = ZrO2(fluid). The Zr4+ concentration in aqueous fluid increases about an order of magnitude upon addition of 1 M NaOH, which reflects the formation of zirconate complexes. The principal solution mechanism is ZrO2 + 4NaOH = Na4ZrO4 + 2H2O with ∆H ~ 200 kJ/mol. Addition of both SiO2 and NaOH to ZrO2-H2O enhances the Zr4+ by an additional factor of about 5 with the formation of partially protonated alkali zircon silicate complexes in the fluid. The principal solution mechanism is 2ZrO2 + 2NaOH + 2SiO2 = Na2Zr2Si2O9 + H2O with ∆H ~ 40 kJ/mol. These results, in combination with other published experimental data, imply that fluid released during high-temperature/high-pressure dehydration of hydrous mineral assemblages in the Earth's interior under some circumstances may carry significant concentrations of Zr and probably other high field strength elements (HFSEs). This suggestion is consistent with the occurrence of Zr-rich veins in high-grade metamorphic eclogite and granulite terranes. Moreover, aqueous fluids transported from dehydrating oceanic crust into overlying mantle source rocks of partial melting also may carry high-abundance HFSE of fluids released from dehydrating slabs and

  1. Redox freezing and melting in the Earth's deep mantle resulting from carbon-iron redox coupling.

    PubMed

    Rohrbach, Arno; Schmidt, Max W

    2011-04-14

    Very low seismic velocity anomalies in the Earth's mantle may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper mantle is likely to be triggered by the presence of small amounts of carbonate. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth's transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ∼250 kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase. This 'redox freezing' process leads to diamond-enriched mantle domains in which the Fe(0), resulting from Fe(2+) disproportionation in perovskites and garnet, is consumed but the Fe(3+) preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe(3+) leading to true carbonatite redox melting at ∼660 and ∼250 kilometres depth to form deep-seated melts in the Earth's mantle.

  2. The North American upper mantle: Density, composition, and evolution

    NASA Astrophysics Data System (ADS)

    Mooney, Walter D.; Kaban, Mikhail K.

    2010-12-01

    The upper mantle of North America has been well studied using various seismic methods. Here we investigate the density structure of the North American (NA) upper mantle based on the integrative use of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity anomalies. The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the three-dimensional density structure of the crystalline crust as determined by seismic observations. Information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the seismic structure of the crust is based on a recent compilation, with layer densities estimated from P-wave velocities. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (-50 to -400 mGal) beneath western North America and the adjacent oceanic region and positive anomalies (+50 to +350 mGal) east of the NA Cordillera. This pattern reflects the well-known division of North America into the stable eastern region and the tectonically active western region. The close correlation of large-scale features of the mantle anomaly map with those of the topographic map indicates that a significant amount of the topographic uplift in western NA is due to buoyancy in the hot upper mantle, a conclusion supported by previous investigations. To separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data. The

  3. The North American upper mantle: density, composition, and evolution

    USGS Publications Warehouse

    Mooney, Walter D.; Kaban, Mikhail K.

    2010-01-01

    The upper mantle of North America has been well studied using various seismic methods. Here we investigate the density structure of the North American (NA) upper mantle based on the integrative use of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity anomalies. The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the three-dimensional density structure of the crystalline crust as determined by seismic observations. Information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the seismic structure of the crust is based on a recent compilation, with layer densities estimated from P-wave velocities. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (−50 to −400 mGal) beneath western North America and the adjacent oceanic region and positive anomalies (+50 to +350 mGal) east of the NA Cordillera. This pattern reflects the well-known division of North America into the stable eastern region and the tectonically active western region. The close correlation of large-scale features of the mantle anomaly map with those of the topographic map indicates that a significant amount of the topographic uplift in western NA is due to buoyancy in the hot upper mantle, a conclusion supported by previous investigations. To separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data

  4. The thermal regimes of the upper mantle beneath Precambrian and Phanerozoic structures up to the thermobarometry data of mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Glebovitsky, V. A.; Nikitina, L. P.; Khiltova, V. Ya.; Ovchinnikov, N. O.

    2004-05-01

    The thermal state of the upper mantle beneath tectonic structures of various ages and types (Archaean cratons, Early Proterozoic accretionary and collisional orogens, and Phanerozoic structures) is characterized by geotherms and by thermal gradients (TG) derived from data on the P- T conditions of mineral equilibria in garnet and garnet-spinel peridotite xenoliths from kimberlites (East Siberia, Northeastern Europe, India, Central Africa, North America, and Canada) and alkali basalts (Southeastern Siberia, Mongolia, southeastern China, southeastern Australia, Central Africa, South America, and the Solomon and Hawaiian islands). The use of the same garnet-orthopyroxene thermobarometer (Theophrastus Contributions to Advanced Studies in Geology. 3: Capricious Earth: Models and Modelling of Geologic Processes and Objects 2000 44) for all xenoliths allowed us to avoid discrepancies in estimation of the P- T conditions, which may be a result of the mismatch between different thermometers and barometers, and to compare the thermal regimes in the mantle in various regions. Thus, it was established that (1) mantle geotherms and geothermal gradients, obtained from the estimation of P- T equilibrium conditions of deep xenoliths, correspond to the age of crust tectonic structures and respectively to the time of lithosphere stabilization; it can be suggested that the ancient structures of the upper mantle were preserved within continental roots; (2) thermal regimes under continental mantle between the Archaean cratons and Palaeoproterozoic belts are different today; (3) the continental mantle under Neoproterozoic and Phanerozoic belts is characterized by significantly higher values of geothermal gradient compared to the mantle under Early Precambrian structures; (4) lithosphere dynamics seems to change at the boundary between Early and Mezo-Neoproterozoic and Precambrian and Phanerozoic.

  5. Trench migration and upper plate strain over a convecting mantle

    NASA Astrophysics Data System (ADS)

    Husson, Laurent

    2012-12-01

    Trench motion and upper plate deformation ultimately respond to mantle flow. Herein I build upon the mantle flow model results of Conrad and Behn (2010) and compute the drag forces underneath all plates, and show that they control the dynamics of plates and plate boundaries. The small misfit angle between between the traction azimuths of mantle traction and absolute plate motion corroborates the idea that convective mantle drag is a prominent driver of plate tectonics. Less intuitive is the fact that the interplay between the drag forces from the upper and lower plates, that amounts to -5 to 8.5 × 1012 N m-1 (per unit trench length), dictates both trench migration rates and upper plate deformation. At odds with the classic view that assigns the prime role to the idiosyncrasies of subduction zones (slab age, interplate friction, water content etc), I find that the intrinsic properties of subduction zones in fact only modulate this behavior. More specifically, the mean value of the integrated trenchward mantle drag force from the lower and upper plates (from -2 to 6.5 × 1012 N m-1) controls upper plate deformation. Conversely, it is the difference between the lower and upper plates mantle drag forces (from -3 to 10 × 1012 N m-1) that controls trench migration rates. In addition, I find that a minimum trenchward force of ˜2.5 × 1012 N m-1 must be supplied by mantle drag before trenches can actually advance, and before upper plates undergo compression. This force results from the default tendency of slabs to rollback when solely excited by their own buoyancy, and is thus the effective tensional force that slab pull exerts on the plate interface.

  6. Seismic structure and heterogeneity in the upper mantle

    NASA Astrophysics Data System (ADS)

    Kenntt, B. L. N.

    The earliest models of the seismic velocity structure of the upper mantle were smooth. But, since the introduction of strong gradients near 400 km depth by Jeffreys to explain the '20° discontinuity" in observed travel times, there has been a steady accumulation of detail in mantle structure. For a particular region, a smoothed and averaged representation of the seismic structure in the upper mantle can be derived from long-period body wave and higher mode surface wave observations. The vertical resolving power of such techniques is limited by the relatively long wavelengths. In contrast short-period observations offer potential resolution, but are susceptible to the influence of lateral heterogeneity. Fortunately the major features of the upper mantle can be discerned but important questions for structural processes such as the detailed nature ofthe transitions near 410 and 660 km are generally inaccessible. There is a natural tendency to overweight those observations on which particularly clear features are seen (as compared with the statistical anonymity of less spectacular data) which can lead to unwarranted generalizationsof specific results. To reconcile different views of mantle structure requires us to address the purpose for which the mantle structures are to be used. For example, fine detail in a velocity model which is insignificant for travel time studies can have a profound effect on amplitudes and short-period seismic waveforms. The variability in the patterns of body wave observations, especially atshort periods, provides strong evidence for 1-2 per cent heterogeneity on scales around 200 km in the upper mantle. Such features are superimposed on larger scale and larger amplitude lateral variations which can be mapped using surface wave studies. Much of the pattern of lateral variability in the upper mantle is likely to be due to thermal processes both directly by the influence of temperature and indirectly by compositional effects induced by flow

  7. Seismic Anisotropy in the Deep Mantle, Boundary Layers and the Geometry of Mantle Convection

    NASA Astrophysics Data System (ADS)

    Karato, S.

    An attempt is made to explore the geodynamical significance of seismic anisotropy in the deep mantle on the basis of mineral physics. The mineral physics observations used include the effects of deformation mechanisms on lattice and shape preferred orientation, the effects of pressure on elastic anisotropy and the nature of lattice preferred orientation in deep mantle minerals in dislocation creep regime. Many of these issues are still poorly constrained, but a review of recent results shows that it is possible to interpret deep mantle seismic anisotropy in a unified fashion, based on the solid state processes without invoking partial melting. The key notions are (i) the likely regional variation in the magnitude of anisotropy as deformation mechanisms change from dislocation to diffusion creep (or superplasticity), associated with a change in the stress level and/or grain-size in the convecting mantle with a high Rayleigh number, and (ii) the change in elastic anisotropy with pressure in major mantle minerals, particularly in (Mg, Fe)O. The results provide the following constraints on the style of mantle convection (i) the SH > SV anisotropy in the bottom transition zone and the SV > SH anisotropy in the top lower mantle can be attributed to anisotropy structures (lattice preferred orientation and/or laminated structures) caused by the horizontal flow in this depth range, suggesting the presence of a mid-mantle boundary layer due to (partially) layered convection, (ii) the observed no significant seismic anisotropy in the deep mantle near subduction zones implies that deformation associated with subducting slabs is due mostly to diffusion creep (or superplasticity) and therefore slabs are weak in the deep mantle and hence easily deformed when encountered with resistance forces, and (iii) the SH > SV anisotropy in the cold thick portions of the D" layer is likely to be due to horizontally aligned shape preferred orientation in perovskite plus magnesiow

  8. The composition of mantle plumes and the deep Earth

    NASA Astrophysics Data System (ADS)

    Hastie, Alan R.; Fitton, J. Godfrey; Kerr, Andrew C.; McDonald, Iain; Schwindrofska, Antje; Hoernle, Kaj

    2016-06-01

    Determining the composition and geochemical diversity of Earth's deep mantle and subsequent ascending mantle plumes is vital so that we can better understand how the Earth's primitive mantle reservoirs initially formed and how they have evolved over the last 4.6 billion years. Further data on the composition of mantle plumes, which generate voluminous eruptions on the planet's surface, are also essential to fully understand the evolution of the Earth's hydrosphere and atmosphere with links to surface environmental changes that may have led to mass extinction events. Here we present new major and trace element and Sr-Nd-Pb-Hf isotope data on basalts from Curacao, part of the Caribbean large igneous province. From these and literature data, we calculate combined major and trace element compositions for the mantle plumes that generated the Caribbean and Ontong Java large igneous provinces and use mass balance to determine the composition of the Earth's lower mantle. Incompatible element and isotope results indicate that mantle plumes have broadly distinctive depleted and enriched compositions that, in addition to the numerous mantle reservoirs already proposed in the literature, represent large planetary-scale geochemical heterogeneity in the Earth's deep mantle that are similar to non-chondritic Bulk Silicate Earth compositions.

  9. Structure of North Atlantic upper mantle based on gravity modelling, regional geochemistry and tectonic history

    NASA Astrophysics Data System (ADS)

    Barantseva, Olga; Artemieva, Irina; Thybo, Hans

    2016-04-01

    We study the link between deep geodynamic processes and their surface expression in the North Atlantic region which has an anomalous, complex structure compared to other oceans. We calculate a model of residual mantle gravity between the Charlie Gibbs Fracture Zone and Svalbard. The calculations are based on GOCE satellite data the regional crustal model EUNAseis (Artemieva and Thybo, 2013) ; for the crustal and topography effects, and the global totpgraphy and bathymetry model ETOPO1 from NOAA (Amante and Eakis, 2009). Results are complemented by sensitivity analysis of the various parameters' effects on the models. Our results identify strong heterogeneity in the upper mantle residual gravity, expressed as a sharp contrasts at the continent-ocean transition, positive mantle gravity below the continental blocks and negative - below oceanic blocks; the MOR has low-gravity anomaly. By introducing regional geochemical data and analysis of the tectonical history, we identify a strong correlation between residual mantle gravity anomalies and geochemical anomalies in ɛNd and Mg#. This analysis identifies three zones of North Atlantic mantle based on the correlation between upper mantle gravity and ocean floor age. In the area around Iceland, the residual mantle gravity is systematically lower than predicted from the half-space cooling model, and we estimate the thermal anomaly that could cause this shift.

  10. Upper mantle fluids involved in diamond formation and mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Sverjensky, D. A.

    2014-12-01

    Diamond formation coupled with metasomatic reactions involving the interaction of fluids with silicate host rocks provides important clues about the deep carbon cycle. However, quantitative modeling of diamond formation with silicate rock metasomatism has not been possible. Here the Deep Water (DEW) model [1] was used to generate equilibrium constants for irreversible chemical mass transfer calculations monitoring evolving fluid chemistry during diamond formation and coupled silicate reactions. Conceptual models for diamond formation in two environments were constructed for the purpose of illustrating the role of pH in diamond-forming systems. For cratonic diamonds, fluid at 900°C and 5.0 GPa was derived in equilibrium with a carbonated mafic part of a subducting slab consisting of pure diopside, enstatite, pyrope, phlogopite, magnesite, diamond and pyrite. The fluid was assumed to infiltrate and react at constant T and P with a model metasedimentary eclogite (jadeite, pyrope, kyanite and coesite). Abundant diamond was predicted to precipitate as reactant silicate minerals were destroyed and secondary pyrope, jadeite and kyanite were precipitated, which could represent the solid inclusions in natural diamonds. The final fluid chemistry was extremely enriched in Si and depleted in Ca relative to the initial fluid, consistent with the worldwide fluid inclusion trend from carbonatitic fluid to silicic fluid. The logfO2 changed by only 0.2, whereas pH continuously decreased as reaction with jadeite and kyanite and precipitation of secondary pyrope removed Mg2+ and added H+ to the fluid. Most of the carbon precipitated as diamond was derived from decreasing concentrations of formate and propionate. In the UHPM scenario, fluid at 600°C and 5.0 GPa in carbonated peridotite (forsterite, antigorite, clinochlore, magnesite, and pyrrhotite) in a subducting slab was assumed to infiltrate and react at constant T and P with a different model metasedimentary eclogite (jadeite

  11. Rayleigh Wave Phase Velocity in the Indian Ocean Upper Mantle

    NASA Astrophysics Data System (ADS)

    Godfrey, K. E.; Dalton, C. A.

    2015-12-01

    Current understanding of the seismic properties of the oceanic upper mantle is heavily weighted toward studies of the Pacific upper mantle. However, global seismic models indicate differences in upper-mantle properties beneath the Pacific, Atlantic, and Indian oceans. Furthermore, factors such as spreading rate, absolute plate motion, and the presence of intraplate volcanism vary between these regions. It is thus important to consider the broad range in parameters when forming ideas about mantle dynamics and lithosphere evolution within ocean basins. We are developing a high-resolution basin-wide seismic model of the Indian Ocean upper mantle. The Indian Ocean contains 16,000 km of mid-ocean ridge, with spreading rates ranging from approximately 14 mm/yr along the Southwest Indian Ridge to 55-75 mm/yr along the Southeast Indian Ridge. It also contains 12 volcanic hotspots, overlies a portion of a large low-shear-velocity province in the lower mantle, and is home to the Australian-Antarctic Discordance and a negative geoid anomaly just south of India, among other features. We measure phase velocity in the period range 30-130 seconds for fundamental-mode Rayleigh waves traversing the Indian Ocean; the data set includes 831 events that occurred between 1992 and 2014 and 769 stations. In order to isolate the signal of the oceanic upper mantle, paths with >30% of their length through continental upper mantle are excluded. Variations in phase velocity in the Indian Ocean upper mantle are explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. Our preliminary results indicate a strong dependence of phase velocity on seafloor age, with higher velocity associated with older seafloor, and perturbations to the age-dependent trend in the vicinity of the Australian-Antarctic Discordance and the Marion and

  12. Postglacial rebound with a non-Newtonian upper mantle and a Newtonian lower mantle rheology

    NASA Technical Reports Server (NTRS)

    Gasperini, Paolo; Yuen, David A.; Sabadini, Roberto

    1992-01-01

    A composite rheology is employed consisting of both linear and nonlinear creep mechanisms which are connected by a 'transition' stress. Background stress due to geodynamical processes is included. For models with a non-Newtonian upper-mantle overlying a Newtonian lower-mantle, the temporal responses of the displacements can reproduce those of Newtonian models. The average effective viscosity profile under the ice-load at the end of deglaciation turns out to be the crucial factor governing mantle relaxation. This can explain why simple Newtonian rheology has been successful in fitting the uplift data over formerly glaciated regions.

  13. Postglacial rebound with a non-Newtonian upper mantle and a Newtonian lower mantle rheology

    NASA Technical Reports Server (NTRS)

    Gasperini, Paolo; Yuen, David A.; Sabadini, Roberto

    1992-01-01

    A composite rheology is employed consisting of both linear and nonlinear creep mechanisms which are connected by a 'transition' stress. Background stress due to geodynamical processes is included. For models with a non-Newtonian upper-mantle overlying a Newtonian lower-mantle, the temporal responses of the displacements can reproduce those of Newtonian models. The average effective viscosity profile under the ice-load at the end of deglaciation turns out to be the crucial factor governing mantle relaxation. This can explain why simple Newtonian rheology has been successful in fitting the uplift data over formerly glaciated regions.

  14. Detailed study of upper mantle anisotropy in the upper mantle of eastern North America

    NASA Astrophysics Data System (ADS)

    Chen, X.; Levin, V. L.; Li, Y.

    2016-12-01

    We collected observations of core-refracted shear waves on a 1300 km long array crossing the eastern part of the North American continent from James Bay to the Fundy Basin. We combine data from the Earthscope Transportable Array, Canadian and US permanent observatories, and the recently completed Earthscope FlexArray QMIII.Past studies found ample evidence for directional dependence (anisotropy) of seismic wave speed in the upper mantle of this region. However, to date the lateral spacing of seismic observatories made direct comparisons between anisotropic structure and tectonic divisions evident on the surface challenging. With instruments spacing 50 km, and less near major tectonic boundaries such as the Grenville Front and the Appalachian Front, we can discriminate between gradual changes in anisotropic properties due to asthenospheric flow variations, and abrupt and localized changes likely to arise from juxtaposition of distinct lithospheric blocks.To insure lateral consistency of measurements we selected core-refracted shear waves that were observed over the entire length of our array. Also, since directional dependence of splitting parameters is a well recognized signature of vertical changes in anisotropic properties we examine observations from different directions, and look for systematic changes.Most locations show evidence for some degree of splitting in observed shear waves. Delays between fast and slow components estimated using rotation-correlation method range from 0.3 to 1.5 s. At most sites delay values vary considerably between individual phases measured. Fast polarizations are predominantly NE-SW, which agrees with numerous past studies of the region. Systematic directional dependence of fast polarization is seen at all sites we studied. Furthermore, the values of fast polarization appear to be similar along the entire array for individual events but vary from event to event. Both of these observations are consistent with the previously proposed

  15. Localized seismic deformation in the upper mantle revealed by dense seismic arrays.

    PubMed

    Inbal, Asaf; Ampuero, Jean Paul; Clayton, Robert W

    2016-10-07

    Seismicity along continental transform faults is usually confined to the upper half of the crust, but the Newport-Inglewood fault (NIF), a major fault traversing the Los Angeles basin, is seismically active down to the upper mantle. We use seismic array analysis to illuminate the seismogenic root of the NIF beneath Long Beach, California, and identify seismicity in an actively deforming localized zone penetrating the lithospheric mantle. Deep earthquakes, which are spatially correlated with geochemical evidence of a fluid pathway from the mantle, as well as with a sharp vertical offset in the lithosphere-asthenosphere boundary, exhibit narrow size distribution and weak temporal clustering. We attribute these characteristics to a transition from strong to weak interaction regimes in a system of seismic asperities embedded in a ductile fault zone matrix. Copyright © 2016, American Association for the Advancement of Science.

  16. Localized seismic deformation in the upper mantle revealed by dense seismic arrays

    NASA Astrophysics Data System (ADS)

    Inbal, Asaf; Ampuero, Jean Paul; Clayton, Robert W.

    2016-10-01

    Seismicity along continental transform faults is usually confined to the upper half of the crust, but the Newport-Inglewood fault (NIF), a major fault traversing the Los Angeles basin, is seismically active down to the upper mantle. We use seismic array analysis to illuminate the seismogenic root of the NIF beneath Long Beach, California, and identify seismicity in an actively deforming localized zone penetrating the lithospheric mantle. Deep earthquakes, which are spatially correlated with geochemical evidence of a fluid pathway from the mantle, as well as with a sharp vertical offset in the lithosphere-asthenosphere boundary, exhibit narrow size distribution and weak temporal clustering. We attribute these characteristics to a transition from strong to weak interaction regimes in a system of seismic asperities embedded in a ductile fault zone matrix.

  17. Carbon-dioxide-rich silicate melt in the Earth's upper mantle.

    PubMed

    Dasgupta, Rajdeep; Mallik, Ananya; Tsuno, Kyusei; Withers, Anthony C; Hirth, Greg; Hirschmann, Marc M

    2013-01-10

    The onset of melting in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic melt could be stable 250 km or less beneath mid-oceanic ridges, owing to the small fraction (∼0.03 wt%) its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present at ∼200 km (refs 3-5) but large melt fractions are thought to be restricted to shallower depths. Here we present experiments on carbonated peridotites over 2-5 GPa that constrain the location and the slope of the onset of silicate melting in the mantle. We find that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and that silicate melting of dry peridotite + CO(2) beneath ridges commences at ∼180 km. Accounting for the effect of 50-200 p.p.m. H(2)O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ∼100 p.p.m. CO(2) becomes as deep as ∼220-300 km. We suggest that, on a global scale, carbonated silicate melt generation at a redox front ∼250-200 km deep, with destabilization of metal and majorite in the upwelling mantle, explains the oceanic low-velocity zone and the electrical conductivity structure of the mantle. In locally oxidized domains, deeper carbonated silicate melt may contribute to the seismic X-discontinuity. Furthermore, our results, along with the electrical conductivity of molten carbonated peridotite and that of the oceanic upper mantle, suggest that mantle at depth is CO(2)-rich but H(2)O-poor. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth's deep upper mantle, and the inventory of carbon, H(2)O and other highly incompatible elements at ridges becomes controlled by the flux of the former.

  18. Double Layering and Bilateral Asymmetry of a Thermochemical Plume in the Upper Mantle beneath Hawaii

    NASA Astrophysics Data System (ADS)

    Ballmer, Maxim D.; Ito, Garett; Wolfe, Cecily J.; Solomon, Sean C.

    2013-04-01

    Classical plume theory describes purely thermal upwellings that rise through the entire mantle, pond beneath the lithospheric plate in a thin "pancake," and generate hotspot volcanism. High-resolution seismic velocity images obtained from the Plume-Lithosphere Undersea Melt Experiment (PLUME) support the concept of a deep-rooted mantle plume beneath the Hawaiian hotspot. However, in detail these images challenge traditional concepts inasmuch as they indicate a broad low-velocity body in the upper mantle that is much thicker and more asymmetric than a thermal pancake predicted from purely thermal plume models. Geochemical observations also argue against a purely thermal (i.e., isochemical) mantle source for Hawaiian lavas and instead indicate a heterogeneous plume involving mafic lithologies such as eclogite. To explore the dynamical and melting behavior of hot plumes that also contain eclogite, we perform three-dimensional numerical simulations of thermochemical convection. The models simulate eclogite with an excess density relative to ambient-mantle peridotite that peaks at depths of 410-300 km due to solid phase changes in the quartz and olivine systems. Because of the effects of these phase transitions, a mantle plume containing >12% eclogite pools as a wide body (hundreds of kilometers wide) at depths of 450-300 km (the "deep eclogite pool", or DEP). From the top of the DEP rises a shallow plume that narrows rapidly due to removal of eclogite by melting. The shallow plume supplies material into a thin sublithospheric pancake and feeds hotspot volcanism. Seismic resolution tests indicate that the double layering of hot plume material (DEP and shallow pancake) can account for the thick low-velocity body imaged by PLUME. A subset of models predicts pulsations in plume ascent and variations in magmatic activity over timescales of ~10 Myr, as are observed. In these models, the complex upper-mantle dynamics of the eclogitic plume moreover give rise to intermittent

  19. Upper mantle reflectors: Modelling of seismic wavefield characteristics and tectonic implications

    NASA Astrophysics Data System (ADS)

    Hansen, T. M.; Balling, N.

    2003-04-01

    In recent years a number of deep seismic experiments have demonstrated the existence of seismic reflectors in the mantle lithosphere. The origin of the reflective structures is a matter of debate. Hypothesis and interpretations include remnant subduction zones, shear zones, fluids and seismic anisotropy. Through forward modelling studies including numerical full wavefield modelling, we have found that both upper mantle shear zones of reduced seismic velocity and density and remnant subduction slabs containing high-density eclogites may have sufficient seismic impedance contrasts to normal mantle peridotites to generate near-normal recidence reflectivity. Wide-angle energy may originate from subduction slabs containing high or low velocity eclogites whereas localized shear zones in mantle peridotite may generally not produce significant wide-angle energy. Analysis of two specific deep-seismic data sets from the North Sea (MONA LISA data) and the Baltic Sea (BABEL data) show good agreement between observations and modelling results for dipping remnant subduction slabs containing small-scale inhomogeneities associated with incomplete transformation of low velocity/low density crustal material to high velocity/high density eclogite. Our modelling results improve our possibilities of distinguishing between two often contrasting tectonic interpretations for dipping upper mantle seismic reflectors, the remnant subduction and extensional shear zone models.

  20. Water Distribution in the Continental and Oceanic Upper Mantle

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.

    2015-01-01

    Nominally anhydrous minerals such as olivine, pyroxene and garnet can accommodate tens to hundreds of ppm H2O in the form of hydrogen bonded to structural oxygen in lattice defects. Although in seemingly small amounts, this water can significantly alter chemical and physical properties of the minerals and rocks. Water in particular can modify their rheological properties and its distribution in the mantle derives from melting and metasomatic processes and lithology repartition (pyroxenite vs peridotite). These effects will be examined here using Fourier transform infrared spectrometry (FTIR) water analyses on minerals from mantle xenoliths from cratons, plume-influenced cratons and oceanic settings. In particular, our results on xenoliths from three different cratons will be compared. Each craton has a different water distribution and only the mantle root of Kaapvaal has evidence for dry olivine at its base. This challenges the link between olivine water content and survival of Archean cratonic mantle, and questions whether xenoliths are representative of the whole cratonic mantle. We will also present our latest data on Hawaii and Tanzanian craton xenoliths which both suggest the intriguing result that mantle lithosphere is not enriched in water when it interacts with melts from deep mantle upwellings (plumes).

  1. Dikes, joints, and faults in the upper mantle

    USGS Publications Warehouse

    Wilshire, H.G.; Kirby, S.H.

    1989-01-01

    Three different types of macroscopic fractures are recognized in upper-mantle and lower-crustal xenoliths in volcanic rocks from around the world: 1. (1) joints that are tensile fractures not occupied by crystallized magma products 2. (2) dikes that are tensile fractures occupied by mafic magmas crystallized to pyroxenites, gabbros or hydrous-mineral-rich rocks, 3. (3) faults that are unfilled shear fractures with surface markings indicative of shear displacement. In addition to intra-xenolith fractures, xenoliths commonly have polygonal or faceted shapes that represent fractures exploited during incorporation of the xenoliths into the host magma that brought them to the surface. The various types of fractures are considered to have formed in response to the pressures associated with magmatic fluids and to the ambient tectonic stress field. The presence of fracture sets and crosscutting relations indicate that both magma-filled and unfilled fractures can be contemporaneous and that the local stress field can change with time, leading to repeated episodes of fracture. These observations give insight into the nature of deep fracture processes and the importance of fluid-peridotite interactions in the mantle. We suggest that unfilled fractures were opened by volatile fluids exsolved from ascending magmas to the tops of growing dikes. These volatile fluids are important because they are of low viscosity and can rapidly transmit fluid pressure to dike and fault tips and because they lower the energy and tectonic stresses required to extend macroscopic cracks and to allow sliding on pre-existing fractures. Mantle seismicity at depths of 20-65 km beneath active volcanic centers in Hawaii corresponds to the depth interval where CO2-rich fluids are expected to be liberated from ascending basaltic magmas, suggesting that such fluids play an important role in facilitating earthquake instabilities in the presence of tectonic stresses. Other phenomena related to the fractures

  2. Upper- Mantle Driven Dynamic Uplift in Central Anatolia

    NASA Astrophysics Data System (ADS)

    Sengul Uluocak, Ebru; Pysklywec, Russell; Hakan Gogus, Oguz

    2016-04-01

    Based on geological and geophysical observations and interpretations of the present-day geodynamics, we propose that mantle structures beneath the crust drive a non-isostatic component of topography in Central Anatolia. Topography residuals for the region were calculated from the isostatic component of topography according to the principle of Airy isostasy while assuming crustal block is in hydrostatic equilibrium in the mantle. For the geodynamic interpretations we ran numerous 2D thermo-mechanical models based on different temperature inputs and viscous creep strength coefficients and using available P-wave tomography data along a N-S directional profile (33oE) through Central Anatolia as an estimate on the regional mantle structure. Our models are uniformly affected by widespread NE-SW oriented mantle flow obtained in the shear-wave azimuthal anisotropy studies and predict dynamic topography based on vertical components of density-driven flow in the upper mantle mainly induced by 2D temperature variations. The dynamic topography results indicate ~1 km instantaneous uplift in concordance with the under-compensated topography in the region. The data and modelling results define the region as a plateau-like uplift, slightly inflated in southern part based on dynamic topography patterns. The dynamic topography induced by upper-mantle flow provides robust new information about the main geodynamic components by showing broad consistency with independent data sets and observables for the area; such as, asthenospheric source of volcanism, gravity data, and high surface heat flow distributions.

  3. On the deep-mantle origin of the Deccan Traps.

    PubMed

    Glišović, Petar; Forte, Alessandro M

    2017-02-10

    The Deccan Traps in west-central India constitute one of Earth's largest continental flood basalt provinces, whose eruption played a role in the Cretaceous-Paleogene extinction event. The unknown mantle structure under the Indian Ocean at the start of the Cenozoic presents a challenge for connecting the event to a deep mantle origin. We used a back-and-forth iterative method for time-reversed convection modeling, which incorporates tomography-based, present-day mantle heterogeneity to reconstruct mantle structure at the start of the Cenozoic. We show a very low-density, deep-seated upwelling that ascends beneath the Réunion hot spot at the time of the Deccan eruptions. We found a second active upwelling below the Comores hot spot that likely contributed to the region of partial melt feeding the massive eruption.

  4. On the deep-mantle origin of the Deccan Traps

    NASA Astrophysics Data System (ADS)

    Glišović, Petar; Forte, Alessandro M.

    2017-02-01

    The Deccan Traps in west-central India constitute one of Earth’s largest continental flood basalt provinces, whose eruption played a role in the Cretaceous-Paleogene extinction event. The unknown mantle structure under the Indian Ocean at the start of the Cenozoic presents a challenge for connecting the event to a deep mantle origin. We used a back-and-forth iterative method for time-reversed convection modeling, which incorporates tomography-based, present-day mantle heterogeneity to reconstruct mantle structure at the start of the Cenozoic. We show a very low-density, deep-seated upwelling that ascends beneath the Réunion hot spot at the time of the Deccan eruptions. We found a second active upwelling below the Comores hot spot that likely contributed to the region of partial melt feeding the massive eruption.

  5. Water percolation in the upper lower mantle: A way to fill the Earth's mantle transition zone?

    NASA Astrophysics Data System (ADS)

    Richard, G. C.; Monnereau, M.; Ricard, Y.; Ingrin, J.

    2003-04-01

    Numerous studies have been devoted to the determination of water solubility in mantle material [1]. They all show strong solubility variations from one mineral phase to another. Principally, water partitioning has made the transition zone a probable trap for water from the Earth's mantle [2]. Nevertheless, previous numerical study [3] suggests the presence of 'free' water in the lower mantle. We have studied the behaviour of this 'free' phase and its importance in the water distribution just below the transition zone (660 km depth) in a 2D cartesian geometry. The model takes into account water partitioning between the mantle's transition zone and the upper-mantle of 10:1 and between the lower-mantle and the transition zone of 1:100 (i.e. respectively between olivine-spinel and spinel-postspinel). We have modelled two possible transport processes for the 'free' water: Diffusion and percolation. Our numerical experiments show that the presence of a ‘free phase’ modifies the water distribution in this area. Percolation process is more efficient to hydrate the transition zone than diffusion one, even in a very high diffusivity case. [1] J. Ingrin, H. Skogby, Hydrogen in nominally anhydrous upper mantle minerals: Concentration levels and implications, Eur. J. Mineral. 12 (2000) 543-570. [2] N. Bolfan-Casanova, H. Keppler, D. C. Rubie, Water partitioning between nominally anhydrous minerals in the MgO-SiO2-H2O system up to 24 GPa: implications for the distribution of water in Earth's mantle, Earth Planet. Sci. Lett. 182 (2000) 209-221. [3] G. C. M. Richard, M. Monnereau, J. Ingrin, Is the transition zone an empty water reservoir? Inferences from numerical model of mantle dynamics, Earth. Planet. Sci. Lett. 205 (2002) 37-51.

  6. The link between Hawaiian mantle plume composition, magmatic flux, and deep mantle geodynamics

    NASA Astrophysics Data System (ADS)

    Harrison, Lauren N.; Weis, Dominique; Garcia, Michael O.

    2017-04-01

    Oceanic island basalts sample mantle reservoirs that are isotopically and compositionally heterogeneous. The Hawaiian-Emperor chain represents ∼85 Myr of volcanism supplied by a deep mantle plume. Two geographically and geochemically delineated trends, Kea and Loa, are well documented within the Hawaiian Islands. Enriched Loa compositions originate from subduction recycled or primordial material stored in deep mantle reservoirs such as the large low shear velocity province (LLSVP) below Hawai'i. Loa compositions have not been observed along the Emperor Seamounts (>50 Ma), whereas lavas on the Hawaiian Islands (<6.5 Ma) sample both Kea and Loa sources. Lead isotopes in shield lavas along the Northwest Hawaiian Ridge (NWHR) spanning ∼42 Myr between the bend in the chain and the Hawaiian Islands record the geochemical evolution of the Hawaiian mantle plume over a time period when many geophysical parameters (volcanic propagation rate, magmatic flux, mantle potential temperature) increased significantly. Along the NWHR, the Loa geochemical component appears ephemerally, which we link to the sampling of different lower mantle compositional domains by the Hawaiian mantle plume. The plume initially sampled only the deep Pacific mantle (Kea component) from outside the LLSVP during the formation of the Emperor Seamounts. Southward migration and anchoring of the plume on the LLSVP led to entrainment of increasing amounts of LLSVP material (Loa component) along the NWHR as documented by an increase in 208Pb*/206Pb* with decreasing age. The correlation between 208Pb*/206Pb* and magmatic flux suggests source composition affects the magmatic flux, and explains why the Hawaiian mantle plume has dramatically strengthened through time.

  7. Estimating Upper Mantle Hydration from In Situ Electrical Conductivity

    NASA Astrophysics Data System (ADS)

    Behrens, J.; Constable, S.; Heinson, G.; Everett, M.; Weiss, C.; Key, K.

    2004-12-01

    The electrical conductivity of 35-40 Ma Pacific plate has been measured in situ; one robust result is the presence of bulk anisotropy in the lithospheric upper mantle. We interpret this anisotropy to be a result of hydrothermal circulation into the upper mantle along spreading-ridge-parallel normal faults: the associated zones of serpentinized peridotite provide the pathways of enhanced electrical conductivity required by the data. Our modeling bounds the range of possible anisotropic ratios, which are then used to estimate the amount of water required to serpentinize the requisite amounts of peridotite. These data sets, however, do not indicate anisotropy in the bulk conductivity of the crust, nor in the asthenospheric mantle. This second point is significant, as recent measurements of sub-continental asthenospheric conductivity have been interpreted to indicate anisotropy aligned with present plate motion, with the diffusion of hydrogen through olivine advanced as an explanation.

  8. Role of the deep mantle in generating the compositional asymmetry of the Hawaiian mantle plume

    NASA Astrophysics Data System (ADS)

    Weis, Dominique; Garcia, Michael O.; Rhodes, J. Michael; Jellinek, Mark; Scoates, James S.

    2011-12-01

    Linear chains of volcanic ocean islands are one of the most distinctive features on our planet. The longest, the Hawaiian-Emperor Chain, has been active for more than 80 million years, and is thought to have formed as the Pacific Plate moved across the Hawaiian mantle plume, the hottest and most productive of Earth's plumes. Volcanoes fed by the plume today form two adjacent trends, including Mauna Kea and Mauna Loa, that exhibit strikingly different geochemical characteristics. An extensive data set of isotopic analyses shows that lavas with these distinct characteristics have erupted in parallel along the Kea and Loa trends for at least 5 million years. Seismological data suggest that the Hawaiian mantle plume, when projected into the deep mantle, overlies the boundary between typical Pacific lower mantle and a sharply defined layer of apparently different material. This layer exhibits low seismic shear velocities and occurs on the Loa side of the plume. We conclude that the geochemical differences between the Kea and Loa trends reflect preferential sampling of these two distinct sources of deep mantle material. Similar indications of preferential sampling at the limit of a large anomalous low-velocity zone are found in Kerguelen and Tristan da Cunha basalts in the Indian and Atlantic oceans, respectively. We infer that the anomalous low-velocity zones at the core-mantle boundary are storing geochemical anomalies that are enriched in recycled material and sampled by strong mantle plumes.

  9. Thermo-compositonal anomalies of the Australian upper mantle

    NASA Astrophysics Data System (ADS)

    Tesauro, M.; Kaban, M. K.; Aitken, A.; Kennett, B. L. N.

    2016-12-01

    The upper mantle of the Australian continent has been deeply investiagted in the last two decades using a variety of geophysical methods. The resulting models have revealed the robust large-scale features of the continental lithosphere of Australia, i.e., faster wave speeds in the Archean and Proterozoic cratons in the West, North and South Australia and slower wave speeds in the eastern Phanerozoic margin. Furthermore, it has been identified an area of low seismic wavespeeds in the uppermost mantle beneath central Australia. The zone of slow wavespeeds is underlain by a region of fast wavespeeds, more typical of continental lithosphere. This layered velocity structure may have a thermal origin, due to the redistribution of high heat producing elements within the crust or reflects the presence of amphibole. To discern temperautre and compositional variations of the Australian upper mantle, we apply an iterative technique, which jointly interprets seismic tomography and gravity data. This technique consists in removing the effect of the crust from the observed gravity field and topography. In the second step, the residual mantle gravity field and residual topography are inverted to obtain a 3-D density model of the upper mantle. The inversion technique accounts for the notion that these fields are controlled by the same factors but in a different way (e.g., depending on depth and horizontal dimension of the heterogeneity.) This enables us to locate the position of principal density anomalies in the upper mantle. Afterwards, the thermal contribution to the density structure is estimated by inverting the seismic tomography model AusREM (http://rses.anu.edu.au/seismology/AuSREM/index.php). We improve the initial thermal and compositional models iteratively. In particular, the negative residual density anomalies is compensated by the density of a rock having a larger Mg# and depleted in garnet and CPX and the temperature is re-estimated according to the new composition.

  10. Geodetic Estimate of Water in the Wharton Basin Upper Mantle

    NASA Astrophysics Data System (ADS)

    Masuti, S. S.; Barbot, S.; Karato, S. I.; Feng, L.; Bannerjee, P.; Natawidjaja, D.

    2015-12-01

    The formation of oceans at the Earth's surface and hence the origin of life can be directly linked to the fate of water after planetary formation, but how much water is now confined in the upper mantle remains elusive. Current estimates of water in olivine from geochemistry are between 200 and 3600 H/106 Si. Here, we exploit the water-sensitive rheology of olivine to estimate the water content in the Wharton Basin asthenosphere in the Indian Ocean. We explore the role of water stratification in the upper mantle and the transient behavior of olivine flow in the context of postseismic deformation following the 2012 Mw 8.6 Wharton Basin earthquake using geodetic data from the Sumatra GPS network. We build a model that incorporates afterslip in the brittle upper mantle and viscous flow in the asthenosphere. We introduce a formulation of the transient rheology of olivine in the form of a flow law coupled to a state evolution that characterizes the internal stress of the mineral before steady-state. We find that the Wharton Basin asthenosphere contains about 1000 H/106 Si, representing 10% of water saturation of olivine at 100 km depth. If these results can be extrapolated to other depths, this indicates an equivalent of about 0.7 ocean mass is now present in the Earth's upper mantle.

  11. Volatile element content of the heterogeneous upper mantle

    NASA Astrophysics Data System (ADS)

    Shimizu, K.; Saal, A. E.; Hauri, E. H.; Forsyth, D. W.; Kamenetsky, V. S.; Niu, Y.

    2014-12-01

    The physical properties of the asthenosphere (e.g., seismic velocity, viscosity, electrical conductivity) have been attributed to either mineral properties at relevant temperature, pressure, and water content or to the presence of a low melt fraction. We resort to the geochemical studies of MORB to unravel the composition of the asthenosphere. It is important to determine to what extent the geochemical variations in axial MORB do represent a homogeneous mantle composition and variations in the physical conditions of magma generation and transport; or alternatively, they represent mixing of melts from a heterogeneous upper mantle. Lavas from intra-transform faults and off-axis seamounts share a common mantle source with axial MORB, but experience less differentiation and homogenization. Therefore they provide better estimates for the end-member volatile budget of the heterogeneous upper mantle. We present major, trace, and volatile element data (H2O, CO2, Cl, F, S) as well as Sr, Nd, and Pb isotopic compositions [1, 2] of basaltic glasses (MgO > 6.0 wt%) from the NEPR seamounts, Quebrada-Discovery-Gofar transform fault system, and Macquarie Island. The samples range from incompatible trace element (ITE) depleted (DMORB: Th/La<0.035) to enriched (EMORB: Th/La>0.07) spanning the entire range of EPR MORB. The isotopic composition of the samples correlates with the degree of trace element enrichment indicating long-lived mantle heterogeneity. Once shallow-level processes (degassing, crystallization, and crustal assimilation) have been considered, we conducted a two-component (DMORB- and EMORB-) mantle melting-mixing model. Our model reproduces the major, trace and volatile element contents and isotopic composition of our samples and suggests that (1) 90% of the upper mantle is highly depleted in ITE (DMORB source) with only 10% of an enriched component (EMORB source), (2) the EMORB source is peridotitic rather than pyroxenitic, and (3) NMORB do not represent an actual

  12. Slab mantle dehydrates beneath Kamchatka—Yet recycles water into the deep mantle

    NASA Astrophysics Data System (ADS)

    Konrad-Schmolke, Matthias; Halama, Ralf; Manea, Vlad C.

    2016-08-01

    The subduction of hydrated slab mantle is the most important and yet weakly constrained factor in the quantification of the Earth's deep geologic water cycle. The most critical unknowns are the initial hydration state and the dehydration behavior of the subducted oceanic mantle. Here we present a combined thermomechanical, thermodynamic, and geochemical model of the Kamchatka subduction zone that indicates significant dehydration of subducted slab mantle beneath Kamchatka. Evidence for the subduction of hydrated oceanic mantle comes from across-arc trends of boron concentrations and isotopic compositions in arc volcanic rocks. Our thermodynamic-geochemical models successfully predict the complex geochemical patterns and the spatial distribution of arc volcanoes in Kamchatka assuming the subduction of hydrated oceanic mantle. Our results show that water content and dehydration behavior of the slab mantle beneath Kamchatka can be directly linked to compositional features in arc volcanic rocks. Depending on hydration depth of the slab mantle, our models yield water recycling rates between 1.1 × 103 and 7.4 × 103 Tg/Ma/km corresponding to values between 0.75 × 106 and 5.2 × 106 Tg/Ma for the entire Kamchatkan subduction zone. These values are up to one order of magnitude lower than previous estimates for Kamchatka, but clearly show that subducted hydrated slab mantle significantly contributes to the water budget in the Kamchatkan subduction zone.

  13. Seismic anisotropy of upper mantle in Longmenshan and adjacent areas

    NASA Astrophysics Data System (ADS)

    Gao, Y.; Chen, L.

    2016-12-01

    Based on GPS results, it's been proposed that the continental collision between the Indian and Eurasian plates and the obstruction of the rigid Sichuan Basin jointly contributed to the steep rise of Longmen Shan (LMS). However, GPS results are insufficient to constrain the feature of the upper mantle deformation and to determine whether it contributes to the topographic relief across the LMS fault belt. To investigate the roles played by the upper mantle in the uplift and expansion processes of the tibetan Plateau and the interactions of the plateau with surrounding blocks, we studied the seismic anisotropy structure of the upper mantle in LMS and adjacent areas by teleseismic shear wave splitting analyses. We obtained the splitting parameters for seismic stations forming two nearly perpendicular linear arrays that cross the eastern margin of the Tibetan Plateau and the Sichuan Basin. We carefully analyzed the features of shear wave splitting and its spatial variations in this region and then delineate the pattern of upper mantle deformation by comparing our results with GPS results, the direction of absolute plate motion (APM), surface structures and crustal anisotropy. Our results show that the fast direction of shear waves in the LMS fault belt is subparallel to the fault strike, consistent with the stress distribution. Near tectonic boundaries, such as the Kunlun fault, and in the Yun-Gui Plateau, the fast directions are also subparallel to the fault strikes and agree with GPS observations. This suggests that in these areas tectonism may have ererted important impacts on the upper mantle deformation and the crust and upper mantle probably deformed in a vertically coherent way. In contrast, double-layer anisotropy was observed in the Qilian orogenic belt, which corroborates previous studies and indicates tectonic decoupling of the crust-mantle system. The fast direction in the Sichuan Basin is dominantly NW, close to the direction of APM. One possibility for

  14. Toward a coherent model for the melting behavior of the deep Earth's mantle

    NASA Astrophysics Data System (ADS)

    Andrault, D.; Bolfan-Casanova, N.; Bouhifd, M. A.; Boujibar, A.; Garbarino, G.; Manthilake, G.; Mezouar, M.; Monteux, J.; Parisiades, P.; Pesce, G.

    2017-04-01

    Knowledge of melting properties is critical to predict the nature and the fate of melts produced in the deep mantle. Early in the Earth's history, melting properties controlled the magma ocean crystallization, which potentially induced chemical segregation in distinct reservoirs. Today, partial melting most probably occurs in the lowermost mantle as well as at mid upper-mantle depths, which control important aspects of mantle dynamics, including some types of volcanism. Unfortunately, despite major experimental and theoretical efforts, major controversies remain about several aspects of mantle melting. For example, the liquidus of the mantle was reported (for peridotitic or chondritic-type composition) with a temperature difference of ∼1000 K at high mantle depths. Also, the Fe partitioning coefficient (DFeBg/melt) between bridgmanite (Bg, the major lower mantle mineral) and a melt was reported between ∼0.1 and ∼0.5, for a mantle depth of ∼2000 km. Until now, these uncertainties had prevented the construction of a coherent picture of the melting behavior of the deep mantle. In this article, we perform a critical review of previous works and develop a coherent, semi-quantitative, model. We first address the melting curve of Bg with the help of original experimental measurements, which yields a constraint on the volume change upon melting (ΔVm). Secondly, we apply a basic thermodynamical approach to discuss the melting behavior of mineralogical assemblages made of fractions of Bg, CaSiO3-perovskite and (Mg,Fe)O-ferropericlase. Our analysis yields quantitative constraints on the SiO2-content in the pseudo-eutectic melt and the degree of partial melting (F) as a function of pressure, temperature and mantle composition; For examples, we find that F could be more than 40% at the solidus temperature, except if the presence of volatile elements induces incipient melting. We then discuss the melt buoyancy in a partial molten lower mantle as a function of pressure

  15. Upper Mantle Discontinuity Structure Beneath the Western Atlantic Ocean and Eastern North America from SS Precursors

    NASA Astrophysics Data System (ADS)

    Schmerr, N. C.; Beghein, C.; Kostic, D.; Baldridge, A. M.; West, J. D.; Nittler, L. R.; Bull, A. L.; Montesi, L.; Byrne, P. K.; Hummer, D. R.; Plescia, J. B.; Elkins-Tanton, L. T.; Lekic, V.; Schmidt, B. E.; Elkins, L. J.; Cooper, C. M.; ten Kate, I. L.; Van Hinsbergen, D. J. J.; Parai, R.; Glass, J. B.; Ni, J.; Fuji, N.; McCubbin, F. M.; Michalski, J. R.; Zhao, C.; Arevalo, R. D., Jr.; Koelemeijer, P.; Courtier, A. M.; Dalton, H.; Waszek, L.; Bahamonde, J.; Schmerr, B.; Gilpin, N.; Rosenshein, E.; Mach, K.; Ostrach, L. R.; Caracas, R.; Craddock, R. A.; Moore-Driskell, M. M.; Du Frane, W. L.; Kellogg, L. H.

    2015-12-01

    Seismic discontinuities within the mantle arise from a wide range of mechanisms, including changes in mineralogy, major element composition, melt content, volatile abundance, anisotropy, or a combination of the above. In particular, the depth and sharpness of upper mantle discontinuities at 410 and 660 km depth are attributed to solid-state phase changes sensitive to both mantle temperature and composition, where regions of thermal heterogeneity produce topography and chemical heterogeneity changes the impedance contrast across the discontinuity. Seismic mapping of this topography and sharpness thus provides constraint on the thermal and compositional state of the mantle. The EarthScope USArray is providing unprecedented access to a wide variety of new regions previously undersampled by the SS precursors. This includes the boundary between the oceanic plate in the western Atlantic Ocean and continental margin of eastern North America. Here we use a seismic array approach to image the depth, sharpness, and topography of the upper mantle discontinuities, as well as other possible upper mantle reflectors beneath this region. This array approach utilizes seismic waves that reflect off the underside of a mantle discontinuity and arrive several hundred seconds prior to the SS seismic phase as precursory energy. In this study, we collected high-quality broadband data SS precursors data from shallow focus (< 30 km deep), mid-Atlantic ridge earthquakes recorded by USArray seismometers in Alaska. We generated 4th root vespagrams to enhance the SS precursors and determine how they sample the mantle. Our data show detection of localized structure on the discontinuity boundaries as well as additional horizons, such as the X-discontinuity and a potential reflection from a discontinuity near the depth of the lithosphere-asthenosphere boundary. These structures are related to the transition from predominantly old ocean lithosphere to underlying continental lithosphere, as while

  16. Search for deep slabs in the Northwest Pacific mantle.

    PubMed

    Zhou, H W; Anderson, D L

    1989-11-01

    A residual sphere is formed by projecting seismic ray travel-time anomalies, relative to a reference Earth model, onto an imaginary sphere around an earthquake. Any dominant slab-like fast band can be determined with spherical harmonic expansion. The technique is useful in detecting trends associated with high-velocity slabs beneath deep earthquakes after deep-mantle and near-receiver effects are removed. Two types of corrections are used. The first uses a tomographic global mantle model; the second uses teleseismic station averages of residuals from many events over a large area centered on the events of interest. Under the Mariana, Izu-Bonin, and Japan trenches, the dominant fast bands are generally consistent with seismicity trends. The results are unstable and differ from the seismicity trend for Kurile events. The predominant fast band for most deep earthquakes under Japan is subhorizontal rather than near vertical. We find little support for the deep slab penetration hypothesis.

  17. The Hales discontinuity and upper mantle anisotropy beneath cratons

    NASA Astrophysics Data System (ADS)

    Musacchio, G.; White, D. J.; Thomson, C. J.

    2003-04-01

    Seismic velocity discontinuities are commonly found within the upper 100 km of the mantle lithosphere, with great variability in their depth, lateral extent, and the polarity of velocity jump. Among the more commonly observed is the Hales discontinuity, identified in a variety of tectonic environments, and commonly associated with a high-velocity, highly reflective and sometimes anisotropic layer. In the Archean Western Superior Province of the Canadian Shield, long range R/WAR profiling gives a high-resolution estimate of the mantle V_P in the shallow upper mantle, providing a more certain determination of the nature of the Hales dicontinuity. Ray-based travel-time inversion of the data, have shown that Vp in the uppermost mantle is 8.0-8.3 km/s. A 15-20 km thick layer (layer-H) with >6% seismic anisotropy (N-S V_P of 8.3 km/s and E-W V_P of 8.8 km/s) dips northward at ˜10^o from a minimum depth of 48-50 km. The attitude of layer-H is consistent with the general tectonic strike; its depth range (50-75 km) falls within that of the Hales discontinuity. If a link between the Hales discontinuity and layer-H can be drawn, observations strengthen the objection that the estimated velocity contrast (0.2 to 0.4 km/s depending on the direction of wave propagation) is relatively high if layer-H represents a phase transition, and thus (re)opens the debate on the nature of shallow upper-mantle boundaries beneath continents. The high V_P and intermediate anisotropy of upper-mantle layer-H requires a harzburgite peridotitic composition with the a-axis of olivine aligned E-W. Layer-H might have emplaced during accretion (2.7 Ga Kenoran orogeny) of the North American proto-craton and be relic oceanic lithosphere. The Hales discontinuity might be an expression of continents accretion and map relic slabs in the shallow upper mantle.

  18. Large gem diamonds from metallic liquid in Earth's deep mantle.

    PubMed

    Smith, Evan M; Shirey, Steven B; Nestola, Fabrizio; Bullock, Emma S; Wang, Jianhua; Richardson, Stephen H; Wang, Wuyi

    2016-12-16

    The redox state of Earth's convecting mantle, masked by the lithospheric plates and basaltic magmatism of plate tectonics, is a key unknown in the evolutionary history of our planet. Here we report that large, exceptional gem diamonds like the Cullinan, Constellation, and Koh-i-Noor carry direct evidence of crystallization from a redox-sensitive metallic liquid phase in the deep mantle. These sublithospheric diamonds contain inclusions of solidified iron-nickel-carbon-sulfur melt, accompanied by a thin fluid layer of methane ± hydrogen, and sometimes majoritic garnet or former calcium silicate perovskite. The metal-dominated mineral assemblages and reduced volatiles in large gem diamonds indicate formation under metal-saturated conditions. We verify previous predictions that Earth has highly reducing deep mantle regions capable of precipitating a metallic iron phase that contains dissolved carbon and hydrogen.

  19. Upper mantle viscosity and dynamic subsidence of curved continental margins.

    PubMed

    Sacek, Victor; Ussami, Naomi

    2013-01-01

    Continental rifting does not always follow a straight line. Nevertheless, little attention has been given to the influence of rifting curvature in the evolution of extended margins. Here, using a three-dimensional model to simulate mantle dynamics, we demonstrate that the curvature of rifting along a margin also controls post-rift basin subsidence. Our results indicate that a concave-oceanward margin subsides faster than a convex margin does during the post-rift phase. This dynamic subsidence of curved margins is a result of lateral thermal conduction and mantle convection. Furthermore, the differential subsidence is strongly dependent on the viscosity structure. As a natural example, we analyse the post-rift stratigraphic evolution of the Santos Basin, southeastern Brazil. The differential dynamic subsidence of this margin is only possible if the viscosity of the upper mantle is >2-3 × 10(19) Pa s.

  20. Double Layering and Bilateral Asymmetry of a Thermochemical Plume in the Upper Mantle beneath Hawaii

    NASA Astrophysics Data System (ADS)

    Ito, G.; Ballmer, M. D.; Wolfe, C. J.; Solomon, S. C.

    2012-12-01

    Classical plume theory describes purely thermal upwellings that rise through the entire mantle, pond beneath the lithospheric plate in a thin "pancake," and generate hotspot volcanism. High-resolution seismic velocity images obtained from the Plume-Lithosphere Undersea Melt Experiment (PLUME) support the concept of a deep-rooted mantle plume beneath the Hawaiian hotspot. However, in detail these images challenge traditional concepts inasmuch as they indicate a broad low-velocity body in the upper mantle that is much thicker and more asymmetric than a thermal pancake predicted from purely thermal plume models. Geochemical observations also argue against a purely thermal (i.e., isochemical) mantle source for Hawaiian lavas and instead indicate a heterogeneous plume involving mafic lithologies such as eclogite. To explore the dynamical and melting behavior of hot plumes that also contain eclogite, we perform three-dimensional numerical simulations of thermochemical convection. The models simulate eclogite with an excess density relative to ambient-mantle peridotite that peaks at depths of 410-300 km due to solid phase changes and lessens at depths of 250-190 km where eclogite is removed by melting. For a plume core with an eclogite content >12%, a moderately buoyant plume stem rises into the upper mantle but pools as a much wider body at depths of 450-300 km (the "deep eclogite pool", or DEP). Out of the top of the DEP rises a shallow plume that feeds hotspot volcanism and supplies material into a thin sublithospheric pancake that supports the hotspot swell. Seismic resolution tests indicate that the double layering of hot plume material (DEP and shallow pancake) can account for the thick low-velocity body as imaged by PLUME. We also find that upwelling fingers of non-eclogitic outskirt material can support magmatism comparable in volume and geographic distribution to Hawaiian rejuvenated stage and arch volcanism. In some models, thermochemical plumes with radial and

  1. Metasomatic oxidation of upper mantle periodotite

    USGS Publications Warehouse

    McGuire, A.V.; Dyar, M.D.; Nielson, J.E.

    1991-01-01

    metasomatism may produce a greater change in the redox state of mantle peridotite than cryptic metasomatism. Comparison of the metasomatized samples with unmetasomatized peridotites reveals that both Fe2+ and Fe3+ cations p.f.u. were increased during metasomatism and 50% or more of iron added was Fe3+. With increasing distance from the dike, the ratio of added Fe3+ to added Fe2+ increases. The high Fe3+/FeT of amphibole and phlogopite in the dikes and in the peridotite, and the high ratios of added Fe3+/added Fe2+ in pyroxenes and spinel suggest that the Fe3+/FeT ratio of the metasomatic silicate fluid was high. As the fluid perolated through and reacted with the peridotite, Fe3+ and C-O-H volatile species were concentrated in the fluid, increasing the fluid Fe3+/FeT. ?? 1991 Springer-Verlag.

  2. Teleseismic array analysis of upper mantle compressional velocity structure. Ph.D. Thesis

    NASA Technical Reports Server (NTRS)

    Walck, M. C.

    1984-01-01

    Relative array analysis of upper mantle lateral velocity variations in southern California, analysis techniques for dense data profiles, the P-wave upper mantle structure beneath an active spreading center: the Gulf of California, and the upper mantle under the Cascade ranges: a comparison with the Gulf of California are presented.

  3. Constraining Upper Mantle Azimuthal Anisotropy With Free Oscillation Data (Invited)

    NASA Astrophysics Data System (ADS)

    Beghein, C.; Resovsky, J. S.; van der Hilst, R. D.

    2009-12-01

    We investigate the potential of Earth's free oscillations coupled modes as a tool to constrain large-scale seismic anisotropy in the transition zone and in the bulk of the lower mantle. While the presence of seismic anisotropy is widely documented in the uppermost and the lowermost mantle, its observation at intermediate depths remains a formidable challenge. We show that several coupled modes of oscillations are sensitive to radial and azimuthal anisotropy throughout the mantle. In particular, modes of the type 0Sl-0T(l+1) have high sensitivity to shear-wave radial anisotropy and to six elastic parameters describing azimuthal anisotropy in the 200 km-1000 km depth range. The use of such data enables us thus to extend the sensitivity of traditionally used fundamental mode surface waves to depths corresponding to the transition zone and the top of the lower mantle. In addition, these modes have the potential to provide new and unique constraints on several elastic parameters to which surface waves are not sensitive. We attempted to fit degree two splitting measurements of 0Sl-0T(l+1) coupled modes using previously published isotropic and transversely isotropic mantle models, but we could not explain the entire signal. We then explored the model space with a forward modeling approach and determined that, after correction for the effect of the crust and mantle radial anisotropy, the remaining signal can be explained by the presence of azimuthal anisotropy in the upper mantle. When we allow the azimuthal anisotropy to go below 400 km depth, the data fit is slightly better and the model space search leads to better-resolved model than when we force the anisotropy to lie in the top 400 km of the mantle. Its depth extent and distribution are, however, still not well constrained by the data due to parameter tradeoffs and a limited coupled mode data set. It is thus clear that mode coupling measurements have the potential to constrain upper-mantle azimuthal anisotropy

  4. Inherited Upper Mantle Structures in the Southeastern United States

    NASA Astrophysics Data System (ADS)

    Wagner, L. S.; Fischer, K. M.; Hawman, R. B.; Biryol, C. B.

    2014-12-01

    The southeastern United States has experienced repeated cycles of terrane accretion, continental suturing, and extensive rifting over the past 1 Ga. Recent data collected from the EarthScope Transportable Array and the EarthScope FlexArray deployment SESAME (South Eastern Suture of the Appalachian Margin Experiment) allow us to image in detail the upper mantle structures associated with these tectonic events using both Rayleigh waves and teleseismic body waves. Of particular interest is the mantle expression of the Suwannee suture. Located across southern Georgia and Alabama, the Suwannee suture represents the boundary between the Gondwanan Suwannee terrane and the Laurentian margin. Previous authors have speculated that this accretion occurred due to southward dipping subduction of the Laurentian lithosphere, though more recent work suggests a possible dextral transpressional plate boundary. We find that the mantle lithosphere across the suture may dip slightly to the south, but the dip angle is much shallower than that of crustal structures seen in scattered wave and active source images. Ongoing work will help to constrain the precise dip angle and will also investigate the nature of lower velocities located between the mantle lithosphere and the overriding crust. Intriguingly, we do image a distinct dipping high velocity feature further to the west. This slab-like structure dips to the east, and has a surface projection located near the 1 Ga Grenville front. While the provenance of this high velocity anomaly is still unclear, a few intriguing observations can be made. Given the location of this anomaly in a tectonically stable area, it is unlikely to be due to an ongoing lithospheric process. This then raises the question of how such a dipping structure can persist over time despite over-riding plate motions and mantle flow patterns. The increase in shear velocities is unlikely due to any remnant thermal effects. Our ability to image this structure at all

  5. The European Continent : Surface Expression of Upper Mantle Dynamics

    NASA Astrophysics Data System (ADS)

    Tondi, M. R.; Schivardi, R.; Molinari, I.; Morelli, A.

    2012-12-01

    images of the European upper mantle isotropic shear-wave speeds and mass densities, recently recovered by combined inversion of surface-wave information and GRACE satellite gravity data (Tondi et al., 2012) are used to select the regions where the residual topography and the residual mantle gravity anomalies are strongly correlated (correlation coefficient is equal to 1). We assume surface uplift processes with negative density anomalies and downward pull with positive anomalies. Our work shows a strong correlation among the areas where, on the basis of our assumptions, the mantle dynamics have surface expression and the areas of low values of radial anisotropy: (1) the southern margins of the East European Craton, (2) the North-Eastern edges of the Arabian Plateau, (3) the northern edge of the CEVP (Central European Volcanic Province), (4) the North-Eastern part of the Atlantic Ocean, between Greenland and Iceland.

  6. Magnesium stable isotope composition of Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Handler, Monica R.; Baker, Joel A.; Schiller, Martin; Bennett, Vickie C.; Yaxley, Gregory M.

    2009-05-01

    The mantle is Earth's largest reservoir of Mg containing > 99% of Earth's Mg inventory. However, no consensus exists on the stable Mg isotope composition of the Earth's mantle or how variable it is and, in particular, whether the mantle has the same stable Mg isotope composition as chondrite meteorites. We have determined the Mg isotope composition of olivine from 22 mantle peridotites from eastern Australia, west Antarctica, Jordan, Yemen and southwest Greenland by pseudo-high-resolution MC-ICP-MS on Mg purified to > 99%. The samples include fertile lherzolites, depleted harzburgites and dunites, cryptically metasomatised ('dry') peridotites and modally metasomatised apatite ± amphibole-bearing harzburgites and wehrlites. Olivine from these samples of early Archaean through to Permian lithospheric mantle have δ25Mg DSM-3 = - 0.22 to - 0.08‰. These data indicate the bulk upper mantle as represented by peridotite olivine is homogeneous within current analytical uncertainties (external reproducibility ≤ ± 0.07‰ [2 sd]). We find no systematic δ25Mg variations with location, lithospheric age, peridotite fertility, or degree or nature of mantle metasomatism. Although pyroxene may have slightly heavier δ25Mg than coexisting olivine, any fractionation between mantle pyroxene and olivine is also within current analytical uncertainties with a mean Δ25Mg pyr-ol = +0.06 ± 0.10‰ (2 sd; n = 5). Our average mantle olivine δ25Mg DSM-3 = - 0.14 ± 0.07‰ and δ26Mg DSM-3 = - 0.27 ± 0.14‰ (2 sd) are indistinguishable from the average of data previously reported for terrestrial basalts, confirming that basalts have stable Mg isotope compositions representative of the mantle. Olivine from five pallasite meteorites have δ25Mg DSM-3 = - 0.16 to - 0.11‰ that are identical to terrestrial olivine and indistinguishable from the average δ25Mg previously reported for chondrites. These data provide no evidence for measurable heterogeneity in the stable Mg isotope

  7. Thermo-chemical plumes rooted in the deep mantle beneath major hotspots: implications for mantle dynamics

    NASA Astrophysics Data System (ADS)

    Romanowicz, B. A.; French, S.

    2015-12-01

    The existence of mantle plumes as a possible origin for hotspots has been the subject of debate for the last 30 years. Many seismic tomographic studies have hinted at the presence of plume-like features in the lower mantle, but resolution of narrow low velocity features is difficult, and ambiguity remains as to the vertical continuity of these features and how distinct they are from other low velocity blobs. We present robust evidence for significant, vertically continuous, low velocity columns in the lower mantle beneath prominent hotspots located within the footprint of the large low shear velocity provinces (LLSVPs), from a recent global, radially anisotropic whole mantle shear-wave velocity (Vs) model, SEMUCB-WM1 (French and Romanowicz, 2014, 2015). This model was constructed by inversion of a large dataset of long period three-component seismograms down to 32s period. Because it includes surface-wave overtones, S-diffracted waves and multiply reflected waves between the surface and the CMB, this dataset provides considerably better illumination of the whole mantle volume than can be obtained with a standard set of travel times alone. In addition, accurate numerical computation of the forward wavefield using the spectral element method at each iteration of the model construction, allows us to better resolve regions of lower than average Vs. The imaged plumes have several common characteristics: they are rooted in patches of very low Vs near the core mantle boundary, some of which contain documented ULVZs, and extend vertically through the lower mantle up to ~1000 km depth, where some are deflected horizontally, or give rise to somewhat thinner conduits that meander through the upper mantle in the vicinity of the target hotpots. Combined with evidence for slab stagnation at ~1000 km depth, this suggests a change in rheology between 660 and 1000 km depth, very high viscosity throughout the bulk of the lower mantle, and lower viscosity plumes, only mildly

  8. Seismic evidence for widespread serpentinized forearc upper mantle along the Cascadia margin

    USGS Publications Warehouse

    Brocher, T.M.; Parsons, T.; Trehu, A.M.; Snelson, C.M.; Fisher, M.A.

    2003-01-01

    Petrologic models suggest that dehydration and metamorphism of subducting slabs release water that serpentinizes the overlying forearc mantle. To test these models, we use the results of controlled-source seismic surveys and earthquake tomography to map the upper mantle along the Cascadia margin forearc. We find anomalously low upper-mantle velocities and/or weak wide-angle reflections from the top of the upper mantle in a narrow region along the margin, compatible with recent teleseismic studies and indicative of a serpentinized upper mantle. The existence of a hydrated forearc upper-mantle wedge in Cascadia has important geological and geophysical implications. For example, shearing within the upper mantle, inferred from seismic reflectivity and consistent with its serpentinite rheology, may occur during aseismic slow slip events on the megathrust. In addition, progressive dehydration of the hydrated mantle wedge south of the Mendocino triple junction may enhance the effects of a slap gap during the evolution of the California margin.

  9. Seismic anisotropy of South African upper mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Long, C.; Christensen, N. I.

    2000-07-01

    Seismic properties of six upper mantle xenoliths from South Africa were determined both by laboratory measurements at elevated pressure and numerical calculations from petrofabric analyses, in order to provide constraints on the structural interpretation of the field seismic data obtained in recent years. The samples studied include harzburgite, garnet harzburgite, mica harzburgite, lherzolite and garnet lherzolite. Average anisotropies calculated from petrofabrics are 5.4% for Vp and 4.4% for Vs. Ultrasonic measured anisotropies from xenolith rock cores are approximately 1% lower than calculated values. This difference is likely due to simplifications in modal mineralogy used in the calculations and small amounts of alteration in the samples used in the velocity measurements. Comparison of laboratory data with the SKS and Pn data yields possible structural orientations in the upper mantle beneath the South Africa continent. Our results suggest that foliation in the upper mantle is likely horizontal, and the stretching lineation (olivine a-axis) is oriented N30°E, which coincides with the absolute plate motion (APM) of the South African continent. It is apparent from our data that the fast S-wave polarization depends on the propagation direction with respect to petrofabric orientations and does not always indicate the stretching lineation.

  10. Investigation of Eurasian seismic sources and upper mantle structure

    NASA Astrophysics Data System (ADS)

    Jordon, T. H.

    1985-06-01

    Complex P-SV wavegroups are sucessfully fit on vertical component seismograms for paths crossing northern Eurasia with a model having a 39 km crust and no asthenospheric low-velocity zone. Between the Moho and the 400 km discontinuity, the shear velocities found from the P-SV waveform analysis are consistently lower than those inferred from the SH waveform modeling of Grand and Helmberger. We suggest that this discrepancy is diagnostic of a polarization anisotropy associated with the olivine rich mineralogy of the thick, basalt depleted chemical boundary layer that characterizes the upper mantle beneath stable continents. The vertical resolving power of source and receiver array are investigated in determining Eurasian crustal and upper mantle structure using waveform inversion technique of Lerner-Lam and Jordan (1983). The results show that good resolution of upper mantle structure can be obtained with this method from sparse arrays of sources and/or receivers. Source arrays are particularly effective in enhancing resolution, provided that the source depths are well distributed and the source centroids and moment tensors are well determined, while receiver arrays contribute to the reduction of the variance of the model estimates.

  11. Superweak asthenosphere in light of upper mantle seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Becker, Thorsten W.

    2017-05-01

    Earth's upper mantle includes a ˜200 km thick asthenosphere underneath the plates where viscosity and seismic velocities are reduced compared to the background. This zone of weakness matters for plate dynamics and may be required for the generation of plate tectonics itself. However, recent seismological and electromagnetic studies indicate strong heterogeneity in thinner layers underneath the plates which, if related to more extreme, global viscosity reductions, may require a revision of our understanding of mantle convection. Here, I use dynamically consistent mantle flow modeling and the constraints provided by azimuthal seismic anisotropy as well as plate motions to explore the effect of a range of global and local viscosity reductions. The fit between mantle flow model predictions and observations of seismic anisotropy is highly sensitive to radial and lateral viscosity variations. I show that moderate suboceanic viscosity reductions, to ˜0.01-0.1 times the upper mantle viscosity, are preferred by the fit to anisotropy and global plate motions, depending on layer thickness. Lower viscosities degrade the fit to azimuthal anisotropy. Localized patches of viscosity reduction, or layers of subducted asthenosphere, however, have only limited additional effects on anisotropy or plate velocities. This indicates that it is unlikely that regional observations of subplate anomalies are both continuous and indicative of dramatic viscosity reduction. Locally, such weak patches may exist and would be detectable by regional anisotropy analysis, for example. However, large-scale plate dynamics are most likely governed by broad continent-ocean asthenospheric viscosity contrasts rather than a thin, possibly high melt fraction layer.

  12. Upper-Mantle Flow Driven Dynamic Topography in Eastern Anatolia

    NASA Astrophysics Data System (ADS)

    Sengul Uluocak, Ebru; Pysklywec, Russell; Eken, Tuna; Hakan Gogus, Oguz

    2016-04-01

    Eastern Anatolia is characterized by 2 km plateau uplift -in the last 10 Myrs-, high surface heat flow distribution, shallow Curie-point depth, anomalous gravity field. Seismological observations indicate relatively high Pn and Sn attenuation and significant low seismic velocity anomalies in the region. Moreover, the surface geology is associated predominantly with volcanic rocks in which melt production through mantle upwelling (following lithospheric delamination) has been suggested. It has been long known that the topographic loading in the region cannot be supported by crustal thickness (~45 km) based on the principle of Airy isostasy. Recent global geodynamic studies carried out for evaluating the post-collisional processes imply that there is an explicit dynamic uplift in Eastern Anatolia and its adjacent regions. In this study we investigate the instantaneous dynamic topography driven by 3-D upper-mantle flow in Eastern Anatolia. For this purpose we conducted numerous thermo-mechanical models using a 2-D Arbitrary Lagrangian Eulerian (ALE) finite element method. The available P-wave tomography data extracted along 10 profiles were used to obtain depth-dependent density anomalies in the region. We present resulting dynamic topography maps and estimated 3D mantle flow velocity vectors along these 2-D cross sections for each profile. The residual topography based on crustal thickness and observed topography was calculated and compared with other independent datasets concerning geological deformation and dynamic topography predictions. The results indicate an upper mantle driven dynamic uplift correlated with the under-compensated characteristic in Eastern Anatolia. We discuss our results combined with 3D mantle flow by considering seismic anisotropy studies in the region. Initial results indicate that high dynamic uplift and the localized low Pn velocities in concurrence with Pn anisotropy structures show nearly spatial coherence in Eastern Anatolia.

  13. Upper-mantle reflectors: modelling of seismic wavefield characteristics and tectonic implications

    NASA Astrophysics Data System (ADS)

    Hansen, T. M.; Balling, N.

    2004-05-01

    Deep seismic experiments on continental lithosphere generally reveal marked reflectivity from structures in the crust and a significant decrease in reflectivity from the upper mantle. However, reflected and refracted energy from mantle lithosphere are observed in both near-normal incidence and wide-angle data. The origin of the reflective structures is a matter of debate. Hypotheses include remnant subduction zones, shear zones, fluids and seismic anisotropy. Through analytical and numerical modelling studies, including full wavefield modelling, we investigate seismic characteristic signatures generated from a variety of geologically plausible models. We have found that both upper-mantle shear zones of reduced velocity and density and remnant subduction slabs containing high-density eclogites may contain sufficient seismic impedance contrasts to normal mantle peridotites to generate near-normal incidence reflectivity. Wide-angle energy originates from subduction slabs containing either high- or low-velocity eclogites, whereas intermediate-velocity eclogites are unlikely to produce significant wide-angle phases. In general, energy of seismic phases originating from upper-mantle zones of anomalous seismic velocities and densities is significantly increased if homogeneous zones are replaced by zones of inhomogeneous petrophysical properties resulting from constructive interference. Maximum wavefield anomalies are generated from sub Moho dipping slabs of incomplete transformation of low-velocity/low-density crustal material to high-velocity/high-density eclogites. Localized shear zones generated in mantle peridotite generally do not produce significant wide-angle energy. Only if highly inhomogeneous structures containing material of marked (ca 10 per cent) velocity and density reduction are present, may shear zones be observed in wide-angle data. Analyses of two specific deep-seismic data sets (MONA LISA data) from the North Sea and (BABEL data) from the Baltic Sea

  14. Deep-focus earthquakes and recycling of water into the earth's mantle

    NASA Technical Reports Server (NTRS)

    Meade, Charles; Jeanloz, Raymond

    1991-01-01

    For more than 50 years, observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic: at these depths, rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces. Laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep-focus seismicity could originate from dehydration and high-pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle. Thus, seismologists may be mapping the recirculation of water from the oceans back into the deep interior of the planet.

  15. Deep-focus earthquakes and recycling of water into the earth's mantle

    NASA Technical Reports Server (NTRS)

    Meade, Charles; Jeanloz, Raymond

    1991-01-01

    For more than 50 years, observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic: at these depths, rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces. Laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep-focus seismicity could originate from dehydration and high-pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle. Thus, seismologists may be mapping the recirculation of water from the oceans back into the deep interior of the planet.

  16. A Global Upper-Mantle Tomographic Model of Shear Attenuation

    NASA Astrophysics Data System (ADS)

    Karaoglu, H.; Romanowicz, B. A.

    2016-12-01

    Mapping anelastic 3D structure within the earth's mantle is key to understanding present day mantle dynamics, as it provides complementary constraints to those obtained from elastic structure, with the potential to distinguish between thermal and compositional heterogeneity. For this, we need to measure seismic wave amplitudes, which are sensitive to both elastic (through focusing and scattering) and anelastic structure. The elastic effects are less pronounced at long periods, so previous global upper-mantle attenuation models are based on teleseismic surface wave data, sometimes including overtones. In these studies, elastic effects are considered either indirectly, by eliminating data strongly contaminated by them (e.g. Romanowicz, 1995; Gung and Romanowicz, 2004), or by correcting for elastic focusing effects using an approximate linear approach (Dalton et al., 2008). Additionally, in these studies, the elastic structure is held fixed when inverting for intrinsic attenuation . The importance of (1) having a good starting elastic model, (2) accurate modeling of the seismic wavefield and (3) joint inversion for elastic and anelastic structure, becomes more evident as the targeted resolution level increases. Also, velocity dispersion effects due to anelasticity need to be taken into account. Here, we employ a hybrid full waveform inversion method, inverting jointly for global elastic and anelastic upper mantle structure, starting from the latest global 3D shear velocity model built by our group (French and Romanowicz, 2014), using the spectral element method for the forward waveform modeling (Capdeville et al., 2003), and normal-mode perturbation theory (NACT - Li and Romanowicz, 1995) for kernel computations. We present a 3D upper-mantle anelastic model built by using three component fundamental and overtone surface waveforms down to 60 s as well as long period body waveforms down to 30 s. We also include source and site effects to first order as frequency

  17. Constraints on the heterogeneity spectrum of Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Mancinelli, Nicholas; Shearer, Peter; Liu, Qinya

    2016-05-01

    We constrain the heterogeneity spectrum of Earth's upper mantle at scales from a few kilometers to tens of thousands of kilometers using observations from high-frequency scattering, long-period scattering, and tomography. Tomography and high-frequency scattering constraints are drawn from previous studies, but constraints on mantle heterogeneity at intermediate scales (5-500 km) are lacking. To address this, we stack ˜15,000 long-period P coda envelopes to characterize the globally averaged scattered wavefield at periods from 5 to 60 s and at ranges from 50 to 98°. To fit these observations, we consider models of random mantle heterogeneity and compute the corresponding global wavefield using both a ray theoretical "seismic particle" approach and full spectral element simulations. Von Kármán random media distributed throughout the uppermost 600 km of the mantle with a = 2000 km, ɛ = 10%, and κ = 0.05 provide a good fit to the time, range, and frequency dependence of the stacks, although there is a trade-off between ɛ and the thickness of the assumed scattering layer. This random media model also fits previously published 1 Hz stacks of P coda and agrees with constraints on long-wavelength structure from tomography. Finally, we explore geodynamically plausible scenarios that might be responsible for the RMS and falloff rate of the proposed spectrum, including a self-similar mixture of basalt and harzburgite.

  18. Upper mantle flow and lithospheric dynamics beneath the Eurasian region

    NASA Astrophysics Data System (ADS)

    Zhang, G.; Jiang, G.; Jia, Z.; Gao, R.; Fu, R.

    2010-12-01

    Evidence from seismic tomography, geothermal and short wavelength geoid anomalies reveals the existence of small-scale convective systems in the upper mantle, with scales ranging from 500 km to 700 km. It is reasonable to suggest that these small-scale convective systems probably control the regional tectonic structure and the dynamical processes of the lithosphere. Here we have calculated the patterns of small-scale convection in the upper mantle for the Eurasian region (20°E~170°E,15°N~75°N), using the anomaly of isostatic gravity. The results show that the regional lithospheric tectonics is strongly correlated with the upper mantle flow in the Eurasian region. Two intensive convective belts against the weak background convection can be recognized from convection patterns in this region: Alpine-Himalayan collision belt and West Pacific island arc-underthrust belt. Alpine-Himalayan belt is caused by the collision between the northern plate (Eurasian plate) and the southern plates (African plate and Indian plate). West Pacific island arc-underthrust belt is caused by the subduction of the Pacific plate beneath the Eurasian plate. Both of them are also seismotectonic belts. The collision and the subduction are two important geological events occurred since Mesozoic era and Cenozoic era in the Eurasian region. Therefore, the mantle flows may be one of the main driving forces of two events. In addition, most plate boundaries in this region can be recognized and the characteristics of upper mantle convection are different completely between the Eurasian plate and the plates around it (African plate, Arabian plate, Indian plate, Philippine Sea plate and Pacific plate). Main structures and geodynamic characteristics of the Eurasian can also be explained by our model results. The Tibet plateau is located in the intensive convective belt. Around the belt, the upwelling materials push the lithosphere to lift unitarily and form the plateau. Towards the north of the Tibet

  19. Plume's buoyancy and heat fluxes from the deep mantle estimated by an instantaneous mantle flow simulation based on the S40RTS global seismic tomography model

    NASA Astrophysics Data System (ADS)

    Yoshida, Masaki

    2012-11-01

    It is still an open question as to how much heat is transported from the deep mantle to the upper mantle by mantle upwelling plumes, which would impose a strong constraint on models of the thermal evolution of the earth. Here I perform numerical computations of instantaneous mantle flow based on a recent highly resolved global seismic tomography model (S40RTS), apply new simple fluid dynamics theories to the plume's radius and velocity, considering a Poiseuille flow assumption and a power-law relationship between the boundary layer thickness and Rayleigh number, and estimate the plume's buoyancy and heat fluxes from the deep lower mantle under varying plume viscosity. The results show that for some major mantle upwelling plumes with localized strong ascent velocity under the South Pacific and Africa, the buoyancy fluxes of each plume beneath the ringwoodite to perovskite + magnesiowüstite ("660-km") phase decomposition boundary are comparable to those inferred from observed hotspot swell volumes on the earth, i.e., on the order of 1 Mg s-1, when the plume viscosity is 1019-1020 Pa s. This result, together with previous numerical simulations of mantle convection and the gentle Clausius-Clapeyron slope for the 660-km phase decomposition derived from recent high-pressure measurements under dehydrated/hydrated conditions in the mantle transition zone, implies that mantle upwelling plumes in the lower mantle penetrate the 660-km phase decomposition boundary without significant loss in thermal buoyancy because of the weak thermal barrier at the 660-km boundary. The total plume heat flux under the South Pacific is estimated to be about 1 TW beneath the 660-km boundary, which is significantly smaller than the core-mantle boundary heat flux. Previously published scaling laws for the plume's radius and velocity based on a plume spacing theory, which explains well plume dynamics in three-dimensional time-dependent mantle convection, suggest that these plume fluxes depend

  20. Differential motion between upper crust and lithospheric mantle in the central Basin and Range

    NASA Astrophysics Data System (ADS)

    Schulte-Pelkum, Vera; Biasi, Glenn; Sheehan, Anne; Jones, Craig

    2011-09-01

    Stretching of the continental crust in the Basin and Range, western USA, has more than doubled the surface area of the central province. But it is unknown whether stretching affects the entire column of lithosphere down to the convecting mantle, if deep extension occurs offset to the side, or if deeper layers are entirely decoupled from the upper crust. The central Basin and Range province is unusual, compared with its northern and southern counterparts: extension began later; volcanism was far less voluminous; and the unique geochemistry of erupted basalts suggests a long-preserved mantle source. Here we use seismic data and isostatic calculations to map lithospheric thickness in the central Basin and Range. We identify an isolated root of ancient mantle lithosphere that is ~125km thick, providing geophysical confirmation of a strong, cold mantle previously inferred from geochemistry. We suggest that the root caused the later onset of extension and prevented the eruption of voluminous volcanism at the surface. We infer that the root initially pulled away from the Colorado Plateau along with the crust, but then was left behind intact during extension across Death Valley to the Sierra Nevada. We conclude that the upper crust is now decoupled from and moving relative to the root.

  1. The mantle transition zone and the upper mantle in Central-Eastern Greenland

    NASA Astrophysics Data System (ADS)

    Anja Kraft, Helene; Thybo, Hans; Vinnik, Lev

    2016-04-01

    We present a receiver function (RF) study of the mantle transition zone (MTZ) and upper mantle in central-eastern Greenland. Our results are based on data from 18 temporary broad-band seismometers and 5 additional stations from the GLISN and GLATIS networks. The stations were operating in the region between Scoresby Sund and Summit (~ 70 ° N) with half of them installed on ice, the other half on bedrock. For our analysis we calculated low frequency PRF and SRF, which use the difference in travel times between converted and not converted phases at discontinuities. We see clear signals from P410s and P660s in most of our PRF and from S410p in the SRF. Their delay times suggest a surprisingly thin MTZ for most parts of the study area with up to 25 km of thinning compared to standard Earth models. The only exception is a small region in the centre of the study area, which shows times close to standard. It is mainly the delay time for P410s, that varies, while P660s is stable throughout our study area. This indicates, that the thinning of the MTZ is mainly due to topography on the 410-discontinuity. We furthermore observe an M-shaped signal for P410s at stations in the western part around Summit. A similar, complicated signal has been observed previously in different settings and is interpreted as a thin low velocity layer between 410 km and 520 km. In addition we jointly inverted the PRF and SRF for upper mantle velocities. These results show velocities slower than IASP91 for the entire study area. Both the low velocities in the upper mantle and the thinning of the MTZ are in contrary to simple models of old continental shields and might indicate a fairly recent heating event.

  2. Grain-Boundary Plasticity and the Strength of the Upper Mantle

    NASA Astrophysics Data System (ADS)

    Raterron, P.; Bollinger, C.; Hilairet, N.; Merkel, S.

    2014-12-01

    The rheology of the Earth's upper mantle is controlled by the plasticity of olivine-rich rocks. Despite considerable efforts to better understand their plasticity, experimental flow laws fail to explain important geodetic and geophysical observations, such as fast post-seismic surface displacements or the "elusiveness" of the lithosphere-asthenosphere boundary beneath cratons. The plasticity of olivine single crystals is well understood and was experimentally quantified to mantle pressures and temperatures (e.g., Bai et al., 1991, JGR, 96, 2441-2463; Raterron et al., 2012, PEPI, 200-201, 105-112). The plasticity of aggregates, however, involves additional mechanisms, and the fundamental question of the amount of strain accommodation at grain boundaries remain unanswered. In this study, we compare the plasticity of olivine aggregates deformed experimentally at mantle conditions (Durham et al., 2009, PEPI, 172, 67-73; Hilairet et al., 2012, JGR, 117, B01203 ; Bollinger et al., 2013, PEPI, 228, 211-219) to that of single crystals and demonstrate that strain at grain boundaries can be orders of magnitude larger than intracrystalline strain. We further show that the proportion of grain-boundary strain decreases with increasing temperature and stress. Applied along mantle geotherms, our results shows that grain boundary plasticity is dominant in the shallow mantle. The strength of olivine in the lithosphere can be more than one order of magnitude lower than predicted by classical flow laws (Hirth and Kohlstedt, 2003, AGU Geophys. Monogr, 138, 83-105). In the deep upper mantle, grain boundary plasticity vanishes and strain is mostly accommodated within the grains.

  3. The African upper mantle: the view from surface waves

    NASA Astrophysics Data System (ADS)

    Fishwick, Stewart

    2014-05-01

    Given the sparse distribution of seismic stations across significant regions of the African continent surface wave tomography is the ideal seismological technique to give a clear picture of the large scale structure for the upper mantle. An updated tomographic model is presented, and reviewed in comparison with other tomography for the region. Cratonic regions are clearly outlined with fast velocities extending to depths of >175km. Areas of slow shear velocity, at depths of 100-150km, show good correlation with long wavelength gravity highs and areas of uplifted topography. The numerous temporary deployments of seismometers along the East African rift system provide strong constraints on the structure in this region. Importantly, many of the seismological features are now converging in a range of tomographic models, adding to the confidence in interpretations. However, significant challenges remain, both for seismology and for the interpretations of the results. Pushing towards smaller features and higher resolution to understand geological problems is still difficult. For example, the mantle structure imaged beneath the Bushveld Complex remains very variable depending on the technique used. Lithospheric thickness can be estimated using a variety of proxies - comparisons of this are shown for southern Africa. But, are the seismic models actually compatible with a mineral physics view of the lithosphere? From a geodynamic perspective, how do localised regions of low velocity in the upper mantle relate to the larger patterns of whole mantle circulation? While seismic imaging is providing an increasingly clear picture of the present velocity structure more integration is still needed to answer many of the questions related to the African continent.

  4. Experimentally determined water storage capacity in the Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Ferot, A.; Bolfan-Casanova, N.

    2010-12-01

    Trace amounts of hydrogen dissolved as defects in nominally anhydrous minerals (NAMs) in the mantle are believed to play a key role in physical and chemical processes in the Earth’s upper mantle. Hence, the estimation of water storage in mantle phases and solubility mechanisms are important in order to better understand the effect of water. Experimental data on water solubility in NAMs are available for upper mantle minerals such as olivine, pyroxenes and garnet. However, the majority of studies are based on the study of single phases, and at temperatures or pressures that are too low for the Earth’s upper mantle. The aim of this study is to constrain the combined effects of pressure, temperature and composition on water solubility in olivine and orthopyroxene under upper mantle conditions. The solubility of water in coexisting orthopyroxene and olivine was investigated by simultaneously synthesizing the two phases at high pressure and high temperature in a multi-anvil press. Experiments were performed under water-saturated conditions in the MSH systems with Fe and Al at 2.5, 5, 7.5 and 9 GPa and temperatures between 1175 and 1400°C. Integrated OH absorbances were determined using polarized infrared spectroscopy on doubly polished thin sections of randomly oriented crystals. Water solubility in olivine increases with pressure and decreases with temperature as has been described previously (Bali et al., 2008). The aluminum content strongly decreases in olivine with pressure from 0.09 wt% at 2.5 GPa and 1250°C to 0.04 wt% at 9 GPa and 1175°C. The incorporation of this trivalent cation in the system enhances water solubility in olivine even if present in trace amounts, however this behavior appears to reverse at high pressure. The effect of temperature on water solubility follows a bell-shaped curve with a maximum solubility in olivine and orthopyroxene at 1250°C. Aluminum is incorporated in orthopyroxene following the Tschermak substitution and strongly

  5. Tomography images of the Alpine roots and surrounding upper mantle

    NASA Astrophysics Data System (ADS)

    Plomerova, Jaroslava; Babuska, Vladislav

    2017-04-01

    Teleseismic body-wave tomography represents powerful tool to study regional velocity structure of the upper mantle and to image velocity anomalies, such as subducted lithosphere plates in collisional zones. In this contribution, we recapitulate 3D models of the upper mantle beneath the Alps, which developed at a collision zone of the Eurasian and African plates. Seismic tomography studies indicate a leading role of the rigid mantle lithosphere that functioned as a major stress guide during the plate collisions. Interactions of the European lithosphere with several micro-plates in the south resulted in an arcuate shape of this mountain range on the surface and in a complicated geometry of the Alpine subductions in the mantle. Early models with one bended lithosphere root have been replaced with more advanced models showing two separate lithosphere roots beneath the Western and Eastern Alps (Babuska et al., Tectonophysics 1990; Lippitsch et al., JGR 2003). The standard isotropic velocity tomography, based on pre-AlpArray data (the currently performed passive seismic experiment in the Alps and surroundings) images the south-eastward dipping curved slab of the Eurasian lithosphere in the Western Alps. On the contrary, beneath the Eastern Alps the results indicate a very steep northward dipping root that resulted from the collision of the European plate with the Adriatic microplate. Dando et al. (2011) interpret high-velocity heterogeneities at the bottom of their regional tomographic model as a graveyard of old subducted lithospheres. High density of stations, large amount of rays and dense ray-coverage of the volume studied are not the only essential pre-requisites for reliable tomography results. A compromise between the amount of pre-processed data and the high-quality of the tomography input (travel-time residuals) is of the high importance as well. For the first time, the existence of two separate roots beneath the Alps has been revealed from carefully pre

  6. Seismic tomography shows that upwelling beneath Iceland is confined to the upper mantle

    USGS Publications Warehouse

    Foulger, G.R.; Pritchard, M.J.; Julian, B.R.; Evans, J.R.; Allen, R.M.; Nolet, G.; Morgan, W.J.; Bergsson, B.H.; Erlendsson, P.; Jakobsdottir, S.; Ragnarsson, S.; Stefansson, R.; Vogfjord, K.

    2001-01-01

    range ??? 100-300 km beneath east-central Iceland. The anomalous body is approximately cylindrical in the top 250 km but tabular in shape at greater depth, elongated north-south and generally underlying the spreading plate boundary. Such a morphological change and its relationship to surface rift zones are predicted to occur in convective upwellings driven by basal heating, passive upwelling in response to plate separation and lateral temperature gradients. Although we cannot resolve structure deeper than ??? 450 km, and do not detect a bottom to the anomaly, these models suggest that it extends no deeper than the mantle transition zone. Such models thus suggest a shallow origin for the Iceland hotspot rather than a deep mantle plume, and imply that the hotspot has been located on the spreading ridge in the centre of the north Atlantic for its entire history, and is not fixed relative to other Atlantic hotspots. The results are consistent with recent, regional full-thickness mantle tomography and whole-mantle tomography images that show a strong, low-wave-speed anomaly beneath the Iceland region that is confined to the upper mantle and thus do not require a plume in the lower mantle. Seismic and geochemical observations that are interpreted as indicating a lower mantle, or core-mantle boundary origin for the North Atlantic Igneous Province and the Iceland hotspot should be re-examined to consider whether they are consistent with upper mantle processes.

  7. Komatiites reveal a hydrous Archaean deep-mantle reservoir

    NASA Astrophysics Data System (ADS)

    Sobolev, Alexander V.; Asafov, Evgeny V.; Gurenko, Andrey A.; Arndt, Nicholas T.; Batanova, Valentina G.; Portnyagin, Maxim V.; Garbe-Schönberg, Dieter; Krasheninnikov, Stepan P.

    2016-03-01

    Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth’s mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth’s history.

  8. Komatiites reveal a hydrous Archaean deep-mantle reservoir.

    PubMed

    Sobolev, Alexander V; Asafov, Evgeny V; Gurenko, Andrey A; Arndt, Nicholas T; Batanova, Valentina G; Portnyagin, Maxim V; Garbe-Schönberg, Dieter; Krasheninnikov, Stepan P

    2016-03-31

    Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth's mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth's history.

  9. Ultradeep (greater than 300 kilometers), ultramafic upper mantle xenoliths.

    PubMed

    Haggerty, S E; Sautter, V

    1990-05-25

    Geophysical discontinuities in Earth's upper mantle and experimental data predict the structural transformation of pyroxene to garnet and the solid-state dissolution of pyroxene into garnet with increasing depth. These predictions are indirectly verified by omphacitic pyroxene exsolution in pyropic garnet-bearing xenoliths from a diamondiferous kimberlite. Conditions for silicon in octahedral sites in the original garnets are met at pressures greater than 130 kilobars, placing the origin of these xenoliths at depths of 300 to 400 kilometers. These ultradeep xenoliths support the theory that the 400-km seismic discontinuity is marked by a transition from peridotite to eclogite.

  10. Seismic velocity, attenuation and rheology of the upper mantle

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.; Minster, J. B.

    1980-01-01

    Seismic and rheological properties of the upper mantle in the vicinity of the low-velocity zone are expressed in terms of relaxation by dislocation glide. Dislocation bowing in the glide plane explains seismic velocities and attenuation. Climbing at higher stresses for longer periods of time give the observed viscosity, and explain the low velocity and high temperature attenuation found at seismic frequencies. Due to differing parameters, separate terms for thermal, seismic and rheological lithospheres are proposed. All three lithospheres, however, are related and are functions of temperature, and must be specified by parameters such as period, stress, and stress duration.

  11. Plate tectonics began in Neoproterozoic time, and plumes from deep mantle have never operated

    NASA Astrophysics Data System (ADS)

    Hamilton, Warren B.

    2011-04-01

    Archean, Paleoproterozoic, and Mesoproterozoic rocks, assemblages, and structures differ greatly both from each other and from modern ones, and lack evidence for subduction and seafloor spreading such as is widespread in Phanerozoic terrains. Most specialists nevertheless apply non-actualistic plate-tectonic explanations to the ancient terrains and do not consider alternatives. This report evaluates popular concepts with multidisciplinary information, and proposes options. The key is fractionation by ca. 4.45 Ga of the hot young Earth into core, severely depleted mantle, and thick mafic protocrust, followed by still-continuing re-enrichment of upper mantle from the top. This is opposite to the popular assumption that silicate Earth is still slowly and unidirectionally fractionating. The protocrust contained most material from which all subsequent crust was derived, either directly, or indirectly after downward recycling. Tonalite, trondhjemite, and granodiorite (TTG), dominant components of Archean crust, were derived mostly by partial melting of protocrust. Dense restitic protocrust delaminated and sank into hot, weak dunite mantle, which, displaced upward, enabled further partial melting of protocrust. Sinkers enriched the upper mantle, in part maintaining coherence as distinct dense rocks, and in part yielding melts that metasomatized depleted-mantle dunite to more pyroxenic and garnetiferous rocks. Not until ca. 3.6 Ga was TTG crust cool enough to allow mafic and ultramafic lavas, from both protocrust and re-enriched mantle, to erupt to the surface, and then to sag as synclinal keels between rising diapiric batholiths; simultaneously upper crust deformed ductily, then brittly, above slowly flowing hot lower TTG crust. Paleoproterozoic and Mesoproterozoic orogens appear to be largely ensialic, developed from very thick basin-filling sedimentary and volcanic rocks on thinned Archean or Paleoproterozoic crust and remaining mafic protocrust, above moderately re

  12. On the slab temperature in the deep lower mantle

    NASA Astrophysics Data System (ADS)

    Komabayashi, T.; Omori, S.; Hirose, K.; Maruyama, S.

    2009-05-01

    Temperature of the subducted cold slab has been one of the important issues in the deep Earth dynamics because it gives thermal anomaly in the mantle. The numerical simulations can estimate the slab temperatures from a number of physical parameters of the slab. It is, however, very difficult to obtain a set of reliable parameters for the calculations. We will discuss the slab temperatures in the deep lower mantle by comparison of phase relations in cold subducted slabs with the seismic observations. In the hydrous peridotite system MgO-SiO2-H2O, seven high-pressure hydrous phases appear after serpentine dehydration (˜150-km depth). These hydrous phases carry water to the deep mantle condition. At the transition zone, a series of dehydration reactions will occur if the slab temperature is above 1300K. In the case of lower temperature, high-P hydrous phases will further carry water into the deep lower mantle. At about 1300-km depth, hydrous phase D will transfer water to high-pressure ice if the temperature is lower than 1300K. After the ice formation, no fluid-forming reaction may occur in the slab except at the core- mantle boundary where the temperature increase is expected. The depth distribution of dehydration reactions in the slab is well consistent with that of seismic event in the subduction zones if the appropriate temperature is assumed. This suggests that the deep-focus seismicity is induced by the dehydration reactions in the slab, and further suggests that the seismic event is an indicator of slab temperature and water transport into the deep mantle. CaSiO3-perovskite undergoes a structural phase transition from tetragonal to cubic symmetry at about 540 K, almost independent of pressure. This transition temperature significantly increases with increasing Al2O3 contents in Ca-perovskite. Unlike in peridotite systems, in a mid-oceanic ridge basalt (MORB) system, Ca-perovskite contains significant amounts of Al2O3 up to about 3 wt% where the structural

  13. Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth's upper mantle.

    PubMed

    Ohuchi, Tomohiro; Kawazoe, Takaaki; Higo, Yuji; Funakoshi, Ken-Ichi; Suzuki, Akio; Kikegawa, Takumi; Irifune, Tetsuo

    2015-10-01

    Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth's upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 10(19.6) to 10(20.7) Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size-sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle.

  14. Upper Mantle Discontinuities Underneath Central and Southern Mexico

    NASA Astrophysics Data System (ADS)

    Perez-Campos, X.; Clayton, R. W.

    2011-12-01

    Central and southern Mexico are affected by the subduction of Cocos plate beneath North American plate. The MesoAmerican Subduction Experiment (MASE) and the Veracruz-Oaxaca (VEOX) project have mapped the geometry of the Cocos slab. It is characterized in central Mexico by a shallow horizontal geometry up to ~300 km from the trench, then it dives steeply (70°) into the mantle, to its apparent end at 500 km depth. In contrast, some 400 km to the south, the slab subducts smoothly, with a dip angle of ~26° to a depth of 150 km. We use receiver functions from teleseismic events, recorded at stations from MASE, VEOX, and the Servicio Sismológico Nacional (SSN, Mexican National Seismological Service) to map the upper mantle discontinuities and properties of the transition zone in central and southern Mexico. We also use data from the Mapping the Rivera Subduction Zone (MARS) Experiment to get a complete picture of the subduction regime in central Mexico and compare the mantle transition zone in a slab tear regime. The 410 discontinuity shows significant variation in topography in central Mexico, particularly where the slab is expected to reach such depth. The 660 discontinuity shows a smoother topography, indicating that the slab does not penetrate this far down. The results will be compared with a ridge regime in the Gulf of California.

  15. The Upper Mantle Flow Field around South-Africa as Reflected by Isotopic Provinciality

    NASA Astrophysics Data System (ADS)

    Meyzen, C.; Blichert-Toft, J.; Ludden, J.; Humler, E.; Mevel, C.; Albarede, F.

    2006-12-01

    Isotopic studies of MORB have established the existence of broad isotopic provinces within the underlying asthenosphere, such as in the Indian Ocean (DUPAL). How these features relate to mantle circulation is, however, still unknown. The steepness of the transition between such isotopic provinces will define the geometry of the velocity field in the upper mantle. In this respect, the transition between the Indian and South Atlantic provinces, two domains that are isotopically contrasted, should be readily identifiable over this long ridge segment. Here, we present Hf isotope data for 60 samples dredged along the SWIR between 35° and 69°E. The new Hf isotope data show that the Indian asthenosphere does not spill directly into the South Atlantic upper mantle: the general decreasing southward gradient observed for ^{176}Hf/^{177}Hf down the mid- Atlantic Ridge, and also for Sr isotopes and model Th/U ratios (derived from Pb isotopes), is overprinted by material with radiogenic Sr, unradiogenic Hf and high Th/U. The Indian domain grades into the South Atlantic around Bouvet, while the South Atlantic collides with the Atlantic province around Tristan. We interpret these features to represent fronts between three adjacent isotopic provinces similar to what has been suggested for the Australian-Antarctic Discordance. The common DUPAL signature of MORB and OIB from the Indian province and the geochemistry of Gulf of Aden MORB and the Afar plume suggest that the source of this distinctive mantle component is deep and lies to the north of the province. This is also what the three-dimensional flow field computed by Behn et al. (2004) from shear-wave splitting shows with a major lower mantle upwelling radiating at the base of the asthenosphere under the Afar plume. Lower mantle gushing out from this source flows southward unimpeded along the Indian ridges, whereas it only reaches the South Atlantic ridge after first having been deflected under the deep roots of the South

  16. Evidence for Primordial Water in Earths Deep Mantle: D/h Ratios in Baffin Island and Icelandic Picrites

    NASA Astrophysics Data System (ADS)

    Hallis, L. J.; Huss, G. R.; Nagashima, K.; Taylor, J.; Hilton, D. R.; Mottl, M. J.; Meech, K. J.; Halldorsson, S. A.

    2016-12-01

    Experimentally based chemical models suggest Jeans escape could have caused an increase in Earth's atmospheric D/H ratio of between a factor of 2 and 9 since the planets formation1. Plate tectonic mixing ensures this change has been incorporated into the mantle. In addition, collisions with hydrogen bearing planetesimals or cometary material after Earth's accretion could have altered the D/H ratio of the planet's surface and upper mantle2. Therefore, to determine Earth's original D/H ratio, a reservoir that has been completely unaffected by these surface and upper mantle changes is required. Most studies suggest that high 3He/4He ratios in some OIBs indicate the existence of relatively undegassed regions in the deep mantle compared to the upper mantle, which retain a greater proportion of their primordial He3-4. Early Tertiary (60-million-year-old) picrites from Baffin Island and west Greenland, which represent volcanic rocks from the proto/early Iceland mantle plume, contain the highest recorded terrestrial 3He/4He ratios3-4. These picrites also have Pb and Nd isotopic ratios consistent with primordial mantle ages (4.45 to 4.55 Ga)5, indicating the persistence of an ancient, isolated reservoir in the mantle. The undegassed and primitive nature6of this reservoir suggests that it could preserve Earth's initial D/H ratio. We measured the D/H ratios of olivine-hosted glassy melt inclusions in Baffin Island and Icelandic picrites to establish whether their deep mantle source region exhibits a different D/H ratio to known upper mantle and surface reservoirs. Baffin Island D/H ratios were found to extend lower than any previously measured mantle values (δD -97 to -218 ‰), suggesting that areas of the deep mantle do preserve a more primitive hydrogen reservoir, hence are unaffected by plate tectonic mixing. Comparing our measured low D/H ratios to those of known extra-terrestrial materials can help determine where Earths water came from. References: [1] Genda and Ikoma

  17. The Upper Mantle Velocity Structure Beneath the Sea of Okhotsk

    NASA Astrophysics Data System (ADS)

    Gomer, B. M.; Lebedev, S.; Okal, E. A.; van der Hilst, R. D.; Mackey, K. G.; Gunbina, L. V.

    2001-12-01

    We use Rayleigh-wave tomography to explore the velocity structure of the upper mantle beneath the Sea of Okhotsk. The Sea of Okhotsk is a complex geologic region, frequently defined as an independent plate, with large variations in crustal structure and an unresolved tectonic history. Upper mantle tomography provides constraints on the tectonic evolution of a region by imaging fast and slow velocity anomalies associated with structures that may result from recent tectonic activity (i.e. remnant slabs). Using long-period waveforms recorded by IRIS, Michigan State University, and the Northeast Interdisciplinary Scientific Research Institute, we find the best-fitting multi-mode synthetics along a variety of regional source-receiver paths. The multi-mode synthetics produce path-averaged velocity profiles that act as model parameter constraints when the paths are combined and inverted to produce a 3-D tomographic model. Our model shows high and low-velocity anomalies, primarily associated with the modern slab and mantle wedge. Both anomalies vary vertically and laterally as the angle of the Wadati-Benioff zone changes. In general, the depth to the Moho discontinuity in our model agrees well with the interpretation of refraction and reflection surveys in this region (e.g. Pavlenkova, 1996). Thicker crustal structures and a deeper Moho boundary are present in the shallow central and northeastern portions of the Okhotsk Sea, while the Moho is shallower in the Kuril Basin. Measurements of residual travel times for multiple ScS waves agree with the velocity structure of the 3-D model. We find positive (slow) residuals in the central portion of the Okhotsk Sea, and negative (fast) residuals in regions where the modern slab is present. The most negative (fastest) ScS residual is located north of Sakhalin Island. As revealed by the 3-D model, this fast residual appears to be the result of a shallower Moho depth and a high-velocity zone extending down from approximately 150 km

  18. Effect of Upper Mantle Heterogeneities on Lithosphere Stresses and Topography

    NASA Astrophysics Data System (ADS)

    Osei Tutu, A.; Steinberger, B.; Rogozhina, I.; Sobolev, S. V.

    2016-12-01

    The orientation and magnitude of lithosphere stresses give us knowledge about most of the processes within the Earth that are not easy to observe. It has been established (Steinberger, Schmeling, and Marquart 2001) that large contribution of the forces producing lithosphere stresses have their source origination from the buoyancies of both the upper and lower mantle acting beneath the lithosphere. The contribution of the crustal thickness to the stresses has been estimated to be less than 10% (Steinberger et al. 2001) in most region and increases in areas with high gravitational potential energy like the Himalayas. In most of these studies, the effect of the crust was determined separately by computing the gravitational potential energy from the crust (Ghosh et al. 2013) and applied as correction. (Artyushkov 1973) showed that the inhomogeneous nature of the crust contribute to the stresses observed as against using constant lithosphere thickness in most studies, due to the complexities for implementing a variable lithosphere. We seek extend the approach of Ghosh et al. (2013) by coupling the Crust 1.0 (Laske et al. 2013) to a varaible lithosphere thickness in our numerical method. Using a 3D global lithosphere-asthenosphere model (Popov and Sobolev 2008) with visco-elasto-plastic rheology, coupled at 300 km depth to a mantle modeled with a spectral technique (Hager and O'Connell, 1981), we compute lithosphere stresses and topography. we compare our model with observations; the World Stress Map, Global Strain Rate Map and the observed topgraphy. We use S40RTS seismic tomography below 300 km depth, with radial viscosity distribution (Steinberger et al 2006). To account for all the heterogeneities in the upper mantle (300 km) we used different 3D temperatures models setups. The first model is the thermal lithosphere model (Artemieva and Mooney, 2001) in continental regions and assumes half-space cooling of sea floor with age (Müller et al. 2008) for oceans. For the

  19. Imaging the Atlantic upper mantle with Rayleigh waves

    NASA Astrophysics Data System (ADS)

    James, E.; Dalton, C. A.; Gaherty, J. B.

    2013-12-01

    The seismic properties of the oceanic upper mantle provide important constraints on the thermal evolution of the lithosphere, the rheological contrast across the lithosphere-asthenosphere boundary, and mantle flow within the asthenosphere. Knowledge of the seismic properties of the oceanic upper mantle comes primarily from regional seismic models of the Pacific basin and the East Pacific Rise. Considerably less is known about the seismic structure beneath the Atlantic, and differences in the spreading rates, plate velocities, and proximity of hotspots to the ridge in the Atlantic and Pacific suggest that different dynamic processes may be occurring beneath the two basins. We conduct a whole-basin study to investigate the seismic velocity structure of the Atlantic upper mantle. A new data set of Rayleigh wave phase delays is measured using waveforms generated by 453 earthquakes with magnitude > 5.5 that occurred within or along the margins of the Atlantic basin between January 1992 and October 2012. We use data from 544 permanent and temporary broadband seismic stations located within or on the margins of the basin; paths with any significant length through continental regions are excluded. The complete dataset, which consists of nearly 10,000 paths, is used to explore the lateral distribution of Rayleigh wave phase velocity in the Atlantic basin. We consider two approaches for parameterizing the phase-velocity variation. One, phase velocity is assumed to vary only as a function of seafloor age, and a pure-path approach is employed to determine age-dependent velocities. Two, we utilize a more general 2-D parameterization in order to capture phase-velocity variations that cannot be incorporated into the underlying age dependence. In both scenarios, phase velocity shows a strong dependence on seafloor age, with the lowest velocities associated with the youngest seafloor and increasing velocity with seafloor age. We find that pure-path phase velocities in the Atlantic

  20. Upper mantle seismic structure beneath central mongolia from body wave traveltime tomography

    NASA Astrophysics Data System (ADS)

    Zhang, F.; Wu, Q.; Gao, M.; Munkhuu, U.; Demberel, S. G.

    2013-12-01

    The Mongolian Plateau (northern Asia) is situated between the Gobi-Altai range and the Siberian craton. The crustal deformations occurring in this area are the northernmost effects of the India-Eurasia collision. There exists evident seismicity and tectonics contrast across the Mongolian Plateau, with strong seismicity and older but more active geological structure in the west in contrast to weak seismcity and younger but less active geological structure in the east. In order to understand the deep structure environmental characteristics and mechanism of strong earthquakes, we need the teleseismic data to map the deep velocity structure beneath the central Mongolia. There were 68 broadband seismic stations supported by ';The Geophysical Investigation and Deep Structure Modeling for Seismic Hazard Assessment in the Far East' project. The average spacing for this temporary network is ~40 km. Thanks to the good instrument quality and low noise level in this region, about 2-year and 1-year long data are recorded by 41 and the other 27 stations, respectively. From this project we used teleseismic events, with epicenter rang from 30° to 90°, to get the upper mantle seismic structure beneath central Mongolia (103.5°-111.5°E, 42°-50°N) from traveltime tomography. We imaged the velocity structure by finite-frequency traveltime theory inversion. Based on this data we test the checkerboard resolution, the smallest anomaly scales which were reliable was about 1°×1°. Most of the earthquakes occurred in southeastern direction, which indicate the southeastern part of the study area were more reliable. From the different horizontal depth slice we found that: the Ulaanbaatar and the southeastern area showed high velocity anomalies at ~100 km depth slice, while at the deeper slice, the southeastern area changed to low velocity anomalies from ~200 km depth to ~300 km depth, and showed a mainly big low velocity zone from ~400 km depth to ~600 km depth, while the Ulaanbaatar

  1. Tracking Silica in the Earth's Subduction Zone and Upper Mantle

    NASA Astrophysics Data System (ADS)

    Chen, T.; Wang, X.; Zou, Y.; Gwanmesia, G. D.; Liebermann, R. C.; Li, B.

    2014-12-01

    The X-discontinuity (~300 km) in the upper mantle has been revealed under some continental or oceanic region by a number of seismic studies, at which depth the P and S wave velocities increase by about 2%. One possible cause for this discontinuity is the coesite-stishovite phase transition. In this study, we conducted ultrasonic interferometry measurements on polycrystalline coesite and stishovite up to 12.6 GPa at ambient temperature and 14GPa 1073K, respectively. While the P wave velocities of coesite continuously increase with pressure, the S wave velocities exhibit a monotonic decrease to the peak pressure of the current experiment followed by a reversible recovery upon release of pressure. As a result, within the pressure range of 8-12 GPa (corresponding to ~250-350 km depths), the velocity contrasts between coesite and stishovite reach as high as ~38% for P wave and 48%-50% for S wave together with impedance contrasts of 71-69% and ~78% for P and S waves, respectively, the highest among all known phase transitions in mantle minerals. With such extreme contrasts, the coesite-stishovite phase transition in the MORB composition with 4-10wt% of SiO2 is sufficient to generate velocity and impedance contrasts comparable to those reported for the X-discontinuity. The current data, together with the seismic X-discontinuity, may provide a geophysical approach to track the ancient subducted oceanic slabs, and place constraints on the amount of silica in the upper mantle.

  2. Anisotropic Structure of the Upper Mantle, Imaged with Surface and S Waveform Tomography

    NASA Astrophysics Data System (ADS)

    Schaeffer, A. J.; Lebedev, S.

    2011-12-01

    The rapid recent expansion of global and regional seismic networks has paved the way for a new generation of tomographic models, with significantly improved resolution at global and regional scales. We present a new global model of shear velocity and azimuthal anisotropy in the upper mantle, down to the base of the transition zone. The model is constrained by an unprecedentedly large waveform dataset collected from over 2000 stations of GSN and affiliates, USArray, VEBSN, CNSN, PASSCAL experiments, and other networks with data available from IRIS, ORFEUS, and GFZ data centers. Applying the accurate and efficient automated multimode inversion of surface- and S-wave forms to this massive dataset, we generated linear constraints on elastic structure within approximate sensitivity volumes between individual source-receiver pairs, with respect to a 3D reference model. The full waveform inversions resulted in more than one million successful fits (one million seismograms), with structural information extracted from both the fundamental and higher modes. The linear equations were then simultaneously solved for a high-resolution, 3D model of shear velocity and azimuthal anisotropy in the upper mantle. In continental domains, clearly identifiable boundaries between different tectonic features such as basins and relic mountain ranges are readily observable, as well as the signature of deep cratonic roots versus juvenile accretionary margins. Both active and fossil subduction zones are marked by slab signatures deep in the upper mantle and extending through the transition zone. In oceanic regions, largest mid-ocean-ridge anomalies indicative of melting terminate at depths of 100-120 km, with evidence for vertical flow in the upper mantle observed through a combination of VSV, VSH, and azimuthal anisotropy. Spatio-temporal evolution (cooling and thickening) of lithosphere away from the spreading ridges matches the signature expected from geodynamic and thermal modeling. The

  3. Wavefront Healing and Tomographic Resolution of Deep Mantle Superplumes

    NASA Astrophysics Data System (ADS)

    Xue, Jing; Zhou, Ying

    2014-05-01

    Seismic tomography revealed two large low shear velocity province (LLSVP) at the bottom of the mantle, one under Africa and the other under the southern Pacific. Traditional tomographic results show strong anti-correlation between bulk sound speed and S wave speed perturbations, supporting at least partially chemical origin of deep mantle superplumes. The core-mantle boundary regions are best sampled by core diffracted waves while wave front healing effects of diffracted waves have been ignored in traditional tomographic studies. To investigate the resolution of deep mantle superplumes as well as the robustness of the anti correlation between bulk sound speed and S wave speed, we use Spectral Element Method (SEM) to simulate global seismic wave propagation in 3-D plume models at periods down to 10s. We measure frequency-dependent P-wave and S-wave travel time anomalies caused by 3-D plume structures using a multi-taper technique, and calculate bulk sound speed perturbations based on measured P-wave and S-wave traveltimes. The comparison between measured delay times and ray-theory predictions shows that different healing rates between P waves and S waves in thermal plume models can lead to significant artifact as anti-correlation between bulk sound speed and S-wave speed perturbations. The strength of this artifact depends on epicenter distance and wave frequency. The artifact in anti-correlation is also confirmed in tomographic inversions based on ray theory using Pdiff and Sdiff traveltimes measured from SEM seismograms. We show that resolutions of superplumes as well as artifacts in the anti-correlation are dependent upon the length scale of the anomalies, the frequency of the wave as well as source-receiver distribution. Finally we compare calculations based on finite-frequency theory and ray theory and show that different healing rates between P waves and S waves can be properly accounted for in finite-frequency tomography.

  4. Seismic tomography of the Colorado Rocky Mountains upper mantle from CREST: Lithosphere-asthenosphere interactions and mantle support of topography

    NASA Astrophysics Data System (ADS)

    MacCarthy, J. K.; Aster, R. C.; Dueker, K.; Hansen, S.; Schmandt, B.; Karlstrom, K.

    2014-09-01

    The CREST experiment (Colorado Rocky Mountains Experiment and Seismic Transects) integrated the EarthScope USArray Transportable Array with portable and permanent stations to provide detailed seismic imaging of crust and mantle properties beneath the highest topography region of the Rocky Mountains. Inverting approximately 14,600 P- and 3600 S-wave arrival times recorded at 160 stations for upper mantle Vp and Vs structure, we find that large Vp perturbations relative to AK135 of 7% and Vs variations of 8% take place over very short (approaching tens of kilometers) lateral distances. Highest heterogeneity is observed in the upper 300 km of the mantle, but well resolved low velocity features extend to the top of the transition zone in portions of these images. The previously noted low velocity upper mantle Aspen Anomaly is resolved into multiple features. The lowest Vp and Vs velocities in the region are found beneath the San Juan Mountains, which is clearly distinguished from other low velocity features of the northern Rio Grande Rift, Taos/Latir region, Aspen region, and below the Never Summer Mountains. We suggest that the San Juan anomaly, and a similar feature below the Taos/Latir region of northern New Mexico, are related to delamination and remnant heat (and melt) beneath these sites of extraordinarily voluminous middle-Cenozoic volcanism. We interpret a northeast-southwest grain in velocity structure that parallels the Colorado Mineral belt to depths near 150 km as being reflective of control by uppermost mantle Proterozoic accretionary lithospheric architecture. Further to the north and west, the Wyoming province and northern Colorado Plateau show high velocity features indicative of thick (∼150 km) preserved Archean and Proterozoic lithosphere, respectively. Overall, we interpret the highly heterogeneous uppermost mantle velocity structure beneath the southern Rocky Mountains as reflecting interfingered chemical Proterozoic lithosphere that has been, is

  5. Numerical modelling of volatiles in the deep mantle

    NASA Astrophysics Data System (ADS)

    Eichheimer, Philipp; Thielmann, Marcel; Golabek, Gregor J.

    2017-04-01

    The transport and storage of water in the mantle significantly affects several material properties of mantle rocks and thus water plays a key role in a variety of geodynamical processes (tectonics, magmatism etc.). The processes driving transport and circulation of H2O in subduction zones remain a debated topic. Geological and seismological observations suggest different inflow mechanisms of water e.g. slab bending, thermal cracking and serpentinization (Faccenda et al., 2009; Korenaga, 2017), followed by dehydration of the slab. On Earth both shallow and steep subduction can be observed (Li et al., 2011). However most previous models (van Keken et al., 2008; Wilson et al., 2014) did not take different dip angles and subduction velocities of slabs into account. To which extent these parameters and processes influence the inflow of water still remains unclear. We present 2D numerical models simulating the influence of the various water inflow mechanisms on the mantle with changing dip angle and subduction velocity of the slab over time. The results are used to make predictions regarding the rheological behavior of the mantle wedge, dehydration regimes and volcanism at the surface. References: van Keken, P. E., et al. A community benchmark for subduction zone modeling. Phys. Earth Planet. Int. 171, 187-197 (2008). Faccenda, M., T.V. Gerya, and L. Burlini. Deep slab hydration induced by bending-related variations in tectonic pressure. Nat. Geosci. 2, 790-793 (2009). Korenaga, J. On the extent of mantle hydration caused by plate bending. Earth Planet. Sci. Lett. 457, 1-9 (2017). Wilson, C. R., et al. Fluid flow in subduction zones: The role of solid rheology and compaction pressure. Earth Planet. Sci. Lett. 401, 261-274 (2014). Li, Z. H., Z. Q. Xu, and T. V. Gerya. Flat versus steep subduction: Contrasting modes for the formation and exhumation of high- to ultrahigh-pressure rocks in continental collision zones. Earth Planet. Sci. Lett. 301, 65-77 (2011).

  6. An upper-mantle S-wave velocity model for East Asia from Rayleigh wave tomography

    NASA Astrophysics Data System (ADS)

    Li, Yonghua; Wu, Qingju; Pan, Jiatie; Zhang, Fengxue; Yu, Daxin

    2013-09-01

    We present a new shear velocity model of the upper mantle beneath the East Asia region derived by inverting Rayleigh wave group velocity measurements between 10 and 145 s combined with previously published Rayleigh wave phase velocity measurements between 150 and 250 s. Rayleigh wave group velocity dispersion curves along more than 9500 paths were measured and combined to produce 2D dispersion maps for 10-145 s periods. The group velocity maps benefit from the inclusion of new data recorded by the China National Seismic Network and surrounding global stations. The increase in available data has resulted in enhanced resolution compared with previously published group velocity maps; the horizontal resolution across the region is about 3° for the periods used in this study. The new shear-wave velocity models indicate varying velocity structure beneath eastern China, which yields estimates of a lithosphere-asthenosphere boundary depth from around 160 km beneath the Yangtze block to approximately 140 km beneath the western part of the North China Craton (NCC), up to depths of 70-100 km beneath the eastern NCC, Northeast China, and the Cathaysia block. The models reveal the subduction of two opposite-facing continental plates under the southern and northern margin of Tibet. An obvious low-velocity anomaly appears in the top 200 km of the upper mantle beneath northern Tibet, which is inconsistent with the presence of subducted Asian or Indian mantle lithosphere beneath northern Tibet. The Cenozoic volcanism fields in the Mongolian plateau are characterized by an obvious upper mantle negative anomaly, but no signature of deep-seated plume was observed.

  7. Double layering of a thermochemical plume in the upper mantle beneath Hawaii

    NASA Astrophysics Data System (ADS)

    Ballmer, M. D.; Ito, G.; Wolfe, C. J.; Cadio, C.; Solomon, S. C.

    2012-04-01

    Volcanism far from plate boundaries has traditionally been explained by "classical" plume theory. Classical plumes are typically described as narrow thermal upwellings that rise through the entire mantle to be deflected into a thin (<100 km) "pancake" beneath the overriding lithosphere. The pancake is thought to be deflected by the drag of the overriding plate and hence to support a hotspot swell that is parabolic in map view and symmetric about the direction of plate motion. Many hotspots and their swells, such as Cape Verde and Iceland, are indeed well explained by near-classical thermal plumes. High-resolution seismic velocity images obtained from the PLUME project support the concept of a deep-rooted mantle plume beneath the Hawaiian hotspot. However, in detail these images challenge traditional concepts inasmuch as they indicate a low-velocity body in the upper mantle that is too thick (~400 km) and asymmetric to be interpreted as a classical pancake. Classical plume theory is, moreover, inconsistent with several geochemical characteristics of Hawaiian magmas, which point to a heterogeneous mantle source involving mafic lithologies such as eclogite and not an exclusively thermal (i.e., isochemical) origin¹. To explore the dynamical and melting behavior of plumes containing a substantial fraction (~15%) of eclogite, we performed three-dimensional numerical simulations of thermochemical convection. Relative to ambient-mantle peridotite, eclogite is intrinsically dense. This density contrast is sensitive to phase changes in the upper mantle; the contrast peaks at 410-300 km and lessens at about 250-190 km depth, where eclogite is subsequently removed by melting. For a plume core with an eclogite content >12%, these effects locally increase the density beyond that of the ambient mantle. Therefore, the upwelling column forms a broad and thick pool at depths of 450-300 km (which we term the deep eclogite pool, or DEP). As the DEP is well supported by the deeper

  8. Plumes do not Exist: Plate Circulation is Confined to Upper Mantle

    NASA Astrophysics Data System (ADS)

    Hamilton, W. B.

    2002-12-01

    Plumes from deep mantle are widely conjectured to define an absolute reference frame, inaugurate rifting, drive plates, and profoundly modify oceans and continents. Mantle properties and composition are assumed to be whatever enables plumes. Nevertheless, purported critical evidence for plume speculation is false, and all data are better interpreted without plumes. Plume fantasies are made ever more complex and ad hoc to evade contradictory data, and have no predictive value because plumes do not exist. All plume conjecture derives from Hawaii and the guess that the Emperor-Hawaii inflection records a 60-degree change in Pacific plate direction at 45 Ma. Paleomagnetic latitudes and smooth Pacific spreading patterns disprove any such change. Rationales for other fixed plumes collapse when tested, and hypotheses of jumping, splitting, and gyrating plumes are specious. Thermal and physical properties of Hawaiian lithosphere falsify plume predictions. Purported tomographic support elsewhere represents artifacts and misleading presentations. Asthenosphere is everywhere near solidus temperature, so melt needs a tensional setting for egress but not local heat. Gradational and inconsistent contrasts between MORB and OIB are as required by depth-varying melt generation and behavior in contrasted settings and do not indicate systematically unlike sources. MORB melts rise, with minimal reaction, through hot asthenosphere, whereas OIB melts react with cool lithosphere, and lose mass, by crystallizing refractories and retaining and assimilating fusibles. The unfractionated lower mantle of plume conjecture is contrary to cosmologic and thermodynamic data, for mantle below 660 km is more refractory than that above. Subduction, due to density inversion by top-down cooling that forms oceanic lithosphere, drives plate tectonics and upper-mantle circulation. It organizes plate motions and lithosphere stress, which controls plate boundaries and volcanic chains. Hinge rollback is the

  9. Study on 3-D velocity structure of crust and upper mantle in Sichuan-yunnan region, China

    USGS Publications Warehouse

    Wang, C.; Mooney, W.D.; Wang, X.; Wu, J.; Lou, H.; Wang, F.

    2002-01-01

    Based on the first arrival P and S data of 4 625 regional earthquakes recorded at 174 stations dispersed in the Yunnan and Sichuan Provinces, the 3-D velocity structure of crust and upper mantle in the region is determined, incorporating with previous deep geophysical data. In the upper crust, a positive anomaly velocity zone exists in the Sichuan basin, whereas a negative anomaly velocity zone exists in the western Sichuan plateau. The boundary between the positive and negative anomaly zones is the Longmenshan fault zone. The images of lower crust and upper mantle in the Longmenshan fault, Xianshuihe fault, Honghe fault and others appear the characteristic of tectonic boundary, indicating that the faults litely penetrate the Moho discontinuity. The negative velocity anomalies at the depth of 50 km in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with the temperature and composition variations in the upper mantle. The overall features of the crustal and the upper mantle structures in the Sichuan-Yunnan region are the lower average velocity in both crust and uppermost mantle, the large crustal thickness variations, and the existence of high conductivity layer in the crust or/and upper mantle, and higher geothermal value. All these features are closely related to the collision between the Indian and the Asian plates. The crustal velocity in the Sichuan-Yunnan rhombic block generally shows normal.value or positive anomaly, while the negative anomaly exists in the area along the large strike-slip faults as the block boundary. It is conducive to the crustal block side-pressing out along the faults. In the major seismic zones, the seismicity is relative to the negative anomaly velocity. Most strong earthquakes occurred in the upper-mid crust with positive anomaly or normal velocity, where the negative anomaly zone generally exists below.

  10. Deep venous thrombosis of the upper extremity.

    PubMed

    Stephens, M B

    1997-02-01

    Deep venous thrombosis (DVT) of the upper extremity is a relatively uncommon but important cause of morbidity, especially in young active persons. The causes of upper extremity DVT may be categorized as catheter-related, spontaneous (effort-related) and miscellaneous (e.g., trauma, intravenous drug use). Diagnosis is based on clinical history and confirmed by either duplex ultrasonography or contrast venography. Significant controversy surrounds the optimal management of upper extremity DVT. Treatment options include conservative therapy, anticoagulation, catheter-directed thrombolysis and surgical intervention to remove intravascular clot or revise the anatomy of the costoclavicular space. Early aggressive treatment of active young patients may decrease long-term morbidity.

  11. Crust upper mantle seismic velocity structure across Southeastern China

    NASA Astrophysics Data System (ADS)

    Zhang, Zhongjie; Badal, José; Li, Yinkang; Chen, Yun; Yang, Liqiang; Teng, Jiwen

    2005-01-01

    One in-line wide-angle seismic profile was conducted in 1990 in the course of the Southeastern China Continental Dynamics project aimed at the study of the contact between the Cathaysia block and the Yangtze block. This 380-km-long profile extended in NW-SE direction from Tunxi, Anhui Province, to Wenzhou, Zhejiang Province. Five in-line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line. We have used two-dimensional ray tracing to model P- and S-wave arrivals and provide constraints on the velocity structure of the upper crust, middle crust, lower crust, Moho discontinuity, and the top part of the lithospheric mantle. P-wave velocity, S-wave velocity and VP/ VS ratio are mapped. The crust is 36-km thick on average, albeit it gradually thins from the northwest end to the southeast end (offshore) of the profile. The average crustal velocity is 6.26 km/s for P-waves but 3.6 km/s for S-waves. A relatively narrow low-velocity layer of about 4 km of thickness, with P- and S-wave velocities of 6.2 km/s and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23-km northwest and 17-km southeast. At the crust-mantle transition, the P- and S-wave velocity change quickly from 7.4 to 7.8 km/s (northwest) and 8.0 to 8.2 km/s (southeast) and from 3.9 to 4.2 km/s (northwest) and 3.9 to 4.5 km/s (southeast), respectively. This result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately. The average VP/ VS ratio ranges from 1.68-1.8 for the upper crust to 1.75 for the middle and 1.75-1.85 for lower crust. With the interpretation of the wide-angle seismic data, Jiangshan-Shaoxin fault is considered as the boundary between the Yangtze and the Cathaysia block.

  12. Numerical Modeling of Deep Mantle Flow: Thermochemical Convection and Entrainment

    NASA Astrophysics Data System (ADS)

    Mulyukova, Elvira; Steinberger, Bernhard; Dabrowski, Marcin; Sobolev, Stephan

    2013-04-01

    One of the most robust results from tomographic studies is the existence of two antipodally located Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle, which appear to be chemically denser than the ambient mantle. Results from reconstruction studies (Torsvik et al., 2006) infer that the LLSVPs are stable, long-lived, and are sampled by deep mantle plumes that rise predominantly from their margins. The origin of the dense material is debated, but generally falls within three categories: (i) a primitive layer that formed during magma ocean crystallization, (ii) accumulation of a dense eclogitic component from the recycled oceanic crust, and (iii) outer core material leaking into the lower mantle. A dense layer underlying a less dense ambient mantle is gravitationally stable. However, the flow due to thermal density variations, i.e. hot rising plumes and cold downwelling slabs, may deform the layer into piles with higher topography. Further deformation may lead to entrainment of the dense layer, its mixing with the ambient material, and even complete homogenisation with the rest of the mantle. The amount of the anomalous LLSVP-material that gets entrained into the rising plumes poses a constraint on the survival time of the LLSVPs, as well as on the plume buoyancy, on the lithospheric uplift associated with plume interaction and geochemical signature of the erupted lavas observed at the Earth's surface. Recent estimates for the plume responsible for the formation of the Siberian Flood Basalts give about 15% of entrained dense recycled oceanic crust, which made the hot mantle plume almost neutrally buoyant (Sobolev et al., 2011). In this numerical study we investigate the mechanics of entrainment of a dense basal layer by convective mantle flow. We observe that the types of flow that promote entrainment of the dense layer are (i) upwelling of the dense layer when it gets heated enough to overcome its stabilizing chemical density anomaly, (ii

  13. Hunting for the Tristan plume - An upper mantle tomography around the volcanic island Tristan da Cunha

    NASA Astrophysics Data System (ADS)

    Schlömer, Antje; Geissler, Wolfram H.; Jokat, Wilfried; Jegen, Marion

    2016-04-01

    Tristan da Cunha is a volcanic island in the South Atlantic close to the Mid-Atlantic Ridge. It is part of an area consisting of widely scattered seamounts and small islands at the western and youngest end of the aseismic Walvis Ridge. Tristan da Cunha together with the Walvis Ridge represents the classical example of a mantle plume track, because of the connection to the Cretaceous Etendeka flood basalt province in NW Namibia. The genesis of the island has so far remained enigmatic. It is hotly debated, if Tristan da Cunha sits actually above a deep mantle plume or if it is only originated by upwelling material from weak (leaky) fracture zones. It also has to be clarified if there are any indications for a plume-ridge interaction. Geochemical investigations have shown complex compositions of magmatic samples from Tristan da Cunha, which could be interpreted as a mixing of plume-derived melts and depleted upper mantle sources. To improve our understanding about the origin of Tristan and to test the mantle plume hypothesis, we deployed 24 broadband ocean-bottom seismometers and 2 seismological land stations around and on the island during an expedition in January 2012 with the German research vessel Maria S. Merian. After acquiring continuous seismological data for almost one year, the seismometers were recovered in early January 2013. We cross-correlated the arrival times of teleseismic P and PKP phases to perform a finite-frequency tomography of the upper mantle beneath the study area. Here we show the 3D mantle structure in terms of velocity variations: We do not image a "classical" plume-like structure directly beneath Tristan da Cunha, but we observe regions of low velocities at the edges of our array that we relate to local mantle upwelling from potentially deeper sources. Additionally we discuss local seismicity within the Tristan da Cunha region, which show processes along the nearby mid-ocean ridge and transform faults. Furthermore, the local seismicity

  14. Shear velocity structure of the crust and upper mantle of Madagascar derived from surface wave tomography

    NASA Astrophysics Data System (ADS)

    Pratt, Martin J.; Wysession, Michael E.; Aleqabi, Ghassan; Wiens, Douglas A.; Nyblade, Andrew A.; Shore, Patrick; Rambolamanana, Gérard; Andriampenomanana, Fenitra; Rakotondraibe, Tsiriandrimanana; Tucker, Robert D.; Barruol, Guilhem; Rindraharisaona, Elisa

    2017-01-01

    The crust and upper mantle of the Madagascar continental fragment remained largely unexplored until a series of recent broadband seismic experiments. An island-wide deployment of broadband seismic instruments has allowed the first study of phase velocity variations, derived from surface waves, across the entire island. Late Cenozoic alkaline intraplate volcanism has occurred in three separate regions of Madagascar (north, central and southwest), with the north and central volcanism active until <1 Ma, but the sources of which remains uncertain. Combined analysis of three complementary surface wave methods (ambient noise, Rayleigh wave cross-correlations, and two-plane-wave) illuminate the upper mantle down to depths of 150 km. The phase-velocity measurements from the three methods for periods of 8-182 s are combined at each node and interpolated to generate the first 3-D shear-velocity model for sub-Madagascar velocity structure. Shallow (upper 10 km) low-shear-velocity regions correlate well with sedimentary basins along the west coast. Upper mantle low-shear-velocity zones that extend to at least 150 km deep underlie the north and central regions of recent alkali magmatism. These anomalies appear distinct at depths <100 km, suggesting that any connection between the zones lies at depths greater than the resolution of surface-wave tomography. An additional low-shear velocity anomaly is also identified at depths 50-150 km beneath the southwest region of intraplate volcanism. We interpret these three low-velocity regions as upwelling asthenosphere beneath the island, producing high-elevation topography and relatively low-volume magmatism.

  15. Variation of the upper mantle velocity structure along the central-south Andes

    NASA Astrophysics Data System (ADS)

    Liang, X.; Sandvol, E. A.; Shen, Y.; Gao, H.; Zhang, Z.

    2013-12-01

    Variations in the subduction angle of the Nazca plate beneath the South American plate has lead to different modes of deformation and volcanism along the Andean active margin. The volcanic gap between the central and southern Andean volcanic zones is correlated with the Pampean flat-slab subduction zone, where the subducting Nazca slab changes from a 30-degree dipping slab beneath the Puna plateau to a horizontal slab beneath the Sierras Pampeanas, and then to a 30-degree dipping slab beneath the south Andes from north to south. The Pampean flat-slab subduction correlates spatially with the track of the Juan Fernandez Ridge, and is associated with the inboard migration of crustal deformation. A major Pliocene delamination event beneath the southern Puna plateau has previously been inferred from geochemical, geological, and preliminary geophysical data. The mechanisms for the transition between dipping- and flat-subduction slab and the mountain building process of the central Andean plateau are key issues to understanding the Andean-type orogenic process. We use a new frequency-time normalization approach to extract very-broadband (up to 300 second) empirical Green's functions (EGFs) from continuous seismic records. The long-period EGFs provide the sensitivity needed to constrain the deep mantle structure. The broadband waveform data are from 393 portable stations of eight temporary networks: PUNA, SIEMBRA, CHARGE, RAMP, East Sierras Pampeanas, BANJO/SEDA, REFUCA, ANCORP, and 31 permanent stations accessed from both the IRIS DMC and GFZ GEOFON DMC. A finite difference wave propagation method is used to generate synthetic seismograms from 3-D velocity model. We use 3-D traveltime sensitivity kernels, and traveltime residuals measured by waveform cross-correlation to directly invert the upper mantle shear-wave velocity structure. The preliminary model shows strong along-strike velocity variations within in the mantle wedge and the subducting NAZCA slab. Low upper

  16. Crust and Upper Mantle Structure of Antarctica from Rayleigh Wave Tomography

    NASA Astrophysics Data System (ADS)

    Wiens, D. A.; Heeszel, D. S.; Sun, X.; Chaput, J. A.; Aster, R. C.; Nyblade, A.; Anandakrishnan, S.; Wilson, T. J.; Huerta, A. D.

    2012-12-01

    We combine data from three temporary arrays of seismometers (AGAP/GAMSEIS 2007-2010, ANET/POLENET 2007-2012, TAMSEIS 2001-2003) deployed across Antarctica, along with permanent stations in the region, to produce a large scale shear velocity model of the continent extending from the Gamburtsev Subglacial Mountains (GSM) in East Antarctica, across the Transantarctic Mountains (TAM) and West Antarctic Rift System (WARS) to Marie Byrd Land (MBL) in West Antarctica. Our combined dataset consists of Rayleigh wave phase and amplitude measurements from 112 stations across the study region. We first invert for 2-D Rayleigh wave phase velocities using the two-plane wave method. These results are then inverted for shear velocity structure using crustal thicknesses derived from ambient noise tomography and teleseismic receiver functions. We refine our shear velocity model by performing a Monte Carlo simulation that explores the tradeoff between crustal thickness and upper mantle seismic velocities. The resulting model is higher resolution than previous studies (~150 km resolution length) and highlights significant differences in crustal and uppermost mantle structure between East and West Antarctica in greater detail than previously possible. East Antarctica is underlain by thick crust (reaching ~55 km beneath the GSM) and fast, cratonic lithosphere. West Antarctica is defined by thinner crust and slow upper mantle velocities indicative of its more recent tectonic activity. The observed boundary in crustal thickness closely follows the TAM front. MBL is underlain by a thicker lithosphere than that observed beneath the WARS, but slow mantle velocities persist to depths greater than 200 km, indicating a 'deep seated' (i.e. deeper than the deepest resolvable features of our model) thermal source for volcanism in the region. The slowest seismic velocities at shallow depths are observed in the Terror Rift region of the Ross Sea along an arc following the TAM front, where the most

  17. Insights on the upper mantle beneath the Eastern Alps

    PubMed Central

    Bianchi, Irene; Miller, Meghan S.; Bokelmann, Götz

    2014-01-01

    Analyses of Ps and Sp receiver functions from datasets collected by permanent and temporary seismic stations, image a seismic discontinuity, due to a negative velocity contrast across the entire Eastern Alps. The receiver functions show the presence of the discontinuity within the upper mantle with a resolution of tens of kilometers laterally. It is deeper (100–130 km) below the central portion of the Eastern Alps, and shallower (70–80 km) towards the Pannonian Basin and in the Central Alps. Comparison with previous studies renders it likely that the observed discontinuity coincides with the lithosphere–asthenosphere boundary (LAB) east of 15°E longitude, while it could be associated with a low velocity zone west of 15°E. PMID:25843967

  18. Insights on the upper mantle beneath the Eastern Alps.

    PubMed

    Bianchi, Irene; Miller, Meghan S; Bokelmann, Götz

    2014-10-01

    Analyses of Ps and Sp receiver functions from datasets collected by permanent and temporary seismic stations, image a seismic discontinuity, due to a negative velocity contrast across the entire Eastern Alps. The receiver functions show the presence of the discontinuity within the upper mantle with a resolution of tens of kilometers laterally. It is deeper (100-130 km) below the central portion of the Eastern Alps, and shallower (70-80 km) towards the Pannonian Basin and in the Central Alps. Comparison with previous studies renders it likely that the observed discontinuity coincides with the lithosphere-asthenosphere boundary (LAB) east of 15°E longitude, while it could be associated with a low velocity zone west of 15°E.

  19. Conditions of the Asthenosphere Layer Appearance during Upper-Mantle Convection

    NASA Astrophysics Data System (ADS)

    Sharapov, V. N.; Perepechko, Y. V.; Sorokin, K.

    2012-12-01

    The upper mantle parameters responsible to the asthenosphere appearance during convection have been studied. Many geophysical models of the earth mantle have a continuous layer of a partially melted upper-mantle material under the lithosphere plate. From the period of the early earth this structure is possible only if there is the upper-mantle convection due to 660 km depth phase transition. The previous research considered hydrodynamic conditions of the general mantle convective systems taking into account phase boundaries within the upper mantle. In this work, based on numerical modeling, we discuss thermodynamic conditions of the asthenosphere layer evolution under a homogeneous lithospheric plate. Convection in the upper mantle is modeled using the Boussinesq approximation provided spatially distributed phase transitions within the upper mantle. The problem was solved using the control volume method, which provides integral conservation laws. The results of modeling show that the convective instability is possible when the temperature at the mantle boundary is Tb~1410÷1450°C. Decompression melting of the upper mantle rocks take place if Tb>1650°C. The starting temperatures at the lower boundary of the upper mantle are 1700÷1950°C result in the appearance of the asthenosphere layer existing for 30÷100 Ma. In this case the development of complex, separated asthenosphere zones with additional convective cells is typical. Further evolution results in decay of the melting elements and their final disappearance. The initial phase of the evolution of the asthenosphere zones lasts for about 30÷100 Ma. The second, longer phase of decompression melting contains periodical melting elements with the sizes coextensive to lava sheets of intraplate volcanoes. If Tb>1950°C, the evolution of the asthenosphere is different: after the decay of large asthenosphere zones we observe a reconstruction of convective cells; in this case spatial 'wandering' of varying melting

  20. Lattice thermal conductivity of minerals in the deep mantle condition

    NASA Astrophysics Data System (ADS)

    Dekura, H.; Tsuchiya, T.; Tsuchiya, J.

    2011-12-01

    Thermal transport property of materials under pressure and temperature is of importance for understanding the dynamics of the solid Earth and the thermal history. Both experimental and theoretical determinations of the thermal conductivity, however, still remain technically challenging particularly at the deep mantle condition. Recent progress in ab initio computational method based on the density-functional theory is now makes it possible to examine the transport phenomena including the lattice thermal conduction. The intrinsic bulk thermal conduction of insulator is caused by lattice anharmonicity owing to phonon-phonon interaction. The key parameter to predict lattice thermal conductivity is thus the anharmonic coupling constant. Earlier theoretical works calculated the lattice thermal conductivity of MgO with ab initio molecular dynamics simulation or finite difference lattice dynamics simulation (Nico de Koker, Phys. Rev. Lett. 103, 125902, 2009; X. Tang and J. Dong, Proc. Natl. Acad. Sci. U.S.A. 107, 4539, 2010). However, in these approaches, the simulation cell size could often be insufficient for accurate description of the long wavelength phonon scattering. This leads to a lack of the decay channels for the phonons. As an alternative approach, the anharmonic coupling strength between phonon modes can be evaluated within the density-functional perturbation theory. In this approach, the higher-order force tensors are calculated through a number of phonon decay channels obtained within the perturbative scheme taking care only of the primitive cell. We have been developing a technique for calculation of the phonon linewidth necessary to obtain the phonon lifetime. Then the lattice thermal conductivity is evaluated combining with additional harmonic-level of propeties. In this presentation, we show the behavior of lattice thermal conductivity in lower mantle minerals, and discuss the effects of pressure and temperature on their conductivities up to the deep

  1. Noble gas isotope signals of mid-ocean ridge basalts and their implication for upper mantle structure

    NASA Astrophysics Data System (ADS)

    Stroncik, Nicole A.; Niedermann, Samuel

    2016-04-01

    The geochemical structure of the upper mantle in general and its noble gas isotopic structure in particular have been the subject of countless studies, as both provide important insights into mantle dynamic processes and are essential for the formulation of mantle geodynamic models. This contribution presents a noble gas study of basaltic glasses derived from the Mid-Atlantic-Ridge (MAR) between 4 and 12° S, an area well known for its high degree of lithophile isotope heterogeneity and exhibiting anomalous crustal thickness. The Sr, Nd, Pb and Hf isotopies along this segment of the MAR range from ultra-depleted (more than NMORB) to highly enriched, and different concepts have been proposed to explain the observed isotopic signatures. Here we show that the high degree of heterogeneity is not confined to the isotopes of the lithophile elements, but is also shown by the noble gas isotopes and noble gas interelement ratios, such as e.g. 3He/22NeM or 4He/40Ar*. 3He/4He, 21Ne/22Neextra and 40Ar/36Ar range from 7.3 to 9.3 RA, from 0.05 to 0.08, and from 346 to 37,400, respectively. Nevertheless, the majority of the Ne isotope data are clearly aligned along a single mixing line in the Ne-three-isotope diagram, represented by the equation 20Ne/22Ne=70.5 x 21Ne/22Ne + 7.782, with a slope distinctly different from that of the MORB line, indicating that the ultra-depleted material is characterised by a significantly more nucleogenic 21Ne/22Ne isotopy than the normal depleted mantle. We show, based on covariations between 3He/4He and 21Ne/22Neextra with 206Pb/204Pb and 178Hf/177Hf, that the ultra-depleted material erupted at this MAR segment was most likely produced by an ancient, deep melting event. This implies that isotopic heterogeneities in the upper mantle are not solely caused by the injection of enriched materials from deep-seated mantle plumes or by crustal recycling but may also be due to differences in the depth and degree of melting of upper mantle material within

  2. Upper Mantle of the Central Part of the Russian Platform by Receiver Function Data.

    NASA Astrophysics Data System (ADS)

    Goev, Andrey; Kosarev, Grigoriy; Sanina, Irina; Riznichenko, Oksana

    2017-04-01

    The study of the upper mantle of the Russian Platform (RP) with seismic methods remains limited due to the lack of broadband seismic stations. Existing velocity models have been obtained by using the P-wave travel-times from seismic events interpreted as explosions recorded at the NORSAR array in 1974-75 years. Another source of information is deep seismic sounding data from long-range profiles (exceeding 3000 km) such as QUARTZ, RUBIN-1 and GLOBUS and peaceful nuclear explosions (PNE) as sources. However, the data with the maximum distances larger than 1500 km have been acquired on the RP and only in the northern part. Being useful, these velocity models have low spatial resolution. This study analyzes and integrates all the existing RP upper mantle velocity models with the main focus on the central region. We discuss the completeness of the RP area of the LITHO 1.0 model. Based on results of our analysis, we conclude that it is necessary to get up-to-date velocity models of the upper mantle using broadband stations located at the central part of the RP using Vp/Vs ratio data and anisotropy parameters for robust estimation of the mantle boundaries. By applying the joint inversion of receiver-function (RF) data, travel-time residuals and dispersion curves of surface waves we get new models reaching 300 km depth at the locations of broadband seismic stations at the central part of the RP. We used IRIS stations OBN, ARU along with MHV and mobile array NOV. For each station we attempt to determine thickness of the lithosphere and to locate LVL, LAB, Lehman and Hales boundaries as well as the discontinuities in the transition zones at the depth of 410 and 660 km. Also we investigate the necessity of using short-period and broadband RF separately for more robust estimation of the velocity model of the upper mantle. This publication is based on work supported by the Russian Foundation for Basic Research (RFBR), project 15-05-04938 and by the leading scientific school NS

  3. Magnetization of lower oceanic crust and upper mantle

    NASA Astrophysics Data System (ADS)

    Kikawa, E.

    2004-05-01

    The location of the magnetized rocks of the oceanic crust that are responsible for sea-floor spreading magnetic anomalies has been a long-standing problem in geophysics. The recognition of these anomalies was a key stone in the development of the theory of plate tectonics. Our present concept of oceanic crustal magnetization is much more complex than the original, uniformly magnetized model of Vine-Matthews-Morley Hypothesis. Magnetic inversion studies indicated that the upper oceanic extrusive layer (Layer 2A of 0.5km thick) was the only magnetic layer and that it was not necessary to postulate any contribution from deeper parts of oceanic crust. Direct measurements of the magnetic properties of the rocks recovered from the sea floor, however, have shown that the magnetization of Layer 2A, together with the observations that this layer could record geomagnetic field reversals within a vertical section, is insufficient to give the required size of observed magnetic anomalies and that some contribution from lower intrusive rocks is necessary. Magnetization of oceanic intrusive rocks were observed to be reasonably high enough to contribute to sea-floor spreading magnetic anomalies, but were considered somewhat equivocal until late 1980Os, in part because studies had been conducted on unoriented dredged and ophiolite samples and on intermittent DSDP/ODP cores. Since ODP Leg 118 that cored and recovered continuous 500m of oceanic intrusive layer at Site 735B, Southwest Indian Ridge with an extremely high recovery of 87 percent, there have been several ODP Legs (legs 147, 153, 176, 179 and 209) that were devoted to drilling gabbroic rocks and peridotites. In terms of the magnetization intensities, all of the results obtained from these ODP Legs were supportive of the model that a significant contribution must come from gabbros and peridotites and the source of the lineated magnetic anomalies must reside in most of the oceanic crust as well as crust-mantle boundary

  4. Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere

    USGS Publications Warehouse

    Kirby, S.H.; Stein, S.; Okal, E.A.; Rubie, David C.

    1996-01-01

    Earth's deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km. This depth interval closely brackets the mantle transition zone, characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher-pressure phases. Deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal, thermodynamic, and mechanical ferment inside slabs. Numerical simulations of reaction rates show that the olivine ??? spinel transformation should be kinetically hindered in old, cold slabs descending into the transition zone. Thus wedge-shaped zones of metastable peridotite probably persist to depths of more than 600 km. Laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting. This instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser, finer-grained phase. Hence in cold slabs, such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite. It is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs. This consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features. It explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone, where minerals in downgoing slabs should transform to spinel and ilmenite structures. The onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab. Even if a slab penetrates into the lower mantle, earthquakes

  5. Volatile-rich Melts in the Earth's Upper Mantle (AGU Kuno Medal)

    NASA Astrophysics Data System (ADS)

    Dasgupta, Rajdeep

    2013-04-01

    The onset of silicate magma generation in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic fluid with variable water content could be stable ≤250 km beneath mid oceanic ridges [1-3], owing to the small fraction (<< 1 wt.%), its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present down to ~200 km [4-6] but large melt fractions is thought to be restricted to shallower depths. In this Kuno Award lecture, I will discuss the recent advancements on our understanding of deeper silicate melt generation induced by CO2-H2O volatiles and the relative stability of silicate versus carbonatitic melt in various tectonic settings. I will present recent experiments on carbonated peridotites that constrain the location and the slope of the onset of silicate melting in the mantle [7]. The new finding is that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and as a consequence the silicate melting of dry peridotite+CO2 beneath ridges commences at ~180 km. Accounting for the effect of 50-200 ppm of mantle H2O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ~100 ppm CO2 becomes as deep as ~220-300 km [7]. This melting generates a kimberlitic magma with ~25 wt.% dissolved CO2 and 1-5 wt.% dissolved H2O. Based on the recent constraints of oxygen fugacity of the mantle in the garnet peridotite field [2, 3], we suggest that on a global scale, carbonated silicate melt generation at ~250-180 km deep redox solidus, with destabilization of metal and majorite in the upwelling mantle, explains oceanic low-velocity zone and electrical conductivity structure of the mantle. In locally oxidized domains (i.e., higher than average Fe3+/Fetotal), deeper carbonated

  6. Mantle Evolution Associated With the Rio Grande Rift: Geochemistry of Upper Mantle Xenoliths

    NASA Astrophysics Data System (ADS)

    Kil, Y.; Wendlandt, R. F.

    2001-12-01

    Upper mantle xenoliths from three locales associated with the southern Rio Grande Rift have been investigated to determine lithosphere composition, chemical processes, and pre-eruptive pressure and temperature conditions. Sample locations, Potrillo and Elephant Butte within the rift axis and Adam's Diggings, located 50 km west of the rift axis, were specifically selected to evaluate spatial differences in mantle evolution. Xenolith suites from all locations included spinel lherzolites, harzburgites, and pyroxenites hosted in basanite and alkali basalt. Thin section, electron microprobe, and LA-ICPMS analyses were used to obtain detailed textural information, mineral compositions, and whole rock geochemistry. Xenoliths are classified as protogranular, porphyroclastic, or equigranular texture types. Equigranular texture types occur in the off-axis site. Recrystallized olivine grains are larger in xenoliths from sites along the rift axis than from the rift shoulder. Geothermal gradients based on mineral compositions, utilizing two-pyroxene and olivine-spinel geothermometers and the Ca-in olivine geothermobarometer, indicate temperatures off the rift axis at Adam's Diggings that are 75o-100oC cooler for a given pressure than under the rift axis. Whole rock chemical data and mineral modes support an early depletion event affecting xenoliths from all locations: Al2O3, CaO, Na2O, TiO2, V, Sc, Yb, and clinopyroxene content decrease with increasing MgO. The average (La/Yb)n of clinopyroxenes are 12.37, 0.95, and 1.14 for Adam's Diggings, Elephant Butte, and Potrillo xenoliths, respectively. This LREE enrichment and the occurrence of phlogopite that is interpreted to be primary in xenoliths from the off-axis site indicate both cryptic and modal metasomatic events. Both LREE-enriched and -depleted lherzolites are present at rift axis sites. Differences in recrystallized olivine size, xenolith textures, composition, and pre-eruptive pressure-temperature conditions between rift

  7. The trapped fluid phase in upper mantle xenoliths from Victoria, Australia: implications for mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Andersen, T.; O'Reilly, Suzanne Y.; Griffin, W. L.

    1984-11-01

    Mantle-derived xenoliths of spinel lherzolite, spinel pyroxenite, garnet pyroxenite and wehrlite from Bullenmerri and Gnotuk maars, southwestern Victoria, Australia contain up to 3 vol.% of fluids trapped at high pressures. The fluid-filled cavities range in size from fluid inclusions (1 100 μm) up to vugs 11/2 cm across, lined with euhedral high-pressure phases. The larger cavities form an integral part of the mosaic microstructure. Microthermometry and Raman laser microprobe analysis show that the fluids are dominantly CO2. Small isolated inclusions may have densities ≥1.19 g/cm3, but most inclusions show microstructural evidence of partial decrepitation during eruption, and these have lower fluid densities. Mass-spectrometric analysis of gases released by crushing or heating shows the presence of He, N2, Ar, H2S, COs and SO2 in small quantities; these may explain the small freezing-point depressions observed in some inclusions. Petrographic, SEM and microprobe studies show that the trapped fluids have reacted with the cavity walls (in clinopyroxene grains) to produce secondary amphiboles and carbonates. The trapped CO2 thus represents only a small residual proportion of an original volatile phase, which has undergone at least two stages of modification — first by equilibration with spinel lherzolite to form amphibole (±mica±apatite), then by limited reaction with the walls of the fluid inclusions. The inferred original fluid was a CO2-H2O mixture, with significant contents of (at least) Cl and sulfur species. Generation of this fluid phase in the garnet-peridotite stability field, followed by its migration to the spinel peridotite stability field, would provide an efficient mechanism for metasomatic enrichment of the upper mantle in LIL elements. This migration could involve either a volatile flux or transport in small volumes of silicate melt that crystallize in the spinel peridotite field. These observations suggest that some portions of the

  8. Crustal and upper mantle velocity structure of the Salton Trough, southeast California

    USGS Publications Warehouse

    Parsons, T.; McCarthy, J.

    1996-01-01

    This paper presents data and modelling results from a crustal and upper mantle wide-angle seismic transect across the Salton Trough region in southeast California. The Salton Trough is a unique part of the Basin and Range province where mid-ocean ridge/transform spreading in the Gulf of California has evolved northward into the continent. In 1992, the U.S. Geological Survey (USGS) conducted the final leg of the Pacific to Arizona Crustal Experiment (PACE). Two perpendicular models of the crust and upper mantle were fit to wide-angle reflection and refraction travel times, seismic amplitudes, and Bouguer gravity anomalies. The first profile crossed the Salton Trough from the southwest to the northeast, and the second was a strike line that paralleled the Salton Sea along its western edge. We found thin crust (???21-22 km thick) beneath the axis of the Salton Trough (Imperial Valley) and locally thicker crust (???27 km) beneath the Chocolate Mountains to the northeast. We modelled a slight thinning of the crust further to the northeast beneath the Colorado River (???24 km) and subsequent thickening beneath the metamorphic core complex belt northeast of the Colorado River. There is a deep, apparently young basin (???5-6 km unmetamorphosed sediments) beneath the Imperial Valley and a shallower (???2-3 km) basin beneath the Colorado River. A regional 6.9-km/s layer (between ???15-km depth and the Moho) underlies the Salton Trough as well as the Chocolate Mountains where it pinches out at the Moho. This lower crustal layer is spatially associated with a low-velocity (7.6-7.7 km/s) upper mantle. We found that our crustal model is locally compatible with the previously suggested notion that the crust of the Salton Trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust. However, we observe an apparently magmatically emplaced lower crust to the northeast, outside of the Salton Trough, and propose that this layer in part

  9. Multiple-frequency tomography of the upper mantle beneath the African/Iberian collision zone

    NASA Astrophysics Data System (ADS)

    Bonnin, Mickaël; Nolet, Guust; Villaseñor, Antonio; Gallart, Josep; Thomas, Christine

    2014-09-01

    During the Cenozoic, the geodynamics of the western Mediterranean domain has been characterized by a complex history of subduction of Mesozoic oceanic lithosphere. The final stage of these processes is proposed to have led to the development of the Calabria and Gibraltar arcs, whose formation is still under debate. In this study, we take advantage of the dense broad-band station networks now available in the Alborán Sea region, to develop a high-resolution 3-D tomographic P velocity model of the upper mantle beneath the African/Iberian collision zone that will better constraint the past dynamics of this zone. The model is based on 13200 teleseismic arrival times recorded between 2008 and 2012 at 279 stations for which cross-correlation delays are measured with a new technique in different frequency bands centred between 0.03 and 1.0 Hz, and for the first time interpreted using multiple frequency tomography. Our model shows, beneath the Alborán Sea, a strong (4 per cent) fast vertically dipping anomaly observed to at least 650 km depth. The arched shape of this anomaly, and its extent at depth, are coherent with a lithospheric slab, thus favouring the hypothesis of a westward consumption of the Ligurian ocean slab by roll-back during Cenozoic. In addition to this fast anomaly in the deep upper mantle, high intensity slow anomalies are widespread in the lithosphere and asthenosphere beneath Morocco and southern Spain. These anomalies are correlated at the surface with the position of the Rif and Atlas orogens and with Cenozoic volcanic fields. We thus confirm the presence, beneath Morocco, of an anomalous (hot?) upper mantle, but without clear indication for a lateral spreading of the Canary plume to the east.

  10. Unified Structural Representation of the southern California crust and upper mantle

    NASA Astrophysics Data System (ADS)

    Shaw, John H.; Plesch, Andreas; Tape, Carl; Suess, M. Peter; Jordan, Thomas H.; Ely, Geoffrey; Hauksson, Egill; Tromp, Jeroen; Tanimoto, Toshiro; Graves, Robert; Olsen, Kim; Nicholson, Craig; Maechling, Philip J.; Rivero, Carlos; Lovely, Peter; Brankman, Charles M.; Munster, Jason

    2015-04-01

    We present a new, 3D description of crust and upper mantle velocity structure in southern California implemented as a Unified Structural Representation (USR). The USR is comprised of detailed basin velocity descriptions that are based on tens of thousands of direct velocity (Vp, Vs) measurements and incorporates the locations and displacement of major fault zones that influence basin structure. These basin descriptions were used to developed tomographic models of crust and upper mantle velocity and density structure, which were subsequently iterated and improved using 3D waveform adjoint tomography. A geotechnical layer (GTL) based on Vs30 measurements and consistent with the underlying velocity descriptions was also developed as an optional model component. The resulting model provides a detailed description of the structure of the southern California crust and upper mantle that reflects the complex tectonic history of the region. The crust thickens eastward as Moho depth varies from 10 to 40 km reflecting the transition from oceanic to continental crust. Deep sedimentary basins and underlying areas of thin crust reflect Neogene extensional tectonics overprinted by transpressional deformation and rapid sediment deposition since the late Pliocene. To illustrate the impact of this complex structure on strong ground motion forecasting, we simulate rupture of a proposed M 7.9 earthquake source in the Western Transverse Ranges. The results show distinct basin amplification and focusing of energy that reflects crustal structure described by the USR that is not captured by simpler velocity descriptions. We anticipate that the USR will be useful for a broad range of simulation and modeling efforts, including strong ground motion forecasting, dynamic rupture simulations, and fault system modeling. The USR is available through the Southern California Earthquake Center (SCEC) website (href="http://www.scec.org).

  11. Upper mantle heterogeneity below the Mid-Atlantic Ridge, 0°-15°N

    NASA Astrophysics Data System (ADS)

    Bonatti, E.; Peyve, A.; Kepezhinskas, P.; Kurentsova, N.; Seyler, M.; Skolotnev, S.; Udintsev, G.

    1992-04-01

    Small-scale variations in composition of mantle-derived peridotites have been investigated in the 0°-15°N portion of the Mid-Atlantic Ridge (MAR), thanks to a relatively close-spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle-equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a "cold" upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter-Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (˜8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogenous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites. Finally, the 15°20' FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°-3°N and 14°-15°N regions is associated spatially with light rare earth element enriched mid-ocean ridge basalt showing a "hot spot"-type geochemical signature. The areal association of refractory

  12. The Effects of Increased Thermal Conductivity and Viscosity on Mixing Rates and Convection Patterns in the Deep Lower Mantle.

    NASA Astrophysics Data System (ADS)

    Naliboff, J. B.; Kellogg, L. H.

    2004-12-01

    Changes in the spin state of iron in both magnetowustite and perovskite at lower mantle conditions may result in increases in radiative thermal transport and viscosity that could suppress convection in the lowermost mantle (Badro et al. 2003, 2004). It has been suggested that such a stagnant layer in the lower mantle could serve as a reservoir for a significant portion of the mantle's incompatible elements, accounting for the isotopic characteristics of hot spots linked to proposed deep-rooted mantle plumes. We investigate the possible effects on mantle dynamics of increases in thermal conductivity and viscosity, using finite-element models of mantle convection in 2-D. Our previous results (Naliboff et al. 2003) showed that increases in thermal conductivity in the lower mantle up to 250 times that in the upper mantle, with otherwise uniform physical properties, fail to isolate a stagnant layer beneath a mid-mantle phase change. When both the viscosity and thermal conductivity increase in the lower mantle, flow velocities through the lower layer and across the boundary decrease. To investigate the rate of mass exchange and mixing in the presence of a partially stagnant layer, we injected tracer particles into the models. We examine mixing in three different classes of models: two models have a viscosity and thermal conductivity change at the mantle mid-point; the third has a viscosity increase at 660 km and a viscosity and thermal conductivity change near 2000 km depth. In models in which the viscosity and thermal conductivity increases by a factor of 10 at the mid-mantle, multi-cell whole-mantle convection rapidly produces a marble cake mantle, leaving no isolated reservoir of material in the lower mantle. Increasing the viscosity and thermal conductivity in the lower mantle by a factor of 50 or 100 produces a relatively stable pattern of convection with a few strong upwellings and downwellings. Although mixing rates decrease and the residence time of material in

  13. Seismic structure of the European upper mantle based on adjoint tomography

    NASA Astrophysics Data System (ADS)

    Zhu, Hejun; Bozdağ, Ebru; Tromp, Jeroen

    2015-04-01

    We use adjoint tomography to iteratively determine seismic models of the crust and upper mantle beneath the European continent and the North Atlantic Ocean. Three-component seismograms from 190 earthquakes recorded by 745 seismographic stations are employed in the inversion. Crustal model EPcrust combined with mantle model S362ANI comprise the 3-D starting model, EU00. Before the structural inversion, earthquake source parameters, for example, centroid moment tensors and locations, are reinverted based on global 3-D Green's functions and Fréchet derivatives. This study consists of three stages. In stage one, frequency-dependent phase differences between observed and simulated seismograms are used to constrain radially anisotropic wave speed variations. In stage two, frequency-dependent phase and amplitude measurements are combined to simultaneously constrain elastic wave speeds and anelastic attenuation. In these two stages, long-period surface waves and short-period body waves are combined to simultaneously constrain shallow and deep structures. In stage three, frequency-dependent phase and amplitude anomalies of three-component surface waves are used to simultaneously constrain radial and azimuthal anisotropy. After this three-stage inversion, we obtain a new seismic model of the European curst and upper mantle, named EU60. Improvements in misfits and histograms in both phase and amplitude help us to validate this three-stage inversion strategy. Long-wavelength elastic wave speed variations in model EU60 compare favourably with previous body- and surface wave tomographic models. Some hitherto unidentified features, such as the Adria microplate, naturally emerge from the smooth starting model. Subducting slabs, slab detachments, ancient suture zones, continental rifts and backarc basins are well resolved in model EU60. We find an anticorrelation between shear wave speed and anelastic attenuation at depths < 100 km. At greater depths, this anticorrelation becomes

  14. Dislocation Damping and Anisotropic Attenuation in the Earth's Upper Mantle

    NASA Astrophysics Data System (ADS)

    Jackson, I.; Farla, R. J.; Fitz Gerald, J. D.; Faul, U.; Zimmerman, M. E.

    2011-12-01

    Seismic anisotropy, attributed to olivine lattice preferred orientation, suggests that tectonic deformation in the Earth's shallow upper mantle involves dislocation creep. Reversible glide of dislocations, generated by the prevailing/fossil tectonic stress, may result in anelastic relaxation that contributes to the reduction of seismic wave speeds and associated attenuation. To test this hypothesis, polycrystalline olivine specimens were synthesised from synthetic (sol-gel) precursors and hot-pressed at high temperature. The hot-pressed material is fully dense, fine-grained and essentially dry and melt-free olivine. Other, coarser-grained material was synthesised from San Carlos olivine powders. These contrasting materials provided the opportunity to distinguish between the influences of grain size and dislocation density. Selected specimens were deformed by dislocation creep either in compression or torsion and characterised for dislocation density via oxidation and backscattered electron imaging. Additionally, the dislocation recovery rate was determined for both olivines at different temperatures and time durations. The results established that a maximum temperature of 1100C should allow a relatively stable dislocation density to be maintained during prolonged mechanical testing (> 50 hours). The shear modulus and associated strain-energy dissipation in both hot-pressed and pre-deformed specimens were subsequently measured at seismic frequencies under conditions of simultaneous high pressure and temperature with torsional forced-oscillation methods. These experiments were carried out with strain amplitudes < 10-5 to permit direct comparison with seismological models. The high-temperature dissipation background, attributed in undeformed fine-grained materials to grain-boundary sliding, and the associated partial relaxation of the shear modulus, are systematically enhanced in the pre-deformed materials - suggesting a role for the dislocations introduced during the

  15. Whole-mantle convection with tectonic plates preserves long-term global patterns of upper mantle geochemistry.

    PubMed

    Barry, T L; Davies, J H; Wolstencroft, M; Millar, I L; Zhao, Z; Jian, P; Safonova, I; Price, M

    2017-05-12

    The evolution of the planetary interior during plate tectonics is controlled by slow convection within the mantle. Global-scale geochemical differences across the upper mantle are known, but how they are preserved during convection has not been adequately explained. We demonstrate that the geographic patterns of chemical variations around the Earth's mantle endure as a direct result of whole-mantle convection within largely isolated cells defined by subducting plates. New 3D spherical numerical models embedded with the latest geological paleo-tectonic reconstructions and ground-truthed with new Hf-Nd isotope data, suggest that uppermost mantle at one location (e.g. under Indian Ocean) circulates down to the core-mantle boundary (CMB), but returns within ≥100 Myrs via large-scale convection to its approximate starting location. Modelled tracers pool at the CMB but do not disperse ubiquitously around it. Similarly, mantle beneath the Pacific does not spread to surrounding regions of the planet. The models fit global patterns of isotope data and may explain features such as the DUPAL anomaly and long-standing differences between Indian and Pacific Ocean crust. Indeed, the geochemical data suggests this mode of convection could have influenced the evolution of mantle composition since 550 Ma and potentially since the onset of plate tectonics.

  16. Signal restoration through deconvolution applied to deep mantle seismic probes

    NASA Astrophysics Data System (ADS)

    Stefan, W.; Garnero, E.; Renaut, R. A.

    2006-12-01

    We present a method of signal restoration to improve the signal-to-noise ratio, sharpen seismic arrival onset, and act as an empirical source deconvolution of specific seismic arrivals. Observed time-series gi are modelled as a convolution of a simpler time-series fi, and an invariant point spread function (PSF) h that attempts to account for the earthquake source process. The method is used on the shear wave time window containing SKS and S, whereby using a Gaussian PSF produces more impulsive, narrower, signals in the wave train. The resulting restored time-series facilitates more accurate and objective relative traveltime estimation of the individual seismic arrivals. We demonstrate the accuracy of the reconstruction method on synthetic seismograms generated by the reflectivity method. Clean and sharp reconstructions are obtained with real data, even for signals with relatively high noise content. Reconstructed signals are simpler, more impulsive, and narrower, which allows highlighting of some details of arrivals that are not readily apparent in raw waveforms. In particular, phases nearly coincident in time can be separately identified after processing. This is demonstrated for two seismic wave pairs used to probe deep mantle and core-mantle boundary structure: (1) the Sab and Scd arrivals, which travel above and within, respectively, a 200-300-km-thick, higher than average shear wave velocity layer at the base of the mantle, observable in the 88-92 deg epicentral distance range and (2) SKS and SPdiff KS, which are core waves with the latter having short arcs of P-wave diffraction, and are nearly identical in timing near 108-110 deg in distance. A Java/Matlab algorithm was developed for the signal restoration, which can be downloaded from the authors web page, along with example data and synthetic seismograms.

  17. Modeling the Crust and Upper Mantle in Northern Beata Ridge (CARIBE NORTE Project)

    NASA Astrophysics Data System (ADS)

    Núñez, Diana; Córdoba, Diego; Cotilla, Mario Octavio; Pazos, Antonio

    2016-05-01

    The complex tectonic region of NE Caribbean, where Hispaniola and Puerto Rico are located, is bordered by subduction zone with oblique convergence in the north and by incipient subduction zone associated to Muertos Trough in the south. Central Caribbean basin is characterized by the presence of a prominent topographic structure known as Beata Ridge, whose oceanic crustal thickness is unusual. The northern part of Beata Ridge is colliding with the central part of Hispaniola along a transverse NE alignment, which constitutes a morphostructural limit, thus producing the interruption of the Cibao Valley and the divergence of the rivers and basins in opposite directions. The direction of this alignment coincides with the discontinuity that could explain the extreme difference between west and east seismicity of the island. Different studies have provided information about Beata Ridge, mainly about the shallow structure from MCS data. In this work, CARIBE NORTE (2009) wide-angle seismic data are analyzed along a WNW-ESE trending line in the northern flank of Beata Ridge, providing a complete tectonic view about shallow, middle and deep structures. The results show clear tectonic differences between west and east separated by Beata Island. In the Haiti Basin area, sedimentary cover is strongly influenced by the bathymetry and its thickness decreases toward to the island. In this area, the Upper Mantle reaches 20 km deep increasing up to 24 km below the island where the sedimentary cover disappears. To the east, the three seamounts of Beata Ridge provoke the appearance of a structure completely different where sedimentary cover reaches thicknesses of 4 km between seamounts and Moho rises up to 13 km deep. This study has allowed to determine the Moho topography and to characterize seismically the first upper mantle layers along the northern Beata Ridge, which had not been possible with previous MCS data.

  18. Formation of harzburgite by pervasive melt/rock reaction in the upper mantle

    USGS Publications Warehouse

    Kelemen, P.B.; Dick, H.J.B.; Quick, J.E.

    1992-01-01

    Many mantle peridotite samples are too rich in SiO2 (in the form of orthopyroxene) and have ratios of light to heavy rare earth elements that are too high to be consistent with an origin as the residuum of partial melting of the primitive mantle. Trace element studies of melt/rock reaction zones in the Trinity peridotite provide evidence for reaction of the mantle lithosphere with ascending melts, which dissolved calcium-pyroxene and precipitated orthopyroxene as magma mass decreased. This process can account for the observed major and trace element compositions of lithospheric mantle samples, and may accordingly be prevalent in the upper mantle.

  19. Upper Mantle Structure Around the Trans-European Suture Zone

    NASA Astrophysics Data System (ADS)

    Janutyte, Ilma; Majdanski, Mariusz; Voss, Peter H.; Kozlovskaya, Elena

    2014-05-01

    The Trans-European Suture Zone (TESZ) is the transition between old Proterozoic lithosphere in Northern and Eastern Europe and the younger Phanerozoic lithosphere in Central and Western Europe. The presented study is a part of the PASSEQ 2006-2008 project which is linked to the TOR project realized during 1996-1997. The PASSEQ and the TOR projects aimed to study the lithosphere and asthenosphere structure around the TESZ, but the latter was focused on the northwestern part of the TESZ between Sweden and Denmark - Germany, while the PASSEQ project was focused on the TESZ mainly beneath Poland. During the PASSEQ project 139 short-period and 49 broadband temporary seismic stations were deployed along the transect stretching from Germany throughout Czech Republic and Poland to Lithuania. The array recorded continuous seismic data from May, 2006 to June, 2008. In our study we used data of all available PASSEQ seismic stations and seismic stations of the national seismological networks of the participating countries and compiled a data set of teleseismic P-wave arrivals. The full data set consists of 8308 manually picked arrivals. Due to limited computational power we used the data of the highest quality only, i.e. 6008 picks. The non-linear teleseismic tomography algorithm TELINV was used to obtain the model of P-wave velocity perturbations in the upper mantle around the TESZ. We recovered the upper mantle structure from 70 km down to 350 km in the study area. The results show ±6.5 % P-wave velocity variations compared to the IASP91 velocity model. We found higher velocities beneath the old East European Craton (EEC) east of the TESZ and lower ones beneath the younger Western Europe west of the TESZ. The thickest litosphere was found beneath the EEC (Lithuania) where the higher velocities continue to about 300 km or even more. To the west of the TESZ under the Variscides the average depth of the lithosphere-asthenosphere boundary (LAB) is about 100 km. The TESZ appears

  20. A kinematic model for the late Cenozoic development of southern California crust and upper mantle

    NASA Technical Reports Server (NTRS)

    Humphreys, Eugene D.; Hager, Bradford H.

    1990-01-01

    A model is developed for the young and ongoing kinematic deformation of the southern California crust and upper mantle. The kinematic model qualitatively explains both the overall seismic structure of the upper mantle and much of the known geological history of the late Cenozoic as consequences of ongoing convection beneath southern California. In this model, the high-velocity upper-mantle anomaly of the Transverse ranges is created through the convergence and sinking of the entire thickness of subcrustal lihtosphere, and the low-velocity upper-mantle anomaly beneath the Salton Trough region is attributed to high temperatures and 1-4 percent partial melt related to adiabatic decompression during mantle upwelling.

  1. A kinematic model for the late Cenozoic development of southern California crust and upper mantle

    NASA Technical Reports Server (NTRS)

    Humphreys, Eugene D.; Hager, Bradford H.

    1990-01-01

    A model is developed for the young and ongoing kinematic deformation of the southern California crust and upper mantle. The kinematic model qualitatively explains both the overall seismic structure of the upper mantle and much of the known geological history of the late Cenozoic as consequences of ongoing convection beneath southern California. In this model, the high-velocity upper-mantle anomaly of the Transverse ranges is created through the convergence and sinking of the entire thickness of subcrustal lihtosphere, and the low-velocity upper-mantle anomaly beneath the Salton Trough region is attributed to high temperatures and 1-4 percent partial melt related to adiabatic decompression during mantle upwelling.

  2. Supercritical Clinopyroxene in Upper Mantle Peridotites and their Bearing on the Composition of Mantle Melts

    NASA Astrophysics Data System (ADS)

    Muntener, O.; Pilet, S.; Ulianov, A.; Vonlanthen, P.

    2011-12-01

    It is generally accepted that a fertile upper mantle compositions consists of the assemblage olivine, clinopyroxene, orthopyroxene and an aluminous phase, either plagioclase, spinel or garnet. Along the solidus, the composition of the constituent phases changes as a function of pressure and temperature. Several experimental studies showed that clinopyroxene-orthopyroxene phase relations are more complex than suggested by residual peridotite mineral assemblages. At pressures less than about 18 kbar and/or low Mg numbers, pigeonite might be stable at solidus temperatures of upper mantle compositions. At higher pressures, once the solidus temperature is exceeding the cpx - pigeonite solvus, the clinopyroxene becomes supercritical and displays very low CaO contents (1,2). Since these complex phase relations occur at high temperatures and pressures, and might be quenchable in high pressure experiments, they are difficult to observe in the natural rock record. In this contribution, we present results from serpentinized spinel harzburgites from ODP leg 210 from the Newfoundland margin that preserve relics of supercritical cpx coexisting with opx, spinel and olivine. Cr numbers reach up to 0.66 for spinels and up to 0.3 for clinopyroxene and are among the most refractory samples from abyssal peridotites observed worldwide. We present textural and chemical data that suggest the presence of exsolved cpx coexisting with opx in highly depleted spinel peridotite. Actual compositions indicate that the final equilibration occurred at temperatures around 870 °C as determined by two-pyroxene thermometry. Combined with geological reasoning these temperatures suggest that the drilled peridotites represent ancient refractory mantle that is unrelated to the Iberia Newfoundland rift (3). Digital images were used to recombine the primary pyroxene composition and we calculate an XCa on the M2-site in cpx of 0.24 to 0.29. Electron backscatter diffraction (EBSD) of exsolved cpx and opx

  3. Density Structure of the Upper Mantle in the Middle East and Surroundings: Interaction of Diverse Tectonic Processes

    NASA Astrophysics Data System (ADS)

    Kaban, M. K.; El Khrepy, S.; Al-Arifi, N. S.

    2015-12-01

    The Middle East is a very complex region combining several tectonic regimes, which are linked together. Density heterogeneity of the upper mantle, which is related to temperature and compositional variations, is one of the principal factors governing tectonic processes. Therefore, a comprehensive density model of the upper mantle is a key for understanding of these processes. Here we use seismic, gravity and tomography data to construct a 3D density model of the lithosphere and upper mantle and to identify main factors responsible for density variations. At the first stage we use a recent crustal model (Stolk et al., 2013) to estimate gravity effect of the crust and to remove it from the observed fields. As a result, the residual mantle gravity anomalies and residual topography are calculated. In addition we remove the impact of deep density variations below 325 km as estimated by a recent instantaneous dynamic model of the mantle (Kaban et al., 2014). We invert the residual fields jointly with seismic tomography data to image density distribution within the crust and upper mantle. The inversion technique accounts for the fact that the residual gravity and residual topography are controlled by the same factors but in a different way, e.g. depending on depth and wavelength. This provides a possibility for remarkably better vertical resolution of the resulting density model. As the initial approximation, we employ the seismic tomography model of Schaeffer and Lebedev (2013). Velocity variations are converted to density by applying mineral physics constrains. This model is adjusted in the inversion to fit both residual mantle gravity and topography. The obtained density variations are very significant; their amplitude somewhere exceeds 60 kg/m3 relative to a reference model. The most pronounced decrease of the mantle density corresponds to the Gulf of Aden spreading axis, the Red sea and the Afar zone. The maximum density of the upper mantle is associated with the

  4. Effect of phase transformations on microstructures in deep mantle materials

    NASA Astrophysics Data System (ADS)

    Merkel, Sébastien; Langrand, Christopher; Rosa, Angelika; Hilairet, Nadège

    2017-04-01

    Phase transformations induce microstructural changes in deep Earth materials, including changes in grain size and orientation distribution. The effect of phase transformations on mineral microstructures is usually studied using electron microscopy on quench products from high P/T experiments. The method allows for a precise evaluation of the microscopic mechanisms involved. It is limited, however, to samples that can be quenched to ambient conditions and allows for investigations at a single P/T point for each experiment. In recent years, we extended the use of multigrain crystallography to samples inside diamond anvil cells under mantle P/T conditions. The method allows for monitoring the orientations of hundreds of grains and grain size variations during various physical processes, such as plastic deformation and successions of phase transformations (Rosa et al 2015, Langrand et al 2017). Here, we will show results concerning hydrous Mg2SiO4 during the series of α-β-γ phase transformations up to 40 GPa and 850 °C. Such results are important to understand the descending behaviour of subducted slabs, observations of seismic anisotropy, and polarity changes for seismic waves reflected of deep Earth interfaces. The data is used to asses the effect of the transformation on grain orientation and grain sizes. In particular, we do not observe orientation relationships between the parent α-phase and the daughter β-phase phase, suggesting an incoherent growth. We also observe significant grain size reductions and only little grain growth within the newly formed phases (Rosa et al 2016). These new results are important for understanding the mechanical behavior of subducting slabs, seismic anisotropy in the Earth's mantle, and phase transformation mechanisms in olivine. Now that it is validated, the method can also be applied to other phases that can not be studied using electron microscopy, such as perovskite and post-perovskite. Langrand, Hilairet, Nisr, Roskosz, Rib

  5. Crust and upper mantle electrical conductivity beneath the Yellowstone Hotspot Track

    NASA Astrophysics Data System (ADS)

    Kelbert, A.; Egbert, G. D.

    2012-12-01

    We have used high-quality electromagnetic data obtained through the EarthScope USArray project to obtain detailed three-dimensional images of electrical resistivity / conductivity in the crust and upper mantle beneath the Snake River Plain/Yellowstone (SRP/Y) volcanic province (Idaho and Wyoming, United States). The lowest resistivities in the area can only plausibly be explained by partial melt and/or fluids, providing valuable new information about the distribution of these phases deep within the Earth beneath the volcanic system. Unexpectedly, in light of the mantle plume models often used to explain Yellowstone volcanism, the electromagnetic data imply that there is no interconnected melt in the lower crust and uppermost mantle directly beneath the modern Yellowstone caldera. Instead, low resistivities consistent with 1-3% melt in the uppermost mantle (depths of 40-80 km) extend at least 200 km southwest of Yellowstone. Shallower areas of reduced resistivity extend upward into the mid-crust around the edges of the seemingly impermeable Snake River Plain province, including beneath Yellowstone. We suggest that the elevated temperatures beneath the active volcanic center have resulted in greater permeability, allowing magma to ascend to shallower depths and pool in the crust. Little melt is entering the system from below at present, perhaps due to intermittency of supply. We describe these results in the context of larger scale electrical resistivity and seismic tomography models of the western US and employ joint interpretation to formulate hypotheses that would explain this unexpected melt distribution beneath the SRP/Y. Our 3-D model is available at http://www.iris.edu/dms/products/emc/models/SRPY-MT.htm

  6. Double layering of thermochemical-plume material can reconcile upper-mantle seismic velocity structure beneath Hawaii

    NASA Astrophysics Data System (ADS)

    Ballmer, M. D.; Ito, G.; Wolfe, C. J.; Laske, G.; Solomon, S. C.

    2011-12-01

    Volcanism far from plate boundaries, in Hawaii and elsewhere, has traditionally been explained by "classical" plume theory. Classical plumes are typically described as narrow thermal upwellings that rise through the entire mantle to be deflected into a thin (<100 km), bilaterally symmetric "pancake" beneath the overriding lithosphere. New high-resolution seismic velocity images obtained from the PLUME seismic experiment indeed support the concept of a deep-rooted mantle plume to feed Hawaiian volcanism. However, in detail these images challenge classical concepts inasmuch as they indicate a low-velocity body in the upper mantle that is too thick (~400 km) and asymmetric to be interpreted as a pancake. Classical plumes are, moreover, inconsistent with geochemical aspects of Hawaiian volcanism, which indicate a heterogeneous mantle source involving mafic lithologies such as eclogite, and not an exclusively thermal (i.e., isochemical) origin. To explore the dynamical behavior and melting of plumes with a substantial fraction of eclogite (10-18%), we performed thermochemical three-dimensional numerical experiments. Relative to the ambient-mantle peridotite, eclogite is intrinsically dense. This chemical density contrast is sensitive to phase changes in the upper mantle peaking at depths of 410-300 km and fading at 250-190 km, where eclogite is removed by partial melting. For models with an eclogite content >12%, these effects cause a complex regime of plume upwelling. The thermochemical plume forms a broad and thick pool at depths of 480-300 km (deep eclogite pool, or DEP), from which one or two secondary plumes rise to feed a hot shallow pancake that supports the seafloor swell. The rising secondary plumes undergo decompression melting at their deflection points to supply shield stage and rejuvenated stage volcanism. Their transience in vigor can reconcile observations of temporal variability of Hawaiian hotspot volcanism. The double layering of hot plume material

  7. Illuminating the upper mantle beneath the Newer Volcanics province, southeast Australia, using seismic body wave tomography

    NASA Astrophysics Data System (ADS)

    Rawlinson, N.; Sandiford, M.

    2012-12-01

    model shows a clear zone of low velocity underlying the NVP (maximum perturbation of -4% relative to AK135). It clearly extends to a depth of just over 200 km, before terminating, with no evidence of reduced velocities down to approximately 300 km, the maximum depth resolution of the seismic data. Furthermore, nearer the surface (~100km depth), there appears to be three distinct regions of low velocity that are distributed E-W between central Victoria and Mt. Gambier near the South Australian border. The lack of evidence for a deep seated anomaly is consistent with the hypothesis that the source of the NVP is confined to the upper mantle, although an important caveat is that plumes are expected to be narrow as they rise through the mantle before broadening out as they encounter the base of the lithosphere; as such, it s possible that the limited spatial resolution of the data (approximately 50 km) is unable to detect narrow vertical structures at depth. However, combined with the observations discussed earlier, our results strengthen the argument for a localized upper mantle anomaly.

  8. Upper mantle P velocities beneath the North America craton

    NASA Astrophysics Data System (ADS)

    Chu, R.; Helmberger, D. V.

    2010-12-01

    In this work, we determined the detailed mechanisms of three earthquakes occurring in Quebec, Texas and Idaho for use in modeling triplication data. The first event provided pure-path triplication recordings at over 400 USArray stations. Although amplitudes of the direct P waves are small, the depth phase sP is clear and displays shadow-zone characteristics indicative a low velocity layer (LVL) in the upper mantle, where the amplitude of the AB branch decreases rapidly at a distance of 16 degrees. Another feature of the LVL is that the AB branch can be seen at distances larger than 23 degrees. Similar to the Canadian Shield velocity model S25 (LeFevre and Helmberger, 1989), we found a LVL between 160 km and 215 km and obtained excellent fits assuming 1D model. The other two events are located near the craton margins and have been recorded by the MOMA array (Texas event) and CANOE array (Idaho event). These mixed paths are mostly craton with modified 1D models producing good fits. We, also, produced 2D modeling results that use tomographic images for correcting the source structures.

  9. Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth’s upper mantle

    PubMed Central

    Ohuchi, Tomohiro; Kawazoe, Takaaki; Higo, Yuji; Funakoshi, Ken-ichi; Suzuki, Akio; Kikegawa, Takumi; Irifune, Tetsuo

    2015-01-01

    Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth’s upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 1019.6 to 1020.7 Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size–sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle. PMID:26601281

  10. Calculations of upper-mantle velocity from published Soviet earthquake data

    USGS Publications Warehouse

    Rodriquez, Robert G.

    1965-01-01

    The lack of information on mantle velocities and crustal structure of the U.S.S.R. has led to a preliminary examination of published Soviet earthquake bulletins in the hope of deriving useful velocity and structure information from the data they contain. Mantle velocities deduced from earthquake data on several Russian earthquakes are in excellent agreement with results of Soviet deep seismic sounding.

  11. Global Transition Zone Anisotropy and Consequences for Mantle Flow and Earth's Deep Water Cycle

    NASA Astrophysics Data System (ADS)

    Beghein, C.; Yuan, K.

    2011-12-01

    The transition zone has long been at the center of the debate between multi- and single-layered convection models that directly relate to heat transport and chemical mixing throughout the mantle. It has also been suggested that the transition zone is a reservoir that collects water transported by subduction of the lithosphere into the mantle. Since water lowers mantle minerals density and viscosity, thereby modifying their rheology and melting behavior, it likely affects global mantle dynamics and the history of plate tectonics. Constraining mantle flow is therefore important for our understanding of Earth's thermochemical evolution and deep water cycle. Because it can result from deformation by dislocation creep during convection, seismic anisotropy can help us model mantle flow. It is relatively well constrained in the uppermost mantle, but its presence in the transition zone is still debated. Its detection below 250 km depth has been challenging to date because of the poor vertical resolution of commonly used datasets. In this study, we used global Love wave overtone phase velocity maps, which are sensitive to structure down to much larger depths than fundamental modes alone, and have greater depth resolution than shear wave-splitting data. This enabled us to obtain a first 3-D model of azimuthal anisotropy for the upper 800km of the mantle. We inverted the 2Ψ terms of anisotropic phase velocity maps [Visser, et al., 2008] for the first five Love wave overtones between 35s and 174s period. The resulting model shows that the average anisotropy amplitude for vertically polarized shear waves displays two main stable peaks: one in the uppermost mantle and, most remarkably, one in the lower transition zone. F-tests showed that the presence of 2Ψ anisotropy in the transition zone is required to improve the third, fourth, and fifth overtones fit. Because of parameter trade-offs, however, we cannot exclude that the anisotropy is located in the upper transition zone as

  12. Measuring Anisotropy in the Oceanic Upper Mantle from Splitting in SS Waveforms

    NASA Astrophysics Data System (ADS)

    Niu, F.; Silver, P. G.; Behn, M. D.

    2004-05-01

    The anisotropic properties of the upper mantle beneath a seismic station are most directly estimated from the splitting in core phases, such as SKS. This approach has been very successful in constraining the anisotropy beneath continents, by exploiting the high density of continental stations available. The relatively small number of oceanic stations makes it more difficult to extract the anisotropic properties of the oceanic upper mantle from splitting. It has nevertheless been shown (Behn et al. 2004) that these ocean-station measurements are directly related to asthenospheric flow and can be used to estimate the subasthenospheric flow velocity field in the oceanic upper mantle. Motivated by the high value of oceanic splitting measurements, we have begun a project to map oceanic mantle anisotropy using shear-wave splitting in the phase SS, whose bounce points provide excellent global coverage of the ocean basins. In order to isolate the anisotropy at the bounce point, however, it is necessary to first account for two influences: (i) Moho reverberations at the bounce point, and (ii) anisotropy beneath the source and receiver. Measurement of splitting in synthetic SS waveforms for a model with an oceanic Moho gives rise to an apparent splitting in SS (transverse leading radial) with a delay time as long as 10s. This apparent splitting is produced by several arrivals that include the precursory reflection at the Moho, followed by several later Moho reverberations. The reflection series, when filtered to the low-frequency band of observed SS waveforms (~20 s), provides an explanation for large observed apparent SS delay times, such as those measured by Wolfe and Silver (1998). If we assume that the crustal structure at the bounce point is known (a good assumption given the simplicity of the oceanic crust), it is possible compute a Moho reverberation operator, and use it to correct the observed seismograms through deconvolution. Regarding source-side and receiver

  13. Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere

    NASA Astrophysics Data System (ADS)

    Mikhail, Sami; Sverjensky, Dimitri A.

    2014-11-01

    Volatile elements stored in the mantles of terrestrial planets escape through volcanic degassing, and thereby influence planetary atmospheric evolution and habitability. Compared with the atmospheres of Venus and Mars, Earth's atmosphere is nitrogen-rich relative to primordial noble gas concentrations. The compatibility of volatile elements in mantle minerals versus melts and fluids controls how readily these elements are degassed. However, the speciation of nitrogen in mantle fluids is not well constrained. Here we present thermodynamic calculations that establish the speciation of nitrogen in aqueous fluids under upper mantle conditions. We find that, under the relatively oxidized conditions of Earth's mantle wedges at convergent plate margins, nitrogen is expected to exist predominantly as N2 in fluids and, therefore, be degassed easily. In contrast, under more reducing conditions elsewhere in the Earth's upper mantle and in the mantles of Venus and Mars, nitrogen is expected predominantly in the form of ammonium (NH4+) in aqueous fluids. Ammonium is moderately compatible in upper mantle minerals and unconducive to nitrogen degassing. We conclude that Earth's oxidized mantle wedge conditions--a result of subduction and hence plate tectonics--favour the development of a nitrogen-enriched atmosphere, relative to the primordial noble gases, whereas the atmospheres of Venus and Mars have less nitrogen because they lack plate tectonics.

  14. The upper-mantle transition zone beneath the Chile-Argentina flat subduction zone

    NASA Astrophysics Data System (ADS)

    Bagdo, Paula; Bonatto, Luciana; Badi, Gabriela; Piromallo, Claudia

    2016-04-01

    The main objective of the present work is the study of the upper mantle structure of the western margin of South America (between 26°S and 36°S) within an area known as the Chile-Argentina flat subduction zone. For this purpose, we use teleseismic records from temporary broad band seismic stations that resulted from different seismic experiments carried out in South America. This area is characterized by on-going orogenic processes and complex subduction history that have profoundly affected the underlying mantle structure. The detection and characterization of the upper mantle seismic discontinuities are useful to understand subduction processes and the dynamics of mantle convection; this is due to the fact that they mark changes in mantle composition or phase changes in mantle minerals that respond differently to the disturbances caused by mantle convection. The discontinuities at a depth of 410 km and 660 km, generally associated to phase changes in olivine, vary in width and depth as a result of compositional and temperature anomalies. As a consequence, these discontinuities are an essential tool to study the thermal and compositional structure of the mantle. Here, we analyze the upper-mantle transition zone discontinuities at a depth of 410 km and 660 km as seen from Pds seismic phases beneath the Argentina-Chile flat subduction.

  15. The Robustness of Tomographically Imaged Broad Plumes in the Deep Mantle: Constraints on Mantle Dynamics

    NASA Astrophysics Data System (ADS)

    Romanowicz, B. A.; Jiménez-Pérez, H.; Adourian, S.; Karaoglu, H.; French, S.

    2016-12-01

    Existing global 3D shear wave velocity models of the earth's mantle generally rely on simple ray theoretical assumptions regarding seismic wave propagation through a heterogeneous medium, and/or consider a limited number of seismic observables, such as surface wave dispersion and/or travel times of body waves (such as P or S) that are well separated on seismograms. While these assumptions are appropriate for resolving long wavelength structure, as evidenced from the good agreement at low degrees between models published in the last 10 years, it is well established that the assumption of ray theory limits the resolution of smaller scale low velocity structures. We recently developed a global radially anisotropic shear wave velocity model (SEMUCB_WM1, French and Romanowicz, 2014, 2015) based on time domain full waveform inversion of 3-component seismograms, including surface waves and overtones down to 60s period, as well as body waveforms down to 30s. At each iteration, the forward wavefield is calculated using the Spectral Element Method (SEM), which ensures the accurate computation of the misfit function. Inversion is performed using a fast converging Gauss-Newton formalism. The use of information from the entire seismogram, weighted according to energy arrivals, provides a unique illumination of the deep mantle, compensating for the uneven distribution of sources and stations. The most striking features of this model are the broad, vertically oriented plume-like conduits that extend from the core-mantle boundary to at least 1000 km depth in the vicinity of some 20 major hotspots located over the large low shear velocity provinces under the Pacific and Africa. We here present the results of various tests aimed at evaluating the robustness of these features. These include starting from a different initial model, to evaluate the effects of non-linearity in the inversion, as well as synthetic tests aimed at evaluating the recovery of plumes located in the middle of

  16. Upper Mantle Structure beneath Afar: inferences from surface waves.

    NASA Astrophysics Data System (ADS)

    Sicilia, D.; Montagner, J.; Debayle, E.; Lepine, J.; Leveque, J.; Cara, M.; Ataley, A.; Sholan, J.

    2001-12-01

    The Afar hotspot is related to one of the most important plume from a geodynamic point of view. It has been advocated to be the surface expression of the South-West African Superswell. Below the lithosphere, the Afar plume might feed other hotspots in central Africa (Hadiouche et al., 1989; Ebinger & Sleep, 1998). The processes of interaction between crust, lithosphere and plume are not well understood. In order to gain insight into the scientific issue, we have performed a surface-wave tomography covering the Horn of Africa. A data set of 1404 paths for Rayleigh waves and 473 paths for Love waves was selected in the period range 45-200s. They were collected from the permanent IRIS and GEOSCOPE networks and from the PASSCAL experiment, in Tanzania and Saudi Arabia. Other data come from the broadband stations deployed in Ethiopia and Yemen in the framework of the French INSU program ``Horn of Africa''. The results presented here come from a path average phase velocities obtained with a method based on a least-squares minimization (Beucler et al., 2000). The local phase velocity distribution and the azimuthal anisotropy were simultaneously retrieved by using the tomographic technique of Montagner (1986). A correction of the data is applied according to the crustal structure of the 3SMAC model (Nataf & Ricard, 1996). We find low velocities down to 200 km depth beneath the Red Sea, the Gulf of Aden, Afars, the Ethiopian Plateau and southern Arabia. High velocities are present in the eastern Arabia and the Tanzania Craton. The anisotropy beneath Afar seems to be complex, but enables to map the flow pattern at the interface lithosphere-asthenosphere. The results presented here are complementary to those obtained by Debayle et al. (2001) at upper-mantle transition zone depths using waveform inversion of higher Rayle igh modes.

  17. Upper mantle dynamics and quaternary climate in cratonic areas (DynaQlim)—Understanding the glacial isostatic adjustment

    NASA Astrophysics Data System (ADS)

    Poutanen, Markku; Ivins, Erik R.

    2010-07-01

    A substantial material flow, deep within the solid Earth, is caused by the periodic ocean-continent water transport of the Quaternary ice ages. That lateral transport is enormous, causing 120-135 m of equivalent global sea-level rise and fall, or about 45-50 Peta tonnes (1 Peta tonne = 10 18 kg) of surface mass transfer. The global manifestation of the slow mantle flow response to this surface load is glacial isostatic adjustment (GIA). Measurements of this phenomenon offer a unique opportunity to retrieve information pertaining to both the Earth's upper mantle and the changing mass of glaciers and ice sheets during the past. The waxing and waning of ice mass is driven by long-term variations in climate. DynaQlim (upper mantle dynamics and quaternary climate in cratonic areas), a regional coordination committee of the International Lithosphere Program (ILP) since 2007, is focused on studying the relations between upper mantle dynamics, its composition and physical properties, temperature, rheology, and Quaternary climate. Combining historical and modern terrestrial and space-borne geodetic observations with seismological investigations, studies of the postglacial faults and continuum mechanical modelling of GIA, the research goal of DynaQlim is to offer new insights into properties of the lithosphere and upper mantle. The joint inversion of different types of observational data is an important step toward providing a better understanding of GIA on all levels of Earth sciences. A primary regional focus of DynaQlim is the study of cratonic areas of northern Canada and Scandinavia. Greenland and Antarctica are also of great interest, as they represent observational examples of ice sheet dynamics and mass change in response to relatively strong present-day climate forcing.

  18. A Preliminary Look at the Crust and Upper Mantle of North Africa Using Libyan Seismic Data

    SciTech Connect

    Pasyanos, M

    2005-08-05

    In recent years, LLNL has been developing methods to jointly invert both surface wave dispersion data and teleseismic receiver functions. The technique holds great promise in accurately estimating seismic structure, including important tectonic parameters such as basin thickness, crustal thickness, upper mantle velocity, etc. We proposed applying this method to some recently available data from several Libyan stations, as we believe the technique has not been applied to any stations in Libya. The technique holds the promise of improving our understanding of the crust and upper mantle in Libya and North Africa. We recently requested seismic data from stations GHAR (Gharyan) and MARJ (Al Marj) in Libya for about 20 events. The events were large events at regional distances suitable for making dispersion measurements. An example of waveforms recorded at the two stations from an earthquake in Italy is shown in Figure 1. The paths traverse the Ionian Sea. Notice the slow short period group velocities of the surface waves across the Mediterranean, particularly to the easternmost station MARJ. However, because of data availability, signal-to-noise ratio, etc. we were unable to make measurements for every one of these events at both stations. Figure 2 shows a map of paths for 20 sec Rayleigh waves in the eastern Mediterranean region. Paths measured at the two Libyan stations are shown in green. Rayleigh wave dispersion measurements at 20 sec period are sensitive to velocities in the upper 20 km or so, and reveal sediment thickness, crustal velocity, and crustal thickness. Tomographic inversions reveal the sharp group velocity contrast between regions with deep sedimentary basins and those without. Figure 3, the result of an inversion made before adding the new dispersion measurements, shows slow group velocities in the Black Sea, Adriatic Sea, and Eastern Mediterranean. In general, these features correspond well with the sediment thickness model from Laske, shown in Figure

  19. Evidence for deep mantle circulation from global tomography

    USGS Publications Warehouse

    Van Der Hilst, R. D.; Widiyantoro, S.; Engdahl, E.R.

    1997-01-01

    Seismic tomography based on P-wave travel times and improved earthquake locations provides further evidence for mantle-wide convective flow. The use of body waves makes it possible to resolve long, narrow structures in the lower mantle some of which can be followed to sites of present-day plate oonvergence at the Earth's surface. The transition from subduction-related linear structures in the mid-mantle to long-wavelength hetorogeneity near the core-mantle boundary remains enigmatic, but at least some slab segments seem to sink to the bottom of the mantle.

  20. Upper Mantle Q and Transmission Studies Using LASA and WWSS Data.

    DTIC Science & Technology

    complicated than the Heiskanen , Pratt, or Airy mechanisms would suggest and extends well into the upper mantle. A long-range seismic refraction study along the axis of the Rocky Mountains supports a double M-discontinuity. (Author)

  1. Peeling linear inversion of upper mantle velocity structure with receiver functions

    NASA Astrophysics Data System (ADS)

    Shen, Xuzhang; Zhou, Huilan

    2012-02-01

    A peeling linear inversion method is presented to study the upper mantle (from Moho to 800 km depth) velocity structures with receiver functions. The influences of the crustal and upper mantle velocity ratio error on the inversion results are analyzed, and three valid measures are taken for its reduction. This method is tested with the IASP91 and the PREM models, and the upper mantle structures beneath the stations GTA, LZH, and AXX in northwestern China are then inverted. The results indicate that this inversion method is feasible to quantify upper mantle discontinuities, besides the discontinuities between 3 h M ( h M denotes the depth of Moho) and 5 h M due to the interference of multiples from Moho. Smoothing is used to overcome possible false discontinuities from the multiples and ensure the stability of the inversion results, but the detailed information on the depth range between 3 h M and 5 h M is sacrificed.

  2. 100 Years of Studies of the Crust and Upper Mantle in Croatia

    NASA Astrophysics Data System (ADS)

    Herak, D.; Herak, M.; Tkalcic, H.

    2009-12-01

    The study of properties of the earth’s crust and upper mantle started in Croatia one century ago, by the seminal paper in which A. Mohorovicic proved the existence of the crust-mantle boundary. Most of Mohorovicic’s work which followed was dedicated to improving travel-time curves of crustal phases. During the 1980-es most of the research—mostly based on interpretation of several deep seismic profiles running across the country—was directed towards determination of the Moho-topography. The interest for the study of elastic properties of the crust was revived in the 1990-es, when several papers appeared which dealt with determination of velocities and attenuation (coda-Q) in the greater circum-Adriatic region. More recently, a large database of Pg-phase arrival times was used to assess the azimuthal anisotropy of the P-wave velocity within the crust in the NW Croatia and in parts of the External Dinarides. It was found that the direction of the fast velocities closely correspond to the direction of predominant current tectonic stress field. Current research based on the data obtained within a large international seismic experiment (ALP2002) lead to new constraints on the crustal structure in northern and western Croatia. Lithospheric structure beneath coastal and continental Croatia is being studied also by broadband teleseismic waveform modeling using receiver functions. The results indicate that the Mohorovicic discontinuity in Dalmatia may lie considerably deeper than presented on recent maps of the Moho topography in Europe.

  3. Stability of Carbonated Eclogite in the Upper Mantle: Experimental Solidus from 2 to 9 GPa

    NASA Astrophysics Data System (ADS)

    Dasgupta, R.; Withers, A. C.; Hirschmann, M. M.

    2003-12-01

    intersects the oceanic geotherm deeper than 400 km. Thus, eclogite cannot host carbonates in the asthenosphere. Carbonated eclogite bodies entering the convecting upper mantle would release carbonate melt in the mantle transition zone. Upon release, this small volume, highly reactive melt could be an effective agent of deep mantle metasomatism. Comparison of our eclogite-CO2 solidus with that of peridotite-CO2 shows a shallower solidus-geotherm intersection for the latter. This implies that carbonated peridotite is a more likely proximal source of magmatic carbon in oceanic provinces. However, carbonated eclogite is a potential source of continental carbonatites, as its solidus crosses the continental shield geotherm at ca. 4 GPa.

  4. A review of crust and upper mantle structure beneath the Indian subcontinent

    NASA Astrophysics Data System (ADS)

    Singh, Arun; Singh, Chandrani; Kennett, B. L. N.

    2015-03-01

    This review presents an account of the variations in crustal and upper mantle structure beneath the Indian subcontinent and its environs, with emphasis on passive seismic results supplemented by results using controlled seismic sources. Receiver function results from more than 600 seismic stations, and over 10,000 km of deep seismic profiles have been exploited to produce maps of average crustal velocities and thickness across the region. The crustal thickness varies from 29 km at the southern tip of India to 88 km under the Himalayan collision zone, and the patterns of variation show significant deviations from the predictions of global models. The average crustal shear velocity (Vs) is low in the Himalaya-Tibet collision zone compared to Indian shield. Major crustal features are as follows: (a) the Eastern Dharwar Craton has a thinner and simpler crustal structure crust than the Western Dharwar Craton, (b) Himalayan crustal thickness picks clearly follow a trend with elevation, (c) the rift zones of the Godavari graben and Narmada-Son Lineament show deeper depths of crust than their surroundings, and (d) most of the Indian cratonic fragments, Bundelkhand, Bhandara and Singhbhum, show thick crust in comparison to the Eastern Dharwar Craton. Heat flow and crustal thickness estimates do not show any positive correlations for India. Estimates of the thickness of the lithosphere show large inconsistencies among various techniques not only in terms of thickness but also in the nature of the transition to the asthenosphere (gradual or sharp). The lithosphere beneath India shows signs of attrition and preservation in different regions, with a highly heterogeneous nature, and does not appear to have been thinned on broader scale during India's rapid motion north towards Asia. The mantle transition zone beneath India is predominantly normal with some clear variations in the Himalayan region (early arrivals) and Southwest Deccan Volcanic Province and Southern Granulite

  5. Structural change in molten basalt at deep mantle conditions.

    PubMed

    Sanloup, Chrystèle; Drewitt, James W E; Konôpková, Zuzana; Dalladay-Simpson, Philip; Morton, Donna M; Rai, Nachiketa; van Westrenen, Wim; Morgenroth, Wolfgang

    2013-11-07

    Silicate liquids play a key part at all stages of deep Earth evolution, ranging from core and crust formation billions of years ago to present-day volcanic activity. Quantitative models of these processes require knowledge of the structural changes and compression mechanisms that take place in liquid silicates at the high pressures and temperatures in the Earth's interior. However, obtaining such knowledge has long been impeded by the challenging nature of the experiments. In recent years, structural and density information for silica glass was obtained at record pressures of up to 100 GPa (ref. 1), a major step towards obtaining data on the molten state. Here we report the structure of molten basalt up to 60 GPa by means of in situ X-ray diffraction. The coordination of silicon increases from four under ambient conditions to six at 35 GPa, similar to what has been reported in silica glass. The compressibility of the melt after the completion of the coordination change is lower than at lower pressure, implying that only a high-order equation of state can accurately describe the density evolution of silicate melts over the pressure range of the whole mantle. The transition pressure coincides with a marked change in the pressure-evolution of nickel partitioning between molten iron and molten silicates, indicating that melt compressibility controls siderophile-element partitioning.

  6. Multi-Scale Imaging of Earth's Deep Interior: New Constraints on Structure and Thermo- chemical Evolution of Earth's Mantle

    NASA Astrophysics Data System (ADS)

    van der Hilst, R. D.; de Hoop, M. V.; Wang, P.; Cao, Q.; Shang, X.

    2008-12-01

    Since seismic tomography began its revolution of global geophysics some 30 years ago we have made tremendous progress in our ability to image and understand structures and processes in Earth's deep interior. The long wavelength (global) structures discovered in pioneering studies in the early 198ies (e.g., Dziewonski and Woodhouse, 1984) have largely survived the test of time, and later studies have pushed the tomographic models to more-and-more detail. As a result, consensus has emerged on the large scale variations in mantle P and S wavespeed, the presence of compositional heterogeneity, and an intermediate style convection that is neither strictly layered nor unobstructed whole-mantle flow. Tomography constrains smooth variations in material properties. To understand better the radial structure of Earth's interior, along with (mass, heat) fluxes across interfaces and boundary layers, we also need constraints on rapid transitions in material properties. Seminal discoveries have been made through analysis of data associated with reflection, refraction, and phase conversion at interfaces. This trend continues, but the explosion in availability of waveforms from broadband seismograph networks all over the world, combined with advances in inverse scattering theory and high-performance computing, has begun to make global "exploration seismics" of deep Earth interfaces possible. We report new results of large-scale, high resolution imaging of the core-mantle boundary region (D") with inverse scattering of ScS and SKKS wavefields (either separately or jointly) and of the upper mantle transition zone with the wavefield that contains SS precursors due to underside reflection at mantle discontinuities. In the future, inverse scattering with wavefields recorded at global networks may enable the systematic scanning of Earth's mantle and CMB region, which in tandem with parallel advances in mineralogy and phase chemistry research may reveal that the mantle that we often think

  7. Global Upper Mantle Structure from Finite-Frequency Surface-Wave Tomography

    NASA Astrophysics Data System (ADS)

    Zhou, Y.; Nolet, G.; Dahlen, F.; Laske, G.

    2004-12-01

    We report global shear-wave velocity structure and radial anisotropy in the upper mantle obtained by finite-frequency surface-wave tomography, based on complete three-dimensional Born sensitivity kernels developed by Zhou et al (2004). Because wavefront healing effects are properly taken into account, finite-frequency surface-wave tomography improves the resolution of small-scale mantle heterogeneities using long-period surface waves. The resulting S-wave velocity models fit the dispersion data better, and show stronger small-scale mantle anomalies compare to traditional ray-theory-based tomographic models. Separate inversions of Love wave (SH-type) and Rayleigh wave (SV-type) dispersion provide insight into the radial anisotropy in the upper mantle. In our model, the globally averaged radial anisotropy is positive (V SH > V SV) (horizontal flow) in the top 220 km, and becomes negative (V SV > V SH) (vertical flow) below 220 km depth. In cratons, both SH and SV velocities show strong fast anomalies down to 250 km depth, and the fast anomalies gradually diminish below 250 km. Radial anisotropy beneath cratons is positive, which largely agrees with a recent global model by Gung et.~al (2003). The old Pacific plate is characterized by strong positive anisotropy with its maximum centered west of Hawaii; this supports an earlier observation on Pacific radial anisotropy by Ekstrom & Dziewonski (1998). The depth extent of mid-ocean ridges and the primary force that drives plate tectonics has been a long-standing question. In our model, ridge anomalies are characterized by strong negative radial anisotropy (vertical flow). Ridge anomalies at fast-spreading centers are stronger than those at slow-spreading centers at shallow depth, but the amount of velocity reduction rapidly decreases below 250 km. However, at slow-spreading centers such as the north Mid-Atlantic ridge (MAR) and East Africa (Red Sea), ridge anomalies extend down at least to the top of the transition zones

  8. Multi-Observable Thermochemical Tomography of the lithosphere and upper mantle

    NASA Astrophysics Data System (ADS)

    Afonso, J. C.; Yang, Y.; Rawlinson, N.; Jones, A. G.; Fullea, J.; Qashqai, M.

    2015-12-01

    Current knowledge of the present-day physical state and structure of the lithosphere and upper mantle essentially derives from four independent sources: i) gravity field and thermal modelling, ii) modelling/inversion of different seismic datasets, iii) magnetotelluric studies, and iv) thermobarometric and geochemical data from exhumed mantle samples. Unfortunately, the integration of these different sources of information in modern geophysical studies is still uncommon and significant discrepancies and/or inconsistencies in predictions between these sources are still the rule rather than the exception.In this contribution we will present a thermodynamically-constrained multi-observable probabilistic inversion method capable of jointly inverting i) surface and body wave datasets, gravity anomalies, geoid height, gravity gradients, receiver functions, surface heat flow, magnetotelluric data, and elevation (static and dynamic) in 3D spherical coordinates. Key aspects of the method are: (a) it combines multiple geophysical observables with different sensitivities to deep/shallow, thermal/compositional anomalies into a single thermodynamic-geophysical framework; (b) it works with thermophysical models of the Earth rather than with parameterized structures of physical parameters (e.g. Vs, Vp, density, etc), (c) it uses a general probabilistic (Bayesian) formulation to appraise the data; (d) no initial model is needed; (e) a priori compositional information relies on robust statistical analyses of a large database of natural mantle samples; (f) it provides a natural platform to estimate realistic uncertainties; (g) it handles multiscale parameterizations and complex physical models, and (h) it includes dynamic (convection) effects on surface observables by solving the complete Stokes flow using multi-dimensional decomposition methods. We will present results for both synthetic and real case studies, which serve to highlight the advantages and limitations of this new

  9. 3D Shear Velocity Structure of Crust and Upper Mantle in China From Ambient Noise Tomography

    NASA Astrophysics Data System (ADS)

    Sun, X.; Song, X.; Zheng, S.; Yang, Y.; Ritzwoller, M.

    2008-12-01

    We perform ambient noise tomography of China using the data from the China National Seismic Network and global and PASSCAL stations in the region. We obtain Rayleigh wave group and phase velocity dispersion maps at 1 by 1 degree grids for periods from 8 to 60 s. The results are combined with longer-period dispersion maps from global earthquake-based measurements. We then obtain the 3D shear velocity structure of the crust and upper mantle in China by inverting the dispersion curves at each grid. The inversion results show remarkable features for continental China and in particular the Tibetan Plateau (TP), including slow sedimentary layers of all the major basins at the shallow depth, striking east-west contrasts in Moho depth variation and lithosphere thickness, fast (strong) mid-lower crust and mantle lithosphere in major basins surrounding the TP (Tarim, Ordos, and Sichuan) (in contrast, Qaidam Basin does not have such a "deep root"). These strong blocks thus seem to play an important role in confining the deformation of the TP to be a triangular shape. The Moho changes from plateau to Tarim and Sichuan Basins are quite sharp. The India lithosphere seems to terminate around the Bangong Nujiang Suture as indicated by the fast-slow velocity contrast in the mantle lithosphere, but it seems to extend further north under E. Tibet. In northwest TP, slow anomalies extend from crust to great depth (200 km). A widespread, prominent low-velocity zone is observed in midcrust in the TP, which are generally connected and seem to reach to the surface near the margins of the TP, consistent with the notion of the growth of the TP by crustal channel flow and the extrusion of channel flow materials at the topographic fronts.

  10. Investigations of Eurasian Seismic Sources and Upper Mantle Structure

    DTIC Science & Technology

    1989-05-25

    occurring as the odd xenolith in kimberlites and other igneous intrusions, or exposed in mountain peridotites and ophiolite sequences-and carry with them...are observed frequently in kimberlites , so it seems safe to assume that the proposed reaction occurs in the Earth’s uppermost mantle. Recent...velocities of garnet lherzolites and their geophysical importance, in The Mantle Sample: Inclusions in Kimberlites and Other Volcanics, edited by .. R. Boyd

  11. Potassium:rubidium ratio in ultramafic rocks: differentiation history of the upper mantle.

    PubMed

    Stueber, A M; Murthy, V R

    1966-08-12

    The increase in K:Rb ratio with decrease in potassium content found in basaltic rocks does not seem to apply to ultramafic rocks. The ratios in a series of alpine ultramafic rocks and ultramafic inclusions in basals and kimberlite pipes are about 200 to 500-significantly lower than those in oceanic tholeiites. This characteristic of ultramafic rocks appears to be consistent with a simplified model in which early differentiation of the primitive mantle led to formation of an upper mantle region enriched in alkali elements and having a low K:Rb ratio. Alpine ultramafic rocks may be residuals from such an upper mantle region.

  12. Upper Extremity Deep Vein Thromboses: The Bowler and the Barista

    PubMed Central

    du Breuil, Anne L.; Close, Jeremy

    2016-01-01

    Effort thrombosis of the upper extremity refers to a deep venous thrombosis of the upper extremity resulting from repetitive activity of the upper limb. Most cases of effort thrombosis occur in young elite athletes with strenuous upper extremity activity. This article reports two cases who both developed upper extremity deep vein thromboses, the first being a 67-year-old bowler and the second a 25-year-old barista, and illustrates that effort thrombosis should be included in the differential diagnosis in any patient with symptoms concerning DVT associated with repetitive activity. A literature review explores the recommended therapies for upper extremity deep vein thromboses. PMID:27800207

  13. Teleseismic Body Wave Attenuation in the Upper Mantle beneath the United States

    NASA Astrophysics Data System (ADS)

    Cafferky, S.; Schmandt, B.

    2014-12-01

    EarthScope seismic data provide opportunities to examine mantle properties on a continental scale as the Transportable Array (TA) nears the end of its traverse across the contiguous United States. We use P- and S-wave amplitude spectra from all >M5.7 deep earthquakes recorded by the TA to examine seismic attenuation patterns in the upper mantle. More than 2 million inter-station P-wave spectral ratios were inverted for maps of relative tP* variations across the U.S. in multiple frequency bands between 0.08 - 2 Hz. We plan to have corresponding S-wave results by meeting time. Maps of tP* are strongly correlated (>0.8) for frequency bands of 0.08 - 2 Hz, 0.25 - 2 Hz, 0.08 - 1 Hz, and 0.25 - 1 Hz. The broader the frequency band examined (e.g. 0.08 - 2 Hz), the lower the magnitude in variations of tP*; however, those broader frequency bands still exhibited geographic patterns similar to the narrow frequency bands. We compare our maps' tP* with seismic velocity models and constraints on crustal scattering to assess the physical origin of apparent attenuation. In the tectonically active and high heat flow domain of the western U.S., tP* variations are moderately correlated with thermal variations predicted by tomography studies of seismic velocity. However, contrast in tP* between western Cordillera and the cratonic interior is weaker than predicted by tomography. Additionally some areas of high attenuation are correlated with Precambrian tectonic boundaries within the Laurentian craton. The weak contrast between the western and eastern U.S. and correlations with Precambrian tectonics suggest that elastic scattering due to small-scale (~10 - 100 km) heterogeneity or compositional variations in the lithosphere are major contributors to tP* estimates from deep earthquake spectral ratios. Moderate correlation of tP* with estimates of mantle temperature within the western U.S. suggests deep earthquake spectral ratios do carry some evidence of intrinsic attenuation, but

  14. Upper-mantle structures beneath USArray derived from waveform complexity

    NASA Astrophysics Data System (ADS)

    Sun, Daoyuan; Helmberger, Don

    2011-01-01

    Tomographic imaging of the crust and upper mantle beneath the western United States has greatly improved with the addition of USArray data. These models display many detailed images of both fast and slow blobs penetrating into the transition zone. To study such features, we apply a newly developed technique, called MultiPath Detector analysis, to the SH waveform data. The method simulates each observed body waveform by performing a decomposition; by [S(t)+C×S(t-ΔLR)]/2, where S(t) is the synthetics for a reference model. Time separation ΔLR and amplitude ratio C are needed to obtain a high cross-correlation between a simulated waveform and data. The travel time of the composite waveform relative to the reference model synthetics is defined as ΔT. A simulated annealing algorithm is used to determine the parameters ΔLR and C. We also record the amplitude ratio (Amp) between the synthetics for the reference model relative to the data. Generally, large ΔLR values are associated with low Amp's. Whereas the conventional tomography yields a travel time correction (ΔT), our analysis yields an extra parameter (ΔLR), which describes the waveform complexity. With the array, we can construct a mapping of the gradient of ΔLR with complexity patterns. A horizontal structure introduces waveform complexity along the distance profile (in-plane multipathing). An azimuthally orientation ΔLR pattern indicates a vertical structure with out-of-plane multipathing. Using such maps generated from artificial data, we can easily recognize features produced by dipping fast structures and slow structures (DSS). Many of these features display organized waveform complexity that are distinctly directional indicative of dipping sharp-edges. Here, we process the array data for events arriving from various azimuths and construct maps of multipathing patterns. The similarity between tomographic features and complexity maps is striking. When features are dipping such as the slab structures

  15. Crust and upper mantle of Kamchatka from teleseismic receiver functions

    NASA Astrophysics Data System (ADS)

    Levin, Vadim; Park, Jeffrey; Brandon, Mark; Lees, Jonathan; Peyton, Valerie; Gordeev, Evgenii; Ozerov, Alexei

    2002-11-01

    Teleseismic receiver functions (RFs) from a yearlong broadband seismological experiment in Kamchatka reveal regional variations in the Moho, anisotropy in the supra-slab mantle wedge, and, along the eastern coast, Ps converted phases from the steeply dipping slab. We analyze both radial- and transverse-component RFs in bin-averaged epicentral and backazimuthal sweeps, in order to detect Ps moveout and polarity variations diagnostic of interface depth, interface dip, and anisotropic fabric within the shallow mantle and crust. At some stations, the radial RF is overprinted by near-surface resonances, but anisotropic structure can be inferred from the transverse RF. Using forward modeling to match the observed RFs, we find Moho depth to range between 30 and 40 km across the peninsula, with a gradational crust-mantle transition beneath some stations along the eastern coast. Anisotropy beneath the Moho is required to fit the transverse RFs at most stations. Anisotropy in the lower crust is required at a minority of stations. Modeling the amplitude and backazimuthal variation of the Ps waveform suggests that an inclined axis of symmetry and 5-10% anisotropy are typical for the crust and the shallow mantle. The apparent symmetry axes of the anisotropic layers are typically trench-normal, but trench-parallel symmetry axes are found for stations APA and ESS, both at the fringes of the central Kamchatka depression. Transverse RFs from east-coast stations KRO, TUM, ZUP and PET are fit well by two anisotropic mantle layers with trench-normal symmetry axes and opposing tilts. Strong anisotropy in the supra-slab mantle wedge suggests that the mantle "lithosphere" beneath the Kamchatka volcanic arc is actively deforming, strained either by wedge corner flow at depth or by trenchward suction of crust as the Pacific slab retreats.

  16. Concentration, behavior and storage of H/sub 2/O in the suboceanic upper mantle: implications for mantle metasomatism

    SciTech Connect

    Michael, P.J.

    1988-02-01

    Mid-ocean ridge basalt glasses from the Pacific-Nazca Ridge and the northern Juan de Fuca Ridge were analyzed for H/sub 2/O by gas chromatography. Incompatible element enriched (IEE) glasses have higher H/sub 2/O contents than depleted (IED) glasses. H/sub 2/O increases systematically with decreasing Mg/Mg + Fe/sup 2 +/ within each group. Near-primary IED MORBs have an average of about 800 ppm H/sub 2/O, while near-primary IEE MORBs (with chondrite normalized Nb/Zr or La/Sm approx. 2) have about 2100 ppm H/sub 2/O. If these basalts formed by 10-20% partial melting then the IED mantle source had 100-180 ppm H/sub 2/O, while the IEE source had 250-450 ppm H/sub 2/O. The ratio H/sub 2/O/(Ce + Nd) is fairly constant at 95 +/- 30 for all oceanic basalts from the Pacific. During trace element fractionation in the suboceanic upper mantle, H/sub 2/O behaves more compatibly than K, Rb, Nb, and Cl, but less compatibly than Sm, Zr and Ti. H/sub 2/O is contained mostly in amphibole in the shallow upper mantle. At pressures greater than the amphibole stability limit, it is likely that a significant proportion of H/sub 2/O is contained in a mantle phase which is more refractory than phlogopite at these pressures. The role of H/sub 2/O in mantle enrichment processes is examined by assuming that an enriched component was added. The modeled concentrations of K, Na, Ti and incompatible trace elements in this component are high relative to H/sub 2/O, indicating that suboceanic mantle enrichment is caused by silicate melts such as basanites and not by aqueous fluids.

  17. Crustal and upper mantle seismic structure of the Australian Plate, South Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Melhuish, Anne; Holbrook, W. Steven; Davey, Fred; Okaya, David A.; Stern, Tim

    2005-01-01

    Seismic reflection and refraction data were collected west of New Zealand's South Island parallel to the Pacific-Australian Plate boundary. The obliquely convergent plate boundary is marked at the surface by the Alpine Fault, which juxtaposes continental crust of each plate. The data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary. Arrival times of wide-angle reflected and refracted events from 13 recording stations are used to construct a 380-km long crustal velocity model. The model shows that, beneath a 2-4-km thick sedimentary veneer, the crust consists of two layers. The upper layer velocities increase from 5.4-5.9 km/s at the top of the layer to 6.3 km/s at the base of the layer. The base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end. The lower layer velocities range from 6.3 to 7.1 km/s, and are commonly around 6.5 km/s at the top of the layer and 6.7 km/s at the base. Beneath the lower layer, the model has velocities of 8.2-8.5 km/s, typical of mantle material. The Mohorovicic discontinuity (Moho) therefore lies at the base of the second layer. It is at a depth of around 30 km but shallows over the south-central third of the profile to about 26 km, possibly associated with a southwest dipping detachment fault. The high, variable sub-Moho velocities of 8.2 km/s to 8.5 km/s are inferred to result from strong upper mantle anisotropy. Multichannel seismic reflection data cover about 220 km of the southern part of the modelled section. Beneath the well-layered Oligocene to recent sedimentary section, the crustal section is broadly divided into two zones, which correspond to the two layers of the velocity model. The upper layer (down to about 7-9 s two-way travel time) has few reflections. The lower layer (down to about 11 s two-way time) contains many strong, subparallel reflections. The base of this reflective zone is the Moho. Bi

  18. Upper mantle anisotropy in western Iran: observations from quasi-Love surface wave scattering

    NASA Astrophysics Data System (ADS)

    Sadeghi-Bagherabadi, Amir; Margheriti, Lucia; Aoudia, Abdelkrim; Sobouti, Farhad; Lucente, Francesco Pio; Baccheschi, Paola

    2017-04-01

    The Iranian plateau is made up of different tectonic and structural provinces such as the Zagros and Alborz orogenic belts, the Sanandaj-Sirjan and Urumieh-Dokhtar magmatic arcs, and the active subduction zone of Makran. We use data from a temporary seismic network in western Iran. The network was deployed in 2013 and 2014 and consisted of 63 broadband seismometers installed along three parallel profiles that crossed the Zagros Mountains, central Iran and the Alborz Mountains. Diverse patterns of upper mantle anisotropy in these regions are revealed by recent studies on shear wave splitting of core-refracted phases. Observation of quasi-Love surface waves is a proxy for the lateral gradients of anisotropy. We quantitatively analyzed the relative presence or absence of coupled Love and Rayleigh waves recorded by the temporary seismic stations. The records were filtered between 70 s and 200 s which are sensitive to structures deeper than 100 km. Assuming a horizontal anisotropic symmetry axis, Love to Rayleigh scattering is expected to be maximized when the incoming surface wave direction is at a 45 orientation to the fast anisotropy axis. The presence of quasi-Love is predicted by the geometric relation between the fast axis as inferred from shear wave splitting measurements, and the surface wave back-azimuths. Our coherent observations of SKS measurements and Love-to-Rayleigh scattering suggest a deep origin of anisotropy and allow us to argue for the existence of an upper mantle anisotropic structure with laterally-variable horizontal symmetry axis. The anisotropic pattern so found puts new constraints on the geodynamic models of the Iranian region of Arabia-Eurasia collision zone.

  19. 3SMAC: an a priori tomographic model of the upper mantle based on geophysical modeling

    NASA Astrophysics Data System (ADS)

    Nataf, Henri-Claude; Ricard, Yanick

    1996-05-01

    We present an a priori three-dimensional 'tomographic' model of the upper mantle. We construct this model (called 3SMAC — three-dimensional seismological model a priori constrained) in four steps: we compile information on the thickness of 'chemical' layers in the Earth (water, sediments, upper and lower crust, etc); we get a 3D temperature distribution from thermal plate models applied to the oceans and continents; we deduce the mineralogy in the mantle from pressure and temperature and we finally get a three-dimensional model of density, seismic velocities, and attenuation by introducing laboratory measurements of these quantities as a function of pressure and temperature. The model is thus consistent with various geophysical data, such as ocean bathymetry, and surface heat flux. We use this model to compute synthetic travel-times of body waves, and we compare them with observations. A similar exercise is performed for surface waves and normal modes in a companion paper (Ricard et al., 1996, J. Geophys. Res., in press). We find that our model predicts the bulk of the observed travel-time variations. Both the amplitude and general pattern are well recovered. The discrepancies suggest that tomography can provide useful regional information on the thermal state of the continents. In the oceans, the flattening of the sea-floor beond 70 Ma seems difficult to reconcile with the seismic observations. Overall, our 3SMAC model is both a realistic model, which can be used to test various tomographic methods, and a model of the minimum heterogeneities to be expected from geodynamical modeling. Therefore, it should be a useful a priori model to be used in tomographic inversions, in order to retrieve reliable images of heterogeneities in the transition zone, which should, in turn, greatly improve our understanding of geodynamical processes in the deep Earth. 3SMAC and accompanying software can be retrieved by anonymous ftp at geoscope.ipgp.jussieu.fr.

  20. Incipient fluid migration through the deep mantle by dissolution-precipitation: crystal growth constraints

    NASA Astrophysics Data System (ADS)

    Shatskiy, Anton; Litasov, Konstantin; Borzdov, Yury; Katsura, Tomoo; Ohtani, Eiji

    2010-05-01

    The mechanism and driving forces for transport of incipient C-O-H-bearing fluid/melt species through the Earth's mantle is a key issue in geosciences. Several mechanisms of the volatile transport, mainly solid state diffusion and fluid/melt porous flow have been considered. The grain boundary diffusion of hydrogen and carbon is in the range of 10-10-10-11 m2/s. These values suggest failure of the solid state diffusion hypothesis to explain melt migration. Since the extensive partial melting of the Earth's mantle seems improbable, the porous flow model of fluid or melt migration, accepted for the shallow upper mantle, can not be applied for the volatile transport through the deep mantle. At the mantle conditions water and carbonates are the excellent silicate solvents. Hence the migration of insulated portions of fluid through the solid matrix should proceed by means of the dissolution-precipitation mechanism. The major driving force for this process would be pressure or temperature gradient, differences in stable and metastable phase solubilities, and stress. In order to estimate the reliability of proposed mechanism we measured migration rate of carbonate, water-carbonate, or water-rich liquid layer through the solid silicate matrix at the upper and lower mantle PT conditions. The thermal gradient was employed as a driving force. The kinetic constant of the migration rates were estimated to be 8×10-8 m/s/K for H2O, 5×10-9 m/s/K for K2Mg(CO3)2+2H2O, and 3×10-10 m/s/K for K2Mg(CO3)2 solvents. In order to extrapolate obtained data to the Earth we assumed that (a) mass transfer of silicate components through the melt layer is limited by diffusion and (b) the thickness of the melt layer is not enough to establish convection. The large lateral thermal gradient, 1-4 oC/km, proposed for mantle plumes reveals lateral fluid migration rate relevant to the plume lifetime (25-50 Ma). Nevertheless, the radial (vertical) migration rate is quite slow, about 1 km in 12.5 Ga

  1. Upper Mantle Responses to India-Eurasia Collision in Indochina, Malaysia, and the South China Sea

    NASA Astrophysics Data System (ADS)

    Hongsresawat, S.; Russo, R. M.

    2016-12-01

    We present new shear wave splitting and splitting intensity measurements from SK(K)S phases recorded at seismic stations of the Malaysian National Seismic Network. These results, in conjunction with results from Tibet and Yunnan provide a basis for testing the degree to which Indochina and South China Sea upper mantle fabrics are responses to India-Eurasia collision. Upper mantle fabrics derived from shear wave splitting measurements in Yunnan and eastern Tibet parallel geodetic surface motions north of 26°N, requiring transmission of tractions from upper mantle depths to surface, or consistent deformation boundary conditions throughout the upper 200 km of crust and mantle. Shear wave splitting fast trends and surface velocities diverge in eastern Yunnan and south of 26°N, indicating development of an asthenospheric layer that decouples crust and upper mantle, or corner flow above the subducted Indo-Burma slab. E-W fast shear wave splitting trends southwest of 26°N/104°E indicate strong gradients in any asthenospheric infiltration. Possible upper mantle flow regimes beneath Indochina include development of olivine b-axis anisotropic symmetry due to high strain and hydrous conditions in the syntaxis/Indo-Burma mantle wedge (i.e., southward flow), development of strong upper mantle corner flow in the Indo-Burma wedge with olivine a-axis anisotropic symmetry (i.e., westward flow), and simple asthenospheric flow due to eastward motion of Sundaland shearing underlying asthenosphere. Further south, shear-wave splitting delay times at Malaysian stations vary from 0.5 seconds on the Malay Peninsula to over 2 seconds at stations on Borneo. Splitting fast trends at Borneo stations and Singapore trend NE-SW, but in northern Peninsular Malaysia, the splitting fast polarization direction is NW-SE, parallel to the trend of the Peninsula. Thus, there is a sharp transition from low delay time and NW-SE fast polarization to high delay times and fast polarization directions that

  2. Relationship between observed upper mantle structures and recent tectonic activity across the Southeastern United States

    NASA Astrophysics Data System (ADS)

    Biryol, C. Berk; Wagner, Lara S.; Fischer, Karen M.; Hawman, Robert B.

    2016-05-01

    The lithospheric structure of the Southeastern United States is a product of earlier episodes of continental collision and breakup. The region is located in the interior of the North American Plate, away from active plate margins. However, there is ongoing tectonism in the region with multiple zones of seismicity, uplifting arches, and Cenozoic intraplate volcanism. The mechanisms controlling this activity and the state of stress remain enigmatic. Two important factors are plate strength and preexisting, inherited structures. Here we present new tomographic images of the upper mantle beneath the Southeastern United States, revealing large-scale structural variations in the upper mantle. Examples include the relatively thick lithospheric mantle of stable North America that abruptly thins beneath the Paleozoic Appalachian orogeny, and the slow upper mantle of the Proterozoic Reelfoot rift. Our results also indicate fast seismic velocity patterns that can be interpreted as ongoing lithospheric foundering. This provides a viable explanation for seismicity, uplifting, and young intraplate volcanism. We postulate that not only tectonic inheritance but also continuing lithospheric foundering may control the ongoing activity of the region long after it became a passive margin. Based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere, we propose that piecemeal delamination has occurred beneath the region throughout the Cenozoic, removing a significant amount of reworked/deformed mantle lithosphere. Ongoing lithospheric foundering beneath the eastern margin of stable North America explains significant variations in thickness of lithospheric mantle across the former Grenville deformation front.

  3. The Asthenosphere Melting Regimes Alteration due to Changing Conditions of Upper Mantle

    NASA Astrophysics Data System (ADS)

    Perepechko, Y. V.; Sharapov, V. N.; Sorokin, K., Jr.

    2014-12-01

    Analyzed in the article are different asthenosphere magma generation regimes above the upper mantle hot spots as thermodynamic and geometric parameters of the upper mantle and the conditions on its boundaries vary. The two-layer mantle model is applied to consider the formation of decompression melting areas. The thickness of metasomatically altered lithospheric mantle is determined by the mantle substance rheology and the location of the upper boundary of asthenosphere. We also take into consideration the principal solid state phase transitions by using the mantle substance state equation. The sizes and distribution of hot spots as well as their maximal temperature were defined by the thermodynamic conditions of the perovskite transition existence. The numerical analysis results demonstrate the manifestation of three main mantle dynamics modes; the conditions necessary to form the partial melting zones are not reached; some melting areas with the 30 to 65 Ma existence time do occur; the melting areas that are formed exist permanently. The permanently existing asthenosphere zones are marked by quasiperiodical variation in thickness and the degree of melting. The typical temperatures of a hot spot sharing these modes are the 1740°С and 2020°С correspondingly. The originally presupposed heating degree and the temperature ratio of the upper mantle do influence the decompression melting degree substantially and - to a lesser extent - they influence the size of melting zones. The primary evolution of the second mode is described by the development of a complex system of asthenosphere zones that lead to the occurrence of additional convectional cells dividing the partial melting zone. The variation in the rheological properties of the mantle substance also contributes to the manifestation of the complex structure of asthenosphere zone. The work was made with support of the Russian Foundation for Basic Research grant #12-05-00625.

  4. Duration of the hydrocarbon fluid formation under thermobaric conditions of the Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Mukhina, Elena; Kolesnikov, Anton; Kutcherov, Vladimir

    2016-04-01

    Deep abiogenic formation of hydrocarbons is an inherent part of the Earth's global carbon cycle. It was experimentally confirmed that natural gas could be formed from inorganic carbon and hydrogen containing minerals at pressure and temperature corresponding to the Earth's upper mantle conditions. Reaction between calcite, wustite and water in the large volume device was studied in several works. It was previously proposed that reaction is possible only after 40 minutes of exposure at high pressure and temperature. In this work similar experiment at P = 60 kbar and T = 1200 K were carried out in "Toroid" type chamber with the 5 seconds duration of thermobaric exposure. Gas chromatographic analysis of the reaction products has shown the presence of hydrocarbon mixture comparable to 5 minutes and 6 hours exposure experiments. Based on this fact it is possible to conclude that the reaction of natural gas formation is instant at least at given thermobaric conditions. This experiment will help to better understand the process of deep hydrocarbon generation, particularly its kinetics.

  5. Stress-driven relaxation of heterogeneous upper mantle and time-dependent afterslip following the 2011 Tohoku earthquake

    NASA Astrophysics Data System (ADS)

    Hu, Yan; Bürgmann, Roland; Uchida, Naoki; Banerjee, Paramesh; Freymueller, Jeffrey T.

    2016-01-01

    Understanding of postseismic deformation following great subduction zone earthquakes is complicated by the combined effects of viscoelastic relaxation of earthquake-induced stresses in the upper mantle and time-dependent afterslip on the megathrust. We integrate geodetic observations and constraints on afterslip from small repeating earthquakes on the megathrust to better distinguish contributions from these two postseismic processes. We have developed a three-dimensional, spherical viscoelastic finite element model to study the postseismic deformation of the 2011 Mw9.0 Tohoku earthquake that has been recorded at unprecedented high resolution in space and time. We model stress-driven afterslip in a 2 km thick weak shear zone away from historic rupture zones on the megathrust. We model both the viscoelastic relaxation of the upper mantle and shear zone deformation with a transient Burgers body rheology. The transient Kelvin viscosity is assumed to be one order of magnitude lower than that of the Maxwell viscosity. Viscoelastic relaxation of the mantle wedge alone causes postseismic uplift and seaward motion in the upper plate, opposite to the pattern from relaxation of just the oceanic upper mantle. Afterslip on the fault produces uplift updip of the afterslip zone and subsidence over its downdip edge and mostly seaward motion above the afterslip zone. The best fit Maxwell viscosity of the shear zone at depths ≤50 km is 1017 Pa s, constrained by afterslip estimates from repeating earthquakes. The optimal viscosities of the deep weak shear zone, continental mantle wedge, and oceanic upper mantle are determined to be 5 × 1017 Pa s, 3 × 1019 Pa s, and 5 × 1019 Pa s, respectively. The stress-driven afterslip in the shear zone is up to ~3.5 m in the first 2 years after the earthquake, equivalent to an Mw8.4. Our model reproduces the first-order pattern of the GPS observations both in horizontal and in vertical directions. Seafloor geodetic observations of subsidence

  6. Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Wang, Ze-Zhou; Liu, Sheng-Ao; Liu, Jingao; Huang, Jian; Xiao, Yan; Chu, Zhu-Yin; Zhao, Xin-Miao; Tang, Limei

    2017-02-01

    compositions of MORB. Therefore, preferential melting of spinel in the peridotites may account for the Zn isotopic difference between spinel peridotites and basalts. By contrast, the absence of Zn isotope fractionation between silicate minerals suggests that Zn isotopes are not significantly fractionated during partial melting of spinel-free garnet-facies mantle. If the studied non-metasomatized peridotites represent the refractory upper mantle, mass balance calculation shows that the depleted MORB mantle (DMM) has a δ66Zn value of +0.20 ± 0.05‰ (2SD), which is lighter than the primitive upper mantle (PUM) estimated in previous studies (+0.28 ± 0.05‰, 2SD, Chen et al., 2013b; +0.30 ± 0.07‰, 2SD, Doucet et al., 2016). This indicates that the Earth's upper mantle has a heterogeneous Zn isotopic composition vertically, which is probably due to shallow mantle melting processes.

  7. Thermally induced phase changes, lateral heterogeneity of the mantle, continental roots, and deep slab anomalies

    NASA Technical Reports Server (NTRS)

    Anderson, Don L.

    1987-01-01

    Factors which influence the lateral heterogeneity in density and seismic velocity with depth in the upper earth mantle are discussed. It is emphasized that most of the increases in density and seismic velocity with depth are caused by pressure-induced solid-solid phase changes in the high-density high-velocity phases of mineral assemblage, due to variations in temperature. In particular, the ilmenite form of MgSiO3 and the gamma-spinel form of Mg2SiO4 have broad stability fields in cold mantle and are not stable in hotter mantle. It is emphasized that the density and velocity anomalies associated with temperature-induced phase changes in mineral assemblage must be taken into account in the thermal models of the slabs; when these effects are accounted for, the geoid and seismic anomalies associated with subducted slabs are consistent with slab confinement to the upper mantle and with layered models of mantle convection.

  8. Crustal and upper mantle structure of central Qiangtang terrane (Tibet Plateau) imaged with magnetotelluric data

    NASA Astrophysics Data System (ADS)

    Zeng, S.; Hu, X.; Li, J.

    2013-12-01

    Since the Tethys Ocean closed, the ongoing collision between India and Aisa continents has created the Tibet Plateau, which is the most spectacular topographic feature on the surface of the earth. In the last decades, a large number of geological and geophysical studies have been undertaken in the Tibet Plateau, but most of these studies were focused on southern Tibet, where the collision of the Indian tectonic plate with Eurasia was occurred, and southeast Tibet, where lateral extrusion of crustal material may be occurred, absent in the central Tibet. As research continues, it has become clear that a complete understanding of the formation and deformation of the Tibet Plateau requires a study of the entire plateau. The Qiangtang terrane is located in the central Tibet Plateau. In 1993-1994, three profiles of broadband MT data (320 Hz to 2000 s) along N-S trending ranges from 86°E to 91°E were collected by China University of Geoscience in central Qiangtang terrane for the purpose of oil and gas exploration, the previous interpretation was focused on the shallow structures. In this study, we reanalyze the three MT profiles to produce more detailed images of the deep electrical structure of the Qiangtang terrane. Dimensionality analysis and geoelectric strike analysis of these data show that they appear to be two dimensional. 2-D inversion model show that there is a pervasive conductivity layer in the mid- to lower crustal and upper mantle, especially in the north Qiangtang terrane, which was considered to be the result of partial melt. The partial melt fraction is sufficient for crustal flow to occur. The similarity of the inversion models of the three profiles show that there is west-east crustal flow along the Jinsha River suture in central Qiangtang terrane, which seems to be western extension of the crustal flow observed in southeast Tibet by Bai et al. (2010). The inversion results also show difference of the electrical structure between the west and east

  9. The effect of water and iron content on electrical conductivity of upper mantle rocks.

    NASA Astrophysics Data System (ADS)

    Wang, D.; Yi, L.

    2008-12-01

    Geophysical observations (MT and GDS) show the conductivity anomaly which may be related to the presence of water and melting. Recently, several researchers have estimated the water content in the transition zone (Huang et al. 2005; Yoshino et al. 2008) and the upper mantle (Wang et al.2006; Yoshino et al. 2006) by electrical conductivity methods. They may underestimate the water content, especially, Yoshino et al did too much underestimate. However, other coexisting phases such as pyroxene and its high-pressure polymorphs may also contribute to the bulk conductivity of the mantle. To test this hypothesis, we measured the electrical conductivity of upper mantle rocks- dunite, pyroxenite and lherzolite at ~ 2-3 GPa and ~1273-1573 K using impedance spectra within a frequency range of 0.1~1000000 Hz. The oxygen fugacity was controlled by a Mo-MoO2 solid buffer. The results show that the electrical conductivity of lherzolite and pyroxenite are ~ half and one order of magnitude higher than that of dunite. These differences were interpreted through a preliminary model involving water and iron content effects on the electrical conductivity. We extrapolated our results and compared the results with some of geophysical observations of the upper mantle. Our results indicate the maximum water content in oceanic upper mantle is as high as ~ 0.09wt % and suggest that pyroxenes dominate the bulk conductivity of upper mantle in hydrous conditions. These results indicated that our model with various water contents could explain the conductivity anomaly in the oceanic upper mantle without involving the presence of partial melt at these depths. This work was supported by national natural science foundation of china (40774036); the special grant from the president of Chinese Academy of Sciences and Graduate University of Chinese Academy Sciences.

  10. Numerical Simulation Analysis of Deformation Effect of The Upper Mantle Flow to Ordos and Its Surroundings

    NASA Astrophysics Data System (ADS)

    Yun, S.; Ping, L. C.; Qi, D.

    2014-12-01

    Ordos block is a typical representative of cratonic lithosphere in North China. It is stable in the block ,but around the block there are a series of faulted basins and folded mountains, the new tectonic movement around the block is intense. Some scholars propose that the upper mantle flow is an important factor to the extension activity of the fault zone around the block. But it has never been discussed in detail that how the upper mantle flow affects the movement and deformation around Ordos block? A 3D viscoelastic modeling is realized for studying the deformation effect of the upper mantle flow to Ordos and its surroundings, based on the comprehensive geological and geophysical data ,such as 3d rheological structure, the active blocks of China, thermal structure, shear wave splitting, et al. The modeling results indicate that in the vertical direction, compared with the local uplift and depression caused by the compression among the plates, the uplifting of Ordos block as a whole is mainly effected by mantle upwelling. In general the upper mantle surrounding of Ordos block is upwelling, Linfen basin goes up more faster. In the horizontal direction, The general flow direction of upper mantle in the study area is NE, basically the same as Qingzang block movement direction. But there is a bifurcation flow along the southwestern margin of Liupanshan. Generally speaking, the regional deformation is drive mainly by the movement of Qingzang block and adjacent blocks pushing into each other,the deformation effect of the upper mantle flow to Ordos and its surroundings is a superposition and partial adjustment.

  11. Radial anisotropy of the North American upper mantle based on adjoint tomography with USArray

    NASA Astrophysics Data System (ADS)

    Zhu, Hejun; Komatitsch, Dimitri; Tromp, Jeroen

    2017-10-01

    We use seismic data from USArray to image the upper mantle underneath the United States based on a so-called `adjoint tomography', an iterative full waveform inversion technique. The inversion uses data from 180 regional earthquakes recorded by 4516 seismographic stations, resulting in 586 185 frequency-dependent measurements. Three-component short-period body waves and long-period surface waves are combined to simultaneously constrain deep and shallow structures. The transversely isotropic model US22 is the result of 22 pre-conditioned conjugate-gradient iterations. Approximate Hessian maps and point-spread function tests demonstrate good illumination of the study region and limited trade-offs among different model parameters. We observe a distinct wave-speed contrast between the stable eastern US and the tectonically active western US. This boundary is well correlated with the Rocky Mountain Front. Stable cratonic regions are characterized by fast anomalies down to 250-300 km, reflecting the thickness of the North American lithosphere. Several fast anomalies are observed beneath the North American lithosphere, suggesting the possibility of lithospheric delamination. Slow wave-speed channels are imaged beneath the lithosphere, which might indicate weak asthenosphere. Beneath the mantle transition zone of the central US, an elongated north-south fast anomaly is observed, which might be the ancient subducted Farallon slab. The tectonically active western US is dominated by prominent slow anomalies with magnitudes greater than -6 per cent down to approximately 250 km. No continuous lower to upper mantle upwellings are observed beneath Yellowstone. In addition, our results confirm previously observed differences between oceans and continents in the anisotropic parameter ξ = (βh/βv)2. A slow wave-speed channel with ξ > 1 is imaged beneath the eastern Pacific at depths from 100 to 200 km, reflecting horizontal shear within the asthenosphere. Underneath continental

  12. Seismic structure of the European crust and upper mantle based on adjoint tomography

    NASA Astrophysics Data System (ADS)

    Zhu, Hejun

    We use adjoint tomography to estimate three-dimensional variations in seismic parameters within the crust and upper mantle beneath Europe and the North Atlantic Ocean. Spectral-element and adjoint methods are used to numerically calculate synthetic seismograms and sensitivity kernels in three-dimensional Earth models. Combined with gradient- based optimization algorithms, e.g., preconditioned conjugate-gradient and L-BFGS methods, we iteratively update seismic models of Earth's interior. A three-stage inversion strategy is designed to estimate variations in elastic wavespeeds, anelastic attenuation and radial & azimuthal anisotropy. In stage one, frequency-dependent phase differences between observed and simulated seismograms are used to determine a new radially anisotropic wavespeed model for the European crust and upper mantle, namely EU30. Long-wavelength structures in EU30 compare favorably with previous body- and surface-wave tomographic models. Some hitherto unidentified features naturally emerge from the smooth starting model. In stage two, frequency-dependent amplitude differences combined with remaining phase anomalies are used to simultaneously constrain elastic and anelastic structures. A new anelastic model, named EU50, is constructed in this stage. We observe several notable features, such as enhanced attenuation within the mantle transition zone beneath the North Atlantic Ocean. In the first two stages, long-period surface waves and short-period body waves in three-component seismograms are combined to simultaneously constrain shallow and deep structures. In stage three, frequency-dependent phase and amplitude anomalies of three-component surface waves are used to construct a radially and azimuthally anisotropic model EU60. We find that the direction of the fast axis is closely tied to the tectonic evolution in this region, such as extension along the North Atlantic Ridge, trench retreat in the Mediterranean, and counterclockwise rotation of the

  13. Deep mantle mineralogy and novel materials synthesis using multianvil high-pressure technology (Robert Wilhelm Bunsen Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Tetsuo, Irifune

    2016-04-01

    Phase relations in mantle and slab materials have been studied using Kawai-type multianvil apparatus (KMA) under pressure and temperature conditions of the mantle transition region and the uppermost lower mantle of the Earth. The associated density and sound velocity changes of these materials have also been determined using the KMA technology combined with synchrotron in situ X-ray and ultrasonic interferometry measurements. The results show that the mantle transition region is made of a pyrolitic composition, while the presence of a harzburgite-rich layer is suggested in the lower parts of this region. Use of sintered diamond anvils for KMA has allowed expansion of these measurements toward deeper region of the lower mantle. Our preliminary results of such measurements indicate that at least upper part of the lower mantle is made of the pyrolitic composition contrary to a recent study based on Brillouin scattering measurements in diamond anvil cell, which concluded a more Si-rich lower mantle. On the other hand, we have been applying KMA technology to synthesis of novel functional materials utilizing its capability of producing very high static pressures and homogeneous temperatures in relatively large sample volumes. These include ultrahard nano-polycrystalline diamond (NPD) directly converted from graphite, which is now being used for applications to abrasive and cutting tools as well as for some scientific applications such as anvils for some high-pressure devices. Another example of such a novel material is hard and tough nano-polycrystalline stishovite (NPS), which is also potentially important for some industrial applications. Moreover, we recently succeeded in making highly transparent nano-polycrystalline garnet (NPG), which is ideal for the measurements of sound velocities by various methods, such as Brillouin scattering and GHz ultrasonic interferometry. Thus, the KMA technology opens the door to the synthesis of transparent nano

  14. Deep mantle forces and the uplift of the Colorado Plateau

    NASA Astrophysics Data System (ADS)

    Moucha, Robert; Forte, Alessandro M.; Rowley, David B.; Mitrovica, Jerry X.; Simmons, Nathan A.; Grand, Stephen P.

    2009-10-01

    We introduce a quantitative model of global mantle convection that reconstructs the detailed motion of a warm mantle upwelling over the last 30 Ma towards the interior of the southwestern USA from observed present-day mantle heterogeneity. The onset and evolution of uplift in the central Basin and Range province and Colorado Plateau during this time is determined by tracking the topographic swell due to this mantle upwelling. We show that: (1) the extension and basaltic volcanism (post 25 Ma) in the central Basin and Range coincides with the arrival and eastward progression of this upwelling, and (2) dynamic uplift of the southern Colorado Plateau, totaling about 1 km, transpired in the last 20 Ma. Since 10 Ma, the center of uplift continued northeastward from the southwestern rim of the plateau consistent with a young Grand Canyon model and eastward sweep of magmatism in the western Colorado Plateau.

  15. Deep mantle forces and the uplift of the Colorado Plateau

    SciTech Connect

    Moucha, R; Forte, A M; Rowley, D B; Mitrovica, J X; Simmons, N A; Grand, S P

    2009-06-23

    Since the advent of plate tectonics, it has been speculated that the northern extension of the East Pacific Rise, specifically its mantle source, has been over-ridden by the North American Plate in the last 30 Myrs. Consequently, it has also been postulated that the opening of the Gulf of California, the extension in the Basin and Range province, and the uplift of the Colorado Plateau are the resulting continental expressions of the over-ridden mantle source of the East Pacific Rise. However, only qualitative models based solely on surface observations and heuristic, simplified conceptions of mantle convection have been used in support or against this hypothesis. We introduce a quantitative model of mantle convection that reconstructs the detailed motion of a warm mantle upwelling over the last 30 Myrs and its relative advance towards the interior of the southwestern USA. The onset and evolution of the crustal uplift in the central Basin and Range province and the Colorado Plateau is determined by tracking the topographic swell due to this mantle upwelling through time. We show that (1) the extension and magmatism in the central Basin and Range province between 25 and 10 Ma coincides with the reconstructed past position of this focused upwelling, and (2) the southwestern portion of the Colorado Plateau experienced significant uplift between 10 Ma and 5 Ma that progressed towards the northeastern portion of the plateau. These uplift estimates are consistent with a young, ca. 6 Ma, Grand Canyon model and the recent commencement of mafic magmatism.

  16. Westward migration of oceanic ridges and related asymmetric upper mantle differentiation

    NASA Astrophysics Data System (ADS)

    Chalot-Prat, Françoise; Doglioni, Carlo; Falloon, Trevor

    2017-01-01

    Combining geophysical, petrological and structural data on oceanic mantle lithosphere, underlying asthenosphere and oceanic basalts, an alternative oceanic plate spreading model is proposed in the framework of the westward migration of oceanic spreading ridges relative to the underlying asthenosphere. This model suggests that evolution of both the composition and internal structure of oceanic plates and underlying upper mantle strongly depends at all scales on plate kinematics. We show that the asymmetric features of lithospheric plates and underlying upper asthenosphere on both sides of oceanic spreading ridges, as shown by geophysical data (seismic velocities, density, thickness, and plate geometry), reflect somewhat different mantle compositions, themselves related to various mantle differentiation processes (incipient to high partial melting degree, percolation/reaction and refertilization) at different depths (down to 300 km) below and laterally to the ridge axis. The fundamental difference between western and eastern plates is linked to the westward ridge migration inducing continuing mantle refertilization of the western plate by percolation-reaction with ascending melts, whereas the eastern plate preserves a barely refertilized harzburgitic residue. Plate thickness on both sides of the ridge is controlled both by cooling of the asthenospheric residue and by the instability of pargasitic amphibole producing a sharp depression in the mantle solidus as it changes from vapour-undersaturated to vapour-saturated conditions, its intersection with the geotherm at 90 km, and incipient melt production right underneath the lithosphere-asthenosphere boundary (LAB). Thus the intersection of the geotherm with the vapour-saturated lherzolite solidus explains the existence of a low-velocity zone (LVZ). As oceanic lithosphere is moving westward relative to asthenospheric mantle, this partially molten upper asthenosphere facilitates the decoupling between lower asthenosphere

  17. Temperature Control of Continental Lithosphere Elastic Thickness: Effective Elastic Thickness Te vs Upper Mantle Velocity Vs

    NASA Astrophysics Data System (ADS)

    Hyndman, R. D.; Currie, C. A.; Mazzotti, S.; Frederiksen, A.

    2006-12-01

    The elastic thickness of continental lithosphere is closely related to its total strength and therefore to its susceptibility to tectonic deformation and earthquakes. Recently it has been questioned whether the elastic thickness and strength are dependent on crust and upper mantle temperatures and compositions in the way predicted by laboratory data. We test this dependence in western North America by a regional comparison of the effective elastic thickness (Te) from topography-gravity coherence, and upper mantle temperatures mapped by tomography shear wave velocities (Vs). We find a good correlation between Te and Vs of the form expected based on the thermal and laboratory data. The Te distribution is strongly bimodal as previously found globally, less than 20 km for the high temperature Cordillera and over 100 km for the adjacent cold stable Canadian Shield. Only intermediate thermal regimes have intermediate Te that suggests a weak layer in the lower crust over a stronger upper mantle. Strength envelopes based on laboratory data correspond to the observed Te for thermal regimes with temperatures at the Moho of 800-900C for the Cordillera and 400-500C for the Shield, in agreement with temperatures from Vs and other estimators. Our study supports the conclusion that lithosphere elastic thickness and strength are controlled primarily by temperature and that laboratory- based rheology provides a good first order estimate of the deformation behaviour of the crust and upper mantle. The Cordillera and other continental backarcs are weak enough to be deformed by plate boundary forces, whereas cratons are generally much too strong. In the Cordillera, the upper mantle is too hot for brittle failure and earthquakes occur only in the upper 10-15 km of the crust. In the cool craton, earthquakes occur rarely in the upper mantle because the total lithosphere strength is too great for significant deformation by plate tectonic forces.

  18. Seismic Evidence for Widespread Serpentinized Forearc Upper Mantle Along the Cascadia Margin

    NASA Astrophysics Data System (ADS)

    Brocher, T. M.; Parsons, T.; Trehu, A. M.; Snelson, C. M.; Fisher, M. A.

    2002-12-01

    Petrologic models for subducting slabs suggest that metamorphism of such slabs releases water that serpentinizes the overlying forearc mantle wedge. We use controlled-source-seismic and earthquake-tomography data from Cascadia to test the hypothesis that a narrow wedge of low velocity, serpentinized upper mantle extends along the Cascadia forearc from Vancouver Island to the Mendocino triple junction. The primary evidence for this wedge is low upper mantle (Pn) velocities (<7.8 km/s) and/or absent or low-amplitude wide-angle reflections from the top of the upper mantle (PmP) in a narrow belt (about 50 to 70 km wide) beneath the Georgia Strait, Puget Lowland, Willamette Valley, and the northern Great Valley. These results are compatible with a recent teleseismic model from central Oregon showing a weak and even inverted velocity contrast across the forearc Moho. Tomography models from Georgia Strait and the northern Puget Lowland show low upper mantle velocities. The absence of near-vertical and wide-angle PmP reflections from the top of the upper mantle in the SHIPS data from Puget Lowland are consistent with a weak continental Moho contrast there. Similarly, wide-angle reflections from the continental Moho (PmP) along a 1995 SW Washington seismic profile are discontinuous in the forearc. No clear PmP reflections or upper mantle refractions are observed from the North America plate, above the subducting Gorda plate, along Line 6 from the 1993 Mendocino seismic experiment. Local seismic reflectivity within the forearc upper mantle wedge may indicate, among other possibilities, that these low-velocity rocks have undergone extensive shearing. These observations are consistent with the suggestion that this wedge of low-velocity, rheologically-weak, serpentinized rocks facilitates slow slip events on the megathrust by lubricating the megathrust and/or accommodating some of the interplate deformation. The presence of a low-velocity forearc upper mantle wedge in Cascadia

  19. Upper-mantle P- and S-wave velocities across the Northern Tornquist Zone from traveltime tomography

    NASA Astrophysics Data System (ADS)

    Hejrani, Babak; Balling, Niels; Jacobsen, Bo Holm; Tilmann, Frederik

    2015-10-01

    This study presents P- and S-wave velocity variations for the upper mantle in southern Scandinavia and northern Germany based on teleseismic traveltime tomography. Tectonically, this region includes the entire northern part of the prominent Tornquist Zone which follows along the transition from old Precambrian shield units to the east to younger Phanerozoic deep sedimentary basins to the southwest. We combine data from several separate temporary arrays/profiles (276 stations) deployed over a period of about 15 yr and permanent networks (31 stations) covering the areas of Denmark, northern Germany, southern Sweden and southern Norway. By performing an integrated P- and S-traveltime analysis, we obtain the first high-resolution combined 3-D VP and VS models, including variations in the VP/VS ratio, for the whole of this region of study. Relative station mean traveltime residuals vary within ±1 s for P wave and ±2 s for S wave, with early arrivals in shield areas of southern Sweden and later arrivals in the Danish and North German Basins, as well as in most of southern Norway. In good accordance with previous, mainly P-velocity models, a marked upper-mantle velocity boundary (UMVB) is accurately delineated between shield areas (with high seismic mantle velocity) and basins (with lower velocity). It continues northwards into southern Norway near the Oslo Graben area and further north across the Southern Scandes Mountains. This main boundary, extending to a depth of at least 300 km, is even more pronounced in our new S-velocity model, with velocity contrasts of up to ±2-3 per cent. It is also clearly reflected in the VP/VS ratio. Differences in this ratio of up to about ±2 per cent are observed across the boundary, with generally low values in shield areas to the east and relatively higher values in basin areas to the southwest and in most of southern Norway. Differences in the VP/VS ratio are believed to be a rather robust indicator of upper-mantle compositional

  20. Spin Transition in the Lower Mantle: Deep Learning and Pattern Recognition of Superplumes from the Mid-mantle and Mid-mantle Slab Stagnation

    NASA Astrophysics Data System (ADS)

    Yuen, D. A.; Shahnas, M. H.; De Hoop, M. V.; Pysklywec, R.

    2016-12-01

    The broad, slow seismic anomalies under Africa and Pacific cannot be explained without ambiguity. There is no well-established theory to explain the fast structures prevalent globally in seismic tomographic images that are commonly accepted to be the remnants of fossil slabs at different depths in the mantle. The spin transition from high spin to low spin in iron in ferropericlase and perovskite, two major constituents of the lower mantle can significantly impact their physical properties. We employ high resolution 2D-axisymmetric and 3D-spherical control volume models to reconcile the influence of the spin transition-induced anomalies in density, thermal expansivity, and bulk modulus in ferropericlase and perovskite on mantle dynamics. The model results reveal that the spin transition effects increase the mixing in the lower regions of mantle. Depending on the changes of bulk modulus associated with the spin transition, these effects may also cause both stagnation of slabs and rising plumes at mid-mantle depths ( 1600 km). The stagnation may be followed by downward or upward penetration of cold or hot mantle material, respectively, through an avalanche process. The size of these mid-mantle plumes reaches 1500 km across with a radial velocity reaching 20 cm/yr near the seismic transition zone and plume heads exceeding 2500 km across. We will employ a deep-learning algorithm to formulate this challenge as a classification problem where modelling/computation aids in the learning stage for detecting the particular patterns.The parameters based on which the convection models are developed are poorly constrained. There are uncertainties in initial conditions, heterogeneities and boundary conditions in the simulations, which are nonlinear. Thus it is difficult to reconstruct the past configuration over long time scales. In order to extract information and better understand the parameters in mantle convection, we employ deep learning algorithm to search for different

  1. A tomographic glimpse of the upper mantle source of magmas of the Jemez lineament, New Mexico

    USGS Publications Warehouse

    Spence, W.; Gross, R.S.

    1990-01-01

    To infer spatial distributions of partial melt in the upper mantle source zones for the Rio Grande rift and the Jemez lineament, the lateral variations of P wave velocity in the upper mantle beneath these features has been investigated. Teleseismic P wave delays recorded at a 22-station network were used to perform a damped least squares, three-dimensional inversion for these lateral variations. Results infer that a large magmatic source zone exists beneath the Jemez lineament but not beneath the Rio Grande rift. This implies that the volcanic potential of the Jemez lineaments continues to greatly exceed that of the Rio Grande rift. The magmatic source zones of the Jemez lineament are modeled as due to clockwise rotation of the Colorado Plateau about a pole in northeastern Colorado. This rotation caused extension of the lithosphere beneath the Jemez lineament, permitting concentration there of partially melted rock in the upper mantle. -from Authors

  2. Teleseismic P wave spectra from USArray and implications for upper mantle attenuation and scattering

    NASA Astrophysics Data System (ADS)

    Cafferky, Samantha; Schmandt, Brandon

    2015-10-01

    Teleseismic P wave amplitude spectra from deep earthquakes recorded by USArray are inverted for maps of upper mantle Δt* for multiple frequency bands within 0.08-2 Hz. All frequency bands show high Δt* regions in the southwestern U.S., southern Rocky Mountains, and Appalachian margin. Low Δt* is more common across the cratonic interior. Inversions with narrower frequency bands yield similar patterns, but greater Δt* magnitudes. Even the two standard deviation Δt* magnitude for the widest band is ˜2-7 times greater than predicted by global QS tomography or an anelastic olivine thermal model, suggesting that much of the Δt* signal is nonthermal in origin. Nonthermal contributions are further indicated by only a moderate correlation between Δt* and P travel times. Some geographic variations, such as high Δt* in parts of the cratonic interior with high mantle velocities and low heat flow, demonstrate that the influence of temperature is regionally overwhelmed. Transverse spectra are used to investigate the importance of scattering because they would receive no P energy in the absence of 3-D heterogeneity or anisotropy. Transverse to vertical (T/Z) spectral ratios for stations with high Δt* are higher and exhibit steeper increases with frequency compared to T/Z spectra for low Δt* stations. The large magnitude of Δt* estimates and the T/Z spectra are consistent with major contributions to Δt* from scattering. A weak positive correlation between intrinsic attenuation and apparent attenuation due to scattering may contribute to Δt* magnitude and the moderate correlation of Δt* with travel times.

  3. Sharpness of upper-mantle discontinuities determined from high-frequency reflections

    USGS Publications Warehouse

    Benz, H.M.; Vidale, J.E.

    1993-01-01

    AN understanding of the nature of seismic discontinuities in the Earth's upper mantle is important for understanding mantle processes: in particular, the amplitude and sharpness of these discontinuities are critical for assessing models of upper-mantle phase changes and chemical layering. So far, seismic studies aimed at determining the thickness and lateral variability of upper-mantle discontinuities have yielded equivocal results, particularly for the discontinuity at 410km depth1,2. Here we present short-period (0.8-2.0 s) recordings of upper-mantle precursors to the seismic phase P???P??? (PKPPKP) from two South American earthquakes recorded by the ???700-station short-period array in California. Our results show that the 410- and 660-km discontinuities beneath the Indian Ocean are locally simple and sharp, corresponding to transi-tion zones of 4 km or less. These observations pose problems for mineral physics models3-5, which predict a transitional thickness greater than 6 km for the peridotite to ??-spinel phase transition. In contrast to the results of long-period studies6,7, we observe no short-period arrivals from near 520 km depth. ?? 1993 Nature Publishing Group.

  4. Sharpness of upper-mantle discontinuities determined from high-frequency reflections

    USGS Publications Warehouse

    Benz, H.M.; Vidale, J.E.

    1993-01-01

    AN understanding of the nature of seismic discontinuities in the Earth's upper mantle is important for understanding mantle processes: in particular, the amplitude and sharpness of these discontinuities are critical for assessing models of upper-mantle phase changes and chemical layering. So far, seismic studies aimed at determining the thickness and lateral variability of upper-mantle discontinuities have yielded equivocal results, particularly for the discontinuity at 410km depth1,2. Here we present short-period (0.8-2.0 s) recordings of upper-mantle precursors to the seismic phase P???P??? (PKPPKP) from two South American earthquakes recorded by the ???700-station short-period array in California. Our results show that the 410- and 660-km discontinuities beneath the Indian Ocean are locally simple and sharp, corresponding to transition zones of 4 km or less. These observations pose problems for mineral physics models3-5, which predict a transitional thickness greater than 6 km for the peridotite to ??-spinel phase transition. In contrast to the results of long-period studies6,7, we observe no short-period arrivals from near 520 km depth.

  5. Constraints on radial anisotropy in the central Pacific upper mantle from the NoMelt OBS array

    NASA Astrophysics Data System (ADS)

    Russell, J. B.; Gaherty, J. B.; Lin, P. P.; Zebker, M.

    2016-12-01

    Observations of seismic anisotropy in ocean basins are important for constraining deformation and melting processes in the upper mantle. The NoMelt OBS array was deployed on relatively pristine, 70-Ma seafloor in the central Pacific with the aim of constraining upper-mantle circulation and the evolution of the lithosphere-asthenosphere system. Azimuthal variations in Rayleigh-wave velocity suggest strong anisotropic fabric both in the lithosphere and deep in the asthenosphere, and we aim to evaluate whether radial anisotropy shows a similar pattern. We use a combination of Love waves from earthquakes (20-100 s) as well as high-frequency ambient noise (5-10 s) to estimate VSH in the upper 300 km beneath the NoMelt array. Waveform fitting of the ambient-noise cross spectra provide phase-velocity estimates that are sensitive to the upper 50 km of the mantle. To constrain structure beneath the lid, we employ an array-based approach to measure Love-wave phase velocities across the array using seven shallow-focus events (< 25 km) with high signal-to-noise ratio and diverse azimuthal coverage. The Love wave phase-velocity measurements suggest strong interference of the first overtone for intermediate periods (20-50 s), while longer periods (>60 s) are mostly dominated by fundamental mode energy. Through forward modeling of Love wave Fréchet kernels, we find an extremely strong nonlinearity in individual mode-branch sensitivity that is dependent on the relative velocity difference between the low-velocity zone (LVZ) and the overlying Pacific lid. For the fundamental mode in the presence of a strong LVZ, intermediate periods (20-50 s) have little sensitivity within the lithospheric mantle with peak sensitivity pushed to the base of the low-velocity zone. This peak sensitivity migrates to much shallower depth as the lid/LVZ contrast is reduced. Therefore, we use a Monte Carlo approach to systematically explore the model space and identify the most robust model features

  6. Consequences of experimental transient rheology. [of earth lower crust and upper mantle

    NASA Technical Reports Server (NTRS)

    Sabadini, Roberto; Smith, Brad K.; Yuen, David A.

    1987-01-01

    Recent analyses of transient creep data for lower crustal and upper mantle substances are used to constrain the viscosity contrast across the 670 km discontinuity, and a 4-layer earth model in which both upper and lower mantles are described by a Burgers body rheology is assumed. Results indicate that the previously inferred viscosity variations would be reduced by a factor of ten using the new transient models. This result is in agreement with a revised viscosity estimate based on long-wavelength geoid anomalies and seismic tomography. Time-dependent perturbations to the gravity field from recent ice movements are shown to be significant.

  7. Structure of the crust and upper mantle in the western United States

    USGS Publications Warehouse

    Pakiser, L.C.

    1963-01-01

    Seismic waves generated by underground nuclear and chemical explosions have been recorded in a network of nearly 2,000 stations in the western conterminous United States as a part of the VELA UNIFORM program. The network extends from eastern Colorado to the California coastline and from central Idaho to the border of the United States and Mexico. The speed of compressional waves in the upper-mantle rocks ranges from 7.7 km/sec in the southern part of the Basin and Range province to 8.2 km/sec in the Great Plains province. In general, the speed of compressional waves in the upper-mantle rocks tends to be nearly the same over large areas within individual geologic provinces. Measured crustal thickness ranges from less than 20 km in the Central Valley of California to 50 km in the Great Plains province. Changes in crustal thickness across provincial boundaries are not controlled by regional altitude above sea level unless the properties of the upper mantle are the same across those boundaries. The crust tends to be thick in regions where the speed of compressional waves in the upper-mantle rocks (and presumably the density) is high, and tends to be relatively thin where the speed of compressional waves in the upper-mantle rocks (and density) is lower. With in the Basin and Range province, crustal thickness seems to vary directly with regional altitude above sea level. Evidence that a layer of intermediate compressional-wave speed exists in the lower part of the crust has been accumulated from seismic waves that have traveled least-time paths, as well as secondary arrivals (particularly reflections). On a scale that includes many geologic provinces, isostatic compensation is related largely to variations in the density of the upper- mantle rocks. Within geologic provinces or adjacent provinces, isostatic compensation may be related to variations in the thickness of crustal layers. Regions of thick crust and dense upper mantle have been relatively stable in Cenozoic

  8. Deep mantle heat flow and thermal evolution of the Earth's core based on thermo-chemical mantle convection

    NASA Astrophysics Data System (ADS)

    Nakagawa, T.; Tackley, P.; Buffett, B.

    2004-12-01

    A coupled core-mantle evolution model that combines the global heat balance in the core with a fully-dynamical thermo-chemical mantle convection [Nakagawa and Tackley, 2004 published in EPSL] is used to investigate the deep mantle heat flow that is required to sustain the magnetic field generated by the geodynamo process. Effects of a radioactive heat source due to potassium in the core are also included in the global heat balance in the Earth??s core. Two important parameters are checked in this study; (1) density variation between depleted hartzbergite and basaltic material (0 to 3 percent) and (2) concentration of radioactive potassium in the core alloy (0ppm to 400ppm). The parameter set that most closely satisfies the criteria of size of the inner core (1220km at present time) is around 2 percent of density difference in a convecting mantle and 200ppm of radioactive heat source in the core. The concentration of potassium in the core is consistent with the geochemical approach [Murthy et al., 2003] but smaller than other successful thermal evolution models [Labrosse, 2003; Nimmo et al., 2004]. Heat flow through the core-mantle boundary and the contribution of radioactive heat sources in the core are consistent with theoretical estimates [e.g. Buffett, 2002] and geochemical constraints [Gessmann and Wood, 2002]. The power available to the geodynamo, based on the predicted heat flow through the core-mantle boundary, is approximately four times greater than the value predicted by numerical models of the geodynamo [Christensen and Kutzner, 2004] but closer to theoretical estimates [e.g. Buffett, 2002].

  9. Using Seismic Discontinuities to Image Melt and Dynamics in the Sub-Continental Upper Mantle

    NASA Astrophysics Data System (ADS)

    Schmerr, N. C.; Courtier, A. M.; Hier-Majumder, S.; Lekic, V.

    2014-12-01

    Continents are assembled from multiple Proterozoic and Archean terranes to form stable cratonic platforms with associated deformation typically localized to margins and/or rift zones. Successive episodes of subsequent extension, compression, magmatism, accretion, and rifting have left the sub-continental upper mantle with a complex signature of thermal and chemical heterogeneity. One key interest is the history of melt production, migration, and storage in sub-continental upper mantle as it provides a window into past and present dynamical processes, including the differentiation and formation of continental structure. Here we examine seismic discontinuities within the mantle that arise from a wide range of mechanisms, including changes in mineralogy, major element composition, melt content, volatile abundance, anisotropy, or a combination of the above. Using a dataset of broadband seismograms of underside reflected S-waves arriving as precursors to the seismic phase SS, we determine the depth and impedance contrast of discontinuities in the depth range of 80-410 km. Our observations are compared to predictions for the seismic moduli from a mineral physics database using the software MuMaP (Multiphase Material Properties). MuMaP modeling allows us to vary the average regional temperature, mantle composition and account for the effects of melt (if present). In our initial study of the western North American plate, we detect the presence of the 410 km discontinuity, a discontinuity at 300 km depth (X), and a G discontinuity at 60-80 km depth. The X is indicative of the coesite to stishovite phase transition in the upper mantle and suggests substantial mixing of subducted basalt with the mantle. The presence of the G may indicate partial melt in the asthenosphere, melt frozen into the lithosphere, and/or anisotropic fabrics preserved beneath the continent. These hypotheses are evaluated against MuMap predictions for melt content and anisotropic structure in the upper

  10. Towards imaging thermal and compositional structure in the global upper mantle

    NASA Astrophysics Data System (ADS)

    Zhou, Y.; Ruan, Y.

    2009-12-01

    Rapid progress has been made in imaging S-wave velocity structure in the global upper mantle in recent surface-wave tomographic studies. Perturbations in S-wave speed alone may have either a thermal or compositional origin, therefore, at least one additional independent observable is required to resolve mantle heterogeneities in temperature and composition. Theoretically, observations of 3-D anelasticity (Q) structure --- together with 3-D S-wave velocity structure --- can be used to constrain temperature and compositional variations in the upper mantle. Despite of their great importance, the coupling between elastic and anelastic effects in surface waves has not been well understood, and large discrepancies exits among global upper-mantle Q models. We investigate the dual dependence of seismic traveltimes and amplitudes upon 3-D velocity and 3-D Q structure by simulating seismic wave propagation in 3-D temperature earth models and show that (1) lateral variations in Q have significant effects on surface-wave traveltimes -— it accounts for about a quarter of the observed surface-wave delay times if mantle seismic anomalies are dominantly thermal; (2) surface-wave amplitude perturbations are dependent upon the geometry and lengthscale of mantle anomalies in both wavespeed and Q structure through elastic and anelastic focusing and defocusing. The common practice of inverting seismic amplitudes for ``attenuation" may have led to large discrepancies as seen among current global Q models. We develop a finite-frequency approach to simultaneously invert for lateral variations in velocity and anelasticity (Q), fully accounting for the coupling between elastic and anelastic effects in seismic traveltimes and amplitudes. This work opens the opportunity for joint diffractional tomography of high-resolution lateral heterogeneities in temperature and composition in the upper mantle.

  11. Crust and upper mantle structure beneath southeast Australia from ambient noise and teleseismic tomography

    NASA Astrophysics Data System (ADS)

    Rawlinson, N.; Pilia, S.; Young, M.; Salmon, M.; Yang, Y.

    2016-10-01

    In the last decade, the lithospheric structure beneath southeast Australia has been intensively studied using passive seismic data from WOMBAT, the largest transportable seismic array in the southern hemisphere. The two primary imaging methods that have been applied are ambient noise tomography for the crust and teleseismic tomography for the upper mantle. Despite these recent studies, no attempt has yet been made to provide an integrated view of the crust-mantle system. Here, we perform teleseismic tomography using WOMBAT data that includes a detailed crustal model from ambient noise tomography in the starting model. A Moho surface from the Australian seismological reference Earth model (AuSREM) is also included. This has the dual benefit of accounting for the unresolved crustal component of the teleseismic arrival time residuals, and producing a model that reveals a high level of detail in both the crust and upper mantle. Our new integrated P-wave model contains a number of noteworthy features, including (i) low velocity anomalies in the lower crust and high velocity anomalies in the lithospheric mantle beneath the Gawler Craton and Curnamona Province, which are of Paleoproterozoic-Archean origin; (ii) a marked velocity transition in the crust and lithospheric mantle near the Moyston Fault, which we interpret as the boundary between the Lachlan and Delamerian orogens; (iii) a rapid eastward decrease in upper mantle velocity 200 km inboard of the east coast of Australia, which is consistent with a marked thinning of the lithosphere; (iv) an increase in upper mantle velocity north of the Gawler Craton and Curnamona Province, which points to the presence of thicker lithosphere associated with the Precambrian shield region of the Australian continent; (v) Cenozoic intraplate basaltic volcanic centres distributed exclusively above the zone of thinner lithosphere inboard of the east coast, with the exception of low volume leucitite volcanics.

  12. Upper Extremity Deep Vein Thrombosis: A Community-Based Perspective

    PubMed Central

    Spencer, Frederick A.; Emery, Cathy; Lessard, Darleen; Goldberg, Robert J.

    2010-01-01

    Purpose The purpose of this study was to examine the magnitude, risk factors, management strategies, and outcomes in a population-based investigation of patients with upper, as compared to lower, extremity deep vein thrombosis diagnosed in 1999. Methods The medical records of all residents from Worcester, Massachusetts (2000 census=478,000) diagnosed with ICD-9 codes consistent with possible deep vein thrombosis at all Worcester hospitals during 1999 were reviewed and validated. Results The age-adjusted attack rate (per 100,000 population) of upper extremity deep vein thrombosis was 16 (95% CI 13, 20) compared to 91 (83,100) for lower extremity deep vein thrombosis. Patients with upper extremity deep vein thrombosis were significantly more likely to have undergone recent central line placement, a cardiac procedure, or an intensive care unit admission than patients with lower extremity deep vein thrombosis. Although short and 1-year recurrence rates of venous thromboembolism and all-cause mortality were not significantly different between patients with upper, versus lower, extremity deep vein thrombosis, patients with upper extremity deep vein thrombosis were less likely to have pulmonary embolism at presentation or in follow-up. Conclusions Patients with upper extremity deep vein thrombosis represent a clinically important patient population in the community setting. Risk factors, occurrence of pulmonary embolism, and timing and location of venous thromboembolism recurrence differ between patients with upper as compared to lower extremity deep vein thrombosis. These data suggest that strategies for prophylaxis and treatment of upper extremity deep vein thrombosis need further study and refinement. PMID:17679126

  13. Carbon Dioxide Carbonates in the Earth;s Mantle: Implications to the Deep Carbon Cycle

    SciTech Connect

    Yoo, Choong-Shik; Sengupta, Amartya; Kim, Minseob

    2012-05-22

    An increase in the ionic character in C-O bonds at high pressures and temperatures is shown by the chemical/phase transformation diagram of CO{sub 2}. The presence of carbonate carbon dioxide (i-CO{sub 2}) near the Earth's core-mantle boundary condition provides insights into both the deep carbon cycle and the transport of atmospheric CO{sub 2} to anhydrous silicates in the mantle and iron core.

  14. Abiogenic hydrocarbons produced under upper-mantle pressure-temperature conditions

    NASA Astrophysics Data System (ADS)

    Goncharov, Alexander F.; Chellappa, Raja S.; Kolesnikov, Anton; Somayazulu, Maddury; Kutcherov, Vladimir G.; Hemley, Russell J.

    2010-05-01

    There are growing evidences for abiogenic pathways of the petroleum production in the deep Earth. The laboratory experiments under extreme pressures and temperatures provide direct information about the chemical reactivity and stability of the natural oil components. Here we present new results on formation of hydrocarbons at high pressures and temperatures generated in diamond anvil cells (DAC). We use in situ Raman spectroscopy in laser heated diamond anvil cells to monitor the chemical reactivity; Raman spectroscopy and synchrotron x-ray diffraction are used to determine the reaction products quenched to ambient temperature. We have explored chemical reactions in the system consisting of CaCO3-H2O with either of the following mantle minerals: San Carlos olivine, peridodite, and Rockport fayalite. We also studied chemical reactivity of methane and ethane to explore possible routes to generate heavier hydrocarbons. The pressure range of the experiments (3-6 GPa) are similar to those studied by Kenney et al. [1] and Scott et al. [2] but temperatures up to 2500 K were generated. At pressures in the 5-6 GPa range, methanogenesis was observed in the olivine-calcite-water, fayalite-calcite-water system at temperatures greater than 2300 K and less than 800 K, respectively. We find that methane at 2-5 GPa and 1000-1500 K partially reacts and forms saturated hydrocarbons (C2-C4alkanes), molecular hydrogen and graphite. Formation of methane in similar experiments on ethane suggests reversibility of hydrocarbon formation. These results support proposals of abiogenic pathways for the formation of hydrocarbons in the Earth's upper mantle. 1 Kenney, J. F., Kutcherov, V. G., Bendeliani, N. A. & Alekseev, V. A. The Evolution of Multicomponent Systems at High Pressures: VI. The Thermodynamic Stability of the Hydrogen-Carbon System: The Genesis of Hydrocarbons and the Origin of Petroleum. Proc. Natl. Acad. Sci. U.S.A. 99, 10976-10981 (2002). 2 Scott, H.P., Hemley, R.J., Mao, H

  15. Dipping Upper Mantle Boundary Beneath Southeastern Superior Province - a Trace of Archean Subduction?

    NASA Astrophysics Data System (ADS)

    Levin, V. L.; Hynes, A.; Servali, A.; VanTongeren, J. A.; Menke, W. H.

    2016-12-01

    Lithosphere of Archean cratons preserves a record of their long and complex evolution. Widely reported examples of dipping seismic boundaries in the shallow upper mantle, including those in the Slave and Superior cratons of the North American continents, were used as evidence for Archean subduction episodes. New broadband seismic data collected in the southeastern Superior Province of Quebec reveal another example of a south-dipping seismic boundary at depths 40-55 km in the upper-most mantle. The boundary is evident in receiver functions from a densely spaced quasi-linear array of seismographs crossing from the Grenville Province into the Abitibi greenstone belt, and extending into the Opatica terrane. The seismic phase we associate with this boundary has the same polarity as the Moho, and thus reflects an abrupt increase in seismic wave speed with depth. This phase appears at 5.5 s beneath central Opatica terrane, and becomes progressively later at sites further south, reaching 6.5 s beneath the Abitibi terrane close to the Grenville Front. Unlike the extremely sharp crust-mantle boundary seen at all sites of the observing array, the dipping boundary is more complex. Its directional signature is less uniform than that of the Moho, and at some locations it is composed of multiple pulses suggesting a complex zone at depth. A major change in the crustal and upper mantle structure just north of the Grenville Front is suggested by the receiver functions from sites close to it. The Moho depth increases significantly, and phases indicative of upper-mantle boundaries, including that we associate with the dipping feature, disappear. The Grenville Front is, however, only the northwestern limit of Grenvillian thin-skinned deformation, not the location of the Grenvillian suture. The newly described inclined seismic boundary in the upper mantle therefore likely originated at a much earlier time.

  16. Shear zones in the upper mantle - relation between geochemical enrichment and deformation in mantle peridotites

    SciTech Connect

    Downes, H. )

    1990-04-01

    Textural variations in mantle-derived spinel peridotites have previously been interpreted as evidence of the existence of asthenospheric mantle diapirs, indicating deformational heterogeneity on a large lateral scale (kilometers to tens of kilometers). However, many volcanic vents entrain both deformed and undeformed xenoliths, and field relations in peridotite massifs show the scale of alternation between deformed and undeformed peridotite to be small (centimeters to meters) because of the presence of numerous lithospheric shear zones. Some rare xenoliths contain both deformed and undeformed peridotite. These is also an apparent relation between deformation and the growth of metasomatic minerals; amphibole is often concentrated in strongly deformed zones in peridotite massifs and deformed xenoliths, although it is also found in crosscutting veins and aureoles around such veins. A relation can also be seen between deformation and indicators of geochemical enrichment. Clinopyroxenes from many deformed spinel peridotites show light rate earth element (REE) enrichment, whereas clinopyroxenes from undeformed spinel peridotites commonly have mid-ocean ridge basalt (MORB)-type light REE-depleted patterns. Sr and Nd radiogenic isotopic compositions of undeformed peridotites are generally MORB-like, and have low {epsilon}Sr and high {epsilon}Nd. In contrast, deformed peridotites, with or without amphibole, often have higher {epsilon}Sr and low {epsilon}Nd values, indicating geochemical enrichment by large ion lithophile (LIL)- and light REE-enriched fluids or melts. These observations can be used to infer that the shallow mantle contains lithospheric ductile shear zones in which metasomatic fluids precipitated amphibole and clinopyroxene.

  17. Structure of the Upper Mantle and Mantle Transition Zone in Central Mongolia

    NASA Astrophysics Data System (ADS)

    Cui, Z.; Meltzer, A.; Stachnik, J.; Fischer, K. M.; Russo, R. M.; Munkhuu, U.; Baasanbat, T.

    2016-12-01

    Located between two major Archean cratons, the Siberian Craton to the north and the Tarim and Sino-Korean Cratons to the south, the lithosphere of Central Mongolia was constructed over an extended period of orogenesis associated with the Central Asian Orogenic Belt. Archean to Early Proterozoic basement was modified by accreted subduction complexes during the Paleozoic and early Mesozoic and basalt magmatism in the Cenozoic. Central and western Mongolia constitute a significant portion of the greater Mongolian plateau, an approximately 2.6 million km2area of Central Asia with an average elevation of 1500 meters. The high topography of the Mongolian Plateau has been attributed to far-field effects of India-Asia convergence, Pacific plate subduction, mantle plume activity, convective mantle flow, and magmatic underplating. The origin and persistence of continental plateaus through time provides insight into the evolution of continents and interactions between mantle dynamics and surface processes. As part of a larger interdisciplinary project to understand the origin of high topography in continental interiors we deployed 112 seismic broadband stations in central Mongolia as three separate subarrays in two separate mobilizations over a four year period (2012-2016). The stations extend from the Hovsgol rift in northern Mongolia, through the Hangay Dome, and into the Gobi Altai in southern Mongolia. We use S wave Receiver functions (SRF) to examine the lithosphere asthenosphere boundary and P wave Receiver functions (PRF) to investigate the mantle transition zone (MTZ). Preliminary SRF results from the subarray in the Hangay show lithospheric thinning and E-W variation. The LAB beneath the Hangay is 100km. It gradually thins to 90 km at the western end of the central Hangay and thins more abruptly to 80km at the eastern end of the central Hangay. These results are in agreement with results from joint inversion of receiver functions and surface waves and teleseismic

  18. Anisotropic Signature of the Afar plume in the Upper Mantle.

    NASA Astrophysics Data System (ADS)

    Sicilia, D.; Montagner, J.; Debayle, E.; Leveque, J.; Cara, M.; Lepine, J.

    2002-12-01

    Plumes remain enigmatic geological objects and it is still unclear how they are formed and whether they act independently from plate tectonics. The role of plumes in mantle dynamics can be investigated by studying their interaction with lithosphere and crust and their perturbations on flow pattern in the mantle. The flow pattern can be derived from seismic anisotropy. An anisotropic surface wave tomography in the Horn of Africa was performed. The choice of the experiment in the Horn of Africa is motivated by the the presence of the Afar hotspot, one of the biggest continental hotspot. In the framework of the mantle degree 2 pattern, the Afar hotspot is the antipode of the Pacific superswell, but its origin at depth and its geodynamic importance are still debated. Data were collected from the permanent IRIS and GEOSCOPE networks and from the PASSCAL experiment in Tanzania and Saudi Arabia. We completed our data base with a French deployment of portable broadband stations surrounding the Afar Hotspot. Path average phase velocities are obtained by using a method based on a least-squares minimization (Beucler et al.,2002). A correction of the data is applied according to the a priori 3SMAC model (Nataf and Ricard, 1996). 3D-models of velocity, radial and azimuthal anisotropies are inverted for. Down to 250km, low velocities are found beneath the Red Sea, the Gulf of Aden, the South East of the Tanzania Craton, the Afar hotspot. High velocities are present in the eastern Arabia and the Tanzania Craton. These results are in agreement with the isotropic model of Debayle et al. (2002). The anisotropy model beneath Afar displays a complex pattern. The azimuthal anisotropy shows that the Afar plume might be interpreted as feeding other hotspots in central Africa. Deeper in the asthenosphere, a wide stem of positive radial anisotropy (VSH > VSV) comes up, where we might expect the reverse sign. The same observation was made below Iceland (Gaherty, 2001) and Hawaii (Montagner

  19. Three-dimensional shear wave velocity structure in the Atlantic upper mantle

    NASA Astrophysics Data System (ADS)

    James, Esther Kezia Candace

    Oceanic lithosphere constitutes the upper boundary layer of the Earth's convecting mantle. Its structure and evolution provide a vital window on the dynamics of the mantle and important clues to how the motions of Earth's surface plates are coupled to convection in the mantle below. The three-dimensional shear-velocity structure of the upper mantle beneath the Atlantic Ocean is investigated to gain insight into processes that drive formation of oceanic lithosphere. Travel times are measured for approximately 10,000 fundamental-mode Rayleigh waves, in the period range 30-130 seconds, traversing the Atlantic basin. Paths with >30% of their length through continental upper mantle are excluded to maximize sensitivity to the oceanic upper mantle. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. Phase velocity shows a strong dependence on seafloor age, and removing age-dependent velocity from the 2-D maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. Depth-dependent variations in vertically-polarized shear velocity (Vsv) are determined with two sets of 3-D models: a layered model that requires constant VSV in each depth layer, and a splined model that allows VSV to vary continuously with depth. At shallow depths (˜75 km) the seismic structure shows the expected dependence on seafloor age. At greater depths (˜200 km) high-velocity lithosphere is found only beneath the oldest seafloor; velocity variations beneath younger seafloor may result from temperature or compositional variations within the asthenosphere. The age-dependent phase velocities are used to constrain temperature in the mantle and show that, in contrast to previous results for

  20. Constraints on lateral variations in upper mantle viscosity from Lake Bonneville shorelines

    NASA Astrophysics Data System (ADS)

    Austermann, Jacqueline; Chen, Christine; Lau, Harriet C. P.

    2017-04-01

    Lake Bonneville is an extinct pluvial lake that formed and catastrophically drained at the onset of the last deglaciation (˜ 20 - 18ka). With a volume of just over 10 000 km3 this lake was comparable in size to present-day Lake Michigan. During its existence the excess load of water stored in Lake Bonneville depressed the crust and upper mantle. After the drainage of the lake this area rebounded by up to 75 m, which is recorded in the paleoshorelines around the lake periphery and on islands within the lake. The rebound pattern has been used to infer the lithospheric thickness and upper mantle viscosity structure of the area (e.g. Bill et al., 1994). In agreement with the tectonic history of the Basin and Range area, the deformed shorelines point to a thin lithosphere (< 30km) and low upper mantle viscosity (˜ 1019 Pa s). This differs from the upper mantle viscosity inferred from post-glacial data in cratonic regions (e.g., Hudson Bay, Fennoscandia), which is one to two orders of magnitude larger (˜ 5 × 1020 Pa s). Direct constraints on the lateral variability of mantle viscosity are invaluable but in order to utilize such constraints it is important to consider the sensitivity range of different observations before comparing the inferred viscosities. In this study we revisit the earlier inversions of shoreline elevations for mantle and lithospheric structure with an updated dataset of paleoshoreline elevations by Chen and Maloof (2017). We construct depth-dependent sensitivity kernels for the lake rebound and compare them to kernels associated with the rebound from glacial ice sheets over Canada and Scandinavia. This comparison along with the inferred viscosities allows us to evaluate the degree to which lateral viscosity variations are required. We additionally compare our results to estimates of lateral viscosity variations based on perturbations in seismic shear wave speed in the respective areas in order to assess the consistency of our results with

  1. P/n/ velocity and cooling of the continental lithosphere. [upper mantle compression waves in North America

    NASA Technical Reports Server (NTRS)

    Black, P. R.; Braile, L. W.

    1982-01-01

    The average upper mantle compressional wave velocity and heat flow figures presently computed for continental physiographic provinces in North America exhibit an inverse relationship, and possess a statistically significant correlation coefficient. A correlation is also demonstrated between compressional wave velocity and material temperature by estimating crust-mantle boundary temperatures from heat flow values. The dependency of compressional wave velocity on temperature implies that the observed geographical distribution in upper mantle seismic velocity may be due to the temperature effect character of upper mantle compressional wave velocity variation.

  2. 3-D upper mantle shear wave speed structure beneath the South Pacific Superswell by a BBOBS array

    NASA Astrophysics Data System (ADS)

    Isse, T.; Suetsugu, D.; Shiobara, H.; Sugioka, H.; Yoshizawa, K.; Kanazawa, T.; Fukao, Y.

    2005-12-01

    Previous seismic tomography studies show a broad low velocity anomaly in the lower mantle, so-called superplume, beneath the South Pacific and there are hotspot chains and large scale topographic high at surface of this region. However, the resolution of seismic tomography is poor, especially in the upper mantle, because of limited spatial distribution of seismic stations. To improve the station coverage, we deployed an array of long-term broadband ocean bottom seismometers (BBOBS) in this region. The quality of the vertical component of seismograms recorded by the BBOBS array is comparable with those by island seismic stations. This observation has enabled us to obtain a more precise 3-D shear wave speed structure in the upper mantle of this region by analyzing Rayleigh waves. We employed a two-station method to determine phase velocity of fundamental mode Rayleigh wave recorded by the BBOBS array and island stations in the Pacific Ocean. We obtained 1025 path-average phase velocity dispersion curves including 188 dispersion curves using the BBOBS data in a period range between 40 and 140 seconds. We then inverted them to a 3-D shear wave speed structure down to a depth of 200 km. At shallow depths the eastern part of the French Polynesia region is in general slower than the western part, which indicates an age-dependence of seismic structure of the uppermost mantle. Slow speed anomalies corresponding to the hotspots are apparently superposed on this age-dependence: Slow speed anomalies can be seen from the surface to a depth of 200 km beneath the Society, Pitcairn, and Macdonald hotspots, but they are limited only to the deep part beneath the Samoa hotspot. The slow speed anomalies beneath the Pitcairn and Society hotspots apparently coalesce at a depth of 100 km, where a single anomaly extending upward from below seems to branch into two directions. A resolution analysis indicates that the BBOBS array data has improved the spatial resolution substantially.

  3. Multi-observable thermochemical tomography of the lithosphere and upper mantle beneath the Western/Central US

    NASA Astrophysics Data System (ADS)

    Afonso, J. C.; Yang, Y.; Rawlinson, N.; Schutt, D.; Fullea, J.; Jones, A. G.

    2013-12-01

    We use a novel multi-observable 3D inversion method (Afonso et al., 2013a; b) to study the present-day thermal and compositional structures of the lithosphere and sublithospheric upper mantle beneath the Western and Central US (between 256-246o long and 33-43o lat). We jointly invert Rayleigh wave phase velocity maps for periods up to 150 sec (from ambient noise and earthquake data), P and S teleseismic travel time residuals (>63000 for P-waves and > 28000 for S-waves), geoid and gravity anomalies, surface heat flow, gravity gradients, and absolute elevation. These observables have different sensitivities to deep/shallow, thermal/compositional anomalies and therefore they provide complementary constraints to the inversion. The method is based on a thermodynamically-constrained, nonlinear probabilistic (Bayesian) approach and includes the effect of potential dynamic contributions from density anomalies in the sublithospheric mantle via full solutions of the Stokes-flow problem. From this joint inversion, we obtain the 3D density, compositional, electrical conductivity and thermal structure for the entire lithosphere (including a multi-layer crust) and sublithospheric upper mantle down to 400 km. The resulting models show a number of robust features that carry important implications for supporting or disapproving current evolutionary models for this region. References: - Afonso, J.C., Fullea J., Griffin, W.L., Yang, Y., Jones, A.G., Connolly, J.A.D., O'Reilly, S.Y. (2013a), 3D multi-observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle I: a priori information and geophysical observables. J. Geophys. Res., 118, 2586-2617, doi:10.1002/jgrb.50124. - Afonso, J.C., Fullea J., Yang, Y., Connolly, J.A.D., Jones, A.G. (2013b), 3D multi-observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle II: General methodology and resolution analysis. J. Geophys. Res

  4. The Interaction Between Supercontinent Cycles and Compositional Variations in the Deep Mantle

    NASA Astrophysics Data System (ADS)

    Lowman, J. P.; Trim, S. J.

    2015-12-01

    Earth is the only planet known to currently feature active plate tectonics. Two features that may influence the Earth's ability to sustain plate-like surface motion are the presence of continents and the inferred chemical piles lying on the core mantle boundary. In our previous study that modelled thermochemical convection in the mantle with evolving plates, it was shown that upwellings that form on top of chemical piles are relatively weak and make a diminished contribution to lithospheric stress. Yet, surface yielding is required in order to maintain plate tectonics and form new plate boundaries. Consequently an intrinsically dense layer in the lower mantle can decrease the vigour of convection and the likelihood of surface failure. In contrast to the mantle upwellings that form above the chemically dense provinces in our models, particularly vigorous plumes form where the ambient mantle lies adjacent to the core mantle boundary and at the edges of the chemically dense piles. Continents also affect surface mobility, due to their inherent buoyancy and their distinct yield strength. In this study we employ numerical models of mantle convection featuring both tectonic plates and compositional variation in the mantle and lithosphere. Plate-like surface motion is dynamically modelled using a force-balance method that determines plate velocities based upon lithospheric stresses. Oceanic and continental margins evolve in response to the plate velocities and specified lithospheric yield stresses. Compositional variations in the deep mantle are tracked using the tracer ratio method. For a range of ratios of the ambient mantle density to the density of the compositionally enriched material, we examine the the impact of mantle compositional variation on plate evolution, the effect of continents on planetary surface mobility and the frequency of supercontinent assembly versus the mobility of compositional provinces.

  5. Geophysical constraints on partial melt in the upper mantle

    SciTech Connect

    Shankland, T.J.; O'Connell, R.J.; Waff, H.S.

    1981-08-01

    This paper adresses the conditions under which partial melt can exist in the mantle in order to be observed as a geophysical 'anomaly'. Typical observed anomalies are high electrical conductivity of the order of 0.1 S/m or greater, velocity decreases of 7--10%, seismic Q values less than 100, and a frequency band for seismic effects in the region mear 1 Hz. Existing theories of electrical conduction in partial melts and of frequency-dependent seismic properties together with recent measurements of melt electrical conductivity, viscosity, and partial melt texture can be used to establish requirements for melt to be observed by geophysical methods. From electrical anomalies, mainly sensitive to melt volume and its interconnection, one can require a minimum melt fraction of several percent at temperatures close to the solidus (1150/sup 0/--1300/sup 0/C). However, seismic models demand only a small volume in very flattened shapes (aspect ratio approx. =0.001, melt fraction approx.0.1%). Further, if melt configuration permits seismic dissipation in bulk, that is, there exist flattened voids intersecting more or less equant voids, then it is possible to infer melt fractions for elastic anomalies that are consistent with the several percent required for electrical anomalies. Observed equilibrium textures of partly melted peridotite together with inferred melt-solid surface energies suggest that melt on a grain size scale in a gravitational field segregates into a strongly anisotropic pattern. Thus if partial melt causes mantle geophysical anomalies, it should exist in a variety of void shapes and probably of sizes. While the association of electrical and elastic anomalies with indications of reduced density, volcanism, and high heat flow makes the hypothesis of partial melting an attractive explanation, the minimum physical requirement is for existence of relatively high temperature.

  6. Accounting for lateral variations of the upper mantle gradient in Pn tomography studies

    NASA Astrophysics Data System (ADS)

    Phillips, W. S.; Begnaud, M. L.; Rowe, C. A.; Steck, L. K.; Myers, S. C.; Pasyanos, M. E.; Ballard, S.

    2007-07-01

    The effect of an upper mantle velocity gradient on regional arrival times has been approximated by a cubic distance term, which can be extended to two dimensions for use in tomographic studies. To demonstrate this, we add a laterally varying upper mantle gradient to the standard P n time-term tomography technique, and apply to a data set from Asia compiled using ground truth, event location criteria. We observe strong lateral variations in the gradient, ranging from -0.001 to 0.003 s-1, with high gradients associated with the Tethys convergence zone. The gradient patterns may reflect lateral variations in the thermal gradient of the mantle lid. Variance reduction is 63% with respect to P n tomography without gradients. Adding gradients allows the use of longer path lengths, improving velocity image definition in high-gradient regions with sparse station distribution, such as Tibet.

  7. Multiscale Modeling of Upper Mantle Plasticity: Integrating Experimental and Theoretical data into Mean-field Schemes

    NASA Astrophysics Data System (ADS)

    Raterron, P.; Castelnau, O.; Detrez, F.; Bollinger, C.; Cordier, P.; Fraysse, G.; Merkel, S.

    2013-12-01

    the hard phase. It shows that, to reproduce the low stresses expected in the deep upper mantle, ';diffusion-related' creep must contribute significantly to deformation. It also suggests that the significant weakening of olivine LPO with increasing depth results from both the P-induced [100]/[001] dislocation-slip transition and the increasing activity with T of ';diffusion-related' plasticity.

  8. Using Receiver Functions to Image the Montana Crust and Upper Mantle

    NASA Astrophysics Data System (ADS)

    Sirianni, R. T.; Russo, R. M.

    2008-12-01

    We determined receiver functions (RFs) at six permanent Advanced National Seismic System (ANSS) stations to examine crust and upper mantle structure of the Wyoming craton (WC) and Medicine Hat block (MHB). The Deep Probe & SAREX projects (Henstock et al., 1998; Clowes et al., 2002; Gorman et al., 2002) used active source seismics to model a high velocity crustal layer (the so-called 7x layer) beneath the WC. This layer exhibits P wave velocities that are high for lower continental crust (~7+ km/s) and extends from 30-55 km below the surface. Interpretations of the active source data indicate that this layer may represent wide scale crustal underplating of the WC, implying post-Archean craton modification with implications for Laurentia assembly. We used 43 earthquakes from a wide azimuthal distribution recorded at the Montana ANSS stations; high signal-to-noise ratios of 25 of these RFs were acceptable for further analysis. Receiver functions constrain crustal velocity structure beneath a seismometer by using P-to-S wave conversions at sharp velocity contrast boundaries. Preliminary results for seismic stations DGMT, EGMT, and LAO, located to the east of the Deep Probe and SAREX seismic line on the Wyoming craton/Medicine Hat block show the influence of sedimentary cover and a strong Ps phase at approximately four seconds after P. At BOZ and MSO, located in the Rocky mountains, the sedimentary cover signal previously noted is absent, and instead we observe a sharp Ps phase at about four and a half seconds after P. RFs at station RLMT (on the WC) are highly anomalous, probably reflecting complex conversions from two differently oriented dipping layers. We will use the RFs to produce suites of acceptable structural models to test for the presence and lateral extent of the 7x layer and other structural features of the Rocky Mountains-craton transition.

  9. Are Deep Seismic Reflections at Volcanic Margins from the Petrological Moho or from within the Mantle?

    NASA Astrophysics Data System (ADS)

    Harkin, C. J.; Kusznir, N. J.; Roberts, A. M.; Bellingham, P.; Manatschal, G.

    2015-12-01

    Deep long-offset seismic-reflection now frequently provide imaging of strong and laterally continuous reflectors in the TWTT range of 10 to 14 seconds. Examples of deep laterally-coherent reflectivity can be seen within the ocean-continent transition of the Argentine, Uruguayan and S Brazilian volcanic margins of the S Atlantic. Qualitative interpretation of the seismic data suggests the presence of deep crustal "keels" or crustal roots underlying well developed seaward dipping reflectors (SDRs). While an initial interpretation might be that these reflectors correspond to the crust-mantle interface, this interpretation may in some cases be incorrect or over-simplistic. Do these deep reflectors correspond to the petrological Moho or could they be located within the mantle? Joint inversion of the PSTM time-domain seismic reflection and gravity anomaly data has been used to determine the average interval density and seismic velocity between base sediment and the deep seismic reflectivity. Joint inversion densities and seismic velocities for this depth interval reach values in excess of 3000 kg/m3 and 7.0 km/sec for the entire thickness of the interval, substantially in excess of densities and velocities observed for normal oceanic and continental crust. The high densities determined from joint seismic-gravity inversion under the SDR regions are also consistent with results from flexural subsidence analysis. We consider two interpretations of these results. One interpretation is that the strong deep reflectivity corresponds to the base of the petrological crust and that the crust has an abnormally high average density and seismic velocity due to high-temperature mantle-plume-related magmatism. An alternative interpretation is that the deep seismic reflectivity is located within the mantle beneath the petrological Moho, and that the high density and seismic velocity result from averaging of both crustal basement (~2850 kg/m3) and mantle (~3300 kg/m3) values. In some

  10. Deep Seismic Reflectivity at Volcanic Margins: Reflections from the Petrological Moho or from within the Mantle?

    NASA Astrophysics Data System (ADS)

    Kusznir, Nick; Roberts, Alan; Bellingham, Paul

    2015-04-01

    Advances in deep long-offset seismic-reflection acquisition and processing now frequently provide imaging of strong and laterally continuous reflectors in the TWTT range of 10 to 14 seconds. While an initial interpretation might be that these reflectors correspond to the crust-mantle interface, this interpretation may in some cases be incorrect or over-simplistic. Do these deep reflectors correspond to the petrological Moho or could they be located within the mantle? Examples of deep laterally-coherent reflectivity can be seen within the ocean-continent transition of the Argentine, Uruguayan and S Brazilian volcanic margins of the S Atlantic. An initial qualitative interpretation of the seismic data suggests the presence of deep crustal "keels" or crustal roots underlying well developed seaward dipping reflectors (SDRs). Joint inversion of the PSTM time-domain seismic reflection and gravity anomaly data has been used to determine the average interval density and seismic velocity between base sediment and the deep seismic reflectivity. Joint inversion densities and seismic velocities for this depth interval reach values in excess of 3000 kg/m3 and 7.0 km/sec for the entire thickness of the interval, substantially in excess of densities and velocities observed for normal oceanic and continental crust. The high densities determined from joint seismic-gravity inversion under the SDR regions are also consistent with results from flexural subsidence analysis. We consider two interpretations of these results. One interpretation is that the strong deep reflectivity corresponds to the base of the petrological crust and that the crust has an abnormally high average density and seismic velocity due to high-temperature mantle-plume-related magmatism. An alternative interpretation is that the deep seismic reflectivity is located within the mantle beneath the petrological Moho, and that the high density and seismic velocity result from averaging of both crustal basement (~2850

  11. Crust and upper mantle resistivity structure at middle section of Longmenshan, eastern Tibetan plateau

    NASA Astrophysics Data System (ADS)

    Wang, Xuben; Zhang, Gang; Fang, Hui; Luo, Wei; Zhang, Wei; Zhong, Qing; Cai, Xuelin; Luo, Haozhong

    2014-04-01

    Longmenshan is located in the eastern edge of the Tibetan Plateau. It is one of the hotspots of geophysical and geological studies in the world for its special tectonic characters and dynamic mechanism, especially after the 2008 Wenchuan earthquake (Ms 8.0). From 2008 to 2010, a 570 km Long-period magnetotelluric (LMT) and broadband magnetotelluric (MT) sounding profile across the Longmenshan fault zone has been carried out to investigate the crust and upper mantle resistivity structure. The coupling relationship of Longmenshan with the Songpan-Ganzi block and with the Sichuan basin of Yangtze block has been studied respectively. The analysis of the deep resistivity structure is of importance for the study of the geodynamic characteristics of the Wenchuan earthquake. The inversion reveals a high-low-high resistivity layer in the Songpan-Ganzi block, a low-high resistivity layer in the Sichuan basin and a complex resistivity structure in the Longmenshan area, respectively. There is a low resistivity layer in the middle and lower crust at the depth about 20-45 km beneath the Songpan-Ganzi block, which has shown that there may be a continuous slip layer in the crust, while there is no such a layer in the crust beneath the Sichuan basin. The thrust imbrication of resistivity structure in the upper crust beneath the Longmenshan indicates that the Songpan-Ganzi block is over thrusting onto the Yangtze block. And the low resistivity layer in the lower crust stretching downward to the bottom of the lithosphere beneath Longmenshan indicates that the Songpan-Ganzi block subducts below the lithosphere of Yangtze block. The complex structure of high resistivity massif in the lithosphere beneath Longmenshan provides some important evidence for the study of the mechanism of the 2008 Wenchuan earthquake.

  12. P, S wave velocity model of the crust and upper most mantle of Albania region

    NASA Astrophysics Data System (ADS)

    Ormeni, Rrapo

    2011-01-01

    This paper describes the one-dimensional (1D) velocity model computed by VELEST in the SEISAN seismic analysis system, inverting re-picked P-wave and S-wave arrival times recorded during 2002-2006 by the Albanian, Montenegro, Thessalonica and Macedonia seismic networks. The re-picked data yield P-wave and S-wave velocities proved to be more suitable compared to bulletin data for this detailed inversion study. Seismic phases recorded by the Albania seismic network and integrated with data from the Montenegro, Thessalonica and Macedonia networks are used to prepare the Albanian seismic bulletin. Earthquake hypocenters from the Albanian bulletins have also location errors that are negligible for civil protection purposes, large scale seismotectonic analyses and more accurate hypocentral determinations which are necessary for detailed seismotectonic and geodynamic studies. It was noted that the smoothness of the velocity variation increased with depth. A velocity of 5.5 km/s was calculated for the upper crust, 6.1 km/s was calculated for the middle crust and 6.9 km/s was computed for the lower crust. P wave velocity was 7.85 km/s at depth of 50 km and for the upper mantle it is 8.28 km/s. Using the improved velocity model, the earthquakes which occurred in Albania in the past 5 years were able to be relocated, achieving constrained hypocentral determinations for events in Albania. The interpretation of the 1 D velocity models infers interesting features of the deep structure of Albania. These results represent an important step towards more detailed seismotectonic analyses.

  13. Osmium Isotopic Compositions of Chondrites and Earth's Primitive Upper Mantle: Constraints on the Late Veneer

    NASA Technical Reports Server (NTRS)

    Walker, R. J.; Horan, M. F.; Morgan, J. W.; Meisel, T.

    2001-01-01

    The 187 Os/188 Os of carbonaceous chondrites averages approximately 2% lower than for enstatite and ordinary chondrites. The primitive upper mantle ratio for the Earth best matches that of ordinary and enstatite chondrites. Additional information is contained in the original extended abstract.

  14. Crust and Upper Mantle of North Africa Using Libyan Seismic Data

    NASA Astrophysics Data System (ADS)

    Pasyanos, M. E.; Eshwehdi, A.

    2005-12-01

    We investigate the crust and upper mantle structure of North Africa using Libyan seismic data. Libya sits at the transition between the relatively aseismic continental crust of the African plate and the seismically active oceanic crust under the Mediterranean Sea which is subducting under the Eurasian Plate along the Calabrian, Hellenic, and Cyprean Arcs. The country also encompasses the Sirte Basin to the north and the smaller Murzuk and Kufra basins in the south. Broadband data from several seismic stations in Libya provide an opportunity for studying the velocity structure of the region. We have made some preliminary dispersion measurements from these stations and have found notable improvements in the group velocity tomography model by incorporating the additional measurements. We will be adding to this analysis by making dispersion measurements from regional events and receiver functions for teleseismic events. Recently, we have been employing methods to jointly invert both surface wave dispersion data and teleseismic receiver functions. The technique holds great promise in accurately estimating seismic structure, including important tectonic parameters such as basin thickness, crustal thickness, upper mantle velocity, as well as more detail about the upper mantle (lithospheric thickness and presence of anisotropy). We propose to apply this method to data from several Libyan stations where we can and, in the absence of receiver functions, invert the dispersion data only. The technique holds the promise of improving our understanding of the crust and upper mantle in Libya and how it fits into the larger tectonic picture of North Africa.

  15. Subduction recycling of continental sediments and the origin of geochemically enriched reservoirs in the deep mantle

    SciTech Connect

    Rapp, R.P.; Irifune, T.; Shimizu, N.; Nishiyama, N.; Norman, M.D.; Inoue, T.

    2008-10-08

    Isotopic and trace element geochemical studies of ocean island basalts (OIBs) have for many years been used to infer the presence of long-lived ({approx} 1-2 Ga old) compositional heterogeneities in the deep mantle related to recycling of crustal lithologies and marine and terrigenous sediments via subduction [e.g., Zindler, A., Hart, S.R., 1986. Chemical geodynamics. Annu. Rev. Earth Planet. Sci. 14, 493-571; Weaver, B.L., 1991. The origin of ocean island basalt end-member compositions: trace element and isotopic constraints. Earth Planet. Sci. Lett. 104, 381-397; Chauvel, C., Hofmann, A.W., Vidal, P., 1992. HIMU-EM: the French Polynesian connection. Earth Planet. Sci. Lett. 110, 99-119; Hofmann, A.W., 1997. Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219-229; Willbold, M., Stracke, A., 2006. Trace element composition of mantle end-members: Implications for recycling of oceanic and upper and lower continental crust. Geochem. Geophys. Geosyst. Q04004. 7, doi:10.1029/2005GC001005]. In particular, models for the EM-1 type ('enriched mantle') OIB reservoir have invoked the presence of subducted, continental-derived sediment to explain high {sup 87}Sr/{sup 86}Sr ratios, low {sup 143}Nd/{sup 144}Nd and {sup 206}Pb/{sup 204}Pb ratios, and extreme enrichments in incompatible elements observed in OIB lavas from, for example, the Pitcairn Island group in the South Pacific [Woodhead, J.D., McCulloch, M.T., 1989; Woodhead, J.D., Devey, C.W., 1993. Geochemistry of the Pitcairn seamounts, I: source character and temporal trends. Earth Planet. Sci. Lett. 116, 81-99; Eisele, J., Sharma, M., Galer, S.J.G., Blichert-Toft, J., Devey, C.W., Hofmann, A.W., 2002. The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot. Earth Planet. Sci. Lett. 196, 197-212]. More recently, ultrapotassic, mantle-derived lavas (lamproites) from Gaussberg, Antarctica have been interpreted as the product

  16. New interpretation of the deep mantle structure beneath eastern China

    NASA Astrophysics Data System (ADS)

    Ma, Pengfei; Liu, Shaofeng; Lin, Chengfa; Yao, Xiang

    2016-04-01

    Recent study of high resolution seismic tomography presents a large mass of high velocity abnormality beneath eastern China near the phase change depth, expanding more than 1600km-wide in East-west cross-section across the North China plate. This structure high is generally believed to be the subducted slab of Pacific plate beneath the Eurasia continent, while its origin and dynamic effect on the Cenozoic tectonic evolution of eastern China remain to be controversial. We developed a subduction-driven geodynamic mantle convection model that honors a set of global plate reconstruction data since 230Ma to help understand the formation and evolution of mantle structure beneath eastern China. The assimilation of plate kinematics, continuous evolving plate margin, asymmetric subduction zone, and paleo seafloor age data enables the spatial and temporal consistency between the geologic data and the mantle convection model, and guarantees the conservation of the buoyancy flux across the lithosphere and subducted slabs. Our model achieved a first order approximation between predictions and the observed data. Interestingly, the model suggests that the slab material stagnated above discontinuity didn't form until 15Ma, much later than previous expected, and the fast abnormality in the mid-mantle further west in the tomographic image is interpreted to be the remnants of the Mesozoic Izanagi subduction. Moreover, detailed analysis suggests that the accelerated subduction of Philippine Sea plate beneath Eurasia plate along the Ryukyu Trench and Nankai Trough since 15Ma may largely contribute to extending feature above 670km discontinuity. The long distance expansion of the slab material in the East-west direction may be an illusion caused by the approximate spatial perpendicularity between the cross-section and the subduction direction of the Philippine Sea plate. Our model emphasizes the necessity of the re-examination on the geophysical observation and its tectonic and

  17. High-pressure orthorhombic ferromagnesite as a potential deep-mantle carbon carrier

    DOE PAGES

    Liu, Jin; Lin, Jung -Fu; Prakapenka, Vitali B.

    2015-01-06

    In this study, knowledge of the physical and chemical properties of candidate deep-carbon carriers such as ferromagnesite [(Mg,Fe)CO3] at high pressure and temperature of the deep mantle is necessary for our understanding of deep-carbon storage as well as the global carbon cycle of the planet. Previous studies have reported very different scenarios for the (Mg,Fe)CO3 system at deep-mantle conditions including the chemical dissociation to (Mg,Fe)O+CO2, the occurrence of the tetrahedrally-coordinated carbonates based on CO4 structural units, and various high-pressure phase transitions. Here we have studied the phase stability and compressional behavior of (Mg,Fe)CO3 carbonates up to relevant lower-mantle conditions ofmore » approximately 120 GPa and 2400 K. Our experimental results show that the rhombohedral siderite (Phase I) transforms to an orthorhombic phase (Phase II with Pmm2 space group) at approximately 50 GPa and 1400 K. The structural transition is likely driven by the spin transition of iron accompanied by a volume collapse in the Fe-rich (Mg,Fe)CO3 phases; the spin transition stabilizes the high-pressure phase II at much lower pressure conditions than its Mg-rich counterpart. It is conceivable that the low-spin ferromagnesite phase II becomes a major deep-carbon carrier at the deeper parts of the lower mantle below 1900 km in depth.« less

  18. High-Pressure Orthorhombic Ferromagnesite as a Potential Deep-Mantle Carbon Carrier

    PubMed Central

    Liu, Jin; Lin, Jung-Fu; Prakapenka, Vitali B.

    2015-01-01

    Knowledge of the physical and chemical properties of candidate deep-carbon carriers such as ferromagnesite [(Mg,Fe)CO3] at high pressure and temperature of the deep mantle is necessary for our understanding of deep-carbon storage as well as the global carbon cycle of the planet. Previous studies have reported very different scenarios for the (Mg,Fe)CO3 system at deep-mantle conditions including the chemical dissociation to (Mg,Fe)O+CO2, the occurrence of the tetrahedrally-coordinated carbonates based on CO4 structural units, and various high-pressure phase transitions. Here we have studied the phase stability and compressional behavior of (Mg,Fe)CO3 carbonates up to relevant lower-mantle conditions of approximately 120 GPa and 2400 K. Our experimental results show that the rhombohedral siderite (Phase I) transforms to an orthorhombic phase (Phase II with Pmm2 space group) at approximately 50 GPa and 1400 K. The structural transition is likely driven by the spin transition of iron accompanied by a volume collapse in the Fe-rich (Mg,Fe)CO3 phases; the spin transition stabilizes the high-pressure phase II at much lower pressure conditions than its Mg-rich counterpart. It is conceivable that the low-spin ferromagnesite phase II becomes a major deep-carbon carrier at the deeper parts of the lower mantle below 1900 km in depth. PMID:25560542

  19. Upper mantle shear wave velocity structure of the east Anatolian-Caucasus region

    NASA Astrophysics Data System (ADS)

    Skobeltsyn, Gleb Anatolyevich

    The Eastern Anatolian-Caucasus region is a relatively young part of the Alpine- Himalayan orogenic belt and has been formed as the result of the ongoing continental collision of Arabia and Eurasia. In spite of a number of geological studies that have been conducted in this area, there is still no consensus within the geoscience community about the regional tectonic settings and a model for the late Cenozoic tectonic evolution of the Anatolian Plateau. Knowledge of the upper mantle velocity structure in this region can provide the geological community with important constraints that are crucial for developing an understanding of the regional geology and the processes associated with early stages of mountain building. In the present dissertation, I describe two studies of the regional upper mantle S wave velocity structure. In order to derive the absolute velocity structure of the upper mantle, I have applied surface wave tomography to model Rayleigh wave phase velocities as a function of period. Then I inverted the Rayleigh phase velocities to obtain S wave velocities as a function of depth. The resulted high-resolution 3-D S wave velocity model of the regional upper mantle is characterized by a better depth resolution than any preexisting tomographic models. I also conducted an S wave splitting analysis using traditional methods and developed a two-layer grid search algorithm in order to infer the upper mantle anisotropic structure. The results of the S wave splitting analysis for the stations located in Azerbaijan are the first in the region. (Abstract shortened by ProQuest.).

  20. Os Isotope Heterogeneity of the Convecting Upper Mantle: The Mayari-Baracoa Ophiolitic Belt (Eastern Cuba)

    NASA Astrophysics Data System (ADS)

    Frei, R.; Gervilla, F.; Meibom, A.; Proenza, J. A.

    2005-12-01

    Chromite separates from a set of historically important chromite deposits from the 90 Ma old Mayarí-Baracoa Ophiolitic Belt in eastern Cuba were inspected for Re-Os isotopic systematics in an attempt to quantify the extent of Os isotopic heterogeneities within a restricted upper mantle portion represented by a single ophiolite. Compositional variations of chromites indicate their crystallization from hydrous melts varying in composition from back-arc basin basalts (Al-rich chromites; Cr# = 0.43-0.55; low Pd/Ir) to boninites (Cr-rich chromites; Cr# = 0.60-0.83; high Pd/Ir) in a supra-subduction zone setting. Initial Os isotopic compositions of the studied chromites can be grouped according to their distribution in 3 regional districts. Results indicate systematically negative calculated initial γOs values varying from -1.06 ± 0.79 (Moa-Baracoa district), -1.77 ± 0.80 (Sagua de Tanamo district) and -2.79 ± 0.31 (Mayari district). These suprachondritic values are distinctly (3.5-5.2%) less radiogenic than the estimated minimum 187Os/188Os composition of the primitive upper mantle of 0.1296 ± 8 and can be explained by Re depletion during ancient partial melting and melt percolation events. Old Os isotope model ages (<2100 Ma)of some of the chromites (or platinum-group minerals included in them) show and confirm previous findings that ancient Os isotopic signatures can survive in the Earth's upper mantle. Our systematically negative initial γOs values do not improve the definition of an already statistically poorly defined present-day Os isotopic composition of the convecting upper mantle, but instead indicate a complex history for the convecting upper mantle which precludes the calculation of a uniform regional Os isotopic signature for this reservoir.

  1. Fault-controlled hydration of the upper mantle during continental rifting

    NASA Astrophysics Data System (ADS)

    Bayrakci, G.; Minshull, T. A.; Sawyer, D. S.; Reston, T. J.; Klaeschen, D.; Papenberg, C.; Ranero, C.; Bull, J. M.; Davy, R. G.; Shillington, D. J.; Perez-Gussinye, M.; Morgan, J. K.

    2016-05-01

    Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems. Serpentinized mantle rocks are found beneath the sea floor at slow- to ultraslow-spreading mid-ocean ridges and are thought to be present at about half the world’s rifted margins. Serpentinite is also inferred to exist in the downgoing plate at subduction zones, where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults. Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that sea water reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth.

  2. Lower crust and upper mantle electrical anisotropy in southeastern Australia

    NASA Astrophysics Data System (ADS)

    Dennis, Zara R. 13Thiel, Stephan 2Cull, James P.

    2012-08-01

    The dominant north-south strike of the Palaeozoic outcrop of central Victoria has been well documented, but to the north, these rocks are covered by the Cainozoic sedimentary deposits of the Murray Basin. Two magnetotelluric surveys were completed to assist in extrapolation of the known structure and to identify possible new targets for mineral discovery. Supporting the results from previous seismic interpretations for the region, the 2D MT inversion models substantiate an intrazone thrust fault system of listric geometries in the Bendigo Zone connected in the mid-crust. With the zone boundary clearly defined the electrical resistivity structure is distinct between the major subdivisions, indicating a different tectonic evolution for the Bendigo and Melbourne Zones. However, the conductive overburden in the region poses complications for the generation of the 2D resistivity models. Static shifts and electrical anisotropy were identified as distortions in the dataset, with further processing needed to attain a complete picture of the underlying geology. The difficulties caused by galvanic distortion were allayed by using the phase tensor response in place of the distorted amplitude response. Phase tensor analysis of MT data has been completed subsequently, the results of which we present here, along with the original 2D inversion models, confirming that electrical anisotropy persists into the mantle.

  3. Origin of Ultra-Deep Diamonds: Chemical Interaction of Ca-CARBONATE and the Earth's Lower Mantle Minerals

    NASA Astrophysics Data System (ADS)

    Spivak, A. V.; Dubrovinsky, L. S.; Litvin, Yu. A.

    2012-04-01

    The main goal of the work is experimental study of physicochemical conditions of origin of ultra-deep diamonds in the substance of the Earth's lower mantle (LM) based on the experimental criterium of syngenesis of diamond and primary inclusions of LM mineral. Magnesiowustite (Mg,Fe)O, Mg-Fe perovskite (Mg,Fe)(Si,Al)O3 and Ca-perovskite CaSiO3 mainly present the LM substance and are frequently disclosed as primary inclusions in ultra-deep diamonds together with Ca-, (Ca, Mg, Fe)-, Na-Ca-carbonates. For the upper mantle conditions, the mantle-carbonatite conception of diamond genesis was developed based on the effects of congruent melting of carbonates and complete liquid miscibility of carbonate-silicate melts. Melting of Ca-carbonate and CaCO3 - (Mg,Fe)O, CaCO3 - (Mg,Fe)(Si,Al)O3 systems, stability of the melts and their decomposition were studied in static high pressure experiments at pressures of 16 to 55 GPa and temperatures of 1600 to 3900 K using diamond anvil cell technique with laser heating. It was determined that melting of Ca-carbonate is congruent at the PT-conditions of the lower mantle and characterized by an expanded field of liquid Ca-carbonate phase. We observed formation of graphite (below 16 GPa) and diamond (between 16 and 43 GPa) on decomposition of the CaCO3 melt at temperatures above 3400 K. At temperatures below 3400 K congruent melting of calcium carbonate was confirmed. Also it was shown that CaCO3 - (Mg,Fe)O - (Mg,Fe)(Si,Al)O3 system is capable to form diamonds together with Ca-carbonate, magnesiowustite and perovskite as syngenesis minerals at PT-conditions of the lower mantle. We observed formation diamond (between 40 and 55 GPa) on decomposition of the CaCO3 from CaCO3 - (Mg,Fe)(Si,Al)O3 melt at temperatures above 2000 K. The experimental data on phase relations at the melting and decomposition of CaCO3 and CaCO3-(Mg,Fe)O-(Mg,Fe)(Si,Al)O3 system as well as diamond crystallization are applied to the problem of formation of natural ultra-deep

  4. Interaction Between the Supercontinent Cycle and the Evolution of Intrinsically Dense Provinces in the Deep Mantle

    NASA Astrophysics Data System (ADS)

    Lowman, J. P.; Trim, S. J.

    2016-12-01

    Shear-wave travel times in the Earth's deep mantle reveal broad steep-sided seismologically distinct provinces lying on the Core-Mantle Boundary (CMB). The longevity and permanence of the two large principal provinces, located below the sites of present-day Africa and the Pacific Ocean, have become a matter of great interest. Examination of the flood basalt record and kimberlite eruption dating suggests the presence of these provinces may disclose a deep mantle component with a compositionally distinct origin playing a role in the generation of mantle plumes at preferred locations. By extension, the presence of these provinces may affect the supercontinent cycle. Implementing a mantle convection model featuring distinct continental lithosphere and a Compositionally Anomalous and Intrinsically Dense (CAID) component, we study the distribution and mobility of naturally forming compositionally distinct provinces and their impact on model supercontinent assembly. In calculations featuring Earth-like convective vigor and global scale we find that an intrinsically dense mantle component generally aggregates into one or two broad provinces. The positions of the provinces are time-dependent but in many of our calculations the province locations are characterized by periods of fixity that reach several hundred million years. Eras of province and associated plume fixity are punctuated by periods of relatively rapid migration. A correlation between supercontinent position and the locations of CAID provinces is not supported by our findings. However, we find the frequency of supercontinent assemblies increases when CAID provinces are present.

  5. Crustal and upper mantle structure of the Slave craton from P- and S- Receiver Functions

    NASA Astrophysics Data System (ADS)

    Barantseva, Olga; Vinnik, Lev; Artemieva, Irina

    2017-04-01

    Teleseismic events recorded by POLARIS array in NW Canada (Slave craton) and Yellowknife station were used to calculate a sufficient number of receiver functions for P (PRF) and S (SRF) waves. Velocity (Vp and Vs) and Vp/Vs profiles from the Earth's surface down to 300 km are obtained through the simultaneous inversion of PRF and SRF with teleseismic travel time residuals for the crust and upper mantle. We observe highly heterogeneous structure of the cratonic upper mantle. The Lehman discontinuity (the bottom of the low velocity zone) is found in the western Slave craton, whereas it is not observed in the eastern part of the Slave craton. At stations located in the southern part of the craton, we observe an increase of S-wave velocities (as compared to IASP91 values) at the depths 45-150 km which is typical for depleted cratonic mantle. Low Vp/Vs ratio, obtained for the uppermost mantle (1.65-1.70) can be explained by a high fraction of Opx. A comparison of our results with available xenoliths data shows a good agreement between seismic velocity change at a depth of ca. 160 km and a decrease in mantle depletion at about the same depth.

  6. An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions

    NASA Astrophysics Data System (ADS)

    Warren, J. M.; Shimizu, N.; Sakaguchi, C.; Dick, H. J. B.; Nakamura, E.

    2009-12-01

    Abyssal peridotites, the depleted solid residues of ocean ridge melting, are the most direct samples available to assess upper oceanic mantle composition. We present detailed isotope and trace element analyses of pyroxene mineral separates from Southwest Indian Ridge abyssal peridotites and pyroxenites in order to constrain the size and length scale of mantle heterogeneity. Our results demonstrate that the mantle can be highly heterogeneous to <1 km and even <0.1 m length scales. Examination of Nd isotopes in relation to modal, trace, and major element compositions indicate that the length scales and amplitudes of heterogeneities in abyssal peridotites reflect both ancient mantle heterogeneity and recent modification by melting, melt-rock reaction and melt crystallization. The isotopic and trace element compositions of pyroxenite veins in this study indicate that they are not direct remnants of recycled oceanic crust, but instead are formed by recent melt crystallization. Combined with existing data sets, the results show that the average global isotopic composition of peridotites is similar to that of mid-ocean ridge basalts, though peridotites extend to significantly more depleted 143Nd/144Nd and 87Sr/86Sr. Standard isotope evolution models of upper mantle composition do not predict the full isotopic range observed among abyssal peridotites, as they do not account adequately for the complexities of ancient and recent melting processes.

  7. Electromagnetic Imaging of the Crust and Upper Mantle across the Continental Margin in Central California

    NASA Astrophysics Data System (ADS)

    Wheelock, Brent David

    Electromagnetic methods, particularly magnetotelluric (MT) sounding, are often used to detect fluids in the crust and upper mantle. Recently, MT studies on land have focused on the deep structure of the San Andreas fault (SAF) in central California. One of these studies found a region of high conductivity, interpreted as a local abundance of pore fluid, southwest of the fault at depths corresponding to the lower crust and upper mantle. The study also identified a change in the connection of this deep conductor to the root of the SAF as their measurements moved along the strike of the fault. It was proposed that a change in access of fluids to the deep extension of the fault, as evidenced by the modeled resistivity of the region, is responsible for the observed change in behavior of both earthquakes and non-volcanic tremor along the central Californian segment of the SAF. As an effort to extend knowledge of the resistivity structure into the offshore environment of central California, as well as to better constrain the interpretations made onshore, the Marine Electromagnetic Research Group at the Scripps Institution of Oceanography collected marine MT and controlled-source electromagnetic (CSEM) data there. Our single profile of data extends an existing profile of land data. In this dissertation I interpret both profiles of MT data, land and marine, jointly. Inversions of the amphibious dataset produce a resistivity model that is similar to previous results. However, upon closer inspection it was found that the marine data and the land data solicit antithetical structures. I suspected this to be caused by distortion from bathymetry that varies in three dimensions (3D), which is a problem because our 2D models assume that the earth is uniform in the horizontal dimension perpendicular to our profile. Few numerical methods exist to calculate the effects of 3D bathymetry in the necessary detail, so I improved upon an established thin-sheet technique to allow for higher

  8. Upper Mantle Qβ Structure beneath the East Pacific Rise from Shear Wave Triplicated Waveforms

    NASA Astrophysics Data System (ADS)

    Wang, Y.; Grand, S. P.; Tang, Y.

    2012-12-01

    A consensus on how mantle flows in the transition zone region of the Earth has not been reached. Some propose a boundary to flow while others prefer a model with essentially no boundary to flow across the upper-lower mantle boundary. It is possible that an intermediate situation exists where flow is intermittent across the boundary. A situation where mantle flow is inhibited near the 660 km discontinuity will result in a thermal boundary layer at that depth and thus high temperature gradients. Such a high temperature gradient zone is difficult to detect using seismic velocities but may be detectable through measurements of Q since attenuation is highly sensitive to temperature. Models of seismic attenuation as a function of depth through the mantle are difficult to determine. Normal mode and surface waves have been used but use long wavelength waves and thus can only resolve broad scale structure. Body waves such as multiple ScS phases have been used for regional mantle attenuation studies but lack vertical resolution. S waves recorded from 15 to 28 degrees distance turn within the upper mantle and due to discontinuities near 410 and 660 km depth are triplicated with multiple arrivals sampling different depths arriving at the same station. The triplicated arrivals can also be seen in SS waves at double the distance and SSS waves at triple the distance …. Here we model dense profiles of broadband S, SS, SSS and SSSS waves recorded mainly by US-Array, Canadian seismic network and other surrounding stations in the North America. Earthquakes along the East Pacific Rise were recorded with the wave paths underneath oceanic crust younger than 15Ma. The distance range covered is from 15 to 105 degrees, consequently triplicated body waves sample the transition zone from S to SSSS. By modeling the relative amplitudes of the triplicated waveforms a model of Qβ (.01-.1 Hz) as a function of depth is determined focused on the mantle transition zone.

  9. Crust and upper mantle electrical structure of Haiyuan-Liupanshan Thrust Belt and its vicinity revealed by magnetotelluric(MT) detection

    NASA Astrophysics Data System (ADS)

    Han, S.; Liu, G.; Han, J.

    2015-12-01

    Under the auspices of SinoProbe Project, an array of 91 broad-band magnetotelluric(MT) sites across the southern segment of the Haiyuan-Liupanshan Thrust Belt (HLTB) was occupied to determine the crust and upper mantle structure of the transition zone between the Ordos Block (OB) and the Qilian Orogenic Belt (QOB).An electrical structure model of the crust and the upper mantle was finally obtained after data processing, qualitative analysis and 2D inversion of the observed data.The model revealed the deep structure of the profile.The upper crust of the HLTB is modelled as resistive while the other two tectonic units are modelled as less resistive.The massive high resistive blocks in the upper crust are seen in the HLTB.On the contrast,the lower crust is revealed as conduvtive on the whole.Middle to lower crustal high conductive layers (HCL) are seen both in the QOB and the OB.A strong lower crust conductor is revealed in the HLTB.Electrical structure of the upper mantle is revealed as resistive,respectively.The wedge structure is seen in the uppermost mantle under the Liupanshan Mountain.According to the electrical structure of the profile,the study region can be divided into three tectonic units:the QOB,the Liupan Transition Zone (LTZ,expansion of the HLTB) and the OB.The tectonic deformation for the QB manifest as thrust nappe in the upper crust and shortening strain in the lower crust.The east-dipping conductor in the west of QOB may represent the accumulated weak material in the form of middle crust flow and the HCL of the QB may be the migration channel.The fluctuation of HCL may indicate interior deformation of the OB.The LTZ is quite different from the adjacent tectonic units that fragment structure exists in the upper crust and deep thrust faults cut through the upper crust.The conductor located in the lower crust is interpreted as partial melt zone as a result of the vertical decompression process.Joint interpretation of the electrical structure and the

  10. Upper mantle structure of central and West Antarctica from array analysis of Rayleigh wave phase velocities

    NASA Astrophysics Data System (ADS)

    Heeszel, David S.; Wiens, Douglas A.; Anandakrishnan, Sridhar; Aster, Richard C.; Dalziel, Ian W. D.; Huerta, Audrey D.; Nyblade, Andrew A.; Wilson, Terry J.; Winberry, J. Paul

    2016-03-01

    The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a two-plane wave tomography method and are inverted for shear velocity using a Monte Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70-100 km thick lithosphere, underlain by a low-velocity zone to depths of at least 200 km. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow-velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher-velocity lithosphere to depths of about 70 km beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90 mW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity.

  11. Kinetics of melt migration in upper mantle-type rocks

    NASA Astrophysics Data System (ADS)

    Riley, G. N.; Kohlstedt, D. L.

    1991-08-01

    Experiments have been performed to determine the permeability of an aggregate of olivine plus a silicate melt, as well as the viscosity of the matrix. Melt migration couples were formed between discs composed of olivine with about 12% of a synthetic potassium-aluminum silicate glass and discs of polycrystalline San Carlos olivine. Four melt infiltration experiments were carried out at temperatures between 1050 and 1255°C at 300 MPa in a gas-medium apparatus; each couple was held at the experimental conditions for 2 h. At temperature, capillary forces cause the molten glass to infiltrate into the dunite along triple junctions because the dihedral angle is less than 60°. In order to analyze the resulting melt migration profiles, the coupled differential equations governing melt migration via "porous flow" driven by capillary forces and resisted by compaction/dilation of the matrix were solved numerically. The effects of dihedral angle, melt fraction exponent and amount of the melt in the source region on the spatial and temporal evolution of the melt distribution were investigated with numerical simulations for the initial and boundary conditions imposed by the experimental geometry. The permeability of the aggregate and the viscosity of the matrix were determined by comparison of the melt migration profiles obtained from the experiments with those generated from simulation. The permeability of the partially molten aggregate increased approximately linearly with increasing melt fraction and, at 1255°C, the permeability of the rock and viscosity of the olivine are about 8 × 10 -16 m 2 and 8 × 10 9 Pa s, respectively, for a grain size of 4.2 μm and a melt fraction of 0.145. These results predict a relatively high permeability at low melt fractions, indicating that only very small amounts of melt (˜ 0.1%) could be maintained in a dunitic mantle.

  12. Mapping Upper Mantle Seismic Discontinuities Using Singular Spectrum Analysis

    NASA Astrophysics Data System (ADS)

    Gu, Y. J.; Dokht, R.; Sacchi, M. D.

    2015-12-01

    Seismic discontinuities are fundamental to the understanding of mantle composition and dynamics. Their depth and impedance are generally determined using secondary seismic phases, most commonly SS precursors and P-to-S converted waves. However, the analysis and interpretation using these approaches often suffer from incomplete data coverage, high noise levels and interfering seismic phases, especially near tectonically complex regions such as subduction zones and continental margins. To overcome these pitfalls, we apply Singular Spectrum Analysis (SSA) to remove random noise, reconstruct missing traces and enhance the robustness of SS precursors and P-to-S conversions from seismic discontinuities. Our method takes advantage of the predictability of time series in frequency-space domain and performs a rank reduction using a singular value decomposition of the trajectory matrix. We apply SSA to synthetic record sections as well as observations of 1) SS precursors beneath the northwestern Pacific subduction zones, and 2) P-to-S converted waves from the Western Canada Sedimentary Basin (WCSB). In comparison with raw or interpolated data, the SSA enhanced reflectivity maps show a greater resolution and a stronger negative correlation between the depths of the 410 and 660 km discontinuities. These effects can be attributed to the suppression of incoherent noise, which tends to reduce the signal amplitude during normal averaging procedures, through rank reduction and the emphasis of principle singular values. Our new results suggest a more laterally coherent 520 km reflection in the western Pacific regions. Similar improvements in data imaging are achieved in western Canada, where strong lateral variations in discontinuity topography are observed in the craton-Cordillera boundary zone. Improvements from SSA relative to conventional approaches are most notable in under-sampled regions.

  13. The fate of sulfide during decompression melting of peridotite - implications for sulfur inventory of the MORB-source depleted upper mantle

    NASA Astrophysics Data System (ADS)

    Ding, Shuo; Dasgupta, Rajdeep

    2017-02-01

    Magmatism at mid ocean ridges is one of the main pathways of S outflux from deep Earth to the surface reservoirs and is a critical step in the global sulfur cycle, yet our understanding of the behavior of sulfide during decompression melting of the upper mantle is incomplete. In order to constrain the sulfur budget of the mantle and reconcile the sulfur and chalcophile element budget of mantle partial melts parental to primitive mid-ocean ridge basalts (MORBs), here we developed a model to describe the behavior of sulfide and Cu during decompression melting by combining the pMELTS thermodynamic model and empirical sulfur contents at sulfide concentration (SCSS) models, taking into account the effect of the presence of Ni and Cu in sulfides on SCSS of mantle-derived melts. Calculation of SCSS along melting adiabat at mantle potential temperature of 1380 °C with variable initial S content in the mantle indicates that the complete consumption or partial survival of sulfide in the melting residue depends on initial S content and degree of melting. Primitive MORBs (Mg# > 60) with S and Cu mostly concentrated in 800-1000 ppm and 80-120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100-200 ppm S. Model calculations to capture the effects of variable mantle potential temperatures (1280-1420 °C) indicate that for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle owing to the effect of temperature in enhancing sulfide solubility in silicate melt, and higher mantle temperature stabilizing partial melt with higher FeO* and lower SiO2 and Al2O3, all of which generally enhance sulfide solubility. However, sulfide can still be exhausted by ∼ 10- 15% melting with bulk S of 100-150 ppm in the mantle when TP is as low as 1300 °C. We also show that although variation of DCuperidotite/ melt and initial Cu in the

  14. Origin of a 'Southern Hemisphere' geochemical signature in the Arctic upper mantle.

    PubMed

    Goldstein, Steven L; Soffer, Gad; Langmuir, Charles H; Lehnert, Kerstin A; Graham, David W; Michael, Peter J

    2008-05-01

    The Gakkel ridge, which extends under the Arctic ice cap for approximately 1,800 km, is the slowest spreading ocean ridge on Earth. Its spreading created the Eurasian basin, which is isolated from the rest of the oceanic mantle by North America, Eurasia and the Lomonosov ridge. The Gakkel ridge thus provides unique opportunities to investigate the composition of the sub-Arctic mantle and mantle heterogeneity and melting at the lower limits of seafloor spreading. The first results of the 2001 Arctic Mid-Ocean Ridge Expedition (ref. 1) divided the Gakkel ridge into three tectonic segments, composed of robust western and eastern volcanic zones separated by a 'sparsely magmatic zone'. On the basis of Sr-Nd-Pb isotope ratios and trace elements in basalts from the spreading axis, we show that the sparsely magmatic zone contains an abrupt mantle compositional boundary. Basalts to the west of the boundary display affinities to the Southern Hemisphere 'Dupal' isotopic province, whereas those to the east-closest to the Eurasian continent and where the spreading rate is slowest-display affinities to 'Northern Hemisphere' ridges. The western zone is the only known spreading ridge outside the Southern Hemisphere that samples a significant upper-mantle region with Dupal-like characteristics. Although the cause of Dupal mantle has been long debated, we show that the source of this signature beneath the western Gakkel ridge was subcontinental lithospheric mantle that delaminated and became integrated into the convecting Arctic asthenosphere. This occurred as North Atlantic mantle propagated north into the Arctic during the separation of Svalbard and Greenland.

  15. New constraints on the upper mantle structure of the Slave craton from Rayleigh wave inversion

    NASA Astrophysics Data System (ADS)

    Chen, Chin-Wu; Rondenay, Stéphane; Weeraratne, Dayanthie S.; Snyder, David B.

    2007-05-01

    Rayleigh wave phase and amplitude data are analyzed to provide new insight into the velocity structure of the upper mantle beneath the Slave craton, in the northwestern Canadian Shield. We invert for phase velocities at periods between 20 s-142 s (with greatest sensitivity at depths of 28-200 km) using crossing ray paths from events recorded by the POLARIS broadband seismic network and the Yellowknife array. Phase velocities obtained for the Slave province are comparable to those from other cratons at shorter periods, but exceed the global average by ~2% at periods above 60 s, suggesting that the Slave craton may be an end member in terms of its high degree of mantle depletion. The one-dimensional inversion of phase velocities yields high upper-mantle S-wave velocities of 4.7 +/- 0.2 km/s that persist to 220 +/- 65 km depth and thus define the cratonic lithosphere. Azimuthal anisotropy is well resolved at all periods with a dominant fast direction of N59°E +/- 20°, suggesting that upper mantle anisotropy beneath the Slave craton is influenced by both lithospheric fabric and sub-lithospheric flow.

  16. Deep mantle melting-solidifying and produced heterogeneities

    NASA Astrophysics Data System (ADS)

    Fomin, Ilya; Tackley, Paul

    2015-04-01

    Model for solid-liquid equilibrium and substance properties in lower mantle conditions is important to understand the early stages of evolution of terrestrial planets, such as core formation and magma ocean crystallization. This model is also necessary to prove theories on some modern seismic features of the Earth (e.g. ultra-low velocity zones) and petrological observations (e.g. lower mantle mineral assemblage inclusions in diamonds). Numerous experimental and numerical studies of the lower mantle phases provide sufficient amount of data to build up a thermodynamic model, which can be used in geophysical fluid dynamics research. Molecular Dynamics modeling provides data on thermodynamic properties of solids and liquids (density, heat capacity, thermal expansion, latent heat of melting etc.). Absence of minor components (iron, alkali etc.) makes it to overestimate melting temperatures significantly (up to 20-30%), so experimental data are also very important. Our model is based on MD data by [de Koker et al., 2013] with evaluation of all important parameters according to classical thermodynamic equations. Melting temperatures (especially at eutectic points) are corrected along Clausius-Clapeyron slopes to agree with modern experimental data ([Andrault et al., 2011], [Andrault et al., 2014], [Fiquet et al., 2010], [Hirose et al., 1999], [Mosenfelder et al., 2007], [Nomura et al., 2014], [Ozawa et al., 2011], [Zerr et al., 1998]). KD value for iron reported by [Andrault et al., 2012] was used. Proposed model was implemented into StagYY software (e.g. [Tackley, 2008]). It is a finite-volume discretization code for advection of solid and liquid in a planetary scale. A principal new feature of the used code modification is that we use separated variables for chemical compounds: SiO2, FeO, MgO and other (list can be extended). So it is possible to trace mantle heterogeneities produced by melting and solidifying events. Calculations predict appearing and disappearing

  17. Rock species formation due to deep-mantle melting

    NASA Astrophysics Data System (ADS)

    Fomin, Ilya; Tackley, Paul

    2017-04-01

    Melting and melting migration are processes leading to chemically distinct rock species from a homogeneous substrate in the Earth mantle. Iron-rich melts and corresponding rock species are proposed to result from magma ocean progressive crystallization [Labrosse et al., 2007], and modern geophysical models of ULVZ (e.g. [Beuchert & Schmeling, 2013]) discuss their presence at around the CMB today. We perform long-term (tens of millions of years) numerical simulations of the Earth's mantle for a plausible range of CMB temperatures to understands the possibility of melting and it's consequences. Our model of melting is based on experimental data and ab initio simulations. Physical properties (liquid-solid density differences) are adjusted with data of [de Koker et al., 2013; Mosenfelder et al., 2007; Stixrude & Lithgow-Bertelloni, 2011; Thomas & Asimow, 2013]. This model is included in StagYY numerical code (e.g. [Tackley, 2008]) to simulate mass and thermal fluxes within the Earth mantle. Melt segregation (rocks' permeability and velocities) is considered using equations listed in [Abe, 1995; Solomatov, Stevenson, 1993; Martin & Nokes, 1989]. Thermal effects (adiabatic heating and viscous dissipation) are considered. Viscous dissipation term includes Darcy flux term, but omits highly non-linear Brinkman contribution [Nield, 2007]. Modeling predicts formation of melt if temperature at CMB exceeds 4000-4050K. It's segregation and reequilibration results in sufficient volumes of slightly iron-enriched melt lighter than solid counterpart and moving upward. However, it's propagation is strongly controlled by temperature. Partial melting atop the molten layer results in formation of refractory iron-poor restite which delaminates and sink down, so that a layer of iron-depleted material forms underneath the molten layer. Our model applied to homogeneous pyrolitic mantle results in formation of layers of iron-depleted material with average FeO around 4.6 mol.% and iron

  18. Evidence for a heterogeneous upper mantle in the cabo ortegal complex, Spain.

    PubMed

    Girardeau, J; Ibarguchi, J I; Jamaa, N B

    1989-09-15

    A well-preserved fragment of a heterogeneous upper mantle is present in the Cabo Ortegal Complex (Spain). This section is made of harzburgite containing a large volume of pyroxenite. The pyroxenite is concentrated in a layer 300 meters thick by 3 kilometers long. In this layer, ultramafic rocks, essentially pyroxenite (massive websterite and clinopyroxenite) and minor dunite, alternate without any rhythmicity. Part of this layering is of primary magmatic origin and possibly resulted from crystallization of magmas in dikes intruded into the host peridotite under mantle conditions.

  19. the P-wave upper mantle structure beneath an active spreading center: The Gulf of California

    NASA Technical Reports Server (NTRS)

    Walck, M. C.

    1983-01-01

    Detailed analysis of short period travel time, and waveform data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km. More than 1400 digital seismograms from earthquakes in Mexico and central America recorded at SCARLET yield 1753 travel times and 58 direct measurements of short period travel time as well as high quality, stable waveforms. The 29 events combine to form a continuous record section from 9 deg to 40 deg with an average station spacing of less than 5 km. First the travel times are inverted. Further constraints arise from the observed relative amplitudes of mantle phases, which are modeled by trial and error.

  20. Large gem diamonds from metallic liquid in Earth’s deep mantle

    NASA Astrophysics Data System (ADS)

    Smith, Evan M.; Shirey, Steven B.; Nestola, Fabrizio; Bullock, Emma S.; Wang, Jianhua; Richardson, Stephen H.; Wang, Wuyi

    2016-12-01

    The redox state of Earth’s convecting mantle, masked by the lithospheric plates and basaltic magmatism of plate tectonics, is a key unknown in the evolutionary history of our planet. Here we report that large, exceptional gem diamonds like the Cullinan, Constellation, and Koh-i-Noor carry direct evidence of crystallization from a redox-sensitive metallic liquid phase in the deep mantle. These sublithospheric diamonds contain inclusions of solidified iron-nickel-carbon-sulfur melt, accompanied by a thin fluid layer of methane ± hydrogen, and sometimes majoritic garnet or former calcium silicate perovskite. The metal-dominated mineral assemblages and reduced volatiles in large gem diamonds indicate formation under metal-saturated conditions. We verify previous predictions that Earth has highly reducing deep mantle regions capable of precipitating a metallic iron phase that contains dissolved carbon and hydrogen.

  1. Early differentiation and volatile accretion recorded in deep-mantle neon and xenon.

    PubMed

    Mukhopadhyay, Sujoy

    2012-06-06

    The isotopes (129)Xe, produced from the radioactive decay of extinct (129)I, and (136)Xe, produced from extinct (244)Pu and extant (238)U, have provided important constraints on early mantle outgassing and volatile loss from Earth. The low ratios of radiogenic to non-radiogenic xenon ((129)Xe/(130)Xe) in ocean island basalts (OIBs) compared with mid-ocean-ridge basalts (MORBs) have been used as evidence for the existence of a relatively undegassed primitive deep-mantle reservoir. However, the low (129)Xe/(130)Xe ratios in OIBs have also been attributed to mixing between subducted atmospheric Xe and MORB Xe, which obviates the need for a less degassed deep-mantle reservoir. Here I present new noble gas (He, Ne, Ar, Xe) measurements from an Icelandic OIB that reveal differences in elemental abundances and (20)Ne/(22)Ne ratios between the Iceland mantle plume and the MORB source. These observations show that the lower (129)Xe/(130)Xe ratios in OIBs are due to a lower I/Xe ratio in the OIB mantle source and cannot be explained solely by mixing atmospheric Xe with MORB-type Xe. Because (129)I became extinct about 100 million years after the formation of the Solar System, OIB and MORB mantle sources must have differentiated by 4.45 billion years ago and subsequent mixing must have been limited. The Iceland plume source also has a higher proportion of Pu- to U-derived fission Xe, requiring the plume source to be less degassed than MORBs, a conclusion that is independent of noble gas concentrations and the partitioning behaviour of the noble gases with respect to their radiogenic parents. Overall, these results show that Earth's mantle accreted volatiles from at least two separate sources and that neither the Moon-forming impact nor 4.45 billion years of mantle convection has erased the signature of Earth's heterogeneous accretion and early differentiation.

  2. Teleseismic wave front anomalies at a Continental Rift: no mantle anomaly below the central Upper Rhine Graben

    NASA Astrophysics Data System (ADS)

    Kirschner, Stephanie; Ritter, Joachim; Wawerzinek, Britta

    2011-08-01

    The deep structure of the Upper Rhine Graben (URG), a continental rift in SW Germany and E France, is still poorly known. This deficit impedes a full understanding of the geodynamic evolution of this prominent rift. We study the lithosphere-asthenosphere structure using teleseismic waveforms obtained from the passive broad-band TIMO project across the central URG. The recovered, crust-corrected traveltime residuals relative to the iasp91 earth model are tiny (mostly less than 0.2-0.3 s). The average measured slowness (<1 s deg-1) and backazimuth (<5°) deviations are also very small and do not show any systematic wave front anomalies. These observed perturbation values are smaller than expected ones from synthetic 3-D ray tracing modelling with anomalies exceeding 2-3 per cent seismic velocity in the mantle. Thus there is no significant hint for any deep-seated anomaly such as a mantle cushion, etc. This result means that the rifting process did not leave behind a lower lithospheric signature, which could be clearly verified with high-resolution teleseismic experiments. The only significant traveltime perturbation at the central URG is located at its western side in the upper crust around a known geothermal anomaly. The upper crustal seismic anomaly with traveltime delays of 0.2-0.3 s cannot be explained with increased temperature alone. It is possibly related to a zone of highly altered granite. In the west of our network a traveltime anomaly (0.6-0.7 s delay) related with the Eifel plume is confirmed by the TIMO data set.

  3. Crust and Upper Mantle Anisotropy of Southern Tibet Revealed by Shear Wave Splitting

    NASA Astrophysics Data System (ADS)

    Zhao, J.; Sun, Y.; Huang, Y.; Pei, S.; Liu, H.; Wang, W.; Xu, Q.; Cheng, H.; Tang, W.; Yang, L.; Zhang, H.

    2011-12-01

    Shear wave splitting is an explicit indicator of anisotropy, but it is often difficult to determine the depth of the anisotropy that produces the observed signals. We analyzed shear waves recorded on horizontal components at the broadband stations of the ANTILOPE-II array located in central-southern Tibet. Splitting of SKS is consistently observed. Fresnel zone analysis suggests the upper mantle (above 175 km depth) is the most likely source of anisotropy. The directions of fast polarization, which is related to the directions of past and present plate motions and delay times, can be divided into two parts with a boundary between suture zones (ITS and BNS). In the northern part of the study region, the orientation of upper mantle deformation closely reflects plate tectonic crustal vectors. While GPS campaigns provide evidence suggesting crustal flow in the study region, the anisotropy in the southern part of our study region argues against such upper mantle flow (i.e., the fast polarization directions are quasi perpendicular to known surface features and geodetic estimates of the crustal displacement field). In contrast to the southern part of the study region where the delay time averages ~0.4 s, average delay times in the northern part of the study area are ~1.0 s. The dissimilar delay times suggests the existence of an upper mantle transitional zone at ~30.5ο N latitude. We suggest that this difference in the observed delay times reflects a fundamental change in the deformation regime across our study region. We suggest that the change in deformation may be related to a lithospheric crush zone, which developed in northern and eastern Tibet between the colliding Eurasian and Indian plate, the existence of which is marked by high temperature, low mantle seismic wave speed, east and southeast oriented displacements, and poor Sn propagation. Understanding the change in deformation regime may be critical for understanding the geotectonic evolution of southern Tibet.

  4. Seismogenic Shear Zones In The Upper Mantle: Some Evidences From Central Italy

    NASA Astrophysics Data System (ADS)

    Creati, N.; Boncio, P.; Lavecchia, G.

    The presence of brittle, brittle-ductile and ductile shear zones down to the upper man- tle is rather well known in the literature and testified by both geophysical (seismic lines) and geological (milonyte fault rocks in exumated lherzolites) data. Moreover, the occurrence of intermediate earthquakes (down to a depth about 150-200 km) which cannot be easily interpreted as Wadati-Benioff zones (High Atlas, Romania, Hindukush, Tibet and Burma) suggests the existence of intra-lithosphere seismogenic shear zones. In this paper, we will illustrate preliminary evidences about a possible seismogenic intra-lithosphere shear zone in Northern-Central Italy. Up to now, the oc- currence in this area of earthquakes down to a depth of about 90 km has been linked to the westward subduction of the Adriatic lithosphere. As a matter of fact, the loca- tion of the deep earthquakes does not fit well with the location of the Apennine Moho doubling zone, that, as classically recognized, would sign the location of the slab. In order to better address the problem, we have performed a detailed analysis of the deep seismicity along two regional sections coinciding with the traces of the deep crust CROP 03 (Punta Ala-Gabicce) and DSS 1978 (Piombino-Ancona) seismic profiles. In both sections, the earthquake distribution defines a seismic area 30 km wide and 100 km long, deepening westward at about 30 and reaching a maximum depth of about 75 km. This area substantially overlaps on the easternmost SW-dipping reverse shear zone shown by the DSS 78 and CROP 03 profiles. This shear zone, named Adriatic Shear Zone (ASZ), emerges in the Adriatic Sea and deepens westward dislocating the base of the crust. The ASZ along-dip distribution of the seismicity is not homogeneous and four main clusters are observed at depths of 5-10 km, 18-25 km, 35-40 km and 60-70 km. The distribution of seismicity has been compared with the rheological strat- ification and structure of the lithosphere. Adopting a not

  5. The influence of deep mantle heterogeneity on the rhythms and scales of surface topography evolution

    NASA Astrophysics Data System (ADS)

    Arnould, Maëlis; Coltice, Nicolas; Flament, Nicolas

    2016-04-01

    Earth's surface, the interface between external processes and internal dynamics (lithosphere motions and mantle convection), is continuously reorganised. A large part of Earth's topography is generated by mantle motions and lithospheric stresses [1], which impacts for instance the global sea-level, the dynamics of sedimentary basins and the geoid. Studying how surface topography evolves in both space and time thus not only provides information on the rhythms and scales of evolution of those processes, but would also be a tool for the study of the mantle motions and properties from which it originates [2]. In this study, we propose to characterise the spatial and temporal scales of evolution of surface topography in 2D spherical annulus numerical models of mantle convection developing a plate-like behaviour. We use the geodynamical code StagYY [3] to first determine a mantle convection regime generating a surface topography with Earth-like amplitudes and realistic mantle dynamics at first order (e.g. high Rayleigh number, reasonable lithosphere thickness, pseudo-plastic lithosphere rheology generating plate tectonics). We then use this convection regime to investigate how the presence of stable deep-rooted thermochemical heterogeneities influence the rhythms of evolution of surface topography. We analyse our results to identify how the timescales of evolution are connected with the lengthscales of topography, in light of the tectonic histories produced by the models. References: [1] M. Gurnis, Long-term controls of eustatic and epeirogenic motions by mantle convection, GSA Today, 2(7):141-157, 1992. [2] B.H. Hager, R.W. Clayton, M.A. Richards, R.P. Comer, and A.M. Dziewonski, Lower mantle heterogeneity, dynamic topography and the geoid, Nature, 313:541-545, 1985. [3] J.W. Hernlund and P.J. Tackley, Modeling mantle convection in the spherical annulus, Phys. Earth Planet. Interiors, 171(1):48-54, 2008.

  6. Hunting for the Tristan mantle plume - An upper mantle tomography around the volcanic island of Tristan da Cunha

    NASA Astrophysics Data System (ADS)

    Schlömer, Antje; Geissler, Wolfram H.; Jokat, Wilfried; Jegen, Marion

    2017-03-01

    The active volcanic island Tristan da Cunha, located at the southwestern and youngest end of the Walvis Ridge - Tristan/Gough hotspot track, is believed to be the surface expression of a huge thermal mantle anomaly. While several criteria for the diagnosis of a classical hotspot track are met, the Tristan region also shows some peculiarities. Consequently, it is vigorously debated if the active volcanism in this region is the expression of a deep mantle plume, or if it is caused by shallow plate tectonics and the interaction with the nearby Mid-Atlantic Ridge. Because of a lack of geophysical data in the study area, no model or assumption has been completely confirmed. We present the first amphibian P-wave finite-frequency travel time tomography of the Tristan da Cunha region, based on cross-correlated travel time residuals of teleseismic earthquakes recorded by 24 ocean-bottom seismometers. The data can be used to image a low velocity structure southwest of the island. The feature is cylindrical with a radius of ∼100 km down to a depth of 250 km. We relate this structure to the origin of Tristan da Cunha and name it the Tristan conduit. Below 250 km the low velocity structure ramifies into narrow veins, each with a radius of ∼50 km. Furthermore, we imaged a linkage between young seamounts southeast of Tristan da Cunha and the Tristan conduit.

  7. African hot spot volcanism: small-scale convection in the upper mantle beneath cratons.

    PubMed

    King, S D; Ritsema, J

    2000-11-10

    Numerical models demonstrate that small-scale convection develops in the upper mantle beneath the transition of thick cratonic lithosphere and thin oceanic lithosphere. These models explain the location and geochemical characteristics of intraplate volcanos on the African and South American plates. They also explain the presence of relatively high seismic shear wave velocities (cold downwellings) in the mantle transition zone beneath the western margin of African cratons and the eastern margin of South American cratons. Small-scale, edge-driven convection is an alternative to plumes for explaining intraplate African and South American hot spot volcanism, and small-scale convection is consistent with mantle downwellings beneath the African and South American lithosphere.

  8. Petrology of lower crustal and upper mantle xenoliths from the Cima Volcanic Field, California

    USGS Publications Warehouse

    Wilshire, H.G.; McGuire, A.V.; Noller, J.S.; Turrin, B.D.

    1991-01-01

    Basaltic rocks of the Cima Volcanic Field in the southern Basin and Range province contain abundant gabbro, pyroxenite, and peridotite xenoliths. Composite xenoliths containing two or more rock types show that upper-mantle spinel peridotite was enriched by multiple dike intrusions in at least three episodes; the mantle was further enriched by intergranular and shear-zone melt infiltration in at least two episodes. Because of their high densities, the gabbros and pyroxenites can occupy the zone immediately above the present Moho (modeled on seismic data as 10-13 km thick, with Vp 6.8 km/s) only if their seismic velocities are reduced by the joints, partial melts, and fluid inclusions that occur in them. Alternatively, these xenoliths may have been derived entirely from beneath the Moho, in which case the Moho is not the local crust-mantle boundary. -from Authors

  9. Constraining upper mantle mass structure below the oceans from seismic and geodetic data

    NASA Astrophysics Data System (ADS)

    Panet, Isabelle; Romanowicz, Barbara; Greff, Marianne; French, Scott

    2016-04-01

    We address the question of understanding the mantle convective structure below the oceans, in the sublithospheric to transition zone depth range. For that, we study how the mantle mass distribution can be constrained from a combination of global seismic tomography, gravity and bathymetry data and models. We focus on oriented patterns, that may arise from interactions between the flows and the plate motions. A directional analysis of the geodetic datasets, over the Pacific and Indian Oceans, shows the presence of elongated anomalies following the direction of the present-day absolute plate motions, correlated with the low shear velocity channels in the upper mantle from the SEMum2 model (French et al., 2013). We derive regional sensitivity kernels relating these observables to the internal mass distribution, and set up an inverse problem to determine the seismic velocity to density conversion factor. We discuss our approach and preliminary results.

  10. Melting experiments on anhydrous peridotite KLB-1: Compositions of magmas in the upper mantle and transition zone

    NASA Astrophysics Data System (ADS)

    Herzberg, Claude; Zhang, Jianzhong

    1996-04-01

    Electron microprobe results are reported for liquid and crystalline phases that were synthesized at 5-22.5 GPa in multianvil experiments on anhydrous peridotite KLB-1 [Zhang and Herzberg, 1994]. The results provide information on the partitioning of TiO2, Al2O3, Cr2O3, FeO, MnO, MgO, Na2O, and NiO among liquid and the crystalline phases olivine, modified spinel, garnet, magnesiowustite, and magnesium perovskite. Uncertainties in these partition coefficients stem from quenching problems and from the effects of thermal migration of liquid in a temperature gradient. We have, however, exploited the temperature gradients by determining how the crystalline phase chemistry varies throughout the melting interval from the liquidus to the solidus. This has permitted new constraints to be obtained on the compositions of liquids along the anhydrous peridotite solidus at low melt fractions and at pressures in the 5-18 GPa range. It is demonstrated that the wide range of Al2O3 and CaO/Al2O3 contents in picrites and komatiites can be explained by melt segregation at upper mantle pressures that ranged from 3 to ˜10 GPa. These magmas could have formed by anhydrous melting in plumes with temperatures that were only 100°-200°C higher than ambient mantle below ridges, demonstrating that unusually hot conditions are not required to form komatiites. Primary igneous MgO contents in excess of 26% should be rare, and those that do exist in some komatiites can be explained by advanced melting during adiabatic or superadiabatic ascent, by low Na2O in the source, or by melting in hot plumes from the transition zone and lower mantle. Evidence for deep melting in hot plumes is rather conjectural, but it may be contained in some 2700 Myr komatiites that have high MgO and mantle-like CaO/Al2O3.

  11. Tectonic denudation of upper mantle along passive margins: a model based on drilling (ODP Leg 103) and diving (Galinaute cruise) results, western Galicia Margin, Spain

    SciTech Connect

    Boillot, G.; Winterer, E.L.; Recq, M.; Girardeau, J.; Kornprobst, J.; Loreau, J.P.; Malod, J.; Mougenot, D.

    1987-05-01

    During ODP Leg 103 (April-June 1985) and the Galinaute cruise (June-July 1986), serpentinized peridotite (clinopyroxene-spinel harzburgite) was recovered within the basement approximately at the boundary between the North Atlantic ocean crust to the west and the thinned continental crust of the Galicia passive margin (Spain) to the east. The exposure of mantle-derived peridotite on the sea floor occurred at the end of the period of rifting, roughly 110 Ma. Ductile shear zones observed in the peridotite are consistent with movements along a deep, low-angle normal fault rooted within the upper mantle and dipping eastward beneath the Galicia margin. To explain the tectonic denudation of the mantle at the ocean-continent boundary, they use a nonuniform stretching model for the lithosphere, set up from Wernicke's model.

  12. Tectonic denudation of the upper mantle along passive margins: a model based on drilling results (ODP leg 103, western Galicia margin, Spain)

    NASA Astrophysics Data System (ADS)

    Boillot, G.; Recq, M.; Winterer, E. L.; Meyer, A. W.; Applegate, J.; Baltuck, M.; Bergen, J. A.; Comas, M. C.; Davies, T. A.; Dunham, K.; Evans, C. A.; Girardeau, J.; Goldberg, G.; Haggerty, J.; Jansa, L. F.; Johnson, J. A.; Kasahara, J.; Loreau, J. P.; Luna-Sierra, E.; Moullade, M.; Ogg, J.; Sarti, M.; Thurow, J.; Williamson, M.

    1987-01-01

    During ODP Leg 103, serpentinized peridotite (clinopyroxene-spinel harzburgite) was cored within the basement approximatively at the boundary between the North Atlantic oceanic curst to the west, and the thinned continental crust of the Galicia passive margin (Spain) to the east. The exposure of mantle derived peridotite on the seafloor occurred at the end of the period of rifting, roughly 110 Ma ago. Ductile shear zones observed in the cored peridotite are consistent with movements along a deep low-angle, normal fault rooted within the upper mantle and dipping eastward, beneath the Galicia margin. To explain the tectonic denudation of the mantle at the ocean-continent boundary, we use a non-uniform stretching model for the lithosphere, set up from the Wernicke's model (1985).

  13. Inferred rheology and upper mantle conditions of western Nevada and southern California-northwest Mexico

    NASA Astrophysics Data System (ADS)

    Dickinson, Haylee L.

    Understanding the viscous strength (rheology) of the mantle is essential for understanding the dynamics and evolution of the Earth. Rheology affects many geologic processes such as mantle convection, the earthquake cycle, and plate tectonics. This study uses tectonic (postseismic) and non-tectonic (lake unloading) events that have induced differential stress changes within the crust and mantle, which in turn, create surface deformation. The viscoelastic relaxation is constrained using geodetic methods, such as GPS, InSAR, or measurements of shoreline rebound. We can use these observed surface displacements to constrain numerical models of the relaxation processes that can be used to infer a viscosity structure. These studies allow us to infer the mechanical nature of the lithosphere and asthenosphere using 3D finite element models. When we combine our inferred viscosity structure with calculations of conductive geothermal gradients and models of mantle melting, we can infer environmental conditions of the upper mantle like water content, mineralogy, and degree of melt. In our first study, we seek to reduce non-uniqueness issues that plague in situ rheology studies by simultaneously modeling the response of the crust and mantle for a single region of western Nevada to multiple processes constrained by multiple observational data sets. Western Nevada has experienced a series of Mw >6.5 earthquakes over the last ~150 years, from the 1872 Owen's Valley earthquake to the 1954 Dixie Valley event, as well as the loading/unloading of Pleistocene-aged Lake Lahontan. Our goal was to answer whether a single Newtonian viscosity structure can explain all of the geodetic constraints. We found a strong lower crust underlain by a relatively weak upper mantle can explain all observational constraints. We also infer the decreases in viscosity we observed are due to hydration possibly from the subduction of the Farallon slab and melt content. In the next study, we investigate the

  14. Synthetic receiver function profiles through the upper mantle and the transition zone for upwelling scenarios

    NASA Astrophysics Data System (ADS)

    Nagel, Thorsten; Düsterhöft, Erik; Schiffer, Christian

    2017-04-01

    We investigate the signature relevant mantle lithologies leave in the receiver function record for different adiabatic thermal gradients down to 800 kilometers depth. The parameter space is chosen to target the visibility of upwelling mantle (a plume). Seismic velocities for depleted mantle, primitive mantle, and three pyroxenites are extracted from thermodynamically calculated phases diagrams, which also provide the adiabatic decompression paths. Results suggest that compositional variations, i.e. the presence or absence of considerable amounts of pyroxenites in primitive mantle should produce a clear footprint while horizontal differences in thermal gradients for similar compositions might be more subtle. Peridotites best record the classic discontinuities at around 410 and 650 kilometers depth, which are associated with the olivin-wadsleyite and ringwoodite-perovskite transitions, respectively. Pyroxenites, instead, show the garnet-perovskite transition below 700 kilometers depth and SiO2-supersaturated compositions like MORB display the coesite-stishovite transition between 300 and 340 kilometers depth. The latter shows the strongest temperature-depth dependency of all significant transitions potentially allowing to infer information about the thermal state if the mantle contains a sufficient fraction of MORB-like compositions. For primitive and depleted mantle compositions, the olivin-wadsleyite transition shows a certain temperature-depth dependency reflected in slightly larger delay times for higher thermal gradients. The lower-upper-mantle discontinuity, however, is predicted to display larger delay times for higher thermal gradients although the associated assemblage transition occurs at shallower depths thus requiring a very careful depth migration if a thermal anomaly should be recognized. This counterintuitive behavior results from the downward replacement of the assemblage wadsleyite+garnet with the assemblage garnet+periclase at high temperatures

  15. Upper mantle electrical resistivity structure beneath the central Mariana subduction system

    NASA Astrophysics Data System (ADS)

    Matsuno, Tetsuo; Seama, Nobukazu; Evans, Rob L.; Chave, Alan D.; Baba, Kiyoshi; White, Antony; Goto, Tada-Nori; Heinson, Graham; Boren, Goran; Yoneda, Asami; Utada, Hisashi

    2010-09-01

    This paper reports on a magnetotelluric (MT) survey across the central Mariana subduction system, providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back-arc spreading ridge. After calculation of MT response functions and their correction for topographic distortion, two-dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab. The resultant isotropic electrical resistivity structure contains several key features. There is an uppermost resistive layer with a thickness of up to 150 km beneath the Pacific Ocean Basin, 80-100 km beneath the Mariana Trough, and 60 km beneath the Parece Vela Basin along with a conductive mantle beneath the resistive layer. A resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge. There is no evidence for a conductive feature beneath the back-arc spreading center. Sensitivity tests were applied to these features through inversion of synthetic data. The uppermost resistive layer is the cool, dry residual from the plate accretion process. Its thickness beneath the Pacific Ocean Basin is controlled mainly by temperature, whereas the roughly constant thickness beneath the Mariana Trough and beneath the Parece Vela Basin regardless of seafloor age is controlled by composition. The conductive mantle beneath the uppermost resistive layer requires hydration of olivine and/or melting of the mantle. The resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three-dimensional and of limited size. In contrast, the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab. The

  16. Deep mantle structure and the postperovskite phase transition

    PubMed Central

    Helmberger, D.; Lay, T.; Ni, S.; Gurnis, M.

    2005-01-01

    Seismologists have known for many years that the lowermost mantle of the Earth is complex. Models based on observed seismic phases sampling this region include relatively sharp horizontal discontinuities with strong zones of anisotropy, nearly vertical contrasts in structure, and small pockets of ultralow velocity zones (ULVZs). This diversity of structures is beginning to be understood in terms of geodynamics and mineral physics, with dense partial melts causing the ULVZs and a postperovskite solid–solid phase transition producing regional layering, with the possibility of large-scale variations in chemistry. This strong heterogeneity has significant implications on heat transport out of core, the evolution of the magnetic field, and magnetic field polarity reversals. PMID:16217029

  17. Seismic structure of the Central US crust and shallow upper mantle: Uniqueness of the Reelfoot Rift

    NASA Astrophysics Data System (ADS)

    Pollitz, Fred F.; Mooney, Walter D.

    2014-09-01

    Using seismic surface waves recorded with Earthscope's Transportable Array, we apply surface wave imaging to determine 3D seismic velocity in the crust and uppermost mantle. Our images span several Proterozoic and early Cambrian rift zones (Mid-Continent Rift, Rough Creek Graben-Rome trough, Birmingham trough, Southern Oklahoma Aulacogen, and Reelfoot Rift). While ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences, the Reelfoot Rift is unique in its association with low mantle seismic velocity. Its mantle low-velocity zone (LVZ) is exceptionally pronounced and extends down to at least 200 km depth. This LVZ is of variable width, being relatively narrow (∼50 km wide) within the northern Reelfoot Rift, which hosts the New Madrid Seismic Zone (NMSZ). We hypothesize that this mantle volume is weaker than its surroundings and that the Reelfoot Rift consequently has relatively low elastic plate thickness, which would tend to concentrate tectonic stress within this zone. No other intraplate ancient rift zone is known to be associated with such a deep mantle low-velocity anomaly, which suggests that the NMSZ is more susceptible to external stress perturbations than other ancient rift zones.

  18. Seismic structure of the central US crust and upper mantle: Uniqueness of the Reelfoot Rift

    USGS Publications Warehouse

    Pollitz, Fred; Mooney, Walter D.

    2014-01-01

    Using seismic surface waves recorded with Earthscope's Transportable Array, we apply surface wave imaging to determine 3D seismic velocity in the crust and uppermost mantle. Our images span several Proterozoic and early Cambrian rift zones (Mid-Continent Rift, Rough Creek Graben—Rome trough, Birmingham trough, Southern Oklahoma Aulacogen, and Reelfoot Rift). While ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences, the Reelfoot Rift is unique in its association with low mantle seismic velocity. Its mantle low-velocity zone (LVZ) is exceptionally pronounced and extends down to at least 200 km depth. This LVZ is of variable width, being relatively narrow (∼50km">∼50km wide) within the northern Reelfoot Rift, which hosts the New Madrid Seismic Zone (NMSZ). We hypothesize that this mantle volume is weaker than its surroundings and that the Reelfoot Rift consequently has relatively low elastic plate thickness, which would tend to concentrate tectonic stress within this zone. No other intraplate ancient rift zone is known to be associated with such a deep mantle low-velocity anomaly, which suggests that the NMSZ is more susceptible to external stress perturbations than other ancient rift zones.

  19. Shear wave polarization anisotropy in the upper mantle beneath Honshu, Japan

    SciTech Connect

    Ando, M.; Ishikawa, Y.; Yamazaki, F.

    1983-07-10

    Shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth's surface is investigated using three-component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in Honshu, Japan. Eighty nine high-quality seismograms were selected from a period of 3 years. The data used in this study are restricted such that incidence angles are smaller than the critical angle of 30/sup 0/ to the earth's surface in order to avoid phase shifts in the shear wave train. To find directions of the maximum and minimum velocities in split shear waves, where shear waves are resolved into two phases with the maximum time separation, each set of the two horizontal component seismograms is rotated in the horizontal plane. The split shear waves thus obtained are again recombined after the correction of anisotropy, and the anisotropy-corrected particle motion is compared with the focal mechanism for a cross-check of the observed anisotropy. Directions of the maximum axes are plotted on azimuth-incidence angle stereograms at each station. The stereograms and the cross sections of seismic ray paths show that (1) the anisotropic material is distributed at intermediate locations between earthquake sources and receiving stations, and (2) the anisotropic region is separated into two parts: one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0/sup 0/ to 30/sup 0/ from the north (north anisotropy) and the other in the south with it trending 90/sup 0/ to 120/sup 0/ (south anisotropy). The maximum time delays between the two shear waves along a vertical seismic ray is about 1 s for both the anisotropic regions. The horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km. perhaps prevalent in west Honshu.

  20. The Role of Deep Mantle Flow in Shaping the Hawaiian-Emperor Bend

    NASA Astrophysics Data System (ADS)

    Hassan, R.; Müller, D.; Gurnis, M.; Williams, S.; Flament, N. E.

    2016-12-01

    Age-progressive volcanic hotspot tracks are typical surface expressions of plate tectonic movement on top of narrow plumes of hot material within Earth's mantle. Seismic imaging reveals that these plumes can be of deep origin, potentially rooted on thermochemical structures in the lower mantle. Although palaeomagnetic and radiometric age data suggest that mantle flow can advect plume conduits laterally, the flow dynamics underlying the formation of the sharp bend occurring only in the Hawaiian-Emperor hotspot track in the Pacific Ocean remains enigmatic. The north Pacific features long-lasting subduction systems, unlike those in the south Pacific. We present palaeogeographically-constrained numerical models of thermochemical convection demonstrating that flow in the deep lower mantle under the north Pacific was anomalously vigorous between 100 Ma and 50 Ma. These models show a sharp bend in the Hawaiian-Emperor hotspot track arising from the interplay of plume tilt and the lateral advection of plume sources. We show that the different trajectories of the Hawaiian and Louisville hotspot tracks arise from asymmetric deformation of thermochemical structures under the Pacific between 100 Ma and 50 Ma. This asymmetric deformation waned just before the Hawaiian-Emperor bend developed, owing to flow in the deepest lower mantle associated with slab descent in the north and south Pacific.

  1. A rapid burst in hotspot motion through the interaction of tectonics and deep mantle flow.

    PubMed

    Hassan, Rakib; Müller, R Dietmar; Gurnis, Michael; Williams, Simon E; Flament, Nicolas

    2016-05-12

    Volcanic hotspot tracks featuring linear progressions in the age of volcanism are typical surface expressions of plate tectonic movement on top of narrow plumes of hot material within Earth's mantle. Seismic imaging reveals that these plumes can be of deep origin--probably rooted on thermochemical structures in the lower mantle. Although palaeomagnetic and radiometric age data suggest that mantle flow can advect plume conduits laterally, the flow dynamics underlying the formation of the sharp bend occurring only in the Hawaiian-Emperor hotspot track in the Pacific Ocean remains enigmatic. Here we present palaeogeographically constrained numerical models of thermochemical convection and demonstrate that flow in the deep lower mantle under the north Pacific was anomalously vigorous between 100 million years ago and 50 million years ago as a consequence of long-lasting subduction systems, unlike those in the south Pacific. These models show a sharp bend in the Hawaiian-Emperor hotspot track arising from the interplay of plume tilt and the lateral advection of plume sources. The different trajectories of the Hawaiian and Louisville hotspot tracks arise from asymmetric deformation of thermochemical structures under the Pacific between 100 million years ago and 50 million years ago. This asymmetric deformation waned just before the Hawaiian-Emperor bend developed, owing to flow in the deepest lower mantle associated with slab descent in the north and south Pacific.

  2. A rapid burst in hotspot motion through the interaction of tectonics and deep mantle flow

    NASA Astrophysics Data System (ADS)

    Hassan, Rakib; Müller, R. Dietmar; Gurnis, Michael; Williams, Simon E.; Flament, Nicolas

    2016-05-01

    Volcanic hotspot tracks featuring linear progressions in the age of volcanism are typical surface expressions of plate tectonic movement on top of narrow plumes of hot material within Earth’s mantle. Seismic imaging reveals that these plumes can be of deep origin—probably rooted on thermochemical structures in the lower mantle. Although palaeomagnetic and radiometric age data suggest that mantle flow can advect plume conduits laterally, the flow dynamics underlying the formation of the sharp bend occurring only in the Hawaiian-Emperor hotspot track in the Pacific Ocean remains enigmatic. Here we present palaeogeographically constrained numerical models of thermochemical convection and demonstrate that flow in the deep lower mantle under the north Pacific was anomalously vigorous between 100 million years ago and 50 million years ago as a consequence of long-lasting subduction systems, unlike those in the south Pacific. These models show a sharp bend in the Hawaiian-Emperor hotspot track arising from the interplay of plume tilt and the lateral advection of plume sources. The different trajectories of the Hawaiian and Louisville hotspot tracks arise from asymmetric deformation of thermochemical structures under the Pacific between 100 million years ago and 50 million years ago. This asymmetric deformation waned just before the Hawaiian-Emperor bend developed, owing to flow in the deepest lower mantle associated with slab descent in the north and south Pacific.

  3. Upper mantle structure of the Tonga-Lau-Fiji region from Rayleigh wave tomography

    NASA Astrophysics Data System (ADS)

    Wei, S. Shawn; Zha, Yang; Shen, Weisen; Wiens, Douglas A.; Conder, James A.; Webb, Spahr C.

    2016-11-01

    We investigate the upper mantle seismic structure in the Tonga-Lau-Fiji region by jointly fitting the phase velocities of Rayleigh waves from ambient-noise and two-plane-wave tomography. The results suggest a wide low-velocity zone beneath the Lau Basin, with a minimum SV-velocity of about 3.7 ± 0.1 km/s, indicating upwelling hot asthenosphere with extensive partial melting. The variations of velocity anomalies along the Central and Eastern Lau Spreading Centers suggest varying mantle porosity filled with melt. In the north where the spreading centers are distant from the Tonga slab, the inferred melting commences at about 70 km depth, and forms an inclined zone in the mantle, dipping to the west away from the arc. This pattern suggests a passive decompression melting process supplied by the Australian plate mantle from the west. In the south, as the supply from the Australian mantle is impeded by the Lau Ridge lithosphere, flux melting controlled by water from the nearby slab dominates in the back-arc. This source change results in the rapid transition in geochemistry and axial morphology along the spreading centers. The remnant Lau Ridge and the Fiji Plateau are characterized by a 60-80 km thick lithosphere underlain by a low-velocity asthenosphere. Our results suggest the removal of the lithosphere of the northeastern Fiji Plateau-Lau Ridge beneath the active Taveuni Volcano. Azimuthal anisotropy shows that the mantle flow direction rotates from trench-perpendicular beneath Fiji to spreading-perpendicular beneath the Lau Basin, which provides evidence for the southward flow of the mantle wedge and the Samoan plume.

  4. Seismic evidence of on-going sublithosphere upper mantle convection for intra-plate volcanism in Northeast China

    NASA Astrophysics Data System (ADS)

    Guo, Zhen; Chen, Y. John; Ning, Jieyuan; Yang, Yingjie; Afonso, Juan Carlos; Tang, Youcai

    2016-01-01

    A 3-D crustal and upper mantle S-wave velocity model of NE China is constructed by inversion of phase velocity dispersion curves at 6-140 s periods from ambient noise tomography and two-plane surface wave tomography. The seismic data used in this study are collected from 120 China Earthquake Administration (CEA) permanent stations and 127 portable stations of NECESSArray. We observe strong low S-wave velocity beneath the Changbaishan volcano in the upper mantle to at least 200-km depth, which is interpreted as a mantle upwelling beneath the Changbaishan volcano that is consistent with the body wave tomographic image. The Songliao Basin is dominated by a high velocity extending to at least 200-km depth. Built upon the observed velocity anomalies, we propose a sub-lithosphere mantle convection model for NE China in which the upwelling of upper mantle materials from the mantle transition zone to the Changbaishan volcano could induce a local sub-lithosphere convection in the upper mantle and the strong high velocity of the upper mantle beneath the Songliao Basin corresponds to the downwelling limb of this convection cell. The downwelling beneath the Songliao Basin could also induce secondary local convection in the asthenosphere to the west, leading to local asthenospheric upwelling beneath the Abaga and Halaha volcanoes in the Xing'an-Mongolia Orogenic Belt.

  5. Towards the Next Generation Upper-Mantle 3D Anelastic Tomography

    NASA Astrophysics Data System (ADS)

    Karaoglu, H.; Romanowicz, B. A.

    2015-12-01

    In order to distinguish the thermal and compositional heterogeneities in the mantle, it is crucial to resolve the lateral variations not only in seismic velocities but also in intrinsic attenuation. Indeed, the high sensitivity of intrinsic attenuation to temperature and water content, governed by a form of Arrhenius equation, contrasts with the quasi-linear dependence of velocities on both temperature and major element composition. The major challenge in imaging attenuation lies in separating its effects on seismic waves from the elastic ones. The latter originate from the wave propagation in media with strong lateral elastic gradients causing (de)focusing and scattering. We have previously developed a 3D upper-mantle shear attenuation model based on time domain waveform inversion of long period (T > 60s) fundamental and overtone surface wave data (Gung & Romanowicz, 2004). However, at that time, resolution was limited to very long wavelength structure, because elastic models were still rather smooth, and the effects of focusing could only be estimated approximately, using asymptotic normal mode perturbation theory.With recent progress in constraining global mantle shear velocity from waveform tomography based on the Spectral Element Method (e.g. SEMUCB_WM1, French & Romanowicz, 2014), we are now in a position to develop an improved global 3D model of shear attenuation in the upper mantle. In doing so, we use a similar time domain waveform inversion approach, but (1) start with a higher resolution elastic model with better constraints on lateral elastic gradients and (2) jointly invert, in an iterative fashion, for shear attenuation and elastic parameters. Here, we present the results of synthetic tests that confirm our inversion strategy, as well as preliminary results towards the construction of the next generation upper-mantle anelastic model.

  6. Upper-mantle water stratification inferred from observations of the 2012 Indian Ocean earthquake

    NASA Astrophysics Data System (ADS)

    Masuti, Sagar; Barbot, Sylvain D.; Karato, Shun-Ichiro; Feng, Lujia; Banerjee, Paramesh

    2016-10-01

    Water, the most abundant volatile in Earth’s interior, preserves the young surface of our planet by catalysing mantle convection, lubricating plate tectonics and feeding arc volcanism. Since planetary accretion, water has been exchanged between the hydrosphere and the geosphere, but its depth distribution in the mantle remains elusive. Water drastically reduces the strength of olivine and this effect can be exploited to estimate the water content of olivine from the mechanical response of the asthenosphere to stress perturbations such as the ones following large earthquakes. Here, we exploit the sensitivity to water of the strength of olivine, the weakest and most abundant mineral in the upper mantle, and observations of the exceptionally large (moment magnitude 8.6) 2012 Indian Ocean earthquake to constrain the stratification of water content in the upper mantle. Taking into account a wide range of temperature conditions and the transient creep of olivine, we explain the transient deformation in the aftermath of the earthquake that was recorded by continuous geodetic stations along Sumatra as the result of water- and stress-activated creep of olivine. This implies a minimum water content of about 0.01 per cent by weight—or 1,600 H atoms per million Si atoms—in the asthenosphere (the part of the upper mantle below the lithosphere). The earthquake ruptured conjugate faults down to great depths, compatible with dry olivine in the oceanic lithosphere. We attribute the steep rheological contrast to dehydration across the lithosphere-asthenosphere boundary, presumably by buoyant melt migration to form the oceanic crust.

  7. A crust and upper mantle model of Eurasia and North Africa for Pn travel time calculation

    SciTech Connect

    Myers, S; Begnaud, M; Ballard, S; Pasyanos, M; Phillips, W S; Ramirez, A; Antolik, M; Hutchenson, K; Dwyer, J; Rowe, C; Wagner, G

    2009-03-19

    We develop a Regional Seismic Travel Time (RSTT) model and methods to account for the first-order effect of the three-dimensional crust and upper mantle on travel times. The model parameterization is a global tessellation of nodes with a velocity profile at each node. Interpolation of the velocity profiles generates a 3-dimensional crust and laterally variable upper mantle velocity. The upper mantle velocity profile at each node is represented as a linear velocity gradient, which enables travel time computation in approximately 1 millisecond. This computational speed allows the model to be used in routine analyses in operational monitoring systems. We refine the model using a tomographic formulation that adjusts the average crustal velocity, mantle velocity at the Moho, and the mantle velocity gradient at each node. While the RSTT model is inherently global and our ultimate goal is to produce a model that provides accurate travel time predictions over the globe, our first RSTT tomography effort covers Eurasia and North Africa, where we have compiled a data set of approximately 600,000 Pn arrivals that provide path coverage over this vast area. Ten percent of the tomography data are randomly selected and set aside for testing purposes. Travel time residual variance for the validation data is reduced by 32%. Based on a geographically distributed set of validation events with epicenter accuracy of 5 km or better, epicenter error using 16 Pn arrivals is reduced by 46% from 17.3 km (ak135 model) to 9.3 km after tomography. Relative to the ak135 model, the median uncertainty ellipse area is reduced by 68% from 3070 km{sup 2} to 994 km{sup 2}, and the number of ellipses with area less than 1000 km{sup 2}, which is the area allowed for onsite inspection under the Comprehensive Nuclear Test Ban Treaty, is increased from 0% to 51%.

  8. New upper mantle P-velocity model of Eurasia resolves connections to main regional tectonic structures

    NASA Astrophysics Data System (ADS)

    Koulakov, I.; Kaban, M. K.; Cloetingh, S.

    2012-12-01

    We present a new model of P-velocity anomalies in the upper mantle beneath Eurasia constructed by merging several existing models and by computing new results for a number of gap areas. The models were computed based on tomographic inversions of travel-time data from the worldwide catalogues (ISC, 2001). The calculations were performed in a series of overlapping circular areas of 700-1000 km size. All data with rays corresponding to sources and/or stations in the areas traveling through the target volume were, at least partly, used in the inversions. In case of lack of stations and events, the calculations were based on PP-rays with reflection points in the target area. The new model of Eurasia resolves connections between upper mantle structures and main tectonic units. Cratonic blocks in Eurasia, such as the East-European, Siberian, Indian and Arabian cratons are detected in terms of high-velocity patterns down to 250-300 km depth. The subduction zones in the western Pacific, Burma and the Mediterranean are robustly resolved, consistent with previous studies. In zones of continental collision, we observe traces of mantle delamination as drops of high-velocity material in the mantle. Sites of intraplate volcanism in Europe, Siberia, Mongolia and Yakutia coincide with low-velocity areas, interpreted as overheated upper mantle. Digital version of the model can be downloaded at www.ivan-art.com/temp/vis_eurasia.zip. P-velocity anomalies beneath Eurasia at 100 km depth from regional tomographic inversion. Polygons indicate possible locations of cratonic lithosphere blocks; stars mark the areas of Cenozoic volcanism. P-velocity anomalies beneath Eurasia at 300 km depth from regional tomographic inversion. Polygons indicate possible locations of cratonic lithosphere blocks.

  9. Upper-mantle water stratification inferred from observations of the 2012 Indian Ocean earthquake.

    PubMed

    Masuti, Sagar; Barbot, Sylvain D; Karato, Shun-Ichiro; Feng, Lujia; Banerjee, Paramesh

    2016-10-20

    Water, the most abundant volatile in Earth's interior, preserves the young surface of our planet by catalysing mantle convection, lubricating plate tectonics and feeding arc volcanism. Since planetary accretion, water has been exchanged between the hydrosphere and the geosphere, but its depth distribution in the mantle remains elusive. Water drastically reduces the strength of olivine and this effect can be exploited to estimate the water content of olivine from the mechanical response of the asthenosphere to stress perturbations such as the ones following large earthquakes. Here, we exploit the sensitivity to water of the strength of olivine, the weakest and most abundant mineral in the upper mantle, and observations of the exceptionally large (moment magnitude 8.6) 2012 Indian Ocean earthquake to constrain the stratification of water content in the upper mantle. Taking into account a wide range of temperature conditions and the transient creep of olivine, we explain the transient deformation in the aftermath of the earthquake that was recorded by continuous geodetic stations along Sumatra as the result of water- and stress-activated creep of olivine. This implies a minimum water content of about 0.01 per cent by weight-or 1,600 H atoms per million Si atoms-in the asthenosphere (the part of the upper mantle below the lithosphere). The earthquake ruptured conjugate faults down to great depths, compatible with dry olivine in the oceanic lithosphere. We attribute the steep rheological contrast to dehydration across the lithosphere-asthenosphere boundary, presumably by buoyant melt migration to form the oceanic crust.

  10. Hydration Mechanisms, Crystal Preferred Orientation, and Anisotropy in the Upper Mantle and Transition Zone

    NASA Astrophysics Data System (ADS)

    Smyth, J. R.; Ye, Y.; Jacobsen, S. D.

    2011-12-01

    Nominally anhydrous silicate minerals of the upper mantle and transition zone incorporate H2O into their structures at mantle temperatures and pressures as ordered hydroxyl defects. These defects can commonly be identified by single-crystal X-ray diffraction and polarized infrared spectroscopy. Regional or large-scale mantle hydration can account for two to ten times the mass of water in Earth's oceans affecting anisotropic elastic properties of the mantle's constituent minerals such that hydration causes distinct patterns of seismic anisotropy. Recent crystallographic studies indicate that the principal hydration mechanism of the nominally anhydrous minerals is by protonation of octahedral (Mg,Fe) cation vacancies. Here we show that in the most abundant minerals of the upper 660km, olivine (alpha), wadsleyite (beta), and ringwoodite (gamma) polymorphs of Mg2SiO4, hydrated point defects order to form planes of weakness that can control lattice preferred orientation and velocity anisotropy in various regions of the upper mantle and transition zone. In olivine, ordering of protonated (Mg,Fe) vacancies in the M1 octahedral site predicts that hydration will enhance c-axis alignment parallel to the direction of shear in the (010) plane, as in type-B LPO with SH > SV by up to 10% under horizontal shear. In wadsleyite, vacancies and protons order into the M3 octahedral sites which form double edge-sharing chains of octahedra parallel to a. This vacancy ordering predicts a slip vector of [100] on {011} planes and, unlike olivine, would produce SH < SV by up to 2-6% under horizontal or vertical shear. In ringwoodite, velocity anisotropy is weak, but shear velocity differences of one or two percent are possible. Velocity and electrical conductivity anisotropy may thus be useful indicators of hydration in planetary interiors.

  11. Crust and Upper Mantle Structure of the Hellenic and Cyprus Subduction Zones from Gravity Data Modelling

    NASA Astrophysics Data System (ADS)

    Alemdar, S.; Mahatsente, R.; Cemen, I.

    2015-12-01

    The neotectonics of the Anatolian and Aegean regions is the result of the African plate subduction along the Hellenic and Cyprus trenches and the Anatolian plate collision with the Eurosian plate. The African slab, as imaged by seismic tomography, penetrates the lower mantle and exhibits two major lateral tears below the Anatolian plate. The tears in the slab are related to low velocity structures in the sub-lithospheric mantle. The presence of low velocity structures in the upper mantle is a clear indication of anomalous asthenosphere (asthenospheric windows). What remains unclear is, however, how and to what extent the crust and upper mantle structure beneath the Anatolian region has been modified by the upwelling hot asthenospheric material. To determine the effects of the upwelling hot asthenospheric material in the region, we developed a 3-D gravity model of the crust and upper mantle structure of the Aegean and Anatolian regions (24°-33° E and 34°-40° N). The gravity model is based on satellite-derived gravity data from GRACE, LAGEOS and GOCE missions (EIGEN 6C2). The results of the gravity modelling, as constrained by seismic tomography, shows that the crust above the asthenospheric window, where the subducted African slab exhibits major lateral tears, is relatively thin. The crustal thickness variation within the asthenospheric window area is between 24 & 29 km. In contrast, the regions outside the asthenospheric window area exhibit by far the largest crustal thickness (30 - 42 km). We therefore conclude that the observed crustal thinning in the asthenospheric window area might be attributed to thermal erosion induced by the upwelling hot asthenospheric material and extensional tectonics related to the Southwest retreating Hellenic trench and westward movement of the Anatolian micro plate. The thinning may also be responsible for the high geothermal gradient in the Denizli graben area where two major grabens (i.e., Alaşehir and Bűyűk Menderes Grabens

  12. Mantle phase changes and deep-earthquake faulting in subducting lithosphere.

    PubMed

    Kirby, S H; Durham, W B; Stern, L A

    1991-04-12

    Inclined zones of earthquakes are the primary expression of lithosphere subduction. A distinct deep population of subduction-zone earthquakes occurs at depths of 350 to 690 kilometers. At those depths ordinary brittle fracture and frictional sliding, the faulting processes of shallow earthquakes, are not expected. A fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics, including the discovery and characterization of transformational faulting, a shear instability connected with localized phase transformations under nonhydrostatic stress. These developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine-rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km. Predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes, the maximum depths of earthquakes in individual subductions zones, and key source characteristics of deep events.

  13. Mantle phase changes and deep-earthquake faulting in subducting lithosphere

    USGS Publications Warehouse

    Kirby, S.H.; Durham, W.B.; Stern, L.A.

    1991-01-01

    Inclined zones of earthquakes are the primary expression of lithosphere subduction. A distinct deep population of subduction-zone earthquakes occurs at depths of 350 to 690 kilometers. At those depths ordinary brittle fracture and frictional sliding, the faulting processes of shallow earthquakes, are not expected. A fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics, including the discovery and characterization of transformational faulting, a shear instability connected with localized phase transformations under nonhydrostatic stress. These developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine-rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km. Predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes, the maximum depths of earthquakes in individual subductions zones, and key source characteristics of deep events.

  14. Low velocities in the oceanic upper mantle and their relation to plumes: insights from SEM-based waveform tomography

    NASA Astrophysics Data System (ADS)

    Lekic, V.; French, S. W.; Romanowicz, B. A.

    2013-12-01

    The exchange of heat, mass and momentum between tectonic plates and mantle convection controls lithospheric evolution and hotspot volcanism, and must occur at a range of spatial scales. Yet, the detailed morphology of the associated convection patterns continues to elude geophysicists. Because seismic velocities are affected by temperature, seismic tomography can be used to map the patterns of flow in the Earth's mantle. Here, we present a global-scale long-period full-waveform seismic tomographic model SEMum2 constructed using the Spectral Element Method, which can very accurately model wave propagation through highly complex structures, and account for phenomena such as scattering, (de)focusing, and wavefront healing. Notably, SEMum2 achieves more realistic amplitudes of lateral heterogeneity - particularly low velocities in the upper 250km - than previous generations of global models, while still retrieving the long-wavelength structure present in earlier tomographic models. Cluster analysis of profiles of shear velocity in the SEMum2 oceanic upper mantle, confirms the presence of a well marked shear wave low velocity zone (LVZ) beneath the lithosphere, with a velocity minimum which deepens progressively as a function of age of the plate. The LVZ minimum in SEMum2 reaches values that are lower than in previous tomographic global models and in agreement with local estimates where available. Interestingly, reaching below this "classical" low velocity zone, the model reveals a pattern of alternating lower and higher velocities organized into elongated bands in the direction of absolute plate motion (APM), with a quasi-regular spacing of ~2000 km perpendicular to the APM. This fingerlike structure, most prominent around 200-250 km and extending down to 350-400 km, is most prominent beneath the Pacific plate, but also present under the eastern Antarctic plate, in the south Atlantic and in parts of the Indian Ocean Below this depth, the low velocities appear organized

  15. Dynamics of upper mantle rocks decompression melting above hot spots under continental plates

    NASA Astrophysics Data System (ADS)

    Perepechko, Yury; Sorokin, Konstantin; Sharapov, Victor

    2014-05-01

    Numeric 2D simulation of the decompression melting above the hot spots (HS) was accomplished under the following conditions: initial temperature within crust mantle section was postulated; thickness of the metasomatized lithospheric mantle is determined by the mantle rheology and position of upper asthenosphere boundary; upper and lower boundaries were postulated to be not permeable and the condition for adhesion and the distribution of temperature (1400-2050°C); lateral boundaries imitated infinity of layer. Sizes and distribution of lateral points, their symmetry, and maximum temperature varied between the thermodynamic condition for existences of perovskite - majorite transition and its excess above transition temperature. Problem was solved numerically a cell-vertex finite volume method for thermo hydrodynamic problems. For increasing convergence of iterative process the method of lower relaxation with different value of relaxation parameter for each equation was used. The method of through calculation was used for the increase in the computing rate for the two-layered upper mantle - lithosphere system. Calculated region was selected as 700 x (2100-4900) km. The time step for the study of the asthenosphere dynamics composed 0.15-0.65 Ma. The following factors controlling the sizes and melting degree of the convective upper mantle, are shown: a) the initial temperature distribution along the section of upper mantleb) sizes and the symmetry of HS, c) temperature excess within the HS above the temperature on the upper and lower mantle border TB=1500-2000oC with 5-15% deviation but not exceed 2350oC. It is found, that appearance of decompression melting with HS presence initiate primitive mantle melting at TB > of 1600oC. Initial upper mantle heating influence on asthenolens dimensions with a constant HS size is controlled mainly by decompression melting degree. Thus, with lateral sizes of HS = 400 km the decompression melting appears at TB > 1600oC and HS

  16. Upper mantle velocity structure beneath the Cameroon Volcanic Line region and implications for the formation of mantle hot lines

    NASA Astrophysics Data System (ADS)

    Adams, A. N.; Wiens, D. A.; Euler, G. G.; Nyblade, A.; Shore, P.

    2013-12-01

    The Cameroon Volcanic Line (CVL) is a 1800km long feature, extending SW-NE from the Gulf of Guinea into Central Africa. Volcanism along the line does not display the typical age progression exhibited by hotspot-related volcanic tracks, leading to speculation over the geodynamic source of this intraplate feature. Numerous models have been proposed to explain the linear nature of the CVL in the absence of age progression, including laterally transported material from a single or multiple plumes, reactivation of the Central African Shear Zone, edge-flow convection associated with the neighboring Congo Craton, and convection driven by lithospheric instabilities at the edge of continental lithosphere. In this study, we calculate Rayleigh wave phase velocities and upper mantle shear wave velocity structure beneath the continental portion of the CVL to investigate the geodynamic source of the CVL. Rayleigh wave phase velocities are measured at periods from 20 to 182 seconds following the two-plane wave methodology developed by Forsyth and Li (2005), and using data from the Cameroon Seismic Experiment, which consists of 32 broadband stations deployed between 2005 and 2007. These phase velocities are then inverted to build a model of shear wave velocity structure in the upper mantle beneath the CVL. We find that phase velocities beneath the CVL are reduced, while velocities beneath the Congo Craton to the south are elevated. This is observed for all periods, but the difference between regions decreases at the longest periods measured in the study. Shear wave velocity structure indicates a tabular low velocity anomaly directly beneath the CVL, extending from 50km to at least 200km depth, with a sharp vertical boundary with the faster velocities beneath the Congo Craton. These observations are most consistent with the edge convection or lithospheric instability models as the source of the continental CVL. Further study of offshore structure will aid in better characterizing

  17. New constraints on upper mantle creep mechanism inferred from silicon grain-boundary diffusion rates

    NASA Astrophysics Data System (ADS)

    Fei, Hongzhan; Koizumi, Sanae; Sakamoto, Naoya; Hashiguchi, Minako; Yurimoto, Hisayoshi; Marquardt, Katharina; Miyajima, Nobuyoshi; Yamazaki, Daisuke; Katsura, Tomoo

    2016-01-01

    The creep in the Earth's interior is dominated either by diffusion creep which causes Newtonian mantle flow, or by dislocation creep which results in non-Newtonian mantle flow. Although previous deformation studies on olivine claimed a transition from dislocation creep to diffusion creep with depth in the upper mantle, they might misunderstand the creep rates due to experimental difficulties. Since creep in olivine is controlled by silicon diffusion, we measured the silicon grain-boundary diffusion coefficient in well-sintered iron-free olivine aggregates as a function of temperature, pressure, and water content, showing activation energy, activation volume, and water content exponent of 220 ± 30 kJ /mol, 4.0 ± 0.7 cm3 /mol, and 0.26 ± 0.07, respectively. Our results based on Si diffusion in forsterite predict that diffusion creep dominates at low pressures and low temperatures, whereas dislocation creep dominates under high pressure and high temperature conditions. Water has negligible effects on both diffusion and dislocation creep. There is a transition from diffusion creep in the shallow upper mantle to dislocation creep in deeper regions. This explains the seismic anisotropy increases at the Gutenberg discontinuity beneath oceans and at the mid-lithosphere discontinuity beneath continents.

  18. Elasticity of Diopside to 8 GPa and 1073K and Implications for the Upper Mantle

    SciTech Connect

    B Li; D Neuville

    2011-12-31

    Simultaneous measurements of elastic wave velocities and density have been conducted on diopside by a combined ultrasonic interferometry and X-ray diffraction methods at pressure and temperature conditions relevant to the Earth's upper mantle. The current study not only yielded the first direct measurement of the shear modulus at high pressures, but also enabled a simultaneous determination of the bulk and shear properties and their pressure and temperature derivatives from the measured density and velocities. Finite strain analysis of the experimental data results in K{sub S0} = 116.4(7) GPa, K'S0 = 4.9(1), ({partial_derivative}K{sub S}/{partial_derivative}T){sub P} = -0.012(1) GPa GPa, G'{sub 0} = 1.6(1) and ({partial_derivative}G/{partial_derivative}T){sub P} = -0.011(1) GPa/K. With these results and the same finite strain equations, the P and S wave velocities of diopsidic mantle clinopyroxene were calculated along a 1600 K adiabatic geotherm. In comparison with other mantle minerals, the seismic velocities of diopsidic clinopyroxene at upper mantle depths are 1-3% higher than those of orthopyroxene, 1-2% and 6-8% lower than those of olivine and majoritic garnet, respectively.

  19. The Deep Mantle Volatile Cycle Revealed in Superdeep Diamonds and their Mineral Inclusions

    NASA Astrophysics Data System (ADS)

    Walter, Michael; Thomson, Andrew; Frost, Jennifer; Bulanova, Galina; Smith, Chris; Kohn, Simon; Burnham, Antony

    2013-04-01

    Diamonds crystallize in the mantle primarily as a consequence of fluid or melt metasomatism. In doing so they sample the fluid-melt-solid equilibria directly by incorporation of carbon and its isotopic flavours, and by entrapping other phases as they grow. Superdeep diamonds from the transition zone and lower mantle provide evidence for crystallization from melts derived from subducted materials [1, 2]. The presence of deeply subducted volatile components such as carbon and water are important because they lower the solidus of subducted materials. The source of carbon may ultimately be via deposition of biogenic or abiogenic carbon in subducted crust, and water may become available via dehydration of high-pressure hydrous phases in the slab (e.g. superhydrous B, Phase D) [3]. Foundering of slabs around 700 km due to density inversion and thermalization with surrounding mantle leads to the generation of low-degree, volatile-charged melts. Melts from subducted oceanic crust may be carbonated, and diamond crystallization occurs as a consequence of 'redox freezing' when the oxidized slab melts react with reducing mantle rocks [4]. Reaction of slab melts with mantle peridotite may precipitate phases such as Ca-perovskite, Mg-perovskite, majorite and ferropericlase. Here we will survey evidence from the chemistry of superdeep mineral inclusions for a record of this deep mantle reactive transport process, and speculate on the role of deep mantle volatiles. 1. Bulanova, G.P., et al., Contributions to Mineralogy and Petrology, 2010. 160: p. 489-510. 2. Walter, M.J., et al., Nature, 2008. 454: p. 622-U30. 3. Harte, B., Mineralogical Magazine, 2010. 74: p. 189-215. 4. Rohrbach, A. and M.W. Schmidt, Nature, 2011. 472: p. 209-212.

  20. Spin and valence dependence of iron partitioning in Earth's deep mantle.

    PubMed

    Piet, Hélène; Badro, James; Nabiei, Farhang; Dennenwaldt, Teresa; Shim, Sang-Heon; Cantoni, Marco; Hébert, Cécile; Gillet, Philippe

    2016-10-04

    We performed laser-heated diamond anvil cell experiments combined with state-of-the-art electron microanalysis (focused ion beam and aberration-corrected transmission electron microscopy) to study the distribution and valence of iron in Earth's lower mantle as a function of depth and composition. Our data reconcile the apparently discrepant existing dataset, by clarifying the effects of spin (high/low) and valence (ferrous/ferric) states on iron partitioning in the deep mantle. In aluminum-bearing compositions relevant to Earth's mantle, iron concentration in silicates drops above 70 GPa before increasing up to 110 GPa with a minimum at 85 GPa; it then dramatically drops in the postperovskite stability field above 116 GPa. This compositional variation should strengthen the lowermost mantle between 1,800 km depth and 2,000 km depth, and weaken it between 2,000 km depth and the D" layer. The succession of layers could dynamically decouple the mantle above 2,000 km from the lowermost mantle, and provide a rheological basis for the stabilization and nonentrainment of large low-shear-velocity provinces below that depth.

  1. Spin and valence dependence of iron partitioning in Earth’s deep mantle

    PubMed Central

    Piet, Hélène; Badro, James; Nabiei, Farhang; Dennenwaldt, Teresa; Shim, Sang-Heon; Cantoni, Marco; Hébert, Cécile; Gillet, Philippe

    2016-01-01

    We performed laser-heated diamond anvil cell experiments combined with state-of-the-art electron microanalysis (focused ion beam and aberration-corrected transmission electron microscopy) to study the distribution and valence of iron in Earth’s lower mantle as a function of depth and composition. Our data reconcile the apparently discrepant existing dataset, by clarifying the effects of spin (high/low) and valence (ferrous/ferric) states on iron partitioning in the deep mantle. In aluminum-bearing compositions relevant to Earth’s mantle, iron concentration in silicates drops above 70 GPa before increasing up to 110 GPa with a minimum at 85 GPa; it then dramatically drops in the postperovskite stability field above 116 GPa. This compositional variation should strengthen the lowermost mantle between 1,800 km depth and 2,000 km depth, and weaken it between 2,000 km depth and the D” layer. The succession of layers could dynamically decouple the mantle above 2,000 km from the lowermost mantle, and provide a rheological basis for the stabilization and nonentrainment of large low-shear-velocity provinces below that depth. PMID:27647917

  2. A global horizontal shear velocity model of the upper mantle from multimode Love wave measurements

    NASA Astrophysics Data System (ADS)

    Ho, Tak; Priestley, Keith; Debayle, Eric

    2016-10-01

    Surface wave studies in the 1960s provided the first indication that the upper mantle was radially anisotropic. Resolving the anisotropic structure is important because it may yield information on deformation and flow patterns in the upper mantle. The existing radially anisotropic models are in poor agreement. Rayleigh waves have been studied extensively and recent models show general agreement. Less work has focused on Love waves and the models that do exist are less well-constrained than are Rayleigh wave models, suggesting it is the Love wave models that are responsible for the poor agreement in the radially anisotropic structure of the upper mantle. We have adapted the waveform inversion procedure of Debayle & Ricard to extract propagation information for the fundamental mode and up to the fifth overtone from Love waveforms in the 50-250 s period range. We have tomographically inverted these results for a mantle horizontal shear wave-speed model (βh(z)) to transition zone depths. We include azimuthal anisotropy (2θ and 4θ terms) in the tomography, but in this paper we discuss only the isotropic βh(z) structure. The data set is significantly larger, almost 500 000 Love waveforms, than previously published Love wave data sets and provides ˜17 000 000 constraints on the upper-mantle βh(z) structure. Sensitivity and resolution tests show that the horizontal resolution of the model is on the order of 800-1000 km to transition zone depths. The high wave-speed roots beneath the oldest parts of the continents appear to extend deeper for βh(z) than for βv(z) as in previous βh(z) models, but the resolution tests indicate that at least parts of these features could be artefacts. The low wave speeds beneath the mid-ocean ridges fade by ˜150 km depth except for the upper mantle beneath the East Pacific Rise which remains slow to ˜250 km depth. The resolution tests suggest that the low wave speeds at deeper depths beneath the East Pacific Rise are not solely due

  3. Some recent advances in understanding the mineralogy of Earth's deep mantle.

    PubMed

    Duffy, Thomas S

    2008-11-28

    Understanding planetary structure and evolution requires a detailed knowledge of the properties of geological materials under the conditions of deep planetary interiors. Experiments under the extreme pressure-temperature conditions of the deep mantle are challenging, and many fundamental properties remain poorly constrained or are inferred only through uncertain extrapolations from lower pressure-temperature states. Nevertheless, the last several years have witnessed a number of new developments in this area, and a broad overview of the current understanding of the Earth's lower mantle is presented here. Some recent experimental and theoretical advances related to the lowermost mantle are highlighted. Measurements of the equation of state and deformation behaviour of (Mg,Fe)SiO3 in the CaIrO3-type (post-perovskite) structure yield insights into the nature of the core-mantle boundary region. Theoretical studies of the behaviour of MgSiO3 liquids under high pressure-temperature conditions provide constraints on melt volumes, diffusivities and viscosities that are relevant to understanding both the early Earth (e.g. deep magma oceans) and seismic structure observed in the present Earth (e.g. ultra-low-velocity zones).

  4. Some recent advances in understanding the mineralogy of Earth's deep mantle

    SciTech Connect

    Duffy, T S

    2008-12-09

    Understanding planetary structure and evolution requires a detailed knowledge of the properties of geological materials under the conditions of deep planetary interiors. Experiments under the extreme pressure-temperature conditions of the deep mantle are challenging, and many fundamental properties remain poorly constrained or are inferred only through uncertain extrapolations from lower pressure-temperature states. Nevertheless, the last several years have witnessed a number of new developments in this area, and a broad overview of the current understanding of the Earth's lower mantle is presented here. Some recent experimental and theoretical advances related to the lowermost mantle are highlighted. Measurements of the equation of state and deformation behaviour of (Mg,Fe)SiO{sub 3} in the CaIrO{sub 3}-type (post-perovskite) structure yield insights into the nature of the core-mantle boundary region. Theoretical studies of the behaviour of MgSiO3 liquids under high pressure-temperature conditions provide constraints on melt volumes, diffusivities and viscosities that are relevant to understanding both the early Earth (e.g. deep magma oceans) and seismic structure observed in the present Earth (e.g. ultra-low-velocity zones).

  5. Sound velocities of olivine at high pressures and temperatures and the composition of Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Zhang, Jin S.; Bass, Jay D.

    2016-09-01

    We present the elastic properties of San Carlos olivine up to P = 12.8(8) GPa and T = 1300(200) K using Brillouin spectroscopy with CO2 laser heating. A comparison of our results with the global seismic model AK135 yields average olivine content near 410 km depth of about 37% and 43% in a dry and wet (1.9 wt % H2O) upper mantle, respectively. These olivine contents are far less than in the pyrolite model. However, comparisons of our results with regional seismic models lead to very different conclusions. High olivine contents of up to 87% are implied by seismic models of the western U.S. and eastern Pacific regions. In contrast, we infer less than 35% olivine under the central Pacific. Strong variations of olivine content and upper mantle lithologies near the 410 km discontinuity are suggested by regional seismic models.

  6. Dislocation damping and anisotropic seismic wave attenuation in Earth's upper mantle.

    PubMed

    Farla, Robert J M; Jackson, Ian; Fitz Gerald, John D; Faul, Ulrich H; Zimmerman, Mark E

    2012-04-20

    Crystal defects form during tectonic deformation and are reactivated by the shear stress associated with passing seismic waves. Although these defects, known as dislocations, potentially contribute to the attenuation of seismic waves in Earth's upper mantle, evidence for dislocation damping from laboratory studies has been circumstantial. We experimentally determined the shear modulus and associated strain-energy dissipation in pre-deformed synthetic olivine aggregates under high pressures and temperatures. Enhanced high-temperature background dissipation occurred in specimens pre-deformed by dislocation creep in either compression or torsion, the enhancement being greater for prior deformation in torsion. These observations suggest the possibility of anisotropic attenuation in relatively coarse-grained rocks where olivine is or was deformed at relatively high stress by dislocation creep in Earth's upper mantle.

  7. Towards Tidal Tomography: Using Earth's Body-Tide Signal to Constrain Deep-Mantle Density Structure

    NASA Astrophysics Data System (ADS)

    Lau, Harriet; Yang, Hsin-Ying; Davis, James; Mitrovica, Jerry; Tromp, Jeroen; Latychev, Konstantin

    2015-04-01

    Luni-solar forcings drive long wavelength deformation at timescales ranging from 8 hours to 18.6 years. We propose that globally distributed GPS estimates of this deformation within the semi-diurnal band provide a new and independent constraint on long-wavelength deep mantle structure. A particular target of "tidal tomography" is the buoyancy structure of LLSVPs, which constitute a large volumetric fraction of the mantle. Constraining this structure is the key to understanding the longevity of the LLSVPs, and indeed the evolution of the entire mantle and Earth system. To this end, we begin by reporting on the development of a new normal-mode theory, based on relatively recent advances in free oscillation seismology, which is capable of predicting semi-diurnal body tides on a laterally heterogeneous, rotating and anelastic Earth. We next present the results of a suite of benchmark tests involving comparisons with predictions based on both classical tidal Love number theory for 1-D Earth models and finite-volume simulations that incorporate 3-D elastic and density structure. We find that body tide deformation is most sensitive to long wavelength, deep mantle structure, and, in particularly, to shear wave velocity and density structure. When combined with results from seismological datasets, this sensitivity provides a powerful tool to investigate the buoyancy structure of the LLSVPs. For example, adopting a variety of seismic tomography models a priori, we perform an extensive parameter search to determine misfits between model predictions based on the new theory and GPS-derived estimates of the semi-diurnal body tide displacements. Preliminary results, focusing only on density structure, have indicated that the observations are best fit when the LLSVPs have a bulk density greater than average mantle, in broad agreement with previous inferences based upon seismic normal mode inversions. In follow-up work, we have mapped out trade-offs related to the adopted seismic

  8. Calibration of Three-Dimensional Upper Mantle Structure in Eurasia Using Regional and Teleseismic Full Waveform Seismic Data

    DTIC Science & Technology

    2007-09-01

    successfully for global and regional mantle tomography at Berkeley since 1995. In the subregion of study, our “N-Born” model is parameterized at...waveforms, Geophys. J. Int. 143: 709–728. Panning, M. and B. Romanowicz (2004). Inferences on flow at the base of Earth’s mantle based on seismic...CALIBRATION OF THREE-DIMENSIONAL UPPER MANTLE STRUCTURE IN EURASIA USING REGIONAL AND TELESEISMIC FULL WAVEFORM SEISMIC DATA Barbara Romanowicz1

  9. Solubility of Nitrogen in Stishovite: A Possible Storage Mechanism for Nitrogen in Earth's Deep Mantle

    NASA Astrophysics Data System (ADS)

    Noble, S. M.; Shim, S. H. D.; Hervig, R. L.; Prakapenka, V.

    2016-12-01

    Although studies have suggested the flux of nitrogen from the mantle as well as the subduction of nitrogen back into the mantle, it is unknown how much nitrogen can be stored in the deep mantle. Some key questions remain unknown: Does nitrogen exist in major mantle minerals or in minor phases? Or does nitrogen form nitrides or oxynitrides in the mantle? We have synthesized stishovite under nitrogen saturated conditions in laser-heated diamond-anvil cells at pressures between 16 and 44 GPa and temperatures centered at 1800 K. Experimental products were recovered and analyzed for nitrogen content via SIMS, SEM and EDX analysis at Arizona State University, while unit cell parameters were measured through synchrotron x-ray diffraction at the GSECARS sector of the Advanced Photon Source. Our SIMS data show that nitrogen solubility in stishovite is 1.54 wt %. The existence of nitrogen was also confirmed through energy-dispersive X-ray spectroscopy in SEM. Diffraction data indicates a slightly higher unit cell volume of stishovite synthesized under a nitrogen saturated environment than pure stishovite at pressures above 28 GPa. Because stishovite is expected to be a major phase in subducting oceanic crust in the mantle (more than 15%), according to our new experiments, stishovite can serve as a host for nitrogen in the mantle. With similar atomic radii, slightly smaller-sized nitrogen may substitute for oxygen atoms in the crystal structure of stishovite. At a storage capacity of 1.54 wt % for N in stishovite, this study indicates a potentially large nitrogen reservoir within the Earth's mantle.

  10. Earth's Deep Carbon Cycle Constrained by Partial Melting of Mantle Peridotite and Eclogite

    NASA Astrophysics Data System (ADS)

    Dasgupta, R.; Hirschmann, M. M.; Withers, A. C.

    2006-05-01

    The mass of carbon in the mantle is thought to exceed that in all Earth's other reservoirs combined1 and large fluxes of carbon are cycled into and out of the mantle via subduction and volcanic emission. Devolatilization is known to release water in the mantle wedge, but release of carbon could be delayed if the relevant decarbonation reactions or solidi of oceanic crust are not encountered along P-T path of subduction. Outgassing of CO2 from the mantle also has a critical influence on Earth's climate for time scales of 108-109 yr1. The residence time for carbon in the mantle is thought to exceed the age of the Earth1,2, but it could be significantly shorter owing to pervasive deep melting beneath oceanic ridges. The dominant influx of carbon is via carbonate in altered ocean-floor basalts, which survives decarbonation during subduction. Our experiments demonstrate that solidi of carbonated eclogite remain hotter than average subduction geotherms at least as deep as transition zone3, and thus significant subducted C is delivered to the deep Earth, rather than liberated in the shallow mantle by melting. Flux of CO2 into the mantle, assuming average estimate of carbon in altered ocean crust of 0.21 wt. % CO24, can amount to 0.15 × 1015 g/yr. In upwelling mantle, however, partial melting of carbonated eclogite releases calcio-dolomitic carbonatite melt at depths near ~400 km and metasomatically implants carbonate to surrounding peridotite. Thus, volcanic release of CO2 to basalt source regions is likely controlled by the solidus of carbonated peridotite. Our recent experiments with nominally anhydrous, carbonate-bearing garnet lherzolite indicate that the solidus of peridotite with a trace amount of CO2 is ~500 °C lower than that of volatile-free peridotite at 10 GPa5. In upwelling mantle the solidus of carbonated lherzolite is ~100-200 km shallower than that of eclogite+CO2, but beneath oceanic ridges, initial melting occurs as deep as 300-330 km. For peridotite

  11. Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming

    USGS Publications Warehouse

    McCallum, M.E.; Eggler, D.H.

    1976-01-01

    Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) Iherzolite at a depth of 130 to 180 kilometers.

  12. Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming.

    PubMed

    McCallum, M E; Eggler, D H

    1976-04-16

    Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) lherzolite at a depth of 130 to 180 kilometers.

  13. Regionalized Temperature Variations in the Upper 400 km of the Earth's Mantle

    NASA Technical Reports Server (NTRS)

    Tralli, D. M.; Ita, J. J.

    1995-01-01

    Tectonically regionalized variations in the temperature of the upper 400 km of the Earth's mantle are estimated from analysis of global seismic travel-time data catalogued by the International Seismological Centre (ISC). Seismic parameter profiles are determined from estimates of P and S velocities obtained by tau inversion, and summary phase diagrams for the olivine and pyroxene- garnet subsystems are constructed in conjunction with a thermodynamic potential formulation.

  14. Compositional effects on the density of volatile-bearing magmatic liquids in the upper mantle

    NASA Astrophysics Data System (ADS)

    Seifert, R.; Malfait, W.; Sanchez-Valle, C.; Petitgirard, S.; Mezouar, N.

    2013-12-01

    The density of silicate liquids is a key control on many magmatic processes, including magma chamber dynamics and volcanic eruptions, melt extraction from residual rocks during partial melting, fractional crystallization processes and crystal settling. However, the experimental data on the density and compressibility of silicate melts relevant for magmatic processes in the crust and upper mantle remain scarce until now, especially for volatile-bearing compositions. In this contribution we review recent experimental studies to determine the density of silicate liquids with magmatic composition at upper mantle conditions (up to 3.5 GPa and 2000 K). The investigated compositions include dry and hydrous granitic (4.5 and 7.7 wt% H2O), alkaline (phonolite, 4.3 wt% H2O) and andesitic (6 wt% H2O) melts. The experiments were performed using a panoramic Paris-Edinbugh and the density was determined from the X-ray absorption contrast between the samples and a diamond capsule used as sample container. The run products were analyzed by electron microprobe, infrared spectroscopy and SIMS to verify the chemical composition and volatile content of the samples. The results, combined with literature data on silicate melt density at atmospheric pressure, provides the first experimentally derived equations of state for magmatic liquids with a broad range of silica contents at crustal and upper mantle conditions. The resulting equations of state allow constraining pressure and compositional effects on the compressibility of dissolved water in silicate melts. The implications of these results for the ascent rate of slab melts in subduction zones, the dynamics of magma chambers and crystal-liquid buoyancy relations and crystal settling velocities in the upper mantle will be discussed.

  15. Rayleigh Wave Phase Velocity in the Upper Mantle Beneath the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Godfrey, K. E.; Dalton, C. A.; Ritsema, J.

    2016-12-01

    Most of what is currently understood about the seismic properties of oceanic upper mantle is based on either global studies or regional studies of the upper mantle beneath the Pacific Ocean. However, global seismic models and geochemical studies of mid-ocean ridge basalts indicate differences in the properties of the upper mantle beneath the Pacific, Atlantic, and Indian oceans. Though the Indian Ocean is not as well studied seismically, it is host to a number of geologically interesting features including 16,000 km of mid-ocean ridge with a range of spreading rates from 14 mm/yr along the Southwest Indian Ridge to 55-75 mm/yr along the Southeast Indian Ridge. The Indian Ocean also contains multiple volcanic hotspots, the Australian-Antarctic Discordance, and a low geoid anomaly south of India, and it overlies a portion of a large low-shear-velocity province. We are using Rayleigh waves to construct a high-resolution seismic velocity model of the Indian Ocean upper mantle. We utilize a global dataset of phase delays measured at 20 periods, between 37 and 375 seconds; the dataset includes between 700 and 20,000 that traverse our study region exclusively, with a larger number of paths at shorter periods. We explore variations in phase velocity using two separate approaches. One, we allow phase velocity to vary only as a function of seafloor age. Two, we perform a damped least-squares inversion to solve for 2-D phase velocity maps at each period. Preliminary results indicate low velocities along the Southeast Indian Ridge and Central Indian Ridge, but the expected low velocities are less apparent along the slow-spreading Southwest Indian Ridge. We observe a region of fast velocities extending from Antarctica northward between the Kerguelen and Crozet hotspots, and lower than expected velocities beneath the Reunion hotspot. Additionally, we find low velocities associated with a region of extinct seafloor spreading in the Wharton basin.

  16. 3D density model of the upper mantle of Asia based on inversion of gravity and seismic tomography data

    NASA Astrophysics Data System (ADS)

    Kaban, Mikhail K.; Stolk, Ward; Tesauro, Magdala; El Khrepy, Sami; Al-Arifi, Nassir; Beekman, Fred; Cloetingh, Sierd A. P. L.

    2016-11-01

    We construct a new-generation 3D density model of the upper mantle of Asia and its surrounding areas based on a joint interpretation of several data sets. A recent model of the crust combining nearly all available seismic data is employed to calculate the impact of the crust on the gravity anomalies and observed topography and to estimate the residual mantle anomalies and residual topography. These fields are jointly inverted to calculate the density variations in the lithosphere and upper mantle down to 325 km. As an initial approximation, we estimate density variations using a seismic tomography model. Seismic velocity variations are converted into temperatures and then to density variations based on mineral physics constraints. In the Occam-type inversion, we fit both the residual mantle gravity anomalies and residual topography by finding deviations to the initial model. The obtained corrections improve the resolution of the initial model and reflect important features of the mantle structure that are not well resolved by the seismic tomography. The most significant negative corrections of the upper mantle density, found in the Siberian and East European cratons, can be associated with depleted mantle material. The most pronounced positive density anomalies are found beneath the Tarim and South Caspian basins, Barents Sea, and Bay of Bengal. We attribute these anomalies to eclogites in the uppermost mantle, which have substantially affected the evolution of the basins. Furthermore, the obtained results provide evidence for the presence of eclogites in the oceanic subducting mantle lithosphere.

  17. Crust and upper mantle structure of the New Madrid Seismic Zone: Insight into intraplate earthquakes

    NASA Astrophysics Data System (ADS)

    Chen, Chuanxu; Zhao, Dapeng; Wu, Shiguo

    2014-05-01

    We determine a 3-D P-wave velocity model of the crust and upper mantle down to 400 km depth to investigate structural heterogeneity and its influences on the generation of intraplate earthquakes in the New Madrid Seismic Zone. We used 4871 high-quality arrival times from 187 local earthquakes and 30,846 precise travel-time residuals from 1041 teleseismic events recorded by the EarthScope/USArray Transportable Array. Our results show that, beneath the Reelfoot rift, a significant low-velocity (low-V) zone exists in the upper mantle down to 200 km depth, with a large volume of 200 × 200 × 150 km3. The origin of the low-V zone may be related to the passage of the Bermuda hotspot and the stalled ancient Farallon slab materials foundering in the mantle transition zone. This low-V zone may have relatively low shear strength and act as a viscously weak zone embedded in the lithosphere, being apt to concentrate tectonic stress and transfer stress to the seismogenic faults in the upper crust, leading to the large intraplate earthquakes in the New Madrid Seismic Zone.

  18. Sensitivity analysis of seismic waveforms to upper-mantle discontinuities using the adjoint method

    NASA Astrophysics Data System (ADS)

    Koroni, Maria; Bozdağ, Ebru; Paulssen, Hanneke; Trampert, Jeannot

    2017-09-01

    Using spectral-element simulations of wave propagation, we investigated the sensitivity of seismic waveforms, recorded on transverse components, to upper-mantle discontinuities in 1-D and 3-D background models. These sensitivity kernels, or Fréchet derivatives, illustrate the spatial sensitivity to model parameters, of which those for shear wave speed and the surface topography of internal boundaries are discussed in this paper. We focus on the boundaries at 400 and 670 km depth of the mantle transition zone. SS precursors have frequently been used to infer the topography of upper-mantle discontinuities. These seismic phases are underside reflections off these boundaries and are usually analysed in the distance range of 110°-160°. This distance range is chosen to minimize the interference from other waves. We show sensitivity kernels for consecutive time windows at three characteristic epicentral distances within the 110°-160° range. The sensitivity kernels are computed with the adjoint method using synthetic data. From our simulations we can draw three main conclusions: (i) The exact Fréchet derivatives show that in all time windows, and also in those centred on the SS precursors, there is interference from other waves. This explains the difficulty reported in the literature to correct for 3-D shear wave speed perturbations, even if the 3-D structure is perfectly known. (ii) All studies attempting to map the topography of the 400 and 670 km discontinuities to date assume that the traveltimes of SS precursors can be linearly decomposed into a 3-D elastic structure and a topography part. We recently showed that such a linear decomposition is not possible for SS precursors, and the sensitivity kernels presented in this paper explain why. (iii) In agreement with previous work, we show that other parts of the seismograms have greater sensitivity to upper-mantle discontinuities than SS precursors, especially multiply bouncing S waves exploiting the S

  19. P wave velocity of Proterozoic upper mantle beneath central and southern Asia

    NASA Astrophysics Data System (ADS)

    Nyblade, Andrew A.; Vogfjord, Kristin S.; Langston, Charles A.

    1996-05-01

    P wave velocity structure of Proterozoic upper mantle beneath central and southern Africa was investigated by forward modeling of Pnl waveforms from four moderate size earthquakes. The source-receiver path of one event crosses central Africa and lies outside the African superswell while the source-receiver paths for the other events cross Proterozoic lithosphere within southern Africa, inside the African superswell. Three observables (Pn waveshape, PL-Pn time, and Pn/PL amplitude ratio) from the Pnl waveform were used to constrain upper mantle velocity models in a grid search procedure. For central Africa, synthetic seismograms were computed for 5880 upper mantle models using the generalized ray method and wavenumber integration; synthetic seismograms for 216 models were computed for southern Africa. Successful models were taken as those whose synthetic seismograms had similar waveshapes to the observed waveforms, as well as PL-Pn times within 3 s of the observed times and Pn/PL amplitude ratios within 30% of the observed ratio. Successful models for central Africa yield a range of uppermost mantle velocity between 7.9 and 8.3 km s-1, velocities between 8.3 and 8.5 km s-1 at a depth of 200 km, and velocity gradients that are constant or slightly positive. For southern Africa, successful models yield uppermost mantle velocities between 8.1 and 8.3 km s-1, velocities between 7.9 and 8.4 km s-1 at a depth of 130 km, and velocity gradients between -0.001 and 0.001 s-1. Because velocity gradients are controlled strongly by structure at the bottoming depths for Pn waves, it is not easy to compare the velocity gradients obtained for central and southern Africa. For central Africa, Pn waves turn at depths of about 150-200 km, whereas for southern Africa they bottom at ˜100-150 km depth. With regard to the origin of the African superswell, our results do not have sufficient resolution to test hypotheses that invoke simple lithospheric reheating. However, our models are not

  20. Investigation of upper mantle seismic discontinuities beneath the Indian Ocean using array seismology methods

    NASA Astrophysics Data System (ADS)

    van Driel, J.; Reiss, A. S.; Thomas, C.

    2016-12-01

    The topography of upper mantle seismic discontinuities can be used to constrain regional variations in composition and temperature of the Earths mantle. The 410 km discontinuity is caused by the solid-solid phase transition from olivine to wadsleyite. Due to its positive Clapeyron slope, the discontinuity is depressed in hot regimes. The phase transition from ringwoodite to bridgemanite and magnesiowüstite in contrast has a negative Clapeyron slope and therefore is elevated when hot material is present. Cold material is expected to yield an opposing topographic signature, culminating in an elevated 410 km and a depressed 660 km discontinuity. As part of the RHUM-RUM project (Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel) we extract relevant geophysical parameters, by investigating the properties of upper mantle seismic discontinuities beneath the Indian Ocean. The topography of the 410 and 660 km discontinuities, which define the upper and lower bounds of the mantle transition zone, have been mapped using PP and SS underside reflections. This study has utilised over 8500 events with Mw ≥ 5.8, distributed over the entire Indian Ocean. Our robust data set yields a dense coverage of points, which are defined by consistently crossing ray paths. Array seismology methods, such as vespagrams and slowness-backazimuth analysis, are used to enhance the signal-to-noise-ratio and detect and identify weak precursor signals. The differential travel times are corrected for crustal features and converted into depth values of the discontinuities by comparing the measured travel times with theoretical ones derived from ray tracing through the 1D reference Earth model ak135. A `travel-time' stacking method has also been applied for 4° radius bins around each of the bounce points. The addition of a secondary method derives greater stability of our results and allows an enhanced error analysis procedure. In order to better constrain the mineralogical processes taking

  1. Three dimensional crust and upper mantle velocity structure of Antarctica from seismic noise correlation (Invited)

    NASA Astrophysics Data System (ADS)

    Sun, X.; Wiens, D. A.; Nyblade, A.; Anandakrishnan, S.; Aster, R. C.; Chaput, J. A.; Huerta, A. D.; Wilson, T. J.

    2013-12-01

    The successful deployment and year-around operation of the AGAP/GAMSEIS and POLENET/ANET arrays in Antarctica, which include more than 50 broadband seismic stations, provides an unprecedented opportunity to study the detailed structure beneath the continent. Using about four years of continuous data from these arrays (from late 2007 through end of 2011), together with data from the previous TAMSEIS array and permanent stations around Antarctica, we acquire empirical Green's functions between all possible pairs of seismographs by cross-correlating seismic ambient noise. We then extract Rayleigh wave group and phase velocities from 8 to 60 s, and velocity maps for each period are determined by tomographic inversion. Finally, shear velocities in the crust and upper mantle, together with Moho depths are determined from the Rayleigh wave dispersion curves at each location. Our results show the crust and upper mantle structure with higher resolution than obtained in previous studies. The general features are: 1) At shallow depths (several to tens of km), fast velocities are seen beneath the Gamburtsev Mountains (GSM), Transantarctic Mountains (TAM), Marie Byrd Land and Ellsworth Mountains, while slow velocities are seen underneath the West Antarctic rift system (WARS) and Ross Embayment. We interpret this result as indicating thick sedimentary deposits in the WARS and Ross Sea. 2) We also find slow velocities in East Antarctica and fast velocities in West Antarctica at about 20-40 km, which is consistent with the thick/thin crust thickness in these two regions. The transition between the fast and slow velocity is along the Transantarctic Mountains front. 3) Beneath the Gamburtsev Mountains in East Antarctica, low crustal velocities extend to about 55 km, suggesting the mountains are supported by thickened crust. 4) There are pronounced slow upper mantle anomalies within the WARS, indicating a mantle thermal anomaly resulting from Cenozoic extension. 5) Clear fast

  2. Deeper Subduction Zone Melting Explains Enrichment of Upper Mantle and Resolves Dehydration Paradox

    NASA Astrophysics Data System (ADS)

    Dixon, Jacqueline; Bindeman, Ilya; Kingsley, Richard

    2017-04-01

    We present new volatile and stable isotope data on oceanic basaltic glasses with a range of enriched compositions. Basalt compositions studied here can be modeled by mixing between depleted mantle and various enriched (EM) and prevalent (PREMA) mantle components. We develop a multi-stage metasomatic and melting model for the origin of the enriched components, extending the subduction factory concept to involve melting of different components at different depths, down to the mantle transition zone (660 km), with slab temperature a key variable. EM components are heterogeneous, ranging from wet and heavy (Arctic Ridges) to dry and light (East Pacific Rise), and are derived from the subducted slab at depths of 150 to 250 km by addition of <1 % carbonated sediment-derived supercritical C-O-H fluids to depleted peridotite. PREMA mantle sources have a limited compositional range, and form at depths at and within the transition zone (410 to 660 km) by addition of <1 % carbonated eclogite ± sediment-derived supercritical fluids to depleted mantle. The model resolves several problems, including the "dehydration paradox," refering to the following conundrum. The enriched "prevalent mantle" (PREMA) end-member in mid-oceanic ridge and ocean island basalts requires involvement of a mostly dehydrated slab component to explain trace element ratios and radiogenic isotopic compositions, but a fully hydrated slab component to explain stable isotope compositions. In our model, thermal parameters of slabs control the timing and composition of subduction-derived components. This includes deep release of fluids from subcrustal hydrous phases that may rehydrate previously dehydrated slab, resolving the paradox.

  3. Average Potential Temperature of the Upper Mantle and Excess Temperatures Beneath Regions of Active Upwelling

    NASA Astrophysics Data System (ADS)

    Putirka, K. D.

    2006-05-01

    The question as to whether any particular oceanic island is the result of a thermal mantle plume, is a question of whether volcanism is the result of passive upwelling, as at mid-ocean ridges, or active upwelling, driven by thermally buoyant material. When upwelling is passive, mantle temperatures reflect average or ambient upper mantle values. In contrast, sites of thermally driven active upwellings will have elevated (or excess) mantle temperatures, driven by some source of excess heat. Skeptics of the plume hypothesis suggest that the maximum temperatures at ocean islands are similar to maximum temperatures at mid-ocean ridges (Anderson, 2000; Green et al., 2001). Olivine-liquid thermometry, when applied to Hawaii, Iceland, and global MORB, belie this hypothesis. Olivine-liquid equilibria provide the most accurate means of estimating mantle temperatures, which are highly sensitive to the forsterite (Fo) contents of olivines, and the FeO content of coexisting liquids. Their application shows that mantle temperatures in the MORB source region are less than temperatures at both Hawaii and Iceland. The Siqueiros Transform may provide the most precise estimate of TpMORB because high MgO glass compositions there have been affected only by olivine fractionation, so primitive FeOliq is known; olivine thermometry yields TpSiqueiros = 1430 ±59°C. A global database of 22,000 MORB show that most MORB have slightly higher FeOliq than at Siqueiros, which translates to higher calculated mantle potential temperatures. If the values for Fomax (= 91.5) and KD (Fe-Mg)ol-liq (= 0.29) at Siqueiros apply globally, then upper mantle Tp is closer to 1485 ± 59°C. Averaging this global estimate with that recovered at Siqueiros yields TpMORB = 1458 ± 78°C, which is used to calculate plume excess temperatures, Te. The estimate for TpMORB defines the convective mantle geotherm, and is consistent with estimates from sea floor bathymetry and heat flow (Stein and Stein, 1992), and

  4. Probability density functions for radial anisotropy: implications for the upper 1200 km of the mantle

    NASA Astrophysics Data System (ADS)

    Beghein, Caroline; Trampert, Jeannot

    2004-01-01

    The presence of radial anisotropy in the upper mantle, transition zone and top of the lower mantle is investigated by applying a model space search technique to Rayleigh and Love wave phase velocity models. Probability density functions are obtained independently for S-wave anisotropy, P-wave anisotropy, intermediate parameter η, Vp, Vs and density anomalies. The likelihoods for P-wave and S-wave anisotropy beneath continents cannot be explained by a dry olivine-rich upper mantle at depths larger than 220 km. Indeed, while shear-wave anisotropy tends to disappear below 220 km depth in continental areas, P-wave anisotropy is still present but its sign changes compared to the uppermost mantle. This could be due to an increase with depth of the amount of pyroxene relative to olivine in these regions, although the presence of water, partial melt or a change in the deformation mechanism cannot be ruled out as yet. A similar observation is made for old oceans, but not for young ones where VSH> VSV appears likely down to 670 km depth and VPH> VPV down to 400 km depth. The change of sign in P-wave anisotropy seems to be qualitatively correlated with the presence of the Lehmann discontinuity, generally observed beneath continents and some oceans but not beneath ridges. Parameter η shows a similar age-related depth pattern as shear-wave anisotropy in the uppermost mantle and it undergoes the same change of sign as P-wave anisotropy at 220 km depth. The ratio between dln Vs and dln Vp suggests that a chemical component is needed to explain the anomalies in most places at depths greater than 220 km. More tests are needed to infer the robustness of the results for density, but they do not affect the results for anisotropy.

  5. High-pressure orthorhombic ferromagnesite as a potential deep-mantle carbon carrier

    SciTech Connect

    Liu, Jin; Lin, Jung -Fu; Prakapenka, Vitali B.

    2015-01-06

    In this study, knowledge of the physical and chemical properties of candidate deep-carbon carriers such as ferromagnesite [(Mg,Fe)CO3] at high pressure and temperature of the deep mantle is necessary for our understanding of deep-carbon storage as well as the global carbon cycle of the planet. Previous studies have reported very different scenarios for the (Mg,Fe)CO3 system at deep-mantle conditions including the chemical dissociation to (Mg,Fe)O+CO2, the occurrence of the tetrahedrally-coordinated carbonates based on CO4 structural units, and various high-pressure phase transitions. Here we have studied the phase stability and compressional behavior of (Mg,Fe)CO3 carbonates up to relevant lower-mantle conditions of approximately 120 GPa and 2400 K. Our experimental results show that the rhombohedral siderite (Phase I) transforms to an orthorhombic phase (Phase II with Pmm2 space group) at approximately 50 GPa and 1400 K. The structural transition is likely driven by the spin transition of iron accompanied by a volume collapse in the Fe-rich (Mg,Fe)CO3 phases; the spin transition stabilizes the high-pressure phase II at much lower pressure conditions than its Mg-rich counterpart. It is conceivable that the low-spin ferromagnesite phase II becomes a major deep-carbon carrier at the deeper parts of the lower mantle below 1900 km in depth.

  6. Constraints on upper mantle Vp/Vs ratio variations beneath eastern North China from receiver function tomography

    NASA Astrophysics Data System (ADS)

    Si, Shaokun; Tian, Xiaobo; Gao, Rui

    2017-05-01

    To detect the thinning, modification, and replacement of the basement of the lithosphere is a key step in understanding the destruction mechanism of the North China lithosphere. The difference of the basement of the lithosphere is mainly displayed by the variation of the peridotite composition and its physical state. Vp/Vs ratio (hereafter referred to as velocity ratio) is more sensitive to this change than Vp or Vs alone. By means of the strong dependence of the travel-time of the wave converted at the 410-km discontinuity (P410s) observed in the receiver function (RF) on the velocity ratio in the upper mantle, we developed a new mapping method to constrain the velocity ratio between the Moho and 410-km discontinuity. Using the RFs extracted from 246 broadband stations beneath the North China Craton (NCC), we obtained a high-resolution velocity ratio image of the upper mantle. The abnormal velocity ratio indicates a strong lateral variation of the mineral composition in the upper mantle beneath North China. Two low-velocity-ratio patches are imaged at the top of the upper mantle and the 410 km depth, respectively. The former may be related to the orthopyroxene enrichment in the lithospheric mantle, and the latter may reflect the stagnant Pacific slab in the mantle transition zone (MTZ). A prominent high-velocity-ratio anomaly is also imaged in the upper mantle beneath the Shaanxi-Shanxi rift system in the central NCC, with the highest anomaly reaching 10%. We speculate that the high velocity ratio of upper mantle is related to convective flow due to slab dehydration in the MTZ. The dehydration of the retained slab in the MTZ results in partial melting and upwelling of upper mantle materials. Such convective flow and their melting are closely related to the Cenozoic basalt eruption in the northern section of the Shaanxi-Shanxi rift system.

  7. Investigating Potential Causes for An Abrupt Change of Thermal State in Earth's Upper Mantle During the Great Oxygenation Event

    NASA Astrophysics Data System (ADS)

    Li, M.; McNamara, A. K.

    2014-12-01

    The oxygenic photosynthesis might have well evolved about 3 billion years ago, but there seems no great increase of atmospheric oxygen until the great oxygenation event (GOE) at about 2.4 Ga. One possibility for the suppressing of atmospheric oxygen level before the GOE is through consumption of oxygen by reduced volcanic gasses. The amount of atmospheric oxygen that could be consumed by volcanic gases depends on the absolute amount of volcanic gases as well as the redox state of the upper mantle. Evidence from the redox sensitive V/Sc ratio have shown that the redox state of the upper mantle have remained constant for the last 3.5 billion years (e.g., Li and Lee, 2004). If so, abrupt changes in thermal state of Earth's upper mantle could explain the rapid changes of degassing rate at the time of GOE. The Earth's lowermost mantle has been shown to be compositionally heterogeneous, which could be caused by the presence of dense, primordial material resulting from early differentiation processes. An important question is how do chemical heterogeneities in the lowermost mantle influence the secular cooling of the upper mantle. Here, we performed numerical calculations to explore the effects of themochemical convection on the thermal evolution of Earth's upper mantle. A large parameter space is explored, with varying Rayleigh number, viscosity, internal heating and density of chemical heterogeneities. We start with an initially hot mantle with a layer of dense material in the lowermost mantle. We found that when the mantle is hot, the dense material remains layered and covers the entire CMB, leading to low CMB heat flux. In this stage, the upper mantle cools down rapidly. However, as the mantle cools, the dense material is swept into discrete thermochemical piles by cold downwellings, leading to increasing CMB heat flux. The cooling rate of the mantle is temporarily reduced as this transition occurs. This occurs at a time consistent with the GOE event. Li, Z. X. A. and

  8. Upper mantle velocity-temperature conversion and composition determined from seismic refraction and heat flow

    NASA Astrophysics Data System (ADS)

    Perry, H. K. C.; Jaupart, C.; Mareschal, J.-C.; Shapiro, N. M.

    2006-07-01

    We compile upper mantle Pn velocities from seismic refraction/wide-angle reflection surveys in the southern Superior Province of the Canadian Shield and compare them with temperatures at the Moho deduced from heat flow data. Calculated Moho temperatures and Pn velocities correlate well, showing that in this area, Pn depends primarily on temperature. The obtained values of ∂V(Pn)/∂T depend weakly on the assumed value of Moho heat flow and are on the order of -6.0 × 10-4 ± 10% km s-1 K-1, within the range of temperature derivatives obtained in laboratory studies of ultramafic rocks. Comparison between observed Pn velocities and predicted values for several mineralogical models at Moho temperatures allows constraints on both the Moho heat flow and the shallow mantle composition. For all Moho heat flows, undepleted (clinopyroxene-rich) mantle compositions do not allow a good fit to the data. For depleted mantle compositions, temperatures consistent with the observed Pn velocities correspond to values of Moho heat flow larger than 12 mW m-2. For our preferred Moho heat flow of 15 mW m-2, the best fit mantle composition is slightly less depleted than models for average Archean subcontinental lithospheric mantle. This may be due to rejuvenation by melt-related metasomatism during the Keweenawan rifting event. The similarity in Pn - T conversion factors estimated from this empirical large-scale geophysical study and those from laboratory data provides confidence in the absolute temperature values deduced from heat flow measurements and seismic studies.

  9. Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge.

    PubMed

    Hanan, Barry B; Blichert-Toft, Janne; Pyle, Douglas G; Christie, David M

    2004-11-04

    The origin of the isotopic signature of Indian mid-ocean ridge basalts has remained enigmatic, because the geochemical composition of these basalts is consistent either with pollution from recycled, ancient altered oceanic crust and sediments, or with ancient continental crust or lithosphere. The radiogenic isotopic signature may therefore be the result of contamination of the upper mantle by plumes containing recycled altered ancient oceanic crust and sediments, detachment and dispersal of continental material into the shallow mantle during rifting and breakup of Gondwana, or contamination of the upper mantle by ancient subduction processes. The identification of a process operating on a scale large enough to affect major portions of the Indian mid-ocean ridge basalt source region has been a long-standing problem. Here we present hafnium and lead isotope data from across the Indian-Pacific mantle boundary at the Australian-Antarctic discordance region of the Southeast Indian Ridge, which demonstrate that the Pacific and Indian upper mantle basalt source domains were each affected by different mechanisms. We infer that the Indian upper-mantle isotope signature in this region is affected mainly by lower continental crust entrained during Gondwana rifting, whereas the isotope signature of the Pacific upper mantle is influenced predominantly by ocean floor subduction-related processes.

  10. Seismic anisotropy beneath La Réunion hotspot track: plume spreading vs deep mantle convection

    NASA Astrophysics Data System (ADS)

    Barruol, G.; Fontaine, F. R.

    2012-12-01

    Seismic anisotropy beneath the Western Indian Ocean is analyzed from temporary and permanent seismological deployments on the Piton de la Fournaise volcano, the active place of La Réunion hotspot, and from the permanent stations in Mauritius, Rodrigues and the Maldives Islands, in order to decipher the sublithospheric spreading signature of La Réunion mantle plume and the large-scale mantle flow pattern induced by the buoyancy-driven upwelling of the African superplume. The comparison of the SKS splitting observations with geodynamic mantle flow models show that the large-scale anisotropy pattern - characterized by fast directions trending NE-SW in the north (Maldives and Seychelles) to EW in the south (Mauritius, Rodrigues and La Réunion) may be largely explained by asthenospheric flow resulting from the combined effects of plate motion and deep mantle circulation. Anisotropy observed at the seismic stations installed on La Réunion Island shows, however, complex backazimuthal variations characterized by numerous "nulls" and by fast split directions trending normal to the plate motion observed within only a small backazimuthal window, that cannot be explained by neither a single nor two anisotropic layers. By testing models of sublithospheric spreading of La Réunion mantle upwelling, we show that this complex anisotropy pattern can be explained by a parabolic asthenospheric plume spreading with a plume conduit located 100 to 200 km north of La Réunion Island. Anisotropy beneath the GEOSCOPE station recently installed in Rodrigues Island does not appear to be influenced by the La Réunion plume-spreading signature but is fully compatible with either a model of large-scale deep mantle convection pattern and/or with a channeled asthenospheric flow beneath the Rodrigues ridge.

  11. Helium in deep circulating groundwater in the Great Hungarian Plain: Flow dynamics and crustal and mantle helium fluxes

    NASA Astrophysics Data System (ADS)

    Stute, M.; Sonntag, C.; Deák, J.; Schlosser, P.

    1992-05-01

    Observed helium concentrations in deep circulating groundwater of the sedimentary basin of the Great Hungarian Plain (GHP), Hungary, cover a range of three orders of magnitude (≈4 ·10 -8 to 4 · 10 -5 ccSTP g-1). 3He /4He ratios and noble gas concentrations are used to separate helium components originating from the atmosphere, tritium decay, crustal production, and mantle degassing. The characteristic distribution of measured helium concentrations and isotope ratios can be reproduced qualitatively by a simple two-dimensional advection/diffusion model. Other simple models isolating parts of the regional flow domain (recharge, discharge, and horizontal flow) are discussed and applied to derive quantitative information on helium fluxes due to degassing of the Earth's crust /mantle and on the dynamics of groundwater flow. The estimated helium flux of 0.7-4.5 · 10 9 atoms 4He m -2 s -1 is lower than values derived from other deep groundwater circulation systems, probably because the relatively young upper few thousand meters of the sedimentary basin (Tertiary to Quaternary age) shield the flux from the deeper crust. The high mantle helium flux of up to 4.2 · 10 8 atoms 4He m -2 s -1 is probably related to the Miocene volcanism or to continuing intrusion accompanying extension. By fitting calculated helium depth profiles to measured data in the discharge area, vertical flow velocities of the order of 1.5 mm y -1 are estimated. Assuming that a flux of 0.7-4.5 · 10 9 atoms 4He m -2 s -1 is representative for the entire basin, the turnover time of the regional groundwater flow system is estimated to be about 10 6 y.

  12. Deep dehydration and physical and chemical nature of the mantle above the stagnant slab (Invited)

    NASA Astrophysics Data System (ADS)

    Ohtani, E.; Zhao, D.; Kuritani, T.; Tajima, F. C.

    2010-12-01

    Recent seismic tomography studies imply that the slab is stagnant in some regions such as beneath Japan and NE China [1]. Dehydration is expected from the slabs due to decomposition of hydrous and nominally anhydrous minerals in the slabs. There are two phase boundaries between the phases with a large contrast of the water contents; i.e., the olivine-wadsleyite boundary and the decomposition boundary of ringwoodite. Dehydration could occur at the boundaries in plumes or slabs crossing the boundaries. The low velocity beneath Eastern China and United State (e.g., [2]) suggests existence of gravitationally stable hydrous melts at the base of the upper mantle. Body waveforms analysis suggested existence of highly localized low velocity anomalies at the base of the transition zone [3], which are consistent with decomposition of hydrous ringwoodite in slabs. Measurement of hydrogen diffusion in wadsleyite and ringwoodite revealed that the diffusion rates of hydrogen are comparable with that of olivine suggesting heterogeneity in hydrogen contents in the transition zone [4]. Based on hydrogen diffusion coefficients together with reported electrical conductivity of mantle minerals [5] the water content in the mantle transition zone and upper mantle can be estimated combining the electrical conductivity observations and seismic tomography data. These analyses indicate that transition zone is generally more hydrous beneath Japan compared to beneath Europe [6], and the water is localized within the wet transition zone [7]. The stagnant slabs have an important effect on the overlying transition zone and upper mantle. A big mantle wedge (BMW) model has been proposed by Zhao [1], in which the stagnant slab in the transition zone could play an essential role in the intra-plate volcanic activities overlying the slab. Water released by the stagnant slab could be important for these igneous activities, such as Changbaishan in Northeast China. The recent isotopic data of basaltic

  13. Developing a Crustal and Upper Mantle Velocity Model for the Brazilian Northeast

    NASA Astrophysics Data System (ADS)

    Julia, J.; Nascimento, R.

    2013-05-01

    Development of 3D models for the earth's crust and upper mantle is important for accurately predicting travel times for regional phases and to improve seismic event location. The Brazilian Northeast is a tectonically active area within stable South America and displays one of the highest levels of seismicity in Brazil, with earthquake swarms containing events up to mb 5.2. Since 2011, seismic activity is routinely monitored through the Rede Sismográfica do Nordeste (RSisNE), a permanent network supported by the national oil company PETROBRAS and consisting of 15 broadband stations with an average spacing of ~200 km. Accurate event locations are required to correctly characterize and identify seismogenic areas in the region and assess seismic hazard. Yet, no 3D model of crustal thickness and crustal and upper mantle velocity variation exists. The first step in developing such models is to refine crustal thickness and depths to major seismic velocity boundaries in the crust and improve on seismic velocity estimates for the upper mantle and crustal layers. We present recent results in crustal and uppermost mantle structure in NE Brazil that will contribute to the development of a 3D model of velocity variation. Our approach has consisted of: (i) computing receiver functions to obtain point estimates of crustal thickness and Vp/Vs ratio and (ii) jointly inverting receiver functions and surface-wave dispersion velocities from an independent tomography study to obtain S-velocity profiles at each station. This approach has been used at all the broadband stations of the monitoring network plus 15 temporary, short-period stations that reduced the inter-station spacing to ~100 km. We expect our contributions will provide the basis to produce full 3D velocity models for the Brazilian Northeast and help determine accurate locations for seismic events in the region.

  14. Upper mantle structure of the Pacific and Philippine Sea plates revealed by seafloor seismic array observations

    NASA Astrophysics Data System (ADS)

    Isse, Takehi; Shiobara, Hajime; Suetsugu, Daisuke; Sugioka, Hiroko; Ito, Aki

    2016-04-01

    Seismic tomography studies have revealed the structure and dynamics of Earth's interior since the 1980s. However, the spatial resolution of the oceanic region is not good enough caused by sparse distribution of the seismic stations. The observations with broadband ocean-bottom seismographs (BBOBSs) since the 2000s enabled us to obtain seismic tomography models with higher spatial resolution. Our Japanese BBOBS group deployed more than 100 BBOBSs in the Pacific Ocean and obtained a high-resolution (300-500 km) three-dimensional shear wave velocity structure in the upper mantle beneath northwestern and south Pacific Ocean by using surface wave tomography technique. In the northwestern Pacific Ocean, where the Pacific plate subducts beneath the Philippine Sea plate, we found that the shear wave structure in the Philippine sea plate is well correlated with the seafloor age in the upper 120 km, three separate slow anomalies in the mantle wedge at depth shallower than 100 km beneath the Izu-Bonin-Mariana arc, which have a close relationship with the three groups of frontal and rear arc volcanoes having distinct Sr, Nd, and Pb isotope ratios, and that the Philippine Sea plate, which is a single plate, shows very large lateral variations in azimuthal and radial anisotropies compared with the Pacific plate. In the South Pacific Ocean, where midplate hotspots are concentrated, we found that the localized slow anomalies are found near hotspots in the upper mantle, estimated thickness of the lithosphere is about 90 km in average and is thinned by ~20 km in the vicinity of hotspots, which may represent thermal erosion due to mantle plumes.

  15. Inversion of Multiple Traveltime Datasets for Crust and Upper Mantle Structure in Southeast Australia

    NASA Astrophysics Data System (ADS)

    Rawlinson, N.

    2015-12-01

    The inversion of seismic traveltimes for 2-D or 3-D velocity structure still represents the most common form of seismic tomography in use today. Studies generally focus on a particular class of data, such as teleseismic arrival time residuals, local earthquake arrival times, refraction and wide-angle reflection traveltimes or the traveltimes of regional or global phases. Group or phase traveltimes extracted from dispersion analysis of surface waves can also be inverted for velocity structure. A much smaller number of studies attempt to combine multiple data types in a single inversion; this can be of benefit when the datasets provide overlapping coverage. For instance, local earthquake and teleseismic datasets are often complementary, because while they can both sample the crust and upper mantle, there are many situations in which the teleseisms offer poor constraint on crustal structure, and local earthquakes do not constrain upper mantle structure particularly well. A joint inversion can therefore provide good recovery throughout the crust and mantle lithosphere. In this study, an updated version of the FMTOMO package will be used to jointly invert refraction, wide-angle reflection, teleseismic and local earthquake traveltime datasets for crustal, Moho and upper mantle structure beneath southeast Australia. The main target region is Tasmania, which lies at the southeastern tip of the Australian continent and represents the southern-most expression of the Tasmanides, a large Paleozoic fold belt that abuts the Pre-Cambrian shield region of central and western Australia. Broadside refraction and wide-angle reflection coverage is provided by off-shore airguns recorded by on-shore stations, and several passive seismic arrays supply teleseismic and local earthquake data. The combined dataset is able to resolve the trade-off between velocity and interface structure in the neighbourhood of the Moho, which results in a detailed picture of the Tasmanian lithosphere.

  16. Dynamics and Upper Mantle Structure Beneath the Northwestern Andes: Subduction Segments, Moho Depth, and Possible Relationships to Mantle Flow

    NASA Astrophysics Data System (ADS)

    Monsalve, G.; Yarce, J.; Becker, T. W.; Porritt, R. W.; Cardona, A.; Poveda, E.; Posada, G. A.

    2014-12-01

    The northwestern South American plate shows a complex tectonic setting whose causes and relationship to mantle structure are still debated. We combine different techniques to elucidate some of the links between slabs and surface deformation in Colombia. Crustal structure beneath the Northern Andes was inferred from receiver functions where we find thicknesses of nearly 60 km beneath the plateau of the Eastern Cordillera and underneath the southern volcanic area of the Central Cordillera. We infer that such crustal thickening resulted from shortening, magmatic addition, and accretion-subduction. Analyses of relative teleseismic travel time delays and estimates of residual surface topography based on our new crustal model suggest that there are at least two subduction segments underneath the area. The Caribbean slab lies at a low angle beneath northernmost Colombia and steepens beneath the Eastern Cordillera. Such steepening is indicated by negative travel time relative residuals in the area of the Bucaramanga Nest, implying a cold anomaly in the upper mantle, and by positive residual topography just off the east of this area, perhaps generated by slab-associated return flow. Results for the western Andes and the Pacific coastal plains are consistent with "normal" subduction of the Nazca plate: travel time relative residuals there are predominantly positive, and the residual topography shows an W-E gradient, going from positive at the Pacific coastline to negative at the Magdalena Valley, which separates the eastern cordillera from the rest of the Colombian Andean system. Azimuthal analysis of relative travel time residuals further suggests the presence of seismically slow materials beneath the central part of the Eastern Cordillera. Azimuthal anisotropy from SKS splitting in that region indicates that seismically fast orientations do not follow plate convergence, different from what we find for the western Colombian Andes and the Caribbean and Pacific coastal plains

  17. Episodic entrainment of deep primordial mantle material into ocean island basalts.

    PubMed

    Williams, Curtis D; Li, Mingming; McNamara, Allen K; Garnero, Edward J; van Soest, Matthijs C

    2015-11-24

    Chemical differences between mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) provide critical evidence that the Earth's mantle is compositionally heterogeneous. MORBs generally exhibit a relatively low and narrow range of (3)He/(4)He ratios on a global scale, whereas OIBs display larger variability in both time and space. The primordial origin of (3)He in OIBs has motivated hypotheses that high (3)He/(4)He ratios are the product of mantle plumes sampling chemically distinct material, but do not account for lower MORB-like (3)He/(4)He ratios in OIBs, nor their observed spatial and temporal variability. Here we perform thermochemical convection calculations which show the variable (3)He/(4)He signature of OIBs can be reproduced by deep isolated mantle reservoirs of primordial material that are viscously entrained by thermal plumes. Entrainment is highly time-dependent, producing a wide range of (3)He/(4)He ratios similar to that observed in OIBs worldwide and indicate MORB-like (3)He/(4)He ratios in OIBs cannot be used to preclude deep mantle-sourced hotspots.

  18. Episodic entrainment of deep primordial mantle material into ocean island basalts

    PubMed Central

    Williams, Curtis D.; Li, Mingming; McNamara, Allen K.; Garnero, Edward J.; van Soest, Matthijs C.

    2015-01-01

    Chemical differences between mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) provide critical evidence that the Earth's mantle is compositionally heterogeneous. MORBs generally exhibit a relatively low and narrow range of 3He/4He ratios on a global scale, whereas OIBs display larger variability in both time and space. The primordial origin of 3He in OIBs has motivated hypotheses that high 3He/4He ratios are the product of mantle plumes sampling chemically distinct material, but do not account for lower MORB-like 3He/4He ratios in OIBs, nor their observed spatial and temporal variability. Here we perform thermochemical convection calculations which show the variable 3He/4He signature of OIBs can be reproduced by deep isolated mantle reservoirs of primordial material that are viscously entrained by thermal plumes. Entrainment is highly time-dependent, producing a wide range of 3He/4He ratios similar to that observed in OIBs worldwide and indicate MORB-like 3He/4He ratios in OIBs cannot be used to preclude deep mantle-sourced hotspots. PMID:26596781

  19. Variation of the upper mantle velocity structure along the central-south Andes

    NASA Astrophysics Data System (ADS)

    Liang, Xiaofeng; Sandvol, Eric; Shen, Yang; Gao, Haiying

    2014-05-01

    Variations in the subduction angle of the Nazca plate beneath the South American plate has lead to different modes of deformation and volcanism along the Andean active margin. The volcanic gap between the central and southern Andean volcanic zones is correlated with the Pampean flat-slab subduction zone, where the subducting Nazca slab changes from a 30-degree dipping slab beneath the Puna plateau to a horizontal slab beneath the Sierras Pampeanas, and then to a 30-degree dipping slab beneath the south Andes from north to south. The Pampean flat-slab subduction correlates spatially with the track of the Juan Fernandez Ridge, and is associated with the inboard migration of crustal deformation. A major Pliocene delamination event beneath the southern Puna plateau has previously been inferred from geochemical and geological and preliminary geophysical data. The mechanisms for the transition between dipping- and flat-subduction slab and the mountain building process of the central Andean plateau are key issues to understanding the Andean-type orogenic process. We use a new frequency-time normalization approach with non-linear stacking to extract very-broadband (up to 300 second) empirical Green's functions (EGFs) from continuous seismic records. The long-period EGFs provide the deeper depth-sensitivity needed to constrain the mantle structure. The broadband waveform data are from 393 portable stations of four temporary networks: PUNA, SIEMBRA, CHARGE, RAMP, East Sierras Pampeanas, BANJO/SEDA, REFUCA, ANCORP, and 31 permanent stations accessed from both the IRIS DMC and GFZ GEOFON DMC. A finite difference waveform propagation method is used to generate synthetic seismograms from 3-D velocity model. We use 3-D traveltime sensitivity kernels, and traveltime residuals measurement by waveform cross-correlation to directly invert the upper mantle shear-wave velocity structure. The preliminary model shows strong along-strike velocity variations within in the mantle wedge and

  20. The sensitivity of GNSS measurements in Fennoscandia to distinct three-dimensional upper-mantle structures

    NASA Astrophysics Data System (ADS)

    Steffen, Holger; Wu, Patrick

    2015-04-01

    This poster will present the results of Steffen & Wu (2014). The sensitivity of GNSS measurements in Fennoscandia to nearby viscosity variations in the upper mantle is investigated using a three-dimensional finite element model of glacial isostatic adjustment (GIA). Based on the lateral viscosity structure inferred from seismic tomography and the location of the ice margin at the last glacial maximum (LGM), the GIA earth model is subdivided into four layers, where each of them contains an amalgamation of about 20 blocks of different shapes in the central area. The sensitivity kernels of the three velocity components at 10 selected GNSS stations are then computed for all the blocks. We find that GNSS stations within the formerly glaciated area are most sensitive to mantle viscosities below and in its near proximity, i.e., within about 250 km in general. However, this can be as large as 1000 km if the stations lie near the center of uplift. The sensitivity of all stations to regions outside the ice margin during the LGM is generally negligible. In addition, it is shown that prominent structures in the second (250-450 km depth) and third layers (450-550 km depth) of the upper mantle may be readily detected by GNSS measurements, while the viscosity in the first mantle layer below the lithosphere (70-250 km depth) along the Norwegian coast, which is related to lateral lithospheric thickness variation there, can also be detected but with limited sensitivity. For future investigations on the lateral viscosity structure, preference should be on GNSS stations within the LGM ice margin. But these stations can be grouped into clusters to improve the inference of viscosity in a specific area. However, the GNSS measurements used in such inversion should be weighted according to their sensitivity. Such weighting should also be applied when they are used in combination with other GIA data (e.g., relative sea-level and gravity data) for the inference of mantle viscosity. Reference

  1. Aspects of upper mantle structure in the Yellowstone Swell, Wyoming Craton, and Yavapai Province

    NASA Astrophysics Data System (ADS)

    Schutt, Derek Leigh

    2000-12-01

    We use seismological methods to investigate mantle structure in two highly deformed regions of the western U.S., the Yellowstone Swell and the Cheyenne Belt. Using shear wave splitting to measure anisotropy, and thus infer mantle flow, we find a single layer of simply sheared mantle under the Yellowstone Swell. Outside the region affected by the passage of the hotspot, we can measure no anisotropy. This result is unusual, and we interpret it to mean that the upper mantle in the western U.S. is not being strained by plate motion, perhaps indicating that plate motion is being accommodated at a much greater depth than previously thought, perhaps in the transition zone. We also measure seismic velocities in a transect across the Yellowstone Swell. We find significant VS and V P/VS structure. This is interpreted as being mostly to the presence of melt beneath the Snake River Plain, probably between .5%--1.5%. This dissertation includes both previously published and unpublished co-authored materials.

  2. Surface Wave Tomography of the European Upper Mantle and its Relationship to Tectonics

    NASA Astrophysics Data System (ADS)

    Barron, Jamie; Priestley, Keith; Debayle, Eric; McKenzie, Dan

    2010-05-01

    We have constructed a new surface-wave shear velocity model of Europe based on the inversion of fundamental and higher-mode Rayleigh waveforms for over 39,000 paths. One-dimensional path average models are calculated from the seismograms, using the linearized inversion technique of Cara and Lévêque, and the average models are used as the input to a tomographic inversion using the continuous regionalization method of Debayle and Sambridge. The tomographic maps and profiles from this analysis provide images of the European mantle down to the top of the mantle transition zone with a horizontal resolution of about 500 km. Our model shows a number of features which can be related to the tectonics of the region. A clear distinction can be seen between the Baltic Shield/Russian platform, with high shear wave velocities in the upper mantle, and the rest of Europe, which has lower shear wave velocities. This feature follows the Tornquist line. Low V s in eastern Spain is consistent with the high elevation of this region, while low V s in southern Greece and across Turkey suggests a warm mantle which is consistent with basaltic volcanism in the region. Finally, a high V s in the Aegean may be related to the low-angle subduction under the Hellenic arc.

  3. Basin-scale redox heterogeneity in MORBs and the upper mantle

    NASA Astrophysics Data System (ADS)

    Kelley, K. A.; Cottrell, E.

    2016-12-01

    Mid-ocean ridge basalt (MORB) geochemistry reveals that Earth's upper mantle is heterogeneous over many spatial scales, ranging from fine, within-segment variations to broad, hemispheric-scale differences. The well-established isotopic and trace element contrast between MORB mantle beneath the Indian and Pacific ocean basins (Hart, 1984), for example, suggests a fundamental, long-lived variation in the composition of the MORB mantle at the largest scale. Here, we present a new, global compilation of Fe3+/∑Fe ratios measured in MORBs from the three major ocean basins (n=182) using X-ray Absorption Near Edge Structure spectroscopy (XANES), which is calibrated against room-temperature Mössbauer spectroscopy (Cottrell et al., 2009). The data set was screened to exclude MORB that show influence from known hotspots, averaged at the segment scale (n=75 segments), and corrected for fractional crystallization to MgO=8 wt.%. Our results show that Indian and Pacific MORB, and by proxy their mantle sources, are also distinct in iron redox and oxygen fugacity (fO2). Indian MORBs are more reduced, with an average Fe3+/∑Fe (8) ratio of 0.154±0.009 (n=19 segments), whereas Pacific MORBs are more oxidized, averaging 0.161±0.005 (n=34 segments). Statistical analysis indicates that the means of the two populations are significantly different (t-value=2.9, p-value=0.008). Atlantic MORBs are intermediate (ave. Fe3+/∑Fe (8)=0.157±0.008; n=22 segments), although the southern mid-Atlantic ridge, which has isotopic Indian affinity, is also more reduced (ave. Fe3+/∑Fe (8)=0.155±0.006; n=5 segments). This contrast adds new constraints to models of the origins of Earth's MORB mantle domains, and requires long-lived heterogeneity not only in chemical composition but perhaps also in oxygen fugacity. Models invoking a recycled sediment component to explain the Indian mantle composition may fit with our observations if the sediment was rich in organic carbon, which could impart

  4. PRISM3D: a preliminary 3D reference seismic model of the crust and upper mantle beneath Iberia

    NASA Astrophysics Data System (ADS)

    Arroucau, Pierre; Custódio, Susana; Civiero, Chiara; Dias, Nuno A.; Silveira, Graça

    2017-04-01

    Earthquake location and moment tensor inversion are usually performed assuming one-dimensional (1D), radially symmetric models of the Earth's seismic velocity field. In a region such as Iberia, that hypothesis may no longer be valid: its location combining a plate boundary context with a passive margin environment indeed results in the presence of mountain ranges and abyssal plains, and juxtaposes sedimentary basins, thickened, normal and thinned continental crust, oceanic material, and even, locally, exhumed mantle. Furthermore, earthquakes in the Gulf of Cadiz and in the Alboran Sea, respectively west and east of Gibraltar Strait, are known to occur in the upper mantle, which also presents lateral velocity variations. A three-dimensional (3D) seismic velocity model of the crust and upper mantle beneath Iberia therefore appears necessary for accurate travel time and waveform modeling, in order to account for lateral variations of topography, Moho depth and lithology, and improve earthquake location and source inversion. Here, we present a new 3D P- and S-wave velocity model for Iberia. The model spans a region ranging from 17W to 6E in longitude and 28N to 47N in latitude, and from 7km above to 250 km below sea level in depth, with a grid node spacing of 10 km in longitude and latitude, and 0.5 km in depth. It results from the compilation of existing -published and unpublished - local earthquake and teleseismic body wave tomographic models, earthquake and ambient noise surface wave as well as receiver function inversion studies, and active source seismic experiments. Moho depth is estimated by interpolation of results from receiver function and deep seismic sounding using the reversible jump algorithm. Crust and mantle velocity field are then obtained separately by weighted average of the compiled models. This new 3D model, which combines the most recent seismic structure studies in Iberia and its surroundings, forms a reliable basis for earthquake location and

  5. Interaction of Cocos and Rivera plates with the upper-mantle transition zone underneath central Mexico

    NASA Astrophysics Data System (ADS)

    Pérez-Campos, Xyoli; Clayton, Robert W.

    2014-06-01

    Receiver functions (RFs) from 224 permanent and temporary stations in central and southern Mexico were used to characterize the upper-mantle transition zone in that region. Discontinuities at 410 and 660 km depth are both deeper compared to iasp91, which reflects a slow velocity anomaly in the upper mantle. They show topography on the interfaces that is consistent with the interaction of the subducted slab or its broken off extension. A low-velocity layer on top of the 410 is identified mainly on the continental side of where the slab pierces it (i.e. in the lee of the slab roll-back). In general the RFs show a complex behaviour where the mantle has been disturbed by the lateral motion of the subducted slab, and are simple where it has not. Complexity on the 660 coincides with the place where the broken off portion of the Farallon Plate would have penetrated this interface or is possibly lying on top of it.

  6. Contrasting upper-mantle shear wave anisotropy across the transpressive Queen Charlotte margin

    NASA Astrophysics Data System (ADS)

    Cao, Lingmin; Kao, Honn; Wang, Kelin

    2017-10-01

    In order to investigate upper mantle and crustal anisotropy along the transpressive Queen Charlotte margin between the Pacific (PA) and North America (NA) plates, we conducted shear wave splitting analyses using 17 seismic stations in and around the island of Haida Gwaii, Canada. Despite the limited station coverage at present, our reconnaissance study does reveal a systematic pattern of mantle anisotropy in this region. Fast directions derived from teleseismic SKS-phase splitting are mostly margin-parallel (NNW-SSE) near the plate boundary but transition to predominantly E-W-trending farther away. We propose that the former is associated with the absolute motion of PA, and the latter reflects a transition from this direction to that of the absolute motion of NA. The broad width of the zone of transition from the PA to NA direction is probably caused by the very obliquely subducting PA slab that travels primarily in the margin-parallel direction. Anisotropy of Haida Gwaii based on local earthquakes features a fast direction that cannot be explained with regional stresses and is probably associated with local structural fabric within the overriding crust. Our preliminary shear wave splitting measurements and working hypotheses based on them will serve to guide more refined future studies to unravel details of the geometry and kinematics of the subducted PA slab, as well as the viscous coupling between the slab and upper mantle in other transpressive margins.

  7. Upper mantle seismic velocity anomaly beneath southern Taiwan as revealed by teleseismic relative arrival times

    NASA Astrophysics Data System (ADS)

    Chen, Po-Fei; Huang, Bor-Shouh; Chiao, Ling-Yun

    2011-01-01

    Probing the lateral heterogeneity of the upper mantle seismic velocity structure beneath southern and central Taiwan is critical to understanding the local tectonics and orogeny. A linear broadband array that transects southern Taiwan, together with carefully selected teleseismic sources with the right azimuth provides useful constraints. They are capable of differentiating the lateral heterogeneity along the profile with systematic coverage of ray paths. We implement a scheme based on the genetic algorithm to simultaneously determine the relative delayed times of the teleseismic first arrivals of array data. The resulting patterns of the delayed times systematically vary as a function of the incident angle. Ray tracing attributes the observed variations to a high velocity anomaly dipping east in the mantle beneath the southeast of Taiwan. Combining the ray tracing analysis and a pseudo-spectral method to solve the 2-D wave propagations, we determine the extent of the anomaly that best fits the observations via the forward grid search. The east-dipping fast anomaly in the upper mantle beneath the southeast of Taiwan agrees with the results from several previous studies and indicates that the nature of the local ongoing arc-continent collision is likely characterized by the thin-skinned style.

  8. Anisotropy in the Pacific upper mantle from inversion of a surface-wave dispersion dataset

    NASA Astrophysics Data System (ADS)

    Eddy, C. L.; Ekstrom, G.; Nettles, M.; Gaherty, J. B.

    2015-12-01

    We present work towards a three-dimensional model of the anisotropic velocity structure of the Pacific upper mantle. Models of seismic anisotropy in oceanic regions provide important constraints on the geometry of strain in the mantle, the nature of the lithosphere-asthenosphere transition, and the possible presence of partial melt in the asthenosphere. The goal of this work is to produce a three-dimensional model of isotropic and anisotropic velocities in the Pacific, which will improve constraints on olivine fabrics and strain geometries in the oceanic upper mantle. Measurements of fundamental-mode dispersion for Rayleigh and Love waves traversing oceanic paths are drawn from the waveform dataset used to construct the global dispersion model GDM52. We develop anisotropic phase-velocity maps of the Pacific basin for Rayleigh and Love waves between 25 s and 250 s and invert the phase-velocity maps for anisotropic velocity structure at depth. The resulting models are radially anisotropic and include the G parameters that are related to the azimuthal anisotropy of vSV. We compare results of these two-step inversions with direct inversions of fundamental-mode phase anomalies for three-dimensional anisotropic structure. In much of the central and western Pacific, vertical gradients in both vS and anisotropy are consistent with the transition from rigid lithosphere to viscously deforming asthenosphere. In future work we plan to incorporate waveform data providing constraints on higher-mode dispersion in the modeling of the three-dimensional anisotropic structure.

  9. The Influence of Water on Seismic Wave Attenuation in the Upper Mantle

    NASA Astrophysics Data System (ADS)

    David, E. C.; Jackson, I.; Faul, U.; Berry, A.

    2014-12-01

    Trace amounts of water, present as protons structurally bound in olivine crystal defects, are inferred to significantly enhance the low-strain solid-state viscoelastic relaxation responsible for attenuation and dispersion of seismic waves in the upper mantle. This inferrence is supported by recent observation of water weakening at moderate compressive strains in synthetic, water-undersaturated aggregates (Faul et al., in preparation). In these fine-grained olivine polycrystals of Fo90 composition, doped with 0.02wt% TiO2, "water" is incorporated in the remarkably stable Ti-clinohumite defect. Such synthetic olivine specimens reproduce the infrared spectra of natural mantle olivines (Berry et al., 2005), and present the advantage of being melt-free and of low dislocation density. The water contents in such synthetic polycrystalline olivine aggregates, which can be quantitatively measured by Fourier Transform Infrared Spectroscopy (FTIR), range up to 90 ppm, and are thus representative of water-undersaturated conditions in the upper mantle. We will report here the outcome of torsional-oscillation tests,in which attenuation and shear modulus were measured at seismic frequencies (mHz-Hz) and various temperatures up to 1300C on Pt-encapsulated, Ti-doped olivine specimens, enclosed within a mild-steel jacket.

  10. Crustal and upper mantle structure beneath the NE Tibetan Plateau and its tectonic implication

    NASA Astrophysics Data System (ADS)

    Li, H.; Zheng, D.; Shen, Y.; Ouyang, L.; Li, X.; Tan, J.

    2015-12-01

    The crustal and upper mantle velocity structures in the northeastern Tibetan Plateau are obtained from joint analysis of receiver functions and Rayleigh wave dispersion curves derived from teleseismic earthquake arrivals and ambient noise seismic data. The resulting velocity model reveals a close correlation between the thick (>60 km) crust and the presence of an intra-crustal low-velocity zone, which is detected beneath the Qiangtang and Songpan-Ganzi terranes as well as the northwestern Qilian orogen. However, the high Vp/Vs ratio is found only beneath the Qiangtang and Songpan-Ganzi terranes. The crustal low-velocity zone is not observed beneath the west Qinling and southeastern Qilian orogens, which have a relatively thin (~50 km) crust, indicating that crustal channel flow is not the primary mechanism by which the northeastern Tibetan plateau grows. In contrast to the widespread low velocities in the mid-to-lower crust beneath the Qiangtang and Songpan-Ganzi terranes, the upper mantle in these two regions shows alternating high and low velocity anomalies. A continuous low-velocity zone from the mid-to-lower crust down to 140 km beneath the eastern Kunlun fault suggests an induced local mantle upwelling after the delamination of the lithosphere.

  11. Saudi Arabian seismic-refraction profile: A traveltime interpretation of crustal and upper mantle structure

    USGS Publications Warehouse

    Mooney, W.D.; Gettings, M.E.; Blank, H.R.; Healy, J.H.

    1985-01-01

    The crustal and upper mantle compressional-wave velocity structure across the southwestern Arabian Shield has been investigated by a 1000-km-long seismic refraction profile. The profile begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, trends southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan, and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, and six shot points were used, including one in the Red Sea. Two-dimensional ray-tracing techniques, used to analyze amplitude-normalized record sections indicate that the Arabian Shield is composed, to first order, of two layers, each about 20 km thick, with average velocities of about 6.3 km/s and 7.0 km/s, respectively. West of the Shield-Red Sea margin, the crust thins to a total thickness of less than 20 km, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust. A major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition. Elsewhere along the profile, several high-velocity anomalies in the upper crust correlate with mapped gneiss domes, the most prominent of which is the Khamis Mushayt gneiss. Based on their velocities, these domes may constitute areas where lower crustal rocks have been raised some 20 km. Two intracrustal reflectors in the center of the Shield at 13 km depth probably represent the tops of mafic intrusives. The Mohorovic??ic?? discontinuity beneath the Shield varies from a depth of 43 km and mantle velocity of 8.2 km/s in the northeast to a depth of 38 km and mantle velocity of 8.0 km/s depth in the southwest near the Shield-Red Sea transition. Two velocity discontinuities occur in the upper mantle, at 59 and 70 km depth. The crustal and upper mantle velocity structure of the Arabian Shield is

  12. Waveform complexity in teleseismic broadband SH displacements: Slab diffractions or deep mantle reflections

    SciTech Connect

    Lay, T.; Young, C.J. )

    1989-07-01

    Transverse component bodywave ground displacements from deep focus earthquakes complexities which are not accounted for by standard radially symmetric Earth models. This anomalous behavior is examined using 123 broadband teleseismic SH displacement recordings, reconstituted from short-and long-period or broadband seismograms for 11 events in the Kurile subduction zone. The greatest complexity, involving an extra arrival between the S and ScS phases at epicentral distances greater than 72, has a strong distance dependence and little azimuthal variation with respect to the source region, favoring an interpretation as a deep mantle triplication rather than a near-source slab diffraction.

  13. Model of the Arctic evolution since the Cretaceous to present, based on upper mantle convection linked with Pacific lithosphere subduction

    NASA Astrophysics Data System (ADS)

    Lobkovsky, Leopold

    2015-04-01

    The present paper comprises a model of Arctic basin evolution since early-mid Cretaceous to present. The model is based on the mechanism of upper mantle substance circulation beneath the Arctic lithosphere linked with Pacific lithosphere subduction. Seismic tomography data obtained for the Pacific-Eurasia-Arctic joint area indicate that Pacific lithosphere slab sinking to the mantle in subduction zone transforms into the horizontal layer upon reaching the upper mantle foot, this layer extending for two or more thousands km beneath the Eurasian continent. This pattern of seismic tomography indicates the presence of a horizontal convective cell where a flow of substance moving along the upper mantle foot from a subduction zone into the continent is compensated by a return flow moving along the lithosphere foot towards the subduction zone. The return mantle flow makes continental lithosphere extension, giving rise to processes of rifting, magmatism and spreading. The convective cell being continuously supplied with new substance which is transported through the subduction zone it is sure to expand horizontally. The above cell expansion occurs first, due to ocean ward movement of subduction zone (roll back) and secondly, due to the cell front propagation into the continent. The given model allows to understand main features for the Arctic evolution since early-mid Cretaceous to present. Numerous seismic profiling data obtained for shelf and deep water sedimentary basins in the Arctic Ocean as well as on land geological investigation reveal that since Aptian up to present the Arctic region has been characterized by sublatitudinal lithosphere extension. This extension is explained by the effect the return mantle flow related to the subduction of the Northern part of the Pacific plate acts on the Arctic lithosphere foot. The model shows the phenomenon of Arctic plume to be caused by the convective cell uprising flow. In fact lower horizontal flow of convective cell moving

  14. Upper mantle P velocity structure beneath the Baikal Rift from modeling regional seismic data

    NASA Astrophysics Data System (ADS)

    Brazier, Richard A.; Nyblade, Andrew A.

    2003-02-01

    Uppermost mantle P wave velocity structure beneath the Baikal rift and southern margin of the Siberian Platform has been investigated by using a grid search method to model Pnl waveforms from two moderate earthquakes recorded by station TLY at the southwestern end of Lake Baikal. The results yielded a limited number of successful models which indicate the presence of upper mantle P wave velocities beneath the rift axis and the margin of the platform that are 2-5% lower than expected. The magnitude of the velocity anomalies and their location support the presence of a thermal anomaly that extends laterally beyond the rift proper, possibly created by small-scale convection or a plume-like, thermal upwelling.

  15. Upper mantle heterogeneity: Comparisons of regions south of Australia with Philippine Basin

    NASA Technical Reports Server (NTRS)

    1982-01-01

    The nature of mass anomalies that occur beneath the regions of negative residual depth anomalies were identified. Residual geoid anomalies with negative residual depth anomalies are identified in the Philippine Basin (negative) and in the region south of Australia (positive and negative). In the latter region the geoid anomalies are eastward and the depth anomaly is northeast. It is suggested that the negative depth anomaly and the compensating mass excess in the uppermost mantle developed in the Eocene as the lithosphere of the west Philippine basin formed. Heating of the deeper upper mantle which causes slow surface wave velocities and negative gravity and geoid anomalies may be a younger phenomenon which is still in progress.

  16. Investigating Transition Zone Thickness Variation under the Arabian Plate: Evidence Lacking for Deep Mantle Upwellings

    NASA Astrophysics Data System (ADS)

    Juliá, J.; Tang, Z.; Mai, P. M.; Zahran, H.

    2014-12-01

    Cenozoic volcanic outcrops in Arabia - locally known as harrats - span more than 2000 km along the western half of the Arabian plate, from eastern Yemen to southern Syria. The magmatism is bimodal in character, with older volcanics (30 to 20 My) being tholeiitic-to-transitional and paralleling the Red Sea margin, and younger volcanics (12 Ma to Recent) being transitional-to-strongly-alkalic and aligning in a more north-south direction. The bimodal character has been attributed to a two-stage rifting process along the Red Sea, where the old volcanics would have produced from shallow sources related to an initial passive rifting stage, and young volcanics would have originated from one or more deep-seated mantle plumes driving present active rifting. Early models suggested the harrats would have resulted from either lateral flow from the Afar plume in Ethiopia, or more locally from a separate mantle plume directly located under the shield. Most recently, tomographic images of the Arabian mantle have suggested the northern harrats could be resulting from flow originating at a deep plume under Jordan. In this work, we investigate the location of deep mantle plumes under the Arabian plate by mapping transition zone thickness with teleseismic receiver functions. The transition zone is bounded by seismic discontinuities, nominally at 410 and 660 km depth, originating from phase transitions in the olivine-normative component of the mantle. The precise depth of the discontinuities is strongly dependent on temperature and, due to the opposing signs of the corresponding Clapeyron slopes, positive temperature anomalies are expected to result in thinning of the transition zone. Our dataset consists of ~5000 low-frequency (fc < 0.25 Hz) receiver function waveforms obtained at ~110 broadband stations belonging to a number of permanent and temporary seismic networks in the region. The receiver functions were migrated to depth and stacked along a ~2000 km long record section

  17. Upper Mantle Seismic Structure Beneath Southern Africa: Constraints on the Buoyancy Supporting the African Superswell

    NASA Astrophysics Data System (ADS)

    Brandt, Martin B. C.; Grand, Stephen P.; Nyblade, Andrew A.; Dirks, Paul H. G. M.

    2012-04-01

    We present new one-dimensional SH-wave velocity models of the upper mantle beneath the Kalahari craton in southern Africa obtained from waveform inversion of regional seismograms from an Mw = 5.9 earthquake located near Lake Tanganyika recorded on broadband seismic stations deployed during the 1997-1999 Southern African Seismic Experiment. The velocity in the lithosphere beneath the Kalahari craton is similar to that of other shields, and there is little evidence for a significant low velocity zone beneath the lithosphere. The lower part of the lithosphere, from 110 to 220 km depth, is slightly slower than beneath other shields, possibly due to higher temperatures or a decrease in Mg number (Mg#). If the slower velocities are caused by a thermal anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by shallow buoyancy from a hot lithosphere. However, a decrease in the Mg# of the lower lithosphere would increase the density and counteract the buoyancy effect of the higher temperatures. We obtain a thickness of 250 ± 30 km for the mantle transition zone, which is similar to the global average, but the velocity gradient between the 410 and 660 km discontinuities is less steep than in global models, such as PREM and IASP91. We also obtain velocity jumps of between 0.16 ± 0.1 and 0.21 ± 0.1 km/s across the 410 km discontinuity. Our results suggest that there may be a thermal or chemical anomaly in the mantle transition zone, or alternatively that the shear wave velocity structure of the transition zone in global reference models needs to be refined. Overall, our seismic models provide little support for an upper mantle source of buoyancy for the unusually high elevation of the Kalahari craton, and hence the southern African portion of the African Superswell.

  18. Receiver function study on the crustal and upper mantle structure beneath southeastern China

    NASA Astrophysics Data System (ADS)

    Xu, M.; Ma, Y.; Wang, L.; Mi, N.; Li, H.; Yu, D.; Huang, H.

    2011-12-01

    Recently, receiver function analysis has been widely used for obtaining the crustal and mantle structure. Since December 2008 to April 2010, 11 broadband portable seismic stations were deployed in Zhejiang province. A total of 246 high quality receiver functions were calculated from teleseismic records. H-k stacking, common conversion point-phase weighted stacking and cross correlation methods were used for studying the crustal and upper mantle structure beneath southeastern China. The results suggest that the crustal thickness ranges from 31.4 to 35.4 km with an average of 33.6 km. From continent to offshore of the Zhejiang Province, the crustal thickness decreases. The corresponding Vp/Vs ratio ranges from 1.676 to 1.782 with an average of 1.723. There's no obvious change in the Poisson's ratio, which may be due to the fact that the widely distributed Mesozoic granite in this region are mainly Late Yanshanian granite with identical chemical and mineral compositions. Generally, there exist three fast shear wave oscillation directions (FSODs), NW and NEE mainly, and then NNE direction. The delay time ranges from 0 to 0.425 s, with an average of 0.17±0.12 s. The depths of 410 and 660 km discontinuities are the same as that in IASP91 model. The 520 km discontinuity is also observed. The stagnant slab imaged by the seismic tomography in the mantle transition zone little affects the above-mentioned upper mantle discontinuities. It suggests that the temperature fields around the discontinuities are normal.

  19. Refining Estimates of the Seismic Velocities of the Crust and Upper Mantle

    NASA Astrophysics Data System (ADS)

    BARMIN, M.; SHAPIRO, N. M.; Ritzwoller, M. H.; Levin, V.; Park, J.

    2001-12-01

    We discuss recent efforts to improve a global shear-velocity model of the crust and upper mantle by advancing surface wave methodology as well as by introducing new types of geophysical data in the inversion. The primary data-set used to construct the model consists of broad-band Rayleigh and Love wave group-velocity (CU-Boulder) and phase-velocity (Harvard, Utrecht) dispersion curves. The first step of the inversion is surface wave tomography in which group and phase velocity maps are constructed. We present a new method of surface wave tomography called "diffraction tomography" that is based on a physical model of the surface wave Fresnel zone rather than on ray-theory and ad hoc regularization. Diffraction tomography accounts for path-length dependent sensitivity, wave-form healing and associated diffraction effects, and provides a more accurate assessment of spatially variable resolution than traditional tomographic methods. The second step is Monte-Carlo inversion of the dispersion maps for an ensemble of acceptable shear velocity models of the crust and uppermost mantle. Because surface waves have limited vertical resolution, we apply constraints on the model derived from other types of geophysical observations. We consider two types of additional data: teleseismic receiver functions and heat flow measurements. Receivers functions are formed by P-S converted waves that arise from sharp boundaries close to the Earth's surface, and thus provide important constraints on the crustal structure. Their use in the inversion mitigates the tradeoff between the crust (where surface waves have poor sensitivity) and the deeper part of the model. Heat-flow data constrain mantle shear velocities through the conversion of heat-flow into temperature and subsequently into shear velocity at the top of the upper mantle. We present results from the joint inversion and discuss how the combination of different types of data reduces both uncertainties and systematic bias in the

  20. Experimental investigation of flow-induced fabrics in rocks at upper-mantle pressures. Application to understanding mantle dynamics and seismic anisotropy

    SciTech Connect

    Durham, William B.

    2016-05-02

    The goal of this collaborative research effort between W.B. Durham at the Massachusetts Institute of Technology (MIT) and D.L. Kohlstedt and S. Mei at the University of Minnesota (UMN) was to exploit a newly developed technology for high-pressure, high-temperature deformation experimentation, namely, the deformation DIA (D-DIA), to determine the deformation behavior of a number of important upper mantle rock types including olivine, garnet, enstatite, and periclase. Experiments were carried out under both hydrous and anhydrous conditions and at both lithospheric and asthenospheric stress and temperature conditions. The result was a group of flow laws for Earth’s upper mantle that quantitatively describe the viscosity of mantle rocks from shallow depths (the lithosphere) to great depths (the asthenosphere). These flow laws are fundamental for modeling the geodynamic behavior and heat transport from depth to Earth’s surface.-

  1. Experimental investigation of flow-induced fabrics in rocks at upper-mantle pressures: Application to understanding mantle dynamics and seismic anisotropy

    SciTech Connect

    Kohlstedt, David L.

    2016-04-26

    The goal of this collaborative research effort between W.B. Durham at the Massachusetts Institute of Technology (MIT) and D.L. Kohlstedt and S. Mei at the University of Minnesota (UMN) was to exploit a newly developed technology for high-pressure, high-temperature deformation experimentation, namely, the deformation DIA (D-DIA) to determine the deformation behavior of a number of important upper mantle rock types including olivine, garnet, enstatite, and periclase. Experiments were carried out under both hydrous and anhydrous conditions and at both lithospheric and asthenospheric stress and temperature conditions. The result was a group of flow laws for Earth’s upper mantle that quantitatively describe the viscosity of mantle rocks from shallow depths (the lithosphere) to great depths (the asthenosphere). These flow laws are fundamental for modeling the geodynamic behavior and heat transport from depth to Earth’s surface.

  2. Upper mantle-derived free gas exhalations in central Europe - an isotope study

    NASA Astrophysics Data System (ADS)

    Braeuer, K.; Kaempf, H.; Niedermann, S.; Strauch, G.; Weise, S. M.

    2003-04-01

    The results of geochemical mapping (about 100 locations) of free gas exhalations in the western Eger Rift (Czech-German border region) are compared with data of free gas exhalations from the surroundings of the Laacher See (East Eifel, Germany) and Le Mont Dore (Massif Central, France). In the western Eger Rift, three gas exhalation centres characterized by high gas flux (> 85 m^3 h-1), CO_2 concentrations > 99 vol. -%, δ13C between -2 and -4 per mil, and ^3He/^4He ratios up to 6 R_a could be distinguished. CO_2-rich gases consist of mantle-derived components with only small admixtures of dissolved air due to sampling after passing a water phase. The scatter in δ13C values of CO_2-rich gases is due to isotope fractionation between gaseous CO_2, dissolved CO_2 and HCO_3, respectively. As a result of long-term monitoring studies at four locations in the western Eger Rift, the natural variation range of the gas and isotope composition could be evaluated and as a consequence of numerous data sets reliable δ15N mean values could be specified. At the Bublak mofette (NW Bohemia), the transport velocity in the upper crust is estimated at about 400 m/d. The Bublak CO_2 is enriched in 13C (δ13C ≈ -2 per mil) relative to MORB values and its CO_2/^3He is ˜6.5x10^9. In addition to the ^3He/^4He ratio of ≈ 5.9 R_a, which is in the range of values for European sub-continental mantle xeno-liths, also the uncorrected δ15N values (-3.2±0.7, n=16) show an upper mantle signature. Corrected for air via O_2 contents and/or 40Ar/36Ar ratios, the 15N/14N ratios yield δ15N <= -5 per mil, corresponding to mantle-derived nitrogen contributions of about 50 %. The channel-like fluid supply at the Bublak mofette related with the high transport velocity indicates that its isotope ratios are not significantly altered during transport through the crust and represent probably the signature of the magmatic source in the European sub-continental mantle. Comparable distribution patterns of

  3. Partitioning of copper between olivine, orthopyroxene, clinopyroxene, spinel, garnet and silicate melts at upper mantle conditions

    NASA Astrophysics Data System (ADS)

    Liu, Xingcheng; Xiong, Xiaolin; Audétat, Andreas; Li, Yuan; Song, Maoshuang; Li, Li; Sun, Weidong; Ding, Xing

    2014-01-01

    Previously published Cu partition coefficients (DCu) between silicate minerals and melts cover a wide range and have resulted in large uncertainties in model calculations of Cu behavior during mantle melting. In order to obtain true DCumineral/melt values, this study used Pt95Cu05 alloy capsules as the source of Cu to experimentally determine the DCu between olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), spinel (spl), garnet (grt) and hydrous silicate melts at upper mantle conditions. Three synthetic silicate compositions, a Komatiite, a MORB and a Di70An30, were used to produce these minerals and melts. The experiments were conducted in piston cylinder presses at 1.0-3.5 GPa, 1150-1300 °C and oxygen fugacities (fO2) of from ∼2 log units below to ∼5 log units above fayalite-magnetite-quartz (FMQ). The compositions of minerals and quenched melts in the run products were measured with EMP and LA-ICP-MS. Attainment of equilibrium is verified by reproducible DCu values obtained at similar experimental conditions but different durations. The results show that DCu for ol/, opx/, spl/ and possibly cpx/melt increase with increasing fO2 when fO2 > FMQ + 1.2, while DCu for cpx/ and spl/melt also increase with increasing Na2O in cpx and Fe2O3 in spinel, respectively. In the investigated P-T-fO2 conditions, the DCumineral/melt values are 0.04-0.14 for ol, 0.04-0.09 for opx, 0.02-0.23 for cpx, 0.19-0.77 for spl and 0.03-0.05 for grt. These results confirm that Cu is highly incompatible (DCu < ∼0.2) in all the silicate minerals and oxides of the upper mantle with the exception of the high-Fe spinel, in which Cu is moderately incompatible (DCu = 0.4-0.8) and thus Cu will be enriched in the derived melts during mantle partial melting and magmatic differentiation if sulfide is absent. These experimental DCu values are used to assess the controls on Cu behavior during mantle melting. The model results suggest that MORBs and most arc basalts must form by sulfide

  4. Improving Ionospheric Source Models for Imaging Upper Mantle/Transition Zone Resistivity

    NASA Astrophysics Data System (ADS)

    Egbert, G. D.; Alken, P.; Maute, A. I.; Richmond, A. D.

    2016-12-01

    Information about deep Earth (> 200 km) resistivity comes almost exclusively from magneto-variational data, a superposition of magnetic fields generated by complex processes in the ionosphere and magnetosphere, and by induction in the spatially heterogeneous conducting Earth. Frequencies of 0.5-10 cycles per day, required to image the upper mantle from the aesthenosphere to transition zone, mostly have their origin in the ionospheric dynamo region at 100-150 km height, controlled by the spatial and temporal varying thermospheric neutral wind and the ionospheric conductivity distribution. To interpret the relatively subtle induced internal signals, these spatially complex ionospheric signals must be properly accounted for. Here we present progress on a collaboration between specialists in ionospheric physics and in EM induction, to develop methods to constrain better external source spatial structure, so as to reveal solid Earth heterogeneity. There are two novel aspects to our approach. First, we use robust frequency domain Principal Components Analysis (PCA) to reduce a large and heterogeneous set of historical and modern geomagnetic observatory data to the dominant spatial modes present in the daily variation band. The PCA scheme, massively reduces the size of the dataset, and allows data from different eras to be merged into more densely sampled spatial modes. Second, we use a mature physics based numerical model, the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) to constrain source spatial structure. TIE-GCM external magnetic field outputs are analyzed in the frequency domain using PCA. Results are used to define a reduced-rank covariance, which is used with optimal interpolation to define global source structure for each data mode. Initially, we allow for internal fields with a global thin-sheet model of ocean/continent conductivity variations. The estimated sources can then be used to invert the data for Earth conductivity, with

  5. Seismic Imaging of the crust and upper mantle beneath Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Hammond, J. O.; Kendall, J. M.; Stuart, G. W.; Ebinger, C. J.

    2009-12-01

    In March 2007 41 seismic stations were deployed in north east Ethiopia. These stations recorded until October 2009, whereupon the array was condensed to 13 stations. Here we show estimates of crustal structure derived from receiver functions and upper mantle velocity structure, derived from tomography and shear-wave splitting using the first 2.5 years of data. Bulk crustal structure has been determined by H-k stacking receiver functions. Crustal Thickness varies from ~45km on the rift margins to ~16km beneath the northeastern Afar stations. Estimates of Vp/Vs show normal continental crust values (1.7-1.8) on the rift margins, and very high values (2.0-2.2) in Afar, similar to results for the Main Ethiopian Rift (MER). This supports ideas of high levels of melt in the crust beneath the Ethiopian Rift. Additionally, we use a common conversion point migration technique to obtain high resolution images of crustal structure beneath the region. Both techniques show a linear region of thin crust (~16km) trending north-south, the same trend as the Red Sea rift. SKS-wave splitting results show a general north east-south west fast direction in the MER, systematically rotating to a more north-south fast direction towards the Red Sea. Additionally, stations close to the recent Dabbahu diking episode show sharp lateral changes over small lateral distances (40° over <30km), with fast directions overlying the Dabbahu segment aligning parallel with the recent diking. This supports ideas of melt dominated anisotropy beneath the Ethiopian rift. The magnitude of splitting in this region is smaller than that seen at the MER, suggesting a thinner region of melt, or less focused melt is causing the anisotropy. Seismic tomography inversions show that in the top 150km low velocities highlight plate boundaries. The low velocity anomalies extend from the main Ethiopian rift NE, towards Djibouti, and from Djibouti NW towards the Dabbahu segment The lowest velocities exist on the rift

  6. Experimental halogen partitioning between earth upper mantle minerals and silicate melt

    NASA Astrophysics Data System (ADS)

    Joachim, Bastian; Pawley, Alison; Lyon, Ian; Henkel, Torsten; Burgess, Ray; Ballentine, Christopher J.

    2013-04-01

    Owing to their incompatibility, halogens have similar geochemical properties to noble gases in many systems and may therefore be used as key tracers of volatile transport processes in the earth. Halogen fractionation may occur during partial melting of the upper mantle, fractional crystallization or partitioning between immiscible fluids. Experimental determination of the halogen partitioning behaviour is the basis for the investigation of the concentration and distribution of halogens in the earth's mantle. High P-T partition experiments were performed in a piston cylinder apparatus using a model primitive mantle composition proposed by Jagoutz et al. (1979) simplified to the four components CaO, MgO, A2lO3 and SiO2 (CMAS) according to the procedure of O'Hara (1968). Defined small amounts of halogens (0.2 wt%) were added as CaF2, CaCl2 and CaBr2. All experiments were first heated up to 1720° C and then cooled slowly to the target temperature to guarantee growth of large homogeneous crystals, following the method of Beyer et al. (2011). Pressures range between 1.0 GPa and 2.5 GPa and final experimental temperatures between 1500° C and 1600° C, thus representing partial melting conditions of the earth upper mantle. Back-scattered electron images of polished samples show euhedral, almost rectangular forsterite grains or a mixture of euhedral forsterite and pyroxene grains with a side length of up to 150 μm, which are embedded in a MORB-like melt. Electron microprobe analysis reveals a homogeneous major element composition of the forsterite and pyroxene single crystals as well as of the melt. Halogen mapping, measured via Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS), shows no concentration gradients within the minerals or within the melt. These observations suggest that the experiments were performed at equilibrium conditions. The fact that we were able to produce large pyroxene and forsterite crystals at equilibrium conditions in a halogen doped

  7. Adjoint tomography of crust and upper-mantle structure beneath Continental China

    NASA Ast