High temperature in the upper mantle beneath Cape Verde as a possible cause for the oceanic lithosphere rejuvenation inferred from Rayleigh-wave phase-velocity measurements.

2020 ◽  
Author(s):  
Joana Carvalho ◽  
Raffaele Bonadio ◽  
Graça Silveira ◽  
Sergei Lebedev ◽  
Susana Custódio ◽  
...  
Keyword(s):  
Phase Velocity ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Oceanic Lithosphere ◽  
Cape Verde ◽  
Seismic Velocities ◽  
Low Velocity ◽  
Period Range ◽  
Rayleigh Wave Phase Velocity ◽  
Central Atlantic

<p>Cape Verde is an intraplate archipelago located in the Atlantic Ocean about 560 km west of Senegal, on top of a ~130 Ma sector of the African oceanic lithosphere. Until recently, due to the lack of broadband seismic stations, the upper-mantle structure beneath the islands was poorly known. In this study we used data from two temporary deployments across the archipelago, measuring the phase velocities of Rayleigh-waves fundamental-modes in a broad period range (8–250 s), by cross-correlating teleseismic earthquake data between pairs of stations. Deriving a robust average, phase-velocity curve for the Cape Verde region, we inverted it for a shear-wave velocity profile using non-linear gradient search.</p><p>Our results show anomalously low velocities of ∼4.2 km/s in the asthenosphere, indicating the presence of high temperatures and, eventually, partial melting. This temperature anomaly is probably responsible for the thermal rejuvenation of the oceanic lithosphere to an age as young as about 30 Ma, which we inferred from the comparison of seismic velocities beneath Cape Verde and the ones representing different ages in the Central Atlantic.</p><p>The present results, together with previously detected low-velocity anomalies in the lower mantle and relatively He-unradiogenic isotopic ratios, also suggest a hot plume deeply rooted in the lower mantle, as the origin of the Cape Verde hotspot.</p><p><span>The author</span><span>s</span><span> would like to acknowledge the financial support FCT through project</span> <span>UIDB/50019/2020</span> <span>– IDL</span><span> and FIRE project Ref. PTDC/GEO- GEO/1123/2014.</span></p>

Central U.S. crust—Upper mantle structure from Love and Rayleigh wave phase velocity inversion

1964 ◽  
Vol 54 (6A) ◽  
pp. 1997-2015
Author(s):  
T. V. McEvilly
Keyword(s):  
Phase Velocity ◽  
Upper Mantle ◽  
Structural Model ◽  
Period Range ◽  
Velocity Inversion ◽  
Wave Phase ◽  
Upper Mantle Structure ◽  
Wave Phase Velocity ◽  
Rayleigh Wave Phase Velocity ◽  
Derivatives Of

abstract Fundamental mode Rayleigh and Love wave phase velocities in the period range 5 to 80 seconds have been measured in the central U. S. Using the concept of constant partial derivatives of phase velocity with respect to layer parameters in a least-squares inversion scheme a structural model for the area was determined. Several parameters were constrained on the basis of other available data. It was not possible to fit satisfactorily both Rayleigh and Love data with a simple isotropic model. Conclusions require either anisotropy in the upper mantle or systematic errors in the phase velocity measurements.

Evidence for high temperature in the upper mantle beneath Cape Verde archipelago from Rayleigh-wave phase-velocity measurements

2019 ◽  
Vol 770 ◽  
pp. 228225 ◽  
Author(s):  
Joana Carvalho ◽  
Raffaele Bonadio ◽  
Graça Silveira ◽  
Sergei Lebedev ◽  
João Mata ◽  
...  
Keyword(s):  
High Temperature ◽  
Phase Velocity ◽  
Rayleigh Wave ◽  
Upper Mantle ◽  
Cape Verde ◽  
Velocity Measurements ◽  
Wave Phase ◽  
Wave Phase Velocity ◽  
Rayleigh Wave Phase Velocity

The formation, preservation and seismic signatures of chemical heterogeneities in the lower mantle

2020 ◽  
Author(s):  
Anna J. P. Gülcher ◽  
Maxim D. Ballmer ◽  
Paul J. Tackley ◽  
Paula Koelemeijer
Keyword(s):  
Physical Properties ◽  
Lower Mantle ◽  
Numerical Models ◽  
Bulk Composition ◽  
Oceanic Lithosphere ◽  
Global Scale ◽  
Seismic Velocities ◽  
Mantle Dynamics ◽  
Wide Range ◽  
Scientific Challenge

<p>Despite stirring by vigorous convection over billions of years, the Earth’s lower mantle appears to be chemically heterogeneous on various length scales. Constraining this heterogeneity is key for assessing Earth’s bulk composition and thermochemical evolution, but remains a scientific challenge that requires cross-disciplinary efforts. On scales below ~1 km, the concept of a “marble cake” mantle has gained wide acceptance, emphasising that recycled oceanic lithosphere, deformed into streaks of depleted and enriched compositions, makes up much of the mantle. On larger scales (10s-100s of km), compositional heterogeneity may be preserved by delayed mixing of this marble cake with either intrinsically-dense or intrinsically-strong materials. Intrinsically dense materials may accumulate as piles at the core-mantle boundary, while intrinsically viscous domains (e.g., enhanced in the strong mineral bridgmanite) may survive as “blobs” in the mid-mantle for large timescales, such as plums in the mantle “plum pudding”<sup>1,2</sup>. While many studies have explored the formation and preservation of either intrinsically-dense (recycled) or intrinsically-strong (primordial) heterogeneity, only few if any have quantified mantle dynamics in the presence of different types of heterogeneity with distinct physical properties.<span> </span></p><p>To address this objective, we use state-of-the-art 2D numerical models of global-scale mantle convection in a spherical-annulus geometry. We explore the effects of the <em>(i)</em> physical properties of primordial material (density, viscosity), <em>(ii)</em> temperature/pressure dependency of viscosity, <em>(iii)</em> lithospheric yielding strength, and <em>(iv)</em> Rayleigh number on mantle dynamics and mixing. Models predict that primordial heterogeneity is preserved in the lower mantle over >4.5 Gyr as discrete blobs for high intrinsic viscosity contrast (>30x) and otherwise for a wide range of parameters. In turn, recycled oceanic crust is preserved in the lower mantle as “marble cake” streaks or piles, particularly in models with a relatively cold and stiff mantle. Importantly, these recycled crustal heterogeneities can co-exist with primordial blobs, with piles often tending to accumulate beneath the primordial domains. This suggests that the modern mantle may be in a hybrid state between the “marble cake” and “plum pudding” styles.<span> </span></p><p>Finally, we put our model predictions in context with recent discoveries from seismology. We calculate synthetic seismic velocities from predicted temperatures and compositions, and compare these synthetics to tomography models, taking into account the limited resolution of seismic tomography. Convection models including preserved bridgmanite-enriched domains along with recycled piles have the potential of reconciling recent seismic observations of lower-mantle heterogeneity<sup>3</sup> with the geochemical record from ocean-island basalts<sup>4,5</sup>, and are therefore relevant for assessing Earth’s bulk composition and long-term evolution.<span> </span></p><p><sup>1</sup> Ballmer et al. (2017), <em>Nat. Geosci</em>., 10.1038/ngeo2898<br><sup>2</sup> Gülcher et al. (in review), <em>EPSL</em>: Variable dynamic styles of primordial heterogeneity preservation in Earth’s lower mantle <br><sup>3</sup> Waszek et al. (2018), <em>Nat. Comm., </em>10.1038/s41467-017-02709-4 <br><sup>4</sup> Hofmann (1997), <em>Nature, </em>10.1038/385219a0; <br><sup>5</sup> Mundl et al. (2017), <em>Science, </em>10.1126/science.aal4179</p>

scholarly journals Surface-wave images of western Canada: lithospheric variations across the Cordillera–craton boundary

2018 ◽  
Vol 55 (8) ◽  
pp. 887-896 ◽  
Author(s):  
Taras Zaporozan ◽  
Andrew W. Frederiksen ◽  
Alexey Bryksin ◽  
Fiona Darbyshire
Keyword(s):  
Surface Wave ◽  
Phase Velocity ◽  
Upper Mantle ◽  
High Velocity ◽  
Western Canada ◽  
Low Velocity ◽  
Phase Velocities ◽  
Slave Province ◽  
Upper Mantle Structure ◽  
Tectonically Active

Two-station surface-wave analysis was used to measure Rayleigh-wave phase velocities between 105 station pairs in western Canada, straddling the boundary between the tectonically active Cordillera and the adjacent stable craton. Major variations in phase velocity are seen across the boundary at periods from 15 to 200 s, periods primarily sensitive to upper mantle structure. Tomographic inversion of these phase velocities was used to generate phase velocity maps at these periods, indicating a sharp contrast between low-velocity Cordilleran upper mantle and high-velocity cratonic lithosphere. Depth inversion along selected transects indicates that the Cordillera–craton upper mantle contact varies in dip along the deformation front, with cratonic lithosphere of the Taltson province overthrusting Cordilleran asthenosphere in the northern Cordillera, and Cordilleran asthenosphere overthrusting Wopmay lithosphere further south. Localized high-velocity features at sub-lithospheric depths beneath the Cordillera are interpreted as Farallon slab fragments, with the gap between these features indicating a slab window. A high-velocity feature in the lower lithosphere of the Slave province may be related to Proterozic or Archean subduction.

scholarly journals Deformation of the Crust and Upper Mantle beneath the North China Craton and Its Adjacent Areas Constrained by Rayleigh Wave Phase Velocity and Azimuthal Anisotropy

2021 ◽  
Vol 14 (1) ◽  
pp. 110
Author(s):  
Xiaoming Xu ◽  
Dazhou Zhang ◽  
Xiang Huang ◽  
Xiaoman Cao
Keyword(s):  
Phase Velocity ◽  
Rayleigh Wave ◽  
Azimuthal Anisotropy ◽  
The Tibetan Plateau ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Fast Axis ◽  
Wave Phase ◽  
Wave Phase Velocity ◽  
Rayleigh Wave Phase Velocity

The North China Craton (NCC) has experienced strong tectonic deformation and lithospheric thinning since the Cenozoic. To better constrain the geodynamic processes and mechanisms of the lithospheric deformation, we used a linear damped least squares method to invert simultaneously Rayleigh wave phase velocity and azimuthal anisotropy at periods of 10–80 s with teleseismic data recorded by 388 permanent stations in the NCC and its adjacent areas. The results reveal that the anomalies of Rayleigh wave phase velocity and azimuthal anisotropy are in good agreement with the tectonic domains in the study area. Low-phase velocities appear in the rift grabens and sedimentary basins at short periods. A rotation pattern of the fast axis direction of the Rayleigh wave together with a distinct low-velocity anomaly occurs around the Datong volcano. A NW–SE trending azimuthal anisotropy and a low-velocity anomaly at periods of 60–80 s are observed subparallel to the Zhangbo fault zone. The whole lithosphere domain of the Ordos block shows a high-phase velocity and counterclockwise rotated fast axis. The northeastern margin of the Tibetan plateau is dominated by a low-velocity and coherent NW–SE fast axis direction. We infer that the subduction of the Paleo-Pacific plate and eastward material escape of the Tibetan plateau mainly contribute to the deformation of the crust and upper mantle in the NCC.

Rayleigh wave particle motion and crustal structure

1969 ◽  
Vol 59 (1) ◽  
pp. 331-346
Author(s):  
David M. Boore ◽  
M. Nafi Toksöz
Keyword(s):  
Phase Velocity ◽  
Rayleigh Wave ◽  
Crustal Structure ◽  
Particle Motion ◽  
Minor Effect ◽  
Low Velocity ◽  
Period Range ◽  
Near Surface ◽  
Earth Structures ◽  
Using Data

Abstract A feasibility study was made concerning the use of the ellipticity of the Rayleigh wave particle motion for determining earth structures. Variational parameters were computed empirically for both the ellipticity and phase velocity of Rayleigh waves in the period range T = 10-50 seconds. It was found that, in general, the ellipticity and phase velocity are about equally sensitive to structural perturbations, but that near-surface low-velocity sedimentary layers influence the ellipticity much more strongly than they do the phase velocity. Anelasticity has a minor effect on the ellipticity, whereas the presence of interfering waves can have a significant influence. A test of the independence between ellipticity and phase velocity indicated that in our period range ellipticity does contribute independent information, and thus provides an additional constraint toward uniqueness. Using data from LASA, both ellipticity and Rayleigh- and Love-wave phase velocities were measured and the results interpreted in terms of a crustal structure. The ellipticity data proved useful when combined with the phase velocity and some structures that fit the phase velocity data could be rejected on the basis of ellipticity.

A note on Rayleigh-wave flattening corrections

1976 ◽  
Vol 66 (6) ◽  
pp. 1873-1879
Author(s):  
R. G. North ◽  
A. M. Dziewonski
Keyword(s):  
Phase Velocity ◽  
Rayleigh Wave ◽  
Wave Dispersion ◽  
Period Range ◽  
Empirical Correction ◽  
Phase Velocity Dispersion ◽  
Wave Phase Velocity ◽  
Earth Models ◽  
Normal Mode Calculations ◽  
Rayleigh Wave Phase Velocity

abstract The effects of sphericity and gravity upon Rayleigh-wave dispersion are examined. The widely used empirical correction of Bolt and Dorman (1961), although originally determined from a limited set of earth models, appears to predict phase-velocity curves in a spherical gravitating earth from flat earth calculations to almost 1 per cent accuracy, as claimed, for five earth models chosen to reproduce the considerable range of observed dispersion. Its application in the past therefore does not seem likely to have introduced large errors in inversion of such dispersion to determine earth structure. The use of spherical gravitating earth normal mode calculations in computing dispersion is strongly urged: for those without access to the computing facilities required by the complexity of the numerical problem a new empirical correction based on flat earth group velocity is proposed. This predicts Rayleigh-wave phase velocity dispersion in a spherical gravitating earth to better than 0.4 per cent in the period range 10 to 200 sec. Even better precision can be obtained by application of the tables of corrections given for different types of crustal and upper mantle structures.

Constraining dynamic models in North America using the static gravity field

2020 ◽  
Author(s):  
Jesse Reusen ◽  
Bart Root ◽  
Javier Fullea ◽  
Zdenek Martinec ◽  
Wouter van der Wal
Keyword(s):  
Gravity Anomaly ◽  
Gravity Field ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Dynamic Models ◽  
Gravity Anomalies ◽  
Mantle Flow ◽  
Seismic Velocities ◽  
Crust And Upper Mantle

<p>The negative anomaly present in the static gravity field near Hudson Bay bears striking resemblance to the area depressed by the Laurentide ice sheet during the Last Glacial Maximum, suggesting that it is at least partly due to Glacial Isostatic Adjustment (GIA), but mantle convection and density anomalies in the crust and the upper mantle are also expected to contribute. At the moment, the contribution of GIA to this anomaly is still disputed. Estimates, which strongly depend on the viscosity of the mantle, range from 25 percent to more than 80 percent. Our objective is to find the contributions from GIA and mantle convection, after correcting for density anomalies in the topography, crust and upper mantle. The static gravity field has the potential to constrain the viscosity profile which is the most uncertain parameter in GIA and mantle convection models. A spectral method is used to transform 3D spherical density models of the crust into gravity anomalies. Density anomalies in the lithosphere are estimated so that isostatic compensation is reached at a depth of 300 km. The dynamic processes of mantle flow are corrected for before isostasy is assumed. Upper and lower mantle viscosities are varied so that the gravity anomaly predicted from the dynamic models matches the residual gravity anomaly. We consider uncertainties due to the crustal model, the lithosphere-asthenosphere boundary (LAB), the conversion from seismic velocities to density and the ice history used in the GIA model. The best fit is found for lower mantle viscosities >10<sup>22</sup> Pa s.</p>

scholarly journals Hydrous SiO2 in subducted oceanic crust and water transport to the core-mantle boundary

2020 ◽  
Author(s):  
Yanhao Lin ◽  
Qingyang Hu ◽  
Jing Yang ◽  
Yue Meng ◽  
Yukai Zhuang ◽  
...  
Keyword(s):  
Partial Melting ◽  
Oceanic Crust ◽  
Lower Mantle ◽  
Oceanic Lithosphere ◽  
Low Velocity ◽  
Water Storage Capacity ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Subducted Oceanic Crust

Abstract Subduction of oceanic lithosphere transports surface water into the mantle where it can have remarkable effects, but how much can be cycled down into the deep mantle, and potentially to the core, remains ambiguous. Recent studies show that dense SiO2 in the form of stishovite, a major phase in subducted oceanic crust at depths greater than ~300 km, has the potential to host and carry water into the lower mantle. We investigate the hydration of stishovite and its higher-pressure polymorphs, CaCl2-type SiO2 and seifertite, in experiments at pressures of 44–152 GPa and temperatures of ~1380–3300 K. We quantify the water storage capacity of these dense SiO2 phases at high pressure and find that water stabilizes CaCl2-type SiO2 to pressures beyond the base of the mantle. We parametrize the P-T dependence of water capacity and model H2O storage in SiO2 along a lower mantle geotherm. Dehydration of slab mantle in cooler slabs in the transition zone can release fluids that hydrate stishovite in oceanic crust. Hydrous SiO2 phases are stable along a geotherm and progressively dehydrate with depth, potentially causing partial melting or silica enrichment in the lower mantle. Oceanic crust can transport ~0.2 wt% water to the core-mantle boundary region where, upon heating, it can initiate partial melting and react with the core to produce iron hydrides, providing plausible explanations for ultra-low velocity regions at the base of the mantle.

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