Global observations of 3D mantle attenuation using normal modes 

2021 ◽  
Author(s):  
Sujania Talavera-Soza ◽  
Arwen Deuss
Keyword(s):  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Velocity Structure ◽  
3D Structure ◽  
Normal Modes ◽  
Seismic Attenuation ◽  
Body Waves ◽  
Splitting Function ◽  
Thermal Origin

<div> <div> <div> <p>Seismic tomographic models based solely on wave velocities are unable to distinguish between a temperature or compositional origin for Earth’s 3D structure variations, such as the Large Low Shear Velocity Provinces (LLSVPs) beneath the lower mantle of Africa and the Pacific. Seismic attenuation or damping is able able to provide additional information that may help to unravel the origin of the LLSVPs, which is fundamental to understand mantle convection evolution. For example, a thermal origin for the LLSVPs will point to them being short-lived anomalies, whereas a compositional origin will point to them being long-lived, forming mantle 'anchors' and influencing the pattern of mantle convection for a large part of Earth’s history. Seismic attenuation is able to make that distinction, because it is directly sensitive to temperature variations. So far, global 3D attenuation models have only been available for the upper mantle, with only two regional body waves studies exploring the lower mantle (Lawrence and Wysession, 2006; Hwang and Ritsema, 2011).<br>Here, we use normal mode data to measure elastic splitting functions (dependent on velocity and density) and anelastic splitting functions (dependent on attenuation). The advantage of normal modes is that they allow us to include focussing and scattering due to the velocity structure without the need for approximations, because we measure the elastic splitting function jointly with the anelastic splitting function. In our measurements for upper mantle sensi- tive modes, we find anti-correlation between the elastic and anelastic splitting functions, suggesting a thermal origin for low velocity spreading ridges, and agreeing with previous studies. On the other hand, for lower mantle sensitive modes, we find correlation, suggesting the averagely attenuating LLSVPs are surrounded by strongly attenuating regions potentially due to the presence of post-perovskite.</p> </div> </div> </div>

A new 3-D mantle density model from recent normal-mode measurements

2021 ◽  
Author(s):  
Rûna van Tent ◽  
Arwen Deuss ◽  
Andreas Fichtner ◽  
Lars Gebraad ◽  
Simon Schneider ◽  
...  
Keyword(s):  
Normal Mode ◽  
Lower Mantle ◽  
Velocity Structure ◽  
Normal Modes ◽  
Shear Velocity ◽  
Boundary Region ◽  
Splitting Function ◽  
The Pacific ◽  
Compositional Variations ◽  
Body Tides

<p>Constraints on the 3-D density structure of Earth’s mantle provide important insights into the nature of seismically observed features, such as the Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle under Africa and the Pacific. The only seismic data directly sensitive to density variations throughout the entire mantle are normal modes: whole Earth oscillations that are induced by large earthquakes (M<sub>w</sub> > 7.5). However, their sensitivity to density is weak compared to the sensitivity to velocity and different studies have presented conflicting density models of the lower mantle. For example, Ishii & Tromp (1999) and Trampert et al. (2004) have found that the LLSVPs have a larger density than the surrounding mantle, while Koelemeijer et al. (2017) used additional Stoneley-mode observations, which are particularly sensitive to the core-mantle boundary region, to show that the LLSVPs have a lower density. Recently, Lau et al. (2017) have used tidal tomography to show that Earth's body tides prefer dense LLSVPs.</p><p>A large number of new normal-mode splitting function measurements has become available since the last density models of the entire mantle were published. Here, we show the models from our inversion of these recent data and compare our results to previous studies. We find areas of high as well as low density at the base of the LLSVPs and we find that inside the LLSVPs density varies on a smaller scale than velocity, indicating the presence of compositionally distinct material. In fact, we find low correlations between the density and velocity structure throughout the entire mantle, revealing that compositional variations are required at all depths inside the mantle.</p>

scholarly journals Feedbacks between a non-Newtonian upper mantle, mantle viscosity structure and mantle dynamics

2020 ◽  
Vol 224 (2) ◽  
pp. 961-972
Author(s):  
A G Semple ◽  
A Lenardic
Keyword(s):  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Mantle Flow ◽  
Mantle Dynamics ◽  
Plate Motions ◽  
Low Viscosity ◽  
Mantle Viscosity ◽  
Flow Feature ◽  
Mantle Rheology

SUMMARY Previous studies have shown that a low viscosity upper mantle can impact the wavelength of mantle flow and the balance of plate driving to resisting forces. Those studies assumed that mantle viscosity is independent of mantle flow. We explore the potential that mantle flow is not only influenced by viscosity but can also feedback and alter mantle viscosity structure owing to a non-Newtonian upper-mantle rheology. Our results indicate that the average viscosity of the upper mantle, and viscosity variations within it, are affected by the depth to which a non-Newtonian rheology holds. Changes in the wavelength of mantle flow, that occur when upper-mantle viscosity drops below a critical value, alter flow velocities which, in turn, alter mantle viscosity. Those changes also affect flow profiles in the mantle and the degree to which mantle flow drives the motion of a plate analogue above it. Enhanced upper-mantle flow, due to an increasing degree of non-Newtonian behaviour, decreases the ratio of upper- to lower-mantle viscosity. Whole layer mantle convection is maintained but upper- and lower-mantle flow take on different dynamic forms: fast and concentrated upper-mantle flow; slow and diffuse lower-mantle flow. Collectively, mantle viscosity, mantle flow wavelengths, upper- to lower-mantle velocities and the degree to which the mantle can drive plate motions become connected to one another through coupled feedback loops. Under this view of mantle dynamics, depth-variable mantle viscosity is an emergent flow feature that both affects and is affected by the configuration of mantle and plate flow.

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 Compositional mantle layering revealed by slab stagnation at ~1000-km depth

2015 ◽  
Vol 1 (11) ◽  
pp. e1500815 ◽  
Author(s):  
Maxim D. Ballmer ◽  
Nicholas C. Schmerr ◽  
Takashi Nakagawa ◽  
Jeroen Ritsema
Keyword(s):  
Numerical Modeling ◽  
Seismic Tomography ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Global Scale ◽  
Compositional Gradient ◽  
Mantle Enrichment ◽  
Geodynamic Models ◽  
Neutrally Buoyant

Improved constraints on lower-mantle composition are fundamental to understand the accretion, differentiation, and thermochemical evolution of our planet. Cosmochemical arguments indicate that lower-mantle rocks may be enriched in Si relative to upper-mantle pyrolite, whereas seismic tomography images suggest whole-mantle convection and hence appear to imply efficient mantle mixing. This study reconciles cosmochemical and geophysical constraints using the stagnation of some slab segments at ~1000-km depth as the key observation. Through numerical modeling of subduction, we show that lower-mantle enrichment in intrinsically dense basaltic lithologies can render slabs neutrally buoyant in the uppermost lower mantle. Slab stagnation (at depths of ~660 and ~1000 km) and unimpeded slab sinking to great depths can coexist if the basalt fraction is ~8% higher in the lower mantle than in the upper mantle, equivalent to a lower-mantle Mg/Si of ~1.18. Global-scale geodynamic models demonstrate that such a moderate compositional gradient across the mantle can persist can in the presence of whole-mantle convection.

scholarly journals Features of the velocity structure of the mantle under the Precambrian structures on the example of the Indian platform (according to seismic tomography)

2021 ◽  
Vol 43 (1) ◽  
pp. 211-226
Author(s):  
L.N. Zaiets ◽  
I.V. Bugaienko ◽  
T.A. Tsvetkova
Keyword(s):  
Transition Zone ◽  
Seismic Tomography ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Velocity Structure ◽  
Initial Approximation ◽  
Low Velocity ◽  
Tectonic Zone ◽  
Lower Velocity ◽  
Northern Border

The paper presents additional data, approaching to understanding the driving forces in the formation of geological structures and the development of the Indian platform. The results of seismic tomography are attracted here and their analysis is presented. A 3-dimensional P-velocity model of the mantle of the Indian platform was obtained according to the Taylor approximation method developed by V. Geyko. The undeniable advantages of the method are independence from the initial approximation (reference model) and the best approximation of nonlinearity. According to the data, the mantle under the Indian platform is influenced by both plumes and fluid systems. The influence of plumes is observed in the form of low-velocity subvertical exits from the lower mantle to the transition zone; fluids — in the form of interbedding of high and low velocity anomalies from the lower mantle (or from the transition zone of the upper mantle) to the upper mantle. An analysis is presented of both general velocity structure of the platform mantle and the velocity structure of the mantle under individual cratons (Bandelkand, Singhbum, Bastar and Darvar), the totality of which forms the Indian platform and the trap provinces. At lower velocity, an area is distinguished in the mantle that corresponds to the surface of the Narmada-Son lineament moving into the Central Indian Tectonic Zone. The mantle high-velocity structures under the Deccan trap province, together with their spreading area in the transitional zone of the mantle, subdivide the platform into two parts at depths of 375 km. Areas in the mantle with inclined layers were identified and analyzed: under the cratons Bandelkand and Singbum, the Rajmahal traps and the northern border of the Deccan traps. According to the model, an area bordering the Himalayas is well distinguished in the mantle. It is shown how, when the Indian platform collides with the Eurasian margin, the upper mantle stratifies into plates capable of independent motions, including subduction.

A mechanism for ocean-floor spreading

1971 ◽  
Vol 268 (1192) ◽  
pp. 731-731 ◽  
Keyword(s):  
Transition Zone ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Garnet Peridotite ◽  
Ocean Floor ◽  
Ocean Floor Spreading ◽  
Melted Layer ◽  
Lithospheric Plates ◽  
Melted Material

I wish to suggest a mechanism for ocean-floor spreading, other than deep mantle convection cells. If the geotherm intersects the mantle solidus at the base of the upper mantle, a partly melted layer will result. It will be less dense than the overlying unmelted garnet peridotite lithosphere and the situation will be gravitationally unstable. It will be more voluminous than the overlying unmelted mantle and will distend the lithosphere which will break up into plates. It will have low rigidity and decouple those lithospheric plates from the underlying transition zone and lower mantle. Melted material, perhaps with crystals, will escape in two ways.

Slab Remnant Recycling and Mantle-Wide Convection: A Separation of Time Scales

2020 ◽  
Author(s):  
Gary Jarvis
Keyword(s):  
Time Scales ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Numerical Models ◽  
High Viscosity ◽  
Mineral Phase ◽  
Core Mantle Boundary ◽  
The Core ◽  
Very High

<p>Two dimensional numerical models of mantle convection in a cylindrical shell provide a possible geodynamic explanation for cold patches in the mantle below India and Mongolia as detected by seismic tomography. We investigate the influence of very high viscosities at mid-mantle and lower-mantle depths, as proposed by Mitrovica and Forte (2004) and Steinberger and Calderwood (2006), on mantle convective flow.  Models are considered with and without mineral phase transitions.  Our viscosity profiles are depth dependent with deep mantle viscosities increasing to values of 300 times the viscosity of the upper mantle, and then decreasing dramatically on approaching the core-mantle boundary.   The decrease of viscosity near the CMB mobilizes the overall mantle-wide flow despite very high mid-mantle viscosities.  However, cold detached slabs sinking below continental collisions become captured by the high viscosity interior and circulate slowly for times exceeding 200 Myr.  The separation of time scales for mantle-wide flow vs slab circulation, is a consequence of the high viscosity of the mid-mantle.</p>

A narrow plume conduit anchored in the lower mantle beneath la Réunion hotspot

2020 ◽  
Author(s):  
Mathurin Dongmo wamba ◽  
Barbara Romanowicz ◽  
Jean-Paul Montagner ◽  
Guilhem Barruol
Keyword(s):  
Indian Ocean ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Deep Structure ◽  
Order Approximation ◽  
Hessian Matrix ◽  
Waveform Inversion ◽  
Normal Modes ◽  
Low Velocity ◽  
The Earth

<pre>The arrival of some plumes and the birth of hotspots at the Earth surface have broken up the Pangea in many continents 200 Ma ago. La Réunion hotspot is known as one of the largest on the Earth. Its birth, 65Ma ago, creating the Deccan volcanic traps in India (almost 2 million km<sup>2</sup>) and the death of more than 90% of life on the Earth including dinosaurs. So far the origin of the mantle plumes and their role in geodynamics are still controversial in Earth sciences. In that respect, we use the dataset from the French-German RHUM-RUM experiment around La Réunion hotspot (2012-2013), from IRIS data center and FDSN to investigate the deep structure of the plume along its complete track from its birth to its present stage. The use of spectral element method allows us to perform forward modelling for several thousand paths across the Indian ocean. The sensitive matrix (first order approximation of the Hessian matrix) is built from the coupling of normal modes along and across the branches, by using nonlinear asymptotic theory. This waveform inversion approach enables us to resolve deep anomalies in the mantle underneath Indian ocean. So far we found out a low velocity zone channel (extended from the west to the east) in the upper mantle beneath the Mascarene bassin, and plume conduit with a broad head in the upper mantle and narrow tail anchored in the lower mantle. The connection between la Réunion hotspot and the African LLSVP is also brought to light by our model.</pre>

scholarly journals A new catalogue of toroidal-mode overtone splitting function measurements

2020 ◽  
Author(s):  
Simon Schneider ◽  
Arwen Deuss
Keyword(s):  
Normal Mode ◽  
Large Scale ◽  
Velocity Structure ◽  
Normal Modes ◽  
Splitting Function ◽  
Data Set ◽  
Toroidal Mode ◽  
Function Coefficient ◽  
Fiji Island ◽  
Shear Wave Velocity Structure

Abstract Spectra of whole Earth oscillations or normal modes provide important constraints on Earth’s large scale structure. The most convenient way to include normal mode constraints in global tomographic models is by using splitting functions or structure coefficients, which describe how the frequency of a specific mode varies regionally. Splitting functions constrain 3D variations in velocity, density structure and boundary topography. They may also constrain anisotropy, especially when combining information from spheroidal modes, which are mainly sensitive to P-SV structure, with toroidal modes, mainly sensitive to SH structure. Spheroidal modes have been measured extensively, but toroidal modes have proven to be much more difficult and as a result only a limited number of toroidal modes have been measured so far. Here we expand the splitting function studies by Resovsky and Ritzwoller (1998) and Deuss et al. (2013), by focusing specifically on toroidal mode overtone observations. We present splitting function measurements for 19 self-coupled toroidal modes of which 13 modes have not been measured before. They are derived from radial and transverse horizontal component normal mode spectra up to 5 mHz for 91 events with MW ≥ 7.4 from the years 1983-2018. Our data include the Tohoku event of 2011 (9.1MW), the Okhotsk event of 2013 (8.3MW) and the Fiji Island event from 2018 (8.2MW). Our measurements provide new constraints on upper and lower mantle shear wave velocity structure and in combination with existing spheroidal mode measurements can be used in future inversions for anisotropic mantle structure. Our new splitting function coefficient data set will be available online.

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