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Monthly Notices of the Royal Astronomical Society

splitting observations are interpreted in terms of the strain-induced lattice preferred orientation of mantle minerals, especially olivine. We consider three hypotheses concerning the origin of the continental anlsotropy: (1) strain associated with absolute plate motion, as in the oceanic upper mantle, (2) crustal stress, and (3) the past and present internal deformation of the subcontinental upper mantle by tectonic episodes. It is found that the last hypothesis is the most successful, namely that the most recent significant episode of internal deformation appears to be the best predictor of q. For stable continental regions, this is interpreted as "fossil" anisotropy, whereas for presently active regions, such as Alaska, the anisotropy reflects present-day tectonic activity. In the stable portion of North America there is a good correlation between delay time and lithospheric thickness; this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant. The acceptance of this hypothesis has several implications for subcontinental mantle deformation. First, it argues for coherent deformation of the continental lithosphere (crust and mantle) during orogenies. This implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent "underplating" or conductive growth of the thermal boundary layer. One of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes. Most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed (by convective instability or delamination) and replaced by warm asthenosphere. These models, however, are incompatible with the evidence for preserved coherent lithospheric deformation; rather, the deformed mantle appears to have been heated in place. We suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation.

/pdf/shear-wave-splitting-and-subcontinental-mantle-deformation-3tk4a7yxgq.pdf

Shear wave splitting and subcontinental mantle deformation

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98RG02075

Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting?

We present a new global model for the Earth's crust based on seismic refraction data published in the period 1948-1995 and a detailed compilation of ice and sediment thickness. An extensive compilation of seismic refraction measurements has been used to determine the crustal structure on continents and their margins. Oceanic crust is modeled with both a standard model for normal oceanic crust, and variants for nonstandard regions, such as oceanic plateaus. Our model (CRUST 5.1) consists of 2592 5°x5° tiles in which the crust and uppermost mantle are described by eight layers: (1) ice, (2) water, (3) soft sediments, (4) hard sediments, (5) crystalline upper, (6) middle, (7) lower crust, and (8) uppermost mantle. Topography and bathymetry are adopted from a standard database (ETOPO-5). Compressional wave velocity in each layer is based on field measurements, and shear wave velocity and density are estimated using recently published empirical V p -V s and V p -density relationships. The crustal model differs from previous models in that (1) the thickness and seismic/density structure of sedimentary basins is accounted for more completely, (2) the velocity structure of unmeasured regions is estimated using statistical averages that are based on a significantly larger database of crustal structure, (3) the compressional wave, shear wave, and density structure have been explicitly specified using newly available constraints from field and laboratory studies. Thus this global crustal model is based on substantially more data than previous models and differs from them in many important respects. A new map of the thickness of the Earth's crust is presented, and we illustrate the application of this model by using it to provide the crustal correction for surface wave phase velocity maps. Love waves at 40 s are dominantly sensitive to crustal structure, and there is a very close correspondence between observed phase velocities at this period and those predicted by CRUST 5.1. We find that the application of crustal corrections to long-period (167 s) Rayleigh waves significantly increases the variance in the phase velocity maps and strengthens the upper mantle velocity anomalies beneath stable continental regions. A simple calculation of crustal isostacy indicates significant lateral variations in upper mantle density. The model CRUST 5.1 provides a complete description of the physical properties of the Earth's crust at a scale of 5° x 5° and can be used for a wide range of seismological and nonseismological problems.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JB02122

CRUST 5.1: A global crustal model at 5° × 5°

[1] Since their development in the 1960s, seismic arrays have given a new impulse to seismology. Recordings from many uniform seismometers in a well-defined, closely spaced configuration produce high-quality and homogeneous data sets, which can be used to study the Earth's structure in great detail. Apart from an improvement of the signal-to-noise ratio due to the simple summation of the individual array recordings, seismological arrays can be used in many different ways to study the fine-scale structure of the Earth's interior. They have helped to study such different structures as the interior of volcanos, continental crust and lithosphere, global variations of seismic velocities in the mantle, the core-mantle boundary and the structure of the inner core. For this purpose many different, specialized array techniques have been developed and applied to an increasing number of high-quality array data sets. Most array methods use the ability of seismic arrays to measure the vector velocity of an incident wave front, i.e., slowness and back azimuth. This information can be used to distinguish between different seismic phases, separate waves from different seismic events and improve the signal-to-noise ratio by stacking with respect to the varying slowness of different phases. The vector velocity information of scattered or reflected phases can be used to determine the region of the Earth from whence the seismic energy comes and with what structures it interacted. Therefore seismic arrays are perfectly suited to study the small-scale structure and variations of the material properties of the Earth. In this review we will give an introduction to various array techniques which have been developed since the 1960s. For each of these array techniques we give the basic mathematical equations and show examples of applications. The advantages and disadvantages and the appropriate applications and restrictions of the techniques will also be discussed. The main methods discussed are the beam-forming method, which forms the basis for several other methods, different slant stacking techniques, and frequency–wave number analysis. Finally, some methods used in exploration geophysics that have been adopted for global seismology are introduced. This is followed by a description of temporary and permanent arrays installed in the past, as well as existing arrays and seismic networks. We highlight their purposes and discuss briefly the advantages and disadvantages of different array configurations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000RG000100

Array seismology: Methods and applications

http://perso.ens-lyon.fr/yanick.ricard/Article/3smac1.pdf

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

For the first time, flow velocity is measured in a vortex of liquid gallium, using the pulsed Doppler shift ultrasonic method. At the top of a copper cylinder filled with liquid gallium, we spin a disk and create a turbulent vortex with a dominant nearly axisymmetric velocity field with little variation in the axial direction. The velocity profiles are shown to be well resolved and in quantitative agreement with earlier observations. Reliable velocity measurements in liquid gallium could be obtained only after serious problems due to the formation of oxides were solved. This work opens the way to performing accurate velocity measurements in other liquid metals; preliminary results for liquid sodium are shown.

Ultrasonic Doppler velocimetry in liquid gallium

A systematic experimental study of rapidly rotating spherical convection in water and liquid gallium

http://sappho.eps.mcgill.ca/~olivia/666/2006/Plumes/sdarticle.pdf

Detection of mantle plumes in the lower mantle by diffraction tomography: Hawaii

Considering the growing number of observations of a seismic discontinuity at the top of D″, some 300 km above the core-mantle boundary, we investigate the scenario of whether this feature could be global. We show that the mean first arrivals of P-waves, as used to build global models, can be fit by the two branches of a discontinuous model. We argue that the intermittence of the detection of the reflector is due to lateral variations of a global discontinuity. These observations put strong constraints on the type of chemical layering that could explain the discontinuity. We speculate that the properties of the D″ discontinuity are better explained in terms of a new phase transition.

Henri-Claude Nataf

Papers

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

Ultrasonic Doppler velocimetry in liquid gallium

A systematic experimental study of rapidly rotating spherical convection in water and liquid gallium

Detection of mantle plumes in the lower mantle by diffraction tomography: Hawaii

Seismic discontinuity at the top of d : a world-wide feature ?