scholarly journals A Bayesian Method to Quantify Azimuthal Anisotropy Model Uncertainties: Application to Global Azimuthal Anisotropy in the Upper Mantle and Transition Zone

2017 ◽  
Author(s):  
Kaiqing Yuan ◽  
Caroline Beghein
Keyword(s):  
Transition Zone ◽  
Upper Mantle ◽  
Bayesian Method ◽  
Azimuthal Anisotropy ◽  
Model Uncertainties ◽  
Anisotropy Model

scholarly journals Seismic anisotropy reveals crustal flow driven by mantle vertical loading in the Pacific NW

2020 ◽  
Vol 6 (28) ◽  
pp. eabb0476
Author(s):  
Jorge C. Castellanos ◽  
Jonathan Perry-Houts ◽  
Robert W. Clayton ◽  
YoungHee Kim ◽  
A. Christian Stanciu ◽  
...  
Keyword(s):  
Pacific Northwest ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Azimuthal Anisotropy ◽  
Mantle Flow ◽  
Near Surface ◽  
Vertical Loading ◽  
The Pacific ◽  
Anisotropy Model ◽  
Lithosphere Dynamics

Buoyancy anomalies within Earth’s mantle create large convective currents that are thought to control the evolution of the lithosphere. While tectonic plate motions provide evidence for this relation, the mechanism by which mantle processes influence near-surface tectonics remains elusive. Here, we present an azimuthal anisotropy model for the Pacific Northwest crust that strongly correlates with high-velocity structures in the underlying mantle but shows no association with the regional mantle flow field. We suggest that the crustal anisotropy is decoupled from horizontal basal tractions and, instead, created by upper mantle vertical loading, which generates pressure gradients that drive channelized flow in the mid-lower crust. We then demonstrate the interplay between mantle heterogeneities and lithosphere dynamics by predicting the viscous crustal flow that is driven by local buoyancy sources within the upper mantle. Our findings reveal how mantle vertical load distribution can actively control crustal deformation on a scale of several hundred kilometers.

scholarly journals An automatically updated S ‐wave model of the upper mantle and the depth extent of azimuthal anisotropy

2016 ◽  
Vol 43 (2) ◽  
pp. 674-682 ◽  
Author(s):  
Eric Debayle ◽  
Fabien Dubuffet ◽  
Stéphanie Durand
Keyword(s):  
Upper Mantle ◽  
Wave Model ◽  
Azimuthal Anisotropy ◽  
S Wave ◽  
Depth Extent

scholarly journals Gravity constraints on structure of the East-West Antarctic lithospheric transition zone

2021 ◽  
Author(s):  
Sam Treweek
Keyword(s):  
Transition Zone ◽  
Upper Mantle ◽  
Geophysical Methods ◽  
East Antarctica ◽  
Gravity Models ◽  
West Antarctica ◽  
Asthenospheric Mantle ◽  
Surface Uplift ◽  
Mantle Viscosity ◽  
Taylor Valley

<p><b>The differing structural evolution of cratonic East Antarctica and younger West Antarctica has resulted in contrasting lithospheric and asthenospheric mantle viscosities between the two regions. Combined with poor constraints on the upper mantle viscosity structure of the continent, estimates of surface uplift in Antarctica predicted from models of glacial isostatic adjustment (GIA) and observed by Global Satellite Navigation System (GNSS) contain large misfits. This thesis presents a gravity study ofthe lithospheric transition zone beneath the Taylor Valley, Antarctica, conducted to constrain the variation in lithological parameters such as viscosity and density of the upper mantle across this region.</b></p> <p>During this study 119 new gravity observations were collected in the ice-free regions of the Taylor Valley and amalgamated with 154 existing land-based gravity observations, analysed alongside aerogravity measurements of southern Victoria Land. Gravity data are used to construct 2D gravity models of the subsurface beneath this region. An eastward gradient in Bouguer anomalies of ~- 1.6 mGal/km is observed within the Taylor Valley. Models reveal thickening of the Moho from 23±5 km beneath the Ross Sea to 35±5 km in the Polar Plateau (dipping at 24.5±7.2°), and lithospheric mantle 100 km thicker in East Antarctica (~200±30 km) than West Antarctica (~90±30 km). </p> <p>Models of predicted surface uplift history are used to estimate an asthenospheric mantle viscosity of 2.1x1020 Pa.s at full surface recovery beneath the Ross Embayment, differing by ~14% from the viscosity at 50% recovery. The temperature contrast between lithospheric and asthenospheric mantle is estimated as ~400°C, equivalent to a viscosity that decreases by a factor of about 30 over the mantle boundary.</p> <p>Results demonstrate that the history of surface uplift in the study area may be complicated, resulting in observations of uplift, or subsidence, at GNSS stations. Future work should incorporate additional geophysical methods, such as seismicity and electrical resistivity, improving constraints on gravity models. A better understanding of the surface uplift (or subsidence) history in the Transantarctic Mountains is critical, with implications in reducing uncertainty in GIA models.</p>

Evidence of electrical anisotropy in limestone formations using the RMT technique

Geophysics ◽  
2004 ◽  
Vol 69 (4) ◽  
pp. 909-916 ◽  
Author(s):  
Niklas Linde ◽  
Laust B. Pedersen
Keyword(s):  
Transfer Functions ◽  
Median Filter ◽  
Azimuthal Anisotropy ◽  
Electrical Anisotropy ◽  
Shallow Subsurface ◽  
Resistivity Method ◽  
1D Model ◽  
Anisotropy Model ◽  
Azimuthal Resistivity ◽  
Wenner Array

Azimuthal resistivity surveys are often applied to complement hydrological information or to improve the location of observation boreholes in pump tests. Symmetric electrode configurations cannot distinguish anisotropy from lateral changes or dipping layers, but asymmetric arrays (e.g., the offset Wenner array) can. Tensor radiomagnetotellurics (RMT) is presented as an alternative method in studies of electrical anisotropy in the shallow subsurface. The electromagnetic and geomagnetic transfer functions provide information about the dimensionality of the data. These transfer functions can also be used to find the directions of anisotropy. Data with an anisotropic signature can be inverted for a one‐dimensional (1D) azimuthal anisotropy model. The method is faster than the azimuthal resistivity method. A 380‐m‐long profile of tensor RMT data (12.7–243 kHz) from limestones that overlie shale on the island of Gotland, Sweden, is used to show the merits of the method. The data have a clear anisotropic signature. The data are inverted for a three‐layer 1D model with azimuthal anisotropy using two different approaches: (1) a moving median filter of five neighboring stations and neglecting static shift parameters; and (2) treating each station separately and including static shifts of the electric field in the inversion. Both inversions show models having a marked anisotropy with anisotropy factors of 3.7 and 4.5, respectively, in the limestones. The second approach has a significantly better data fit. However, the first approach is preferred because the models are smoother from station to station.

Radial Anisotropy and it's Relation to Fast and Slow Moving Tectonic Plates

2021 ◽  
Author(s):  
Tak Ho ◽  
Keith Priestley ◽  
Eric Debayle
Keyword(s):  
Upper Mantle ◽  
Large Scale ◽  
Oceanic Lithosphere ◽  
Azimuthal Anisotropy ◽  
Plate Motion ◽  
Moho Depth ◽  
Dispersion Characteristics ◽  
Anisotropic Models ◽  
Ocean Ridges ◽  
Average Dispersion

&lt;p&gt;We present a new radially anisotropic (&lt;strong&gt;&amp;#958;)&lt;/strong&gt;&amp;#160;tomographic model for the upper mantle to transition zone depths derived from a large Rayleigh (~4.5 x 10&lt;sup&gt;6&amp;#160;&lt;/sup&gt;paths) and Love (~0.7 x 10&lt;sup&gt;6&lt;/sup&gt;&amp;#160;paths) wave path average dispersion curves with periods of 50-250 s and up to the fifth overtone. We first extract the path average dispersion characteristics from the waveforms. Dispersion characteristics for common paths (~0.3 x 10&lt;sup&gt;6&lt;/sup&gt;&amp;#160;paths) are taken from the Love and Rayleigh datasets and jointly inverted for isotropic V&lt;sub&gt;s&amp;#160;&lt;/sub&gt;and&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;. CRUST1.0 is used for crustal corrections and a model similar to PREM is used as a starting model. V&lt;sub&gt;s&lt;/sub&gt;&amp;#160;and&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;&amp;#160;are regionalised for a 3D model. The effects of azimuthal anisotropy are accounted for during the regionalisation. Our model confirms large-scale upper mantle features seen in previously published models, but a number of these features are better resolved because of the increased data density of the fundamental and higher modes coverage from which our&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;(z) was derived. Synthetic tests show structures with radii of 400 km can be distinguished easily. Crustal perturbations of +/-10% to V&lt;sub&gt;p&lt;/sub&gt;, V&lt;sub&gt;s&lt;/sub&gt;&amp;#160;and density, or perturbations to Moho depth of +/-10 km over regions of 400 km do not significantly change the model. The global average decreases from&amp;#160;&lt;strong&gt;&amp;#958;~&lt;/strong&gt;1.06 below the Moho to&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;~1 at ~275 km depth. At shallow depths beneath the oceans&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;&gt;1 as is seen in previously published global mantle radially anisotropic models. The thickness of this layer increases slightly with the increasing age of the oceanic lithosphere. At ~200 km and deeper depths below the fast-spreading East Pacific Rise and starting at somewhat greater depths beneath the slower spreading ridges,&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;&lt;1. At depths &amp;#8805;200 km and deeper depths below most of the backarc basins of the western Pacific&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;&lt;1. The signature of mid-ocean ridges vanishes at about 150 km depth in V&lt;sub&gt;s&lt;/sub&gt;&amp;#160;while it extends much deeper in the&amp;#160;&lt;strong&gt;&amp;#958;&lt;/strong&gt;&amp;#160;model suggesting that upwelling beneath mid-ocean ridges could be more deeply rooted than previously believed. The pattern of radially anisotropy we observe, when compared with the pattern of azimuthal anisotropy determined from Rayleigh waves, suggests that the shearing at the bottom of the plates is only sufficiently strong to cause large-scale preferential alignment of the crystals when the plate motion exceeds some critical value which Debayle and Ricard (2013) suggest is about 4 cm/yr.&lt;/p&gt;

scholarly journals An Overview of the Experimental Studies on the Electrical Conductivity of Major Minerals in the Upper Mantle and Transition Zone

Materials ◽  
2020 ◽  
Vol 13 (2) ◽  
pp. 408 ◽  
Author(s):  
Lidong Dai ◽  
Haiying Hu ◽  
Jianjun Jiang ◽  
Wenqing Sun ◽  
Heping Li ◽  
...  
Keyword(s):  
Electrical Conductivity ◽  
Transition Zone ◽  
Oxygen Fugacity ◽  
Upper Mantle ◽  
Activation Enthalpy ◽  
Small Polaron ◽  
Experimental Studies ◽  
Transport Processes ◽  
Structural Water ◽  
Nominally Anhydrous Minerals

In this paper, we present the recent progress in the experimental studies of the electrical conductivity of dominant nominally anhydrous minerals in the upper mantle and mantle transition zone of Earth, namely, olivine, pyroxene, garnet, wadsleyite and ringwoodite. The main influence factors, such as temperature, pressure, water content, oxygen fugacity, and anisotropy are discussed in detail. The dominant conduction mechanisms of Fe-bearing silicate minerals involve the iron-related small polaron with a relatively large activation enthalpy and the hydrogen-related defect with lower activation enthalpy. Specifically, we mainly focus on the variation of oxygen fugacity on the electrical conductivity of anhydrous and hydrous mantle minerals, which exhibit clearly different charge transport processes. In representative temperature and pressure environments, the hydrogen of nominally anhydrous minerals can tremendously enhance the electrical conductivity of the upper mantle and transition zone, and the influence of trace structural water (or hydrogen) is substantial. In combination with the geophysical data of magnetotelluric surveys, the laboratory-based electrical conductivity measurements can provide significant constraints to the water distribution in Earth’s interior.

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