scholarly journals Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere

2016 ◽  
Vol 113 (38) ◽  
pp. 10503-10506 ◽  
Author(s):  
Lars N. Hansen ◽  
Chao Qi ◽  
Jessica M. Warren
Keyword(s):  
Upper Mantle ◽  
Plate Tectonics ◽  
Large Scale ◽  
Flow Model ◽  
Seismic Anisotropy ◽  
Current Debate ◽  
Tectonic Plates ◽  
Midocean Ridges ◽  
Wide Range ◽  
Lithospheric Plates

Tectonic plates are a key feature of Earth’s structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which lithospheric plates couple to the convecting interior. Current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins. Agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the lithosphere, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary.

scholarly journals Large-scale variation in seismic anisotropy in the crust and upper mantle beneath Anatolia, Turkey

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Cédric P. Legendre ◽  
Li Zhao ◽  
Tai-Lin Tseng
Keyword(s):  
Upper Mantle ◽  
Large Scale ◽  
Seismic Anisotropy ◽  
Crust And Upper Mantle ◽  
Wave Splitting ◽  
Mantle Processes ◽  
Open Question ◽  
Subduction System ◽  
Anisotropic Phase ◽  
Regional Tectonics

AbstractThe average anisotropy beneath Anatolia is very strong and is well constrained by shear-wave splitting measurements. However, the vertical layering of anisotropy and the contribution of each layer to the overall pattern is still an open question. Here, we construct anisotropic phase-velocity maps of fundamental-mode Rayleigh waves for the Anatolia region using ambient noise seismology and records from several regional seismic stations. We find that the anisotropy patterns in the crust, lithosphere and asthenosphere beneath Anatolia have limited amplitudes and are generally consistent with regional tectonics and mantle processes dominated by the collision between Eurasia and Arabia and the Aegean/Anatolian subduction system. The anisotropy of these layers in the crust and upper mantle are, however, not consistent with the strong average anisotropy measured in this area. We therefore suggest that the main contribution to overall anisotropy likely originates from a deep and highly anisotropic region round the mantle transition zone.

scholarly journals Principles of structural geology on rocky planets

2019 ◽  
Vol 56 (12) ◽  
pp. 1437-1457
Author(s):  
Christian Klimczak ◽  
Paul K. Byrne ◽  
A.M. Celâl Şengör ◽  
Sean C. Solomon
Keyword(s):  
Structural Geology ◽  
Plate Tectonics ◽  
Large Scale ◽  
The Other ◽  
Tectonic Structures ◽  
Tectonic Plates ◽  
Tectonic Processes ◽  
Planetary Tectonics ◽  
Rocky Planets ◽  
Mechanisms Of Deformation

Although Earth is the only known planet on which plate tectonics operates, many small- and large-scale tectonic landforms indicate that deformational processes also occur on the other rocky planets. Although the mechanisms of deformation differ on Mercury, Venus, and Mars, the surface manifestations of their tectonics are frequently very similar to those found on Earth. Furthermore, tectonic processes invoked to explain deformation on Earth before the recognition of horizontal mobility of tectonic plates remain relevant for the other rocky planets. These connections highlight the importance of drawing analogies between the rocky planets for characterizing deformation of their lithospheres and for describing, applying appropriate nomenclature, and understanding the formation of their resulting tectonic structures. Here we characterize and compare the lithospheres of the rocky planets, describe structures of interest and where we study them, provide examples of how historic views on geology are applicable to planetary tectonics, and then apply these concepts to Mercury, Venus, and Mars.

On the forcing mechanism for the H2-driven deep biosphere

2008 ◽  
Vol 7 (2) ◽  
pp. 157-167 ◽  
Author(s):  
Helge Hellevang
Keyword(s):  
Hydrogen Production ◽  
Ferrous Iron ◽  
Plate Tectonics ◽  
Large Scale ◽  
Ocean Water ◽  
Deep Biosphere ◽  
The Earth ◽  
Hydrothermal Convection ◽  
Lithospheric Plates ◽  
Reduced State

AbstractHeat produced in the mantle and core of the Earth by the decay of radioactive elements and mineral fusion results in large-scale mantle convection. The outer shell of the Earth that floats on the convective mantle is divided into rigid lithospheric plates. Subduction of dense cold plates into the mantle leads to plate tectonics. At divergent plate margins, heat is dissipated through hydrothermal convection cells. As ocean water is entrained into hydrothermal cells it interacts with fresh magmatic rocks and liberates ferrous iron. This iron reduces the ocean water to such an extent that it decomposes and forms hydrogen. Molecular hydrogen, as the most reduced component in the system, forms a basal component to a deep dark biosphere powered by metastable redox gradients. In this paper we review the driving force behind a hydrogen-driven deep biosphere. We present abundant observations of hydrogen produced at natural hydrothermal settings as well as in laboratory experiments. The key mineral reactions responsible for the bulk of this hydrogen production are then presented. A division of the reaction progression into an oxidized state and a reduced state is suggested. The amount of hydrogen produced is insignificant in the oxidized state whereas it becomes proportional to the amount of ferrous iron oxidized in the reduced state. The bulk of basalt-hosted aquifers are expected to reside in the oxidized state because of the low content of ferrous minerals, whereas abundant olivine in ultramafic-hosted systems is responsible for large-scale hydrogen production. Today the majority of the seafloor is basaltic. The Archean seafloor on the other hand consisted of fewer ultramafic exposures, but was dominated by ultramafic magnesium-rich lavas with a higher potential for hydrogen production than the present seafloor.

The formation of viscous anisotropy in the asthenosphere and its effect on plate tectonics

2020 ◽  
Author(s):  
Agnes Kiraly ◽  
Clinton P. Conrad ◽  
Lars N. Hansen ◽  
Menno Fraters
Keyword(s):  
Upper Mantle ◽  
Effective Viscosity ◽  
Plate Tectonics ◽  
Large Scale ◽  
Plate Motion ◽  
Shear Direction ◽  
Viscosity Change ◽  
Macro Scale ◽  
Geodynamic Processes ◽  
Anisotropic Viscosity

<p>Developing an appropriate characterization of upper mantle viscosity structure presents one of the biggest challenges for understanding geodynamic processes in the upper mantle. This is because different creep mechanisms become activated depending on depth, accumulated strain, and applied stress, and other factors such grain size and anisotropic fabric can change as the deformation develops, changing the effective viscosity. Here we focus on the relationship between anisotropic fabric development and viscous anisotropy.</p><p>Under applied shear, olivine crystals, which form a large proportion of the asthenosphere, rotate towards the shear direction and accumulate a lattice preferred orientation (LPO) parallel to the macroscopic deformation. On a large scale, LPO can be observed through the propagation of seismic waves because of the anisotropic elastic properties of olivine. As olivine is anisotropic in its viscous properties, this developing texture within the asthenosphere can affect the macro-scale viscosity of the asthenosphere. This behavior has been detected in rock mechanics measurements on pure olivine aggregates, showing more than an order magnitude of viscosity change between shear parallel to the olivine aggregate’s LPO versus shear across this fabric (Hansen et al., EPSL 2016a, JGR 2016b).</p><p>Here, we use numerical models developed first in MATLAB and then implemented into the mantle convection code ASPECT. These models incorporate both anisotropic fabric development and anisotropic viscosity, both calibrated according to laboratory measurements on slip system activities of olivine aggregates (Hansen et al., JGR 2016b), to better understand the coupling between the large-scale formation of LPO textures and changes in asthenospheric viscosity.</p><p>The modeling results allows us to discuss the role of anisotropic viscosity on the processes of plate tectonics. An asthenosphere with a well-developed LPO becomes weak parallel to its texture, allowing for increasing plate velocity at the surface, for a given applied driving force.  On the other hand, this fabric resists abrupt changes in the direction of plate motion because the effective viscosity is elevated for shear perpendicular to the developed LPO. This increased resistance to fabric-perpendicular shear also decreases strain rates, which slows texture development. This means that asthenospheric fabric can impede changes in plate motion direction for periods of over 10 Myrs. However, the same well-developed texture in the asthenosphere could enhance the initiation of subduction or lithospheric gravitational instabilities as vertical deformation is favored across a sub-lithospheric olivine fabric, and the sheared fabric can quickly rotate into a vertical LPO. These end-member cases examining shear-deformation across a formed asthenospheric fabric illustrate the importance of olivine fabrics, and their associated viscous anisotropy, for a variety of geodynamic processes.</p>

Concluding remarks

1978 ◽  
Vol 288 (1350) ◽  
pp. 235-236
Keyword(s):  
Large Scale ◽  
Impact Craters ◽  
Small Scale ◽  
Triple Junctions ◽  
Tectonic Plates ◽  
Wide Range ◽  
Lunar Impact

The idea of this meeting came to the organizers from the type of considerations set out by Dr Kelly in his opening remarks, namely that we see many apparent similarities between phenomena of large scale in geophysics and phenomena of small scale in metallurgy and we would like to understand their significance. Dr Kelly mentioned triple junctions between tectonic plates and between metal grains. Another example, which goes back many years, is that structures in mountains can be simulated by flow in materials such as plasticine, and Dr King has showed us how patterns of faulting look much the same over a wide range of scales. Yet another, by now well known, example is that lunar impact craters look almost indistinguishable over a range of sizes from a few micrometres up to hundreds of kilometres. Dr Kelly remarked to me this morning that the similarity between geophysics and metallurgy extends to terminology, for the geophysicist has his fault and the metallurgist his defect.

Performance Study of Load Flow Algorithms in Well and Ill-Conditioned Systems

2017 ◽  
Vol 8 (2) ◽  
pp. 414
Author(s):  
Lea Tien Tay ◽  
Kai Seng Chieng
Keyword(s):  
Large Scale ◽  
Flow Model ◽  
Load Flow ◽  
Second Order ◽  
Performance Study ◽  
Large Scale System ◽  
Wide Range ◽  
Flow Algorithm ◽  
Ill Conditioning ◽  
Newton Raphson

Existing transmission systems are classified as either ill systems or healthy systems. Most of the load flow algorithms works proficiently under well-conditioned systems. However, some of those algorithms fail to produce the accurate results for ill-conditioned systems. This paper investigates the performance of eight load flow algorithms based on the conventional Newton-Raphson, Fast-decoupled and Second-order Load Flow methods for a wide range of electrical bus system sizes. Tests are carried out for each load flow algorithm on six different standard bus systems, each with five different ill-conditioning levels. The results show that improved load flow model with constant Jacobian has advantages over the conventional load flow approach in both well and ill-conditioned system, especially for large-scale system.

scholarly journals Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations

2021 ◽  
Vol 225 (3) ◽  
pp. 2097-2119
Author(s):  
Brandon P VanderBeek ◽  
Manuele Faccenda
Keyword(s):  
Subduction Zone ◽  
Upper Mantle ◽  
Large Scale ◽  
Seismic Anisotropy ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Zone Model ◽  
Upper Mantle Anisotropy ◽  
Delay Times ◽  
Mantle Anisotropy

SUMMARY Despite the well-established anisotropic nature of Earth’s upper mantle, the influence of elastic anisotropy on teleseismic P-wave imaging remains largely ignored. Unmodelled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities. Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy. However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as synthetic testing has been restricted to the recovery of simplified block-like structures using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm). Here, we present a modified parametrization for imaging arbitrarily oriented hexagonal anisotropy and test the method by reconstructing geodynamic simulations of subduction. Our inversion approach allows for isotropic starting models and includes approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Synthetic seismic data are created by propagating teleseismic waves through an elastically anisotropic subduction zone model created via petrologic-thermomechanical modelling. Delay times across a synthetic seismic array are measured using conventional cross-correlation techniques. We find that our imaging algorithm is capable of resolving large-scale features in subduction zone anisotropic structure (e.g. toroidal flow pattern and dipping fabrics associated with the descending slab). Allowing for arbitrarily oriented anisotropy also results in a more accurate reconstruction of isotropic slab structure. In comparison, models created assuming isotropy or only azimuthal anisotropy contain significant isotropic and anisotropic imaging artefacts that may lead to spurious interpretations. We conclude that teleseismic P-wave traveltimes are a useful observable for probing the 3-D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies particularly in subduction settings.

The effect of composite rheology on mantle convection models with plate-like behavior

2020 ◽  
Author(s):  
Maelis Arnould ◽  
Tobias Rolf
Keyword(s):  
Deformation Mechanism ◽  
Mantle Convection ◽  
Plate Tectonics ◽  
Large Scale ◽  
Seismic Anisotropy ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Mantle Flow ◽  
Dynamic Topography ◽  
Lattice Preferred Orientation

<p>The coupling between mantle convection and plate tectonics results in mantle flow patterns and properties which can be characterized with different seismic methods. In particular, the presence of mantle seismic anisotropy in the uppermost mantle suggests the existence of mineral Lattice-Preferred Orientation (LPO) caused by asthenospheric flow. Dislocation creep, which implies non-Newtonian mantle rheology, has been identified as a deformation mechanism responsible for such LPO leading to seismic anisotropy. While it has been proposed that the use of a composite rheology (with both diffusion and dislocation creep) significantly impacts the planform of convection and thus the resulting tectonic behavior at the surface, large-scale mantle convection studies have typically assumed diffusion creep (Newtonian rheology) as the only deformation mechanism, due to computational limitations.</p><p>Here, we investigate the role of composite rheology on mantle convection with self-consistent plate-like behavior using the code StagYY in 2D annulus (Hernlund and Tackley, 2008). We quantify the spatial distribution of dislocation creep in the mantle in models characterized by different transitional stresses between Newtonian and non-Newtonian rheology. Such models are built on previous viscoplastic cases featuring Earth-like plate velocities, surface heat flow and topography with Newtonian rheology (Arnould et al., 2018). We then investigate how composite rheology impacts the planform of convection and the style of plate-like behavior.</p><p> </p><p><strong>References:</strong></p><p>Hernlund, J. W., & Tackley, P. J. (2008). Modeling mantle convection in the spherical annulus. Physics of the Earth and Planetary Interiors, 171(1-4), 48-54.</p><p>Arnould, M., Coltice, N., Flament, N., Seigneur, V., & Müller, R. D. (2018). On the scales of dynamic topography in whole‐mantle convection models. Geochemistry, Geophysics, Geosystems, 19(9), 3140-3163.</p>

Long-lived low Th/U Pacific-type isotopic mantle domain

2021 ◽  
Author(s):  
Xijun Liu ◽  
Zhiguo Zhang ◽  
Pengde Liu ◽  
Yujia Song ◽  
Yao Xiao
Keyword(s):  
Upper Mantle ◽  
Plate Tectonics ◽  
Large Scale ◽  
Mafic Rock ◽  
Global Scale ◽  
Data Set ◽  
Provenance Data ◽  
The Pacific ◽  
Long Time ◽  
Material Circulation

<p>    The presence of Pacific-type and Indian-type mid-ocean ridge (MORB) isotopic source domains in the upper mantle is a clear manifestation of global-scale mantle compositional heterogeneities. The Indian-type mantle domain is a long-lived feature that can be traced back to, at least, the Palaeozoic Tethyan mantle domain. Little temporal constraints currently exist, however, regarding the longevity of Pacific-type mantle domain. The extinct Paleo-Asia Ocean (PAO), a subsidiary ocean of the Panthalassic Ocean that formed during the breakup of the Rodinia Supercontinent in Mesoproterozoic to Neoproterozoic, can provide a solution to this dilemma. Here, we report the first complete geochemical and Sr, Nd and high-precision Pb isotopic data set for representative mafic rock samples from ophiolites representing remnants of the PAO basement ranging in age from 275 to 624Ma to constrain the composition of their mantle provenance. Data suggest that the sub-PAO mantle has a similar long time-integrated, high Sm/Nd ratio as the global depleted upper mantle, but also shows typical Pacific MORB-like Pb isotopic compositions with lower <sup>207</sup>Pb/<sup>204</sup>Pb<sub>(t) </sub>and <sup>208</sup>Pb/<sup>204</sup>Pb<sub>(t)</sub> for given <sup>206</sup>Pb/<sup>204</sup>Pb<sub>(t)</sub> ratios, and low radiogenic <sup>208</sup>Pb*/<sup>206</sup>Pb*, indicating a long time-integrated, low Th/U ratios. Thus, the Pacific-type mantle domain, like the Indian-type mantle domain, is a long-lived secular mantle domain that can be traced back to early Paleozoic or even to the Neoproterozoic. Data further indicate that the Nd and Pb isotopic distinction between such two large-scale and long-term mantle domains is due to the different evolutionary and tectonic histories of the circum-Pacific (PAO, Paleo- and modern Pacific) and sub-Tethys-Indian oceanic mantle realms. The Panthalassic-Pacific ocean realm had remarkable permanency existing as a big ocean at lease throughout the Phanerozoic, that implies that continental materials were limit to recycle into underlying mantle, thus the underlying mantle was relative free of the continental material contamination and then produce the low time-integrated Th/U Pacific-type mantle domain. In contrast, the break-up of the Gondwana supercontinent makes the Tethys realms to experience repeated opening and closures, which transferred large volume of continental materials into the underlying mantle and then produce the high Th/U Indian-type mantle domain. Our results indicate that the high Sm/Nd and low Th/U ratio of Pacific-type mantle domain most likely are an inherited, long-standing intrinsic feature of the depleted upper mantle derived from the Earth's primordial mantle with less contamination of continental materials. In contrast, the large-scale and long-lived Indian-type mantle heterogeneity is produced by plate tectonic-driven continental material circulation in the upper mantle. Such a genetic link between plate tectonics and mantle chemical geodynamics is crucial to our understanding of how the Earth system works.</p><p>    This study was financially supported by the National Natural Science Foundation of China (92055208，41772059) and the CAS “Light of West China” Program (2018-XBYJRC-003).</p>

A Discussion on natural strain and geological structure - Magnetic, seismic, and other anisotropic properties of rock fabrics

1976 ◽  
Vol 283 (1312) ◽  
pp. 55-68 ◽  
Keyword(s):  
Upper Mantle ◽  
Large Scale ◽  
Seismic Anisotropy ◽  
Geological Structure ◽  
Magnetic Fabric ◽  
Dielectric Anisotropy ◽  
Bulk Property ◽  
Crust And Upper Mantle ◽  
Fabric Analysis ◽  
Individual Grains

Magnetic fabric, as a resultant property, summed over individual grains in a rock, stands apart from other, bulk property measurements (e.g. seismic, thermal, dielectric anisotropy), which treat the rock as a continuum. Thus, magnetic anisotropy can be more directly related to preferred orientation of grains in a rock than can bulk property measurements. The latter, however, may permit remote, geophysical determinations of large scale ‘fabric’ features. These contrasting aspects of fabric determination are discussed, drawing examples of magnetic fabric analysis from studies on naturally and experimentally deformed rocks, and seismic anisotropy from refraction studies of the crust and upper mantle.

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