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

Mapping Intimacies ◽

10.5194/egusphere-egu21-13788 ◽

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

We present a new radially anisotropic (&#958;)&#160;tomographic model for the upper mantle to transition zone depths derived from a large Rayleigh (~4.5 x 106&#160;paths) and Love (~0.7 x 106&#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 106&#160;paths) are taken from the Love and Rayleigh datasets and jointly inverted for isotropic Vs&#160;and&#160;&#958;. CRUST1.0 is used for crustal corrections and a model similar to PREM is used as a starting model. Vs&#160;and&#160;&#958;&#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&#160;&#958;(z) was derived. Synthetic tests show structures with radii of 400 km can be distinguished easily. Crustal perturbations of +/-10% to Vp, Vs&#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&#160;&#958;~1.06 below the Moho to&#160;&#958;~1 at ~275 km depth. At shallow depths beneath the oceans&#160;&#958;>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,&#160;&#958;<1. At depths &#8805;200 km and deeper depths below most of the backarc basins of the western Pacific&#160;&#958;<1. The signature of mid-ocean ridges vanishes at about 150 km depth in Vs&#160;while it extends much deeper in the&#160;&#958;&#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.

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

10.5194/egusphere-egu2020-11957 ◽

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

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.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&#8217;s LPO versus shear across this fabric (Hansen et al., EPSL 2016a, JGR 2016b).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.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.&#160; 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.

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

Geophysical Journal International ◽

10.1093/gji/ggab081 ◽

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.

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Integrated teleseismic studies of the southern Alberta upper mantle

Canadian Journal of Earth Sciences ◽

10.1139/e01-084 ◽

2002 ◽

Vol 39 (3) ◽

pp. 399-411 ◽

Cited By ~ 26

Author(s):

J Shragge ◽

M G Bostock ◽

C G Bank ◽

R M Ellis

Keyword(s):

Upper Mantle ◽

Velocity Structure ◽

Broad Band ◽

Receiver Functions ◽

P Wave ◽

Crustal Thickening ◽

Plate Motion ◽

Moho Depth ◽

Hydrous Minerals ◽

South Central

This paper presents results from a teleseismic experiment conducted across the Hearne Province in south-central Alberta. Data from an array of nine portable broad-band seismographs deployed along a 500 km NWSE array have been supplemented with recordings from two Canadian National Seismograph Network stations. P-wave delay times from 293 earthquakes have been inverted for upper-mantle velocity structure below the array. The recovered model reveals high velocities beneath much of the southern Hearne Province to depths of 200250 km, which are interpreted as deep-seated lithospheric structure. Contrary to recent tectonic models, these results suggest that the Hearne lithosphere has remained intact. In particular, it appears unlikely that evidence for extensive, lower crustal melting derives from lithospheric delamination. However, the results admit the possibility that high mantle conductivity, as revealed in magnetotelluric studies, originates through small volumes of connected hydrous minerals or other conductive species introduced during subduction. Decreased upper-mantle velocities at the northern end of the Medicine Hat block also pose challenges for the interpretation of differential subsidence across the region which may manifest distant forcing due to more recent subduction. Multievent SKS-splitting analysis yields an average polarization direction that is broadly consistent with both the orientation of fossil strain fields, related to ~ 1.8 Ga NWSE shortening, and North American absolute plate motion. Moho depth estimates from receiver functions are fairly uniform (~ 38 km) beneath northern stations but show crustal thickening (>40 km) within the Medicine Hat block to the south and are consistent with values from active-source profiling.

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Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling

Science Advances ◽

10.1126/sciadv.1601107 ◽

2016 ◽

Vol 2 (12) ◽

pp. e1601107 ◽

Cited By ~ 11

Author(s):

David B. Rowley ◽

Alessandro M. Forte ◽

Christopher J. Rowan ◽

Petar Glišović ◽

Robert Moucha ◽

...

Keyword(s):

East Pacific Rise ◽

Oceanic Lithosphere ◽

Plate Motion ◽

Mantle Flow ◽

Plate Boundaries ◽

Tectonic Plates ◽

Ocean Ridges ◽

East Pacific ◽

The Pacific ◽

Negative Buoyancy

Earth’s tectonic plates are generally considered to be driven largely by negative buoyancy associated with subduction of oceanic lithosphere. In this context, mid-ocean ridges (MORs) are passive plate boundaries whose divergence accommodates flow driven by subduction of oceanic slabs at trenches. We show that over the past 80 million years (My), the East Pacific Rise (EPR), Earth’s dominant MOR, has been characterized by limited ridge-perpendicular migration and persistent, asymmetric ridge accretion that are anomalous relative to other MORs. We reconstruct the subduction-related buoyancy fluxes of plates on either side of the EPR. The general expectation is that greater slab pull should correlate with faster plate motion and faster spreading at the EPR. Moreover, asymmetry in slab pull on either side of the EPR should correlate with either ridge migration or enhanced plate velocity in the direction of greater slab pull. Based on our analysis, none of the expected correlations are evident. This implies that other forces significantly contribute to EPR behavior. We explain these observations using mantle flow calculations based on globally integrated buoyancy distributions that require core-mantle boundary heat flux of up to 20 TW. The time-dependent mantle flow predictions yield a long-lived deep-seated upwelling that has its highest radial velocity under the EPR and is inferred to control its observed kinematics. The mantle-wide upwelling beneath the EPR drives horizontal components of asthenospheric flows beneath the plates that are similarly asymmetric but faster than the overlying surface plates, thereby contributing to plate motions through viscous tractions in the Pacific region.

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The structure of the oceanic lithosphere

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1971.0016 ◽

1971 ◽

Vol 268 (1192) ◽

pp. 619-619

Keyword(s):

Oceanic Crust ◽

Upper Mantle ◽

Oceanic Lithosphere ◽

Ocean Floor ◽

Mass Motion ◽

Sea Floor ◽

Ocean Ridges ◽

Velocity Gradients ◽

The Earth ◽

Sea Floor Spreading

Sea-floor spreading requires that new ocean floor be generated at mid-ocean ridges and that along with the underlying oceanic crust it move laterally away from its site of generation. In so far as it is unlikely that the 5 km thick oceanic crust moves independently of the underlying upper mantle, the horizontal mass motion associated with spreading extends at least some way into the mantle. The lithosphere is the crust and that part of the upper mantle to which it is mechanically coupled; together they form the brittle and relatively ‘strong’ outermost part of the Earth; velocity gradients within the lithosphere are negligible.

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Global imaging of the lithosphere-asthenosphere system

10.5194/egusphere-egu2020-6919 ◽

2020 ◽

Author(s):

Eric Debayle ◽

Yanick Ricard ◽

Stéphanie Durand ◽

Thomas Bodin

Keyword(s):

Large Scale ◽

Seismic Velocity ◽

3D Models ◽

Azimuthal Anisotropy ◽

Plate Motion ◽

Effect Of Temperature ◽

Mantle Flow ◽

Simultaneous Interpretation ◽

Velocity Models ◽

Scale Properties

Massive surface wave datasets constrain upper mantle seismic heterogeneities with horizontal wavelengths larger than 1000 km, allowing us to investigate the large-scale properties and alignment of olivine crystals in the lithosphere and asthenosphere. The azimuthal anisotropy projected onto the direction of present plate motion shows a very specific relation with the plate velocity. Plate-scale present-day deformation is remarkably well and uniformly recorded beneath plates moving faster than &#8764;4 cm/yr. Recent geodynamic models suggest that cold sinking instabilities tilted in the direction opposite to plate motion below fast plates could produce a pattern of large-scale azimuthal anisotropy consistent with our observations. Beneath slower plates, plate-motion aligned anisotropy is only observed locally, which suggests that the lithospheric motion does not control mantle flow below these plates.Radial anisotropy extends deeper beneath continents than beneath oceans, but we find no such difference for azimuthal anisotropy, suggesting that beneath most continents, the alignment of olivine crystal is preferentially horizontal and azimuthally random at large scale. As most continents are located on slow moving plates, this supports the idea that azimuthal anisotropy aligns at large scale with the present plate motion only for plates moving faster than &#8764;4 cm/yr.The same inversion also provides 3D models of seismic velocity and attenuation. The simultaneous interpretation of global 3D shear attenuation and velocity models has a great potential to decipher the effect of temperature, melt and composition on seismic observables. We will discuss our findings from the simultaneous interpretation of our latest models.

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Mantle-derived helium released through the Japan trench bend-faults

Scientific Reports ◽

10.1038/s41598-021-91523-6 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Jin-Oh Park ◽

Naoto Takahata ◽

Ehsan Jamali Hondori ◽

Asuka Yamaguchi ◽

Takanori Kagoshima ◽

...

Keyword(s):

Upper Mantle ◽

Large Scale ◽

Japan Trench ◽

Helium Isotope ◽

Normal Faults ◽

Circulation System ◽

Deep Penetration ◽

Oceanic Plate ◽

Seismic Reflection Data ◽

Reflection Data

AbstractPlate bending-related normal faults (i.e. bend-faults) develop at the outer trench-slope of the oceanic plate incoming into the subduction zone. Numerous geophysical studies and numerical simulations suggest that bend-faults play a key role by providing pathways for seawater to flow into the oceanic crust and the upper mantle, thereby promoting hydration of the oceanic plate. However, deep penetration of seawater along bend-faults remains controversial because fluids that have percolated down into the mantle are difficult to detect. This report presents anomalously high helium isotope (3He/4He) ratios in sediment pore water and seismic reflection data which suggest fluid infiltration into the upper mantle and subsequent outflow through bend-faults across the outer slope of the Japan trench. The 3He/4He and 4He/20Ne ratios at sites near-trench bend-faults, which are close to the isotopic ratios of bottom seawater, are almost constant with depth, supporting local seawater inflow. Our findings provide the first reported evidence for a potentially large-scale active hydrothermal circulation system through bend-faults across the Moho (crust-mantle boundary) in and out of the oceanic lithospheric mantle.

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

Communications Earth & Environment ◽

10.1038/s43247-021-00142-6 ◽

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.

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