Constraints on Seismic Models from Other Disciplines - Constraints on 3-D Seismic Models from Global Geodynamic Observables: Implications for the Global Mantle Convective Flow

Forte, A. M.; Simmons, N. A.; Grand, S. P.

doi:10.1016/b978-0-444-53802-4.00028-2

Cited by 32 publications

(68 citation statements)

References 178 publications

(265 reference statements)

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“…Figures c and d show the surface velocity predictions and asthenospheric match to azimuthal anisotropy, respectively, for my reference, dynamically consistent mantle flow model. The correlation of plate motions with the observations is fairly good at the 0.916 level, as noted, though improvements could likely be achieved by adjusting the density or viscosity structure [cf., Becker and O'Connell , ; Alisic et al ., ; Forte et al ., ]. The match of the LPO inferred from the dynamically consistent model to seismic anisotropy is somewhat worse (increase of average, angular misfit in oceanic plate regions,

{〈 Δ α 〉}_{o c}

, of 7

°

) compared to the prescribed plate motion model of Becker et al .…”

Section: Resultsmentioning

confidence: 99%

“…In the case of buoyancy driven flow, velocity amplitudes depend on the ratio of density anomalies and viscosity. Density anomalies in mantle circulation models are somewhat uncertain, for reasons including the possibly inaccurate representation of anomaly amplitudes by tomography, complexities of the mineral physics relationships as a function of temperature, pressure, and phase, and other effects of compositional heterogeneity [e.g., Forte et al, 2015]. Slab-only tests can therefore provide some insights into the slab-pull related, plate-scale flow independent of mantle density anomalies outside subduction zones [cf., Hager, 1984].…”

Section: 1002/2017gc006886mentioning

confidence: 99%

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Superweak asthenosphere in light of upper mantle seismic anisotropy

Becker

2017

Geochem Geophys Geosyst

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Earth's upper mantle includes a ∼200 km thick asthenosphere underneath the plates where viscosity and seismic velocities are reduced compared to the background. This zone of weakness matters for plate dynamics and may be required for the generation of plate tectonics itself. However, recent seismological and electromagnetic studies indicate strong heterogeneity in thinner layers underneath the plates which, if related to more extreme, global viscosity reductions, may require a revision of our understanding of mantle convection. Here, I use dynamically consistent mantle flow modeling and the constraints provided by azimuthal seismic anisotropy as well as plate motions to explore the effect of a range of global and local viscosity reductions. The fit between mantle flow model predictions and observations of seismic anisotropy is highly sensitive to radial and lateral viscosity variations. I show that moderate suboceanic viscosity reductions, to ∼0.01–0.1 times the upper mantle viscosity, are preferred by the fit to anisotropy and global plate motions, depending on layer thickness. Lower viscosities degrade the fit to azimuthal anisotropy. Localized patches of viscosity reduction, or layers of subducted asthenosphere, however, have only limited additional effects on anisotropy or plate velocities. This indicates that it is unlikely that regional observations of subplate anomalies are both continuous and indicative of dramatic viscosity reduction. Locally, such weak patches may exist and would be detectable by regional anisotropy analysis, for example. However, large‐scale plate dynamics are most likely governed by broad continent‐ocean asthenospheric viscosity contrasts rather than a thin, possibly high melt fraction layer.

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{〈 Δ α 〉}_{o c}

, of 7

°

) compared to the prescribed plate motion model of Becker et al .…”

Section: Resultsmentioning

confidence: 99%

Section: 1002/2017gc006886mentioning

confidence: 99%

Superweak asthenosphere in light of upper mantle seismic anisotropy

Becker

2017

Geochem Geophys Geosyst

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“…In Figure we compare the splitting orientations with mantle flow predictions [ Forte et al , ] based on the seismic‐geodynamic global tomography model TX2008 [ Simmons et al , ], using two different radial viscosity profiles, “V1” [ Mitrovica and Forte , ] and “V2” [ Forte et al , ]. The main difference between the two profiles in the upper mantle is the thickness of the high‐viscosity lithospheric layer: ∼100 km for V1 and ∼200 km for V2.…”

Section: Discussionmentioning

confidence: 99%

Variability and origin of seismic anisotropy across eastern Canada: Evidence from shear wave splitting measurements

et al. 2015

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Measurements of seismic anisotropy in continental regions are frequently interpreted with respect to past tectonic processes, preserved in the lithosphere as “fossil” fabrics. Models of the present‐day sublithospheric flow (often using absolute plate motion as a proxy) are also used to explain the observations. Discriminating between these different sources of seismic anisotropy is particularly challenging beneath shields, whose thick (≥200 km) lithospheric roots may record a protracted history of deformation and strongly influence underlying mantle flow. Eastern Canada, where the geological record spans ∼3 Ga of Earth history, is an ideal region to address this issue. We use shear wave splitting measurements of core phases such as SKS to define upper mantle anisotropy using the orientation of the fast‐polarization direction ϕ and delay time δt between fast and slow shear wave arrivals. Comparison with structural trends in surface geology and aeromagnetic data helps to determine the contribution of fossil lithospheric fabrics to the anisotropy. We also assess the influence of sublithospheric mantle flow via flow directions derived from global geodynamic models. Fast‐polarization orientations are generally ENE‐WSW to ESE‐WNW across the region, but significant lateral variability in splitting parameters on a ≤100 km scale implies a lithospheric contribution to the results. Correlations with structural geologic and magnetic trends are not ubiquitous, however, nor are correlations with geodynamically predicted mantle flow directions. We therefore consider that the splitting parameters likely record a combination of the present‐day mantle flow and older lithospheric fabrics. Consideration of both sources of anisotropy is critical in shield regions when interpreting splitting observations.

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“…Various studies have examined the dependence of seismic velocities on density in the mantle, and several of them have argued for a nonlinear relation between them (cf. Cammarano et al, 2003;Forte et al, 2015;Moulik & Ekstrom, 2016). We have tested a few cases where we use depth-dependent scalings (Model 2 of Steinberger and Calderwood (2006) with scaling reduced by a factor 2 in the uppermost 220 km; preferred scaling used by Cadek and Fleitout (1999)) and a radial viscosity structure to compute the geoid.…”

Section: Methodsmentioning

confidence: 99%

The Importance of Upper Mantle Heterogeneity in Generating the Indian Ocean Geoid Low

Ghosh

Thyagarajulu

Steinberger

2017

Geophysical Research Letters

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One of the most pronounced geoid lows on Earth lies in the Indian Ocean just south of the Indian peninsula. Several theories have been proposed to explain this geoid low, most of which invoke past subduction. Some recent studies have also argued that high‐velocity anomalies in the lower mantle coupled with low‐velocity anomalies in the upper mantle are responsible for these negative geoid anomalies. However, there is no general consensus regarding the source of this particular anomaly. We investigate the source of this geoid low by using models of density‐driven mantle convection. Our study is the first to successfully explain the occurrence of this anomaly using a global convection model driven by present‐day density anomalies derived from tomography. We test various tomography models in our flow calculations with different radial and lateral viscosity variations. Some of them produce a fairly high correlation to the global geoid, but only a few (SMEAN2, GyPSuM, SEMUCB, and LLNL‐JPS) could match the precise location and pattern of the geoid low in the Indian Ocean. The source of this low stems from a low‐density anomaly stretching from a depth of 300 km down to ∼900 km in the northern Indian Ocean region. This density anomaly potentially originates from material rising along the edge of the African Large Low Shear Velocity Province and moving toward the northeast, facilitated by the movement of the Indian plate in the same direction.

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Constraints on Seismic Models from Other Disciplines - Constraints on 3-D Seismic Models from Global Geodynamic Observables: Implications for the Global Mantle Convective Flow

Cited by 32 publications

References 178 publications

Superweak asthenosphere in light of upper mantle seismic anisotropy

Superweak asthenosphere in light of upper mantle seismic anisotropy

Variability and origin of seismic anisotropy across eastern Canada: Evidence from shear wave splitting measurements

The Importance of Upper Mantle Heterogeneity in Generating the Indian Ocean Geoid Low

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