scholarly journals Feedbacks between a non-Newtonian upper mantle, mantle viscosity structure and mantle dynamics

2020 ◽  
Vol 224 (2) ◽  
pp. 961-972
Author(s):  
A G Semple ◽  
A Lenardic
Keyword(s):  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Mantle Flow ◽  
Mantle Dynamics ◽  
Plate Motions ◽  
Low Viscosity ◽  
Mantle Viscosity ◽  
Flow Feature ◽  
Mantle Rheology

SUMMARY Previous studies have shown that a low viscosity upper mantle can impact the wavelength of mantle flow and the balance of plate driving to resisting forces. Those studies assumed that mantle viscosity is independent of mantle flow. We explore the potential that mantle flow is not only influenced by viscosity but can also feedback and alter mantle viscosity structure owing to a non-Newtonian upper-mantle rheology. Our results indicate that the average viscosity of the upper mantle, and viscosity variations within it, are affected by the depth to which a non-Newtonian rheology holds. Changes in the wavelength of mantle flow, that occur when upper-mantle viscosity drops below a critical value, alter flow velocities which, in turn, alter mantle viscosity. Those changes also affect flow profiles in the mantle and the degree to which mantle flow drives the motion of a plate analogue above it. Enhanced upper-mantle flow, due to an increasing degree of non-Newtonian behaviour, decreases the ratio of upper- to lower-mantle viscosity. Whole layer mantle convection is maintained but upper- and lower-mantle flow take on different dynamic forms: fast and concentrated upper-mantle flow; slow and diffuse lower-mantle flow. Collectively, mantle viscosity, mantle flow wavelengths, upper- to lower-mantle velocities and the degree to which the mantle can drive plate motions become connected to one another through coupled feedback loops. Under this view of mantle dynamics, depth-variable mantle viscosity is an emergent flow feature that both affects and is affected by the configuration of mantle and plate flow.

scholarly journals The impact of rheological uncertainty on dynamic topography predictions

Solid Earth ◽  
2019 ◽  
Vol 10 (6) ◽  
pp. 2167-2178 ◽  
Author(s):  
Ömer F. Bodur ◽  
Patrice F. Rey
Keyword(s):  
Mantle Convection ◽  
Numerical Experiments ◽  
Mantle Flow ◽  
Dynamic Topography ◽  
Density Anomaly ◽  
Low Viscosity ◽  
Local Reduction ◽  
Dynamic Components ◽  
Mantle Rheology ◽  
The Impact

Abstract. Much effort is being made to extract the dynamic components of the Earth's topography driven by density heterogeneities in the mantle. Seismically mapped density anomalies have been used as an input into mantle convection models to predict the present-day mantle flow and stresses applied on the Earth's surface, resulting in dynamic topography. However, mantle convection models give dynamic topography amplitudes generally larger by a factor of ∼2, depending on the flow wavelength, compared to dynamic topography amplitudes obtained by removing the isostatically compensated topography from the Earth's topography. In this paper, we use 3-D numerical experiments to evaluate the extent to which the dynamic topography depends on mantle rheology. We calculate the amplitude of instantaneous dynamic topography induced by the motion of a small spherical density anomaly (∼100 km radius) embedded into the mantle. Our experiments show that, at relatively short wavelengths (<1000 km), the amplitude of dynamic topography, in the case of non-Newtonian mantle rheology, is reduced by a factor of ∼2 compared to isoviscous rheology. This is explained by the formation of a low-viscosity channel beneath the lithosphere and a decrease in thickness of the mechanical lithosphere due to induced local reduction in viscosity. The latter is often neglected in global mantle convection models. Although our results are strictly valid for flow wavelengths less than 1000 km, we note that in non-Newtonian rheology all wavelengths are coupled, and the dynamic topography at long wavelengths will be influenced.

Mantle convection beneath East Asia under global mantle convection spherical framework

2020 ◽  
Author(s):  
Qunfan Zheng ◽  
Huai Zhang
Keyword(s):  
Tibetan Plateau ◽  
East Asia ◽  
Upper Mantle ◽  
Mantle Convection ◽  
Mantle Flow ◽  
The Tibetan Plateau ◽  
Mantle Structure ◽  
Lithospheric Deformation ◽  
Mantle Dynamics ◽  
Flow Models

&lt;p&gt;East Asia is a tectonically active area on earth and has a complicated lithospheric deformation due to the western Indo-Asian continental collision and the eastern oceanic subduction mainly from Pacific plate. Till now, mantle dynamics beneath this area is not well understood due to its complex mantle structure, especially in the framework of global spherical mantle convection. Hence, a series of numerical models are conducted in this study to reveal the key controlling parameters in shaping the present-day observed mantle structure beneath East Asia under 3-D global mantle flow models. Global mantle flow models with coarse mesh are firstly applied to give a rough constraint on global mantle convection. The detailed description of upper mantle dynamics of East Asia is left with regional refined mesh. A power-law rheology and absolute plate field are applied subsequently to get a better constraint on the related regional mantle rheological structure and surficial boundary conditions. Thus, the refined and reasonable velocity and stress distributions of upper mantle beneath East Asia at different depths are retrieved based on our 3-D global mantle flow simulations. The derived large shallow mantle flow beneath the Tibetan Plateau causes significant lithospheric shear drag and dynamic topography that result in prominent tectonic evolution of this area. And the Indo&amp;#8211;Asian collision may have induced mantle flow beneath the Indian plate and the different velocity structures between the asthenosphere and lithosphere indicate the shear drag of asthenospheric mantle. That may explain the reason that Indo&amp;#8211;Asian collision has occurred for 50 Ma, and this collision can still continue to accelerate uplift in the Tibetan plateau. Finally, we also consider the possible implementations of 3-D numerical simulations combined with global lithosphere and deep mantle dynamics so as to discuss the relevant influences.&lt;/p&gt;

Constraining dynamic models in North America using the static gravity field

2020 ◽  
Author(s):  
Jesse Reusen ◽  
Bart Root ◽  
Javier Fullea ◽  
Zdenek Martinec ◽  
Wouter van der Wal
Keyword(s):  
Gravity Anomaly ◽  
Gravity Field ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Dynamic Models ◽  
Gravity Anomalies ◽  
Mantle Flow ◽  
Seismic Velocities ◽  
Crust And Upper Mantle

&lt;p&gt;The negative anomaly present in the static gravity field near Hudson Bay bears striking resemblance to the area depressed by the Laurentide ice sheet during the Last Glacial Maximum, suggesting that it is at least partly due to Glacial Isostatic Adjustment (GIA), but mantle convection and density anomalies in the crust and the upper mantle are also expected to contribute. At the moment, the contribution of GIA to this anomaly is still disputed. Estimates, which strongly depend on the viscosity of the mantle, range from 25 percent to more than 80 percent. Our objective is to find the contributions from GIA and mantle convection, after correcting for density anomalies in the topography, crust and upper mantle. The static gravity field has the potential to constrain the viscosity profile which is the most uncertain parameter in GIA and mantle convection models. A spectral method is used to transform 3D spherical density models of the crust into gravity anomalies. Density anomalies in the lithosphere are estimated so that isostatic compensation is reached at a depth of 300 km. The dynamic processes of mantle flow are corrected for before isostasy is assumed. Upper and lower mantle viscosities are varied so that the gravity anomaly predicted from the dynamic models matches the residual gravity anomaly. We consider uncertainties due to the crustal model, the lithosphere-asthenosphere boundary (LAB), the conversion from seismic velocities to density and the ice history used in the GIA model. The best fit is found for lower mantle viscosities &gt;10&lt;sup&gt;22&lt;/sup&gt; Pa s.&lt;/p&gt;

scholarly journals Impact of upper mantle convection on lithosphere hyper-extension and subsequent convergence-induced subduction

2020 ◽  
Author(s):  
Lorenzo G. Candioti ◽  
Stefan M. Schmalholz ◽  
Thibault Duretz
Keyword(s):  
Upper Mantle ◽  
Mantle Convection ◽  
Numerical Models ◽  
Unit Length ◽  
Mantle Flow ◽  
Suction Force ◽  
Continuous Simulation ◽  
Passive Margins ◽  
Low Viscosity ◽  
Lower Viscosity

Abstract. We present two-dimensional thermo-mechanical numerical models of coupled lithosphere-mantle deformation, considering the upper mantle down to a depth of 660 km. We consider visco-elasto-plastic deformation and for the lithospheric and upper mantle a combination of diffusion, dislocation and Peierls creep. Mantle densities are calculated from petrological phase diagrams (Perple_X) for a Hawaiian pyrolite. The model generates a 120 Myrs long geodynamic cycle of subsequent extension (30 Myrs), cooling (70 Myrs) and convergence (20 Myrs) in a single and continuous simulation with explicitly modelling convection in the upper mantle. During lithosphere extension, the models generate an approximately 400 km wide basin of exhumed mantle bounded by hyper-extended passive margins. The models show that considering only the thermal effects of upper mantle convection by using an effective thermal conductivity generates results of lithosphere hyper-extension that are similar to the ones of models that explicitly model the convective flow. Applying a lower viscosity limit of 5 × 1020 Pa s suppresses convection and generates results different to the ones for simulations with a low viscosity asthenosphere having minimal viscosity of approximately 1019 Pa s. During cooling without far-field deformation, no subduction of the exhumed mantle is spontaneously initiated. Density differences between lithosphere and mantle are too small to generate a buoyancy force exceeding the mechanical strength of the lithosphere. The extension and cooling stages generate self-consistently a structural and thermal inheritance for the subsequent convergence stage. Convergence initiates subduction of the exhumed mantle at the transition to the hyper-extended margins. The main mechanism of subduction initiation is thermal softening for a plate driving force (per unit length) of approximately 15 TN m−1. If convection in the mantle is suppressed by high effective thermal conductivities or high, lower viscosity limits, then subduction initiates at both margins leading to divergent double-slab subduction. Convection in the mantle assists to generate a single-slab subduction at only one margin, likely due to mantle flow which exerts an additional suction force on the lithosphere. The first-order geodynamic processes simulated in the geodynamic cycle of subsequent extension, cooling and convergence are applicable to orogenies that resulted from the opening and closure of embryonic oceans bounded by magma-poor hyper-extended passive margins, which might have been the case for the Alpine orogeny.


The interplay between recycled and primordial heterogeneities: predicting Earth's mantle dynamics via numerical modeling

2021 ◽  
Author(s):  
Matteo Desiderio ◽  
Anna J. P. Gülcher ◽  
Maxim D. Ballmer
Keyword(s):  
Lower Mantle ◽  
Mantle Convection ◽  
Large Scale ◽  
Numerical Models ◽  
Bulk Composition ◽  
Oceanic Lithosphere ◽  
Mantle Dynamics ◽  
Density Anomaly ◽  
Mantle Mineral ◽  
Intrinsic Strength

&lt;p&gt;According to geochemical and geophysical observations, Earth's lower mantle appears to be strikingly heterogeneous in composition. An accurate interpretation of these findings is critical to constrain Earth's bulk composition and long-term evolution. To this end, two main models have gained traction, each reflecting a different style of chemical heterogeneity preservation: the 'marble cake' and 'plum pudding' mantle. In the former, heterogeneity is preserved in the form of narrow streaks of recycled oceanic lithosphere, stretched and stirred throughout the mantle by convection. In the latter, domains of intrinsically strong, primordial material (enriched in the lower-mantle mineral bridgmanite) may resist convective entrainment and survive as coherent blobs in the mid mantle. Microscopic scale processes certainly affect macroscopic properties of mantle materials and thus reverberate on large-scale mantle dynamics. A cross-disciplinary effort is therefore needed to constrain present-day Earth structure, yet countless variables remain to be explored. Among previous geodynamic studies, for instance, only few have attempted to address how the viscosity and density of recycled and primordial materials affect their mutual mixing and interaction in the mantle.&lt;/p&gt;&lt;p&gt;Here, we apply the finite-volume code &lt;strong&gt;STAGYY&lt;/strong&gt; to model thermochemical convection of the mantle in a 2D spherical-annulus geometry. All models are initialized with a lower, primordial layer and an upper, pyrolitic layer (i.e., a mechanical mixture of basalt and harzburgite), as is motivated by magma-ocean solidification studies. We explore the effects of material properties on the style of mantle convection and heterogeneity preservation. These parameters include (i) the intrinsic strength of basalt (viscosity), (ii) the intrinsic density of basalt, and (iii) the intrinsic strength of the primordial material.&lt;/p&gt;&lt;p&gt;Our preliminary models predict a range of different mantle mixing styles. A 'marble cake'-like regime is observed for low-viscosity primordial material (~30 times weaker than the ambient mantle), with recycled oceanic lithosphere preserved as streaks and thermochemical piles accumulating near the core-mantle boundary. Conversely, 'plum pudding' primordial blobs are also preserved when the primordial material is relatively strong, in addition to the 'marble cake' heterogeneities mentioned above. Most notably, however, the rheology and the density anomaly of basalt affect the appearance of both recycled and primordial heterogeneities. In particular, they control the stability, size and geometry of thermochemical piles, the enhancement of basaltic streaks in the mantle transition zone, and they influence the style of primordial material preservation. These results indicate the important control that the physical properties of mantle constituents exert on the style of mantle convection and mixing over geologic time. Our numerical models offer fresh insights into these processes and may advance our understanding of the composition and structure of Earth's lower mantle.&lt;/p&gt;

scholarly journals Numerical Modeling of Mantle Flow Beneath Madagascar to Constrain Upper Mantle Rheology Beneath Continental Regions

2020 ◽  
Vol 125 (2) ◽  
Author(s):  
T. A. Rajaonarison ◽  
D. S. Stamps ◽  
S. Fishwick ◽  
S. Brune ◽  
A. Glerum ◽  
...  
Keyword(s):  
Numerical Modeling ◽  
Upper Mantle ◽  
Mantle Flow ◽  
Mantle Rheology

Deep Mantle Dynamics in East Asia: Numerical Simulation of Mantle Convection Based on Seismic Tomography

2020 ◽  
Author(s):  
Huai Zhang ◽  
Qunfan Zheng ◽  
Zhen Zhang
Keyword(s):  
East Asia ◽  
Seismic Tomography ◽  
Upper Mantle ◽  
Continental Collision ◽  
Indian Plate ◽  
Mantle Flow ◽  
Lithospheric Deformation ◽  
Mantle Dynamics ◽  
Asthenospheric Mantle ◽  
Deep Mantle

&lt;p&gt;East Asia is a tectonically active area on earth and has a complicated lithospheric deformation due to the western continental collision from the cratonic Indian plate and the eastern oceanic subduction mainly from Pacific plate. Studies have suggested that the Indo&amp;#8211;Asian continental collision may have driven significant lateral mantle flow, but the velocity, range and effect of the mantle flow remain uncertain. Hence, a series of 3-D numerical models are conducted in this study to reveal the impacts of the Indo&amp;#8211;Asian collision on mantle dynamics beneath the East Asia, especially on the asthenospheric mantle. Global model domain encompasses the lithosphere, upper mantle and the lower mantle with different viscosity for each layer. A global temperature structure built from seismic tomography and absolute plate field are applied subsequently to get a better constraint of the initial temperature condition and surficial velocity boundary condition. Thus, the reasonable velocity and temperature distributions of upper mantle beneath East Asia at different depths are retrieved based on our 3-D global mantle flow simulations, and the key controlling parameters in shaping the present-day observed mantle structure are investigated. The results show different scales of convection beneath East Asia.&lt;/p&gt;&lt;p&gt;Our results suggest that Indo&amp;#8211;Asian collision may have induced mantle flow beneath the Indian plate and the different velocity structures between the asthenosphere and lithosphere indicate the shear drag of asthenospheric mantle. That may explain the reason that Indo&amp;#8211;Asian collision has occurred since 50 Ma, and this collision can still continue to accelerate in the Tibetan Plateau. The simulation results also show the lithospheric delamination and the induced mantle upwelling, which is consistent with the general understanding from previous observations. The Indian lithosphere and its asthenosphere move northward, while the Yunnan lithosphere and its asthenosphere move southward, that may reflect the differences in deep mantle dynamics between the eastern and western Himalayan Syntaxis.&lt;/p&gt;

Upper mantle viscosity structure and lithospheric thickness of Antarctica inferred from recent seismic models

2020 ◽  
Author(s):  
Douglas Wiens ◽  
Andrew Lloyd ◽  
Weisen Shen ◽  
Andrew Nyblade ◽  
Richard Aster ◽  
...  
Keyword(s):  
Antarctic Peninsula ◽  
Upper Mantle ◽  
Ice Sheet ◽  
West Antarctica ◽  
Lithospheric Thickness ◽  
Isostatic Adjustment ◽  
Low Viscosity ◽  
Mantle Viscosity ◽  
Seismic Models ◽  
The Antarctic

&lt;p&gt;Upper mantle viscosity structure and lithospheric thickness control the solid Earth response to variations in ice sheet loading. These parameters vary significantly across Antarctica, leading to strong regional differences in the timescale of glacial isostatic adjustment (GIA), with important implications for ice sheet models. &amp;#160;We estimate upper mantle viscosity structure and lithospheric thickness using two new seismic models for Antarctica, which take advantage of temporary broadband seismic stations deployed across Antarctica over the past 18 years. Shen et al. [2018] use receiver functions and Rayleigh wave velocities from earthquakes and ambient noise to develop a higher resolution model for the upper 200 km beneath Central and West Antarctica, where most of the seismic stations have been deployed. Lloyd et al [2019] use full waveform adjoint tomography to invert three-component earthquake seismograms for a radially anisotropic model covering Antarctica and adjacent oceanic regions to 800 km depth. We estimate the mantle viscosity structure from seismic structure using laboratory-derived relationships between seismic velocity, temperature, and rheology. Choice of parameters for this mapping is guided in part by recent regional estimates of mantle viscosity from geodetic measurements. We also describe and compare several different methods of estimating lithospheric thickness from seismic constraints.&lt;/p&gt;&lt;p&gt;The mantle viscosity estimates indicate regional variations of several orders of magnitude, with extremely low viscosity (&lt; 10&lt;sup&gt;19&lt;/sup&gt; Pa s) beneath the Amundsen Sea Embayment (ASE) and the Antarctic Peninsula, consistent with estimates from GIA models constrained by GPS data. &amp;#160;Lithospheric thickness is also highly variable, ranging from around 60 km in parts of West Antarctica to greater than 200 km beneath central East Antarctica. In East Antarctica, several prominent regions such as Dronning Maude Land and the Lambert Graben show much thinner lithosphere, consistent with Phanerozoic tectonic activity and lithospheric disruption. Thin lithosphere and low viscosity between the ASE and the Antarctic Peninsula likely result from the thermal effects of the slab window as the Phoenix-Antarctic plate boundary migrated northward during the Cenozoic. Low viscosity regions beneath the ASE and Marie Byrd Land coast connect to an offshore anomaly at depths of ~ 250 km, suggesting larger-scale thermal and geodynamic processes that may be linked to the initial Cretaceous rifting of New Zealand and Antarctica. Low mantle viscosity results in a characteristic GIA time scale on the order of several hundred years, such that isostatic adjustment occurs on the same time scale as grounding line retreat.&amp;#160; Thus the associated rebound may lessen the effect of the marine ice sheet instability proposed for the ASE region.&amp;#160;&lt;/p&gt;

scholarly journals An investigation into the sensitivity of postglacial decay times to uncertainty in the adopted ice history

2019 ◽  
Author(s):  
Joseph Kuchar ◽  
Glenn Milne ◽  
Alexander Hill ◽  
Lev Tarasov ◽  
Maaria Nordman
Keyword(s):  
Sea Level ◽  
Decay Time ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Solution Space ◽  
Hudson Bay ◽  
James Bay ◽  
Mantle Viscosity ◽  
Viscosity Models ◽  
Decay Times

Abstract At the centers of previously glaciated regions such as Hudson Bay in Canada and the Gulf of Bothnia in Fennoscandia, it has been observed that the sea level history follows an exponential form and that the associated decay time is relatively insensitive to uncertainty in the ice loading history. We revisit the issue of decay time sensitivity by computing relative sea level histories for Richmond Gulf and James Bay in Hudson Bay and Ångerman River in Sweden for a suite of reconstructions of the North American and Fennoscandian Ice Sheets and Earth viscosity profiles. We find that while some Earth viscosity models do indeed show insensitivity in computed decay times to the ice history, this is not true in all cases. Moreover, we find that the location of the study site relative to the geometry of the ice sheet is an important factor in determining ice sensitivity, and based on our set of ice sheet reconstructions, conclude that the location of James Bay is not well-suited to a decay time analysis. We describe novel corrections to the RSL data to remove the effects associated with the spatial distribution of sea level indicators as well as for other signals unrelated to regional ice loading (ocean loading, rotation and global mean sea-level changes) and demonstrate that they can significantly affect the inference of viscosity structure. We performed a forward modelling analysis based on a commonly adopted 2-layer, sub-lithosphere viscosity structure to determine how the solution space of viscosity models changes with the input ice history at the three study sites. While the solution spaces depend on ice history, for both Richmond Gulf and Ångerman River there are regions of parameter space where solutions are common across all or most ice histories, indicating low ice load sensitivity for these mantle viscosity parameters. For example, in Richmond Gulf, upper mantle viscosity values of (0.3–0.5)x1021 Pa s and lower mantle viscosity values of (5–50)x1021 Pa s tend to satisfy the data constraint consistently for most ice histories considered in this study. Similarly, the Ångerman River solution spaces contain a solution with an upper mantle viscosity of 0.3 × 1021 Pa s and lower mantle viscosity values of (5–50)x1021 Pa s common to 9 of the 10 ice histories considered there. However, the dependence of the viscosity solution space on ice history suggests that joint estimation of ice and Earth parameters is the optimal approach.

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