scholarly journals The impact of rheological uncertainty on dynamic topography predictions: Gearing up for dynamic topography models consistent with observations

2019 ◽  
Author(s):  
Ömer F. Bodur ◽  
Patrice F. Rey
Keyword(s):  
Mantle Convection ◽  
Numerical Models ◽  
Mantle Flow ◽  
Dynamic Topography ◽  
Dynamic Component ◽  
Density Anomaly ◽  
Low Viscosity ◽  
Local Reduction ◽  
Non Linear ◽  
The Impact

Abstract. Much effort has been given on extracting the dynamic component of the Earth’s topography, which is 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 topographies generally larger by a factor of ∼2 compared to dynamic topographies estimated from residual topography after extraction of the isostatically compensated topography. Our 3D thermo-mechanical numerical experiments suggest that this discrepancy can be explained by the use of a viscosity model, which doesn’t account for non-linear viscosity behaviour. In this paper, we numerically model the dynamic topography induced by a spherical density anomaly embedded into the mantle. When we use non-linear viscosities, our numerical models predict dynamic topographies lesser by a factor of ∼2 than those derived from numerical models using isoviscous rheology. This reduction in dynamic topography is explained by either the formation of a low viscosity channel beneath the lithosphere, or a decrease in thickness of the mechanical lithosphere due to induced local reduction in viscosity. Furthermore, we show that uncertainties related to activation volume and fluid activity, lead to variations in dynamic topography of about 20 %.

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.

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.


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.

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;

The importance of phase morphology for rheology of ferropericlase-bridgmanite mixtures

2021 ◽  
Author(s):  
Marcel Thielmann ◽  
Gregor Golabek ◽  
Hauke Marquardt
Keyword(s):  
Lower Mantle ◽  
Mantle Convection ◽  
Effective Viscosity ◽  
Large Scale ◽  
Numerical Models ◽  
Phase Morphology ◽  
Strong Impact ◽  
Analytical Approximations ◽  
The Impact ◽  
Strong Control

&lt;p&gt;The rheology of the Earth&amp;#8217;s lower mantle is poorly constrained due to a lack of knowledge of the rheological behaviour of its constituent minerals. In addition, the lower mantle does not consist of only a single, but of multiple mineral phases with differing deformation behaviour. The rheology of Earth&amp;#8217;s lower mantle is thus not only controlled by the rheology of its individual constituents (bridgmanite and ferropericlase), but also by their interplay during deformation. This is particularly important when the viscosity contrast between the different minerals is large. Experimental studies have shown that ferropericlase may be significantly weaker than bridgmanite and may thus exert a strong control on lower mantle rheology.&lt;/p&gt;&lt;p&gt;Here, we thus explore the impact of phase morphology on the rheology of a ferropericlase-bridgmanite mixture using numerical models. We find that elongated ferropericlase structures within the bridgmanite matrix significantly lower the effective viscosity, even in cases where no interconnected network of weak ferropericlase layers has been formed. In addition to the weakening, elongated ferropericlase layers result in a strong viscous anisotropy. Both of these effects may have a strong impact on lower mantle dynamics, which makes is necessary to develop upscaling methods to include them in large-scale mantle convection models. We develop a numerical-statistial approach to link the statistical properties of a ferropericlase-bridgmanite mixture to its effective viscosity tensor. With this approach, both effects are captured by analytical approximations that have been derived to describe the evolution of the effective viscosity (and its anisotropy) of a two-phase medium with aligned elliptical inclusions, thus allowing to include these microscale processes in large-scale mantle convection models.&lt;/p&gt;

Dynamic topography controls on source-to-sink systems: Example of the last 40 Ma in South Africa.

2020 ◽  
Author(s):  
Claire Mallard ◽  
Tristan Salles ◽  
Sabin Zahirovic ◽  
Xuesong Ding
Keyword(s):  
South Africa ◽  
South African ◽  
Surface Processes ◽  
Source Surface ◽  
Mantle Flow ◽  
Dynamic Topography ◽  
The South ◽  
Geological Record ◽  
Source To Sink ◽  
The Impact

&lt;p&gt;Over deep time, mantle flow-induced dynamic topography drives deposition moderated by higher-frequency fluctuations in climate and sea level. The effects of deep mantle convection impact all the segment of the source to sink systems at different wavelengths and over various scales which remains poorly quantified. Field observations and numerical investigations suggest that the long-term stratigraphic record along continental margins contains essential clues on the interactions between dynamic topography and surface processes. However, it remains challenging to isolate the fingerprints of dynamic topography in the geological record.&lt;/p&gt;&lt;p&gt;We use the open-source surface evolution code Badlands (badlands.readthedocs.io), to quantify the impact of different timings and wavelengths of dynamic topography migration on the South African landscape responses.&lt;/p&gt;&lt;p&gt;We test three different dynamic topography scenarios obtained by both backwards advection and forwards modelling of mantle flow. We investigate their influence on landscape dynamics, stratal geometries and depositional patterns of South Africa over the past 40 Ma. We compare the evolution of the drainage organization, sediments flux, and stratigraphy obtained with the models with seismic, geochronological, and thermochronological data. We demonstrate that inland incision, spatial sediment accumulation, and depocenter migration strongly depend on the direction of sediment transport relative to the direction of dynamic topography propagation. It allows to identify realistic evolutions of mantle flow associated with the South African uplift history. Our results suggest that our source-to-sink numerical workflow can be used to explore, in a systematic way, the interplay between dynamic topography and surface processes and can provide insights into recognizing the geomorphic and stratigraphic signals of dynamic topography in the geological record.&lt;/p&gt;

Plate speeds modulated by sediment subduction: insights from numerical models

2020 ◽  
Author(s):  
Whitney Behr ◽  
Adam Holt ◽  
Thorsten Becker ◽  
Claudio Faccenna
Keyword(s):  
Subduction Zones ◽  
Convergence Rates ◽  
Numerical Models ◽  
Finite Width ◽  
Dynamic Topography ◽  
Plate Interface ◽  
Interface Models ◽  
Low Viscosity ◽  
Uniform Interface ◽  
Sinking Velocities

&lt;p&gt;Tectonic plate velocities predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere, and slab&amp;#8211;upper plate interface. A range of observations suggest that slabs may be weak, implying a more prominent role for plate interface dissipation than previously thought. Behr &amp; Becker (2018) suggested that the deep interface viscosity in subduction zones should be strongly affected by the relative proportions of sedimentary to mafic rocks that are subducted to depth, and that sediment subduction should thus facilitate faster subduction plate speeds. Here we use fully dynamic 2D subduction models built with the code ASPECT to quantitatively explore how subduction interface viscosity influences: a) subducting plate sinking velocities, b) trench migration rates, c) convergence velocities, d) upper plate strain regimes, e) dynamic topography, and f) interactions with the 660 km mantle transition zone.&amp;#160; We implement two main types of models, including 1) uniform interface models where interface viscosity and slab strength are systematically varied, and 2) varying interface models where a low viscosity sediment strip of finite width is embedded within a higher viscosity interface. Uniform interface models indicate that low viscosity (sediment-lubricated) slabs have substantially faster sinking velocities prior to reaching the 660, especially for weak slabs, and also that they achieve faster &amp;#8216;steady state&amp;#8217; velocities after 660 penetration. Even models where sediments are limited to a strip on the seafloor show accelerations in convergence rates of up to ~5 mm/y per my, with convergence initially accommodated by trench rollback and later by slab sinking. We discuss these results in the context of well-documented plate accelerations in Earth&amp;#8217;s history such as India-Asia convergence and convergence rate oscillations along the Andean margin.&lt;/p&gt;&lt;p&gt;References: Behr, W. M., &amp; Becker, T. W. (2018). Sediment control on subduction plate speeds. &lt;em&gt;Earth and Planetary Science Letters&lt;/em&gt;,&amp;#160;&lt;em&gt;502&lt;/em&gt;, 166-173.&lt;/p&gt;

scholarly journals Impact of upper mantle convection on lithosphere hyperextension and subsequent horizontally forced subduction initiation

Solid Earth ◽  
2020 ◽  
Vol 11 (6) ◽  
pp. 2327-2357
Author(s):  
Lorenzo G. Candioti ◽  
Stefan M. Schmalholz ◽  
Thibault Duretz
Keyword(s):  
Upper Mantle ◽  
Mantle Convection ◽  
Driving Forces ◽  
Numerical Models ◽  
Passive Margin ◽  
Continuous Simulation ◽  
Subduction Initiation ◽  
Lithosphere Dynamics ◽  
The Impact

Abstract. Many plate tectonic processes, such as subduction initiation, are embedded in long-term (>100 Myr) geodynamic cycles often involving subsequent phases of extension, cooling without plate deformation and convergence. However, the impact of upper mantle convection on lithosphere dynamics during such long-term cycles is still poorly understood. We have designed two-dimensional upper-mantle-scale (down to a depth of 660 km) thermo-mechanical numerical models of coupled lithosphere–mantle deformation. We consider visco–elasto–plastic deformation including a combination of diffusion, dislocation and Peierls creep law mechanisms. Mantle densities are calculated from petrological phase diagrams (Perple_X) for a Hawaiian pyrolite. Our models exhibit realistic Rayleigh numbers between 106 and 107, and the model temperature, density and viscosity structures agree with geological and geophysical data and observations. We tested the impact of the viscosity structure in the asthenosphere on upper mantle convection and lithosphere dynamics. We also compare models in which mantle convection is explicitly modelled with models in which convection is parameterized by Nusselt number scaling of the mantle thermal conductivity. Further, we quantified the plate driving forces necessary for subduction initiation in 2D thermo-mechanical models of coupled lithosphere–mantle deformation. Our model generates a 120 Myr long geodynamic cycle of subsequent extension (30 Myr), cooling (70 Myr) and convergence (20 Myr) coupled to upper mantle convection in a single and continuous simulation. Fundamental features such as the formation of hyperextended margins, upper mantle convective flow and subduction initiation are captured by the simulations presented here. Compared to a strong asthenosphere, a weak asthenosphere leads to the following differences: smaller value of plate driving forces necessary for subduction initiation (15 TN m−1 instead of 22 TN m−1) and locally larger suction forces. The latter assists in establishing single-slab subduction rather than double-slab subduction. Subduction initiation is horizontally forced, occurs at the transition from the exhumed mantle to the hyperextended passive margin and is caused by thermal softening. Spontaneous subduction initiation due to negative buoyancy of the 400 km wide, cooled, exhumed mantle is not observed after 100 Myr in model history. Our models indicate that long-term lithosphere dynamics can be strongly impacted by sub-lithosphere dynamics. The first-order processes in the simulated geodynamic cycle are applicable to orogenies that resulted from the opening and closure of embryonic oceans bounded by magma-poor hyperextended rifted margins, which might have been the case for the Alpine orogeny.

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

&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Hernlund, J. W., &amp; Tackley, P. J. (2008). Modeling mantle convection in the spherical annulus. Physics of the Earth and Planetary Interiors, 171(1-4), 48-54.&lt;/p&gt;&lt;p&gt;Arnould, M., Coltice, N., Flament, N., Seigneur, V., &amp; M&amp;#252;ller, R. D. (2018). On the scales of dynamic topography in whole&amp;#8208;mantle convection models. Geochemistry, Geophysics, Geosystems, 19(9), 3140-3163.&lt;/p&gt;

Sign in / Sign up

Export Citation Format

Share Document