Investigating the role of plate tectonics on the stability and plume generating capacity of deep mantle structures.

Abigail Plimmer¹ , J. Huw Davies¹, James Panton^1
1Cardiff University, UK

The Earth's surface and deep interior are closely linked by transfer of energy and material in the form of downwelling slabs and upwelling plumes. While there is a strong feedback between plate tectonics and dynamics of the upper mantle, the dynamics near the core-mantle boundary (CMB) are less well understood. At present, two large low shear-wave velocity provinces (LLSVPs) exist beneath Africa and the Pacific Ocean, covering roughly 20-30% of the CMB. Some studies suggest a spatial relationship between LLSVPs and subduction zones at the surface, whilst other studies propose that LLSVPs have remained stable for hundreds of millions of years. Additionally, the edges of LLSVPs have often been proposed as plume generation zones, highlighting the importance of these deep mantle structures across the a mantle circulation cycle.

We present 3D spherical global circulation models which aim to contribute to the discussion regarding LLSVP stability and capacity to generate plumes. These simulations are driven at the surface by plate motion histories which utilise different reference frames, which we modify to systematically vary the plate velocities. Additionally, we vary CMB temperature, mantle viscosity and, in some of these models, implement a primordial layer of varying densities. Through this we aim to better understand the importance of plate tectonics relative to intrinsic mantle properties on shaping deep mantle structures and, in turn, the extent to which plate tectonics may control the location of upwellings.

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Mantle structures beneath an evolving supercontinent: the interaction of slabs and plumes in respose to variable lithosphere structure.

Abigail Plimmer¹ , J. Huw Davies¹, James Panton^1
1Cardiff University, UK

The relationship between the lithosphere and the mantle during the supercontinent cycle is complex and poorly constrained. The processes which drive dispersal are often simplified to two end members: slab pull and plume push. We aim to explore the interaction between slab and plume processes throughout the supercontinent cycle and consider how subduction processes during supercontinent assembly may affect the way in which a supercontinent breaks apart. We consider lithosphere structure as one of many potential processes which may affect the evolution of upwellings and downwellings and therefore systematically vary the properties of oceanic, continental, and cratonic lithosphere, respectively within our 3D spherical simulations.

The viscosity and thickness of the lithosphere alters the trajectory of downwelling material beneath the supercontinent as it assembles. Where slabs and plumes interact, the cold slabs can reduce the magnitude of an evolving plume, which may limit the contribution of plume push forces on supercontinent breakup.

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Mantle structures beneath an evolving supercontinent: the interaction of slabs and plumes in respose to variable lithosphere structure.

Abigail Plimmer¹ , J. Huw Davies¹, James Panton^1
1Cardiff University, UK

The relationship between the lithosphere and the mantle during the supercontinent cycle is complex and poorly constrained. The processes which drive dispersal are often simplified to two end members: slab pull and plume push. We aim to explore the interaction between slab and plume processes throughout the supercontinent cycle and consider how subduction processes during supercontinent assembly may affect the way in which a supercontinent breaks apart. We consider lithosphere structure as one of many potential processes which may affect the evolution of upwellings and downwellings and therefore systematically vary the properties of oceanic, continental, and cratonic lithosphere, respectively within our 3D spherical simulations.

The viscosity and thickness of the lithosphere alters the trajectory of downwelling material beneath the supercontinent as it assembles. Where slabs and plumes interact, the cold slabs can reduce the magnitude of an evolving plume, which may limit the contribution of plume push forces on supercontinent breakup.

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Slab dynamics in the mantle: a back-to-basics approach

Abigail Plimmer¹ , J. Huw Davies¹, James Panton^1
1Cardiff University, UK

Subduction is one of the most fundamental processes on Earth, linking the lithosphere and mantle and is a key driving force in mantle circulation. Despite this, and the advancement of geophysical methods which allow us to better understand mantle dynamics, our understanding of slab behaviour is still limited. The Earth is a very complex system, and so conclusions regarding slab dynamics are also sensitive to the interplay between countless processes acting within the mantle.

There is much to learn about slab sinking in the mantle from considering a single 'perfect' plate, such that the dynamics can be isolated from any pre-established or distal processes. We present a range of 3D spherical mantle circulation models which evolve from the initial condition, driven by a 'perfect' plate at the surface. Each of these plates comprises a rectangular geometry, bound by a subduction zone on one side, a spreading ridge on the opposite side, and two transform faults on the adjacent edges. We vary the geometry of the plate, both in terms of the length of the subducting trench, and the distance from the trench to the ridge, and vary the plate velocity.

We will report the slab behaviour in terms of plate geometry, mantle properties, and plate velocity, focussing on the evolution of downwellings, upwellings and other mantle structures in response to mantle circulation models driven solely by a single plate at the surface.

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Mantle upwellings and supercontient breakup in response to lithosphere heterogeneity

Abigail Plimmer¹ , J. Huw Davies¹, James Panton^1
1Cardiff University, UK

The processes which drive supercontinent dispersal have long been discussed, with two end members often being the focus; subduction at the edges of the supercontinents (trench pull) and plumes rising beneath the continental landmass to drive break up from the middle (plume push). Plume push models often consider that a supercontinent acts to shield the mantle from cold downwellings, therefore insulating the mantle and increasing sub-supercontinent mantle temperatures, which in turn drives upwellings.

Using the 3D mantle convection code, TERRA, we aim to test the evolution of mantle downwellings beneath an assembling supercontinent when the lithosphere properties are systematically varied. We present 12 models which represent 500 Ma of evolution, in which the lithosphere is segregated into oceanic, continental and cratonic regions, each of varying viscosity and thickness, respectively.

We find that the viscosity and thickness of the lithosphere can alter the trajectory of downwelling material. Where there is an increase in viscosity laterally, downwelling material sinks through the mantle near-vertically. In cases where the continents are weaker than the surrounding lithosphere, there is a greater lateral component to the slab sinking in the upper mantle.

In our models, the trajectory of the slab affects the evolution of a sub-supercontinental upwelling. Slabs sinking vertically interact with the upwelling, restricting its evolution whilst those with a lateral motion act to sweep the edges of the upwellings whilst maintaining the temperature anomaly. We suggest that this could have important implications for the breakup of supercontinents. The restriction of upwellings beneath the centre of a supercontinent which has a viscous continental landmass may indicate the limited contribution of the plume push mode of breakup in these circumstances. Conversely, where continents are weaker than the surrounding lithosphere, the role of plume push may be more significant, as upwellings migrate to shallower depths.

Assessing the role of lithosphere heterogeneity on mantle circulation models.

Abigail Plimmer¹ , J. Huw Davies¹, James Panton^1
1Cardiff University, UK

The relationship between the mantle and the lithosphere is complex and the feedback between the two is poorly constrained throughout geological time. Mantle circulation models have often been utilised to better understand these interactions, but implement a homogeneous lithosphere which cannot accurately reflect the heterogeneity of oceanic, continental and cratonic regions at the Earth’s surface.

Using the 3D mantle convection code, TERRA, we aim to test the evolution of mantle downwellings beneath an assembling supercontinent when the lithosphere properties are systematically varied. We present 12 models which represent 500 Ma of evolution, in which the lithosphere is segregated into oceanic, continental and cratonic regions, each of varying viscosity and thickness, respectively.

We have developed a workflow by which we can implement these different lithospheric domains into the 3D mantle convection code, TERRA (Baumgardner, 1985), consistent with the plate motion reconstructions of Müller et al. (2022). We evaluate the importance of lithosphere heterogeneity on mantle circulation models and suggest that lateral viscosity variations at the Earth’s surface can induce edge-driven convective instabilities with significant convective wavelength to interact with well-established upwellings and downwellings within the upper and lower mantle. These smaller-scale features may help to solve some of the problems with the geodynamics field, especially the complex interactions between the mantle and lithosphere away from plate boundaries.

References

Baumgardner, J. R. (1985). Three-dimensional treatment of convective flow in the earth’s mantle. Journal of Statistical Physics, 39 , 501-511. doi: http://doi.org/10.1007/BF01008348
Müller, R. D., Flament, N., Cannon, J., Tetley, M. G., Williams, S. E., Cao, X., Merdith, A. (2022). A tectonic-rules-based mantle reference frame since 1 billion years ago–implications for supercontinent cycles and plate–mantle system evolution. Solid Earth, 13 , 1127-1159. doi: http://doi.org/10.5194/se-13-1127-2022

Assessing the role of oceanic, continental, and cratonic lithosphere in mantle circulation

Abigail Plimmer¹ , J. Huw Davies¹, James Panton^1
1Cardiff University, UK

Supercontinent cycling is a pervasive process which has endured in its current Wilson tectonics style for at least 1 Ga (Stern, 2008; Ernst, 2017). Despite this, the degree of feedback between the surface and CMB boundary layers during the supercontinent cycle remains elusive.

Historically, mantle models have treated the lithosphere as a homogenous rigid layer, which deforms in response to the imposed plate velocities or thermal instabilities. However, to properly assess the degree of coupling between the surface and the mantle it is crucial to represent the complexity of the Earth’s lithosphere since the intrinsic properties of different lithospheric domains may affect the dynamics of the upper mantle.

We use the plate motion reconstructions of Merdith et al., (2021), Cao et al., (2021), and Müller et al (2022) to define regions of oceanic, continental, and cratonic lithosphere which can then be implemented into our models with variable buoyancies, viscosities, and depths. We assess upper mantle thermal perturbations in each simulation and consider the implications of this on slab sinking and plume upwelling. We suggest that where the upper mantle is shielded by thicker, viscous cratons, we may observe slab and plume dynamics reminiscent of those proposed in the mantle insulation model proposed by Anderson (1982).

We utilise the 3D mantle convection code, TERRA (Baumgardner 1983), to model the interaction between slabs subducted around the edges of supercontinents and deep mantle structures. Our initial models track and measure the sinking times of slabs with varying physical properties and geometries throughout the mantle. Slab buoyancies and viscosity profiles within the mantle are varied to constrain the implications of each parameter. We find that thermal processes are critical factors controlling the slab sinking times since the thermal and density structure of the slab, as well as the upper mantle viscosity ultimately control the slab velocity. We show the effects of composition are secondary and relatively minor.

As such, the lithospheric complexities at the surface may be considered alongside plate density and strength as significant controls on slab dynamics and as one of many possible explanations of global slab heterogeneity.

References

Anderson, D.L., 1982. Hotspots, polar wander, Mesozoic convection and the geoid. Nature, 297(5865), pp.391-393.
Cao, X., Flament, N. and Müller, R.D., 2021. Coupled evolution of plate tectonics and basal mantle structure. Geochemistry, Geophysics, Geosystems, 22(1), p.e2020GC009244.
Ernst, W.G., 2017. Earth’s thermal evolution, mantle convection, and Hadean onset of plate tectonics. Journal of Asian Earth Sciences, 145, pp.334-348.
Merdith, A.S., Williams, S.E., Collins, A.S., Tetley, M.G., Mulder, J.A., Blades, M.L., Young, A., Armistead, S.E., Cannon, J., Zahirovic, S. and Müller, R.D., 2021. Extending full-plate tectonic models into deep time: Linking the Neoproterozoic and the Phanerozoic. Earth-Science Reviews, 214, p.103477.
Müller, R.D., Flament, N., Cannon, J., Tetley, M.G., Williams, S.E., Cao, X., Bodur, Ö.F., Zahirovic, S. and Merdith, A., 2022. A tectonic-rules-based mantle reference frame since 1 billion years ago–implications for supercontinent cycles and plate–mantle system evolution. Solid Earth, 13(7), pp.1127-1159.
Stern, R.J., 2008. Modern-style plate tectonics began in Neoproterozoic time: An alternative interpretation of Earth’s tectonic history. When did plate tectonics begin on planet Earth, 440, pp.265-280.

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Sensitivity of slab sinking times to mantle viscosity and slab buoyancy.

Abigail Plimmer¹ , J. Huw Davies¹, James Panton^1
1Cardiff University, UK

For decades, researchers have studied supercontinents and their important implications for the evolution of climates and biological systems through geological time (Nance et al. 2014). Despite this, the mechanisms which control the breakup and amalgamation of these supercontinents remain enigmatic.

We consider a mantle convection ‘cycle’, comprising downwellings, deep mantle structures, upwellings, and the continents. This cycle encompasses the critical features and processes involved in the breakup and assembly of supercontinents. Whilst it is possible to constrain the present-day geometries and localities of mantle features using seismic tomographic models, understanding the mechanisms and mantle dynamics which give rise to them is more complex. We aim to constrain the degree of feedback between the surface and CMB boundary layers by modelling the mutual interaction of their respective instabilities, slabs and plumes.

As such, we present the following research questions:

• On what timescales do LLSVPs respond to subduction and what are the possible implications of this for upwellings?
• How much stress is exerted on a supercontinent by slab pull if the slab (a) stalls at the transition zone, (b) penetrates the upper region of the lower mantle, or (c) penetrates straight through to the CMB?
• What role does mantle inheritance play in the supercontinent cycle? Can each cycle be considered in isolation or are the structures predetermined by previous cycles?
• How has the feedback between the surface and CMB changed since the onset of plate tectonics, specifically through the pre-Wilson Cycle era?

We utilise the 3D mantle convection code, TERRA (Baumgardner 1983), to model the interaction between slabs subducted around the edges of supercontinents and deep mantle structures. Our initial models track and measure the sinking times of slabs with varying physical properties and geometries throughout the mantle. Slab buoyancies and viscosity profiles within the mantle are varied to constrain the implications of each parameter. We find that thermal processes are critical factors controlling the slab sinking times since the thermal and density structure of the slab, as well as the upper mantle viscosity ultimately control the slab velocity. We show the effects of composition are secondary and relatively minor.

Subsequently, we will track the centre of thermochemical piles to understand the timescales on which they interact with downwelling slabs and ask, when considered in relation to proposed plume ascent times, whether this cycle can occur within the lifetime of a single supercontinent.

References

Baumgardner, J. R. (1985). Three-dimensional treatment of convective flow in the earth’s mantle. Journal of Statistical Physics, 39 , 501-511. doi: http://doi.org/10.1007/BF01008348
Nance, R. D., Murphy, J. B., & Santosh, M. (2014). The supercontinent cycle: a retrospective essay. Gondwana Research, 25 , 4-29. doi: http://doi.org/10.1016/j.gr.2012.12.026

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Sensitivity of slab sinking times to mantle viscosity and slab buoyancy.

Abigail Plimmer¹ , J. Huw Davies¹, James Panton^1
1Cardiff University, UK

For decades, researchers have studied supercontinents and their important implications for the evolution of climates and biological systems through geological time (Nance et al. 2014). Despite this, the mechanisms which control the breakup and amalgamation of these supercontinents remain enigmatic.

We consider a mantle convection ‘cycle’, comprising downwellings, deep mantle structures, upwellings, and the continents. This cycle encompasses the critical features and processes involved in the breakup and assembly of supercontinents. Whilst it is possible to constrain the present-day geometries and localities of mantle features using seismic tomographic models, understanding the mechanisms and mantle dynamics which give rise to them is more complex. We aim to constrain the degree of feedback between the surface and CMB boundary layers by modelling the mutual interaction of their respective instabilities, slabs and plumes.

As such, we present the following research questions:

• On what timescales do LLSVPs respond to subduction and what are the possible implications of this for upwellings?
• How much stress is exerted on a supercontinent by slab pull if the slab (a) stalls at the transition zone, (b) penetrates the upper region of the lower mantle, or (c) penetrates straight through to the CMB?
• What role does mantle inheritance play in the supercontinent cycle? Can each cycle be considered in isolation or are the structures predetermined by previous cycles?
• How has the feedback between the surface and CMB changed since the onset of plate tectonics, specifically through the pre-Wilson Cycle era?

We utilise the 3D mantle convection code, TERRA (Baumgardner 1983), to model the interaction between slabs subducted around the edges of supercontinents and deep mantle structures. Our initial models track and measure the sinking times of slabs with varying physical properties and geometries throughout the mantle. Slab buoyancies and viscosity profiles within the mantle are varied to constrain the implications of each parameter. We find that thermal processes are critical factors controlling the slab sinking times since the thermal and density structure of the slab, as well as the upper mantle viscosity ultimately control the slab velocity. We show the effects of composition are secondary and relatively minor.

Subsequently, we will track the centre of thermochemical piles to understand the timescales on which they interact with downwelling slabs and ask, when considered in relation to proposed plume ascent times, whether this cycle can occur within the lifetime of a single supercontinent.

References

Baumgardner, J. R. (1985). Three-dimensional treatment of convective flow in the earth’s mantle. Journal of Statistical Physics, 39 , 501-511. doi: http://doi.org/10.1007/BF01008348
Nance, R. D., Murphy, J. B., & Santosh, M. (2014). The supercontinent cycle: a retrospective essay. Gondwana Research, 25 , 4-29. doi: http://doi.org/10.1016/j.gr.2012.12.026

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