The Generation of Plate Tectonics From Grains to Global Scales: A Brief Review

Mulyukova, Elvira; Bercovici, David

doi:10.1029/2018tc005447

Cited by 23 publications

(17 citation statements)

References 117 publications

(241 reference statements)

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“…Volatiles on Earth and in particular the presence of free‐surface water are thought to be an important factor in establishing plate tectonics and also associated geohazards such as earthquakes, tsunamis, landslides, and volcanoes, in contrast to the stagnant lid convection exhibited on other similar planets, such as Venus (Kaula, 1990; Landuyt & Bercovici, 2009; Phillips, 1990). This could be because Earth's volatiles directly weaken the lithosphere allowing it crack and deform or because free surface water reduces surface temperatures of the planet which in turn leads to more localized deformation (Bercovici, 1998; Bercovici & Ricard, 2014; Korenaga, 2007, 2020; Landuyt & Bercovici, 2009; Lenardic & Kaula, 1994; Moresi & Solomatov, 1998; Mulyukova & Bercovici, 2019; Regenauer‐Lieb et al, 2001; Tozer, 1985). Plate tectonic processes also create mountain chains and continents required for our survival on the planet above water.…”

Section: Geodynamic Implicationsmentioning

confidence: 99%

The Nature of the Lithosphere‐Asthenosphere Boundary

et al. 2020

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Plate tectonic theory was developed 50 years ago and underpins most of our understanding of Earth's evolution. The theory explains observations of magnetic lineations on the seafloor, linear volcanic island chains, large transform fault systems, and deep earthquakes near deep sea trenches. These features occur through a system of moving plates at the surface of the Earth, which are the surface expression of mantle convection. The plate consists of the chemically distinct crust and some amount of rigid mantle, which move over a weaker mantle beneath. However, exactly where the transition between stronger and weaker mantle occurs and what determines and defines the plate are still debated. In the classic definition the plate is defined thermally, by the geotherm-adiabat intersection, where the plate is the conductively cooling part of the mantle convection system. Many observations such as heat flow, seafloor bathymetry, seismic imaging, and magnetotelluric (MT) imaging are consistent with general lithospheric thickening with age, which suggests that temperature is an important factor in determining lithospheric thickness. However, while age averages give a good indication of overall properties, the range of lithospheric thicknesses reported is large for any given tectonic age interval, suggesting greater complexity. A number of observations including sharp discontinuities from teleseismic scattered waves and active source reflections and also strong anomalies from surface and body wave tomography and MT imaging cannot be explained by a purely thermal model. Another property or process is required to explain the anomalies and sharpen the boundary. Many subsolidus models have been proposed, although none can universally explain the variety of independent global observations. Alternatively, a small amount of partial melt can easily satisfy a range of observations. The presence of melt could also weaken the mantle over geologic timescales, and it would therefore define the lithosphere-asthenosphere boundary (LAB). The location of melt is important to mantle dynamics and the LAB, although exactly where and exactly how much melt exists in the mantle are debated. Asthenospheric melt interpretations include a variety of forms: in small or large melt triangles beneath spreading ridges, in channels, in layers, along a permeability boundary leading to the ridge, at a depth of neutral buoyancy, punctuated, or pervasively over broad areas and either sharply or gradually falling off with depth. This variability in melt character or geometry may explain the previously described variability in LAB depths. The LAB is likely highly variable laterally as are the locations, forms, and amounts of melt, and the LAB is likely dynamic, dictated by small-scale convection and the dynamics of melt generation and migration. A melt-defined, dynamic LAB and a weak asthenosphere have broad implications for our understanding of Earth systems and planetary habitability. A weak asthenosphere caused by volatiles or melt could enable plate tectonic ...

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Section: Geodynamic Implicationsmentioning

confidence: 99%

The Nature of the Lithosphere‐Asthenosphere Boundary

et al. 2020

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“…If we consider a single grain with radius R 1 , its interface curvature without the effect of secondary particles would equal R 1 . However, due to the presence of secondary particles with radius R 2 , this interface is significantly distorted and its mean interface curvature, or roughness, is quantified by r. In the "pinned state" limit, the system is assumed FIGURE 2 | Sketch of the impact of Zener pinnning on grain interface roughness (after Bercovici and Ricard, 2012;Mulyukova and Bercovici, 2019). Shown is a sketch of a grain surrounded by secondary particles, with a zoom on the interface around one of the secondary particles.…”

Section: Grain Size Evolutionmentioning

confidence: 99%

Contributions of Grain Damage, Thermal Weakening, and Necking to Slab Detachment

Thielmann

Schmalholz

2020

Front. Earth Sci.

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We investigate the impact of three coupled weakening mechanisms on the viscous detachment of a stalled lithospheric slab: structural weakening due to necking, material weakening due to grain size reduction, using a two-phase grain damage model, and thermal weakening due to shear heating (thermal damage). We consider a combined flow law of dislocation and diffusion creep. To understand and quantify the coupling of these three nonlinear weakening processes, we derive a mathematical model, which consists of three coupled nonlinear ordinary differential equations describing the evolution of slab thickness, grain size, and temperature. With dimensional analysis, we determine the dimensionless parameters which control the relative importance of the three weakening processes and the two creep mechanisms. We derive several analytical solutions for end-member scenarios that predict the detachment time, that is the duration of slab detachment until slab thickness becomes zero. These analytical solutions are then tested against numerical solutions for intermediate cases. The analytical solutions are accurate for end-member scenarios where one of the weakening mechanisms and one of the creep mechanisms is dominant. Furthermore, we use numerical solutions of the system of equations to systematically explore the parameter space with a Monte Carlo approach. The numerical approach shows that the analytical solutions typically never deviate by more than 50% from the numerical ones, even for scenarios where all three weakening and both creep mechanisms are important. When both grain and thermal damage are important, the two damage processes generate a positive feedback loop resulting in the fastest detachment times. For Earth conditions, we find that the onset of slab detachment is controlled by grain damage and that during later stages of slab detachment thermal weakening becomes increasingly important and can become the dominating weakening process. We argue that both grain and thermal damage are important for slab detachment and that both damage processes could also be important for lithosphere necking during continental rifting leading to break-up and ocean formation.

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“…Earth is unique among the other planets of our solar system in that it exhibits plate tectonic behavior, whereby narrow, weak zones of deformation accommodate the relative motion of internally-rigid tectonic plates. Within these weak plate boundary shear zones, fine-grained, polymineralic, well-mixed ultramylonites are ubiquitous (e.g., Jaroslow et al, 1996;Jin et al, 1998;Linckens et al, 2015;Mehl & Hirth, 2008;Platt & Behrmann, 1986;Skemer et al, 2010;Warren & Hirth, 2006;White et al, 1980)-a finding that has led many authors to conclude that ultramylonites both enable and arise from plate tectonics (Gueydan et al, 2014;Mulyukova & Bercovici, 2019;Norris & Toy, 2014;Platt, 2015;Précigout et al, 2007;White et al, 1980). Numerous processes can induce rheological weakening during mylonitic deformation, including dynamic recrystallization (Cross & Skemer, 2019;Etheridge & Wilkie, 1979;Jin et al, 1998;Urai et al, 1986), fluid-rock interactions (Griggs, 1967;Kronenberg et al, 1990), metamorphic reactions (Dixon & Williams, 1983;Drury et al, 1991), phase transformations (Poirier, 1982;White & Knipe, 1978), partial melting (Cooper & Kohlstedt, 1986;Holtzman et al, 2012;Kohlstedt, 2002), shear heating (Brun & Cobbold, 1980;Foley, 2018), the alignment of viscously anisotropic crystal lattices Skemer et al, 2013), and mineral phase rearrangement (Handy, 1994;Holyoke & Tullis, 2006).…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

How Does Viscosity Contrast Influence Phase Mixing and Strain Localization?

et al. 2020

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Ultramylonites—intensely deformed rocks with fine grain sizes and well‐mixed mineral phases—are thought to be a key component of Earth‐like plate tectonics, because coupled phase mixing and grain boundary pinning enable rocks to deform by grain‐size‐sensitive, self‐softening creep mechanisms over long geologic timescales. In isoviscous two‐phase composites, “geometric” phase mixing occurs via the sequential formation, attenuation (stretching), and disaggregation of compositional layering. However, the effects of viscosity contrast on the mechanisms and timescales for geometric mixing are poorly understood. Here, we describe a series of high‐strain torsion experiments on nonisoviscous calcite‐fluorite composites (viscosity contrast, ηca/ηfl ≈ 200) at 500°C, 0.75 GPa confining pressure, and 10−6–10−4 s−1 shear strain rate. At low to intermediate shear strains (γ ≤ 10), polycrystalline domains of the individual phases become sheared and form compositional layering. As layering develops, strain localizes into the weaker phase, fluorite. Strain partitioning impedes mixing by reducing the rate at which the stronger (calcite) layers deform, attenuate, and disaggregate. Even at very large shear strains (γ ≥ 50), grain‐scale mixing is limited, and thick compositional layers are preserved. Our experiments (1) demonstrate that viscosity contrasts impede mechanical phase mixing and (2) highlight the relative inefficiency of mechanical mixing. Nevertheless, by employing laboratory flow laws, we show that “ideal” conditions for mechanical phase mixing may be found in the wet middle to lower continental crust and in the dry mantle lithosphere, where quartz‐feldspar and olivine‐pyroxene viscosity contrasts are minimized, respectively.

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The Generation of Plate Tectonics From Grains to Global Scales: A Brief Review

Cited by 23 publications

References 117 publications

The Nature of the Lithosphere‐Asthenosphere Boundary

The Nature of the Lithosphere‐Asthenosphere Boundary

Contributions of Grain Damage, Thermal Weakening, and Necking to Slab Detachment

How Does Viscosity Contrast Influence Phase Mixing and Strain Localization?

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