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Asthenosphere

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Adrian Lenardic – One of the best experts on this subject based on the ideXlab platform.

  • Toward a boot strap hypothesis of plate tectonics: Feedbacks between plates, the Asthenosphere, and the wavelength of mantle convection
    Physics of the Earth and Planetary Interiors, 2019
    Co-Authors: Adrian Lenardic, Tobias Höink, M. B. Weller, J. Seales

    Abstract:

    Abstract The solid Earth system is characterized by plate tectonics, a low viscosity zone beneath plates (the Asthenosphere), and long wavelength flow in the convecting mantle. We use suites of numerical experiments to show: 1) How long wavelength flow and the operation of plate tectonics can generate and maintain an Asthenosphere, and 2) How an Asthenosphere can maintain long wavelength flow and plate tectonics. Plate subduction generates a sub-adiabatic temperature gradient in the mantle which, together with temperature-dependent viscosity, leads to a viscosity increase from the upper to the lower mantle. This allows mantle flow to channelize in a low viscosity region beneath plates (an Asthenosphere forms dynamically). Flow channelization, in turn, stabilizes long wavelength convection. The degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet. The plate margin strength needed to initiate that transition increases for long wavelength cells (long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength). The coupled feedbacks allow for a linked causality between plates, the Asthenosphere, and the wavelength of mantle flow, with none being more fundamental than the others and the existence of each depending on the others. Under this hypothesis, the Asthenosphere is defined by an active process, plate tectonics, which maintains it and is maintained by it and plate tectonics is part of an emergent, self-sustaining flow system that bootstraps itself into existence.

  • Plug flow in the Earth’s Asthenosphere
    Earth and Planetary Science Letters, 2018
    Co-Authors: Alana G. Semple, Adrian Lenardic

    Abstract:

    Abstract Recent seismic observations, focused on mantle flow below the Pacific plate, indicate the presence of two shear layers in the Earth’s Asthenosphere. This is difficult to explain under the classic assumption of Asthenosphere flow driven by plate shear from above. We present numerical mantle convection experiments that show how a power law rheology, together with dynamic pressure gradients, can generate an Asthenosphere flow profile with a near constant velocity central region bounded above and below by concentrated shear layers (a configuration referred to as plug flow). The experiments show that as the power law dependence of Asthenosphere viscosity is increased from 1 to 3, maximum Asthenosphere velocities can surpass lithosphere velocity. The wavelength of mantle convection increases and Asthenosphere flow transitions from a linear profile (Couette flow) to a plug flow configuration. Experiments in a 3D spherical domain also show a rotation of velocity vectors from the lithosphere to the Asthenosphere, consistent with seismic observations. Global mantle flow remains of whole mantle convection type with plate and Asthenosphere flow away from a mid-ocean ridge balanced by broader return flow in the lower mantle. Our results are in line with theoretical scalings that mapped the conditions under which Asthenosphere flow can provide an added plate driving force as opposed to the more classic assumption that Asthenosphere flow is associated with a plate resisting force.

  • Depth-dependent viscosity and mantle stress amplification: implications for the role of the Asthenosphere in maintaining plate tectonics
    Geophysical Journal International, 2012
    Co-Authors: Tobias Höink, Adrian Lenardic, Mark A. Richards

    Abstract:

    SUMMARY Boundary layer theory is used to derive scaling relationships for plate stresses in a mantle convection system with a low-viscosity Asthenosphere. The theory assumes a plate tectonic like mode of mantle convection withflow driven by an active upper boundary layer. The theory predicts that the confinement of horizontal mantle flow within a low-viscosity, sublithospheric channel can lead to an increase in plate stress compared to the case lacking a channel (even if the absolute viscosity of the sublithosphere mantle does not change between the two cases). Thetheoryfurtherpredictsincreasingshearstresswithdecreasinglow-viscositychannelthickness. If the thickness of tectonic plates is determined dominantly by a dehydrated chemical lithosphere, then the plate normal stress is predicted to also increase with decreasing channel thickness. We use 3-D spherical shell simulations of mantle convection with temperature-, depth- and stress dependent rheology to test scaling trends. The simulations and theoretical scalings demonstrate that a low-viscosity layer (Asthenosphere) can amplify convective stresses. If the level of convective stress plays a role in maintaining and/or reactivating plate boundaries, this suggests that a relatively thin low viscosity layer may help to maintain plate tectonics. The numerical simulations support this suggestion as they show that an increase in the thickness of a low viscosity channel can cause the system to transition from an active-lid modeofconvectiontoastagnantlidstate.Collectively,thesimulationsandtheoreticalscalings lead to the conclusion that the role of the Asthenosphere in maintaining plate tectonics does not come principally from a basal lubrication effect, associated with a low absolute Asthenosphere viscosity, but, instead, from a mantle flow channelization effect, associated with a high viscosity contrast from the Asthenosphere to the mantle below.

Tobias Höink – One of the best experts on this subject based on the ideXlab platform.

  • Toward a boot strap hypothesis of plate tectonics: Feedbacks between plates, the Asthenosphere, and the wavelength of mantle convection
    Physics of the Earth and Planetary Interiors, 2019
    Co-Authors: Adrian Lenardic, Tobias Höink, M. B. Weller, J. Seales

    Abstract:

    Abstract The solid Earth system is characterized by plate tectonics, a low viscosity zone beneath plates (the Asthenosphere), and long wavelength flow in the convecting mantle. We use suites of numerical experiments to show: 1) How long wavelength flow and the operation of plate tectonics can generate and maintain an Asthenosphere, and 2) How an Asthenosphere can maintain long wavelength flow and plate tectonics. Plate subduction generates a sub-adiabatic temperature gradient in the mantle which, together with temperature-dependent viscosity, leads to a viscosity increase from the upper to the lower mantle. This allows mantle flow to channelize in a low viscosity region beneath plates (an Asthenosphere forms dynamically). Flow channelization, in turn, stabilizes long wavelength convection. The degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet. The plate margin strength needed to initiate that transition increases for long wavelength cells (long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength). The coupled feedbacks allow for a linked causality between plates, the Asthenosphere, and the wavelength of mantle flow, with none being more fundamental than the others and the existence of each depending on the others. Under this hypothesis, the Asthenosphere is defined by an active process, plate tectonics, which maintains it and is maintained by it and plate tectonics is part of an emergent, self-sustaining flow system that bootstraps itself into existence.

  • Depth-dependent viscosity and mantle stress amplification: implications for the role of the Asthenosphere in maintaining plate tectonics
    Geophysical Journal International, 2012
    Co-Authors: Tobias Höink, Adrian Lenardic, Mark A. Richards

    Abstract:

    SUMMARY Boundary layer theory is used to derive scaling relationships for plate stresses in a mantle convection system with a low-viscosity Asthenosphere. The theory assumes a plate tectonic like mode of mantle convection withflow driven by an active upper boundary layer. The theory predicts that the confinement of horizontal mantle flow within a low-viscosity, sublithospheric channel can lead to an increase in plate stress compared to the case lacking a channel (even if the absolute viscosity of the sublithosphere mantle does not change between the two cases). Thetheoryfurtherpredictsincreasingshearstresswithdecreasinglow-viscositychannelthickness. If the thickness of tectonic plates is determined dominantly by a dehydrated chemical lithosphere, then the plate normal stress is predicted to also increase with decreasing channel thickness. We use 3-D spherical shell simulations of mantle convection with temperature-, depth- and stress dependent rheology to test scaling trends. The simulations and theoretical scalings demonstrate that a low-viscosity layer (Asthenosphere) can amplify convective stresses. If the level of convective stress plays a role in maintaining and/or reactivating plate boundaries, this suggests that a relatively thin low viscosity layer may help to maintain plate tectonics. The numerical simulations support this suggestion as they show that an increase in the thickness of a low viscosity channel can cause the system to transition from an active-lid modeofconvectiontoastagnantlidstate.Collectively,thesimulationsandtheoreticalscalings lead to the conclusion that the role of the Asthenosphere in maintaining plate tectonics does not come principally from a basal lubrication effect, associated with a low absolute Asthenosphere viscosity, but, instead, from a mantle flow channelization effect, associated with a high viscosity contrast from the Asthenosphere to the mantle below.

  • Viscous coupling at the lithosphere‐Asthenosphere boundary
    Geochemistry Geophysics Geosystems, 2011
    Co-Authors: Tobias Höink, A. Mark Jellinek, Adrian Lenardic

    Abstract:

    Tectonic plate motions reflect dynamical contributions from subduction processes (i.e., classical “slab-pull” forces) and lateral pressure gradients within the Asthenosphere (“Asthenosphere-drive” forces), which are distinct from gravity forces exerted by elevated mid-ocean ridges (i.e., classical “ridge-push” forces). Here we use scaling analysis to show that the extent to which Asthenosphere-drive contributes to plate motions depends on the lateral dimension of plates and on the relative viscosities and thicknesses of the lithosphere and Asthenosphere. Whereas slab-pull forces always govern the motions of plates with a lateral extent greater than the mantle depth, Asthenosphere-drive forces can be relatively more important for smaller (shorter wavelength) plates, large relative Asthenosphere viscosities or large Asthenosphere thicknesses. Published plate velocities, tomographic images and age-binned mean shear wave velocity anomaly data allow us to estimate the relative contributions of slab-pull and Asthenosphere-drive forces for the motions of the Atlantic and Pacific plates. Whereas the Pacific plate is driven largely by slab pull, the Atlantic plate is predicted to be strongly driven by basal forces related to viscous coupling to strong asthenospheric flow, consistent with recent observations related to the stress state of North America. In addition, compared to the East Pacific Rise (EPR), the relatively large lateral pressure gradient near the Mid-Atlantic Ridge (MAR) is expected to produce significantly steeper dynamic topography. Thus, the relative importance of this plate-driving force may partly explain why the flanking topography at the EPR is smoother than at the MAR. Our analysis also indicates that this plate-driving force was more significant, and heat loss less efficient, in Earth’s hotter past compared with its cooler present state. This type of trend is consistent with thermal history modeling results which require less efficient heat transfer in Earth’s past.

J. C. Savage – One of the best experts on this subject based on the ideXlab platform.

  • Equivalent strike‐slip earthquake cycles in half‐space and lithosphere‐Asthenosphere earth models
    Journal of Geophysical Research, 1990
    Co-Authors: J. C. Savage

    Abstract:

    By virtue of the images used in the dislocation solution, the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike-slip fault in an Earth model consisting of an elastic plate (lithosphere) overlying a viscoelastic half-space (Asthenosphere) can be duplicated by prescribed slip on a vertical fault embedded in an elastic half-space. For the case in which each earthquake ruptures the entire lithosphere (thickness H), the half-space equivalent slip rate is as follows: Depth interval 0-H, slip identical to that in lithosphere-Asthenosphere model (i.e., abrupt coseismic slip and no subsequent slip); depth interval (2n−1)H to (2n+1)H (n = 1,2,…), slip rate uniform in space and dependent upon time as Fn(t) exp (−t/τa) where Fn is a (n – 1) degree polynomial in t, τa is twice the Asthenosphere relaxation time (η/μ), and t is measured from the instant after the preceding earthquake. The slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion. For reasonable values of τa, slip rates below 5H do not vary much from that mean value and can be treated as constant. Thus the surface deformation due to the earthquake cycle in the lithosphere-Asthenosphere model can be calculated very simply from the half-space model with time-dependent slip in the two depth intervals H−3H and 3H−5H, and uniform slip at a rate equal to the secular relative plate velocity below depth 5H. Inversion of 1973–1988 geodetic measurements of deformation across the segment of the San Andreas fault in the Transverse Ranges north of Los Angeles for the half-space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm/yr below 30 km. One equivalent lithosphere-Asthenosphere model would have a 30-km thick lithosphere and an Asthenosphere relaxation time greater than 33 years, but other models are possible.

  • equivalent strike slip earthquake cycles in half space and lithosphere Asthenosphere earth models
    Journal of Geophysical Research, 1990
    Co-Authors: J. C. Savage

    Abstract:

    By virtue of the images used in the dislocation solution, the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike-slip fault in an Earth model consisting of an elastic plate (lithosphere) overlying a viscoelastic half-space (Asthenosphere) can be duplicated by prescribed slip on a vertical fault embedded in an elastic half-space. For the case in which each earthquake ruptures the entire lithosphere (thickness H), the half-space equivalent slip rate is as follows: Depth interval 0-H, slip identical to that in lithosphere-Asthenosphere model (i.e., abrupt coseismic slip and no subsequent slip); depth interval (2n−1)H to (2n+1)H (n = 1,2,…), slip rate uniform in space and dependent upon time as Fn(t) exp (−t/τa) where Fn is a (n – 1) degree polynomial in t, τa is twice the Asthenosphere relaxation time (η/μ), and t is measured from the instant after the preceding earthquake. The slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion. For reasonable values of τa, slip rates below 5H do not vary much from that mean value and can be treated as constant. Thus the surface deformation due to the earthquake cycle in the lithosphere-Asthenosphere model can be calculated very simply from the half-space model with time-dependent slip in the two depth intervals H−3H and 3H−5H, and uniform slip at a rate equal to the secular relative plate velocity below depth 5H. Inversion of 1973–1988 geodetic measurements of deformation across the segment of the San Andreas fault in the Transverse Ranges north of Los Angeles for the half-space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm/yr below 30 km. One equivalent lithosphere-Asthenosphere model would have a 30-km thick lithosphere and an Asthenosphere relaxation time greater than 33 years, but other models are possible.