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On the dynamics of subduction and the effect of subduction zones on mantle convection

Abstract : Subduction is one of the principal surface expressions of mantle convection and it represents a key ingredient of global geodynamics. It affects Earth processes ranging from the generation of mega-earthquakes and explosive volcanoes at thesurface to the recycling of volatile species back into the deep interior. Yet despite its obvious importance, several aspects of subduction remain to be clarified.In this work we endeavored to shed light on the mechanics and the energetics of the phenomenon adopting of a 2-D numerical model of “free subduction” based on the Boundary-Element Method. Systematically interpreting our numerical solutions in the light of thin viscous-sheet theory, we determined various scaling laws describing the physical mechanisms underlying different aspects of the phenomenon. Two dimensionless parameters stand out for their recurrence in suchscaling laws: i) the (dimensionless) strength of the subduction interface, which controls the shear stress acting at the interface between the two plates and ii) the flexural stiffnes of the subducting plate, which describes the mechanical resistance opposed by such plate to bending. This latter parameter is particularly important as it highlights the length scale that properly describes the bending deformation of the subducting plate (bending length).For what concerns the energetics of subduction, we also investigated the effect of the dissipation of energy occurring at subduction zones on large-scale mantleconvection. Our results seem to suggest that the classical scaling law found in the study of the steady-state Rayleigh-Bénard convection of an isoviscous fluid layer remains generally valid also for Earth’s mantle convection.To conclude, we ran a convection experiment based on the drying of a colloidal suspension of silica nanoparticles. As preliminary results have shown, thanksto its particular rheology, this material seems to be a promising candidate for effective laboratory modeling of mantle convection.
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Gianluca Gerardi. On the dynamics of subduction and the effect of subduction zones on mantle convection. Mechanics of the fluids [physics.class-ph]. Université Paris-Saclay, 2018. English. ⟨NNT : 2018SACLS437⟩. ⟨tel-01982285v2⟩

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