Newtonian versus non-Newtonian upper mantle viscosity : implications for subduction initiation

Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 32 (2005): L19304, doi:10.1029/2005GL023457.The effect of rheology on the evolution of the slab-tip during subduction initiation is analyzed using 2-D numerical flow models. Experimentally determined flow laws have both strong temperature- and stress-dependence, which leads to large local variations in viscosity with direct consequences for subduction initiation. We find that models with Newtonian viscosity lead to flat or coupled subduction due to hydrodynamic stresses that pull the slab-tip up towards the overriding plate. Non-Newtonian rheology reduces these hydrodynamic stresses by decreasing the wedge viscosity and the slab coupling to wedge-corner flow, rendering the small negative-slab buoyancy of the slab-tip sufficient to maintain its dip during the early stages of subduction

Analytical Parametrization of Self-Consistent Polycrystal Mechanics: Fast Calculation of Upper Mantle Anisotropy

Progressive deformation of upper mantle rocks via dislocation creep causes their constituent crystals to take on a non-random orientation distribution (crystallographic preferred orientation or CPO) whose observable signatures include shear-wave splitting and azimuthal dependence of surface wave speeds. Comparison of these signatures with mantle flow models thus allows mantle dynamics to be unraveled on global and regional scales. However, existing self-consistent models of CPO evolution are computationally expensive when used in 3-D and/or time-dependent convection models. Here we propose a new method, called ANPAR, which is based on an analytical parameterisation of the crystallographic spin predicted by the second-order (SO) self-consistent theory. Our parameterisation runs approximately 2-6x10^4 times faster than the SO model and fits its predictions for CPO and crystallographic spin with a variance reduction > 99%. We illustrate the ANPAR model predictions for the deformation of olivine with three dominant slip systems, (010)[100], (001)[100] and (010)[001], for three uniform deformations (uniaxial compression, pure shear, simple shear) and for a corner-flow model of a spreading mid-ocean ridge

Defining a proxy for the interpretation of seismic anisotropy in non-Newtonian mantle flows

International audienceSeismic anisotropy can provide unique insights on convection in the upper mantle. Here we study the link between seismic anisotropy and mantle flow using a non-Newtonian rheology consistent with deformation by dislocation creep. Using analytical first-order flow models underneath a ridge and in subduction zones, we find that finite strain ellipsoid (FSE) is a robust proxy of seismic anisotropy, both in terms of orientation and strength, for natural strains smaller than ≈ 1. At larger strains, anisotropy aligns with the "infinite strain axis" (ISA), defined as the orientation of the long axis of the FSE in the limit of infinite strain, and its percentage reaches a plateau. Anisotropy aligns with the flow direction only when the product of the inverse strain rate with the timescale of ISA rotation within the flow is smaller than 0.1