Trench-parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge

autho

A benchmark comparison of subduction models

Numerically modelling the dynamics of a self-consistently subducting lithosphere is a challenging task because of the decoupling problems of the slab from the free surface. We address this problem with a benchmark comparison between various numerical codes (Eulerian and Lagrangian, Finite Element and Finite Difference, with and without markers) as well as a laboratory experiment. The benchmark test consists of a prescribed setup of viscous flow, driven by compositional buoyancy, and with a low viscosity, zero-density top layer to approximate a free surface. Alternatively, a fully free surface is assumed. Our results indicate that the convergence of the subduction behaviour with increasing resolution strongly depends on the averaging scheme for viscosity near moving rheological boundaries. Harmonic means result in fastest subduction, arithmetic means produces slow subduction and geometric mean results in intermediate behaviour. Complete convergence of results appears mostly beyond presently feasible grid resolution. Analysing the behaviour reveals that this problem is caused by the entrainment of the weak zero-density material into a lubrication layer on top of the subducting slab whose thickness turns out to be smaller than even the finest grid resolution. Agreement between the free surface runs and the weak top layer models is satisfactory only if both approaches use high resolution. Comparison of numerical models with a free surface laboratory experiment shows that (1) Lagrangian-based free-surface numerical models can closely reproduce the laboratory experiments provided that sufficient numerical resolution is employed and (2) Eulerian-based codes with a weak surface layer reproduce the experiment if harmonic averaging of viscosity is used. The harmonic mean is also preferred if circular high viscosity bodies with or without a lubrication layer are considered. We conclude that modelling the free surface by a weak zero-density layer gives good results but care has to be taken in 1) handling the associated entrainment and formation of a lubrication layer and 2) choosing the appropriate averaging scheme for viscosity at rheological boundaries

Development of texture and seismic anisotropy during the onset of subduction

How reliable are shear wave splitting measurements as a means of determining mantle flow direction? This remains a topic of debate, especially in the context of subduction. The answer hinges on whether our current understanding of mineral physics provides enough to accurately translate between seismic observations and mantle deformation. Here, we present an integrated model to simulate strain- history-dependent texture development and estimate resulting shear wave splitting in subduction environments. We do this for a mantle flow model that, in its geometry, approximates the double-sided Molucca Sea subduction system in Eastern Indonesia. We test a single-sided and a double-sided subduction case. Results are compared to recent splitting measurements of this region by Di Leo et al. (2012a). The setting lends itself as a case study, because it is fairly young and, therefore, early textures from the slab’s descent from the near surface to the bottom of the mantle transition zone—which we simulate in our models—have not yet been overprinted by subsequent continuous steady state flow. Second, it allows us to test the significance of the double-sided geometry, i.e., the need for a rear barrier to achieve trench-parallel subslab mantle flow. We demonstrate that although a barrier amplifies trench- parallel subslab anisotropy due to mantle flow, it is not necessary to produce trench-parallel fast directions per se. In a simple model of A-type olivine lattice-preferred orientation and one-sided subduction, trench-parallel fast directions are produced by a combination of simple shear and extension through compression and pure shear in the subslab mantle