Résumé
Observations of shear wave splitting are often interpreted as being due to strain-induced crystal alignment of olivine in the convecting upper mantle, and the polarization of the fast shear wave is frequently taken to directly indicate the direction of mantle flow. Caution must be exercised when making such inferences, as the relationship between olivine lattice-preferred orientation (LPO) and fast direction is dependent on many factors, including the entire deformation history. This is especially the case in regions where complex time-dependent mantle flow is expected, e.g., subduction zones. Observations of shear wave splitting at subduction zones are varied, ranging from trench-perpendicular to -parallel fast directions, or a combination of both. Rigorously interpreting this variety of observations requires modeling which properly accounts for LPO development in the near-slab mantle environment. To this end, we simulate olivine LPO evolution caused by deformation of polycrystalline aggregates as they deform and move along pathlines extracted from a 3-D mantle flow model at a subduction zone (Li & Ribe, 2012). The model is based on 3-D boundary-element numerical simulations of a dense fluid sheet (representing the slab) with a geometry approximating that of the Sangihe subduction zone in Indonesia, where trench-parallel fast directions have recently been measured and ascribed to trench-parallel sub-slab mantle flow (Di Leo et al., 2012). This subduction zone is unique in that it is part of the only double-sided subduction system on Earth. At the Sangihe Trench, the Molucca Sea plate is subducting westwards beneath the Eurasian Plate. However, this microplate is also subducting eastwards at the nearby Halmahera Trench. To test whether the measured trench-parallel fast directions are due to sub-slab mantle flow, and whether this is only possible due to the double-sided geometry, we use two different flow models: one with single- and one with double-sided subduction. The effect of deformation and LPO development is simulated assuming the deformation of each crystal is governed by the motion of dislocations. Interactions between crystals are described using the visco-plastic self-consistent (VPSC) approach. Unlike previous studies, we consider the entire subduction history from subduction initiation onwards. This approach is necessary for the Sangihe subduction zone, as it is fairly young and the slab has only just reached the bottom of the mantle transition zone. In older subduction zones, early textures may eventually be destroyed. After calculating elastic properties associated with LPO at multiple depths, we estimate the resulting splitting parameters (fast direction phi , delay time delta t) for synthetic SKS phases. We compare these measurements with splitting observations in the Sangihe subduction zone. Our models show that complex behavior in phi appears in even apparently simple models of subduction zone mantle flow and that making robust dynamic inferences requires proper consideration of the geometry of subduction.