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Menke, W, Zha Y, Webb SC, Blackman DK.  2015.  Seismic anisotropy indicates ridge-parallel asthenospheric flow beneath the Eastern Lau Spreading Center. Journal of Geophysical Research-Solid Earth. 120:976-992.   10.1002/2014jb011154   AbstractWebsite

Seismic anisotropy beneath the Eastern Lau Spreading Center (ELSC) is investigated using both Rayleigh waves and shear waves, using data from the 2009-2010 ELSC ocean bottom seismograph experiment. Phase velocities of Rayleigh waves determined by ambient noise cross correlation are inverted for azimuthally anisotropic phase velocity maps. Splitting of S waves from five intermediate and deep focus earthquakes was determined by waveform analysis. Taken together, Rayleigh wave and S wave data indicate that significant (similar to 2%) anisotropy extends to at least 300km depth. Both data sets indicate a fast direction aligned within a few degrees of the N10 degrees E striking ELSC and somewhat oblique to the N25 degrees E strike of the neighboring volcanic arc. We therefore describe the fast direction as spreading perpendicular, not convergence perpendicular and interpret it as due to ridge-parallel flow of the asthenosphere. However, the region arcward (east) of the ELSC has the stronger anisotropy, suggesting that the strongest flow gradients may occur in the region between the spreading center and the arc, in contrast to being centered beneath the ELSC. Fluids released from the underlying plate may produce anisotropic hydrous materials, but more importantly lower the viscosity, thus enhancing along-strike flow. Both could contribute to an along-strike fast direction signature. Seafloor spreading diminishes south of the seismic array, ceasing altogether south of latitude 25 degrees S (500km south of the array center), a region dominated by much slower tectonic extension, suggesting that asthenosphere is inflowing from the north to accommodate the increase in asthenospheric volume associated with the seafloor spreading.

Castelnau, O, Blackman DK, Becker TW.  2009.  Numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow. Geophysical Research Letters. 36   10.1029/2009gl038027   AbstractWebsite

The development of Lattice Preferred Orientations (LPO) within olivine aggregates under flow in the upper mantle leads to seismic and rheological (or viscoplastic) anisotropies. We compare predictions from different micromechanical models, applying several commonly used theoretical descriptions to an upwelling flow scenario representing a typical oceanic spreading center. Significant differences are obtained between models in terms of LPO and associated rheology, in particular in regions where the flow direction changes rapidly, with superior predictions for the recently proposed Second-Order approach. This highlights the limitations of ad hoc formulations. LPO-induced rheological anisotropy may have a large effect on actual flow patterns with 1-2 orders of magnitude variation in directional viscosities compared to the isotropic case. Citation: Castelnau, O., D. K. Blackman, and T. W. Becker (2009), Numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow, Geophys. Res. Lett., 36, L12304, doi:10.1029/2009GL038027.

Castelnau, O, Blackman DK, Lebensohn RA, Castaneda PP.  2008.  Micromechanical modeling of the viscoplastic behavior of olivine. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005444   AbstractWebsite

Efforts to couple mantle flow models with rheological theories of mineral deformation typically ignore the effect of texture development on flow evolution. The fact that there are only three easy slip systems for dislocation glide in olivine crystals leads to strong mechanical interactions between the grains as the deformation proceeds, and subsequent development of large viscoplastic anisotropy in polycrystals exhibiting pronounced Lattice Preferred Orientations. Using full-field simulations for creep in dry polycrystalline olivine at high temperature and low pressure, it is shown that very large stress and strain rate intragranular heterogeneities can build up with deformation, which increase dramatically with the strength of the hard slip system (included for the purpose of enabling general deformations). Compared with earlier nonlinear extensions of the Self-Consistent mean-field theory to simulate polycrystal deformation, the "Second-Order'' method is the only one capable of accurately describing the effect of intraphase stress heterogeneities on the macroscopic flow stress, as well as on the local stress-and strain rate fluctuations in the material. In particular, this approach correctly predicts that olivine polycrystals can deform with only four independent slip systems. The resistance of the fourth system (or accommodation mechanism), which is likely provided by dislocation climb or grain boundary processes as has been observed experimentally, may essentially determine the flow stress of olivine polycrystals. We further show that the "tangent'' model, which had been used extensively in prior geophysical studies of the mantle, departs significantly from the full-field reference solutions.

Blackman, DK.  2007.  Use of mineral physics, with geodynamic modelling and seismology, to investigate flow in the Earth's mantle. Reports on Progress in Physics. 70:659-689.   10.1088/0034-4885/70/5/r01   AbstractWebsite

Seismologists and mineral physicists have known for decades that anisotropy inherent in mantle minerals could provide a means to relate surface seismic measurements to deformation patterns at great depth in the Earth, where direct geologic observations would never be possible. Prior to the past decade, only qualitative relationships or simple symmetry assumptions between mantle flow (deformation), mineral alignment and seismic anisotropy were possible. Recent numerical methods now allow quantitative incorporation of constraints from mineral physics to flow/deformation models and, thereby, direct estimates of the resulting pattern of seismic anisotropy can be made and compared with observed signatures. Numerical methods for simulating microstructural deformation within an aggregate of minerals subjected to an arbitrary stress field make it possible to quantitatively link crystal-scale processes with largescale Earth processes of mantle flow and seismic wave propagation, on regional (100s of kilometres) and even global scales. Such linked numerical investigations provide a rich field for exploring inter-dependences of micro and macro processes, as well as a means to determine the extents to which viable seismic experiments could discern between different models of Earth structure and dynamics. The aim of this review is to provide an overview of why and how linked numerical models are useful for exploring processes in the mantle and how they relate to surface tectonics. A brief introduction to the basic concepts of deformation of mantle minerals and the limits of knowledge currently available are designed to serve both the subsequent discussions in this review and as an entry point to more detailed literature for readers interested in pursuing the topic further. The reference list includes both primary sources and pertinent review articles on individual aspects of the combined subjects covered in the review. A series of flow/texturing models illustrate the differences that can arise when different methods or different flow parameters are employed. Representative seismic results illustrate the types of studies done to date and the inferences possible using their anisotropy measurements. Trade-offs involved in the modelling assumptions and seismic data processing methods are touched on. A final example illustrates the effects, relative to a 2D model of mantle flow near a subduction zone, that flow in a third dimension can have on anisotropy patterns.

Becker, TW, Schulte-Pelkum V, Blackman DK, Kellogg JB, O'Connell RJ.  2006.  Mantle flow under the western United States from shear wave splitting. Earth and Planetary Science Letters. 247:235-251.   10.1016/j.epsl.2006.05.010   AbstractWebsite

We show that SKS splitting in the westernmost United States (polarization of the fastest shear waves and splitting times, including their back-azimuthal dependence) can be explained by a geodynamic model that includes a continuum-mechanics description of plate motions and underlying asthenospheric circulation. Models that include a counterflow at depths of similar to 300 km are preferred, which may indicate a far-field effect of the Farallon slab anomaly sinking underneath the central continental United States. This finding is broadly consistent with earlier suggestions, and we demonstrate that a mechanically coupled system, though with a strong viscosity contrast with depth, is consistent with the data. We explore the depth dependence of predicted anisotropy by means of computing seismogram synthetics and comparing synthetic splits with observations. Some patterns in the data, including null observations, are matched well. Linked models of geodynamic flow and mineral alignment in the mantle provide a means to test the relationship between strain and the saturation of texturing. Lower fabric saturation strains than for global models are preferred by the data, which may reflect the relatively active tectonic setting and thin asthenosphere of the study region. In general, our results show that seismic anisotropy, when interpreted jointly with mineral physics theories, may be used to quantitatively constrain the spatial character of flow, and the degree of force coupling, at depth. (c) 2006 Elsevier B.V. All rights reserved.

Hall, CE, Fischer KM, Parmentier EM, Blackman DK.  2000.  The influence of plate motions on three-dimensional back arc mantle flow and shear wave. Journal of Geophysical Research-Solid Earth. 105:28009-28033.   10.1029/2000jb900297   AbstractWebsite

Both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back are basins of western Pacific subduction zones. The goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting. We calculated three-dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates. Shear wave splitting was calculated for polymineralic anisotropy within the back are mantle wedge assuming that the anisotropy was created by flow-induced strain. Predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain. A parameter study shows that: both finite strain and textural anisotropy developed within three-dimensional, plate-coupled asthenospheric flow models are very heterogeneous when back are shearing occurs within the overriding plate. Therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back are asthenosphere. Flow models incorporating plate motion boundary conditions for the Tonga, southern Kuril, and eastern Aleutian subduction zones produce splitting parameters consistent with observations from each region. Trench-parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge.