Shear wave splitting characteristics of vertically aligned partial melt discs in a subduction zone back-arc setting

Abstract

Could the strength of observed upper mantle anisotropy in regions of volcanic past be significantly influenced by partial melt inclusions? Patterns of SKS shear wave splitting (SWS), defined via fast polarization direction, ϕ, and delay time, δt, are commonly interpreted in terms of lattice-preferred orientation (LPO) of anisotropic minerals in the upper mantle. However, shape-preferred orientation (SPO) of elastically distinct materials may influence SWS observations as well. We carry out global wavefield simulations using AxiSEM3D to understand the effects of vertically aligned partial melt discs on shear waveforms and derived SWS observations. We confirm earlier findings that the amount of splitting depends, for example, on melt fraction and aspect ratio and demonstrate that the presence of melt SPO (MPO) can significantly increase δt. We explore to what extent a combination with E-type olivine fabric can explain the occurrence of exceptionally high δt in the southern Cascadia Subduction Zone (SCSZ) back-arc and evaluate the effect of dehydration-related fabric transition to A-type on SWS. In this modeling, we assume that the presence of vertical melt disc inclusions is related to continuous upwelling and decompression melting that led to the formation of dykes in the uppermost mantle. For each model we examine the spatial variation and statistical distribution of splitting parameters. For a model considering multiple melt regions, we further evaluate their directional dependence. Stations above the melt inclusions tend to a unimodal δt distribution and explain the high δt values and the wide range of δt observed in the SCSZ back-arc.

Plain language summary

Could mantle deformation caused by volcanic activity be detected by the directional dependence of seismic wave propagation referred to as seismic anisotropy? Shear waves that pass through an anisotropic medium split into two waves with different orientations of wave vibration. These two waves travel at different speeds and accumulate a time delay δt, which can be measured from a seismogram along with the orientation of the faster wave, ϕ. These parameters represent the strength and orientation, respectively, of anisotropy beneath a seismometer. Seismic anisotropy in Earth's upper mantle is often interpreted to be due to the deformation linked alignment of individual grains of olivine, the most common mineral to occur in this depth region. However, the alignment of pockets of partial melt in the upper mantle may also contribute to the observed anisotropy. In this study, we evaluate the potential of vertically aligned partial melt discs to explain unusual previous observations of δt in the southern Cascadia subduction zone, which include exceptionally high delay times with significant variability over small length scales. We computationally simulate the propagation of seismic waves for a variety of models of upper mantle anisotropy that include aligned partial melt and explore the spatial and statistical effects on anisotropy measurements.