Constraints on olivine deformation from SKS shear-wave splitting beneath the southern Cascadia subduction zone back-arc

Eric Löberich, Maureen D. Long, Lara S. Wagner, Ehsan Qorbani, Götz Bokelmann

Publications: Contribution to journalArticlePeer Reviewed

Abstract

Shear-wave splitting observations of SKS and SKKS phases have been used widely to map azimuthal anisotropy, as caused by the occurrence of olivine, to constrain the dominant directions of upper mantle deformation. While SK(K)S splitting measurements at individual seismic stations are often averaged before interpretation, it is useful to consider additional information, for example, based on the variation of splitting parameters with azimuth due to the non-vertical incidence of core-phases. These constraints in theory enable a differentiation between various types of olivine and may allow us to infer otherwise poorly known upper mantle parameters such as stress, temperature, and water content. In this study, we predict the azimuthal variation of splitting parameters for A-, C-, and E-type olivine fabrics and match them with observations from the High Lava Plains, Northwestern Basin and Range, and Western Yellowstone Snake River Plain in the Pacific Northwest US. This helps to constrain the amount of water in the upper mantle in the back-arc of the Cascadia subduction zone, known for its consistent E-W oriented seismic anisotropy, and particularly large splitting delay times. Our investigation renders the C-type olivine mechanism improbable for this location; A- and E-type fabrics match the observations, although differentiating between them is difficult. However, the agreement of the amplitude of backazimuthal variation of the fast orientation, plus the potential to explain large splitting delay times, suggest the occurrence of E-type olivine, and thus the likely presence of a moderately hydrated upper mantle beneath Cascadia's back-arc.

Original languageEnglish
Article numbere2021GC010091
Number of pages25
JournalGeochemistry, Geophysics, Geosystems
Volume22
Issue number11
DOIs
Publication statusPublished - Nov 2021

Austrian Fields of Science 2012

  • 105122 Seismic

Keywords

  • AZIMUTHAL ANISOTROPY
  • Cascadia subduction zone
  • EVOLUTION
  • LATTICE-PREFERRED ORIENTATION
  • MANTLE FLOW BENEATH
  • ORIGIN
  • PACIFIC-NORTHWEST
  • SEISMIC ANISOTROPY
  • SNAKE RIVER PLAIN
  • WATER
  • WESTERN
  • olivine fabric type
  • seismic anisotropy
  • shear-wave splitting
  • upper mantle deformation
  • water content

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