Olivine fabric transitions and shear wave anisotropy in the Ryukyu subduction system

نویسندگان

  • Erik A. Kneller
  • Maureen D. Long
  • Peter E. van Keken
چکیده

Recent shear wave splitting measurements from the fore-arc region of the Ryukyu subduction system show large magnitude (0.3–1.6 s) trenchparallel splitting in both local and teleseismic phases. The similarity of splitting parameters associated with shallow local-S and teleseismic phases suggests that the source of anisotropy is located in the fore-arc mantle. One explanation for this pattern of shear wave splitting involves a transition from commonly observed high-temperature olivine fabrics with flow-parallel seismically fast directions to a flow-normal B-type olivine fabric in the cold fore-arc mantle of the Ryukyu wedge. We test the B-type fabric hypothesis by comparing observed splitting parameters to those predicted from geodynamic models that incorporate olivine fabric development. The distribution of olivine fabric is calculated with high-resolution thermomechanical models of the Ryukyu subduction zone that include realistic slab geometry and an experimentally based wet olivine rheology. We conclude that B-type fabric can explain the magnitude and trench-parallel orientation of deep local-S phases that sample the core of the forearc mantle. However, our calculations show that B-type fabric alone cannot account for large magnitude trench-parallel splitting associated with teleseismic phases that sample the shallow tip of the fore-arc mantle. Alternative models for trench-parallel teleseismic splitting in the shallow tip of the fore-arc mantle involve the addition of crustal or slab anisotropy and highly anisotropic foliated antigorite serpentinite. © 2008 Elsevier B.V. All rights reserved.

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تاریخ انتشار 2008