Subduction zone anisotropy beneath Corvallis, Oregon

a serpentinite skid mark of trench-parallel terrane migration?

Jeffrey Park*, Huaiyu Yuan, Vadim Levin

*Corresponding author for this work

Research output: Contribution to journalArticle

69 Citations (Scopus)

Abstract

We studied the back azimuth dependence of Ps converted phases at GSN station COR (Corvallis, Oregon) using broadband P receiver functions computed from 602 teleseismic earthquakes. The amplitudes and polarities of the transverse Ps phases are largely two-lobed, which indicates anisotropy with a tilted symmetry axis. A double-peaked Ps conversion at 4.5-6.5 s delay has the moveout of a dipping slab but is not consistent with simple deflection of the Ps converted phase by the dipping interface. A polarity flip on the transverse receiver function near north-south back azimuth indicates an anisotropic symmetry axis aligned north-south, far from the convergence direction (N68°E). The Ps phase is modeled using reflectivity synthetics with a highly anisotropic layer of depressed wave speed (Vp ∼ 6.0-6.5 km/s) near 40 km depth, at the slab interface with the overriding North American plate. One-dimensional modeling suggests 10% anisotropy in the supraslab layer, with a slow symmetry axis oriented N5°W at a 60° tilt from the vertical. Adjustments for the effect of slab interface dip on Ps amplitude suggest a somewhat lower 7% anisotropy. We infer a thin (∼ 7 km) anisotropic detachment zone for the northward slippage of the Siletz forearc terrane along the top of the descending slab, a motion consistent with GPS measurements and models of regional lithospheric dynamics. Serpentinite is a likely constituent for the deep anisotropic layer, owing to its reduced VP and high Poisson ratio. The ductile rheology and hydrated composition of serpentinite make it a plausible lithology for a mechanical detachment zone.

Original languageEnglish
Article numberB10306
Pages (from-to)1-15
Number of pages15
JournalJournal of Geophysical Research
Volume109
Issue numberB10
DOIs
Publication statusPublished - 10 Oct 2004
Externally publishedYes

Keywords

  • anisotropy
  • Cascadia
  • subduction

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