Anisotropy and Flow in Pacific Subduction Zone Back-arcs

Abstract

Abstract-We have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the Tonga, southern Kuril, Japan, and Izu-Bonin subduction zones using shear-wave splitting in S phases from local earthquakes and in teleseismic core phases such as SKS. The observed splitting in all four subduction zones is consistent with a model in which the lower transition zone (520-660 km) and lower mantle are isotropic, and in which significant anisotropy occurs in the back-arc upper mantle. The upper transition zone (410 -520 km) beneath the southern Kurils appears to contain weak anisotropy. The observed fast directions indicate that the geometry of back-arc strain in the upper mantle varies systematically across the western Pacific rim. Beneath Izu-Bonin and Tonga, fast directions are aligned with the azimuth of subducting Pacific plate motion and are parallel or sub-parallel to overriding plate extension. However, fast directions beneath the Japan Sea, western Honshu, and Sakhalin Island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing. Models of back-arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in Tonga and Izu-Bonin, but further three-dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern Kurils and Japan. The fast directions in the southern Kurils and Japan may require strain in the back-arc mantle that is driven by regional or global patterns of mantle flow.

Relation between mantle shear zone deformation and metasomatism in spinel peridotite xenoliths of Jeju Island (South Korea): Evidence from olivine CPO and trace elements

“…Dashed yellow lines depict the major tectonic boundaries of the Korean Peninsula (after Chough et al, 2000). Results of SKS-wave splitting measurements are compiled after Fouch and Fischer (1996), Fischer et al (1998), Long and van der Hilst (2006), Salah et al (2008), Wirth and Long (2008), Chang et al (2009), Kang andShin (2009), Salah et al (2009), where solid bars indicate the fast polarization direction and red circles are scaled to denote the delay time. Satellite image downloaded from GoogleEarth.…”

Seismic anisotropy in the wedge above the Philippine Sea slab beneath Kanto and southwest Japan derived from shear wave splitting

“…It can provide us with useful information about the in-situ stress state in the crust. The presence of anisotropy in the crust and upper mantle beneath subduction zones has also been well established (e.g., Ando et al, 1983;Kaneshima and Ando, 1989;Kaneshima, 1990;Fischer and Yang, 1994;Yang et al, 1995;Fouch and Fischer, 1996;Hiramatsu and Ando, 1996;Iidaka and Obara, 1995;Okada et al, 1995;Fischer et al, 1998Fischer et al, , 2000van der Hilst, 2005, 2006;Nakajima et al, 2006, and many others). A diverse range of splitting behavior has been associated with different subduction zones.…”

“…A diverse range of splitting behavior has been associated with different subduction zones. Both trench-parallel and trench-perpendicular fast directions have been recorded in many subduction zones (e.g., Russo and Silver, 1994;Yang et al, 1995;Fouch and Fisher, 1996;Levin et al, 2004;Nakajima and Hasegawa, 2004;Long and van der Hilst, 2005), whereas in some areas of other subduction zones no splitting is observed (Fischer et al, 1998).…”

Theory and Observations - Seismic Anisotropy

“…Fast polarization in the supraslab mantle wedge, as revealed by the splitting of S waves from earthquakes in downgoing lithosphere, shows a variable pattern. It is either trench-normal (Fischer et al, 1998), consistent with shear-coupled corner flow or back-arc extension; trench-parallel (Cassidy and Bostock, 1996;Long and van der Hilst, 2006), consistent with 3-D flow or B-type fabric; or splitting is absent (Fischer et al, 1998;Wiemer et al, 1999), consistent with poorly developed wedge flows in tank simulations of slab rollback (Buttles and Olson, 1998).…”

“…Trenchnormal fast polarization in the back-arc has been associated with the entrainment of mantle by shear coupling to the slab Schutt and Humphreys, 2001;Wang and Zhao, 2010), but Fischer et al (1998) found that fast-axis alignment in several Pacific subduction zones parallels backarc spreading rather than subduction azimuth. Fast polarization in the supraslab mantle wedge, as revealed by the splitting of S waves from earthquakes in downgoing lithosphere, shows a variable pattern.…”