Mantle flow at a slab edge: Seismic anisotropy in the Kamchatka Region

Abstract

Abstract.The

Shear wave splitting from local events beneath the Ryukyu arc: Trench-parallel anisotropy in the mantle wedge

“…Trench-parallel flow beneath the subducting plate has been invoked to explain trench-parallel fast directions in South America (Russo and Silver, 1994;Anderson et al, 2004) and Kamchatka (Peyton et al, 2001); however, we reject such a mechanism here because our local splitting measurements suggest that the trench-parallel anisotropy in the Ryukyu arc has its origin in the mantle wedge, that is, above the plate. Instead, we consider the possibility of trench-parallel flow in the wedge itself.…”

Rock seismic anisotropy of the low‐velocity zone beneath the volcanic front in the mantle wedge

“…The thickness (T) of an anisotropic layer is given by T = (100dthVsi)/AVs, where dt is the delay time of S-waves, hVsi is the average velocity of the fast and slow velocities, and AVs is the anisotropy for a specific propagation direction expressed as a percentage [e.g., Pera et al, 2004]. Accordingly, we estimated an anisotropic layer of 13-38 km thickness to explain the observed local-S fastpolarization axes with splitting delays of 0.1-0.3 s (at PET in Figure 1) [Peyton et al, 2001;Levin et al, 2004], indicating that the intrinsic rock seismic anisotropy is sufficient to generate the observed delay time.…”

“…[3] On shear-wave polarization anisotropy in the mantle wedge, whereas the orientation of the fast direction perpendicular to the trench axis on the back-arc region is likely to reflect a-axis slip olivine fabrics (A-type) in mantle [e.g., Nicolas and Christensen, 1987], the orientation of the fast direction commonly parallel to the trench axis on the forearc region might reflect a number of possible mechanisms: deformation of olivine via c-axis slip (B-type fabric) [Jung and Karato, 2001;Karato, 2003], trench-parallel flow [e.g., Smith et al, 2001;Peyton et al, 2001], crack induced anisotropy in the crust and/or slab [Currie et al, 2001], or highly anisotropic foliated antigorite serpentine [Kneller and van Keken, 2007]. Here, we present that peridotite xenoliths derived from the Avacha frontal volcano, Kamchatka, preserve a-axis slip fabrics, comparable with fabrics in xenoliths from the back-arc region.…”

Upper mantle anisotropy beneath Japan from shear wave splitting

“…It is worth noting that both trench-perpendicular and trench-parallel fast directions associated with subduction zone anisotropy have been observed in different parts of the world (e.g., Fouch and Fischer, 1996;Smith et al, 2001;Levin et al, 2004). Three possible mechanisms have been suggested to generate trench-parallel fast directions above a subduction zone: trench-parallel flow above the slab, due to transpression of the overlying mantle wedge (Mehl et al, 2003), trench-parallel flow beneath the slab due to slab rollback or a similar mechanism (Peyton et al, 2001) or corner flow in the mantle wedge induced by viscous coupling with the downgoing slab, along with the presence of an "exotic" deformation-induced LPO pattern (Jung and Karato, 2001;Holtzman et al, 2003).…”

“…First, the magnitude of the splitting times for F-net stations (often larger than 1 s) may be difficult to explain with lithospheric anisotropy alone, but would be consistent with the large deformation that can be expected at subduction zones. Second, there is significant observational evidence for an asthenospheric component in subduction zones worldwide (e.g., Fischer et al, 1998;Fischer and Wiens, 1996;Fouch and Fischer, 1996;Smith and Fouch, submitted for publication;Peyton et al, 2001, and others). Furthermore, for stations that are consistent with a simple anisotropic model (Fig.…”