Complex mantle flow in the Mariana subduction system: evidence from shear wave splitting

Abstract

S U M M A R YShear wave splitting measurements provide significant information about subduction zone mantle flow, which is closely tied to plate motions, lithospheric deformation, arc volcanism, and backarc spreading processes. We analyse the shear wave splitting of local S waves recorded by a large 2003-2004 deployment consisting of 58 ocean-bottom seismographs (OBSs) and 20 land stations and by nine OBSs from a smaller 2001-2002 deployment. We employ several methods and data processing schemes, including spatial averaging methods, to obtain stable and consistent shear wave splitting patterns throughout the arc-backarc system. Observed fast orientation solutions are dependent on event location and depth, suggesting that anisotropic fabric in the mantle wedge is highly heterogeneous. Shear waves sampling beneath the northern island arc (latitudes 17.5 • -19 • N) and between the arc and backarc spreading centre show arcparallel fast orientations for events shallower than 250 km depth; whereas, fast orientations at the same stations are somewhat different for deeper events. Waves sampling beneath the central island arc stations (latitudes 15.5 • -17.5 • ) show fast orientations subparallel to both the arc and absolute plate motion (APM) for events <250 km depth and APM-parallel for deeper events. Ray paths sampling west of the spreading centre show fast orientations ranging from arc-perpendicular to APM-parallel. Arc-parallel fast orientations characterize the southern part of the arc with variable orientations surrounding Guam. These results suggest that the typical interpretation of mantle wedge flow strongly coupled to the downgoing slab is valid only at depths greater than ∼250 km and at large distances from the trench. We conclude that the arc-parallel fast orientations are likely the result of physical arc-parallel mantle flow and are not due to recently proposed alternative lattice preferred orientation mechanisms and fabrics. This flow pattern may result from along-strike pressure gradients in the mantle wedge, possibly due to changes in slab dip and/or convergence angles.

Weakening of the subduction interface and its effects on surface heat flow, slab dehydration, and mantle wedge serpentinization

“…[59] The second issue is how to explain the presence of arc-parallel fast direction observed in the arc-back arc mantle wedge of some subduction zones, such as Tonga [Smith et al, 2001], Ryukyu [Long and van der Hilst, 2006], and Mariana [Pozgay et al, 2007]. Beneath the arc and the back arc, temperatures are too high and stresses are too low to allow the formation of B-type olivine fabric.…”

Deformation microstructures of olivine and chlorite in chlorite peridotites from Almklovdalen in the Western Gneiss Region, southwest Norway, and implications for seismic anisotropy

“…In contrast, for the high dipping angle (θ > 50º) of slab in the subduction zone ( Figure 14(b)), LPO of chlorite will cause trenchparallel seismic anisotropy for the vertically propagating fast S-waves. For example, the slab under both Tonga and Mariana is dipping at a high angle and a strong trenchparallel seismic anisotropy was observed under the forearc and arc areas (Smith et al 2001;Pozgay et al 2007), which may be attributed to the LPO of chlorite in addition to the B-type LPO of olivine in the mantle wedge. Figure 14(c) illustrates the change of seismic anisotropy when a dipping angle of slab in a subduction zone changes from low to high angle.…”

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

“…As with the sub-slab case, the pattern that would be predicted for a simple corner flow model with A-type olivine fabric-dominantly trenchperpendicular fast directions-is virtually absent from the global database. Arc-parallel flow in localized channels (e.g., Pozgay et al 2007), the presence of B-type fabric in the forearc wedge (e.g., Nakajima and Hasegawa 2004), complex slab morphology (Kneller and van Keken 2007), and the foundering of lower crust ) have all been invoked to explain the complex splitting patterns associated with anisotropy in the wedge.…”

“…For example, array studies of shear wave splitting in subduction zones have led to the detailed characterization of splitting patterns in the mantle wedge above subducting slabs, which typically show dramatic lateral variations (e.g., Fischer et al 1998;Smith et al 2001;Levin et al 2004;Pozgay et al 2007). Analysis of the length scales over which splitting changes can also provide insight about the processes that produce short-scale changes in anisotropic structure, and about the lateral and vertical sensitivity of shear wave splitting measurements.…”