Arc‐parallel flow within the mantle wedge: Evidence from the accreted Talkeetna arc, south central Alaska

Abstract

Residual mantle exposures in the accreted Talkeetna arc, Alaska, provide the first rock analog for the arc‐parallel flow that is inferred from seismic anisotropy at several modern arcs. The peridotites exposed at the base of the Jurassic Talkeetna arc have a Moho‐parallel foliation and indicate dislocation creep of olivine at temperatures of ∼1000° to >1100°C. Slip occurred chiefly on the (001)[100] slip system, which has only rarely been observed to be the dominant slip system in olivine. Stretching lineations and olivine [100] slip directions are subparallel to the long axis of the Talkeetna arc for over 200 km, indicating that mantle flow was parallel to the arc axis. The alignment of the olivine [100] axes yields a calculated S wave anisotropy with the fast polarization direction parallel to the arc. Thus (1) the fast polarization directions observed parallel to some modern arcs now have an exposed geological analog; (2) arc‐parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones, fluid‐filled pockets, or glide on the (010)[001] H2O‐induced slip system; (3) seismic anisotropy beneath modern arcs may be due to slip on (001)[100] with a horizontal foliation rather than slip on (010)[100] with a vertical foliation; and (4) the observed dominance of the (001)[100] slip system may be due to high H2O concentrations, suggesting that strain in the oceanic upper mantle may be accommodated dominantly by (001)[100] olivine slip.

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).…”

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

“…An astonishing diversity in splitting patterns has been identified in different regions from both local S phases (from earthquakes originating in the slab) and teleseismic phases such as SKS and direct S, including both trench-parallel and trenchperpendicular fast directions (with some oblique directions as well) and widely variable dt values. A variety of models has been proposed to explain shear wave splitting observations in specific regions, including those that invoke 2-D corner flow (e.g., Fischer et al 2000;Hall et al 2000), B-type olivine fabric (e.g., Nakajima and Hasegawa 2004;Long et al 2007b;Kneller et al 2008;Jung et al 2009), trench-parallel flow above (e.g., Smith et al 2001;Conder and Wiens 2007;Hoernle et al 2008) or below (Russo and Silver 1994;Long and Silver 2008) the slab, foundering of lower crust beneath the island arc , transpression due to oblique subduction (Mehl et al 2003), anisotropy due to aligned hydrated faults in the slab (Faccenda et al 2008), or some combination of these mechanisms. It has proven difficult, however, to identify a unique, synoptic model for subduction zone anisotropy that can explain the global variability in splitting observations.…”

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

“…Both trench-parallel and trench-perpendicular fast directions have been observed in subduction zone settings (for a detailed review, see Wiens and Smith, 2003), but trench-parallel fast directions seem to be more common and have been observed or inferred in New Zealand (Marson-Pidgeon et al, 1999), the Aleutians (Yang et al, 1995;Mehl et al, 2003), Japan (Fouch and Fischer, 1996), and Tonga (Fischer and Wiens, 1996), among other regions. These observations of trench-parallel fast directions contradict the predictions of simple cornerflow models for flow in the mantle wedge, in which viscous coupling between the downgoing slab and the overlying wedge material induces flow that is parallel to the convergence direction (e.g., Fischer et al, 2000).…”