Microstructures and petro-fabrics of lawsonite blueschist in the North Qilian suture zone, NW China: Implications for seismic anisotropy of subducting oceanic crust

We conducted a detailed study on themicrostructures and petro-fabrics ofmassive and foliated lawsonite blueschist (LBS) in North Qilian suture zone, NWChina. The lattice preferred orientation (LPO) of glaucophane and lawsonite in foliated lawsonite blueschist (LBS) is considered to be dominantly formed by the deformation mechanism of dislo-cation creep and rigid-body rotation, respectively. The LPO of glaucophane is mainly characterized by the [001] axis aligning parallel to lineation and the [100] axis and (110) pole plunging perpendicular to foliation. In contrast, the LPO of lawsonite features the maximum [010] axis concentrated close to lineation and the [001] axis strongly clus-tered normal to foliation. The preferred orientation of [010] axis of lawsonite parallel to lineation is supported by a two-dimensional numerical modeling using the nite-volumemethod(FVM).ThemineralLPOsaremuchstronger in foliated LBS than inmassive LBS. In addition, a kinematic vorticity analysis suggests that both pure shear dominant (Wm =0.18-0.26) and simple shear dominant (Wm =0.86-0.93) deformation regimes are present in foliated LBS. The [001] axis and (010) pole of glaucophane, and the [100] and [010] axes of lawsonite, tend to distribute in a fo-liation-parallel girdle in the pure shear dominant samples, but simple shear dominant samples display more linea-tion-parallelconcentrationsofa[001]axisofglaucophaneanda[010]axisoflawsonite.Becausethewhole-rock seismic anisotropies in foliated LBS are signicantly higher than those in massive LBS and a counteracting effect on seismic anisotropies occurs between glaucophane and lawsonite, the delay time of fast S-wave polarization an-isotropy induced by an actual subducting oceanic crustwith a high subducting angle (>45-60°) is expected to range from0.03 to 0.09 s (lower bound formassive LBS) and from0.1 to 0.3 s (upper bound for foliated epidote blueschist). © 2014 Elsevier B.V.