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

Abstract

We conducted a detailed study on the microstructures and petro-fabrics of massive and foliated lawsonite blueschist (LBS) in North Qilian suture zone, NW China. 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 dislocation 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 clustered normal to foliation. The preferred orientation of [010] axis of lawsonite parallel to lineation is supported by a two-dimensional numerical modeling using the finite-volume method (FVM). The mineral LPOs are much stronger in foliated LBS than in massive 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 foliation–parallel girdle in the pure shear dominant samples, but simple shear dominant samples display more lineation–parallel concentrations of a [001] axis of glaucophane and a [010] axis of lawsonite. Because the whole-rock seismic anisotropies in foliated LBS are significantly 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 anisotropy induced by an actual subducting oceanic crust with a high subducting angle (>45–60°) is expected to range from 0.03 to 0.09s (lower bound for massive LBS) and from 0.1 to 0.3s (upper bound for foliated epidote blueschist).