Seismic anisotropy has been widely observed in crust and mantle materials and plays a key role in the understanding of structure and flow patterns. Although seismic anisotropy can be explained by the crystal preferred orientation (CPO) of highly anisotropic minerals in the crust, that is, amphibole, experimental studies on the CPO of amphibole are limited. Here we present the results of novel experiments on simple shear deformation of amphibolite at high pressure and temperatures (1 GPa, 480–700 °C). Depending on the temperature and stress, the deformed amphibole produced three types of CPOs and resulted in a strong seismic anisotropy. Our data provide a new understanding of the observed seismic anisotropy. The seismic data obtained from the amphibole CPOs revealed that anomalous seismic anisotropy observed in the deep crust, subducting slab and mantle wedge can be attributed to the CPO of amphibole.

The potential seismological contributions of metamorphosed and deformed oceanic crust in a subduction zone environment were studied in a detailed petro‐fabric analysis of blueschist and eclogite in the North Qilian suture zone, NW China. The calculated whole‐rock seismic properties based on the measured lattice preferred orientations of the constituting minerals show increasing P‐wave and S‐wave velocities and decreasing seismic anisotropies from blueschist to eclogite, mainly due to the decreasing volume proportion and deformation extent of glaucophane. The low velocity of the upper layer in the subducting oceanic crust can be explained by the existence of blueschist and foliated eclogite, which induces a 3–12% reduction in velocity compared to that induced by the surrounding mantle rocks. This low‐velocity layer may gradually disappear when blueschist and foliated eclogite are replaced by massive eclogite at a depth in excess of 60–75 km for the paleo North Qilian subduction zone. Trench‐parallel seismic anisotropy with a moderate delay time (0.1–0.3 s) can only effectively contribute to deformed blueschist and eclogite in a high‐angle (>45–60°) subducting slab, regardless of the direction of slab movement. The calculated reflection coefficients (R_c = 0.04–0.20) at the lithologic interfaces between eclogite and blueschist imply that it may be possible to detect eclogite bodies in shallow subduction channels using high‐resolution seismic reflection profiles. However, the imaging of eclogite bodies located in deep subduction zones could be challenging.

The fore-arc mantle above a subducting slab is a unique site where complex partial melting, melt/fluid–rock interaction, and deformation of mantle rocks occur. To constrain these geological and geodynamic processes we analyzed the deformation microstructures, crystal preferred orientations (CPOs or fabrics), and water content in natural harzburgites that occur as exhumed massifs in the North Qilian suture zone, NW China. These harzburgites are very fresh, and have mineral assemblages of olivine (∼81–87 vol. %), orthopyroxene (∼11–17 vol. %), clinopyroxene (∼1–2 vol. %), and spinel (∼1 vol. %). Detailed analyses of mineral textures, CPO patterns, and rotation axis distributions suggest that the plastic deformation of olivine and pyroxene was accommodated by activating a series of slip systems of dislocation. The olivine (A-/D-type fabric) shows dominant (010)[100] and/or (001)[100] slip systems, as well as other minor [100]-glide, {0kl}[100], and [001]-glide slip systems. The orthopyroxene shows dominant (100)[001] and subordinate (010)[001] slip systems, with minor (100)[010] and (100)[0vw] slip systems. The water content is extremely low in the orthopyroxene (38–44 ppm by weight), equilibrated olivine (4–7 ppm), and bulk-rock samples (9–14 ppm). Previously published refractory mineral and whole-rock compositions, as well as estimated low-pressure (∼1–2 GPa), high-temperature (∼1100–1300°C), low-stress (∼1–4 MPa) and water-poor conditions of deformation, suggest that these harzburgites represent a remnant of fossil fore-arc lithospheric mantle that was probably both formed (experienced partial melting and high-temperature melt/fluid–rock reaction) and deformed in a young and warm fore-arc mantle setting (i.e. juvenile subduction zone). Based on these results, a refined schematic model of olivine fabric distributions in subduction zones is proposed. In this model, the A-/D-type olivine fabrics are inferred to be prevalent in fore-arc lithospheric mantle. The opposing polarizing directions of A-/D-type olivine fabrics with other underlying anisotropic sources in the mantle wedge (e.g. B-type olivine fabrics and oriented serpentinite layers) may weaken the trench-parallel fast S-wave anisotropy contributed by the deformed fore-arc mantle, and thus provide an alternative explanation for the short or nearly null delay times of local shear-wave splitting (sourced from intra-slab earthquakes) that have been detected in some fore-arc regions. In addition, topotactic antigorite fabrics after the A-/D-type olivine fabric might play a minor role in contributing to the low P- and S-wave velocities, high V_p/V_s ratios, and large seismic anisotropies that are typically observed in fore-arc mantle.