Seismic anisotropy produced by serpentine in mantle wedge

Abstract

Trench-parallel seismic anisotropy has been observed in many subduction zones in the upper mantle. In this study, the crystal-preferred orientation (CPO) and seismic anisotropy of natural serpentine from Val Malenco in northern Italy and Punta Bettolina in western Italy are studied. It is found that the [001] axes are aligned subnormal to the foliation but the [010] axes of the serpentine are aligned subparallel to the lineation, which is significantly different from that produced in a recent high-pressure experiment. CPOs of serpentine found herein can be used to explain the trench-parallel seismic anisotropy in a cold mantle wedge for serpentine deformed not only at angles greater than 45° but also at low angles such as in horizontal shear from the surface, indicating the broader implications of the CPO of serpentine for interpreting seismic anisotropy than previously thought. It is also found that seismic anisotropy caused by the CPO of serpentine depends on the degree of serpentinization and flow geometry. The seismic anisotropy increases with antigorite volume fraction. The seismic anisotropy of shear waves is large, i.e., 23–36% for samples with an antigorite volume fraction of 40–93%. Current data suggest that the trench-parallel seismic anisotropy with a delay time of 1–2 s in the forearc mantle wedge can be attributed to the CPO of antigorite having [010] axes aligned subparallel to the flow direction and [001] axes subnormal to the flow plane.