Seismology in Ryukyu arc, Japan reveals the distribution and orientation of serpentine minerals suggesting convection and low viscosity of forearc mantle

Abstract:

Refining available estimates of the amount, distribution and alignment of serpentinite in the forearc wedge is needed to develop a better understanding of the seismic anisotropy, strength and fluid transport in this region. Mantle dominantly consists of olivine. However, petrological studies and thermal modeling of convergent margins predict that olivine will be replaced by hydrous mineral phases in fluid-rich and relatively cold forearc mantle. The dominant hydrous mineral will be antigorite. Lower seismic velocities (Vp < ~8 km/s and Vs < ~4 km/s) and higher Vp/Vs values (> ~1.8) of serpentine minerals than those of olivine are commonly used as to detect the distribution of antigorite and estimate its proportion compared to olivine.

However, antigorite is highly anisotropic and this anisotropy can disguise the presence of antigorite in seismic tomography; the apparent Vp/Vs ratio of antigorite can vary from 1.2–3.4 (Vp = 5.6–8.9 km/s and Vs = 2.5–5.1 km/s) depending on the propagation path of the seismic wave relative to the crystal orientation. Here, we take advantage of this anisotropy and perform an analysis of seismic anisotropy that takes into account ray path measured above the forearc mantle of the Rykuyu arc subduction zone. The measured shear wave splitting delay time above this subduction zone is very large, suggesting the presence of aligned antigorite.

Comparing the results of modeling to observed shear wave splitting for both local-S and teleseismic SKS phases, we conclude that the mantle wedge consists of 65 % antigorite and that the antigorite must be aligned along the subducting slab in the deepest part of the wedge but aligned vertically at intermediate depths. This distribution of different orientations strongly suggests the presence of convective mantle flow in the forearc mantle. Physical modeling of the dynamics of the mantle wedge shows that a bulk long-term viscosity of less than 10¹⁹ Pa s is required to maintain this large-scale flow.

This analysis reveals the presence of otherwise invisible antigorite bearing domains, allows an estimate of the volume percent of serpentinite in the forearc mantle to be made and indicates the pattern of large scale induced flow in this domain.