Subduction Factory 5. Unusually low Poisson’s ratios in subduction zones from elastic anisotropy of peridotite

[1] The growing capability to measure seismic velocities in subduction zones has led to an unusual observation: VP/VS ratios as low as 1.65 with VS 4.7 km/s in the mantle wedge of some subduction zones. This is difficult to explain because most minerals have VP/VS ratios>1.75, and some of the likely alteration phases in mantle rocks, like antigorite, phlogopite, clinohumite and chlorite have isotropic high VP/VS ratios. It is possible that these measurements are biased by anisotropy in rock fabric or by the raypaths through these regions, leading to relatively high VS estimates and/or relatively low VP estimates compared with isotropic averages. Strong anisotropy has been documented in several mantle wedges, but its effect on velocity estimates are typically ignored. Anisotropic peridotites may produce the observed VP/VS ratios if olivine [100] axes are perpendicular rather than parallel to raypaths, consistent with typical seismic sampling geometries and with fabric predictions for wedge corner flow. Hence, low VP/VS ratios may be an indicator of strong anisotropy, rather than unusual composition, and may provide a useful additional constraint on orientation and strength of the rock fabric