Open AccessSubduction Factory 5: Unusually low Poisson's ratios in subduction zones from elastic anisotropy of peridotite
Author(s) -
Hacker Bradley R.,
Abers Geoffrey A.
Publication year - 2012
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2012jb009187
Subject(s) - geology , subduction , peridotite , seismic anisotropy , mantle (geology) , anisotropy , mantle wedge , olivine , isotropy , petrology , geophysics , seismology , mineralogy , tectonics , physics , quantum mechanics
The growing capability to measure seismic velocities in subduction zones has led to an unusual observation: V P /V S ratios as low as 1.65 with V S ∼ 4.7 km/s in the mantle wedge of some subduction zones. This is difficult to explain because most minerals have V P /V S ratios >1.75, and some of the likely alteration phases in mantle rocks, like antigorite, phlogopite, clinohumite and chlorite have isotropic high V P /V S 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 V S estimates and/or relatively low V P 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 V P /V S 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 V P /V S 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.