A. Tommasi, FORWARD MODELING OF THE DEVELOPMENT OF SEISMIC ANISOTROPY IN THE UPPER-MANTLE, Earth and planetary science letters, 160(1-2), 1998, pp. 1-13
Development of seismic anisotropy in response to upper mantle flow is
approached through an integrated numerical model. This model allows to
predict the splitting parameters for a shear wave propagating across
an upper mantle which deformed in response to a given geodynamic proce
ss. It consists of (1) thermo-mechanical modeling of the finite strain
field, (2) modeling olivine lattice-preferred orientation (LPO) gener
ated by this strain held, (3) calculation of the 3-D elastic propertie
s associated with this LPO, and (4) estimation of the shear-wave split
ting parameters: the time lag between the fast and slow split shear wa
ve arrivals (delta t) and the polarization azimuth of the fast wave (p
hi). Modeled olivine LPO are constrained relative to LPO measured in n
aturally and experimentally deformed peridotites. Comparison of predic
ted shear-wave splitting parameters with seismological data allows us
to quantify the possible contribution of the modeled upper mantle flow
to the measured splitting values and, hence, to constrain the interpr
etation of shear-wave splitting data in terms of upper mantle flow. We
use this forward model to investigate the seismic anisotropy generate
d in ocean basins by a velocity gradient between the plate and the dee
p mantle. Fast-shear wave polarizations calculated assuming a constant
plate motion are in good agreement with both the SKS polarization and
the fast propagation direction for P and Rayleigh waves observed in t
he Pacific and Indian oceans, suggesting that, away from mid-ocean rid
ges, seismic anisotropy in oceanic basins primarily results from asthe
nospheric deformation by resistive drag beneath the plate. Delay times
are, however, overestimated. This may be ascribed to a stronger strai
n localization in nature or to partial erosion of the anisotropic laye
r by hotspots. Indeed, hotspot activity may explain the short length s
cale variations of delta t in the southern Pacific. Finally, two-layer
models that simulate a change in Pacific plate motion as suggested by
the bend in the Hawaii-Emperor chain fail to reproduce the observed s
hear-wave splitting. This is consistent with previous suggestions that
the Emperor chain track may not faithfully record the Pacific plate a
bsolute motion before 43 Ma. (C) 1998 Elsevier Science B.V. All rights
reserved.