Upper mantle anisotropy

Surface waves also provide a means for estimating upper-mantle anisotropy. There are three canonical ways of detecting anisotropy using surface waves. One comes from discrepancies in isotropic inversions of Love and Rayleigh phase velocities (e.g. Anderson 1961) and leads to transversely isotropic models or polar anisotropy. Another comes from azimuthal variations in phase velocities (e.g. Forsyth 1975) and leads to models of azimuthal or radial anisotropy. 

LPO) of olivine suggest that upper-mantle anisotropy may be locally much stronger in magnitude and more complex in symmetry (Blackman e( al. 1996 Tommasi 1998). There have also been suggestions for other mechanisms for anisotropy in the mantle, such as preferentially aligned inclusions (e.g. melt) (Schlue Knopoff 1977 Kendall 1994) and grain shape fabric or foliation (Wendt et al. 1999). 

Anderson, D. L. Dziewonski, A. M. 1983. Upper mantle anisotropy evidence from free oscillations. Geophysical Journal of the Royal Astronomical Society, 69, 383-404. 

Upper-mantle anisotropy and shear-wave splitting... 

Rochette P, Pillion G (1988) Identification of multicomponent anisotropies in rocks using various field and temperature values in a cryogenic magnetometer. Phys Earth Planet Interiors 51 379-386 Ronov AB, Yaroshevsky AA (1969) The Earth s Crast and Upper Mantle. American Geophysical Uiuon, Washington, DC... 

Anisotropy in the upper mantle is primarily attributed to the preferred alignment of olivine and, to a lesser extent, the alignment of other upper-mantle minerals. Evidence comes from measurements on mantle xenoliths (Mainprice Silver 1993 Ben Ismail Mainprice 1998) and ophiolites (Nicolas Christensen 1987) and laboratory deformation studies (Zhang Karato 1995). These measurements help guide the interpretation of seismic observations. It is conventionally assumed that the anisotropy is c.4-5% in magnitude and has hexagonal symmetry with a horizontal synunetry axis (azimuthal anisotropy) (Mainprice Silver 1993). Numerical simulations of the lattice preferred orientation... 

Ben Ismail, W. Mainprice, D. 1998. An olivine fabric database an overview of upper mantle fabrics and seismic anisotropy. Tectonophysics, 296, 145-157. 

Forsyth, D. W. 1975. The early structural evolution and anisotropy of the oceanic upper mantle. Geophysical Journal of the Royal Astrological Society, 43, 103-162. 

Nicolas, A. Christensen, N. 1.1987. Formation of anisotropy in upper mantle peridotites - a review. In Fuchs, K. Froidevaux, C. (eds) Composition, Structure and Dynamics of the Lithosphere-Asthenosphere System. Geodynamics Series, American Geophysical Union, 16,111-123. 

Simons, F. J., van der Hilst, R. D., Montagner J.-P. ZiELHUls, A. 2001. Multimode Rayleigh wave inversion for shear wave speed heterogeneity and azimuthal anisotropy of the Australian upper mantle. Geophysical Journal Interrmtional, 203, 1-17. 

Sutherland, F. H. 2000. Characterising upper-mantle seismic anisotropy in Archaean and Proterozoic Provinces beneath eastern Canada. MGeophy thesis. University of Leeds. 

Tommasi, a. 1998. Forward modelling of the development of seismic anisotropy in the upper mantle. Earth and Planetary Science Letters, 160, 1-13. 

Debayle, E. 1999. SV-wave azimuthal anisotropy in the Australian upper mantle preliminary results from automated Rayleigh waveform inversions. Geophysical Journal International, 137, 747-754. 

Debayle, E. Kennett, B. J. N. 2000h. Anisotropy in the Australian upper mantle from Love and Rayleigh waveform inversion. Earth and Planetary Science Letters, 184, 339-351. 

Nataf, H. C., Nakanishi, I. Anderson, D. L. 1984. Anisotropy and shear-velocity heterogeneities in the upper mantle. Geophysical Research Letters, 11, 109-112. 

Barruol, G. Kern, H. 1996. Seismic anisotropy and shear-wave splitting in lower crust and upper-mantle rocks from the Ivrea Zone-experimental and calculated data. Physics of the Earth and Planetary Interiors, 95, 175-194. 

Blackman, D. K., Wenk, H.-R. Kendall, J.-M. 2002. Seismic anisotropy of the upper mantle 1. Factors that affect mineral texture and effective elastic constants. Geochemistry, Geophysics, Geosystems, (in press). 

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