Uppermantle anisotropy and the oceanic lithosphere

Abstract

Rayleigh wave and Love wave dispersion data for oceanic paths cannot be satisfied by an isotropic uppermantle, and it is incorrect to invert these two datasets independently for separate isotropic structures. Available phase and group velocity data are inverted for oceanic structure as a function of age taking into account anelastic dispersion, sphericity, and anisotropy. The resulting models are quite different from previous results which ignore the above effects and the extra parameters involved in proper anisotropic inversion. The models have a high-velocity nearly isotropic layer at the top of the mantle that thickens with age, and an anisotropic low-velocity zone with an age-dependent anisotropy. The LID, or seismic lithosphere, reaches a maximum thickness of 50 km which is about one-half the thickness obtained in previous isotropic or pseudo-isotropic inversions. The seismic lithosphere therefore may be comparable in thickness to the elastic or flexural lithosphere, raising the possibility that both are controlled by effects other than temperature, stress and time, such as mineralogy, crystal orientation or partial melting. The velocities in the low-velocity zone are higher than inferred by previous surface wave studies.

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