Measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy: 3. Inversion

Abstract

Lateral heterogeneity in the earth's upper mantle is investigated by inverting dispersion curves of long‐period surface waves (100–330 s). Models for seven different tectonic regions are derived by inversion of regionalized great circle phase velocity measurements from our previous studies. We also obtain a representation of upper mantle heterogeneities with no a priori regionalization from the inversion of the degree 6 spherical harmonic expansion of phase and group velocities. The data are from the observation of about 200 paths for Love waves and 250 paths for Rayleigh waves. For both the regionalized and the spherical harmonic inversions, corrections are applied to take into account lateral variations in crustal thickness and other shallow parameters. These corrections are found to be important, especially at low spherical harmonic order the “trench region” and fast velocities down to 250 km under shields. Below 200 km under the oceans, both S velocity and S anisotropy support a model of small‐scale convection in which cold blobs detach from the bottom of the lithosphere when its age is large enough. The spherical harmonic models clearly demonstrate (a posteriori) the relation between surface tectonics and S velocity heterogeneities in the first 250 km: all shields are fast; most ridges are slow; below 300 km, a belt of fast mantle follows the Pacific subduction zones. However, at greater depths, large‐scale heterogeneities that seem to bear no relationship to surface tectonics are observed. The most prominent feature at 450 km is a fast‐velocity region under the South Atlantic Ocean. Smaller‐scale heterogeneities that are not related to surface tectonics are also mapped at shallower depths: an anomalously slow region centered in the south central Pacific is possibly linked to intense hot spot activity; a very fast region southeast of South America may be related to subduction of old Pacific plate. Between 200 and 400 km, a belt of SV>SH anisotropy follows part of the ridge and subduction systems, indicating vertical mantle flow in these regions. The spherical harmonic results open new horizons for the understanding of convection in the mantle. Perspectives for the improvement of the models presented are discussed.