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

Abstract

Abstract. Lateral heterogeneity in the earth'~-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 aoout 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 spheric<;J.l 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 c],early 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 fastvelocity 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 pre sen ted are discussed.'Now at

Global mapping of the uppermantle by surface wave tomography

“…In general the perturbations across discontinuities is highly correlated. In both Nataf et al (1984Nataf et al ( , 1986 and Woodhouse and Dziewonski (1984) the character of the perturbations changes at 220 and 400 km:…”

“…Perturbations are assumed to be smooth between the discontinuities in PREM (60, 220, 400 and 670 km) and to be loosely coupled across these discontinuities (Nataf et al, 1986). Thus, the radial variation in perturbation can change rapidly at physical discontinuities.…”

“…Techniques have recently been developed to simultaneously invert data over many great cicle paths to obtain global maps of lateral heterogeneity (Nakanishi and Anderson, 1982, 1983a, b, 1984, Woodhouse and Dziewonski, 1984, (Fig. 1) azimuthal anisotropy (Tanimoto and Anderson, 1983, 1984, 1985 and polarization anisotropy (Nataf et al, 1984(Nataf et al, , 1986. Although the results are currently of relatively low resolving power they can be used to discuss the regional structure of the uppermantle on a scale which is comparable to the sizes of the plates and the characteristic lengths which are thought to be important in plate tectonics.…”

“…Upper Mantle Models Nataf et al (1984Nataf et al ( , 1986 inverted the data sets of Nakanishi and Anderson to obtain the lateral structure and polarization anisotropy of the mantle to depths of 670 km. The resolution of the data is best between about 200 and 400 km.…”

The temperature sensitivity of elastic wave velocity at high pressure: New results for molybdenum

“…Lateral variability of longitudinal and shear wave velocity has been extensively documented throughout the Earth including the upper mantle [Natal et al, 1986], subducting slabs [Engdahl and Gubbins, 1987], the lower mantle [Dziewonski and Woodhouse, 1987], and the D" region at the base of the mantle [Wysession et al, 1992;Silver and Bina, 1993]. Tomographic studies yield compressional wave velocity (Vp) anomalies with RMS amplitudes of ~0.3-0.5% in the upper mantle and the D" layer and ~0.1-0.2% in the lower mantle [Dziewonski and Woodhouse, 1987;Hager and Clayton, 1989].…”