JOINT INVERSIONS OF SEISMIC AND GEODYNAMIC DATA FOR MODELS OF 3-DIMENSIONAL MANTLE HETEROGENEITY

The seismic models of three-dimensional (3-D) mantle heterogeneity may be interpreted in terms of the density perturbations which drive mant le flow and thus provide important predictions of flow-related observa bles, such as the nonhydrostatic geoid. The current models of global-s cale shear velocity heterogeneity provide reasonably good fits to the long-wavelength nonhydrostatic geoid data (60-70% variance reductions) but rather poor fits to the corresponding free-air gravity anomalies (30-40% variance reductions). This major difference is due to two fact ors: (1) the very different amplitude spectrum of the gravity anomalie s, which is nearly flat for degrees l < 8, and (2) the relatively poor match between the individual harmonic components of the predicted and observed gravity anomalies for degrees l > 3. The largest mismatch be tween the pattern of predicted and observed gravity anomalies is in th e southern hemisphere. This observation suggests that one reason for t he poor overall match between the two fields may be that the global se ismic data are not resolving sufficiently well the heterogeneity in th e southern hemisphere portion of the deep mantle. In contrast, the gra vity data provide accurate and geographically uniform constraints on t he vertically integrated heterogeneity in the mantle. We therefore per form a series of experiments in which we simultaneously invert a large set of seismic data (which includes long-period waveforms, SS - S and ScS - S differential travel times, and normal-mode structure coeffici ents) and the long-wavelength gravity anomaly data. We thus determine whether it is possible to derive new 3-D heterogeneity models which sa tisfy both data sets. The gravity anomaly data are interpreted in the context of spherically-symmetric viscous flow models of the mantle. In these inversion experiments we test several radial viscosity and delt a ln rho/delta ln upsilon profiles and thereby assess their plausibili ty. The joint seismic-geodynamic inversions reveal that it is indeed p ossible to greatly improve the fit to the free-air gravity anomalies ( with variance reductions of 80-90% readily accessible) while preservin g the fit to the seismic data. This improvement is achieved with some adjustment to the heterogeneity in the depth range 1500-2500 km, where the seismic data constraints appear to be weakest. The joint inversio ns also reveal new structures in the southern hemisphere portion of th e lower mantle which apparently are not resolved by the seismic data a lone.