Comparison between tomographic structures and models of convection in the upper mantle

Gruneisen's and third-order finite-strain theories are used to compute the density and seismic-wave velocities of minerals. Assuming the mineralogical model of Ito & Takahashi (1987), seismic velocities of the upper mantle are calculated using the Hashin-Shtrikman averaging procedure. 1-D profiles are first obtained along adiabats, and compared to the IASP91 model. Different adiabats are considered in order to take into account the thermal effect of phase transitions. The best results are found with adiabats initiated at 1473, 1573 and 1613 K for alpha-olivine, beta- and gamma-spinel, respectively. The incorporation of thermal effects resulting from phase transitions gives velocity jumps at discontinuities close to those of IASP91. Next, a model of convection constructed by Dupeyrat, Sotin & Parmentier (1995), incorporating plate tectonics, is used to compute 1-D profiles and 2-D fields of seismic anomalies in the upper mantle. Averaged profiles show seismic-velocity gradients very close to those of IASP91, but individual values are much too high, suggesting that the mean temperature profile of the convection model is too cold by 400 K. When lowpass filtered to the resolution scale of presently available tomographic models, both the amplitude and shape of the computed seismic anomalies are consistent with the results of tomographic studies. The amplitude of the anomalies ranges between -2.7 and 3.8% for P-wave slownesses, and from -3.3 to 4.5% for S-wave slownesses. These anomalies correspond to lateral temperature variations of -465 to 520 K. These calculations are used (1) as an aid to the interpretation of global tomographic models, for instance by computing spectra of lateral heterogeneities, and (2) to test the adequacy of the basic assumptions used in the computation of numerical models of mantle convection, and to build a theoretical temperature profile that would give the best fit to IASP91. In the uppermost mantle this theoretical model has a shape close to both the convection model and the 1473 K adiabat, but in the transition zone the profile is highly subadiabatic. The spectra obtained for the synthetic seismic anomalies resemble that of tomographic studies, with most of the energy contained within gravest angular orders l, and a fast decrease of energy with increasing l. The spatial filtering has clearly different effects on heterogeneities, depending on their respective wavelengths. It is suggested that the change of decreasing rate observed in tomographic models at l = 7 is closely related to the filter wavelength and may correspond at a lesser extent to a characteristic wavelength of mantle heterogeneities.