Читать книгу Mantle Convection and Surface Expressions - Группа авторов - Страница 26

We analyzed the long‐wavelength structure of four recent global tomographic models and compared the features of these models with predicted structures in five geodynamic models that incorporate surface velocity constraints from plate reconstructions. The long‐wavelength radial correlation functions of SEMUCB‐WM1 and SEISGLOB2 show strong evidence for a change in radial correlation structure near 1,000 km depth, whereas the most abrupt change in the RCFs for S362ANI+M and GLAD‐M15 occur at 650 km depth. The change in the RCF reflects a change in the pattern of long‐wavelength structure between the lower mantle and transition zone. The transition zone structure is correlated with more recent subduction history, whereas the long‐wavelength, lower‐mantle structure is more similar to ancient subduction history, in agreement with previous work. The long‐wavelength transition zone structure is dominated by seismically fast anomalies in the Western Pacific, and especially beneath the Philippine Sea Plate. This suggests that the change in the pattern of long‐wavelength heterogeneity in the transition zone and shallow lower mantle is controlled by subduction history and by the interaction of slabs with mantle phase transitions and rheological changes. The depth of changes in the pattern of heterogeneity in the models (and the associated RCFs) is sensitive to the data used to constrain the inversions and the radial parameterizations. Future studies that investigate whether these changes in long‐wavelength structure occur at 650 km depth or somewhat deeper within the lower mantle will have important implications for our understanding of mantle structure and dynamics. All of the tomographic models show a local minimum in the spectral slope at or slightly above 650 km, indicating concentration of power at longer wavelengths within the transition zone. This feature is most consistent with a global geodynamic model that includes a weakly endothermic (–2 MPa/K) phase transition at 660 km depth and a low viscosity channel below 660 km and a viscosity increase in the shallow lower mantle. New inferences of the viscosity profile (Figure 1.6) using both a whole‐mantle density model from full‐spectrum tomography (Moulik and Ekström, 2016) and a scaled V_S model (SEMUCB‐WM1, French and Romanowicz (2014)), recover viscosity profiles that are compatible with the presence of a low‐viscosity channel below 660 km depth and a viscosity maximum in the mid‐mantle.