Sidorin, I.A., 1999, Dynamically consistent interpretation of the seismic structure at the base of the mantle (convection, D'' Layer, discontinuity): Pasadena, California, California Institute of Technology, Ph.D. dissertation, 153 p., illust.
There is increasing evidence of large degrees of heterogeneity in the seismic structure of the lowermost 200 - 300 km of the mantle constituting the D' layer. One of the most diagnostic seismologically observed features in the D' region is an apparent seismic velocity discontinuity 200-300 km above the core-mantle boundary, generally referred to as the D' discontinuity. We use a combination of dynamic and seismic waveform modeling to provide tighter constraints on this structural feature of D' and reduce the tradeoffs that exist in both seismological studies and dynamic modeling. The dynamic models are based on the adiabatic model that is computed in Chapter 2 by integrating available mineral physics data. In Chapter 3 we demonstrate that a model with a chemical layer at the base of the mantle does not provide a consistent explanation for the seismological observations of the D' discontinuity. We propose that the strength of the triplication is conditioned by both the abrupt velocity increase at the D' discontinuity and the local velocity structure accompanying the discontinuity. We also show that purely thermal gradients computed from convection models do not produce a sufficiently strong Scd phase. In Chapter 4 we suggest that the observed regional patterns in the strength of the D' triplication are most compatible with a phase change model of the D' discontinuity. In Chapter 5 a variety of convection models with a basal phase transition are tested to obtain the characteristics of the phase transition most compatible with observations. We find that the best value for the ambient elevation above the core-mantle boundary is about 150 km and the best value for the Clapeyron slope is about 6 MPa/K. In Chapter 6 this model is further tested by placing a discontinuity in context of the global shear velocity structure recovered by Grand's [1994] tomography model. We find that such a synthetic velocity model with a phase change characterized by a shear velocity contrast of 1.5%, ambient elevation [special characters omitted]; 200 km and Clapeyron slope [special characters omitted]; 6 MPa/K predicts the observed differential travel times patterns for the D' triplication beneath Alaska, Eurasia and Central America. The model also provides an explanation for the apparent intermittance of the D' discontinuity by predicting very weak triplication for Central Pacific and north-eastern Caribbean where convincing evidence for the D' triplication is lacking.
Theses and Dissertations
