Dynamics of large-scale lower mantle seismic structure elucidated through computational models, plate evolution, and mineral physics
|Location||International Geological Congress,oslo 2008|
|Author||Bower, Dan; Tan, Eh; Sun, Daoyuan; Gurnis, Michael; Helmberger, Don|
|Holding Date||21 September 2008|
Seismic tomography reveals two Large Low Velocity Provinces (LLVPs) under the Pacific and Africa atop the core mantle boundary. Various observations suggest that compositional heterogeneity exists in the LLVPs: the anti-correlation of shear and bulk sound velocity, the sharp edge of African LLVP, the spatial correlation of surface hot spots, the spatial and temporal correlation with eruption of Large Igneous Provinces over the past 200 million years, and possible density anomaly in LLVPs. We propose that the LLVPs are made of "anomalous" material with both higher bulk modulus and density than surrounding "normal" material. Under this scenario, we explored a suite of 2-D models to understand the dynamics and seismic implications. The results show that the anomalous material can form large chemical domes at the base of the mantle with characteristic shape and longevity similar to the LLVPs. The dynamic models were converted to seismic velocity anomalies. Synthetic seismograms of S, ScS, SKS, P, PcP, and PKP phases were generated from the derived seismic models. The travel time of the synthetics matches observed travel times closely.
We are developing global thermo-chemical convection models to find a better match of the dynamics with the geography and morphology of the LLVPs. Using finite element methods in a spherical geometry, we have been solving system with multiple compositions using both incompressible and compressible convection equations. The models incorporates a layer of compositionally distinct material at the core-mantle boundary (CMB) and refined time-dependent plate kinematic models as a boundary condition. In general, the models are integrated from 140 Ma to the present. It is suggested that the kinematic model will promote the development of a ridge structure beneath Africa and a rounded pile structure beneath the Pacific Ocean.
Our results differ from previous studies and demonstrate that the dynamics of LLVS are complex and strongly dependent on model parameters. Comparison with seismic tomographic images reveals geographically and morphologically correlated structures in the mantle at depths of around 2500 km. Correlation at other depths, however, is less convincing, implying that the geodynamic model requires further refinement. In particular, the structure of the lower thermal boundary layer requires revision, as currently it is dominating the thermal evolution of the CMB region. Due to the high temperatures within the lower thermal boundary layer, and the suggested positive Clapeyron slope of the perovskite to post-perovskite phase change, our prediction of the depth to the phase change does not correlate well with seismic evidence. This is because the model temperatures in the CMB region are large and the phase change is suppressed in many regions of the mantle. Evidence from seismic waveform modeling suggests a more ubiquitous post-perovskite layer occurring approximately 250 km above the CMB.