CSEDI: Fine Structure of the Lower Mantle

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Intellectual Merit Seismologists have known for many years that the lowermost mantle is complex. Attempts at modeling seismic waveforms sampling this region produced ultra-low velocity zones, sharp horizontal discontinuities, strong zones of anisotropy (SH>SV), and in some cases near vertical walls. It now appears that this myriad of fine structure is beginning to be understood in terms of mineral physics with dense melts from hot perovskite and a new post-perovskite structure. These new discoveries can explain many of the above features. In particular, the investigators earlier dynamic 2D model of slabs folding at the CMB generates a strong positive velocity gradient, which in turn produces synthetic seismograms containing a small triplication, if a 1% velocity jump is allowed. Assuming that this jump results from a phase-change with a positive Clapeyron slope, and transforming tomographic images through such a hybrid global model, generates some of the geographic behavior of the observed S-wave triplication: where the post-perovskite layer is thickest beneath fast regions and disappears beneath slow regions. The team's latest 3D dynamic models explain the locations of the fastest regions based on the history of plate subduction. Results from 3D dynamics are significantly different from those in 2D, and may help us explain more of the variability in seismic waveform data sampling the lower mantle when seismic and dynamic models are merged. In order to make accelerated progress toward understanding the lower mantle and D" propose research to resolve the 3D nature of these structures. They will address the following questions: o How compatible are 3D dynamic predictions of slab histories with lower mantle structure? o Can we separate velocity gradient effects caused by subduction from the phase-boundary effects and update a global map? o How different is the mid-Pacific lower mantle structure compared to the African structure and what does this imply in terms of the stability of upwelling? o What dynamic models of thermo-chemical boundary layers are consistent with the proposed post-perovskite structure and seismic data? Broader Impact Answers to the above questions will be of great use to the entire Earth Science community, especially mineral physicists who need fundamental seismic information about the global properties of the newly discovered phase boundary. Making the direct connection between the detailed fast structures in the lower mantle with respect to the history of subduction will answer a key question in plate tectonic dynamics. These multi-disciplinary efforts should help the involved students become leaders in future CSEDI developments.