Institute of Geophysics and Tectonics (IGT) PhD Projects
Large Low Shear Velocity Provinces – chemical or thermal mantle anomalies?
Supervisors: Dr Sebastian Rost, Prof Graham Stuart & Dr Stephen Stackhouse
Tomographic images of the velocity of the lowermost mantle are dominated by a strong degree 2 reduction in seismic velocities centered below the Pacific and Africa. These structures are often referred to as Large-Low Shear Velocity Provinces (LLSVPs). Other geophysical data such as seismic waveforms indicate that LLSVPs have sharp sides and geoid anomalies might indicate increased density within the LLSVPs. Simulation of thermo-chemical convection indicates that LLSVPs might be dense thermo-chemical piles of unknown origin which are internally convecting and denser and hotter than the surounding mantle. On the other hand, several studies show that LLSVPs can be explained as purely thermal features. Identifying the origin of the LLSVPs will contribute to our understanding of the structure and evolution of the Earth’s deep interior.
Fig. 1: Cross section through seismic tomography model by grant showing potential source receiver locations for the project and the location of the LLSVP
Fig2: Crystal structure of perovskite. The dominant mineral of the lower mantle
The structure of LLSVPs at the core- mantle boundary (CMB) is well sampled in many regions using P- and S-wave waveforms and the identification of seismic multi-pathing indicates sharp edges to LLSVPs at the CMB. This is partly in agreement with many S-wave tomography models but the structure of LLSVPs in the mid mantle is less well known. Especially, a measurement of the density at the top of the LLSVPs could decide whether they are dense thermo-chemical piles or of purely thermal origin. This project will use data from networks in Ethiopia that were installed by the University of Leeds and collaborators during several field experiments to resolve the upper mantle structure beneath the Afar region of Ethiopia. Using earthquakes from the Mid Atlantic ridge allows sampling of the top of the LLSVP beneath western Africa. Rays turning above and within the LLSVPs will show multi-pathing and reflection effects that will allow the development of velocity and density models of the LLSVP in this region. Paths from the western.
Pacific into the same station network not sampling a LLSVP will act as control experiment. In addition, we will use ab-initio calculations of material properties (velocity and density) of different materials that can explain the seismic observations to determine the extent of a possible chemical anomaly within the LLSVP. This project combines seismic observation with mineral-physical modelling. It will require detailed waveform analysis of P- and S-waves and their modelling using 2D and 3D wavefront propagation techniques. The student will be trained in seismic data analysis, high-performance computing, waveform modelling and mineral-physics. An involvement in field work in one of our current deployments in Ethiopia and Eastern Europe or planned experiments (e.g. Turkey) is possible. A background in physics or geophysics and strong interest in deep Earth structure is desirable.