School of Earth and Environment

Rebecca Smith

Postgraduate Researcher

Email address:

Affiliation: Institute of Geophysics and Tectonics

Project details

Project title

Mapping and characterisation of ultra-low velocity zones - Deciphering the most enigmatic deep structures of the Deep Earth


Dr. Sebastian Rost

Dr. Stephen Stackhouse

Project outline

Tomographic images of Earth's lowermost mantle are dominated by two nearly antipodal regions of reduced seismic velocities. These areas are now dubbed Large Low Shear Velocity Provinces (LLSVPs) and are proposed to be thermochemical piles of unknown origin, although purely thermal models are also being discussed. The purpose of this study is to determine the location of the boundaries of African LLSVP, specifically along its north-western margin. Although LLSVP boundaries are usually constrained by a specific drop in seismic velocities in tomographic models it remains unclear which velocity reduction is indicative for LLSVP existence. Here we aim to define the north-western boundary of the Africa LLSVP from direct seismic observations.

The African LLSVP is a ridge-like structure that extends from the Indian Ocean through Central Africa along to the Atlantic where it turns northwards and runs towards Iceland. Using a combination of data from European and African seismic stations we aim to better constrain the north western boundary of the African LLSVP and especially determine its extent towards Iceland and a possible connection to the Icelandic hotspot. Our dataset will allow the largest lateral extent of the north-western margin to be examined to date. The vertical extent of the LLSVP could potentially be examined as well in this region through traveltime and waveform analysis providing further insight into the origin of the African LLSVP.

This study will combine three main strands of investigation: firstly travel-time anomalies, secondly wave-form complexities and thirdly anisotropy changes across the LLSVP boundary. We are using traveltime anomalies for a variety of seismic phases including P, PcP, Pdiff, S and SKS and across different frequencies and therefore will be able to determine the boundary for P and S waves. The combined P- and S- wave study will help to elucidate any chemical contributions to the velocity anomalies. Ultra-Low Velocity Zones will be investigated along the newly defined boundaries using travel-time delays. Wave-form complexities will also be used in conjunction with 3D waveform modelling to determine the sharpness of the boundaries detected. Using anisotropy, this study, will look at how mantle flow is altered at the LLSVP boundary potentially increasing anisotropy at the LLSVP boundary. Combining constraints from all three methods will provide evidence for the location of the north-western boundary African LLSVP and its origin. It will also provide an insight into how the surrounding mantle is affected by the African LLSVP.