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School of Earth and Environment

Understanding small-scale heterogeneities through Earth’s mantle and crust

Dr. Sebastian Rost

s.rost@leeds.ac.uk

http://earth.leeds.ac.uk/~earsro

The Earth shows evidence for lateral heterogeneities on many scale lengths throughout the Earth’s crust, mantle and core. The heterogeneities of the crust are indicated by the diversity of the surface geology and include variations from large scales (i.e. differences between oceanic and continental crust) down to crystal scales. Heterogeneities deeper in the Earth are likely the result of mantle convection and plate tectonics continuously introducing chemical heterogeneities in form of subducted slabs into the mantle which are slowly and likely incompletely mixed into the mantle. These heterogeneities are more difficult to observe and are best studied using geophysical methods. Understanding the origin of these heterogeneities and their generation and evolution throughout Earth’s history is important to decipher the Earth’s dynamic systems from the subduction process to plume generation and the feedback mechanism between lower mantle and the surface.

Subduction is a dominant feature of our planet. The continuous production of oceanic crust at oceanic ridges and their recycling through the subduction process indicates a very dynamic interior of our planet. The subducted slabs are chemically different from the undisturbed mantle and have to be mixed back into the mantle material over time. The mixing process and the longevity of these heterogeneities is not yet well understood.

High frequency seismic waves from earthquakes can be used to resolve the chemical heterogeneities that are introduced by subducted slabs. Using seismic information it is possible to detect, map and interpret the nature of these heterogeneities.

Text Box: Figure 1: Schematic of large scale mantle convection processes introducing chemical heterogeneity into Earth’s mantle. Depending on physical parameters such as density the heterogeneities are mixed into the mantle material, but might survive in the mantle for a long time.

The interaction of the seismic wavefield with heterogeneities on the order of one wavelength leads to the generation of scattered energy which is mainly observed in the short-period seismic wavefield. This seismic information it is crucial in understanding the range and distribution of heterogeneities throughout the Earth’s interior.

This project will use the data from seismic arrays and networks to study the scattered wavefield related to several seismic phases such as PKP, PKKP and PKPPKP (P’P’). Using the sensitivity of these phases for small-scale heterogeneities, the project will develop vertical profiles of heterogeneities in different locations, e.g. potential plume regions and subduction zones. The comparison of these areas will allow constraints on vertical mantle flow, the longevity of heterogeneities in the mantle and their distribution through mantle convection currents. The student will be part of an active research institute with many international contacts and will work with large seismic datasets. Training will be provided for the development of appropriate time series analysis tools. Additionally, the student will learn to process and interpret seismic data and to work with seismic

wave propagation algorithms.