Dr Stewart Fishwick

Stewart FishwickAssociate Professor in Geophysics

Direct contact:
T
: 0116 252 3810
E: sf130@le.ac.uk

Research Group: Geophysics and Borehole Research

Personal details

  • Director of Learning and Teaching

Research

My primary research has involved using seismology to understand the structure and evolution of the lithosphere and upper mantle.  Interests cross the whole geological timespan - from contrasts in lithospheric structure within Precambrian Shields, through to understanding controls on present day plate tectonics.

The Australian and African continents have been a focus for many of my studies, although recently, through students work, we're exploring the structure beneath the oceans as well. With colleagues here in Leicester and elsewhere, I have particular interests in how seismological data and models can be combined with magnetotelluric studies to give a more holistic view of the physical properties of the mantle.

Outside of seismology I'm involved in a range of other work.  Recently this has included inversions of magnetic datasets targeting archaeological data sets; thermochronology studies in the Canadian Cordillera, and involvement in two NERC funded petrophysics projects.

Seismic Structure of the Upper Mantle

My main research interest is understanding the variations in seismic wavespeed within the upper mantle.  Can we link the observed velocities to large scale tectonic processes and the geology observed at the surface, and how are the variations related to changes in the physical properties of the lithosphere and asthenosphere?  Research has been focussed on continental scale studies (Australia and Africa), however, I believe that incorporating data at various length-scale is an important challenge, and will lead to an improved understanding of the structure of the Earth.

Surface wave tomography is an ideal tool for investigating the upper mantle and my work has focussed on looking at ways to improve the reliability of the tomographic models.  Multiple starting models can be used in the inversion process to get a better estimate on the reliability and uncertainty related to the data. One of the problems with tomographic images is that what is observed is strongly dependent on the colour scale and reference model, for this reason I have also looked at using images of the horizontal gradient to define where there are rapid variations in wavespeed (link).

The focus of my present research is the relationship between the tomographic models and physical processes within the Earth:

  • The results from a number of tomographic studies in cratonic regions suggest a positive velocity gradient in the upper mantle, which is not compatible with simple physically based models of constant composition and cratonic geotherms.  Investigating the internal structure of the lithospheric mantle is a major target and the subject of a NERC New Investigator proposal.
  • Beneath Africa there is a long-standing interest in the relationship between mantle convection, volcanism, and topography. Using the results from my recent tomographic study this has become an area of focus for my research, with collaborations with Dr Nicky White (University of Cambridge), and as an international collaborator on a recent proposal by a group at the University of Rennes.
  • For further discussion and links to the tomographic models for either Australia or Africa see the following:

Australia

(Tomographic models to follow)

The Australian continent is in an ideal location for surface wave tomography.  The subduction zones to the north and east provide a large number of events which give good fundamental and higher mode coverage.  The mid ocean ridges to the South and West provide additional data to improve the azimuthal variation in path coverage.

As has been observed in previous surface wave studies (and in some very early work from travel time data) there is a very strong contrast in wavespeed between the region beneath the Precambrian shield of western and central Australia and the Phanerozoic terranes to the east.  Within the shield region there are also clear differences in structure, which appear to have some relationship to the surface geology (link to paper to follow).

Using both the tomographic models and the gradient maps it appears that there are a series of steps in the lithospheric thickness beneath Australia (third link). Similar structures have been observed across the Trans European Suture Zone. At the eastern margin of Australia a very strong gradient in wavespeed is observed inland of the continent-ocean transition, and is correlated with both recent volcanism and the location of the great dividing range.  Understanding the temporal evolution of this feature and the present thermal structure remains a challenge.

Africa

(Tomographic models to follow)

A similar method of surface wave tomography has been applied to the African continent.  One of the limitations in this region is that the majority of the events used have been quite small shallow mid ocean ridge earthquakes, and there is less higher mode information than available for the Australian study.  However a very large data set has been processed giving over 8200 path-averaged models which can be used to constrain the wavespeed structure within the region.

The tomographic models again show the large differences in wavespeed beneath the cratons and the surrounding regions.  Work is in progress looking at specific features in the models: The structure of the Congo Basin; Slow wavespeeds beneath the Angolan dome; The relationship between azimuthal anisotropy and flow directions.  More details will be added as this work evolves.

Publications

  • Civiero, C., Hammond, J.O.S., Goes, S., Fishwick, S., et al., 2015, Multiple mantle upwellings in the transition zone beneath the northern East‐African Rift system from relative P‐wave travel‐time tomography, Geochem, Geophys, Geosys., 16, doi:10.1002/2015GC005948
  • Cheyney, S., Fishwick, S., Hill, I.A., Linford, N.T, 2015, Successful adaptation of three-dimensional inversion methodologies for archaeological scale, total field magnetic datasets. Geophys. J. Int. 202, 1271-1288.
  • Jones, A. G., Fishwick, S., Evans, R.L., Muller, M.R. & Fullea J. 2013, Velocity-conductivity relations for cratonic lithosphere and their application: Example of southern Africa, Geochem. Geophys. Geosyst., 14, doi:10.1002/ggge.20075
  • Fishwick, S. & Rawlinson, N. 2012. 3-D structure of the Australian lithosphere from evolving seismic datasets. Australian Journal of Earth Sciences, 59, 809-826.
  • Fishwick, S. & Bastow, I.D. 2011. Towards a better understanding of African topography, a review of passive-source seismic studies of the African crust and upper mantle. In: Van Hinsbergen, D. J. J., Buiter, S. J. H., Torsvik, T. H., Gaina, C. & Webb, S. J. (eds) The Formation and Evolution of Africa: A Synopsis of 3.8 Ga of Earth History. Geological Society, London, Special Publications, 357, 343–371. DOI: 10.1144/SP357.19

Teaching

  • GL1020 - Micro to Macro (Module Coordinator)
  • GL1021 - Introductory Field Course - Arran
  • GL3073 - Planetary Science (Module Coordinator)
  • GL4023 - Global and Crustal Seismology (Module Coordinator)

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