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E317: Thermochemical Tomography of the Tibetan Plateau (Lead Supervisor: Nicholas Rawlinson, Earth Sciences)

Supervisors: Nicholas Rawlinson (Earth Sciences), Keith Priestley (Earth Sciences) and Juan-Carlos Afonso (Macquarie University)

Importance of the area of research:

The Tibetan Plateau formed in response to the collision of Eurasia and India, and despite many decades of study by Earth Scientists, still defies a unified explanation of its deep structure, development and on-going evolution. Numerous seismic studies of the region have been carried out, and a variety of often conflicting findings have been published: for example, some believe that low shear-wave velocities in the upper mantle beneath northern Tibet is evidence of lithospheric delamination, while others regard it as a side-effect of radiogenic heating of the crust. One of the major limitations of traditional seismic methods is that they only image properties such as seismic wavespeed and attenuation, yet interpretation requires a non-unique conversion to physical parameters such as temperature and composition. To better understand this important region of the Earth, new imaging techniques which integrate a variety of observables to directly constrain physical properties are required.

Project summary:

The aim of this project is to apply multi-observable thermochemical tomography to directly image temperature, composition and pressure in the crust and upper mantle beneath Tibet. Datasets to be utilised (many currently available in Cambridge) include surface wave dispersion, body wave arrival times, gravity anomalies, geoid heights, elevation, gravity gradients and heat flow. Inversion is performed within a Bayesian probabilistic framework which allows for the inclusion of carefully selected prior information, such as reliable petrological composition data from xenoliths. The new modelling results will provide, amongst other things, constraints on lithospheric thickness, dynamic topography and the presence of melt.

What the student will do:

The student will be involved in the acquisition and quality control of various pre-existing geophysical datasets (seismic, gravity, elevation, heat flow) that span Tibet, and the compilation of independent information that can be used as prior constraints on composition and temperature. The student will also be involved in the application of the tomographic scheme, which requires the mobilisation of significant computational resources owing to the probabilistic nature of the inversion framework, which avoids linearisation and thus requires the forward solution of some complex problems (e.g. calculation of body wave traveltimes and stokes flow in 3-D media) to be computed many thousands of times. The inversion process produces an ensemble of models that sample the posterior probability distribution, and summary information can then be extracted to describe particular features, such as lithospheric thickness. Interpretation and dissemination of results via publications and presentations at conferences forms an integral part of the project.

Please contact the directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Afonso, J. C., Fullea. J, Griffin, W. L., Yang, Y., Jones, A. G., Connolly, J. A. D. and O'Reilly S. Y., 3-D multi-observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle. I: A priori petrological information and geophysical observables, Journal of Geophysical Research: Solid Earth 118, 25862617 (2013).

Afonso, J. C., Rawlinson, N., Yang, Y., Schutt, D. L., Jones, A. G., Fullea, J. and Griffin, W. L., 3-D multiobservable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle III: Thermochemical Tomography in the Western-Central US. Journal of Geophysical Research: Solid Earth, in press (2016).

McKenzie, D. and Priestley, K., The influence of lithospheric thickness variations on continental evolution. Lithos 1, 1-11 (2008).

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