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This project is eligible for support from Cardiff Universtiy through the School of Earth & Ocean Sciences.

To apply, please visit the Cardiff University Postgraduate Research portal: http://www.cardiff.ac.uk/for/prospective/postgraduate/applying.html

 Description

The subduction thrust interface separates downgoing oceanic crust from the overlying continental or oceanic hanging wall. Although all recorded earthquakes of magnitude 9.0 or greater have occurred on subduction thrusts, some subduction thrust faults are almost aseismic. The influence of the underthrust oceanic crust on the behaviour of the subduction thrust interface at seismogenic depths, in terms of its seismic vs. aseismic slip style, strength, and fluid pressure state, is largely unknown. In particular, the thickness of the oceanic crust, and any deformation fabric(s) and mineral alterations developed before subduction, may vary between different locations and influence the rheology of the oceanic crust. We therefore propose to study the hydration state of incoming oceanic crust, the dehydration reactions that occur in oceanic crust within the seismogenic interval, and deformation styles within subducting oceanic crust at seismogenic depths.

Whereas normal undeformed oceanic crust is relatively well studied, the Hikurangi subduction margin, along the North Island of New Zealand, involves subduction of an oceanic plateau, and provides an end-member example of thick oceanic crust. Rocks of the Hikurangi Plateau are available from dredge samples and can be used as an example of the hydration and deformation state of thick oceanic crust before subduction. The blueschists of Anglesey provide an exhumed analogue of subducted oceanic crust. Mapping of these blueschists, at micro- to mesoscales, can elucidate the deformation history of oceanic crust undergoing subduction, and be related to deformation observed geophysically in active subduction zones.

Training

1) Field mapping at Anglesey.

2) Optical microscopy of rocks sampled during field mapping, optical microscopy of samples from the Hikurangi Plateau (provided by N. Mortimer, GNS Science).

Mapping at all scales is to investigate deformation structures in subducted oceanic crust (facilities at Cardiff University) – such mapping is excellent practice for work in both natural hazards and in exploration for fault-hosted mineral deposits.

3) Thermodynamic modelling of dehydration reactions in oceanic crust (in collaboration with Johann Diener, University of Cape Town).

4) The student should visit GNS Science, New Zealand, to choose samples from the Hikurangi Plateau for study, to access geophysical data, and to meet with experts on the New Zealand margin.

Contact Details

Dr Ake Fagereng - ake.fagereng@uct.ac.za