PhD Research Project: NERC CENTA - Network analysis of fault and fractures above serpentinized mant

Thin crust at the feather edge of magma-poor margins is commonly underlain by mantle characterised by a velocity well below 8 km/s and a strong vertical velocity gradient up to normal mantle velocities (see Figure). This anomalous mantle is generally accepted to result from its hydration and serpentinization. However, it is not clear exactly how the water reaches the mantle to cause serpentinization. The volume of water required implies that it comes from the overlying ocean, which implies that it passed through fractured brittle crust. Observations that reduced mantle velocities occur where the crust was entirely brittle during rifting, and that the degree of serpentinization correlates with extension of the overlying crust, both support the idea that a fracture network within the crust is a critical element in magma-poor margin evolution (Perez-Gussinye & Reston, 2001; Bayrakci et al., 2016).

This project will investigate the nature of the crustal fracturing. It is likely that seawater reached the mantle through a connected network of fractures through both crustal basement and the early syn-rift sediments that make up the tilted fault blocks. The project will determine the connectivity of faults within the block at both seismic and a sub-seismic scales. It will then investigate the role of the fault network in transporting seawater to depth by considering how the network responds to the evolving stress regime. Implications for the transport of water through fractured mantle massifs exposed at slow and ultraslow spreading ridges will also be considered. These mantle massifs are possible places where serpentinization and mineral carbonation resulting from the uptake of water might be stimulated to capture carbon (Kelemen and Matter, 2008).

The project will build on current work interpreting the 3D seismic volume, by interpreting the intrablock faults in terms of their connectivity, orientation, and dimensions, using the technique of network analysis. The connectivity will then be compared with the seismic properties of the underlying mantle. A second aspect of the work will be to then consider the effect on the intra-block fault networks of stress before, during, and after slip on the block-bounding faults. Of particular interest will be the changing dilatancy of these faults, providing constraints on the efficiency of the faults as conduits for fluid flow.

Funding Notes

In addition to completing an online application form, you will also need to complete and submit the CENTA studentship application form available from www.centa.org.uk.

CENTA studentships are for 3.5 years and are funded by the Natural Environment Research Council (NERC). In addition to the full payment of their tuition fees, successful candidates will receive the following financial support.

Annual stipend, set at £14,296 for 2016/17
Research training support grant (RTSG) of £8,000

CENTA students are required to undertake from 45 days training throughout their PhD including a 10 day placement.

References

Magma-poor margins
Bayrakci et al., 2016. Fault controlled hydration of the upper mantle during continental rifting'. Nature Geoscience, March 2016. DOI: 10.1038/NGEO2671
Kelemen, P and Matter J, 2008. In situ carbonation of peridotite for CO2 storage. PNAS, 105, 17295-17300
Perez-Gussinye, M, and Reston, TJ, 2001, Rheological evolution during extension at passive non-volcanic margins: onset of serpentinization and development of detachments to continental break-up. J. Geophys. Res., 106, 3691-3975
Reston, TJ, 2009. The structure, evolution and symmetry of the magma-poor rifted margins of the North and Central Atlantic: a Synthesis, Tectonophysics 468, 6–27.