NSFGEO-NERC: Quantifying evolution of magmatism and serpentinisation during the onset of seafloor spreading

Over periods of hundreds of millions of years, Earth's surface is recycled via the fragmentation of continents to form new oceans and elsewhere the sinking of oceanic plates into the mantle beneath. The breakup of continents involves progressive stretching and thinning prior to final breakup and the formation of new oceanic crust from molten rock that rises from below, flanked by continental margins comprised of thinned continental crust. There is a range of continental margin types, varying from those where the underlying mantle starts to melt very early in the process and very large volumes are added to the crust, to those "magma-poor" margins where there is little evidence for such melting until the very end of the process. At these magma-poor margins, which are common globally, it has been found that the crust can thin to nothing and mantle rocks can be exposed at the seabed, where they react with seawater in a process called serpentinisation. This serpentinisation plays an important role in exchange of chemicals between the Earth's interior and the ocean, and may be particularly intense around geological faults. While the final stages of thinning of the continental crust have been studied extensively over the past three decades, the transition from exposing mantle at the seabed through to forming new oceanic crust by the eruption of molten rock has been less well studied. Even designing such a study can be challenging because it is often unclear how wide this transition is. Also, because such mantle exposure has also been found in the middle of the oceans, this transition may be more complicated than often assumed.

Our project will use a novel combination of geophysical techniques to study this final stage of continental breakup at a magma-poor continental margin southwest of the UK. There, crust that seems from all available data to be "normal" oceanic crust lies within about 150 km of crust confirmed by drilling to be continental. A region of serpentinised mantle, now overlain by up to around 1 km of mud, lies in between. For the first time in such a location, we will use electromagnetic waves, generated from a towed source, to measure the electrical resistivity of the crust and serpentinised mantle. Electromagnetic waves are strongly attenuated by seawater, so the source must be powerful and must be towed close to the seabed. We will use a combination of towed sensors, that are most sensitive to structures just below the seabed, and seabed detectors that can measure tiny fluctuations in electrical and magnetic fields at distances of up to tens of kilometres from our source, and thus allow us to probe deeper. We will also use some of the same seabed receivers to detect sound waves travelling through the crust from a source towed close to the ship, and to detect lower-frequency electromagnetic waves that are generated by natural sources and penetrate deeper into the Earth.

The data that we collect will allow us, via the use of powerful computer programmes, to construct models of the variation of both sound speed and electrical resistivity in the crust and in the upper few tens of kilometres of the mantle beneath. These parameters provide a powerful combination because they are sensitive in different ways to the nature of the rocks. The electrical resistivity is particularly sensitive to the presence of water, and also of a mineral called magnetite that can be formed during the process of serpentinisation. The sound velocity is less sensitive to the presence of water but can be more sensitive to variations in the minerals present. From our models, we expect to be able to distinguish the continental crust and mantle, the oceanic crust and mantle, and the nature of the materials in between. We will then link these observations to computer models of the physical and chemical processes occurring as continents break apart. Thus we will find out how the formation of new oceanic crust actually starts.

a. Who could potentially benefit from the proposed research over different timescales?

Identified user groups for the new knowledge that we will create are:

1. The hydrocarbon industry - in particular companies holding exploration licenses on the Goban Spur margin.

2. Government agencies dealing with licensing of hydrocarbon exploration.

3. Geophysical contractors.

4. The general public.

b. How might the potential beneficiaries benefit?

1. The Goban Spur margin is an area of active hydrocarbon exploration, with several companies holding exploration licenses in the Irish sector just to the north of our proposed survey area. Our project is focused on more distal parts of the margin than those that would be of direct exploration interest, and therefore unlikely to attract funding from the industry. However, the nature of the continent-ocean transition and associated processes of magmatism and serpentinisation are relevant to the thermal history of the margin and thus to evaluation of exploration risk. For this reason, contractors acquiring multi-client datasets often extend their profiles into this region. Thus our results will be of broad interest to companies that work on distal margins worldwide, and of particular interest to companies working on the Goban Spur margin.

2. For the reasons outlined in 1) above, government exploration licensing agencies will also gain some benefit from our work, through the a broader understanding of the prospectivity of the margin.

3. Geophysical contractors will gain insights from our approaches to data acquisition and analysis, and in particular the way we combine information from seismic and electromagnetic datasets.

4. The public will benefit through outreach activities that focus on continental breakup as a fundamental process in the evolution of our planet.

Specific activities for engagement of these target groups are covered in our Pathways to Impact document.