Crustal accretion and transform margin evolution at ultraslow spreading rates

The crust that underlies the world's oceans forms as a result of seafloor spreading - a process that sees the rigid oceanic plates pulled apart at fast (>100 mm/yr), intermediate (100-55 mm/yr) or slow (55-20 mm/yr) rates. As plates separate the mantle beneath rises to fill the gap and as it does so it melts due to the lower pressure. This molten rock, or magma, solidifies to form the ~6-8 km thick oceanic crust, comprising a layer of erupted and rapidly cooled magma (basalt) at the top and a layer of slowly cooled magma (gabbro) beneath.

Over the last decade, observations have shown that the crust created where oceanic plates are pulled apart at slower rates, does not form by such a simple process of symmetrical, magmatic construction as our current models predict, but instead the magmatic construction is interspersed with periods of apparent magma-starvation. During these amagmatic phases plate separation is accommodated by large-offset faults along which rocks from the lower crust and the upper mantle beneath are brought to the surface. These regions of exhumed lower crust and upper mantle rocks are called oceanic core complexes (OCCs).

About 25% of the Earth's mid-ocean ridges spread at very slow rates of less than 20 mm/yr. However, most of these ultraslow ridges are located in remote areas that have poor weather or ice cover that impedes their investigation. Consequently, how the crust forms and ages at these slowest spreading centres, which current models predict should be predominantly magma-starved and cold, remains poorly understood. Recent seabed imaging and sampling studies of the ultraslow Mid-Cayman Spreading Centre (MCSC) in the Caribbean, have observed the deepest and hottest black smoker hydrothermal vents on Earth, and regions of exhumed lower crust and upper mantle juxtaposed against volcanically erupted rocks of the "normal" upper oceanic crust. Here we will establish the crustal context of these contrasting observations that challenge the predictions of traditional models, and we will determine the time and space interplay between magmatic construction and amagmatic extension and the controls on, and relationship between, faulting and hydrothermal activity.

As part of a British, German and American partnership, we will use sub-seabed seismic imaging to study the structure and lithology of the crust at the Mt Dent OCC on the MCSC and determine the relationship between this and the adjacent volcanic domain that also hosts hydrothermal vents. We will also investigate how the crust changes as it cools and ages as it spreads away from the ridge axis. Using the pattern of local earthquakes we will map sub-seabed fault geometries and whether or not these faults are connected at depth. As the southern tip of the MCSC also abuts against the continental crust of the Caribbean plate across the Swan Island Transform Zone, this also provides a unique opportunity to determine not only how the mantle rises up and melts beneath the ridge and how this melt is distributed along-ridge, but also if this process is impeded by the cooling affect of adjacent thick, cold continental lithosphere.

To achieve our goals we will deploy ocean-bottom seismographs (OBSs) onto the seabed to determine the variation in velocity associated with, and the interfaces between the different rock types deep into the crust and upper mantle using man-made seismic signals. We will also use the OBSs to record the signals that occur naturally when faults move. We will measure the gravity field to determine crustal density as a test of our seismic models, and to image deeper into the mantle to depths beyond which our seismic signals will penetrate. Finally, we will measure reversals in the magnetic field to reveal seafloor spreading rate and crustal age and, jointly with the seismic data, determine how frequently phases of amagmatic extension have occurred from the current time to at least 20 million years ago.

The primary beneficiaries of this research will be other researchers studying the structure and geodynamics of mid-ocean ridges and ocean-continent transitions, and those working on hydrothermal circulation and water-rock interactions. Although it will not generate "direct-to-market" products, there is likely to be interest by the deep ocean minerals industry, since the results and data acquired will be directly applicable to the development, assessment and exploitation of the metal-rich sulphide deposits which surround hydrothermal vent systems.

Beyond academia and industry, we anticipate a significant interest from the general public, given the popularity of Earth-science-related television programmes in general, and the media as demonstrated by the BBC's interest in the PI's most recent sea-going campaign. Reports of scientific achievements from this cruise were the main science item on the main BBC national news bulletin and received the highest weekly science topic hit-rates on their web portal - e.g.:

BBC National News - 6th December 2011
https://www.dur.ac.uk/christine.peirce/louisville_media/louisville_BBC_news_061211.mov

BBC News web site - 23rd April 2011 - Quake mission to 'ring of fire'
http://www.bbc.co.uk/news/science-environment-13161058

BBC News web site - 6th December 2011 - Undersea mountains march into the abyss
http://www.bbc.co.uk/news/science-environment-16056192

BBC News web site - 13th May 2012 - Rise and fall of underwater volcano revealed
http://www.bbc.co.uk/news/science-environment-18040658

Plate tectonics is a subject taught in the UK National Curriculum and understood at some level by the general public. We will be testing some of its fundamental principles in one of the deepest and harshest environments on Earth. This is something that makes our work accessible and of interest to many, and which makes this a great opportunity to demonstrate Earth science "in action". We will enthuse and inform the next generation of scientists by demonstrating Earth processes to school students during Schools Science Weeks, and for the more senior members of society we will present our work at regional geological and natural history societies.

We will submit our data to appropriate data centres (e.g. BODC), and make it available to other scientists upon request. We will use conferences and papers in peer-reviewed journals to exchange our knowledge with the academic in industrial community, together with articles in appropriate newsletters. We will publish in academic journals with the largest possible and most appropriate readership, and we will present our results and developing models at the annual AGU Fall and at the EGU meetings, as these international meetings attract the largest cross-section of likely interested parties. We will also participate in special meetings of the Geological Society of London and meetings supported by the British Geophysical Association. We will create a dedicated web site for the project, with sections for academic researchers, industry, the media, the general public and schools. We will blog during the cruise, explaining the science behind our project as it proceeds, describing working and living at sea, and describing the data and results "hot off the press" - science as it happens. Finally, we will use our cruise blog to interact with school students and teachers via the Earth Sciences in Schools undergraduate module run in Durham, and we will use the acquired data for undergraduate research-led teaching and for MSci research projects.