Continental extension leading to breakup: determining the 3D structure of the west Galicia rifted margin

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It has been widely accepted since the 1960s that the Earth's continental landmasses are not fixed, and that over periods of millions of years continents can separate and coalesce, for that for example the landmasses that make up the United Kingdom were once joined to Greenland and North America. The breakup of continents is commonly preceded by a long period of stretching, during which the continental crust is thinned and sediments accumulate in thick piles which may become future sources of oil and gas. During the last decade, researchers have discovered that, at least in some places, the nature of this stretching changes dramatically when the crust becomes thinner than 8-10 km. The top few kilometers of the crust are brittle, and initially stretch by movement along steeply dipping faults, but once this thickness threshold is crossed, the extension is instead accommodated on faults that are close to horizontal. This type of behaviour is also seen in other geological settings, such in the western United States, where the crust may be unusually weak, and close to volcanic mid-ocean ridges. The best-studied example of such behaviour is off the west coast of the Iberian peninsula. In such locations, faults lying deep beneath the seafloor can be seen in images derived from sound waves / the 'seismic reflection' technique. The application of this technique has revealed beneath the broken, fractured brittle crust the presence of a sub-horizontal fault covering an area of at least 1000 square kilometers (the size of the West Midlands) that was active when Iberia broke away from Newfoundland over 100 million years ago. Such faults can have very complicated shapes and interactions with the broken crustal blocks above, and because the existing images are just along a series of lines several kilometers apart, it is very difficult to work out how the fault moved. We will solve this problem by collecting a series of very closely spaced seismic reflection images that can be combined on a computer into a three-dimensional picture. This approach has been widely used by the oil industry in areas thought to contain economically viable oil and gas deposits, but never before to study the first order process of continental breakup itself. To make the most of the image, we will also collect another type of seismic data that allows us to work out how the speed of sound varies beneath the seafloor and hence what type of rocks may be present. As well us providing a detailed picture of how this particular pair of continents broke apart, the experiment will provide the most detailed and complete image ever of a large fault surface, and reveal how this fault and other more steeply-dipping faults above it have moved over millions of years. These observations will help us to understand more generally how the crust stretches and how large faults work.