Origin, structure and deformation of low-magmatic oceanic lithosphere in the vicinity of ODP Leg 209, Mid-Atlantic Ridge 14oN - 16oN.

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When tectonic plates separate, the peridotite rock in the Earth's upper mantle rises and melts, then the melt cools to form oceanic crust. Generally this crust is 6-7 km thick, but in some areas it is much thinner and may be missing altogether. Such a situation occurs around the Fifteen Twenty Fracture Zone on the Mid-Atlantic Ridge. Here, peridotite crops out extensively for tens of kilometres along the ridge axis, in zones marked by complex topography and faulting. This provides a superb opportunity to study both rocks from the mantle, and the processes by which seafloor spreading takes place in the absence of substantial melt or a thick crust. The ocean drilling programme has already drilled eight deep holes in the area, and we will complement that work with very detailed mapping and taking many more shallow samples. We will image the seafloor using sonar, and use this to map the faults and different rock types. We will then choose sites where we will take up to 60 shallow cores of rock. These samples will be oriented in space, allowing us to determine properties such as the directions of flow, stretching and fracturing as the mantle rocks were emplaced, and subsequent rotations by faulting. Chemical analysis will enable us to unravel details of where these rocks originated and how they melted and subsequently evolved.