| Abstract |
Two-thirds of the Earth's surface is paved by oceanic crust formed by seafloor spreading at the 60,000 km-long global mid-ocean ridge (MOR) system. As the rigid ocean plates are pulled apart, at rates varying from <10 to 160 mm/year, the Earth's mantle is drawn up from beneath, partly melting as it does so. The melt separates from the mantle and rises to the surface to form a continuous layer of 'magmatic' crust, typically about 6 km thick, made of basalt at the surface and gabbro, its slowly cooled equivalent, beneath.
How can we distinguish these very different hypotheses about the mechanism of seafloor spreading? The key data needed are: (1) the sub-surface geometry and extent of the detachments beneath the ridge axis, (2) the amount and detailed distribution of magmatic crust, and (3) the asymmetry of spreading rates associated with OCCs and volcanic seafloor (they should be similar in the regional and differ in the local detachment models).
We propose to obtain these data in a comprehensive seismic and seabed magnetic survey of the MAR in the 13N region, where detachment faults are active at the ridge axis today. We will use a large array of ocean-bottom seismographs (OBSs) to image 3D velocity variations related to different rock types using 'seismic tomography' - akin to medical CT scanning - and conduct a multi-channel reflection survey, which will image sub-surface discontinuities - like a simple X-ray. We will then leave the OBSs (to be recovered on a later cruise) to record the locations of natural micro-earthquakes in the region. These will show directly the 3D geometry and linkage of active faults. Finally, we will deploy the autonomous robot vehicle Autosub 6000, which will be programmed to make very detailed maps of magnetic field reversals (yielding seafloor age and spreading rate) and seafloor topography (helping structural interpretations) while we perform the seismic experiments.
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