Normal faults play a dominant role in the morphology and internal structure of crust formed at slow-spreading mid-ocean ridges. The surface expression of these faults has been studied well, but few direct controls are available on their shape at depth. Multichannel seismic reflection profiles of old oceanic lithosphere potentially contain a wealth of structural information, but the origin of many of the features observed in these profiles remains controversial. Sedimented rifts provide a rare opportunity to study present-day tectonics of mid-ocean ridges with conventional seismic reflection profiles. We have used two such profiles and a recently developed inverse method to infer, from the deformation of sediments in the hanging wall, the geometry of a normal fault bounding the axis of the Juan de Fuca Ridge at Middle Valley. The fault is listric in cross section and can be traced about 5 km below the sea bed. Its shape is similar to whole-crust reflectors imaged in old oceanic lithosphere.

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