Abstract

In the FAMOUS area on the Mid-Atlantic Ridge, four rift-valley segments trend slightly east of north and are separated by short right-lateral transform faults. From north to south, the valleys are designated rift valleys 1, 2, 3, and 4, respectively. Of these, rift valley 2 has been studied extensively by manned submersibles. We describe the characteristics of rift valley 3 on the basis of detailed bathymetric charts. Rift valley 3 has an 8-km-wide inner floor with a central volcanic ridge and is bordered by steep inner walls, terraces, and outer walls. Although the structure and dimensions are similar to those of rift valley 2, the inner floor is significantly wider and contains more trains of volcanic hills. One of these is lobate and irregular and defines the position of the ridge axis. Individual volcanoes, formed at the axis, appear to rift on either one or the other side and are then transported by sea-floor spreading, while being modified by along-strike faulting. At the walls, the hills are uplifted in units. Reconstruction of the evolution of the inner rift shows a history similar to that of rift valley 2, with minor oscillation of the axis over a 1-km-wide zone with transient transforms. In rift valleys 2 and 3, as well as in other segments of the Mid-Atlantic Ridge, the distance from the axis to the inner walls is correlated with the spreading rate, suggesting that crustal age and associated properties of the lithosphere control the beginning of uplift. The differences between rift valleys 2 and 3 are caused mainly by differences in spreading-rate asymmetry and by some historic events.

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