In situ structural observations along Transform Fault A in the FAMOUS area, Mid-Atlantic Ridge

Abstract

Two areas of Transform Fault A were explored in situ during the FAMOUS project. Ten dives were conducted with the diving saucer Cyana in the median part and five dives with the bathyscaph Archimede near the western intersection with the rift valley. The resulting field observations are the first made along an oceanic transform fault with a resolution similar to the one obtained with morphotectonic observations on land. The existence of left-lateral motion predicted by the plate-tectonics model is demonstrated with the help of micro-tectonic observations on fault scarps. However, the surface expression of the transform fault does not consist of a unique or major fault. Instead, the fault pattern is complex and variable, in the north-south as well as the east-west directions, although the width of the active zone of transform faulting is small (300 to 1,000 m). A comparison of the north-south sections of the transform fault made in the two areas leads to the conclusion that its dynamics have evolved in a complex way through time. The significance of the axial deep and its associated rubble cover is discussed in this context.