A long-range sonar study of the mid-atlantic ridge crest near 37°N (FAMOUS area) and its tectonic implications

Abstract

In July 1973 the whole FAMOUS (French-American Mid-Ocean Undersea Study) area was surveyed using a long-range side-scan sonar (GLORIA). The mosaic of sonographs which was produced shows two sets of mainly linear targets. These targets can be related to locally steep scarps on the sea-floor and ate interpreted as the scarps of steep faults. The fault scarps comprising the larger set are tens of kilometres long and subparallel to the median valley but tend to be curved in plan near fracture zones. The most satisfactory explanation for these is that they are formed by caldera-collapse over an elongate cigar-shaped magma chamber which lies beneath the inner floor of the median valley. The fault scarps of the other set occur only in fracture zone valleys and run obliquely to the general trend of the transform fault zone. They appear to be subsidiary strike-slip and normal faults generated within the zone of shear which characterizes a transform fault. Our results indicate that the upper crust of a slow-spreading ridge is faulted into long narrow parallel-sided slices within the inner floor, probably by the faults which produce the inner walls of the median valley.