On the structure and formation of fracture zones

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Abstract
Studies of the oceanic fracture zones, as well as field observations of the on‐land parts of a fracture zone in Iceland, show that there are numerous tension fractures, normal faults, small‐scale grabens and dykes within, and trending subparallel with, the fracture zones. These structures indicate that, in addition to the shear displacement, there is considerable extension associated with the development of fracture zones and that many of them may be regarded as complex graben structures. The ridges surrounding the continents of Africa and Antarctica are examples of mid‐ocean ridges that are moving away from the continental margins where they originated and therefore expanding. Here it is suggested that as the perimeter of an expanding ridge increases, the tensile stresses associated with the ridge extension may contribute to the formation of fracture zones.