Abstract

A morphology is presented for the typical cross-section of an inactive fracture zone in the Atlantic. Inactive fracture zones typically consist of an asymmetric valley to the young side of the fracture zone plane and a high wall or scarp to the old side. Large-offset fracture zones may be accompanied by a marginal valley on the other side of the high wall. This topography is superimposed on the depth-age step due to lithospheric cooling.

A model is developed which accounts for this morphology. The model relates the inactive fracture zone morphology to the topography found at present-day intersections of the spreading axis with transform faults. The asymmetry of the median valley near fracture zones plays an important role in explaining the typical fracture zone morphology. The existence of a median valley is related to the viscous delay of the upwelling mantle material at the spreading axis. Its asymmetry near fracture zones can be accounted for by modelling the viscous drag exerted by the lithosphere on the asthenosphere. If the viscosity is low, as under Reykjanes Ridge and the East Pacific Rise, no median valley develops and a different morphology may be expected. The occurrence of marginal valleys is interpreted as the result of lithospheric warping when the graben walls in the transform domain, which are caused by tension due to horizontal thermal contraction. are excessively high.

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