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Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
The major faults that bound the median valley on slow spreading ridges form a staircase that migrates upwards and outwards from the valley floor like a slow escalator. Each boundary wall fault is generated at the margin of the median valley floor at about 2 km from the spreading axis. Boundary wall faults that appear to be single scarps on the relatively low resolution Seabeam multibeam echosounder maps are shown by high resolution TOBI sidescan sonar images in some cases to be single faults, but in others to be complex anastomosing fault zones. The major faults in such zones are often scalloped in plan view. There is no systematic relation between fault zone geometry and segment type.
We conclude that during the growth of boundary wall faults the strain is first accommodated over a wide area, in which an array of small faults dissects the volcanic surface. However, with time, and with continued extension, the strain becomes organized into increasingly narrower zones of deformation until a major boundary wall fault develops. Faults grow by propagation and linkage with other nearby faults. The early stages of linkage include tilted ramps, but at later stages brittle faults cut across the ramps, and eventually become part of a scalloped fault trace. In some places, linkage is through zones of small-scale diffuse faulting, rather than through a single fault. Fault capture and linkage can occur over lateral distances (perpendicular to the mean azimuth of the faults) of up to 1.5 km.
This deformation takes place within a fault growth window fixed relative to the spreading axis, through which the lithosphere passes as seafloor spreading continues. Most of the growth of each boundary fault takes place before the next fault is initiated. We conclude that the location and size of the narrow fault growth window is controlled by the changing mechanical properties of the ocean lithosphere as it spreads away from the axis. Its inner boundary represents the position at which a normal fault reaches the sea floor after originating at the base of the lithosphere at the spreading axis. Its outer boundary is interpreted as the point at which the strength of the boundary wall fault, increasing as the lithosphere ages and thickens, becomes greater than the strength of the lithosphere at the spreading axis, when a new fault is generated. The process of fault generation at mid-ocean ridges has relevance to fault generation in other extensional environments.
