Oceanic crust on opposite sides of a fracture zone is generally of different ages (t) and is therefore at different depths dependent on √t. Subsidence rates are proportional to 1/√t, so that the younger, higher side of a fracture zone is subsiding faster than the older, lower side. Thus, over all of its length, including the aseismic extensions, a fracture zone has a small component of dip-slip motion. Calculated dip-slip rates vary from 2 mm/yr in the vicinity of a spreading ridge with an axial valley to 10−3 mm/yr at a distance from the ridge where the younger side of the fracture zone is 50 m.y. old. A similar qualitative argument suggests that strike-slip motion may also occur because of differential thermal contraction of oceanic crust parallel with the fracture zone. Cumulative effects of this very slow dip-slip and strike-slip motion could contribute to (1) off-ridge seismicity, (2) a recognizable pattern of scarp height versus age that could be useful in identifying ambiguous ocean-floor features as fossil fracture zones, (3) distinctive fabrics on the younger side of the aseismic portions of fracture zones, which have never passed through the transform domain, and (4) development of complex stratigraphic and structural relationships within fracture-zone troughs.

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