Fault scarp statistics at the Galapagos spreading centre from Deep Tow data

Much of the relief of the abyssal hills covering the ocean basins is believ ed to originate from faulting of oceanic crust at mid-ocean ridges. The tim escale over which faults grow is controversial, however, with some authors arguing that faults continue to grow in places for 0.5 m.y. or more based o n increasing relief of fault scarps with distance from ridge axes. We exami ne Deep Tow profiler records of the Galapagos Spreading Centre, in which ba sement reflections allow scarp relief to be measured beneath the sediment c over, and find that relief does not increase but decreases systematically t o 40 km off-axis (1.5 Ma seafloor). Since reversal of fault offsets is unli kely in this tectonic setting, we interpret this result as indicating that variations in fault statistics could reflect temporal variations in the tec tonic or volcanic state of the ridge crest, not necessarily progressive fau lt growth with age as previously assumed. Resolving the issue of fault long evity will therefore require independent data on the timing of fault growth and distribution of present growth activity. We suggest some possible alte rnative indicators of fault longevity and discuss more generally the implic ations of volcanic flows to studies of faulting at ridges.