Abstract

A marked variation in fault-facing direction with spreading rate is observed on the flanks of mid-ocean ridges. On slow- and intermediate-rate spreading centers inward-facing faults predominate, whereas at fast-spreading centers, large numbers of both inward- and outward facing faults are observed. The distribution and character of inward- and outward-dipping faults derived from SeaMARC II data from the flanks of the East Pacific Rise and Galapagos spreading center are compared with observations from previously published data. We suggest that the fault-facing direction variation observed may reflect relative differences in the mean stress on potential fault planes through the oceanic lithosphere which might favor inward- vs. outward-facing faults at different spreading rates. For example, due to the thermal structure of a slow-spreading ridge, a fault dipping toward the axis extends to shallower depths than one dipping away from the axis, thus favoring the occurrence of inward-dipping faults. On a fast-spreading ridge, the difference in fault length is less, so both inward- and outward-facing faults are likely to occur.

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