EVIDENCE FROM EARTHQUAKES FOR BOOKSHELF FAULTING AT LARGE NON-TRANSFORM RIDGE OFFSETS

MIGRATING non-transform offsets, which occur along mid-ocean ridges when a propagating segment gradually elongates and takes over spreading from a neighbouring 'doomed' rift segment1-3, represent a significant modification of the plate tectonic paradigm4. The migrating offset zone between the two spreading segments is clearly a locus of significant deformation, but the mechanism of this deformation has been controversial. Here we use the source parameters and locations of earthquakes at such offsets to discriminate between previously proposed models2,4-7 . Earthquakes at large non-transform offsets at medium and fast spreading rates show strike-slip mechanisms, with nodal planes rotated relative to the expected transform fault orientation. One set of nodal planes (presumably corresponding to the fault planes) is parallel to curved sea-floor lineaments which show increased rotation as a function of position in the deforming zone. A bookshelf faulting model in which initially ridge-parallel faults are rotated by simple shear is consistent with the observed lineament orientations, focal mechanisms and earthquake distributions.