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Institute of Oceanographic Sciences, Wormley, Godalming, Surrey GU8 5UB, UK
The tectonic patterns of some twenty spreading centre offsets have been determined using the long range sidescan sonar GLORIA and are reviewed in this paper. Offsets of less than about 20 km fail to produce transform faults but are accommodated by short sections of oblique spreading (with overlapping spreading centres in the fast spreading case). At slow slip rates such offsets can be associated with topographic fracture zones that are indistinguishable from those produced at true transform faults. Such features may account for a significant proportion of spreading centre offsets on slowly spreading mid-ocean ridges. Offsets larger than about 30 km produce a true 'transform fault'. This comprises a 1 to 5 km-wide band of individual faults that are parallel or subparallel to the spreading direction. The band corresponds to the 'Transform Fault Zone' (TFZ) of continental wrench faulting. It includes the 'Principal Transform Displacement Zone' (PTDZ) and secondary structures such as Riedel shears. A narrow TFZ is normally associated with a prominent PTDZ, which appears to be either a narrow furrow or a single scarp; on the other hand wider zones often fail to show a clear PTDZ on GLORIA. Both types are common on the slowly spreading Mid-Atlantic Ridge, and may even occur at different points along the same large offset transform (such as Romanche), although the narrow type appears to occur preferentially where the local Ridge axis is oblique to the spreading direction. Narrow TFZs and prominent PTDZs appear to be more common on the fast spreading East Pacific Rise. It is suggested that broader TFZs and less prominent PTDZs may form in response to the complicating effects of small components of compression across the transform fault, perhaps as a result of departure of the transform fault trace from a true small circle. Such compression would be more easily supported in the thicker, stronger lithosphere of slowly spreading ridges, but could be relieved by tension due to the ridge-push force where the ridge is oblique to the spreading direction.
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