Multibeam bathymetric and geophysical data reveal a major strike-slip fault that extends along the summit of the Puysegur Ridge east of the Puysegur Trench. The northward structural development of this ridge-trench system illustrates the evolution of an incipient subduction zone along a transform plate boundary that has been subjected to increasing transverse shortening during the past 10 m.y. At the southern end of the trench, where subduction has not yet started, the Puysegur Ridge has a narrow (<50 km) steep-sided cross section, and the axial strike-slip fault separates a shallow (125–625 m), flat-topped eastern crest from a deeper (400–1600 m) western crest; these characteristics indicate differential uplift during the initial stage of shortening. On the lower plate an incipient, 5.2-km-deep trench developed in conjunction with normal and reverse faults, suggesting strong interplate coupling across the trench. Northward, the ridge broadens linearly to 80 km wide, its western flank has locally collapsed, and the ridge summit has subsided, possibly by 1.5 km, suggesting that the interplate coupling decreases and that a Benioff zone is being formed. Concomitant to the northward ridge evolution, the trench deepens to 6.2 km and normal fault throws increase along its outer wall, indicating greater flexure of the downgoing plate.

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