MORPHOSTRUCTURE OF AN INCIPIENT SUBDUCTION ZONE ALONG A TRANSFORM PLATE BOUNDARY - PUYSEGUR RIDGE AND TRENCH

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-tr ench 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 h as a narrow (<50 km) steep-sided cross section, and the axial strike-s lip fault separates a shallow (125-625 m), flat-topped eastern crest f rom a deeper (400-1600 m) western crest; these characteristics indicat e differential uplift during the initial stage of shortening, On the l ower plate an incipient, 5.2-km-deep trench developed in conjunction w ith normal and reverse faults, suggesting strong interplate coupling a cross the trench. Northward, the ridge broadens linearly to 80 km wide , its western flank has locally collapsed, and the ridge summit has su bsided, possibly by 1.5 km, suggesting that the interplate coupling de creases and that a Benioff zone is being formed. Concomitant to the no rthward ridge evolution, the trench deepens to 6.2 km and normal fault throws increase along its outer wall, indicating greater flexure of t he downgoing plate.