Abstract. Spatial differences in late Quaternary structural style and deformation rates indicate a complex pattern of strain partitioning within the transition area from oblique subduction beneath southern North Island to oblique continental collision in northern South Island, New Zealand. The late Quaternary structure of the offshore southern Hikurangi margin is determined here using seismic reflection profiles, MR1 side-scan sonar swath images, EM 12Dual multibeam swath bathymetry and backscatter images, and sediment core and dredge samples. Where oblique (50 ø ) subduction of the oceanic Hikurangi Plateau is occurring beneath southern Wairarapa region, the steep, dissected, upper margin consists of a wedge of rocks that are being deformed by strike-slip and reverse faults. Beneath the lower margin an accretionary wedge of Quaternary age has developed rapidly by frontal accretion of a

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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 (4 0 km) steepsided cross section, and the axial strike-slip fault separates a shallow (125-625 m), flattopped 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 kni and normal fault throws increase along its outer wall, indicating greater flexure of the downgoing plate.

[1] Associated with Pacific-Australia plate convergence, the Lau Basin -Havre Trough is an active backarc basin that has been opened since $5.5 Ma by rifting and southward propagating oceanic spreading. Current back-arc opening rates decrease from 159 mm yr À1 in the northern Lau Basin to 15 mm y À1 in the southern Havre Trough. Major tectonic changes occur at the transition between Havre Trough rifting and full oceanic spreading of the Eastern Lau Spreading Center (ELSC), where the oblique-to-trench, westward subducting Louisville Seamount Chain (LSC) sweeps southwards along the Tonga trench. New swath bathymetry, seismic reflection data, and limited rock sampling in this area constrain a tectonic and kinematic back-arc model that incorporates the effects of LSC subduction. The ELSC, which extends southward to 24°55 0 S, forms a deep rift valley propagating southward through older, rifted arc basement. Present-day seismicity and fresh and fractured pillow lavas at 23°42 0 S are consistent with rift valley neovolcanism. Conversely, the northern Havre Trough has low seismicity and rifted volcanic basement ridges trending 25-45°oblique to the basin axis consistent with low levels of extensional tectonism and volcanism. This latter structural fabric is interpreted as an early stage of rifting that is now ''locked'' due to compression on the arc exerted by LSC subduction, while in the Lau Basin such effects have passed as the LSC swept along the Tonga Trench. It is proposed that the Lau-Havre back-arc opening is controlled by tectonic constraints exerted at the limits of the system by the LSC subduction, which determines the southward migration of the Tonga Arc pole of rotation and associated Lau Basin opening. A discrete threestage back-arc opening evolution is proposed, comprising: (1) an initial phase of back-arc rifting along the whole length of the plate boundary, beginning at $6-5 Ma; (2) a subsequent phase, mostly present in the southern part of the back-arc domain and still active in the Havre Trough, of transpression and transtension, starting at $4 Ma in the north, as the LSC starts to subduct and sweeps southward along the Tonga trench; and (3) a renewed opening phase in the northern segment of the back-arc domain, with rifting and spreading, starting at $3.5 Ma, as subduction of the LSC along the northern Tonga trench is progressively completed.