Crustal structure and neotectonics of the Puysegur oblique subduction zone, New Zealand

Eocene-Oligocene rifting, followed by Miocene-Quaternary; strike-slip and obliquely convergent tectonics, are responsible for crustal thinning in the Solander Basin and subsequent development of the Puysegur oblique subduction zone, southwest New Zealand. Highly oblique convergence at a rate of 35 mm/yr is being partitioned between oblique subduction at the Puysegur trench, thrusting within the trench slope, strike-slip faulting on the Puysegur Bank, and minor shortening within the Solander Basin. Subduction is young and a volcanic are is not yet well-developed. We present seismic reflection data recorded across the Puysegur subduction zone and Solander Basin. Three strike-slip fault zones are recognised on the basis of seismic and bathymetric data in the Puysegur Bank region. North of the profile, some displacement on the-eastern fault is transferred eastward to the Hauroko Fault, but most strike-slip motion is inferred to be west of Fiordland. South of the profile, strike-slip faults merge with faults of the Puysegur Ridge. Active thrusting dominates the trench slope, where a thick sedimentary section is evident and bottom simulating reflectors are common. The relatively steep average slope angle of similar to 6 degrees suggests the sediments are moderately well cemented, possibly similar to sediments found in the Solander Basin. The small amount of sediment found on the subducting plate is consistent with little or no accretion at the margin. Patches of reflectivity we interpret as sediment at the top of the subducted plate suggest that the slab has a moderate dip of 10-20 degrees to 20 km depth. In the Solander Basin, seismic reflection data clearly image the sedimentary section, and show well developed crustal and upper mantle reflections to depths of 30-35 km (12-13 s TWT). On the basis of lower crustal reflectivity and gravity modelling, the base of the crust beneath the adjacent Stewart Island shelf is at similar to 30 km (similar to 9 a TWT), and rises to a depth of similar to 20 km (-8 s TWT) beneath the Solander Basin. Prominent dipping reflections which we interpret as an active fault can be traced to similar to 30 km depth (similar to 12 s TWT), where they merge into a zone of strong reflectivity in the upper mantle. (C) 1999 Elsevier Science B.V. All rights reserved.