Quaternary faulting in the offshore Flaxbourne and Wairarapa Basins, southern Cook Strait, New Zealand

Marine seismic reflection profiles, bathymetric data, and seabed samples reveal the stratigraphy and Quaternary structure of the southern Wairarapa and Flaxbourne Basins in southeastern Cook Strait and eastern Marlborough. These SW-NE-trending basins began forming before the late Miocene (>10 Ma), but their development has been mainly during and since that time and continues today within the Pacific-Australia plate boundary zone. Recently active structures deforming and bounding the basins are recognised by growth strata and deformation of Quaternary sediments. Observed structural geometries reflect Pliocene-Recent changes in the kinematics of faulting in central New Zealand. The 12-22 km wide southern Wairarapa Basin contains up to c. 2.9 km of strata and is deforming between offshore segments of the dextral strike-slip Wairarapa Fault and associated Wharekauhau Thrust on the western margin, and offshore extensions of the Aorangi Mountains range-front reverse faults on the eastern margin. To the southwest, in the eastern Marlborough Fault System, the 15-20 km wide, 80 km long Flaxbourne Basin contains >4.5 km of strata and is deforming by strike-slip and oblique-slip faults including offshore sections of the Hope and Kekerengu Faults. A new set of strike-slip faults, probably younger than 1 Ma, strike parallel (c. 080 +/- 10 degrees) to the current Pacific-Australian plate motion vector and obliquely to inherited structural trends. Three of these faults are possibly separating the Flaxbourne and Wairarapa Basins in central, southern Cook Strait. Curved traces of the Needles and Wairarapa Faults on the western margins of the basins are aligned, and may cut across disrupted Miocene structures to link part of the eastern Marlborough Fault System with the North Island Dextral Fault Belt.