3-D imaging of Marlborough, New Zealand, subducted plate and strike-slip fault systems

Abstract

SUMMARY We present a 3-D seismic velocity model of the northern South Island, New Zealand, using several combined passive and active-source data sets. This area is the transition from Hikurangi subduction in the North Island to oblique continental collision along the Alpine fault and the Southern Alps, and geodetically observed deformation is consistent with plate motion being primarily taken up on a series of rotating strike-slip faults. We carry out simultaneous inversion for Vp and Vp/Vs and hypocentres using earthquake data recorded by the permanent seismic network and three temporary seismic arrays, also using active-source marine airgun data recorded at onshore stations. We image the subducted Hikurangi slab as a continuous unbroken feature extending from the active subduction zone to the north. Beneath the Marlborough region, the 3-D hypocentres define the subducted slab as a smooth curved feature to over 250-km depth, whereas below the junction of the Awatere and Alpine faults, the seismicity abruptly ends at 100-km depth. The subducting Pacific plate has a thick crust, including greywacke upper crust, which slows subduction. The interaction of the subducted plate with the translating overlying plate forms a 50-km-long zone of apparent crustal thickening with Vp < 7.0 km s−1 observed to 40-km depth. The Australian Plate has a high-velocity block to 65-km depth, aligned with the Separation Point pluton and trending oblique to the subducted slab. This zone of thicker Australian lithosphere may also hinder subduction. The Marlborough faults trend about 25° to the strike of the underlying subducted slab, so that the relation of faults to the slab varies along the fault strikes, and the overlying plate ranges in thickness from 20 to 50 km along the faults. Thus, the fault interaction with the subducted slab varies, from translating material directly above the subducting plate, to bottoming into a thick ductile region. The lower crust has high Vp/Vs under the Marlborough faults (except for a region of Haast schist), which may indicate excess fluid released by the subducted plate.