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Seismic analysis and models from a rifted submarine plateau of continental origin: Great South Basin and Bounty Trough

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Citation:
Grobys, J. , Gohl, K. , Uenzelmann-Neben, G. , Davy, B. , Barker, D. and Deen, T. (2004): Seismic analysis and models from a rifted submarine plateau of continental origin: Great South Basin and Bounty Trough , Geo³, 6-9 Dec., Taupo, New Zealand .
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Abstract:

The Campbell Plateau and Chatham Rise are large submarine plateaux of continental origin forming part of the submarine New Zealand continent. Bounty Trough lies parallel to the former Gondwana subduction zone along the northern margin of Chatham Rise. At its western end, Bounty Trough (BT) connects with the Great South Basin (GSB). The whole region formed part of Gondwana until extension and subsequent seafloor spreading formed the Southern Ocean in the Late Cretaceous. Prior to the break-up of Gondwana, New Zealand was situated at the proto-Pacific plate boundary of Gondwana. Although the timing of break-up is relatively well defined, processes of break-up and the development of the continental fragments forming Campbell Plateau and Chatham Rise are not yet understood. It is expected that these processes played a key role in the development of Gondwanas plate boundary from a convergent margin to continental rifting. One model suggests that the Cretaceous rift evolved from an already existing back-arc basin, a proto-Bounty Trough. It may also be possible that the trough first developed as a rift system of the Southern Pacifics early opening as plate-kinematic reconstructions of the South Pacific region suggest.To investigate the evolution processes of these submarine plateaux, a geophysical and geolo-gical survey was conducted across the Campbell Plateau and Bounty Trough in 2003. The survey carried out two deep seismic transects with an ocean-bottom seismograph (OBS) spacing of 10-20 km in combination with a series of crustal multichannel seismic (MCS) reflection lines across the GSB and BT. The OBS records were bandpass-filtered and deconvolved. Magnetic and gravity data provide further constraints on the crustal models.The MCS profiles across the Bounty Trough show about 1.2 s (TWT) thick layered sediments with an incised valley. In most parts, basement can be identified in the reflection data, how-ever, in places only the OBS model allows discrimination of basement from older sediments. Internal basement reflections are clearly visible in some areas, but Moho is not observed on any of the MCS lines. The travel-time inversion model of the Bounty Trough implies a reduc-tion of crustal thickness to some 14 km (b.s.f.) in the area of the Bounty Channel, including 2-3 km of sedimentary cover, while crustal thickness increases to 20-25 km (b.s.f.) underneath the Bounty Platform and the southern Chatham Rise. P-wave velocities are about 2-3 km/s for sediments and range from 5 km/s for the upper basement to 7.5 km/s for the lower crust. The MCS profile across the Great South Basin shows generally well-layered sedimentary sequences. The travel-time model shows a crustal thickness of about 20-23 km (b.s.f.) including a sedimentary cover of up to 8 km under the channel. Crustal thickness increases to 27 km towards the South Island of New Zealand and 26 km with only 1 km of sedimentary cover towards the Campbell Plateau. P-wave velocities range from 2-4 km/s for sediments and increase from 5.5 to 7.5 km/s for the lower crust. Velocity models for both the Bounty Trough and Great South Basin are consistent with the gravity-anomaly models.Crustal models of both Bounty Trough and Great South Basin infer an extremely thinned crust beneath the Bounty Trough and the Great South Basin. This information related to the magnitude and style of rifting along Bounty Trough and Great South Basin, helps constrain models of the break-up process between New Zealand and Antarctica.

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