Geological Framework of the Southern Lord Howe Rise/West Norfolk Ridge Region : Project 121.30 : Post Cruise Report

Brief description

Project 121.30 - "Lord Howe Rise and Norfolk Ridge 'Law of Sea' Study" -was initiatedto improve our understanding of the geological framework of the southern part of theLord Howe Rise region, in the vicinity of the Australia/New Zealand seabed boundary zone.

The major objectives of the project are:
To investigate the structure, stratigraphy and basin development of the southern LordHowe Rise, southern New Caledonia Basin and the West Norfolk Ridge.

To assess the resource potential of the major structural features within the region.

To determine the tectonic framework, crustal characteristics and evolution of the region, and attempt to understand the processes that have produced narrow strips of thinned and extended continental lithosphere ("ribbon continents") separated bynarrow ocean basins.

To acquire data to assist with the definition of Australia's 'legal' Continental Shelf on the southwestern margin of the Lord Howe Rise.

Because of the presence of Lord Howe Island and Norfolk Island, Australia has laid claim to a large part of the seabed in the region, enveloped by a 200 nautical mile Exclusive Economic Zone (EEZ) around each island, plus areas of 'legal' ContinentalShelf beyond the EEZ. The total area of Australia's 'legal' Continental Shelf in the Lord Howe Rise region would be about 1.65 million km2. While the seabed boundary in the northern part of the region has been negotiated between Australia and France, there is not, as yet, a negotiated boundary between Australia and New Zealand in the southernLord Howe Rise/West Norfolk Ridge region.

The proposal for Survey 114 was to acquire approximately 3250 km of multichannelseismic and associated geophysical data. The data was acquired mainly on longregional lines, two of which extended from the Tasman Basin to the Norfolk Basin, using a 3000 m streamer (120 x 25 m groups), 49 litre sleeve gun arrays, 16 secondrecords, and 30-fold coverage.

The ship departed Sydney at 2000 hrs on Friday, 6 November 1992, and, after transitingthrough rough seas, arrived in the study area on the evening of 9 November. The cablewas deployed and balanced by midnight, 10 November. The next twenty one days wasspent shooting deep crustal (16 sec.) seismic . During this period a total of 3191 km ofseismic was shot, over 70% with two gun arrays operating. This was achieved in spiteof 3.5 days loss as a result of bad weather and 20 hours loss for cable repairs as a resultof fish bites. Seismic acquisition ceased on the afternoon of 1 December. The shipreturned to Sydney at 1730 hrs on Thursday, 3 December.

Overall, the quality of the seismic data was very good. When shooting the two verylong lines, the weather remained reasonably calm, and so noise levels were withinspecifications.

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