We have written these conclusions right after the end of the cruise and they are to be considered preliminary.
During S23-AB06 cruise in the Andrew Bain Fracture Zone we obtained:
The data is under detailed processing and analysis, and we expect to have new insights into the Andrew Bain FZ geology and geochemistry.
The major accomplishments were (a) to complete a full multi-beam survey of the southern section of the Andrew Bain transform fault and (b) to dredge in the southern spreading centers and along the south western wall of the transform fault.
In the southern spreading center we found basalts, gabbros and a few ultra mafics on the segment closest to the Du Toit Transform Fault. We tried but did not complete successful dredges within the trough that marks the spreading center segment closest to the Andrew Bain. However at the presumed spreading center/transform fault intersection (SC/TFI) we dredged significant quantities of peridotite with very little to no basalt or gabbro. On the western wall we had a series of successful dredges on successively older crust before we encounter significant sediment and we recovered no more material. The dredges, which started in greater than 6000 m of water depth within the transform valley, recovered almost entirely ultra mafic rocks. We plan to examine the transient nature of the temperature of the intrusion zone with these samples.
Completing the survey of the southern part of the transform permitted us to identify the major transform valley from the SC/TFI north to about 50°S. Clearly it follows the western portion of the transform valley with at least one small offset to the west. With our preliminary understanding of the morphology we do not see how to account for two high angle dip slip events reported by Sclater et al., (2005). Motion along the southern section of the transform domain appears to be taken up by a series of linear offsetting transform faults. This is very different to the complex relay basin type motion observed in the northern section of the domain.
In addition to the work to the southern section of the transform fault we also completed a number of north-south lines both within, and to the west and east of, the transform domain. These lines filled in, and extended to west and east of the domain, our previous survey in the area. We obtained identifiable magnetic lineations on the normal ocean floor created at the southern spreading center. In addition we established the morphological limit of the south eastern extent of the transform domain. A final success was a 48 channel seismic line across the center of the transform domain. This line identified significant thicknesses of sediment within the central transform valley and dipping and truncated reflectors within the en echelon topographic highs that dominate the morphology of the central portion of the domain. These reflectors appear to indicate a tectonic instead of intrusive origin for these features. Unfortunately bad weather prevented the completion of the two additional seismic lines that we had planned. All in all we had much success on this cruise and will be able to modify and extend many of the preliminary conclusions that resulted from the initial survey to the Andrew Bain from R/V Knorr in 1996.