Expedition Strakhov 23, Cape Town – Cape Town
February 10 – March 17 2006

Conclusions

We have written these conclusions right after the end of the cruise and they are to be considered preliminary. Peyve et al, (2007) present an English version of the summary of the results of the cruise. A pdf version of this paper can be found in John SclaterÕs list of publications.

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 6000m of water 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 uplifted horizontal reflectors within the en echelon topographic highs that dominate the morphology of the central portion of the domain. These reflectors appear to indicate a faulting as against an 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 moodily and extend many of the preliminary conclusions that resulted from the initial survey to the Andrew Bain from R/V Knorr in 1996.

February 25, 2006

Second report from the R/V Strakhov. Most of the past week has been taken up with dredging. We completed the dredging of the spreading center between the Du Toit and Andrew Bain transform faults. We recovered basalts, gabbros and ultra-mafics from the ridge segment further from the Andrew Bain. This segment has shallow depths, a negative mantle Bouguer anomaly and a pronounced high in the magnetization. It is clearly a region of magmatic intrusion and the basalts and gabbros were expected. The second segment to the east connects the spreading center to the Andrew Bain transform fault. It consists of a deep rift valley with no obvious axial volcanic zone. It has almost no magnetic anomalies and no mantle Bouguer anomaly. We expected to dredge ultra-mafic rocks from this valley. The first two dredges on the western end of the rift valley came up empty: but we recovered ultra-mafics from a curved ridge in the center of the rift valley where it entered the Andrew Bain transform fault and from another pronounced ridge at the junction of the northern rift valley with the transform fault. Clearly the eastern segment of the spreading center appears to be the site of amagmatic extension.

After finishing this first dredging program we started onto to a detailed multibeam and magnetic survey of the southern portion of the transform fault preliminary to starting our second dredging program. This consisted of a series of dredges starting at the rift valley that marks the spreading center and spaced at 3 million years intervals along the deepest portion of the western transverse ridge. The objective of this program is to investigate differences in the ultra-mafic rocks with time at a specific spreading center. The first two dredges along this line yielded abundant ultra-mafics but the next three recovered only erratic rocks and some gabbros. It appears that as the ocean floor ages to the north more and more sediment has accumulated making the recovery of fresh rock difficult. We will continue some more on this program before breaking off to continue our survey of the southern end of the Andrew Bain.

February 18, 2006

We have now been at sea for eight days. The first four days involved a transit from Cape Town to the northern end of the Andrew Bain. The transit was uneventful as the weather was good and the ship rides well except with a tendency to roll because of her flat bottom. On the fifth day, we started a long multibeam, magnetometer and single channel seismic line down the western edge of the Andrew Bain. Even though we had a spell of bad weather on this long line we were able to complete it within three days. For the last day we have been dredging the rift valley that marks the spreading center at the southwestern end of the Andrew Bain.

The objective of the dredging is to determine if the intrusion zone changes from basalts to ultra mafics as we approach the intersection of the spreading center and the transform fault. It has been cold but the weather has not been much of an impediment.

February 11, 2006

Chris and I met the R/V Strakhov when the ship arrived in Cape Town on February 7. The weather in Cape Town was beautiful and we were able to have a very pleasant trip around the top of Table Mountain before meeting the ship. After the scientists from the previous leg had left, we moved on board on February 8. The ship left Cape Town at noon on February 10 2006.

February 6, 2006

I am part of a group of three American scientists, graduate student Chris Takeuchi, I from Scripps Institution of Oceanography and Nancy Gridlay from the Center for Marine Sciences of the University of North Carolina at Wilmington, who have joined an Italian/Russian expedition to the Southwest Indian Ocean. This expedition on the R/V Strakhov, a Russian research vessel will survey and dredge the Andrew Bain, a mega-transform fault that offsets the Southwest Indian Ridge some 750 km to the northeast almost sue south of the western edge of southern Africa. This program follows up a preliminary survey that Nancy and I carried out of the Andrew Bain ten years ago on the American research vessel, R/V Knorr.

February 2, 2006

Russian/Italian Expedition to Andrew Bain Transform Fault/Southwest Indian Ocean

R/V “Akademik Nikolaj Strakhov”

I am leaving on February 4 2006 to spend five weeks at sea on the R/V Strakhov to study the composition and tectonic structure of the Andrew Bain transform on the Southwest Indian Ridge.

The Andrew Bain transform lies almost due south of the western coast of South Africa and splits the Southwest Indian ridge almost exactly in half. The trace of this fractures zone marks the direction of separation of Africa away from Antarctica after the break up of Gondwanaland some 140 million years ago.

A preliminary survey was carried out of the Andrew Bain from the R/V Knorr in 1996. However, no rock sampling was carried out during this expedition. The purpose of the current expedition is to recover dredge samples from the floor and the side of the transform fault and to fill in the gaps in the preliminary survey.

The Andrew Bain transform is classified as a mega-transform fault. With a width of 100 km it is the widest active transform in the oceans. It has a length of 750 km and is the second longest transform fault in the oceans. Only the 900 km Romanche transform fault in the central Atlantic is longer. The Southwest Indian Ridge and the Andrew Bain transform fault separate the Somalian and Nubian plates from the Antarctic plate. The great width of the Andrew Bain transform has been attributed to a change in motion within the last 20 million year of the Nubian plate with respect to the Antarctic plate. This plate motion caused transtensional stresses along the Andrew Bain that in turn created oblique extension within the transform fault.

The fault has been the site of some major strike slip earthquakes and includes the magnitude 8.2 1942 earthquake which is the largest strike slip earthquake on a transform fault in the oceans.

Figure. Southwest Indian Ridge: Relocated earthquakes (black dots), focal mechanism solutions (size indicates relative magnitude), the dominant rock type in dredge hauls (circles: red basalt, yellow-gabbro, green-peridotite, magneta-mixture peridotite and basalt, superimposed on a chart of the predicted bathymetry from Sandwell and Smith (1997) combined with the multibeam bathymetry from KN145L16 . DT, AB, M and PE identify respectively the Du Toit, Andrew Bain, Marion and Prince Edward transform faults. The inset presents crest of the SWIR (thick black line) and the traces of the prominent fracture zones (shaded lines) superimposed upon the 4000 m contour (thin black line) in the Southwest Indian Ocean.