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Geology & Geophysics Group

RRS Charles Darwin cruise CD167

Geophysical Study of Hydrothermal Venting at the Saldanha Massif, Mid-Atlantic Ridge

Charles Darwin 167 was a joint research cruise carried out by UK and Portuguese scientists from the National Oceanography Centre Southampton, the University of Durham and the University of Lisbon. The objective was to carry out geophysical and geological studies at the Mid-Atlantic Ridge. Our work area is a ridge offset at 36º 45' North - about 200 n.m. south of the island of Flores in the Azores. At this location, the median valley of the ridge is offset by about 20 km right-laterally, forming a non-transform discontinuity. The region of the offset is tectonically complex. Of particular interest is a site known as the 'Saldanha Massif', where previous studies have revealed an area of exposed, tectonically-unroofed mantle rocks. Unexpectedly, this same area also hosts a significant hydrothermal circulation system with venting documented near the summit of the massif. Such circulation systems have in the past been believed to be associated with volcanic, rather than non-volcanic, ridge sites - so this combination, while not unique, represents an aspect of sea floor spreading and ocean-lithosphere interaction that has not been well studied or documented in the past.

Study area map

Above: The location of the study area, on the Mid-Atlantic Ridge southwest of the Azores.

Our primary objective was to carry out a controlled-source electromagnetic sounding survey of the seafloor at and around the Saldanha massif. We deployed an array of 18 seafloor recording instruments for this purpose, together with an array of three current meter moorings to provide information on near-sea-bed hydrographic conditions. All eighteen seafloor geophysical instruments were recovered without loss; seventeen of them having recorded data. All three current meter moorings were also recovered successfully; and the data from these was supplemented by a sound velocity meter profile and several XBT stations. The DASI deep-towed transmitter system was used to provide the controlled source signals. We had some technical difficulties with the DASI system, arising from ship-to-towed-vehicle communications problems and the operation of the deep-towed altimeter. This resulted in the loss of some of the planned DASI tow lines, and in some other lines being towed at a greater height above the sea floor than would have been ideal. The CSEM survey objective was therefore only partly achieved. Nonetheless we expect to be able to analyse the data collected to provide new information about the target area.

Our second objective was to collect a set of bathymetry, gravity and magnetic profiles across the ridge through, and N and S of, the Saldanha massif, to complement and add to existing French and Portuguese compilations. This objective was achieved, with high quality bathymetry data provided by the EM-12 swath system. The magnetometer worked properly for only parts of the cruise. At other times the results were very noisy, although we may be able to extract useful data by filtering.

Our third objective was to collect a series of short gravity cores of sediments on and around the massif, for the purpose of investigating the history of hydrothermal activity and assessing the extent of hydrothermal circulation through and reactions within the sediments. A total of 11 cores were collected, with nine of these providing useful materials.

Our fourth objective was to dredge for hard rock samples from sites around the Saldanha massif. Two dredges were completed, and both of these returned useful samples.

In summary: our primary objective was partly met, and our supplementary objectives were fully met. All of the 21 sets of seafloor instruments/moorings that we deployed were recovered safely.

CSEM survey map

Above: The layout of the CSEM survey within the non-transform offset of the MAR.  Red circles denote ocean bottom electric field receiver locations. White lines indicate DASI-2 transmitter tow lines.

 

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