RRS Charles Darwin Cruise 167, 23 Nov - 21 Dec 2004. Sub-seafloor physical properties at Saldanha Seamount, Mid-Atlantic Ridge, and controls on the spatial distribution of hydrothermal venting

Abstract

Charles Darwin 167 (CD167) was a joint research project carried out by UK and Portuguese scientists from the National Oceanography Centre, Southampton, the University of Durham and the University of Lisbon. The task of CD167 was to carry out geophysical and geological studies at the Mid-Atlantic Ridge. The work area was a ridge offset centred on 36º 34’ N, 33º 26’ W - 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. Of particular interest is a site known as the Saldanha Massif, where previous studies have revealed an area of exposed, tectonically-unroofed mantle rocks and unexpectedly significant hydrothermal circulation with venting near the summit of the massif.

A three-dimensional controlled source electromagnetic (CSEM) survey of sub-sea-floor electrical resistivity was carried out over a 10 km2 area centred on the Saldanha Massif. Following CSEM data analysis, the resulting images of electrical structure will be translated into constraints on porosity distribution, interconnectedness and pore fluid properties by means of geophysical effective medium modelling methods. This in turn will address the questions of whether the Saldanha vent site owes its existence to the presence of a deep fracture network, whether this network completely penetrates the thin crustal carapace within the ridge offset, and how far it extends downwards into the underlying mantle rocks.

A secondary objective was to collect a series of ridge-perpendicular (approximately east-west) profiles of bathymetry and of gravity and magnetic anomalies, to contribute to regional tectonic studies through improving an international European compilation of such data. This was successfully achieved, and the data have been passed on to our international collaborators. A third objective was to collect sea bottom geological samples, by gravity coring and dredging, for analysis at CREMINER in Lisbon. This objective was also achieved.

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