AISTEK-II: Mozambique Ridge and Mozambique Basin

Expedition SO183 with RV Sonne, 20th of May – 7th of July, 2005

Background:

Continental break-up is in general connected with extensive onshore volcanism before, during and also after the plates split. The relationship between pre-break-up magmatism and the onset of the formation of true oceanic crust is a matter of debate. More and more data, however, indicate that between both events there is a time span of 20-30 million years. This is also true for the volcanism prior to the separation of Africa from Antarctica. Massive volcanism occurred around 183 Ma onshore on both continents, but the drift of the plates did not start before 160 Ma. Several geological structures like the Astrid, Gunnerus, Mozambique and Madagascar ridge as well the Agulhas Plateau might have been affected by volcanism during or after the break-up of Gondwana. In order to better understand the distribution, relative timing and age of the plate separation and the associated volcanism, the nature of the different features must be better understood. Extensive marine magnetic investigations as well as seismic refraction and reflection data in critical areas are needed to provide the necessary information.

Objectives:

To better constrain reconstructions for the break-up of Gondwana between Africa and Antarctica the expedition SO183 with the RV Sonne was carried out off the coast of SE-Africa (Fig. 1) between May and July 2005. The major objectives of the cruise were:

• Acquire new magnetic data south of the southernmost tip of the Mozambique Ridge to better describe the drift history of this continental fragment.
• Retrieve dredge samples at critical locations to better describe the nature of the basement in order to relate it to the onshore geology of SE-Africa.
• Acquire new marine magnetic data in the Mozambique Basin to allow a more detailed description of the opening history. Several models predict that the eastern part of the Mozambique Basin is older and/or that the western Mozambique Basin might have a different opening history. Did the basin opened step wise? If yes, how?

Data acquisition:

For the magnetic measurements a ship towed magnetometer system (Type: SeaSpy) was used. At a speed of 10 knots the sensor was towed about 200 m behind the ship. A three-component fluxgate magnetometer system has been installed onboard the ship as a backup system for the duration of the cruise. Marine gravity measurements were performed with a LaCoste & Romberg S56 Land & Sea gravity meter. Bathymetric data were acquired by the use of the SIMRAD Multibeam Echo Sounder System (EM 120). Additionally, a Parasound sub-bottom profiler has been used to identify internal structures of the sedimentary cover along the ships track.

Results:

During 46 days of data acquisition a total of about 18000 km magnetic and bathymetric profile data were acquired (Fig 1). Additionally, at five selected locations basement rocks were dredged on the flanks of the Mozambique Ridge and in the Mozambique Channel next to the Jaguar seamount.

Figures: