Background:

The Mesozoic opening history of the Southern Ocean between South America, Africa and Antarctica is still a matter of ongoing debate with completely different models being published in literature in the last couple of years. A key to this problem is the opening of the Weddell Sea in the southwestern corner of Gondwana. Until recently, no clear constraints existed to define the age and structure of the oldest ocean floor in the Weddell Sea basin which caused large uncertainties in any model dealing with the early opening between South America and Antarctica. The same holds for the opening between Africa and Antarctica. While some constraints on the age of the ocean floor existed for the Mozambique Basin (Africa) for its conjugate the Riiser-Larsen Sea (Antarctica) almost no information on the age of the basin was available. To close these gaps of knowledge, the Alfred Wegener Institute conducted a set of high resolution aeromagnetic surveys along the coast of Dronning Maud Land within the East Antarctic Margin Aeromagnetic and Gravity Experiment (EMAGE) (Fig. 1).

Data acquisition and processing:

The EMAGE project was accomplished in the years 1996 to 2002 along the coast of Dronning Maud Land (Fig, 1). Within six aeromagnetic campaigns a total of 90000 km of magnetic data were acquired covering an area of about 680000 km2. Additionally, two helicopter borne magnetic surveys were performed and resulted in 20000 km of magnetic profile data. Most of the aeromagnetic measurements were carried out with the German research aircraft Polar-2, a Dornier-228. The helicopter surveys were conducted with a BO-105 operating from the German research vessel and icebreaker Polarstern.

After a standard processing of each individual campaign (despiking, IGRF correction, diurnal correction) the data were upward continued to a common flight level of 1500 m and merged into one database. For leveling a systematic adjustment procedure was applied. Finally, for visualization, the data were gridded and interpolated to a grid cell size of 2x2 km (Fig. 2).

From a detailed interpretation of the clearly identifiable coast parallel magnetic anomalies along the west coast of Dronning Maud Land and the anomalies in the Riiser-Larsen Sea new constraints on the age of the ocean floor in the Weddell Sea and the Riiser-Larsen Sea could be derived. The new data and magnetic anomaly identifications were combined with already existing ship track and airborne magnetic data to extend the age information further west into the central and southern Weddell Sea. Based on these interpretations and additional magnetic, gravimetric and geometric constraints form other ocean basins along the continental margins of Africa, Antarctica and South America a new set of finite rotations for the break-up of Gondwana and the opening of the South Atlantic Ocean were calculated. An animation visualizing the new model for the break-up of Gondwana between Africa, Antarctica and South America is shown below (Fig. 3).

Results:

According to the new Gondwana break-up model, first rifting between Africa and Antarctica starts at around 167 Ma in the conjugate Mozambique Basin – Riiser-Larsen Sea while extensional forces are already active between the Antarctic Peninsula and the East Antarctic craton and within the Falkland Plateau. With the Somali Basin being also active at the same time there are three separate basins (Weddell Sea Basin, Mozambique Basin, Somali Basin) developing independently since the early beginning of Gondwana break –up. Subsequently, the opening of the Weddell Sea starts at around 147 Ma in the southernmost part of the Weddell Sea in a NNW direction at slow spreading rates (~12-14 mm/yr). At about 122 Ma (M2) the spreading rate drops down to ultra-slow rates causing the formation of Anomaly-T. Between 140-130 Ma the Falkland Plateau clears the coast of central Dronning Maud Land resulting in the complete separation of South America from the East Antarctic craton. This new model indicates that the Karoo and Dronning Maud Land magmatism (both 183 Ma) occurred well before any new ocean floor in the Weddell Sea was created and therefore allows no correlation between the formation of new oceanic crust and the occurrence of any plume related events. The formation of the Explora Wedge (sequence of seaward dipping reflectors along the west coast of Dronning Maud Land) is dated to 150-138 Ma. This is about 30 million years later than previously proposed by other authors.

Figures: