From 19th November to 19th December 2004 BGR conducted a marine geophysical cruise between 34°S and 36°S off Uruguay and between 46°S and 50°S off Argentine. The main research objective was to contribute to a better understanding of the initial breakup and the early opening of the South Atlantic. In continuation of our former work on the South Atlantic continental margins off Argentina, Brazil, Uruguay, Namibia and South Africa marine geophysical research (multi-channel seismics, refraction-/wide-angle reflection seismics, magnetics and gravity) was performed in close cooperation with the Argentine and Uruguayan authorities Comisión Nacional del Límite Exterior de la Plataforma Continental (COPLA) of Argentina and Servicio de Oceanograficia, Hidrograficia y Meteorologia de la Armada (SOHMA) of Uruguay. Multi-channel seismic lines with a total length of 3,754 km and additional 3540 km with the other geophysical methods were acquired . Along two lines refraction-/wide-angle reflection seismic work was carried out. The preliminary analyses of the new seismic data show different images of the crustal structures between Uruguay and southern Argentine with regard to the distribution and volume of offshore volcanic rocks (seaward dipping reflector sequences, SDRS) along the South American Atlantic margin. On the northern profiles between 34°S and 36°S one single well developed wedge of SDRS is present. Although the landward termination (‘feather edge’) on most of the lines is masked by multiples the average total width of the wedge across the margin seems to be 90 – 100 km and is very constant for this margin segment. This is strong contrast to the results from former cruises (BGR87, SO85 and BGR98) which covered the area between 38°S and 45°S. There, the SDRS showed distinct multiple wedges which in some places extend over 120 km across the continental slope. The investigation of the sedimentary section yielded that in the area off Uruguay widespread bottom simulating reflectors (BSR) are present. This indications for stable gas hydrates cover a total area of 7000 km2. One major aim of the cruise was to cover the transition between a volcanic passive margin and a non-volcanic passive resp. sheared margin. This was accomplished in the southern part of the investigated area. Two EW-trending profiles across the Argentine shelf into the Argentine Basin still show indications for SDRS but these structures are only 25 – 30 km wide. The profiles which extend from the NE to the SW crossing the Agulhas-Falkland Fracture Zone (AFFZ) onto the Falkland Plateau show the typical trend of a sheared margin. At the northern rim of the Falkland Plateau a set of small pre-rift half grabens were found indicating pre-rift extensional tectonic phases. The magnetic data in the area off Uruguay show lineations which are preliminary interpreted as chrons M0 to M3. This might indicate that the first (oldest) oceanic crust was created at a time around the magnetic polarity reversal between the normal interval M4 and the reversed interval M3 (126-127 Ma). Together with existing data from previous cruises this indicates that the breakup of the South Atlantic started further South because there magnetic chrons back to M9 (130 Ma) were identified. In the southernmost part of the margin at 47°S only the magnetic lineations M0 to M4 were identified in the oceanic domain Nevertheless, it is likely that between M4 and the assumed position of the continent ocean boundary/transition (COB/COT) older oceanic crust exists that for some reasons does not show correlatable lineations. The the free-air gravity map is dominated by the main topographic and structural features in the survey area. Rifted continental margins are characterized by prominent free-air gravity anomalies elongated parallel to the ocean-continent transition. The continental slope is considerably steeper in the North off Uruguay than in the South and thus the gravity high is much more pronounced in the North than in the South. The simple Bouguer anomaly map also shows the difference between the more gentle and wider continental slope in the South and the steeper slope in the North. The lowest Bouguer gravity values are found in the area of the basins on the continental shelf. Especially the Salado Basin in the prolongation of the Rio de la Plata and the Colorado Basin at about 40°S are indicated by Bouguer gravity anomaly highs. The interpretation by forward density modelling shows, however, the presence of SDRS units in the North of relative high density in the area of the continental slope. Whereas the modelling shows no indications for such volcanic bodies in the South. Although the MCS data indicate a small SDRS wedge but this body may be too small to cause an anomaly.From 17th April to 6th June 2003 BGR conducted a marine geophysical cruise between 30°S and 38°S off the Atlantic coast of South Africa. The main research objective was to contribute to a better understanding of the initial breakup and the early opening of the South Atlantic. In continuation of our former work on the South Atlantic continental margins off Argentina, Brazil, Uruguay and Namibia marine geophysical research (multi-channel seismics, wide-angle refraction seismics, magnetics and gravity) was performed in cooperation with the Petroleum Agency South Africa (PASA). Multi-channel lines with a total lenght of 3,260 km, and additional 1,365km, with the other geophysical methods were acquired. Combined onshore/offshore refraction seismic work in cooperation with GeoForschungsZentrum Potsdam (Germany) and the Council for Geoscience (South Africa) was also part of the program. Im November und Dezember 2004 hat die BGR eine marine geophysikalische Forschungsfahrt vor Uruguay (34°S bis 36°S) und Argentinien (46°S bis 50°S) durchgeführt. Das Hauptziel der Untersuchungen war ein besseres Verständnis für die frühe Öffnungsgeschichte des Südatlantiks. In der Tradition früherer Projekte vor Argentinien, Brasilien, Uruguay, Nambia und Südafrika wurden auch diese Arbeiten in enger Kooperation der argentinischen Institution COPLA (Comisión Nacional del Límite Exterior de la Plataforma Continental) und der uruguayischen Institution SOHMA (Servicio de Oceanograficia, Hidrograficia y Meteorologia de la Armada) durchgeführt. Insgesant wurden 3754 Profil-km reflexionsseismisch vermessen, weitere 3540 Profil-km wurden mit Magnetik und Gravimetrie erfasst. Zusätzlich wurden zwei weitwinkelseismische Linien akquiriert. Eine erste Analyse der seismischen Daten hat grundlegende Unterschiede der kontinentalen Ränder vor Uruguay und Süd-Argentinien in Bezug auf die Verteilung und das Volumen von vulkanischen Gesteinen gezeigt. Im nördlichen Bereich zwischen 34°S und 36°S findet sich nur ein ausgeprägter Keil mit seewärts einfallenden Reflektorfolgen (SDRs). Dieser Keil ersteckt sich über ca. 90-100 km senkrecht zum Kontinentrand. Dies steht im Kontrast zu Ergebnissen von früheren Expeditionen im Bereich 38°S bis 45°S. Hier erstrecken sich die SDRs über bis zu 120 km und sind in mehrere Keile unterteilt. In der Sedimentsäule im nördlichen Untersuchungsgebiet werden gehäuft meeresboden-simulierende Reflektoren (BSRs) angetroffen. Diese sind Indikatoren für stabile Gashydrate und bedecken eine Fläche von 7000 km2. Ein Hauptziel der Arbeiten im südlichen Gebiet war es, den Übergang zwischen einem vulkanischen und einem gescherten Kontinentrand zu untersuchen. Am südlichen Ende des argentinischen Schelfs beträgt die Breite der SDRs nur noch ca. 25-30 km. Weiter süd-östlich – im Bereich der Agulhas-Falkland Bruchzone und dem Falkland Plateau – findet sich ein klassisches Abbild eines gescherten Randes. Am nördlichen Rand des Falkland Plateaus zeugen kleine Halb-Gräben von früherer Extensionstektonik. Magnetische Anomalien im nördlichen Teilgebiet zeugen von einer beginnenden Ozeanbildung bei Anomalie M4 und M4 (126-127 Ma). Im südlichen Bereich werden ältere Anomalien als M4 vermutet, diese zeigen sich jedoch nicht im bestehenden Datensatz. Die gemessene Freiluft-Schwere wird durch die Bathymetrie dominiert. Kontinentränder zeigen im Bereich der Kontinent-Ozean-Grenze ausgeprägte Anomalien. Da der Kontinenthang im nördlichen Teilbereich wesentlich steiler verläuft als im südlichen, ist dort die Freiluft-Anomalie auch deutlich ausgeprägter.