ALBERT

Description: The southern African continental transform margin is of great interest for the understanding of processes related to continental breakup, transform fault formation and vertical plate motion. Open questions include the cause and consequences for the high topography of southern Africa, neotectonic activity along the Agulhas-Falkland Fracture Zone and the formation of the Outeniqua Basin. As a component of the project Inkaba yeAfrica, the 900 km long Agulhas-Karoo Geoscience Transect was carried out in order to shed light on the tectonic structure, evolution and processes along this margin. Two onshore-offshore seismic refraction/wide-angle reflection profiles are part of this transect, which extends from the Karoo Province, across the Cape Fold Belt, the Outeniqua Basin and the Agulhas-Falkland Fracture Zone to the Agulhas Plateau. Thinning of the continental crust begins landward of the coast line and continues beneath the shelf which has a thickness of 28-30 km. The transition from stretched continental to normal oceanic crust in the Agulhas Passage occurs at the Agulhas-Falkland Fracture Zone. The oceanic crust south of the Agulhas Passage is significantly thickened to 24 km, forming the Agulhas Plateau where high seismic velocities of greater than 7 km/s are modelled for the lower 50 % of the crust.

Description: The southern African continental transform margin is of great interest for the understanding of processes related to continental breakup, transform fault formation and vertical plate motion. Open questions include the cause and consequences for the high topography of southern Africa, neotectonic activity along the Agulhas-Falkland Fracture Zone and the formation of the Outeniqua Basin. As a component of the project Inkaba yeAfrica, the 900 km long Agulhas-Karoo Geoscience Transect was carried out in order to shed light on the tectonic structure, evolution and processes along this margin. Two onshore-offshore seismic refraction/wide-angle reflection profiles are part of this transect, which extends from the Karoo Province, across the Cape Fold Belt, the Outeniqua Basin and the Agulhas-Falkland Fracture Zone to the Agulhas Plateau. Thinning of the continental crust begins landward of the coast line and continues beneath the shelf which has a thickness of 28-30 km. The transition from stretched continental to normal oceanic crust in the Agulhas Passage occurs at the Agulhas-Falkland Fracture Zone. The oceanic crust south of the Agulhas Passage is significantly thickened to 24 km, forming the Agulhas Plateau where high seismic velocities of greater than 7 km/s are modelled for the lower 50 % of the crust.

Description: A number of geophysical on-shore and off-shore experiments were carried out in a profile across the southern margin of the African continent in the framework of the Inkaba yeAfrica project. Refraction seismic experiments have shown that the crustal thickness decreases rapidly from over 40 to around 30 km well inland of the present coast, before gently thinning out towards the Agulhas Falkland Fracture Zone, which marks the transition zone between continental and oceanic crust. This is consistent with a non-volcanic mode of breakup due to shear along the Agulhas-Flakland Transform Fault. In region of the abruptly decreasing Moho depth inland from the coast, lower crustal P-wave velocities up to 7.4 km/s are observed. We interpret these to represent metabasic lithologies of the Mesoproterozoic Namaqua-Natal Metamorphic Complex, or intrusions of gabbroic material added to the base of the crust by younger magmatism. This magmatism could be the result of the mid-Jurassic Karoo-Ferrar-Chon Aike event. The velocity model for the upper crust has excellent resolution, and is consistent with the known geological record. A comparison of the velocity model with electrical conductivity models shows that a zone of high seismic velocities north of the centre of the Beattie Magnetic Anomaly, one of the largest magnetic anomalies, correlates well with a resistive zone. Contrary to existing interpretation, the Beattie Magnetic Anomaly does not origin from an area which is electrically conductive but seems to correlate with a source region which is resistive and has high seismic velocities.

Description: Southern Africa and its southern continental margin offer an unrivalled region, where continental accretion, continental breakup and magmatic processes over a period of more than 3.5 billion years can be studied. The Agulhas-Karoo Geoscience Transect is part of the South African - German cooperative research project Inkaba yeAfrica, which aims to investigate this part of the continent and ocean in a cone-shaped sector from core to space. Geophysical and geological data and samples were collected along this transect which spans from the Agulhas Plateau across the Agulhas-Falkland fracture zone, the Outeniqua Basin, the Cape Fold Belt, the Namaqua-Natal Belt onto the Karoo Craton. A combined onshore-offshore deep crustal seismic reflection and refraction survey as well as several magnetotelluric surveys provide information about tectonic and magmatic structures and constraints for physical parameters from sedimentary sequences to the upper mantle. The main objectives include an understanding of the crustal nature of the Agulhas Plateau, the processes accompanying and succeeding the crustal shearing process along the Agulhas-Falkland Transform/Fracture Zone, the offshore basin formation in relation to breakup, the deep-seated tectonics of the Cape Fold Belt, and the geometries and sources of the Beattie Magnetic Anomaly and the Southern Cape Conductivity Belt. The geophysical data are integrated with geological, petrological and geochemical analysis on rock composition, age and alteration history to form an overarching geodynamic model of the evolution of this region and its sedimentary, tectonic and magmatic units.

Description: The southern African continental transform margin is of great interest for the understanding of processes related to continental breakup, transform fault formation and vertical plate motion. Open questions include the cause and consequences for the high topography of southern Africa, neotectonic activity along the Agulhas-Falkland Fracture Zone and the formation of the Outeniqua Basin. As a component of the project Inkaba yeAfrica, the 900 km long Agulhas-Karoo Geoscience Transect was carried out in order to shed light on the tectonic structure, evolution and processes along this margin. Two onshore-offshore seismic refraction/wide-angle reflection profiles are part of this transect, which extends from the Karoo Province, across the Cape Fold Belt, the Outeniqua Basin and the Agulhas-Falkland Fracture Zone to the Agulhas Plateau. Thinning of the continental crust begins landward of the coast line and continues beneath the shelf which has a thickness of 28-30 km. The transition from stretched continental to normal oceanic crust in the Agulhas Passage occurs at the Agulhas-Falkland Fracture Zone. The oceanic crust south of the Agulhas Passage is significantly thickened to 24 km, forming the Agulhas Plateau where high seismic velocities of greater than 7 km/s are modelled for the lower 50 % of the crust.

Description: Southern Africa and its southern continental margin offer an unrivalled region, where continental accretion processes over a period of more than 3.5 billion years can be studied. Along a geoscientific transect from the offshore Agulhas Plateau across the Agulhas Fracture, the Outeniqua Basin, the Cape Fold Belt, the Namaqua-Natal Belt into the Karoo Province, geophysical and geological data and samples have been collected in order to build a model of the evolution and crustal accretion as well as the continental break-up of this region. With this transect, which is a component of the German - South African project Inkaba ye Africa, objectives are addressed such as the Mesoproterozoic accretion processes along the southern margin of the Kaapvaal Craton, the extent of Pan-African inliers in the Cape Fold Belt, the formation of the Cape Fold Belt, the sources for the Beattie Magnetic Anomaly and the Southern Cape Conductivity Belt, the continental/oceanic origin of the Agulhas Plateau, the formation of the Agulhas Fracture Zone and its consequences for basin formation and uplift processes. A combined land-sea deep crustal seismic reflection and refraction survey as well as a magnetotelluric survey along the transect provides detailed structures and constraints for physical parameters from the upper crust to the upper mantle which will be integrated with geological, petrological and geochemical analysis on rock composition, age and alteration history to form an overarching geodynamic model for the evolution of the region and its tectonic units.

Description: The concept of South America and Africa as rigid continents during the formation, growth and motion of their respective plates has frustrated reconstruction of a tight, geologically economic fit between these two fragments in their Gondwana framework. We recognize that (1) internal strains released during and following Gondwana break-up have distorted their actual shapes within Gondwana and (2) these two continents comprise mosaics of smaller microblocks, or platelets, of relatively undistorted Precambrian terrains that experienced modest, episodic relative motions along rift zones that separate them. This permits a fresh approach to quantitative reconstructions of palaeo-continents. Former geological ties forged at the time of Gondwana amalgamation, now exposed at the continental margins of the South Atlantic as piercing points, provide robust anchors for new paleo-cartographic experiments. We present two new tectonic maps of the Brasiliano and Pan-African structures of West Gondwana on which we identify ten piercing points that, if re-joined simultaneously, could facilitate quantification of a well-substantiated Gondwana fit and help retrace the evolution of its continental margins with greater accuracy than has been achieved until now. This has significant bearing on understanding the origin and evolution of passive continental margins, and the geodynamics of Gondwana break-up.