SUMMARY Large Igneous Provinces (LIP) are of great interest due to their role in crustal generation, magmatic processes and environmental impact. The Agulhas Plateau in the southwest Indian Ocean off South Africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity. With new geophysical data from seismic refraction and reflection profiling, we are able to present improved evidence for its crustal structure and composition. The velocity–depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km, where three major units can be identified in the crust. In our seismic reflection records, evidence for volcanic flows on the Agulhas Plateau can be observed. The middle crust is thickened by magmatic intrusions. The up to 10 km thick lower crustal body is characterized by high seismic velocities of 7.0–7.6 km s−1. The velocity–depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic LIPs such as the Ontong‐Java Plateau or the northern Kerguelen Plateau. The total volume of the Agulhas Plateau was estimated to be 4 × 106 km3 of which about 10 per cent consists of extruded igneous material. We use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material. The Agulhas Plateau was formed as part of a larger LIP consisting of the Agulhas Plateau itself, Northeast Georgia Rise and Maud Rise. The formation time of this LIP can be estimated between 100 and 94 (± 5) Ma.

The southern margin of South Africa developed as a consequence of shear motion between the African and South American plates along the Agulhas-Falkland Transform Fault during the Early Cretaceous break-up of Gondwana. The Agulhas-Karoo Geoscience Transect crosses this continental margin, and includes two combined offshore-onshore seismic reflection/refraction profiles. We present results from the western offshore profile. Using ocean-bottom seismometers and a dense airgun shot pattern, a detailed image of the velocity -depth structure of the margin from the Agulhas Passage to the Agulhas Bank was derived. Modelling reveals crustal thicknesses of between 7 km and 30 km along the profile. The upper crust has P-wave velocities of between 5.6 and 6.6 km/s and the lower crust has velocities that lie between 6.4 and 7.1 km/s. Uppermost mantle velocities range from 7.8 to 8.0 km/s. The 52 km wide continent-ocean-transition zone, where the Moho rises steeply, occurs at the Agulhas-Falkland Fracture Zone.Beneath the Southern Outeniqua Basin and the Diaz Marginal Ridge, a zone of relatively low velocities (~5 km/s) with a thickness of up to 3 km, can be discerned in the upper crust. We interpret this zone as an old basin filled with pre-break-up metasediments, which may be related to the Cape Fold Belt. We suggest that the origin of the Diaz Marginal Ridge is bound up with the tectonic history of this basin as it exhibits a similar velocity structure. Almost no stratified sediment cover exists in the Agulhas Passage because of strong erosion due to ocean currents. The velocity structure and seismic reflection results indicate the presence of alternating layers of volcanic flows and sediments, with a mean velocity of about 4 km/s. We suggest these volcanic flows were an accompaniment of possible re-activation events of parts of the Agulhas-Falkland Fracture Zone. Tectonic motion seems to be sub vertical instead of strike-slip along the re-activated part of the fracture zone. Therefore, a relation to the uplift of the South African crust may be possible.

[1] Dynamic processes at sheared margins associated with the formation of sedimentary basins and marginal ridges are poorly understood. The southern African margin provides an excellent opportunity to investigate the deep crustal structure of a transform margin and to characterize processes acting at these margins by studying the Agulhas-Falkland Fracture Zone, the Outeniqua Basin, and the Diaz Marginal Ridge. To do this, we present the results of the combined seismic land-sea experiments of the AgulhasKaroo Geoscience Transect. Detailed velocity-depth models show crustal thicknesses varying from 42 km beneath the Cape Fold Belt to 28 km beneath the shelf. The Agulhas-Falkland Fracture Zone is embedded in a 50 km wide transitional zone between continental and oceanic crust. The oceanic crust farther south exhibits relatively low average crustal velocities (6.0 km/s), which can possibly be attributed to transform-ridge intersection processes and the thermal effects of the adjacent continental crust during its formation. Crustal stretching factors derived from the velocity-depth models imply that extension in the Outeniqua Basin acted on regional as well as more local scales. We highlight evidence for two episodes of crustal stretching. The first, with a stretching factor b of 1.6, is interpreted to have influenced the entire Outeniqua Basin. The stresses possibly originated from the beginning breakup between Africa and Antarctica (169-155 Ma). The second episode can be associated with a transtensional component of the shear motion along the Agulhas-Falkland Transform from 136 Ma. This episode caused additional crustal stretching with b = 1.3 and is established to only have affected the southern parts of the basin. Crustal velocities directly beneath the Outeniqua Basin are consistent with the interpretation of Cape Supergroup rocks underlying most parts of the basin and the Diaz Marginal Ridge. We propose that the formation of this ridge can be either attributed to a transpressional episode along the Agulhas-Falkland Transform or, more likely, to thermal uplift accompanying the passage of a spreading ridge to the south.

[1] A number of geophysical onshore and offshore experiments were carried out along a profile across the southern margin of the African Plate in the framework of the Inkaba yeAfrica project. Refraction seismic experiments show that Moho depth decreases rapidly from over 40 km inland to around 30 km at the present coast before gently thinning out toward the Agulhas-Falkland Fracture Zone, which marks the transition zone between the continental and oceanic crust. In the region of the abruptly decreasing Moho depth, in the vicinity of the boundary between the Namaqua-Natal Mobile Belt and the Cape Fold Belt, lower crustal P-wave velocities up to 7.4 km/s are observed. This is interpreted as metabasic lithologies of Precambrian age in the Namaqua-Natal Mobile Belt, or mafic intrusions added to the base of the crust by younger magmatism. The velocity model for the upper crust has excellent resolution and is consistent with the known geological record. A joint interpretation of the velocity model with an electrical conductivity model, obtained from magnetotelluric studies, makes it possible to correlate a high-velocity anomaly north of the center of the Beattie magnetic anomaly with a highly resistive body.

The Amundsen Sea embayment of West Antarctica is in a prominent location for a series of tectonic and magmatic events from Paleozoic to Cenozoic times. Seismic, magnetic and gravity data from the embayment and Pine Island Bay (PIB) reveal the crustal thickness and some tectonic features. The Moho is 24-22 km deep on the shelf. NE-SW trending magnetic and gravity anomalies and the thin crust indicate a former rift zone that was active during or in the run-up to breakup between Chatham Rise and West Antarctica before or at 90 Ma. NW-SE trending gravity and magnetic anomalies, following a prolongation of Peacock Sound, indicate the extensional southern boundary to the Bellingshausen Plate which was active between 79 and 61 Ma.

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The Amundsen Sea embayment of West Antarctica is in a prominent location for a series of tectonic and magmatic events from Paleozoic to Cenozoic times. Seismic, magnetic and gravity data from the embayment and Pine Island Bay (PIB) reveal the crustal thickness and some tectonic features. The Moho is 24-22 km deep on the shelf. NE-SW trending magnetic and gravity anomalies and the thin crust indicate a former rift zone that was active during or in the run-up to breakup between Chatham Rise and West Antarctica before or at 90 Ma. NW-SE trending gravity and magnetic anomalies, following a prolongation of Peacock Sound, indicate the extensional southern boundary to the Bellingshausen Plate which was active between 79 and 61 Ma.

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