Tectonic and magmatic processes along the transform margin of southern Africa

Abstract

Giant shear processes shaped Africa's southern margin during the break-up of the supercontinent Gondwana. They acted along a more than 1000 km long transform fault, whose remnant structure - the Agulhas-Falkland Fracture Zone - stretches today from the Falkland Plateau to the southeastern margin of Africa. A study of the processes which initiated such a long-offset transform fault and acted during their active phase is essential to understand how this sheared margin developed. Africa's southern margin is not only one of the best examples to study the sharp continent-ocean-transition zones, the marginal ridge, fracture zone, and basin structures usually associated with transform margins, but it provides the unique opportunity to study how excessive magmatic processes acted which formed a Large Igneous Province at a sheared margin. In this thesis, I use seismic refraction, seismic reflection data and plate-tectonic reconstructions to investigate the structure and dynamics of this margin. These are ideal methods as they lead to high-quality velocity-depth models showing the present-day structure across the margin and provide timing and geometries by means of plate kinematics. The Agulhas-Falkland Fracture Zone bounds the Outeniqua Basin and the Diaz Marginal Ridge to the south. The early formation of Outeniqua Basin is a result of an intra-continental stretching episode in Jurassic times which acted well before the strike-slip motions along the Agulhas-Falkland Transform occurred in the Early Cretaceous. The transform itself has caused crustal thinning in the southern parts of this basin in a second stretching episode. Beneath this sedimentary basin, an as-yet unknown region with relatively low velocities was discovered and interpreted as a pre-break-up metasedimentary basin. It is possible that the Diaz Marginal Ridge was uplifted from this basin in a transpressional episode or is a result of thermal uplift due to a passing spreading ridge. In either case the formation of the Diaz Marginal Ridge is linked with the shear process itself. The active shear episode of the southern African margin ceased in late Cretaceous times when the spreading ridge passed to the west. In this thesis evidence was provided that the margin was not inactive in this post-shear phase as previously thought. Deep faults in the sediments show that the Agulhas-Falkland Fracture Zone may still be a zone of crustal or even lithospheric weakness. This means for southern Africa - a region, which was and still is affected by plume activity - that uplift is possibly accommodated by sub-vertical motion at the fracture zone. The resultant anomalously high topography is often attributed to the African superplume.The seismic and plate-tectontic investigations of this thesis showed that plume activity and interactions between the Bouvet plume and Bouvet triple junction were responsible for the evolution of the Agulhas Plateau. They indicate that the Agulhas Plateau consists of overthickened oceanic crust and formed as a Large Igneous Province (LIP). Seismic refraction modelling revealed a thick high-velocity lower crustal body at the base of the plateau and evidenced together with seismic reflection records that extensive volcanic flows build the cover of the plateau. Both of these observations are consistent with the typical structure of oceanic LIPs. Plate-tectonic reconstructions of this thesis showed that the Agulhas Plateau developed together with Northeast Georgia Rise and Maud Rise as part of a huge LIP which erupted between 100 and 94 Ma. It broke apart along the ridges of the Bouvet triple junction already in the final stages of its emplacement. The large amount of carbon dioxide released during its eruption was suspected to have had an impact on the climate. However, it is shown here that the carbon dioxide emission was not enough to have a significant impact on the climate. The presented results of the crustal structure of the Agulhas region shed more light on the complex interactions between the uplift of southern Africa, plume activity and plate motion as well as provide first quantitative estimates of the climate impact of LIPs with regard to carbon dioxide emission into the atmosphere