Tectonic and magmatic processes along the transform margin of southern Africa

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

Tektonische und magmatische Prozesse am gescherten Kontinentalrand des südlichen Afrikas

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

Multiple Scattering and Coda Localization at Merapi Volcano

AB: Due to their eruptive history the cones of strato volcanoes consist of different materials such as hardened lava, tephra, and volcanic ash. Additionally, due to their rough topography, e.g. caused by erosion, deposition is irregular and the volcanic structure cannot be described by a simple 1D layered model. The 3D small scale heterogeneities with large impedance contrast cause multiple scattering of seismic waves and are important features for the modelling of seismic wave propagation in strato volcanoes. Active seismic experiments at Merapi and Vesuvius volcanoes have shown that the transport mean free path of strato volcanoes is as small as some hundreds of meters and, therefore, is about three orders of magnitude smaller than the transport mean free path of usual Earth's crust. Moreover, the transport mean free path is at least one order of magnitude smaller than the characteristic scale length of intrinsic attenuation. Finally, the transport mean free path is in the same order as the inverse of the wave number. This indicates, that in strato volcanoes heterogeneity is so strong that we approach the regime of strong scattering where the classical theories such as radiative transfer and diffusion become invalid. All this makes strato volcanoes a natural laboratory for the application of multiple scattering theories. One important recent observation at Merapi volcano is an abnormal spatial concentration of coda energy in the summit region. This observed coda localization can be interpreted as an indication of Anderson localization, which is a theoretically predicted effect of strong scattering beyond the validity of diffusion theory. We show that the Anderson localization model better fits the data observed at Merapi than a standard half space diffusion model. However, we also show that, alternatively, the observation can also be explained within the classical diffusion approach by assuming leakage of energy from the strongly scattering volcanic edifice into the much more homogeneous underlying earth crust. Similar to Anderson localization the leakage results in an inhomogeneous distribution of energy in space, where the energy is low near the volcano-crust boundary and large inside the strongly scattering volcano far from that boundary. Additionally to the Anderson localization model, we use two classical models to explain coda localization: The first one is based on an analytical solution of the diffusion equation for a scattering cylinder (representing the volcano) embedded in a homogeneous half-space (representing the surrounding crust). The second model is based on a Monte-Carlo simulation of the acoustic equation of radiative transfer. In this simulation we take into account multiple scattering inside the volcanic edifice as well as leakage at the bottom of the volcano into the less heterogeneous crust. Additionally, in this model we also consider the true topography of the volcano by simulating reflections at the free surface, where we use a digital elevation model of the volcano and the Kirchhoff tangent plane method