Structure of the Bushveld Complex from resistivity measurements
Knowledge of the deep structure of the arc-like bodies constituting the Bushveld Complex (South Africa) is essential not only to understand the emplacement history of this huge layered intrusion, but also for success in the exploration for the platiniferous Merensky Reef and chromitite layers occurring near its base. Despite many studies the structure of the complex has remained obscure. Here we describe new insights into the structure and additional geophysical boundary conditions, based on a geoelectrical study. This has provided a geoelectrical stratigraphy according to which a conductive layer below the granitic rocks, of both the eastern and western compartments of the Complex, is correlated with sedimentary rocks of the Transvaal Sequence over large areas, rather than with the mafic sequence as conventionally assumed. A new feature of this model is that the mafic sequence in both compartments occurs as segments of an inward-dipping sheet transgressing the layering of the sedimentary rocks into which it intruded. The geoelectrical model supports earlier gravity models in that mafic rocks are absent from the central part of the Complex, but shows that here the Bushveld granite directly overlies Archaean basement rather than Transvaal Sequence rocks as previously accepted.
Making and Breaking of a Continent: Following the Scent of Geodynamic Imprints on the African Continent Using Electromagnetics
The African continent inherits a long history of continental accretion and breakup. The stage of “making” a continent goes back to the Archean, when the first continental masses formed cratons which mostly remained stable ever since. Subsequent collision of weaker continental masses was followed by several extension and compression episodes that resulted in the formation of super-continents. After the assemblage of Gondwana, a period of predominantly “breaking” , i.e., the breakup of super-continents, took over. The modern-day African continent exhibits different types of margins; continental rifting occurs side by side with recent collision. Since the late 1960s, magnetotelluric (MT) experiments have played an important role in studies of the electrical conductivity structure of Africa. The early results significantly shaped the MT community’s understanding of continental-scale conductivity belts and basic characteristics of cratons and mobile belts on both crustal and lithospheric mantle scales for some decades. Modern MT studies in Africa have generally supported earlier results with high resistivities observed on cratons and low resistivities observed across mobile belts. Advances in instrumentation, data processing and interpretation resulted in higher-resolution images of the lithosphere, which in consequence induce an improved understanding of tectonic processes and geological prerequisites for the occurrence of natural resources. The high electrical conductivity of mobile belts and their relation to reactivated fault and detachment zones were often interpreted to characterize mobile belts as tectonic weak zones, which can accommodate stress and constitute zones along which continents can break. Recent breaking of the African continent can be studied on land across the East African rift; however, the lack of amphibian MT experiments across today’s margins does not allow for good resolution of remnants of continental breakup processes. Naturally, the regions and the focus of the MT studies in Africa are diverse, but they all contribute to the story of making and breaking a continent.
Geomagnetic micropulsations due to the motion of ocean waves
When sea waves move in the magnetic field of the earth, electric currents are generated in the water volume. These currents give rise to a small magnetic field which perturbs the earth's field. For plane trochoidal waves, this perturbing field decays exponentially with altitude and lies in a vertical plane which is oriented in the direction of wave propagation. The magnitude of the perturbing field also depends upon the magnitude and dip angle of the earth's field, and upon the height, wavelength, and propagation direction of the sea waves. For north-south waves, sea state 5, and an altitude of 100 feet, the perturbation is of the order of 0.1 gamma.