Making and Breaking of a Continent: Following the Scent of Geodynamic Imprints on the African Continent Using Electromagnetics

Abstract

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 supercontinents. After the assemblage of Gondwana, a period of predominantly "breaking" , i.e. the breakup of supercontinents, took over. The modern day African continent exhibits different types of margins; continental rifting occurs side by side with recent collision. Since the late 1960's 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 scale 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 break-up 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.

Implications for the lithospheric geometry of the Iapetus suture beneath Ireland based on electrical resistivity models from deep-probing magnetotellurics

“…High conductivity at crustal depths can be caused by many factors, such as saline water, partial melt, sulphides, iron oxides and graphite (Frost et al 1989;Jones 1992;Duba et al 1994). High conductivity in the tectonically non-active paleo-orogenic regions is generally correlated with magnetite-rich serpentinization or metamorphosed graphitic/sulphidic sediments (Weckmann 2012). It seems most likely that the high conductivity we observe is due to the occurrence of low-grade metamorphosed graphitic sediments in Palaeozoic suture zones.…”

Structures and geometries of the Tajo Basin crust, Spain: Results of a magnetotelluric investigation compared to seismic and thermal models

“…MT has been successfully applied in numerous tectonic studies (recently reviewed by Unsworth [], Weckmann [], and Selway []), and here only a very brief general overview about basic MT principles is given; for more detailed discussion of theoretical aspects of the MT methods, the interested reader is referred to standard literature [e.g., Schmucker , ; Vozoff , ; Berdichevsky , ; Simpson and Bahr , ; Becken et al , ; Berdichevsky et al , ] and the new book by Chave and Jones [], as well as the review papers from the biennial IAGA EM Induction Workshops. In MT, the depth of investigation is directly dependent on the period of the utilized electromagnetic waves and the resistivity of the subsurface, so penetration to all depths is assured, albeit with broader and broader resolution kernels.…”

Structure of the Lithosphere Beneath the Barotse Basin, Western Zambia, From Magnetotelluric Data

“…Knowledge of the lithospheric structure beneath and surrounding the Barotse Basin in western Zambia (Figure b) is critical to understanding the extent of the SWB and its relationship with the underlying Precambrian structure. The MT method has the ability to map variations in lithospheric structure (e.g., Jones et al, ) and is thus a useful tool in understanding rifting processes (e.g., Weckmann, ). In this paper we report on the results inferred from a MT data set collected in western Zambia as part of the larger PRIDE project studying the incipient stages of continental rifting along the EARS.…”