Die elektrische Leitfähigkeitsstruktur der archaischen Suturzonen im Barberton Greenstone Belt, Südafrika (ELIBABA)
Zusammenfassung der Projektergebnisse
The Archean eon marks the transition of the Earth from an hostile environment to a place where the earliest life forms could evolve. Only few remnants of the early crust have been preserved from the Archean until today to study the first stages of the evolution of our planet; one of them is the Barberton Greenstone Belt (BGB) in South Africa. The geological setting of the BGB including ages of rocks, mineral content, structural properties, surface traces of major faults is well known. This detailed knowledge, however, is limited to the surface, deeper structures remain enigmatic and instigate controversial discussion on the tectonic evolution in the Archean, i.e. if vertical mass movement or plate tectonic activity formed this continental block initially. Within a fossil tectonic setting, faults often correlate with zones of high conductivity due to mineralisation along shear planes. We therefore applied the magnetotelluric (MT) method, the only geophysical technique available to probe the earth’s deep interior on crustal and upper mantle scales for its electrical conductivity distribution. Within the project ELIBABA, a MT field experiment was conducted in 2010 along a > 100 km profile and several smaller parallel profiles in the southern Barberton Greenstone Belt (BGB) in South Africa. The measurement area covers the main rock formations of the BGB as well as major shear zones within and consists of 193 broad band Mt stations. Due to the high level of severe cultural electromagnetic (EM) noise, the interpretable data after standard processing was restricted to high frequencies (shallow depth) and not very helpful for crustal scale studies. Therefore Kuetter et al. (2012) developed an advanced filtering scheme using an adopted Wiener filter and sucessfully improved signal to noise ratios for additional 1-2 decades of frequency data. Main results of 2D and 3D inversion studies are pronounced zones of high electrical conductivity correlating with the surface traces of the Inyoka, Kromberg, Maanhaar and Komatii Faults. Except of the Inyoka fault, thses tectonic structure have only shallow (∼ 5 km) depth extent. The Inyoka fault, however, is located in vicinity to gold mines and exhibit a deep reaching channel along which fluids required for the formation of orogenic gold deposits. The main rocks of the Barberton Greenstone Belts (Tonalite, Trondhjemite and Granodiorite (TTG)) reach partly down to > 15 km depth and continue beneath the Transvaal Group Sediments in the northwestern part. The Maanhaar Fault seems to be the southeastern margin of the BGB. Structures at greater depths are dominated by highly resistive plutonic intrusions (not sheet-like and shallow), which favour a geological model with a strong influence of vertical tectonics. However, based on the 3D model, the Mpuluzi Batholith is not intersected by faults, as previously assumed, and we therefore have no indication that the faults are younger than the surrounding rock. The inversion results partly support tectonic models by Heubeck & Lowe (1994), although main features such as the shallow detachment zone beneath the BGB rocks are not supported. In summary, electrical conductivity images show remnants of vertical mass transport as well as fault zones which indicate early tectonic processes.
Projektbezogene Publikationen (Auswahl)
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(2012) Making and Breaking of a Continent: Following the Scent of Geodynamic Imprints on the African Continent Using Electromagnetics, Surv. Geophys, 33, 107–134
Weckmann, U.
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(2012). Magnetotelluric measurements across the southern Barberton greenstone belt, South Africa: data improving strategies, Proceedings of the 24th Schmucker-Weidelt-Kolloquium, ISSN 0946-7467
Kütter, S.; Weckmann, U.
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(2014): Das Entstehen und Auseinanderbrechen von Kontinenten: mit Geophysik auf den Spuren geodynamischer Prozesse im südlichen Afrika. - System Erde, 4(2), 6-13
Weckmann, U., Kütter, S., Ritter, O., Ryberg, T. and Weber, M.
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(2015). Magnetotelluric measurements across the southern Barberton Greenstone Belt, South Africa. University of Potsdam, Germany
Kütter, S.
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A deep electrical conductivity structure of the southern Barberton Greenstone Belt, South Africa, derived from magnetotelluric measurements. South African Journal of Geology (2016) 119 (1): 273-290
Kütter, S., Weckmann, U. & de Wit, M.
