Magnetotelluric measurements across the southern Barberton Greenstone Belt, South Africa

The Barberton Greenstone Belt (BGB) in the northwestern part of South Africa belongs to the few well-preserved remnants of Archean crust. Over the last centuries, the BGB has been intensively studied at surface with detailed mapping of its surfacial geological units and tectonic features. Neverthele...
Ausführliche Beschreibung

The Barberton Greenstone Belt (BGB) in the northwestern part of South Africa belongs to the few well-preserved remnants of Archean crust. Over the last centuries, the BGB has been intensively studied at surface with detailed mapping of its surfacial geological units and tectonic features. Nevertheless, the deeper structure of the BGB remains poorly understood. Various tectonic evolution models have been developed based on geo-chronological and structural data. These theories are highly controversial and centre on the question whether plate tectonics - as geoscientists understand them today - was already evolving on the Early Earth or whether vertical mass movements driven by the higher temperature of the Earth in Archean times governed continent development. To get a step closer to answering the questions regarding the internal structure and formation of the BGB, magnetotelluric (MT) field experiments were conducted as part of the German-South African research initiative Inkaba yeAfrica. Five-component MT data (three magnetic and two electric channels) were collected at ~200 sites aligned along six profiles crossing the southern part of the BGB. Tectonic features like (fossil) faults and shear zones are often mineralized and therefore can have high electrical conductivities. Hence, by obtaining an image of the conductivity distribution of the subsurface from MT measurements can provide useful information on tectonic processes. Unfortunately, the BGB MT data set is heavily affected by man-made electromagnetic noise caused, e.g. by powerlines and electric fences. Aperiodic spikes in the magnetic and corresponding offsets in the electric field components impair the data quality particularly at periods >1 s which are required to image deep electrical structures. Application of common methods for noise reduction like delay filtering and remote reference processing, only worked well for periods <1 s. Within the framework of this thesis two new filtering approaches were developed to handle the severe noise in long period data and obtain reliable processing results. The first algorithm is based on the Wiener filter in combination with a spike detection algorithm. Comparison of data variances of a local site with those of a reference site allows the identification of disturbed time series windows for each recorded channel at the local site. Using the data of the reference site, a Wiener filter algorithm is applied to predict physically meaningful data to replace the disturbed windows. While spikes in the magnetic channels are easily recognized and replaced, steps in the electric channels are more difficult to detect depending on their offset. Therefore, I have implemented a novel approach based on time series differentiation, noise removal and subsequent integration to overcome this obstacle. A second filtering approach where spikes and steps in the time series are identified using a comparison of the short and long time average of the data was also implemented as part of my thesis. For this filtering approach the noise in the form of spikes and offsets in the data is treated by an interpolation of the affected data samples. The new developments resulted in a substantial data improvement and allowed to gain one to two decades of data (up to 10 or 100 s). The re-processed MT data were used to image the electrical conductivity distribution of the BGB by 2D and 3D inversion. Inversion models are in good agreement with the surface geology delineating the highly resistive rocks of the BGB from surrounding more conductive geological units. Fault zones appear as conductive structures and can be traced to depths of 5 to 10 km. 2D models suggest a continuation of the faults further south across the boundary of the BGB. Based on the shallow tectonic structures (fault system) within the BGB compared to deeply rooted resistive batholiths in the area, tectonic models including both vertical mass transport and in parts present-day style plate tectonics seem to be most likely for the evolution of the BGB. Ausführliche Beschreibung

Autor*in:	Kütter, Sissy - 1983- [verfasserIn]
Hochschulschrift:	Dissertation ; Universität Potsdam ; 2015

Format:	E-Book
Sprache:	Englisch

Erschienen:	Potsdam: 2015

Schlagwörter:	Barberton Greenstone Belt / Grünstein / Metabasit / Magnetotellurik / Elektromagnetische Messung / Elektromagnetisches Verfahren Südafrika / Archaikum, Geologie / Grünsteingürtel / Tiefenstruktur, Geologie / Elektromagnetische Tiefensondierung / Elektrische Leitfähigkeit Datenanalyse / Datenauswertung / Störsignal / Filter, Mathematik / Methode
Formangabe:	Hochschulschrift

Umfang:	1 Online-Ressource (xix, 156 Seiten) ; Illustrationen, Diagramme, Karten

Weitere Ausgabe:	Erscheint auch als Druck-Ausgabe Kütter, Sissy, 1983 -: Magnetotelluric measurements across the southern Barberton Greenstone Belt, South Africa - 2015 Erscheint auch als Druck-Ausgabe Kütter, Sissy, 1983 -: Magnetotelluric measurements across the southern Barberton Greenstone Belt, South Africa - Potsdam, 2015

Links:	Link aufrufen Volltext Volltext Link aufrufen Link aufrufen Link aufrufen

DOI / URN:	urn:nbn:de:kobv:517-opus4-83198

Katalog-ID:	857554905