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Geobulletin: News from Stellenbosch Earth Sciences
Author: JD Clemens
Published: 25/08/2016

 

Contribution for Geobulletin from Stellenbosch Earth Sciences

 

 

For this issue, we have many interesting and significant things to report.

 

Staffing Matters

After two years, the Faculty of Science finally caught up with Professor Alex Kisters and, on August 2nd, he delivered a typically lucid and fascinating inaugural lecture in front of an audience of friends, staff, students, former colleagues and government and industry associates. Alex's talk was entitled "What Lies Beneath Table Mountain or All Models are Wrong, but Some are Useful". Alex reviewed the various models for the development of the Malmesbury Group, concluding that it contains no really exotic terranes but instead represents an accretionary wedge, deformed initially in oblique, southeast-directed subduction. Alex reminded us that models are critical for understanding the geological development of a region but that we must always bear in mind the limitations of the models that we present. We all congratulate Alex on his full professorial appointment and wish him every success for his future at Stellenbosch. The photo below shows Alex (right) with Prof. Eugene Cloete (Vice Rector for Research, Innovation and Postgraduate Studies) and Prof. Louise Warnich (Dean of the Faculty of Science).

 

 

Science Highlights

1. THE VREDEFORT PSEUDOTACHYLYTES: A CENTENNIAL REAPPRAISAL

Almost precisely a century ago, venerable Stellenbosch University Professor S. J. Shand applied the term 'pseudotachylyte' to widespread, highly irregular, commonly clast-loaded, dark, flinty veins in granites and gneisses (Fig. 1A & C) within the Vredefort dome of South Africa (Shand, 1916). Following in his footsteps, Dr Martin B. Klausen (from the same Earth Science Department) has, together with a colleague from the Geological Survey of Denmark and Greenland (Garde and Klausen, 2016), introduced seismic shaking as a key process in the formation of its enigmatically voluminous pseudotachylytes within what is now known to be a circa 2.023 Ga meteorite impact structure.

 

The process of seismic shaking is well known from meteoritic impacts on the Moon (e.g. Fig. 1B) and asteroids, from terrestrial earthquakes and rock slides, as well as from nuclear tests, but has been largely overlooked in terrestrial cratering, except in the theoretical concept of acoustic fluidisation (Melosh, 1979). Previous struggles to convincingly explain Vredefort's large pseudotachylyte volumes through frictional, or instantaneous 'shock', melting/cataclasis, are now elegantly resolved by repeated comminution of blocks, initially loosened within a dendritic fracture system that was generated during the early, high-amplitude stage of seismic shaking (Fig. 1A). Continued high-frequency oscillations of these loosened blocks led to size reduction, rounding and further comminution (Fig. 1C), and eventually even frictional melting of K-feldspar and biotite in the comminuted mass. Most pseudotachylyte was thus not injected from elsewhere but produced in situ. Furthermore, microstructural analysis shows that most of the pseudotachylyte is actually finely comminuted material rather than former melt.

 

 

Figure 1: (A) Thin pseudotachylyte veins within a dendritic fracture system, generated by the initial shock waves from the c. 2.023 Ga Vredefort meteorite impact, as well as along a more regular, and presumed pre-existing, fracture/fault that cuts diagonally across the photo. (B) An approximately 90 min. three-component seismographic record of a Lunar meteorite impact (13 May 1972; modified from Nakamura et al., 1982) illustrating the amount of seismic shaking following initial impact. (C) A thicker, more voluminous, pseudotachylyte with characteristic rounded clasts, typically formed in situ through prolonged seismic shaking of loosened block corners (i.e. at intersections between fracture sets in A) and repeated comminution of dismembered clasts. Both field photos are from the Esperanza quarry and show approximately the same field of view, with a R5 coin for scale in (A).

Garde, A.A. and Klausen, M.B. 2016. A centennial reappraisal of the Vredefort pseudotachylytes: shaken, not stirred by meteorite impact. Journal of the Geological Society (London) in press. doi: 10.1144/jgs2015-147.

Melosh, H.J. 1979. Acoustic fluidization: a new geologic process? Journal of Geophysical Research 84: 7513-7520.

Nakamura, Y., Latham, G.V. and Dorman, J. 1982. Apollo lunar seismic experiments – final summary. Journal of Geophysical Research 87, A117-A123.

Shand, S.J. 1916. The pseudotachylyte of Parijs (Orange Free State) and its relation to 'trap-shotten gneiss' and 'flinty crush rock'. Quarterly Journal of the Geological Society of London 72: 198-217.

 

 

2. CONCENTRATION AND DISSOLUTION OF TRACE METALS FROM KNOWN DUST SOURCES IN NAMIBIA (HOW NAMIBIAN DESERTS FERTILIZE THE OCEAN FROM ABOVE)

Our Environmental Geochemistry Masters student, Ismael Kangueehi took part in a Summer-school program hosted at the University of Namibia's Sam Nujoma Campus and Marine Research Centre in Henties Bay, from the 2nd of May to 3rd of June 2016. The Regional Graduate Network in Oceanography (RGNO) for Southern Africa (in Namibia) offers research-driven and practical learning opportunities for researchers in oceanography and integrated disciplines. Sailing on the Mirabilis Research Vessel for two weeks with RGNO, participants carry out research in the Namibian Ocean by collecting sediment cores and water samples. This course provided Ismael with an excellent opportunity to study the North-South flowing Benguela Current, which creates cells along the coast where cold, nutrient- and CO2-rich upwelling water leads to one of the world's most productive marine ecosystems.

 

The theoretical part of the program involved lecture sessions by various guest speakers and course instructors, on subjects ranging from physical and chemical oceanography, sedimentary organic and inorganic geochemistry to molecular biology and environmental geobiochemical modelling. Stellenbosch Earth Science department senior lecturer Dr Susanne Fietz was one of the guest lecturers, addressing the group on Fe fertilization and the use of the software package Ocean Data View (https://odv.awi.de/) in marine research and exploration. As a collaboration program with scientists from ETH Zurich, the University of Minnesota, the AGOURON Institute, the University of Namibia, the Scientific Committee of Oceanic Research (SCOR) and the Leibniz Institute for Baltic Sea Research, this course attracted students from across the globe.

 

Ismael's practical work on board the R/V Mirabilis included sampling of sediment cores along the Namibian coastline, and mapping and investigating the rock outcrops around Henties Bay. The practical work carried out during the RGNO course will form part of Ismael's thesis which focuses on the dissolution of dust particles that originate from Southern African sources.

 

 

 

Ismael Kangueehi handling a sediment core, with the assistance of (Daniel Montluçon, ETH Zurich) while Stephen Broccardo (North West University, South Africa) looks on.

 

 

 

Some of the RGNO workshop participants after the two-week cruise along the Namibian coastline on the Research Vessel Mirabilis. From left: Daniel Montluçon (ETH Zurich, Switzerland), Ismael Kangueehi (University of Stellenbosch, South Africa), Phillip Eickenbusch (ETH Zurich, Switzerland), Blanca Ausin (ETH Zurich, Switzerland), Kurt Hanselmann (Course coordinator; ETH Zurich, Switzerland), Thomas Andreas (driver), Said Mohamed Hashim (University of Nairobi, Kenya), Zimkhita Gebe (University of Cape Town, South Africa), Leanne Grace Hancock (University of California, Riverside, USA) and Beth Wangui Waweru (Kenya Marine and Fisheries Research Institute).

 

 

More Doings of Environmental Geochemistry Students

Four of our department's students attended the South African National Antarctic Programme (SANAP) meeting in Pretoria in July – Jean Loock (MSc; see below), Ryan Cloete (MSc), Ismael Kangueehi (MSc; see above) and Johan Viljoen (BSc Hons). The MSc students gave oral presentations, and the Honours student a poster.

 

Top National Awards for Stellenbosch Postgraduate Students

We are particularly pleased to report that two of our students received national awards, from the Geological Society of South Africa, for the best theses in Earth Sciences in 2015. These are:

 

Mr Jean Loock – Best Fourth-year Student Award

Jean's thesis examined climate change by studying bio-active trace metal concentrations in the Southern Ocean and defining the growth stress induced by low concentrations on marine phytoplankton. He graduated cum laude and is now working on his MSc at Stellenbosch, under the supervision of Prof. Alakendra Roychoudhury. He presented results from his thesis at a symposium of the South African National Antarctic Program (SANAP) at the end of July and again at the International Ocean Science Conference of the Scientific Committee on Antarctic Research (SCAR), in Malaysia, in August.

 

Mr Matthew Mayne – The John Handley Award for the best MSc thesis in Geology

As part of his thesis, Matt created a software tool, called Rcrust, which enables a new concept in modelling known as 'path dependence'. This allows researchers to investigate multi-step processes that are crucial to the formation of metamorphic rocks. Matt also graduated cum laude and is also registered at Stellenbosch and now working on his PhD, under the supervision of Prof. Gary Stevens. Based on his MSc work, he recently published an article in the Journal of Metamorphic Geology. In July he presented at the Workshop on the Origin and Evolution of Plate Tectonics in Locarno, Switzerland, and will also present in August at the International Geological Congress in Cape Town.

 

Prof. J. D. Clemens

(Departmental Communications Coordinator)