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Neoarchaean Tectonic History of the Witwatersrand Basin and Ventersdorp Supergroup: New Constraints from High-Resolution 3D Seismic Reflection Data

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Neoarchaean Tectonic History of the Witwatersrand Basin and Ventersdorp Supergroup: New Constraints from High-Resolution 3D Seismic Reflection Data Tectonophysics 590 (2013) 94–105 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Neoarchaean tectonic history of the Witwatersrand Basin and Ventersdorp Supergroup: New constraints from high-resolution 3D seismic reflection data Musa S.D. Manzi a,⁎, Kim A.A. Hein a,1, Nick King b,2, Raymond J. Durrheim a,c a School of Geosciences, University of the Witwatersrand Johannesburg, PBag 3, WITS, 2050, Republic of South Africa b South Deep Gold Mine, Gold Fields Limited, Republic of South Africa c Council for Scientific and Industrial Research (CSIR) and University of the Witwatersrand, Johannesburg, Republic of South Africa article info abstract Article history: First-order scale structures in the West Wits Line and West Rand goldfields of the Witwatersrand Basin (South Received 6 November 2012 Africa) were mapped using the high-resolution 3D reflection seismic method. Structural models constrain the Received in revised form 11 January 2013 magnitude of displacement of thrusts and faults, the gross structural architecture and Neoarchaean tectonic evo- Accepted 18 January 2013 lution of the West Rand and Bank fault zones, which offset the gold-bearing reefs of the basin. Available online 28 January 2013 The merging of several 3D seismic surveys made clear the gross strato-structural architecture of the gold- fields; a macroscopic fold-thrust belt is crosscut by a macroscopic extensional fault array. These are dissected, Keywords: Neoarchaean eroded and overlain by the Transvaal Supergroup above an angular unconformity. Witwatersrand Basin The seismic sections confirm that the West Rand Group (ca. 2985–2902 Ma) is unconformably overlain by Ventersdorp Supergroup the Central Rand Group (ca. 2902–2849 Ma), with tilting of the West Rand Group syn- to post-erosion at Seismics ca. 2.9 Ga. The seismic sections also confirm that an unconformable relationship exists between the Central Structure Rand Group and the auriferous Ventersdorp Contact Reef (VCR), with an easterly-verging fold-thrust belt Tectonics being initiated concomitant to deposition of the VCR at approximately 2.72 Ga. Fold-thrust formation includ- ed development of the (1) newly identified first-order scale Libanon Anticline, (2) Tandeka and Jabulani thrusts which displace the West Rand Group, and (3) parasite folds. The fold-thrust belt is crosscut by a macroscopic extensional fault array (or rift-like system of faults) which incepted towards the end of extrusion of the Ventersdorp lavas, and certainly during deposition of the Platberg Group (2709–2643 Ma) when a mantle plume may have heated the lithosphere. The West Rand and Bank fault zones formed at this time and include (1) the West Rand and Bank faults which are scissors faults; (2) second and third-order scale normal faults in the immediate footwall and hanging wall of the faults; (3) drag synclines, and (4) rollover anticlines. © 2013 Elsevier B.V. All rights reserved. 1. Introduction surprisingly little regional-scale kinematic and structural analysis has taken place across the Witwatersrand Basin despite more than The West Wits Line and West Rand goldfields of the Mesoarchaean 100 years of mining of gold. However, it is generally agreed that the Witwatersrand Basin (Fig. 1) have been the focus of renewed interest Witwatersrand Basin underwent inversion tectonics during and after in recent years in terms of their strato-structural and geophysical archi- deposition of the quartzite and conglomerate units (some auriferous) tecture (Beach and Smith, 2007; Dankert and Hein, 2010; Frimmel and of the Central Rand Group, from extension to compression i.e. positive Minter, 2002; Mambane et al., 2011; Manzi et al., 2012a; Mashabella, inversion (c.f., Beach and Smith, 2007; Dankert and Hein, 2010), and 2011; Mohale, 2010). The studies undertaken by Coward et al. (1995), then compression to extension (i.e. negative inversion) during deposi- Beach and Smith (2007), Dankert and Hein (2010), Jolley et al. (2007) tion of the Ventersdorp Supergroup (Van der Westhuizen et al., 1991). and others confirm that the basin experienced more than one episode Listric faults in the Witwatersrand Basin were recognized by of deformation although Dankert and Hein (2010) pointed out that Coward et al. (1995) and Vermaakt and Chunnet (1994) through studies in gold mines in the West Wits Line and West Rand goldfields, and by Beach and Smith (2007), Gibson et al. (2000, 2004) and Gibson (2005) through the interpretation of 3D seismic reflection ⁎ Corresponding author. Tel.: +27 11 717 6623. data. Beach and Smith (2007) showed that the dominant structural E-mail addresses: [email protected] (M.S.D. Manzi), styles, including major normal fault zones and their related drag [email protected] (K.A.A. Hein), nick.king@goldfields.co.za (N. King), folds, are those attributed to extensional tectonics that occurred dur- [email protected] (R.J. Durrheim). 1 – Tel.: +27 11 717 6623. ing deposition of the Ventersdorp Supergroup (ca. 2709 2643 Ma) 2 Tel.: +27 83 657 9518. possibly during deposition of the Platberg Group. 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.01.014 M.S.D. Manzi et al. / Tectonophysics 590 (2013) 94 – 105 Fig. 1. Location map of the West Rand and West Wits Line (Carletonville) goldfields of the Witwatersrand Basin in South Africa, showing historical 3D seismic reflection surveys (modified after Dankert and Hein, 2010; historical surveys after Gibson et al., 2000). Generalized seismic stratigraphy and tectonic events, with dates and ages are derived from Armstrong et al. (1991), Dankert and Hein (2010), Kositcin and Krapež (2004) and Myers et al. (1989). Note that Driefontein and Kloof gold mines have been re-named KDC WEST and KDC EAST, respectively. KDC: Kloof-Driefontein Complex; WUDLs: Western Ultra Deep levels; VCR: Ventersdorp Contact Reef; BLR: Black Reef Formation. aUnpublished; bMartin et al. (1998; zircon U–Pb SHRIMP); c,eArmstrong et al. (1991; single zircon U–Pb SHRIMP); dKositcin and Krapež (2004; zircon U–Pb SHRIMP). 95 96 M.S.D. Manzi et al. / Tectonophysics 590 (2013) 94–105 Further to this, previous structural models of the Witwatersrand The upper part of the Central Rand Group is unconformably overlain Basin were constrained by low resolution 3D seismic surveys that by the Ventersdorp Contact Reef (VCR) of the Venterspost Formation, covered the region to the west of the Bank Fault (Beach and Smith, which has the maximum age of 2729±19 Ma (Kositcin and Krapež, 2007; Gibson et al., 2000; Jolley et al., 2007). Little or no coverage of 2004;U–Pb detrital zircon SHRIMP). The Venterspost Formation con- the eastern side of the fault (West Rand goldfields) was incorporated. sists of a thin fluvial auriferous conglomerate. The VCR represents an In this study, the interpretation of high-resolution 3D seismic reflec- erosional surface that formed at the end of deposition of the Central tion data that covers the West Wits Line and West Rand goldfields is Rand Group (Frimmel et al., 1999; Gartz and Frimmel, 1999; Gibson, presented, and includes studies across the Driefontein Block (now 2004, 2005; McCarthy, 2006; Tankard et al., 1982). It unconformably termed KDC West), the Kloof Block (now termed the KDC East), and overlies and truncates approximately 2500 m stratigraphic thickness the South Deeps Block (Fig. 1). The structural studies of the entire of Witwatersrand Supergroup sediments (Myers et al., 1989; Robb et reprocessed and merged dataset are unique and are herein reported al., 1991; Spencer, 1992; Winter, 1994). The depth of the VCR unconfor- for the first time. The data was used to study the structural setting, mity is approximately 3000–4200 m below surface. The conglomerates mainly the first-order scale structures, but also their associated of the VCR are too thin (less than 1.5 m) to be directly resolved seismi- second- and third-order faults and folds. Focus was given to (1) fault ge- cally because they fall below the tuning thickness or resolution limit, ometry, fault form, and fault-throw analysis, and (2) the relative chro- which is given by ¼ seismic dominant wavelength (λ), where λ=v/f nology of tectonic events. The results were integrated with borehole (Widess, 1973). Our data is characterized by the frequency (f)of data and underground maps to validate the seismic and Neoarchaean 65 Hz and a mean velocity (v) of 6500 m/s, thus providing the tuning tectonic interpretations. thickness of 25 m. This implies that the top and bottom of any reef The data has also been used by Manzi et al. (2012a) to map poten- with a thickness less than 25 m may not be resolved. However, the tial conduits of water and methane in the deep gold mines covered by VCR unconformity coincides with a reflective interface or strong seismic the seismic volume. It has been used in the application of 3D seismic reflector resulting from a major acoustic impedance contrast between technique in evaluation of ore resources and is reported by Manzi et metamorphosed basalts of the Klipriviersberg Group (~6400 m/s veloc- al. (2012b), and for mapping the distribution and timing of meso- ity) and quartzite units at the top of the Central Rand Group (~5700 m/s to mega-scale structures and provide constraints on the ore genetic velocity). models (Malehmir et al., in press). Unconformably overlying the VCR is the Neoarchaean Ventersdorp Supergroup (ca. 2.72–2.63 Ga), which comprises ultramaficandmafic 2. Geological setting metavolcanic rocks of the Klipriviersberg Group, and metasedimentary rocks and bimodal metavolcanic rocks of the Platberg Group (Crow and The Mesoarchaean Witwatersrand Basin, one of the world's premier Condie, 1988; Van der Westhuizen et al., 1991). The Klipriviersberg gold regions and the greatest known source of gold on Earth, was de- Group is divided into five formations, i.e.
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