Lunar and Planetary Science XXXI 1621.pdf

RECENT STRUCTURAL STUDIES ALONG THE RAND ANTICLINE, NORTHERN MARGIN OF THE BASIN, SOUTH AFRICA. Tony Jamison: Centre for Applied Mining and Exploration Geology (CAMEG),and Impact Cratering Research Group, Department of Geology, University of the Witwatersrand, Private Bag 3, P.O.Box Wits 2050, , South Africa. ([email protected])

Introduction: Structural investigations detailed mapping has encountered north-verging have been undertaken along the Rietfontein Fault thrusts intersecting the typical flower structure of south of the Johannesburg Dome, in the West the left-lateral wrench, and right-lateral Rand at Lindum Reefs Gold Mine in transverse faults. Major shearing is found to Province; as well as on the northern flank of the occur at the bedding contacts of the brittle Rand Anticline at Ventersdorp and Drylands in quartzitic units. The latter are intensely Northwest Province. These sites are situated on shattered and are rotated into blocks, slivers and the northern margin of the late archaean boudins. Dating of individual events and their Witwatersrand Basin , a part of the basin that has sequence will require further systematic been preserved from erosion by the combination investigation by students currently mapping the of the Vredefort impact event and the meso- area. proterozoic Kibaran Namaqua-Natal mobile belt At Lindum Reefs opencast goldmine in the West compression within the Kaapvaal craton. The Rand, complex tectonic episodes affect the sites are being mapped and structural data Turffontein Subgroup and the unconformably recorded as well as specimens obtained for study overlying dolomites and Black Reef of the 2.6- of age relationships and analysis of paragenetic 2.25 Ga Supergroup. These episodes sequences. The geodynamic model of Friese, are characterised by significant normal faults Charlesworth and McCarthy; (EGRU 292, 1995) overprinted by post Transvaal thrust faulting suggests that previously observed thrust with a westerly and northerly vergence. Some of tectonics concentric to the Vredefort Dome this thrust movement can be dated as pre- (2025 Ma) may be related more to the later Transvaal in age. (~1250 Ma) northwest directed compression of Some 100 km further west near the town of the Kibaran age orogenic process. The Ventersdorp, twenty trenches straddling the significance of the mapped data are discussed. Black Reef outcrop on the northern flank of the West Rand granite ridge have been mapped. These trenches of some 3 meters depth and 10 to 15 meters length revealed a little-known tectonic event which resulted in an imbricate south- verging thrust duplex of the thin Basal Contact Reef of the Black Reef Quartzite. The thrust zone interacts with both strike and dip faults, considerably modifying the geometry of the mineralised Black Reef channels. Additional crush and shear breccias have been created at low angles to bedding and could have acted as fluid pathways for late hydrothermal redistribution of gold.

Results: The Rietfontein Fault zone[ 2,3], in the Florida- area west of Johannesburg, shows several tectonic events over a wide range of ages. Igneous and magmato-hydrothermal events have also contributed to fluid penetration and local mineralisation within the fault system. Recent

Lunar and Planetary Science XXXI 1621.pdf

Recent Structural Studies Along the Rand Anticline: A. A. Jamison

The Drylands gold deposit, similarly situated on the northern flank of the West Rand granite ridge differs in its close association with a sill-like ultramafic intrusive and intense alteration of the overlying dolomites and shales to a manganiferous wad, with stockwork quartz veining containing gold. Tectonic deformation within this mineralized sequence and its age are not well understood but are very clearly inherent in the geometry of the deposit, which still requires detailed analysis. Thrust faults have been noted with an apparent northerly vergence, imbrication and folding of the stockwork veins Conclusion: These four localities are regarded as important windows into understanding the tectonic episodes that contribute to the geodynamic model for this region. At issue are the age and sequence of wrench tectonics, thrusting and back-thrusts. The Vredefort impact event, large-scale cratonic intrusion episodes such as the Bushveld Complex, and their attendant shock and gravity- induced outward-verging compressional tectonics and the overprinting of later craton- wide Kibaran (Grenville) northwest-directed thrust tectonics, should provide decipherable signatures in these relatively well exposed localities References: [1] Friese A.E.W, Charlesworth E.G. and McCarthy T.S.(1995) Econ. Geol. Res. Unit, Univ Witwatersrand, Johannesburg, Inf. Circ. No. 292.; [2] Mellor E.T. (1913), Trans. Geol. Soc. S. Afr. 16, 1-32 ; [3] Stanistreet I.G, McCarthy T.S, et al,(1986) Tectonophysics, 131, 53-74.