A Structural analysis of the Vaalputs waste disposal site and environs, Bushmanland, South Africa

Abstract

Within the Gari.es Terrane of the Namaqualand Metamorphic Complex (NMC) the V aalputs sub-Terrane is recognised as bounded to the north by the Kamiebees Shear Zone and to the south by the Platbakkies Shear Zone. These structures are interpreted as lateral-ramps related to a regional thrust surface that has emplaced the lithologies of the Syntectonic Granite Suite from east to west. A zone of tectonised pegmatite in the west of the study area has been interpreted as the frontal ramp to this thrust sheet. The aeromagnetic data indicates an angular discordance between the structural grain of the Vaalputs region and the areas north and south. This discordant region is here termed the Vaalputs sub-Terrane. The various rock units could be readily "mapped" as they all possessed distinctively different magnetic susceptibilities or remanent magnetization directions. An interpreted regional geological map of Vaalputs and environs was produced. Faults show up clearly and two shear zones could be clearly identified due to the change in the direction of the aeromagnetic trends. A quartz petrofabric study, utilising material from all major rock units in the study area, indicates that the last regional deformation relates to E-W simple shear in a vertical plane. This deformation gave rise to a well developed linear fabric recognisable in all major litho-types including the intrusive Koperberg suite. Kinematic indicators related to this regional shearing allows the subdivision of the Vaalputs sub-terrane into 4 quadrants. Palaeomagnetic measurements were made on all major rock types in the area using stepwise alternating frequency demagnetization. Only 3 of the 5 units sampled yielded stable primary magnetization direction which indicated the following: 1. The virtual geomagnetic pole position of the Okiep noritoids is compared with that of its lithological equivalent on Vaalputs, the Vaalputs enderbite. The Okiep noritoids have been dated at 1070 + 20 Ma and the Vaalputs enderbite at 1088 + 40 Ma. The Vaalputs antipole plots significantly northward of the Okiep antipole implying a northward rotation of the Vaalputs antipole. This is interpreted as indicating that the Garies Terrane rotated during strike-slip faulting along the Buffels River Shear Zone with respect to the Okiep Terrane. 2 The above rotation indicates that shearing took place on the as yet undated Buffels River Shear Zone subsequent to the setting of the 1070 + 20 Ma Okiep pole. 3 In a similar manner the Noncaip pole north of the Buffels River Shear Zone is compared with the Wolfkraal pole to the south of it, where the same northward rotation is observed. 4 The charnockites possess a fairly strong remanent magnetization which opposes the earth's field yielding a negative total field and reversed polarization which enables them to be easily identified by aeromagnetic techniques. 5 The palaeomagnetic antipole obtained in the hanging wall of the Riembreek thrust sheet plotted in the same position as the Port Edward antipole. It is proposed that this antipole was set at the time of thrusting thus dating this movement at 987 + 19 Ma, this being the age of the Port Edward charnockite.