The Lithological and Structural Characterisation of the Sperlingputs Shear Zone in Southern Namibia
Thesis (MSc)--Stellenbosch University, 2017.
ENGLISH ABSTRACT: The Richtersveld Magmatic Arc (RMA) forms a major ~200km wide Palaeoproterozoic block within the Mesoproterozoic Namaqua Metamorphic Province (NMP). The RMA consist of rafts of Orange River Group volcanic rocks intruded by voluminous coeval Vioolsdrif Suite granitoids (1905-1865 Ma, ). The RMA is further subdivided into two domains with equivalent stratigraphic units but different metamorphic grade and deformation. In the west are low grade greenschist-facies rocks affected only by D1 which have been termed the Vioolsdrif Domain. These rocks are separated by a km wide tectonic zone termed the Southern Namaqua Front from the amphibolite-facies Pella Domain in the NE. The Pella Domain is strongly transposed by the main ductile D2 phase of the Namaqua Orogeny at ~1215Ma. As part of a regional scale mapping program by the Namibian Geological Survey and the Council for Geoscience, the Southern Namaqua Front has been re-examined and redefined. The Southern Namaqua Front in fact coincides with the northern margin of a ~15 km deformation zone termed the Sperlingputs Shear Zone System (SPSZS). The SPSZS consists of three main WNW-trending, steeply dipping shear zones that cross-cut the Vioolsdrif Domain and truncate the Haib porphyry Cu deposit. Collectively the SPSZS represents an anastomosing zone of deformation that mainly follows the less competent Orange River Group volcanic rocks that are sheared into fissile cataclasites and mylonites and wrap around largely unsheared blocks of bedded lava and, more often, granitoids of the Vioolsdrif Suite. The northernmost shear zone termed Fontal-SPSZS reworks the Southern Namaqua Front and cross-cuts the penetrative Pella Domain fabrics and structures, and thus post-dates D2. The rocks on the north of the F-SPSZS have a gneissic and schistose texture, with an overall mid to upper amphibolite-facies mineral assemblage, whilst those on the south of the F-SPSZS, are weakly deformed with a lower greenschist-facies mineral assemblage. The dominant steeply south plunging lineation along with various shear sense indicators suggest a significant a dextral sense of movement, which is the transpression regime. However, in some areas, the shear zones are accompanied by the development of vertical lineation with a suggesting a vertical component to the shear with an overall top to the south sense of movement. The SPSZS is intruded by pegmatite dykes and large plugs, both deformed and undeformed, which suggests intrusion during and soon after shearing - a relationship similar to that in other large shear zones in the area (Marshall Rocks-Pofadder Shear Zone (MRPSZ), Eureka shear Zone(ESZ)). Assuming that these pegmatites are equivalent to those in other parts of the Pella Domain (MRPSZ and ESZ), the SPSZS developed during the late Namaqua D4-dextral shearing event between ca. 1005 and 950 Ma, together with the also NW-trending Marshall Rocks-Pofadder and Eureka Shear Zones.
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