Geological mapping and tectonic evolution of the Blaauwberg Member in the Tygerberg Formation of the Malmesbury Group: A revision using joint orientation analysis
The Blaauwberg Strand member is part of a Neoprotorozoic metamorphic belt (the Saldania Belt) found in western South Africa. It forms part of a basement sequence of rocks that have poor exposure within the Cape Peninsula. Previous work has had little focus on the joints of the outcrop to deduce an evolutionary history for the area. This study uses joint plane data and other structural measurements and to add it to previous work done to deduce a structural evolutionary history of the Blaauwberg Strand member. The most prominent joint type of the Blaauwberg Strand member is shear type joints, that when plotted on an equal area net produce principle stress orientations of σ1 - 042/02, σ2 – and σ3 - 133/14 which implies that these joints were formed in a strike-slip stress environment. The foliations of the outcrop that is only found on the eastern section of the study area plots around a central maximum and implies that it part of a bigger structure that can be seen due to its submergence beneath the Atlantic Ocean. Folding of an outcrop adjacent to the study area produces a compressional sense of stress. The Tygerberg formation, the formation which contains the Blaauwberg Strand member is composed of turbiditic type deposits and the Blaauwberg Strand member is an intermediate igneous extrusive rock which means that they both formed in a submerged continental shelf environment and aged between 750Ma. Compression on a continental margin most likely forms in a collisional environment. This initial collision deformed the Tygerberg and Blaauwberg lithologies to a lower greenschist facies of metamorphism and produced large isoclinal folds. The stress field then changed to produce a transverse environment. Transpressional features and strain partitioning were developed during the transition from a regional compressional stress field to a regional strike-slip stress field. During the transition into a strike-slip environment shear zones started to develop with a sinistral sense of rotation. The emplacement of the Cape Granite Suite soon followed and marked the end of structural events of the Cape basement series at 560 – 540 Ma.
Environmental and Water Science 211
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