A structural and geochemical traverse across the NW outcrop of the Colenso Fault Zone, Saldanha, South Africa

Master Thesis


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University of Cape Town

The Colenso Fault Zone marks the boundary between the Tygerberg and Swartland Terranes in the Pan-African Saldania Orogenic Belt. The fault zone comprises several discrete shear discontinuities that in places cross-cut relatively undeformed granites of the Cape Granite Suite. It is NW-SE striking, ~150 km long, and ~7km wide, but poorly exposed except in coastal exposures at the NW end. The deformation sequence can be divided into 2 sequences: The oldest deformation started with the emplacement of the G1, G2 and then the G3 of the Cape Granite Suite, followed by formation of aplite veins and strike-slip faulting generating cataclasites. The initial strike-slip sense of movement along the fault was sinistral, followed by dextral strike-slip shearing and finally late stage jointing. The Colenso Fault Zone is host to 3 large zones of cataclasis that are in the order of up to a few hundred metres in exposed down-dip and along-strike lengths. The cataclasites are composed of quartz and plagioclase clasts, in a phyllosilicate matrix. Both the wide cataclasites (several metres) and small cataclasite zones (tens of centimeters) show a decrease in shear intensity away from the core of the fault zone. The cataclasites have the same bulk chemical composition as the surrounding granite.