A study of the structural geology of an area between the Neusspruit shear zone and the Brakfontein shear zone near Kakamas, Northern Cape.

Abstract

The study area Koekoeb B is a farm that falls under the Kakamas Terrane which in turn falls under the Gordonia Subprovince in the Namaqua-Natal Metamorphic Province, South Africa. This area was chosen due to lack in literature about its lithology. Koekoeb B is comprised of metasedimentary rocks of the Biesje Poort Subgroup and granitoids of the Keimoes Suite. The Kakamas Terrane was deposited in an intracratonic basin between the Kaapvaal Craton and the Namaqua continental mass. The sediments were buried with time and experienced metamorphism due to burial pressures and temperatures. The area experienced folding as a result of the collision of the newly formed Kakamas Terrane and the Bushmanland segment with the Kaapvaal Craton during a Wilson Cycle. During subduction and collision the metasedimentary rocks were intruded by what is known as the Keimoes Suite. The most abundant intrusive rock in Koekoeb B is the Friersdale Charnockite. It is considered the youngest with Rb-Sr ages around 1080-1090 Ma. The Gordonia Subprovince experienced such intense deformation that continuous folds formed and there is even evidence of parasitic folds. Northwest striking shear zones developed as a result of the continued compression of the Namaqua mass with the adjacent north easterly Kaapvaal Craton. The folds and shear zones formed under four major deformational events Two months were spent acquiring orientation data (direction of dip and dip) in the field. A Clar compass was used to measure the dip direction and dip readings of bedding, cleavage, joints and lineations. The orientation data was imported into Move® software to create a geological map. Samples collected from the field were used to produce thin sections for petrography studies using the petrographic microscope. Conclusions were drawn from the analysis of the data. Koekoeb B experienced regional metamorphism and folding when the Kakamas Terrane collided with the Kaapvaal Craton. The area was subdivided into four subareas based on the strike and dip data generated on the geological map. The synoptic β-axis diagram determined that the subareas are of the same generation but the fold axes orientations vary slightly. Because the study area did not include the shear zones no conclusive reason can be given but it can be assumed that the variation is due to movement along the shear zone or as a result of the intrusion of the Keimoes Suite. The area later experienced brittle deformation which is evident from the large number of joints found; the joints cut across the folds and show a different stress regime from the folds. Conjugate joints were observed on the field and plotted on stereonets. The results showed a vertical sigma two which confirmed that Koekoeb had been affected not only by compression tectonics but by the strike-slip movement on the shear zone.