Characterization of joints in the Keimoes suite with respect to Namaqua deformation events
The Keimoes Suite is a group of poorly defined granitoids that characterize the Namaqua Front and Foreland zones. There is a lack of knowledge on its content and distribution. A significant amount of work has been done on the geochemical and geochronological aspects of the Keimoes Suite but no structural analysis using a comparison between joint occurrences in the suite and the country rock has been found in the literature. This study provides insight on whether these joints formed as a result of the emplacement and subsequent cooling of the granitoids or whether they are the result of later deformation processes. This was achieved through remote sensing, detailed field mapping and structural analysis of joint data to determine the type of stress regime associated with their deformation. Eleven granitoids of the Keimoes Suite were mapped in the Kakamas-Keimoes area in the Northern Cape, South Africa. Up to four joint sets were mapped and characterized according to orientation, abutting relationship, in-filling material and spacing properties. The orientation analysis revealed two prominent joint sets (NNW and NE) that are consistent throughout the Keimoes Suite granite. However after careful analysis of their abutting relationship it has been concluded that these joints are the youngest joints formed in the Keimoes Suite. The fourth set is the E-W set which does not occur at a wide spread scale. The oldest joint set (NNE) is defined by the quartz and feldspar filling and these joints only occur in the oldest granite of the suite. Field observation revealed shear displacement, forming a conjugate joint set. This conjugate set closes at an acute angle of 60˚ and the joints displace each other. The presence of en echelon sigmoidal veins suggests these joints formed as mode II fractures and that they are tectonic joints. The dominant joint set NNW is parallel to the regional foliation, shear zones and faults which were formed during the D2 deformational vent of the Namaqua Orogeny. This NNW joint set post-dates the D2 deformational event and was formed during the D3 deformational environment of the Namaqua Orogeny. Principal stress analysis of all the joints in the study area suggests a strike-slip environment, which coincides with the D3 deformation event of the Namaqua Natal Province. Even though the country rock and the Keimoes Suite granites were subjected to same stress field during the D3 event, the analysis of principal stresses between the Keimoes Suite granites and the country rock reveals a slight difference in the orientation of the principal stresses. This is caused by the difference in competency between the Keimoes Suite granites and the country rock thus caused the refraction. In conclusion Structural evidence on various members of the Keimoes Suite indicates three episodes of intrusions with respect to the D2 Namaqua deformation event based on foliation and mineral filled joints. The Vaalputs Granite is pre tectonic relative to the main D2 deformation event and the thermal peak M2 metamorphisms, while Louisvale, Kanon Eiland and Klip Kraal Granites are syn-tectonic to these deformation events. The granites that lack foliation are classified as post-tectonic granites and these are the Keboes, Kleinbegin, Gemsbokbult, Colstone and Cnydas Granites as well as the Friesdale Charnockite. However the presence of foliation in some granites suggests that a compressional episode existed for a period of time and ended sometime before the onset of the post-tectonic granites. Therefore the D2 pre-dates the last episode of Keimoes Suite granite emplacement. This study partly validates the work done by previous workers on the Keimoes Suite, although there are some slight differences which are subjected to change. Conclusion can be drawn that this study provided additional insight in the findings of other workers nonetheless also differs with some of their findings regarding the timing of emplacement of the Keimoes Suite.