A stratigraphic-structural investigation of an area south of Pofadder, Namaqua Mobile Belt, South Africa
| dc.contributor.advisor | Colliston, W. P. | |
| dc.contributor.author | Nel, Wayne Justin | |
| dc.date.accessioned | 2022-05-25T13:28:14Z | |
| dc.date.available | 2022-05-25T13:28:14Z | |
| dc.date.issued | 2021-12 | |
| dc.description.abstract | The area of the Namaqua Mobile Belt investigated is situated in the eastern part of the Aggeneys Terrane (2-1.06 Ga), south of Pofadder in the Northern Cape. The mapping campaign covered an area of 1,350 kmĀ², in the eastern Aggeneys Terrane which is juxtaposed against the Pofadder (2Ga) and Bladgrond Terranes. The terranes are separated by well-defined inter-terrane thrust boundaries, namely, the Groothoek Thrust that separates the Aggeneys and Pofadder Terranes in the north, and the Geelvloer Thrust separating the Bladgrond and Aggeneys Terranes in the south of the study area. The study area is defined by kilometre scale sheet intrusives hosting 23 stacked sequences of sheath fold nappes characterised by double vergence geometries, representing four well defined structural domains; the Witkoppies, Samoep, Lekdam and Houmoed Structural Domains. Three of the four domains contain supracrustal sequences, that can be divided into prominent structural successions and sequences. One structural domain however, the Samoep Structural Domain, is defined by a well foliated medium quartz-feldspar gneiss and contains no isolated sheath fold structures. The structural domains are separated by intra-terrane thrust sheet boundaries that have been juxtaposed on top of one another. The stratigraphic sequences can be correlated with the upper succession of the Wortel Formation, defined in the west, together with a total new set of supracrustal rocks not yet identified in the Aggeneys Terrane. The stacked sequences of isolated double vergent isoclinal fold structures can be classified as sheath folds using listed criteria. Three prominent planar fabrics were identified within the study area namely: S0, S1 and S2. The S2 fabric is however only identified in the hinges pf macroscopic sheath folds as a sillimanite grade axial planar fabric and forms the regional fabric, defining the XY plane of the regional strain ellipse. The S2 fabric is also identified on a mesoscopic scale, by F2 Model 1 and 2 flow perturbation folds, seen in the YZ and XZ planes of the regional foliation, all planar fabrics are co-planar. The macroscopic sheath folds mapped during this study contain co-linear L fabrics defined by stretching lineations, fold axes of various fold phases and the long axes of the sheath folds and represents the X-direction of the regional strain ellipse. The co-linear linear fabrics have a mean east-northeasterly trend and plunge indicating the southwesterly directed tectonic transport overthrust direction. The planar fabrics have a mean dip toward the north-northeast across the study area. A progressive shear deformation model is proposed for the study area and is defined by six phases of deformation affecting all four structural domains, interpreted to take place under deep crustal conditions of upper amphibolite grade of metamorphism. The D1 phase is defined by the intrusion of kilometre scale sheet intrusives that define an S1 fabric, associated with inter- and intra-terrane thrusting. The D2 phase is defined by the development of macroscopic F2 sheath folds that fold an S0-1 foliation and define an S2 regional axial planar foliation. The D2 phase is also defined by F2 Model 1 and Model 2 folds. The D3 phase is defined by macroscopic F3 folds that refold D2 sheath fold traces and define localised S3 foliations that are also co-planar with S2. The D4 phase is defined by macroscopic F4 open folds that define a characteristic Z asymmetry that refolds all earlier structures, including kilometre scale sheet intrusives. The study area itself can be defined as a macroscopic D4 antiformal-synformal structure. The D5 phase is defined by east-west shears that are associated with the reactivated movements of thrust boundaries, and movements associated with flexural slip on the limbs of major D2, D3 and D4 structures. The D5 phase also rotates linear and planar fabrics in the north-western sector of the study area on the limbs of the major D4 synformal structure. The D6 phase is associated with northwest-southeast oblique trending shears that effect the eastern sector of the study area and deforms all earlier D2-D5 structures. It is concluded that the structures identified across the eastern Aggeneys Terrane formed progressively during a single long-lived event, formed by a process of sub-simple/general shear conditions, under deep crustal conditions. | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/11660/11635 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | University of the Free State | en_ZA |
| dc.rights.holder | University of the Free State | en_ZA |
| dc.subject | Dissertation (M.Sc. (Geology))--University of the Free State, 2021 | en_ZA |
| dc.subject | Namaqua Mobile Belt | en_ZA |
| dc.subject | Shear deformation model | en_ZA |
| dc.subject | Tectonic framework | en_ZA |
| dc.subject | Structural framework | en_ZA |
| dc.subject | Mesoscopic structures | en_ZA |
| dc.subject | Macroscopic structures | en_ZA |
| dc.title | A stratigraphic-structural investigation of an area south of Pofadder, Namaqua Mobile Belt, South Africa | en_ZA |
| dc.type | Dissertation | en_ZA |