Structural-stratigraphic investigation of an area near Kakamas and environs, Namaqua mobile belt, South Africa
Mathee, Hendrik Lukas Marthinus
MetadataShow full item record
The study area is near Kakamas in the Northwest Cape and is located in the tectonostratigraphic Grünau Terrane - an accreted crustal fragment associated with the Mesoproterozoic Namaqua Province. The mapping campaign covered an area of some 6,500 km2 comprising of highly deformed and metamorphosed pre-tectonic supracrustals and syntectonic sheet intrusives emplaced and tectonised during the 1.2 to 1 Ga Namaqua Orogeny. The granulite grade Grünau Terrane is juxtaposed against the amphibolite facies Bladgrond Terrane and transported south-westwards along the inter-terrane Hartbees River Thrust (HBRT). The study area incorporates the north-western section of the Riemvasmaak-Kenhardt Mega Sheath Fold (RK-MS) which contains a series of sheath fold complexes divided into five structural domains. Macroscopic sheath folds have been recognised and documented in the western Namaqua Province for both the ~2Ga Pofadder Terrane and the ~1.6Ma Aggeneys Terrane: this study reports for the first time, the details of large scale sheath fold complexes in the Eastern Namaqua sector. In the Aggeneys Terrane the Aggeneys Mountain consist of a series of stacked sheath folds and Gamsberg Mountain represents a single macroscopic sheath fold formed under a compressive simple shear regime associated with south-west accretion of terranes. The dominant stratigraphic features are suites of sheeted granitoids interdigitating pre-tectonic supracrustals that consist of metasedimentary and volcaniclastic rocks. The oldest of the supracrustals is the Blouputs Formation with a provenance age of c.1800 Ma. The intrusives are a combination of leucocratic to granodioritic granites and the product of kilometre scale anatexis. Both the supracrustal and intrusive rocks are confined to the five structural domains. The Vaaldrift sheath fold is the only structure (Domain 4) that does not have an inter-sheeted granite associated with the supracrustals. The intrusive rocks has an intrusive age distribution ranging from the oldest, Eendoorn gneiss (1200 Ma) to the youngest, Friersdale charnockite (1080 ±13 Ma). The sheath fold complexes are bounded by intra-terrane thrusts along which thrust sheets (both supracrustals and granites) are cut out. The boundary of the RK-MS is defined by the Waterval thrust which is the sole thrust to the intra-terrane thrust system. The macroscopic sheath folds which are mapped during this study contains co-linear L-fabrics consisting of meso- and macroscopic fold axes of various fold phases and mineral stretching lineations plunging towards the north-north-east, which indicates a south-south-westerly tectonic transport direction. Three main fabrics are defined during this study, namely: S0 (compositional banding), S1 and the regional S2; both foliations are caused by shear processes and are also recognisable in the sheeted intrusives (e.g. Rooipad, Eendoorn and Harpersputs). The foliation and axial planes of the macroscopic sheath folds have a co-planar relationship and trend north-west. On a mesoscopic scale, two types of folds have been defined as model 1 and model 2 folds, which formed simultaneously during a flow perturbation process. North-west trending shear zones are mapped as the last stage of deformation during which the intra-terrane thrust was reactivated as sub-vertical shear zones. A progressive shear deformation model is proposed for the structures in the study area. Four deformation phases were recognised with the first of them having two separate sub-phases (D1a and D1b). The initial phase of the first event (D1a) resulted in the mesoscopic model 1 and model 2 folds during terrane assembly. The main deformation event was the second D1 event (D1b), characterised by macroscopic scale sheath folds (F1) formed during flow perturbation under general shear. The D1(b) event consisted of two phases of sheath folds (F1 and F2), the F2 being localised refolding of the F1 structures during a similar process. The D1(b)F1 structures are characterised by folded S0/S1 with S2 as an axial planar cleavage. Two metamorphic events are recorded by previous authors for the area: the first event was during terrane amalgamation at ~1200Ma and the second event during the last stages of deformation (1018±11 to 1024±14Ma; D4 north-west shear event). The second deformation phase (D2) is characterised by the intrusion of the Oranjekom Complex (~1100Ma) which is simultaneously deformed into a sheath fold; it defined the end of a progressive shear model which initiated at D1a. The Grünau Terrane underwent two phases of kilometre scale anatectic melting producing two of the most prominent lithological units, namely: Eendoorn gneiss (~1200Ma) and Witwater gneiss (~1123±6). The third deformation phase (D3) resulted in the intrusion of the Friersdale Charnockite into pre-existing macroscopic D1(b)F1 and F2 sheath fold hinge zones. This emplacement resulted in the D3 folds which are associated with D4 shearing. The D4 shear event caused reactivation of intra-terrane thrusts as sub-vertical shear zones and shears such as the Cnydas, Neusberg and Duiwelsnek shear zones along the limbs of the macroscopic sheath folds. The D4 shear zones trend north-westerly with an associated oblique movement resulting in both a lateral and vertical displacement of strata and structures. The dominant lateral displacement is predominantly sinistral with East-up; the sigmoidal rotation (on km-scale) of F1 axial traces of the macroscopic sheath folds are prominent features of this late shear event. It is concluded that a dynamic model combining progressive shear deformation during flow perturbation (layer-normal differential and layer-parallel shear) from a mesoscopic to a macroscopic scale resulted in the intricate structures mentioned above.