The structural development of part of the Namaqua Mobile Belt between Springbok and Vioolsdrif

Abstract

Abstract by author:

The area is comprised lithologically of subordinate volcano-sedimentary, units enveloped in a sea of granitoid material of various ages

The Eenriet and Khurisberg Subgroups represent the base of the Bushmanland succession and consist of a metaquartzite/schist association. The relative stratigraphic position between the two subgroups, is unknown. The Haib Subgroup overlies the Eenriet and Khurisberg Subgroups and represents the volcanic member of the Bushmanland Group. It consists of an upper mafic unit (Nous Formation) and a lower felsic unit (Tsams Formation). The mafic unit outcrops in the Richtersveld Province, while the felsic unit is also present in the northern parts of the Namaqua Province (Transition Zone) in the form of leucocratic biotite gneiss and muscovite-cordierite-sillimanite schist

The igneous rocks are subdivided into the 1730-2000 Ma and 1000-1200 Ma groups, which represent the two periods of igneous activity. The Vioolsdrif Suite is part of the former and, together with the volcanic rocks of the Orange River sequence, constitute the calc-alkaline Orange River igneous belt. The Vioolsdrif Suite has a large range in compositional varieties ranging from peridotite to adamellite. The Gladkop Suite intrudes both the Eenriet and Khurisberg Subgroups and constitutes the bulk of the Steinkopf Domain. It represents a suite of intrusives with compositions ranging from granodiorite to granite. Isotopic characteristics suggest a time-equivalence with the Vioolsdrif Suite, while structural and metamorphic characteristics point to a spatial separation of the two intrusive suites. Both the Vioolsdrif and Gladkop Suites, pre-date the Namaqua tectonogenesis

The Little Namaqualand and Spektakel Suites represent the 1000 - 1200 Ma age group and intrude the above units. The Spektakel Suite in turn intrudes the Little Namaqualand Suite. The Little Namaqualand Suite represents the big feldspar augen-gneisses, which are the dominant rock-type in the Copper District. It was emplaced pre to syntectonic with respect to the Namaqua tectonogenesis, while the Spektakel Suite is late-syntectonic

The Kinderlé Metamorphic Complex constitutes a variety of rock types which include both intrusive and metasedimentary rocks and is commonly referred to as pink gneiss

The present-day disposition of the Namaqua and Richtersveld Provinces is the result of extensive thrusting during the Namaqua Orogeny

The Richtersveld Province is bounded to the north and south by ramp - structures, i. e. the Lower Fish River and Groothoek Thrusts. The Orange River event in the Richtersveld Province is manifested in macroscopic fold structures, penetrative axial plane foliations, extension lineations and a greenschist grade metamorphic imprint, which all pre-date the emplacement of the Vioolsdrif Suite. Strain analyses showed that most of the volcanoclastic material (which constitutes a reasonable proportion of these rocks), is not ideally suited as strain gauges except for some accretionary lapilli tuffs from which good results were obtained. The amount of strain ranges from undetectable in the massive lavas to strains with axial ratios (X:Y:Z) of the order of 2, 5 : 1 : 0, 65 in the pyroclastic units. Ratios as high as 5, 9 : 1 : 0, 7 have been recorded

Except for the pre-Vioolsdrif Gladkop deformation (with associated high-grade metamorphism), little is known of the pre-Namaqua imprints beyond the Richtersveld Province

The variety of structures of the Namaqua deformation are the result of extensive. thrusting from the north-east. These are discussed below

The Groothoek Thrust is an oblique ramp and is a major feature of the Namaqua Orogeny along which the Richtersveld Province was transported to its present position. Vioolsdrif granitoids constitute the hanging wall of the Groothoek Thrust. The granites are undeformed in the Richtersveld Province, but become gneissic towards the south under the influence of the thrusting. The main locus of movement is situated in the Groothoek and Kabina Formations (Eenriet Subgroup), which are essentially schist and schist/metaquartzite successions, often mylonitic in character. The fold axes of the contemporary folds were rotated towards a well defined transport direction. The pervasive dextral sense of rotation of the fold axes is the result of the pre-thrust attitude of the folded layers with respect to the kinematic axes of the Groothoek Thrust. The shear plane dips 30° towards the north-north-east and the extension direction has a down-dip plunge on the shear plane. The sense of movement along the Groothoek Thrust is north-east overriding south-west. Strain analyses reveals that conditions of plane-strain prevailed during the thrusting, with the complete reorientation of the fold axes, suggesting shear strains in excess of y = lO. A minimum displacement of 55 km is suggested for the Groothoek Thrust. Fold profiles (in shear regimes) as gauges to quantify strain are unsuited and the rotation of axial planes of contemporary folds towards the shear plane, although restricted, are more useful

The combined effects of the Gladkop deformation and the Namaqua Orogeny on the Gladkop Suite resulted in its gneissic finite strained state and the near complete obliteration of the primary igneous features

The Skelmfontein Thrusting represents a later phase of subhorizontal thrusting related to the Namaqua Orogeny. It placed high-grade metamorphic rocks of the Copper District (augen gneisses of the Little Namaqualand Suite) on top of rocks of the Steinkopf Domain. It is present in the Steinkopf Domain in the form of minor subhorizontal shears. The Spektakel Suite was emplaced syntectonic with respect to the Skelmfontein age shearing

The thrusting is followed by the development of open folds with subvertical axial planes and fold axes subparallel to the transport direction. These folds fold the thrust fabrics. The superposition of a group of less prominent cross folds resulted in the formation of domes and basins. The equivalents of both these directions of folding in the Steinkopf Domain are found as the subvertical Dabbiknik crenulations. The origin of these folds is poorly understood. These structures formed due to shortening normal to the transport direction which is probably the result of a bottleneck-effect created when large volumes of ductile material in the form of thrust sheets are wedged between two or more lateral ramps of regional extent. Lateral ramps of this magnitude have not been recognized. Similar longitudinal folds (folds with fold axes parallel to the transport direction) are present in glaciers of salt and ice, when the valley in which it is transported, narrows. A horizontal component of shear is manifested by the en echelon distribution of the pegmatite - filled fractures and the dextral sense of movement associated with the late subvertical shears such as the Ratelpoort Shear. In the latter case the movement direction is oblique; the Copper District moved down with respect to the Steinkopf Domain. The dominant movement, however, was in a horizontal plane. Large scale open folds often formed between two adjacent shear zones. These late shears probably represent lateral ramps

The last imprint of the Namaqua Orogeny is the development of subvertical shear zones in the Richtersveld Province. The structures flatten at depth into parallelism with the Groot-hoek Thrust and have a subvertical movement direction, with the northern blocks invariably thrown down. These structures represent the listric normal faults commonly associated with thrusting

The metamorphic history and zonation are complex, with major retrograde metamorphism. A model of gravity spreading is suggested as the driving force for the thrusting of the Namaqua Orogeny