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A Microstructural Kinematic Study of Selected Shear Zones in the Hartbees River Thrust Belt, Northeastern Namaqua Tectonic Province

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A Microstructural Kinematic Study of Selected Shear Zones in the Hartbees River Thrust Belt, Northeastern Namaqua Tectonic Province A MICROSTRUCTURAL KINEMATIC STUDY OF SELECTED SHEAR ZONES IN THE HARTBEES RIVER THRUST BELT, NORTHEASTERN NAMAQUA TECTONIC PROVINCE Thesis submitted in fulfilment of the requirements for the Degree of MASTER OF SCIENCE of Rhodes University I Grahamstown by Christopher Jackson November 1992 Abstract The Hartbees River Thrust Selt (HRTS) is a 40-60 km wide, southwest-vergent zone of complex structure, lithostratigraphy and high-grade metamorphism in the northeastern part of the mid-Proterozoic Namaqua Tectonic Province. The HRTS comprises the boundary zone separating the Sushmanland and Gordonia Subprovinces of the Namaqua Province. A knowledge of the movement histories of major ductile shear zones within the HRTS is fundamental to understanding the tectonic development of the belt, and Namaqua tectogenesis as a whole. In spite of this, no detailed microstructural kinematic studies have been attempted and the movement histories and age relationships of these shear zones have not been described in detail. This thesis represents a detailed microstructural kinematic study of a representative suite of orientated samples of mylonitic rocks, collected from five ductile shear zones within the HRTS. These shear zones include the Neusspruit Lineament, the Kakamas shear zone (KSZ), the Hugosput shear system (HSS)' the Rozynenbosch-Ganzenmond shear zone (RGSZ) and the Graafwater shear system (GSS)' Accepted modern methods of microstructural kinematic analysis were applied to samples of mylonite from these shear zones, in order to determine the precise orientation of the kinematic vectors, and the sense and relative ages of movements on each of the shear zones. Shear sense criteria, including composite SoC planar fabrics and shear band foliations, asymmetrical porphyroclast systems, mica-fish, oblique grain-shape and subgrain fabrics, asymmetrical microfolds, and the displacement of fractured rigid grains, together with a well-developed mylonite elongation lineation, conclusively indicate that SSW-directed thrusting occurred along the HSS, RGSZ, GSS and possibly along the Neusspruit Lineament, while normal, top-tooNE movements occurred on the Neusspruit Lineament, KSZ and HSS. Rare transposition criteria, and textural and paragenetic contrasts between syn-kinematic fabrics, strongly suggest that the phase of normal, top-tooNE movement seen in the northeastern HRTS shear zones is younger than the more widespread top-to-SW thrusting event. On the basis of mesoscopic structural criteria, SSW-directed thrusting is correlated with the 02 deformation event in the HRTS. The mylonite zones have been refolded by ENE-SSW trending F3 crossfolds, whose demonstrated coaxial relationship to the mylonite elongation lineation precluded reorientation of primary kinematic vectors. In the southwestern HRTS, primary thrust vectors have been reoriented by right-lateral, strike-slip shearing adjacent to the Pofadder Lineament during 04' Simple shear dispersion of mylonite lineations related to normal movement, suggests that they too have been modified by 04 shearing, and this constrains the timing of extensional movements to post-02 and pre- or syn-04. Syn-kinematic mineral assemblages, rheological criteria and the annealing states of the mylonites, provide insight into the thermotectonic evolution of the shear zones. A model is proposed in which the movement histories of shear zones within the HRTS are explained in terms of a typical orogenic cycle, involving crustal thickening by thrusting during a compressional orogenic phase, followed by collapse of the thickened crust during an extensional taphrogenic phase. Table of Contents Abstract Table of Contents ...................................................................................................... 1 List of Figures ..........................................................................................................VI List of Mineral Abbreviations .................................................................................... VII Acknowledgements ................................................................................................ VIII Chapter 1. - Introduction ............................................................................................ 1 1 .1 Background to study ....................................................................................... 1 1.2 Regional geological setting ............................................................................... 2 1 .2.1 The Bushmanland Subprovince .................................................................. 2 1.2.2 The Gordonia Subprovince ........................................................................ 4 1.3 The study area ................................................................................................ 5 1.3.1 Lithostratigraphy ...................................................................................... 8 1.3.1.1 Pretectonic units ............................................................................... 8 1.3.1.2 Syntectonic intrusive units ................................................................. 8 1.3.2 Structural synthesis ................................................................................ 11 1.3.2.1 0, regional fabric forming event ........................................................ 11 1.3.2.2 02 folding and ductile shearing event ................................................. 11 1.3.2.3 03 crossfolding event ...................................................................... 12 1.3.2.4 04 transpressive event .................................................................... 13 1.3.3 Metamorphism ...................................................................................... 14 1.3.3.1 Granulite facies event ...................................................................... 14 1.3.3.2 Amphibolite facies retrograde event ................................................... 15 1.3.3.3 Granulite facies thermal event ........................................................... 16 1.3.3.4 Greenschist facies retrogression ........................................................ 16 1.4 Scope of the project ...................................................................................... 16 1.4.1 Motivation ............................................................................................ 17 1.4.2 Review of kinematic models for shear zones of the HRTB ............................ 17 1.4.2.1 The Neusspruit Lineament ................................................................ 17 1.4.2.2 The Kakamas shear zone ................................................................. 20 1.4.2.3 Shear zones of the southern HRTB .................................................... 20 1.4.3 Aims of the project ................................................................................ 23 II Chapter 2. - Mylonites: Tectonic settings, terminology and methods of microstructural kinematic analysis .............................................................................. 24 2.1 Ductile shear zones and mylonites ................................................................... 24 2.2 Tectonic settings of mylonitic rocks ................................................................. 24 2.3 Deformation mechanisms in crustal fault zones ................................................. 26 2.4 Historical review of fault-rock classification ....................................................... 27 2.5 Microstructural kinematic analysis of ductile shear zones .................................... 31 2.5.1 Determination of displacement vectors and movement sense ........................ 31 2.5.1.1 Elongation lineations and the kinematic vector .................................... 31 2.5.2 Microstructural shear sense indicators ...................................................... 34 2.5.2.1 Asymmetrical porphyroclast systems and rolling structures ................... 35 2.5.2.2 Pressure shadows ........................................................................... 38 2.5.2.3 Spiralled inclusion fabrics ................................................................. 38 2.5.2.4 S-C composite planar fabrics and shear band foliations ........................ 39 2.5.2.5 Mica fish ....................................................................................... 42 2.5.2.6 Grain-shape and subgrain-shape fabrics ............................................. 43 2.5.2.7 Asymmetrical microfolds .................................................................. 44 2.5.2.8 Displacement of fractured rigid grains ................................................ 45 2.5.2.9 Crystallographic fabric asymmetry ..................................................... 46 2.6 Methods of microstructural kinematic analysis employed in this study .................. 47 2.6.1 Selection of shear zones for analysis ........................................................ 47 2.6.2 Sample collection ................................................................................... 47 2.6.3 Sample preparation ................................................................................ 48 2.6.4 Movement sense determination ................................................................ 51 2.6.5 Confidence-index for microstructural kinematic interpretations ...................... 51 2.6.6 Calculation of kinematic vector means and the mean mylonite plane .............. 52 2.6.7 Definition of the dip-slip to strike-slip ratio ................................................
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