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Abstract

This paper examines progressive evolution of fault architectures through late orogenic compression- to post-orogenic extensional deformation in the Witwatersrand Basin, South Africa. The results indicate that rapid extrusion of mafic lavas of the lower Klipriviersberg Group formed a rigid ‘lid’ over the thrust front, changing its mechanical character and thereby driving a change of structural style from fold growth to passive roof duplex. Flexural tightening of folds in the core of the triangle zones at this time may have helped provide the dynamic permeability for distributed ingress of hydrothermal fluids and consequent gold mineralization. Shortly afterwards, the kinematic environment changed to become extensional. However, this study shows sharp lateral partitioning of the duration of kinematic style and structural amplification, such that thrusting and extension coexisted along strike in the upper Klipriviersberg Group. Thus the switch from thrusting to extension was progressive within the region, but locally very rapid. As the local kinematic environment became extensional, the fault system evolved progressively, with the early stages of kinematic changes being dominated by a process of reactivation by architectural scavenging, in which new extensional structures developed by selectively reusing and incorporating geometrical segments of earlier formed thrust and normal faults. Three basic stages can be identified in this evolution: broad extension above underlying detachments, involving reactivation of lateral structures; a period of intensive reactivation and kinematic reworking incorporating frontal structures; and an abandonment stage when the detailed influence of the earlier architecture diminished and the fault system developed larger through-going normal faults. The interaction of the newly developing fault system with the pre-existing architecture constitutes pre-programming of the final geometry, in which individual large faults are composed of a reticulated network of new and inherited segments. The observations are consistent with fault scale being a key control on the fault reactivation involved. This study has involved full integration of a dataset comprising 2D and 3D seismic reflection data, geological mine plans, logging of over 120 km of drill core and underground mapping in deep mine workings that pass 3 km into the seismic volume at 2–3 km depth.

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