- © 2016 Geological Society of South Africa
Quartzitic sandstones and shales of the upper Witteberg Group (Cape Supergroup) near Grahamstown show hydraulic fracturing manifested in syntectonic quartz veins associated with tension gashes, reverse and strike-slip faults, and layer-parallel thrusts. These structures formed in the latest stages of, or post-date, large-scale regional folding of the Cape Supergroup and parts of the overlying Karoo Supergroup metasedimentary rocks. Fluid inclusions in all hydraulic structures show similar compositions and physical characteristics. They contain essentially two fluid types; (i) a hydrous weakly saline liquid associated with hydrous gas and (ii) a CH4-CO2 bearing hydrous fluid. The latter is not present in quartz along strike-slip faults. Microthermobarometry indicates that hydrothermal quartz in all structures was precipitated in the lowermost greenschist facies at ~280 to 340°C and ~170 to 200 MPa, corresponding to a depth of ~6500 to 7500 m. Secondary fluid inclusions were captured at slightly lower temperatures on the retrograde P-T path. All hydraulic fractures and faults are compatible with stress vector orientations plunging shallowly towards the southwest and northwest, and steeply towards the northeast, with periodically changing positions of the maximum, minimum and intermediate stress vector positions, creating differently oriented structures. We consider the swapping of stress vectors possible without significant geodynamic changes in a contractional crustal environment where small differential stresses, supported with high fluid pressure, facilitated fracturing and faulting by tensile or transitional tensile failure.
The investigated exposure lies close to the boundary of Cape and Karoo Supergroups and the regional structural inventory does not change across this boundary, indicating that all secondary structural features formed after deposition of at least the lower parts of the Karoo Supergroup. This implies that much of the pressure at which these structures, regional folds and subsequently formed hydrothermal faults, was exerted by depositional sediment loading. The estimated 6500 to 7500 m thick rock column of Karoo Supergroup rocks overlying the Witteberg Group was deposited at the end of the Permian, which we consider the most likely age of hydraulic faulting in the Grahamstown region.