Structural and kinematic analysis of a pseudotachylite-bearing fault system within the upper Critical Zone of the Bushveld Complex, South Africa

Hales, Matthew Paul
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Two SSW-dipping, pseudotachylite-bearing faults are exposed within the northern portion of the Bushveld Complex Eastern Limb Critical Zone. Both faults display S-plunging slickenlines on fault-parallel shear fracture surfaces as well as several planar to sigmoidal macroscopic and microscopic fractures and fabrics (south-dipping P shear fractures and quasi-ductile sigmoidal fabrics and north-dipping R, R’ and P’ shear fractures) that are consistent with top-to-north, reverse-sense displacement. These fractures and fabrics display a mutually crosscutting relationship with E and W-dipping normal-sense shear fractures. From the contemporaneous N-S compression and E-W extension, oblate strain with principal stress axes orientations of subhorizontal N-S σ1, subhorizontal E-Wσ2 and subvertical σ3 is deduced. The lower fault crosscuts the LG-6 chromitite and the footwall and hangingwall pyroxenite layers is steeply SSW-dipping. It displays a reverse, dip-slip displacement of 1.8 m. Field observations show a single pseudotachylite injection vein extending from the fault while petrographic studies show both quasi-ductile cataclasite and cataclasite within the fault. The fault zone displays pervasive epidote alteration. Petrographic studies revealed strong intracrystalline fracturing of pyroxenes, however, these fractures do not propagate into interstitial plagioclase. This phenomenon was observed in all pyroxene-plagioclase lithologies in the vicinity of the faults. The upper fault is a shallowly SSW-dipping, layer-parallel fault at the contact of the MG-4chromitite and underlying anorthosite near the boundary between the lower and upper Critical Zone. Owing to the layer-parallel orientation of the fault and homogeneity of the hangingwall and footwall rocks, the amount of slip for this fault is undetermined. Several fault-rock types were observed along strike for approximately 6 km. These fault rocks include quasi-ductile cataclasite, pseudotachylite and cataclasite. The quasi-ductile cataclasite is defined by highly altered and comminuted anorthosite with comminuted chromite aggregates defining a S-dipping sigmoidal fabric. The quasi-ductile cataclasite is crosscut by cogenetic pseudotachylite and cataclasite which have mutually crosscutting relationships. Pseudotachylite dykes form both parallel and almost orthogonal to the fault fabric, which is controlled by magmatic layering and foliation. The layer-parallel dykes are up to 5 cm thick and may contain more than one pseudotachylite generation. Up to four of these dykes may be exposed in outcrop. The layer-oblique dykes, interpreted as injection veins, extend up to 20 cm from the layer-parallel dykes preferentially into the footwall. Using crosscutting relationships between several dykes as well as mutually crosscutting relationships between pseudotachylite, cataclasite and oblique shear fractures, at least three pseudotachylite generations are identified, indicating that the fault underwent several seismic-slip events. SEM and X-ray element mapping work shows extensive chloritization of the quasi-ductile cataclasite. Crosscutting relationships between chloritization textures within cataclasite and pseudotachylite suggests the ingress of fluids during interseismic periods, with the younger fault-rock generations displaying less chloritization. Chloritization during interseismic periods may have facilitated subsequent seismic-slip events that led to renewed friction melting and pseudotachylite generation. Field and petrographic analysis of the upper fault zone indicates that strain was initially partitioned into the anorthosite along chromite aggregates that enhanced the propagation of fractures owing to the strong competency contrast between chromite and plagioclase. Increasing slip and fracture density led to the widening of the fault zone downward into the anorthosite from the MG-4chromitite contact. The widening of the fault zone formed quasi-ductile cataclasite which was subsequently crosscut by pseudotachylite and cataclasite as the faulting mechanism transitioned from continuous cataclastic flow to seismic stick-slip faulting. Owing to the transition in the faulting mechanism, the ambient fault zone temperature is deduced to have decreased below 300°C. Despite ambient greenschist temperatures, the fault is presumed to have formed outside the greenschist facies and typical seismogenic zone owing to the emplacement depth and mineralogy of the Bushveld Complex. The initiation and progressive deformation of the two faults are attributed to the development of a S-type flanking fold in the Transvaal and Bushveld Complex rocks abutting the Thabazimbi-Murchison Lineament during the sinistral-sense reactivation of the lineament during a broadly N-S compressional deformation event following the emplacement of the Bushveld Complex, ca. 2.04-1.95 Ga
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science Geology, 2020