Structural controls on the interaction between basin fluids and a rift flank fault: constraints from the Bwamba Fault, East African Rift

We present petrographic and structural analyses of a basement-hosted border fault in the East African Rift. Understanding the mechanical evolution and fluid-rock interaction of rift-flank faults is integral to developing models of fluid flow in the crust, where hydraulic connections may occur between basement faults and basin sediments. The Bwamba Fault forms the flank of the Rwenzori Mountains Horst in western Uganda, and has locally reactivated older mylonitic fabrics in the basement gneisses. The fault core features discrete mineralised and veined units. Shear fabrics and fault scarp striations indicate predominately normal kinematics, with minor strike-slip faulting and fabrics. Transient brittle failure was accompanied by two phases of fluid ingress, associated with veining and mineralisation. The first was localised and strongly influenced by host lithology. The second involved widespread Fe-oxide and jarosite mineralisation. The latter signals the onset of a hydraulic connection between Fe- and S-rich sedimentary rocks in the adjacent Semliki Rift Basin and the Bwamba Fault, involving co-seismic and or post-seismic fluid injection into the fault at ca. 150–200 °C, and 2.5–3 km depth. Such evolving permeability connections between basin sediments and basement faults are important for local hydrocarbon and geothermal systems, and may be typical of active rifts

Deformation-induced and reaction-enhanced permeability in metabasic gneisses, Iona, Scotland: controls and scales of retrograde fluid movement

The spatial distribution of greenschist-facies retrograde reaction products in metabasic gneisses from Iona, western Scotland, has been investigated. The retrograde products may be broadly accounted for by a single reaction, but their different spatial and temporal development indicates that a series of reactions occur with significantly different scales of metasomatic transfer. After initial fluid influx linked to deformation-induced high permeability, reaction-enhanced permeability, coupled to cycling of fluid pressure during faulting, strongly controls the pervasive retrogression. Ca-plagioclase and pyroxene in the gneisses are replaced by albite and chlorite in pseudomorphic textures, and this is followed by localized epidotization of the albite. Two main generations of epidote are formed in the gneisses. Epidosite formation is associated with prominent zones of cataclasite indicating a strong link between faulting and fluid influx. In contrast, complete alteration of albite to epidote in the host metabasic gneisses is spatially complex, and areas of pervasive alteration may be constrained by both epidote-rich veins and cataclasites. In other instances, reaction fronts are unrelated to structural features. Volume changes associated with individual stages of the reaction history strongly control the localized distribution of epidote and the earlier more widespread development of chlorite and albite. Such behaviour contrasts with adjacent granitic gneisses where epidotization is restricted to local structural conduits. Many small-scale mineralized fractures with evidence of having previously contained fluids do not enhance the pervasive retrogression of the metabasic gneisses and represent conduits of fluid removal. Retrogression of these basement gneisses is dominated by a complex combination of reaction-enhanced and reaction-restricted permeability, kinetic controls on the nucleation of reaction products, changes in fluid composition buffered by the reactions, and periodic local migration of fluids associated with fault movements. This combination generates spatially complex patterns of epidotization that are limited by cation supply rather than fluid availability and alternations between focused and pervasive types of retrogression

Rheological and permeability evolution of crystalline basement fault zones

Fault zones are dynamic sites of deformation and reactivation that partition strain throughout the Earth’s crust. Their nucleation and growth in the upper crust is often associated with economically important fluid flow, which is of particular importance in low-permeability host rocks that require brittle deformation to create interconnected permeability networks. Fault zones in gneissose basement complexes have been increasingly recognised as economically significant structures in the hydrocarbon and geothermal sectors due their ability to act as fluid conduits and/ or barriers. Their hydraulic properties and permeability structures are also important in environmental and alternative energy sectors. Carbon-capture and storage will be vital and necessary to mitigate the effects of the climate crisis. Joint industry, academia and government-funded projects such as Carbfix are currently exploring the potential for basement gneiss complexes to sequester CO2. With regards to alternative energy such as nuclear power, which generates significant amounts of hazardous waste, many deep geological repositories in gneissose basement and granitic intrusions are currently operational or are being considered by many countries. Comprehensively characterising the structural framework of complexes with long-lived deformation histories is therefore crucial for assessing their attributes as reservoirs and seals of fluids in the subsurface. There is a need to develop a conceptual model that describes attributes of fault zones that cross-cut basement gneisses. The key questions that inspired this research are: i) What are predictable fault zone characteristics of basement-hosted faults throughout the upper crust? ii) How do fluids influence basement fault rheology and deformation processes, throughout the upper crust? iii) What effect does fluid-rock interaction have on host rock and basement fault zone properties? Two well-exposed major fault zones, the Outer Hebrides Fault Zone (OHFZ), Scotland, and the Bwamba Fault Zone (BFZ), Rwenzori Mountains, Uganda, have been studied. Additionally, minor faults that populate the basement gneiss complex that forms the Rwenzori Mountains were studied. These fault zones are all characterised by similar host-rock compositions and are known to be important with regards to upper crustal fluid flow. The OHFZ represents a “deep” upper crustal fault zone, whilst the BFZ represents a shallower upper crustal fault zone, and the investigation presents a comparison between fault zones at different crustal levels. The BFZ is juxtaposed against a sedimentary basin in it’s hanging wall, and so provides a useful analogue for hydrocarbon systems, both locally in the East African Rift System, and for general buried hill-type hydrocarbon plays. Minor faults of the Rwenzori Mountains have developed at various times, in both deep and shallow upper crustal conditions. This network of variably oriented faults are investigasted to assess the potential for fluid migration throughout interconnected fault networks in gneissose basement complexes. A field-based approach to developing kinematic models of basement faults in both the OHFZ and the BFZ is coupled with microstructural and geochemical analyses to determine structural and fluid-rock interaction textures. Optical and scanning electron microscopy is used to analyse the petrography, structure and geochemistry of the fault rocks of the BFZ. Furthermore, X-ray computed microtomography allows fault zone mineralisation and porosity structure to be visualised in three dimensions, and provides insight into large scale fluid pathways within the BFZ. Stable isotope analysis is used determine the source and temperature of fluid ingress to the BFZ, and constrain hydraulic connectivity between the basement fault and the hanging-wall basin. Optical microscopy and quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) is undertaken in order to characterise ultra-fine-grained samples of the OHFZ. Results indicate that in deep upper crustal fault zones in the OHFZ, and some minor faults studied in western Uganda, basement faults are composed of interconnected cataclasite networks composed of greenschist-facies mineral assemblages of albite, epidote, muscovite and actinolite. These phases also form within altered wall-rock, and replace significant volumes of the original mineralogy of the gneissose host. High strain zones are composed of albite, epidote, actinolite and quartz cataclasites with very fine grain sizes <20 µm. Matrix-forming albite often shows strong shape preferred orientations, and epidote, actinolite, muscovite, chlorite and illite are aligned along S-C` fabrics. A significant result from the OHFZ is that rocks traditionally thought to be phyllonites contain <6 vol % of phyllosilicate phases, and are instead composed largely of strongly foliated albite, quartz, epidote and actinolite cataclasites. The BFZ is considered to be representative of deformation at shallower formation depths in the upper crust between 4-7 km. It is composed of banded quartzo-feldspathic cataclasites that have well-developed Riedel, Y- and P-foliations. These are sites of localised grain size reduction and commonly host haematite mineralisation. Mineralisation of these structural fabrics has resulted in the formation of rhombohedral compartments, within which fault rocks have undergone variable amounts of haematite and jarosite mineralisation. Many compartments remain porous. The host gneisses do not contain significant Fe- or S-bearing phases, but adjacent hanging wall basins are rich in gypsiferous, iron-rich mudstones. Calculations of sulphur and oxygen isotopic signatures of mineralising fluids were derived from haematite (δ18O) and jarosite (δ34) isotopic analyses from the BFZ. These are consistent with a rock buffered diagenetic fluid origin. Faults analyses from minor upper crustal faults of the northern Rwenzori ridge indicate fault orientations and kinematics both consistent with the regional extensional stress field of the East African Rift System, and faults oriented at high angle to the rift axis that host strike-slip and reverse motion. These faults are characterised by loose breccias and strongly foliated clay-gouges with Riedel, Y-, and P- foliations, or by slickenside-surfaces that lack fault rock development. Basement gneiss complexes analysed in this study experience a complex history of faulting and reactivation during exhumation, with broad similarities in fluid rock-interaction and rheological evolution of the faults: i) Deformation and structural fabrics such is Riedel, P- and Y-shears, and S-C` fabrics represent primary fluid pathways, as indicated by the spatial distribution of fluid-rock interaction textures in optical and scanning electron microscopy, and through XCT analyses. Brittle deformation facilitates both grain size reduction and fluid ingress. This results in compartmentalisation due to mineralisation of these fabrics, or strain-softening under periodic high-fluid pressures. ii) Intra-granular and grain boundary fluid influx promotes widespread alteration of the wall rock, and clasts within the fault core and represents an effective secondary control. Fluid-rock interaction in the host rock triggers mineral replacement reactions and products may either crystallise within pore-fluid, or are removed in solution via fractures. iii) Rheology changes are strongly linked to fluid-influx in basement faults. These reflect changes in mineralogy, e.g. due to the formation phyllosilicate networks, but also high fluid pressures, the development of fault fabrics, and grain size reduction. iv) Basement faults with adjacent hanging-wall sedimentary rocks act as transient basin fluid conduits in tectonically active basins. Petrographic and microstructural analyses of fault zone mineralisation products coupled with stable isotope analyses can constrain the origins of fluids and highlight hydraulic connections with sedimentary rocks in hanging-wall basins. v) Basement gneiss complexes respond to both regional and local stresses in syn-rift settings and may host cross-cutting faults that create interconnected permeability networks. Such networks may be of importance for regional fluid flow in economically important basins. Basement gneiss complexes present many exciting opportunities for meeting the energy demands of a growing global population, and have potential to offer solutions in storage of hazardous waste and CO2 as humanity addresses the energy transition away from fossil fuels and the climate crisis. It is therefore integral that comprehensive knowledge of basement fault systems is understood in order to meet these challenges, and this project contributes something toward these goals

The value of teaching observations for the development of GTA educator identity

No abstract available