Browsing by Author "Sloan, Alastair"

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    An investigation into the geomorphology of the Hebron Fault, Namibia, using a satellite-derived, high-resolution digital elevation model (DEM)
    (2019) Salomon, Guy; Sloan, Alastair
    The Hebron fault scarp in southern Namibia is 45 km in length with an average height of 5.5 m and a maximum height of 8.9 m. Namibia is a Stable Continental Region (SCR) — a slowly deforming area within a continental plate. The country also has little recorded seismicity with the largest earthquake on the International Seismological Center (ISC) catalogue being MW 5.4. If the Hebron fault scarp was formed in a single event, this would represent a MW 7.3 earthquake. SCRs do occasionally experience large earthquakes, however, the recurrence intervals between these events is much larger than in rapidly deforming areas. Consequently, studying palaeo-earthquakes allows the record of seismicity to be extended and the characteristics of SCR events to be better understood. These studies may help refine the Mmax estimates required for seismic hazard assessment. Previous work on Hebron has been limited to field descriptions and theodolite survey scarp heights. Furthermore, there have been several interpretations of the fault mechanism and number of rupture events. This study produces a high-resolution Digital Elevation Model (DEM) via stereophotogrammetry using pan-sharpened Worldview-3 satellite imagery (0.31 m resolution). The DEM was used for several geomorphological analyses. These included measuring the scarp height at 160 locations along its length, measuring river channel displacements and identifying knickpoints along river profiles. Results indicate that the scarp formed from a normal, dip-slip fault that ruptured in a single event. This scenario would imply a high slip-to-length ratio. A comparison of other SCR fault scarps in the literature was made which shows that Hebrons’ slip-to-length ratio falls within the values found on other SCR faults. This study also discusses the implications of results for seismic hazard assessment in the region. Due a poor seismic record, probabilistic seismic hazard analysis (PSHA) will calculate a low seismic risk for Namibia. As large earthquakes can occur in SCRs, deterministic seismic hazard analysis (DSHA) can be used to inform policy makers of the worst case scenarios.
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    Contributions to the Study of Lithospheric Deformation and Seismicity in Stable Continental Regions
    (2022) New, Thomas; Sloan, Alastair
    Recently, the field of geophysics has seen increasing recognition of the unique character of deformation and seismicity in stable continental regions (SCRs). However several important questions remain understudied. What controls the locations of earthquakes in SCRs? How well do observations, in SCRs, of elastic strain accumulation and release correlate with each other? How well do they correlate with stresses and geological proxies for rheological variation? The ultimate goal of this study was to better understand stable continental regions like southern Africa, where large earthquakes occur despite not being near plate boundaries, for example the 2017 Mw 6.5 earthquake in Moiyabana, Botswana. One way of studying the stress and strain in stable continental regions is by understanding the surface deformation of the region. This deformation is easily studied using global navigation satellite system (GNSS) velocity data. One of the biggest difficulties when it comes to GNSS data is that it isn't collected on a regular grid, but rather as irregular data points that need to be interpolated. This research investigated multiple interpolation methods and recommended two methods that best replicate the original velocity field (using a well populated dataset from Southeast Asia). These interpolated GNSS data can then be used to determine deviatoric strain in a region, which can in turn be fed into numerical stress models. However, limited GNSS data exist across southern Africa, and therefore topographic data was used to calculate the gravitational potential energy, and in turn the body stress and deviatoric stress for the region. This study also investigated how this deviatoric stress (or deviatoric strain) can be more accurately calculated on a spherical rather than a flat surface, which is particularly important over large study areas. Across southern Africa, data show that deviatoric stress lined up with stress data within mobile belts. This suggests that in these weaker mobile belt crust (such as the Namaqua-Natal and Damara-Chobe belts), gravitational collapse is the dominant driver of deformation, which is in line with conclusions that have been made in previous literature. In other regions, deviatoric stress vectors and stress data do not coincide and therefore there are other forces at play. These observations are obviously restricted by limited data coverage; it remains an open question if areas that have increased deviatoric stress due to gravitational collapse, which are also aligned with the orientation of weak zones, will have elevated strain in the long term.
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    Evolution of tectonic and stratigraphic geometries in the Southern Outeniqua Basin
    (2019) Smithard, Travis; Kahle, Beth; Sloan, Alastair
    The evolution of the Southern Outeniqua Basin has been analysed within a tectono-stratigraphic framework by the interpretation of 3,700km of 2D seismic data constrained by five exploration wells, drilled on the adjacent shelf. The mapping of different fault families into distinct structural orientations, NE-SW, NNE-SSW and NW-SE, was done and the basin sub-divided into four sectors (far-west, west, central and east) based on the geographical position of these faults. Sequence stratigraphic analysis of the basin lead to the identification of seven secondorder unconformities which have been further defined into five main seismo-stratigraphic units. The variation in major structural orientations suggest the evolution of the basin developed during three phases of extension, which have been related to the broader plate tectonic context during the Mesozoic break-up of Gondwana. Early Jurassic (170-150Ma) rifting between West and East Gondwana is proposed to have created weaknesses in the crust along the Southern African margin in a NNE-SSW orientation leading to the development of small rift basins and the deposition of syn-rift sediments. A series of intra-continental strike slip faults, including the proto-Agulhas Falklands Fracture Zone, are proposed to have developed along the southern margin during the incipient formation of the Weddell Sea (147Ma) between Antarctica and West Gondwana. Later mid-Jurassic (150-130Ma) extension along the North-Falkland rift zone lead to the oblique extension along the southern margin and the development of normal faults orientated NE-SW. Continued rifting (134Ma) in the proto-South Atlantic lead to translation along the southern African margin and the coalescence of the individual segments of the intra-continental strike slip faults and ultimately the formation of the Agulhas Falklands Fracture Zone. This phase of wrench faulting (134- 130Ma) is characterised by the strike-slip deformation of syn-tectonic sediments defined by second-order unconformities BR and B - late Valanginian - Hauterivian (134-130Ma). The Diaz Marginal Ridge (DMR), which bounds the Southern Outeniqua Basin and AFFZ to the south and north respectively, shows a marked variation is seismic character along strike. In the eastern and western sectors of the basin the DMR is made of syn-rift sediments. The discontinuous and non-parallel seismic character of the DMR in the central sector of the basin is interpreted to be continental basement. Significant bevelling of the top of the DMR in this sector, along with on-lapping reflector geometries of horizon B, Hauterivian (130Ma), is proposed to signify that the continental block existed prior to the formation of the Southern Outeniqua Basin and remained a basement promontory until the development of horizon G - Tertiary (66Ma). The Southern Outeniqua Basin is therefore proposed to have formed through four periods of tectonism: 1) early Jurassic (170-150Ma) orthogonal rifting of West from East Gondwana; 2) late Jurassic to early Cretaceous (150-130Ma) oblique extension and translation along intra-continental strike-slip faults; 3) early-late Cretaceous deformation associated with transverse motion along the AFFZ and development of an active transform margin; 4) mid-late Cretaceous thermal uplift of the Southern Outeniqua Basin associated with migration of the Natal Valley spreading centre along the AFFZ. Thermal subsidence and true passive transform margin conditions developed in the Southern Outeniqua Basin from early Tertiary to present day.
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    The oxygen isotope composition of the country rock of the Koegel Fontein igneous complex
    (2018) Whitehead, Benjamin; Harris, Chris; Sloan, Alastair
    The Koegel Fontein Complex is a Cretaceous anorogenic complex located in southern Namaqualand, west of Bitterfontein, South Africa. The complex comprises a large granite intrusion (the Rietpoort Granite), a series of NW-striking quartz porphyry dykes, and numerous other minor igneous bodies. A breccia plug, and the quartz porphyry dykes have δ18O values as low as -4.1‰, and these are believed to be the result of selective dehydration and melting of previously 18O-depleted rock (Curtis et al., 2013; Olianti and Harris, 2018). This thesis investigates the oxygen isotope composition of the country rock of the Koegel Fontein Complex, which consists of Namaqua-Natal Belt granulite facies rock and minor remnants of overlying Gariep Supergroup metasedimentary rock. The Brak Fontein Shear Zone (BFSZ) is investigated as a protolith for low δ18O (<6 ‰) units of the Koegel Fontein Complex. Geological mapping combined with oxygen isotope data of the country rock confirms that 18Odepletion occurred prior to the emplacement of the Koegel Fontein Complex and is localised in the ~545 Ma BFSZ. Generation of rock with δ18O values as low as -4 ‰ requires a combination of high temperature water-rock interaction, a high water-rock ratio and very low δ18O values in the alteration fluid. Deformation in the BFSZ coincides with the Pan-African Orogeny and a 549-545 Ma global glaciation, and it is reasonable to assume that ambient meteoric water had a very low δ18O value, perhaps as low as -30 ‰. Assuming a possible Δrock-water range from 0.5 to 2.8, a δ18O value of 8‰ for the rock before alteration, a δ18O value of -2‰ for the rock after alteration, a δ18O value of -30‰ for the fluid before fluid-rock exchange and exchange at a temperature range of 400°C to 700°C, a waterrock ratio of ~0.31 to ~0.40 is required to create the observed 18O-depletion in the BFSZ. The calculated water-rock ratio suggests that a significant amount of meteoric water passed through the BFSZ, into the brittle-ductile transition zone. A cold climate only coincides with a suitable tectonic setting during the Pan-African Orogeny, suggesting that the 18O-depletion is syn-tectonic. Furthermore, strong recrystalisation and annealing after deformation and a lack of brittle overprint or cross-cutting fractures and/or veins suggest a low permeability in the BFSZ after the Pan-African deformation. Therefore 18O-depletion in the BFSZ must have occurred during the Pan-African Orogeny and may be evidence for the infiltration of meteoric water into the ductile crust, possibly requiring variable strain rates or seismic pumping.