Browsing by Author "Rheeder, Christie George"

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    Characterisation of regional strains in the western Saldania Belt and implications for the structural evolution of Pan-African Rocks in the Western Cape, South Africa
    (Stellenbosch : Stellenbosch University, 2024-03) Rheeder, Christie George; Kisters, Alexander; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The western Saldania Belt records the closure of the Adamastor Ocean and the amalgamation of southwest Gondwana during the late Neoproterozoic and early Phanerozoic. Still, uncertainties regarding its geodynamic setting and tectonic evolution remain. The lack of outcrop and deceptive monotony of the belt added to the uncertainty about the Saldania Belt and has seen the belt linked to the well-understood northern Pan-African belts. Structural mapping included numerous type localities, made up of widespread surface mapping and 3D exposures in quarries throughout the region, together with historical data, which formed an integrated regional geodatabase to reveal the juxtaposition of structural domains characterised by distinct fabric development and kinematics. Two structurally overlying domains, namely an upper and a lower domain, form the main regions within the study area. The rocks in the upper domain exhibit transpressional strain partitioning into (1) broad regions of north to northwesterly-trending folds (F2) indicating horizontal, mainly coaxial, east-northeast to west-southwest-directed shortening at high angles to the continental margin, and (2) margin-parallel, non-coaxial deformation along the sinistral strike-slip Colenso Fault Zone (ca. 555 and 520 Ma). Non-coaxial strain was further promoted by strain localisation into synkinematic plutons of the Cape Granite Suite. Structures within the belt record Pan-African strains and signify sinistral transpression dominated by strike-slip motion during southeast-directed subduction and convergence at shallow angles to the continental margin (< 20°). In contrast, lower domain rocks present a pervasively transposed, mélange-like sequence that documents episodes of vertical, coaxial shortening alternating with non-coaxial top-to-the-west and -northwest thrusting. The coaxial shortening strains are linked to the thickening and gravitational loading of the succession in response to thrusting and tectonic underplating. In contrast, non-coaxial strains signify episodes of seismic slip during underthrusting. The distinct differences in the fabrics observed in the contrasting domains represent a deformed fore- arc region, made-up of a deeper-level accretionary prism (lower domain), overlain by a folded fore- arc basin fil (upper domain). The transfer of lower-domain phyllites, utilising thrusts, into the overlying fore-arc sequence is indicated by regional-scale klippen structures. The proposed fore-arc setting of the Saldania Belt suggests the subduction of the Adamastor Ocean beneath the leading edge of the Kalahari Craton, challenging pre-existing stratigraphic and structural correlations.