The Stratigraphy and Structure of the Western Saldania Belt, South Africa and Geodynamic Implications

Abstract

The western Saldania belt in South Africa exposes a section through a fore-arc region that records deformation, sedimentation and plutonism in the late Neoproterozoic and Cambrian (>560 to <510 Ma) along the obliquely convergent continental margin of the Kalahari Craton. The belt comprises two main and structurally overlying units. Imbricated and pervasively transposed marine metasediments and relics of oceanic crust constitute the structurally lower Swartland complex. Kinematics and strains indicate formation of the accretionary complex during tectonic underplating and top-to-the west and northwest thrusting related to the southeast-directed subduction of the Adamastor ocean below the Kalahari Craton. The Swartland complex is unconformably overlain by low-grade metamorphic metasediments and minor metavolcanic rocks of the Malmesbury group that represent the late-Neoproterozoic to Cambrian fore-arc basin fill. Sedimentary facies suggest the presence of a volcanic arc in the east, succeeded in the west by metaturbidites of the inner fore-arc basin and deeper-water metapelitic successions in the northwest interpreted to form slope-apron deposits overlying the toe of the prim and facing the ocean basin. The regional deformation of the fore arc is characterized by partitioned sinistral transpression related to oblique convergence. The absence of collisional structures and very limited exhumation of the rocks suggest a soft collisional event, probably as a result of slab break-off. This break-off may also account for the voluminous, syn- to late-tectonic plutonism of the Cape Granite Suite in the fore-arc region, peaking around 540–530 Ma and accompanying the waning stages of regional tectonism.