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Abstract

We analyse a crust-scale 3D structural model of the Western Bredasdorp Basin (WBB) located offshore southern South Africa to assess both the spatial and temporal development of geothermal gradients in the Southern African ‘passive-transform’ continental margin. The results of simulations of the present-day conductive thermal field were used to constrain the heat flow evolution during 3D numerical modelling of the paleo-thermal regime. Besides conforming to the regional geotectonic framework, the ensuing models are largely consistent with measured temperatures. The largest control on present-day temperatures is exerted by the basin's present-day geometry and distribution of thermal properties which, in turn, trace back to the earliest tectonic event involving crustal thinning and significant syn-rift sedimentation. Implying from the modelled shallow and deep geotherms, we find that the variations of present-day surface heat fluxes in the WBB are much more dictated by the long-term crustal radiogenic heat production in concert with the interaction of low thermally conductive sediments and high thermally conductive crustal rocks rather than by the present-day mantle heat flow. In addition, model calibration with measured vitrinite reflectance trends suggests that three major phases of thermal disequilibrium resulting in high heat flow during syn-rift and post-rift times characterise the distribution of maximum paleo-temperatures and paleo-geotherms. In terms of subsidence, the syn-rift tectonic event plays the most significant role in basin evolution. The height and width of the syn-rift heat flow anomalies and their lateral variations reflect a direct consequence of instantaneous crustal stretching leading to rapid burial and lateral differences in crustal thicknesses and amounts of radiogenic heat produced. The post-rift heat flow peaks are interpreted to be related to a Late Cretaceous mantle-related hotspot event and a late Miocene episode of margin uplift. As indicated by validated vitrinite trends and the modelled distribution of eroded sediments, the hotspot event represents a more localised thermal event, while the late Cenozoic erosion and inferred uplift depict a regional character. The modelled evolution of deep heat flow further suggests that ancient thermal perturbations have little influence on the present-day thermal field.