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Abstract

The sedimentary basin infill of continental passive margins is considered as geo-reactors with temperature-related physical and chemical processes. The aim of this study is to derive regional variations in geothermal gradient at depth for two differently aged passive margins and to explore the controlling factors for these variations. Hence, we analyzed two previously published 3D conductive and lithospheric-scale thermal models of the Southwest (SW) African and the Norwegian passive margins. These 3D models differentiate various sedimentary, crustal and mantle units and integrate different geophysical data such as seismic observations and the gravity field. We extracted the temperature-depth distributions in 1 km intervals down to 6 km below the upper thermal boundary condition (UBC). The geothermal gradient was then calculated for these intervals between the UBC and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the UBC). According to our results, the geothermal gradient decreases with increasing depth and shows different trends and values for these two different margins. It has the least lateral variations over the area at 6 km below the UBC by a range of 16°C/km for the SW African margin and 26°C/km for the Norwegian margin. In the onshore parts of the SW African margins (variably covered by sediments) the geothermal gradient differs by 28-34°C/km and varies with depth. In contrast, the Norwegian onshore domains (with outcropping basement) show a variation as low as 15-17°C/km throughout the different depth intervals. Offshore, at the Norwegian margin, the geothermal gradient increases oceanward. However, at the SW African margin, the geothermal gradient declines from the sedimentary basins towards the distal parts of the shelf and reaches the minimum value in the oceanic crustal domain. These results indicate the ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old SW African passive margin that seems to be thermally equilibrated.