Variability of the geothermal gradient across two differently aged magma-rich continental rifted margins of the Atlantic Ocean: the Southwest African and the Norwegian margins

Abstract

Abstract. The aim of this study is to investigate the shallow thermal field differences for two differently aged passive continental margins by analyzing regional variations in geothermal gradient and exploring the controlling factors for these variations. Hence, we analyzed two previously published 3-D conductive and lithospheric-scale thermal models of the Southwest African and the Norwegian passive margins. These 3-D 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. The geothermal gradient was then calculated for these intervals between the upper thermal boundary condition and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the upper thermal boundary condition). According to our results, the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for these two different margins. We compare the 3-D geological structural models and the geothermal gradient variations for both thermal models and show how radiogenic heat production, sediment insulating effect, and thermal lithosphere–asthenosphere boundary (LAB) depth influence the shallow thermal field pattern. The results indicate an 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 in the present day.

Formation and geophysical character of transitional crust at the passive continental margin around Walvis Ridge, Namibia

“…After eliminating this source of error, we interpret the two clusters' differentiation to result mainly from the sediment layer's electrical resistivity, which indicates a change in sediment type or sea bottom heat flow (Grayver, 2021). The previously described effect of "thermal blanketing" in the southern Namibe basin (Gholamrezaie et al, 2018;Maystrenko et al, 2013), is likely a factor responsible for the differentiation of the two sediment clusters. This would mean, that the increased resistivity values of cluster sed-B are linked to a decrease in temperature away from the major depositional centre of Walvis Basin.…”

“…These sediments may have been transported along the shelf by bottom and surface currents and likely originate from cyclic erosion-deposition phases linked to the Orange, Kuiseb, and Swakop rivers (Dingle, 1992;Dingle et al, 1987;Goslin et al, 1974). Furthermore, Gholamrezaie et al (2018) and Maystrenko et al (2013) observed increased geothermal gradients in the sediment basins along the margin, which are explained with the effect of "thermal blanketing" through the thick insulating cover. Therefore, increased heat flow in these basins can also be regarded as a potential reason for the high electrical conductance values of cluster sed-A.…”

Lithospheric strength variations and seismotectonic segmentation below the Sea of Marmara

“…Overall, temperature observations from deep well logs would be a more suitable to validate lithospheric-scale 3-D thermal models in comparison to shallow geothermal gradients and surface heat-flow (e.g. Scheck-Wenderoth et al, 2014;Sippel et al, 2017;Gholamrezaie et al, 2018). Moreover, a good fit between observed and modeled temperature is found within the deeper parts of the 2,500-m-deep onshore well (Fig.…”

Formation and geophysical character of transitional crust at the passive continental margin around Walvis Ridge, Namibia

“…These sediments may have been transported along the shelf by bottom and surface currents and likely originate from cyclic erosion-deposition phases linked to the Orange, Kuiseb, and Swakop rivers (Dingle, 1992;Dingle et al, 1987;Goslin et al, 1974;Garzanti et al, 2018). Furthermore, Gholamrezaie et al (2018) and Maystrenko et al (2013) observed increased geothermal gradients in the sediment basins along the margin, which are explained with the effect of "thermal blanketing" through the thick insulating cover. Therefore, increased heat flow in these basins can also be regarded as a potential reason for the high electrical conductance values of cluster sed-A.…”