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.
Highlights
Comprehension of the lithosphere-scale thermal state is a key to unraveling the evolution, strength, and physical and chemical processes of the lithosphere (e.g., Davies, 1980; Chapman, 1986; Artemieva and Mooney, 2001; ScheckWenderoth and Lamarche, 2005; McKenzie et al, 2005; Ebbing et al, 2009)
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
By comparing the calculated geothermal gradients of these margins, we address the consequences of the lateral heterogeneities for the thermal field and test the hypothesis that the present-day thermal field is different for the two settings and determined by the lithospheric mantle characteristics
Summary
Comprehension of the lithosphere-scale thermal state is a key to unraveling the evolution, strength, and physical and chemical processes of the lithosphere (e.g., Davies, 1980; Chapman, 1986; Artemieva and Mooney, 2001; ScheckWenderoth and Lamarche, 2005; McKenzie et al, 2005; Ebbing et al, 2009). The lithospheric thermal field generally depends on the thermal thickness and the thermal properties of the lithosphere This has been deduced from continental crustal geotherm (Pollack, 1986; McKenzie and Bickle, 1988; Rudnick and Nyblade, 1999; Kaminski and Jaupart, 2000; Artemieva and Mooney, 2001; Artemieva, 2006; Jaupart and Mareschal, 2007; Mareschal and Jaupart, 2013) and from plate cooling models explaining oceanic heat flow patterns and seafloor depth evolution (Parsons and Sclater, 1977; Johnson and Carlson, 1992; Stein and Stein, 1992; Goodwillie and Watts, 1993; DeLaughter et al, 1999; Watts and Zhong, 2000; Crosby et al, 2006; Crosby and McKenzie, 2009). Gholamrezaie et al.: The Southwest African and the Norwegian margins ent lithospheric layers (summary in Allen and Allen, 2005; Turcotte and Schubert, 2014) The interaction of these controlling factors complicates predictions of temperature increase with depth. One well-established strategy to investigate the present-day thermal field of a certain area is to integrate existing geophysical and geological data into 3-D structural models that provide the basis for numerical modeling, which simulates heat transport processes after setting boundary conditions and thermal properties according to the geological structure (e.g., Scheck-Wenderoth and Lamarche, 2005; Noack et al, 2013; Scheck-Wenderoth et al, 2014; Sippel et al, 2015; Balling et al, 2016)
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