Thermal regime and evolution of the Congo Basin as an intracratonic basin

Abstract

The Congo Basin (CB) lies over a thick (200-250 km) and cold lithosphere: We estimate the present-day surface heat-flow to 40 ± 5 mW m−2, from the BHT temperatures, lithology and porosity recorded in two oil wells and in agreement with the only measurement in this area (44 mW m−2). This value is consistent with the thickness of the lithosphere inferred from seismic tomography, assuming stationary conditions. The paleo-thermal regimes can be constrained by additional information, such as the pressures and temperatures derived from kimberlites studies, the variations of vitrinite reflectance with burial or the reconstructed subsidence history. The pressures and the temperatures derived from kimberlites xenocrysts suggest that the conditions were similar for at least 120 Myr. The long-term subsidence can be interpreted by the thermal relaxation of a thick lithosphere after a Neo-Proterozoic rifting stage (ca. 700-635 Ma) with a thinning factor β = 1.4 and a possible reactivation during the Karoo period (ca. 320 Ma). Because the magnitude of the crustal thinning is small, the past thermal conditions throughout the Phanerozoic were probably not very different from the present-day. Additional short term variations (~20-40 Myr) of the subsidence are interpreted by dynamic subsidence or uplift caused by sublithospheric mantle instabilities at the transition between litho-spheres of different thicknesses. These short term variations should not be associated with significant thermal changes. In order to explain the observed maturation of the vitrinite as well as angular uncomformities on seismic lines, one should assume one or two stages of erosion in the basin, representing at least 4 km of removed material. Heat advection by hydrothermal or volcanic fluids can conversely reduce the magnitude of this erosion.