Gravimetry and petrophysics for defining the intracratonic and rift basins of the Western-Central Africa zone

Abstract

The global gravity field obtained from the observations of the satellite Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite offers new opportunities in defining density variations of earth’s crust and mantle, allowing new insights into the structure of specific geologic features. The Central African Rift is a key feature in understanding the dismemberment of Gondwana, and we contribute to defining the crustal density structure underlying the rift. The presence of a narrow and up to 12 km deep basin implies crustal stretching allowed the sediment to accumulate, but a key question is whether the stretching processes also affected the deeper layers of the crust or was limited to the upper crust. The study area includes a subbasin of the greater Chad sag basin, which extends over 1500 × 1500 km and occupies the center of North-Central Africa, shared between the countries of Chad, Sudan, Nigeria, Niger, Algeria, Libya, and Cameroon. We find that the rifting affected the lower crust of the West African Rift, and we evaluate evidence for a 1500 km long and several km thick magmatic crustal intrusion presumably associated with underplating and crustal thinning. We estimate that the stretching factor must be at least 1.5 and had affected the entire crust. To our knowledge, the identification of a continuous body of magmatic intrusions is new and has been only possible through the recent global gravity field. The magmatism has altered the thermal conditions from the time of emplacement on, and it is relevant for the maturation of hydrocarbons present in the sediments. The timing of the magmatism is presumably tied to two pulses of volcanism documented in the rift, associated with the first postrift phase from 96 to 88 Ma and the second postrift phase from 23 Ma up to the Quaternary.