Use of the magnetotelluric method for a better understanding of the West African Shield

Abstract

Magnetotelluric soundings have been carried out in the Republic of Upper Volta and in the Niger Republic to gain a better understanding of the structure of the West African shield. The sounding stations are situated from east to west on a sedimentary basin on the Central African mobile belt (Pan‐African orogeny at 550±100 m.y.) and on the West African craton (Eburnean orogeny at 1850±250 m.y.). The results show that the craton is characterized by a zone of high resistivity in the crust and uppermost mantle. The absence of a conductive zone at the interface between the crust and the uppermost mantle is consistent with the hypothesis of Hyndman and Hyndman (1968) on the dehydration of the crust of the stable shields. The first conductive layer of the craton is situated at a depth of 130 km with a temperature of about 860°C. In the mobile belt and the basin the presence of a low resistivity layer at a depth of 30–40 km has been established. If water is present in the lower crust, it can explain the origin of this conductive zone. A plausible explanation is that this layer may have been formed by the slow infusion of water from the mantle during the last thermal reactivation. Regional differences in electrical conductivity structure between the Central African mobile belt and West African craton appear to extend deeper than 200 km. Modeled conductivity variations with depth in the upper mantle obtained from magnetotelluric results are compared to other continental models (thermal model‐seismic structure) in order to define a lower lithospheric boundary for West Africa. At the present stage the problem is not yet resolved (mobile crustal plate or anchorage in the African plate).