The structure of a Mesozoic basin beneath the Lake Tana area, Ethiopia, revealed by magnetotelluric imaging

The northwestern Plateau of Ethiopia is almost entirely covered with extensive Tertiary continental flood basalts that mask the underlying formations. Mesozoic and Tertiary sediments are exposed in a few locations surrounding the Lake Tana area suggesting that the Tana depression is an extensional basin buried by the 1–2 km thick Eocene–Oligocene flood basalt sequences in this region. A magnetotelluric survey has been carried out to investigate the deep structure of the Tana area. The objectives were to estimate the thickness of the volcanics and anticipated underlying sedimentary basin. We have collected 27 magnetotelluric soundings south and east of Lake Tana. Two-dimensional inversion of the data along a 160 km long profile gives a model consistent with a NW–SE trending sedimentary basin beneath the lava flows. The thickness of sediments overlying the Precambrian basement averages 1.5–2 km, which is comparable to the Blue Nile stratigraphic section, south of the area. A 1 km thickening of sediments over a 30–40 km wide section suggests that the form of the basin is a half-graben. It is suggested that electrically resistive features in the model are related to volcanic materials intruded within the rift basin sediments through normal faults. The results illustrate the strong control of the Precambrian fracture zones on the feeding of the Tertiary Trap series.