Geoelectrical signature of the Central Mauritanides Deep Structure, Mauritania, West Africa

Abstract

Magnetotelluric soundings have been carried out in southern Mauritania along a 225‐km long profile, extending from the Senegal‐Mauritania coastal basin through the Mauritanide orogenic belt to the West African craton. Results across the belt show a good correlation between upper electrical layers and the main lithostructural units, whose lateral and deep extensions are now better estimated. Soundings reveal a highly anomalous lower crustal unit involving two well defined conductors, one at 35 km in the eastern part of the belt and its foreland, and the second, more conductive and thicker, at a depth of 22 km in its western part. The latter has a resistivity of less than 15 ohm m and a thickness of the order of 14 km, and is probably associated with the thrust zone of mylonites and crush rocks with trapped pore water, representing the deep roots of the Mauritanide nappes. This zone is overlain by a westward‐dipping resistor, separating two tectonic blocks differing in their electrical characteristic. This resistor is inferred to be basic or ultrabasic rocks and may reflect the vestigal trace of a suture between two collided continental blocks. The geoelectrical model derived from magnetotelluric data alone is not unique, but the Mauritanide belt has a characteristic gravimetric signature which is compatible with a gravity model based upon the magnetotelluric model geometry. From the available geological and geophysical data, a schematic cross section for the Central Mauritanide crustal structure is presented.