The Tuareg Shield, to which Hoggar (southern Algeria) belongs, has a swell-shaped morphology of lithospheric scale of ~1000 km in diameter linked to Cenozoic volcanism occurring in several regions, including Atakor, the center of the swell, which reaches nearly 3000 m in altitude. The lack of high-resolution geophysical data for constraining its deep structure is at the origin of a controversy about its innermost nature and about the origin of the Cenozoic volcanism. During the course of this study, magnetotelluric (MT) broadband data were collected at 18 sites forming a northeast-southwest profile 170 km long within the Atakor region. The electrical resistivity model obtained by inverting the magnetotelluric data reveals lithospheric structure down to a depth of ~100 km. From this depth to the surface, the model does not show any regional anomaly that may result from a metasomatized lithosphere or from an asthenospheric upwelling, including a mantle plume. MT data reveal rather a lithosphere affected by a set of rather thin subvertical conductors that can be attributed to the electrical signature of some known shear zones resulting from the Pan-African evolution of the LATEA metacraton, which globally corresponds to the uplifted Central Hoggar swell. The main anomaly is deeply rooted in the lithosphere and underlies the Atakor-Manzaz volcanic districts. As a whole, MT data are therefore properly integrated within the hypothesis of the reactivation of shear zones due the intraplate deformation related to the collision between Africa and Europe since the Eocene, applied onto the metacratonic region.

The Atakor massif is a part of the Hoggar volcanic province, which was emplaced on top of a basement swell initiated during the Cretaceous. There have been three main episodes of volcanic activity since the Miocene, separated by long periods of quiescence. The lava flows and domes were emitted along lithosphere-scale fault zones. With its famous scenery, the Atakor massif is one of the largest (2150 km 2 ) volcanic districts of the province. Mafic volcanic rocks are abundant in the center of the massif, but become scarce to the south, where only few scarps are observed. Phonolites occur only in the Assekrem area, whereas trachytes occur everywhere, with a marked enrichment in quartz to the south and the southeast (Tahifet area), where rhyolites are also exposed. Two magmatic groups have been identified based on field and petrological observations. The mafic group has a basanite-phonotephrite association, forming uplifted plateaus, scoria cones, and valley-filling lava flows. The presence of mantle-derived amphibole ± biotite megacrysts and peridotite mantle xenoliths together with the nonprimary chemical compositions of the magmatic rocks suggest that magmatic differentiation may have occurred within the upper mantle. The felsic group is composed of two diverging trends, a silica-saturated benmoreite-trachyte-rhyolite trend and a silica-undersaturated trachyte-phonolite trend. The primary magmas are considered to have been produced as a consequence of lithospheric mantle delamination along linear megashear zones inducing low degrees of decompression partial melting at variable depths (110–40 km) in the upwelling asthenosphere. The discrete volcanic episodes correspond to periods of reactivation of the major fault zones in response to discrete Neogene extensional tectonic events associated with Alpine orogenesis in the Western Mediterranean region induced by Africa-Eurasia collision.