Electrical conductivity constraints on the geometry of the western LATEA boundary from a magnetotelluric data acquired near tahalgha volcanic district (Hoggar, Southern Algeria)

Abstract

The lithospheric structure of the Hoggar massif remains relatively unknown. The lack of high-resolution geophysical studies devoted to it is the source of controversial debates about its underlying structure and its geodynamics. This study targets the western edge of the LATEA microcontinent at the intersection of the 4°50′ sub-meridian major fault and the 4°35′ fault with the oued Amded NE–SW lineament. The study area also includes the northern flank of the Tahalgha Cenozoic Volcanic District. The analysis and modeling of magnetotelluric data collected at 12 sites forming an EW profile of 75 km length made it possible to build a resistivity model over a hundred km of depth. The magnetotelluric data reveal a heterogeneous upper crust made up of probably very compact and mechanically strong, electrically resistive structures corresponding to hypovolcanic granitoid and batholiths, and others more conductive probably more inhomogeneous, weak and more fractured corresponding to the Paleoproterozoic metamorphic basement. On the contrary, the lower crust and the lithospheric mantle down to a depth of about 100 km are fairly homogeneous. The 4°50′ mega-fault is rooted into the lithospheric mantle to a depth of about 70 km. This is corroborated by potential field data to at least the base of the crust. By comparison, the 4°35′ fault does not appear as important. The fault network highlighted by the magnetotelluric data has probably been used to transport melt from the asthenosphere up to the surface to give rise to the Tahalgha volcanic district. The fluids released or the precipitated mineralization are at the origin of the strong fall of the resistivity which gives a signature so peculiar to these faults.