Geophysics as an Aid to Geological Mapping – A Case History from the Hoggar Region of Algeria

The Hoggar Region of Algeria, situated in the central part of the Sahara, is geologically complex and logistically difficult. Programmes of geological mapping and mineral exploration are being conducted by Algerian Government Agencies in which geophysical studies are playing useful roles. The region is covered by aerogeophysical surveys at regional scale. The interpretation of this data, integrated with the results of ground magnetic, radiometric and gravity traverses and of measurements of geophysical properties of outcrop samples is the first stage in the mapping programmes. Examples of results from the Timissao-Timgaouine region are presented. The several episodes of granite were identified and mapped by their radiometric expressions. The sequence of granites is interpreted, from older (early Pan African) to younger (Eocambrian), as Timgaouine granite, migmatitic granite, Abankor and Timissao magmatic granites and finally the intrusive granites of Taourirt and Bahouinet. The various magnetic bodies (conformable metavolcanics, diorite and gabbro intrusives, effusive complexes and mafic dikes) show characteristic aeromagnetic signatures by which they can be mapped. Source body parameters are calculated from magnetic and gravity data. The evolution of the Pharusian Chain west of the great 4°50' submeridional fault has been re-interpreted in the light of modern theories of global tectonics. In the Timgaouine region, the computed structures, the distribution of magnetic units and the systems of faults all support the hypothesis that the Pharusian Chain evolved as a back-arc basin during a period of separation of the West African Craton from the Touareg Shield as a result of the Pan African Orogeny.