Geodynamic evolution of the Pan-African orogenic belt: A new interpretation of the Hoggar shield (Algerian Sahara)

Abstract

The Pan-African orogenic belt of Hoggar, 800 km wide, represents an extremely tightened complex and composite mobile zone. In the western part, the earlier thick meta-sedimentary units and alkaline-peralkaline intrusives, both of middle Proterozoic age, account for the early mobility of a N-S trending ensialic domain. Later, large scale upper-mantle contribution was responsible for many magmatic complexes of basic to ultrabasic rocks intruded at c. a. 800 m. y. in cratonic platform sediments 1000 to 800 m. y. old. A very important volume of volcanoclastic deposits, andesites to dacites, and widespread calc-alkaline batholiths are supposed to derive from two assemblages of island-arc type upon basic crust and of Andean type upon granulite basement and this newly accreted material 800 to 650 m. y. old, may be related to subduction zones dipping East. A cryptic suture ist postulated along the margin of the West African craton. The central Hoggar is mainly composed of pre-Pan-African gneisses belonging to the Eburnean cycle andpro parte formed during a “Kibaran” cycle, and ensialic upper Proterozoic schist belts, which were subjected at varying degrees to the Pan-African deformation and metamorphism. Eastern Hoggar includes also pre-Pan-African gneisses and granites, and the narrow ensialic upper Proterozoic Tiririne belt formed during the late Pan-African along a N-S trending major shear zone. High structural level nappes, recumbent folding, anatexis and syn-kinematic granites emplaced at c. a. 650 m. y. define the early Pan-African, related to crustal thickening and crustal melting during the early stage of a continental collision. The late Pan-African E-W compression produced in the whole shield N-S trending folds geometrically linked to a mega-system of shear belts and strike-slip faults, which dissect the earlier edifices into N-S trending branches and blocks of many hundred kilometres lateral displacement. This pattern is the result of an extreme E-W tightening of the belt between two rigid plates: the West African craton and the East Saharian craton colliding with a sinistral confrontation. During this stage (600–550 m. y.) an unquantified E-W shortening may have been absorbed by lateral movements along conjugate shear zones with a main sinistral component. Mineral ages and diagenesis of molassic deposits point out to a post-Cambrian uplift of the belt.