Structural and tectonic evolution of the Marampa Group fold thrust belt, northwestern Sierra Leone, West Africa

The structural evolution of the Marampa Group, a poorly described 60 km wide NNW trending fold thrust belt in west central Sierra Leone is presented in detail for the first time. The Marampa Group is flanked by the Rokel River Group to the east and gneisses and amphibolites of the Kasila Group to the west. Previous workers have interpreted this mobile belt as a Klippe of the Kasila Group derived from somewhere west of its present outcrop or as an autochthonous volcano-sedimentary deposit engulfed by granitic basement. The upper age of deformation is constrained by the Precambrian to Ordovician Saionya Scarp Group sedimentary rocks which lie unconformably on the Rokel River Group. The earliest structures consist of flat lying thrusts which transported Marampa Group metasediments and basal metavolcanics eastward over a granitic gneiss basement. The thrusting was associated with the development of a pervasive L-S fabric containing a greenschist facies metamorphic assemblage in the foreland but developing an amphibolite facies metamorphic assemblage closer to the hinterland due to late thrust stacking. This early event was followed by extensional collapse and intrusion of porphyroblastic granites. Erosion of the uplifted mass sourced the Rokel River Group deposited in flanking basins marking a renewed phase of extension. Renewed east-west crustal shortening ascribed to the Pan-African event inverted earlier extensional structures thrusting the Rokel River Group westward over the Marampa Group and leading to local E-W facing confrontations. This compressive phase was accompanied by metamorphic changes of a lesser intensity showing a similar east to west gradient characterised by very low greenschist facies metamorphism in the east and upper greenschist facies metamorphism in the west, this time because a deeper level of the crustal pile was thrust faulted over the Marampa Group in the west in comparison to the eastern areas. Subsequently the eastern flanks of the mobile belt were subject to a NNW trending ductile and brittle sinistral strike-slip regime that locally produced ENE oriented folds and thrusts and low greenschist facies metamorphic reconstructions in the vicinity of late shear zones.