Tectonic wedging, back-thrusting and basin development in the frontal parts of the Mesoproterozoic Karagwe-Ankole belt in NW Tanzania

Abstract

Structural complexities in the Mesoproterozoic Karagwe-Ankole fold belt in northwest Tanzania have led to conflicting interpretations of regional kinematics and the geodynamic significance of the belt. Structural mapping of an eastern portion of the belt indicates that the regional-scale (>100km) Mugera-Nyakahura basement inlier may be considered a forethrusted tectonic wedge. Tectonic wedging in the frontal parts of the belt occurred during top-to-the southeast thick-skinned thrusting of the gneissic Archaean basement. The diagnostic feature of tectonic wedging is the reversal of vergence directions of kinematic fabrics on either side of the basement wedge, resulting in hinterland-directed, top-to-the northwest kinematics in front and on top of the wedge. Strain is localised into the often graphitic metapelitic rocks of the Upper Muyaga Group. The mainly coarse-grained clastic Mesoproterozoic sediments of the Bukoba Group represent the foreland, molasse-type deposits of the Karagwe-Ankole fold belt. The only gently folded Bukoba Group is separated from the underthrusted, highly deformed Muyaga Group by a passive roof thrust. This corresponds to the regional-scale asymmetry of the synclinal structure of the Bukoba basin in the frontal parts of the belt. The gentle folding is the result of the underthrusting and lifting of the Bukoba sediments above the basement wedge creating a triangle zone. The kinematics and geometry of the frontal parts of the Karagwe-Ankole belt described here confirm the belt to represent a top-to-the-east and -southeast verging foreland fold-and-thrust belt. The actual timing of deformation is, at present, unknown, but regional-scale kinematics and the metamorphic zonation are compatible with an origin of the belt during convergence between the Congo and Tanzania Cratons in the west.