Stratigraphy and tectonic evolution of the palaeoproterozoic usagaran belt in south-eastern tanzania

A.H. MRUMA, Department of Geology, University of Dar es SalaamFollow

Section

Physical Sciences

Abstract

The palaeoproterzoic Usagaran Belt is bordered by the Archaean Tanzanian Craton to the NW and the Mozambique Belt to the east. It is made up of the following three stratigraphic units:- (i) The lithodemic sillimanite, kyanite and cordierite gneiss lithodemes and pyroxene-garnet amphibolite lithodemes (ii) The lithostratigraphic Konse Group : greenschist to lower amphibolite facies metasedimentary and metavolcanic formations. (iii) The metavolcanic Ndembera Group; a possible lateral equivalent of the konse. Furthermore, the belt contains a post-tectonic gabbro-granitoid suite. The mutual stratigraphic relationship of the konse and Isimani has been a matter of dispute. However, the field observations described in this paper proves that the Konse was deposited unconformably upon the Isimani. The Usagaran Orogen was affected by three major phases of deformation (D₁, D₂ and D₃). The initial deformation (D₁) occurred prior to the deposition of the Konse Group and affected only the Isimani Suite in which a compositional banding (S₁) was formed. The second phase of deformation (D₂) which postdates Konse deposition is regarded as the main deformation even of the belt and it produced characteristic isoclinal folds (F₂) which deform the S₁ of the Isimani Suite and the bedding surfaces of the Konse Group. It also produced pervasive S₂-mineral foliation to the Isimani Suite parallel to the axial planes of the F₂-folds. The D₂ event also produced large scale shear and thrust zones and faults trending sub-parallel the S₂ foliation. The third phase (D₃) gave birth to ENE-and NNW –striking conjugate faults with associated large scale drag folds and crenulations in micaceous rocks. The geotectonic evolution of the Usagaran Belt is explained by the rifting, delamination and A-subduction model of Kroner (1983). In this model the Isimani Suite represents a volcanic-sedimentary assemblage formed in an extensional basin situated between the Tanzania Craton to the north and the Zimbabwe craton to the south. At the beginning of the collision stage, the Isimani Suite, together with the Zimbabwe Craton, overrode the Tanzania Craton leading to the flexuring of the latter to form a peripheral foreland basin on its south-eastern margin. The Konse and Ndembera Groups were then deposited in this basin. Total collision (the locking stage) is correlated with the main phase (D₂) of deformation in the Usagaran Belt. After locking, the change of orientation of 1-principal stress axis from SE to NE gave rise to the D₃ conjugate fault system.

Recommended Citation

MRUMA, A.H. (1995) "Stratigraphy and tectonic evolution of the palaeoproterozoic usagaran belt in south-eastern tanzania," Tanzania Journal of Science: Vol. 21: Iss. 1, Article 1.
Available at:https://doi.org/10.65085/2507-7961.2419