Hot granulite nappes - Tectonic styles and thermal evolution of the Proterozoic granulite belts in East Africa

A section through the Neoproterozoic Mozambique Belt of Tanzania exposes western foreland (Archaean Tanzania Craton and Palaeoproterozoic Usagaran Belt), marginal (Western Granulites) and eastern, internal (Eastern Granulites) portions of the orogen. The assembly of granulite nappes at ca. 620 Ma displays westward emplacement along an eastward deepening basal decollement and forward propagation of thrusts, climbing from the deep crust to the surface. This goes along with eastward increase of syntectonic temperatures, derived from prevalent deformation mechanisms, and eastward decrease of the kinematic vorticity number. Distinctly different pressure - temperature paths with a branch of isothermal decompression (ITD) in Western Granulites and isobaric cooling (IBC) in Eastern Granulites reflect residence times of rocks within lower crustal levels. Western Granulites, exhumed rapidly at the orogen margin, display ITD and non-coaxial fabrics. Eastern Granulites in the internal orogen portions escaped from rapid exhumation and show IBC and co-axial flow fabrics. The vertical variation of structural elements, i.e. basement - cover relations within the Eastern Granulites, shows decoupling between lower and middle crust with horizontal west - east stretching in the basement and horizontal west - east shortening in the cover. A model of hot fold nappes [Beaumont, C., Nguyen, M.H., Jamieson, R.A., Ellis, S., 2006. Crustal flow modes in large hot orogens. in: Law, R.D., Searle, M.P., Godin, L. (eds). Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications. vol. 268, 91-145] is adopted to explain flow diversity in the deep crust. The lower crust represented by Eastern Granulite basement flowed coaxially outwards (westward) in response to thickened crust and elevated gravitational forces, supported by a melt-weakened, viscous channel at the crustal base. Horizontal flow with rates faster than thermal equilibration gave rise to isobaric cooling. Simultaneously the mid crust (Eastern Granulite cover) was shortened when hot fold nappes moved along upward climbing thrust planes. Western Granulites preserved isothermal decompression through exhumation by thrusting and coeval erosion at the orogen front. Two different styles define the Neoproterozoic East African Orogen between northern Egypt and southern Mozambique. The Arabian Nubian Shield in the north is classified as small and cold orogen in which thin - skinned thrusting was associated with lateral extrusion. The Central Mozambique Belt in Tanzania/Southern Kenya is classified as large and hot orogen characterized by thick-skinned thrusting and assembly of large granulite nappes. (C) 2009 Elsevier B.V. All rights reserved.