International Geologiical Congress - Oslo 2008

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HPP-07 Late Neoproterozoic orogenic belts and assembly of Gondwana

 

Mechanically versus thermally driven orogens - contrasting styles along the Neoproterozoic East African Orogen

 

Harald Fritz, Department of Earth Sciences (Austria)
Christoph Hauzenberger, Department of Earth Sciences (Austria)
Veronika Tenczer, Department of Earth Sciences (Austria)
Sospeter Muhongo, ICSU Regional Committee for Africa (South Africa)
Georg Hoinkes, Department of Earth Sciences (Austria)
Eckart Wallbrecher, Department of Earth Sciences (Austria)
 

 

The East African Orogen (EAO) between Egypt and Mozambique is classically subdivided into the northern Arabian Nubian Shield (ANS) and the southern Mozambique Belt (MB). The orogen consolidated during the late Neoproterozoic by island arcs accretion (ANS) and collision of West and East Gondwana fragments (MB). Both portions of the EAO differ significantly in crustal structure and tectonic styles. The ANS comprises mainly juvenile Neoproterozoic crust with large volumes of island arc magmatics and remnants of ophiolites whereas the MB may be defined as recycled orogen involving Archean and Paleoproterozoic fragments. Transcurrent motion with orogen parallel strike-slip and northward extrusion tectonics dominates the ANS (Stern 2002) whereas orthogonal collision with westward thrusting characterizes the MB (Fritz et al. 2005).

Both, tectonic style and degree of crustal recycling determine the general style of the orogen when displayed in an orogen temperature versus orogen magnitude diagram (TM: Beaumont et al. 2006). Since orogen magnitude translates into operational availability of gravitational forces and orogen temperature into bulk viscosity the TM diagram includes information about dominant mechanisms driving the orogen. The style of cold and small orogens is largely determined by external mechanical forces, the style of large and hot orogens by internal body forces. This in turn relates with different tectonic styles, timing between metamorphism and deformation, shapes of P-T loops and dominant deformation mechanisms. For the ANS between Egypt and northern Kenya, an example for a small and cold trancurrent orogen, we suggest: (1) Extensional structures are dominantly developed and associated with magmatic activity. The temperature was rising during decay of the orogen. (2) Dominant structural imprint occurred after peak metamorphism. (3) Characteristic is a beta-shaped P-T loop that developed as a consequence of temperature rise during extension. (4) Deformation is localized into distinct domains and continues till final cooling of rocks. (5) Deformation mechanisms in quartz range from grain boundary migration to cataclasis and show rarely static annealing. Orogenesis in the juvenile ANS is mechanically driven by oblique motion of Arabian derived fragments relative to the Meta-Sahara Craton (external forces).

Tanzania, an example for a large and hot collisional orogen, we suggest: (1) A thick crust evolved by stacking of crustal nappes. The temperature rises coevally with crustal thickening. (2) Dominant structural imprint is coeval with peak metamorphism. (3) Granulites with a characteristic branch of isobaric cooling typify the lower crust. (4) Deformation is highly penetrative. (5) Melt enhanced deformation followed by static annealing defines a low viscose lower crust. Orogenesis in the MB shows low viscosity flow in the lower crust due to enhanced gravitational forces (internal forces).

 

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