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Styles of Paleoproterozoic and Neoproterozoic orogens Kenya / Tanzania.

From 2002 onwards our group accomplished three projects founded by the Austrian Science Foundation (FWF) in Kenya and Tanzania. Focus is to distinguish Paleoproterozoic and Neoproterozoic orogenic cycles and to characterize styles of continental collision tectonics during West- and East-Gondwana collision. This work contributes to the understanding of tectonics in the lower crust by combing structural and rheological data with petrology and geochronology.

Some outcomes - Central Tanzanian Tectonic Map:
On the basis of a compilation of geological maps from central Tanzania coupled with structural and petrological studies a distinction is made between the tectonic evolution of the 1.8–2.0 Ga Usagaran orogeny and the 650–580 Ma Pan-African orogeny in the Mozambique Belt. The geometry of both orogens is determined by displacement partitioning around the indenting Tanzania Craton. The Usagaran Belt formed by strike-slip tectonics in an island arc regime. By contrast, the Mozambique Belt formed by westward thrust propagation during oblique collision of east and west Gondwana. This resulted in a first stage of lower crustal strike slip with isobaric cooling in the eastern hinterland. Continuous forward propagation of thrusts and coeval hinterland extension accompanied an isothermal decompression phase in all units. Displacement partitioning along the oblique continental margin triggered the formation of two crustal-scale Neoproterozoic shear belts, the dextral Central Tanzanian Shear Belt and the sinistral Kiboriani Shear Belt.

Tectonic map of Central Tanzania (as file for CorelDraw Version 9)

Sequence of thrusting and palinspastic reconstruction of Tanzanian Nappes.

Some outcomes - Central Tanzanian Shear Belt:
The Central Tanzanian Shear Belt (CTSB) constitutes a 300-km long W-E trending dextral shear belt that was active during Paleoproterozoic and Neoproterozoic orogenies. It formed along the southern margin of the Archean Tanzania Craton that acted as rigid indenter during both orogenies. Based on structural and microtextural methods several deformational stages have been identified. Paleoproterozoic shear is recorded in the 1.8-2.0 Ga old magmatic Usagaran Belt. This shear was accompanied by vast melt intrusions in a hot and soft crust leading to low strain and coaxial flow. It ceased in an exhumation phase with localized non-coaxial shear followed by the deposition of nonconform sediments dated around 1.9 Ga. In the Neoproterozoic, the CTSB is found as reactivated shear zones in the Usagaran Orogen and as megascale shear zone in the metamorphic Mozambique Belt. It is a release (or counterflow) structure that evolved in the course of crustal thickening in a strong crust around 0.6 Ga. Along the strike of the Neoproterozoic CTSB syntectonic conditions change from localized brittle shear along the Tanzania Craton to distributed high temperature coaxial shear in the eastern section of the orogen’s root. This goes along with a significant change in microstructures and LPO patterns.

The geometry of the Neoproterozoic CTSB in map view showing changes in tectonic features (temperature, stress field, structural elements) from the Paleoproterozoic Usagaran Belt in the west to the Neoproterozoic Mozambique Belt in the east. s1, s3 are directions of principal stress axes, SR-subgrain rotation, GBM – grain boundary migration, Diff – diffusion mechanisms in quartz.

Selected publications.

For further papers see publications

Central Tanzanian tectonic map: A step forward to decipher Proterozoic structural events in the East African Orogen. H. Fritz, V. Tenczer, C. A. Hauzenberger, E. Wallbrecher, and G. Hoinkes.- TECTONICS, Vol. 24, TC6013, doi:10.1029/2005TC001796, 2005.

Two orogens - One shear belt: 1 Ga of repeated deformation along the Central Tanzanian Shear Belt: Veronika Tenczer, Harald Fritz, Andreas Bauernhofer, Christoph Hauzenberger.- Journal of Structural Geology 29 (2007) 1632-1649

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