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AFR-01 Pan-African orogeny in Africa and adjacent regions
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Decoupled deformation in the deep crust - constraints from the Eastern Granulites of northern Tanzania
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Harald Fritz, Department of Earth Sciences (Austria)
Veronika Tenczer, Department of Earth Sciences (Austria)
Christoph Hauzenberger, Department of Earth Sciences (Austria)
Sospeter Muhongo, ICSU Regional Committee for Africa (South Africa)
Eckart Wallbrecher, Department of Earth Sciences (Austria)
Georg Hoinkes, Department of Earth Sciences (Austria)
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The Eastern Granulites in Tanzania constitute a deep crustal section within the Neoproterozoic Mozambique Belt. They are exposed along a North -South trending chain and referred to as Pare-Usambara Granulites (north), Uluguru Granulites (central) and Mahenge Granulites (south). All occurrences display a structural basement, largely composed of metamorphosed magmatic rocks and a metasedimentary cover containing marbles and schists. A conspicuous feature is that basement rocks display a rather uniform penetrative deformation which contrasts cover rocks that display complex folding. These structural heterogeneities elaborated from the northern granulites are used to present a model of flow diversity in the deep crust.
The structural basement in the northern Eastern Granulites is exposed in the Pare-Usambara Mountains and extends further west to the Massai Steppe. Homogenous high temperature West-East stretching is associated with vast melt extraction. Deformation in eastern domains occurs at major coaxial flow, high temperature thrusting associated with westward emplacement of large granulite nappes dominates the west where the Eastern Granulites override the Western Granulites. By contrast to basement domains, the structural cover displays complex folding arising in fold interference pattern best visible in the Lelatema marble fold exposed in the Massai Steppe. Geometric modelling suggests polyphase folding with (1) isoclinal intrafolial folding of variable orientation and (2) tight folding with North - South trending axes and vertical axial planes. The axial plane cleavage formed during this event developed at amphibolite facies metamorphic conditions associated with pronounced fluid flow and is likely responsible for Tanzanite gemstone formation. (3) Open folds with NNW plunging axes and vertical axial planes and (4) open folds with West-East axes and vertical axial planes overprint former folding phases. Superposition of all folding phases resulted in an oblique dome and basin geometry known as Ramsey type 1-2 fold interference pattern. The main folding phases responsible for the map scale geometry of the Lelatema fold is result of West-East shortening.
A consistent model for lower to mid-crustal deformation within the Eastern Granulites has to explain contrasting styles between structural basement and cover, namely West-East horizontal stretching in the structural basement and West-East shortening in the structural cover. We explain this situation with the model of hot fold nappe evolution (Beaumont et al. 2006). The lower crust flows coaxially outwards (westward) as response to thickened crust and elevated gravitational forces supported by a melt weakened, viscous channel at the crustal base. Simultaneously the mid crust is shortened when hot fold nappes move along upward climbing thrust planes. A pronounced decollement level is suggested to have developed along the basement - cover boundary to accommodate contrasting deformational styles.
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