Identifying the tectono-metamorphic overprints of a Gondwana forming collision: a structural and thermobarometric transect of the Southern Irumide and Zambezi belts, Zambia
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Abstract The Irumide Belt of eastern Zambia is one of a number of complex Proterozoic orogenic belts found in the African continent. The belt lies along the southeastern flank of the Tanzania-Bangweulu Craton and to the NW of the Mozambique Belt of East Africa. The dominant tectonic features of the belt are ductile shear zones, thrusts and folds related to a Mesoproterozoic, NW-SE crustal shortening event, and late-tectonic granite emplacement. Later tightening of Irumide structures and localized deformation of granites is also widespread. The timing of post-Irumide deformation is poorly constrained but is believed to be a mix of Pan-African and Late Palaeozoic reactivation and thermal reworking. This complex history is recently overprinted by significant Tertiary rifting. The Irumide deformation is evident in three tectonic zones stretching from the NW foreland of Mporokoso to the SE hinterland of Lundazi: (1) the NW-facing basement shear zones and associated cover deformation of the Luongo Fold Belt; (2) the Foreland Fold Belt of large upright folds defined by the kilometre-thick quartzite ridges of the Irumi, Shiwa Ngandu and Isoka hills; (3) an intensely deformed Internal Zone of basement and granite gneisses folded into major domes and associated tight synclines. Post-tectonic granites intrude the last of these zones and define an end to the Irumide deformation. The linking of these three zones is based on geometric, kinematic and geochronological evidence. The main Irumide deformation and metamorphism is believed to have occurred between 1100-950 Ma with a metamorphic peak between 1050 and 1000 Ma. Termination of Irumide deformation is dated by the emplacement of the post-tectonic granite and K/Ar biotite and hornblende closure dates of c. 950 Ma. Similar K/Ar dates from the Luongo shear zone indicate biotite closure and cooling during the period 1020-950 Ma. Together with parallel kinematic indicators, this supports a coeval development and tectonic link between the Irumide Belt and the Luongo Fold Belt. The intracontinental Irumide Belt deformation was driven by an accretionary collisional margin, to the SE of the present-day Irumide Belt, in southern Malawi and Mozambique. This accretionary margin is today represented by arc and island-arc magmatism and the complex terranes of the Mozambique Belt of East Central Africa.
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Abstract Gondwana amalgamated along a suite of Himalayan‐scale collisional orogens, the roots of which lace the continents of Africa, South America, and Antarctica. The Southern Granulite Terrane of India is a generally well‐exposed, exhumed, Gondwana‐forming orogen that preserves a record of the tectonic evolution of the eastern margin of the East African Orogen during the Ediacaran‐Cambrian (circa 600–500 Ma) as central Gondwana formed. The deformation associated with the closure of the Mozambique Ocean and collision of the Indian and East African/Madagascan cratonic domains is believed to have taken place along the southern margin of the Salem Block (the Palghat‐Cauvery Shear System, PCSS) in the Southern Granulite Terrane. Investigation of the structural fabrics and the geochronology of the high‐grade shear zones within the PCSS system shows that the Moyar‐Salem‐Attur shear zone to the north of the PCSS system is early Paleoproterozoic in age and associated with dextral strike‐slip motion, while the Cauvery shear zone (CSZ) to the south of the PCSS system can be loosely constrained to circa 740–550 Ma and is associated with dip‐slip dextral transpression and north side‐up motion. To the south of the proposed suture zone (the Cauvery shear zone), the structural fabrics of the Northern Madurai Block suggest four deformational events (D 1 –D 4 ), some of which are likely to be contemporaneous. The timing of high pressure‐ultrahigh temperature metamorphism and deformation (D 1 –D 3 ) in the Madurai Block (here interpreted as the southern extension of Azania) is constrained to circa 550–500 Ma and interpreted as representing collisional orogeny and subsequent orogenic collapse of the eastern margin of the East African Orogen. The disparity in the nature of the structural fabrics and the timing of the deformation in the Salem and the Madurai Blocks suggest that the two experienced distinct tectonothermal events prior to their amalgamation along the Cauvery shear zone during the Ediacaran/Cambrian.
Transpression
geodynamics
Geochronology
Orogeny
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Summary The Irumide Belt of Zambia is often quoted as being typical of African intracratonic mobile belts which involve no crustal shortening or major displacements. This belt is shown here to include the NW-facing foreland fold and thrust zone of the Southern Moçambique Belt and to have involved considerable crustal shortening. A crustal scale ‘pop-up’ structure separates this foreland from the internal higher-grade granulite-facies rocks of Malawi and Moçambique where major SE-facing nappe structures of Irumide age have been described. Several possible suture zones are identified and the Irumide-Moçambique Belt is interpreted as resulting from collisional processes during the Mid-Proterozoic. The adjacent Ubendian Belt represents a continental transform fault zone associated with crustal shortening across the Irumide Belt. The implications of these interpretations regarding the onset of plate tectonics in the Proterozoic and Archaean are briefly discussed.
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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.
Orogeny
Transpression
continental collision
Mountain formation
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