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Geological Society, London, Special Publications; 2003; v. 206; p. 363-379;
DOI: 10.1144/GSL.SP.2003.206.01.17
© 2003 Geological Society of London

The East African Oragen

Neoproterozoic deformation in central Madagascar: a structural section through part of the East African Orogen

Alan S. Collins1, Simon Johnson2,4, Ian C. W. Fitzsimons1, Chris McA. Powell2, Bregje Hulscher2, Jenny Abello2 & Théodore Razakamanana3

1 Tectonics SRC, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia alanc{at}lithos.curtin.edu.au
2 Tectonics SRC, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA 6907, Australia
3 Département des Sciences de la Terre, Université de Toliara, Toliara, Madagascar
4 Institute for Frontier Research on Earth Evolution, Japan Marine Science Technology Centre, 2-15 Natsushima-cho, Yohosuka, Kanagaua, 237-0061, Japan

The Itremo region of central Madagascar has an importance in the evolution of the East African Orogen (EAO) that belies its size. Unusually for the southern EAO (Mozambique Belt), it is made up of low-grade metasedimentary rocks and therefore preserves an almost unique window into upper crustal deformation during this key period of the Gondwana supercontinent cycle.

In this paper new field mapping of three linked regions in the Itremo Sheet and in the upper part of the underlying mid-crustal Antananarivo Block are presented. From these a complete structural section through the eastern Itremo Sheet is produced and the complex deformation record preserved there is then discussed.

An early deformation (D1) consists of 10 km scale recumbent isoclinal folds that predate intrusion of a c. 780–800 ma igneous suite. Metamorphic aureoles around these plutonic bodies overprint D1-related fabrics. Local deformation accompanies intrusion of the c. 780–800 Ma, suite (D2). Extensive E-W contractional deformation occurs between 780 and c. 570 Ma, that is here amalgamated as a composite D3 event, which includes thrusts and at least two phases of upright folds. Post-551 Ma, normal shearing (D4) marks the boundary between the Itremo Sheet and the underlying Antananarivo Block (the Betsileo Shear Zone), and may have also been responsible for formation of the Saronara Shear Zone. Finally, E-W open folding and dextral shear zone development marks a late N-S contractional event that is interpreted as a far-field response to collision between the northern Bemarivo Belt and central Madagascar.