A laterally extensive, Neoproterozoic extensional detachment (the Betsileo shear zone) is
recognized in central Madagascar separating the Itremo sheet (consisting of Palaeoproterozoic to
Mesoproterozoic sediments and underlying basement rocks) from the Antananarivo block
(Archaean/Palaeoproterozoic crust re-metamorphosed in the Neoproterozoic). Non-coaxial deformation
gradually increases to a maximum at a lithological contrast between the granitoids and gneisses of
the footwall and the metasedimentary rocks of the hangingwall. Ultramylonites at this highest-strained
zone show mineral-elongation lineations that plunge to the southwest.
σ-, δ- and C/S-type fabrics imply top-to-the-southwest extensional shear sense. Contrasting metamorphic
grades are found either side of the shear zone. In the north, where this contrast is greatest,
amphibolite-grade footwall rocks are juxtaposed with lower-greenschist-grade hangingwall rocks. The
metamorphic grade in the hangingwall increases to the south, suggesting that a crustal section is preserved.
The Betsileo shear zone facilitated crustal-scale extensional collapse of the East African Orogeny,
and thus represents a previously poorly recognized structural phase in the story of Gondwanan amalgamation.
Granitic magmatism and granulite/amphibolite-grade metamorphism in the footwall are all
associated with formation of the Betsileo shear zone, making recognition of this detachment important
in any attempt to understand the tectonic evolution of central Gondwana.