The Precambrian crystalline basement is divided into important regions by the major thrusting fault of Bongolava/Ranotsara (Rolin, 1991). This fault system has been active probably during Pan African orogenesis.
The reactivations of the faults inherited from the basement have initiated the breaking up of the Gondwanaland, occurring from the Paleozoic to the Mesozoic.
The geology of the basement is a subject to research which remains in process during these last decades.
Bésairie (1964) assumed that the basement would be a pilling up of sediments which would have been metamorphosed, but the present concept defines that the basement is fashioned by tectono-metamorphic events:
Bésairie has defined three major system which are from the top : Vohibory , Graphite and Androy.
Collins has put forward three tectono-metamorphic entities which are from the North to the South: Antongil block, Antananarivo block and Bekily block.
Madagascar crystalline basement is recognized by:
- a meridian major shear zone (Angavo), in which rock foliations became totally vertical and the areas of high intensity of deformation correspond to alkaline granites.
|MADAGASCAR BASEMENT GEO-TECTONICAL MAP
( Collin & al, 2000)
The shear zones in the southern Madagascar : Vohibory, Ampanihy and Tranomaro (Martelat, 2000).
The Betsimisaraka suture which bounds in the East the Lower Archean craton of Darw-har