Pár slov o geologii Madagaskaru
Madagascar can be conveniently described as comprising two geological entities:
the Precambrian crystalline formations (the 'crystalline basement') and the
Phanerozoic, non-metamorphosed sedimentary formations (the 'sedimentary cover').
1. Crystalline basement
The crystalline basement of Madagascar is composed of three blocks:
* Antongil block, comprising early Archaean granites, granodior ites,
migmatites and gneisses exposed in the north-east and extreme north of the
crystalline basement, and constituting the Bemarivo sheet.
* Bekily block in the extreme south of the crystalline basement, to the
south of Ranotsara structure. This is mainly composed of granulites and
six magmatic belts chacterised by phlogopite mica.
* Antananarivo block. This forms the remainder of the crystalline basement
and comprises migma tites, gneisses and granitoids, and two schist belts -
Tsaratanana and Itremo. Within the former, there are three greenstone
belts, evident in the Maevatanana, Andriamena, Befandriana, Alaotra and
Brickaville regions.
There are two major fault systems: Betsimisaraka and Betsileo, and, in terms
of the main structures, we should note:
* Angavo-lfanadiana shear-zone.
* Over-lapping fault structure of Bongolava-Ranotsara.
* Three shear-zones in the south (Bekily,Tranomaro and Arnpanihy/Vohibory).
2. Sedimentary cover
The sedimentary cover is essentially confined to a western coastal portion,
the extreme south of the island (in three areas:Ambilobe, Majunga and
Morondava, the latter being a very small area close to the eastern coast);
and areas of Neogene sediments away from the coast (including Ankay-Alaotra,
Antanifotsy, Sambaina and Vinanikarena).
Madagascar can be conveniently described as comprising two geological entities: the Precambrian crystalline formations (the 'crystalline basement') and the Phanerozoic, non-metamorphosed sedimentary formations (the 'sedimentary cover').
- Crystalline basement
The crystalline basement of Madagascar is composed of three blocks:- Antongil block, comprising early Archaean granites, granodior ites, migmatites and gneisses exposed in the north-east and extreme north of the crystalline basement, and constituting the Bemarivo sheet.
- Bekily block in the extreme south of the crystalline basement, to the south of Ranotsara structure. This is mainly composed of granulites and six magmatic belts chacterised by phlogopite mica.
- Antananarivo block. This forms the remainder of the crystalline basement and comprises migma tites, gneisses and granitoids, and two schist belts - Tsaratanana and Itremo. Within the former, there are three greenstone belts, evident in the Maevatanana, Andriamena, Befandriana, Alaotra and Brickaville regions.
There are two major fault systems: Betsimisaraka and Betsileo, and, in terms of the main structures, we should note:- Angavo-lfanadiana shear-zone.
- Over-lapping fault structure of Bongolava-Ranotsara.
- Three shear-zones in the south (Bekily,Tranomaro and Arnpanihy/Vohibory).
- Sedimentary cover
The sedimentary cover is essentially confined to a western coastal portion, the extreme south of the island (in three areas:Ambilobe, Majunga and Morondava, the latter being a very small area close to the eastern coast); and areas of Neogene sediments away from the coast (including Ankay-Alaotra, Antanifotsy, Sambaina and Vinanikarena).
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