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Nouvelles méthodes de cartographie sur le socle protérozoïque du Sud de Madagascar. Nature et géométrie de la croûte continentale d'un domaine orogénique en convérgence, implications économiques.

Abstract : The objective of this thesis is to investigate the petrologic nature and geometry of the outcropping Madagascar granulitic crust. We have created a geological database of the southern third of Madagascar (in the Madagascar Laborde 1923 coordinate system). The data used are varied: field observations, satellite images, topographic, gravity, radiometric and aeromagnetic data. The analysis, processing, comparison and integration of these different datasets (after being made compatible) have enabled us to create precise and reliable new geological maps of the region, as well as new map representations. We will highlight particular the areas of major and minor vertical shear zones (SZ), fractures, dykes and lithological boundaries.
Thus, we clarify the nature of the structural-lineaments, the so-called Bongolava – Ranotsara shear zone: the ductile structure oriented 140 °N is on a regional scale due to the North SZ deflections oriented N ° 157, and the “Ranotsara” fractures (orientation 140 ° N) does not exist in terms of regional fault, but is the results of faults relays with a size ≤ 60 km long which are oriented 115 °N, 140 °N and 160 °N.
SZ are the sites of localisation of anomalies rich in incompatible elements (U-Th). They are also the location of concentration of oxides of magnetite and titanomagnetite, together with the disappearance of biotite. These observations emphasize the concentration of magmas and the merger within the SZ. Thus, the SZ are focus the concentration of the magmas that provide heat by advection. In southern Madagascar, the conditions of high temperature (800 ° C) were homogenous over a wide area.
We establish a new lithological map, highlighting competency contrasts: SZ grow preferentially in the aluminous orthogneiss; aluminous metasediments and ampibolo-migmatites are more resistant and form boudins at different scales; all of the granites, syenites and stratoïdes amphibolo-migmatites are the most rigid blocks. We identify kilometers of dykes: basalt, dolerites and microgranite, they're preferentially oriented at 25 °N and 120 °N.
By cross-correlating our data, we show that the position of economic mineralization is controlled by the kilometer scale anisotropies (SZ, hinges folds, lithological boundaries) that localize interchange and magmatic fluids. The primary mineralization types (metamorphic or metasomatic: corundum, iron and graphite) are located within the SZ. The secondary mineralization types (uranothorianite, gold) are explained by remobilizations in the SZ. The final mineralization is controlled by the geometry of river systems and faults. On the scale of Gondwana, the geometry of Neoproterozoic major SZ controls the geometries of faults and dykes (170 Ma-90 Ma) which are oriented in 160 °N.
In four regions: Satrokala, Fenoarivo, Analavoka, Lavaraty; we identified magnetic and gravity "bulls eye" type anomalies which are located at the intersection of 160 °N oriented dykes with other ones oriented 30°N, 140 °N and 170 °N; and sometimes with SZ. These points correspond to circular topographic depressions (10 m - 2.5 km in diameter) isolated or in clusters. These areas are potential kimberlite intrusions. Their presence at the intersection of kilometric anisotropies supports the rapid ascent of silicate magmas, resulting in the possible presence of diamondiferous kimberlites.
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https://tel.archives-ouvertes.fr/tel-00404948
Contributor : Elisa Brice Randrianasolo <>
Submitted on : Monday, July 20, 2009 - 10:13:20 AM
Last modification on : Thursday, November 19, 2020 - 3:54:26 PM
Long-term archiving on: : Saturday, November 26, 2016 - 10:29:03 AM

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  • HAL Id : tel-00404948, version 1

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Elisa Brice Randrianasolo. Nouvelles méthodes de cartographie sur le socle protérozoïque du Sud de Madagascar. Nature et géométrie de la croûte continentale d'un domaine orogénique en convérgence, implications économiques.. Géologie appliquée. Université Joseph-Fourier - Grenoble I, 2009. Français. ⟨tel-00404948⟩

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