Analyses de la dégradation du lac Kinkony pour la conservation du Complexe des Zones Humides Mahavavy-Kinkony, Région Boeny, Madagascar

The Mahavavy-Kinkony wetland complex is an ecologically diverse and economically critical habitat assemblage. Despite recently receiving national protection, the ecological integrity of Lake Kinkony is threatened by the conversion of adjacent, unprotected lands into rice paddies. Conversion to aqua-cultural lands eliminates reed beds which provide favoured habitat for numerous endemic and endangered avian, fish and reptilian species, including Amaurornis olivieri, Paretroplus dambabe, P. kieneri and Erymnochelys madagascariensis. This research identified the physio-chemical sources and extent of degradation and evaluated the associated impacts on endangered wildlife. Employing digital environmental data within a Geographic Information System, the historical extent of reed habitat (circa 1949) was compared to distribution identified through fieldwork in 2008, indicating an areal loss of 80%. Results indicate the primary contributor to reed loss was increased turbidity associated with erosion. The continuing decrease in marsh habitat facilitated by an increase in erosion poses significant threats to already endangered wildlife. However, while the reduction in marsh area was shown to reduce habitat availability for Amaurornis olivieri, observations imply this species does not occupy all potential reed habitats and that territorial preferences and tolerance towards turbidity need to be understood to accurately determine the magnitude of threat. Despite the need for continued research, information representing agents of change and their associated ramifications on fauna is essential for developing regional conservation and natural resource management strategies. In particular, anti-erosion management of the most vulnerable water catchment areas and restoration activities within the most severely degraded marshes are prescribed. RÉSUMÉ Le lac Kinkony fait partie des habitats clefs pour la biodiversité du Complexe des Zones Humides Mahavavy-Kinkony, une des Nouvelles Aires Protégées prioritaires qui doit être mise en place à Madagascar. Malgré les pressions anthropiques comme la conversion des zones marécageuses en rizières et la surpêche qui s’exercent sur le lac, le lac joue un rôle économique et écologique important dans la région. La présente recherche a permis d’identifier les changements physico-chimiques importants de ce biotope lacustre afin d’évaluer les menaces qui pèsent sur sa faune afin de fournir aux décideurs les éléments nécessaires à l’élaboration des stratégies de conservation de ses ressources biologiques. Les analyses ont été basées sur l’utilisation des outils d’investigation spatiale comme le Système d’Information Géographique et la Télédétection, appliquées plus particulièrement sur le lac Kinkony, à savoir le plan d’eau et les zones de phragmites de son pourtour marécageux. Les résultats obtenus, nous ont permis d’estimer une forte réduction de la superficie occupée par les phragmites, de l’ordre de 80% entre 1949 et 2008. Les principales sources de sédiments qui contribuent à la turbidité de l’eau du lac proviennent de l’érosion des bassins versants situés au Sud du lac. Les dépôts apportés par les rivières au cours des temps ont provoqué l’envasement du lac et la réduction progressive de la profondeur des zones d’entrée, au niveau de la confluence du plan d’eau avec ces rivières. La réduction de la superficie des phragmitaies couplée à l’érosion constituent ainsi les principales menaces écologiques qui affectent le lac Kinkony. Si l’évaluation précise du degré d’impact de ces changements et leur éventuelle synergie sur la biologie de la faune menacée requièrent de plus amples recherches, des aménagements anti-érosifs sur les quatre bassins environnants les plus vulnérables et des restaurations de phragmitaies sont proposés pour la conservation de la biodiversité du lac Kinkony

Geological evolution of the Neoproterozoic Bemarivo Belt, northern Madagascar

The broadly east-west trending, Late Neoproterozoic Bemarivo Belt in northern Madagascar has been re-surveyed at 1: 100 000 scale as part of a large multi-disciplinary World Bank-sponsored project. The work included acquisition of fourteen U-Pb zircon dates and whole-rock major and trace element geochemical data of representative rocks. The belt has previously been modelled as a juvenile Neoproterozoic arc and our findings broadly support that model. The integrated datasets indicate that the Bemarivo Belt is separated by a major ductile shear zone into northern and southern “terranes”, each with different lithostratigraphy and ages. However, both formed as Neoproterozoic arc/marginal basin assemblages that were translated southwards over the north-south trending domains of “cratonic” Madagascar, during the main collisional phase of the East African Orogeny at ca. 540 Ma. The older, southern terrane consists of a sequence of high-grade paragneisses (Sahantaha Group), which were derived from a Palaeoproterozoic source and formed a marginal sequence to the Archaean cratons to the south. These rocks are intruded by an extensive suite of arc-generated metamorphosed plutonic rocks, known as the Antsirabe Nord Suite. Four samples from this suite yielded U-Pb SHRIMP ages at ca. 750 Ma. The northern terrane consists of three groups of metamorphosed supracrustal rocks, including a possible Archaean sequence (Betsiaka Group: maximum depositional age approximately 2477 Ma) and two volcano-sedimentary sequences (high grade Milanoa Group: maximum depositional age approximately 750 Ma; low grade Daraina Group: extrusive age = 720 to 740 Ma). These supracrustal rocks are intruded by another suite of arc-generated metamorphosed plutonic rocks, known as the Manambato Suite, 4 samples of which gave U-Pb SHRIMP ages between 705 to 718 Ma. Whole-rock geochemical data confirm the calc-alkaline, arc-related nature of the plutonic rocks. The volcanic rocks of the Daraina and Milanoa groups also show characteristics of arc-related magmatism, but include both calc-alkaline and tholeiitic compositions. It is not certain when the two Bemarivo terranes were juxtaposed, but ages from metamorphic rims on zircon suggest that both the northern and southern terranes were accreted to the northern cratonic margin of Madagascar at about 540 to 530 Ma. Terrane accretion included the assembly of the Archaean Antongil and Antananarivo cratons and the high-grade Neoproterozoic Anaboriana Belt. Late- to post tectonic granitoids of the Maevarano Suite, the youngest plutons of which gave ca. 520 Ma ages, intrude all terranes in northern Madagascar showing that terrane accretion was completed by this time

Analyse des facteurs de l'érosion en lavaka par télédétection et système d'information géographique (rôle du drainage le long des cours d'eau)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Neoproterozoic extension in the Greater Dharwar Craton: a reevaluation of the "Betsimisaraka suture" in Madagascar

International audienceThe Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U-Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil-Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo-Mesoarchean (3.32-3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55-2.48 Ga), involving emplacement of juvenile igneous rocks, high-grade metamorphism, and the juxtaposition of disparate belts of mafic gneiss and schist (metagreenstones). The concept of the ''Betsimisaraka suture'' is dispelled and the zone is redefined as a domain of Neoproterozoic metasedimentary (Manampotsy Group) and metaigneous rocks (Itsindro-Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Younger orogenic convergence (560-520 Ma) resulted in east-directed overthrusting throughout south Madagascar and steepening with local inversion of the domain in central Madagascar. Along part of its length, the Manampotsy Group covers the boundary between the eastern and central Archean domains and is overprinted by the Angavo-Ifanadiana high-strain zone that served as a zone of crustal weakness throughout Cretaceous to Recent times

Cover sequences at the Northern Margin of the Antongil Craton, NE Madagascar.

The island of Madagascar is a collage of Precambrian, generally high-grade metamorphic basement domains, that are locally overlain by unmetamorphosed sedimentary rocks and poorly understood lowgrade metasediments. In the Antalaha area of NE Madagascar, two distinct cover sequences rest on high-grade metamorphic and igneous basement rocks of the Archaean Antongil craton and the Neoproterozoic Bemarivo belt. The older of these two cover sequences, the Andrarona Group, consists of low-grade metasedimentary rocks. The younger sequence, the newly defined Ampohafana Formation, consists of unmetamorphosed sedimentary rocks. The Andrarona Group rests on Neoarchaean granites and monzogranites of the Antongil craton and consists of a basal metagreywacke, thick quartzites and an upper sequence of sericite-chlorite meta-mudstones, meta-sandstones and a volcaniclastic meta-sandstone. The depositional age of the volcaniclastic meta-sandstone is constrained in age by U–Pb laser-ablation ICP-MS analyses of euhedral zircons to 1875 ± 8 Ma (2). Detrital zircons of Archaean and Palaeoproterozoic age represent an input from the Antongil craton and a newly defined Palaeoproterozoic igneous unit, the Masindray tonalite, which underlies the Andrarona Group, and yielded a U–Pb zircon age of 2355 ± 11 Ma (2), thus constraining the maximum age of deposition of the basal part of the Andrarona Group. The Andrarona Group shows a low-grade metamorphic overprint in the area near Antalaha; illite crystallinity values scatter around 0.17◦2 CuK, which is within the epizone. The Ampohafana Formation consists of undeformed, polymict conglomerate, cross-bedded sandstone, and red mudstone. An illite crystallinity value of >0.25◦2 CuK obtained from the rocks is typical of the diagenetic zone. Occurrences of rhyodacite pebbles in the Ampohafana Formation and the intrusion of a basaltic dyke suggest a deposition in a WSW-ENE-trending graben system during the opening of the Indian Ocean in the Upper Cretaceous, that was characterized by extensive rhyolitic to basaltic magmatism along Madagascar’s eastern coast