Extension et sédimentation au paléozoïque terminal et au Mésozoïque dans le bassin de Majunga (Nord-Ouest de Madagascar)

The Majunga Basin is located in the northwestern part of Madagascar with a N45–60°E trending axis. It was filled by almost exclusively continental Karoo Supergroup sediments, which are Permian to Early Jurassic in age, and by younger sequences, mainly marine, that were deposited from the Middle Jurassic to the present.The Karoo Basin geometry is deduced from the analysis of seismic sections. A central northeast trending horst is flanked by two sub-basins. Deposition of the Karoo sequences was controlled by these northeast trending faults. On the contrary, the Middle Jurassic to present sequences witness only a slight tilting of the basement towards the northwest.The development of the Majunga Basin includes, therefore, two successive stages. In the synrift episode, from Permian to Early Jurassic times, the sedimentation was syntectonic, controlled by synsedimentary faulting and the creation of a horst and graben extensive pattern. The postrift episode started during the Middle Jurassic.These two stages of the Majunga Basin development correspond to the geodynamic evolution recorded elsewhere in this part of the Gondwana.     

Compositional diversity of soluble mineralizing matrices in some recent coral skeletons compared to fine-scale growth structures of fibres: discussion of consequences for biomineralization and diagenesis

 In contrast to the purely crystalline model of the coral fibre, a transversal zonation is made visible within fibres by specific preparations (enzymatic or light acidic etchings). This growth zonation that corresponds to the successive positions of the secretory ectoderm during septal development shows that crystallization of coral fibre is controlled not only by an external organic coating of fibres, but also includes intra-fibrous organic components. This results in a micron-scaled internal structure of fibres, with a noticeable diversity depending on species. In parallel, soluble matrices also exhibit significant differences, although the global high content in acidic amino acids is always observed. Gathering these two sets of data allows the current ideas concerning structures and growth modalities of coral septa to be discussed. Basically, the respective role of centres of calcification and fibres during septal development appears clearly different from the current ideas, resulting in significant change in interpretation of coral structures, with concerns in both skeleton formation and understanding of diagenetic processes. Received: 2 November 1998 / Accepted: 10 May 1999     

喜马拉雅的两个造山体系和一个转换-转移断层:证据及推论

位于中喜马拉雅和巴基斯坦境内西喜马拉雅的两个相互结合的剖面在一级单元、断层中展现出不同的构造形式;并在不同时期,以不同速率发育了二级构造。沿两剖面岩性单元的显著差异显示通常指的圆柱状喜马拉雅带并没有越过喀喇昆仑山断层。与此同时,在近来许多区域研究中显示出来的构造轮廓强调主中央逆冲断层是一个貌似与中喜马拉雅断层带和越过西部山脉的西喜马拉雅断层带有联系的独立部分。上述两个地区展现出不同的碰撞历史。这些不同之处揭示喀喇昆仑山断层是西部岛弧保留造山带与东部岛弧俯冲造山带之间转移/转换断层的再活动或衍变。     

Facies and architecture of an estuarine tidal bar (the Trompeloup bar, Gironde Estuary, SW France)

This study describes the sedimentary facies and internal architecture of a modern estuarine tidal bar present in the bay-head delta of a wave- and tide-dominated estuary, the Gironde Estuary, in southwest France. Based on 51 cores (2 to 5 m long), this work demonstrates that such bars are composite sedimentary bodies made up of individual sigmoidal sand units isolated from one another by thick muddy layers. The bar appeared in the study area around 1874. Since then, its evolution has been characterized by phases of rapid downstream bar migration alternating with periods of bar abandonment. During the periods of active bar progradation, which commonly last between 10 and 20 years, the bar progrades up to 7 km seaward and forms a narrow (200–800 m wide) sand ridge up to 6 m thick. During abandonment phases which also last around 10–20 years, the bar is starved of sand and a mud layer (1 to 3 m thick) is deposited. The vertical succession formed by the seaward progradation of the tidal bar records these alternating phases of growth and abandonment. A typical vertical succession in the bar system is 8 m thick and comprises several superimposed, upward-coarsening units of cross-stratified sand, deposited during phases of seaward progradation, separated by a thick layer of thinner-bedded mud and sand laminae deposited during phases of bar abandonment. When a tidal bar builds up to the supratidal zone, marshes accumulate and the bar becomes a permanent island in the estuary. The large-scale architecture of the bar is thus made of an alternation of potential sandy reservoir units and muddy internal seals. The reservoir units are characterized by a sigmoidal shape. The muddy layer, which is deposited between the two sigmoidal units, is a thick, continuous potential internal seal. This complex internal architecture is due to the fact that there is a considerable amount of mud present in the bay-head delta of the Gironde Estuary. It is thought to be representative of the architecture of tidal bars in the inner part of wave- and tide-dominated estuaries.     

Numerical modelling of Cretaceous Pyrenean Rifting: The interaction between mantle exhumation and syn-rift salt tectonics

The preshortening Cretaceous Pyrenean Rift is an outstanding geological laboratory to investigate the effects of a pre-rift salt layer at the sedimentary base on lithospheric rifting. The occurrence of a pre-rift km-scale layer of evaporites and shales promoted the activation of syn-rift salt tectonics from the onset of rifting. The pre- and syn-rift sediments are locally affected by high-temperature metamorphism related to mantle ascent up to shallow depths during rifting. The thermo-mechanical interaction between décollement along the pre-existing salt layer and mantle ascent makes the Cretaceous Pyrenean Rifting drastically different from the type of rifting that shaped most Atlantic-type passive margins where salt deposition is syn-rift and gravity-driven salt tectonics has been postrift. To unravel the dynamic evolution of the Cretaceous Pyrenean Rift, we carried out a set of numerical models of lithosphere-scale extension, calibrated using the available geological constraints. Models are used to investigate the effects of a km-scale pre-rift salt layer, located at the sedimentary cover base, on the dynamics of rifting. Our results highlight the key role of the décollement layer at cover base that can alone explain both salt tectonics deformation style and high-temperature metamorphism of the pre-rift and syn-rift sedimentary cover. On the other hand, in the absence of décollement, our model predicts symmetric necking of the lithosphere devoid of any structure and related thermal regime geologically relevant to the Pyrenean case.     

粤北晚泥盆世天子岭组遗迹组构及其环境解释

遗迹组构是由生物成因改造的沉积岩组构,它是物理过程和生物过程相互作用的产物。本文在广东韶关晚泥盆世天子岭组碳酸盐岩沉积中建立了4个遗迹组构,即反映局限台地或封闭海湾环境的Planolies montanus遗迹组构,反映台地潮下低能环境的Thalassinoides—Planolites遗迹组构,反映潮上-潮间环境的．Skolithos遗迹组构和受固底底质控制的Thalassinoides遗迹组构。对遗迹化石和生物扰动构造进行阶层分析,从中识别出受固底底质控制的遗迹化石Thalassinoides.sp,这类遗迹化石常常代表层序地层学中的重要界面。     

Do river discharge rates drive the overall functioning of the pelagic ecosystem over the continental shelf of the Bay of Biscay (NE Atlantic)? A comparison of two contrasting years with special reference to anchovy (<Emphasis Type="Italic">Engraulis encrasicolus</Emphasis> L.) nutritional state

In the 2000 and 2001 springtime periods, mesozooplankton samples were systematically taken during research cruises devoted to the study of the distribution and abundance of small pelagic fishes in the French part of the Bay of Biscay, NE Atlantic, continental shelf. Simultaneous activities of three enzymes were measured in these mesozooplankton samples, to characterize the main features of ecosystem function at a regional scale. The aim was to assess assimilation rates of carbohydrates, through pyruvate kinase activity, and protein, through trypsin activity, by the mesozooplankton community and the resulting overall productivity, through aspartate transcarbamylase activity. The highest influence on the pelagic environment was the strong difference in the freshwater outflow rates from the two main large rivers, the Loire and the Gironde, which was principally responsible for the inter-annual fluctuations of the nutrients in the Bay. This overall influence of river flow results in a characteristic pattern of sea-surface salinity, which was reflected by the phytoplankton biomass distribution. Mesozooplankton enzyme activities showed high variations between the two years. The contrast in the dynamics of the pelagic food web processes between the two consecutive years, with higher activity in 2001, affected the nutritional condition of the zooplanktivorous breeding anchovy Engraulis encrasicolus.     

Optical long baseline intensity interferometry: prospects for stellar physics

More than sixty years after the first intensity correlation experiments by Hanbury Brown and Twiss, there is renewed interest for intensity interferometry techniques for high angular resolution studies of celestial sources. We report on a successful attempt to measure the bunching peak in the intensity correlation function for bright stellar sources with 1 meter telescopes (I2C project). We propose further improvements of our preliminary experiments of spatial interferometry between two 1 m telescopes, and discuss the possibility to export our method to existing large arrays of telescopes.