The role of water,mixing processes and metamorphic fabric in the genesis of the Baume migmatites (Ardèche,France)

The Baume migmatites arose from melting of a high-grade metamorphic sequence comprising quartzofeld-spathic and biotite-plagioclase gneisses. Geochemical data and REE modelling suggest that melting occurred under open-system conditions and was caused mainly either by the infiltration along shears of an alkali-rich (K, Rb) aqueous fluid phase or by the injection along foliation planes of granitic magma; some leucosomes result from mixing between in situ partial melts and externally derived granitic materials. Melting appears to be almost independent of the paleosome composition but rather is dependent on the amount of introduced material, itself controlled by the metamorphic fabrics of parent gneisses and/or tectonic structures.     

An evolutionary model of the western churchill province and western margin of the superior province in Canada and the North-Central United States

Regional-scale geophysical information, which includes aeromagnetic, gravity, seismic refraction, multi-channel seismic reflection and electromagnetic induction data, is used to extend our knowledge of the Canadian Shield beneath the Phanerozoic Williston basin of south-central Canada and the north-central United States. A new tectonic map based on this information shows the Proterozoic Flin Flon-Snow Lake and La Ronge-Lynn Lake volcanic island arcs and their associated fore-arc (Kisseynew belt) and back-arc (Reindeer-South Indian Lakes belt) basins wedged between the Archean Superior craton on the east and the Archean parts of the Churchill and Wyoming cratons on the west. Along the western margin of the Superior craton the Thompson nickel belt, including its extension southwards beneath the Williston basin, is interpreted to have been successively the site of continental rifting and rupturing, an evolving continental margin, a continent-volcanic island arc “suture” zone and eventually a continental-scale strike-slip fault. The North American Central Plains electrical conductivity anomaly and closely related seismic low-velocity zones are explained by the presence in the lower crust of buried slices of hydrated oceanic-type material, situated within the southward extension of the Reindeer-South Indian Lakes remnant back-arc basin and adjoining tectonic units. A new plate tectonic model is proposed for this region that involves the rifting and rupturing of the Archean continents and the opening and closing of one or more oceanic basins. This model is shown to be consistent with most of the geological, geophysical and geochronological data that pertains to the Proterozoic evolution of the exposed Shield and similar geophysical data and subsurface geochronological information from further south.     

Main aspects of structural change in the northern regions of the GDR

The northern Bezirke (Rostock, Schwerin and Neubrandenburg) occupy about a quarter of the territory of the GDR but account for only one-eight of its population. These Bezirke form a geographical region which in the past (before 1945) had many characteristics of an agrarian periphery in the then German Reich. After 1945 this situation changed radically in the context of the development of the GDR. Besides the development of modern agriculture, characteristic features have been a rapid process of industrialization and the construction of a versatile maritime and port economy. Recreation functions on the coast have taken on new dimensions and the recreative potential of inland Mecklenburg, rich in woods and lakes, has also been increasingly developed. On this economic basis an intensive process of urbanization has taken place, which has on the whole raised the living and working conditions of the population of the northern counties to the level of those in the south of the GDR. In view of the large number of small and very small settlements this development is at present not without its problems.     

Enregistrement de la dernière remontée du niveau marin dans l'architecture interne d'une vallée incisée : le pertuis Breton (Charente-Maritime)

Recent very high-resolution seismic profiles ground-truthed by vibrocores allow us to evidence an atypical incised valley fill in a drowned valley segment, the ‘Pertuis Breton’ (Bay of Biscay, France). The sedimentary valley-fill architecture mainly includes five superimposed progradational wedges composed by marine sands. Sandbodies show a landward migration of their depocentres upward and are topped by almost flat unconformities extended by submarine terraces. This sedimentary infill pattern is similar to backstepping wedges, described on continental shelfs. It suggests that this valley fill records sea-level rise during the last transgression. To cite this article: N. Weber et al., C. R. Geoscience 336 (2004).     

On shear-wave triplication in transversely isotropic media

The exact solution to the problem of qSV triplication in homogenous transversely isotropic media has been long known, but the result is algebraically complex and is seldom applied in practice. We present an appropriate approximation (not assuming weak qSV-anisotropy) that simplifies the conditions for the onset of off-axis triplication as anisotropy is increased, identifying the anisotropy parameter σ as the controlling parameter. It follows that commonly reported surface-seismic P-wave move-out measurements imply that many formations in the earth's sedimentary crust support off-axis qSV triplications. For typical Vp/Vs velocity ratios and a horizontally stratified earth, however, off-axis qSV triplications appear to only occur for shear-wave incidence angles too far from the vertical to be sampled by surface-seismic converted-wave survey geometries.     

Crustal make-up of the northern Andes: evidence based on deep crustal xenolith suites, Mercaderes, SW Colombia

Samples of the deep crust and upper mantle in the Northern Andes occur as abundant xenoliths in the Granatífera Tuff, a late Cenozoic vent in the Mercaderes area of SW Colombia. The lower crustal assemblage includes granulites, hornblendites, pyribolites, pyroxenites and gneisses; mafic rocks predominate, but felsic material is also common. P–T conditions for the pyribolite assemblages (i.e. Hbl+Fs/Scp+Grt+Cpx+Qtz±Bt), which are the best constrained, are 720–850 °C and 10–14 kbar, consistent with a deep-to-lower crustal origin. A notable feature of this xenolith suite is that it is dominated by hornblende. However, mineral reactions within the suite show that there is a transition from amphibolite to granulite facies, and there is a probable restite–melt relationship represented within the suite. However, the latter appears to be dominated by hornblende and garnet.The mafic rocks mostly lack the high Cr and Ni that would be expected of cumulates. Neither do they possess the positive Sr and Eu anomalies that would be consistent with resite or cumulate models for the lower crust. They bear greatest similarity to oceanic basalts (s.l.). The Rb contents of the xenoliths, whether mafic or silicic, are very low, and the more silicic members of the suite tend to have small positive Sr and Eu anomalies, which are transitional to adakitic compositions. The Sr isotopic compositions of the xenoliths lie between 0.704 and 0.705; however, the Nd isotopic compositions are much more variable, indicating considerable long-term heterogeneity. Few of the xenoliths can be compositionally recognised as metasedimentary; however, a sedimentary component is evident in the Pb isotopic compositions. Within these constraints, our favoured model is a deep crust formed by basaltic components (subduction–accretion?), and minor sediment, which is subject to an increase in thermal gradient to produce the granulites, any melting being dominated by hornblende-out reactions involving garnet. However, there is no evidence of any pervasive crustal melting, leading to the conclusion that the voluminous Andean magmatism arises from the mantle wedge.     

Évolution temporelle et architecture interne d'un banc sableux estuarien : la Longe de Boyard (littoral atlantique,France)

The evolution and the internal architecture of an estuary type sand ridge has been studied with a set of bathymetric data recorded during the last two centuries and with a dense grid of recent very high resolution seismic profiles. Bathymetric data of the so-called, Longe de Boyard sand ridge, displays sand losts due to wave and tide erosion. Internal geometry, through seismic profile analysis, indicates two main phases of deposition recording both, a recent high energy environment and an older low energy one, respectively. Such an evolution is believed to record changes in sedimentation processes mainly related to the end of the Holocene transgression (8 000–5 000 yr BP). To cite this article: É. Chaumillon et al., C. R. Geoscience 334 (2002) 119–126.