Nucleation and growth of metamorphic minerals in contact aureoles

THESIS ABSTRACT Nucleation and growth of metamorphic minerals are the consequence of changing P-T-X-conditions. The thesis presented here focuses on processes governing nucleation and growth of minerals in contact metamorphic environments using a combination of geochemical analytics (chemical-, isotope-, and trace element composition), statistical treatments of spatial data, and numerical models. It is shown, that a combination of textural modeling and stable isotope analysis allows a distinction between several possible reaction paths for olivine growth in a siliceous dolomite contact aureole. It is suggested that olivine forms directly from dolomite and quartz. The formation of olivine from this metastable reaction implies metamorphic crystallization far from equilibrium. As a major consequence, the spatial distribution of metamorphic mineral assemblages in a contact aureole cannot be interpreted as a proxy for the temporal evolution of a single rock specimen, because each rock undergoes a different reaction path, depending on temperature, heating rate, and fluid-infiltration rate. A detailed calcite-dolomite thermometry study was initiated on multiple scales ranging from aureole scale to the size of individual crystals. Quantitative forward models were developed to evaluate the effect of growth zoning, volume diffusion and the formation of submicroscopic exsolution lamellae (<1 µm) on the measured Mg-distribution in individual calcite crystals and compare the modeling results to field data. This study concludes that Mg-distributions in calcite grains of the Ubehebe Peak contact aureole are the consequence of rapid crystal growth in combination with diffusion and exsolution. The crystallization history of a rock is recorded in the chemical composition, the size and the distribution of its minerals. Near the Cima Uzza summit, located in the southern Adamello massif (Italy), contact metamorphic brucite bearing dolomite marbles are exposed as xenoliths surrounded by mafic intrusive rocks. Brucite is formed retrograde pseudomorphing spherical periclase crystals. Crystal size distributions (CSD's) of brucite pseudomorphs are presented for two profiles and combined with geochemistry data and petrological information. Textural analyses are combined with geochemistry data in a qualitative model that describes the formation periclase. As a major outcome, this expands the potential use of CSD's to systems of mineral formation driven by fluid-infiltration. RESUME DE LA THESE La nucléation et la croissance des minéraux métamorphiques sont la conséquence de changements des conditions de pression, température et composition chimique du système (PT-X). Cette thèse s'intéresse aux processus gouvernant la nucléation et la croissance des minéraux au cours d'un épisode de métamorphisme de contact, en utilisant la géochimie analytique (composition chimique, isotopique et en éléments traces), le traitement statistique des données spatiales et la modélisation numérique. Il est montré que la combinaison d'un modèle textural avec des analyses en isotopes stables permet de distinguer plusieurs chemins de réactions possibles conduisant à la croissance de l'olivine dans une auréole de contact riche en Silice et dolomite. Il est suggéré que l'olivine se forme directement à partir de la dolomie et du quartz. Cette réaction métastable de formation de l'olivine implique une cristallisation métamorphique loin de l'équilibre. La principale conséquence est que la distribution spatiale des assemblages de minéraux métamorphiques dans une auréole de contact ne peut pas être considérée comme un témoin de l'évolution temporelle d'un type de roche donné, puisque chaque type de roche suit différents chemins de réactions, en fonction de la température, la vitesse de réchauffement et le taux d'infiltration du fluide. Une étude thermométrique calcite-dolomite détaillée a été réalisée à diverses échelles, depuis l'échelle de l'auréole de contact jusqu'à l'échelle du cristal. Des modèles numériques quantitatifs ont été développés pour évaluer l'effet des zonations de croissance, de la diffusion volumique et de la formation de lamelles d'exsolution submicroscopiques (<1µm) sur la distribution du magnésium mesuré dans des cristaux de calcite individuels. Les résultats de ce modèle ont été comparés ä des échantillons naturels. Cette étude montre que la distribution du Mg dans les grains de calcite de l'auréole de contact de l'Ubehebe Peak (USA) résulte d'une croissance cristalline rapide, associée aux processus de diffusion et d'exsolution. L'histoire de cristallisation d'une roche est enregistrée dans la composition chimique, la taille et la distribution de ses minéraux. Près du sommet Cima Uzza situé au sud du massif d'Adamello (Italie), des marbres dolomitiques à brucite du métamorphisme de contact forment des xénolithes dans une intrusion mafique. La brucite constitue des pseudomorphes rétrogrades du périclase. Les distributions de taille des cristaux (CSD) des pseudomorphes de brucite sont présentées pour deux profiles et sont combinées aux données géochimiques et pétrologiques. Les analyses textorales sont combinées aux données géochimiques dans un modèle qualitatif qui décrit la formation du périclase. Ceci élargit l'utilisation potentielle de la C5D aux systèmes de formation de minéraux controlés par les infiltrations fluides. THESIS ABSTRACT (GENERAL PUBLIC) Rock textures are essentially the result of a complex interaction of nucleation, growth and deformation as a function of changing physical conditions such as pressure and temperature. Igneous and metamorphic textures are especially attractive to study the different mechanisms of texture formation since most of the parameters like pressure-temperature-paths are quite well known for a variety of geological settings. The fact that textures are supposed to record the crystallization history of a rock traditionally allowed them to be used for geothermobarometry or dating. During the last decades the focus of metamorphic petrology changed from a static point of view, i.e. the representation of a texture as one single point in the petrogenetic grid towards a more dynamic view, where multiple metamorphic processes govern the texture formation, including non-equilibrium processes. This thesis tries to advance our understanding on the processes governing nucleation and growth of minerals in contact metamorphic environments and their dynamic interplay by using a combination of geochemical analyses (chemical-, isotope-, and trace element composition), statistical treatments of spatial data and numerical models. In a first part the thesis describes the formation of metamorphic olivine porphyroblast in the Ubehebe Peak contact aureole (USA). It is shown that not the commonly assumed succession of equilibrium reactions along a T-t-path formed the textures present in the rocks today, but rather the presence of a meta-stable reaction is responsible for forming the olivine porphyroblast. Consequently, the spatial distribution of metamorphic minerals within a contact aureole can no longer be regarded as a proxy for the temporal evolution of a single rock sample. Metamorphic peak temperatures for samples of the Ubehebe Peak contact aureole were determined using calcite-dolomite. This geothermometer is based on the temperature-dependent exchange of Mg between calcite and dolomite. The purpose of the second part of this thesis was to explain the interfering systematic scatter of measured Mg-content on different scales and thus to clarify the interpretation of metamorphic temperatures recorded in carbonates. Numerical quantitative forward models are used to evaluate the effect of several processes on the distribution of magnesium in individual calcite crystals and the modeling results were then compared to measured field. Information about the crystallization history is not only recorded in the chemical composition of grains, like isotope composition or mineral zoning. Crystal size distributions (CSD's) provide essential information about the complex interaction of nucleation and growth of minerals. CSD's of brucite pseudomorphs formed retrograde after periclase of the southern Adamello massif (Italy) are presented. A combination of the textural 3D-information with geochemistry data is then used to evaluate reaction kinetics and to constrain the actual reaction mechanism for the formation of periclase. The reaction is shown to be the consequence of the infiltration of a limited amount of a fluid phase at high temperatures. The composition of this fluid phase is in large disequilibrium with the rest of the rock resulting in very fast reaction rates. RESUME DE LA THESE POUR LE GRAND PUBLIC: La texture d'une roche résulte de l'interaction complexe entre les processus de nucléation, croissance et déformation, en fonction des variations de conditions physiques telles que la pression et la température. Les textures ignées et métamorphiques présentent un intérêt particulier pour l'étude des différents mécanismes à l'origine de ces textures, puisque la plupart des paramètres comme les chemin pression-température sont relativement bien contraints dans la plupart des environnements géologiques. Le fait que les textures soient supposées enregistrer l'histoire de cristallisation des roches permet leur utilisation pour la datation et la géothermobarométrie. Durant les dernières décennies, la recherche en pétrologie métamorphique a évolué depuis une visualisation statique, c'est-à-dire qu'une texture donnée correspondait à un point unique de la grille pétrogénétique, jusqu'à une visualisation plus dynamique, où les multiples processus métamorphiques qui gouvernent 1a formation d'une texture incluent des processus hors équilibre. Cette thèse a pour but d'améliorer les connaissances actuelles sur les processus gouvernant la nucléation et la croissance des minéraux lors d'épisodes de métamorphisme de contact et l'interaction dynamique existant entre nucléation et croissance. Pour cela, les analyses géochimiques (compositions chimiques en éléments majeurs et traces et composition isotopique), le traitement statistique des données spatiales et la modélisation numérique ont été combinés. Dans la première partie, cette thèse décrit la formation de porphyroblastes d'olivine métamorphique dans l'auréole de contact de l'Ubehebe Peak (USA). Il est montré que la succession généralement admise des réactions d'équilibre le long d'un chemin T-t ne peut pas expliquer les textures présentes dans les roches aujourd'hui. Cette thèse montre qu'il s'agirait plutôt d'une réaction métastable qui soit responsable de la formation des porphyroblastes d'olivine. En conséquence, la distribution spatiale des minéraux métamorphiques dans l'auréole de contact ne peut plus être interprétée comme le témoin de l'évolution temporelle d'un échantillon unique de roche. Les pics de température des échantillons de l'auréole de contact de l'Ubehebe Peak ont été déterminés grâce au géothermomètre calcite-dolomite. Celui-ci est basé sur l'échange du magnésium entre la calcite et la dolomite, qui est fonction de la température. Le but de la deuxième partie de cette thèse est d'expliquer la dispersion systématique de la composition en magnésium à différentes échelles, et ainsi d'améliorer l'interprétation des températures du métamorphisme enregistrées dans les carbonates. Des modèles numériques quantitatifs ont permis d'évaluer le rôle de différents processus sur la distribution du magnésium dans des cristaux de calcite individuels. Les résultats des modèles ont été comparés aux échantillons naturels. La composition chimique des grains, comme la composition isotopique ou la zonation minérale, n'est pas le seul témoin de l'histoire de la cristallisation. La distribution de la taille des cristaux (CSD) fournit des informations essentielles sur les interactions entre nucléation et croissance des minéraux. La CSD des pseudomorphes de brucite retrograde formés après le périclase dans le sud du massif Adamello (Italie) est présentée dans la troisième partie. La combinaison entre les données textorales en trois dimensions et les données géochimiques a permis d'évaluer les cinétiques de réaction et de contraindre les mécanismes conduisant à la formation du périclase. Cette réaction est présentée comme étant la conséquence de l'infiltration d'une quantité limitée d'une phase fluide à haute température. La composition de cette phase fluide est en grand déséquilibre avec le reste de la roche, ce qui permet des cinétiques de réactions très rapides.

A estação arqueológica de Salamansa (ilha de São Vicente, República de Cabo Verde)

Apresentam‑se os principais resultados obtidos nas duas missões arqueológicas patrocinadas pelo Centro Português de Actividades Subaquáticas (CPAS) à ilha de São Vicente (República de Cabo Verde), em 1998 e em 2005. Em 1998, confirmou‑se o efectivo interesse arqueológico do sítio, localizado sobre o mar, em local abrigado da vasta baía de Salamansa, situada na parte setentrional da ilha, tendo‑se registado a respectiva extensão e estratigrafia e procedido à colheita de amostras para datação. Embora os resultados dessa campanha tivessem sido publicados, indicando estação de carácter habitacional, revelada pela notável acumulação de conchas, acompanhada de abundantes fragmentos de cerâmicas manuais, de produção africana, mantinha‑se indefinida a sua verdadeira natureza. Impunha‑se, assim, proceder à escavação integral da área que ainda subsistia da estação — sujeita de forma contínua a forte erosão marinha — bem como à colheita de novos materiais para datação, de forma a confirmar as conclusões preliminares anteriormente obtidas, objectivos que se concretizaram em 2005. Deste modo, foi possível concluir que, contrariando a hipótese, de início considerada, de poder corresponder a um testemunho da ocupação da ilha em época anterior à chegada dos Portugueses — hipótese que já as primeiras datas de radiocarbono contradiziam — se trata de um sítio onde uma unidade habitacional construída por muros de pedra seca, de planta ortogonal, revela inspiração europeia, aliás sublinhada pelos materiais exumados, onde estão representados produtos com tal origem, como cachimbos de caulino, vidros, faianças portuguesas, e projécteis de armas de fogo, a par de objectos oriundos do Extremo Oriente, num quadro dominado pelas produções cerâmicas africanas. Esta situação evidencia um estabelecimento cuja ocupação se centrou no século XVII, conforme indicam os materiais recolhidos e os resultados das datações obtidas, francamente aberto aos contactos de longa distância, apesar do isolamento do local escolhido. Os restos faunísticos recolhidos, com a presença deburro e de boi, sugerem um estacionamento sedentário, sendo a alimentação assegurada essencialmente pela captura de tartarugas, pela pesca e pela recolecção de moluscos marinhos (especialmente grandes lapas) e complementada pelo consumo de cabra, que poderia ser doméstica ou caçada, dado o estado selvagem a que retornou ali esta espécie. na última parte do trabalho, discutem‑se as diversas hipóteses susceptíveis de explicar esta estação — desde um entreposto comercial relacionado com a exploração agro‑pecuária da ilha de Santo Antão, passando por pequeno estabelecimento especializado de apoio à navegação, com a produção de carne salgada de tartaruga, até ter constituído refúgio relacionado com a intensa pirataria vigente à época no arquipélago, tendo presente os elementos históricos conhecidos, que, aliás, indicam que o início da ocupação permanente de São Vicente só se produziu a partir da segunda década do século XIX. Seja como for, a forte componente cultural africana revelada pelo espólio destes primeiros ocupantes da ilha expressa‑se também pelos rituais que terão envolvido o abandono do estabelecimento, com o enterramento de dois vasos emborcados sob o chão da habitação explorada, e a deposição de uma pequena taça, nas mesmas circunstâncias, junto à parede da mesma, do lado externo.

Oscil.lacions del nivell del mar i sediments associats durant l'Holocè a la Costa Brava Meridional (Girona)

Several marine deposits in the southern Costa Brava shoreline have been studied. They appear at different heights above and below the present sea level. Three groups are defined in relation to their origin: conglomerate levels at the bottom of the cliffs, not-cementedemerged beaches and cemented submerged beaches. The age of the emerged beaches has been accurately determined by means of the ceramic content and radiocarbon dating. Chronological succession of the deposits and their stratigraphic and paleontological characteristics allow to define a sequence of the sea level changes during the latest stages of the versiliane transgression in the studied area

Late Pan-African magmatism in the Himalaya: new geochronological and geochemical data from the Ordovician Tso Morari metagranites (Ladakh, NW India)

Two granitic plutons, the Tso Morari gneiss and the Rupshu metagranite, crop out in the Tso Morari area. The Polokongka La granite, classically interpreted as a young intrusion in the Tso Morari gneiss, has been recognized as the undeformed facies of the latter. Conventional isotope dilution U-Pb zircon dating on single-grain and small multi-grain fractions yielded magmatic ages of 479 +/- 2 Ma for the Tso Morari gneiss and the Polokongka La granite, and 482.5 +/- 1 Ma for the Rupshu granite. There is a great difference in zircon morphology between the Tso Morari gneiss (peraluminous type) and the Rupshu granite (alkaline type). This difference is confirmed by whole-rock chemistry. The Tso Morari gneiss is a typical deformed S-type granite, resulting from crustal anatexis. On the other hand, the Rupshu granite is an essentially metaluminous alkali-calcic intrusion derived from a different source material. Data compilation from other Himalayan Cambro-Ordovician granites reveals huge and widespread magmatic activity all along and beyond the northern Indian plate between 570 and 450 Ma, with a peak at 500-480 Ma. A major, continental-scale tectonic event is required to generate such a large magmatic belt; it has been tentatively compared to the Variscan post-orogenic extensional regime of Western Europe, as a late evolution stage of a Pan-African orogenic event.

Global radiolarian zonation for the Pliensbachian, Toarcian and Aalenian

Jurassic radiolarians from 220 samples in Queen Charlotte Islands, B.C., Williston Lake, B.C., east-central Oregon, Baja California Sur, southern Spain, Austria, Slovenia, Turkey, Oman, Japan and Argentina were studied in order to construct global zonation for the Pliensbachian, Toarcian and Aalenian stages. Well-preserved faunas from continuous stratigraphic sections in Queen Charlotte Islands provide the most detailed record for this time interval, and all collections are tied to North American ammonite zones or assemblages. Collections from nearly all other areas lack independent dating except for early Toarcian carbon-isotope dating in Slovenia and late Aalenian ammonites in Spain. A database of 197 widely distributed updated taxonomic species was used to construct a Unitary Association (UA) zonation for the interval. A global sequence of 41 UAs was obtained for the top of the Sinemurian to the base of the Bajocian. The first and the last UAs represent the Late Sinemurian and the Early Bajocian respectively. The remaining 39 UAs were merged into nine zones (four Early Pliensbachian, one Late Pliensbachian, one Early Toarcian, one Middle-Late Toarcian, and two Aalenian) according to prominent radiolarian faunal breaks and ammonite data. The new zones are the Canutus tip pen - Katroma clara Zone (latest Sinemurian/earliest Pliensbachian); Zartus mostleri - Pseudoristola megaglobosa, Hsuum mulleri - Trillus elkhornensis and Gigi fustis - Lantus sixi zones (Early Pliensbachian); Eucyrtidiellum nagaiae - Praeparvicingula tlellensis Zone (Late Pliensbachian); Napora relica - Eucyrtidiellum disparile Zone (Early Toarcian); Elodium pessagnoi - Hexasaturnalis hexagonus Zone (Middle and Late Toarcian); Higumastra transversa - Napora nipponica Zone (early Aalenian); and Mirifusus proavus - Transhsuum hisuikyoense Zone (late Aalenian). These zones can be correlated worldwide and link previously established UA zonations for the Hettangian-Sinemurian and the Middle to Upper Jurassic. The new zonation allows high-resolution dating in the studied interval and provides a solid basis for analyzing faunal turnovers and the paleobiogeography of Jurassic radiolarians. (C) 2010 Elsevier B.V. All rights reserved.

Population consequences of environmental sex reversal.

When sex determination in a species is predominantly genetic but environmentally reversible, exposure to (anthropogenic) changes in the environment can lead to shifts in a population's sex ratio. Such scenarios may be common in many fishes and amphibians, yet their ramifications remain largely unexplored. We used a simple model to study the (short-term) population consequences of environmental sex reversal (ESR). We examined the effects on sex ratios, sex chromosome frequencies, and population growth and persistence after exposure to environmental forces with feminizing or masculinizing tendencies. When environmental feminization was strong, X chromosomes were driven to extinction. Analogously, extinction of normally male-linked genetic factors (e.g., Y chromosomes) was caused by continuous environmental masculinization. Although moderate feminization was beneficial for population growth in the absence of large viability effects, our results suggest that the consequences of ESR are generally negative in terms of population size and the persistence of sex chromosomes. Extreme sex ratios resulting from high rates of ESR also reduced effective population sizes considerably. This may limit any evolutionary response to the deleterious effects of ESR. Our findings suggest that ESR changes population growth and sex ratios in some counter-intuitive ways and can change the predominant factor in sex determination from genetic to fully environmental, often within only a few tens of generations. Populations that lose genetic sex determination may quickly go extinct if the environmental forces that cause sex reversal cease.

Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): Geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith

For the first time, an albite orthogneiss has been recognised and dated within the HP-LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U-Pb age of 480.8 +/- A 4.8 Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6-647.4 Ma. Zircon and titanite U-Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U-Pb age of 366 +/- A 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP-LT peak metamorphism.

Initial Results on the Composition of Fingerprints and its Evolution as a Function of Time by GC/MS Analysis

Determining the time since deposition of fingermarks may prove necessary to assess their relevance to criminal investigations. The crucial factor is the initial composition of fingermarks, because it represents the starting point of any aging model. This study mainly aimed to characterize the initial composition of fingerprints, which show a high variability between donors (inter-variability), but also to investigate the variations among fingerprints from the same donor (intra-variability). Solutions to reduce this initial variability using squalene and cholesterol as target compounds are proposed and should be further investigated. The influence of substrates was also evaluated, and the initial composition was observed to be larger on porous surface than nonporous surfaces. Preliminary aging of fingerprints over 30 days was finally studied on a porous and a nonporous substrate to evaluate the potential for dating of fingermarks. Squalene was observed to decrease in a faster rate on a nonporous substrate.

Transformations in Deuteronomistic and Biblical Historiography. On "Book-Finding" and other Literary Strategies

L'a. cherche à dater l'historiographie deutéronomiste et à établir une transformation à l'intérieur de cette historiographie. Les écrits de propagande de l'époque de Josias (Rois, Deutéronome, Josué, Exode et Nombres) ont été changés après la catastrophe de 597/87 en récit historique prenant une distance et présentant la Tora prenant la relève de l'autorité perdue du Temple. L'A. peut ainsi pencher pour une rédaction de ce courant historiographique datant de l'époque post-éxilique.

Temporal and spatial origin of Gesneriaceae in the New World inferred from plastid DNA sequences

Gesneriaceae are represented in the New World (NW) by a major clade (c. 1000 species) currently recognized as subfamily Gesnerioideae. Radiation of this group occurred in all biomes of tropical America and was accompanied by extensive phenotypic and ecological diversification. Here we performed phylogenetic analyses using DNA sequences from three plastid loci to reconstruct the evolutionary history of Gesnerioideae and to investigate its relationship with other lineages of Gesneriaceae and Lamiales. Our molecular data confirm the inclusion of the South Pacific Coronanthereae and the Old World (OW) monotypic genus Titanotrichum in Gesnerioideae and the sister-group relationship of this subfamily to the rest of the OW Gesneriaceae. Calceolariaceae and the NW genera Peltanthera and Sanango appeared successively sister to Gesneriaceae, whereas Cubitanthus, which has been previously assigned to Gesneriaceae, is shown to be related to Linderniaceae. Based on molecular dating and biogeographical reconstruction analyses, we suggest that ancestors of Gesneriaceae originated in South America during the Late Cretaceous. Distribution of Gesneriaceae in the Palaeotropics and Australasia was inferred as resulting from two independent long-distance dispersals during the Eocene and Oligocene, respectively. In a short time span starting at 34 Mya, ancestors of Gesnerioideae colonized several Neotropical regions including the tropical Andes, Brazilian Atlantic forest, cerrado, Central America and the West Indies. Subsequent diversification within these areas occurred largely in situ and was particularly extensive in the mountainous systems of the Andes, Central America and the Brazilian Atlantic forest. Only two radiations account for 90% of the diversity of Gesneriaceae in the Brazilian Atlantic forest, whereas half of the species richness in the northern Andes and Central America originated during the last 10 Myr from a single radiation.

Rate and processes of river network rearrangement during incipient faulting: The case of the Cahabon river, Guatemala

Deeply incised river networks are generally regarded as robust features that are not easily modified by erosion or tectonics. Although the reorganization of deeply incised drainage systems has been documented, the corresponding importance with regard to the overall landscape evolution of mountain ranges and the factors that permit such reorganizations are poorly understood. To address this problem, we have explored the rapid drainage reorganization that affected the Cahabon River in Guatemala during the Quaternary. Sediment-provenance analysis, field mapping, and electrical resistivity tomography (ERT) imaging are used to reconstruct the geometry of the valley before the river was captured. Dating of the abandoned valley sediments by the Be-10-Al-26 burial method and geomagnetic polarity analysis allow us to determine the age of the capture events and then to quantify several processes, such as the rate of tectonic deformation of the paleovalley, the rate of propagation of post-capture drainage reversal, and the rate at which canyons that formed at the capture sites have propagated along the paleovalley. Transtensional faulting started 1 to 3 million years ago, produced ground tilting and ground faulting along the Cahabon River, and thus generated differential uplift rate of 0.3 +/- 0.1 up to 0.7 +/- 0.4 mm . y(-1) along the river's course. The river responded to faulting by incising the areas of relative uplift and depositing a few tens of meters of sediment above the areas of relative subsidence. Then, the river experienced two captures and one avulsion between 700 ky and 100 ky. The captures breached high-standing ridges that separate the Cahabon River from its captors. Captures occurred at specific points where ridges are made permeable by fault damage zones and/or soluble rocks. Groundwater flow from the Cahabon River down to its captors likely increased the erosive power of the captors thus promoting focused erosion of the ridges. Valley-fill formation and capture occurred in close temporal succession, suggesting a genetic link between the two. We suggest that the aquifers accumulated within the valley-fills, increased the head along the subterraneous system connecting the Cahabon River to its captors, and promoted their development. Upon capture, the breached valley experienced widespread drainage reversal toward the capture sites. We attribute the generalized reversal to combined effects of groundwater sapping in the valley-fill, axial drainage obstruction by lateral fans, and tectonic tilting. Drainage reversal increased the size of the captured areas by a factor of 4 to 6. At the capture sites, 500 m deep canyons have been incised into the bedrock and are propagating upstream at a rate of 3 to 11 mm . y(-1) deepening at a rate of 0.7 to 1 5 mm . y(-1). At this rate, 1 to 2 million years will be necessary for headward erosion to completely erase the topographic expression of the paleovalley. It is concluded that the rapid reorganization of this drainage system was made possible by the way the river adjusted to the new tectonic strain field, which involved transient sedimentation along the river's course. If the river had escaped its early reorganization and had been given the time necessary to reach a new dynamic equilibrium, then the transient conditions that promoted capture would have vanished and its vulnerability to capture would have been strongly reduced.

Emplacement of Jurassic - Lower Cretaceous radiolarites of the Nicoya Complex (Costa Rica)

We present a new model to explain the origin, emplacement and stratigraphy of the Nicoya Complex in the NW part of the Nicoya Peninsula (Costa Rica) based on twenty-five years of field work, accompanied with the evolution of geochemical, vulcanological, petrological, sedimentological and paleontological paradigms. The igneous-sedimentary relation, together with radiolarian biochronology of the NW-Nicoya Peninsula is re-examined. We interpret the Nicoya Complex as a cross-section of a fragment of the Late Cretaceous Caribbean Plateau, in which the deepest levels are exposed in the NW-Nicoya Peninsula. Over 50% of the igneous rocks are intrusive (gabbros and in less proportion plagiogranites) which have a single mantle source; the remainder are basalts with a similar geochemical signature. Ar39/Ar40 radioisotopic whole rock and plagioclase ages range throughout the area from 84 to 83 Ma (Santonian) for the intrusives, and from 139 to 88 Ma (Berriasian-Turonian) for the basalts. In contrast, Mn-radiolarites that crop out in the area are older in age, Bajocian (Middle Jurassic) to Albian (middle Cretaceous). These Mn-radiolaritic blocks are set in a "matrix" of multiple gabbros and diabases intrusions. Chilled margins of magmatites, and hydrothermal baking and leaching of the radiolarites confirm the Ar39/Ar40 dating of igneous rocks being consistently younger than most of the radiolarian cherts. No Jurassic magmatic basement has been identified on the Nicoya Peninsula. We interpret the Jurassic-Cretaceous chert sediment pile to have been disrupted and detached from its original basement by multiple magmatic events that occurred during the formation of the Caribbean Plateau. Coniacian-Santonian (Late Cretaceous), Fe-rich radiolarites are largely synchronous and associated with late phases of the Plateau.

Synorogenic extension: Quantitative constraints on the age and displacement of the Zanskar shear zone (northwest Himalaya)

subsequent extension-induced exhumation. Geochronological dating of various Structural, thermobarometric, and geochronological data place limits on the age and tectonic displacement along the Zanskar shear zone, a major north-dipping synorogenic extensional structure separating the high-grade metamorphic sequence of the High Himalayan Crystalline Sequence from the overlying low-grade sedimentary rocks of the Tethyan Himalaya, A complete Barrovian metamorphic succession, from kyanite to biotite zone mineral assemblages, occurs within the I-km-thick Zanskar shear zone. Thermobarometric data indicate a difference In equilibration depths of 12 +/- 3 km between the lower kyanite zone and the garnet zone, which is Interpreted as a minimum estimate for the finite vertical displacement accommodated by the Zanskar shear zone. For the present-day dip of the structure (20 degrees), a simple geometrical model shows that a net slip of 35 +/- 9 km is required to regroup these samples to the same structural level. Because the kyanite to garnet zone rocks represent only part of the Zanskar shear zone, and because its original dip may have been less than the present-day dip, these estimates fur the finite displacement represent minimum values. Field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the Zanskar shear zone occurred as a continuous profess starting at the Barrovian metamorphic peak and lasting throughout the subsequent extension-induced exhumation. Geochronological dataing of various leucogranitic plutons and dikes in the Zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19.8 Ma and that extension most likely initiated shortly before 22.2 Ma.

The abrupt climate change at the Eocene-Oligocene boundary and the emergence of South-East Asia triggered the spread of sapindaceous lineages.

Background and Aims Paleoclimatic data indicate that an abrupt climate change occurred at the Eocene-Oligocene (E-O) boundary affecting the distribution of tropical forests on Earth. The same period has seen the emergence of South-East (SE) Asia, caused by the collision of the Eurasian and Australian plates. How the combination of these climatic and geomorphological factors affected the spatio-temporal history of angiosperms is little known. This topic is investigated by using the worldwide sapindaceous clade as a case study. Methods Analyses of divergence time inference, diversification and biogeography (constrained by paleogeography) are applied to a combined plastid and nuclear DNA sequence data set. Biogeographical and diversification analyses are performed over a set of trees to take phylogenetic and dating uncertainty into account. Results are analysed in the context of past climatic fluctuations. Key Results An increase in the number of dispersal events at the E-O boundary is recorded, which intensified during the Miocene. This pattern is associated with a higher rate in the emergence of new genera. These results are discussed in light of the geomorphological importance of SE Asia, which acted as a tropical bridge allowing multiple contacts between areas and additional speciation across landmasses derived from Laurasia and Gondwana. Conclusions This study demonstrates the importance of the combined effect of geomorphological (the emergence of most islands in SE Asia approx. 30 million years ago) and climatic (the dramatic E-O climate change that shifted the tropical belt and reduced sea levels) factors in shaping species distribution within the sapindaceous clade.

Artérite á cellules géantes et vitesse de sédimentation normale: plus qu'une exception [Giant cell arteritis and normal sedimentation rate: more than an exception!]

BACKGROUND: Giant cell arteritis (GCA) is a systemic segmental vasculitis of unknown etiology, typically affecting elderly patients. Elevated erythrocyte-sedimentation rate (ESR) is usually found in such patients. PATIENTS AND METHODS: One hundred and twenty three patients underwent temporal artery biopsy in our institution between 1977 and 1995. Among them, 66 (53.7%) biopsies were positive (i.e. histologic findings were very suggestive of GCA). The clinical charts from all patients with positive biopsies were retrieved and 47 were eligible for our study (inadequate data in 19 cases). RESULTS: Seven of the 47 patients with positive biopsies (15%) had a normal ESR and 70% (33/47 cases) had neuro-ophthalmic complications including anterior ischemic optic neuropathy, central retinal artery occlusion, choroidal ischemia and extraocular muscle and/or cranial nerve palsy (III, IV, VI). No differences were found between the groups with normal or elevated ESR as 87.5% (6/7 cases) of the group with normal ESR exhibited neuro-ophthalmic complications. CONCLUSIONS: ESR was normal in 15% of our GCA patients and these patients had the same frequency of neuro-ophthalmic complications as the GCA patients with elevated ESR. Thus, our study does not support the previous concept that patients with higher ESR are more at risk for neuro-ophthalmic complications. GCA with normal ESR is not rare and such patients should be investigated with other blood studies (C-reactive protein) and with fluorescein angiography.