Equilibrium state of diopside-bearing harzburgites from ophiolites: Geobarometric and geodynamic implications

The bulk compositions of coexisting enstatite and diopside in basal lherzolites and clinopyroxene-bearing harzburgites from ophiolitic complexes are typical of solidus/subsolidus equilibria, but for a few texturally distinct magmatic diopsides. They would presumably reflect the state of equilibrium at the time they last coexisted with liquid as the rocks reentered subsolidus conditions. The total lack of correlation between Al and Ca concentrations shows that the compositional scatter observed for any given massif, results from analytical errors related to extensive exsolution and serpentinization, rather than from differences in equilibrium conditions. However, significant differences are found between the residual ophiolitic lherzolites from Hare Bay, Newfoundland, and from Xigaze, Tibet, two massifs selected for their distinct structural and textural features. As for thermobarometry techniques relevant to these rocks, the best barometer found is an empirical relation for the expression of pressure as a virtually temperature-independent function of the ratioK _f=(X _Di ^opx )/(1 –X _Di ^cpx ), in agreement with semi-quantitative models based on natural solid solutions. Temperatures are then simply derived from a surface-fitting expression relating pressure, temperature and diopside-solvus compositions, according to a regularX _En ^cpx solution model (CMS) corrected for the effect of Al in the spinel facies. Application of these techniques yield pressures of 0.4 and 1.4 GPa, i.e. depths from sea-bottom of about 13 and 43 km, for temperatures of 1,170 and 1,300° C for the ophiolitic lherzolites of Tibet and New-foundland, respectively, in good agreement with dry-solidus data by radioactive tracing and with geothermal-model estimates for ridges.     

The pre-Cretaceous deep-seated tectonics of the Abukuma massif and its place in the structural framework of Japan

The structures and microstructures of the Takanuki and Hitachi areas in the Abukuma massif, Northeast Japan are described. In the Takanuki area, the basic Gosaisho series thrusts the pelitic Takanuki ones in a HP metamorphic context. The nappe structure is afterwards refolded by a migmatitic dome: the Samegawa dome, in a HT metamorphic context. Microtectonic analysis shows that the nappe was transported from south to north along the stretching lineation. Geometric features suggest that the Samegawa dome was emplaced by diapirism. The role of the thrust surface as an instable interface promoting the doming is emphasized. The Hitachi metamorphic rocks composed of basic schist, limestone and sandstone shist thrust the pelitic rocks of the western Hitachi gneisses. As for the Takanuki area, the thrusting occurred in ductile synmetamorphic conditions with a north or northeastward displacement. Owing to lithologic, petrologic, structural similitudes, the nappe of the Hitachi metamorphic rocks and that of the Gosaisho series are unified into a unique nappe with a northward motion. The emplacement occurred between late Permian and late Cretaceous likely in late Jurassic. The allochthonous units of the Abukuma massif are correlated with the Green Schist nappe described in Southwest Japan, since they are surrounded by the same zones, namely the Tanba zone and the Kurosegawa-Kitakami one. Moreover both in Southwest and Northeast Japan, the emplacement of the Green Schist nappes is due to a shear deformation inducing rotational structures along the stretching lineation indicating the same sense of transport, that is eastward in Southwest Japan and northward in Northeast Japan, owing to the late bending of the Japanese Islands. The late Jurassic nappe structure is obliquely overprinted by a HT metamorphism, Ryoke in Southwest Japan, Abukuma in Northeast Japan, and afterwards cut by late faults as the Median Tectonic Line or the Tanakura fault, giving rise to the present complexity.     

Rocking strong earthquake accelerations

In this paper, the method presented by Lee and Trifunac (1985) for generating synthetic torsional accelerograms has been extended to the estimation of synthetic rocking accelerograms and of their response spectra. Results from our previous regression analyses for the characterization of strong shaking in terms of (1) earthquake magnitude and epicentral distance, or (2) Modified Mercalli Intensity at the site are utilized here again. The effects of geologic environment, in terms of site parameters or the representative depth of sediments, which influence amplification, and the dispersive properties of ground motion are also included. The synthetic rocking accelerogram is then constructed from the horizontal and vertical acceleration components.     

Petrologic evolution of Krakatau (Indonesia): Implications for a future activity

Krakatau Volcano is located along a N35E volcanic lineament running through the Sunda straits (Indonesia). Its last activity has been characterized by successive phases, each beginning with the construction of a cone, and ending with its destruction and the formation of a caldera. The two last (pre- and post-1883) cycles are well known, but the more ancient ones are not so clearly defined.Lavas of Krakatau belong to an andesitic series, in which fractional crystallization plays the most important role. The petrologic evolution is characterized by a cyclicity in good agreement with the structural evolution: the succession is regular: basalts, basic andesites, acid andesites, dacites. A gap between acid and basic andesites occurs in each cycle. The destructive stages correspond to the occurrence of dacitic terms.The Anak cycle was characterized from 1927 to 1979 by basalts and basic andesites; the 1981 eruption involved a more differentiated magma (close to dacitic). Detailed study of the petrologic evolution since 1883 emphasizes the predominant role of fractional crystallization. This process occurred during a very short period, between 1979 and 1981. Separation of labradorite, augite, olivine and magnetite from parental basic andesite may generate the dacitic descendant, in a shallow reservoir (P_H2O estimated about 0.5 kbar). Implications for a future activity are considered.     

Eruptive history of the Colima volcanic complex (Mexico)

The evolution of the Colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes: emission of andesitic to dacitic lava flows, acid-ash and pumice-flow deposits, fallback nuées ardentes leading to pyroclastic flows with heterogeneous magma, plinian air-fall deposits, scoriae cones of alkaline and calc-alkaline nature. Four caldera-forming events, resulting either from major ignimbrite outbursts or Mount St. Helens-type eruptions, separate the main stages of development of the complex from the building of an ancient shield volcano (25 × 30 km wide) up to two summit cones, Nevado and Fuego.The oldest caldera, C1 (7–8 km wide), related to the pouring out of dacitic ash flows, marks the transition between two periods of activity in the primitive edifice called Nevado I: the first one, which is at least 0.6 m.y. old, was mainly andesitic and effusive, whereas the second one was characterized by extrusion of domes and related pyroclastic products. A small summit caldera, C2 (3–3.5 km wide), ended the evolution of Nevado I.Two modern volcanoes then began to grow. The building of the Nevado II started about 200,000 y. ago. It settled into the C2 caldera and partially overflowed it. The other volcano, here called Paleofuego, was progressively built on the southern side of the former Nevado I. Some of its flows are 50,000 y. old, but the age of its first outbursts is not known. However, it is younger than Nevado II. These two modern volcanoes had similar evolutions. Each of them was affected by a huge Mount St. Helens-type (or Bezymianny-type) event, 10,000 y. ago for the Paleofuego, and hardly older for the Nevado II. The landslides were responsible for two horseshoe-shaped avalanche calderas, C3 (Nevado) and C4 (Paleofuego), each 4–5 km wide, opening towards the east and the south. In both cases, the activity following these events was highly explosive and produced thick air-fall deposits around the summit craters.The Nevado III, formed by thick andesitic flows, is located close to the southwestern rim of the C3 caldera. It was a small and short-lived cone. Volcan de Fuego, located at the center of the C4 caldera, is nearly 1500 m high. Its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls.The evolution of the primitive volcano followed a similar pattern leading to formation of C1 and then C2. The analogy between the evolutions of the two modern volcanoes (Nevado II–III; Paleofuego-Fuego) is described. Their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir, or two independent magmatic chambers, after the evolution of a common structure represented by the primitive volcano.     

Identification and age of neoformed Paleozoic feldspar (adularia) in a Precambrian basement core from Scioto County,Ohio, USA

Fourteen core samples of Precambrian granitic gneisses from a well drilled in the Green Township, Scioto County, Ohio were studied to determine the origin of alkali feldspar in these rocks. The well intersected the basement at a depth of 1,700 m and penetrated 11.3m of Precambrian crystalline rocks. Petrographically the samples in the upper 6.4 m of the basement core show evidence of severe alteration by the presence of hematite, limonite and chlorite and by the absence of plagioclase. Alkali feldspars from this part of the core are turbid, have a low 2 V of about 10°, are highly enriched in K, have low Na and Rb concentrations, lack cathode luminescence, and form a straight line on a Rb-Sr isochron diagram yielding a date of 599±69 Ma. Core samples from below 6.4 m appear relatively fresh and unaltered. Alkali feldspar from this portion of the core is orthoclase, shows uniform blue luminescence and gives a Rb-Sr date of 1,162±11 Ma. These results indicate that feldspars in the lowest part of the core are primary minerals that crystallized during the Grenville Orogeny, whereas the K-feldspar in the top of the core is of low-temperature secondary origin. The formation of this feldspar is explained as a consequence of chemical weathering of primary feldspar during late Precambrian time to clay minerals that were later reconstituted under low-temperature hydrothermal conditions as K-feldspar (adularia) by reactions with brines derived from the overlying Mt. Simon Formation of Cambrian age.Laboratory for Isotope Geology and Geochemistry (Isotopia), Contribution No. 68     

Application of the hydrologic visibility concept to estimate rainfall measurement quality of two planned weather radars

This paper presents a practical application of the “hydrologic visibility” concept to select the future site of two planned weather radars of the French national network ARAMIS. This selection was realised by simulating the errors in radar rainfall measurement due to interactions of the radar beam with relief, and to the vertical variation of the radar reflectivity with altitude. Results show the interest of these simulations to optimise the radar location according to the objectives of radar coverage. Beyond these results, this paper highlights aspects interesting for hydrology: this type of simulation can be used to assess the radar measurement quality before initiating a quantitative exploitation of radar data, and before making a comparison or a combination with rain gauge data.     

The junction of Fensch and Moselle rivers,France; mineralogy and composition of river materials

The Moselle river flows in the north-east of France, from Vosges Mountains to neighboring countries Luxembourg and Germany. One of its tributaries, the Fensch river, drains a highly industrialized watershed, strongly impacted by mining, smelting and surfacing activities. The objective of this work, part of a general research program on Moselle watershed (Zone Atelier Moselle) was to assess the impact of the polluted Fensch river on the global quality of the Moselle river. For that purpose, water, sediments and suspended particulate matter were sampled in both rivers, upstream and downstream the junction. Four main sampling campaigns were carried out, in winter during a flood event and in spring at low water level. On a first step, mineralogical analyses (XRD and FTIR) and chemical analyses (ICP-MS, ICP-AES), were performed on sediments, suspended particulate matters and filtered waters. Major and trace elements concentrations were obtained on two different granulometric fractions (0–2 mm and 0–50 μm) revealing the enrichment in heavy metals of fine particles. From one collecting campaign to another, seasonal variations could be evidenced on suspended matter composition even though major minerals (calcite, quartz and kaolinite) were always present. Furthermore, spatial variations were evidenced for Fensch and Moselle downstream sediments. Thus, very fine-grained sediments, poorly crystallized, displaying at the same time higher metal concentrations and higher organic matter content than in Fensch river material, were collected downstream, in a low hydrodynamic conditions zone, assumed as a preferential sedimentary zone or even as a placer. Strong correlations could be revealed between iron content and contaminant concentrations, confirming the origin of polluted material.     

Multiple geographies of the Arab Internet

The Arab world plays a relatively minor role in the rapidly changing geographies of global cyberspace. This paper explores the multiple geographies of the Arab Internet. First, it addresses Internet penetration rates, which averaged 7.8 per cent in 2006, although these varied widely among and within the region's countries. Between 2000 and 2006, the number of users jumped by 830 per cent, indicating these geographies are in rapid flux. It then examines the telecommunications infrastructure of the Middle East and North Africa, including fixed and mobile telephone networks and Internet cafes. Third, it turns to the reasons why the Internet has experienced relatively late adoption among Arab countries, including the dominance of the Latin alphabet, high access costs reflecting state-owned telecommunications monopolies, low Arab literacy rates, and restrictive gender relations that keep the proportion of female users low. The paper pays special attention both to government censorship of the Arab Internet as well as resistance to such controls and attempts to utilize the Internet counter-hegemonically. Finally, it explores the impacts of the Internet on some Arab societies, including the opening of discursive communities of politics, the Palestinian–Israeli conflict, Iraq and electronic commerce.