Rhyodacites,andesites, ferro-basalts and ocean tholeiites from the galapagos spreading center

Volcanic rocks, dredged from depths greater than 1000 m on the Galapagos spreading center, show extreme chemical diversity, including rhyodacites, andesite, ferro-basalts, and low-K oceanic tholeiite. All samples have fresh glassy margins. The ferro-basalts contain up to 18.5% total iron as FeO and up to 3.75% TiO₂, while the oceanic tholeiites are as low as 0.02% K₂O. The ferro-basalts correlate with the previously proposed zone of high magnetic anomaly amplitudes which flank the Galapagos hot spot, and are consistent with a genesis by shallow fractional crystallization.     

La sismicité en corse: Revue historique et etude macrosismique de quelques evénéments récents

Résumé L'étude des documents (archives, presse et catalogues sismiques) que nous avons consultés, nous a permis de remonter jusqu'à la fin du XVIIIe siècle dans l'histoire sismologique corse. Le nombre des séismes ressentis (épicentres en mer ou sur l'île) recensés pendant ces deux siècles et demi est relativement faible. Les intensités maximales ont parfois atteint le degré VI, plus rarement VII. L'activité sismique touche principalement une région située au Nord-Est de la Corse, au voisinage de failles au rejeu récènt et séparent le domaine de la Corse schisteuse (alpin) du domaine de la Plaine Orientale (Quaternaire, Néogène). Une crise sismique récente (1978–1979) a de nouveau secoué cette même région où on a suggéré l'activité actuelle de certaines de ces failles. Les plus importants de ces séismes (I _max=VI et V) ontifait l'objet d'enquêtes macrosismiques qui permettent d'en préciser les épicentres. Deux d'entre-eux ont été étudiés en détail (3 avril 1978 et 27 mars 1979); en utilisant la relation de Kövesligethy, les hypocentres se situent respectivement à 4,5±0.5 km et vers 5 km, pour des coefficients d'absorption de 0,020 et 0,031. Ces profondeurs indiquent que ces deux séismes ont mis en jeu des éléments superficiels de la croûte corse (30 km).

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The documents that we have compiled (archives, newspaper reports and seismic catalogues) enabled us to go back to the end of the 18th century in the seismic history of Corsica. The number of felt earthquakes (epicentres off shore or on the island) which were documented during the two and a half centuries is relatively low. Maximum intensities have reached VI or rarely VII. The seismic activity is mainly located in the North-East of Corsica, near faults which have recently been active between the Corse schisteuse (Alpine domain) and the Plaine Orientale (Quaternary, Neogene). A seismic crisis happened recently (1978–1979) in this area where present activity of some of these faults has been suggested. Macroseismic studies have been conducted for the main shocks (I _max=VI and V) in order to locate them accurately. Two among them (April 3, 1978 and March 27, 1979) are studied here in detail. Using the Kövesligethy formula hypocenters are respectively found at 4.5±0.5 km and around 5 km, whereas the corresponding absorption factors are 0.020 and 0.031. Such depths show that the two events affected the upper crust of Corsica (30 km).

Contribution n^o 321 de l'Institut de Géophysique, Ecole Polytechnique Fédérale de Zürich.     

Volcano spacing,fractures, and thickness of the lithosphere

Both “hot-spot” type and possibly island-arc volcanoes may form at the intersections of fractures whose spacing is near the thickness of the lithosphere and increases with increasing thickness. An approximate equality between layer thickness and spacing of major fractures observed in some sedimentary rocks and clay cake models may thus extend to the “mega-joints” that have fractured the lithosphere and controlled volcano spacing on the earth, and possibly on Mars. If the hot-spot fractures are interpreted as due to shear, many hot-spot fracture systems suggest roughly north-south least principal stress, or, alternatively in some instances, a 90° rotation of this pattern.     

Pollutant dispersion close to an urban surface – the BUBBLE tracer experiment

Summary In this publication first results of an urban tracer experiment are reported. This experiment was realized in the framework of the Basel UrBan Boundary-Layer Experiment (BUBBLE) in an area with abundant information on turbulence and flow conditions available. Release height was close to roof level and so was the height of the concentration samplers. The meteorological conditions during the experiments were mainly convective, but due to the rough character of the underlying surface also the mechanical turbulence was substantial.The concentration distribution is found to be essentially Gaussian in the horizontal plane and some commonly used methods to estimate the plume widths in applied dispersion models are compared to the observations. From measurements at one site downwind of the source it is found that for a near-roof level source, only an insignificant vertical gradient in tracer concentration is present within a street canyon. Using a Lagrangian Particle Dispersion Model the tracer experiments are simulated. It is shown that the exact form of the parameterization for the flow and turbulence structure within the urban roughness sublayer is of great importance for the simulation results. Also the numerical simulation results underline the necessity (and difficulty) to describe the vertical profile of the dissipation rate of turbulent kinetic energy close to an urban surface.     

Genetic implications of geochemical factor analysis in a sediment-hosted Cu-Pb-Ba mineralization

Factor analysis has been applied to geochemical data obtained from Abra, a sediment-hosted Cu-Pb-Ba mineralization in the Late Proterozoic (1,100 Ma) Bange-mall Basin in Western Australia. From some 5,000 m of drill core 57 samples have been analyzed for 26 elements by ICP/AAS. Statistical analysis was performed on 19 elements (W, Zn, P, Pb, Co, Ni, Ba, Fe, Mn, Mg, V, Al, Ca, Cu, Ti, Sr, Na, K, Si). The data have been treated first by statistical standard methods, not considered here, then by representative graphic display of the spatial distributions of elements in a section through the mineralization, and finally by factor analysis. Three groups of elements can be distinguished according to their mode of spatial distribution in section, i.e., strata-bound, discordant, and equivocal. Eight factors have been found to explain 90% of the total variability, and the spatial distribution of these factors is discussed with respect to their geological plausibility.These findings were confirmed by previous geological and mineralogical studies. In addition, a second mineralizing fault system at the southern margin of the Abra Sub-Basin could be identified by means of the spatial distribution of factor scores.     

Genesis of the Kinabalu (Sabah) granitoid at a subduction-collision junction

The Kinabalu batholith is a late Neogene granitoid in northwestern Sabah (East Malaysia) apparently marking the locus at which subducted South China Basin lithosphere interacted with roots of the northern Sabah collision suture. The exposed batholith comprises a relatively small core of biotite quartz monzodiorite (BQM) grading out to dominant hornblende quartz monzonite (HQM). Both lithologies contain mafic igneous and metasedimentary inclusions and are cut by late-stage aplite dikes. Major element data indicate the BQM (K₂O/Na₂O ratios ranging 0.72–1.03) represents a low-K type while HQM (K₂O/Na₂O ratios ranging 1.35–5.58) is a distinct high-K type. Magmaphile element distributions support this distinction, HQM showing higher K, Rb, LREE and lower Ta and Nb contents than BQM, indicating the more extensive interaction of HQM with sial. Least-squares mass balance models suggest that HQM evolved through the combined effects of fractional crystallization and crustal assimilation while BQM was dominated by fractional crystallization. However, similar plagioclase zoning patterns in both lithologies suggest they are comagmatic rather than generated by melting of separate sources. It is concluded that low-K type melts, formed by subduction-induced remelting of underplated lower crust, underwent high pressure sialic contamination with development of high-K character. These provided access to later-formed low-K melts which were less contaminated and consolidated to form the pluton core. The unusual zonation from inner low-K to outer high-K type compositions may indicate that the cessation of subduction prevented upward migration of the melting anomaly and thermal maturation of the pluton.     

Seismogeological and hydrological criteria for the New European Macroseismic Scale (MSK-92)

In 1988, an ESC Working Group Macroseismic Scales started upgrading the MSK-81 Intensity Scale. This paper presents the background and decisions made with respect to the so-called seismogeological effects. Discussion has pointed out that they cannot be treated and used in the same way as the effects on humans, objects and buildings, for many reasons. Therefore, the WG adopted the solution of using such effects as a side tool for intensity assessment, providing a comprehensive table where the experimental relations between seismogeological effects and intensity degrees - assessed by means of other effects - are presented.     

Self-diffusion studies of pore fluids in unconsolidated sediments by PFG NMR

The self-diffusion of water and hexadecane in medium and coarse sands from glacial sand deposits in central Germany were investigated by pulsed field gradient nuclear magnetic resonance (PFG NMR). Due to the restriction of the diffusion path at the pore/grain interface, the measured apparent self-diffusion coefficients (D(Δ)) in the pore space depend on the observation time (Δ) in the PFG NMR experiment. Although the bulk self-diffusion coefficients of water and hexadecane differ by about one order of magnitude, the apparent self-diffusion coefficients in the pore space obey the same characteristic time-behaviour, which depends only on geometrical properties of the pore system. Using the “short-time diffusion” model, surface-to-volume (S/V) ratios and inherent self-diffusion coefficients (D₀) of the pore fluids were extracted from these diffusion measurements. The S/V ratios obtained are independent of the pore fluid used and agree with known geometrical properties of the sand grains. Moreover, the D₀ values are consistent with the corresponding bulk self-diffusion coefficients measured separately. In contrast to these results of PFG NMR, simultaneous investigations of longitudinal (T₁) nuclear magnetic relaxation reveal that the relaxation time of the pore fluid is a less suitable parameter for a quantitative estimation of geometrical properties of the pore/grain interface in these unconsolidated sediments since it depends on chemical properties of the fluid/grain interface.     

Alfvén wave coupling in the auroral current circuit

Auroral phenomena are controlled by the geomagnetic field.Since the terrestrial field lines connect the auroral oval to the equatorial region at large distances, the collisionless plasma in this remote space environment can act as a power supply for the high-latitude upper atmosphere where auroral emissions take place. The coupling process is intimately linked to currents which flow across the local magnetic field direction both in the equatorial part and at the atmospheric end of the auroral field lines. These two auroral key regions are connected through currents flowing along the terrestrial field lines, thereby completing the auroral current circuit. Such field-aligned currents are carried by Alfvén waves, that is, magnetohydrodynamic shear waves, which are thus a means to exchange momentum and energybetween rather remote parts of the geomagnetically controlledspace environment. Auroral dynamics is further affected by a third key region in the auroral current circuit, namely the auroral acceleration region, where parallel electric fields accelerate particle to keV energies. This review focuses on key region coupling through Alfvén waves. Continuity requirements for currents and electric fields provide a convenient means to describe the interaction of Alfvén waves with different plasma regimes. Basic coupling aspects can be demonstrated with the help of a simplified model. Inhomogeneities and nonlinear feedback can lead to resonance effects and instabilities.