Tectonic setting of Western Pacific marginal basins

Global kinematics as well as magnetic anomalies of marginal basins, with continental geology and paleomagnetic data as additional constraints, are used to present a set of reconstructions of the Western Pacific marginal basins between 56 Ma and the Present at key periods (56, 43, 32, 20, 12 and 3 Ma). Our model accounts for the rapid motion of “exotic terranes” along the whole of the Western Pacific convergent zone.

Marginal basins appear to open in a great variety of tectonic settings, the two extreme examples being the Mariana trough where trench suction may be the predominant driving force and the South China Sea where intracontinental deformation appears to be the major mechanism. The study of marginal basins is a 3-D problem which must take into account the whole tectonic context (subduction related tectonics in cross-section and upper plate deformation in map view) and not only a 2-D problem (the classical trench-arc-back-arc problem).     

Lipid geochemistry of a post-glacial lacustrine sediment

The post-glacial environmental history of Voua de la Motte, a small pond, was studied by the lipid geochemistry of a 6 m long core. Palynological studies show that the deepest part of the core goes back to 10,000 yr BP corresponding to the time of formation of the lake following the retreat of the Rhodanian glacier. Hydrocarbons, aldehydes, linear alcohols, sterols, monocarboxylic and monohydroxy fatty acids, were determined throughout the core both in the free and bound lipid fractions, as well as in the tightly bound fraction for the carboxylic acids. There is no clear evidence of a transformation from the unbound to the bound form, except perhaps for the α-hydroxy acids.Qualitative as well as quantitative fluctuations in the distributions of these lipid classes were observed upward in the core. An attempt is made to correlate these fluctuations with the bioenvironmental and climatological evolution of the Basin on the basis of information obtained from palynology.     

New age constraints on metamorphism,metasomatism and gold mineralisation at Plutonic Gold Mine,Marymia Inlier,Western Australia

The Plutonic Well Greenstone Belt (PWGB) is located in the Marymia Inlier between the Yilgarn and Pilbara cratons in Western Australia, and hosts a series of major Au deposits. The main episode of Au mineralisation in the PWGB was previously interpreted to have either accompanied, or shortly followed, peak metamorphism in the late Archean at ca 2650 Ma with a later, minor, event associated with the Capricorn Orogeny. Here we present new Pb isotope model ages for sulfides and Rb–Sr ages for mica, as well as a new ²⁰⁷Pb–²⁰⁶Pb age for titanite for samples from the Plutonic Gold Mine (Plutonic) at the southern end of the PWGB. The majority of the sulfides record Proterozoic Pb isotope model ages (2300–2100 Ma), constraining a significant Au mineralising event at Plutonic that occurred >300 Myr later than previously thought. A Rb–Sr age of 2296 ± 99 Ma from muscovite in an Au-bearing sample records resetting or closure of the Rb–Sr system in muscovite at about the same time. A younger Rb–Sr age of 1779 ± 46 Ma from biotite from the same sample may record further cooling, or resetting during a late-stage episode of metasomatism in the PWGB. This could have been associated with the 1820–1770 Ma Capricorn Orogeny, or a late-stage hydrothermal event potentially constrained by a new ²⁰⁷Pb–²⁰⁶Pb age of 1725 ± 26 Ma for titanite in a chlorite–carbonate vein. This titanite age correlates with a pre-existing age for a metasomatic event dated at 1719 ± 14 Ma by U–Pb ages of zircon overgrowths in a sample from the Marymia Deposit. Based on the Pb-isotope data presented here, Au mineralising events in the PWGB are inferred to have occurred at ca 2630, 2300–2100 Ma, during the Glenburgh and Capricorn orogenies, and 1730–1660 Ma. The 2300–2100 Ma event, which appears to have been significant based on the amount of sulfide of this age, correlates with the inferred age for rifting of the Marymia Inlier from the northern margin of the Yilgarn Craton. The texturally-later visible Au may have been deposited during the Glenburgh and Capricorn orogenies.     

Genesis of the Dordogne (France) high-purity silica placers

This article describes the formation of the principal French high-purity silica placer (source for electrometallurgy of silicon and ferrosilicon), the Boudeau deposit in Dordogne, and of one of its subordinate deposits, the Pays Brûlé deposit. Boudeau has always been considered to be a colluvial deposit, formed in a trap produced by extensional tectonism. The present work suggests a new genetic interpretation, based on a combined study of the deposits and their underlying rocks. It shows that the quartz gravel deposits, laid down by stream flow within braided channels, were reworked by karstic progressive sinking, and that the mechanism responsible for the increase in the thickness of the economic formations is cryptokarstic corrosion. This undercovers weathering process, which involves dissolution of karstifiable rocks (oolitic and bioclastic limestones) at their interface with a permeable, non-karstifiable cover composed of chemically pure quartz gravels in a sandy-argillaceous matrix. A typological study of the gravels provides a basis for tackling the problem of their origin and their purity. They seem to be derived mainly from the synmetamorphic quartz mobilisates of the stripped metamorphic units (the lower and upper gneiss units forming the western edge of the Massif Central). Lastly, a dynamic model is suggested for the genesis of the high-purity silica deposits of Dordogne.     

Towards realistic simulations of lava dome growth using the level set method

The level set method has been implemented in a computational volcanology context. New techniques are presented to solve the advection equation and the reinitialisation equation. These techniques are based upon an algorithm developed in the finite difference context, but are modified to take advantage of the robustness of the finite element method. The resulting algorithm is tested on a well documented Rayleigh–Taylor instability benchmark [19], and on an axisymmetric problem where the analytical solution is known. Finally, the algorithm is applied to a basic study of lava dome growth.     

Automatic identification of continuous or non-continuous evolution of landslides and quantification of deformations

Landslides - Nowadays, the 3D modelling of surfaces is widely used to provide point clouds in geosciences. To study the evolution of landslides, many point clouds are available but post-processing...     

Disaster prevention,disaster preparedness and local community resilience within the context of disaster risk management in Cameroon

This paper presents a simulation of three components of near-field ground shaking recorded during the main shock at three stations of the September 16, 1978, Tabas (M _w = 7.4), Iran, earthquake, close to the causative fault. A hybrid method composed of a discrete wavenumber method developed by Bouchon (Bouchon in Bull Seismol Soc Am 71:959–971, 1981; Cotton and Coutant in Geophys J Int 128:676–688, 1997) and a stochastic finite-fault modeling based on a dynamic corner frequency proposed by Motazedian and Atkinson (Bull Seismol Soc Am 95:995–1010, 2005), modified by Assatourians and Atkinson (Bull Seismol Soc Am 97:935–1949, 2007), is used for generating the seismograms at low (0.1–1.0 Hz) and high frequencies (1.0–20.0 Hz), respectively. The results are validated by comparing the simulated peak acceleration, peak velocity, peak displacement, Arias intensity, the integral of velocity squared, Fourier spectrum and acceleration response spectrum on a frequency-by-frequency basis, the shape of the normalized integrals of acceleration and velocity squared, and the cross-correlation with the observed time-series data. Each characteristic is compared on a scale from 0 to 10, with 10 being perfect agreement. Also, the results are validated by comparing the simulated ground motions with the modified Mercalli intensity observations reported by reconnaissance teams and showed reasonable agreement. The results of the present study imply that the damage distribution pattern of the 1978 Tabas earthquake can be explained by the source directivity effect.     

Protracted,coeval crust and mantle melting during Variscan late-orogenic evolution: U–Pb dating in the eastern French Massif Central

International Journal of Earth Sciences - The late stages of the Variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt,...     

Easterly wave regimes and associated convection over West Africa and tropical Atlantic: results from the NCEP/NCAR and ECMWF reanalyses

 NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude 17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from about 3000 km/8 m s^-1 north of the jet to 5000 km/12 m s^-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics. The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength is higher, about 5000 km long, and the average phase speed is about 7 m s^-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component. The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR reanalyses, especially for the 3–5-day regime over the Atlantic. Received: 28 May 1998 / Accepted: 2 May 1999