Etude des mecanismes de condensation des gaz magmatiques — exemple de l’Etna (Italie)

Sublimates were collected on Mt Etna in the gas escaping from the surface of an active lava flow. Deposition occurred on the wall of a silica tube. Temperatures of deposition for the different sublimates collected were recorded continuously with 6 Thermocouples along the tube. At high temperature (300 to 860 °C) the sulfates of sodium, potassium and iron deposited. Halogen were collected at low temperature with copper, zinc and nickel chloride; copper was also associated with sulfates.     

Premières mesures françaises de gravimétrie absolue dans les régions polaires Antarctique (terre Adélie) et Arctique (Spitzberg)First French measurements of absolute gravity in the Antarctic (Terre Adélie) and Arctic (Spitsbergen) polar regions

This study is devoted to the first French determinations of absolute gravity in the Antarctic (Dumont-d'Urville, Terre Adélie) and Arctic (Ny-Alesund, Spitsbergen) polar regions. The measurements in Dumont-d'Urville were performed in 2000 with the help of the FG5#206 absolute gravimeter belonging to the French geodetic community since beginning 1997; they show a strong noise that causes an uncertainty close to 11 μGal in the determination of the mean gravity value, which will be compared, to a new determination planned for 2005. The Ny-Alesund measurements show on the contrary a noise that, although very variable in time, leads to a gravity determination of better than 5 μGal. The comparison of our value with a previous one two years before suggests a gravity decrease of about 4 μGal which is related to the vertical uplift measured by Very Large Baseline Interferometry (VLBI) at the same site. However, the uncertainty does not allow discriminating between height-to-gravity conversion factors originating from different models. To cite this article: J. Hinderer et al., C. R. Geoscience 334 (2002) 819–826.     

Volcanic vents rooted on extension fractures and their geodynamic implications in the Ethiopian Rift

Most of the extension fractures located in the Ethiopian rift are related to rift dynamics, and they have consistently similar orientations for hundreds of kilometers. The orientation of extension fracture gives the local extension direction which is perpendicular to its horns (end segments of an extension fracture). We have established clear geometrical relationships between tectonics and volcanism in the Main Ethiopian Rift by interpreting high-resolution images obtained from Landsat-TM and SPOT satellites and by quantitatively analyzing the geometry of extension fractures, elongated vents and linear volcanic clusters. Applying the relationships obtained, we show that extension fractures in the rift served as channels for magma rising to the surface, and that extension fractures underlie most of the elongated volcanic vents and linear volcanic clusters. The geometry of extension fractures beneath volcanic edifices can be deduced from the shape of elongated vents and pattern of linear volcanic clusters rooted on them. Utilizing the orientations of extension fractures directly observed and those inferred from elongated volcanic vents and linear clusters, we found that the extension direction of the Main Ethiopian Rift is northwest–southeast and that the direction has been rotated clockwise for about 20° in the time interval 2.83 to 0.023 Ma. The Recent axis of rift opening is oriented N40° and located closer to the southeastern escarpment giving an asymmetric geometry to the Main Ethiopian Rift (MER).     

Simulating strong ground motion with the `k -2 ' kinematic source model: An application to the seismic hazard in the Erzincan basin, Turkey

We present a seismic hazard application of a kinematic broad-bandrupture model. This model is based on the k-square dislocation distribution concept (Herrero and Bernard, 1994).Synthetic seismograms are calculated in the far-field approximation with alayered velocity medium for the 13 March 1992 Erzincan earthquake.With a parametrization of the source constrained by other studies,the far-field contribution correctly fits the recorded strong ground motion, which presents a 0.5 to 2 Hz dominant frequency range.As the k-square model is particularly well adapted to synthetize realistic strong-motion at short distances from the fault,it is a reliable tool for calculating seismic hazard maps around active faults. We thus present a synthetic peak ground acceleration map associatedwith the 13 March 1992 activated fault, for a 60 km × 60 km regionaround the epicenter taking into account a smoothed velocity structure ofthe basin in agreement with the absence of significant site effects related to1D resonance deduced from the aftershock records study.This map is compared with several post seismic reports: macroseismicintensities, detailed distribution of damage, and soil cracking andliquefaction. Our model shows that the values of the peak acceleration andvelocity can explain the dominant spatial distributionof these effects, which concentrates in a narrow band along theactivated segment fault, and in particular at its southernextremity. These results enable us to present such maps forhypothetical future earthquake ruptures, located on the major visible orinferred active fault segments in and around the basin. The effects ofthese potential sources are analyzed in relation to the 1992 eventeffects in order to eliminate unknown site responses. We show that thesouthern part of the basin is particularly exposed because of thepresence of strike-slip faults,and that the western part of the basin would suffer a significantlyhigher strong motion levelthan during the 1992 event with the activation of moderate sizednormal faults evidenced on thesouthwestern edge of the basin.     

Evidence for Non-Conservative Behaviour of Chlorine in Humid Tropical Environments

The knowledge of the biogeochemical cycle of chlorine (Cl)is important since this element is used as a tracer of geochemical and hydrological processes in oceanic or continental environments. More specifically, Cl can be used to correct surface water composition from atmospheric contribution in order to calculate precise chemical weathering rates in watersheds. Beyond the problem of potential Cl sources in a given watershed, which is directly related to the lithology, vegetation, and industrial activities, the Cl normalization is based on the assumption that this element behaves conservatively during surface processes (e.g., chemical weathering, adsorption/desorption processes).The purpose of the present study is to forecast the geochemical behavior of Cl in a forested ecosystem located under humid tropical environment.For this reason, we have analyzed the Cl (and also Ca and Na) concentrations ofsurface waters (rainwater, groundwater, river water) over a two-year period in the Nsimi–Zoetele watershed (Cameroon).The Cl mass balance for the watershed appears to be equilibrated over the studied period (1995–1996) but Cl behavior in Mengong River draining the watershed suggests a non-conservative behavior. Indeed, Cl concentrationsin the Mengong River are low during dry seasons and high during wet seasons, which is the reverse tendency to what is usually observed taking into account dilution and evaporation processes. As Cl concentrations in the Mengong River are lower than those measured in all the feeding reservoirs, Cl should be adsorbed onto the soils of the watershed. However, as the Cl mass balance is equilibrated over the whole-year, Cl should be adsorbed and releasedat a seasonal scale. The results we obtained for this small watershed were not generalized for a larger studied basin (i.e., Nyong River basin). Even if these results should be followed by further investigations, this study suggests that Cl normalization should be used with caution to avoid under- or over-estimation of chemical weathering rates.     

Projective ordination by SIMCA: A dynamic strategy for cost-efficient environmental monitoring around offshore installations

A method for environmental monitoring using benthic species profiles as input is developed in this work. The method, referred to as projective ordination, utilises local principal component modelling (SIMCA) to obtain a cross-validated model which spans the natural variation in a region around offshore oil-producing installations. The borderline between regions with disturbed and non-disturbed species communities is subsequently decided from the residual distribution. This distribution is used to design an approximate F-test for assessing whether a community at a particular sampling location is disturbed or not. If so, the nature of the disturbance is determined by projecting the data on the PC model.Projective ordination utilises information from previous surveys to define the permissible variation in species communities, i.e. the limit of the natural variation. In addition, the method is dynamic, in a sense that the sampling locations may vary from survey to survey.Furthermore, our analysis shows that the customary number of replicate samples per station can be reduced. Modelling with only four randomly chosen replicates out of the complete set of five for each sampling location, does not affect the model significantly. On the other hand, taking only 3 replicates into account leads to significant divergences.A model based on the 1990 and 1993 surveys at the Statfjord field is presented as an example of the technique.     

From oblique subduction to intra-continental transpression: Structures of the southern Kermadec-Hikurangi margin from multibeam bathymetry,side-scan sonar and seismic reflection

The southern Kermadec-Hikurangi convergent margin, east of New Zealand, accommodates the oblique subduction of the oceanic Hikurangi Plateau at rates of 4–5 cm/yr. Swath bathymetry and sidescan data, together with seismic reflection and geopotential data obtained during the GEODYNZ-SUD cruise, showed major changes in tectonic style along the margin. The changes reflect the size and abundance of seamounts on the subducting plateau, the presence and thickness of trench-fill turbidites, and the change to increasing obliquity and intracontinental transpression towards the south. In this paper, we provide evidence that faulting with a significant strike-slip component is widespread along the entire 1000 km margin. Subduction of the northeastern scrap of the Hikurangi Plateau is marked by an offset in the Kermadec Trench and adjacent margin, and by a major NW-trending tear fault in the scarp. To the south, the southern Kermadec Trench is devoid of turbidite fill and the adjacent margin is characterized by an up to 1200 m high scarp that locally separates apparent clockwise rotated blocks on the upper slope from strike-slip faults and mass wasting on the lower slope. The northern Hikurangi Trough has at least 1 km of trench-fill but its adjacent margin is characterized by tectonic erosion. The toe of the margin is indented by 10–25 km for more than 200 km, and this is inferred to be the result of repeated impacts of the large seamounts that are abundant on the northern Hikurangi Plateau. The two most recent impacts have left major indentations in the margin. The central Hikurangi margin is characterized by development of a wide accretionary wedge on the lower slope, and by transpression of presubduction passive margin sediments on the upper slope. Shortening across the wedge together with a component of strike-slip motion on the upper slope supports an interpretation of some strain partitioning. The southern Hikurangi margin is a narrow, mainly compressive belt along a very oblique, apparently locked subduction zone.     

Impact of spatial resolution on simulated surface water mass transformations in the Atlantic

We analyze the water mass transformation in coarse (1°) and high (1/6°) resolution ocean simulations with the identical configuration of the CLIPPER model and interannual ERA15 forcing function. Climatological characteristics of surface water mass transformation in the two experiments are quite different. The high resolution experiment exhibits a stronger surface transformation in equatorial and tropical regions, in the Gulf Stream area and in the location of the formation of Subtropical Mode Water (STMW), associated with high levels of eddy kinetic energy. The coarse resolution experiment shows a better representation of the transformation rates corresponding to the densest subpolar mode waters and Labrador Sea Water (LSW). This is explained by the differences in lateral mixing procedures between high and coarse resolution experiments. The high resolution 1/6° run is eddy-resolving only in the tropics and mid-latitudes. In these areas eddies are found to enhance the process of water mass transformation compared to the isopycnal diffusion used to parameterized the eddies in the 1° model. Despite its 1/6° resolution, the high resolution model does not adequately represent eddies in the subpolar gyre and Labrador Sea. In these areas the high resolution model fails to correctly simulate water mass transformation because the lateral mixing (provided through the bi-harmonic sub-gridscale parameterization) of newly ventilated waters with surrounding waters is not efficient enough. In contrast in the coarse 1° resolution model, the strong lateral mixing and the unrealistically broad boundary currents imposed by the high diffusivity required for numerical stability mixes newly formed LSW waters with the warmer and saltier waters of the rim current. Finally, it results in a more effective representation of the surface water mass transformation in high latitudes in the 1° model. A possible impact of the increased lateral diffusion in high resolution experiment on the representation of re-stratification in the Labrador Sea was studied in sensitivity experiments with different lateral diffusion coefficients compared to the regional eddy-resolving 1/15° simulation in the subpolar North Atlantic. If the eddies are not resolved in subpolar latitudes (as in the case of 1/6° model), the GM90 parameterization with the coefficient close to 800 m² s⁻¹ provides the closest agreement with the solution of eddy-resolving 1/15° model.     

Geometry and structure of the Vitiaz Trench Lineament (SW Pacific)

Swath bathymetric, sonar imagery and seismic reflection data collected during the SOPACMAPS cruise Leg 3 over segments of the Vitiaz Trench Lineament and adjacent areas provide new insights on the geometry and the stuctural evolution of this seismically inactive lineament. The Vitiaz Trench Lineament, although largely unknown, is one of the most important tectonic feature in the SW Pacific because it separates the Cretaceous crust of the Pacific Plate to the north from the Cenozoic lithosphere of the North Fiji and Lau Basins to the south. The lineament is considered to be the convergent plate boundary between the Pacific and Australian Plates during midde to late Tertiary time when the Vitiaz Arc was a continuous east-facing are from the Tonga to the Solomon Islands before the development of the North Fiji and Lau Basins. Progressive reversal and cessation of subduction from west to east in the Late Miocene-Lower Plioene have been also proposed. However, precise structures and age of initiation and cessation of deformation along the Vitiaz Trench Lineament are unknown.The lineament consists of the Vitiaz Trench and three discontinuous and elongated troughs (Alexa, Rotuma and Horne Troughs) which connect the Vitiaz Trench to the northern end of the Tonga Trench. Our survey of the Alexa and Rotuma Troughs reveals that the lineament is composed of a series of WNW-ESE and ENE-WSW trending segments in front of large volcanic massifs belonging to the Melanesian Border Plateau, a WNW trending volcanic belt of seamounts and ridges on Pacific crust. The Plateau and Pacific plate lying immediately north of the lineament have been affected by intense normal faulting, collapse, and volcanism as evidenced by a series of tilted blocks, grabens, horsts and ridges trending N 120° to N100° and N60°–70°. This tectonism includes several normal faulting episodes, the latest being very recent and possibly still active. The trend of the fault scarps and volcanic ridges parallels the different segments of the Vitiaz Trench Lineament, suggesting that tectonics and volcanism are related to crustal motion along the lineament.Although the superficial observed features are mainly extensional, they are interpreted as the result of shortening along the Vitiaz Trench Lineament. The fabric north of the lineament would result from subduction-induced normal faulting on the outer wall of the trench and the zig-zag geometry of the Vitiaz Trench Lineament might be due to collision of large volcanic edifices of the Melanesian Border Plateau with the trench, provoking trench segmentation along left-lateral ENE-WSW trending transform zones. The newly acquired bathymetric and seismic data suggest that crustal motion (tectonism associated with volcanism) continued up to recent times along the Vitiaz Trench Lineament and was active during the development of the North Fiji Basin.