Apport de la modélisation géophysique pour la compréhension de la structuration du pas de Calais

This study is based on an analysis of seismic, gravimetric and magnetic data to model the structure of the Dover Straits area. The Dover Straits are set in a transitional position within the Brabant para-autochthon, which lies between the Ardennes allochthon to the south and the Brabant Massif to the north. The seismic profiles show that this area is characterised by several superimposed Mesozoic basins, whose evolution has been controlled by the reactivation of Lower Palaeozoic faults. In common with the adjacent Weald–Wessex and southern North Sea basins, the Dover Straits area records a Jurassic extensional phase and a post-Cretaceous tectonic inversion. The geophysical modelling reveals the presence of a basement to these Mesozoic basins that exhibits Ordovician to Silurian structures, which cannot be detected on the seismic reflection profiles. In contrast to earlier interpretations, the modelling presented here suggests that the Brabant Massif continues southward beyond the Dover Straits and under the Brabant para-autochthon. Gravity data also provides evidence for the presence of granitic body at depth, in the vicinity of Km 27 on the seismic profile, which may have driven the uplift of the Silurian rocks in the area of the Straits. Similar granitic intrusions have been proposed to explain gravity anomalies along the Rhenohercynian Margin in Belgium, eastern and southern England. The geophysical modelling demonstrates the continuity of the basement and Mesozoic basin structures across the Dover Straits, suggesting a common geological evolution of the adjacent southern England–northern France regions. To cite this article: B. Minguely et al., C. R. Geoscience 337 (2005).     

Paleoproterozoic Crustal Evolution of the São Luís Craton, Brazil: Evidence from Zircon Geochronology and Sm-Nd Isotopes

The São Luís Craton, northern Brazil, is composed of a few granitoid suites and a metavolcano-sedimentary succession. New single zircon Pb evaporation ages and Nd isotope data, combined with other available information, show that the metavolcano-sedimentary succession developed from 2240 Ma to approximately 2200-2180 Ma from juvenile protoliths. The subduction-related calc-alkaline suites of granitoids, spatially associated with the metavolcano-sedimentary sequence, formed in an oceanic island arc setting between 2168-2147 Ma. Most of these granitoids are tonalitic and formed from juvenile, mantle- or oceanic plate-derived protoliths, whereas minor true granites are the product of the reworking of the juvenile island arc material. These arc-related successions represent an accretionary event around 2.20±0.05 Ga, which is coincident with one of the main periods of crustal growth in the South American Platform. This accretionary orogen has subsequently been involved in a collision episode, at ca. 2100-2080 Ma, which is mainly recorded in the nearby Gurupi Belt. The rock associations, inferred geological settings, and the crustal evolution detected in the São Luís Craton are similar to what is described in Paleoproterozoic domains of major geotectonic units of the South American Platform, such as part of the São Francisco Craton, southeastern Guyana Shield, and of the West African Craton.     

The influence of H2O-H2 fluids and redox conditions on melting temperatures in the haplogranite system

Solidus temperatures of quartz–alkali feldspar assemblages in the haplogranite system (Qz-Ab-Or) and subsystems in the presence of H₂O-H₂ fluids have been determined at 1, 2, 5 and 8 kbar vapour pressure to constrain the effects of redox conditions on phase relations in quartzofeldspathic assemblages. The hydrogen fugacity (f _H2) in the fluid phase has been controlled using the Shaw membrane technique for moderately reducing conditions (f _H2 < 60 bars) at 1 and 2 kbar total pressure. Solid oxygen buffer assemblages in double capsule experiments have been used to obtain more reducing conditions at 1 and 2 kbar and for all investigations at 5 and 8 kbar. The systems Qz-Or-H₂O-H₂ and Qz-Ab-H₂O-H₂ have only been investigated at moderately reducing conditions (1 and 5 kbar) and the system Qz-Ab-Or-H₂O-H₂ has been investigated at redox conditions down to IW (1 to 8 kbar). The results obtained for the water saturated solidi are in good agreement with those of previous studies. At a given pressure, the solidus temperature is found to be constant (within the experimental precision of ± 5°C) in the f _H2 range of 0–75 bars. At higher f _H2, generated by the oxygen buffers FeO-Fe₃O₄ (WM) and Fe-FeO (IW), the solidus temperatures increase with increasing H₂ content in the vapour phase. The solidus curves obtained at 2 and 5 kbar have similar shapes to those determined for the same quartz - alkali feldspar assemblages with H₂O-CO₂- or H₂O-N₂-bearing systems. This suggests that H₂ has the behaviour of an inert diluent of the fluid phase and that H₂ solubility in aluminosilicate melts is very low. The application of the results to geological relevant conditions [HM (hematite-magnetite) > f _O2 > WM] shows that increasing f _H2 produces a slight increase of the solidus temperatures (up to 30 °C) of quartz–alkali feldspar assemblages in the presence of H₂O-H₂ fluids between 1 and 5 kbar total pressure. Received: 4 March 1996 / Accepted: 22 August 1996     

Neogene–Quaternary structures in the border zone between Alps, Dinarides and Pannonian Basin (Hrvatsko zagorje and Karlovac Basins, Croatia)

Analysis of Neogene-Quaternary structures from seismic lines, surface measurements and geological-mapping is presented from the border zone between the Alps, Dinarides and Pannonian Basin. First, Early Miocene extension was possibly characterised by ENE directed extension. It was partly synchronous with NW-SE shortening. Second, Middle Miocene extension was possibly characterised by NW-SE to WNW-ESE directed extension. Again, this event was followed by a new generation of thrusts related to end-Sarmatian shortening. The last, Late Miocene E-W to WNW-ESE directed extension was followed by a final shortening that created major, map-scale folds, basement pop-ups and inverted former basins. Geometry, onlap and thickness patterns of the youngest syn-tectonic basin fill indicate that this last, N-S to NW-SE directed shortening started in Late Pontian and continued up to the present time. When taking into account the wider surrounding area, it seems that the structures related to this latest shortening are arranged in often perpendicular directions, centred at the eastern end of the Periadriatic lineament. To explain this fan-like pattern of synchronous shortenings a kinematic model is proposed combining counter-clockwise rotation with north- or northwestward shift of the Dinaridic block with respect to the more stable Alpine buttress.     

Concentrated flow erosion in cultivated catchments: Influence of soil surface state

Concentrated flow erosion is the dominant form of winter erosion in northern France. This study correlates the ephemeral rill and gully volumes measured in 20 cultivated catchments (4–95 ha) for three consecutive winters with the size of the potential runoff-contributing areas. These areas were identified by characterizing soil surface state through crust development stage, importance of surface wheel tracks and roughness grade. A single and significant relationship was found between the size of runoff-contributing areas, estimated by this criterion, and the rill and gully volumes. This identified the proportion of the catchment area occupied by fields with a degraded surface structure as the main factor controlling the variability of erosion in a context of concentrated flow erosion on cultivated land. The extension of degraded areas was shown to be controlled by dynamic interactions between weather, land occupation and soil physical properties. This criterion accounts for the uneven distribution of rainfall in space and time. Morphological factors, such as talweg length and slope, are believed to determine part of the residual variability.     

Crystal structure of Di50CaTs50 synthetic clinopyroxene (CaMgo0.50AlSil1.5O6). Crystal chemistry along the Di-CaTs join

Summary The crystal structure of the Di₅₀CaTs₅₀ clinopyroxene Ca(Mg_0.5Al_0.5) (Al_0,5Si_1.5)O₆, synthesized at T=1 100°C andP=14 kb, was refined in the C 2/c space group. Cell parameters are:a=9.655,b=8.771,c= 5.278 Å, =106.22°. The finalR value is: 2.1%. Comparison with the Di and CaTs structures shows that the variation of the M 2-0, M 1-0 and T-0 bond lengths is continuous and almost linear in the clinopyroxene solid solutions along the Di-CaTs join. The modifications caused in M 2 and M 1 polyhedra by shifting in tetrahedral chains along z are significant. The different values of shifts with composition account for the anomalous trend for the angle in the DiCaTs solid solution.

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La structure cristalline du clinopyroxène synthétique CaMgo_0.5AlSil_1.5O₆ (Di₅₀CaTs₅₀). La cristallochime le long du joint Di-CaTs

Résumé La structure cristalline du clinopyroxáne Ca(Mg_0.5Al_0.5)(Al_0.5Si_1.5)O₆ (Di₅₀CaTs₅₀), cristallisé á T= 1100° etP=14 kb, a été affinée dans le groupe spatial C 2/c. Les paramétres de la maille sont:a=9.655,b=8.771,c=5.278 Å, =106.22°. La valeur finale deR est 2.1%. La comparaison avec les structures de Di et de CaTs montre que le variation des longueures de liaison M 2-0, M 1-0 et T-0 est continue et presque linéaire pour les clinopyroxénes le long du joint Di-CaTs. Le déplacement des chaines tétraédriques le long de l'axe z cause de remarquables modifications dans les polyèdres M 2 et M 1. Les différentes valeurs du déplacement le long de l'axe z avec la composition expliquent la tendance anomale du paramétre de la maille le long du joint.

     

Analysis of seismic patterns observed at Nevado del Ruiz volcano, Colombia during August – September 1985

The seismic activity of the Nevado del Ruiz volcano was monitored during August–September 1985 using a three-component portable seismograph station placed on the upper part of the volcano. The objective was to investigate the frequency content of the seismic signals and the possible sources of the volcanic tremor. The seismicity showed a wide spectrum of signals, especially at the beginning of September. Some relevant patterns from the collected records, which have been analyzed by spectrum analysis, are presented. For the purpose of analysis, the records have been divided into several categories such as long-period events, tremor, cyclic tremor episodes, and strong seismic activity on September 8, 1985.The origin of the seismic signals must be considered in relation to the dynamical and acoustical properties of fluids and the shape and dimensions of the volcano's conduits.The main results of the present experiment and analysis show that the sources of the seismic signals are within the volcanic edifice. The signal characteristics indicate that the sources lie in fluid-phase interactions rather than in brittle fracturing of solid components.     

Badrinath-Gangotri plutons (Garhwal, India): petrological and geochemical evidence for fractionation processes in a high Himalayan leucogranite

The Gangotri leucogranite is the western end of the Badrinath granite, one of the largest bodies of the High Himalayan Leucogranite belt (HHL). It is a typical fine grained tourmaline + muscovite ± biotite leucogranite. The petrography shows a lack of restitic phases. The inferred crystallization sequence is characterized by the early appearance of plagioclase, quartz and biotite and by the late crystallization of the K-feldspar. This suggests that, in spite of being of near minimum melt composition, the granite probably had long crystallization or melting interval, in agreement with previous experimental studies. Tourmaline and muscovite have a mainly magmatic origin. Even though the major element composition is homogeneous, there are several geochemical trends (when CaO decreases there is an increase in Na₂O, Rb, Sn, U, B, F and a decrease in K₂O, Fe₂O₃, TiO₂, Sr, Ba, Zr, REE, Th) which are best explained by a fractionation process with early crystallizing phases. Experimental solubility models for zircon and monazite in felsic melt support a magmatic origin for these two accessory phases as well.Rb/Sr isotope data show this granite to have, like other HHL, heterogeneous isotopic values for Sr (initial ⁸⁷Sr/⁸⁶Sr ratios, calculated at 20 Ma, range between 0.765 and 0.785). Therefore no mixing (i.e. no convection) occurred between the different batches of magma. In contrast ¹⁸O data show little variation (13.04% ± 0.25), implying a source with homogeneous ¹⁸O values. Differences in timing between fluid infiltration and the onset of melting, related to differences in temperature of the source, could explain why source homogenization occurred for the Gangotri and not for the Manaslu granite.The use of experimental results for solubility and the position of the accessory minerals during melting, predict a low viscosity for the melt during its extraction. This in turn explains the lack of restitic phases (major and accessory) in the granite as well as some field features (lensoid shape, pronounced magmatic layering). Based on the petrographic and isotopic studies, it is suggested that the mechanism of ascent was not diapiric but rather that the melt ascended along several fractures and the level of emplacement was partialy controlled by the density contrast between the melt and host rocks.     

Oxygen diffusion in amphiboles

The kinetics of oxygen isotope self-diffusion in natural samples of hornblende, tremolite, and richterite have been measured. Samples were run under hydrothermal conditions using ¹⁸O enriched water. Profiles of ${}^{18}\text{O}\text{(}{}^{16}\text{O +}{}^{18}\text{O)}$vs depth into the crystal were obtained using an ion microprobe; the depths of sputtered holes were measured using an optical interferometer. At 1000 bars (100 MPa) water pressure, the activation energies (Q) and pre-exponential factors (D₀) for diffusion parallel to c are: D₀(cm²/sec) Q (kcal/gm-atom) T (°C) Hornblende 1⁺²⁰_?1 × 10^?741 ± 6 650–800 Tremolite 2⁺³⁰_?2× 10^?8 39 ± 5 650–800 Richterite 3⁺⁵_?2 × 10^?4 57 ± 2 650–800The diffusion coefficient (D) for hornblende at 800°C and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions. An increase in water pressure from 200 to 2000 bars increases D by a factor of 2.7 for diffusion parallel to c at 800°C. The D value for hornblende at 800°C is about 0.01 that for quartz and 0.001 that for anorthite. As a result, closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars. It is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions.     

Lead-strontium isotopic variations along the East Pacific Rise and the Mid-Atlantic Ridge: a comparative study

We have determined the Pb and Sr isotopic compositions in a number of fresh young oceanic basalts from the East Pacific Rise (between 20°N and 21°S latitudes), and from the Mid-Atlantic Ridge (between 65°N and 10°N).The results confirm the PbSr isotopic correlation for mid-ocean ridges basalts obtained by Allègre et al. [1], Cohen et al. [2], Dupréand Allègre [3], and the correlation between isotopic variation and the compatible trace elements ratios variation [1].A comparison between the Atlantic and Pacific results reveals that there is a wider range of values for the Atlantic than for the Pacific. After filtering the short wavelengths, a good correlation is obtained between long-wavelength bathymetric and isotopic variations for the Atlantic.The preferred model proposed to explain these differences involves the constant presence of hot spots under ridges. On slow-spreading ridges like the Atlantic, the hot spots signature is clearly visible in both bathymetry and isotopic ratios. On fast-spreading centers, the hot spot signature in both the bathymetry and isotopic signature may be diluted by the rapid supply of material coming from the asthenosphere.However, an alternative explanation for which no hot spot influence is found on the East Pacific Rise cannot be definitely ruled out.In two occurrences, south of the Hayes fracture zone (Atlantic), large isotopic heterogeneities are observed within a single dredge. This does not contradict the concept of regional isotopic regularities, but suggests that blob injection and source mixing may be observed at very different scales under the ridges.