Le projet gradio et la determination a haute resolution du geopotentiel

Résumé Une des techniques de détermination fine et globale du champ de gravitation terrestre U est la gradiométrie spatiale, dans laquelle on mesure à bord d'un satellite sur orbite basse certaines combinations linéaires des composantes du tenseur ∂² U/∂x_i ∂x_j dans des axes {x _i } liés au satellite. Un tel projet, appelé GRADIO, est actuellement à l'étude en France et pourrait aboutir à partir de 1990. Après avoir rappelé les objectifs scientifiques d'une telle mission, nous en donnons les spécifications—étayées par une série d'études analytiques; nous définissons ensuite le satellite porteur et ses caractères techniques, en insistant sur les points délicats de la faisabilité (facteurs d'échelle des micro-accéléromètres constituant l'appareil, connaissance de l'attitude...) et en présentant des idées de solution en cours d'approfondissement.

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Summary Satellite gradiometry arises as one of the methods for improving our knowledge of the global Earth gravity field at high resolution: by means of micro-accelerometers on board a low orbiting spacecraft, linear combination of the gravity tensor components ∂² U/∂x_i ∂x_j are measured in a satellite-fixed reference frame {x _i }. Based on this technique, a project named GRADIO is presently under study in France and could fly in 1990 at the earliest. After the scientific objectives of that experiment have been reviewed, the measurement specifications are given as coming from various analytical studies. The platform and its characteristics are then defined: the critical realization problems (scale factors of the micro-accelerometers, spacecraft attitude control and restitution) are pointed out together with some ideas for their solution which are under analysis and require further study.

     

Geoid,Pangea and convection

We show that the present geoid has a simple low-order configuration with an axis of symmetry in the equatorial plane. We show further that it is a “tennis-ball” pattern, with an equatorial high belt and a polar low one, which is clearly controlled by the rotation of the Earth. Finally, we show that the outline of Pangea between at least 200 Ma and 125 Ma ago lay along a great circle passing through the paleo-poles of rotation. Thus, it also had an axis of symmetry in the equatorial plane. This hemispheric super-continent configuration ended in Middle Cretaceous time during a major geologic catastrophe which was accompanied by high rates of spreading, hotspot outbreaks and high sea-level stands. We interpret this evidence in terms of separate steady state lower mantle convection, responsible for the present geoid, weakly coupled to the upper mantle one. This weak coupling leads to the hemispheric continent configuration which ends when excessive heating of the upper mantle due to the insulating continental cap leads to continent dispersal. The complete cycle, from one supercontinent to the next, might be of the order of 400 Ma.     

The eastern and western ends of Nankai Trough: results of Box 5 and Box 7 Kaiko survey

Seabeam mapping and detailed geophysical surveying have been conducted over the Nankai Trough where the fossil Shikoku Ridge is subducted below southwest Japan. The geometry of the oceanic lithosphere bending under the margin as well as the three-dimensional structure of the accretionary prism have thus been determined in detail. Three 350° trending, probably transform faults have been identified in the area of the survey. They do not extend further south and appear to be limited to the last phase of spreading within the Shikoku Basin, probably between 15 and 12 Ma; this last phase of spreading would then have been accompanied by a sharp change in spreading direction from east-west to N 350°. The two eastern transform faults limit a zone of reduced Nankai trench fill of turbidites opposite to the Tosa Bae Embayment. This observation suggests that the Tosa Bae Embayment actually results from this reduced supply of trench fill to the imbricate thrusting process. The accretionary prism can be divided into three different tectonic provinces separated by continuous mappable thrusts, the Lower and Upper Main Thrusts. Surface shortening is limited to the lower accretionary prism south of the Upper Main Thrust (UMT) whereas uplift with possible extension characterizes the prism above the UMT. Deformation, due to the relative plate motion, mostly affects the lower accretionary prism south of the UMT.     

Deep basins of the Black Sea and Caspian Sea as remnants of Mesozoic back-arc basins

We review the geological and geophysical structural framework of the deep Black Sea and Caspian Sea basins. Based on seismic evidence and subsidence history, we conclude that the deep basins have an oceanic crust formed in a marginal sea environment. We propose that the present deep basins are remnants of a much greater marginal sea formed during three separate episodes during the Mesozoic: in the Middle Jurassic, Upper Jurassic and Late Cretaceous. A tentative sketch of the geologic evolution of the area is presented. The marginal sea reached its greatest extent in the Early Tertiary when it was about 900 km wide and 3000 km long. The central part of the marginal sea has since disappeared during the collision between the Arabian promontory and the Eurasian margin.     

Surface destabilization and laboratory-induced non-stoichiometry in San Carlos olivine

Annealing experiments on natural olivine (Mg_1-x Fe _x )₂SiO₄ (with x≈0.11) crystals (San Carlos, Arizona, spinel-lherzolite context) have been performed between T=1,100° C and 1,500° C for oxygen partial pressures pO₂=10^?3 to 10^?13 bar and times of 1 to 140 h in CO/CO₂ or H₂/H₂O gas mixtures. Even specimens annealed within the T-pO₂ theoretical stability field (TSF) calculated for stoichiometric olivine (Nitsan 1974) show systematic alterations developed within the first few microns of the surface of the crystals. Pyroxene crystals or melt form on the original olivine surface even at T=1,100° C, with preference of pyroxene when T<1,350° C and SiO₂-rich glass if T>1,350° C. This glass (rhyolite-like) can concentrate calcium from the starting olivine, and aluminum when Cr-Al spinels are present as inclusions. These observations are in contradiction with the TSF. They are obviously due to the presence of platinum used as a container of our samples, even if the contact between olivine and platinum is very weak. Rapid surficial diffusion of iron toward platinum (or via the gas phase) induces a Fe-depleted surface. According to the TSF, this more forsteritic surface should have a wider pO₂ range of stability. This is not the case, just because this situation is largely out of equilibrium. This iron loss induces a departure from cationic stoichiometry: (Mg, Fe)_2(1?δ), SiO₄ with δ small and positive. We extended the model that Nakamura and Schmalzried (1983) (N.S.) developed for fayalite (x=1) to our natural olivine composition, under the assumption that the majority defects are magnesium vacancies, Fe³⁺ occupying octahedral and tetrahedral sites, and the more complex neutral defect corresponding to Coulombic attraction between neighboring Fe³⁺ ions. We have recalculated the olivine stability field in pO₂ vs. δ space at T=1,300° C using this model for x≈0.1 (its extreme limit of validity) and conclude that olivine is stable only in a very narrow range in pO₂ which depends on δ. The calculation shows also that when olivine has nearly cationic stoichiometry (δ=0) as we believe for our starting material, the pO₂ range of stability is narrower than indicated by the TSF. In particular, it explains why Fe precipitates from the olivine (δ=0) (in absence of any other precipitation of SiO₂-rich phases) between 10^?11 and 10^?13 bar at 1,300° C; this was not predicted by the TSF. Magnetite or iron precipitates also coexist with SiO₂-rich exsolutions or pyroxene when pO₂ is close to the upper or lower boundaries of the TSF, respectively. The N.S. model may have important implications for the interpretation of the existence of partial melting and/or the low-viscosity/low velocity zone in the upper mantle.     

Controls on the organic chemical composition of settling particles in the Northeast Atlantic Ocean

The organic matter of sinking particulate material collected in the Northeast Atlantic Ocean (ca. 49°N, 16°W) was investigated in order to determine temporal and depth-related variability in its composition. Three sediment traps were deployed at nominal depths of 1000 m (below the permanent thermocline), 3000 m (representing the deep-water fluxes) and at 4700 m, about 100 m above the seafloor (just above the benthic boundary layer). The samples span a 28-month sampling period from October 1995 until February 1998, each sample representing a period of between 7 and 28 days.Total organic carbon and total nitrogen contents decrease with depth, as did the absolute concentrations of most biochemicals measured in this study, such as intact proteins and individual lipids. However, concentrations of proteins relative to total organic carbon and total nitrogen did not show any significant change with depth, implying that they are not being rapidly degraded and so may provide an important supply of nitrogen to the benthos. Fluxes of protein, TN and TOC are significantly correlated at all depths.Lipid compositions vary temporally. During periods of high flux, particularly in the summer, the lipids are richer in ‘labile components’, namely unsaturated fatty acids and low molecular weight alcohols. During periods of low flux other compounds, such as sterols, steroidal ketones and a trisnorhopan-21-one are more abundant. One sample, taken close to the seafloor, was highly enriched in lipids, sterols and fatty acids in particular; this may represent detritus derived from bottom-dwelling invertebrates.     

Relationship between the fractal dimension of orthopyroxene distribution and the temperature in mantle xenoliths

As rock textures reflect the physical conditions and the mechanisms of formation of the rocks, new approaches are used for improving texture analyses, both qualitatively and quantitatively. Pioneer work has recently boosted interest in fractal analysis for quantifying and correlating patterns. Fractal‐like patterns relate to a degree of multiscale organization, and fractal dimensions (FD) and their potential variations can be used to infer the physical conditions of rock formation at various scales of observation. Here, we characterize quantitatively the shape and distribution of orthopyroxene grains in ultramafic xenoliths in terms of FD and their relation with temperature of equilibration. Fractal analysis has been applied to several populations of mantle xenoliths: 7 xenoliths collected in the vicinity of Pico Santa Isabel on Bioko Island, an alkaline basaltic volcano in oceanic domain (Gulf of Guinea, Equatorial Atlantic), 9 samples from Sangilen, in the Agardag alkaline lamprophyre dyke (Russia), and 11 samples form Śnieżnik (Lutynia, Poland), in the continental domain. Fractal analysis has been conducted to characterize the degree of complexity of the petrographic textures: it is indeed known that large values of FD are associated to more complex textures. The correlation here observed between the orthopyroxene fractal dimension and the temperature of equilibration suggests that FD captures a significant textural feature directly related to the temperature (i.e. generated by a temperature‐controlled process). The significant difference in the FD–T correlation observed for the continental and oceanic mantle domains suggests that the mechanical and rheological behaviour is distinct in the oceanic and continental lithospheres. These first promising results should be confirmed by analysing other mantle suites of rocks in different geodynamic settings. Copyright © 2015 John Wiley & Sons, Ltd.     

Plate convergence measured by GPS across the Sundaland/Philippine Sea Plate deformed boundary: the Philippines and eastern Indonesia

The western boundary of the Philippine Sea (PH) Plate in the Philippines and eastern Indonesia corresponds to a wide deformation zone that includes the stretched continental margin of Sundaland, the Philippine Mobile Belt (PMB), extending from Luzon to the Molucca Sea, and a mosaic of continental blocks around the PH/Australia/Sunda triple junction. The GPS GEODYSSEA data are used to decipher the present kinematics of this complex area. In the Philippines, the overall scheme is quite simple: two opposing rotations on either side of the left-lateral Philippine Fault, clockwise to the southwest and counterclockwise to the northeast, transfer 55 per cent of the PH/Sundaland convergence from the Manila Trench to the northwest to the Philippine Trench to the southeast. Further south, 80 per cent of the PH/Sunda convergence is absorbed in the double subduction system of the Molucca Sea and less than 20 per cent along both continental margins of northern Borneo. Finally, within the triple junction area between the Sundaland, PH and Australia plates, from Sulawesi to Irian Jaya, preferential subduction of the Celebes Sea induces clockwise rotation of the Sulu block, which is escaping toward the diminishing Celebes Sea oceanic space from the eastward-advancing PH Plate. To the south, we identify an undeformed Banda block that rotates counterclockwise with respect to Australia and clockwise with respect to Sundaland. The kinematics of this block can be defined and enable us to compute the rates of southward subduction of the Banda block within the Flores Trench and of eastward convergence of the Makassar Straits with the Banda block. The analysis made in this paper confirms that this deformation is compatible with the eastward motion of Sundaland with respect to Eurasia determined by the GEODYSSEA programme but is not compatible with the assumption that Sundaland belongs to Eurasia, as was often assumed prior to this study.     

Evolution of a Paleoproterozoic “weak type” orogeny in the West African Craton (Ivory Coast)

The Paleoproterozoic domain of the Ivory Coast lies in the central part of the West African Craton (WAC) and is mainly constituted by TTG, greenstones, supracrustal rocks and leucogranites. A compilation of metamorphic and radiometric data highlights that: i) metamorphic conditions are rather homogeneous through the domain, without important metamorphic jumps, ii) HP-LT assemblages are absent and iii) important volumes of magmas emplaced during the overall Paleoproterozoic orogeny suggesting the occurrence of long-lived rather hot geotherms. Results of the structural analysis, focused on three areas within the Ivory Coast, suggest that the deformation is homogeneous and distributed through the Paleoproterozoic domain. In details, results of this study point out the long-lived character of vertical movements during the Eburnean orogeny with a two folds evolution. The first stage is characterized by the development of “domes and basins” geometries without any boundary tectonic forces and the second stage is marked by coeval diapiric movements and horizontal regional-scale shortening. These features suggest that the crust is affected by vertical movements during the overall orogeny. The Eburnean orogen can then be considered as an example of long-lived Paleoproterozoic “weak type” orogen.