Extension et sédimentation au paléozoïque terminal et au Mésozoïque dans le bassin de Majunga (Nord-Ouest de Madagascar)

The Majunga Basin is located in the northwestern part of Madagascar with a N45–60°E trending axis. It was filled by almost exclusively continental Karoo Supergroup sediments, which are Permian to Early Jurassic in age, and by younger sequences, mainly marine, that were deposited from the Middle Jurassic to the present.The Karoo Basin geometry is deduced from the analysis of seismic sections. A central northeast trending horst is flanked by two sub-basins. Deposition of the Karoo sequences was controlled by these northeast trending faults. On the contrary, the Middle Jurassic to present sequences witness only a slight tilting of the basement towards the northwest.The development of the Majunga Basin includes, therefore, two successive stages. In the synrift episode, from Permian to Early Jurassic times, the sedimentation was syntectonic, controlled by synsedimentary faulting and the creation of a horst and graben extensive pattern. The postrift episode started during the Middle Jurassic.These two stages of the Majunga Basin development correspond to the geodynamic evolution recorded elsewhere in this part of the Gondwana.     

Crystal chemistry of suspended matter in a tropical hydrosystem, Nyong basin (Cameroon, Africa)

Suspended matter (SM) from the Nyong basin (Cameroon, Africa), a tropical watershed, was collected by tangential flow ultrafiltration to separate particulate (>0.45 μm) and colloidal (<0.45 μm; >20 kDa) fractions. In this basin, two distinctive systems in a selected small catchment (Nsimi–Zoétélé) of the Nyong river basin have been considered: (i) colourless water (groundwater and spring) with a low suspended load (<3 mg/l) and a low total organic carbon content (TOC<1 mg/l) and (ii) coloured water (Mengong brook and Nyong river), which is organic rich (TOC>10 mg/l) and contains higher amounts of SM (10–20 mg/l) than the colourless water. Freeze-dried samples of SM have been analysed by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), electron paramagnetic resonance spectroscopy (EPR), and visible diffuse reflectance spectroscopy (DRS).

Colourless water mainly contains mineral phases, such as poorly ordered kaolinite, plus quartz and goethite in the particulate fraction, and euhedral kaolinite plus amorphous iron oxyhydroxides in the colloidal fraction. In contrast, the SM in coloured water is mainly organic in nature. The mineral phases in the particulate fraction are similar to those from clear water, but with additional phytoliths and diatom frustules composed of biogenic opal. In the colloidal fraction, complexation of Fe³⁺ and Mn²⁺ with organic matter is evidenced by EPR, together with significant occurrence of Fe oxyhydroxides associated with organic matter.

The sites of Al, Si, Fe, Mn in colloidal fractions derived from spectroscopic analyses are discussed with reference to chemical analyses performed by inductively coupled plasma mass spectrometry. Most of the observed solid phases or species correspond to those expected from published thermodynamic calculations for the same hydrosystem, except the colloidal iron oxyhydroxides in the coloured water. The presence of such iron phases is emphasised since they are expected to have large sorption capacities for numerous trace elements.

The crystal chemistry of SM is used to discuss the origin of the mineral particles transported from the soil to the main rivers in terms of mechanical and chemical erosion processes.     

Magmatic source enrichment by slab-derived melts in a young post-collision setting, central Mindanao (Philippines)

Central Mindanao was the locus of a Pliocene (4–5 Ma old) arc–arc collision event followed by basaltic to dacitic magmatism starting at 2.3 Ma, representing the most voluminous volcanic field in the Philippines. Lava compositions range from calc-alkaline to shoshonitic. Adakites and Nb-enriched basalts are among the magmatic products. All the lavas are Na-rich (up to 4.88%), with Na₂O/K₂O ratios from 2.5 to 6.5. Sr, Nd and Pb isotopic compositions are similar to MORB, except for some shoshonitic lavas that have slightly less radiogenic Nd ratios. K-enrichment in basalts can be related to both fractional crystallization (FC) at moderate pressures and to partial melting of an enriched source. Trace element systematics indicate that the sub-central Mindanao mantle is characterized by the presence of garnet, phlogopite, amphibole, and perhaps some titanate phase. The enrichment of this source is attributed to the interaction of slab-derived melts, i.e., adakites, with the arc mantle. This would explain the presence of Nb-enriched basalts, transitional adakites and high-magnesium andesites, as well as the bulk Na-enrichment and relatively unradiogenic character of the central Mindanao lavas. We envision an ion-exchange type of enrichment, in which the HFSE, LILE and LREE, mobilized during slab melting, are preferentially enriched in the metasomatized mantle, resulting in a diversity of post-collision magma compositions. The MORB-like isotopic signatures of the central Mindanao lavas preclude important contributions of slab-derived hydrous fluids, sediments, continental crust or an OIB-type contaminant. Slab melting after cessation of subduction is deemed possible by thermal rebound of previously depressed geotherms. Initial contributions to mantle enrichment in post-collision sites may thus come from slab melts. In most other cases of post-collision magmatism, however, this signature can be easily masked by enrichments coming from other sources, e.g., the continental lithosphere.     

The origin of chaotic behaviour in the Miranda-Umbriel 3 : 1 resonances

We have investigated the pericentric resonances through which Miranda and Umbriel are believed to have passed when, due to tidal evolution, their orbital mean motions reached a 3 : 1 commensurability. Our investigation is based upon a perturbative treatment. The predictions of this theory are in good agreement with the results of numerical integrations concerning both the extend of the chaotic layers generated by the separatrices of the primary resonances and the location of the secondary resonances. The effect of tidal evolution is discussed on the bases of the adiatatic invariant theory and its extension to separatrix crossing. We recover qualitatively the mean features of the numerical experiments of Tittermore and Wisdom (1988–1989), Dermott et al (1988) and Malhotra and Dermott (1989).     

Hill's Problem in lunar theory

It is shown that the appearance of higher order resonances (that produce higher order islands on a surface of section), is not an indication of non-integrability. Examples are given and a method is described for constructing integrable Hamiltonians with higher order resonances.     

A perturbation method for problems with two critical arguments

We develop a semi-numerical perturbation method for problems with two critical arguments. We apply it to a truncated model of the restricted, elliptic three body problem in case of a resonance 2/1 We identify regions of the phase space where chaotic motion is expected because of the presence of homoclinic orbits. One of these regions, the largest one, sits at the entrance to the resonance zone and is associated with a 2/1 resonance between the two critical arguments. The results are compared with numerical results due to Murray (1986)     

Epiphyte-grazer relationships in seagrass meadows: Consequences for seagrass growth and production

Studies of seagrass meadows have shown that the production of algal epiphytes attached to seagrass blades approaches 20% of the seagrass production and that epiphytes are more important as food for associated fauna than are the more refractory seagrass blades. Since epiphytes may compete with seagrasses for light and water column nutrients, excessive epiphytic fouling could have serious consequences for seagrass growth. We summarize much of the literature on epiphytegrazer relationships in seagrass meadows within the context of seagrass growth and production. We also provide insights from mathematical modeling simulations of these relationships for a Chesapeake BayZostera marina meadow. Finally we focus on future research needs for more completely understanding the influences that epiphyte grazers have on seagrass production.     

Collision and stress trajectories in Taiwan: a finite element model

Field analyses of Plio-Quaternary compressional deformations in Taiwan have enabled us to reconstruct the paleostress trajectories resulting from the collision of the Luzon arc (Philippine Sea plate) with the Chinese continental margin (Eurasian plate). The direction of the maximum compressional stress σ₁ shows a fan-shaped pattern that we interpret as resulting from the collision of a rigid body (the Luzon arc) indenting a more deformable material (the thick sediments of the Chinese continental margin). Simple analytical models qualitatively explain the fan-shaped pattern, but the influence of various parameters such as boundary conditions and rheology cannot be quantitatively accounted for by this approach. Consequently, we have used a finite element technique to compute the stresses and strains induced by the push of a rigid body against a two-dimensional, viscous material. The boundary conditions are the velocities based on plate kinematics. A motion in the N300°E direction best explains the stress trajectories observed in central Taiwan. Viscosity contrasts as well as small changes in the shape of the northern edge of the indenter have little influence on the computed stress pattern. The most important parameter is the direction of convergence. Our model quantitatively explains the general pattern of the stress trajectories observed in the collision zone of Taiwan, between the Philippine Sea plate and Eurasia.     

Seismic detection of underwater volcanism: The example of French Polynesia

We present a review of the principal methods used for the seismic detection and identification of active underwater volcanism, based on our experience in French Polynesia. In particular, we descrobe the 5-year activity in the Tahiti-Mehetia area, during which more than 32000 earthquakes were detected by the Polynesian network. We discuss the use of the following three types of seismic waves: conventional (mostly body waves), seismic tremor, andT waves propagated in the low-velocity acoustic channel of the ocean. For each of these waves, we discuss the principal characteristics of the signals, their spectral content, the type of information they provide on the activity of the volcano, and the various limitations faced by their use in detection or monitoring of underwater volcanic edifices. We present a review of the principal swarms monitored by the Polynesian network, and discuss their characterization as either volcanic or tectonic.