East Asia plate tectonics since 15 Ma: constraints from the Taiwan region

15 Ma ago, a major plate reorganization occurred in East Asia. Seafloor spreading ceased in the South China Sea, Japan Sea, Taiwan Sea, Sulu Sea, and Shikoku and Parece Vela basins. Simultaneously, shear motions also ceased along the Taiwan–Sinzi zone, the Gagua ridge and the Luzon–Ryukyu transform plate boundary. The complex system of thirteen plates suddenly evolved in a simple three-plate system (EU, PH and PA). Beneath the Manila accretionary prism and in the Huatung basin, we have determined magnetic lineation patterns as well as spreading rates deduced from the identification of magnetic lineations. These two patterns are rotated by 15°. They were formed by seafloor spreading before 15 Ma and belonged to the same ocean named the Taiwan Sea. Half-spreading rate in the Taiwan Sea was 2 cm/year from chron 23 to 20 (51 to 43 Ma) and 1 cm/year from chron 20 (43 Ma) to 5b (15 Ma). Five-plate kinematic reconstructions spanning from 15 Ma to Present show implications concerning the geodynamic evolution of East Asia. Amongst them, the 1000-km-long linear Gagua ridge was a major plate boundary which accommodated the northwestward shear motion of the PH Sea plate; the formation of Taiwan was driven by two simple lithospheric motions: (i) the subduction of the PH Sea plate beneath Eurasia with a relative westward motion of the western end (A) of the Ryukyu subduction zone; (ii) the subduction of Eurasia beneath the Philippine Sea plate with a relative southwestward motion of the northern end (B) of the Manila subduction zone. The Luzon arc only formed south of B. The collision of the Luzon arc with Eurasia occurred between A and B. East of A, the Luzon arc probably accreted against the Ryukyu forearc.     

Water Geochemistry of Three Mountain Streams from Carbonate Watersheds in the Southern French Alps

The chemical composition of water from three streamsflowing through a carbonate watershed wasinvestigated. Although the study area is not spatiallyvery large (a few km²), local inhomogeneitieswithin the lithology appreciably affect the chemicalcomposition of the water and the geochemical gradientas a function of elevation. The main chemical processwhich leads to the observed water chemistry is thedissolution of calcium carbonate by atmospheric andmetabolic CO₂. In the stream La Sigouste, thewater dissolves nearly pure calcite and this reactionproceeds until an equilibrium with respect to calciteis reached. In Le Lauzon stream, local inhomogeneitiesin the lithologic composition prevent theestablishment of an unambiguous weathering budgetwhile, for Le Rif de l'Arc a stoichiometric modelshows that the weathering process is adequatelyrepresented by the dissolution of calcite associatedwith a minor incongruent dissolution of chloritepresent in marls. For the two last streamssupersaturation with respect to calcite is observed.For both streams, supersaturation with respect toatmospheric CO₂ is nearly permanent. The observedaltitudinal gradients of sulfate are interpretedthrough a set of biogeochemical redox reactions.     

Oxygen isotope study of Cayenne duricrust paleosurfaces: implications for past climate and laterization processes over French Guiana

Oxygen isotopic compositions of weathering goethite, hematite, kaolinite and gibbsite in duricrust paleosurfaces of the Cayenne region, northeastern French Guiana, were investigated. Samples were microdrilled from different horizons and different pedogenetic microfacies of three well-characterized duricrust units of Eocene (unit 1) and Miocene (units 2 and 3) age. The results provided valuable information on duricrust formation processes and on climate change over French Guiana.The pristine original isotopic signal, suited for reconstruction of past climates, appears to be best preserved in the massive duricrust facies (middle part of the duricrust). In the overlying pisolotic duricrust and in the underlying mottled zone, the isotopic signal is affected by subsequent re-activation of weathering processes. Oxygen isotope data from massive duricrust indicate that Cayenne duricrusts formed from meteoric waters increasingly depleted in ¹⁸O with age. The youngest duricrust paleosurface (unit 3) formed under humid climate similar to present-day, while the two older paleosurfaces formed from meteoric paleo-water distinctly depleted in ¹⁸O, reflecting a “moonsonal” contrasted ancient climate for unit 2 added to the ¹⁸O-depleted nature of the pre-Miocene ocean for unit 1. The results further suggest that the major climatic change, from tropical wet-and-dry to present-day humid regime, recorded in Amazonia most likely occurred in the Late Miocene, and that Miocene laterization events may be responsible for the secondary bauxitization of the Eocene duricrust paleosurface.The oxygen isotopic composition of weathering minerals occurring in massive duricrust horizons, and in particular in hematite nodules and matrix, may be used as a tool to discriminate among the different duricrust paleosurfaces encountered in Amazonia and constrain the evolution of past climates.     

Origin and age of Paleoproterozoic conglomerates and sandstones of the Tarkwaian Group in Burkina Faso, West Africa

The Tarkwaian rocks of Burkina Faso overlie the Birimian Supergroup which is considered to be part of a juvenile crust formed during the Eburnean orogen. They comprise fluviatile pebbles (quartz, rhyolite, chert and schist), embedded in an acid volcaniclastic matrix. During a sinistral transpressive shearing they underwent a low-grade metamorphism characterized by a paragonitic muscovite assemblage formed during isoclinal folding. Fold axes are nearly horizontal, parallel to a stretching lineation and oriented 035–215°.A typologic study of zircons in the volcaniclastics, coupled with radiometric datings, enables the volcanism to be characterized and its age to be determined.The typologic study has shown that: (1) zircons from the rhyolitic pebbles, the matrix of the conglomerates and from the sandstone are basically the same; (2) the zircons' morphology characterizes a calc-alkaline to alkaline evolutionary trend implying three main episodes of volcanism; (3) the morphology of this zircon population is very similar to those usually observed in post-orogenic metasediments or sediments. The tectonic setting is one of intracontinental extension associated with rifting and crustal thinning in the course of a major period of crustal growth around 2.1 Ga. The continental volcaniclastics accumulated in the rift at the same time as the sediments.The radiometric study has shown that there are no reworked Archean rocks. The ages cover the period 2170 to 2124 Ma. Three main volcanic episodes may be defined 2170, 2150 and 2124 Ma; the last one was probably contemporaneous with the sedimentation. Deformation of Tarkwaian rocks took place between 2124 ± 9 Ma (the age of the youngest inherited zircon) and 1991 ± 12 Ma (the age of a post-Tarkwaian granite). This period, 2170 to 2124 Ma, fits well with the main Birimian crustal growth and the ages reported for the Tarkwaian rocks of Ghana. The same period of accretion is also recognized in South America.The Tarkwaian conglomeratic rocks of Burkina Faso do not have gold concentrations similar to those in Ghana and Guyana. With respect to the Tarkwaian rocks of Ghana, where gold is concentrated in the quartz pebbles, they also differ because: (1) there is a volcaniclastic contribution; (2) the zircons from the Tarkwaian rocks in Burkina Faso are euhedral, indicating that transport distances were short; and (3) they have been deformed by shearing.With respect to Guyana, the rocks are very similar and probably have the same origin; the short transport distance may not allow a secondary concentration of gold. These differences, at least, may help explain the lack of economic gold concentration in the Tarkwaian metasediments.     

Residues of C-4n-nonylphenol in mosquitofish (Gambusia holbrooki) oocytes and embryos during dietary exposure of mature females to this xenohormone

In order to assess in fish the maternal transfer of alkylphenolic compounds to the progeny, the identification and quantification of the labelled compounds present in oocytes and embryos was conducted after dietary exposure of mature female mosquitofish to ¹⁴C-4n-nonylphenol during vitellogenesis and embryogenesis respectively. Radioactivity found in bile and liver extracts accounted for 0.9–0.6 and 0.2–0.1% of ingested radioactivity for females exposed during vitellogenesis and embryogenesis respectively. The amount of extractable radioactivity present in oocytes and embryos was 0.19 and 0.07% of the ingested dose respectively. The radio-HPLC profiles obtained from bile, liver, oocyte and embryo extracts were similar. They showed the presence of 4n-NP-glucuronide as the major metabolite and traces of unchanged 4n-NP. The other metabolites corresponded to 8-hydroxynonylphenol, 9-(4-hydroxyphenyl)-nonanoic acid and para-hydroxybenzoic acid which is the final product of the alkyl chain oxidation. Our results indicate that exposure of ovoviviparous female fish to 4-NP during vitellogenesis and embryogenesis leads to the contamination of the progeny by 4-NP and its metabolites.     

A Deep-sea Channel in the Northwestern Mediterranean Sea: Morphology and seismic structure of the Valencia Channel and its surroundings

The 400 km long Valencia Channel occupies the axis of the Valencia Trough in the Northwestern Mediterranean. Four different types of seismic reflection profiles were used to analyze the morphology and structure of the Valencia Channel with regard to the role played by both margins, Balearic and Iberian, of the Valencia Trough. From a detailed morphoseismic analysis of the Valencia Channel, its upper, middle, and lower courses can be characterized as follows: (1) in the upper course, tributaries are short and only slightly incised, with recent mass-transport deposits occurring on the adjacent continental slopes; (2) in the middle course, the channel deepens, and tributary valleys merge into it; and (3) the lower course begins after a sudden change in the direction of the channel, has a meandering path, is flanked by levees, and is fed by some valleys.During the Pliocene and Quaternary, at least four erosional and filling phases are observed in seismic profiles of the lower course of the Valencia Channel. The varying intensity of mass-transport processes and associated retrogressive slumping, which are related with fluctuations in sediment supply and relative sea-level changes, have played a major role in the formation, maintenance and deepening of the Valencia Channel. In addition to these sedimentary processes, a basement tectonic control and some morphostructural features affect the direction of the Valencia Channel locally.     

A study of a two-dimensional cloudiness transition during a cold air outbreak event

A two-dimensional version of a hydrostatic mesoscale model with a partial cloudiness scheme is used to study a cold air outbreak event during MASEX (Mesoscale Air-Sea Exchange experiment). The model produces a weak mesoscale circulation which is slightly advected offshore and simulates well an observed cloudiness transition zone of 80 km where the cloud cover ranges between 0 and 100%. It is shown that the cloud top entrainment instability criterion can explain the observed cloudiness phenomenon i.e., the transition from a cumulus field to a solid stratocumulus cloud layer.The usefulness of the partial cloudiness scheme is demonstrated by comparing the results to those obtained with a simple all or nothing condensation scheme. A model sensitivity study shows that the sea surface temperature and the horizontal advection speed control the surface heat and moisture fluxes and so explain the structure and horizontal extent of the cloudiness transition zone.     

La stratigraphie oligo-miocène et la surface d'érosion messinienne en mer Noire,stratigraphie sismique haute résolution

The stratigraphy of the Black Sea western margin is revisited through seismic data acquired during two French-Romanian surveys. These data are calibrated by industrial and DSDP drillings; they display several major discontinuities regarded as Tertiary erosional surfaces. The major seismic discontinuity underlines the base of Miocene formations and corresponds to a composite surface including at least three erosional phases ranging from Oligocene to Pontian times. Moreover, a Messinian erosional surface is clearly identified. This is in agreement with the Hsü's proposition [Palaeogeogr. Palaeoclimatol. Palaeoecol. 29 (1979) 75–93] suggesting a Messinian sea-level drop of Black Sea related to the Messinian Salinity Crisis described in the Mediterranean Sea. To cite this article: H. Gillet et al., C. R. Geoscience 335 (2003).     

Photoelastic and numerical investigation of stress distributions around fault models under biaxial compressive loading conditions

This paper presents the results of a photoelastic and numerical study of stress distributions (contours and trajectories) around fault models of various geometries, submitted to a biaxial compressive load. It aims to describe typical biaxial stress behaviours and emphasize the existing differences with the well-known uniaxial compressive load case. Stress trajectories are sometimes shown by joint sets acting as markers of a paleostress field, and they can be interpreted by particular shallow tension–compression situations. At depth fractures can be reactivated, or can dilate under conditions of triaxial compression, and behaviour is essentially controlled by a high stress ratio (high σ₃/σ₁ ratio). In spite of the potential importance of such stress states on fracture permeability and fluid flow, and although they are frequently found at depth in a reservoir context, such stress conditions are poorly investigated, particularly in terms of stress perturbations.The presented analogue experiments consisted of compression tests done on polymethylmethacrylate (PMMA) plates; this material has mechanical properties comparable to those of brittle rocks in the upper crust, and presents birefringence. The samples contained open defects acting as faults, and the stress trajectories around these faults were investigated using a photoelastic device. Comparable numerical experiments were realised with a finite-element code (Franc 2D), using frictionless fault models.First, the effect of an increasing biaxial compressive load ratio σ₃/σ₁ on stress trajectories around an isolated open defect was explored. It was shown that the stress trajectories were drastically modified when σ₃/σ₁ increased from 0.2 to 0.4, this result being consistent with previous studies. In particular, when σ₃/σ₁ was superior or equal to 0.4, external isotropic points around where trajectories diverged (called repulsive isotropic points) appeared near the tips, but away from the defects. They tended to move away from the defects towards the main load direction when σ₃/σ₁ increased. The described isotropic points were points of stress trajectory divergence, i.e. points where stress decreased, implying that zones around them were strongly unfavourable to shear reactivation.Second, stress trajectories around fault models of various geometries (oblique isolated defect, dilational jog, compressive jog, and complex patterns) were studied, the applied biaxial compressive load ratio being 0.7. These biaxial stress trajectories were compared with similar uniaxial stress trajectories in order to explore the existing differences between the two regimes. It enabled new stress trajectory geometries to be described and interpreted. In particular, numerous external repulsive isotropic points were observed, and defect tips were shown to be zones of high convergence of stress trajectories. Furthermore, in contrast to the uniaxial compressive load case, stress trajectories and stress contours were geometrically similar within dilational and compressive jogs under biaxial compressive load. For both jog types, the centre of the overlapping zone, and the areas along the wall of a jog-defining fault and facing the overlapping tip of the other fault, proved to be zones of low mean stress, implying that fluids may migrate towards them from the tips of the faults, in response to mean stress gradients. Furthermore, the centre of the jogs exhibiting a high differential stress was a favourable area in terms of fracture reactivation, which may facilitate fluids transfer and storage.It was also observed that for both the uniaxial and biaxial compressive loads, isotropic zones were localised at the acute angle between branching defects.