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.     

Active deformation of a segment of arc: the strait of Kythira,Hellenic arc,Greece

A field analysis of Late Cenozoic faults has been carried out in the southwest Hellenic arc, on the islands of Kythira and Antikythira as well as on both sides of the strait, in southeast Peloponnisos and western Crete. The previously unknown geological structure of Antikythira is briefly described. Together with accurate bathymetric data obtained using a multi-beam echo sounder on submarine fault scarps, the results of studies on land lead to a new interpretation of the neotectonic deformation of this segment of the Hellenic arc. The submersion of the strait is a consequence of extension related to normal faulting, the extension rate being faster than in adjacent segments of the arc. The direction of extension related to Late Pliocene-Quaternary normal faulting is determined from on-land information. The existence of oblique en échelon normal faults indicates that dextral deformation plays a role within the Kythira strait, in connection with the opening of the adjacent Cretan Sea triggered by subduction beneath the Hellenic arc. The whole pattern of fault systems in southeast Peloponnisos, as inferred from teledetection and field studies, is consistent with the clockwise rotation that has been independently proposed and verified by palaeomagnetic studies. The Kythira strait thus provides an example of a complex transform-extensional deformation and rotation between two major segments of external arc.     

The longevity of subcontinental lithospheric mantle beneath Jiangsu-Anhui Region ——The Os isotope model age of mantle-derived peridotite xenoliths

The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in two laboratories of USTC, China and CRPG, France, respectively. The 187Os/188Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC)and from 0.117 to 0.131 (17 samples, CRPG). The Os isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good correlation with the major element compositions. With the 187Os/188Os-Al2O3 alumichron, the samples yield a model age of 2.5 ± 0.1 Ga (data of USTC) and 1.9 ± 0.1 Ga (data of CRPG), late Archean to early Proterozoic, The two samples with the lowest 187Os/188Os ratio (0.119 and 0.117) have the TRD (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle, but residual fractions of Proterozoic mantle.     

Behavior of Re and Os during low-temperature alteration: Results from Himalayan soils and altered black shales

Re-Os analyses were performed on five soil profiles developed on silicate lithologies in the Narayani drainage basin of the Himalayas, as well as on several altered black shales and associated alteration phases. The results indicate that all of these samples have lost large amounts of Re. This Re loss approached 100% in the black shale-derived soils, which also most probably lost substantial quantities of Os. The near constancy of Os and Re concentrations with depth indicates that this loss occurred quite early in the alteration process, rather than progressively during the course of soil development. A slight decrease in ¹⁸⁷Os/¹⁸⁸Os was observed toward the surface of the two black shale soil profiles, possibly indicating a minor preferential mobility of radiogenic ¹⁸⁷Os for this lithology. In the black shale-derived saprolite and soils, the loss of Re and Os was related to the loss of organic carbon. The importance of this phase is underscored by the fact that organic material separated from a black shale sample accounted for nearly all of the Os contained in the whole rock. Analysis of alteration phases from the surface of a weathered black shale showed that Os liberated during alteration was partially reprecipitated in Fe oxides, with Os isotopic compositions very similar to that of the original rock, whereas Re was removed with the fluid phase.These results demonstrate the disproportionately large role that black shale weathering may play in determining the Re and Os contents of continental runoff and, thus, ultimately of seawater, but they also emphasize the possible complexity of this process. The data suggest that a large fraction of the Os contained in black shales is released during chemical alteration, which is apparently not the case for most other silicate rocks. Therefore, the potential contribution of black shales to the dissolved Os budget of runoff and river water may be even greater than that expected from the high Os concentrations of these rocks. The extent to which this Os will affect the seawater Os composition depends on how efficiently it is recaptured by secondary phases such as Fe-Mn oxides in situ or during transport. These results also underscore the high mobility of Re with respect to Os at every stage in the erosional process, thus explaining the order of magnitude increase in Re/Os ratio from the black shale source rocks to seawater.     

Late-Glacial Vegetation and Climate Change in Western Oregon

Pollen records from two sites in western Oregon provide information on late-glacial variations in vegetation and climate and on the extent and character of Younger Dryas cooling in the Pacific Northwest. A subalpine forest was present at Little Lake, central Coast Range, between 15,700 and 14,850 cal yr B.P. A warm period between 14,850 and 14,500 cal yr B.P. is suggested by an increase inPseudotsugapollen and charcoal. The recurrence of subalpine forest at 14,500 cal yr B.P. implies a return to cool conditions. Another warming trend is evidenced by the reestablishment ofPseudotsugaforest at 14,250 cal yr B.P. Increased haploxylonPinuspollen between 12,400 and 11,000 cal yr B.P. indicates cooler winters than before. After 11,000 cal yr B.P. warm dry conditions are implied by the expansion ofPseudotsuga.A subalpine parkland occupied Gordon Lake, western Cascade Range, until 14,500 cal yr B.P., when it was replaced during a warming trend by a montane forest. A rise inPinuspollen from 12,800 to 11,000 cal yr B.P. suggests increased summer aridity.Pseudotsugadominated the vegetation after 11,000 cal yr B.P. Other records from the Pacific Northwest show an expansion ofPinusfrom ca. 13,000 to 11,000 cal yr B.P. This expansion may be a response either to submillennial climate changes of Younger Dryas age or to millennial-scale climatic variations.     

Organic compounds in radiation fogs in Davis (California)

New stainless steel active fogwater collectors were designed and used in Davis (CA, USA) to collect fogwater for the speciation of organic matter. Organic compounds in fog samples were extracted by liquid–liquid extraction and analyzed by gas chromatography coupled to mass spectrometry. Numerous organic compounds, including various alkanes, polycyclic aromatic hydrocarbons (PAH) and alkanoic acids, have been identified in the fogwater samples. Higher molecular weight (MW) compounds are preferentially associated with an insoluble phase inside the fog drops, whereas lower molecular weight and more polar compounds are found predominantly in the dissolved phase. Concentrations in the dissolved phase were sometimes much higher than estimated by the compounds' aqueous solubilities.     

The two stage collision of Taiwan: An active orogen

The active Taiwan orogen is the product of a two stage collision, that included first the collision of the Hengchun ridge, an accretionary wedge, with the Chinese continental margin, and second the collision of the Luzon trough and volcanic arc, from the Philippine Sea plate, with the Central Range of Taiwan. During the first stage, the strength of the continental margin induced a decrease of the convergence rate that controlled the final Central Range orientation and induced the second stage of the collision. Taking into account the kinematics of the plate interaction, a reconstruction of the Taiwan collision during the last 4 Ma is proposed.     

Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling

The Northern Mediterranean Current is the return branch of the cyclonic circulation of the northwestern Mediterranean Sea. Because of geostrophic constraints, this warm and oligotrophic current is forced to flow westward along the continental slope of the Gulf of Lion. But, occasionally, it penetrates on the shelf and strongly impacts the local biogeochemistry and in turn the primary production. By combining in situ observations and high-resolution modelling, it is shown that intrusions on the eastern part of the gulf are mainly forced by easterly or northwesterly wind events, through physical mechanisms that are very different in nature. Easterlies induce a piling of water along the Gulf of Lion coast that drives, through geostrophy, an alongshore shelf-intruding current. This intrusive current occurs independently of the stratification and is concomitant with the wind forcing. On the other hand, intrusions due to northwesterlies only occur during stratified conditions and are related to the development of upwellings along the Gulf of Lion coasts. When the upwelling develops, a northwestward alongshore pressure force balances the Coriolis force associated with the onshore flow at depth. When the winds drop, the upwelling relaxes and the onshore flow weakens. Consequently, the Coriolis force no longer counterbalances the pressure force that ultimately dominates the momentum balance, causing the displacement of the Northern Current on the Gulf of Lion shelf approximately 1 day after the wind relaxation. This time lag between the northwesterlies decrease and the intrusions permits to anticipate possible changes in the biogeochemistry of the Gulf of Lion.     

Northern fulmars as biological monitors of trends of plastic pollution in the eastern North Pacific

Marine plastic debris is a global issue, which highlights the need for internationally standardized methods of monitoring plastic pollution. The stomach contents of beached northern fulmar (Fulmarus glacialis) have proven a cost-effective biomonitor in Europe. However, recent information on northern fulmar plastic ingestion is lacking in the North Pacific. We quantified the stomach contents of 67 fulmars from beaches in the eastern North Pacific in 2009-2010 and found that 92.5% of fulmars had ingested an average of 36.8 pieces, or 0.385g of plastic. Plastic ingestion in these fulmars is among the highest recorded globally. Compared to earlier studies in the North Pacific, our findings indicate an increase in plastic ingestion over the past 40years. This study substantiates the use of northern fulmar as biomonitors of plastic pollution in the North Pacific and suggests that the high levels of plastic pollution in this region warrant further monitoring.