Upper-mantle flow beneath French Polynesia from shear wave splitting   总被引：1，自引：0，他引：1  

Fabrice R. Fontaine  Guilhem Barruol  réa Tommasi  Götz H. R. Bokelmann 《Geophysical Journal International》2007,170(3):1262-1288

Upper-mantle flow beneath the South Pacific is investigated by analysing shear wave splitting parameters at eight permanent long-period and broad-band seismic stations and 10 broad-band stations deployed in French Polynesia from 2001 to 2005 in the framework of the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). Despite the small number of events and the rather poor backazimuthal coverage due to the geographical distribution of the natural seismicity, upper-mantle seismic anisotropy has been detected at all stations except at Tahiti where two permanent stations with 15 yr of data show an apparent isotropy. The median value of fast polarization azimuths (N67.5°W) is parallel to the present Pacific absolute plate motion direction in French Polynesia (APM: N67°W). This suggests that the observed SKS fast polarization directions result mainly from olivine crystal preferred orientations produced by deformation in the sublithospheric mantle due to viscous entrainment by the moving Pacific Plate and preserved in the lithosphere as the plate cools. However, analysis of individual measurements highlights variations of splitting parameters with event backazimuth that imply an actual upper-mantle structure more complex than a single anisotropic layer with horizontal fast axis. A forward approach shows that a two-layer structure of anisotropy beneath French Polynesia better explains the splitting observations than a single anisotropic layer. Second-order variations in the measurements may also indicate the presence of small-scale lateral heterogeneities. The influence of plumes or fracture zones within the studied area does not appear to dominate the large-scale anisotropy pattern but may explain these second-order splitting variations across the network.  相似文献   

Estimation of velocity in the uppermost crust in a part of the western Gulf of Corinth,Greece, from the inversion of <Emphasis Type="Italic">P</Emphasis> and <Emphasis Type="Italic">S</Emphasis> arrival times using the neighbourhood algorithm     

Jaromír Janský  Vladimír Plicka  Helene Lyon-Caen  Oldřich Novotný 《Journal of Seismology》2007,11(2):199-204

We determine the velocities in an upper crustal model, composed of three homogeneous layers, for one subregion of the western part of the Gulf of Corinth, NE of the town of Aigion, Greece. We have used local events that occurred there in the year 2001 and were recorded by the Corinth Rift Laboratory Network. Weighted P and S arrival time residuals are minimized using the Neighbourhood Algorithm of Sambridge (1999), combined with the grid search for source locations. The resolution of the inversion is tested by delete-one jackknifing. The model obtained is compared with some other models derived or applied to the subregion. A fast velocity increase between depths of 5 and 7 km is confirmed as the major structural element.  相似文献   

The effect of mineralogical and cationic heterogeneities on rheological properties of suspensions with Li-smectites, extracted from Volclay MX-80     

Christine Malfoy  Claude Fontaine  Anne Pantet  Philippe Monnet 《Comptes Rendus Geoscience》2007,339(16):960-969

The rheological study of colloidal suspensions contributes to the understanding of the aggregation phenomena of colloidal particles within a suspended fluid. The infra-2-μm (equivalent diameter) fraction of a bulk material, Volclay MX-80 bentonite was intensively analysed in order to understand the relative rheological data scattering observed with Li-smectite suspensions, prepared with the usual saturation procedure. A good mineralogical characterisation, especially for secondary minerals, coupled to a verification of cation exchanges on clayey fraction allowed us to propose a new way for investigating the observed differences of the bentonites rheological behaviour. Indeed, for a well Li-saturated smectite obtained from the purified infra-2-μm fraction, rheological data proved to be very accurate and perfectly reproducible. This emphasises the role of other minerals as well as the commonly observed discrepancy between the rheological behaviours of natural and industrial bentonites.  相似文献   

Late Weichselian environmental change in southern Sweden and Denmark     

Bjrn E. Berglund  Svante Bjrck  Geoffrey Lemdahl  Helene Bergsten  Kjell Nordberg  Else Kolstrup 《第四纪科学杂志》1994,9(2):127-132

A synthesis is presented of the envronmental and climatic variations that are inferred to have occurred in southern Sweden (up to latitude 59°N) and Denmark during the Weichselian Late-glacial (14-9 ka BP). The chronology and characteristics of the main phases of deglaciation, sea-level change, periglacial activity, soil development, vegetation cover and climate change are summarised. A curve representing the main changes in temperature, including quantitative estimates based upon beetle data and using the ‘mutual climatic range’ method, is presented.  相似文献   

Investigation of spheroidal weathering and twinning     

Robert Sarracino Dr.  Gisela Prasad Dr. 《GeoJournal》1989,19(1):77-83

What determines the geometry of spheroidal weathering patterns? Spheroidal weathering rings appear to arise from the leaching of fresh rock, rather than from reprecipitation of dissolved minerals diffusing out of weathered rock. In previous work the authors developed a mathematical model to explain the geometry of spheroidal weathering structures, and showed how some of the features of weathering patterns, otherwise difficult to explain, were natural consequences of the model. In describing the geometrical aspect of these shapes, we hope to understand how natural forces acted upon the unweathered rock and why and under what conditions spheroidal weathering structures were formed. In this paper we review the main features of our model and examine and discuss twinning of spheroidal weathering structures, concluding that the structures were formed during a distinct period in the past. Twinning is an apparent anomaly in that its natural occurrence, as part of the weathering process, cannot be explained by our model without additional ad hoc assumptions. Careful examination of twins reveals an explanation which does not require revision of the model.  相似文献   

Book reviews     

El-Sayed  El-Bushra  J. A. Hellen  Hong-key Yoon  A. Wallner  C. A. Halstead  Gisela Prasad  Ernst H. Doerpinghaus/Altea  Ulrik Lohm  Ernst H. Doerpringhaus  Helmut Vidal  John M. Reynolds  Matthias Kuhle 《GeoJournal》1989,18(4):461-466

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Selective chemical extraction of carbonate-associated metals from recent lacustrine sediments     

Robert Deurer  Ulrich Förstner  Gisela Schmoll 《Geochimica et cosmochimica acta》1978,42(4):425-427

Acidic ion-exchanger was applied to the selective extraction of heavy metals in carbonate phases of recent lake sediments. Cr was not detected and Fe and Cu contents were reduced by factors of three to ten, relative to non-carbonate fractions of the sediment samples. Values obtained for Mn and Zn exemplify the effects of dilution was well as enrichment, both originating from increased carbonate percentages. Determinations on grain-sized fractions from Bodensee samples suggest that elevated inputs of dissolved Zn, which partly originate from human activities, may be immobilized by authigenic carbonate precipitation.  相似文献   

Enhanced ion acoustic fluctuations and ion outflows     

F. R. E. Forme  D. Fontaine 《Annales Geophysicae》1999,17(2):182-189

A number of observations showing enhanced ion acoustic echoes observed by means of incoherent scatter radars have been reported in the literature. The Received power is extremely enhanced by up to 1 or 2 orders of magnitude above usual values, and it is mostly contained in one of the two ion acoustic lines. This spectral asymmetry and the intensity of the received signal cannot be resolved by the standard analysis procedure and often causes its failure. As a result, and in spite of a very clear spectral signature, the analysis is unable to fit the plasma parameters inside the regions of ion acoustic turbulence. We present European Incoherent Scatter radar (EISCAT) observations of large ion outflows associated with the simultaneous occurrence of enhanced ion acoustic echoes. The ion fluxes can reach 10¹⁴ m^–2 s^–1 at 800 km altitude. From the very clear spectral signatures of these echoes, a method is presented to extract estimates of the electron temperature and the ion drift within the turbulent regions. It is shown that the electron gas is strongly heated up to 11 000 K. Also electron temperature gradients of about 0.02 K/m exist. Finally, the estimates of the electron temperature and of the ion drift are used to study the possible implications for the plasma transport inside turbulent regions. It is shown that strong electron temperature gradients cause enhancement of the ambipolar electric field and can account for the observed ion outflows.  相似文献   

Diel feeding, daily food consumption and the predatory impact of whitemouth croaker (Micropogonias furnieri) in an estuarine environment     

Gisela Mandali de  Figueiredo & João Paes Vieira 《Marine Ecology》2005,26(2):130-139

Diel feeding pattern and food consumption of whitemouth croaker (Micropogonias furnieri) were examined and related to seasonal variations in an estuarine environment. To determine diel feeding pattern whitemouth croaker were collected from the Patos Lagoon Estuary over 24 h (～3‐h intervals) and their stomach contents were examined. Food consumption of whitemouth croaker was determined from evacuation rates estimated experimentally and in the field. The results suggested that whitemouth croaker in the Patos Lagoon estuary feed all day long. However, whitemouth croaker showed higher feeding intensity and food consumption during daylight hours when the water transparency increased than in others when water transparency was low. Thus, whitemouth croaker seemed to intensify their feeding when they could see their prey. Daily food consumption was between 0.9 and 5.3% of body wet weight (these being the lowest and highest consumption rates during the winter and summer, respectively). The maximum predation impact of whitemouth croaker on their preferential prey, the tanaid Kalliapseudes schubartii, was 27indiv·m^?2·day^?1. The results suggested that the changes in water transparency seem to affect the diel feeding pattern and food consumption of whitemouth croaker. This could have important consequences for fish growth.  相似文献   

Nifte: The near infrared faint-object telescope experiment     

James J. Bock  Andrew E. Lange  T. Matsumoto  Peter B. Eisenhardt  Perry B. Hacking  Helene R. Schember 《Experimental Astronomy》1994,3(1-4):119-120

The high sensitivity of large format InSb arrays can be used to obtain deep images of the sky at 3–5 m. In this spectral range cool or highly redshifted objects (e.g. brown dwarfs and protogalaxies) which are not visible at shorter wavelengths may be observed. Sensitivity at these wavelengths in ground-based observations is severely limited by the thermal flux from the telescope and from the earth's atmosphere. The Near Infrared Faint-Object Telescope Experiment (NIFTE), a 50 cm cooled rocket-borne telescope combined with large format, high performance InSb arrays, can reach a limiting flux < 1 Jy (1) over a large field-of-view in a single flight. In comparison, ISO will require days of observation to reach a sensitivity more than one order of magnitude worse over a similar area of the sky. The deep 3–5 m images obtained by the rocket-borne telescope will assist in determining the nature of faint red objects detected by ground-based telescopes at 2 m, and by ISO at wavelengths longer than 5 m.  相似文献