Rare earth element geochemistry of Thetford Mines ophiolite complex,Northern Appalachians,Canada     

Luc Harnois  Jacques Trottier  Maurice Morency 《Contributions to Mineralogy and Petrology》1990,105(4):433-445

The Thetford Mines complex is a complete ophiolite which is part of an ultramafic-mafic belt within Québec Appalachians. These allochtonous bodies were emplaced during the Early Ordovician. The Thetford Mines complex comprises a lower unit of metamorphic harzburgite (in which tabular, dyke-like, dunitic bodies occur) overlain successively by ultramafic cumulates, mafic cumulates, ophitic gabbros, diabase sills and dykes, and basaltic volcanic rocks. Field evidence, petrography and chemical data indicate that the tabular dunitic bodies formed when fractures in the metamorphic harzburgite (which constituted the floor of the magma chamber) filled with early cumulates (i.e., olivine±chromite). Representative rocks from all units were analyzed for major and rare earth elements (REE). Metamorphic harzburgite samples from Thetford Mines complex have U-shaped chondrite-normalized REE patterns. Pyroxenites and wehrlites of the cumulate sequence are all strongly light-REE depleted and have heavy REE ranging from 0.4 to 1.5 times chondrite. REE data from ultramafic and volcanic rocks of Thetford Mines complex and geochemical modelling indicate that the metamorphic harzburgite has the chemical characteristics of depleted upper mantle residues with U-shaped patterns, and that the ultramafic cumulates crystallized from magmas having different La/Yb ratios.  相似文献   

Multiple transit times and least squares for gyroscopic azimuth     

Luc M. A. Jeudy 《Journal of Geodesy》1986,60(4):288-296

Principles of North determination using suspended gyrocompasses are reviewed. Accuracy is evaluated and a procedure with two series of measurements symmetrical with respect to the zero torsion tape position is mathematically proven to be the “best” (minimum variance). Our purpose is to prove that a 20″ accuracy (1 σ) instrument was brought to a level of accuracy four times better by using multiple transit times and least squares fit. Over a total of 15 North determinations based on more than a thousand transit times, an external standard error of 4″.4 was obtained using a WildGAK-1.  相似文献   

Linear demultiple solution based on the concept of bottom-multiple-generator (BMG) approxiamtion: some new results     

Abiola O. Oladeinde  Luc T. Ikelle 《Geophysical Prospecting》2006,54(4):425-431

Recent advances in the demultiple technique have shown that a multidimensional convolution of a portion of data containing only primaries with the whole data (containing both primaries and multiples) can allow us to predict and attenuate all orders of free‐surface multiples that are relevant for practical purposes. One way of constructing the portion of the data containing only primaries is by muting the actual data just above the first free‐surface multiple to arrive. The location of the mute is generally known as the bottom‐multiple‐generator (BMG) reflector; the portion of the data containing only primaries required for constructing the free‐surface multiples is located above the BMG. The outstanding question about this method is how effective can the technique be when the BMG cuts through several seismic events, as is the case in long‐offset data or in very complex shallow geology. We present new results which demonstrate the fact that the BMG location may cut through several seismic events without affecting the accuracy or the cost of demultiple.  相似文献   

Tropical high Andes lakes: A limnological survey and an assessment of exotic rainbow trout (Oncorhynchus mykiss)     

Ximena Aguilera  Steven Declerck  Luc De Meester  Mabel Maldonado  Frans Ollevier 《Limnologica》2006,36(4):258-268

Tropical high Andes lakes are aquatic ecosystems with peculiar limnological characteristics that are related to their geographical location and high altitude, yet they remained understudied. We present the results of a standardized survey of morphometric, physico-chemical and biotic variables in 32 high altitude lakes of the Cordillera del Tunari (Eastern Andes of Bolivia). Based on the variables measured, we identified three lake types. One group of lakes differed from the other lakes by a relatively high pH and biological productivity (as evidenced from higher densities of phytoplankton, zooplankton, and fish). A second group of lakes consisted of relatively large and deep water bodies with neutral to slightly acid pH and with a relatively high occurrence of the large cladoceran Daphnia pulex. The third group contained relatively small, shallow, and acid lakes with no Daphnia. Rainbow trout occurred in more than half of the lakes and catch yields were very variable. Overall, the abundances of different organism groups tended to be positively associated (e.g. phytoplankton, copepods, rotifers, fish) indicating the existence of a major productivity gradient. We found no negative associations between trout catches and densities of any of the major zooplankton groups, suggesting moderate to low top-down effects of trout on the zooplankton communities.  相似文献   

An evaluation of the role of daphnids in controlling phytoplankton biomass in clear water versus turbid shallow lakes     

Koenraad Muylaert  Steven Declerck  Jeroen Van Wichelen  Luc De Meester  Wim Vyverman 《Limnologica》2006,36(2):69-78

Phytoplankton and zooplankton were monitored during 2 years in four eutrophic shallow lakes (two turbid and two clear water) from two wetland reserves in Belgium. In each wetland, phytoplankton biomass was significantly higher in the turbid lake than in the clear water lake. Although total macrozooplankton biomass and the contribution of daphnids to total zooplankton biomass was comparable in the clear water and the turbid lakes, the grazing pressure of macrozooplankton on phytoplankton as estimated from zooplankton to phytoplankton biomass ratios was higher in the clear water lakes. Estimated grazing by daphnids in the clear water lakes was always high in spring. In summer, however, daphnid biomass was low or daphnids were even absent during prolonged periods. During those periods phytoplankton was probably controlled by smaller macrozooplankton or by submerged macrophytes through nutrient competition, allelopathic effects or increased sedimentation rates in the macrophyte vegetation.  相似文献   

Negative bending in seismic reflection associated with time-advanced and time-retarded fields     

Luc T. Ikelle  Anthony F. Gangi 《Geophysical Prospecting》2007,55(1):57-69

We present an analysis of scattering diagrams (i.e., Feynman-like diagrams for wave scattering) of the correlation-type representation theorem for ordinary inhomogeneous media with both positive stiffnesses and positive Poisson's ratios. This analysis reveals scattering events whose scattering diagrams include “negative” bending (i.e., bending in the opposite direction of that of scattering diagrams in ordinary inhomogeneous media). Unlike common scattering events, these events are inconsistent with the current interpretation of some of the basic physical laws, such as Snell's law, just like the so-called “negative refraction” in optics. Yet we find them very useful, for instance, in suppressing some undesired events from scattering data.  相似文献   

Using Geomorphological Indicators to Predict Earthquake Magnitude (MOb‒Max): A Case Study from Cao Bang Province and Adjasent Areas (Vietnam)     

Trong  C. D.  Hoang  N.  Bach  M. X.  Luc  N. M.  Dung  L. V.  Trieu  C. D.  Syrbu  N. S.  Hai  D. Th.  Tuan  Th. A.  Toan  N. Q.  Thanh  D. V. 《Geotectonics》2022,56(3):321-338

Geotectonics - This paper presents analysis of geomorphological indices for predicting the maximum observed earthquake (MOb?Max) in research region, conducted in three steps: (i)...  相似文献   

On the role of pre-existing discontinuities on the micromechanical behavior of confined rock samples: a numerical study     

Gao  Ge  Meguid  Mohamed A.  Chouinard  Luc E. 《Acta Geotechnica》2020,15(12):3483-3510

The deformation process and failure mechanism of rock mass with increased density of initial joints subjected to confined stress state are investigated in this study using discrete element method (DEM). A numerical model of standard size granite samples is developed and validated using experimental data for both intact and jointed rocks. The micro-parameters of the rock material are first determined, and the effects of the rock discontinuity on strength, deformability, stress–strain relationship, and failure modes are then investigated at the macro-scale level. Analyses are also performed to examine the tensile and shear crack distributions, fragmentation characteristics, particle kinematics, and energy dissipation to advance the current understanding of the deformation processes and failure mechanisms of jointed rock masses. The microscopic evolutions in the fabric and force anisotropy during loading and distributions of contact forces provide insights into the influence of increasing initial jointing on the macroscopic deformational behavior of the rock. The results show how the deceleration in the growth of fabric and contact force anisotropies develops and confirms that the increase in initial jointing and the associated changes in microstructure can restrain the development of anisotropy, thereby reducing significantly the strength of the rock samples.

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Chronology of Upper Pleistocene sequences at Sidi Messaoud (wadi Noun, southwestern Morocco) based on C, optical and U-series dating     

Norbert Mercier  Christine Hatt  Michel Fontugne  Jean-Louis Reyss  Hlne Valladas  Luc Wengler  Jean-Philip Brugal  Abderrahmane Ouammou  Andr Weisrock 《Quaternary Geochronology》2009,4(4):326-334

The Sidi Messaoud site provides a 40 m thick sedimentary infilling of great importance for the study of environmental changes and landscape evolution during the Upper Pleistocene in the Western Sahara. In this study, chronological results obtained by the ¹⁴C, U-series and optical dating techniques are compared. Each individual age, depositional environment and post-deposition history of the various samples are evaluated, in order to explain the methodological reasons for discordances and concordances. Stratigraphic and palaeoenvironmental implications for the last 120 ka history in southwestern Morocco are also inferred: the wadi Noun gorges had been incised before MIS 5, and later, during MIS 3, the valley has been filled in with a mean sedimentation rate of 1 m/ka.  相似文献   

Plateau reduction by drainage divide migration in the Eastern Cordillera of Colombia defined by morphometry and 10Be terrestrial cosmogenic nuclides          下载免费PDF全文

Lucía Struth  Antonio Teixell  Lewis A. Owen  Julien Babault 《地球表面变化过程与地形》2017,42(8):1155-1170

Catchment‐wide erosion rates were defined using ¹⁰Be terrestrial cosmogenic nuclides for the Eastern Cordillera of the Colombian Andes to help determine the nature of drainage development and landscape evolution. The Eastern Cordillera, characterized by a smooth axial plateau bordered by steep flanks, has a mean erosion rate of 11 ± 1 mm/ka across the plateau and 70 ± 10 mm/ka on its flanks, with local high rates >400 mm/ka. The erosional contrast between the plateau and its flanks was produced by the increase in the orogen regional slope, derived from the progressive shortening and thickening of the Eastern Cordillera. The erosion rates together with digital topographic analysis show that the drainage network is dynamic and confirms the view that drainage divides in the Eastern Cordillera are migrating towards the interior of the mountain belt resulting in progressive drainage reorganization from longitudinal to transverse‐dominated rivers and areal reduction of the Sabana de Bogotá plateau. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献