New constraints on granulite facies metamorphism and melt production in the Lewisian Complex,northwest Scotland          下载免费PDF全文

Yves Feisel  Richard W. White  Richard M. Palin  Tim E. Johnson 《Journal of Metamorphic Geology》2018,36(6):799-819

In this study, we investigate the metamorphic history of the Assynt and Gruinard blocks of the Archean Lewisian Complex, northwest Scotland, which are considered by some to represent discrete crustal terranes. For samples of mafic and intermediate rocks, phase diagrams were constructed in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (NCKFMASHTO) system using whole‐rock compositions. Our results indicate that all samples equilibrated at similar peak metamorphic conditions of ~8–10 kbar and ~900–1,000°C, consistent with field evidence for in situ partial melting and the classic interpretation of the central region of the Lewisian Complex as representing a single crustal block. Melt‐reintegration modelling was employed in order to estimate probable protolith compositions. Phase equilibria calculated for these modelled undepleted precursors match well with those determined for a subsolidus amphibolite from Gairloch in the southern region of the Lewisian Complex. Both subsolidus lithologies exhibit similar phase relations and potential melt fertility, with both expected to produce orthopyroxene‐bearing hornblende granulites, with or without garnet, at the conditions inferred for the Badcallian metamorphic peak. For fully hydrated protoliths, prograde melting is predicted to first occur at ~620°C and ~9.5 kbar, with up to 45% partial melt predicted to form at peak conditions in a closed‐system environment. Partial melts calculated for both compositions between 610 and 1,050°C are mostly trondhjemitic. Although the melt‐reintegrated granulite is predicted to produce more potassic (granitic) melts at ~700–900°C, the modelled melts are consistent with the measured compositions of felsic sheets from the central region Lewisian Complex.  相似文献   

Fluvial incisions in the North‐Western Pyrenees (Aspe Valley): Dissection of a former planation surface and some tectonic implications     

Jessica Uzel  Bertrand Nivire  Yves Lagabrielle 《地学学报》2020,32(1):11-22

Dissected remnants of a high elevation low relief surface (HELRS) are encountered all along the Pyrenean range. All authors agree that the HELRS was shaped during the post‐shortening evolution of the belt, but doubt remains on its original elevation. Notably, whether a post‐shortening uplift event occurred after the generation of the Pyrenean planation surface is still debated. Based on a geomorphological study of the entrenchment of the Aspe River in the North‐Western Pyrenees, we describe a post‐Oligocene multi‐stage dissection of the HELRS linked to a regional post‐shortening uplift. Each incision stage is recorded by erosional triangular facets and associated stepped remnants of former smooth topographies. Compared analysis of patterns of incision in the Axial Zone and its northern border allows to evidence differential vertical motions. We discuss the forcing mechanism(s) that controlled this morphological evolution.  相似文献   

Neogene upper‐crustal cooling of the Olympus range (northern Aegean): major role of Hellenic back‐arc extension over propagation of the North Anatolia Fault Zone     

Isabelle Coutand  Michael Walsh  Bertha Louis  Frank Chanier  Jacky Ferrière  Jean‐Yves Reynaud 《地学学报》2014,26(4):287-297

The North Anatolian Fault Zone (NAFZ) is one of the most hazardous active faults on Earth, yet its Pliocene space‐time propagation across the north Aegean domain remains poorly constrained. We use low‐temperature multi‐thermochronology and inverse thermal modelling to quantify the cooling history of the upper crust across the Olympus range. This range is located in the footwall of a system of normal faults traditionally interpreted as resulting from superposed Middle–Late Miocene N–S stretching, related to the back‐arc extension of the Hellenic subduction zone, and a Pliocene‐Quaternary transtensional field, attributed to the south‐westward propagation of the NAFZ. We find that accelerated exhumational cooling occurred between 12 and 6 Ma at rates of 15–35 °C Ma^?1 and decreased to <3 °C Ma^?1 by 8–6 Ma. The absence of significant Plio‐Pleistocene cooling across Olympus suggests that crustal exhumation there is driven by late Miocene back‐arc extension, while the impact of the NAFZ remains limited.  相似文献   

On the age calibration of the geomagnetic polarity timescale     

Pierre Agrinier  Yves Gallet  & Eric Lewin 《Geophysical Journal International》1999,137(1):81-90

Knowledge of the age of undated events is not null if a time-order relationship can be found among these events. The knowledge of such a time-ordered sequence can be formalized by using non-informative (uniform) prior probability densities for the ages of undated events and Bayes' theorem to introduce the time-order relationship condition. We show that the conditional probability densities of the ages of events of unknown age are given by various forms of Euler's beta distribution. These distributions yield an estimate of the probability for an undated event to occur in a given age interval.
  We use this method to propose appropriate probabilistic representations of our actual knowledge of the dating of the magnetic polarity reversals during the Cenozoic. These representations take into account the uncertainties arising from irregularities in accretion process and from the quality of a few calibration points. Both types of uncertainties generate large ambiguities in the age of magnetic reversals, which should be taken into consideration when the geomagnetic polarity timescale is used for dating purposes. We propose to use the entropy function to quantify these ambiguities.  相似文献   

Dense collection areas and terrestrial alteration of meteorites in the Atacama Desert     

Gabriel A. Pinto  Adrien Tavernier  Jérôme Gattacceca  Alexandre Corgne  Millarca Valenzuela  Béatrice Luais  Laura Flores  Felipe Olivares  Yves Marrocchi 《Meteoritics & planetary science》2024,59(2):351-367

In the last 15 years, more than 2700 meteorites have been recovered and officially classified from the Atacama Desert. Although the number of meteorites collected in the Atacama has risen, the physical and climatic properties of the dense collection areas (DCAs) have not been fully characterized. In this article, we compiled the published data of all classified meteorites found in the Atacama Desert to (i) describe the distribution by meteorite groups, (ii) compare the weathering degree of chondrites among different Atacama DCAs and other hot and cold deserts, and (iii) determine the preservation conditions of chondrites in the main Atacama DCAs in relation with the local climatic conditions. The 35 DCAs so far identified in the Atacama Desert are located in three main morphotectonic units: The Coastal Range (CR), Central Depression (CD), and Pre-Andean Range/Basement. A comparison with reported weathering data from other cold and hot deserts indicates that the mean terrestrial weathering of Atacama chondrites (W1–2), displays less alteration than other hot deserts (W2–3) and resembles the weathering distribution of the Antarctic meteorites (W1–2). The highest abundance of Atacama chondrites with low weathering (≤W2) is localized in the CD (78.8%, N = 1435), which is protected from the coastal fog influence and seasonal rainfalls and displays the oldest surfaces in the Atacama Desert. The morphogenetic classification based on present-day temperatures and precipitations of the main Atacama DCAs reveals similar regional/subregional climatic conditions in the most productive areas and a truly productive surface for meteorite recovery between 5% and 58% of the quadrangles formally defined for each Atacama DCA. Our morphogenetic classification lacks consideration of some meteorological parameters such as the coastal fog, so it cannot fully explain the differences in weathering patterns among CR chondrites. Future studies of chondrite preservation in the Atacama DCAs should consider other meteorological variables such as relative humidity, specific humidity, or dew point, in combination with exposure ages of meteorites and its surfaces.  相似文献   

A Review on Forearc Ophiolite Obduction,Adakite-Like Generation,and Slab Window Development at the Chile Triple Junction Area: Uniformitarian Framework for Spreading-Ridge Subduction     

Jacques Bourgois  Yves Lagabrielle  Hervé Martin  Jérôme Dyment  Jose Frutos  Maria Eugenia Cisternas 《Pure and Applied Geophysics》2016,173(10-11):3217-3246

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Hydrogen emissions from Erebus volcano, Antarctica     

Yves Moussallam  Clive Oppenheimer  Alessandro Aiuppa  Gaetano Giudice  Manuel Moussallam  Philip Kyle 《Bulletin of Volcanology》2012,74(9):2109-2120

The continuous measurement of molecular hydrogen (H₂) emissions from passively degassing volcanoes has recently been made possible using a new generation of low-cost electrochemical sensors. We have used such sensors to measure H₂, along with SO₂, H₂O and CO₂, in the gas and aerosol plume emitted from the phonolite lava lake at Erebus volcano, Antarctica. The measurements were made at the crater rim between December 2010 and January 2011. Combined with measurements of the long-term SO₂ emission rate for Erebus, they indicate a characteristic H₂ flux of 0.03?kg s^–1 (2.8?Mg? day^–1). The observed H₂ content in the plume is consistent with previous estimates of redox conditions in the lava lake inferred from mineral compositions and the observed CO₂/CO ratio in the gas plume (～0.9 log units below the quartz–fayalite–magnetite buffer). These measurements suggest that H₂ does not combust at the surface of the lake, and that H₂ is kinetically inert in the gas/aerosol plume, retaining the signature of the high-temperature chemical equilibrium reached in the lava lake. We also observe a cyclical variation in the H₂/SO₂ ratio with a period of ～10?min. These cycles correspond to oscillatory patterns of surface motion of the lava lake that have been interpreted as signs of a pulsatory magma supply at the top of the magmatic conduit.  相似文献   

Comparison of LiDAR waveform processing methods for very shallow water bathymetry using Raman,near‐infrared and green signals     

Tristan Allouis  Jean‐Stéphane Bailly  Yves Pastol  Catherine Le Roux 《地球表面变化过程与地形》2010,35(6):640-650

Airborne light detection and ranging (LiDAR) bathymetry appears to be a useful technology for bed topography mapping of non‐navigable areas, offering high data density and a high acquisition rate. However, few studies have focused on continental waters, in particular, on very shallow waters (<2 m) where it is difficult to extract the surface and bottom positions that are typically mixed in the green LiDAR signal. This paper proposes two new processing methods for depth extraction based on the use of different LiDAR signals [green, near‐infrared (NIR), Raman] of the SHOALS‐1000T sensor. They have been tested on a very shallow coastal area (Golfe du Morbihan, France) as an analogy to very shallow rivers. The first method is based on a combination of mathematical and heuristic methods using the green and the NIR LiDAR signals to cross validate the information delivered by each signal. The second method extracts water depths from the Raman signal using statistical methods such as principal components analysis (PCA) and classification and regression tree (CART) analysis. The obtained results are then compared to the reference depths, and the performances of the different methods, as well as their advantages/disadvantages are evaluated. The green/NIR method supplies 42% more points compared to the operator process, with an equivalent mean error (?4·2 cm verusu ?4·5 cm) and a smaller standard deviation (25·3 cm verusu 33·5 cm). The Raman processing method provides very scattered results (standard deviation of 40·3 cm) with the lowest mean error (?3·1 cm) and 40% more points. The minimum detectable depth is also improved by the two presented methods, being around 1 m for the green/NIR approach and 0·5 m for the statistical approach, compared to 1·5 m for the data processed by the operator. Despite its ability to measure other parameters like water temperature, the Raman method needed a large amount of reference data to provide reliable depth measurements, as opposed to the green/NIR method. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Statement to comment by K. Föllmi on ,,Neubenennungen lithostratigraphischer Einheiten in der helvetischen Kreide“     

Schweizerisches Komitee für Stratigraphie  Thierry Adatte  Jean-Pierre Berger  Hansruedi Bläsi  Reto Burkhalter  Stephan DallAgnolo  Danielle Decrouez  Hanspeter Funk  Heinz Furrer  Yves Gouffon  Hansruedi Graf  Werner Heckendorn  Wilfried Winkler 《Swiss Journal of Geoscience》2009,102(3):519-519

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Quantitative reconstruction of climatic variations during the Bronze and early Iron ages based on pollen and lake-level data in the NW Alps, France     

M. Magny  O. Peyron  E. Gauthier  Y. Rouche  A. Bordon  Y. Billaud  E. Chapron  A. Marguet  P. Ptrequin  B. Vannire 《Quaternary International》2009,200(1-2):102

Vegetation and lake-level data from the archaeological site of Tresserve, on the eastern shore of Lake Le Bourget (Savoie, France), are used to provide quantitative estimates of climatic variables over the period 4000–2300 cal BP in the northern French Pre-Alps, and to examine the possible impact of climatic changes on societies of the Bronze and early Iron Ages. The results obtained indicate that phases of higher lake level at 3500–3100 and 2750–2350 cal BP coincided with major climate reversals in the North Atlantic area. In west-central Europe, they were marked by cooler and wetter conditions. These two successive events may have affected ancient agricultural communities in west-central Europe by provoking harvest failures, more particularly due to increasing precipitation during the growing season. However, archaeological data in the region of Franche-Comté (Jura Mountains, eastern France) show a general expansion of population density from the middle Bronze Age to the early Iron Age. This suggests a relative emancipation of proto-historic societies from climatic conditions, probably in relation to the spread of new modes of social and economic organisation.  相似文献