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91.
Lucie Mathieu Paul Byrne Damien Guillaume Benjamin van Wyk de Vries Bertrand Moine 《Journal of Volcanology and Geothermal Research》2011,199(3-4):206-215
The Kerguelen Archipelago is part of an oceanic plateau with a complex history. Little work has been done on the tectonics of the onshore areas, even though the extensive outcrop renders the islands especially good for structural work. We present the results of three field campaigns and remote sensing analysis carried out in the main Kerguelen Island, around Val Travers valley and Mt Ross volcano (Central Plateau) and in the Rallier du Baty peninsula (SW part of the archipelago). We have mapped faults, fracture sets, and the location and geometry of intrusive bodies. We found that the plateau basalt lavas that make up most of the area are densely fractured, crossed by many veins, dykes and some small faults. This work provides a general framework for the structure of Kerguelen Archipelago that is dominated by 110°-striking faults and veins, dyke swarms and an alignment of recent central volcanoes, which have formed in N-S to NNW-SSE directed extensional stress field. The other structures are fractures, veins and dykes which strike 130–150°, 000° and 030–050°. They are likely related to transform faults of the Indian oceanic crust and to faults of the north Kerguelen Plateau (offshore basement of the archipelago). These buried structures were likely re-activated by a low magnitude stress field. 相似文献
92.
Beno?t Joseph Mbassa Emmanuel Njonfang Mathieu Benoit Pierre Kamgang Michel Grégoire Stephanie Duchene Pierre Brunet Bekoa Ateba Félix M. Tchoua 《Mineralogy and Petrology》2012,106(3-4):217-242
The Mbengwi recent magmatic formations consist of volcanics and syenites belonging to the same magmatic episode. Lavas form a bimodal basanite-rhyolite alkaline series with a gap between 50 and 62?wt.% SiO2. Mafic lavas (basanite-hawaiite) are sodic while felsic rocks (trachyte-rhyolite-syenites) are sodi-potassic, slightly metaluminous to peralkaline. The geochemical and isotopic characteristics (0.7031?<?(87Sr/86Sr)initial?<?0.7043; 1.03?<?εNdi?<?5.17) of these rocks are similar to those of other rocks from the CVL. The main differentiation process is fractional crystallization with two trends of fractionation. Their Rb/Sr isochron age of 28.2?Ma, almost similar to 27.40?±?0.6?Ma?K/Ar age obtained in a trachyte from neighboring Bamenda Mountains system, precludes any local age migration of an hypothetic hotspot. Mafic lavas have OIB features displaying an isotopic signature similar to that of HIMU mantle source different from FOZO known as source of most parental magmas along the CVL. 相似文献
93.
This study aimed at analysing the effects of biological and meteorological factors on stemflow generation in a temperate mixed oak (Quercus petraea Liebl.) and beech (Fagus sylvatica L.) stand. A statistical model was developed to predict single‐event individual stemflow volume from trunk circumference and rainfall depth allowing mechanistic stemflow parameters to be deduced from the model, namely stemflow rates (SFrate), storage of water on tree organs (St) and rainfall thresholds for stemflow (RFmin). SFrate and St increased with increasing trunk circumference while RFmin was not significantly influenced by tree size. RFmin and, for a given tree size, St were higher for oak than for beech, and inversely for SFrate. For each species, RFmin was higher for the leaved season than for the leafless period, while the opposite was found for SFrate, and St was not significantly affected by the season. Increasing wind speed during rain increased SFrate, lowered RFmin and did not influence St. In contrast, St and RFmin tended, respectively, to decrease and to increase with increasing values of the ratio between the cumulated potential evaporation during the dry period preceding the rain event and the volume of the preceding rainfall (Eva pADP/Rprevious). Stemflow volume, which results from the combined effects of the previous parameters, was higher for beech than for oak and also higher during the leafless period than during the leaved period; these differences were large for the smallest events but decreased rapidly as rainfall depth increased. In addition, an enhancing and a depressing effect on stemflow volume were shown for the average wind speed during rain and for the ratio Eva pADP/Rprevious, respectively. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
94.
Mathieu?Le?CozEmail author Pierre?Genthon Pierre?M.?Adler 《Mathematical Geosciences》2011,43(7):861-878
Multiple-point statistics are used to model facies heterogeneities in the vadose zone of the Komadugu-Yobe River valley (southeastern
Niger) which is presently submitted to an undergoing intensive agricultural development; therefore, increasing quantitative
and qualitative pressures are exerted on groundwater resources. The sand–clay heterogeneities are analyzed by means of a Landsat
image acquired during a high flow period over a 160 km stretch in the downstream part of the valley and a set of 50 boreholes
drilled near the town of Diffa (4 km×4 km area). The horizontal variograms of heterogeneities are characterized by a noticeably
constant length scale of 380 m and clayey objects are shown to be randomly distributed in space according to a Poisson process.
A set of two-dimensional vertical images is built based on a Boolean procedure and the Snesim algorithm is used to simulate
synthetic three-dimensional media. When the vertical correlation length is fitted, the three-dimensional model satisfactorily
reproduces the second order statistics of heterogeneities and the specific facies patterns. 相似文献
95.
We argue that the first stars may have spanned the conventional mass range rather than be identified with the very massive objects (∼100–103 M⊙ ) favoured by numerical simulations. Specifically, we find that magnetic field generation processes acting in the first protostellar systems suffice to produce fields that exceed the threshold for magneto-rotational instability (MRI) to operate, and thereby allow the MRI dynamo to generate equipartition-amplitude magnetic fields on protostellar mass scales below ∼50 M⊙ . Such fields allow primordial star formation to occur at essentially any metallicity by regulating angular momentum transfer, fragmentation, accretion and feedback in much the same way as occurs in conventional molecular clouds. 相似文献
96.
Mathieu de Naurois 《Astrophysics and Space Science》2007,309(1-4):277-284
Recent observations of the binary system LS 5039 with the High Energy Stereoscopic System (H.E.S.S.) revealed that its Very
High Energy (VHE) γ-ray emission is modulated at the 3.9 days orbital period of the system. The bulk of the emission is largely confined to half
of the orbit, peaking around the inferior conjunction epoch of the compact object. The flux modulation provides the first
indication of γ-ray absorption by pair production on the intense stellar photon field. This implies that the production region size must
be not significantly greater than the gamma-gamma photosphere size (∼1 AU), thus excluding the large scale collimated outflows
or jets (extending out to ∼1000 AU). A hardening of the spectrum is also observed at the same epoch between 0.2 and a few
TeV which is unexpected under a pure absorption scenario and could rather arise from variation with phase in the maximum electron
energy and/or the dominant VHE γ-ray production mechanism. This first-time observation of modulated γ-ray emission allows precise tests of the acceleration and emission models in binary systems.
Mathieu de Naurois for the H.E.S.S. Collaboration. 相似文献
97.
Alessandro Ielpi Mathieu G.A. Lapôtre Alvise Finotello Massimiliano Ghinassi 《地球表面变化过程与地形》2021,46(2):357-370
River bends occasionally meander to the point of cutoff, whereby a river shortcuts itself and isolates a portion of its course. This fundamental process fingerprints a river's long-term planform geometry, its stratigraphic record, and biogeochemical fluxes in the floodplain. Although meander cutoffs are common in fast-migrating channels, timelapse imagery of the Earth surface typically does not offer a long enough baseline for statistically robust analyses of these processes. We seek to bridge this gap by quantifying cutoff kinematics along the Humboldt River (Nevada) – a stream that, from 1994 to 2019, hosted an exceptionally high number of cutoffs (specifically, 174 of the chute type and 53 of the neck type). A coincidence between major floods and cutoff incidence is first suggestive of hydrographic modulation. Moreover, not just higher sinuosity but also upstream planform skewness is associated with higher cutoff incidence and channel widening for a sub-population of chute cutoffs. We propose a conceptual model to explain our results in terms of channel-flow structure and then examine the distances between adjacent cutoffs to understand the mechanisms governing their clustering. We find that both local and nonlocal perturbations together trigger the clustering of new cutoffs, over distances capped by the backwater length and over yearly to decadal timescales. Our research suggests that planform geometry and backwater controls might sway the occurrence of cutoff clusters – both local and nonlocal – thereby offering new testable hypotheses to explore the evolution of meandering-river landscapes that have significant implications for river engineering and stratigraphic modelling. © 2020 John Wiley & Sons, Ltd. 相似文献
98.
Thierry Calmus Carlos Pallares Ren�� C. Maury Alfredo Aguill��n-Robles Herv�� Bellon Mathieu Benoit Fran?ois Michaud 《Pure and Applied Geophysics》2011,168(8-9):1303-1330
The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ??regular?? calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ??no-slab?? regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle. 相似文献
99.
We have produced detailed maps of U and Th isotopes for three cross-sections of an Early Pleistocene equid tooth from the archaeological site of Fuente Nueva-3 (Orce, Andalusia, Spain). This permits us to visualise, for the first time, U migration processes in 3 dimensions. The tooth shows a concentration gradient from the top to the base, indicating the U profile had not equilibrated after >1 Ma. The spatial pattern of 230Th/234U and 234U/238U indicates complex U-mobilisation processes over the last 100 ka, dominated by small-scale redistribution of U. Leaching from the tooth through the pulp cavity started at least 93 ka ago with several later phases in various domains of the dentine and cement. This leaching event could have been triggered by changes in the local hydrological regime associated with periods of increased erosion in the Guadix-Baza basin. The results illustrate the difficulty of dating faunal material from Early Pleistocene sites. They also demonstrate that dental tissues can neither be considered as homogeneous media for U-diffusion, nor behave as closed systems for U-series isotopes because diagenetic alterations seem to trigger U-migration. The results do not support the notion that U-uptake into dental tissues is necessarily of short duration. Nevertheless, rapid laser ablation scanning can be used to identify suitable samples for dating as well as domains within the teeth that may have preserved original isotopic signatures, i.e. domains that have not been affected by recent U-mobilisation process. 相似文献
100.
The Mt Cameroon volcano is the highest and most active volcano of the Cameroon Volcanic Line. Little geological information
is available for improving the understanding of the structure of this large volcanic system and its relationship to regional
tectonics. After reviewing the tectonic evolution of the region, the analysis of a Digital Elevation Model and results from
a field campaign dedicated to mapping geological structures in the summit area and at the SE base of Mt Cameroon are presented.
Mt Cameroon is a lava-dominated volcano with long steep (over 30°) flanks. It is elongate parallel to its well defined rift
zone. The summit plateau is bordered by 10 m high cliffs formed by summit subsidence along normal faults. Geological profiles
were measured along rivers cutting through a topographic step at the SE base of Mt Cameroon. This step is associated with
deformed Miocene sediments from the Douala basin that are overlain by volcanic products. Weak sediments of this area are deformed
by 050°–060° and 020°–030° trending asymmetrical folds verging toward the SE, and thrusts faults related to the spreading
of the volcano over its mechanically weak substratum. Combined remote sensing and field observations suggest that spreading
is accommodated by summit subsidence and flanks sliding. Both slow spreading movements and catastrophic collapses of the steep
flanks are interpreted to result from complex interactions between the growing edifice, repeated dyke intrusions, the weak
sedimentary substratum and tectonic structures. 相似文献