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71.
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
72.
Marie Lefrancq Paul Van Dijk Victor Jetten Matthieu Schwob Sylvain Payraudeau 《水文研究》2017,31(6):1408-1423
Predicting runoff hot spots and hot‐moments within a headwater crop‐catchment is of the utmost importance to reduce adverse effects on aquatic ecosystems by adapting land use management to control runoff. Reliable predictions of runoff patterns during a crop growing season remain challenging. This is mainly due to the large spatial and temporal variations of topsoil hydraulic properties controlled by complex interactions between weather, growing vegetation, and cropping operations. This interaction can significantly modify runoff patterns and few process‐based models can integrate this evolution of topsoil properties during a crop growing season at the catchment scale. Therefore, the purpose of this study was to better constrain the event‐based hydrological model Limburg Soil Erosion Model by incorporating temporal constraints for input topsoil properties during a crop growing season (LISEM). The results of the temporal constraint strategy (TCS) were compared with a classical event per event calibration strategy (EES) using multi‐scale runoff information (from plot to catchment). The EES and TCS approaches were applied in a loess catchment of 47 ha located 30 km northeast of Strasbourg (Alsace, France). A slight decrease of the Nash–Sutcliffe efficiency criterion on runoff discharge for TCS compared to EES was counterbalanced by a clear improvement of the spatial runoff patterns within the catchment. This study showed that limited agronomical and climatic information added during the calibration step improved the spatial runoff predictions of an event‐based model. Reliable prediction of runoff source, connectivity, and dynamics can then be derived and discussed with stakeholders to identify runoff hot spots and hot‐moments for subsequent land use and crop management modifications. 相似文献
73.
Matthieu E. Galvez Isabelle Martinez Olivier Beyssac Karim Benzerara Pierre Agrinier Nelly Assayag 《Contributions to Mineralogy and Petrology》2013,166(6):1687-1708
Multiple pieces of geologic evidence suggest that interfaces between contrasted lithologies exert a strong control on the fate of volatiles in subduction zones. Here we present results from a contact between serpentinites and sediments, located in Corsica and metamorphosed in the blueschist facies during the alpine orogeny. It was shown previously that carbonates in the sediments have been reduced to graphitic carbonaceous material within a 5–10-cm-thick reaction zone at the contact with serpentinites. In an effort to investigate the mechanisms governing this unusual process, bulk rock geochemical analyses incorporating a statistical analysis of compositional data are presented. Observations show that the fate of C was decoupled from that of other elements such as O, H, and large-ion lithophile elements—e.g. K, Sr, Ba...,As—that were extensively leached from the reaction zone. Notably, Na is strongly enriched in the reaction zone and structurally linked to pectolite. Reducing conditions, manifested by the depletion of O in the reaction zone compared to the bulk metasediment, were likely maintained by the presence of Fe(II) in the serpentinite. Moreover, thermodynamic calculations show that the low solubility of carbon in COH fluids at high-pressure and low-temperature conditions was the main driver for graphite precipitation synchronously with carbonate destabilization. This may have been kinetically favored by the presence of already existing graphitized carbonaceous material and phengite in the metasediment. Limited lateral flow might have contributed as well to the geochemical and petrological patterns observed in these rocks. 相似文献
74.
75.
Matthieu Kervyn Gerald G. J. Ernst Jurgis Klaudius Jörg Keller François Kervyn Hannes B. Mattsson Frederic Belton Evelyne Mbede Patric Jacobs 《Bulletin of Volcanology》2008,70(9):1069-1086
The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to
April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the
crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed
by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO2). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer
(MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity.
The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 ± 3.0 × 105 m3. The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion
from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this
unique natrocarbonatite volcano. 相似文献
76.
Matthieu Kretzschmar Thierry Dudok de Wit Jean Lilensten Jean-Francois Hochedez Jean Aboudarham Pierre-Olivier Amblard Frederic Auchère Said Moussaoui 《Acta Geophysica》2009,57(1):42-51
The knowledge of solar extreme and far ultraviolet (EUV) irradiance variations is essential for the characterization of the
Earth’s upper atmosphere. For a long time, this knowledge has been based on empirical models, which are themselves based on
proxies of the solar activity. However, the accurate modeling and prediction of the Earth’s upper atmosphere necessitate to
improve the precision on the irradiance and its variations below about 200 nm. Here, we present a review of recent works made
by the authors that aim at quantifying the irradiance variability at these wavelengths, and that lead to new way of monitoring
the solar EUV/FUV irradiance spectrum. In more details, it is shown that the quantification of the high level of redundancy
in the solar spectrum variability allows to envisage measuring only a small portion of the spectrum without losing essential
knowledge. Finally, we discuss what should and could be measured in order to retrieve the solar extreme and far ultraviolet
spectrum. 相似文献
77.
The development of coastal ocean modeling in the recent years has allowed an improved representation of the associated complex
physics. Such models have become more realistic, to the point that they can now be used to design observation networks in
coastal areas, with the idea that a “good” network is a network that controls model state error. To test this ability without
performing data assimilation, we set up a technique called Representer Matrix Spectra (RMS) technique that combines the model
state and observation error covariance matrices into a single scaled representer matrix. Examination of the spectrum and the
eigenvectors of that matrix informs us on which model state error modes a network can detect and constrain amidst the observation
error background. We applied our technique to a 3D coastal model in the Bay of Biscay, with a focus on mesoscale activity,
and tested the performance of various altimetry networks and an in situ array deployment strategy. It appears that a single
nadir altimeter is not efficient enough at capturing coastal mesoscale physics, while a wide swath altimeter would do a much
better job. Testing various local in situ array configurations confirms that adding a current meter to a vertical temperature
measurement array improves the detection of secondary variability modes, while shifting the array higher on the shelf break
would obviously enhance the model constraint along the coast. The RMS technique is easily set up and used as a “black box,”
but the utility of its results is maximized by previous knowledge of model state error physics. The technique provides both
quantitative (eigenvalues) and qualitative (eigenvectors) tools to study and compare various network options. The qualitative
approach is essential to discard possibly inconsistent modes. 相似文献
78.
Touron A Berthe T Gargala G Fournier M Ratajczak M Servais P Petit F 《Marine pollution bulletin》2007,54(9):1441-1450
The Seine estuary, one of the largest estuaries of the European northwest continental shelf, is subjected to numerous anthropogenic influences. Here we present an assessment of the microbial faecal contamination of the estuary water. The most vulnerable areas were defined on the basis of the fluxes of indicator organisms and the occurrence of Salmonella and Cryptosporidium sp. and Giardia sp. (oo)cysts. The microbial quality of the water changes from upstream to downstream: in the upstream area, contamination by faecal-indicator bacteria and Salmonella occurs during periods of high flow; in the urbanized area, mid-way between the uppermost areas of the estuary and its mouth, discharge from a wastewater treatment plant and a tributary degrade water quality; at the estuary mouth, the accumulation of microorganisms attached to particles in the maximum turbidity zone, particularly Clostridium perfringens spores and oocysts of Cryptosporidium, is accompanied by inputs of ThC and Escherichia coli from tributaries. In some areas, significant strong relations are observed between Salmonella, (oo)cysts of protozoan, and levels of faecal indicators. 相似文献
79.
Jean-Paul Toutain Margot Munoz Jean-Louis Pinaud Stéphanie Levet Matthieu Sylvander Alexis Rigo Jocelyne Escalier 《Pure and Applied Geophysics》2006,163(4):723-744
Groundwater coseismic transient anomalies are evidenced and characterized by modelling the mixing function F characteristic
of the groundwater dynamics in the Ogeu (western French Pyrénées) seismic context. Investigations of water-rock interactions
at Ogeu indicate that these mineral waters from sedimentary environments result from the mixing of deep waters with evaporitic
signature with surficial karstic waters. A 3-year hydrochemical monitoring of Ogeu springwater evidences that using arbitrary
thresholds constituted by the mean ± 1 or 2σ, as often performed in such studies, is not a suitable approach to characterize transient anomalies. Instead, we have used
a mixing function F calculated with chemical elements, which display a conservative behavior not controlled by the precipitation of a mineral
phase. F is processed with seismic energy release (Es) and effective rainfalls (R). Linear impulse responses of F to Es and R have been calculated. Rapid responses (10 days) to rainwater inputs are evidenced, consisting in the recharge of the shallow
karstic reservoir by fresh water. Complex impulse response of F to microseismic activity is also evidenced. It consists in a 2-phase hydrologic signal, with an inflow of saline water in
the shallow reservoir with a response delay of 10 days, followed by an inflow of karstic water with a response delay of 70
days, the amount being higher than the saline inflow. Such a process probably results from changes in volumetric strain with
subsequent microfracturation transient episodes allowing short inflow of deep salted water in the aquifer. This study demonstrates
that groundwater systems in such environments are unstable systems that are highly sensitive to both rainfall inputs and microseismic
activity. Impulse responses calculation of F to Es is shown to be a powerful tool to identify transient anomalies. Similar processing is suggested to be potentially efficient
to detect precursors of earthquakes when long time-series (5 years at least) are available in areas with high seismicity. 相似文献
80.
Alexis?RigoEmail author Annie?Souriau Noalwenn?Dubos Matthieu?Sylvander Christian?Ponsolles 《Journal of Seismology》2005,9(2):211-222
Two temporary seismological networks have been set up in 2000 and 2002 in the central part of the Pyrenees, in a region, which appears as a transition between two domains where both the seismic activity and the tectonic regime are different. Together with the permanent networks, they allowed us to obtain precise hypocenter locations for more than 400 events with local magnitudes ranging from 1.5 to 4.6, as well as 30 new focal mechanisms. The seismicity is distributed in several clusters, which are not located along the North Pyrenean Fault, considered as the major tectonic accident resulting from the suture of the Iberian and Eurasian convergent plates when the range formed. Several small fault segments dipping to the north are identified. The maximum focal depth varies from 10 to 20 km, with variations which are roughly parallel to those of the Moho, indicating a thickening of the seismogenic layer to the east of the studied area. The obtained focal solutions reveal a predominance of normal faulting to the West and reverse faulting to the East, with strike-slip motions in between. The largest fault segment to the East, with a length of about 20 km, could possibly be related to a large historical event which occurred in 1660, with intensity IX, close to cities which have become since then important touristic centres. 相似文献