Mechanics of low-angle extensional shear zones at the brittle-ductile transition

Present address : Institut des Sciences de la Terre d'Orléans UMR 7327, Université d'Orléans, CNRS/INSU, BGRMInternational audienceLow-angle midcrustal ductile shear zones and the related microseismic activity recorded below regions of active extension are seen here as two consequences of strain localization. The feldspar-to-mica reaction which occurs once feldspar grains are fractured is the destabilizing mechanism selected to explain the strain localization. The model problem considered to substantiate these claims is solved by numerical means and combines the simple shear due to the rigid gliding of the upper crust (at the velocity of V s ) and the stretch resulting from the extension of the whole crust (at the velocity V e ). The rheological model accounts for dislocation creep of quartz, feldspar, and mica, the feldspar-to-mica reaction, and its prerequisite, which is the feldspar fracturing detected by the Mohr-Coulomb criterion. The one-dimensional (1-D) solution, which constrains shear bands to be horizontal, shows the depth partitioning in deformation mode between the simple shear of the low-viscosity deep crust and the stretching of the highly viscous midcrust. Strain localization occurs during rapid increase of the shearing velocity V s , corresponding to low values of the velocity ratio V e /V s . The 2-D solution (for V e /V s = 10−3) reveals the development of a periodic system of extensional shear bands, dipping at 30° toward the shearing direction at a depth of 12 to 14 km. Shear bands are formed after less than half a million years at the base of the reaction zone defined by the region where feldspar-to-mica reaction is completed. Shear bands do not propagate to greater depths because the pressure prevents the feldspar from fracturing and thus the reaction to occur. The periodic system of shear bands defines a midcrustal flat weakened zone within which the equivalent shear stress is enhanced by at least a factor of three at the shear band tips. Brittle fracture could thus occur within the midcrustal flat weakened zone, explaining therefore the microseismicity monitored at these depths in regions of active extension

An evaluation of components manufactured from a range of materials, fabricated using PolyJet technology

With advances in additive manufacturing, particularly in the area of three dimensional (3D) printing, it is becoming possible to generate small quantities of functional components. As with all manufacturing technologies, the ability to reproducibly generate parts with defined dimensional accuracy and surface finish is of key importance. This work characterises the ability of one such technology, PolyJet 3D printing, to produce such parts. In order to characterise the output from the manufacturing technique, a test part was designed and manufactured in three alternative materials representative of alternative grades across a commercial range. The designed test part consisted of positive and negative feature types printed both in the direction of, and normal to, the print head traverse direction. The test parts were characterised in terms of dimensional accuracy in three dimensions and also in terms of surface finish. Differences in dimensional accuracy and surface profile were observed depending on the orientation of the feature relative to the print head travel direction

Remote sensing and urban malaria: radar Envisat contribution for the determination of potential Anopheles breeding site in Antananarivo (Madagascar)

International audienceMost studies of anopheline mosquito larval ecology have been done in rural settings. However, latest data based on two cross-sectional surveys in Antananarivo, the capitol of Madagascar, shown low rate of malaria cases among febrile episodes but autochthonous malaria cases exist. Anopheles funestus constitutes the main vector of malaria in the highlands of Madagascar. This paper described the determination of their potential breeding site using remotely sensed data. A supervised classification by the classical method of maximum likelihood was used for enhanced thematic mapper image of Landsat 7. Overall accuracy of the classification was 86% and kappa index was 0.835. Determination of landscape change by subtraction of images acquired on January and July was carried out for the Advanced Synthetic Aperture Image Precision images of Envisat. Increased backscatter coefficient between the two periods made possible to raise ambiguity between rice fields and other vegetation. That may improve the determination of potential anopheles breeding sites

Elucidating the crystal-chemistry of Jbel Rhassoul stevensite (Morocco) by advanced analytical techniques

The composition of Rhassoul clay is controversial regarding the nature of the puremineral clay fraction which is claimed to be stevensite rather than saponite. In this study, the raw and mineral fractions were characterized using various techniques including Fourier transform infrared spectroscopy and magic angle spinning nuclear magnetic resonance (MAS NMR). The isolated fine clay mineral fraction contained a larger amount of Al (>1 wt.%) than that reported for other stevensite occurrences. The 27Al MAS NMR technique confirmed that the mineral is stevensite in which the Al is equally split between the tetrahedral and octahedral coordination sites. The 29Si NMR spectrum showed a single unresolved resonance indicating little or no short-range ordering of silicon. The chemical composition of the stevensite from Jbel Rhassoul was determined to be ((Na0.25K0.20 (Mg5.04Al0.37Fe0.20&0.21)5.61(Si7.76Al0.24)8O20(OH)4). This formula differs from previous compositions described from this locality and shows it to be an Al-bearing lacustrine clay mineral

Neogene strike-slip faulting in Sakhalin and the Japan Sea opening

Laurent Jolivet est Professeur à l'Université d'Orléans au 1er Septembre 2009International audienceWe describe structural data from a 2000 km N-S dextral strike-slip zone extending from northern Sakhalin to the southeast corner of the Japan Sea. Satellite images, field data, and focal mechanisms of earthquakes in Sakhalin are included in the interpretation. Since Miocene time the deformation in Sakhalin has been taken up by N-S dextral strike-slip faults with a reverse component and associated en e'chelon folds. Narrow en échelon Neogene basins were formed along strike-sup faults and were later folded in a second stage of deformation. We propose a model of basin formation along extension al faults delimitating dominos between two major strike-slip faults, and subsequent counterclockwise rotation of the dominos in a dextral transpressional regime, basins becoming progressively oblique to the direction of maximum horizontal compression and undergoing shortening. The association of both dextral and compressional focal mechanisms of earthquakes indicates that the same transpressional regime still prevails today in Sakhalin. We present fault set measurements undertaken in Noto Peninsula and Yatsuo Basin at the southern end of the Sakhalin-East Japan Sea strike-slip zone. Early and middle Miocene formations recorded the same transtensional regime as observed along the west coast of NE Honshu. During the early and middle Miocene the strike-slip regime was transpressional to the north in Sakhalin and Hokkaido, and transtensional to the south along the west coast of NE Honshu as far as Noto Peninsula and Yatsuo basin. Dextral motion accommodated the opening of the Japan Sea as a pull-apart basin, with the Tsushima fault to the west. The opening of the Japan Sea ceased at the end of the middle Miocene when transtension started to change to E-W compression in the Japan arc. Subduction of the Japan Sea lithosphere under the Japan arc started 1.8 Ma ago. The evolution of the stress regime from transtensional to compressional in the southern part of the strike-slip zone is related to the inception of the subduction of the young Philippine Sea Plate lithosphere under the Japan arc during the late Miocene. Subduction related extension is a necessary condition for the opening of the Japan Sea. Two possible mechanisms can account for dextral shear in this area: (1) counterclockwise rotation of crustal blocks due to the collision of India with Asia, (2) extrusion of the Okhotsk Sea block squeezed between the North America and Eurasia plates

The Burst-Like Behavior of Aseismic Slip on a Rough Fault: The Creeping Section of the Haiyuan Fault, China

Recent observations suggesting the influence of creep on earthquakes nucleation and arrest are strong incentives to investigate the physical mechanisms controlling how active faults slip. We focus here on deriving generic characteristics of shallow creep along the Haiyuan fault, a major strike‐slip fault in China, by investigating the relationship between fault slip and geometry. We use optical images and time series of Synthetic Aperture Radar data to map the surface fault trace and the spatiotemporal distribution of surface slip along the creeping section of the Haiyuan fault. The fault trace roughness shows a power‐law behavior similar to that of the aseismic slip distribution, with a 0.8 roughness exponent, typical of a self‐affine regime. One possible interpretation is that fault geometry controls to some extent the distribution of aseismic slip, as it has been shown previously for coseismic slip along active faults. Creep is characterized by local fluctuations in rates that we define as creep bursts. The potency of creep bursts follows a power‐law behavior similar to the Gutenberg–Richter earthquake distribution, whereas the distribution of bursts velocity is non‐Gaussian, suggesting an avalanche‐like behavior of these slip events. Such similarities with earthquakes and lab experiments lead us to interpret the rich dynamics of creep bursts observed along the Haiyuan fault as resulting from long‐range elastic interactions within the heterogeneous Earth’s crust

Estimation de stocks de carbone organique des sols à différentes échelles d'espace et de temps

National audienceCet article traite de l’estimation des stocks de carbone organique des sols à différentes échelles d’espace et de temps. Dans une première partie nous passons en revue les principales échelles de variabilité des stocks de carbone des sols et nous en tirons des recommandations quant aux techniques à utiliser pour leur estimation et des considérations sur les incertitudes associées à ces estimations. Nous analysons ensuite les conséquences de cette variabilité sur la faisabilité de la détection de changements au cours du temps et sur l’optimisation des dispositifs de surveillance in situ. Dans une dernière partie nous comparons différentes techniques analytiques et nous développons l’apport potentiel que pourraient représenter les techniques de spectrométrie infra-rouge

Cenozoic intracontinental dextral motion in the Okhotsk-Japan Sea Region

Laurent Jolivet est Professeur à l'Université d'Orléans au 1er Septembre 2009International audienceA right-lateral shear zone trending northerly along more than 2000 km is recognized from central Japan to northern Sakhalin. It was active mainly during the Neogene and has accommodated several hundreds of kilometers of displacement. The whole structure of Sakhalin is built on this shear zone. En échelon sigmoidal folds and thrusts, en échelon narrow Miocene basins, and a major discontinuity which is observed along more than 600 km, the Tym-Poronaisk fault, characterize the deformation there. In Hokkaido, en échelon folds and thrusts and a ductile shear zone with high-temperature metamorphism constitute the southern extension of this transpressional shear zone. It continues to the south as a zone of transtensional deformation along the eastern margin of Japan Sea, as en échelon basins and dextral transfer faults observed as far south as Noto peninsula and Yatsuo basin. The style of the shear zone thus evolves from transpressional in the north far from the subduction zone, to transtensional in the south in the back-arc region. Strike-slip motion along this shear zone was primarily responsible for the dextral pull-apart opening of Japan Sea during the early and middle Miocene. Dextral motion is still active in the north along the Tym-Poronaisk fault in Sakhalin as well as on the continental margin of Japan Sea (Korea and Asia mainland). Active E-W compression replaced the dextral motion along the eastern margin of Japan Sea in late Miocene time, and incipient subduction began in the early Quaternary

Strain localisation in mechanically layered rocks beneath detachment zones: insights from numerical modelling.

International audienceFully dynamic numerical simulations have been designed in order to asses how the orientation of mechanical layering in rocks controls the orientation of shear bands and the depth of penetration of strain in the footwall of detachment zones. Two parametric studies are presented. In the first one, the influence of stratification orientation on the occurrence and mode of strain localisation is tested. The second parametric study shows that results are length-scale independent and that orientation of shear bands is not sensitive to the viscosity contrast or the strain rate. Based on the results, a conceptual model for strain localisation under detachment faults is presented. In the early stages, strain localisation occurs at slow rates by viscous shear instabilities but as the layered media is exhumed, the temperature drops and the strong layers start yielding plastically, forming shear bands and localising strain at the top of the shear zone. Once strain localisation has occured, the deformation in the shear band becomes extremely penetrative but the strength cannot drop since the shear zone has a finite thickness

En reconstruction mammaire : intérêt du dépistage du portage de Staphylococcus aureus dans la prévention de l’infection du site opératoire

National audienceThe incidence of prosthesis infections after breast reconstruction is of the order of 4% to 13% according to the literature. In surgical patients, Staphylococcus aureus (S. aureus) is the bacterial species most often responsible for surgical site infections. In cardiac surgery, screening for carriage of S. aureus and preoperative decontamination are carried out routinely before prosthetic surgery. We retrospectively reviewed data from patients at our institution between January 2011 and December 2013. Our series showed that the prosthesis infection rates were in the range of 5.92% in 2008 with an ISO rate of S. aureus 3.61%. Routine screening for prosthetic reconstructions was performed to assess the impact of preoperative decontamination patients in carriers of S. aureus. This screening was done in 381 patients: 17.8% of patients were carriers of S. aureus ; 11 patients have an ISO (or an incidence rate of 2.88%) ; 5 patients have an ISO S. aureus (an incidence of S. aureus ISO 1.3%). The introduction of the screening process, allowed a drop of 5.92% ISO rate at 1.46% with a passage of S. aureus SSI rates of 3, 60% to 0.72%. In the near future, studies are needed to confirm these encouraging results, to demonstrate the efficacy of preoperative decontamination in carriers of S. aureus patients before laying prosthesis