Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Wallace (1951) and Bott (1959) were the first to introduce the idea that the slip on each fault surface has the same direction and sense as the maximum shear stress resolved on that surface. However, this simplified hypothesis is questionable since fault mechanical interactions may induce slip reorientations. Earlier numerical geomechanical models confirmed that the slickenlines (slip vectors) are not necessarily parallel to the maximum resolved shear stress but are consistent with local stress perturbations. This leads us to ask as to what extent the Wallace and Bott simplifications are reliable as a basis hypothesis for stress inversion from fault slip data. Here, a geomechanical multi-parametric study using a 3D boundary element method, covering (i) fault geometries such as intersected faults or corrugated fault surfaces, (ii) the full range of Andersonian state of stress, (iii) fault friction, (iv) fault fluid pressure, (v) half space effect and (vi), rock properties, is performed in order to understand the effect of each parameter on the misfit angle between geomechanical slip vectors and the resolved shear stresses. It is shown that significant misfit angles can be found under specific configurations invalidating the Wallace and Bott assumptions, even though fault friction tends to minimize the misfit. We therefore conclude that in such cases, stress inversions based on fault slip data should be interpreted with care.
ISSN: 0040-1951

hal-01218418
https://hal.archives-ouvertes.fr/hal-01218418
DOI : 10.1016/j.tecto.2015.06.030

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We conducted triaxial compression experiments to investigate the effects of pressure, temperature and strain rate on the rheology of talc rocks. The tests were carried out at T from 25 to 860 degrees C, P from 0.1 to 300 MPa, and <(epsilon) over dot> from 0.3 to 30 x 10-5 s(-1). In addition to being very weak, there are other important differences between the mechanical behavior of talc and that of most other silicates. Deformation at all temperatures up to dehydration was accommodated by a combination of crystal plasticity, frictional sliding, and cataclasis. A transition from localized to distributed deformation occurred at P = 300 MPa and T = 400 degrees C, but this transition is ill-defined as both distributed and localized deformation were observed at P = 300 MPa and T = 600 degrees C. Unlike most silicates, both the coefficient of internal friction and the coefficient of (sliding) friction are very low and nearly equal to each other. Temperature enhances plastic deformation (kinking), and inter- and intra-granular microcracks are observed parallel to the (001) planes at all conditions. Voids are created by delamination along (001) planes at kink-band boundaries. Full crystal plasticity was not achieved under any of the conditions tested, i.e., the von Mises criterion was never satisfied. However, the strength of the aggregates remained much less than the confining pressure. These unusual properties are likely a manifestation of the pronounced mechanical anisotropy of talc at the grain scale. At T >= 750 degrees C dehydration is observed, but only along shear zones. These results suggest that feedbacks between deformation and reaction kinetics could control fluid flow in fault zones. The presence of talc can promote significant weakening and strain localization in the oceanic lithosphere, the subducting plate and the overlying mantle wedge. (c) 2008 Elsevier B.V. All rights reserved.
ISSN: 0012-821X

hal-00492706
https://hal.archives-ouvertes.fr/hal-00492706
DOI : 10.1016/j.epsl.2008.02.004

Éditeur(s) : HAL CCSD
Résumé : International audience
In boundary lubrication regime, friction reduction and antiwear processes are associated to the presence of additives in the lubricating oils or greases. These processes are due to the formation of protective tribofilms resulting from chemical reactions between the additives and the sliding surfaces, in the physico-chemical conditions of the sliding contact. Conventional antiwear additives mainly consist of transition metal organo phosphate or thiophosphates which present a remarkable efficiency in the case of contacts between ferrous alloys. In the case of non reacting surfaces, these additives become inactive. Recently developped lubrication strategies consist in the use of dispersion in oils of nano additives able to build the protective tribofilm in the sliding contact without reaction with the surfaces. Carbon fluorinated phases, due to their lamellar structure and their high chemical stability even at relatively high temperature (400°C) represent interesting candidates as lubricant nano-additives subjected to present friction reduction, anti wear and anti corrosion actions. This work presents the tribologic behaviour of some carbon fluorinated derivatives such as graphite fluorides, fluorinated carbon nanofibers, fluorinated carbon nanodiscs and fluorinated carbon blacks. The influence, on the tribologic performances, of the structure of the initial carbon phases, of the fluorination rate (0
World Tribology Congress 2013
Torino, Italy

Droits : info:eu-repo/semantics/OpenAccess
hal-00926514
https://hal.archives-ouvertes.fr/hal-00926514
https://hal.archives-ouvertes.fr/hal-00926514/document
https://hal.archives-ouvertes.fr/hal-00926514/file/Delbe_9783.pdf
OATAO : 9783

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The 2004 Aceh and 2005 Nias events are the two greatest earthquakes of the past 40 years with a total rupture of 1700 km long and a coseismic slip reaching up to 25 m. These two earthquakes have caused large stress perturbations which significantly altered seismic activity in the Sumatra-Andaman region. Using both detailed mapping of failure planes and various slip distributions, we calculate this stress change along the Sumatra-Andaman-Sagaing fault system from central Sumatra to southern Myanmar. The static Coulomb stress change Delta CFF and the observed seismic activity are in very good agreement with a Coulomb index similar to 20% greater than the one obtained for random events. Compared to previous studies, this high Coulomb Index confirms two important issues on the use of static stress change criterion: unsuited to study near-field aftershocks and only relevant for aftershocks analysis on large and mature faults at a time scale of several months. The calculated Delta CFF distribution suggests that the 2004 and 2005 earthquakes inhibit failure on the North Andaman rift and on the Sagaing fault, while failure is encouraged along the transform Andaman zone, the central Andaman rift, the West Andaman fault, the Sumatra fault system, and the offshore thrust faults west of Sumatra Island. The maximum value of similar to 15-20 bar (1.5-2 MPa) for Delta CFF is reached in the northern part of the Sumatra fault system. This high value together with the lack of major earthquake in the last 170 years result in a high seismic hazard for this region. Our results are also consistent with temporal evolution of both earthquakes' location and focal mechanism prior to and after the events. In particular, we explain the occurrence and the mechanism of seismic swarms observed in the central Andaman rift and along the west Andaman fault. Finally, our calculations reveal that the seismicity in the Andaman rift zone can only be explained if mu' > 0.5. This result leads to two end-member models: one with a constant and high fault friction and one with spatial variations, for which friction may depend on either the nature of the lithosphere (oceanic versus continental) or the fault type.
ISSN: 1525-2027

hal-00456027
https://hal.archives-ouvertes.fr/hal-00456027
DOI : 10.1029/2008GC002167

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Intrinsic tribological properties of lamellar compounds are usually attributed to the presence of van der Waals gaps in their structure through which interlayer interactions are weak. The controlled variation of the distances and interactions between graphene layers by intercalation of electrophilic species in graphite is used in order to explore more deeply the friction reduction properties of low-dimensional compounds. Three graphite intercalation compounds with antimony pentachloride, iron trichloride and aluminium trichloride are studied. Their tribological properties are correlated to their structural parameters, and the interlayer interactions are deduced from ab initio bands structure calculations.
ISSN: 1023-8883

hal-00767639
https://hal.archives-ouvertes.fr/hal-00767639
https://hal.archives-ouvertes.fr/hal-00767639/document
https://hal.archives-ouvertes.fr/hal-00767639/file/Delbe_6330.pdf
DOI : 10.1007/s11249-012-9981-0

Éditeur(s) : HAL CCSD Geological Society of America
Résumé : International audience
Fault pseudotachylytes form by frictional melting during seismic slip and therefore are widely interpreted as “earthquake fossils.” Rapid movement along a rupture surface typically forms a pseudotachylyte generation vein, the thickness of which increases with earthquake magnitude. The direction and sense of seismic slip cannot always be determined due to the generally complex geometry of pseudotachylyte veins. Here we show, for the first time, that the orientation of the magnetic fabric of fault pseudotachylytes indicates both direction and sense of seismic slip. The magnetic fabric, acquired in a manner similar to that of other magmas, arises in this case from the asymmetric preferred orientation of paramagnetic grains during viscous shear of the friction melt. This kinematic information, coupled with fault plane orientation and generation vein thickness, provides new and critical insight for the earthquake focal mechanism. The magnetic fabric of pseudotachylytes therefore not only constitutes a valuable kinematic criterion for these fault rocks, but also could expand our knowledge of prehistoric seismic events.
ISSN: 0091-7613

hal-01217133
https://hal.archives-ouvertes.fr/hal-01217133
DOI : 10.1130/G36587.1

Éditeur(s) : HAL CCSD
Résumé : National audience
New experimental data on electrical and anelastic properties of oceanic rocks at high pressures and temperatures are compared with the field data of seismic and electrical anomalies observed at oceanic spreading centers. The laboratory results of electrical conductivity and internal friction on gabbro and peridotites permit to estimate a degree of partial melting and maximum temperature in axial magma chambers (12-15% of melting at 1155-1165°C) and at the upper boundary of a source zone at the depth of 40-90 km (less than 5% of melting in the absence of pyroxene crystals and up to 11-14% in the pyroxene reach rocks at 1360-1370°C) at mid-oceanic ridges.
ISSN: 0934-6554

hal-01322920
http://hal.univ-reunion.fr/hal-01322920

Éditeur(s) : HAL CCSD EDP Sciences
Résumé : International audience
In the setting of a real Hilbert space H, we investigate the asymptotic behavior, as time t goes to infinity, of trajectories of second-order damped evolution equations, which are governed by the sum of the gradient operator of a convex differentiable potential function , and a maximal monotone operator which is assumed to be cocoercive. Under a sharp condition involving the viscous and the cocoercive parameters, it is proved that each trajectory weakly converges to an equilibrium. Passing from weak to strong convergence is obtained by introducing an asymptotically vanishing Tikhonov-like regularizing term. As special cases, we recover the asymptotic analysis of the heavy ball with friction dynamic attached to a convex potential, the second-order gradient-projection dynamic, and the second-order dynamic governed by the Yosida approximation of a general maximal monotone operator. The breadth and flexibility of the proposed framework is illustrated through applications in the areas of constrained optimization, dynamical approach to Nash equilibria for noncooperative games, and asymptotic stabilization in the case of a continuum of equilibria.
ISSN: 1292-8119

hal-00803774
https://hal.archives-ouvertes.fr/hal-00803774
https://hal.archives-ouvertes.fr/hal-00803774/document
https://hal.archives-ouvertes.fr/hal-00803774/file/ATTOUCH-MAINGE-09-04-2010-COCV.pdf
DOI : 10.1051/cocv/2010024

Éditeur(s) : HAL CCSD De Gruyter
Résumé : International audience
This study aims at better understanding the respective influences of specific gravity (γ), microfibril angle (MFA), and cell-wall matrix polymers on viscoelastic vibrational properties of wood in axial direction. The wide variations of properties between normal wood (NW) and compression wood (CW) are in focus. Three young bent trees (Picea abies, Pinus sylvestris and Pinus pinaster) that recovered verticality were sampled. Several observed differences between NW and CW were highly significant in terms of anatomical, physical (γ, shrinkage, CIELab colorimetry), mechanical (compressive strength), and vibrational properties. Specific dynamic modulus of elasticity (E'/γ) decreases with increasing MFA, and Young's modulus (E') can be satisfactorily explained by γ and MFA. Apparently, the type of the cell wall polymer matrix is not influential in this regard. The damping coefficient (tanδ) does not depend solely on MFA of NW and CW. The tanδ - E'/γ relationship evidences that, at equivalent E'/γ, the tanδ of CW is approx. 34% lower than that of NW. This observation is ascribed to the more condensed nature of CW lignins, and this is discussed in the context of previous findings in other hygrothermal and time/frequency domains. It is proposed that the lignin structure and the amount and type of extractives, which are both different in various species, are partly responsible for taxonomy-related damping characteristics.
ISSN: 0018-3830

hal-00808062
https://hal.archives-ouvertes.fr/hal-00808062
https://hal.archives-ouvertes.fr/hal-00808062/document
https://hal.archives-ouvertes.fr/hal-00808062/file/Changes_viscoelastic_vibrational_Bremaud_al.pdf
DOI : 10.1515/hf-2011-0186

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Four decades after their introduction, the validity of fault slip inversion methods based on Wallace (1951) and Bott (1959) hypothesis, which states that the slip on each fault surface has the same direction and sense as the maximum resolved shear stress, is still a subject of debate. According to some authors, this hypothesis is questionable since fault mechanical interactions induce slip reorientations as confirmed by geomechanical models. This leads us to ask as to what extent the Wallace-Bott simplifications are reliable as a basis hypothesis for stress inversion from fault slip data. In this paper, we compare two inversion methods; the first is based on the Wallace-Bott hypothesis, and the second relies on geomechanics and mechanical effects on fault heterogeneous slip distribution. In that context, a multi-parametric stress inversion study covering (i) the friction coefficients (μ), (ii) the full range of Andersonian state of stress and (iii) slip data sampling along the faults is performed. For each tested parameter, the results of the mechanical stress inversion and the Wallace-Bott (WB) based stress inversion for slip are compared in order to understand their respective effects. The predicted discrepancy between the solutions of both stress inversion methods (based on WB and mechanics) will then be used to explain the stress inversions results for the chimney Rock case study. It is shown that a high solution discrepancy is not always correlated with the misfit angle (ω ) and can be found under specific configurations (View the MathML sourceR−, θ, μ, geometry) invalidating the WB solutions. We conclude that in most cases the mechanical stress inversion and the WB based stress inversion are both valid and complementary depending on the fault friction. Some exceptions (i.e. low fault friction, simple fault geometry and pure regimes) that may lead to wrong WB based stress inversion solutions are highlighted.
ISSN: 0040-1951

hal-01467165
https://hal.archives-ouvertes.fr/hal-01467165
DOI : 10.1016/j.tecto.2016.11.039

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We use 3-D mechanical modeling representing faults as planar surfaces with frictional properties that obey Coulomb-failure process to explore the long-term slip rates of the Altyn Tagh fault and Kunlun faults in the north Tibetan plateau. Crustal theology is simplified as an elastoplastic upper crust and a viscoelastic lower crust. Far-field GPS velocities and late Quaternary fault slip rates are used to constrain the model results. Rheological tests show that effective fault friction lower than 0.1-0.08 leads to high slip rates that fit with geologically and geodetically determined slip rates of the Kunlun fault (10-11.7 +/- 1.5 mm/yr). Meanwhile, the modeled Altyn Tagh fault reaches slip rates similar to 13.7 mm/yr to similar to 7.8 mm/yr in its central portion, between ranges of the geological slip rates. Associated with high slip rates, our model predicts that central Tibet (similar to 84 degrees E-95 degrees E) from the Altyn Tagh fault to the north of the Himalayan arc accommodates north-south shortening and east-west extension rates of similar to 10-12 mm/yr and similar to 8-10 mm/yr, respectively. We also question the widely accepted idea that interseismic strain is driven at the base of the seismogenic zone by a screw dislocation. If this assumption fails, the presented model implies that interseismic strain around large strike-slip faults could be distributed in a much broader way if the lithosphere deforms as a thin elastic plate rather than an elastic half-space with an embedded dislocation. If this distributed deformation is ignored, and the instantaneous surface deformation field modeled as that resulting from slip on a dislocation below a specified depth embedded in an elastic half-space, the estimated slip rate will inevitably be lower than the true long-term slip rate. This appears to explain why geodetic slip rates proposed for the Altyn Tagh fault (510 mm/yr) are lower than some of the geological slip rates. (C) 2008 Elsevier B.V. All rights reserved.
ISSN: 0012-821X

hal-00411926
https://hal.archives-ouvertes.fr/hal-00411926
DOI : 10.1016/j.epsl.2008.06.049

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Both oedometric and plane-strain tests were performed with parallelepipedic samples made of synthetic granular, cohesive, frictional and dilatant rock analogue material GRAM2. For the first time parallel sets of fractures that have all the characteristics of natural joints were reproduced in the laboratory. The fractures are regularly spaced, normal to σ3, and have plumose morphology very similar to that of natural joints. These fractures can form at tensile stress σ3 much smaller in magnitude than the tensile strength of material and even at slightly compressive σ3. When mean stress σ exceeds a certain value, the fractures become oblique to σ1 (the obliquity increases with σ), forming networks of conjugate shear bands/fractures. These results of plane-strain experiments are in good agreement with those of better controlled conventional axisymmetric tests on a similar material in Chemenda et al. (2011b) and are closer to real geological situations. Both types of experiments are complementary. Their results lead to the conclusion that at least certain categories of natural fractures (including joints, and conjugate shear fractures/bands) were initiated as deformation localization bands. The band orientation is defined by the constitutive properties/parameters (notably the dilatancy factor) that are sensitive to σ.
ISSN: 0040-1951

hal-00795555
https://hal.archives-ouvertes.fr/hal-00795555
DOI : 10.1016/j.tecto.2011.11.021

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The structural parameters of fluorinated nanocarbons presenting different shapes, i.e. spherical, tubular and discotic are investigated and correlated to their tribological properties. Different fluorination rates of graphitized carbon blacks (0D), carbon nanofibres (1D) and a mixture of carbon nanodiscs and nanocones (2D) were achieved under pure molecular fluorine gas flow (direct fluorination). Raman spectrometry, X-ray diffraction and 19F solid state nuclear magnetic resonance underline similar structure and nature of the C-F bonds (covalent) for equivalent fluorine contents. In spite of the similarities of physical-chemical properties at equivalent fluorine contents, the tribological properties of the fluorinated nanocarbons differ. Those properties are discussed taking into account the role of the fluorine content and location of the fluorine atoms in the decrease of the interparticle interactions and in the cleavage of the external fluorocarbon layers to form the tribofilm. Finally, the effect of the shape is discussed.
ISSN: 0022-1139

hal-00787267
https://hal.archives-ouvertes.fr/hal-00787267

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We review the evolution of concepts on and methods of estimating the state of stress from fault movements. Theories of failure in isotropic materials suggested a simple geometrical construction of optimal principal stress directions from a fault plane and its associated slip. These optimal directions align shear stress and slip directions and maximize the difference between shear stress and frictional resistance on the fault plane. Optimal stress directions for calcite twinning are obtained by a similar construction, with the difference that they maximize shear stress. Force representation of seismic sources independently introduced pressure, P, and tension, T, axes at positions that also maximize shear stress on both nodal planes. Frictional slip theory and the constraint that slip and shear stress directions be parallel allowed to address reactivation of pre-existing faults. This suggested that stress could also be inverted from reactivated fault and slip data or earthquake focal mechanisms. Early methods relied on geometrical constructions as a substitute for calculations, whereas later methods relied on software as these calculations became tractable with the help of computers. Similar methods were developed for the inversion of stress from crystal twin gliding with non-optimal geometry, with a different criterion that relies on a threshold of the component of shear stress along the gliding line. Even though these methods seek a common stress tensor compatible with fault and slip data, their main use is to separate polyphase data into homogeneous subsets and help deciphering complex tectonic histories. Fault and slip data can also be analyzed to constrain the strain rather than the stress tensor. In most cases this involves a summation and yields an average strain for the considered rock volume. Stress inversion thus appears better suited for differentiating heterogeneous data whereas strain analysis appears better suited for homogenizing them.
ISSN: 0040-1951

hal-00759043
https://hal.archives-ouvertes.fr/hal-00759043
DOI : 10.1016/j.tecto.2012.02.009

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The majority of the largest subduction megathrust earthquakes share the common characteristic of rupturing more than one asperity along strike of the margin. Understanding the factors that control coseismic failure of multiple asperities, and thus maximum magnitude, is central for seismic hazard assessment. To investigate the role of asperities size and spacing on maximum magnitude, seismicity rate, and percentage of synchronized ruptures, we use analog models simulating along-strike rupture behavior of megathrust earthquakes. We found negative correlations between the barrier-to-asperity length ratio Db/Da and maximum magnitude and seismicity rate. Db/Da also controls the process of asperities synchronization along the megathrust. A permanent barrier behavior is observed for Db/Da > 0.5. Comparing our experimental results to the Nankai Trough historical seismicity, we propose that the distribution of megathrust frictional heterogeneities likely explains the diversity of earthquakes which occurred there.
ISSN: 0094-8276

hal-01685546
https://hal.archives-ouvertes.fr/hal-01685546
https://hal.archives-ouvertes.fr/hal-01685546/document
https://hal.archives-ouvertes.fr/hal-01685546/file/Corbi_et_al-2017-Geophysical_Research_Letters.pdf
DOI : 10.1002/2017GL074182

Éditeur(s) : HAL CCSD European Geosciences Union
Résumé : International audience
Earth deformation is a multi-scale process ranging from seconds (seismic deformation) to millions of years (tectonic deformation). Bridging short- and long-term deformation and developing seismotectonic models has been a challenge in experimental tectonics for more than a century. Since the formulation of Reid's elastic rebound theory 100 years ago, laboratory mechanical models combining frictional and elastic elements have been used to study the dynamics of earthquakes. In the last decade, with the advent of high-resolution monitoring techniques and new rock analogue materials, laboratory earthquake experiments have evolved from simple spring-slider models to scaled analogue models. This evolution was accomplished by advances in seismology and geodesy along with relatively frequent occurrences of large earthquakes in the past decade. This coincidence has significantly increased the quality and quantity of relevant observations in nature and triggered a new understanding of earthquake dynamics. We review here the developments in analogue earthquake modelling with a focus on those seismotectonic scale models that are directly comparable to observational data on short to long timescales. We lay out the basics of analogue modelling, namely scaling, materials and monitoring, as applied in seismotectonic modelling. An overview of applications highlights the contributions of analogue earthquake models in bridging timescales of observations including earthquake statistics, rupture dynamics, ground motion, and seismic-cycle deformation up to seismotectonic evolution.
ISSN: 1869-9510

hal-01553120
https://hal.archives-ouvertes.fr/hal-01553120
https://hal.archives-ouvertes.fr/hal-01553120/document
https://hal.archives-ouvertes.fr/hal-01553120/file/rosenau2017.pdf
DOI : 10.5194/se-8-597-2017

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Fractures or discontinuities perpendicular to the least stress σ3 were generated in a synthetic rock analogue (granular, frictional, cohesive, and dilatant) material in axisymmetric extension tests. These fractures are of two types defined by the mean stress σ. When σ is very small, the fractures form through the mode I cracking at tensile σ3 equal to the material tensile strength. The fracture walls have smooth surfaces. At higher σ, these surfaces become rougher, with the topography features forming faint or delicate plumose patterns. The amplitude of the plumose topography increases with σ. The axial stress σ3 at fracturing reduces in magnitude and changes in sign with the σ increase. Thus σ3 orthogonal discontinuities can form at compressive σ3. SEM observations show that these discontinuities are deformation localization bands where the material is characterized by the heterogeneous decohesion and volume and porosity increase due to dilatancy. The band thickness is several grain sizes. At formation, the bands are not opened, so they are not mode I fractures. They become fractures with plumose fractography after the separation of the sample parts along the band. The formation mechanism of these discontinuities or fractures is not completely clear, but it is suggested that it represents a running constitutive instability in the form of dilatancy banding (with further σ increase the bands become inclined to σ1, i.e., shear). The similarity between the experimentally generated plumose surface fractures and natural joints is discussed, and it is suggested that they can be formed as propagating constitutive instabilities.
ISSN: 0148-0227

hal-00585846
https://hal.archives-ouvertes.fr/hal-00585846
DOI : 10.1029/2010JB008104

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We present Contact Dynamics discrete element simulations of the earthquake-triggered Jiufengershan avalanche, which mobilized a 60 m thick, 1.5 km long sedimentary layer, dipping similar to 22 degrees SE toward a valley. The dynamic behavior of the avalanche is simulated under different assumptions about rock behavior, water table height, and boundary shear strength. Additionally, seismic shaking is introduced using strong motion records from nearby stations. We assume that seismic shaking generates shearing and frictional heating along the surface of rupture, which, in turn, may induce dynamic weakening and avalanche triggering; a simple "slip-weakening'' criterion was adopted to simulate shear strength drop along the rupture surface. We investigate the mechanical processes occurring during triggering and propagation of an avalanche mobilizing shallowly dipping layers. Incipient deformation forms a pop-up structure at the toe of the dip slope. As the avalanche propagates, the pop-up deforms into an overturned fold, which overrides the surface of separation along a decollement. Simultaneously, uphill layers slide at high velocity (125 km/h) and are folded and disrupted as they reach the toe of the dip slope. The avalanche foot forms a wedge that is pushed forward as deformed rocks accrete at its rear. We simulated five cross sections across the Jiufengershan avalanche, which differ in the geometry of the surface of separation. Topographic and simulated surface profiles are similar. The friction coefficient at the surface of separation determined from back analysis is abnormally low (mu(SS) = 0.2), possibly due to lubrication by liquefied soils. The granular deposits of simulated earthquake- and rain-triggered avalanches are similar.
ISSN: 0148-0227

hal-00420882
https://hal.archives-ouvertes.fr/hal-00420882
DOI : 10.1029/2008JF001075

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
A San Carlos olivine polycrystal has been deformed under uppermost mantle conditions, by compression at 900 °C, at a strain rate of 1.1 × 10−5 s−1, under a confining pressure of 300 MPa, using the Paterson press. Transmission electron tomography of dislocations has been performed by scanning transmission electron microscopy, by conventional transmission electron microscopy using the weak-beam dark-field technique, associated with precession or not, in order to determine the glide planes of [001] screw dislocations. This recent technique is the most suitable one since most [001] dislocations exhibit straight screw segments due to the high lattice friction on this character at low temperature. We find that [001] dislocations glide in (100), (010) and {110} as already reported, but also more unexpectedly in {120} and {130}. We show that at 900 °C, [001] {110} glide is dominant in polycrystals. We have, however, noted and characterized numerous cross-slip events in the specimen.
ISSN: 0342-1791

hal-01054330
https://hal.archives-ouvertes.fr/hal-01054330
DOI : 10.1007/s00269-014-0665-1

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Deep-seated gravitational spreading (DSGS) affects the slopes of formerly glaciated mountain ridges. On Mars, DSGS has played a key role in shaping the landforms of the giant Valles Marineris troughs. Though less spectacular, DSGS is common in terrestrial orogens, where understanding its mechanics is critical in the light of the ongoing climate change because it is a potential source of catastrophic landslides in deglaciated valleys. We conducted parametric numerical studies in order to identify important factors responsible for DSGS initiation. DSGS models are computed using an elastoviscoplastic finite element code. Using ADELI's software, we reproduce topographic ridge spreading under the effect of valley unloading. Two types of spreading topographic ridges are investigated, homogeneous or with horizontal rheological layering. We find that gravitational instabilities are enhanced by high slopes, which increase gravitational stress, and low friction and cohesion, which decrease yield stress. In the unlayered ridge, instability is triggered by glacial unloading with plastic strain concentration inside the ridge and at the base of the high slopes. Vertical fractures develop in the upper part of the slope, potentially leading to fault scarps. Ridge homogeneity promotes a deformation mode controlled by uphill-facing normal faulting and basal bulging. In the second case, the ridge encompasses horizontal geological discontinuities that induce rock mass anisotropy. Discontinuity located at the base of the slope accumulates plastic strain, leading to the formation of a sliding plane evolving into a landslide. The presence of a weak layer at ridge base therefore promotes another slope deformation mode ending up with catastrophic failure. Mechanical conditions and slope height being equal, these conclusions can probably be extrapolated to Earth. Compared with Mars, DSGS on Earth is inhibited because terrestrial topographic gradients are lower than in Valles Marineris, an effect counterbalanced by increased gravitational stress, where the intensity of deformation is enhanced because of the Earth gravity potential.
ISSN: 0169-555X

hal-01420007
https://hal.archives-ouvertes.fr/hal-01420007
DOI : 10.1016/j.geomorph.2016.06.011