Éditeur(s) : HAL CCSD
Résumé : The evolution of mountain belt piedmont is controlled by interactions between tectonics, erosion and sedimentation. To study coupling between these processes, we analyze morphometric data collected in the eastern Tian Shan piedmont (NW China) and develop a new analogue modelling technique. The modeling approach uses an experimental set-up and analog material specifically designed to study simultaneously tectonic structures (faults, folds), detailed morphology (drainage basins, channels, alluvial fans, etc.) and stratified sedimentation. Measurement techniques based on laser interferometry and image correlation are also developed to quantify the topographic and kinematic evolution of models.
Our experiments allow a new approach for the study of piedmont morphotectonic evolution. Our results indicate that tectonics mainly controls this evolution. The propagation of deformation and the formation of relief both significantly influence the development of drainage networks, the evolution of drainage basins and the formation of morphotectonic markers. Particularly, a study of alluvial fans and watershed metrics suggests that there is a dynamical equilibrium between uplift, erosion and morphology. In addition, our results show that terrace nucleation is controlled by thrust activity and hydrography adjustments. Folding of terraces responds to a homothetic deformation pattern.
This new methodology brings promising perspectives in the fields of geomorphology, seismotectonics and sedimentology.
L'évolution d'un piémont de chaîne de montagnes est contrôlée par les interactions entre la tectonique, l'érosion et la sédimentation. Pour étudier ces processus, nous avons analysé des données morphométriques issues des piémonts orientaux du Tian Shan (NW Chine) et développé un nouveau type de modélisation analogique. Cette approche utilise un dispositif et un matériau expérimental spécialement conçus pour modéliser simultanément des structures tectoniques (failles, plis), une morphologie détaillée (bassins versants, rivières, cônes alluviaux, etc.) et une sédimentation stratifiée. Des techniques de mesure optique ont été également mises en oeuvre afin de quantifier l'évolution topographique et cinématique des modèles.
Les expériences réalisées offrent une opportunité inédite d'étudier l'évolution morphotectonique d'un piémont. Nos résultats montrent que la tectonique exerce un contrôle de premier ordre sur cette évolution. La propagation de la déformation et la formation des reliefs influencent en effet significativement le développement du réseau de drainage, l'évolution des bassins versants et la formation des marqueurs morphotectoniques. En particulier, l'analyse de la morphométrie des cônes alluviaux et de leurs bassins versants suggère qu'un équilibre dynamique s'installe entre la surrection, l'érosion et la morphologie. Nos résultats montrent également que l'apparition des terrasses est guidée par l'activité des chevauchements et les adaptations de l'hydrographie. Leur modèle de plissement répond à une déformation homothétique.
Cette nouvelle approche ouvre des perspectives d'étude prometteuses en géomorphologie, sismotectonique et stratigraphie.
https://tel.archives-ouvertes.fr/tel-00339145

tel-00339145
https://tel.archives-ouvertes.fr/tel-00339145
https://tel.archives-ouvertes.fr/tel-00339145v2/document
https://tel.archives-ouvertes.fr/tel-00339145/file/These_Graveleau08.pdf

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Rifting in a cratonic lithosphere is strongly controlled by several interacting processes including crust/mantle rheology, magmatism, inherited structure and stress regime. In order to better understand how these physical parameters interact, a 2 yr long seismological experiment has been carried out in the North Tanzanian Divergence (NTD), at the southern tip of the eastern magmatic branch of the East African rift, where the southward-propagating continental rift is at its earliest stage. We analyse teleseismic data from 38 broad-band stations ca. 25 km spaced and present here results from their receiver function (RF) analysis. The crustal thickness and Vp/Vs ratio are retrieved over a ca. 200 × 200 km2 area encompassing the South Kenya magmatic rift, the NTD and the Ngorongoro-Kilimanjaro transverse volcanic chain. Cratonic nature of the lithosphere is clearly evinced through thick (up to ca. 40 km) homogeneous crust beneath the rift shoulders. Where rifting is present, Moho rises up to 27 km depth and the crust is strongly layered with clear velocity contrasts in the RF signal. The Vp/Vs ratio reaches its highest values (ca. 1.9) beneath volcanic edifices location and thinner crust, advocating for melting within the crust. We also clearly identify two major low-velocity zones (LVZs) within the NTD, one in the lower crust and the second in the upper part of the mantle. The first one starts at 15–18 km depth and correlates well with recent tomographic models. This LVZ does not always coexist with high Vp/Vs ratio, pleading for a supplementary source of velocity decrease, such as temperature or composition. At a greater depth of ca. 60 km, a mid-lithospheric discontinuity roughly mimics the step-like and symmetrically outward-dipping geometry of the Moho but with a more slanting direction (NE–SW) compared to the NS rift. By comparison with synthetic RF, we estimate the associated velocity reduction to be 8–9 per cent. We relate this interface to melt ponding, possibly favouring here deformation process such as grain-boundary sliding (EAGBS) due to lithospheric strain. Its geometry might have been controlled by inherited lithospheric fabrics and heterogeneous upper mantle structure. We evidence that crustal and mantle magmatic processes represent first order mechanisms to ease and locate the deformation during the first stage of a cratonic lithospheric breakup.
ISSN: 0956-540X

hal-01622078
https://hal.archives-ouvertes.fr/hal-01622078
https://hal.archives-ouvertes.fr/hal-01622078/document
https://hal.archives-ouvertes.fr/hal-01622078/file/plasmanGJI2017.pdf
DOI : 10.1093/gji/ggx177

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
Résumé : International audience
In most rock mechanical testing devices, the texture developed during deformation is only observed after the failure, and the detailed mapping of cracks and pore structure evolution, just before or after rupture, remains speculative. Computerized X-ray tomography (CT X-ray) offers a way to observe 'in vivo' some characteristics of a tested sample in the course of its deformation. Therefore we developed an X-ray transparent tri-axial cell that was included in a medical purpose X-ray scanner with a volumetric resolution of the order of 1 mm3. This technique is described and applied to a porous calcarenite, which is regularly scanned for porosity evolution during mechanical testing. The experiment is performed at low confining pressure (2-10 MPa). During testing, an axial load is applied until failure and is then progressively decreased. In these experiments, the peak stress is preceded by phase of diffuse dilatancy. After failure localization, the axial stress is unloaded until the stress reaches isotropy. During this unloading phase, the previous diffuse dilatancy is lost due to an elastic relaxation. Then, the stress is completely relaxed. At this moment the dilatancy increases again and the residual dilatancy is larger than that reached at the peak stress. When observed "post mortem" at microscopic scale, the dilatant structures are comprised of cataclastic deformation bands and cracks in some larger crystals.
International Journal of Rock Mechanics and Mining Science

hal-00767563
https://hal.archives-ouvertes.fr/hal-00767563
DOI : 10.1016/j.ijrmms.2012.07.020

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In oblique collision settings, parallel and perpendicular components of the relative plate motion can be partitioned into different structures of deformation and may be localized close to the plate boundary, or distributed on a wider region. In the Southern Alps of New Zealand, it has been proposed that one-third of the regional convergence is distributed in a broad area along the Southern Alps orogenic wedge. To better document and understand the regional dynamics of such systems, reliable markers of the horizontal tectonic motion over geological time scales are needed. River networks are able to record a large amount of distributed strain and they can thus be used to reconstruct the mode and rate of distribution away from major active structures. To explore the controls on river resilience to deformation, we develop an experimental model to investigate river pattern evolution over a doubly-vergent orogenic wedge growing in a context of oblique convergence. We use a rainfall system to activate erosion, sediment transport and river development on the model surface. At the end of the experiment, the drainage network is statistically rotated clockwise, confirming that rivers can record the distribution of motion along the wedge. Image analysis of channel time-space evolution shows how the fault-parallel and fault-perpendicular components of motion decrease toward the fault and impose rotation to the main trunk valleys. However, rivers do not record the whole imposed rotation rate, which suggest that the natural lateral channel dynamics can alter the capacity of rivers to act as passive markers of deformation.
ISSN: 0040-1951

hal-01467564
https://hal.archives-ouvertes.fr/hal-01467564
DOI : 10.1016/j.tecto.2016.04.016

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We analyze the kinematics of fault tip folding at the front of a fold-and-thrust wedge using a sandbox experiment. The analog model consists of sand layers intercalated with low-friction glass bead layers, deposited in a glass-sided experimental device and with a total thickness h = 4.8 cm. A computerized mobile backstop induces progressive horizontal shortening of the sand layers and therefore thrust fault propagation. Active deformation at the tip of the forward propagating basal décollement is monitored along the cross section with a high-resolution CCD camera, and the displacement field between pairs of images is measured from the optical flow technique. In the early stage, when cumulative shortening is less than about h/10, slip along the décollement tapers gradually to zero and the displacement gradient is absorbed by distributed deformation of the overlying medium. In this stage of detachment tip folding, horizontal displacements decrease linearly with distance toward the foreland. Vertical displacements reflect a nearly symmetrical mode of folding, with displacements varying linearly between relatively well defined axial surfaces. When the cumulative slip on the décollement exceeds about h/10, deformation tends to localize on a few discrete shear bands at the front of the system, until shortening exceeds h/8 and deformation gets fully localized on a single emergent frontal ramp. The fault geometry subsequently evolves to a sigmoid shape and the hanging wall deforms by simple shear as it overthrusts the flat ramp system. As long as strain localization is not fully established, the sand layers experience a combination of thickening and horizontal shortening, which induces gradual limb rotation. The observed kinematics can be reduced to simple analytical expressions that can be used to restore fault tip folds, relate finite deformation to incremental folding, and derive shortening rates from deformed geomorphic markers or growth strata.
ISSN: 2169-9313

hal-00138061
https://hal.archives-ouvertes.fr/hal-00138061
https://hal.archives-ouvertes.fr/hal-00138061/document
https://hal.archives-ouvertes.fr/hal-00138061/file/Bernard_et_al-2007-Journalsur Geophysical_Research__Solid_Earth_%281978-2012%29.pdf
DOI : 10.1029/2005JB004149

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Orogenic wedges locally present chaotic tectonostratigraphic units that contain exotic blocks of various size, origin, age and lithology, embedded in a sedimentary matrix. The occurrence of ophiolitic blocks, sometimes huge, in such “mélanges” raises questions on (i) the mechanisms responsible for the incorporation of oceanic basement rocks into an accretionary wedge and (ii) the mechanisms allowing exhumation and redeposition of these exotic elements in “mélanges” during wedge growth.To address these questions, we present the results of a series of analogue experiments performed to characterize the processes and parameters responsible for accretion, exhumation and tectonosedimentary reworking of oceanic basement lithospheric fragments in an accretionary wedge.The experimental setup is designed to simulate the interaction between tectonics, erosion and sedimentation. Different configurations are applied to study the impact of various parameters, such as irregular oceanic floor due to structural inheritance, or the presence of layers with contrasted rheology that can affect deformation partitioning in the wedge (frontal accretion vs basal accretion) influencing its growth. Image correlation technique allows extracting instantaneous velocity field, and tracking of passive particles. By retrieving the particle paths determined from models, the pressure-temperature path of mélange units or elementary blocks can be discussed. The experimental results are then compared with observations from ophiolite-bearing mélanges in Taiwan (Lichi and Kenting mélanges) and Raman spectroscopy of carbonaceous material (RSCM) Thermometry data on rocks from the northern Apennines (Casanova mélange). A geological scenario is proposed following basic observations. The tectonic evolution of the retroside of doubly vergent accretionary wedges is mainly controlled by backthrusting and backfolding. The retro wedge is characterized by steep slopes that are prone to gravitational instabilities. It triggers submarine landslides inducing huge mass transfers. This erosion combined with backthrusting could favour exhumation of the ophiolitic fragments formerly accreted at the base of the wedge along the rough seafloor-sediments interface. Such an exhumed material can be reworked and deposited as debris- flows in proximal basins located at the foot of the retrowedge slope forming a tectono-sedimentary mélange. These syntectonic basins are continuously deformed and involved in prograding backthrusting-induced deformation.
ISSN: 0264-3707

hal-01395127
https://hal.archives-ouvertes.fr/hal-01395127
DOI : 10.1016/j.jog.2016.05.008

Éditeur(s) : HAL CCSD
Résumé : We present an experimental study of relations between hydraulic properties of clayey rocks and their deformation at bulk and micro scale. The experimentation is based on triaxial deformation of kaolinitic core samples, on permeability measurements, and on several methods of analysis of the microstructure. To produce the samples, remoulded kaolinite material is consolidated by axial loading up to 20 MPa in an oedometric cell. Then, in a triaxial cell, an undrained stress path is applied in order to induce shearing damage in the sample. The permeability of the sample is measured by fluid flow across the sample. Before and after these hydro-mechanical tests, the evolution and the homogeneity of the pore space and moreover the texture of the material are analysed by X-ray tomography, environmental scanning electron microscopy and thermic neutron diffraction. Contrary to expectations, the permeability at the sample scale does not vary significantly, although quite large deformation is reached. The final deformation is heterogeneous, with well-developed shear bands. It appears that the development of compacting and dilating zones generates a heterogeneous permeability field, which results in a negligible variation of the bulk permeability at the sample scale.
International Journal of Rock Mechanics and Mining Science

hal-00496407
https://hal.archives-ouvertes.fr/hal-00496407
DOI : 10.1016/j.ijrmms.2010.04.007

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Seismic reflection profiles from the sediment rich Alaska subduction zone image short, frontally accreted, imbricate thrust slices and repeated sequences of long, underthrust sheets. Rapid landward increases in wedge thickness, backthrusting, and uplift of the forearc are observed, suggesting underthrusting beneath the wedge. These features and a widely varying frontal wedge morphology are interpreted to be caused by different modes of accretion active concurrently along the trench at different locations. Episodic wedge growth is observed in high basal friction experiments using sand as an analog material. Two phases of an accretionary cycle can be distinguished: frontal accretion of short imbricate thrust slices, alternating with underthrusting of long, undeformed sheets. The phase is shown experimentally to depend upon the surface slope of the wedge. Mechanical analysis of the forces at work predicts these two modes of deformation due to the varying frictional forces and yield strengths for a temporally varying wedge geometry. Maximum length of thrust slices is calculated for experimental conditions and confirmed by the observations. For a steep frontal slope (at the upper limit of the Mohr-Coulomb taper stability field) the overburden is too great to permit underthrusting, and failure occurs repeatedly at the wedge front producing short imbricate slices. The wedge grows forward, lowering the surface angle to the minimum critical taper. For a shallow frontal slope the reduced overburden along an active roof thrust permits sustained underthrusting, causing frontal erosion and backthrusting, steepening the wedge and thus completing the cycle.
ISSN: 0148-0227

hal-01261538
https://hal.archives-ouvertes.fr/hal-01261538
https://hal.archives-ouvertes.fr/hal-01261538/document
https://hal.archives-ouvertes.fr/hal-01261538/file/gutscherJGR1998.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Melt generation in the continental crust is thought to significantly influence seismic bulk velocities and anisotropy, although existing laboratory data provide limited constraints on such seismic attributes. In this study we measured ultrasonic compressional wave speeds (Vp) and anisotropy during sample compaction, mica-breakdown and melt-generation in synthetic, foliated quartz–muscovite aggregates. Measurements were performed at peak conditions of 300 MPa hydrostatic confining pressure and 750 °C, over a six hour time period, for three separate samples where wave propagation directions were aligned at 0°, 45° and 90° with respect to the foliation plane. The experiments are initially marked by sample compaction and rapid reduction in porosity (from 25% to <2 vol%), with a corresponding increase in Vp. During initial stages of the experiment Vp ranges from 4.65 to 5.45 km/s, depending on the direction of the P wave propagation, resulting in up to ∼16% anisotropy, which originates mainly from the preferred orientation of muscovite and possible incipient melt lenses oriented parallel to foliation. As the experiment progresses the bulk Vp decreases gradually and Vp anisotropy drops from ∼16% to ∼2% after five hours into the experiment. The drop in anisotropy is caused by muscovite-breakdown and increasing amount of partial melting of the aggregate. The bulk velocities are considerably lower than predicted for a mixture of muscovite and β-quartz (stable at 300 MPa and 750 °C). Thermodynamic modeling indicates that mica breakdown and production of melt with ∼2 wt% dissolved water in the aggregate can explain the low Vp, even though β-quartz is stable; the modeled Vp is ∼4.75 km/s at 300 MPa and 750 °C. These ultrasonic attributes signify movement of the melt to fill vacant pores and coating grain boundaries, effectively connecting melt films and reducing seismic anisotropy. The results may apply to regions where crustal melting is on-going, in particular regions of elevated temperature that are only weakly deforming, with coupled anomalously low bulk velocities and seismic anisotropy.
ISSN: 0012-821X

hal-01243569
https://hal.archives-ouvertes.fr/hal-01243569
DOI : 10.1016/j.epsl.2015.05.039

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
Nowadays, technological advances in satellite imagery measurements as well as the development of dense geodetic and seismologic networks allow for a detailed analysis of surface deformation associated with active fault seismic cycle. However, the study of earthquake dynamics faces several limiting factors related to the difficulty to access the deep source of earthquake and to integrate the characteristic time scales of deformation processes that extend from seconds to thousands of years. To overcome part of these limitations and better constrain the role and couplings between kinematic and mechanical parameters, we have developed a new experimental approach allowing for the simulation of strike-slip fault earthquakes and analyze in detail hundreds of successive seismic cycle. Model rheology is made of multilayered visco-elasto-plastic analog materials to account for the mechanical behavior of the upper and lower crust and to allow simulating brittle/ductile coupling, postseismic deformation phase and far-field stress transfers. The kinematic evolution of the model surface is monitored using an optical system, based on subpixel spectral correlation of high-resolution digital images. First, results show that the model succeed in reproducing the deformation mechanisms and surface kinematics associated to the main phases of the seismic cycle indicating that model scaling is satisfactory. These results are comforted by using numerical algorithms to study the strain and stress distribution at the surface and at depth, along the fault plane. Our analog modeling approach appears, then, as an efficient complementary approach to investigate earthquake dynamics.
ISSN: 0278-7407

hal-01174176
https://hal.archives-ouvertes.fr/hal-01174176
DOI : 10.1002/2014TC003701

Éditeur(s) : HAL CCSD Taylor & Francis: STM, Behavioural Science and Public Health Titles
Résumé : International audience
The dislocation microstructures in a single crystal of olivine deformed experimentally in uniaxial compression at 1090 °C and under a confining pressure of 300 MPa, have been investigated by transmission electron tomography in order to better understand deformation mechanisms at the microscale relevant for lithospheric mantle deformations. Investigation by electron tomography reveals microstructures, which are more complex than previously described, composed of and dislocations commonly exhibiting 3D configurations. Numerous mechanisms such as climb, cross-slip, double cross-slip as well as interactions like junction formations and collinear annihilations are the source of this complexity. The diversity observed advocates for microscale deformation of olivine significantly less simple than classic dislocation creep reported in metals or ice close to melting temperature. Deciphering mechanism of hardening in olivine at temperatures where ionic diffusion is slow and is then expected to play very little role is crucial to better understand and thus model deformation at larger scale and at temperatures (900–1100 °C) highly relevant for the lithospheric mantle.
ISSN: 1478-6443

hal-01667146
https://hal.archives-ouvertes.fr/hal-01667146
DOI : 10.1080/14786435.2017.1367858

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Homogenization models are widely used to predict the evolution of texture (crystal preferred orientations) and resulting anisotropy of physical properties in metals, rocks, and ice. They fail, however, in predicting two main features of texture evolution in simple shear (the dominant deformation regime on Earth) for highly anisotropic crystals, like olivine: (1) the fast rotation of the CPO towards a stable position characterized by parallelism of the dominant slip system and the macroscopic shear and (2) the asymptotical evolution towards a constant intensity. To better predict CPO-induced anisotropy in the mantle, but limiting computational costs and use of poorly-constrained physical parameters, we modified a viscoplastic self-consistent code to simulate the effects of subgrain rotation recrystallization. To each crystal is associated a finite number of fragments (possible subgrains). Formation of a subgrain corresponds to introduction of a disorientation (relative to the parent) and resetting of the fragment strain and internal energy. The probability of formation of a subgrain is controlled by comparison between the local internal energy and the average value in the polycrystal. A two-level mechanical interaction scheme is applied for simulating the intracrystalline strain heterogeneity allowed by the formation of low-angle grain boundaries. Within a crystal, interactions between subgrains follow a constant stress scheme. The interactions between grains are simulated by a tangent viscoplastic self-consistent approach. This two-level approach better reproduces the evolution of olivine CPO in simple shear in experiments and nature. It also predicts a marked weakening at low shear strains, consistently with experimental data.
ISSN: 0012-821X

hal-01243503
https://hal.archives-ouvertes.fr/hal-01243503
DOI : 10.1016/j.epsl.2015.08.018

Éditeur(s) : HAL CCSD
Résumé : National audience
Les flux sédimentaires dans les bassins d'avant-pays dépendent de l'activité érosive des rivières qui drainent en amont les chaînes de montagnes. Les variations du flux sédimentaire sont généralement attribuées à des variations de l'activité érosive en réponse à la tectonique ou aux changements climatiques. À l'aide d'un dispositif expérimental nous étudions les interactions entre la déformation, la pluviométrie et le comportement intrinsèque d'un système géomorphologique que constitue un prisme orogénique soumis à l'érosion. Nous montrons que le réseau de drainage dans un tel prisme évolue toujours depuis un réseau dominé par des drains longitudinaux (parallèles aux structures tectoniques) vers un réseau dominé par des drains transversaux. Au cours de cette réorganisation, les captures de drains longitudinaux par les drains transversaux provoquent des modifications des pentes locales et des flux d'eau. Ces captures entraînent une augmentation subite de la vitesse d'érosion des drains transversaux provoquant celle-là même du flux de sédiment à la sortie du prisme (figure 1), alors que la vitesse de raccourcissement et la pluviométrie sont restées constantes. L'analyse de systèmes naturels montre que cette réorganisation progressive du système de drainage semble être une conséquence inéluctable de la croissance des chaînes de montagnes (Babault et al 2012). On doit par conséquent s'attendre à ce que le flux sédimentaire dans les chaînes soit contrôlé, non seulement par la tectonique et/ou le climat, mais aussi par la dynamique du réseau de drainage.
14 ème congrès français de sédimentologie, Paris, 4-8 novembre 2013, livre des résumés. Publication ASF
Paris, France

insu-00913436
https://hal-insu.archives-ouvertes.fr/insu-00913436

Éditeur(s) : HAL CCSD
Résumé : The purpose of this work is to specify the deformation offshore East of Taiwan by characterizing major active faults in particular at the level of the Ryukyu forearc. The major problem associated with the study of this region is that earthquakes are located outside the permanent networks. Consequently, this led to some unprecise earthquake locations limiting tectonic and statistical analyses, and therefore the correct estimation of the seismic hazard. This work is included in the project ANR ACTS (Active Tectonics and Seismic Hazard in Taiwan). To achieve this goal, we worked on three different timescales for which a different localization method has been proposed to obtain a clear image of seismic deformation and highlight major faults offhore East of Taiwan. First, at the scale of the century (from the analysis of a seismicity catalogue with homogeneous magnitude), the instrumental historical earthquakes (1897-2007) show that the Ryukyu margin was affected by 6 earthquakes of magnitude greater than 7 with 4 of them for which the responsible fault is unknown. We propose a method of relative location to relocate the historic events from the time difference of P and S arrival times (Ts-Tp) at each station. The location is obtained by searching for "analogue" earthquakes in the recent instrumental catalogue (1991-2008) for which Ts-Tp at each station is close. This method has been applied to the largest (in terms of magnitude) earthquake ever recorded in Taiwan, the earthquake of June 5, 1920 (M7.7 +/- 0.2). A rupture along the interplate of the Ryukyu subduction with a possible nucleation at the downdip limit of a splay-fault is certainly responsible of this earthquake. Then, at the scale of the decade, the improvement of the method of absolute location based on MAXimum of Intersection of EDT (MAXI, EDT: Equal Differential Time) allows to better extract the erroneous arrival times and prevents the "trade-off" between depth and origin time, and between depth and epicentral position when the azimuthal coverage is low (gap Azimuthal > 180degre). Synthetic tests show the effectiveness of MAXI (using P-wave only) to determine the parameters x, y and z even when the azimuthal gap is important. In the latter case, the quality of the results is dependent on the velocity model to represent the 3D structure of the Earth. We propose an approach involving the use of a 3D a priori P-wave velocity model to locate earthquakes which are lateral and remote to the seismic network. An application to the case of Taiwan and Ecuador validates this approach. Finally, at the level of a few months, the marine geophysical campaign RATS (Ryukyu Arc: Tectonics and Seismology) was conducted in two stages. A passive seismology experiment from July to October, 2008 (RATS1) has been conducted over the forearc of the Ryukyus and then active seismic experiment (refraction and reflection) was conducted in May 2009 in a NNE - SSW line through the Ryukyu margin. These two combined experiments allow improving our knowledge about the crustal structure of the margin. At the level of the forearc, the basement of the forearc is characterized by vertical backstop and a very deformed basis certainly associated with a significant out of sequence deformation. In depth, the downgoing plate is certainly affected by a tear that controls the seismicity in the region of transition between the subduction and collision.
Le but de ce travail est de préciser la déformation en mer à l'est de Taïwan et de notamment caractériser les failles actives majeures en particulier au niveau de l'avant-arc des Ryukyus. Le problème majeur associé à l'étude de cette région est que les séismes sont localisés en dehors des réseaux permanents et que cela conduit à des localisations peu précises limitant les analyses statistiques, tectoniques et donc l'estimation correcte de l'aléa sismique. Ce travail est inclus dans le projet ANR ACTS (Active Tectonics and Seismic Hazard in Taiwan). Pour atteindre cet objectif, nous travaillons à trois échelles de temps différentes pour lesquelles une méthode de localisation différente a été proposée pour obtenir une image précise de la déformation sismique et mettre en évidence les failles majeures en mer à l'est de Taïwan. D'abord, à l'échelle du siècle (à partir de l'analyse d'un catalogue de sismicité homogène en magnitude), les séismes historiques instrumentaux (1897-2007) montrent que la marge des Ryukyus a été affectée par 6 séismes de magnitude supérieure à 7 dont 4 pour lesquels la faille responsable n'est pas connue. Nous proposons une méthode de localisation relative qui permet de relocaliser les évènements historiques à partir des différences de temps d'arrivées P et S (Ts-Tp) à chaque station. La localisation est obtenue en recherchant les séismes analogues dans le catalogue instrumental récent (1991-2008) pour lesquels Ts-Tp à chaque station est proche. Cette méthode a été appliquée au plus gros séisme (en terme de magnitude) jamais enregistré à Taïwan, le séisme du 5 juin 1920 (M7.7 $pm$ 0.2). Ce séisme a certainement pour origine une rupture sur l'interplaque sismogène de la subduction des Ryukyus avec une possible nucléation à la base d'une faille hors-séquence. Ensuite, à l'échelle de la dizaine d'années, l'amélioration de la méthode de localisation absolue du MAXimum d'Intersection EDT (MAXI, EDT: Equal Differential Time) permet de mieux extraire les temps d'arrivées erronés et empêche les "trade-off" entre profondeur et temps d'origine et, entre profondeur et position épicentrale lorsque la couverture azimutale est faible (gap azimutal > 180°). Des tests synthétiques montrent l'efficacité de MAXI (en utilisant les ondes P uniquement) à déterminer les paramètres x, y et z même lorsque le gap azimutal est important. Dans ce dernier cas, la qualité des résultats reste dépendant du modèle de vitesse à représenter la structure 3D de la Terre. Nous proposons une démarche associant l'utilisation d'un modèle de vitesse des ondes P 3D a priori avec la méthode MAXI pour localiser les séismes qui sont latéraux et éloignés du réseau. Une application au cas de Taïwan et en Équateur valide cette approche. Enfin, à l'échelle de quelques mois, la campagne de géophysique marine RATS (Ryukyu Arc: Tectonics and Seismology) a été menée en deux temps. Une expérience de sismologie passive de juillet à octobre 2008 (RATS1) a été menée au-dessus de l'avant-arc des Ryukyus puis une expérience de sismique active (réfraction et réflexion) a été menée en mai 2009 sur une ligne NNE-SSW à travers la marge des Ryukyus. Ces deux expériences combinées permettent d'améliorer notre connaissance de la structure crustale de la marge. Au niveau de l'avant-arc, le socle de l'avant-arc est caractérisé par une extrémité formant un butoir vertical et une base très déformée certainement associée à une importante déformation hors-séquence. En profondeur, la plaque plongeante est certainement affectée par une déchirure qui contrôle la sismicité dans cette région de transition entre la subduction et la collision.
https://tel.archives-ouvertes.fr/tel-00660737

tel-00660737
https://tel.archives-ouvertes.fr/tel-00660737
https://tel.archives-ouvertes.fr/tel-00660737/document
https://tel.archives-ouvertes.fr/tel-00660737/file/Thesis_small.pdf

Résumé : L’objet de cette thèse est l’étude des comportements mécanique et à la corrosion d’un nouveau système de fondation, dit pieux SPIRMEC, adapté aux structures légères. Le travail s’est focalisé sur deux grands aspects. Le premier aspect concerne l’analyse du comportement sous chargement axiale des pieux SPIRMEC sur la base de l’expérimentation et de la modélisation numérique dans un sol mou non cohésif. Quant au deuxième aspect, il traite du problème de la corrosion de l’acier constituant le pieu dans un environnement tropical humide. Le comportement des pieux SPIRMEC sous une charge axiale de traction, dans un sol fin non cohésif, a montré le développement d’une surface cylindrique de rupture. Celle-ci ne dépend pas de la configuration géométrique des pieux tel que : l’espacement S, le diamètre d’hélice Dh et le diamètre du tube d. En adoptant ce critère de rupture et à l’aide d’une analyse statistique des données, nous avons proposé quatre méthodes analytiques de prédiction de la capacité portante des pieux SPIRMEC à partir des essais in-situ et au laboratoire qui sont: •Méthode CPT basée sur la résistance de pointe pénétromètrique (essai CPT), •Méthode MPT basée sur la pression limite pressiomètrique (essai MPT), •Méthode du couple d’installation, •Méthode analytique basée sur les paramètres de cisaillement de sol notamment l’angle de frottement interne (essais triaxial et boîte de cisaillement). Ces méthodes tiennent compte de la géométrie du pieu, du procédé de mise en oeuvre et de la compacité du sol. L’analyse de performance des méthodes proposées a montré que les méthodes analytique et CPT représentent mieux le comportement des pieux testés. La modélisation numérique du comportement des pieux SPIRMEC sous charge axiale de traction a été également étudiée à l’aide du code de calcul Plaxis 2D. Un modèle axisymétrique a été adopté pour modéliser la géométrie de la structure. Le modèle de comportement hardening soil a été retenu pour modéliser le comportement élastoplastiq des couches de sol. Un modèle élastique linéaire a été retenu pour le pieu. Quant au comportement de l’interface sol-structure, il a été pris en compte par un modèle élastoplastique basé sur les critères de Mohr-Coulomb. L’expansion du sol résultant de l’installation des pieux a été approchée par une cavité cylindrique. Cette dernière a été modélisée par des déformations volumiques anisotropes. Nous avons constaté, qu’appliquer une déformation volumique de l’ordre de 40% permet une bonne concordance entre les résultats numériques et expérimentaux. L’étude paramétrique nous a également permis de constater l’existence d’une variation linéaire entre la capacité portante et l’espacement S. Le comportement de l’acier galvanisé dans le sol a été également étudié au moyen de trois approches: électrochimique, visuelles grâce au microscope électronique et gravimétrique. Les tests électrochimiques de corrosion du zinc ont été réalisés dans la solution de sol artificielle pour deux périodes bien distinctes : saison des pluies et après 8 mois de contact sol-pieu. Une forte dépendance est constatée de la vitesse de corrosion de zinc au pH et à la conductivité de la solution. La vitesse de corrosion est plus importante en saison des pluies qu’après 8 mois de contact de pieu avec le sol. Les fortes concentrations en sulfates semblent augmenter la vitesse de corrosion. Les analyses menées au MEB montrent que la zone de fluctuation de nappe et en surface externe (sol/air) sont les zones les plus corrodées. La comparaison entre les différentes techniques d’évaluation de la vitesse de corrosion a montré que l’eau de nappe peut être utilisée comme un milieu représentatif du profil chimique d’un sol non cohésif et légèrement granulaire.
The purpose of this thesis is to study the both behaviors mechanical and of the corrosion of a new foundation system, called spirec piles, for lightweight structures. The work was based on two main aspects. The first aspect concerns the analysis of behavior of the spirmec piles under axial loading on the basis of experimental and numerical modeling in a non-cohesive soft soil. On the second aspect, it addresses the problem of corrosion of steel forming the pile in a humid tropical environment. The behavior of the tested piles under axial traction load, in a non-cohesive soil, showed the development of a cylindrical surface of failure. This does not depend on the geometrical configuration of the tested piles such that: the spacing s, the helix diameter dh and the diameter of the shaft. By adopting this failure criterion and using statistical data analysis, we proposed four analytical methods for predicting the bearing capacity of the piles from in-situ and laboratory tests, which are: cpt method based on tip resistance penetrometer (cpt test), mpt method based on pressuremeter limit pressure (test mpt), method of installation torque and analytical method based on the shear parameters of soil (triaxial tests and shear box). These methods take into account the geometry of the pile, the method and implementation of soil compaction. Performance analysis of the proposed methods showed that the analytical methods and cpt represent better the behavior of piles tested. An axisymmetric condition was assumed to model this geometry in two-dimensional space. In this study, we proposed to model the pipe screw pile with plates elements available in plaxis 2d. The helixes were modeled as circular disks. Interface elements were incorporated along the pile to simulate the soil-pile interaction. Hardening soil model was chosen to describe the soil with drained behavior was assigned for all layers. Linear elastic for pile and elastic plastic model based on the mohr-coulomb criterion was used to describe the interface behavior. The lateral expansion generated by the pile shaft was modeled by anisotropic volumetric strains (ɛxx=ɛzz, ɛyy=0). The pile behavior under tensile loading at different volumetric strains, it is clear that the best fit was obtained at volumetric strain of 40%. The parametric study also allowed us to establish the existence of a linear variation between the bearing capacity and spacings. The behavior of the galvanized steel in the ground has also been studied using three approaches such as: electrochemical, microscopic imaging and gravimetrically. Electrochemical tests of the corrosion of the zinc have been made in artificial soil solution into two periods, the rainy season and after eight months of contact soil-pile. Is a strong dependence of the observed corrosion rate of the zinc at the ph and conductivity of the solution. The corrosion rate is higher in the rainy season compared to eight months of contact of the pile with soil. High sulfate concentrations increase the rate of corrosion. The meb analysis shows that the fluctuation zone of the water table is the most corroded area. Comparison between the different methods of evaluating the corrosion rate showed that the ground water may be used as a representative profile of the chemical environment of a non-cohesive soil and slightly granular.

http://www.theses.fr/2014AGUY0790/document

Éditeur(s) : HAL CCSD Pergamon
Résumé : This paper presents an experimental study of the deformation of Beaucaire marl subjected to drained triaxial tests under a wide range of confining pressure. Several methods of observation have been used: X-ray computed tomography, scanning electronic microscopy, quantitative image analysis of thin sections and mercury porosimetry. The analysis at the grain scale shows the existence of various deformation modes depending on the mean stress level. These modes are classified with reference to the brittle and ductile regimes. In specimens deformed under low confining pressure, macro-cracks are localised in a band more contracting than the rest of the specimen. In specimens deformed under moderate confining pressure, a series of micro-scale shear bands characterised by a grain re-orientation towards the direction of macro-shear is detected in the zones of localised deformation. The material in between the micro-scale shear bands is contracting more than the rest of the specimen in some cases; in other cases, a schistosity is detected also in the rest of the specimen outside the localisation zones. In the specimen deformed under high confining pressure, no localisation occurs, and a schistosity is observable throughout the specimen.
ISSN: 1365-1609

hal-00411208
https://hal.archives-ouvertes.fr/hal-00411208
DOI : 10.1016/j.ijrmms.2007.08.007

Éditeur(s) : HAL CCSD
Résumé : International audience
Metamorphic soles correspond to m to 500 m thick highly strained metamorphic rock units found beneath myloniticbanded peridotites at the base of large-scale ophiolites, as exemplified in Oman. Metamorphic soles aremainly composed of metabasalts deriving from the downgoing oceanic lithosphere and metamorphosed up togranulite-facies conditions by heat transfer from the mantle wedge. Pressure–temperature peak conditions areusually estimated at 1.00.2 GPa and 800100ºC. The absence of HP–LT metamorphism overprint implies thatmetamorphic soles have been formed and exhumed during subduction infancy. In this view, metamorphic soleswere strongly deformed during their accretion to the mantle wedge (corresponding, now, to the base of the ophiolite).Therefore, metamorphic soles and banded peridotites are direct witnesses of the dynamics of early subductionzones, in terms of thermal structure, fluid migration and rheology evolution across the nascent slab interface.Based on fieldwork and EBSD analyses, we present a detailed (micro–) structural study performed on samplescoming from the Sumeini window, the better-preserved cross-section of the metamorphic sole of Oman. Largedifferences are found in the deformation (CPO, grain size, aspect ratio) of clinopyroxene, amphibole and plagioclase,related to mineralogical changes linked with the distance to the peridotite contact (e.g., hardening due to theappearance of garnet and clinopyroxene).To model the incipient slab interface in laboratory, we carried out 5 hydrostatic annealing and simple-shear experimentson Griggs solid-medium apparatus. Deformation experiments were conducted at axial strain rates of 10-6s-1. Fine-grained amphibolite was synthetized by adding 1 wt.% water to a (Mid-Ocean Ridge) basalt powder asa proxy for the metamorphic sole (amphibole + plagioclase + clinopyroxene garnet assemblage). To synthetizegarnet, 2 experiments were carried out in hydrostatic conditions and with deformation at 800C with confiningpressure of 2 GPa. Another simple-shear experiment has been carried out at 800C and 1 GPa with fined-grainednatural garnet. With the aim of mimicking the early slab interface (between the metamorphic sole and bandedperidotites at the base of the ophiolite), 2 simple-shear deformation experiments with 2 layers have been carriedout at 800C and confining pressure of 1 GPa. The bottom layer was made of hydrated basalt powder and the toplayer was made of olivine.Fined-grained garnet-free amphibolite is significantly weaker than dunite but the appearance of harder mineralsin the amphibolite (garnet and clinopyroxene) has major implications on its rheological evolution. These resultsallow liking field observations of strain localization at the interface to the metamorphic sole formation
European Geosciences Union General Assembly 2016
Vienne, Austria

insu-01309394
https://hal-insu.archives-ouvertes.fr/insu-01309394

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
We report new textural and chemical data for 10 garnet peridotite xenoliths from the Udachnaya kimberlite and examine them together with recent data on another 21 xenoliths from the 80-220 km depth range. The samples are very fresh (LOI near zero), modally homogeneous and large (>100 g). Some coarse-grained peridotites show incipient stages of deformation with <10 % neoblasts at grain boundaries of coarse olivine. Such microstructures can only be recognized in very fresh rocks, because fine-grained interstitial olivine is strongly affected by alteration, and may have been overlooked in previous studies of altered peridotite xenoliths in the Siberian and other cratons. Some of the garnet peridotites are similar in composition to low-opx Udachnaya spinel harzburgites (previously interpreted as pristine melt extraction residues), but the majority show post-melting enrichments in Fe and Ti. The least metasomatized coarse peridotites were formed by 30-38 % of polybaric fractional melting between 7 and 4 GPa and ≤1-3 GPa. Our data together with experimental results suggest that garnet in these rocks, as well as in some other cratonic peridotites elsewhere, may be a residual mineral, which has survived partial melting together with olivine and opx. Many coarse and all deformed garnet peridotites from Udachnaya underwent modal metasomatism through interaction of the melting residues with Fe-, Al-, Si-, Ti-, REE-rich melts, which precipitated cpx, less commonly additional garnet. The xenoliths define a complex geotherm probably affected by thermal perturbations shortly before the intrusion of the host kimberlite magmas. The deformation in the lower lithosphere may be linked to metasomatism.
ISSN: 0010-7999

hal-00941252
https://hal.archives-ouvertes.fr/hal-00941252
DOI : 10.1007/s00410-013-0855-8

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Upper-mantle flow beneath the South Pacific is investigated by analysing shear wave splitting parameters at eight permanent long-period and broad-band seismic stations and 10 broad-band stations deployed in French Polynesia from 2001 to 2005 in the framework of the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). Despite the small number of eventsand the rather poor backazimuthal coverage due to the geographical distribution of the natural seismicity, upper-mantle seismic anisotropy has been detected at all stations except at Tahiti where two permanent stations with 15 yr of data show an apparent isotropy. The median value of fast polarization azimuths (N67.5°W) is parallel to the present Pacific absolute platemotion direction in French Polynesia (APM: N67°W). This suggests that the observed SKS fast polarization directions result mainly from olivine crystal preferred orientations produced by deformation in the sublithospheric mantle due to viscous entrainment by the moving Pacific Plate and preserved in the lithosphere as the plate cools. However, analysis of individualmeasurements highlights variations of splitting parameters with event backazimuth that imply an actual upper-mantle structure more complex than a single anisotropic layer with horizontal fast axis. A forward approach shows that a two-layer structure of anisotropy beneath French Polynesia better explains the splitting observations than a single anisotropic layer. Second ordervariations in the measurements may also indicate the presence of small-scale lateral heterogeneities. The influence of plumes or fracture zones within the studied area does not appear to dominate the large-scale anisotropy pattern but may explain these second-order splitting variations across the network.
ISSN: 0956-540X

hal-01249275
https://hal.archives-ouvertes.fr/hal-01249275
https://hal.archives-ouvertes.fr/hal-01249275/document
https://hal.archives-ouvertes.fr/hal-01249275/file/GJI2007SKS.pdf
DOI : 10.1111/j.1365-246X.2007.03475.x