Éditeur(s) : HAL CCSD Geological Society of London
Résumé : We have applied transmission electron microscopy (TEM) analyses coupled with viscoplastic self-consistent (VPSC) numerical modelling to identify the active slip systems and to better understand the crystal preferred orientation (CPO) development of the Torridon quartz mylonite (NW Scotland). TEM analyses showed evidence of activation of 1/3〈a〉{π′}, 1/3〈a〉{z} and possible 〈a〉(c) slip systems, as well as dislocation climb and dynamic recrystallization. All the CPOs generated by VPSC models share common characteristics with the Torridon quartz mylonite, but only Models 2 and 3 reproduce the [c]-axes maxima at low angle (<20°) to the foliation pole along the YZ plane, as observed in the mylonite. In Model 2, this concentration only occurs at γ≥2.6, whereas in Model 3 this maxima occurs at lower shear strains. The models that start with a previous preferred orientation acquire very strong CPOs after small-imposed strains, followed by the rapid rotation of the fabric in relation to the new imposed finite strain axes. The combined activation of 〈a〉{π′}, 〈a〉{z} and possibly 〈a〉(c) slip systems, as demonstrated by TEM analyses, suggests that the VPSC model that best predicts CPO development in the Torridon quartz mylonite is Model 2, where the critical resolved shear stress (CRSS) of 〈a〉{π/π′} is assumed to be slightly stronger than 〈a〉(c).
Deformation Mechanisms, Rheology and Tectonics: Microstructures, Mechanics and Anisotropy

hal-00688840
https://hal.archives-ouvertes.fr/hal-00688840

Éditeur(s) : HAL CCSD
Résumé : This work provides new constraints on the development and on the distribution of the deformation in the upper mantle and particularly beneath transform plate boundaries. USArray experiment and the remarkable increase of the dataset in California for the past ten years allowed us to scrutinize the lateral variations of the anisotropy in the vicinity of the San Andreas Fault zone. We have confirmed and increased the detection of two layers of anisotropy beneath this plate boundary. The first layer, located in the lithosphere, is related to the deformation induced at the fault, and the other one, located in the asthenosphere, is coherent with the anisotropy observed far from it, its origin is however less clear. We show that the deformation zone associated both to the San Andreas, Calaveras and Hayward Faults, is likely 40 km wide at 70 km depth. We then performed numerical thermomechanical modeling (ADELI) of the displacement of a transform plate boundary associated with the computation of the development of crystallographic fabrics using a viscoplastic selfconsistent approach (VPSC). We analyzed the distribution of the deformation in the model ant looked after the possible interactions at depth between deformation caused at surface by the strike-slip dynamic of the fault and the shearing at the base of the lithosphere caused by the horizontal displacement of the plates. Elastic properties derived from the crystallographic fabrics modeled, show that such interactions exist and induce, beneath the fault zone, a progressive rotation of the crystallographic fabrics with depth. Seismological signature of these smooth rotations is however not relevant with the presence of two anisotropic layers as proposed beneath California. We thus consider that a decoupling zone exists between the lithosphere and the asthenosphere beneath the California to account for the sharp separation between a lithospheric and an asthenospheric deformation. We furthermore estimate that anisotropy observed far form the San Andreas Fault in California cannot be explained only by the drag of the asthenosphere by the North America lithosphere as proposed in our article. Indeed, we can only expect few tenths of second of splitting delay from the anisotropic properties derived from the numerical modeling of a plate moving in the same direction and in the same velocity than the North American lithosphere only for 10 Ma of displacement. As delays observed in California rather reach 1.5 s, anisotropy in this region thus requires the existence of an active asthenospheric flow to be explained.
Le travail réalisé pendant cette thèse a permis d'apporter de nouvelles contraintes sur le développement et la distribution de la déformation dans le manteau supérieur et plus particulièrement au niveau des grandes limites de plaques décrochantes. Grâce à l'apport de l'expérience USArray et d'une dizaine d'années d'enregistrements sismologiques supplémentaires, nous avons pu étudier, de manière précise, les variations d'anisotropie dans le voisinage de la Faille de San Andreas. Nous avons confirmé et étendu l'observation de deux couches anisotropes sous cette limite de plaque. On y observe une première couche localisée dans la lithosphère marquant la déformation induite à la limite de plaque, et une autre, asthénosphérique, cohérente avec l'anisotropie observée loin de la faille et d'origine plus discutée. Nous avons montré que la zone de déformation associée aux failles de San Andreas, Calaveras et d'Hayward a, vraisemblablement, une largeur d'au moins 40 kilomètres en base de lithosphère, sous chacune de ces failles. Nous avons ensuite procédé à la modélisation thermomécanique (ADELI) de la migration d'une limite de plaques décrochante couplée à une modélisation du développement de fabriques cristallographiques par une approche viscoplastique auto-cohérente (VPSC). Ceci nous a permis d'y observer le développement de la déformation et les conséquences des possibles interactions entre la déformation décrochante en surface et le cisaillement en base de lithosphère dû au déplacement horizontal des plaques. Les propriétés élastiques déduites des fabriques cristallographiques modélisées montrent que de telles interactions existent et provoquent, sous la limite de plaques, une rotation des orientations cristallographiques avec la profondeur. Le signal associé à ces rotations progressives n'est toutefois pas cohérent avec la présence de deux couches d'anisotropie comme proposée sous la faille de San Andreas. Nous pensons par conséquent qu'il existe, sous la Californie, une zone de découplage entre la lithosphère et l'asthénosphère, permettant d'individualiser une déformation lithosphérique d'une déformation asthénosphérique. Nous estimons, en outre, que l'anisotropie observée dans l'asthénosphère sous la Californie ne peut être expliquée seulement par le cisaillement induit par le déplacement de la lithosphère Nord Amérique. En effet, les propriétés anisotropes obtenues par modélisation à partir d'une plaque se déplaçant dans une direction et une vitesse proche de celle de la plaque Amérique du Nord montrent qu'on ne peut espérer guère plus que quelques dixièmes de seconde de délai au bout de 10 Ma de déplacement. Les déphasages mesurés en Californie étant de l'ordre de 1,5 s, il est donc nécessaire d'invoquer la présence d'écoulements mantelliques actifs sous cette région.
https://tel.archives-ouvertes.fr/tel-00767124

tel-00767124
https://tel.archives-ouvertes.fr/tel-00767124
https://tel.archives-ouvertes.fr/tel-00767124/document
https://tel.archives-ouvertes.fr/tel-00767124/file/theseBonnin.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Quartz is a common crustal mineral that deforms plastically in a wide range of temperatures and pressures, leading to the development of different types of crystallographic preferred orientation (CPO) patterns. In this contribution we present the results of an extensive modeling of quartz fabric transitions via a viscoplastic self-consistent (VPSC) approach. For that, we have performed systematic simulations using different sets of relative critical resolved shear stress of the main quartz slip systems. We have performed these simulations in axial compression and simple shear regimes under constant Von Mises equivalent strain of 100% (γ = 1.73), assuming that the aggregates deformed exclusively by dislocation glide. Some of the predicted CPOs patterns are similar to those observed in naturally and experimentally deformed quartz. Nevertheless, some classical CPO patterns usually interpreted as result from dislocation glide (e.g. Y-maxima due to prism < a > slip) are clearly not developed in the simulated conditions. In addition we reported new potential preferred orientation patterns that might happen in high temperature conditions, both in axial compression and simple shear. We have demonstrated that CPOs generated under axial compression are usually stronger that those predicted under simple shear, due to the continuous rotation observed in the later simulations. The fabric strength depends essentially on the dominant active slip system, and normally the stronger CPOs result from dominant basal slip in < a >, followed by rhomb < a > and prism [c] slip, whereas prism < a > slip does not produce strong fabrics. The opening angle of quartz [0001] fabric used as a proxy of temperature seems to be reliable for deformation temperatures of ~ 400 °C, when the main slip systems have similar behaviors.
ISSN: 0040-1951

hal-01115674
https://hal.archives-ouvertes.fr/hal-01115674
DOI : 10.1016/j.tecto.2014.08.011

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In order to estimate the seismic anisotropy of subducted crust, polycrystalline samples of KA1Si(3)O(8) K-lingunite (25% of the total subducted transformed sediments), have been synthesized and deformed under the temperature and pressure conditions of the subducted slabs. Transmission electron microscopy (TEM) characterizations of the recovered samples reveal that the microstructures are clearly dominated by [001] glide involving screw dislocations. For this reason, only {100} could be identified as glide planes, the question of [001] slip on {110} remains open. Few 1/2 < 111 > dislocations were observed gliding on {110} planes, which implies that 1/2 < 111 > {110} is a harder slip system than those involving [001] slip. The occurrence of sub-grain boundaries suggests that diffusion and climb might be active under these conditions.;To assess the texture of polycrystalline K-lingunite, the crystal preferred orientations (CP05) were calculated using visco-plastic self-consistent (VPSC) polycrystalline plasticity model in simple shear using the slip systems identified by TEM. Finally, the seismic properties of K-lingunite aggregates were calculated from the CPO and single crystal elasticity tensor. K-lingunite is predicted to have a high seismic anisotropy, which could combine constructively with one of the stishovite (same proportion as K-lingunite at the transition zone depth ranges).
ISSN: 0031-9201

hal-00534050
https://hal.archives-ouvertes.fr/hal-00534050
DOI : 10.1016/j.pepi.2010.06.013