Numerical modeling of the onset of localised deformation : example of lithospheric buckling
Modélisation numérique de la naissance des déformations localisées : exemple du flambage lithosphérique
Résumé
several classical tectonic and mecanical situations are modelled numerically in this study, illustrating the first order importaqnce of non-associated plasticity in the onset and propagation of localised deformation in brittle domain. The efficiency of the numerical code Parovoz (based on a FLAC method, Cundal & Board, 1988) is shown to be able to reproduce different mechanical contexts of lithospheric rocks. This approach is specifically developed to the lithospheric scale folding that affects the Central Indian Basin, which displays present-day active seismicity. While several analytical, numerical and analog models have described the process of lithospheric scale buckling, we show here for the first time numerically, the development of failure in association to the development of buckling instabilities. The modeled oceanic lithosphere has an elasto-visco-plastic behavior (Coulomb failure criterion and temperature-dependent power-law creep). Buckling becomes significant only when the entire resistant portion of the lithosphere reaches its yield stress. Faults (shear bands) then stabilize progressively at the inflexion points of the folds, compatible with the the growth of buckling amplitudes. The wavelength to thickness ratio of this buckling is close to 5, whereas the folds growth rate is linked to the compression velocity, and the strength contraste between competent and ductile layers (friction and cohesion controle the locally sustainable gravitational load). Frictional weakening allows to form a single major shear zone (subduction initiation). lithospheric-scale buckling also appears to occur in intracontinental plates, as in Central Asia and Central Australia. Numerical models show several modes of buckling modes, crustal only, biharmonic or coupled with the mantle lithosphere, depending on the thermo-tectonic age and compositional ability to have a ductile lower crust in between stronger upper crust and upper mantle. buckling deformation may evolve into a localised thickenineg and orogenesis: the characteristic width, the flowing capacity of the lower crust, and surface processes all favor orogenesis development. Accounting for diffusional erosion and sedimentation processes aid the growth of instabilities whereas they limit the relief growth. Large intracontinental basins form, with faulting geometries that evolve through time, and erosion rates and surface uplift rates (0.1 to 1mm/yr) being comparable to commonly observed values.
La plasticite non-associee est un facteur important si ce n'est primordial dans le processus de localisation de la deformation cassante. Un code numerique particulierement adapte a reproduire les comportements mecaniques tres varies des roches nous permet de modeliser efficacement l'initiation et la propagation des deformations localisees a travers la lithosphere. La region de l'Ocean Indien est un cas spectaculaire de flambage lithospherique , accompagne d'une activite sismique intense. Plusieurs modeles analytiques, analogiques et numeriques rendent compte du processus de ambage. On developpe un modele numerique qui pour la premiere fois reproduit le developpement de ambage simultan'avec celui de failles. En utilisant une rheologie elasto-visco-plastique (critere de Mohr-Coulomb et loi de puissance dependant du geotherme), la fracturation di use a ecte d'abord l'ensemble de la lithosphere competente, est suivie de ambage et simultanement de failles qui se localisent progressivement aux points d'inflexion des plis. Les modeles numeriques testes dans ce memoire renseignent sur le comportement rheologique de la lithosphere, en particulier sur l'importance de l'angle de friction interne, qui permet de supporter les de ections liees au ambage. Les contraintes reelles sur les amplitudes montrent une dependance combinee du taux de raccourcissement, de la friction, et de l'epaisseur de la couche inferieure ductile. Des domaines intracontinentaux tels que l'Ouest du Gobi et le centre de l'Australie presentent les caracteristiques d'un ambage d'echelle lithospherique, malgre les heterogeneites pre-existantes. Une modelisation numerique confirme le role de l'age thermotectonique et de la rheologie crustale dans le developpement d'un mode de flambage. La dimension caracteristique des couches qui ambent determine l'evolution du modele vers un epaississement localise et une orogenese. Dans ce cas la lithosphere s'epaissit sous de larges bassins intracontinentaux de tailles comparables a la realite. L'insertion d'un modele d'erosion confirme son role stabilisant dans l'evolution de la topographie, alors qu'en profondeur le ambage est ampli e, et la localisation facilitee. L'elevation du relief obtenu varie de 0.1 a 1mm/an.
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