arXiv subjects

Andréa Tommasi

Publications and source records attributed to Andréa Tommasi.

3 recordsLinked to original sources

arXiv2026

Modelling ductile strain localization with evolutive stochastic rheologies

Plate Tectonics requires strain localization over the entire thickness of the plates. However, modelling strain localization in the deep sections of the plates, which deform by ductile processes, remains a challenge, prompting the use of ad hoc schemes to model plate boundaries. We posit that the bottleneck for self-consistent generation of ductile strain localization in geodynamical models is poor representation of the intrinsic mechanical heterogeneity of rocks, in particular at small scales. This prevents its effects from being accounted for at larger scales, notably emergent properties that arise during upscaling, like anisotropy. To model this heterogeneity and its evolution, we adopt a stochastic description of the rheology, which evolves in time and space as a function of the local work-rate. This approach enables to reproduce the full variety of responses observed in nature, from heterogeneous deformation at the local scale, but homogeneous at the system-scale, with or without softening, to spontaneous development of system-scale shear zones. It enables, thereby, the construction of regime diagrams for ductile strain localization using three adimensional parameters. These parameters describe the degree of heterogeneity and potential for evolution of the rheology, function of (1) the constitutive equation, which represents the active deformation processes, (2) the evolution law for the rock mechanical properties, (3) the initial properties, and (4) the energy input to the system. This approach also enables the prediction of the intensity of localization and the resulting system-scale anisotropic softening, paving the way for self-consistent modelling of plate boundaries in geodynamics.

physics.geo-ph↗

arXiv2021

An effective parameterization of texture-induced viscous anisotropy in orthotropic materials with application for modeling geodynamical flows

In this article, we describe the mathematical formulation and the numerical implementation of an effective parametrization of the viscous anisotropy of orthorhombic materials produced by crystallographic preferred orientations (CPO or texture), which can be integrated into 3D geodynamic and materials science codes. Here, the approach is applied to characterize the texture-induced viscous anisotropy of olivine polycrystals, the main constituent of the Earth's upper mantle. The parameterization is based on the Hill (1948) orthotropic yield criterion. The coefficients of the Hill yield surface are calibrated based on numerical tests performed using the second order Viscoplastic Self-consistent (SO-VPSC) model. The parametrization was implemented in a 3D thermo-mechanical finite-element code developed to model large-scale geodynamical flows, in the form of a Maxwell rheology combining isotropic elastic and anisotropic non-linear viscous behaviors. The implementation was validated by comparison with results of the analytical solution and of the SO-VPSC model for simple shear and axial compression of a homogeneous anisotropic material. An application designed to examine the effect of texture-induced viscous anisotropy on the reactivation of mantle shear zones in continental plates highlights unexpected couplings between localized deformation controlled by variations in the orientation and intensity of the olivine texture in the mantle and the mechanical behavior of the elasto-viscoplastic overlying crust. Importantly, the computational time only increases by a factor 2-3 with respect to the classic isotropic Maxwell viscoelastic rheology.

cond-mat.mtrl-sci↗

arXiv2021

Deformation of upper mantle rocks with contrasting initial fabrics in axial extension

To explore the role of the olivine grain size and crystal preferred orientation (CPO) on the evolution of the microstructure and the mechanical behaviour of upper mantle rocks up to large strains, we performed axial extension experiments at 1200C, 300MPa confining pressure, and constant displacement rate on three natural peridotites: a fine-grained mylonitic harzburgite with a weak CPO and two coarse-grained well-equilibrated dunites with CPO of variable intensity. Initial flow stresses show a limited range of variation (115-165MPa), with the fine-grained sample showing the highest initial strength. However, the evolution of both mechanical behavior and microstructure differs between fine and coarse-grained peridotites. In the fine-grained harzburgite, necking resulted in decrease in the apparent differential stress. Focusing of strain and stress produced increase of the olivine recrystallized fraction and decrease of the recrystallized grain size in the neck. Analysis of the final stress and strain in the neck indicates softening due to evolution of the microstructure and CPO. In contrast, necking of the coarse-grained samples produced weak or no decrease in the apparent differential stress. This implies hardening, consistently with the increase in bulk intragranular misorientation and final stresses in the neck similar or higher than initial ones. Coarse-grained dunites deformed heterogeneously. Crystals well oriented to deform by dislocation glide became elongated, whereas those in hard orientations remained almost undeformed. In the neck, stress and strain concentration resulted in formation of kinks in hard crystals and dynamic recrystallization in soft crystals. We interpret the more effective strain-induced softening of the fine-grained peridotite as due to easier dynamic recrystallization, probably due to the higher proportion of grain boundaries acting as nucleation sites.

physics.geo-ph↗