A free surface capturing discretization for the staggered grid finite difference scheme

International audienceThe coupling that exists between surface processes and deformation within both the shallowcrust and the deeper mantle-lithosphere has stimulated the development of computationalgeodynamic models that incorporate a free surface boundary condition. We introduce a treatmentof this boundary condition that is suitable for staggered grid, finite difference schemesemploying a structured Eulerian mesh. Our interface capturing treatment discretizes the freesurface boundary condition via an interface that conforms with the edges of control volumes(e.g. a ‘staircase’ representation) and requires only local stencil modifications to be performed.Comparisons with analytic solutions verify that the method is first-order accurate. Additionalintermodel comparisons are performed between known reference models to further validateour free surface approximation. Lastly, we demonstrate the applicability of a multigrid solverto our free surface methodology and demonstrate that the local stencil modifications do notstrongly influence the convergence of the iterative solver

The Minimized Power Geometric model: An analytical mixing model for calculating polyphase rock viscosities consistent with experimental data

International audienceHere we introduce the Minimized Power Geometric (MPG) model which predicts the viscosity of any polyphase rocks deformed during ductile flow. The volumetric fractions and power law parameters of the constituting phases are the only model inputs required. The model is based on a minimization of the mechanical power dissipated in the rock during deformation. In contrast to existing mixing models based on minimization, we use the Lagrange multipliers method and constraints of strain rate and stress geometric averaging. This allows us to determine analytical expressions for the polyphase rock viscosity, its power law parameters, and the partitioning of strain rate and stress between the phases. The power law bulk behavior is a consequence of our model and not an assumption. Comparison of model results with 15 published experimental data sets on two-phase aggregates shows that the MPG model reproduces accurately both experimental viscosities and creep parameters, even where large viscosity contrasts are present. In detail, the ratio between experimental and MPG-predicted viscosities averages 1.6. Deviations from the experimental values are likely to be due to microstructural processes (strain localization and coeval other deformation mechanisms) that are neglected by the model. Existing models that are not based on geometric averaging show a poorer fit with the experimental data. As long as the limitations of the mixing models are kept in mind, the MPG model offers great potential for applications in structural geology and numerical modeling

Taiwan mountain building: insights from 2-D thermomechanical modelling of a rheologically stratified lithosphere

International audienceThe Taiwan orogen has long been regarded as a case example for studying mountain building in association with subduction processes. In this paper, we present a fully coupled thermomechanical modelling of the Taiwan collision based on a realistic viscous-elastic­plastic rheology. It satisfactorily reproduces available thermochronometric data, long-/short-term deformation patterns, heat flux and erosion/sedimentation distribution across the Taiwan orogeny. We found that a deep seated flux of Asian crustal material into the orogenic wedge should be invoked to counter-balance observed exhumation and erosion in the Central Range. However, in contrast with recent thermokinematic models of exhumation and deformation suggesting that underplating plays a significant role, we show that most constraints on exhumation and deformation can be more straightforwardly interpreted by the frontal accretion of the rheologically layered Asian crust. We finally infer that such a model is in better agreement with the basic expectation that the hot/young and buoyant Chinese continental margin should hardly be subducted beneath the cold/old and dense oceanic plate of the Philippines Sea

Comportement mécanique des roches et dynamique de la lithosphère: Des données de terrain aux modèles numériques

non renseignéCe mémoire dresse le bilan de mes activités de recherche depuis ma thèse de 3ème cycle. Il s’agit làd’une tentative de synthèse des travaux effectués à Rennes pendant 2 ans de post-doctorat (2007-2008), à Zürich durant 1 an (en 2008-2009), puis à Rennes depuis mon recrutement en septembre 2009en tant que maître de conférences

Subduction interface processes recorded by eclogite-facies shear zones (Monviso, W. Alps)

International audienceThe Monviso ophiolite Lago Superiore Unit constitutes a well-preserved, almost continuous upper fragment of oceanic lithosphere subducted at c. 80 km depth, thereby providing a unique opportunity to study mechanical coupling processes and meter-scale fluid-rock interactions occurring at such depths in present-day subduction zones. It is made of (i) a variably thick (50-500 m) section of eclogitized basaltic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body and of (ii) a c. 1 km thick serpentinite sole. We herein focus on the three major eclogite-facies shear zones found at the top of the unit, at the boundary between basalts and gabbros, and between gabbros and serpentinites, respectively. Strain localization occurred at lithological interfaces, irrespective of material strength. While ductile deformation dominates along the shear zones, local brittle behaviour is demonstrated by the existence of numerous eclogite breccias of Fe-Ti metagabbros and widespread garnet fractures, possibly linked with intermediate-depth eclogite-facies (micro)seismicity. These m- to hm-sized fragments of Fe-Ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro-serpentinite boundary (Lower Shear zone; LSZ). Pervasive and focused fluid flow is attested in the LSZ by significant alteration of bulk rock compositions, weakening of the rocks and widespread crystallization of hydrous parageneses. By contrast, the Intermediate Shear zone (ISZ) shows evidence for more restricted, short-range fluid flow. The activity of both the ISZ and LSZ ceased during early lawsonite eclogite-facies exhumation, when deformation localized deeper within the serpentinite sole, allowing for the detachment (and preservation) of this large ophiolitic fragment

Episodic slab rollback fosters exhumation of HP—UHP rocks

The burial—exhumation cycle of crustal material in subduction zones can either be driven by the buoyancy of the material, by the surrounding flow, or by both. High pressure and ultrahigh pressure rocks are chiefly exhumed where subduction zones display transient behaviours, which lead to contrasted flow regimes in the subduction mantle wedge. Subduction zones with stationary trenches (mode I) favour the burial of rock units, whereas slab rollback (mode II) moderately induces an upward flow that contributes to the exhumation, a regime that is reinforced when slab dip decreases (mode III). Episodic regimes of subduction that involve different lithospheric units successively activate all three modes and thus greatly favour the exhumation of rock units from mantle depth to the surface without need for fast and sustained erosio

Influence of surrounding plates on 3D subduction dynamics

International audienceOur 3D modelling study shows that the presence of lithospheric plates around a subducting plate has a significant influence on subduction dynamics, in particular on trench retreat rate, slab dip, and lateral shortening of the subducting plate. Neighbouring plates prevent unrealistic plate behaviour with no need for complex rheologies. Because, at the Earth's surface, plates form a continuous shell, they should not be neglected

Burial and exhumation in a subduction wedge : mutual constraints from thermo-mechanical modelin and natural P-T-t data (Sch. Lustrés, W. Alps)

The dynamic processes leading to synconvergent exhumation of high-pressure low-temperature (HP-LT) rocks at oceanic accretionary margins, as well as the mechanisms maintaining nearly steady state regime in most accretion prisms, remain poorly understood. The present study aims at getting better constraints on the rheology, thermal conductivity, and chemical properties of the sediments in subduction zones. To reach that goal, oceanic subduction is modeled using a forward visco-elasto-plastic thermomechanical code (PARA(O)VOZ-FLAC algorithm), and synthetic pressure-temperature-time (P-T-t) paths, predicted from numerical experiments, are compared with natural P-T-t paths. The study is focused on the well constrained Schistes Lustrés complex (SL: western Alps) which is thought to represent the fossil accretionary wedge of the Liguro-Piemontese Ocean. For convergence rates comparable to Alpine subduction rates (∼3 cm yr−1), the best-fitting results are obtained for high-viscosity, low-density wedge sediments and/or a strong lower continental crust. After a transition period of 3-5 Ma the modeled accretionary wedges reach a steady state which lasts over 20 Ma. Over that time span a significant proportion (∼35%) of sediments entering the wedge undergoes P-T conditions typical of the SL complex (∼15-20 kbar; 350-450°C) with similar P-T loops. Computed exhumation rates (<6 mm yr−1) are in agreement with observations (1-5 mm yr−1). In presence of a serpentinite layer below the oceanic crust, exhumation of oceanic material takes place at rates approaching 3 mm yr−1. In all experiments the total pressure in the accretionary wedge never deviated by more than ±10% from the lithostatic component

Episodic slab rollback fosters exhumation of HP-UHP rocks

International audienceThe burial-exhumation cycle of crustal material in subduction zones can either be driven by the buoyancy of the material, by the surrounding flow, or by both. High pressure and ultrahigh pressure rocks are chiefly exhumed where subduction zones display transient behaviours, which lead to contrasted flow regimes in the subduction mantle wedge. Subduction zones with stationary trenches (mode I) favour the burial of rock units, whereas slab rollback (mode II) moderately induces an upward flow that contributes to the exhumation, a regime that is reinforced when slab dip decreases (mode III). Episodic regimes of subduction that involve different lithospheric units successively activate all three modes and thus greatly favour the exhumation of rock units from mantle depth to the surface without need for fast and sustained erosion

Subducting slabs: jellyfishes in the Earth mantle

International audienceThe constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations