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

Formation of metamorphic core complex in inherited wedges: A thermomechanical modelling study

International audienceMetamorphic Core Complexes (MCCs) form when a thickened domain with a low-strength lower crust is submitted to extension. These structures are characteristic of post-orogenic extension, and field observations suggest that several MCCs rework a crustal nappe-stack emplaced before extension begins. These MCCs therefore develop within heterogeneous crusts that contain pre-existing dipping heterogeneities, such as thrust faults and dipping nappes in a crustal wedge. Although very common, this first order structural inheritance has never been considered in studies modelling MCCs. Our contribution therefore investigates the effect of an inherited crustal wedge structure on the dynamics and kinematics of formation of the MCCs, using fully coupled thermomechanical modelling. The wealth of petrological, structural and time informations available in the Cycladic MCCs (Aegean domain) allows setting up more realistic initial conditions for the experiments than usual flat-lying setups. It also allows the results of the numerical computation to be directly validated with final geometries, P-T paths and exhumation rates. The experiments using dipping heterogeneities are characterised by a much more complex evolution and final structure than their flat-lying layered equivalents. Dipping heterogeneities drive lateral strength contrasts and help to re-localise the deformation on successive detachments. The dip of the inherited wedge structures imposes kinematic constraints on the flow, which provides a model that explains the regional scale asymmetry of the Cycladic MCCs. The P-T paths, the exhumation rates and the final crustal structure that come out of an initial shallow-dipping wedge model provide a much more realistic comparison with their natural counter-parts than common flat-lying models. Other parameters, like crustal-scale density inversion, thermal structure and creep law parameters are of second order when compared to the initial wedge structure. Being little dependent on these second order parameters, the proposed model for the formation of MCCs within inherited crustal wedges is likely to be applied to other areas where the MCCs formed in a nappe stack involving continental basement

Post-orogenic extension and metamorphic core complexes in a heterogeneous crust: the role of crustal layering inherited from collision. Application to the Cyclades (Aegean domain)

International audienceThe development of metamorphic core complexes (MCC) corresponds to a mode of lithospheric continental stretching that follows collision. In most of the models that explain the formation of the MCC, high thermal gradients are necessary to weaken the lower crust and to induce its ascent. Such models fail to explain the exhumation of high pressure-low temperature metamorphic rocks in metamorphic core complex structures as observed in the Cycladic Blueschists in the Aegean domain. Besides, account for the lithological crustal stratification induced from collision has never been tested. In this paper, we use fully coupled thermomechanical modelling to investigate the impact of structural heritage and initial thermal gradient on the behaviour of the post-orogenic continental lithosphere. The models are designed and validated by petrological, structural and time data from the Cyclades. As a result, high thermal gradients (Moho temperature higher than 800°C) are neither necessary nor always sufficient to induce the development of a metamorphic core complex. At the contrary, the rheological layering of the crust inherited from collision is a first-order parameter controlling the development of extensional structures in post-orogenic settings. 'Cold' MCC can develop if the crust is made of a strong nappe thrust on top of weaker metamorphic cover and basement units, as observed in the Cyclades

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

Strain localisation in the continental lithosphere, a scale-dependent process

International audienceStrain localisation in continents is a general question tackled by specialists of various disciplines in Earth Sciences. Field geologists working at regional scale are able to describe the succession of events leading to the formation of large strain zones that accommodate large displacement within plate boundaries. On the other end of the spectrum, laboratory experiments provide numbers that quantitatively describe the rheology of rock material at the scale of a few mm and at deformation rates up to 8-10 orders of magnitude faster than in nature. Extrapolating from the scale of the experiment to the scale of the continental lithosphere is a considerable leap across 8-10 orders of magnitude both in space and time. It is however quite obvious that different processes are at work for each scale considered. At the scale of a grain aggregate diffusion within individual grains, dislocation or grain boundary sliding, depending on temperature and fluid conditions, are of primary importance. But at the scale of a mountain belt, a major detachment or a strike-slip shear zone that have accommodated tens or hundreds of kilometres of relative displacement, other parameters will take over such as structural softening and the heterogeneity of the crust inherited from past tectonic events that have juxtaposed rock units of very different compositions and induced a strong orientation of rocks. Once the deformation is localised along major shear zones, grain size reduction, interaction between rocks and fluids and metamorphic reactions and other small-scale processes tend to further localise the strain. Because the crust is colder and more lithologically complex this heterogeneity is likely much more prominent in the crust than in the mantle and then the relative importance of "small-scale" and "large-scale" parameters will be very different in the crust and in the mantle. Thus, depending upon the relative thickness of the crust and mantle in the deforming lithosphere, the role of each mechanism will have more or less important consequences on strain localisation. This complexity sometimes leads to disregard of experimental parameters in large-scale thermo-mechanical models and to use instead ad hoc "large-scale" numbers that better fit the observed geological history. The goal of the ERC RHEOLITH project is to associate to each tectonic process the relevant rheological parameters depending upon the scale considered, in an attempt to elaborate a generalized "Preliminary Rheology Model Set for Lithosphere" (PReMSL), which will cover the entire time and spatial scale range of deformation

Modélisations numériques thermo-mécaniques de subduction continentale

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocFONTAINEBLEAU-MINES ParisTech (771862302) / SudocSudocFranceF