The effective elastic thickness (Te) of continental lithosphere: What does it really mean?

International audienceIt is well accepted that the lithosphere may exhibit nonzero mechanical strength over geological time and space scales, associated with the existence of non-lithostatic (deviatoric) stress. The parameter that characterizes the apparent strength of the lithosphere is the flexural rigidity D, which is commonly expressed through the effective elastic thickness (Te) of the litho-sphere. Estimates of Te for oceanic lithosphere approximately follow the depth to a specific iso-therm (-600°C), which marks the base of the mechanical lithosphere. The physical meaning and significance of the effective elastic thickness for continents are still enigmatic, because for continental lithosphere estimates of Te bear little relation to specific geological or physical boundaries. Although high observed values of Te (70-90 km for cratons) can be partly explained by the present day temperature gradients, the low values (10-20 km), in general, cannot. In addition, the elastic plate models are self-inconsistent in that they mostly predict intraplate stresses high enough to lead to inelastic (brittle or ductile) deformation, according to data of rock mechanics. To provide a basis for a physically consistent unified interpretation of the observed variations of Te for continental and oceanic lithosphere, we developed an analytical and numerical approach that allows direct treatment of Te in terms of the lithospheric rheology, thermal structure, and strain/stress distribution. Our technique is based on finding true inelastic and equivalent (effective) elastic solutions for the problem of deformation of the lithosphere with realistic brittle-elasto-ductile rheology. We show that the thermal state (thermotectonic age) of the lithosphere is only one of at least three equally important properties that determine apparent values of Te. These other properties are the state of the crust-mantle interface (decoupling of crust and mantle), the thickness and proportions of the mechanically competent crust and mantle, and the local curvature of the plate, which is directly related to the bending stresses. The thickness of the mechanically competent crust and the degree of coupling or decoupling is generally controlled by composition of the upper and lower crust, total thickness of the crust, and by the crustal geotherm. If decoupling takes place, it permits as much as 50% decrease of Te, compared with Te implied from conventional thermal profiles. Comparison of the theoretically predicted Te with inferred values for different regions suggests that the lower crust of most continental plates has a low-temperature activation rheology (such as quartz) which permits crust and mantle decoupling. The curvature of the plate depends on the theological structure and on the distribution of external loads applied to the plate (e.g., surface topography, sediment fill, and plate-boundary forces). Bending stresses created by major mountain belts are large enough to cause inelastic deformation (brittle failure and a ductile flow) in the underlying plate, which, in turn, leads to a 30 to 80% decrease of Te beneath such belts and less beneath the adjacent regions. The boundary forces and moments (e.g., due to the slab pull, etc.) lead to more localized but even stronger reductions in Te (e.g., plate necking in subduction zones). Our approach provides a feedback between the "observed" Te and rheology, allowing to constrain the lithospheric structure from estimates of Te

Flexure of the continental lithosphere with multilayered rheology

International audienceIn this paper, a model of flexure of the continental lithosphere is derived taking into account crustal and mantle rheologies. Bending of the continental lithosphere is modelled with a double yield stress envelope: three layers (brittle, elastic and ductile) for the crust and three analogous layers for the mantle portion. The deformation of the layers is controlled by the rheological properties of quartz-rich crustal rocks and olivine-rich mantle rocks. The influence of various factors such as the depth of Moho, strain rates, thermal structure of the lithosphere, boundary conditions, and topographic load, is examined. Results show that the mechanical strength of the continental lithosphere in the horizontal and vertical directions is primarily controlled by the present thermal structure of the plate, boundary forces and moments, and the applied topographic load. This explains why mountainous regions may be more locally compensated than adjacent regions. We also thus are able to explain why many continental plates have apparent effective rigidities much smaller than those predicted on the basis of their geological ages. The model is then applied to the Tien Shan-Tarim area (Central Asia), and original topography and gravity data are used to constrain parameters of the model. We found that the model satisfactorily matches the data and is also able to predict the thermal state of the plate and the location of the deep seismicity

A broken plate beneath the North Baikal rift zone revealed by gravity modelling

International audienceWe modelled a 1200 km long gravimetric profile in the North Baikal rift to assess the mechanical behaviour of the lithosphere, using a numerical model that accounts for realistic brittle-elasto-ductile rheology. We use published seismicity and re-fraction data, a new 5'x7.5' free-air/Bouguer gravity and topography data set, and a detailed map of faults obtained from high resolution SPOT imagery. Analysis of the gravity field over the North Baikal rift zone indicates significant asymmetry of the mechanical processes governing the deformation of the diverging sides of the rift. These anomalies cmmot be explained by a conventional continuous plate undergoing extension beneath the rift zone, whereas a strong mechanical discontinuity (wedge shaped detachment zone beneath the rift axis) is able to reproduce observations. Such a discontinuous model provides a good fit to the gravity and crustal thickness data and explains the deep seismicity reported there

Lithospheric folding and sedimentary basin evolution: a review and analysis of formation mechanisms

Lithospheric folding is an important mode of basin formation in compressional intraplate settings. Basins formed by lithospheric folding are characterized by distinct features in subsidence history. A comparison with extensional basins, foreland basins, intracratonic basins and pull-apart basins provides criteria for the discrimination between these modes of basin formation. These findings are important in deciphering the feedbacks between tectonics and surface processes. In addition, inferences on accommodation space and thermal regime have important consequences for hydrocarbon maturity. Lithospheric folding is coupled to compressional reactivation of basins and faults, and therefore, strongly affects reservoir characteristics of sedimentary basins. © 2010 The Authors. Basin Research © 2010 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists

Ductile crustal flow in Europe's lithosphere

Potential gravity theory (PGT) predicts the presence of significant gravity-induced horizontal stresses in the lithosphere associated with lateral variations in plate thickness and composition. New high resolution crustal thickness and density data provided by the EuCRUST-07 model are used to compute the associated lateral pressure gradients (LPG), which can drive horizontal ductile flow in the crust. Incorporation of these data in channel flow models allows us to use potential gravity theory to assess horizontal mass transfer and stress transmission within the European crust. We explore implications of the channel flow concept for a possible range of crustal strength, using end-member 'hard' and 'soft' crustal rheologies to estimate strain rates at the bottom of the ductile crustal layers. The models show that the effects of channel flow superimposed on the direct effects of plate tectonic forces might result in additional significant horizontal and vertical movements associated with zones of compression or extension. To investigate relationships between crustal and mantle lithospheric movements, we compare these results with the observed directions of mantle lithospheric anisotropy and GPS velocity vectors. We identify areas whose evolution could have been significantly affected by gravity-driven ductile crustal flow. Large values of the LPG are predicted perpendicular to the axes of European mountain belts, such as the Alps, Pyrenees-Cantabrian Mountains, Dinarides-Hellenic arc and Carpathians. In general, the crustal flow is directed away from orogens towards adjacent weaker areas. Gravitational forces directed from areas of high gravitational potential energy to subsiding basin areas can strongly reduce lithospheric extension in the latter, leading to a gradual late stage inversion of the entire system. Predicted pressure and strain rate gradients suggest that gravity driven flow may play an essential role in European intraplate tectonics. In particular, in a number of regions the predicted strain rates are comparable to tectonically induced strain rates. These results are also important for quantifying the thickness of the low viscosity zones in the lowermost part of the crustal layers. © 2011 Elsevier B.V

A new thermal and rheological model of the European lithosphere

We present a new thermal and rheological model of the European lithosphere (10°W-35°E; 35°N-60°N), which is based on a combination of recently obtained geophysical models. To determine temperature distribution we use a new tomography model, which is principally improved by an a-priori correction of the crustal effect, by using EuCRUST-07, a new digital model of the European crust. The inversion approach is similar to those used in previous studies, but the employment of a more robust tomography model essentially improves the result. The uppermost mantle under western and central Europe is mostly characterized by temperatures in a range of 900°-1100 °C, with the hottest areas corresponding to the basins, which have experienced recent extension (e.g. Tyrrhenian Sea and Pannonian Basin). By contrast, the mantle temperatures under eastern Europe are about 550°-750 °C at the same depth and the minimum values are found in the northeastern part of the study area. The new temperature estimates are used to trace the lithosphere-asthenosphere thermal boundary, as a depth of the isotherm of 1200 °C. The lithospheric thickness is less than 100 km beneath the hottest part of western and central Europe, while the maximum values are observed beneath the East European Platform (200-230 km), the Alps and the Dinarides-Hellenic Arc (150-180 km). EuCRUST-07 and the new thermal model are used to calculate the strength distribution within the European lithosphere. Differently from previous estimates, the new model adopts lateral variations of lithology and density, which are derived from the crustal model. According to these estimates, in western and central Europe the lithosphere is more heterogeneous than in eastern Europe, the latter being generally characterized by higher strength values. These strength variations are also in a good agreement with other geophysical characteristics of the lithosphere such as residual mantle gravity anomalies. © 2009 Elsevier B.V. All rights reserved