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

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

Structural trends in the Southern Cook and Austral archipelagoes (South Central Pacific) based on an analysis of SEASAT data : geodynamic implications

Cette étude permet de caractériser deux directions structurales présentes dans les archipels des Iles Australes et Cook. Dans le cadre de la tectonique globale, les auteurs en discutent les implications géodynamiques

Exploring Global Connections through Art

What role can works of art play in encouraging perspective-taking and exploring issues of global significance? How can thinking routines, developed by Harvard Project Zero, support rich conversations around works of art and global topics? In this interactive workshop, teachers will engage deeply with a work of art, using a combination of discussion and creative writing. They will deepen their understanding of art, history, and broader issues that affect them, their communities, and the world. Participants will practice using thinking routines to investigate the work of art and global themes. (Thinking routines are pedagogical tools that support and encourage key thinking moves, like reasoning with evidence, leading to understanding). Creative writing will be used as a tool to help participants consider various perspectives within and around the work of art. There will be time at the end to reflect on and debrief the participants’ experience and to consider classroom application

Influence of Some Rheological Parameters On Flexure of the Oceanic Lithosphere

International audienceS U M M A R Y Rheological parameters describe properties of the lithosphere, but it is generally impossible to measure them in situ. Thus a model has to be developed to relate parameters to observables. We consider flexure of the oceanic lithosphere. A quantitative representation of a brittle-elastic-ductile rheology is based on a semi-empirical model. The non-linear problem of plate flexure is solved by the multiple-shooting algorithm using a Runge-Kutta procedure from analytical solutions for an elastic model. The bending moment and its derivatives have been deduced analytically to improve the accuracy and speed of computations. A family of numerical solutions was built that describes flexure of the oceanic lithosphere at subduction zones for various ages, strain rates, and activation energies. Relative contributions of rheological parameters are discussed. The model is then used to evaluate rheological parameters of the Kuril trench from gravity data. The strain rate estimates are found to be about 3. 10-('6*1)s-1 assuming creep activation energies between 520 kJ/mole and 550 kJ/mole for lithospheric age ranging from 90 Ma to 110 Ma

Continuous gravity recording with Scintrex CG-3M meters: a promising tool for monitoring active zones

International audienceWe acquired continuous series of microgravity measurements using several Scintrex CG-3M gravity meters for several weeks in 1997. The meters with 1 mGal resolution were installed side by side in a stable reference station at the ORSTOM research centre to perform identical data acquisition. We present and compare the instrumental responses obtained for the various gravity meters (measurement series of gravity field, standard deviation, internal temperature, tilts) and analyse their correlation with simultaneous recordings of meteorological parameters. The data have been processed in order to (1) establish the mid-to long-term relative stability and the accuracy of the instruments, (2) estimate the contribution of instrumental effects to gravity data measurements and (3) quantify the amplitude of the time variations of the gravity field that might be detected with such instruments. This study emphasizes the sensitivity of some instrumental responses of the Scintrex CG-3M gravity meters (such as internal temperature or tilt) to local atmospheric-pressure variations. This sensitivity can lead to non-negligible perturbations of the gravity measurements through automatic corrections applied in real-time mode by the integrated software. We show that most of these instrumental artefacts can be easily removed in data post-processing by using simultaneous atmospheric-pressure data. After removal of an accurate Earth tide model, the instrumental drift and the instrumental effects, the temporal series are compared by computing differential signals. These residual signals obtained over a period of several weeks exhibit the following characteristics: (1) the gravity residuals have a maximum amplitude ranging from 5 to 10 mGal and from 10 to 15 μGal for filtered and unfiltered data, respectively; and (2) the standard error, tilts and internal temperature measurements of the various gravity meters are very consistent; their respective residual amplitudes are ±2 mGal, ±3 arcsec and ±0.05 mK. In order to calibrate the gravity meters precisely in the measurement range used in this study, we have measured a calibration line established in the framework of the fourth intercomparison of absolute and relative gravity meters. This calibration was achieved with an accuracy of 5 μGal. This result is consistent with other field tests already performed with such gravity meters. In addition, we also checked the accuracy of the tilt sensors by increasing the electronic read-out by a factor of 10. The tilt response of the whole gravity meter to a small induced inclinometric variation indicates that the precision of the tilt measurements is about a few tenths of an arc second. This study reveals that temporal variations of the gravity field could potentially be detected in the field with an accuracy of about 5–15 mGal by permanent networks of Scintrex CG-3M gravity meters set up a few kilometres apart. This result is of particular interest in field surveys of temporal gravity changes related to some environmental or geodynamical processes, where the expected gravity variations are greater than a few tens of mGal. In particular, in volcanological applications, the continuous monitoring of active volcanoes with such permanent networks of gravity meters co-located with subcentimetre-accuracy GPS receivers should be very helpful to understand internal magmatic processes better and to detect possible gravity and inclinometric signals occurring during pre-eruptive phases. In this field, continuous microgravity recordings associated with classical reiteration networks will probably improve hazard mitigation in the near futur