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

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

Modelling of compression and extension of the continental lithosphere: towards rehabilitation of the necking-level model

International audienceWe present a dynamic model of continental lithosphere deformation under extension or compression, focusing on the role of an effective mechanical parameter called "necking level" or "necking depth", a widely used concept in basin modelling studies. Though it has generally been assumed that "necking depth" depends strongly upon the rheological structure of the lithosphere (especially the depth distribution of its strong layers), such a dependency has never been demonstrated. Our model, which accommodates small deformations of a thin inhomogeneous plate induced by in-plane as well as by mantle boundary forces (applied to the model sides and base, respectively), shows that "necking depth" is a function of the horizontal position and depends mainly on the relative thicknesses and strengths of the rigid layers in the uppermost crust and below the Moho. Using different yield strength envelopes we demonstrate that the final structure of the lithosphere formed as a result of deformation and its consequent isostatic adjustment can be closely approximated by a model with a flat necking level. In the process of extension and compression of the continental lithosphere all boundaries, including the topographic surface and the Moho, deform. As a result, the total disturbance of the isostatic equilibrium state (specified as a load) is only a part of the topographic weight. Estimates of the correct load can be made using the depth to the necking level inferred from lithosphere structure, composition and thermal state. The final topography of lithospheric interfaces depends on both necking depth and effective flexural rigidity. Any attempt to estimate simultaneously strain distribution, necking depth and effective flexural rigidity, however, represents an ill-posed problem and is not possible without reliance upon strong independent assumptions constraining lithosphere structure

Contrasting geophysical and geochemical signatures of a volcano at the axis of the Wharton Fossil Ridge (N-E Indian Ocean)

International audienceThe Styx volcano is a prominent seamount (20-30 km diameter, 2500 m high) located at the axis of the Wharton fossil spreading center (N-E Indian Ocean), where seafloor spreading stopped 40 Ma ago. Gravity modeling shows that it was emplaced on a weak lithosphere, in agreement with an on axis origin when seafloor spreading was active. The rocks dredged at the summit of the volcano are extreme typical alkaline basalts, indicating a deep enriched mantle source. We show here that most of this seamount was emplaced during the final stage of the seafloor spreading in the Wharton Basin. We then discuss how alkaline magmatism can be emplaced at the summit of the Styx

Tensor deconvolution: A method to locate equivalent sources from full tensor gravity data

International audienceWe present a method dedicated to the interpretation of full tensor ͑gravity͒ gradiometry ͑FTG͒ data called tensor decon-volution. It is especially designed to benefit from the simultaneous use of all the FTG components and of the gravity field. In particular, it uses tensor scalar invariants as a basis for source location. The invariant expressions involve all of the independent components of the tensor. This method is a ten-sor analog of Euler deconvolution, but has the following advantages compared to the conventional Euler deconvolution method: ͑1͒ It provides a solution at every observation point, without the use of a sliding window. ͑2͒ It determines the structural index automatically; as a consequence, the structural index follows the variations of the field morphology. ͑3͒ It uses all components of the measured full gradient tensor and gravity field, thus reducing errors caused by random noise. It is based on scalar invariants that are by nature insensitive to the orientation of the measuring device. We tested our method on both noise-free and noise-contaminated data. These tests show that tensor solutions cluster in the vicinity of the center of causative bodies, whereas Euler solutions better outline their edges. Hence, these methods should be combined for improved contouring and depth estimation. In addition , we use a clustering method to improve the selection of solutions, which proves advantageous when data are noisy or when signals from close causative bodies interfere

Seismicity of the Sunda Strait: Evidence for crustal extension and volcanological implications

International audienceThe Sunda Strait is located in the transitional zone between two different modes of subduction: the Java frontal subduction and the Sumatra oblique subduction. This setting implies that the Sunda Strait region is a key to the understanding of the geodynamic processes involved. In order to study the shallow seismicity, a microearthquake survey was carried out in that region. Twelve stations, accurately located by the aim of satellite positionning, recorded about 300 local events in the summer 1984. From this set, 174 shallow earthquakes have been precisely located. The results of this study reveal that the crustal earthquakes in the Sunda Strait area occurs in three main areas: (1) beneath the Krakatau complex, where earthquakes are generated by double-couples and are of tectonic origin; (2) inside a graben in the western part of the strait; and (3) in a more diffused zone to the south of Sumatra. The individual and composite focal mechanisms from the events inside the strait show an extensional regime. A stress tensor, which have been deduced from the individual focal mechanisms of earthquakes of the Krakatau group shows that the tensional axis is oriented N130øE. This study confirms that the Sunda Strait is in an extensional tectonic regime as a result of the northwestward movement of the Sumatra sliver plate along the Semangko fault zone

Aftershock sequence of the 1994, Mw 6.8, Liwa earthquake (Indonesia): seismic rupture process in a volcanic arc

International audienceWe present the aftershock activity following the February 15, 1994, Mw 6.8 earthquake which was strongly felt in southern Sumatra, Indonesia, near the Great Sumatran Fault (GSF). At this place, the slip rate is supposed to be low; neverthless, three M>6 events occurred along this segment during this century. No significant instrumental microseismi-city has ever been recorded there. We use data from both the regional Indonesian network and a local seismic array operating 11 days after the mainshock during one month. Aftershocks mostly locate in a broad zone of 55x20 km 2 near two active NW-trending strike-slip segments of the GSF separated by a recent caldera, Suwoh. During the experiment, the NW segment (from Suwoh up to 15 km SE of the Ranau lake caldera, an old right-stepover pull-apart) was very active. As first suggested by the aftershock distribution and the lack of coseismic rupture at the surface, the 20 focal mechanisms determined provide evidence for various post-seismic stress adjustments on secondary faults located in the Ranau-Suwoh paleo-pull-apart graben. Less than 20% of the aftershocks are directly linked to the main rupture, a nearly pure right-lateral strike-slip faulting reaching 25 km depth. A narrow seismic gap underlines the active volcanic area of Suwoh. We conclude that the rupture process along the GSF is controlled both by volcanism and structures, and that the volcanic activity affects the mechanical properties of the crust only in a narrow zone