Marc Daignieres

A simple parameterization of strain localization in the ductile regime due to grain size reduction: A case study for olivine

We propose a simple parameterization of the transition between dislocation creep and grain-size-sensitive creep under conditions characteristic of the lithospheric mantle and derived from the results of laboratory experiments on olivine-rich rocks. Through numerical modeling and linear stability analysis, we determine the conditions under which this transition takes place and potentially leads to strain localization. We pay particular attention to the effect of cooling rate and strain rate which are likely to be dominant parameters in actively deforming tectonic areas. We conclude that at constant temperature, strain localization can only take place if the rheology of the material is nonlinearly related to grain size; that strain localization is facilitated by syndeformation cooling; that there is only a narrow region in the strain rate versus cooling rate parameter space where localization is likely to take place; and that grain growth inhibits strain localization at fast cooling rates but may lead to “grain growth localization” at low cooling rates. We draw attention to the potential consequences of our analysis of strain localization for the style of plate motions at the Earths surface.

Do faults trigger folding in the lithosphere

A number of observations reveal large periodic undulations within the oceanic and continental lithospheres. The question if these observations are the result of large-scale compressive instabilities, i.e. buckling, remains open. In this study, we support the buckling hypothesis by direct numerical modeling. We compare our results with the data on three most proeminent cases of the oceanic and continental folding-like deformation (Indian Ocean, Western Gobi (Central Asia) and Central Australia). We demonstrate that under reasonable tectonic stresses, folds can develop from brittle faults cutting through the brittle parts of a lithosphere. The predicted wavelengths and finite growth rates are in agreement with observations. We also show that within a continental lithosphere with thermal age greater than 400 My, either a bi-harmonic mode (two superimposed wavelengths, crustal and mantle one) or a coupled mode (mono-layer deformation) of inelastic folding can develop, depending on the strength and thickness of the lower crust.

Prediction of faulting from the theories of elasticity and plasticity: what are the limits?

Abstract Elasticity, rigid-plasticity and elasto-plasticity are the simplest constitutive models used to describe the initiation and evolution of faulting. However, in practice, the limits of their application are not always clear. In this paper, we test the behaviour of these different models using as examples tectonic problems of indentation of a die, compression with basal shear, bending of a plate and normal faulting around a dike. By comparing the results of these tests, we formulate some guidelines that may be useful for the selection of an appropriate constitutive model of faulting. The theory of elasticity reasonably predicts the initiation of the fault pattern but gives erroneous results for large strains. The theory of rigid-plasticity is more appropriate for large deformations, where the geometry of faults can be found by the method of characteristics. This method works well for zones of failure that are not severely constrained by elastic material outside e.g. when faults are connected to the free-surface, a viscous substratum or a zone of weakness. Non-associated elasto-plasticity is the most complete theory among those considered in this paper. It describes the evolution of faults from the initiation of localized deformations to the formation of a complicated fault network.

Evidence for a 20° tilting of the Earth’s rotation axis 110 million years ago

True polar wander (TPW), the shift of the Earths rotation axis with respect to the entire globe, is most probably due to mass redistribution in the Earths mantle as a result of convection. Using a new rigorously selected palaeomagnetic database gathering only directions obtained from magmatic rocks, we find that TPW has been clearly intermittent over the last 200 Ma with two long periods of strict standstill from the present to 80 Ma and from approximately 150 to 200 Ma. A single period of shifting is observed, between 80 and about 150 Ma ago. This period culminates around 110 Ma ago in an 20° abrupt tilting during which an angular speed exceeding 5°/Ma (0.5m/yr) may have been reached. Assuming that the time-averaged geomagnetic field is axial, our results indicate that the changes in the position of the rotation axis, and therefore in the inertia tensor of the Earth are intermittent. We suggest that a major reorganization of the mass distribution in the Earths mantle occurred in the Lower Cretaceous. This event, concomitant with plume hyperactivity at the Earths surface and probable drastic changes at the core/mantle boundary attested by the inhibition of geomagnetic reversals, suggests unmixing of upper and lower mantle by avalanching of upper mantle material down to the core/mantle boundary. The astonishingly strict stability of the time-averaged position of the rotation axis before and after this episode of shifting implies the existence of some steady convection which does not modify the large scale distribution of mass within the mantle. Given the intermittence of mantle avalanching, we suggest that these long periods of stability correspond to the temporary reestablishment of a basically two-layered convection system within the mantle.

A model for the evolution of the axial zone of mid-ocean ridges as suggested by icelandic tectonics*

Abstract In Iceland tectonic activity in the neovolcanic zone leads to the formation of three kinds of parallel structures: open fissures, emissive fissures, and normal faults. This observation is used to built a kinematic model which is based on the superposition of lava flows generated from an active central belt of finite width. The results are in good agreement with the recent results in magnetism and tectonic observations of both subaerial and underwater active ridges.

Comment on ''Stability of the Earth with respect to the spin axis for the last 130 million years'' by J.A. Tarduno and A.Y. Smirnov (Earth Planet. Sci. Lett. 184 (2001) 549^553)

In a recent issue of Earth and Planetary Science Letters, Tarduno and Smirnov [1] claim that no true polar wander (TPW) occurred over the last 130 Ma. This claim is in contradiction with the conclusion of several studies devoted to this topic [2^8], including our recent EPSL paper [9]. In this paper [9] we showed that some 110 þ 10 Ma ago the Earth’s spin axis shifted by 18 þ 4‡ (at a rate larger than 1‡/Ma, or possibly even larger than 5‡/ Ma) with respect to the main hotspots presently located in the Atlantic and Indian oceans. We interpreted this shift as a displacement of the Earth’s spin axis with respect to the global solid Earth (i.e. TPW). Tarduno and Smirnov [1] contest this conclusion and a⁄rm that TPW is an artifact. Their argument relies on the fact that a few North American paleomagnetic poles not included in our selection do not ¢t the trends predicted by two simplistic models. However, we show below that these additional poles very nicely ¢t our TPW model. Tarduno and Smirnov’s reasoning [1] is based upon the consideration of seven North American poles labeled A^G (¢gure 2 in [1]). Note that Pre¤vot et al.’s whole database [9] is now available through anonymous FTP at ftp.dstu.univmontp2.fr ; use your email address as the password and type cd pub/paleomag/epls2000. Our own selection criteria, discussed in detail in [9], are particularly stringent, which led us to reject, in a completely objective manner, three of the seven poles of Tarduno and Smirnov: especially we rejected poles B, D and G on the basis of too large Fisher concentration parameter (U), poles obtained mainly from syenitic intrusions (not granites, as mentioned in [1]), and Sierra Nevada batholith, respectively. In particular, Sierra Nevada obviously is not part of the North American craton, and this pole cannot be considered a reliable datum for the present purpose. We list in Table 1 the selected poles of Pre¤vot et al. [9] for the period between approximately 80 and 130 Ma. Tarduno and Smirnov [1] believe they can check the reality of the TPW shift observed around 110 Ma [9] by comparing, in the 40 Ma long bounding interval between 90 and 130 Ma, the paleolatitudes computed from these seven paleomagnetic poles with the paleolatitude curves predicted from two distinctly diierent models: (i) Model 1 (curve 1 in the present Fig. 1) assumes that the North American plate moved with respect to the hotspot system as described by Muller et al. [10] while the Earth’s rotation axis remained in its present posi-