Ph. Laurent

Variscan intracontinental deformation: The Coimbra—Cordoba shear zone (SW Iberian Peninsula)

The Coimbra—Cordoba shear zone is a major lineament roughly parallel to the NW—SE Variscan structures of the southern Iberian Peninsula. Inside the shear zone evidence for a non-coaxial strain path exists from structural, microstructural and lattice-preferred orientation data on quartz. Plane strain is suggested by the study of microstructures in the (XY) and (YZ) sections in all formations occurring in the shear zone. With increasing strain, a mylonitic series develops. The deformation, contemporaneous with epizonal to mesozonal metamorphism, is responsible for: 1. (1) the regional subvertical foliation oblique to the shear zone 2. (2) the well defined subhorizontal lineation striking NW—SE 3. (3) the discrete planes of shear roughly parallel to the shear zone. The data are interpreted as resulting from a left-lateral slip motion with a displacement of at least 72 km. In the light of plate tectonics it is inferred that the Coimbra—Cordoba lineament could have been a suture zone which evolved into an intracontinental left-lateral shear zone during the Variscan orogeny of the Ibero-Armorican arc.

Search for variations of fundamental constants using atomic fountain clocks.

Over five years, we have compared the hyperfine frequencies of 133Cs and 87Rb atoms in their electronic ground state using several laser-cooled 133Cs and 87Rb atomic fountains with an accuracy of approximately 10(-15). These measurements set a stringent upper bound to a possible fractional time variation of the ratio between the two frequencies: d/dt ln([(nu(Rb))/(nu(Cs))]=(0.2+/-7.0)x 10(-16) yr(-1) (1sigma uncertainty). The same limit applies to a possible variation of the quantity (mu(Rb)/mu(Cs))alpha(-0.44), which involves the ratio of nuclear magnetic moments and the fine structure constant.

Strain analysis of a shear zone in a granodiorite

Abstract A ductile shear zone in a late Precambrian granodiorite, from the Rouergue (southwest part of the French Massif Central) has been studied. A single episode of deformation is responsible for the formation of a foliation and a well-defined lineation which are localized into an elongated zone, a few decimeters wide. The strain features can be attributed to a simple-shear mechanism (Ramsay and Graham, 1970), so that the main parameters of the deformation are defined. At stages of increasing deformation, the quartz isotropic sub-fabric of the undeformed host rock is progressively transformed into an anisotropic fabric composed of a single oblique girdle while the subgrain size progressively decrease and the dislocation density remains constant. It is suggested that the gliding planes of quartz are the basal plane (0001) and a predominant prismatic plane 1010 the slip directions may be 〈a〉 for both glide-planes. The results obtained in this investigation provide a basis for a high voltage electron microscope (H.V.E.M.) study which shows that the fabrics development may be related to dislocation processes. The difference of strain rates in the host rock and in the shear zone is calculated from the dislocation microstructures.

A cesium fountain frequency standard: preliminary results

Laser cooling of atoms has opened up new possibilities in the field of atomic frequency standards. A cesium atomic fountain, first proposed by Zacharias in 1953, is now feasible: the atoms, first cooled by six laser beams, are launched upward using laser light, pass once through a microwave cavity, continue their ballistic flight and then fall through the same cavity. The long time between the two microwave interactions leads to a Ramsey resonance much narrower than in conventional Cs clocks using thermal atomic beams. The stability and accuracy of such a cesium fountain are very attractive. The use of diode lasers to cool, launch and detect cesium atoms in a low cesium pressure cell allows the construction of a simple and reliable atomic fountain frequency standard. A fountain frequency standard is now in operation at LPTF. A Ramsey resonance as narrow as 0.8 Hz has been obtained. A few days of continuous operation are routinely obtained. In closed loop operation the fountain frequency standard is continuously monitored against a H maser allowing an evaluation of the accuracy of the device. The short-term frequency stability is about 5.10/sup -13/ /spl tau//sup - 1/2 / limited only by the frequency noise of the microwave source. We intend to present a preliminary evaluation of this new standard with a discussion of the major systematic effects which determine the accuracy. The expected accuracy is expected to be at 10/sup -14/ level. In addition, a description of the whole design of the cesium fountain is presented. >

Determination of deviatoric stress tensors based on inversion of calcite twin data from experimentally deformed monophase samples: preliminary results

Abstract Following previous successful (paleo)stress determinations from numerically generated aggregates and naturally deformed samples, preliminary results about stress reconstructions based on inversion of calcite twin data from experimentally deformed monophase samples are reported. This new study carried out on samples of Carrara marble and crinoidic limestone from eastern France provides a test of the validity of stress reconstructions using inverse methods, which had not been properly calibrated before. The percentages of correlated twins are discussed for each stress tesnor, and the range of uncertainties on the determination of stress orientations and differential stress magnitudes is evaluated.

Evolution of the stress field pattern in the south of the Rhine Graben from the Eocene to the present

Abstract A stress field evolution from the Eocene to the present is proposed for the southern part of the Rhine Graben. Stress tensor determinations are supported by striated microfaults and calcite twin lamellae analyses. Both methods lead to the determination of: 1. (a) an E-W extension contemporaneous with the opening of the graben, and 2. (b) a N-S compressive event which may be related with Alpine tectonics. It is shown that a N-S compressive event occurred prior to the Oligocene extension. A permutation between the σ1 and σ2 axes is proposed to explain the transition between compression and extension. A σ 1 σ 2 inversion during the Oligocene is compatible with compression in the southern part of the Rhine Graben infilling (horizontal σ1), and extension (vertical σ1) more to the North. During the Miocene, the Jura emplacement corresponds to a N-S compression in this area. There is no significant variation of the σ1 direction south of the Rhine Graben from the Eocene to the Late Miocene. The σ1 direction only turns to a N140° direction during the Pliocene or the Quaternary.

BNM-SYRTE fountains: recent results

This paper describes several recent improvements of the BNM-SYRTE fountain ensemble. A new method for controlling the cold collision shift with improved accuracy has been proposed and demonstrated. A thorough investigation of some cold collision properties of 133Cs is presented, including the observation of molecular Feshbach resonances. Finally, a new microwave synthesis scheme based on a fully operational cryogenic oscillator is presented. With this, a fractional frequency instability below 2 times 10-14tau-frac12 is obtained routinely

Determination of principal stress magnitudes using calcite twins and rock mechanics data

Abstract This paper proposes a new method for determining principal stress magnitudes, based on the use of the deviatoric stress tensor determined from computer inversion of calcite twin data. Additional evaluation of the vertical stress (weight of overburden) or of the mean stress is enough to calculate the complete stress tensor. We emphasize that the best reasoning consists of combining calcite twin analysis with rock mechanics data (failure/friction criteria), because the determination of the six parameters of the absolute stress tensor is overconstrained.

Joint analyses of calcite twins and fault slips as a key for deciphering polyphase tectonics: Burgundy as a case study

Abstract Determination of regional paleostresses based on calcite twin analysis has been successfully applied for the first time in the Burgundy platform, France. Paleostress tensors obtained with this method are internally consistent along the profile studied. Moreover, the same directions of paleostresses are independently inferred from fault striation analysis. The results clearly show that the late Mesozoic-Cenozoic evolution of the area has involved polyphase deformation, including 1. (1) NNE-SSW extension, probably late Mesozoic in age, 2. (2) N-S compression related to a major orogenic event in the Pyrenees and Provence, late Eocene in age, 3. (3) E-W extension related to the Oligocene rifting event (the Rhine-Saone rift system), 4. (4) WNW-ESE compression related to the Late Miocene westward thrusting of Jura. Calcite twin and striated fault analyses are two complementary tools for paleostress determination: their combined application will allow accurate mapping of paleostress trajectories in intraplate tectonics setting. Moreover, calcite twins can allow stress and paleostress determination when macroscopic features are not observable, which is often the case in very weakly deformed areas and in drill holes.

Determining paleostress orientations from faults and calcite twins: a case study near the Sainte-Victoire Range (southern France)

Abstract This paper presents the results of the analyses of striated faults and calcite twins near a highly deformed, polyphase range, the Sainte-Victoire Mountain (southern France). We thus show that combining both paleostress indicators provides a reliable way to reconstruct polyphase tectonic evolution involving homoaxial compressional stresses. From a regional point of view, the paleostress orientations have been accurately determined. These orientations correspond to three main stages of the stress field evolution since the late Cretaceous: N–S compression (late Cretaceous to late Eocene), E–W extension (Oligocene) and ENE-WSW compression (early Miocene). During the Pyrenean N–S compression, several substages of folding, strike-slip or reverse faulting have been detected. These early tectonic events account for compressional deformations older than the late Eocene major shortening of the Provence sedimentary cover. From the methodological point of view, tectonic investigations based on both paleostress indicators (macroscopic and microscopic) were consistent and complementary. Calcite twins often allowed determination of paleostress tensors in sites where microfaults are absent for a given tectonic event. The greater sensibility of twinning to stress is related to the lower differential stress required to activate twin gliding, compared with brittle failure. Despite local stress deviations related to structural inhomogeneities, the regional stress field remains homogeneous, and provides a basis for interpreting deformation at all scales.