Noise limitations in the core-mode band of superconducting gravimeter data

In recent years, several claims of detection of weak harmonic signals in the sub-tidal band of frequencies of high-quality gravimeter data have been made. Here, we review our attempts at confirming Smylie's claim of detection of the Slichter mode of inner-core oscillation using the same four data sets he and his colleagues used. We further examine the homogeneous 2 year data sets obtained from the superconducting gravimeters sited in Strasbourg and at Cantley, Quebec. We show that the power spectra of tidal-reduced, pressure and slew-corrected gravity residuals from these data are indistinguishable from a random walk process except in narrow bands dominated by residual earth tides and by harmonics of the diurnal thermal-atmospheric tide. Such a random ‘brown’ residual could result from mechanical instabilities and electronic noise in the instruments themselves, from site-specific tectonic noise, from local hydrological variations and pier instabilities or it could derive from unaccounted for variability in the atmospheric pressure in the vicinity of the instruments. We argue that the homogeneous 2 year data sets show no evidence whatever of Slichter harmonics even though our preprocessing methods reduce ‘apparent noise levels’ in the core-mode band by almost an order magnitude in comparison with the four data sets originally employed by Smylie et al. and, in their first attempted confirmation of his claimed discovery, by Jensen et al.     

Late holocene level fluctuations of the Lake Ilay in Jura,France: sediment and mollusc evidence and climatic implications

The level fluctuations of Lake Ilay, Jura (France) during the last three millennia are reconstructed from sedimentological and malacological analyses of a core that is well-dated by tree-ring, radiocarbon and pollen datings. Changes in sediment facies, in carbonate concretion assemblages and in mollusc assemblages highlight a major lowering phase atc. 1550 BP and minor lowering phases atc. 2800 BP and shortly before AD 1394. Rises in lake level developed during the early Subatlantic and betweenc. 1500 and 1000 BP. These data are in good agreement with other proxy data from higher European and American latitudes. These correlations support the climatic origin of the level fluctuations of the Lake Ilay during the late Holocene. They suggest that the mediaeval climatic optimum is centred rather in the early than the late Middle Age.     

Immature and mature transform zones near a hot spot: The South Iceland Seismic Zone and the Tjörnes Fracture Zone (Iceland)

We describe and compare the two transform zones that connect the Icelandic rift segments and the mid-Atlantic Ridge close to the Icelandic hot spot, in terms of geometry of faulting and stress fields. The E–W trending South Iceland Seismic Zone is a diffuse shear zone with a Riedel fault pattern including N0°–N20°E trending right-lateral and N60°–N70°E trending left-lateral faults. The dominant stress field in this zone is characterised by NW–SE extension, in general agreement with left-lateral transform motion. The Tjörnes Fracture Zone includes three major lineaments at different stages of development. The most developed, the Húsavík–Flatey Fault, presents a relatively simple geometry with a major fault that trends ESE–WNW. The stress pattern is however complex, with two dominant directions of extension, E–W and NE–SW on average. Both these extensions are compatible with the right-lateral transform motion and reveal different behaviours in terms of coupling. Transform motion has unambiguous fault expression along a mature zone, a situation close to that of the Tjörnes Fracture Zone. In contrast, transform motion along the immature South Iceland Seismic Zone is expressed through a more complicate structural pattern. At the early stage of the transform process, relatively simple stress patterns prevail, with a single dominant stress field, whereas, when the transform zone is mature, moderate and low coupling situations may alternate, as a function of volcanic–tectonic crises and induce changes in stress orientation.     

Motion near the 3/1 resonance of the planar elliptic restricted three body problem

The global semi-numerical perturbation method proposed by Henrard and Lemaître (1986) for the 2/1 resonance of the planar elliptic restricted three body problem is applied to the 3/1 resonance and is compared with Wisdom's perturbative treatment (1985) of the same problem. It appears that the two methods are comparable in their ability to reproduce the results of numerical integration especially in what concerns the shape and area of chaotic domains. As the global semi-numerical perturbation method is easily adapted to more general types of perturbations, it is hoped that it can serve as the basis for the analysis of more refined models of asteroidal motion. We point out in our analysis that Wisdom's uncertainty zone mechanism for generating chaotic domains (also analysed by Escande 1985 under the name of slow Hamiltonian chaotic layer) is not the only one at work in this problem. The secondary resonance _p = 0 plays also its role which is qualitatively (if not quantitatively) important as it is closely associated with the random jumps between a high eccentricity mode and a low eccentricity mode.     

The anatomy of an active slide: the Gascons rockslide,Québec,Canada

A railroad crosses the coastal Gascons rockslide, in Quebec, Canada. This study improves the understanding of the rockslide’s failure mechanism and post-failure behaviour responsible for the deformation in the railroad. The slide is an asymmetrical wedge failure of 410,000 m³, in rocks made up of centimetre-scale beds of nodular calcilutite alternating with sandstones and limestone. The post-failure stage of the rockslide is characterised by continuous movement of blocks with speeds ranging from 6 to 110 mm/year. The main water table is just below the sliding surface, but precipitation and snowmelt can raise it above the sliding surface in the upper part of the slide. We propose a model for the rockslide, its failure mechanism, geometry and hydrogeology to provide a baseline for interpreting the near-real-time data collected since 2009.     

Le facies granulite en Norvege Meridionale II. Les inclusions fluides

Liquid inclusions in quartz found in rocks of amphibolite and of granulite facies were studied. In the former water is predominant, in the latter CO₂ (probably juvenile). Estimates of PT conditions during metamorphism based on the study of inclusions gave values (800°C, 8 kb) comparable to those obtained by the petrological study (7–800 °C, 6–8 kb) presented in part I.     

The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny

Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized by Archaean and Palaeoproterozoic lithologies. Nearly 10 years ago, the Tuareg shield was shown to be composed of 23 displaced terranes [Geology 22 (1994) 641] whose relationships were deciphered in Aïr to the SE [Precambr. Res. 67 (1994) 59]. The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean/Palaeoproterozoic and Neoproterozoic lithologies.We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Egéré-Aleksod) belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early-Middle Neoproterozoic but was partly destabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogen.An early Pan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P–T–t paths have been established from>15 kbar––790 °C (eclogite) to 4 kbar––500 °C (greenschist retrogression) through 12 kbar––830 °C (garnet amphibolite) and 8 kbar––700 °C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm–Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobility of REE, particularly LREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite-facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean–Palaeoproterozoic basement was only locally affected by the Pan-African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust early onto LATEA: the Iskel island arc at ≈850 Ma to the west of LATEA, the Serouenout terrane in the 700–620 Ma age range to the east. No subduction-related magmas have intruded LATEA during this epoch, which behaved as a passive margin.During the main Pan-African phase (625–580 Ma), LATEA was dissected by mega-shear zones that induced several hundreds km of relative displacement and allowed the emplacement of high-K calc-alkaline batholiths. Smaller movements continued till 525 Ma, accompanied by the emplacement of subcircular plutons with alkaline affinity. Here is dated the Ounane granodiorite (624 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.70839 ± 0.00016; 6WR, MSWD=0.87) and the Tisselliline granite (552 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.7074 ± 0.0001; 5WR, MSWD=1.4). Nd isotopes indicate a preponderant Palaeoproterozoic crustal source for these two plutons: _Nd=−14 to −21 at 624 Ma and T_DM=1650–2320 Ma for Ounane and _Nd=−13 to −15 at 555 Ma and T_DM=1550–1720 Ma for Tisselliline. Our model links these intrusions to a linear lithospheric delamination along mega-shear zones, allowing the hot asthenosphere to rise, melt by adiabatic pressure release and inducing the melting of the Palaeoproterozoic and Archaean lower crust.The LATEA cratonic microcontinent remained however sufficiently rigid to preserve Archaean and Palaeoproterozoic lithologies as well as Middle Neoproterozoic oceanic thrust sheets. This corresponds to the notion of metacraton [J. African Earth Sci. 34 (2002) 119], i.e. a craton that has been remobilized during an orogenic event but is still recognizable dominantly through its rheological, geochronological, isotopic and sometimes petrological characteristics.     

郯庐断裂带（安徽部分）动力学演化及其构造意义

依据区域构造层划分，从最新地层内的断裂动力学分析开始，层层深入，层层筛选，采用计算机程序对所获的断层面上的擦痕统计分析计算，求得每个点上的古应力场状态，继而求得区域古构造应力场状态。确定发生于各个不同时代断裂活动的古应力场特征。最后确定了一个连续的、完整的古应力场演化序列表，结合区域构造变形特征分析，阐述断裂带的构造事件演化序列及其构造意义。