[en] The Tarat Formation is composed at Arlit of three sequences consisting of sandstones and black mudstones rich in organic matter of continental origin. The three of them are interpreted as estuarine sediments deposited in different tectonic frameworks. The Arlit Unit consists of motley argillaceous sandstones deposited in a very flat lacustrine or alluvial environment and under an arid climate. These formations are separated by a fundamental discontinuity linked to the structuration of the Tim Mersoi Trough. At regional scale, the sediments lying below this discontinuity are grouped in tectono-sedimentary supersequences, which are interpreted as biorhexistasic rhythms; their deposition was horizontally directed by complex evolving synsedimentary tectonics controlled by the N-S In Azawa Lineament, and was influenced by the permanent proximity of a northern sea. The sediments lying above the fundamental discontinuity belong to the ''Continental Intercalaire'' Supergroup; their deposition was influenced by the Tim Mersoi Trough. The whole Carboniferous sedimentation took place in a climatic context which evolved from cold (Lower Visean) to warm and arid (Upper Carboniferous) through temperate climates

[en] U-Pb analysis on zircons has pointed out that the alkaline orthogneiss of Plaisance in the axial zone of the 'Montagne Noire', dates from 532+-13 m.y. this corresponds closely to previous data of Rb-Sr analysis (515+-11 m.y.). Although as a recent petrographical work has shown that this orthogneiss presents characters of an alkaline granite complex, the observed initial ratio ⁸⁷Sr/⁸⁶Sr which equals to 0.710, does not necessarily imply the partial melting of an old Precambrian basement in this area. Besides, the datation found from U-Pb analysis confirms the existence of a major episod of crustal thinning, characterized by the occurrence of an alkaline magmatism (530 m.y.) followed by one which is bimodal (500-480 m.y.)

[en] Nine K/Ar datations of some basaltic lavas from the Atakor mountains allow to date, for the first time, an arid period at 1.5 m.y. +- 0.1. This arid period takes place after the drying of the Plio-Pleistocene tropical lakes and after the first cool Pluvial, during an important turning phase of the geomorphologic and climatic evolution. These environmental changes are similar to those pointed out in Eastern Africa

[en] A 615+-12 m.y. age is obtained by U-Pb method on zircons extracted from the granodiorite of Bleida (Anti-Atlas, Morocco). This age which is in good agreement with U-Pb and Rb-Sr previous data, argues for the occurrence of the panafrican orogeny in the Anti-Atlas and contributes to define the chronology of the typical features of this orogenic belt

[en] Seismic hazard assessment relies on the knowledge of the source characteristics of past earthquakes. Unfortunately, seismic waveform analysis, representing the most powerful tool for the investigation of earthquake source parameters, is only possible for events occurred in the last 100-120 years, i.e., since seismographs with known response function were developed. Nevertheless, during this time significant earthquakes have been recorded by such instruments and today, also thanks to technological progress, these data can be recovered and analysed by means of modern techniques. In this paper, aiming at giving a general sketch of possible analyses and attainable results in historical seismogram studies, I briefly describe the major difficulties in processing the original waveforms and present a review of the results that I obtained from previous seismogram analysis of selected significant historical earthquakes occurred during the first decades of the 20. century, including (A) the December 28, 1908, Messina straits (southern Italy), (B) the June 11, 1909, Lambesc (southern France) - both of which are the strongest ever recorded instrumentally in their respective countries - and (C) the July 13, 1930, Irpinia (southern Italy) events. For these earthquakes, the major achievements are represented by the assessment of the seismic moment (A, B, C), the geometry and kinematics of faulting (B, C), the fault length and an approximate slip distribution (A, C). The source characteristics of the studied events have also been interpreted in the frame of the tectonic environment active in the respective region of interest. In spite of the difficulties inherent to the investigation of old seismic data, these results demonstrate the invaluable and irreplaceable role of historical seismogram analysis in defining the local seismo-genic potential and, ultimately, for assessing the seismic hazard. The retrieved information is crucial in areas where important civil engineering works are planned, as in the case of the single-span bridge to be built across the Messina straits and the ITER nuclear fusion power plant to be built in Cadarache, close to the location of the Lambesc event, and in regions characterized by high seismic risk, such as southern Appennines. (author)

[en] Shells extracted from loess deposits at Achenheim (Rhine Graben) have been investigated for AMS ¹⁴C and for U-Th dating by the isochron method on single shells. The two chronometers provide results that are not in agreement : the obtained ¹⁴C date is 38 ka B.P. In contrast, U-Th results provide much older ages. The ²³⁸U/²³²Th - ²³⁰Th/²³²Th isochron diagram gives an age of 69 kyr, whereas a slightly younger age of 64 kyr is derived from the other, ²³⁴U/²³²Th - ²³⁰Th/²³²Th, diagram. ²³⁴U/²³⁸U ratios have not been found to vary from a shell to another: the average value is 1.199, slightly higher than radioactive equilibrium. For both isochrones, the Y-axis intercept is slightly negative, which indicates that there is no evidence for a significant detrital component. Thorium 232 that is present in the shells is probably incorporated in the material. The negative intercepts suggest that the two ages have been probably 'aged' 'resulting from a slight mobilization of uranium. By constraining the Y-axis intercepts at 0, ages that are in good agreement from one to another, can be calculated at 60.9 kyr and 60.4 kyr, respectively. In both cases, the statistical quality of the alignment is only slightly deteriorated. Since U-Th ages are in agreement with stratigraphic data, in contrast to the ¹⁴C date, one may believe that they date the end of early diagenesis, with uranium trapping. In contrast, the AMS ¹⁴C date is either thought to reflect analytical limitations of AMS ¹⁴C dating for such type of material, or to date a younger geologic event. In this latter case, a possible hypothesis is that this event has triggered the slight opening of the ²³⁸U - ²³⁴U system. In any case, this study demonstrates once again that surface formations often tell such complex stories that only one single dating method cannot usually he sufficient to describe precisely these histories. Instead, informations derived from different independent chronometers have to be compared. (authors)

[en] Coral limestones sampled from marine terraces along the Afar coast have been dated by the ²³⁰Th/²³⁴U method. The ages confirm the stratigraphic unity of these formations and the existence of the paleo sea level dated 124 000 years ago in this region. These results permit to deduce the uplift rates of this littoral

[en] A laser probe has been used in order to fuse terrestrial minerals and to date them with the ³⁹Ar/⁴⁰Ar method. The results are compared with those obtained by other radiometric methods. This new technique is specially useful when several generations of minerals are present in the rock, or in case of excess of argon

[en] The hydrothermal input of elements to the ocean was largely underestimated before the discovery of the high temperature hydrothermal vents along active ridges. One of the first steps to quantify this input is to know the duration of such systems, their possible frequency and the chemical composition of the hydrothermal fluid. We studied some hydrothermal deposits from 21⁰N East Pacific Rise active vents and from the fossil deposit found in the vicinity. The active vents deposit exhibit ages from 20 to 60 years, and the fossil deposit an age around 4000 years. This last age may indicate that at 21⁰N, the accretion rate is 6 times faster when calculated at a thousand years scale than the one calculated at million years scale. The temperature of precipitation of amorphous silica found in the material forming the vents has been evaluated through the isotopic composition of oxygen. This temperature is 50-70⁰C which allows to calculate a silica content of 1700 ppm for the initial hydrothermal fluid

[en] In the different plutons of the medio-armorican variscan batholith, the uranium occurrences are preferentially localized along or near lineaments. The lineaments are interpreted as one of the major metallotects for uranium: source of the metal by destabilization of the uraniferous minerals in the tectonites: guide of a thermic flow which has drawn along the leaching and the transport of the uranium; lastly traps in drains made by late transverse fractures