L’éclipse de soleil du 20 mars 2015, depuis l’Université de Bourgogne
Publication du papier sur l’utilisation de la LIBS afin de déterminer les roches volcanique en Islande.
- •Portable LIBS applied to field geology
- •Fast semi-quantitative geochemical analysis of volcanic rocks and magmatic series
- •Discriminant analysis and statistical treatments for LIBS compositional data
This study evaluates the potentialities of a lab-made pLIBS (portable Laser-Induced Breakdown Spectroscopy) to sort volcanic rocks belonging to various magmatic series. An in-situ chemical analysis of 19 atomic lines, including Al, Ba, Ca, Cr, Cu, Fe, Mg, Mn, Na, Si, Sr and Ti, from 21 sampled rocks was performed during a field exploration in Iceland. Iceland was chosen both for the various typologies of volcanic rocks and the rugged conditions in the field in order to test the sturdiness of the pLIPS. Elemental compositions were also measured using laboratory ICP-AES measurements on the same samples. Based on these latter results, which can be used to identify three different groups of volcanic rocks, a classification model was built in order to sort pLIBS data and to categorize unknown samples. Using a reliable statistical scheme applied to LIBS compositional data, the classification capability of the pLIBS system is clearly demonstrated (90–100% success rate). Although this prototype does not provide quantitative measurements, its use should be of particular interest for future geological field investigations.
Publication de l’article sur l’origine des vallées tunnel
- •We compared processes involved in the formation of two Upper Ordovician tunnel valleys.
- •Models of tunnel valley formation are driven by porewater pressures or meltwater flows.
- •The distribution of ice streams controls tunnel valley formations and morphologies
Several Ordovician tunnel valleys are exposed in the Moroccan Anti-Atlas Mountains, including the Alnif and the Foum Larjamme tunnel valleys, located 150 km away from each other. Sedimentological and deformational analyses of these two glacial troughs reveal that differing processes lead to their formations.
The Alnif tunnel valley contains numerous deformation structures within sediments both below and above the main glacial erosion contact surface. Ball-structures and clastic dykes occur within preglacial sediments down to 35 m below glacial incisions while overlying glacial sediments contain fluted surfaces, clastic dykes, dewatering structures, folds and radial step normal faults. The characteristics of the Alnif tunnel valley can be explained by a porewater pressure-driven model of formation where the localized increase of basal shear stress and porewater pressure underneath subglacial deforming zones lead to the development of a dense hydrofracture network in the preglacial bed. These processes of hydraulic brecciation promoted subglacial remobilization of the preglacial material and contributed to the formation of the tunnel valley.
The Foum Larjamme tunnel valley displays undisturbed preglacial sediments and few dewatering structures at the base of the glacial sedimentary infill which suggests relatively low porewater pressures within the tunnel valley during formation. This second type of tunnel valley where porewater pressure remained relatively low appears to have been formed by meltwater erosion. The undulating base of the Foum Larjamme tunnel valley implies progressive erosion by a stable subglacial braided network of Nye-channels, or alternatively by channels migrating laterally during episodic minor subglacial outbursts.
These two tunnel valleys highlight the regional variability of processes involved in the formation of tunnel valleys. The distribution of palaeo-ice streams in North Africa illustrate that morphologies and processes involved in the formation of tunnel valleys vary between ice stream and inter-ice stream zones due to variations in meltwater availability, the topography and bed lithological properties.