Stratigraphy and age of the Cappadocia ignimbrites, Turkey: reconciling field constraints with paleontologic, radiochronologic, geochemical and paleomagnetic data

The stratigraphy and age of the Neogene Cappadocia ignimbrites (Central Turkey) have been inferred in previous studies from fieldwork and K–Ar age determinations. The resulting stratigraphic schemes, however, differed from each other, suggesting that further studies were required to produce a reliable succession. In this paper, we examine the chronostratigraphy of mammalian remains recovered in the continental sediments interbedded with the Cappadocia ignimbrites. Using recent advances in mammalian chronostratigraphy, we evaluate selected taxa and faunal associations to place new and independent constraints on the ignimbrite ages. The biostratigraphically bracketed ages concur with some published radiometric dates, but they disagree with others, principally at localities where major stratigraphic discrepancies have arisen in the literature. In order to reconcile these apparent inconsistencies, we combine, at selected sites, our field observations with the biostratigraphic and radiometric age limits, and we compare these with the available geochemical and magnetic data. This allows us to present revised age estimates, and a revised stratigraphy which includes the correlation of the local Sofular deposits with the large-volume Gördeles ignimbrite. The issues faced in this study apply to other ignimbrite provinces in the world. For instance, ignimbrite eruption frequency in Cappadocia is higher than the resolution of many published K–Ar ages. Furthermore, different K–Ar ages have led to the description of individual and distinct ignimbrites that fieldwork and geochemical data allow to merge into a single ignimbrite. Argon loss from pumice samples leading to radiometric “rejuvenation” provides a likely explanation for most stratigraphic discrepancies. Cappadocia is the only documented ignimbrite field in which the chronostratigraphy of vertebrate remains provides better constraints on some ignimbrite ages than scattered K–Ar dates. We further argue that K–Ar dates from the Cappadocia area are too imprecise to establish a reliable magnetostratigraphic scheme for the ignimbrite succession, with the exception of the not, vert, similar2.8 Ma Valibaba Tepe ignimbrite

Links between topography, wind, deflation, lakes and dust: The case of the Bodélé Depression, Chad

The Bodélé Depression, Chad is the planet's largest single source of dust. Deflation from the Bodélé could be seen as a simple coincidence of two key prerequisites: strong surface winds and a large source of suitable sediment. But here we hypothesise that long term links between topography, winds, deflation and dust ensure the maintenance of the dust source such that these two apparently coincidental key ingredients are connected by land-atmosphere processes with topography acting as the overall controlling agent. We use a variety of observational and numerical techniques, including a regional climate model, to show that: 1) contemporary deflation from the Bodélé is delineated by topography and a surface wind stress maximum; 2) the Tibesti and Ennedi mountains play a key role in the generation of the erosive winds in the form of the Bodélé Low Level Jet (LLJ); 3) enhanced deflation from a stronger Bodélé LLJ during drier phases, for example, the Last Glacial Maximum, was probably sufficient to create the shallow lake in which diatoms lived during wetter phases, such as the Holocene pluvial. Winds may therefore have helped to create the depression in which erodible diatom material accumulated. Instead of a simple coincidence of nature, dust from the world's largest source may result from the operation of long term processes on paleo timescales which have led to ideal conditions for dust generation in the world's largest dust source. Similar processes plausibly operate in other dust hotspots in topographic depressions

Constraints on the composition and thermal structure of the upper mantle beneath NW Turkey: Evidence from mantle xenoliths and alkali primary melts

Ultramafic xenoliths entrained in the Late Miocene alkali basalts and basanites from NW Turkey include refractory spinel-harzburgites and dunites accompanied by subordinate amount of spinel-lherzolites. Whole-rock major element and mineral chemical characteristics indicate that the xenoliths are mostly the solid residues of varying degrees of partial melting (similar to 3 to similar to 15%), but some have mineral chemical signatures reflecting processes of melt/rock interaction. The mineral compositions of the undisturbed xenoliths have been used to estimate temperatures and, where possible, pressures of equilibration, and to construct a geothermal framework to interpret the thermal structure of the upper mantle beneath the region at the time of basaltic melt extraction. Most of the peridotite xenoliths have chemically homogeneous mineral phases reflecting ambient equilibrium pressure-temperature conditions. The combination of results from Ca-in-orthopyroxene geothermometer and Ca-in-olivine geobarometer yields pressure-temperature estimates for spinel-peridotites that fall in experimentally determined spinel-lherzolite field in CaO-Fe-M-O-Al2O3-SiO2-Cr2O3 (CFMASCr) system. These pressure-temperature data sets have been used to construct the Late Cenozoic geotherm of NW Turkey, which defines a depth-temperature array from about 1.4 GPa at 810 degrees C to 2.1 GPa at 1180 degrees C. Extrapolation of this array to the adiabatic upwelling curve of normal temperature asthenospheric mantle indicates an apparent lithospheric thickness of similar to 80 km. The geothermal gradient of NW Turkey is only slightly higher than that of the conventional conductive models (similar to 90 MW m(-2)) and may be interpreted as being a consequence of the thermal perturbation by the heat input into the relatively thin mantle lithosphere via the passage and emplacement of magma(s). In contrast, the calculations, based on the melt products of experimental phase equilibria, show that the compositions of the host magmas precipitating the most magnesian olivine (with 88.5 to 91.4% forsterite; Mg2SiO4) phenocrysts of alkaline melts reveal significantly higher anhydrous liquidus temperatures that range from about 1290 to 1410 degrees C. The results are inconsistent with melt generation by either deep-seated thermal anomalies associated with mantle plumes (T-p > 1450 degrees C) or melting of thermally perturbed (wet) mantle lithosphere, but are consistent with the adiabatic upwelling of normal temperature (T-p similar to 1300 degrees C) mantle asthenosphere probably along the lithospheric scale major shear zones

The Cenozoic volcanic province of Tibesti (Sahara of Chad): major units, chronology, and structural features

International audienceUsing both field relationships and some absolute ages, the sequence of volcanic units in the Cenozoic Tibesti Volcanic Province (TVP) (Chad) is established as follows: (1) plateau volcanism, between at least 17 and 8 Ma, consisting of flood basalts and silicic lava plugs, with intercalated ignimbritic sheets in the upper basalt succession increasing in amount upwards. Ages decrease from NE to SW, following the migration of the small NW-SE flexures concentrating the feeding dike swarms; (2) Late Miocene large central composite volcanoes exhibiting diverse and original structures. Some of them (Tarso Toon, Ehi Oyé, and Tarso Yéga) are located along a major NNE fault, representing the main tectonic direction in Tibesti since Precambrian times; (3) construction of three large ignimbritic volcanoes, associated with significant updoming of the basement, ending with the collapse of large calderas: Voon (about 5–7 Ma), Emi Koussi (2.4–1.33 Ma), and Yirrigué (0.43 Ma); (4) basaltic activity, starting at about 5–7 Ma, and essentially consisting of cinder cones and associated lava flows (Tarso Tôh, Tarso Ahon, and Tarso Emi Chi); and (5) final volcanic activity represented by post-Yirrigué caldera activity in the Tarso Toussidé Volcanic Complex, and especially Ehi Toussidé (the only active volcano in Tibesti), plus Ehi Timi and Ehi Mousgou volcanoes, similar to Ehi Toussidé. The two tectonic directions controlling some volcanic features of the province correspond to the major old lithospheric structures delimiting the volcanic province, namely, the great NW-SE Tassilian flexure to the SW and a major NE-NNE fault zone to the E. Unusual conditions of uplift and erosion in the TVP enable exceptional exposure of the internal structure of its volcanoes

Structure of Puy de Dôme volcano (Chaîne des Puys, France): towards a revised model

The project TOMUVOL, aimed at developing a method for muonic tomography of volcanic edifices, has chosen the Puy de Dôme as an experimental test site. In fact, this volcano with a simple external shape has a complex internal structure that may allow testing the performances of this new method. The Puy de Dôme is a composite lava dome, about 11 ka old. It is composed of two distinct morphological units. The first one would consist of a classic bristled lava-dome whereas the second one would be a dome erected on the eastern flank of the first one, after a large flank collapse. However, between 2004 and 2011, fresh outcrops revealed by the building of touristic facilities on the summit and at the basis of the mountain, afforded the collection of new data inducing new questions. The field was then more systematically explored and the results of a LiDAR survey centred on the Puy de Dôme and recorded in March 2011 were examined

Géodynamique andine : résumés étendus = Andean geodynamics : extended abstracts

L'éruption plinienne (VEI 6) du petit centre volcanique du Huaynaputina, le 19 février 1600, a mis en place une retombée ponceuse > 8500 km2, des ignimbrites, des déferlantes et enfin des lahars. L'édifice pré-1600 a été détruit en grande partie, formant le cratère complexe, l'effondrement partiel de la paroi nord de la caldera d'avalanche pré-existante, et enfin trois cratères et tuf-cônes accolés. Deux processus de déclenchement sont suggérés par la petite quantité de magma basaltique dans la retombée et par plusieurs indices d'hydromagmatisme. (Résumé d'auteur

Unique CO-tolerance of Pt–WOx materials

Well-defined tungsten-oxide-supported platinum nanoparticles (Pt/WOx) were elaborated by impregnation-reduction of a platinum salt onto commercial monoclinic WO3. Field-emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM) revealed that the Pt particles are well-distributed on the oxide support, present a narrow particle size distribution centered on ca. 2–3 nm and a low degree of agglomeration. Carbon black was added to ensure electronic percolation in the electrodes during the electrochemical measurements. COads electrooxidation currents were monitored at potentials as low as 0.1 V vs. RHE on Pt/WOx, demonstrating high CO-tolerance compared to carbon-supported Pt or PtRu catalysts. Keywords: CO-tolerance, Nanoparticle, Platinum, Tungsten oxide, PEMF

Géodynamique andine : résumé étendus = Andean geodynamics : extended abstracts

La Laguna Salinas (département d'Arequipa) est une dépression fermée d'origine volcano-tectonique, qui constitue un piège sédimentaire et un salar. Un sondage de reconnaissance a traversé les formations lacustro-palustres supérieures jusqu'à un niveau de matériau grossier non identifié. La séquence prélevée couvre les quinze derniers millénaires et regroupe sept téphras. La recherche pluridisciplinaire réalisée concerne la sédimentation, la corrélation et la géochimie des téphras, le diagramme de pollen, les diatomées, ainsi que 4 datations C14. (Résumé d'auteur