Plio-Quaternary megasequence geometry and its tectonic controls within the Maghrebian thrust belt of south-central Sicily

Sequence stratigraphy in marine foredeep and thrust-top basins is controlled by the conventional variations in eustatic sea-level and sedimentation rate together with tectonics. Vertical motions reflect combinations of subsidence due to regional flexure and uplift on local thrust anticlines which act to modify the volume and shape of accommodation space together with syn-depositional slopes. Plio-Pleistocene successions on Sicily were deposited in thrust-top and foredeep basins, above and ahead of evolving structures of the Maghrebian fold and thrust belt. Collectively the sediments represent a single megasequence defined at its base by a maximum flooding surface of earliest Pliocene age following reconnection with global sea-level at the end of the Messinian. The internal stratigraphy of this megasequence consists of Trubi chalks, blue marls and a coastal calcarenite package with subordinate silciclastic sand. Plankton biostratigraphy allows these facies to be placed in a chronostratigraphic framework. Regionally the upper assemblage progrades away from the orogenic hinterland, recording a tectonically forced regression in response to regional uplift from late Pliocene times. This uplift may be associated with isostatic unloading in the orogenic hinterland due to tectonic collapse of the more internal thrust sheets. Prior to this, flexure from orogenic loading is inferred to have been sufficient for regional subsidence locally to outstrip uplift associated with the growth of some thrust structures. For shallow-water facies the competition between thrust-related uplift and flexural subsidence can be investigated from the stacking patterns of parasequence sets. For structures developed at greater palaeobathymetries receiving fine-grained pelagic sediment, active tectonics may be recognized from depositional hiatuses.     

The lack of thermical equilibrium as a consequence on the universe expansion

The time-asymmetry of the universe is essentially produced by its low-entropy, unstable states. Using quantitative arguments Paul Davies has demonstrated that the expansion of the universe coupled with nuclear reaction within the stars may produce a decrease in entropy, thereby explaining its low entropy states. This idea is implemented in a qualitative way in a simple homogeneous model. Some rough coincidence with observational data is found.     

SHORT COMMUNICATION PATTERN RECOGNITION IN CLINICAL CHEMISTRY:TUMOR MARKERS

Pattern recognition techniques(PCA,Bayesian analysis,LDA and KNN)are applied to a set of datacomposed of cancer patients and controls.A rather good separation between different populations isobtained using four tumor markers determined for each patient.     

Refining the noble gas record of the Réunion mantle plume source: Implications on mantle geochemistry

We report isotope analyses of helium, neon, argon, and xenon using different extraction techniques such as stepwise dynamic and static crushing, and high-resolution stepwise heating of three mantle xenoliths from Réunion Island. He and Ne isotopic compositions were similar to previously reported Réunion data, yielding a more radiogenic composition when compared to the Hawaiian or Icelandic mantle plume sources. We furthermore observed correlated ¹²⁹Xe/¹³⁰Xe and ¹³⁶Xe/¹³⁰Xe ratios following the mantle trend with maximum values of 6.93 ± 0.14 and 2.36 ± 0.06, respectively. High-resolution argon analyses resulted in maximum ⁴⁰Ar/³⁶Ar ratios of 9000–11,000, in agreement with maximum values obtained in previous studies. We observed a well-defined hyperbolic mixing curve between an atmospheric and a mantle component in a diagram of ⁴⁰Ar/³⁶Ar vs. ²⁰Ne/²²Ne. Using a mantle ²⁰Ne/²²Ne of 12.5 (Ne–B) a consistent ⁴⁰Ar/³⁶Ar value of 11,053 ± 220 in sample ILR 84-4 was obtained, whereas extrapolations to a higher mantle ²⁰Ne/²²Ne ratio of 13.8 (solar wind) would lead to a much higher ⁴⁰Ar/³⁶Ar ratio of 75,000, far above observed maximum values. This favours a mantle ²⁰Ne/²²Ne of about 12.5 considered to be equivalent to Ne–B. Extrapolated and estimated ⁴⁰Ar/³⁶Ar ratios of the Réunion, Iceland, Loihi, and MORB mantle sources, respectively, tend to be linearly correlated with air corrected ²¹Ne/²²Ne and show the same systematic sequence of increasing relative contributions in radiogenic isotopes (Iceland–Loihi–Réunion–MORB) as observed for ⁴He/³He. In general, He–Ne–Ar isotope systematics of the oceanic mantle can be explained by following processes: (i) different degree of mixing between pure radiogenic and pure primordial isotopes generating the MORB and primitive plume (Loihi-type) endmembers; (ii) relatively recent fractionation of He relative to Ne and Ar, in one or both endmembers; (iii) after the primary fractionation event, different degrees of mixing between melts or fluids of MORB and primitive plume affinity generate the variety of observed OIB data, also on a local scale; (iv) very late-stage secondary fractionation during magma ascent and magma degassing leads to further strong variation in He/Ne and He/Ar ratios.     

Time-dependent geochemistry of clinopyroxene from the Alban Hills (Central Italy): Clues to the source and evolution of ultrapotassic magmas

We investigated chemical and isotopic compositions of clinopyroxene crystals from well age-constrained juvenile scoria clasts, lava flows, and hypoabyssal magmatic ejecta representative of the whole eruptive history of the Alban Hills Volcanic District. The Alban Hills is a Quaternary ultra-potassic district that was emplaced into thick limestone units along the Tyrrhenian margin of Italy. Alban Hills volcanic products, even the most differentiated, are characterised by low SiO₂ content. We suggest that the low silica activity in evolving magmas can be ultimately due to a decarbonation process occurring at the magma/limestone interface. According to the liquid line of descent we propose, the differentiation process is driven by crystallisation of clinopyroxene + leucite ± apatite ± magnetite coupled with assimilation of a small amount of calcite and/or with interaction with crustal CO₂. By combining age, chemical data, strontium and oxygen isotopic compositions, and REE content of clinopyroxene, we give insights into the evolution of primitive ultrapotassic magmas of the Alban Hills Volcanic District over an elapsed period of about 600 kyr. Geochemical features of clinopyroxene crystals, consistent with data coming from other Italian ultrapotassic magmas, indicate that Alban Hills primary magmas were generated from a metasomatized lithospheric mantle source. In addition, our study shows that the ⁸⁷Sr / ⁸⁶Sr and LREE/HREE of Alban Hills magmas continuously diminished during the 600–35 ka time interval of the Alban Hills eruptive history, possibly reflecting the progressive depletion of the metasomatized mantle source of magmas.     

Contribution de la gravimétrie et de la sismique réflexion pour une nouvelle interprétation géodynamique des fossés d'effondrement en Tunisie : exemple du fossé de Grombalia

Troughs in Tunisia are interpreted as Plio-Quaternary structures associated to normal faults (grabens) or to flexure faults. Gravity data and seismic sections are used in this study to clarify the structure and the geodynamic evolution of an example of trough: the Grombalia trough (northeastern Tunisia), since the Upper Miocene to the Quaternary. A high residual negative gravity anomaly, which reaches ?15 mGal, is interpreted as being related to the thickening of Mio-Plio-Quaternary deposits (and probably older), as illustrated by seismic data. This subsidence has been the result of a negative flower structure related to strike-slip faults that have been reactivated with normal component during the Upper Miocene and with reverse component during the Pliocene. Seismic and gravity data demonstrate that the fault system is rooted, and more than four kilometres deep. The Grombalia example outlines the association between troughs and strike-slip faults; such a system is recognized in Tunisia, in the Ionian Sea and in the Pelagian Sea. To cite this article: M. Hadj Sassi et al., C. R. Geoscience 338 (2006).     

On the estimation of hysteretic energy demands for SDOF systems

Based on a statistical study of the dynamic response of single degree of freedom (SDOF) systems subjected to earthquake ground motions, a rule to estimate hysteretic energy demands is proposed. Expressions for elastic–perfectly plastic, stiffness degrading and pinching SDOF systems were obtained. The proposed rule does not explicitly depend on the period of the system; instead, it depends on the elastic pseudo‐acceleration and elastic velocity spectra. It is shown that the proposed rule can be applied to compute hysteretic energy demands for systems located at different soil conditions. In addition, information about scatter and bias of the proposed rule is also presented. Copyright © 2007 John Wiley & Sons, Ltd.     

Endogenous and nonimpact origin of the Arkenu circular structures (al‐Kufrah basin—SE Libya)

Abstract– The twin Arkenu circular structures (ACS), located in the al‐Kufrah basin in southeastern Libya, were previously considered as double impact craters (the “Arkenu craters”). The ACS consist of a NE (Arkenu 1) and a SW structure (Arkenu 2), with approximate diameters of about 10 km. They are characterized by two shallow depressions surrounded by concentric circular ridges and silica‐impregnated sedimentary dikes cut by local faults. Our field, petrographic, and textural observations exclude that the ACS have an impact origin. In fact, we did not observe any evidence of shock metamorphism, such as planar deformation features in the quartz grains of the collected samples, and the previously reported “shatter cones” are wind‐erosion features in sandstones (ventifacts). Conversely, the ACS should be regarded as a “paired” intrusion of porphyritic stocks of syenitic composition that inject the Nubia Formation and form a rather simple and eroded ring dike complex. Stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite–hematite horizons (typical of iron oxide copper‐gold deposits). Their origin was nearly coeval with the development of silicified dikes in the surroundings. Plugs of tephritic‐phonolitic rocks and lamprophyres (monchiquites) inject the Nubian sandstone along conjugate fracture zones, trending NNW–SSE and NE–SW, that crosscut the structural axis of the basin.     

Aquatic and terrestrial proxy evidence for Middle Pleistocene palaeolake and lake-shore development at two Lower Palaeolithic sites of Schöningen,Germany

The archaeological sites in the open-cast mine of Schöningen, Germany, represent outstanding archives for understanding Middle Pleistocene interglacial–glacial transitions and human adaption. Aquatic microfossil and pollen assemblages from the ‘Reinsdorf sequence’, likely correlated to Marine Isotope Stage 9, document environmental changes from a thermal maximum to succeeding glacial conditions recorded in two sequences of excavation sites 12 II and 13 II. Multi-proxy analyses enable detailed reconstruction of lake-shore and landscape developments despite variable microfossil preservation in changing carbonate- and organic-rich deposits. Rich aquatic vegetation with abundant charophytes suggests repeated phases with water depths of 0.5–2 m at site 13 II, while even greater temporary depths are deduced for 12 II DB. Mesorheophilic and mesotitanophilic ostracod species indicate stream inflows with medium–low calcium contents of >18 mg Ca L^–1 originating from nearby springs. Diatoms point to meso-eutrophic conditions and an alkaline pH of the lake water. Interglacial conditions with thermophile forests but no aquatic microfossils preserved, suggesting a dry or only temporarily flooded site, mark the beginning of the sequence. Continuous presence of aquatic organisms and overall dominance of small tychoplanktonic diatoms during a subsequent cool steppe phase provide evidence for increased water depths and unstable habitats characterized by erosion and probably prolonged periods of lake ice cover. During the succeeding boreal forest-steppe phase, surface runoff into the productive, shallow lake decreased due to a more extensive vegetation cover. Concurrently, intensified groundwater input in contact with the nearby salt wall caused elevated salinities. Following a lake level drop, stream inflows and lake levels increased again towards the end of the Reinsdorf sequence and promoted development of a diverse fauna and flora at the lake shore; thereby maintaining an attractive living and hunting environment for early humans during a phase of generally cooler temperatures and landscape instability at the transition into a glacial period.     

Petrological,geochemical, isotopic,and geochronological constraints for the Late Devonian–Early Carboniferous magmatism in SW Gondwana (27–32°LS): an example of geodynamic switching

International Journal of Earth Sciences - We report a study integrating 13 new U–Pb LA-MC-ICP-MS zircon ages and Hf-isotope data from dated magmatic zircons together with complete...