Mise en évidence d'un nouveau front de chevauchement dans l'Atlas tunisien oriental de Tunisie par sismique réflexion. Contexte structural régional et rôle du Trias salifère

Structural interpretations of newly acquired seismic lines in northeastern Tunisia allow us to highlight a new thrust front for the Atlasic range of Tunisia, in contrast to the previously Zaghouan fault thrust Dorsale zone. This new thrust front takes place on weakness tectonic zones, materialized by inherited faults anchored on the pre-Triassic basement. This front seems to be a paleogeographic trend controlling structural style and basin fill with a synsedimentary activity. The front is expressed by reverse faults, thrust faults, back thrusting, and decollement structures. To cite this article: S. Khomsi et al., C. R. Geoscience 336 (2004).     

Geochemical characteristics, Ar–Ar ages and original tectonic setting of the Band-e-Zeyarat/Dar Anar ophiolite, Makran accretionary prism, S.E. Iran

The Makran accretionary prism in southeastern Iran contains extensive Mesozoic zones of melange and large intact ophiolites, representing remnants of the Tethys oceanic crust that was subducted beneath Eurasia. To the north of the Makran accretionary prism lies the Jaz Murian depression which is a subduction-related back-arc basin. The Band-e-Zeyarat/Dar Anar ophiolite is one of the ophiolite complexes; it is located on the west side of the Makran accretionary prism and Jaz Murian depression, and is bounded by two major fault systems. The principal rock units of this complex are a gabbro sequence which includes low- and high-level gabbros, an extensive sheeted diabase dike sequence, late intrusive rocks which consist largely of trondhjemites and diorites, and volcanic rocks which are largely pillow basalts interbedded with pelagic sedimentary rocks, including radiolarian chert. Chondrite- and primitive-mantle-normalized incompatible trace element data and age-corrected Nd, Pb, and Sr isotopic data indicate that the Band-e-Zeyarat/Dar Anar ophiolite was derived from a midocean ridge basalt-like mantle source. The isotopic data also reveal that the source for basalts was Indian-Ocean-type mantle. Based on the rare earth element (REE) data and small isotopic range, all the rocks from the Band-e-Zeyarat/Dar Anar ophiolite are cogenetic and were derived by fractionation from melts with a composition similar to average E-MORB; fractionation was controlled by the removal of clinopyroxene, hornblende and plagioclase. Three ⁴⁰Ar–³⁹Ar plateau ages of 140.7±2.2, 142.9±3.5 and 141.7±1.0 Ma, and five previously published K–Ar ages ranging from 121±4 to 146±5 Ma for the hornblende gabbros suggest that rocks from this ophiolite were formed during the Late Jurassic–Early Cretaceous. Plate reconstructions suggest that the rocks of this complex appear to be approximately contemporaneous with the Masirah ophiolite which has crystallization age of (150 Ma). Like Masirah, the rocks from the Band-e-Zeyarat/Dar Anar ophiolite complex represent southern Tethyan ocean crust that was formed distinctly earlier than crust preserved in the 90–100 Ma Bitlis-Zagros ophiolites (including the Samail ophiolite).     

A multiphase approach for evaluating the horizontal and rocking impedances of pile group foundations

This paper advocates the use of a multiphase model, already developed for static or quasi‐static geotechnical engineering problems, for simulating the behaviour of piled raft foundations subject to horizontal as well as rocking dynamic solicitations. It is shown that such a model, implemented in a FEM code, yields appropriate predictions for the foundation impedance characteristics, provided that shear and bending effects in the piles are taken into account, thus corroborating the findings of the asymptotic homogenization theory. Besides, it is notably pointed out that such a multiphase‐based computational tool makes it possible to assess the dynamic behaviour of pile groups in a much quicker way than when using direct numerical simulations, which may face oversized problems when large pile groups are concerned. Copyright © 2016 John Wiley & Sons, Ltd.     

Seismic stability analysis of piled embankments: A multiphase approach

The seismic stability analysis of an embankment lying over a soft foundation soil reinforced by a group of vertical piles is performed within the framework of the upper bound kinematic approach of yield design. The analysis is based on a previously developed ‘multiphase’ model of the reinforced ground, which explicitly accounts for the shear and bending resistances of the piles. Making use of appropriate failure mechanisms involving shear zones across which the reinforcements are continuously deforming, along with ‘plastic hinge’ surfaces, upper bound estimates to the critical seismic coefficient of the structure are derived. The results, which are confirmed by the simulations obtained from a finite element elastoplastic code, give clear evidence of the key role played by the bending strength capacities of the piles in ensuring the stability of the pile reinforced embankment. Copyright © 2009 John Wiley & Sons, Ltd.     

Mineral chemical composition and geodynamic significance of peridotites from Nain ophiolite,central Iran

The peridotites from north of the town of Nain in central Iran consist of clinopyroxene-bearing harzburgite and lherzolite with small lenses of dunite and chromitite pods. The lherzolite contains aluminous spinel with a Cr number (Cr^# = Cr/[Cr + Al]) of 0.17. The Cr number of spinels in harzburgite and chromitite is 0.38–0.42 and 0.62, respectively. This shows that the lherzolite and harzburgite resulted from <18% of partial melting of the source materials. The estimated temperature is 1100 ± 200 °C for peridotites, the estimated pressure is <15 ± 2.3 kbar for harzburgites and >16 ± 2.3 kbar for lherzolites and estimated fo₂ is 10^?1±0.5 for peridotites. Discriminant geochemical diagrams based on mineral chemistry of harzburgites indicate a supra-subduction zone (SSZ) to mid-oceanic ridge (MOR) setting for these rocks. On the basis of their Cr^#, the harzburgite and lherzolite spinels are analogous to those from abyssal peridotites and oceanic ophiolites, whereas the chromites in the chromitite (on the basis of Cr^# and boninitic nature of parental melts) resemble those from SSZ ophiolitic sequences. Therefore, the Nain ophiolite complex most likely originated in an oceanic crust related to supra-subduction zone, i.e. back arc basin. Field observations and mineral chemistry of the Nain peridotites, indicating the suture between the central Iran micro-continent (CIM) block and the Sanandaj–Sirjan zone, show that these peridotites mark the site of the Nain–Baft seaway, which opened with a slow rate of ocean-floor spreading behind the Mesozoic arc of the Sanandaj–Sirjan zone as a result of change of Neo Tethyan subduction régime during middle Cretaceous.     

Experimental study on the mechanical behaviour of a heat exchanger pile using physical modelling

This study aims to provide knowledge on the thermo-mechanical behaviour of heat exchanger piles, through a laboratory scale model. The model pile (20 mm in external diameter) was embedded in dry sand. The behaviour of the axially loaded pile under thermal cycles was investigated. After applying the axial load on the pile head, the pile temperature was varied between 5 and 30 °C. Seven tests, corresponding to various axial loads ranging from 0 to 70 % of the pile estimated bearing capacity, were performed. The results on pile head displacement show that heating under low axial load induced heave and cooling induced settlement; the pile temperature-displacement curve was found to be reversible and compatible with the thermal expansion curve of the pile. However, at higher axial loads, irreversible settlement of the pile head was observed after a few thermal cycles. The axial load profile measured by the strain gauges evidenced that the pile head load was mainly transferred to the pile toe. Nevertheless, thermal cycles modified significantly the mobilised skin friction along the pile. The total pressure measured at various locations in the soil mass was also slightly influenced by the thermal cycles.     

Hydrogeological investigation of groundwater artificial recharge by treated wastewater in semi-arid regions: Korba aquifer (Cap-Bon Tunisia)

Korba aquifer is one of the most typical examples of overexploited coastal aquifer in the Mediterranean countries. In fact, from 1985, a considerable piezometric level drop, water salinization, and seawater intrusion were registered in the aquifer. In December 2008, Tunisian authorities initiated a general plan to groundwater management in order to augment groundwater resources, restore the piezometric levels, and improve water quality. The plan consists of artificial recharge of groundwater used treated wastewater through three infiltration basins. During the first 4 years (from December 2008 to December 2012), 1.41 Mm³ of treated wastewater was injected to the Korba aquifer. This study presents a hydrogeological assessment of groundwater evolution during the recharge processes. In this study, 32 piezometric and chemical surveys of 70 piezometers and observed wells are used to present hydrogeological investigation and water quality evolution of wastewater reuse through artificial recharge in Korba coastal aquifer. The piezometric evolution maps are used to specify the positive effect in groundwater level that exceeding 1.5 m in some regions. The interpretation of salinity evolution maps are used to indicate the improving of groundwater quality.     

Spatial distribution of uranium and vanadium in the upper phosphorite member in Eshidiyya basin, southern Jordan

Geostatistical analysis of geochemical exploration data can provide useful information for evaluating the mineralization potential of geologic bodies. The main objective of this study was to evaluate the spatial distribution of U and V over an area in the upper phosphorite member, the A0 unit, in Eshidiyya basin in southern Jordan. Drill-hole samples were analyzed. The geochemistry of the analyzed elements was assessed. The spatial variability of U and V was also assessed using semivariance analysis. Both U and V were found to exhibit a normal distribution as indicated by the constructed histograms and the calculated skewness and kurtosis coefficients. Exponential models with nugget effects were fitted to the experimental semivariograms. Ordinary kriging was performed to generate geochemical maps. The applied interpolation technique proved to be the best in producing geochemical exploration maps for both U and V in Eshidiyya phosphorites. The constructed geochemical maps helped visualize a WNW–ESE U mineralization trend in the studied phosphorites. This trend should be taken into consideration in any future exploration programs for U in south Jordan.     

Finite element implementation of a homogenized constitutive law for stone column-reinforced foundation soils,with application to the design of structures

Making use of the homogenization method for periodic media, developed in the context of elastoplasticity, a simplified constitutive law is proposed for a stone column-reinforced soil, regarded as a homogeneous but anisotropic medium. The closed-form expressions derived for such a constitutive law allow for its implementation into a f.e.m-based numerical procedure. The computational code so obtained is then applied to simulating the response of a foundation soil reinforced by a group of floating columns, expressed in terms of load–settlement curves drawn up to the ultimate bearing capacity.