Imagery of the metamorphic bedrock roof of the Sahel active fault in the Sablettes (Algiers) reclaimed area by ambient vibration HVSR

The Sablettes (Algiers) coastal reclaimed fringe region, located on the hanging wall of the Sahel active fault, is subject to different types of geological hazard such as flood and tsunami, coastal uplift, earthquake, liquefaction, landslide, and site effects. In this present work, we used ambient vibration HVSR for imaging the bedrock. The thickness of the sedimentary column under the backfill layer is unknown, and the coastal reclaimed areas are prone to strong amplification of seismic waves. The determination of the depth of the metamorphic base allowed us to establish a mapping of the bedrock roof surface. The 3D representation of this surface enabled us to present models of tectonic structures in this basement (i.e., fault, fold). This analysis will make it possible to make better evaluation of the amplification after having determined the depth of the metamorphic basement exceeding 240 m, which is supposed to have velocities close to those of the seismological basement, as well as the thicknesses of the different layers surmounting it.     

Planning and systematic management of water resources by the WEAP model,case of the Mabtouh watershed (northwestern Algeria)

Water management remains a complex task due to the multiplicity of users-consumers, which necessitates the use of planning tools and decision support in a rigorous manner. The objective of the present study is to find an alternative solution to the water use conflict applied to a scale of a hydrographic unit. Our choice focused on the Mebtouh River watershed (1306 km²), where there is a chronic water shortage. With an irrigated perimeter of 8200 ha and an urban area with a regional dimension, the hydraulic system is based on a Dam (Cheurfa) with a capacity of 83 hm³ and diversions by pumping over the river. The methodology adopted is the application of the WEAP software, with a view to constructing a water resource allocation model by 2050. The data set consists of the physical parameters of the study area, hydroclimatic records for the reference year (2015), and the estimation of urban and agricultural water demand sites. The results obtained by the WEAP model applied to our study area confirm the validation of the data used. Therefore, it is timely to develop a water resources management tool to reduce user-consumer tensions at the planned planning timeline.     

Granulitic metamorphism in the Laouni terrane (Central Hoggar, Tuareg Shield, Algeria)

In the Laouni terrane, which belongs to the polycyclic Central Hoggar domain, various areas contain outcrops of formations showing granulite-facies parageneses. This high-temperature metamorphism was accompanied by migmatization and the emplacement of two types of magmatic suite, one of continental affinity (garnet pyroxenites and granulites with orthoferrossilite–fayalite–quartz), and the other of arc affinity (layered metanorites). Paragenetic, thermobarometric and fluid-inclusion studies of the migmatitic metapelites and metabasites make it possible to reconstruct the P–T–aH₂O path undergone by these formations. This path is clockwise in the three studied areas, being characterized by a major decompression (Tamanrasset: 10.5 kbar at 825 °C to 6 kbar at 700 °C; Tidjenouine: 7.5 kbar at 875 °C; to 3.5 kbar at 700 °C; Tin Begane: 13.5 kbar at 850 °C; to 5 kbar at 720 °C), followed by amphibolitization that corresponds to a fall of temperature (from 700 to 600 °C) and an increase in water activity (from 0.2–0.4 to almost 1).The main observed features are in favour of petrogenesis and exhumation related to the Eburnean orogeny. However, the lacks of good-quality dating work and a comparison with juvenile Pan-African formations having undergone high-pressure metamorphism, in some cases reaching the eclogite facies, do not rule out the possibility that high-temperature parageneses are locally due to Pan-African events.     

Discrimination between primary magmatic biotites, reequilibrated biotites and neoformed biotites

The ternary diagram TiO₂–FeO*–MgO (FeO* = FeO + MnO) is proposed as a quantitative objective tool for distinguishing between primary magmatic biotites and those that are more or less reequilibrated, or possibly neoformed, by or within a hydrothermal fluid. The limit of the domains of the primary magmatic biotites, the reequilibrated biotites and the neoformed biotites were determined on the basis of optical, paragenetic and chemical criteria. To cite this article: H. Nachit et al., C. R. Geoscience 337 (2005).     

Application of 3D Euler deconvolution and improved tilt angle to the aeromagnetic data of In Ouzzal terrane,western Hoggar,Algeria

The study area is located in the western Hoggar Shield (southern Algeria). It includes the In Ouzzal terrane, which consists of Archaean metamorphic rocks. By contrast to other rocks of the Hoggar Shield, the In Ouzzal terrane represents an exception of being neither deformed nor metamorphosed during the Pan-African event, remaining as a rigid block since 2 Ga. Although, previous geophysical works in the area include an airborne magnetometer and gamma-ray spectrometric survey as well as ground gravity and magnetotelluric survey structurally, the study area has not been very well understood. In this paper, we present the interpretation results of the airborne magnetic data by using the 3D Euler deconvolution and the improved Tilt-angle methods. These results reveal the existing of fault systems (FS) occurring within the center of the study area and along the latitude of 22°; the results also suggested that the deepest fault system is oriented NE–SW and is represented by parallel major faults splitting the In Ouzzal terrane into two different parts: northern and southern. The northern part moved northwards, whereas the southern part moved southwards colliding with the Iforas unit. The interpretation confirms that the In Ouzzal terrane and the surrounding Pan-African structures are bound two by two sub-vertical lithospheric faults with the existence of dextral and sinistral faults in the west and east of the terrane, respectively.     

Modélisation magnétique de la suture ophiolitique de Bou Azzer–El Graara (Anti-Atlas central,Maroc). Implications sur la reconstitution géodynamique panafricaine

Aeromagnetic data of the Anti-Atlas Mountains show an important magnetic anomaly along the ‘Major Anti-Atlas Fault’, produced by different mafic and ultramafic rocks of a Neoproterozoic ophiolite complex. The magnetic modelling of Bou Azzer–El Graara ophiolitic suture shows a deep-seated anomaly through the upper continental crust corresponding to a north-dipping subduction. The polarity of the Pan-African subduction in the Anti-Atlas is therefore compatible with the contemporaneous Pan-African orogenic belts, where polarity of subduction dipped away from the West African Craton during the amalgamation of Western Gondwana. To cite this article: A. Soulaimani et al., C. R. Geoscience 338 (2006).     

Statistical categorization geochemical modeling of groundwater in Ain Azel plain (Algeria)

Water analysis data of 54 groundwater samples from 18 uniformly distributed wells were collected during three campaigns (June, September and December 2004). Q-mode hierarchical cluster analysis (HCA) was employed for partitioning the water samples into hydrochemical facies. Interpretation of analytical data showed that the abundance of major ions was identified as follows: Ca ? Mg > Na > K and HCO₃ ? Cl > SO₄. Three major water facies are suggested by the HCA analysis. The samples from the area were classified as recharge area waters (Ca–Mg–HCO₃ water), transition area waters (Mg–Ca–HCO₃–Cl water), and discharge area waters (Mg–Ca–Cl–HCO₃ water). Inverse geochemical modeling suggests that relatively few phases are required to derive the water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into two categories: (1) evaporite weathering reactions and (2) precipitation of carbonate minerals.     

The polymetallic Au–Ag-bearing veins of Bou Madine (Jbel Ougnat, eastern Anti-Atlas, Morocco): tectonic control and evolution of a Neoproterozoic epithermal deposit

The Bou Madine ore deposit is located SW of Jbel Ougnat, the easternmost inlier of the Anti-Atlas Pan-African belt in Morocco. The host rocks are high-K calc-alkaline volcanic rocks, that are part of the Neoproterozoic Tamerzaga-Timrachine Formation (TTF, lower PIII). The TTF consists of ignimbrites of rhyolitic to dacitic compositions, andesite flows and hypovolcanic bodies (andesite dykes and rhyolite chonoliths) emplaced along N160°E tension gashes associated with a regional N30°E sinistral fault system. The mineralization is related to a high enthalpy geothermal system, eventually evolving into a low temperature epithermal system. A regional propylitisation (T around 260 °C) overprinted the TTF rocks prior to the emplacement of the mineralization. There were two main hydrothermal stages. During the first stage, massive veins with pyrite, arsenopyrite and minor pyrrhotite and cassiterite were formed. The veins were emplaced along N160°E-trending en echelon joints related to N120°E dextral arrays. A quartz-sericite-pyrite alteration overprinted the propylites around the veins (“bleached haloes”), at temperatures up to 300–310 °C. The second stage of mineralization was coeval with dextral re-activation of the N160°E veins, in relation with a NE-ward shift of the shortening direction. First, polymetallic sulphides (sphalerite, chalcopyrite, stannite, galena) were deposited at temperatures 260 °C. Younger quartz veinlets contain arsenopyrite and minor micrometre-size sulphides and sulpho-salts, hosting the precious metals. This was the low temperature epithermal stage (≈150 °C), in relation with invading meteoric water.     

Vulnerability evaluation of the strategic buildings in Algiers (Algeria): a methodology

Algeria is a country with a high seismic activity. During the last decade, many destructive earthquakes occurred, particularly in the northern part, causing enormous losses in human lives, buildings, and equipments. In order to reduce this risk in the capital and avoid serious damages to the strategic existing buildings, the government decided to invest in seismic upgrade, strengthening, and retrofitting of these buildings. To do so, seismic vulnerability study of this category of buildings has been considered. Structural analysis is performed based on a site investigation (inspection of the building, collecting data, materials characteristics, general conditions of the building, etc.) and existing drawings (architectural plans, structural design, etc.). The aim of these seismic vulnerability studies is to develop guidelines and a methodology for rehabilitation of existing buildings. This paper presents the methodology followed in our study and summarizes the vulnerability assessment and strengthening of one of the strategic buildings according to the new Algerian Seismic Design Code RPA 99/version 2003. As a direct application of this methodology, both static equivalent method and nonlinear dynamic analysis are performed and presented in this paper.