Le Volcanisme du Kouh-e-Tchah-e-Shahi au nord du Makran(Iran)

The recent fissural volcanism of Kouh-e-Tchah-e-Shahi is undifferentiated and consists mainly in andesitic basalt, rich in olivine, diopside, pigeonite, and labradorbytownite. The evolution, which leads to residual liquids, shows glasses unmiscible between one another. The volcanic arc of Bazman-Taftan and Kouh-e-Tchah-e-Shahi is interpreted as a subducting volcanism of an oceanic crust of the Indian Ocean, deeply diving from Jaz Murian behind the accretional prism of Makran.     

Earthquake prediction activities and Damavand earthquake precursor test site in Iran

Iran has long been known as one of the most seismically active areas of the world, and it frequently suffers destructive and catastrophic earthquakes that cause heavy loss of human life and widespread damage. The Alborz region in the northern part of Iran is an active EW trending mountain belt of 100 km wide and 600 km long. The Alborz range is bounded by the Talesh Mountains to the west and the Kopet Dagh Mountains to the east and consists of several sedimentary and volcanic layers of Cambrian to Eocene ages that were deformed during the late Cenozoic collision. Several active faults affect the central Alborz. The main active faults are the North Tehran and Mosha faults. The Mosha fault is one of the major active faults in the central Alborz as shown by its strong historical seismicity and its clear morphological signature. Situated in the vicinity of Tehran city, this 150-km-long N100° E trending fault represents an important potential seismic source. For earthquake monitoring and possible future prediction/precursory purposes, a test site has been established in the Alborz mountain region. The proximity to the capital of Iran with its high population density, low frequency but high magnitude earthquake occurrence, and active faults with their historical earthquake events have been considered as the main criteria for this selection. In addition, within the test site, there are hot springs and deep water wells that can be used for physico-chemical and radon gas analysis for earthquake precursory studies. The present activities include magnetic measurements; application of methodology for identification of seismogenic nodes for earthquakes of M ≥ 6.0 in the Alborz region developed by International Institute of Earthquake Prediction Theory and Mathematical Geophysics, IIEPT RAS, Russian Academy of Science, Moscow (IIEPT&MG RAS); a feasibility study using a dense seismic network for identification of future locations of seismic monitoring stations and application of short-term prediction of medium- and large-size earthquakes is based on Markov and extended self-similarity analysis of seismic data. The establishment of the test site is ongoing, and the methodology has been selected based on the IASPEI evaluation report on the most important precursors with installation of (i) a local dense seismic network consisting of 25 short-period seismometers, (ii) a GPS network consisting of eight instruments with 70 stations, (iii) magnetic network with four instruments, and (iv) radon gas and a physico-chemical study on the springs and deep water wells.     

Geochemical characteristics of an intracontinental shoshonitic association: the example of the Damavand volcano,IranCaractéristiques géochimiques d'une association shoshonitique intracontinentale : exemple du Damavand,Iran

Lavas of the Quaternary Damavand volcano (Central Alborz, Northern Iran) have been analysed for major and trace elements. The shoshonitic character of this magmatic series is definitively confirmed and most samples correspond to banakites. The Nb, Ta and Ti negative anomalies observed in all lavas suggest that they derive from of a sub-continental lithosphere metasomatised in a subduction setting. Their variable trace element enrichments are explained both by mineral fractionation and by crustal contamination in a shallow magmatic chamber. To cite this article: H. Mehdizadeh et al., C. R. Geoscience 334 (2002) 111–117.     

Hydrogeochemical investigation in an arid region of Iran (Tabas, Central Iran)

Groundwater is the most important source of water supply in Iran and understanding the geochemical evolution of groundwater is important for sustainable development of the water resources in Tabas area. A total of 29 samples of groundwater in Tabas area have been analyzed for ions and major elements. Groundwater of the study area is characterized by the dominance of Na–Cl water type. Groundwater was generally acidic to high alkaline with pH ranging from 5.42 to 10.75. The TDS as a function of mineralization characteristics of the groundwater ranged from 479 to 10,957 mg/l, with a mean value of 2,759 mg/l. The Ca²⁺, Mg²⁺, SO₄ ^2? and HCO₃ ^? were mainly derived from the dissolution of calcite, dolomite and gypsum. The Cu, Pb and Zn ions are not mobile in recent pH–Eh, but these conditions controlled dissolved Se, V and Mo in groundwater. The As is released in groundwater as a result of the weathering of sulfide minerals like arsenopyrite.     

Les péridotites et pyroxénolites à grenat du Bois des feuilles (Monts du lyonnais) (France)

The author describes a new occurrence of garnet peridotite and garnet pyroxenite interlayered in the biotite-sillimanite-garnet gneisses at the top of the granulitic serie of the Monts du Lyonnais (Massif Central français). Its dimensions are rather significant for a crustal gisement (500×100 m). It is only composed of forsterite, enstatite, chromiferous diopside, pyrope and spinel peridotites with their products of retrograde transformations as kelyphites, amphiboles, chlorites, lizardite, ores, etc. The petrographic studies show the heterogeneity of the massif and the anteriority of the red spinel upon the garnet which always forms a corona around the spinel. The peridotites are intermingled with numerous streched and dislocated layers of garnet websterites with rare centimetric levels. These pyroxenites would be derived of particular magmatic processes (partial anatectic melting followed by cristallisation) developped from an upper mantle level in a primary pyrolitic lherzolitic (s. l) or garnet peridotitic material. The garnet peridotite of “Le Bois des Feuilles” would be, in fact, a “secondary garnet lherzolite” derived: - either from a spinel lherzolite intermingled with garnet websterite layers and their “dunitic” remnants, to form a “pseudo-garnet lherzolite” like this of Beni Bouchera described by Kornprobst; - or from a spinel lherzolite associated with garnet websterites and submitted temporarily, at the time of its diapiric rising movement from the mantle towards the crust to the conditions of the spinel garnet lherzolite facies. The plastic deformations and intense laminations form blastomylonites of mixed rocks recristallised ultimately under granulitic facies conditions. These rocks are, pro parte, not very different from the other crustal garnet peridotites, in spite of the frequency of the spinel inclusions in garnet. In corollary, it seems that numerous crustal garnet peridotites would have the same origin.     

Eocene continental dyke swarm from Central Iran (Khur area)

The Eocene dyke swarm with east-west general trend intrudes the Cretaceous sedimentary rocks in ～25 km north of the Khur city (Central Iran). Some of the studied dykes can be followed for over 7 km, but the majority of exposures in the area are less than 5 km long. The dykes commonly exhibit a chilled contact with the wall rocks. These dykes are trachybasalt and basalt in composition. The trachybasalt dykes are much more abundant. The basaltic dykes cross cut the trachybasalt dykes in some locations, indicating that trachybasalt dykes are older than the basaltic ones. Primary igneous minerals of the basaltic dykes are olivine (chrysolite), clinopyroxene (diopside, augite), plagioclase (labradorite), sanidine, magnetite, orthopyroxene (enstatite), spinel and phlogopite, and secondary minerals are zeolite (natrolite and mesolite), chlorite (diabantite), calcite and serpentine. The trachybasalt dykes are composed of clinopyroxene (diopside), plagioclase (labradorite), sanidine, mica (biotite and phlogopite), amphibole (magnesio-hastingsite) and magnetite as primary minerals, and chlorite and calcite as secondary ones. Whole rocks geochemical data of the studied dykes indicate their basic and calc-alkaline nature and suggest that these two set of dykes were derived from the same parental magma. The chondrite-normalized REE patterns and the primitive mantle-normalized multi-elemental diagram of the Khur dykes show enrichment of light rare earth elements (LREE) relative to heavy rare earth elements (HREE), and negative anomalies of high field strength elements (HFSE) (e.g. Ti, Nb and Ta). These rocks show enrichment of the large ion lithophile elements (LILE) (e.g. Cs, Ba, Th and U) and depletion of the HREE and Y relative to MREE, Zr and Hf. In the chondrite-normalized REE diagram, the basalts show elevated REE abundances relative to the trachybasalt samples. Geochemical analyses of the studied samples suggest a spinel lherzolite from the mantle as the source rock and confirm the role of subduction in their generation. The chemical characteristics of the Khur dykes resemble those of continental arc rocks, and they were possibly formed by subduction of the Central-East Iranian microcontinent (CEIM) confining oceanic crust and decompression melting of a lithospheric subcontinental mantle spinel lherzolite enriched by subduction.     

Contribution à l'étude géologique du gisement de fer du Marquesado (Espagne)

The iron ore deposit of Marquesado (Spain) has been considered as a typical metasomatic substitution deposit (Type eight of Blondel's classification). Recent investigations have completely modified this point of view: stratigraphic analysis and paleogeographic data allow to consider the ore as a syngenetic formation. The author examines the possibility of a detritical sedimentary and of a volcanic-sedimentary origin. The conclusion is reached to propose a paleokarst origin of the orebody.

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Zusammenfassung Das Eisenerzvorkommen in Marquesado (Spanien) wurde von jeher als typisches Vorkommen metasomatischen Ersatzes betrachtet (Achter Typ der Klassifizierung von Blondel). Neuere Untersuchungen haben diesen Gesichtspunkt vollständig verändert: Stratigraphische Analyse und paleogeographische Daten gestatten, das Erz als eine syngenetische Bildung zu betrachten. Der Verfasser prüft die Möglichkeit eines detritischen und eines vulkanischen Ablagerungsursprungs des Eisenerzes. Die beste Erklärung liegt wohl in der Deutung des Vorkommens als Paläokarst.

     

Les stalactites de galène et de jordanite du gisement du Djebel Hallouf (Tunisie)

Sans résumé     

Existence de Mineralisations Plombo-Zinciferes Syngenetiques du Substratum Cambrien du District des Malines (Gard,France)

Résumé L'essentiel du minerai actuellement exploité dans le district minier des Malines (Gard, France) provient d'amas plombo-zincifères apparamment constitués par le remplissage de cavités paléo-karstiques post-stéphaniennes développées dans la dolomie cambrienne, à proximité de la surface de discordance qui la sépare de sa couverture mésozoïque. L'étude détaillée des minéralisations du substrat carbonaté révèle l'existence de concentrations également plombo-zincifères, mais stratoïdes et syngénétiques de la dolomie qui les supporte. En effet, ces minéralisations ont enregistré tous les phénomènes ayant affecté la dolomie encaissante: figures de sédimentation et de déformation synmétamorphe (orogénie varisque), fracturation post-schisteuse antérieure à la discordance permo-triasique, ... Il est proposé de considérer que ce type de minéralisation alimenta en métaux de base, voire en soufre, les pièges karstiques post-hercyniens: on en trouve notamment remanié à l'état d'éléments brèchiques dans les remplissages paléo-karstiques.

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Most of the ores extracted from the Les Malines mining district come from Pb-Zn deposits apparently formed by the in-filling of paleo-karstic caves. These cavities derive from cambrian dolomites solution by meteoric water circulations below the unconformity separating the triassic and/or mesozoic strata from the hercynien basement. Detailed study of the mineralizations located in the carbonate basement reveals besides the karstic in-filling stratoid ore bodies apparently syngenetic of their dolomitic bearing rock. The mineralizations record all the main geological events which affected the dolomitic rock since its deposition i.e. sedimentary features, synmetamorphic deformation, post-schistosity fracturation prior to the permo-triassic unconformity ... It is proposed to consider that these early stratoid mineralizations supplied the post-hercynian karstic traps with base metals and sulfur. It is to be noticed that these cambrian ores are found reworked as breccia elements in the karstic infilling.

     

Leymeriellidae (Cretaceous ammonites) from the lower Albian of Esfahan and Khur (Central Iran)

Leymeriellid ammonite faunas are described from northeast of Esfahan and the Khur area (Central Iran). The faunas comprise Leymeriella (L.) tardefurcata, L. (L.) germanica and L. (L.) acuticostata? as well as L. (Neoleymeriella) regularis, L. (N.) diabola and L. (Neoleymeriella) pseudoregularis?. The leymeriellids of Iran are closely related to faunas from northwest Europe, the Vocontian Basin and Transcaspia (Mangyschlak) and thus allow for biostratigraphic correlations, indicating the earliest Albian Leymeriella tardefurcata Zone with its superimposed L. acuticostata and L. regularis subzones. We suggest that the sudden appearance of common representatives of the genus Leymeriella in the northern and Central Iranian basins (Leymeriella acme) can be used as a useful proxy marker to define the base of the Albian Stage in Iran.     

Sur la nature et la signification des dolerites du bassin precambrien de sembe-ouesso (republique du congo)

Three sequences are recognized in the Sembé-Ouesso basin in the north of the Congo Republic: a schist and quartzite sequence (the oldest), a pelitic and sandstone sequence and the Dja tillitic sequence (upper Precambrian: 950 Ma?) This basin is divided by long sub-meridian strike-slip faults. These epimetamorphic sequences are folded in the NE-SW direction.Dolerites are intruded in all the formations. Two types of dolerites occur: alkaline quartz dolerites older than Dja tillitic formation and post-tillitic olivine dolerites of tholeiitic composition.The dolerites are intruded during crustal distension leading to basin subsidence. The geotectonic context is a failed rift, late deformed (aulacogen).     

La limite Aptien-Albien dans le secteur du Mompichel (Albacete)

The object of this paper is the study of the Cretaceous deposits in the Mompichel area, Albacete (SE Spain) and to describe the lithostratigraphic units. In the same way the different species of spores and pollen are shown to occur in two levels belonging to unit D, which has allowed for the dating of these terrigenous deposits as Early Albian.     

Composition des Sphalerites du District Minier de Hualgayoc (Perou)

Microprobe analysis of mineralisations from the district of Hualgayoc, Peru, shows a zonation of sphalerite compositions all around a rhyodacitic body. The differences in the iron content of sphalerites in veins inside the rhyodacite and in sedimentary strata may be explained by a contamination of the hydrothermal solutions by those strata.     

Extension at the coast of the Makran subduction zone (Iran)

In the Makran subduction zone, earthquake focal mechanisms and geodetic data indicate that the deforming prism currently experiences N–S compression. However, palaeostress inversions performed on normal faults observed along the coast reveal local stress components consistent with N‐S extension. Previously proposed mechanisms such as gravitational collapse are not favoured by N–S compression and surface uplift. We propose that the observed kinematics result from transient stress reversals following large earthquakes. During the interseismic period (now), the region experiences N–S compression. However, following a large reverse rupture on the subduction interface, stresses in the inner wedge relax, enabling a brief period of extensional faulting before a compressive stress state is re‐established. This mechanism, also observed in other subduction zones, requires low overall stresses in the upper plate and that the margin ruptures in large megathrust earthquakes that result in nearly complete stress drops.     

Microfossiles (Acritarches) du Proterozoique superieur dans les shales de la formation d'Atar (Mauritanie)

Earlier studies of Stromatolites had permitted the dating of the Atar Formation (Mauritania) as late Riphean by comparing these structures with those of the U.S.S.R. Geochronological data (Rb/Sr) obtained subsequently on illite had given ages ranging between 890 ± 36 and 595 ± 43 Ma for the whole Atar Group, the Atar Formation being dated at 890 ± 36 Ma (late Riphean). The microfossils (Acritarchs) studied here come from black shales cored while drilling for water near Atar. These shales are equivalent to the intercalations between the stromatolitic limestones of the Atar Formation. They have yielded 20 species of Acritarchs, six of which are most important as they provide a diagnostic assemblage of the late Riphean (1000 ± 50–650 ± 10 Ma) of the U.S.S.R. and northern Europe. These are: Chuaria circularis, Kildinosphaera chagrinata, K. verrucata, Leiosphaeridia asperata, Stictosphaeridium cf. sinapticuliferum, Trematosphaeridium holtedahli. The late Riphean age of the Atar Formation is thus confirmed by this microfossil assemblage.     

The role of blocking system in heavy precipitation of Iran (a case study: southeast of Iran January 2008)

This article concerns the analysis of the heavy precipitation, which allows investigating the effect of the blocking system on the unusual precipitation and temperature occurrence in Iran. The days of January 2008 have been the coldest days during the history of recorded data in Iran. Variation of precipitation during January 2008 compared with long-term data (30 years) shows the maximum positive anomaly in the stations located in southeast of Iran. However, the precipitation in consecutive days, 14–15 and 15–16 of January, produce a more important proportion of the heavy precipitation in this region. In order to study the role of the blocking system related to heavy precipitation in January 2008, the position and movement of the atmospheric systems including cyclones, anticyclones, fronts, and wind fields have been analyzed by the use of synoptic maps by the environment to circulation approach. Consequently, the weather maps indicated that the blocking system over the north of the Caspian Sea has caused the relatively deep low trough on January 5, 7, 14, 15, and 16, 2008, while the thermal and moisture gradients in the warm section of air masses have produced heavy precipitation. As a result, wind field of low levels (850 hPa) provided remarkable moisture fed by the Arabian Sea, Oman Sea, and Persian Gulf in the study area. Furthermore, the speed of wavelength and the position of the blocking system associated with the heavy precipitation can be clearly identified.     

Episyénitisation dans le Gisement d'Uranium du Bernardan (Marche): Comparaison avec des Gisements Similaires du Nord-Ouest du Massif Central Français

Résumé Le gisement d'uranium du Bernardan exploité dans le granite à deux micas de la Marche Occidentale appartient au groupe des gisements «d'imprégnation». La minéralisation uranifère (pechblende, coffinite, «produits noirs», autunite, ...) est disseminée dans une roche vacuolaire appelée «épisyénite». Cette dernière résulte d'une altération hydrothermale du granite par des solutions chaudes (370–260°C; 2–5,7% poids équiv. NaCl). Le quartz est dissout; les feldspaths (principalement les plagioclases) et les biotites sont partiellement transformés en phengites. Les muscovites et orthoses granitiques se réequilibrent avec perte de sodium. Cette altération est guidée par un réseau de fractures à différentes échelles. Une altération comparable a été observée dans d'autres gisements du Nord Ouest du Massif Central français, comme Margnac et Fanay dans le massif de St Sylvestre et Hyverneresse dans celui du Millevaches. Les transformations minéralogiques, à l'exception de la quantité de phengites secondaires, les changements chimiques (lessivage de silice et métasomatisme potassique), les conditions P-T et le contrôle tectonique sont les mêmes dans tous ces gisements.

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The Bernardan uranium deposit, mined in the Marche Occidentale two-mica granite, is a disseminated type deposit. Similar deposits are known in hercynian belt. Uraniferous mineralizations (pitchblende, coffinite, black minerals, autunite, ...) are disseminated in a vuggy rock called episyenite. It results from an hydrothermal alteration of the granite by hot aqueous solutions (370 to 260°C, 2–5.7 wt% equiv. NaCl). Quartz is dissolved; feldspars (mainly plagioclases) and biotites are partially transformed into phengites. Granitic muscovites and orthoclases are reequilibrated with loss of sodium. This alteration is controlled by fractures at several scales. Such an alteration can be observed in other deposits of the North West part of the French Massif Central, as Margnac and Fanay in the St Sylvestre massif and Hyverneresse in the Millevaches massif. Mineralogical transformations except the content in secondary phengites, chemical changes (SiO₂ decrease and potassic metasomatism), P-T conditions and tectonic control are the same in all these deposits.

     

Application of integrated geoelectrical methods in Marand (Iran) manganese deposit exploration

This paper addresses the application of integrated geoelectrical methods including induced polarization (IP), resistivity (RS), and self-potential (SP) for exploration and evaluation of the Marand (Iran) manganese deposit. The SP method was used for reconnaissance, in which five profiles were designed and surveyed with a reference electrode that was placed in a distance of at least 800 m far from the measurement points. For the conduct of RS and IP, a new array proposed by Ramazi (2005a), named combined resistivity profiling and sounding (CRSP) was used along nine profiles that made it possible to do sounding survey and produce apparent resistivity pseudo-sections. Some anomalies were detected by means of SP surveys along the profiles and some other were located from IP and RS pseudo-sections, and there was good consistency between anomalies detected by IP and RS and SP method. To check the efficiency of CRSP and in comparison with dipole–dipole array, in the location of profile no. 6, a profile with the dipole–dipole array was designed and surveyed. Finally, by integration of the results from the geoelectrical methods, some locations were suggested for borehole drilling. After drilling in those locations, cores were studied and compared with the results obtained from the geoelectrical methods that resulted in confirmation of the geoelectrical findings. The results obtained by the CRSP array have a match better than the drilling results.     

Landslides induced by the 2017 Mw7.3 Sarpol Zahab earthquake (Iran)

Landslides are the main secondary effects of earthquakes in mountainous areas. The spatial distribution of these landslides is controlled by the local seismic ground motion and the local slope stability. While gravitational instabilities in arid and semi-arid environments are understudied, we document the landslides triggered by the Sarpol Zahab earthquake (November 12, 2017, Mw7.3, Iran/Iraq border), the largest event ever recorded in the semi-arid Zagros Mountains. An original earthquake-induced landslide inventory was derived, encompassing landslides of various sizes and velocities (from rapid disrupted rockfalls to slow-moving coherent landslides). This inventory confirms the low level of triggered landslides in semi-arid environments. It also displays clear differences in the spatial and volumetric distributions of earthquake-induced landslides, having 386 rockfalls of limited size triggered around the epicenter, and 9 giant (areas of ca. 10⁶ m²) active and ancient deep-seated landslides coseismically accelerated at locations up to 180 km from the epicenter. This unusual distant triggering is discussed and interpreted as an interaction between the earthquake source properties and the local geological conditions, emphasizing the key role of seismic ground motion variability at short spatial scales in triggering landslides. Finally, the study documents the kinematics of slow-moving ancient landslides accelerated by earthquakes, and opens up new perspectives for studying landslide triggering over short (~?1–10 years) and long-time (~?1000–10,000 years) periods.