Textures and phase relationships in ferrous granulites from Tidjenouine (Hoggar, Algeria): fayalite–ferrossilite–quartz secondary assemblage

Ferrous granulites in the area of Tidjénouine (Central Hoggar) exhibit a remarkable mineralogical composition characterized by the association orthoferrossilite–fayalite–quartz. These granulites are metamorphosed mafic igneous rocks showing the juxtaposition of different metamorphic parageneses. Peak paragenesis with garnet–clinopyroxene–amphibole–plagioclase–quartz reach to assemblage with orthopyroxene–plagioclase₂. Secondary orthopyroxene reacted with garnet to produce symplectites with fayalite + plagioclase + quartz. The latest stage corresponds to an orthopyroxene–fayalite–quartz–plagioclase assemblage. The metamorphic history of the ferrous granulites is inferred by combining the study of phase relations with the construction of a petrogenetic grid and pseudosection in the CFMASH and CFAS systems using the Thermocalc program of [J. Metamorph. Geol. 6 (1988) 173]. The evolution of paragenetic minerals indicates a metamorphic P–T path through the following conditions: 7.1 ± 1 kbar at 880 °C, 4.9 ± 1.6 kbar at 750 °C and 3–4 kbar at 700 °C, which is consistent with a clockwise P–T path recorded throughout the area.     

On the optically biaxial character and heterogeneity of anthracites

The results of the study of optical properties of 13 anthracites from different parts of the world are presented in this paper. Measurements of reflectance values were made on non-oriented vitrinite grains for a minimum of 300 points per sample. The reconstruction of Reflectance Indicating Surfaces (RIS) were made by Kilby's method [Kilby, W.E., 1988. Recognition of vitrinite with non-uniaxial negative reflectance characteristics. Int. J. Coal Geol. 9, 267–285; Kilby, W.E., 1991. Vitrinite reflectance measurement — some technique enhancements and relationships. Int. J. Coal Geol. 19, 201–218]. It was found that the use of Kilby's method for strongly anisotropic materials like anthracites did not give unambiguous results. Some improvement in Kilby's method, consisting of the division of the cumulative cross-plot into several elemental components, is suggested. Each elemental cross-plot corresponds to a textural class of anthracite, which is characterized by the values of RIS main axes R_MAX^(k), R_INT^(k) and R_MIN^(k) (k=1,2,…n; n — number of classes). The global texture of anthracite is characterized as a RIS with main axes calculated as the weighted means of , and for each class of this anthracite.The division of cumulative Kilby's cross-plot on elemental components makes possible the calculation of new coefficients H_t and H₁₀ characterizing the heterogeneity of the structure and texture of anthracites. The results of our study show that all anthracites have biaxial negative textures, but their heterogeneity varies in a wide range of H_t and H₁₀ coefficients depending upon the individual coal basin.     

Transfer functions under no-analog conditions: Experiments with Indian Ocean planktonic Foraminifera

This paper documents and investigates an important source of inaccuracy when paleoecological equations calibrated on modern biological data are applied downcore: fossil assemblages for which there are no modern analogs. Algebraic experiments with five calibration techniques are used to evaluate the sensitivity of the methods with respect to no-analog conditions. The five techniques are: species regression; principal-components regression [e.g., Imbrie, J., and Kipp, N. G. (1971). In “The Late Cenozoic Ages,” 71–181]; distance-index regression [Hecht, A. D. (1973). Micropaleontology19, 68–77]; diversity-index regression (Williams, D. F., and Johnson, W. C. (1975). Quaternary Research5, 237–250]; weighted-average method [Jones, J. I. (1964). Unpublished Ph. D. Thesis, Univ. of Wisconsin]. The experiments indicate that the four regression techniques extrapolate under no-analog conditions, yielding erroneous estimates. The weighted-average technique, however, does not extrapolate under no-analog conditions and consequently is more accurate than the other techniques. Methods for recognizing no-analog conditions downcore are discussed, and ways to minimize inaccuracy are suggested. Using several equations based on different calibration techniques is recommended. Divergent estimates suggest that no-analog conditions occur and that estimates are unreliable. The value determined by the weighted-average technique, however, may well be the most accurate.     

Isostasie flexurale et érosion différentielle : modélisation numérique appliquée au seuil du PoitouFlexural isostasy and differential erosion: numerical modelling applied at the ‘Seuil du Poitou’

One uses the principle of flexural isostasy to represent the uplift of the ‘Seuil du Poitou’ in response to erosion. The model indicates that a different uplift of on the last five millions years resulted from an erosion paradoxically stronger in the crystalline basement than in the limestones bedrock. To cite this article: J.-C. Maurin, K. Renaud, C. R. Geoscience 334 (2002) 1149–1155.     

Stress field in the Ryukyu–Kyushu Wadati–Benioff zone by inversion of earthquake focal mechanisms

The study addresses the space distribution of the stress field in the Kyushu–Ryukyu Wadati–Benioff zone based on homogeneous data of earthquake focal mechanisms and the inverse technique by Gephart and Forsyth [J. Geophys. Res. 89 (1984) 9305]. The used data set consists of 148 Harvard CMT solutions and 22 earthquake focal mechanisms listed in previous studies. The stress field parameters are determined for 0–40, 41–100 and h>100 km depth ranges. The top 100-km layer of the Wadati–Benioff zone (WBZ) is characterized by strike normal maximum compression σ₁ and steeper than the slab minimum compression σ₃, the last indicating for unbalanced slab pull force. The Tokara channel ‘divides’ the subduction into two parts of different stress regime at depth greater than 100 km. To the south of the channel the slab is under slab parallel σ₁ and slab normal σ₃ while its northern part, beneath Kyushu, is under slab parallel extension and slab normal compression. The results of recent studies on the regional velocity structure and geochemistry of the volcanic lava indicate that the most plausible reason for the observed stress field difference below 100 km in the northern and rest part of the arc is the presence of hot low viscosity upper mantle west of Kyushu.The results of this study indicate that the forces involved in the contemporary subduction dynamics in the Ryukyu–Kyushu Wadati–Benioff zone are related to the convergence between the Philippine Sea Plate and the Eurasian plate, the trench suction force, slab pull, the slab anchor force and, in the southern-central part of the arc, mantle resistance.     

Le passage Ordovicien–Silurien et la partie inférieure du Silurien (Sud-Est du Massif armoricain, France)The Ordovician–Silurian transition and the lower part of the Silurian (southeastern Armorican Massif, France)

A new section in the Silurian graptolitic ‘phtanites’ (black cherts) of Les Fresnaies at Chalonnes-sur-Loire (SE Armorican Massif) shows for the first time that these rocks (1) succeed conformably to Uppermost Ordovician (Hirnantian) glaciomarine deposits and (2) contain successive graptolite assemblages that characterise the base of the Silurian, the whole Rhuddanian and Aeronian stages and the lower part of the Telychian. To cite this article: J.M. Piçarra et al., C. R. Geoscience 334 (2002) 1177–1183.     

Crustal assimilation in basalt and jotunite: Constraints from layered intrusions

To constrain the amount and rate of crustal contamination that is possible in basaltic and jotunitic magma, and to gain an insight into the physical and thermal processes of assimilation in crustal magma chambers, we have modelled published Sr and Nd isotopic data from three layered intrusions. Well-exposed sequences of cumulates with no evidence of magma recharge provide direct records of concurrent assimilation and fractional crystallization (AFC). The key to the modelling is that F, the mass fraction of magma remaining in the chamber, can be estimated from the thicknesses of the studied cumulate sequences. This allows AFC model curves to be fitted to the isotopic data by varying r, the ratio of the rate of mass assimilated to the rate of mass crystallized. The results of modelling show that r is nearly constant in 800 to 2000 m thick sequences of cumulates displaying up-section decreases in anorthite content of plagioclase, increases in whole-rock Sr₀ (initial ⁸⁷Sr/⁸⁶Sr) and decreases in whole-rock εNd₀ (initial εNd). The r-values of the layered sequences range from 0.12 in the Fongen–Hyllingen Intrusion, over 0.20 in the Bjerkreim–Sokndal Intrusion, to 0.27 in the Hasvik Intrusion. The total amount of assimilation, the bulk crust/magma ratio, reaches values of 0.08, 0.19 and 0.28 at the level of the most contaminated samples after 60% to 80% crystallisation, whereas the instantaneous crust/magma ratio of the most contaminated magmas were respectively 0.14, 0.46, and 0.70, for the three intrusions.Innumerable country rock xenoliths occur in the three layered intrusions and played a crucial role in the assimilation process. The xenoliths spalled off the roofs of the magma chambers during magma emplacement and their initial temperature and composition relate to r in the intrusions. In the Hasvik Intrusion (r = 0.27), the initial temperature of the country rocks was 450 °C and the xenoliths were fusible metasediments and therefore produced a high fraction of partial melt that could be assimilated. In the Bjerkreim–Sokndal Intrusion (r = 0.20), the country rocks were initially at temperatures of 640–880 °C but included both refractory massif-type anorthosite and fusible gneisses. In the Fongen–Hyllingen Intrusion (r = 0.12), the country rocks were cooler (300 °C) and the xenoliths include refractory metabasalt (dominant) and fusible metapelite. We argue that the refractory metabasalt and anorthosite xenoliths acted mainly as heat sinks, resulting in reduced r-values in Fongen–Hyllingen and Bjerkreim–Sokndal Intrusions.Heating of refractory and fusible xenoliths, and melting of fusible xenoliths absorbed sensible and latent heat of the magma. Energy-balanced modelling shows that up to 75% of the heat available was absorbed by xenoliths within the magma chambers, promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the envelope of country rocks alone. The high r-values reflect the amount of heat absorbed by heating and melting country rock within the magma chambers themselves, and their constancy reflects the ready availability of fusible xenoliths.     

Rb-Sr-und K-Ar-Evidenz für eine intensive alpidische Beeinflussung der Paragesteine in Kor-und Saualpe,SE-Ostalpen, Österreich

Zusammenfassung Von den Paragesteinen der Kor-und Saualpe werden 30 Gesamtgesteins-Analysen, 21 Rb–Sr-Glimmer-Analysen und 27 K–Ar-Glimmer-Analysen mitgeteilt. Die Rb–Sr-Altersdaten (Biotit und Hellglimmer) geben ausnahmslos frühalpidische Abkühlalter an: 14 Biotite: 72–84 M.J., 7 Hellglimmer: 82–115 M.J.Die K–Ar-Bestimmungen geben einen deutlichen Hinweis auf Ar-Überschuß bei den 14 Biotiten mit Altersdaten von 76–122 M.J., während, dies bei den Hellglimmern mit 80–95 M.J. nicht so deutlich ist.Gesamtgesteins-Isochronen an drei verschiedenen Gesteinen der Decke (Pilger undWeissenbach, 1965) zeigen eine alpidische Öffnung des Rb–Sr-Systems für die Gesteine an. Die Gesteine aus dem Klieningfenster und die aus dem Wolfsberger-Fenster (Morauf, 1980) zeigen keine alpidische Öffnung an. Damit wird der Befund vonPilger undWeissenbach (1965), daß die Decke höher metamorph ist als das Liegende, voll bestätigt, sowie transportierte Metamorphose (Pilger undWeissenbach, 1971) nachgewiesen.Mit 3 Abbildungen     

Beiträge zur Entwicklung der Kriegsgeologie

Ohne ZusammenfassungTeil I (Einleitung) und II (England) vgl. Geol. Rundschau,11, 1920, S. 329–349.     

Lower Devonian tentaculite bed in the Satun area, southern peninsular Thailand

A Lower Devonian (Emsian) tentaculite fauna including Nowakia acuaria [Richer, R., 1854. Thüringische Tentaculiten. Zeitschr. Deutsch. Geol. Gesellsch. 6, 275–290] occurs in black shale in the basal part of a siliciclastic sequence exposed north of Satun, southern peninsular Thailand. Similar tentaculite beds with Nowakia have been reported from several areas in the Fang, Sri Sawat, and Trang areas of Thailand, the Langkawi Islands and the Mahang–Baling areas of Malaysia. The depositional environments in which the tentaculite-bearing black shale accumulated extended from modern northern Thailand to northwestern Malaysia during the Early Devonian (Emsian).     

Die neue Studien- und Prüfungsordnung für Studierende der Geologie in Deutschland

Ohne ZusammenfassungZ. Deutsch. Geol. Ges.93, S. 47–59, 1941.     

The role of kerogen constituents in oil generation in the Gevar'am Formation (Lower Cretaceous), Israel

Kerogen from various samples of the Gevar'am Formation (Early Cretaceous, Israel) were analyzed for their elemental composition, and by I.R. and ESR spectroscopy. The results were compared with other organochemical characteristics of the basin (Amit, 1978, Bull. Am. Assoc. Pet. Geol., V. 62 p. 827–836). The types of organic material and their role in the formation of oil were investigated. It appears that the kerogen is mostly constituted of terrestrially derived organic matter. A small amount of marine-derived organic matter is also inferred. The kerogen of terrestrial origin is a poor oil producer and is expected to generate oil only at a higher stage of maturation than the Gevar'am Formation has attained. Any hydrocarbons that have been generated in this formation were probably generated from that part of the kerogen thought to be marine derived. Thus the Gevar'am Formation is considered as a potential source rock which has generated only a small amount of oil.     

Comparison between thermal–viscous coupling and frictional sliding

We investigated the similarity between thermal–viscous coupling (TVC) and frictional sliding, proposed by Kameyama and Kaneda [Pure Appl. Geophys. 159 (2002) 2011]. We consider a one-dimensional layer composed of viscous material, which is sandwiched and sheared by two thick elastic layers. The rate of viscous deformation depends on the temperature T_c in the viscous layer as well as shear stress τ. The temperature T_c changes owing to heating by viscous dissipation and conductive cooling. We carried out velocity-stepping tests for the steady-state deformation both numerically and analytically, and compared the temporal evolution of small perturbations with that of the spring-block model with rate- and state-dependent friction (RSF). We found that, as is the case of frictional slip stability, the manner of temporal evolution is classified into four regimes depending on whether it is stable or not and whether it is monotonous or oscillatory with time. By further interpreting TVC in terms of general RSF theory by Ruina [J. Geophys. Res. 88 (1983) 10359], we obtained the relations between the parameters appearing in the phenomenological RSF law and the nondimensional parameters which characterize the nature of TVC. A further improvement of this approach might be important for estimating the actual values of frictional constitutive parameters at the deeper portion of seismogenic faults of interplate or inland earthquakes where a ductile deformation is expected to be significant.     

Focal parameters of seismic sources during the 1981 and 1983 eruption at Mt. Etna volcano (Sicily,Italy)

In this study 50 seismic events, preceding and accompanying the eruptions occurring in 1981 and 1983, have been considered. Seismic moments, fault radii, stress drops and seismic energies have been calculated using Brune’s model (J Geophys Res 75:4997–5009, 1970; J Geophys Res 76:5002, 1971); site, anelastic attenuation along the propagation path, geometrical spreading and interaction with the free surface effects are taken into account. For each event we have also estimated the equivalent Wood–Anderson magnitude (MWAeq) (Scherbaum and Stoll in Bull Seism Soc Am 73:1321–1343, 1983); relations among all these source parameters have been determined. Furthermore, the hypothesis of self-similarity (Aki in J Geophys Res 72:1217–1231, 1967) is not verified for events with seismic moments <10¹² N-m: in fact the relationship between log-stress drop and log-moment is linear up to a moment of 10¹² N-m (events of 1981 eruption), while for higher moments (events of 1983 eruption) the slope of the regression line is not significantly different from zero. We suppose that such a behaviour is related to a heterogeneous medium with barriers on the faults. Finally, the main conclusion is that eruptions of 1981 and 1983 differ from one another both in eruptive and seismic aspects; analysis of seismic energies indicates an increase in Mt. Etna’s activity, confirmed by studies performed on the following lateral eruption of 1991–1993 (Patanè et al. in Bull Volcanol 47:941–952, 1995), occurring on the same structural trend.     

Stresses and velocities in orogenic wedges with power-law rheology and linearly varying longitudinal strain rate

A two-dimensional analytical solution for stress, strain rate, and velocity is obtained for parallel-sided and wedge-shaped blocks with generalized viscous rheology (linearly viscous and power-law) deforming in plane strain. The main assumptions used in the derivation of the solution are that the material is incompressible, the longitudinal gradient in shear stress is much less than the vertical gradient of vertical normal stress, and the longitudinal strain rate varies linearly in the horizontal direction. Velocity boundary conditions are specified at the top of the block, and shear stress boundary conditions at the base of the block. In the one-dimensional case (where stress and strain rate do not vary in the longitudinal direction), the solution reduces to a well-known solution for the deformation of parallel-sided ice sheets [Nye, J. F. (1957) The distribution of stress and velocity in glaciers and ice sheets. Proceedings of the Royal Society of London A-239, 113–133]. The stress equilibrium for tapered wedges [Platt, J. P. (1986) Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks. Geological Society of America Bulletin 97, 1037–1053] is a special case of the present stress solution. Implementation of the solution requires the subdivision of the wedge into vertical segments, and yields the tectonic normal and shear stresses that must be applied to the rear of a block with specified rheology in order to maintain a given longitudinal strain rate. The solution makes it possible to model deformation patterns analytically with longitudinally varying strain rate (including coeval compression and extension) and with vertical components of velocity reflecting the effects of underplating.     

Late Pleistocene eolian features in southeastern Maryland and Chesapeake Bay region indicate strong WNW–NW winds accompanied growth of the Laurentide Ice Sheet

Inactive parabolic dunes are present in southeastern Maryland, USA, along the east bank of the Potomac River. More elongate and finer-grained eolian deposits and paha-like ridges characterize the Potomac River–Patuxent River upland and the west side of Chesapeake Bay. These ridges are streamlined erosional features, veneered with eolian sediment and interspersed with dunes in the low-relief headwaters of Potomac- and Patuxent-river tributaries. Axis data for the dunes and ridges indicate formation by WNW–NW winds. Optically stimulated luminescence and radiocarbon age data suggest dune formation from  33–15 ka, agreeing with the 30–13 ka ages Denny, C.S., Owens, J.P., Sirkin, L., Rubin, M., 1979. The Parsonburg Sand in the central Delmarva Peninsula, Maryland and Delaware. U.S. Geol. Surv. Prof. Pap. 1067-B, 16 pp. suggested for eolian deposits east of Chesapeake Bay. Age range and paleowind direction(s) for eolian features in the Bay region approximate those for late Wisconsin loess in the North American midcontinent. Formation of midcontinent loess and Bay-region eolian features was coeval with rapid growth of the Laurentide Ice Sheet and strong cooling episodes (δ¹⁸O minima) evident in Greenland ice cores. Age and paleowind-direction coincidence, for eolian features in the midcontinent and Bay region, indicates strong mid-latitude WNW–NW winds for several hundred kilometers south of the Laurentide glacial terminus that were oblique to previously simulated anticyclonic winds for the last glacial maximum.     

Enregistrement de phénomènes de resédimentation au cours du Maastrichtien supérieur et du Paléocène supérieur (J. Serj, Tunisie atlasique) : apport micropaléontologique et signature tectono-eustatique

Sedimentological and biostratigraphic analysis of Campanian–Maastrichtian reduced series of the J. Serj permitted to characterize two episodes of submarine erosion. The first is pre-Upper Maastrichtian (Rosita contusa zone), the second is pre-Upper Palaeocene (Morozovella velascoensis zone). Thus, two type-1 boundary surfaces are distinguished. These surfaces limit three stratigraphic intervals in apparent continuity. Syn-sedimentary tectonic activity is attested by gravity deposits or processes (conglomerates, slumps). To cite this article: A. Amri et al., C. R. Geoscience 337 (2005).