AMS record of brittle dilation,viscous-stretching and gravity-driven magma ascent in area of magma-rich crustal extension (Vosges Mts., NE France)

Orogenic compression-related fabrics (~340–335 Ma) were reworked during regional extensional deformation (~328–325 Ma) in a large anatectic crustal domain of the Central Vosges (NE France). The extension was first accommodated by brittle dilation affecting vertically anisotropic high-grade rocks associated with emplacement of subvertical granitic sheets. The AMS fabric of granitoids is consistent with highly partitioned transtensional deformation marked by alternations of flat and steep foliations and development of orthogonal lineations. This deformation passes to top-to-the-southwest ductile shearing expressed in southerly migmatitic middle crust. The AMS fabric revealed moderately west-dipping foliations bearing subhorizontal NNW–SSE-trending lineations and predominantly plane strain to prolate shapes. This fabric pattern is interpreted as a viscous response of stretched partially molten crust during continuous ductile extension. Vertical ascent of voluminous granites and stoping of the upper crust occurs further south. This gravity ascent triggered by extension leads to development of south-dipping AMS foliations, south-plunging lineations and oblate fabrics in various crustal granites. Vertical shortening related to ascent of these (~325 Ma) granitoids and persistent N–S stretching is responsible for reworking and remelting of originally vertical compression-related fabric in roof supracrustal granites (~340 Ma) and development of highly prolate fabrics in these rocks. This work shows that the finite shape of AMS fabric ellipsoid is highly sensitive to both strain regime and superpositions of orthogonal deformation events.     

Biochemogenic Calcite as an Important Component of Matter Turnover in the Eutrophic Mozhaisk Reservoir

Microcrystals of biochemogenic calcite were found for the first time in the water of the slightly mineralized and eutrophic Mozhaisk Reservoir in Moscow region. Photographs of eight samples of such particles of autochthonous mineral suspension, magnified several tens of thousand times are presented, and the conditions of their crystallization in the trophogenic layer of the reservoir are described in detail. Analysis of data collected during seven-year hydroenvironmental observations of diurnal variations in the characteristics of the state of aquatic ecosystem was used to demonstrate the role of various physical, chemical, and biological processes, biochemic calcite, and mineral phosphorus in the minor cycle of matter. This cycle in the upper part of the stratified water mass serves as a natural mechanism regulating the intensity of blooming in eutrophic lakes and reservoirs.__________Translated from Vodnye Resursy, Vol. 32, No. 4, 2005, pp. 477–488.Original Russian Text Copyright © 2005 by Edel’shtein, Ershova, Zaslavskaya.     

An Andean type Palaeozoic convergence in the Bohemian Massif

The geological inventory of the Variscan Bohemian Massif can be summarized as a result of Early Devonian subduction of the Saxothuringian ocean of unknown size underneath the eastern continental plate represented by the present-day Teplá-Barrandian and Moldanubian domains. During mid-Devonian, the Saxothuringian passive margin sequences and relics of Ordovician oceanic crust have been obducted over the Saxothuringian basement in conjunction with extrusion of the Teplá-Barrandian middle crust along the so-called Teplá suture zone. This event was connected with the development of the magmatic arc further east, together with a fore-arc basin on the Teplá-Barrandian crust. The back-arc region – the future Moldanubian zone – was affected by lithospheric thinning which marginally affected also the eastern Brunia continental crust. The subduction stage was followed by a collisional event caused by the arrival of the Saxothuringian continental crust that was associated with crustal thickening and the development of the orogenic root system in the magmatic arc and back-arc region of the orogen. The thickening was associated with depression of the Moho and the flux of the Saxothuringian felsic crust into the root area. Originally subhorizontal anisotropy in the root zone was subsequently folded by crustal-scale cusp folds in front of the Brunia backstop. During the Visean, the Brunia continent indented the thickened crustal root, resulting in the root's massive shortening causing vertical extrusion of the orogenic lower crust, which changed to a horizontal viscous channel flow of extruded lower crustal material in the mid- to supra-crustal levels. Hot orogenic lower crustal rocks were extruded: (1) in a narrow channel parallel to the former Teplá suture surface; (2) in the central part of the root zone in the form of large scale antiformal structure; and (3) in form of hot fold nappe over the Brunia promontory, where it produced Barrovian metamorphism and subsequent imbrications of its upper part. The extruded deeper parts of the orogenic root reached the surface, which soon thereafter resulted in the sedimentation of lower-crustal rocks pebbles in the thick foreland Culm basin on the stable part of the Brunia continent. Finally, during the Westfalian, the foreland Culm wedge was involved into imbricated nappe stack together with basement and orogenic channel flow nappes.     

Sedimentation of suspension in the Mozhaisk Reservoir

Sedimentation fluxes of suspension were determined in experimental studies on the Mozhaiskoe Reservoir in summer. The leading factors of formation and the rate of accumulation of sedimentary material and organic mater in the reservoir in different phases of synoptic cycle in the channel and shallow zones of the bed in the central part of the reservoir.     

Synoptic variability of the thermodynamical state of Rybinsk Reservoir water masses

Presented is a brief characteristic of structurally new grid-box mathematical model for calculation of characteristics of hydrological regime of reservoirs of any size and residence time, morphological class and hydroeconomic purpose, with time resolution of 24 h and depth increment of 1 m. Presented are the results of its verification, as well as the fragments of results of diagnostic calculation of changes in vertical distribution of temperature, mineralization, and discharges of katabatic, density, wind drift and compensated flows in separate segments of four reaches of the Rybinsk Reservoir in case of weather changing for all seasons of two years with extreme flow.     

Late Hercynian remagnetization of Tournaisian series from the Laval syncline,Armorican Massif,France

Paleomagnetic investigations have been conducted on Tournaisian volcanics and sediments from the Laval syncline, in order to evaluate the consequences of the Late Variscan tectonic and thermic phases. On the southern flank of the syncline, anisotropy measurements have yielded maximum susceptibility in the schistosity planes. Thermal demagnetization exhibits only remagnetizations, characterized by a large range of blocking temperatures (from 350° to 670°C). Two phases of remagnetization seem to follow each other. The first may have occurred during the Tournaisian major orogenic phase. The second has taken place during the latest Stephanian/earliest Permian, in relation with the latest Variscan thermal event. All results are in favour of a clockwise rotation by 15–40° of the Laval syncline after the latest overprints. According to results from other regions of Brittany, this clockwise rotation involved the whole Armorican Massif. This motion may be related to the latest compression phase which has tightened the Ibero-Armorican arc, well before the Permo-Triassic opening of the Bay of Biscay.     

Paleomagnetic evolution of the Central Massif (France) during the Carboniferous

The present paper aims to synthesize results of a systematic paleomagnetic investigation performed on metamorphic, plutonic and volcanic series from the Central Massif. Detailed, thermal and alternating field demagnetizations yield a large set of paleomagnetic directions. Several groups of directions corresponding to different age intervals are identified. The group D mean direction: D = 288°, I = 57° (37°S, 110°E), characterizes Late Devonian/Early Carboniferous metamorphic and plutonic rocks from Limousin. The group C′ directions: D = 301°, I = 24° (30°S, 79°E), represent Late Visean/Namurian magnetizations, present in the major investigated areas. The group B directions: D = 249°, I = 7° (12°N, 111°E), exist not only in the whole Central Massif, but also in other Paleozoic outcrops of the Variscan belt. They were acquired during the Namurian/Westphalian. The group A′-A directions are the only typically “European” magnetic directions. They have taken place in Stephanian/Autunian times, mainly during the Kiaman reversed interval. Interpretation of these directions in terms of geodynamics leads to a probable large S-N drift of the massif during the Latest Devonian/Early Carboniferous followed by two important rotation phases, first in the Middle Carboniferous, then at the end of the Westphalian. These rotations have also affected other massifs of the Variscan belt.     

Mise en evidence par le paleomagnetisme d'une importante rotation antihoraire des Vosges meridionales entre le Viseen terminal et le Westphalien superieur

The volcano-sedimentary formations from the southern Vosges are subdivided in two main series: a lower Visean series characterized by a volcanism of spilite-keratophyre type, and an upper Visean series which includes a normal volcanic association of shoshonitic tendency. Paleomagnetic study of 50 sites sampled in both series, but mostly in the upper one, yields three types of directions of characteristic magnetizations. The first type corresponds to Tertiary and Quaternary remagnetizations with low apparent blocking temperatures (350°–500°C, titano-maghemites?). The second group is formed by remagnetizations which have taken place during late Carboniferous-early Permian times, and which show high blocking temperatures of magnetite and mostly titano-haematites. The mean direction is D = 16°, I = 7°, α₉₅ = 9° for 13 sites, (λ = 43°N, φ = 165°E). The last group is represented by primary magnetizations of latest Visean age and post-Sudetic remagnetizations, with blocking temperatures of magnetite and haematite. The mean direction D = 323°, I = −17°, α₉₅ = 9° for 18 sites, (λ = 25°N, φ = 228°E), deviates from about 60° from the theoretical direction, calculated with the early Carboniferous, European pole position. This deviation is interpreted as resulting from a counterclockwise rotation of the southern Vosges between late Visean and Westphalian times. One consequence may be the formation of the variscan “V”, due to the anticlockwise rotation of the eastern branch of the chain. The northwesterly directions show a variation of the inclinations which may indicate that the rotation was preceded by a relatively significant drift of the Vosges to the north.

Résumé

Les terrains volcano-sédimentaires des Vosges méridionales se subdivisent en deux séries principales: la série du Viséen inférieur caractérisée par un volcanisme du type spilite-kératophyre et la série du Viséen supérieur qui comporte une association volcanique normale à tendance shoshonitique. L'étude paléomagnétique de 50 sites échantillonnés dans les deux séries, avec une prédominance dans la série supérieure, met en évidence trois types de directions d'aimantations caractéristiques, Le premier type correspond à des réaimantations d'áge Tertiaire à Quaternaire, à températures de blocage apparentes basses (350°–500°C, titano-maghemites?). Le second groupe est f'orme par des réaimantations mises en place au Carbonifère supérieur-Permien inférieur, à température de blocage haute de magnétite et surtout de titanohématites. La direction moyenne est D = 16°, I = 7°, α₉₅ = 9° pour 13 sites. (λ = 43°N, φ = 165°E). Le dernier groupe est représenté par des aimantations primaires, d'âge Viséen supérieur et des réaimantations post phase Sudète II, à température de blocage de magnetite et d'hématite. La direction moyenne D = 323°, I = −17°, α₉₅ = 9° pour 18 sites (λ = 25 °N, φ = 228°E), dévie de prés de 60° de la direction théorique calculée à partir du pôle européen au Carbonifère inférieur. Cette déviation est interprétée comme résultant d'une rotation antihoraire des Vosges méridionales entre le Viséen supérieur et le Westphalien. Une des conséquences en serait la formation du “V” varisque. par suite de la rotation antihoraire de la branche orientale de la chaîne. Les directions nord-ouest présentent une variation en inclinaison qui semble indiquer que la rotation antihoraire était précédée par une dérive relativement importante des Vosges vers le Nord.