Insight into the structure of the Upper Rhine Graben and its basement from a new compilation of Bouguer Gravity

A compilation of gravity data from the Upper Rhine Graben (URG) is presented that includes all the main data sources from its German and French parts. This data is used to show that the URG consists of, at least, two arc-shaped and asymmetric rift units that tectonically are the basic building blocks of the graben. In this sense the URG does not differ from other continental rifts, such as the African rifts. This division should replace the now classical geomorphologic division of the URG into three segments, based on their different trends. Moreover, the gravity suggests that the faults in the central and southern segments are continuous and have the same trend, appearing to respond as a single kinematic unit. Changes in the gravity field in the graben are shown to reflect not only the structure of the graben, but also the highly variable composition of the basement. In this respect, the URG is quite different from some other Tertiary continental rifts, where possible changes in the composition of the basement are mostly masked in the gravity field by the effect of the overlying low-density sediments. This characteristic is used to study the extent of some of the main basement units that underlie the graben.     

A 140 Ma K/Ar age for the Msissi norite (Morocco): new geochemical and paleomagnetic data

The plutonic intrusion known as the Msissi norite (southern Morocco), which represents the single paleomagnetic reference pole for Africa in Devonian times, has been investigated. This Msissi intrusion is in fact an alkaline gabbro, a teschenite. K/Ar dating on separate biotites yields a 140 Ma age. Paleomagnetic results reveal a large scatter of isolated directions that precludes the computation of any Late Jurassic/Early Cretaceous paleopole. All these data lead us to reject this intrusion as a reference pole for the Devonian in Africa. The paleoposition of Africa in Devonian times remains, therefore, largely unknown.     

Cadomian terranes,wrench faulting and thrusting in the central Europe Variscides: geophysical and geological evidence

Sixty five per cent of the Paleozoic basement of western and central Europe is hidden by a sedimentary cover and/or sea. This work aims to remove that blanket to detect new structures which could used to build a more comprehensive model of the Variscan orogeny. It is based on the interpretation of various forms of data: (a) published gravity maps corrected for the effects of the crust-mantle boundary topography and light sedimentary basins; (b) aeromagnetic maps; (c) measurements of densities; and (d) induced and remanent magnetizations on rocks from Paleozoic outcrops of the upper Rhenish area. From the northern Bohemian Massif to the eastern Paris Basin, the Saxothuringian is characterized by a 500 km long belt of gravity highs, the most important being the Kraichgau high. Most of the corresponding heavy bodies are buried under a post-early Viséan cover. They are interpreted as relics of Late Proterozoic terranes overlain by an Early to Middle Paleozoic sequence, equivalent to the Bohemian terrane in the Bohemian Massif. The most probable continuation of these dense Bohemian terranes toward the west is the Southern Channel-Northern Brittany Cadomian terrane. The gravity lows are correlated with Variscan granites and pre- and early Variscan metagranites.Gravity and magnetic maps demonstrate large-scale displacement in Devonian-Early Carboniferous times along the parallel and equidistant, NW-SE striking, Vistula, Elbe, Bavarian, Bray and South Armorican dextral wrench faults. In the Vosges-Schwarzwald and Central Massif the faults continue with the east-west striking Lalaye-Lubine-Baden-Baden and Marche faults and with south vergent thrusts. The Bavarian faults shift the Kraichgau terrane by 150 km relative to the Bohemian terrane, whereas the offset of the Northern Brittany Cadomian relative to the Northern Vosges-Kraichgau terranes is estimated at 400 km along the Bray fault. Sinistral wrench faults are the NE-SW striking Sillon Houiller, Rheingraben, Rodl, Vitis and Diendorf faults. The southern Vosges-Schwarzwald Devonian-Dinantian basin is interpreted as a pull-apart basin at the south-easterly extremity of the Bray fault. The Bohemian and Kraichgau body form allochthonous terranes which were thrust over the Saxothuringian crust. Thrusting to the north-west was accompanied by back-thrusting and led to the formation of pop-up structures. Contemporaneous dextral and sinistral wrench faulting resulted in transpressive strain during collision. The zonal structure of the Variscides in the sense of Kossmat (1927) is relevant only to the Rhenohercynian Foreland Belt. Kossmat (1927) already spoke of a Moldanubian Region because it displays no real zonal structure. The Saxothuringian Zone was formed by terrane accretion. Their apparent zonal structure is not a pre-collisional feature, but only the result of accretion and collision.     

Bottom sediments of the oldest reservoir in Moscow region

The methodological features and results of two soil surveys of the Senezh Reservoir carried out in 1992 and 2006 as a part of multidisciplinary limnological studies of this large recreation water body with the aim to monitor its ecological state and prepare recommendations for water protection measures are analyzed     

Paleomagnetic evidence for a large counterclockwise rotation of northern Greece prior to the Tertiary clockwise rotation

Abstract

This study aims at unravel the geotectonic evolution of northern Greece prior to the already established Tertiary clockwise rotation. Therefore, Mesozoie sediments, Early Mesozoie ophiolites and Carboniferous granites were sampled. While the metamorphosed and/or too weakly magnetized limestones had to be rejected, the gabbros and serpentinites of the 80 km long Chalkidiki belt (40.4°N, 23.3”E), and the granites of the northern Pelagonian zone (40.8°N, 21.2°E) have yielded similar results interpretable in terms of geoleetonies. In both areas the demagnetizing process has revealed a poh phased magnetic evolution.

The oldest magnetizations, labelled M (D=311°, I=20°, a₉₅, = 15°; VGP: 37°N, 272.5°, for the ophiolites; D=320.5°, I = 26°, a₉₅ =11°; VGP : 46°N, 264.5”E, for the granites) are interpreted as overprints acquired in Late Jurassic-Cretaceous times. The younger magnetizations, called C2 (D = 66°, I = 28°, a₉₅ = 9°; VGP : 28°N, 117°E, in the ophiolites ; D=64°, I = 2° a₉₅, = 11°; VCP : 20°N, I28°E, in the granites) are Tertiary overprints. Northeasterly C’ directions with negative inclinations (and conversely) are considered as overprints empiaceli prior to the Ca magnetizations ; they are interpreted as due to a barkthrusting of the ophiolilic belt of Chalkidiki and of the N. Pelagonian granitic belt, during the Early - Middle Tertiary convergence phase. The large deviation from the M to the C₂ directions, also observed by other authors in Mesozoic volcanics and sediments, results from a counterclockwise rotation of the Hellenides, probably in the Late Cretaceous as it is the case for the counterclockwise rotations of the western Mediterranean microplates. The deviation from the C₂ to the present field direction is due to a clockwise rotation of all Hellenic zones, probably in several phases.     

The Variscan tectonic inheritance of the Upper Rhine Graben: evidence of reactivations in the Lias,Late Eocene–Oligocene up to the recent

Processing of gravity and magnetic maps shows that the basement of the Upper Rhine Graben area is characterized by a series of NE–SW trending discontinuities and elongated structures, identified in outcrops in the Vosges, Black Forest, and the Odenwald Mountains. They form a 40 km wide, N30–40° striking, sinistral wrench-zone that, in the Visean, shifted the Variscan and pre-Variscan structures by at least 43 km to the NE. Wrenching was associated with emplacement of several generations of plutonic bodies emplaced in the time range 340–325 Ma. The sub-vertical, NE–SW trending discontinuities in the basement acted as zones of weakness, susceptible to reactivation by subsequent tectonism. The first reactivation, marked by mineralizations and palaeomagnetic overprinting along NE–SW faults of the Vosges Mountains, results from the Liassic NW–SE extension contemporaneous with the break-up of Pangea. The major reactivation occurred during the Late Eocene N–S compression and the Early-Middle Oligocene E–W extension. The NE–SW striking basement discontinuities were successively reactivated as sinistral strike-slip faults, and as oblique normal faults. Elongated depocenters appear to form in association with reactivated Variscan wrench faults. Some of the recent earthquakes are located on NE–SW striking Variscan fault zones, and show sinistral strike-slip focal mechanisms with the same direction, suggesting also present reactivation.     

Magnetic overprints in granitoids and metamorphic rocks from Limousin (France): evidence for Late Variscan rotations, crustal folding and tilting

Generally, the lack of bedding criteria in basement units hampers the interpretation of paleomagnetic results in terms of geotectonics. Nevertheless, this work demonstrates that successive remagnetizations recorded in Early Carboniferous metamorphic and plutonic units, without clear bedding criteria, can be used to constrain a polyphased tectonic evolution consisting of a regional clockwise rotation, followed by a folding phase, a tilting phase and a second regional clockwise rotation.Metamorphic, ultrabasic, tonalitic and granitic rocks from different parts of Limousin (western French Massif central; 45.5°N/1.25°E), which underwent metamorphism during Devonian–Early Carboniferous or were intruded in the Early–Middle Carboniferous, were sampled in order (a) to identify the magnetic overprinting phases and the related tectono-magmatic events and (b) to constrain the regional and plate tectonic evolution of Limousin. Paleomagnetic results from 32 new and 26 sites investigated previously show that at least 90% of the magnetization isolated in rocks older than 330 Ma are overprints. In agreement with results from adjacent areas of the Variscan belt, the major overprinting phases occurred: (a) in the last stages of the major exhumation phase [332–328 Ma; mean Virtual Geomagnetic Pole (VGP) “Cp”: 37°N/70.5°E], (b) during the post-collisional syn-orogenic extension (325–315 Ma; VGP “B”: 11°N/114°E), (c) in the Latest Carboniferous and Early Permian (VGP “A1”: 27°N/149°E) and (d) in the Late Permian (VGP “A”: 48°N/146°E). The Middle–Late Carboniferous overprints “Cp” and “B” are contemporaneous with emplacement of leucogranitic, crustal derived plutons, and probably result from the hydro-thermal activity related to the magmatism. The drift from “Cp” directions to “B” directions implies that after 330 Ma, Limousin underwent a clockwise rotation by 65°, together with the Central Europe Variscides. The “Bt” components, the VGPs of which deviate from the mean apparent polar wander path (APWP) of the belt, are interpreted as “B” overprints tilted during Late Variscan tectonics, that is, in the time range 325–315 Ma. The first and most important generation of “Bt” overprints was tilted during NW–SE folding associated with NE–SW shortening, updoming and emplacement of leucogranitic plutons. The second generation reveals southeastward tilting due to NE-striking normal faulting. The drift from “B” to “A1” directions implies that Limousin has participated to the second clockwise rotation by 40° of the whole belt in Westphalian times.     

The Rate of Oxygen Release from Freshwater Bodies

The annual regimes of estimated rates of oxygen and carbon dioxide exchange between water surface and the atmosphere was compared for two water bodies with different sizes and hydroecological conditions: the deep oligotrophic Lake Baikal and the small mesotrophic–eutrophic Mozhaisk Reservoir with water exchange rate 150 times greater than that of Baikal. The obtained, very large, differences between the rates of gas evasions and invasions in the lake and the reservoir allow the rates of these processes to be used as integral characteristics for the parameterization of the self-purification capacity of freshwater bodies from organic pollutants.     

Variations in the Hydroecological Characteristics of Water Masses in the Central Pool of the Mozhaisk Reservoir

Results of a special experiment conducted in the summer of 2002 in the Central pool of the Mozhaisk Reservoir are discussed. The experiment was aimed at studying short-period and small-scale variations in water temperature, electric conductivity, transparency, dissolved oxygen content, total and mineral phosphorus, phyto-, microzoo- and zooplankton. Statistical characteristics of the above-mentioned environmental parameters are presented.__________Translated from Vodnye Resursy, Vol. 32, No. 3, 2005, pp. 352–360.Original Russian Text Copyright © 2005 by Datsenko, Edelshtein, Goncharov, Puklakov, Chernega.