ALBERT

Notes: This paper demonstrates how magnetic overprints of a geological series can provide information over a long time period, which can be interpreted in terms of geotectonic evolution. According to these new results, the Late Carboniferous-Early Permian rhyolites from the northern Vosges have recorded the magnetic field over a major part of the Permian. Recent radiometric dating assigns a Late Carboniferous-Early Permian age (298 Ma) to the previously ‘Middle Permian’ rhyolitic volcanism of the northern Vosges. A palaeomagnetic study was undertaken on rhyolites of the Nideck-Donon massif and on neighbouring Permian basins (Villé, Saint-Dié) in order to obtain a new pole for the Early Permian and to detect overprints. Standard palaeomagnetic and rock magnetic experiments demonstrate that over 90 per cent of the characteristic directions of magnetizations are of secondary origin. The high-temperature magnetizations carried by secondary haematite and haematized titano-magnetite fail the fold test, and display directions consistent with the Saxonian-Thuringian directions from Variscan Europe (Nideck-Donon: 186°/−17°, k = 167 for 26 sites, VGP: 50°N/178°E; S.aint-Dié basin: 184°/−20.5° for two sites, VGP: 53°N/180°E; Villé basin: 191.5°/−25° for two sites, VGP: 53°N/168 E; Vosges dikes: 197.5°/−26.5° for two sites, VGP: 53°N/158°E). Part of the magnetizations with intermediate unblocking temperatures of magnetite consists also of post-tectonic overprints. Two consistent mean directions with lower inclinations, obtained in the Nideck-Donon rhyolites (182°/−7°, k= 459 for 13 sites, VGP: 45°N/184°E) and pyroclastic deposits from the Villébasin (178°/−9, k = 519 for three sites, VGP: 46° N/190° E), were probably acquired during Late Autunian-Early Saxonian times. In rhyolites the pre- and post-tectonic characteristic mag netizations are carried by magnetite, while in the ash-fall deposits the carrier is allogenic haematite. A few older magnetizations (191/−3, two sites, VGP: 42°N/173°E), consistent with magnetizations carried by magnetite from rhyolites of the central Black Forest, and results from coeval volcanics as well as from Autunian sediments have been observed in tuffs from the Villi basin (188°/1.5°, two sites, VGP: 40°N/177°E. The oldest magnetizations (207°/10°, k = 329, VGP: 32°N/155°E), presumably of Stephanian age, were identified in two rhyolitic stocks within the granitic basement from the central Vosges and central Black Forest. The distribution of the palaeomagnetic directions suggests that, from the Late Autunian to the Late Thuringian, overprinting due to low-temperature alteration of titano-magnetite and crystallization of secondary haematite was more or less continuous.The apparent polar wander path (APWP) computed with the new results and the published poles shows a hairpin, which implies a drastic change of the European plate motion during the Permian. The clockwise rotation of Europe initiated in the Late Visean-Namurian stopped in the Late Autunian-Early Saxonian. This event corresponds to the end of the Variscan convergence and of the Appalachian orogeny. In the northern Vosges, the hinge of the APWP is also associated with the tectonic phase responsible for the tilting of the volcanic layers. The motion of the European plate was then converted into a counterclockwise rotation and a northward drift until the Late Triassic.

Notes: The investigated material comprises rocks of the Palaeozoic ophiolitic assemblage exposed around the Sowie Góry Mountains Massif in the Sudetes in the form of the three units: ślza, Zαbkowice and Nowa Ruda. The radiometric age (U-Pb) of the rocks, stated as 420 ± 20 Ma, places them in the Silurian. A palaeomagnetic study reveals the presence of four groups of palaeomagnetic poles. One of these is derived from an isolated component representing Permian overprinting. The second remanence component fits with the Silurian results from Baltica and may therefore be regarded as being of Silurian age. According to this interpretation, the ophiolitic complex would not have been involved in tectonic action after Silurian times. On the other hand, this component does not agree with the results of the neighbouring Bohemian Massif but is consistent with Late Carboniferous data of the European Variscans. In this case the other two components should be interpreted as Early- to Mid-Carboniferous overprints. This interpretation implies important strike-slip movements. In order to exclude one of these hypotheses, more data are still needed.

Notes: Abstract  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.

Notes: Abstract 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.