Blackwell Publishing Journal Backfiles 1879-2005
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.
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