A paleomagnetic cross-section through the Ardenne and the Brabant Massifs (France-Belgium)

In order to constrain the motions of the allochthonous Ardenne during the Paleozoic, standard paleomagnetic techniques have been applied on acidic and basic sills of the Cambrian Rocroi massif and on Cambrian-to-Visean sedimentary series sampled along the Meuse valley. Ordovician–Silurian volcanics and Devonian–Visean limestones have also been collected in the autochtonous Brabant and Namur basin, in order to get paleomagnetic reference poles. In the Ardenne, two groups of in situ paleomagnetic directions arise. The A components (mean: D = 212°, I = -11°, α95 = 6°, for 10 sites), that are characterized by unblocking temperatures around 330° C, represent Late Variscan (Stephanian–Permian) overprints. The B components (mean: D = 236°, I = 2°, α95 = 11°, for 13 sites) display higher unblocking temperatures, in the range 400°-580° C. They show a large scatter in inclination (–25° < I < 35°) that is not reduced by tectonic correction. The B components are interpreted as post- and partly synfolding overprints of Middle-Late Carboniferous age. In the Brabant, all series display Late Variscan remagnetizations (mean: D = 204°, I = -7°, α95 = 9°, for 5 sites), consistent with the Stephanian- Permian pole of Europe. The existence of Middle-to-Late Variscan B directions in the Armorican Massif, Central Massif, Vosges and Black Forest indicates that in Namurian–Westphalian times the whole investigated Variscan belt, including the Ardenne, was trending N-S.  A 45° clockwise rotation relative to the paleomeridian, during the latest Westphalian–Stephanian, has lead the massif to its Permian position.           ARK: https://n2t.net/ark:/88439/y010025 Permalink: https://geophysicsjournal.com/article/140 &nbsp

Deep structure of the southern Rhinegraben area from seismic refraction investigations

A joint interpretation of all seismic-refraction profiles in the southern part of the Rhinegraben area is presented. A time-term analysis of all Pg-arrivals reveals the topography of the crystalline basement and provides an average velocity of 6.0 km/s for the uppermost crust. The crust-mantle boundary is clearly elevated in the Rhinegraben rift system forming an arch with a span of 150-180 km and reaching a depth of only 25 km at the flanks of the graben proper. The velocity of P-waves in the uppermost mantle is 8.0-8.1 km/s. Below the flanks of the graben, the crust-mantle boundary is formed by a first-order discontinuity. Within the graben proper it is replaced by a transition zone of 4 km thickness with the strongest velocity gradient at a depth of 21 km. This transition zone is regarded as region of crust-mantle interaction and seems to be confined to the graben proper.         ARK: https://n2t.net/ark:/88439/y074159 Permalink: https://geophysicsjournal.com/article/71 &nbsp

Crustal influx, indentation, ductile thinning and gravity redistribution in a continental wedge: Building a Moldanubian mantled gneiss dome with underthrust Saxothuringian material (European Variscan belt)

27 p.International audience[1] The contribution of lateral forces, vertical load, gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated. In the Orlica-Snieznik dome (Moldanubian zone, European Variscan belt), the polyphase tectono-metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium- to high-grade metamorphic conditions in different levels of the crust. It reflects the eastward influx of a Saxothuringian-type passive margin sequence below a Teplá-Barrandian upper plate. The ongoing influx of continental crust creates a thick felsic orogenic root with HP rocks and migmatitic orthogneiss. The orogenic wedge is subsequently indented by the eastern Brunia microcontinent producing a multiscale folding of the orogenic infrastructure. The resulting kilometre-scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms. These localized vertical exchanges of material and heat are coeval with a larger crustal-scale folding of the whole infrastructure generating a general uplift of the dome. It is exemplified by increasing metamorphic conditions and younging of 40Ar/39Ar cooling ages toward the extruded migmatitic subdomes cored by HP rocks. The vertical growth of the dome induces exhumation by pure shear-dominated ductile thinning laterally evolving to non-coaxial detachment faulting, while erosion feeds the surrounding sedimentary basins. Modeling of the Bouguer anomaly grid is compatible with crustal-scale mass transfers between a dense superstructure and a lighter infrastructure. The model implies that the Moldanubian Orlica-Snieznik mantled gneiss dome derives from polyphase recycling of Saxothuringian material

A new upper bound for the cross number of finite Abelian groups

In this paper, building among others on earlier works by U. Krause and C. Zahlten (dealing with the case of cyclic groups), we obtain a new upper bound for the little cross number valid in the general case of arbitrary finite Abelian groups. Given a finite Abelian group, this upper bound appears to depend only on the rank and on the number of distinct prime divisors of the exponent. The main theorem of this paper allows us, among other consequences, to prove that a classical conjecture concerning the cross and little cross numbers of finite Abelian groups holds asymptotically in at least two different directions.Comment: 21 pages, to appear in Israel Journal of Mathematic

Palaeomagnetic and structural constraints on 90° anticlockwise rotation in SW Mongolia during the Permo-Triassic: Implications for Altaid oroclinal bending. Preliminary palaeomagnetic results.

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