The Variscan belt of Northern France–Southern Belgium: geodynamic implications of new palaeomagnetic data

• Published in: Tectonophysics 2000-09, Vol.324 (1), p.57-80
• Main Authors: Márton, Emő, Mansy, Jean-Louis, Averbuch, Olivier, Csontos, László
• Format: Article
• Language: English
• Subjects: geodynamic implication; palaeomagnetism; Palaeozoic; Variscan belt
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/S0040-1951(00)00109-8

Palaeomagnetic investigations were carried out in Devonian–early Carboniferous rocks of the Variscan foreland chain of Northern France–Southern Belgium in order to reveal the origin of its arcuate shape. The Brabant Parautochthon was sampled in the Boulonnais (near Calais) and near Tournai, while the Ardenne Allochthon was sampled near Maubeuge and in the Givet area. All the sampled localities yielded characteristic remanent magnetization as a result of stepwise demagnetization and component analysis. Fold or tilt tests were possible for three localities, with negative results indicating pervasive remagnetization. The tectonic position was sub-horizontal at two localities, while the tilt was monoclinal for the rest. Therefore, the acquisition time of the magnetic signals was estimated by comparing the palaeolatitude computed from each magnetic component to the palaeolatitudes of Variscan Europe calculated after Van der Voo (1993). Three components showing: A, a southern B, a near-Equatorial, and C, a northern palaeolatitude are recognized from our data. Since a pre-Variscan age of component A (observed only in Boulonnais, at 10 sites) is not supported by data, it is assigned to an early phase of deformation. Component B (16 sites) was acquired during the peak of the Variscan tectonics (late Westphalian), while component C (five sites) originated during Permian times. Regardless of the palaeolatitudes, declinations fall between 190 and 210°, thus being conformable with the declinations expected for Variscan Europe. The declinations show no correlation with the arcuate shape of the belt, neither are they different in the Paraauthochthon and in the Allochthon, nor in the different components. Arc formation by moulding of the Allochthon on the Brabant Parautochthon is, therefore, not supported by these data, since this mechanism requires substantial (opposed) rotations on both wings of the arc. The available palaeomagnetic data are conformable with a pre-formed arc, simply docking to the Brabant obstacle of similar shape. Variable offsets along a main thrust make possible a third model, which slightly unfolds the former passive arc.