Paleomagnetic Evidence for Crustal and Thin-skinned Rotations in the European Hercynides

Devonian and Carboniferous paleoArmorican to the central European Variscan strucmagnetic data from the European Hercynides show a rural trend (Figure 1) and its potential imporcoherent pattern in inclination but significant tance for geodynamics is only one aspect of this dispersion in declination. The declination anomaplate tectonic enigma. The bending around the Bay lies with respect to expected declinations for of Biscay (the Ibero-Armorican arc) , originally stable Europe indicate a strong correlation with recognized and discussed by Argand [1924] and the regional changes in the structural trend of Carey [1958], totals at least 165 ø [Ries and the mountain belt. Detailed analysis of the paleShackleton, 1976] in a pre-Mesozoic reconstrucomagnetic and structural data revealed the hightion [Van der Voo, 1969], whereas the curvature est degree of correlation between changes in of the orogen in central Europe is approximately strike and declination anomalies for external 40 o. parts of the orogen, where thin-skinned nappe emTo test the validity of several possible modplacement has been most prevalent. The correlaels proposed to explain the curvatures [e.g. Pertion between changes in strike and declination roud and Bonhommet, 1981, 1983], it is very imanomalies from interior partes of the Hercynides portant to reconstruct the direction of the reis less well defined and displays a greater degional stress field during Variscan times. One gree of scatter. NevertheleSs the partly secpossible approach is the determination of strucondary character of the arcuate shape can be tural geometries involved in possibly bent fold demonstrated. Thus the inter•al Hercynides are at belts and a test of primary versus secondary orileast in part a true orocline with secondary gin as proposed by Carey [1958] in his orocline bending in the sense of S.W. Carey. We in. fear that concept. Paleomagnetic data, especially declinathe bending of the European Hercynides resulted tions, are ideal for such a determination. from the combination of multiple d•eep-reaching deformations of the European lithosphere as well The paleomagnetic declinations as the effects of thin-skinned rotations. The decreasing degree of rotations toward the crysA substantial paleomagnetic data base (Table talline interior of the belt appears to favor a 1) now exists for Devonian and Carboniferous geodynamic model involving indentations and/or rocks from different parts of the Hercynian mounbuttressing effects. The indentation of Hercynian tain belt. Positive fold tests and/or inclination Europe by a microplate or an African promontory values, in agreement with the apparent polar during the Hercynian orogeny might be one of the wander path for stable Europe [Duff, 1980, principal causes for the secondary bending and French, 1976], constrain the ages of magnetizathe associated nappe rotations. tion to the Devono-Carboniferous. To calculate