Paleomagnetic results in support of a model for the origin of the Asturian arc

Abstract

Paleomagnetic directions have been determined at 17 Cambrian and 28 Carboniferous limestone sites distributed along the Asturian arc of the Hercynian fold belt in northwestern Spain. The stable vectors are carried by hematite in all cases, and also by magnetite in some grey Carboniferous limestones. A secondary component of magnetization is usually removed well below 400°C, and the higher-temperature component is taken to be the characteristic direction. The structure at each site was carefully evaluated in order to make optimum local tectonic corrections. In addition the data must be corrected for rotations about a vertical axis to allow for a phase of radial folding superimposed on earlier structures. The corrected paleomagnetic declinations are found to vary systematically along the arc of the fold belt. Consequently, paleomagnetic data from the Asturian arc should not be included in compilations used for the construction of a Paleozoic apparent polar wander path. The paleomagnetic data allow us to distinguish between different tectonic models for the evolution of the Asturian arc. A two-stage model for the development of the present curvature is favored. Part of the curvature appears to be primary, preceding the Hercynian deformation. The first stage of the model involved rotations of thrust sheets during their emplacement, producing a more tightly curved arc than the original form. In a second stage, the development of radial folds further tightened the curvature of the arc. Both stages result in clockwise rotations in the north and anticlockwise rotations in the south.