Magnetic fabric and deformation in the Navia–Alto Sil slate belt, northwestern Spain

• Published in: Tectonophysics 2000-04, Vol.320 (1), p.1-16
• Main Authors: Hirt, A.M., Julivert, M., Soldevila, J.
• Format: Article
• Language: English
• Subjects: AMS; deformation; finite strain; magnetic fabric; slates
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/S0040-1951(00)00047-0

Ries and Schackleton (Ries, A.C., Schackleton, R.M., 1976. Patterns of strain variation in arcuate fold belts. Philos. Trans. R. Soc. London A. 283, 281–288) discussed the distribution of finite strain in the Ibero-Armorican arc and suggested that the arc was produced by tectonic bending of a straight or gently curved fold belt. Later investigations by others revealed different strain patterns in the fold-thrust belt, and so its evolution continues to be controversial. This study focuses on the strain pattern in the Navia–Alto Sil slate belt. The low-field anisotropy of magnetic susceptibility (AMS) was measured in Ordovician black slates and Cambrian to lower Ordovician quartzites, sampled around the arcuate belt. The magnetic fabric is controlled primarily by the slaty cleavage in both lithologies. The AMS was also measured with a high-field torsion magnetometer, and the results indicate that paramagnetic minerals are responsible for the magnetic fabric in the slates and diamagnetic and paramagnetic minerals in the quartzites. Strain markers, i.e. pressure shadows around pyrite grains and rutile distribution, are found in the slates, and the orientation of the principal axes of strain ellipsoids agrees with those of the AMS ellipsoid. A systematic variation in the orientation of the strain ellipsoid and anisotropy ellipsoid suggests that there is strain heterogeneity along the slate belt, and that the arc did not form by tectonic bending.