Strain removal applied to paleomagnetic directions in an orogenic belt: the Permian red slates of the Alpes Maritimes, France

Abstract

Detailed investigations of relationships between strain and directions of characteristic remanent magnetization (ChRM) have been conducted on the Permian red series of the Alpes Maritimes, France. The analysis of the results obtained by structural and paleomagnetic studies on 14 sites leads to the following observations: in most sites, the mean direction of the strain shortening axis is close to the expected Permian paleomagnetic direction. In this case we observe a large scatter of the ChRM directions within each site, which is interpreted as an increase of an initial dispersion due to strain. Moreover this dispersion more or less increases with strain intensity. In other cases, where the angle between the shortening axis and the paleomagnetic direction is high, the whole population is deflected towards the cleavage ( XY) plane. These observations lead us to assume a material line-like behaviour of the ChRM vectors and consequently an attempt has been made to recalculate these directions by strain removal. These calculations give positive results at two levels: (1) a statistically significant improvement in the clustering of the within-site distributions, and (2) after tilt correction, using unstrained bedding planes, a significant improvement in the clustering of the between-site distribution. The formation mean direction, after strain removal, is in good agreement with known Permian results for stable Europe.