Tectonic strain and paleomagnetism: experimental investigation

Abstract

A porous, clay-rich sandstone with a natural remanent magnetisation shows complex changes in the orientation of the remanence vector when the sandstone is subjected to experimental deformation, at 100 MPa, room temperature and constant natural strain rates of 1 × 10−5 s−1. The paleomagnetic vector does not simply rotate away from the axis of compression. When the rock is shortened perpendicular to bedding the remanence vector rotates toward the bedding, as expected, but the remanence also rotates toward bedding when the rock is shortened parallel to bedding. Moreover, in some cases the remanence vector changes azimuth within the specimen during experimental deformation. Thus bedding anisotropy and the accompanying mineral-fabric anisotropy are more influential than strain in controlling deflections of the paleomagnetic vector in this study. This anisotropy also influences the post-deformational behaviour of remanence: 4 months after deformation, some cores shortened perpendicular to bedding relaxed their remanence vectors back towards the pre-deformational attitude whereas some other cores continued to show progressive changes. Intragranular, recoverable elastic strains in the magnetic grains may be partly responsible for the experimentally induced deflections of remanence.