The clay fabric of shales is a strain gauge

Abstract

Shales with their large clay fraction are prone to deform in an almost ductile way under tectonic loading. Here, we propose to monitor and quantify the evolution of strained shales clay fabric in the field of illite diagenesis by using magnetic fabric. In the footwall of a south Pyrenean thrust, 281 shales samples have been collected along a 2 km long exceptional exposure. The cleavage evolves from pencil structures at ~1 km of the thrust, to pencil cleavage at ~850 m and slaty cleavage at ~170 m. The average magnetic foliation of illite remains nearly parallel to bedding over a distance of ~600 m despite development of pencil cleavage. It is at ~600 m of the thrust that illite foliation shifts gradually from bedding to cleavage. By contrast, the magnetic fabric scalar data reflect a more continuous evolution of the deformation. The application of a rigid rotation model of the clays allows to quantify the strain magnitude using Eigen values of tensor. This strain magnitude is consistent with distance to the thrust and conforms favorably with a trishear modelling of this thrust damage. We demonstrate therefore that the magnetic fabric of illite-rich shales is a strain gauge.