Localization effect on AMS fabric revealed by microstructural evidence across small-scale shear zone in marble

Anisotropy of magnetic susceptibility (AMS) is regularly applied as a tool to infer structural analysis of deformation and flow in rocks, particularly, with low anisotropy. AMS integrates the magnetic signature of crystallographic and shape preferred orientation of all mineral grains present in the rock microstructure. Those preferred orientations result from multiple processes affecting the rock during its evolution, therefore the desirable AMS-strain relationship is not straightforward. Here we show that due to localization of deformation, AMS is indirectly dependent on the magnitude and character of deformation. In order to decipher the AMS-strain relationship, AMS studies should be accompanied by microstructural analyses combined with numerical modelling of magnetic fabric. A small-scale shear zone produced by single deformation event was studied. The resultant AMS fabric is “inverse” due to the presence of Fe-dolomite and controlled by calcite and dolomite crystallographic preferred orientations. The localized deformation resulted in the angular deviation between macroscopic and magnetic fabric in the shear zone, systematically increasing with increasing strain. This is a result of the presence of microstructural subfabrics of coarse porphyroclasts and fine-grained recrystallized matrix produced by localization.The localization of deformation is a multiscale and widespread process that should be considered whenever interpreting AMS in deformed rocks and regions.

Localization of deformation and its effects on power-law singularity preceding catastrophic rupture in rocks

Three distinct length scales are involved in the deformation evolution and catastrophic rupture of heterogeneous rocks in general: two essential ones are the specimen size macroscopically and the grain size at micro-scale respectively, the other is the emerging localized band of deformation and damage. The band initiates almost nearby the peak load, and the rupture eventually occurs afterwards within the localized band. In this paper, we report that with the evolution of concentrated high strain and damage in the localized band, a power-law singularity emerges within the localized band preceding the eventual rupture. The localization of deformation imposes a spatial non-uniqueness on the power-law singularity, and then leads to a trans-scale characteristic of the power-law singularity. Based on this characteristic, it is demonstrated that the singularity presented by the global response of a whole specimen comes from the singularity of local response in the localized band. The localization and the power-law singularity are associated precursory events, spatially and temporally, respectively, before macroscopic rupture. In particular, based on the power-law singularity exhibited in the zonal areas near or across the rupture surface, a prediction of the occurrence time of catastrophic rupture can be made accordingly. This provides a practically helpful approach to the prediction of rupture, merely by means of monitoring the zonal areas adjacent to the localized band.