Title: Effect of deformation localization on the anisotropy of magnetic susceptibility in natural shear zone
Vladimír Kolář Kusbach, Matěj Machek &Zuzana Roxerová
Institute of Geophysics of the Czech Academy of Sciences, Czech Republic
Event: Abstract GeoUtrecht2020
We derived the relationship between AMS and strain in a marble shear zone by combining rock magnetic studies, detailed microstructural analysis and CPO-based numerical modelling of the AMS. AMS ellipsoids are characterised by k1 orientation as being at a high angle to the primary fabric and experience gradual rotation towards the SZ plane with increasing strain. The k1 orientation is consistent with the calcite c-axes preferred orientation, which is considered to represent an "inverse" AMS fabric, because the "normal" AMS fabric should show k3 parallel to c-axes. Moreover, the AMS shows an angular deviation from the local macroscopic fabric observed in the shear zone. The microstructural evolution related to the shear zone development is characterised by dynamic recrystallization of a primary coarse-grained calcite microstructure. The increasing strain is accommodated by increasing the amount of recrystallized matrix at the porphyroclasts expense. To interpret inverse magnetic fabric and observed strain-AMS relationship we have implemented numerical modelling. Models are constructed based on microstructure, CPO, modal and chemical composition of constituting minerals.
The localization of deformation at P-T conditions of dislocation creep leads to the contemporaneous evolution of two microstructural subfabrics in the marble. The combination of their respective magnetic signals results in distinct orientation of total AMS and local macroscopic fabric. This means that neither strain magnitude nor its orientation derived from macroscopic fabric orientation would correspond to estimates deduced from AMS. The combination of magnetic signal of distinct subfabrics also influences the shape and strength of magnetic anisotropy.
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.