THE MAGNETIC FABRIC OF HIGH STRAIN SHEAR ZONES: A REVIEW

The low-field Anisotropy of Magnetic Susceptibility (AMS) is a popular petrofabric tool used as a strain proxy. The success of this method has been established on various rock types and different settings. The dimensionless magnetic susceptibility (K) depends on the nature of magnetic minerals and their abundance. The arrangement of magnetic grains is responsible for the spatial anisotropy of K. The AMS scalar parameters used to quantify strain are the corrected degree of anisotropy (Pj > 1), a mesure of fabric intensity, and the shape factor (-1 < T < +1), an indicator of the strain ellipsoid prolateness-oblateness.

A positive correlation exists between Pj and strain (measured independently). Thus, the AMS has a great potential to study large strain gradient structures such as shear zones. In spite of the good agreement between AMS and strain, a number of caveats deserve to be addressed:

1) The increase of Pj with strain cannot be solely attributed to deformation because Pj also increases with K regardless of strain.

2) Strain across shear zones is heterogeneous and often localized along units of different lithology, thus masking the effect of strain only. Strain commonly takes place with mineral seggregation or fluid-rock interaction which induces changes in magnetic mineralogy.

3) Even if the undeformed lithology was constant across the shear zone, variations in strain rate or temperature would likely result in different deformation mechanisms. The relationship between Pj and strain depends strongly on the mineral carriers of the AMS and on deformation mechanisms.

4)Composite fabrics, resulting from S-C structures, are not easily resolved by AMS because the bulk AMS fabric is the bissector of the S- and the C- planes.

In spite of these problems, AMS studies in shear zones still provide a fast and reproducible measurement of strain. Recent developments in magnetic fabrics allow to separate the respective contributions of different minerals to the bulk fabric. For example, measurement of AMS in high-field (HF-AMS) isolate the contribution of the paramagnetic silicates to the AMS from the bulk. In such cases, the obliquity between two magnetic subfabrics (low-field AMS carried mostly by magnetite and HF-AMS carried mostly by biotite), can be used as a kinematic indicator.