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Magnetic Anisotropy


The study of the magnetic anisotropy of rocks is one of the most promising research issues of rock-magnetism. Particularly, the study of the low-field magnetic susceptibility (AMS) has become during the last years a quick, non-destructive and efficient method to determine the rock fabric and has been used to solve a variety of geologic problems.
The magnetic susceptibility (K, that represents the ratio between the induced magnetization M and the applied magnetic field H) of a rock sample depends on the direction of measurement (fig. 1): it is an anisotropic physical property; K, is expressed mathematically by a second order tensor and, geometrically, is represented by a triaxial ellipsoid (fig. 2).

Figure 1 Magnetic susceptibility variations according to different measuring directions.

 

Figure 2 The magnetic susceptibility ellipsoid.

  
The study of the AMS allows the reconstruction of the bulk fabric of a rock, to which all constituent mineral fractions contribute depending on their intrinsic susceptibility. The definition of a “magnetic susceptibility ellipsoid” identifies two important elements: a “magnetic foliation” (the orthogonal plane to the minimum magnetic susceptibility direction) and a “magnetic lineation” (the direction of maximum susceptibility in a rock). The potential applications of the AMS research include:

  • determining the orientation of the sedimentation paleohorizontal and eventual paleo flow directions in non-deformed sediments;
  • determining the orientation of the plane and flux direction of the magma in igneous rocks
  • determining the orientation of the stress ellipsoid in deformed rocks.

Particularly, studies of the AMS carried in clayey and weakly deformed sediments in Italy have underlined a significant relationship between the tectonic history of the sedimentary basins, including both extensional and compressional settings, and the fabric of clays. In such sediments the magnetic foliation generally (exceptions may be due to peculiar mineral magnetic content) indicates the sedimentation-compaction plane, whereas the magnetic lineation is perpendicular to the shortening direction in the compressional settings and parallel to the extension direction in the extensional settings.
Conversely to the AMS approach, the studies on the anisotropy of remanent magnetization of a rock, allow determining the fabric related to the ferromagnetic fraction. The knowledge of the preferred orientation of the magnetic minerals contained in a rock has important implications in the paleomagnetic studies.
 


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