Modelling relationship between bulk susceptibility and AMS in rocks consisting of two magnetic fractions represented by ferromagnetic and paramagnetic minerals — Implications for understanding magnetic fabrics in deformed rocks

Abstract

Measurement of Anisotropy of Magnetic Susceptibility (AMS) has become an important tool for Structural Geological analysis in the past few decades. In the past, AMS data have been used for petrofabric analysis of deformed rocks as well as for gauging strain. However, the AMS of some rocks can be carried by both ferromagnetic and paramagnetic minerals. Separating effects of these mineral groups on the rock’s AMS is difficult because of expensive and commercially less available instrumentation. On the other hand, instrumentation is available in most rock magnetic and palaeomagnetic laboratories for resolving bulk susceptibility into ferromagnetic and paramagnetic components. Mathematical modelling was made of the relationship between bulk susceptibility and AMS. If the contribution of the ferromagnetic or the paramagnetic fraction to the rock susceptibility is dominant (let us say higher than 80%), the resultant AMS is relatively near to the AMS of the dominating fraction in all aspects, the degree of AMS, shape parameter and orientation of principal susceptibilities. In the interpretation of the AMS of rocks with dominating one fraction, the resolution of the AMS into paramagnetic and ferromagnetic components is not necessary, the resolution of bulk susceptibility into components is sufficient that can be made using the instrumentation available in most rock magnetic and palaeomagnetic laboratories.