Anisotropy of magnetic susceptibility - an efficient technique of structural analysis of rocks
Professor in Petrology and Structural geology,University of Praha 2, Czech Republic
Anisotropy of Magnetic Susceptibility (AMS) has become a unique method to investigate preferred orientation of magnetic minerals, called the magnetic fabric, in different varieties of metamorphic, sedimentary and igneous rocks, because it can be measured with sufficient precision in virtually all rock types. In addition, this technique is extremely fast, an order of magnitude faster than classical methods of structural analysis. The magnetic fabric develops during various geological processes, such as by water flow in sediments, by lava or magma flow in volcanic and plutonic rocks, or by ductile deformation in metamorphic rocks, and in turn, these processes can be assessed from it.
The AMS in sedimentary rocks provides information on the deposition and compaction processes. In natural sedimentary rocks unaffected by later deformation, the relationship between the magnetic foliation and lineation and the sedimentary external structures (sole markings, flute casts, groove casts) and internal structures (cross-bedding, current-ripple lamination, symmetric and asymmetric ripples) is primarily investigated. During the process of diagenesis, the originally sedimentary magnetic fabric may be slightly modified due to ductile deformation accompanying this process.
The AMS of volcanic rocks is, in general, very weak, reflecting a very poor dimensional orientation of magnetic minerals (mostly titanomagnetites) in these rocks. In spite of this, their AMS can, in general, be measured precisely because the volcanic rocks are often strongly magnetic, and the AMS seems to be the quickest method to be able to reliably investigate the weakly preferred orientation of the minerals. Since the first investigations of the AMS of volcanic rocks it has been clear that it reflects the dimensional orientation of magnetic minerals created during a lava flow.
In plutonic rocks, the AMS can efficiently measure the magnetic fabric even in massive granites that are isotropic at the first sight. The AMS of plutonic rocks varies from very weak (typical of volcanic rocks) to extraordinarily strong (typical of metamorphic and/or strongly deformed rocks). The plutonic rocks that have suffered no post-intrusive deformation show the magnetic fabric created during the process of magma emplacement. Some plutonic rocks have suffered tectonic ductile deformation after their emplacement. In this process, the originally intrusive magnetic fabric is overprinted or even obliterated by the deformational magnetic fabric. The degree of AMS of such rocks is often much higher, because ductile deformation is a relatively efficient mechanism for the reorientation of magnetic minerals.
The AMS of metamorphic rocks in general and even of low-grade metamorphic rocks is considerably higher than the AMS in undeformed sedimentary and volcanic rocks. Hence, the mechanism orienting magnetic grains during metamorphism is very effective. It probably involves ductile deformation or recrystallization in an anisotropic stress field operating during metamorphism. The AMS therefore enables these processes to be studied.