Examples of tectonic overprints of magnetic fabrics in rocks of the Bohemian Massif and Western Carpathians

Abstract

The magnetic fabrics in sedimentary, volcanic, and plutonic rocks primarily originate during deposition, lava or ash flow, and magma flow, respectively. During later rock development, these magnetic fabrics can be overprinted by various processes among which regional metamorphism and ductile deformation tectonic in origin are probably the most frequent and important. During the overprinting, the degree of anisotropy in general increases with progressing deformation and the magnetic foliations and magnetic lineations are reoriented from the primary directions towards parallelism to the doubtless deformational fabric elements in the body considered or in surrounding rocks. Because of the second rank tensor character of the anisotropy of magnetic susceptibility (AMS) it is often difficult to recognize whether a particular magnetic fabric was overprinted or not. The identification techniques of the magnetic fabric overprints on the site scale were developed, which are based on the investigation of the relationship between the magnetic fabric and orientations of mesoscopic fabric elements whose origin is known (bedding, cleavage, metamorphic foliation, mineral lineation, fold axis, etc.) and on the investigation of fabric relationship between geological units of different origins, because the AMS can be measured with sufficient precision in virtually all rock types.