TECTONIC LINEATION AND MAGNETIC FABRICS IN DEFORMED ROCKS

Tectonic lineation is a component in structural geology analysis that may reflect the stretching lineation, shear transport and fold axes among other properties of rock deformation. In many situations, tectonic lineations are marked by preferred grain orientation of, for example, phyllosilicates. Thus, magnetic fabrics can provide an powerful tool to determine lineation in deformed rocks. Magnetic lineation in rocks is given by a cluster of the principal axes of maximum susceptibility, Kmax, as determined from the Anisotropy of Magnetic Susceptibility (AMS). Determination of AMS in deformed rocks as well as modeling indicate that magnetic lineation typically tracks the intersection between two competing subfabrics, such as bedding and cleavage. This situation where Kmax parallels fabric intersection has been commonly interpreted as an intermediate deformational stage, where upon incremental deformation, Kmax will eventually align with track the tectonic extension direction. We have carried out room- and low-temperature magnetic fabric analyses in a variety of deformed rocks with variable bulk strain to further explore the development of lineation in rocks. The studied rocks are from the Pyrenees and the Valley and Ridge Province. We selected samples from different tectonic environments, including mudstone with spaced cleavage, slate, mylonitic schist and granite, and gneiss. Our results indicate that AMS does not track the finite strain. In many cases, the magnetic lineation does not track the tectonic lineation that is recorded by the AMS fabric. The orientation of the AMS axes, and in particular the magnetic lineation, depends on several competing factors, including the original anisotropy of the rock, bulk strain and relative orientation between the predeformational fabric and tectonic fabric.