Anisotropy of magnetic susceptibility fabrics in syntectonic plutons as tectonic strain markers: the example of the Canso pluton, Meguma Terrane, Nova Scotia

Abstract

The anisotropy of magnetic susceptibility (AMS) is widely and routinely used to measure the preferred orientations of Fe-rich minerals in undeformed and weakly deformed granite plutons. The interpretation of the mapped AMS fabrics depends on rock-textural observations, on the map patterns of the fabrics in plutons, and on comparisons of the pluton fabrics to tectonic structures in the country rocks. The AMS may document emplacement-flow related fabrics, but the emplacement fabrics may be reworked or completely overprinted by rather weak tectonic strains of the magma mush or the cooling pluton, especially in syntectonic intrusions. The Late Devonian Canso granite pluton is an excellent example of overprinting of emplacement fabrics by weak tectonic strains. The Canso pluton was emplaced ca. 370 Ma along the boundary between the Meguma and Avalon tectonic terranes, in the northern Appalachian orogen. The AMS was mapped along two traverses that cross the pluton and that are perpendicular to the terrane boundary. Textural evidence suggests the rocks underwent very modest post-full crystallisation strains. The AMS records the dextral transcurrent shearing that occurred on the terrane boundary during emplacement and cooling of the Canso pluton, supporting interpretations that weakly deformed syntectonic granites can be used as indicators of regional bulk kinematics. AMS fabrics in Late Devonian granites of the Meguma Terrane suggest partitioning of the non-coaxial shearing into the terrane bounding fault, with pure-shear dominated deformation further from the fault. Numerical simulations suggest that the kinematics recorded by the fabrics in the Canso pluton was simple-shear, or transpression or transpression with small components of pure shear oriented perpendicular to the bounding shear zone.