Anisotropy of Magnetic Susceptibility (AMS) in Pelham Gneiss Dome, Northern Massachusetts

The Pelham gneiss dome in northern Massachusetts ultimately developed during the Appalachian Mountains orogenesis. Gneiss domes can be excellent reflectors of finite-strain patterns, and are quite pervasive structures in areas where there has been exhumation, particularly due to orogeny. Although gneiss domes can display deformation fabrics, determining the exact associated deformational process that caused the genesis of the gneiss dome is poorly understood and difficult to do. However, although the actual method of gneiss dome formation may be hard to assess, the deformational fabrics within these gneiss dome rock units are a great resource for determining the type of strain that has occurred in the area. In metamorphic rocks, the anisotropy of magnetic susceptibility (AMS) is usually related to the deformation fabrics, lineation and foliation. This purpose of this study is to characterize the anisotropy of magnetic susceptibility in metamorphic rocks from the Pelham gneiss dome. Measuring the strength of deformation fabric is difficult to do quantitatively, but AMS may provide a method of quantifying fabric strength in some metamorphic rocks, as long as the relationships between the AMS tensors and deformation fabric is consistent. This study aims to prove that AMS tensors can be quantitatively related to deformation fabric in some metamorphic rocks, as long as certain mineralogical conditions are met. To test this hypothesis, we collected 70 oriented samples of various rock types from around the Pelham dome. Low field AMS was measured on all 70 samples. Results suggest that the AMS tensors of the samples primarily produce minimum susceptibility axis symmetry ellipsoids and orthorhombic symmetry ellipsoids when samples are oriented with respect to lineation and foliation. These AMS ellipsoids further suggest that rocks of the Pelham gneiss dome have been subjected to pure flattening as well as plane strain. AMS data is an excellent resource for flow and strain direction reconstruction, and for geologists to better understand metamorphic rocks and the formation and deformation of gneiss domes and gneiss dome systems