EVALUATING ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) AS A PROXY FOR STRAIN MEASUREMENT IN SEDIMENTARY ROCKS

Because it is fast and easy to measure, many studies have attempted to correlate anisotropy of magnetic susceptibility (AMS) to strain in sedimentary rocks. However, success has been limited. Most of these studies have used finite strain, which measures the cumulative or end result of all strain events that a rock has experienced. However, at the grain scale, strain is distributed among one or more different deformation mechanisms. The lithology of a rock, specifically the composition and grain size distribution, controls how strain is accumulated in the rock, by controlling the deformation mechanisms operative under a specific set of pressure and temperature conditions. Each mechanism affects the shape and orientation of ferromagnetic, paramagnetic, and diamagnetic grains in the rock in a different manner, and controls how the magnetic fabric is modified during deformation. In addition, each mechanism contributes differently to the finite strain and has a different role during the multiple events of the deformation history. Strain partitioning is useful in evaluating the contribution of deformation mechanisms to the AMS. In this study, strain was partitioned in 35 limestone samples and 65 sandstone samples from the central Appalachians. Each of the different AMS fabrics was found to be a composite fabric resulting from the overprinting of four components: 1) a primary depositional fabric attributed to preferentially oriented phyllosilicates in the rock matrix; 2) a compaction fabric due to preferentially oriented phyllosilicates in bed-parallel solution structures; 3) a tectonic fabric preferentially oriented phyllosilicates in bed-normal solution structures; and 4) an inverse fabric resulting from the deformation twinning of ferroan calcite. In a section of rocks where there is a variety of different lithologies based on mineralogy, grain size and clay content, each lithology responds differently to a particular set of deformation conditions. Therefore, although all the rocks have experienced the same deformation events, they will have markedly different partitioned strain distributions and AMS fabrics. In contrast, similar AMS ellipsoid shapes and magnitudes may result from different mechanisms and partitioned strains. Based on this study, AMS is not a reliable strain proxy.