STRAIN PARTITIONING OF DEFORMATION MECHANISMS IN LIMESTONES: EXAMINING THE RELATIONSHIP OF STRAIN AND ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS)

Strain partitioning in limestones is useful in evaluating the contribution of calcite deformation mechanisms to the anisotropy of magnetic susceptibility (AMS). 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.

Silurian Tonoloway Fm. and the Devonian Helderberg Fm. limestone samples were collected from 27 sites located in two macroscale folds in the central Appalachian Valley and Ridge province. AMS measurements were made on 2 to 18 specimens per site and strain analysis was done on three mutually perpendicular thin sections. Strain was partitioned into three deformation mechanisms. First, grain boundary sliding, which should not have a significant effect on AMS fabric because there is no grain distortion involved. Second, mechanical twinning of calcite (up to 4.3% bed parallel shortening), where the rotation of twinned material results in a rotation of the axis of maximum susceptibility toward the maximum compression direction causing an inverse magnetic fabric. Third, pressure solution, which results in flattening of the AMS ellipsoid into the plane of shortening by the mechanical alignment of mineral grains and growth of new minerals. Pressure solution was in-turn partitioned into compaction (up to 35% bed normal volume loss) and tectonic (up to 13% bed parallel volume loss). In any rock one or more of the mechanisms contributes to the total strain, and hence the AMS fabric.

Subtle differences in limestone lithology have significant effects on the shape and magnitude of the AMS ellipsoid. For example, the initial depositional magnetic fabric of fine-grained mudstones and wackstones is modified during deformation primarily by diffusion-accommodated grain boundary sliding and pressure solution. In contrast, the magnetic fabric of coarser-grained packstones and grainstones is modified by mechanical twinning, dislocation mechanisms and pressure solution.