Strain partitioning of deformation mechanisms in limestones: examining the relationship of strain and anisotropy of magnetic susceptibility (AMS)

Abstract

In order to investigate the relationship between rock strain and anisotropy of magnetic susceptibility (AMS), strain partitioning and AMS analysis was conducted at 35 sites from two stratigraphically adjacent Paleozoic limestone units in the Patterson Creek and Wills Mountain anticlines in the central Appalachian orogen of West Virginia. In addition, anisotropy of anhysteretic remanent magnetization (AARM) was conducted on selected samples to examine the role of preferentially oriented magnetite on the AMS fabric. Strain is partitioned into bed-normal shortening due to compaction solution strain (≤35.0% shortening), bed-parallel shortening due to tectonic solution strain (≤13.3% shortening), calcite twinning strain (≤5.8% shortening), and grain-boundary-sliding (≤26.7% shortening). The AMS fabrics in the rocks were found to be a result of a complex interaction between rock lithology, deformation mechanisms, and strain magnitude. Although all the rocks have experienced the same deformation conditions, six different AMS fabrics are exhibited. Each of the different AMS fabrics is a composite fabric resulting from the overprinting of three components: (1) an inherent primary depositional AMS fabric that is attributed to preferentially oriented phyllosilicates in the rock matrix; (2) a diagenetic and/or compaction AMS fabric formed during burial that is due to preferentially oriented phyllosilicates in solution structures and in the rock matrix; and (3) a tectonic AMS fabric that was imparted on the rocks by layer-parallel-shortening deformation prior to folding, and is also attributed to preferentially oriented phyllosilicates in solution structures and in the rock matrix, as well as twinning of ferroan calcite.