Strain partitioning of deformation mechanisms in limestones: examining the relationship of strain and anisotropy of magnetic susceptibility (AMS) M.A. Evans a, * , M.T. Lewchuk b , R.D. Elmore b a Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260 USA b School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019 USA Received 1 September 2001; received in revised form 5 January 2002; accepted 11 June 2002 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. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Magnetic susceptibility; Strain partitioning; Limestone 1. Introduction The anisotropy of magnetic susceptibility (AMS) is a physical property of rocks that is used for petrofabric and structural studies (see Hrouda, 1982; Borradaile 1988; Lowrie, 1989; Jackson and Tauxe, 1991; Rochette et al., 1992; Tarling and Hrouda, 1993; Borradaile and Henry, 1997 for reviews). The promise of AMS data is that it often shows a relationship with rock strain, both in terms of magnitude and fabric. Because it is relatively easy and fast to measure, as opposed to most methods of determining rock strain, many workers have attempted to directly correlate AMS to strain (e.g. Kligfield et al., 1982; Borradaile and Mothersill, 1984; Cogne and Perroud, 1987; Lu ¨neburg et al., 1999; Hirt et al., 2000). However, success has been limited, and no reliable correlation has been established. This attempt at a correlation has been complicated by: (1) the inherent inhomogeneity of strain at the grain scale; (2) the fact that whole rock magnetic anisotropy is primarily a function of the diamagnetic and paramagnetic minerals of the rock matrix plus inhomogeneously distributed high susceptibility ferromagnetic grains; (3) the pre-deformation anisotropy fabric is generally unknown; and (4) the relationship between the final AMS shape fabric and multiple deformation events is difficult to establish. Most studies to date that have attempted to relate AMS to strain have used finite strain (e.g. Borradaile and Mothersill, 1984; Lu ¨neburg et al., 1999; Hirt et al., 2000), 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 defor- mation mechanisms. Each mechanism contributes differ- ently to the finite strain and has a different role during the multiple events of the deformation history. For example, in 0191-8141/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. PII: S0191-8141(02)00186-4 Journal of Structural Geology 25 (2003) 1525–1549 www.elsevier.com/locate/jsg * Corresponding author. Tel./fax: þ 1-412-624-3914. E-mail address: mae6@pitt.edu (M.A. Evans).