Deformation mechanisms and kinematics of a soft sedimentary bed beneath the Scandinavian Ice Sheet, north-central Poland, revealed by magnetic fabrics

Strain Localization: Analog Modeling and Anisotropy of Magnetic Susceptibility

“…The magnetic fabrics of diverse origin across SZs in the sedimentary rocks exhibit the most striking similarity with performed experiments (e.g., Haerinck et al., 2013; Marcén et al., 2018; McCarthy et al., 2015; Narloch et al., 2021; Parés, 2004, 2015). The deformation mechanisms that are thought to explain the AMS evolution with increasing shear strain in sediments differs based on studied lithology.…”

“…The attempts of direct correlation between AMS and strain (Ferré et al., 2014; Humbert et al., 2014) are in general based on assumption of homogeneous reorientation/rotation of the magnetic particles such as magnetite or platy sheet silicates (Humbert et al., 2014) and the anisotropy degree to strain correlation is limited only by magnetocrystalline or shape anisotropy of AMS carriers. However, as shown before (Kusbach et al., 2019; Narloch et al., 2021; Parés, 2015; Peč et al., 2016) and documented by this study, deformation of the rock system is often inhomogeneous and localized on many size scales. It follows that the deformation is accommodated by formation of several subfabrics whose relationship and therefore also resulting AMS is determined by rheological properties of the rock.…”

“…The deformation mechanisms that are thought to explain the AMS evolution with increasing shear strain in sediments differs based on studied lithology. However, it is always associated with the observation of primary fabric intersection and transposition (e.g., Debacker et al., 2004; Hirt et al., 2004; Housen et al., 1993; Kligfield et al., 1977; Parés, 2015; Satolli et al., 2020), synkinematic C/S like subfabrics evolution (Haerinck et al., 2013; Marcén et al., 2018; Satolli et al., 2020), cleavage or crenulation formation (Haerinck et al., 2013; Housen & van der Pluijm, 1991; Pares & van der Pluijm, 2003) or Riedel (R‐, Rʹ‐, Y‐, P‐) shears occurrence (Narloch et al., 2021). In magmatic rocks rich in phenocryst content in submagmatic state, the flow/deformation of crystal mush accommodated by mechanisms of conjugate crystal alignment domains formation leads to the origin of prolate AMS fabric at high angle to expected flow (Závada, Schulmann, et al., 2009; Závada et al., 2017), the AMS fabric counterintuitive orientation (Silva et al., 2014; Závada, Schulmann, et al., 2009) or to the gradual evolution of strong AMS fabric caused by the PS deformation (Machek et al., 2019).…”

“…Numerous sedimentary as well as subsolidus SZs exhibit similar evolution of the AMS ellipsoid across the strain gradient (e.g., Almqvist et al, 2011;Haerinck et al, 2013;Kusbach et al, 2019;Machek et al, 2014;Marcén et al, 2018;Narloch et al, 2021;Parés, 2004;Till et al, 2015). The evolution is characterized by a gradual decrease of magnetic anisotropy, and by a change from primary oblate to prolate shape of the AMS ellipsoid on the SZ sides and then again an increase of magnetic anisotropy and transition to oblate shapes of the AMS ellipsoids in the high strain core of the SZ.…”

“…Therefore, as noted by Borradaile (1988) “magnetic fabrics should not be used for routine methods of ‘strain analysis’ without further study” as for example, composite fabrics of sedimentary, compaction and tectonic origin can result in distinct AMS due to lithology based differences in strain partitioning (Evans et al., 2003). All together this establishes the necessity to combine magnetic studies with microstructural, mineral chemistry and geochemical characterization as the relationship between fault or SZ kinematics and dynamics can be individual (Kusbach et al., 2019; Narloch et al., 2021; Robustelli Test & Zanella, 2021; Yang et al., 2020).…”