Localization effect on AMS fabric revealed by microstructural evidence across small-scale shear zone in marble

Abstract

Anisotropy of magnetic susceptibility (AMS) is regularly applied as a tool to infer structural analysis of deformation and flow in rocks, particularly, with low anisotropy. AMS integrates the magnetic signature of crystallographic and shape preferred orientation of all mineral grains present in the rock microstructure. Those preferred orientations result from multiple processes affecting the rock during its evolution, therefore the desirable AMS-strain relationship is not straightforward. Here we show that due to localization of deformation, AMS is indirectly dependent on the magnitude and character of deformation. In order to decipher the AMS-strain relationship, AMS studies should be accompanied by microstructural analyses combined with numerical modelling of magnetic fabric. A small-scale shear zone produced by single deformation event was studied. The resultant AMS fabric is “inverse” due to the presence of Fe-dolomite and controlled by calcite and dolomite crystallographic preferred orientations. The localized deformation resulted in the angular deviation between macroscopic and magnetic fabric in the shear zone, systematically increasing with increasing strain. This is a result of the presence of microstructural subfabrics of coarse porphyroclasts and fine-grained recrystallized matrix produced by localization.The localization of deformation is a multiscale and widespread process that should be considered whenever interpreting AMS in deformed rocks and regions.

Strain Localization: Analog Modeling and Anisotropy of Magnetic Susceptibility

“…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).…”

Rock Magnetic Signature of Heterogeneities Across an Intraplate Basal Contact: An Example From the Northern Apennines

“…In well‐developed shear‐fabric, the magnetic foliation can be aligned with the shear plane and k 1 can be either parallel or perpendicular to the transport direction (Casas‐Sainz et al., 2018; Román‐Berdiel et al., 2019; Satolli et al., 2020; Ujiie et al., 2000). Moreover, coexisting magnetic subfabrics can result from the combination of different modal mineral composition, grain‐size, and strain partitioning (Borradaile, 1988; Evans et al., 2003; Kusbach et al., 2019; Pueyo Anchuela et al., 2010, 2012). The mineralogical carriers of the AMS, both paramagnetic and ferromagnetic minerals, may form at different deformation stages and respond differently to the deformation mechanisms (e.g., Borradaile & Alford, 1988; Ferré et al., 2004; Martín‐Hernández & Ferré, 2007).…”

Effects of Temperature on Structural Properties of Hydrated Montmorillonite: Experimental Study and Molecular Dynamics Simulation

“…Structural changes in rocks and soils under environmental change are often accompanied by damage and even disasters, posing potential hazards to human and property. In recent years, increasing numbers of scholars have been focusing on components of geotechnical systems that are sensitive to environmental changes, because these materials play a determining role in changing the system [1][2][3][4][5][6][7][8][9][10][11]. Macrobehaviour is the cumulative manifestation of microstructural changes.…”

“…e breakdown of aggregates leads to the expansion of existing cracks, the formation of new microcracks, and the structural collapse of minerals, thereby changing geotechnical properties. Environmental changes frequently trigger and accelerate this process [1][2][3][4][5][6][7]. Humidity and temperature are environmental triggers that significantly impact the structure and physicochemical properties of many rocks and soils.…”