Magnetic fabric in carbonatic rocks from thrust shear zones: A study from the Northern Apennines (Italy)

We present the results of an integrated structural and anisotropy of magnetic susceptibility (AMS) investigation in calcareous and marly rocks from thrust shear zones. A total of 17 sites from six localities along the frontal and oblique ramp of the Olevano-Antrodoco-Sibillini thrust and back thrust (Northern Apennines, Italy) were studied to investigate both magnetic fabric and structural characteristics of Cretaceous to Neogene calcareous and marly rocks. In most of the sites AMS is controlled by the paramagnetic minerals (prevailingly phyllosilicates), with minor contribution of ferromagnetic (sensu latu) ones. Structural analysis shows the presence of SC- and S-tectonites associated to predominant simple and pure shear, respectively. The combination of density diagrams and cluster analysis allowed discriminating the contributions of sedimentary and tectonic processes to the bulk magnetic fabric. Six different subfabrics were distinguished, related to the structural data and associated to deformation stages and regimes: A. sedimentary fabric; B. intersection lineation; C. magnetic foliation parallel to S planes with k1 parallel to the S/C intersection; D. k1 moving toward the slip vector; E. magnetic foliation parallel either to the S or the C planes, with k1 parallel to the slip vector or the transport direction; F. k1 parallel to extensional planes intersection. The magnetic foliation has a double tendency to parallelize to pressure solution cleavage (S) and shear planes (C), while the magnetic lineation tends to progressively align with the slip vector, except for pure-shear-dominated sites at less than 15–20 cm from the thrust, where it aligns with the transport direction. The magnetic fabric is dominated by simple shear deformation. The protocol applied for AMS analysis shows a great potential to unravel mixed sedimentary and/or tectonic features in magnetic fabrics. AMS can be considered as a useful tool in unravelling the variation of simple-pure shear deformation regime in shear zones.