DOI: 10.1594/pangaea.899435
title: AMS data and EBSD maps of small scale marble shear zone - Estremoz, Portugal

description: We studied two structural profiles across subvertical thin shear zone in marble from quarry in Estremoz (Portugal) to clarify a relationship between AMS and strain in natural rocks. The mesoscopic fabric can be described as changing from the subhorizontal coarse-grained foliation towards the ∼2cm-wide shear zone center with subvertical fine-grained foliation. In microstructure, the shear zone records dynamic recrystallization of calcite aggregate which resulted in development of porphyroclastic microstructure with increasing proportion of fine-grained recrystallized matrix towards the shear zone center. Two distinct crystallographic preferred orientations of calcite were recorded. One related with porphyroclasts, characterized by subvertical orientation of calcite axes and another associated with recrystallized matrix showing subhorizontal calcite axes orientation. The majority of the rock mass is diamagnetic, corresponding well with the thermomagnetic curves, with local paramagnetic accumulations in form of thin bands.
description: Update 2019-07-10: columns 'Specimen orientation', 'F-statistics' and 'Fitting error' were added. AMS ascidata-zip folder was added to the datasetProject: Czech Science Foundation project no. 16-25486Y: How does AMS reflect the microstructure? Natural and experiment shear zones. Czech Academy of Sciences institutional support to the Institute of Geophysics of the CAS, v.v.i. (RVO 67985530).The low field anisotropy of magnetic susceptibility (AMS) was measured with a MFK1-FA Kappabridge (Jelínek and Pokorný, 1997) in the field of 423 A/m and for a frequency of 976 Hz (Institute of Geophysics of the Czech Academy of Sciences).File columns: Sample label, rock type, x and y coordinates, magnetic field in A/m (Field), operating frequency (Freq.), measured bulk magnetic susceptibility (Km), anisotropy factors L, F, P, Pj, T, U (for explanation please see Jelínek, 1981) , orientation of principal magnetic susceptibilities (orientation of principal axes of magnetic susceptibility tensor K1&gt;K2&gt;K3) dec - declination (0 - 360°), inc - inclination (0 - 90°) in geographic coordinates (North = 0, horizontal = 0), K11- K13 - description of symmetrical AMS tensor.The small-scale shear zone in marble has been studied by combination of detailed microstructural and rock magnetic analysis to explore the relationship between anisotropy of magnetic susceptibility and strain produced by single deformation event. The microstructural and rock magnetic data has been interpreted and discussed based on numerical modeling of magnetic fabric. The structural record in the area of Estremoz is dominated by pervasive isoclinal folding resulting in steep SW dipping axial plane cleavage. Based on the observed structural superposition, the studied shear zone belongs to the family of subhorizontal mostly brittle to ductile shear zones to shear fractures crosscutting the SW-dipping axial plane cleavage. The studied marble is characterized by alternating bands of white almost pure calcite up to 30cm wide and thinner gray bands parallel to coarse-grained foliation. In the shear zone, foliation is continuously curved towards the center where most of the strain is localized in fine-grained shear plane. The studied shear zone belongs to the class of ductile shear zones where no macroscopic fracture is involved, i.e. deformation preserves continuity of preexisting markers. This implies that ductile shear zones show a continuous displacement gradient across the zone. The SZ is a product of simple shear deformation as markers of non-simple shear are missing (non-planar shear zone walls, non-parallel shear-strain contours, porphyroclasts showing conflicting sense of rotation and sets of differently oriented shear bands showing opposite senses of shear). Displacement (~41.5cm) on the SZ is approximately 3 times width of the SZ (~13cm). Studied were two structural profiles across the SZ. The profile A is located in the pure calcite rocks and the profile B is realized in the grey zone enriched by accessory minerals. The maximal strain in the SZ core is achieved in the B10 sample (γaverage ~3, γlocal ~14). Crystalographyc prefered orientation maps of marble (ShearZoneEBSDdata.zip) in Channel text files *.ctf obtained using NORDLYS II (HKL Technology) EBSD system mounted on TESCAN Vega scanning electron microscope (Institute of Petrology and Structural Geology of the Faculty of Science at Charles University in Prague) in manual (B5-clasts, B8-clasts, B10-clasts) and in the mapping mode (B8-map with 5 µm step size; B10-map1, B10-map4, B10-map5 with 3 µm step size; B10-map2, B10-map3 with 2.5 µm step size). The .ctf file is a text file containing header with information about mineral phases measured and the data consists of phase orientation in each point measured. The .ctf file can be read by commercial HKL Technology (Oxford Instruments) software or better using the MATLAB® Toolbox for Quantitative Texture Analysis (MTEX) (Bachmann et al., 2010; 2011; https://code.google.com/p/mtex/) licensed under GNU General Public License v2.0.
description: Supplement to: Kusbach, Vladimír K; Machek, Matěj; Roxerová, Zuzana; Racek, Martin; Silva, Pedro F (2019): Localization effect on AMS fabric revealed by microstructural evidence across small-scale shear zone in marble. Scientific Reports, 9(1)