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Highlights:-Fine-grained rock fabric is investigated by X-ray tomography and SEM-EDS.-Samples cover all stages of cleavage development along a strain gradient.-Shape preferred orientation of quartz, calcite and pores is studied.-Deformation imprints differently on the grain shape fabric of quartz and calcite.-Grain fracturing, rotation and pressure-solution identified as dominant processes.

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<p>We focus on calcareous homogenous shales featuring different degrees of damage along a km-long strain gradient, marked by cleavage development. In a previous study, we used high-resolution X-ray computed tomography (µCT) to document the evolution of the 3D fabric of the fine-grained shales along the strain gradient (Saur et al., JSG, 2020). Our conclusions were based on samples of ~ 2.5 mm<sup>3</sup> containing over 10’000 quartz and calcite grains. The objective of the current study is to assess the representativeness of analyses on such small rock samples. To that extent, we first repeat the µCT analysis on multiple samples of the same, limited, volume and assess the variability of the results. These results are then compared to both macroscopic field observations and bulk fabric measurements obtained with AMS (Anisotropy of Magnetic Susceptibility) on larger samples (~ 10 cm<sup>3</sup>). AMS provides a statistical description of the magnetic susceptibility tensor, and particularly the confidence angle of axis orientation. Generally, this confidence angle is the result of matrix organization and rock magnetism. In this study, AMS is only controlled by the presence of illite particles which reflect the matrix organization. Finally, we perform a subvolume analysis on the µCT images to determine the smallest representative volume characterizing the fine-grained fabric. In light of these analyses we discuss the representativeness of investigated volume of fine grained shales, subjected to different degrees of deformation.</p>

<p>With the objective of mapping strain around a thrust front in an orogenic context (Pyrenean Range), 757 shale fragments (0.7-6.2 g) have been collected in 49 sites. Scalar data (degree of anisotropy P and shape parameter T), together with ellipse of confidence of individual axes provide a proxy of strain acquired by shales in the footwall of the main thrust (Saur et al. 2020).</p><p>Normally, sampling is done by two methods: collecting oriented decimetric hand specimens; or drilling 2.5 cm diameter cylinders. This presents the advantage to deal with oriented samples. However, those techniques are time consuming and it is difficult to collect numerous samples in loose materials like shales. On the contrary, collecting rock fragments present the net advantage to have a much better statistical description of the site. We are restricted by the dimensions of AGICO holders (8cm<sup>3</sup> for cubes, or 10 cm<sup>3</sup> for cylinder). It is possible to use an empty 10 cm<sup>3</sup> cylinder, which could be filled with smaller fragments of rock. The homogeneity of magnetic field of MFK2 Kappabridge (AGICO) allows to measure sample with no distortion due to irregular shape. In addition, the automatic rotator allow a fast and precise description of the AMS tensor.</p><p>All samples belong to the Hecho Group (Eocene from Jaca Basin), consisting of cleaved or stratified marls. Rock fragments are mostly fractured according to the bedding and/or cleavage surfaces. Then we set the rock “horizontally” with the main surface parallel to the bottom of the box, to keep a geometrical reference. We assume that the anisotropy parameters P and T will maintain their values, regardless the shape and size of fragments. Rock magnetism indicates that AMS is primarily governed by illite, with little contribution of magnetite. AMS provides therefore a proxy of illite organisation within the matrix.</p><p>It is noticeable the speed with which data can be acquired in a well-known regional geological setting (757 samples, 49 sites) during 5 field work days and 17 laboratory days. About 15 fragments per site, covering few square meters, display homogenous pattern of P, T, and ellipse of confidence. The data visualization is done thanks to Anisoft 5.1 Software (Chadima, M.). We removed from analysis low susceptibility samples which are carbonate-rich and with more varieties of magnetic minerals. All sites present homogenous results at the site scale, but with significant differences with respect to strain. P and T parameters are very sensitive to strain as illite is the dominant carrier. In addition, the ellipse of confidence of minimum AMS axis (K3) provide a sensitive proxy to characterize the competition between bedding and cleavage.</p><p>This new approach is very promising, and allows much more detailed sampling in difficult area, with much more robust statistical description of scalar AMS data. Aubourg et al. (EGU, TS7.3 session) will use these data to show the pattern of strain in a ramp-related fold.</p>