No abstract

<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>

<p>The Sigues fold (Aragon, Spain) presents an exceptional outcrop where 1) the footwall is largely exposed, 2) it is constituted of homogenous shales, 3) the strain varies at distance from the emergent thrust, with all steps of cleavage development. The best model to explain the strain distribution is the trishear propagation of a thrust with a P/S ratio of 1.  However, from East to West, the thrust geometry is changing progressively from blind thrust to flat ramp. The topographic surface as well as the position of the emerging part of the thrust determine the geometry of the structure. This is, therefore, a place with variable geometries, which allow us to describe the different geometric stages of the ramp-and-flat model that we are used to find in major orogenic thrusts.</p><p>To map the strain, we measured the magnetic fabric of hundreds of shale fragments (weighting a few grams) in dozens of localities. The magnetic fabric is governed primarily by illite. Hence, the magnetic fabric represents a 3D view of illite organization, i.e. the matrix of those shales (see Gracia-Puzo et al., EGU, EMRP3.8). The measurement of 3D fabric of illite takes about a minute per fragment and is non-destructive.</p><p>Magnetic fabric of shale fragments provides three useful parameters, the degree of anisotropy, the shape parameter from oblate to prolate, and the length of the confidence angle of the minimum axis of tensor. We show that all these three parameters are highly sensitive to strain. While each locality provides homogenous results from ~15 fragments (covering few square meters each), it is statistically different from one site to the other, with trends consistent with distance to the main thrust. Assuming rigid rotation of illite particles, we calculate the strain using Eigen values of magnetic fabric tensor.</p><p>Our preliminary maps shows: 1) that the strain increases considerably (from units to tens in %) when approaching the main thrust, 2) at a distance of more than 1 km, several strain gradients are detected, suggesting that blind thrusts propagate in the footwall. Serial N-S cross-sections are expected to describe the lateral variability on the structure, the deformation accumulated on the footwall and also establishing the portion of the hanging wall which is being affected and the décollement of the thrust. Our approach is thus promising to map strain in shales from deformed regions, both from natural outcrops, or from boreholes.</p>

<p><span>Interpretation at depth of geological structures and cross-section reconstruction of fold and thrust belts requires either (1) constraints derived from geophysical exploration (seismic, gravimetric or, in some cases, magnetic) or borehole data, or, alternatively, (2) assumptions about the geometrical model that help to accept or discard, or, eventually, to evaluate the feasibility of possible solutions. In this sense, 3-D reconstructions can help to correct and modify the reconstruction at depth of the main structural traits of a structure or a set of structures. Factors to take into consideration include the consistency in shortening figures along strike for each thrust sheet and the whole set of thrust sheets, and the deformation associated with thrust fronts, together with the consistency in constraints referred to the relative chronology between the different thrust slices. In this work we present the results of a 3-D high-resolution modelling of the Leyre thrust (Southern Pyrenees), confronting different possible models of its structure at depth, and showing the usefulness of 3-D reconstruction. The interest for its study lies in the strong along-strike changes observed, that must be linked to the particular kinematics of this sector of the Pyrenean chain. The proposed geometrical reconstruction benefits from the outstanding outcrops along the Esca valley transect and the existence of geophysical low quality data that, nevertheless, allow to establish some limits to the maximum depth of particular horizons.</span></p><p><span>The Leyre thrust is a plurikilometric, E-W striking, shallow-dipping, South-verging thrust located within the Eocene Jaca-Pamplona basin and detached at depth in the Upper Triassic evaporites (the regional décollement for many thrust systems in this area). The overall geometry of the outcropping segment of the Leyre thrust is a low-angle ramp of the Cretaceous-Paleocene competent units (folded and cut with high-angle ramp geometry), onto the Eocene marls that show pervasive slaty cleavage related to the thrust front. A second thrust sheet can be inferred at depth also involving the Cretaceous-Paleocene sequence. Furthermore, a back-thrust linked to a box-fold anticline appears in the hangingwall of the main thrust. This box folds shows a strong eastwards plunge, and disappears laterally towards the East. Finally, a slightly oblique thrust (WNW-ESE) ramps up the box-fold, with increasing displacement from West to East. The connection between this latter thrust and the back-thrust at the rear front of the box-fold is probably related with the warping of the fault surface and (possibly) a clockwise rotation of the uppermost thrust sheet.</span></p><p><span>All in all, the 3-D reconstruction proposed allows to update and contrast some of the tectonic models classically proposed for the area (see e.g. Labaume et al., 1985), reducing the number of superimposed thrust sheets and relating their geometry with an overall break-back (or hanging-wall-sequence) kinematics triggered by the blocking of movement at particular thrusts and the upward steepening of thrust surfaces. Development of (hardly-to-detect) thrust surfaces in the marls located in the footwall of the frontal thrust would be the manifestation of the last movements of the thrust system.</span></p>