Abstract:Evaluation of karst hazards benefits from the integration of different techniques, methodologies and approaches. Each one presents a different signature and is sensitive to certain indicators related to karst hazards. In some cases, detailed analysis permits the evaluation of representativeness either from isolated approaches or by means of integrated analyses.In this study, we present the evaluation of an area with high density of karstic collapses at different evolutionary stages through the integration of surficial, historical, geomorphological and geophysical data in order to finally define the evolutionary model for karst activity development. The obtained dataset permits to identify different steps in sinkhole evolution: (i) cavities and open sinkholes, (ii) filling of these cavities, with materials having different signatures, (iii) the progression from collapses to subsidence sinkholes and (iv) enlargement through collapses in marginal areas of previous sinkholes. The presence of different stages of this evolutionary model permits to determine their own signatures that can be of application in contexts where analysis cannot be so systematic and also to evaluate the definition of the marginal areas of previous sinkholes as the most hazardous sectors.
<p>With the objective of mapping strain on the footwall of a thrust in an orogenic context (Leyre thrust, South Pyrenean Range), more than 1500 unoriented shale fragments (0.7-6.2 g) have been collected. 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 such as shales. On the contrary, collecting rock fragments presents the net advantage to provide a much better statistical characterization of the site.</p><p>All samples belong to the Eocene shaly formations from the Jaca Basin. Rock fragments are mostly fractured according to the bedding and/or cleavage surfaces. We demonstrate that the anisotropy parameters P and T 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>In the footwall of the Sierra de Leyre we have defined up to 7 parallel sampling sections, whose traces are perpendicular to the direction of the main thrust. On average, each section is made up of about 10 sampling sites and about 15 fragments are collected per site, covering a few square meters.</p><p>We are restricted by the dimensions of AGICO holders (8cm<sup>3</sup> for cubes, or 10 cm<sup>3</sup> for cylinders). It is possible to use an empty 10 cm<sup>3</sup> cylinder, which can be filled with smaller fragments of rock. The automatic rotator allows a fast and precise description of the AMS tensor. We removed from analysis low susceptibility, carbonate-rich samples, that show a higher variety 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 the minimum AMS axis (K3) provides a sensitive proxy to characterize the competition between bedding and cleavage.</p><p>The comparison between the different sections allows to map the areas of damage linked to the propagation of faults associated with the folds. 5 stages of development of the magnetic fabric allows the detection of damage gradients. The mapping has allowed the identification of hidden faults.&#160;&#160;&#160;&#160;</p><p>This new approach is very promising, and allows much more detailed samplings in difficult areas, providing more robust statistical description of scalar AMS data. This methodology could be useful for the study of outcrops that are difficult to access, and more interestingly, from borehole cuttings.</p>
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