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El análisis de la Anisotropía de la Susceptibilidad Magnética (ASM) constituye una técnica rápida y no destructiva muy valiosa en geología estructural y tectónica, ya que es capaz de proporcionar información relacionada con la orientación e intensidad del elipsoide de deformación en rocas deformadas. Parte de sus ventajas son, su capacidad para (i) caracterizar rocas muy débilmente deformadas, y (ii) integrar gran cantidad de datos provenientes de grandes áreas. Desde los trabajos pioneros de los años 1950s, gran cantidad de trabajos han utilizado esta técnica en diferentes áreas de las Ciencias de la Tierra y aplicada a, prácticamente, todos los tipos de rocas existentes tanto en edad como litología. En este trabajo exploramos y mostramos el uso actual de esta técnica en geología estructural y tectónica, así como sus beneficios y limitaciones.

The Pyrenees constitutes an exceptional example of an Alpine orogenic belt characterized by basement thrust sheets involving Paleozoic rocks and Mesozoic and Cenozoic cover units detached on the Triassic evaporites, the main décollement level. This work is located in the Central Pyrenees, where gravity data help to better constrain the internal architecture of the upper crust of the southern half of the Axial Zone and the northern part of the South Pyrenean Zone, a key area to understand the orogenic evolution of the chain. Previous and new gravity, petrophysical and geological data have been used to obtain the Bouguer and residual anomaly maps of the study area and six serial gravity-constrained cross sections perpendicular to the main structural trend. The residual anomaly map shows a good correlation between basement units involved in thrust sheets of the study area and gravity highs whereas negative anomalies are interpreted to correspond with Mesozoic/Cenozoic basins, Triassic evaporites, Late Variscan igneous bodies, and Ordovician gneisses. The six gravity-constrained cross sections highlight strong along-strike variations on the gravity signal due to lateral differences of the superficial and subsurface occurrence of Triassic evaporites, different geometry at depth of the Late Variscan igneous bodies outcropping in the study area, and geometric lateral variations of the basement thrust sheets and their relationship with the Mesozoic-Cenozoic units., This work was funded by projects CGL2017-84901-C2-2-P funded by MCIN/AEI/10.13039/501100011033 and “ERDF A way of making Europe”, PID2020-114273GB-C22 funded by MCIN/AEI/10.13039/501100011033 from the Spanish Ministry of Science and Innovation, and "Severo Ochoa” extraordinary grants for excellence IGME-CSIC (AECEX2021).

The Small Circle (SC) tools analyse the stereographic tracks (small circles) followed by the palaeomagnetic vectors during folding processes. Working with interfolding and synfolding remagnetizations, the Small Circle Intersection (SCI) method allows finding the best solution of grouping that should correspond with the remagnetization direction. Once this is known, it is possible to determine the magnetization age as well as the degree of bed tilting at this moment. The SC tools are based on some assumptions, among which the coaxiality between the different deformation events is the one addressed in this work (i.e. absence of vertical axis rotations, VARs, or differential horizontal axis rotations, dHARs). This assumption is based on the necessity of knowing the rotation axis for folding after the acquisition of the remagnetization, and SC tools consider the bedding strike as this axis, something that is only accomplished under coaxial folding. In order to explore how non-coaxiality affects the solutions derived from the SC methods, we first (i) identify the variables that control these errors through simple models that only consider two theoretical palaeomagnetic sites, after that it is possible (ii) to derive the mathematical relationships between them. Finally, we (iii) simulate errors derived from the use of SC tools using a population of 30 palaeomagnetic sites recreating different possible scenarios with VARs and dHARs in nature., Research financed by the projects DR3AM- CGL2014-55118 and CGL2016-77560-C2 from the Spanish Ministry of Science (MINECO) and by the Applied Geology (GeoAP-E0117R) and Geotransfer groups of the Government of Aragon, as well as by the project BU235P18 (Junta de Castilla y Leon, Spain) and the European Regional Development Fund (ERD). It has also benefited from the MAGIBER-II network (CGL2017-90632-REDT) funded by the Ministry of Science, Innovation and Universities.

Centrifuge analog modeling incorporating thickness variations of décollement and overlying sedimentary cover is used to study thrust-wedge evolution on salt-detached contractional systems involving tapered sedimentary cover. From simpler, layer-cake to 3D tapered décollement-cover sequences, our models constrain wedge evolution: a more advanced front, where the cover is thicker, promotes differential frontal translation and triggers vertical axis rotation of several hinterland anticlines which appear towards the thinner cover area. In all cases, the décollement level is strongly sheared and tends to migrate laterally toward the thinner cover areas. Models are compared with previous analog models under normal gravity and the natural example of the South Pyrenean Central salient. All in all, we conclude that a 3D tapered décollement-cover basin architecture, as a sole factor, can determine the formation of a thrust salient formed by increasing structural curvature (progressive arc).

The aim of this work is the application of the anisotropy of magnetic susceptibility (AMS) to the study of volcanic materials from the Late Carboniferous-Permian Cadí basin (southern Pyrenees) in order to (i) explore the limits of this technique in ancient volcanic materials that have undergone strong chemical alteration and a complex tectonic evolution and (ii) provide new insights on the deposition of volcanic materials in this tectonically controlled sedimentary basin. To establish a comparison with the petrofabric of rocks, AMS was complemented with observations and orientation analyses at the outcrop scale, in oriented polished blocks and in thin sections under the microscope. High resolution sampling (620 specimens from 56 sites located in five transects along a sequence 150 m thick) allowed to characterize the volcaniclastic and lava flow series. The AMS results show a fairly constant average magnetic lineation oriented WNW-ESE (in situ and tilt-corrected) and two secondary maxima (N-S to NE-SW in some areas). The magnetic mineralogy analyses indicate the contribution of paramagnetic and ferromagnetic (magnetite, Tc 580 °C) phases to susceptibility. The magnetic fabric has a close relationship with the spatial distribution and orientation of the mineral and grain components of the studied rocks, mainly controlled by depositional processes, with a negligible contribution of the tectonic imprint. Therefore, the magnetic lineation (i.e. K1 axes) can be reliably used as a paleoflow indicator. Nevertheless, it was not possible to establish a clear correspondence between lithological and magnetic fabric types. The results obtained indicate that the AMS approach represents a powerful tool to infer the paleoflow pattern in ancient volcanic and volcaniclastic rocks, even under a priori unfavourable conditions., This work was supported by projects grants CGL2017-84901-C2-2-P, PID2020-114273GB-C22, PID2019-108753GB-C22 and FPU19/02353 funded by MCIN/AEI/10.13039/501100011033, “ERDF A way of making Europe” and “ESF Investing in your future”.

AMS and structural analysis are here applied to study the deformed zone associated with a large-scale, active normal fault in the central Betic Cordillera (Spain), namely the Baza fault system, to determine: i) the kinematics of structures and their relation with fault zone architecture and segmentation degree, ii) the correlation between deformational structures and the different types of magnetic fabrics and iii) the evolution of magnetic fabrics patterns, from sedimentary to shear-related, associated with normal faults. Five outcrops (969 samples) were analysed along the fault trace, which shows different degrees of segmentation along strike and strong localization of deformation along narrow fault zones. A first, main set of magnetic fabric data corroborates the normal kinematics of the Baza fault, showing magnetic lineations parallel to the dip-slip, transport direction. A second, secondary set of magnetic lineations, is parallel to the intersection lineation, and can be related to less intense deformation in the fault rocks. Furthermore, a detailed study (523 samples) of a trench excavated across the fault zone, where two fault splays tend to coalesce in a linkage relay zone indicates that i) lithology and distance to fault planes are two factors that control the development of extension-related magnetic fabrics in weakly deformed sediments, ii) the development of shear-related fabrics in fault zones entails the mechanical rotation of minerals, iii) different orientation of magnetic lineations are related to different intensity of bulk deformation and iv) magnetic lineation is useful to define local deviations of deformation axes produced by changes in the local extension direction (from fault-perpendicular to fault-parallel extension) in the linkage zone between adjacent fault splays.

This work presents preliminary paleomagnetic results from the Cadí basin, one of the Pyrenean Late-Carboniferous-Permian basins.These basins are the consequence of a complex tectonic evolution. At the outset, they were controlled by an extensional or transtensional regime during the progressive dismantling of the Variscan chain that gradually changed to a strike-slip fault regime. Afterwards, during Pyrenean compression (Late Cretaceous to Cenozoic) most of these basins were inverted and transported southwards as a part of the basement units. All of them are characterized by a similar subsidence pattern interpreted as graben or half-graben continental troughs. In a close relationship with its genesis, during Late-Carboniferous-Permian times, several magmatic events took place. Because of that, fluvio-lacustrine sediments present lateral changes to volcanic and volcanoclastic deposits and materials are sometimes cut by intrusions.The studied zone, the Cadí basin, is located at the eastern Pyrenees, south of the Axial Zone, and shows a high variability of Late-Carboniferous-Permian volcanic products in E-W continuous outcrops that reach several hundred meters of thickness with a good preservation and exposition. The Late Carboniferous-Permian evolution of the Cadí basin was strongly controlled by tectonics and volcanism. In order to characterize this stage and its volcanic rocks, we applied in a previous work the anisotropy of magnetic susceptibility (AMS) technique to determine the primary magnetic fabric of lava flows and volcanoclastic materials. A dominant WNW-ESE direction of the magnetic lineation was recognized related to the paleoflow direction. Paramagnetic minerals and magnetite were recognized as the main carriers of the AMS.In this work we present the paleomagnetic results from 15 sites (about 150 specimens) in volcanic, volcanoclastic and intrusive materials sampled along four N-S cross sections. The obtained results indicate that the carrier of the magnetization are both magnetite and haematite. The thermal demagnetization of samples shows a paleomagnetic component with unblocking temperatures from 480ºC with reversed polarity and very low inclination after bedding correction. In some sites a normal polarity component is also recognized. These results seem to be coherent with a magnetization coetaneous with the emission and deposition of these materials during Permian times. In cases where the samples record a normal component is important to take into account the complex structural situation of these outcrops.

The characterization of the basement architecture of the Pyrenean Axial Zone, backbone of the chain, is crucial to understand its geodynamic evolution and the interplay between tectonism and magmatism. In this work, a new gravity-constrained cross section was built along the Central Pyrenees, between two of the largest Pyrenean Late Variscan granitic complexes, La Maladeta and Andorra-Mont Louis granites, to infer the geometry at depth of the basement host rocks. This cross section is ca. 65 km long and extends from the Mesozoic Bóixols basin in the South to the Late Variscan Bassiès granite to the North, close to the northern end of the Axial Zone. It is based on available geological maps, previous published works and new geological field data; together with newly acquired gravimetric stations (1141), to improve the existent spatial resolution of the gravity data from the databases of the Spanish and Catalan Geological Surveys, and density values from 65 rock samples covering all different lithologies in the cross section. Thus, its geometry at depth is constrained by means of an integrated 2.5D gravity/structural/petrophysical modelling.The La Maladeta and Andorra-Mont Louis granites appear aligned in a WNW-ESE direction and both lie within the same Alpine basement unit, the Orri thrust sheet. They are separated about 40 km by the WNW-ESE-oriented Llavorsí syncline, formed by Devonian and Silurian rocks and limited to the north and south by south vergent thrusts. This syncline is located between two large Cambro-Ordovician anticlinorium structures, the La Pallaresa and Orri massifs to the north and south respectively, formed by a monotonous alternation of shales and sandstones with some intercalations of limestones and conglomerates affected by very low to medium grade of metamorphism. Most structures show southern vergence along the cross section, and its southern part is characterized by the occurrence of Triassic evaporites, a significant detachment level decoupling deformation between the Paleozoic basement and the Mesozoic-Cenozoic cover rocks.The observed residual anomaly along the cross section shows a relative maximum, coinciding with the southern edge of the Axial Zone (Nogueras Zone) and southern half of the Orri massif, followed to the north by a relative large minimum. This gravity minimum in the core of the Axial Zone coincides with the northern half of the Orri massif, the Llavorsí syncline and southern half of the La Pallaresa massif and must be related at depth with rocks of lower density with respect to rocks located to the North and South. Two possible solutions have been postulated to explain the presence of lower density rocks: (i) the presence of Triassic evaporites at depth as a continuation to the North of the Triassic evaporites outcropping in the Rialp window located to the South and/or (ii) the presence of buried granitic bodies equivalent to the adjacent La Maladeta and Andorra-Mont Louis granites.

The presence of multiple evaporite levels strongly influence the structural style and kinematics of fold-and-thrust belts. Particularly (but not exclusively) in their fronts, it is common for these décollements to favor the formation of triangle zones. In the central portion of the Pyrenees, the South Pyrenean Triangle Zone represents the frontal part of this chain, that involves the Oligocene-Miocene Ebro Basin foreland deposits. We have focused on its western termination, characterized by a salt-cored anticline that laterally passes to a backthrust which dies out to the west. These structures are detached on the Upper Eocene-Lower Oligocene syntectonic evaporite Barbastro Formation (and lateral equivalents) that acted as a multidetachment unit. To the north, the south-directed Pyrenean thrust unit detached on Middle-Upper Triassic evaporites to finally glide along the Upper Eocene-Lower Oligocene décollement horizons.In this contribution, we present a detailed structural and stratigraphic model of this triangle zone termination, constructed accordingly to two major approaches (1) constraining the geometry and structural architecture based on surface geology, interpretation of seismic lines (>900 km) and wells and, (2) obtaining the 3D density distribution of the detachment level (Barbastro Fm. and lateral equivalents as well as deeper, Triassic evaporites) using gravity stochastic inversion by means of more than 7000 gravity stations and 1500 actual density data from surface rocks. All in all, this multidisciplinary approach allows us to characterize the western termination of the South Pyrenean Triangle zone as the transition from a ramp-dominated and multiple triangle zone to a detachment-dominated one whose geometry, kinematics, and location were controlled by the distribution and heterogeneity of the Upper Eocene-Lower Oligocene syntectonic décollements and the southern pinch-out of the basal detachment of this unit.

We introduce a field trip to the southwestern termination of the Pyrenean sole thrust: the Santo Domingo anticline. The field trip is articulated in three main stops with panoramic views. We pursue to emphasize some outstanding characteristics of this structure: (A) the large-scale progressive (laterally angular) unconformity that crops out in its southern flank, fully records its folding kinematics and witnesses for two distinct deformation periods in relation to the basement activity (Gavarnie gentle folding and Guarga paroxism) (stop #1). (B) The remarkable conical geometry of the fold termination (San Marzal area) that is completely documented by structural and palaeomagnetic analyses (stop #2). (C) The rotational kinematics of this structure recorded in its northern flank. Both the folding and the rotational kinematics can be wholly time-bracketed by means of seven long magnetostratigraphic sections and numerous additional palaeomagnetic sites within the syn-tectonic (syn-rotational) sequence (stop #3). We also describe the overall structure of the Western Pyrenees from a panoramic view from the summit of the anticline (1520 m). All these stops are accessible by car (dirt roads). Finally, an additional field stop (#4) is devoted to the sedimentology of the Lutetian-Bartonian carbonates following a hiking path (3 km) from stop #3.

The South Pyrenean triangle zone represents the southernmost front of the Pyrenees at its central portion deforming the Upper Eocene‐Miocene Ebro Basin deposits. Two main structures characterize its western termination, the Barbastro anticline and the San Román backthrust, which detached on the Barbastro Formation (and lateral equivalents), an Upper Eocene‐Lower Oligocene syntectonic evaporite‐rich formation that acted as a multidetachment unit. Northward, the south directed Pyrenean thrust unit (i.e., Gavarnie‐Sierras thrust sheet) detached along the Middle‐Upper Triassic evaporitic rocks to finally ramp up and glide along the same Upper Eocene‐Lower Oligocene multidetachment unit. A multidisciplinary approach allowed constructing a detailed structural and stratigraphic model of the study area. The workflow consisted of (1) constraining the geometry and structural architecture based on surface geology, interpretation of seismic lines (>900 km), and wells and (2) obtaining the 3‐D density distribution of the multidetachment unit using gravity stochastic inversion (more than 7,000 gravity stations and 1,500 density data). The geometry of the sole thrust of the Gavarnie‐Sierras thrust sheet was controlled by the distribution of the evaporite‐rich units of the Barbastro Fm. Weak detachments promoted thrust salient formation and thrust flat geometries. The western termination of the South Pyrenean triangle zone is defined as a westward transition from a ramp‐dominated and multiple triangle zone to a detachment‐dominated one. Its geometry, kinematics, and location were controlled by the heterogeneous lithology of the Barbastro Fm. and its basal, halite‐based detachment southern pinch‐out., This work was financed by the DR3AM (CGL2014‐54118‐C2‐2‐R) and E‐28(GeoApp Research Group) projects by the Spanish Ministry of Science and the Aragón Government, respectively. This work has received support from and it also aligns with methodological goals of the GeoERA project 3DGeoEU (ERANET Cofund action 731166 [H2020], Project code GeoE.171.005) and GeoPiri3D project (from the Spanish Ministry of Education and Universities, project code CGL2017‐84901‐C2‐2‐P). We acknowledge the TopoIberia project for providing the gravity data from SITOPO databases.The Academic agreement with MVE is also acknowledged. The IHS is also acknowledged for providing the Kingdom Suite software. José María Llorente and Agustín Gonzalez are here acknowledged for gravity data acquisition and processing. We also want to thank reviewers Christoph v. Hagke and Jonas Kley and Editor Laurent Jovilet for their productive suggestions and comments that helped to enrich our work and improve the manuscript.

The Jurassic carbonates of the Central High Atlas (CHA) are affected by a widespread and homogeneous chemical remagnetization. This is an interfolding remagnetization (dated in ca. 100 Ma by comparison with the GAPWP of the African plate) that separates two deformational events; the first one is related to the basinal period in the Atlas (Triassic and Jurassic times) that is responsible of the thick Jurassic sequence that crops out in the CHA. The second one is associated with the tectonic inversion during the Cenozoic caused by the African and European plates convergence, which resulted in the uplift of the cordillera.Using the Small Circle tools, it is possible (i) to obtain the remagnetization direction and then (ii) use it as reference to obtain the paleodips of each site (i.e. the paleodips at the remagnetization time). This methodology applied to interfolding remagnetizations allows restoring the present-day geometry to the one at ca. 100 Ma and therefore separating the structure associated to the extensional and compressional periods.This work is framed on an ambitious research project (CGL2016-77560-C2-P, Spanish Ministry of Science and Innovation) in which 700 AMS/paleomagnetic sites and additional 1000 bedding data are integrated to unravel the Mesozoic and subsequent Cenozoic evolution of the CHA. Based on the aforementioned datasets, field work, geophysical (gravimetric and magnetic surveys) constraints and the construction and restoration (at the remagnetization time) of cross-sections, the ultimate goal of the project is to reconstructed and restored the 3-D geometry of the CHA fold-and-thrust belt. Here in particular, we present the paleomagnetic data supporting the calculated paleodips. We also analyze how the deformation associated with each of the two deformational events is distributed along the study area. The comparison of dip-domains maps showing the present day attitude and also the pre-inversional one allows analyzing how extensional deformation is more or less associated with particular structures and to understand the importance of the different processes that acted during this period (i.e. deformation associated with extensional faults, salt walls, igneous intrusions, etc.).

Magnetic fabric has become a first-order tool for the study of the evolution of inverted sedimentary basins, as has been demonstrated in the last decade (García-Lasanta et al. 2018 and references therein). Its application is based on its broad and reliable applicability to characterize the structural context of a region where structural markers are often punctually located or scarce. Determining the contribution of basinal (extensional) and compressional (inversion) deformation to the total magnetic fabric is a major issue in understanding the internal deformation underwent by the basin fill.The main goal of this work is to integrate the available data of anisotropy of magnetic susceptibility (AMS) performed during the last ten years in the Mesozoic series of the Central High Atlas. It has a total of 645 sites (7477 standard specimens), 484 of them (5657 standard specimens) are measured in the framework of the actual CGL2016-77560-C2-P research project (Spanish Ministry of Science and Innovation), and it has been integrated with 161 sites (1820 standard specimens) obtained in the precedent research projects (CGL2012-38481, CGL2009-08969 and CGL2009-10840). Samples were measured in a KLY3-S Kappabridge (AGICO) susceptometer at the Zaragoza University. Magnetic subfabric analysis were also done (AMS-LT and AARM) for representative selected sites, that allow us to identify anomalous fabrics. Magnetic carriers were determined by carrying out temperature-dependent susceptibility curves (from 40 to 700ºC) combining the susceptibility bridge with a CS-3 furnace, an also by means of the acquisition curves of isothermal remanent magnetization (IRM), backfield curves and hysteresis loops using a variable field translation balance MMVFTB at the Paleomagnetic Laboratory of the Burgos University. Rock magnetic experiments indicate the presence of paramagnetic behavior in most samples, the presence of magnetite as main ferromagnetic contribution, and of hematite in the red beds.The application of the ASM has made it possible to obtain data of well-defined foliations and magnetic lines from the analysis of a large number of samples, and therefore representative of the Mesozoic rocks that emerge in the High Central Atlas. Viewing the data as a whole, magnetic ellipsoids can be divided into three main types depending on the orientation of the main axes, and can be related with the kinematic evolution of the Central High Atlas: 1) kmin normal to bedding and sub-horizontal kmax with a NW-SE main maximum, which is mainly associated with gentle synclines and can be related to Mesozoic extensional tectonic; 2) kint normal to bedding and sub-horizontal kmax with a NE-SW main maximum, which can be interpreted as modified by compressional tectonics; 3) kmax normal to bedding, which are located near thrust planes or near the core of narrow and tight anticlines and can be interpreted as related with transport direction or salt tectonics and re-tightening of structures. The predominance of one or another type of fabric varies spatially; so that in the Western and Eastern sectors type 1 fabric dominates (more tan 60% of the samples), whereas in the central sector this percentage decreases to 48% of the samples.

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).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 (8cm3 for cubes, or 10 cm3 for cylinder). It is possible to use an empty 10 cm3 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.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.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.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.

Chemical widespread remagnetizations are especially frequents during superchrons. An interesting issue is whether this relationship is due to especial requirements of the mechanism of acquisition of the remagnetizations and their timing. One example of this type of remagnetizations during a superchron is the one recorded by the Jurassic carbonates from the Central High Atlas (CHA) in Morocco. This normal polarity overprint has been dated ca 100 Ma, comparing the remagnetization direction with the African APWP, i.e. during the Cretaceous Normal Superchron (CNS) and also during the extensional stage of these inverted basins.After several paleomagnetic studies performed in this area in the framework of a big research project, paleomagnetic and rock magnetic data from a set of more than 600 paleomagnetic sites distributed over an area of 10000 km2 are available. The CHA cretaceous remagnetization has been observed in all these sites with the same magnetic properties: a viscous paleomagnetic component with maximum unblocking temperatures of 200-250ºC and the remagnetization normal polarity component up to 450–500ºC. Both are carried by authigenic uniaxial SSD magnetite. The paleomagnetic direction calculated by small circle intersection method (SCI) is also similar in the different locations of this wide area.The mechanism proposed for this type of widespread remagnetization is the generation of magnetite grains due to the heating related with burial. The homogeneous direction of the remagnetization seems to suggest an acquisition for a short event at 100 Ma. However, the extensional stage of these basins lasts tens of millions years keeping the necessary burial conditions for growth of magnetite grains covering several polarity chrons including the CNS.In this work we address the question of timing under with these processes happened, i.e. short vs. long remagnetization periods. We propose the hypothesis that the ca. 100 Ma paleomagnetic direction shows by the remagnetization is just the average of magnetic moments of the entire SSD magnetite population that grow from the Middle Jurassic up to the Cenozoic. Grains block the magnetic moments when they grow above their critical volume, keeping the magnetic polarity generating over time a distribution of grains in normal and reverse polarity groups. To test this hypothesis we develop 1) simulations for the calculation of magnetization directions assuming a homogeneous and constant growth of magnetite crystals and 2) rock magnetic experiments to demonstrate the presence of SSD magnetite grains with opposed magnetic moments. These experiments intend to assess the effectiveness of the SSD grains carrying the remagnetization by comparing the NRM and the ARM signal through the pseudo-Thellier approach.

The Atlas system, an ENE-WSW intracontinental chain in the NW of Africa, grew because of the inversion of Mesozoic extensional basins during the Cenozoic convergence between the African and European plates. The Central High Atlas (CHA) is located in the mid-western sector of the chain and is characterized by (i) the presence of an Upper Triassic décollement, (ii) thick Lower-Middle Jurassic sedimentary sequences, and (iii) the occurrence of diapirs and igneous bodies which are especially common in the central part of the chain. The northern and southern borders are characterized by fold-and-thrust-systems involving the Paleozoic basement and the Mesozoic cover and showing significant displacements, especially towards the South.Framed on a multidisciplinary structural project aiming to reconstruct the 4-D structure of the CHA, the purpose of this work is to gather a vast constraining dataset into a present-day, regional, 3-D geological model of the CHA fold-and-thrust belt. This 3-D reconstruction gives special weight to along- and across-strike variations of the geometry of the basement and cover structures and the distribution of salt and igneous bodies. The 3-D model is founded by 23 serial cross-sections, constrained by surface geology and more than 1900 structural data and complemented by geophysical modelling. The model considers regional structures having enough lateral continuity and so we ruled out minor, local features. Stratigraphically, we considered 5 horizons: (1) the top of the Triassic located below the detachment level, and partially equivalent to the top of the basement, (2) the base of the Jurassic succession (i.e. the top of the detachment level), (3) the Lower-Middle Jurassic limit and, towards the southern and northern fronts and foreland basins, (4) the bases of the Cretaceous and (5) the Cenozoic succession.The reconstruction of the 3-D model entailed a strong feedback between the model and the cross-sections. The incipient 3-D model helped to refine the lateral consistency between cross-sections regarding branch and tip lines, cut-offs, fault angles, etc., and so to improve and further constrain them.Thick to thin skinned deformation dominates the eastern, central and northwestern areas of the chain while thick skinned deformation occurs in the westernmost transects. The chain is defined as an asymmetric, doubly verging fold-and-thrust belt. A north-dipping, basement regional fault represents the main rooting structure of the CHA. Its geometry varies from and almost horizontal (West), to a 10°-12° (Centre) and 15° (East) thrust ramp surface. As this fault intersects the cover, it splits into a regional thrust front with several thrust branches. To the north, antithetic basement faults change to a thrust relay system as they intersect the Mesozoic sequence. Along the core of the chain, the structural style is characterized by open salt bodies, welded diapirs and steep thrusts having relatively limited lateral continuation. The Toundoute Unit, located in the central-western sector represents a basement-and-cover thrust stack where the basement is exhumed and crops out.This 3-D structural model provides the bases for further 4-D reconstruction of the CHA and, at the same time, served as a constraining approach for cross-section construction.

Multiple constraints, including poorly known parameters, determine along-strike changes of frontal thrust structures in fold-and-thrust belts. Along the 400 km long, continuous Central Moroccan Atlas belt, structural style shows significant changes, preserving similar figures of shortening. This implies the absence of large-scale vertical-axes rotations, as demonstrated by paleomagnetic studies accomplished during the development of this project. The main factors controlling thrust geometry are:- the geometry of Triassic-Jurassic extensional basins subsequently inverted during Cenozoic compression, with especial mention to changes of cover thickness and orientation of structures- transfer of displacement between the northern and southern thrust systems- transfer of displacement between the basement (Paleozoic) units and the Mesozoic cover through the Upper Triassic detachment. This factor strongly determines the width of the belt in each transect, as it occurs in other basement-and-cover fold-and-thrust belts- cover/detachment thickness ratio.- localization and partitioning of deformation between different structures in the inner part and the borders of the massif- amount of superposition between different cover thrust sheets, including folded thrusts- structural style, changing from thin-skinned style to large recumbent folds along strike, probably depending on P-T conditions and cover thickness- backthrusts related to low cover thickness/detachment thickness ratio, especially frequent in the northern Atlas thrusts- differential shortening between sections related to layer-parallel shortening and folds associated with cleavage development in the central part of the chain- influence of previous structures, such as individual diapirs, salt walls or igneous intrusions that modify the pre-compressional geometry of the detachment level, nucleate structures and favor buttressing. This feature can also be a source of errors in the calculation of shortening. All these factors result in strong along-strike changes such as branching of thrust surfaces, progression of deformation towards the foreland and differential cleavage development. Influence of structures developed during the basinal/diapiric/igneous stage results in a variability of trends that varies between from less than 10° to more than 30°, what allows in some cases to distinguish between structures controlled by basinal features and newly formed thrusts.In spite of the different techniques for cross-sections reconstruction, and in some cases, the different interpretations for the origin of structures, the shortening figures obtained along the chain are remarkably constant, on the range of 35 km, thus implying a 18 to 30% of shortening for most of the transects what attests for the reliability of the results.Recognition and quantification of factors controlling the development of structures is the fundamental step to determine the main thrust surfaces, and the secondary backthrusts in a region where basin inversion is one of the main constraints. Structural criteria point to a dominant southward vergence and secondary northwards-directed thrusts. Minor strike-slip components were probably localized in the core of the chain. Present-day 3-D reconstruction of the Atlas is currently being done considering all these inputs as well as those obtained from merging the vast dataset obtained.

New residual magnetic map is presented to help decipher the magnetic imprints in the Central High Atlas (CHA) fold-and-thrust belt. The total intensity map shows a main direction mimicking the N070 trend which features the Atlas range. Detailed structural and paleomagnetic studies performed in the selected area demonstrate that similar shortening figures are observed in western and eastern portions. Differences in structural style are the consequence of (i) the inherited structure from the Triassic-Jurassic rifting stage, to Cenozoic inversion, (ii) the differential displacement through the Upper Triassic detachment level and (iii) superposition of cover thrust sheets. Remarkable magnetic anomalies are recorded, from negative values (500 nT) in the core of the range. The western sector of the chain is defined by intermediate to high anomalies, probably related to NE-SW basement structures, which favored the emplacement of Triassic CAMP basalts and/or Jurassic gabbro bodies, within the syn-rift sequence. The central part of the range is characterized by high and very high positive anomalies with an irregular distribution, probably linked to Middle-Late Jurassic diapirism produced during extension and intrusion of gabbroid bodies at the core of diapirs, whose structure nucleated NE-SW anticlinal ridges. The eastern sector is characterized by intermediate to low intensity anomalies, likely associated to thick series of basinal Jurassic limestones, whose sequences were stacked (during the Cenozoic compressional stage) by means of kilometer-scale thrusts. Very high positive, linear anomalies seem to be related to Jurassic gabbro intruding directly into the carbonate facies in the eastern sector. Widespread negative anomalies are detected in the foreland southern basin. In this case, the remanent signature could be related to the Paleozoic magmatic provinces.

Geophysical surveying (both gravity and magnetic) is of great help in 3D modeling of granitic bodies at depth. As in any potential-field geophysics study, petrophysical data (density [r], magnetic susceptibility [k] and remanence) are of key importance to reduce the uncertainty during the modeling of rock volumes. Several works have already demonstrated that ∂18O or [SiO2] display a negative correlation to density and to magnetic susceptibility. These relationships are particularly stable (and linear) in the so-called “non-magnetic” granites (susceptibilities falling within the paramagnetic range; between 0 and 500 10-6 S.I.) and usually coincident with calc-alcaline (CA) compositions (very common in Variscan domains). In this work we establish robust correlations between density and magnetic susceptibility at different scales in CA granites from the Pyrenees. Other plutons from Iberia were also considered (Veiga, Monesterio). The main goal is to use the available and densely sampled nets of anisotropy of magnetic susceptibility (AMS) data, performed during the 90’s and early 2000’s, together with new data acquired in the last few years, as an indirect measurement of density in order to carry out the 3D modelling of the gravimetric signal. We sampled some sections covering the main range of variability of magnetic susceptibility in the Mont Louis-Andorra, Maladeta and Marimanha granite bodies (Pyrenees), all three characterized by even and dense nets of AMS sites (more than 550 sites and 2500 AMS measurements). We performed new density and susceptibility measurements along two main cross-sections (Maladeta and Mont Louis-Andorra). In these outcrops, numerous measurements (usually more than 50) were taken in the field with portable susceptometers (SM20 and KT20 devices). Density data were derived from the Arquimedes principle applied on large hand samples cut in regular cubes weighting between 0.3 and 0.6 kg (whenever possible). These samples were subsampled and measured later on with a KLY-3 susceptibility bridge in the laboratory. Additionally, some density data were derived from the geometry and weighting of AMS samples. After the calibration of portable and laboratory susceptometers, density and magnetic susceptibility were plotted together. Regressions were derived for every granite body and they usually followed a linear function similar to: r = 2600 kg/m3 + (0.5 * k [10-6 S.I.]). As previously stated, this relationship is only valid in CA and paramagnetic granites, where iron is mostly fractioned in iron-bearing phyllosilicates and the occurrence of magnetite is negligible (or at least its contribution to the bulk susceptibility). These relationships allow transforming magnetic susceptibility data into density data helping in the 3D modelling of the gravimetric signal when density data from rock samples are scarce. Given the large amount of AMS studies worldwide, together with the quickness and cost-effectiveness of susceptibility measurements with portable devices, this methodology allows densifying and homogenizing the petrophysical data when modelling granite rock volumes based on both magnetic and gravimetric signal.

The Atlas system is an ENE-WSW intracontinental chain that extends from Morocco to Tunisia. It is the result of the Cenozoic inversion of a set of intraplate extensional basins that started its development during the Triassic and continued during the Jurassic. The Central High Atlas (CHA) is located at the Moroccan part of the Atlas System, characterised by NE-SW to ENE-WSW tight anticlines that limit wide synclines with the same orientation.In this work, we present a high resolution structural and paleomagnetic study in a representative area with a tectonic evolution characteristic of the CHA. The study area is formed mainly by the Anemzi syncline, a structure of about 28 km long and 12 km wide. This structure is filed in by lower to mid Jurassic marine carbonates, which gradually change upwards to continental red beds. Towards the south, the Anemzi syncline limits with a vertical set of Jurassic intrusive bodies together with Triassic shales and basalts. On the other hand, towards the north crops out Lower Jurassic carbonates in the north limb of the syncline, which overthrust Middle Jurassic rocks.Alongside with other areas of the CHA, in the study area can be identified a widespread remagnetization that has been dated ca 100 Ma. This remagnetization happened after the extensional period, and before the Cenozoic deformation started. The fact that it is an inter-folding record, allows using an already proved method in the CHA to[o1] restore the structures of the area, and so, erase the Cenozoic deformation to better understand all the structural evolution of the area.Samples from 90 palaeomagnetic sites were collected from sedimentary rocks, together with 170 bedding sites. The paleomagnetic results can be divided depending on the lithology. (1) Jurassic[o2] limestones show, in addition to a viscous component, the remagnetization typical from the CHA: a component with maximum unblocking temperatures between 450⁰C and 550⁰C carried by magnetite. Also in this lithology, in few samples a component carried by pyrrhotite can be observed. (2) Red beds show also a Cretaceous overprint, but carried by hematite.By applying Small Circles methods to the Cretaceous remagnetization, we have obtained the paleobedding at the remagnetization acquisition time (ca. 100 Ma). These results allow us to restore two geological cross-sections at the remagnetization time and compare their with the present-day geometry. Besides, two maps of dip domains have been done, one showing the present day structure and the other one using the dips at the remagnetization. This methodology is a remarkable tool to assess the evolution of singular structures and to separate the deformation related with the basinal period with those related with the subsequent inversion. This restoration is part of a bigger project, whose objective is to build two 3D model of the Anemzi syncline with both the present-day and the restored structure at ca. 100 Ma using the palaeomagnetic data.

Shear zones, or their counterparts in near-surface conditions, the brittle fault zones, constitute crustal-scale, narrow, planar domains where deformation is strongly localized. The variation with depth of deformation conditions (P-T), rheology and strain rates entails a wide range of fault rock types, characterized by different petrofabrics and classically grouped into mylonitic (fault rocks undergoing crystalline plasticity) and cataclasitic (fault rocks undergoing frictional deformation) series. Magnetic fabric methods (most frequently anisotropy of magnetic susceptibility, AMS) have been established as a useful tool to determine fault rock petrofabrics in shear/fault zones, being interpreted as kinematic indicators with a considerable degree of success. However, mylonites and cataclasites show remarkable differences in magnetic carriers, shape and orientation of the fabric ellipsoid. Here, we present a study of ten brittle fault zones (one of them at the plastic-brittle transition) located in various locations in the Iberian Plate, with an aim to interpret patterns of AMS in cataclasites.Reviewing AMS studies dealing with SC mylonites, three fundamental features can be drawn: i) the presence of composite magnetic fabrics with shape and lattice-preferred orientations, ii) the fabric is carried predominately by ferromagnetic minerals and iii) surprisingly in composite fabrics, the absolute predominance of magnetic lineations parallel to (shear) transport direction (88% of the reviewed sites), independently of fabrics being defined by paramagnetic or ferromagnetic carriers. Based on our study, magnetic fabrics in cataclasites: i) are mainly carried by paramagnetic minerals and ii) show a strong variability in magnetic lineation orientations, which in relation with SC deformational structures, are either parallel to transport direction (44% of sites) or parallel to the intersection lineation between shear (C) and foliation (S) planes (41%). Furthermore, changes between the two end-members can be frequently observed in the same fault zone. Sub-fabric determinations (LT-AMS; AIRM and AARM) also indicate that the type of magnetic lineation cannot be consistently related with a specific mineralogy (i.e. paramagnetic vs ferromagnetic minerals).The wide range of deformation conditions and fault rocks covered in our study allowed us to analyse the factors that control these different magnetic lineation orientations, especially in brittle contexts. Plastic deformation results into a mineral stretching parallel to transport direction which can be directly correlated with the development of transport-parallel magnetic lineation. In brittle fault zones, the degree of shear deformation can be directly correlated with the type of magnetic lineation. The fault cores, where strain and slip are localized, show a predominance of transport-parallel magnetic lineations, most probably related with the development of lineated petrofabrics. Furthermore, the minor development of shear-related petrofabrics enhance the frequency of intersection-parallel magnetic lineations, also contributing the presence of inherited, host rock petrofabrics in the fault rocks.

Cenozoic contractional deformation in the Central Pyrenees generated several basement thrust sheets involving Paleozoic rocks and decoupled Mesozoic and Cenozoic cover units detached on the main décollement level, the Triassic evaporites. The overall geometry and structural architecture of the chain have already been established based on numerous geological and geophysical data obtained during several decades. This work aims to validate the overall accepted geometry of the Central part of the chain by the construction of six serial cross-sections constrained by gravity data and 2.5D gravity modelling. The study area comprises the southern half of the Axial Zone between La Maladeta and Andorra-Mont Louis granites and its southern leading edge as well as the northernmost part of the South-Pyrenean Zone.New gravity data were acquired and combined with previous existing databases to obtain Bouguer anomaly and residual anomaly maps of the study area. Six serial gravity-constrained cross sections have been built using available geological maps, previous published works, new geological and gravity data and 2.5D gravity modelling. Density values for gravity modelling were derived from 231 laboratory measurements of rock samples collected in the field from non-weathered outcrops that include all rock types outcropping in the study area. The residual anomaly map shows a good correlation between basement thrust sheets and gravity highs whereas negative anomalies seem to correspond to (1) Mesozoic basins, (2) Triassic evaporites and (3) Late Variscan igneous bodies. The 2.5D gravity modelling along the six cross sections highlights: (i) strong along-strike variations on the gravity signal due to lateral differences of the surficial and subsurface occurrence of Triassic evaporites, (ii) different geometry at depth of the Late Variscan igneous bodies outcropping in the study area and (iii) geometric lateral variations of the basement thrust sheets and their relationship with the Mesozoic-Cenozoic units.

Positive tectonic inversion of sedimentary basins has been recognized as one of the primary mechanisms of mountain building and intraplate deformation. Reconstructing the tectonic history of basins is relatively easy for the inversion stage but becomes more difficult for the basinal stage, especially when strong deformation involving cleavage development is associated with the subsequent compressional tectonics. Since tectonic markers for the extensional episodes are not commonly well developed, Anisotropy of Magnetic Susceptibility (AMS) has provided recently a tool for analyzing early stages in the evolution of sedimentary basins, even in the absence of other outcrop-scale mesostructures. Here, we expose and discuss the applicability of magnetic fabrics (by means of AMS) to different types of intra-plate sedimentary basins in the Western Tethys region formed under extensional or transtensional regimes and which underwent different inversion styles (total or partial inversion, with or without cleavage development, forming part of compressional thrust sheets, etc.) owing to specific particular p-T conditions and structural controls. Factors such as lithology, magnetic mineralogy, position within the sedimentary pile and deformation intensity are key to interpret the obtained magnetic fabrics in terms of tectonic evolution. A basin classification is proposed according to inversion styles and magnetic fabrics: Where inversion did not involve cleavage development, magnetic lineation is parallel to the stretching direction corresponding to the extensional stage. The transition between non-cleaved to inversion-related cleaved units is marked by the switch of magnetic lineations from parallel to extension to parallel to the intersection lineation between cleavage and bedding. These relationships are enhanced when extension and compression are roughly coaxial, then favoring the clustering of axes of the magnetic ellipsoid. Even when extreme inversion occurs and the early, extensional fabric is obliterated, magnetic fabrics provide information about the interaction between preferred deformation directions associated with the main stages in basin evolution.

A paleomagnetic and magnetic fabric study is performed in Upper Jurassic gabbros of the central High Atlas (Morocco). These gabbros were emplaced in the core of preexisting structures developed during the extensional stage and linked to basement faults. These structures were reactivated as anticlines during the Cenozoic compressional inversion. Gabbros from 19 out of the 33 sampled sites show a stable characteristic magnetization, carried by magnetite, which has been interpreted as a primary component. This component shows an important dispersion due to postemplacement tectonic movements. The absence of paleoposition markers in these igneous rocks precludes direct restorations. A novel approach analyzing the orientation of the primary magnetization is used here to restore the magmatic bodies and to understand the deformational history recorded by these rocks. Paleomagnetic vectors are distributed along small circles with horizontal axes, indicating horizontal axis rotations of the gabbro bodies. These rotations are higher when the ratio between shales and gabbros in the core of the anticlines increases. Due to the uncertainties inherent to this work (the igneous bodies recording strong rotations), interpretations must be qualitative. The magnetic fabric is carried by ferromagnetic (s.s.) minerals mimicking the magmatic fabric. Anisotropy of magnetic susceptibility (AMS) axes, using the rotation routine inferred from paleomagnetic results, result in more tightly clustered magnetic lineations, which also become horizontal and are considered in terms of magma flow trend during its emplacement: NW-SE (parallel to the general extensional direction) in the western sector and NE-SW (parallel to the main faults) in the easternmost structures.

Remagnetizations are common in intraplate basins. When remagnetizations occur at an intermediate stage between different tectonic processes, they can be used for paleo-geometrical reconstructions and relative dating of different structures. This has a particular interest in geological frameworks where other geological time markers are absent. In order to apply this methodology, it is necessary to calculate the regional remagnetization direction and subsequently to use this reference direction to restore the attitude of the beds at the moment of remagnetization acquisition. In this work, we use this methodology for dating a pervasive cleavage (whose time of formation is controversial) and the associated structures in the Central High Atlas (Morocco). Paleomagnetic directions from 64 sites were used to calculate the remagnetization direction (D = 330.9°, I = 35.1°, A/n = 6.107) which is coincident with the Albian-Cenomanian (ca. 100 M.a.) expected direction for NW Africa. This direction was used to restore the Mesozoic paleo-geometry of beds allowing us to analyze bedding orientation, cleavage and folding relationships between the present day and the Cretaceous geometry. After restoration we conclude that the development of cleavage post-dates remagnetization, being in relation with Cenozoic basin inversion. However, the paleo-geometry shows incipient folds at Cretaceous times, which can be related to an intra-Mesozoic compressional event.

The Santo Domingo Anticline (External Sierras, Southern Pyrenees), which separates the Jaca piggyback basin from the Ebro foreland basin, is a key structure of the Pyrenees. Its geometry has been interpreted both as a detachment fold and as a hangingwall anticline associated with an underlying thrust. In this paper, we present the results from a gravity survey and 2.5D gravity modelling carried out around the Santo Domingo Anticline. Density measurements indicate a sharp density contrast between the Triassic evaporites-mudstones in the core of the anticline and the sedimentary sequence (limestones and sandstones) at its limbs. Gravity anomalies together with 2.5D gravity models allow to discern the along-strike structural changes. From east to west, we document a change from the ramp-associated fold to the detachment anticline. The capabilities and limitations of the gravimetric method for the determination of fold geometry are also discussed. Copyright © 2017 John Wiley & Sons, Ltd.

EGU2020: Sharing Geoscience Online, 4-8 May 2020, The aim of this work is to apply the anisotropy of magnetic susceptibility (AMS) to determine the primary and tectonic fabrics of lava flows and volcanoclastic materials in one of the Pyrenean Stephano-Permian basins. The Pyrenean Range is a double vergence orogen located at the northern end of the Iberian Peninsula. During Carboniferous-Early Permian times the extensional or transtensional regime dominant during the progressive dismantling of the Variscan belt resulted in the development of E-W elongated intra-mountainous basins. This process was coeval with an exceptional episode of magmatic activity, both intrusive and extrusive. The Cadí basin represents a good example of these structures were Stephano-Permian rocks are aligned along an E-W continuous outcrop and reach thickness of several hundreds of meters. The stratigraphy of the study area is characterized by fluviolacustrine sediments changing laterally to volcanoclastic and pyroclastic rocks with interbedded andesitic lava flows. A total of 75 sites (733 standard specimens) were studied and analysed throughout the volcanoclastic, volcanic and intrusive materials of the Stephano-Permian outcrops in the Cadí basin. Samples were drilled in the field along 5 sections with N-S or NW-SE direction in the Grey and Transition Unit. Afterwards, standard specimens were measured in a Kappabridge KLY-3 (AGICO) at the Zaragoza University to characterise the magnetic fabric. The susceptibility bridge combined with a CS-3 furnace (AGICO) was used for the temperature-dependent magnetic susceptibility curves (from 20 to 700 °C) to recognize the magnetic mineralogy. In addition, textural and mineralogical recognition in thin-sections of the samples was carried out in order to recognize the relationship between magnetic and petrographic fabrics. The results shows that the bulk magnetic susceptibility of the specimens ranges between 118 and 9060·10-6 SI but most of the values are bracketed between 160 to 450·10-6 SI. Taking into account magnetic parameters (Km, Pj and T) there is no correlation between magnetic fabrics and magnetic mineralogy and there is a dominance of triaxial and prolate ellipsoids. Thermomagnetic curves indicate the dominance of paramagnetic behaviour in all the samples and except in one case there is a ferromagnetic contribution due to the generalised presence of magnetite. Magnetic ellipsoids can be divided into four main types depending on the orientation of the main axes and associated with the lithologic types: 1) Kmax vertical and Kint and Kmin horizontal for small intrusive bodies (no restoring); 2) Kmax horizontal or subhorizontal and Kint and Kmin included in a subvertical plane (before and after restitution) for volcanic breccias; 3) Kmin vertical (after restoring) and three directional maxima for lava flows and 4) non-defined fabric for the explosive materials (probably due to their complex depositional mechanisms). In general, a dominant E-W magnetic lineation is observed in many sites, resulting either from dominant flow direction, or to secondary processes. This is the case for some of the magnetic ellipsoids, that seems to be affected by deformation, Kmin is not normal to bedding and therefore, they do not become vertical after bedding restitution.

A detailed magnetic survey combined with the study of magnetic properties and spectral analysis in the Tusculum archaeological site (Alban Hills, Italy) indicates the existence of magnetic anomalies (total field and gradient, showing amplitudes up to thousands of nT) resulting from combined geological and archaeological features. In this paper we propose a two-fold analysis based on both manual and automatic procedures for a precise characterization of those elements from total magnetic field and magnetic gradient profiles. The first, automated step consists in spectral analysis of a long (1.2 km) profile to define the dominant wavelengths related to geological and/or archaeological features. The second, manual step consists of the definition of a hierarchical pattern of magnetic anomalies, according to their amplitude and pervasiveness, in the selected (90 × 90 m) area. The defined anomaly pattern includes 1st-order, total field, magnetic anomaly (1000 nT, decreasing towards the North) related to the geological (volcanic) background, 2nd order anomalies (tens to hundreds of nT), showing a marked periodicity, revealed by spectral analysis, in the magnetic gradient in the 1.2 km long profile, and, finally, superimposed 3rd order anomalies (on the order of several nT). Most second-order and some of the third order anomalies are normal dipoles arranged in an orthogonal pattern in plan view, and can be interpreted as linked to the main roman walls of public buildings or roads. The magnetic susceptibility of most materials used in Roman constructions (volcanic tuff and basalt , whose lithology was characterized by means of thin sections) ranges from 600 to 110,000 × 10 −6 S.I., contrasting with the overall lower susceptibility of soil fillings between walls, what explains most of the anomalies found. Magnetite and other iron oxides and sulphides are the main ferromagnetic phases in construction materials. Koenigsberger ratios of roman construction materials vary between 0.25 and 107 what explains for magnetic remanence to have played a certain (but limited) role in some particular magnetic anomalies.

Epiglyptic thrusts can be misinterpreted as paleolandslides since they underwent displacement on the Earth surface. The Daroca Thrust has been object of controversial interpretations owing to (i) the particular style of its frontal structure (a thin slab of Cambrian dolostones overthrusting syntectonic Miocene deposits), and (ii) its relatively young age with respect to the overall compressional building of the Iberian chain. While several authors have interpreted its frontal flat as a synsedimentary landslide, a number of key features evince that it represents an epiglyptic thrust: (i) recumbent folds in competent Cambrian dolostones; (ii) thick damage zone with foliated fault rocks; (iii) physical continuity between the frontal flat and the main thrust ramp; (iv) transport directions towards NNE and E consistently recorded in both of them. Geometric reconstruction in cross section indicates that the syntectonic deposits of the Daroca area lie below the well-dated succession of the Aragonian stratotype (Middle Miocene). Nevertheless, by extrapolating the age model resulting from the complete succession of mammal sites, it can be inferred that the slip on the Daroca Thrust still took place during Middle Aragonian time.

Thrusting at shallow depths often precludes analysis by means of structural indicators effective in other geological contexts (e.g., mylonites, sheath folds, shear bands). In this paper, a combination of techniques (including structural analysis, magnetic methods, as anisotropy of magnetic susceptibility and paleomagnetism, and paleothermometry) is used to define thrusting conditions, deformation, and transport directions in the Cameros–Demanda thrust (North Spain). Three outcrops were analyzed along this intraplate, large-scale major structure having 150 km of outcropping length, 30 km of maximum horizontal displacement, and 5 km of vertical throw. Results obtained by means of the different techniques are compared with data derived from cross sections and stratigraphic analysis. Mixed-layer illite–smectite and vitrinite reflectance indicating deep diagenetic conditions and mature stage of hydrocarbon generation suggests shallow depths during deformation, thus confirming that the protolith for most of the fault rocks is the footwall of the main thrust. Kinematic indicators (foliation, S/C structures, and slickenside striations) indicate altogether a dominant NNW movement of the hanging wall in the western zone and NE in the eastern zone of the thrust, thus implying strain partitioning between different branches of the main thrust. The study of AMS in fault rocks (nearly 400 samples of fault gouge, breccia, and microbreccia) indicates that the strike of magnetic foliation is oblique to the transport direction and that the magnetic lineation parallelizes the projection of the transport direction onto the k max/k int plane in sites with strong shear deformation. Paleomagnetism applied to fault rocks indicates the existence of remagnetizations linked to thrusting, in spite of the shallow depth for deformation, and a strong deformation or scattering of the magnetic remanence vectors in the fault zone. The application of the described techniques and consistency of results indicate that the proposed multidisciplinary approach is useful when dealing with thrusts at shallow crustal levels.

New magnetic fabric data from 48 sites in Upper Triassic red beds from the Argana, Asni and Tizi n’Tichka areas in the western High Atlas, in combination with rock magnetic analyses, SEM observations and qualitative chemical analyses, reveal that mineralization processes can affect the primary (extensional) or secondary (post-depositional) magnetic fabrics. Twenty out of the 48 analyzed sites show tectonic-related fabrics consistent with the rifting stage (primary). Their orientation suggests that the extensional Atlasic (for the Asni area) and Atlantic (for Argana area) distinct directions prevailing during Liassic times are already present in the Upper Triassic sediments. The other 28 sites show axes switching (including different possibilities, k max -k min or k int -k min ), indicating their secondary development related to mineralogical changes after deposition. However, orientation of magnetic susceptibility axes (without considering their relative value) is consistent with the main directions obtained for the rifting stage. This magnetic fabric study also suggests that (i) extension had a small transtensional component and (ii) there is a limited influence of compressional inversion tectonics.

Contractional deformation in the transition between the Iberian and Catalan Coastal Ranges (Linking Zone) generated both thin-skinned structures detached in low-strength Triassic units and basement-involved structures. To evaluate their extent and relative contribution to the overall structure, we carried out a study combining structural geology and gravimetry. New gravity data (938 stations) and density determinations (827 samples) were acquired and combined with previous existing databases to obtain Bouguer anomaly and residual Bouguer anomaly maps of the study area. Seven serial and balanced cross sections were built, their depth geometries being constrained through the 2.5-D gravity modeling and the 3-D gravity inversion that we accomplished. The residual Bouguer anomaly map shows a good correlation between basement antiforms and gravity highs whereas negative anomalies mostly correspond to (i) Meso-Cenozoic synclines and (ii) Neogene-Quaternary basins. Cross sections depict a southern, thick-skinned domain where extensional, basement faults inherited from Late Jurassic-Early Cretaceous times were inverted during the Cenozoic. To the north, we interpret the existence of both Triassic-detached and basement-involved deformation domains. The two deformation styles are vertically overlapped in the southernmost part of the Catalan Coastal Ranges but relay both across and along strike in the Eastern Iberian Range. These basement and cover relationships and their along-strike variations are analyzed in terms of the interplay between structural inheritance, its obliquity to the shortening direction, and the continuity and effectiveness of Triassic décollements in the study area., This study has been supported by the Instituto para la Reestructuracion de la Mineria del Carbon y el Desarrollo Alternativo de las Comarcas Mineras Ministry of Energy and Industry, Spain. Additional support has also come from the DR3AM and SALTCRES Projects (CGL2014-54118) from the Science National Plan of Spain (MINECO). The authors also acknowledge Midland Valley Exploration for providing Move software academic licenses. The contribution of Jose Maria Llorente, Agustin Gonzalez, and Israel Perez was essential for the acquisition of the geophysical data and the density measurements. The financial support of the Aragonian Goberment is also acknowledged (Applied Geology Group-GeoAp, E01_17R). The data used in this work can be found in the manuscript itself or in previous works listed in the references. Elevation reference data from the Instituto Geografico Nacional (http://www.ign.es/ign/main/index.do) were used in the gravity processing as well as digital terrain models from the Spanish Army Geographical Survey (http://www.ejercito.mde.es/unidades/Madrid/ceget/).Geological maps are available at the IGME website (http://info.igme.es/cartografiadigital/), whereas well data from the Water Authority (CHE) are accessible at the IBER website (http://iber.chebro.es/sitebro/sitebro.aspx?hydrogeoebro).

Large‐scale faults in the continental crust are significant features that control the evolution of sedimentary basins and intraplate mountain chains. Deciphering their evolution is a significant task because faults slip and reactivate in a variety of geological settings. In this work, clay gouges of two major orogen‐scale, long‐lived faults in northern Iberia, the Río Grío and Vallès‐Penedès Faults, were investigated by X‐ray diffraction and K‐Ar isotopic analysis. Illite polytype determinations of 44 subfractions (from

During the Late Jurassic and Early Cretaceous, Iberia experienced extensional and transtensional stresses leading to a complex rifting time interval. Africa–America–Europe relative motions determined the definition of the Iberian plate boundaries and the generation of rifted sedimentary basins and sub-basins along its continental margins and in the plate interior. Complex extensional and salt tectonics controlled the geometry and subsidence of those basins, filled mainly by clastic sediments derived from emerged Variscan massifs and by marine carbonates. The chapter considers the North Iberian Continental Margin, including the Basque-Cantabrian and the Pyrenean basin, the South Iberian Continental Margin, and the Iberian Basin Rift System, including South-Iberian, Central-Iberian, Cameros, Maestrazgo/Maestrat, and Garraf Basins.

A global view of the Mesozoic-Cenozoic geodynamic evolution of the Iberian microplate is necessary to determine the role of inherited Variscan structures in the configuration of the Alpine Cycle. Furthermore, it is important to reconstruct the extensional Mesozoic geometries of the Iberian margins, presently inverted during the Cenozoic compression. We discuss briefly in this chapter, thanks to a large number of innovative scientific studies, the evolution of the Iberian microplate and of its margins, and the geology of Iberia mountain ranges and foreland basins: the Betic-Rif system along the southern plate boundary and the Pyrenees-Bay of Biscay along the northern Iberian plate boundary. In addition, the renewed interest in the role of Triassic salt tectonics both during extension and compression is also discussed.

19 pages, 7 figures, The Alpine orogeny is well recorded onshore and offshore by tectonic inversion of the Mesozoic rift basins. Large scale linear seamounts (more than 250 km long and with up to 5 km of uplift) involving oceanic and continental lithosphere were carried on top of thrusts, such as the Gorringe seamount and the Estremadura Spur in the SouthWest and West Iberia Margin, respectively. The SouthWest Iberia Margin also recorded the westward migration of the Gibraltar Oceanic slab as the westwards propagation of the Neo-Tethys subduction. Rotation of the tectonic compression from NW-SE to WNW-ESE in Pliocene times caused the development of large scale dextral wrench faults as the present day Africa-Iberia plate boundary. Neotectonics of this plate boundary caused large to mega-scale destructive earthquakes and tsunamis

During the Triassic, Iberia and western North Africa displayed a unique situation in relation with the Central and North Atlantic opening and westward expansion of the Tethys. Unravelling the stretching direction in Triassic deposits of the studied area can help in our understanding of this scenario. The tectonic setting is characterized by localized basins with strong thickness variations greatly influenced by previous post-Variscan mechanical discontinuities. In this work, we revise and compile magnetic fabric data from eight Triassic depocentres in terms of defining the stretching direction (i.e. magnetic lineation), resulting from extensional deformation of this period. Data show the importance of the opening of the Atlantic rift as the leading process during the Triassic. Dextral transtension can explain the deflection of the extensional direction observed in most studied depocentres that is caused by the activity of previous major oblique faults.