Introduction: Palaeomagnetism in fold and thrust belts: new perspectives

Palaeomagnetism, that is, the study of the ancient magnetic field recorded in rocks, is the only vectorial indicator in the Earth sciences that is capable of associating geological bodies with their original location (primary vectors) or with intermediate locations (secondary vectors) during their geological history. For this reason, palaeomagnetism has played a key role in supporting continental drift theory. Beyond tectonic plate-scale applications, palaeomagnetism has become a fundamental tool for assessing the evolution of mountain ranges owing to its unique potential for quantifying vertical axis rotations (VAR). Since the pioneering applications of authors such as Norris & Black (1961) and Tarling (1969), palaeomagnetism has been applied to problems at a variety of scales in many orogenic systems (e.g. Elredge et al. 1985; Kissel & Laj 1989; Weil & Sussman 2004; Elmore et al. 2012). In particular, palaeomagnetic data have been increasingly used as key quantitative information for determining the timing, distribution and magnitude of vertical axis rotations (Van der Voo & Channell 1980; McCaig & McClelland 1992; Allerton 1998)...

Virtual directions in paleomagnetism: A global and rapid approach to evaluate the NRM components

We introduce a method and software to process demagnetization data for a rapid and integrative estimation of characteristic remanent magnetization (ChRM) components. The virtual directions (VIDI) of a paleomagnetic site are “all” possible directions that can be calculated from a given demagnetization routine of “n” steps (being m the number of specimens in the site). If the ChRM can be defined for a site, it will be represented in the VIDI set. Directions can be calculated for successive steps using principal component analysis, both anchored to the origin (resultant virtual directions RVD; m* (n2 +n)/2) and not anchored (difference virtual directions DVD; m* (n2 -n)/2). The number of directions per specimen (n2) is very large and will enhance all ChRM components with noisy regions where two components were fitted together (mixing their unblocking intervals). In the same way, resultant and difference virtual circles (RVC, DVC) are calculated. Virtual directions and circles are a global and objective approach to unravel different natural remanent magnetization (NRM) components for a paleomagnetic site without any assumption. To better constrain the stable components, some filters can be applied, such as establishing an upper boundary to the MAD, removing samples with anomalous intensities, or stating a minimum number of demagnetization steps (objective filters) or selecting a given unblocking interval (subjective but based on the expertise). On the other hand, the VPDprogramalso allows the application of standard approaches (classic PCA fitting of directions a circles) and other ancillary methods (stacking routine, linearity spectrum analysis) giving an objective, global and robust idea of the demagnetization structure with minimal assumptions. Application of the VIDI method to natural cases (outcrops in the Pyrenees and u-channel data from a Roman dam infill in northern Spain) and their comparison to other approaches (classic end-point, demagnetization circle analysis, stacking routine and linearity spectrum analysis) allows validation of this technique. The VIDI is a global approach and it is especially useful for large data sets and rapid estimation of the NRM components

Structural and paleomagnetic evidence for non-rotational kinematics of the South Pyrenean Frontal Thrust at the western termination of the External Sierras (southwestern central Pyrenees)

The definition of the structure and kinematics of the South-Pyrenean Frontal thrust, to the west of its westernmost outcrop in the External Sierras is the goal of this work. The methodology used is based on the construction and restoration of three balanced cross-sections. In addition to that, paleomagnetic analyses are applied to unravel possible vertical axis rotations linked to thrust kinematics. Stepwise thermal demagnetizations of 22 new sites together with previously published data from 25 sites (sampled in Bartonian-Priabonian sediments) define reliable primary directions in the region allowing for potential vertical axis rotations estimation. The comparison between the deformed and the pre-deformational states in the cross-sections agrees with the paleomagnetic data in that neither gradient of shortening, nor significant vertical axis rotations can be invoked to explain the along-strike changes of the main structures (folds and thrusts) linked to the South-Pyrenean Frontal thrust, west of the western termination of the External Sierras. Therefore, these changes are here interpreted as the result of a wedge thrust in the Paleozoic basement, whose displacements in transferred to the Mesozoic-Tertiary cover through the Upper Triassic detachment level. This non-rotational kinematics of deformation implies a change of deformational style with respect to the External Sierras, where clockwise vertical axis rotations and gradient of shortening linked to rotational kinematics are found to be controlled by the Upper Triassic detachment level

Structural and paleomagnetic evidence for non-rotational kinematics of the South Pyrenean Frontal Thrust at the western termination of the External Sierras (southwestern central Pyrenees)

The definition of the structure and kinematics of the South-Pyrenean Frontal thrust, to the west of its westernmost outcrop in the External Sierras is the goal of this work. The methodology used is based on the construction and restoration of three balanced cross-sections. In addition to that, paleomagnetic analyses are applied to unravel possible vertical axis rotations linked to thrust kinematics. Stepwise thermal demagnetizations of 22 new sites together with previously published data from 25 sites (sampled in Bartonian-Priabonian sediments) define reliable primary directions in the region allowing for potential vertical axis rotations estimation. The comparison between the deformed and the pre-deformational states in the cross-sections agrees with the paleomagnetic data in that neither gradient of shortening, nor significant vertical axis rotations can be invoked to explain the along-strike changes of the main structures (folds and thrusts) linked to the South-Pyrenean Frontal thrust, west of the western termination of the External Sierras. Therefore, these changes are here interpreted as the result of a wedge thrust in the Paleozoic basement, whose displacements in transferred to the Mesozoic-Tertiary cover through the Upper Triassic detachment level. This non-rotational kinematics of deformation implies a change of deformational style with respect to the External Sierras, where clockwise vertical axis rotations and gradient of shortening linked to rotational kinematics are found to be controlled by the Upper Triassic detachment level

A Paleomagnetic Inspection of the Paleocene-Eocene Thermal Maximum (PETM) in the Southern Pyrenees

Magnetic properties of rocks can be useful for determining paleoenvironmental changes. A dramatic climate change that occurred in the Paleocene-Eocene Thermal Maximum (PETM) modified the environment and, hence, the magnetic properties recorded in the sediments. New paleomagnetic data from marine records of the PETM in the Southern Pyrenean zone displays a variation of the magnetic parameters in four different sections. The magnetic signal reveals a positive excursion of magnetic values starting before the onset of the marly interval of La Faja de las Flores Mb (beginning of the PETM record, Ilerdian) in the Carriata section with a maximum value in the marly interval. A similar magnetic signal is observed in the Bujaruelo section (similar to 10 km south of Carriata at PETM times) that is related directly to the marly interval of Faja de las Flores Mb. However, towards the south, the PETM interval does not appear in the sedimentary record; therefore, in the southern Gallisue section, no magnetic excursion occurs. In the southernmost-studied Entremon section, a positive magnetic excursion occurs in a thin marly interval unrelated to the PETM, and in two lower intervals of the column. All sections were later subjected to deformation during the pyrean orogeny and the three northernmost sections in the regional cleavage front, where pressure solution and remagnetizations have been described. A post-folding remagnetization component is found in the three northern sections

On the influence of magnetic mineralogy in the tectonic interpretation of anisotropy of magnetic susceptibility in cataclastic fault zones

Of the several factors involved in the development of magnetic fabrics in fault zones at shallow crustal levels, lithology and deformation intensity have probably the most important consequences for the reconstruction of their kinematic history. The basement-involved Cenozoic thrusts in the Demanda Massif (N Spain) provide the opportunity for testing the applicability of anisotropy of magnetic susceptibility (AMS) to the study of deformation in cataclastic fault rocks belonging to shallow fault zones. The Rastraculos thrust is a relatively minor basement thrust (dip-slip movement of 2km defined from cross-sections and geological maps) of Cenozoic age. This thrust contains a re-activated fault zone involving different rock types both belonging to its hangingwall (Palaeozoic) and its footwall (Triassic sandstones and dolostones and Jurassic limestones). AMS results show magnetic foliations parallel or slightly oblique to the fault zone, and both transport-parallel (projected onto the foliation plane) and transport-perpendicular (parallel to the observed intersection lineation) magnetic lineations. The two types of strain/magnetic fabric relationships can be related to deformational and mineralogical features inferred from the direct analysis of thin and polished sections under the microscope and the naked eye, respectively. Analysis of fault rocks in the Rastraculos fault zone indicates that in cataclasites, magnetic fabrics are particularly dependent on lithology and hence magnetic mineralogy. The results obtained prove the usefulness of AMS in fault zones where kinematic indicators are scarce and also give clues on the number of samples necessary to define magnetic susceptibility axes, depending on grain size, ellipsoid shapes and magnetic mineralogy

A short guide for the study of Anisotropy of Magnetic Susceptibility (AMS) in deformed rocks

The analysis of the Anisotropy of Magnetic Susceptibility (AMS) constitutes a fast and non-destructive technique that has gained the acceptance of structural geologists because it provides valuable information related to the orientation and intensity of the strain ellipsoid of deformed rocks. Part of its strength results from the possibility of (i) characterizing very subtle rock deformation, and (ii) integrating a large number of data, widespread across large areas. Since the pioneer works in the 1950s, a considerable amount of papers applying this technique to a wide variety of issues in Earth Sciences and to almost all rock types (considering both age and lithology) has been published. Here we explore and expose the use of this valuable technique in modern structural geology and tectonics, as well as its benefits and limits. 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 limitacione

Primary vs. secondary curved fold axes: Deciphering the origin of the Aït Attab syncline (Moroccan High Atlas) using paleomagnetic data

The Aït Attab syncline, located in the Central High Atlas, displays a curved geometry in plan view, and is considered as one of the most spectacular fold shapes in the Central High Atlasic belt. We conducted a paleomagnetic study in Jurassic-Cretaceous red beds to investigate the origin of this geometry. The Natural Remanent Magnetization (NRM) is dominated by a secondary magnetization carried by haematite with unvarying normal polarity that has been dated at about 100 Ma. The regional fold test performed in both limbs of the syncline is positive and the paleomagnetic vectors (after tectonic correction) are parallel throughout the curvature, indicating a negative oroclinal bending test. These results are inconsistent with previous works that consider the bent geometry of this syncline to result from subsequent distortion of originally NE–SW trending structures by rotation about a vertical axis. We interpret the NRM data to demonstrate that the changing trend of the Aït Attab syncline is a primary feature, resulting from the influence of pre-existing, NE–SW and E-W-striking extensional faults that developed during a strike-slip regime. Paleomagnetic results also reveal that the tilting observed in the sampled red beds is post Albian, probably linked to the Cenozoic inversion of the High Atlasic belt

Tethyan versus Iberian extension during the Cretaceous period in the Eastern Iberian Peninsula: insights from magnetic fabrics

This work investigates how anisotropy of magnetic susceptibility (AMS) recorded the strain related to the Early Cretaceous extensional processes in synrift sediments of the Maestrat basin (eastern Spain). Forty-two sites, distributed throughout the Lower Cretaceous sequence with dominant gentle dips, were sampled. Minerals contributing to the AMS are mainly phyllosilicates. The parallelism between magnetic and sedimentary foliation seems to indicate that a primary (synsedimentary and early diagenetic) magnetic fabric was preserved at 84% of sites. Consequently, preferred orientations of magnetic lineations are interpreted to record the effect of extensional processes coeval with sedimentation and diagenesis during this period. At these 35 sites, two main magnetic lineation orientations are found, delimiting two large domains: a NE–SW orientation prevailing in the NW sector of the basin (parallel to the extension direction of the Iberian basin), and NW–SE to NNW–SSE orientations to the SE (parallel to the extension direction controlling the western Tethys margin). Directional variability demonstrates that the Maestrat basin is located at the boundary between two domains (Iberian and Tethyan) undergoing different plate-scale extensional processes. The subsequent Cenozoic tectonic inversion affected the synsedimentary magnetic fabrics at only a few sites at the borders of the basin, where compressive features are more developed

Magnetic fabric from Quaternary volcanic edifices in the extensional Bransfield Basin: Internal structure of Penguin and Bridgeman islands (South Shetlands archipelago, Antarctica)

Studying the magnetic fabric in volcanic edifices, particularly lava flows from recent eruptions, allows us to understand the orientation distribution of the minerals related to the flow direction and properly characterize older and/or eroded flows. In this work, the magnetic fabric from recent (Quaternary) lava flows (slightly inclined in seven sites and plateau lavas in two sites), pyroclastic deposits (two sites from a scoria cone) and volcanic cones, domes and plugs (three sites) from Penguin and Bridgeman islands, located in the Bransfield backarc basin, are presented. The volcanism in the two islands is related to rifting occurring due to the opening of the Bransfield Strait, between the South Shetlands archipelago and the Antarctic Peninsula. The direction of flow of magmatic material is unknown. Rock magnetic analyses, low temperature measurements and electron microscope observations (back-scattered electron imaging and Energy Dispersive X-ray analyses) reveal a Ti-poor magnetite (and maghemite) as the main carrier of the magnetic fabric. Hematite may be present in some samples. Samples from the centre of the lavas reveal a magnetic lineation either parallel or imbricated with respect to the flow plane, whereas in the plateau lavas the magnetic lineation is contained within the subhorizontal plane except in vesicle-rich samples, where imbrication occurs. The magnetic lineation indicates a varied flow direction in Bridgeman Island with respect to the spreading Bransfield Basin axis. The flow direction in the plateau lavas on Penguin Island is deduced from the imbrication of the magnetic fabric in the more vesicular parts, suggesting a SE-NW flow. The volcanic domes are also imbricated with respect to an upward flow, and the bombs show scattered distribution