Thirty years (1988-2018) of advances in the knowledge of the structural evolution of the south-central Pyrenees during the Cenozoic collision, a summary

La cadena Pirenaica ha atraído una gran atención, lo que ha producido una gran cantidad de artículos científicos y tesis doctorales sobre las relaciones de la estructura-sedimentación durante la colisión. En años recientes, la aplicación de técnicas geofísicas y geoquímicas han permitido cuantificar (en ocasiones de manera cualitativa) los procesos que han estado activos durante la convergencia de las placas Ibérica-Euroasiática desde el Santoniense tardío hasta el Mioceno. Esta convergencia produjo una cadena de dirección ONO-ESE estrecha, de ~ 1000 km de longitud que se extiende desde el Golfo de León hasta el margen de Galicia. La parte centro-occidental del Pirineo ístmico es donde se centra este trabajo, donde se agrupa un resumen de los trabajos realizados en las últimas tres décadas (1988-2018). El agrupamiento se centra en las técnicas utilizadas a lo largo de estos años: cortes geológicos compensados, limitaciones a partir de anomalías magnéticas del fondo oceánico, gravimetría…, que contribuyen a calcular el acortamiento que se ha producido durante la convergencia; datos geoquímicos que proporcionan información sobre paleotemperaturas y edades de levantamiento/exhumación, paleomagnetismo y fábricas magnéticas que permiten calcular rotaciones de eje vertical, edades de sedimentos sintectónicos y la deformación de las rocas; y modelos analógicos que apuntan factores que influyen en el desarrollo de la deformación. Se han añadido unos comentarios breves sobre los estadios pre-convergencia y sobre la evolución post-orogénica de la cuenca de antepaís del Ebro. Finalmente se muestra un resumen con la cronología más relevante sobre la actividad de los distintos cabalgamientos y la exhumación de la parte interna de la cadena. A pesar de la gran cantidad de datos, aún queda por resolver la aparente inconsistencia entre algunos de ellos. The Pyrenean Chain has drawn an extensive attention which has produced a plethora of research papers and doctoral theses about the structure-sedimentation relationships, and in recent years, geophysical and geochemical techniques have allowed quantifying the processes active during the convergence of Iberian-Eurasian plates from late Santonian to Miocene times. This convergence produced a WNW-ESE striking, narrow asymmetric chain of ~1000 km in length from the Gulf of Lion to the Galicia margin. The west-central part of the Pyrenean isthmus is the focus of this work, where a summary of some of the last 30 years’ publications are grouped by technique(s): geological balanced cross-sections and geophysical constraints from oceanic magnetic anomalies, gravimetry…, which contribute to calculate shortening; geochemical data that provide information about paleotemperatures and ages of uplift; paleomagnetism and magnetic fabrics which allow calculating vertical axis rotations, dating syntectonic sediments and getting information about the strain in rocks; and analog models and their input in the understanding of the Pyrenean deformation. Short comments about the pre-convergence stages and post-orogenic evolution of the Ebro foreland basin are also included. Finally, a chronostratigraphic chart with a summary of the main events (activity of thrust sheets and uplifted areas) is shown. Despite many data have been collected, some inconsistencies are not yet resolved

The case of a southern European glacier which survived Roman and medieval warm periods but is disappearing under recent warming

Artículo escrito por un elevado número de autores, solo se referencian el que aparece en primer lugar, el nombre del grupo de colaboración, si le hubiere, y los autores pertenecientes a la UA

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)...

The Use of Magnetic Susceptibility as a Technique to Measure the Impact of Wildfires on Archaeological Heritage

Wildfires are one of the main threats of natural areas and often fires can affect protected or heritage areas and properties, in which the preservation requirements demand the use of non-destructive techniques (NDTs). The magnetic susceptibility is an NDT that provides information on the mineralogical composition of the materials but has never been applied to the evaluation of fires. Here, we combine laboratory with field analysis to test the applicability of the magnetic susceptibility for the assessment of the impacts of wildfires. The laboratory results showed an increase in the magnetic susceptibility with the temperature, more evident in the samples heated to 600 °C and above. The in situ measures revealed a spatial variation in the magnetic susceptibility, which was related to the behaviour of the fire in the area. The samples were later analysed with other magnetic destructive techniques that were used to confirm the mineralogical processes that occurred in the materials. The increase in the susceptibility values were due to the formation of iron oxides. The destructive analysis also showed the presence of minerals such as hematite and magnetite in the samples. Overall, the study allowed a first approach to test the magnetic susceptibility as a simple and fast way to measure the impacts of wildfires

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 (~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 Gallisué section, no magnetic excursion occurs. In the southernmost-studied Entremón 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