Contribución de la modelización analógica al estudio de procesos tectónicos de gran escala

La tectónica experimental o modelización analógica de procesos tectónicos es una técnica experimental que estudia los agentes que intervienen en los procesos tectónicos que se producen en la naturaleza mediante modelos dimensionados realizados en el laboratorio. El objetivo de estos modelos no es simplemente reproducir observaciones naturales, sino comprobar, mediante experimentos controlados, hipótesis sobre los mecanismos y las fuerzas que impulsan los procesos tectónicos. Un modelo correctamente dimensionado permite observar las condiciones necesarias para la formación de las estructuras tectónicas, así como su evolución en el tiempo. En este trabajo se presentan los fundamentos de dicha técnica y se muestran algunos ejemplos de modelos analógicos de procesos tectónicos a gran escala que han permitido avanzar en el conocimiento y comprender los mecanismos causantes de los mismos. Estos ejemplos incluyen la colisión de la India con el continente euroasiático, el inicio de un proceso de subducción, el plegamiento litosférico de la placa Iberia, la extensión litosférica y las grandes fallas de desgarre y cuencas de pull-apart asociadas

Los procesos geológicos internos: aprendizaje basado en fenómenos y observaciones cotidianas

El presente trabajo recoge una propuesta didáctica adaptada a un aula de biología y geología de 1º de bachillerato, centrada en los contenidos del currículo relacionados con los procesos geológicos internos. La propuesta didáctica parte de un análisis de las dificultades de aprendizaje e ideas previas detectadas en diversos estudios bibliográficos y considera el nivel y las dificultades concretas del aula de trabajo durante el desarrollo del practicum II. Partiendo de las dificultades, características y carencias del grupo específico , el trabajo plantea una serie de (i) objetivos didácticos, acordes con los criterios de evaluación recogidos en el currículo oficial y (ii) actividades. Entre dichas actividades, la propuesta considera numerosos trabajos experimentales, de trabajo en grupo y de aprendizaje cooperativo, y utiliza los recursos cotidianos y la observación de los fenómenos comunes como hilo conductor. Parte de las actividades propuestas fueron realizadas con el grupo de referencia durante el practicum II. Su puesta en práctica, aunque parcial, permite realizar un análisis de los resultados, una evaluación del grado de logro de los objetivos didácticos y una serie de propuestas de mejora.<br /

Structure of the Internal Sierras in the South Pyrenean Zone (Aragón and Osia valleys, Central Pyrenees). Implications in the timing of deformation

La estructura del sector Norte de la Zona Surpirenaica está caracterizada por la presencia de foliación asociada a pliegues. Los pliegues tienen una dirección ONO-ESE, vergencia Sur y planos axiales verticales o con buzamientos elevados (generalmente mayores de 50º) hacia el Norte. Este estilo estructural contrasta con la existencia en el margen Norte de las Sierras Interiores de un sinclinal volcado a tumbado, de escala kilométrica, cuyo flanco invertido aflora entre los valles de los ríos Aragón y Osia. La cartografía de su superficie axial se traduce en trazas cerradas que aíslan el flanco invertido. Este pliegue está asociado con una foliación de plano axial con un buzamiento hacia el Norte (entre subhorizontal y 35ºN), que es reducido en comparación con la inclinación de la foliación en el resto del corte geológico. El pliegue se interpreta como una estructura temprana en relación al emplazamiento de la lámina de Gavarnie, que produciría el basculamiento de la mismaThe structure of the northern sector of the South Pyrenean Zone is characterized by fold-related cleavage. The folds are WNW-ESE-trending and South-verging, with axial planes that are subvertical or steeply dipping to the North (dips are generally steeper than 50º). This structural style contrasts with the presence in the northern margin of the Internal Sierras of a kilometric- scale, overturned to recumbent syncline, whose overturned limb crops out between the valleys of the Aragon and Osia rivers. The mapping of its axial surface is represented by closed curves that isolate the overturned limb. This fold is related to a shallowly North-dipping, axial plane cleavage (subhorizontal to 35ºN). Its dip is low in comparisson with the cleavage in the rest of the geological cross section. The described fold is interpreted as an early structure with respect to the emplacement of the Gavarnie thrust sheet, that would be responsible for its tiltin

Análisis de la fábrica magnética en modelos analógicos de arcillas

Este estudio presenta el análisis de la fábrica magnética en una serie de modelos analógicos realizados con arcillas rojas comerciales. A partir de la disgregación de la arcilla en agua y su posterior decantación conseguimos reproducir la adquisición de una fábrica magnética deposicional (geometría oblata del elipsoide magnético con el eje mínimo de susceptibilidad perpendicular al plano de depósito). En un modelo de deformación de desgarre (experimento de Riedel), la lineación magnética se dispone paralela a la dirección de estiramiento. En el modelo de acortamiento, la lineación magnética se orienta perpendicular a la dirección de acortamiento. Los resultados obtenidos ponen de manifiesto el gran potencial de la arcilla roja comercial para ser utilizada en este tipo de modelos analógicos, así como su capacidad de registrar una fábrica magnética coherente en cada caso. This study shows the magnetic fabric analysis applied to several analogue models developed with common commercial red clays. By dispersing the clays in water and after allowing their subsequent decantation, we were able to reproduce the acquisition of a depositional magnetic fabric (oblate geometry of the magnetic ellipsoid with the minimum susceptibility axis perpendicular to the deposit plane). In a strike-slip deformation model (Riedel experiment), magnetic lineation is parallel to the stretching direction. In the shortening model, magnetic lineation orients perpendicular to the shortening direction. The obtained results illustrate the great potential of these commercial red clays to be used in this type of analogue models, as well as their capacity to register a coherent magnetic fabric in each case

Multi-episodic remagnetization related to deformation in the Pyrenean Internal Sierras

The Internal Sierras (IS) in the southern margin of the Western and Central Axial Zone (Southern Pyrenees) are affected by a syn-orogenic remagnetization that provides information to reconstruct deformation geometries at the time of acquisition of magnetization. Furthermore, the IS structure changes strike along its structural trend, from ∼N120 to 130°E in the western and eastern margins to ∼N070–090°E in the central part. Palaeomagnetic techniques have been used to (i) accurately define the timing of remagnetization with regard to deformation and (ii) determine if the along-strike trend variation in the IS was induced by deformation and thrust emplacement during the Pyrenean compression or, on the contrary, was the result of a primary orientation controlled by structures inherited from pre-orogenic times. From 23 new palaeomagnetic sites, collected in Upper Cretaceous marls and marly limestones, two meaningful and stable palaeomagnetic components were resolved, principally carried by magnetite: (1) a lower-temperature component (B) that unblocks between 200 °C and 325–400 °C and (2) a higher-temperature component (C) that has been successfully isolated by means of combined thermal (up to 400 °C) and AF demagnetization (generally up to 50–100 mT). The B component is a late remagnetization that post-dates folding and emplacement of basement thrust sheets in the IS (mainly the Gavarnie thrust). It supports small but statistically significant clockwise rotations in the western part of the IS (from +18 to +26°). These rotations can be attributed to the westwards shortening decrease in the thrust system below the Gavarnie unit that results from its along-strike structural change, with a higher number of basement thrusts to the east. The C component has been interpreted as an early remagnetization, based on the results of conglomerate and fold tests. This component predates basement thrusting and is diachronous across the study area: reverse and normal polarities dominate in the eastern and western margins of the IS, respectively. New and previous palaeomagnetic data point out that curvature in the IS is probably a primary feature and the along-strike change in their trend could be interpreted as the result of basement geometrical features inherited from Variscan, Late Variscan or Mesozoic times. A complex, multi-episodic remagnetization probably related to burial and deformation processes occurred during Eocene times

Multi-episodic Remagnetization Related to Deformation in the Pyrenean Internal Sierras

The Internal Sierras (IS) in the southern margin of the Western and Central Axial Zone (Southern Pyrenees) are affected by a syn-orogenic remagnetization that provides information to reconstruct deformation geometries at the time of acquisition of magnetization. Furthermore, the IS structure changes strike along its structural trend, from ∼N120 to 130°E in the western and eastern margins to ∼N070–090°E in the central part. Palaeomagnetic techniques have been used to (i) accurately define the timing of remagnetization with regard to deformation and (ii) determine if the along-strike trend variation in the IS was induced by deformation and thrust emplacement during the Pyrenean compression or, on the contrary, was the result of a primary orientation controlled by structures inherited from pre-orogenic times. From 23 new palaeomagnetic sites, collected in Upper Cretaceous marls and marly limestones, two meaningful and stable palaeomagnetic components were resolved, principally carried by magnetite: (1) a lower-temperature component (B) that unblocks between 200 °C and 325–400 °C and (2) a higher-temperature component (C) that has been successfully isolated by means of combined thermal (up to 400 °C) and AF demagnetization (generally up to 50–100 mT). The B component is a late remagnetization that post-dates folding and emplacement of basement thrust sheets in the IS (mainly the Gavarnie thrust). It supports small but statistically significant clockwise rotations in the western part of the IS (from +18 to +26°). These rotations can be attributed to the westwards shortening decrease in the thrust system below the Gavarnie unit that results from its along-strike structural change, with a higher number of basement thrusts to the east. The C component has been interpreted as an early remagnetization, based on the results of conglomerate and fold tests. This component predates basement thrusting and is diachronous across the study area: reverse and normal polarities dominate in the eastern and western margins of the IS, respectively. New and previous palaeomagnetic data point out that curvature in the IS is probably a primary feature and the along-strike change in their trend could be interpreted as the result of basement geometrical features inherited from Variscan, Late Variscan or Mesozoic times. A complex, multi-episodic remagnetization probably related to burial and deformation processes occurred during Eocene times

Imaging the crustal structure of the Central Pyrenees using Seismic Interferometry

The Pyrenees constitute a natural laboratory where hundreds of geological and geophysical data have been acquired during the last decades. It represents a roughly E-W oriented doubly vergent orogen formed during the Alpine Orogeny. Deep seismic reflection data obtained during the 80s revealed its crustal architecture that resulted from the subduction of the Iberian plate under the European lithosphere at its central part. In this work we applied seismic interferometry to the same passive dataset through two different techniques aiming to construct two independent images of the Central Pyrenean lithosphere, to enhance the current knowledge of the area. The main objectives are to compare them and correlate the obtained results with previous data. Data were acquired within the IMAGYN project along a NE-SW 70 km-long profile extending from the Southern Pyrenees (Pedraforca and Cadí Units, northern Iberia) to the northern part of the Axial Zone, close to Ax-les-Thermes (France). Data came from three to five months of continuous recording from an almost linear array of 43 seismic stations (being 17 and 26 broadband and short-period stations, respectively). The two applied techniques are (1) the global-phase seismic interferometry (GloPSI), using continuous recordings of teleseismic (30¿ 120¿ epicentral distance), and (2) the use of continuous ambient seismic noise recordings through autocorrelation. Despite both methods rely on different energy sources, they are complementary and use static receivers. In the first method (GloPSI), we extracted global phases (PKP, PKiKP and PKIKP) and their reverberations within the lithosphere. The selected phases were autocorrelated and stacked to construct a high-resolution pseudo zero-offset reflection image. The second approach provided an approximation to the zero-offset reflection response of a single station. Results reveal features that can be correlated in both reflection images. The crust-mantle boundary is mapped as a relative flat interface at approximately 35-40 km depth. Crustal interfaces detected at 15 and 25 km depth can be related to the Conrad discontinuity and other compositional changes within the crust.This work is part of the project ¿High-resolution imaging of the crustal-scale structure of the Central Pyrenees and role of Variscan inheritance on its geodynamic evolution¿ (IMAGYN), PID2020-114273GB-C22 funded by MCIN/AEI/10.13039/50110001103

Anisotropy of magnetic susceptibility of the Pyrenean granites

In this paper, we report on a compilation of more than 2200 sites (more than 10,000 individual measurements) where anisotropy of magnetic susceptibility (AMS) was studied in granites from the Variscan Pyrenees. The standardization and homogenization of this information has allowed us to produce three Main Maps that synthesize all the information related with the AMS of the Pyrenean granites. We also describe the problems found during the construction of the database (variable geo-positioning, different published information, etc.). The information derived from 21 granite bodies, the database, and the synthesis maps (magnetic susceptibility, Km, and the orientation of the magnetic foliation, plane perpendicular to k3, and of the magnetic lineation, k1) allow us to see for the first time a complete image of this important kinematic and petrographic indicator

Structure of the Central High Atlas (Morocco): constraints from Potential Field Data and 3D Models

[EN] From a 3-D reconstruction based on serial cross-sections, structural observations, and potential field geophysical data we characterize the major structural frame of the Moroccan Central High Atlas. Its overall structure shows two fold-and-thrust systems (each one showing particular features) at the northern and southern borders of the chain, resulting from the inversion of the Mesozoic basin. Five more zones can be distinguished in the inner part of the chain, according to their structural features, ranging from basement-involved structures in the westernmost sector to completely detached structures in the eastern part. Most compressional structures show an ENE-WSW (atlasic) trend, also pinpointed by the orientation of pressure-solution and slaty cleavage associated with the compressional stage. Large-scale faults of crustal or lithospheric reach can be inferred from the aeromagnetic maps, that show that the gabbro intrusions (some of them showing sigmoidal shape) are aligned in an E-W direction, oblique to the main trend. The overall compressional structure is controlled by a main, low-angle, South-verging thrust involving both the Mesozoic cover and the Paleozoic basement thrusting over the southern foreland of the chain. Branching on this surface there is a series of back-thrusts and faults, some of them resulting from re-activation of extensional structures.This research was financed by projects CGL2016-77560-C2 (C21 and C22) and PID2019-108753GB-C2 (C21 and C22), funded by MICIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”.Peer reviewe

Mapa de anomalía de Bouguer del Pirineo Suroccidental

Las relaciones geométricas y cinemáticas entre las unidades de basamento de los Pirineos Suroccidentales, específicamente entre la terminación de la Zona Axial y el afloramiento de los Macizos Vascos, son desconocidas y potencialmente muy complejas (Oliva-Urcia, 2018). La gravimetría se ha aplicado con éxito en otras regiones pirenaicas para resolver la geometría de las unidades de cobertera y basamento debido al contraste de densidad existente en las rocas involucradas. Con el objetivo de resolver dichas relaciones y sus implicaciones en la comprensión 4D de esta parte de la cadena, hemos llevado a cabo un exhaustivo programa de adquisición de datos gravimétricos (2018-2020) entre los valles de Salazar (al oeste) y del Aragón Subordán (hacia el este) en el marco del proyecto 3DGeoEU (GeoERA-H2020). Más de 1500 nuevas estaciones (varias de ellas en alta montaña) junto con > 1700 puntos de la base de datos SITOPO, ¿ 1200 de campañas mineras, y > 500 de anteriores proyectos del IGME hacen un total de > 5000 estaciones gravimétricas armonizadas. Además, se han recopilado > 500 datos de densidad de bases de datos o muestreados en el campo. En este trabajo presentaremos los mapas de anomalías Bouguer y de anomalía residual obtenidos, junto con una revisión de las principales estructuras cartográficas en la zona