Análisis de estrías en pliegues N-S del Pirineo central meridional

Este trabajo muestra el análisis de indicadores cinemáticos (estrías) tomados sobre planos de estratificación de varios pliegues N-S con inmersión N de la vertiente meridional del Pirineo Central. Para explicar la orientación no perpendicular al eje del pliegue de varias estrías encontradas se han propuesto dos modelos geométricos relacionados con pliegues cilíndricos formados por mecanismo de flexural-slip: (1) dos etapas de plegamiento e inmersión sucesivas, y (2) plegamiento e inmersión simultáneos. This work studies the use of slickenfibres measured on bedding surfaces as kinematic indicators in N-S trending, N-plunging folds located in the Southern Pyrenees. To explain oblique to fold axis slickenfibres, two geometrical models are derived from flexural-slip mechanism of cylindrical folding: (1) two sequential stages of folding and plunging, and (2) simultaneous folding and plunging

Emplacement and Deformation of Mesozoic Gabbros of the High Atlas (Morocco): Paleomagnetism and Magnetic Fabrics

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

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

Internal characterization of embankment dams using ground penetrating radar (GPR) and thermographic analysis: A case study of the Medau Zirimilis Dam (Sardinia, Italy)

The stability of embankment dams without an impermeable core depends on the characteristics of the face slab that prevents internal erosion, piping and eventual collapse of the structure. Under a Mediterranean climate, the impermeable asphaltic face slab is subjected to high solar radiation and consequent temperature changes, which can generate the creation of cracks and joints. The Medau Zirimilis Dam, located in the Casteddu River (Sardinia), is an embankment dam that has undergone seepage and continuous repairs in its asphalt face slab. These reparations have been conducted because of the occurrence of cracks and relative movement of different segments of the slab. To evaluate if seepage endangers the integrity of the dam, GPR was used, with different antennas (100, 250 and 500 MHz), along its crest and upstream and downstream faces, and the data were integrated with infrared thermographic images. Although geophysical data do not show structural changes affecting the main dam structure, deformation structures at shallow levels and in particular in the upstream face and along the crest of the dam have been identified. Such deformation affects the road atop the crest, the face slab and underlying levels, resulting in landslides that include material from several meters below the surface. The analysis permitted the identification of the origin of surficial cracks and their effects on the face slab. These sectors, independent of current movement, define the most unstable areas against water level changes that can affect the dam integrity. GPR analysis at the embankments usually has the handicap of high clay content that precludes electromagnetic wave penetration; however, in this case, the obtained resolution and extent of penetration using the different antennas was sufficient, due to the absence of an inner waterproof unit, and permitted the evaluation of the inner structure of the dam and the application of GPR for construction quality surveillance, internal structural characterization and dam monitoring

La presa de Enciso. Certificado de defunción para el valle del Cidacos

El proyecto de presa de Enciso en el valle del Cidacos es ya una realidad: 100 metros de pared de hormigo´n (94 metros desde el cauce, 105 metros sobre cimientos) que se alzan amenazadores sobre Enciso para retener 37 Hm3 (tres cuartas partes del volumen anual del ri´o). Lejos de aportar soluciones para el desarrollo del alto Cidacos, la presa, tal como esta´ disen~ada, constituye un grave riesgo y una hipoteca para el futuro de todas las poblaciones situadas entre Yanguas y Arnedo. En este arti´culo se ex- ponen los riesgos asociados a la construccio´n y llenado de la presa de Enciso, cuyas consecuencias afectara´n a la calidad, estilo de vida y futuro de las gentes que vivan aguas abajo de la misma

Evaluación multidisciplinar 3D de una estructura kárstica asociada a colapso y subsidencia (Cuenca del Ebro)

La propagación de cavidades de disolución localizadas por debajo de series aluviales genera una serie de evidencias cuya representación superficial, en ocasiones, puede no ser indicativa de su extensión real en profundidad. La propagación de cavidades a través de medios heterogéneos, en los que pueden existir niveles cementados, puede producir cambios en la progresión de dichas cavidades y la identificación en superficie de grietas, colapsos, o zonas de subsidencia que pueden involucrar decenas de miles de metros cúbicos de material inestable. Cualquier tipo de evaluación de soluciones mitigadoras en estos contextos debe considerar la amplitud de estos volúmenes y el origen del proceso con independencia de la actividad registrada en superficie. En este trabajo se aborda un estudio integrado con el objetivo de determinar la estructura de un sector con evidencias de actividad kárstica superficial y evaluar su extensión y desarrollo tridimensional integrando el análisis geomorfológico, de inspección de campo, geofísica (GPR) y sondeos. The propagation of solution cavities located below alluvial deposits produce surficial evidences that, in some cases, cannot be representative of their actual extension at depth. Cavity propagation through heterogeneous media, for example with interbedded cemented levels, can produce changes in the propagation of deformation producing the identification of cracks, collapses or subsidence zones at the surface. These processes can involve thousands of cubic meters of unstable materials. Any evaluation of mitigation tasks, in this context, needs to consider: i) the amplitude of the affected volumes and ii) the in depth origin with independence of the surficial recorded activity In this work an integrated analysis is developed to evaluate the size and 3D structure of a sector with surficial evidences of karstic activity integrating geomorphological analysis, field inspection, geophysics (GPR) and boreholes

El embalse de Mularroya (Zaragoza): problemas geológicos de una obra en avanzado estado de construcción.

El embalse de Mularroya (más de 100 hm3 de capacidad), situado sobre el cauce del río Grío, uno de los afluentes del Jalón (afluente a su vez del Ebro por su margen derecha) es una obra en avanzado estado de construcción. Desde la fase de estudios previos (mediados de los años 90) hasta la actualidad, se han constatado problemas geológicos que ponen en entredicho la oportunidad de la realización de esta obra píblica. En este trabajo se revisan y se aportan nuevos datos sobre los problemas más significativos: (i) más de 1/5 de la superficie del vaso del embalse se asienta sobre unidades carbonatadas permeables del acuífero regional de las unidades del tránsito Triásico-Jurásico (formaciones Imón, Cortes de Tajuña y Cuevas Labradas), que forman una estructura sinclinal con inmersión NNO; (ii) su situación sobre un cauce cuya aportación anual es 20% la capacidad del embalse, lo que requiere el trasvase de agua desde el río Jalón por un tínel de desviación que atraviese estructuras complejas del macizo paleozoico que constituye la Rama Aragonesa de la Cordillera Ibérica; y (iii) los riesgos geológicos que conlleva, fundamentalmente por su localización sobre el sistema de fallas de río Grío, que incluye varios segmentos activos durante el Cuaternario capaces de generar sismos de magnitud entre 6,2 y 6,8. De acuerdo con la simulación realizada mediante el programa IBER, el riesgo de inundación asociado a un posible fallo geotécnico de la presa afectaría significativamente a poblaciones importantes, como Ricla o Calatorao, situadas aguas abajo de la presa. In this work we analyze the geological hazard associated with the Mularroya reservoir (whose total volume is more than 100 hm(3)), located on the Grio river, a tributary of the Jalon river, one of the right-bank main tributaries of the Ebro river. Its construction, about to be finished, is problematic because of (i) the particular geological setting of the reservoir, located on the permeable carbonate units belonging to the Triassic-Jurassic transition (Imon, Cortes de Tajuna and Cuevas Labradas formations), defining a NNW-plunging syncline; (ii) its location on a river whose streamflow per year is 20% of the total reservoir volume, thus needing a gravity fed tunnel from the Jalon river cutting accross complex structures of the Paleozoic massifs of the Aragonian Branch of the Iberian Chain; and (iii) geological risks associated with its particular location, close to the rio Grio fault system, that includes faults that were active during the Quaternary, and are able to generate earthquakes with magnitudes 6.2-6.8. According to our simulations by means of IBER program, flooding risk associated with a possible geotechnical fault of the Mularroya dam would cause severe damage in villages located downstream (Ricla and Calatorao)

Basement-Cover Relationships and Their Along-Strike Changes in the Linking Zone (Iberian Range, Spain): A Combined Structural and Gravimetric Study

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

Caracterización de la estructura constructiva de la fachada de las escuelas mayores de la universidad de salamanca. interpretación por combinación de técnicas de georradar y sondeos mecánicos

La caracterización de la fábrica constructiva de edificaciones previa a su restauración o evaluación de estabilidad requiere estudios históricos previos y análisis del estilo constructivo del momento de construcción. En ocasiones dicha información es limitada o requiere ser contrastada por métodos de caracterización directa. En este trabajo se aborda el análisis de la fachada principal de las Escuelas Mayores de Salamanca (“Fachada Rica”) a través de la caracterización geofísica por medio de georradar, de 3 sondeos horizontales y los resultados obtenidos del análisis constructivo con criterios histórico-artísticos. Los resultados obtenidos permiten identificar la correlación entre las unidades geofísicas identificadas en los sondeos, y evaluar la variabilidad lateral de dichas unidades. Constructive fabric characterization previous to restauration is required for the design and stability analysis. These studies include the integration of historical data and knowledge of building style at the construction period. Sometimes this information is limited or requires to be contrasted with present-day construction features. In this work the Escuelas Mayores de Salamanca main façade (“ Fachada Rica ”) is evaluated through the integration of three horizontal boreholes and the obtained results from the geophysical characterization by means of GPR and considering the artistic-historical construction criteria since its construction moment. The obtained results permit to identify the correlation between the de?ned geophysical units and the layers identi?ed in boreholes and to evaluate the lateral correlation of these boreholes along the whole analyzed façade

De la gran urbe a la modesta villa: aplicación de la prospección magnética a la arqueología romana

La caracterización de yacimientos arqueológicos es una de las aplicaciones más extendidas de la prospección magnética, por su carácter no invasivo, que permite explorar sin destruir superficies de extensión considerable, y debido al carácter ferromagnético de muchos elementos asociados a la actividad humana (instrumentos de hierro, componentes minerales de tejas, ladrillos, hornos de cocción, etc...). Habitualmente, en prospección magnética se mide la intensidad total del campo magnético terrestre (realizando las correcciones de las oscilaciones debidas a la variación diurna y las micropulsaciones) y el gradiente horizontal o vertical (diferencia entre las medidas tomadas simultáneamente en posiciones cercanas mediante dos sensores, ver figura 1), que resulta muy útil cuando se trata de detectar anomalías debidas a cuerpos cercanos a la superficie. Frente a la sistemática de trabajo en prospecciones geológicas, el mallado de la prospección magnética en arqueología suele ser muy denso, con 1 m o 0,5 m de espaciado entre perfiles en muchas ocasiones