When working with geological maps and cross-sections, and no kinematic criteria on fault motion a... more When working with geological maps and cross-sections, and no kinematic criteria on fault motion are available, we usually classify the type of fault (dip-slip, strike-slip, oblique) according to the observed separation of planar markers, usually bedding. Although in many cases the separation agrees with fault slip, and therefore, the deduced type of fault is correct, these deductions can be completely wrong in certain circumstances. Amongst other parameters, the observed separation depends on the angular relationship between the cutoff line of the planar marker on the fault plane and the slip vector. In this paper, we elaborate on this relationship and present a tool to facilitate classifications of faults whose motion does not involve rotation of the fault blocks, and cut and offset previously tilted or folded surfaces. This tool consists of graphs that predict how the observed separation will be in cross sections and maps. In addition, the influence of different parameters on the magnitude of the separation in a geological section across a fault is discussed. The validity of the presented tool is demonstrated through its application to two field examples of strike-slip faults offsetting inclined layers and a fold train in the Asturian Basin. Our graphs can be used to improve geological interpretations, in a predictive way when constructing geological maps and cross-sections, and to decipher the existence of layers tilted or folded before fault development.
The first steps to study natural structures are data collection, their representation and their g... more The first steps to study natural structures are data collection, their representation and their geological interpretation. There is no doubt that the development of digital techniques in recent times has facilitated these tasks. Here we present an inventory of virtual outcrop models from the Cantabrian Zone and Asturian Basin, North-Northwest Iberian Peninsula, and the procedure employed to obtain the data, construct the models and interpret them geologically. These models correspond to contractional folds and faults of Palaeozoic age, and to Mesozoic extensional structures affected by Cenozoic tectonic inversion in the form of folds and thrusts. The digital techniques and data employed are fieldwork, as well as Google Earth images, orthophotographs, stereoscopical pairs of photographs and virtual outcrop models, constructed using Structure from Motion photogrammetry based on images extracted from Google Earth or from photographs taken in the field using a tripod or unmanned aerial vehicles. The analysis of these models has provided us with geological information that would have been difficult to obtain using traditional techniques. Apart from their scientific interest, the examples shown may be helpful for structural geologists who wish to obtain 3D geological models, maps and sections, and additional structural information from field examples, as well as elements to prepare a virtual fieldtrip and/or for educational purposes.
The Schmidt hammer, widely used in Engineering Geology, has also been used in Structural Geology ... more The Schmidt hammer, widely used in Engineering Geology, has also been used in Structural Geology as a tool to quantify the mechanical properties of rocks affected by different structures and the damage caused to rocks by fracturing. However, how the impact resistance of rocks (rebound) varies in layers located in different positions of a fold has never been tested. Here, the Schmidt hammer is used to quantify this parameter in a syncline involving a Carboniferous carbonate sequence located in the Cantabrian Zone, the foreland fold-and-thrust belt of the Variscan orogen in western Iberia. The variations in the Schmidt-hammer rebound value along a folded layer are consistent with other indicators such as variations of dip and thickness. These findings have also implications on obtaining representative rebound values of stratigraphic units, and on deriving parameters such as the uniaxial compressive strength and the Young modulus from the Schmidt-hammer rebound values applied to folded regions.
An in-house software for Computer Assisted Virtual Environments (CAVE TM), called 3D Stereo Virtu... more An in-house software for Computer Assisted Virtual Environments (CAVE TM), called 3D Stereo Virtual Drawing Tool (3D Stereo VDT), is presented through the interpretation of bedding and structural elements in two virtual outcrops. The main purpose of this software is to perform geological interpretations of point clouds through a friendly interface. 3D trackers allow the drawing of points and lines. Such elements may represent all sorts of geological features: bedding, unconformities, fractures, fold axial surfaces, axes, etc. One of the outcrops is a small, close-range point cloud constructed using field photographs of a beach cliff, whereas the other is a bigger, aerial point cloud built with Google Earth images of a valley slope. Both irregular and partly inaccessible outcrops consist of folded and faulted Palaeozoic rocks in the northwest portion of the Iberian Peninsula. The 3D analysis of these outcrops allowed us to supply new orientation data as well as ideas regarding the characteristics, origin and evolution of the mapped structures in relation to their setting.
ABSTRACT A detailed structural analysis was carried out on the Jurassic rocks cropping out along ... more ABSTRACT A detailed structural analysis was carried out on the Jurassic rocks cropping out along the cliffs of La Conejera Inlet (Asturias, Spain). It includes a geological map and a distortion-free cross-section constructed via photogrammetric methods. La Conejera Inlet is located within the Asturian Basin, a Permian–Mesozoic extensional basin partially formed during the opening of the Bay of Biscay. It suffered selective basin inversion during a Cenozoic contraction responsible for the Pyrenees and its western prolongation along the north margin of the Iberian Peninsula. The study of the structures (folds, faults, joints and veins) of the hangingwall of two normal faults with opposite dip senses reveals that it underwent a later compressional stage in which one fault block acted as a buttress. The contractional deformation in the hangingwall, interpreted as a deformed rollover anticline with an associated antithetic fault, diminishes on moving away from one of the main faults. The positive inversion tectonics produced not only a buttressing effect, but it also involved a certain amount of reverse reactivation of one of the main faults that still preserves a normal displacement. The original normal motion would have taken place during the Middle?–Late Jurassic, related to an embryonic stage of the opening of the Bay of Biscay. The later contractional stage would have been caused by the Cenozoic Alpine shortening. The good outcrop quality allows a relative chronology for the observed structures to be established. Employing all the available information we tried to reconstruct the structure at depth and predict the detachment depth, and to estimate the amounts of extension (the present-day value and that before the compression) and compression.
A number of geometrical techniques allow estimating amounts of horizontal extension and depth to ... more A number of geometrical techniques allow estimating amounts of horizontal extension and depth to detachment in simple rollover anticlines over listric normal faults given one or more horizons, the portion of the fault between the hanging wall and foot wall cut off points, and the depth to detachment (if the extension is to be estimated) or the extension (if the depth to detachment is to be estimated). These techniques assist in the construction and validation of sections across listric normal faults, but currently, it is unclear which ones predict correct amounts of horizontal extension and detachment depth and why is that. To sort this out, these techniques are evaluated using physical experiments and subsurface seismic examples of simple listric normal faults.
Resumen: Las estructuras estudiadas se sitúan en el flanco N del anticlinal de Villasecino (Zona ... more Resumen: Las estructuras estudiadas se sitúan en el flanco N del anticlinal de Villasecino (Zona Cantábrica). Dichas estructuras, de dimensiones métricas, afloran en un talud de carretera y deforman capas de caliza carbonífera. Aplicando un método fotogramétrico, se ha elaborado un corte geológico sin distorsión y perpendicular a las estructuras. Para ello, se han tomado 3 pares estereoscópicos de fotografías, se han registrado las coordenadas precisas de varios puntos de control del afloramiento fotografiado, se han rectificado las fotografías a fin de formar pares estereoscópicos, se ha realizado la interpretación geológica de las imágenes en 3D y, finalmente, se han proyectado las coordenadas tridimensionales de los elementos interpretados en un corte geológico perpendicular al eje de los pliegues y paralelo a la dirección de transporte tectónico. El afloramiento presenta fallas subparalelas a la estratificación, pliegues asociados y fallas subperpendiculares a la estratificación...
Sedimentation coeval with growth of the Santaren anticline provides an excellent opportunity to s... more Sedimentation coeval with growth of the Santaren anticline provides an excellent opportunity to study the relationships between sedimentation and anticline uplift through time. The Santaren anticline is a kilometre-scale, NW -SE trending fold offshore of Cuba, in the Bahamas foreland of the Cuban fold and thrust belt. The growth strata associated with this anticline consist of a thick package of carbonate sediments that were deposited without major interruptions from Neogene (and perhaps before) to present day. The excellent seismic resolution and age control of a number of seismic horizons within the growth strata allowed us to define 25 growth beds, each of them representing between 0.1 and 3.2 Ma. An analysis of the thickness of these beds allowed us to determine accurate quantitative values of cumulative decompacted thickness and crestal structural relief at the time of their deposition. In addition, for the same periods, sedimentation and fold uplift rates were calculated. Moreover, some information on relationships between sedimentation and fold uplift rates was inferred from the growth stratal geometry. Growth beds that overlap the fold crest and thin over it indicate that sedimentation rates outpaced fold growth rates during their deposition. Some overlapping beds have constant thickness indicating that no fold uplift occurred during their sedimentation. The rest of the growth beds exhibit onlap/offlap geometries that do not indicate a unique sedimentation/fold uplift rate relationship. Only in those cases in which the geometry of the underlying bed at the end of its deposition is known is it possible to infer a specific sedimentation/fold uplift rate relationship. As a result of this analysis, we have been able to (1) illustrate that the geometry of the growth strata associated with the Santaren anticline results from competition between sedimentation and tectonic fold uplift, (2) document the episodic and non-steady nature of fold growth, and show that short-term rates (at the scale of hundreds of thousands years) provide much insight into the interplay between sedimentation and tectonic fold uplift that control the growth stratal patterns. D
A strategy to predict strain across geological structures, based on previous techniques, is modif... more A strategy to predict strain across geological structures, based on previous techniques, is modified and evaluated, and a practical application is shown. The technique, which employs cross-section restoration combined with kinematic forward modelling, consists of restoring ...
ABSTRACT A detailed structural analysis was carried out on the Jurassic rocks cropping out along ... more ABSTRACT A detailed structural analysis was carried out on the Jurassic rocks cropping out along the cliffs of La Conejera Inlet (Asturias, Spain). It includes a geological map and a distortion-free cross-section constructed via photogrammetric methods. La Conejera Inlet is located within the Asturian Basin, a Permian–Mesozoic extensional basin partially formed during the opening of the Bay of Biscay. It suffered selective basin inversion during a Cenozoic contraction responsible for the Pyrenees and its western prolongation along the north margin of the Iberian Peninsula. The study of the structures (folds, faults, joints and veins) of the hangingwall of two normal faults with opposite dip senses reveals that it underwent a later compressional stage in which one fault block acted as a buttress. The contractional deformation in the hangingwall, interpreted as a deformed rollover anticline with an associated antithetic fault, diminishes on moving away from one of the main faults. The positive inversion tectonics produced not only a buttressing effect, but it also involved a certain amount of reverse reactivation of one of the main faults that still preserves a normal displacement. The original normal motion would have taken place during the Middle?–Late Jurassic, related to an embryonic stage of the opening of the Bay of Biscay. The later contractional stage would have been caused by the Cenozoic Alpine shortening. The good outcrop quality allows a relative chronology for the observed structures to be established. Employing all the available information we tried to reconstruct the structure at depth and predict the detachment depth, and to estimate the amounts of extension (the present-day value and that before the compression) and compression.
To check the behaviour of physical experiments of simple rollover anticlines over listric normal ... more To check the behaviour of physical experiments of simple rollover anticlines over listric normal faults, several fault-slip, bed-length and area parameters are measured for different beds in two stages of evolution of two physical experiments. Given a certain amount of extension, the heave and dip of the displacement vary for different beds, whereas the throw is approximately constant for small amounts of extension and the displacement remains constant for high amounts of extension. Neither bed lengths nor areas beneath beds remain constant with increasing extension. A number of techniques allow amounts of extension and detachment depth to be estimated given one or more marker horizons, the portion of the fault between hanging-wall and footwall cut off points, the depth to detachment and/or the shear angle. These techniques are applied to the physical experiments of listric normal faults analysed. Since many of these techniques rely on parameters measured on the physical experiments such as fault slip, bed length and area, the influence of these parameters on the different magnitudes of extension and detachment depths estimated using different techniques is discussed and the accuracy of the results is compared. D
Several authors have proposed different methods to predict the subsurface geometry of thrust-rela... more Several authors have proposed different methods to predict the subsurface geometry of thrust-related folds from shallow baseline data. Given certain bed dips of a backlimb, crest and forelimb of a fold model, we reconstructed the structure at depth as a fault-bend, a fault-propagation and a detachment fold following these methods and we checked that the resulting geological cross-sections were balanced. In addition, we modified the position of the surface dips so that the resulting fold models had different half-wavelengths and different structural relief and, therefore, allowing chevron folds and box folds to be constructed. In order to compare the different balanced interpretations, we elaborated several graphs in which various geometrical parameters (shortening, detachment depth and dimensions of the thrust ramp) are plotted versus the half-wavelength and the structural relief. We analyzed the evolution of the geometrical parameters with the rise in half-wavelength and with the r...
When working with geological maps and cross-sections, and no kinematic criteria on fault motion a... more When working with geological maps and cross-sections, and no kinematic criteria on fault motion are available, we usually classify the type of fault (dip-slip, strike-slip, oblique) according to the observed separation of planar markers, usually bedding. Although in many cases the separation agrees with fault slip, and therefore, the deduced type of fault is correct, these deductions can be completely wrong in certain circumstances. Amongst other parameters, the observed separation depends on the angular relationship between the cutoff line of the planar marker on the fault plane and the slip vector. In this paper, we elaborate on this relationship and present a tool to facilitate classifications of faults whose motion does not involve rotation of the fault blocks, and cut and offset previously tilted or folded surfaces. This tool consists of graphs that predict how the observed separation will be in cross sections and maps. In addition, the influence of different parameters on the magnitude of the separation in a geological section across a fault is discussed. The validity of the presented tool is demonstrated through its application to two field examples of strike-slip faults offsetting inclined layers and a fold train in the Asturian Basin. Our graphs can be used to improve geological interpretations, in a predictive way when constructing geological maps and cross-sections, and to decipher the existence of layers tilted or folded before fault development.
The first steps to study natural structures are data collection, their representation and their g... more The first steps to study natural structures are data collection, their representation and their geological interpretation. There is no doubt that the development of digital techniques in recent times has facilitated these tasks. Here we present an inventory of virtual outcrop models from the Cantabrian Zone and Asturian Basin, North-Northwest Iberian Peninsula, and the procedure employed to obtain the data, construct the models and interpret them geologically. These models correspond to contractional folds and faults of Palaeozoic age, and to Mesozoic extensional structures affected by Cenozoic tectonic inversion in the form of folds and thrusts. The digital techniques and data employed are fieldwork, as well as Google Earth images, orthophotographs, stereoscopical pairs of photographs and virtual outcrop models, constructed using Structure from Motion photogrammetry based on images extracted from Google Earth or from photographs taken in the field using a tripod or unmanned aerial vehicles. The analysis of these models has provided us with geological information that would have been difficult to obtain using traditional techniques. Apart from their scientific interest, the examples shown may be helpful for structural geologists who wish to obtain 3D geological models, maps and sections, and additional structural information from field examples, as well as elements to prepare a virtual fieldtrip and/or for educational purposes.
The Schmidt hammer, widely used in Engineering Geology, has also been used in Structural Geology ... more The Schmidt hammer, widely used in Engineering Geology, has also been used in Structural Geology as a tool to quantify the mechanical properties of rocks affected by different structures and the damage caused to rocks by fracturing. However, how the impact resistance of rocks (rebound) varies in layers located in different positions of a fold has never been tested. Here, the Schmidt hammer is used to quantify this parameter in a syncline involving a Carboniferous carbonate sequence located in the Cantabrian Zone, the foreland fold-and-thrust belt of the Variscan orogen in western Iberia. The variations in the Schmidt-hammer rebound value along a folded layer are consistent with other indicators such as variations of dip and thickness. These findings have also implications on obtaining representative rebound values of stratigraphic units, and on deriving parameters such as the uniaxial compressive strength and the Young modulus from the Schmidt-hammer rebound values applied to folded regions.
An in-house software for Computer Assisted Virtual Environments (CAVE TM), called 3D Stereo Virtu... more An in-house software for Computer Assisted Virtual Environments (CAVE TM), called 3D Stereo Virtual Drawing Tool (3D Stereo VDT), is presented through the interpretation of bedding and structural elements in two virtual outcrops. The main purpose of this software is to perform geological interpretations of point clouds through a friendly interface. 3D trackers allow the drawing of points and lines. Such elements may represent all sorts of geological features: bedding, unconformities, fractures, fold axial surfaces, axes, etc. One of the outcrops is a small, close-range point cloud constructed using field photographs of a beach cliff, whereas the other is a bigger, aerial point cloud built with Google Earth images of a valley slope. Both irregular and partly inaccessible outcrops consist of folded and faulted Palaeozoic rocks in the northwest portion of the Iberian Peninsula. The 3D analysis of these outcrops allowed us to supply new orientation data as well as ideas regarding the characteristics, origin and evolution of the mapped structures in relation to their setting.
ABSTRACT A detailed structural analysis was carried out on the Jurassic rocks cropping out along ... more ABSTRACT A detailed structural analysis was carried out on the Jurassic rocks cropping out along the cliffs of La Conejera Inlet (Asturias, Spain). It includes a geological map and a distortion-free cross-section constructed via photogrammetric methods. La Conejera Inlet is located within the Asturian Basin, a Permian–Mesozoic extensional basin partially formed during the opening of the Bay of Biscay. It suffered selective basin inversion during a Cenozoic contraction responsible for the Pyrenees and its western prolongation along the north margin of the Iberian Peninsula. The study of the structures (folds, faults, joints and veins) of the hangingwall of two normal faults with opposite dip senses reveals that it underwent a later compressional stage in which one fault block acted as a buttress. The contractional deformation in the hangingwall, interpreted as a deformed rollover anticline with an associated antithetic fault, diminishes on moving away from one of the main faults. The positive inversion tectonics produced not only a buttressing effect, but it also involved a certain amount of reverse reactivation of one of the main faults that still preserves a normal displacement. The original normal motion would have taken place during the Middle?–Late Jurassic, related to an embryonic stage of the opening of the Bay of Biscay. The later contractional stage would have been caused by the Cenozoic Alpine shortening. The good outcrop quality allows a relative chronology for the observed structures to be established. Employing all the available information we tried to reconstruct the structure at depth and predict the detachment depth, and to estimate the amounts of extension (the present-day value and that before the compression) and compression.
A number of geometrical techniques allow estimating amounts of horizontal extension and depth to ... more A number of geometrical techniques allow estimating amounts of horizontal extension and depth to detachment in simple rollover anticlines over listric normal faults given one or more horizons, the portion of the fault between the hanging wall and foot wall cut off points, and the depth to detachment (if the extension is to be estimated) or the extension (if the depth to detachment is to be estimated). These techniques assist in the construction and validation of sections across listric normal faults, but currently, it is unclear which ones predict correct amounts of horizontal extension and detachment depth and why is that. To sort this out, these techniques are evaluated using physical experiments and subsurface seismic examples of simple listric normal faults.
Resumen: Las estructuras estudiadas se sitúan en el flanco N del anticlinal de Villasecino (Zona ... more Resumen: Las estructuras estudiadas se sitúan en el flanco N del anticlinal de Villasecino (Zona Cantábrica). Dichas estructuras, de dimensiones métricas, afloran en un talud de carretera y deforman capas de caliza carbonífera. Aplicando un método fotogramétrico, se ha elaborado un corte geológico sin distorsión y perpendicular a las estructuras. Para ello, se han tomado 3 pares estereoscópicos de fotografías, se han registrado las coordenadas precisas de varios puntos de control del afloramiento fotografiado, se han rectificado las fotografías a fin de formar pares estereoscópicos, se ha realizado la interpretación geológica de las imágenes en 3D y, finalmente, se han proyectado las coordenadas tridimensionales de los elementos interpretados en un corte geológico perpendicular al eje de los pliegues y paralelo a la dirección de transporte tectónico. El afloramiento presenta fallas subparalelas a la estratificación, pliegues asociados y fallas subperpendiculares a la estratificación...
Sedimentation coeval with growth of the Santaren anticline provides an excellent opportunity to s... more Sedimentation coeval with growth of the Santaren anticline provides an excellent opportunity to study the relationships between sedimentation and anticline uplift through time. The Santaren anticline is a kilometre-scale, NW -SE trending fold offshore of Cuba, in the Bahamas foreland of the Cuban fold and thrust belt. The growth strata associated with this anticline consist of a thick package of carbonate sediments that were deposited without major interruptions from Neogene (and perhaps before) to present day. The excellent seismic resolution and age control of a number of seismic horizons within the growth strata allowed us to define 25 growth beds, each of them representing between 0.1 and 3.2 Ma. An analysis of the thickness of these beds allowed us to determine accurate quantitative values of cumulative decompacted thickness and crestal structural relief at the time of their deposition. In addition, for the same periods, sedimentation and fold uplift rates were calculated. Moreover, some information on relationships between sedimentation and fold uplift rates was inferred from the growth stratal geometry. Growth beds that overlap the fold crest and thin over it indicate that sedimentation rates outpaced fold growth rates during their deposition. Some overlapping beds have constant thickness indicating that no fold uplift occurred during their sedimentation. The rest of the growth beds exhibit onlap/offlap geometries that do not indicate a unique sedimentation/fold uplift rate relationship. Only in those cases in which the geometry of the underlying bed at the end of its deposition is known is it possible to infer a specific sedimentation/fold uplift rate relationship. As a result of this analysis, we have been able to (1) illustrate that the geometry of the growth strata associated with the Santaren anticline results from competition between sedimentation and tectonic fold uplift, (2) document the episodic and non-steady nature of fold growth, and show that short-term rates (at the scale of hundreds of thousands years) provide much insight into the interplay between sedimentation and tectonic fold uplift that control the growth stratal patterns. D
A strategy to predict strain across geological structures, based on previous techniques, is modif... more A strategy to predict strain across geological structures, based on previous techniques, is modified and evaluated, and a practical application is shown. The technique, which employs cross-section restoration combined with kinematic forward modelling, consists of restoring ...
ABSTRACT A detailed structural analysis was carried out on the Jurassic rocks cropping out along ... more ABSTRACT A detailed structural analysis was carried out on the Jurassic rocks cropping out along the cliffs of La Conejera Inlet (Asturias, Spain). It includes a geological map and a distortion-free cross-section constructed via photogrammetric methods. La Conejera Inlet is located within the Asturian Basin, a Permian–Mesozoic extensional basin partially formed during the opening of the Bay of Biscay. It suffered selective basin inversion during a Cenozoic contraction responsible for the Pyrenees and its western prolongation along the north margin of the Iberian Peninsula. The study of the structures (folds, faults, joints and veins) of the hangingwall of two normal faults with opposite dip senses reveals that it underwent a later compressional stage in which one fault block acted as a buttress. The contractional deformation in the hangingwall, interpreted as a deformed rollover anticline with an associated antithetic fault, diminishes on moving away from one of the main faults. The positive inversion tectonics produced not only a buttressing effect, but it also involved a certain amount of reverse reactivation of one of the main faults that still preserves a normal displacement. The original normal motion would have taken place during the Middle?–Late Jurassic, related to an embryonic stage of the opening of the Bay of Biscay. The later contractional stage would have been caused by the Cenozoic Alpine shortening. The good outcrop quality allows a relative chronology for the observed structures to be established. Employing all the available information we tried to reconstruct the structure at depth and predict the detachment depth, and to estimate the amounts of extension (the present-day value and that before the compression) and compression.
To check the behaviour of physical experiments of simple rollover anticlines over listric normal ... more To check the behaviour of physical experiments of simple rollover anticlines over listric normal faults, several fault-slip, bed-length and area parameters are measured for different beds in two stages of evolution of two physical experiments. Given a certain amount of extension, the heave and dip of the displacement vary for different beds, whereas the throw is approximately constant for small amounts of extension and the displacement remains constant for high amounts of extension. Neither bed lengths nor areas beneath beds remain constant with increasing extension. A number of techniques allow amounts of extension and detachment depth to be estimated given one or more marker horizons, the portion of the fault between hanging-wall and footwall cut off points, the depth to detachment and/or the shear angle. These techniques are applied to the physical experiments of listric normal faults analysed. Since many of these techniques rely on parameters measured on the physical experiments such as fault slip, bed length and area, the influence of these parameters on the different magnitudes of extension and detachment depths estimated using different techniques is discussed and the accuracy of the results is compared. D
Several authors have proposed different methods to predict the subsurface geometry of thrust-rela... more Several authors have proposed different methods to predict the subsurface geometry of thrust-related folds from shallow baseline data. Given certain bed dips of a backlimb, crest and forelimb of a fold model, we reconstructed the structure at depth as a fault-bend, a fault-propagation and a detachment fold following these methods and we checked that the resulting geological cross-sections were balanced. In addition, we modified the position of the surface dips so that the resulting fold models had different half-wavelengths and different structural relief and, therefore, allowing chevron folds and box folds to be constructed. In order to compare the different balanced interpretations, we elaborated several graphs in which various geometrical parameters (shortening, detachment depth and dimensions of the thrust ramp) are plotted versus the half-wavelength and the structural relief. We analyzed the evolution of the geometrical parameters with the rise in half-wavelength and with the r...
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Papers by Mayte Bulnes