Shear zones

Punta del Peñón Blanco shear zone (Fuerteventura, Canary Islands). Folded basic dikes within a carbonatite matrix. Model of triclinic transpression with oblique extrusion (Carlos Fernández and Manuel Díaz Azpiroz, work in progress).

The content of my Ph.D. dissertation was focused on the structural analysis of the Hiendelaencina shear zone (central part of the Variscan Iberian Massif). Since then, the study of natural ductile shear zones has been one of my priorities. Areas of interest include the Iberian Variscan Massif (e.g., the Hiendelaencina and the South Iberian shear zones) and the Basal Complex of Fuerteventura, Canary Islands (e.g., the Caleta de la Cruz and the Punta del Peñón Blanco shear zones, see an example below). More recently, I am fascinated by the complexities of transpression and transtension zones. Manuel Díaz Azpiroz (Universidad Pablo de Olavide) and I are involved in the fight against transpression with oblique extrusion. The fabric patterns resulting from this particular kinematic setting are very similar to those observed in natural examples such as the South Iberian shear zone. Detailed map and structural data from the Punta del Peñón Blanco shear zone (Fuerteventura, Canary Islands; see field photograph to the left). Taken from Fernández et al. (1997, JSG). Detailed field sketch of the Punta del Peñón Blanco shear zone (Fuerteventura, Canary Islands). Structural data from the Punta del Peñón Blanco shear zone (Fuerteventura, Canary Islands). Left: Equal area, lower hemisphere projection of mylonitic foliation planes (great circles), stretching lineation (yellow squares) and minor asymmetric folds (Z-shaped, red circles; S-shaped, orange circles). Right: Equal area, lower hemisphere density plot of calcite [c] axes from a sheared carbonatite.

Migmatites and granites

Mountains: Left, structural map of the central part of the Gredos batholith (central Spain); right, the northern Patagonian batholith (Argentina) Outcrops: Left, frozen interaction between mafic and felsic magmas in Gerena (southwestern Spain); right, magmatic foliation in the Gredos batholith (central Spain). Microscope: Left, quartz crystallographic fabrics (EBSD), subsolidus deformation in a granodiorite; right, zircons from a granodiorite. Spanish Central System.

My first contact with the study of granites was immediately afer my arrival to Huelva. I am in debt with Antonio Castro, the leader of the Spanish granitologists, whose enthusiasm for granites defeated my initial skepticism (What on earth a structural geologist can do with granite plutons? I said). The brilliant words of Hans Cloos (Gespräch Mit Der Erde) also helped. Not to mention the impressive amount of papers published on the topics of melt segregation, ascent and emplacement appeared since the 90´s. My own contribution to this research field up to now has been made possible thanks to the collaborative research effort of the Huelva group: Antonio Castro, Iñaki Moreno-Ventas, Jesús de la Rosa, Hassan El-Hmidi, Mohammed El-Biad, Susana López Aparicio, Helena Chacón and Juan Díaz Alvarado. Recent collaboration with portuguese colleagues has allowed the enlargement of our research interest to migmatites and zircon geochronology. M. Francisco Pereira and Patricia Moita (Universidade de Évora), Martim Chichorro (Universidade Nova de Lisboa), and José Brandão Silva (Universidade de Lisboa) constitute the core of the portuguese team. In Argentina we work in close cooperation with researchers of the Universities of Buenos Aires (Víctor Ramos, Graciela Vujovich), Córdoba (Roberto Martino), Río Cuarto (Juan Otamendi) and Salta (Raúl Becchio). Permanent or occasional contacts with colleagues around the world has been of great help. I can make mention here of Guillermo Corretgé, Jean Luc Vigneresse, Alberto Patiño Douce, Ed Stephens, Graham Rogers, Ian Williams and many others. Areas of interest include the Variscan Iberian Massif (the Gredos batholith, the Central Extremadura batholith, the Castilblanco de los Arroyos batholith, the Sanabria region and the Évora-Aracena metamorphic belt), the Lewisian Complex, and, recently, the northern Patagonian batholith. Click the publications link of this page or the personal page of Antonio Castro to get additional information on the results of this line of research. And you don't want to miss the book here below! Understanding granites

Volcanism and tectonics

The Canary Archipelago. Northeasterly trade wind flow deflected by the islands (image: NASA). Dike swarms are one of the main structures in the basal complexes of the Canary Islands. Left: Dike swarm in Fuerteventura. Right: folded dike swarm in La Gomera. Ductile and brittle deformation structures are also common. Left: Caleta de la Cruz ductile shear zone (Fuerteventura). Right: slickenside striations and associated steps, La Gomera.

Relationships between tectonics and volcanism are fascinating and their study constitute a research field that has experienced a considerable development over the past few decades. I have been working in the Canary Islands since 1993. Deciphering the complex processes that took place during the growth and emersion of these islands is not an easy task. Fortunately, we rely on a multidisciplinary research group involving structural geologists (Encarnación García Navarro, Manuel A. Camacho and Carlos Fernández, Universidad de Huelva), petrologists and volcanologists (Ramón Casillas, Agustina Ahijado, Margarita Gutiérrez, Julio de la Nuez and Cande Martín, Universidad de La Laguna), sedimentologists (J. Ramón Colmenero, Universidad de Salamanca), and geochronologists and geochemists (Kadosa Balogh, Attila Demény, Geza Nagy, Hungarian Academy of Sciences, Budapest). Contributions to the knowledge of the geological evolution of the Canary Islands have been presented in several papers and abstracts (see publications). As an example, here you can find some photographs and figures illustrating our contribution. I hope that these results would also help to clarify some particular topics concerning the structural evolution of oceanic volcanic islands. The southern part of Fuerteventura (looking southward). Selected fault measurement sites and deformation maps for the Miocene deformation phases of Fuerteventura. A detailed structural study (more 2990 than dikes and 1780 faults measured and analysed) allowed us (Fernández et al., 2006, Tectonics) to define three Miocene tectonic phases in Fuerteventura. Our deformation group has proposed a new model for the first stages of evolution of oceanic islands (Left, model for the submarine and emersion stages of Fuerteventura, Gutiérrez et al., 2006, GSA Bulletin). The mature, subaerial evolution of the islands was also associated with the tectonic activity (Right, initial stage of Miocene rifting of Fuerteventura, Fernández et al., 2006, Tectonics).

Geoplanetology

The little companion of our planet (left) has still many secrets to reveal to the experimented geologist. Consider, for instance, the astonishing discovery of water on Mars (right). The old good maps of Mars were still in use a few decades ago (map of Schiaparelli, 1877). Yet important advances have been made in our present-day space probe era. Structural map of the northern and central segments of Tempe Fossae (northern hemisphere of Mars) by Fernández and Anguita (2007, JGR).

Spring, 2003. Francisco (Paco) Anguita (Universidad Complutense de Madrid) sent me an intriguing Viking image of a rather unusual graben floored by small folds and located to the south of Olympus Mons. This was the starting point of my own space race. I was quickly captivated by the elegance of the Martian structures (grabens and rifts, wrinkle ridges, lobate scarps, strike slip faults, etc. See the figure bottom left). The low erosion rates and the declining tectonic activity with time offer a fine preservation of very old structures (> 3.5 Ga). They are directly comparable with the results of analogue experiments, a task that is not so easy on Earth due to vegetal cover, active erosion and sedimentation processes, and the activity of successive tectonic phases. The quality and variety of the images yielded by the modern NASA and ESA Mars missions, and the availability of good topographical coverage (MOLA), are making Mars a paradise for the structural geologists. Come on, a whole planet is waiting for you! Collaboration with my colleagues of the Seminario de Ciencias Planetarias (Universidad Complutense de Madrid, I would like to make mention in particular of Paco Anguita and Javier Ruiz) has been fruitful. We have investigated the similarities between Thaumasia Highlands and terrestrial orogenic belts (see below), the depth to the brittle-ductile transition in the Amenthes and Southern Thaumasia regions, and its implications for the estimation of the heat flow, and the crustal rheology and stratification during the Late Noachian and Early Hesperian periods (≈ 4.0-3.5 Ga). Last, but not least, Paco has finally convinced me to start studying a lowland region of Venus. I hope to have something to show about Venus soon, before I get mad (Venus, Mars and Earth are so distinct and so similar at the same time, that no one can jump from one to another without going mentally ill). Comparison between the structures observed at Nectaris Fossae (eastern Tharsis region, Mars) and terrestrial examples (from Anguita et al., 2006, Icarus). Top: geological cross-sections showing the suggested lateral transition from thin-skinned to thick-skinned tectonics in Nectaris, similarly to the well documented architecture of terrestrial orogenic belts (e.g., Pyrenees). Down: rhomb-horst structures in Nectaris (left) and Earth (right). The geometries are interchangeable, which strongly emphasizes the importance of strike-slip displacements on Mars.

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