Changes in dip and frictional properties of the basal detachment controlling orogenic wedge propagation and frontal collapse: the External central Betics case

Thin-skinned fold-and-thrust belts (FTBs) have been extensively studied through both field examples and modelling. The overall dynamics of FTBs are, therefore, well understood. One less understood aspect is the combined influence of across-strike changes in the detachment properties and the basement topography on the behaviour of an orogenic wedge. In this paper, we use field data together with reflection seismic interpretation from the External Zones of the Central Betics FTB, S Spain, to identify a significant increase in the wedge basal dip (a basement "threshold") coinciding with the pinch-out of a weak substrate. This induced both changes to the wedge geometry and to the basal friction, which in turn influenced the wedge dynamics. The changing dynamics led to a transient “stagnation” of the FTB propagation, topographic build-up and subsequent collapse of the FTB front. This in turn fed an important Langhian depocenter made up of mass transport deposits. Coevally with the FTB propagation, extension took place both parallel and perpendicular to the orogenic trend. This case study illustrates how across-strike changes in wedge basal properties can control the detailed behaviour of a developing FTB front, but questions remain regarding the time-space interaction and relative importance of the basal parameters

El contacto entre las zonas de Ossa Morena y Sudportuguesa. Características y significado de la banda metamórfica de Aracena, en su sector central entre Aroche y Aracena (Huelva)

El contacto entre las zonas de Ossa Morena y Sudportuguesa está subrayado por la banda metamórfica de Aracena. Las características principales de esta región del macizo Ibérico pueden considerarse desde distintos puntos de vista: litológico, estructural, metamórfico, magmático, geoquímica, isotópico y experimental, y así se presentan en este artículo. La banda metamórfica de Aracena está dividida en dos dominios principales: el dominio oceánico está formado por metabasitas derivadas de un MORB (las metabasitas de Acebuches) y por un antiguo prisma de acrección. El dominio continental incluye gneises y migmatitas alumínicos, rocas de silicatos cálcicos, gneises leucocráticos, mármoles, anfibolitas, intrusiones sin/post-tectónicas de noritas ricas en Mg (con afinidad boninítica), así como rocas intrusivas post-tectónicas de composición ácida a básica. Las metabasitas de Acebuches sufrieron, en primer lugar, un metamorfismo de alta temperatura/baja presión que, en la actualidad, presenta un gradiente metamórfico invertido, y que estaba relacionado con un cabalgamiento vergente al SO. El pico térmico asociado a este evento metamórfico muestra un gradiente de edad, de forma que las edades más antiguas han sido obtenidas en el extremo oeste. La mitad inferior de la pila metabasítica de Acebuches fue afectada posteriormente por una deformación milonítica y un retrometamorfismo asociados a la zona de cizalla Sudibérica. Cuatro fases de deformación dúctil han sido definidas en el dominio continental: la fase CD-D1 estuvo relacionada con la generación de pliegues recumbentes de escala kilométrica. La fase CD-D2 se puede asociar a un colapso extensional, y es contemporánea con un metamorfismo de alta temperatura/baja presión que afectó al dominio continental y generó diversos complejos migmatíticos. La fase CD-D3 produjo pliegues simétricos verticales, mientras que la fase CD-D4 dio lugar a cabalgamientos vergentes al sur a los que se asociaron pliegues de propagación.Las principales características de la banda metamórfica de Aracena se interpretan como el resultado de la evolución de un punto triple de tipo fosa-fosa-dorsal. De acuerdo con el modelo propuesto, la interacción entre la dorsal y la zona de subducción dio como resultado la formación de una ventana astenosférica bajo el margen continental cabalgante, lo cual provocó el ascenso de la astenosfera y el consecuente rebote térmico. Esta unión triple migro a lo largo del margen continental hacia el este, lo que generó un cinturón metamórfico de alta temperatura/baja presión en el contacto entre las zonas de Ossa Morena y Sudportuguesa

Arc-parallel vs back-arc extension in the Western Gibraltar arc : is the Gibraltar forearc still active?

Extremely tight arcs, framed within the Eurasia-Africa convergence region, developed during the Neogene on both sides of the western Mediterranean. A complex interplate deformation zone has been invoked to explain their structural trend-line patterns, the shortening directions and the development of back-arc basins. Updated structural and kinematic maps, combined with earthquake data covering the complete hinge zone of the western Gibraltar arc help us to explore the mode of strain partitioning from 25My ago to present. During the Miocene, the strain partitioning pattern showed arc-perpendicular shortening in the active orogenic wedge -assessed from the radial pattern of tectonic transport directions- accompained by subhorizontal stretching. Structures accommodating stretching fall into two categories on the basis of their space distribution and their relationships with the structural trend-line pattern: i) arc-parallel stretching structures in the external wedge (mainly normal faults and conjugate strike-slip faults); and ii) extensional faults developed in the hinterland zone in which transport directions are centripetal towards the Alborán back-arc basin. Pliocene to Recent deformational structures together with focal solutions from crustal earthquakes (n=167; 1.

The Guadiaro-Baños contourite drifts (SW Mediterranean). A geotechnical approach to stability analysis

Two Quaternary plastered contourite drifts, with terraced and low-mounded morphologies, make up the continental slope and base-of-slope in the northwestern Alboran Sea, respectively, between the Guadiaro and Baños turbidite systems, close to the Strait of Gibraltar. Considering their significant lateral extent, the link between the contourite drift deposits and landslides may be particularly important for hazard assessment. The physical properties, composition and geometry of contourite drifts have been proposed as key factors in slope stability, although this relationship still needs to be better constrained. In this work, new in-situ geotechnical data (cone penetration tests; CPTu) has been combined with morphostratigraphic, sedimentological, and (laboratory) geotechnical properties to determine the stability of the Guadiaro-Baños drifts. For the depositional domains of both drifts, the resulting sedimentary and geotechnical model describes low-plasticity granular and silty sands on the erosive terraced domain that evolve seawards to silty and silty-clay deposits with a higher plasticity and uniform geomechanical properties. For the shallower coarse-grained contourite sediments, the cohesion (c') and internal friction angle (ϕ') values are 0–9 kPa and 46–30°, respectively, whereas for the distal fine contourites the undrained shear strength gradient (∇Su) is 2 kPa/m. These properties allow us to establish high factors of safety for all the scenarios considered, including seismic loading. Slope failure may be triggered in the unlikely event that there is seismic acceleration of PGA > 0.19, although no potential glide planes have been observed within the first 20 m below the seafloor. This suggests that the contourite drifts studied tend to resist failure better than others with similar sedimentary characteristics. The interplay of several processes is proposed to explain the enhanced undrained shear strength: 1) the geometry of the drifts, defined by an upper contouritic terrace and lower low-mounded shapes; 2) recurrent low-intensity earthquakes with insufficient energy to trigger landslides, favouring increased strength due to dynamic compaction; and 3) cyclic loading induced by solitons/internal waves acting on the sediment.En prens

Microfabric analysis of quartz in quartzite resisters of the «Garganta de las Pozas» ductile shear zone central part of the Gredos Massif

43ª Sesión Científica. Móstoles, Noviembre de 2007The NE-SW «Garganta de las Pozas» ductile shear zone goes along the contact between one of the granodioritic sheets mapped and the «Refugio del Rey» anatectic complex. As well as the descriptive work, it has been determined the microfabric and the lattice preferred orientation (OCP) by EBSD in a cuarcitic sample, both studies confirm a lateral dextral movement in the shear zone. The dynamic recristalization in quartz has been produced by three different mechanism: subgrain rotation, grain boundaries migration and dauphine twin, which correspond to a deformation temperature of 575ºC.Este trabajo ha sido financiado con los proyectos CGL2004-06808-CO4-01 y CGL2004-06808-CO4-02.Peer reviewe

Structural trend line pattern and strain partitioning around the Gibraltar Arc accretionary wedge: Insights as to the mode of orogenic arc building

Mediterranean tectonics results in tight orogenic arcs enclosing back-arc basins of oceanic or thinned continental lithosphere. The Gibraltar Arc cannot be explained solely by the Europe-Africa plate convergence; therefore complementary mechanisms have been proposed. Most of them imply a westward motion of the arc and a general transpressive regime on both branches (Betic and Rif chains). A structural revision made along the western Gibraltar Arc allows us to generate a detailed kynematic map and to introduce new constraints on the possible arc formation mechanisms. Our results suggest that the strain partitioned into two main types of structures: structures accommodating suborthogonal shortening (folds and thrusts) and structures accommodating arc-parallel stretching (normal faults, conjugate strike-slip faults, and distributed minor structures). On the basis of the fan pattern depicted by the slip direction of contractional structures and the homogeneous distribution of arc-parallel stretching, an arc formation mode close to the piedmont glacier type is suggested.Peer reviewe

Cambrian ensialic rift-related magmatism in the Ossa-Morena Zone (Évora–Aracena metamorphic belt, SW Iberian Massif): Sm–Nd isotopes and SHRIMP zircon U–Th–Pb geochronology

International audienc

Analogue modelling of inclined, brittle–ductile transpression: Testing analytical models through natural shear zones (external Betics)

The combination of analytical and analogue models gives new opportunities to better understand the kinematic parameters controlling the evolution of transpression zones. In this work, we carried out a set of analogue models using the kinematic parameters of transpressional deformation obtained by applying a general triclinic transpression analytical model to a tabular-shaped shear zone in the external Betic Chain (Torcal de Antequera massif). According to the results of the analytical model, we used two oblique convergence angles to reproduce the main structural and kinematic features of structural domains observed within the Torcal de Antequera massif (α = 15° for the outer domains and α = 30° for the inner domain). Two parallel inclined backstops (one fixed and the other mobile) reproduce the geometry of the shear zone walls of the natural case. Additionally, we applied digital particle image velocimetry (PIV) method to calculate the velocity field of the incremental deformation. Our results suggest that the spatial distribution of the main structures observed in the Torcal de Antequera massif reflects different modes of strain partitioning and strain localization between two domain types, which are related to the variation in the oblique convergence angle and the presence of steep planar velocity – and rheological – discontinuities (the shear zone walls in the natural case). In the 15° model, strain partitioning is simple and strain localization is high: a single narrow shear zone is developed close and parallel to the fixed backstop, bounded by strike-slip faults and internally deformed by R and P shears. In the 30° model, strain partitioning is strong, generating regularly spaced oblique-to-the backstops thrusts and strike-slip faults. At final stages of the 30° experiment, deformation affects the entire model box. Our results show that the application of analytical modelling to natural transpressive zones related to upper crustal deformation facilitates to constrain the geometrical parameters of analogue models

Miocene deformation front propagation and strain partitioning within the fold-and-thrust belt of the Central Betics

The External Zones of migrating orogenic arcs can be deformed as fold-and-thrust belts where the advance of the deformation front toward the foreland is contemporary to the arc-lengthening. The Sierra de Cabra, in the Central Betics, is made up of Subbetic units surrounded by synorogenic, Middle to Upper Miocene rocks. The combined data from surface and seismic profiles show the presence of partially coetaneous structures during the Middle Miocene: (i) a NNW-verging thrust system detached within Triassic evaporites accommodating arc-perpendicular shortening, (ii) N-S, NW-SE and NNE-SSW normal faults accommodating WSW-ENE stretching and (iii) WSW-ENE normal faults. These results allow us to interpret the Sierra de Cabra as the Langhian Betic fold-and-thrust belt front. Additionally, the WSW-ENE stretching contributes to the structural and topographic segmentation along-strike the Betics. The Miocene strain partitioning here proposed is similar to that found in the Western Betics

Arc-parallel vs back-arc extension in the Western Gibraltar arc : is the Gibraltar forearc still active?

Extremely tight arcs, framed within the Eurasia-Africa convergence region, developed during the Neogene on both sides of the western Mediterranean. A complex interplate deformation zone has been invoked to explain their structural trend-line patterns, the shortening directions and the development of back-arc basins. Updated structural and kinematic maps, combined with earthquake data covering the complete hinge zone of the western Gibraltar arc help us to explore the mode of strain partitioning from 25My ago to present. During the Miocene, the strain partitioning pattern showed arc-perpendicular shortening in the active orogenic wedge -assessed from the radial pattern of tectonic transport directions- accompained by subhorizontal stretching. Structures accommodating stretching fall into two categories on the basis of their space distribution and their relationships with the structural trend-line pattern: i) arc-parallel stretching structures in the external wedge (mainly normal faults and conjugate strike-slip faults); and ii) extensional faults developed in the hinterland zone in which transport directions are centripetal towards the Alborán back-arc basin. Pliocene to Recent deformational structures together with focal solutions from crustal earthquakes (n=167; 1.