Strain partitioning in the eastern-end of the Torcal Shear Zone (Betics, S Spain)

The eastern end of the Torcal Shear Zone (TSZ) is made up of two structural domains that accommodate variable proportions of the strain partitioning associated to the TSZ dextral transpressive deformation. The Sierra de las Cabras Domain is a dextral transpressive band at an angle to the TSZ giving way southeastward to the Sierra de Camorolos Domain, whose thrusts and folds accommodate mainly coaxial strain, though some dextral component must be present. Kinematic analysis carried out on these structures permit to establish a range between N120º-160ºE for the far-field displacement vector in this sector, in contrast to the WNW-ESE orientation obtained for the central TSZ. Given that the structural organization is coeval in the whole ZCT, the difference in the far-field displacement vector orientation may point to a decreasing simple shear component toward the end of the dextral transpressive bandLa terminación oriental de la Zona de Cizalla del Torcal (ZCT) está definida por dos dominios estructurales que acomodan el reparto de la deformación transpresiva dextra de la ZCT en proporciones variables. El Dominio de la Sierra de las Cabras es una banda transpresiva dextra, oblicua a la ZCT, que se releva en su sector SE con el Dominio de Sierra de Camorolos, cuyos cabalgamientos y pliegues acomodan una deformación principalmente coaxial, aunque con cierta componente dextra. El análisis cinemático de estas estructuras permite estimar un vector de desplazamiento con una orientación comprendida entre N120º-N160ºE, frente a la orientación ONO-ESE obtenida de los macizos centrales de la ZCT. Dado que la organización estructural de los dos dominios es coetánea con la que registra el resto de la ZCT, podría concluirse que la diferencia en la orientación de los vectores de desplazamiento responde a una disminución de la componente de cizalla simple de la deformación transpresiva dextra hacia la zona de terminación de la band

Analog Models of Fold-and-Thrust Wedges in Progressive Arcs: A Comparison With the Gibraltar Arc External Wedge

The timing and kinematics of the different types of structures and the associated vertical-axis rotations that permit an arcuate external wedge to acquire progressively its curved shape throughout its deformation history—known as progressive arcs—are key questions in natural cases of arcuate fold-and-thrust belts that we want to address through analog modeling. We present laboratory models of fold-and-thrust belts formed with a backstop that deforms in map view to simulate progressive arcs in a thin-skinned tectonic regime. Our setup makes use of a deformable backstop rigid enough to push from behind the initial parallelepiped but deformable in map view. This innovative design permits us to increase the amplitude of the arc indenting in the model as its radius of curvature decreases, that is, it simulates a progressive arc. Taking the Gibraltar Arc external wedge situated in the western Mediterranean to scale our models in terms of rheology, velocities, and sizes, four types of experiments were made. We varied the type of substratum (sand or silicone), the silicone thickness, and the width and length of the initial analog pack in order to test the influence of each of these parameters on the resulting fold-and-thrust belts. All experiments led to the formation of arcuate wedges where strain was partitioned into: (a) arc-perpendicular shortening, accommodated by thrusts which main structural trend is broadly subparallel to the indenter shape and with divergent transport directions, and (b) arc-parallel stretching, accommodated by normal and conjugate strike-slip faults. The normal and strike-slip faults contributed to the fold-and-thrust belt segmentation and the formation of independent blocks that rotated clockwise and counterclockwise depending on their position within the progressive arc. Our experiments allow to simulate and understand the finite deformation mode of the external wedge of the Gibraltar Arc. Accordingly, they shed light on how an arcuate fold-and-thrust belt can develop progressively in terms of structural trend and transport directions, types and distribution of the structures accommodating strain partition, and timing of vertical-axis rotations.This study was supported by projects RNM-0451, EST1/00231, CGL2017-89051-P, PGC2018-100914-B-I00, and UPO 1259543

A "core-complex-type structure" formed by superposed ductile and brittle extension followed by folding and high-angle normal faulting. The Santi Petri dome (western Betics, Spain)

The Santi Petri dome (western Betics, southern Spain) shows a core-complex-like structure, where migmatitic gneisses and schists outcrop below low-grade slates and phyllites, all of which form the basement of the Neogene Málaga basin. The migmatites and schists suffered a coaxial-flattening event during isothermal decompression and were later exhumed by ductile ESE non-coaxial stretching. Further exhumation was achieved by W- to SW-transport brittle low-angle normal faulting. Subsequently these extensional structures were gently folded in the core of a NE/SW-oriented antiform during the Tortonian. Finally the Santi Petri domal geometry was accentuated by the interference of orthogonal high-angle faults with ENE–WSW and NNW–SSE orientation. This core-complex-like structure, formed by superposition of extensional and compressive tectonic events, does not represent a classical, purely extensional core complex, which shows that metamorphic structure and geometry are not decisive criteria to define a core-complex

Gravitational dismantling of the Miocene mountain front of the Gibraltar Arc system deduced from the analysis of an olistostromic Complex (Western Betics)

A mélange complex seals the internal-external zone boundary of the western part of the Gibraltar Arc orogenic belt and constitutes a key element to establish milestones of the Betic-Rif tectonic evolution. The blocks and olistoliths embedded in this mélange provide constraints on the geological history of the main tectonic units involved in the Miocene mountain front. We mapped and analysed the blocks and olistoliths included in this mélange in order to understand its age and genesis, which have long been a matter of debate. The relationships of this mélange La Joya Olistostromic Complex (LaJOC) with the basement units together with the high variability of the block lithologies suggest a sedimentary origin for this mélange. Two large-scale olistoliths retain their original structure prior to their emplacement in the LaJOC Basin. The sedimentological and structural analysis allowed us to correlate these olistoliths with the folded and thrust sequence belonging to the Miocene Betic-Rif accretionary prism (Flysch Trough units), and to constrain the age of deposition of the La Joya Olistostromic Complex. The age of the matrix of the mélange deposits is poorly known because of the lack of in-place fauna. Indeed, the formation of the inherited fold-and-thrust structure of these olistoliths is well-known in the western Betics. Accordingly, the LaJOC should have been deposited during middle Miocene times and the blocks and olistoliths included within the mélange would derive from the gravitational dismantling of the Gibraltar Arc mountain front. The data presented help us to understand the formation of reliefs and basins in the western part of the Gibraltar Arc orogenic system

Strike-slip faults in the south Haouz (Calcareous Dorsal, Internal Rif, N of Tetouan, Maroc). Comparison with the Betics (Spain)

4 páginas, 4 figuras.-- Trabajo presentado en la 41ª Sesión Científica, Barcelona, 2006.Different sets of strike-slip faults of the Haouz Mountains (from Tetouan to the strait of Gibraltar) deform the tectonic slices of the Calcareous Dorsal there present. In this paper we study those faults at the southern part of the Haouz. Three fault formation stages can be distinguished and correspond to the progressive s1 rotation during the Neogene from a near E-W position to approximately N-S, in a progressive anticlockwise rotation. The faults and other related structures are compared with similar ones existing in the Betic Cordillera, in equivalent areas of the arc of Gibraltar.Este artículo es una contribución del grupo de Travaux de Recherches de l’Unité Risques Géologiques et Télédetection, 2006, Rabat y de los proyectos BTE2001-5230-E, CGL200401636/BTE, CGL2004-03333/ BTE, y el grupo RNM 217 de la Junta de Andalucía.Peer reviewe

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

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.