I. Expósito

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 Alboran back-arc basin. Pliocene to Recent deformational structures together with focal solutions from crustal earthquakes (n=167; 1.5<Mw<6.3) support that this strain partitioning pattern still occurs. By contrast, the eastern end zones of the western Gibraltar arc, especially during the last 5My, underwent intense transpression tectonics with a NW to NNW main shortening axis. These results agree with a still active Gibraltar forearc, governed by westward migrating subduction retreat or subcontinental mantle delamination.

Metamorphic and deformational imprint of Cambrian -Lower Ordovician rifting in the Ossa-Morena Zone (Iberian Massif, Spain)

Abstract The high-temperature metamorphism recorded in the Valuengo and Monesterio areas constitutes a rare occurrence in the Ossa-Morena Zone of Southwest Iberia, where low-grade metamorphism dominates. The metamorphism of the Valuengo area has been previously considered either Cadomian or Variscan in age, whereas that of Monesterio has been interpreted as a Cadomian imprint. However, these areas share important metamorphic and structural features that point towards a common tectonometamorphic evolution. The metamorphism of the Valuengo and Monesterio areas affects Late Proterozoic and Early Cambrian rocks, and is syn-kinematic with a top-to-the-north mylonitic foliation, which was subsequently deformed by early Variscan folds and thrusts. The U–Pb zircon age (480±7 Ma) we have obtained for an undeformed granite of the Valuengo area is consistent with our geological observations constraining the age of the metamorphism. We propose that this high-temperature metamorphic imprint along a NW–SE ductile extensional shear zone is related to the crustal extension that occurred in the Ossa-Morena Zone during the Cambro-Ordovician rifting. In the same way, the tectonothermal effect of the preorogenic rifting stage may have been wrongly attributed to orogenic processes in other regions as well as in this one.

La Orogenia Cadomiense y el rifting del Paleozoico Inferior en el Sudoeste de Iberia

La intensidad y la duracion de la Orogenia Cadomiense en el sudoeste de Iberia han sido temas controvertidos. El magmatismo calcoalcalino del Vendiense superior, que se encuentra ampliamente representado en la Zona de Ossa Morena, es el efecto orogenico cadomiense mas significativo. Ademas, en las rocas del Proterozoico superior que afloran en Peraleda del Zaucejo (sur de la Zona Centroiberica) se ha establecido firmemente la existencia de foliacion y metamorfismos tardiprecambricos. En la Zona de Ossa Morena, sin embargo, la deformacion penetrativa de las rocas precambricas parece corresponder casi exclusivamente a pliegues y cabalgamientos de edad varisca. En conjunto, la intensidad de la orogenia cadomiense en el sudoeste de Iberia es, como en el resto del Macizo Iberico, moderada. Los datos geocronologicos existentes aparentan la existencia de un magmatismo initerrumpido desde el Proterozoico superior hasta el Ordovicico; sin embargo, se sugiere que tal continuidad no es real. Del analisis de los datos estratigraficos se deduce que la orogenia cadomiense se habria desarrollado exclusivamente en el Proterozoico terminal, siendo el Cambrico inferior un periodo de transicion entre el ciclo orogenico cadomiense y el ciclo varisco. Desde el Cambrico Inferior alto, se desarrollo en la Zona de Ossa Morena el rifting pre-varisco, cuyo reflejo estratigrafico es la sustitucion de carbonatos de plataforma (Cambrico Inferior) por sedimentos terrigenos y rocas volcanicas. Localmente, el rifting pre-varisco origino una fabrica milonitica y un metamorfismo de grado medio-alto, como el que se observa en Valuengo y en Monesterio. Como consecuencia del rifting cambro-ordovicico, la Zona de Ossa-Moreno debio de ser en el Silurico un fragmento aislado de corteza continental.

Crescent-like large-scale structures in the external zones of the western Gibraltar Arc (Betic–Rif orogenic wedge)

The external wedge of the Gibraltar Arc orogenic system (western Mediterranean) is a natural case of a fold-and-thrust salient. Although at the scale of the arc a swing of the structural trend can be observed, the presence of closed, crescent-like structures, some of them more than 25 km long and scattered over an area of 3500 km2, characterize the Gibraltar Arc external wedge. This feature makes it very different from a ‘classical’ arc-shaped fold-and-thrust belt. Detailed structural maps of representative crescent-like structures are presented. Synorogenic sediments reveal that they probably formed during a short time interval, most probably during a single shortening event. The crescent-like structures are localized in front of a recess formed by the internal zones that acted as backstop. This backstop shape and a viscous substrate (essentially Triassic evaporites below the fold-and-thrust external wedge) constrain the experimental setting of an analogue model in which we were able to reproduce structures that developed originally with a highly non-cylindrical shape during a single, straight convergence. Moreover, the tectonic transport directions along the thrusts are broadly comparable in the natural case and the model.

La estructura sísmica de la corteza de la Zona de Ossa Morena y su interpretación geológica

El experimento de sismica de reflexion profunda IBERSEIS ha proporcionado una imagen de la corteza del Orogeno Varisco en el sudoeste de Iberia. Este articulo se centra en la descripcion de la corteza de la Zona de Ossa Morena (OMZ), que esta claramente dividida en una corteza superior, con reflectividad de buzamiento al NE, y una corteza inferior de pobre reflectividad. Las estructuras geologicas cartografiadas en superficie se correlacionan bien con la reflectividad de la corteza superior, y en la imagen sismica se ven enraizar en la corteza media. Esta esta constituida por un cuerpo muy reflectivo, interpretado como una gran intrusion de rocas basicas. La imagen de las suturas que limitan la OMZ muestra el caracter fuertemente transpresivo de la colision orogenica varisca registrada en el sudoeste de Iberia. La Moho actual es plana y, en consecuencia, no se observa la raiz del orogeno.

Strain partitioning and relief segmentation in arcuate fold-and-thrust belts: a case study from the western Betics

PurposeWithin arcuate orogenic belts, strain is commonly partitioned between arc-parallel stretching and arc-perpendicular shortening. Arc-parallel stretching can be accommodated by arc-oblique strike-slip faults and arc-perpendicular normal faults, whose localization in fault systems may provoke significant along-strike structural relief drops.MethodsIn this work, we have studied the Ubrique area, located in a frontal segment of the external western Betics (northern branch of the Gibraltar Arc), where one of the most significant relief discontinuities along the orogenic grain is defined.ResultsWe have found that this discontinuity is determined by two main types of structures: (1) arc-parallel, kilometric-scale folds and reverse faults that control the conformable relief of the fold-and-thrust belt; (2) the tectonic lineation related to the SW segmentation of this relief, which is composed of the Colmenar fault and the Ubrique Normal Fault Zone (UNFZ).ConclusionsThis tectonic lineation seems to have localized arc-parallel extension. Qualitative and quantitative geomorphological analyses together with the age of the deformed rocks indicate that these structures have been active from the Tortonian to Holocene. Our results suggest that post-Serravallian outward radial thrusting and arc-parallel stretching accommodate a strain partitioning typical of progressive arcs, thus suggesting that the Gibraltar Arc is still protruding.ResumenObjetivoEn cinturones orogénicos arqueados, la deformación está normalmente repartida entre estiramiento paralelo y acortamiento perpendicular al arco. El estiramiento paralelo al arco puede estar acomodado por fallas de salto en dirección oblicuas y fallas normales perpendiculares al arco, cuya localización en sistemas puede provocar importantes bajadas bruscas del relieve a lo largo de las directrices estructurales.MétodosEn este trabajo, hemos estudiado el área de Ubrique, localizada en un segmento frontal de las Béticas externas (rama norte del Arco de Gibraltar), donde una de las discontinuidades más importantes del relieve a lo largo de las directrices del orógeno ha sido definida.ResultadosHemos encontrado que esta discontinuidad está determinada por dos tipos principales de estructuras: (1) pliegues de escala kilométrica y paralelos al arco y fallas inversas que controlan el relieve conforme del cinturón del pliegues y cabalgamientos; (2) la lineación tectónica relacionada con la segmentación del relieve hacia el SO, la cual está compuesta por la falla de Colmenar y la Zona de Falla Normal de Ubrique (UNFZ).ConclusionesEsta lineación tectónica parece haber localizado la extensión paralela al arco. Análisis geomorfológicos cualitativos y cuantitativos junto con la edad de las rocas deformadas indican que estas estructuras han sido activas con posterioridad al Serravaliense. De hecho, nuestros resultados sugieren que este modo de reparto de la deformación, típico de arcos progresivos, ha sido activo hasta el Holoceno, sugiriendo así que el Arco de Gibraltar continúa protruyéndose.