J. C. Balanyá

Alternating contractional and extensional events in the Alpujarride nappes of the Alboran Domain (Betics, Gibraltar Arc)

In the western Alpine system, Neogene extensional tectonics triggered the development of marine basins on the concave side of tight orogenic arcs, as happened within the Alboran Crustal Domain, the hinterland of the Gibraltar Arc. A detailed analysis of the structural and metamorphic records of one of the main Alboran Domain complexes, however, plainly reveals a complex tectonic evolution prior to the development of the Miocene arc/back arc system, which includes a major intraorogenic extensional event. This large-scale subvertical shortening, that can be assessed from the PT paths of representative tectonic units, was subsequent to the continental crust subduction inferred from high pressure-low temperature mineral asssemblages. The crustal section was thinned in nearly isothermal conditions, its thickness being reducted to at least 1/3 of the initial value. Yet still before the Miocene, a second contractional event led to the overthrusting of high-grade metamorphic rocks over other low-grade rocks, accompanied by subordinate overturning of the metamorphic zones. Since migration of the Gibraltar Arc is roughly balanced by crustal spreading in the back arc, available data concerning Miocene extension suggest that the Alboran Domain can be restored to its appropriate position several hundred kilometers to the east. Thus a collision belt that underwent significant intraorogenic extension must have existed in what is now the western South-Balearic basin.

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.

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.