Julia Cuevas

High-temperature emplacement of the Los Reales peridotite nappe (Betic Cordillera, Spain)

West of Malaga (Southern Spain), the Sierra Alpujata belongs to the Alpujarride Complex in the Internal Zone of the Betic Cordillera, and is composed of migmatites underlying alpine-type peridotites. The migmatite-peridotite contact zone is a ductile zone of shear with highly strained rocks. This is demonstrated using field structural data and systematic analysis of the microstructures and the preferred orientations of enstatite and olivine. We establish that the peridotites were thrust over continental crust from the WSW to the ENE, in relatively low temperature conditions for the olivine deformation (porphyroclastic and mylonitic textures: T ⋍ 900°C) but high enough to lead to syntectonic migmatization in crustal rocks, with identical kinematic signatures on both sides of the contact.

Sequential development of the metamorphic aureole beneath the Ronda peridotites and its bearing on the tectonic evolution of the Betic Cordillera

Abstract At Sierra Alpujata (western Betic Cordillera, Spain), the Ronda peridotites overlay a metamorphic sole of felsic composition formed during the hot overthrusting of the ultrabasic rocks over the Ojen nappe. The metamorphic sole evolved from an initial eclogite facies metamorphism ( P ≥ 1.7 GPa; T = 790°C) to low-pressure ( P ≤ 300 MPa) high-temperature ( T ∼ 600°C) conditions. The active movement zone migrated downwards in the metamorphic sole as temperatures decreased during this exhumation process: high temperatures led to strain localization along the 200-m-thick ductile shear zone that forms the uppermost level of the metamorphic sole, and brittle to ductile deformations achieved at temperatures below 500°C are concentrated in the basal contact of the metamorphic sole. The maintenance of a common kinematics, with ENE-trending stretching lineations and top-to-the-ENE shearing, along the high-temperature and the low-temperature shear zones suggests that the main part of the exhumation process was performed before the overall north-south convergence of the Iberian and African plates. The subduction of the Ojen nappe, a wedge of subcontinental lithosphere, below the orogenic lherzolites of Ronda probably gave rise to the syn-collisional exhumation of the metamorphic sole and then to the slab breakoff and sinking of the subducted lithospheric mantle.

Tectonic evolution of the Alpujárride Complex (Betic Cordillera, southern Spain)

Abstract The Alpujarride Complex, belonging to the Internal Zones of the Betic Cordillera, is an E-W elongated domain of metamorphic nappes, which extends over more than 400 km in southern Spain. The Alpujarride Complex suffered two shearing episodes defined by different kinematic and physical conditions. The first one involved ductile thrusting from the south-west towards the north-east, related to the regional metamorphism of the Alpujarride nappes. This process led to crustal thickening, related to which the hot thrusting of the Ronda peridotites occurred. The second shearing episode, directed northwards, occurred after the metamorphism. It corresponds to a deformation in the brittle-ductile transition that led to the thinning of the crust thickened in the previous episode. Previous tectonic models of the Betic Cordillera have only considered the northwards shearing, which reflect the N-S convergence of Europe and Africa from the Oligocene onwards. The present work clearly points to the existence of previous episodes of ductile thrusting towards the north-east that reflect an earlier process of continental collision and that in future should be considered in tectonic models of the Betic orogen.

Lithospheric anisotropy beneath the Pyrenees from shear wave splitting

We investigate upper mantle anisotropy beneath the Pyrenean range along three N-S profiles across the mountain belt. The results of a first profile that operated in 1993 in the central part of the belt have been presented elsewhere. We present the results of two other profiles that ran in 1995-1996 and 1996-1997 in the eastern and western part of the belt, respectively and propose an interpretation of the whole results. Teleseismic shear waves (SKS, SKKS, and PKS) are used to determine splitting parameters: the fast polarization direction φ and the delay time δt. Teleseismic shear wave splitting in the eastern Pyrenees displays homogeneous φ values trending N100°E and δt values in the range 1.1 to 1.5 s. A station located in the southern Massif Central, 100 km north of the range, is characterized by different splitting parameters (φ = N70°E, δt = 0.7 s). In the western part of the belt, anisotropy parameters are similar across the whole belt (φ = N110°E and δt = 1.3 to 1.5 s). Most of the measured delay times, including those obtained in the central part of the range, are above the global average of the SKS splitting (around 1 s). At the belt scale, φ is generally poorly correlated with recent estimations of the absolute plate motion, which predicts a fast direction ranging between N50°E and N80°E. Instead, the orientation of φ (N100°E) is parallel to the trend of the Pyrenean belt but also to Hercynian preexisting structures. This parallelism supports an anisotropy primarily related to frozen or active lithospheric structures. We show that a signature related to the Pyrenean orogeny is likely for the stations located in the internal domains of the belt. By contrast, the anisotropy measured at the stations located on the external parts of the belt could reflect a pre-Pyrenean (Hercynian) deformation. We suggest that a late Hercynian strike-slip deformation is responsible for this frozen upper mantle anisotropy and that the Pyrenean tectonic fabric developped parallel to this preexisting fabric. Finally, no particularly strong splitting is related to the North Pyrenean Fault, commonly believed to represent the plate boundary between Iberia and Eurasia.

The Los Pedroches Batholith (Southern Spain): Polyphase Interplay between Shear Zones in Transtension and Setting of Granites

The Los Pedroches batholith (LPB) in the Variscan belt of southern Spain is a 200 kilometres long magmatic body which has intruded low-grade metamorphic rocks of the Central Iberian Zone. The LPB is composed of granodiorites, a dyke swarm, and granites emplaced during three successive magmatic events. Field structural data from both the country rocks and the LPB indicate that the northern boundary of the LPB is a transtensional shear zone, the Conquista shear zone, dipping 50° to the NE. This shear zone is sealed by the Cerro Mogabar granite which was emplaced during the last magmatic event. A detailed magnetic susceptibility study, using 2236 oriented samples from 559 stations, of the main part of the batholith yields flat magmatic foliations, and magmatic lineations with a mean east-west trend. The structural, kinematic and magnetic susceptibility data indicate that the emplacement of the LPB was controlled by a crustal scale and dextral transtensional shear zone, parallel to the batholith elongation. The eastern part of the granodiorite takes the shape of a large laccolith prolonged westwards by four NW-SE elongate domes. These contrasted geometries reflect the upwelling of magma through the transtensional zone that vanishes towards the west. Intrusion of dykes along Riedel shears related to the regional transtension took place in a second magmatic event. The main transtensional shear zone is interpreted as a lateral ramp that accommodated the crustal thinning developed in the Central Iberian zone during the collapse of the Iberian Variscan belt.

Tectonic evidence in the Ronda peridotites, Spain, for mantle diapirism related to delamination

The contribution of asthenospheric diapirs to the exhumation of orogenic lherzolites from the mantle to Earths surface is a major issue in the evolution of orogenic belts. Detailed maps of the trajectories of the foliation in the Carratraca massifs of the Ronda peridotites (Betic Cordillera, southern Spain) provide evidence for a narrow mantle diapir that was formed in early Miocene time. When set in its geologic and petrologic context, this diapir documents the injection of hot asthenosphere into older and cooler lithospheric mantle, possibly in response to the delamination of the lithosphere that had been thickened during the Mesozoic and Cenozoic convergence of the African and Iberian plates from Cretaceous time onward.

Composite magnetic fabrics from S-C mylonites

Magnetic anisotropy data are presented for samples of S-C mylonites from a shear zone developed in the Veiga granodiorite, a late-kinematic pluton located in the northern part of the Iberian Variscan belt (Spain). The magnetic susceptibility is geometrically represented by an ellipsoid whose principal axes, K,,, 2 Kint 1 Kmi,, often show a one-to- one correlation with the principal directions-X,Y and 2-f finite strain. We report an exception to this rule, since our results reflect composite magnetic fabrics arising from the contribution of both the C- and S-structures. The magnetic foliation, defined as the plane normal to K,i,, shows an intermediate orientation between the C- and S-planes, and rotates towards the orientation of the C-planes as finite strain increases. The magnetic lineation, parallel to K,,,, and the stretching lineation present a similar trend, but different values of plunge. Copyright 0 1996 Elsevier Science Ltd

A revised Aquitanian age for the emplacement of the Ronda peridotites (Betic Cordilleras, southern Spain)

The hot emplacement of the Ronda peridotites (Betic Cordilleras) developed a dynamothermal aureole and partial melts that led to the intrusion of granite dykes in the peridotites. Previous geochronological data place rather broad limits for this event between 22 and 19 Ma. Analyses of neocrystalline zircon rims from large zircon populations yield a U–Pb SHRIMP age of 22.3±0.7 Ma for the dynamothermal aureole formation, and intrusion ages of granite dykes between 22.6±1.8 and 21.5±3.8 support that conclusion. Therefore, these new ages provide a more robust constraint on the hot emplacement of the Ronda peridotites at middle crustal levels.

Structures et cinématique liées á la mise en place des péridotites de Ronda (Cordilléres Bétiques, Espagne)

ResumeLes massifs ultrabasiques de Ronda (Cordilleres Betiques) appartiennent aux peridotites alpines du subtype lherzolitique. Ils portent l’empreinte de deux deformations plastiques liees a leur mise en place. Le cœur du massif de la Sierra Alpujata montre des lherzolites a plagioclase dont les fabriques permettent de caracteriser une deformation de basse contrainte et haute temperature (~ 1100 0C). Elle est obliteree par une deuxieme, de haute contrainte (1-2 Kb) et basse temperature (~ 800 °C), observee dans les contacts limitant la lame peridotitique. Ces deformations temoignent de deux etapes de mise en place pour les peridotiques de Ronda. La premiere peut etre rattachee au flux du manteau sous une zone de croute continentale en extension, et la deuxieme a la fermeture ulterieure de cette zone et a l’apparition de cisaillements ductiles responsables du chevauchement d’une ecaille du manteau.L’analyse cinematique de ces tectonites montre que les sens de mouvements dans les peridotites de Ronda sont ...

The Maláguide-Los Reales Nappe: an example of crustal thinning related to the emplacement of the Ronda peridotites (Betic Cordillera)

Abstract The Malaga region in southern Spain represents a cross-section through the continental crust and subcontinental mantle of the westernmost portion of the Internal Zone of the Betic Cordillera. Recent petrological and structural studies carried out in this region show a link between the Malaguide Complex and the underlying Los Reales Nappe, where the Ronda peridotites were incorporated as allochthonous slabs. The Ronda peridotites are covered by the metamorphic sequence of the Los Reales Nappe, which consists of acidic granulites, migmatites and schists. The tectonometamorphic evolution of these granulite facies rocks involves a first high-temperature (800°C)-high-pressure (10 kbar) metamorphism replaced by high-temperature (750°C)-low-pressure (3.5 kbar) conditions. This strong thermal event, which is also recognized in biotite-bearing schists of the Los Reales Nappe and even in the overlying Malaguide Complex, developed during an extensional deformation related to the diapiric stage of the Ronda peridotite emplacement. Stretching lineations from the mylonites related to this extensional event are N 110°E in orientation and kinematic criteria show that low-grade metamorphic rocks (Malaguide Complex) move down on to higher grade ones (Los Reales Nappe) from west to east. Consequently, we interpret that the Malaguide Complex and Los Reales Nappe form a single domain of continental crust which was thinned during the emplacement of the Ronda peridotites. Stretching lineations are subparallel to the trend of the Betic Cordillera and can be integrated in a model of lithospheric transcurrent thinning.