José María Tubía

Dominant c slip in naturally deformed quartz: Implications for dramatic plastic softening at high temperature

A combined microstructural, X-ray texture goniometry and transmission electron microscopy study has been undertaken to document rare examples of c direction of slip in naturally deformed quartz. The presence of optically visible basal (0001) subgrain boundaries and strong concentrations of c axes parallel to the inferred shearing direction (close to the stretching lineation) are considered characteristic of c slip. Dominant c slip appears to be limited to high-temperature (>650 °C) and possibly hydrous conditions. The possibility of plastic softening associated with the relatively easy glide on this system is discussed.

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.

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

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 ...

Experiments on granite intrusion in transtension

Abstract Granite intrusion in transtensional regime is modelled by injecting a Newtonian fluid into a sand pack containing a ductile layer. The transtensional regime is obtained using two plastic sheets sliding along two rigid horizontal plates, and diverging from two narrow spaces (two fixed velocity discontinuities). The injection tube is located in a central space between these plates. Both symmetric experiments (when the two sheets were displaced with equal and opposite velocity vectors) and asymmetric experiments (in which only one sheet was displaced) were performed. Transtension was applied with a systematic variation (every 15°) of the divergence angle (α), between 15° and 90°. Experiments showed that: intrusions localize strain from the first stages of deformation; intrusions result in partially conformable laccoliths with bowler-hat geometry in cross-section; intrusions show an important offset towards the mobile basal plate for asymmetric transtensional regime, and are more symmetric and centred on the injection point for symmetric transtensional regime; the geometry of intrusions is controlled by the faults developed in the overburden. The significance of this control depends upon the angle of divergence α. Examples of the Hombreiro and Los Pedroches granites of the Variscan belt of Spain have been addressed to test the applicability of these experimental results.

Interprétation des cisaillements ductiles vers le NE dans les gneiss de Torrox (Complexe Alpujarride, Cordillères Bétiques)

ResumenLes Gneiss de Torrox font partie de l’Unite de Sayalonga, appartenant au Complexe Alpujarride des Cordilleres Betiques (Espagne). Ils ont subi une deformation heterogene par cisaillement simple, caracterisee par le developpement de zones mylonitiques a pendage subhorizontal et de lineations d’etirement de direction N070°E. Nous interpretons ces lineations comme la direction de transport dans un contexte de chevauchements ductiles vers l’Kst-Nord-est. Ce sens de mouvement est tres repandu dans les Alpujarrides, et permet de mieux comprendre que les nappes Alpujarrides occidentales soient plus metamorphiques que celles des Alpujarrides centrales et orientales.

Structural Evolution of the Sierras Interiores (Aragón and Tena Valleys, South Pyrenean Zone): Tectonic Implications

The Sierras Interiores preserve an outstanding record of the alpine deformation along the northern border of the South Pyrenean Zone, in northern Spain. New mapping and structural data illustrate the existence of two thrust systems developed from competent sandstones and limestones of Cenomanian to Paleocene age. A deeper thrust system duplicates the massive limestones of the Lower Calcareous Section (Cenomanian to Early Campanian) at the base of the Mesozoic cover of the Sierras Interiores that rests unconformable over Paleozoic rocks of the Pyrenean Axial Zone. There is an upper thrust system that comprises a duplex leading to duplications of the Marboré Sandstones (Campanian to Maastrichtian) and the Light Limestones and Dolostones (Paleocene). From the regional point of view, the relevance of the deeper thrust system is evidenced by its great lateral continuity and because it constitutes the higher thrust of a large antiformal stack made up by at least four Paleozoic-involving allochthonous sheets associated with the emplacement of the Gavarnie nappe, a main tectonic nappe in the southern part of the Pyrenees. Footwall ramps with footwall synclines are common along the thrusts of the upper thrust system, supporting the development of smooth trajectory thrusts that climb through the cores of break-thrust folds. Complex duplex structures of hectometric length are observed along some thrust contacts, preventing the interpretation of the thrusts as simple slip décollements. Later, S-vergent folds with axial plane foliation deform the thrust systems. All of these structural features raise issues about the application of cross-section balancing methods.