Manuel Díaz-Azpiroz

Variscan intra-orogenic extensional tectonics in the Ossa-Morena Zone (Évora-Aracena-Lora del Rı́o metamorphic belt, SW Iberian Massif): SHRIMP zircon U-Th-Pb geochronology

Abstract Following a Middle–Late Devonian (c. 390–360 Ma) phase of crustal shortening and mountain building, continental extension and onset of high-medium-grade metamorphic terrains occurred in the SW Iberian Massif during the Visean (c. 345–326 Ma). The Évora–Aracena–Lora del Rı́o metamorphic belt extends along the Ossa–Morena Zone southern margin from south Portugal through the south of Spain, a distance of 250 km. This major structural domain is characterized by local development of high-temperature–low-pressure metamorphism (c. 345–335 Ma) that reached high amphibolite to granulite facies. These high-medium-grade metamorphic terrains consist of strongly sheared Ediacaran and Cambrian–early Ordovician (c. 600–480 Ma) protoliths. The dominant structure is a widespread steeply-dipping foliation with a gently-plunging stretching lineation generally oriented parallel to the fold axes. Despite of the wrench nature of this collisional orogen, kinematic indicators of left-lateral shearing are locally compatible with an oblique component of extension. These extensional transcurrent movements associated with pervasive mylonitic foliation (c. 345–335 Ma) explain the exhumation of scarce occurrences of eclogites (c. 370 Ma). Mafic-intermediate plutonic and hypabyssal rocks (c. 355–320 Ma), mainly I-type high-K calc-alkaline diorites, tonalites, granodiorites, gabbros and peraluminous biotite granites, are associated with these metamorphic terrains. Volcanic rocks of the same chemical composition and age are preserved in Tournaisian–Visean (c. 350–335 Ma) marine basins dominated by detrital sequences with local development of syn-sedimentary gravitational collapse structures. This study, supported by new U–Pb zircon dating, demonstrates the importance of intra-orogenic transtension in the Gondwana margin during the Early Carboniferous when the Rheic ocean between Laurussia and Gondwana closed, forming the Appalachian and Variscan mountains.

Significance of graphite occurrences in the Aracena Metamorphic Belt, Iberian Massif

The significance of syngenetic and epigenetic graphite occurrences from the Variscan high-temperature/low-pressure Aracena Metamorphic Belt is discussed in the framework of the tectono-thermal evolution of this southern zone of the Iberian Massif. Syngenetic graphite is associated with both low- to medium-grade metamorphic rocks (La Umbria series, Precambrian in age) and high-grade, granulite facies rocks (the Precambrian Fuente del Oro series and a Cambrian calc-silicate series). Epigenetic, fluid-deposited occurrences correspond to overgrowths on existing metamorphic graphite grains and vein-type mineralization. Two types of graphitized particles with remarkable differences in reflectance, anisotropy and size can be distinguished in the Precambrian metapelites of the La Umbria series. Large, >150 μm length, platy crystals with high reflectance and anisotropy are interpreted as detrital and are considered indirect evidence of an old orogenic cycle prior to the Cadomian Orogeny, during which metamorphism exceeded greenschist facies. The coexistence of two types of particles explains the scattering of values of the c parameter of graphite determined by XRD (c = 6.72–6.74 A), and the anomalously high temperatures of the DTA exothermic peak (close to 600 °C) of graphite with respect to that inferred from mineral assemblages in these rocks. The presence of graphite-rich quartzites and gneisses within the Fuente del Oro series and the calc-silicate series is evidence of sedimentation under reducing conditions in a continental shelf. The characteristics of graphite reflect the high-grade metamorphic conditions attained in the southern area of the Aracena Metamorphic Belt. Pervasive flow of fluids related to a major Variscan extensional event resulted in overgrowths on the pre-existing graphite in the gneisses and quartzites of the calc-silicate series, as evidenced by the heterogeneous isotopic composition of graphite single crystals in these rocks. A later stage of graphite precipitation is represented by scarce vein-type occurrences in mafic granulites that document channelled flow of fluids.

Inclined transpression in the Neka Valley, eastern Alborz, Iran

Three major nappes in the Neka Valley in the eastern Alborz Mountains of Iran allow the Cimmerian to present convergence following the oblique collision between Iran and the southern margin of Eurasia. This work reports the identification of an inclined transpression zone recognized by field investigations and strain analyses of the geometries of formations and detailed mesoscopic structural analyses of multiple faults, folds and a cleavage. The main structures encountered include refolded recumbent asymmetric fold nappes, highly curved fold hinges, in a transpression zone that dips 37° to the NW between boundaries thrusts striking from N050° to N060°. The β angle (the angle between the zone boundary and direction of horizontal far-field shortening) is about 80°. The north-west and south-east boundaries of this zone coincide with the Haji-abad thrust and the Shah-Kuh thrust, respectively. Fold axes generally trend NE–SW and step to both right and left as a result of strike–slip components of fault displacements. Strain analyses using Fry’s method on macroscopic ooids and fusulina deformed into oblate ellipsoids indicate that the natural strain varies between 2.1 and 3.14. The estimated angle between the maximum instantaneous strain axis (ISAmax) and the transpression zone boundary (θ′) is between 6° and 20°. The estimated oblique convergence angle (α), therefore, ranges between 31° and 43°. The average kinematic vorticity number (Wk) is 0.6, in a zone of sinistral pure shear-dominated inclined triclinic transpression. These results support the applicability of kinematic models of triclinic transpression to natural brittle–ductile shear zones.

Deformation mechanisms of plagioclase and seismic anisotropy of the Acebuches metabasites (SW Iberian massif)

Abstract Samples of the Acebuches metabasites (SW Spain), deformed under low-pressure/medium-to-high temperature metamorphic conditions, have been analysed via electron backscattered diffraction (EBSD) to obtain their plagioclase crystal lattice preferred orientations (LPO). Plagioclases from the highest temperature amphibolites show moderate LPO and a good correlation between 180° misorientation angles and both the crystal and the kinematic coordinate systems, which is attributed to dislocation glide accommodated by mechanical albite+pericline twinning. Plagioclases from medium-temperature amphibolites exhibit well-developed LPO, suggesting that dislocation creep was active during plagioclase deformation. Plagioclases from the more intensively deformed mafic schists exhibit weak LPO, indicating the activity of LPO-destroying deformation mechanisms. Evidence points to grain-boundary sliding accompanied by limited fracturing. The observed LPO are characterized by the alignment of [100] parallel to the kinematic X-direction. This association suggests that [100] was the preferential slip direction during dislocation creep of plagioclase, with (010) and/or (001) appearing to have acted as the dominant slip planes. The observed plagioclase LPO is combined with hornblende LPO to define the seismic fabric of the Acebuches metabasites. In samples with strong plagioclase LPO, the resulting seismic fabrics are highly influenced by this phase.

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.