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Alpine olivine- and titanian clinohumite-bearing assemblages in the Erro-Tobbio peridotite (Voltri Massif, NW Italy) (1991)

SCAMBELLURI, M. ; STRATING, E. H. HOOGERDUIJN ; PICCARDO, G. B. ; [et al.]

Oxford, UK : Blackwell Publishing Ltd

Journal of metamorphic geology 9 (1991), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: The microstructures in the Erro-Tobbio peridotite indicate several stages of recrystallization of olivine + titanian clinohumite-bearing assemblages. The development of these assemblages is closely associated with serpentinite mylonites, in which they occur in shear bands and foliations and are inferred to have grown synkinematically, in veins, and as post-kinematic radial aggregates. In the peridotite wall-rock adjacent to these mylonites, the same assemblages have recrystallized statically at the expense of original olivine and pyroxenes, mesh-textured chrysolite and antigorite veins. In addition, the olivine-bearing assemblage occurs in widespread vein systems. The brittle deformation of the peridotite resulting in the development of these vein systems is closely related to ductile deformation of metagabbroic dykes in the peridotite. Although early metasomatism resulted in extensive rodingitization of the gabbros, some dykes show an eclogitic assemblage of Na-clinopyroxene + garnet + chloritoid + chlorite ± talc. These observations, the microstructures and the mineral chemistry all suggest that the assemblages in the ultramafic rocks and metagabbros developed during a prograde evolution towards high pressures (〉13–16 kbar, 450–550° C), and during subsequent decompression. This metamorphic evolution is considered to be related to Late Cretaceous intraoceanic subduction in the Alps-Apennine system and closure of the Piedmont-Ligurian basin.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1525-1314.1991.tb00505.x

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Porphyroblast rotation: eppur si muove? (1992)

PASSCHIER, C. W. ; TROUW, R. A. J. ; ZWART, H. J. ; [et al.]

Oxford, UK : Blackwell Publishing Ltd

Journal of metamorphic geology 10 (1992), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: In a number of recent papers, the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates, and can be used to determine the original orientation of older foliations. Complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis, involving several alternating phases of horizontal shortening and extension. Critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws. Millipede structures, used as proof for flow partitioning, can also form by other flow geometries. Evidence quoted to support irrotational behaviour of porphyroblasts is unsound. Porphyroblasts do occur in sets with a preferred orientation of the internal foliation trace, but these cannot be shown to represent original orientations. Microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis. They can, however, also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation. Finally, porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1525-1314.1992.tb00083.x

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A zone of mylonite and related ductile deformation beneath the alpujarride nappe complex, betic cordilleras, S. Spain (1984)

Platt, J. P. ; Behrmann, J. H. ; Martínez, J. -M. M. ; [et al.]

Springer

International journal of earth sciences 73 (1984), S. 773-785

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ISSN: 1437-3262

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Description / Table of Contents: Abstract A zone of mylonite is commonly developed beneath the Alpujarride nappe complex. The contact with the underlying Nevado-Filabride Complex is marked by a zone of calc mylonite, dolomitic breccia, iron-rich carbonate rocks, and marble. These lie on a thin zone of ultramylonite derived from Nevado-Filabride schist. Related ductile deformation decreases downwards, but may extend up to 400 m beneath the contact. This deformation produces a characteristic platy foliation, a strong elongation lineation, and a proto-mylonitic microstructure. Kinematic analysis of these rocks may help determine the direction of nappe transport. Mylonitic or protomylonitic microstructures are not developed in the Alpujarride rocks, although these are strongly deformed adjacent to the contact. Microstructural evidence suggests that deformation occurred by pressure-solution in these rocks, and not by the crystal-plastic processes that operated in the Nevado-Filabride protomylonitic schists.

Abstract: Resumen Bajo el complejo de mantos de los Alpujárrides se observa comunmente el desarrollo de una zona milonitica. El contacto de dicho complejo con el de Nevado-Filabride está señalado por una zona de calcomilonitas, brechas dolomiticas, rocas carbonatadas ricas en hierro y mármoles. Estos se situan en una zona estrecha de ultramilonitas que se derivan de los esquistos del Nevado-Filabride, La deformación dúctil asociada decrece en sentido descendente, pero puedo extenderse hasta 400 m por debajo del contacto. Esta deformation produce una foliación planar characterística, una lineación de extensión muy pronunciada y una microestructura protomilonítica. Los análisis cinemáticos de éstas rocas pueden ayudar a determinar la dirección de transporte de los mantos. Las microestructuras miloníticas y protomyloníticas no se desarrollan en las rocas Alpujárrides, aunque dichas rocas están fuertemente deformadas junto al contacto. Las observaciones microestructurales sugieren que la deformación tuvo lugar por «pressure-solution» y no por procesos de plasticidad cristalina como los operantes en los esquistos protomiloníticos del Nevado-Filabride.

Notes: Zusammenfassung Im Liegenden des Deckengebäudes der Alpujarriden findet sich an vielen Stellen eine Mylonitzone. Der Kontakt mit den unterlagernden nevado-filabriden Gesteinen ist durch Kalkmylonite, Dolomitbrekzien, eisenreiche Karbonate und Marmore markiert. Diese liegen auf einer dünnen Zone von Ultramylonit, der sich aus nevado-filabriden Glimmerschiefern entwickelt hat. Diese plastische Verformung nimmt zum Liegenden hin ab, kann aber bis zu 400 m unterhalb des Deckenkontaktes reichen. Sie erzeugt eine charakteristische, plattige Schieferung, eine stark ausgeprägte Streckungslineation und eine protomylonitische Mikrostruktur. Mit einer kinematischen Analyse dieser Gesteine kann die Transportrichtung der Deckeneinheiten bestimmt werden. Die Glimmerschiefer und Phyllite alpujarrider Zuordnung besitzen keine mylonitische Mikrostruktur, obwohl sie stark deformiert sind. Dies deutet darauf hin, daß, im Gegensatz zum intrakristallinen Gleiten in den nevado-filabriden Protomyloniten, vor allem Drucklösung als Deformationsmechanismus anzusehen ist.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01824981

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Data on the tectonic and metamorphic evolution of the central Sierra De Los Filabres, Betic Cordilleras, se Spain (1977)

Vissers, R. L. M.

Springer

International journal of earth sciences 66 (1977), S. 81-90

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ISSN: 1437-3262

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Description / Table of Contents: Abstract Within the area investigated three tectonic units are distinguished which can be referred to the Nevado-Filabride, Ballabona-Cucharón, and Alpujarride complex, respectively. During the Alpine orogeny the rocks considered were subjected to various phases of folding and overthrusting. This tectonic evolution was accompanied by a metamorphism of a plurifacial character. The relation between tectonic events and metamorphic recrystallization is schematically outlined for each of the tectonic units distinguished. The units show marked differences in their degree of metamorphic recrystallization, caused by differences between the metamorphic conditions in each of the units during the earlier stages of metamorphism. The younger stages of metamorphism postdate important overthrust movements and have affected the whole pile of tectonic units. The Palaeozoic (and older?) rock sequences do not yield conclusive evidence of prealpine metamorphism, and definite proof of pre-alpine deformation is wanting. Consequently the difference in metamorphic grade between the Palaeozoic (and older?) and the Permo-Triassic rocks of the Alpujarride complex has to be explained by differences in metamorphism of Alpine age.

Abstract: Resumen En el área objeto de este estudio afloran tres unidades tectónicas pertenecientes a los complejos Nevado-Filäbride, Ballabona-Cucharón y Alpujárride, respectivamente. Durante la orogenia alpina, las rocas de que se trata han sufrido varias fases de plegamiento y corrimiento. Esta evolución tectónica ha sido acompanada por un metamorfismo de carácter plurifacial. Se ha representado en un esquema la relación entre las fases tectónicas y las etapas de recristalización metamórfica para cada unidad. Dichas unidades muestran entre si una notable diferencia en grado de recristalización metamórfica, causada por condiciones metamórficas distintas, en cada una de ellas, durante las etapas más tempranas del metamorfismo. Las etapas tardias son esencialmente postmantos y han afectado al conjunto de unidades ya empiladas. El estudio de las rocas paleozoicas (¿y más antiguas?) no ha revelado ninguna prueba acerca de la existencia de fases de deformación o etapas de metamorfismo prealpinos. Por consiguiente, la diferencia en grado de metamorfismo entre las rocas paleozoicas (¿y más antiguas?) y permotriásicas del complejo Alpujárride debe ser explicada por diferencias en metamorfismo de edad alpina.

Notes: Zusammenfassung Im vorliegenden Gebiet sind drei tektonische Einheiten zu unterscheiden, die man beziehentlich dem Nevado-Filabriden, Ballabona-Cucharón, und Alpujarriden Komplex zuschreiben kann. Während der alpinen Orogenese wurden die bezüglichen Gesteine von mehreren Phasen von Faltung und Überschiebung betroffen, begleitet von Metamorphose plurifaziellen Charakters. Der Zusammenhang von tektonischen Phasen und metamorpher Umkristallisation ist für jede tektonische Einheit schematisch wiedergegeben. Die Einheiten weisen beachtliche Unterschiede in Metamorphosegrad auf, die von Unterschieden der metamorphen Verhältnisse während der früheren Stadien der Metamorphose bedingt wurden. Die jüngeren Stadien der Metamorphose sind post-Überschiebung und beeinflußten das ganze Deckenpaket. Die Existenz einer starken prä-alpinen Metamorphose in den paläozoischen (und der älteren?) Gesteinen ist nicht bewiesen worden, indem eine prä-alpine Deformation sich nicht leicht erweisen läßt. Folglich ist der Unterschied in Metamorphosegrad zwischen den paläozoischen (und älteren?) und permo-triadischen Gesteinen des Alpujarriden Komplexes von einem Unterschied in alpiner metamorpher Entwicklung zu erklären.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01989565

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Tectonic and magmatic evolution of the mantle lithosphere during the rifting stages of a fossil slow-ultraslow spreading basin: insights from the Erro-Tobbio peridotite (Voltri Massif, NW Italy) (2014)

Padovano, M., Piccardo, G. B., Vissers, R. L. M.

Geological Society (of London)

In: Special Publications / Geological Society London

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Publication Date: 2014-12-05

Description: We investigate the structural, petrological and compositional features recorded by strongly deformed and melt-percolated Erro–Tobbio peridotites (Voltri Massif, Ligurian Alps, NW Italy), in order to demonstrate that the processes of shear-zone formation and melt percolation are intimately linked by a positive feedback. We focus on spinel and plagioclase peridotites, and extensional shear zones that underwent infiltration by upwelling asthenospheric melts. Shear and porosity bands, which developed during extension prior to melt infiltration, represent important structural and rheological pathways to facilitate and enhance melt infiltration into the extending lithosphere and the ascent of such melts to shallower levels. Our results lend strong support to numerical models addressing the physical processes underlying extensional systems. These show that, in the case of slow–ultraslow continental extension and the subsequent formation of slow–ultraslow spreading oceans, porosity and shear-localization bands may develop in a previously unstructured lithosphere, prior to melt infiltration. Our studies on the Erro–Tobbio peridotites allow a model for the inception of continental extension and rifting to drifting of slow–ultraslow spreading oceans to be proposed. We suggest that integrated studies of on-land peridotites, coupled with geophysical–structural results from modern oceans, may provide clues to the geodynamic processes governing continental extension and passive rifting.

Print ISSN: 0305-8719

Electronic ISSN: 2041-4927

Topics: Geosciences

Published by Geological Society (of London)

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Middle Jurassic shear zones at Cap de Creus (eastern Pyrenees, Spain): a record of pre-drift extension of the Piemonte-Ligurian Ocean? (2017)

Vissers, R. L. M., van Hinsbergen, D. J. J., Wilkinson, C. M., Ganerod, M.

Geological Society of London

In: Journal of the Geological Society

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Publication Date: 2017-03-04

Description: The Cap de Creus peninsula in NE Spain consists of greenschist- to amphibolite-facies metasediments and granitoid bodies of the Variscan Axial Zone of the Pyrenees, overprinted in the north by anastomosed greenschist-facies shear zones. Current tectonic interpretations ascribe these shear zones to the waning stages of the Variscan orogeny. We present muscovite 40 Ar/ 39 Ar data from the shear zones, yielding Middle Jurassic ages between 159.33 ± 0.43 and 175.18 ± 1.10 Ma and one Tertiary age of 58.57 ± 0.55 Ma. We suggest that the present-day structure at Cap de Creus resulted from Variscan deformation and HT–LP metamorphism, followed during the Jurassic by crustal stretching and development of ductile normal faults reflecting pre-drift continental extension related to opening of the Piemonte–Ligurian basin east of Iberia. Tilting during Alpine convergence caused steepening in the northern part of the penisula, with the ductile normal faults rotated to their present orientations appearing as dextral reverse shear zones. The shear zone yielding a Tertiary age could reflect either an Alpine structure or reactivation of an earlier, presumably Jurassic shear zone. The Cap de Creus structure may thus represent a continental margin that has undergone ductile stretching equivalent to the now-buried west Iberian or Newfoundland margin.

Print ISSN: 0016-7649

Topics: Geosciences