Fluid-assisted large strains in a crustal-scale décollement (Hercynian Belt of South Brittany, France)

Hebel, Florence Le; Gapais, Denis; Fourcade, Serge; Capdevila, Ramon Buxó i

doi:10.1144/gsl.sp.2001.200.01.06

Cited by 10 publications

(7 citation statements)

References 74 publications

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“…The large water amount measured in quartz ribbons by FT-IR is thus directly related to crack-healing. An analogue of quartz ribbons could be the "shear or bedding veins", commonly found in low grade deformed meta-sedimentary rocks (Cox, 1987;Labaume et al, 1991;Cosgrove, 1993;Le Hebel et al, 2002;Fagereng et al, 2010Fagereng et al, , 2011. The strong CPO, with c-axes parallel to the main stretching direction, developed as only favorably oriented crystals grew by this cyclic process.…”

Section: Hyuga Tectonic Mélangementioning

confidence: 99%

Deformation processes at the down-dip limit of the seismogenic zone: The example of Shimanto accretionary complex

Palazzin

Raimbourg

Famin³

et al. 2016

Tectonophysics

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International audienceIn order to constrain deformation processes close to the brittle-ductile transition in seismogenic zone, we have carried out a microstructural study in the Shimanto accretionary complex (Japan), the fossil equivalent of modern Nankai accretionary prisms. The Hyuga Tectonic Mélange was sheared along the plate interface at mean temperatures of 245 °C ± 30 °C, as estimated by Raman spectroscopy of carbonaceous material (RSCM). It contains strongly elongated quartz ribbons, characterized by very high fluid inclusions density, as well as micro-veins of quartz. Both fluid inclusion planes and micro-veins are preferentially developed orthogonal to the stretching direction. Furthermore, crystallographic preferred orientation (CPO) of quartz c-axes in the ribbons has maxima parallel to the stretching direction. Recrystallization to a small grain size is restricted to rare deformation bands cutting across the ribbons. In such recrystallized quartz domains, CPO of quartz c-axes are orthogonal to foliation plane. The evolution of deformation micro-processes with increasing temperature can be further analyzed using the Foliated Morotsuka, a slightly higher-grade metamorphic unit (342 ± 30 °C by RSCM) from the Shimanto accretionary complex. In this unit, in contrast to Hyuga Tectonic Mélange, recrystallization of quartz veins is penetrative. CPO of quartz c-axes is concentrated perpendicularly to foliation plane. These variations in microstructures and quartz crystallographic fabric reflect a change in the dominant deformation mechanism with increasing temperatures: above ~300 °C, dislocation creep is dominant and results in intense quartz dynamic recrystallization. In contrast, below ~300 °C, quartz plasticity is not totally activated and pressure solution is the major deformation process responsible for quartz ribbons growth. In addition, the geometry of the quartz ribbons with respect to the phyllosilicate-rich shear zones shows that bulk rheology is controlled by quartz behavior. Consequently, below 300 °C, the application of quartz pressure-solution laws, based on realistic geometry derived from Hyuga microstructures, results in strongly lowering the overall strength of the plate interface with respect to the classical brittle envelop

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Section: Hyuga Tectonic Mélangementioning

confidence: 99%

Deformation processes at the down-dip limit of the seismogenic zone: The example of Shimanto accretionary complex

Palazzin

Raimbourg

Famin³

et al. 2016

Tectonophysics

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“…480 Ma in Vendée by dating the emplacement age of the por-phyroids [Ballèvre et al, 2012]. P-T estimates for the Belle-Ile group rocks yielded HP/LT metamorphic peakcon-ditions of 7-9 kbar and 350-400 o C [Le Hébel et al, 2002a];…”

Section: Geological Settingmentioning

confidence: 99%

“…after the youngest age of further strain localisation at the roof of leucogranite subduction-relation HP metamorphism recognised within laccoliths was subsequently recorded by ca. 310-300 Ma the uppermost units [Le Hébel et al, 2002a, 2002b Ar/Ar datings ( fig. 12) [Ruffet in Le Hébel, 2002;Turrillot, et al, 2005] and ca.…”

Section: The Ca 325-320 Ma Turning-point In the Evolution Of The Coamentioning

confidence: 99%

A turning-point in the evolution of the Variscan orogen: the ca. 325 Ma regional partial-melting event of the coastal South Armorican domain (South Brittany and Vendée, France)

Augier

Choulet

Faure

et al. 2015

Bulletin De La Société Géologique De France

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International audienceBy drastically reducing the bulk strength of crustal materials, partial-melting is one of the main parameter controlling the rheological behaviour of the continental crust. With more than ca. 50% of the outcropping surface characterised by migmatites and granites, the coastal South Armorican domain, offers an opportunity to study deep-orogenic processes and more particularly, to understand the role of partial-melting for the late-evolution of the Variscan belt. To date, time-constraints are scarce hindering the understanding of this crucial stage in the Variscan belt evolution. This paper provides 29 new U-Th/Pb chemical ages on monazite collected over five sampling areas consisting in migmatite domes and late regional classic plutons. Based on structural, textural and chemical criteria, three main U-Th/Pb age-groups are distinguished. The first group, settled at ca. 335–330 Ma concerns samples of restites and core-domains of the monazite crystals for most of the granite massifs. Its significance is ascribed to inherited crystallisation ages probably recording the crossing of prograde monazite forming reactions (i.e. metamorphic isograds) during increasing P-T conditions in an overall nappe-stacking context. The second group that clusters at ca. 325–320 Ma corresponds to newly formed monazite grains that crystallised from juvenile silicate melts. Ages of this group are interpreted as crystallisation ages of leucosomes after a major partial-melting event that affected the whole domain. The last ca. 320 Ma group corresponds to rim-domains of monazite crystals. It is interpreted as the emplacement age of most of the large-scale granite massifs and therefore fixes the end of the partial-melting event.The inception and drastic generalisation of partial-melting at peak-P conditions therefore coincides with a major change in the tectonic regime recorded at regional-scale. In the lights of these results, this implies that (1) either continuous stacking of continental crustal units, rich in radiogenic elements, led to an increase of temperature within the orogenic wedge provoking partial-melting, the resulting drop in the crustal strength inducing collapse and lateral expansion of the belt, or (2) a drastic change of the boundary conditions has induced hot asthenospheric upwelling which in turn led to coeval extension and partial-melting. At a more local scale, strain benefited of the low-strength of the magmatic bodies prior to complete crystallisation promoting intense strain localisation within the South Armorican domain large-scale laccoliths often referred to as synkinematic plutons

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“…Reaction softening assisted by advection of hydrous fluids is a mechanism often invoked for promoting localized deformation in granitoids at the brittle–ductile transition (e.g. Bos & Spiers, 2001 and references therein) in contrast to the closed system fluid conditions which may allow the existence of distributed large strains within large volumes even at low‐temperature conditions (Le Hébel et al ., 2002).…”

Section: Deformation Mechanismsmentioning

confidence: 99%

“…Reaction softening assisted by advection of hydrous fluids is a mechanism often invoked for promoting localized deformation in granitoids at the brittleductile transition (e.g. Bos & Spiers, 2001 and references therein) in contrast to the closed system fluid conditions which may allow the existence of distributed large strains within large volumes even at lowtemperature conditions (Le He´bel et al, 2002). No evidence of plagioclase or clinopyroxene recrystallization has been identified in the Kettara shear zones; such a mechanism has however, been described in gabbroic rocks deformed at high temperatures in ÔdryÕ ductile shear zones (Bonatti et al, 1975;Helmstaedt & Allen, 1976;Me´vel, 1984).…”

Section: Deformation Mechanismsmentioning

confidence: 99%

Hydrothermal alteration, fluid flow and volume change in shear zones: the layered mafic–ultramafic Kettara intrusion (Jebilet Massif, Variscan belt, Morocco)

Essaifi

Capdevila

Fourcade

et al. 2004

Journal Metamorphic Geology

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During emplacement and cooling, the layered mafic-ultramafic Kettara intrusion (Jebilet, Morocco) underwent coeval effects of deformation and pervasive fluid infiltration at the scale of the intrusion. In the zones not affected by deformation, primary minerals (olivine, plagioclase, clinopyroxene) were partially or totally altered into Ca-amphibole, Mg-chlorite and CaAl-silicates. In the zones of active deformation (centimetre-scale shear zones), focused fluid flow transformed the metacumulates (peridotites and leucogabbros) into ultramylonites where insoluble primary minerals (ilmenite, spinel and apatite) persist in a Ca-amphibole-rich matrix. Mass-balance calculations indicate that shearing was accompanied by up to 200% volume gain; the ultramylonites being enriched in Si, Ca, Mg, and Fe, and depleted in Na and K. The gains in Ca and Mg and losses in Na and K are consistent with fluid flow in the direction of increasing temperature.When the intrusion had cooled to temperatures prevailing in the country rock (lower greenschist facies), deformation was still active along the shear zones. Intense intragranular fracturing in the shear zone walls and subsequent fluid infiltration allowed shear zones to thicken to metre-scale shear zones with time. The inner parts of the shear zones were transformed into chlorite-rich ultramylonites. In the shear zone walls, muscovite crystallized at the expense of Ca-Al silicates, while calcite and quartz were deposited in Ôen echelonÕ veins. Mass-balance calculations indicate that formation of the chlorite-rich shear zones was accompanied by up to 60% volume loss near the centre of the shear zones; the ultramylonites being enriched in Fe and depleted in Si, Ca, Mg, Na and K while the shear zones walls are enriched in K and depleted in Ca and Si. The alteration observed in, and adjacent to the chlorite shear zones is consistent with an upward migrating regional fluid which flows laterally into the shear zone walls. Isotopic (Sr, O) signatures inferred for the fluid indicate it was deeply equilibrated with host lithologies.

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Fluid-assisted large strains in a crustal-scale décollement (Hercynian Belt of South Brittany, France)

Cited by 10 publications

References 74 publications

Deformation processes at the down-dip limit of the seismogenic zone: The example of Shimanto accretionary complex

Deformation processes at the down-dip limit of the seismogenic zone: The example of Shimanto accretionary complex

A turning-point in the evolution of the Variscan orogen: the ca. 325 Ma regional partial-melting event of the coastal South Armorican domain (South Brittany and Vendée, France)

Hydrothermal alteration, fluid flow and volume change in shear zones: the layered mafic–ultramafic Kettara intrusion (Jebilet Massif, Variscan belt, Morocco)

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