Ile de Groix: retrogression and structural developments in an extensional régime

Shelley, David; Bossière, Gérard

doi:10.1016/s0191-8141(99)00092-9

Cited by 22 publications

(17 citation statements)

References 26 publications

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“…There are different geodynamic settings in which the X-axis of the finite strain ellipsoid is parallel to the fold axis. Examples where deformation structures might result from bulk plane-strain with the Y-axis perpendicular to the layer are: (1) folding during crustal extension in rocks of the Austroalpine nappes in eastern Switzerland (Froitzheim 1992); (2) folding in eclogite facies rocks of the Ile de Groix (Shelley and Bossie`re 1999) and (3) folding and boudinage of rocks of the Phyllite-Quartzite unit of Crete during subduction . In order to verify the present assumption that these structures do really result from bulk plane-strain with the Y-axis perpendicular to the competent layer, the amount of layer thickening should be constrained using common techniques of strain analysis.…”

Section: Interpretation and Discussionmentioning

confidence: 99%

Coeval folding and boudinage under plane strain with the axis of no change perpendicular to the layer

Mengong

Zulauf

2005

Int J Earth Sci (Geol Rundsch)

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Plane-strain coaxial deformation of a competent plasticine layer embedded in an incompetent plasticine matrix was carried out to improve our understanding about the evolution of folds and boudins if the layer is oriented perpendicular to the Y-axis of the finite strain ellipsoid. The rock analogues used were Beck's green plasticine (matrix) and Beck's black plasticine (competent layer), both of which are strain-rate softening modelling materials with a stress exponent n=ca. 8. The effective viscosity g of the matrix plasticine was changed by adding different amounts of oil to the original plasticine. At a strain rate _ e of 10 À3 s À1 and a finite strain e of 10%, the effective viscosity of the matrix ranges from 1.2·10 6 to 7.2·10 6 Pa s. The effective viscosity of the competent layer has been determined as 4.2·10 7 Pa s. If the viscosity ratio is large (ca. 20) and the initial thickness of the competent layer is small, both folds and boudins develop simultaneously. Although the growth rate of the folds seems to be higher than the growth rate of the boudins, the wavelength of both structures is approximately the same as is suggested by analytical solutions. A further unexpected, but characteristic, aspect of the deformed competent layer is a significant increase in thickness, which can be used to distinguish plane-strain folds and boudins from constrictional folds and boudins.

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Section: Interpretation and Discussionmentioning

confidence: 99%

Coeval folding and boudinage under plane strain with the axis of no change perpendicular to the layer

Mengong

Zulauf

2005

Int J Earth Sci (Geol Rundsch)

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“…Thus, according to Robardet (2000), the faunal relationships between all the Gondwanan areas now represented by terranes in Armorica and southern France are too close for an ocean to have been between them. As we pointed out in Shelley and Bossière (2000), the volcanic arcs , the thrusting on east-west lines (Brun and Burg, 1982), and the subduction to >70 km depth followed by rapid exhumation (Shelley and Bossière, 1999), are all evident in South Armorica, but it seems impossible to explain them in the general setting of stable platform sedimentation. Eastwest thrusting and extension are also difµcult to reconcile with the opening and closing of an ocean at a high angle to that direction.…”

Section: Oceanic Terranes: Fragments Of the Rheic Ocean Or Oceanic Sumentioning

confidence: 86%

Megadisplacements and the Hercynian orogen of Gondwanan France and Iberia

Shelley¹,

Bossière²

2002

Variscan-Appalachian Dynamics: The Building of the Late Paleozoic Basement

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A model is presented for the formation of the principal Iberian and Armorican fault-bound terranes during the Hercynian orogeny, and their displacement and shuf×ing along an anastomosing complex of dextral shears along the northwest African boundary of Gondwana. Displacements are thousands of kilometers, and relate to movement of Laurentia around Gondwana and formation of Pangea. The terranes in Brittany seem reversed with respect to Gondwana, with the most proximal Paleozoic shelf sediments in the north and oceanic terranes in the south (though Gondwana lay to the south). This may be explained by movements around extensional and contractional bends on the dextral faults. The terranes of Brittany may have been sheared around Iberia along the Porto-Tomar shear zone with Iberia a promontory on the broad Gondwanan shelf, southwest of its present position. The BadajozCórdoba shear zone, a Precambrian suture, formed the southwestern edge of protoIberia, oblique to the Hercynian dextral shears, and it became a zone of sinistral bookshelf-type faulting, underthrusting, and high-pressure metamorphism as displacements were transferred from one dextral shear to another. Similarly, underthrusting and sinistral shearing occurred along the Léon terrane-Northern terrane boundary in France. The µnal Hercynian development of Iberia-Armorica is related to a 90° Permian swing in the shortening direction when the µnal bending of the IberoArmorican arc was imposed together with dextral shearing along the northern and southern coasts of present-day Iberia. The principal purpose of this paper is to show that such a model is reasonably possible.

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“…10a,b). Higher maximal pressures exceeding 20 kbar at 500 C for zone-I rocks have been estimated by equilibria involving garnet, clinopyroxene, rutile, titanite and phengite (Barrientos 1992;Müller et al 1995;Shelley and Bossire 1999). However, mineral±chemical compositions and related maximal P±T conditions are similar in the three samples (Amer, Que, PoMo) from different locations.…”

Section: Discussionmentioning

confidence: 96%

“…Thermobarometry performed from divariant assemblages involving zoned amphiboles appears to be the only method to reconstruct detailed prograde and retrograde P±T variations within such stability fields. Shelley and Bossire (1999) discussed the significance of a complete breakdown of lawsonite to pseudomorph aggregates of white mica, chlorite, albite, quartz and epidote group minerals. Several generations of these pseudomorphs can be controlled by structural criteria.…”

Section: Discussionmentioning

confidence: 99%

“…S1 and L1 structures are parallel in metabasites and metapelites. Shear-sense criteria from inclusion trails in garnets and C' planes indicates a top-to-NW sense of shear during D1 (Quinquis and Choukroune 1981); however, this latter observation is not solitary because Shelley and Bossire (1999) reported top-to-SE shear senses as equally important. Deformation episode D2 is represented by millimetre-to metre-scale upright open folds F2 with axes mostly parallel to L1.…”

Section: Regional Geologymentioning

confidence: 92%

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Two-stage prograde and retrograde Variscan metamorphism of glaucophane-eclogites, blueschists and greenschists from Ile de Groix (Brittany, France)

Schulz

Triboulet

Audren

et al. 2001

International Journal of Earth Sciences

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Two-stage prograde and retrograde Variscan metamorphism of glaucophane-eclogites, blueschists and greenschists from Ile de Groix (Brittany, France) Abstract On Ile de Groix, Variscan metamorphic former tholeiitic and alkaline basalts occur as glaucophane-eclogites, blueschists and greenschists in isolated lenses and layers within metapelites. Whole-rock d 18 O SMOW values of the metabasites show limited variations (10.4±12.0½) and no systematic differences among rock types and metamorphic grades. This provides no argument for large-scale blueschist-to-greenschist transformation driven by infiltration of externally derived fluids. Metamorphic mineralogical changes should have been triggered by internal fluids. Element variations in interlayered blue-and greenschists can be attributed to magmatic fractionation. Assemblages with garnet, clinopyroxene and glaucophane of a high-pressure/low-temperature (HP±LT) metamorphism M1, and NaCa-amphiboles (barroisite, magnesiohornblende, actinolite) of a medium-pressure/medium-temperature metamorphism M2 crystallized during deformation D1. Detailed core-rim zonation profiles display increasing and then decreasing Al IV in glaucophane of M1. NaCa-amphiboles of M2, mantling glaucophane and crystallized in porphyroblasts, show first increasing, then decreasing, Al IV and Al VI . Empirically calibrated thermobarometers allowed P±T path reconstructions. In glaucophane-eclogites of a metamorphic zone I, a prograde evolution to M1 peak conditions at 400±500 C/10±12 kbar was followed by a retrograde P±T path within the glaucophane stability field. The subsequent M2 evolution was again prograde up to >600 C at 8 kbar and then retrograde. Similarly, in metamorphic zones II and III, prograde and retrograde paths of M1 and M2 at lower maximal temperatures and pressures exist. The almost complete metamorphic cycle during M2 signalizes that the HP±LT rocks escaped from an early erosion by a moderate second burial event and explains the longlasting slow uplift with low average cooling rates.Keywords P±T paths´High-pressure metamorphismÁ mphibole mineral chemistry´Amphibole thermobarometry´Oxygen isotopes´Uplift´South Armorican Domain´Variscan orogeny B. Schulz ( ) )

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Ile de Groix: retrogression and structural developments in an extensional régime

Cited by 22 publications

References 26 publications

Coeval folding and boudinage under plane strain with the axis of no change perpendicular to the layer

Coeval folding and boudinage under plane strain with the axis of no change perpendicular to the layer

Megadisplacements and the Hercynian orogen of Gondwanan France and Iberia

Two-stage prograde and retrograde Variscan metamorphism of glaucophane-eclogites, blueschists and greenschists from Ile de Groix (Brittany, France)

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