Granitoid emplacement during a thrusting event: structural analysis, microstructure and quartz c-axis patterns. An example from Hercynian plutons in the French Massif Central (original) (raw)
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Geodinamica Acta, 2002
In the southwestern area of the Belledonne massif (external crystalline massifs, French western Alps), the finite strain pattern reflects the geometry and kinematics of Late Devonian-Late Carboniferous deformational events. Previous works suggested that the nappe stacking occurred during Late Devono-Dinantian times. New observations allow us to constrain the subsequent tectono-metamorphic evolution of the various stacked units. Westward thrusting is responsible for the early dismembering of the Rioupéroux-Livet plutono-volcanic units. Micaschist slices have also been inserted within the Rioupéroux-Livet units during this Dinantian tectonics, and have recorded metamorphic conditions of about 7-8 kbar, 600-650°C. The local NW direction of shortening is similar to the overall shortening direction recorded at this time in the Hercynian belt of western Europe. During the Late Visean, the Early Paleozoic Chamrousse ophiolite was thrust towards the northeast on top of the Rioupéroux-Livet units. The inverted geometry of the Chamrousse ophiolite may be interpreted as a consequence of this tectonics. Northeastward thrusting, also observed in the northeastern part of the French Massif Central is interpreted as the result of lateral escape in a global convergent context. Finally, the Late Carboniferous period is characterized by two extensional events: one leading to southwest dipping low-angle extensional shear zones, during the Westphalian, and a second one during the Stephanian. This latter is characterized by ductile/brittle normal faults, with a northwest-southeast direction of extension and related to the gravitational collapse of the whole Hercynian belt.
The S. Lucia Nappe (Alpine Corsica, France) is located in the Corte area. This tectonic unit recorded a polyphase Alpine evolution developed under lower greenschist facies conditions. In addition, the basement recorded an older (i.e. late-Paleozoic) tectono-metamorphic history, which mainly developed under granulite to amphibolite facies conditions. The S. Lucia basement represents an example of Ivrea-type lower crustal section, including a segment of a Km-scale granulite-facies shear zone made up of sheared Permian gabbros and enclosed metasediments. The occurrence of a thick (i.e. at least 1 Km-wide) high-grade shear zone indicates that the late-Palaeozoic deformation in the lower crust was homogeneously distributed on a large scale. The non-coaxial character of the deformation and the invariable sinistral shear sense, generalized along the whole section, recalls the models of uniformsense, crustal-scale normal shear zones. Two distinct granitoid plutons were emplaced at the margins of the S. Lucia shear zone. Field relations, microstructures, quartz LPO fabric and mineral chemistry data document the syntectonic emplacement respect to the shear zone. Indeed, both plutons experienced a nearly continuum down-temperature deformation, during emplacement and subsequent syndeformational cooling. The observed structural style suggests that the granitoids probably accommodated large strains through magmatic to submagmatic flow. In both cases, the pattern of magmatic and subsequent solid-state fabric was strongly controlled by the stress field imposed by the S. Lucia shear zone. Moreover, the bulk composition of the two plutons likely played a fundamental role in the tectonic evolution, influencing the structural style and the spatial distribution of the solid-state overprint. Finally, the occurrence of magmas along the shear zone may have promoted strain partitioning within the melt-rich areas, allowing the exhumation and the preservation of granulite-facies rocks. The Mesozoic syn-rift exhumation of the S. Lucia basement is suggested by the occurrence of undeformed MORB-type dolerites, by the Middle Jurassic thermal anomaly and by the Cretaceous sedimentary cover. However, although the P-T-t evolution of the S. Lucia section is only partly constrained, several lines of evidence suggest that the Permian granulites had already been incorporated into the upper crust prior to the Mesozoic. Several paleogeographic reconstructions propose that, in the Alpine region, Permian granulites were intimately associated with the Mesozoic ophiolites at the ocean-continent transition. However, the case study proposed here points out that the occurrence of granulites alone cannot be taken as an indication of a syn-rift exhumation of the lower crust along the continental margin.
International Journal of Earth Sciences, 2001
This microstructural and anisotropy of magnetic susceptibility study of the internal structures of the Hercynian NØouvielle granite pluton (100 km 2) provides new data indicating that the pluton was emplaced during the main Hercynian tectonic event recognized in the Pyrenees. It also provides new data about the later Alpine deformation localized along narrow mylonitic bands. These bands acted as reverse faults and have not rotated the Hercynian structures that define the main part of the pluton. The pluton is composed of two structural domains: the northern half of the pluton displays a beak shape in map view, with subhorizontal E±W trending lineations of magmatic origin; the southern half is semi-circular and displays rather steeply northward plunging lineations corresponding to magmatic and high temperature (HT) solid-state microstructures. These features are associated with magma deformation during emplacement. Magma deformation corresponds, in the northern half of the pluton, to an E±W strike-slip deformation recognized in the enveloping pelitic metasediments of Carboniferous age and, in the southern half of the pluton, to southward overthrusting recognized in the enveloping quartzites of Devonian age. Juxtaposition in a single granite body of transcurrent and compressive domains is viewed as a strain partitioning in the magma. This strain partitioning is linked to both the transpressive character of the main regional deformation event and the rheological contrast between the pelitic country rocks and quartzose country rocks.
Journal of Structural Geology, 2009
Magnetic fabric analyses, gravity inversion associated with chemical U-Th-Pb dating and structural observations are carried out to elucidate relationships between faulting and magmatic processes. This multidisciplinary study has been undertaken on Late Carboniferous plutons, situated in the southern part of the Variscan French Massif Central (FMC). The Glénat, Omps and Boisset plutons crop out on both sides of the crustal-scale Sillon Houiller Fault (SHF). The Anisotropy of Magnetic Susceptibility (AMS) measurements and structural observations show that (i) the plutons acquired their final structure during the magma crystallization and record a NW-SE maximum stretching trend; (ii) in the Boisset pluton, post magmatic fabrics predominate with a
Geophysical Journal International, 2000
Thermomagnetic experiments, isothermal remanent magnetization (IRM), hysteresis loops, X-ray reflection analyses, optic microscopic observations, bulk magnetic susceptibility and natural remanent magnetization (NRM) show that the susceptibility signal of 56 sites in the Pont-de-Montvert-Borne pluton is derived mostly from biotite with a very small proportion of ferromagnetic material. Low anisotropy degree (P parameter) and consistent AMS orientations among monzonite, enclaves and aplitic dykes indicate that the AMS was acquired during pluton emplacement in the subsolidus phase. Magnetic fabrics demonstrate that linear deformation is prominent in the area where the granite is in direct contact with micaschists, and that planar deformation becomes more important in the area where the pluton is surrounded by non-porphyritic peraluminous granites. AMS measurements also show that shallow plunging E-Wstretching lineations are the dominant structure over most of the studied area. Foliation orientations follow the pluton contour in the western part, indicating that the Pontde-Montvert-Borne pluton is probably rooted in the west and extruded towards the east. These AMS results agree to some extent with fabric inferred from mineral preferred orientation. The AMS data support an E-W extensional tectonic setting during the Pont-de-Montvert-Borne pluton emplacement, which resulted from the Late Carboniferous crustal thinning of the Hercynian orogeny.
International Journal of Earth Sciences, 2007
This work establishes the relative timing of pluton emplacement and regional deformation from new dating and structural data. (1) Monazite and (2) zircon dating show Tournaisian ages for the Guéret granites [Aulon granite 352 ± 5 Ma (1), 351 ± 5 Ma (2) and Villatange tonalite 353 ± 6 Ma (1)] and Viseo-Namurian ages for the north Millevaches granites [Chavanat granite 336 ± 4 Ma (1), Goutelle granite 336 ± 3 Ma (1), Royère granite 323 ± 2 Ma (1) and 328 ± 6 Ma (2), Courcelles granite 318 ± 3 Ma (1)]. The Guéret and Millevaches granites are separated by the N110 Arrènes–la Courtine Shear Zone (ACSZ), composed from West to East by the Arrènes Fault (AF), the North Millevaches Shear Zone (NMSZ) and the la Courtine Shear Zone (CSZ), respectively. Tournaisian Guéret granites experienced a non-coaxial dextral shearing (NMSZ) recorded by the Villatange granite while the Aulon granite (Guéret granite) cuts across this dextral shear zone which thus stopped shearing during Tournaisian time. Visean to Namurian Millevaches granites experienced a coaxial deformation. Therefore, low displacements along the NMSZ and the CSZ occurred at the emplacement time of Chavanat and Pontarion-Royère granites (336–323 Ma). The structural analyses of Goutelle granite emphasizes a deformation related to the dextral Creuse Fault System (CFS) oriented N150–N160. From 360 to 300 Ma, the Z strain axis is always horizontal inferring a wrench setting for these granite emplacements. During this tectonic evolution, the Argentat zone acted as a minor normal fault and is related with a local Middle Visean (340–335 Ma) syn-orogenic extension on the western border of the Millevaches massif.