La tectonique cisaillante polyphasee du Sud Limousin (Massif central francais) et son interpretation dans un modele d'evolution polycyclique de la chaine hercynienne

2000 ◽  
Vol 171 (3) ◽  
pp. 295-307 ◽  
Author(s):  
Jean-Yves Roig ◽  
Michel Faure
Keyword(s):  
Middle Devonian ◽  
Early Carboniferous ◽  
Evolution Model ◽  
Ductile Deformation ◽  
Late Devonian ◽  
French Massif Central ◽  
The South ◽  
Massif Central ◽  
Mafic Rocks

Abstract Structural, kinematics and thermo-barometric analyses of the ductile deformation of the south-Limousin metamorphic formations show a polyphase shear tectonics corresponding to two different thrusting events. The older one, is a to the top-to-the-SW thrusting during middle Devonian. This deformation occurs under minimum PT conditions of 7 Kbar/700 degrees C simultaneously to anatexis. The second event is a top-to-the-NW shearing which occurred in late Devonian-early Carboniferous under Barrovian conditions (5 kbar/600 degrees C). Diorites bodies and non-eclogitized mafic rocks allow us to argue for an extensional phase between the two thrusting events. These two ductile and syn-metamorphic deformations take place in a polycyclic evolution model of the Hercynian belt of the French Massif Central.

Successive shearing tectonics during the Hercynian collisional evolution of the southwestern French Massif Central

2004 ◽  
Vol 175 (1) ◽  
pp. 49-59 ◽  
Author(s):  
Manuel Duguet ◽  
Michel Faure
Keyword(s):  
Early Carboniferous ◽  
French Massif Central ◽  
The South ◽  
Medium Temperature ◽  
Massif Central ◽  
Ductile Shearing ◽  
Pressure Medium ◽  
Dextral Strike Slip Fault ◽  
Montagne Noire ◽  
Dextral Strike Slip

Abstract In the French Massif Central, the Devonian-Carboniferous tectonic evolution of the Rouergue-Albigeois area is characterized by three phases of low-angle ductile shearing. The first event D1, which occurred probably in the Lower Devonian, is responsible for the south-westward thrusting of the high metamorphic Lévézou nappe which belongs to the Upper Gneiss Unit above the Lower Gneiss Unit overlying itself the Para-autochthonous Unit, (locally called the St-Serninsur-Rance nappe). In the late Devonian-early Carboniferous, this stack of nappes is reworked by a second event, D2, characterized by a top-to-the-NW shearing of the Para-autochthonous Unit upon the Lower Gneiss Unit developed under medium pressure/medium temperature metamorphism. The contact between the Lower Gneiss Unit and the Para-autochthonous Unit is a top-to-the NW low-angle fault that progressively evolues into a dextral strike-slip fault from west to east. The D2 event is followed by a top-to-the south D3 thrusting dated around 330–340 Ma. The main feature of the D3 compressional stage is the emplacement of the whole stack of nappes previously structured by D1 and D2 events upon the Albigeois series. The D3 event produced south-verging recumbent folds in the Albigeois, Mont-de-Lacaune and Montagne Noire. The significance of the D2 event either as thrusting or normal faulting is discussed.

Geothermal Survey of the South-East Margin of the French Massif-Central

1980 ◽  
pp. 202-212
Author(s):  
B. Bosch ◽  
P. Degranges ◽  
J. Demange ◽  
M. Leleu ◽  
A. Marce ◽  
...  
Keyword(s):  
French Massif Central ◽  
The South ◽  
Massif Central

Monazite U-Th/Pb chemical dating of the Early Carboniferous syn-kinematic MP/MT metamorphism in the Variscan French Massif Central

2009 ◽  
Vol 180 (3) ◽  
pp. 283-292 ◽  
Author(s):  
Jérémie Melleton ◽  
Michel Faure ◽  
Alain Cocherie
Keyword(s):  
Electron Probe ◽  
Early Carboniferous ◽  
Variscan Belt ◽  
French Massif Central ◽  
Massif Central ◽  
Close Range ◽  
Chemical Dating

AbstractIn situ U-Th-Pb geochronology on monazite using Electron Probe Micro Analyser, constrained by structural and textural observations, has been performed on four samples from the Limousin area (northwest part of the French Massif Central) in order to date the syn-kinematic MP/MT metamorphism related to the top-to-the-NW shearing that deformed the stack of nappes in this zone of the Variscan belt. All the analyzed samples lead to a mean age at 360 ± 4 Ma. The close range of ages obtained during this study (360 Ma) and with the previous 40Ar-39Ar ones (360–350 Ma) suggests fast processes of cooling and exhumation during the Early Carboniferous in internal zones of the Variscan belt. The geodynamic significance of this Early Carboniferous event is discussed at the scale of the Ibero-Armorican orocline.

scholarly journals The South Millevaches Middle Carboniferous crustal melting and its place in the French Variscan belt

2009 ◽  
Vol 180 (6) ◽  
pp. 473-481 ◽  
Author(s):  
Michel Faure ◽  
Eugène Be Mezeme ◽  
Alain Cocherie ◽  
Jérémie Melleton ◽  
Philippe Rossi
Keyword(s):  
Crustal Melting ◽  
Variscan Belt ◽  
French Massif Central ◽  
The South ◽  
Massif Central ◽  
Rock Types ◽  
Fold And Thrust Belt ◽  
Three Samples ◽  
Middle Carboniferous ◽  
Montagne Noire

AbstractSeveral episodes of crustal melting are now well identified in the Variscan French Massif Central. Middle Devonian (ca 385-375 Ma) migmatites are recognized in the Upper and Lower Gneiss Units involved in the stack of nappes. Late Carboniferous migmatites (ca 300 Ma) are exposed in the Velay Massif only and Middle Carboniferous migmatites crop out in the Para-autochthonous Unit and southern Fold-and-Thrust Belt. In the SW part of the Massif Central, the South Millevaches massif exposes migmatites developed at the expense of ortho- and paragneiss. They form kilometer-sized septa within the foliated Goulles leucogranitic pluton, which is in turn intruded by the non-foliated Glény two micas granite pluton. Monazite grains extracted from these three rock-types have been dated by the EPMA chemical method. Three samples of migmatite yield a late Visean age (ca 337-328 Ma), the Goulles and Glény granitic plutons yield ages at 324-323 Ma and 324-318 Ma, respectively. These new results enlarge the evidence of a Middle Carboniferous crustal melting imprint that up to now was only reported in the eastern part of the French Massif Central, in the northern Cévennes and in the Montagne Noire axial zone. At the scale of the French Variscan massifs, the Visean crustal melting event is conspicuously developed since it is recognized from the Massif Armoricain (Vendée and south coast of Brittany) to the Central Vosges. This episode is synchronous with the huge thermal event responsible for the “Tuffs anthracifères” magmatism of the northern Massif Central and Vosges, and took place immediately after the last thickening phase recorded both in Montagne Noire and Ardennes, that is on the southern and northern outer zones of the Variscan Belt, respectively. However, the geodynamic significance of this major event is not fully understood yet.

Conditions et ages 40 Ar/ 39 Ar de mise en place des granitoides de la zone externe sud du Massif central francais; exemple des granodiorites de St-Guiral et du Liron (Cevennes, France)

2000 ◽  
Vol 171 (5) ◽  
pp. 495-510 ◽  
Author(s):  
Khalid Najoui ◽  
Andre Francois Leyreloup ◽  
Patrick Monie
Keyword(s):  
Gravitational Collapse ◽  
Regional Metamorphism ◽  
Low Grade ◽  
French Massif Central ◽  
The South ◽  
Massif Central ◽  
Laser Probe ◽  
Thermal Structures ◽  
Minimum Age ◽  
Contact Aureoles

Abstract The St-Guiral and the Liron laccolithic granodiorites outcropping in the southeastern area of the French Massif Central intrude the Cambro-Ordovician low grade series of the western Cevennes. The petrological study of their contact aureoles shows that these plutons emplaced at low depth (1-2 kb; 690-700 degrees C). The laser probe 40 Ar/ 39 Ar allows us to suggest a minimum age of 324 Ma for the emplacement of the St-Guiral granodiorite. This emplacement seems synchronous with the LP-HT regional metamorphism in the western Cevennes for which the cooling ages are identical. The Liron granodiorite (310 Ma) is younger and crosscuts the thermal structures of the LP-HT regional metamorphism. Accordingly these plutons emplaced at the Namurian-early Westphalian during the extensional phases related to the generalised gravitational collapse of the previously thickened Hercynian belt. The detachment of the subducted lithosphere (slab detachment) could be related to these phenomena and could explain the granitization of the south external area of the Hercynian belt as well as the migration of the magmatism towards the south of the belt.

40Ar/39Ar dating in the Lochaber–Mulgrave area, northern mainland Nova Scotia: implications for timing of regional metamorphism and sediment provenance in the Late Devonian – Early Carboniferous Horton Group

2004 ◽  
Vol 41 (8) ◽  
pp. 987-996 ◽  
Author(s):  
P H Reynolds ◽  
S M Barr ◽  
C E White ◽  
P J Ténière
Keyword(s):  
Nova Scotia ◽  
Regional Metamorphism ◽  
Early Carboniferous ◽  
Fault System ◽  
Sediment Provenance ◽  
Late Devonian ◽  
The South ◽  
Two Samples ◽  
Meguma Terrane ◽  
Fusion Analysis

40Ar/39Ar dating of whole-rock samples and muscovite separates using age spectrum analysis, and of single muscovite grains using total fusion analysis, yields new insights into the timing of regional metamorphism and sediment provenance in the Late Devonian – Early Carboniferous Horton Group in the Lochaber–Mulgrave area of Nova Scotia. The time of regional metamorphism is constrained to ca. 340–335 Ma by whole-rock spectra from well-cleaved slate and shale samples from the lowermost Clam Harbour River and overlying Tracadie Road formations of the Horton Group. This ca. 340–335 Ma event may have been the result of burial and deformation of the Horton Group by older volcanic and sedimentary rocks of the Guysborough Group, which were overthrust from the south as the result of development of a positive flower structure at a restraining bend along the Cobequid–Chedabucto fault system, the boundary between the Meguma and Avalon terranes. Detrital muscovite ages of ca. 410–380 and ca. 500 Ma were obtained from single-grain analysis and from spectral analysis of separated grains. Whole-rock spectra for two samples from a mylonitic metasedimentary unit in the Cape Porcupine Complex yielded plateau ages of 364 ± 4 and 367 ± 4 Ma, providing a likely source for ca. 370–360 Ma detrital muscovite, ages that may be reflected in some of the age spectrum data. However, the Meguma terrane to the south is the most likely source for most of the detrital muscovite.

The Middle to Late Devonian Høybakken detachment, central Norway: 40Ar–39Ar evidence for prolonged late/post-Scandian extension and uplift

2004 ◽  
Vol 141 (3) ◽  
pp. 329-344 ◽  
Author(s):  
M. A. KENDRICK ◽  
E. A. EIDE ◽  
D. ROBERTS ◽  
P. T. OSMUNDSEN
Keyword(s):  
Middle Devonian ◽  
Early Carboniferous ◽  
White Mica ◽  
Late Devonian ◽  
Brittle Deformation ◽  
Southwestern Part ◽  
Early Devonian ◽  
Southern Margin

The regionally significant 0.5–2 km thick Høybakken detachment in central Norway bounds the southern margin of the Central Norway Basement Window and exhibits a well-developed top-to-the-WSW fabric characteristic of late Scandian, Devonian ductile extension. 40Ar–39Ar data obtained from hornblende, mica and K-feldspar mineral separates of rocks collected in a transect through the Høybakken detachment yield well-defined plateau and isochron mineral ages. Early Devonian exhumation and cooling of the Høybakken detachment footwall is recorded by hornblende ages of ∼ 400 Ma and mica ages of ∼ 390 Ma. The mylonitic fabric overlying the footwall records younger Middle Devonian mica crystallization ages of 384–381 Ma that are among the youngest extensional ductile fabrics dated in the Caledonides and suggest prolonged extensional activity on the Høybakken detachment. After inferred cessation of ductile extension at 381 Ma, the rate of uplift and cooling was reduced, and the footwall records Late Devonian to Early Carboniferous K-feldspar ages of 371–356 Ma. Prolonged extensional activity at Høybakken is compatible with recent U–Pb ages of deformed titanite crystals and established Rb–Sr ages of white mica in shear-related pegmatites, both from the southwestern part of the Fosen Peninsula, and 40Ar–39Ar ages of syn-tectonic mica overgrowth from the adjacent Hitra–Snåsa Fault. Together, these ages suggest the onset of ductile extension soon after ∼ 401 Ma, and with the Middle Devonian crystallization ages determined here, suggest that ductile extension on the Høybakken detachment had a duration of 11–20 Ma. The youngest age of 320 Ma was obtained from a K-feldspar in a cataclastic granite of the Høybakken detachment's hangingwall and is considered to date a phase of post-Scandian brittle deformation that overprinted the mylonitic shear fabric.

EMP-monazite age controls on P-T paths of garnet metapelites in the Variscan inverted metamorphic sequence of La Sioule, French Massif Central

2009 ◽  
Vol 180 (3) ◽  
pp. 271-282 ◽  
Author(s):  
Bernhard Schulz
Keyword(s):  
Early Carboniferous ◽  
Garnet Growth ◽  
Variscan Orogeny ◽  
Structural Domain ◽  
French Massif Central ◽  
Tectonic Event ◽  
Massif Central ◽  
Individual Character ◽  
The Individual ◽  
Low Pressures

Abstract In the La Sioule region (northern French Massif Central), a Variscan inverted metamorphic sequence is made up by high-grade Upper Gneiss (UGU) and Lower Gneiss Units (LGU) which overlie amphibolite-facies micaschists of a Parautochthonous Unit (PAU). Growth-zoned garnets crystallized in gneisses and micaschists and display different Mg-Fe-Mn-Ca evolution trends in the metamorphic units. Microstructurally-controlled geothermobarometry based on cation exchange and net transfer reactions was used to reconstruct syndeformational prograde P-T paths. The P-T paths do not pass eclogite-facies conditions. Different metamorphic peak temperatures confirm the individual character of the evolution in each of the units. EMP-monazite dating (CHIME method) was used to evaluate the age of the P-T-evolution in the garnet-bearing samples. Monazite ages compiled from numerous matrix grains range between 327 and 343 Ma in single samples. Unimodal distributions of ages signalize only slightly younger (320–330 Ma) monazites in the UGU as in the LGU and PAU (330–340 Ma). The inverted metamorphic stratigraphy can be explained by contractional crustal tectonics. In a northern structural domain this involved a top-to-WNW shearing followed by a first thrusting event at the end of garnet growth in UGU and PAU. In a southern domain, the nappe pile was overprinted by a second event with top-to-SE shearing at low pressures. The bulk of monazites should postdate garnet crystallization and the first thrusting event observed in UGU and PAU. At best the ~340 Ma monazites in the LGU to the south may have crystallized during the second tectonic event. The bulk monazite crystallization appears to be related to a Late-Visean thermal event when the three units were already juxtaposed in a nappe pile. In consequence, metapelite garnet and monazite crystallization in the La Sioule series belong to a Late Devonian – Early Carboniferous prograde-retrograde metamorphism. This metamorphic evolution appears as a second metamorphic cycle subsequent to the Silurian – Early Devonian HP and HT events within the complex frame of the Variscan orogeny in the French Massif Central.

Silurian and Devonian base maps

1985 ◽  
Vol 309 (1138) ◽  
pp. 57-77 ◽  
Keyword(s):  
Great Britain ◽  
North America ◽  
Middle Devonian ◽  
Northern Europe ◽  
Late Devonian ◽  
The South ◽  
Early Devonian ◽  
Red Sandstone ◽  
Palaeomagnetic Data

During the Silurian and Devonian, the sequence of continental collisions that were ultimately to result in the formation of the supercontinent of Pangaea had begun. By the Early to Middle Devonian North America (Laurentia), Acadia, Great Britain, and Northern Europe (Baltica) had collided to form the ‘Old Red Sandstone’ continent (Laurussia). Palaeomagnetic data, however, indicate that the configuration of the continents that made up Laurussia did not resemble the pre-breakup, Mesozoic reassembly. Rather, Britain, Baltica, and Acadia were displaced 10—20° to the south with respect to Laurentia. New palaeomagnetic data for Laurentia and Gondwana, suggests that the ocean separating the northern and southern continents was relatively narrow during the early Devonian, and may have been nearly closed by the late Devonian.

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