Variscan thrust nappes, detachments, and strike-slip faults in the French Massif Central: Interpretation of the lineations

2007 ◽  
pp. 391-402 ◽  
Author(s):  
Philippe Matte
Keyword(s):  
Strike Slip ◽  
French Massif Central ◽  
Massif Central

Structure and emplacement in a transcurrent regime and late magmatic rotation of the Variscan granite plutons of Veinazès and Marcolès (Châtaigneraie, French Massif Central)

2002 ◽  
Vol 173 (3) ◽  
pp. 207-218 ◽  
Author(s):  
Philippe Olivier ◽  
Laurent Améglio
Keyword(s):  
Magnetic Susceptibility ◽  
Strike Slip ◽  
French Massif Central ◽  
Good Correspondence ◽  
Plan View ◽  
Massif Central ◽  
North East ◽  
The North ◽  
South West ◽  
The Mean

Abstract Introduction. – The Variscan basement of the French Massif Central is considered [Faure, 1995] to have suffered (i) a southwestward nappe stacking during the early Carboniferous ; (ii) a NW-SE trending extension during the late Visean and granite emplacement during Namurian and Westphalian time ; (iii) a NE-SW trending extension during late Carboniferous-early Permian. The structure of the Veinazès and Marcolès monzogranitic plutons, located in the Châtaigneraie (southern French Massif Central), is studied in this paper through the anisotropy of magnetic susceptibility technique (AMS) to better understand the effects of the Namurian-Westphalian phase which is poorly documented in this region. Magnetic and structural study. – The long axis K1 of the AMS ellipsoid represents the magnetic lineation and the short axis K3 is normal to the magnetic foliation. This magnetic fabric may be equated with the mineral fabric. Km, the mean magnetic susceptibility, corresponds to the arithmetic mean (K1+K2+K3)/3. Pp %, the magnetic anisotropy ratio, corresponds to ((K1 – D/K3 – D) – 1) × 100, D being the diamagnetic component. Magnetic susceptibility (K) values (fig. 3 ; table I) range from 3.5 to 18.9 × 10−5 SI (mean 12.3 × 10−5 SI) for Veinazès, and from 0.7 to 13.1 x 10−5 SI (mean 8.1 × 10−5 SI) for Marcolès. Such values are typical of dominantly paramagnetic rocks. This is confirmed by the very good correspondence between these measured values and the magnetic susceptibilities calculated from sites for which chemical analyses are available (table II). For Veinazès the highest K values are mainly located in the western part of the pluton, whereas the weakest are in the northeastern part (fig. 3). This corresponds roughly to the distribution of the main petrographic facies determined by Joubert [1978] and Bogdanoff et al. [1989a]. The anisotropy of the magnetic susceptibility ratio (Pp %) (fig. 4 ; table I) ranges from 0.9 % to 5.3 % (mean 2.2 %) for Veinazès and from 0.8 % to 4 % (mean 2.2 %) for Marcolès. The part of the Veinazès pluton located to the south-west of the Sansac-Montsalvy line displays a mean anisotropy (2.7 %) clearly higher than the northeastern part of the pluton (1.6 %). For Marcolès, the map of Pp % does not display significant distribution heterogeneities. Three types of microstructures were determined in thin-sections in the Veinazès and Marcolès granites (fig. 5) : 1–undeformed magmatic textures ; 2– late magmatic weakly deformed textures ; 3– late magmatic deformed textures. There is a good correspondence between the mapped distribution of these microstructures (fig. 6) and the Pp % values (fig. 4). Magnetic foliations (fig. 7 & 8 ; table I). Veinazès : to the north-east of the Sansac-Montsalvy line, the foliations are roughly organized in a dome. To the south-west of this line, the foliations are mainly WNW-ESE trending with a medium to steep northward dip. Near the borders of the pluton the foliations become WSW-ENE. Marcolès : the foliations are concentrated around a NW-SE trending mean direction, with generally steep (≥ 60o) and northeast dips. Near the extremities of the pluton the foliations are closer to an E-W trend. In both plutons the pattern of foliations tends to form Z-sigmoids. Magnetic lineations (fig. 7 & 9 ; table I). Veinazès : to the north-east of the pluton, the lineations display generally shallow plunges and N-S trending directions. In the central part of the pluton they display very shallow plunges and are mainly E-W trending. To the west, the lineations display shallow to steep plunges mainly trending either E-W or NW-SE. Marcolès : most lineations are NW-SE to E-W trending, with middle SE or eastward plunges. These lineations form a Z-sigmoid pattern in plan view. Discussion and interpretation. – The shallow plunging lineations carried by rather steep foliations and the sigmoidal features observed in plan view, show that most magmatic fabrics of both plutons may correspond to strike-slip movements. Moreover, the organization of the main cleavage in the country rocks around the Veinazès pluton could correspond to an E-W-trending left-lateral transcurrent zone (fig. 1). In the western part of the Châtaigneraie, the mean direction of the cleavage becomes N150oE-165oE, with eastward middle to steep dips, indicating a clockwise rotation with respect to the regional trend. Dextral shearings are linked to this NNW-SSE band. Gravimetric data indicate that the Veinazès pluton is characterized by a rather flat floor and a single zone of relative deepening (2.5 km) between Ladinhac and Sansac-Veinazès (fig. 10), interpreted as a possible root. For the Marcolès pluton the maximum depth (1.25 km) is located beneath the village of Marcolès. We propose the following model (fig. 11) for the emplacement of the Veinazès and Marcolès plutons during the late Namurian-early Stephanian time-span: initial opening of the crust corresponding to the present deepest parts of the plutons floor, led by ENE-WSW-trending left-lateral strike-slip faults (fig. 11a) ; magmatic ascent and accretion of the plutons (fig. 11b) ; about 40o clockwise rotation of the Marcolès pluton, the SW end of the Veinazès pluton and their country rocks, linked to a NNW-SSE right-lateral strike-slip band (fig. 11c). We consider that these events predate the low temperature sinistral movements along the NNE-SSW-trending Sillon Houiller during the Stephanian time. Conclusion. – The magmatic fabrics measured in the Veinazès and Marcolès plutons, combined with previous data from the country rocks, show the probable importance of transcurrent movements for the emplacement of these plutons. The rotation suffered by the western part of this region after granites were emplaced would need to be better contrained by palaeomagnetic measurements on the Marcolès pluton.

Famenno-Carboniferous (370-320 Ma) strike slip tectonics monitored by syn-kinematic plutons in the French Variscan belt (Massif Armoricain and French Massif Central)

2009 ◽  
Vol 180 (3) ◽  
pp. 231-246 ◽  
Author(s):  
Patrick Rolin ◽  
Didier Marquer ◽  
Michel Colchen ◽  
Charles Cartannaz ◽  
Alain Cocherie ◽  
...  
Keyword(s):  
Large Scale ◽  
Western Europe ◽  
Shear Zones ◽  
Strike Slip ◽  
Variscan Belt ◽  
French Massif Central ◽  
Massif Central ◽  
Granite Emplacement ◽  
Regional Deformation ◽  
Dextral Shear

AbstractThe Variscan continental collision has led to the development of large strike-slip shear zones in western Europe. Our study focuses on the regional deformation and shear zone patterns in the Massif Armoricain and the French Massif Central. The synthesis of granite emplacement ages associated to granite deformation fields, allow us to propose a geodynamic model for the tectonic evolution of this part of the Variscan belt between 370 Ma – 320 Ma (Late Devonian – Namurian).After the first steps of the continental subduction-collision, leading to high temperature and anatexis associated with N-S shortening at 380-370 Ma (Frasnian to Famennian), the southern part of the Massif Armoricain and western part of French Massif Central underwent large dextral shearing along N100-N130 trending shear zones up to early Visean time. These large-scale displacements progressively decreased at around 350-340 Ma, during the first emplacements of biotite bearing granites (Moulins-les Aubiers-Gourgé massif and Guéret massif intrusions).During middle Visean times, the shortening axis direction rotated towards a NNE-SSW direction implying changes in the regional deformation field. The occurrence of N070-N100 sinistral and N110-N130 dextral conjugate shear zones within leucogranites are related to that time. Finally, new N150-N160 dextral shear zones appeared in middle to late Visean times: as for examples, the Parthenay and the Pradines shear zones in the SE Massif Armoricain and the Millevaches massif, respectively. These shear zones were conjugated to the sinistral N020 Sillon Houiller in the French Massif Central. They reflect large scale brittle continental indentation in the French Variscan belt during the middle to late Visean.

scholarly journals Potassic late orogenic Stephanian volcanism in the Southwest French Massif central (Decazeville, Figeac, Lacapelle-Marival basins): an example for mantle metasomatism along strike-slip faults?

2008 ◽  
Vol 179 (5) ◽  
pp. 491-502 ◽  
Author(s):  
Henriette Lapierre ◽  
Christophe Basile ◽  
Thomas Berly ◽  
Emmanuel Canard
Keyword(s):  
Partial Melting ◽  
Melting Process ◽  
Horizontal Displacement ◽  
Mantle Metasomatism ◽  
Strike Slip ◽  
Primitive Mantle ◽  
French Massif Central ◽  
Southwestern Part ◽  
Massif Central ◽  
Ree Patterns

AbstractIn the southwestern part of the French Massif central (Decazeville basin, at the Sillon Houiller fault termination; Figeac and Lacapelle-Marival basins along the Argentat fault), Stephanian volcanism exhibits shoshonitic affinities. Their chondrite-normalized rare earth element (REE) patterns are enriched in light REE, but almost flat for heavy REE, with marked negative Eu anomalies. Primitive mantle-normalized element spectra show negative Nb, Ta, P, Sm, Ti, and positive Th, U, Pb anomalies, respectively. εNd values are negative and homogeneous (−6 to −4). This volcanism shares the same geochemical patterns as the late-orogenic Stephanian-Permian magmatism from the southern part of the Variscan belt (Pyrénées, Alps, Sardinia). We explain these geochemical characteristics as resulting from the partial melting of a metasomatised mantle. We propose a new mechanism to explain this melting process: horizontal displacement along the main late-orogenic strike-slip faults might bring into contact a hydrated lower crust with the lithospheric mantle. Mantle metasomatism within the strike-slip fault zone may then induce partial melting.

Carboniferous magmatism related to progressive pull-apart opening in the western French Massif Central

2014 ◽  
Vol 185 (3) ◽  
pp. 171-189 ◽  
Author(s):  
Patrick Rolin ◽  
Didier Marquer ◽  
Charles Cartannaz ◽  
Philippe Rossi
Keyword(s):  
Shear Zones ◽  
Strike Slip ◽  
Fault System ◽  
Normal Faults ◽  
Variscan Belt ◽  
French Massif Central ◽  
Massif Central ◽  
Granite Emplacement ◽  
Regional Deformation ◽  
Northern Border

AbstractThe Variscan continental collision induced the development of large crustal melting in the western French Massif Central, accompanied by emplacement and deformation of syn- to post-tectonic granites spatially related to normal and strike slip faulting. Our study focuses on the regional deformation and shear zone patterns in the Millevaches massif, one of the largest magmatic area of the French Massif Central. In this massif, the syn-tectonic intrusions are related i) to the dextral wrenching along the Treignac-Pradines shear zones and the Creuse faults system, and ii) to the coeval extension along the N000°–N020° normal faults on the western edge of the Millevaches massif (Bourganeuf and Argentat faults). The analysis of deformation and kinematics correlated to new datations of granites allow us to propose a pull-apart model to explain the tectono-magmatic evolution of this part of the Variscan belt from 350 Ma to 325 Ma. At that time, these granites intruded a “pull-apart” system bounded by two major N140°–160° dextral strike-slip zones operating in the middle continental crust during a bulk N020° regional shortening.From 325 Ma to 320 Ma, a clockwise rotation of the regional shortening axis was responsible for the late reactivation of the N020° eastern Millevaches tectonic border as a dextral fault system (Felletin-Ambrugeat fault system). This NE-SW shortening displaced the N140°–160° Creuse fault system and induced a reverse motion along the northern border of the Millevaches massif (St-Michel-de-Veisse fault). This Visean tectono-magmatic event induced the late exhumation of the Millevaches massif with respect to surrounding units and favoured the widespread granite emplacement in this part of the Variscan belt.

Evaluating Holocene natural hazards in the French Massif Central from a regional lake sediment approach

2021 ◽  
Author(s):  
E. Chapron ◽  
A. Foucher ◽  
L. Chassiot ◽  
W. Fleurdeus ◽  
V. Arricau ◽  
...  
Keyword(s):  
Natural Hazards ◽  
Lake Sediment ◽  
French Massif Central ◽  
Massif Central

Sellaphora labernardierei (Sellaphoraceae, Bacillariophyta), a new epilithic species from French spring and four new combinations within the genus Sellaphora

Phytotaxa ◽  
2016 ◽  
Vol 260 (3) ◽  
pp. 235 ◽  
Author(s):  
AUDE BEAUGER ◽  
CARLOS E. WETZEL ◽  
OLIVIER VOLDOIRE ◽  
ALEXANDRE GARREAU ◽  
LUC ECTOR
Keyword(s):  
Electron Microscopy ◽  
Scanning Electron Microscopy ◽  
Central Area ◽  
Mineral Waters ◽  
New Combinations ◽  
French Massif Central ◽  
Small Species ◽  
Massif Central ◽  
Floristic Survey ◽  
Scanning Electron

During a floristic survey of freshwater springs in the French Massif Central region, a new Sellaphora species (Bacillariophyceae) was found and is hereby described: Sellaphora labernardierei sp. nov. This new small species, with linear to linear-elliptical valves, slightly inflated in the middle, is formally described using light and scanning electron microscopy. The species has a single H-shaped chloroplast and striae usually biseriate tending to be uniseriate near the centre of the valve. The valve dimensions, the valve, pole and central area shapes, and the striae density and pattern allow their separation from similar taxa belonging to the genera Eolimna, Naviculadicta and Sellaphora. The ecological preferences of Sellaphora labernardierei are mineral waters with high nitrate concentrations. The examination of similar taxa belonging to the genera Eolimna and Naviculadicta and their characteristics lead to propose four new combinations of Sellaphora species.

scholarly journals Tephras in lacustrine sediments of the Sarliève marsh (French Massif Central): age and preservation

2006 ◽  
Vol 338 (16) ◽  
pp. 1141-1149 ◽  
Author(s):  
Agathe Fourmont ◽  
Jean-Jacques Macaire ◽  
Jean-Gabriel Bréhéret ◽  
Jacqueline Argant ◽  
Béatrice Prat ◽  
...  
Keyword(s):  
Lacustrine Sediments ◽  
French Massif Central ◽  
Massif Central
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