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.

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.

scholarly journals The Borborema Strike-Slip Shear Zone System (NE Brazil): Large-Scale Intracontinental Strain Localization in a Heterogeneous Plate

Lithosphere ◽  
2021 ◽  
Vol 2021 (Special 6) ◽  
Author(s):  
Sergio P. Neves ◽  
Andréa Tommasi ◽  
Alain Vauchez ◽  
Thais Andressa Carrino
Keyword(s):  
Shear Zone ◽  
Large Scale ◽  
Shear Zones ◽  
Strike Slip ◽  
Northeastern Brazil ◽  
Main Stage ◽  
Plate Boundaries ◽  
Deformation Localization ◽  
Continental Scale ◽  
Dextral Shear

Abstract Large-scale strike-slip faults are fundamental tectonic elements of the continental lithosphere. They constitute plate boundaries (continental transforms), separate terranes with contrasting geological histories within accretionary orogens, or accommodate heterogeneous deformation in intracontinental settings. In ancient orogens, where deeper levels of the crust are exposed, these faults are expressed as shear zones materialized by up to tens of km-wide mylonitic belts. The Borborema shear zone system in northeastern Brazil is one of the largest and best-exposed intracontinental strike-slip shear zone systems in the world, cropping out over 250,000 km2. Here, we review its main geophysical, structural, petrologic, and geochronologic characteristics and discuss the factors controlling its development. This complex continental scale shear zone system is composed of a set of NE- to NNE-trending dextral shear zones from which there are two major E-trending dextral shear zones with horse-tail terminations into the transpressional belt branch, as well as several smaller E-trending dextral and NE-trending dextral and sinistral shear zones. The major shear zones are marked by extensive linear or curvilinear magnetic gradients, implying their continuation at depth. The major shear zones are materialized by migmatite to amphibolite-facies mylonites, but the entire system shows evidence of late deformation at lower temperatures. The system developed during the late stages of the Neoproterozoic Brasiliano (Pan-African) orogeny (mainly from 590 to 560 Ma), postdating by more than 20 Ma the main stage of contractional deformation. Localization of strike-slip shearing in this intraplate setting was controlled by rheological contrasts between blocks with distinct Paleoproterozoic histories, the presence of preorogenic Neoproterozoic rifts, the craton geometry, and zones of enhanced magmatic activity, highlighting the importance of rheological heterogeneity in controlling shear zone nucleation and evolution.

scholarly journals Coupling RSCM paleothermometry with 40Ar/39Ar analysis in the south Variscan belt (Cévennes, France): new constraints on the late-orogenic metamorphic gradient in an orogen outer domain.

2020 ◽  
Author(s):  
Clément Montmartin ◽  
Michel Faure ◽  
Stéphane Scaillet ◽  
Hugues Raimbourg
Keyword(s):  
Cross Sections ◽  
Crustal Thickening ◽  
Early Event ◽  
Variscan Belt ◽  
French Massif Central ◽  
Raman Spectrometry ◽  
Massif Central ◽  
Temperature Jumps ◽  
Outer Domain ◽  
The One

<p>In the SE part of the Variscan French Massif Central, the Cévennes area belongs to the para-autochthonous unit of the southern Variscan belt. This area underwent three metamorphic events (Faure et al., 2001).  I) A green schist to low amphibolite facies one (500°C, 4.5Kb Arnaud, 1997) developed in micaschists and quartzites. These rocks were stacked as south-directed nappes during the final stage of the Variscan crustal thickening dated at ca 340 Ma by <sup>40</sup>Ar/<sup>39</sup>Ar on biotite (Caron, 1994). This early event was responsible for the flat-lying foliation, the N-S striking stretching lineation, and intrafolial foliation. II) A high temperature event (680°C, 4.5kb Rakib, 1996) dated at ca 325 Ma (<sup>40</sup>Ar/<sup>39</sup>Ar on two biotites, Najoui et al, 2000) overprinted the early one. On the basis of the mineral assemblages of this event, a NE-ward increase of the T conditions was interpreted as a remote effect of the Velay Dome (Rakib, 1996). III) Finally, the M<sup>t</sup>-Lozère and Aigoual-S<sup>t</sup>-Guiral-Liron monzogranitic plutons intruded the Cévennes para-autochthonous unit. Monazite and biotite yield U-Pb, and <sup>40</sup>Ar/<sup>39</sup>Ar ages at 315-303Ma and 306 Ma , respectively (Brichaud et al. 2008). The pluton emplacement conditions are determined at 695°C, 1.5Kb (Najoui et al, 2000).</p><p>We report Raman Spectrometry of Carbonaceous Matter (RSCM) paleotemperature data acquired on more than 100 samples throughout the entire Cévennes area. These show a regional homogeneous thermal distribution with a 535 ± 50 °C mean temperature without any geometric correlation with the nappes structure, nor the granitic intrusions. Moreover, no thermal increase towards the NE can be documented. SW of the Aigoual-S<sup>t</sup>-Guiral-Liron massif, our RSCM data document a temperature jumps between the overlying Cévennes micaschists and the underlying epimetamorphic rocks belonging the the Fold-and-Thust belt unit of the French Massif Central.</p><p>In order to constrain the age of this regional thermal event, we <sup>40</sup>Ar/<sup>39</sup>Ar dated 25 new regionally-distributed syn- and post-folial muscovites by step heating along two N-S cross sections within the Cévennes micaschists series. In areas distant from the plutons, the muscovite yields a ca 325 Ma age interpreted as the one of the HT event recorded by the RSCM measurements. However, young muscovite ages at ca 305Ma are observed around the plutons. We assume that the heat supplied by the plutons reset these muscovites at around 400°C while the organic matter cannot record the contact metamorphic peak lower than the regional one. Moreover, <sup>40</sup>Ar/<sup>39</sup>Ar in-situ analyses carried out on 5 mm-sized post folial (but deformed) biotites in the central part of the micaschist series provide ages around 320Ma. The presence of a hidden dome, underneath the Cévennes micaschists, similar to the pre-Velay migmatites exposed in the northern part of the Cévennes area (Faure et al., 2001, Be et al., 2006) is discussed.</p>

French and Belgian Uplands

2005 ◽  
Author(s):  
Bernard Etlicher
Keyword(s):  
Continental Crust ◽  
Rift Valley ◽  
Sedimentary Basins ◽  
Shear Zones ◽  
Metamorphic Complex ◽  
French Massif Central ◽  
Massif Central ◽  
Montagne Noire ◽  
Oceanic And Continental Crust ◽  
Moderate Elevation

The French Uplands were built by the Hercynian orogenesis. The French Massif Central occupies one-sixth of the area of France and shows various landscapes. It is the highest upland, 1,886 m at the Sancy, and the most complex. The Vosges massif is a small massif, quite similar to the Schwarzwald in Germany, from which it is separated by the Rhine Rift Valley. Near the border of France, Belgium, and Germany, the Ardennes upland has a very moderate elevation. The largest part of this massif lies in Belgium. Though Brittany is partly made up of igneous and metamorphic rocks, it cannot be truly considered as an upland; in the main parts of Brittany, altitudes are lower than in the Parisian basin. Similarities of the landscape in the French and Belgian Uplands derive from two major events: the Oligocene rifting event and the Alpine tectonic phase. The Vosges and the Massif Central are located on the collision zone of the Variscan orogen. In contrast, the Ardennes is in a marginal position where primary sediments cover the igneous basement. Four main periods are defined during the Hercynian orogenesis (Bard et al. 1980; Autran 1984; Ledru et al. 1989; Faure et al. 1997). The early Variscan period corresponds to a subduction of oceanic and continental crust and a highpressure metamorphism (450–400 Ma) The medio- Variscan period corresponds to a continent–continent collision of the chain (400–340 Ma). Metamorphism under middle pressure conditions took place and controlled the formation of many granite plutons: e.g. red granites (granites rouges), porphyroid granite, and granodiorite incorporated in a metamorphic complex basement of various rocks. The neo-Variscan period (340–320 Ma) is characterized by a strong folding event: transcurrent shear zones affected the units of the previous periods and the first sedimentary basins appeared. At the end of this period, late-Variscan (330–280 Ma), autochthonous granites crystallized under low-pressure conditions related to a post-collision thinning of the crust. Velay and Montagne Noire granites are the main massifs generated by this event. Sediment deposition in tectonic basins during Carboniferous and Permian times occurred in the Massif Central and the Vosges: facies are sandstone (Vosges), shale, coal, and sandstone in several Stephanian basins of the Massif Central, with red shale and clay ‘Rougier’ in the south-western part of the Massif Central.

Proterozoic geological evolution of the northern Vestfold Hills, Antarctica

1991 ◽  
Vol 128 (4) ◽  
pp. 307-318 ◽  
Author(s):  
C. W. Passchier ◽  
R. F. Bekendam ◽  
J. D. Hoek ◽  
P. G. H. M. Dirks ◽  
H. de Boorder
Keyword(s):  
Large Scale ◽  
Structural Evolution ◽  
Shear Zones ◽  
Granulite Facies ◽  
Strike Slip ◽  
Amphibolite Facies ◽  
Tectonic Event ◽  
Vestfold Hills ◽  
Two Phases ◽  
Brittle Faulting

AbstractThe presence of polyphase shear zones transected by several suites of dolerite dykes in Archaean basement of the Vestfold Hills, East Antarctica, allows a detailed reconstruction of the local structural evolution. Archaean and early Proterozoic deformation at granulite facies conditions was followed by two phases of dolerite intrusion and mylonite generation in strike-slip zones at amphibolite facies conditions. A subsequent middle Proterozoic phase of brittle normal faulting led to the development of pseudotachylite, predating intrusion of the major swarm of dolerite dykes around 1250 Ma. During the later stages and following this event, pseudotachylite veins were reactivated as ductile, mylonitic thrusts under prograde conditions, culminating in amphibolite facies metamorphism around 1000–1100 Ma. This is possibly part of a large-scale tectonic event during which the Vestfold block was overthrust from the south. In a final phase of strike-slip deformation, several pulses of pseudotachylite-generating brittle faulting alternated with ductile reactivation of pseudotachylite.

scholarly journals The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution

2001 ◽  
Vol 342 (3-4) ◽  
pp. 207-237 ◽  
Author(s):  
P. Ledru ◽  
G. Courrioux ◽  
C. Dallain ◽  
J.M. Lardeaux ◽  
J.M. Montel ◽  
...  
Keyword(s):  
French Massif Central ◽  
Massif Central ◽  
Granite Emplacement ◽  
Melt Generation

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.

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