Hot metamorphic core complex in a cold foreland

The Montagne Noire forms the southernmost part of the French Massif Central. Carboniferous flysch sediments and very low-grade metamorphic imprint testify to a very external position in the orogen. Sedimentation of synorogenic clastic sediments continued up to the Viséan/Namurian boundary (≤320 Ma). Subsequently, the Palaeozoic sedimentary pile underwent recumbent folding and grossly southward thrusting. An extensional window exposes a hot core of Carboniferous HT/LP gneisses, migmatites and granites (Zone Axiale), which was uplifted from under the nappe pile. After the emplacement of the nappes on the Zone Axiale (Variscan D₁), all structural levels shared the same tectonic evolution: D₂ (extension and exhumation), D₃ (refolding) and post-D₃ dextral transtension. HT/LP-metamorphism in the crystalline rocks probably started before and continued after the emplacement of the nappes. Peak metamorphic temperatures were attained during a post-nappe thermal increment (M₂). M₂ occurred during ENE-directed bilateral extension, which exhumed the Zone Axiale and its frame as a ductile horst structure, flanked to the ENE by a Stephanian intra-montane basin. Map patterns and mesoscopic structures reveal that extension in ENE occurred simultaneously with NNW-oriented shortening. Combination of these D₂ effects defines a bulk prolate strain in a “pinched pull-apart” setting. Ductile D₂ deformation during M₂ dominates the structural record. In wide parts of the nappes on the southern flank of the Zone Axiale, D₁ is only represented by the inverted position of bedding (overturned limbs of recumbent D₁ folds) and by refolded D₁ folds. U–Pb monazite and zircon ages and K–Ar muscovite ages are in accord with Ar–Ar data from the literature. HT/LP metamorphism and granitoid intrusion commenced already at ≥330 Ma and continued until 297 Ma, and probably in a separate pulse in post-Stephanian time. Metamorphic ages older than c. 300 Ma are not compatible with the classical model of thermal relaxation after stacking, since they either pre-date or too closely post-date the end of flysch sedimentation. We therefore propose that migmatization and granite melt generation were independent from crustal thickening and caused, instead, by the repeated intrusion of melts into a crustal-scale strike-slip shear zone. Advective heating continued in a pull-apart setting whose activity outlasted the emplacement of the Variscan nappe pile. The shear-zone model is confirmed by similar orogen-parallel extensional windows with HT/LP metamorphism and granitoid intrusion in neighbouring areas, whose location is independent from their position in the orogen. We propose that heat transfer from the mantle occurred in dextral strike-slip shear zones controlled by the westward propagating rift of the Palaeotethys ocean, which helped to destroy the Variscan orogen.     

The Montalet granite, Montagne Noire, France: An Early Permian syn-extensional pluton as evidenced by new U-Th-Pb data on zircon and monazite

Dating the magmatism in the Montagne Noire gneiss dome in the southern French Massif Central is a key point for understanding the Late Palaeozoic evolution of this part of the Variscan belt, which is characterised by compressive tectonics during the Carboniferous and extensional tectonics during Stephanian-Permian times. The Montalet granite crops out in the north-western part of the dome and was first considered as an early syntectonic intrusion related to compressive deformation. More recently, it has been dated at 327 Ma and considered as contemporaneous with the diapiric ascent of the Montagne Noire gneiss dome before the Stephanian-Permian extension. We show that in fact, this pluton was emplaced 294 ± 1 Ma ago and is therefore contemporaneous with the Stephanian-Permian extension. This age is consistent with the interpretation of the Montagne Noire Massif as an extensional gneiss dome.     

Tectonic evolution of the Montagne Noire (french Massif Central): a model of extensional gneiss dome

Abstract

The tertonic interpretation of the Montagne Noire Gneiss Dome (southern French II Massif (Central) has been controversial for a long time. Several models have been proposed : diapirie uplift, wreneching and diapirism, compressive anticline, and metamorphie core complex. Evidence for extensional tectonics in the French Varisean Belt favours the latter interpretation. Strain and metamorphism patterns in the eastern part of the Montagne Noire result from two successive extensional deformations during Late Carboniferous to Permian times. The occurrence of a major detachment zone along the northern edge of the Montagne Noire Gneiss Dome as well the presence of sedimentary) basias to the north point to the asymmetry) of the Stephanian-Permian extensional system. We propose a new model of gneiss dome involving isostatie uprising and consecutive tectonic denudation of the duetile lower crust. This process results in asymmetrical extensional systems characterized by roll-under folding of the footwall and development of basins in the hangingwall as in the Montagne Noire. The model is finally discussed in comparison to previous interpretations.     

Première datation UPb des orthogneiss œillés de la zone axiale de la Montagne noire (Sud du Massif central) : nouveaux témoins du magmatisme ordovicien dans la chaı̂ne Varisque

We present new U–Pb results on felsic Augen orthogneisses from the Axial Zone of the Montagne Noire (French Massif Central). The data indicate Ordovician ages, 456±3 and 450±6 Ma for two samples collected at ‘Pont-de-Larn’ and ‘Gorges d'Héric’, respectively. These ages are interpreted as the igneous emplacement age of the granitic protolith. To cite this article: F. Roger et al., C. R. Geoscience 336 (2004).     

Middle Carboniferous crustal melting in the Variscan Belt: New insights from U–Th–Pbtot. monazite and U–Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central)

In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pb_tot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale.     

Middle Carboniferous crustal melting in the Variscan Belt: New insights from U–Th–Pbtot. monazite and U–Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central)

In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pb_tot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale.     

U-Pb systematics of detrital zircons from some unmetamorphosed to slightly metamorphosed sediments of Central Europe

In an attempt to find the premetamorphic discordance pattern of detrital zircons extracted from Central European metasediments, unmetamorphosed or only slightly metamorphosed sediments were collected from two areas: (1) from the Montagne Noire, the southernmost part of the French Central Massif and (2) from the innerbohemian Algonkian (= Proterozoic) in the CSSR.The generally accepted hypothesis that zircons from Central European metasediments must have plotted close to or at the corresponding upper intercept between discordia trajectory and concordia curve prior to the metamorphism of the host rocks could not be supported. The zircon populations from sediments of both areas are similar in discordance to those of the numerous populations extracted from metasediments of the Central European basement complexes. However, in contrast to the latter, the data points of size fractions scatter considerably and reliable intercept ages cannot be calculated.In the case of the Cambro-Ordovician sand- and siltstones of the Montagne Noire, the ages of detrital muscovites strongly argue for a Cadomian (550–700 m.y.) provenance of the detritus. Thus, the strong discordance of the analyzed fractions most probably is caused by zircons newly formed and/or partly or completely reset during a Cadomian event in the provenance of the detritus. In addition, lattice unit parameters indicate that the detrital zircons must have been recrystallized after their primary formation more than 1.7 b.y. ago.The Algonkian sediments of Bohemia (CSSR) can be taken as the very low-grade metamorphic equivalents of the Moldanubian paragneisses from which discordia trajectories between about 2 b.y. and 460–320 m.y. are known (Gebauer and Grünenfelder, 1974; Grauert et al., 1974). Nevertheless, all analyzed zircon fractions are strongly discordant indicating that they probably recrystallized during the Assyntian (=Cadomian) very low-grade metamorphism of the host rock loosing most of their accumulated radiogenic lead. If such an interpretation is correct, the low-temperature recrystallization model of Gebauer and Grünenfelder (1976) can be applied to metamict zircons in host rocks formed at temperatures as low as 300 ° C. In our 1976-paper we gave temperatures of 350–400 ° C for the maximum temperature necessary to recrystallize metamict zircons in chlorite-grade quartzphyllites in agreement with the experimental results of Pidgeon et al. (1973).In contrast to the zircons of the Montagne Noire it can be shown that the U-Pb systems of the Algonkian zircons must have been re-opened in post-Assyntian time, probably recently or in the Tertiary. However, no plausible explanation can be given to account for this.     

Terrane character of the north-east margin of the Moldanubian Zone: the Kutná Hora Crystalline Complex,Bohemian Massif

Three major allochthonous units have been distinguished on the north-eastern border of the Moldanubian Zone, which differ each from other in lithology and structural and metamorphic evolution. Their present day position displays significant metamorphic and structural inversion resulting from progressive nappe stacking during the Variscan orogeny. The uppermost-Gföhl Unit consists of anatectic rocks containing high temperature/high pressure relics, i.e. granulites, eclogites and garnet peridotites. The rocks of the Gföhl Unit were strongly mylonized during uplift and later also extensively migmatized in the kyanite stability field. The Kouiim Nappe is built up by a sequence of fine-grained leucocratic migmatites with preserved relics of a pre-Variscan deformation event. This event was terminated by the intrusion of coarse-grained porphyritic granites, converted into augen orthogneisses by the Variscan orogeny. The lowermost Micaschist Zone was formed from a sequence of metapelites intercalated with diopsidic amphibolites.During uplift from deep crustal zones the Gföhl Unit cut off a thick slice of the basement crustal material represented by the Kourim Nappe. The quartzo-feldspathic material of the Kourim Nappe acted as a major shear interface because of its extreme ductility under the conditions found in the middle crust. This process occurred under amphibolite facies metamorphism. The continuous uplift of the nappe pile induced another crustal segment in the nappe stack, represented by the Micaschist Zone. The whole nappe sequence was then thrust over the Moldanubian Zone. A westward sense of shear is suggested for the whole uplift history. The kinematic pattern was complicated by later strike-slip ductile faults which finished the recent geological configuration.Correspondence to: J. Synek     

Variations de l’âge des sédiments calcaires et « Culm » carbonifère dans la chaîne varisque du Sud de la France : migration de l’orogenèse varisque

Résumé

Dans le Sud de la France et les Pyrénées espagnoles, les calcaires sous-jacents aux roches détritiques du Culm carbonifère ont fait l’objet d’une datation systématique par conodontes. Il en ressort que le dépôt de ces niveaux carbonates s’est poursuivi de plus en plus tardivement de l’Est vers l’Ouest : du Viséen inférieur (V3b) en Montagne Noire, au Namurien supérieur (G2) en Pays basque.

Les roches détritiques à caractère flyschoïde du Culm remanient, localement, des éléments de calcaire de plate-forme. La datation de ces éléments par foraminifères, algues, pseudo-algues et végétaux supérieurs montre qu’ils sont de plus en plus récents d’Est en Ouest : leur âge s’échelonne du Viséen 3bp-Namurien A en Montagne Noire, au Westphalien C (Kachirien) au Pays basque (Massif des Cinco-Villas).

Ces divers résultats montrent que les manifestations tectoniques, responsables des premiers épandages détritiques du Culm puis de la resédimentation des calcaires de plate-forme dans le bassin, ont commencé de plus en plus tardivement de la Montagne Noire au Pays basque.

Dans sa branche Sud, l’orogenèse varisque, si l’on s’en tient à la position actuelle de ses segments observables, a migré progressivement du Nord vers le Sud, en Montagne Noire et dans le Massif de Mouthoumet, puis vers l’Ouest, des Pyrénées centrales aux Pyrénées orientales.     

Along-strike shear-sense reversal in the Vals-Scaradra Shear Zone at the front of the Adula Nappe (Central Alps,Switzerland)

The Adula Nappe in the Central Alps comprises pre-Mesozoic basement and minor Mesozoic sediments, overprinted by Paleogene eclogite-facies metamorphism. Peak pressures increase southward from ca. 1.2 GPa to values over 3 GPa, which is interpreted to reflect exhumation from a south-dipping subduction zone. The over- and underlying nappes experienced much lower Alpine pressures. To the north, the Adula Nappe ends in a lobe surrounded by Mesozoic metasediments. The external shape of the lobe is simple but the internal structure highly complicated. The frontal boundary of the nappe represents a discontinuity in metamorphic peak temperatures, between higher T in the Adula Nappe and lower T outside. A shear zone with steeply dipping foliation and shallowly-plunging, WSW-ENE oriented, i.e. orogen-parallel stretching lineation overprinted the northernmost part of the Adula Nappe and the adjacent Mesozoic metasediments (Vals-Scaradra Shear Zone). It formed during the local Leis deformation phase. The shear sense in the Vals-Scaradra Shear Zone changes along strike; from sinistral in the W to dextral in the E. Quartz textures also vary along strike. In the W, they indicate sinistral shearing with a component of coaxial (flattening) strain. A texture from the middle part of the shear zone is symmetric and indicates coaxial flattening. Textures from the eastern part show strong, single c-axis maxima indicating dextral shearing. These relations reflect complex flow within the Adula Nappe during a late stage of its exhumation. The structures and reconstructed flow field indicate that the Adula basement protruded upward and northward into the surrounding metasediments, spread laterally, and expelled the metasediments in front towards west and east.     

The northwest-directed “Bretonian phase” in the French Variscan Belt (Massif Central and Massif Armoricain): A consequence of the Early Carboniferous Gondwana–Laurussia collision

In the Variscan French Massif Central and Armorican Massif, the tectonic significance of a widespread NW–SE-trending stretching lineation, coeval with medium pressure–medium temperature metamorphism, is an open question. Based on a structural analysis in the southern part of the Massif Central, we show that this top-to-the-NW shearing is a deformation event, referred to as D2, which followed a D1 top-to-the-south shearing Devonian phase, and was itself re-deformed by a Late D3 Visean–Serpukhovian southward-thrusting event. We date the D2 phase at 360 Ma (Famennian–Tournaisian boundary). In the Armorican Massif, D2 is the “Bretonian phase” recorded in the metamorphic series and sedimentary basins. Geodynamically, D2 is related to a general northwestward shearing during the Laurussia–Gondwana collision, which occurred after the closure of the Rheic Ocean, as indicated by the emplacement of the Lizard ophiolitic nappe in Britain. The left-lateral Nort-sur-Erdre fault accommodated the absence of ductile shearing in Central Armorica.     

Barite deposits of the “Montagne Noire”, Southern France

Several small barite deposits of Devonian age are known in the Monts de Cabrières region, Montagne Noire (southern France). A field and laboratory investigation of these stratabound deposits showed their possible diagenetic origin and a limited economic value.

---

Zusammenfassung De petits gisements de barytine du Dévonien dans les Monts de Cabrières (Montagne Noire, France) ont été étudié en détail sur le terrain et au laboratoire. Les observations ont apporté des critères pour une explication génétique de ces gisements «stratiformes». Ils indiquent que les concentrations minérales se sont effectuées par un processus de sécrétion au cours de la diagenèse. L'importance économique de ces gisements est limitée.

---

---

...es ist zwar nicht sinnlos, aber doch etwas unlogisch, wenn man von vornherein nicht versucht, eine Lagerstätte in ihre Umgebung auf die wahrscheinlichste Art einzugliedern, sondern sie unbedingt einem ganz fremden, zunächst nicht von selbst verständlichen Bildungsvorgang zuordnen will. H. Schneiderhöhn (1954)     

The Léon Domain (French Massif Armoricain): a westward extension of the Mid-German Crystalline Rise? Structural and geochronological insights

The Léon Domain in the NW part of the French Massif Armoricain is a stack of synmetamorphic nappes displaced from south to north in ductile conditions. From bottom to top, an orthogneissic basement is overthrusted successively by (1) a Lower Nappe of gneiss including mafic eclogites, (2) an Intermediate Nappe of biotite–garnet–staurolite micaschists with mafic blocks, and (3) an upper nappe made up of Neoproterozoic phyllites covered by unmetamorphosed Paleozoic sedimentary series. This microstructural study documents a polyphase evolution with firstly a top-to-the-N shearing, secondly followed by upright folding of the stack of nappes coeval with migmatization, and lastly, a dextral wrenching along the North Armorican Shear Zone associated with emplacement of synkinematic plutons. New U–Th/Pb chemical dating of monazites from biotite–garnet–staurolite micaschists, migmatites, and granitoids argue for 340–335 Ma, 335–327 Ma, and about 320 Ma ages for synthrusting metamorphism, anatexis, and wrenching, respectively. A metagabbro from Le Conquet yields a zircon LA-ICP-MS age of 478 ± 4 Ma, which corresponds to magma emplacement time. The Léon Domain is interpreted as a microcontinent separated from Armorica by the Le Conquet-Penzé suture to the south and east, and from Laurussia by the Rheic suture to the north. A possible correlation with the Mid-German Crystalline Rise of Central Europe is discussed.     

Geothermobarometry, P–T paths and metamorphic field gradients of high-pressure rocks from the Adula Nappe, Central Alps

Metabasic rocks from the Adula Nappe in the Central Alps record a regional high‐pressure metamorphic event during the Eocene, and display a regional variation in high‐pressure mineral assemblages from barroisite, or glaucophane, bearing garnet amphibolites in the north to kyanite eclogites in the central part of the nappe. High‐pressure rocks from all parts of the nappe show the same metamorphic evolution of assemblages consistent with prograde blueschist, high‐pressure amphibolite or eclogite facies conditions followed by peak‐pressure eclogite facies conditions and decompression to the greenschist or amphibolite facies. Average PT calculations (using thermocalc ) quantitatively establish nested, clockwise P–T paths for different parts of the Adula Nappe that are displaced to higher pressure and temperature from north to south. Metamorphic conditions at peak pressure increase from about 17 kbar, 640 °C in the north to 22 kbar, 750 °C in the centre and 25 kbar, 750 °C in the south. The northern and central Adula Nappe behaved as a coherent tectonic unit at peak pressures and during decompression, and thermobarometric results are interpreted in terms of a metamorphic field gradient of 9.6 ± 2.0 °C km^?1 and 0.20 ± 0.05 kbar km^?1. These results constrain the peak‐pressure position and orientation of the nappe to a depth of 55–75 km, dipping at an angle of approximately 45° towards the south. Results from the southern Adula Nappe are not consistent with the metamorphic field gradient determined for the northern and central parts, which suggests that the southern Adula Nappe may have been separated from central and northern parts at peak pressure.     

The tectono‐metamorphic evolution of the very low‐grade hangingwall constrains two‐stage gneiss dome formation in the Montagne Noire (Southern France)

The Montagne Noire in the southernmost French Massif Central is made of an ENE‐elongated gneiss dome flanked by Palaeozoic sedimentary rocks. The tectonic evolution of the gneiss dome has generated controversy for more than half a century. As a result, a multitude of models have been proposed that invoke various tectonic regimes and exhumation mechanisms. Most of these models are based on data from the gneiss dome itself. Here, new constraints on the dome evolution are provided based on a combination of very low‐grade petrology, K–Ar geochronology, field mapping and structural analysis of the Palaeozoic western Mont Peyroux and Faugères units, which constitute part of the southern hangingwall of the dome. It is shown that southward‐directed Variscan nappe‐thrusting (D₁) and a related medium‐P metamorphism (M₁) are only preserved in the area furthest away from the gneiss dome. The regionally dominant pervasive tectono‐metamorphic event D₂/M₂ largely transposes D₁ structures, comprises a higher metamorphic thermal gradient than M₁ (transition low‐P and medium‐P metamorphic facies series) and affected the rocks between c. 309 and 300 Ma, post‐dating D₁/M₁ by more than 20 Ma. D₂‐related fabrics are refolded by D₃, which in its turn, is followed by dextral‐normal shearing along the basal shear zone of both units at c. 297 Ma. In the western Mont Peyroux and Faugères units, D₂/M₂ is largely synchronous with shearing along the southern dome margin between c. 311 and 303 Ma, facilitating the emplacement of the gneiss dome into the upper crust. D₂/M₂ also overlaps in time with granitic magmatism and migmatization in the Zone Axiale between c. 314 and 306 Ma, and a related low‐P/high‐T metamorphism at c. 308 Ma. The shearing that accompanied the exhumation of the dome therefore was synchronous with a peak in temperature expressed by migmatization and intrusion of melts within the dome, and also with the peak of metamorphism in the hangingwall. Both, the intensity of D₂ fabrics and the M₂ metamorphic grade within the hangingwall, decrease away from the gneiss dome, with grades ranging from the anchizone–epizone boundary to the diagenetic zone. The related zonation of the pre‐D₃ metamorphic field gradients paralleled the dome. These observations indicate that D₂/M₂ is controlled by the exhumation of the Zone Axiale, and suggest a coherent kinematic between the different crustal levels at some time during D₂/M₂. Based on integration of these findings with regional geological constraints, a two‐stage exhumation of the gneiss dome is proposed: during a first stage between c. 316 and 300 Ma dome emplacement into the upper crust was controlled by dextral shear zones arranged in a pull‐apart‐like geometry. The second stage from 300 Ma onwards was characterized by northeast to northward extension, with exhumation accommodated by north‐dipping detachments and hangingwall basin formation along the northeastern dome margin.     

Lu–Hf garnet systematics of a polymetamorphic basement unit: new evidence for coherent exhumation of the Adula Nappe (Central Alps) from eclogite-facies conditions

The Adula Nappe in the Central Alps is a mixture of various pre-Mesozoic continental basement rocks, metabasics, ultrabasics, and Mesozoic cover rocks, which were pervasively deformed during Alpine orogeny. Metabasics, ultrabasics, and locally garnet–mica schists preserve eclogite-facies assemblages while the bulk of the nappe lacks such evidence. We provide garnet major-element data, Lu profiles, and Lu–Hf garnet geochronology from eclogites sampled along a north–south traverse. A southward increasing Alpine overprint over pre-Alpine garnets is observed throughout the nappe. Garnets in a sample from the northern Adula Nappe display a single growth cycle and yield a Variscan age of 323.8 ± 6.9 Ma. In contrast, a sample from Alpe Arami in the southernmost part contains unzoned garnets that fully equilibrated to Alpine high-pressure (HP) metamorphic conditions with temperatures exceeding 800 °C. We suggest that the respective Eocene Lu–Hf age of 34.1 ± 2.8 Ma is affected by partial re-equilibration after the Alpine pressure peak. A third sample from the central part of the nappe contains separable Alpine and Variscan garnet populations. The Alpine population yields a maximum age of 38.8 ± 4.3 Ma in line with a previously published garnet maximum age from the central nappe of 37.1 ± 0.9 Ma. The Adula Nappe represents a coherent basement unit, which preserves a continuous Alpine high-pressure metamorphic gradient. It was subducted as a whole in a single, short-lived event in the upper Eocene. Controversial HP ages and conditions in the Adula Nappe may result from partly preserved Variscan assemblages in Alpine metamorphic rocks.     

Position of the Blyb Complex in the pre-Mesozoic crustal structure of the Front Range Zone in the Northern Caucasus

A number of Variscan nappe complexes were recognized in the Late Mesozoic structure of the Front Range Zone of the Greater Caucasus in the 1970s. They consist predominantly of greenstone units and override one another in a consecutive order. The only exception is the upper, Atsgara Nappe, which is composed of crystalline schists, amphibolites, and microgneisses. Crystalline schists, gneisses, amphibolites, and other rocks of the so-called Blyb Complex occur at the base of the nappe packet. The affinity of crystalline rocks of the Blyb Complex to one of the upper Variscan nappes is substantiated in this paper. The Middle Paleozoic rocks, which originally were located below the Blyb Complex in the Front Range structure, overrode its rocks along the surface of the Blyb Thrust Fault in the Early Triassic. Since that time, the crystalline rocks of the Blyb Complex have occupied the lowermost position in the structure of the Front Range. The absence of Upper Paleozoic rocks in the footwall of the thrust fault is due to the fact that, in the Late Paleozoic, the area underlain by the Blyb Complex was an inlier and a source of clastic material. The hanging wall of the Blyb Thrust Fault may be traced farther southward into the Main Range Zone, where it most likely consists of the Laba Group and other rocks. As has been established previously, the lower portion of the Laba Group consists of analogues of the Middle Paleozoic successions of the Front Range Zone, while its upper portion consists of crystalline schists of the Lashtrak Nappe, which occupy a position similar to that of the Atsgara Nappe metamorphic rocks. These relationships suggest that the rock complexes of the Front Range Zone could have undergone repeated displacements due to post-Variscan (Indosinian) tectonic events and overrode crystalline rocks in the Main Range Zone and more easterly areas. Owing to the uplift of the Central Caucasus, they are now eroding. The difference in the metamorphic grade of the Blyb Complex and the rocks of the Atsgara and Marukha nappes is due to the fact the Blyb Complex lies close to the root zones of nappes or belongs to different nappe sheets. The Blyb Thrust Fault pertains to the Indosinian faults that played the main role in the formation of the Front Range structure.     

Late‐stage deformation in a collisional orogen (Western Alps): nappe refolding,back‐thrusting or normal faulting?

ABSTRACT Nappe refolding, back-thrusting and normal faulting frequently cause severe late-stage overprinting of the architecture of an orogen. A combined investigation of nappe stack polarity, kinematics of shearing and metamorphic gradients in the Western Alps develops criteria for distinguishing between these three modes of late-stage deformation. This distinction is a prerequisite for any retro-deformation necessary for understanding the main tectonic and metamorphic evolution of collisional orogens. In the case of the Western Alps overprint was by mega-scale nappe refolding in the Oligocene. This implies exhumation of the HP-rocks prior to postnappe folding, i.e. during nappe stacking and by foreland-directed ascent within a subduction channel.     

Peak‐temperature patterns of polyphase metamorphism resulting from accretion,subduction and collision (eastern Tauern Window,European Alps) – a study with Raman microspectroscopy on carbonaceous material (RSCM)

Raman microspectroscopy on carbonaceous material (RSCM) from the eastern Tauern Window indicates contrasting peak‐temperature patterns in three different fabric domains, each of which underwent a poly‐metamorphic orogenic evolution: Domain 1 in the northeastern Tauern Window preserves oceanic units (Glockner Nappe System, Matrei Zone) that attained peak temperatures (T_p) of 350–480 °C following Late Cretaceous to Palaeogene nappe stacking in an accretionary wedge. Domain 2 in the central Tauern Window experienced T_p of 500–535 °C that was attained either within an exhumed Palaeogene subduction channel or during Oligocene Barrovian‐type thermal overprinting within the Alpine collisional orogen. Domain 3 in the Eastern Tauern Subdome has a peak‐temperature pattern that resulted from Eo‐Oligocene nappe stacking of continental units derived from the distal European margin. This pattern acquired its presently concentric pattern in Miocene time due to post‐nappe doming and extensional shearing along the Katschberg Shear Zone System (KSZS). T_p values in the largest (Hochalm) dome range from 612 °C in its core to 440 °C at its rim. The maximum peak‐temperature gradient (≤70 °C km^?1) occurs along the eastern margin of this dome where mylonitic shearing of the Katschberg Normal Fault (KNF) significantly thinned the Subpenninic‐ and Penninic nappe pile, including the pre‐existing peak‐temperature gradient.     

Crust-mantle relationships in the French Variscan chain: the example of the Southern Monts du Lyonnais unit (eastern French Massif Central)

Garnet lherzolite from the Lyonnais area (eastern French Massif Central) occurs as several lenses elongated within the regional foliation of garnet-biotite-sillimanite gneisses. Within the peridotites a mylonitic foliation can be observed which clearly is oblique to the regional foliation of the surrounding gneisses. Petrological and thermobarometric studies emphasize a tectonometamorphic re-equilibration for both crustal and mantle rocks characterized by a prograde metamorphic stage followed by retrograde evolution. During the burial stage, interpreted as lithospheric subduction, the peridotites underwent their mylonitic deformation, under high-pressure conditions (23–30 kbar). In contrast, the paragneisses have suffered their deformation during the retromorphic evolution under mesozonal conditions (6–8 kbar, 700°C). Our thermobarometric investigations allow us to interpret the granulitic/ultramafic association from the Monts du Lyonnais area as a lithospheric section buried into a Palaeozoic subduction zone, laminated during continental collision and uplifted by erosion processes.