Pavel Pitra

Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French–Spanish project

Abstract A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.

Crustal influx, indentation, ductile thinning and gravity redistribution in a continental wedge: Building a Moldanubian mantled gneiss dome with underthrust Saxothuringian material (European Variscan belt)

[1] The contribution of lateral forces, vertical load, gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated. In the Orlica-Snieznik dome (Moldanubian zone, European Variscan belt), the polyphase tectono-metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium- to high-grade metamorphic conditions in different levels of the crust. It reflects the eastward influx of a Saxothuringian-type passive margin sequence below a Tepla-Barrandian upper plate. The ongoing influx of continental crust creates a thick felsic orogenic root with HP rocks and migmatitic orthogneiss. The orogenic wedge is subsequently indented by the eastern Brunia microcontinent producing a multiscale folding of the orogenic infrastructure. The resulting kilometre-scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms. These localized vertical exchanges of material and heat are coeval with a larger crustal-scale folding of the whole infrastructure generating a general uplift of the dome. It is exemplified by increasing metamorphic conditions and younging of 40Ar/39Ar cooling ages toward the extruded migmatitic subdomes cored by HP rocks. The vertical growth of the dome induces exhumation by pure shear-dominated ductile thinning laterally evolving to non-coaxial detachment faulting, while erosion feeds the surrounding sedimentary basins. Modeling of the Bouguer anomaly grid is compatible with crustal-scale mass transfers between a dense superstructure and a lighter infrastructure. The model implies that the Moldanubian Orlica-Snieznik mantled gneiss dome derives from polyphase recycling of Saxothuringian material.

Late Variscan strike-slip tectonics between the Teplá-Barrandian and Moldanubian terranes (Czech Bohemian Massif): petrostructural evidence

In the Variscan Bohemian Massif (Czech Republic), no agreement has been reached on the nature of the relationship between the Teplá-Barrandian and the Moldanubian terranes. However, the Teplá-Barrandian is interpreted as a central segment separating the Moldanubian and Saxothuringian terranes and its position is therefore essential for any reconstruction of this part of the Variscan belt. Our petrological and structural study in the Domazlice-Klatovy region spans the boundary between the Teplá-Barrandian and Moldanubian terranes. There is no conformity of ductile structures in both domains near their interface and their P–T evolution is separate in time. The development of major structures and associated metamorphism in the southwestern part of the Barrandian was accompanied by syntectonic intrusions of granitoid and diorite bodies, dated at about 520 Ma. The Moldanubian migmatites are characterized by a LP-HT metamorphism that possibly took place at about 320 Ma. The metamorphic record of the Barrandian rocks shows no influence of the Moldanubian partial melting. The contacts are marked by semi-brittle to brittle structures: sinistral strike-slip faulting developed in the Central Bohemian Shear Zone, whereas the West Bohemian Shear Zone acted as a semi-brittle normal fault with a dextral component. Both shear zones may be nearly contemporaneous, accommodating a bulk N-S compression. The Central Bohemian Shear Zone was later reactivated in a dextral regime. Although timing is uncertain, the juxtaposition of both terranes took place probably in the waning stages of the Variscan orogeny, earlier than the late Stephanian. We argue that the late-orogenic (semi)brittle strike-slip tectonics modified considerably the face of the Bohemian Massif and the geodynamic models of the Bohemian Massif have to be reviewed with reference to this essential point.

Oligo-Miocene exhumation of the Beni-Bousera peridotite through a lithosphere-scale extensional shear zone

New structural data and P–T estimates of syn-deformational assemblages within the Beni Bousera peridotites and their crustal envelope are used to explain their Alpine exhumation. The Beni Bousera peridotites occur as thin sheets within high grade crustal units of the lower Sebtides (inner Rif, Morocco) and are composed of weakly deformed spinel lherzolite in the core of the massif and garnet-spinel mylonite at the rim. The main foliation trajectories in both the peridotites and overlying crustal units show systematic rotation towards their mutual contact, indicating a kilometer-scale top to the NW shearing with a dextral component along this crust/mantle contact. Widespread top to the NW shear criteria within the crustal units overlying the peridotite support this feature. Available ages constrain the development of the main foliation in both the peridotites and crustal rocks between 25 and 20 Ma. New P–T data from the peridotites show that deformation occurs during decompression from ≈ 22 kbar, 1050°C to ≈ 9-15 kbar, 800°C. As a consequence, exhumation of the Beni Bousera peridotites takes place during the Oligo-Miocene lithosphere thinning in the footwall of a lithospheric extensional shear zone. The exceptional preservation of garnet within the mylonitic peridotites results from rapid cooling of the border of the massif due to the juxtaposition with colder crustal rocks along this shear zone. Uplifting of the hot mantle rocks simultaneously induces high temperature metamorphism in the overlying crustal units. These new findings allow us to reconstruct the deformation history of the Beni Bousera region and the Alboran domain in the framework of the western Mediterranean geodynamics during the last 40 Myrs.

Late orogenic extension in the Bohemian Massif: petrostructural evidence in the Hlinsko region

AbstractThe tectonic contact between low-grade metase-dimentary series and high-grade rocks in the Hlinsko region (Bohemian Massif) is commonly interpreted as a thrust of the Barrandian sediments over the upper Moldanubian nappe.The sediments occur in an E-facing synform that contains a tonalitic laccolith on its eastern boundary with the Moldanubian, and is truncated by a granodiorite pluton to the west. The synform represents a late deformational folding event related to the granodiorite intrusion. NW-oriented normal shear in the tonalite is indicated by S-C microstructures. Kinematic criteria associated with the major foliation and lineation development in the metasediments also indicate a north-westward, normal shear. In addition, Moldanubian gneiss display late shear bands due to north-westward, normal shear. Consequently, the presumed thrust is a low-angle, normal shear zone.Low-pressure type metamorphism (3 < P < 4 x 102 MPa) coeval with the major deformational phase in pelites of the Hlinsko synfo...

Wagnerite in a cordierite-gedrite gneiss: Witness of long-term fluid-rock interaction in the continental crust (Ile d’Yeu, Armorican Massif, France)

Abstract We describe the first occurrence in the Variscan Belt of Western Europe of the relatively rare phosphate wagnerite, ideally Mg2PO4F. It occurs in albite-rich, cordierite-gedrite-bearing gneisses on the island of Ile d’Yeu, southern Armorican Massif, France. These gneisses are associated with a network of shear zones that crosscut granitoid orthogneisses of calc-alkaline affinity. Wagnerite is zoned and displays a rimward decrease of Fe/(Fe + Mg) from 0.16 to 0.08 and a concomitant increase in F. The F content ranges 0.46-1.05 apfu, but critically depends on the choice of the analytical standard. Based on phase diagrams calculated with THERMOCALC, we infer that the wagnerite-bearing orthoamphibole + cordierite + biotite + chlorite paragenesis equilibrated at ca. 550 °C, and pressures lower than 4 kbar. The presence of staurolite relics requires similar temperatures, but pressures higher than 4 kbar, implying an evolution dominated by decompression. On the basis of whole-rock chemistry and stable isotopes, we suggest that superimposed periods of metasomatic alteration throughout the metamorphic history led to the prograde stabilization of the cordierite-gedrite gneiss at the expense of the orthogneiss. This alteration involved aqueous fluids in isotopic equilibrium with local rocks and caused significant loss of Ca, K, and Si, and gain of Mg and Na. We argue that the Na-enrichment is the most significant difference between wagnerite-bearing and wagnerite-free Mg-rich, Ca-poor rocks on Ile d’Yeu. This emphasizes the possible importance of Na metasomatism for the formation of wagnerite. In light of comparisons with other wagnerite occurrences, we conclude that a long-term fluid-rock interaction, typically associated with shear-zones, may be the rule rather than the exception for the formation of wagnerite in metamorphic rocks unaffected by anatexis.

Viscous collision in channel explains double domes in metamorphic core complexes. Comment

[Rey et al. (2011)][1] propose an appealing numerical model for the development of gneiss domes in an extending crust. They suggest that extension in the upper brittle crust triggers oppositely verging ductile flows in the deep crust, resulting in the rising of two domes of foliation separated by a

Oligo-Miocene thinning of the Beni Bousera peridotites and their Variscan crustal host rocks, Internal Rif, Morocco†

Deciphering Variscan versus Alpine history in the internal Rif system is a key to constraining the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and LA-ICP-MS U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western internal Rif. The regional foliation (S2) is characterized by LP/HT mineral assemblages and obliterates a former foliation (S1) developed along a barrovian (MP/MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP/HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab roll-back in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidences for a Tertiary HP/LT metamorphism have been identified in the studied area

The Variscan belt: correlations and plate dynamics

an increasing volume of high-quality information from metamorphic petrology and structural geology. It becomes obvious that the evolution of the European Variscan belt is diachronous in various senses. As an example, the main paroxysmal event, characterised by the HT-LP metamorphism and associated granitic magmatism, culminated at c. 340 Ma in the north-east and c. 320–300 Ma to the southwest. A new geodynamic scenario emerges that takes into account the important Ordovician extensional event, and the mid-Ordovician Sardic unconformity in various regions of the belt as well as more precise data of igneous events coming from northern Gondwana and southern Laurussia. Altogether, new results show that the progress in Variscan orogenic research is converging towards a new paradigm involving more precise plate configurations and dynamics in Palaeozoic times. This volume of the International Journal of Earth Sciences is an evidence of this tendency and presents several studies documenting the fundamental importance of detailed geochronology, coupled with solid geological field data, for unravelling the complex geological history of the Variscan belt. Racek et al. use a novel approach to decipher the polyphase Variscan kinematics on the eastern margin of the Bohemian massif via kinematic modelling coupled with detailed Ar/Ar geochronology. They demonstrate the frontal character of the thrusting of the Moldanubian hot nappe over the easterly Brunia basement. The hornblende Ar/Ar data, yielding two peaks at c. 342 and 332 Ma, document the associated progressive erosion of the nappe during the thrusting and are correlated with published Ar/Ar ages on detrital muscovite from the adjacent foreland basement. Laurent et al. propose an impressive data set of geochronological and geochemical data on various types of both granitoid and mafic rocks from the eastern French Massif Central. They document that all these magmatic rocks The new series of international conferences focused on the Palaeozoic evolution of the European lithosphere started in 2007 in Orléans, followed by the successful meeting in 2012 in Sassari, and the last one in 2015 in Rennes. The first conference was dedicated to mechanical aspects of the Variscan orogeny and resulted in a highly successful Special Issue of Comptes Rendus Geoscience published in 2009. The second, Sassari conference was devoted to the extent and time scales of formation of the European crust and resulted in a number of excellent papers published in the form of a Special Publication of the Geological Society of London (# 405, 2014). The last, Rennes conference, was dedicated to the correlations and plate dynamics across the European Variscan belt and resulted in the current special issue of the International Journal of Earth Sciences. The papers published in this volume clearly show that the Variscan orogenic research evolves to become more modern and quantitative based in particular on detailed stateof-the-art geochronology and geochemistry, and the advent of numerical modelling. The community is critically using large and new data sets of high-resolution geochronological data combined with modern geochemistry and