Tepla Barrandian Research Articles

Restoration of early-Variscan structures exposed along the Teplá shear zone in the Bohemian Massif: constraints from kinematic modelling

Perpendicular or high-angle relationship of lineations is common in shear zones where synchronous fabric transposition and shearing occur. This paper documents a polyphase early-Variscan structural record exposing the transition from orogenic superstructure to infrastructure along a major detachment shear zone at the western margin of the Tepla–Barrandian domain, Bohemian Massif. On the scale of tens of kilometers this region shows major but continuous reorientation of S2/S3 intersection lineation from its NNE–SSW trends in the superstructure to the ESE–WNW trends in the infrastructure. A simple kinematic numerical model, which allows deformation parameters such as fabric attractor, deformation symmetry, and proportion of simple shear, to be varied, was formulated to explain the rotation of fabrics observed in the field. In line with the results of our kinematic modelling, these rotations are interpreted as a result of progressive transposition of the originally subvertical S2 foliation into the gently SE-dipping S3 marked by stretching at high angle to the incipient S2–S3 intersection. The mineral–fabric relations suggest that the S2 was associated with crustal thickening and is well preserved in the superstructure, while S3 was associated with exhumation of deeper parts of the infrastructure. The kinematic model confirmed the operation of major detachment shear zone and allowed a set of the most probable deformation parameters leading to successful simulation of the fabric geometries to be identified. In addition, the successful model parameters were used to restore the vertical structure of the studied region prior to exhumation on the basis of available pressure–temperature data. We believe that unlike the commonly used balanced cross-section restoration, our restoration technique can be successfully applied in orogen interiors if the mineral–fabric relations are well-constrained and high-quality pressure–temperature estimates exist.

Precambrian basement in the Rheic suture zone of the Central European Variscides (Odenwald)

Detrital zircon age spectra of Ediacaran paragneiss from the Rheic suture between the Rhenohercynian Zone and Saxothuringian Zone suggest that they originated from different parts of peri-Gondwana. The paragneiss from the Northern Phyllite Belt displays an age spectrum of detrital zircons with a high amount of Neoproterozoic (82%) and Mesoproterozoic zircons (11%) typical for Amazonian provenance, whereas the spectrum from the metagreywacke of the Odenwald (Mid-German Crystalline Zone) shows a Mesoproterozoic age gap which is correlated with the West African Craton. The metagreywacke of the Odenwald contains 20% Paleoproterozoic and 32% Archean zircons, whereas the paragneiss of the Northern Phyllite Belt (Wartenstein Crystalline) contains only 6% Paleoproterozoic and no Archean zircons. The paleoposition of the basement of Northern Phyllite Belt was proximal to the Avalonian magmatic arc of the London–Brabant high. The Armorican metagreywacke of the Odenwald occupied a distal position to a Neoproterozoic magmatic arc, probably in a back-arc basin related to the West African Craton. Such a U–Pb age spectrum of detrital zircons together with a Mesoproterozoic age gap is typical for sediments of Armorica in Europe during the Ediacaran to Carboniferous. Neoproterozoic igneous rocks extruded at 566 ± 2 Ma forming a volcano-clastic sequence of the Cadomian magmatic arc which is the wall rock of the Silurian to Carboniferous plutons of the entire West Odenwald. This is the first occurrence of an extensive Cadomian crystalline basement in the Mid-German Crystalline Zone. Metagranite dykes crosscut the foliation of the Cadomian para- and orthogneiss at 542 ± 3 Ma. The deformation and migmatization of the Cadomian basement are bracketed between 566 and 542 Ma. A similar late Cadomian event is known from the Bohemian Massif and the Armorican Massif. Large Cadomian plutons with an age around 540 Ma, like that of the northern Odenwald, are common for Armorica. Silurian to Devonian granitoids (434 ± 4 Ma, 411 ± 5 Ma) are witness to an active margin along the northern boundary of Armorica. The Cadomian basement of the Odenwald together with the Palaeozoic granitoids is overprinted by a high-grade metamorphism at 384 ± 4 Ma (U–Pb on zircon) and cooled down below ca. 500 °C at 370 Ma (K–Ar ages of amphibole; U–Pb on titanite). Such a combination of late Cadomian and early Variscan ages could be correlated with the Munchberger Nappe, the Tepla–Barrandian Unit, Central Armorican Domain and the Massif Central.

First U–Pb LA-ICP-MS zircon ages assessed from a volcano-sedimentary complex of the mid-European Variscides (Pennsylvanian, Flöha Basin, SE Germany)

Nine radiometric ages constrained by U–Pb isotopes from magmatic zircon grains are provided for different volcanic rocks from the Floha Basin and its surroundings. For most of them, this is the first reliable data to correlate with the International Chronostratigraphic Chart. Measurements were obtained with LA-ICP-MS and revealed middle Pennsylvanian ages (311–308 Ma) allowing first consistent correlation to other Variscan volcano-sedimentary basins of the Saxo-Thuringian and Tepla Barrandian zones. The Obermuhlbach Volcano, formerly considered to be early Permian in age, is shown to be of middle Pennsylvanian age. In contrast, the dykes from Metzdorf village and Oederan have shown to be not Pennsylvanian, but rather of early Permian age. Biostratigraphic data obtained from the Floha Formation macroflora coincide with the new isotopic evidence which encompasses a likely time span from the middle to late Bolsovian (Westphalian C) up to the Bolsovian–Asturian boundary. The results not only yield a modified picture of the Floha Formation and their intercalated pyroclastics. As new insights challenge previous geological mapping they contribute to the understanding of the complex volcano-tectonic processes in the type area of the Saxo-Thuringian zone of the European Variscides.

Failed Silurian continental rifting at the NW margin of Gondwana: evidence from basaltic volcanism of the Prague Basin (Teplá–Barrandian Unit, Bohemian Massif)

The Silurian volcanic rocks of the Prague Basin represent within-plate, transitional alkali to tholeiitic basalts, which erupted in a continental rift setting through the thick Cadomian crust of the Tepla–Barrandian Unit (Bohemian Massif). Despite the variable, often intense alteration resulting in post-magmatic replacement of the basalt mass due to carbonatization, the geochemical signatures of Silurian basalts are still sufficiently preserved to constrain primary magmatic processes and geotectonic setting. The studied interval of Silurian volcanic activity ranges from Wenlock (Homerian, ~431 Ma) to late Ludlow (Gorstian, ~425 Ma) with a distinct peak at the Wenlock/Ludlow boundary (~428 Ma). Trace-element characteristics unambiguously indicate partial melting of a garnet peridotite mantle source. Wenlock basalts are similar to alkaline OIB with depleted radiogenic Nd signature compared to Ludlow basalts, which are rather tholeiitic, EMORB-like with enriched radiogenic Nd signature. The correlation of petrogenetically significant trace-element ratios with Nd isotopic compositions points to a mixing of partial melts of an isotopically heterogeneous, possibly two-component mantle source during the Wenlock–Ludlow melting. Lava eruptions were accompanied by intrusions of doleritic basalt and meimechite sills. The latter represent olivine-rich cumulates of basaltic magmas of probably predominantly Ludlow age. Meimechites with dolerites and, to a lesser extent, some lavas were subject to alteration due to wall-rock–fluid interaction. The trigger for the Wenlock-to-Ludlow (431–425 Ma) extension and related volcanism in the Prague Basin is related to far-field forces, namely slab-pull regime due to progressive closure of the Iapetus Ocean. The main stage of the Baltica–Laurentia collision then caused the Prague Basin rift failure at ca. 425 Ma that has never reached an oceanic stage.

Pressure–temperature estimates of the blueschists from the Kopina Mt., northern Bohemian Massif, Poland – constraints on subduction of the Saxothuringian continental margin

The blueschist occurrence at the Kopina Mt. is situated at the eastern margin of the Karkonosze–Izera Massif in the West Sudetes of the northern Bohemian Massif. Metabasic rocks with continental crust affinity occur as lenses within the prevailing metasediments. The metabasites consist mainly of garnet, blue amphibole, epidote, chlorite-I, titanite, hematite and quartz forming the high-pressure assemblage. Synkinematic garnet exhibits prograde zoning, a feature allowing for assessment of the prograde course of the pressure–temperature ( P–T ) path. Phase equilibrium modelling has been used to estimate prograde and peak metamorphic conditions. Based on the P–T pseudosections, calculated in the system Na 2 O–CaO–K 2 O–FeO–MgO–MnO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –O 2 (NCKFMMnASHTO), the garnet + glaucophane + epidote + chlorite + white mica(?) + Fe oxide + quartz ± titanite assemblage was formed between 12–15 kbar and 480–520 °C. These results are based on garnet and Na-amphibole compositional isopleths. Garnet shows a prograde zoning expressed by the rapid rim-ward decrease of spessartine, moderate increase of almandine and as light increase of other components. It is inferred here that white mica must have decomposed to secondary albite–chlorite-II–K-feldspar. The obtained results point to the formation of the Kopina blueschists along a low-temperature P–T gradient of 8–10 °C/km, typical of rocks from a subduction–exhumation channel. We postulate that the Kopina blueschists were formed in a subduction system developed along the eastern termination of the Saxothuringian Ocean and operating beneath the Tepla–Barrandian upper plate. When combined with previous data on continental crust affinity for the protolith, our results demonstrate a derivation of the Kopina blueschists from subducted part of the Saxothuringian margin.

Nd and Sr isotopic evidence for provenance of clastic material of the Upper Triassic rocks of Silesia, Poland

Nd and Sr isotope data were used to characterize the sources of the Upper Triassic (Keuper) siliciclastic rocks of Silesia in southern Poland. This continental succession, consisting predominantly of fine-grained mud- stones and siltstones, yields a remarkably uniform Nd isotopic composition. Nd model ages T 2DM vary from 1.56 to 1.69 Ga and e Nd values are in the range from –8.9 to –11.2, documenting old crust contribution in the provenance. In contrast, the Sr isotopic composition ( 87 Sr/ 86 Sr) of the clastics exhibits a relatively large variation from 0.710 to 0.723. The isotopic compositions indicate that the southern part of the Germanic Basin in Silesia was supplied with clastic material from the Bohemian Massif. The axis of the drainage area must have crossed from SW to NE the Saxothuringian units of the East Sudetes and most probably also the area of the Tepla–Barrandian Unit. There is no indication of any sediment transport from the Moravo-Silesian Belt and the Fore-Sudetic Block. It seems, that the Palaeozoic rocks of the latter domain must have been buried completely during Late Triassic times.

Resolving the Variscan evolution of the Moldanubian sector of the Bohemian Massif: the significance of the Bavarian and the Moravo-Moldanubian tectonometamorphic phases

*Corresponding author The Variscan evolution of the Moldanubian sector in the Bohemian Massif consists of at least two distinct tectonometamorphic phases: the Moravo–Moldanubian Phase (345–330 Ma) and the Bavarian Phase (330–315 Ma). The Moravo–Moldanubian Phase involved the overthrusting of the Moldanubian over the Moravian Zone, a process which may have followed the subduction of an intervening oceanic domain (a part of the Rheiic Ocean) beneath a Moldanubian (Armorican) active continental margin. The Moravo–Moldanubian Phase also involved the exhumation of the HP–HT rocks of the Gfohl Unit into the Moldanubian middle crust, represented by the Monotonous and Variegated series. The tectonic emplacement of the HP–HT rocks was accompanied by intrusions of distinct magnesio-potassic granitoid melts (the 335–338 Ma old Durbachite plutons), which contain components from a strongly enriched lithospheric mantle source. Two parallel belts of HP–HT rocks associated with Durbachite intrusions can be distinguished, a western one at the Tepla–Barrandian and an eastern one close to the Moravian boundary. The combined occurrence of Durbachite plutons and HP rocks would be difficult to understand in terms of the previous tectonic models, in which the Gfohl Unit was viewed as a large flat nappe on top of the Moldanubian Zone. In recent studies it has been suggested that Saxothuringian crust was subducted eastwards under the Moldanubian Zone during the Early Carboniferous. We discuss here an alternative tectonic scenario, in which the south-eastern Bohemian Massif is tentatively interpreted as an accretionary wedge successively underplated by material of a Gfohl and a Moravian terrane. It is suggested that parts of the HP–HT rocks of the Gfohl Terrane were exhumed along the Moravian–Moldanubian plate contact, while earlier subducted portions were steeply uplifted close to the Tepla–Barrandian block, which may have functioned as a rigid backstop of the accretionary wedge. Final stages of the Moravo–Moldanubian Phase were characterised by a strong LP–HT regional metamorphism at c. 335–340 Ma, which may be an expression of increased mantle heat flow after slab break-off, and is seen mainly in the Ostrong Unit along the central axis of the Moravo–Moldanubian Fold Belt. As indicated from palaeomagnetic data, the (already established) Moravo–Moldanubian Fold Belt has then (around 330 Ma) rotated by about 90° clockwise, while the palaeogeographic position of Baltica remained widely unchanged. This implies that the Moravian Zone lost its former contact to Baltica and that a major Late Variscan fragmentation of the Old Red continental margin must have occurred in the Moravo–Silesian area at that time. Also within the Bohemian Massif, this rotation event may have caused a significant Late Variscan (fault bounded) disturbance of previous terrane relationships. The Bavarian Phase (330–315 Ma) represents a fully independent stage of the Variscan orogeny in the Bohemian Massif. It is defined by a significant reheating (LP–HT regional metamorphism combined with voluminous granitic plutonism) and a tectonic remobilisation of crust in the south-western sector of the Bohemian Massif. These processes were most likely triggered by a Late Variscan delamination of mantle lithosphere. The Bavarian Phase overprinted western parts of the (widely cooled) Moravo–Moldanubian Fold Belt and transformed these rocks into various anatexites (metablastites, metatexites and diatexites). The HP–HT rocks of the Gfohl Unit, the Durbachite plutons, the LP–HT rocks of the Ostrong Unit and other typical constituents of the Moravo–Moldanubian Fold Belt can be followed from the Czech Republic southwards into eastern Bavaria and western Upper Austria (Muhl and Sauwald Zone), but are difficult to identify there due to the strong anatectic overprint. The LP–HT regions further west (Oberpfalz and western Bavarian Forest, Sumava Mts.?) may include former continuations of Tepla–Barrandian or Saxothuringian crust.

Mantle lithosphere control of crustal tectonics and magmatism of the western Ohře (Eger) Rift

*Corresponding author This paper summarizes our recent research on the relationship between tectonics of the mantle lithosphere and of the upper crust in the western Bohemian Massif. The rejuvenated Variscan junction of three mantle domains, the Saxothuringian, Moldanubian and Tepla–Barrandian, delimited by different orientation of seismic anisotropy reflecting a consistent fossil olivine orientation and belonging to originally separated microplates, plays important role in the crustal architecture of the region. The crust above the mantle junction is the most active part of the BM characterized by periodic earthquake swarms and mofettes emitting gases of mantle origin. Detected offsets of surface boundaries of the three units from their mantle counterparts indicate a detachment of the rigid upper crust from the mantle lithosphere. Locations of major tectonic features like the Ohře (Eger) Rift, the West Bohemian Shear Zone and the Marianske Lazně Fault were probably predisposed by the boundaries of the mantle lithosphere domains. We have suggested that the sublithospheric mantle of the western BM acted as a major provider for heat and partial melts and the Variscan mantle boundaries served as magma conduits for the late-orogenic granitic magmatism and, after their rejuvenation, also for the Cenozoic volcanism. Similarly, the boundaries may have pre-determined the subsequent locations of sedimentary basins. It is possible that also in other regions some of these so called intra-plate phenomena can be related to partly healed up and later rejuvenated microplate boundaries. List of abbreviations: BM – Bohemian Massif, BOHEMA – International project “BOhemian Massif HEterogeneity and Anisotropy”, BP – Bor Pluton, CHB – Cheb Basin, DEKORP Deutsches Kontinentales Reflexionsseismisches Programm, DP – Doupovske hory Mts., DU – Dyleň Unit, ECC – Erzgebirge (Krusne hory) Crystalline Complex, ECRIS – European Cenozoic Rift System, ER – Ohře (Eger) Rift, HSU – Horni Slavkov Unit, KH – Komorni Hůrka, KHF – Krusne Hory (Erzgebirge) Fault, KU – Kladska Unit, KVP – Karlovy Vary Pluton, MD – Moldanubian Unit, MLC – Marianske Lazně Complex, MLF – Marianske Lazně Fault, PF – Plesna Fault, SB – Sokolov Basin, SP – Smrciny (Fichtelgebirge) Pluton, ST – Saxothuringian Unit, T-junction – “triple” junction of the ST/TB/MD mantle lithospheres, TB – Tepla–Barrandian Unit, TCC – Tepla Crystalline Complex, TP – Tirschenreuth Pluton, WBSZ – West Bohemian Shear Zone, ZEV – Zone of Erbendorf–Vohenstrauss, ZH – Železna Hůrka, ŽP – Žandov (also Lesný–Lysina or Kynžvart) Pluton.

Pre-Late Carboniferous geology along the contact of the Saxothuringian and Teplá-Barrandian zones in the area covered by younger sediments and volcanics (western Bohemian Massif, Czech Republic)

The boundary between the Saxothuringian and the Tepla–Barrandian zones at the western margin of the Bohemian Massif represents an important tectonic suture of the Central European Variscides. However, most of this boundary is covered by Late Carboniferous and younger sedimentary and volcanic rocks, which prevent direct observation of particular geological units. We present a compilation of geological and depth measurement data from 12,134 exploration boreholes that reached the basement of the volcanic and sedimentary infill in the area of the Eger Graben in the north-western Bohemia, and correlate covered geological units with those exposed on the present-day surface. The resulting compilation reveals the relief of the sedimentary basins basement and interprets the real extent of the basement geological units in the western part of the Bohemian Massif. It also shows the position of the contact between units with the Saxothuringian and the Tepla–Barrandian affinities and suggests the boundary between rocks with Devonian metamorphic record and those metamorphosed during the Early Carboniferous period of the Variscan tectonometamorphic cycle.

Comparison of lithology and tectonometamorphic evolution of units at the northern margin of the Moldanubian Zone: implications for geodynamic evolution in the northeastern part of the Bohemian Massif

*Corresponding author Multidisciplinary research evaluates structural, metamorphic and petrochemical data of selected rock types in different units located in the northeastern part of the Bohemian Massif, Czech Republic: (1) the Stražek Unit in the northeastern part of the Moldanubian Zone, (2) the Svratka and Kutna Hora units correlated with the Orlice–Sněžnik Unit in Western Sudetes as well as (3) the Policka, Hlinsko and Zabřeh units belonging to the Tepla–Barrandian Zone. Petrochemical data for metasediments of the Policka, Hlinsko and Zabřeh units are mutually comparable and confirm a lithologi cal affinity to the upper-crustal Tepla–Barrandian Zone. The FeO t /MnO ratios in metasediments of the Stražek Unit and the Svratka Unit indicate differences in the origin of sedimentary protolith rocks. Relict pre-Variscan structures, including extensive migmatization, and high-grade mineral assemblages with peak metamorphic pressures of ~1.4 GPa in skarn bodies, are preserved in the Svratka Unit. The evidence for Palaeo-Variscan (390–355 Ma) HP and UHP events, recorded in the high-grade Kutna Hora and Orlice–Sněžnik units, was observed neither in the NE part of the Moldanubian Zone (Stražek Unit) nor the Tepla–Barrandian Zone (Policka and Zabřeh units). The Variscan orogenic event imprinted in the Svratka, Policka and Zabřeh units was the right-lateral slip along WNW–ESE trending shear zones. This deformation was accompanied by metamorphism at ~580–650 °C and ~0.5–0.7 GPa (352–343 Ma) and intrusion of numerous small bodies of syn-deformation calc-alkaline granitoids in the Policka and Zabřeh units. The younger metamorphic fabrics in the northeastern part of the Moldanubian Zone reflect a fast exhumation of deep-seated high-grade complexes at ~340 Ma, which was constrained by post-tectonic emplacement of durbachites at ~339 Ma. Metamorphic development in felsic granulites of the Stražek Unit, metamorphosed c. 340 Ma ago at 850 °C and 1.8 GPa, was followed by decompression to T ≅ 790 °C and P = 1.3 GPa and finally T = 700 °C and P = ~0.4 GPa. In contrast, Běst vina granulite in the Kutna Hora Unit, with the ~360 Ma high-grade metamorphism at 800–920 °C and 1.8–2.1 GPa, is free of such a HT–LP overprint. Thus the data indicate that the Svratka and Kutna Hora units, exhibiting numerous mutual differences, should not be considered as belonging to the Moldanubian Zone as they evolved as independent entities. The geochemical data on garnet–clinopyroxene skarns from the Moldanubian Zone, the Svratka and Kutna Hora units do not provide mutually distinguishing features. This is largely due to a very wide compositional variation in rocks interpreted as metamorphosed exhalite with detrital material admixture. Skarns from the Svratka Unit preserve clinopyroxenes with elevated jadeite component (0.5–24 mol. %) and prograde compositional zoning in garnet – features obliterated in samples from the Moldanubian Zone and the Kutna Hora Unit.

Tectonic setting of the Ohře/Eger Graben between the central part of the České středohoří Mts. and the Most Basin, a regional study

*Corresponding author The paper presents the results of a new detailed survey in the SW part of the Ceske středohoři Mts. (a section of the Ohře/Eger Graben/Rift), focused on its tectonic setting. Courses of the SE marginal graben faults are traced in detail. This limiting fault zone is described as a horst, with the graben fill being structured into rhomboidal blocks; older known faults are specified, and new faults are depicted. Respecting the age ranges of the preserved stratigraphic units, fault movements are classified into those older than 36 Ma, younger than 24 Ma and younger than 16 Ma (with a possible age less than 9 Ma), based on different lithologies and abrupt lithological changes (i.e., the rock boundaries). The faults are compared with the earlier defined paleostress fields. Fault reversals were detected on several faults. An important role of crystalline basement ridges during the graben formation was identified: three complex tectonic structures are spatially associated with these ridges. Two areas with distinct tectonic and geological development were defined within the Ohře/Eger Graben. The Saxothuringian – Tepla–Barrandian terrane boundary was found to be less important for graben formation during the Cenozoic. This disagrees with simplified riftogenic model of the Ohře/Eger Graben. The supposed “Litoměřice Deep-seated Fault”, possibly representing a first-order terrane boundary, could not be confirmed in the post-Paleozoic rocks – this structure did not function as a fault during Cenozoic times.

Reply to the discussion on “Emplacement, structural and P-T evolution of the ˜346 Ma Miřetín Pluton (eastern Teplá-Barrandian Zone, Bohemian Massif): implications for regional transpressional tectonics”

First of all we would like to appreciate all the remarks made by P. Pitra, which allow wider discussion of emplacement of the Miřetin Pluton and Variscan geodynamic evolution of its host metamorphic rocks in the northeastern Bohemian Massif. We believe that our results made by a combination of several methods of structural analysis, thermobarometry, new geochronological data and regional geological context of eastern part of the Bohemian Massif allow an alternative interpretation of the emplacement and geodynamic evolution of the Miřetin Pluton and uppercrustal host rocks in the eastern Tepla–Barrandian Zone as published by Vondrovic et al. (2011).

Gravity response of igneous rocks in the northwestern part of the Bohemian Massif

A new cross-border gravity map on the scale of 1:200,000 covering 14,900 km2 of the SE Saxony and NW Bohemia was compiled. It is limited by the sites of Grimma (NW), Karlovy Vary (SW), Neratovice (SE) and Bautzen (NE). Three positive gravity regions – (a) Lusatian Anticline, (b) SE part of the North Saxon Syncline and (c) Tepla–Barrandian Unit were delimited. Separation of the Bouguer anomalies into the regional and residual components together with the Linsser filtering provided three types of derived gravity maps (regional, residual and density boundaries) for geological interpretation. Eighteen negative residual anomalies mostly pertaining to partially buried granite or acid volcanic bodies and ten positive residual anomalies mostly caused by metamorphic complexes were identified. The map of the Linsser indications showing the density boundaries at three depth levels (1, 3 and 6 km) introduces not only numerous disjunctions but also indicates an internal structure of the individual regions. A new cross-border magnetic map covering the same area is also presented. A “central” circular gravity low (−61 mGal) delineates the Altenberg–Teplice Caldera extending to 10 km depth. Variscan igneous bodies produce only negative gravity anomalies regardless their size. Pre-Variscan igneous bodies cause either weak negative or positive anomalies. A chain of gravity and magnetic anomalies follows the Litoměřice Deep Fault and a large pronounced magnetic anomaly between Doupov volcanic complex (SW) and the Elbe Zone (NE) delineates the Saxothuringian/Tepla–Barrandian Suture Zone.

Deciphering the Variscan tectonothermal overprint and deformation partitioning in the Cadomian basement of the Teplá–Barrandian unit, Bohemian Massif

The Tepla–Barrandian unit (TBU) has long been considered as a simply bivergent supracrustal ‘median massif’ above the Saxothuringian subduction zone in the Variscan orogenic belt. This contribution reveals a much more complex style of the Variscan tectonometamorphic overprint and resulting architecture of the Neoproterozoic basement of the TBU. For the first time, we describe the crustal-scale NE–SW-trending dextral transpressional Krakovec shear zone (KSZ) that intersects the TBU and thrusts its higher grade northwestern portion severely reworked by Variscan deformation over a southeastern very low grade portion with well-preserved Cadomian structures and only brittle Variscan deformation. The age of movements along the KSZ is inferred as Late Devonian (~380–370 Ma). On the basis of structural, microstructural, and anisotropy of magnetic susceptibility data from the KSZ, we propose a new synthetic model for the deformation partitioning in the Tepla–Barrandian upper crust in response to the Late Devonian to early Carboniferous subduction and underthrusting of the Saxothuringan lithosphere. We conclude that the Saxothuringian/Tepla–Barrandian convergence was nearly frontal during ~380–346 Ma and was partitioned into pure shear dominated domains that accommodated orogen-perpendicular shortening alternating with orogen-parallel high-strain domains that accommodated dextral transpression or bilateral extrusion. The synconvergent shortening of the TBU was terminated by a rapid gravity-driven collapse of the thickened lithosphere at ~346–337 Ma followed by, or partly simultaneous with, dextral strike-slip along the Baltica margin-parallel zones, driven by the westward movement of Gondwana from approximately 345 Ma onwards.

Structure, emplacement, and tectonic setting of Late Devonian granitoid plutons in the Teplá–Barrandian unit, Bohemian Massif

The Stěnovice and Cista granodiorite–tonalite plutons are small (~27 and ~38 km2, respectively) intrusions that are largely discordant to regional ductile structures in the center of the upper-crustal Tepla–Barrandian unit, Bohemian Massif. Their whole-rock and trace-element compositions are consistent with medium-K calc-alkaline magma, generated above a subducted slab in a continental margin arc setting. The U–Pb zircon age of the Stěnovice pluton, newly determined at 375 ± 2 Ma using the laser ablation ICP-MS technique, is within the error of the previously published Pb–Pb age of 373 ± 1 Ma for the Cista pluton. The two plutons also share other characteristics that are typical of concentrically expanded plutons (CEPs), such as elliptical cross-section in plan view, steep contacts, inferred downward-narrowing conical shape, faint normal zoning, and margin-parallel magmatic foliation decoupled from the regional host-rock structures. We interpret the Stěnovice and Cista plutons as representing the initial Late Devonian stage of much more voluminous early Carboniferous arc-related plutonism (represented most typically by the Central Bohemian Plutonic Complex) in the upper crust of the central Bohemian Massif. These two plutons are important tectonic elements in that they indicate an overall shift of the arc-related plutonic activity from the ~NW to the ~SE, accompanied with a general compositional trend of the magmas from medium-K calc-alkaline to shoshonitic/ultrapotassic. Such a pattern is compatible with SE-directed subduction of the Saxothuringian Ocean beneath the Tepla–Barrandian overriding plate as a cause of arc-related magmatism in this part of the Bohemian Massif.

Links between the structure of the mantle lithosphere and morphology of the Cheb Basin (Eger Rift, central Europe)

The Cheb Basin (CHB), located in the western part of the Eger Rift (ER) and the western Bohemian Massif, is characterized by earthquake swarms, neotectonic crust movements and emanations of CO2 dominated gases of mantle origin. Deep structure of the region can be characterized as junction of three domains of mantle lithosphere with different olivine fabrics revealed by consistent orientations of seismic anisotropy. The domains represent mantle components of the major tectonic units (micro-plates): Saxothuringian (ST), Tepla-Barrandian (TB) and Moldanubian (MD), which were assembled during the Variscan orogeny. The ST-TB boundary, reactivated during the Cenozoic extension, controlled the position and development of the ER and the CHB. We show that the CHB originated above the rejuvenated mantle suture between the ST and TB. Though the basin is located within the ST crust domain, which is thrust over the mantle junction, it is the mantle suture that controls the CHB shape and its development through the allochthonous ST crust. The seismically active Marianske Lazně Fault limits the basin against the uplifted block of the Erzgebirge Crystalline Complex. The most subsided parts of the ER and CHB developed above the centre of the mantle transition, whereas a well expressed morphology developed above its flanks. Our study documents a long memory of the mantle lithosphere assembly inherited from the Variscan orogeny. It is possible that other continental regions also contain some of intra-plate basins that originated above healed palaeo-plate mantle boundaries.

The “Donauplatin”: source rock analysis and origin of a distal fluvial Au-PGE placer in Central Europe

The mineral assemblage and the sedimentological characteristics of the “Donauplatin” (Danubian fluvial placer containing platinum-group elements (PGE) and gold (Au)) are described for the first time in connection with upstream reference placer deposits near the potential source area in tributaries of the River Danube/Donau. Granulometric and morphometric data have been obtained using the CCD-based CAMSIZER technique. The platinum-group minerals (PGM; iridium, osmium, unknown iridium-osmium-sulfide, ruthenium-osmium-iridium alloys, platinum-iron alloys, iridium-bearing platinum, sperrylite) have been derived from ultramafic magmatic rocks, probably belonging to the ophiolitic series in the Tepla Barrandian unit of the Bohemian Massif. The Au-Pd-Cu compounds in the placer originated from dynamo-metamorphogenic processes in a sulfur-deficient environment at the SW edge of the Bavarian Basement. Gold in the “Donauplatin” has been reworked from a “secondary” or intermediate repository of lateritic gold (Boddington-type). Its primary source is supposed to be of orogenic origin. Provenance analyses of the associated non-heavy minerals point to high-pressure metamorphic rocks, igneous rocks (monazite) and high-temperature metamorphic rocks (750° to 850°C, zircon morphology). Garnet compositions indicate that meta(ultra)basic igneous rocks, calc-silicate rocks and skarns prevailed over paragneisses in the provenance area. Extraterrestrial processes creating the well-known Ries impact crater in the environs of Nordlingen during the Miocene have a minor share in the PGE budget by delivering molybdenum-ruthenium-osmium-iridium alloys and iridium solid-solution series (s.s.s.) minerals. Judging by the heavy mineral suites, Saxothuringian source rocks of the NE Bavarian Basement connected with the Donau River via the Naab River drainage system have not contributed to the element budget of the “Donauplatin” under study. Stream sediments which have been derived from this provenance area are characterized by low-temperature (LT) crystalline rocks and a considerable proportion of pegmatitic and metabauxitic material lacking in the Holocene sediments of the “Donauplatin”.

Bottom to top lithosphere structure and evolution of western Eger Rift (Central Europe)

We present model of the structure and development of the entire lithosphere beneath the western Eger Rift (ER). Its crustal architecture and paths of volcanic products are closely related to sutures/boundaries of uppermost mantle domains distinguished by different orientations of olivine fabric, derived from 3-D analysis of seismic anisotropy. Three different fabrics of the mantle lithosphere belong to the Saxothuringian (ST), Tepla-Barrandian (TB) and Moldanubian (MD) microplates assembled during the Variscan orogeny. Dipping fossil (pre-assembly) olivine orientations, consistent within each unit, do not support any voluminous mantle delamination. The variable rift structure and morphology depend on the character of the pre-rift suture between the northern ST unit and the TB/MD units in the southern rift flank. The proper rift with typical graben morphology has developed above the steep lithosphere-scale suture between the ST and TB units. This subduction-related boundary originated from the closure of the ST Ocean. Parts of the crust and mantle lithosphere were dragged there into asthenospheric depths and then rapidly uplifted. The suture is marked by abrupt change in the mantle fabric and sharp gradients in regional gravity field and in metamorphic grade. The secular TB-side-down normal movement is reflected in deep sedimentary basins, which developed since the Carboniferous to Cenozoic and in topography. The graben morphology of the ER terminates above the “triple junction” of the ST, TB and MD mantle lithospheres. The junction is characterized by offsets of surface boundaries of the tectonic units from their mantle counterparts indicating a detachment of the rigid upper crust from the mantle lithosphere. The southwest continuation of the rift features in Bavaria is expressed in occurrences of Cenozoic sediments and volcanics above an inclined broad transition zone between the ST and MD lithospheres. Schematic scenario of evolution of the region consists mainly of a subduction of the ST lithosphere to depths around 140 km, exhumation of HP-HT rocks and the post-tectonic granitoid plutonism.

Elevator tectonics and orogenic collapse of a Tibetan-style plateau in the European Variscides: the role of the Bohemian shear zone

Variscan collision of peri-Gondwanan terranes led to a doubly vergent crustal wedge that was thicker than 55 km in the area of the Bohemian Massif. This crustal thickness resulted in a highly elevated Bohemian plateau with a topographic height >3–4 km. The Bohemian plateau was covered with unmetamorphic Paleozoic strata, all of which are today well preserved in the Tepla–Barrandian unit because of crustal-scale vertical slip along the Bohemian shear zone (BSZ). The BSZ forms a subvertical, ca. 500-km long and up to 2-km wide belt of dip–slip mylonites which show several 90° deflections in map view. Tepla-Barrandian-down movements were active under retrograde metamorphic conditions, starting with granulite and ceasing with greenschist facies conditions. As slip along the BSZ was largely vertical and led to a minimum throw of 10 km, this type of crustal-scale deformation is referred to as elevator tectonics. The elevator-style movements caused the juxtaposition of the supracrustal Tepla–Barrandian lid (the “elevator”) against high-grade rocks of the extruding orogenic root. The BSZ has further governed the foci of mantle-derived plutonism. New U–Pb zircon and monazite TIMS dating of six plutons suggest that emplacement of mantle-derived melts along the BSZ lasted for at least 20 m.y., starting with the emplacement of the Klatovy granodiorite at 347 +4/−3 Ma and ceasing with the emplacement of the Drahotin pluton at 328 ± 1 Ma. When taking into account the new ages of synkinematic plutons, the simultaneous vertical slip along the individual segments of the BSZ (North, West, and Central Bohemian shear zone) is bracketed to the period 343–337 Ma. Elevator tectonics was probably controlled by delamination of thickened mantle lithosphere that caused a dramatic thermal turnover and heating-up of the orogenic root. The overheated lower crust was thermally softened by anatexis and diffusion creep resulting in channel flow, vertical extrusion, fast uplift, and exhumation of the orogenic root.

Control of paths of quaternary volcanic products in western Bohemian Massif by rejuvenated Variscan triple junction of ancient microplates

Our objective is to look for deep paths of Cenozoic volcanism and migration routes of active mantle volatiles through the lithosphere of the western Bohemian Massif. We show that the rejuvenated junction of three mantle domains, delimited by different orientation of seismic anisotropy and belonging to originally separated microplates — the Saxothuringian (ST), Moldanubian (MD) and Tepla-Barrandian (TB) — can provide the easiest upward routes of fluids through the deep lithosphere. Geographic distribution of mantle-fluid escapes at the surface suggests fluid migration through the ductile lower crust and through partly open faults in the rigid upper crust, which is locally detached and shifted from its lower part and from the mantle lithosphere. Present-day escapes of mantle-derived helium and CO2 concentrate mainly in two tectonically different crust edifices — in the Cheb Basin (CHB) and in an allochtonous block called the Marianske Lazně Complex (MLC). Crystalline basement of the CHB developed above the Variscan ‘triple junction’ of the mantle lithosphere domains. The basement was extended during the Cenozoic and dissected by systems of faults into small partly sunken blocks. Thanks to buoyancy the mantle fluids migrate upwards along the lithosphere junction into the faulted basement of the CHB. The highest CO2 flow and the highest 3He/4He ratios are observed at intersections of major normal faults and along the southern boundary of the Smrciny (Fichtelgebirge) granite Pluton. The fluid escapes are separated from the earthquake swarm epicentres. Routes of the fluids to the MLC are longer and more complicated. Surface escapes tap the mantle fluids mainly from the Marianske Lazně Fault (MLF) and from the tectonic boundaries along which the MLC block of the TB lower crust was thrust over the ST complexes. Hypocentres of earthquake swarms of the two major focal areas at Nový Kostel and Lazy, located mainly at depths of 6–13 km, reside either in granite or in underlying gneiss, while the escapes of mantle fluids follow major faults or boundaries of crystalline units outside the Smrciny and Karlovy Vary granite Plutons. We suggest that primarily those parts of faults in the upper crust, which is strengthened by granite magmatism and rigid enough to selectively accumulate stresses, are seismoactive. On the other hand, other parts of the faults tapping ascending mantle volatiles are ‘lubricated’ by the fluids and secondary mineralogical changes, and thus they cannot accumulate sufficient stresses to be released by earthquakes. A comparison of the most probable paths of the mantle fluids with the space-time distribution of the Nový Kostel hypocentres does not seem to support the model of the earthquake swarms triggered by pressurized fluids of mantle origin.