Earthquake hazard of the northern parts of the Bohemian Massif and Western Carpathians

The new procedure of earthquake hazard evaluation developed by Kijko and Sellevoll is tested and applied for the border region of Czechoslovakia and Poland. The new method differs from the conventional approach. It incorporates the uncertainty of earthquake magnitudes, and accepts mixed data containing only large historical events and recent, complete catalogues. Seismic hazard has been calculated for nine regions determined in the border area. In the investigated area, data of historical catalogues are uncertain or, in many cases, the epicentral intensities are unknown. Thus, a number of assumptions have to be adopted in data preparation of catalogues since the year 1200. The calculated values of parameters b in the Gutenberg-Richter frequency-intensity relation as well as the return periods, seem to be reasonable and are generally confirmed by the results obtained from catalogues for the last 80–130 years.     

The Precambrian tectogenesis in the Bohemian Massif

The Cadomian and Variscan tectogeneses are two distinctive and easily distinguished cycles of a long-term geological process leading to the almost complete crustal consolidation of the Bohemian Massif. The internal parts of the massif are those of the main development of the Upper Proterozoic geosyncline and the intensive Cadomian metamorphism and plutonism.The Precambrian of the Bohemian Massif is divided into two principal regional chronostratigraphic units: the Moldanubian and the Brioverian; the age of the boundary between them is estimated at about 1000 Ma B. P. The Brioverian is subdivided into three units: the Lower, Middle and Upper. The Moldanubian is provisionally subdivided into two units: the Lower and the Upper.The maximum of the metamorphic activity of the Cadomian cycle falls approximately within the sedimentation interval of the Middle Brioverian, and that of the Variscan cycle in the Devonian. Compared with the Cadomian regional metamorphism which attained mostly amphibolite to granulite facies, the Variscan metamorphism generally did not exceed greenschist facies.The origin of the granitoid rocks of the massif is closely associated with the metamorphic processes of the two above-mentioned cycles. Cadomian granitoids are represented mostly by rocks of granodiorite-tonalite and durbachite types whereas the Variscan intrusives are dominated by rocks of granitic composition.

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Zusammenfassung Die assyntische und variszische Orogenese sind die wesentlichen strukturprägenden Ereignisse im Böhmischen Massiv. Die internen Teile des Massives sind aus der jung-proterozoischen Geosynklinale hervorgegangen und während der assyntischen Gebirgsbildung metamorphisiert und von Magmatiten durchdrungen worden. Das Präkambrium des Böhmischen Massives ist in zwei chronostratigraphische Einheiten zu gliedern: das Moldanubium und das Brioverian. Die Grenze liegt etwa bei 1000 Mill. Jahren. Das Brioverian läßt sich in drei Einheiten gliedern und das Moldanubium in zwei Einheiten.Das Maximum der metamorphen Überprägung während der assyntischen Orogenese fällt in die Sedimentationslücke im mittleren Brioverian; im variszischen Zyklus erfolgt die Metamorphose im Devon. Die assyntische Metamorphose erreicht Amphibolit- bis Granulitfazies, die variszische geht nicht über eine Grünschieferfazies hinaus.Die magmatischen Gesteine sind eng mit den beiden Orogenesen verbunden. Die assnytischen Magmatite sind vorwiegend Granodiorit-Tonalite und Durbachite, während die variszischen Magmatite aus Graniten bestehen.

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Résumé Les tectogenèses cadomienne et varisque sont deux cycles différents, bien discernables, d'un processus géologique de longue durée, qui a conduit à la cosolidation presque complète du Massif de Bohème. La partie interne du Massif est celle du développement principal du géosynclinal protérozoïque supérieur et du métamorphisme et du plutonisme cadomiens.Le Précambrien du Massif de Bohème se divise en deux unités chronostratigraphiques régionales: le Moldanubien et le Briovérien. On estime l'âge de la limite entre elles à 1000 millions d'années environ. Le Briovérien est subdivisé en trois unités: inférieure, moyenne et supérieure. Le Moldanubien est divisé provisoirement en deux unités: inférieure et supérieure.La culmination de l'activité métamorphique du cycle cadomien tombe à peu près dans l'intervalle de sédimentation du Briovérien moyen, celle du cycle varisque dans le Dévonien. Comparé avec le métamorphisme régional cadomien qui le plus souvent a atteint le faciès amphibolite, le métamorphisme varisque, en général, n'a pas dépassé le faciès schistes verts.L'origine des roches granitoïdes du Massif et associée avec les processus métamorphiques des deux sus-dits cycles. Les granitoïdes cadomiens sont représentés pour la plupart par des roches du type des granodiorites-tonalites et durbachites, tandis que parmi les roches intrusives varisques prédominent des roches de composition granitique.

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, , . - , , , . : Moldanubium Brioverian. 1 . Brioverian , a Moldanubium . Brioverian; . ; . . - , — qg.

     

An Andean type Palaeozoic convergence in the Bohemian Massif

The geological inventory of the Variscan Bohemian Massif can be summarized as a result of Early Devonian subduction of the Saxothuringian ocean of unknown size underneath the eastern continental plate represented by the present-day Teplá-Barrandian and Moldanubian domains. During mid-Devonian, the Saxothuringian passive margin sequences and relics of Ordovician oceanic crust have been obducted over the Saxothuringian basement in conjunction with extrusion of the Teplá-Barrandian middle crust along the so-called Teplá suture zone. This event was connected with the development of the magmatic arc further east, together with a fore-arc basin on the Teplá-Barrandian crust. The back-arc region – the future Moldanubian zone – was affected by lithospheric thinning which marginally affected also the eastern Brunia continental crust. The subduction stage was followed by a collisional event caused by the arrival of the Saxothuringian continental crust that was associated with crustal thickening and the development of the orogenic root system in the magmatic arc and back-arc region of the orogen. The thickening was associated with depression of the Moho and the flux of the Saxothuringian felsic crust into the root area. Originally subhorizontal anisotropy in the root zone was subsequently folded by crustal-scale cusp folds in front of the Brunia backstop. During the Visean, the Brunia continent indented the thickened crustal root, resulting in the root's massive shortening causing vertical extrusion of the orogenic lower crust, which changed to a horizontal viscous channel flow of extruded lower crustal material in the mid- to supra-crustal levels. Hot orogenic lower crustal rocks were extruded: (1) in a narrow channel parallel to the former Teplá suture surface; (2) in the central part of the root zone in the form of large scale antiformal structure; and (3) in form of hot fold nappe over the Brunia promontory, where it produced Barrovian metamorphism and subsequent imbrications of its upper part. The extruded deeper parts of the orogenic root reached the surface, which soon thereafter resulted in the sedimentation of lower-crustal rocks pebbles in the thick foreland Culm basin on the stable part of the Brunia continent. Finally, during the Westfalian, the foreland Culm wedge was involved into imbricated nappe stack together with basement and orogenic channel flow nappes.     

Kinematics of crustal stacking and dispersion in the south-eastern Bohemian Massif

The south-eastern Bohemian Massif consolidated during the Late Variscan orogeny by the oblique collision of two continental crustal blocks after closure of an oceanic realm. One microcontinent comprises portions which are now distributed among Moravian and Moldanubian units and which are characterized by Late Proterozoic tectonothermal events, especially by granitoid intrusions. The other microcontinent includes the Gföhl gneiss and granulites (Gföhl nappe) of probable Early Palaeozoic protolith ages. Both continental blocks are separated by an ophiolite-like assemblage, which is preserved in portions of the Raabs unit.Oblique crustal stacking is accompanied by north-eastward propagation of nappes in a dextral transpressive regime. Exhumation of previously thickened crust is achieved by equally oriented bulk extension but partitioned in distinct displacement paths. Coeval stacking and extension at different crustal levels is suggested.Correspondence to: H. Fritz     

Some late precambrian microfossils from the Bohemian Massif and their correlation

A brief account is given of the main biostratigraphical conclusions resulting from the micropalaeontological study of the Bohemian Upper Proterozoic. The relatively rich microfossil material presently known from the Barrandian area enables comparison to be made both with Middle and Upper Brioverian microfossils of the West European Brioverian complex and also with those from the Upper Riphean and Vendian platform sediments. A correlation between the relatively close geographical areas of Bohemia, Lusatia and Saxony on the basis of microorganisms is evident. Biocommunities from siliceous rocks display algal-mat assemblages. Their features are very similar to those of biogenic rocks reported from other regions, especially Australia and North America. In the Moldanubian Supergroup, the ?eský Krumlov Formation, in which graphitized phytoclasts with anatomical structures of primitive land plants have recently been found, was examined. The question of the age of this formation remains open and its study is still in progress. The correlation of the sedimentary complex of the East Sudeten (the presumably Proterozoic Záb?eh Formation) with the Palaeozoic assemblage is possible due to the finds of Chitinozoa. Remains of megascopic algae were recently found in this assemblage, along with chilinozoan chambers. This association is highly specialized and comprises new taxa of higher Thallophytes.     

Climate change in Central Europe and the sensitivity of the hard rock aquifer in the Bohemian Massif to decline of recharge: case study from the Bohemian Massif

Climate change in Central Europe is manifested by periods of drought during the summer months of years when there is a reduction in atmospheric precipitation and temporary accumulation of water in the form of snow during winter period. The shallow sub-surface aquifer in crystalline rocks is very vulnerable to this decline in rainfall. Monitoring and subsequent model simulations revealed that the groundwater level in recharge areas had been lowered by almost 10 m, but the study also showed that levels in discharge areas are significantly more stable. The transmissivity of hard rock aquifers evidently shows changes in both space and time. The temperature rise results in increasing values of evapotranspiration that is manifested by a gentle but long-lasting groundwater-level decline.     

Phosphorus in regionally metamorphosed skarns of the Bohemian Massif and its genetic meaning

Concentrations of phosphorus have been determined in 130 specimens o regionally metamorphosed skarn rocks, including their enclosing rocks Average P₂O₅-values found were 0.11% in the pyroxene skarns from the Core of the Bohemian Massif and 0.07% in those of the Kruné hory Mountains, which correspond to those commonly present in primary, i.e. contact metamorphic skarns, but are quite different from the high P₂O₅-values usually found in sedimentary iron ores. The P-contents of the rocks from the cores of the skarn bodies studied equal those determined in relic carbonate rocks, from which they probably originated, whereas the P-contents of the rocks from the outer parts of the skarn bodies correspond to those found in the enclosing rocks. The distribution of P in migmatites surrounding the skarn bodies suggests that P did not migrate during the regional metamorphism, so that the P-contents in the skarns may be considered as primary (premetamorphic). Thus, a study of P-percentages in regionally metamorphosed skarns may be useful in the investigation of skarn genesis.

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Zusammenfassung Die Phosphorgehalte von 130 Proben regionalmetamorpher Skarne, sowie ihrer Hüll- und Begleitgesteine wurden ermittelt. Der P-Durchschnittsgehalt der Pyroxenskarne beträgt im Kern der Böhmischen Masse 0,11%, im Erzgebirge 0,07%. Diese Gehalte entsprechen denjenigen kontaktmetamorpher Skarne, sind aber von den großen gewöhnlich in sedimentogenen Eisenerzen vorkommenden P-Gehalten verschieden. Die Phosphor-Gehalte der Skarngesteine aus den Kernpartien der Skarnkörper entsprechen denjenigen der reliktischen Carbonatgesteine, die die Skarne begleiten und auf deren Kosten sich die Pyroxenskarne wahrscheinlich entwickelt haben. Demgegenüber entsprechen die P-Gehalte der Gesteine aus den äußeren Partien der Skarnkörper denen der Hüllgesteine. Die Verteilung des Phosphors zwischen Metatekt und Paläosom in Migmatiten, die die Skarnkörper umhüllen, zeigt, daß Phosphor während der Regionalmetamorphose kaum migrierte. Seine Gehalte in Gesteinen lassen sich also für primär, d.h. prämetamorph halten. Wie ersichtlich, können Kenntnisse über die P-Verteilung zur Lösung der Fragen der Skarngenese beitragen.

     

The geochemistry of formation waters in the molasse basin of upper Austria

The geochemistry of formation waters in the Molasse basin of Upper Austria has been investigated to ascertain the extent of meteoric water replacement of the connate interstitial fluids in these sediments. The chemistry, isotopic composition, and dissolved gas contents of the groundwaters and of oil and gas associated brines have been determined. The most superficial sediments of the basin, the Innviertel (Miocene), have been completely flushed by meteoric waters within the last 200 ka. The underlying Hall and Puchkirchen formations (Miocene/Oligocene) form gas reservoirs for biogenic methane, and the associated formation waters are chemically and isotopically modified connate brines of the original marine deposition. In the northeastern part of the basin, the connate brines of the deeper sediments (Cretaceous/Jurassic) have been partially or completely replaced by meteoric waters, whereas in the south of the basin these sediments contain high salinity fluids which are substantially of connate origin. These conclusions are supported by the stable isotope composition of the various brines. Oil-associated brines from the Eocene sediments contain large amounts of dissolved radiogenic⁴⁰Ar, which suggests that the oils have migrated from high-temperature environments. This is in contrast with the Puchkirchen, for which the observed absence of radiogenic⁴⁰Ar suggests that the biogenic methane has been formed in situ. The⁴He contents of these brines and of the Cretaceous/Jurassic groundwaters are, however, less than those in the overlying Puchkirchen formation and suggest that He has been removed from the deeper sediments as a result of flushing by meteoric water. The ratios of dissolved methane and nitrogen to argon increase with increasing ammonium content of the formation waters. All of these parameters may be used as indices for the maturation of the system. The heavy noble gases, Kr and Xe, are abnormally abundant in the dissolved gases, and this is attributed to geochemical concentration of these gases by adsorption onto shales in the sediment sequence. The overall geochemical situation confirms the existence of separate hydraulic systems with little interconnection in the several overlying geological horizons.     

Conventional and in situ geochronology of the Teplá Crystalline unit, Bohemian Massif: implications for the processes involving monazite formation

The Teplá Crystalline unit (TCU), western Bohemian Massif, proves highly suitable for studying the effects of differential metamorphic reworking on the U–Th–Pb systematics in monazite, as the overprint of Variscan regional metamorphism onto high-grade Cadomian paragneisses intensifies progressively towards the northwest. Although variably hampered by scarcity, small size, and low uranium contents of monazite, isotope dilution–thermal ionisation mass spectrometry of monazite from paragneisses from the garnet, staurolite, and kyanite zones of the TCU gives a narrow ²⁰⁶Pb/²³⁸U age range from 387 to 382 Ma for Variscan peak metamorphism. These data are supported by 382–373 Ma monazite ages derived from electron microprobe analyses. Inheritance of older components in grains from the central TCU imply major “resetting” of pre-Variscan monazite around 380 Ma, possibly due to widespread garnet growth during Variscan metamorphism, which led to the consumption of pre-Variscan high-Y monazite and subsequent growth of new low-Y monazite. Concordant 498–494 Ma monazite ages in a migmatitic paragneiss close to the adjacent Mariánské Lázně Complex (MLC) grew in response to metagabbro emplacement in the MLC from 503 to 496 Ma and not during either Cadomian or Variscan regional metamorphism. Backscatter imaging and electron microprobe analyses reveal that discordant monazite of the migmatite comprises a mix of various age domains that range from ca. 540 to 380 Ma. Combined evidence presented here suggests that instead of Pb loss by volume diffusion, the apparent resetting of the U–Th–Pb systematics in monazite rather involves new crystal growth or regrowth by recrystallisation and dissolution/reprecipitation.     

Early metamorphic evolution and exhumation of felsic high-pressure granulites from the north-western Bohemian Massif

Summary Several granulite terrains are exposed in the Bohemian Massif of Central Europe. These were metamorphosed at pressures close to 12 kbar and temperatures of more than 800 °C c. 340 Ma ago. The corresponding penetrative deformation almost totally erased the record of the preceding metamorphic evolution. Nevertheless, rare relics such as mineral inclusions in large garnet grains are witness of this earlier evolution, which was previously related to significantly higher pressures and, thus, to a subduction-related event. The exemplary investigation of such mineral relics in a felsic granulite from the Granulitgebirge rather points to pressures of 13–14 kbar only at relatively low temperatures of 620 °C and, thus, to considerable, nearly isobaric heating before the exhumation of the granulites started at 800 °C or somewhat higher temperature. The inferred P–T evolution is compatible with a geodynamic model of lithospheric delamination, with crustal material having been involved. The delamination at c. 340 Ma ago followed long-lasting, continuous collision of Gondwana and Laurussia forming the Variscan orogen. Within the thickened continental crust, the delamination concerned mainly the dense basic material in the lower crust. This event also caused upwelling of the mantle asthenosphere. Both processes resulted in heating of the more felsic lower portion of the continental crust, thinner than before delamination. Heating by 200 °C or more caused prograde mineral reactions and created buoyancy forces, as the overlying crust became denser than the underlying hot and felsic granulites. As a consequence, considerable volumes of felsic granulite could have reached shallow crustal levels (corresponding to 3 to 4 kbar), conditions documented in granulite bodies in the north-western Bohemian Massif.     

Age and origin of detrital zircons from the pre-Permian basements of the Bohemian Massif and the Alps

U-Pb isotopic analyses were made on detrital zircon populations from sandstones and quartzites of the pre-Permian basement in an attempt to shed light on the presedimentary history of the zircons and the age of their primary source rocks. Eight rock samples were collected from the Saxothuringian and Moldanubian parts of the Bohemian Massif, the western part of the Upper Austroalpine Nappes, and the Southern Alps. The heterogeneous populations were separated into fractions of different size, magnetic susceptibility, color, and shape. Because of their typically pitted surface all zircon grains from the sandstones and quartzites appear to be detrital. Only in three samples from the Alps—one from a contact metamorphic aureole—the zircons show surface recrystallization and minor new growth. With the exception of some euhedral crystals in the Saxothuringian quartzites all zircon fractions have highly discordant U-Pb ages. On a concordia diagram their data points scatter slightly around best-fit lines with upper intersections between 2000 and 2300 m.y. From this pattern the following conclusions are reached:

1. A large proportion of the material of the metasedimentary basement rocks in the Bohemian Massif as well as in the Alps derives from one or more sources, about 2000 to 2300 m.y. old.
2. The estimated proportion of detrital zircons with primary ages of 700 to 1500 m.y. is less than 10%.
3. The existence of a regional high-grade metamorphism in the Bohemian Massif as well as in the Alps during 700 to 1500 m.y. can be excluded. From Rb-Sr isotopic data, a metamorphism for the time prior to 1500 m.y. is very unlikely.

The lower intersections of the best-fit lines with the concordia curve cannot be clearly correlated with an episodic disturbance of the U-Pb systems during weathering and sedimentation and/or during regional metamorphism. For the zircons of the Bohemian Massif a disturbing event, about 550 to 600 m.y. ago, is likely. Clear, euhedral, but nevertheless detrital zircons found among the zircon populations of two Saxothuringian quartzites (“Plattenquarzit” of the pre-Ordovician “Arzberger Serie” and Lower Ordovician “FrauenbachQuarzit”) crystallized most probably during the Upper Proterozoic and/or the Assyntian petrogenesis. The highly discordant age pattern of the detrital zircons from the Alps is likely to be the result of the Caledonian and/or Hercynian (=Variscan) metamorphism. Differences in concentration levels of common lead in detrital zircons and the problem of red zircons as indicators of Precambrian origin are discussed.     

Variscan Sm-Nd and Ar-Ar ages of eclogite facies rocks from the Erzgebirge, Bohemian Massif

Abstract The Erzgebirge Crystalline Complex (ECC) is a rare example where both‘crustal’eclogites and mantle-derived garnet-bearing ultramafic rocks (GBUs) occur in the same tectonic unit. Thus, the ECC represents a key complex for studying tectonic processes such as crustal thickening or incorporation of mantle-derived material into the continental crust. This study provides the first evidence that high-pressure metamorphism in the ECC is of Variscan age. Sm-Nd isochrons define ages of 333 ± 6 (Grt-WR), 337± 5 (Grt-WR), 360± 7 (Grt-Cpx-WR) (eclogites) and 353 ± 7 Ma (Grt-WR) (garnet-pyroxenite). ⁴⁰Ar/³⁹Ar spectra of phengite from two eclogite samples give plateau ages of 348 ± 2 and 355 ± 2 Ma. The overlap of ages from isotopic systems with blocking temperatures that differ by about 300 ° C indicates extremely fast tectonic uplift rates. Minimum cooling rates were about 50° C Myr-¹. As a consequence, the closure temperature of the specific isotopic system is of minor importance, and the ages correspond to the time of high-pressure metamorphism. Despite textural equilibrium and metamorphic temperatures in excess of 800° C, clinopyroxene, garnet and whole rock do not define a three-point isochron in three of four samples. The metamorphic clinopyroxenes seem to have inherited their isotopic signature from magmatic precursors. Rapid tectonic burial and uplift within only a few million years might be the reason for the observed Sm-Nd disequilibrium. The ε_Nd values of the eclogites (+4.4 to +6.9) suggest the protoliths were derived from a long-term depleted mantle, probably a MORB source, whereas the isotopically enriched garnet-pyroxenite (ε_Nd–2.9) might represent subcontinental mantle material, emplaced into the crust prior to or during collision. The similarity of ages of the two different rock types suggests a shared metamorphic history.     

Apatite fission-track dating and low-temperature history of the Bavarian Forest (southern Bohemian Massif)

Apatite fission-track (AFT) dating applied to uplifted Variscan basement blocks of the Bavarian Forest is employed to unravel the low-temperature history of this segment of the Bohemian Massif. Twenty samples were dated and confined track lengths of four samples were measured. Most samples define Cretaceous APT ages between 110 and 82 Ma (Albian to Campanian) and three samples give older ~148–140 Ma (Jurassic–Cretaceous boundary) ages. No discernible regional age variations exist between the areas north-east and south-west of the Pfahl shear zone, but >500 m post-Jurassic and post-Cretaceous vertical offsets along this and other faults can be inferred from elevation profile analyses. The AFT ages clearly postdate the Variscan exhumation history of the Bavarian Forest. Thermal modeling reveals that the ages are best explained by a slight reheating of the basement rocks to temperatures within the apatite partial annealing zone during the middle and late Jurassic and/or by late Cretaceous marine transgression causing burial heating, which affected marginal low-lying areas of the Bohemian Massif and the Bavarian Forest. Late Jurassic period was followed by enhanced cooling through the 120–60 °C temperature interval during the subsequent exhumation phase for which denudation rates of ~100 m myr^?1 were calculated. On a regional scale, Jurassic–Cretaceous AFT ages are ubiquitous in marginal structural blocks of the Bohemian Massif and seem to reflect the exhumation of these zones more distinctly compared to central parts.     

Exhumation, strain localization, and emplacement of granitoids along the western part of the Central Bohemian shear zone (Bohemian Massif)

Emplacement of granitoid magmas and simultaneous exhumation of deeply buried rocks has been investigated along the western part of the Central Bohemian shear zone (CBSZ, Bohemian Massif). Combined structural, petrological and geochronological data of the steeply dipping shear zone suggest complex uplift and exhumation of deeply buried, high-temperature Moldanubian rocks, resulting in the juxtaposition against the supracrustal Teplá-Barrandian unit. Uplift of Moldanubian rocks from depths of probably more than 30 km was initiated after crustal stacking in Upper Devonian times. Syntectonic Lower Carboniferous emplacement of the Klatovy pluton into the pre-existing shear zone led to melt-controlled strain softening and localization. However, the major part of the total displacement of the CBSZ was accommodated within a late- to post-intrusive high-temperature shear zone in the uprising Moldanubian unit and a post-intrusive unexposed fault zone in the Klatovy pluton, respectively. During uplift of the Moldanubian rocks, strain was strongly partitioned into melt-bearing zones (Klatovy pluton, migmatites of the Moldanubian unit) resulting in a repeated shift of deformation in space and time.     

Tectonometamorphic evolution of the Bohemian Massif: evidence from high pressure metamorphic rocks

The Variscan orogenic belt, of which the Bohemian Massif is a part, is typically recognized for its characteristic low pressure, high temperature metamorphism and a large volume of granites. However, there are also bodies of high pressure rocks (eclogites, garnet peridotites and high pressure granulites) which are small in size but widely distributed throughtout the Massif. Initially the high pressure rocks were considered to be relicts of a much older orogenic event, but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have Palaeozoic protoliths. Metamorphic ages from the high pressure rocks define no single event. Instead, a number of discrete clusters of ages are found between about 430 Ma and the time of the dominant low pressure event at around 320–330 Ma.Most of the eclogite and granulite facies rocks are assigned to allochthonous nappes that arrived close to the end of the low pressure event, but before final granite intrusion. The nappes contain a mixture of different units and the relationship between rocks with high pressure relicts and host gneisses with no apparent signs of deep burial is still problematic. Some of the high pressure rocks retain evidence of multiple stages of partial re-equilibration during uplift. Moreover, it can be shown in certain instances that host gneisses also endured a multistage metamorphic development but with a peak event convergent with one of the breakdown stages in the enclosed rocks with high pressure relicts. It thus appears that the nappe units are composite bodies probably formed during episodic intracrustal thrusting. Fluids derived from prograde dehydration reactions in the newly under thrusting slab are taken to be the catalysts that drove the partial re-equilibrations.On the scale of the whole Massif it can be seen within the units with high pressure relicts that the temperature at the peak recorded pressure and that during the breakdown are variable in different locations. It is interpreted that regional metamorphic gradients are preserved for given stages in the history and thus the present day dismembered nappe relicts are not too far removed from their original spatial distribution in an original coherent unit. From the temperature information alone it is highly probable that the refrigerating underthrusting slab was situated in the north-west. However, this north-west to south-east underthrusting probably represents the major 380–370 Ma event and is no guide to the final thrusting that emplaced the much thinned nappe pile with high pressure relicts.Granite genesis is attributed to the late stage stacking, during the final Himalayan-type collision stage, of thinned crust covered by young, water-rich, sediments — erosion products of the earlier orogenic stages. Regional metamorphism at shallow depths above the voluminous granites was followed by final nappe emplacement which rejuvenated the granite ascent in places. Correspondence to: P. J. O'Brien     

Adjusting stream-sediment geochemical maps in the Austrian Bohemian Massif by analysis of variance

The Austrian portion of the Bohemian Massif is a Precambrian terrane composed mostly of highly metamorphosed rocks intruded by a series of granitoids that are petrographically similar. Rocks are exposed poorly and the subtle variations in rock type are difficult to map in the field. A detailed geochemical survey of stream sediments in this region has been conducted and included as part of the Geochemischer Atlas der Republik Österreich,and the variations in stream sediment composition may help refine the geological interpretation. In an earlier study, multivariate analysis of variance (MANOVA) was applied to the stream-sediment data in order to minimize unwanted sampling variation and emphasize relationships between stream sediments and rock types in sample catchment areas. The estimated coefficients were used successfully to correct for the sampling effects throughout most of the region, but also introduced an overcorrection in some areas that seems to result from consistent but subtle differences in composition of specific rock types. By expanding the model to include an additional factor reflecting the presence of a major tectonic unit, the Rohrbach block, the overcorrection is removed. This iterative process simultaneously refines both the geochemical map by removing extraneous variation and the geological map by suggesting a more detailed classification of rock types.     

Brittle deformation events at the western border of the Bohemian Massif (Germany)

Investigations of brittle deformation structures, present within the crystalline rocks of the Bavarian Oberpfalz, reveal a complex late to post-Variscan crustal evolution. Upper Carboniferous (mainly Westphalian) granites were emplaced into semibrittle to brittle rocks of the ZEV (zone of Erbendorf-Vohenstrauß) and the EGZ (Erbendorf greenschist unit), respectively. From both the alignment of the granites and the direction of granite-related tension gashes a north-east-south-west extension must be assumed for the period of magmatic activities. Apart from the granite intrusions, rapid crustal uplift (about 1.5 km/my) led to an increase in the geothermal gradient from < 30 °C/km (late Variscan pre-granitic) to > 40 °C/km (late Variscan post-granitic). The increased geothermal gradient persisted during the succeeding reverse faulting which results from late Carboniferous (probably Stephanian) east-west and northeast-south-west compression. Although not evidenced directly in the area considered, strike-slip faults seem to have played an important part during the late Variscan crustal evolution, particularly in the Early Permian. The strike-slip events indicate further crustal shortening and indentation under north-south compression.A similar indentation was present in Cretaceous time. After a weak phase of Early Cretaceous reverse faulting, which results from north-south compression, strike-slip faults formed under north-west-south-east and north-south compression. All these faults, in particular the strike-slip faults, seem to be related to the Cretaceous and lowermost Tertiary convergence of the Alpine/Carpathian orogeny.A late stage of crustal extension, characterized by a radial stress tensor (₂ = ₃), is indicated through high angle normal faults which probably formed during the subsidence of the adjacent Neogene Eger Graben.     

Zircon dating of North Bohemian granulites, Czech Republic: further evidence for the Lower Carboniferous high-pressure event in the Bohemian Massif

U-Pb zircon and rutile multigrain ages and ²⁰⁷Pb/²⁰⁶Pb zircon evaporation ages are reported from high-pressure felsic and metapelitic granulites from northern Bohemia, Czech Republic. The granulites, in contrast to those from other occurrences in the Bohemian Massif, do not show evidence of successive HT/MPLP overprints. Multigrain size fractions of nearly spherical, multifaceted, metamorphic zircons from three samples are slightly discordant and yield a U-Pb Concordia intercept age of 348 ± 10 Ma, whereas single zircon evaporation of two samples resulted in ²⁰⁷Pb/²⁰⁶Pb ages of 339 ± 1.5 and 339 ± 1.4 Ma, respectively. A rutile fraction from one sample has a U-Pb Concordia intercept age of 346 ± 14 Ma. All ages are identical, within error, and a mean age of 342 ± 5 Ma was adopted to reflect the peak of HP metamorphism. Because rutile has a lower closing temperature for the U-Pb isotopic system than zircon, the results and the P-T data imply rapid uplift and cooling after peak metamorphism. The above age is identical to ages for high-grade metamorphism reported from the southern Bohemian Massif and the Granulite Massif in Saxony. It can be speculated that all these granulites were part of the same lower crustal unit in early Carboniferous, being separated later due to crustal stacking and subsequent late Variscan orogenic collapse.