Western Bohemian Massif Research Articles

A Protocol for Electron Probe Microanalysis (EPMA) of Monazite for Chemical Th-U-Pb Age Dating

A protocol for the monazite (LREE,Y,Th,U,Si,Ca)PO4 in situ Th-U-Pb dating by electron probe microanalyser (EPMA) involves a suitable reference monazite. Ages of several potential reference monazites were determined by TIMS-U-Pb isotope analysis. The EPMA protocol is based on calibration with REE-orthophosphates and a homogeneous Th-rich reference monazite at beam conditions of 20 kV, 50 nA, and 5 µm for best possible matrix matches and avoidance of dead time bias. EPMA measurement of samples and repeated analysis of the reference monazite are performed at beam conditions of 20 kV, 100 nA, and 5 µm. Analysis of Pb and U on a PETL crystal requires YLg-on-PbMa and ThMz-on-UMb interference corrections. Offline re-calibration of the Th calibration on the Th-rich reference monazite, to match its nominal age, is an essential part of the protocol. EPMA-Th-U-Pb data are checked in ThO2*-PbO coordinates for matching isochrones along regressions forced through zero. Error calculations of monazite age populations are performed by weighted average routines. Depending on the number of analyses and spread in ThO2*-PbO coordinates, minimum errors <10 Ma are possible and realistic for Paleozoic monazite ages. A test of the protocol was performed on two garnet metapelite samples from the Paleozoic metamorphic Zone of Erbendorf-Vohenstrauß (NE-Bavaria, western Bohemian Massif).

Radiogenic heat production of Variscan granites from the Western Bohemian Massif, Germany

Radiogenic heat production of Variscan granites from the Western Bohemian Massif, Germany

Cenozoic volcanism in the Bohemian Massif in the context of P‐ and S‐velocity high‐resolution teleseismic tomography of the upper mantle

AbstractNew high‐resolution tomographic models of P‐ and S‐wave isotropic‐velocity perturbations for the Bohemian upper mantle are estimated from carefully preprocessed travel‐time residuals of teleseismic P, PKP and S waves recorded during the BOHEMA passive seismic experiment. The new data resolve anomalies with scale lengths 30–50 km. The models address whether a small mantle plume in the western Bohemian Massif is responsible for this geodynamically active region in central Europe, as expressed in recurrent earthquake swarms. Velocity‐perturbations of the P‐ and S‐wave models show similar features, though their resolutions are different. No model resolves a narrow subvertical low‐velocity anomaly, which would validate the “baby‐plume” concept. The new tomographic inferences complement previous studies of the upper mantle beneath the Bohemian Massif, in a broader context of the European Cenozoic Rift System (ECRIS) and of other Variscan Massifs in Europe. The low‐velocity perturbations beneath the Eger Rift, observed in about 200km‐broad zone, agree with shear‐velocity models from full‐waveform inversion, which also did not identify a mantle plume beneath the ECRIS. Boundaries between mantle domains of three tectonic units that comprise the region, determined from studies of seismic anisotropy, represent weak zones in the otherwise rigid continental mantle lithosphere. In the past, such zones could have channeled upwelling of hot mantle material, which on its way could have modified the mantle domain boundaries and locally thinned the lithosphere.

Origin of earthquake swarms in the western Bohemian Massif: Is the mantle CO2 degassing, followed by the Cheb Basin subsidence, an essential driving force?

We propose a new model of the origin of earthquake swarms in West Bohemia/Vogtland (central Europe) by extensive CO2 degassing of carbonates in the metasomatized junction of three mantle domains. The associated volume change of the decarbonation reaction accounts for the continuing subsidence of the Cheb Basin adjacent to the major focal zone. The local stress perturbation created by the subsidence, in combination with the regional stress field, may account for the recurring swarm seismicity. The largest earthquake energy has been continuously released along a steep contact between orthogneisses of the uplifting Krušné Hory/Erzgebirge domain and granites of the subsiding Smrčiny/Fichtelgebirge domain, forming boundary between two lithospheric segments. The physical parameters of both lithologies, Poisson's ratio and bulk modulus, derived from the P- and S-wave velocities at different depths indicate that this high-friction suture might be able to accumulate deformation energy that is being released as periodically recurring seismic swarms. The proposed model represents an alternative to prevailing considerations suggesting that the earthquake swarms were triggered by pressurized fluids of mantle origin, whose sources are however separated from the earthquake foci.

Lithogeochemistry and Sr-Nd isotopic composition of Neoproterozoic metasedimentary rocks of the Teplá Crystalline Complex, western Bohemian Massif: a geotectonic interpretation

Clastic metasedimentary rocks from the Tepla Crystalline Complex (western Bohemian Massif) were analysed for major and trace elements, Sr and Nd isotopes. The metamorphic grade of these rocks of presumed Neoproterozoic protolith age increases from SE to NW from very low-grade to amphibolite-facies conditions. Geochemistry indicates that the sedimentary protoliths for the whole sequence consisted of immature (pelitic) greywackes chiefly derived from an ensialic island arc. No significant changes in composition from the lowest to the highest grade or across the strike of isograds were observed. Chemical variations between original slates and greywackes within a single locality often considerably exceed the variation among samples of different metamorphic grades or of different geographic positions. The prevailing REE spectra with distinct negative Eu anomalies show a close similarity with those of modern turbidites from ensialic island arcs. Several samples without any Eu anomaly resemble the REE patterns of less differentiated island arc andesites. LREE leaching under oxidizing conditions is suggested by several REE patterns with positive Ce anomalies. The Sm–Nd model ages TDM of samples with Ce positive anomalies are higher (TDM 1.8–2.0 Ga) than those of all other samples (TDM = 1.1–1.5 Ga). Initial Sr isotopic ratios for all samples are fairly constant and compatible with an assumed dominance of isotopically less evolved detrital material. Geochemical characteristics of the clastic metasediments of the Tepla Crystalline Complex are thus consistent with a model of incorporation and preservation of arc-derived sediments in a Cadomian accretionary wedge.

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.

Receiver function search for a baby plume in the mantle transition zone beneath the Bohemian Massif

SUMMARY The western Bohemian Massif is known for geodynamic phenomena such as earthquake swarms, CO2 dominated free gas emanations of upper-mantle origin, and Tertiary/ Quaternary volcanism. Among other explanations, a small-scale mantle plume has been suggested. We used data from the international passive seismic experiment BOHEMA (2001–2004) and of a previous seismic experiment to investigate the structure of the upper-mantle discontinuities at 410km and 660km depth (the ‘410’ and the ‘660’) beneath the Bohemian Massif with the P receiver function method. More than 4500 high-quality receiver function traces could be utilized. Two stacking techniques were used: stacking by station (common station method, CSM) and stacking by piercing points in the mantle transition zone (common conversion point method, CCM).Sincethestationspacingisveryclose,raysfromdifferentstationshavesimilarpiercing points in the mantle transition zone. Therefore CCM is sensitive in the transition zone and CSM issensitive totheuppermost structureof themantle. TheCSM showsdelayed conversion times from the 410km discontinuity beneath the western Bohemia earthquake region, which indicate a slow uppermost mantle. When stacking our data by CCM, we observe thickening of the transition zone towards the Alpine foreland, which agrees with tomographic results by Piromallo and Morelli. The thickness of the mantle transition zone beneath the western Bohemian Massif is normal, with a faint hint to thinning in the northern part. Ourconclusionisthataplume-likestructuremayexistintheuppermantlebelowthewestern Bohemia earthquake region, but with no or only weak imprint on the 410km discontinuity.

Shear wave crustal velocity model of the Western Bohemian Massif from Love wave phase velocity dispersion

We propose a new quantitative determination of shear wave velocities for distinct geological units in the Bohemian Massif, Czech Republic (Central Europe). The phase velocities of fundamental Love wave modes are measured along two long profiles (~200 km) crossing three major geological units and one rift-like structure of the studied region. We have developed a modified version of the classical multiple filtering technique for the frequency-time analysis and we apply it to two-station phase velocity estimation. Tests of both the analysis and inversion are provided. Seismograms of three Aegean Sea earthquakes are analyzed. One of the two profiles is further divided into four shorter sub-profiles. The long profiles yield smooth dispersion curves; while the curves of the sub-profiles have complicated shapes. Dispersion curve undulations are interpreted as period-dependent apparent velocity anomalies caused both by different backazimuths of surface wave propagation and by surface wave mode coupling. An appropriate backazimuth of propagation is found for each period, and the dispersion curves are corrected for this true propagation direction. Both the curves for the long and short profiles are inverted for a 1D shear wave velocity model of the crust. Subsurface shear wave velocities are found to be around 2.9 km/s for all four studied sub-profiles. Two of the profiles crossing the older Moldanubian and Tepla-Barrandian units are characterized by higher velocities of 3.8 km/s in the upper crust while for the Saxothuringian unit we find the velocity slightly lower, around 3.6 km/s at the same depths. We obtain an indication of a shear wave low velocity zone above Moho in the Moldanubian and Tepla-Barrandian units. The area of the Eger Rift (Tepla-Barrandian–Saxothuringian unit contact) is significantly different from all other three units. Low upper crust velocities suggest sedimentary and volcanic filling of the rift as well as fluid activity causing the earthquake swarms. Higher velocities in the lower crust together with weak or even missing Moho implies the upper mantle updoming.

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.

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.

Mantle fabric of western Bohemian Massif (central Europe) constrained by 3D seismic P and S anisotropy

New teleseismic data from a dense network of temporary and permanent seismic stations were analyzed to investigate large-scale fabric of the lithosphere–asthenosphere system beneath the western part of the Variscan Bohemian Massif. We study three-dimensional orientation and strength of seismic anisotropy and model a fabric of the mantle lithosphere from shear-wave splitting and directional terms of relative P residuals. Fast shear-wave polarizations have mostly E–W orientation and split-delay times δt are around 1.2 s, on average. Small but systematic changes of the fast S split polarizations show a difference at about 20° to 30° between the Teplá-Barrandian (TBU)/Moldanubian (MD) tectonic units and the adjacent part of the Saxothuringian (ST), and about 40° between two parts of the ST, separated by a suture identified in the upper crust by different authors and by other methods. On the other hand, P-velocity anisotropy clearly indicates three different orientations of fossil olivine fabrics with inclined axes of symmetry in the mantle lithospheres of three tectonic units (ST, TBU, MD). The seeming contradiction between the results of the two independent data sets can be explained by different 3D anisotropic models: one with hexagonal ‘slow’ symmetry axis b and divergently dipping ( a, c) foliations (ST, MD) and the other with hexagonal or orthorhombic symmetry with dipping high-velocity lineation a (TBU). These 3D self-consistent anisotropic models are compatible for both the P- and S-anisotropy observables. Our results suggest that a directionally varying constituent of the anisotropic signal is “frozen” in the mantle lithosphere. Regional changes in seismic anisotropy thus map tectonic boundaries, which cut the whole lithosphere, though with shifted crustal and mantle parts (TBU). Depending on symmetry and orientation of fabrics of the lithosphere domains, lateral changes are reflected in either P-velocity anisotropy and/or shear-wave polarizations. About half of the shear-wave split-time delays with predominantly E–W polarization azimuths represent a constant anisotropic signal which we associate with an olivine preferred orientation due to present-day flow in the asthenosphere. Our study emphasizes the importance of combining different methods of analysis and using complementary data sets in the three-dimensional analysis of mantle fabrics.

Age and emplacement of late-Variscan granites of the western Bohemian Massif with main focus on the Hauzenberg granitoids (European Variscides, Germany)

The Variscan Hauzenberg pluton consists of granite and granodiorite that intruded late- to postkinematically into HT-metamorphic rocks of the Moldanubian unit at the southwestern margin of the Bohemian Massif (Passauer Wald). U–Pb dating of zircon single-grains and monazite fractions, separated from medium- to coarse-grained biotite-muscovite granite (Hauzenberg granite II), yielded concordant ages of 320 ± 3 and 329 ± 7 Ma, interpreted as emplacement age. Zircons extracted from the younger Hauzenberg granodiorite yielded a 207Pb– 206Pb mean age of 318.6 ± 4.1 Ma. The Hauzenberg granite I has not been dated. The pressure during solidification of the Hauzenberg granite II was estimated at 4.6 ± 0.6 kbar using phengite barometry on magmatic muscovite, corresponding to an emplacement depth of 16-18 km. The new data are compatible with pre-existing cooling ages of biotite and muscovite which indicate the Hauzenberg pluton to have cooled below T = 250–400 °C in Upper Carboniferous times. A compilation of age data from magmatic and metamorphic rocks of the western margin of the Bohemian Massif suggests a west- to northwestward shift of magmatism and HT/LP metamorphism with time. Both processes started at > 325 Ma within the South Bohemian Pluton and magmatism ceased at ca. 310 Ma in the Bavarian Oberpfalz. The slight different timing of HT metamorphism in northern Austria and the Bavarian Forest is interpreted as being the result of partial delamination of mantle lithosphere or removal of the thermal boundary layer.

Metamorphic rocks of the Samerberg complex, Eastern Alps: 2. P-T paths and the problem of a geodynamic model

This paper continues the study of Perchuk et al. (2004), who reported a detailed petrographic description of peridotites and host gneisses composing the ultrahigh-pressure Samerberg complex. Part 2 presents the calculation of the P-T parameters of local equilibria and derivation of P-T paths: counterclockwise for spinel-gamet metaultrabasic rocks and decompression cooling for the host two-mica gneisses and migmatites. A possible clinoenstatite-orthoenstatite transition (characteristic twins in orthopyroxene) suggests high-temperature but shallow-depth, almost near-surface crystallization (quenching) of the ultrabasic protolith. During the subsequent burial of the already quenched rocks to great depths, metamorphic temperature increased but remained below the quenching temperature, which must be reflected in a clockwise P-T loop. Since the subsidence rate was higher than the rates of chemical reactions, complete reequilibration of the rocks upon heating was reached at maximum depths. Using these data, a geodynamic model was proposed for the formation and exhumation of the complex. It was found, in particular, that the P-T path of the gneisses closely reproduces the P-T path of the Erzgebirge gneiss dome in the western Bohemian Massif. This suggests that the thermal and dynamic Hercynian evolution of the entire Bohemian Massif can be adequately described by the model of lower crustal delamination.

Seismic evidence for whole lithosphere separation between Saxothuringian and Moldanubian tectonic units in central Europe

The Bohemian Massif is part of the Variscan belt of central Europe. We carried out a high resolution mapping of lithospheric thickness beneath central Europe by investigating 264 teleseismic events recorded at 80 broad band stations in the western Bohemian Massif with the method of S receiver function analysis. A negative phase beneath the Saxothuringian and northeastern Teplá‐Barrandian units at about 9–10 s before the S onset is interpreted as caused by the lithosphere‐asthenosphere boundary (LAB) at 80–90 km depth. In the Moldanubian unit, the negative phase occurs at 13–15 s before the S onset, corresponding to lithospheric thickness of 120–130 km. The boundary between the domains is oriented E‐W and probably marks the northern extension of Moldanubian lithosphere. The Moho also deepens from the Saxothuringian to the Moldanubian unit. The observed crustal/lithospheric domains could represent two distinct microplates with a relatively sharp boundary cutting through the whole lithosphere.

Upper mantle beneath the Eger Rift (Central Europe): plume or asthenosphere upwelling?

SUMMARY We present the first results of a high-resolution teleseismic traveltime tomography and seismic anisotropy study of the lithosphere–asthenosphere system beneath the western Bohemian Massif. The initial high-resolution tomography down to a depth of 250 km did not image any columnar low-velocity anomaly which could be interpreted as a mantle plume anticipated beneath the Eger Rift, similar to recent findings of small plumes beneath the French Massif Central and the Eifel in Germany. Alternatively, we interpret the broad low-velocity anomaly beneath the Eger Rift by an upwelling of the lithosphere–asthenosphere transition. We also map lateral variations of seismic anisotropy of the mantle lithosphere from spatial variations of P-wave delay times and the shear wave splitting. Three major domains characterised by different orientations of seismic anisotropy correspond to the major tectonic units—Saxothuringian, Moldanubian and the Tepla-Barrandian—and their fabrics fit to those found in our previous studies of mantle anisotropy on large European scales.

Intraplate seismicity in the western Bohemian Massif (central Europe): A possible correlation with a paleoplate junction

Locations of the Eger Rift, Cheb Basin, Quaternary volcanoes, crustal earthquake swarms and exhalation centers of CO2 and 3He of mantle origin correlate with the tectonic fabric of the mantle lithosphere modelled from seismic anisotropy. We suggest that positions of the seismic and volcanic phenomena, as well as of the Cenozoic sedimentary basins, correlate with a “triple junction” of three mantle lithospheres distinguished by different orientations of their tectonic fabric consistent within each unit. The three mantle domains most probably belong to the originally separated microcontinents – the Saxothuringian, Tepla-Barrandian and Moldanubian – assembled during the Variscan orogeny. Cenozoic extension reactivated the junction and locally thinned the crust and mantle lithosphere. The rigid part of the crust, characterized by the presence of earthquake foci, decoupled near the junction from the mantle probably during the Variscan. The boundaries (transitions) of three mantle domains provided open pathways for Quaternary volcanism and the ascent of 3He- and CO2-rich fluids released from the asthenosphere. The deepest earthquakes, interpreted as an upper limit of the brittle–ductile transition in the crust, are shallower above the junction of the mantle blocks (at about 12 km) than above the more stable Saxothuringian mantle lithosphere (at about 20 km), probably due to a higher heat flow and presence of fluids.

Seismic evidence for asthenospheric updoming beneath the western Bohemian Massif, central Europe

[1] Detailed images of the lithosphere beneath the western Bohemian Massif were obtained by analysis of more than 8500 P receiver functions. At the intersection of Regensburg-Leipzig-Rostock zone and Eger Rift, crustal thickness decreases to 26 km from approx. 31 km in the surroundings. The receiver functions display a positive phase at about 6 s delay time and a strong negative phase at 7 to 8 s, which coincides with an area of Moho updoming, CO2 mantle-derived degassing and earthquake swarm activity. These phases can be modeled by a velocity increase at 50 km and a velocity decrease at 65 km depth. The velocity decrease, observed over an area of 5300 km2, gives evidence for local asthenospheric updoming and/or a confined body of partial melt, which might be the cause for high CO2 mantle fluid flow and earthquake swarm activity in this recently nonvolcanic, intracontinental rift area.

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

The Tepla 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 Marianske Lazně 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.

Seismic structure and location of a CO2 source in the upper mantle of the western Eger (Ohře) Rift, central Europe

P‐SV conversions provide new insights into the lithosphere of the western Eger (Ohře) Rift, a presently active CO2 emanation area, Quaternary volcanic field, and earthquake swarm region in central Europe. Gas and isotope (He and C) mapping of free gas phases in mineral springs and mofettes proved the origin of CO2‐ dominated gases from a subcrustal magmatic fluid reservoir. Analyzing teleseismic data from several seismic networks in the western Bohemian Massif, the source region of these gases was investigated. Moho Ps conversions have 3 to 4.5 s delay. Crustal thicknesses vary between 27 and 38 km; vp/vs ratios vary between 1.63 and 1.81. Beneath the western Eger Rift an approximately 40 km wide Moho updoming up to 27 km exists. Locally observed weak conversions indicate a complex Moho transition zone in this area. A local “6 s phase” possibly originates at a discontinuity in approximately 50 to 60 km depth or may represent multiples from velocity inversions at the base of the upper crust. Moho updoming and the distribution of the “6 s phase” coincide with the CO2 degassing fields and the positions of Quaternary volcanoes at the surface. We hypothesize the release of CO2‐dominated fluid/magma from isolated melt reservoirs in the depth range of 60 to 30 km, separation of CO2 from the melt at 29 to 21 km depths, and CO2 transport through the crust. The geophysical indications may point to presently active magmatic underplating beneath the study area, supporting the results of gas geochemical and isotope investigations. This is the first attempt that combines seismic and gas geochemical data for a tectonic model. Our model may be transferable to other continental rift areas worldwide.