extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

topographic inversion and changes in the sediment routing systems in the variscan orogenic belt as revealed by detrital zircon and monazite upb geochronology in post collisional continental basins abstract the basal alluvial and fluvial siliciclastic successions within the late carboniferous intracontinental basins of the west central bohemian massif provide an intriguing sedimentary record of changes in paleotopography and paleodrainage systems during and after orogenic collapse we analyze this record using the u pb laser ablation icp ms detrital zircon and monazite geochronology five distinct age groups have been identified across the examined stratigraphic succession i archean paleoproterozoic ii late neoproterozoic iii cambro ordovician iv late devonian and v mid to late carboniferous these age groups indicate mixing of both local and distal extra basinal sources and also point to a two stage post convergent geodynamic evolution of the variscan belt the first stage followed collapse of the central bohemian orogenic plateau at around 346 337 ma and involved rapid cooling at ca 340 320 ma and removal of ca 2 3 km of sedimentary overburden from the top of the plateau no sedimentary record of this erosional event suggests that the river systems transported detritus radially outwards from the inner part of the orogen to its forelands a paleorelief inversion and thus major reorganization of the sediment routing systems were then associated with uplift in the southerly and southwesterly peripheral domains of the orogen at ca 330 310 ma in turn these uplifted domains occupied by rapidly unroofed voluminous granite plutons shed detritus into the bohemian sink this was represented by the former plateau collapsed to relatively low elevations but still with significant internal paleorelief dominated by a system of tens of kilometers long nne ssw trending paleovalleys some of the material was further transported in extensive river systems crossing the former plateau to the northerly variscan foreland basin thus the radial sediment routing system changed into axial unidirectional finally we suggest that these major inversions of paleorelief and changes in the surface processes and paleodrainage systems may have been linked to distinct late orogenic lithospheric mantle delamination events first stage delamination occurred beneath the center of the bohemian massif to cause the plateau collapse denudation and radial sediment transport whereas the second stage delamination occurred along its periphery to cause uplift sourcing the intracontinental basins and unidirectional sediment transport end of deposition and magmatic shutdown at around 305 ma may then define the very cessation of late orogenic vertical movements and the transition to a post orogenic intra plate setting

variscan tectonics of central europe postaccretionary intraplate deformation of weak continental lithosphere the area occupied by the variscan belt of central europe forms part of gondwana derived microplates essentially avalonia and armorica that according to paleomagnetic data juxtaposed baltica and laurentia from the upper ordovician to the lower devonian the actual structure of this area is a puzzle of rather small fault bounded crustal blocks different in p t t histories which was created by later tectonic processes at around 360 to 320 ma according to a common view and by contrast to paleomagnetic data the structural record of fault bounded blocks is interpreted in terms of mid devonian to lower carboniferous amalgamation of narrow continental plates represented by the fault bounded blocks themselves however this is not supported in this article instead it will be shown that fault systems and fault bounded blocks were created by intraplate deformation of a weak domain of gondwana derived continental lithosphere significantly after its accretion deformation was partitioned into newly formed deep seated strike slip and kinematically connected reverse or normal detachment systems large translation magnitudes caused juxtaposition of crustal areas of variable thickness until they were emplaced along the fault systems the various categories of fault bounded complexes record different tectonic events category 1 metamorphic complexes represent deep seated parts of thickened crust some time before unroofing on the detachment systems between about 340 360 and 320 ma however the record of subduction and extreme crustal thickening during earlier stages of the collision history was erased to a high degree and is only preserved by mineral and structural relics strike slip and detachment systems propagated across and dismembered tectonic boundaries formed during these early events category 2 metamorphic complexes and the barrandian basin recording predominant pre late devonian metamorphic or stratigraphic events represent the brittle upper crustal parts during continuing metamorphism in the category 1 complexes weakly or unmetamorphosed lower paleozoic category 3 basins represent areas of thinned continental crust and record subsidence before and to some degree during activity of detachment systems the complex overall kinematic pattern of block bounding strike slip and detachment systems indicates compression and extension acting simultaneously this probably reflects the effects of continuing oblique plate convergence rotation and wsw extrusion of fault bounded blocks in addition to heating and extensional flow of previously thickened crustal areas

evolution of the middle westphalian river valley drainage system in central bohemia czech republic and its palaeogeographic implication abstract the basal unit of the kladno rakovnik basin krb in central and western bohemia the radnice member bolsovian is interpreted here as the fill of a combined incised and tectonically formed system of river valleys having a palaeotopography of the depocentre of up to 200 m restoration of the palaeotopography on basement surface is used to reconstruct the palaeo drainage system of this unit consisting of streams of four or five orders the main valleys form conjugated system of narrow straight and more then 15 km long tectonically controlled nw se and nne ssw striking valleys entered by incised valleys these erosional valleys consist of several orders and display a dendritic pattern the fill of this valley system the radnice member consists of two tectonically controlled sequences separated by a basin wide erosional surface with a relief of at least 20 m both units differ in their architecture as a consequence of base level changes expressed by the a s ratio accommodation sediment supply tectonics and sediment supply are considered as the most important controls on deposition whereas palaeotopography controlled the distribution of sediments within a valley system restorated palaeotopography indicates the existence of two orographically separated drainage systems in the krb the eastern part of the kladno rakovnik basin was drained to the nw possibly to the north variscan foredeep whilst its western part was probably drained through a central tectonic valley to the south southwest to the radnice and plze basins and then further ssw perhaps to the naab basin in se bavaria the krb was probably located about 1000 m above the carboniferous sea level

late variscan tectonic evolution by thinning of earlier thickened crust an 40ar39ar study of the montagne noire southern massif central france abstract 40ar 39ar age determinations on samples selected on deformational criteria form the basis of a proposal of a new detailed tectonometamorphic history of the montagne noire the southern part of the massif central france this range is classically divided into a cover and an axial gneissic and migmatitic core which was metamorphosed under high t medium p conditions the basement is structurally overlain by epimetamorphic paleozoic sediments in the north in the south equivalent rocks form a well known pile of recumbent nappes the variscan history of this area comprises early shortening and thickening of the continental crust the climax of this event took place around 320 ma ago as is shown by previous rb sr data and by 40ar 39ar measurements new developments in structural analyses have led to a better understanding of the late evolution of the area in terms of diapiric uplift of the core south southwestwards verging recumbent nappes have been emplaced and were weakly metamorphosed in their lower parts this movement was accompanied and followed by dextral extensional wrenching of the paleozoic cover along the northern and southern margins of the axial zone attendantly ductile shear zones were formed in order to decipher the complex history three structural levels have been dated with the 40ar 39ar method these levels are 1 the migmatitic and gneissic series in the axial core where micas yield ages of c 315 ma 2 mylonites from the northern and southern shear zones where biotites and muscovites yield ages of c 310 ma 3 muscovites and biotites from the epimetamorphic nappe structures yielding ages of c 297 ma the later ages may represent a younger thermal pulse these data are compared and discussed in the light of previous radiometric studies of the area and precise biostratigraphic constraints a detailed geodynamic evolution of the studied segment is proposed

thermotectonic evolution of the barrandian czech republic as revealed by apatite fission track analysis abstract a fission track ft study was carried out on apatite from upper proterozoic to upper carboniferous sedimentary and volcanic rocks in the barrandian czech republic apatite ft ages vary between 324 8 ma lower carboniferous and 196 14 ma lower jurassic mean confined track lengths range from 11 68 1 76 to 13 30 1 41 m the temperature time path of selected samples was simulated by applying the laslett annealing model and independent geological constraints from the barrandian the first stage of increased temperature that is recorded in nearly all samples occurred at the upper devonian lower carboniferous boundary the change in temperature is due to the thickening of the crust by increased sediment load and stacking of sedimentary rocks along thrusts ft data indicate a limitation of the extension of thrust sheets towards the west cooling during the carboniferous is attributed to exhumation of parts of the barrandian deposition of upper carboniferous to lower permian sediments caused a second increase in temperature in lower permian time variation in maximum temperature indicates variable thickness of the sedimentary cover the second cooling during the mesozoic is linked to the long term slow to medium exhumation process of the central variscan belt

extending a thickened crustal bulge toward a new geodynamic evolution model of the paleozoic nw bohemian massif german continental deep drilling site se germany abstract fault bounded tectonic metamorphic complexes assembling the nw bohemian massif around the german continental deep drilling ktb site are seen to be extremely heterogeneous in tectonic and metamorphic histories in current models the different complexes were supposed to reflect a puzzle of small pre devonian microplates and the related collision events supposedly lasted until the carboniferous opposed to these models it will be shown that all the boundaries among the complexes were formed by detachment late in a prolonged overall geodynamic history of a thickened crustal bulge during extensional tectonics and associated thermal events that outlasted the onset of collision in the silurian lower devonian by about 70 80 ma micro structures petrological and geochronological data of individual complexes predominantly preserve the late stages rather than the unbroken record of their tectonometamorphic histories such partial histories strongly different among individual complexes depict diverse snapshots taken at different places in the evolving thickened crustal bulge and at different instants in its overall evolution and do not define different precollisional microplates predominantly p t and deformation episodes after terrane juxtaposition are preserved this article presents an integrated view of the structural geology microscopic fabrics p t data and geochronology of such diverse metamorphic complexes this integrated view provides a new understanding of 1 the tectonic evolution during upper silurian devonian collision of the gondwana derived central european lithosphere with laurussia 2 the postaccretionary events that lasted through the upper carboniferous and 3 the earlier lower ordovician metamorphic and magmatic history which is only locally recorded metamorphic complexes occupying the structurally highest position upper tectonic complexes record devonian and earlier tectonometamorphic and magmatic events after the mid devonian such complexes did not experience any metamorphism the recorded devonian events consist of subduction and exhumation of hp rocks and their exhumation involved thrusting and extensional tectonics upper tectonic complexes comprise fragments of both the plate that was subducted in this period and the overriding plate in the footwall carboniferous extension has brought upper tectonic complexes against metamorphic complexes that essentially record lower and upper carboniferous tectonometamorphic and magmatic events lower tectonic complexes in the lower tectonic complexes such events are 1 consecutive extensional stages that created at least three sets of ductile normal detachment systems intersecting each other and various associated thermal pulses as well as 2 predetachment events that are only recorded as petrologic and structural relics transposed during extension inferred as predetachment events are crustal subduction as well as stacking that outlasted thrusting and exhumation of the upper tectonic complexes in the deeper portion of the thickened crustal bulge represented by the lower tectonic complexes spatial variations of p t t d histories during decompression occurred as a result of continued differential exhumation on the three sets of detachment systems and also from various thermal pulses that have affected different parts of this section during progressive shearing and exhumation to various degrees whereas the lower tectonic complexes of the erzgebirge gneiss dome preserve the record of a lower carboniferous history of hp metamorphism and crustal thickening followed by extension in those of the oberpfalz region ktb site a major upper carboniferous thermal pulse mostly erased the pre upper carboniferous strain fabrics and metamorphic record in the oberpfalz region ongoing extension emplaced mid crustal rocks that pervasively equilibrated in the upper carboniferous at high temperatures and low pressures low p t ratio against rocks not exposed to high temperatures at that time in summary a prolonged postaccretionary history concealed large scale structural relationships linked to crustal subduction and thickening associated with former juxtaposition of gondwana lithosphere to laurussia polyphase crustal extension and strike slip created the variety of different metamorphic tectonic complexes observed in the nw bohemian massif

potential field imaging of palaeozoic orogenic structure in northern and central europe abstract the trans european suture zone tesz is the most fundamental lithospheric boundary in europe separating the ancient crust of the fennoscandian shield east european craton from the younger crust of central europe and extending deep into the mantle geophysical potential field images provide an overview of the entire palaeozoic orogenic system of northern and central europe for the first time the tesz is largely concealed by sedimentary basins of permian cenozoic age geological observations are largely restricted to local basement highs and deep boreholes and the coverage of deep seismic surveys is widely spaced despite experiments recently acquired within the europrobe programme by contrast the potential field data offer a relatively detailed coverage of standardised observations throughout the tesz while some features of the images may be sourced in the near surface particularly in the gravity image much of their content reflects the structure of the underlying palaeozoic basement at the scale presented the images highlight the most fundamental features of the crustal structure of the tesz these include the strong contrast between the highly magnetic crust of the east european craton and the less magnetic palaeozoic accreted terranes of central europe the lateral continuity of terranes and their internal structure particularly where arc magmatic complexes are involved and the location and geometry of the terrane boundaries oceanic sutures and strike slip zones that separate them

strain and strain rate in a synkinematic trondhjemitic dike evidence for melt induced strain softening during shearing bohemian massif czech republic abstract emplacement related deformation of a cambrian trondhjemitic dike located at the western border of the tepla barrandian unit central european variscides is studied in detail the trondhjemitic melt was emplaced at t 750 c into an active low temperature ca 350 c east northeast trending dextral transcurrent shear zone thermal modelling indicates that the rheologically critical melt fraction and the solidus of the dike were achieved after 1 and 3 days of cooling respectively the major part of the shearing occurred within 8 days after the melt emplaced the zonation of the dike in strain magnitude mineralogy and geochemistry can be explained by the laterally varying rheology from margin to centre during cooling and shearing the deformation was volume constant and the strain data plot in the apparent constrictional field the unusually high value of the calculated longitudinal strain rate 2 8 10 6 s 1 equivalent to a displacement rate of 2 cm h 1 is probably related to the intruding melt that lubricated the shear zone and thus enhanced its displacement velocity consequently melts that intrude as dikes into active transcurrent shear zones may significantly weaken the strength of the middle and upper continental crust

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

ductile normal faulting along the west bohemian shear zone moldanubian tepla barrandian boundary evidence for late variscan extensional collapse in the variscan internides structural and kinematic investigations of the west bohemian shear zone wbs clearly indicate late variscan orogen parallel wsw ene extension within the variscan internides along the wbs the western part of the tepla barrandian tb was downthrown to the east against the adjacent moldanubian according to seismic data the steeply east dipping wbs flattens with depth forming a prominent detachment zone the western part of the tb was tilted along this zone producing the patterns of metamorphic isograds the age of which is probably cadomian cross cutting relationships of wbs mylonites and carboniferous granites as well as the overall cooling ages of hornblende and mica suggest that ductile normal faulting along the wbs was active from about 330 to 310 ma

crustal thinning recorded by the shape of the namurian westphalian leucogranite in the variscan belt of the northwest massif central france in the western part of the variscan belt of the french massif central the gueret massif is an extensional allochthon emplaced to the southeast during namurian westphalian time the internal structure of the coeval leucogranites surrounding the western edge of the gueret massif consists of the plateau d9aigurande brame st sylvestre and st goussaud massifs these plutons are asymmetric diapirs rooted in the marche bussiere madeleine and arrenes faults respectively the plutons indicate diverging extravasation from the gueret massif whatever their shapes the plutons have a northwest trending stretching lineation that is also marked in the granite host rocks by retrograde metamorphic minerals the same extension direction is also inferred from namurian westphalian leucogranites and dikes intruding the gueret massif extension related granitoids present typical structural patterns such as asymmetric shapes extravasation from the collapsing area and persistence of a stretching lineation parallel to the regional extension direction these features may be used to diagnose crustal thinning

forearc deformation and strain partitioning during growth of a continental magmatic arc the northwestern margin of the central bohemian plutonic complex bohemian massif abstract the late devonian subduction followed by the early carboniferous continental collision of the saxothuringian and overriding upper crustal tepla barrandian units led to the growth of a large magmatic arc the 354 337 ma central bohemian plutonic complex in the central part of the bohemian massif far field tectonic forces resulting from the collision produced wnw ese to nw se regional shortening across the forearc upper crust above the subduction zone the shortening was accommodated by predominantly top to the ese tectonic transport along the southeastern flank of the tepla barrandian unit approaching the magmatic arc margin the regional structural pattern changes and exhibits significant across and along strike variations interpreted as a result of strain partitioning where the saxothuringian tepla barrandian convergence interacted in different ways with the intruding magma pulses around the voluminous northeasterly 354 ma sazava pluton the principal shortening was at high angle to the forearc facing intrusive contact and the host rocks were significantly thermally softened the regional top to the ese tectonic transport converted here into arc parallel ductile flow within the structural aureole around and above the pluton in contrast a narrow to nonexistent ductile strain aureole is preserved in the host rocks around discordant sheet like plutons the southwesterly pre 354 ma marginal granite and the milin granodiorite of unknown radiometric age our ams study of the marginal granite and milin granodiorite and mapping of mesoscopic magmatic foliations and lineations in another neighboring sheet like pluton the 346 ma kozarovice granodiorite reveals sigmoidal map scale fabric patterns consistent with dextral transpression we thus suggest that the thin sheet like plutons were oriented obliquely to the principal shortening and were rheologically weaker than the host rocks prior to final crystallization producing dextral transpression recorded by the internal fabrics of these plutons our study shows that the far field plate kinematics during pluton emplacement is not the only factor that controls strain partitioning in continental magmatic arcs the two contrasting styles of pluton emplacement documented here indicate that the pluton shape orientation of intrusive contacts with respect to the background plate convergence vector and heat budget of intrusions magma volume composition and proportion of hot mantle component may also govern strain partitioning

a study of retrogression in eclogites of the oberpfalz forest north east bavaria west germany and their significance in the tectonic evolution of the bohemian massif

pore fluids and the lgm ocean salinity reconsidered abstract pore fluid chlorinity salinity data from deep sea cores related to the salinity maximum of the last glacial maximum lgm are analyzed using estimation methods deriving from linear control theory with conventional diffusion coefficient values and no vertical advection results show a very strong dependence upon initial conditions at 100 ky earlier inferences that the abyssal southern ocean was strongly salt stratified in the lgm with a relatively fresh north atlantic ocean are found to be consistent within uncertainties of the salinity determination which remain of order 1 g kg however an lgm southern ocean abyss with an important relative excess of salt is an assumption one not required by existing core data none of the present results show statistically significant abyssal salinity values above the global average and results remain consistent apart from a general increase owing to diminished sea level with a more conventional salinity distribution having deep values lower than the global mean the southern ocean core does show a higher salinity than the north atlantic one on the bermuda rise at different water depths although much more sophisticated models of the pore fluid salinity can be used they will only increase the resulting uncertainties unless considerably more data can be obtained results are consistent with complex regional variations in abyssal salinity during deglaciation but none are statistically significant

magmatic to solid state fabrics in syntectonic granitoids recording early carboniferous orogenic collapse in the bohemian massif abstract the 354 336 ma central bohemian plutonic complex is a variscan magmatic arc that developed in the central bohemian massif in response to subduction of the saxothuringian lithosphere beneath the tepla barrandian microplate magmatic to solid state fabrics in the most voluminous portion of this arc the 346 ma blatna pluton record two superposed orogenic events dextral transpression associated with arc parallel stretching and arc perpendicular shortening and normal shearing associated with exhumation of the high grade core of the orogen moldanubian unit this kinematic switch is an important landmark in the evolution of this segment of the variscan belt for it marks the cessation of subduction related compressive forces in the upper crust giving way to gravity driven normal movements of the tepla barrandian hanging wall block relative to the high grade moldanubian footwall we use thermal modeling to demonstrate that the emplacement of huge volumes of arc magmas and their slow cooling produced a thermally softened domain in the upper crust and that the magmatic arc granitoids may have played a major role in initiating the orogenic collapse in the bohemian massif through lubrication and reactivation of a pre existing lithospheric boundary and decreasing the overall strength of the rigid orogenic lid

hercynian low pressure high temperature regional metamorphism and subhorizontal foliation development in the canigou massif pyrenees france evidence for crustal extension a detailed study has been made of the structural and metamorphic history of part of the hercynian low pressure high temperature regional metamorphic terrane exposed in the canigou massif france microtextural relations between and compositional zoning within porphyroblast phases in pelitic rocks indicate that the pressure temperature time p t t trajectory followed by the massif during metamorphism involved decompression under prograde and retrograde conditions clockwise p t t trajectory the geometric relations between inclusion trails within these porphyroblasts and the structures in the adjacent matrix indicate that metamorphism was synchronous with the development of the regional subhorizontal foliation s3 the s3 foliation formed in response to noncoaxial bulk strain the absence of a marked hiatus in sedimentation in the pyrenean region during hercynian metamorphism and deformation and the widespread preservation of low grade hercynian metasedimentary rocks suggest that low p high t metamorphism occurred in an extensional tectonic environment and that extension manifested itself in asymmetric top to the northwest shear during d3

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

palaeozoic terranes and their lithospheric boundaries within the trans european suture zone tesz a review the trans european suture zone tesz is a broad and complex zone of terrane accretion separating ancient lithosphere of the baltic shield and east european craton eec from the younger lithosphere of western and southern europe there is debate about the number of terranes involved and their origins the most significant terrane boundaries originally oceanic sutures are poorly exposed and are defined using faunal provinciality prominent geophysical lineaments may represent sub terrane boundaries within composite terranes the avalonia composite terrane was amalgamated to baltica in latest ician time the provenance of the malopolska and moravo silesian terranes whose neoproterozoic basement is suspect with respect to baltica but with which they nonetheless share faunal affinities is still debated the extent of the variscide rhenohercynian zone defines the southern limit of the early palaeozoic accreted terranes terranes within the bohemian massif exhibit hp metamorphism recording subduction driven collision prior to incorporation in the tesz collage itself associated with a hp event in romania the saxothuringian zone and rheic suture are directly juxtaposed with the eec and the crustal structure of the tesz has been much modified by post palaeozoic tectonism deep seismic reflection data where available show that the oceanic sutures are frequently associated with inclined reflectivity zones irz in the lithospheric mantle here inferred to mark relict subduction zones interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the irz resulting from early orogenic lithospheric delamination at the moho interface and or post orogenic reordering processes the mantle irz appear to indicate a n directed polarity for early palaeozoic subduction apparently incompatible with the geological evidence which indicates subduction of the tornquist sea beneath avalonia and a large amount of underthrusting of avalonia by baltica possible explanations for these apparently incompatible observations are evaluated

late variscan strike slip tectonics between the tepla barrandian and moldanubian terranes czech bohemian massif petrostructural evidence in the variscan bohemian massif czech republic no agreement has been reached on the nature of the relationship between the tepla barrandian and the moldanubian terranes however the tepla barrandian is interpreted as a central segment separating the moldanubian and saxothuringian terranes and its position is therefore essential for any reconstruction of this part of the variscan belt our petrological and structural study in the domazlice klatovy region spans the boundary between the tepla barrandian and moldanubian terranes there is no conformity of ductile structures in both domains near their interface and their p t evolution is separate in time the development of major structures and associated metamorphism in the southwestern part of the barrandian was accompanied by syntectonic intrusions of granitoid and diorite bodies dated at about 520 ma the moldanubian migmatites are characterized by a lp ht metamorphism that possibly took place at about 320 ma the metamorphic record of the barrandian rocks shows no influence of the moldanubian partial melting the contacts are marked by semi brittle to brittle structures sinistral strike slip faulting developed in the central bohemian shear zone whereas the west bohemian shear zone acted as a semi brittle normal fault with a dextral component both shear zones may be nearly contemporaneous accommodating a bulk n s compression the central bohemian shear zone was later reactivated in a dextral regime although timing is uncertain the juxtaposition of both terranes took place probably in the waning stages of the variscan orogeny earlier than the late stephanian we argue that the late orogenic semi brittle strike slip tectonics modified considerably the face of the bohemian massif and the geodynamic models of the bohemian massif have to be reviewed with reference to this essential point

a 7000 year record of environmental change including early farming impact based on lake sediment geochemistry and pollen data from county sligo western ireland detailed chronologically tightly constrained lake sediment based geochemical and pollen records have enabled local changes in soil erosion woodland cover and composition and prehistoric farming impact to be reconstructed in considerable detail the profile opens shortly after 7800 bc when tall canopy trees were well established and presumably in equilibrium with their environment a distinct perturbation that involved an increase in pine and birch a decrease in oak and a minor opening up of the woodland is regarded as the local expression of the 8 2 ka climate anomaly lack of response in the geochemical erosional indicators is interpreted as evidence for drier conditions a short lived over compensation in climate recovery followed the 8 2 ka event neolithic farming impact is clearly expressed in both the pollen and geochemical data both datasets indicate that neolithic impact was concentrated in the early neolithic 3715 3440 bc in the interval 3000 2700 bc there appears to have been a break in farming activity the pollen data suggest substantially increased farming impact both arable and pastoral in the bronze age with maximum farming and woodland clearances taking place in the late bronze age 1155 935 bc these developments are poorly expressed in the geochemical record possibly due to within lake changes

the petrology of retrograded eclogites of the oberpfalz forest northeastern bavaria west germany abstract in the bavarian margin of the moldanubian basement of the bohemian massif retrograded eclogitic metabasites are found within cordierite gneisses associated with garnet free serpentinised peridotites the primary eclogite assemblage gar omph qz rut ky is rarely retained typically garnet pyr 12 37 alm 38 56 gro 14 29 is prograde zoned with thin retrograde rims and a kelyphitic corona of orthopyroxene plagioclase rare or mg hastingsite plagioclase an 60 common primary omphacitic clinopyroxene jd 25 35 is preserved within garnet groundmass omphacite jd 15 25 always showing an early exsolution of plagioclase is usually replaced by a vermicular di plag an 20 symplectite itself overgrown by a diablastic hastingsite plagioclase an 40 aggregate quartz is rimmed by hypersthene with an outer margin of diopside kyanite never preserved in the metabasites is altered anisochemically to a symplectite of hercynite anorthite commonly only anorthite aggregates remain geothermobarometric studies indicate equilibration around 710 c minimum pressure 15 kb for the primary eclogite assemblage the granulite facies overprint occurred at probably slightly higher temperatures but lower pressures with later retrogression into the amphibolite fades as mostly shown by the surrounding gneisses the p t t path preserved within these metabasic rocks is that expected from a region having undergone crustal thickening by subduction and crustal imbrication their present day surface exposure underlain by crust of normal thickness would indicate the presence of a basal thrust complex that has driven under the imbricate crustal pile aiding uplift and erosion a geometry confirmed by recent geophysical studies

geochemistry of shear zone hosted uranium mineralisation at the zadni chodov uranium deposit bohemian massif abstract samples from the zadni chodov uranium deposit of the bohemian massif were analysed using optical and chemical methods organic matter and brannerite were studied in detail to estimate conditions that prevailed in association with uranium enriched shear zones four major events are known in the high grade metasediments the pre ore stage is associated with the formation of shear zones during the uranium ore deposition stage temperatures ranged from 122 c to 258 c and coffinite and brannerite appeared two generations of post ore stage minerals are distinguishable a detailed study of a brannerite mineral grain revealed variations in its interior and surroundings but these are related to the composition of brannerite which is not a nominal mineral si and al are presented as chlorites but the main mineral phases are mixtures of anatase oxidized brannerite with non stoichiometric uranium oxide and carbonaceous matter carbonaceous matter in the samples studied appeared as a mixture of substances of different composition origin and age optical and structural characteristics showed variations in three coexisting types of metamorphosed carbonaceous matter including fine grained and larger lamella shaped particles that represent anthracite meta anthracite and semi graphite stages lamellas of crystalline graphite and pyrobitumen corresponding to impsonite the structure of organic particles was not affected by uranium minerals which is an interesting difference from changes that minerals undergo in for example coal biomarkers compounds used in assessing sources of organic matter were present in extracts but they were not a remnant of the original shear zone organic material they are likely products from a later impact probably of tertiary and younger sediments

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

lateral displacement of crustal units relative to underlying mantle lithosphere example from the bohemian massif abstract we propose a mechanical model of deformation of the entire lithosphere of the bohemian massif bm whose core is formed by an asymmetric block of the tepla barrandian tb unit in between the saxothuringian st and moldanubian md units for the modelling we have re processed p wave travel times recorded during the last two decades at dense networks of seismic stations installed in the bm during several passive seismic experiments we also use previous results of anisotropic studies based on splitting of teleseismic shear waves this allows us to refine estimates of the lithosphere thickness and delimit deep margins of the individual mantle lithosphere domains the domains are rigid enough to preserve pre orogenic olivine fabrics differently oriented in each of the units shapes and dips of the mantle boundaries representing major zones of weakness inherited from the variscan amalgamation of independent microplates indicate that north westward subductions beneath the tb unit dominated tectonic development of the core of the bm two mantle lithosphere domains with different fabric orientations separated by a wsw ene striking shear zone underlie the tb crust the nw domain is the tb mantle lithosphere while the se domain is the md mantle lithosphere thrust under the tb crust lithosphere of the north western tb domain compressed between early variscan subductions of the st continental lithosphere from the northwest and the md continental lithosphere from the southeast was pushed south westward by about 50 km though the crust of the south westerly tb promontory is commonly attributed to the md unit apparently it preserves the tb mantle lithosphere the shifted tb lithosphere provides compelling evidence in support of older views suggesting that the zone erbendorf vohenstrauss zev originally belonged to the tilted western rim of the tb unit during the final phase of the assemblage of the bm the rigid tb lithosphere was disrupted by the southward pushing st lithosphere along the newly formed nw se striking jachymov fault zone jfz this lithosphere scale process most likely changed the tectonic regime released subduction related forces and started the gravity dominated tectonics

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

accretion and exhumation at a variscan active margin recorded in the saxothuringian flysch the saxothuringian flysch basin on the north flank of the central european variscides was fed and eventually overthrust by the northwestern active margin of the tepla barrandian terrane clast spectra mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge the earliest clastic sediments preserved are of early famennian age ca 370 ma they are exposed immediately to the nw of the suture and belong to the par autochthon of the foreland besides ultramafic ophiolite material these rocks contain clasts derived from early paleozoic continental slope sediments originally deposited at the nw margin of the saxothuringian basin these findings together with the paleogeographic position of the famennian clastics debris on the northwestern passive margin indicate that the saxothuringian narrow ocean had been closed by that time microprobe analyses of detrital hornblendes suggest derivation from the randamphibolit unit now present in the middle part of the saxothuringian allochthon munchberg nappes detrital zircons of metamorphic rocks formed a little earlier ca 380 ma indicate rapid recycling at the tectonic front the middle part of the flysch sequence ca early to middle visean both in the par autochthon and in the allochthon contains abundant clasts of paleozoic rocks derived from the northwestern slope and rise together with debris of cadomian basement 500 ma granitoids and 380 ma early variscan crystalline rocks all of these source rocks were still available in the youngest part of the flysch c middle to late visean but some clasts record in addition accretion of the northwestern shelf our findings permit deduction of minimum rates of tectonic shortening well in excess of 10 30 mm per year and rates of exhumation of ca 3 mm a and possibly more

structural model for the saxothuringian moldanubian suture in the variscan basement of the oberpfalz northeastern bavaria f r g interpreted from geophysical data abstract the moldanubian saxothuringian boundary marks a prominent subduction zone in the internal parts of the european variscides in the oberpfalz area deep reflection seismic profiles show a high density of various reflectors mainly in the upper crust indicating a complex polyphase deformation during the collision process seismic structures and related data from surface geology suggest that the moldanubian saxothuringian collision consisted of two main phases the first phase is marked by extensive nw directed overthrusting of moldanubian onto saxothuringian crust along se dipping master decollements some of which can be traced down to the moho during an advanced stage progressive overthrusting led to the development of wedge structures associated with se directed backthrusting and backfolding in the upper structural levels subsequently some of these wedges and their related decollements were folded to form the main ne striking anticlines the first phase of crustal imbrication correlates with a regional low pressure high temperature metamorphism at about 320 ma decreasing in grade northwestward the second phase is characterized by a reorientation of the main direction of crustal shortening from nw se to sw ne which is interpreted as the result of the n directed indentation of the vindelician terrane into the moldanubian this event is recorded by conjugate systems of overthrusting and subsequent strike slip faulting in the oberpfalz and the bavarian forest and the black forest vosges area in the area of the southern german block between these basement outcrops the indentation front is marked by the arc shaped axes of gravimetric and magnetic anomalies and permo carboniferous troughs

rapid extensional unroofing of a granite migmatite dome with relics of high pressure rocks the podolsko complex bohemian massif the podolsko complex bohemian massif is a high grade dome that is exposed along the suprastructure infrastructure boundary of the variscan orogen and records snapshots of its protracted evolution the dome is cored by leucocratic migmatites and anatectic granites that enclose relics of high to ultrahigh pressure rocks and is mantled by biotite migmatites and paragneisses whose degree of anatexis decreases outwards our new u pb zircon ages indicate that the leucocratic migmatites were derived from early ordovician c 480 ma felsic igneous crust the same age is inferred for melting the proto source of the metapelitic migmatites the relics of high to ultrahigh pressure rocks suggest that at least some portions of the complex witnessed an early variscan subduction to mantle depths followed by high temperature anatexis and syntectonic growth of the podolsko dome in the middle crust at c 340 339 ma subsequently the dome exhumation was accommodated by crustal scale extensional detachments similar c 340 ma ages have also been reported from other segments of the variscan belt yet the significance of this tectonothermal event remains uncertain here we conclude that the 340 ma age post dates the high pressure metamorphism the high temperatures required to cause the observed isotopic resetting and new growth of zircon were probably caused by heat input from the underlying mantle and finally the extensional unroofing of the complex requires a minimum throw of about 8 10 km we use this as an argument for significant early carboniferous palaeotopography in the interior of the variscan orogen

microstructural evolution and geologic significance of garnet pyriclasites in the hoher bogen shear zone bohemian massif germany ductile extensional movements along the steeply inclined hoher bogen shear zone caused the juxtaposition of tepla barrandian amphibolites granulites and metaperidotites against moldanubian mica schists and paragneisses garnet pyriclasites are well preserved within low strain domains of this shear zone their degree of metamorphism is significantly higher than that of the surrounding rocks microstructural and mineral chemical data suggest in situ formation of the garnet pyriclasite by dehydration of pyroxene amphibolite at t 750 840 c and p 10 13 kbar including recrystallization accommodated grain size reduction of plagioclase and clinopyroxene nucleation of garnet and breakdown of amphibole into garnet clinopyroxene rutile subsequent decompression and retrograde extensional shearing led to the formation of mylonitic epidote amphibolite the presence of lower crustal and mantle derived slices within the hoher bogen shear zone supports the view that a in upper devonian times the tepla barrandian unit was thrust over moldanubian rocks as a complete crustal unit and b that during the subsequent lower carboniferous orogenic collapse the garnet pyriclasite and metaperidotite were scraped off from the basal parts of the tepla barrandian unit being dragged into the hoher bogen shear zone due to dramatic and large scale elevator style movements

a lifetime of the variscan orogenic plateau from uplift to collapse as recorded by the prague basin bohemian massif the ordovician to middle devonian prague basin bohemian massif represents the shallowest crust of the variscan orogen corresponding to c 1 4 km palaeodepth the basin was inverted and multiply deformed during the late devonian to early carboniferous variscan orogeny and its structural inventory provides an intriguing record of complex geodynamic processes that led to growth and collapse of a tibetan type orogenic plateau the northeastern part of the prague basin is a simple syncline cross cut by reverse thrust faults and represents a doubly vergent compressional fan accommodating c 10 19 nw se shortening only minor syncline axis parallel extension and significant crustal thickening the compressional structures were locally overprinted by vertical shortening kinematically compatible with ductile normal shear zones that exhumed deep crust in the orogen s interior at c 346 337 ma on a larger scale the deformation history of the prague syncline is consistent with building significant palaeoelevation during variscan plate convergence based on a synthesis of finite deformation parameters observed across the upper crust in the centre of the bohemian massif we argue for a differentiated within plateau palaeotopography consisting of domains of local thickening alternating with topographic depressions over lateral extrusion zones the plateau growth involving such complex three dimensional internal deformations was terminated by its collapse driven by multiple interlinked processes including gravity voluminous magma emplacement and thermal softening in the hinterland and far field plate boundary forces resulting from the relative dextral motion of gondwana and laurussia

some late precambrian microfossils from the bohemian massif and their correlation abstract 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 cesk 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 zab 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

rare earth element geochemistry of standard sediments a study using inductively coupled plasma spectrometry the rare earth elements ree and y have been determined in 15 international standard sediments from france german democratic republic canada and the u s a samples were prepared using conventional rock dissolution techniques and the ree and y were separated from other constituents using cation exchange chromatography the ree la ce pr nd sm eu gd dy ho er yb lu and y were analysed simultaneously by inductively coupled plasma atomic emission spectrometry results indicate that u s g s standard shale sco 1 cody shale is a good reference material to use for the normalisation of the ree distributions of sediments data normalised to sco 1 are more readily interpreted than those normalised to existing estimates of average shale the ree patterns of standard sediments vary considerably in shape span an order of magnitude in absolute concentration and provide valuable reference values for the ree analysis of other sedimentary materials

structural evidence for the relationship between thrusts extensional faults and granite intrusions in the variscan belt of galicia spain abstract new structural and petrofabric data are presented from the hombreiro pluton a synkinematically emplaced granite located in the northern part of the variscan belt of spain this pluton shows the imprint of two plastic deformations the first is related to the motion of the mondonedo nappe with a top to the east shear sense and the later one to the vivero fault an extensional shear zone with a top to the west motion these deformations initiated at high temperatures close to the granite solidus as shown by the preservation of c slip fabrics in quartz this fact indicates that extensional faulting was activated soon after the thrusting of the mondonedo nappe during which the hombreiro granite was emplaced we propose that the position of the vivero fault was controlled by a crustal instability induced by the intrusion of a number of plutons that delineate the footwall block of this fault

10 km minimum throw along the west bohemian shear zone evidence for dramatic crustal thickening and high topography in the bohemian massif european variscides the west bohemian shear zone wbsz forms a steep collapse structure along which east side down normal movements led to the juxtaposition of the relatively cold cadomian basement of the tepla barrandian unit against high grade moldanubian rocks synkinematic plutons straddle the wbsz the mut nin pluton intruded into moldanubian crust at a depth of 23 4 km as derived by using al in hornblende barometry the tepla barrandian babylon pluton intruded at 12 km depth as indicated by phengite barometry and petrogenetic considerations both emplacement depths together with mineral cooling ages result in a minimum vertical displacement of 10 km between 340 and 320 ma this large throw could be explained by over thickened crust that was weakened from below the alkaline signature of the mut nin diorite indicates that mantle melting was important to thermally weaken the crust at 340 ma the cold tepla barrandian upper crust sank into its weak partly molten moldanubian substratum resulting in elevator style movements not only along the wbsz but also along the hoher bogen and central bohemian shear zone all these ductile normal shear zones were active simultaneously during the lower carboniferous and dip steeply towards the tepla barrandian unit that probably formed a highly elevated plateau at this time

comment on dorr and zulauf elevator tectonics and orogenic collapse of a tibetan style plateau in the european variscides the role of the bohemian shear zone int j earth sci geol rundsch 2010 99 299 325 the present comment disproves the tectonic model of a late devonian early carboniferous tibetan style collisional plateau in the tepla barrandean tb part of the bohemian massif which later collapsed by thermal weakening of the underlying crust contrary to this model the tb neither reveals major crustal thickening nor uplift and erosion and eastern continuations of the tb were during the relevant time span areas of open marine sedimentation late devonian early carboniferous marine sediments widespread also in the armorican and central massifs of france testify to low topography in central parts of the variscan orogen notional traces of a permo carboniferous ice cap on the french massif central do not support the plateau model because they are questionable and much younger than the inferred plateau stage of the tb the relative uplift of high grade metamorphic rocks to the nw and the se of the tb is not due to sinking of an elevated tb but instead to the hydraulic and buoyant expulsion of hp material from the saxo thuringian and moldanubian subduction channels the rise of lower grade ht rocks along the southwestern margin of the bohemian massif was effected by late carboniferous transpression the high temperature and the resulting low viscosity of the rising materials were probably not caused by variscan mantle delamination but relate to lithospheric thinning and heating at the tip of the westward propagating tethys rift

alteration of granitoids and crystalline rocks and uranium mineralisation in the bor pluton area bohemian massif czech republic abstract the bor pluton belongs to the variscan granitoid plutons of the moldanubian zone of the central european variscides along with similar granitoid plutons of the western part of the bohemian massif leuchtenberg babylon fichtelgebirge it is close in its composition to low f biotite granites of the saxothuringian zone this paper investigates the hydrothermal alteration of the bor pluton in relation to uranium mineralisation uranium mineralisation of the bor pluton is associated with shear zones occurring on the western margin of the pluton on its boundary with high grade metasediments of the moldanubian zone zadni chodov and with metasomatic mineralisation evolved in hydrothermally altered biotite granites of the bor pluton vitkov ii lhota uranium mineralisation in altered granites is accompanied by their intense hematitisation albitisation chloritisation and carbonatisation hydrothermal alterations of granites were accompanied by the enrichment in u na p ti mg ca and depletion in si and k the altered high grade metasediments occurring in shear zones on the western boundary of the bor pluton are enriched in u ca mg and p and carbonaceous matter in the bor pluton area with typical coffinite uraninite association the presence of brannerite is significant for uranium mineralisation the observed coffinite is enriched in y up to 4 3 wt y 2 o 3 and zr up to 2 0 wt zro 2 unaltered brannerite displays low ca 0 9 4 5 wt cao al 0 0 0 5 wt al 2 o 3 fe 0 7 2 2 wt feo pb 0 0 1 4 wt pbo and th 0 0 2 8 wt tho 2 concentrations the highly altered brannerite is depleted in u enriched in ti si and al the temperature of the ore stage was estimated using chlorite thermometry and ranged from 122 c to 258 c

multiproxy evidence for terrestrial and aquatic ecosystem responses during the 8 2 ka cold event as recorded at hojby so denmark abstract a sediment succession from hojby so a lake in eastern denmark covering the time period 9400 7400 cal yr bp was studied using high resolution geochemistry magnetic susceptibility pollen macrofossil diatom and algal pigment analysis to investigate responses of the terrestrial and aquatic ecosystems to the 8 2 ka cold event a reduced pollen production by thermophilous deciduous tree taxa in the period c 8250 8000 cal yr bp reveal that the forest ecosystem was affected by low temperatures during the summer and winter early spring seasons this finding is consistent with the timing of the 8 2 ka cold event as registered in the greenland ice cores at hojby so the climate anomaly appears to have started 200 250 yr earlier than the 8 2 ka cold event as the lake proxy data provide strong evidence for a precipitation induced distinct increase in catchment soil erosion beginning around 8500 cal yr bp alteration of the terrestrial environment then resulted in a major aquatic ecosystem change with nutrient enrichment of the lake and enhanced productivity which lasted until c 7900 cal yr bp

k ar geochronology of different tectonic units at the northwestern margin of the bohemian massif abstract the polymetamorphic moldanubian mo of the northeasthern margin of the bohemian massif has been thrust to the north onto the mainly paleozoic sedimentary saxothuringian of the fichtelgebirge fg these two units have undergone polyphase deformation and the last regional event to affect both units was a low pressure metamorphism in which temperatures decreased towards the north in contrast the nappe units of the erbendorf vohenstrauss zone zev and the erbendorf greenschist zone egz which partly cover the border of the moldanubian and the saxothuringian and the munchberg nappe pile mm which lies on the saxothuringian were in parts subjected to a late medium pressure metamorphic event the zev the egz the mo and the fg are intruded by late carboniferous granites conventional k ar analyses mainly of hornblendes and muscovites from the autochthonous fg and mo the units beneath the nappes have yielded exclusively carboniferous dates the oldest dates point to a regional cooling of the rocks which outcrop at the present day surface at about 330 320 ma i e at the early late carboniferous boundary the late carboniferous cooling history was largely governed by the thermal influence of the post kinematic granites 320 295 ma especially in the fg and the northern mo the high grade metamorphic rocks in the western part of the zev and in the upper three nappes of the mm mostly yield dates around 380 ma i e early devonian the results show a relatively wide scatter moreover biotites frequently appear to be older than the coexisting muscovites both observations indicate that the rocks underwent a later thermal influence whether some groups of older dates e g 400 ma are due to excess argon or to inherited argon is still open to discussion slightly scattered muscovite dates around 366 ma were obtained for the prasinite phyllite series one of the lower nappes of the mm a single hornblende from the egz gave the same age these two nappes have therefore probably been affected by a late devonian thermal and or tectonic event the muscovite dates obtained from the paleozoic bavarian lithofacies the lowermost nappe of the mm and the hornblende dates from the eastern part of the zev are indistinguishable from those of the autochthonous units fg and mo

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

coronitic metagabbros of the marianske lazně complex and tepla crystalline unit inferences for the tectonometamorphic evolution of the western margin of the tepla barrandian unit bohemian massif

emplacement depths and radiometric ages of paleozoic plutons of the neukirchen kdyně massif differential uplift and exhumation of cadomian basement due to carboniferous orogenic collapse bohemian massif abstract the igneous complex of neukirchen kdyn is located in the southwestern part of the tepla barrandian unit tbu in the bohemian massif the tbu forms the most extensive surface exposure of cadomian basement in central europe cambrian plutons show significant changes in composition emplacement depth isotopic cooling ages and tectonometamorphic overprint from ne to sw in the ne the vsepadly granodiorite and the smr ovice diorite intruded at shallow crustal levels t 20 km the teufelsberg cert v kamen diorite on the other hand forms an unusual intrusion dated at 359 2 ma concordant u pb zircon age k ar dating of biotite of the teufelsberg diorite yields 342 4 ma these ages together with published cooling ages of hornblende and mica in adjacent plutons are compatible with widespread medium to high grade metamorphism and strong deformation fabrics suggesting a strong variscan impact under elevated temperatures at deeper structural levels the plutons of the neukirchen area are cut by the steeply ne dipping hoher bogen shear zone hbsz which forms the boundary with the adjacent moldanubian unit the hbsz is characterized by top to the ne normal movements which were particularly active during the lower carboniferous a geodynamic model is presented that explains the lateral gradients in cambrian pluton composition and emplacement depth by differential uplift and exhumation the latter being probably related to long lasting movements along the hbsz as a consequence of lower carboniferous orogenic collapse

deciphering the variscan tectonothermal overprint and deformation partitioning in the cadomian basement of the tepla 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

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research