the mesozoic and tertiary basins of the bohemian massif and their evolution abstract the bohemian massif is a major positive element within the structural framework of the alpine carpathian foreland its basement became consolidated during the variscan orogeny crustal thicknesses range between 32 5 km and 40 km during the late carboniferous and early permian the bohemian massif became dissected by a conjugate system of wrench faults which controlled the accumulation of a continental coal bearing series late permian red beds were deposited on intramontane topographic depressions in triassic to mid jurassic times the bohemian massif formed a high from which clastics were shed into adjacent basins during the late jurassic and late cretaceous it became transected by nw se trending sedimentary basins the sub hercynian and laramide phases of foreland compression resulted in deformation and uplift of the bohemian massif the eger graben a 350 km long volcanotectonic zone cutting across the bohemian massif in an ene wsw direction came into evidence during the oligocene and is at present inactive the crust of the bohemian massif forms the autochthonous substratum of the outer carpathians which are paralleled by the neogene carpathian foredeep the neogene intramontane neo autochthonous vienna basin is superimposed on the alpine carpathian nappe systems and developed under a tensional setting

provenance of neoproterozoic and early paleozoic siliciclastic rocks of the tepla barrandian unit bohemian massif evidence from u pb detrital zircon ages abstract new la icp ms u pb detrital zircon ages from ediacaran and paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the tepla barrandian unit tbu in the centre of the bohemian massif central europe czech republic the samples taken span the period from 635 ma to 385 ma and permit recognition of provenance changes that reflect changes in geotectonic regime detrital zircon age spectra of two ediacaran one lower cambrian and three upper ordovician samples resemble the ages known from the nw african proportion of gondwana particularly the trans saharan belt while three rocks from higher lower cambrian to lowermost ordovician strata contain detritus that may have been derived exclusively from local sources the age spectrum of the devonian rock is a combination of the nw gondwanan and local features these new findings in combination with a wide range of published data are in agreement with a neoproterozoic subduction related setting at the margin of gondwana followed by a cambrian early ordovician rifting stage and an ordovician passive margin setting furthermore the data are in favour of a position of the tbu at the gondwanan margin throughout pre variscan times

plate and intraplate processes of hercynian europe during the late paleozoic abstract it is speculated that until late carboniferous time the region of hercynian europe was occupied by an elongated island arc system underlain by a segment of continental crust in the upper carboniferous two subduction zones are assumed to have extended from the north and south beneath hercynian europe an extensive zone of hot partially molten upper mantle lay above and between these and diapiric uprise of portions of this material led to separation of mafic magmas widespread partial melting in the lower and middle crust high temperature low pressure metamorphism in crustal rocks and regional uplift and extension of the crust as indicated by intermontane troughs and their associated volcanic rocks in visean to westphalian time hercynian europe collided with both the large neighbouring plates north america europe and africa during these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of hercynian europe after the collision processes had been terminated processes of upper mantle activity continued causing further crustal uplift and even enhanced crustal extension for several tens of million years into the lower permian decline of the upper mantle activity beneath hercynian europe is indicated by crustal subsidence and formation of a peneplain in permian time followed by the upper permian transgression of both the zechstein sea and the tethys sea which mark the end of the hercynian geodynamic cycle

transition from island arc to passive setting on the continental margin of gondwana u pb zircon dating of neoproterozoic metaconglomerates from the se margin of the tepla barrandian unit bohemian massif abstract sedimentological petrological and geochemical data combined with shrimp and la icp ms u pb ages of pebbles of granitic rocks and detrital zircons from neoproterozoic vletice metaconglomerates of the se margin of the tepla barrandian unit were used to characterize the regional geotectonic setting at the proterozoic paleozoic boundary the metaconglomerates form several metres thick intercalations in metasediments of the svrchnice formation roof pendants of central bohemian plutonic complex which is considered to have been deposited in neoproterozoic cambrian times a sedimentological study indicates that the metaconglomerates are products of debris flows deposited in submarine canyons or proximal parts of submarine fans geochemistry of metaconglomerate matrix and petrography of pebbles revealed an obvious difference from the western and central part of the tepla barrandian unit proterozoic where pebbles eroded from uplifted parts of the basin and island arc sources are most common the abundance of quartz quartzite granite and crystalline limestone in pebbles together with the more mature composition of host metagreywackes indicate the presence of relatively evolved continental crust in the source area for the vletice metaconglomerates and shallowing of the neoproterozoic basin to the se in the present geographic co ordinate system the neoproterozoic to lowermost cambrian age of the svrchnice formation suggested by its comparison with similar rocks in the central part of the tepla barrandian unit was confirmed by u pb dating ages of detrital zircons from the metaconglomerate matrix cluster around two values archaean to paleoproterozoic u pb ages between ca 2580 and 1780 ma and neoproterozoic to early cambrian ages between 660 and 520 ma with a significant peaks at 600 and 544 ma suggesting local volcanic activity extending shortly beyond the proterozoic paleozoic boundary zircons separated from two trondhjemite pebbles from the metaconglomerate yielded a mean u pb age of 610 17 ma arguing for active arc magmatism during the latest neoproterozoic the overall age distribution of detrital zircons and the absence of grenvillian zircon ages favour a location of the tepla barrandian unit close to the west african craton at the neoproterozoic paleozoic boundary the rapid onset of a transtensional geotectonic regime and maturing of sediments is evidenced by the overlying cambrian shallow water clastics alternating with predominantly felsic calc alkaline to alkaline volcanics during the cambrian oblique plate convergence the active continental margin was transformed into belts of horsts and narrow pull apart or small rift basins after the ordovician break off of armorican terrane assemblage blocks from gondwana the extensional regime in the tepla barrandian unit continued until the beginning of variscan orogeny in the middle devonian

evolution of the rheic ocean the rheic ocean which separated laurussia from gondwana following the closure of iapetus is arguably the most important ocean of the palaeozoic its suture extends from mexico to turkey and its closure produced the climactic variscan alleghanian ouachita orogeny that assembled the supercontinent pangaea following protracted cambrian rifting that represented a continuum from neoproterozoic orogenic processes the rheic ocean opened in the early ordovician with the separation of several neoproterozoic arc terranes from the continental margin of northern gondwana separation occurred along the line of a former neoproterozoic suture following the onset of subduction in the outboard iapetus ocean the timing of rift drift transition and drive for subsequent spreading was likely governed by slab pull accounting for the rapid rate 8 10 cm yr at which the rheic ocean widened during the ordovician the ocean broadened at the expense of iapetus and attained its greatest width 4000 km in the silurian by which time baltica had sutured to laurentia and the neoproterozoic arc terranes had accreted to laurussia closing iapetus in the process closure of the rheic ocean began in the devonian and was facilitated by northward subduction beneath southern baltica and southward subduction beneath northwest gondwana closure was largely complete by the mississippian as gondwana and laurussia sutured to build pangaea north africa colliding with southern europe to create the variscan orogen in the devonian carboniferous and west africa and south america suturing to north america to form the alleghanian and ouachita orogens respectively during the carboniferous permian the rheic ocean consequently plays a dominant role in the basement geology of southern europe in the appalachian ouachita orogeny of north america and in the palaeozoic sedimentary structural and tectonothermal record from middle america to the middle east with its closure the ocean brought about the assembly of pangaea and brought the palaeozoic era to an end

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

sr nd isotopic constraints on the petrogenesis of the central bohemian pluton czech republic the sr nd isotopic data for selected granitoids of the central bohemian pluton show a broad negative correlation with the total range of 87sr 86sr 330 0 7051 0 7129 and nd330 0 2 to 8 9 the older intrusions have more depleted sr nd compositions and calc alkaline geochemistry sazava suite whereas the younger intrusions shift towards k rich calc alkaline blatna suite and shoshonitic rocks icany and certovo b emeno suites with more evolved isotopic signatures the distribution of the data is interpreted as reflecting a diversity of sources and processes rather than a single progressive crustal contamination trend the sazava suite could have originated by partial melting of metabasites or of a mantle source with an isotopic composition close to bulk earth or by hybridization of crustally derived tonalitic and mantle derived magmas variation within the blatna suite is modelled by mixing between a moderately enriched 87sr 86sr 330 0 708 nd330 3 mantle component with either an isotopically evolved metasedimentary component or with more evolved magmas of the suite the icany suite was most probably produced by partial melting of peraluminous lithologies possibly of the adjacent moldanubian unit the certovo b emeno suite evolved from strongly enriched mantle derived magmas 87sr 86sr 330 0 7128 end330 7 either through closed system fractional crystallization or interaction with magma corresponding to leucogranites of the central bohemian pluton

neoproterozoic to cenozoic magmatism in the central part of the bohemian massif czech republic isotopic tracking of the evolution of the mantle through the variscan orogeny abstract the evolution of the mantle source beneath the tepla barrandian tbb and the adjacent southern part of the saxo thuringian s stb and northern part of the moldanubian n mb blocks of the bohemian massif czech republic is tracked from the late neoproterozoic to the cenozoic in order to examine the coupling between the crust and underlying continental lithospheric mantle clm during the late paleozoic variscan orogeny in the tbb late neoproterozoic mafic rocks have highly radiogenic nd but moderately radiogenic sr isotopes and were derived from a spinel peridotite mantle these within plate rocks were emplaced in an intra oceanic back arc basin early paleozoic ordovician to early devonian mafic rocks are rift extension related enriched in lree and nb relative to hree and were probably derived from a garnet peridotite mantle source compared to neoproterozoic basalts their nd t values are slightly lower but tdm model ages are indistinguishable suggesting derivation from a deeper portion of the same mantle source thus the weak deformation and low grade metamorphism associated with the cadomian orogeny did not significantly decouple the crust from the underlying mantle source beneath the tbb early paleozoic rift related mafic rocks of the s stb and n mb have comparable isotopic signatures indicating a similar mantle source late paleozoic mafic magmatism occurred during the extensional collapse phase of the variscan orogen in the aftermath of the collision between gondwana and laurussia the tbb and s stb basaltic rocks are within plate and transitional between alkaline and tholeiitic compositions but show a relative depletion of nb their isotopic characteristics contrast with older basalts in that they have negative nd t values high initial 87sr 86sr ratios and older tdm model ages 1050 to 1300 ma late paleozoic mantle derived potassic ultrapotassic magmas occurring as lamprophyre dikes and small volume high mg k intrusions in n mb s stb and tbb have similar isotopic characteristics suggesting that all these late paleozoic rocks were derived from an old clm contaminated by fluids or silicic melts derived from subducted precambrian crustal material or alternatively the region was underthrust by gondwanan mantle during the variscan collision cenozoic volcanic rocks in the bohemian massif are local representatives of the regionally extensive cenozoic european volcanic province the rocks are within plate alkaline basalts enriched in lree and nb and were derived from garnet peridotite mantle they are significantly more juvenile than carboniferous early permian mafic rocks with higher nd t values typically 3 to 5 lower initial 87sr 86sr ratios and significantly younger tdm age these data suggest significant input from upwelling asthenospheric mantle implying that at least a part of the permian clm mantle was re fertilized

palaeozoic terrane amalgamation in central europe a ree and sm nd isotope study of the pre variscan basement ne bohemian massif

structure and stratigraphy of the tepla barrandian neoproterozoic bohemian massif a new plate tectonic reinterpretation abstract the tepla barrandian unit tbu of the bohemian massif exposes a section across the once extensive avalonian cadomian belt which bordered the northern active margin of gondwana during late neoproterozoic this paper synthesizes the state of the art knowledge on the cadomian basement of the tbu to redefine its principal component units to revise an outdated stratigraphic scheme and to interpret this scheme in terms of a recent plate tectonic model for the cadomian orogeny in the bohemian massif the main emphasis of this paper is on an area between two newly defined fronts of the variscan pervasive deformation to the nw and se of the barrandian lower paleozoic overlap successions this area has escaped the pervasive variscan late devonian to early carboniferous ductile reworking and a section through the cadomian orogen is here superbly preserved the nw segment of the tbu consists of three juxtaposed allochthonous belts of unknown stratigraphic relation the kralovice rakovnik radnice kralupy and zbiroh sarka belts differing in lithology complex internal strain patterns and containing sedimentary and tectonic melanges with blocks of diverse ocean floor meta basalts we summarize these three belts under a new term the blovice complex which we believe represents a part of an accretionary wedge of the cadomian orogen the se segment of the tbu exposes the narrow picin belt which is probably a continuation of the blovice complex from beneath the barrandian lower paleozoic and a volcanic arc sequence the davle group their stratigraphic relation is unknown flysch units the st chovice group and svrchnice formation overlay the arc volcanics and both units contain material derived from volcanic arc the former was also sourced from the nw segment whereas the latter contains an increased amount of passive margin continental material in contrast to the blovice complex the flysch experienced only weak cadomian deformation the new lithotectonic zonation fits the following tectonic scenario for the cadomian evolution of the tbu well the s to se directed cadomian subduction beneath the tbu led to the involvement of turbidites chaotic deposits and 605 39 ma ocean floor in the accretionary wedge represented by the blovice complex the accretionary wedge formation mostly overlapped temporally with the growth of the volcanic arc the davle group at 620 560 ma upon cessation of the arc igneous activity the rear of the wedge and some elevated portions of the arc were eroded to supply the deep water flysch sequences of the st chovice group whereas the comparable svrchnice formation 560 to

a review of the middle late pennsylvanian west european regional substages and floral biozones and their correlation to the geological time scale based on new u pb ages abstract lower moscovian duckmantian to lower asselian ash beds in the continental basins of the central and western czech republic have yielded 15 new high precision u pb single crystal zircon ca id tims ages these are used to improve the calibration of the west european regional terrestrial stages and their correlation to the geological time scale they also allow an improved correlation between the macrofloral biozones that are used for biostratigraphy in the european pennsylvanian coalfields the marine biozones on which the global geological time scale was partly established and the fresh water and terrestrial faunal biozones most of the floral biozones have proved to be essentially isochronous although the ranges of some individual but stratigraphically important taxa may differ between basins

continental mafic magmatism of different ages in the same terrane constraints on the evolution of an enriched mantle source the antigonish highlands nova scotia are within the avalon terrane of the northern appalachians and contain four distinct episodes of rift related magmatism neoproterozoic cambrian middle ordovician and late devonian all four magmatic suites are composed of basalts and subordinate crustally derived felsic rocks the mafic rocks of these suites which do not appear to be significantly contaminated by continental crust display similar geochemical and sm nd isotopic characteristics consistent with an enriched mantle source that was metasomatically enriched prior to the oldest rifting event probably between 0 8 and 1 1 ga these data also imply that the avalonian crust and its subcontinental lithospheric mantle remained coupled during four magmatic events because the earliest phases of magmatism occurred when avalonia was located along the gondwanan margin and the latest phases after avalonia accreted to laurentia the coupling of crust and mantle in the antigonish highlands suggests that the migration of avalonia did not completely detach its lithospheric mantle from its crustal cover this study demonstrates the importance of comparing magmatism of different ages in the same terrane

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

two stage collision exploring the birth of pangea in the variscan terranes abstract the variscan suture exposed in nw iberia contains a stack of terranes including two allochthonous units with continental affinity and gondwanan provenance upper and basal units separated by an ophiolite belt where the most common units show protolith ages at c 395 ma recent lu hf zircon data obtained from these ophiolites indicate interaction between the gabbroic magmas and old continental crust hence the ophiolites could not have originated in a deep ocean basin associated with a mature mid ocean ridge or intraoceanic subduction the tectonothermal evolution of the continental terranes bounding the suture zone records two consecutive events of deep subduction the upper units record an initial high p ultra high p metamorphic event that occurred before 400 390 ma while the basal units were affected by a second high p low to intermediate t metamorphic event dated at c 370 ma continental subduction affected the most external margin of gondwana and developed in a setting of dextral convergence with laurussia development of the two high p events alternated with the opening of an ephemeral oceanic basin probably of pull apart type in early devonian times this ephemeral oceanic domain is suggested as the setting for the protoliths of the most common ophiolites involved in the variscan suture current ideas for the assembly of pangea advocate a single collisional event between gondwana and laurussia in the carboniferous however the new evidence from the allochthonous terranes of the variscan belt suggests a more complex scenario for the assembly of the supercontinent with an interaction between the colliding continental margins that started earlier and lasted longer than previously considered based on modern analogs of continental interaction the development of complex collisions as here suggested for gondwana and laurussia during the assembly of pangea could have been the norm rather than the exception throughout earth history

origin of the silicic volcanic rocks of the early permian panjal traps kashmir india abstract the panjal traps of kashmir india represent a significant outpouring of mafic and silicic volcanic rocks during the early permian and are synchronous with the opening of the neotethys ocean previous studies have suggested that the silicic volcanic rocks are derived by differentiation of mafic magmas dacites and rhyolites collected from the lower portions of the volcanic pile near pampore kashmir are peraluminous anck 1 0 in composition their calculated whole rock i sr values are variable i sr 0 69307 to 0 71825 and indicate open system behavior of either rb or sr or both whereas their nd isotope compositions end t 8 6 to 8 9 are nearly uniform the end t values and trace element th nb pm 4 nb u 8 ratios suggest the rocks are derived partially or wholly from the crust least squares and melts modeling indicate that the likely source of the silicic rocks is the middle crust rather than the lower or upper crust furthermore the silicic panjal traps have trace element compositions similar to some felsic volcanic rocks and a type granitic rocks from other large igneous provinces e g karoo parana and camp the paleoproterozoic t dm ages t dm 1836 to 1937 ma indicate that the rocks were likely derived from an ancient crustal source which experienced multiple episodes of recycling in contrast to silicic volcanic rocks from other lips the silicic panjal traps are unusual in that they did not involve mantle source material the heat required to melt the crust was likely due to the continuous injection of contemporaneous basaltic magmas which formed the majority of the mafic panjal traps

evidence for high temperature diffusional creep preserved by rapid cooling of lower crust north bohemian shear zone czech republic diffusional creep structures in orthogneisses and granulites of the north bohemia shear zone eastern part of the erzgebirge crystalline complex imply high interfacial free energy that has been preserved from equilibration as a result of rapid cooling u pb dating of monazite and ar ar dating of muscovite of kadan orthogneisses result in a high cooling rate this is consistent with a rapid tectonic movements resulting in the juxtaposition of upper crustal rocks of the tepla barrandian unit against lower crustal rocks of the erzgebirge crystalline complex

ultrapotassic dykes in the moldanubian zone and their significance for understanding of the post collisional mantle dynamics during variscan orogeny in the bohemian massif abstract we report mineral textures geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the western part of the moldanubian zone at contact to the tepla barrandian block the dykes have orientation almost perpendicular to the nne ssw trending central bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites based on mineral compositions and geochemistry the dykes show close relations to mg k syenite plutons in the moldanubian zone the two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine grained matrix that consists of k feldspar phlogopite with small amounts of clinopyroxene and accessory quartz apatite titanite and sulphides of fe cu ni the syenite porphyry dyke cross cuts the vaugnerite it contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs the vaugnerite and syenite porphyry have high k 2 o 6 to 7 wt and mg number 0 66 0 74 but low cao and na 2 o contents the vaugnerite is markedly rich in p 2 o 5 2 wt and comparing to syenite porphyry has higher amount of mantle incompatible elements e g rb cs ba pb th u v and cr id tims analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338 59 0 68 and 337 87 0 21 ma respectively mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post collisional mantle dynamics initiated by slab break off and asthenosphere upwelling into the moldanubian accretionary complex during the variscan orogeny

self subduction of the pangaean global plate one of the most striking and rare occurrences in the earth s history is the amalgamation of most of the continental lithosphere into one supercontinent the most recent supercontinent pangaea lasted from 320 to 200 million years ago here we show that after the continental collisions that led to the formation of pangaea plate convergence continued in a large wedge shaped oceanic tract we suggest that plate strain at the periphery of the supercontinent eventually resulted in self subduction of the pangaean global plate when the ocean margin of the continent subducted beneath the continental edge at the other end of the same plate our scenario results in a stress regime within pangaea that explains the development of a large fold structure near the apex of the palaeotethys ocean extensive lower crustal heating and continental magmatism at the core of the continent as well as the development of radially arranged continental rifts in more peripheral regions of the plate

new data on the neoproterozoic cambrian geotectonic setting of the tepla barrandian volcano sedimentary successions geochemistry u pb zircon ages and provenance bohemian massif czech republic the tepla barrandian unit tbu of the bohemian massif was a part of the avalonian cadomian belt at the northern margin of gondwana during neoproterozoic and early cambrian times new detrital zircon ages and geochemical compositions of late neoproterozoic siliciclastic sediments confirm a deposition of the volcano sedimentary successions of the tbu in a back arc basin a change in the geotectonic regime from convergence to transtension was completed by the time of the precambrian cambrian boundary the accumulation of around 2 500 m lower cambrian continental siliciclastics in a basin and range type setting was accompanied by magmatism which shows within plate features in a few cases but is predominantly derived from anatectic melts displaying the inherited island arc signature of their cadomian source rocks the geochemistry of clastic sediments suggests a deposition in a rift or strike slip related basin respectively a marine transgression during middle cambrian times indicates markedly thinned crust after the cadomian orogeny upper cambrian magmatism is represented by 1 500 m of subaerial andesites and rhyolites demonstrating several geochemical characteristics of an intra plate setting zircons from a rhyolite give a u pb shrimp age of 499 4 ma the cambrian sedimentary and magmatic succession of the tbu records the beginning of an important rifting event at the northern margin of gondwana

physical models of regional fold superposition the role of competence contrast abstract physical models of superposed folds demonstrate that rheologic contrast strongly controls the style of fold interference rheologic contrast affects strain partitioning between layer parallel shortening and buckling and affects the development of structural anisotropies parallel to first generation folds laminates with insignificant competence contrast are characterized by circular to elliptical domes and basins type 1 interference in addition to folding the layers accommodate shortening by layer parallel thickening preferentially in the fold culminations laminates with significant competence contrast are characterized by buckled fold hingelines and axial surfaces type 2 interference the less competent layers thicken in the hinges the more competent layers maintain their initial thickness in plan view the f 1 hingelines are refolded in a lobate cuspate to box style whose axial traces form conjugate pairs the models are dynamically scaled to represent folds with km scale wavelengths the evolution of surface structures map patterns at different depths and equal area projections synthesized from structure contour maps were used for analysis comparable to regional analysis common to field studies gravitational body forces effectively damp the vertical amplitudes of both the first and second generation folds thereby enhancing the formation of type 2 interference

provenance and tectonic evolution of ganderia constraints on the evolution of the iapetus and rheic oceans we provide estimates for the width timing and rates of opening and closing of the iapetus and rheic oceans the evolution of which profoundly infl uenced paleozoic global paleogeography these estimates are primarily derived from the transfer of ganderia and avalonia from gondwana to laurentia which led to closure of the iapetus ocean and opening of the rheic ocean ganderia a long lived arc terrane separated from the paleo caribbean margin of amazonia at 505 ma with a latitudinal speed of 9 cm a northward initiating the rheic ocean as a backarc basin ganderia s trailing edge was reduced to 5 cm a following opening of a 600 800 km wide backarc basin within ganderia at 475 ma opening and closing of the iapetus ocean was largely driven by far fi eld stresses slab pull in some places and slab rollback in others

mafic rocks in a deep crustal segment of the variscides the gory sowie sw poland evidence for crustal contamination in an extensional setting coronitic metagabbronorites so called hyperites and metabasites occur within gneisses migmatites and minor granulites in the high grade metamorphic gory sowie block in the sudetes sw poland incompatible trace element and end data combined with field and petrographic evidence suggest that three groups may be distinguished 1 enriched amphibolites zr nb ca 8 end500 1 2 5 2 depleted amphibolites zr nb 30 end500 5 6 and 3 metagabbronorites zr nb 13 19 end500 scattered between 5 3 and 1 these characteristics favour an extensional within plate setting consistent with their field occurrence as small bodies scattered within metasedimentary and felsic metaigneous gneisses in that setting enriched mantle sources were partially melted to produce a range of basic magmas showing affinities with continental tholeiites within plate type basalts presumably in a more advanced stage of rifting more depleted varieties transitional to n morb were emplaced these mafic magmas were variably contaminated during their ascent in the crust the metagabbronorites with their well preserved igneous textures were previously interpreted as having been intruded after the main deformation and after the peak of amphibolite facies metamorphism some authors suggesting a relation to the circum gory sowie ophiolites however the geochemical characteristics and p t estimates suggest that the metagabbronorites are unrelated to the ophiolitic gabbros found in the neighbourhood of the gory sowie block and in contrast to them have experienced a complex polybaric p t path

neoproterozoic to early cambrian history of an active plate margin in the tepla barrandian unit a correlation of u pb isotopic dilution tims ages bohemia czech republic abstract the tepla barrandian unit tbu of the bohemian massif shared a common geological history throughout the neoproterozoic and cambrian with the avalonian cadomian terranes the neoproterozoic evolution of an active plate margin in the tepla barrandian is similar to avalonian rocks in newfoundland whereas the cambrian transtension and related calc alkaline plutons are reminiscent of the cadomian ossa morena zone and the armorican massif in western europe the neoproterozoic evolution of the tepla barrandian unit fits well with that of the lausitz area saxothuringian unit but is significantly distinct from the history of the moravo silesian unit the oldest volcanic activity in the bohemian massif is dated at 609 17 19 ma u pb upper intercept subduction related volcanic rocks have been dated from 585 7 to 568 3 ma lower intercept rhyolite boulders which pre dates the age of sedimentation of the cadomian flysch st chovice group accretion uplift and erosion of the volcanic arc is documented by the neoproterozoic dob is conglomerate of the upper part of the flysch the intrusion age of 541 7 8 ma from the zgorzelec granodiorite is interpreted as a minimum age of the neoproterozoic sequence the neoproterozoic crust was tilted and subsequently early cambrian intrusions dated at 522 2 ma t sovice granite 524 3 ma vsepadly granodiorite 523 3 ma smr ovice tonalite 523 1 ma smr ovice gabbro and 524 0 8 ma orlovice gabbro were emplaced into transtensive shear zones

tectonic controls on carbonate platform evolution in southern papua new guinea passive margin to foreland basin the middle oligocene collision of the northern margin of the australian craton with a complex subduction system resulted in emplacement of a thrust mass and formation of a foreland basin that extended from the coral sea to the indian ocean the distribution of carbonate platform facies in southwestern papua new guinea reflects the transition from an eocene passive margin setting to the early stages of foreland basin evolution the initial basin configuration with terrigenous sedimentation confined to the proximal foredeep allowed carbonate deposition in the shallow environment adjacent to the peripheral forebulge subsequent southward migration of the basin resulted in a rapid increase in the area and thickness of carbonate platform deposition when the proximal foredeep became filled by detritus shed from the emerging orogen clastic sediments buried the platform and terminated carbonate deposition the history of the southern papua new guinea carbonate platform illustrates the paradox of carbonate deposition within the foreland basin whereby basin configuration initially encourages thick and extensive carbonate deposition but inevitably leads to terrigenous inundation and the demise of the carbonate platform

anatomy of a diffuse cryptic suture zone an example from the bohemian massif european variscides the fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late paleozoic variscan belt in continental europe in particular the bohemian massif at its eastern extremity preserves well the evolution of an andean type orogen involved in continental collision this process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafic lower crust of the upper plate this led to gravity driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials in the studied example this zone of interaction repeatedly reappears within the orogen forming a so called diffuse cryptic suture zone

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

failed silurian continental rifting at the nw margin of gondwana evidence from basaltic volcanism of the prague basin tepla barrandian unit bohemian massif the silurian volcanic rocks of the prague basin represent within plate transitional alkali to tholeiitic basalts which erupted in a continental rift setting through the thick cadomian crust of the tepla barrandian unit bohemian massif despite the variable often intense alteration resulting in post magmatic replacement of the basalt mass due to carbonatization the geochemical signatures of silurian basalts are still sufficiently preserved to constrain primary magmatic processes and geotectonic setting the studied interval of silurian volcanic activity ranges from wenlock homerian 431 ma to late ludlow gorstian 425 ma with a distinct peak at the wenlock ludlow boundary 428 ma trace element characteristics unambiguously indicate partial melting of a garnet peridotite mantle source wenlock basalts are similar to alkaline oib with depleted radiogenic nd signature compared to ludlow basalts which are rather tholeiitic emorb like with enriched radiogenic nd signature the correlation of petrogenetically significant trace element ratios with nd isotopic compositions points to a mixing of partial melts of an isotopically heterogeneous possibly two component mantle source during the wenlock ludlow melting lava eruptions were accompanied by intrusions of doleritic basalt and meimechite sills the latter represent olivine rich cumulates of basaltic magmas of probably predominantly ludlow age meimechites with dolerites and to a lesser extent some lavas were subject to alteration due to wall rock fluid interaction the trigger for the wenlock to ludlow 431 425 ma extension and related volcanism in the prague basin is related to far field forces namely slab pull regime due to progressive closure of the iapetus ocean the main stage of the baltica laurentia collision then caused the prague basin rift failure at ca 425 ma that has never reached an oceanic stage

review of nd isotopic data and xenocrystic and detrital zircon ages from the pre variscan basement in the eastern bohemian massif speculations on palinspastic reconstructions

the central sudetic ophiolites sw poland petrogenetic issues geochronology and palaeotectonic implications abstract the central sudetic ophiolites cso including the largest and best studied leza ophiolite are relatively well preserved complete palaeozoic ophiolitic sequences at the ne margin of the bohemian massif near the eastern edge of the variscan belt in central europe despite a detailed study and a well documented morb type affinity the geochemical complexity of these ophiolites has only recently been appreciated allowing construction of petrogenetic models we review here petrological and geochemical data indicating a contrast between the plutonic and volcanic members of the leza ophiolite this contrast was previously interpreted in terms of a model of two stage incremental melting of a depleted asthenospheric source producing basaltic magmas and subsequent melts crystallizing as gabbros however we argue that the geochemical differences between the plutonic and volcanic members can be explained by differentiation and cumulation processes of broadly similar parental magmas preliminary interpretation of new shrimp zircon age data from plutonic and volcanic members are consistent with earlier u pb zircon ages and provide evidence for magmatic crystallization of the leza ophiolite at c 400 ma the cso interpreted as traces of the rheic ocean are key for reconstructing the early to mid palaeozoic rifting processes distinct similarities exist with time equivalent ophiolitic units in the western variscan segment of the nw iberian massif the cso represent c 400 ma oceanic crust fragments and their structural position delineates likely major variscan tectonic sutures and tectonic mosaic domains within the variscan accretionary prism