structural architecture of the southern and middle urals foreland from reflection seismic profiles structural architecture of the southern and middle urals foreland from reflection seismic profiles

paleozoic laurentia gondwana interaction and the origin of the appalachian andean mountain system laurentia the rift bounded precambrian nucleus of north america may have broken out from a neoproterozoic supercontinent between east and west gondwana several lines of evidence suggest that the appalachian margin of laurentia subsequently collided with the proto andean margin of the amalgamated gondwana supercontinent in different relative positions during early and mid paleozoic time in route to final docking against northwest africa to complete the assembly of pangea hence the appalachian and andean orogens may have originated as a single mountain system the overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the paleozoic era laurentia gondwana collisions may help to explain contemporaneous unconformities in the paleozoic sedimentary cover of the laurentian gondwanan and baltic cratons

shallow water anoxia in the mesoproterozoic ocean evidence from the bashkir meganticlinorium southern urals abstract the apparent lag between the first permanent rise of atmospheric oxygen to appreciable levels and oxygenation of the deep ocean has focused efforts in deciphering the evolution of seawater chemistry across the proterozoic eon 2 5 0 542 ga it is generally accepted that from 1 85 ga oxic shallow marine waters were widespread while the deep ocean remained dominantly ferruginous fe ii rich with episodic euxinia confined to productive continental margins and intracontinental basins the geochemical record that informs this picture however is currently sparse and further studies are required to adequately evaluate temporal and spatial variability in ocean redox conditions across this vast expanse of time here we report fe s c systematics alongside major al mn and trace metal mo u data for 1 6 0 8 ga marine sediments from the bashkir meganticlinorium bma southern urals russia our fe speciation data reveal a water column dominated by ferruginous conditions no evidence for euxinia and oxygenated waters confined to only the shallowest near coastal settings trace metal data support these findings with mo and u enrichment factors in anoxic sediments implying the operation of a particulate fe oxyhydr oxide shuttle under ferruginous conditions as the main mechanism for generating moderate authigenic mo enrichments sulfur isotope systematics imply that predominantly low dissolved sulfate concentrations prevailed throughout the deposition of the bma succession which in combination with a low organic carbon loading promoted the development of ferruginous rather than euxinic water column conditions the restriction of oxic conditions to the shallowest near coastal depositional settings within the bashkir basin contrasts to other redox studies from this period such discrepancies highlight major uncertainty in our understanding of the temporal and spatial evolution of water column redox chemistry across the mesoproterozoic era and signify the need for further detailed work to constrain the chemical evolution of the oceans during this crucial time period

40 ar 39 ar thermochronology west of the main uralian fault southern urals russia west of the main uralian fault the main suture in the southern urals 40ar 39ar apparent ages of amphibole muscovite and potassium feldspar are interpreted as cooling ages a fast exhumation of the metamorphic complex of kurtinsky during upper carboniferous time is indicated by the small age difference 15 ma between cogenetic amphibole and muscovite differentiated movement in the footwall of the main uralian fault along strike is indicated by the age difference of 70 ma between the metamorphic complexes of kurtinsky north and maksyutov south no upper paleozoic uralian medium to high temperature event is recorded in 40ar 39ar data from the metamorphic complex of beloretzk mcb an amphibole age of 718 5 ma and the occurrence of mafic intrusions might signal the break up of rodinia and therefore indicate the rifting period followed by the separate movement of the beloretzk terrane muscovite ages of approximately 550 5 ma the unique pre ordovician tectonometamorphic evolution of the mcb and the late vendian sedimentary history of the western bashkirian megaanticlinorium bma imply the existence of a neoproterozoic orogeny at the eastern margin of baltica this orogeny might have been initiated by the accretion of the beloretzk terrane the metamorphic grade of the overlain silurian shales and the k ar microcline ages from the beloretzk terrane give evidence for a new thermal event at approximately 370 ma a microcline age of 530 550 ma obtained for the vendian conglomerate in the western bma suggests that a maximum temperature of approximately 200 c was reached in cambrian or vendian times an orthoclase age 590 630 ma of the vendian zigan flysch deposits might be inherited from the eastern source area the cadomian orogen an orthoclase age 910 950 ma from the riphean zilmerdak conglomerate coincides with a documented decrease in the subsidence rate of the upper riphean basin

structural framework of the bashkirian anticlinorium sw urals the bashkirian anticlinorium of the southwestern urals shows a much more complex structural architecture and tectonic evolution than previously known pre uralian proterozoic extensional and compressional structures controlled significantly the uralian tectonic convergence a long lasting proterozoic rift process created extensional basement structures and a riphean basin topography which influenced the formation of the western fold and thrust belt with inversion structures during the uralian deformation a complete orogenic cycle during cadomian times including terrane accretion at the eastern margin of the east european platform resulted in a high level cadomian basement complex which controlled the onset of uralian deformation and resulted in intense imbrication and tectonic stacking in the subjacent footwall of the main uralian fault the uralian orogenic evolution can be subdivided into three deformation stages with differently oriented stress regimes tectonic convergence started in the late devonian with ophiolite obduction tectonic accretion of basin and slope units and early flysch deposits zilair flysch the accretionary complex prograded from the se to the nw continuous nw se directed convergence resulted finally in the formation of an early orogenic wedge thrusting the cadomian basement complex onto the east european platform the main tectonic shortening was connected with these two stages and although not well constrained appears to be of late devonian to carboniferous age in the permian a final stage of e w compression is observed throughout the sw urals in the west the fold and thrust belt prograded to the west with reactivation of former extensional structures and minor shortening in the east this phase was related to intense back thrusting the east european platform was subducted beneath the magnitogorsk magmatic arc during the late paleozoic collision the thick and cold east european platform reacted as a stable rigid block which resulted in a narrow zone of intense crustal shortening tectonic stacking and high strain at its eastern margin whereas the first orogenic wedge is of thick skinned type with the involvement of crystalline basement even the later west directed wedge is not typically thin skinned as the depth of the basal detachment appears below 15 km and the involvement of archean basement can be assumed

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

geometric and kinematic evolution of the foreland thrust and fold belt in the southern urals the first balanced and restored cross section through the foreland thrust and fold belt of the southern urals along the belaya river of bashkortostan is presented this cross section is used to discuss the paleozoic structure and kinematic evolution of the area and to provide quantitative estimates of fault displacement and shortening the belaya river thrust stack can be divided into a late imbricate thrust system nygush thrust system the burzyan thrust sheet and an early duplex system timirovo thrust system overprinting relationships indicate two deformation events during which the foreland thrust and fold belt evolved by a forward propagating thrust sequence the first deformation event involved emplacement of the low metamorphic grade timirovo thrust system and the overlying zilair nappe during accretion of the magnitogorsk volcanic arc with the east european craton at the end of the devonian a deformational hiatus followed until the end of the carboniferous at which time a second deformation event related to final closure of the uralian ocean resulted in development of the nygush thrust system the calculated shortening in the belaya river area 16 7 km or 17 is significantly less than that in other foreland thrust and fold belts the basal thrust in the belaya river section is located within the basement and cuts continuously up section with a staircase like geometry instead of following an easy glide horizon

k ar dating of magmatic sericite crystallites for determination of cooling paths of metamorphic overprints a combined optical and x ray characterisation of fine grained micas in granodioritic to tonalitic dikes and intrusions of the eastardennes eifel nappe linksrheinisches schiefergebirge germany belgium was completed with k ar dating of separated size fractions from these rocks in each sample the k ar ages decrease with decreasing size fraction but the same size fractions of the different samples yield different ages application of the closure temperature concept to the k ar ages of the various size fractions indicates that the crystallite size expressed as cis fwhmser001ad and coherent scattering domain csd thickness acts as the effective radius of ar diffusion a linear model may describe the correlation between k ar ages and crystallite thickness the ages younger than the model prediction are caused by the occurrence of microcline as an accessory k bearing mineral phase the obtained k ar ages were interpreted as representative of the cooling path of the magmatic components in the eastardennes eifel nappe below 330 c the k ar age of the largest size fraction of the tonalitic dike v11 313 6 7 2 ma is the closest value for the age of the variscan cleavage development the oldest age of the hydrothermally altered tonalitic dike 300 2 8 4 ma might represent an episode of hydrothermal fluid migration after cleavage development it represents at least a minimum age the microcline age 242 8 6 0 ma of the hill intrusion may represent the closure temperature of this mineral at about 140 c

the structure of the south urals foreland fold and thrust belt at the transition to the precaspian basin the structural architecture of the south urals foreland fold and thrust belt at the transition into the precaspian basin is that of a wedge shaped west vergent thrust stack with a significant amount of basement involvement the surface geology of this part of the thrust belt is dominated by the south urals accretionary complex whose internal structure was primarily developed during late devonian to early carboniferous arc continent collision and which was subsequently incorporated into the thrust belt a number of thrusts identified in the field and in seismic profiles breach the accretionary complex suggesting that they post date its emplacement and may be related to the development of the foreland fold and thrust belt a basal detachment can be confidently defined only beneath the frontal part of the thrust belt making it difficult to construct balanced and restored cross sections and therefore to calculate the amount of shortening nevertheless the amount of shortening appears to be of the order of only a few tens of kilometres there is a major change in the structural style along the frontal part of the thrust belt that may be related to basement topography and or basement response to the deformation across a basement fault oriented at a high angle to the structural grain of the thrust belt

contrasting provenance signals in riphean and vendian sandstones in the sw urals russia constraints for a change from passive to active continental margin conditions in the neoproterozoic abstract two contrasting provenance areas for neoproterozoic sandstones can be distinguished in the southwestern urals by light and heavy mineral analyses as well as by mineral chemistry these reflect a dramatic change of geotectonic conditions at the eastern border of the baltica protocontinent at around 610 620 ma detritus from riphean and lower vendian sandstones representing about 1 ga of continuous sedimentation is characterised by a continental platform provenance reflecting its derivation from the proterozoic basement of the east european platform source rocks include mainly minerals and few lithic clasts from granitoids and high grade metamorphic rocks as well as reworked clastic sediments and silicic volcanic rocks the provenance signal changes abruptly with upper vendian detritus representing a recycled orogenic provenance this includes mineral and lithic clasts of low grade siliciclastic metasediments and mylonites containing phengites with a high pressure signature as well as clasts of bimodal volcanic rocks and reworked siliciclastic sediments the composition of phengites contrasts strongly with those of detrital white mica in the riphean rocks also the composition of tourmalines derived from a metapsammopelitic source indicates a mainly al poor metasediment provenance whereas riphean tourmalines were derived from mainly al rich metapsammopelites zircon morphology tourmaline zoning and a reduced heavy mineral spectrum provide evidence for polycyclic sedimentation during riphean and vendian upper vendian sedimentary rocks were deposited in a foreland basin derived from a proximal uplift to the east provenance characteristics of the upper vendian detritus are consistent with areas affected by a high pressure low temperature metamorphism during a pre uralian orogenic event most likely from the metamorphic complex of beloretsk which was emplaced and exhumed during the upper vendian the change of geotectonic conditions in the upper vendian reflects a change from a passive continental margin in the south urals throughout the entire riphean since at least 1350 ma to a convergent continental margin within a presumed transpressional setting

high temperature memory in calcic amphiboles and constraints on compositional control of their 40ar 39ar ages well characterized calcic amphiboles from an incompletely mixed magmatic rock association of mainly intermediate dioritic composition were studied by incremental heating experiments of 40 ar 39 ar the rocks in question were thermally overprinted by late hercynian granites microprobe and scanning electron microscopy analyses show that the amphibole grains consist of two different calcic hornblende varieties 1 a si and mg rich variety and 2 a variety enriched in al ti fe k and na the mg si rich hornblende can be further characterized by its higher cao k 2 o ratios 30 50 compared to the al ti fe k na rich hornblende 12 15 three hornblende separates that previously had yielded conventional k ar ages of 320 317 and 302 ma have been dated by the 40 ar 39 ar step heating technique the low and medium temperature parts of the 40 ar 39 ar age spectrum show discordant ar systematics with single step ages generally younger than 325 ma the gas released at the highest temperature steps preserves an exceptional record of old and remarkably similar integrated ages 342 346 and 344 ma the high temperature ages correspond to phases with higher apparent cao k 2 o ratios 30 45 and are interpreted to result from ar release from mg si hornblende domains that are assumed to have been very resistent to ar loss during the late hercynian thermal events the results of this study suggest that ar retentivity differs among calcic amphiboles and mg si rich hornblendes are more ar retentive than their al ti fe k na rich counterparts

the main tectonic events depositional history and the palaeogeography of the southern urals during the riphean early palaeozoic abstract investigation of former and new sections in the classic riphean region of the bashkirian anticlinorium ba revealed that this palaeozoic segment of the passive margin of the east european platform eep during its proterozoic history was initially formed as an intracratonic aulacogen basin which developed as an inter cratonic half structure only after the early ordovician opening of the uralian palaeo ocean the originally opposite half structure of this riphean basin may have been located either on the afro european part of gondwana or along the margins of the siberian craton or elsewhere but definitely not along the conjugate permian collision suture with the kazakh block or the mugodzhary microcontinent the ca 1150 ma 1635 to 488 ma depositional history of the bashkirian basin was punctuated several times due to major tectonic events folding downfaulting uplift magmatic intrusions metamorphism which may have caused extended periods of erosion and non deposition furthermore numerous cyclothems of unknown regional extent are incorporated into the enormous time span involved in the evolution of the ca 12 15 km thick riphean sequence the riphean vendian sedimentary assemblages reflect alternating shallow marine to peritidal and continental fluviatile environments the regional distribution of which is illustrated in sets of palaeogeographic maps angular unconformities are seen at the base of the lower riphean ai formation dated at 1635 ma at the base of the middle riphean mashak formation dated at 1340 ma at the base of the upper riphean zilmerdak formation approx 1000 ma and below the middle ordovician quartzites containing shelly fossils which overlie steeply folded and slightly metamorphosed proterozoic suites along the southeastern margin of the bashkirian anticlinorium during the vendian to ordovician interval major eastern parts of the bashkirian anticlinorium as well as regions further north along the european margin of the urals were subjected to strong diastrophism probably related to the cadomian or baikalian orogeny later on during the ordovician the western half part of the bashkirian basin turned into a marginal basin of the east european platform and thus was modified by the early stages of the uralian oceanic development

thermotectonic evolution of the western fold and thrust belt southern uralides russia as revealed by apatite fission track data abstract the uralides a linear n s trending palaeozoic fold belt reveals an intact well preserved orogen with a deep crustal root within a stable continental interior in the western fold and thrust belt of the southern uralides devonian to carboniferous siliciclastic and carbonate rocks overlay mesoproterozoic to neoproterozoic sedimentary rocks deformation in the devonian carboniferous and permian caused thick skinned tectonic features in the western and central parts of the western fold and thrust belt a stack of several nappes characterizes the deformation in the eastern part along the e w transect ac ts 96 that crosses the western fold and thrust belt apatite fission track data record various stages of the geodynamic evolution of the uralide orogeny such as basin evolution during the palaeozoic synorogenic movements along major thrusts synorogenic to postorogenic exhumation and a change in the regional stress field during the upper jurassic and lower cretaceous the palaeozoic sedimentary cover and the neoproterozoic basement of the ala tau anticlinorium never exceed the upper limit of the paz since the devonian a temperature gradient similar to the recent one 20 c km would account for the ft data reactivation of the neoproterozoic zilmerdak thrust was time equivalent to the onset of the devonian and carboniferous collision related deformation in the east west directed movement along the tashli thrust occurred in the lower permian the devonian and carboniferous exhumation path of the neoproterozoic siliciclastic units of the tirlyan synclinorium mirrors the onset of the uralian orogeny the emplacement of the tirlyan nappe and the continuous west directed compression the five main tectonic segments inzer synclinorium beloretzk terrane ala tau anticlinorium yamantau anticlinorium and zilair synclinorium were exhumed one after another to a stable position in the crust between 290 and 230 ma each segment has its own t t path but the exhumation rate was nearly the same final denudation of the western fold and thrust belt and exhumation to the present surface probably began in late tertiary in jurassic and cretaceous south directed movements along w e trending normal faults indicate a change in the tectonic regime in the southern uralides

the cooling history of the late pliocene eldzhurtinskiy granite caucasus russia and the thermochronological potential of grain size age relationships abstract isotopic age investigations have been carried out on samples from the late pliocene eldzhurtinskiy granite great caucasus russia in order to elucidate the cooling history of this kilometere sized plutonic body the samples were taken from a horizontal profile from the edge to the centre of the granite and from a vertical profile from the top to a depth of 4570 m the 40ar 39ar results on biotites ranging from 2 5 to 1 2 ma indicate a vertical age zoning of the granite caused by regional uplift an uplift rate of 4 mm a is calculated from the age differences together with the results of fission track dating on apatite and zircon and the present day rock temperatures the biotite ages can be used to evaluate cooling rates below 400 c the cooling rates of about 180 c ma are relatively high and spatially and temporarily fairly uniform throughout the granite a geothermal gradient of 45 c km is derived from uplift and cooling rates this is identical to the present day borehole gradient the 40ar 39ar ages of distinct biotite grain fractions from one and the same sample specimen are discordant generally they show clear age grain size correlation as expected from ar diffusion theory for cooling systems thus the biotite results support the additional detailed thermochronological information the cooling rates in the temperature range of biotite closure based on the grain size age correlations of the biotites are close to 180 c ma they provide evidence that within a certain temperature interval the time temperature path of a rock may be determined exclusively by isotopic dating of different grain size fractions of only one type of mineral

carbon isotope chemostratigraphy of the middle riphean type section avzyan formation southern urals russia signal recovery in a fold and thrust belt abstract the avzyan formation of the southern ural mountains russia forms part of the middle riphean 1300 1000 ma type section its age is constrained to be younger than 1348 ma and older than 1080 ma and thus may preserve all or part of the mid mesoproterozoic isotopic shift this shift represents a significant reorganization of the carbon cycle and separates pre 1300 ma carbonates with 13c near 0 from post 1200 ma mesoproterozoic successions having average 13c values greater than 3 5 because the avzyan formation is a significant reference section for the mesoproterozoic and because it has the potential to record this mid mesoproterozoic shift a chemostratigraphic profile was constructed for this strongly folded and thrust faulted succession best preserved 13c values from carbonates of the avzyan formation lie mainly between 0 and 2 but the unit contains a single excursion represented by several data points to values greater than 4 the chemostratigraphy of the avzyan formation does not provide a definitive match to a better dated succession however the character of the carbon isotopic curve is most similar to that of the dismal lakes group arctic canada suggesting that the avzyan formation was deposited during the mid mesoproterozoic isotopic shift and may have an age near 1270 ma best preserved sr isotopic values of 0 70587 are also consistent with a later mesoproterozoic age

insights from petrography mineralogy and u pb zircon geochronology into the provenance and reservoir potential of cenozoic siliciclastic depositional systems supplying the northern margin of the eastern black sea the eastern black sea is one of the few remaining underexplored hydrocarbon basins in europe reservoir quality is a key risk factor in the basin the analysis of up dip portions of oligocene to pliocene depositional systems exposed around its northern and eastern margins enable the composition and provenance of sandstones within the basin to be predicted two major sources of quartz rich sandstone are present the russian western greater caucasus which became a subaerial sediment source in the oligocene and the east european craton scythian platform these are predicted to form better quality reservoirs at shallow and moderate burial depths than their rock fragment rich counterparts due to lesser amounts of compactional porosity loss oligo miocene sandstones from the russian western greater caucasus were deposited as turbidites in the tuapse trough sandstones from the east european craton scythian platform were largely trapped within the indolo kuban basin north of the greater caucasus they only entered the eastern black sea in large volumes from latest miocene or pliocene time in the region of the kerch taman shelf at the western tip of the greater caucasus however it is possible that a precursor to this system supplied the oligocene to early miocene reservoir sandstones within the subbotina field the dziruli massif in western georgia also generated quartz rich sandstones but these are not thought to have entered the black sea

the metamorphic complex of beloretzk sw urals russia â a terrane with a polyphase meso to neoproterozoic thermo dynamic evolution abstract an integrated geological study of the tectono metamorphic evolution of the metamorphic complex of beloretzk mcb which is part of the eastern bashkirian mega anticlinorium bma sw urals russia shows that the main lithological units are neoproterozoic riphean and vendian age siliciclastic to carbonate successions granitic syenitic and mafic intrusions together with subaerial equivalents comprise the neo and mesoproterozoic magmatic rocks the metamorphic grade ranges from diagenetic and very low grade in the western bma to high grade in the mcb the n s trending zuratkul fault marks the change in metamorphic grade and structural evolution between the central and eastern bma structural data pb pb single zircon ages 40 ar 39 ar cooling ages and the provenance signature of riphean and vendian siliciclastic rocks in the western bma give evidence of mesoproterozoic grenvillian rifting deformation and eclogite facies metamorphism in the mcb and a neoproterozoic cadomian orogenic event in the sw urals three pre ordovician deformation phases can be identified in the mcb the first sse vergent isoclinal folding phase d 1 is younger than the intrusion of mafic dykes pb pb single zircon 1350 ma and older than the eclogite facies metamorphism high p low t eclogite facies metamorphism is bracketed by d 1 and the intrusion of the achmerovo granite pb pb single zircon 970 ma an extensional sinistral top down to nw directed shearing d 2 is correlated with the first exhumation of the mcb e vergent folding and thrusting d 3 occurred at retrograde greenschist facies metamorphic conditions the tremolite 40 ar 39 ar cooling age 718 5 ma of amphibolitic eclogite and muscovite 40 ar 39 ar cooling ages about 550 ma of mica schists indicate that a maximum temperature of 500 50 c was not reached during the neoproterozoic orogeny the style and timing of the neoproterozoic orogeny show similarities to the cadomian aged timan range nw of the polar urals geochronological and thermochronological data together with the abrupt change in structural style and metamorphism east of the zuratkul fault suggest that the mcb is exotic with respect to the se margin of the east european platform thus the mcb is named the beloretzk terrane recognition of the beloretzk terrane and the neoproterozoic orogeny at the eastern margin of baltica has important implications for neoproterozoic plate reconstruction and suggests that the eastern margin of baltica might have lain close to the avalonian cadomian belt

kinetics of pb release during the zircon evaporation technique abstract the release of silica and pb from zircon by the thermal evaporation technique is shown to relate to the conversion of zircon to a porous baddeleyite structure the zircon baddeleyite reaction occurs at a sharp reaction front which progressively migrates towards the centre from the grain margin significant reaction in non metamict zircon commences in the mass spectrometer at temperatures of the order of 1500 c the progressive migration of zircon baddeleyite transformation towards grain interiors explains why the evaporation technique with its inherent advantage of not requiring chemical separation of pb and u progressively samples pb in zones towards the centre of the grain and gives 207 pb 206 pb ages as accurate as conventional u pb dating methods the rates of pb and silica loss are primarily determined by the rate of advance of this de silicification reaction front towards the interior of the grain most estimates of pb diffusion coefficients in zircon imply that diffusion distances in the unreacted zircon are negligible 10 2 m at the temperatures and duration of the pb evaporation experiments isothermal experiments demonstrate the rate of zircon baddeleyite transformation with time and temperature and provide an estimate of 130 10 1 kcal mol 1 for the activation energy of the process significantly higher than estimates of pb diffusion activation energy

geodynamic regimes in the precambrian of the urals abstract of all precambrian rocks in the urals the late proterozoic is best represented the archaean and lower proterozoic are reconstructed from separate blocks massifs which are roughly correlated with the crystalline basement of the adjacent platforms radiometric dates for the majority of early precambrian blocks exceed 2 ga features of very old 2 9 ga protocrustal material have been identified with grey gneiss complexes archaean blocks represent remnants of charnockite granulite and migmatite amphibolite complexes and the geodynamic regime of their generation is the most primitive and does not occur in later periods of geological history the karelian age blocks appear to be relics of nongeosynclinal mobile belts their petrochemical characteristics and ree patterns suggest rift related conditions of their formation late proterozoic deposits 1650 590 ma comprise thick terranes both at the western slope and in the axial zone of the urals revision of rock associations disclosed that these terranes were previously wrongly interpreted as geosynclinal formations they consist of platform sediments and include magmatic rocks which are typical of zones of diaschysis tectonomagmatic activation and rifting recurrent rifting of the elsonian grenville and baikal assynthian cycles of diastrophism was responsible for crustal extension and thinning followed by complete continental breakup and development of oceanic basins in the early palaeozoic only extension in the lower crust during rifting was responsible for dynamometamorphism under different temperature conditions and dropping lithostatic pressure blastomylonites and schists produced during this process were previously mistaken for products of orogenic metamorphism they were considered as evidence of geosynclinal cycles occurring in the late proterozoic and similar to those of the palaeozoic diaschysis and rifting were probably predominant geodynamic regimes in the late precambrian of the urals they resulted in destruction or extensive reworking of the original archaean and early proterozoic crust to variable degrees and in the accumulation of volcano sedimentary and terrigenous rifted depressional successions there is no reliable evidence for the existence of an oceanic basin in the late proterozoic of the urals

main uralian thrust and main uralian normal fault non extensional palaeozoic high p rock exhumation oblique collision and normal faulting in the southern urals the crustal architecture of the southern urals is dominated by an orogenic wedge thrusted westward upon the subducted east european continental margin the n s trending wedge constitutes an antiformal stack composed mainly of the high p maksyutov complex the overlying suvanyak complex and the allochthonous synformal zilair flysch further west these tectono metamorphic units are separated by tectonic contacts and record discontinously decreasing metamorphic conditions from bottom to top in the east the e dipping main uralian normal fault cross cuts the metamorphic footwall and juxtaposes the non metamorphic magnitogorsk island arc this syncollisional normal fault compensated crustal thickening and exhumation of the high p rocks orogenic shortening was accommodated by the main uralian thrust a w vergent crustal scale shear zone at the base of the wedge geological investigations and reflection seismics urseis 95 argue in favour of a geodynamic evolution integrating subduction and basal accretion of high p rocks during sinistral oblique thrusting along the main uralian thrust and coeval normal faulting along the main uralian normal fault

the structural architecture of the footwall to the main uralian fault southern urals abstract the uralide orogen is a linear collisional belt formed during the upper paleozoic as a result of convergence between the east european craton and outboard terranes and accretion of intervening island arcs and obduction of oceanic crust onto the east european craton this paper presents a new structural subdivision of the southern urals to the west of the main suture zone that puts this part of the mountain belt in a context similar to that of other collisional orogenic belts this subdivision consists of a west vergent thrust stack that is made up of an autochthonous to parautochthonous foreland basin and a parautochthonous foreland thrust and fold belt flanked to the east by a metamorphic thrust stack the latter units are structurally overlain by allochthonous rocks of an accretionary complex slope sediments oceanic crust and high grade eclogite blueschist bearing gneiss where possible these units are defined on the basis of their internal stratigraphy and structure and on the nature of their bounding faults

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

diagenesis and incipient metamorphism in the western fold and thrust belt sw urals russia in the western fold and thrust belt of the southern urals the kubler and arkai indices determined on shales slates and phyllites record an increase from lower late diagenetic to epizonal grade from west to east the metamorphic grade varies strongly within the different tectonic segments which are separated by major thrusts the increase of diagenetic and incipient metamorphic grade from the footwall to the hanging wall of all major upper palaeozoic thrusts indicates a pre permo triassic origin west of the avzyan thrust zone the diagenetic to incipient metamorphic grade is related to the palaeozoic basin development and reached the final grades in late carboniferous to early permian times east of the first avzyan thrust in the yamantau anticlinorium the diagenetic to lower greenschist metamorphic grade is possibly of neoproterozoic origin and might be related to the development of the neoproterozoic basin at the eastern margin of the east european craton the eastern part of the yamantau anticlinorium was exhumed below 200 c in the late carboniferous or early permian the diagenetic grade of the autochthonous palaeozoic sedimentary units increases toward the stack of palaeozoic nappes and might partly be caused by the deformational process due to the emplacement of the palaeozoic nappes within the timirovo thrust sheet the decrease of metamorphic grade with stratigraphic age developed prior to the emplacement of the nappes the upper anchizonal metamorphic grade of the upper devonian slates of the zilair nappe results from the deformation process related to the lower carboniferous nappe emplacement

quaternary paleoceanography of the deep arctic ocean based on quantitative analysis of ostracoda abstract ostracodes were studied from deep arctic ocean cores obtained during the arctic 91 expedition of the polarstern to the nansen amundsen and makarov basins the lomonosov ridge morris jesup rise and yermak plateau in order to investigate their distribution in arctic ocean deep water aodw and apply these data to paleoceanographic reconstruction of bottom water masses during the quaternary analyses of coretop assemblages from arctic 91 boxcores indicate the following ostracodes are common at all depths between 1000 and 4500 m and species distribution is strongly influenced by water mass characteristics and bathymetry quantitative analyses comparing eurasian and canada basin assemblages indicate that distinct assemblages inhabit regions east and west of the lomonosov ridge a barrier especially important to species living in lower aodw deep eurasian basin assemblages are more similar to those living in greenland sea deep water gsdw than those in canada basin deep water two upper aodw assemblages were recognized throughout the arctic ocean one living between 1000 and 1500 m and the other having high species diversity at 1500 3000 m downcore quantitative analyses of species abundances and the squared chord distance coefficient of similarity reveals a distinct series of abundance peaks in key indicator taxa interpreted to signify the following late quaternary deep water history of the eurasian basin during the last glacial maximum lgm a gsdw aodw assemblage characteristic of cold well oxygenated deep water 3000 m today inhabited the lomonosov ridge to depths as shallow as 1000 m perhaps indicating the influence of gsdw at mid depths in the central arctic ocean during termination 1 a period of high organic productivity associated with a strong inflowing warm north atlantic layer occurred during the mid holocene several key faunal events indicate a period of warming and or enhanced flow between the canada and eurasian basins a long term record of ostracode assemblages from kastenlot core ps2200 5 1073 m water depth from the morris jesup rise indicates a quasi cyclic pattern of water mass changes during the last 300 kyr interglacial ostracode assemblages corresponding to oxygen isotope stages 1 5 and 7 indicate rapid changes in dissolved oxygen and productivity during glacial interglacial transitions

variscan orogeny in the black sea region two gondwana derived paleozoic belts rim the archean paleoproterozoic nucleus of the east european platform in the black sea region in the north is a belt of paleozoic passive margin type sedimentary rocks which extends from moesia to the istanbul zone and to parts of the scythian platform the mois block this belt constituted the south facing continental margin of the laurussia during the late paleozoic this margin was deformed during the carboniferous by folding and thrusting and forms the variscan foreland in the south is a belt of metamorphic and granitic rocks which extends from the balkanides through strandja sakarya to the caucasus bassac block the protoliths of the metamorphic rocks are predominantly late neoproterozoic granites and paleozoic sedimentary and igneous rocks which were deformed and metamorphosed during the early carboniferous there are also minor eclogites and serpentinites mostly confined to the northern margin of the bassac block typical metamorphism is of low pressure high temperature type and occurred during the early carboniferous visean 340 330 ma coevally with that observed in the central europe volumetrically more than half of the crystalline belt is made up of carboniferous earliest permian 335 294 ma granites the type of metamorphism its concurrent nature over 1800 km length of the bassac block and voluminous acidic magmatism suggest that the thermal event probably occurred in the deep levels of a continental magmatic arc the bassac arc collided with laurussia in the mid carboniferous leading to the foreland deformation the ensuing uplift in the permian resulted in the deposition of continental red beds which are associated with acidic magmatic rocks observed over the foreland as well as over the bassac block in the black sea region there was no terminal collision of laurussia with gondwana during the late paleozoic and the laurussia margin continued to face the paleo tethyan ocean in the south

preserved collisional crustal structure of the southern urals revealed by vibroseis profiling the vibroseis reflection profiling component of the urseis 95 experiment provides a high resolution crustal scale image of the unextended southern uralide orogen a marked lateral and vertical variation of reflectivity throughout the entire crust differentiates the former margin of the east european craton to the west from accreted terranes to the east between these regions is a less reflective zone corresponding to the magnitogorsk volcanic arc and the crustal root zone continuous reflections and reflective domains that correlate at the surface with major tectonic features define a bivergent orogen in which the paleozoic collisional structure has been largely preserved