the development of the central european continental crust since the early archaean based on conventional and ion microprobe dating of up to 3 84 b y old detrital zircons abstract archaean components up to 3 84 b y old have been detected by ion microprobe dating of zircon from various sediments and metasediments of the european hercynides moldanubian of northeastern bavaria and the massif central the early archaean age of 3 84 b y was obtained within the euhedral core of a detrital grain from a moldanubian paragneiss the same detrital zircon grain reveals at least three metamorphic overprints at 2 59 b y b p 1 94 b y b p and 460 m y b p and at least two cycles of mechanical abrasion during surface recycling in the early proterozoic and after the pan african cycle it is thus the crystal with the most complex geological history ever detected further archaean ages for the crystallization of primary magmatic zircons probably in the course of continental crust formation in the first cycle provenances of the two studied areas were found at 3 15 2 9 2 76 and 2 65 b y b p and in a few cases between 2 6 and 2 5 b y b p early proterozoic crust forming events between about 1 75 and 2 15 b y b p play an important role especially for the sediments and metasediments of the montagne noire at the southern rim of the massif central middle and late proterozoic ages cluster around 1 0 b y b p and 600 m y b p for all analysed samples therefore the deposition of the metasedimentary precursors took place after the pan african cycle which is in line with the rb sr whole rock systematics of central european metasediments the ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains the following sequence coinciding with times of magmatic events has been so far established 2 59 1 94 and 1 0 b y b p and 600 m y b p thus ion probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles the ubiquitous presence of pan african detrital zircons in european metasediments is a strong indication that the detrital material was derived from gondwana however from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m y and 330 m y ago it is probable that various gondwana derived microcontinental plates contributed to the different basin fillings the gondwana derived detritus is also supported by the observation of two periods of relative quiescence or non activity around 2 3 b y ago and especially around 1 5 b y ago which is a typical feature observed in gondwana cahen et al 1984 but not in laurasia the other possible source of the detritus the lack of zircons crystallized between 3 84 and 3 15 b y ago might have different causes thus the continental crust of the european hercynides probably developed since the early archaean and grew via many crustal accretion events to its present state

collapse of the thickened variscan crust in the french massif central mont pilat extensional shear zone and st etienne late carboniferous basin abstract a large scale extensional shear zone is described from the mont pilat area in the northeastern part of the french massif central this shear zone is about 2 km thick and is characterized by well developed n dipping foliation and stretching lineations deformation occurred under conditions of retrograde metamorphism from high temperature conditions to greenschist facies it is superimposed on a composite foliation related to the earlier variscan 380 350 ma compressional events associated with southward emplacement of nappes and hp mt metamorphism regional n s lineations and shear criteria observed at a regional scale indicate a northward displacement of the hanging wall during extension large strains locally recorded in thick mylonitic zones suggest that displacement was substantial some tens of kilometres late superimposed extensional brittle deformation and pseudotachylites show that northward displacement on normal faults continued at a shallower depth a large granitic pluton granite du velay intruded late in the the variscan orogeny was associated with extension some small leucogranitic syntectonic melts were emplaced within the shear zone and are strongly deformed geochronological rb sr data on these leucogranite bodies give an age of 320 ma and 40 ar 39 ar dating of synkinematic biotites from gneissose rocks in the shear zone yields a plateau age of 313 ma these data show that extensional ductile deformation and magmatism were still active during the latest late carboniferous thus extension occurred immediately after the well known late palaeozoic collision type compressional tectonics which had significantly thickened the crust we propose that the extensional tectonism described here is due to the gravitational collapse of the initially thick variscan crust the stephanian intracontinental sedimentary basin of st etienne was formed along the hanging wall of the normal shear zone during the late stages of this post orogenic crustal extension

multiple variscan nappes in limousin western massif central france geophysical constraints to the geological model and geodynamic implications abstract the upper crustal structure of limousin western massif central france results from tectonic superposition of major variscan high grade metamorphic crystalline nappes one of them containing well preserved relicts of an early paleozoic oceanic lithosphere three of these nappes outcrop in the studied area as evidenced by surface geological observations their overall present synclinorium structure is confirmed by the gravimetric data results from three short refraction seismic profiles recorded in this area are totally compatible either with the local tectonic model or with the global one recently proposed for limousin although it would be an over interpretation to derive a geological model from the only seismic profiles presented this study shows that short seismic profiles are critical for 3 d extrapolation of the geological surface data in old orogenic regions as ophiolites are located within a peculiar unit representing an early major thrust contact the origin of the various nappes above and below may be defined in terms of plate tectonics

retrogressive development of glaucophane in some eclogites from massif armoricain east of nantes france a petrological and mineralogical study using an electron microprobe of a blue amphibole eclogite occurring near nantes massif armoricain france has enabled us to characterize this amphibole as glaucophane resulting from a secondary reaction in the rock this sodic amphibole was formed at the expense of primary eclogite paragenesis including omphacite garnet and quartz according to a sliding reaction which it was possible to study quantitatively 3 24 omphacite 0 90 sio2 0 76 garnet 1 08 h2o 1 glaucophane 0 55 grossular s s in the garnet 0 04 paragonite

u pb zircon and monazite dating of a mafic ultramafic complex and its country rocks u pb data on zircons from the largest mafic ultramafic body 6 2 km of the french central massif sauviat sur vige yield the following age results primary magmatic crystallization of the gabbroic and peridotitic protoliths took place in the cambro ordovician 496 25 17 m y variable transformation under eclogite facies conditions was hercynian 320 29 36 m y the same age pattern derived by u pb monazite analyses was found also for the immediate country rocks i e kyanite bearing coarse grained metagranites occurring to the w and n of the sauviat massif due to the fact that there is no regional hercynian high grade metamorphism in this part of the french central massif e g duthou 1977 bernard griffiths 1975 both mafic ultramafic complex as well as immediate felsic country rocks must have been emplaced tectonically into pre hercynian acadian caledonian crustal rocks the cause for such a hercynian tectonism is thought to be due to continent continent collision of the spanish with the armorican plate

palaeomagnetism and the age of the cracow volcanic rocks s poland summary permian rhyodacites melaphyres and tuffs from the cracow area south poland were sampled for the palaeomagnetic and isotope studies single grain u pb dating of most zircon grains separated from the rhyodacites gave mean age of magma emplacement of 294 2 2 1 ma some zircons however displayed younger ages 268 7 3 4 ma probably related to the metasomatic alterations of these rocks two permian components of magnetizations related to these processes were isolated and together with previously defined late carboniferous permian palaeomagnetic poles from south poland were used for construction of the regional apparent polar wander path apwp the early permian segment of this apwp shows a certain departure from the coeval part of the fennoscandian apwp due to anticlockwise rotations of studied rocks most probably caused by mid permian sinistral tectonic movements along reactivated prominent variscan faults of central europe this sense of tectonic mobility does not support the hypothesis about transformation from pangea b to pangea a along an intra pangea dextral megashear during the permian older than previously assumed ages of the post variscan igneous rocks of central europe reduce overlap of gondwana s and laurussia s parts of the early permian pangea a

precambrian u pb zircon ages in eclogites and garnet pyroxenites from south brittany france an old oceanic crust in the west european hercynian belt abstract u pb zircon ages have been determined for two eclogites from the vendee and for two garnet pyroxenites from the baie d audierne in an episodic pb loss model the two discordia would give upper intercept ages around 1300 1250 ma and lower intercepts ages of 436 384 ma two interpretations are proposed 1 the 1250 1300 ma ages may reflect an unspecified upper mantle event or process the paleozoic ages correspond to the tectonic emplacement of an eclogitic mantle fragment into the continental crust 2 the protolith may have been extracted from the upper mantle 1250 1300 ma ago and stored in a crustal environment until it was metamorphosed under high pressure conditions around 400 ma ago this latter model is favoured by available geologic and isotopic data consequently we propose that a 1300 ma old oceanic crust was tectonicly incorporated into a sialic basement during the proterozoic this mixture was subsequently subducted during the paleozoic

early palaeozoic palaeobiogeography of the variscan regions abstract the palaeobiogeographic method appears to be an attractive alternative for reconstruction of the palaeogeography of the variscan regions during the palaeozoic the evolution of faunal assemblages and climatic characteristics from the cambrian up to the end of the devonian demonstrates that baltica and north gondwana represent two major and independent palaeogeographic entities separated by a mid european rheic ocean which opened as early as the cambrian and closed progressively from the devonian the south armorican ocean whose closure generated the so called ligerian orogen is regarded as a branch of the rheic ocean both a tornquist s sea separating baltica from its western extension avalonia during the ordovician and a proto tethys ocean extending between southwestern europe and north africa during the devonian are excluded from our reconstructions

evidence of an imbricate crustal thrust belt in the southern british variscides contributions of southwestern approaches traverse swat deep seismic reflection profiling recorded through the english channel and the celtic sea the southwestern approaches traverse swat seismic reflection profiles recorded through the celtic sea and the english channel provide unexpected data concerning notably the deep structure of the irish and south british variscan crust one of the most significant results is the recognition of prominent deep southerly dipping reflectors regarded as large scale foreland directed variscan thrusts which constitute in the southern innermost zones a crustal stacking wedge the resulting overthickened crust may have induced the petrogenesis of the cornubian granitic batholith by an anatectic melting process the roots of this crustal duplex are not imaged on the swat lines because of their subsequent assimilation by the more recent layering of the present day lower crust upward the deep crustal ramps are assumed to flatten out and to join a roof thrust which acts as a major midcrustal decoupling zone not revealed by the swat profiles and which underlies the thick devonian allochthonous high strained units of cornwall northward this shallow northerly verging shear zone is overstepped by the tintagel antithetic back thrusts which are closely related to deeper northward dipping basement faults considered as caledonian features southward the imbricate crustal thrust unit is overridden by the lizard ophiolitic suture which appears as a 8 km thick gently southerly dipping sheared zone crosscuttting the entire crust and rooting deep beneath the present day moho its hanging wall is constituted by the channel cadomian block characterized by an almost seismically featureless upper crust which is not involved in the main variscan thrust stacking northward the imbricate thrust unit is delineated by a moderately southerly dipping ramp which penetrates straight down into the upper crust without any evidence of an intervening flat lying decollement level this frontal ramp emerges along a broadly 110 n trending discontinuous thrust front located above the northern downfaulted margins of the munster ireland and pembroke wales devonian basins the swat deep seismic data relate the overall variscan crustal shortening undergone by the south british domain to an oblique converging process involving a continental shelf pinched between the rigid allochthonous channel block and the northern autochthonous foreland this crustal shortening occurred during a carboniferous intracontinental collisional context subsequent to the closure and the obduction of a small oceanic crust floored basin lizard

variszische duktile tektonik im bohmischen massiv die verformungsanalyse und richtungsverteilung der streckungslineationen in den duktilen bereichen des bohmischen massiv zusammen genommen mit der lage der foliationsflachen deuten darauf hin das die blattverschiebungen wahrend der variszischen deformation die entscheidende rolle gespielt haben die kompatibilitat der deformation im gesamtbereich wurde durch uberschiebungen und transpression in den durch die blattverschiebungen begrenzten blocke erreicht das variszische tektogen wurde als blattverschiebungsorogen auf einer kontinentalen oberproterozoischen kruste angelegt die zonen mit intensiver krustenverkurzung und damit groster krustenmachtigkeit sind durch weitraumige anatexis und granitische intrusionen gekennzeichnet

the late ordovician diamictic formations from southwestern europe north gondwana glaciomarine deposits abstract this paper presents a review and a discussion of the clast bearing formations diamicts which occur in the uppermost part of the ordovician succession of numerous regions in southwestern europe lithology facies stratigraphic setting and age are discussed the strong similarities of these formations with contemporaneous glaciomarine deposits of the northern and northwestern margins of the african ice sheet and the late ordovician paleogeographic pattern based upon paleomagnetic data lead to the conclusion that the south european clast bearing formations are glaciomarine deposits supplied with clasts by floating icebergs released from the african ice sheet the proposed paleogeographic model confirms the close connection between southwestern europe and gondwana during early palaeozoic times

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

importance of hercynian tectonics within the framework of the southern alps abstract the hercynian remnants present within the alpidic structural zones of the southern alps are reviewed the pattern of hercynian metamorphism is zonal from granulites in the western area to anchimetamorphic facies in the eastern one in the folded zone at the eastern margin a severe hercynian folding phase took place during the westphalian thrust sheets comprising sequences of different facies ranging in age from caradocian to early westphalian are sutured by late westphalian molasse deposits the assumption that the southern alps and the external dinarides remained outside the hercynian folding front is contradicted by field evidence

isograds and mineral assemblages in the eastern axial zone montagne noire france implications for temperature gradients and p t history detailed petrography across a metapelitic sequence in the eastern axial zone of the montagne noire france is the basis for a sequence of isograds marking the first appearance of biotite cordierite staurolite andalusite and sillimanite the juxtaposition of low grade biotite free rocks against medium grade rocks at the gently dipping biotite cordierite isograd is attributed to tectonic telescoping of the metamorphic sequence study of mineral assemblages with respect to an afm reaction sequence indicates the staurolite isograd is related to changes in rock composition and complex assemblages in the sillimanite zone may be the result of unstable persistence of minerals formed when metamorphic grade was lower these assemblages are interpreted to contain a record of part of the p t history during which pressure decreased as temperature increased p t profiles show that temperature gradients of 200 300 c km suggested by previous workers are not required to explain the isograd pattern gradients of 37

concepts of the evolution of the austroalpine basement complex eastern alps during the caledonian variscan cycle the austroalpine basement complex has a complicated pre alpidic history which begins with the caledonian era in the late precambrian and early paleozoic a magmatic sedimentary rock sequence is formed presumably in an island arc or active continental margin environment subduction with eclogite formation is followed by collision high grade metamorphism and anatexis in the ordovician this caledonian basement is preserved in parts of the austroalpine crystalline mass the post caledonian deposits are mainly shelf type sediments with intercalated volcanics although there is evidence for an oceanic basin to the south the variscan facies zones are arranged in sw ne direction oblique to the alpidic trend in a first stage of variscan orogeny in the carboniferous south east vergent decollement nappes syntectonic flysch deposits and granitoids are formed along with regional metamorphism this is followed by a second stage in the permian with north west vergent thrusting renewed granite formation and metamorphism the variscan nappe pile is today exposed in a deeper level in the west or northwest than in the east or southeast

aeromagnetic data a tool for studying the variscan arc of western europe and its correlations with transatlantic structures abstract a new compilation of magnetic data from france with the french continental shelf and portugal allows the delineation of the geodynamic features of the variscan domain of western europe specific techniques such as the use of shadowgrams provide a better definition of the variscan structures in france especially of those below the mesozoic basins and offshore on the western continental shelf in addition hypotheses on the extension of the western part of the variscan arc may be considered comparison of the magnetic trends using a permian palaeogeographic reconstruction suggests new constraints on the interpretation of the ibero armorican arc and hitherto unknown relations hips between the main variscan faults and sutures defined in europe and their possible extensions into north america

inverted metamorphic zonation and variscan thrust tectonics in the rouergue area massif central france p t t record from mineral to regional scale

response of precambrian zircon to the chemical abrasion ca tims method and implications for improvement of age determinations abstract electron backscatter imaging raman spectroscopy and u pb geochronology have been applied to precambrian zircon grains that were annealed at 1000 and 1450 c for various times then leached with hf to constrain the conditions for healing radiation damage and attaining primary u pb zircon ages using the chemical abrasion ca tims method sem images reveal a variety of textures for zro2 overgrowths on 1450 c annealed and leached zircon surfaces that depend on the degree of radiation damage and annealing history highly damaged zircon produces finer textures than zircon with less damage raman spectroscopy indicates that crystals with different levels of radiation damage are only partially restored by annealing at 1000 c for 2 3 days longer annealing periods of 20 days are not noticeably more effective annealing at 1450 c for 1 h results in partial breakdown of zircon but restores raman peak widths and wave numbers to values characteristic of undamaged zircon after zro2 overgrowths are removed by hf raman spectra are much less sensitive to polarization angle for annealed highly damaged grains than for weakly damaged zircon u pb isotopic analyses of low to moderately damaged zircon alpha fluence ranging up to 1019 g corresponding to an amorphization volume fraction of 80 or more yield almost concordant data 0 3 0 5 discordance after high temperature annealing at 1450 c followed by hf leaching at 195 c analyses of cracked zircon annealed at 1450 c and leached may remain discordant but those of uncracked grains are concordant most analyses show primary 207pb 206pb ages although cracked grains annealed at 1450 c may produce discordant data with 207pb 206pb ages that are too young after leaching the solubility of highly damaged very disordered zircon amorphization level of 99 is only slightly reduced by annealing and analyses of leach residues are strongly discordant although primary 207pb 206pb ages are obtained annealing of highly damaged zircon under any conditions apparently results in a mass of randomly oriented micro crystals that pseudomorph the original grain this could explain the fine scale pattern observed on etched crystal surfaces reduced anisotropy at the 5 m scale of the raman laser beam and high solubility in hf it may be impossible to restore primary u pb isotopic ages in such cases but precise ages can still potentially be determined from 207pb 206pb ratios or by application of the air abrasion method

geological interpretation of dekorp 2 s a deep seismic reflection profile across the saxothuringian and possible implications for the late variscan structural evolution of central europe abstract a 250 km long deep seismic section the first of its kind in west germany s dekorp project is interpreted in terms of late variscan collisional tectonics the structural model used is based on the intracontinental thrust tectonics concept of modern collision orogens the crustal profile across the saxothuringian zone seems to indicate north facing ramps antiformal stacks and culminations within the crystalline basement of the internal mid european variscides that have been only partially overprinted by permo mesozoic crustal stretching

u pb und k ar datierungen des uranvorkommens hohenstein oberpfalz k ar datierungen an glimmern des fichtelgebirges und der nordlichen oberpfalz uberdecken das intervall von 330 ma bis weniger als 300 ma zusammen mit rb sr gesamtgesteinsdatierungen an post deformativen graniten lassen sich vier variskische ereignisse herausarbeiten 1 regionalmetamorphose vor rund 330 ma 2 die intrusion der alteren post deformativen granite vor 320 ma 3 die intrusion der jungeren granite vor rund 300 ma 4 lokale einflusse auf hellglimmer noch nach 290 ma auf kluften sogar bis 150 ma

seismic signature and tectonic cross section of the variscan crust in western france deep seismic reflection profiling ecors program vertical and wide angle survey coupled with field and subsurface geology are used to propose a complete nne ssw section of the variscan belt in western france from belgium to southern brittany on this 800 km long profile which probably crosses two paleozoic sutures the variscan belt appears as a broad fanlike orogen characterized by large 100 km northward and southward overthrusts with polyphase deformation and metamorphism the central part of the belt central armorican zone suffered a simpler dextral shearing parallel to the strike of the belt with only green schist metamorphism deep reflection profiling shows that all the large thrusts visible at the surface root deeply in the lower crust or even crosscut the moho in the same way all the large strike slip faults parallel to the belt appear at depth as narrow discontinuities which crosscut the whole crust and in some cases the moho the seismically well layered crust develops essentially at depth in the most deformed and metamorphic internal parts of the belt in contrast with the relatively transparent brabant foreland basement which escaped the variscan deformation the moho appears as a relatively flat boundary at an average depth of 35 km with different characters it is sharp and strongly reflective both by wide angle and vertical seismics below the internal thrust zones much less distinct in the central armorican zone and only defined by low frequency signals at wide angle below the brabant foreland all these characteristics led us to consider the layering of the lower crust and the moho itself as initially tectonometamorphic features essentially produced by shearing and crust mantle decollement during the variscan stacking of the crust by intracontinental lithospheric subduction

geochemical and isotopic characteristics of blueschist facies rocks from the ile de groix armorican massif northwest france abstract nine samples of blueschist facies metabasic rocks and four samples of associated metasedimentary lithologies were analysed for ree contents and nd isotopic composition sampling includes the main rock types exposed on groix concentrating on those metabasic rocks with least dispersed rb sr systems the ree abundance patterns appear not to have suffered significant alteration since emplacement of the protolith magma despite subsequent blueschist facies metamorphism the data when integrated into a wider geochemical data base allow the identification and characterization of both tholeiitic and alkaline magma types in the original igneous suite which was differentiated from depleted mantle sources the basalts probably originated in an oceanic context some sediments are predominantly volcaniclastic in which a continental crustal component may be recognized other sediments are closer in composition to continental crust with variable additions of tholeiitic or alkali basalt debris the volcanic sedimentary assemblage on groix is suggestive of ocean island magmatic activity associated with a sedimentary sequence derived from a nearby continent

palaeoenvironmental changes at col des tribes montagne noire france a reference section for the famennian of north gondwana related areas we present detailed biostratigraphy based on conodonts and palaeoenvironmental trends deduced from microfacies and conodont abundance through the famennian late devonian at col des tribes montagne noire france the succession is characterized by micritic limestones deposited in settings oscillating between mid to outer ramp facies contain poor fauna widely dominated by nektonic organisms this section is complete and one of the most conodont rich for the famennian of the north gondwana related area the upper kellwasser event frasnian famennian boundary and the hangenberg devonian carboniferous boundary have been lithologically identified they are characterized by decimetre thick black dysoxic to anoxic argillaceous sediments the condroz and annulata events although not materialized by lithological changes have been positioned due to the precise stratigraphy the first event occurred during the deposition of condensed ferruginous facies griotte limestones and the second event during the deposition of micrites barren of benthic fauna the combination of information from both facies and conodont biofacies changes allows a general sea level curve through the entire famennian for north gondwana to be proposed for the first time at col des tribes the general trend is a slight deepening upwards from triangularis to trachytera zones then a pronounced shallowing upwards trend from upper trachytera to praesulcata zones this curve correlates with the well known reference curve from euramerica concerning the late famennian trachytera to praesulcata zones there are some discrepancies in minor cycles which can be explained by tectonical phenomena at the onset of the edification of the variscan belt in europe

accretionary history and crustal evolution of the variscan belt in western europe abstract the variscan belt of western europe is part of a large intra paleozoic belt extending on both sides of the atlantic from the ouachitas in the us and the mauritanides in west africa to the bohemian massif in czechoslovakia and poland the belt was constructed between 500 and 250 ma as a result of convergence and collision between two main continents laurentia baltica and africa after the closure of various oceanic basins iapetus rheic and galicia massif central now recorded as discrete in part cryptic sutures which form the roots of large crystalline nappes in which are found dismembered remnants of oceanic crust and mantle the accretionary history of the variscan belt in western europe is related to the progressive closure of probably two oceanic basins the rheic in the north and another the galicia massif central in the south by respectively southward and northward intraoceanic subduction followed by obduction and intracontinental lithospheric subduction the result is a broad 700 800 km fan like divergent belt which must be considered as a classical obduction collision orogen with a long intracontinental history of about 100 150 ma its arcuate shape is probably due to the northwestward impingement of a promontory of the african continent into laurentia baltica

metamorphism of an obducted island arc example of the kohistan sequence pakistan in the himalayan collided range in northern pakistan the extraordinary 40 km thick kohistan sequence of metamorphosed mafic ultramafic and calc alkaline layered plutonic and volcanic mainly andesitic to rhyodacitic rocks has been recently reinterpreted as the only complete vertical section of an intra oceanic island arc presently exposed anywhere in the world plate scale models have been suggested to explain the origin and the tectonic evolution of this arc in the himalayan collision range despite some noticeable differences in the models there is general agreement that the kohistan sequence represents the crust of an arc obducted onto the northern edge of the indian plate before the oligocene collision of india against asia new data on the tectonometamorphic evolution of the kohistan sequence have led to the conclusion that a first major d 1 event developed with increasing metamorphic grades oriented downward the kohistan pile but also toward ensandwiched basic ultrabasic granulitic rocks rare geochronological data and petrological evidence indicate that the latter granulites pyriclasites and various plagiopyrigarnites pyrigarnites metatroctolites metawebsterites metadunites etc were parts of enormous lower to middle cretaceous layered calc alkaline plutons emplaced during the arc building stages as the d 1 metamorphic event is correlated with the upper cretaceous obduction process of the kohistan onto india it is proposed that the d 1 thermal structure was strongly controlled by remnant magmatic heat source s within the obducted arc this interpretation fits nicely with recent theoretical thermal models since the obduction of the arc was probably shortly after 10 20 ma the emplacement of the former plutonic arc rocks a blueschist tectonic melange underlying the obducted arc was possibly synchronous to the obduction and not clearly linked to the older subductive process the oligocene collision of india against asia was associated with a barrovian overprinting metamorphism in the kohistan arc during this second event the dynamo thermal structure was much more classical and not controlled by the occurrence of the cretaceous metaplutonic rocks a tentative model of the thermal structures during the major events which affected the kohistan arc is proposed

thin skinned tectonics in the northern rhenish massif germany a network of profiles in the hercynian mountain system geo traverse rhenoherzynikum is needed to determine details of the geological structure and find the reasons for tectonic movements outstanding features within the mountain system such as the large fault zone along the southern boundary of the hunsruck1 and the left flank of the rhinegraben have already been investigated by reflection seismology2 3 we describe here a new reflection refraction experiment carried out in 1978 in the most northern part of the rhenish massif across the stavelot venn massif near aachen fig 1 while layers in the lower crust dip to the nnw a strong upper reflector at 3 4 km depth shows a slight dip to the sse and indicates a prominent fault zone near aachen known in belgium as the faille du midi here a comparable and apparently identical reflector was found some 100 and 175 km to the west of our profile we interpreted this reflector as a prominent and well lubricated thrust fault along which a huge horizontal nappe displacement took place during the last stages of the variscan orogeny

paleozoic evolution of continental crust in the beaujolais lyonnais area northeastern part of the massif central france in the beaujolais lyonnais area of the northeastern massif central accretion of continental and possibly oceanic crustal fragments occurred between cambrian and early carboniferous time three distinct lithotectonic units terranes have been recognized the southern lyonnais unit consists of medium to high grade metamorphics and includes eclogites it formed in the early paleozoic the brevenne unit to the north contains low to medium grade metamorphic mafic and felsic volcanics and subordinate sedimentary rocks which possibly originated during the early paleozoic until devonian time in a sialic back arc environment or along an active continental margin the beaujolais unit is represented by volcanics on the south and predominantly shallow marine clastics and carbonates on the north it developed in a late devonian or early carboniferous ensialic marginal basin the peak of metamorphism in the lyonnais unit hp ht was reached in silurian time subsequent nw se to e w oriented convergence produced mylonitic foliation structural imbrication of the lyonnais basement rocks with the brevenne unit and se vergent folds accompanied by low to medium grade metamorphism late visean to namurian n s to nw se directed n vergent thrusting produced tectonic imbrication of the metamorphic northern brevenne unit with the nonmetamorphic beaujolais unit in the southern brevenne unit and in the lyonnais unit updoming along right lateral high angle normal faults was followed by emplacement of voluminous granitic plutons of crustal origin late carboniferous to early permian crustal thinning in the beaujolais lyonnais area was associated with n s trending left lateral strike slip faults and e w to ne sw trending right lateral strike slip faults basins that developed along these faults contain continental red beds

archean protoliths within early proterozoic granulitic crust of the west european hercynian belt possible relics of the west african craton granulites from the bay of biscay area have been studied by using u pb and sm nd methods results on four granulites suggest two major events of crustal growth and high grade metamorphism the first is archean 2 7 ga and the second is early proterozoic 1 9 ga in the hercynian belt the oldest basement remnants known so far are about 2 0 ga and are located in the northern armorican massif the 2 7 ga orthogneiss protolith age from the bay of biscay is the first direct evidence of archean crust involved in the hercynian belt major events at 2 7 1 8 and 0 6 ga in the hercynian belt suggest that european precambrian microplate s were part of the west african craton where the same sequence of events is observed

datation u pb sur zircons de l eclogite de la borie haut allier france et consequences sur l evolution ante hercynienne de l europe occidentale a crystallization age of 432 10 20 ma has been determined for the la borie eclogite haut allier france by u pb dating on zircons this age is yielded by the upper intercept with the concordia curve in agreement with recent dates obtained in the southern part of the massif central in brittany and in vendee these data indicate that the high pressure high temperature metamorphic phase representative of the pre variscan evolution of the western europe took place at the silurian ordovician boundary this high pressure high temperature metamorphism could be significant of subduction processes more or less synchronous preceding the main continental collision responsible of the variscan belt s s

new and revisited paleomagnetic data from permian triassic red beds two kinematic domains in the west central pyrenees abstract new paleomagnetic results of permian triassic red beds from bielsa aure and somport anayet sectors in the pyrenean axial zone are presented and combined with revisited and reviewed paleomagnetic data from the west central pyrenees the paleomagnetic data from the red beds vary between different sectors that share similar structural position i e whilst all sectors share the existence of a characteristic prefolding component a cenozoic postfolding component is found only in one area but not in other areas with similar structural position and kinematics previous paleomagnetic data to the west of the studied zone reveal a pre turonian remagnetization component but this secondary component is not found in an area that shares a similar structural position aure this variability suggests that the paleomagnetic behavior is the result of a series of factors related to the particular tectonic history of the region sedimentation burial and deformation during basin inversion and therefore paleomagnetic data demands careful assessment in order to unravel the kinematics of areas with equivalent structural positions in orogens however important kinematic implications that hold in the pyrenees can be inferred from the restoration of the permian triassic characteristic magnetizations to positions previous to the paleomagnetic rotations recorded by cretaceous or cenozoic paleomagnetic data the restoration reveals on one hand the lack of large rotations in the south pyrenenan zone except in the nogueras area according to bates 1989 and on the other hand the contrasting rotations recorded to the west of the studied zone in the paleozoic basque massifs pbm and the south pyrenean zone indicating the existence of two different tectonic domains in the west central sector of the pyrenees the strong clockwise rotations postdating the early cretaceous remagnetization that are recorded in the pbm and in the north pyreneean zone suggest a late dextral shear deformation affecting areas near the north pyrenean fault zone

mapping of crustal scale tectonic boundaries in the ossa morena zone using reprocessed iberseis reflection seismic data the iberseis deep seismic reflection profile imaged crustal scale structures in the sw iberian variscan belt crossing the south portuguese zone the ossa morena zone and the central iberian zone in spain two subsets of the profile corresponding to the south portuguese zone ossa morena zone and the ossa morena zone central iberian zone tectonic contacts have been reprocessed with the aim of investigating the influence of cross dip and to better image steeply dipping features alternative strategies for binning midpoints into common depth point cdp bins using different azimuths were examined for synthetic data we show that the choice of the cdp processing line and the bin azimuth orientation has a significant impact on the normal moveout and dip moveout velocities and is crucial to optimizing the quality of the stacked seismic image along the crooked profile multi azimuth binning normal moveout dip moveout and migration velocity analysis on synthetic and real data show the presence of clear sub vertical upper crustal structures near the south portuguese zone ossa morena zone suture the aroche fault this sub vertical reflectivity that was not imaged earlier projects into a location in the lower crust with low reflectivity

low frequency sea waves generated by atmospheric convection cells 1 the atmospheric origin of low frequency sea waves that cause seiches in the port of rotterdam is investigated using hydrological and meteorological observations these observations combined with weather charts show that all significant seiche events coincide with the passage of a low pressure area and a cold front following these front passages increased wind speed fluctuations occur with periods on the order of 1 hour the records show that enhanced low frequency wave energy at sea and the seiche events in the harbor occur more or less simultaneously with these strong wind speed fluctuations these oscillatory wind speed changes are due to convection cells that arise in an unstable lower atmosphere in the area behind a cold front where cold air moves over the relatively warm sea surface it is shown that the moving system of a cold front and trailing convection cells generates forced low frequency waves at sea that can cause seiche events inside the harbor the occurrence of such events may be predictable operationally on the basis of a criterion for the difference in temperature between the air in the upper atmosphere and the water at the sea surface

seiche characteristics of rotterdam harbour recently the main mechanism of the generation of seiches in the port of rotterdam has been identified it comprises atmospheric convection cells that arise over the relatively warm water of the north sea which cause fluctuations in wind speed and atmospheric pressure that can generate long waves at sea as these cells move towards the coast the new insights have been used in this study together with recent data to make a detailed analysis of the seiche characteristics of the rotterdam harbour it is found that the generating mechanism that has been identified can excite basins with different natural frequencies the ratios of seiche amplitudes at different locations inside the harbour have been derived from measurements moreover theoretical amplitude ratios have been determined based on a parameterised excitation spectrum at the harbour mouth and computed amplification spectra the latter had been calculated in previous studies with a numerical harbour model the results confirm the validity of this method moreover the amplitude ratio that has been used for the design of the main storm surge barrier of rotterdam has now been confirmed to within 5 finally the possibility of prediction of the occurrence of these seiche events has been studied a prediction method based on criteria for the occurrence of convection cells has been tested in a hindcast setting results indicate that presently the occurrence of seiche events in the port of rotterdam can be predicted with high accuracy

variscan intracontinental deformation the coimbra cordoba shear zone sw iberian peninsula the coimbra cordoba shear zone is a major lineament roughly parallel to the nw se variscan structures of the southern iberian peninsula inside the shear zone evidence for a non coaxial strain path exists from structural microstructural and lattice preferred orientation data on quartz plane strain is suggested by the study of microstructures in the xy and yz sections in all formations occurring in the shear zone with increasing strain a mylonitic series develops the deformation contemporaneous with epizonal to mesozonal metamorphism is responsible for 1 1 the regional subvertical foliation oblique to the shear zone 2 2 the well defined subhorizontal lineation striking nw se 3 3 the discrete planes of shear roughly parallel to the shear zone the data are interpreted as resulting from a left lateral slip motion with a displacement of at least 72 km in the light of plate tectonics it is inferred that the coimbra cordoba lineament could have been a suture zone which evolved into an intracontinental left lateral shear zone during the variscan orogeny of the ibero armorican arc

upper paleozoic oceanic crust in the polish sudetes ndsr isotope and trace element evidence the two main mafic ultramafic complexes outcropping in the polish sudetes around the sowie gory high grade massif mt sleza and nowa ruda have been studied for trace elements and nd sr isotopes upper paleozoic sm nd whole rock isochron ages are obtained for both massifs 353 21 ma 2 with i 8 8 0 1 mt sleza and 351 16 ma with i 8 5 0 1 nowa ruda the high initial nd point to a source characterized by a strong time integrated lree depletion and along with incompatible trace element patterns they substantiate the oceanic derivation of these massifs although a subduction related marginal basin setting cannot be totally precluded trace element and isotopic data rather suggest a normal mid ocean ridge origin for the sudetic ophiolites while these results are in marked contrast with previously inferred precambrian or early paleozoic ages remnants of oceanic crust as young as the early carboniferous are consistent with the local sedimentary record of pelagic facies from the frasnian to the tournaisian and they may provide a suitable explanation for the contrasted evolution displayed by different domains of the sudetes the sudetic ophiolites might tentatively be correlated with the upper paleozoic prasinites of the saxon lineament if a late hercynian ca 150 km dextral offset is assumed along the elbe fracture in that case they might represent the latest stage remnants of an oceanic suture zone between the saxo thuringian and moldanubian zones together with several other examples throughout europe these ophiolites provide compelling evidence for the involvement of oceanic crust and plate tectonic processes in the variscan belt

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

paleozoic evolution of the armorica plate on the basis of paleomagnetic data new paleomagnetic data support the contention that armorica and gondwana formed a coherent block until ordovician time however by early devonian time armorica collided with the north american northern european assembly to form the old red continent and the collision itself was responsible for the acadian orogeny not for the earlier taconic orogeny as postulated previously the carboniferous collision of gondwana with the old red continent subsequently formed pangaea and produced the appalachian hercynian orogeny

paleozoic accretionary and collisional tectonics of the eastern tianshan china implications for the continental growth of central asia this paper deals with the various tectonic units in the chinese eastern tianshan orogenic collage in the central asian orogenic belt and discusses the paleozoic geological history of the several periods of accretion and collision of archipelago systems lying between the tarim and southern angaran continental margins the late ordovician silurian to early devonian eastern tianshan archipelago was characterized by a the harlik dananhu subduction system with a s dipping polarity in the north b a southerly n dipping subduction system beneath the central tianshan arc in the middle and c the south tianshan ocean against tarim in the south during the devonian to early carboniferous n dipping subduction led to the harlik dananhu arc and the kanggurtag forearc basin accretionary complex in the early to mid carboniferous the magmatic front associated with the n dipping subduction beneath the dananhu harlik arc migrated southwards forming the yamansu arc constructed upon the kanggurtag accretionary forearc by the late carboniferous the dananhu harlik arc was attached northwards to the angaran margin resulting in lateral enlargement of the angaran continent in the latest carboniferous to early permian a multiple soft collision left wide suture zones in the south that include the ophiolite strewn aqikkuduk shaquanzi and kumishi accretion collision complexes which were stitched by early permian post collisional plutons by re defining and re interpreting the various tectonic terranes this paper presents a new improved model for the paleozoic evolution of this part of central asia

recycling processes in granite granodiorite complex genesis the querigut case studied by ndsr isotope systematics abstract using the isotopic rb sr and sm nd petrogenetic tracers we have studied the calc alkaline plutonic complex of querigut pyrenees france which is famous for its acid to mafic rock association and is a typical post tectonic hercynian granitoid its age of 280 m y is already known 2 23 24 and our purpose is to examine the relationship between its different petrographic units and the problem of their source composition the sm nd isotopic results rule out any genetic process involving a unique homogeneous source and imply the presence of old recycled continental materials in contrast the rb sr isotope system seems to have been homogenized by hot fluids maybe post magmatic this is also true for pb isotopes as shown previously by michard vitrac et al 18 on the basis of the isotopic results and the major element data a model of combined fractional crystallization assimilation of country rocks is proposed for the basic part of the complex in contrast acid parts are mainly the result of the continental anatexis this study also constrains the basement s age 1200 1900 m y of the area and suggests a scheme for the gological history of the eastern pyrenees

decollements in slate belts examples from the european variscides and the qin ling belt of central china decollements are conspicuous features of collision belts as a result of shortening and shearing of rocks of different competencies in response to intracontinental subduction and crustal stacking when these decollements occur at upper levels the classical thin skin tectonics of the foredeeps without important internal strain results at mid crustal levels slaty cleavage and foliation development occurs in the more internal parts of the chains in the lower crust near the crust mantle boundary the more severe conditions granulitic metamorphism result in ductile deformation in the deepest parts of the mountain belts examples of middle and lower crust are well documented in central china and the variscan belt of europe from field evidence and deep seismic profiling from these examples it appears more and more likely that most of the mid crustal slate belts showing vertical slaty cleavage at the erosion surface overly flat decollements at greater depth large decollements must occur as well in the lower continental crust and at crust mantle boundary typical features of the deep seismic vertical profiles such as the strongly layered lower crust may represent the image of such deep decollements

field and petrological evidence for the development of an ensialic marginal basin related to the hercynian orogeny in the massif central france six lithologic units in tectonic contact with each other have been defined during mapping of the devonian in the beaujolais area of the northeastern massif central five main igneous suites have been recognized 1 a transitional basaltic suite restricted to a single unit 2 an acid volcanic plutonic suite the members of which are related by fractional crystallization and magma mixing 3 low tio2 volcanic rocks with calc alkaline affinities 4 a tio2 rich tholeiitic suite related to an ophiolitic complex 5 a plutonic suite with close resemblances to alaskantype intrusions

40ar 39ar geochronology of carboniferous permian volcanism in the midland valley scotland twenty one new 40 ar 39 ar step heating experiments on mineral separates from intrusive and extrusive carboniferous and permian igneous rocks in the midland valley of scotland yielded 17 concordant experiments with a relative age precision better than 1 2 these ages resolve inconsistencies between existing k ar dates on the same samples and their stratigraphical constraints correlated to recently published timescales the precise 40 ar 39 ar dates are stratigraphically constrained to stage level and can contribute to carboniferous timescale tie points at the tournaisian visean boundary within the visean and at the carboniferous permian boundary situated in the extending variscan foreland two distinct phases of extension related transitional alkaline volcanism have been resolved in the dinantian the garleton hills volcanic formation in the eastern midland valley near the tournaisian visean boundary 342 1 1 3 and 342 4 1 1 ma and the clyde plateau volcanic formation in the western midland valley during the mid visean 335 2 329 2 1 4 ma alkaline basic sills near edinburgh previously thought to be namurian appear to be coeval with the clyde plateau volcanic formation at 331 8 1 3 329 3 1 5 ma the new ages allow correlation between these short lived dinantian magmatic pulses and extensional and magmatic phases in the northumberland solway and tweed basins to the south after late westphalian end variscan compression and a regionally important tholeiitic intrusive phase at c 301 295 ma alkaline magmatism related to post variscan extension occurred in the central and western midland valley during the latest carboniferous or permian from 298 3 1 3 to 292 1 1 1 ma this correlates well with post varsican extension and magmatism observed across the nw european foreland from 300 to 280 ma

zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the ivrea verbano and strona ceneri zones nw italy abstract in order to improve the understanding of thermal tectonic evolution of high grade terranes we conducted a systematic study of textures ree content and u pb ages of zircon and monazite grains extracted from migmatitic metapelites across the amphibolite to granulite facies metamorphic gradient exposed in the ivrea verbano and strona ceneri zones italy this study documents the behaviour of these accessory minerals in the presence of melt the absence of relict monazite grains in the metasediments and the gradual decrease in the size of inherited zircon grains from amphibolite to granulite facies cores indicate partial to total dissolution of accessory minerals during the prograde path and partial melting the retrograde path is marked by i growth of new zircon rims r1 and r2 around inherited cores in the mesosome ii crystallisation of stubby zircon grains in the leucosome especially at granulite facies and iii crystallisation of new monazite in the mesosome stubby zircon grains have a distinctive fir tree zoning and a constant th u ratio of 0 20 together these features reflect growth in the melt conversely the new zircon grains with r1 rims have dark prismatic habits and th u ratios in the ivrea verbano zone the interquartile range iqr of u pb ages on zircon and monazite are interpreted to represent the length of zircon and monazite crystallisation in the presence of melt accordingly they provide an indication on the minimum duration for high temperature metamorphism and partial melting of the lower crust 20 ma and 30 ma in amphibolite and granulite facies respectively in amphibolite facies zircon crystallisation between 310 and 294 ma iqr is interpreted to reflect metamorphic peak condition and earlier retrograde history conversely monazite crystallisation between 297 and 271 ma iqr reflects cooling under 750 c to a temperature close to the solidus in granulite facies zircon crystallisation between 295 and 265 ma iqr is interpreted to reflect high temperature conditions which were attained after peak of metamorphism during isothermal decompression and subsequent cooling under 850 950 c the observed decrease of u pb ages in metamorphic zircon and monazite from amphibolite to granulite facies i e from the middle to the lower crust is interpreted to record slow cooling and crystallisation of the variscan orogenic root at the transition from orogenic collapse to opening of the tethys ocean

high frequency sea level oscillations in the mediterranean and their connection to synoptic patterns abstract this paper contains a basin wide assessment of high frequency sea level oscillations in the mediterranean sea for the period from 2010 to 2014 sea level series with temporal resolutions of 1 min were taken from 29 tide gauges that had been operational for at least 4 years with high quality time series the contribution of high frequency sea level variance 2 min 6 h to the total and residual sea level variance was estimated to vary between 0 4 and 9 5 and between 0 6 and 12 1 respectively but to become dominant at some stations during extreme high frequency events a total of 36 high frequency sea level events were extracted from the series with some occurring locally in one of the four selected regions of spain sardinia sicily and the greece ionian sea 6 events some occurring over two or three regions 19 events and some occurring in all of the selected regions within a period of 1 3 days 11 events the basin wide events normally propagate from the western to the eastern parts of the basin with an average velocity of approximately 30 km h synoptic patterns associated with high frequency events were analysed and a high resemblance to patterns associated with meteotsunamis was observed i a cyclone with a centre w nw from the affected area ii a strong thermal front in the lower troposphere and iii a forefront of an unstable strong mid tropospheric jet placed over the affected area the strong coherence between high frequency sea level events and synoptic patterns introduces the possibility of a timely forecast of these events

crustal scale thrusting and extension in the hercynian schwarzwald and vosges central europe the hercynian outcrop areas in the schwarzwald of sw germany and vosges of easternmost france consist from north to south of three major lithotectonic complexes very low to medium grade metamorphics of the saxothuringian belt the polymetamorphic central gneiss complex and the tectonic melange of the southern complex juxtaposition of these three complexes occurred by oblique convergent motions in early carboniferous time the saxothuringian belt is part of a major se dipping accretionary wedge that consists of lower paleozoic meta sedimentary and metaigneous rocks and superimposed upper devonian to lower carboniferous synorogenic volcanics and elastics the central gneiss complex consists of polymetamorphic paragneisses orthogneisses metabasites and meta ultramafics dipping to the nw the southern complex consists of an imbricated melange of north to nw dipping synorogenic elastics volcanics and blocks of ultramafics and gneissic basement the central gneiss complex cgc experienced pervasive hercynian low pressure high temperature metamorphisrn before it was emplaced against and thrust over the sedimentary and volcanic fill of the adjacent synorogenic basins this convergence 340 330 ma produced mylonitic cataclastic shear zones in the cgc the most significant shear zones are the se verging todtnau fault zone between the cgc and the southern complex and the nw verging lubine fault zone between the central gneiss complex and the rocks of the saxothuringian belt these two fault zones enclose a bivergent pop up of the polymetamorphic basement the todtnau fault zone probably extends for more than 100 km along strike and can be traced to a depth of 10 km by deep seismic reflection studies detachment probably occurred along a mid crustal zone of anatexis expressed by a domain of low seismic reflectivity between about 10 and 15 km crustal thickening during early carboniferous convergence led to an extensional phase of crustal reordering between about 330 and 280 ma that was accompanied by the massive rise of posttectonic granitic plutons from middle to high crustal levels simultaneous creation of a structural lamination in the refractory lower crust neglected the main convergent tectonic boundaries it produced a basin and range type lower crust and mantle crust transition in early permian time and involved a major advective input of heat

a sm nd isotopic study of 500 ma old oceanic crust in the variscan belt of western europe the chamrousse ophiolite complex western alps france we report the results of a sm nd isotopic study of whole rock samples from the chamrousse ophiolite complex in the variscan basement of the western alps france our data point to a 497 24 2 ma age in excellent agreement with u pb results menot et al 1984 on subordinate plagiogranites strong isotopic heterogeneity and time evolution of magmas from relatively less depleted nd 5 to very depleted nd 9 sources are also implied available data favour a mixing model between n morb and e morb type end members but a third component with distinct supra subduction zone affinity is also required these results preclude any tectonic setting such as intracontinental rift ensialic marginal basin or major ocean ridge but substantiate an oceanic back arc environment they also demonstrate that the well documented upper cambrian lower ordovician extensional phase here reached a true oceanic stage

u pb dating of an early paleozoic bimodal magmatism in the french massif central and of its further metamorphic evolution in the marvejols area southern french massif central the gneissic marvejols supergroup is overthrust on the metasedimentary serie du lot deposited in part prior to 540 ma the allochtonous terranes are characterized by the occurrence of a leptyno amphibolitic group a complex association of mafic and felsic rocks of igneous and sedimentary derivation a 480 10 ma age has been obtained by u pb dating of zircons for the crystallization of both mafic and felsic meta igneous rocks these rocks were emplaced during an important extensional tectonics relics of eclogites pyrigarnites coronite gabbros and hp trondhjemites are clear evidence for a further hp ht event dated at 415 6 ma on zircons from a hp trondhjemite subsequently the marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345 10 ma

glaucophane bearing assemblage overprinted by greenschist facies metamorphism in the variscan kaczawa complex sudetes poland an early palaeozoic ordovician metamudstone sequence near wojcieszow kaczawa mts western sudetes poland contains numerous metabasite sills up to 50 m thick these subvolcanic rocks are of within plate alkali basalt type primary igneous phases in the metabasites clinopyroxene salite and kaersutite are veined and partly replaced by complex metamorphic mineral assemblages particularly the kaersutite is corroded and rimmed by zoned sodic sodic calcic and calcic amphiboles the matrix is composed of actinolite pycnochlorite albite an 0 5 epidote ps 27 33 titanite calcite opaques and occasionally biotite phengite and stilpnomelane the sodic amphiboles are glaucophane to crossite in composition with nab from 1 9 to 1 6 they are rimmed successively by sodic calcic and calcic amphiboles with compositions ranging from magnesioferri winchite to actinolite no compositions between nab 0 92 and nab 1 56 have been ascertained the textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist or blueschist greenschist transitional assemblage the presence of glaucophane and no traces of a jadeitic pyroxene quartz association indicate pressures between 6 and 12 kbar during the high pressure episode temperature is difficult to assess in this metamorphic event the replacement of glaucophane by actinolite chlorite albite with associated epidote allows restriction of the upper pressure limit of the greenschist recrystallization to 8 kbar between 350 and 450 c the mineral assemblage representing the greenschist episode suggests the p t conditions of the high pressure part of the chlorite or lower biotite zone the latest metamorphic recrystallization under the greenschist facies may have taken place in the visean

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

geochemical and geochronological cross section of the deep variscan crust the cabo ortegal high pressure nappe northwestern spain the high pressure rocks of the cabo ortegal nappe and related massifs are represented by several tectonometamorphic units which underwent separate histories before regional overthrusting and amphibolite facies recrystallization at 380 ma the occurrence of final cooling is recorded by 40ar 39ar and rb sr mica ages of up to 350 ma u pb zircon dating and sm nd and rb sr isotope data together with ree and major and trace element geochemistry provide constraints on the timing of earlier tectonothermal events and the nature of the protoliths the earliest tectonothermal event occurred at 490 480 ma and corresponds to the emplacement of lree enriched magmas of probable calc alkaline affinity and their metamorphism in the granulite facies this occurred at approximately the same time as the eclogite facies metamorphism of morb like protholiths associated with basic to intermediate granulites together this suggests formation of syn accretion granulites in an active margin setting a second high grade metamorphic event is recorded at 420 ma in metasedimentary formations associated with eclogite like assemblages garnet omphacite plagioclase this event is possibly related to a collisional plate tectonic setting in a back arc basin the age and origin of the associated ultrabasic rocks remains unclear but a late pyroxenite vein has yielded an emplacement age of 390 ma

regional shear variation in relation to diapirism and folding abstract regional structural trends in the levezou area southern massif central france delineate an antiformal structure deformed granitoids display changes along strike in s c plane relationships and therefore in sense of shear these variations are interpreted to be strain accommodation features that developed during the diapiric emplacement and folding of the granitoid sheets and so cannot be used as kinematic indicators on a regional scale

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana

mesoscale shallow convection in the atmosphere this paper is a review of the observational experimental theoretical and numerical studies of mesoscale shallow convection msc in the atmosphere typically msc is 1 to 2 km deep has a horizontal length scale of a few to a few tens of kilometers and takes distinctive planforms linear and hexagonal the former is called a cloud street roll or band while the latter is called mesoscale cellular convection mcc comprising three dimensional cells msc is characterized by its shape horizontal extent convective depth and aspect ratio the latter is the ratio of the horizontal extent to that in the vertical for cells the horizontal extent is their diameter whereas for rolls it is their spacing rolls usually align along or at angles of up to 10 from the mean horizontal wind of the convective layer with lengths from 20 to 200 km widths from 2 to 10 km and convective depths from 2 to 3 km the typical value of aspect ratio ranges from 2 to 20 rolls may occur over both water surface and land surfaces mesoscale convective cells may be divided into two types open and closed open cell circulation has downward motion and clear sky in the cell center surrounded by cloud associated with upward motion closed cells have the opposite circulation both types of cell have diameters ranging from 10 to 40 km and aspect ratios of 5 to 50 and both occur in a convective layer with a depth of about 1 to 3 km both the magnitude and direction of horizontal wind in the convective layer change little with height msc results from a complex and incompletely understood mix of processes these processes are outlined and their interplay is examined through a review of theoretical and laboratory analyses and numerical modeling of msc