on the trace of the iapetus suture in ireland and britain the iapetus suture in ireland and britain is that line which separates caledonian rocks of the laurentian and avalonian continents the suture is cryptic nowhere is there an exposed fault zone containing ophiolite remnants blue schist melanges or trench deposits instead a suture line may be traced with varying degrees of confidence through a series of faults that traverse the iapetus suture zone which contains two or more tectonostratigraphic terranes several data sets are utilized to constrain this trace the distribution of faunal provinces caused by oceanic separation can be used most reliably to define early ordovician terranes fauna1 intermingling in the mid ordovician reduces provinciality and the confidence of terrane identification using faunal data the late ordovician and particularly silurian tectonostratigraphic histories of terranes either side of the suture suggest that amalgamation of the terranes elimination of the iapetus ocean and development of the suture zone had begun by the ashgill therefore the resolution of the suture trace becomes unreliable where ordovician rocks do not crop out because silurian turbidite fans of laurentian provenance may have dispersed across a significant width of avalonian crust similarly the precision of structural identification of the suture zone by correlating the suture with a major fold facing confrontation may be weakened by late overthrusting tectonic interleaving of crustal flakes in the zone is supported by geophysical evidence which suggests that some major boundaries in the lower crust do not always coincide with their inferred projections in outcrop tectonic and sedimentary mixing of crustal blocks and sediments in the zone is further reflected by nd and pb isotopic patterns the rb sr patterns of granites record a stronger distinction between north and south of the suture despite these difficulties we depict a possible suture trace in ireland that departs significantly from the traditionally placed line by following a ne sw aligned trace through the slane fault and the navan tipperary lineament and thence through a dog leg to pass north of the dingle peninsula

mobility and fractionation of rare earth elements in argillaceous sediments implications for dating diagenesis and low grade metamorphism abstract we report sm nd and rb sr data for the fine fractions of lower paleozoic argillaceous rocks from wales uk and new york usa spanning the range of low grade metamorphic conditions from the diagenetic zone zeolite facies to the epizone greenschist facies in all cases leaching of the fine fractions results in a high 147 sm 144 nd 0 09 0 29 acid soluble component and a complementary low 147 sm 144 nd 0 05 0 14 residual component the observed fractionation is an ancient feature related to diagenesis burial and metamorphism the magnitude of sm nd fractionation between leachates and residues as well as the resulting sm nd ages vary as a function of grain size and metamorphic grade uncleaved welsh mudrocks of the diagenetic zone yield sm nd leachate residue ages of 453 484 ma in agreement with their llanviian to caradocian biostratigraphic ages whereas higher grade rocks of the anchizone and epizone yield sm nd ages as young as 413 ma these ages are transitional between the time of deposition and the time of regional deformation related to the acadian orogeny at 390 ma distinct convex upward rare earth element ree patterns of the leachates suggest that the precipitation of early diagenetic apatite controls the trace element budget of the rock forcing a depletion of middle rees on the subsequently formed diagenetic phyllosilicates the amount of organic matter present and the extent of later prograde reactions are probable modifiers of this fractionation process ordovician and devonian clastic rocks associated with the trenton and onondaga limestones of new york yield single sample and multi sample sm nd isochron ages that agree well with their biostratigraphic ages of 454 ma and 390 ma respectively the ree fractionation observed in shale leachates of the ordovician utica formation is related to ca mg of the bulk rock and hence to the composition of the diagenetic carbonate cement in all cases the sm nd system remained closed subsequent to the peak of diagenesis or metamorphism including the north american rocks that show no evidence of being isotopically reset during widespread remagnetization of the subjacent limestone units in the late paleozoic

short paper palaeontological constraints on the definition and development of irish caledonide terranes the irish caledomde terranes occupied a variety of sites marginal to the cratons of laurentia baltica and gondwana palaeontological data provide age constraints on the timing of sedimentary linkage and the age of overlap sequences the northwestern terrane is characterized by the development of toquima table head faunas in the early ordovician in contrast to the celtic faunas of the bellewstown and rosslare terranes during the middle ordovician the northwestern terrane and the scottish equivalent of the central terrane were occupied by the scoto appalachian fauna the leinster terrane was dominated by anglo welsh elements with varying proportions of the scoto appalachian and baltic faunas across the terrane in the iapetus suture zone in the east of ireland the grangegeeth terrane is characterized by a predominantly scoto appalachian fauna

provenance constraints from detrital zircon u pb ages in the nw iberian massif implications for palaeozoic plate configuration and variscan evolution detrital zircons from pre orogenic upper ordovician to devonian low grade metasedimentary rocks have been dated by laser ablation inductively coupled plasma mass spectrometry la icp ms to 1 constrain the variations of zircon age populations within the autochthonous sequences of nw iberia 2 compare u pb detrital zircon ages with previous data 3 test the hypothesis of the armorica microplate as a peri gondwanan terrane separated from gondwana the similarity in the ages of detrital zircons found in the palaeozoic samples studied here to those published for the upper proterozoic lower cambrian and lower ordovician sediments of nw iberia argues against the separation of armorica and points to a common source area for all of the palaeozoic detrital formations of nw iberia the west african craton and the surrounding pan african belts la icp ms u pb dating of zircons from a synorogenic flysch preserved in klippen in the core of a syncline establishes a maximum depositional age for this deposit as early namurian this age is also a maximum age limit for thrusting and refolding in this part of the iberian massif correlation of the zircon age populations with published ages confirms the link between the emplacement of the allochthonous complexes of nw iberia and synorogenic sedimentation

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

early palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms abstract stylophorans cornutes mitrates represent one of the most diverse classes of cambro ordovician echinoderms they were free living benthic non radiate forms closely related to asterozoans and crinoids taphonomic sedimentological and palaeosynecological data provide useful information on key aspects of stylophoran palaeoecology such a combined approach suggests that the rarity of stylophorans in proximal environments above storm wave base was probably original and does not exclusively result from the possession of a loosely articulated polyplated calcitic test conversely stylophorans were relatively abundant in deeper settings below storm wave base where they frequently occur in low diversity dense beds several tens to hundreds of individuals per square metre such assemblages were either largely dominated by stylophorans only middle cambrian darriwilian or by the association of both stylophorans and ophiuroids latest darriwilian silurian comparison with ecological requirements of similar modern and fossil dense populations of ophiuroids suggests that stylophoran beds occurred in well oxygenated oligotrophic waters with low predatory pressure and that sea temperature played a key role in the palaeobiogeographic distribution of stylophorans seven main stylophoran biofacies are identified and described the associated environments and tropical distribution of primitive heavily plated forms suggest that the initial radiation of the class probably took place in warm to temperate shallow settings earliest middle cambrian late cambrian however from this primitive stock stylophorans radiated several times towards deeper and or colder environments possibly because of the lower predation pressure in such settings cothurnocystid cornutes in the middle cambrian mitrates in the late cambrian hanusiids in the tremadocian as a consequence the fact that most cambro ordovician cornutes and mitrates were psychrospheric i e cold water adapted probably explains their relatively wide palaeobiogeographic distribution in high palaeolatitudes cambro ordovician stylophorans were abundant and diverse in shallow to deep environments but they were generally restricted to deep settings in low palaeolatitudes the great caradocian transgression favoured the extension of stylophorans from circum polar regions e g western margin of gondwana towards external settings at lower palaeolatitudes avalonia baltica and eastern margin of laurentia where one group of mitrates underwent a major radiation anomalocystitids and extended towards shallower and warmer palaeoenvironments in contrast all silurian stylophoran assemblages known so far are rare poorly diversified and restricted to shallow warm to temperate environments in tropical regions this distribution partly results from 1 the probable eradication of psychrospheric forms e g cornutes because of the prolonged anoxia of deep environments in the late hirnantian early silurian of west gondwana 2 the radiation of some mitrates anomalocystitids in shallow tropical settings in the late ordovician and 3 of strong taphonomic and or sampling biases numerous ghost lineages

mode of mixture of oceanic fragments and terrigenous trench fill in an accretionary complex example from southern sakhalin abstract four modes of mixture of oceanic materials such as cherts limestones and basalts and terrigenous trench fill sediments are recognized in the cretaceous accretionary complexes in southern sakhalin far eastern pacific margin of the u s s r the first mode is a thin skinned stacking of pelagic bedded cherts with black shale due to duplex formation in association with shallow underplating or off scraping the second mode is a mixture of the uppermost part of the oceanic crust with terrigenous trench fill sediments due to olistostrome formation reconstruction of the original sequence in ascending order suggests that the basement is pillow basalt with overlying pelagic sediments of chert and limestone olistostrome caused by collapse of oceanic crust due to its bending before encountering a trench and finally trenchfill sediments of turbidite this sequence is imbricated by thrusting which appears to have occurred at the time of underplating the third mode is a tectonic mixing of oceanic fragments and terrigenous sediments due to shear which is related to underthrusting of the oceanic plate a reduction in the supply of trench fill sediments causes copious accretion of the upper part of oceanic crust the fourth mode is in situ basalt volcanism at the trench where thick baked margins of terrigenous shale are formed around the injected basalts this mode appears to represent a ridge trench encounter these four different modes of mixing seem to be usual in ancient accretionary complexes in orogenic belts

acritarch marine microphytoplankton diversity in an early ordovician deep water setting the skiddaw group northern england implications for the relationship between sea level change and phytoplankton diversity early ordovician acritarch microphytoplankton assemblages from an outer shelf to slope facies in northern england show changes in diversity that parallel changes in sea level with high diversity occurring during lowstands and low diversity during highstands the pattern is the reverse of that recorded in the literature in which high acritarch diversity in continental platform successions has been related to periods of transgression and highstand and low diversity to periods of regression and lowstand the apparent contradiction can be explained if high diversity assemblages move further onto continental platforms during transgression and highstand but towards basins during regression and lowstand the hypothesis that high diversity acritarch assemblages track onshore and offshore during episodes of sea level change implies that there is a direct causal link between sea level and microphytoplankton diversity at least in so far as changes in sea level influence the location of high diversity the direct physical influence of sea level change on phytoplankton diversity might have been restricted to relatively small scales however its most obvious effect being perhaps to shift the locus of maximum diversity rather than affect the overall level of diversity at larger temporal and spatial scales the direct influence of sea level change on acritarch diversity is less certain parallel large scale changes in sea level and phytoplankton diversity have been documented for the phanerozoic as a whole but possibly reflect either independent responses to other factors such as the effects of tectonic cycles and changes in palaeogeography ocean circulation and climate or else indicate an indirect influence in the sense that sea level rise created conditions under which phytoplankton diversity could increase in contrast to the results discussed in this paper other analyses of ordovician acritarch diversity at larger spatial scales than those considered here have concluded that there is no evidence for any causal link between sea level change and diversity if however the controls on diversity exerted directly by changes in sea level are scale dependent and only apparent at relatively small scales analyses at larger spatial scales that compile measures of diversity across a range of depositional environments i e that sample both onshore and offshore environments and combine the results might not evince a correlation between changes in sea level and diversity in addition to producing shifts in the location of high diversity assemblages sea level changes also appear to be accompanied by at least local turnover in species composition during the early ordovician in northern england the high diversity assemblages associated with lowstands are very different in composition elsewhere in cratonic settings maximum flooding intervals within successive early palaeozoic stratigraphic sequences are characterised by markedly different acritarch assemblages hence changes in sea level appear to be associated with local extinctions and originations as well as changes in the locus of high diversity possibly as a result of ecological stresses arising from changes in the location of acritarch assemblages

origin and global tectonic significance of early archean cherts from the marble bar greenstone belt pilbara craton western australia abstract five sections of bedded chert in mafic ultramafic rocks of the archean warrawoona group in the marble bar greenstone belt pilbara craton were analyzed in order to understand their depositional environment and to provide some constraints on early archean tectonics the sections are divisible into two types based on their field occurrence mineralogy and geochemistry thicker ones a and b that overlie fe rich low k tholeiites and thinner ones c1 c2 and c3 overlying komatiitic basalts the thickest ferruginous section a 45 m thick is the marble bar chert of the towers formation and is interpreted to have been an in situ precipitate derived from a high t hydrothermal solution emanating from a mid oceanic ridge mor the following geochemical features are similar to those of modern hydrothermal iron rich sediments at a mor i p v zn and y are positively correlated with fe ii a positive eu anomaly normalized to chondrite decreases from 6 6 to 1 3 and the magnitude of a negative ce anomaly decreases from 0 6 to 1 0 as ree and lree hree increase the 13 m thick b section in the apex basalt dominated by sio2 and containing significant amounts of ba up to 4330 ppm originated from a low t mor hydrothermal solution this section is characterized by an association with massive black gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center geochemical evidence from the greenstones underlying the b section indicates that they are of morb origin in contrast to a and b c1 c2 and c3 sections are intercalated conformably with komatiitic basalts of the apex basalt and are 3 6 m thick the geochemical signatures of these three sections suggest that they were most likely formed by low t weak hydrothermal activity that may have been associated with hot spot volcanism they show strong enrichment of cr and ni reflecting a significant contribution of komatiitic basaltic detritus during sedimentation the presence of volcaniclastic chert in their uppermost beds indicates a decrease of hydrothermal silica precipitation due to waning hydrothermal activity among the three sections the uppermost c3 exhibits greater enrichment in zr nb hf and th and higher th sc and la yb n relative to the lower c1 and c2 sections this could mean that the depositional environment of c3 was relatively closer to a continent or to island arcs composed of granitoids and or their volcanic equivalents siliceous mudstones and sandstones of the uppermost clastic rocks t section panorama formation have geochemical signatures analogous to those of felsic plutonic volcanic rocks high th sc negative eu anomalies and high ree in some siliceous mudstones from this unit imply that differentiated granitoids or volcanic equivalents were already exposed erupted at least locally in the pilbara at this time this study shows these warrawoona group cherts were deposited in a variety of environments ranging from a mid oceanic spreading center to a convergent plate boundary via a hot spot this variation was most likely due to horizontal plate motions which accordingly support the operation of plate tectonics in the early archean

relationships between syn orogenic sedimentation and nappe emplacement in the hinterland of the variscan belt in nw iberia deduced from detrital zircons flysch type syn orogenic deposits of carboniferous age occur in relation to the emplacement of a large allochthonous nappe stack in the variscan belt of nw iberia new u pb age populations of detrital zircons obtained using laser ablation inductively coupled plasma mass spectrometry la icp ms are considered together with others from previously dated samples to establish the relationships between sedimentation and thrusting the age populations of four syn orogenic formations are compared with those of the pre orogenic sequence in the autochthon and parautochthon representing the gondwanan passive margin and in the allochthon formed by peri gondwanan and oceanic terranes in addition a new structural study has been carried out to understand the relationships between the syn orogenic deposits and the development of variscan structures the aims are to identify the sources of sediments and to establish the relationship between variscan structural evolution and syn orogenic sedimentation development of a forebulge outwards from the allochthonous front deduced from the structural study suggests the existence of depocentres that hosted the syn orogenic sediments together with the trend shown by the more recent zircons in each formation that are younger towards the external zones the data suggest that sedimentation occurred in progressively migrating depocentres formed in front of the allochthonous wedge during its emplacement the zircon age populations point to the allochthon as the main source of detritus for the syn orogenic basins with perhaps a limited participation of the parautochthon and autochthon in the younger formations

marginal faunal belts and their structural implications with examples from the lower palaeozoic fossils provide an independent criterion for testing structural hypotheses generated from field evidence or geochemistry as well as functioning in their traditional role in the dating of rocks or determining biogeographic realms on a global scale the identification of exterior oceanic biofacies may be of use in discriminating the course of former continental margins the isograptid graptolite biofacies early to middle ordovician can be distinguished from contemporary epicratonic graptolite biofacies its worldwide occurrence corresponds with continent margins reconstructed from other criteria hence a new isograptid record from irian jaya e new guinea although based on a small sample suggests that there may have been an ordovician oceanic site there on a continental scale the tectonic juxtaposition of different biofacies or of different stratigraphical packages contributes to an understanding of tectonics and can serve to quantify displacement the accretionary prism hypothesis for the southern uplands of scotland is supported and contrasted with the rearrangement of biofacies in the cambro ordovician cow head group of western newfoundland which reveals not only the original shelf margin slope topography now vanished but also provides an estimate of the movement involved in the emplacement of one taconic allochthon on a more local scale faunal evidence is used in assessing the original position of 9suspect9 marginal areas using the simplest possible model this shows for example that anglesey was not greatly separated from gondwana in the early ordovician but strike slip movement cannot be disproved local changes in relative sea level are determinable from sediments and faunas in marginal belts these may differ from the global eustatic curve and when this happens it is a measure of local tectonic activity particular sea level curves may serve to define structurally coherent units within mobile belts new sea level curves are contrasted for a segment of n wales north of the bala fault with that for central sw wales arenig llandovery the s welsh area mostly responded passively to the eustatic curve which can adequately account for shifts in facies and faunas while n wales had a curve often at odds with eustasy and reflecting its structural history

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

the ordovician silurian boundary at the eastern end of anticosti island a hirnantia fauna is recorded for the first time from anticosti island situated 6 m below the beds interpreted here as representing the maximum regression at the eastern end of the island the latter coinciding with the local top of the revised ellis bay formation the ordovician silurian boundary on the island is discussed

nature of accretion related to paleo tethys subduction recorded in northern thailand constraints from melange kinematics and illite crystallinity abstract we reconstructed the accretion process related to paleo tethys subduction recorded in northern thailand based on melange and thrust structures and metamorphic temperatures derived from illite crystallinity data melange formation was characterized by hydrofracturing and cataclastic deformation with mud injection under semi lithified conditions followed by shear deformation and pressure solution illite crystallinity data suggest metamorphic temperatures below 250 c during melange formation the combined structural and metamorphic data indicate that during melange formation the accretionary complex related to paleo tethys subduction developed at shallow levels within an accretionary prism asymmetric shear fabrics in melange indicate top to south shear after correction for rotation associated with collision between the indian and eurasian continents the trend of the paleo tethys subduction zone is estimated to have been n80 e we conclude that the paleo tethys was subducted northward beneath the indochina block from the permian to triassic

early devonian pre acadian magnetization directions in lower old red sandstone of south wales uk summary two components of magnetization have been resolved from the lower old red sandstone of south wales all 39 sampled sites lie north of the variscan front and all but three are largely unaffected by variscan folding thirteen sites yield a high unblocking temperature magnetization component which pre dates acadian mid devonian folding the site mean direction for this component dec 232 2 inc 31 9 95 8 5 yields a pole position lat 7 3 s long 306 7 e which can be assigned an early devonian age a lower unblocking temperature component is present at 38 of the sampled sites this component post dates both the acadian and variscan folding has reversed polarity and a mean direction dec 193 8 inc 7 2 95 2 6 which corresponds to a pole position lat 40 4 s long 338 0 e consistent with a latest carboniferous or early permian age the early devonian pole position confirms the existence of a siluro devonian inflexion in the apparent polar wander path apwp for southern britain which coincides with an inflexion in the apwp for scotland britain north of the iapetus suture indicating the closure of the iapetus ocean by siluro devonian time the early devonian palaeolatitude 17 s 5 is consistent with an early devonian configuration in which gondwana laurentia baltica and avalonia were part of a single supercontinent

early ordovician orogenic event in galicia nw spain evidence from u pb ages in the uppermost unit of the ordenes complex new u pb data zircon monazite and rutile obtained from rocks of the uppermost allochthon in the variscan belt of nw spain indicate that the hangingwall to the suture includes an allochthonous unit with a pre variscan tectonothermal evolution this evolution is characterised by an early ordovician 498 500 ma bimodal magmatism followed almost immediately 493 498 ma by a barrovian style metamorphism up to the granulite facies the metamorphism subsequent to the igneous intrusions requires convergence and crustal thickening in order to generate the barrovian facies pattern shown by the lithologies of the uppermost allochthon the almost coeval magmatism and metamorphism and the chemistry of the metabasites suggest an accretionary complex probably related to a volcanic arc as the most probable setting for the origin of the uppermost unit the implications of the existence of early ordovician convergent plate boundaries are discussed in the context of gondwana laurentia avalon interactions

upper ordovician palaeogeography of the bohemian massif implications for armorica summary the palaeogeographic position of armorica in the upper ordovician is still rather unclear due mainly to the lack of reliable palaeomagnetic data to help resolve this a palaeomagnetic study of caradocian and ashgillian sediments and volcanics of the barrandian basin in the central bohemian massif czech republic has been carried out a total of 29 sites were sampled including four from a diabase sill intruded into hirnantian age sediments thus providing a baked contact test three components of magnetization labelled a b and c have been resolved in addition to a low temperature component which corresponds to the present day direction of the local geomagnetic field d i 360 67 component a southerly and shallow before bedding correction is only sporadically present and corresponds to a remagnetization event of permo carboniferous age which is well known throughout hercynian europe component b present as stable endpoints in the sill and as a secondary component in the host rock sandstones yields an overall mean direction of 360 39 95 9 3 k 52 3 in situ and 355 21 95 9 3 k 52 3 after bedding correction component c identified as stable endpoints in sedimentary and volcanic rocks passes both a fold test and a baked contact test and is interpreted as being the primary direction of magnetization the overall mean direction for c is 183 59 95 9 5 k 30 3 nine sites after bedding correction which corresponds to a palaeopole position of 80 s 360 e this translates into palaeolatitudes of 40 s for the area studied in latest ordovician times but indicates large amounts of rotation up to 170 of the bohemian massif before final consolidation of hercynian europe although such large amounts of rotation are difficult to accommodate they are consistent with palaeomagnetic results obtained from lower ordovician and upper silurian rocks elsewhere in the barrandian basin if the bohemian massif is interpreted as being an integral part of the armorican microplate then these results indicate that prior to caradoc times armorica rifted away from the northern margin of gondwana and that by ashgillian times it was located at intermediate palaeolatitudes

a reassessment of facies and faunas in the type llandeilo ordovician wales abstract detailed analysis of the lithological and faunal successions of the type llandeilo forms the basis of an evaluation of the sequence of depositional environments and a quantitative description of the entombed faunas comprising mainly brachiopods trilobites and ostracods recorded distribution patterns confirm the validity of the existing trilobite trinucleid biostratigraphic zones but emphasize the importance of benthic bivalved taxa which are particularly abundant in the lower llandeilo groups of assemblages comprising localized faunules or associations despite exhibiting evidence of transportation are readily identified as representative of more widespread contemporary palaeocommunities recently described from wales and the welsh borderland the relationship between these palaeocommunities and the transgressive llandeilo sequence indicates a shoreface to open shelf zonation of ecosystems controlled primarily by substrate conditions rather than depth

new palaeomagnetic constraints on the position of central bohemia during early ordovician times summary apparently conflicting data within the palaeomagnetic database suggest that bohemia was at much shallower palaeolatitudes than the armorican and iberian massifs in ordovician times in order to resolve this controversy a palaeomagnetic study of arenig and llanvirn volcanic and sedimentary rocks from the barrandfan basin central bohemia has been carried out this basin comprises a sequence of unmetamorphosed lower palaeozoic rocks the main folding of which occurred in late devonian to early carboniferous times the samples collected l sites 153 samples were subjected to stepwise thermal and alternating field treatment eleven sites yielded reliable magnetic directions with analysis of the results using standard principal component and great circle analyses up to three directions of magnetiza tion can be identified in these rocks the first termed a is generally removed below 200 c although in some cases it persists up to 450 oc in the volcanic rocks it is approximately parallel to the present day earth s magnetic field in the study area dec inc 360 67 and is thus thought to be of recent origin the second direction identified termed b is isolated at intermediate blocking temperatures 150 350 c and yields an overall in situ mean direction of 195 8 k 22 3 a 95 13 1 seven sites these directions fail the fold test of mcfadden 1990 and correspond to a palaeopole position of 34 s 356 e this coincides with the late carboniferous sector of the european apparent polar wander path and thus the b direction is interpreted as being a secondary overprint of this age the highest blocking temperature direction termed c is identified at temperatures between 350 and 450 600 c it passes the fold test resulting in an overall mean direction of 312 83 k 21 9 a 95 14 6 six sites after bedding correction this direction is interpreted as being representative of the early ordovician palaeomagnetic field direction in the bohemian massif and yields a palaeopole position of 58 n 355 e this translates into palaeolatitudes of 76 s for bohemia thus demonstrating that the bohemian massif was at similar peri polar latitudes to the rest of armorica during early mid ordovician times and formed part of the northern margin of gondwana

density structure of the cratonic mantle in southern africa 1 implications for dynamic topography abstract the origin of high topography in southern africa is enigmatic by comparing topography in different cratons we demonstrate that in southern africa both the archean and proterozoic blocks have surface elevation 500 700 m higher than in any other craton worldwide except for the tanzanian craton an unusually high topography may be caused by a low density high depletion of the cratonic lithospheric mantle and or by the dynamic support of the mantle with origin below the depth of isostatic compensation assumed here to be at the lithosphere base we use free board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern africa our analysis takes advantage of the sase seismic experiment which provided high resolution regional models of the crustal thickness we calculate the model of density structure of the lithospheric mantle in southern africa and show that it has an overall agreement with xenolith based data for lithospheric terranes of different ages density of lithospheric mantle has significant short wavelength variations in all tectonic blocks of southern africa and has typical spt values of ca 3 37 3 41 g cm 3 in the cape fold and namaqua natal fold belts ca 3 34 3 35 g cm 3 in the proterozoic okwa block and the bushveld intrusion complex ca 3 34 3 37 g cm 3 in the limpopo belt and ca 3 32 3 33 g cm 3 in the kaapvaal and southern zimbabwe cratons the results indicate that 0 5 1 0 km of surface topography with the most likely value of ca 0 5 km cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities and requires the dynamic or static contribution from the sublithospheric mantle given a low amplitude of regional free air gravity anomalies ca 20 mgal on average we propose that mantle residual dynamic topography may be associated with the low density region below the depth of isostatic compensation a possible candidate is the low velocity layer between the lithospheric base and the mantle transition zone where a temperature anomaly of 100 200 c in a ca 100 150 km thick layer may explain the observed reduction in vs velocity and may produce ca 0 5 1 0 km to the regional topographic uplift

definition of the tremadoc series and the series of the ordovician system in britain the tremadoc series and series of the ordovician system are widely used in international correlations and this review explains and summarizes how the base of each of these series is recognized in britain no type section in which the base of any one of them is displayed has ever been selected and formally designated hence the basal horizon of each series is a matter of general understanding rather than precise definition this general understanding is of particular biozones or of assemblages of fossils and their relative ages the geographical separation between type sections of series and differences in facies complicate the understanding of relative ages of biozones brachiopod and trilobite faunas of the llandeilo caradoc and ashgill series have been studied in detail in recent years and biozones proposed these biozones have proved difficult or impossible to recognize outside their type areas in britain and brachiopod trilobite faunas of the tremadoc arenig and llanvirn series are likewise highly provincial thus for correlation within and outside britain attention has been focused on the standard graptolite biozones particularly since such conodont faunas as have been found have not proved useful yet there are problems associated with these standard biozones for example the tetragraptus approximatus biozone is unknown in wales or england the relationship between the base of the glyptograptus teretiusculus biozone and the earliest llandeilo shelly faunas requires clarification and at what level the base of the caradoc series may lie in the nemagraptus gracilis biozone is unknown thus to bring precision to definitions of ordovician series in britain the identification of continuous fossiliferous sequences that will serve as stratotypes is required such a section for the basal portion of the tremadoc series has recently been proposed systematic studies related to particular biozones are needed for example early caradoc and early ashgill brachiopods and graptolites of the didymograptus bifidus and glyptograptus teretiusculus biozones the stratigraphical ranges and abundances of species in particular sections require documentation so that the type of biozone assemblage acme concurrent range etc may be recognized such work could lead towards chronostratigraphy the provision of a standard stratigraphical scale in britain for the tremadoc series and ordovician series

periodicity in the early palaeozoic marine realm evidence from the british isles suggests that during 3 intervals in the early palaeozoic conditions approximating to those of a non uniformitarian greenhouse g state earth were realized middle early upper cambrian st davids merioneth ages 30 ma mid ordovician llandeilo caradoc ages 32 ma and early silurian llandovery early wenlock ages 12 ma during these times sea levels were high and ocean waters below the zone of surface overturning poorly aerated and at times euxinic pelagic biotas graptoloids were diverse and in aerated portions of the highly productive shelf seas benthic biotas brachiopods flourished opposing conditions marking a well oxygenated o state earth occurred to varying degrees in the intervening intervals early cambrian comley age 20 ma late cambrian mid ordovician tremadoc late llandeilo ages 46 ma late ordovician ashgill age 4 ma and mid to late silurian post early wenlock age 15 ma within the constraints of the present data perturbations in the early palaeozoic marine realm do not appear to follow the same regular periodicity as in the mesozoic and caenozoic among a number of likely causes for this the greater tendency for the early palaeozoic seas to become anoxic seems the most important

ordovician provincial signals from appalachian caledonian terranes statistical analysis of mainly mid to deep shelf ordovician brachiopod faunas from localities within and marginal to the appalachian caledonian orogen reveals biogeographical patterns not entirely consistent with those presumed for the progressive orthogonal closure of the iapetus ocean correspondence analysis of faunas of late arenig early llanvirn late llandeilo earliest caradoc and late ashgill age emphasize the importance of marginal and island faunas the role of regressions and transgressions on faunal distributions and the effects of climatic change in the definition of biogeographical trends during the ordovician

la repartition geographique des mollusques bivalves du paleozoique cambrien a devonien une revue abstract paleozoic bivalves have long been neglected in faunal studies the reasons for this are considered and the conclusion reached that this neglect has no real foundation recent and growing interest in paleozoic bivalve faunas has accentuated their potential importance in paleogeographic faunal reconstructions current knowledge of bivalve faunas from the cambrian to devonian is reviewed in a framework of global plate tectonics

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

extinction and migration in silurian trilobites and conodonts of northwestern canada appearances and disappearances lineage turnover of trilobite and conodont evolutionary species clades in silurian shelf marginal sections of the mackenzie mountains are concentrated at particular stratigraphical levels a depth energy curve generated for this region shows three major and several minor transgressive regressive cycles in the llandovery and wenlock series a comparison of this curve with conodont and trilobite distributional data supports the thesis that there are cause effect relationships between changes in sea level and migration evolution and extinction events of these marine organisms the rate amount and direction of change in depth were important factors in the migration and extinction of clades the depth energy curve for the region fits the pattern expected for a passive downdropping shelf responding to global eustatic sea level changes emigrations and extinctions are associated with rapid transgressive episodes and concentrated at three levels lower llandovery lower wenlock and middle or lower part of the upper wenlock the early wenlock event was the most impressive a further concentration of extinctions and or migrations not considered here occurred later in the ludlow associated with a regional regression phyletic evolution is most convincingly demonstrated in fossil collections from intervals of gradual regression a model is proposed that predicts that a similar pattern of extinctions occurred around the silurian world in regions of similar palaeotectonic regime and that global extinction patterns of clades between episodes of mass extinction were not random episodes of high extinction turnover pulses were caused by perturbations of the environment foremost of which were rapid changes in the depth of the seas on the continental shelves because taxonomic loss during these extinction emigration episodes was generally at low below generic levels resolution of silurian turnover patterns requires data collected at the evolutionary species level

caledonian deformation upon southwest baltica and its tectonic implications alternatives and consequences the continuation of the basement of the southwestern baltic shield toward the sw is of major interest for the caledonian evolution in the southwestern baltic sea and northern germany a reinterpretation of various marine seismic data suggests a reconsideration of the caledonian collision pattern the sorgenfrei tornquist zone forming the northwestern extension of the tornquist zone is regarded as the northeasterly boundary fault and the elbe lineament is regarded as the southwestern limit of an area which suffered from the docking of east avalonia to baltica it shows many northwestsoutheast striking structures anomalies and fault patterns from our studies two alternative models emerge one which considers the caledonian deformation front as the northeastern limit of east avalonia and a zone of major transpression collision and subduction of the tornquist sea another model considers the elbe lineament in schleswig holstein nw germany as a major dextral strike slip fault which is interpreted as the northeasterly limit of east avalonia and the southwest boundary of the baltic shield along the elbe lineament the tornquist sea plus rotating avalonia were displaced to the northwest a large part of the area to be studied might have developed into a passive margin toward the upper ordovician a decision which of the two models is to be prefered has to be the major goal of future geophysical experiments

evidence for archean ocean opening and closing in the southern slave province archean mafic volcanic and plutonic rocks of the cameron and beaulieu river greenstone belts outcrop in the southern slave structural province of the canadian shield lower portions of the greenstone belts have ophiolitic characteristics but young felsic units may represent superimposed arc magmatism the greenstone belts are separated from older circa 2 7 3 2 ga quartzo feldspathic gneisses of the sleepy dragon complex by a regional decollement zone patterson lake structural complex consisting of deformed mafic volcanic rocks thin slivers of gneiss ultramafic rocks and banded iron formation mixed in a highly deformed phyllonitic and phacoidally cleaved metapelitic matrix high synmetamorphic strains are indicated by abundant intrafolial folds transposed layering rare sheath folds and by the parallelism of elongated primary features and metamorphic mineral lineations the patterson lake structural complex is similar to melanges of phanerozoic mountain belts adjacent quartzo feldspathic gneisses are strongly mylonitized the melange and deeper mylonites formed when the greenstone belts were emplaced as thrust sheets wildflysch shed from tips of emergent thrusts was in part incorporated into the basal melange as the allochthon overrode its own detritus continued compression involved the basement in the deformation uplifting the sleepy dragon complex in a structural culmination mantled by the imbricated greenstone stack late granodioritic to granitic intrusions formed regional thermal aureoles which overprint earlier metamorphic fabrics

paleomagnetism of the moreton s harbour group northeastern newfoundland appalachians evidence for an early ordovician island arc near the laurentian margin of iapetus paleomagnetic results have been obtained from mafic volcanic units in the upper part of the moreton s harbour group which is part of an accreted island arc terrane that is preserved in the notre dame bay subzone of the central mobile belt of the newfoundland appalachians detailed thermal and alternating field demagnetization reveals a stable characteristic component of magnetization carried by magnetite at a large number of sites in pillow basalts and coeval basaltic dikes the intrusives contain both polarities and a primary age for the characteristic magnetization is indicated by a positive contact test for one of the dikes and by a positive structural test involving a correction for block rotations strike correction the overall mean direction for the characteristic component after tectonic tilt and strike correction flows and intrusives d 171 i 22 k 22 6 95 6 5 pole 29 n 135 e flows d 166 i 22 k 33 2 95 6 5 corresponds to an early ordovician paleolatitude for the arc of 11 s which is indistinguishable from the expected paleolatitude of the north american margin this implies that the arc formed at or near the margin of the craton in contrast the avalon block which formed the other margin of iapetus was widely separated from the arc and the craton at this time the early ordovician paleolatitude of the arc terrane supports a tectonic model in which coeval ophiolitic sequences inboard of the arc were formed in a narrow ocean basin between the arc and the craton subsequent convergence between the arc and laurentia in middle ordovician time resulted in closure of this narrow ocean basin and obduction of the back arc basin oceanic crust onto the ancient margin of the craton thus giving rise to the taconic orogenic pulse in the newfoundland appalachians

a lancefieldian graptolite from the lake district large dictyonema species including d pulchellum t s hall have been collected from beds in the uldale fells northern lake district below strata yielding didymograptus protobalticus monsen they suggest the presence there of graptolitic beds transitional from lancefieldian to bendigonian in age approximately tremadoc arenig and suggest that the area may yet provide an internationally correlatable base to the arenig series in britain

the significance of a reliable ordovician age for some highland border rocks in central scotland a lower arenig fauna with north american affinities has been rediscovered at dounans aberfoyle central scotland this discovery makes part of the highland border complex younger than the adjacent dalradian rocks and suggests a provenance from the s

large scale melange formation due to seamount subduction an example from the mesozoic accretionary complex in central japan deformation of forearc wedges due to seamount subduction in modern trenches is related to the origin of mesozoic melanges in central japan and a hypothetical model for large scale melange formation due to seamount subduction is proposed recent studies of seamount subduction in modern trenches have revealed that seamounts pass under forearc wedges retaining their approximate original shape but the forearc wedges themselves are swelled in front of subducting seamounts and indented behind them this strongly suggests that forearc wedges are widely disrupted and transformed into large scale melanges that include seamount fragments the mesozoic accretionary complex in the mino terrane of central japan is comprised of large scale melanges including seamount fragments and coherent tectonically stacked sedimentary successions of chert siliceous shale and turbidite in ascending order melanges are widespread in a belt 20 to 40 km wide and about 100 km long and seamount fragments of permian basalts and ree