pre folding magnetization reconfirmed for the late ordovician early silurian dunn point volcanics nova scotia johnson r j e and van der voo r 1990 pre folding magnetization reconfirmed for the late ordovician early silurian dunn point volcanics nova scotia tectonophysics 178 193 205 paleomagnetic results have been obtained for three new sites in steeply inclined subarea1 volcanic flows of the dunn point formation from the avalon terrane of nova scotia demagnetization analysis reveals a characteristic magnetization carried by hematite which is similar to that reported in previous studies of this unit a new and improved fold test for the characteristic component combining paleomagnetic and structural data from the present study with paleomagnetic data for vertically dipping flows and laterites studied previously by the authors is significantly positive the new fold test reconfirms the prefolding age of the characteristic component a primary age of magnetization is inferred based on field evidence for early oxidation of the flows the overall mean direction for the characteristic component of d 344o i 60 k 68 6 ass 4 1 paleopole at 2on 130 e corresponds to a late ordovician earliest silurian paleolatitude for avalon of 41 s the latter is much higher than that predicted for the area on the basis of the cratonic north america apparent polar wander path implying a substantial post ordovician displacement 1700 km of avalon with respect to the craton the dunn point result supports previous paleogeographic reconstructions that show avalon together with the continental blocks of hercynian europe and northwestern gondwana

geochemistry and sm nd isotopic systematics of ediacaran ordovician sedimentary and bimodal igneous rocks in the western acatlan complex southern mexico evidence for rifting on the southern margin of the rheic ocean abstract ordovician igneous rocks in the western acatlan complex olinala area of southern mexico include a bimodal igneous suite that intrudes quartzites and gneisses of the zacango unit and all these rocks were polydeformed and metamorphosed in the amphibolite facies during the devono carboniferous the ordovician igneous rocks consist of the penecontemporaneous amphibolites megacrystic granitoids and leucogranite the latter dated at ca 464 ma geochemical and sm nd data indicate that the amphibolites have a differentiated tholeiitic signature and that its mafic protoliths formed in an extensional setting transitional between within plate and ocean floor the amphibolites are variably contaminated by a mesoproterozoic crustal source inferred to be the oaxacan basement exposed in the adjacent terrane the most primitive samples have endt t 465 ma values significantly below that of the contemporary depleted mantle and were probably derived from the sub continental lithospheric mantle the megacrystic granites were most probably derived by partial melting of an arc crustal source similar to the oaxacan complex and triggered by the ascent of mafic magma from the lithospheric mantle sm nd isotopic signatures suggest that metasedimentary rocks from zacango unit were derived from adjacent oaxacan complex trace elements relationships e g la th vs hf and ree patterns suggest provenance in felsic intermediate igneous rocks with a calc alkaline signature the ordovician bimodal magmatism is inferred to have resulted from rifting on the southern flank of the rheic ocean and is an expression of a major rifting event that occurred along much of the northern gondwanan margin in the ordovician

high temperature deformation during continental margin subduction exhumation the ultrahigh pressure western gneiss region of norway abstract a new dataset for the high pressure to ultrahigh pressure western gneiss region allows the definition of distinct structural and petrological domains much of the study area is an e dipping homocline with e plunging lineations that exposes progressively deeper more strongly deformed more eclogite rich structural levels westward although eclogites crop out across the wgr scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area the scandian reworking increases westward culminating in strong scandian fabrics with only isolated pockets of older structures the dominant scandian deformation was coaxial e w stretching the sinistrally sheared more trondelag fault complex and nordfjord mylonitic shear zone bound these rocks to the north and south there was moderate top e amphibolite facies deformation associated with translation of the allochthons over the basement along its eastern edge and the nordfjord sogn detachment zone underwent strong lower amphibolite facies to greenschist facies top w shearing a northwestward increase in exhumation related melting is indicated by leucosomes with hornblende plagioclase and scandian sphene in the western 2 3 of the study area exhumation related amphibolite facies symplectite formation in quartzofeldspathic gneiss postdated most scandian deformation further deformation was restricted to slip along biotite rich foliation planes and minor local folding that the western gneiss region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation implies that continental crust in general need not undergo pervasive deformation during subduction

reconstruction of eroded and deposited sediment volumes of the embanked river waal the netherlands for the period ad 1631 present in the last few centuries humans have modified rivers and rivers have responded with noticeable changes in sedimentary dynamics the objective of this study is to assess these responses of the sedimentary dynamics therefore we calculated a sediment budget for eroded and deposited sediment volumes in a similar to 12 km long floodplain section of the largest semi natural embanked but still dynamic lower rhine distributary for similar to 50 years time slices between ad 1631 and present this is the period during which embanked floodplains were formed by downstream migration of meander bends between confining dykes our sediment budget involves a detailed reconstruction of vertical and lateral accretion rates and erosion rates of floodplain sediment to do so we developed a series of historical geomorphological maps and lithogenetic cross sections based on the maps and cross sections we divided the floodplain into building blocks representing channel bed and overbank sediment bodies chronostratigraphy within the blocks was estimated by interpretation of heavy metal profiles and from optically stimulated luminescence osl dating results sediment budgets were hence calculated as a change of volume of each building block between time steps the amount of lateral accretion initially increased as a result of island and sand bar formation following embankment from the eighteenth century onwards there was a decrease of lateral processes in time which is a result of straightening of the river by human activities and a reduction of water and sediment supply due to the construction of a new upstream bifurcation with straightening of the river the floodplain area grew artificial fixation of the channel banks after ad 1872 prevented lateral activity from then on overbank deposition became the main process leading to a continuous increase of floodplain elevation and inherent decrease of flooding frequency and sediment accumulation rate copyright c 2014 john wiley sons ltd

evaluating controls on crevasse splay size implications for floodplain basin filling abstract although crevasse splays are often considered important contributors to floodplain basin filling evidence from modern and ancient fluvial deposits indicates that they may not play a significant role in all river systems in order to better understand controls on the abundance and scale of crevasse splays produced in fluvial systems we used aerial photography to observe crevasse splay deposits along three rivers and conducted a targeted numerical modeling study to explore factors which promote widespread crevasse splay deposition on fluvial floodplains results demonstrate that splay size does not always scale directly with channel size or discharge and that hydrodynamic factors primarily sediment size and floodplain drainage conditions play a significant role in determining whether a fluvial system is likely to produce large floodplain filling splays this work suggests that widespread crevasse splays demand both a large supply of relatively coarse suspended sediment and a steep water surface slope away from the channel across the floodplain without both these conditions the role of crevasse splays in flood basin filling is limited

longitudinal facies architectural changes of a middle holocene anastomosing distributary system rhine meuse delta central netherlands abstract longitudinal changes in channel planform and facies architecture occur over a 22 km long reach of an anastomosing middle holocene distributary system in the rhine meuse delta central netherlands a geological map and a sequence of cross sections show that the channel belt is characterized by sand body width thickness ratios which decrease within the study area from approximately 25 upstream to well below 10 downstream accompanied by a change in channel planform as indicated by a downstream increase of the number of crevasse splays the lithological composition of the sand body is relatively homogeneous but there is always a well developed fining upward sequence from sandy channel deposits to clayey natural levee deposits within channel vertical aggradation appears to have been restricted during most of the period of activity of the distributary system and since lateral accretion was also limited especially in the downstream part rapid accumulation of channel deposits seems to have occurred predominantly during the avulsion stage leaving a narrow and shallow residual channel as the last remnant the longitudinal change in channel pattern is mainly attributed to a downstream increase in the rate of ground water level rise that resulted from holocene sea level rise in the rhine meuse delta a rate of ground water level rise of 1 5 mm yr 1 is required for the creation of fully developed anastomosing fluvial systems ribbons with a width thickness ratio of

the formation of a tidal intrusion front at the mouth of a macrotidal partially mixed estuary a field study of the dart estuary uk high resolution measurements of density and velocity were made before during and after the formation of a transient tidal intrusion front at the mouth of a narrow partially mixed estuary during spring tides detailed acoustic doppler current profiler adcp and density measurements throughout 1 km longitudinal transects at hourly intervals during spring tides indicated that the front formed when the barotropic inflow was sufficient to arrest the buoyant outflow from the estuary this has been shown to be driven by a constriction of the channel at the mouth and a depression in the seabed just landward of the mouth with these changes in topography shown to interact with the flow in a similar manner to theoretical two layer flow over the lee side of a sill a detailed analysis of the flow and stratification across the narrow frontal interface found a high degree of shear and mixing just prior to the plunge point which together with a hydraulic jump at the seabed depression eventually led to the destruction of the density interface and the total intrusion of seawater as the flooding tide strengthened there was no surface manifestation of this front during neap tides and further measurements have confirmed that the relatively weak barotropic flow at this time was not sufficient to overcome the stratification in a two layer regime an evaluation of the key frontal discriminators has confirmed the validity of their use in such an estuary

timing styles and kinematics of cambro ordovician extension in the tepla barrandian unit bohemian massif and its bearing on the opening of the rheic ocean this paper describes late cambrian dikes and early ordovician volcano sedimentary successions of the prague basin bohemian massif to discuss the timing and kinematics of breakup of the northern margin of gondwana andesitic dikes indicate minor e w crustal extension in the late cambrian whereas the tremadocian to dapingian lithofacies distribution and linear array of depocenters suggest opening of this rheic ocean rift related basin during nw se pure shear dominated extension this kinematic change was associated with the onset of basic submarine volcanism presumably resulting from decompression mantle melting as the amount of extension increased we conclude from these inferences and from a comparison with other avalonian cadomian terranes that the rifting along the northern gondwana margin was a two stage process involving one major pulse of terrane detachment in the early cambrian and one in the early ordovician while the geodynamic cause for the former phase remains unclear but still may include effects of cadomian subduction roll back slab break off isostatic rebound or mantle plume the incipient stage of the latter phase may have been triggered by the onset of subduction of the iapetus ocean at around 510 ma followed by advanced extension broadly coeval tremadocian to darriwilian in large portions of the avalonian cadomian belt unequal amounts of extension resulted in the separation and drift of some terranes while other portions of the belt remained adjacent to gondwana

geochemical background in polluted river sediments how to separate the effects of sediment provenance and grain size with statistical rigour abstract defining anomalies from geochemical background is a principal step in anthropogenic contamination studies and environmental risk assessment and yet the complexity of processes affecting the composition of sediments makes this task extremely difficult we address grain size and sediment provenance as two parameters influencing the geochemistry of contaminated fluvial sediments we analysed 576 sediment samples from channels floodplains oxbow lakes and dam reservoirs of the morava river czech republic the geochemical data are compared with grain size and statistically processed using primary observations and log ratios an effective way to remove the grain size and provenance effect is to use enrichment factors calculated from least trimmed squares regression of pb zn cu and cr data vs normalizing ti which is sensitive to grain size based on empirical testing titanium proved more suitable for normalizing than did al and rb isometric log ratio coordinates of the pb zn cu and cr data show a strong misfit with those defined by the enrichment factors a log ratio approach considering just subcompositions of elements with similar geochemical behaviour provides much better results

human induced changes in bed shear stress and bed grain size in the river waal the netherlands during the past 900 years human impact has changed the flow characteristics and bed sediment characteristics of lowland rivers affecting channel stability flood risk navigability and biodiversity we analysed the effect of human activities on the flow and bed sediment characteristics of the river waal the netherlands the objectives were a to reconstruct the historical change in bed shear stress during the past 900 years b to reconstruct the contemporary change in bed grain size and c to identify the main causes of these changes various data types were used such as borehole descriptions historical river maps and modern hydraulic data it was found that the bed shear stress in the river waal strongly increased during the past 900 years in the same period the gravel content of the sandy river bed increased causing a coarsening of the river bed before ad 1870 the shear stress increase and bed coarsening were mainly due to embankment artificial levee construction after ad 1870 the shear stress increase and bed coarsening were mainly due to river narrowing and dredging the systematic grain size difference between present day and historical rivers should be taken into account in river restoration projects copyright 2009 john wiley sons ltd

natural levee evolution in the rhine meuse delta the netherlands during the first millennium ce this paper presents reconstructions on natural levee development in the rhine meuse delta the netherlands during the first millennium ce covering the full delta plain it is the first study that performs this on a delta scale which allows seeing the delta wide trends on levee forming controls and their feedbacks we mapped the levee morphology and elevation by combining lidar imagery lithological borehole data soil mapping radiocarbon dates archaeological data and gis reconstruction techniques from the detailed levee reconstructions we quantified natural levee dimensions and evaluated the temporal changes therein the dimensions and the changes therein were then linked to external forcings increasing suspended sediment load variable flooding intensity and to natural preconditions e g delta plain width flood basin configuration we show that natural preconditions are an important control on levee shape this is demonstrated for the upper delta where the relatively narrow delta plain combined with strong compartmentation i e the occurrence of many alluvial ridges and enclosed flood basins caused the flood levels to be amplified allowing the natural levees to grow relatively high compartmentation also seems to have stimulated trapping of coarse grained overbank sediments explaining the clear downstream trend in levee width this effect was probably further aided by the clearance of the riparian forests mainly in the upstream and central delta which caused the coarser fraction of the suspended load to be further dispersed into the flood basin leading to wider levees in the first millennium ce several new river courses formed that avoided the areas of natural levee relief of abandoned alluvial ridges on these fossil alluvial ridges the topographical expression gradually reduced because of widespread flood basin trapping of overbank sediment which led to topographic levelling the natural levees that formed during this period along the new courses appear to be relatively high compared to precursor generations in the upper and central delta this is most likely related to the increased suspended sediment supply and intense flooding regime during their formation the hypotheses generated with this new delta wide overview help to better understand the controls in the development of levees which are important elements in river landscapes and in fluvial sedimentary records 2017 elsevier b v

ordovician back arc foreland and ocloyic thrust belt development on the western gondwana margin as a response to precordillera terrane accretion 1 the famatina belt southern central andes records a circa 470 m y shortening episode ocloyic orogeny affecting the peri gondwanan back arc basin in response to the accretion of the precordillera terrane collision created distinct features across and along the margin some of which persisted into the present andean structure from east to west the ocloyic deformation is recorded across the telescoped region of famatina represented by 1 an east vergent fold thrust belt evftb involving a major angular unconformity between early and middle ordovician rocks 2 a highly deformed volcano sedimentary belt and 3 an east vergent ductile shear zone affecting famatinian granites axis attitudes of ordovician refolded anticlines in the evftb show west east tectonic transport in agreement with kinematics of ocloyic high strain belts indicating shortening orthogonal to the present pacific margin eastward displacement of the ordovician volcanic arc at 27 s latitude together with a documented sinistral slip zone across the proto andean margin suggests tectonic indentation

the structure and timing of lateral escape during the scandian orogeny a combined strain and geochronological investigation in finnmark arctic norwegian caledonides abstract determining the timing duration and mechanism of tectonic events within an orogenic cycle such as ocean subduction continent continent collision or gravitational collapse is challenging especially in ancient orogenic belts variations in the tectonic transport direction however can be used as a guide to these stages of orogeny while thrust sheets within the caledonian allochthon in north norway were emplaced broadly eastwards perpendicular to the trend of the orogen many features indicate material transport in other orientations one dominant feature of the mageroy nappe sitting above and infolded with the kalak nappe complex is the development of a strong n s lineation orthogonal to the main transport direction strain measurements in part determined by a new method are used in the context of the regional structural data to identify the critical stage in orogeny when compressional forces are balanced by orogen parallel lateral escape quantitative 3 d strain estimation in the mageroy nappe indicates prolate deformation with c 50 horizontal shortening parallel to the thrusting direction e w and c 200 extension along the orogenic strike n s with c 30 vertical shortening temporal constraint on this fabric is provided by ar ar isotopic analysis of undeformed white mica in cross cutting granitic pegmatites these data show that prolate deformation occurred before the white mica cooling age of 416 4 ma while the previously determined depositional age of the hellefjord schist indicates that it occurred after 438 4 ma a granitic pegmatite that intruded the hellefjord schist after an initial deformation phase but during or prior to a later deformation has been dated at 431 2 ma by u pb zircon a previous lower age constraint on this deformation of 428 5 ma is given by metamorphic zircon overgrowths on fractured grains these results constrain the continental collision between baltica and laurentia in finnmark to the interval c 431 428 ma placed in a regional context these results indicate that lateral escape was orthogonal to the thrusting direction and occurred during the continent continent collision stage in the scandian orogeny before gravitationally driven collapse

channel morphology changes and their relationship to valley bottom geology and human interventions a case study from the vistula valley in warsaw poland the aim of the study was to determine the role of the geological setting of channel zone in the morphological variability of a river reach situated between engineered and seminatural channel sections investigations were carried out in the vistula river channel in warsaw the study included the drilling of 60 geological boreholes in the river channel and the registering of sedimentary structures of modern river channel sediments the morphology of the channel was defined based on a series of bathymetric measurements of different river levels to identify the reasons for the variation in the channel bed morphology numerical modelling of river flow was performed calculations were made using the fluent cfd computational fluid dynamics model the stabilization of bottom erosion along the engineered reach of the river was found to be associated with a shallow depth to the top of the basement of modern alluvial sediments composed of erosion resistant glacial deposits the suballuvial basement in this part of the river channel also serves as a local base level in relation to the river reach upstream

timing of eclogite facies metamorphism in the southernmost scandinavian caledonides by lu hf and sm nd geochronology the scandinavian caledonides contain several non cogenetic eclogite bearing terranes that were metamorphosed before the main orogenic stage in scandian time 430 395 ma although petrological and geochronological data from these terranes have provided essential information on the geodynamic history of the caledonian orogenic cycle the general picture is still patchy to refine existing geodynamic models we have dated the eclogite occurrence in the jaeren nappe sw norway by lu hf and sm nd geochronology five out of the six studied samples provide a weighted mean lu hf age of 469 9 1 2 ma 2 one sample provided a significantly younger age of 457 9 2 4 ma garnet from the younger sample grew exclusively at eclogite facies conditions in contrast garnet from the other samples comprises prograde cores and peak metamorphic rims age estimates that take lu contributions of each zone into account provide an age of 471 0 0 9 ma for the cores and suggest a ca 455 ma age for the rims which is identical to the bulk garnet age of the younger sample the same pattern is indicated by sm nd ages although these are relatively imprecise and reflect isotopic disturbance during thermal overprinting upon exhumation the data define a new high pressure age population for the scandinavian caledonides which allows more detailed insight into the subduction history that affected the baltoscandian margin before scandian continental collision furthermore this study highlights the potential complexities involved in garnet geochronology and shows the strength of lu hf dating for unraveling the geochronological record of hp rocks

the cambrian to devonian odyssey of the brabant massif within avalonia a review with new zircon ages geochemistry sm nd isotopes stratigraphy and palaeogeography abstract this study provides an up to date and comprehensive review of the early palaeozoic evolution of the brabant massif belonging to the anglo brabant deformation belt situated at the southeastern side of avalonia microplate it is the only well known part of the northern passive margin of the rheic ocean the cambrian silurian sedimentary pile is 13 km thick with 9 km for the cambrian only the unraveling of this continuous registration reflects the successive rifting and drifting of avalonia from the gondwana mainland followed by soft collisional processes with baltica and finally the formation of laurussia based on recently established detailed stratigraphy sedimentology and basin development on u pb la icp ms analyses of igneous and detrital zircon grains along with geochemical data including sm nd isotopes a new geodynamic and palaeogeographic evolution is proposed brabant megasequence 1 lower cambrian to lowermost ordovician 9 km thick represents an embayment of the peri gondwanan rift from which the rheic ocean has evolved detrital zircon ages demonstrate that the brabant is a typical peri gondwanan terrane with a major pan african neoproterozoic age and a mixed west african and amazonian source palaeoproterozoic archaean and some mesoproterozoic age the transition towards the avalonia drifting is marked by an unconformity and a short volcanic episode the northward drift of avalonia towards baltica is recorded by the megasequence 2 middle to upper ordovician 1 3 km thick the source for mesoproterozoic zircons vanished as the result of the rheic ocean opening and the isolation from amazonian sources the transition to megasequence 3 is marked by a drastic change in palaeobathymetry and an important sub volcanic episode during a tectonic instability period 460 430 ma reflecting the avalonia baltica soft docking as also shown by the reappearance of mesoproterozoic detrital zircons typical of baltica unradiogenic nd isotope signature end 4 5 and tdm model ages 1 3 1 7 ga for brabant magmatic rocks indicate an old recycled component megasequence 3 uppermost ordovician to lowermost devonian 3 5 km thick includes the onset of a silurian foreland basin that reflects the tectonic inversion of the core of the massif brabantian orogeny in response to the baltica avalonia laurentia collision finally the comparison with the strikingly similar cambrian successions of the harlech dome wales avalonia and the meguma terrane nova scotia peri gondwana allows the construction of a new early cambrian palaeogeographic model for the whole avalonia microplate in which the meguma terrane is included

timing and kinematics of caledonian thrusting and extensional collapse southern norway evidence from 40ar 39ar thermochronology 40ar 39ar dating methods have been applied to rocks across the caledonian orogen in southwestern norway in the eastern nappe area k feldspar thermochronological modeling microfabric characteristics and conodont color alteration together indicate that temperatures were not above the closure temperature of muscovite for any significant period of time two groups of muscovite and biotite ages from this area 415 408 for most samples with top to the se fabrics and 402 394 for samples with top to the nw fabrics are therefore interpreted as ages of contractional thrusting and extensional hinterland directed nappe translation deformation respectively in the west hinterland peak caledonian temperatures were higher and 40ar 39ar plateau ages are generally interpreted as cooling ages the western basement and lower nappes cooled rapidly through 500 c basement at 404 ma and 350 c basement and lower nappes shortly after i e during extensional top to the nw transport of the orogenic wedge in addition tectonostratigraphically higher nappes in the hinterland show evidence of earlier cooling probably following ordovician orogenic activity prior to the main collisional stage the new 40ar 39ar data conform to kinematic observations that contraction and extension in the caledonian nappe region were sequential and that the change from contraction convergence to extension divergence was quick between 408 and 402 ma

the margins of laurussia in central and southeast europe and southwest asia the continuation of the laurussian margin into central and southeast europe and southwest asia is defined by the rheic suture which becomes harder to trace to the southeast because of complex post variscan strike slip displacements and tectonic stacking linked to the cimmerian and alpine cycles the uncertain extent of the neoproterozoic baikalian orogeny fringing baltica ambiguous early paleozoic paleobiogeographic data and strike slip activity complicate the distinction of the tornquist suture from the transeuropean suture zone there is growing evidence for the incorporation of the brunovistulian malopolska and west moesian terranes into the late neoproterozoic cambrian baltican margin the istanbul zonguldak terrane along with the bitesh and east moesia terranes may have been part of the avalonian terrane assemblage although an arabian nubian shield or baltica provenance cannot be excluded the terranes of the armorica terrane assemblage bordering the rheic suture include the outer carpathian tatra getic and balkan terranes in southeast europe and sakarya and the anatolide tauride terrane in turkey further east the rheic suture may be present in the caucasus in all probability between the greater caucasus and the scythian platform or alternatively between the greater caucasus and transcaucasia alborz in northern iran is interpreted as a distal part of transcaucasia the location of the rheic suture in the southeast part of the turan domain of central asia is poorly constrained

paleomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain abstract in order to shed light on the early paleozoic paleogeography of iapetus bordering continents samples for paleomagnetic study were collected from middle ordovician llanvirn volcanic rocks of the shelve inlier welsh borderland the folding within the inlier is constrained to late ordovician ashgill time thus offering a fold test with tight age brackets after stepwise thermal demagnetization a pre folding characteristic magnetization component was isolated from 95 specimens the mean of 11 site mean directions for the characteristic magnetization gives d i 116 4 67 9 k 89 4 95 4 9 and the paleomagnetic pole lies at 26 6 n 36 1 e p 6 9 m 8 2 the fold test is positive at the 99 confidence level indicating that the magnetization was acquired during the middle or late ordovician we infer from geological considerations that this magnetization is not a primary thermoremanence but was acquired during late ordovician time as a result of a thermochemical disturbance of that age our results indicate that southern britain was near 51 south latitude during the later ordovician since middle to late ordovician paleolatitudes for gondwana and laurentia are well determined our result can be used to test various scenarios for the closure of iapetus we conclude that 1 the iapetus ocean in britain still had a latitudinal width of some 30 at this time thus rendering a postulated ordovician closure unlikely 2 southern britain avalon and hercynian europe all occupied high latitudes during the later ordovician consistent with a paleoposition near the northern margin of gondwana and 3 the acadian orogeny in britain and north america could have been the result of the collision of gondwana and laurentia during early devonian time

reconstruction of floodplain sedimentation rates a combination of methods to optimize estimates reconstruction of overbank sedimentation rates over the past decades gives insight into floodplain dynamics and thereby provides a basis for efficient and sustainable floodplain management we compared the results of four independent reconstruction methods optically stimulated luminescence osl dating caesium 137 cs 137 dating heavy metal analysis and flood bed interpretation applied at three embanked floodplain sites along lower rhine river distributaries in the netherlands all methods indicate significant sedimentation rates on the floodplains varying between 2 7 mm a in the distal zones and 3 9 mm a in the proximal zones on a rapidly developing sand bar along a natural levee sedimentation rates of 9 to 25 mm a were found except for some minor inconsistencies in cs 137 dating results all methods show decreasing sedimentation rates with increasing distance from the river channel intercomparison of the results of the different dating methods revealed the potential errors associated with each method particularly where disagreement among the results were found uncertainties may arise due to 1 grain size dependent downward migration of cs 137 2 smoothing of the vertical heavy metal and cs 137 profiles 3 delayed sediment associated input of cs 137 in addition to direct atmospheric fall out 4 overestimation of the burial age in os l dating due to incomplete resetting of the osl signal or 5 non linear relationships between sediment deposition and flood magnitude in the count from the top correlation between sediment lamination and past observed flood records still taking the uncertainties associated with each method into account the results are generally in good agreement using the results we indicate the optimal spatial range of application of each method depending on sediment texture and sedimentation rate the optimal spatial and temporal ranges differ for each method but show significant overlap a combination of the methods will thus provide maximum information for accurate estimation of sedimentation rates on a decadal time scale

pre mesozoic alpine basements their place in the european paleozoic framework prior to their alpine overprinting most of the pre mesozoic basement areas in alpine orogenic structures shared a complex evolution starting with neoproterozoic sediments that are thought to have received detrital input from both west and east gondwanan cratonic sources a subsequent neoproterozoic cambrian active margin setting at the gondwana margin was followed by a cambrian ordovician rifting period including an ordovician cordillera like active margin setting during the late ordovician and silurian periods the future alpine domains recorded crustal extension along the gondwana margin announcing the future opening of the paleotethys oceanic domain most areas then underwent variscan orogenic events including continental subduction and collisions with avalonian type basement areas along laurussia and the juxtaposition and the duplication of terrane assemblages during strike slip accompanied by contemporaneous crustal shortening and the subduction of paleotethys under laurussia thereafter the final pangea assemblage underwent triassic and jurassic extension followed by tertiary shortening and leading to the buildup of the alpine mountain chain recent plate tectonic reconstructions place the alpine domains in their supposed initial cambrian ordovician positions in the eastern part of the gondwana margin where a stronger interference with the chinese blocks is proposed at least from the ordovician onward for the visean time of the variscan continental collision the distinction of the former tectonic lower plate situation is traceable but becomes blurred through the subsequent oblique subduction of paleotethys under laurussia accompanied by large scale strike slip since the pennsylvanian this global collisional scenario has been replaced by subsequent and ongoing shortening and strike slip under rising geothermal conditions and all of this occurred before all these puzzle elements underwent the complex alpine reorganization

agro industrial alluvium in the swale catchment northern england as an event marker for the anthropocene foulds s a macklin m g brewer p a 2013 agro industrial alluvium in the swale catchment northern england as an event marker for the anthropocene the holocene 23 4 587 602

geomorphological facies reconstruction of late quaternary alluvia by the application of fluvial architecture concepts abstract this paper investigates the methodical implications the benefits and the constraints of applying the fluvial architecture concept to late quaternary shallow subsurface alluvial deposits the focus is upon a typical small to mesoscale valley floor of the temperate zone the studied reach is part of the river wetter catchment 517 km 2 in central germany a large number of studies on late quaternary terrestrial paleoenvironmental change of the temperate zone refer to such small to mesoscale catchments which are mostly occupied by mixed to suspended load rivers as in many cases mostly coring based evidence is available the observation and measurement is limited to lithology and lithofacies boundaries facies geometry must be inferred moreover in those environments differences in lithofacies are obscured by a significantly reduced range of grain size distributions an example of an ancient channel belt of late glacial to holocene age serves to delineate the methodical practice and utility of the fluvial architecture approach field evidence is obtained from detailed cross sectional surveys and comprises descriptions of lithofacies structural pedogenetical biotic features munsell color and total organic carbon magnetic volume susceptibility cross sectional lithofacies information is represented by spatially attributed scaled borehole logs the example also accentuates the need for applying additional stratigraphical methods such as physical age determination macrofossil analysis and tephrological stratigraphy these methods form the basis to discern stacked channel facies and derive a diachrony of channel forms thus the adapted architecture approach provides a significant surplus of information on channel dimensions ages and channel floodplain interconnectedness distinct fluvial landforms such as channels levees abandoned channels swamps and floodplain flats can be highlighted a number of methodical constraints are discussed in detail e g the sharpness of channel element geometry and the underlying ordering of bounding surfaces moreover it is shown that the analytical process resembles an iterative looping process that is led by deduction many geomorphologists and multi disciplinary floodplain researchers collect sedimentary data but still neglect utilizing the potential of architectural analysis the study makes clear that sedimentological approaches used in big river floodplains also apply to small valleys in fact the procedures for facies reconstruction need to be adjusted to each individual case study alluvial architecture analysis provides the tools to reveal interconnectedness or disconnectedness of channel near channel and overbank fluvial landforms which is essential for a geomorphological understanding of floodplain evolution

ordovician faunas island arcs and ophiolites in the scandinavian caledonides ordovician faunal data from the scandinavian caledonides is tested with new geochemical information from zircons to give u pb ages and source origins of volcanic arc and ophiolite sequences early ordovician arenig llanvirn low latitude toquima table head faunas from the upper upper allochthon are associated with an island arc system formed adjacent to laurentia contemporaneous mafic magmas were contaminated by crustal material during subduction and associated granites contain inherited zircons of archaean age the nearest source for such rocks is on the laurentian rather than the baltic side higher latitude celtic province faunas from the upper upper allochthon are from one insular site accessible to forms from both laurentia and baltica the late ordovician low latitude holorhynchus and subtropical hirnantia faunas occur in overstep sequences above deeply eroded early ordovician arc complexes the transgression appears to be coeval with a second generation of spreading related complexes single detrital zircons from sediments show sources from archaean proterozoic and early ordovician terranes this suggests that deposition was in a basin situated along the same continental margin laurentia to which the early ordovician ophiolite arc sequences had already become accreted the late ordovician faunas link both laurentia and baltica at a time of narrowing of lapetus the new geochemical data together with the faunal information is supported by recent palaeomagnetic studies

observations of a tidal intrusion front in a tidal channel abstract a visible front indicated by a surface colour change and sometimes associated with foam or debris lines was observed in a tidal channel during neap tide this is an example of a tidal intrusion front occurring in the absence of sudden topographical changes or reversing flows typically reported to be associated with such fronts detailed acoustic doppler current profiler and conductivity temperature depth measurements were taken on repeated transects both with fronts apparent and with fronts absent the results indicated that the front occurred as a result of stratification which was sustained by the buoyancy flux and the weak tide induced mixing during neap ebb tide the stronger tide induced mixing during spring tide restrained stratification leading to the absence of a front the mechanism of the frontogenesis was the density gradient between the stratified water formed during neap ebb tide and the more mixed seawater during neap flood tide thus the water on the landward southwestern side of the front was stratified and that on the seaward side northeastern of the front was vertically well mixed gradient richardson number estimates suggest that the flow between the stratified and mixed water was near the threshold 0 25 for shear instability meanwhile the density gradient would provide an initial baroclinic contribution to velocity convergence which is indicated by the accumulation of buoyant matter such as foam grass and debris into a sharply defined line along the surface the front migrates with the flood current with a local maximum towards the eastern side of the channel leading to an asymmetrical shape with the eastern side of the front driven further into the tiaozhoumen tidal channel

overbank sand deposition in relation to transport volumes during large magnitude floods in the dutch sand bed rhine river system using aerial photographs and field measurements sandy overbank deposits formed by the large magnitude floods of 1993 94 and 1995 were quantified along two branches of the dutch rhine river system the waal 1993 94 and 1995 and the ijssel 1995 these deposits were laid down intermittently all along the length of these rivers on the top and landward slope of the natural levees and covered about 4 per cent of the embanked floodplain on the waal and about 1 per cent on the ijssel the overbanks and transport mechanism is basically convective by nature the spatial variability of overbank sedimentation points to the important role played by helicoidal currents in determining overbank deposition the presence of embankments and training works appears to influence the sand transport to and morphological development of the floodplains along the dutch rhine river system overbank deposition volumes about equal present estimates of sand transport during a large magnitude flood it appears that studies on sand transport in the dutch rhine carried out so far underestimate sand transport during floods 1998 john wiley sons ltd

how was the iapetus ocean infected with subduction because subduction in the iapetus ocean began only 35 m y after the end of rifting spontaneous foundering of mature passive margins is an unlikely subduction initiation mechanism subduction is more likely to have entered the iapetus from the boundary with the external paleo pacific similar to the incursion of the scotia caribbean and gibraltar arcs into the modern atlantic the subduction zone probably became sinuous entraining fragments of the gondwanan margin along its complex sinistral southern boundary where oblique collision caused monian penobscottian deformation following taconian grampian collision of part of the subduction system with laurentia remaining parts of the iapetus were progressively infected with subduction leading to silurian closure

lower rhine historical flood magnitudes of the last 450years reproduced from grain size measurements of flood deposits using end member modelling abstract objective the objective of this research was to unlock the potential of fluvial archives to create flood chronologies based on grain size characteristics of flood deposits located in two recently formed fluvio lacustrine sequences methods grain size data was compared with contemporaneous discharge measurements for the lower rhine the netherlands regression relations between coarse tail grain size parameters and measured discharge were established for the last 240 years and applied to the older parts of both cores resulting in peak discharge estimates back to ad 1550 results grain size descriptive parameters such as the 95th percentile and end member modelling outcomes correlate well with discharge and turn out to be sensitive proxies for inferring flood magnitudes locally geomorphological changes influence the relation between peak discharge magnitudes and flood bed coarseness but these can be assessed using continuous flood layer and background sedimentation measurements to standardize grain size descriptive parameters conclusion implications flood records distilled from sedimentary archives hold great potential for extending existing records of observed discharge and aid establishment of design discharge for flood protection measures and assessment of non stationarity of the flooding regime due to climatic and anthropogenic forcing

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

cambrian palaeomagnetic data from baltica implications for true polar wander and cambrian palaeogeography a reliable early cambrian c 535 ma and a preliminary late cambrian c 500 ma palaeomagnetic pole from baltica sweden overlap within uncertainty and they are also broadly compatible with vendian c 583 ma palaeomagnetic data apparent polar wander for baltica amounts to less than 25 between 583 and 500 ma and therefore negates recent speculations that the earth tipped 90 during the early cambrian true polar wander throughout vendian and cambrian times baltica lay at southerly latitudes c 30 60 s baltica was geographically inverted and present day northern baltica faced the nw margin of gondwana which covered the south pole laurentia eastern baltica and laurentia west gondwana were separated by the iapetus ocean while the aegir sea separated western baltica from the taimyr region of siberia during the cambrian baltica probably moved eastward along the gondwana margin and by c 515 520 ma subduction in the aegir sea was initiated a major event is recognized in late cambrian or early ordovician times c 500 478 ma when baltica must have undergone a 55 counter clockwise rotation in c 22 million years 3 ma we relate this to the early caledonian finnmarkian orogeny which involved arc continent collision following subduction

some consequences of a flood tide front in loch creran abstract loch creran is a small scottish sea loch with two basins which are connected by a shallow narrows a sharp front that forms just inside the narrows during flood tides producing a simple mechanism for flushing the top brackish layer of the inner basin the mechanism is shown to be consistent with temperature and velocity data conservation equations are used with salinity data to estimate mixing and entrainment rates and thus the energy used a bottom drag coefficient for the narrows is estimated from the tidal signals of velocity and pressure gradient and the turbulent kinetic energy entering the inner loch from the narrows is calculated so that finally a value for the flux richardson number the fraction of available energy used by mixing and entrainment can be given

deep mantle structure as a reference frame for movements in and on the earth earth s residual geoid is dominated by a degree 2 mode with elevated regions above large low shear wave velocity provinces on the core mantle boundary beneath africa and the pacific the edges of these deep mantle bodies when projected radially to the earth s surface correlate with the reconstructed positions of large igneous provinces and kimberlites since pangea formed about 320 million years ago using this surface to core mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander we have developed a model for absolute plate motion back to earliest paleozoic time 540 ma for the paleozoic we have identified six phases of slow oscillatory true polar wander during which the earth s axis of minimum moment of inertia was similar to that of mesozoic times the rates of paleozoic true polar wander 1 my are compatible with those in the mesozoic but absolute plate velocities are on average twice as high our reconstructions generate geologically plausible scenarios with large igneous provinces and kimberlites sourced from the margins of the large low shear wave velocity provinces as in mesozoic and cenozoic times this absolute kinematic model suggests that a degree 2 convection mode within the earth s mantle may have operated throughout the entire phanerozoic

impact of historic land use change on sediment delivery to a chesapeake bay subestuarine delta historic land use in the chesapeake bay drainage basin induced large fluxes of fluvial sediment to subestuarine tributaries stratigraphic and palaeoecologic analyses of deltaic deposits may be used to infer changes on the landscape but are not sufficient to quantify past sediment supply when viewed as an inverse boundary value problem reconstruction of the sediment supply function may be achieved by combining deltaic sedimentation chronologies with an equation governing delta progradation we propose that the diffusion equation is appropriate for simulating delta progradation and obtaining the sediment supply function provided a suitable diffusion constant d can be determined three new methods for estimating d are presented for the case of estuarine deltas when the inverse boundary value technique was applied to otter point creek a tidal freshwater delta at the head of bush river in upper chesapeake bay d values ranged from 3763 to 6199 m2 a 1 delta growth simulations showed a 1740 1760 initial pulse a 1760 1780 erosive redistributive interval a 1780 1920 growth period and a 1920 present erosive redistributive era coupling of simulated delta elevations with an empirical plant habitat predictive equation allowed for comparison of predicted versus actual relative habitat areas also the model yielded reconstructed watershed erosion rates and stream suspended sediment concentrations that could be useful for development of water quality regulations copyright 2001 john wiley sons ltd

u pb ages of ophiolites and arc related plutons of the norwegian caledonides implications for the development of iapetus u pb zircon ages are reported for four ophiolites and three crosscutting arc related plutons from the norwegian caledonides plagiogranite differentiated from gabbro of the karmoy ophiolite is dated at 493 7 4 ma whereas arc related trondhjemite cutting this ophiolite crystallized at 485 2 ma a crosscutting clinopyroxene phyric gabbro intrusion is dated at 470 9 5 ma by near concordant magmatic titanite sphene and discordant u rich 2903 6677 ppm zircon lower intercepts of 247 68 and 191 70 ma defined by the plagiogranite and clinopyroxene phyric gabbro best fit lines may reflect a real low t alteration rift related event

morphology of fluvial levee series along a river under human influence maros river hungary abstract the development and morphometry of fluvial levees reflect the connection between channel and overbank processes which can be altered by various human activities the aims of this study are to investigate the morphology and spatial characteristics of fluvial levees and evaluate the role of some local and catchment scale human activities on their medium term 150 years development this study applies lidar data along a 53 km long reach of the maros river in hungary six fluvial levee types are identified based on the beginning and end of their evolution these levee types were generated by local nineteenth century channel regulation works cutoffs and mid twentieth century channel narrowing which was caused by gravel mining and water impoundment in the upstream sections however other human activities also influenced the development of active fluvial levees because their horizontal evolution could have been limited by embanked flood protection levees or the widening of low lying floodplain benches that were generated by channel narrowing additionally revetment constructions influenced their vertical parameters as higher fluvial levees developed along the fixed banks generally the older active fluvial levees are wider while the younger active levees are narrower with steeper slopes but not always lower on the low lying floodplain levels benches the youngest fluvial levees evolved quite rapidly and consist of coarser material currently only 9 8 to 38 year return period floods could cover the fluvial levees contributing to their evolution this fact and the development of fluvial levee series with two three members reflect a gradual decoupling of the channel from the floodplain

the dynamics of a bathymetrically arrested estuarine front abstract the mixing and circulation associated with a bathymetrically arrested estuarine front was studied using hydrographic and current data a quasisteady front exhibiting strongly convergent surface flows is formed along the steeply sloping inner margins of the flood tide delta during each semidiurnal tide cycle this front separates the brackish ambient water within a deep estuarine basin from the incoming oceanic tidal water the position of the front is dependent on the local water depth and the difference in density between the two water masses beneath the surface there is an inclined frontal interface where static stability is very low and vertical mixing intense a vertically integrated horizontal momentum equation was derived for flow in the upper layer and an estimate made as to the value of the associated entrainment coefficient

first late ordovician paleomagnetic pole for the cuyania precordillera terrane of western argentina a microcontinent or a laurentian plateau abstract time and tectonic processes involved in docking of the argentine precordillera cuyania terrane against sw gondwana has been a matter of much debate a paleomagnetic study on the early caradoc pavon formation exposed in the san rafael block province of mendoza argentina is presented after detailed thermal and alternating field demagnetizations two geologically significant magnetic components were defined a widespread post tectonic component a is present in most sites of the pavon formation with dual polarities and is coincident with the characteristic remanence isolated from a permo triassic rhyolitic dome intruding the sediments its pole position 83 7 s 271 0 e dp 6 8 dm 9 0 n 11 sites falls on the late permian early triassic south american reference poles suggesting that this component was acquired during the choiyoi magmatic phase a second component b also shows dual polarities and a positive fold test suggesting a primary origin unblocking temperatures and rock magnetic experiments indicate that b is carried either by hematite or magnetite at different sites anisotropy of magnetic susceptibility results suggest a depositional fabric and no remanence distortion due to deformation or compaction a paleomagnetic pole computed from this remanence pv falls on 3 6 n 346 4 e dp 2 9 dm 4 6 n 22 samples it indicates a paleolatitude around 26 s for deposition of pavon sediments and constrains the paleogeographic evolution of cuyania during the ordovician which was still at subtropical latitudes by the early caradoc pv is consistent with the laurentian late ordovician reference pole if cuyania remains attached to se laurentia for the early caradoc while it shows a significant cw rotation with no paleolatitude anomaly respect to the gondwana reference pole when kept in its present position in sw south america these comparisons are interpreted in three possible alternatives for the paleogeographic and tectonic setting of cuyania in the late ordovician

textural analysis and modelling of a flood deposit river severn u k samples of sediment collected from the severn floodplain between worcester and gloucester following the severe flooding in january and february 1990 were analysed for their grain size distribution the results show that most sand was deposited within 20 m of the channel bank but that fine sand may contribute to flood sediment across the width of the floodplain james 1985 numerical model of overbank sedimentation attempts to predict the transfer of sediment to the floodplain during flooding geometrical and hydraulic data relating to the severn flood are used as input for a computer program of james 1985 model the pattern of sediment concentrations predicted by the model was compared with that obtained from statistical analysis of the flood sediment the patterns were found to be similar so james 1985 model was considered to predict in a relative sense the distribution of flood sediment

roman and early medieval habitation patterns in a delta landscape the link between settlement elevation and landscape dynamics settlement locations in delta landscapes change through time because of cultural and natural dynamics we assessed the impact of natural landscape dynamics on settlement location shifts for the rhine meuse delta in the netherlands during the roman and early medieval periods 12 bce 450 ce and 450 1050 ce respectively during this time interval major landscape and cultural changes occurred in this area with river avulsions and changes in flooding frequency coinciding with changing settlement patterns in the delta plain the relatively high and dry alluvial ridges of abandoned or active rivers were most favourable for habitation settlement location and elevation patterns were reconstructed in these landscape units using a high resolution elevation map of the alluvial ridges by integrating high resolution palaeo environmental and archaeological datasets for this period we were able to spatially analyse the trends and to assess the effect of environmental changes on habitation results show that settlements progressively shifted towards higher areas between 250 and 750 ce on average by 20 cm over this period deltawide which was coeval with an increased frequency of severe rhine floods the observed spatial differences demonstrate that this trend is most notable in the least elevated segments of the study area in areas where new large river branches developed settlements show a strong shift towards higher elevated parts of the landscape or even became completely abandoned the river probably caused floods to be more frequent and more severe in these areas despite the clear link between changing settlement positions and floods during the studied time interval floods do not seem to have caused long term abandonment of major parts of the study area

human induced sedimentation patterns of a channelized lowland river channelization of the severely polluted odra and vistula rivers in poland induced intensive accumulation of fine grained deposits rich in organic matter and heavy metals these sediments have been identified in vertical profiles in a narrow zone along river banks both in groyne created basins and on the floodplain grain size organic matter zinc zn lead pb copper cu content and cesium 137 137cs was used for sediment dating and stratigraphy and chemistry have been diagnostic features for these deposits named industrial alluvium in the most polluted river reaches stabilized by bank reinforcements and groynes 2 m thick slack water groyne deposits are composed of uniform strata of polluted silts with organic matter content over 10 zn content over 1000 mg kg and average cu and pb over 100 mg kg the average rate of sediment accretion in groynes is higher than on the floodplain and reaches 5 cm yr stratification which appears at higher levels in the groyne fields and on the levees reflects a change from in channel to overbank deposition and is typified by dark layers separated by bright sandy and less polluted strata stratified 4 m thick sediment sequences have been found in groyne fields of incised river reaches the average rate of sediment accretion in these reaches is of the order of 5 cm yr in stable and relatively less polluted river reaches vertical accretion organic deposits are finely laminated and the average rate of deposition amounts to a few millimeters per year investigations indicate that groyne construction favors conditions for long term storage of sediments at channel banks for this reason groynes should be considered as structures that efficiently limit sudden release of sediment associated heavy metals stored in channels and in floodplains of the historically polluted rivers copyright 2014 john wiley sons ltd

the thor suture zone from subduction to intraplate basin setting the crustal seismic velocity structure of northwestern europe shows a low p wave velocity zone lvz in the lower crust along the caledonian thor suture zone tsz that cannot be easily attributed to avalonia or baltica plates abutting the tsz the lvz appears to correspond to a hitherto unrecognized crustal segment accretionary complex that separates avalonia from baltica explaining well the absence of avalonia further east consequently the northern boundary of avalonia is shifted 150 km southward our interpretation based on analysis of deep seismic profiles places the lvz in a consistent crustal domain interpretation a comparison with present day examples of the kuril and cascadia subduction zones suggests that the lvz separating avalonia from baltica is composed of remnants of the caledonian accretionary complex if so the present day geometry probably originates from pre variscan extension and eduction during devonian carboniferous backarc extension the reinterpretation of deep crustal zonation provides a crustal framework in which the northern limit of avalonia corresponds to the southern limit of the deep north german basin and the northern limit of prolific gas reservoirs and late mesozoic inversion structures 2016 geological society of america

paleomagnetic confirmation of the laurentian origin of the argentine precordillera abstract several recent tectonic models have portrayed the argentine precordillera has been recently portrayed as an early paleozoic laurentian derived exotic terrane now found in southwestern south america these models have primarily been based on strong biogeographic and stratigraphic evidence however no paleomagnetic data have previously been available to independently test them a paleomagnetic study was therefore carried out on the early cambrian cerro totora formation exposed in the northern reaches of the argentine precordillera after stepwise thermal demagnetization a pre folding remanence was identified in ten sites of this formation yielding a paleomagnetic pole ct at 37 0 n 314 1 e a95 5 8 this pole is not consistent with the latest proterozoic early paleozoic apparent polar wander path for gondwana but it agrees with the early cambrian section of the laurentian path if the argentine precordillera is positioned as the conjugate margin of the ouachita embayment in southeast laurentia this result confirms that the precordillera is an allochthonous terrane derived from laurentia in cambrian times that was later accreted to gondwana probably in middle ordovician times

a plate tectonic scenario for the iapetus and rheic oceans abstract the tectonics dynamics and biogeographic landscape of the early paleozoic were dominated by the opening and expansion of one large ocean the rheic and the diminution to terminal closure of another iapetus an understanding of the evolution of these oceans is thus central to an understanding of the early paleozoic but their chronicle also presents a rich temporal profile of the wilson cycle illustrating continental scale rifting microcontinent formation ocean basin development arc accretion and continent continent collision nevertheless contemporary paleogeographic models of the iapetus and rheic oceans remain mostly schematic or spatiotemporally disjointed which limits their utility and hinders their testing moreover many of the important kinematic and dynamic aspects of the evolution of these oceans are impossible to unambiguously resolve from a conceptual perspective and the existing models unsurprisingly present a host of contradictory scenarios with the specific aim to resolve some of the uncertainties in the evolution of this early paleozoic domain and a broader aim to instigate the application of quantitative kinematic models to the early paleozoic i present a new plate tectonic model for the iapetus and rheic oceans the model has realistic tectonic plates which include oceanic lithosphere that are defined by explicit and rigorously managed plate boundaries the nature and kinematics of which are derived from geological evidence and plate tectonic principles accompanying the presentation and discussion of the plate model an extensive review of the underlying geological and paleogeographic data is also presented

the allochthony of the argentine precordillera ten years later 1993 2003 a new paleobiogeographic test of the microcontinental model abstract recent and new faunal data from the cambrian to silurian rocks of the precordillera famatina and northwest argentina basins are used to discriminate between different paleogeographic models and especially to establish to what extent they are compatible with a previous conclusion that the precordillera is a laurentian derived microcontinent there is no paleontological evidence to support a para autochthonous gondwanan origin of the precordillera the strong differences in the cambrian trilobite faunas and lithologic successions preclude a common origin of the precordillera terrane eastern antarctica and south africa recent discoveries of brachiopods and organisms of the phylum agmata strengthened laurentian affinities during the cambrian the latest cambrian early ordovician faunas that inhabited the autochthonous northwest argentina basin including the western puna volcaniclastic successions are mostly peri gondwanan the early ordovician brachiopods ostracods and trilobites display mixed laurentian baltic and avalonian biogeographical links supporting a drifting of the precordillera across the iapetus ocean increasing gondwanan elements during the llanvirn along with varied geological evidence indicate that the first stages of collision may have begun at that time involving a major change in the plate kinematics the distribution of facies and faunas basin development and timing of deformation are interpreted as resulting from a north to south diachronous closing of the remnant basin during the last phases of convergence and oblique collision of the precordillera terrane with the gondwana margin the high level of endemism of caradoc faunas may be a consequence of the rearrangement and partial isolation of sedimentary areas during the strike slip movement of the colliding precordillera plate with respect to the gondwana margin suggested relationships between facies distribution geographic barriers and faunal migrations before and during the collision are depicted in a series of schematic reconstructions at five time slices from late cambrian to silurian

high resolution observations of a shear front in a tidal channel abstract the characteristics of a shear front system in a tidal channel have been investigated through a field study high resolution sampling including three dimensional measurements of current temperature salinity and turbidity was performed on repeated transects across the frontal zone both with fronts apparent and with fronts absent the density measured using conductivity temperature depth sensors provides a field proof that the shear front is present whether a density gradient exists it is distinguishable from conventional buoyant shear fronts which require a strong along channel density gradient the velocities measured using an acoustic doppler current profiler adcp and trapping global position system gps drifters indicate that shear front evolution has a four stage dynamic process in a tidal cycle 1 a directional shear front of shoal flood and channel ebb during the slack before flood and early flood 2 a speed shear front of shoal quick and channel slow after the early flood 3 a directional shear front of shoal ebb and channel flood during the slack before ebb and early ebb and 4 a speed shear front of shoal slow and channel quick ebb during the late ebb thus the velocity shear a variation in the current speed and direction on either side of the front is the generation mechanism of the observed shear front system it is caused by the unequal tidal durations between deep channels and shallow shoals in the channel discussed herein the coriolis effect is found to be negligible through the momentum balance analysis the unequal tidal durations can be caused by the following alternative mechanisms according to the analysis and model simulation 1 differential friction owing to the steep bathymetry between the shallow shoal and the deep channel and 2 circulation owing to the coastline bend of the shallow shoal

sedimentary products of avulsions in the holocene rhine meuse delta the netherlands abstract the main objective of this study was to describe the characteristics of deposits related to avulsion to determine if avulsion locations can be recognized by characteristic sediment architecture in the rhine meuse delta the majority of avulsion sites is accompanied by crevasse splay deposits with a relatively large areal extent these crevasse splay complexes occur near the base of the overbank deposits of the newly formed channel belt and are similar to avulsion belt deposits described by smith et al sedimentology 36 1989 1 can j earth sci 35 1998 453 the lithology of all investigated crevasse splay deposits ranges from coarse sand to silty clay sand is concentrated in crevasse splay channels and silty and sandy clay occurs as distal deposits and as levee deposits between the channels the total volume of sand is small relative to clay and silt of a total of 91 avulsion sites three sites were studied in detail one each representing the sedimentary architecture of a full nodal avulsion a failed avulsion and a crevasse splay complex that did not lead to avulsion it is concluded that dating evidence of the abandoned and newly formed paleo channel is always required to verify if avulsion really occurred although the presence of large scale sandy crevasse splay deposits in combination with two main meandering channels often indicates avulsion based on a 2 d outcrop or just one cross section it is impossible to determine whether a splay complex was associated with an avulsion or not because the architecture and lithology in both cases can be identical not all avulsion locations are characterized by crevasse splay deposits there are two possible explanations for this 1 splay deposits were eroded by the newly formed channel or 2 splay deposits were never formed this may be a result of reoccupation of an old channel in this case avulsive flow is immediately appropriated by the pre existing channel and no large scale splay formation occurs alternatively splay deposits were not formed because avulsion resulted from headward erosion

alluvial architecture of the human influenced river rhine the netherlands abstract the alluvial architecture of the embanked rhine distributaries in the netherlands is largely controlled by human interference with fluvial processes embankment straightening and stabilization of the rivers have influenced both in channel and overbank sedimentation embanked river floodplain deposits are lithologically and sedimentologically different from natural not human influenced floodplain deposits after the embankments between 1050 and 1350 ad the dominant process remained lateral within channel accretion however the morphology of the point bars changed in the natural situation the bars had a convex shape as seen from the channel whereas in the embanked situation they are concave as seen from the channel the latter is due to the development of bars along the downstream end of a previous formed bar deposits associated with lateral within channel accretion are typically very fine to medium or coarse sand the infill of residual channels is characterized by the absence of peat after the normalization of the river channel to a fixed width 1850 ad concave bank erosion and development of scroll bars were constricted and overbank deposition became the dominant embanked floodplain forming process thick layers of sandy and clayey deposits were deposited on the embanked floodplain splay like sand lenses are present in the overbank deposits in case of dyke failures a deep scour hole was formed typical deposits associated with dyke breaches are sandy dyke breach deposits and scour hole fills consisting of silty to sandy clay within the embanked floodplain and gyttja within the polder

contemporaneous faulting in the upper ordovician of the oslo asker district norway and its significance in the development of the oslo basin abstract the position of syn sedimentary upper ordovician fault lines are recognised in the oslo asker district through the identification of discrete changes in the thickness of chronostratigraphic units stages 5a and 5b and coincident changes in facies three fault zones are recognised named the nesoya line the bronnoya line and the bunnefjord line and these can be shown to have been actively moving at various times during the period directions of downthrow and minimum value of the amount of downthrow can be determined the faulting is associated with basement block tectonics and it is pointed out that fault trends would have been similar to that of the modern oslo graben this trend parallels the margin of the lower palaeozoic iapetus ocean and it is suggested that fault movement was caused by plate tectonic activity at this margin analogies are drawn with tectonic features associated with the opening of the atlantic ocean and it seems likely that the fault pattern which controlled the development of the palaeozoic oslo basin was rejuvenated at that time to form the mesozoic oslo graben

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

paleomagnetism and 40ar 39ar ages from the carolina slate belt albemarle north carolina implications for terrane amalgamation with north america paleomagnetic and sup 40 ar sup 39 ar dating studies of late proterozoic metamorphic rocks from the carolina slate belt were undertaken to gain insight into the history of this suspect lithostratigraphic terrane geochronological data suggest a peak metamorphic age of ca 455 ma and a later thermal overprint at ca 185 ma thermal demagnetization indicates two magnetization components a low temperature component yields a pole close to 180 190 ma segments of the north american apparent polar wander path a high temperature component postdates folding and is inferred to have been acquired at ca 455 ma corresponding paleomagnetic pole is lat 20 sup 0 n long 80 sup 0 e although paleomagnetic data from the carolina slate belt remain too ambiguous for construction of a unique polar wander path consideration of available paleolatitudes suggests that the terrane may have been proximal to cratonic north america after ca 455 ma this result contrasts markedly with high southerly ordovician paleolatitudes inferred from paleomagnetic studies within central and northern appalachian terranes

characteristics of the great whale river plume observations of the motion field and dilution effects associated with the plume of great whale river in hudson bay are presented for both open water and ice covered conditions in the summer months a distinct plume of about 100 km2 in area is formed offshore which is characterized by a 1 2 m thickness and large velocities directed away from the river mouth in contrast to slower currents parallel to the shore in the ambient waters underneath surface drifter results suggest that the outer boundary of plume may be a zone of frontal convergence under ice covered conditions the plume was significantly thicker and extended much farther offshore in spite of a marked reduction in river runoff at this time

u pb chronology and duration of late ordovician magmatism in the english lake district u pb zircon ages are presented from igneous rocks in the english lake district for which a wide range of ages had been obtained previously by other methods the little stand and crinkle tuffs from the borrowdale volcanic group have ages of 451 6 1 4 and 452 8 0 7 ma 2 respectively the threlkeld microgranite 451 0 1 1 ma 2 and the carrock microgranite 452 4 4 1 ma 2 these dates are identical to the previously published dates for the ennerdale and eskdale granites and together are considered to be a good estimate for both the age and length of caradoc magmatism in the lake district this is the first of three magmatic events recognized in the lake district the other two occurring during ashgill and early devonian times regional comparisons with the leinster terrane in se ireland and the largely concealed english midlands basement emphasize the magnitude and significance of the caradoc event along the eastern avalonia margin

the tidal intrusion front an estuarine convergence zone freshwater discharge in a small estuary flows seaward as a density current which is forced back into the estuary during the flood phase of the tidal streams the development of the two layer flow and the associated frontal structure is described the front exhibits a v configuration with an isolated point convergence at its apex and an associated gyre system

fluvial responses to climate and sea level change a review and look forward summary fluvial landforms and deposits provide one of the most readily studied quaternary continental records and alluvial strata represent an important component in most ancient continental interior and continental margin successions moreover studies of the long term dynamics of fluvial systems and their responses to external or allogenic controls can play important roles in research concerning both global change and sequence stratigraphy as well as in studies of the dynamic interactions between tectonic activity and surface processes these themes were energized in the final decades of the twentieth century and may become increasingly important in the first decades of this millennium this review paper provides a historical perspective on the development of ideas in the fields of geomorphology quaternary geology vs sedimentary geology and then summarizes key processes that operate to produce alluvial stratigraphic records over time scales of 103 106 years of particular interest are changes in discharge regimes sediment supply and sediment storage en route from source terrains to sedimentary basins as well as changes in sea level and the concept of accommodation late quaternary stratigraphic records from the loire france mississippi usa colorado texas usa and rhine meuse the netherlands rivers are used to illustrate the influences of climate change on continental interior rivers as well as the influence of interacting climate and sea level change on continental margin systems the paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea level change at present empirical field based research on fluvial response to climate and sea level change lags behind a the global change community s understanding of the magnitude and frequency of climate and sea level change b the sequence stratigraphic community s desire to interpret climate and especially sea level change as forcing mechanisms and c the modelling community s ability to generate numerical and physical models of surface processes and their stratigraphic results a major challenge for the future is to catch up which will require the development of more detailed and sophisticated quaternary stratigraphic sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings there is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter 102 103 years and longer 104 106 years time scales than has commonly been the case to date as well as in larger river systems from source to sink studies of quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre quaternary rock record

rift related volcanism predating the birth of the rheic ocean ossa morena zone sw iberia abstract two very different periods of magma emplacement in the crust of the ossa morena zone early and main events in sw iberia have been previously interpreted to record a cambrian early ordovician rifting event that is thought to have culminated in the opening of the rheic ocean during the early ordovician new stratigraphic petrographic geochemical and sm nd isotope data from cambrian volcanic rocks included in six key low grade sections in both portugal and spain considerably improve our understanding of these events these data 1 confirm the existence of two rift related magmatic events in the cambrian of the ossa morena zone 2 demonstrate that the early rift related event was associated with migmatite and core complex formation in the mid upper crust and is represented by felsic peraluminous rocks the parent magmas of which were derived mainly from crustal sources and 3 show the main rift related event to be represented by a bimodal association of felsic and mafic rocks with minor amounts of intermediate rocks some of the mafic rocks show n morb affinity whereas others have oib or e morb affinities suggesting different heterogeneous mantle sources depleted and enriched asthenospheric and lithospheric plume like and non plume like the acid and intermediate rocks appear to represent hybrid mixtures of crust and mantle derived magmas this new data supports the hypothesis that the onset of rifting was associated with a process of oblique ridge trench collision we interpret the significant differences between the early and main events as reflecting the evolution from a wide rift stage with passive extension mainly accommodated by lower crust flow in a high heat flow setting to a narrow rift stage with active extension characterized by extension rates that outpaced thermal diffusion rates

palaeomagnetism of the early devonian lower old red sandstones of south wales implications to variscan overprinting and differential regional rotations component structures of magnetic remanence in the old red sandstones of south wales are related to tectonic history by detailed study of three contrasting areas 1 remote from 2 bordering and 3 within the variscan orogenic belt a uniform southerly shallow directed component dominates remanence in each sector in area 1 where it is considered most representative of the regional field this magnetisation has a mean di 191 1 8 0 151 samples in situ 95 2 equivalent to a pole position at 341 1 e 41 2 s fold tests show that it was acquired at an intermediate stage of orogenesis and during the carboniferous permian reversed superchron it is interpreted as a pervasive magnetic overprint imparted by northward fluid migration consequent upon loading of the orogen to the south remote from the orogenic front a higher blocking temperature dual polarity remanence of probable lower devonian age has sporadically survived di 246 38 27 samples tilt adjusted 95 7 2 corresponding to a pole position at 297 e 3 n this remanence is entirely replaced close to the frontal zone of the orogen within the orogen low blocking temperature components are more prominent and chemical alteration obscures the highest blocking temperature portion of the remanence spectrum this region was deformed into upright folds before the magnetising event following remanence acquisition continuing deformation produced asymmetry in fold structure producing a plunge and steepening of the southern limbs structures in the south were already more deformed than those in the north at the time of magnetisation implying that the causative fluid front moved northwards with time the palaeomagnetic vectors are rotated clockwise to the west in south wales and in part record the effects of regional dextral transpression the largest rotation however is observed within the same thrust block and apparent differential rotation here is probably a consequence of strain

a sveconorwegian terrane boundary in the caledonian hardanger ryfylke nappe complex the lost link between telemarkia and the western gneiss region abstract magmatic and metamorphic events in two of the nappes of the hardanger ryfylke nappe complex in the caledonides in sw norway and in the intervening thrust zone have been investigated by means of id tims u pb zircon and titanite data orthogneiss protoliths in the upper kvitenut nappe are dated at 1615 6 ma showing analogies to the gothian terrane including the western gneiss region by contrast the dyrskard nappe is composed of metasedimentary rocks and metarhyolites with a 1508 4 ma extrusion age and shows an affinity to rocks of the telemarkia terranes we argue that the time of thrusting and juxtaposition of the two nappes along the shear zone is constrained by the age of 999 5 ma of a syndeformational granite body and co genetic pegmatitic leucosomes with late sveconorwegian movements and fluid activity being recorded by titanite at 924 6 ma both nappes behaved as one block during the silurian emplacement in the caledonian nappe stack sharing a 434 1 ma metamorphic peak and later overprints as young as 414 2 ma related to retrogression the distinct origin and sveconorwegian age of coupling of the dyrskard and the kvitenut nappes suggest that in their pre caledonian location to the west northwest they represent the now hidden boundary zone between the western gneiss region and telemarkia

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

upper cambrian to upper ordovician peri gondwanan island arc activity in the victoria lake supergroup central newfoundland tectonic development of the northern ganderian margin the exploits subzone of the newfoundland appalachians comprises remnants of cambro ordovician peri gondwanan arc and back arc complexes that formed within the iapetus ocean the exploits subzone experienced at least two accretionary events as a result of the rapid closure of the main portion of the iapetus tract the penobscot orogeny c 480 ma which juxtaposed the penobscot arc c 513 486 ma with the gander margin and c 450 ma collision of the victoria arc c 473 454 ma with the annieopsquotch accretionary tract that juxtaposed the peri laurentian and peri gondwanan elements along the red indian line the newly recognized pats pond group forms a temporal equivalent to other lower ordovician intra oceanic complexes of the penobscot arc the pats pond group c 487 ma has a geochemical stratigraphy that is consistent with rifting of a volcanic arc an ensialic setting is indicated by low end values end 0 3 to 0 5 near the stratigraphic base and its abundant zircon inheritance c 560 ma and 0 9 1 2 ga the spatial distribution of tremadocian arc back arc complexes indicates that the penobscot arc is best explained in terms of a single east dipping subduction zone this model is favored over west dipping models in that it explains the distribution of the penobscot arc elements continental arc magmatism and the obduction of back arc penobscot ophiolites without requiring subduction of the gander margin or subduction reversal the newly recognized wigwam brook group c 454 ma disconformably overlies the pats pond group and records the youngest known phase of ensialic arc volcanism end 4 1 in the victoria arc which is also related to east dipping subduction thus the penobscot and the overlying victoria arc are reinterpreted in terms of a single relatively long lived east dipping subduction zone beneath the peri gondwanan microcontinent of ganderia the cessation of arc volcanism towards the top of the wigwam brook group and the subsequent syn tectonic sedimentation in the badger group constrain the arrival of the leading edge of ganderia with the ensialic arc complexes to the laurentian margin to c 454 ma

bathymetrically controlled velocity shear front at a tidal river confluence nonbuoyant front formation at the confluence of nanjemoy creek and the main potomac river md channel is examined terra satellite aster imagery reveals a sediment color front emerging from nanjemoy creek when the potomac is near maximum ebb nearly contemporaneous aster and landsat etm imagery are used to extract surface velocities which suggest a velocity shear front is collocated with the color front in situ velocities measured by riverray traverses near the nanjemoy creek mouth confirm the shear front s presence a finite element simulation using adcirc replicates the observed velocity shear front and is applied to decipher its physics three results emerge 1 the velocity shear front forms confined to a shoal downstream of the creek river confluence for most of the tidal cycle 2 a simulation with a flat bottom in nanjemoy creek and potomac river i e no bathymetry variation indicates the velocity shear front never forms hence the front cannot exist without the bathymetry and 3 an additional simulation with a blocked off creek entrance demonstrates that while the magnitude of the velocity shear is largely unchanged without the creek shear front formation is delayed in time without the creek there is no advection of the m6 tidal constituent generated by nonlinear interaction of the flow with bottom friction onto the shoals only a locally generated contribution a tidal phase difference between nanjemoy and potomac causes the ebbing nanjemoy creek waters to intrude into the potomac as far south as its deep channel and draw from a similar location in the potomac during nanjemoy flood

tectonic significance of a llanvirn age for the dunn point volcanic rocks avalon terrane nova scotia canada implications for the evolution of the iapetus and rheic oceans abstract the paleozoic evolution of avalonia is crucial to the understanding of the development of the appalachian orogen and the iapetus and rheic oceans paleomagnetic faunal structural and isotopic data indicate that avalonia had accreted to laurentia by the early silurian in the avalon terrane of nova scotia ordovician early devonian rocks consist of bimodal volcanic rocks at the base dunn point formation disconformably overlain by a thick sequence of fossiliferous siliciclastics arisaig group which contain llandoverian to lochkovian fossils until now the only published age data for the volcanic rocks yielded an imprecise rb sr whole rock isochron age of 421 15 ma and the volcanism has been interpreted to reflect local extension and basin development related to the oblique collision between avalonia and laurentia after closure of the iapetus ocean we present u pb zircon data from a rhyolite which indicate an age of 460 0 3 4 ma for the dunn point formation an age that requires a re interpretation of its tectonic setting the dunn point volcanism probably developed on the avalonian microcontinent outboard from both laurentia and gondwana possibly in a rifted arc setting analogous to the modern taupo zone in northern new zealand a llanvirn age for these felsic rocks also reconciles apparently conflicting paleomagnetic data and inferred paleolatitude for avalonia in the ordovician silurian and implies 10 northward movement of avalonia between 460 and 440 ma and about 5 5 cm year for the latitudinal component of the convergence between avalonia and laurentia i e the destruction of the iapetus ocean and divergence between gondwana and avalonia development of the rheic ocean during this interval

the tornquist sea and baltica avalonia docking early ordovician late arenig limestones from the sw margin of baltica scania bomholm have multicomponent magnetic signatures but high unblocking components predating folding and the corresponding palaeomagnetic pole latitude 19degreesn longitude 051degreese compares well with arenig reference poles from baltica collectively the arenig poles demonstrate a midsoutherly latitudinal position for baltica then separated from avalonia by the tomquist sea tornquist sea closure and the baltica avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for avalonia and baltica during the mid late ordovician by the caradoc avalonia had drifted to palaeolatitudes compatible with those of sw baltica and subduction beneath eastern avalonia was taking place we propose that explosive vents associated with this subduction and related to andean type magmatism in avalonia were the source for the gigantic mid caradoc c 455 ma ash fall in baltica i e the kinnekulle bentonite avalonia was located south of the subtropical high during most of the ordovician and this would have provided an optimum palaeoposition to supply baltica with large ash falls governed by westerly winds in scama we observe a persistent palaeomagnetic overprint of late ordovician ashgill age pole latitude 4degreess longitude 012degreese the remagnetisation was probably spurred by tectonic derived fluids since burial alone is inadequate to explain this remagnetisation event this is the first record of a late ordovician event in scania but it is comparable with the shelveian event in avalonia low grade metamorphism in the north sea basement of ne germany 440 450 ma and sheds new light on the baltica avalonia docking c 2002 elsevier science b v all rights reserved less