palaeomagnetic study of llandovery lower silurian red beds in north west england high unblocking temperature magnetization components carried by haematite have been isolated from 12 sites of llandovery lower silurian red mudstones from nw england the principal deformation phase in this area is acadian mid devonian however some faults with related folds were also active during the later variscan orogeny the site mean magnetization directions pass a regional fold test at the 95 per cent confidence level implying that the magnetization components pre date acadian folding the overall mean direction declination 43 4d inclination 24 1d 95 12 4d gives a pole position at 13 6ds 313 9de and a palaeolatitude of 12ds the low palaeolatitude implies closure of the iapetus ocean between eastern avalonia and laurentia by late llandovery time and is consistent with a recently published palaeolatitude 13ds for the wenlock mid silurian of southern britain comparison of these values with the published palaeolatitude from nw england of 43ds for caradoc late ordovician time implies very rapid at least 15 cm yr 1 northward drift of eastern avalonia during late ordovician early silurian time the new pole position from the llandovery red beds lies close to late silurian and early devonian poles from scotland and wales but is significantly different from a group of mid silurian poles comprising data from scottish intrusions and from swedish sediments this discrepancy may be due to late silurian early devonian pre acadian remagnetization of our sampling sites or local clockwise rotation of the sampling sites during acadian and or variscan deformation however the positive fold test presence of reversals and local deformation history tend to mitigate against these possibilities the more likely interpretation is that although baltica scotland were one continental block from the mid silurian scandian orogeny eastern avalonia did not become part of this block until latest silurian

late paleozoic remagnetization in limestones of the craven basin northern england and the rock magnetic fingerprint of remagnetized sedimentary carbonates samples of lower carboniferous chatburn and pendleside limestones collected from 27 sites in the craven basin northern england carry a syn hercynian remagnetization residing in magnetite the paleomagnetic directions are all of reversed polarity consistent with a kiaman superchron remagnetization the partially 62 tilt corrected mean direction has the minimum correlation statistic implying a synfolding magnetization the resulting pole appears to be late carboniferous in age and there is no indication of any magnetization component that predates the remagnetization event the magnetization appears to be of the same type as the synfolding remagnetizations documented in several appalachian carbonate units jackson 1990 reported that remagnetized appalachian carbonates have distinctive hysteresis properties and we have obtained similar hysteresis loops from remagnetized paleozoic carbonates from the craven basin england the great basin nevada alaska and new york state the hysteresis loops are all wasp waisted and yield highly anomalous hysteresis ratios compared to data from synthetic or typical magnetite bearing samples in contrast unremagnetized limestones from italy show hysteresis behavior which is more typical for magnetite bearing rocks channell and mccabe this issue these results suggest that remagnetization in magnetite bearing carbonates can be readily detected on the basis of hysteresis properties prior to a full scale paleomagnetic investigation

the lower middle ordovician palaeofield of scandinavia southern sweden revisited abstract we present a palaeomagnetic pole for the arenig llanvirn swedish orthoceras limestones based on a new analysis of some earlier reported palaeomagnetic data this new pole 18 n and 046 e differs from the original estimate of 30 n and 46 e the revised pole probably represents the most reliable ordovician pole from baltica although some uncertainties remain concerning the relative importance of diagenetic depositional controls on remanence acquisition based on the new pole from the swedish orthoceras limestone and recent palaeomagnetic results from the northern scandinavian caledonides it is now evident that baltica lay within southerly latitudes of between 30 and 60 s in cambro ordovician times subsequent anticlockwise rotation relative to eastern avalonia southern britain may indicate dextral shear across the tornquist suture prior to mid late ordovician amalgamation of eastern avalonia and baltica

cenozoic evolution of asian climate and sources of pacific seawater pb and nd derived from eolian dust of sediment core ll44 gpc3 1 the large diameter piston core ll44 gpc3 from the central north pacific ocean records continuous sedimentation of eolian dust since the late cretaceous two intervals resolved by nd and pb isotopic data relate to dust coming from america prior to 40 ma and dust coming from asia since 40 ma the intertropical convergence zone itcz separates these depositional regimes today and may have been at a paleolatitude of 23 n prior to 40 ma such a northerly location of the itcz is consistent with sluggish atmospheric circulation and warm climate for the northern hemisphere of the early to middle eocene since 40 ma correlations between nd 7 55 end t 10 81 and pb 18 625 206 4pb 18 879 15 624 207 4pb 15 666 38 611 208 4pb 38 960 0 8294 207 6pb 0 8389 2 0539 208 6pb 2 0743 isotopes reflect the progressive drying of central asia triggered by the westward retreat of the paleo tethys comparisons between the changes with time in the isotopically well defined dust flux and nd and pb isotopic compositions of pacific deep water allow one to draw two major conclusions 1 dust bound nd became a resolvable contribution to pacific seawater only after the one order of magnitude increase in dust flux starting at 3 5 ma therefore eolian nd was unimportant for pacific seawater nd prior to 3 5 ma 2 the lack of a response of pacific deep water pb to this huge flux increase suggests that dust bound pb has never been important instead mobile pb associated with island arc volcanic exhalatives probably consists of a significant contribution to pacific deep water pb and possibly to seawater elsewhere far away from landmasses

comment on palaeomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain by c mccabe and j e t channell in a recent paper mccabe and channell 1 report a late ordovician palaeomagnetic pole from the shelve inlier wales and argue for a iapetus ocean of 30 degrees latitudinal width between laurentia and southern britain in late ordovician times they further suggest that southern britain lay in high latitudes adjacent to the northern margin of gondwana at this time unlike mccabe and channell we contend a primary llanvirn mid ordovician remanence age for the shelve volcanics additionally several important points regarding the palaeomagnetism of southern britain may have been overlooked by mccabe and channell in particular re examination of a previous palaeomagnetic study of the shelve inlier 2 reveals a dual polarity remanence which passes a fold test at 95 confidence and indicates temperature southerly palaeolatitudes of 32s 8

further paleomagnetic results from the builth wells ordovician inlier wales a characteristic magnetization has been isolated from 18 sites in middle ordovician igneous and sedimentary rocks from the builth inlier wales two conglomerate tests demonstrate that the characteristic magnetization in two of the volcanic units was acquired early perhaps during initial emplacement six sites from the northern and central parts of the inlier give a mean direction of d 153 8 i 69 8 k 45 8 95 10 that passes a regional fold test however 12 sites from the southern end of the inlier in and near the llanelwedd quarry yield a significantly different direction after correction for local tilt d 176 4 i 44 5 k 42 4 95 6 7 the llanelwedd quarry data are anomalous with respect to both results from the northern parts of the inlier and those obtained from approximately coeval rocks from the shelve inlier we have searched for an explanation for the discrepancy between the magnetization directions of the two areas and have now ruled out some possibilities including superimposed magnetization components at one of the two areas the llanelwedd quarry lies within a wedge of dominantly andesitic volcanic rocks that appear only locally and which are overlain by two unconformities that are also not present elsewhere in the inlier one possible explanation for the anomalous nature of the llanelwedd quarry magnetization is that the strata were not horizontal at the time of magnetization either due to early volcano tectonic tilting or primary dips on the flanks of a volcano another possibility is that the llanelwedd quarry rocks were extruded rapidly enough that the geocentric coaxial dipole field hypothesis cannot be applied results from cenozoic volcanic rocks from the western united states suggest that even highly anomalous fields can be preserved virtually unchanged over a number of separate flow events we currently regard this to be the most viable explanation for the anomalous nature of the llanelwedd quarry magnetization we conclude that the data from the 6 northern sites represent the best estimate of the middle ordovician axial dipole field for the inlier

the phytoplankton bloom in the northwestern arabian sea during the southwest monsoon of 1979 the biological variability of the northwestern arabian sea during the 1979 southwest monsoon has been investigated by the synthesis of satellite ocean color remote sensing with analysis of in situ hydrographic and meteorological data sets and the results of wind driven modeling of upper ocean circulation the phytoplankton bloom in the northwestern arabian sea peaked during august september extended from the oman coast to about 65 e and lagged the development of open sea upwelling by at least 1 month in total the pigment distributions hydrographic data and model results all suggest that the bloom was driven by spatially distinct upward nutrient fluxes to the euphotic zone forced by the physical processes of coastal upwelling and offshore ekman pumping coastal upwelling was evident from may through september yielded the most extreme concentrations of phytoplankton biomass and along the arabian coast was limited to the continental shelf in the promotion of high concentrations of phytoplankton upward ekman pumping to the northwest of the somali jet axis stimulated the development of a broad open sea phytoplankton bloom oceanward of the oman shelf vertical mixing during the 1979 southwest monsoon was apparently not a primary cause of the regional scale phytoplankton bloom

the lower palaeozoic apparent polar wander path for baltica palaeomagnetic data from silurian limestones of gotland sweden summary well dated and undeformed silurian lower wenlock limestones from gotland southern sweden yield two stable remanence components following detailed thermal and alternating field demagnetization studies 1 a low blocking temperature coercivity magnetization termed l delineated below 250dc 10mt is oriented parallel to the present earth s field dec 001 inc 67 n 4 sites a95 16d 2 a higher blocking temperature coercivity magnetization termed h unblocked between 250 400dc 10 35mt has a nne declination and shallow negative inclination dec 025 inc 19 n 5 sites a95 5d this h component direction compares favourably with a previous result from gotland based upon blanket cleaning given a lack of evidence for subsequent geological heating conodont alteration index 1 1 5 or pervasive palaeomagnetic overprinting the h palaeopole is regarded as reliable and primary early diagenetic in origin 19ds 352de dp dm 3 5 the only other well constrained mid silurian pole from baltica that from the ringerike sandstone of the oslo district is in excellent agreement with the gotland data these combined poles resolve previous problems regarding the shape and time calibration of silurian apparent polar wander relative to baltica

environmental rock magnetism of cenozoic red clay in the south pacific gyre nonfossiliferous red clay can be used for elucidating long range environmental changes although such studies were limited so far because of the difficulty in precise age estimation and extremely low sedimentation rates we conducted an environmental rock magnetic study of cenozoic red clay at the integrated ocean drilling program site u1365 in the south pacific gyre magnetostratigraphy could be established only above 6 m below the seafloor mbsf 5 ma below 6 mbsf the ages of the cores were transferred from the published ages of nearby deep sea drilling project site 596 which is based mainly on a constant cobalt flux model by intercore correlation using magnetic susceptibility and rare earth element content variation patterns rock magnetic analyses including first order reversal curve diagrams the ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization sirm and irm component analyses revealed that magnetic minerals consist mainly of biogenic magnetite and terrigenous maghemite and that the proportion of the terrigenous component increased since 23 ma we consider that the increase reflects a growth of eolian dust flux associated with a northward shift of australia and the site to an arid region of the middle latitudes the increase of the terrigenous component accelerated after 5 ma which may be associated with a further growth of the antarctic glaciation at that time this is coeval with the onset of the preservation of magnetostratigraphy suggesting that the primary remanent magnetization is carried by the terrigenous component

paleomagnetism of the pennington mountain terrane a near laurentian back arc basin in the maine appalachians paleomagnetic studies of volcanic terranes in the appalachians provide quantitative data on the ordovician paleogeography of the iapetus ocean new paleomagnetic results from submarine volcanics of the middle to late ordovician winterville formation of maine further constrain the evolution of iapetus ten sites yield a tilt corrected direction of d i 327 21 95 9 3 k 27 9 the corresponding paleomagnetic pole calculated for the average site location 46 8 n 291 3 e is located at 26 n 148 e d p 5 d m 10 a 95 7 7 a silurian conglomerate frenchville formation test is inconclusive due to a strong present day field overprint which obscures any primary remanence the presence of a positive tilt test however supports the conclusion that the characteristic magnetization of the winterville formation is a prefolding primary magnetization an ordovician paleolatitude of 111 5 for the pennington mountain terrane of northern maine is indistinguishable from that of the laurentian margin 15 20 during the middle to late ordovician the paleolatitude of the pennington mountain terrane is also similar to but slightly more equatorial than that of the previously studied bluffer pond formation of the nearby munsungun terrane 18 9 we conclude that the ordovician pennington mountain and munsungun volcanic terranes of maine were formed and acquired their characteristic magnetization near the laurentian margin the paleolatitudes of these terranes are also similar to that obtained from the stacyville volcanics of the more outboard lunksoos terrane 20 8 but they contrast strongly with the ordovician miramichi terrane of northern new brunswick that yielded a paleolatitude of 51 21 16 these results from maine support the presence of one or more backarc basins adjacent to the laurentian margin during the middle to late ordovician

carbon isotopic evidence for organic matter oxidation in soils of the old red sandstone silurian to devonian south wales uk petrographic and calcrete carbon isotope data from seasonally waterlogged upper silurian p idoli to lower devonian pragian palaeo vertisols of the old red sandstone south wales uk are presented the 13 c values mostly range from 9 to 12 vpdb suggesting that the soils were inhabited by abundant vegetation that when oxidized perhaps with microbial assistance resulted in co 2 rich soils such soils would favour calcrete precipitation through equilibration of soil zone co 2 with the relatively lower atmospheric p co 2 however reliably estimating palaeoatmospheric p co 2 using these carbon isotope data is a challenge supplementary material detailed information on stable isotope measurement methods descriptions of the samples and carbon and oxygen isotope data are available at www geolsoc org uk sup18754

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

paleomagnetics of ringerike old red sandstone and related rocks southern norway implications for pre carboniferous separation of baltica and british terranes abstract paleomagnetic study of two stratigraphic sections of the upper silurian lower devonian ringerike sandstone oslo area norway indicate a straightforward multicomponent nrm for these rocks a lower temperature magnetization component gives a southwest and up reversed polarity in situ direction yields a negative fold test i e post folding at the 99 confidence level and is nearly identical to directions obtained from overlying permo carboniferous lavas a high temperature magnetization component exhibits both normal and reversed polarity directions passes a fold test at the 95 confidence level and is supported by a baked contact test which indicates a pre intrusion age of magnetization this characteristic high temperature component was successfully isolated in 79 samples from 19 sites within the ringerike succession results indicate a tilt corrected characteristic magnetic direction of declination 26 inclination 16 which is consistent with earlier work this direction corresponds to a time integrated paleopole position of 19 n latitude 164 longitude d p 4 8 d m 9 3 when compared to previous results from rocks of similar age from the british isles an approximately 20 latitudinal displacement between britain and baltica during late silurian early devonian time is suggested

palaeomagnetism of the ross of mull granite complex western scotland lower palaeozoic apparent polar wander of the orthotectonic caledonides the ross of mull granite complex was emplaced near the western margin of the orthotectonic caledonides of scotland in early mid silurian times anisotropy of magnetic susceptibility reflects the post tectonic character of the pluton and contrasts with magnetic fabrics in adjoining country rocks which record the regional d3 deformation palaeomagnetic study identifies two dominant magnetizations an a component is magnetite resident with distributed blocking temperatures 100 500 c and normal polarity d i 339 50 30 samples 95 5 6 palaeomagnetic pole at 12 e 1 s dp dm 5 0 7 5 it was probably acquired during initial cooling a b magnetization unblocking temperatures 500 580 c d i 195 21 40 samples 95 4 3 palaeomagnetic pole at 339 e 22 s dp dm 2 4 4 5 is of dual polarity and comparable to magnetizations widely recorded in the younger granite suite of the scottish caledonides it does not correlate with the timing of regional isotopic closure as defined by k ar thermochron ages and appears to have been imparted during a regional thermochemical event at low temperatures 250 150 c and a late stage of cooling of the orogen 420 410 ma metamorphic facies in the aureole and adjoining country rocks record examples of both a and b magnetizations together with a low blocking temperature 0 350 c component acquired during late palaeozoic early mesozoic rifting in the hebridean basin palaeomagnetic poles from the orthotectonic caledonides define a late to post tectonic magnetization record equivalent to a general east to west apparent polar wander trend of late ordovician lower devonian age a short term reversal of this trend is identified during the earlier part of silurian times so that mid late silurian poles are located in a similar position to late ordovician poles the overall path coincides with the contemporaneous record from the paratectonic caledonides of england and wales from c 455 ma it is therefore concluded that the three plate model of the caledonides incorporating the collision of laurentia and baltica in mid ordovician times to produce the orthotectonic caledonides and the closure of the iapetus ocean between baltica laurentia and avalonia in late ordovician times to produce the paratectonic caledonides was unified prior to the oldest magnetizations preserved in the orthotectonic caledonides certain second order discrepancies remain and may either reflect the incompleteness of the present database or the effects of subsequent regional rotation and strike slip movements prior to full welding of the orogen by late lower devonian times

environmental magnetism of pleistocene sediments in the north pacific and ontong java plateau temporal variations of detrital and biogenic components 1 an environmental magnetic study was conducted to investigate temporal variations of the sources of magnetic minerals in the pacific ocean using sediment cores of pleistocene age the proportion of interacting i to noninteracting n i single domain components was estimated from first order reversal curve diagrams and the proportion of middle m to low l coercivity components was determined from decomposition of isothermal remanent magnetization irm acquisition curves the i and m components are interpreted to be carried by terrigenous maghemite while the n i and l components represent biogenic magnetite an inverse correlation between the ratio of anhysteretic remanent magnetization susceptibility arm to saturation irm sirm and the i n i ratio was confirmed for sediments across wide expanses in the pacific ocean this implies that arm sirm can be used to estimate the relative abundances of biogenic and terrigenous components in sediments in the north pacific the relative abundance of the i and m components increases in glacial periods variations of arm sirm and s ratio s 0 1t resemble each other and decrease in the same time intervals these variations reflect increases of terrigenous input in glacial periods as eolian dust on the ontong java plateau in the western equatorial pacific on the other hand minima in arm sirm and s 0 1t occur at glacial to interglacial transitions and the relative abundance of the i and m components increases in these periods terrigenous material in this region is considered to be transported mainly from the new guinea and solomon islands by the equatorial undercurrent the higher proportion of the terrigenous component at glacial to interglacial transitions is coeval with carbonate preservation maxima reported from the ontong java plateau suggesting a linkage between the two

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

deflection versus reflection of sediment gravity flows in the late llandovery rhuddnant grits turbidite system welsh basin the rhuddnant grits turbidite system was deposited in an elongate fault bounded trough an apparently ideal locality for the reflection of sediment gravity flows as solitons however 280 palaeocurrent readings from turbidite sandstones do not show the expected high angle discrepancies between palaeocurrent directions from sole structures and those from cross lamination palaeocurrents orthogonal to the bounding lineament are not generally observed instead there is a gradual progressive anticlockwise deflection of the currents with distance from source this may be explained by banking of the currents against an intrabasinal slope by the coriolis effect or by a combination of the two processes

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

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

a palaeomagnetic study of the builth wells llandrindod wells ordovician inlier wales palaeogeographic and structural implications summary discrepancies exist in the southern british ordovician palaeomagnetic data set with individual studies indicating either high southerly or temperate southerly palaeolatitudes although previous palaeomagnetic investigations from the builth inlier have delineated a three component remanence structure doubts have been raised concerning i the magnetization age of the supposed primary component p ii the structural setting of the sampling area llanelwedd quarry within a zone of strike slip deformation and iii whether detailed step wise demagnetization has fully separated the various magnetization components in addressing these problems we present new palaeomagnetic results as follows i a positive palaeomagnetic conglomerate test establishing a pre late llanvirn age for component p ii an enlarged geographic sampling embracing several tectonic domains indicating that relative rotations linked to strike slip tectonism are minor within the builth sampling area and iii evidence that component p is uncontaminated by overprints at some sites suggesting the resulting palaeolatitude of 35 s to be representative for the builth inlier we also present evidence for local syn volcanic deformation within llanvirn volcanics and for intrusion related deformation of llanvirn shales when the revised llanvirn pole from the inlier 3 s 4 e is combined with other data from southern britain in a statistical spline analysis a mean palaeolatitude of 44 5 s is calculated for builth in llanvirn times a palaeoreconstruction for 468 474 ma c mid llanvirn is presented utilizing this analysis

magnetostatic interactions in deep sea sediments inferred from first order reversal curve diagrams implications for relative paleointensity normalization 1 understanding magnetostatic interactions in sediments is important for paleomagnetism and rock magnetism magnetostatic interactions are known to affect significantly the efficiency of anhysteretic remanent magnetization arm acquisition and hence the ratio of arm susceptibility arm to saturation isothermal remanent magnetization sirm a magnetic grain size proxy widely used for environmental applications can be controlled by magnetostatic interactions relative paleointensity estimations may also be influenced by magnetostatic interactions because arm is often used as a normalizer to correct for the difference in magnetizability of sediments in this study characterization of magnetic grains in deep sea sediments from the north pacific ocean was conducted using first order reversal curve forc diagrams and irm acquisition curves the forc diagrams indicate that the magnetic grains consist mainly of a noninteracting single domain sd component and an interacting sd component and additionally of a multidomain md component the relative abundances of these components were semiquantitatively estimated by curve fitting of cross sections along a line parallel to the axis of local interaction fields hu and through a peak in the coercivity hc three components were fitted assuming a gaussian distribution of interaction fields hu the irm acquisition curves can be described by two dominant components assuming a log gaussian distribution the mean coercivities of the two components are 40 and 100 mt respectively it is estimated that the former component is carried by biogenic magnetite and roughly corresponds to the noninteracting sd component derived from the forc diagrams and that the latter component is carried by eolian maghemite corresponding to the interacting sd component the arm sirm ratio decreases with increasing concentration of the interacting sd component this implies that the arm sirm ratio is significantly affected by magnetostatic interactions and that it does not necessarily reflect magnetic grain size the effect of magnetostatic interactions on natural remanent magnetization acquisition in sediments is not well understood but if it is similarly sensitive to the sirm normalization by arm in relative paleointensity estimations would overcompensate for the concentration of magnetic grains and cause the significant coherence between the normalized intensity and the normalizer that has been reported in the literature

palaeomagnetic data from the borrowdale volcanic group volcano tectonics and late ordovician palaeolatitudes nineteen palaeomagnetic sampling sites from the borrowdale volcanic group between borrowdale and langdale english lake district yield high unblocking temperature magnetization components which postdate the structural tilt and therefore postdate the formation of the broad block faulted syncline which characterizes this part of the central lake district the mean magnetization direction before tilt correction dec 347 2 inc 61 9 95 6 9 corresponds to a palaeolatitude of 43 s 5 7 the magnetization is associated with pervasive late ordovician hydrothermal alteration and is certainly pre devonian based on comparison with available palaeomagnetic data from southern britain this implies that the block faulted synclinal structure of the borrowdale langdale region is pre acadian in age consistent with recent interpretations of late ordovician volcano tectonics based on mapping and volcanological studies

the pontesford lineament welsh borderland the ne trending pontesford lineament centres on the pontesford linley and adjacent faults in the shelve longmynd inlier to the ne it is usually drawn along the se edge of the cheshire basin and to the sw along a variety of lines parallel to or converging with the church stretton fault new mapping and reassessment of published data suggest that sw of the pontesford linley fault the lineament is logically drawn along the clun forest disturbance a post pridoli late silurian fold fault belt then across the nw side of the llandrindod inlier to flank the tywi lineament the lineament was intermittently active at least from mid ordovician through to triassic time with both strike slip and dip slip displacements that involved the upper part of the precambrian basement late ordovician and early silurian strike slip was particularly important the lineament controlled the sites of several silurian sedimentary basins ordovician contrasts across the lineament can be explained either by contemporary fault control or by subsequent large displacements juxtaposing unlike terranes the long history of reactivation and basement control on the pontesford lineament is typical of lineaments throughout wales and the borderland it suggests that both caledonian and variscan structural trends n of the variscan front may be inherited from older pre silurian or even precambrian basement structures

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

tectonic rotation within the british paratectonic caledonides and early palaeozoic location of the orogen magnetizations of late ordovician caradoc early ashgill age in the paratectonic caledonides of wales and the lake district are directed westerly and northerly respectively they identify c 55 of relative rotation of probable acadian age southward subduction of iapetus ocean crust occurred in ordovician times beneath a latitudinally oriented orogen sited in mid southerly latitudes with the borrowdale and snowdonia volcanic provinces forming parallel fore arc and back arc lineaments respectively mid ordovician to mid devonian palaeomagnetic poles from the orogen correlate with contemporaneous gondwana data to identify a former location sited west of the south american perimeter and not near to north africa as assumed in previous reconstructions

sea ice conditions in the okhotsk sea during the last 550 kyr deduced from environmental magnetism 1 reconstructing past sea ice conditions in the okhotsk sea is important because sea ice conditions vary in response to global climate changes which in turn may affect global ocean circulation through intermediate water mass formation we conducted an environmental magnetic study of six cores from three stations in the central okhotsk sea to better understand temporal and spatial sea ice variations intercore correlations and age estimations are based mainly on geomagnetic paleointensity an oxygen isotope stratigraphy is available for one station magnetic susceptibility ms minima are accompanied by maxima in color b the ratio of the anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization karm sirm and the s ratio which indicates a higher proportion of biogenic to terrigenous magnetic components this reflects enhanced ocean productivity first order reversal curve diagrams and irm component analyses support the dominance of biogenic magnetite at ms minima in contrast color b karm sirm and s ratio values are low when ms is high which indicates an increased proportion of the terrigenous component that was probably transported as ice rafted debris ird for the southern two stations ird accumulation increased in glacial and deglacial periods which implies mobile sea ice conditions even in full glacials this was succeeded by extremely enhanced ocean productivity in early interglacials which suggests nearly ice free conditions for the northernmost station on the other hand ird accumulation was low in glacials and increased in early interglacials which indicates perennial sea ice coverage with little mobility in glacials succeeding ocean productivity enhancement was delayed compared to the southern stations

silurian paleogeography of armorica new paleomagnetic data from central bohemia due to the lack of reliable paleomagnetic data from armorica for mid ordovician to late devonian times the drift history for this tectonic element is poorly understood in order to help rectify this problem a paleomagnetic study of upper silurian volcanic intrusive and sedimentary rocks from the barrandian basin of the central bohemian massif czech republic has been carried out this basin comprises a sequence of unmetamorphosed lower paleozoic rocks the folding of which has an uppermost age limit of early carboniferous stepwise thermal demagnetization of the samples collected 182 samples from 21 sites and principal component analysis of both great circle and stable endpoint data reveal a total of three directions of magnetization the first labelled a is in the direction of the present day local earth s magnetic field d i 360 67 and is thus interpreted as being of recent origin the other two directions termed b and c comprise the intermediate and high blocking temperature directions respectively the overall in situ mean direction obtained for b identified between 150 and 320 c is 195 8 d i k 18 5 95 13 2 eight sites it fails the fold test and yields an in situ paleo south pole position of 42 s 354 e this coincides with the late carboniferous segment of the apparent polar wander apw path for stable europe and is considered to be a secondary overprint of this age the third magnetization termed c is identified between 320 and 600 c gives an overall mean direction of 208 40 k 96 8 95 5 7 eight sites after bedding correction it passes fold and reversal tests on both local and regional levels and is interpreted as representative of the late silurian paleofield direction in the bohemian massif it yields a paleo pole position of 55 s 325 e assuming a normal polarity the inclination value obtained translates into paleolatitudes of 23 s for this part of armorica and implies a position adjacent to the southern margin of baltica this therefore indicates closure of the intervening ocean by late silurian times however adopting this polarity also implies large scale up to 140 anticlockwise rotations of the bohemian massif prior to deformation in the early carboniferous

a new silurian palaeolatitude for eastern avalonia and evidence for crustal rotations in the avalonian margin of southwestern ireland summary palaeomagnetic data are presented from mid silurian homerian upper wenlock 425 ma sediments from the dingle peninsula sw ireland which forms part of thenorthern margin of the palaeozoic microcontinent of avalonia three remanencecomponents were recognized after removal of a low temperature component l oriented parallel to the present earth eld at the sampling area two higher stabilitycomponents were isolated an intermediate unblocking temperature component i with mean in situ d 196 9 i 11 0 a95 10 8 with a corresponding palaeopole at330 0 e 30 6 s dp 5 6 dm 11 0 and a high unblocking temperature component h with mean tilt corrected d 218 6 i 22 1 a95 7 9 with a correspondingpalaeopole at 309 5 e 18 3 s dp 4 4 dm 8 4 a primary wenlock age is indicatedfor the h component by a positive intraformational conglomerate test whereas the i component is thought to be a secondary mid carboniferous partial remagnetization these data con rm that the sector of the iapetus ocean between avalonia andlaurentia was essentially closed within the limits of palaeomagnetic resolution by thewenlock there is still however a discrepancy between the declinations recorded bysimilar aged sequences to the north and south of the iapetus suture these point toeither an approximately 30 clockwise rotation of the entire avalonian microcontinentrelative to laurentia during closure or local vertical axis rotations of the samplingsites in southern britain

is the welsh borderland fault system a terrane boundary the welsh borderland fault system forms one of the most prominent tectonic lines in southern britain and along with the menai straits and malverns fault systems is a candidate for a terrane boundry a review of stratigraphic linkage across the system reveals significant contrasts in pre ashgill sequences these contrasts occur particularly across its north west edge the pontesford and tywi lineaments rather than across the church stretton lineament the ashgill event produced strike slip dominated structure and an unconformity that expands from the welsh basin towards the platform an important linkage sequence was produced by the subsequent late ashgill to llandovery marine transgression across the fault system silurian facies changes are marked but gradational and no more than expected across a basin margin from stable to thinned crust devonian acadian sinistral transpression and later phanerozoic movements reactivated the fault system but did not engineer major terrane amalgamation across it this reactivation has obscured the early history of the fault system the remaining evidence admits though does not prove major ashgill or earlier transcurrent movements broader considerations suggest that the welsh borderland fault system originated either as a cambro ordovician extensional system on the south eastern margin of the welsh basin or during late precambrian early cambrian transcurrent movements occurring within avalonia inboard from the menai strait line

superimposed recent permo carboniferous and ordovician palaeomagnetic remanence in the builth volcanic series wales the builth volcanic series of llanvirnian age in llanelwedd quarries mid wales carries three components of natural remanent magnetisation component p regarded as primary is a thermochemical remanence directed at d 181 7 i 54 5 95 4 4 relative to bedding component s is a secondary component with in situ d 178 7 i 6 7 95 5 4 and is believed to be a low temperature chemical remanence crm of permo carboniferous age component r is directed close to the present geomagnetic field and is believed to be a recent viscous remanence vrm the results are of interest for three reasons first they are an unusually good example of multi component nrm analysis the three components being so clearly discriminated by thermal demagnetisation because they have almost completely separate blocking temperature ranges second they provide evidence of a permo carboniferous event possibly a mild thermal or hydrothermal pulse promoting crm acquisition some 40 km north of the hercynian orogenic front third they illustrate very clearly the importance of detailed demagnetisation this work revises the pole position for these rocks by 10 and removes an obstacle to the palaeomagnetic recognition of the 1000 km wide iapetus ocean cutting britain in ordovician time

a revised palaeozoic apparent polar wander path for southern britain eastern avalonia summary a revised apparent polar wander path apwp is presented for palaeozoic southern britain the new path based on a structural reinterpretation of existing data combined with new data from wales differs significantly from previous estimates in the following ways 1 the 1 ocus of the ordovician path segment is extended by approximately 25 degrees of arc 2 revised time calibration of the apwp suggests more ordovician apw than previously calculated and 3 the new path implies middle ordovician separation across the british sector of iapetus to be greater than previous palaeomagnetic estimates based on apwp analyses c 3300 km northern britain c 133 southern britain c 45s

palaeomagnetic results from the lower devonian llandstadwell formation dyfed wales discussion discussion sur l article de carola stearns et rob van der voo paru dans tectonophysics vol 143 no 4 p 335 336

discordant silurian paleolatitudes for central newfoundland new paleomagnetic evidence from the springdale group ancient remanences are retained by the early silurian 429 6 5 ma mafic volcamcs of the sprmgdale caldera five sites and the overlying red beds seven sites dual polarity magnetizations are obtained by thermal demagnehzation of samples from the red beds whereas single polarity d rectlons are observed m the volcamcs high unblockmg temperatures indicate hematite as the remanence carrier m both the volcamcs and sedxments these h gh temperature characteristic remanences are easily solated and pass both the tilt and conglomerate tests they are likely to be of primary silurian age characteristic dechnatlons are predominantly northerly and northeasterly and md cate s gmficant structural rotahons on a local scale when the results of the red beds and the voleamcs are combined they show characteristic mchnatlons that are shallower than those of the correlative botwood group ca 36 vs 43 but not nearly as shallow as those reported from the king george iv lake area 0 5 mean mchnatlons obtained from the sprmgdale red beds are however slgmficantly shallower than those of the sprmgdale volcanlcs the same difference can be seen m other previous silurmn paleomagnehc studies of central newfoundland we infer that an mcllnat on error affects the red bed magnetizations of the sprmgdale group botwood group wigwam formation and rocks of the king george iv lake area therefore the results from sdurlan red beds should not be used to determine paleolahtudes for central newfoundland the mean paleolatltude of the sprlngdale group volcamcs is 30 the mean paleolatltudes for both the sprlngdale volcanlcs and botwood volcamcs lawrenceton formation are md stmgmshable w thm paleomagnetlc error hm ts from the predicted paleolatltude of newfoundland on the northeast trendi ng north american margin thus no detectable post silurian displacement ts shown by the volcamcs of the sprmgdale group with respect to cratomc north america

reply to comment of a trench and t h torsvik on paleomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain

palaeomagnetic studies of fold development and propagation in the pembrokeshire old red sandstone abstract the geometry of two folds in lower devonian old red sandstone in pembrokeshire deformed in the hercynian has been defined at intermediate stages during their evolution by this preliminary palaeomagnetic study this is possible because remagnetization appears to have occurred in discrete episodes during deformation and the shape can be reconstructed by restoring the fold limbs to an angle at which the remagnetization directions coincide the plunge of the folds is shown to vary considerably during evolution one fold having an early easterly plunge which changed to the present westerly plunge the other fold having an early plunge which was more steeply to the west than its present attitude clockwise rotation of the whole of southern pembrokeshire by 40 towards the end of or since the hercynian deformation is indicated by the palaeomagnetic data and a structural model for this is proposed

paleomagnetic results from the lower devonian llandstadwell formation dyfed wales abstract a paleomagnetic investigation of the lower devonian llandstadwell formation in southwest wales has yielded a characteristic direction declination 196 inclination 5 that passes a fold test comparison of the corresponding virtual geomagnetic pole 334 e 39 s to previously published poles for great britain indicates that the magnetization is secondary and likely to be of carboniferous age no evidence for post hercynian rotation of pembrokeshire is indicated by our data

paleomagnetism of the dunn point formation nova scotia high paleolatitudes for the avalon terrane in the late ordovician volcanic flows of the late ordovi clan dunn point formation contain a generally univectorial magnetization which passes the fold test its steep direction d 335 o i 61 o k 79 95 4 2o paleopole at 2os 136oe is un like any known for north america or for the ava lon terrane for post ordovician time so that a primary age of the magnetization appears to be very likely the paleoiatitude 42os for the avalon terrane derived from this result is much higher than that predicted for the area on the basis of the cratonic north american apparent polar wander path and a substantial post ordo vician displacement 3500 km of avalon with respect to the craton can be deduced in all likelihood the avalon terrane did not collide with north america until middle devonian time this collision produced the acadian orogeny high ordovician paleolatitudes have also been obtained for northern africa and for several localities in

british paleomagnetism iapetus ocean and the great glen fault new paleomagnetic data for britain establish the width of the iapetus ocean as 1000 km in mid ordovician time but do not yet allow precise estimates of the rate of its subsequent closure the new lower paleozoic data from northern scotland are similar to data from the rest of britain but are not compatible with north america data on the bullard et al reassembly if this discrepancy is correct lines of major transcurrent dislocation have yet to be identified either in the north atlantic continental shelves and margins or in eastern north america and the great glen fault was not the main locus of offset between britain and north america the tectonic complexities associated with this configuration require further evaluation

palaeomagnetism of late ordovician igneous intrusions from the northern welsh borderlands implications to motion of eastern avalonia and regional rotations the palaeomagnetism of a late ordovician dolerite suite widely distributed in the northern welsh borderlands has been investigated to evaluate i regional rotations associated with later deformations and ii lower palaeozoic latitudes of eastern avalonia only local effects of acadian and variscan overprinting are observed in this region and most bodies appear to preserve a primary cooling related magnetization magnetic properties in the breidden dolerite are related to primary igneous differentiation and a dual polarity characteristic remanence d i 333 1 71 6 6 sites 95 10 4 predates late ordovician folding dolerites elsewhere in the breidden hills inlier record the same reversal of remanence and the combined mean d i 314 3 72 8 is rotated clockwise from the mean remanence d i 291 5 59 5 21 sites 95 6 0 in a similar dolerite suite within the adjoining shelve inlier which is also of dual polarity and predates late ordovician folding the difference in magnetic declinations is similar to the rotation of late ordovician fold axes and fracture systems between the two inliers and is probably the resultant effect of local rotation possibly about a constraining bend in the severn valley fault system the moel y golfa andesite of caradocian soudleyan age has a remanence d i 294 3 50 4 13 samples 95 2 8 which study of an adjoining volcanic conglomerate shows to be of primary cooling related origin the equivalent palaeolatitude is 31 s in early caradocian times a similar estimate is derived from a probable diagenetic overprint in the contemporaneous castle hill conglomerate at montgomery the mynydd bryn andesite in the berwyn inlier of probable similar origin and age to moel y golfa has a magnetization of opposite polarity but similar inclination d i 239 1 47 4 14 samples 86 5 5 declination differences between these volcanic outcrops are a possible signature of later block rotations between the lower palaeozoic inliers and identify the importance of terrane definition in analysis of lower palaeozoic palaeomagnetic results from the caledonides a dolerite sheet complex at hendre in the berwyn hills has a younger remanence d i 182 3 3 0 20 samples 95 7 8 probably of mid carboniferous age the collective upper ordovician results define a rapid movement of the avalonian plate probably spanning no more than 10 ma from 30 s in early caradocian times to 57 s prior to folding of the dolerites in late caradocian to early ashgill times this motion into higher southerly palaeolatitudes marks a reversal of the motion of avalonia into more shallow southerly palaeolatitudes between arenig and llanvirn times and shows that ordovician closure of the iapetus ocean was more complex than recognized hitherto

palaeozoic palaeomagnetic studies in the welsh basin recent advances in the last two years new palaeomagnetic data fro m wale s have resulted i n radical revision of the ordovician palaeogeography of eastern avalonia part of the southern margin of the iapetus ocean combined with palaeozoic palaeomagnetic data from laurentia and gondwana these data suggest that eastern avalonia was a peri gondwanide high latitude continental fragment during at least part of ordovician time with a palaeolatitude of about 62 s and 51 s in arenig and llanvirn time respectively this implies a latitudinal width of the early ordovician iapetus ocean between eastern avalonia and laurentia of at least 30 geological evidence for the proximity of eastern avalonia and laurentia suggests that the intervening iapetus ocean closed during silurian time from late llandovery to early ludlow recent palaeolatitude data from the iapetus bordering continents are consistent wit h closure b y middle to late silurian time new pre acadian early devonian palaeomagnetic data from the old red sandstone places the welsh basin at about 17 s consistent with a palaeogeography in which laurentia baltica avalonia armorica and possibly gondwana wer e part o f a singl e supercontinent pervasiv e lat e carboniferous early permia n remagnetization affect s the welsh basin the remagnetization i s probably associated with fluids emanating from th e variscan thrust front w e do not observe remagnetization associate d with acadian orogen y and the remagnetizations which have been studied in more detail in north america appear to be a unique feature of the variscan hercynian alleghenian orogeny

multiphase reddening and weathering events in upper carboniferous red beds from the english west midlands sediments of the halesowen and salop formations westphalian d stephanian in the west midlands were deposited on the southern margin of the pennine basin during a period of relative tectonic quiescence during the variscan orogeny the clastic framework of the predominantly red sandstones and mudstones in the salop formation includes large detrital hematite grains these yield a primary component of magnetization indicating that sediments were deposited close to the carboniferous palaeo equator variscan uplift erosion and subsequent deep oxidation in an arid climate during the permian resulted in the precipitation of finer hematite in rocks of both formations which are now found at depths of over 150 m below the present day ground level these grains carry secondary magnetizations which indicate formation at palaeolatitudes of between 5 and 15 n the position that britain occupied during the permian the pattern of permian remagnetization particularly in borehole samples suggests that the secondary fine grained hematite was precipitated as a result of lateral flow of meteoric water through connected permeable sandstone units rather than downward percolation through the entire succession present day surface weathering has resulted in the dissolution of the finer grains of hematite and in some cases where permeabilities are relatively high it has also affected the coarser fraction consequently the record of late carboniferous and permian magnetization events is commonly incomplete in strata close to the present day ground level