reliability of palaeozoic palaeomagnetic poles and apwp of gondwanaland abstract a review of reliable palaeomagnetic data from gondwana palaeozoic rocks supports the apparent polar wander path apwp initially proposed by morel and irving this path or versions of it has recently gained favour with a number of groups the apwp suggests that during the mid palaeozoic gondwanaland was very mobile no palaeomagnetic pole position s has been yet reported that confirms the contentious segment of apwp from the late ordovician to the early silurian the apwp implies that the south pole moved rapidly from north africa in the ordovician to a position off southern south america by the silurian back to central africa by the early carboniferous and across gondwanaland to australia by the late carboniferous the distribution of palaeozoic tillites independently supports such a mobilistic gondwanaland the palaeomagnetic and tectonic evidence are compatible with the lachlan fold belt of australia having been in place since the mid devonian

an early carboniferous paleomagnetic pole for gondwanaland new results from the mount eclipse sandstone in the ngalia basin central australia paleomagnetism of fourteen diamond drill cores from the uppermost devonian lower carboniferous mount eclipse sandstone in the ngalia basin central australia has been studied the samples were oriented using the borehole directions bedding planes identified in the drill cores and regional bedding attitudes three magnetic remanent components have been revealed 1 a drilling induced low to moderate temperature remanence c1 from all the drill cores which is oriented parallel to the drill cores and is most likely a combination of isothermal remanent magnetization and piezoremanent magnetization 2 a moderate to high temperature syndeformational overprint c2 d 226 7 i 82 3 95 6 2 k 68 9 in 65 samples from nine drill cores which was probably acquired during the mid to late carboniferous mount eclipse movement and 3 a moderate to high temperature likely primary remanence c3 d 057 1 i 36 9 95 9 6 k 29 6 from 64 samples in nine drill cores which exhibits a positive fold test c2 and c3 are found only in reddish siltstones and sandstones whereas c1 is best developed in grey reduced sandstones and siltstones a palynological examination gives an early to mid visean early carboniferous age of deposition for samples carrying the interpreted primary component c2 gives a mid to late carboniferous pole at 32 1 s 119 5 e with a95 11 9 which is in agreement with results of a previous study on surface outcrops c3 when combined with results from one drill core in a previous study gives a visean pole at 37 6 s 52 6 e with a95 8 7 this pole suggests a south polar position for central africa during the early carboniferous and therefore indicates the existence of a large ocean between gondwanaland and laurussia at that time the paleo south pole shifted from central africa in the early carboniferous 340 5 ma to east antarctica in the mid to late carboniferous 310 10 ma with a minimal continental drifting rate of 15 20 cm yr

palaeomagnetism of devonian ring complexes from the bayuda desert sudan new constraints on the apparent polar wander path for gondwanaland summary the approach of gondwana and or microplates derived from its northern margins towards laurentia and the closure of the intervening palaeo tethys is a controlling factor in the geodynamic evolution of the atlantic bordering continents during the early and mid palaeozoic however the currently available palaeomagnetic data for gondwana especially for the geodynamically critical silurian to devonian interval are still rather sparse and are compatible with a number of palaeogeographic scenarios in order to address this issue 114 samples 18 sites have been collected from a series of high level acidic intrusive rocks rhyolitic lavas and tuffs of mid devonian age 377 5 ma rb sr along the southern escarpment of the gilif hills sudan 17 83 n 32 67 e detailed demagnetization experiments revealed the presence of three directionally distinct components of magnetization labelled a b and c the overall mean direction of component a dec 0 9 inc 29 8 95 4 6 k 91 1 n 12 sites is indistinguishable from either the direction of the present field or the geocentric axial dipole field in northern sudan mixed polarities suggest a recent or pleistocene age of this component component b also displays mixed polarities the site mean directions for this component average to 6 5 declination 40 2 inclination with 95 7 0 and k 48 1 based on 10 sites and yield a palaeomagnetic pole at 48 8 s 23 5 e dp 5 1 1 dm 8 4 maghaemite and haematite have been identified to be the predominant magnetic phases in samples which carry component a and or b both a and b are the probable results of low temperature oxidation of magnetite although the age of component b can not be directly determined it is inferred by correlation with published data to reflect a late carboniferous overprint component c identified in 11 sites 39 samples is characterized by a very steep downward magnetization dec 296 2 inc 79 3 95 10 8 k 18 7 n 11 sites which corresponds to a palaeomagnetic pole position in the southwestern corner of libya 25 9 n 11 6 e dp 19 6 dm 20 6 maximum blocking temperatures of component c generally below 580 c are indicative of magnetite as carrier of this remanence the results of thermal remanent magnetization experiments demonstrate that component c is a thermo remanent magnetization probably primary and suggest this component to be representative for the direction of the earth s magnetic field during emplacement of the gilif hills volcanic rocks in mid devonian times the position of the resulting palaeomagnetic pole lends new support to geodynamic scenarios advocating the existence of a wide approx 5000 km pre hercynian ocean during devonian times the simplest reconciliation of component b with the reference apparent polar wander path is achieved by assuming it to be of carboniferous age and by invoking a westward cusp in the carboniferous part of the apparent polar wander path taken at face value the age of this cusp coincides with large scale post orogenic right lateral strike slip movements in hercynian europe and north africa this interpretation does not require the apparent polar wander apw loop and very high rates of plate motion implied by some alternative models however if it is valid it will require reinterpretation of the remanence age of one west african intrusive suite and of some eastern australian palaeozoic rocks

late devonian carbonate magnetostratigraphy from the oscar and horse spring ranges lennard shelf canning basin western australia a b s t r a c t the late devonian was a time of major evolutionary change encompassing the fifth largest mass extinction the frasnian famennian event in order to establish a chronological framework for global correlation before during and following the frasnian famennian mass extinction we carried out a coupled magnetostratigraphic and biostratigraphic study of two stratigraphic sections in the upper devonian carbonate reef complexes of the lennard shelf in the canning basin western australia magnetostratigraphy from these rocks provides the first high resolution definition of the late devonian magnetic polarity timescale a 581 m reference section and an 82 m overlapping section through the marginal slope facies napier formation of the oscar range as well as a 117 m section at horse spring virgin hills formation were sampled at decimeter to meter scale for magnetostratigraphy conodont biostratigraphy was used to correlate both sections and link magnetostratigraphic polarity zones to a globally established biostratigraphy as table characteristic remanent magnetization chrm with dual polarities ne shallowly upward and sw shallowly downward is recovered from 60 of all samples with magnetite inferred to be the chief magnetic carrier from thermal demagnetization characteristics these directions define a geomagnetic pole at 49 5 s 285 8 e and 95 2 4 n 501 placing the canning basin at 9 9 s during the late devonian consistent with carbonate reef development at this time ac onservative interpretation of the magnetostratigraphy shows the recovery of multiple reversals from both sections not including possible cryptochrons and short duration magnetozones field tests for primary remanence include positive reversal tests and matching magnetozones from an overlapping section in the oscar range as trong correlation was found between magnetic polarity stratigraphies of the oscar range and horse spring sections and we correlate 12 magnetostratigraphic packages the relative stratigraphic thicknesses of the isochronous sediments from these two sections indicate that carbonate accumulation was 4 5 faster in the middle slope deposits at oscar range than in the more distal lower slope horse spring deposits for the middle frasnian through famennian the magnetic field during the late devonian underwent a relatively high reversal frequency with good potential for regional and global correlation and should prove useful in deciphering a high resolution chronostratigraphy across the lennard shelf to enable higher confidence examination of reef development across a major biotic crisis

fish from the upper devonian of the shan thai terrane indicate proximity to east gondwana and south china terranes we report the first discovery of abundant devonian vertebrates from thailand an unusual form of coronodontid shark tooth with six equal sized cusps occurs only in the upper devonian of thailand shan thai terrane and south china and a new species of phoebodus is reported that occurs elsewhere only in south china and australia east gondwana terrane the occurrence of the chondrichthyan harpagodens thrinacodus ferox turner 1982 in the late devonian of thailand australia and south china predates the appearance of this species in euramerica early carboniferous these data suggest late devonian proximity of the shan thai south china and east gondwana terranes and is in accord with recent paleomagnetic data

plate reconstructions using stromatolite heliotropism principles and applications fossilized columnar stromatolites and other microbialites when heliotropic incline toward averaged incident solar radiation and preserve information useful for plate reconstructions some fast growing small diameter living heliotropic microbialites form sinusoidal columns by recording seasonal changes in solar inclination others simply incline toward the equator fossilized heliotropic stromatolites have the same potential for recording information about past positions of the sun relative to the strata they are preserved in and can be used to infer lines of paleolongitude plane of column inclination and or sinusoidal oscillation and paleolatitude amount of net inclination to reconstruct past configurations of continents using heliotropic stromatolites the apparent deviation from vertical growth in individual columns is measured corrected for refraction and strain and used as a minimum paleolatitude the plane perpendicular to beds and containing the inclined stromatolite columns is equated with

paleomagnetism of upper devonian reefal limestones canning basin western australia two conflicting models have been proposed for the middle paleozoic position of gondwana with respect to euramerica a pole from morocco msissi norite that may be of late devonian age suggests that a broad ocean 3 000 km separated the northern continents from gondwana whereas results from mauritania gneiguira supergroup indicate that this ocean had closed by devonian time detailed thermal and alternating field demagnetization experiments have been carried out on red and gray limestones from seven sites 89 samples in an essentially undeformed late devonian reef complex northern canning basin western australia after minor tilt correction the site mean characteristic directions average 42 7 28 9 declination inclination with k 61 6 and 95 7 8 these results position northwestern australia at 15 s latitude in the late devonian a paleolatitude consistent with the presence of a well developed reefal sequence an early age is inferred for these magnetizations on the basis of the occurrence of mixed polarities and the fact that no post devonian directions for australia resemble the observed declinations and inclinations if the smith and hallam reconstruction for gondwana is used the paleomagnetic pole calculated for the late devonian of western australia agrees with the msissi norite result from morocco and suggests that an ocean still separated gondwana from euramerica in the late devonian the pole measured from the gneiguira supergroup in mauritania may be based on remagnetizations of late paleozoic age the directions determined from canning basin limestones are similar in inclination to but differ by 40 in declination from paleomagnetic results from the upper devonian comerong volcanics lachlan fold belt southeastern australia canning basin samples are from the stable craton and comerong volcanics samples are from the tasman fold belt the discrepancy between the two australian results may be best explained by tectonic rotation of the comerong volcanics locality

δ18o values 87sr86sr and sr mg ratios of late devonian abiotic marine calcite implications for the composition of ancient seawater abstract late devonian frasnian abiotic marine calcite has been microsampled and analyzed for 87 sr 86 sr ratios 18o and 13c values and minor element concentrations portions of marine cement crystals from the alberta and canning basins have escaped diagenetic alteration and preserve original marine 18o values 4 8 0 5 pdb 13c values 2 0 to 3 0 pdb 87 sr 86 sr ratios 0 70805 3 and sr mg weight ratios 0 04 to 0 05 marine 87 sr 86 sr ratios are globally consistent and can be correlated within the alberta basin and among the alberta canning and williston basins correlation of isotopic and chemical data strengthen the conclusion that marine cements from the leduc formation preserve original marine 18o values which are 3 to 4 lower than those of modern marine cements these low 18o values are best explained by precipitation from 18o depleted seawater and not by elevated seawater temperature or diagenetic alteration for comparison with devonian data analogous data were collected from holocene mg calcite and aragonite marine cements from enewetak atoll marshall islands mg calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater and mg calcite cements are homogeneous with respect to sr and mg contents empirically derived homogeneous distribution coefficients for mg and sr in modern abiotic mg calcite from enewetak atoll are 0 034 and 0 15 respectively an equation describing the dependence of dsr on mg content was based on a compilation of sr and mg data from holocene abiotic marine calcite dsr 3 52 10 6 ppm mg 6 20 10 3 unlike that derived from experimental data this sr mg relation is consistent over a range of 4 to 20 mol mgco3 and may represent precipitation phenomena which are minimally controlled by kinetic effects comparison of sr and mg contents of analogous devonian and holocene marine cements suggests that the mg ca ratio of late devonian seawater was significantly lower and that the sr ca ratio was significantly higher than that of modern seawater

a model for stress controlled pipe growth abstract the rock mechanics theory for deformation of underground mining excavations under high stress conditions can be used to explain the growth and geometry of volcanic pipes in an underground excavation stress concentrates greatest on the sides of an excavation perpendicular to the principal vector of compression if the stress is high enough fractures will develop causing scaling of the tunnel sidewalls and tunnel growth perpendicular to the principal vector of compression pre existing structures aid the physical mechanisms of pipe growth such as gravitational collapse explosive fragmentation and turbulent erosion and by reducing the strength of the rock mass should also aid stress induced scaling universal distinct element code numerical modelling demonstrated in this study reproduces the stress conditions around a circular pipe under uniaxial compression and simulates pipe growth as wedges bounded by failed pre existing joints form around the pipe and are assimilated into the pipe the results show how a volcanic pipe will tend to grow perpendicular to the principal vector of compression if the internal magma pressure is low or absent the orientations of the pre existing joints affect the exact direction of pipe growth in a predictable manner examples from other publications demonstrate that the model is consistent even in extensional tectonic environments case studies from kimberlite occurrences in the limpopo belt at finsch mine and the gross brukkaros volcanic complex demonstrate that dykes and magmatic bodies of kimberlite with high overpressures during emplacement normally have geometries trending near parallel to the principal vector of compression yet pipes or parts of pipes that underwent strong underpressures during emplacement often ending up comprising fragmental volcaniclastic infill have an elongation near perpendicular to the same vector thus the stress induced pipe growth model is demonstrated to be important for some pipes the study of volcanic pipe and dyke shapes can therefore be used to determine the stress regime at the time of emplacement and to distinguish between kimberlite occurrences formed at different times within different stress tensors

polar wander and paleomagnetic reference pole controversies the purpose of this paper is to review the progress made over the past four years in studies of phanerozoic and precambrian polar wander with an emphasis on the contributions made by u s scientists we perceive a general theme embracing the notion that the accuracy of cratonic reference poles and therefore of apparent polar wander apw paths is not as good as was generally believed four years ago a renewed interest in research concerning cratonic reference poles is largely the result of paleomagnetic studies in orogenic belts namely the western cordillera and appalachians of north america the andes of south america and elsewhere which seek to document the existence or non existence of displacements of suspect terranes i e rotation and or translation with respect to their presently associated cratons some of the results of this research are reviewed in this volume by hillhouse and mcwilliams an important realization of which paleomagnetists have long been aware but which was demonstrated recently by a number of sobering examples for both the paleozoic and mesozoic apw paths for north america is that the reliability of concordance discordance parameters that define terrane displacement is dependent not only on the precision and accuracy of the individual paleomagnetic study but also on the precision and accuracy of the appropriate cratonic reference pole

syn deformational remanent magnetization of the mount eclipse sandstone central australia summary the palaeomagnetism of the latest devonian early carboniferous mount eclipse sandstone in central australia reveals samples from 16 out of 31 sites having a consistent hightemperature component of natural remanent magnetization site mean directions from the two limbs of the most detailed studied syncline attain their best grouping after 75 per cent of unfolding and further unfolding causes a cross over feature mineralogical studies using microscope and electron microprobe show that this remanance is most likely to be carried by secondary haematite formed by alteration of fe rich minerals during the midcarboniferous deformation four different models are examined to reconstruct the palaeohorizontal for the syn deformational remanence the best estimate of the palaeomagnetic south pole position is lat 33 8 s and long 121 2 e dp 19 2 drn 19 7 close to australia this result indicates a rapid polanvard movement of eastern gondwanaland during the first half of the carboniferous we also suggest that the widespread alice springs orogeny related overprints in central australia were acquired during the beginning of the main phase of deformation in the mid carboniferous

paleomagnetic results indicate pre permian counter clockwise rotation of the southern tamworth belt southern new england orogen australia 1 visean ignimbrites from three fault bounded blocks within the southern tamworth belt southern new england orogen australia retain a prefolding high temperature component of remanence carried mainly by magnetite paleomagnetic data indicate that the southern tamworth belt occupied low to moderate paleolatitudes during the early carboniferous normal and reversed polarities with low to moderate inclinations suggest that the steep reverse polarity latest carboniferous to earliest permian overprint is absent from the high temperature component data set the mean characteristic remanence direction of three early to middle visean ignimbrite flows 32 sites from the rouchel block is declination dec 154 2 and inclination inc 38 8 95 12 5 k 98 8 which corresponds to a paleo south pole at latitude 64 9 s longitude 258 1 e dp 5 5 dm 9 2 where dp is the angular length of the semiaxis of the ellipse of confidence of the calculated paleopole that lies along the pole to site great circle and dm is the angular length of the semiaxis perpendicular to dp six middle to late visean ignimbrite flows 29 sites from the gresford block retain a mean remanence of dec 159 6 and inc 54 1 95 13 7 k 24 7 corresponding to a paleo south pole at latitude 72 8 s longitude 228 4 e dp 13 4 dm 19 2 despite widespread overprinting throughout much of the myall block reliable information was obtained from three late visean ignimbrite flows five sites with a mean direction of dec 118 1 and inc 42 5 95 12 0 k 41 6 from which a paleo south pole of latitude 35 7 s longitude 233 4 e dp 9 1 dm 14 7 is calculated the calculated paleo south poles do not fall on the early carboniferous segment of the published apparent polar wander path for australia and suggest that the southern tamworth belt has undergone significant counter clockwise rotation relative to cratonic australia inferred rotations for the rouchel gresford and myall blocks are approximately 80 80 and 120 respectively stratigraphic and geological evidence suggest rotation took place between the mid namurian and latest carboniferous and is thought to have resulted from sinistral strike slip during compression at the eastern margin of australia

paleomagnetic study of siluro devonian volcanic rocks from the central lachlan orogen implications for the apparent pole wander path of gondwana 1 the apparent pole wander apw path for gondwana is still not clearly established in particular for silurian devonian times a controversial debate places authors who argue for an x path running directly through africa on a reconstruction of gondwana against those who advocate a large loop passing by southern south america the y path most of the paleomagnetic data used to draw this loop come from the lachlan orogen australia a paleomagnetic study was carried out in the well dated ambone and ural volcanics in the central subprovince of lachlan orogen new south wales anisotropy of magnetic susceptibility measurements confirms detailed mapping of the region and shows that these massive dacitic sills and or lava flows are flat lying among the different localities studied only one yields interpretable paleomagnetic results two components of magnetization can be identified a midtemperature direction yielding a corresponding pole in australian coordinates 67 9 s 084 4 e b 5 n 21 dp 17 5 dm 23 1 and a high temperature direction with a corresponding vgp 24 4 s 060 6 e b 5 n 25 dp 1 4 dm 2 5 the first is interpreted as corresponding to an early carboniferous pole position and can be regarded as an overprint probably related to the early carboniferous kanimblan orogenic event the second does not correspond to any expected silurian devonian or younger pole position this magnetization is thought to be primary in origin however secular variation has apparently not been averaged out in the single lava flow sampled therefore the earliest devonian paleopole position probably lies in a 30 cone around the obtained vgp and this position can only match the x type apw path for gondwana it is in particular very different from coeval poles obtained in the eastern subprovince of the lachlan orogen and it is mostly used as key poles supporting the silurian devonian loop for the apw path of gondwana therefore some poles from the lachlan orogen must be affected by rotation to explain these results the lachlan orogen was hence not stable up to the mid paleozoic and data from this region should not be used as representative for gondwana

amplitude interpretation of 3 d reflection data large 3 d seismic surveys are now routinely used to delineate and assist in the development of hydrocarbon reservoirs during the acquisition and processing of modern reflection data special attention is given to recording and preservation of amplitude values the reflected seismic wave amplitude depends on three groups of factors

biogenic magnetite from surface holocene carbonate sediments great bahama bank magnetic separates and rock magnetic tests from holocene carbonate surface sediments of the bahamas indicate that fine grained magnetite maghemite is common at or near the sediment water interface and forms a major portion of the magnetic fraction magnetite crystals range from 400 to 1000 a in diameter as observed under the transmission electron microscope tem and often occur as elongate chains or multi grain clusters crystals are very similar in dimension and grain habit to those of known biogenic origin mainly bacteria all measured grains are within the single domain stability field for magnetite based on crystal dimensions coercivity distributions are fairly broad 5 100 mt or greater likely the result of partial oxidation of the magnetic minerals no large multidomain grains were observed in the tem study individual grains occur in several shapes in the microscope image mainly hexagonal and cuboidal prismatic widespread occurrence of single domain magnetite in this isolated carbonate setting suggests that biogenic precipitation is a major contributor of early in situ depositional magnetization a contribution of magnetite from authigenic precipitation during burial has not been documented in these near surface environments

palaeomagnetism of the brewer conglomerate in central australia and fast movement of gondwanaland during the late devonian summary palaeomagnetic analysis of 11 diamond drill cores from the late devonian brewer conglomerate in the northeastern part of the amadeus basin central australia yields three magnetic components a drilling induced remanence c1 acquired during the industrial drilling process a mid to late carboniferous syndeformational overprint c2 acquired during the alice springs orogeny and a primary remanence c3 c3 passes a fold test and gives a latest devonian pole bc at 47 1ds 041 0de with a95 6 4d the revised apwp for gondwanaland requires west gondwanaland to drift across the south pole during the late devonian with a speed of 20cm yr 1 while east gondwanaland remained at low latitude positions it is suggested that fast plate movement occurred far more often during the palaeozoic than has previously been thought

tertiary remagnetization of paleozoic rocks from the eastern cordillera and sub andean belt of bolivia paleomagnetic samples were collected from 98 sedimentary horizons in eight different devonian to permian sedimentary units at eight localities in the eastern cordillera and the sub andean belt of bolivia for 77 sites thermal demagnetization allowed determination of a characteristic magnetization chrm with site mean 95 confidence limit 95 15 the chrm is carried predominantly or entirely by hematite fold and reversal tests from two of the sampled localities indicate that the characteristic magnetization is synfolding likely acquired during the earliest stages of deformation additionally a modified conglomerate test at one locality and the nearly uniform direction of chrm across the devonian to permian age units clearly reveals the secondary nature of the characteristic magnetization finally the chrm directions are discordant from any expected paleozoic directions paleomagnetic poles calculated from the chrm directions fall near the cenozoic portion of the apparent polar wander path for south america we interpret these observations to indicate widespread chemical remagnetization of these paleozoic strata during but prior to completion of cenozoic andean folding

paleomagnetism of the western cape fold belt south africa and its bearing on the paleozoic apparent polar wander path for gondwana in order to test two different proposals for the poorly defined african paleozoic apparent polar wander path apwp a paleomagnetic study was carried out on ordovician through carboniferous clastic sediments from the cape fold belt west of the 22nd meridian one proposal involves a relatively simple apwp connecting the ordovician gondwana poles in north africa with the late paleozoic poles to the east of south africa in a more or less straight line crossing the present equator in the devonian the other proposal adds a loop to this path connecting ordovician poles in north africa with poles to the southwest of south africa and then returning to central africa this loop would occur mainly in silurian time new results reported herein yield paleopoles in northern and central africa for ordovician to lowermost silurian and lower to middle devonian formations the best determined paleopole of our study is for the early ordovician graafwater formation and falls at 28 n 14 e k 25 0t95 8 8 n 28 samples the other paleopoles are not based on sufficient numbers of samples but can help to constrain the apparent polar wander path for gondwana our results give 0nly paleopoles well to the north of south africa and we observe no directions within the proposed loop hence if the loop is real it must have been of relatively short duration 60 70 ma and be essentially of silurian early devonian age implying very high drift velocities for gondwana with respect to the pole during that interval

paleomagnetism of paleozoic strata brabant and ardennes massifs belgium implications of prefolding and postfolding late carboniferous secondary magnetizations for european apparent polar wander paleozoic carbonate rocks of the brabant massif namur basin and dinant synclinorium belgium are characterized by two secondary late carboniferous magnetizations of uniform reverse polarity a postfolding low unblocking temperature 300 375 c component lt and a prefolding high unblocking temperature 400 550 c component ht in the namur basin lt magnetizations postdate late westphalian deformation 305 ma in the dinant synclinorium ht magnetizations predate early westphalian deformation 316 ma lt magnetizations for the combined brabant massif and namur basin sites give a south paleopole at 42 3 s 339 6 e dp 1 9 dm 3 8 consistent with the european cratonic reference the lt magnetizations in the dinant synclinorium give a paleopole at 36 2 s 321 0 e dp 2 4 dm 4 7 the ht magnetization gives paleopoles at 34 1 s 333 4 e dp 2 6 dm 5 1 brabant and namur and at 25 9 s 321 5 e dp 3 5 dm 6 8 dinant synclinorium declinations indicate 15 clockwise rotation of the dinant synclinorium with respect to the brabant massif after acquisition of the lt magnetization devonian red beds yield near univectorial apparent synfolding magnetizations of distributed unblocking temperature that we interpret as artifacts of the remanence acquisition or deformation processes the ht paleopole for the brabant massif is indistinguishable from other early late carboniferous european paleopoles and is similar to poles interpreted to be derived from remagnetizations in the british isles age constraints on the time of remanence acquisition of secondary magetizations allow a refined calibration of the european apparent polar wander path inclinations of the lt and ht components indicate about 6 8 700 km northward motion during the late carboniferous

an analysis of the effects of thermal demagnetization on magnetic carriers we have attempted to determine why samples of the mio pliocene purisima formation are not amenable to thermal demagnetization analyses of the changes in direction intensity susceptibility color and saturation magnetization as well as examination of individual magnetic grains using a scanning electron microscope have all contributed to the development of a consistent hypothesis for the fate of iron during thermal demagnetization in this particular case we believe that oxidation exsolution of ilmenite and then direct dissolution of nearly pure magnetite result from progressively higher heating levels our approach should be useful in understanding the origin and nature of the magnetization of many lithologies including those for which thermal demagnetization is the method of choice

paleomagnetism of the carboniferous tepuel group central patagonia argentina samples of the carboniferous glaciogenic sequences of the tepuel group western central patagonia argentina were collected from 23 sites in two localities sierra de tepuel 43 7 s 70 5 w and las salinas 43 2 s 70 3 w most were fine sandstones to siltstones of late visean to westphalian age a characteristic remanent magnetization was determined in most sites by alternating field and or thermal cleaning at sierra de tepuel all samples were partially overprinted by a secondary magnetization apparently related to mesozoic magmatic activity in this area however site mean directions of the characteristic remanence from both localities passed a tilt test indicating a pretectonic pre early permian magnetization isothermal remanent magnetization experiments showed magnetite as the main and often the only magnetic carrier while thin section analyses indicated a detrital origin for this mineral suggesting a depositional or postdepositional magnetization the paleomagnetic pole for the tepuel group namurian westphalian at 316 1 e 31 7 s number of sites n 16 minor and major semi axes of confidence p 15 0 m 16 0 is considered a key pole for the middle carboniferous of south america and gondwana confirming the apparent polar wander path for the supercontinent for which the pole moved from central to southern africa in the early carboniferous followed by a rapid motion from southern africa to antarctica during the late westphalian the agreement of the tepuel pole with this gondwanan path confirms that patagonia was already part of south america by the middle carboniferous

the sabaloka ring complex revisited palaeomagnetism and rockmagnetism summary the isotopically dated lower to middle devonian sabaloka complex 16 3 n 32 6 e in northern sudan for which a palaeomagnetic pole position close to the present geomagnetic pole had been derived in a previous investigation has been revisited the formerly determined direction of remanence is shown to be of secondary origin possibly due to recent or pleistocene weathering as shown by ore microscopy and a negative conglomerate test and a further component of magnetization is detected at several sites rockmagnetic studies give strong arguments in favour of a primary origin of the newly found remanence direction in this paper the palaeomagnetic results site means of the two laboratories munich and leeds for the sabaloka complex have been combined yielding d 340 1 i 43 9 n 15 k 20 6 ag5 8 0 this gives a palaeomagnetic south pole position site mean at 42 0 s and 55 7 e which is discussed in the context of other palaeozoic pole positions of gondwana

oxygen isotope and mg ca composition of late visean mississippian brachiopod shells from sw iberia palaeoclimatic and palaeogeographic implications in northern gondwana abstract latest visean brachiopod shells gigantoproductus sp from the pedroches basin of sw spain were sampled in order to obtain chemical and isotopic evidence that may help first to assess if primary geochemical and isotopical signals are preserved in these shells and second to reconstruct temporal variations in palaeoceanographic and palaeoclimatic conditions in northern gondwana during this time span microtexture cathodoluminescence cl and ultrastructure under sem of the brachiopods were examined to avoid sampling of shell portions altered after deposition shell portions classified as non luminescent nl and non luminescent to slightly luminescent nl sl present geochemical signatures within the ranges considered normal for recent brachiopods that have not undergone significant diagenetic modification these include low to undetectable fe and mn contents high sr values and 18o values similar to those of carbonates in equilibrium with seawater on the other hand valve areas that are luminescent l or moderately luminescent with luminescent portions ml l present higher fe and mn contents lower sr concentrations and lower 18o values than those corresponding to nl and nl sl shells this suggests a greater diagenetic alteration of these portions and that they acted as microzones of reaction with the diagenetic fluids temporal trends through the studied interval show a spread in 13c values much lower than that observed for contemporaneous samples up to 1 6 thus obscuring the 13c global signal at the time scale resolution of this study in contrast the 18o record of brachiopod calcite 18oc shows an overall increase of 1 relative to v pdb from the oldest to the youngest samples this is accompanied by a 30 decrease in mg ca ratios of the brachiopod calcites confirming that at least part of this shift in 18oc was related to cooling combination of mg ca and 18oc proxies established in this paper although not full proof for brachiopods yet allowed for the calculation of a temperature drop of 3 3 c and a change in seawater 18ow that becomes 0 4 more positive during this interval the data are consistent with regional seawater cooling that accompanied an increase in continental ice volume over a relatively short time period of

paleomagnetism of the hervey group central new south wales and its tectonic implications paleomagnetic work on the hervey group in southeast australia reveals a predeformational magnetization as suggested by the retention of a predeformational compaction produced magnetic fabric and the effect of deformation on both the magnetic fabric and the direction of the magnetic remanence in cleaved rocks the formation mean direction d 29 2 i 19 3 with 95 15 5 gives a latest devonian early carboniferous palaeomagnetic pole at 54 4 s 24 1 e with dp 8 4 dm 16 2 comparing this pole with existing poles from australia and north america it is suggested that 1 the lachlan fold belt of southeast australia has been part of cratonic australia since at least the late devonian and consequently the late palaeozoic palaeomagnetic data from the lfb may be applied to the whole of gondwanaland 2 cratonic australia and the new england fold belt were all in an equatorial position during the early carboniferous 3 there was probably a rapid anticlockwise rotation of gondwanaland during the late devonian around an axis close to australia 4 a v shaped ocean existed between laurussia and the northern margin of western gondwanaland during the late devonian early carboniferous the appalachian hercynian orogenic belt is probably the result of the closure of this ocean

palaeomagnetic investigation of middle devonian limestones of algeria and the gondwana reconstruction summary we have carried out a palaeomagnetic investigation of the upper middle devonian marly limestone of the hazzel matti formation outcropping at meredoua in the northern ahaggar in the sahara desert the givetian age of the formation is well constrained by palaeontological arguments the formation has been deformed later than the moscovian upper middle carboniferous in the altered yellowish samples and in the heterogeneous samples made of mixed fresh and altered zones several obviously secondary magnetization components are present in the freshest bluegrey samples a se down dipping component can be isolated particularly when a combined thermal and af treatment is applied this component observed in 53 samples from three sites passes the fold test of mcfadden lowes 1981 and mcfadden jones 1981 at the 95 per cent probability level the mean direction for the three sites after tilt correction is ds 118 9 is 34 2 k 378 95 6 3 which corresponds to a south pole mer situated at op 61 7 p 16 2 a95 4 20 and places the site area under a palaeolatitude of 18 8 however the best grouping of the directions is achieved after 85 per cent of unfolding using a statistical method based on a bivariate extension of the fisher s statistics legoff 1990 legoff henry daly 1992 such a tilt correction changes the pole position by 1 5 as the 95 and k curves obtained from stepwise unfolding display a broad minimum and maximum value respectively and because there was probably slight original syn sedimentary dips it is likely that 85 per cent of tilt correction is not significantly different from complete unfolding the magnetic behaviour of the samples upon thermal and af demagnetization the evolution of the initial susceptibility upon thermal treatment hysteresis cycles and investigation of the magnetic mineralogy through microscope examination and electron microprobe analyses lead to the following conclusion greigite was probably a primary magnetic mineral formed during the diagenesis of the rock however its primary magnetization has been totally replaced by a vrm except perhaps in rare samples in which the se component can be found in the narrow range of 300 350 c magnetite is thought to carry the se magnetization component and may have formed early during the diagenesis or soon after greigite to consider the fold test negative would assign the mer pole a post moscovian age and place it in a situation inconsistent with the other poles of the west gondwanian apwp unless all the middle carboniferous and probably lower permian african poles are rejuvenated we rather favour the interpretation of a positive fold test which however does not constrain the magnetization to be of primary givetian age the se component may as well have been acquired any time between the givetian and at least the late moscovian assuming that the mer pole has a givetian age implies that the rheic ocean which separated laurussia from gondwana in the early palaeozoic was almost closed in the middle devonian in agreement with palaeobiogeographic arguments but it increases the discrepancy between the apwp of respectively west and east gondwana an intermediate lower carboniferous age might be in better agreement with both african and australian data clearly however more gondwanian poles are needed to test the reliability of both west and east gondwanian apwp and check simultaneously the possibility of relative displacements between these two blocks during the devono carboniferous time period

paleomagnetic results from the upper carboniferous of the shan thai malay block of western yunnan china the upper carboniferous basaltic woniusi formation was sampled at 21 sites from two limbs of a syncline near baoshan city 25 2 n 99 3 e and from a monocline near yongde county 23 9 n 99 2 e for paleomagnetic study the study area is generally thought to be part of the shan thai malay microplate thermal demagnetization reveals a single polarity characteristic remanent magnetization chrm which is carried by both magnetite and hematite with the former being the dominant mineral in basalt and the latter being the principal carrier in associated tuffaceous basalt and sandstone the tilt corrected formation mean from baoshan d 209 2 i 61 0 95 6 7 confirms the results reported by previous workers and a positive fold test constrains the chrm acquisition to be prefolding the paleomagnetic inclinations determined from this study are similar to that derived from the devonian rocks from the same area by other workers indicating a paleolatitude of about 42 for the baoshan area during the devono carboniferous time the paleomagnetic declinations however present a large difference between the two sampling localities for the upper carboniferous as well as between the sampling sites for the devonian and for the upper carboniferous the paleo declination difference can be partly explained by the apparent polar wander of gondwana if the shan thai malay microplate was rigidly attached to gondwanaland during this time interval the large part of it has to be ascribed to tectonic rotations between the sampling sites this fact suggests that extreme caution should be exerted when trying to establish an apparent polar wander path for and to reconstruct the paleo geographic position of the shan thai malay microplate

paleomagnetism of the silurian devonian andreas redbeds evidence for an early devonian supercontinent two components of magnetization were isolated in the silurian devonian andreas redbeds of the central appalachians of pennsylvania lat 40 75 n long 75 78 w a thermally distributed synfolding b component and a thermally discrete pre alleghenian age folding c component the c component mean direction and associated pole position correspond to a silurian devonian paleolatitude for the andreas location of about 35 s which in conjunction with early devonian results from gondwana is consistent with an early devonian supercontinent configuration

australian palaeozoic palaeomagnetism and tectonics ii a revised apparent polar wander path and palaeogeography abstract new palaeomagnetic data from mid to late palaeozoic rocks in australia have enabled us to revise the palaeozoic apparent polar wander path apwp this modified australian apwp is supported by data from other parts of gondwanaland the palaeomagnetic poles indicate that during the early and mid palaeozoic australia underwent rapid rotation first clockwise during the mid ordovician to the early silurian then counterclockwise from the mid silurian until the end of the devonian while it remained at low to equatorial latitudes this was succeeded by a rapid southward movement during mid carboniferous times the implications of the palaeomagnetic data for the tectonic relationship between the lachlan fold belt lfb and cratonic australia are consistent with the tectonic evidence that the lfb has been in place since the mid devonian