rb sr systematics of permian volcanites in the schwarzwald sw germany from the quartz porphyry occurrences of haigerach and geisberg central schwarzwald which were dated by minerals and whole rock samples the whole rock samples generally yielded lower ages than the minerals this result agrees with other studies on apparently rejuvinated whole rock ages of acid volcanic rocks compare mckerrow et al 1980 petrographic and geochemical observations show that two processes disturbed the rb sr relations 1 fast alterations immediately after extrusion such as sericitization during autohydrothermal bleaching in the pipe area of the volcanoes gain of rb addition of common sr 2 loss of radiogenic 87sr due to long lasting exchange processes with groundwaters

topographic inversion and changes in the sediment routing systems in the variscan orogenic belt as revealed by detrital zircon and monazite upb geochronology in post collisional continental basins abstract the basal alluvial and fluvial siliciclastic successions within the late carboniferous intracontinental basins of the west central bohemian massif provide an intriguing sedimentary record of changes in paleotopography and paleodrainage systems during and after orogenic collapse we analyze this record using the u pb laser ablation icp ms detrital zircon and monazite geochronology five distinct age groups have been identified across the examined stratigraphic succession i archean paleoproterozoic ii late neoproterozoic iii cambro ordovician iv late devonian and v mid to late carboniferous these age groups indicate mixing of both local and distal extra basinal sources and also point to a two stage post convergent geodynamic evolution of the variscan belt the first stage followed collapse of the central bohemian orogenic plateau at around 346 337 ma and involved rapid cooling at ca 340 320 ma and removal of ca 2 3 km of sedimentary overburden from the top of the plateau no sedimentary record of this erosional event suggests that the river systems transported detritus radially outwards from the inner part of the orogen to its forelands a paleorelief inversion and thus major reorganization of the sediment routing systems were then associated with uplift in the southerly and southwesterly peripheral domains of the orogen at ca 330 310 ma in turn these uplifted domains occupied by rapidly unroofed voluminous granite plutons shed detritus into the bohemian sink this was represented by the former plateau collapsed to relatively low elevations but still with significant internal paleorelief dominated by a system of tens of kilometers long nne ssw trending paleovalleys some of the material was further transported in extensive river systems crossing the former plateau to the northerly variscan foreland basin thus the radial sediment routing system changed into axial unidirectional finally we suggest that these major inversions of paleorelief and changes in the surface processes and paleodrainage systems may have been linked to distinct late orogenic lithospheric mantle delamination events first stage delamination occurred beneath the center of the bohemian massif to cause the plateau collapse denudation and radial sediment transport whereas the second stage delamination occurred along its periphery to cause uplift sourcing the intracontinental basins and unidirectional sediment transport end of deposition and magmatic shutdown at around 305 ma may then define the very cessation of late orogenic vertical movements and the transition to a post orogenic intra plate setting

late carboniferous late permian paleomagnetic overprinting of carboniferous granitoids in southern bohemian massif austria paleomagnetic investigations on early middle carboniferous granitoids of the austrian part of the south bohemian pluton 48 5 n 14 2 e yielded two groups of characteristic remanent magnetization chrm directions one being characterized by ssw declinations and positive inclinations and labelled a1 d 200 i 14 k 67 n 9 virtual geomagnetic pole vgp 31 n 170 e the other group having the same declinations but with negative inclinations d 201 i 21 k 88 n 18 vgp 48 n 162 e both magnetizations are considered as magnetic overprints owing to distinct recrystallization processes that occurred in different hydrothermal and tectonic conditions in late carboniferous and late permian times in some cases the original magnetizations may remain after the remagnetization processes but as a whole this study confirms that the paleomagnetic history of a magmatic crystalline basement in this case of late carboniferous to late permian age maybe differ significantly from its intrusion history in this case early to middle or late carboniferous

structure and emplacement of a rhyolitic obsidian flow little glass mountain medicine lake highland northern california many rhyolitic obsidian flows show consistent stratigraphic relations among textural units exposed in the flow fronts of undissected flows and in cross sections of older flows the stratigraphy of the holocene little glass mountain rhyolitic obsidian flow consists of from bottom to top air fall tephra deposits basal breccia coarsely vesicular pumice obsidian finely vesicular pumice and surface breccia slightly crystalline rhyolite occurs near the vent areas a model of obsidian flow emplacement based on these textural relations is presented this stratigraphy may reflect the distribution of volatiles within the magma source region with the interlayered contact between coarsely vesicular pumice and overlying obsidian indicating stratification of volatiles in the magma body the distribution of pumiceous and glassy zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows this complex structure as mapped on the little glass mountain flow reflects both the initial flow stratigraphy and subsequent deformational processes during emplacement of the lava three processes disrupt the initial flow configuration the rise of coarse pumice diapirs from the base of the flow the inward propagation of fractures in areas of extension and surface folding in sites of flow parallel compression subvertical flow banding in vent areas indicates that fracturing accompanies the emergence of lava most of the observed upper surface of a dome originates as a fracture plane the structure of domes that form over vent areas may reflect the orientation of dike like conduits as well as the local state of stress during extrusion

what was the paleogene latitude of the lhasa terrane a reassessment of the geochronology and paleomagnetism of linzizong volcanic rocks linzhou basin tibet the paleogene latitude of the lhasa terrane southern tibet can constrain the age of the onset of the india asia collision estimates for this latitude however vary from 5 n to 30 n and thus here we reassess the geochronology and paleomagnetism of paleogene volcanic rocks from the linzizong group in the linzhou basin the lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes we report consistent 40ar 39ar and u pb zircon dates of 52 ma for the upper linzizong and 40ar 39ar dates 51 ma from the lower linzizong are significantly younger than u pb zircon dates 64 63 ma suggesting that the lower linzizong was thermally and or chemically reset paleomagnetic results from 24 sites in lower linzizong confirm a low apparent paleolatitude of 5 n compared to the upper part 20 n and to underlying cretaceous strata 20 n detailed rock magnetic analyses end member modeling of magnetic components and petrography from the lower and upper linzizong indicate widespread secondary hematite in the lower linzizong whereas hematite is rare in upper linzizong volcanic rocks of the lower linzizong have been hydrothermally chemically remagnetized whereas the upper linzizong retains a primary remanence we suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower linzizong we estimate that the paleogene latitude of lhasa terrane was 20 4 n consistent with previous results suggesting that india asia collision likely took place by 52 ma at 20 n

the rotations of the variscides during the carboniferous collision paleomagnetic constraints from the vosges and the massif central france abstract this study aims at completing and refining results from previous paleomagnetic investigations carried out in various outcrops of the european variscan belt sampling was performed in visean effusive and intrusive rocks of the southern vosges basin and the northern massif central all emplaced in the time range 345 340 ma stepwise thermal demagnetization of 254 specimens from 13 sites of the doller valley and nine sites of the massif central has yielded a very similar paleomagnetic record for both areas with the exception of post carboniferous overprints isolated in the southern vosges two groups of directions with both normal and reversed polarity have been evidenced the first group of directions labelled c occurs in both areas and is characterized by scarce pre tectonic magnetizations two sites and by syn tectonic overprints five sites acquired during the late visean convergence i e in the time range 340 328 ma the mean pole co is 28 s 45 e k 139 95 5 the main group labelled b shows a negative fold test and a positive reversal test indicating that the magnetizations are post tectonic overprints acquired after the main tightening phase i e after 328 ma at a few sites in both areas were evidenced syn tectonic b magnetizations that are in favour of an overprinting in the context of post visean extension tectonics the mean post tectonic b direction of the doller valley southern vosges 62 25 k 52 95 6 n 11 vgp 8 n 127 e is consistent with the mean post tectonic b direction 72 23 k 126 95 5 n 6 vgp 3 n 114 e obtained from the tuffs the grey microgranite and the dyke intrusive into the microgranite of the massif central the occurrence of similar paleomagnetic records in the eastern and central variscides implies that from the massif central to the sudetes the belt has probably moved as one block since early carboniferous the change in inclination of the c components suggests a southward drift of the moldanubian and saxothuringien variscides in the late visean 340 328 ma according to geological features in the same time the rhenohercynian crust was underplating the saxothuringian arc and fore arc zone subsequent to 328 ma the drift turned into a rapid clockwise rotation by about 70 around a pole located on southeastern corner of baltica leading to the closure of the rhenohercynian oceanic basin in the namurian large scale overprinting b components likely related to post rotation extension affected the internal belt the convergence continuing the variscides underwent a second clockwise rotation by about 45 in the late namurian westphalian 320 305 ma stacking of the rhenohercynian sedimentary cover and thrusting over the london brabant basement may be related with this motion

plate and intraplate processes of hercynian europe during the late paleozoic abstract it is speculated that until late carboniferous time the region of hercynian europe was occupied by an elongated island arc system underlain by a segment of continental crust in the upper carboniferous two subduction zones are assumed to have extended from the north and south beneath hercynian europe an extensive zone of hot partially molten upper mantle lay above and between these and diapiric uprise of portions of this material led to separation of mafic magmas widespread partial melting in the lower and middle crust high temperature low pressure metamorphism in crustal rocks and regional uplift and extension of the crust as indicated by intermontane troughs and their associated volcanic rocks in visean to westphalian time hercynian europe collided with both the large neighbouring plates north america europe and africa during these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of hercynian europe after the collision processes had been terminated processes of upper mantle activity continued causing further crustal uplift and even enhanced crustal extension for several tens of million years into the lower permian decline of the upper mantle activity beneath hercynian europe is indicated by crustal subsidence and formation of a peneplain in permian time followed by the upper permian transgression of both the zechstein sea and the tethys sea which mark the end of the hercynian geodynamic cycle

a palaeomagnetic investigation of a permian redbed sequence from a mining drill core summary samples spanning 66 m of permian redbeds were collected from a vertical but azimuthally unoriented drill core obtained during a uranium exploration programme in the lodeve basin of southern france inclinations are well grouped 6 5 k 121 5 and consistent with the known permian palaeolatitude of europe declinations have been estimated by using a sedimentological model involving tectonically rotated cross bed laminations the results are very encouraging and suggest a pole at 151 e 53 n p 1 m 2 n 43 which agrees well with the eurasian apparent polar wander curve the small polar discrepancy could well be due to secular variation which is consistent with the small variance and presumed short time spanned by the sequence fully oriented surface samples confirm these findings and support the validity of the orientation model temporal variations revealed by the stratigraphic ordering can be interpreted as slow dipole wobble on a time scale of 105 yr or more on which is superimposed a more rapid nondipole secular variation it appears that the core stores of the world could become at least under favourable circumstances an important source of palaeomagnetic information

guadalupian middle permian paleobiogeography of the neotethys ocean abstract a matrix of presence absence data of guadalupian middle permian brachiopod genera from sicily tunisia oman turkey north iran central afghanistan karakoram salt range and south thailand has been analyzed by multivariate methods cluster analysis principal coordinate analysis minimum spanning trees and parsimony analysis of endemicity the application of these different and independent paleobiogeographical methods has led to the individuation of three bioprovinces the cathaysian sicily and tunisia cimmerian oman turkey north iran salt range south thailand and transhimalayan karakoram and central afghanistan provinces these provinces have been placed on a paleogeographical reconstruction based on paleomagnetic data which we used to decipher the principal factors that governed brachiopod distribution in the neotethys ocean during the guadalupian as a conclusion the pattern of biotic provinces at this time resulted from the complex interplay between latitudinal thermal gradient oceanic paleocurrents and the continental drift of the cimmerian terranes across zonal climate belts

triassic magnetic overprints related to albitization in granites from the morvan massif france the morvan massif located in the northern part of the french massif central shows widespread unusual deep alteration profiles in the palaeozoic basement rocks gneisses granites and rhyolites they are characterised by overall albitization chloritisation and haematisation albite and chlorite constitute their characteristic mineral paragenesis haematite pigmentation comes with this paragenesis and gives an overall pink coloration to the weathered rocks a paleomagnetic analysis was conducted after detailed petrographical and mineralogical studies of the alteration that lead to the albitized profiles special attention was devoted to the occurrence of haematite the main paleomagnetism carrier with the though to date this palaeoweathering event petrographical checking after heating treatments showed that deshydroxylation of chlorite also led to the formation of secondary haematite this haematite formed by recrystallization acquired remanence properties and thus superimposed and hided the original magnetic signal this problem has prevented the extraction of the original magnetic signal especially in the sections made up of rhyolites virtual geomagnetic poles vgps obtained from the characteristic directions carried by haematite have been compared to the european apparent polar wander path apwp an early triassic magnetic overprint has been recorded in the albitized basement rocks of the morvan massif the age obtained for this albitization phenomena strengthens the hypothesis that albitization developed in relation with palaeoweathering processes linked with the triassic palaeosurface therefore large scale continental weathering could be the main mechanism for late permian early triassic remagnetizations which are widespread throughout the palaeozoic formations of western europe as far as we know this is the first time that deep palaeoweathering phenomena affecting hard and non disaggregated weathered rocks of the basement have been directly dated palaeomagnetism appears as a precious tool to date alteration events developed in widely and long lasting emerged areas of the basement

diffuse flow hydrothermal manganese mineralization along the active mariana and southern izu bonin arc system western pacific 1 abundant ferromanganese oxides were collected along 1200 km of the active izu bonin mariana arc system chemical compositions and mineralogy show that samples were collected from two deposit types fe mn crusts of mixed hydrogenetic hydrothermal origin and hydrothermal mn oxide deposits this paper addresses only the second type mn oxides cement volcaniclastic and biogenic sandstone and breccia layers mn sandstone and form discrete dense stratabound layers along bedding planes and within beds stratabound mn the mn oxide was deposited within coarse grained sediments from diffuse flow systems where precipitation occurred below the seafloor deposits were exposed at the seabed by faulting mass wasting and erosion scanning electron microscopy and microprobe analyses indicate the presence of both amorphous and crystalline 10 a and 7 a manganate minerals the fundamental chemical difference being high water contents in the amorphous mn oxides alternation of amorphous and crystalline laminae occurs in many samples which likely resulted from initial rapid precipitation of amorphous mn oxides from waxing pulses of hydrothermal fluids followed by precipitation of slow forming crystallites during waning stages the chemical composition is characteristic of a hydrothermal origin including strong fractionation between fe mean 0 9 wt and mn mean 48 wt for the stratabound mn generally low trace metal contents and very low rare earth element and platinum group element contents however mo cd zn cu ni and co occur in high concentrations in some samples and may be good indicator elements for proximity to the heat source or to massive sulfide deposits for the mn sandstones fe mean 8 4 and mn 12 4 are not significantly fractionated because of high fe contents in the volcaniclastic material however the proportion of hydrothermal fe nondetrital fe to total fe is remarkably constant 49 58 for all the sample groups regardless of the degree of mn mineralization factor analyses indicate various mixtures of two dominant components hydrothermal mn oxide for the stratabound mn and detrital aluminosilicate for the mn cemented sandstone and two minor components hydrothermal fe oxyhydroxide and biocarbonate biosilica our conceptual model shows that mn mineralization was produced by hydrothermal convection cells within arc volcanoes and sedimentary prisms that occur along the flanks and within calderas the main source of hydrothermal fluid was seawater that penetrated through fractures faults and permeable volcanic edifices the fluids were heated by magma enriched in metals by leaching of basement rocks and sediments and mixed with magmatic fluids and gases dikes and sills may have been another source of heat that drove small scale circulation within sedimentary prisms

paleomagnetic investigations in the thuringer forest g d r abstract in the thuringer forest the whole rotliegende is well exposed and was chosen as beginning of a permian traverse via czechoslovakia austria into italy rockmagnetic experiments established haematite as main carrier of the nrm except a few localities where magnetite was found to be present sediments and volcanites show consistent chrm directions the comparison with krs s 1978 compilation proves a good permian pole position

the apparent polar wander path of the european plate in upper triassic lower jurassic times and the liassic intraplate fracturing of pangaea new palaeomagnetic constraints from nw france and sw germany summary in order to constrain the european plate motions during triassic lower jurassic times new palaeomagnetic data were acquired from various triassic liassic sediments of northeastern france and southwestern germany a carnian palaeopole 49 n 131 e k 193 a95 7 n 4 was obtained for keuper marls with consistent directions of normal and reversed polarities rhaetian sandstones also with both polarities and carnian dolomites with a magnetization of probable secondary origin provide a mean pole 51 n 112 e k 109 a95 6 n 6 more westerly than that given above the lower hettangian sampled in limestones of the xeuilley quarry near nancy shows a n r n polarity sequence with well defined anti parallel directions the mean hettangian pole is 55 n 100 e k 123 a95 11 n 3 induan sandstones from the northern vosges show two distinct overprints one consistent with the ladinian carnian magnetizations already observed in the same region the second consistent with the recalculated mean lower jurassic pole of europe five other sites with different rock types also exhibit late overprints with lower jurassic directions the mean pole calculated from these overprints is 74 n 124 e k 125 a95 6 n 6 the revised apparent polar wander path apwp presented here shows that for the mid permian to the earliest jurassic the palaeopoles of europe stayed around 50 latitude while the longitudes drifted from 185 e to 100 e the previously established upper triassic lower jurassic loop of the apwp is shifted westwards by about 25 the revised apwp involves the following plate motions after the late alleghanian tectonic phase which marked the end of the variscan orogeny central western europe rotated counterclockwise during about 70 myr up to the earliest jurassic the south north drift in latitude by 35 nearly 4000 km remained regular from 320 to 205 ma with a constant rate of 3 4cm yr 1 around 205 ma this motion ceased probably because of a collision somewhere in the north of eurasia then in the lower liassic the plate underwent a rapid counterclockwise rotation with a eulerian pole located in northeastern asia synchronous with fracturing in the atlantic and the neotethys areas it is proposed that the intraplate fracturing of pangaea was initiated by the rapid change of the megacontinent motion at the end of the trias

lacustrine sedimentation in a permian intermontane basin the ville graben vosges france abstract the permian ville graben located on a major fault of the vosges basement has several stages of fluvial fill interrupted by a period of lacustrine sedimentation the lacustrine deposits of the autunian 2 series include various facies marginal and fan delta clastics and permanent lacustrine carbonate facies evolving into evaporitic playa facies differential subsidence of tectonic blocks controlled the facies patterns from the margin to the centre of the basin the sedimentation is determined by three components terrigenous volcanosedimentary and carbonate fan delta progradation with resedimentation processes provided the clastic material in the upper part of the lacustrine formation volcanosedimentary material was deposited primary ash fall sedimentation was followed by sub aqueous erosion and redeposition towards the edge of the basin the lacustrine facies is thin and was subject to frequent emergence sediments were occasionally oxidized and iron was redistributed to give mottled horizons cabonate crusts were precipitated and some silicification took place in the centre of the basin sapropelic carbonate sedimentation develops into a eutrophic lake with anoxic bottom waters the sapropels are massive to finely laminated they include monospecific ostracod faunas type ii to type iii kerogens and a total organic carbon content reaching 2 7 cyanobacterial and microbial activity in these organic rich sediments produced early diagenetic carbonate phosphate and sulfide phases at the top of the formation important sedimentological changes follow a lacustrine regression a restricted and evaporative environment leads to the development of an evaporitic playa lake with stromatolites

palaeomagnetism of the permian volcanic rocks of moissey french jura implications for the palaeofield and tectonic evolution rocks from the massif de la serre in the french jura latitude 47 3 n longitude 5 6 e belonging to an ignimbritic assemblage dominated by vitrophyric rhyolites and whose age of formation is probably permian autunian to saxonian have been studied by applying thermal and alternating field demagnetization the characteristic magnetization has a mean direction derived from 89 samples of d 170 i 16 k 26 2 95 3 and a corresponding north palaeopole at 41 n 172 e a95 5 the pole which is very close to the permian european poles can thus be considered as a new contribution some samples are found to carry a unique normal polarity magnetization others carry both normal and reverse polarities it therefore seems that similar to permian series in the ussr these west european rocks have registered a normal event in the kiaman interval from a structural point of view we may conclude that during the alpine tectonic phases the massif de la serre has not been subjected to substantial rotation

paleoposition of the northern margin of armorica in late devonian times paleomagnetic and rock magnetic results from the frankenstein intrusive complex mid german crystalline rise a detailed rockmagnetic and paleomagnetic study of the frankenstein intrusive complex fic 363 7 ma in age southwestern germany has been carried out in order to test whether the paleomagnetic declinations within the internal parts of the european variscides follow a common trend controlled by the structural grain of the orogen or whether the distribution of declinations follows a random pattern in addition new paleomagnetic data will contribute to further refine the drift history of the armorican terrane assemblage during late paleozoic times the unmetamorphosed frankenstein igneous complex forms part of the mid german crystalline rise within the armorican terrane assemblage amalgamated to avalonia and baltica during the variscan orogeny stepwise thermal and alternating field af demagnetization experiments identify four components a b c n and c r of magnetization component a is a recent viscous overprint that parallels the present day geomagnetic field the high unblocking temperatures of a in some samples can be explained by the presence of multidomain magnetite component b is carried by hematite and is considered to be a late carboniferous remagnetization components c r and c n found in 110 samples from 20 sites are antiparallel and pass a class c reversal test their primary origin is further supported by a contact test c r and c n have maximum unblocking temperatures of up to 580 c and occasionally above over 600 c indicating magnetite and hematite to carry the characteristic remanent magnetization ore microscopy revealed that primary hematite lamellae in ilmenite carry c r and c n and that a secondary generation of hematite is the carrier of b the overall site mean direction of c r and c n 20 sites based on endpoints and remagnetization circle analysis of 198 40 declination inclination with a 95 confidence limit 95 of 4 9 and a precision parameter k of 45 0 defines a paleopole at 15 s 9 w when comparing to the apparent polar wander path for stable europe this result indicates up to 27 counterclockwise rotation of the fic region this is significantly less than reported previously for the odenwald spessart black forest and the vosges massifs of the central european variscides furthermore the resulting paleolatitude indicates significant separation between baltica and this sector of the armorican terrane assemblage in the latest devonian

the late carboniferous to early triassic geodynamic evolution of variscan europe in the light of magnetic overprints in early permian rhyolites from the northern vosges france and central black forest germany summary this paper demonstrates how magnetic overprints of a geological series can provide information over a long time period which can be interpreted in terms of geotectonic evolution according to these new results the late carboniferousearly permian rhyolites from the northern vosges have recorded the magnetic field over a major part of the permian recent radiometric dating assigns a late carboniferous early permian age 298 ma to the previously middle permian rhyolitic volcanism of the northern vosges a palaeomagnetic study was undertaken on rhyolites of the nideck donon massif and on neighbouring permian basins villc saint die in order to obtain a new pole for the early permian and to detect overprints standard palaeomagnetic and rock magnetic experiments demonstrate that over 90 per cent of the characteristic directions of magnetizations are of secondary origin the high temperature magnetizations carried by secondary haematite and haematized titano magnetite fail the fold test and display directions consistent with the saxonian thuringian directions from variscan europe nideckdonon 186 17 k 167 for 26 sites vgp 50 n 178 e saint dic basin 184 20 5 for two sites vgp 53 n 180 e villc basin 191 5 25 for two sites vgp 53 n 168 e vosges dikes 197 5 26 5 for two sites vgp 53 n 158 e part of the magnetizations with intermediate unblocking temperatures of magnetite consists also of post tectonic overprints two consistent mean directions with lower inclinations obtained in the nideck donon rhyolites 182 7 k 459 for 13 sites vgp 45 n 1840 e and pyroclastic deposits from the villc basin 178 9 k 519 for three sites vgp 46 n 190 e were probably acquired during late autunianearly saxonian times in rhyolites the pre and post tectonic characteristic mag netizations are carried by magnetite while in the ash fall deposits the carrier is allogenic haematite a few older magnetizations 191 3 two sites vgp 42 n 173 e consistent with magnetizations carried by magnetite from rhyolites of the central black forest and results from coeval volcanics as well as from autunian sediments have been observed in tuffs from the villi basin 188 1 5 two sites vgp 40 n 177 e the oldest magnetizations 207 10 k 329 vgp 32 n 155 e presumably of stephanian age were identified in two rhyolitic stocks within the granitic basement from the central vosges and central black forest the distribution of the palaeomagnetic directions suggests that from the late autunian to the late thuringian overprinting due to low temperature alteration of titano magnetite and crystallization of secondary haematite was more or less continuous the apparent polar wander path apwp computed with the new results and the published poles shows a hairpin which implies a drastic change of the european plate motion during the permian the clockwise rotation of europe initiated in the late visean namurian stopped in the late autunian early saxonian this event corresponds to the end of the variscan convergence and of the appalachian

character of the alleghanian orogeny in the southern appalachians part iii regional tectonic relations geological and geochronological studies in the eastern piedmont of the southern appalachians have documented a polyphase late paleozoic deformational chronology d 2 d 4 that is contemporaneous with the alleghanian orogeny recorded in the western appalachian foreland the scale and vergence of d 3 anticlinoria in the eastern piedmont are similar to those of anticlinoria in the western appalachians which are related to ramping of underlying thrust surfaces we suggest that the appalachian decollement extends southeastward beneath the entire piedmont province and that all of the piedmont rocks were displaced at least 175 km northwestward relative to north america during the alleghanian orogeny late paleozoic deformational effects in the eastern piedmont thus are considered cogenetic with alleghanian foreland deformation we interpret the alleghanian orogeny to be the result of oblique dextral convergence and collision of gondwana and laurentia

comparison of time averaged acoustic backscatter concentration profile measurements with existing predictive models time averaged concentration profiles over bedforms measured using acoustic backscatter abs instrumentation on a macrotidal beach at whitsand bay cornwall uk were dominated by vortex type suspension profiles wave heights were typically in the range 0 25 1 0 m with wave periods of between 5 and 8 s mean currents measured at 0 2 m above the bed were generally weak 0 1 m s 1 near bed concentrations were compared with two reference concentration cref models smith j d mclean s r 1977 boundary layer adjustments to topography and suspended sediment mem soc r sci liege 11 123 151 nielsen p 1986 suspended sediment concentrations under waves coastal eng 10 23 31 the resuspension coefficient 0 showed a strong dependence on the maximum skin friction shields parameter the smith and mclean smith j d mclean s r 1977 boundary layer adjustments to topography and suspended sediment mem soc r sci liege 11 123 151 model implies that cref z0 increases with excess skin friction at the bed excess however field data showed no such correlation and the inclusion of a variable 0 gave poor model predictions that tended to have cref z0 decreasing with increasing excess reference concentrations at the bed c0 were determined from the time averaged concentration profiles using regression analysis the nielsen nielsen p 1986 suspended sediment concentrations under waves coastal eng 10 23 31 model predicts that c0 increases rapidly with the modified skin friction shields parameter r however extrapolated values of c0 showed no dependence on r in addition model predictions were orders of magnitude higher than extrapolated values of c0 the influence of the bedforms on the magnitude of near bed concentration measurements has been examined we conclude that the observed variations in cref over different bedforms vortex ripples lunate ripples lunate mega ripples will need to be taken into account if model predictions of cref as a function of the skin friction shields parameter are to improve the mixing height l predictions of the nielsen nielsen p 1992 combined convection diffusion modelling of sediment entrainment proc 3rd int conf on coastal engineering venice 4 9 oct 1992 pp 3202 3215 and the vincent and osborne vincent c e osborne p d 1995 predicting suspended sand concentration profiles on a macrotidal beach cont shelf res 15 1497 1514 models have been compared with measured data although the predictions of both models were typically within a factor of two of l measurements the vincent and osborne vincent c e osborne p d 1995 predicting suspended sand concentration profiles on a macrotidal beach cont shelf res 15 1497 1514 model is believed to be more appropriate for modelling l over bedforms as field data showed a strong dependence of l on and model predictions do not rely on bedform data

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

a middle triassic paleomagnetic pole for the eurasian plate from heming france the paleomagnetic study of middle triassic limestones 91 cores 160 samples from a 40 m thick section in heming france yields a paleomagnetic pole for stable europe at lat 53 5 n lon 140 8 e dp 2 3 dm 3 9 this pole fills an important gap of the apparent polar wander path apwp of the eurasian plate the comparison of this pole with available data shows a standstill of the eurasian plate from the late paleozoic to the middle triassic followed by a rapid drift until the lower jurassic the beginning of the rapid drift is contemporaneous with the initiation of main rifting processes in the tethyan oceans

remagnetization and chemical alteration of sedimentary rocks remagnetization is a common phenomenon in rocks and developing a greater understanding of its mechanisms has several benefits acquisition of a secondary magnetization is usually tangible evidence of a diagenetic or thermal event which can be dated using palaeomagnetic techniques this is important because the timing of diagenetic and thermal events is commonly difficult to determine remagnetization can also obscure primary magnetizations and a better understanding of remagnetization could improve our ability to uncover primary magnetizations many chemical remagnetization mechanisms have been proposed including those associated with chemical alteration by a number of different fluids e g orogenic weathering mineralizing hydrocarbons and burial diagenetic processes e g clay diagenesis maturation of organic matter this book contains case studies and review articles that focus on remagnetization chemical remagnetization mechanisms and magnetic changes associated with chemical alteration by hydrocarbons

can a primary remanence be retrieved from partially remagnetized eocence volcanic rocks in the nanmulin basin southern tibet to date the india asia collision paleomagnetic dating of the india asia collision hinges on determining the paleogene latitude of the lhasa terrane southern tibet reported latitudes range from 5 n to 30 n however leading to contrasting paleogeographic interpretations here we report new data from the eocene linzizong volcanic rocks in the nanmulin basin which previously yielded data suggesting a low paleolatitude 10 n new zircon u pb dates indicate an age of 52 ma negative fold tests however demonstrate that the isolated characteristic remanent magnetizations with notably varying inclinations are not primary rock magnetic analyses end member modeling of isothermal remanent magnetization acquisition curves and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously previously reported paleomagnetic data from the nanmulin basin implying low paleolatitude should thus not be used to estimate the time and latitude of the india asia collision we show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization we tentatively propose a new method to recover a primary remanence with inclination of 38 1 35 7 40 5 95 significance and a secondary remanence with inclination of 42 9 41 5 44 4 95 significance the paleolatitude defined by the modeled primary remanence 21 n 19 8 n 23 1 n is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper linzizong group corrected for inclination shallowing the start of the tibetan himalaya asia collision was situated at 20 n and took place by 50 ma

paleomagnetic evolution of the armorican massif in late paleozoic times in the light of overprints recorded in cadomian and paleozoic units abstract previous investigations have shown that in paleozoic outcrops of the variscides the major part of the paleomagnetic record consists of overprints after identification and dating of these overprints the magnetic record can be used to constrain the geotectonic evolution of the investigated area after the major tectonic event in order to test this hypothesis a paleomagnetic investigation was carried out on various volcanic and plutonic units from northern brittany 48 5 n 3 w emplaced and metamorphosed in the late proterozoic during the cadomian orogeny to calibrate the overprints in terms of their ages doleritic dykes intruded into the cadomian basement in the early paleozoic as well as early carboniferous intrusive and extrusive rocks were also sampled thermal demagnetization often shows multivectorial behaviour of most nrms and indicates a wide range of magnetic components characterized by various unblocking temperatures a large majority of the components isolated in cadomian units display directions that are consistent whith the results from the younger paleozoic units with exception of the d components mean vgp 7 s 152 e which are interpreted as being ordovician in age all other mean directions poles agree with results obtained from visean volcanic plutonic and metamorphic rocks of the central part of the variscan belt this means that no cadomian magnetization has survived the tectonic magmatic and metamorphic phases that have affected the cadomian basement during the paleozoic the major part of the magnetic overprints were acquired during the variscan orogeny particularly in relation to visean and westphalian magmatism the early carboniferous 340 328 ma cn 7 s 53 e co 29 s 60 e and cp 51 s 62 e components the middle carboniferous 328 310 ma b components 11 n 124 e the late carboniferous 310 300 ma a1 30 n 151 e the late permian a 51 n 154 e and the jurassic j1 overprints 72 n 107 e delineate an apparent polar wander path that can be interpreted in terms of geotectonic evolution since the late visean this evolution was the same as for the massif central the vosges the black forest and the odenwald spessart the variable inclinations of the c components suggest a northwestward tilting of the area north of saint brieuc and a possible latitudinal drift of the whole massif in late visean times the c b and b a1 segments of the apwp reveal two successive clockwise rotations by 70 80 in the latest visean and by 30 in the late namurian early westphalian the first rotation closed the rhenohercynian basin during the last rotation the variscan belt was tied to northern europe the a and j1 overprints give evidence of hydrothermal activity in northern brittany in the late permian and the early jurassic

an upper carboniferous lower permian 280 ma palaeomagnetic pole from the western margin of the bohemian massif summary in the area of the drill site of the german continental deep drilling project ktj3 on the western margin of the bohemian massif 15 sites of uppermost carboniferous to lowermost permian rocks quartz porphyries granites and a siltstone geographic mean coordinates 12 e 49 8 n have been studied in order to obtain a reliable pole position for this time interval the investigated rocks are syngenetic or only slightly younger than the youngest granites of the fichtelgebirge for which a k ar and rb sr age of 280 ma has been determined by besang ef al 1976 while four sites quartz porphyries showed too weak intensities of the natural remanent magnetization nrm the results of two sites with otherwise good rock magnetic properties had to be discarded because of uncertain tectonic position in a shear zone the remaining nine sites have all reversed polarity as expected for a remanence age within the reversed kiaman interval and a mean pole position at 165 9 e 42 3 n with n 9 r 8 8776 a 6 4 this is in good agreement with the mean upper carboniferous lower permian european pole position of stable europe according to irving irving 1982 and of the bohemian massif krs 1968 rock magnetic investigations revealed that the chrm of the siltstone is due to primary magnetite while the mostly single component characteristic remanent magnetization chrm of the magmatic rocks is carried by primary magnetite and by primary haematite produced by syngenetic high temperature oxidation of primary magnetite

paleomagnetic evidence for rotation during stephano permian extension in southern massif central france we present results of a preliminary paleomagnetic study devised as a test of a tectonic model proposed by van den driessche and brun 1 to interpret the main structural features of the montagne noire south massif central france this model suggests that crustal extension during stephanian to permian times is responsible for a major fiat lying detachment the inferred geometrical relationships between fault azimuth and extension direction is befieved to induce a counter clockwise rotation of the paleozoic cover during its northward motion in order to test this model we have sampled autunian lower permian grey and red sandstone beds within two basins opened during the extension the lod ve and the st affrique basins the paleomagnetic study of six sites three in each basin using thermal demagnetization procedures reveals the presence of 3 magnetization components a first one a which is unblocked below 400 o c is interpreted as a recent overprint due to weathering of the sandstones the b component is unblocked between 400 o and 575 o c this b component is assumed to be primary although the gentle dip of the beds does not allow a fold test it fits the lower permian field direction in the lod ve basin in situ d m 194 i m 6 o k 50 tilt corrected d m 194 i m 10 o k 53 in contrast it shows a systematic difference in declination in the st affrique basin in situ d m 173 i m 2 5 o k 101 tilt corrected d m 172 lm 6 o k 37 this is interpreted as a post autunian counter clockwise rotation of at least 20 o of the st affrique basin thus supporting one of the main implications of the tectonic model finally red sediments from two sites of the lod ve basin yield a third component c with unblocking temperatures up to 670 c and carried by hematite its direction is also consistent with the permian field direction showing that hematite crystallisation occurred early in the diagenesis history of the sediments

evidence for mid permian compressive tectonics in western europe supported by a comparison with the alleghanian geodynamic evolution abstract the final continental collision of gondwana with the appalachian variscan fold belt during mid permian times is evident from 1 the simultaneous deformation of the entire fold belt marked by a thin skin structural style in the appalachians and in parts of the variscan front midi fault 2 the disruption of the hercynian zones in western europe and 3 left lateral transcurrent displacement visible in features such as the leucogranite belt in france and associated with thrusting of the ales saint etienne and saar lorraine basins the line along which the main transcurrent movement occurs stretches through western europe from ales to ireland and it nearly coincides with the paris basin magnetic anomaly it is consistent with the movement along the biscay fault and with the existence of stable areas iberia aquitaine and armorica where local extension occurred there are symmetrical movements and deformations on both the american and european sides the alleghanian orogeny is interpreted as being the result of oblique dextral convergence and collision of gondwana with laurasia on the european side this collision resulted in sinistral displacement laterally associated with thrusting such a geometrical pattern can be achieved through a frontal intracontinental compression

magnetic overprints in granitoids and metamorphic rocks from limousin france evidence for late variscan rotations crustal folding and tilting abstract generally the lack of bedding criteria in basement units hampers the interpretation of paleomagnetic results in terms of geotectonics nevertheless this work demonstrates that successive remagnetizations recorded in early carboniferous metamorphic and plutonic units without clear bedding criteria can be used to constrain a polyphased tectonic evolution consisting of a regional clockwise rotation followed by a folding phase a tilting phase and a second regional clockwise rotation metamorphic ultrabasic tonalitic and granitic rocks from different parts of limousin western french massif central 45 5 n 1 25 e which underwent metamorphism during devonian early carboniferous or were intruded in the early middle carboniferous were sampled in order a to identify the magnetic overprinting phases and the related tectono magmatic events and b to constrain the regional and plate tectonic evolution of limousin paleomagnetic results from 32 new and 26 sites investigated previously show that at least 90 of the magnetization isolated in rocks older than 330 ma are overprints in agreement with results from adjacent areas of the variscan belt the major overprinting phases occurred a in the last stages of the major exhumation phase 332 328 ma mean virtual geomagnetic pole vgp cp 37 n 70 5 e b during the post collisional syn orogenic extension 325 315 ma vgp b 11 n 114 e c in the latest carboniferous and early permian vgp a1 27 n 149 e and d in the late permian vgp a 48 n 146 e the middle late carboniferous overprints cp and b are contemporaneous with emplacement of leucogranitic crustal derived plutons and probably result from the hydro thermal activity related to the magmatism the drift from cp directions to b directions implies that after 330 ma limousin underwent a clockwise rotation by 65 together with the central europe variscides the bt components the vgps of which deviate from the mean apparent polar wander path apwp of the belt are interpreted as b overprints tilted during late variscan tectonics that is in the time range 325 315 ma the first and most important generation of bt overprints was tilted during nw se folding associated with ne sw shortening updoming and emplacement of leucogranitic plutons the second generation reveals southeastward tilting due to ne striking normal faulting the drift from b to a1 directions implies that limousin has participated to the second clockwise rotation by 40 of the whole belt in westphalian times

late paleozoic orogenies in western africa and eastern north america the diachronous closure of theic ocean the carboniferous evolution in western morocco southeastern canada and new england was dominated by intracontinental processes the subsidence of pull apart sedimentary basins and their subsequent deformation during middle carboniferous to early permian times with the development of autochthonous structures and mainly low to medium grade metamorphism on the other hand the structural style of the late carboniferous deformation in the southern appalachians and to a substantial degree also in new england that is related to a continental collision is characterized by deep seated northwest vergent thrusts and medium to high grade metamorphism the mauritanides structures are symmetrically east vergent thrusts this pattern is explained by the diachronous closure of the theic ocean that separated the avalon and carolina microcontinental blocks from paleo gondwana the closure of the theic ocean occurred during devonian times between the northern appalachians and the northwestern edge of paleo gondwana morocco whereas it was achieved only at the end of carboniferous times between the southern appalachians and western africa mauritania senegal the late devonian to early carboniferous counterclockwise rotation of gondwana induced the opening of the pull apart basins of canada new england and morocco as well as later the oblique collision in the southern appalachians and the generalized late paleozoic dextral transcurrent faulting along the northern edge of paleo gondwana

a magnetotectonic study of the hercynian montagne noire france a paleomagnetic study of eleven sites of devonian limestones sampled along the flanks and hinge of a polyphase hercynian recumbent fold in the montagne noire reveals well defined directions of characteristic remanent magnetization chrm the directions have good within site precision but a large between site scatter which fails to be reduced by a classical tilt correction this is readily understood if widespread remagnetization occurred between the two major hercynian phases of folding a and b which are known to have shaped the montagne noire this remagnetization appears to be an instance of a continent wide chemical event that wiped out much of the magnetic memory of devonian and lower carboniferous rocks in laurasia baltica and northern gondwana possibly due to groundwater circulation following the major hercynian tectonic phases comparison of observed and predicted hercynian magnetic directions allows one to determine through a fairly simple geometric construction the tectonic elements of the later phase b it is found that different rotations affected the flanks and hinge zone of the folds generated by phase a thus demonstrating how a pre existing structure can control the development of deformation in a later tectonic phase the effects of the non cylindrical phase b can next be removed to reveal the broad tectonic features of phase a deformations the reversed kiaman polarity of the chrm allows a determination of the age of phase b which is found to be about 300 310 ma and of the age of the remagnetization itself at about 310 320 ma the search for the age of remagnetization included paleomagnetic measurements at two lower permian sites from the nearby lodeve basin these provide new data for the permian pole of europe and point out the limitations of some earlier studies due to insufficient magnetic cleaning finally the principal shortening directions of the two phases can be determined from the magnetic data and are found to have rotated counterclockwise by 30 between phase a and phase b this feature already observed in the collision of india against eurasia may be related to the progressive suturing of former subduction zones during the hercynian collision of gondwana against laurasia this study demonstrates the potential of remagnetizations for the solution of rather complex tectonic problems we propose the term magnetotectonics for this rapidly expanding field of studies in which the methods of paleomagnetism and structural analysis are closely associated

suspended sediment concentrations under waves abstract a predictive formula is derived for calculation of the magnitude of time averaged suspended sediment concentrations under waves by taking into account the effect of flow contraction near ripple crests the formula applies equally well for rippled beds and sheet flow conditions the formula is in qualitative agreement with the bed load flux formula of madsen and grant 1976

paleopositions of the western europe hercynides during the late carboniferous deduced from paleomagnetic data consequences for stable europe abstract paleomagnetic investigations have been carried out in different areas of the variscan belt armorican massif central massif ardenne vosges and the black forest in order to unravel the paleogeographic evolution of the hercynides during the carboniferous subsequent to important southerly or northerly drifts the central massif and vosges underwent a large rotation of a still undetermined direction in latest visean namurian times during the namurian westphalian the whole belt was oriented n s all investigated areas yield wsw ene directions for that period these directions question the validity of carboniferous mean poles available for stable europe a european plate rotated counterclockwise by about 40 relative to its permian position would be more consistent with these results during the latest westphalian stephanian period i e within the 310 290 ma timespan the western europe hercynides rotated clockwise by 40 50 perhaps together with the whole of europe late variscan strike slip faulting and overthrusting probably result from that rotation

a geomagnetic polarity timescale for the carboniferous the geomagnetic polarity pattern for the carboniferous is incompletely known but with the best resolved parts in the serpukhovian and bashkirian hence data from both igneous and sedimentary units are also used in an additional polarity bias evaluation in the tournaisian to mid visean interval polarity is mainly derived from palaeopole type palaeomagnetic studies allowing identification of polarity bias chrons seven polarity bias chrons exist in the mississippian mi1nb to mi4nb with an additional 33 conventional magnetochrons and submagnetochrons mi4r to mi9r the moscovian and gzhelian polarity is best resolved in magnetostratigraphic studies from the donets basin and the southern urals dispute about the reliability of these data is ill founded since an assessment of supporting data from palaeopole type studies suggests that these datasets currently provide the best magnetic polarity data through the pennsylvanian polarity bias assessment indicates a normal polarity bias zone in the kasimovian in the pennsylvanian there are 27 conventional magnetochrons and submagnetochrons pe1n to ci1r and one normal polarity bias chron pe8nb the kiaman superchron begins in the mid bashkirian with clear data indicating brief normal polarity submagnetochrons within the superchron the magnetochron timescale is calibrated using 31 u pb zircon dates and a quantitative bayesian based age scaling procedure

paleomagnetic investigations and k ar dating on the variscan plutonic massif of the champ du feu and its volcanic sedimentary environment northern vosges france abstract paleomagnetic investigations have been carried out on samples from the plutonic series of the champ du feu massif and the devonian ordovician host rocks in order to date the different phases of magnetization a series of samples has been selected for k ar analysis on micas amphiboles and feldspars the diorites granodiorites diabases and hornfelses are characterized by high susceptibilities of up to 10 1 si thermal demagnetization reveals several groups of directions of characteristic magnetization the a direction d 202 i 23 49 n 153 e is consistent with the permian virtual geomagnetic pole for europe the b 1 direction d 59 i 22 11 n 128 5 e was obtained from the effusive parts of the kagenfels granite dated at 284 ma diorites microdiorites and volcanics emplaced a short time after the intrusion of the natzwiller granite dated at 344 and 351 ma display three groups of directions the c 1 direction d 328 i 15 27 n 223 5 e the c 2 d 323 i 38 13 s 222 e and the c 3 d 272 i 41 16 s 260 e the c 1 and c 2 directions agree well with those obtained previously for visean volcanics from the southern vosges in the host rocks we have identified b and c overprintings despite the lack of tectonic control the paleomagnetic data demonstrate at least two important motions of the vosges first a rotation of undetermined direction relative to stable europe which affected the variscan belt between the late visean and 320 ma and second a clockwise rotation which occurred during the stephanian autunian in late variscan time

paleopositions of the saxothuringian northern vosges pfalz odenwald spessart in variscan times paleomagnetic investigation abstract in the saxothuringian zone thermal demagnetization of 581 specimens of late devonian early carboniferous gabbros diorites granodiorites gneisses and schists yields different time dependent paleomagnetic directions the oldest c n directions vgp northern vosges 19 n 222 e spessart 27 5 n 227 e were emplaced during crystallization of the visean plutonics in the time range 340 330 ma c p directions n v 47 n 241 e mostly characterized by low unblocking temperatures are interpreted as slightly younger overprints or as resultant vectors of c n and present day directions the b directions northern vosges 14 n 132 e odenwald 6 n 128 e exist as primary magnetizations in middle late carboniferous rhyolites and as overprint in the late devonian early carboniferous plutonics the a directions sp 28 n 175 e od 26 n 168 e correspond to overprints emplaced later in the late carboniferous the a directions northern vosges 49 n 155 e pfalz 45 n 154 e odenwald 48 n 151 e spessart 47 n 165 e are consistent with the permian early triassic european directions and are present as primary directions in permian volcanics the late carboniferous permo triassic overprints are mainly characterized by high unblocking temperatures due to the presence of secondary hematite the deviations from c to b and from b to a are interpreted as due to two successive block rotations the first 90 clockwise or counterclockwise the second 40 clockwise and the deviation from a to a to a 2500 km northward drift of the saxothuringian