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

a middle cambrian trilobite faunule from the meguma group of nova scotia the meguma group of southern nova scotia is a thick succession of mostly deep water siliciclastic sedimentray rocks of uncertain age previous fossil finds have been in the halifax formation of the upper part of the sequence and have provided early ordovician ages a newly discovered trilobite bearing bed from the underlying goldenville formation yields middle cambrian taxa of acado baltic affinity specimens belonging to paradoxides s l agraulos dorypyge and ellipsocephalidae indicate that the meguma group is an exotic succession with faunal similarity to the adjacent avalon terrane and to middle cambrian successions in europe the middle east and west africa

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

palaeomagnetic and rock magnetic reliability criteria in ophiolitic rocks a case study from the palaeozoic ballantrae ophiolite scotland abstract palaeomagnetic studies of the ballantrae ophiolite southwest scotland are assessed in the light of palaeomagnetic and rock magnetic reliability criteria the remanence directions are palaeomagnetically reliable but have only limited significance for apparent polar wander path construction rock magnetic reliability criteria are found to be ambiguous when treated in isolation original grain size effects are detected in magnetic properties across three sections through pillow lavas we propose that a systematic study of spatial magnetic property variations within pillow lavas can provide an additional field test in the assessment of reliability thermomagnetic analyses and microscopic observations suggest three remanence carrying magnetic phases exist within the extrusive layer of the ophiolite these are almost pure magnetite hematite and titanomaghemite palaeomagnetic rock magnetic and geological observations are best satisfied by a pre obduction magnetisation age this study links the remagnetisation of an ultra oceanic conglomerate to local hydrothermal circulation rather than to a pervasive remagnetisation event as previously suggested

formation and palaeozoic evolution of the gorny altai altai mongolia suture zone south siberia zircon u pb constraints on the igneous record abstract the altai sayan fold belt epitomises how phanerozoic island arc continental crust contributed to the growth of the palaeo eurasian continent its formation is dominated by the closure of the palaeo asian ocean pao when island arc systems and gondwana derived terranes accreted to the siberian margin relics of the pao related tectonic units and associated granitoids occur along the ophiolitic charysh terekta ulagan sayan suture ctuss between gorny altai and altai mongolia zircon la icp ms u pb dating of this igneous record constrains the multi stage geodynamic pao evolution primitive kuznetsk altai island arc crust formed at the siberian margin during the ediacaran early cambrian 525 555 ma this island arc matured during the middle late cambrian 510 ma and was consumed by pao subduction in the late cambrian early ordovician 480 490 ma forming the siberian early caledonian accretion collision belt south of the ctuss granitic magmatism occurred within gondwana derived altai mongolia during the middle late ordovician 450 470 ma palaeo kazakhstan assembly and during a silurian early devonian 400 425 ma andean type collision middle late devonian 360 395 ma granitoids were emplaced as a result of the collision of altai mongolia with siberia during final pangaea amalgamation the suture was strike slip reactivated associated magmatism 295 ma the youngest 220 255 ma sampled granitoids originated in an intra plate setting

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

tectonic evolution of nw iberia during the paleozoic inferred from the geochemical record of detrital rocks in the cantabrian zone the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges in age from ediacaran to early permian its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana ii an active continental margin or iii the margin of a drifting ribbon continent and iv a combination of the three aforementioned possibilities in this paper we present geochemical data from 21 samples taken in detrital rocks in the cantabrian zone from ediacaran to early permian age the results together with previously published detrital zircon ages from these strata allow a comprehensive analysis of changing tectonic scenarios and provenance through time collectively these data indicate that this portion of iberia was an active margin during the ediacaran that evolved to become part of the passive margin of gondwana at least from ordovician to late devonian times changes in geochemistry zircon age distribution and sm nd isotopes during the carboniferous are attributed to the far field effects of the onset of the collision between gondwana and laurussia and related processes such as recycling of older sedimentary sequences as well as the involvement of the rocks formed during the variscan orogeny and oroclinal buckling latest carboniferous and permian show a more juvenile sm nd and higher values of illite cristallinity that may support the hypothesis of lithospheric foundering after oroclinal buckling

les crises du devonien superieur par l etude des faunes d ostracodes dans leur cadre paleogeographique abstract ostracod evolution studied at a global scale can contribute to an evaluation of the palaeobilogical crises during the late devonian the mostly pelagic entomozoidae are not affected data on benthic neritic ostracods come from the western canadian platform europe the russian platform north africa and the middle east species extinction rates vary according to provinces and areas extinction occur at the givetian frasnian lower upper frasnian and frasnian famennian boundaries only five families seem to disappear during the late devonian the appearance of new assemblages since the earlisst famennian is worldwide on platforms the thuringian ecotype ostracod assemblages underwent a major change since the early famennian it is characterized by many species with spinous carapaces and is known in the paleotethys during famennian dinantian times it is related to a glacial event in the north of the gondwana which generated cold deep water currents invading the paleotethysian basins the thuringian ecotype allows to propose a paleobathymetric model its species composition shows a double origin by evolution of early devonian roots and with lineages proceeding from circalittoral species causes of these late devonian events can be understood in a particular paleogeographic reconstruction where the paleotethys has no western faunal communication since the givetian frasnian boundary warming water associated with an oxygen deficiency eliminated several species at the givetian frasnian and lower upper frasnian boundaries a first cooling at the frasnian famennian time was sufficient to decimate species strongly adapted to the frasnian conditions the generalized regression at this time probably increased this last phenomenon

neodymium isotopic composition of ordovician conodonts as a seawater proxy testing paleogeography 1 to evaluate recent ordovician paleogeographic reconstructions end values were determined in conodonts as a proxy for the isotopic variations in ancient seawater using samples from the major cratons and microplates isotopic variations reveal the existence of distinct oceanic masses and epeiric seas that constrain the position of plates and terranes relative to these water masses and hence constrain models of regional and global paleogeography the isotopic patterns show a consistent picture of the changes in the ordovician oceans the end values for water masses associated with laurentia are strongly negative 28 to 18 in the early ordovician evolving over time to higher values in the range 13 to 5 the early ordovician signature of laurentia is in marked contrast to other cratons and microplates which have a range of values from 10 to 5 samples from south china show interesting signals that reflect a greater similarity to laurentia than other peri gondwana terranes the isotopic variations are a function of both regional geology and global tectonic processes the most obvious being the taconic orogeny and onset of the closure of the iapetus ocean regional and global models of paleogeography are considered in light of these proxy signals this study also reveals that conodonts are powerful geochemical tools for obtaining information on ancient water masses the use of the nd isotopic signatures from conodonts provides a method independent of paleomagnetism and biogeography to test paleogeographic models by integrating such information a more multidisciplinary approach is possible

early palaeozoic palaeobiogeography of the variscan regions abstract the palaeobiogeographic method appears to be an attractive alternative for reconstruction of the palaeogeography of the variscan regions during the palaeozoic the evolution of faunal assemblages and climatic characteristics from the cambrian up to the end of the devonian demonstrates that baltica and north gondwana represent two major and independent palaeogeographic entities separated by a mid european rheic ocean which opened as early as the cambrian and closed progressively from the devonian the south armorican ocean whose closure generated the so called ligerian orogen is regarded as a branch of the rheic ocean both a tornquist s sea separating baltica from its western extension avalonia during the ordovician and a proto tethys ocean extending between southwestern europe and north africa during the devonian are excluded from our reconstructions

the adaptation of pearce element ratio diagrams to complex high silica systems pearce element ratios pers of pearce 1968 express geochemical data in a form where variations in absolute compositions of an igneous suite can be evaluated generally the denominator value in the ratio is taken as a major element abundance but it is argued here that zr provides a more suitable choice zr remains incompatible in magmatic systems up to 68 wt sio2 because zircon fractionation can be suppressed by high melt temperatures and increased volatile contents the use of zr thus permits per modelling to be extended to much higher levels of silica than previously investigated however such systems are more complex than those just involving simple basaltic magmas besides fractionation the processes of magma mixing combined assimilation and fractional crystallization and the initial degree of partial melting in the mantle source must also be considered to distinguish and evaluate these processes a set of example suites are investigated from a complex synextensional calc alkaline province in the western usa samples within most individual suites can be modelled by fractionation however a significant trend orthogonal to the main fractionation vector is also apparent and open system processes are inferred successful modelling is achieved on an inter suite basis using diagrams with axis functions of 4 ca na 0 5 fe mg zr versus si al zr potential open system evolution paths between mafic end members and crustal contaminants are also displayed and evaluated on these same diagrams the encouraging results suggest that such per diagrams may be employed as a versatile tool for investigating the systematics of related igneous suites over a wide area

cambro ordovician paleogeography of the southeastern new england avalon zone implications for gondwana breakup paleomagnetic measurements and u pb geochronology of alkalic igneous rocks intruding the ediacaran cambridge argillite and the lower cambrian weymouth formation north of boston at nahant massachusetts yield a well constrained paleopole the oldest intrusive phases are a syenodiorite sill and a granite dike with 206 pb 238 u crystallization ages of 490 19 0 90 ma and 489 03 0 82 ma respectively this sequence along with undated mafic sills penetrating both the argillite and weymouth strata are cut by 488 53 0 81 ma gabbro locally associated with 488 48 0 79 ma syenite forming a steep sided intrusive plug ten paleomagnetic sampling sites within the mafic sills and the gabbro contain a stable a magnetic component with a mean direction of d 278 5 i 76 7 a 95 3 9 n 10 this is considered to be the primary magnetization direction because it passes both fold and reversals tests the corresponding paleopole at 320le 34 n a 95 7 2 n 10 places the southeastern new england avalon zone at a southerly paleolatitude of 65 slightly removed from the african margin of gondwana and suggests that bimodal nahant magmatism is linked with latest cambrian early ordovician rifting of west avalonia to form the rheic ocean stable b and c magnetic components in nahant syenodiorite and syenite give rise to virtual geomagnetic poles that track late silurian through carboniferous segments of the north american apparent polar wander path indicating docking of west avalonia by mid paleozoic time

silurian extension in the upper connecticut valley united states and the origin of middle paleozoic basins in the quebec embayment pre silurian strata of the bronson hill arch bha in the upper connecticut valley nh vt are host to the latest ludlow comerford intrusive suite consisting east to west of a mafic dike swarm with sheeted dikes and an intrusive complex the rocks are mostly mafic but with compositions ranging from gabbro to leucocratic tonalite the suite is truncated on the west by the monroe fault a late acadian thrust that carries rocks of the bha westward over silurian devonian strata of the connecticut valley gaspetrough cvgt dikes intrude folded strata with a pre intrusion metamorphic fabric taconian but they experienced acadian deforma tion twenty fractions of zircon and baddeleyite from three sample sites of gabbro diorite spanning nearly 40 km yield a weighted 207 pb 206 pb age of 419 1m a greenschist facies dikes sampled over a strike distance of 35 km were tholeiitic basalts formed by partial melting of asthenospheric mantle with little or no influence from mantle or crustal lithosphere the dike chemistry is similar to mid ocean ridge within plate and back arc basin basalts parent magmas originated in the astheno sphere and were erupted through severely thinned lithosphere adjacent to the cvgt extensive middle paleozoic basins in the internides of the appalachian orogen are restricted to the quebec embayment of the laurentian rifted margin and include the cvgt and the central maine trough cmt separated from the bha by a silurian tectonic hinge the ne trending comerford intrusions parallel the cvgt cmt and the tectonic hinge and indicate nw se extension during post taconian convergence the irregular margins of composite laurentia and avalon permitted continued colli sion in newfoundland st lawrence promontory and coeval extension in the quebec embayment extension may be related to hinge retreat of the northwest directed brunswick subduction complex and rise of the asthenosphere following slab break off an alternative hypothesis is that the basins originated as pull apart basins between northwest trending left stepping sinistral strike slip faults along the southern flanks of the new york and st lawrence promontories

lithospheric sources of north florida usa tholeiites and implications for the origin of the suwannee terrane abstract sr nd and pb isotopic data for mesozoic tholeiites from six drillholes in northern florida suwannee terrane indicate derivation of the magmas from a source that was slightly incompatible element enriched with respect to bulk earth probably continental lithospheric mantle variably contaminated by more enriched crust initial isotopic compositions of the florida tholeiites show a considerable range 87 sr 86 sr 0 7059 to 0 7097 end 4 6 to 0 1 206 pb 204 pb 18 30 to 18 65 207 pb 204 pb 15 57 to 15 66 and 208 pb 204 pb 38 22 to 38 35 these compositions are distinctly different from those of mesozoic alkali basalts from the subsurface of southwestern florida heatherington a l mueller p a 1991 geochemical evidence for triassic rifting in southwestern florida tectonophysics 188 291 302 it is suggested that the north florida tholeiites were derived from within the continental lithosphere in contrast with mesozoic volcanic rocks from southwestern florida which have been previously suggested to be related to a mantle plume sm nd isotope systematics suggest derivation of the tholeiites from a 1 ga source a similar age to that proposed by pegram pegram w j 1990 development of continental lithospheric mantle as reflected in the chemistry of the mesozoic appalachian tholeiites usa earth and planetary science letters 97 316 331 for the source of tholeiitic basalts within the carolina terrane this supports proterozoic tectonic models which suggest that both the suwannee and carolina terranes had affinities with south american portions of gondwana containing 1 ga crust specifically the sunsas rondonian province nance murphy 1996 mueller p a kozuch m heatherington a l wooden j l offield t w koeppen r p klein t l nutman a p 1996 evidence for mesoproterozoic basement in the carolina terrane and speculations on its origin in nance r d thompson m d eds avalonian and related peri gondwanan terranes of the circum north atlantic gsa special paper 304 pp 207 218

3d imaging of subducting and fragmenting indian continental lithosphere beneath southern and central tibet using body wave finite frequency tomography abstract we perform a finite frequency tomographic inversion to image 3d velocity structures beneath southern and central tibet using teleseismic body wave data recorded by the tibet 31n passive seismic array as well as waveforms from previous temporary seismic arrays high velocity bodies dip 40 northward beneath the himalaya and the lhasa terrane we interpret these high velocity anomalies as subducting indian continental lithosphere icl the icl appears to extend further north in central tibet than in eastern tibet reaching 350 km depth at 31 n along 85 e but at 30 n along 91 e low p and s wave velocity anomalies extend from the lower crust to 180 km depth beneath the tangra yum co rift yadong gulu rift and the cona rift suggesting that rifting in southern tibet may involve the entire lithosphere the anomaly beneath tangra yum co rift extends down to about 180 km whereas the anomalies west of the yadong gulu rift and east of the cona rift extend to more than 300 km depth the low velocity upper mantle west of the yadong gulu rift extends furthest north and appears to connect with the extensive upper mantle low velocity region beneath central tibet thus the northward subducting indian plate is fragmented along north south breaks that permit or induce asthenospheric upwellings indistinguishable from the upper mantle of northern tibet

magnetic stratigraphy of cenozoic terrestrial sediments and mammalian dispersal the existing literature on the correlation of the magnetic polarity timescale to vertebrate mammalian chronology for the cenozoic is reviewed for all continents except the soviet union the magnetostratigraphic data from north america is the most complete followed by the indian sub continent ten criteria for rating magnetostratigraphic studies are established and the papers reviewed are rated from 1 10 with 10 being the maximum score the timing of mammalian dispersal is examined against the background of information from magnetostratigraphic studies allied with radiometric dating north american land mammal ages are partly defined on immigrant taxa and the boundaries of the nalma are based on dispersal events it has been suggested that dispersal and evolutionary episodes observed in siwalik sediments were correlative to sea level change deduced from delta 18 o data from benthic foraminifera to test this possibility in the general case dispersal events into north america were determined by us

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

accretionary history and crustal evolution of the variscan belt in western europe abstract the variscan belt of western europe is part of a large intra paleozoic belt extending on both sides of the atlantic from the ouachitas in the us and the mauritanides in west africa to the bohemian massif in czechoslovakia and poland the belt was constructed between 500 and 250 ma as a result of convergence and collision between two main continents laurentia baltica and africa after the closure of various oceanic basins iapetus rheic and galicia massif central now recorded as discrete in part cryptic sutures which form the roots of large crystalline nappes in which are found dismembered remnants of oceanic crust and mantle the accretionary history of the variscan belt in western europe is related to the progressive closure of probably two oceanic basins the rheic in the north and another the galicia massif central in the south by respectively southward and northward intraoceanic subduction followed by obduction and intracontinental lithospheric subduction the result is a broad 700 800 km fan like divergent belt which must be considered as a classical obduction collision orogen with a long intracontinental history of about 100 150 ma its arcuate shape is probably due to the northwestward impingement of a promontory of the african continent into laurentia baltica

sequestration of 12c in the deep ocean during the early wenlock silurian positive carbon isotope excursion abstract marine carbonates from three biostratigraphically well dated silurian sections from the mid continent of north america were analyzed for carbon isotopic 13 c carb study the early wenlock positive carbon isotope excursion ireviken excursion is recorded in three sections from tennessee iowa and ohio with values of 4 5 this global shift in the carbon isotopic ratio of marine waters began during a protracted extinction event ireviken event that spanned the llandovery wenlock boundary although several oceanographic models have been developed to account for the coincident changes in lithology biology and carbon isotope stratigraphy during this interval of the silurian their proposed causal connections among glaciation oceanography primary productivity and the global carbon cycle remain a matter of debate this investigation provides carbon isotopic and lithologic evidence that the ireviken excursion occurred during a time of relatively oligotrophic conditions a third order transgressive regressive cycle and increased carbonate production throughout the mid continent of north america the ireviken excursion is recorded in relatively pure carbonate sequences and is immediately preceded by a significant unconformity in each of the measured sections it is proposed that the ireviken excursion was the product of sequestration and burial of 12 c in deep water as a result of altered deep ocean circulation

bathymetry and paleoecology of silurian bethic assemblages late llandoverian new york state late llandoverian marine shales in new york state are highly fossiliferous and provide and opportunity to examine gradients of biofacies the williamson and willowvale formations llandoverian c6 were deposited during a major transgressive episodes in and adjacent to the axis of a narrow ne sw trending central appalachian foreland basin the williamson shale is a black to dark green graptolite bearing unit of the upper clinton group of western new york state it grades eastward into gray fossiliferous shales and coquinoidlimestones of the willowvale formation orientation analysis of graptolites monograptus clintonensis indicates paleoccurents dominantly from the northeast to east the presence of rippled cross laminated sandstone beds tempestites within dark gray graptolitic facies of the lower williamson indicates that the darl shale facies was deposited in dysaerobic to marginally aerobic water slightly below storm wave base poorly fossiliferous green shales higher in the willimason appear to slow deposition in dysaerobic water and suggest a gradual deepening graded brachipod coquinoid limestones interpreted as storm layers occur near the middle and top of the williamson and in the willovale shale and indicate deposition of these sediments above maximum storm wave base the faunas of the williamson and willowvale shales are subdivisible into five vertically and laterally adjacent associations as follows a ichnofossil chondrites b monograptid chonetes cornutus c eopletodonta atrypta d palaeocyclus bryozoan and e eocoelia chonetes cornutus associations this spectrum of fossil associations appears to parallel the increasing frequency of bottom disturbance by waves and currents from deeper basinal areas to higher energy outer and low energy inner shallow shelf settings the fossil associations of the williamson a c probably belong to the deeper benthic assemblages 4 5 but developed in environments within to slightly below storm wave base about 50 80 m deep the willowvale fauna c d representing benthic assemblages 3 4 inhabited somewhat shallower better oxygenated environments of the photic zone probably at depths of approximately 20 50 m the easternmost to slightly southeastward equivalent of williamson willowvale fauna e evidently represents shallow water benthic assemblage 2 these conclusions indicate the need for a critical reevaluation of the bathymetric relations of benthic assemblages 3 5 a new interpretations of approximate depths for this range is presented herein

polybaric geochemical evolution of two shield volcanoes from santorini aegean sea greece evidence for zoned magma chambers from cyclic compositional variations

climatic implications of surface domains in arrays of δd and δ18o from hydroxyl minerals goethite as an example abstract paired d h and 18o 16o analyses of natural goethites range from 224 to 93 for d and from 15 5 to 2 8 for 18o the isotopic systematics of these goethites seem to be most closely mimicked by the goethite water d h and 18o 16o fractionation curves of yapp 1987 yapp 1990 most of these goethites are of unknown age but the large range of their isotopic compositions illustrates the possible paleoenvironmental significance of surface domains calculated for hydroxyl minerals a modern surface domain msd for goethite in coordinates of d and 18o was calculated from the following information a scatter in the modern relationship between surface temperatures and 18o of meteoric water rozanski et al 1993 b average annual surface air temperatures from 0 c to 30 c for the meteoric water sample sites c the meteoric water line mwl of craig 1961 and d the goethite water isotopic fractionation factors of yapp 1987 yapp 1990 goethites that had formed in continental environments in approximate thermal equilibrium with the earth s modern atmosphere would be expected to lie within the msd and 16 of 31 goethites do so the remaining 15 are outliers of the 16 msd samples 12 including the three samples from active soil or bog environments are concordant the calculated isotopic temperatures of the concordant goethites range from 6 c to 27 c and are the same within analytical uncertainty as the modern average surface air temperatures at the respective sites calculated temperatures of the four discordant msd samples range from 7 c to 25 c and are higher than corresponding modern surface temperatures a warm earth surface domain wesd was calculated with an assumed temperature of 35 c for the tropics eight of the 15 outlier samples plot within the portion of the wesd that does not overlap the msd thus these eight goethites may have formed at times when the earth s global climate was warmer than at present temperatures calculated from the eight wesd samples range from 20 c to 33 c and suggest lower latitudinal temperature gradients as predicted by climate models of a warmer earth calculated temperatures of the seven samples that lie outside both the msd and the wesd non sd samples range from 19 c to 69 c the non sd status of these seven samples suggests that they formed in environments dominated by subsurface heat sources surface domains can be calculated for any hydroxyl mineral for which both the hydrogen and oxygen isotope mineral water fractionation factors are known the concept of surface domains provides a criterion for assessing the likelihood that a particular sample records information on ancient climatic temperature

partition coefficients of hf zr and ree between zircon apatite and liquid concentration ratios of hf zr and ree between zircon apatite and liquid were determined for three igneous compositions two andesites and a diorite the concentration ratios of these elements between zircon and corresponding liquid can approximate the partition coefficient although the concentration ratios between apatite and andesite groundmass can be considered as partition coefficients those for the apatite in the diorite may deviate from the partition coefficients the hree partition coefficients between zircon and liquid are very large 100 for er to 500 for lu and the hf partition coefficient is even larger the ree partition coefficients between apatite and liquid are convex upward and large d 10 100 whereas the hf and zr partition coefficients are less than 1 the large differences between partition coefficients of lu and hf for zircon liquid and for apatite liquid are confirmed these partition coefficients are useful for petrogenetic models involving zircon and apatite

palaeomagnetism and 40ar 39ar dating of the tregor dolerites armorican massif france abstract the tregor doleritic dike swarm intrudes the cadomian north tregor batholith armorican massif france alternating field a f and thermal demagnetization performed on 158 samples from 18 dolerite dikes and 37 samples from 4 baked contact sites reveal a multi component magnetization and a complex magnetic mineralogy which was thoroughly investigated with thermomagnetic experiments and polished thin section examinations three magnetization components were recognized in addition to a low temperature component with a viscous origin the most frequently observed component named b d 214 i 10 was isolated at 12 sites independent upon their magnetic mineralogy and appears to be a remagnetization negative baked contact test a carboniferous age of acquisition is inferred from comparison of the b component palaeopole 29 s 318 e with an armorican apwp at some sites after demagnetization of the b component higher temperature and coercivity components were isolated their directions fall into two groups named a and c positive baked contact tests were obtained for both the c component d 174 i 50 whose magnetic carrier is magnetite yields a palaeopole 11 s 2 e which is similar to some ordovician palaeopoles from the british isles south of the lapetus suture or eocambrian palaeopoles from the tregor area the a component d 191 i 33 whose magnetic carrier is magnetite partly transformed to leucoxene yields a palaeopole 58 s 336 e and a palaeolatitude 17 9 n not previously observed for the armorican massif in palaeozoic time comparison between this palaeolatitude and those for northern spain from ordovician to carboniferous time suggests a late silurian remanence age this interpretation supports the model of apparent polar wander for gondwana between ordovician and devonian time proposed by van der voo and bachtadse and briden 40 ar 39 ar dating of mineral separates from the dikes do not allow us to place constraints on the acquisition age of the a and c components

seismicity and tectonics of the bay of bengal evidence for intraplate deformation of the northern indian plate abstract seismicity and fault plane solutions of several earthquakes in the bay of bengal have been studied to focus attention on the nature of intraplate deformation in the northern indian plate a widely dispersed diffuse seismicity provides evidence that the intraplate region of the entire bay of bengal and peninsular india of the indian plate are seismically active of which the gangetic delta and the eastern flank of the ninety east ridge show somewhat higher seismicity in comparison to the thickly populated epicenters at the eastern plate boundary predominance of thrust faulting earthquake mechanisms with consistent north dipping fault planes and more or less n s pressure axes imply that n s compression is more prevalent in the intraplate region of the bay of bengal which is analogous to the himalayan arc this intraplate seismicity and n s compressive deformation is presumed to be the result of collisional resistance to the north moving indian plate with the eurasian plate at the himalayas where subduction of the indian lithosphere in mantle has stopped recently as a result to compensate the horizontal compressive force by ridge push at the carlsberg ridge the intraplate region of the northern indian plate in continuation to the central indian ocean basin no longer remains passive due to a strong repulsive force at the collisonal plate boundary himalaya that causes stress transmission towards south and makes the intraplate region seismically active

petrologic hypothesis testing with pearce element ratio diagrams derivation of diagram axes in petrology pearce element ratio per diagrams have been used i to determine whether members of a rock suite are co genetic ii to identify the minerals involved in differentiation processes and iii to evaluate the extent to which those mineral are involved the axis coefficients of each diagram are chosen such that sorting of minerals or combinations of minerals will generate unique and predictable trends unfortunately selection of the optimal combination of axis coefficients is a difficult task especially if the system being investigated has a large number of phases or complicated solid solution minerals our work has established a formal set of rules and matrix operations which facilitate the determination of per diagram axes coefficients this methodology can be used to determine the unit molar vector displacement caused by the addition or subtraction of a specific mineral given a set of axis coefficients it can also be used to create per diagrams on which minerals have predetermined vector displacements by designating all vector displacements to be parallel axis coefficients for assemblage test diagrams can be determined to test the following hypothesis the observed chemical variation is due to the addition or removal of a specific set of minerals alternatively by designating all vector displacements to be mutually perpendicular phase discrimination diagrams can be created which test whether the observed chemical variations require a specific phase to be involved in differentiation phase discrimination diagrams also provide a means to estimate the extent of that involvement this methodology facilitates construction of powerful yet simple per diagrams which provide an effective means of testing alternative differentiation hypotheses

petrologic and geochemical attributes of fracture related dolomitization in ordovician carbonates and their spatial distribution in southwestern ontario canada abstract middle ordovician trenton group carbonates are fractured and extensively dolomitized along the axis of the algonquin arch in southwestern ontario hydrocarbon reservoirs formed where these dolomitized fracture zones penetrate otherwise impermeable host limestones three different types of dolomite d1 d2 and d3 are distinguished petrographic characteristics and 18 o values indicate that d1 formed during early diagenesis from middle ordovician seawater and recrystallized during progressive burial whereas fracture related replacive matrix dolomite d2 formed by hydrothermal fluids 68 99 c late stage saddle dolomite d3 and calcite c3 cements occlude fractures based on petrographic fluid inclusion and stable isotope data d3 dolomite and c3 calcite formed from warm 68 144 c saline 22 24 wt nacl cacl 2 hydrothermal fluids the least radiogenic 87 sr 86 sr values 0 70830 0 70842 of d2 are consistent with estimated values of devonian and silurian seawater whereas the slightly enriched 87 sr 86 sr ratios of d2 and d3 0 70902 0 70918 suggest their precipitation from fluids similar in composition to oil field brines rare earth element ree results of d2 d3 and c3 indicate enrichment in rees content of these mineral phases relative to undolomitized host rock the similarity in the average rees pattern of d2 d3 and c3 and the overlying blue mountain shale and basement rocks suggest progressive water rock interaction magnesium required for dolomite precipitation was supplied by mg rich seawater derived silurian and or devonian saline waters from dissolution of silurian evaporites which descended along faults and fractures to reservoir depths at the center of the basin while being heated hot basinal brines migrated laterally through basal sandstones and ascended into the network of faults and fractures and precipitated fracture related dolomite the abundance of fracture related dolomite in the periphery of michigan basin in southwestern ontario suggests that dolomitizing fluids originated from the michigan basin rather than appalachian basin

magmatic stratigraphy of the tilted tottabetsu plutonic complex hokkaido north japan magma chamber dynamics and pluton construction abstract the steeply tilted tottabetsu plutonic complex tpc provides an exceptional cross sectional view of a frozen felsic magma chamber the entire pluton is apparently cylindrical in shape but the active magma chamber at any one time was much thinner tabular in shape the pluton is stratified with a topmost granitic unit zone iii an underlying dioritic unit zone ii and a lowermost gabbroic unit zone i field petrographic and geochemical data suggest that zones i and ii developed at the aggrading floor of a felsic magma chamber only part of zone iii appears to have crystallized from the roof successive stoping events recorded as included country rock blocks in zones i and ii apparently prohibited growth of the roof solidification zone the complex record of hotter replenishments and fractional crystallization is preserved in the stratigraphy of zones i and ii as either alternating comagmatic sheets and cumulate layers or an upward fractionation trend with a major compositional reversa

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

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

isotopic and trace element constraints on the origin and evolution of alkaline and calc alkaline magmas in the northwestern mexican volcanic belt a wide variety of rock types has been produced by pliocene to recent volcanism in the northwestern portion of the mexican volcanic belt composite cones erupted rocks typical of calc alkaline volcanic belts associated with subduction monogenetic cinder cones surrounding the composite cones erupted mildly alkaline basalts and related rocks one larger center las navajas produced basalts trachytes and peralkaline rhyolites such alkaline rocks are typically associated with crustal extension related to rifting both subduction and rifting appear to be taking place in western mexico the most basic calc alkaline rocks although somewhat fractionated show depletion in high field strength elements hfse and enrichment in alkaline earth elements relative to other incompatible elements a characteristic typical of magmas in subduction related volcanic arcs the basic alkaline rocks show no depletion in hfse and show higher concentrations of incompatible elements than the basic calc alkaline rocks the calc alkaline rocks show a tight cluster of 87sr 86sr and 143nd 144nd ratios both of which correlate weakly with sio2 basic alkaline rocks have lower sr and higher nd isotopic ratios whereas the more differentiated alkaline rocks have isotopic ratios that overlap with and extend beyond the range of values found in calc alkaline rocks trace element data indicate that calc alkaline magmas could not have been derived by crystal fractionation from the alkaline magmas nor by assimilation of crustal materials by alkaline magmas the evolution of both calc alkaline and alkaline rocks requires a process of crystal fractionation accompanied by assimilation afc of crustal rocks to account for changes in isotopic ratios and trace element concentrations in the more differentiated members of each suite assuming a contaminant located in the lower or middle crust afc modeling shows that the amounts of assimilation required for alkaline magmas are somewhat higher than those for calc alkaline magmas calc alkaline and alkaline rocks appear to have resulted from melting of two distinct sources alkaline from an oib type source and calc alkaline from a mixed mantle and slab derived source

swell dominated carbonates on a mississippian ramp in the canadian rocky mountain front ranges abstract the livingstone formation and the middle opal member of the mount head formation of western canada are mississippian carbonate deposits tens to hundreds of meters thick that are interpreted to have accumulated in a mid ramp wave dominated sub thermocline environment at water depths up to 100 m or more the rocks are predominantly pelmatozoan grainstones and packstones that contain abundant physical sedimentary structures the most common sedimentary structures are planar lamination swaly and hummocky cross stratification and normally graded event beds that are interpreted as having been formed by oscillatory wave currents or oscillatory dominated combined flows the abundant oscillatory currents are interpreted to have been generated by swell waves and to a lesser extent by local storms high angle planar tabular cross bedding is also present in rare discrete beds produced by unidirectional offshore directed currents the unidirectional currents that produced the dunes are interpreted to have been generated by coastal set up that produced downwelling storm currents during the rare storms that occurred along the mississippian coastline it is hypothesized that local storms were not common due to the west coast location and the tropical latitude of the study area instead it is postulated that swell waves approaching from the broad panthalassa ocean to the west were responsible for most sediment remobilization and deposition across the mid and outer ramp swell waves are believed to have resuspended sediment to greater depths than short period less organized storm waves these mississippian deposits suggest that in situ grain production on the ramp without need for the offshore transport of sediment from the shoreface resulted in two major differences from siliciclastic wave dominated settings 1 swaly cross stratification was not restricted to environments shallower than hummocky cross stratification on the upper shoreface because sand size sediment was generated across a large portion of the ramp and 2 aggradation by means other than sediment supply from shallower water caused preservation of deep water swell dominated deposits

state of stress and its bearing on tectonics under the tibetan plateau abstract using the composite plot of pressure and tension axes of the available focal mechanism solutions of earthquakes this paper investigates the nature and process of stress systems and their impression on tectonics under the tibetan plateau a composite plot shows that the tibetan plateau is now experiencing a nne ssw directed compressive stress with an almost ese wnw tensile stress and it confirms a wrench movement towards the east along a prominent strike slip fault transcurrent tectonics appear to be the principal phase of deformation in tibet in response to the northward push of india this study also suggests that the extensional tectonics of the tibetan plateau primarily involve a combination of compressive and shear stresses as a consequence of continued continental convergence rather than merely a gravitational body force

changes in silicic melt structure between the two bandelier caldera forming eruptions new mexico usa evidence from zirconium and light rare earth elements

baltica a synopsis of vendian permian palaeomagnetic data and their palaeotectonic implications torsvlk t h smethurst m a van der voo r trench a abrahamsen n and halvorsen e 1992 baltlca a synopsis of vendlan permmn palaeomagnetic data and their palaeotectonlc implications earth sci rev 33 133 152 in light of recent additions to the palaeozoic palaeo magnetlc data base particularly for the ordovlclan era a revised apparent polar wander apw path for baltica has been constructed following a rigorous synthesis of all late precambrlan perm lan data the apw path is characterized by two prominent loops firstly a late precambrlan camb rlan loop probably relating to a rifting event and secondly a younger loop relating to a mid silurian scandlan colhsion event these features imply major change in plate tectonic reconfiguratlon baltlca probably represented an individual continental unit m early palaeozoic times and was positioned m high southerly latitudes in an inverted geographic orientation in such a reconstruction baltlca was separated from the northern margin of gondwana by the tornqulst sea and from laurentla by the iapetus ocean the tornqulst zone is thus interpreted as a passive or dextral transform margin during the early palaeozoic while undergoing counter clockwise rotations up to 1 6 ma baltica drifted northward through most ol the palaeozoic except for a short period of southerly movement in late silurian early devonian times after colhslon with laurentla rapid movements in latitude up to 9 cm yr are noted m late precambrlan earl y palaeozoic times and slgmficant decrease in velocities throughout palaeozoic time probably reflect the progressive amalgamation of a larger continent by early devonian euramerlca and permian pangea times the tornqulst sea had a principal component of palaeo east west orientation hence it is difficult to be precise in the timing of when micro continents such as eastern avalonla and the european masslfs ultimately collided along the southwestern margin of baltica these micro continents are considered to have been peripheral to gondwana m high southerly latitudes during the early ordovlclan eastern avalonla clearly had rifted off gondwana by llanvlrn llanded o times and may have collided with baltlca during late ordovlclan times although the present available sdunan palaeomagnetlc data from eastern avalonia may suggest colhslon in late silurian times across the lapetus facing margin of baltica laurentla was s tuated in equatorml to southerly latitudes during most of the lower palaeozoic these continents collided in mid silurian times l e a first collision between southwestern norway and greenland scotla nd which gave rise to the early scandlan orogeny 425 ma in southwestern norway possible followed by a later but less dramatic scandlan event in northern norway at around 410 ma since baltlca was geographically reverted m early palaeozoic times the colhsional margin could not have been a margin that once rifted off laurentm as assumed in a number of plate tectonic models

modelling of igneous fractionation and other processes using pearce diagrams geochemical data can be quantitatively modelled by means of pearce diagrams these are graphs of a z vs b z where a b and z are compositional abundances e g wt sio2 wt mgo and ppm la and z has the additional property of having constant absolute abundance in the terminology of igneous petrology z the common denominator variable could be an incompatible element the numerators a and b may be complex algebraic combinations of elements or even cipw normative abundances the utility of pearce diagrams lies in the fact that slopes of data distributions equal the bulk a b ratio of minerals lost or gained from a suite of cogenetic rocks there is no distortion because these plots correct for data closure terms of the form ai z0 zi where z0 is the abundance in a reference sample remove the scaling to ai caused by the abundance of a particular choice of z subtraction of these terms for different samples e g ai z0 zi aj z0 zj quantifies mineral losses and gains mathematical analysis shows that limited compatibility of the denominator variable is permitted a bulk partition value d of 0 1 introduces an error of only 10 in values of ai z0 zi and 10 in slope angle on pearce diagrams over a crystallization interval of 50 for d 0 01 the error is minimal for a crystallization interval over 90

deformation of the ne basin and range province the response of the lithosphere to the yellowstone plume summary tectonic deformation in the ne basin and range province of the western united states is concentrated in two zones the n s trending northern part of the intermountain seismic belt in northern utah se idaho and sw wyoming and the e w trending central idaho seismic zone which converge at yellowstone in nw wyoming accepting the popular view that the yellowstone volcanic field is above an upwelling plume from the deep mantle i explore the possibility that deformation of the brittle upper crust in these two seismic zones may be caused by return flow from this plume this return flow is assumed sheared sw in the direction of motion of the north american plate relative to yellowstone the snake river plain srp a linear outcrop of neogene and quaternary basalt that trends sw from yellowstone for more than 400 km bisecting the angle between the two seismic zones may be a trail formed as the plate moves sw across the upwelling plume beyond 150 km from yellowstone slip vectors on active normal faults in both deforming zones are oriented inwards at 45 towards the sw azimuth along the srp normal slip rates on these faults are typically 1 mm yr 1 implying extensional strain rate 10 15 s 1 proportions of normal slip and strike slip suggest faults in the intermountain seismic belt are rotating anticlockwise at 2 myr 1 around vertical axes and faults in the central idaho seismic zone are rotating clockwise at similar rates within 100 km of yellowstone observed sense of rotation around vertical axes in both zones appears reversed tilt rates of hanging wall beds suggest most faults in both zones are rotating at 2 myr 1 also 10 15 s 1 around horizontal axes oriented nw the relative size of rotation rate around vertical axes and horizontal extensional strain rate is shown to imply that velocity in the underlying deforming fluid is small in the direction perpendicular to the axis of the srp in an initial model this velocity is set to zero enabling the two dimensional pattern of flow at the horizontal upper boundary of this deforming fluid to be solved straightforwardly this approximation enables observed rates and senses of horizontal extension and rotation around a vertical axis to be explained as consequences of the pattern of horizontal gradients of velocity parallel to the axis of the srp in the deforming fluid beneath the brittle layer a refinement suggests that velocity perpendicular to the srp near the upper boundary of the deforming fluid is small but inward towards the srp from both sides and is accompanied by downwelling beneath the srp beyond 100 km sw of yellowstone this pattern of flow will act to move magma in the uppermost mantle beneath the srp from its sides it may thus reconcile petrological observations that srp basalts do not show deep mantle characteristics expected had they been extruded from the yellowstone plume with the existence of this plume nearby conservation of angular momentum around horizontal axes perpendicular to the srp for the combined system of the plume interacting with the north american plate leads to the deduction that the plume radius is 50 km consistent with results of other investigations and its upwelling velocity is 60 mm yr 1

basalts of the north fiji basin the generation of back arc basin magmas by mixing of depleted and enriched mantle sources active spreading ridges in the north fiji basin range from well developed stable ridges where largescale mantle upwelling is in progress to proto ridges where spreading is incipient south of 17 s where the central ridge of the north fiji basin has a bathymetric profile normally expected of a fast spreading steadystate mid ocean ridge basalts are evolved n type morbs north of 17 s where the central ridge is propagating northward into old north fiji basin crust and spreading is in the initial stages two types of basalt have been recovered n type morbs from this northern arm of the central ridge are believed to be samples of older north fiji basin crust basalts with transitional alkalic chemistry up to 0 5 ne in the norm and characterized by strong relative enrichments in rb ba k nb la ce sr p zr and ti are believed to be associated with incipient rifting among the latter group are compositions that are intermediate between transitional alkalic types and morbs and these are geochemically similar to the back arc basin babb magma type defined by sinton and fryer 1987 from a study of mariana back arc basin basalts dredges along the south pandora ridge a transform zone characterized by short spreading segments are dominated by basalts that are enriched in large ion lithophile and high field strength minor and trace elements and compositions range from types resembling ocean island tholeiites to transitional alkalic varieties basalts from rotuma are regarded as alkalic end members of the south pandora ridge magmatic spectrum in areas of the north fiji basin where relatively fast spreading must be accompanied by largescale asthenospheric upwelling depleted n type morbs dominate whereas in areas of slow mantle upwelling or where some other tectonic effect e g a transform fault causes a transient thermal disturbance within the lithosphere or upper asthenosphere enriched alkalic magmas either dominate or make a significant and noticeable contribution to the overall chemical characteristics of basalts being erupted the morbs have a depleted asthenospheric source and the alkalic component is believed to derive from an enriched lithospheric or shallow asthenospheric source the babb magma type may simply be part of the spectrum of mixed magmas that can occur in the transitional tectonic settings represented by the early development of most back arc basins

tectonic significance of late ordovician silicic magmatism avalon terrane northern antigonish highlands nova scotia12 1 this article is one of a series of papers published in cjes special issue in honour of ward neale on the theme of appalachian and grenvillian geology 2 contribution to international geological correlation programme igcp project 497 avalonia was a microcontinent during most of the ordovician separating the iapetus ocean to the north from the rheic ocean to the south in the northern antigonish highlands nova scotia volcanic rocks dunn point and mcgillivray brook formations underlie early silurian early devonian strata arisaig group and were thought to represent extension that heralded the development of the basin into which arisaig group strata were deposited however recent u pb zircon thermal ionization mass spectrometry data from rhyolite in the dunn point formation dpf yielded an age of 460 0 3 4 ma and here we report a concordant age of 454 5 0 7 ma for an ignimbrite in the overlying mcgillivray brook formation mbf these data confirm a ca 10 million year gap between volcanism and onset of arisaig group deposition which occurred after accretion of avalonia to baltica the dpf and mbf both resemble a type sio2 rich magmas the mbf has very high concentrations of zr 745 1965 ppm y 65 213 ppm nb 57

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana

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

shear wave splitting across the ailao shan red river fault zone sw china 1 a broad band seismic array was set up across the ailao shan red river asrr fault zone sw china the shear wave splitting parameters on sks and skks phases are calculated using the method of silver and chan 1991 and indicate the polarization direction of 105 6 and the delay time of 1 68 0 44 s beneath the asrr fault zone combined with previous studies the average polarization direction is 110 19 and the delay time is 1 58 0 44 s to the sw side of the fault while the values are 83 30 and 1 29 0 29 s to the ne side this result implies that the upper mantle anisotropy to two sides of the asrr fault zone are different compared with the se polarization direction observed in the p s phase and the n to s surface movement in gps data our results suggest decoupling of the crust and upper mantle beneath the study region

astronomical forcing of meandering river behaviour milankovitch cycles in devonian of east greenland abstract a more than 1100 m thick succession dominated by single and multistorey sandstone bodies from the upper devonian kap graah group in east greenland is interpreted as a meandering river and associated overbank deposits the sandstone bodies display well developed lateral accretion bedding and are interpreted to be deposited as point bars in a well exposed section 215 m thick individual sandstone bodies have been analized quantitatively in order to compare the individual palaeochannel characteristics the results indicate cyclic variations in discharge channel slope and channel size the cycles are of the order of c 20 m thick with superimposed c 100 m mega cycles both are interpreted as reflecting cyclic variations in precipitation rate the cycles are interpreted as being governed by astronomical forcing of the climate reflecting milankovitch type precession cycles in the order of 20 000 years with modulation by c 110 000 years eccentricity mega cycles this study confirms that the precession and eccentricity cycles exhibit the main forcing on low latitude monsoonal climate

a cambrian pole for the moroccan coastal meseta summary middle cambrian volcanics of sidi said maachou east of el jadida western morocco 158 cores 23 sites sampled in two localities north of sidi said maachou were subjected to progressive alternating field and thermal demagnetization thermomagnetic cycles isothermal remanent magnetization acquisition experiments as well as microscopic observations the magnetic mineralogy of these volcanic rocks is dominated by magnetite and titanomagnetites the palaeomagnetic analysis has revealed two characteristic components of magnetization the first direction d 300 i 31 95 6 pole position at 14 n 231 e p 4 m 7 palaeolatitude 16 7 s is carried by magnetite and yields a positive fold test it is interpreted using the results of a baked contact test as the original magnetization of middle cambrian age 520 ma the second direction d 121 i 20 95 6 pole position at 18 s 55 e p 3 m 6 is nearly anti parallel to the first magnetization but the fold test is not conclusive and it is interpreted as an overprint that occurred a short time after this volcanism this study provides the first detailed palaeomagnetic result from cambrian rocks in the moroccan meseta

storm effects on siliciclastic and carbonate shelf sediments in the medial silurian succession of pennsylvania abstract upper llandoverian through wenlockian strata in the pennsylvania part of the northern appalachian basin formed on a gentle epeiric ramp that extended from marginal tidal flats through coarser grained mid shelf shoals to a muddy basin center over approximately 10 million years the composition of the accumulating sediment evolved from siliciclastic to carbonate while the ramp maintained a relatively similar configuration and position below sea level and while storm generated processes continued as the major producers of sediment characteristics in both siliciclastic and carbonate parts of the succession sedimentary structures and their sequences are generally similar with only a small number of dissimilar features resulting from the differences in composition small scale coarsening and shallowing upward sequences are present in both mid shelf bar and basin transition deposits although specific sequence characteristics are different in siliciclastic and carbonate parts such sequences record cyclic fluctuations of relative sea level while the composition of the sediment evolved over this extended period the gentle ramp gradient was maintained largely by the processes generated by storms such processes were more effective because of the fluctuations of relative sea level

the palaeogeographic evolution of southern britain during early palaeozoic times a reconciliation of palaeomagnetic and biogeographic evidence abstract palaeomagnetic biogeographic and palaeoclimatic evidence together suggest that in the early ordovician southern britain lay off the west african margin of gondwana at around 60 s the combination of these disciplines reduces latitude uncertainties to around 5 to 10 degrees rifting of avalonia from gondwana was followed during most of the ordovician by northward drift as avalonia and baltica converged with subduction of tornquist sea crust both continents had similar moderate northward drift rates 3 6 cm yr so their convergence probably had a considerable longitudinal component silurian and early devonian data indicate that northward drift of avalonia and baltica continued after their collision and that southern britain reached a latitude of around 20 s by c 400 ma

paleomagnetism of the navan zn pb deposit ireland for the past quarter century the navan mine has exploited europe s premier zn pb deposit its genesis is controversial in part because it has not proved amenable to radiometric dating the ore is epigenetic and is hosted in lower carboniferous platform carbonates of the early courceyan navan group 351 4 ma and of the late chadian early arundian boulder conglomerate 338 1 ma paleomagnetic analyses were done on 282 specimens from 26 sites in the mine using alternating field and thermal step demagnetization plus isothermal remanence analyses paleomagnetic fold tests on navan group ore and arundian carbonate strata and conglomerate tests on mineralized and unmineralized clasts were negative meaning that the clasts and adjacent rocks were magnetized at some time after clast deposition and that the magnetization postdates folding further a contact test using a tertiary dike that cut ore was positive meaning that the remanence predates intrusion of the dike both the ore and host rocks retain a stable characteristic remanence direction in single to pseudosingle domain magnetite at d 196 0 i 6 0 n 21 95 2 6 k 147 that defines a pole position of 25 6 w 31 8 s p 1 3 m 2 6 comparison of the pole to the rotated laurentian and unrotated eastern avalonian apparent polar wander paths yields a late arundian to early asbian age of 333 4 ma for the chemical remanent magnetization that is primary in the ore minerals and a secondary remagnetization in the host rocks the result implies that the ore mineralization event followed deposition and tilting of the overlying arundian holkerian lucan formation at navan

along strike acadian structural variations in the quebec appalachians consequence of a collision along an irregular margin the quebec appalachians have been shaped by two major orogenies the middle to late ordovician taconian and the middle devonian acadian thrust faults and nappe structures characterized the taconian deformation throughout the quebec appalachians whereas structural styles pertaining to the acadian orogeny differ from southern quebec to the gaspe peninsula structural analysis of late ordovician to middle devonian supracrustal rocks shows two different acadian deformational regimes strike slip tectonics in the gaspe peninsula and thrust or dip slip tectonics in southern quebec in southern quebec acadian regional deformation produced folds which vary from upright to overturned and tight to isoclinal from the nw to the se major faults e g la guadeloupe are northwestward directed thrust faults marked by highly ductile shear zones outlined by mylonite in the gaspe peninsula structural trend is ne and major e w dextral strike slip faults transect this trend folds are open and upright inclined and tighter near the major faults where they have a clockwise rotation like the regional cleavage major faults e g grand pabos follow corridors delimiting high strain zones where the fabrics developed are indicative of a ductilebrittle regime of deformation in the temiscouata region major structural features of both southern quebec and the gaspe peninsula are recognized structures of this region consist of ene trending dextral strike slip faults and high angle wnw verging reverse faults parallel to major nne folds these structures reflect the transition from purely horizontal movement along major strike slip faults of the gaspe peninsula to vertical movement along thrust faults in southern quebec acadian structural variations within the quebec appalachians are interpreted in terms of a continental collision of gondwana along the irregular margin of laurentian and its taconian accreted terranes

drip instabilities of continental lithosphere acceleration and entrainment by damage summary geological evidence indicates large volume lithospheric drip instabilities form under regions of continental lithosphere the size of these drips suggests that either the base of the lithosphere is volumetrically increased or the highly viscous upper lithosphere participates in the drip previous theoretical models using simple rheologies have been unable to produce unforced large volume drip instabilities thus large drip instabilities are typically induced by thickening or destabilizing the mobile base of the lithosphere through tectonic forcing such as convergent thickening or dense magmatic emplacement following extension here we propose a theory of lithospheric drip instabilities that may arise naturally from the thermal and rheological structure of earth independent of specific tectonic forcings using damage physics relevant for earth we find a large portion of the lithosphere may be mobilized and entrained into growing drip instabilities for a critical amount of damage the growth is accelerated sufficiently that large volume drip instabilities may form within geologically feasible time frames we therefore suggest large volume lithospheric drip instabilities may arise independently of tectonic settings through damage assisted mobilization and entrainment of the highly viscous lithosphere

ordovician metre scale cycles implications for climate and eustatic fluctuations in the central appalachians during a global greenhouse non glacial to glacial transition abstract metre scale shallowing upward cycles in ordovician carbonates of the central appalachian basin record climate and eustatic fluctuations during a transition from early ordovician global greenhouse to late ordovician icehouse conditions peritidal facies and shale abundance suggest a long term trend in this area from semi arid early ordovician to more humid middle to early late ordovician conditions with a return to semi arid conditions during the late ordovician the climatic fluctuations were most likely produced by tectonics uplift and erosion related to taconic orogenesis plate motion of north america and the areal extent of water covering the shelf peritidal cyclic facies indicate that high frequency relative sea level fluctuations were of small amplitude 20 m relative sea level fluctuations that decreased into the later ordovician if these sea level changes are eustatic then the increased amplitude may mark the early initiation of continental glaciation on gondwana in the late middle ordovician followed by waning of ice sheets prior to the latest ordovician glaciation

late devonian glacial deposits from the eastern united states signal an end of the mid paleozoic warm period abstract a late devonian polymictic diamictite extends for more than 400 km from northeastern pennsylvania across western maryland and into east central west virginia the matrix supported unbedded locally sheared diamictite contains subangular to rounded clasts up to 2 m in diameter the mostly rounded clasts are both locally derived and exotic some exhibit striations faceting and polish the diamictite commonly is overlain by laminated siltstone mudstone facies associations laminites the laminites contain isolated clasts ranging in size from sand and pebbles to boulders some of which are striated the diamictite laminite sequence is capped by massive coarse grained pebbly sandstone that is trough cross bedded a stratigraphic change from red calcic paleo vertisols in strata below the diamictite to non calcic paleo spodosols and coal beds at and above the diamictite interval suggests that the climate became much wetter during deposition of the diamictite the diamictite deposit is contemporaneous with regressive facies that reflect fluvial incision during the late devonian of the appalachian basin these deposits record a late devonian episode of climatic cooling so extreme that it produced glaciation in the appalachian basin evidence for this episode of climatic cooling is preserved as the interpreted glacial deposits of diamictite overlain by glaciolacustrine varves containing dropstones and capped by sandstone interpreted as braided stream outwash the appalachian glacigenic deposits are contemporaneous with glacial deposits in south america and suggest that late devonian climatic cooling was global this period of dramatic global cooling may represent the end of the mid paleozoic warm interval that began in the middle silurian

18o16o and 13c12c in lower paleozoic articulate brachiopods implications for the isotopic composition of seawater three hundred and eleven articulate brachiopods with extensive geographic coverage spanning the cambrian to silurian stratigraphic interval were analyzed for oxygen and carbon isotopic composition cambrian samples have 18o 7 ordovician samples 2 4 and silurian samples 1 9 confirming the previously established trend towards lighter isotopic compositions with increasing age throughout the lower paleozoic forty nine best preserved ordovician and silurian samples were identified based on scanning electron microscopy and trace element analysis they were found to bracket the isotopic compositions of over 85 of specimens from these stratigraphic intervals supporting widespread preservation of the isotopic signal some latest ordovician and lower ludlovian samples associated with shale sequences are apparently enriched in 18o these are interpreted as an environmental phenomenon perhaps related to water temperature and glaciation a number of silurian samples of varying genera and stratigraphic levels are highly enriched in 13c up to 6 some are shale related but some are associated with carbonate bearing basins these are also thought to represent near original compositions but a single environmental cause is unknown the present data show that luminescence is not a decisive criterion for evaluating the degree of brachiopod preservation whole shell values were isotopically similar to their nonluminescent portions for both oxygen and carbon

comparative position of bateny and biya katun terrains altai sayan folded area russia in cambrian based on combined paleomagnetic lithologic and paleontological data abstract cambrian events in the history of the different terranes comprising mosaic block geological structures of the altai sayan folded area is discussed on the basis of the complex analysis of the paleontological lithological and paleomagnetic data the probable territorial proximity of the biya katun gorny altai and bateny kuznetsky alatau blocks is proved

production and preservation of organic matter during deposition of the bakken formation late devonian and early mississippian williston basin abstract the organic rich up to 35 toc lower and upper black mudstone members of the bakken formation late devonian and early mississippian accumulated in response to both increased productivity and enhanced preservation that resulted from unique palaeogeographic palaeoceanographic and palaeoclimatic conditions during bakken deposition the semi isolated williston basin was located close to the equator 5 to 10 n in a zone of prevailing east to west winds a pattern of estuarine like marine circulation between the williston basin and open ocean conditions at the western craton edge of north america is proposed surface water discharged from the basin was replaced by nutrient rich deeper water sourced from the equatorial undercurrent in the pacific basin undercurrent flow into the williston basin upwelled providing a potentially sustainable source of nutrients to enhance productivity in the surface photic zone during black mudstone deposition in response to increased productivity that led to a high rate of organic sedimentation and quiescent deep water conditions due primarily to the geographic isolation of the basin and the restriction of estuarine like circulation to the upper water column anoxic bottom water developed on the basin floor as sea level rose during lower and upper black mudstone deposition anoxic bottom water expanded across the basin enhancing organic matter preservation conditions in the williston basin demonstrate that both increased productivity and improved preservation can be essential and complementary factors affecting organic rich mud deposition these results have potentially important implications for the accumulation of similarly organic rich deposits across the north american interior during late devonian and early mississippian time

les bassins d avant pays de la chaine hercynienne au carbonifere inferieur the evolution of marine sedimentary basins deformed by the hercynian orogeny was studied in germany france iberia northwest africa and north america in the lower carboniferous these basins opened along strike slip faults and were filled with turbidites and shelf deposits until the main compressive phase at the end of visean and the namurian the infilling was interrupted with frequent volcanic episodes and gravity induced movements according to sedimentary vergence these basins were set into two festoons on both sides of an elongated and emergent swell structured during the devonian and corresponding to the internal zones of the orogen these basins were filled during a general tectonic compression phase they correspond to the foreland basins of the hercynian belt

devonian paleomagnetism of yunnan province across the shan thai south china suture a paleomagnetic study of devonian sedimentary sequences from western and eastern yunnan china yields contrasting paleolatitudes for these two areas they indicate an equatorial position for eastern yunnan as part of the yangtze paraplatform or south china block and a paleolatitude of about 40 for western yunnan as part of the shan state or shan thai block which probably was part of the gondwana supercontinent the majority of samples is strongly overprinted by a present day field magnetization but thermal demagnetization has been successful in isolating ancient dual polarity characteristic directions of magnetization in rocks from eastern and western yunnan with results from the latter passing the fold test the paleopole from eastern yunnan is located at 8 9 n 10 4 e declination inclination 279 2 2 7 k 13 alpha 95 6 9 n 33 samples from 7 sites the paleopole from western yunnan is located at 66 5 n 133 8 e declination inclination 18 0 62 0 k 21 alpha 95 5 8 n 31 samples from 7 sites yielding a paleolatitude of 42 the two blocks studied are separated by the red yuan river and lancang mekong river fault zones one of which is inferred to be an ancient suture

paleobiogeographic patterns in the middle and late devonian emphasizing laurentia abstract the relative importance of tectonism and sea level change in driving middle to late devonian biogeographic patterns in marine invertebrates is investigated using phylogenetic biogeography taxa considered include two clades of brachiopods one bivalve clade and a suborder of phyllocarid crustaceans these organisms colonized different depositional environments and played different roles in the devonian ecosystem which allows biogeographic patterns to be considered in a cross faunal analysis vicariance patterns are well resolved and may reflect biogeographic relationships developed in the early to early middle devonian due to the uplift of intracratonic arches uplift of tectonic arches is inferred to occur during times of tectonic quiescence during the second and third and possibly also the first tectophase after ettensohn f r 1985 the catskill delta complex and the acadian orogeny a model in woodrow d l sevon w d eds the catskill delta geological society of america boulder co special paper 201 pp 39 49 of the acadian orogeny geo dispersal patterns are not as well resolved but may indicate dispersal across tectonic arches during downwarping associated with increased compressional tectonism during ettensohn s 1985 ettensohn f r 1985 the catskill delta complex and the acadian orogeny a model in woodrow d l sevon w d eds the catskill delta geological society of america boulder co special paper 201 pp 39 49 tectophases ii and iii geo dispersal across relatively low arches may also be coincident with transgressive events iia and iic of johnson et al 1985 johnson j g klapper g sandberg c a 1985 devonian eustatic fluctuations in euramerica geological society of america bulletin 96 567 587 the overall lack of congruence between the vicariance and geo dispersal patterns suggests that singular tectonic events exerted more profound influence on devonian biogeography than cyclical changes in sea level furthermore the offset in perceived timing between vicariance and geo dispersal patterns may indicate a fundamental change in the style of biogeographic patterns during the middle devonian and may have played a major factor in regulating biodiversity dynamics during the late devonian biodiversity crisis

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

late palaeozoic heavy mineral and clast modes from the betic cordillera southern spain transition from a passive to an active continental margin abstract silurian to late carboniferous sediments constitute most of the uppermost nappe malaguides in the internal zone of the betic cordillera the sediments represent continuous deep water environments dominated by flysch deposition repeated changes between acid plutonic recycled cratonic and metamorphic sedimentary input are evidenced by heavy mineral spectra and psephitic clast modes of the devonian carboniferous formations oscillations and amplitudes especially of metamorphic detrital influx become more frequent and larger respectively near the variscan disconformity permo triassic redbeds are clearly post orogenic sediments according to lithofacies and heavy mineral content the joint development of lithofacies clast modes and heavy mineral spectra demonstrates the transition from a passive to an active continental margin during the palaeozoic the silurian devonian sediments of the malaguide realm were deposited during a divergent cratonic stage north of the betico rifean block which is a detached sliver of the african plate the visean upper carboniferous formations record the collision orogenic stage between the betico rifean block and the southern margin of the southern europe plate the collision processes induced strong uplift and erosion of successively shallowing carbonate shelf realms and the connected crystalline mostly metamorphic hinterland the source area was the betico rifean block south of the malaguide basin palaeogeographic connections of the malaguides especially with minorca baleares are shortly discussed

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

petrogenesis of rhyolites and trachytes from the deccan trap sr nd and pb isotope and trace element evidence trachytes and rhyolites from salsette island north of bombay have distinctive trace element and isotope features which mark them out from typical crustal melts their highly incompatible trace element and sr nd and pb isotope ratios are similar to those of the associated deccan flood basalts thus the rhyolites and trachytes are closely related to the basalts and a striking compositional gap between 50 and 65 sio2 suggests that the high sio2 rocks evolved by 10 15 partial melting followed by variable amounts of fractional crystallisation the source material could have been basalt within the deccan trap or related gabbroic rocks in deep crustal sill complexes the rhyolites yield an rb sr whole rock age of 61 5 1 9 ma with a slightly high initial 87sr 86sr 0 7085 18 it is argued that crustal extension provides a suitable regime for the generation of acid magmas by partial melting of associated basic rocks

measuring the seismic properties of tibetan bright spots evidence for free aqueous fluids in the tibetan middle crust seismic bright spots are commonly interpreted to mark fluid concentrations but their nature melt or aqueous is usually inferred only from circumstantial evidence of the geologic setting a band of bright spot reflections has been imaged by project indepth international deep profiling of tibet and the himalayas at about 15 km depth along 150 km of the northern yadong gulu rift southern tibet we use indepth three component wide angle seismic data to measure seismic velocities at the bright spot reflector and theoretical rock physics bounds to constrain the nature of the fluids merging of data from multiple bright spots allows us to use a one dimensional approximation travel time modeling yields average p and s velocities for the upper crust above the bright spots of 5 3 0 2 and 3 2 0 2 km s 1 respectively reflection amplitude variation with offset avo modeling constrains the p and s velocities of the bright spots to 3 0 0 8 and 1 6 0 8 km s 1 respectively multiple modeling procedures suggest these velocities are not model dependent our results imply that of the order of 10 volume of free aqueous fluids in the tibetan middle crust produces the observed bright spot reflections the presence of relatively large quantities of free aqueous fluids presumably mostly saline supercritical h2o does not preclude the presence of melt but does constrain the maximum temperature at the bright spots to the wet granite solidus about 650 c and thus the maximum surface heat flow to 110 mw m 2 the observed bright spots can alternatively be explained as a result of transient flow of aqueous fluids through a lower temperature and lower heat flow southern tibetan crust

a new siluro devonian paleopole from early paleozoic rocks of the franklinian basin north greenland fold belt demagnetization of dolomitic shales dolomites and carbonate breccias of the cambro ordovician kap stanton formation within the franklinian basin sequence of the north greenland fold belt yields a characteristic direction declination inclination is 16 16 tilt corrected from 14 of the 28 sites a negative breccia test and positive local and regional fold tests bracket the age of the magnetization as postdepositional but pre ellesmerian pre carboniferous after rotating the corresponding paleopole 1 n 302 e to account for the cretaceous tertiary opening of the labrador sea the greenland pole remains removed by approximately 20 from the existing north american apparent polar wander path apwp however the rotated paleopole 2 s 284 e in north american coordinates is similar to british silurian and devonian paleopoles in a bullard et al 1965 reconstruction pole is 2 s 322 e in british coordinates we interpret our new paleopole as late silurian to early devonian in age requiring a modification of previously published apwps for laurentia if the age assignment is correct the new paleopath for north america greenland and that for great britain can be brought into good agreement by closing the atlantic ocean thus indicating that north america greenland and great britain were assembled in a supercontinent in late silurian to early devonian time

the identification and assessment of spurious trends in pearce type ratio variation diagrams a discussion of some statistical arguments spurious trends in pearce type ratio variation diagrams are constrained to go through the origin and the average composition of the data set a valid trend has a significant nonzero y intercept and a meaningful slope the two types of trends are thus easily distinguished numerical simulation is a useful perhaps even necessary method of checking the effects of random variations calculations of probability suggest that the odds against purely random data mimicking an olivine trend in komatiites are great perhaps as much as 100 00 0000 to 1 that such original igneous trends are descernible through the veil of metamorphic overprinting is a powerful demonstration of the usefulness of this technique

geochemical and isotopic characteristics of early silurian clastic sequences in antigonish highlands nova scotia canada constraints on the accretion of avalonia in the appalachian caledonlde orogen avalonia is a terrane that accreted to laurentia baltica during the development of the appalachian caledonide orogen interpretations of the timing of accretion have been constrained by comparing faunal affinities overstep sequences age and kinematics of inferred accretionary deformational events and controversial paleomagnetic data we show that the time of accretion of avalonia may also be constrained by contrasts in the geochemical and isotopic signatures of its igneous rocks which reflect the characteristics of the underlying continental basement and mantle and sedimentary rocks which reflect provenance early silurian clastic sedimentary rocks of the beechill cove formation antigonish highlands nova scotia were deposited on avalonian crust the formation predominantly consists of approximately 80 m of siltstones and shales deposited in a nearshore environment and derived from the northeast their age is constrained by paleontological data and by directly underlying late ordovician early s

the first record of cooksonia from south america in silurian rocks of bolivia plant megafossils resembling cooksonia caledonica lang aredescribed from the kirusillas formation in southern bolivia faunal and lithological evidence together with field relation ships advocate a late silurian ludlow age palaeocontinentalreconstructions suggest that the plants grew on the western edgeof gondwana at 50 60 s and indicate that cooksonia hadspread to higher latitudes soon after its appearance in the wenlock

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

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

provenance analysis of the paleozoic sequences of the northern gondwana margin in nw iberia passive margin to variscan collision and orocline development abstract the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges from neoproterozoic to early permian in age its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana or ii an active continental margin or iii a drifting ribbon continent in this paper we present detrital zircon u pb laser ablation age data from 13 samples taken in detrital rocks from the cantabrian zone sequence ranging from early silurian to early permian in depositional age the obtained results together with previously published detrital zircon ages from ediacaran ordovician strata allow a comprehensive analysis of changing provenance through time collectively these data indicate that this portion of iberia was part of the passive margin of gondwana at least from ordovician to late devonian times zircon populations in all samples show strong similarities with the sahara craton and with zircons found in libya suggesting that nw iberia occupied a paleoposition close to those regions of present day northern africa during this time interval changes in provenance in the late devonian are attributed to the onset of the collision between gondwana and laurussia additionally the middle carboniferous to permian samples record populations consistent with the recycling of older sedimentary sequences and exhumation of the igneous rocks formed before and during the variscan orogeny late devonian to permian samples yield zircon populations that reflect topographic changes produced during the variscan orogeny and development of the lithospheric scale oroclinal buckling

sequences cycles and basin dynamics in the silurian of the appalachian foreland basin abstract field stratigraphic analysis of the early and medial silurian strata of ontario new york and pennsylvania permits recognition of six major sequences reflecting 3rd order cycles in the niagaran llandoverian ludlovian of new york and adjacent regions these sequences correspond approximately to the medina lower clinton middle clinton upper clinton two sequences and lockport groups each is bounded by widespread unconformities produced by the interplay of eustatic sea level drop and local tectonic uplift in general erosional sequence boundaries and transgressive surfaces are merged with no record of the lowstand wedge each sequence is subdivisible into two to five sub sequences that are marked by sharp though generally non erosional basal discontinuities investigation of the clinton and lockport groups corroborates earlier suggestions that carbonate and siliciclassic tongues extended basinward from opposite margins synchronously in response to sea level drops analysis of the thickness and basinward extent of the progradational tongues reveals the existence of multi scale sedimentary cycles sequences and component sub sequences are correlative basinwide and are synchronous within the resolution of bio and event stratigraphy major drops in relative sea level are represented by the sequence bounding discontinutiies these surfaces are overlain by siliciclastic wedges near terrigenous source areas or winnowed carbonate pack and grainstones on the tectonically passive margin overlying highstand deposits typically shales calcareous mudstones and limestones are separated from lowstand or transgressive deposits by thin condensed intervals commonly marked by phosphatic or glauconitic material and or surfaces of maximum sediment starvation the relative highstand deposits of sequences and sub sequences are developed in relatively deep water facies and are at most slightly progradational hence most sequences and sub sequences are sharply bounded roughly upward deepening to slightly shallowing successions of strata in turn sub sequences are further divisible into widespread minor parasequences and parasequence sets probably corresponding to 6th and 5th order cycles respectively the smaller scale units 0 2 5 mthick commonly display an upward shallowing motif the nested relationship of these cycles supports the hierarchical model of transgressive regressive allocycles proposed by busch and rollins 1984 niagaran cycles discussed herein are equivalent to third fourth and fifth order cycles and are traceable circumbasinally depocenters of successive sequences and sub sequences display a pattern of eastward westward eastward basin axis migration through the silurian during early llandoverian the taconic queenston deltaic complex began to subside and the basin axis depocenter and eastern shoreline shifted eastward approximately 200 km over a 6 million year period as the basin axis migrated eastward the western basin ramp forebulge was repeatedly upwarded and eroded resulting in the generation of regionally angular unconformities at the bases of sequences ii iv and vi the pattern of unconformity greatest to the west suggests rise of the algonquin arch concomitant with eastward basin axis migration during the late llandoverian to early wenlockian the algonquin arch subsided slightly concurrently with a reversal in direction of basin axis migration during deposition of upper clinton and lockport vernon strata sequences v vi the basin axis shifted approximately 300 km back to the west eastward migration of the basin axis corresponds with a time of tectonic quiescence and probable thrust load relaxation the abrupt reversal of migration in the late llandoverian may be a signal of renewed thrusting in the hinterland at the onset of the salinic disturbance

analysis of petrologic hypotheses with pearce element ratios pearce element ratios can test whether the members of a rock suite are comagmatic and can illustrate the causes of chemical diversity in comagmatic suites comagmatic rocks have constant ratios for elements conserved in the system during changes that led to the chemical diversity in basaltic systems the incompatible elements ti k and p are often conserved the slope of the trend on a pearce element ratio diagram is sensitive to the stoichiometry of the crystallizing and segregating phases a judicious choice of ratios as axes for the diagram provides a signature for the phases involved and estimates of their compositions in basaltic rocks diagrams with ti k vs p k can provide a test of the comagmatic hypothesis diagrams with 0 5 mg fe k vs si k have trends that are distinct for each comagmatic suite and different mineral assemblage different suites are distinguished by the intercepts in diagrams whereas mineral assemblages are recognized by the slopes of the trends for example if olivine is the sole crystallizing and segregating phase the trend will have a slope of 1 diagrams with 2ca na k vs al k distinguish plagioclase from augite assemblages and in conjunction with 0 5 mg fe k diagrams unravel the crystallization sequences of suites that have suffered three phase crystallization and segregation analyses from the uwekahuna laccolith kilauea the 1955 and 1967 68 eruptions of kilauea diamond craters volcanic field oregon and experimental data on morb glasses provide illustrations of the interpretations that can be obtained from pearce element ratios

paleomagnetic data from alaska reliability interpretation and terrane trajectories virtually the entire body of paleomagnetic data collected from southern alaska depicts a clear decrease in paleolatitude with increasing age strongly suggesting that southern alaska represents a displaced terrane in this paper paleomagnetic studies from southern alaska have been classified with respect to a quality index that is based on four criteria these criteria are the presence of both polarities of magnetic remanence stepwise thermal or alternating field demagnetization of specimens principal component analysis of demagnetization data and a successful fold test of the 51 studies compiled only four from southern alaska and one from western canada are demonstrated to satisfy all criteria and fall therefore in the category of group 3 highly reliable two studies from southern alaska satisfy three of the four criteria lacking both polarities of characteristic remanence and are judged to be of group 2 reliable two of these paleomagnetic studies constrain the accretion time of the southern alaska terrane to the relatively stationary region of central alaska north of the denali fault four paleomagnetic studies from the southern alaska terrane show a distinct paleolatitude anomaly when compared with their expected paleolatitudes from the north american apparent polar wander path using the model of engebretson et al 1984 a series of models are presented to best fit these highly reliable and reliable paleomagnetic studies the model preferred in this article assumes an accretion time with north america of 50 ma and documents pre 50 ma displacement of the southern alaska terrane on the kula plate if the ghost rocks paleomagnetic magnetizations plumley et al 1983 are assumed to be of earliest tertiary age this model fits all of the low paleolatitudes observed in southern alaska models incorporating coastwise translation of the southern alaska terrane along the western boundary of the north america plate and a 50 ma suturing age of this terrane to north america do not satisfy the paleomagnetic constraints four of the reliable studies can be modeled assuming strike slip motion of the southern alaska terrane along the western boundary of the north america plate if a 40 ma suturing age is assumed in this model however the paleomagnetic data must be presumed to represent the highest permissible paleolatitude and the ghost rocks formation paleolatitudes are problematically lower than is possible to model with coastwise translation

crustal volume balance during the india eurasia collision and altitude of the tibetan plateau a working hypothesis a working hypothesis is developed for the two stages of evolution of the tibetan plateau during the continental collision between india and eurasia the first phase was dominated by crustal shortening and thickening in what is now tibet while indochina was expelled southeastward away from eurasia at the end of this phase at 17 ma the estimated tibetan crustal thickness was 51 km the second phase has been dominated by thickening of tibet by the influx of crustal material from india with no associated crustal shortening the crustal thickening that is inferred to have occurred by this process is only feasible because leakage of crust from beneath the plateau is restricted by neighboring archean cratons that block lower crustal flow this leakage to the east and south from tibet to beneath yunnan is estimated as 20 30 of the influx at present but was much smaller at the start of this second phase of deformation it is suggested that at 11 ma the force exerted against the tibetan upper crust by this flowing material first exceeded the threshold that this brittle layer could resist and tibet began to extend eastward any satisfactory working hypothesis for the tibetan plateau must explain its dimensions its 17 ma ending of shortening and 11 ma start of extension and volcanism the eastward slip sense on its active normal and strike slip faults and the ratios of their displacements to present day slip rates which imply that their slip rates have gradually increased this working hypothesis accounts for these features and predicts a smooth and continuing increase in altitude

extensional tectonics in central and eastern asia a brief summary although most of the neotectonics of eastern and central asia seems to be due to the collision and continued convergence between india and eurasia extensional tectonics prevails in four portions of this region tibet the baikal rift system the shansi graben and yunnan we think that the convergence between india and eurasia is responsible for crustal extension in all four regions but the physical mechanisms responsible for the extension are not the same in all of these cases fault plane solutions of earthquakes and clearly defined escarpments and adjacent basins seen on the landsat imagery indicate widespread normal faulting in tibet molnar tapponnier 1975 1978 ni york 1978 the direction of extension appears to be east west but the poorly determined fault plane solutions and the lack of field studies allow some variation in the direction of extension the normal faulting seems to be confined to the areas of highest elevation and some of the normal faults cross the indus zangbu suture into the high himalaya