two views of the cold filament abstract two views of the cold filament first described by weatherly and kelley are presented the first is a local view near 40 n 62 w there its upslope edge is found to be a front which by benthic standards is large its downslope edge was not sampled what distinguishes this benthic front from others is that it is a permanent feature in the abyssal ocean above the cold filament relatively murky detached bottom layers were observed and tracked to where they separated from the bottom at the benthic front apparently these detached layers entrain overlying water a density jump at their base apparently restricts entrainment of underlying water primarily during the detachment process with comparably less entrainment thereafter the second view a regional one comes from examining historical hydrographic sections these indicate that the cold filament extends from the newfoundland ridge westward then southward to 24 n and possibly to 20 n along the base of the continental rise the cold filament i

a simulation of the abyssal circulation in the north pacific ocean part i flow field and comparison with observations abstract a simulation was made of the abyssal circulation in the north pacific ocean the model covers the whole pacific has realistic coastal geometry and bottom topography and is driven by the annual mean surface wind stress and the annual mean differential heating at the surface and the southern boundary about 6 5 sv sv 106 m3 s 1 of bottom water crosses the equator from the south to the north pacific below 3500 m and rises there with 2 8 sv of the 6 5 sv returning to the south pacific in the layer between 1750 and 3250 m the abyssal water below 1750 m is divided into three layers the upper deep layer 1750 3250 m the lower deep layer 3250 4250 m and the bottom layer below 4250 m based on the large scale horizontal circulation pattern in the bottom layer the western boundary current flows to the north without any stagnation point and the interior flow is essentially zonal and eastward especially over the gentle slope of the western flank of the east pacific rise in the upper and

on the average absolute transport of the deep western boundary currents east of abaco island the bahamas abstract data obtained from a line of absolute current profiler pegasus and ctd stations at 26 5 n east of abaco island in the northern bahamas are used to estimate the average transport and overall level of variability of the deep below 800 m southward flow associated with the so called deep western boundary current dwbc at this latitude from april 1985 to september 1987 a total of 11 sections were made along a line extending from near the island boundary to 85 km offshore as part of the subtropical atlantic climate studies stacs program in all but two cases when pegasus data were obtained over the total depth absolute velocities were determined from 0 to about 3000 db while ctd derived relative velocities were determined over the total depth using the combined datasets it is possible to reference the relative geostrophic velocity with pegasus data thereby obtaining an estimate of the absolute southward transport of the deep flow the average value of transport determined in this man

abyssal circulation driven by near boundary mixing water mass transformations and interior stratification the emerging view of the abyssal circulation is that it is associated with bottom enhanced mixing which results in downwelling in the stratified ocean interior and upwelling in a bottom boundary layer along the insulating and sloping seafloor in the limit of slowly varying vertical stratification and topography however boundary layer theory predicts that these upslope and downslope flows largely compensate such that net water mass transformations along the slope are vanishingly small using a planetary geostrophic circulation model that resolves both the boundary layer dynamics and the large scale overturning in an idealized basin with bottom enhanced mixing along a midocean ridge we show that vertical variations in stratification become sufficiently large at equilibrium to reduce the degree of compensation along the midocean ridge flanks the resulting large net transformations are similar to estimates for the abyssal ocean and span the vertical extent of the ridge these results suggest that boundary flows generated by mixing play a crucial role in setting the global ocean stratification and overturning circulation requiring a revision of abyssal ocean theories

the flux and mixing rates of antarctic bottom water within the north atlantic calculation was made of the northward flow of antarctic bottom water into the western north atlantic basin through a passageway approximately 300 km wide between the ceara rise and the mid atlantic ridge at about 4 n two moorings carrying four current meters each were placed in the region where currents were expected to be the greatest and were left for 360 days the current was not steady but exhibited large surges with approximately a 60 day time scale whose cause is unknown current meters in the fastest flowing region all exhibited a net northward flow it is estimated that approximately 0 8 106 m3 s 1 of water colder than 1 9 c potential temperature flowed on the average into the north atlantic with this number possibly being too small by 0 3 106 m3 s 1 or too big by 0 1 106 m3 s 1 depending upon estimates of the width of the region which the current meters sample however these numbers differ from geostrophic estimates of 1 98 106 m3 s 1 the disagreement coming almost entirely from the fact that the geostrophic estimates give a sizeable flux of water colder than 1 2 c while the current meters do not based upon the two different flux rates residence times and mixing coefficients are calculated for antarctic bottom water for each 0 1 c potential temperature interval from 1 0 c to 1 9 c finally some comments on the dynamics of the northward flow are made

permian continental basins in the southern alps italy and peri mediterranean correlations the late carboniferous to permian continental successions of the southern alps can be subdivided into two main tectono sedimentary cycles separated by a marked unconformity sealing a middle permian time gap generally estimated at over 10 ma the lower cycle 1 between the variscan crystalline basement and the early permian is mainly characterised by fluvio lacustrine and volcanic deposits of calc alkaline acidic to intermediate composition which range up to a maximum thickness of more than 2 000 m the upper cycle 2 which is devoid of volcanics is mostly dominated through the mid late permian by alluvial sedimentation which covered the previous basins and the surrounding highs giving rise to the subaerial verrucano lombardo val gardena groden red beds up to about 800 m thick the palaeontological record from the terrigenous deposits of both the above cycles consists mainly of macro and microfloras and tetrapod footprints the age of the continental deposits is widely discussed because of the poor chronological significance of a large number of fossils which do not allow reliable datings however some sections are also controlled by radiometric calibrations the comparison with some selected continental successions in southern europe allows to determine their evolution and set up correlations a marked stratigraphic gap shows everywhere between the above mentioned cycles 1 and 2 as in the southern alps the gap reaches the greatest extent during the mid permian near the illawarra reversal geomagnetic event 265 ma in western europe however such as in provence and sardinia the discussed gap persists upwardly to late permian and early triassic or slightly younger times i e to the onset of the alpine sedimentary cycle even though in northeastern spain iberian ranges balearic islands this gap results clearly interrupted by late guadalupian lopingian deposits the above two major tectonosedimentary cycles reflect in our view two main geodynamic events that affected the southern europe after the variscan orogenesis the late carboniferous early permian transformation of the gondwana eurasia collisional margin into a diffuse dextral transform margin and the middle late permian opening of the neotethys ocean with the onset of a generalised extensional tectonic regime and the progressive westward marine ingression

post collisional rapid exhumation and erosion during continental sedimentation the example of the late variscan salvan dorenaz basin western alps the salvan dorenaz intramontane basin formed between ca 308 293 ma as an asymmetric graben along crustal scale transtensional fracture zones within the aiguilles rouges crystalline massif western alps and represents a feature of the post collisional evolution of the variscan orogens it contains 1 5 1 7 km of continental clastic deposits which were eroded from granitic volcanic and metamorphic rocks textural and compositional immaturity of the sandstones and the numerous lithic fragments with low chemical and physical stability suggest only short range transport 40ar 39ar analyses of detrital muscovite are interpreted to represent cooling of the crystalline basement below the respective closure temperatures ages from detrital muscovite range between ca 280 330 ma 40ar 39ar white mica plateau ages from granitic boulders range between 301 312 ma and suggest rapid cooling the very short time interval recorded between the 40ar 39ar cooling ages and the stratigraphic age of the host sediment suggests that considerable portions of the upper crust were removed prior to the formation of the basin late variscan granitic boulders document surface exposure and erosion of late carboniferous granites during early stages of the infilling of the basin therefore unroofing of basement units magmatic activity and formation of the fault bounded salvan dorenaz basin were acting concomitantly and are highly suggestive of extensional tectonics when compared with other orogens this situation seems specific to the variscan especially the exclusively young ages of detrital material however modern analogous may exist

eastward flow through the mid atlantic ridge at 11 n and its influence on the abyss of the eastern basin abstract the dilute antarctic bottom water of the north atlantic eastern trough is supplied from the western trough through fractures in the mid atlantic ridge in particular the influence on eastern trough property distributions of flow through the romanche and vema fracture zones near the equator and 11 n respectively has been noted previously here new observations are reported that document the abyssal circulation of the northeastern atlantic basins gambia abyssal plain south canary basin and north canary basin in particular the dominance of vema influence the absence of romanche influence and the existence of a system of deep western boundary currents and estimated transport deep isopycnals slope steeply across the vema s eastern end near 39 w corresponding to a geostrophic transport through the vema of 2 1 to 2 3 106 m3s 1 colder than 2 0 c this is half or more of the estimated bottom water that flows north across the equator into the subtropical western north atlantic this transp

the transition between the two major permian tectono stratigraphic cycles in the central southern alps results from facies analysis and u pb geochronology the val daone conglomerate vdc is a continental clastic unit that crops out eastwards of the central southern alps from the ne sector of the collio basin to the w as far as the tione basin to the e this significant but as yet relatively unknown formation lies just above the regional unconformity that marks the boundary between the two permian major tectono sedimentary cycles tsu1 and tsu2 and grades upwards paraconformably to the fluvial red beds of verrucano lombardo val gardena sandstone generally associated with late permian times recent palynological investigations on the vdc suggested a guadalupian age late roadian early wordian owing to the remarkable presence of diversified pollen associations therefore this sedimentary unit is to date the first one ascribed on a palaeontological basis to the middle permian in the entire southern alps domain a detailed facies analysis of vdc shows deposition in amalgamated alluvial fan braided and fluvial environments with wide channels and longitudinal bars in its type area and val rendena the vdc rests unconformably above the last volcanic episode of the tsu1 la icp ms u pb dating on zircon from two samples of such topmost lower permian volcanic rocks known as ponte murandin dacitic lava and malga plan rhyodacitic ignimbrites tione basin provided concordia ages of 278 2 ma mswd 0 01 and 279 2 ma mswd 0 16 respectively as well as allowing us to better define the duration of the time gap between the two permian megacycles in the central southern alps in almost 10 ma these radiometric age determinations are also significant because they enable us to regionally link the coeval volcanic bodies which crop out in the collio basin to the sw and in the athesian volcanic group to the ne respectively

circulation and nutrients in middepth atlantic waters isopycnal analyses of distributions of salinity oxygen apparent oxygen utilization nitrate and silica in the depth range 800 3000 m in the north and tropical atlantic ocean are carried out using data from the transient tracers in the oceans meteor cruise 56 leg 5 atlantis ii cruise 109 legs 1 and 3 and the geosecs atlantic study the results are summarized in isopycnal distribution maps and property property plots the layer between 800 and 1500 m is oxygen poor and nutrient rich and is poorly ventilated below this is a better aerated layer with the deep western boundary current part of which separates along the equator regeneration of nutrients and consumption of oxygen persist into the deep ocean in regions off the west coast of africa the mediterranean outflow water apparently is causing significant cross isopycnal mixing through salt fingering the resultant cross isopycnal velocity may be large enough to cause significant vorticity stretching

filling sequence in late paleozoic continental basins a chimera of climate change a new light shed given by the graissessac lodeve basin se france abstract the characteristics of sedimentary deposits in continental basins especially those controlled by extension are often used to infer global climatic changes we question the relevance of such an approach because the stratigraphic pattern of those basins are mostly similar whatever their latitudes paleolatitudes i e their general climatic conditions namely the transition from 1 an ancestral drainage network to 2 deep lake with a dominance of black and grey shale deposits 3 shallow oxic lake with a dominance of red pelites deposits and to eventually 4 a new drainage network such transitions observed in late palaeozoic continental basins throughout laurasia and gondwana are generally interpreted as evidence of global change from a wet climate to an arid climate from the sedimentological and structural analysis of a late palaeozoic basin the graissessac lodeve basin in the south of the variscan french massif central we proposed a new tectono stratigraphic sequence of this basin from the late carboniferous to the early trias we estimate the paleo area from 295 450 km 2 to 880 1150 km 2 of the basin and we deduce a subsidence curve from the late carboniferous to the late permian from 600 200 m my to 100 10 m my we conclude that the graissessac lodeve basin stratigraphic pattern corresponds to a classical continental rift sequence controlled by tectonics in order to assess a possible control of the stratigraphic sequence by the climate we have reconstructed the paleo depth of the lake with 1 the morphometric analysis of the wave ripples and 2 with an estimation of the lacustrine slope considering the tectonic origin of the lake we find that the maximum lacustrine depth has evolved from a range of 80 300 m during the late carboniferous to 40 45 m during the late permian from our estimates of the different configurations of the basin we show that the volume of the water in the lake has remained nearly constant between 10 and 30 km 3 from the late carboniferous 300 my to the late permian 250 my eventually we conclude that the sedimentological indicators of shallowing and episodic drying observed from the late carboniferous to late permian in this basin and probably in all the pangea basins of the same type are the consequence of the basin enlargement by tectonics and its sedimentary filling rather than climatic change

a simple model of mass driven abyssal circulation over a general bottom topography abstract the classic stommel arons problem is revisited in the context of a basin with a general bottom topography containing an equator topography is taken to be smoothly varying and as such there are no vertical side walls in the problem the perimeter of the abyssal basin is thus defined as the curve along which the layer depth vanishes because of this it is not required that the component of horizontal velocity perpendicular to the boundary curve vanish on the boundary planetary geostrophic dynamics leads to a characteristic equation for the interface height field in which characteristics typically originate from a single point located on the eastern edge of the basin at the equator for a simple choice of topography it is possible to solve the problem analytically in the linear limit of weak forcing the solution exhibits an intensified flow on the western edge of the basin this flow is pan of the interior solution and is thus not a traditional dissipative western boundary current when the

on deep western boundary currents the stommel and arons 1960a b theory of the deep circulation of the world ocean does not directly address the physics of deep boundary currents in the present paper a maximally simple steady state physics for deep western boundary currents is developed and integrated into the stommel arons model this is accomplished by modifying the assumptions of the latter to include effective horizontal eddy viscous terms everywhere in the deep water including in the interior the resulting solution for the meridional velocity in a rectangular ocean on a beta plane is mathematically analogous to that of munk 1950 for the wind driven circulation in the interior the meridional velocity reduces identically to that of stommel and arons among other benefits the solution eliminates the theoretical difficulties associated with the crossing of the equator by the deep western boundary current as an initial test of the theory s consistency with observation theoretical plots of pressure and meridional velocity as a function of longitude are qualitatively compared with hydrographic sections and inferred meridional velocity fields in the eastern and western basins of the abyssal north atlantic also the theoretical profile for the meridional velocity as a function of longitude is qualitatively compared with observations of longshore velocity as a function of offshore distance in the deep western boundary current of the north atlantic deep water

topography and grounding in a simple bottom layer model a reduced gravity model for a dense bottom layer under a motionless deep layer is used to investigate bottom layer flow with topography the two competing effects in the model are topography which introduces a topographic beta effect and strong layer thickness variation which allows for the possibility of zero layer thickness or grounding the goal is to arrive at a simple observationally relevant characterization of the model which can help to distinguish different dynamical mechanisms operating in the ocean several regimes are identified including situations for which the layer depth goes to zero with and without topography ways of distinguishing observationally between competing effects are suggested the study is motivated by the observed shift of antarctic bottom water in the western north atlantic ocean from the west to the east this phenomenon is used to discriminate key elements of the model

zircon growth and recycling during the assembly of large composite arc plutons abstract concordant u pb zircon dates have been interpreted traditionally to date the crystallization ages of plutons because until recently analytical uncertainties have generally been large enough to encompass the anticipated duration of pluton growth advances in zircon tims analysis and geochronological studies by sims on zircon from young volcanic rocks along with evolving views of the evolution of magmatic systems makes it necessary to revaluate this interpretation assignment of unique pluton crystallization ages based on zircon dating is complicated by the propensity of zircon to survive multiple intrusive events that culminate in a large pluton distinctions must be made between true inherited zircon xenocrystic zircon derived from host rocks during magma ascent and or emplacement and multi stage growth of zircon during waxing and waning of magma reservoirs that solidify to form large plutons these complications are exemplified by recent high precision u pb zircon age dating from the cretaceous tuolumne and mt stuart batholiths which clearly show that the timeframe of pluton assembly was long 5 ma for mt stuart and 8 ma for tuolumne zircon crystals from samples from both batholiths exhibit appreciable concordant age dispersion for zircon several 10 5 yr up to 2 10 6 yr and assignment of statistically valid rock solidification ages is not possible from these data low zr in the rocks of each batholith indicates that magmas were initially strongly undersaturated in zircon when emplaced and inherited zircon is rare or absent recycling of zircon antecrysts during successive magmatic injections is the primary cause of the modest age dispersion of concordant zircon ages and is compatible with progressive growth of a large long lived crystal mush body because eruption of magma requires low to moderate crystal fraction and high temperatures relative to stored crystal mushes zircon crystals are more likely to be eradicated during thermal rejuvenation preceding major eruptions

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

revealing exhumation of the central alps during the early oligocene by detrital zircon u pb age and fission track double dating in the taveyannaz formation

encounter of a deep western boundary current with the equator a numerical spin up experiment the encounter of a deep western boundary current with the equator is studied using an ultra high resolution numerical model of the equatorial ocean in a study of the origin of the deep equatorial current system the integration was carried out from a state of rest to 3500 days the spin up experiment shows that the deep western boundary current upon encountering the equator first turns along the equator the time scale of spin up is fast along the equator and the boundaries about 250 days but is considerably slower off the equator after this initial response the western boundary current slowly begins to penetrate into the opposite hemisphere as expected from the linear theory the eastward equatorial current gradually slows down the total water mass transport in the equatorial branch decreases steadily until by day 3000 it has dwindled to zero and all the mass transport in the western boundary current crosses into the other hemisphere however a vestige of the equatorial branch remains until the end of the integration with the speed of less than 3 cm s and with intermittent meandering an analysis of the momentum balance indicates that this meander may be a free equatorial inertial oscillation we postulate that a permanent steady bifurcation of the mid depth western boundary current that carries substantial mass does not occur and eventually all of the mass transport will continue into the opposite hemisphere the asymptotic flow field seems incompatible with the tracer field observed in the atlantic which clearly shows a splitting at the equator of the tracer signal associated with the upper north atlantic deep water we speculate that an inherently time dependent explanation of the tracer tongue is called for

the evolution of the polymetamorphic basement in the central alps unravelled by precise u pb zircon dating the u pb age determinations of zircon and rutile from the aar massif reveal a complex evolution of the central alpine basement the oldest components are found in zircons of metasediments which bear cores of archean age the u pb age of discordant prismatic zircons of the same rocks ranges between 580 and 680 ma an age that is typical for pan african metamorphism the zircons are interpreted as pan african detritus with archean inheritance the provenance region of the pan african zircons is assumed to be a terrane of gondwana affinity i e the w african craton or the pentevrian microplate the caledonian metamorphism left a pervasive structural imprint in amphibolite facies on the rocks of the aar massif it is dated at 456 2 and 445 ma by zircons of a layered migmatitic gneiss and a migmatitic leucosome respectively both occurring in the northernmost zones of the massif hercynian metamorphism never exceeded greenschist facies conditions and is recorded by zircon in a garnet amphibolite and by rutile in a meta psammite that yield an age of 330 ma both zircon and rutile are considered to be products of retrograde mineral reactions and therefore do not date the peak conditions of hercynian metamorphism the gastern granite at the western end of the aar massif is a contaminated granite that intruded at 303 4 ma contemporaneously with the wide spread late hercynian post collisional i type magmatism the study demonstrates the potential of isotope dilution u pb dating of single grains and microfractions in deciphering complex evolutionary histories of polymetamorphic terrains

permo carboniferous climate early pennsylvanian to late permian climate development of central europe in a regional and global context a well justified stratigraphical correlation of continental successions and new palaeogeographic reconstruction of pangaea reveal new insights into the northern pangaean climate development influenced by palaeogeography palaeotopography glacio eustatic sealevel changes and ocean currents the overall permo carboniferous aridization trend was interrupted by five wet phases these are linked to the gondwana icecap the aridization and weakening of wet phases over time were not only caused by the drift of northern pangaea to the arid climatic belt but also by the successive closure of the rheic ocean which caused the expansion of arid semi arid environments in the lower middle permian the end of the gondwana glaciation rearranged ocean circulation leading to a cold coast parallel ocean current west of northern pangaea blocking moisture coming with westerly winds the maximum of aridity was reached during the roadian wordian the trans pangaean mountain belt was non existent its single diachronous parts never exceeded an average elevation of 2000 m the maximum elevation shifted during time from east to west the hercynian orogen never acted as an orographic east west barrier and the inter tropical convergence zone was widely displaced causing four seasons dry summer winter wet spring autumn at the equator and a strong monsoon system the climate history of the european realm during the late carboniferous pennsylvanian and permian is stored in many solitary basins fig 1 within the hercynian orogen and the foreland basin the story of westphalian climate is well known because of the numerous investigations of the coal bearing variscan foredeep the younger westphalian is characterized by a slight aridization abbink van kronijnenburgvan cittert 2003 oplustil 2004 which was accompanied by an increase of seasonality nevertheless the environment was strongly influenced by the ocean and epi continental seas with multiple transgression events the last extensive marine ingression was the aegir mansfield band westphalian b c the post westphalian climate development is more differentiated and

crossing of the equator by the deep western boundary current in the western atlantic ocean abstract property distributions and geostrophic shear from a hydrographic section near 37 w in the atlantic ocean show the deep western boundary current dwbc in the north atlantic deep water nadw established against the western boundary of the brazil basin immediately south of the equator between 2 and 5 s the dwbc thus has directly crossed the equator to the south atlantic following the east southeast trend of the continental slope isobaths the estimated dwbc transport of nadw is 35 106 m3 s 1 similar to other estimates from the tropics discussed here these large dwbc transports are opposed by flow of deep water to the north atlantic immediately offshore of the dwbc with as much as two thirds of the dwbc transport being returned as these recirculations one recirculation center is the guiana basin north of the equator but extends at least a few hundred kilometers south of the equator another is visible at 11 s in the brazil basin the degree of connection of these two observed recirculatio

bottom water circulation in the western north atlantic abstract antarctic bottom water flows into the western north atlantic across the equator shifting from the western side to the eastern side of the trough between the american continents and the mid atlantic ridge as it continues north this is puzzling because such large scale motion is thought to be controlled by dynamics that disallows an eastern boundary current previous explanations for the transposition involve a necessarily small scale density current that changes sides because of the change in sign of rotation across the equator or a topographic effect that changes the sign of the effective mean vorticity gradient and thus requires an eastern boundary current here an alternative explanation for the overall structure of bottom flow is given a source of mass to a thin bottom layer is assumed to upwell uniformly across its interface into a less dense layer at rest a simple formula for the magnitude of the upwelling and thickness of the layer is derived that depends on the source strength to th

a paleomagnetic study of tectonically deformed red beds of the lower glarus nappe complex eastern switzerland a paleomagnetic study at 15 sites 315 samples in permo triassic red beds of the helvetic nappes of eastern switzerland isolated stable characteristic and secondary remanent directions thermal demagnetization and isothermal remanent magnetization experiments suggest pigmentary hematite carries the characteristic direction secondary directions are scattered but may have been acquired during the alpine deformation in the oligocene and miocene the characteristic directions also show large between site scatter anisotropy of magnetic susceptibility indicates that the hematite grains are flattened within the slaty cleavage and that there is a north south lineation this anisotropy pattern implies that the magnetic grains were realigned by the alpine deformation it is not possible to explain the characteristic remanent magnetization deviations on a site by site basis but the site mean directions are streaked along a great circle the down dip direction of which is approximately parallel to the regional fold axes 60 20 this spread could be explained by a penetrative simple shear deformation associated with the nappe internal deformation

thermal stresses in subducting lithosphere can explain double seismic zones the discovery of double planed zones of intermediate depth seismicity associated with subducting oceanic lithosphere refs 1 7 and l s h and k h j in preparation has generated much interest in the origin and distribution of stresses in descending plates several mechanisms have been proposed to explain why double zones occur8 here we investigate the role of thermal stresses for the state of stress in a descending plate and test whether they can account for the observed characteristics of double seismic zones by modelling the descending slab as a thin elastic plate and using the temperatures in the slab estimated by toksoz et al 9 we find that the polarities of thermally generated stresses agree with those obtained from the focal mechanisms of earthquakes in the two zones thermal stresses also can explain the separation between the two zones of seismicity at shallow depths and their gradual merging at greater depth

provenance of the silurian elder sandstone north central nevada one of the principal clastic units in the roberts mountains allochthon of north central nevada is the elder sandstone which may have been deposited in or about early silurian time rock fragments in the elder sandstone were derived from paleozoic carbonate rock argillite chert quartzite and intermediate to basic volcanic rock feldspar zircon and quartz probably were derived initially from pre cambrian crystalline rocks and were recycled into the elder sandstone from pre silurian supracrustal deposits u pb isotopic data indicate that detrital zircons were derived initially from a source that may have included rocks 2 2 b y old the location of the source of detritus in the elder sandstone is problematical and there is nothing in the data presented here that precludes the possibility that detritus was derived from a foreign continent

the volcano sedimentary evolution of a post variscan intramontane basin in the swiss alps glarus verrucano as revealed by zircon u pb age dating and hf isotope geochemistry the late palaeozoic glarus verrucano basin gvb glarus alps eastern switzerland formed as an intramontane graben in the aftermath of the variscan orogeny its fill the glarus verrucano consists of immature alluvial fan and playa lake deposits with intercalated bimodal volcanics basalts and rhyolites despite its importance for local and regional geology no modern sedimentologic or stratigraphic studies on the gvb exist by means of sedimentologic and geochronologic studies we reconstruct the volcano sedimentary evolution of the gvb it developed at the carboniferous permian boundary and experienced a first bimodal volcanic phase around 285 ma for the same time indications for temporarily humid climate in the otherwise rather arid early permian are demonstrated e g pyrite bearing sandstones during the middle and early late permian increasing aridity is indicated by playa deposits fanglomerates and subaerial ignimbrites which mark a second silicic volcanic phase at 268 ma the detrital zircon age spectra are dominated by late variscan ages and thus demonstrate that older sedimentary and metamorphic rocks once forming the variscan nappe edifice were already mostly eroded at that time finally some larger scale speculations are given which could indicate a causal connection between the widespread tectono magmatic mid permian episode and the local development of the glarus verrucano basin

the baroclinic structure of the abyssal circulation abstract a simple baroclinic model of the abyssal ocean circulation is formulated in which the reversals of the meridional velocity with depth and hence the layering of the abyss is explained by the longitudinal variation of upwelling into the main thermocline since the barotropic meridional velocity is connected to the local upwelling velocity by the sverdrup relation regions of weak upwelling have meridional velocity fields that are essentially baroclinic the baroclinic velocities are driven by thermal anomalies that propagate westward by stationary diffusive rossby waves from regions of vertical strong upwelling in the eastern portion of the basin these dynamically driven internally generated vertical velocities produce the layered baroclinic structure in the western interior of the basin a simple linear model continuous in the vertical is developed to illustrate these elements of the conceptual picture

u pb zircon ages for post variscan volcanism in the ligurian alps northern italy abstract ages of permian volcanic rocks from the ligurian brianconnais domain western italian alps have been determined by laser ablation inductively coupled plasma mass spectrometry u pb dating of zircon three major volcanic units yielded zircons that were dated calc alkaline rhyolites 285 6 2 6 ma andesites with inherited cores yielded ages c 476 ma and older and voluminous rhyodacites rhyolites 272 7 2 2 ma following an amagmatic sediment starved time gap of c 14 ma alkaline volcanic activity is recorded at the top of the sequence by k alkaline rhyolites dated at 258 5 2 8 ma the ligurian segment of the southern variscan belt records transtensional and then extensional tectonics associated with the volcanic activity the switch from calc alkaline to alkaline activity corresponds to the transition from a post orogenic to an anorogenic setting in the southern variscides it may represent progressive and increasing delamination of the continental lithosphere accompanied by partial melting of the lithospheric mantle supplementary material technical and data acquisition parameters u th pb isotope analyses and calculated ages of zircons from samples and standard and trace element compositions of selected zircons are available at http www geolsoc org uk sup18322

episodic and long lasting paleozoic felsic magmatism in the pre alpine basement of the suretta nappe eastern swiss alps the suretta nappe of eastern switzerland contains a series of meta igneous rocks with the rofna porphyry complex rpc being the most prominent member we present la icp ms u pb zircon data from 12 samples representing a broad spectrum of meta igneous rocks within the suretta nappe in order to unravel the pre alpine magmatic history of this basement unit fine grained porphyries and coarse grained augengneisses from the rpc give crystallization ages between 284 and 271 ma which either represent distinct magma pulses or long lasting magmatic activity in a complex magma chamber there is also evidence for an earlier variscan magmatic event at 320 310 ma mylonites at the base of the suretta nappe are probably derived from either the rpc augengneisses or another unknown carboniferous permian magmatic protolith with a crystallization age between 320 and 290 ma two polymetamorphic orthogneisses from the southern suretta nappe yield crystallization ages of 490 ma inherited zircon cores are mainly of late neoproterozoic age with minor neo to paleoproterozoic sources we interpret the suretta nappe as mainly representing a gondwana derived crustal unit which was subsequently intruded by minor cambrian ordovician and major carboniferous permian magmatic rocks finally the suretta nappe was thrust into its present position during the alpine orogeny which hardly affected the u pb system in zircon

a simulation of the abyssal circulation in the north pacific ocean part ii theoretical rationale abstract a theoretical rationale is given for the simulated abyssal circulation in the north pacific detailed in part i a simple theory clarifies the importance of the vertical change in area of the horizontal cross section of the basin owing to the existence of bottom topography or basin shape with general upward vertical transport in the basin a water column horizontally diverges and vertically shrinks as it rises in spite of the general upwelling resulting in the tendency to produce an anticyclonic circulation this hypsometric effect is detailed by rhines and maccready in the present case this effect is present in the lower deep layer 3250 4250 m and in the bottom layer below 4250 m of the north pacific in the lower deep layer a single anticyclonic circulation owing to this effect appears explicitly in the bottom layer however this effect almost balances the cyclogenesis owing to the bottom water influx resulting in essentially eastward interior flow on the other hand an anticycloni

the influence of distributed sources and upwelling on the baroclinic structure of the abyssal circulation abstract the study of a simple continuously stratified model of the abyssal ocean driven by upwelling out of the abyss into the main thermocline and by source fluid entering the basin through northern western and southern bound aries is reported our approach divides the deep mean into an inviscid interior and frictional boundary layer regions the interior circulation is fully determined by the horizontal distribution of upwelling and by the net vertical distribution of sources entering the basin forcing by the local upwelling drives a barotropic velocity field and remote forcing by both the upwelling and sources generates an underlying baroclinic flow which can be considerably stronger and of opposite sign at some depths the boundary current functions to redistribute around the perimeter fluid entering the boundary regions either through the basin walls or from the interior in contrast to the interior flow it depends also on the geographical location of sources the boundary current is divided

facies and basin architecture of the late carboniferous salvan dorenaz continental basin western alps switzerland france the salvan dorenaz basin formed during the late palaeozoic within the aiguilles rouges crystalline basement western alps as an asymmetric intramontane graben elongated in a ne sw direction and bounded by active faults at least 1700 m of fluvial alluvial fan and volcanic deposits provide evidence for a strong tectonic influence on deposition with long term average subsidence rates of 0ae 2m m yr 1 the early basin fill was associated with coarse grained alluvial fans that were dominated by braided channels unit i these issued from the south western margin of the basin the fans then retreated to a marginal position and were overlain by muddy floodplain deposits of an anastomosed fluvial system unit ii that drained towards the ne deposition of thick muds resulted from a reduction in the axial fluvial gradient caused by accelerated tectonic subsidence overlying sand rich meandering river deposits unit iii document a reversal in the drainage direction from the ne to the sw caused by synsedimentary tectonism reflecting large scale topographic reorganization in this part of the variscides with subsidence now preferentially in the w and sw and uplift in the e and ne coarse grained alluvial fan deposits unit iv repeatedly prograded into and retreated from the basin as documented by coarsening upward cycles tens of metres thick reflecting smaller scale tectonic cycles volcanism was active throughout the evolution of the basin and u pb isotopic dating of the volcanic deposits restricts the time of basin development to the late carboniferous 308 295 ma 40 ar 39 ar ages of detrital white mica indicate rapid tectonic movements and exhumation of the nearby basement in unit i youngest ages are close to that of the host sediment but the age spectrum is wide in unit ii high subsidence and or sedimentation rates coincide with very narrow age spectra indicating small homogeneous catchment areas in unit iii age spectra became wider again and indicate growing catchment areas

deep cross equatorial flow in the atlantic measured with sofar floats neutrally buoyant sofar floats at nominal depths of 800 1800 and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the atlantic near 6 n and at several sites near 11 n as well as along the equator trajectories at 1800 m show a swift 50 cm s narrow 100 km wide southward flowing deep western boundary current dwbc extending from 7 n to the equator the average transport per unit depth in the dwbc was estimated to be 13 8 103 m2 s coupling this value with mean velocities measured in the dwbc by current meters gave a volume transport of 15 106 m3 s between depths of 900 m and 2800 m approximately 6 106 m3 s recirculated northward between the dwbc and the mid atlantic ridge leaving 9 106 m3 s as cross equatorial transport no obvious dwbc nor swift equatorial current was observed by the 3300 m floats a low mean velocity at this depth lay between f 11 and higher velocity cores above and below the 1800 m trajectories also suggest that at times february march 1989 the north atlantic deep water in the dwbc turned eastward and flowed along the equator and at other times august september 1990 the dwbc crossed the equator and continued southward the velocity near the equator calculated by grouping floats in a box along the equator was eastward at 4 1 cm s from february 1989 to february 1990 and westward at 4 6 cm s from march 1990 to november 1990 thus the amount of cross equatorial flow in the dwbc appeared to be linked to low frequency variability of the structure of the equatorial current system floats in antarctic intermediate water at 800 m revealed a northwestward western boundary current although flow patterns were complicated three floats that significantly contributed to the northwestward flow looped in anticyclonic eddies that translated up the coast at 8 cm s six 800 m floats drifted eastward along the equator between 5 s and 6 n at a mean velocity of 11 cm s one reached 5 w in the gulf of guinea suggesting that the equatorial currents at this depth extended at least 35 40 along the equator three of these floats reversed direction near the end of the tracking period

early permian pangea b to late permian pangea a the pre drift wegenerian model of pangea is almost universally accepted but debate exists on its pre jurassic configuration since tedirving introd ucedpangea b by placing gond wana farther to the east by v3000 km with respect to laurasia on the basis of paleomagnetic data new paleomagnetic data from radiometrically dated early permian volcanic rocks from parts of adria that are tectonically coherent with africa gondwana integrated with published coeval data from gondwana and laurasia again only from igneous rocks fully support a pangea b configuration in the early permian the use of paleomagnetic data strictly from igneous rocks excludes artifacts from sedimentary inclination error as a contributing explanation for pangea b the ultimate option to reject pangea b is to abandon the geocentric axial dipole hypothesis by introducing a significant non dipole zonal octupole component in the late paleozoic time averaged geomagnetic field we demonstrate however by using a dataset consisting entirely of paleomagnetic directions with low inclinations from sampling sites confined to one hemisphere from gondwana as well as laurasia that the effects of a zonal octupole field contribution would not explain away the paleomagnetic evidence for pangea b in the early permian we therefore regard the paleomagnetic evidence for an early permian pangea b as robust the transformation from pangea b to pangea a took place during the permian because late permian paleomagnetic data allow a pangea a configuration we therefore review geological evidence from the literature in support of an intra pangea dextral megashear system the transformation occurred after the cooling of the variscan mega suture andlasted v20 myr in this interval the neotethys ocean opened between india arabia and the cimmerian microcontinents in the east while widespread lithospheric wrenching and magmatism took place in the west around the adriatic promontory the general distribution of plate boundaries and resulting driving forces are qualitatively consistent with a right lateral shear couple between gondwana and laurasia during the permian transcurrent plate boundaries associated with the pangea transformation reactivated variscan shear zones andwere subsequently exploitedby the opening of western neotethyan seaways in the jurassic

magmatic flare up at the carboniferous permian boundary in the ne german basin revealed by shrimp zircon ages shrimp ages of 12 volcanic samples indicate intense magmatic activity to have occurred at the carboniferous permian boundary throughout much of the ne german basin rhyolitic crystal rich samples have been taken from quarries in the halle volcanic complex hvc and in the flechtingen ignimbrite and from drill cores of the kotzen mirow friedland and penkun areas ten samples yielded 206pb 238u ages between 302 and 297 ma 3 ma respectively which indicate that the magmatic activity took place concentrated in a relatively short time span throughout much of the ne german basin two hvc samples have ages of 307 and 294 ma this remarkably synchronous magmatic activity occurred during the initial phase of the basin development considering published volume estimates 48 000 km3 extrusion rates during this time period probably were in the order of 0 01 km3 a 1 implying magma production rates of 0 1 km3 a 1 the calc alkaline sio2 rich volcanic rocks originated from magmas that presumably formed during anatexis and subordinate magma mixing in an intra continental transtensional setting the basaltic magma which must have provided the thermal input into the lower crust probably formed during decompressional melting of lithospheric mantle fertilised by previous magmatic processes the dated volcanic rocks occur in three geotectonic provinces namely the a mid german crystalline rise which forms the northern part of the internal variscides the b external variscides and the variscan foreland which is considered as c eastern avalonia many of the old zircons found in the hvc samples reflect the magmatic activity of the mid german crystalline rise 325 400 ma however cadomian 500 650 ma and older gondwanian elements 1674 2373 ma are also present although the flechtingen and kotzen areas are located within the external variscides it seems more likely that the middle to lower crust that experienced anatexis formed part of the eastern avalonia plate overridden by the variscan front thus the old zircons found in flechtingen 350 and 538 ma and kotzen 345 ma but certainly those present in the penkun drill cores 1483 ma represent eastern avalonia tectonic history the anatectic magmas in the friedland area probably formed partly from baltic crust thrusted below eastern avalonia during the caledonian orogeny among the old zircons from the friedland samples in addition to proterozoic ages 1456 ma we found testimony of caledonian 443 ma and of clearly post caledonian 387 ma magmatism