a vertically averaged circulation model using boundary fitted coordinates abstract a two dimensional vertically averaged circulation model using boundary fitted coordinates has been developed for predicting sea level and currants in estuarine and shelf waters the basic idea of the approach is to use a set of coupled quasi linear elliptic transformation equations to map the physical domain to a corresponding transformed plane such that all boundaries are coincident with coordinate lines and the transformed mesh is rectangular the hydrodynamic equations are then solved by a multi operation finite difference technique in the rectangular mesh transformed grid comparisons of the circulation model predictions for tidally forced flows in a wedge section with both flat and quadratic bottom topography and in a flat channel with exponential variation in width were in excellent agreement with corresponding analytic solutions simulation of steady state wind induced setup in a closed basin formed using elliptic cylindrical coordinates also was in excellent agreement with the analytic

ordovician k bentonites issues in interpreting and correlating ancient tephras abstract prominent upper ordovician k bentonites include the deicke and millbrig beds in the late ordovician of eastern north america and the kinnekulle k bentonite in the uppermost sandbian of baltoscandia these beds are thought to represent some of the largest ash fall deposits known in the phanerozoic record this report presents a comprehensive study of biotite compositions from the three beds covering a far more extensive geographic range than has previously been described these data show that the kinnekulle and millbrig are multiple event ash beds some parts of which are indistinguishable from one another tectonomagmatic discrimination diagrams combined with mg number data indicate that the deicke millbrig kinnekulle sequence represents the progressive transformation from calc alkaline to peraluminous magmatic sources consistent with a model of progressively evolving magmatism during the closure of the iapetus ocean it is concluded that the millbrig and kinnekulle beds are coeval and represent primarily simultaneous episodes of explosive volcanism although it cannot be excluded that some portions of both beds were produced by the same event

calculating tidal current profiles from vertically integrated models near the critical latitude in the barents sea the method of extracting current profiles from vertically integrated hydrodynamic models is extended to incorporate more general boundary conditions and the consistency of the method is investigated using this method the tidal current profiles for semidiurnal and diurnal constituents are studied in high latitudes near the critical latitude where the coriolis frequency equals the tidal frequency for the semidiurnal tide the current profiles are computed at stations in the barents sea where current measurements exist at up to six different depths the observation stations are located close to the critical latitude for the largest semidiurnal constituents and the current profiles are therefore sensitive to variations of magnitude and vertical distribution of the eddy viscosity at this latitude the current is strongly depth dependent and in deep water the tidal ellipse changes direction of rotation near the seabed at four of the stations there are under ice measurements and the effects of an ice covered ocean upon the tidal current profiles are also investigated

on the importance of time varying eddy viscosity in generating higher tidal harmonics by using a spectral modal expansion in the vertical a method is developed for determining tidal current profiles subject to either a slip or no slip bottom boundary condition an arbitrary vertical profile of eddy viscosity can be used in the calculation the magnitude of which can be related to the flow field the nature of the modal solution is such that the high shear bottom boundary layer can be accurately resolved using a few modes of order five compared with the order of 45 grid boxes on a logarithmic transformation which would be required in a finite difference approach calculations show the importance of a time varying eddy viscosity in generating higher harmonics when a no slip condition is applied at the seabed by relating the current one meter above the bed to the bed stress a value of friction coefficient c100 in reasonable agreement with observations was obtained from these no slip calculations however the value of friction coefficient was strongly dependent upon the height above the bed at which the current was calculated with friction coefficient increasing rapidly in the near bed region current profiles on the continental shelf determined using the modal method developed in the paper were in good agreement with observations with computed profiles in the celtic sea giving the observed midwater maximum

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

a simple three dimensional model of the circulation off newfoundland a simple numerical model is developed to study the three dimensional structure of the steady circulation off newfoundland the model uses linear dynamics a vertical eddy viscosity to parameterize the vertical mixing and a linear parameterization of bottom friction in which the bottom stress is related to the bottom velocity realistic geometry and bottom topography are included the results presented here are for an ocean of uniform density the model is driven by specifying the sea level along the northern boundary the inflow corresponding to the inshore and offshore branches of the labrador current the model can reproduce the circulation pattern found by greenberg and petrie 1988 who used a vertically integrated nonlinear primitive equation model it also enables dynamical interpretation to be given in terms of flow along f h contours and frictional diffusion across these contours in this connection we investigate the dependence on the friction parameters of the transport through several sections we also examine the onshore offshore mass exchange over the southeastern edge of the grand banks we find that the model labrador current flowing southward along the shelf break leads to an onshelf flux in the upper part of the water column and an offshelf flux in the bottom ekman layer this is caused by the frictional torque and leads to an along slope decay of the current amplitude finally the limitations of the model and plans for its further development are discussed

on formulating two dimensional vertically integrated hydrodynamic numerical models with an enhanced representation of bed stress a convolution method is developed for computing the bottom stress in a vertically integrated hydrodynamic numerical model by this means the bottom stress is no longer related to the depth mean current as in a conventional vertically integrated model and an enhanced representation of the bottom stress is possible an enhanced vertically integrated model consequently the enhanced model can reproduce phase lags and differences in direction between bed stress and depth mean current which cannot be taken into account in conventional vertically integrated models a linear or quadratic frictional formulation can be used with the method idealized calculations of wind induced flow in a rectangular basin show that the bed stress computed by the convolution method is in excellent agreement with that derived using a full three dimensional model however significant differences in the bed stress between the three dimensional and conventional two dimensional model are evident also in this paper a method is developed for extracting current profiles from vertically integrated models current profiles extracted from the enhanced two dimensional model for a range of frictional effects are found to be in good agreement with those computed using a full three dimensional model

hurricane storm surge simulations comparing three dimensional with two dimensional formulations based on an ivan like storm over the tampa bay florida region 1 we provide a dynamics based comparison on the results from three dimensional and two dimensional simulations of hurricane storm surge we begin with the question what may have occurred in the tampa bay florida vicinity had hurricane ivan made landfall there instead of at the border between alabama and florida this question is explored using a three dimensional primitive equation finite volume coastal ocean model the results show that storm surges are potentially disastrous for the tampa bay area especially for landfalls located to the north of the bay mouth the worst case among the simulations considered is for landfall at tarpon springs such that the maximum wind is positioned at the bay mouth along with such regional aspects of storm surge we then consider the dynamical balances to assess the importance of using a three dimensional model instead of the usual vertically integrated two dimensional approach to hurricane storm surge simulation with hurricane storm surge deriving from the vertically integrated pressure gradient force tending to balance the difference between the surface and bottom stresses we show that three dimensional structure is intrinsically important two dimensional models may overestimate or underestimate bottom stress necessitating physically unrealistic parameterizations of surface stress or other techniques for model calibration our examination of the dynamical balances inherent to storm surges over complex coastal topography suggests that three dimensional models are preferable over two dimensional models for simulating storm surges

assembly of the armorica microplate a strike slip terrane delivery evidence from u pb ages of detrital zircons abstract detrital zircons separated from the ordovician armorican quartzite in the foreland of the variscan belt of iberia were dated by laser ablation inductively coupled plasma mass spectrometry the age groups found correspond to proterozoic and archean crust formation events the proterozoic events reflected by zircon ages are avalonian cadomian pan african 550 800 ma grenvillean sunsas 900 1300 ma and eburnian transamazonian icartian 1800 2200 ma archean zircon ages are comprised between 2500 and 2800 ma central amazonian liberian events the presence of mesoproterozoic grenvillean zircons in the armorican quartzite which was deposited in a shallow platform bordering the nw african craton north gondwana margin suggests that the nw iberian terrane drifted from the south american margin where it was located during the neoproterozoic toward africa the drift of the iberian block likely achieved via transform faults was arrested by its docking and amalgamation with the peri nw africa

early variscan visean granites in the core of central pyrenean gneiss domes implications from laser ablation u pb and th pb studies abstract laser ablation u pb and th pb single zircon and monazite geochronology was applied to seven samples from two variscan gneiss domes in the central pyrenean axial zone detrital zircon ages obtained from a paragneiss in the western aston dome range from neoarchean ca 2 57 ga to late neoproterozoic ca 560 ma suggesting that late ediacaran sediments were deposited along the northern margin of gondwana cadomian magmatism is recorded for the first time in the central pyrenees from a biotite metagranite ca 545 ma in the central aston dome a second sample of an orthogneiss from the aston dome yields a middle ordovician protolith age ca 467 ma similar in age to other dated orthogneisses of the axial zone three samples from granite stocks situated in the core of the aston and bossost domes have crystallization ages of ca 339 337 ma providing the first evidence of visean igneous activity in the axial zone in contrast to the larger and significantly younger ca 312 305 ma granite plutons along the margins of the gneiss domes visean magmatism caused lp ht contact metamorphism m 2 migmatization throughout the aston dome though less well constrained by overgrowth ages on zircon ca 331 ma and monazite ages ca 340 320 ma is likely to be related to visean intrusions the main s 2 schistosity in the aston and bossost domes is coeval with the visean intrusions inclusion trails in porphyroblasts nucleating in the contact aureoles document the continuous development of s 2 related to local extension crystal plastic deformation of the granites indicates that the d 2 deformation event continued after their crystallization visean magmatism played a crucial role in early stages of dome formation by thermally weakening the metasedimentary mantle and to a certain degree the orthogneiss as well enhancing the development of orogen parallel shear zones in an overall n s compressive setting

volcanic winter and accelerated glaciation following the toba super eruption the eruption of toba in sumatra 73 500 years ago was the largest known explosive volcanic event in the late quaternary1 it could have lofted about 1015 g each of fine ash and sulphur gases to heights of 27 37 km creating dense stratospheric dust and aerosol clouds here we present model calculations that investigate the possible climatic effects of the volcanic cloud the increase in atmospheric opacity might have produced a volcanic winter 2 a brief pronounced regional and perhaps hemispheric cooling caused by the volcanic dust followed by a few years with maximum estimated annual hemispheric surface temperature decreases of 3 5 c the eruption occurred during the stage 5a 4 transition of the oxygen isotope record a time of rapid ice growth and falling sea level3 we suggest that the toba eruption may have greatly accelerated the shift to glacial conditions that was already underway by inducing perennial snow cover and increased sea ice extent at sensitive northern latitudes as the onset of climate change may have helped to trigger the eruption itself4 we propose that the toba event may exemplify a more general climate volcano feedback mechanism

modeling ice dynamics of coastal seas a coupled sea ice barotropic ocean model with a 1 km resolution and a seaward domain of 200 km quantifies three coastal processes coupling of ice motion to wind driven coastal currents ice thickness redistribution under compaction at the coast and formation of coastal shear zones the model consists of an ice momentum balance mass concentration and two parameter ice thickness distribution and equations for horizontal water motion and continuity using vertical structure functions an appropriate constitutive law appears to be a hardening plastic based on qualitative observations from alaskan continental shelves for first year sea ice strength is taken to be proportional to the square of ice thickness a north wind example of 10 m s with the coastline to the west shows the depth dependence of rotational shear in the sea ice ocean boundary layer and sea surface tilt which contributes an alongshore slope current there is slight convergence of sea ice over the shelf a coastal shear zone of 4 km and an alongshore ice speed seaward of the shear zone of 6 of the wind speed caused by the combination of an under ice shear layer and an alongshore slope current for an onshore wind ice is near free drift at 3 of the wind seaward of a ridging front which propagates seaward a square dependence of ice strength on thickness is required for the rubble field to approach a limiting thickness consistent with observations the hardening plastic interpretation of the rubble field has the stress state at the yield limit in contrast with a rigid plastic of high constant strength that yields only at the coast we conclude that 1 ice thickness motion feedback is important on scales less than 10 km 2 the observational base to discriminate between mesoscale constitutive laws is not yet available and 3 the relation of ice velocity to wind stress is variable because the ocean slope current responds only to the alongshore component of the wind

in situ u pb zircon ages for early ordovician magmatism in the eastern pyrenees france the canigou orthogneisses u pb age determinations with an ion probe on zircons within orthogneisses from the canigou massif eastern pyrenees give an emplacement age for their granitic protolith of 475 10 ma arenigian this invalidates the previous interpretation of basement cover relationships between the orthogneisses and the overlying lower cambrian canaveilles metasedimentary series and corroborates the intrusive character of the orthogneisses which is further supported by field evidence the data also define two groups of inherited zircons paleo meso proterozoic and cadomian in addition to partial lead loss during the hercynian late carboniferous high temperature low pressure metamorphic event these results are in agreement with ages reported from other orthogneissic bodies across the entire variscan chain of western europe and indicate the existence of a widespread cambro ordovician magmatic event

precambrian protoliths and early paleozoic magmatism in the french massif central u pb data and the north gondwana connection in the west european variscan belt abstract u pb geochronological data were collected on zircon by la mc icp ms on orthogneiss and paragneiss from the limousin area in the french massif central fmc in order to investigate the connection between the west european variscan belt and the northern margin of gondwana fifteen samples were collected in the four main tectonostratigraphic units of the fmc namely the para autochthonous unit the lower gneiss unit lgu the upper gneiss unit ugu and the thiviers payzac unit orthogneiss yield intrusion age between 521 7 and 446 6 ma considering all the results from both magmatic and metasedimentary samples two peaks at 531 and 473 ma are recognized rifting processes taking place along the north gondwana margin during the lower paleozoic went on until the end of the ordovician as suggested by the magmatic event recorded around 450 ma several maximum depositional ages were ascertained in the metasedimentary formations of the fmc as determined by the youngest detected detrital zircon crystal ranging from 604 16 ma for metasediments of the para autochthonous unit 573 12 ma for the lgu 564 9 ma for the thiviers payzac unit and 523 4 ma in the ugu minimum depositional ages are given by magmatic emplacement ages obtained in the crosscutting orthogneiss there is some evidence for a decrease of this maximum age upwards in the tectonostratigraphy detrital zircon in metasedimentary formations and inherited zircon in orthogneiss display a wide spectrum of ages with significant peaks at around 590 ma and 560 ma archean paleoproterozoic and neoproterozoic detrital zircons suggest a west african craton source of the sedimentation the large amount of neoproterozoic and lower paleozoic ages obtained in this study suggests that these periods played a significant role in the continental crustal growth history of western europe

the east variscan shear zone geochronological constraints from the capo ferro area ne sardinia italy abstract the exhumation of the variscan high grade metamorphic complex of sardinia was driven by large scale asymmetric shear zones which reached middle crustal levels at 325 1 3 u pb monazite ma exhumation was accompanied by heating and partial melting the migmatization process was followed by the emplacement of syntectonic granites dated with u pb on zircon at 318 3 ma and 317 2 ma which produced contact metamorphism at 315 1 3 ma u pb monazite in mylonitic silurian orthogneisses the nw se dextral shear zone of the capo ferro area ne sardinia is affected by partial melting by granite emplacement and shows remarkable petrotectonic analogies to and can thus be related to the east variscan shear zone a major intra continental shear zone which was active in the southern european variscides from 325 ma to 315 ma and that has been precisely dated with the u pb monazite system the emplacement of migmatitic gneisses along large shear zones within this time interval is not restricted to sardinia but has also been observed in southern corsica in the french massif central in the western alps and in central spain this pattern of shear zones is related to a complex geodynamic scenario linked to the oblique convergence of the gondwana and laurussia plates during the middle upper carboniferous

tidal spectroscopy of the english channel with a numerical model abstract the possibility of reproducing the complexity of tides in shallow water areas with a classical finite difference numerical model is examined this hydrodynamic model is two dimensional but incorporates topography nonlinear advection and quadratic bottom friction particular care is taken to prescribe sea surface elevations at the open boundaries a one month simulation of real tides is run with a simplified spectrum restricted to only 24 constituents corresponding to the nine main astronomical tides and their nonlinear significant interactions the results are analyzed by spectral decomposition elevations and vertically integrated currents and compared with observational data from the tide gages and current meters and with other solutions produced in the literature it is found that the dominant m2 constituent greatly influences the damping of the other constituents so that it is necessary to run them together for any correct simulation however the quadratic friction law introduced in

3d analysis of an ordovician igneous ensemble a complex magmatic structure hidden in a polydeformed allochthonous variscan unit the basal units of the allochthonous complexes of nw iberia represent a fragment of the external edge of gondwana subducted and subsequently exhumed during the variscan collision the structural analysis carried out in orthogneissic massifs of the southern part of one of these the malpica tui unit reveals the generation of recumbent folds and associated axial planar foliation during their exhumation these folds nucleated in irregular igneous bodies that were initially deformed during the subduction event and show east to southeast vergence down plunge projection of surface data and a series of regularly spaced cross sections have been used to build 3d models of the two main bodies of orthogneiss of calc alkaline and peralkaline compositions respectively the first is presently a lens shaped body folded in a recumbent syncline whereas the peralkaline gneiss also affected by a train of asymmetric recumbent folds in the south exhibits in the north a fold like structure which is interpreted as inherited from its primary geometry that of a ring dike pluton

on extracting tidal current profiles from vertically integrated two dimensional hydrodynamic models a numerically accurate and computationally efficient method is developed for extracting tidal current profiles from vertically integrated two dimensional models the method can incorporate an arbitrary profile of eddy viscosity and determines tidal current profiles satisfying a no slip condition at the sea bed comparisons with observations and current profiles computed with full three dimensional models confirm the accuracy of the method bottom boundary layer thicknesses determined from these current profiles clearly show regions of the european continental shelf where the bottom boundary layer extends to the sea surface the boundary between these regions and other areas where the bottom layer is less than the water depth corresponds quite closely to the known position of tidal fronts

vertical flow structure during sandy duck observations and modeling abstract observations of the vertical distribution of time averaged cross shore and alongshore flows during the sandy duck field experiment are compared with model predictions to assess the parameters governing the flow behaviour the measurements were obtained with a vertical stack of eight two component current meters with the lowest and highest sensor at respectively o 0 1 and o 2 7 m above the bed observations under breaking wave conditions within the surfzone show that the maximum return flow velocities occur in the lower part of the water column consistent with laboratory observations under non breaking conditions outside the surfzone the maximum return flow velocities are observed closer to the water surface again in line with laboratory results analogous to previous observations the measured longshore current velocity profiles are logarithmic under non breaking conditions and become more depth uniform under breaking conditions the model description of the vertical structure of the flow includes the presence of wind stresses wave stresses pressure gradients turbulent eddy viscosity and a wave boundary layer the model utilizes parabolic shape functions to describe the vertical distribution of the turbulent eddy viscosity in the middle layer and within the bottom boundary layer eddy viscosity is enhanced in regions where turbulence is produced i e near the surface in the case of breaking waves and within the bottom boundary layer estimates of the wave breaking induced turbulent eddy viscosity and bottom friction are obtained by minimizing the model measurement discrepancies predictions utilizing calibrated expressions for both the turbulent eddy viscosity and bottom friction are in general agreement with the observations provided the wave transformation and associated mass flux are modeled correctly and a parabolic eddy viscosity distribution is used using a piecewise constant eddy viscosity distribution generally results in a degrading of the agreement between measurements and model results

zircon ages of the metavolcanic rocks and metagranites of the ollo de sapo domain in central spain implications for the neoproterozoic to early palaeozoic evolution of iberia dating the pre middle ordovician metavolcanic rocks and metagranites of the ollo de sapo domain has historically been difficult because of the small compositional variation the effects of the variscan orogeny and as revealed in this paper the unusually high fraction of inherited zircon components the first reliable zircon data u pb ion microprobe and pb pb stepwise evaporation indicate that the ollo de sapo volcanism spanned 495 5 ma to 483 3 ma and was followed by the intrusion of high level granites from 483 3 ma to 474 4 ma in both metavolcanic rocks and metagranites no less than 70 80 of zircon grains are either totally precambrian or contain a precambrian core overgrown by a cambro ordovician rim about 80 90 of inherited zircons are early ediacaran 602 614 ma and derived from calc alkaline intermediate to felsic igneous rocks generated at the end of the pan african arc continent collision in the villadepera region located to the west both the metagranites and metavolcanic rocks also contain meso archaean zircons 3 0 3 2 ga which ultimately originated from the west african craton in the hiendelaencina region located to the east both the metagranites and metavolcanic rocks lack meso archaean zircons but they have two different inherited zircon populations one cryogenian 650 700 ma and the other tonian 850 900 ma which suggest older than ediacaran additional island arc components the different proportion of source components and the marked variation of the 87 sr 86 sr init suggest at least tentatively that the across arc polarity of the remnants of the pan african arc of iberia trended east west with respect to the current coordinates during cambro ordovician times and that the passive margin was situated to the west

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

geochemical characterization and isotopic age of caradocian magmatism in the northeastern iberian peninsula insights into the late ordovician evolution of the northern gondwana margin this paper presents new geochronological geochemical and isotopic data for late ordovician magmatism 455 2 ma from the variscan massifs in the northeastern iberian peninsula this magmatism is mainly composed of aluminous and metaluminous plutonic rocks and a sequence of calc alkaline ignimbrites and epiclastic volcanic rocks which were deformed and metamorphosed during the variscan orogeny the metaplutonic rocks are located in the lower part of a neoproterozoic early cambrian metasedimentary sequence in the canigo massif eastern pyrenees aluminous orthogneisses have isotopic signatures that indicate a crustal origin whereas metaluminous orthogneisses originate from a mixture of juvenile and crustal melts the metavolcanic rocks define a calc alkaline ignimbritic sequence of crustal origin located in the lower part of the late ordovician sequence in the les gavarres massif catalan coastal ranges tdm ages for all the studied late ordovician rocks range from mesoproterozoic to paleoproterozoic 1 3 ga 1 5 ga 1 7 ga and 2 2 ga suggesting a neoproterozoic crust with paleoproterozoic components and an arc signature as the most probable source for the crustal melts the isotopic similarities between the studied samples and other early ordovician magmatic rocks of the central and western iberian massif suggest repeated melt extractions from a common basement as the source for the early and late ordovician magmatism in the iberian realm however the studied massifs are more closely linked to other massifs from the mediterranean realm the most probable tectonic setting for the emplacement of the late ordovician volcanic and plutonic bodies is that of an extensional regime which postdates a previous ordovician contractional event and the opening of the rheic ocean

estimating the volume of tephra deposits a new simple strategy volume determination of tephra deposits is necessary for the assessment of the dynamics and hazards of explosive volcanoes several methods have been proposed during the past 40 years that include the analysis of crystal concentration of large pumices integrations of various thinning relationships and the inversion of field observations using analytical and computational models regardless of their strong dependence on tephra deposit exposure and distribution of isomass isopach contours empirical integrations of deposit thinning trends still represent the most widely adopted strategy due to their practical and fast application the most recent methods involve the best fitting of thinning data using various exponential segments or a power law curve on semilog plots of thickness or mass area versus square root of isopach area the exponential method is mainly sensitive to the number and the choice of straight segments whereas the power law method can better reproduce the natural thinning of tephra deposits but is strongly sensitive to the proximal or distal extreme of integration we analyze a large data set of tephra deposits and propose a new empirical method for the determination of tephra deposit volumes that is based on the integration of the weibull function the new method shows a better agreement with observed data reconciling the debate on the use of the exponential versus power law method in fact the weibull best fitting only depends on three free parameters can well reproduce the gradual thinning of tephra deposits and does not depend on the choice of arbitrary segments or of arbitrary extremes of integration

a three dimensional model of the northwest european continental shelf with application to the m4 tide abstract the solution of the fully nonlinear three dimensional hydrodynamic equations in polar coordinates for tides is developed using a finite difference grid in the horizontal and the galerkin method in the vertical using this method a three dimensional model of the northwest european continental shelf is used to investigate the spatial distribution of the m4 tidal elevations and currants over the shelf computed m4 currents are compared with observations and found to he in good agreement in the celtic sea although there are significant errors in the north sea the reasons for these discrepancies and the spatial and depth variations in the m4 currants are discussed in detail

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

application of a three dimensional turbulence energy model to the determination of tidal currents on the northwest european shelf a three dimensional turbulence energy tidal model employing a sigma coordinate system through the vertical with a logarithmic type transformation to enhance near bed resolution is used to examine m2 tidal currents on the european continental shelf a no slip bottom boundary condition is applied in the model and vertical eddy viscosity is computed from a turbulence energy closure scheme comparisons of observed and computed current profiles determined using the present model and an earlier model employing a spectral representation davies 1986 in the vertical and a slip bottom boundary condition are made in this spectral model a simple relationship between eddy viscosity and depth mean current was employed these comparisons of current profile suggest that above the near bed layer the spectral model can reproduce tidal currents with a comparable accuracy to that of the present turbulence energy model however in the near bed region the turbulence energy model is significantly more accurate computed spatial distributions of current magnitude turbulence energy intensity and eddy viscosity show similar distributions over the model region suggesting that a relationship between eddy viscosity and current intensity the form used in the spectral model is a valid alternative to using a turbulence energy model turbulence energy intensity in particular exhibits significant spatial variations both horizontally and vertically with surface contours of turbulence corresponding to known positions of tidal fronts

large magnitude middle ordovician volcanic ash falls in north america and europe dimensions emplacement and post emplacement characteristics abstract middle ordovician k bentonites represent some of the largest known fallout ash deposits in the phanerozoic era they cover minimally 2 2 106 km2 in eastern north america and 6 9 105 km2 in northwestern europe and represents the coeval accumulation of plinian and co ignimbrite ash on both laurentia and baltica during the closure of the iapetus ocean the three most widespread beds are the deicke and millbrig k bentonites in north america and the kinnekulle k bentonite in northwestern europe the vents were located near the laurentian margin of iapetus on an arc or microplate undergoing collision with laurentia the volume of ash preserved in the stratigraphie record converted to dense rock equivalent dre of silicic magma is minimally estimated to be 943 km3 for the deicke 1509 km3 for the millbrig and 972 km3 for the kinnekulle the millbrig and kinnekulle beds are coeval and possibly equivalent yielding a combined dre volume of nearly 2500 km3 some unknown but probably large amount of additional ash fell into oceanic regions of the iapetus but these areas became subducted and the ash is not preserved in the geologic record the symmetry of the thickness contours is suggestive that one or more ash clouds interacting with equatorial stratospheric and tropospheric wind patterns dispersed pyroclastic material to both the northwest and southeast in terms of ordovician paleogeography based on grain size measurements and thickness area 1 2 plots we conclude the three beds were each formed from co ignimbrite or possibly phreatoplinian eruption columns analyses of melt inclusions in primary quartz crystals indicate the parental magma contained approximately 4 dissolved water at the time of the eruption this water provided the explosive energy during the initial gas thrust phase the implied fragmentation pressure on the magma would have reduced much of the ejecta to small particles forming a deposit composed largely of single crystals and glassy dust conversion of the ash to k bentonite resulted in a mass loss of approximately 35 mostly in the form of si with lesser amounts of na and k

multiple early paleozoic volcanic events at the northern gondwana margin u pb age evidence from the southern variscan branch sardinia italy abstract in sardinia one of the southernmost remains of the european variscan belt a crustal section through different gondwanan paleodomains is largely preserved laser ablation icp ms u pb ages on undoubtedly igneous sites of the zircons were determined on the lower palaeozoic volcanic rocks constrained by defined field relationships thus evidencing three subsequent volcanic events i intermediate and felsic 491 7 3 5 ma 479 9 2 1 concordia ages transitional volcanic rocks embedded within a cambro ordovician terrigenous succession that occurs with continuity in external and inner nappes bounded to the top by the sardic unconformity ii this cambrian lower ordovician succession is cut by calc alkalic rhyodacites which yielded a concordia age of 465 4 1 4 ma confirming their pertinence to the huge bimodal mid ordovician arc volcanism commonly interpreted as the widespread marker of the rheic ocean subduction iii alkalic metaepiclastites in the external nappe within the post caradocian transgressive sequence dated at 440 1 7 ma likely related to rifting and collapse of the mid ordovician volcanic arc in the reshaped lower palaeozoic stratigraphy of sardinia the timing of the early steps of the variscan wilson cycle can be inferred

storm surges 1967 1982 summary an earlier review of storm surges covering the forty years up to 1967 is updated again the emphasis is on storm surges around the british isles but as before references are also made to surges which occur in other areas it is shown that the main advance in the work during the period considered has been the widespread development and use of two dimensional numerical storm surge models the application of these models to operational storm surge forecasting is described research evaluating storm surge currents using three dimensional models is considered

u pb zircon dating and nature of metavolcanics and metarkoses from the monte grighini unit new insights on late ordovician magmatism in the variscan belt in sardinia italy in the external units of the sardinian variscides nappe zone volcanic and volcanoclastic successions of middle ordovician age follow lower paleozoic calc alkaline magmatism developed at the northern gondwana margin we present geochemical and zircon u pb isotopic data for the truzzulla formation a low to medium grade metamorphic volcanic volcanoclastic succession belonging to the monte grighini unit the deepest unit in the nappe zone geochemical and radiometric data allow us to define a late ordovician katian magmatic volcanic event of calc alkaline affinity these new data in conjunction with previously published data indicate that in the sardinian variscides the age of lower paleozoic andean type calc alkaline magmatism spans from middle to late ordovician moreover the age distribution of calc alkaline volcanics and volcanoclastic rocks in the nappe zone is consistent with a diachronous development of middle late ordovician andean type magmatic arc through the portion of the northern gondwanian margin now represented by the sardinian variscides this reconstruction of the sardinian variscides reflects the complex magmatic and tectonic evolution of the northern margin of gondwana in the lower paleozoic

review of recent developments in tidal hydrodynamic modeling i spectral models in this paper we present a brief overview with a comprehensive set of references to recently published papers for detail of a range of techniques used in tidal hydrodynamic modeling and recent progress in the field although the paper aims to introduce the topic to someone new to the area the main emphasis is on new developments in near coastal shallow sea tidal modeling and consequently we concentrate on three dimensional modeling which has progressed significantly in recent years the paper gives a brief account of methods used to discretize the hydrodynamic equations in the horizontal namely finite difference finite element and boundary fitted coordinates and through time the representation of tidal current structure in a three dimensional model using a spectral method in the vertical is also described in this method the subgrid scale vertical diffusion of momentum which has a major influence on tidal current profiles is parameterized using a flow dependent eddy viscosity recent work on

probing crustal and mantle lithosphere origin through ordovician volcanic rocks along the iberian passive margin of gondwana abstract northwestern iberia preserves one of the most complete paleozoic sequences that document the origin and development of a passive margin along the southern gondwanan flank of the rheic ocean in addition to a well preserved sedimentary record there is widespread ordovician volcanic activity that can be used to probe the nature of the lower crust and mantle lithosphere that sourced the volcanic rocks during the rheic ocean opening the ordovician rift related volcanic sequences provide first order constraints on the early evolution of the rheic ocean in addition to published and new lithogeochemical data we provide sm nd isotopic data which together constrain the mantle or crustal source and allow an assessment on the role of the basement in rheic ocean magmatism the data imply that the mafic rocks are derived from a variety of sources including juvenile mantle that was contaminated by subduction coeval with early ordovician magmatism suggesting the importance of arc activity in northwest iberia during the opening of the rheic ocean other basalts were derived from a subcontinental lithospheric mantle that was enriched at about 1 0 ga basalts derived from a mantle enriched at ca 1 0 ga occur along other parts of the gondwanan margin avalonia oaxaquia and so the iberian basalts may be a local representation of a regionally significant enriched mantle the sm nd isotopic characteristics permit a genetic connection between this mantle source and the basement rocks recently identified in northwest iberia felsic magmas are predominantly intra crustal magmas derived from melting a mesoproterozoic crust lending support to other lines of data that the gondwanan margin of northwest iberia was predominantly underlain by a south american rio negro source

three dimensional modal model of wind induced flow in a sea region abstract the solution of the linear three dimensional hydrodynamic equations describing wind induced flow in a sea region is developed using the galerkin method through the vertical a basis set of b splines is shown to have some computational advantages over a set of eigenfunctions vertical modes however a basis set of modes leads to a system of essentially uncoupled equations and current profiles can be interpreted in terms of vertical modes the influence of wind induced surface turbulence and turbulence at depth due to tidal motion upon current profiles in both deep 260 m and shallow 35 m sea regions is examined the variation in the angle between surface current and surface wind for different viscosity profiles and the effect of bottom friction upon it is considered the magnitude and direction of the surface current is significantly influenced by surface eddy viscosity however viscosity at depth due to tidal motion also has an important effect upon the surface current the time evolution of current structure following the sudden onset of a wind is examined using the modal model calculations show that the rate of damping of the internal modes is inversely proportional to the square of the depth consequently wind induced current structure takes longer to reach a steady state in a deep sea region than a shallow area the influence of sea surface elevation gradients in determining the direction of surface current is also considered

precise timing of the late ordovician sandbian super eruptions and associated environmental biological and climatological events two of the largest known eruptions in the phanerozoic produced the ordovician millbrig k bentonite of north america and the kinnekulle k bentonite of scandinavia which have been previously suggested to be coeval the millbrig k bentonite from kentucky usa and the kinnekulle k bentonite from bornholm denmark yielded chemical abrasion thermal ionization mass spectrometry u pb zircon dates of 452 86 0 29 and 454 41 0 17 ma 2 analytical uncertainty respectively thus showing significant age differences contrary to what is generally held these data and four additional newly dated k bentonites directly establish the first radioisotopic age constraints for the ordovician katian sandbian global stage boundary refine global stratigraphic correlations date associated chemostratigraphic events and suggest an alternative volcanic climate hypothesis for the late ordovician supplementary material u pb radioisotopic data table and analytical methods are available at www geolsoc org uk sup18636

was there a super eruption on the gondwanan coast 477 ma ago precise zircon and monazite id tims u pb dating of three lower ordovician altered ash fall tuff beds k bentonites in the cantabrian zone of nw iberia yielded coeval ages together with an equivalent previously studied sample 477 5 1 gutierrez alonso et al 2007 of 477 1 3 ma 477 2 1 1 ma and 477 3 1 ma with a pooled concordia age all analyses in the four samples of 477 2 0 74 ma a conservative estimation of the volume and mass of the studied k bentonite beds using exclusively the cz data yields a volume for the preserved deposits of ca 37 5 km3 volcanic explosivity index vei 6 colossal when considering other putative equivalent beds in iberia and neighboring realms i e armorica sardinia the volume of ejecta associated to this event would make it reach the supervolcanic apocalyptic status vei 8 1000 km3 at variance with most known cases of this kind of gigantic eruption events geological observations indicate that the studied magmatic event was related to continental margin extension and thinning and not to plate convergence we speculate that a geochronologically equivalent large caldera event recognized in the geological record of nw iberia could be ground zero of this super eruption

on improving the bed stress formulation in storm surge models a method of improving the representation of bottom stress in storm surge models an enhanced bed stress model is tested using a fine mesh model of the celtic and irish seas and is compared with results from a conventional two dimensional model the approach uses a convolution method to determine the bed stress rather than relating it directly to the depth mean current as in a conventional storm surge model for the region considered comparisons of surge elevations computed with both models and observations show only a slight improvement of surge elevation in the near coastal region with the enhanced bed stress model however spatial distributions of bed stresses from the two models do exhibit significant differences which are important in considering sediment movement paths during major storms the magnitude and direction of the bed stress vectors computed from the enhanced bed stress model are in excellent agreement with calculations using a full three dimensional model of the area this suggests that the enhanced bed stress model can compute a bed stress that is not significantly different from that in a full three dimensional model at a fraction of the computational cost