a far field record of the end ordovician glaciation the ellis bay formation anticosti island eastern canada abstract the end ordovician was a critical period in earth history it was a time of global climatic change with large scale continental glaciations a wealth of recent work has documented the stratigraphical records from the near field areas i e from various gondwanan continental relics where glaciation was more severely felt there is an urgent need to re examine the stratigraphic record from the far field areas at greater levels of resolution than it has been hitherto possible in order to understand the timing duration and character at the glaciation centred around the ordovician silurian o s boundary one of the best exposed and most complete stratigraphic records from paleotropical areas near the boundary is on anticosti island in eastern canada the east west trending 200 km long ellis bay hirnantian outcrop belt with superb coastal exposures at both ends of anticosti island is slightly oblique to the paleoshoreline with near shore siliciclastic dominated facies restricted to the eastern part of the island and more offshore carbonate dominated facies present along the central and western parts these mixed carbonate siliciclastic facies accumulated along a storm dominated beach to offshore profile they are organized into a stacked set of shallowing upward offshore to shoreline facies successions typically coarsening upward separated by deepening upward facies successions typically fining upward formed during intervening transgressions high frequency glacio eustatic fluctuations were the dominant control on the development of tr sequences observed in the ellis bay succession of anticosti island although tectonic subsidence had a substantial secondary effect on thickness five tr sequences are recognized allowing for precise correlation along the ellis bay outcrop belt in the eastern sections relatively thin tr sequences are locally bounded by unconformities a typical sequence begins with thin transgressive limestone overlain by a maximum flooding calcareous shale grading up into regressive storm influenced proximal to transitional zone fine sandstone grainstone and shale which are sharply overlain by shoreface to foreshore sandstone grainstone in the central and western sections the thicker tr sequences have more symmetrical transgressive regressive components reflecting greater subsidence compensated by a higher carbonate sediment supply two discrete glacial periods are recognized during the deposition of the hirnantian ellis bay succession with the younger one recording large sea level changes 50 m in magnitude caused by the development of very large ice sheets over western gondwana abundant ice conditions on the globe during the end ordovician and the resulting high amplitude eustasy appear to have left distinctive stratigraphical imprints on mixed paleotropical siliciclastic carbonate ramps

stress and timing relationships of a fault related paleohydrothermal system in central new hampshire record of a mesozoic stress change in new england a mineral assemblage comprised of chalcedony drusy quartz and very fine grained iron hydroxide limonite with or without blocky calcite qfc coats joint surfaces fills fractures and commonly occurs as pressure shadow encrustations on brittle fault surfaces in central new hampshire but only occurs sporadically in other areas of new england locally qfc includes silicified breccias of highly altered host and euhedral quartz crystal fragments suspended in a matrix of chalcedony and limonite one hundred and four brittle faults from central new hampshire mineralized with qfc can be separated into two families set n is composed of primarily north northeast to northeast trending steeply dipping normal faults n 61 and set s is composed of north northwest to north northeast trending steeply dipping primarily right lateral strike to normal oblique slip faults n 43 these two sets of faults are thought to record two paleostress conditions the extension direction for both tensors plunges gently east southeast but the compression direction is near vertical for set n and trends roughly northeast plunging 30 for set s fault slip inversion suggests that set n faults are best fit with 1 most deviatoric 2 3 set s is best fit with 3 most deviatoric 1 2 and low values for fault plane coefficient of friction and or high fluid pressure qfc occurs in the 158 ma belknap mountain complex of the white mountain magma series and is thought to have developed at relatively shallow crustal levels based on the assemblage nature of fluid inclusions and association of qfc as brittle fault and fracture filling mineralization occurrence of qfc in the 158 ma belknap mountain complex paleostress implications of qfc mineralized brittle faults and range and character of fault fabrics in the belknap units suggests that qfc developed around the time of belknap intrusion and cooling and that late jurassic to early cretaceous is the approximate time at which a regional change in paleostress conditions occurred in new england this age and nature of the paleostress change is in accord with a change postulated by de boer et al 1988

can long wavelength dynamical signatures be compatible with layered mantle convection analyses of the long wavelength geoid with seismic tomographic models have been providing for a long time important estimates of mantle viscosity these estimates have nearly been derived under the assumption of whole mantle flow it has been commonly held that a fully impermeable boundary at 660 km depth is incompatible with the long wavelength gravity signal on the other hand models with whole mantle circulation which can explain a large portion of the geoid signal usually produce excessive amplitudes of the dynamical topography especially for long wavelengths using recent tomographic models together with genetic algorithm we have successfully demonstrated that the layered convection model can also produce a reasonable fit to the geoid which is comparable in quality with that obtained for the whole mantle model the layered model can simultaneously yield realistic amplitudes of the dynamical topographies of the surface and the 660 km discontinuity

geoid anomalies and the structure of continental and oceanic lithospheres geoid anomalies in the wavelength range corresponding to spherical harmonies 6 to 30 are used to bring new constraints on the deep structure of the continental lithosphere on the nature of upper mantle seismic anomalies and on the thermal evolution of the oceanic lithosphere for the shallow lithospheric density anomalies considered here the geoid anomaly is shown to follow approximately the moment law derived from isostasy by a procedure of least squares inversion the geoid over old cratons is found to be equal to that over relatively young 50 myr oceanic lithosphere the oceanic geoid as a function of age follows the expected linear trend with flattening at old ages these results are independent of the degree at which the spherical harmonics are truncated if the tectosphere consisted only of a thick thermal boundary layer marked geoid lows with an amplitude larger than 20 m should be present over cratons in disagreement with the present observation the predicted geoid agrees with the observations if the negative density anomalies associated with a depleted peridotitic layer are present to the bottom of the lithosphere this analysis of the geoid confirms the petrological inferences that the deep roots observed seismically in the top upper mantle form a chemically differentiate reservoir the density of these cold roots is similar to that of mantle material elsewhere because the increase in density resulting from their lower temperature is largely balanced by the decrease in density caused by their depletion analysis in wavelength range corresponding to harmonics 6 to 30 in the oceans confirms that the geoid flattens at old ages and thus supports the view that an important amount of heat is transferred at the base of the lithosphere by small scale convection

radial profile of mantle viscosity results from the joint inversion of convection and postglacial rebound observables we present new inferences of the radial profile of mantle viscosity that simultaneously fit long wavelength free air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions hudson bay arctic canada and fennoscandia the relative sea level variation at these latter sites is constrained by age height pairs obtained by geological survey rather than the subjective trends which are commonly used in glacial isostatic adjustment gia studies our viscosity inferences are generated using two approaches first we adopt a relative viscosity profile which is known to provide a good fit to the free air gravity harmonics and determine an absolute scaling which yields a best fit to the gia decay time constraints second we perform an iterative nonlinear joint inversion of the two data sets in both cases our inferred profiles are characterized by a significant increase of viscosity 2 orders of magnitude with depth to values of 1022 pa s in the bottom half of the lower mantle the new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both fennoscandia the classic haskell 1935 number and hudson bay which have commonly been invoked to argue for an isoviscous mantle furthermore the models are used to predict a set of long wavelength signatures of the gia process these include predictions of gia induced variations in 1 the length of day over the late holocene period 2 the earth s precession constant and obliquity over the last 2 6 myr and 3 the present day zonal harmonics of the geopotential jl l 7 the predictions 1 and 3 bound the late holocene and ongoing mass flux between the large polar ice sheets greenland and antarctic and the global oceans to small values 0 4 mm yr equivalent eustatic sea level rise

late neoproterozoic to permian tectonic evolution of the quebec appalachians canada abstract due to the unique location of the quebec appalachians in a re entrant of the paleo north american margin and to the fact that the area largely escaped late paleozoic deformation this segment of the orogen preserves evidence of the early orogenic stages that are partly obscured elsewhere despite diachronicity both along and across strike of the orogen five tectonic events of regional significance are recognized 1 the earliest event is mainly recorded by neoproterozoic to early ordovician normal faults associated with extension of the laurentian craton and the establishment of a passive continental margin 2 the second event the ordovician taconian orogeny is marked by foreland directed deformation and corresponds to the principal deformational and metamorphic events recorded by the laurentian margin 3 the third event early salinic is early silurian it is restricted to the southern part of the quebec appalachians and consists of hinterland se directed thrusts folds and retrograde metamorphism 4 the fourth event late salinic is recorded by regional scale late silurian to early devonian normal or transtensional faults and is associated with the formation of the connecticut valley gaspe trough and 5 the last event the devonian acadian orogeny affects with variable intensity all the pre carboniferous lithologies this tectonic synthesis implies that the subdivision of the peri laurentian oceanic tracts proposed in other segments of the appalachians should be applied with caution in quebec and that the structural style the number of oceanic domains and possibly the vergence of subduction change along the strike of the orogen in particular the similarities of the thetford mines and boil mountain ophiolitic complexes and sedimentary covers and the lack of evidence for a major intervening suture are consistent with the maximum allochtony hypothesis i e the assumption that the southern quebec ophiolites are rooted east of the chain lake massif a geodynamic model involving the taconian obduction of a large ophiolitic nappe s the docking of a peri gondwanan gander medial new england terrane in early salinic the formation of large sedimentary basins connecticut valley gaspe and merrimack troughs due to trench rollback and or delamination during the late salinic event and collision at the outer margin of composite laurentia during the acadian orogeny can account for the protracted structural and metamorphic history of the quebec appalachians

episodic sedimentation on an early silurian storm dominated carbonate ramp becscie and merrimack formations anticosti island canada the 150 160 m thick lowermost silurian rhuddanian becscie and merrimack formations of anticosti island canada represent continuous deposition on a shallow open marine carbonate ramp several rock types are identified a laminated and homogenous mudstone b laminated and homogenous packstone c argillaceous mudstone and packstone d calcareous shale e laminated calcisiltite f medium to fine grained grainstone and g bio intraclastic rudstone these rock types are arranged into five distinct lithofacies lf1 calcareous mudstone shale lf2 laminated homogenous mudstone lf3 calcareous grainstone shale lf4 laminated mudstone grainstone and lf5 laminated calcisiltite grainstone the sequence reflects deposition on a low energy muddy carbonate to argillaceous ramp subject to short lived episodic high energy storms these events produced fining upwards storm units 5 80 cm thick or tempestites consisting of a sharp scoured base overlain by intra bioclastic rudstone grading upwards into medium grained grainstone finely laminated calcisiltite and mudstone or shale these are interbedded with low energy fairweather mudstones and calcareous shales deposition progressed from a carbonate mud dominated ramp in the becscie formation to an argillaceous mud dominated ramp in the merrimack formation lateral tempestite proximality trends and lithofacies distribution indicate that the anticosti basin deepened to the south east into the iapetus ocean and shallowed towards a sw ne orientated shoreline to the north west vertical tempestite proximality trends and lithofacies changes identify third order eustatic sea level changes after an initial deepening at the base of the formation a shallowing deepening event dominated the sequence several higher order fluctuations defined by lithofacies and tempestite proximality trends are superimposed on these changes the fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by either lithofacies or palaeontological methods

melting reactions and trace element relationships in selected specimens of migmatitic pelites from new hampshire and maine leucosomes and melanosomes in selected specimens of migmatitic sillimanite zone pelitic schists are modal and chemical complements formed by segregation within originally homogeneous paleosomes systematic bulk chemical and modal variations in melanosomes can be used to infer the reactions by which leucosomes were generated

the extent of equilibration between melt and residuum during regional anatexis and its implications for differentiation of the continental crust a study of partially melted metapelitic enclaves thisstudy presentsthefirst precise evaluation of the extent ofchem ical equilibrium between felsic melt and crystalline residuumduring the crustal anatexis of metasediments the high precision ofthese resultsstemsfrom thefactthat in thiscase the meltsare repre sented by pristine glasses that occur in the matrices of partiallymelted metapelitic enclaves and as abundant glassy melt inclusionstrapped by different minerals in the same enclaves in previous stu dies of anatexis the composition of the melt has been approximatedby that of leucosomes in migmatites which do not necessarily repre sent the composition of the melt very well the present study isbased on laser ablation analyses of 35 trace elements in the glassand principal minerals of enclaves included within neogene peralu minous dacites exposed at el hoyazo se spain the enclaves which contain the assemblage plagioclase biotite sillimanite garnet glass ilmenite graphite k feldspar cordieritequartz represent fragments of metapelitic continental crust thatwaspartiallymeltedatpressuresof5 7kbarduringaregionalmeta morphic event matrix melt and melt inclusions within the enclaveswere quenched to glass upon eruption of the dacite the glasses inthe melt inclusions are interpreted as the remains of melt producedby an initial melting reaction involving a muscovite rich assemblageat 700 7508c whereasthe matrixglassesare consistent with con tinuedmeltingand oralatermeltdominatedbytheincipientmeltingof biotite at 800 8508c the distribution of trace elements be tween residuum and melt duringanatexis of the enclaves isdifferentfrom that predicted by disequilibrium models that consider effectivedistribution coefficients close to unity instead the bulk melt andthe major minerals plagioclase biotite alkali feldspar cordierite were close to equilibrium except for the case of garnet duringthe generation of the melt inclusions and matrix glasses thequenchedmelts meltinclusionsandparticularlythematrixglasses were not in equilibrium with accessory zircon and monazite andare depleted in zirconium and the light rare earth elements recrystallization of mineralsand diffusion in the melt were import ant controls during the redistribution of trace elements betweenthe solids and the bulk melt local disequilibrium processes alsotook place at mineral melt interfaces during the crystallizationof peritectic minerals and the recrystallization of residual phases as shown by the depletion of compatible elements in the meltinclusions

admittance for a convection in a layered spherical shell summary a formalism based on an analytical approximation for convection in a layered spherical shell leads to the computation of the green kernels for topography and gravity and to the admittance of gravity over topography for the harmonics of the earth we study the role of increasing viscosity with depth of two layers of convection and of the coupling with lithospheric plates it is found that a planet with constant viscosity will have positive values for the admittance for all wavenumbers however when a step of viscosity between the lower and upper mantle is assumed negative values for the first harmonics are achieved this result explains some long wave observations and provides evidence for an increase of viscosity in the lower mantle

mantle dynamics constrained by degree 6 surface topography seismic tomography and geoid inference on the origin of the south pacific superswell abstract the relationship between global geophysical observables i e seismic tomography geoid and earth surface topography is investigated this study focuses on the high correlation at degree 6 which in addition to degree 2 characterizes these global fields surface topography is found to be highly correlated at degree 6 to seismic velocity anomalies between the 150 km and 450 km depths in the upper mantle several highs of the degree 6 topography coincide with particular provinces of the earth surface among others the south pacific superswell and the afar region these provinces overlie low velocity hot upper mantle by developing a global circulation model in a compressible radially viscosity stratified mantle we show that the degree 6 global fields can be explained as the result of the mantle convective flow driven by density anomalies the viscosity structure that best fits the degree 6 observables also explains the low degree 2 and 3 components of the dynamical topography and geoid the best fit model has a viscosity increase of factor 30 at the upper mantle lower mantle discontinuity and a low viscosity layer extending from the 100 to the 400 km depth in the upper mantle with a viscosity drop of factor 30 with respect to the transition zone these values are in good agreement with most previously published results based on the low degree geoid alone from this study we infer that the south pacific superswell which lies over a broad zone of hot upper mantle extending down to the 450 km depth is generated by the degree 6 flow of the mantle circulation

glacial isostatic adjustment and earth rotation refined constraints on the viscosity of the deepest mantle we explore the extent to which two properties of earth s present day rotational state may be applied to constrain the viscosity of the lower regions of the lower mantle the analysis builds upon recent advances in understanding the radial resolving power of the relative sea level data of postglacial rebound which have demonstrated anew that no significant increase of viscosity across the 660 km seismic discontinuity appears to be allowed by these data such observations do not constrain the viscosity of the mantle below about 1400 km depth however and here we estimate the average viscosity in this deepest region by invoking observations of the present day rate and direction of polar wander and the present day magnitude of the nontidal acceleration of the rate of planetary rotation since the rotational response to the glaciation cycle depends only upon the degree 2 spherical harmonic components of the induced deformation this response is expected to provide the most specific sampling possible of deep mantle properties our analysis is based upon the use of glacial excitation functions calculated from complete gravitationally self consistent solutions of the sea level equation for the most recent refinement of the history of ice sheet thickness variations across the last glacial interglacial transition namely the ice 4g model this analysis shows that when earth rotation observations are combined with relative sea level constraints the viscosity of the deepest mantle is required to be 1 order of magnitude higher than the viscosity of the upper part of the lower mantle such significant elevation of viscosity in the lowermost region of the lower mantle is also in accord with recent inferences based upon analysis of the nonhydrostatic geoid the analyses presented herein therefore suffice to effect a reconciliation between the requirements of these distinct geophysical observations

tempestite frequency curves a key to late ordovician and early silurian subsidence sea level change and orbital forcing in the anticosti foreland basin quebec canada following partial closure of the northern iapetus ocean along the newfoundland segment of the st lawrence promontory subsidence along the anticosti platform was influenced by residual thermal subsidence renewed tectonic loading by thrust sheets to the south and sediment loading basement subsidence calculated by removing the effects of sediment loading was between 2 and 5 cm ka in the caradocian increasing to 8 6 cm ka in the pusgillian and cautleyan and reaching a maximum of 17 7 cm ka in the rawtheyan during deposition of the princeton lake and vaureal formations a marked decline in subsidence beginning in the hirnantian and continuing into the early silurian may reflect decoupling of thrust loads to the south although a further stage of thrust loading may have occurred in the aeronian during deposition of the lower part of the jupiter formation storm frequency curves produced using tempestite abundance thickness and grain size through more than a kilometre of carbonate strata on antico

low angle faulting in strike slip dominated settings seismic evidence from the maritimes basin canada abstract the maritimes basin is an upper paleozoic sedimentary basin centered in the gulf of st lawrence canada early phases of basin formation included the development of partly connected sub basins bounded by high angle faults in an overall strike slip setting interpretation of reprocessed seismic reflection data indicates that a low angle detachment contributed to the formation of a highly asymmetric sub basin this detachment was rotated toward a lower angle and succeeded by high angle faults that sole into the detachment or cut it this model bears similarities to other highly extended terranes and appears to be applicable to strike slip and or transtensional settings

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

chemical exchange in migmatites during cooling abstract the chemistry of the leucosome components in limited size migmatitic associations is generally believed to reflect the composition of the partial melt that could have been extracted to yield an anatectic magma several well documented studies of migmatites indicate that the distributions of some trace elements in leucosome melanosome pairs are inconsistent with equilibrium partial melting but are rather in agreement with chemical reequilibration during cooling in migmatites the liquid fraction does not leave the system this may induce an alteration of the chemical composition of the whole migmatite system by extensive chemical exchange between the liquid and the solid residue during cooling if this is the case the leucosome and melanosome components are not representative of the chemical composition of the liquid and solid residue respectively at the time of partial melting a rough evaluation of the residences time of the melt fractions in cooling in situ migmatitic bodies yields minimum estimates of 1 to 10 ma for such time spans the characteristics distances of diffusion distances of diffusion induced chemical transfer exceed the sizes of individual mineral grains which is consistent with a re equilibration process the release of latent heat of crystallization in cooling migmatitic systems and the presence of an interstitial liquid phase are critical in determining the extent and scale of chemical exchange the mechanism involves 1 reducing the cooling rate and 2 promoting the efficiency of chemical diffusion through the liquid phase thus some doubt is cast on the ability of small scale migmatitic systems to provide relevant information about chemical mass balance during partial crustal melting

on the transient nature of mantle plumes 1 we present new experiments focussing on the transient behaviour of thermal plumes in a fluid heated from below plumes develop once the hot thermal boundary layer tbl reaches a critical thickness howard 1964 they rise through the fluid owing to their thermal buoyancy and comprise tbl material which empties itself into the plumes as the tbl becomes exhausted plumes start disappearing from the bottom up sometimes even before reaching the upper boundary depending on the convection intensity then they finally fade away by thermal diffusion this sequence of events shows that time dependence is a key factor when interpreting present day tomographic images of mantle upwellings in particular it could be erroneous to identify the depth of a present day slow seismic anomaly with the depth of its origin or to interpret the absence of a long tail as the absence of a plume

measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy 3 inversion lateral heterogeneity in the earth s upper mantle is investigated by inverting dispersion curves of long period surface waves 100 330 s models for seven different tectonic regions are derived by inversion of regionalized great circle phase velocity measurements from our previous studies we also obtain a representation of upper mantle heterogeneities with no a priori regionalization from the inversion of the degree 6 spherical harmonic expansion of phase and group velocities the data are from the observation of about 200 paths for love waves and 250 paths for rayleigh waves for both the regionalized and the spherical harmonic inversions corrections are applied to take into account lateral variations in crustal thickness and other shallow parameters these corrections are found to be important especially at low spherical harmonic order the trench region and fast velocities down to 250 km under shields below 200 km under the oceans both s velocity and s anisotropy support a model of small scale convection in which cold blobs detach from the bottom of the lithosphere when its age is large enough the spherical harmonic models clearly demonstrate a posteriori the relation between surface tectonics and s velocity heterogeneities in the first 250 km all shields are fast most ridges are slow below 300 km a belt of fast mantle follows the pacific subduction zones however at greater depths large scale heterogeneities that seem to bear no relationship to surface tectonics are observed the most prominent feature at 450 km is a fast velocity region under the south atlantic ocean smaller scale heterogeneities that are not related to surface tectonics are also mapped at shallower depths an anomalously slow region centered in the south central pacific is possibly linked to intense hot spot activity a very fast region southeast of south america may be related to subduction of old pacific plate between 200 and 400 km a belt of sv sh anisotropy follows part of the ridge and subduction systems indicating vertical mantle flow in these regions the spherical harmonic results open new horizons for the understanding of convection in the mantle perspectives for the improvement of the models presented are discussed

maturation thermique de la matiere organique dans un bassin du paleozoique inferieur basses terres du saint laurent quebec canada the organic matter om sampled in 15 oil exploration wells and 102 outcrops of cambrian ordovician rocks in the st lawrence lowlands consists of zooclasts chitinozoans graptolites and scolecodonts and solid bitumen mainly pyrobitumen the reflectance of pyrobitumen transformed in vitrinite equivalent ro std b indicates that the upper part of the platform sequence is mature condensate zone in the quebec area but overmature in the montreal area the platform is divided into three domains based on optical texture of om and types of bitumen domain 1 south of montreal contains a highly reflecting and coked pyrobitumen showing alteration rims domain 3 east of trois rivieres contains low reflecting late solid bitumen commonly associated with oil impregnations domain 2 located midway contains a pyrobitumen with moderate reflectance and locally fine mosaic texture the thermal maturation in the autochthonous sequences of the st lawrence lowlands and in the appalachian allochthons increas

carbonate sedimentation in an extensional tectonic regime the lower devonian upper gaspe limestones quebec appalachians carbonate sedimentation occurred over most of eastern gaspe peninsula in early devonian time the resulting succession known as the upper gaspe limestones is a 500 1800 m thick unit outcropping in the connecticut valley gaspe synclinorium detailed stratigraphic and sedimentologic work allows recognition of important thickness and lithofacies variations in a north south oriented transect in the eastern part of the peninsula variations are spatially associated with acadian middle devonian dextral strike slip faults three lithotectonic domains are proposed and interpreted they are from north to south 1 a proximal outer shelf 2 a distal gently sloping outer shelf and 3 a slope or slope toe field mapping and reinterpretation of seismic profiles from this area have shown that the faults bounding the three domains are reactivated normal listric faults early devonian vertical movement along these faults caused accelerated subsidence of the central and southern segments of the sedimentary ba

suspect terranes and accretionary history of the appalachian orogen the north american connection or the grenvillian continental crust that separated from eastern north america during the initiation of the appalachian orogenic cycle is poorly defined within the present orogen a variety of terranes occur outboard of the north american miogeocline all structurally uncoupled and therefore suspect stratigraphic analysis indicates that appalachian accretion progressed from the miogeocline outward the boundaries of the earliest accreted western terranes are marked by melange and ophiolite complexes later boundaries between eastern terranes are steep mylonitic zones and brittle faults accretionary events defined by stratigraphic analysis correspond to times of major deformation plutonism and metamorphism in the history of the orogen

liquid segregation parameters from amphibolite dehydration melting experiments we have experimentally obtained some of the parameters necessary for understanding the segregation of silicate liquid from amphibolite undergoing dehydration melting at 1 gpa and 750 1000 c the solidus for this calcic amphibolite 68 mg hb 32 pl an 90 is 750 c amphibolites begin to melt at relatively high temperatures in the garnet absent field but the solidus appears to backbend at 1 gpa coincident with the garnet in boundary hornblende breakdown due to garnet formation releases h 2o and causes melting thus in the garnet present field 1 gpa the amphibolite dehydration melting solidus may be coincident with the h 2o saturated solidus liquid interconnectivity may be achieved at 900 c and 5 vol liquid based on both physical and chemical data from solid rock runs mass balance calculations from powdered rock runs suggest that small amounts 5 15 vol of hydrous 4 wt h 2o low viscosity 10 3 10 4 pa s heavy rare earth element depleted felsic liquid may be segregated during amphibolite dehydration melting at 875 c the rapid breakdown of coarse grained hb cores may lead to the formation of transient h 2o saturated liquids with even lower viscosities 10 2 pa s although comprising only a small portion of the melting cycle these h 2o rich conditions may enhance the segregation of liquid by reduction of liquid viscosities and by mechanical effects on the restite e g increased deformation and liquid fracturing during anatexis of coarse grained natural rocks transient conditions may control the initial stages of liquid segregation

the corossol structure a possible impact crater on the seafloor of the northwestern gulf of st lawrence eastern canada we report on a 4 1 0 2 km diameter and 185 m deep circular submarine structure exposed on the seabed in 40 m water depths in the northwestern gulf of st lawrence eastern canada from the analysis of high resolution multibeam bathymetric and seismic data the presence of a circular form characterized by a central uplift and concentric rings resembles the morphology and geometry of complex meteorite impact structures also other origins such as kimberlites intrusions karsts or diapirs can be eliminated on geological criteria a single 4 cm long breccia fragment recovered from the central uplift has numerous glassy droplets of fluorapatite composition assumed to be impact melts and a single quartz grain with planar intersection features thought to be shock induced planar deformation features pdfs the absolute age of this possible impact structure is unknown but its geological setting indicates that it was formed long after the mid ordovician and before regional pre quaternary sea level lowstands present results outline the need for further examination to confirm an impact origin and to precisely date the formation of the structure

detection of mantle plumes in the lower mantle by diffraction tomography hawaii abstract a new method is proposed to detect mantle plumes in the lower mantle the method is akin to diffraction tomography and relies on the scattering of long period seismic body waves by nearly vertical heterogeneities the theory is described in a companion paper ying ji h c nataf earth planet sci lett this issue here we apply this method to an actual set of long period digital seismograms recorded on the world wide network of seismic stations between 1980 and 1994 we select seismograms that illuminate the lower mantle in a 20 20 region around hawaii we construct an image of plume like heterogeneities using a cell size of 1 1 by a lsqr inversion of the scattered waves this image shows a strong slow anomaly about 200 km northwest of hawaii although the image is somewhat noisy resolution tests indicate that this feature is fairly robust the amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model built with a 600 k maximum temperature excess and a gaussian horizontal cross section with an 1 e diameter of 250 km it is the first time that such a method is applied to the mantle while we think that the existence of this anomaly is real we do not know how to explain such a large amplitude if real this observation has a number of important geodynamical consequences it indicates that a mantle plume is indeed responsible for the hawaii hotspot as speculated by morgan w j morgan nature 230 1971 42 43 the plume is nearly vertical it originates from the d region at the base of the lower mantle the amplitude of the anomaly suggests that partial melt or and a chemical anomaly must be present the plume source is to the northwest of its surface expression as predicted by some models of plume advection in the mantle wind

diachronous tectonic collapse of the ordovician continental margin eastern canada comparison between the quebec reentrant and st lawrence promontory the upper ordovician trenton group of southern quebec represents the last taconian foreland basin carbonate unit in the quebec reentrant prior to final collapse of laurentia s continental margin and its burial under synorogenic flysch the trenton group either conformably or unconformably overlies the black river group and is in turn conformably overlain by the utica shales the tripartite trenton carbonate unit records progressive deepening 1 very shallow to shallow subtidal 2 shallow to deep carbonate ramp and 3 shallow to deep outer shelf regional facies distribution lithotectonic elements and thickness variations indicate that the trenton shelf was dissected by extensional faults delineating blocks subsiding at various rates this scenario compares favourably with taconian foreland basin development in the middle ordovician table head group at the st lawrence promontory newfoundland a similar stratigraphic succession and tectono sedimentary history occurring 10 15 ma earlier at the s

subducted slabs and the geoid 1 numerical experiments with temperature dependent viscosity one of the most powerful constraints on mantle viscosity comes from the correlation of the long wavelength degree 4 9 geoid with that predicted by a density model for subducted slabs the effect of lateral variations of viscosity should be most pronounced at subduction zones due to the strong effect of temperature on viscosity an idealized slab model with temperature dependent viscosity is considered with various lateral and vertical viscosity structures using a two dimensional finite element formulation the viscosity parameterization affects the amplitude of the long wavelength geoid anomaly but not the sign of the correlation between the geoid and density anomalies depth dependent viscosity models with a high viscosity lithospheric layer do not completely match the temperature dependent laterally varying results suggesting that the rheology of the slab does have an effect on the long wavelength surface topography and geoid however this affect is minor suggesting that the radial models of mantle viscosity inferred from surface observables are correct to first order in contrast the short wavelength features are dramatically affected by the rheology of the region surrounding the downwelling further study of the shorter wavelength geoid over subduction zones may provide better insight into subduction zone processes

the subsolidus segregation of layer parallel quartz feldspar veins in greenschist to upper amphibolite facies metasediments layer parallel i e parallel to foliation or bedding vein formation in the graywackes and pelites of the quetico metasedi mentary belt occurred during synchronous prograde metamorphism and regional dj com pression in a traverse across metasediments which change in metamorphic grade from green schist to upper amphibolite migmatite facies layer parallel veins show the following trends 1 an increase in thickness and internal corn plexity the latter due to successive boudinage 2 low grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks but at higher grades other sites are also used and 3 a systematic increase in plagioclase quartz ratio in the veins towards higher grade adjacent mafic selvedges first exhibit quartz depletion then in the amphibolite facies plagioclase depletion mineralogical zoning is often preserved in a single vein older parts are more quartz rich than younger mass balance calculations and whole rock geochemistry based on veins mafic selvedges and country rock are consistent with a closed system subsolidus segregation origin the layer parallel veins are syntectonic and migration of the mobile components required to form their mineralogy is a stress induced mass transfer the source of these components appears to be dominantly pressure solution of the same minerals in the host rocks although metamor phic reactions may also have contributed veins nucleated first at those sites where initial sedimentary heterogeneites such as fine scale graded bedding provided gradients of normal stress across grain boundaries and hence of chemical potential necessary to drive the sub solidus segregation process the earliest veins are thus parallel to bedding later nucleation of the veins could also occur along more randomly distributed sites within the metasediments and these veins grew parallel to the schistosity rather than bedding if the two were distinct once formed the veins themselves which are more competent than the surrounding rock provide the stress heterogeneity required for their further growth the increasing plagioclase quartz ratio in the veins may be due to a tem perature dependent increase in plagioclase com ponent mobility relative to quartz alterna tively the increasing transfer distances for silica resulting from prior quartz depletion in the inner parts of the mafic selvedge may increase the relative mobility of plagioclase component

fluids and melting under upper amphibolite facies conditions internal buffering paths on temperature water activity diagrams for the pelitic model system naaio 2 kaio 2 caai 2 o 4 al 2 sio 5 sio 2 h 2 o give rise to sequences of assemblages which correspond to those observed in upper amphibolite facies terranes in particular the persistence of the assemblage quartz alkali feldspar plagioclase muscovite sillimanite across a zone and the rapidly changing composition of plagioclase across this zone can be accounted for in a natural way

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 1 presented here are self consistent three dimensional simulations of mantle convection some of which display an approximation of plate tectonic behavior that is continuous in space and time plate behavior arises through a reasonable material description of silicate deformation with a simple yield stress being sufficient to give first order plate like behavior however the required yield strength or fault frictional coefficient is much less than experimentally determined values toroidal poloidal ratios are within geologically observed limits the sensitivity of the system to yield strength and the form of strength envelope is systematically investigated optimum plate character is obtained in a narrow range of yield strength below which diffuse boundaries and above which episodic behavior and eventually a rigid lid are observed models with mobile lids develop very long wavelength horizontal structure the longest wavelength possible in the domain two dimensional models display much greater time dependence than three dimensional models

stratigraphy and structure of the laurentian rifted margin in the northern appalachians a low angle detachment rift system the neoproterozoic early cambrian rifted margin of eastern laurentia is framed by promontories and embayments defined by northeast trending rifts offset by northwest trending transforms a regional first order synthesis of the available stratigraphic data in the northern appalachians reveals significant along strike variations in the thickness composition age and facies of synrift and early postrift stratigraphy these variations are consistent with models for low angle detachment rift systems and allow for resolution of regional structures specific to low angle detachments including upper plate margins lower plate margins and transform faults that bound zones of oppositely dipping low angle detachments

a model for the layered upper mantle abstract numerical modeling of mantle convection by liu 1994 science 264 1904 1907 favors a two layer convection if the results are reinterpreted for the correct phase relations in mg fe 2sio4 the resulting chemical isolation of the upper and lower mantle suggests a highly differentiated and layered upper mantle to account for the discrepancy between the observed compositions of mantle xenoliths and the cosmic abundances of elements it is shown that a layered upper mantle with a hidden reservoir can have a structure consistent with the observed seismic velocity profiles and an average bulk composition corresponding to the cosmic abundances the evolution of the upper mantle and the origin of komatiites are discussed in the context of the proposed model

gaspe belt subsurface geometry in the northern quebec appalachians as revealed by an integrated geophysical and geological study 1 potential field mapping abstract together recent gravity and high resolution aeromagnetic datasets are used to qualitatively investigate the upper and middle crustal geometry of the middle paleozoic gaspe belt in the northern appalachians long wavelength potential field anomalies define two sub basins that are divided by northeast trending gravity highs for each sub basins gravity lows correlate with the youngest rock units maps that highlight anomalies associated with near surface features at the expense of those related to deeper sources provide an important supplement to the spatially discrete observations derived from bedrock mapping analysis of such maps indicates that the sub basins are characterized by different structural patterns and that faults trending obliquely compared to the main structural grain have been previously underestimated the geometry of the gaspe belt as revealed by this integrated geophysical and geological study bears similarities with orogens exhibiting lateral extrusion this geometry is interpreted as the result of a heterogeneous strain regime in front of an indenter corresponding to the early paleozoic gander dunnage crustal block the indentation tectonic model is supported by 1 the various strike and kinematic of faults that suggest a strongly heterogeneous strain regime 2 the greater geological complexity and the occurrence of faults with a significant thrust component in front of the indenter 3 the predominance of dextral strike slip faults in the eastern gaspe peninsula that result in lateral material transport away from the indenter 4 the location of abundant devonian magmatic dykes sills and stocks in a fault bounded zone that experienced local extension 5 the occurrence of block rotation

sedimentation bioturbation and hg uptake in the sediments of the estuary and gulf of st lawrence sedimentation rates and mixing depths have been estimated from the application of a two layer biodiffusion model to 210 pb profiles in sediment cores collected in the laurentian trough in the estuary and gulf of st lawrence sedimentation rates decrease exponentially with distance seaward from 0 70 cm yr 21 0 47 g cm 22 yr 21 near the head of the trough to 0 04 cm yr 21 0 03 g cm 22 yr 21 in the gulf of st lawrence mercury hg sediment profiles exhibit subsurface maxima that decrease from concentrations of 0 6 m gg 21 near the entrance to the saguenay fjord to 0 04 m gg 21 in the central part of the gulf hg release data for a chloralkali plant on the saguenay river were used as the input function for the biodiffusion model and provided accurate simulation of measured profiles when mixing was permitted to extend into the deep 10 cm sediment layer estimates of hg sediment inventories indicate that 100 130 tonnes t of hg have been retained within the sediments of the st lawrence estuary which combined with an hg inventory of 30 t estimated for the sediments of the saguenay fjord approximately balance the 136 t of hg released from the chloralkali plant since 1947 137 cs and

hinterland directed transtensional faulting at an orogen structural front the example of the cap chat melange quebec appalachians the cap chat melange is a regional scale dismembered unit that occurs close to the quebec appalachian front the melange records early d 1 taconian and late d 3 acadian regional deformations the greater degree of disruption of the melange compared with surrounding units is due to the superimposition of d 2 structures that are rare outside the melange d 2 structures include both layer extensional features such as block disruption along shear zones and faults and layer contractional features such as folds that record the transtensional collapse of the taconian orogenic wedge along a rheologically weak deformation zone the cap chat melange is interpreted as the westernmost remnant of a zone of mechanical decoupling in the upper crust which allows normal or transtensional faulting in the inner parts of the orogen this paper presents evidence that beyond the general complexity of melange units geological observations may still contribute to unraveling their tectonic history and that transtensional reactivation of original thrust faults is a viable mechanism in the frontal parts of orogens

carbonate platform to foreland basin revised stratigraphy of the table head group middle ordovician western newfoundland carbonates of the table head group and associated strata were deposited along the western margin of a foreland basin during initial stages of the taconian orogeny and record collapse and cannibalization of a long lived carbonate platform the stratigraphy of klappa opalinski and james is here revised to reflect better understanding of lithologic units within this complex tectono stratigraphic assemblage the table head group now contains only three formations the table point and table cove formations as originally defined and the cape cormorant formation redefined and restricted to western port au port peninsula black cove formation shales are removed from the table head distinctive conglomerates once placed in the cape cormorant are now recognized as separate units within the overlying flysch and called the daniel s harbour member the foreland basin developed in three stages 1 fragmentation uplift and erosion of the platform and subsequent deposition of shallow water limestones table point

melt segregation in migmatites anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity features that have not been explained satisfactorily but which are related to physical processes of melt segregation we evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith compaction by gravity driven two phase flow results in asymmetric separation yields calculated segregation times that are slow or geologically unreasonable and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites although calculated segregation times for very wet granite might allow segregation by compaction some driving force in addition to gravity is needed to cause widespread melt segregation in the crust for segregation at low volume fraction of melt we develop a two dimensional two phase matrix and fluid viscous flow cell model length much greater than thickness with phase changes which predicts either expansion or contraction convection depending on whether volume change vr is positive or negative convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale at moderate volume fraction of melt segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress melt migration is by porous medium flow driven by differences in mean normal stress between the layers as a consequence of differences in rheology and shear enhanced matrix collapse calculated segregation rates are fast and the model yields adequate volume of leucosome the positive vr for volatile phase absent melting reactions under crustal pressures promotes melt escape unless extensional deformation facilitates melt accumulation if the rate of melt production exceeds the rate of melt escape then the increase in melt pressure may lead to melt enhanced embrittlement and fracture and melt may migrate out of the system in contrast water rich volatile phase present melt producing reactions have a negative vr which promotes melt retention after segregation unless deformation generates melt escape pathways evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy and magma may become mobile and entrain residual material restite

mantle heterogeneities geoid and plate motion a monte carlo inversion seismic tomography in both the upper and the lower mantle as well as subducting oceanic slabs defined by seismicity has been translated into density heterogeneities to generate models of mantle circulation these models can predict both the surface velocities and the geoid which can be compared with plate tectonics and gravity data a given model is specified by 6 parameters related to the viscosities of 3 mantle layers and the absolute amplitudes of density variations in the upper and lower mantle as well as in the slabs the values of these parameters are chosen at random within an acceptable range each model is submitted to an appropriate test comparing observations with predictions the results of the most successful models selected by this monte carlo inversion are displayed they yield preferred mantle viscosity structures exhibiting large variations at depth with a physical interface between upper and lower mantle i e with the possibility for the circulation to penetrate the 650 km discontinuity two classes of viscosity profiles stand out the first one implies a regular increase of the viscosity in the sublithospheric mantle with reasonable values for the density parameters the second one is unexpected in the sense that it predicts a very stiff bottom for the upper mantle it also requires vanishingly small amplitudes for the upper mantle density heterogeneities defined by tomography which would thus have to be of lithological rather than thermal origin with a chemical interface at 650 km the outcome is very similar the same two classes of viscosity structures do yield a satisfactory geoid prediction however only the class of models with a stiff layer at midmantle depths predicts acceptable surface velocities altogether the best models out of some 60 000 which have been tested only explain one third of the geoid and two thirds of the surface divergence for spherical harmonic degrees 1 to 6 nevertheless the main features of these two observed patterns are present in the computed maps and 4 out of 6 correlation coefficients lie close to the 90 confidence level this is true for the geoid as well as for the surface divergence of the displacement velocity however as the internal viscosity structure has been assumed to have spherical symmetry the rotational component of the surface velocities cannot be predicted

lower mantle thermal structure deduced from seismic tomography mineral physics and numerical modelling abstract the long wavelength thermal anomalies in the lower mantle have been mapped out using several seismic tomographic models in conjunction with thermodynamic parameters derived from high pressure mineral physics experiments these parameters are the depth variations of thermal expansivity and of the proportionality factor between changes in density and seismic velocity the giant plume like structures in the lower mantle under the pacific ocean and africa have outer fringes with thermal anomalies around 300 400 k but very high temperatures are found in the center of the plumes near the base of the core mantle boundary these extreme values can exceed 1500 k and may reflect large hot thermal anomalies in the lower mantle which are supported by recent measurements of high melting temperatures of perovskite and iron extremely cold anomalies around 1500 k are found for anomalies in the deep mantle around the pacific rim and under south america numerical simulations show that large negative thermal anomalies of this magnitude can be produced in the lower mantle following a catastrophic flushing event cold anomalies in the mid lower mantle have modest magnitudes of around 500 k a correlation pattern exists between the present day locations of cold masses in the lower mantle and the sites of past subduction since the cretaceous results from correlation analysis show that the slab mass flux in the lower mantle did not conform to a steady state nature but exhibited time dependent behavior

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

geoid and topography over subduction zones the effect of phase transformations 1 the association between local maxima in the geoid and subduction zones is examined while it is well known that subduction zones are associated with broad local maxima in the geoid at spherical harmonic degrees 4 9 there is an impressive correlation between back arc geoid maxima at the 5000 km length scale i e spherical harmonic degrees 9 and subduction zones with the exception of the scotia and caribbean arcs geoid maxima are also observed in the forearc regions of the aleutian and central american subduction zones numerical modeling of compressible convection with multiple phase transformations indicates that an increase in viscosity at or near the 660 km phase transformation is necessary to explain the pattern of geoid maxima associated with subduction zones the addition of an endothermic phase transformation at 660 km depth does not provide sufficient resistance to the slab to eliminate the need for a viscosity increase at or near 660 km short wavelength geoid and topography profiles might provide an additional constraint on the deformation at subduction zones however it may be necessary to incorporate geologic history into subduction calculations to further improve the results presented here

a 40ar 39ar study of oceanic and continental deformation processes during an oblique collision taconian orogeny in the quebec reentrant of the canadian appalachians received 7 december 2006 revised 9 january 2008 accepted 10 march 2008 published 1 july 2008 1 two phases of penetrative deformation are documented in the taconian hinterland of the appalachian orogen in the gaspepeninsula d1 is associated with the obduction of the mont albert ophiolite onto the paleozoic laurentian margin whereas d2 corresponds to later transport of allochthons across the margin in the metamorphic sole s1 is a se dipping mylonitic fabric with a downdip lineation in underlying metabasalts d1 is characterized by nw overturned and recumbent folds and a subhorizontal s1 schistosity with an ene trending orogen parallel lineation d2 is characterized by a s2 steeply dipping penetrative axial planar crenulation cleavage and ne trending f2 folds the intraoceanic thrusting of ophiolite is dated at 465 ma early d1 whereas emplacement of ophiolite andsubsequentdeformationofthemarginwasrecorded by isotopic signatures between 459 and 456 ma late d1 d2 is dated at 448 ma throughout the hinterland taconian transpressive deformation is related to an oblique collision within the quebec reentrant of the canadian appalachians during the ordovician citation malo m g ruffet a pincivy and a tremblay 2008 a 40 ar 39 ar study of oceanic and continental deformation processes during an oblique collision taconian orogeny in the quebec reentrant of the canadian appalachians tectonics 27 tc4001 doi 10 1029 2006tc002094

global plate motion and the geoid a physical model summary the effect of lateral variations in the sub lithospheric viscosity i e in the lithosphere asthenosphere coupling is incorporated in dynamic earth models driven by plate velocities if the coupling is stronger under fast moving continents it is shown that a geoid low will develop in their wake and a high in front of them thus the well known low in the indian ocean could be explained at least in part in terms of induced upper mantle dynamics similarly if the thickness of oceanic plates increases with age the models show that trenches should be associated with marked geoid highs whereas ridges could correspond to much weaker geoid lows this also seems to agree with some features of the observed geoid at very long wavelengths the mathematical framework of such dynamic earth models is developed extensively here these models are characterized by lateral viscosity variations inside an outer shell and a purely radial viscosity structure at greater depth their internal flow patterns are only driven by imposed surface velocities not by internal loads

inferences of mantle viscosity from tectonic plate velocities the present day velocities of the earth s tectonic plates provide direct constraints on the absolute value of the mantle viscosity we present the results of a formal inversion of plate motion data and demonstrate its utility in constraining the depth variation of absolute viscosity

viscosity of the lower mantle as inferred from rotational data until recently most estimates of the deep mantle viscosity were based on analyses of postglacial uplifts and there were no serious estimates of the sensitivity of the derived viscosity solutions to variations of the input parameters such as those associated with the surface loading in this paper the viscosity of the lower mantle is arrived at by making comparisons of the observed secular motions of the earth s rotation axis with theoretical results from a layered viscoelastic rotating earth which has been subjected to glacial forcing our model consisting of an elastic lithosphere a two layer adiabatically stratified viscoelastic mantle and an inviscid core is essentially analytical and this makes it economically feasible to use as an aid in conducting an extensive sensitivity analysis of the lower mantle viscosity from changes in the parameters connected with the deglaciation phenomenon both sets of rotational data polar wander and nontidal deceleration of the length of the day have been employed to constrain the viscosity structure on the basis of this type of investigation the viscosity of the lower mantle is found invariably to be larger than that of the upper mantle and lies between 1 and 4 1022 p our results which just rely on the second degree harmonic of the strain field also lead to a determination of an upper bound to the thickness of the globally averaged lithosphere this value ranging between 130 and 190 km suggests that strong lateral heterogeneities between oceanic and continental plates may extend a few hundred kilometers into the upper mantle

what controls partial melting in migmatites the layers of six stromatic migmatites from northern western and central europe display small but systematic chemical and mineralogical differences at least five of these migmatites do not show any signs of largescale metamorphic differentiation metasomatism or segregation of melts it is concluded therefore that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks almost isochemical partial melting seems to be the most probable process transforming layered paragneisses metavolcanics or schists into migmatites the formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers the neosomes have less biotite and more k feldspar if k feldspar is present at all than the adjacent mesosomes these differences are small but systematic and seem to control the access of different amounts of water to the various rock portions petrographical observations chemical data and theoretical considerations indicate a close relationship between rock composition rock deformation transport of water partial melting and formation of layered migmatites

textural relations in melanosomes of selected specimens of migmatitic pelitic schists implications for leucosome generating processes melanosome textures in several specimens of migmatitic pelitic schists are evaluated to test leucosome generating reactions inferred independently on the basis of mineralogic and chemical relationships in the melanosomes systematic variations in grain numbers per unit volume grain sizes size distributions and shapes and grain contact frequencies within melanosomes can be attributed to leucosome generation these variations support independent inferences of reactions based on modes and chemistry and in one case suggest a refinement of reactions inferred on these bases

eustatic sea level fluctuations induced by polar wander the observation of apparently simultaneous episodes of deposition or non deposition of marine sediments in different parts of the world has led to the proposal that at least some sea level rises and falls must be global or eustatic in character1 3 here we show that polar wander of a viscoelastic stratified earth can induce global sea level fluctuations comparable to the short term component in the eustatic sea level curves the sign of these fluctuations which are very sensitive to the rheological stratification depends on the geographical location of the observation point rises and falls in sea level can thus be coeval in different parts of the world this finding is in distinct contrast to the main assumption underlying the reconstruction of eustatic curves namely that global sea level events produce the same depositional sequence everywhere this apparent contradiction is due to the poor time resolution of the stratigraphie records in the distant past which is comparable to the timescale of polar motion4 6 and to non uniform data coverage we propose that polar wander should be added to the list of geophysical mechanisms the others are glacial instabilities plate tectonic mechanisms sea floor spreading and thermal and compaction induced subsidence that can control the third order cycles in sea level

lower mantle viscosity constrained by seismicity around deglaciated regions knowledge of the viscosity structure of the earth s mantle is important for constraining models of mantle convection and isostatic rebound here we show that seismicity around the margins of deglaciated areas provides a constraint on the viscosity of the lower mantle in addition to those previously proposed1 2 calculations using a spherical viscoelastic earth model show that the present day magnitude of the stress fields induced in the lithosphere beneath the now disappeared laurentide and fennoscandian ice sheets is very sensitive to the value of the lower mantle viscosity stresses of 100 bar sufficient to cause seismicity can still remain in the lithosphere for lower mantle viscosities greater than 1022 pa s for lower mantle viscosities of 1021 pa s only a few tens of bars of stress persist in the lithosphere today this influence of lower mantle viscosity on the state of stress in the lithosphere also has implications for the migration of stress from earthquakes and hence for earthquake recurrence times

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

influence of mantle compressibility and ocean warping on dynamical models of the geoid summary the influence of mantle compressibility and warping of the ocean surface on dynamical models of the geoid has been investigated topography and geoid kernels show that compressibility modifies the mantle flow pattern and hence the geoid whereas the deflection of the oceanic surface amplifies the geoid signal without affecting the global flow the combined influence of these two effects is significant and depends on the viscosity profile in the mantle comparisons with incompressible dynamical geoids show that including the effects of compressibility and ocean warping improves the fit to the observed geoid thus these effects must not be neglected in dynamical computations of the geoid

regional metamorphism of the appalachian humber zone of gaspe peninsula 40ar 39ar evidence for crustal thickening during the taconian orogeny geochronological studies on the timing of deformation and metamorphism along the laurentian margin have shown that the ages of metamorphic events change along strike within the newfoundland southern quebec segment of the canadian appalachians the gaspe peninsula is located at mid point of the two extremities of this segment new single grain laser 40ar 39ar plateau ages solely reflect latest middle late ordovician metamorphism samples taken within the internal humber zone in the shickshock group rocks yield 40ar 39ar muscovite and hornblende ages ranging from 457 to 454 ma samples from the amphibolite du diable the metamorphic sole of the mont albert ophiolite yield 40ar 39ar muscovite and hornblende ages ranging from 459 to 457 ma ordovician ages of the internal humber zone are consistent with 40ar 39ar ages from southern quebec and are interpreted as the result of the emplacement onto the margin of both the ophiolite and its metamorphic sole

ophiolitic source rocks for taconic age flysch trace element evidence flysch generated along the appalachian orogen during the ordovician taconic orogeny ranges in age from late arenig to early caradoc allochthonous flysch and from late llanvirn to ashgill autochthonous flysch sandstones in these units are dominantly lithic arenites with a heavy mineral suite containing common to abundant mg rich chromite north of the canada united states border the only reasonable sources for the chromite were the lower ultramafic units of obducted ophiolites trace element analyses cr ni v cu ti zr y of flysch sandstones indicate a striking increase in cr and ni concentrations from the united states appalachians to the canadian appalachians in the united states units cr is in the range of 10 to 50 ppm canadian flysch units average an order of magnitude more cr these results suggest that ophiolite obduction either did not occur or was of minor importance during the taconic orogeny in the united states appalachians the difference in ophiolite abundance between the canadian and united states appalachians suggests a fundamental difference in tectonic style along the orogen a feature that must be accommodated by plate tectonic models new models will probably need to consider both obliquity of subduction and geological complexity of the ordovician collision zone in order to explain the spatial distribution of obducted ophiolite sheets

apatite dissolution into peraluminous haplogranitic melts an experimental study of solubilities and mechanisms apatite ap dissolution and solubility in the peraluminous haplogranite system were investigated in three sets of experiments at 750 c and 200 mpa ph2o as functions of ap grain size abundance melt ap ratio and melt asi alumina saturation index mol al2o3 na2o k2o cao between an asi of 1 1 1 metaluminous to mildly peraluminous the solubility of ap as measured by p2o5 in the melt is low 0 1 wt p2o5 which agrees with previous work in equilibrium runs with more strongly peraluminous compositions however ap solubility increases linearly with asi to p2o5 0 63 wt in melt at asi 1 3 and can be denned by a simple equation p2o5 3 4 3 1 asi r 0 833 which agrees within error with previous calculations reported in the literature in experiments where ap grains are sparce and widely separated i e simulating ap distribution in an ap bearing peraluminous protolith undergoing anatexis the dissolution of ap involves transient disequilibrium that promotes higher solubility of p2o5 up to nearly six times the equilibrium values adjacent to ap at a given bulk asi of melt and this increases the al si of melt in the ap dissolution aureole the approach to equilibrium between ap and melt is hastened by increasing the ap melt ratio within the local melt pool e g the capsule or by decreasing ap grain size for a similar melt ap ratio diffusivity calculations using inverse error function methods for the normal diffusion of p and ca yield dp 10 11 cm2 s and dca 10 10 cm2 s confirming prior results that diffusion of p away from ap surfaces is the rate limiting factor in ap dissolution these results combined with previous work at higher temperatures but on similar hydrous melts yield an activation energy e for p diffusion of 16 5 kcal mol and a frequency factor d0 of 1 4 10 3 cm2 s phosphorus diffusion through the melt is accompanied by uphill diffusion of al resulting in an increase of up to 2 wt al2o3 in melt near the ap within the ap dissolution aureole melt contains one additional al cation for every p cation i e al p 1 1 which is suggestive of an associated alpo4 complex the exchange reaction in melt is represented by alpsi 1 in which 2 to4 clusters are created in the place of 1 this exchange should be accompanied by discernable changes in melt properties e g density molar volume viscosity the results have applications to equilibrium and local disequilibrium effects during anatexis of apbearing metapelites and to the saturation limits of ap during crystallization of peraluminous magmas the dissolution of flux bearing minerals such as ap e g those containing p b or f may influence the stability and solubility of other trace element bearing minerals during anatexis of metapelites texture e g grain size and distribution and spacial relationship to other minerals plays an important role in reaction kinetics and at least temporarily in mineral solubilities the disequilibrium behavior of accessory minerals has critical consequences for ree modelling of granites

basin development and inversion at the appalachian structural front port au port peninsula western newfoundland appalachians in the humber zone of the newfoundland appalachians cambro ordovician shelf and foreland basin successions are affected by middle ordovician taconian orogeny and devonian acadian orogeny deformation on port au port peninsula the presence of the late ordovician to late silurian long point clam bank succession allows these episodes to be separated the taconian foreland basin stratigraphy on port au port peninsula is highly variable on the west coast platform carbonates are overlain by megaconglomerates of the cape cormorant formation which record progressive exposure of 1 km of the platform succession the conglomerates are restricted to a narrow zone consistent with derivation from a fault scarp originally immediately west of the outcrops in palinspastic restoration farther east at victors brook the cape cormorant formation is absent but the overlying almost undeformed goose tickle group contains conglomerate derived both from the upper part of the platform succession and from the tacon

forward potential field modeling of the appalachian orogen in the gaspe peninsula quebec canada implications for the extent of rift magmatism and the geometry of the taconian orogenic wedge the nature and extent of potential source bodies associated with prominent regional scale gravity and magnetic anomalies in the northern quebec appalachians are investigated through forward modeling potential field anomalies are interpreted to be the geophysical expression of basalt flows extruded during the neoproterozoic to cambrian rifting of laurentia prior to opening of iapetus geophysi cal modeling provides substantial new constraints on seismic and structural cross sections and the map distribution of volcanism this study indicates that the volcanism was significantly more widespread than previously recognized basaltic rocks form a sheet up to 4 kilometers thick transported on the main taconic decollement most of this sheet is deeply buried and overlain by a thick pile of paleozoic sediments later tectonic modifications by backthrusting and hinterland directed normal faulting as well as by acadian strike slip faulting substantially changed the geometry of the transported volcanic rocks and the structural style both across and along the strike of the orogen the extension of the gravity anomaly under the st lawrence river also suggests that the ordovician deformation front is 20 to 30 kilometers further to the north than previously interpreted

the st lawrence platform and appalachian deformation front in the st lawrence estuary and adjacent areas quebec canada structural complexity revealed by magnetic and seismic imaging new magnetic and seismic datasets offer a unique opportunity to document the geometry of the appalachian front in a poorly studied segment of the orogen interpretation of high resolution magnetic data allows for the first time the documentation of the contact between the autochthonous st lawrence platform and the appalachians and highlights the regional significance of previously poorly documented ene faults that experienced post ordovician strike slip motion seismic data reveal tectonic slices in the foreland domain underlying the appalachians and show that the depth of the decollement at the base of the appalachian tectonic wedge varies significantly taken together geological magnetic and seismic data suggest that the geometry of the appalachian front exhibits significant variations in map and cross section views and recorded a polyphased structural history

geochemical modeling of low melt fraction anatexis in a peraluminous system the pena negra complex central spain abstract a study was made of the chemical fractionation associated with four cases of anatectic segregation of low melt fraction cordieritic granites from migmatized meta greywackes the aims of the study were to 1 reveal the fractionation patterns of major and trace elements 2 compare the major element chemistry of leucogranites and the quantitative behavior of source minerals during anatexis inferred by mass balance adjustment with available experimental data for peraluminous systems and 3 discuss the behavior of trace elements in crustal melting by comparing the chemically determined composition of leucogranites with the results of three fractionation models two of these assume a perfect diffusive behavior of trace elements within residual solids but they use a different set of distribution coefficients the third assumes a perfect nondiffusive behavior in relation to their source rocks the leucogranites are strongly depleted in li transition elements and light rare earth elements but enriched in k2o sio2 and ba the k rb ratio in leucogranites is higher than in sources and reaches values typical of high grade metamorphism mass balance analysis using the anatexis mixing model shows that the chemistry of cordierite leucogranites is compatible with its having originated by closed system water undersaturated anatexis on previously migmatized meta greywackes leaving a residue enriched in cordierite plus biotite and exhausted in k feldspar biotite melts congruently unless important amounts of sillimanite were also present in the source compared with experimental melts obtained from sources with the same chemical composition but with a different femic mineralogy biotite sillimanite instead of cordierite biotite the pena negra leucogranites are richer in k2o and mgo with a lower fe fe mg ratio the differences in magnesium are believed to result from the changes in the mineral assemblage of the source rocks however the differences in potassium contents are still not well understood the behavior of trace elements during anatexis cannot be modeled by trace element fractionation equations because of the sluggish diffusion kinetics of trace elements at the temperature of anatexis making the melting rate of minerals greater than the diffusion rate of trace elements within minerals the behavior of trace elements essentially consists in following that of the minerals containing them

basin evolution in western newfoundland new insights from hydrocarbon exploration the humber zone is the most external zone of the appalachian orogen in western newfoundland it records multiphase deformation of the cambrian ordovician passive margin and of the ordovician to devonian foreland basins by the taconian salinian and acadian orogenic events the recent phase of exploration drilling has provided new evidence for structural stratigraphic reservoir and source rock maturation models of western newfoundland the first well port au port 1 supported the hypothesis that the round head thrust had an earlier extensional history prior to the acadian compressional inversion that created the present day structural high of the port au port peninsula the well tested a small anticline formed in a footwall shortcut fault of the round head thrust the second well long point m 16 was drilled at the northern tip of long point to test a triangle zone identified by previous workers this well demonstrates that the frontal monocline at the western edge of the triangle zone is elevated by a stack of imbricate thrusts composed of rocks of the taconian allochthon and compressional basement involved faults that have uplifted the cambrian ordovician carbonate platform the structural model developed in the port au port area with the aid of these wells has been extended throughout the humber zone in western newfoundland changes in structural style illustrated by regional cross sections suggest that prospective trap geometries are only developed in the southern and central parts of the region the reservoir model proposed invokes exposure and karsting of the footwalls of extensional faults formed as the carbonate platform collapsed during a middle ordovician hiatus the st george unconformity structural relief became more pronounced as extensional collapse continued through the middle ordovician begin page 394 these structurally high fault footwalls became the foci for dolomitizing and mineralizing fluids that used major faults as fluid conduits during the devonian fluids deposited sulphide ores and created zebra and sparry dolomite and some sucrosic hydrothermal dolomites in the st george group and the table point formation the reservoir model maturity and source rock data and the structural models have been combined with seismic and onshore surface geology this enables the prospectivity of the western newfoundland cambrian ordovician play trend to be evaluated for further exploration

revisiting the appalachian structural front and offshore anticosti basin northern gulf of st lawrence canada by integrating old and new geophysical datasets abstract the anticosti basin is a large predominantly marine sedimentary basin centered within the northern gulf of st lawrence poor quality vintage seismic reflection data collected more than 25 years ago in the offshore part of the basin failed to depict its geometry however recently acquired multibeam bathymetry and magnetic data can be used in combination with high resolution seismic lines to map the paleozoic succession within the honguedo strait the upper part of the offshore sedimentary succession consists of approximately 1140 m of sediments younger than those exposed on anticosti island the offshore part of the basin includes a 40 km wide fold and fault belt sub parallel to the coastline of the gaspe peninsula the structural style is characterized by broad open synclines narrow anticlines and nw striking faults with dextral strike separations which bears similarities to that of the gaspe belt that formed mainly during the middle devonian acadian orogeny this study indicates that the appalachian deformation front is located to the north of its classically inferred location moreover it suggests that the curvature of the early paleozoic assemblages in the gulf of st lawrence is partly a late devonian feature an improved understanding of the offshore architecture of the basin may help to develop new paleogeographic reconstructions and hydrocarbon exploration models in this under explored frontier basin

bayesian inference of mantle viscosity from whole mantle density models the long wavelength geoid is sensitive to earth s mantle density structure as well as radial variations in mantle viscosity we present a suite of inversions for the radial viscosity profile using

mantle convection plates and hotspots abstract more than 20 years after their discovery plates and hotspots are still awaiting a consistent dynamic model both imply large viscosity variations in the mantle sustained efforts have been targeted on modeling on computer as well as in the laboratory convection in fluids with temperature and pressure dependent viscosity the main results of these studies are reviewed and summarized in a few simple rules several properties of plates such as ridge push or heat flow distribution are well accounted for by convection models with temperature dependent viscosity however the process of subduction is not properly explained by these models one of the simple rules deduced from the study of convection and temperature dependent viscosity states that the viscosity drop across the lower boundary layer adjusts to a low value less than 10 this constrasts strongly with the viscosity variations required by current hotspot formation models which are typically 100 times larger it is suggested that this paradox can be solved if subduction is properly included in the convection models

topography and geoid due to lithospheric mass anomalies summary a model of lithospheric thickness and a recent compilation of moho depths are used to compute the earth s isostatic surface topography and associated gravity anomalies the results are strongly influenced by the uncertainties in lithospheric depth and crustal density profiles the preferred models explain most of the observed topography and are highly correlated with observed gravity anomalies for harmonic degrees larger than 10 comparisons of our residual topography with geodynamical calculations of dynamic topography based on mantle circulation are rather poor

low temperature thermochronology of anticosti island a case study on the application of conodont u th he thermochronology to carbonate basin analysis abstract the ordovician to silurian carbonate succession preserved on anticosti island is a near continuous record of the changing depositional and tectonic history of the early paleozoic laurentian margin these strata are a prime natural laboratory to test the applicability of conodont apatite u th he thermochronology cahe for carbonate basin analysis in a setting with a well understood burial history and abundant paleotemperature constraints we report 63 cahe dates from seven samples collected from two boreholes x ray micro computed tomography xr ct was used to assess individual conodont element size volume and alpha ejection correction cahe dates are negatively correlated with effective uranium concentration eu with single conodont dates enigmatically older than the age of deposition to conodont dates as young as 2 7 0 2 ma we rationalize this dispersion as a product of several factors including diagenetic processes and pervasive conodont crystalline porosity and permeability additionally one basement and two sandstone samples were collected for apatite and zircon u th he ahe zhe and apatite fission track aft thermochronology in the precambrian basement zhe dates range from 581 7 46 5 ma to 761 2 60 9 ma n 4 and ahe dates range from 26 7 1 6 to 48 4 2 9 ma n 4 ahe dates from two lower ordovician sandstones n 8 are positively correlated with eu increasing from 13 7 0 8 ma to 105 2 6 3 ma over a range of 186 ppm eu thermal history modeling of these data suggests two plausible thermal histories both of which are novel as they imply that far field effects related to the opening of the atlantic ocean have influenced the thermal history of anticosti island ultimately these data highlight the complexities associated with and necessity of low temperature thermochronology in carbonate basins

deformation styles at the appalachian structural front western newfoundland implications of new industry seismic reflection data new seismic reflection data gathered during hydrocarbon exploration in and adjacent to the external humber zone western newfoundland have important implications for the interpretation of structural style at the appalachian front these new data indicate that the structural front is influenced by both thin skinned and thick skinned structures where the structural front is thin skinned it is characterized by a triangle zone or tectonic wedge similar to structures observed at the southeastern margin of the canadian cordillera and at other orogenic fronts the thin skinned tectonic wedge is overridden locally by thick skinned thrusts which are generally emergent but are locally blind forming a thick skinned tectonic wedge timing relationships indicate that although initial motion occurred during the early to middle ordovician taconian orogeny the thin skinned allochthonous slices in western newfoundland were not emplaced until devonian time the acadian orogeny thick skinned deformation which p

excitation of true polar wander by subduction true polar wander the global motion of the mantle relative to the earth s rotation axis is known from the analysis of palaeomagnetic data hotspot tracks and plate motions to have occurred at velocities of up to 0 5 myr 1 since the late cretaceous period1 4 we address here the longstanding question of how fast episodes of true polar wander tpw can be excited5 by analysing the impact of the distribution and activity of subduction zones on polar motion using nonlinear liouville equations which allow us to treat large excursions of the polar axis we show that unrealistically fast tpw is excited by subduction episodes unless the lower mantle has a viscosity at least 10 times that of the upper mantle this need for a viscosity increase with depth in the mantle reinforces the conclusions of previous studies on post glacial rebound and geoid anomalies theoretical creep laws and some preliminary results on tpw induced by density anomalies embedded in the mantle6 10 the lower viscosity in the upper mantle means that upper mantle density anomalies are most effective in exciting tpw changes in the pattern of subduction through time may be responsible for both episodes of fast tpw and times of quiescence in polar motion

stratigraphy of the upper ordovician upper vaureal and ellis bay formations eastern anticosti island quebec marked facies changes occur in late ordovician strata assigned to the uppermost vaureal and ellis bay formations ashgill rawtheyan hirnantian on anticosti island quebec western anticosti features shales and carbonates whereas outcrops along the eastern coast contain prominent discontinuous mixed siliciclastic carbonate units detailed section measurement along the northeast coast allows for the first time accurate definition of seven new members within this uninterrupted sequence sands present in the upper vaureal and lower ellis bay formations in the east appear to have deterred the growth of muddy bottom brachiopod communities comparable to those in the western and central regions of anticosti sand units within the upper vaureal formation contain 1 m diameter colonies of paleofavosites coeval small coral patch reefs are found in the central part of the island where sands are absent the uppermost ellis bay formation of northeast anticosti is marked by a shallow subtidal coral algal onco

mantle viscosity derived by genetic algorithm using oceanic geoid and seismic tomography for whole mantle versus blocked flow situations abstract we have applied the genetic algorithm ga technique a nonlinear global optimization method to determine the radial viscosity structure of the mantle from regional geoidal patterns from numerical simulations of 2 d mantle convection we examine the horizontal spectra of the vertical mass flux at 660 km depth and find that for long wavelengths there are minor differences between partially layered convection induced by the phase transitions and mantle convection without any phase transition the differences in the spectra of the vertical mass flux become more prominent at shorter wavelengths this result has led us to study mantle viscosity for the intermediate wavelength geoid from the whole mantle and blocked flow situations in which the appropriate boundary condition is imposed on the radial velocity at 660 km depth in order to confirm the robustness of this study two different density models have been used which were constructed from three tomographic models and appropriate velocity to density scaling relations based on recent results from mineral physics we have analyzed only oceanic geoid spanning between spherical harmonic degree l 12 25 the correlation of the predicted geoid with the observations over the atlantic indian and pacific oceans have been employed as the fitting function in our ga approach which has been modified from the common algorithm in constructing the families of suitable viscosity profiles we have used 100 parents which have been iterated for 100 generations and have been started with 10 different sets of initial parents convergence to acceptable viscosity solutions is obtained for all the three oceans and for both the whole mantle and layered models in some cases multiple viscosity solutions are found acceptable by using the correlation criteria the outstanding feature of these models is the nearly ubiquitous presence of two low viscosity zones one lying under the lithosphere the other right under the bottom of the spinel to perovskite phase change the solutions for the whole mantle model can fit better and are preferred over the solutions with the layered boundary condition which generally result in unrealistic viscosity profiles our results would suggest a more complex mantle viscosity structure which has not been detected previously from geoid signals with longer wavelengths and also reveal the potential difficulties in treating the dynamical boundary condition at the 660 km discontinuity

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

maturation thermique et histoire de l enfouissement et de la generation des hydrocarbures du bassin de l archipel de mingan et de l ile d anticosti canada carbonate platform sequences of anticosti island and the mingan archipelago are early ordovician to early silurian in age with the exception of the macasty formation the sequences are impoverished in dispersed organic matter which is chiefly composed of zooclasts zooclast reflectances suggest that the upper ordovician and silurian sequences outcropping on anticosti island are entirely in the oil window but that the lower to middle ordovician beds of the mingan archipelago and their stratigraphic equivalents in the subsurface of most of anticosti island belong to the condensate zone only the deeper sequences of the southwestern sector of anticosti island are in the diagenetic dry gas zone the maximum depth of burial of sequences below now eroded silurian to devonian strata increases from 2 3 km on southwestern anticosti island to 4 5 km in the mingan archipelago a late upwarp of the precambrian basement likely allowed deeper erosion of the paleozoic strata in the vicinity of the mingan archipelago t

the age of the sept iles layered mafic intrusion canada implications for the late neoproterozoic cambrian history of southeastern canada the large sept iles layered mafic intrusion of eastern canada has been precisely dated by u pb zircon methods at 565 4 ma consideration of other magmatic events at that time in southeastern canada reveals three different phases of magmatism the first phase was composed of two tholeiitic dike swarms at 615 ma and 590 ma the second including the sept iles intrusion comprised a widespread series of mostly alkaline intrusions emplaced around 575 565 ma along the rift faults of the st lawrence graben the last phase was composed predominantly of alkali and transitional basalts now exposed in the appalachians erupted during the latest neoproterozoic and early cambrian the second and third phases are best accounted for by the arrival of a major mantle plume near the town of sept iles 565 million years ago the widespread but volumetrically minor alkali magmatism that peaked 10 m y earlier is typical of other plumes elsewhere the plume model is reinforced by sedimentological evidence for a crusta

viscous gravitational relaxation summary this paper is concerned with a detailed examination of the response of maxwell models of the planet to surface mass loads particular attention is devoted to an examination of the factors which determine the isostatic response since the understanding of this response is crucial in a number of different geodynamic problems one particular example which we discuss in detail is concerned with the prediction of free air gravity anomalies produced by large scale deglaciation events using the methods developed here we are able to provide the first direct assessment of the importance of initial isostatic disequilibrium on the observed relative sea level variations and free air gravity anomalies forced by the melting of the laurentide ice sheet we are therefore able to estimate the extent to which such initial disequilibrium might influence the inference of mantle viscosity from isostatic adjustment data our calculations establish that free air gravity data although they are sensitive to the degree of initial disequilibrium provide an extremely high quality constraint upon the viscosity of the lower mantle

rocky shoreline deposits in the lower silurian upper llandovery telychian chicotte formation anticosti island quebec the upper llandovery telychian chicotte formation is a regionally extensive crinoid rich unit exposed in the south central part of anticosti island in the gulf of st lawrence the chicotte encrinites represent an inner ramp crinoidal sand shoal complex about 80 m thick that prograded over deeper middle to outer ramp facies of the underlying jupiter formation in response to a long term sea level fall the typical depositional unit in the chicotte formation is a metre scale subtidal cycle indicating that higher frequency sea level changes were also present metre scale cycles are typically characterized by coarsening upward locally cross bedded encrinitic material capped by a sharp erosional surface these bounding surfaces change progressively upsection from simple planar to low relief scalloped erosional surfaces to complex polyphase surfaces formed by distinct but superimposed erosional events three dimensional paleomorphological features with local relief up to 50 m associated with erosional sur

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

dynamical effects of a hard garnet layer in the transition zone on geophysical signatures geoid anomalies and postglacial rebound abstract recent studies on the structure systematics of dense oxides have shown that the garnet structure has extremely high creep strength these results suggest that the transition zone between 400 and 670 km depth may be stronger than the lower mantle perovskite which in turn is sturdier than the upper mantle above 400 km we have compared two models 1 one with two layers separated at 670 km 2 the other with three layers having thicknesses of 400 km olivine layer 270 km hard layer and 2270 km perovskite layer the viscosity contrast between the olivine and garnet layers varies between o 10 and o 10 2 the viscosity contrast between the olivine and perovskite layer varies similarly we have employed two modelling approaches 1 cartesian model for calculating geoid anomalies and surface topographies due to internal loading 2 a spherical shell model based on analytical functions for analysing postglacial rebound the existence of the garnet layer induces distinct effects in both the geoid and postglacial responses geoid responses as a consequence of the hard layer become much stronger due to excitation of long wavelength density anomalies in the interior of the three layer model to obtain similar magnitudes of geoid signatures a larger viscosity contrast between the lower and upper mantle is required for postglacial rebound and shallow seated density heterogeneities a smaller viscosity variation between the upper and lower mantle is needed in the presence of the garnet layer these findings may reconcile some of the discrepancies in lower mantle viscosity inferred from the two data sets

plate tectonics and aspherical earth structure the importance of poloidal toroidal coupling we derive poloidal flow internal loading green functions for incompressible fluid models of the mantle consisting either of a single constant viscosity spherical shell or of two adjacent spherical shells having different viscosities from the green function we obtain kernels connecting the surface divergence the geoid and the surface topography fields to the lateral density heterogeneity inferred on the basis of the application of seismic tomographic imaging techniques and with these kernels we argue that both the surface divergence and geoid fields consisting of harmonic degrees 2 5 may be reasonably fit with only a factor of 8 viscosity increase at a depth of 1200 km we point out however that the coupling from poloidal to toroidal flow which is required to understand surface velocity spectra may allow these observations to be understood in terms of a viscosity increase at depth which is smaller than required by nonhydrostatic geoid data in the context of pure poloidal models using the observed kinetic energy in the surface plate motions as a constraint allows us to infer a value for the steady state upper mantle viscosity of 2 0 0 5 1021 pa s we comment on the implication of the difference between this number and the somewhat lower value 1 1021 pa s which has been derived on the basis of analyses of signatures of the glacial isostatic adjustment process

an ornithurine bird from the late cretaceous of alberta canada the partial carpometacarpus of a basal ornithurine bird from the late campanian of dinosaur provincial park is described complete proximal fusion of the wrist and the presence of a pisiform process place this taxon in ornithothoraces ornithurine synapomorphies include the large ventral ridge of the carpus and the concave proximal margin of the alular metacarpal but the primitive structure of the pisiform process and extensor process preclude placement in neornithes the relatively thick walls of the bone and the distal placement of the extensor process are consistent with diving habits