the thermal structure of mantle plumes axisymmetric or triple junction summary a mantle plume is probably a complex 3 d thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below but followed down towards the base of the layer probably consists of a triple junction or quadruple junction of connected hot sheets a relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes these conclusions are suggested by numerical experiments on a 3 d constant viscosity plane layer with stress free boundaries which detail the gradational change in the planform of a convecting layer from the top of the layer to its base the planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer these anomalies are the main sources of positive for hot fluid or negative for cold fluid buoyancy and therefore they drive the convective flow they may appear in cross section as structures with either axial symmetry columns planar symmetry sheets or some complex asymmetric form when convection is driven at least partially by basal heating the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns the hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface near the upper surface the apparent planform of this experiment is analogous to that of mantle convection the cold sheets compared to subduction zones and the hot columns compared to mantle plumes the hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies surface uplift and extensional stress fields however the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume formed by the junction of three or four hot sheets on the base of the layer it seems likely that the commonly occurring triple junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath

u pb geochronological constraints for paleoproterozoic evolution of the core zone southeastern churchill province northeastern laurentia abstract the core zone of the southeastern churchill province northeastern laurentia is a composite of paleoproterozoic lithotectonic domains assembled in the vise between obliquely colliding archean superior and north atlantic cratons between ca 1860 and 1810 ma detailed geological and u pb geochronological studies of domains in the southern core zone highlight significant differences in archean and paleoproterozoic geology between domains and constrain timing and models for paleoproterozoic assembly in the south central part of the core zone crossroads and orma domains consist of late archean granite greenstone terrane crust crossroads domain also contains a significant number of granite charnockite intrusions belonging to the 1840 1810 ma de pas batholith magmatism pre dates and overlaps with ca 1820 1775 ma high grade metamorphism and attendant deformation in marked contrast orma domain which occurs to the east of crossroads domain contains only local evidence of paleoproterozoic de pas related intrusions and appears to have mainly escaped paleoproterozoic metamorphism and penetrative deformation a model compatible with available data suggests the de pas batholith is a continental magmatic arc formed above an east dipping subduction zone paleoproterozoic metamorphism and deformation is concentrated in regions contiguous with arc magmatism west of crossroads domain and separated from it by the lac tudor shear zone mckenzie river domain is dominated by archean orthogneiss 80 m y older than gneisses in crossroads and orma domains minor amounts of paleoproterozoic supracrustal rocks absent in domains to the east and 1815 ma tonalite it does not contain de pas intrusions u pb geochronology suggests juxtaposition of the mckenzie river domain with domains to the east may have occurred around 1810 ma one possible model envisages the mckenzie river domain as a piece of reassembled superior craton crust incompletely rifted from the superior margin during ca 2 17 ga rifting domains occurring east of the lac tudor shear zone are interpreted to be exotic with respect to the superior craton

a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

variabilities of low latitude migrating and nonmigrating tides in gps tec and timed saber temperature during the sudden stratospheric warming event of 2013 the global positioning system gps deduced total electron content tec data at 15 n geomagnetic which is the crest region of equatorial ionization anomaly are used to study tidal variabilities during the 2013 sudden stratospheric warming ssw event the results from space time spectral analysis reveal that the amplitudes of migrating diurnal dw1 and semi diurnal sw2 tides are larger than those of non migrating tides after the ssw onset the amplitudes of dw1 sw2 sw1 and ds0 increase moreover they show 16 day variations similar to the periodicity of the high latitude stratospheric planetary wave pw suggesting that the non migrating tides sw1 and ds0 are possibly generated due to non linear interaction of migrating tides with pw similar spectral analysis on temperature at 10 n obtained from the sounding of atmosphere by broadband emission radiometry saber shows that the sw2 enhances at stratospheric heights and the sw2 is more dominant at 80 90 km but its amplitude decreases around 100 km the amplitudes of non migrating tides become comparable to those of sw2 around 100 km and their contribution become increasingly important at higher heights this suggests that the non linear interaction between migrating tides and pw occur at low latitude upper mesospheric heights as sw2 exhibits 16 day periodicity in saber temperature at 100 km as observed in tec besides it is observed that the eastward propagating tides are less dominant than westward propagating tides in both tec and saber temperature

evidence for transition from a continental forearc to a collisional pro foreland basin in the eastern trans hudson orogen detrital zircon provenance analysis in the labrador trough canada abstract the new quebec orogen nqo is the best preserved supracrustal belt of the deeply eroded paleoproterozoic trans hudson orogen and thus constitutes an exceptional window to investigate one of the most ancient wilson cycle sedimentary records within the manikewan paleo ocean it consists of a paleoproterozoic supracrustal belt that underwent reworking when the superior craton collided with an archean to paleoproterozoic microcontinent core zone terrane during the trans hudsonian orogeny within the nqo the western and central parts of the labrador trough comprise greenschist facies sedimentary and volcanic sequences kaniapiskau supergroup inferred to represent the rifted margin of the superior craton whereas the laporte group eastern part composed of similar successions but metamorphosed to higher grades remains of unclear origin competing models purport that the laporte group is either a deeper higher grade exposure of the kaniapiskau supergroup or an exotic terrane large scale sampling petrography and shrimp u pb detrital zircon geochronology provide insights to elucidate the sedimentary provenance and depositional environments for both zones kaniapiskau supergroup detrital zircons yield ages older than 2 5 ga showing a signature typical of the archean superior craton with a main peak at 2 72 ga there is a gap of more than 150 ma between detrital zircon ages and the interpreted depositional age suggesting a divergent environment like a rift and passive margin in contrast detrital zircons from the laporte group yield ages as young as 1834 2 4 ma and a dominant age peak at 1 84 ma the presence of age peaks at 2 2 2 4 ga suggests that the core zone exotic terrane is the main source area and the metamorphic monazite ages previously dated at 1793 2 ma point to a relative short depositional time the presence of an andean type magmatic arc in the western part of the core zone terrane supports the interpretation of an eastward subduction at 1860 1830 ma furthermore a lag time between crystallization and deposition ages within the laporte group points to convergent and collisional settings with a 40 ma transition from continental forearc basin to collisional pro foreland basin marking the consolidation of the columbia supercontinent in northeastern laurentia

viscosity and thickness of the sub lithospheric low viscosity zone constraints from geoid and depth over oceanic swells abstract the medium wavelength geoid to depth anomalies ratio gdr at oceanic hotspot swells has been found to increase from 0 5 m km to 5 m km according to the age of the lithosphere they occur on in order to interpret this trend the geoid and topography anomalies associated with mantle convective plumes crossing a sublithospheric low viscosity zone lvz have been derived from numerical models and a systematic investigation of the gdr dependence on the viscosity and depth extent of the lvz on the thickness and thermal structure of the lithosphere and on the rayleigh number has been conducted it is shown that for viscosity drops across the base of the lvz greater than one order of magnitude the gdr is strongly dependent on the depth of shallow interfaces such as the lithosphere athenosphere boundary and on the lvz s thickness consequently the empirical trend can be accounted for by the thickening of the lithosphere with age provided it occurs at the expense of a lvz whose base is at a fixed depth around 200 km in such a frame no significant variation with age of the lvz s viscosity is required by the gdr data best fit with the empirical trend is found for a lvz about 50 times less viscous than the underlying mantle the mantle flow starts to fluctuate when the local rayleigh number of the low viscosity layer exceeds the rayleigh number of the underlying mantle the fluctuations are initiated in the upper boundary layer in the diverging part of the plume at a distance of a few hundreds of kilometers from the main ascending current for viscosity contrasts in the range of 40 60 deduced from the present study the conditions for the development of these small scale instabilities are realized only where the lithosphere has not yet grown significantly downwards ages

the deep structure of ocean ridges in a convecting mantle abstract plate tectonic models have frequently assumed that mid ocean ridges are the surface expression of the hot rising limb of a convection cell measured half spreading rates show that in any reference frame the horizontal transverse velocity of some ridges is comparable to the individual plate velocities if the above assumption is correct this observation implies that the thermal structure beneath the ridge can adjust rapidly to the changing surface configuration of the plates numerical experiments in a two dimensional convecting layer of constant viscosity fluid show that such a rapid adjustment is probably unrealistic a symmetrically spreading ridge moving horizontally is simulated by specifying a velocity boundary condition on the upper surface of the layer the other boundary conditions are uncertain so several different combinations are used in the experiments including periodic or reflecting lateral boundaries and stress free or no slip lower boundary in all the experiments the rising sheets of the convection cells are anchored firmly in the mean horizontal flow of the layer unless the mean flow coincides approximately with the ridge velocity the ridge quickly moves away from a rising sheet which is initially beneath it if the aspect ratio width depth of the convection cells is significantly greater than one this conclusion is even more evident in the earth it seems unlikely that there is a systematic deep of the order of the convecting layer thickness structure beneath the worldwide network of ocean ridges

differential mobility of elements in burial diagenesis of siliciclastic rocks abstract mass balance calculations involving smectite rich siliciclastic mudstones sampled from boreholes show that significant k2o addition and cao loss correlate strongly with the progress of illitization of smectite and with the dissolution of calcite the calculations are based on the immobility of al2o3 and require a chemically homogeneous sediment at deposition such that the shallower samples represent protolith compositions of deeper samples constancy of ratios of tio2 al2o3 are used to 1 establish that both tio2 and al2o3 have similarly low differential mobilities and 2 identify stratigraphic sections available in the literature that are sufficiently homogeneous to allow the calcu ations our calculations show that most major oxide components are immobile at the 5 limit of resolution of the calculations exceptions are k2o which is almost universally added to mudstones and cao which is commonly removed from mudstones during diagenesis the addition of up to 50 k2o correlates with percent illite in mixed layer illite smectite indicating that illitization of smectite involves fixation of k2o from diagenetic fluids removal of 40 cao from some sections correlates with decreasing calcite mode and is attributed to dissolution of calcite together these reactions lead to a 10 15 loss of mineral volume a volume loss that must be added to porosity loss when modeling compaction illitization and carbonate dissolution may be linked because k fixation during illitization decreases the ph of the local fluid a reverse weathering reaction which enhances carbonate dissolution addition of up to 20 na2o in some rocks may be caused by albitization of plagioclase but na2o addition is not pervasive the apparent lack of mass transfer associated with dissolution of kaolinite and feldspars or precipitation of chlorite implies that these reactions are balanced by other reactions such that their oxide components in mudstones are conserved limited data from sandstone mudstone pairs are consistent with transfer of k2o from sandstones to mudstone via rising diagenetic fluids except for cao and the alkalis diagenesis of siliciclastic mudstones appears isochemical 5 relative for most major oxide components the much larger changes recorded in some reservoir sandstones are clearly exceptions and may not reflect changes occurring at the scale of kilometers

numerical experiments on the onset of convective instability in the earth s mantle summary a simplified model of convection in the mantle is used to investigate the transient effect of cooling a fluid layer from above the model representing the mantle overlain by the lithosphere consists of a two dimensional fluid layer overlain by a solid conducting lid the initial temperature of both layers is the same with the top surface of the lid kept at 0 c throughout we observe the onset of small scale flow in the model in the absence of internal heating the behaviour of the system is controlled by the rayleigh number r and the ratio of the thicknesses of the two layers a the onset time of convection as defined by reference to conduction temperature profiles is related simply to a boundary layer critical rayleigh number the mean temperature profiles for the convection model are also compared with the observed depth age relation for oceanic lithosphere and the results are used to estimate the viscosity of the mantle

paleoproterozoic lithotectonic divisions of the southeastern churchill province western labrador the southeastern churchill province secp is a paleoproterozoic system of orogens that formed during collision of the nain and superior cratons with a composite lithotectonic terrane that now forms the medial metamorphic plutonic core zone of the secp in western labrador the core zone consists of reworked archean gneisses paleoproterozoic supracrustal rocks and variably deformed 1 83 1 81 ga granitic plutons it is subdivided into three paleoproterozoic lithotectonic domains mckenzie river crossroads and orma which are separated from each other by dextral transpressive high strain zones crossroads and orma domains are thought to be derived from archean high grade granite greenstone terrane crust whereas mckenzie river domain is inferred to have been part of an archean orthogneiss terrane dominated by the ca 2776 ma flat point gneiss u pb geochronological data indicate that the igneous precursor of the flat point gneiss is 80 ma older than the oldest tonalite granite intrusions in crossroad

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

sm nd and trace element characterization of shales from the abitibi belt labrador trough and appalachian belt consequences for crustal evolution through time nd isotopic compositions and sm nd li k rb sr ba ni and cr abundances are reported for 25 shale samples from the canadian shield late archean abitibi greenstone belt and the mid proterozoi

review of oceanic transform activity and development oceanic transforms generally have a complex morphology and structure and they do not conform to the ideal concept of simple fractures that exactly follow small circles centered on euler poles of relative plate motions it is proposed that the special properties of transforms arise because they are inherently leaky i e new plate area forms along them as transforms have finite widths the adjacent ridge tips are at different distances from the euler poles and the plate boundaries along transforms are slightly oblique to the direction of plate motions such that new plate area forms along them however along transforms the normal process of seafloor spreading is interrupted and new plate area forms by a different process transforms are underlain by crust consisting of variable proportions of a material emplaced in the transform domains and b stretched ridge generated crust the transform structure is further modified by shearing deformation related to irregularities of lateral faults and because of propagation of adjacent ridges transform leakiness is not considered to result merely from changes of plate motions though such changes will influence transform structure but it reflects the segmented structure of mid oceanic ridges the persistence of transforms and their migration in the absolute reference frame with a component perpendicular to their strikes shows that the ridge structure is decoupled from flow in deeper levels thermal anomalies along transforms especially enhanced heat losses stabilize the segmented ridge structure

convection in three dimensions with surface plates generation of toroidal flow this work presents numerical calculations of mantle convection that incorporate some of the basic observational constraints imposed by plate tectonics the model is three dimensional and includes surface plates it allows plate velocity to change dynamically according to the forces which result from convection we show that plates are an effective means of introducing a toroidal component into the flow field after initial transients the plate motion is nearly parallel to transform faults and in the direction that tends to minimizes the toroidal flow field the toroidal field decays with depth from its value at the surface the poloidal field is relatively constant throughout the layer but falls off slightly at the top and bottom boundaries layered viscosity increasing with depth causes the toroidal field to decay more rapidly effectively confining it to the upper low viscosity layer the effect of viscosity layering on the poloidal field is relatively small which we attribute to its generation by temperature variations distributed throughout the system the generation of toroidal flow by surface plates would seem to account for the observed nearly equal energy of toroidal and poloidal fields of plate motions on the earth a low viscosity region in the upper mantle will cause the toroidal flow to decay significantly before reaching the lower mantle the resulting concentration of toroidal flow in the upper mantle may result in more thorough mixing there and account for some of the geochemical and isotopic differences proposed to exist between the upper and lower mantles

dolomitization on an evaporitic paleoproterozoic ramp widespread synsedimentary dolomite in the denault formation labrador trough canada abstract the denault formation 2 1 1 9 ga crops out in the labrador trough northeastern quebec and western labrador rocks surrounding the town of schefferville quebec contain textural characteristics consistent with deposition on the middle and outer portions of a storm influenced shallow ramp mid ramp facies consist of intraclastic grainstones with hummocky cross stratification hcs swaley cross stratification scs current ripples and graded event beds further outboard grainstones grade into deeper water laminites that are composed of even mm scale couplets of flat lying organic and dolomudstone laminae scours within the laminites suggest periodic storm activity laminites gradually grade into outer ramp deep water shales an isolated eastern stromatolitic buildup is separated from these ramp facies by 50 km present day this succession can be interpreted as the remnant of a near continuous margin or may simply represent an isolated accumulation that developed on a pre existing topographic high the presence of gypsum pseudomorphs in all lithofacies indicates that the denault margin was restricted and evaporitic four paragenetic stages are recognized in the diagenetic evolution of the denault formation 1 carbonate deposition contemporaneous marine cementation authigenic gypsum growth and precipitation of authigenic chert 2 synsedimentary mimetic dolomite precipitation 3 pore rimming and pore occluding shallow burial dolomite cement and 4 fabric destructive sutured anhedral burial dolomite gypsum crystals occur in all lithofacies form the nuclei of interstitial dolomite rhombs average 10 m in length and often display swallowtail twinning paleoproterozoic ocean water had very low concentrations of dissolved sulfate and evaporation in restricted settings would have been required to form gypsum formation of microcrystalline gypsum across this restricted ramp facilitated dolomite precipitation by increasing pore water mg ca ratios and lowering its dissolved sulfate concentrations such an interpretation may explain why there is an abundance of synsedimentary dolostone in the precambrian and the relative paucity of phanerozoic analogs

the influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern united states this paper reports systematic changes in mudrock composition through time on a single con tinental cmstal block the changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton the major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study major elements record sediment recycling processes as well as changing proportions of sedi mentary and first cycle source rocks with the exceptions of kzo which tends to increase and sioz and a o3 which show no trend most major oxides tend to decline in relative abundance in younger mud rocks patterns shown by the index of compositional variability fe03 kro naao cao mgo mno tiojialro and by k20 a1203 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material excursions from progressive trends marked by increases in mgo k20 and cao reflect episodes of large scale input of nonclay first cycle minerals from crystalline source rocks due to large scale basement uplift the chemistry of low solubility trace elements in contrast is not sensitive to recycling effects and reflects the composition of first cycle input incompatible elements are progressively enriched relative to compatible elements in younger mudrocks and values for chondrite normalised rare earth elements also increase in addition the eu anomaly becomes systematically more negative in younger samples these trends cmnot be explained by diagenetic or weathering processes and therefore indicate that the proportion of fnc tionated granitic first cycle detritus being added to the sedimentary system becomes greater with time these results confirm the importance of tectonic setting in controlling mudrock chemistry and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle

the southeastern churchill province synthesis of a paleoproterozoic transpressional orogen the paleoproterozoic southeastern churchill province secp is located in the northeastern canadian shield of labrador and quebec the secp formed through the oblique collisions of the archean nain and superior cratons with a third intervening archean block the core zone the belt has a tripartite structure comprising the torngat orogen to formed by nain craton core zone collision in the east the core zone in the centre and the new quebec orogen nqo formed by superior craton core zone collision in the west the secp thus records transpressional development on the flanks of the superior and nain cratons as they indented northwards to form the larger trans hudson nagssugtoqidian orogenic belt to the north principal stages of tectonic development were 1 2 2 2 1 ga crustal rifting of nain and superior cratons 2 ca 1 9 ga subduction under eastern nain craton 3 ca 1 87 1 85 ga collision of nain craton and core zone to form the to 4 1 845 1 820 ga sinistral transpression in the to and subduction under the western core zone and 5 1 82 1 77 ga collision of superior craton and core zone to form the nqo in association with dextral transpression crustal scale cross sections of the secp have been developed from reflection and refraction seismic data the western part of the nqo is dominantly west vergent and associated with an imbricate thick skinned thrust stack that ramps from the base of the crust the core zone is characterized by a 35 40 km thick crust and pervasive east dipping fabrics related to westerly thrusting the to is a narrow doubly vergent belt associated with a 48 km thick crust that forms a crustal root with a moho relief of 12 km the root is interpreted to result from attempted subduction of the core zone under the nain craton possibly as a result of mid crustal wedging by the nain craton the to was the site of intense convergence that resulted in excision of juvenile crust possibly including tectonic removal of the axial magmatic arc as a result the middle to lower levels of the secp consist largely of refractory archean lithosphere this may account for the lack of widespread post collisional plutonism in the secp and the preservation of the to root

mantle convection as a boundary layer phenomenon summary the boundary layer nature of vigorous thermal convection is explored using high resolution numerical solutions to the governing hydrodynamic equations these solutions are obtained for a series of idealized models of the earth s mantle in which the viscosity is assumed to be constant a detailed analysis of the local energy balance within the horizontal and vertical thermal boundary layers is presented in terms of which a test of the fundamental assumptions of boundary layer theory is provided the results of this test have important geophysical consequences since the asymptotic predictions of boundary layer theory have been employed extensively in the context of thermal history modelling although boundary layer theory closely predicts the correct power law behaviour of various quantities it does not determine their absolute values accurately vertical advection is shown to play an important role in the energy balance within horizontal boundary layers at all rayleigh numbers horizontal and vertical advection dominate the energy balance within vertical plumes while horizontal diffusion plays a very minor role when heating is partially from within the fluid vertical advection into the upper thermal boundary layer can produce significant departures in the thermal structure from that found when heating is entirely from below for a free upper boundary this results in a relative flattening of the variations of surface topography and heat flow across the convection cells for a constant velocity upper boundary similar to plate motion the bathymetry flattens but the heat flow does not this result agrees with marine observations rigidity of the thermal boundary layer below the upper surface is not included explicitly in the model and it is not known whether the inclusion of this feature in future models would significantly alter the topographic expression if not the observed departure of the oceanic bathymetry from a 6 dependence at old ocean floor ages could be attributed to a small amount of internal heating in a mantle wide convective circulation

u pb ages for magmatism and deformation in the new quebec orogen the new quebec orogen is a paleoproterozoic fold and thrust belt active from 2 17 to 1 77 ga bordering the eastern margin of the archean superior province the orogen developed on a long lived passive continental margin and comprises two volcanic sedimentary sequences the ages presented in this study help to bracket the magmatic and deformational history of the orogen a glomeroporphyritic gabbro from the hellancourt formation was dated at 1874 3 ma and a rhyodacite from the upper murdoch formation yielded an identical age of 1870 4 ma these data allow the correlation of the two units and indicate that they are 10 14 ma younger than the estimated age for the sokoman formation these ages together with already published work indicate that magmatism related to the younger magmatic event occurred at 1884 1870 ma albitized pelites from the murdoch formation contain euhedral detrital zircons 2707 2690 ma old indicating that the source rocks were archean an undeformed monzonitic intrusion occurr

u pb geochronology of reactivated archean basement and of hudsonian metamorphism in the northern labrador trough the labrador trough is a proterozoic orogenic belt bordering the eastern margin of the archean superior province the volcano sedimentary sequences of the trough were deformed and metamorphosed during the hudsonian orogeny ca 1800 ma the eastern basement present as domal inliers was remobilized during this orogeny whereas the western basement remained unaffected in this study we present u pb ages of the western and eastern basement of the proterozoic labrador trough orogen between leaf river and koksoak river and of the northernmost extension of the depas batholithic complex farther east from the western basement the leaf bay granodiorite has an age of 2721 3 ma zircon from the gneiss domes of the eastern basement the ages are as follows lac moyer 2883 6 ma zircon 1793 5 ma monazite and 1746 ma minimum age rutile lac boulder an upper intercept age of 2868 8 ma and a lower intercept age of 1783 11 ma zircon titanite regression and an age of 1740 5 ma rutile lac olmst

two billion year old isotopically heavy carbon evidence from the labrador trough canada forty eight samples of bulk carbonates were studied from the dunphy portage alder uve denault and abner formations and the undivided pistolet subgroup in the labrador trough quebec canada the carbonate units occur as a number of thick beds up to 300 m within 100 600 m thick formations they are interbedded with siliciclastic sediments and are composed of crystalline sparry micritic allochemical and stromatolitic dolostones the carbonate rocks are recrystallized and metamorphosed up to amphibolite grade they are chemically dolostones and include chemical or biochemical laminated and massive dolostones dolorudites dolarenites and dololitites which were deposited in shallow water dunphy denault and abner formations to relatively deep water uve formation marine environments reef facies are present in the denault and abner formations the dunphy portage alder and uve formations deposited between 2 17 and 2 14 ga yield isotopically heavy values of 13c pdb ranging from 5 3 to

on some consequences and possible causes of layered mantle convection laboratory and numerical experiments are used to study convection confined to discrete layers the laboratory experiments which use the water content of a predominantly glycerine solution as a way of establishing chemical density variations show that convection will remain confined to superimposed layers when the chemical density contrast between layers is greater than the density change associated with the greatest temperature difference within the convecting system the numerical experiments show a double thermal boundary layer at the interface between layers a feature that would result in an average temperature increase of about 500 c in the mantle near 700 km depth if the lower mantle is isolated as suggested by recent geochemical models the possible causes of a layered mantle convection system include major element chemistry variations the combined effect of major element chemistry and a phase change or a phase change alone if it has a sufficiently negative clapeyron slope

on the forces driving plate tectonics inferences from absolute plate velocities and intraplate stress summary absolute plate motions and intraplate stress both serve as tests of models for the forces acting on plate boundaries plate velocities relative to a presumedly fixed underlying mantle are calculated from the hypothesis that no net torque is exerted on the lithosphere intraplate stress is calculated by solving the equilibrium equations for thin elastic shells in the membrane state of stress the absolute velocity fields predicted from a wide assortment of physical and geological models are all very similar while the global pattern of absolute velocity is probably close to those predicted by such models the absolute motions do not therefore provide a strong test of the driving mechanism comparison of predicted intraplate stress with the long wavelength features of the global stress field however as determined by in situ measurements earthquake mechanisms and stressinduced geological structures does prove to be a powerful test of possible driving forces all absolute velocity models have several interesting properties lithosphere in the equatorial half of the earth is moving significantly faster than lithosphere in the polar half some connection to the earth s rotation is implied since this statement is demonstrably untrue for co ordinate poles much different from the geographic pole subducted slabs are characterized by a slow horizontal translation perpendicular to strike that is independent of the plate convergence rate confounding attempts to explain the dip angles of benioff zones in terms of a uniform vertical sinking and a variable absolute velocity for the overthrust plate ridges must migrate at a wide range of velocities relative to their underlying source of new lithosphere such rapid migration may be a necessary but is not a sufficient condition for ridge jumps force models considered in velocity and stress calculations include driving forces at spreading centres and subduction zones and various parameterizations of drag at the base of the lithosphere from the rms absolute velocities of individual plates there is a weak indication that pull by subducted lithosphere at trenches is an important driving force and that drag may be greater beneath continental than oceanic lithosphere the predicted intraplate deviatoric stress cannot match the well determined stress fields in north america and europe unless the driving force exerted at ridges is at least comparable in magnitude to other forces in the system the mid plate stresses are very sensitive to the nature of drag at the base of the lithosphere and thus measured stresses may ultimately provide a sensitive test of absolute plate velocities

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

aphebian mafic ultramafic magmatism in the labrador trough new quebec its age and the nature of its mantle source the magmatic events occurring within the two main cycles in the south central part of the labrador trough new quebec have been dated in cycle 1 a granophyre dike related to the cramolet lake gabbro sill which intrudes the seward subgroup has a u pb zircon age of 2169 2 ma in cycle 2 the tholeiitic basalts of the willbob hellancourt formation and the related mafic ultramafic sills are dated at ca 1900 ma by the pb pb method these data confirm the existence of at least two main magmatic cycles separated by about 270 ma the magma source was depleted upper mantle and the magma did not experience any significant crustal contamination as indicated by the 1 ratio 7 9 and 4 for the tholeiitic basalts the 1 value for the intrusive rocks 8 03 and the average value for the gabbroic rocks of cycle 2 2 8 and for the granophyre of cycle 1 1 05 could be the result of slight crustal contamination or of mantle heterogeneity whatever the cause of these values the data indicate the prolong

petrography and geochemistry of granitoid and metamorphite pebbles from the early archaean moodies group barberton mountainland south africa pebbles of potassic granitoids and metamorphites constitute up to 5 of the basal conglomerate of the moodies group in a ratio of 2 1 the granitoid pebbles frequently show micrographic quartz feldspar intergrowth whereas the metamorphites of a modal composition similar to that of the granitoids are characterized by large quartz grains which could represent original quartz phenocrysts in felsic volcanic precursors the granitoids show high k2o sr k2o na2o and k rb small enrichment of light ree large negative eu anomalies and slightly depleted and fractionated heavy ree compared to the granitoids the metamorphites show higher fe2o3 tio2 and cr concentrations greater enrichment of light ree and also large negative eu anomalies there is little similarity between the moodies pebbles and the majority of the rocks of the ancient gneiss complex of swaziland agc there is only some similarity of the ree distribution patterns between the pebbles and the mkhondo metamorphic suite possibly an areally restricted phase of the agc the geochemical data and especially the large negative eu anomalies suggest that the moodies pebbles were derived from granites which represent residual magmas from which much plagioclase had been removed the granites crystallized at depths of 7 km from magmas with low h2o pressures in a rather thick sialic crust it appears possible that the pre moodies granitoids originated through partial melting of low al2o3 siliceous gneisses of the agc a chronologic connection of the formation of the granitoids with the late onverwacht group volcanicity is possible

petrography geochemistry and nd isotope systematics of metaconglomerates and matrix rich metasedimentary rocks implications for the provenance and tectonic setting of the labrador trough canada the new quebec orogen consists of a supracrustal belt that was reworked when the superior craton collided with the core zone terrane during the paleoproterozoic trans hudson orogeny within the new

early eocene sedimentary recycling in the kailas area southwestern tibet implications for the initial india asia collision abstract syncollisional sedimentary rocks in the himalayan orogen record important information about the early history of the india asia collision this article presents a combined stratigraphic sedimentologic micropaleontologic and provenance data for the early eocene clastic strata dajin formation in the kailas area southwestern tibet the dajin formation comprises ungraded and normally graded matrix and clast supported conglomerates ungraded and cross stratified sandstones and massive to poorly laminated mudstones deposition of the rocks is characterized by subaerial and subaqueous debris flows waning gravity flows and suspension fallout the larger benthic foraminifera and the youngest ages of the detrital zircons constrain the depositional age to the earliest eocene 56 54 ma the dajin formation contains abundant sandstone and mudstone clasts indicating significant sedimentary recycling in the source area u pb ages of detrital zircons are mostly clustered at 120 54 ma and have positive e hf t values suggesting a gangdese affinity clasts of the dajin formation are interpreted as having recycled from the forearc strata that were originally derived from the gangdese magmatic arc highly immature texture and recycled provenance lead us to propose that deposition of the dajin formation was a result of the development of fold thrusts in the gangdese forearc our new results and published data from the coeval strata in the himalayan orogen indicate that an underfilled foreland basin was initiated soon after the initial india asia collision and the dajin formation records the wedge top depozone of the basin system

how valid are dynamic models of subduction and convection when plate motions are prescribed a detailed comparison between fully dynamic and kinematic plate formulations has been made in models of mantle convection plate velocity is computed self consistently from fully dynamic plate models with temperature and stress dependent viscosity and preexisting mobile faults in fully dynamic models the flow is driven solely by internal buoyancy while in kinematic models the flow is driven by a combination of the prescribed surface velocity and internal buoyancy only a temperature dependent viscosity close to the effective viscosity determined from the fully dynamic models is used in the kinematic models the two types of models give very similar temperature structures and slab evolutionary histories when the effective viscosity and surface velocity are nearly identical in kinematic plate models the additional work introduced by the prescribed velocity boundary condition is apparently dissipated within the lithosphere and has little influence on the convection under the lithosphere in models with periodic lateral boundary conditions slabs sink into the lower mantle at an oblique angle and this contrasts with the vertical sinking which occurs with reflecting boundary conditions models show that we can simulate fully dynamic models with kinematic models under either periodic boundary conditions or reflecting boundary conditions

samarium neodymium elemental and isotopic systematics in sedimentary rocks abstract trace element sm nd th sc and nd isotopic data for sedimentary rocks are examined in order to evaluate 1 1 possible secular trends in the composition of the sedimentary mass and upper continental crust 2 2 the role of tectonic setting in controlling the chemical and nd isotopic composition of the sedimentary mass when plotted against either sedimentation age or average provenance age determined mainly from nd model ages sm nd in sedimentary rocks decreases abruptly from about 0 21 to 0 19 at the archean proterozoic transition consistent with a change towards a more felsic upper crustal composition the change is only somewhat less pronounced for turbidite sediments than for those of continental affinity a trend toward increasing sm nd approaching or exceeding archean values is observed for phanerozoic turbidites this latter change is interpreted as indicating undifferentiated young crust as a component in the sediment which has been lost from the older post archean record due to sedimentary and igneous intracrustal recycling processes when plotted against sedimentation age th sc ratios exhibit a trend directly comparable to that of sm nd in this case th sc increases abruptly at the archean proterozoic boundary from about 0 5 to 1 0 and then decreases to about 0 7 during the phanerozoic nd isotopic composition of modern sediments is affected by tectonic association with those deposited at active margin settings tending to have higher nd reflecting younger provenance and higher and more scattered sm nd reflecting less homogenized and differentiated provenance it is likely that these distinctions can be traced back for about 1 2 ga but with the current data base are not readily resolved in older sedimentary rocks the degree to which the preserved sedimentary mass is open to new additions from the mantle either through replacement during sedimentary recycling or sediment mass growth is highly variable but averages about 15 25 during the phanerozoic

rare earth element geochemistry of australian paleozoic graywackes and mudrocks provenance and tectonic control abstract in the paleozoic graywackes of eastern australia there is an increase in the total abundance of rare earth elements ree a light to heavy ree ratio and a decrease in the chondrite normalized eu anomaly with the increase in sio2 al2o3 and k2o na2o ratios this is due to the change in the dominant source rocks from andesite to dacite to granite gneiss and sedimentary rocks similarly an increase in total ree and a decrease in eu anomaly is also seen in the associated mudrocks with the increase in total clay content this correlation in paleozoic sedimentary rocks suggests that the ree characteristics of terrigenous sedimentary rocks reflect the depositional tectonic setting of the basin the oceanic island arc type graywackes are characterised by their lower total ree abundance only slight enrichment of lree over hree and the absence of a negative eu anomaly on chondrite normalised plots the continental island arc type graywackes are discriminated by their higher ree abundance and la yb ratio the presence of a small negative eu anomaly on the chondrite normalised plots and the significant enrichment of eu compared to paas post archean average australian shale the sedimentary rocks deposited on andean type active continental margins passive margins platform and cratonic basins are all characterised by the high enrichment of lree over hree and the presence of a pronounced negative eu anomaly on chondrite normalised plots and can not be discriminated from each other by ree patterns alone

quantification of the effects of secondary matrix on the analysis of sandstone composition and a petrographic chemical technique for retrieving original framework grain modes of altered sandstones abstract most studies of sandstone provenance involve modal analysis of framework grains using techniques that exclude the fine grained breakdown products of labile mineral grains and rock fragments usually termed secondary matrix or pseudomatrix however the data presented here demonstrate that when the proportion of pseudomatrix in a sandstone exceeds 10 standard petrographic analysis can lead to incorrect provenance interpretation petrographic schemes for provenance analysis such as qfl and qfr should not therefore be applied to sandstones containing more than 10 secondary matrix pseudomatrix is commonly abundant in sandstones and this is therefore a problem for provenance analysis the difficulty can be alleviated by the use of whole rock chemistry in addition to petrographic analysis combination of chemical and point count data permits the construction of normative compositions that approximate original framework grain compositions provenance analysis is also complicated in many cases by fundamental compositional alteration during weathering and transport many sandstones particularly shallow marine deposits have undergone vigorous reworking which may destroy unstable mineral grains and rock fragments in such cases it may not be possible to retrieve provenance information by either petrographic or chemical means because of this pseudomatrix rich sandstones should be routinely included in chemical petrological provenance analysis because of the many factors both pre and post depositional that operate to increase the compositional maturity of sandstones petrologic studies must include a complete inventory of matrix proportions grain size and sorting parameters and an assessment of depositional setting