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

a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

gravity analysis of parga and hecate chasmata implications for rift and corona formation 1 the two largest rift systems on venus parga and hecate chasmata contain one third of all coronae we map variations in elastic thickness and apparent depth of compensation adc in these two regions using the admittance function for gravity and topography we examine the relationship between rifting and coronae by comparing lithospheric structure with corona characteristics including volcanism topographic shape fracture pattern diameter and stratigraphic age at hecate chasmata both adc and elastic thickness correlate with two main rift branches the fracture style and corona characteristics hecate chasma thus appears to be dominated by extensional processes parga chasma shows little correlation between lithospheric properties and the location of coronae or rift segments the adc at hecate and parga chasmata is mostly less than 75 100 km implying that there is not large scale upwelling underlying the rift systems the variations in the corona population sizes between parga chasma which has 131 coronae and hecate chasma with 50 coronae suggest differences in the evolution of the two rift systems the much larger population of coronae at parga contains more small coronae many of which have relatively low volcanism and post date the rift further many of the small coronae occur in chains and have similar diameters as predicted by rayleigh taylor type instabilities these characteristics suggest that the smaller coronae may form via a later secondary decompression melting phase thus hecate chasma may be younger than parga chasma and could experience a secondary stage of corona formation in the future

intraplate volcanism at the edges of the colorado plateau sustained by a combination of triggered edge driven convection and shear driven upwelling although volcanism in the southwestern united states has been studied extensively its origin remains controversial various mechanisms such as mantle plumes upwelling in response to slab sinking and small scale convective processes have been proposed but have not been evaluated within the context of rapidly shearing asthenosphere that is thought to underlie this region using geodynamic models that include this shear we here explore spatiotemporal patterns of mantle melting and volcanism near the colorado plateau we show that the presence of viscosity heterogeneity within an environment of asthenospheric shearing can give rise to decompression melting along the margins of the colorado plateau our models indicate that eastward shear flow can advect pockets of anomalously low viscosity toward the edges of thickened lithosphere beneath the plateau where they can induce decompression melting in two ways first the arrival of the pockets critically changes the effective viscosity near the plateau to trigger small scale edge driven convection second they can excite shear driven upwelling sdu in which horizontal shear flow becomes redirected upward as it is focused within the low viscosity pocket we find that a combination of triggered edge driven convection and sdu can explain volcanism along the margins of the colorado plateau its encroachment toward the plateau s southwestern edge and the association of volcanism with slow seismic anomalies in the asthenosphere geographic patterns of intraplate volcanism in regions of vigorous asthenospheric shearing may thus directly mirror viscosity heterogeneity of the sublithospheric mantle

structural history and granite emplacement in the rottenstone domain during closure of the trans hudson orogen davin lake northern saskatchewan the rottenstone domain at davin lake northern saskatchewan exhibits structural and granite emplacement evidence for crustal thickening and possible himalayan style extrusion overprinted by transpressional strain increasing toward the contact with the wathaman batholith three discrete rottenstone subdomains parallel the regional strike of the trans hudson orogen i the southeast rottenstone subdomain sersd with gently northwest dipping migmatitic straight gneiss s1 and white granitoid rocks with pinch and swell structures parallel to the straight gneissosity ii the central rottenstone subdomain crsd which partly preserves the same nw dipping fabric s1 but is overprinted at its margins by tight upright f2 folds and includes a stockwork of pink monzogranitic aplites and pegmatites and iii the northwest rottenstone subdomain nwrsd in which the f2 folds are generally tighter and penetrative and its network of narrow white granitoid rocks is deformed and transposed by the f2 folds but in

seismic tomographic imaging of the eastern mediterranean mantle implications for terminal stage subduction the uplift of anatolia and the development of the north anatolian fault

age constraints on the paleoproterozoic tectonometamorphic history of the committee bay region western churchill province canada evidence from zircon and in situ monazite shrimp geochronology in situ u pb sensitive high resolution ion microprobe shrimp analyses of monazite from upper amphibolite facies paragneiss of the committee bay supracrustal belt central rae domain canada reveal three age populations ca 2350 1850 and 1780 ma the ca 1850 ma age also corresponds to growth of low th u zircon as indicated by u pb shrimp analyses of zircon separates from melanosome and leucosome the contextual advantage of the in situ monazite analysis allows evaluation of the geochronological data in terms of the regional structural and metamorphic evolution the region is dominated by a northeast striking s2 s1 fabric axial planar to tight northwest vergent f2 folds early garnet is enveloped by this biotite sillimanite cordierite s2 fabric garneti hosts ca 1850 ma monazite inclusions with ca 2350 ma cores placing a maximum age on garneti growth and s2 development d2 metamorphic conditions progressed through 3 5 kbar 1 kbar 100 mpa and 600 c to near peak conditions of 5 kbar and 675 c a minimum age for s2 is provided by unstrained ca 1820 ma monzogranite that locally and regionally truncates s2 dominantly ca 1780 ma matrix monazite is interpreted to date post s2 garnetii and cordierite which record 5 kbar and 675 c these data indicate that the committee bay region experienced penetrative d2 tectonometamorphism at ca 1850 1820 ma with a subsequent static overprint the absence of a ca 1 85 ga plutonic suite in the region suggests that low pressure metamorphism was a response to thick skinned crustal thickening initiated at ca 1870 ma the new data highlight the importance of paleoproterozoic reworking of the central rae domain in the hinterland of the trans hudson orogen

nd sr and o isotopic constraints on the petrogenesis of mafic intrusions in the proterozoic trans hudson orogen of central canada sm nd isotopic data from five mafic intrusions in the la ronge lynn lake domain indicate crystallization between 1 88 and 1 79 ga the data from a single intrusion in the glennie lake domain suggest a slightly younger age of about 1 71 ga these results indicate a cogenetic origin of the mafic intrusions with the associated volcanic and plutonic rocks initial end values vary from 4 5 to 0 6 and can be modeled by mixing proterozoic mantle derived magmas with 17 archaean crust or subduction of 3 sediment similar to pelites from the wollaston domain the initial end value of 0 for a late proterozoic mackenzie dike is compatible with an origin from mantle sources affected by hudsonian subduction processes rb sr data indicate variable and generally high initial 87sr 86sr isotopic ratios which reflect in part isotopic redistribution during metamorphism 18o values of coexisting clinopyroxene orthopyroxene and plagioclase indicate isotopic disequilibrium plagioclase samples show variable degrees o

buoyant melting instabilities beneath extending lithosphere 1 numerical models buoyant decompression melting instabilities in regions of partially molten upper mantle have been proposed to be an important process that might account for some characteristics of intraplate volcanism on earth and other terrestrial planets the instability is driven by variations in the melting rate within a partially molten layer whenever a relative decrease in density accompanies decompression melting of ascending mantle here the development of buoyant decompression melting instabilities in a plane layer of passively upwelling and partially melting mantle beneath diffusely extending lithosphere is studied using numerical convection models covering a wide range of physical parameters we find that the occurrence and nature of these instabilities in such a scenario is strongly affected by the rate of extension and melt percolation as well as depth distribution of solid density variations arising from melt depletion in some cases instabilities do not occur during extension but only develop after extension has slowed or stopped completely this behavior creates two pulses of magma generation due to passive upwelling accompanying extension followed by the subsequent instability and is favored by a faster rate of extension higher mantle viscosity higher rate of melt percolation and smaller amount of solid residuum depletion derived buoyancy larger degrees of solid density changes accompanying melt depletion can enhance the instability of partially molten mantle during extension but decrease the cumulative volume of generated melt this kind of behavior modifies the conventional expectation of spatially and temporally correlated volcanism and extension and may lend insight into the observed increase in localized volcanic activity following miocene basin and range extension in the western united states

shear driven upwelling induced by lateral viscosity variations and asthenospheric shear a mechanism for intraplate volcanism abstract volcanism occurring away from ridges and subduction zones does not have an obvious plate tectonic explanation but instead must arise from sub lithospheric processes that generate upwelling flow and decompression melting several convective processes such as mantle plumes convective instability edge driven convection and richter rolls produce upwelling via the action of gravity on density heterogeneities in the mantle here we investigate an alternative mechanism the shear driven upwelling sdu which instead generates upwelling solely through the action of asthenospheric shear flow on viscosity heterogeneity using a numerical flow model we examine the effect of viscosity heterogeneity on viscous shear flow induced within an asthenospheric layer we demonstrate that for certain geometries and viscosity ratios circulatory flow develops within a cavity or step embedded into the lithospheric base or within a low viscosity pocket embedded within the asthenospheric layer for asthenosphere shearing at 5 cm yr we estimate that sdu can produce upwelling rates of up to 0 2 cm yr within a continental rift 0 5 cm yr along the vertical edge of a craton or 1 0 cm yr within a pocket of low viscosity asthenosphere in the last case the pocket must feature an aspect ratio of more than 5 occupy 20 60 of the asthenosphere s thickness and be at least 100 times less viscous than the surrounding asthenosphere such viscosity heterogeneity may be associated with thermal chemical melting volatile or grain size anomalies and is consistent with tomographic constraints on asthenospheric variability we estimate that sdu may generate up to 2 5 km myr of melt that is potentially eruptible as surface volcanism this is faster than eruption rates observed at some locations of continental basaltic volcanism we conclude that sdu could provide an explanation for intraplate volcanism occurring above rapidly shearing asthenosphere for example in the basin and range region of north america

buoyant mantle upwelling and crustal production at oceanic spreading centers on axis segmentation and off axis melting numerical experiments are used to examine the effect of buoyant mantle flow on melt generation near spreading centers buoyancy results from 1 the depletion of residual mantle in fe relative to mg mantle depletion buoyancy 2 the presence of low density melt melt retention buoyancy and 3 thermal expansion thermal buoyancy thermal buoyancy drives off axis convective rolls that develop closer to the spreading axis at slower spreading rates melt retention buoyancy drives focused upwelling into isolated melting centers with increasing spreading rate the amplitude of on axis crustal segmentation decreases the melting region being essentially two dimensional at full spreading rates greater than 100 km m y these numerical experiments predict centers of mantle upwelling that are more widely spaced than the actual wavelength of inferred crustal thickness variations this discrepancy indicates that short wavelength segmentation of slow spreading centers requires some process not included in our models of mantle flow at fast spreading rates when the lithosphere is thinner melt retention buoyancy gives rise to isolated focused centers of off axis upwelling and melt production off axis melting centers begin to form at about 300 km from the axis with a characteristic along axis spacing of 200 km successive off axis melting centers are shifted along axis so that crustal thickness far from the axis is increased at a 100 km wavelength

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

rb sr study of metavolcanic rocks from the la ronge and flin flon domains northern saskatchewan two whole rock suites of metavolcanic rocks from separate volcanic belts of the churchill province in northern saskatchewan have been dated by rb sr samples from the amisk group of the flin flon snow lake domain provide an isochron date of 1784 44 ma suites from the waddy lake and devil lake areas of the la ronge lynn lake domain yield isochron dates of 1814 26 and 1854 100 ma respectively all are regarded as minima for but close approximations to emplacement ages the maximum crustal age of any suite cannot greatly exceed 1850 ma previous rb sr and u pb isotopic dates together with these new determinations confirm the contemporaneous existence of two volcanic arcs active during the late aphebian 1875 1784 ma in the churchill province low initial 87 sr 86 sr ratios 0 7017 0 7022 are consistent with a petrochemically inferred subduction related origin for the volcanic rocks with no closed system reworking of archean crust and a linear evolution of 87 sr 86 sr ratio in the magmatic arc mantle source region from 4 55 ga to the present

nd and sr isotopic geochemistry of mafic layered intrusions in the eastern baltic shield implications for the evolution of paleoproterozoic continental mafic magmas nd and sr isotopic data are presented for the 2449 2441 ma olanga and burakovka layered mafic complexes in the eastern baltic shield these complexes have similar tectonic position but differ in two aspects the age of the enclosing crust and the post crystallization metamorphic history the sm nd isotopic results for the kivakka and lukkulaisvaara intrusions olanga complex are consistent with the model of closed system crystallization of a single magma without significant wallrock assimilation the rb sr systems of minerals were disturbed by late rb addition during 1 75 1 50 ga metamorphism the nd and sr isotopic systems in the burakovka complex show no metamorphic disturbance and indicate mixing of at least four isotopically distinct components isotopic variations in the burakovka complex can be explained by a 4 20 per cent contamination of a primary komatiitic or picritic magma with a mesoarchean crust similar to that exposed in the region a similar model applied to the olanga complex using a neoarchean crustal isotopic composition cannot reproduce the observed isotopic signature the nearly uniform initial nd values between 1 and 2 3 observed in the kivakka and lukkulaisvaara intrusions of the olanga complex as well as in the other 2 50 2 44 ga layered mafic intrusions throughout the eastern baltic shield are better explained by a mantle plume model with small amounts of crustal contamination and minor involvement of asthenospheric material this model is also consistent with the geological observations and the temporal distribution of the paleoproterozoic mafic magmatism in the eastern baltic shield as an alternative the enriched isotopic characteristics may be explained by melting of a metasomatically modified lithospheric mantle source

diagenetic overprinting of the sphaerosiderite palaeoclimate proxy are records of pedogenic groundwater delta18o values preserved meteoric sphaerosiderite lines msls defined by invariant d 18 o and variable d 13 c values are obtained from ancient wetland palaeosol sphaerosiderites millimetre scale feco3 nodules and are a stable isotope proxy record of terrestrial meteoric isotopic compositions the palaeoclimatic utility of sphaerosiderite has been well tested however diagenetically altered horizons that do not yield simple msls have been encountered well preserved sphaerosiderites typically exhibit smooth exteriors spherulitic crystalline microstructures and relatively pure 95 mol feco3 compositions diagenetically altered sphaerosiderites typically exhibit corroded margins replacement textures and increased crystal lattice substitution of ca 2 m g 2 and mn 2 for fe 2 examples of diagenetically altered cretaceous sphaerosideritebearing palaeosols from the dakota formation kansas the swan river formation saskatchewan and the success s2 formation saskatchewan were examined in this study to determine the extent to which original early diagenetic d 18 o and d 13 c values are preserved all three units contain poikilotopic calcite cements with significantly different d 18 o and d 13 c values from the co occurring sphaerosiderites the complete isolation of all carbonate phases is necessary to ensure that inadvertent physical mixing does not affect the isotopic analyses the dakota and swan river samples ultimately yield distinct msls for the sphaerosiderites and mcls meteoric calcite lines for the calcite cements the success s2 sample yields a covariant d 18 o vs d 13 c trend resulting from precipitation in pore fluids that were mixtures between meteoric and modified marine phreatic waters the calcite cements in the success s2 formation yield meteoric d 18 o and d 13 c values a stable isotope mass balance model was used to produce hyperbolic fluid mixing trends between meteoric and modified marine end member compositions modelled hyperbolic fluid mixing curves for the success s2 formation suggest precipitation from fluids that were 25 sea water

lead isotopic evidence for the origin of paleo and mesoproterozoic rocks of the colorado province u s a abstract lead isotopic ratios of k feldspars and whole rocks from 1 7 and 1 4 ga plutonic rocks of the colorado province are relatively non radiogenic for 207 pb 204 pb plotting below the average crust model curve of stacey and kramers 1975 indicating that the terrane was derived primarily from juvenile mantle material slightly more radiogenic ratios in the northern part of the terrane near the archean wyoming province suggest minor inclusion of an older component the data from 1 7 ga plutons plot in a broad field suggesting two episodes of re equilibration with whole rock pb probably related to heating events in the mesoproterozoic 1 4 ga and cretaceous 70 ma possible differences in calculated whole rock th u coupled with slight pb isotopic variations along the north south transect suggest either a terrane boundary through central colorado near salida and gunnison or fundamental differences in source rocks metasedimentary vs metavolcanic u pb analyses of multigrain splits of detrital zircons from quartzites throughout the colorado province have failed to identify archean detritus the oldest 207 pb 206 pb ages found in two samples of quartzite from northern colorado are about 2 0 ga perhaps derived from rocks of the trans hudson orogen in contrast to 2 75 ga detrital zircon in a paleoproterozoic quartzite from the southern part of the wyoming province while we are not yet able to discern if these ages are true provenance ages or mixtures of archean and paleoproterozoic components the absence of easily recognizable archean zircons supports other isotopic data and a conclusion that most of the paleoproterozoic crust of the colorado province was ultimately derived from a juvenile at 1 8 ga mantle reservoir

organic rich sediments in ventilated deep sea environments relationship to climate sea level and trophic changes sediments on the namibian margin in the se atlantic between water depths of 1000 and 3600 m are highly enriched in hydrocarbon prone organic matter such sedimentation has occurred for more than 2 million years and is geographically distributed over hundreds of kilometers along the margin so that the sediments of this region contain a huge concentrated stock of organic carbon it is shown here that most of the variability in organic content is due to relative dilution by buried carbonates this reflects both export productivity and diagenetic dissolution not differences in either water column or bottom water anoxia and related enhanced preservation of organic matter these observations offer a new mechanism for the formation of potential source rocks in a well ventilated open ocean in this case the south atlantic the organic richness is discussed in terms of a suite of probable controls including local wind driven productivity upwelling trophic conditions transfer efficiency diagenetic processes and climate related sea level and deep circulation the probability of past occurrences of such organic rich facies in equivalent oceanographic settings at the edge of large oceanic basins should be carefully considered in deep offshore exploration

production data as an indicator of gas reservoir heterogeneity in the vicksburg s sandstones oligocene mcallen ranch field hidalgo county texas abstract

metavolcanic rocks of the la ronge domain in the churchill province saskatchewan geochemical evidence for a volcanic arc origin the la ronge domain in the churchill province of the canadian shield is a volcanic sedimentary plutonic belt of early proterozoic age geochemical data supported by geological evidence strongly suggest that the la ronge metavolcanics were emplaced in an island arc setting the metavolcanic rocks range from basalt through andesite to dacite and rhyolite calcalkaline and tholeiitic types are dominant with minor alkaline varieties classification based on the major elements produces ambiguous results because of mobility of these elements during metamorphism however relatively immobile minor and trace elements provide reliable characterization of the basalts in particular the latter display distinctive trace element properties and morb normalized geochemical patterns similar to those of recent volcanic arc basalts being enriched in incompatible elements of low ionic potential and depleted in those of high ionic potential the calcalkaline basalts are in addition enriched in th p ce and sm a minor within plate component is also displayed by some metabasalts suggesting that magma genesis in parts of the la ronge arc involved sub continental lithosphere rare earth element patterns show enrichment in the lree with little or no eu anomaly interpretation by means of trace element discrimination diagrams incorporating elements such as ti y nb zr cr th yb and ce provides clear evidence for emplacement in a volcanic arc tectonic setting

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

mantle upwelling and melting beneath slow spreading centers effects of variable rheology and melt productivity abstract we examine the effects of non uniform rheology and melting models on the 3d structure of mantle upwelling beneath spreading centers our numerical models identify a range of conditions for which 3d upwelling can be considered as a possible mechanism for magmatic segmentation of slow spreading ridges in a highly viscous shallow region due to dehydration of the solid by melt extraction flow results essentially from the plate spreading this contrasts with a deeper buoyant low viscosity region where melt is present but upwelling solid is not yet dehydrated and where both solid and melt flows acquire their 3d nature the thickness of this buoyant region depending mainly on the temperature interval between the wet and dry solidi results in a specific characteristic length scale for the upwelling smaller segmentation wavelengths occur for smaller values of this thickness in contrast to isoviscous models where minimum segmentation wavelengths were large 150 km minimum wavelengths obtained in models with non uniform rheology and melt production 40 70 km are comparable to smallest observed segment lengths a single wavelength may be preferred for a given set of parameters but such a preferred wavelength is difficult to achieve because a range of initially prescribed wavelengths can persist for long times the persistence of 3d solutions for tens of millions of years of model time indicate that segment lengths observed at slow mid ocean ridges may be inherited from the initial stages of spreading

buoyancy effects on mantle flow under mid ocean ridges results of a series of two dimensional numerical experiments of mantle flow melting and melt migration under a spreading center are reported the model predicts the distribution of melt in the subridge mantle the width over which most melt is delivered to the crust and the thickness of crust the sources of buoyancy considered are thermal expansion compositional variation caused by melt extraction and the phase change of solid to melt we infer that the steady state average viscosity of the mantle below a ridge cannot be much less than about 1019 pa s for a lower average viscosity thermal convection causes rapid cooling of a large region under a ridge raising the viscosity results imply that transient increases in mantle temperature should lead to larger increases in the oceanic crustal thickness for slow spreading ridges than for fast spreading ridges we assume that the viscosity is proportional to exp 0 where is melt fraction we parameterize the permeability in terms of the reference velocity for percolation of the melt vr where the relative velocity of melt to solid is vr analytic approximations are used to extrapolate the model results to large values of permeability and small values of 0 if vr is less than 1 m yr then more melt would be retained in the subridge mantle than is estimated from analysis of topography and gravity data at fast spreading centers for vr greater than about 100 m yr so little melt would be retained in the mantle that it is difficult to explain the gravity data and the low shear wave velocity structure close to the east pacific rise estimated from seismic surface waves buoyancy effects can lead to a region of mantle upwelling and melting that is as narrow as the observed zone of oceanic crustal accretion for most melt to be added to the crust within a few kilometers of a fast spreading center requires that 0 be less than 0 015 if vr equals 1 m yr and less than 0 003 if vr is 100 m yr

buoyant decompression melting a possible mechanism for intraplate volcanism 1 volcanic activity away from plate boundaries occurs in a variety of settings linear age progressive volcanic chains have been explained as the manifestation of fixed hot spots possibly generated by plumes originating deep in the mantle however important examples of oceanic intraplate volcanism cannot be explained in this way including short lived chains and chains violating the predicted age distance behavior furthermore a significant fraction of volcanism does not occur in linear chains and observations suggest control by lithosphere structure in many cases in this study melting in mantle upwellings that result from the buoyancy associated with the melting itself is considered as a mechanism of intraplate volcanism numerical models of this buoyant decompression melting in a layer that is initially at its melting temperature over some depth range are used to determine the duration and amount of melting in upwellings which organize from an initially small random perturbation for a range of model parameters a predicted inverse correlation between the amount of melt produced and the duration of melting may be diagnostic of the buoyant melting process buoyant melting could occur in a number of geologic settings since the oceanic upper mantle may have previously melted beneath a spreading center and subsequently cooled spontaneous buoyant decompression melting may be possible only in regions where a large scale mantle upwelling can counteract conductive cooling keeping the mantle at its solidus temperature over some depth range where the mantle is at or sufficiently near its solidus buoyant decompression may spontaneously generate convective storms of melting this may explain the abundance of volcanism associated with the south pacific superswell where mantle is slightly cooler than its melting temperature buoyant melting may not occur spontaneously but may be triggered by some initial upwelling this may provide a physical explanation for volcanism that is controlled by lithosphere structure

the age and origin of the north shore plutons in the rae province goldfields area saskatchewan the north shore plutons are peraluminous granitoids emplaced within the murmac bay group supracrustals exposed on the north shore of lake athabasca saskatchewan 207pb 206pb ages obtained using the single zircon pb evaporation technique indicate emplacement at 1952 18 ma coeval with taltson magmatism that occurred during the 1 9 2 0 ga thelon orogeny which influenced the rae province the granitoids inherited zircons from archean source terranes dated at approximately 2 5 and 3 0 ga detrital zircons from the younger source terrane were also identified in the murmac bay group thus constraining the maximum age of the group to the paleoproterozoic rare earth element and incompatible element data and rb sr and sm nd isotope systematics from the north shore plutons and adjacent host rocks indicate that the plutons are crustal melts generated from melting of murmac bay group rocks and archean crust at deeper levels granitic gneisses described as older granites represent mixtures of variable amounts of

intraplate volcanism due to convective instability of stagnant slabs in the mantle transition zone the study of volcanism can further our understanding of earth s mantle processes and composition continental intraplate volcanism commonly occurs above subducted slabs that stagnate in the mantle transition zone mtz such as in europe eastern china and western north america here we use two dimensional numerical models to explore the evolution of stagnant slabs in the mtz and their potential to sustain mantle upwellings that can support volcanism we find that weak slabs may go convectively unstable within tens of million years upwellings rise out of the relatively warm underbelly of the slab are entrained by ambient mantle flow and reach the base of the lithosphere the first and most vigorous upwellings rise adjacent to lateral heterogeneity within the slab ultimately convective instability also acts to separate the compositional components of the slab harzburgite and eclogite from each other with harzburgite rising into the upper mantle and eclogite sinking into the lower mantle such a physical filtering process may sustain a long term compositional gradient across the mtz

comportement du systeme u pb dans la monazite et chronologie de la deformation et du metamorphisme des metasediments du domaine de kisseynew orogene trans hudsonien manitoba canada the kisseynew domain is the central unit of the reindeer zone of the paleoproterozoic trans hudson orogen in manitoba and saskatchewan the southern flank of the domain is a transition zone betwee

the wathaman batholith largest known precambrian pluton the early proterozoic wathaman batholith in northern saskatchewan and manitoba is a 900 km long megacrystic granite granodiorite intrusion that straddles the junction between ensialic miogeoclinal and probably ensimatic eugeoclinal island arc terranes of the trans hudson orogen of the western churchill province although the largest precambrian batholith known it is apart from marginal complexities remarkably homogeneous throughout and unlike comparably sized and situated phanerozoic batholiths shows no evidence of multiple intrusion nor does it have comagmatic early mafic phases however it may be considered as just one phase of a larger batholithic belt that also includes numerous smaller plutons taken as a whole the composite batholithic belt is similar in many aspects to mesozoic pacific rim batholithic belts and like them probably was emplaced during plate collision the batholith is affected by pervasive internal deformation is bounded on the northwest by major blastomylonite zones a

high and highly anisotropic electrical conductivity of the asthenosphere due to hydrogen diffusion in olivine experimental studies on electrical conductivity in hydrated olivine had been made only at relatively low temperatures the extrapolation of these data to asthenosphere temperatures shows that explaining high and highly anisotropic conductivity by hydrogen is difficult we present new experimental results on the electrical conductivity in hydrated olivine single crystals measured under a broader temperature range than before t temperature 573 1373 k at p pressure 4 gpa the new results agree well with the previous results at low temperatures t 1000 kt 1000 k substantial deviations from the previous results are observed i anisotropy in conductivity increases with temperature and ii activation energies at high temperature regime are substantially higher than those at lower t the high temperature behavior is consistent with a model invoking diffusion of two protons at m site vacancy whereas the low temperature behavior suggests conduction by more mobile species such as free proton the present results i explain the discrepancy between different previous studies conducted under different temperature regimes and ii show that most of geophysical observations on the electrical conductivity in the asthenosphere including the high and highly anisotropic conductivity can be explained by high water content in the asthenosphere that is consistent with the geochemical model combining with a previous model of low seismic wave velocity of the asthenosphere a subsolidus model invoking the role of hydrogen provides a unified explanation for the geophysical anomalies of the asthenosphere

the role of mantle depletion and melt retention buoyancy in spreading center segmentation abstract numerical experiments are used to examine the structure of mantle flow beneath the axis of a spreading center buoyancy results from the depletion of residual mantle in iron relative to magnesium as melt is extracted mantle depletion buoyancy and from the presence of low density melt melt retention buoyancy 3 d buoyant mantle flow arises spontaneously from an initially 2 d solution for low mantle viscosity and low spreading rate at high viscosity and high spreading rate initially 2 d solutions remain 2 d this may explain the fundamentally different structure of fast and slow spreading centers in a uniform viscosity halfspace the along axis wavelength of 3 d buoyant upwelling scales with the maximum depth of melting the only length scale in this system for reasonable maximum depths of melting 60 90 km along axis wavelengths of 200 300 km are preferred longer than the 50 100 km segmentation length of slow spreading centers in a viscosity layered halfspace the thickness of the asthenosphere introduces another length scale a wavelength of segmentation comparable to the asthenosphere thickness also develops in our numerical experiments this suggests the possibility that a wavelength corresponding to the spacing between gravity lows may be controlled by the asthenosphere thickness while the spacing of major fracture zones corresponds to the longer wavelength 3 times the maximum depth of melting intrinsic to the melting region the along axis structure of 3 d flow varies from narrow focused upwellings at low spreading rates and mantle viscosities to broad regions of upwelling at high spreading rates and mantle viscosity to allow an along axis crustal thickness variation no larger than that which is observed 3 4 km the highly focused upwelling and crustal production predicted at slow spreading rates requires appreciable along axis transport of melt

late jurassic to early cretaceous black shale formation and paleoenvironment in high northern latitudes examples from the norwegian greenland seaway 1 the late jurassic to early cretaceous volgian ryazanian was a period of a second order sea level low stand and it provided excellent conditions for the formation of shallow marine black shales in the norwegiangreenland seaway ngs iku petroleum research drilling cores taken offshore along the norwegian shelf were investigated with geochemical and microscopic approaches to 1 determine the composition of the organic matter 2 characterize the depositional environments and 3 discuss the mechanisms which may have controlled production accumulation and preservation of the organic matter the black shale sequences show a wide range of organic carbon contents 0 5 7 0 wt and consist of thermally immature organic matter of type ii to ii iii kerogen rock eval pyrolysis revealed fair to very good petroleum source rock potential suggesting a deposition in restricted shallow marine basins well developed lamination and the formation of autochthonous pyrite framboids further indicate suboxic to anoxic bottom water conditions in combination with very low sedimentation rates it seems likely that preservation was the principal control on organic matter accumulation however a decrease of organic carbon preservation and an increase of refractory organic matter from the volgian to the hauterivian are superimposed on short term variations probably reflecting milankovitch cycles various parameters indicate that black shale formation in the ngs was gradually terminated by increased oxidative conditions in the course of a sea level rise index terms 1055 geochemistry organic geochemistry 4802 oceanography biological and chemical anoxic environments 9609 information related to geologic time mesozoic 8105 tectonophysics continental margins and sedimentary basins 1212 keywords black shale formation depositional environment norwegian greenland seaway mesozoic organic petrography citation langrock u r stein m lipinski and h j brumsack late jurassic to early cretaceous black shale formation and paleoenvironment in high northern latitudes examples from the norwegian greenland seaway paleoceanography 18 3 1074 doi 10 1029 2002pa000867 2003

detrital zircon evidence for non laurentian provenance mesoproterozoic ca 1490 1450 ma deposition and orogenesis in a reconstructed orogenic belt northern new mexico usa defining the picuris orogeny detrital zircon and igneous zircon u pb ages are reported from proterozoic metamorphic rocks in northern new mexico these data give new insight into the provenance and depositional age of a 3 km thick metasedimentary succession and help resolve the timing of orogenesis within an area of overlapping accretionary orogens and thermal events related to the proterozoic tectonic evolution of southwest laurentia three samples from the paleoproterozoic vadito group yield narrow unimodal detrital zircon age spectra with peak ages near 1710 ma igneous rocks that intrude the vadito group include the cerro alto metadacite the picuris pueblo granite and the penasco quartz monzonite and yield crystallization ages of 1710 10 ma 1699 3 ma and 1450 10 ma respectively within the overlying hondo group a metamorphosed tuff layer from the pilar formation yields an age of 1488 6 ma and represents the first direct depositional age constraint on any part of the proterozoic metasedimentary succession in northern new mexico detrital zircon from the overlying piedra lumbre formation yield a minimum age peak of 1475 ma and 60 grains 25 yield ages between 1500 ma and 1600 ma possibly representing non laurentian detritus originating from australia and or antarctica detrital zircons from the basal metaconglomerate and the middle quartzite member of the marquenas formation yield minimum age peaks of 1472 ma and 1471 ma consistent with earlier results we interpret the onset of ca 1490 1450 ma deposition followed by tectonic burial regional al 2 sio 5 triple point metamorphism and ductile deformation at depths of 12 18 km to reflect a mesoproterozoic contractional orogenic event possibly related to the final suturing of the mazatzal crustal province to the southern margin of laurentia we propose to call this event the picuris orogeny

geochemical oxygen and neodymium isotope compositions of metasediments from the abitibi greenstone belt and pontiac subprovince canada evidence for ancient crust and archean terrane juxtaposition abstract the abitibi greenstone belt agb and pontiac subprovince ps in the southwestern superior province are adjacent greenstone plutonic and metasedimentary dominated terranes respectively separated by a major fault zone metasediments from these two contrasting terranes are compared in terms of major and trace element and o and nd isotope compositions and detrital zircon ages the following two compositional populations of metasediments are present in the low grade abitibi southern volcanic zone 1 1 a mafic element enriched population mep characterized by flat depleted ree patterns enhanced mg cr co ni and sc low incompatible element contents and minor or absent normalized negative troughs at nb ta and ti and 2 2 a low mafic element population lmep featuring lree enriched patterns enhanced rb cs ba th and u contents and pronounced normalized negative troughs at nb ta and ti these geochemical features are interpreted to indicate that the mep sediments were derived from an ultramafic and mafic dominated oceanic provenance whereas the lmep sediments represent mixtures of mafic and felsic arc source rocks the ps metasediments are essentially indistinguishable from abitibi lmep on the basis of majorelement and transition metal abundances suggesting comparable types of source rocks and degrees of maturity but are distinct in terms of some trace elements and o isotope compositions the pontiac metasediments are depleted in 18o and enriched in cs ba pb th u nb ta hf zr and total ree and also have higher ratios of rb k cs rb ba rb ta nb th la and ba la relative to the abitibi lmep two subtypes of ree patterns have been identified in ps metasediments both have moderately fractionated distributions with la yb n 3 9 17 and 7 8 13 6 respectively but the first has minor eu anomalies eu eu 0 89 1 16 whereas the second features pronounced negative anomalies eu eu 0 52 0 81 coupled with high concentrations of incompatible elements such as k rb th u nb and ta the first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks whereas the eu depleted type has geochemical features that are typical of post archean sediments or archean k rich granites and volcanic equivalents abitibi metasediments have nd 2 7 ga values of 0 71 to 1 83 corresponding to model tchur 2 82 2 55 ga with the majority around 2 65 ga close to the depositional age of 2 7 ga in contrast ps metasediments are characterized by nd 2 7 ga 0 95 to 3 51 corresponding to t dhur 2 7 to 3 1 ga consistent with a detrital zircon age spectrum of 2 69 ga to 3 1 ga pontiac subprovince and abitibi metasediments have contrasting provenances suggesting their separate initial geodynamic settings and later tectonic juxtaposition major and trace elements o and nd isotopic compositions and detrital zircon ages collectively suggest that some of the ps metasediments were derived mainly from juvenile rocks whereas others require various degrees of contribution from older 3 0 3 5 ga evolved crust mixed with juvenile 2 7 ga rocks older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components if the ps represents in part an accretionary prism to the abitibi southern volcanic zone then oblique subduction may have been involved which allowed the pontiac subprovince to juxtapose or sample exotic source rocks

dynamics and structure of a stirred partially molten ultralow velocity zone abstract the seismic properties of thin 5 40 km thickness patches exhibiting low seismic velocities termed ultralow velocity zones or ulvz just above the core mantle boundary cmb might be explained by the presence of partially molten rock where a liquid phase occupies interstices within a skeletal network of solid grains however a key problem with this explanation is that in the absence of improbably strong surface tension effects partial melt is expected to drain by percolation over geological time scales to form a dense melt rich layer at the cmb with physical properties that are inconsistent with seismic and other geophysical constraints here we consider whether stirring within ulvz driven by viscous coupling to convective motions in the overlying mantle can inhibit the production of such stratification and maintain a partially molten region with a structure and constitution comparable to what is inferred seismically we use two dimensional numerical simulations of the response of a melt solid mixture to stirring imposed from above and scaling analysis to identify conditions leading to melt separation retention and drainage over a broad range of parameters we find that melt migration at plausible ulvz conditions is governed predominantly by dynamic pressure gradients arising from the viscous deformation related to mantle stirring rather than by the buoyancy effects driving melt percolation in particular dense melt that would otherwise drain downward and accumulate at the cmb is expected to remain in suspension as a result of the stirring driven within ulvz in addition our model predicts that partially molten ulvz patches will be characterized by a positive gradient in seismic shear velocity i e increasing with depth consistent with seismic inferences and may persist in this state over geological time scales

short lived 1 9 ga continental margin and its destruction wopmay orogen northwest canada a 1 9 ga continental margin in the northwest of the canadian shield evolved in less than 15 m y from a zone of crustal stretching through a phase of passive margin subsidence to attempted subduction beneath an exotic microcontinent reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m y following microcontinental accretion terminal collision occurred 90 m y at most after initial rifting certain differences in detail between this collided margin and those of phanerozoic age may be related to subduction of newly rifted lithosphere rapid evolution of the orogen the contracted passive margin phase in particular is consistent with but does not prove more rapid recycling of oceanic plates at that time

age and source of detrital zircons from the missi formation a proterozoic molasse deposit trans hudson orogen canada the missi formation in the flin flon basin forms part of a discontinuous series of molasse type sediments found throughout the early proterozoic trans hudson orogen in northern saskatchewan and manitoba the flin flon basin contains a sequence of proximal fan to braided stream fluvial conglomerates and sandstones which unconformably overlie subaerially weathered amisk group volcanic rocks stratigraphic way up indicators have been preserved even though these rocks have undergone greenschist facies metamorphism and polyphase deformation the sedimentary rocks are crosscut by intrusive rocks which provide a minimum age of sedimentation of 1840 7 ma detrital zircons from each of the six stratigraphic subdivisions of the flin flon basin were analyzed using the single zircon pb evaporation technique euhedral to slightly rounded zircons dominate each sample and these zircons give ages of between about 1854 and 1950 ma the missi sediments were thus deposited between 1840 and 1854 ma possible sources for

accretion history of the trans hudson orogen in manitoba and saskatchewan from paleomagnetism lithoprobe s sponsorship has led to the acquisition of paleomagnetic data from 20 units throughout the paleoproterozoic trans hudson orogen tho of saskatchewan and manitoba essentially the first such data for the region discussed summarily in this paper they provide an initial framework for the tho s tectonic evolution they show that the archean hearne and superior cratons were at subtropical and subpolar paleolatitudes respectively at 1875 ma with the lynn lake laronge arc midway between them in the manikewan ocean by 1855 ma this ocean was still 5500 700 km wide and its seafloor was subducting northwestward under the hearne craton and pericratonic peter lake domain margin with the coeval development of an andean type continental magmatic arc the wathaman chipewyan batholith between 1855 and 1810 ma coalescing apparent polar wander path apwp segments record closing of the ocean at a rate of 12 cm a trapping and accreting the several separate intervening terranes flin flon hanson lake lynn lake laronge rottenstone and presumably also other tho terranes from 1815 to 1775 ma the assembled terranes drifted as a coherent craton yielding a stillstand and hairpin in the apwp

intraplate volcanism with complex age distance patterns a case for small scale sublithospheric convection 1 many volcano chains in the pacific do not follow the most fundamental predictions of hot spot theory in terms of geographic age progressions one possible explanation for non hot spot intraplate volcanism is small scale sublithospheric convection ssc and we explore this concept using 3 d numerical models that simulate melting with rheology laws that account for the effects of dehydration ssc spontaneously self organizes beneath relatively mature oceanic lithosphere whenever this lithosphere is sufficiently young and thin ssc replaces the shallow layer of harzburgite which was formed by partial melting at the mid ocean ridge with fresh peridotite this mechanism enables magma generation without any preexisting thermochemical anomalies however the additional effect of melting induced dehydration to stiffen the harzburgite requires lower background viscosities to allow for vigorous ssc overturn of the compositional stratification and related magmatism the intrinsic stiffness of the dehydrated harzburgite furthermore restricts penetration of ssc into very shallow and cooler levels on the one hand such a restriction precludes high degrees of melting but on the other hand it slows asthenospheric cooling and thus prolongs the duration of melting to 25 ma volcanism over such an elongated melting anomaly continues for at least 10 20 ma and occurs on seafloor ages of 20 to 60 ma these seafloor ages increase with increasing mantle temperature due to the effect of forming a thicker harzburgite layer from more extensive mid ocean ridge melting the long durations of volcanism predicted reconcile observations of extended activity of individual seamounts and synchronous activity over great distances along some volcanic chains ssc thus gives an explanation for previously enigmatic volcano ages along the line islands and the gilbert and pukapuka ridges as well as along the individual subchains of the wakes marshalls and cook australs

a cordilleran type batholitic belt in the churchill province in northern saskatchewan abstract the rottenstone and western la ronge domains of the south central churchill province in saskatchewan consist mostly of hudsonian batholithic complexes spatially and structurally related to an aphebian hudsonian volcanic arc the batholithic belt is of comparable dimensions and compositional variation to cordilleran type phanerozoic batholiths and appears to be emplaced in close proximity to a prior late archean early aphebian continental crustal margin a quartz diorite line separating dominant quartz diorite tonalite granodiorite plutonic rocks from dominant granodiorite quartz monzonite granite intrusives can be defined these general characteristics as well as more detailed features of aphebian deposition and hudsonian thermotectonism within and on either side of the rottenstone domain suggest a continental margin rather than an intracontinental location for evolution of the hudsonian orogenic belt of the south central churchill province and provide some insights into the possible development of this part of the canadian shield as part of a hudsonian wilson cycle

western u s upper mantle structure regional three dimensional inversions of upper mande p wave velocity structure are created from teleseismic p wave travel time residuals recorded by many of the high density high frequency regional arrays operated within the western united states these inversions are adjusted to a global international seismic centre reference and merged to obtain an image of the upper mantle beneath western united states the p wave velocities in the upper mantle are slow on average and the structure is very heterogeneous where resolution is good coherent upper mantle structures are imaged that extend as deep as 200 km the juan de fuca and gorda slabs which penetrate to greater depths are exceptions and deviate from the average velocity at a given depth by as much as 4 lateral resolution of these structures usually is very good although the magnitude of the actual seismic variations is probably greater than that imaged the long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200 500 km within 250 km of the pacific coast these structures have a wavelength of 250 km and trend parallel to the surface physiography and young tectonic structures beneath the continental interior where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage structures have a wavelength of 500 km and trend northeasterly this northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity

geochemical and nd isotopic compositions of the metasedimentary rocks in the la ronge domain trans hudson orogen canada implications for evolution of the domain abstract metasedimentary rocks of greenschist to amphibolite facies occur in various belts in the paleoproterozoic la ronge domain of the trans hudson orogen metasedimentary rocks in the central metavolcanic greenstone belt are low to intermediate in sio2 contents have nd values of 2 3 to 4 5 with two rare earth element ree patterns of la yb n 8 5 respectively the predominant component meta arkoses of the overlying mclennan lake belt have distinctly high k2o and sio2 contents high la yb n ratios 11 46 large negative eu anomalies eu eu 0 1 0 58 and uniformly low nd values 0 8 to 0 2 the maclean lake belt is comprised of a variety of metasedimentary gneisses the volumetrically minor amphibole and diopside rich gneisses have high ferromagnesian element sc and co concentrations high nd values of 4 1 to 5 7 and flat ree patterns with no eu anomalies the predominant lithological unit in the maclean lake belt the psammitic gneisses is geochemically more variable the mafic psammitic gneiss samples in the maclean lake belt and the metasedimentary rocks in the central metavolcanic belt are similar in trace and major element geochemistry and nd isotope composition whereas those more felsic psammitic gneisses are similar to the mclennan lake meta arkoses geochemical data combined with lithological and geochronological evidence indicate that sediments in the central metavolcanic belt were derived mainly from contiguous volcanic debris with minor addition of felsic components from exotic sources in contrast the mclennan lake meta arkoses were derived from a distinct felsic source whereas the psammitic gneisses in the maclean lake belt are mixtures of volcanic materials in the central metavolcanic belt and meta arkoses like felsic components nd isotopic data and ages of detrital zircon suggest that proterozoic juvenile materials were dominant sources for metasedimentary rocks in the la ronge domain comparison of geochemical signatures of the metasedimentary rocks with those of the felsic plutonic rocks in the central metavolcanic belt suggests that the felsic materials in the mclennan lake and maclean lake belts could not have been derived predominantly from the plutonic rocks implying more distal sources topographic changes during collision may have resulted in a shift from mixing of both local and distal sources for the rocks in the maclean lake belt to the dominance of a more distal source for the mclennan lake meta arkoses

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

an early proterozoic vgp from an oriented drillcore into the precambrian basement of southern alberta summary we report paleomagnetic results for 25 samples from a fully oriented pancanadian petroleum drillcore which penetrated the precambrian basement of southern alberta at a depth of 2550 m samples were taken at approximately 1 m intervals over a 7 m section of core intensities of natural remanent magnetization nrm were 0 5 14 a m over the section which consists mainly of granodiorite with one short granite interval x ray and curie temperature data indicated magnetite as the only important nrm carrier most of the nrm was an intense steeply inclined drilling remanence with coercivities 20 mt and distributed low to intermediate unblocking temperatures after magnetic cleaning a weak but stable primary nrm with high coercivities and high unblocking temperatures was isolated with d 157 i 52 k 118 mg5 3 n 20 the corresponding virtual geomagnetic pole vgp is 4 s 93 w dp 3 dm 4 which falls on the north american apparent polar wander path around 1850 ma the basement of this part of alberta is probably an extension of the churchill structural province of the laurentian shield

thermochronological constraints on the timing and magnitude of miocene and pliocene extension in the central wassuk range western nevada apatite fission track and u th he thermochronological data provide new constraints on the timing of faulting and exhumation of the wassuk range western nevada where east dipping normal faults have accommodated large magnitude ene wsw oriented extension extensional deformation has resulted in the exhumation of structurally coherent fault blocks that expose sections of preextensional mostly granitic upper crust in the grey hills and central wassuk range these fault blocks display westward tilts of 60 and expose preextensional paleodepths of up to 8 5 km based on the structural reconstruction of tilted preextensional tertiary andesite flows that unconformably overlie mesozoic basement rocks apatite fission track and u th he thermochronological data from the fault blocks constrain the onset of rapid footwall exhumation at 15 ma fission track modeling results indicate rapid fault block exhumation occurred between 15 and 12 ma which is in agreement with miocene volcanic rocks that bracket the tilting history in addition fission track and u th he data suggest reduced rates of cooling following major extension as well as renewed cooling related to active high angle faulting along the present day range front starting at 4 ma thermochronological data from structurally restored fault blocks indicate a preextensional miocene geothermal gradient of 27 5 c km the thermochronological constraints on the timing of extensional faulting and the eruptive history in the wassuk range imply a model for extension where crustal heating and volcanism precede the onset of rapid large magnitude extension and where synextensional magmatism is suppressed during the highest rates of extension

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

mantle heterogeneity and off axis volcanism on young pacific lithosphere plate tectonics and mantle plumes explain most volcanism on earth but there are numerous actively forming linear volcanic chains in the middle of tectonic plates that are not explained by these theories using the multidisciplinary geophysical dataset of the melt and glimpse experiments we show that associated with 3 volcanic chains west of the east pacific rise there are low seismic velocities and densities in the asthenosphere that extend to the east pacific rise spreading center analogous to the hawaiian swell the low density anomalies produce swells beneath the volcanoes on young seafloor the associated gravity anomalies are part of a set of gravity lineaments that have been previously interpreted as being due to thermo elastic cracking of the lithosphere or small scale convection the correlation between the surface volcanism and subsurface density and velocity anomalies and their extension to the spreading center suggest that pre existing buoyant or fertile asthenospheric mantle heterogeneities are stretched in the direction of plate motion by shear between the plate and the underlying mantle these heterogeneities seed small scale convection producing upwelling and pressure release melting forming volcanic chains that extend nearly to the ridge axis

space time patterns and tectonic controls of tertiary extension and magmatism in the great basin of the western united states structural and stratigraphic relations in the great basin indicate widespread pre middle miocene crustal extension that appears to define two north trending belts most extension in these belts was oligocene age but locally it began earlier or lasted into early miocene time the eastern belt straddles the nevada utah border and includes the snake range nevada area with its southern end near 37 5 n and its western edge at the seaman butte mountains breakaway the southern boundary of the eastern belt is occupied by the 26 15 ma caliente caldera complex and approximately coincides with the present east west trending margin of the great basin north of saint george utah crust north of this boundary extended approximately east west before volcanism began at 30 32 ma but to the south extension began after about 15 ma this boundary may have been a rooted zone of left slip faults that allowed the footwall of the stampede detachment to move west relative to unextended terrain to the south the eastern margin of the eastern belt is probably located near the present eastern edge of the great basin but its northern end is poorly defined the western belt runs from the funeral and grapevine mountains california to the ruby mountains nevada and north northeast to the albion range idaho tens of kilometers of crustal extension occurred at least locally in both belts but magnitude of extension is poorly known for large areas of each tertiary volcanism in the great basin began in the north in eocene time with predominantly effusive volcanism and swept southward ending with voluminous oligocene miocene ignimbrite eruptions from calderas in an irregular discontinuous belt between marysvale utah and reno nevada a result of the south ward migration of volcanism is that the onset of extension in both belts was syn or post volcanic in the north but was pre volcanic in the south late paleogene extension and crustal magmatism coincided in both time and space only locally where south migrating magmatism overlapped active north south trending extensional belts most calderas in the southern great basin formed in previously extended belts or on their margins southward migration of ignimbrite sources was apparently blocked by unextended crust to the south in contrast volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to or during crustal extension this is consistent with observations in the southern basin and range where volcanism and crustal extension were generally synchronous and volcanism was dominantly effusive thus caldera formation may be controlled by the distribution of upper crustal extension although the physical mechanism for this control remains speculative the space time patterns of late paleogene extension in the great basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin but being driven by gravitational collapse of thick crust space time patterns of tertiary volcanism in the basin and range also appear to conform to patterns of thermal weakening or destruction of the subducted slab both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts because both predict close spatial and temporal association of extension and magmatism which is not generally observed

buoyant melting instabilities beneath extending lithosphere 2 linear analysis 1 in a companion paper numerical models reveal that buoyant melting instabilities can occur beneath extending lithosphere for a sufficiently small mantle viscosity extension rate and rate of melt percolation however in some cases instabilities do not develop during extension but only occur after extension slows or stops these results are suggestive of a critical behavior in the onset of these kinds of instabilities and motivate a linear analysis to study the onset of instability in a partially melting passively upwelling plane layer of mantle beneath extending lithosphere the model we employ includes the effects of buoyancy arising from thermal expansion the presence of a retained fraction of partial melt and depletion of the solid by melt extraction we find a critical behavior in the onset of instability controlled by melt retention buoyancy that is characterized by a rayleigh number m such that m must exceed some critical value mcrit which depends on the efficiency of stokes rise of a partially molten body relative to the rate of background percolation comparison of this theory to the numerical results in the companion paper yields a close quantitative agreement we also find that solid depletion buoyancy can either stabilize or destabilize a partially melting layer depending upon both the distribution of preexisting depletion and the magnitude of density changes with depth this theory is compared with previous studies of buoyant melting instabilities beneath mid ocean ridges where similar behavior was reported and it suggests that the stability of passively upwelling partially melting mantle underlying both narrow and wide rift settings is controlled by similar processes

u pb zircon ages from the lynn lake and rusty lake metavolcanic belts manitoba two ages of proterozoic magmatism two ages of magmatism have been determined from zircon in felsic flows and plutons in the churchill province of manitoba a rhyolite flow from the lynn lake metavolcanic belt has a u pb age of and a rhyolite flow from the adjacent rusty lake metavolcanic belt has an age of tonalite and quartz diorite from two composite plutons emplaced into the volcanic rocks at lynn lake have ages of and indistinguishable from the age of the rusty lake belt rhyolite the arcuate domain of metavolcanic rocks that includes the rusty lake belt in the southeast the lynn lake belt in the north and the la ronge belt saskatchewan in the southwest has previously been considered a single structural sub province with similar ages throughout our results and published u pb ages from saskatchewan indicate that an older magmatism is represented by volcanic rocks in the lynn lake belt a younger magmatism by volcanic rocks in the rusty lake and la ronge belts and plutons in the lynn lake belt at lynn lake the older magmati

sublithospheric small scale convection a mechanism for collision zone magmatism we studied the effect of increased water content on the dynamics of the lithosphere asthenosphere boundary in a postsubduction setting results from numerical mantle convection models show that the resultant decrease in mantle viscosity and the peridotite solidus produce small scale convection at the lithosphere asthenosphere boundary and magmatism that follows the spatially and temporally scattered style and volumes typical for collision magmatism such as the late cenozoic volcanism of the turkish iranian plateau an inherent feature in small scale convection is its chaotic nature that can lead to temporally isolated volcanic centers tens of millions of years after initial continental collision without evident tectonic cause we also conclude that water input into the upper mantle during and after subduction under the circum mediterranean area and the tibetan plateau can account for the observed magmatism in these areas only fractions 200 600 ppm of the water input need to be retained after subduction to induce small scale convection and magmatism on the scale of those observed from the turkish iranian plateau

three dimensional flow in the subslab mantle three dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench parallel subslab seismic anisotropy subslab flow becomes inherently three dimensional when slab driven flow interacts with a wide variety of ambient background mantle flow conditions this interaction depends on slab geometries mechanical coupling parameters and lower mantle viscosities deflection of subslab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient background mantle flow background mantle flow can become trench perpendicular or trench parallel subslab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or toward the edge of the slab the first case is especially prominent in models with short slabs that do not penetrate into the lower mantle the second case is especially prominent in models with long steep slabs the results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it high levels of coupling create a boundary layer of trench perpendicular entrained flow pushing the deflection due to the obstructing slab away from the slab we compare our subslab flow model predictions with a global set of seismic anisotropy fast directions in the subslab mantle and find generally good agreement between the anisotropy observations dominantly trench parallel or trench perpendicular and the mantle flow directions predicted for decoupled systems

does partial melting explain geophysical anomalies abstract the existence of partial melt is frequently invoked to explain geophysical anomalies such as low seismic wave velocity and high electrical conductivity i review various experimental and theoretical studies to evaluate the plausibility of this explanation in order for a partial melt model to work not only the presence of melt but also the presence of appropriate amount of melt needs to be explained using the mineral physics observations on the influence of melt on physical properties and the physics and chemistry of melt generation and transport i conclude that partial melt model for the asthenosphere with homogeneous melt distribution does not work one needs to invoke inhomogeneous distribution of melt if one wishes to explain observed geophysical anomalies by partial melting however most of models with inhomogeneous melt distribution are either inconsistent with some geophysical observations or the assumed structures are geodynamically unstable and or implausible therefore partial melt models for the geophysical anomalies of the asthenosphere are unlikely to be valid and some solid state mechanisms must be invoked the situation is different in the deep upper mantle where melt could completely wet grain boundaries and continuous production of melt is likely by dehydration melting at around 410 km in the ultralow velocity zone in the d layer where continuous production of melt is unlikely easy separation of melt from solid precludes the partial melt model for low velocities and high electrical conductivity unless the melt density is extremely close to the density of co existing solid minerals or if there is a strong convective current to support the topography of the ulvz region compositional variation such as fe enrichment is an alternative cause for the anomalies in the d layer

structural evolution of basement gneisses and hadrynian cover bulldog creek area rocky mountains british columbia two gneiss bodies are contained in thrust sheets on the west edge of the rocky mountain main ranges near valemount british columbia the bulldog gneiss comprises aphebian or older paragneiss and amphibolitic gneiss intruded by aphebian orthogneiss sheets the yellowjacket gneiss is granodioritic orthogneiss of unknown age both gneiss bodies are basement highs with thin hadrynian metasediment cover sequences the cover sequences are assigned to the lower miette group and are correlated with horsethief creek group internal shortening of gneiss thrust sheets is expressed by recumbent folding and stacking of thin thrust sheets of gneiss and cover the bulldog gneiss and its cover were carried on the postmetamorphic purcell thrust the yellowjacket gneiss and its cover were carried on the pre to synmetamorphic bear foot thrust northeast and northwest displacement is documented on the moderately southwest dipping bear foot thrust and a dextral oblique slip thrust model is proposed to explain the duality

post laramide removal of the farallon slab western united states i propose that post laramide removal of the subhorizontally subducting farallon slab occurred by buckling downward along an approximately east northeast trending axis this process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the united states and propagation of the separated edges toward the central axis of downwelling accompanied by aesthenosphic upwelling behind the trailing edges initial buckling probably began near 50 ma and slab removal was complete by 20 ma this model is based primarily on the space time evolution of the ignimbrite flare up a major mid tertiary igneous event of mantle origin which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central nevada from the north and southeast post laramide uplift extension establishment of the cascadia subduction zone and active magmatism may be consequences of lithosphere scale modifications caused by the laramide removal of the slab and the resulting asthenospheric upwelling

tectonothermics of the north american great plains basement abstract heat flow and heat generation data from the superior churchill and wyoming provinces of western north america contribute to our understanding of the nature of the crust beneath the sedimentary cover of the great plains a major structural boundary defined by gravity gradient data thomas et al 1987 splits the terrains between the cratons into two distinct geothermal provinces the wyoming and churchill provinces are characterised by higher heat flow than the superior province and the eastern part of the intervening hudsonian terrain the proterozoic mobile belt pmb heat flow in the pmb is typical of that of the superior craton this suggests on the basis of a model by ballard and pollack 1987 that the pmb is underlain by archaean cratonic lithosphere the similarity in heat flow between the pmb and the superior craton suggests that the crust of the pmb is akin to that of the superior craton rather than to that of the more radiogenic wyoming and churchill cratons geochronologic gravity aeromagnetic and geothermal data suggest that the structural boundary defining the western boundary of the pmb is also the eastern boundary of the wyoming craton geothermal data alone suggest that the wyoming craton might extend as far north as the cratonic segment of the churchill province or that the wyoming and churchill provinces may gradually merge together

the depositional evolution of the svecofennian supracrustal sequence in finland and sweden abstract on the basis of six new zircon ages from metavolcanic rocks together with other isotopic geophysical and geological data the chronological and tectono stratigraphic evolution of the early proterozoic svecofennian supracrustal rocks in finland and sweden are discussed these data suggest that a nw se displacement from the bothnian bay to lake ladoga and a sinking of the southwestern block was initiated 1950 2000 ma ago accumulation of pelitic sediments and greywackes on the subsiding archaean basement continued until about 1860 ma in south central and northern sweden shallow submarine and subaerial volcanics were extruded 1880 1890 ma ago the thick sedimentary piles and volcanic rocks were subsequently intruded by plutonic rocks forming large batholiths 1860 1890 ma ago

metamorphism of high grade pelites in the brabant lake area eastern la ronge domain northern saskatchewan the brabant lake area mostly comprises early proterozoic clastic metasediments of the maclean lake belt eastern la ronge domain reindeer zone the belt underwent upper amphibolite facies metamorp

formation of 1 9 ga old trans hudson continental crust pb isotopic data pb isotopic data are reported for whole rock samples and feldspars from two parts of the 1 9 1 8 ga old trans hudson orogen in canada the reindeer zone in saskatchewan and the ottawa and belcher islands in hudson bay the first area contains a variety of volcanic granitoid and sedimentary rocks in several oceanic and magmatic arcs aligned parallel to the margin of the archean hearne craton the second is part of the circum superior belt a belt of oceanic basalts and sedimentary rocks that surrounds the superior craton in several regions pb pb isochrons gave ages generally consistent with available geochronological or geological data flin flon 1822 44 ma 2 peter lake complex 2710 220 ma ottawa islands 1790 80 ma belcher islands 1960 160 ma in other regions the pb pb age was too low reflecting disturbance of the pb system e g la ronge belt 1730 115 ma or too high perhaps because of contamination of certain samples with older crust e g wathaman batholith 2200 200 ma model values and th u ratios calculated from the pb data are interpreted in terms of variations in the compositions of mantle sources complicated by contamination with material from the upper or lower continental crust in the southern part of the reindeer zone and in volcanic rocks from the ottawa islands model values vary from 8 0 to 8 1 and correlate inversely with initial nd values this variation is thought to result from contamination of juvenile material 8 0 with material from archean upper crust which had low nd but relatively high in the northern part of the zone and in eskimo formation basalts from the belcher islands values remain constant or decrease as nd declines and this is attributed to contamination with u depleted lower crust calculated th u ratios vary from 1 34 to 6 67 the lowest values 1 4 1 8 come from mafic to felsic volcanic rocks of the flin flon and western la ronge belts these low th u values are like those in modern island arc volcanics and are attributed to enrichment of u in the source slightly higher values are calculated for oceanic basalts from the circum superior belt morb like basalts from the ottawa islands have a low th u of 2 5 reflecting depletion of th along with other incompatible elements higher values of 3 9 characterize the more enriched flaherty basalts the highest th u values which are found in samples with the lowest nd and low values wathaman batholith and eskimo basalts are thought to reflect the high th u of the u depleted lower crustal contaminant these results have several interesting implications the similarity between the isotopic and chemical characteristics inferred for these 1 9 ga old rocks and those of volcanics in modern convergent margin settings provides further indication that the tectonic and petrogenetic processes peculiar to modern subduction environments operated in the middle proterozoic the apparent presence of material from the lower and upper crust in the sources of the granitoids indicates that these rocks were not derived from subducting oceanic plate or the mantle wedge but probably formed and evolved within the crust itself

geochronology at the namew lake ni cu orebody flin flon area manitoba canada thermal history of a metamorphic terrane intermediate composition orthogneisses of the southward extension of the flin flon greenstone belt surround the ultramafic host rock of the namew lake ni cu deposit in northern manitoba the gneiss

spreading rate dependence of three dimensional structure in oceanic spreading centres segmentation by transform faults and other types of along axis discontinuity1 is a well described but poorly explained characteristic of oceanic spreading centres here we use numerical experiments to explore the dynamics of mantle flow and melting beneath a mid ocean ridge buoyant upwelling driven by compositional density variations resulting from the extraction of the melt that forms the ocean crust exhibits a spreading rate dependent transition between two dimensional and three dimensional upwelling structures for low spreading rates and mantle viscosities an initial two dimensional structure transforms into a three dimensional one at high spreading rates an initially two dimensional structure remains two dimensional these results suggest that the origin of spreading centre segmentation may be different at fast and slow spreading rates

thermal structure beneath the snake river plain implications for the yellowstone hotspot basaltic magmatism associated with the yellowstone hotspot has been widely attributed to upwelling of a mantle plume yet the temporal and spatial distribution of these magmas and their compositional characteristics are distinctive from oceanic hotspot magmatism fundamental questions concern the influence of continental cratonic lithosphere in producing the differences and the extent to which upper plate processes contribute to magma production to better understand scenarios of melt generation p t conditions are estimated for segregation of primitive snake river plain srp basalts from the mantle combined with analysis of trace element and seismic constraints we conclude from this that 1 melt production was concentrated at depths between roughly 70 100 km 2 mantle temperature was only slightly higher than ambient conditions with a maximum potential temperature of 1450 c and 3 the mantle source was relatively fertile mg 90 these results suggest that the seismically imaged plume below yellowstone is significantly cooler than upwellings beneath hawaii iceland and many other oceanic hotspots our findings in combination with other geochemical and geodynamic considerations are permissive of magma generation within the ancient lithospheric mantle keel associated with the wyoming craton plume contributions while not excluded involve physical and geochemical implications that suggest they are subordinate

u pb geochronology in the trans hudson orogen northern saskatchewan canada we have obtained u pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern trans hudson orogen in northern saskatchewan these data constrain the timing of early proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the trans hudson orogen to other early proterozoic orogens in north america with one exception all units studied so far yield zircon ages of 1890 1835 ma most of which are systematically earlier than previously reported rb sr isochron ages on the same or similar units suggesting open system behavior in the rb sr systems five metarhyolites from volcanic sequences in the la ronge domain glennie domain and hanson lake block give ages ranging from 1888 to 1876 ma most of the plutons we dated ranging from gneissic syntectonic tonalites and granodiorites to less deformed late intrusions such as the wathaman batholith and other smaller bodies yield ages of 18