automatic conversion of magnetic data to depth dip and susceptibility contrast using the spi tm method the source parameter imaging spi method computes source parameters from gridded magnetic data the method assumes either a 2 d sloping contact or a 2 d dipping thin sheet model and is based on the complex analytic signal solution grids show the edge locations depths dips and susceptibility contrasts the estimate of the depth is independent of the magnetic inclination declination dip strike and any remanent magnetization however the dip and the susceptibility estimates do assume that there is no remanent magnetization image processing of the source parameter grids enhances detail and provides maps that facilitate interpretation by nonspecialists the spi method tests successfully on synthetic profile and gridded data spi maps derived from aeromagnetic data acquired over the peace river arch area of northwestern canada correlate well with known basement structure and furthermore show that the ksituan magmatic arc can be divided into several susceptibility subdomains

convection within the inner core and thermal implications estimation of the rayleigh numbers of the inner core leads to the conclusion that even with low internal heating rates this region of the earth is convecting this work presents an axisymmetrical model able to investigate convection in a sphere and its applications to the earth s inner core rayleigh numbers ranging from the critical onset of the convection to ten thousand times the critical have been explored the thermal implications of a convecting inner core have been pointed out in all the cases the temperature profiles are very close to the adiabat leading to the possibility of a partially molten regions in the inner core indeed the most important effects appear in the first tens of kilometers just below the inner core surface and could be at the origin of the low quality factors q and q revealed by seismological studies

heat producing elements and the thermal and baric patterns of metamorphic belts the character of sedimentary basins before they are deformed and metamorphosed may strongly influence the thermal and baric patterns of metamorphic belts crustal thickening of anoxic sedimentary basins and subsequent thermal reequilibration may produce large areas of high grade metamorphic rocks and granites because the concentrations of the heat producing elements are high in such basins in new england there is a spatial association among granites and high grade metasedimentary rocks rich in u and th that now form the central maine terrane the high content of heat producing elements in these rocks is attributed to fixing of u and th in highly reduced sediments that were deposited in an anoxic basin that formed in the silurian when the basin was thickened during the devonian acadian orogeny the thermal energy generated by the u and th rich sediments produced the observed broad zone of high grade rocks and anatectic granites this hypothesis was tested with thermal calculations that reproduce most of the first order thermal and baric patterns of the acadian appalachians if pretectonic lateral variations in heat production are assumed

late archean quetico accretionary complex superior province canada the 1200 km long quetico subprovince consists of monotonous metagraywacke with derived migmatite and granite in thrust and or transcurrent fault contact with the adjacent wabigoon and wawa metavolcanic subprovinces imbrication of sedimentary wedges derived from volcanic arcs to the north and south produced a 10 100 km wide stratigraphically north facing pile thermal relaxation in the 20 m y following accretion of the southern arc resulted in melting of the accretionary pile to produce peraluminous granite and associated low pressure metamorphism

role of plutonism in low pressure metamorphic belt formation wickham and oxburgh1 recently proposed that low pressure high temperature low p high t metamorphism in the eastern pyrenees and possibly all low p high t metamorphic belts resulted from anomalously high mantle heat flow brought about by rifting their model is largely constrained by the presence of nearby synmetamorphic rift related sedimentary rocks and the interpretation that the migmatites and granites are the product of in situ melting in the presence of an anomalously steep geotherm here we present an alternative model in which low p high t metamorphism pro grade reactions at pressures near or below the al2sio5 triple point results from contact effects near sill like igneous intrusions at intermediate crustal levels low p high t conditions can be achieved through this process in regions of continent continent collision with normal mantle heat flux as well as in zones of extension our model is based on studies of the low p high t metamorphic terrane in the new england appalachians

probing two low velocity regions with pkp b caustic amplitudes and scattering summary we probe low velocity regions of the lower mantle with pkp waves using south pacific earthquakes recorded at stations from the temporary ethiopia afar geoscientific lithospheric experiment array in ethiopia and the german regional seismic network grsn germany the ethiopian stations show strong scattering of pkpab waves which arrive as precursors to pkpdf they also show anomalously large amplitude diffractions from the pkp b caustic in contrast the events recorded at the grsn show scattering of pkpab as precursors to pkpdf but more normal amplitudes in the pkp b caustic diffractions to understand the cause for the observed amplitudes we test a large set of 1 d models with anomalies in p and s wave velocities and density in the lowermost mantle and outer core structures that can cause the observed large pkp b caustic amplitudes are low velocity regions just above the core mantle boundary of the order of 50 km thick for the south pacific to ethiopia path with velocity reductions of around 5 per cent for p and 25 per cent for the s wave velocity alternatively a 30 km thick lamella starting at 50 km above the core mantle boundary with 10 per cent p wave velocity reduction and 25 per cent s wave velocity reduction also produces a good fit to the data we find that the low velocity regions act as waveguides enhancing the amplitude of the pkp b caustic diffracted waves and causing superposition of pkp b diffractions with pkpdf the low velocity regions cannot be smooth layers but must show a certain degree of small scale heterogeneity to produce the high frequency scattering observed in the data the path to germany needs small scatterers in the lowermost mantle but the low velocity region is less strong compared with the south pacific ethiopia path a plausible explanation for these low velocity regions is the presence of melt inclusions perhaps entrained in small scale convective systems that act as high frequency wavefield scatterers in regions where these inclusions or convective systems are abundant they appear as a large scale low velocity zone to lower frequency waves these observations support ideas that ultra low velocity zones at the base of the mantle may be heterogeneous regions of melt rich material

structure of the earth s inner core jacobs1 proposed that the earth s inner core is growing through the freezing of outer core material as the earth gradually cools2 recent studies have shown that compositional effects associated with this freezing process can release energy at a rate sufficient to power the geodynamo3 4 and may be crucial in determining the dynamic state of the outer core5 here we investigate the effects of composition on the freezing process itself drawing on metallurgical experience and speculate on the structure and state of the inner core we propose that the interface separating the inner and outer core is dendritic and argue that the region in which freezing takes place may extend throughout the entire inner core consequently the compositionally driven convective motions which stir the outer core and sustain the geodynamo also occur within the interdendritic spaces of the inner core the seismic evidence which corroborates this proposal is briefly reviewed

a regional perspective of the quetico metasedimentary belt superior province canada alternating greenstone granite and metasedimentary gneiss belts are a first order tectonic feature of the southern superior province the tectonic development of the quetico metasedimentary belt is reviewed with regard to depositional structural and metamorphic plutonic history over its 1200 km length the belt consists of marginal metasedimentary schists of turbiditic origin and interior metasedimentary migmatite and peraluminous leucogranite polyphase deformation has resulted in a steep easterly striking foliation and regional gently east plunging stretching lineation metamorphic grade varies in a low p facies series from greenschist at the belt margins to upper amphibolite and local granulite in the central migmatite intrusive granite zone mineral assemblages in the central zone yield estimates of metamorphic pressure that increase systematically eastward over 800 km from about 250 mpa 2 5 kbar near the canada united states border to 600 mpa 6 kbar in granulites adjacent to the kapuskasi

contribution of mantle plumes crustal thickening and greenstone blanketing to the 2 75 2 65ga global crisis abstract assuming that the period 2 75 2 65 ga corresponds to a single but global geodynamic event we investigate through numerical experiments the mechanisms that could have led to the profound continental reworking that occurred at that time although the extent of the crisis at the earth s surface pledges in favour of the involvement of mantle plumes our numerical experiments suggest that the thermal impact of mantle plumes is unlikely to explain both the amplitude and timing of the thermal anomaly as observed in the superior province canada and the yilgarn craton australia similarly moderate crustal thickening can not lead to significant reworking of the continental crust within the observed time constraint crustal thickening with a factor 1 5 is also unlikely because it is not consistent with the moderate metamorphic grade observed at the surface of many archaean cratons burial of a radiogenic crust under a 10 km thick greenstone cover also falls short of explaining not so much the amplitude and the extent but the timing of the thermal anomaly in contrast the combination of the thermal anomaly related to the greenstone blanketing effect with the heat transfer from a plume head spreading at the top of the thermal boundary layer can adequately explain the amplitude the timing and the extent of the 2 75 2 65 ga crisis our favoured model involves a global rearrangement of convection cells in the deep mantle and formation of multiple mantle plumes the greenstones emplaced at the surface and the plumes that spread in the thermal boundary layer contributed to heat the crust from both above and below this produced massive crustal partial melting that reached its climax ca 40 myr after the emplacement of the plumes and associated greenstone cover rocks this led to gravitational instabilities in the crust as dense greenstone cover rocks began to sink into the thermally softened crust and granite domes rose in response the extraction of heat producing elements toward the upper part of the crust has contributed to the cooling and stabilisation of the cratons this succession of events which is not incompatible with plate tectonic processes may have profoundly changed the nature of the crust exposed at the surface and could explain the contrasting geochemical signatures of archaean and post archaean shales

overview of regional geophysical studies in manitoba and northwestern ontario this paper presents an overview of six geophysical projects seismic reflection and refraction gravity and magnetic anomaly interpretation specific gravity and magnetic property measurements car

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

observations of pkp df and pkp bc across the united kingdom implications for studies of attenuation in the earth s core summary we demonstrate that pkp df and pkp bc recorded by a network of broad band stations in the united kingdom from earthquakes in the southwest pacific show considerable spatial variation in amplitude and frequency content we suggest that strong heterogeneity in apparent attenuation at the base of the mantle related to ultra low velocity zones ulvz beneath the southwest pacific and or possibly to the east of iceland may explain the anomalous observations of df and bc across the uk the length scale of attenuating ulvzs required to explain the observations is similar to that estimated recently for a southwest pacific ulvz by wen helmberger 1998 that is low wave speed low apparent q regions of 80 km height separated by steep sided valleys of width 40 km with typical lowermost mantle wave speeds high apparent q if the apparent attenuation is due to anelasticity alone disregarding the possible contribution from scattering the minimum ulvz q values required to explain the amplitude of 2 hz bc are about 15 30 consistent with the interpretation of the ulvzs as zones of partial melt at the base of the mantle this suggests that estimates of inner core anelastic attenuation inferred from df bc spectral ratios will be biased if df and or bc cross ulvzs at the base of the mantle under the source and or receiver further the presence of attenuating ulvzs may explain the large scatter in estimates of inner core q from global studies

co 2 melts and granulite metamorphism the discovery that granulites contain co2 rich fluid inclusions while amphibolites in the same terrane contain h2o rich or mixed co2 h2o inclusions1 has led to a debate over the processes that produce granulite metamorphism one theory2 4 maintains that granulites form as the result of a massive influx of co2 from lower crustal or mantle sources such an influx could be responsible for the marked depletion many granulite terranes show in large ion lithophile elements2 5 others however maintain that granulite facies metamorphism is a necessary complement of crustal melt ing6 9 recently lamb and valley10 11 have argued that granulite metamorphism may be a product of any one of three processes co2 streaming partial melting or recrystallization of originally dry rocks here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust

high geothermal gradient metamorphism during thermal subsidence the burial of a basement sequence enriched in heat producing elements during thermal subsidence following rifting produces two concomitant changes in the thermal structure of the crust firstly the burial of the enriched layer produces high geothermal gradients in the overlying sedimentary succession with the high gradients propagating down into but not through the enriched basement sequence secondly the lithospheric thickening that drives thermal subsidence reduces the heat flowing into the deeper crust from the mantle because the process of thermal subsidence promotes burial it naturally increases the depth extent of the high geothermal gradients in the upper crust potentially inducing significant temperature increases in the mid upper crust during burial the lowering of the thermal gradients in the deep crust accompanying burial severely limits the temperature changes affecting the moho potentially allowing moho cooling while the mid upper crust heats these effects can promote high geothermal gradient 40 c km metamorphism in the mid upper crust without inducing significant melting in the lower crust providing the basement heat production contributes 70 mw m 2 to the surface heat flow and that the horizontal length scale for the basement heat production anomaly is 50 km these conditions appear to be met in several australian intermediate to high temperature low pressure metamorphic terranes where the thermal causes of metamorphism have hitherto remained enigmatic one of these terrains the mt painter province in the northern flinders ranges south australia is used to illustrate some of the attributes of the model

neoarchean evolution of differing convergent margin assemblages in the wabigoon subprovince geochemical and geochronological evidence from the lake of the woods greenstone belt superior province northwestern ontario abstract the neoarchean lake of the woods greenstone belt is subdivided into three lithostratigraphic assemblages a lowermost mafic volcanic assemblage a compositionally diverse predominantly volcanic middle assemblage and a predominantly sedimentary uppermost assemblage the secular evolution and geodynamic settings of these assemblages are constrained with detailed geochronological and geochemical data the lower keewatin is the oldest assemblage with a zircon u pb age of 2738 2 0 ma it consists of mg and fe tholeiitic basalts mg number 30 65 with flat to slightly enriched rare earth elements ree la yb cn 0 9 1 6 elevated large ion lithophile elements lile and undepleted high field strength elements hfse the tholeiites do not appear to have exact modern analogs but do have some similarities to primitive arc tholeiites and back arc basin basalts suggesting an archean destructive margin setting the upper keewatin assemblage disconformably overlies the lower keewatin assemblage it consists of volcanic and sedimentary groups two geochemically distinct volcanic suites are recognized the dominant suite consists of compositionally diverse calc alkalins basalt to rhyolite felsic end members have yielded zircon u pb ages ranging from 2712 to 2723 ma the suite exhibits highly fractionated ree la yb cn 4 35 elevated lile and nb and ta depletion anomalies which are very similar to modern island arc volcanic rocks developed in proximity to continental margins a coeval suite of mafic to ultramafic volcanic rocks consists of mg tholeiite komatiitic basalt and ultramafic komatiite mg numbers 45 80 the suite exhibits depleted lree la yb cn 0 6 1 0 slightly elevated lile and undepleted to slightly enriched hfse suggesting that this geochemically primitive suite was produced by partial melting of a refractory mantle source perhaps as second stage melts of the same mantle source from which the lower keewatin assemblage basalts were previously derived the electrum assemblage unconformably overlies the upper keewatin assemblage in the northern part of the belt it consists of conglomerates sandstones and minor volcanic rocks having detrital ages indicating deposition after 2699 ma the volcanic rocks consists of andesite to rhyolite with shoshonitic to calc alkaline geochemical affinity as indicated by highly fractionated ree la yb cn 12 112 highly enriched lile and pronounced nb ta and ti depletion anomalies samples from the syenitic falcon island pluton appear to be coeval with electrum assemblage volcanics having an u pb isotopic age of 2694 ma and similar but even more alkaline chemistry consisting of highly fractionated lree and lile trace elements with very pronounced nb ta and ti depletion anomalies these late tectonic suites are geochemically similar to modern shoshonites with a continental influence formed in the late stages of a mature island arc or a continental arc setting

stratigraphy of the archean western superior province from p and s wave receiver functions further evidence for tectonic accretion the archean western superior province in canada represents the nucleus of the north american continent whose origin has been speculated to be the result of widespread crustal accretion some 2 7 ga ago in this paper crustal and upper mantle seismic discontinuities beneath the western superior province of the canadian shield are imaged with teleseismic p to s and s to p converted phases using the receiver function method three crustal discontinuities are observed the moho ranging in depth between 38 and 47 km and dipping to the south and two intra crustal discontinuities having depths of approximately 15 and 30 km the crustal discontinuities undulate laterally and often lose continuity possibly indicating an imbricated structure and or regions of velocity gradients in the shallow lithosphere a positive discontinuity is imaged at approximately 65 km depth and is consistent with earlier refraction and wide angle reflection results additionally two zones of negative receiver function amplitudes at 55 km depth are observed and are coincident with a region of anomalous tomographic low p and s wave velocities as well as a zone of high electrical conductivity the images for the crust and shallow upper mantle when integrated with previous geophysical studies are consistent with ideas of continental root formation due to imbrication of archean subducted material and accretion of island arcs observed in surface geology

seismic evidence for preservation of the archean uchi granite greenstone belt by crustal scale extension abstract in the westernmost superior province of canada the east west alignment of granite greenstone belts and the adjacent highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2 5 ga ago with the belts thrust against one another by east west oriented subduction zones here we present seismic reflection data which demonstrate that in this region the present juxtaposition of the uchi granite greenstone belt and the north caribou gneiss terrane occurred along a late southeast dipping extensional shear zone that extends from the surface into the lower crust the preservation of the uchi belt and probably the english river metasedimentary belt is directly related to their dropping along extensional shear zones which limited subsequent erosion the relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust nor preserved by vertical crustal movements as might occur in the absence of plate tectonics extension may have been associated with the emplacement of mantle derived magmas at 2700 ma which has been linked to slab break off or lithospheric delamination making the extension approximately coeval with local gold mineralisation since crustal scale faults can facilitate the circulation of gold bearing fluids we suggest that greenstone rocks preserved in the hanging walls of syn to post accretion extensional shear zones may preferentially host archean lode gold deposits in the westernmost superior province our seismic observations imply that some of the late structures in the well developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes

seismic anisotropy of the earth s inner core resulting from flow induced by maxwell stresses seismological observations indicate that the inner core of the earth is elastically anisotropic1 anisotropic structures are likely to be formed by dynamic processes and therefore such observations have the potential to provide constraints on flow in the inner core and on the geodynamo itself but in addition to the difficulties in estimating the relevant physical properties of iron under inner core conditions2 3 4 even the macroscopic processes responsible for generating seismic anisotropy in this region have yet to be determined5 6 7 8 9 as a result the geodynamic significance of seismic anisotropy in the inner core has remained unknown here i propose based on geodynamic and mineral physics considerations that flow induced by the stress due to the magnetic field the maxwell stress near the inner core boundary produces an axisymmetric fabric responsible for the observed seismic anisotropy the resultant seismic anisotropy reflects the geometry of the magnetic field near the inner core boundary and therefore seismological observations might provide constraints on the geodynamo this flow also causes non uniform release of energy at the inner core boundary associated with solidification and melting which may affect the pattern of convection in the outer core

subprovince accretion tectonics in the south central superior province development of tectonic subprovinces as shear bounded granite greenstone and sediment dominated terranes during the late archaean is reviewed and interpreted from relationships between portions of

uranium series disequilibrium in a young surficial uranium deposit northeastern washington u s a a recently discovered ore grade accumulation of u in organic rich sediments of late quaternary age provides an opportunity for studying the early association of u u daughters and organic matter in a natural setting the u occurs in valley fill sediments of peat peaty clay silt and sand along the north fork of flodelle creek stevens county washington radiometric techniques delayed neutron high resolution gamma ray spectrometry thin source alpha spectrometry were employed to determine the abundance and distribution of u series nuclides the extent of secular equilibrium within the u decay series and the apparent u series ages of u incorporation sixteen lithologically distinct intervals were sampled from a 292 cm core uranium contents range from 140 to 2790 ppm and are positively correlated with organic contents measured alpha activity ratios of 234u 238u 1 31 1 38 are very similar to those reported in coexisting waters suggesting a rather constant isotopic composition of introduced u much lower th contents of 10 40 ppm are controlled by the type and abundance of silicate detritus the youth of the host sediments 15 000 a and the paucity of associated radioactivity suggested large excesses of u relative to radioactive daughters and such excesses were observed particularly in the shallowest intervals apparent ages of u emplacement determined by the alpha activity ratio of 230th daughter to 234u parent show a general increase with depth and fair agreement with estimated depositional ages this observation suggests dominantly syndepositional or early post depositional emplacement of u followed by decay generated buildup of 230th daughter with time however interval by interval comparisons of the relative abundances of other daughters particularly 226ra and 210pb indicate variability caused by processes other than closed system growth and decay probably because chemically diverse daughters that are decay generated in situ have differing mobilities and because upwelling ground water continuously adds more u and minor amounts of daughters if 230th is considered the least susceptible to these modifications the data suggest some addition of 234u in the deepest intervals and some loss of 226ra and or gain of 222rn throughout the studied core

depth dependent attenuation in the uppermost inner core from the taiwan short period seismic array pkp data inner core attenuation properties from two spots were examined in detail using pkikp and pkp bc phases with a distance range of between 147 and 154 as recorded by both the short period regional seismic array in taiwan and the broadband stations of the german regional seismic network the ray paths from the six south american events to taiwan sampled the inner core region beneath the eastern part of the northern pacific ocean while those from fiji to europe reflected the inner core boundary under the western part of the northern pacific ocean results of this study show the depth dependent attenuation of qp beneath the eastern part of the northern pacific ocean from 241 21 8 at depths 270 km with a weighted mean of 305 25 4 within the frequency band 0 5 2 0hz the average qp under the western part of the northern pacific ocean is 160 33 5 0 5 1 5hz which is consistent with that previously reported it is found that qp under the western part of the northern pacific is clearly lower than that under the eastern part this discrepancy might be related to regional variations in the uppermost inner core

seismological evidence for differential rotation of the earth s inner core the travel times of seismic waves that traverse the earth s inner core show a small but systematic variation over the past three decades this variation is best explained by a rotation of the inner core that moves the symmetry axis of its known seismic anisotropy the inferred rotation rate is on the order of 1 per year faster than the daily rotation of the mantle and crust

seismic properties of rocks with distributions of small cracks summary hudson s theory of multiple scattering for the dynamic properties of cracked rock is applied to media containing cracks of varying orientation radius and aspect ratio angular variation of velocity and attenuation due to parallel and nearly parallel cracks and cracks with normals at a fixed angle to a given axis are presented anisotropy gradually decreases as the distribution of orientations is widened from exactly parallel to random increasing the aspect ratio of thin saturated cracks generally causes the elastic properties to approach those due to dry gas filled cracks for fluid filled cracks the line singularity where the birefringent shear waves have equal velocity moves closer to the symmetry axis as either the distribution of orientations is widened or the aspect ratio is increased calculated attenuations due to rayleigh scattering and viscous dissipation in crack filling fluids are many orders of magnitude less than reported attenuations in the earth viscous dissipation varies linearly with frequency and aspect ratio of thin fluid filled cracks a power law distribution of crack radii intended to represent that found in the earth s crust has no effect on the angular variations of seismic velocities or viscous attenuation but scattering attenuation is sensitive to the power law exponent lognormal distributions of aspect ratio also corresponding to observations in real rock affect velocity as well as attenuation velocities derived from the theory are compared with those measured in laboratory specimens with known geometries of cracks

garnet orthopyroxene plagioclase quartz barometry refinement and application to the english river subprovince and the minnesota river valley recent experimental theoretical and thermodynamic studies permit better calibration of two reactions for geobarometry grossular pyrope quartz anorthite enstatite grossular almandine quartz anorthite ferrosilite if both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet pyroxene plagioclase quartz assemblage application to a variety of high grade terrains generally yielded excellent results however poor results have been obtained for high fe rocks which can be traced to errors in activity models for garnet and or pyroxene either a two site ideal mixing model for orthopyroxene cf wood and banno 1973 underestimates enstatite activity for high fe orthopyroxenes or the ganguly and saxena 1984 model overestimates pyrope activity in low mg garnets

high radiogenic heat producing granites and metamorphism an example from the western mount isa inlier australia the origins of metamorphism 600 c and 3 4 kbar in the western mount isa inlier australia remain controversial for a number of reasons 1 no synmetamorphic intrusive bodies can be recognized 2 high temperatures appear to be sustained for periods 100 m y and 3 metamorphism follows an extended phase of thermal subsidence we show that the burial of granite batholiths enriched in radiogenic elements beneath the thick insulating sedimentary succession of the mount isa group deposited in response to repeated rift sag cycles was capable of generating steep upper crustal thermal gradients immediately prior to the isan orogeny these gradients are appropriate to peak metamorphic conditions such that the ensuing isan orogeny required no significant additional heat input this result is significant in that it may provide a mechanism for understanding the origins of high temperature metamorphism in other terranes where the involvement of transient heating is not obvious

differential response of u pb systems in coexisting accessory minerals winnipeg river subprovince canadian shield implications for archean crustal growth and stabilization the u pb isotopic systems of zircon monazite titanite and some apatite and the pb isotopic composition of k feldspar have been investigated in three areas of the winnipeg river subprovince wrs of the superior province canada in order to define the timing of magmatic and metamorphic processes in this archean gneissic granitoid terrain

integrated potential field and seismic constraints on the structure of the archean metasedimentary english river belt western superior craton canada the english river subprovince is a prominent belt of metasedimentary rocks in the archean western superior province the structure of its western half was investigated by using techniques of enhancement and automatic interpretation of magnetic data and integration of magnetic derived information with seismic and gravity data the results indicate that a suite of exposed felsic plutons that intruded the belt at ca 2698 ma extends under most of the metasedimentary rocks that are exposed at the surface the thickness of the metasedimentary rocks is interpreted to be less than 1 km in areas where it is underlain by the members of this intrusive suite in other areas the metasedimentary rocks attain thicknesses of 3 4 km and appear to be underlain by rocks similar to the gneissic rocks that are exposed in the adjacent metaplutonic winnipeg river subprovince the integration of enhanced magnetic data with gravity data indicates that the large gravity anomaly that extends along the english river belt correlates well spatially and morphologically with the extensive suite of felsic intrusions that underlies the belt suggesting that the crustal component of the gravity anomaly is related to this suite of intrusions we interpret the source of the gravity anomaly as a dense unit comprising anhydrous mineral assemblages that formed within these felsic intrusions in response to low pressure high temperature metamorphism that affected the belt at ca 2691 ma on the basis of geochronological geological and geophysical constraints we propose that this metamorphic episode is linked to the continuation of magmatism at depth after the emplacement of the ca 2698 ma felsic plutons being ultimately related to the advection of mantle heat into the crust during a period of regional extension 2005 elsevier b v all rights reserved

q in the inner core short period vertical seismograms from ten earthquakes and one nuclear explosion in the distance range of 140 170 degrees are examined for a study of the anelastic properties of the inner core the depth of the earthquake foci varies between 116 and 625 km the phases of interest are pkp bc branch and pkikp with bottoming depths in the upper few hundred kilometers of the inner core the data consist of high quality digital data at sampling rates 20 40 samples per second selected after an extensive search of the 1985 1986 gdsn event tapes for short period p waves with a high signal to noise ratio the results are based primarily upon observations between distances of 147 and 154

mapping heterogeneity of the uppermost inner core using two pairs of core phases two pairs of core phases pkpcd pkpdf and pkpbc pkpdf are analysed to map the p wave velocity vp of the uppermost inner core waveform analysis of pkpcd pkpdf shows that vp relative to prem vp within 100 km from the inner core boundary icb is laterally heterogeneous by 0 5 to 0 1 with a wavelength of nearly 1000 km differential travel times of pkpbc pkpdf relative to prem tbc df are positive negative for the regions of positive negative vp near the icb this correlation indicates that the lateral vp variation exists predominantly in the inner core side of the icb absolute values of tbc df for both positive and negative vp regions are too large to be explained with models in which lateral vp variation is restricted within the upper 200 km of the inner core the lateral variation appears to extend at least to 300 km below the icb

gravity driven continental overflow and archaean tectonics whether modern tectonic processes differ substantially from those in archaean times 2 500 myr ago remains controversial one view1 is that archaean tectonic processes were some combination of modern ones occurring faster or more shallowly because of the larger heat output of the early earth but others2 have proposed that significantly different processes operated here i argue that gravitational spreading of archaean continents would have caused them continuously and pervasively to overflow onto adjacent ocean basins and that this process would have naturally ceased at the end of the archaean era because modern continental crust is believed to be ductile rather than brittle below a depth corresponding to a temperature of about 350 400 c ref 3 it seems likely that such a ductile zone was universally present within the hotter archaean continental crust if the mean geothermal gradient of the continents had exceeded 25 30 c km 1 then the resulting ductile zone would have caused continental overflow to occur and such a process can account for many of the distinctive peculiarities observed in the archaean geological record the cessation of continental overflow corresponds naturally to the stabilizing cratonization which marked the end of the archaean era with its timing dependent on the evolution of both the geothermal gradient in the continents and the depth of the ocean basins

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

regionally heterogeneous uppermost inner core observed with hi net array studies of velocity structure in the earth s inner core and its volumetric variations illuminate our understanding of inner core dynamics and composition here we use an extensive number of seismograms recorded by the hi net array to construct complete empirical curves of pkpbc pkpdf differential traveltimes the nature of these curves implies that significant variations in traveltimes are accumulated during the passage of pkpdf waves through the uppermost inner core and rules out outer core structure effects uniform cylindrical anisotropy of a plausible strength in the uppermost inner core can also be ruled out as a cause of the observed traveltime variations because the range of sampled ray angles is too narrow the configuration and strength of inhomogeneities from a recent tomographic model of the lowermost mantle cannot account for the observed traveltime variations therefore we infer that either variations of p wave isotropic velocity on the scale of about hundred km and less are present in the uppermost inner core or the material may be organized in distinctive anisotropic domains and both of these features may be superimposed on long wavelength hemispherical structure if the former holds true the absolute magnitude of required p wave velocity perturbations from referent values is 0 60 0 10 in the quasi eastern and 1 55 0 15 in the quasi western hemisphere 0 85 0 05 and 1 10 0 10 respectively with the lowermost mantle correction the existence of these variations is a plausible physical outcome given that vigorous compositional convection in the outer core and variations in heat exchange across the inner core boundary may control the process of solidification

evaluation of biotite garnet geothermometers application to the english river subprovince ontario in recent years there have been numerous calibrations of the biotite garnet fe mg exchange geothermometer the eastern lac seul region of the english river subprovince ontario provides an excellent field area in which to compare these calibrations

inner core rotation rate from small scale heterogeneity and time varying travel times the time it takes seismic waves to propagate from south atlantic earthquakes through the inner core to station col in alaska has decreased systematically over the past 30 years travel times from three earthquakes in 1991 to an array of 37 seismometers in alaska suggest that lateral gradients in seismic wavespeeds are steep in this part of the inner core this combination of observations can be explained by postulating that the inner core is rotating 0 2 to 0 3 per year faster than the mantle

anisotropy in inner core attenuation a new type of data to constrain the nature of the solid core during the last decade the presence of velocity anisotropy inside the inner core with a fast axis parallel to the earth s rotation axis has been well established the quantitative analysis of the amplitudes of waves which sample a particular region of the inner core under africa but exhibit various orientations allows us to document and characterize the presence of anisotropy in attenuation the analysis is based on a comparison of both amplitudes and travel times of the pkp df wave which samples the inner core and the pkp bc wave which has nearly the same path but bottoms inside the liquid core the data reveal that the direction of strong attenuation correlates with that of fast velocity when referred to the same epicentral distance and focal depth the pkp df pkp bc amplitude ratio are about five times lower for paths tilted by 25 with respect to the earth rotation axis than for nearly equatorial paths a clear negative correlation is observed between travel time and amplitude residuals when the angle to earth rotation axis varies this first quantitative analysis combined with experimental results on oriented crystals and mushy media may bring important constraints on the mechanism responsible for anisotropy in the inner core

joint inversion for inner core rotation inner core anisotropy and mantle heterogeneity observational evidence for differential rotation of the inner core has recently been reported from time dependent travel times of the seismic inner core phase pkp df these analyses are hindered by potential biases from the mantle heterogeneity and systematic earthquake location errors which have raised doubts about the reality of the inferred differential rotation here we report time dependent pkp df pkp bc differential travel times of a total of 611 measurements obtained from 92 earthquakes in the south sandwich islands ssi over the past nearly half a century recorded at the college alaska station col and alaska seismic network asn the events are relocated using the joint hypercenter determination jhd technique dewey 1971 travel time residuals at col from ssi earthquakes over a period of 45 years and at three additional stations from ssi earthquakes over periods of about 22 years show steady increases with time the temporal trends are demonstrated to be statistically very significant with dense samples of the pathway at different time periods we use a joint inversion technique that allows us to separate the time dependent inner core structure from the time independent mantle biases surprisingly the inner core rotation seems resolvable even from digital data observed at asn in the 1990s spanning over 8 years alone in such a joint inversion the results provide strong support for a differential inner core rotation the inferred rotation rate ranges from 0 3 to 1 1 per year faster than the mantle when we take into account the uncertainty in the tilt of the anisotropy axis of the patch of the inner core that was sampled a westward inner core rotation can be ruled out

seismic structure near the inner core boundary in the south polar region 1 it is crucial to obtain good spatial coverage of seismic data points for better understanding the earth s core but the core beneath the polar regions remains largely unexplored we analyzed differential traveltimes and amplitude ratios of core phases whose raypaths run beneath antarctica for determining the vp and qp structure near the inner core boundary in the south polar region the model we obtained south polar region model spr is described relative to the preliminary reference earth model prem as follows a 0 05 km s lower vp value at the top of the inner core a 1 5 times steeper vp gradient in the upper 300 km of the inner core a smaller qp 300 in the upper 300 km of the inner core and a 0 04 km s lower vp at the bottom of the outer core the vp values of spr in the lowermost outer core lie between those of prem and ak135 being closer to those of ak135 the lowermost outer core vp inside the tangent cylinder is thus close to the global average in the upper inner core spr has lower vp than ak135 and prem the spr vp profile is close to that of previous models for the western hemisphere although most of our data sample the eastern hemisphere of the inner core our results indicate that the inner core does not have a simple hemispherical variation as usually supposed our data support an eyeball shaped high vp anomaly with compressional velocity higher than in 1 d reference earth models concentrated to a smaller region beneath eastern asia

thermal modelling in low pressure high temperature metamorphic belts abstract low pressure high temperature metamorphic belts occur throughout the world they display a characteristic metamorphic style which has remained unchanged with time they are found in disparate tectonic settings including magmatic arcs regions of extension regions of thickened crust in continent continent collision zones and accretionary wedges they are commonly characterized by an abundance of granitoid plutons pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point approximately 4 kbar while temperatures are in the range 500 750 c with rare exceptions inferred ptt paths are nearly isobaric and recrystallization was rapid and prograde metamorphic thermal gradients are commonly in the range 60 150 c km these conditions necessitate the presence of a large component of advective heat transfer and or anomalously high basal heat flow during metamorphism the large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism lpm lithospheric extension is likely to produce lpm only if the degree of thinning is large 3 and there is additional heat from magmas the generation of lpm by crustal thickening requires either a large component of advective heating by magmas anomalously high basal heat flow or pre thickening lithospheric extension lpm in this environment must necessarily be accompanied by plutonic activity advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but acting along is unlikely to produce lpm regional lpm induced by advective sources rarely occurs along a geotherm which increases monotonically with depth or synchronously across the region at a given depth it occurs over time as a series of overlapping temporally separate and spatially localized thermal events

mechanical consequences of granite emplacement during high t low p metamorphism and the origin of anticlockwise pt paths high temperature t low pressure p metamorphic belts preserve evidence of metamorphism under extremely perturbed and hence transient thermal regimes pt paths are commonly anticlockwise that is the maximumt was attained prior to or at the same time as the maximump with cooling at near constant or increasingp and in many terrains peak metamorphic temperatures prevailed during convergent deformation using a simple coupled thermal mechanical model that assumes a strongly temperature dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high t low p terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent the model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation tm crit the depth of emplacement of the magmas and most importantly the granite forming mechanism during the prograde cycle material points attain maximum temperatures during a transient 3 ma high strain rate pulse withe up to 10 14 s 1 at levels near the site of granite emplacement the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression

a seismic model of a partially molten inner core fearn et al 1981 recently proposed a mechanism whereby the inner core contains a significant fraction of liquid we construct a model of a partially molten inner core compatible with seismic observation specifically we develop an improved model of bulk anelasticity including the effect of thermal and material diffusion and show that it is quantitatively capable of explaining the observed anelasticity also studies of the reflection of seismic waves from the inner core boundary are used to estimate the mass fraction of liquid in the inner core

long lived transpression in the archean bird river greenstone belt western superior province southeastern manitoba abstract the archean bird river greenstone belt brgb is located on the southwestern edge of the superior province between the 3 2 ga old winnipeg river subprovince to the south and the metasedimentary belt of the english river subprovince ersp to the north this position between two major subprovinces makes the brgb a primary target for investigating the geodynamic and kinematic evolution of a major structural boundary new structural and geochronological data have allowed us to present an evolutionary framework for the southern boundary of the north caribou superterrane the brgb underwent 3 main deformation phases the d1 event took place ca 2698 ma and displays a north side up shearing the d2 event occurring at ca 2684 ma in a transpressive context presents a complex structural pattern mixing vertical tectonics in the brgb and strike slip tectonics along the boundaries of the greenstone belt with other subprovinces between the brgb and the ersp the 2832 2858 ma old maskwa batholith acted as a rigid passive block during the collision and marks the boundary between pure dextral strike slip tectonics along his northern boundary with the ersp and vertical south side up motion in the brgb the brgb can be considered as a pop up structure with anastomosed shear zones displaying different horizontal offset according to the orientation of the shear zones the southern boundary with the winnipeg river subprovince is represented by a sinistral south side up shear zone the same pattern is found at the regional scale where major shear zones acted as a conjugate set in the horizontal plane at ca 2640 ma the d3 event occurred in a general dextral transpressive tectonic regime coeval with the emplacement of rare elements pegmatitic plutons in a still hot 400 500 c country rock the geodynamical and mechanical significance of the partitioning between pure strike slip tectonics in the english river subprovince and vertical motion in the brgb can be explained by the rheological behaviour of a hot and weak lithosphere undergoing transpressive strain the structural framework of the brgb is the result of strong interactions between hot and weak domains coeval with widespread plutonism and a rigid older domain maskwa batholith during the d2 transpressive event

core structure re examined using new teleseismic data recorded in antarctica evidence for at most weak cylindrical seismic anisotropy in the inner core we present a significant addition to the data set of traveltimes of seismic pkp waves that sample the earth s lowermost mantle and core along the earth s rotation axis recorded at permanent global seismic network gsn and temporary sscua deployment broad band seismographic stations in antarctica the new data improve the previously poor and biased coverage that underlies the seismic constraints on recent models of inner core structure and anisotropy on the one hand new differential pkp traveltime measurements improve the sampling of predominantly the eastern inner core hemisphere pkpab df and pkpbc df differential traveltime residuals with respect to the spherically symmetric model ak135 are consistently smaller than two seconds along the north south paths sampled axially symmetric models of inner core seismic anisotropy with fast axis parallel to the earth s rotation axis require a weak anisotropy of 0 7 0 1 per cent to be consistent with our pkpbc df observations pkpbc df residuals from the quasi eastern hemisphere indicate 0 4 0 1 per cent anisotropy if only pkpbc df observations from the top 200 km of this hemisphere are considered this is reduced to 0 1 0 2 per cent consistent with an isotropic layer on the other hand new absolute pkp traveltime measurements add to the sampling of both hemispheres of the inner core but it is difficult to use them with more confidence to assess structure of the core since they are affected by crustal and mantle structure and source uncertainties the newly collected data set also increases constraints on d structure beneath the south pole in contrast to previous inferences based on data from northern stations we find no evidence of a velocity heterogeneity in the outer core near the inner core boundary associated with the cylinder tangent to the inner core in the southern hemisphere

heterogeneity and anisotropy of seismic attenuation in the inner core 1 we evaluate seismic attenuation in the inner core from the spectral ratio between the seismic phase pkp df and either pkp ab or pkp bc at the epicentral distances from 150 to 170 the global data set is divided into two groups according to the position of the turning point of pkp df either in the eastern from 50 e to 120 w or the western hemisphere in the eastern hemisphere the parameter t shows strong dependence on the epicentral distance and no dependence on the angle between the earth s spin axis and pkp df ray at the turning point in the western hemisphere t is weakly dependent on the distance and varies between approximately 0 7 s and 0 1 s for the angles between 0 and 90 respectively our results allow us to reconcile seemingly incompatible results of many previous studies the anisotropy of attenuation is correlated with the anisotropy of p velocity

the age and provenance of metasedimentary rocks in the quetico subprovince ontario from single zircon analyses implications for archean sedimentation and tectonics in the superior province abstract u pb isotopic analyses have been performed on single detrital zircon grains from greywacke samples collected along the northern boundary of the quetico subprovince in northwest ontario secondary lead loss was virtually eliminated by choosing crack free grains and removing the outer surface using air abrasion thus the 207 pb 206 pb age can be considered a good approximation to the age of crystallization of each grain zircons in the quetico metasediments show a range of ages from 2698 3 ma to 3009 4 ma deposition and metamorphism of the metasediments is bracketed within a 10 5 ma time span between crystallization of the youngest detrital zircon grain and a late tectonic intrusion the blalock pluton which cuts the sediments and has a zircon age of 2688 4 ma the most easterly metasediment sample contains zircons with ages which cluster around 3007 6 ma which is within error of the age of a batholith dated in the adjacent marmion lake area of the wabigoon subprovince this along with the fact that most detrital zircons were only slightly abraded by sedimentary transport suggests that the source of many zircons may have been proximal from the north there is also a significant cluster of young zircon ages in the range 2698 2709 ma surprisingly only a few ages correspond to the time of most intense arc type felsic volcanism 2710 2745 ma in the main greenstone belt of the western wabigoon subprovince sedimentary deposition occurred at a time of extensive deformation in the adjacent wabigoon subprovince the pattern of detrital grain ages indicates that this involved uplift of a complex older sialic landmass petrographic evidence for the presence of substantial felsic volcanic detritus as well as the fact that many of the zircons only slightly pre date deposition indicates that much of the sediment was probably derived from erosion of active felsic volcanic centers these data combined with regional geochronological and field studies favour deposition in a basin formed by late orogenic collisional events

inner core anisotropy in three dimensions the purpose of this paper is the investigation of cylindrical anisotropy in the inner core based on the travel time anomalies of the pkikp phase we use the arrival times reported in the international seismological centre bulletins for years 1964 1990 we select only earthquakes which have a good azimuthal coverage and a sufficiently large number of reporting stations the earthquakes are relocated using corrections for lateral heterogeneity computed for our most recent three dimensional mantle model we use a total of 313 422 observations of the df branch of pkp travel time anomalies within the epicentral distance ranges of 120 140 and 150 180 reported by 2335 stations for 26 377 earthquakes we process the data using an averaging procedure which we call cylindrical anisotropy stacking that enhances the effects of anisotropy but is expected to suppress those due to lateral heterogeneity and random errors the processed residuals show a remarkably consistent pattern this confirms the dominance of the cylindrical elastic anisotropy in the inner core with approximately a constant axis of symmetry this axis of symmetry is found to be tilted 10 5 1 from the earth s rotation axis in the direction 160 e 5 in the northern hemisphere in this new coordinate system we determine a four layer axisymmetric model of transverse anisotropy with each layer approximately 300 km thick the model shows that the anisotropy is strongest 3 within the innermost part of the core the travel time anomalies show significant 1 5 s longitudinal variations even when the tilt of the axis of symmetry is considered there is a substantial increase in the amplitude of the longitudinal variations which are dominated by the second and fourth harmonics for rays with bottoming depths exceeding 400 km below the inner core boundary the measurable tilt of the axis of symmetry and the presence of significant nonaxisymmetric signal may provide important clues with respect to the possible causes of anisotropy the increase in the anisotropy in the innermost part of the core departs from earlier inferences that anisotropy may be limited to the outermost 200 300 km of the earth s inner core

a detailed gravity profile across the english river subprovince northwestern ontario this paper presents the results of a detailed gravity profile measured across the two lithotectonic domains of the english river subprovince from vermilion bay to red lake a distance of 190 km in northwestern ontario along the profile 283 fresh rock samples were collected for density measurements the density data clearly suggest that there is a measurable and significant density difference between the migmatized metasediments and plutonic rocks a crustal model based on the seismic data along the profile has been used for computing a regional gravity field which in turn has been used in isolating the residual anomalies from the bouguer anomaly field a strong correlation has been found to exist between the residual anomalies the rock densities and the surface lithologies the northern supracrustal domain which is at its widest 60 km along the profile is dominated by a pronounced 15 mgal 150 m s 2 positive residual anomaly believed to be caused by outcropping anomalously dense metasediments

hemispherical structure in inner core velocity anisotropy 1 we investigate the hemispherical pattern in inner core velocity anisotropy using a new and independent high quality data set of pkpbc pkpdf and pkpab pkpdf body wave observations our data show no evidence for a tilted anisotropy axis with respect to the earth s rotation axis and is significantly better fit by a model with 4 4 anisotropy in the western hemisphere and only 1 0 in the eastern hemisphere than by a model of uniform anisotropy we carry out variance minimization and find the boundaries between the hemispheres at lines of constant longitude at 14 e and 151 w variance minimization enables us to extract the imprint of hemispherical structure from all of the data and not just polar paths resulting in boundaries which are nearly 30 from the results of previous studies the high quality of the data allows us to provide robust evidence that the isotropic velocity in the eastern hemisphere is 0 2 lower than in the western hemisphere in the top 660 km of the inner core our data set also suggests an increase in inner core anisotropy in the eastern hemisphere from 1 to around 6 at depths deeper than 660 km indicating that the hemispherical pattern in anisotropy may disappear at greater depths the presence of hemispherical structure rules out mechanisms for creating anisotropy which are unable to sustain longitudinal variations in the inner core furthermore steady inner core superrotation of the order of 0 1 year would eradicate the hemispherical differences though inner core oscillation would still be permissible

topography of the crust mantle interface under the western superior craton from gravity data the moho undulations beneath the western part of the archean superior province have been investigated with a three dimensional gravity inversion algorithm for a single interface of constant density contrast inversion of the complete gravity data set produces unreal effects in the solution due to the ambiguity in the possible sources of some crustal gravity anomalies to avoid these effects a censored gravity data set was used instead the inversion results are consistent with reflection and refraction seismic data from the region and therefore provide a basis for the lateral correlation of the moho topography between parallel seismic lines the results indicate the existence of a major linear east west trending rise of the moho below the metasedimentary english river subprovince which is paralleled by crustal roots below the granite greenstone uchi and wabigoon subprovinces this correlation between the subprovincial structure at the surface and deep moho undulations suggests that the topography of the crust mantle boundary is related to the tectonic evolution of the western superior belts although certain features of the crust mantle boundary are likely inherited from the accretionary and collisional stages of the western superior craton gravity driven processes triggered by subsequent magmatism and crustal softening may have played a role in both the preservation of those features as well as in the development of new ones

crustal structure and implications for the tectonic evolution of the archean western superior craton from forward and inverse gravity modeling 1 the distribution of mass anomalies within the crust of the archean western superior province has been investigated with forward and inverse gravity modeling routines the gravity models indicate that in most of the western superior province significant mass anomalies occur only within the top 10 km of the crust where they are generally related to dense metavolcanic rocks and low density granitoid plutons and at the crust mantle boundary where they are linked to undulations of this interface an exception is the region encompassing the central wabigoon subprovince and the segments of the quetico and wawa belts south of it where implied large intracrustal mass anomalies are associated with a segment of denser lower crust and with overlying depressions of the interfaces between the main crustal layers we interpret the predominant lateral mass homogeneity observed at deep crustal levels as evidence of mass redistribution processes associated with a major postaccretionary episode of thermal softening of the western superior crust the incongruent presence of a denser lower crustal segment may reflect the subsequent modification of the lower crust through magmatic intra and underplating possibly in relation with the mesoproterozoic midcontinent rifting event

structure of the archean english river subprovince implications for the tectonic evolution of the western superior province canada the english river subprovince is one of two metasediment dominated terranes in the western superior province it has been interpreted as an accretionary complex a foreland or a fore arc basin that developed and was subsequently deformed between the metavolcanic rich uchi subprovince and the orthogneiss and metaplutonic dominated winnipeg river subprovince during a prolonged transpressive orogeny to test these hypotheses we combined a satellite image aeromagnetic image and lithoprobe reflection seismic profile interpretation with detailed structural mapping to better characterize the internal geometry and significance of structural features in the western part of the subprovince in ontario northward directed subduction and collision of the winnipeg river subprovince with the uchi subprovince at ca 2713 2698 ma can account for the deposition of the sedimentary rocks initial metamorphism and the main phase of deformation in the subprovince whereas the subduction of wabigoon crust generated exten

images of a lower crustal oceanic slab direct evidence for tectonic accretion in the archean western superior province the archean western superior province in canada is the type area for proposed archean plate tectonics seismic images from this region provide direct evidence for assembly of the craton by terrane accretion and for a large slab of remnant oceanic crust preserved at the base of the crust this slab with inferred garnet amphibolite composition adds a critical piece of evidence to previous suggestions that archean subduction was at a shallow angle and that some neoarchean tonalite trondhjemite granodiorite suites distinct from most modern day suprasubduction magmas are melts primarily derived directly from subducted slabs

convection and anisotropy of the inner core estimation of the rayleigh number of the inner core suggests that this region is convecting the flow pattern is likely to produce crystallographic preferred orientation of the elastically anisotropic hexagonal closest packed phase of iron such that compressional wave velocities would be greater in the axial relative to the equatorial direction by roughly 1 percent this result is in agreement with seismological evidence that the inner core is elastically anisotropic

u pb geochronology of the central uchi subprovince superior province u pb zircon and titanite ages for rocks of the central uchi subprovince in northwestern ontario indicate a late archean magmatic and tectonic development spanning over 200 ma an early period at 2900 2800 ma formed volcano plutonic complexes presumably linked to 3 1 2 8 ga terrains of the northwestern superior province a later period of southward growth by magmatic and tectonic accretion occurred at 2750 2710 ma and was concluded by large scale compression and plutonism at 2700 ma the oldest 2890 2860 and 2840 2820 ma components occur in the pickle lake and meen dempster greenstone belts and as gneisses in the seach achapi and the lake st joseph batholiths in northern and central sectors of the region together with distinct 2750 2740 ma volcano plutonic complexes they form a collage assembled by multiple episodes of tectonic juxtaposition and magmatic accretion plutons of 2730 2710 ma age are intrusive into these older northern domains whereas their volcanic counterparts compose the lake st joseph and miminiska fort hope greenstone belts to the south late tectonic to posttectonic granitoid rocks intruded a region extending from the northern berens river subprovince to the southern english river subprovince at 2700 ma these plutons were cut by regional scale faults formed by residual north northwest directed shortening the timing of this movement seems to be recorded by titanite ages of 2690 2670 ma reactivation of the same faults may account for proterozoic pb loss observed in some of the zircon populations the age patterns are consistent with crustal growth along a continental margin in a north dipping subduction environment

u pb geoehronology and evolution of the english river subprovince an archean low p high t metasedimentary belt in the superior province u pb isotopic data for zircon monazite and titanite are used to assess the evolution of the late archean english river subprovince ers and adjacent domains of the winnipeg river subprovince in the northern superior province ontario the ers consists of turbiditic metasedimentary rocks deposited during the final stages of magmatic and tectonic accretion within the greenstone granite uchi subprovince to the north at around 2720 to 2710 ma the sedimentary rocks were intruded by a suite of calc alkalic plutons at 2698 ma major regional deformation amphibolite to granulite facies metamorphism anatexis and emplacement of an extensive peraluminous granitic suite culminated at 2691 ma late episodes of metamorphism metasomatism and emplacement of pegmatites occurred locally at circa 2680 and 2669 ma in contrast the winnipeg river subprovince to the south consists of widespread tonalitic suites formed mainly between 3200 and 2800 ma and granodioritic to granitic intrusions formed between 2710 and 2690 ma metamorphism occurred at around 2710 ma in southern domains and a distinct granulite facies event at around 2680 ma affected central and northern domains of the winnipeg river subprovince the rapid sequence of events recorded in the ers supports the notion that metamorphism was caused by thermal perturbations related to injection of granitic magmas these melts were generated through partial melting of the lower crust by a thermal anomaly presumably caused by mantle upwelling related to collapse of the inactivated subducting plate the short lived nature of the metamorphic events and their cyclicity follow the behaviour predicted by recent models linking the evolution of strain rates to episodes of segregation and upward magma transfer in the course of prolonged orogenic events

a local crossing path study of attenuation and anisotropy of the inner core 1 we report results from studying a region under the north pacific using 46 ray paths along east west and north south directions to study the nature of inner core anisotropy and find an anisotropy signal in differential pkpbc pkpdf at about the resolution of the data with north south paths faster than east west but no dependence of attenuation on wavespeed or depth in this part of the inner core inner core q is 130 52 255 between 140 340 km depth the observations also provide constraints on the degree of homogeneous meridional anisotropy possibly present in the core to between 0 1 and 0 6 velocity differences in the fast and slow directions significantly smaller than the 2 4 axi symmetric anisotropy in the inner core the small meridional component to anisotropy argues against significant contributions to anisotropy from low order inner core convection or stresses imposed on the inner core by the outer core field

aspherical structural heterogeneity within the uppermost inner core insights into the hemispherical boundaries and core formation abstract lateral heterogeneities at the top of the inner core are investigated using earthquakes that occurred in indonesia and southeast asia and were recorded in the southeastern caribbean using seismic observations of attenuation and seismic velocity we were able to constrain the characteristics of the boundary between the inner and outer core to further investigate the dynamics and evolution of the earth s core our seismic observations from core phases confirm that the outermost inner core is asymmetrically heterogeneous and we are able to further constrain the morphology and physical properties of this layer comparison of data from earthquakes with ray paths traversing from east to the west versus those with ray paths from west to east allow us to map the aspherical heterogeneity of the boundary layer and specifically image the boundary between the proposed quasi eastern and western hemispheres of the inner core the variation of differential travel times between pkp df and pkp bc attenuation in terms of q factor and latitudinal changes for both of these observations can be attributed to localized heterogeneity at the quasi hemispherical boundaries of the inner core we constrain the change in the thickness of outermost core boundary layer from 100 to 250 km within a distance of a few 10s of kilometers at 45 e 2 for the western boundary with an overall p wave velocity decrease in the western hemisphere of 0 5 and increase of 0 5 in the eastern hemisphere we constrain the eastern boundary at latitudes greater than 45 n to 173 e 4 with an overall p wave velocity decrease in the western hemisphere of 1 0 in the uppermost 200 km of the inner core the eastern boundary at equatorial latitudes is constrained to a region

hemispherical variations in seismic velocity at the top of the earth s inner core knowledge of the seismic velocity structure at the top of the earth s inner core is important for deciphering the physical processes responsible for inner core growth1 2 3 previous global seismic studies4 5 6 7 8 9 have focused on structures found 100 km or deeper within the inner core with results for the uppermost 100 km available for only isolated regions10 11 12 here we present constraints on seismic velocity variations just beneath the inner core boundary determined from the difference in travel time between waves reflected at the inner core boundary and those transmitted through the inner core we found that these travel time residuals observed on both global seismograph stations and several regional seismic networks are systematically larger by about 0 8 s for waves that sample the eastern hemisphere of the inner core 40 e to 180 e compared to those that sample the western hemisphere 180 w to 40 e these residuals show no correlation with the angle at which the waves traverse the inner core this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0 8 higher than that of the western hemisphere

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt