provenance of plumes in global convection models in global convection models constrained by plume motions and subduction history over the last 230 myr plumes emerge preferentially from the edges of thermochemical structures that resemble present day large low shear velocity provinces llsvps beneath africa and the pacific ocean it has been argued that large igneous provinces lips erupting since 200 ma may originate from plumes that emerged from the edges of the llsvps and numerical models have been devised to validate this hypothesis although qualitative assessments that are broadly in agreement with this hypothesis have been derived from numerical models a quantitative assessment has been lacking we present a novel plume detection scheme and derive monte carlo based statistical correlations of model plume eruption sites and reconstructed lip eruption sites we show that models with a chemically anomalous lower mantle are highly correlated to reconstructed lip eruption sites whereas the confidence level obtained for a model with purely thermal plumes falls just short of 95 a network of embayments separated by steep ridges form in the deep lower mantle in models with a chemically anomalous lower mantle plumes become anchored to the peaks of the chemical ridges and the network of ridges acts as a floating anchor adjusting to slab push forces through time the network of ridges imposes a characteristic separation between conduits that can extend into the interior of the thermochemical structures this may explain the observed clustering of reconstructed lip eruption sites that mostly but not exclusively occur around the present day llsvps

a geodynamic model of mantle density heterogeneity using cenozoic and mesozoic plate motion reconstructions we derive a model of present day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle the thermal buoyancy of these slabs is estimated from the observed thermal subsidence cooling of oceanic lithosphere slab velocities in the upper mantle are computed from the local convergence rate we assume that slabs cross the upper lower mantle interface and continue sinking into the lower mantle with a reduced velocity for a velocity reduction factor between 2 and 5 our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other we have also computed a synthetic geoid from our density model for a viscosity increase of about a factor of 40 from the upper to lower mantle our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95 and explains 84 of the observed geoid assuming that the model c21 and s21 terms are absent due to a long relaxation time for earth s rotational bulge otherwise 73 of the geoid variance is explained the viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle since downwelling velocities are expected to scale roughly as the logarithm of viscosity loge 40 3 7 these results show that the history of plate tectonics can explain the main features of the present day structure of the mantle the dynamic topography induced by this heterogeneity structure consists mainly of about 1 km amplitude lows concentrated along the active continental margins of the pacific basin our model can also be used to predict the time variation of mantle heterogeneity and the gravity field we find that the age of the geoid defined as the time in the past before which the geoid becomes uncorrelated with the present geoid is about 50 m y our model for the history of the degree 2 geoid which is equivalent to the history of the inertia tensor should give us a tool to study the variations in earth s rotation pole indicated in paleomagiietic studies

interaction of mantle dregs with convection lateral heterogeneity at the core mantle boundary preliminary numerical models indicate that chemically denser material dregs at the base of the mantle would have substantial lateral variations in thickness induced by convection of the overlying mantle and might well form discontinuous aggregations below mantle upwellings a model with a density contrast of about 2 per cent and an initial uniform thickness of the denser layer of 100 km yields a discontinuous distribution with maximum thickness 230 km and bottom topography of several kilometers amplitude in reasonable accord with recent seismological observations of vertical and lateral structure heat flux out of the core is probably strongly modulated laterally by mantle convection while mantle dregs will complicate and possibly amplify this effect such modulation may be relevant to long term 10 7 10 8 year variations in the magnetic field

a review of antarctic granulite facies rocks abstract precambrian granulite facies rocks occur in significant proportion in the east antarctic precambrian shield ages of metamorphic and deformational events range from 2500 m y to about 500 m y but some rocks are much older notably the approximately 3500 m y ages for crust formation in enderby land mineral assemblages over most of the area are typical of the hornblende granulite facies and sparse temperature pressure estimates indicate metamorphism at 700 800 c and 5 8 kbar at reduced water pressures a terrane of exceptional interest is the napier complex of enderby land where sapphirine quartz garnet sillimanite orthopyroxene osumilite and inverted pigeonite are associated with pyroxene granulite facies rocks metamorphic conditions are estimated to have reached 900 980 c 7 9 kbar and ph2o

charnockite and granite formation and influx of co 2 at kabbaldurga the discovery at kabbaldurga quarry karnataka south india1 3 of patches of charnockite apparently in an arrested state of development has refocused attention on the mechanism of charnockite formation in particular the relative roles of co2 and h2o during high grade metamorphism and charnockite development can be studied in such an area during an influx of a co2 rich volatile phase h2o may be liberated as a result of the reduction in the state of hydration by the breakdown of hydrous minerals4 the p t conditions in which this occurs are important because with an increase in xh2o conditions which may allow anatexis could be initiated in advance of such a co2 rich phase indeed weaver5 has suggested that the k rich acid charnockites found at pallavaram madras6 developed due to changes in volatile composition before the onset of charnockite conditions the exposure at kabbaldurga quarry is interpreted here as displaying evidence that such a process has taken place

the geochronology structure and metamorphism of early archaean rocks at fyfe hills enderby land antarctica abstract the fyfe hills of enderby land antarctica are composed of archaean cratonic rocks of the napier complex initial crustal formation possibly of plutonic volcanic and sedimentary origin was in the early archaean but probably significantly later than the 4000 ma ages previously reported at 3100 ma the rocks underwent granulite facies metamorphism and an intense regional strain d1 which rotated into parallelism pre existing discordances and produced a recumbent gneissic pile approximate sr isotopic homogenisation over at least a metre and new zircon growth and or major resetting apparently occurred at this time subsequent ductile deformation d2 also characterised by tight to isoclinal folding produced characteristic microstructure only within f2 folds but no axial plane foliation was developed peak granulite facies metamorphic conditions close to 900 c and 9 11 kbars were reached during d1 d2 considerable pb loss occurred in zircon during asymmetric open to tight folding d3 and upper amphibolite to granulite facies metamorphism 650 c 7 kbars at 2500 ma rb sr equilibration on the scale of centimetres occurred at that time after which the napier complex was essentially cratonised a subsequent semiductile deformation did not affect the whole napier complex but produced upright shear zones in the fyfe hills area which lies close to the boundary between the napier complex and the younger rayner complex to the south

a pyroxene bearing meta ironstone and other pyroxene granulites from tonagh island enderby land antarctica further evidence for very high temperature 980 c archaean regional metamorphism in the napier complex a suite of granulites including a meta ironstone pyroxenites and spinel lherzolites from east tonagh island enderby land antarctica preserve exsolution recry stallization features consistent with a shared metamorphic evolution that involves marked cooling from initial metamorphic temperatures of nearly 1000 c reintegrated pre exsolution and pre reaction grain compositions in the meta ironstone indicate the former coexistence of metamorphic pigeonite wo12en38fs50 and ferroaugite wo35en31fs34 at temperatures in excess of 980 c for pressures of 7 kbar 0 7 gpa using pyroxene quadrilateral thermometry lindsley 1983 intra grain lamellae relationships indicate the exsolution of a second pigeonite wo12en35fs53 from the ferroaugite at temperatures in the range 930 970 c prior to the c 720 600 c exsolution of orthopyroxene and clinopyroxene 100 lamellae and later partial recrystallization at similar temperatures although pyroxenitic and iherzolitic granulites preserve a much less complete history reintegrated porphyroclast compositions in these yield temperature estimates which approach those inferred from the metaironstone pyroxene thermometry based on neoblast compositions suggests that recrystallization post dating a late low intensity deformation phase d3 occurred at temperatures greater than 600 c these results are consistent with the independent evidence obtained from studies of metapelitic and felsic rock types for very high temperature metamorphism throughout the napier complex followed by near isobaric cooling and later deformation under lower grade granulite facies conditions comparison with similar pyroxene data from fyfe hills sandiford powell 1986 demonstrates further the regional significance of these high temperatures and implies broadly isothermal metamorphic conditions over a large area 5000 km2 and thickness 6 9 km of lower crust at c 3070 ma

analysis of historical seismograms root mean square lg magnitudes yields and depths of explosions at the semipalatinsk test range summary magnitudes mb lg based on rms amplitudes of lg waves recorded at nine internal soviet stations for 83 underground nuclear explosions at the semipalatinsk test range were analysed network magnitudes based on a combination of station magnitudes were formed as weighted averages over the station magnitudes after adjusting for variations among station magnitudes in scaling with source strength combining the station magnitudes into network magnitudes although the number of stations was small less than six appeared to reduce scatter in magnitude yield relations the correlation between network mb lg and announced explosion yields was very high and mb lg closely approximated simple proportionality with the logarithm of yield throughout the yield interval 2 165 kt the error of misfit was about 0 04 magnitude units excluding data for the explosion of 1972 august 16 8 kt at 139m the yield of which appeared high compared to mb lg the close correlation between network mb lg and explosion yield together with the fact that several of the explosions were not at scaled depths indicates that mb lg may not be critically dependent on depth of burial at least for depth ranges within a factor of two

plumes in a convecting mantle models and observations for individual hotspots the motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented a new list of 44 possible hotspots with associated tracks has been compiled for all of them calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow plume source depth has usually been assumed to be the top of d but an alternative source depth at the 670 km discontinuity has also been considered using models of relative plate motions and boundaries hotspot tracks on plates have been calculated and compared with age data ocean floor topography and distribution of volcanics on continents absolute plate motions have been redetermined under consideration of hotspot motion using a new least squares method for the hawaiian and yellowstone hotspots source locations and hotspot motion have been computed for a total of up to 23 different models the results show plume conduits being tilted with source regions at the d moving in the lowermost mantle flow generally toward large scale upwellings under southern africa and the south central pacific hotspot surface motion often represents the horizontal component of midmantle flow which is frequently opposite to plate motion toward ridges and away from subduction zones in particular almost all models tested predict southward motion of the hawaii and kerguelen hotspots and westward motion of the iceland hotspot for models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as or better than for the fixed hotspot model but sometimes fixed hotspots give the best fit in some cases where the track ends at a subduction zone e g for the bowie hotspot results can give indications about the otherwise unknown age of the hotspot in other cases especially for the tahiti hotspot results suggest an origin shallower than d and in yet other cases particularly east africa the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes

thermal history of uht metamorphism in the napier complex east antarctica insights from zircon monazite and garnet ages abstract high grade gneisses from mt riiser larsen east antarctica have been dated by whole rock mineral sm nd and shrimp zircon and monazite u pb to help define the thermal history of ultrahigh temperature uht metamorphism in the napier complex both the monazite and youngest zircon yield a range of apparent ages 2 51 2 47 ga consistent with crystallization during an extended period of metamorphism some zircon also preserves an isotopic record of earlier events placing an upper limit of a few million years on the duration of peak metamorphic conditions the similarity of the monazite and zircon u pb ages implies rapid initial postpeak cooling to below the blocking temperature of these minerals 900 c consistently lower sm nd whole rock mineral isochron ages 2 38 ga indicate that cooling slowed before the temperature reached 650 c the history of the uht metamorphism is interpreted to be 1 protracted high temperature 800 c conditions 2 51 2 47 ga 2 peak conditions up to 1100 c

regional seismic event location with a sparse network application to eastern kazakhstan ussr three component data from a sparse three station seismic network in eastern kazakhstan surrounding the soviet nuclear test site have been analyzed to determine location estimates for regional events recorded by two or three stations included among these events are the september 1987 chemical explosions whose locations are known locations are calculated using arrival times of p and s phases and arrival azimuths from first p location uncertainties are estimated using a combination of a priori and a posteriori data uncertainties a layered p wave velocity model adapted from soviet deep seismic sounding surveys is employed for calculating travel times and two s models are tried location results for the chemical explosions are excellent even if only two stations are used absolute location errors are less than 10 km and estimated 90 confidence uncertainties are only a few kilometers the data are also adequate to determine correctly their depth i e focus at the surface the other regional events include numerous suspected mine blasts and two earthquakes from the tien shan the calculated locations of the latter events agree well with a teleseismic location for one of them falling within a belt of regular seismic activity nearly all of the presumed blasts can be associated with mapped mines and we have been able to identify the source areas for two sets of blasts in high resolution satellite images before and after photographs allow us to identify specific active mines our location estimates agree quite well with the observed active mines

lg magnitudes and yield estimates for underground novaya zemlya nuclear explosions abstract lg wave amplitudes of 30 novaya zemlya underground nuclear explosions as recorded by short period seismographs in denmark finland germany norway spitsbergen and sweden are used to determine m b lg values assuming that the m b lg versus explosion yield relation derived from nevada test site data applies to all continental areas the yields of the explosions are estimated they cover a large range of values from 2 5 to 4900 kt the largest explosion since april 1976 had an estimated yield of 145 kt the m b lg values when subtracted from the m b p values can be used to estimate the m b p bias between two test sites in this way the m b p bias between novaya zemlya and the nevada test site is estimated to be 0 20 magnitude units

the effects of chemical density differences on convective mixing in the earth s mantle subducted lithosphere and crust are chemically distinct from ambient mantle and some have hypothesized that the resulting anomalous buoyancy may affect mixing and in particular may cause chemical heterogeneity to persist to explore this effect two dimensional convective mixing calculations have been carried out boundary conditions mimicking features of plate kinematics have been imposed and negatively buoyant tracers placed beneath converging plate margins to simulate subduction the principal parameter of interest is the density anomaly ratio o t the ratio of the chemical to the thermal buoyancy for a density anomaly ratio appropriate to complete separation of the oceanic crust from the rest of the lithosphere it is marginally possible for the tracers to segregate into the warm region near the bottom and neutralize the thermal buoyancy there for internally heated flows and to segregate to the base of the uprising limbs of convection for bottom heated flows however for the more likely scenario of oceanic crust and lithosphere maintaining their layering after subduction there is neither segregation nor neutralization of thermal buoyancy the average residence time is the average time tracers remain in the box from subduction to sampling beneath ridges and could be increased by 4 from the passive case at the very most if the crust and lithosphere remain layered even in the more extreme case of separation of crust from lithosphere the residence times are only increased by 15 from the passive case

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

on the existence of a second scale of convection in the upper mantle summary this paper is concerned with an examination of the possibility that there might exist a small scale of convective circulation beneath the oceanic lithosphere recent suggestions that this might be the case have been made in an effort to understand why the bathymetry of the sea floor deviates from the prediction of boundary layer theory for ages in excess of about 100 ma the energy source which sustains the secondary motion is supposed to be found in the steep temperature gradient near the planetary surface which is itself presumably maintained by the large scale convective circulation associated with plate creation and destruction here we investigate the extent to which the temperature dependence of viscosity may act so as to stabilize the upper boundary layer against disruption by such secondary instability if the viscosity profile is monotonie and the asymptotic upper mantle viscosity is about 1022poise as suggested by post glacial rebound data then the existence of the second scale is extremely unlikely on the other hand if a sufficiently pronounced low viscosity zone does exist under old sea floor then the development of such a second scale cannot be ruled out completely some recently obtained geophysical evidence is reviewed which suggests that this is unlikely to be the case

a bifurcation in the coupled magmatism mantle convection system and its implications for the evolution of the earth s upper mantle abstract a numerical model is presented for the coupled magmatism mantle convection system in the upper mantle to study how the coupled system controls the thermochemical state of the upper mantle depending upon the internal or basal heating rate the solid state convection in the upper mantle is modeled as a convection of a binary eutectic material with constant viscosity in a two dimensional rectangular box uniformly heated from the bottom boundary or by an internal heat source the density of the material depends on the composition and melt content as well as temperature magmatism is modeled as a permeable flow of the melt generated by a pressure release partial melting of the material the permeable flow is driven by the buoyancy of the melt there are two branches in the thermochemical state controlled by the coupled system for both of the basal and internal heating cases on one branch called the tc branch the solid state convection occurs dominantly as a thermal convection the box remains chemically homogeneous as a whole and magmatism occurs only slightly at most the tc branch is stable only when the heating rate is lower than a threshold a bifurcation occurs on the tc branch at the threshold and the thermochemical state falls on another branch called the cs branch above the threshold an episodic magmatism actively occurs a chemically stratified structure develops well in spite of the homogenizing effect of convective stirring the temperature becomes as high as the solidus temperature at depth and the convection is strongly affected by the buoyancy of melt and the chemical buoyancy that accompanies the chemical stratification on the cs branch the cs branch is stable even at heating rates slightly lower than the threshold a hysteresis in the thermochemical state occurs between the two branches as the heating rate changes around the threshold the observations of tectosphere suggest that the thermochemical state on the cs branch occurred in the upper mantle in the archean and early proterozoic when the mantle was strongly heated by radioactive elements and that the thermochemical state jumped to the tc branch at the end of the early proterozoic

influence of co2 on melting of model granulite facies assemblages a model for the genesis of charnockites it is shown that partial melting studies at crustal pressures in sio2 rich portions of the system kalsio4 mg2sio2 h2o co2 can be used in modeling the anatectic origin of charnockites the univariant reaction phlogopite sanidine quartz vapor enstatite liquid yields an sio2 rich melt granite analog at 3 kbar the vapor composition at the solidus is buffered to high h2o contents by virtue of the coexistence of phlogopite with its breakdown products with higher pressures 8 and 15 kbar the fluid phase is buffered to higher co2 contents and the melt composition becomes enriched in k2o and mgo charnockite analog melting relations are controlled through the expansion of the quartz liquidus field relative to the enstatite and sanidine fields with increasing pressure it is noted that partial melts generated at the base of the crust in the presence of co2 rich fluid will be of an alkaline nature and will crystallize enstatite at lower pressures co2 saturated melting of similar sio2 rich bulk compositions phlogopite absent through the reaction enstatite sanidine quartz co2 liquid occurs at temperatures above 1000 c to about 15 kbar

palaeoproterozoic assembly of the napier complex southern india and western australia implications for the evolution of the cuddapah basin abstract a detailed examination of lithostratigraphic tectonothermal geochronologic and palaeomagnetic database of the dharwar bastar cratons of south india the yilgarn craton of western australia and the east antarctic shield has shown broad similarity two palaeomagnetic key poles of 2400 ma age from the dharwar and yilgarn cratons suggest near neighbor positions matching patterns of craton outlines orientation of mafic dyke swarms and palaeo north direction for 2400 ma have established a continental assembly of siwa an acronym for south india si and western australia wa at 2400 ma in this assembly the yilgarn craton fits against the bastar craton and the singhbhum craton the available geological data from the napier complex of the eastern antarctica and the dharwar craton of south india were used to prepare the barcode style reconstruction of archaean tectonostratigraphic events of both the cratons the method of matching continental outlines the internal features of two continental blocks and the palaeomagnetic data demonstrate that the napier complex was also a part of siwa during the period 2200 1900 ma and was located at the position of the cuddapah basin of india adjacent to the dharwar craton and the yilgarn craton the separation of the napier complex from the south indian block led to the development of the cuddapah basin at 1950 ma

lg magnitudes of selected east kazakhstan underground explosions abstract the amplitude of 1 sec period lg waves as recorded on seismograms of wwssn stations in southern asia and scandinavia are used to determine m b lg values for 62 underground nuclear explosions in the eastern portion of the east kazakhstan test site the explosions were selected to be of m b p 5 5 or greater in the period 1965 to 1985 the yields of the explosions are estimated assuming that a nevada test site derived relation between m b lg and yield for water saturated rock is also applicable to the east kazakhstan test site assuming that lateral variations in crustal structure and in depth of emplacement of the explosion do not affect m b lg values the lg data indicate that the m b p bias between the nevada test site and east kazakhstan is 0 35 magnitude units

testing event discrimination over broad regions using the historical borovoye observatory explosion dataset we test the performance of high frequency regional p s discriminants to differentiate between earthquakes and explosions at test sites and over broad regions using a historical dataset of explosions recorded at the borovoye observatory in kazakhstan we compare these explosions to modern recordings of earthquakes at the same location we then evaluate the separation of the two types of events using the raw measurements and those where the amplitudes are corrected for 1 d and 2 d attenuation structure we find that high frequency p s amplitudes can reliably identify earthquakes and explosions and that the discriminant is applicable over broad regions as long as propagation effects are properly accounted for lateral attenuation corrections provide the largest improvement in the 2 4 hz band the use of which may successfully enable the identification of smaller distant events that have lower signal to noise at higher frequencies we also find variations in p s ratios among the three main nuclear testing locations within the semipalatinsk test site which due to their nearly identical paths to brvk must be a function of differing geology and emplacement conditions

the north sea lg blockage puzzle summary the north sea lg blockage for wave paths across crustal graben structures is a well established observational fact analysis of such observations implies that lg blockage takes place in graben areas associated with sedimentary basin formation and crustal thinning these intriguing observations have triggered many theoretical studies aimed at highlighting specific lg loss mechanisms albeit so far with only moderate success our approach to this problem is to simulate seismic wavefield propagation through the crustal waveguide using 2 d finite difference techniques the graben structures are known in detail from oil exploration works in the north sea which has enabled us to use realistic crustal models in our lg synthetics in the most extreme model tested the crystalline crust thickness beneath the graben amounted to only 5 km while the overlying sedimentary pile is nearly 10 km thick at the base of the crust in the graben area the moho is elevated nearly 10 km this model has similarities to the oceanic crustal waveguide where total lg blockage is claimed for path lengths exceeding 100 km the synthetic wavefields are displayed in terms of snapshots semblance velocity analysis and time space rms amplitudes the dominant structural lg loss mechanisms are the delay of the lg waves in the thick sediments lg to rg conversions scattering by lateral heterogeneities in the sediments and s wave leakage out of the crustal wave guide and into the upper mantle a fraction of these upper mantle s waves return to the crust and appear as sn coda observationally strong sn phases of long duration are often associated with weak lg phases and vice versa our synthetics produced lg amplitude decay amounting at most to 6 10 db while observational data imply blockage amounting to 15 20 db the latter is equivalent to a pn lg magnitude difference of nearly one magnitude unit the main outcome of this study is therefore that lg wave propagation is very robust and that a dominant blockage effect associated with intrinsic attenuation that is q values of the order of 100 at 2 hz for a path length of minimum 100 km is necessary to conform to observations

determination of seismic energy from lg waves seismic energy is a physical concept related to broadband information on the source radiation this is different from seismic moment which is related to low frequency information improved estimates of seismic moment and radiated energy can improve the scientific framework for interpreting observed seismicity in terms of physical source processes this study evaluates the utility of regional lg waves the dominant feature of seismograms for regional earthquakes for the estimation of radiated seismic energy the rms lg which is defined as the root mean square of lg wave ground velocity within a certain group velocity range is proportional to the square root of lg wave radiated energy flux we find that rms lg is a good quantity for estimating the energy release of earthquakes because a large fraction of the total seismic energy is radiated as lg waves we scale the network averaged rms lg value to a reference distance of 100 km for a set of 52 earthquakes in the northeastern united states using an empirically derived relation between rms lg and epicentral distance we apply the source parameters estimated for these earthquakes to boatwright s deceleration source model and calculate the total seismic energy expected to have been radiated by each event assuming it can be modeled by this source we then obtain for earthquakes in the northeastern united states the scaling of the total radiated seismic energy of earthquakes with network averaged rms lg values from synthetic seismograms it is found that different velocity structures have an effect on the efficiency of lg wave radiation using measurements of rms lg made from synthetics we find that the vertical strike slip source tends to radiate less lg wave energy than other source mechanisms manuscript received 4 february 1998

crustal structure in eastern kazakh u s s r from teleseismic receiver functions broadband receiver functions determined from teleseismic p waveforms recorded at two seismic stations in eastern kazakh u s s r are inverted for the vertical velocity structure beneath the stations the detailed broadband receiver functions are obtained by stacking source equalized radial components of teleseismic p waveforms a time domain inversion of the radial receiver function is used to determine the structure assuming a crustal model parameterized by flat lying homogeneous layers the general features of the inversion results are a complex shallow crust velocities less than 6 km s in the upper crust a high velocity 6 9 7 5 km s lower crust and a moho that varies between 47 and 57 km depth these results compare favorably with a composite velocity model from deep seismic sounding data but show lower velocities in the upper crust and higher velocities in the lower crust the results indicate that the crust in this region is relatively uniform however the nature and depth of the moho changes significantly across the region the receiver function inversion structure for the kazakh sites is similar to a published receiver function structure for a site on the canadian shield

rheological control of oceanic crust separation in the transition zone mineral physics observations suggest that distinct density and rheological differences exist between the crustal component of oceanic lithosphere and the underlying mantle we have conducted numerical experiments to investigate the influence of both density and viscosity on the effectiveness of recycling of oceanic crust into the lower mantle confirming previous results the density inversion at 670 km depth alone is not sufficient to prevent crustal recycling however a soft layer may exist between the strong garnet crust and cold slab interior models employing a simplified newtonian sandwich model show that this thin weak layer can effectively decouple the crust and slab once entrained into the lower mantle the then lighter crust can rise sufficiently fast as a rayleigh taylor instability to avoid further entrainment these results suggest that the crustal component of slabs may be trapped at 670 km depth leading to a garnet enriched transition zone

on the separation of relatively buoyant components from subducted lithosphere subducted oceanic lithosphere consists of a sandwich of component layers morb underlain by depleted parent mantle these layers give rise to intrinsic density contrasts within the slab which have been hypothesized to cause component separation after subduction we explore this process through numerical models of simple layered slabs and through convection models which incorporate large local chemical density contrasts we show that the components of positive and negative buoyancy density contrast component volume must be almost equal in order to cause significant component separation or to selectively block deep penetration of buoyant slab components in the transition zone published petrological constraints on component densities and relative volumes indicate that such buoyancy neutralization is not achieved in subducted slabs therefore component separation should not occur on the timescale associated with the sinking of slabs through the upper mantle and transition zone

the origin of archaean gneisses in the fyfe hills region enderby land field occurrence petrography and geochemistry sandiford m and wilson c j l 1986 the origin of archaean gneisses in the fyfe hills region enderby land field occurrence petrography and geochemistry precambrian res 31 37 68 granulite and amphibolite facies gnelsses at fyfe hills in enderby land east antarctica form part of an early archaean c 3 8 ga terrain termed the napier complex the gneissic sequence at fyfe hills is composed of both supracrustal and intrusive rocks the oldest lithological association the layered gnelssic series is defined by interlayered felsic and basic metavolcanics and metasediments derived from psammitic pelitic ferruginous and calcareous precursors felsic gneisses in the layered gneissic series have low y ce low ga a1 and are low in incompatible elements and therefore correspond to sheraton and black s depleted granulites the layered gneissic series has been intruded by ultrabasic rocks and by subconcordant sheets of charnockite enderbite and pyroxene granulite which form the massive gneissic series the undepleted felsic gneiases of the massive gneissic series probably derive from crustal anatexis associated with granulite facies metamorphism of the napier complex dehydration during this unusually high grade metamorphism was achieved by the removal of partial melts rather than by co2 flushing as metamorphism occurred either during vapour absent conditions or if vapour was present then the composition of the vapour phase was internally buffered anatectic pegmatites generated during granulite facies metamorphism differ little in chemistry from the felsic granulites from which they derive chemical modification accompanied rehydration during waning granulite facies metamorphism and during the development of retrograde shear zones the latter are associated with a magmatic episode in which numerous pegmatites were intruded into the napier complex

effect of mantle plumes on the growth of d by reaction between the core and mantle the core mantle boundary is a fundamental compositional discontinuity in the earth where molten iron alloy from the core meets solid silicate minerals from the mantle heat flow from the core to the mantle creates a thermal boundary layer at the base of the mantle at the same time chemical reactions may create a layer of different composition and density than the overlying mantle using a finite element model of convection in a spherical shell that includes formation of dense material at the base of the shell we investigate how a layer forms at the base of the mantle development of a layer at the base of the mantle by this method depends both on the composition of the material forming at the core mantle boundary and on the rate at which the material diffuses into the mantle if the material is less than 3 6 denser than the overlying mantle assuming reasonable choices of lower mantle thermodynamic parameters the reactant material will be swept away by upwelling plumes more dense material on the other hand forms an internally convecting layer that entrains material from above the varying thickness and composition of the layer is consistent with geophysical observations of the characteristics of the deep mantle

the evolution of the mantle s chemical structure abstract the geochemistry of flood basalts and their associated picrites and of komatiites and their associated basalts combined with a theoretical model for the structure of mantle starting plumes can be used to decipher key elements of the geochemical structure of the deep mantle and show how it has varied through time we argue that the thermal boundary layer above the core consisted mainly of depleted mantle similar to the present morb source during the archaean and this was largely replaced between 2 7 and 2 0 billion years ago by enriched mantle to form the oib source we suggest that this change in the nature of the hotspot source reflects a fundamental change in the dominant component of downward convection from cold plumes breaking away from beneath a stable lithosphere during the pre archaean to subduction of lithosphere in the archaean and post archaean mantles

regional mb ms lg pg amplitude ratios and lg spectral ratios as criteria for distinguishing between earthquakes and explosions a theoretical study summary theoretical seismograms for point explosion and double couple sources in a uniform plane layered earth are used to evaluate the likely effectiveness of three criteria that have been proposed for distinguishing between earthquakes and explosions the criteria are regional ms ms lg pg amplitude ratios and lg spectral ratios for explosions in material with a high wave speed the near regional mb ms discriminant gives only marginal separation between the two types of source ranging from zero for strike slip earthquakes to on average 0 4 magnitude units for 45d dip slip mechanisms only for explosions in material with a low wave speed do the theoretical data show an overall separation lg pg amplitude ratios successfully separate the source types except for strike slip earthquakes where there is overlap with shallow explosions theoretical lg spectra for deep 1 5 km explosions are depleted in high frequencies when compared with those for earthquakes and shallow explosions discrimination between earthquakes and these deep explosions is therefore possible using the spectral ratio method an important mechanism underlying these lg based discriminants is the interaction of the spherical p wavefront from an explosion with the free surface which generates shear waves with amplitude depending both on frequency and source depth of burial

geochronology and geological evolution of metamorphic rocks in the field islands area east antarctica detailed geochronological structural and petrological studies reveal that the geological evolution of the field islands area east antarctica was substantially similar to that of the adjacent archaean napier complex though with notable differences in late and post archaean times these differences reflect the area s proximity to the proterozoic rayner complex and consequent vulnerability to tectonic process involved in the formation of the latter distinctive structural features of the field islands are 1 consistent development of a discordant pervasive s3 axial plane foliation 2 re orientation of s3 axial planes to approximate to the subsequent e w tectonic trend of the nearby rayner complex 3 selective retrogression by a post d3 static thermal overprint and 4 relatively common development of retrogressive e w trending mylonitic shear zones peak metamorphic conditions in excess of 800 c at 900 100 m pa 9 kbar were attained at one locality following but probably close to the time of d2 folding d3 took place in late archaean times when metamorphic temperatures were about 650 c and pressures were about 600 mpa 6 kbar later temperatures of 600 50 c and pressures of 700 mpa 7kbar were attained in an amphibolite facies event presumably associated with the widespread granulite to amphibolite facies metamorphism and intense deformation involved in the formation of the rayner complex at about 1100 ma the area was subsequently subjected to near isothermal uplift rb sr isotopic data indicate that the pervasive d3 fabric developed at about 2400 2500 ma and this age can be further refined to 2456 8 5 ma by concordant zircon analyses from a syn d3 pegmatite all zircons were affected by only minor 7 10 pb loss and or new zircon growth during the rayner event at about 1100ma thus the 450 850 g gu concentrations of these zircons were too low to cause sufficient lattice damage over the 1350 ma from 2450 ma for excessive pb to be lost during the 1100 ma event the emplacement of pegmatite at 522 10 ma substantially changed the rb sr systematics of the only analysed rock that developed a penetrative fabric during the 1100 ma event monazite in this pegmatite contains an inherited pb component which probably resides in small opaque inclusions a good correlation is found between rb sr total rock ages and rock fabric u pb zircon intercepts with concordia also mostly correspond to known events however in one example a near perfect alignment of zircon analyses probably developed by mixing of unrelated components produced concordia intercepts that appear to have no direct geochronological significance

gradual entrainment of a chemical layer at the base of the mantle by overlying convection summary a dense chemical layer may exist at the base of the mantle the overlying convection tends to entrain some of this layer reducing its thickness through time the lower more dense fluid is dragged into cusps beneath upwelling limits of the overlying convection and some material is entrained it is obvious that a high density of intrinsically more dense material retards this entrainment an upper limit and probably a reasonable estimate for the amount of entrainment is obtained by considering the carrying capacity of the vertical flow above cusps and ignoring the complications within the cusps themselves flow through conduits formed by the reduced viscosity of hot material is less effective at entrainment than broad scale flow controlled by the background viscosity of the normal mantle cylindrical plumes are less effective at entrainment than 2 d flow intrinsic density contrasts of around 6 per cent are needed for the layer to persist through geological time assuming the thermal expansion coefficients is 2 10 5 in addition the chemical layer may convect internally this causes the layer to entrain less dense material from above and thus aids entrainment from above by reducing the excess density of the layer convection in the chemical layer and stripping of the layer into cusps by the overlying flow also cause much of the temperature contrast to be in the overlying mantle rather than the chemical layer the difficulty of having more than a few 100 k temperature contrast across the layer unless it is internally stratified negates a reason for supposing the layer exists

a numerical study of thermal and chemical structures at the core mantle boundary abstract the core mantle boundary cmb represents the lower boundary layer of the actively convecting earth s mantle and is structurally very complex thermal and chemical structures such as thermal plumes and thermochemical piles have been considered to explain the complexities both affect the dynamics of the earth s mantle and its temporal evolution but also the surface plates are an essential aspect of mantle convection that strongly influence the dynamics of the interior we use numerical thermochemical models of mantle convection to study the structure and dynamics of the lowermost mantle our approach allows for the investigation of plumes and piles in combination with plate like surface motion and deep subduction the models show that the presence of a dense cmb layer generally reduces the mobility of the surface plates but that during plate evolution a variety of plume classes occur leaving a complex structure at the cmb the cmb topography shows large elevated areas with sharp edges and a flat or slightly dented top for piles with plumes atop also plume clusters can cause large elevated areas with sharp edges but a flat top with a few smaller peaks smaller scale patterns often close to the edges of the large structures cause smaller peaks in cmb topography and can be explained by either line plumes or wind driven thermals the latter often arise when heat is trapped beneath subducted slabs that spread over the cmb

compressible thermochemical convection and application to lower mantle structures a new finite element code for compressible thermochemical convection is developed to study the stability of a chemical layer at the base of the mantle using composition dependent compressibility and a density difference between compositions at a reference pressure a composition dependent density profile is derived together with depth dependent thermal expansion this combination of parameters yields a wide range of dynamic evolutions for the chemical layer the chemical structures are classified into five major categories classical plumes mushroom shaped plume domes ridges and continuous layers and a few abnormal cases such as hourglass shaped plumes and columnar plumes several models have a chemical structure morphologically similar to the african low v s structure in the lower mantle at least at a single time guided by our models several dynamic scenarios are proposed for the dynamic nature of the lower mantle low velocity structures a k a superplumes including plumes at an early stage plume clusters ridges passive piles sluggish domes and high bulk modulus domes we predict seismic velocity anomalies from these dynamic models the thermoelastic parameters used in the conversion are additional constraints we compare the density structure with normal mode inversion the predicted seismic signature observations and the required thermoelastic parameters with mineral physics data among the proposed scenarios only the scenario of high bulk modulus domes satisfies all constraints simultaneously the implication on the geochemistry and mineralogy of lower mantle chemical structures is discussed

osumilite in the sapphirine quartz terrane of enderby land antarctica implications for osumilite petrogenesis in the granulite facies

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

geochemical processing in a three dimensional regional spherical shell model of mantle convection 1 a three dimensional regional spherical model of processing of oceanic crust through subduction stirring and remelting confirms the main results from previous two dimensional models and from three dimensional models with low rayleigh number and only depth dependent viscosity the new model features high rayleigh number strongly temperature dependent viscosity a set of realistic surface plates and oceanic crust simulated by passive tracers it confirms that the processing timescale is the main determinant of the age distributions and mean ages of mid ocean ridge basalt morb and ocean island basalt oib samples and also of the present degree of processing of the mantle which is 99 in this model the melting depth decreases with time accounting for its temperature dependence and so the processing timescale increases with time an approximate processing theory reveals that morb and oib ages depend mainly on the recent processing timescale whereas the degree of processing depends mainly on the initial processing timescale

the stability of rms lg measurements and their potential for accurate estimation of the yields of soviet underground nuclear explosions data on underground nuclear explosions have recently become available from modern digital seismic stations installed within the soviet union and china observations of root mean square rms lg wave signals for soviet underground nuclear explosions at the shagan river test site in east kazakhstan show that the relative amplitudes of the rms signals at stations in norway the ussr and china are very similar for different explosions the standard deviation of the differences being only about 0 03 in logarithmic units i e magnitude units this is consistent with earlier observations comparing norsar and graefenberg array data and the observed scatter is significantly lower than has been reported for lg data from nevada test site explosions in view of the excellent correspondence found by nuttli 1986 and patton 1988 for lg versus yield at nevada this indicates that rms lg has a potential for yield estimation with very high accuracy at shagan river our study has shown that a selected stations in the ussr and china situated at regional distances provide a much improved signal to noise ratio of the lg phase for events at shagan river as compared to norsar array data b the scaling of rms lg amplitudes between different sized events recorded at the same single station site appears to be consistent with that of norsar indicating a remarkable degree of precision in single station measurements of lg signal c rms lg amplitude measurements for the best of these stations may be made at 1 5 to 2 0 magnitude units lower than at norsar or graefenberg allowing a much lower threshold for lg based yield determination and d the p wave detection capabilities of these single stations do not match those of the noress and arcess arrays thus teleseismic signals continue to be important for detection of small nuclear explosions our conclusion is that lg signals appear to provide an excellent basis for supplying estimates of the yields of nuclear explosions even down to below 1 kt when such signals are recorded at high quality digital in country seismic stations and when calibrated by access to independent nonseismic yield information for a few nuclear explosions at the test sites of interest in the context of monitoring a low yield threshold test ban treaty it will in addition be important to take into consideration various environmental conditions in the testing area such as the possible presence of cavities and to devise appropriate procedures for on site observations in this regard

sr and nd isotope systematics of polymetamorphic archean gneisses from southern west greenland and northern labrador sr and nd isotopic data for middle to late archean polymetamorphic felsic gneisses from localities in the nuuk area west greenland are compared and contrasted with new isotopic results for early archean amitsoq gneisses and with data for isotopically reworked kiyuktok gneisses from the saglek area labrador sr isotopic data for individual suites of felsic gneisses record the time integrated effect of variable rb sr fractionation during prograde and retrograde events as well as the effect of source inhomogeneity contrasting petrologic and sr nd isotopic characteristics are the result of differences in level of exposure caused partially by juxtaposition of terranes of different metamorphic character by movement on ductile shear zones and post shearing folding deformation sm nd systematics of felsic gneisses from nordafar ikerasakitsup akornga tinissaq and kangimut sammisoq qasigianguit define a geologically meaningless ca 3280 ma nd isochron which is the result of mixing of samples from unrela

is subducted lithosphere trapped at the 670 km discontinuity recent work1 2 on phase equilibria has shown that subducted oceanic lithosphere which is differentiated into a basaltic crust and a harzburgitic residue layer could be significantly denser than undifferentiated peridotite at depths between 650 and 700 km but would be less dense in the range 700 740 km it might thus be trapped in a gravitational stable layer between upper and lower mantle3 here using numerical experiments of isoviscous convec tion i explore the stability of such a layer assuming that it had previously been formed the results suggest that the survival of the intermediate layer is at best marginally feasible provided that new subducted lithosphere is efficiently trapped

isotopic evolution of the mantle a model we investigate a model of mantle evolution which acknowledges the importances in early earth history of large degrees of partial melting and crystal fractionation rather than as in recent models small degrees of partial melting operating on primary mantle the major source regions are in the upper mantle and evolve from a primitive mantle melt by crystal fractionation and the formation of chemically distinct cumulate layers the rb sr and sm nd ratios in melts are relatively unfractionated in processes involving large 15 degrees of partial melting the ratios are even less fractionated in orthocumulates forming from these melts small degrees of partial melting or removal of residual fluids after nearly complete crystalization of cumulate layers are the processes that lead to significant fractionation the importance of these processes increases as the upper mantle cools the observation of nearly primitive isotopic ratios does not necessarily imply the presence of ancient primitive undifferential reservoirs the transfer of intercumulus fluids from a crystalizing cumulate layer gives time dependent rb sr and sm nd ratios in both the donor and receiving reservoirs since the late stage fluids become more enriched as crystalization proceeds enrichment in such a system is progressive with time time integrated nd sm and rb sr ratios therefore need not depart much from primitive ratios even though current values for lile concentration rb sr nd sm etc may be large the time constant for lile enrichment appears to be 2 gyr about the same as required for the u pb system and the same as the characteristics time associated with mantle cooling the mantle reservoirs may be much more ancient than generally assumed

geochemistry and petrogenesis of granitoids around the kolar schist belt south india constraints for the evolution of the crust in the kolar area the peninsular gneiss surrounding the n s trending kolar schist belt in the dharwar craton includes several components of granitic rocks that range in composition from tonalite to granite on the west the different phases are referred to as the dod the dosa and the banded gneiss and the patna granite the gneisses on the east are dominantly of one type and are referred to as the kambha gneiss the champion gneiss occurring within the eastern part of the belt is compositionally similar to the plutonic dod gneiss and probably represents a near surface intrusion late aplites are present on both sides of the belt the banded gneiss was formed by partial melting of tonalitic rocks and the aplites were formed by the action of aqueous fluids during amphibolite grade metamorphism excepting these two rocks all other granitoids seem to have been formed from silica oversaturated parent magmas derived from lile enriched mantle sources by fractionation processes including liquid immiscibility and fractional cry

smnd isotopic systematics of enderby land granulites and evidence for the redistribution of sm and nd during metamorphism measurements of143nd 144nd and147sm 144nd are reported for whole rocks and mineral separates from granulites of the napier complex at fyfe hills charnockites leuconorites and gabbros yield a whole rock smnd isochron age of3060 160m y and an initial143nd 144nd ratio of0 50776 10 nd 3060m y 2 0 1 8 the negative nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of sm and nd and partial re equilibration of143nd 144nd ratios this probably occurred during the upper granulite facies metamorphism which has also been dated at 3100m y by rbsr and upb zircon studies 1 coexisting clinopyroxene apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 300 m y this array indicates that redistribution of sm and nd and re equilibration of143nd 144nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at 2450m y due to the resetting of the smnd system on both whole rock and mineral scales the primary crystallization age of the igneous protolith is not well constrained by the present data although it is clearly3100m y if it is assumed that the complex was derived initially from a depleted mantle reservoir nd t 2 evolution of the negative nd value of 2 0 with the observed sm nd ratios requires a prehistory of 380m y this implies a primary age of 3480m y however substantially older primary ages can be inferred if the source reservoirs had nd t 2 and or substantial reductions in the sm nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m y such an inferred reduction in the sm nd ratio may have been the result of preferential loss of sm relative to nd or introduction of a low sm nd fluid with nd 0 during granulite facies metamorphism

geochemistry and geochronology of proterozoic tholeiite dykes of east antarctica evidence for mantle metasomatism two episodes of tholeiite dyke emplacement have been identified in archaean high grade metamorphics of the napier complex in enderby land middle proterozoic amundsen dykes are typical continental tholeiites and most of the chemical variation in individual suites can be explained in terms of different degrees of partial melting and low pressure crystal fractionation group i amundsen tholeiites were derived from a relatively homogeneous source region 1 190 200 m y ago whereas that of the group ii amundsen tholeiites was chemically and isotopically heterogeneous group ii dykes have various degrees of enrichment in incompatible elements and commonly show normalised trace element abundance patterns with negative nb anomalies these features imply variable metasomatism of the source region by a volatile rich fluid phase rather than a melt of any observed igneous composition enriched in k rb ba th and possibly la and ce early proterozoic 2 350 48 m y tholeiites were emplaced at considerable depths in the crust during the waning stages of granulite facies metamorphism and include a high mg suite of possible komatiitic affinity ranging in composition from hypersthene rich tholeiite norite to quartz rich tholeiite they tend to have higher ratios of highly to moderately incompatible elements e g k zr k ce and larger nb anomalies i e higher k nb compared with middle proterozoic tholeiites suggesting derivation from more enriched source regions isotopic data are not compatible with significant crustal contamination but constrain source metasomatism to a time immediately before emplacement metasomatism of the source region of the much younger group i tholeiites may have been contemporaneous with that of the high mg suite

thermal cooling of the oceanic lithosphere new constraints from geoid height data abstract up to now tests of thermal models of the oceanic lithosphere as it cools and moves away from the ridge crest have been based mainly on topography and heat flow data however large areas of the ocean floor deviate from the normal subsidence due to thermal contraction and heat flow data are not very sensitive to the form of the model cooling of the lithosphere causes a short wavelength step in the geoid across fracture zones that can also be used to constrain thermal models we have analyzed geoid data at fracture zones from the seasat altimeter measurements in the entire pacific ocean and redetermined parameters of the cooling models we find that the data reveal two distinct regimes of cooling one for seafloor ages in the range 0 30 ma the other beyond 30 ma this does not appear to be correlated with particular fracture zones but rather it is representative of the whole area studied i e the entire south pacific and northeast pacific ocean these two trends may be interpreted in terms of two different asymptotic thermal thicknesses of the plate model the smaller thermal thickness 65 km found for ages from the results obtained in this study we conclude that the half space cooling model is unable to explain the data that beyond 30 ma a simple plate model gives a satisfactory fit to the data but in the younger plate portion ages

geochemistry of precambrian gneisses relevance for the evolution of the east antarctic shield archaean granulite facies orthogneisses of the napier complex in enderby land metamorphosed 3070 maago comprise two chemically distinct suites the more abundant mainly of tonalitic to granodioritic composition shows strong y depletion explicable by hydrous partial melting of a garnet bearing source garnet amphibolite or possibly eclogite it apparently represents new continental crust other gneisses predominantly of trondhjemitic to granitic s s composition do not show y depletion and have higher tio2 zr nb la ce and ga al and lower cao sr and mg mg total fe they probably originated by relatively dry melting of predominantly felsic crystal rocks both suites show evidence for loss of rb relative to k th and u during metamorphism late archaean 2800 ma amphibolite facies gneisses of macrobertson land are of undepleted type and may be representative of a higher crustal level than those of enderby land late proterozoic 1000 ma granulite facies gneisses of enderby land rayner complex are to a large extent remetamorphosed napier complex rocks of igneous derivation in contrast gneisses of similar age in macrobertson land include a much higher proportion derived either directly or by partial melting from sedimentary protoliths

some dynamical consequences of partial melting in earth s deep mantle we use regional scale numerical models of mantle convection to investigate the simple hypothesis that seismically anomalous thin patches of earth s lowermost mantle termed ultralow velocity zones or ulvz are derived from partial melting of ordinary mantle the models span the lower 500 km of earth s mantle employ both temperature and melting related contributions to buoyancy and include a cold randomly moving downwelling introduced from above to maintain a thermal boundary layer partial melting of ordinary mantle introduces a ubiquitous partially molten layer above an isothermal core mantle boundary as a consequence of its isothermal and isobaric conditions although it naturally develops variations in thickness greater than two orders of magnitude with the thickest portions occurring at the base of upwelling plumes and a thin layer elsewhere we find that only a dense partially molten mixture produces partial melt distributions that are compatible with seismic observations of ulvz however if such a melt percolates downward a dense basal liquid layer accumulates above the core mantle boundary the apparent requirement of a volumetrically dense and non percolating melt phase in the lowermost mantle presents serious problems for the hypothesis that ulvz arise from melting of ordinary mantle and suggests that such features likely form as a consequence of more complex processes furthermore these considerations suggest that the solidus of ordinary mantle is a reasonable upper bound on the present day temperature of the cmb 2007 published by elsevier b v

exploiting regional amplitude envelopes a case study for earthquakes and explosions in the korean peninsula abstract we introduce a new method to use narrowband regional amplitude envelopes for event analysis building on the success of the coda wave method we construct synthetic template envelopes that attempt to fit the entire waveform including multiple direct phases and their coda across a broad frequency band the method makes use of our understanding of earthquake and explosion source models regional wave propagation and the relationship between direct amplitudes and their respective codas we demonstrate the power of the method by examining earthquake and nuclear explosions in the korean peninsula at regional distance stations mdj mudangjing china and tjn taejon south korea in order to implement the method however we need to account for propagation through the use of an attenuation model for the region which we have developed in addition to an empirical correction to provide for unaccounted effects in the direct to coda transfer functions under the assumption that our explosion and attenuation models and the empirically obtained p to p coda and s to s coda transfer functions are correct we determine that the 2006 test by the democratic people s republic of korea dprk is consistent with a yield between 200 and 800 tons and a depth between 20 and 300 m with our best fit at 500 tons at a depth of 100 m similarly the 2009 dprk test is consistent with a yield range of 1 5 kt and a depth range of 70 600 m with our best fit at 2 kt at a depth of 200 m

chemical th u total pb dating by electron microprobe analysis of monazite xenotime and zircon from the archean napier complex east antarctica evidence for ultra high temperature metamorphism at 2400 ma abstract the chemical th u total pb isochron method chime of dating was carried out on monazite xenotime zircon and polycrase in six samples of ultra high temperature uht granulites and one of pegmatite from five localities in the napier complex enderby land east antarctica despite the chronological heterogeneity in many grains the chime ages overall show strong evidence for a major event near 2420 ma analyzed monazite and xenotime grains except monazite from mt riiser larsen have a narrow range of chime ages 2412 42 ma from beaver island and 2429 36 2421 18 and 2431 22 ma from reference peak these ages are indistinguishable from chime ages on zircon 2436 17 and 2420 20 ma and on central older domains of monazite 2404 54 ma from mt riiser larsen and on a zircon rim 2430 15 ma from mt cronus we conclude that the ca 2420 ma ages relate to the uht metamorphism but we do not exclude the possibility that lead loss has lowered the ages by some 50 ma this interpretation is consistent with a preponderance of ages obtained by other methods on the napier complex particularly the ca 2475 25 ma age for a generation of pegmatites coeval with the uht event and containing the uht mineral assemblage beryllian sapphirine khmaralite quartz events prior to 2500 ma and after 2420 ma have also left their imprint on the chime ages for example a 3646 ma chime age suggests that a zircon core is inherited whereas ca 2200 and 2000 ma chime ages in zoned monazite could be due to further lead loss during post uht activity the much younger chime age of 1094 67 ma for monazite from a pegmatite at zircon point casey bay confirms 1000 1100 ma lead loss ages reported for this region and could result from fluid activity in an outlier of rayner activity near the boundary between the rayner and napier complexes approximately 2500 ma ages have been reported from granulite facies but not uht rocks in the madras and nilgiri blocks in south india these blocks and the napier complex could have constituted a single structural unit by late archean time

mantle convection under simulated plates effects of heating modes and ridge and trench migration and implications for the core mantle boundary bathymetry the geoid and benioff zones summary numerical convection models are presented in which plates are simulated by imposing piecewise constant horizontal velocities on the upper boundary a 4 1 box of constant viscosity fluid and two dimensional 2 d flow is assumed four heating modes are compared the four combinations of internal or bottom heating and prescribed bottom temperature or heat flux the case with internal heating and an isothermal base is relevant to lower mantle or whole mantle convection and it yields a lower thermal boundary layer which is laterally variable and can be locally reversed corresponding to heat flowing back into the core locally when scaled to the whole mantle the surface deflections and gravity and geoid perturbations calculated from the models are comparable to those observed at the earth s surface for models with migrating ridges and trenches the flow structure lags well behind the changing surface plate configurations this may help to explain the poor correlation between the main geoid features and plate boundaries trench migration substantially affects the dip of the cool descending fluid because of induced horizontal shear in the vicinity of the trench such shear is small for whole mantle convection but is large for upper mantle convection and would probably result in the tonga benioff zone dipping to the se opposite to the observed dip for the case of upper mantle convection

the metamorphic evolution of granulites at fyfe hills implications for archaean crustal thickness in enderby land antarctica granulites at fyfe hills in enderby land antarctica crystallized at temperatures in excess of 850 c and possibly as high as 1000 c and at pressures of 8 10kbar during the mid to late archaean a number of features including repeated retrograde metamorphism at 5 5 8kbar retrograde reaction textures and rimward zoning in pressure sensitive systems suggest that following peak metamorphism the granulites stabilized at a depth of 18 26 km after stabilization the granulites cooled near isobarically to temperatures of 600 700 c assuming a total crustal thickness of 35 40 km during this late archaean interval of isobaric cooling the peak metamorphic crustal thickness is estimated at 35 56 km this estimate is significantly less than the 60 70 km obtained by summing the depths of the present levels of exposure 26 34 km and the thickness of the crust presently beneath fyfe hills approxi mately 35km and is therefore consistent with independent evidence for extensive post archaean thickening of the enderby land crust

the origin of retrograde shear zones in the napier complex implications for the tectonic evolution of enderby land antarctica abstract retrograde shear zones rszs in granulites at fyfe hills in the napier complex east antarctica formed during a late proterozoic or early palaeozoic reactivation event these rszs preserve a structural and metamorphic record of excavation from depth equivalents of 6 8 kbar 20 26 km to depth equivalents of 3 5 kbar 10 17 km excavation was essentially isothermal suggesting that the rszs formed in an allochthonous terrain during crustal thickening by low angle thrusting rehydration of the rszs was accompanied in part by large ion lithophile element enrichment and reduction in k rb features which are attributed to 1 the retrograde fluid having originated in continental crust that had not previously undergone granulite facies metamorphism and 2 the fluid having been introduced into the rszs by the intrusion of granitic dykes and pegmatites the saturation of these rsz pegmatites in components characteristically depleted in deep crustal and mantle sources such as boron and beryllium supports a low grade source for the retrograde fluid furthermore a rehydration mechanism involving intrusion of pegmatites suggests that the fyfe hills granulites are underlain by a significantly lower grade hydrous terrain

yield estimates of nevada test site explosions obtained from seismic lg waves a methodology is presented for determining the yield of underground nuclear explosions from lg wave amplitudes the methodology is applied to nevada test site nts explosions for which the data from short period vertical component analog seismographs at three stations are used to develop calibration curves for unsaturated material and water saturated rock source conditions the latter curves are found to provide reasonably accurate estimates of the yields of explosions in other areas of the united states and in the french sahara suggesting that they may be applicable to all continental areas if so they also can provide an estimate of the bias of mb p magnitudes between different continental sites for example the lg data from nts explosions indicate a 0 31 0 02 magnitude unit bias between nts and eastern north america similar to the approximately 0 33 unit bias found between western and eastern north america previously by use of earthquake data

rb sr isotopic and geochemical evolution of a recrystallized shear mylonite zone at broken hill relatively narrow 1 to 1 000 metres planar zones of intense shear deformation and retrograde metamorphism retrograde shist zones rsz are common in the willyama complex around broken hill a rb sr isotopic study of one of these zones has revealed an unexpected isotopic pattern all of the analysed points lie to the left of and above the 1665 ma isochron of the potosi gneiss host this result indicates a different isotopic composition of the zone as a whole rather than a chemical redistribution of potosi gneiss within a closed system the isochron plot scatter is similar to that of ubiquitous metamorphic pegmatites in the basement of the broken hill region the shear zone observation however is shown to be unrelated to the pegmatite segregation process and requires the introduction by elemental exchange of a more radiogenic strontium whilst maintaining relatively fixed rb sr ratios significant proportions of sio2 k2o and ba were removed at the same time na2o al2o3 tio2 mgo p2o5 zr ni sc y nb rb remain essentially constant after allowing for volume loss of the removed elements cao and sr increase slightly overall

u pb geochronology from tonagh island east antarctica implications for the timing of ultra high temperature metamorphism of the napier complex abstract ion microprobe u pb zircon geochronology of an orthopyroxene bearing felsic orthogneiss from central tonagh island enderby land east antarctica provides insight into the chronological metamorphic evolution of the archaean napier complex the details of which have been the source of debate for over two decades the orthogneiss crystallised at 2626 28 ma predating peak ultra high temperature uht metamorphism and development of an intense regional s1 gneissosity two subsequent episodes of zircon growth resetting can be identified a minor period of zircon growth occurred at 2546 13 ma the regional significance and geological nature of which is unclear this was followed by an episode of abundant zircon growth as mantles on 2626 ma cores and as anhedral grains partly characterised by high th u 1 2 at 2450 2480 ma this age coincides with both lower and upper concordia intercept ages from other u pb zircon studies and several rb sr and sm nd whole rock isochron ages from the napier complex we conclude that uht metamorphism occurred at 2450 2480 ma and find no compelling evidence that uht occurred much earlier as has been postulated the zircon u pb data from this study also indicates a lower intercept age of 500 ma which coincides with the emplacement of early palaeozoic pegmatite swarms and synchronous infiltration of aqueous fluids into the southwestern regions of the napier complex

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas