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

relationship between magnetic fabrics and shear directions in melange within the miyama formation shimanto accretionary complex japan article i nfo the magnetic fabrics of tectonic melange in the miyama formation shimanto accretionary complex of kii peninsula japan are examined using anisotropy of magnetic susceptibility ams data and they are com pared with shear directions estimated by traditional measurements of p y intersections in the melange in most cases ams data are consistent with the estimated shear directions and they can be divided into two types one with kmax the maximum magnetic susceptibility axes oriented parallel to the shear direction on the shear planes and another with kmax axes oriented parallel to the p y intersections in both types the kmin the minimum magnetic susceptibility axes are oriented perpendicular to the melange foliation however some sites exhibit unexpected kmax axes orientations trending perpendicular to the melange foli ation analyses of texture and magnetic properties indicate that such unexpected orientations are caused by a predominance of single magnetic domains of magnetite and or the existence of ankerite and siderite veins in which the long axes of crystals are oriented perpendicular to the foliation in the melange as a consequence the restored shear direction deduced from ams data improved the credibility of the kinematics of ancient plate motions although ams data provide useful information on shear directions in melanges it is essential to examine the magnetic carriers and their magnetic domain structures for accuracy of the analyses

strain decoupling across the decollement in the region of large slip during the 2011 tohoku oki earthquake from anisotropy of magnetic susceptibility abstract the integrated ocean drilling program iodp expedition 343 japan trench fast drilling project jfast drilled and cored across the plate boundary decollement at site c0019 near the japan trench where large slip occurred during the 2011 tohoku oki earthquake m w 9 0 anisotropy of magnetic susceptibility ams data obtained from core samples show a striking change in magnetic fabric across the decollement in the frontal prism above the decollement the maximum ams axes display a strong preferred orientation in the northeast southwest direction with the intermediate and minimum ams axes distributed in the vertical plane in the underthrust sediments below the decollement the maximum and intermediate ams axes are subhorizontal but variable in direction and the minimum ams axes display an approximately vertical preferred orientation the ams ellipsoids for all samples have an oblate component but the ams ellipsoids of the underthrust sediments are generally less oblate than those of the frontal prism the magnetic fabric of the sediments in the prism are consistent with horizontal tectonic shortening nearly parallel to the plate convergence direction in the japan trench while that in the underthrust sediments represents a vertical uniaxial strain the magnetic fabrics of the frontal prism and underthrust sediments indicate an abrupt change in strain across the decollement at shallow depths and imply that large co seismic slip occurred along a weak plate boundary decollement that is mainly decoupled over the long term

precise u pb ages and geochemistry of palaeoproterozoic mafic dykes from southern west greenland linking the north atlantic and the dharwar cratons palaeoproterozoic magmatism throughout the north atlantic craton nac has produced dyke swarms of diverse orientations and emplacement ages in southern west greenland previously identified swarms include the ca 2500 ma kilarsaarfik dykes ca 2215 ma boninitic norite bn dykes and the abundant ca 2040 ma kangamiut and coeval 2050 2030 ma md3 dykes additional insight into the distribution of swarms is provided by new baddeleyite u pb id tims dates interpreted as emplacement ages for six dykes the geochemistry for a total of 42 samples on 40 dykes including the dated dykes provides some constraints on their petrogenesis two e w trending dykes yield ages of 2365 2 ma and 2374 4 ma representing a hitherto unknown magmatic event we propose the name graedefjord dykes for dykes of this age in southern west greenland one ne sw trending dyke is dated at 2209 5 ma close to a previously published 2214 10 ma age for a n s trending dyke member of bn dykes three dykes yield md3 ages of 2053 2 ma e w trending 2049 6 ma n s trending and 2042 2 ma wnw ese trending ca 2 21 ga old and predominantly ne se and nne sse trending dolerites some of which are high al basalts were derived from a relatively depleted mantle and injected during a boninite dolerite magmatic event together with nearby bn dykes derived from a harzburgitic yet highly lree and lile enriched subcontinental lithospheric mantle sclm ca 2 05 ga kangamiut md3 dolerites were derived from a similar ambient asthenospheric mantle and subsequently experienced crustal assimilation during fractional crystallisation the ca 2370 ma dykes in the nac are coeval with the dharwar giant dyke swarm of the dharwar craton dc and based on this unique age match it is proposed that the dharwar and nac were neighbours in the palaeoproterozoic furthermore both blocks share 222 2 21 ga dykes and matching the trends of 2 2 2 1 and 2 37 ga swarms between the two blocks allows several plausible reconstructions c 2012 published by elsevier b v

slabs hotspots cratons and mantle convection revealed from residual seismic tomography in the upper mantle to further investigate the fate of slabs the structure of hotspots and the style of mantle convection we calculate residual tomographic maps in the upper mantle by excluding from the tomography the first order effects of conductive cooling of oceanic plates deep cratonic roots and cooling or partial melting associated with subducted lithosphere no correlation is found between residual tomography in the upper mantle and the 30 130 ma subduction the good correlations between residual tomography in the transition zone 400 650 km and 0 30 ma subduction at spherical harmonic degree l 2 can be explained either by slab accumulation in the transition zone beneath some subduction zones or by the poor lateral resolution of seismic tomography hotspots correlate with the residual tomography in the shallow 100 400 km and the lowermost mantle at degree l 2 however correlations of hotspots and seismic velocities in the middle mantle and correlations between residual tomography at shallow depths and lowermost mantle tomography are poor therefore the connection between residual tomography at shallow depths and seismic tomography in the lowermost mantle is not clear craton distribution correlates with distribution of hotspots and residual topography very well at degree l 6 this might indicate that cratonic roots may affect the locations of upwellings and thus modulate upper mantle convection or that the degree l 6 could be intrinsic in upper mantle convection with cratons and and other near surface features establishing the pattern

systematic variation of shear induced physical properties and fabrics in the miura boso accretionary prism the earliest processes during off scraping abstract the interrelations between deformation and variations in the physical properties and fabrics of a shallow level accretionary prism have been clarified using measurements of porosity pore size distribution and the anisotropy of magnetic susceptibility ams within the miura boso accretionary prism central japan based on the mode of deformation this onland example of an off scraped body can be divided into three parts from bottom to top imbricate thrusts corresponding to a decollement zone a thrust unit in the lower part of the off scraped body and an upper coherent unit in the top part of the off scraped body physical properties and fabrics are quite variable in each of the three parts even within a particular lithology these observations reflect each structural unit s individual strain history during shallow off scraping and accretion prior to the accretion the sediments underwent vertical compaction when horizontal compressional stresses subsequently began to deform the sediments precursory shear disturbed the previous compaction fabric accompanied by a pervasive porosity decrease over a zone several tens to hundreds of meters in thickness the porosity decrease occurred by large pore collapse which led to the development of an impermeable over consolidated horizon and inevitable high pore fluid pressures below that controlled the position of the subsequent decollement zone displacement along imbricate thrusts within the decollement zone took place just below the over consolidated horizon where high fluid pressure had been generated here a flattening fabric parallel to the imbricate thrust and associated further porosity and pore size reductions occurred in association with the fault movement these findings imply unrecoverable pre fractured strain information of sediments before being included in the decollement zone and suggest that the precursory formation of an over consolidated zone via grain rearrangement and pore size reduction is an essential control on subsequent large deformation i e within a decollement zone in the shallow part of an accretionary prism especially within lithologically homogeneous material

structural characteristics of shallowly buried accretionary prism rapidly uplifted neogene accreted sediments on the miura boso peninsula central japan 1 the upper miocene misaki and nishizaki formations on the miura and boso peninsulas in central japan preserve the deformation features of an off scraped accretionary prism the spatial distribution geometry and style of accretion related deformation with paleotemperature and burial depth estimation are elucidated in this study the deformation structures and textures are similar to those of modern accretionary prisms the low maximum paleotemperature 50 c and high preserved porosity of the sediments 30 50 imply a maximum burial depth of less than 1000 m on the basis of the mode of deformation this off scraped body is divided into an imbricate thrust a thrust unit and an upper coherent unit in ascending order the imbricate thrust corresponds to a branch from the basal decollement zone and is subdivided into a brecciated zone a main gouge zone and a shear band zone the thrust unit hosts a concentration of thrust systems that form various orders of duplex structures while the upper coherent unit is characterized by gravitational instability and earthquake induced deformation without thrust faulting the duplex distribution shear strain and fluid migration associated with the off scraping processes are clearly localized within the imbricate thrust and thrust unit this accretion related deformation occurred before lithification of the sediment and under high fluid pressure induced by shear deformation thickening of the sedimentary sequence and earthquake induced liquefaction these processes are inferred to control the effect on the mode of deformation and the location of thrusting during early deformation of the accretionary prism

metasomatic mantle source and crustal contamination for the formation of the neoproterozoic mafic dike swarm in the northern yangtze block south china abstract in the huangling region of the northern yangtze block south china the 820 ma huangling felsic pluton was intruded by 800 ma mafic dikes these dikes are composed of fine to medium grained diabase porphyritic diabase and olivine gabbro quartz xenocrysts occur locally in the olivine gabbro the rocks are alkaline to sub alkaline in composition and have sio2 ranging from 43 3 to 57 2 wt mgo from 3 7 to 9 2 wt and total alkali from 3 0 to 7 3 wt they exhibit lree enrichment la yb n 3 3 28 5 with slightly negative eu anomalies eu eu 0 6 1 0 their primitive mantle normalized trace element patterns are characterized by enrichment of large ion lithophile elements rb ba th and u depletion of high field strength elements nb ta zr and hf and positive pb and negative ti anomalies they have relatively high initial 87sr 86sr ratios ranging from 0 7052 to 0 7080 and variably negative end t values from 4 2 to 10 9 the rocks have a large range of whole rock ehf values 0 35 to 11 34 and a relatively narrow range of pb isotope ratios 206pb 204pb 16 96 to 17 97 207pb 204pb 15 41 to 15 58 and 208pb 204pb 38 08 to 39 66 initial 87sr 86sr ratios correlate negatively with 206pb 204pb ratios whereas 143nd 144nd and 208pb 204pb ratios and ehf values correlate positively with 206pb 204pb ratios these features are indicative of an em2 source region suggesting a lithospheric mantle source modified by subducted components during the neoproterozoic the quartz xenocrysts and linear isotopic variations indicate strong crustal contamination during magma emplacement the energy constrained assimilation and fractional crystallization ec afc modeling calculations reveal that the parental magmas were contaminated by continental crust including archaean amphibolites granitic gneisses metasedimentary rocks and granitic rocks the mafic dikes are 20 myrs younger than the huangling felsic pluton which was produced by partial melting of a thickened continental crust suggesting that the mafic dikes were a consequence of post orogenic extension the huangling region may have originally been located at the centre of the yangtze block the possible heat source for the generation of the huangling felsic pluton and mafic dikes was asthenospheric wedging resulting from oceanic slab subduction beneath the northern margin of the yangtze block this conclusion rules out the possibility that the yangtze block was located in the central part of the rodinia supercontinent the block was more likely situated along the margin of rodinia

geochemical constraints on the late stage evolution of basaltic magma as revealed by composite dikes within the kangâmiut dike swarm west greenland abstract the kang miut dike swarm in west greenland contains numerous composite dikes with mafic margins and andesitic centers internal chilled margins show that the andesitic centers intruded into the middle of the mafic dikes major element systematics indicate that the fractionation of olivine clinopyroxene plagioclase and fe ti oxides drove the evolution of the kang miut parental magma during its transition from mafic to andesitic compositions incompatible trace elements show a marked relative decrease in middle and heavy rare earth elements ree between the mafic margins and the andesitic centers the decrease in the ree is not explicable by olivine clinopyroxene plagioclase and fe ti oxide fractionation or by the fractionation of the accessory phases apatite zircon or garnet rb sr and sm nd isotopes from margin and center pairs from these composite dikes are nearly identical indicating that crustal contamination had little to no affect on their evolution trace element modeling utilizing the mixing of evolved kang miut magmas and low degree melts derived from partial melting of garnet lherzolite produce excellent fits with the trace element patterns for the andesitic centers these models suggest that the late stage evolution of the kang miut dikes included input of mantle melts produced during the end stages of rifting

inferring the viscosity and the 3 d density structure of the mantle from geoid topography and plate velocities summary in a dynamic earth mantle mass heterogeneities induce gravity anomalies surface velocities and surface topography these lateral density heterogeneities can be estimated on the basis of seismic tomographic models recent papers have described a realistic circulation model that takes into account the observed plate geometry and is able to predict the rotation vectors of the present plates the relationship between the surface observables and the heterogeneities is sensitive to the viscosity stratification of the mantle here we use this model combined with a generalized least squares method in order to infer the viscosity profile of the earth from the surface observations and to get some new insight into the 3 d density structure of the mantle the computed radial viscosity profile presents a continuous increase of more than two orders of magnitude the asthenosphere has a viscosity close to 2 x lo2 pa s no sharp discontinuity is requested at the upper lower mantle interface the largest viscosity 7 x 10 pas is reached in the middle of the lower mantle at greater depth approaching the core mantle boundary the viscosity decreases by one order of magnitude the model suggests that the well known degree 2 and order 2 anomaly in the transition zone of the upper mantle is merely the signature of the slabs it also slightly increases the degree 2 and order 0 in the lower mantle and decreases it in the upper mantle in other words the inversion requests a hotter lower mantle beneath the equator and a colder upper mantle at the same latitudes

evidence from the rare earth element record of mantle melting for cooling of the tertiary iceland plume widespread flood basalt volcanism and continental rifting in the northeast atlantic in the early tertiary period 55 myr ago have been linked to the mantle plume now residing beneath iceland1 2 3 4 5 although much is known about the present day iceland plume6 7 8 9 its thermal structure composition and position in the early tertiary period remain unresolved estimates of its temperature for example range from 1 600 c in some plume models3 to 1 500 c based on the volume and composition of basaltic crust10 11 12 several recent studies4 have emphasized similarities in the thermal and chemical structure of the tertiary and present day plumes to argue for stability of the mantle anomaly whereas others12 13 relate variations in basalt volumes and compositions to changes in plume flux moreover some authors1 2 13 have assumed that the plume was rift centred for its entire history whereas others argue that it became ridge centred only after plate separation14 15 here we report compositional data for 6 000 metres of flood basalts erupted in east greenland close to the inferred plume axis that we use to constrain the tertiary plume structure rare earth element systematics place limits on the pressures and extents of mantle melting and show that the mantle was initially moderately hot 1 500 c but that its temperature declined during flood volcanism these observations are difficult to reconcile with current plume head models and call for important lithospheric control5 10 16 17 18 on actively upwelling mantle along the rifted margin

mineral chemistry and density of subducted basaltic crust at lower mantle pressures subducted slabs are less dense than the surrounding mantle near the base of the transition zone 660 km depth because of the survival of garnet in former basaltic crust by this depth mantle peridotite has transformed to denser perovskitite1 2 the buoyancy of the former basaltic crust may contribute to the observed accu mulation or horizontal displacement of many slabs at the base of the transition zone3 here we report experimental confirmation of the widely held belief that the basaltic crust of slabs eventually transforms to a dense perovskititic lithology stable in the lower mantle synthetic mid ocean ridge basalt more glass subjected to pressures of 45 80 and 100 gpa in a laser heated diamond anvil cell transforms to an assemblage of aluminous mg fe silicate perovskite non quenchable casio3 perovskite stishovite and a sodic aluminous phase with the ca ferrite structure fig 1 per ovskititic more is about 0 06 g cm 3 more dense than a model lower mantle prem derived from seismological data thus even thermally equilibrated perovskititic slabs should encounter no sig nificant hindrance to subduction and convection in the lower mantle

the scourie dyke suite petrogenesis and geochemical nature of the proterozoic sub continental mantle the petrogenesis of bronzite picrite olivine gabbro norite and quartz tholeiite dykes which make up the 2 39 b y scourie dyke swarm cutting the archaean lewisian gneisses of n w scotland is interpreted on the basis of their major and trace element geochemistry most of the dykes bear primary amphibole and or phlogopite and with one exception are all hypersthene or quartz normative apart from one tholeiite dyke which shows relative light rare earth element depletion all the dykes show enrichment in light rare earths and large ion lithophile elements they do not however show an equivalent enrichment in other incompatible high field strength ions such as nb and ta and in this respect resemble island arc and calc alkaline basalts the different dyke types have distinctive rare earth patterns and other trace element ratios which are maintained over a range of major element compositions

mineralogy and composition of the upper mantle seismic velocities are calculated for two petrological models of the upper mantle an olivine rich assemblage pyrolite and a garnet clinopyroxene rich olivine bearing assemblage piclogite these are compared with recent seismic profiles for various tectonic provinces the shield data is most consistent with a cold olivine and orthopyroxene rich lid the seismic lithosphere extending to 150 km followed by a high temperature gradient and or a change in mineralogy that serves to decrease the velocity from 200 to 400 km the velocities follow a 1400 c adiabat the rise tectonic mantle is much slower presumably hotter and is likely to be above the solidus to depths of at least 300 km the high v p v s ratio of the lower oceanic lithosphere in the western pacific is most consistent with eclogite

mantle temperature anomalies along the past and paleoaxes of the galapagos spreading center as inferred from gravity analyses to better understand the effects of hot spots on mid ocean ridge thermal structure we investigate the subsurface density structure of the galapagos spreading center and nearby lithosphere using shipboard gravity and bathymetry data we obtain maps of mantle bouguer anomalies mba by removing from the free air gravity the attractions of seafloor topography and a 6 km thick model crust comparison of observed and theoretical mba profiles along isochrons for ages 0 0 7 7 ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle this result is consistent with the notion that seafloor topography along the galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs along the ridge axis the mba decreases from the east and west toward the galapagos hot spot by 90 mgal reaching a minimum nearest the hot spot at 91 w seafloor topography mirrors the mba along axis increasing by 1 1 km toward the hot spot these variations in mba and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50 25 c toward the hot spot the predicted crustal thickening of 2 4 km nearest the hot spot accounts for 70 75 of the along axis mba and bathymetry anomalies mantle density variations account for the rest of the anomalies from the crustal isochron of age 7 7 ma to the present day axis the along isochron amplitudes of mba decrease from 150 to 90 mgal the corresponding along isochron bathymetry anomalies decrease from 1 7 to 1 1 km these observations along the paleoaxes of the galapagos spreading center indicate that the axial temperature anomaly was 70 hotter in the past 86 25 c and has steadily decreased to 50 25 c as the ridge axis migrated away from the galapagos hot spot these along isochron temperature anomalies however have remained well below that estimated for the hot spot itself 200 c indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10 20 times greater than that along the ridge axis over the past 7 7 m y

comment on magnetostratigraphic study of the kuche depression tarim basin and cenozoic uplift of the tian shan range western china baochun huang john d a piper shoutao peng tao liu zhong li qingchen wang rixiang zhu earth planet sci lett 2006 doi 10 1016 j epsl 2006 09 020 abstract the recent publication of magnetostratigraphic study of the kuche depression tarim basin and cenozoic uplift of the tian shan range western china by b c huang j d a piper s t peng t liu z li q c wang r x zhu earth planet sci lett 2006 doi 10 1016 j epsl 2006 09 020 discusses the cenozoic uplift history of the tianshan mountains by studying the magnetostratigraphy of paleogene to neogene continental sediments from two sections located in the kuche basin at the northern edge of the tarim basin to support their conclusion they reinterpreted a magnetostratigraphic study of the yaha section which lies 10 km south of their sections we previously published j charreau s gilder y chen s dominguez j p avouac s sen m jolivet y li and w wang magnetostratigraphy of the yaha section tarim basin china 11 ma acceleration in erosion and uplift of the tianshan mountains geology 34 3 2006 181 184 here 1 we argue that the interpretations of the sedimentation rate changes they proposed for the kuche sections are partially invalid 2 we disagree with their reinterpretation of the age of the yaha section and 3 we think that the way they interpret their ams data is incorrect

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

regional variations in subsidence rate of oceanic plates a global analysis a global analysis of the dependence of seafloor depth on crustal age has been performed for each large oceanic plate independently each plate has been divided into regions bounded by major tectonic features a total of 32 regions have been considered bathymetry corrected for sediment loading has been plotted as a function of crustal age for each region except for three regions western pacific between 10 s and 40 n north american plate between 24 n and 38 n and african plate between 10 n and 24 n where ocean floor older than 80 100 ma flattens with age in other regions depth increases linearly with age1 2 subsidence rate over each region has been computed by linear regression large variations up to a factor 2 are reported from one region to another asymmetrical subsidence is also observed in a number of areas however the largest asymmetries are less than the regional variations in subsidence rate occurring on individual plates regional variations in subsidence rate appear linearly related to regional variations in ridge crest topography shallow ridge subsiding quickly and deep ridge subsiding slowly hence seafloor depth subsides at a rate modulated by the value of the initial axial depth this observation suggests that regional variations in subsidence rate reflect the thermal regime of the mantle beneath mid ocean ridges rather than lateral temperature variations inside the asthenosphere involving small scale convection

effects of lateral viscosity variations on long wavelength geoid anomalies and topography models for the earth s geoid and dynamically supported topography usually account only for radial vertical variations in mantle rheology and flow structure however expected lateral variations in viscosity due to temperature or stress dependent rheology can degrade our ability to infer mantle structure from geoid models based on seismically determined density contrasts in the mantel theoretically the main difficulty is that the harmonic components of the flow stress deformation field are no longer independent and exact analytical methods cannot account for this mode coupling for large thermal density contrasts of small lateral dimension e g subducted slabs it is difficult to form any general theory for these effects however we believe that the largest geoid anomalies which occur at the lowest harmonic degrees l 2 3 are due to a global pattern of relatively mild temperature variations for convection with mainly broadscale thermal and rheological changes some relatively straightforward ideas can be developed combining results from simple perturbation theory and numerical models for convective flow we find the following 1 if we are correct in inferring viscosity variations of less than one order of magnitude globally the longest wavelength geoid anomalies l 2 3 are not seriously contaminated by lateral viscosity variations 2 contamination of the higher degrees l 4 is expected and in particular self coupling between l 2 heterogeneity and viscosity variations may be observable at degree 4 the most severe contamination occurs at the doubled harmonic l 4 of the dominant heterogeneity harmonic l 2 3 perturbation techniques are adequate to model global scale lateral viscosity variations of an order of magnitude or less 4 the steady state surface deformation problem at the heart of our geoid models may be more robust with respect to broadscale lateral viscosity variations than are models for transient deformations e g postglacial rebound 5 for loading by broadscale density heterogeneity the effects of stress dependent rheology are probably secondary to the effects of radial viscosity stratification

basin evolution in the arc arc izu collision zone mio pliocene miura group central japan a comparative study of the present collision zone between the izu bonin ridge island arc with mainland japan honshu arc and the mio pliocene of onshore se japan suggests that arc arc collision processes and the resulting stratigraphic successions may be repetitious and predictable arc arc collision has led to the incremental accretion of segments of delaminated izu bonin arc crust onto the honshu arc associated with the sequential southward migration in jumps of the plate boundary and trench prior to accretion of a segment of izu bonin arc crust the leading edge underwent uplift to generate an approximately trench parallel topographic high the zenisu ridge being the present example with the hayama mineoka uplift zone as a mio pliocene example the ridge separated a northern trench or trough from a southern intra oceanic arc basin during collision accretion the trench received both honshu arc derived terrigenous and izu bonin arc derived volcaniclastic sediments whereas the arc basin tended to receive only arc deposits during the final stages of accretion the arc basin began to receive ever increasing volumes of terrigenous honshu arc derived detritus fed through basement controlled canyons the accretionary process was accompanied by intense deformation and the residual deep marine basin was then infilled above an angular unconformity

reply to the comment by j charreau et al on magnetostratigraphic study of the kuche depression tarim basin and cenozoic uplift of the tian shan range western china earth planet sci lett 2008 doi 10 1016 j epsl 2008 01 025

petrogenetic relationship between palaeoproterozoic tholeiitic dykes and associated high mg noritic dykes labrador canada abstract trace element and sr nd isotopic studies of the palaeoproterozoic 2235 ma kikkertavak dyke swarm in labrador provide information on early sub continental lithosphere development there are three distinct chemical dyke types two are tholeiitic the third of high mg norite composition petrogenetically it is not possible to relate any of the chemical groups one to another by simple fractional crystallisation processes nor can they be derived by contaminating asthenospheric magmas with local crustal components from their sr nd isotopic characteristics in relation to their rare earth and multielement patterns it is clear that significant heterogeneity existed in the source regions of the dyke magmas at the time they were generated and that the mantle processes responsible can be linked with the phase of crustal growth in the region at 3 0 ga modelling suggests that the observed compositions can be accounted for by lower lithosphere asthenosphere mantle sources which have been variably depleted by previous episodes of basalt magma extraction and then re enriched by a lamproitic melt component and possibly by progressive infiltration of melts from a subducted sediment or subduction component high mg noritic dykes are relatively common worldwide in the palaeoproterozoic and are often broadly coeval with tholeiitic dykes but are much less common in younger dyke suites most have compositions which require a refractory major element composition coupled with significant re enrichment in incompatible lithophile elements recent models for cyclic mantle overturns with related deep mantle plumes begin to provide the right thermal and compositional environment beneath immature lithosphere which can generate the dyke types observed

regionalized models for the thermal evolution of the earth abstract traditional models for the heat loss in oceanic and continental regions are combined into a regionalized model for the thermal evolution of the earth the need for regionalization is obvious when one considers that the mantle loses 3 to 4 times as much heat per unit area in oceanic regions than in continental areas the present day rate of heat loss together with a geochemical estimate of the concentration of heat producing elements in the earth fixes the response time of the thermally convecting mantle the response time in turn can be used to select the most reasonable representation for mantle convection in terms of the sensitivity of viscosity on temperature and layering versus mantle wide circulation present geochemical estimates of the bulk composition of the earth are most easily reconciled with the observed heat flow if the mantle is layered and its rheology is slightly less temperature dependent than generally assumed the layered system can produce sufficiently high temperatures to explain the high magnesian komatiites of the archean one difficulty with the models is that they predict widespread melting at shallow depth in the early stages of earth history but do not address how such melting affects and alters the heat transfer mechanisms

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

effects of trench migration on fall of stagnant slabs into the lower mantle abstract global seismic tomography has recently revealed horizontally lying slabs near the upper and lower mantle boundary beneath the northwestern pacific region although physical mechanisms that could produce such slab stagnation have been proposed based on numerical simulations there has been little research into what occurs after slab stagnation we proposed trench advance and trench jumps as effective mechanisms related to the fall of stagnant slabs into the lower mantle and our numerical simulations of temperature and fluid flow associated with slab subduction in a 2 d box model confirmed these mechanisms our results indicate that a supply of slab material associated with further slab subduction after slab stagnation plays an important role in differentiating further slab stagnation from the falling of slabs into the lower mantle a shortage of material supply would produce extended slab stagnation near the 660 km boundary for ringwoodite to perovskite magnesiowustite phase transformation whereas downward force due to further slab subduction on a stagnant slab would enhance its fall into the lower mantle the behaviors of falling stagnant slabs were not affected by clapeyron slope values associated with phase equilibrium transformation within the range from 3 0 to 0 0 mpa k compared with models of normal mantle viscosity a high viscosity lower mantle played a role in hindering the fall of slabs into the lower mantle resulting in complicated shapes and slow falling velocities lower mantle viscosity structure also affected slab behavior slabs tended to stagnate when a low viscosity zone lvz existed just below a depth of 660 km because friction between the slab and the lvz was weak there slab stagnation around a depth of 660 km also occurred when a high viscosity zone existed below a depth of 1200 km and acted as a resistive force against a slab even if the slab existed in the lower part of the upper mantle

experimental deformation of synthetic magnetite bearing calcite sandstones effects on remanence bulk magnetic properties and magnetic anisotropy we have quantified effects of experimental deformation on the magnetic properties of a set of synthetic calcite sandstone samples containing magnetite the deformation was carried out in a microcomputer controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10 5 s 1 most samples were deformed under dry conditions but a few were deformed with a pore fluid present the samples deformed under dry conditions required substantially higher differential stresses macroscopically ductile shortening strains of up to 25 produced the following irreversible changes in magnetic properties 1 increased bulk coercivity remanence coercivity and mean anhysteretic remanence susceptibility 2 decreased mean low field susceptibility 3 decreases in the component of remanence parallel to shortening 4 smaller decreases for most samples in the component normal to shortening resulting in a net rotation of the remanence away from the shortening axis 5 larger decreases in the normal component in a few samples resulting in a net rotation of the remanence towards the shortening axis 6 increased magnetic anisotropy and 7 increased deformation of initial magnetic ellipsoids a comparison of data for samples deformed under dry and wet conditions higher and lower differential stresses respectively indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain i e rotation and displacement of particles whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress

predicting and testing continental vertical motion histories since the paleozoic abstract dynamic topography at the earth s surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry sea level change continental flooding sedimentation and erosion these observations provide important constraints on and test of mantle dynamic models based on global mantle convection models coupled with the surface plate motion history we compute dynamic topography and its history for the last 400 ma associated with pangea assembly and breakup with particular focus on cratonic regions we propose that burial unroofing histories of cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models our models show that there are currently two broad dynamic topography highs in the pacific and africa for the present day earth that are associated with the broad warm structures i e superplumes in the deep mantle consistent with previous proposals of dynamical support for the pacific and african superswells our models reveal that pangea assembly and breakup by affecting subduction and mantle upwelling processes have significant effects on continental vertical motions our models predict that the slave craton in north america subsides before pangea assembly at 330 ma but uplifts significantly from 330 ma to 240 ma in response to pre pangea subduction and post assembly mantle warming the kaapvaal craton of africa is predicted to undergo uplift from 180 ma to 90 ma after pangea breakup but its dynamic topography remains stable for the last 90 ma the predicted histories of elevation change for the slave and kaapvaal cratons compare well with the burial unroofing histories inferred from thermochronology studies thus supporting our dynamic models including the development of the african superplume mantle structure the vertical motion histories for other cratons can provide further tests of and constraints on our mantle dynamic models

geohazards in coastal areas near the northernmost sagami trough central japan review of neotectonic activity in onshore and offshore areas of the izu island arc collision subduction zone

paleoproterozoic mafic dike swarms of northeast laurentia products of plumes or ambient mantle abstract the occurrence of mafic dike swarms in archean and paleoproterozoic terranes is often cited as evidence for mantle plume activity associated with abnormally high mantle potential temperatures t p quantitative modeling of rare earth element ree abundances for the 2 04 ga kang miut dike swarm in west greenland and comparisons among other paleoproterozoic dike swarms in northeast laurentia indicates that the conditions for mantle melt generation were similar beneath much of laurentia and only ca 120 c hotter than present day ambient mantle t p ca 1300 c these thermal conditions correspond closely to estimates of ambient mantle potential temperatures during the paleoproterozoic era based on secular cooling models and continental freeboard constraints thus the geochemistry of most paleoproterozoic dikes of northeast laurentia can be accounted for by partial melting of upwelling mantle accompanying passive rifting and do not necessitate plume activity ambient mantle conditions during the paleoproterozoic t p ca 1420 c are similar to those inferred for the paleogene volcanic rift margins of the north atlantic igneous province where the rate of crustal production associated with early oceanic spreading was two to three times higher than at the modern ridge likewise rifting under ambient mantle conditions in the precambrian could have resulted in similar rates of crustal productivity and thus contributed significantly to continental growth

sh velocity and compositional models near the 660 km discontinuity beneath south america and northeast asia 1 we constrain sh wave velocity structures near the 660 km discontinuity beneath south america and northeast asia using triplicated phases near the discontinuity recorded in the epicentral distance range of 10 35 for three deep events we then explore mineralogical and compositional models appropriate for explaining the inferred seismic structures between the two regions sh velocity structures near the 660 km discontinuity are found to be different in the two regions beneath south america the velocity gradient above the 660 km discontinuity is larger than that of preliminary earth reference model prem while the velocity jump across the discontinuity is the same as prem beneath northeast asia the velocity gradient above the 660 km discontinuity is the same as that of prem while the velocity jump across the discontinuity is larger than prem both regions are characterized by a large velocity gradient extending about 80 km deep below the 660 km discontinuity the different velocity structures require different mineralogical models in the transition zone in the two regions the larger velocity gradient above the 660 km discontinuity beneath south america requires existence of the ilmenite phase in the bottom of transition zone while that beneath northeast asia can be explained by the temperature and pressure dependence of elastic properties of the major mantle mineral assemblages the observed large velocity gradients in the top of the lower mantle can be explained by gradual transformation of garnet to perovskite persisting to greater depths the velocity jump across the 660 km discontinuity beneath south america in the presence of the ilmenite phase in the bottom of the transition zone requires a bulk composition of more garnet than the pyrolite model while a larger velocity jump across the discontinuity beneath northeast asia requires a larger fraction of garnet transforming to perovskite across the discontinuity than what is required for explaining prem these different mineralogical models can be caused by different mantle temperature or composition especially the aluminum content in mantle composition the presence of garnet 80 km below the 660 km discontinuity in the two regions may be explained by a uniform composition in the lower mantle with an aluminum content of 3 4 the existence of ilmenite in the bottom of the transition zone beneath south america and the absence of ilmenite beneath northeast asia can be explained by either a difference in mantle temperature of about 100 c with that beneath south america being lower between the two regions assuming a uniform mantle composition or alternatively a difference in aluminum content of about 1 with that beneath south america being lower between the two regions without invoking a temperature difference between the two regions we also discuss conditions of mantle composition and temperature that double discontinuities may occur near the 660 km depth as well as the depth separation and velocity jumps of the double discontinuities for the inferred mantle temperature and composition beneath south america and northeast asia the maximum separation of the double discontinuities is 20 km and cannot be resolved by the sh wave data

an inversion for radial viscosity structure using seismic tomography using a uniform velocity density scaling we have inverted the seismically inferred 3 dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid we are able to obtain 60 72 variance reductions for three different s wave tomographic models the resulting viscosity structures are remarkably similar showing a high viscosity lid a low viscosity zone in the transition region and a high viscosity lower manfie a resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data however the resolution in the lower manfie is poorer our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400 670 km as opposed to 100 400 kin

regional variations in subsidence rate of lithospheric plates implication for thermal cooling models abstract although simple thermal models of lithospheric cooling predict to first order the general behaviour of observed seafloor depth with increasing age important regional variations in seafloor subsidence in the range 250 400 m ma 1 2 are reported for several lithospheric plates such variations cannot be accounted for by classical cooling models unless implausible variations in asthenospheric temperature of 550 c are assumed here we present an alternative cooling model which assumes that at the ridge axis the temperature may deviate from the mean asthenospheric temperature such a model satisfactorily explains the data provided that the temperature deviation is 100 c only

collision of the izu arc with honshu and the effects of oblique subduction in the miura boso peninsulas abstract collision of the izu arc in central japan is discussed with a focus on its tectonic effects to the east of the arc in the miura boso peninsulas of honshu the tectonics are the combination of the following events philippine sea plate spreading in the late oligocene to early miocene opening of the sea of japan in the middle miocene obduction of ophiolitic rocks in the northeasternmost corner of the philippine sea plate and forearc sedimentation between the honshu and izu arcs oblique subduction has shifted the plate boundary from northeast to southwest from the present mineoka tectonic belt through the miura fold belt to the sagami trough since the miocene remarkable right lateral transpressional deformation occurred throughout this period of the oblique collision and subduction

u pb baddeleyite ages for the scourie dyke swarm scotland evidence for two distinct intrusion events precise and accurate radiometric ages for continental mafic dyke swarms are a prerequisite for global correlation of mafic magmatic events calibration of apparent polar wander paths and deciphering mechanisms of dyke emplacement precambrian dyke swarms such as the scourie dyke swarm in north west scotland are invaluable time markers so that a precise and accurate knowledge of their emplacement age is critical when unravelling the complex geological evolution of many terrains precise dating is often difficult however because magma interaction with country rock and subsequent metamorphic events can severely perturb some isotopic systems recent advances in u pb geochronology1 3 combined with the discovery that some mafic dykes contain trace amounts of uranium bearing minerals such as baddeleyite zro2 and or zircon have made it possible to obtain u pb ages for these rocks with a precision typically on the order of 1 2 myr4 5 some of the first detailed u pb studies of precambrian mafic dyke swarms5 6 have shown that large volumes of mafic magma 100 000 km3 were emplaced into the continental crust in surprisingly short periods of time 2 myr here we report the results from a u pb study of three members of the scourie dyke swarm and find at least two periods of dyke emplacement at 2 418 and 1 992 myr bp we speculate on a potential global correlation of early proterozoic mafic magmatism and hence on the origin of dyke swarms

magnetic fabric development in the tertiary accretionary complex in the boso and miura peninsulas of central japan it has been demonstrated that the anisotropy of magnetic susceptibility ams is a sensitive indicator for distinguishing various processes that had acted during and after the deposition of sediments recently much attention has been focused on proving its usefulness in distinguishing early deformation signatures in sediments which show no visible grain scale deformation here we present the ams results of the neogene miura group of central japan which was formed by arc arc collision tectonics with subdivisions of two contrasting tectonic provinces the southern province a highly deformed accretionary prism and slope basin and the northern province a gently deformed forearc basin samples were collected from 21 sites in both tectonic settings we obtained from axes orientations three typical fabric domains considered to represent sedimentary tectonically deformed and their intermediate types the shape parameters of ams also revealed distinctions clearly sedimentary fabrics are more foliated and anisotropic deformed fabrics show a more prolate shape and are closer to being isotropic the sedimentary fabrics predominate in the northern forearc basin sequence whereas the intermediate and deformed fabrics occur predominantly in the southern province the general trend of the shape of the magnetic fabric in studied sediments reveals a clear evolution in the pattern of the isotropic tectonic fabric constructed from foliated sedimentary fabric

seismic constraints on mantle flow and topography of the 660 km discontinuity evidence for whole mantle convection recent seismic models of three dimensional mantle structure and topography on the transition zone seismic discontinuities permit the direct evaluation of the buoyancy forces that drive large scale mantle flow this buoyancy distribution coupled with radial viscosity models that are consistent with the observed geoid predicts flow patterns with significant vertical flow through the 660 km phase boundary the observed topography on the 660 km discontinuity acts to inhibit convection across the boundary but is two to four times too small to prevent whole mantle convection

strain partitioning across a subduction thrust fault near the deformation front of the hikurangi subduction margin new zealand a magnetic fabric study on iodp expedition 375 site u1518 abstract understanding the distribution of strain along thrust and splay faults in active accretionary systems is crucial to understand the mechanical properties of the sediments and the strength of the fault zone and its slip behavior this paper investigates the distribution of strain through sediment compaction and texture development across the p paku fault a major splay fault near the deformation front of the hikurangi subduction margin new zealand using the anisotropy of magnetic susceptibility technique ams international ocean discovery program site u1518 penetrated hanging wall fault zone and footwall sequences to a maximum depth of 484 9 meters below seafloor a total of 330 discrete samples was subjected to ams measurements and magnetic remanence data used to reconstruct the axial orientation of each sample in a geographic reference frame the ams display distinct fabric differences between hanging wall through the fault zone and footwall domains demonstrating that strain is partitioned across the fault zone hanging wall sequences show a strike parallel northeasterly lineation of kmax and weakly prolate shapes typical for a component of northeast southwest lateral shortening in contrast footwall sequences are more oblate and show a clustering of kmax in northerly direction this demonstrates that strain in the footwall is dominated by gravitational loading however a component of sub horizontal east westerly strain parallel to the convergence direction of the pacific plate exists strain decoupling between hanging and footwall sequences occurs near the top of the p paku fault zone differences in the degree of magnetic susceptibility between footwall sediments incorporated into the fault zone and the underlying undeformed footwall sequences are indicative for the progressive dewatering of the underconsolidated footwall sequences

upper mantle sh and p velocity structures and compositional models beneath southern africa abstract we constrain sh and p wave velocity structures in the upper mantle beneath southern africa using triplicated phases recorded in the epicentral distance range of 11 28 for one shallow event we then explore thermal and compositional models appropriate for explaining the inferred seismic structures in the region both sh and p wave data suggest presence of a low velocity zone with velocity reductions of at least 5 for s waves and 2 for p waves beneath a 150 km thick high velocity lithospheric lid seismic observations also suggest that the p s ratio is larger 1 88 in the transition zone than in the lithospheric lid 1 70 the inferred p wave velocity jump across the 660 km discontinuity is small

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

the excess temperature of plumes rising from the core mantle boundary we study the rise of conduit type thermal plumes from the core mantle boundary cmb in a simplified numerical model viscosity is a strong function of temperature and pressure assuming that all plumes start with the same excess temperature at the cmb we find that their temperature in the upper mantle is controlled by the plume flux and is insensitive to variations in viscosity or other material parameters or to the kind of flow law newtonian or non linear the reduction of excess temperature with height is stronger for weaker plumes plumes with a buoyancy flux of less than 1000 kg s should have cooled so much that they would not melt beneath old lithosphere the existence of such hot spots possibly indicates that not all plumes rise from the cmb

melange and its seismogenic roof decollement a plate boundary fault rock in the subduction zone an example from the shimanto belt japan 1 the mugi melange located in western shikoku of the shimanto belt shows systematic y p deformation fabrics formed by microshear and pressure solution that penetrate throughout the melange pile magnetic susceptibility ellipsoids obtained from the anisotropy of magnetic susceptibility ams are highly oblate maximum and minimum axes of the ellipsoids are consistent with the shear orientation of the melange and the mean pole of p surfaces respectively this agreement suggests that the mugi melange was formed as a result of underthrusting of trench filling sediment vitrinite reflectance ranges from 2 52 to 3 08 which corresponds to a maximum paleotemperature of 180 200 c pseudotachylyte evidence of a seismogenic slip was found in the upper boundary roof fault of the mugi melange however there is not a thermal gap between the melange and the overlying coherent piles and the temperature from vitrinite reflectance gradually rises downward from the coherent piles to the melange beyond the boundary fault which suggests that paleoisotherms parallel the boundary fault orientation the isotherms in the seismogenic zone are estimated as subparallel to the plate boundary decollement therefore the setting of the cataclastic boundary fault which includes pseudotachylyte appears to be a major plate boundary thrust or a subhorizontal splay fault a probable geologic setting that accounts for the mugi melange and the seismogenic roof fault is partitioning of the slip along the plate boundary fault in space and time interseismic slip in the melange and seismic slip along the roof fault

global relationship between oceanic geoid and topography in order to compare model predictions of upper mantle convection with surface observations we have computed global residual depth and geoid anomalies on a 5 5 grid over all oceans after removal of lithospheric cooling effects and long wavelength geoid components 4000 km related to lower mantle sources we have derived the transfer function admittance of geoid over topography as a function of wavelength in the range 1000 4000 km the observed admittance is positive at all and presents a maximum of 2 5 3 5 m km at 2750 km comparing this result with the theoretical admittance computed for the lithospheric cooling plate model we ascribe most of the admittance peak at 2750 km to errors in the depth age and geoid age correction this suggests that a standard age correction applied to all seafloor is not valid at shorter wavelengths the admittance remains roughly constant on the order of 1 5 m km and can be explained in terms of sediment loading and crustal thickening effects comparing spatially the residual geoid and depth anomaly maps we note a positive correlation over a few areas coinciding with hot spot swells volcanic plateaus or seafloor deeps the geoid versus depth ratio amounts then to 6 8 m km in agreement with the values predicted above upwellings and downwellings by models of constant viscosity convection however the global admittance analysis leads to much lower values at all wavelengths suggesting that these areas are too limited in number and in extent to produce any significant signal

age and origin of early proterozoic dolerite dykes in south west greenland samples of three dolerite dykes from south west greenland give a pooled weighted mean rb sr whole rock isochron age of 2 130 65 ma initial 87sr 86sr ratios are 0 70155 0 00018 for one of the dykes naujat and 0 70277 0 00012 for the other two torssut and akinaq a fourth dyke qaqarssuaq did not yield an isochron torssut samples have significantly lower initial 207pb 204pb ratios than samples from the naujat dyke indicating that the torssut dyke has been affected by crustal contamination

effects of phase transions and possible compositional changes on the seismological structure near 650 km depth if the lower mantle is enriched in fe si relative to the upper mantle the compositional boundary is expected to be an interface with a density jump of 5 2 and with depth varying laterally by 100 300 km but with changes in seismic velocities that are nearly unobservable 0 to 1 0 2 5 discontinuities in compressional vp and shear vs wave velocities in contrast the polymorphic transformation of transition zone minerals to the perovskite bearing assemblage of the lower mantle is estimated to produce a jump of 5 10 percent in density vp and vs near 650 km depth and the depth of the seismological discontinuity due to the phase transition is relatively independent 50 150 km variation of temperature or horizontal position if present a compositional boundary is not in general expected to occur at the identical depth 650 km as the phase transition to the perovskitebearing assemblage although the depths of the two may be linked

a sharp and flat section of the core mantle boundary the transition zone between the earth s core and mantle plays an important role as a boundary layer for mantle and core convection1 this zone conducts a large amount of heat from the core to the mantle and contains at least one thermal boundary layer2 3 the proximity of reactive silicates and molten iron leads to the possibility of zones of intermediate composition4 here we investigate one region of the core mantle boundary using seismic waves that are converted from shear to compressional waves by reflection at the boundary the use of this phase known as scp the large number of receiving stations and the large aperture of our array all provide higher resolution than has previously been possible5 7 for the 350 km long section of the core mantle boundary under the northeast pacific sampled by the reflections the local boundary topography has an amplitude of less than 500 m no sharp radial gradients exist in the 400 km above the boundary and the mantle to core transition occurs over less than 1 km the simplicity of the structure near and above the core mantle boundary argues against chemical heterogeneity at the base of the mantle in this location

measuring dynamic topography an analysis of southeast asia 1 theoretical models of dynamic topography generated by subducting slabs and consistent with the observed geoid predict 1 2 km of subsidence on wavelengths of 102 103 km the demonstrable existence of such subsidence has important implications for understanding sedimentary basin formation relative sea level changes as well as the earth s viscosity and density structure here we present the results of a combined analysis of subsidence data from oceanic marginal basins and their fringing continental shelves we have chosen to analyze a large area of southeast asia where dynamical predictions are believed to be well constrained by the history of subduction our joint analysis constrains the maximum amplitude of dynamic subsidence to be 300 m with a range of 0 500 m significantly less than predicted actively spreading marginal basins have no resolvable dynamic subsidence but after removing the effects of crustal thickness variation a few inactive marginal basins are consistently 200 500 m deeper than normal adjacent continental sedimentary basins record a possible dynamic subsidence of 0 500 m although wavelengths are shorter than expected 102 km there is also disagreement between the predicted and observed distribution of anomalous subsidence as well as inconsistencies in initiation and duration these results suggest that mass excess within the mantle represented by subducted slabs has little discernible effect on surface topography instead subsidence and marine inundations throughout southeast asia are predominantly controlled by lithospheric extension and or by foreland loading the spatial and temporal distribution of anomalous subsidence suggests that it might not be caused by dynamic topography associated with subducting slabs a large discrepancy between predicted and observed dynamic topography could be reconciled by changing the density structure of subducted slabs by supporting mass excess with a marked increase in viscosity at depth or by decoupling slabs from the surface with a very low viscosity layer

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

the fate of slabs inferred from seismic tomography and 130 million years of subduction the volume and location of subducted plate since 130 ma are reconstructed from magnetic anomalies and updated finite rotation parameters describing relative motions in the ocean basins and between plates and hotspots the area of subducted plate is calculated from the relative motions between overriding and overridden plates along the length of convergence in the hotspot reference frame in 5 ma intervals ridge locations are reconstructed by rotating magnetic anomalies from their present to their former positions the distances between trenches and ridges at a certain point of time in the spreading direction are thus estimated and are used to determine the ages of trenches according to the spreading history recorded in present day oceans the thickness of the oceanic plate is obtained from the age based on the half space cooling model about 3 45 10 10 km 3 of oceanic plate has been subducted during the past 130 ma with maximum accumulation beneath southeast eurasia at spherical harmonic degree l 2 where subduction flux peaks excellent correlation is found between four seismic tomographic models at the top of the lower mantle about 800 1100 km deep and predicted slab locations for some seismic models excellent correlation is also found at other degrees in that depth range some models also have good correlations with predicted slab locations in the deep mantle at l 2 direct comparison between subduction and seismic tomographic patterns at 800 1100 km shows that subduction history correlates with tomography very well in terms of location and amplitude of anomalies cold downwellings which may be related to slabs appear to be trapped in the mesosphere or middle mantle correlations between seismic tomography and subduction over different time periods support this conclusion there are also some correlations in the upper mantle and in the deep lower mantle although they are generally not as significant as those in the 800 1100 km depth range there may be a significant boundary in the mantle near the 800 1100 km depth from a geodynamic and chemical point of view the lower mantle may start at a depth closer to 1000 km than to 670 km

fabric based criteria to distinguish tectonic from sedimentary melanges in the shimanto accretionary complex yakushima island sw japan abstract fabric analyses based on the anisotropy of magnetic susceptibility ams provide a criterion with which to distinguish tectonic from sedimentary melanges in the tertiary shimanto accretionary complex yakushima island sw japan sedimentary melanges are characterized by fabric overprinting associated with compaction and diagenesis the magnetic ellipsoids for such melanges represent a cleavage bedding parallel magnetic foliation similar to that obtained for coherent sedimentary layers in nearby rocks the tectonic melanges and tectonically formed duplex structure are characterized by grain rotation whereby the grain short axis is rotated about axes oriented parallel to the intersection between s and c planes and the long axis of pencil cleavage perpendicular to the shear direction the ams data obtained for tectonic melanges and duplex structure show a less intense magnetic foliation compared with the sedimentary melanges with kmax the maximum magnetic susceptibility axes oriented parallel to the intersection between s and c planes and the long axis of pencil cleavage and kmin the minimum magnetic susceptibility axes showing a girdle distribution about kmax these contrasting paths of fabric development may reflect different stages of melange formation during the overall history of sediment deposition burial accretion and uplift sedimentary melanges are likely to be strongly compacted given that they formed soon after sedimentation in contrast tectonic melanges and duplex structure contain a shear induced fabric because they develop at a relatively late stage once the sediments have been sufficiently compacted for such a fabric to develop possibly at levels near the maximum burial depth

topography of the ocean floor thermal evolution of the lithosphere and interaction of deep mantle heterogeneities with the lithosphere topography associated with large wave length geoid is derived by two methods first the bathymetry corrected for sediment loading is decomposed using a least squares method as a function of age plus terms proportional to the geoid the topography at old ages is flatter predicted by the law at degrees 2 and 3 very little topography is associated with the geoid the transfer function is not even certain in sign but is never smaller than 0 6 more topography seems associated with higher degree harmonics 6 and 7 second the function of age alone best fitting the topography is determined the correlation coefficients of the residual topography with the geoid are then computed a slight correlation appears for degree 6 but not for degrees 2 and 3 these results suggest a mechanical decoupling between the lower mantle mass heterogeneities and the lithosphere

dynamic surface topography a new interpretation based upon mantle flow models derived from seismic tomography the very long wavelength i e in the degree range l 1 8 dynamic topography at the earth s surface estimated by isostatic reduction of the observed topography is shown to be in good agreement with the topography predicted by a model of viscous flow in the mantle this flow model was previously derived by independent consideration of the observed nonhydrostatic geoid and is based upon a recent model of large scale seismic shear velocity heterogeneity in the mantle our estimate and prediction of the long wavelength dynamic surface topography reveals significant depressions 2 3 km of the continental shields relative to their hydrostatic reference positions

magnetostratigraphy of the kelasu section in the baicheng depression southern tian shan northwestern china abstract in order to better constrain chronology of the cenozoic sediments in the foreland basin of the southern tian shan we carried out a magnetostratigraphic study along the kelasu river in the baicheng depression northwestern china this is the basis for future studies of the tectonic shortening history and paleoclimatic changes stepwise thermal demagnetization was used to isolate the high temperature characteristic component chrm from 1521 oriented samples collected along two overlapped sections the chrm directions are interpreted to be acquired at or close to the time of rock formation a composite magnetostratigraphic column composed of 86 45 normal and 41 reversed polarity chrons can correlate with gpts ck95 from 54 ma to 7 6 ma the basal ages of the kumugeliemu suweiyi jidike and the kangcun formations are 54 ma 46 ma 34 ma and 9 7 ma respectively the changes of anisotropy of magnetic susceptibility ams parameters p j and t cannot be used to reflect the cenozoic uplift of the southern chinese tian shan due to the parameters have a significant linear positive correlation with the bulk magnetic susceptibility k m suggesting a sedimentary provenance control based on the alignments of the ams we concluded that the paleo river channel flowed from north to south being similar to the present river flowing direction suggesting that there was still a residue relief of the tian shan orogen after the long term mesozoic denudation or the reactivation of the tian shan may have been initiated as early as 54 ma

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

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

three dimensional seismic models of the earth s mantle accurate models of the distribution of elastic heterogeneity in the earth s mantle are important in many areas of geophysics the purpose of this paper is to characterize and compare quantitatively a set of recent three dimensional models of the elastic structure of the earth to assess their similarities and differences and to analyze their fit to one class of data in order to highlight fruitful directions for future research the aspherical models considered are the following m84c woodhouse and dziewonski 1984 lo2 56 dziewonski 1984 mdlsh tanimoto 1990a sh 10c 17 masters et al 1992 and s12 wm13 su et al 1994 through much of the discussion m84c and lo2 56 are combined into a single whole mantle model m84c lo2 56 the fit of each model to previously tabulated even degree normal mode structure coefficients taken from smith and masters 1989a and ritzwoller et al 1988 for multiplets along the normal mode fundamental and first second and fifth overtone branches is also presented rather than concentrating on detailed comparisons of specific features of the models analyses of these models are general and statistical in nature in particular we focus on a comparison of the amplitude and the radial and geographical distribution of heterogeneity in each model and how variations in each affect the fit to the normal mode observations in general the results of the comparisons between the models are encouraging especially with respect to the geographical distribution of heterogeneity and in the fit to the normal mode data sensitive to the upper mantle and lowermost lower mantle there remain however significant discrepancies in amplitude and in the radial distribution of heterogeneity especially near the top of the upper mantle and near the top of the lower mantle the confident use of these models to constrain compositional and dynamical information about the mantle will await the resolution of these discrepancies the factors that may be responsible for the differences in the models and or for the misfit between the observed and predicted normal mode data are divided into two types intrinsic or procedural and extrinsic or structural we discuss only three extrinsic factors at length here including errors in the reference crustal models unmodeled topography on discontinuities in the interior of the mantle and errors in the assumed relationships between shear s and compressional p heterogeneity

strain removal applied to paleomagnetic directions in an orogenic belt the permian red slates of the alpes maritimes france abstract detailed investigations of relationships between strain and directions of characteristic remanent magnetization chrm have been conducted on the permian red series of the alpes maritimes france the analysis of the results obtained by structural and paleomagnetic studies on 14 sites leads to the following observations in most sites the mean direction of the strain shortening axis is close to the expected permian paleomagnetic direction in this case we observe a large scatter of the chrm directions within each site which is interpreted as an increase of an initial dispersion due to strain moreover this dispersion more or less increases with strain intensity in other cases where the angle between the shortening axis and the paleomagnetic direction is high the whole population is deflected towards the cleavage xy plane these observations lead us to assume a material line like behaviour of the chrm vectors and consequently an attempt has been made to recalculate these directions by strain removal these calculations give positive results at two levels 1 a statistically significant improvement in the clustering of the within site distributions and 2 after tilt correction using unstrained bedding planes a significant improvement in the clustering of the between site distribution the formation mean direction after strain removal is in good agreement with known permian results for stable europe

magnetostratigraphic study of the kuche depression tarim basin and cenozoic uplift of the tian shan range western china abstract magnetostratigraphic and rock magnetic results from 1006 horizons in paleogene and neogene sediments between the upper kumugeliemu formation and the base of the kuche formation within the kuche depression of the tarim basin nw china are used to evaluate postcollisional uplift and deformation in the tian shan range characteristic remanences with both normal and reversed polarity were isolated by thermal demagnetization generally between 300 and 690 c and a composite magnetostratigraphy is constructed from data at 969 accepted levels correlating with polarity chrons c12n to c3r dated between 31 0 and 5 5 ma on the ck95 geomagnetic polarity time scale observed height dependent changes of rock magnetic parameters km and ams ellipsoid parameters indicate that these sediments were influenced by weak deformation with the succession accumulated before 15 ma recording the effects of compressive deformation this finding together with two substantial increases in accumulation rate at 16 17 and 7 ma define a framework for cenozoic uplift and deformation of the tian shan range we argue that thrusting in the south of the range was probably initiated at 20 ma and followed by rapid uplift at 16 17 ma on the assumption that sediment accumulation in western china was still predominately tectonically controlled at 7 ma another later pulse of rapid uplift with a much larger regional influence is also recognized in the tian shan the kuche depression was subjected to two postcollisional phases of rotational deformation with clockwise rotation prior to 12 ma 1 36 ma succeeded by counterclockwise rotation at 1 64 ma after 12 ma and possibly continuing to the present day this rotational deformation is not temporally related to the history of basin formation resolved from the magnetostratigraphy and may be linked to block interactions with adjoining terranes

early proterozoic mafic dykes in the north atlantic and baltic cratons field setting and chemistry of distinctive dyke swarms

oligo miocene magnetostratigraphy and rock magnetism of the xishuigou section subei gansu province western china and implications for shallow inclinations in central asia magnetostratigraphy of 222 remanent directions together with late oligocene to early miocene mammal and charophyte paleontology suggest that 2179 m of the xishuigou section subei gansu province china were deposited from 26 to 19 ma stratigraphic patterns of bulk susceptibility anisotropy of magnetic susceptibility parameters and natural and anhysteretic remanent magnetization intensities demonstrate that 1 faulting does not significantly affect the record 2 sediment deposition was relatively continuous 3 sediment source changed around 23 ma and 4 rapid uplift near subei occurred at 21 ma subei rotated 27 5 counterclockwise with respect to the 20 ma pole from the eurasian synthetic apparent polar wander path folding and rotation of the section took place after 19 ma the paleolatitude of subei is 14 less than at present and 19 3 less than predicted from the reference pole both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing a phenomenon which has likely affected other paleomagnetic data from central asia

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

tectonic record of convergence changes in a collision area the boso and miura peninsulas central japan abstract late cenozoic formations in boso and miura have been affected by several tectonic events tectonic analysis enables us to reconstruct six different paleostress types 1 early extension affecting the oligocene early miocene mineoka group 2 and 3 relatively minor compressional and extensional events probably early pliocene in age 4 major nne ssw compression dominating prior to 2 or 3 ma ago and 5 and 6 more recent major nnw sse compression to the west and wnw ese extension to the east both types affecting the pleistocene and prevailing since 1 2 ma ago the counterclockwise change from nne ssw to nnw sse compression is not accurately dated but very likely occurred between 2 and 3 ma ago it is compared to similar evolutions in other areas of the izu collision zone we conclude that it corresponds to a major counterclockwise change in the direction of plate convergence from sse nnw to se nw the relationships between the directions of convergence and the distributions of plio quaternary compressional paleostresses in and around the collision zone are described through a simple analogy for the two stages of plio quaternary collision this counterclockwise change in stress fields and relative motions also described in the taiwan collision zone along the same philippine sea plate eurasia boundary is interpreted as a major event at the scale of the plate the possible significances of the other paleostress types identified in boso are discussed we conclude that tectonic analysis in and along collision boundaries provides a key for understanding kinematic evolution

layered mantle convection a model for geoid and topography the long wavelength geoid and topography are dynamic effects of a convecting mantle the long wavelength geoid of the earth is controlled by density variations in the mantle and has been explained by circulation models involving whole mantle flow however the relationship of long wavelength topography to mantle circulation has been a puzzling problem in geodynamics we show that the dynamic topography is mainly due to density variations in the upper mantle even after the effects of lithospheric cooling and crustal thickness variation are taken into account layered mantle convection with a shallow origin for surface dynamic topography is consistent with the spectrum small amplitude and pattern of the topography layered mantle convection with a barrier about 250 km deeper than the 670 km phase boundary provides a self consistent geodynamic model for the amplitude and pattern of both the long wavelength geoid and surface topography

seismic shear wave reflectivity structure of the mantle beneath northeast china and the northwest pacific 1 using multiple scs reverberations we examine the mantle reflectivity structure beneath northeast china and the northwest pacific we find several upper mantle discontinuities including a melt layer with a mean thickness of 64 km atop the 410 km discontinuity present on both sides of the subducting slab near the nankai trench the transition zone contains a split 520 km discontinuity in several paths and tomographic images show stagnant slabs at this depth we believe that this may be slab related based on experimental work a negative reflector is found in one path beneath the northeast china craton at a depth of 598 km mid mantle reflectors are found in all of our paths and are present throughout a wide depth range 750 1600 km

constraining the potential temperature of the archaean mantle a review of the evidence from komatiites the maximum potential temperature of the archaean mantle is poorly known and is best constrained by the mgo contents of komatiitic liquids which are directly related to eruptive temperatures however most archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid relatively fresh lavas from the saskmar suite belingwe greenstone belt zimbabwe 2 7 ga include chills of 25 6 wt mgo and olivines ranging to fo93 6 implying eruption at around 1520 c a chill sample from alexo township ontario also 2 7 ga is 28 wt mgo and associated olivines range to fo94 1 implying eruption at 1560 c however inferences of erupted liquids containing 32 33 wt mgo from lavas in the barberton greenstone belt south africa 3 45 ga and from the perseverance complex western australia 2 7 ga may be challenged on the grounds that they contain excess cumulate olivine or were enriched in mg during alteration or metamorphism re interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt mgo corresponding to an eruption temperature of 1580 c this composition is the highest liquid mgo content of an erupted lava that can be supported with any confidence the hottest modern magma on gorgona island 0 155 ga contained 18 20 mgo and erupted at circa 1400 c if 1580 c is taken as the temperature of the most magnesian known eruption then the source mantle from which the liquids rose would have been at up to 2200 c at pressures of 18 gpa corresponding to a mantle potential temperature of 1900 c these temperatures are in excess of the mantle temperatures predicted by secular cooling models and thus komatiites can only be formed in hot rising convective jets in the mantle this result requires that archaean mantle jets may have been 300 c hotter than the archaean ambient mantle temperature this temperature difference is similar to the 200 300 c temperature difference between present day jets and ambient mantle temperatures an important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions

dynamic topography compared with residual depth anomalies in oceans and implications for age depth curves ocean depth is affected by dynamic topography caused by mantle flow regional bathymetric deviations from the cooling plate age depth curve called residual depth anomalies can therefore be used as indicators of dynamic topography in oceans in this work we first evaluated the oceanic residual depth anomalies we then compute the dynamic topography for the flow induced by density perturbations converted from seismic tomography models by assuming we found that the predicted dynamic topography correlates with the depth anomalies when the density perturbations in the shallow part of the upper mantle were inferred from slab densities not from tomographic models we estimated a new age depth curve based on depths corrected for dynamic topography this corrected age depth curve shows that the corrected depths for old seafloor 70 110 ma are a few hundred meters deeper than those uncorrected

geologic evidence for a mantle superplume event at 1 9 ga 1 both preserved and restored areal distributions of proterozoic marine intracratonic passive margin and platform sediments show a prominent peak at 1 9 ga indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time the chemical index of alteration in shales deposited at this time was high suggesting warm climates possibly due to enhanced co2 levels in the atmosphere high sea level and warms climate may also explain an abundance of black shale banded iron formations and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1 9 ga all of these observations are consistent with a 1 9 ga superplume event the occurrence of only a minor positive carbon isotope shift in marine carbonates at 1 9 ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present slightly low 87sr 86sr isotopic ratios in seawater at 1 9 ga reflect increased mantle input of sr from the proposed superplume event whereas higher ratios at 1 85 1 75 ga may reflect increased input of continental sr from a growing supercontinent the first massive sulfate evaporites in the geologic record at 1 8 1 6 ga follow the possible 1 9 ga superplume event this may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume related volcanism wanes

neogene uplift of the tian shan mountains observed in the magnetic record of the jingou river section northwest china the tian shan mountains constitute central asia s longest and highest mountain range understanding their cenozoic uplift history thus bears on mountain building processes in general and on how deformation has occurred under the influence of the india asia collision in particular in order to help decipher the uplift history of the tian shan we collected 970 samples for magnetostratigraphic analysis along a 4571 m thick section at the jingou river xinjiang province china stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary from this component a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between 1 ma and 23 6 ma with some uncertainty below 21 ma this correlation places precise temporal control on the neogene stratigraphy of the southern junggar basin and provides evidence for two significant stepwise increases in sediment accumulation rate at 16 15 ma and 11 10 ma rock magnetic parameters also undergo important changes at 16 15 ma and 11 10 ma that correlate with changes in sedimentary depositional environments together with previous work we conclude that growth history of the modern tian shan mountains includes two pulses of uplift and erosion at 16 15 ma and 11 10 ma middle to upper tertiary rocks around the tian shan record very young 5 ma counterclockwise paleomagnetic rotations on the order of 15 to 20 which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont

modes of mantle convection and the removal of heat from the earth s interior earth thermal histories based on two layer and whole mantle parameterized convection models have been explored to determine the influence of the mode of mantle convection on the content and distribution of mantle radioactive heat sources and on the temperatures and heat transfer properties of the mantle two layer mantle convection thermally insulates the lower mantle and is less efficient at heat removal than whole mantle convection consequently two layer mantle convection requires a lower mantle depleted of radioactive heat sources contrary to the undepleted lower mantles of recent geochemical models whole mantle convection removes 3 times more primordial heat from the earth s interior and 6 times more heat from the core in 4 5 billion years than does two layer convection the average mantle volumetric heat generation rates for both modes of convection are comparable to that of a potassium depleted chondrite thus no potassium is necessary in the core to balance the observed present day surface heat flux however most of the radioactives are concentrated in the upper mantle for two layer convection model geotherms for whole mantle convection compare well with independent estimates of temperature at various depths including the core mantle boundary while the lower mantle is too hot for two layer convection models thermal histories of the upper mantle in two layer models are similar to those of whole mantle models thermal histories for the lower mantle in two layer models are similar to those of the core in whole mantle convection models this is because the lower mantle is thermally insulated by the upper mantle if it convects separately just as the core is thermally insulated by the whole mantle the differences between whole and two layer mantle convection are mostly due to the thermal insulation of the lower mantle and the smaller efficiency of heat removal of the upper mantle compared to that of the whole mantle which is much larger in volume a whole mantle convection model reconciling the need for distinct geochemical reservoirs with the physics of mantle convection is presented

models of the core mantle boundary and the travel times of internally reflected core phases topographic maps of the core mantle boundary cmb obtained by inverting different subsets of international seismological centre arrival time data involving either waves refracted through the boundary pkp or waves reflected from the topside pcp do not agree we report on the implications for models of cmb structure when data from bottomside reflections notably pkkp are included in the inversion travel time residuals with respect to a reference earth model prem of all core phases are positively biased but the mean residual is different for different phases and must be accounted for prior to inversion the new data illustrate that models of cmb topography can explain only a small fraction of the data variance the new cmb model involves both topography and a laterally varying boundary layer the cmb itself is smooth compared to previous results

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved

continent ocean chemical heterogeneity in the mantle based on seismic tomography seismic models of global scale lateral heterogeneity in the mantle show systematic differences below continents and oceans that are too large to be purely thermal in origin an inversion of the geoid based on a seismic model that includes viscous flow in the mantle indicates that the differences beneath continents and oceans can be accounted for by differences in composition in the upper mantle superposed on mantle wide thermal heterogeneities the net continent ocean density differences integrated over depth are small and cause only a low flux of mass and heat across the asthenosphere and mantle transition zone

noritic dykes of southern west greenland early proterozoic boninitic magmatism two major swarms of early proterozoic ca 2 1 ga basic dykes occur within the archaean craton of southern west greenland one swarm comprises ophitic and sub ophitic tholeiitic dolerites while the other the bn dyke swarm constitutes mainly norites in which pyroxenes and olivine are enclosed by plagioclase oikocrysts the close geochemical similarity between a quenched norite and the coarser grained varieties indicates that the composition of the latter type has not been significantly modified by crystal accumulation the bn dykes are geochemically distinctive most having high mgo ca 16 cr and ni contents in conjunction with relatively high sio2 light rare earth ree and large ion lithophile lil element concentrations the texture mineral chemistry and petrochemistry of the quenched noritic dyke all bear strong resemblances to those features in modern boninitic lavas the bn dykes also correspond to proposed parental liquids of the bushveld complex and other major layered basic igneous intrusions the two dyke swarms are petrogenetically distinct the tholeiitic dolerites were derived from a relatively undepleted primordial mantle while the noritic dykes originated from a metasomatized harzburgitic source the wide spread distribution of similar proterozoic intrusions suggests crustal underplating by harzburgitic mantle on a world wide scale at this time

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

locally layered convection inferred from dynamic models of the earth s mantle the structure of convection in the mantle is still the subject of considerable debate the now standard modelling of the convective flow as driven in a viscous mantle by density anomalies derived from seismic tomography has successfully explained the longest wavelength degree 2 to 8 geoid anomalies and provided important information concerning the viscosity structure of the mantle1 8 with this approach however the predicted response of surface topography to convective stresses the dynamic topography has a typical magnitude of several kilometres which does not conform with observations9 11 a possible source of this discrepancy lies in the severe underestimation by tomography of density anomalies due to deflections of the boundary between the upper and lower mantle at 660 km depth here we model the mantle flow implied by seismically derived density heterogeneities using an empirical method to account for the 660 km boundary topography the predicted dynamic surface topography thus obtained is significantly reduced to values that conform with the observations in addition the 660 km boundary topography appears to have a strong influence on the computed mantle circulation inducing local layering of the convective flow