lateral variations of the upper mantle structure in eurasia from group velocities of surface waves abstract group velocities of both rayleigh and love waves in the 20 70 s period range along paths across asia and the eastern europe are used to obtain group velocity maps of the region for this purpose we developed a method for tomographic inversion on spherical surface which is practically free of a priori assumptions checker board test shows that velocity anomalies of 10 10 size can be resolved the local dispersion curves are inverted to velocity depth curves up to 160 km depth along three profiles 1 tibet the south china platform and the southern part of the japan sea 2 the west siberian platform the tienshan and the pamir 3 the east european platform and the black sea basin lateral s wave velocity variations are presented at the depths of 60 90 and 120 km a low velocity zone in asthenosphere under japan sea is extended under the south china and the north china korean platforms the upper mantle structure of the russian east european and the west siberian platforms is different a low velocity zone at 120 140 km depths is observed under the west siberian platform whereas the velocity under the russian platform increases monotonously with depth a low velocity layer appears under the black sea indicating an oceanic structure of the basin low surface wave velocities in tibet are caused not only by a thick crust but also by low s wave velocities in the upper mantle the precambrian west siberian north indian and north china korean platforms are characterized by high velocities at the depth of 120 km unlike the younger tectonic units

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

the south siberia central mongolia transect abstract studies of geological and geophysical data on the south siberia central mongolia geoscience transect which runs approximately along 100 e show that the asian continent was formed in the phanerozoic by accretion of terranes most of which were microcontinents with precambrian basement suture zones separating the terranes are deformed and deeply eroded magmatic arcs of different widths likewise classified as specific terranes in these sutures magmatic arc rocks are locally associated with fore and back arc basin series most of the oceanic crust has been subducted the observed ophiolite belts are as a rule attributed to the base of oceanic island arcs the fragment of the asian continent traversed by the transect was formed through early middle and late paleozoic stages of accretion

the decompensative gravity anomaly and deep structure of the region of the rio grande rift an isostatic correction is commonly made to bouguer anomaly gravity data to remove the gravity effect of isostatic compensation of topographic loads in the ussr a decompensative correction has then been made to the isostatic gravity anomaly to remove the gravity effect of isostatic compensation of geologic loads as well under an hypothesis of local as opposed to regional compensation the gravity effect of a shallow geological body can be separated from that of its inferred deep compensating root by deconvolution by contrast with the procedure originally defined by zorin we employ here calculations in the wave number domain leading to an efficient and exact solution the decompensative transfer function acts to increase amplitude of the isostatic anomaly at all wavelengths but especially at the low end of the isostatic anomaly spectrum in the 200 600 km wavelength region where anomaly amplitudes may increase by as much as a factor of 2 in a 1200 1200 km region centered on the rio grande rift the decompensative correction ranges from about 35 to 25 mgal the rift is not particularly apparent in the decompensative correction nor is the rift any more apparent in the decompensative anomaly than in the isostatic anomaly the decompensative anomaly however highlights an arcuate gravity low and a system of gravity highs inferred to reflect prerift welts of mass concentration which have indirectly influenced the position of the rift and its segmentation and zones of accommodation under the assumptions made if the decompensative anomaly is subtracted from the bouguer anomaly then the residual is the gravity anomaly field of deep structure without gravity effects of shallow sources in the upper crust we suppose this gravity field to be dominated by the gravity effects of the density contrast across the crust mantle boundary moho and the density contrast at the lithosphere asthenosphere boundary using available seismic data to weakly constrain the moho surface we invert the residual gravity field for topography of the base of the lithosphere lithosphere is found to be 200 km thick in the high plains 40 50 km in the eastern great basin 75 100 km in the colorado plateau and as thin as 40 km in the southern rio grande rift in the area studied the thickness of the lithosphere is everywhere greater than that of the crust an asthenosphere bulge closely tracks the axis of the rift and is symmetrical to it by contrast with asymmetry that might be inferred from broad features of the bouguer anomaly the separation of gravity effects made possible by the decompensative correction shows how the rift is fundamentally controlled by thinning of the lithosphere yet in detail is deflected by long lived tectonic welts in the shallow brittle crust

the baikal rift zone the effect of mantle plumes on older structure abstract the main chain of sw ne striking cenozoic half grabens of the baikal rift zone brz follows the frontal parts of early paleozoic thrusts which have northwestern and northern vergency most of the large rift half grabens are bounded by normal faults at the northwestern and northern sides we suggest that the rift basins were formed as a result of transformation of ancient thrusts into normal listric faults during cenozoic extension seismic velocities in the uppermost mantle beneath the whole rift zone are less than those in the mantle beneath the platform this suggests thinning of the lithosphere under the rift zone by asthenosphere upwarp the geometry of this upwarp and the southeastward spread of its material control the crustal extension in the rift zone this nw se extension cannot be blocked by sw ne compression generated by pressure from the indian lithospheric block against central asia the geochemical and isotopic data from late cenozoic volcanics suggest that the hot material in the asthenospheric upwarp is probably provided by mantle plumes to distinguish and locate these plumes we use regional isostatic gravity anomalies calculated under the assumption that topography is only partially compensated by moho depth variations variations of the lithosphere asthenosphere discontinuity depth play a significant role in isostatic compensation we construct three dimensional gravity models of the plume tails the results of this analysis of the gravity field are in agreement with the seismic data the group velocities of long period rayleigh waves are reduced in the areas where most of the recognized plumes are located and azimuthal seismic anisotropy shows that these plumes influence the flow directions in the mantle above their tails the baikal rift formation like the kenya rio grande and rhine continental rifts achauer u granet m 1997 complexity of continental rifts as revealed by seismic tomography and gravity modeling in jacob a w b delvaux d khan m a eds lithosphere structure evolution and sedimentation in continental rifts proceedings of the igcp 400 meeting dublin march 20 22 1997 institute of advanced studies dublin pp 161 171 is controlled by the three following factors i mantle plumes ii older prerift linear lithosphere structures favorably positioned relative to the plumes and iii favorable orientation of the far field forces

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

depositional and tectonic framework of the rift basins of lake baikal from multichannel seismic data recent multichannel seismic reflection data from lake baikal located in a large active continental rift in central asia image three major stratigraphic units totaling 3 5 to 7 5 km thick in four subbasins a major change in rift deposition and faulting between the oldest and middle rift units probably corresponds to the change from slow to fast rifting in early pliocene time inferred from on land studies a minor modification of fault patterns characterizes the youngest unit a brief comparison of the basins of lake baikal with those of the east african rift system highlights differences in structural style that can be explained by differences in age and evolution of the surrounding basement rocks

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

baikal rift zone structure and geodynamics abstract the cenozoic baikal rift zone is superimposed on the caledonian baikal fold belt representing the suture between the precambrian siberian craton and several microcontinents the baikal rift zone consists of a system of disconnected fault bounded basins and extensional and wrench faults that straddles a major arch having a topographic relief of 2 3 km rifting activity commenced during the oligocene and is still active as evident from high seismicity of the baikal rift zone across the baikal rift zone upper crustal extension is considerably smaller than would be expected from its crustal thickness which decreases by 8 11 km across the baikal basins as compared to the adjacent unextended areas the discrepancy between upper and lower crustal attenuation could be attributed to intrusion of a major diapir indeed geophysical data indicate that the sayan baikal dome coincides with such a diapir the top of which is located at the crust mantle boundary this intrusion is held responsible for oligocene and quaternary doming of the rift zone development of the intra continental baikal rift system is thought to be related to asthenospheric diapirism active rifting however intra plate stresses in conjunction with the himalayan collision may have played some role

geodynamics of the western part of the mongolia okhotsk collisional belt trans baikal region russia and mongolia abstract after the western edge of the mongolian microcontinent joined the siberian continent in the region of central mongolia in the earliest permian these two continental blocks remained turned at an angle of about 120 with respect to each other and separated on greater extent of their present day boundary by an enormous gulf of the paleopacific called the mongolia okhotsk ocean closure of this ocean at the early middle jurassic boundary led to the complete collision of siberia and mongolia which by then had already become part of the mongolia north china continent this main collisional episode which lasted through the middle and late jurassic involved thrusting folding and magmatism and produced the mongolia okhotsk belt the onon island arc which was located in the mongolia okhotsk ocean was squeezed between the two major continents inasmuch as the third element the island arc was involved in the collision it is reasonable to distinguish two branches of the mongolia okhotsk suture these branches control the spatial distribution of gold mineralization in the trans baikal region on the southeastern periphery of siberia the crust thickened considerably after the collision and a plateau like uplift formed in the early cretaceous when compression ceased the collisional uplift collapsed and the thrusts were transformed into low angle normal faults the motions on which were responsible for the formation of rift basins and exhumation of metamorphic core complexes

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

cenozoic normal faulting and regional doming in the southern hangay region central mongolia implications for the origin of the baikal rift province abstract the hangay dome in central mongolia is a mountainous region covering more than 200 000 km2 with numerous flat topped peaks over 3000 m that preserve a late cretaceous paleogene erosion surface doming began in the middle oligocene producing more than 2000 m of regional topographic uplift the range represents an important kinematic link between the baikal rift province to the north and altai transpressional ranges to the south and west structural field investigations of major faults visible on satellite imagery indicate that the southern hangay dome region is dominated by late cenozoic normal faults that bound small half graben most faults are ne striking and faults at the highest elevations are the most recently active most graben appear to be isolated systems that constitute small sediment sinks perched on the flanks and crests of the dome the first order feature is the dome itself and most sediment eroded off of the dome is deposited in the mongolian valley of lakes or is carried northwards by the selenga river and its extensive tributaries the basement of the dome is a precambrian craton although the shape and dimensions of the craton are poorly constrained to the north and east late cenozoic uplift of the southern dome region appears to be confined to the area underlain by cratonic basement whereas the altai region to the south and west is underlain by mechanically weaker palaeozoic arc and accretionary belts with respect to the regional northeast directed shmax the hangay craton appears to have acted as a rigid passive indentor focusing late cenozoic transpressional deformation around its west and southern margins models invoking a late cenozoic plume as the driving force for doming and widespread alkaline volcanism on the dome are not strongly supported by geochemical and isotopic data on neogene recent volcanics and the spatial correlation between areas that are domed and older cratonic crust appears too coincidental to be ignored convective removal of an overthickened lithospheric root leading to adiabatically decompressed asthenosphere could explain regional doming and volcanism however major crustal thickening last occurred in the permian in the southern hangay region and the time lag between thickening and postulated root removal and plateau uplift 200 myr appears too long an alternative model is explored based on speculated lithospheric mantle flow patterns driven by india s continued northeastward indentation it is suggested that lithospheric mantle flow diversion around the overthickened hangay craton crustal keel could cause lithospheric thinning beneath the craton and passive asthenospheric upwarp leading to regional topographic uplift and decompression melting alkali volcanism in general the angular relationship between precambrian craton boundaries and the prevailing northeasterly shmax appears to control the kinematics of late cenozoic deformation throughout the hangay altai sayan and baikal regions

thickness of the lithosphere beneath the baikal rift zone and adjacent regions abstract methods of separation of gravity anomalies related to mantle density inhomogeneity are developed based on the inversion of these anomalies with regard to some constraints obtained from seismic data the map of lithospheric thickness beneath the baikal rift zone and adjacent regions was made the lithospheric thickness beneath the baikal rift zone is estimated to be 40 50 km i e the lithosphere is thinned here to a crustal thickness beneath the siberian platform the lithospheric thickness increases to 200 km and underneath the trans baikal region of moderate cenozoic tectonic activity it ranges from 75 to 160 175 km therefore a wide asthenospheric upwelling was revealed beneath the rift zone the data available on the configuration of the baikal depression and that of the crust and the lithosphere as a whole made it possible to estimate the magnitudes of extension at different lithospheric levels the significant increase in extension with depth implies that rifting in the baikal zone was caused by asthenospheric diapirism

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

tomography on pp p waves and its application for investigation of the upper mantle in central siberia abstract we develop the method of tomographic inversion called rr r scheme which is based on joint use of teleseismic p or s refracted rays and corresponding pp or ss rays with bounce points located within a study region this scheme allows imaging the deep seismic structure beneath blank areas where there are neither recording stations nor earthquakes utilization of differential travel times makes it possible to avoid the difficulty of source and station corrections which cause problems in teleseismic tomography the rr r scheme has been applied to more than 10 000 ray pairs from the isc database to investigate a large region from the north arctic ocean to the northern part of china and mongolia inversion was carried out in several separate windows covering the study area down to the depth of 530 km velocity anomalies were computed in grid nodes distributed in the study 3d area according to ray density the tomography results from different windows were combined into one general map of the lithospheric structure in central siberia principal positive p wave velocity anomalies 1 3 down to 400 km are associated with the precambrian siberian craton negative velocity anomalies are related to the thin lithosphere of the west siberian plate the altai mountains and the hangai plateau 3 1 in the depth range 30 to 430 km a strong negative anomaly beneath the precambrian hangai block is apparently associated with a mantle plume positive velocity anomalies in sedimentary basins at southern siberia are believed to have the crustal origin

geodynamics of the baikal rift zone and plate tectonics of asia the baikal rift zone stretches from the southern termination of lake baikal as far as 118 e and includes the baikal basin and the barguzin upper angara and muysk grabens that form its easterly continuation coincident with the rift zone is a narrow seismic belt which extends further east along the stanovoy range to the okhotsk sea this belt can be considered as the boundary between the eurasian and amurian lithospheric plates earthquake focal mechanism solutions that show a consistent orientation of extensional axes across the strike of the rift zone indicate the behaviour of the eurasian and amurian plates as rigid bodies from 83 focal mechanism solutions the pole of rotation of the eurasian and amurian plates was computed to lie at the point 56 95n 117 45e the angular rate estimated from the amount of baikal opening is about 1 10 7 deg year the position of the pole at the eastern termination of the rift zone accounts for the dying out of the rift zone and for the appearance east of the pole of compressive structures in the stanovoy range to the southwest the baikal rift zone is replaced by the mosaic of microplates in central asia that occupy the space between the eurasian and indian lithospheric plates prominent are the mongolian dzungarian alashan tarim tibetan and pamir microplates earthquakes in central asia are dispersed although higher concentrations of earthquakes associated with microplate boundaries are prominent the tectonics of central asia are considered to be due to the interaction of rigid microplates assuming that plastic deformations occur within the relatively wide zones along the microplate boundaries by considering the strike slip faults to define the trajectories of microplate motions the parameters of relative movements of the microplates have been computed the microplate mosaic of central asia is moving northeasterly as a result of the collision and continued convergence of india and eurasia the mosaic acts as a wedge splitting off the amurian plate from eurasia causing it to rotate counterclockwise so opening the baikal rift

upper mantle anisotropy beneath the altai sayan region of central asia abstract the altai sayan region of central asia is actively deforming despite its being located several thousands of kilometers away from the collisional front between india and eurasia to add constraints to the role of the upper mantle in this type of tectonic environment we analyzed intermediate period analogue records of sks and skks to determine the state of mantle anisotropy in this region we find that for all stations located in actively deforming areas the fast direction of anisotropy correlates well with indicators of cenozoic to the present day deformation and correlates with older structures only when they happen to align with younger ones the general east northeast trend of the fast direction of anisotropy in active regions also aligns with the principle extensional strain axis determined from earthquake moment tensors at one station nvs located outside of any actively deforming region the n s fast direction may correspond to the long wavelength strain field of the mantle beneath much of siberia these correlations along with the size of the delay times 1 2 s suggest that the anisotropy in the upper mantle beneath the altai sayan is responding to recent tectonic activity

baikal rift active or passive comparison of the baikal and kenya rift zones abstract the comparison of geological and geophysical data from the kenya and baikal rift zones permits us to consider the formation of these zones to be associated with the development of wide asthenospheric upwellings beneath them in this sense both rifts are active differences between them in amounts of volcanics and geological features may be explained by a higher degree of partial melting of asthenospheric material beneath the kenya rift

broad terrane jurassic flood basalts across northeastern north america early jurassic tholeiitic lavas that flowed across mesozoic basins of northeastern north america were comagmatic as shown by their geochemistry petrography paleomagnetism radiometric ages and stratigraphy the basin basalts are correlated with diabase dikes that intersect the basins but also extend far outside them if the dikes represent feeders to fissure vents a vast flood basalt province must have extended across the rifted terranes of eastern north america in early jurassic time the initial member of the basalt flood was the most uniform and widespread among present basin remnants of the basalts but subsequent flows may have covered some areas farther from the basins other younger jurassic flood basalts have been described along submarine sections of the continental shelf and shelf break and beneath cretaceous coastal plain sediments in the southeastern united states across the inland mesozoic rift terranes the initial fissure eruptions produced a temporary basaltic plain perhaps 1600 km long and 200 to 400 km wide or roughly 500000 km 2 rivaling other great continental flood basalt provinces

on the structure and mechanical behaviour of the extending lithosphere in the baikal rift from gravity modelling abstract the crustal structure and lithospheric flexure of the baikal rift zone siberia are examined by means of gravity modelling we model the bouguer anomaly ba along five 1200 km long gravity profiles we first evidence that continuous elastic plate flexure due to surface loading cannot explain the observed ba then we introduce plate discontinuities coupled with a realistic brittle elasto ductile plate rheology which allow us to model of external tectonic forces acting on the plates and determination of the moho geometry we show that the clearest expression of extensional processes occurs in the central part of the rift which exhibits the highest crustal thinning it evolves southwards to a rapidly increasing compression resulting in an overthickening of the southern plate s crust and in the long wavelength flexure of the siberian plate north of the central rift crustal thinning which is always less than 7 km gives way to a more diffuse zone of deformation inside the sayan baikal folded belt based on plate flexure models we propose that the rift shoulders surrounding the central and north baikal basins are not supported by upward bending plate but have a deep crustal root caused by a downward flexure the other parts of the rift depict two adjacent plates with antithetic flexures we also infer that the axial mantle material upwarping is not related to a large scale asthenospheric upwelling since the lithosphere rheological interfaces are not significantly disturbed our results favour the role of horizontal forces and motions resulting from the india asia collision combined with the effect of inherited tectonic structures and especially the paleozoic suture bounding the siberian craton for explaining the crustal structure and plate flexure modeled

asymmetric upwarp of the asthenosphere beneath the baikal rift zone siberia in the summer of 1991 we installed 27 seismic stations about lake baikal siberia aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the baikal rift zone and adjacent regions sixty six teleseismic events with high signal to noise ratio were recorded travel time and q analysis of teleseisms characterize an upwarp of the lithosphere asthenosphere boundary under baikal theoretical arrival times were calculated by using the international association of seismology and physics of the earth s interior 1991 earth model and travel time residuals were found by subtracting computed arrival times from observed ones a three dimensional downward projection inversion method is used to invert the p wave velocity structure with constraints from deep seismic sounding data our results suggest that 1 the lithosphere asthenosphere transition upwarps beneath the rift zone 2 the upwarp has an asymmetric shape 3 the velocity contrast is 4 9 in the asthenosphere 4 the density contrast is 0 6 and 5 the p wave attenuation contrast t is 0 1 s

geoid height versus topography for a plume model of the hawaiian swell abstract we use a three dimensional variable viscosity convection model of a stationary plume beneath a drifting lithosphere to study the factors that control the geoid to topography ratio gtr of the hawaiian swell the rate of melting in the plume is predicted using a batch melting parameterization and the melt is assumed to migrate to the surface where it builds a volcanic edifice equivalent to the hawaiian island chain viscous stresses elastic deformation of the lithosphere and optionally the volcanic material deposited on the ocean floor are included in the calculation of surface topography and the corresponding geoid the derivation of the gtr from the model imitates methods that have previously been used to estimate the observed gtr for the hawaiian swell the first method we use here is that of marks and sandwell j geophys res 96 1991 8045 8055 which applies bandpass filters to retain only wavelengths from 400 to 4000 km as most characteristic of the swell topography and geoid and the gtr is estimated from the slope of the regression line of geoid versus topography another group of methods analyzes the data along profiles drawn across the hotspot swell and eliminates the unwanted signals e g the volcanic islands and the flexural moat around them by cutting out parts of the sections the gtr is then calculated from curves which best fit the topography and geoid profiles on the swell flanks only in our plume model when the effects of the volcanic surface loading are included marks and sandwell s method yields 4 4 m km for the gtr while profile fitting on the swell flanks gives 7 8 5 m km ignoring the volcanic load leads to 7 8 m km in all processing methods the observed gtr for the hawaiian swell has been reported to lie between 4 and 5 m km analysis of the data processing methods shows that the applied bandpass filters cannot completely remove the signal due to the volcanic edifice and lithospheric flexure and this causes apparent gtrs around 4 m km the pure swell model containing no volcanic load on the surface demonstrates that the hawaiian swell may have a proper gtr near 7 m km

accurate increment identification and the spatial extent of the common signal in five arctica islandica chronologies from the fladen ground northern north sea 1 the creation of networks of shell based chronologies which can provide regionally extensive high resolution proxies for the marine environment depends on the spatial extent of the common environmental signal preserved in the shell banding and on the reliability of the dating model here arctica islandica chronologies from five neighboring sites in the north sea are compared and the strength of the common environmental signal across distances up to 80 km is analyzed using statistical techniques derived from dendrochronology the signal is found to be coherent across these distances in a linked study chronologies based on one of the same sites but constructed by two different research teams are compared methodological differences in increment interpretation are found to lead to slippage in the dating models systematic inclusion or exclusion of intermittently occurring increments results in the two chronologies becoming misaligned by 4 years over a 70 year period comparisons with neighboring chronologies indicate that such increments can generally be regarded as genuine annual increments even if they are not visible in all shells