seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

lithospheric thickness beneath the dabie shan central eastern china from s receiver functions summary p and s receiver functions obtained from a portable array of 34 broad band stations in east central china provide a detailed image of the crust mantle and lithosphere asthenosphere boundaries lab in the dabie shan and its adjacent areas clear s to p converted waves produced at the lab show a thin lithosphere beneath the whole study area based on our results the thickest lithosphere of 72 km is observed beneath the southern part of the area within the yangtze craton whereas beneath the north china platform the lithosphere is only 60 km thick s receiver functions also reveal in good agreement with p receiver functions a maximum depth of the moho beneath the dabie shan orogen at approximately 40 km furthermore we interpret the structural difference at 32 latitude as the probable location of

an s receiver function analysis of the lithospheric structure in south america 1 we make use of s receiver function to investigate the structure and thickness of the crust and mantle lithosphere in the south american continent and adjacent areas the moho discontinuity has been detected at all stations and goes from 18 km beneath the coast of the continent to 80 km in the andean region the depth of the lithosphere asthenosphere boundary can be clearly detected below those stations that are located on stable areas from 50 km to 160 km the identification of this phase becomes more difficult when the stations are located near subduction zones we also observed the base of the subducted oceanic lithosphere down to depths of 220 km

mapping the lithosphere asthenosphere boundary through changes in surface wave anisotropy abstract the alignment of olivine crystals is considered as the dominant source of seismic anisotropy in the subcrustal lithosphere and asthenosphere different components of large scale anisotropy can be traced in depth distributions of the radial and azimuthal anisotropy of surface waves we propose a global model of the lithosphere asthenosphere boundary lab as a transition between a frozen in anisotropy in the lithosphere to anisotropy in the sublithospheric mantle related to the present day flow due to different orientations of velocity maxima in the anisotropic subcrustal lithosphere and the asthenosphere the velocity contrast related to the lab can increase in particular directions because of their long wavelengths and horizontal propagation surface waves suffer from poor lateral resolution however surface waves with various wavelengths allow us to map gross features of the lab with a good vertical resolution we estimate depths to the lab to be between 200 and 250 km for the precambrian shields and platforms around 100 km for the phanerozoic continental regions and 40 70 km beneath oceans from the world wide depth distribution of the radial and azimuthal anisotropy of surface waves

the lithosphere asthenosphere boundary in the north west atlantic region abstract a detailed knowledge of the thickness of the lithosphere in the north atlantic is an important parameter for understanding plate tectonics in that region we achieve this goal with as yet unprecedented detail using the seismic technique of s receiver functions clear positive signals from the crust mantle boundary and negative signals from a mantle discontinuity beneath greenland iceland and jan mayen are observed according to seismological practice we call the negative phase the lithosphere asthenosphere boundary lab the seismic lithosphere under most of the iceland and large parts of central greenland is about 80 km thick this depth in iceland is in disagreement with estimates of the thickness of the elastic lithosphere 10 20 km found from postglacial rebound data in the region of flood basalts in eastern greenland which overlies the proposed iceland plume track the lithosphere is only 70 km thick about 10 km less than in iceland which is located directly above the proposed plume at the western greenland coast the lithosphere thickens to 100 120 km with no indication of the iceland plume track identified below jan mayen the lithospheric thickness varies between 40 and 60 km

summary of project tor delineation of a stepwise sharp deep lithosphere transition across germany denmark sweden project tor teleseismic tomography across the tornquist zone in germany denmark sweden is now producing results we are able to detect very significant deep lithosphere differences and we can now discuss the sharpness laterally in 1996 1997 our 120 seismographs constituted the largest seismic antenna ever in europe the tor area was chosen along a well studied crustal profile of an earlier project and the inversion efforts are concentrated on the deep lithosphere and asthenosphere differences to depths around 300 km the tor investigation can be called two and a half dimensional as it has a 900 km profile length with 100 km width plus a few seismographs off the profile the tor data have been subjected to p wave travel time tomography surface wave and receiver function analysis as well as anisotropy and scattering measurements through ray tracing in a compiled crustal model and subtraction of the modelled travel time anomalies the influence of the lower lithosphere asthenosphere on the seismic rays from distant earthquakes is being established travel time tomography results confirm very large lateral lithosphere differences of 4 6 in p wave velocity for several events of the large data base it is demonstrated that the observed p wave travel time anomalies of 1 2 s can be divided almost equally between known crustal effects and lower lithosphere asthenosphere differences which then must account for about 1 s of the travel time differences the transition is interpreted to be sharp and steep in two places it goes all the way through the lithosphere at the northern rim of the tornquist zone near the border between sweden and denmark and here the lithosphere difference is large a smaller lithosphere difference is found near the southern edge of the ringkobing fyn high just north of the border between denmark and germany also this transition is sharp and steep and goes all through the lithosphere these two sharp transitions divide the tor region into three different lithosphere structures distinguishable in p wave travel time tomography surface wave dispersion p and s wave anisotropy and partly in p wave scattering

seismic anisotropy a key to resolve fabrics of mantle lithosphere of fennoscandia abstract by merging different regional sets of body wave data we compile a relief of the lithosphere asthenosphere boundary in one of the oldest continental regions on the planet the fennoscandian shield a maximum lithospheric thickness of 200 220 km is observed in the central part of the shield and a significant thinning at its margins particularly in the southwest at the trans european suture zone fabric of the mantle lithosphere is modelled from independent sets of anisotropic parameters relative p residual spheres and shear wave splitting the mantle lithosphere of fennoscandia consists of several domains consistent olivine fabrics of dipping foliations in 3d anisotropic models of the proterozoic lithosphere or predominantly dipping lineations in models of the archean lithosphere the strength of seismic anisotropy agrees with findings from mantle xenoliths we interpret the anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric which was created before these micro continents assembled we envisage an early form of plate tectonic processes such as successive subduction episodes stacking of oceanic plates and accretion of magmatic arcs such processes could create the generally observed fossil dipping fabrics of the thick precambrian mantle lithosphere starting as early as the archean

thickness of the central and eastern european lithosphere as seen by s receiver functions summary s receiver functions obtained from seismograms of teleseismic events recorded at 78 european permanent broad band stations are used to estimate the thickness of the european lithosphere our results provide new independent information about the lithospheric thickness beneath the precambrian platform of eastern europe and the phanerozoic platform of central europe detailed high resolution images of the lithosphere asthenosphere boundary lab reveal indications for a typical continental lithosphere of about 100 km thickness beneath a majority of stations within central europe whereas in the vicinity of the trans european suture zone tesz the lithosphere thickens to about 130 km a relatively thin lithosphere of 80 km was found beneath the upper rhine graben region suggesting that the cenozoic extension affects the whole lithosphere no clear signal from the lab was detected beneath the alps and carpathians the lab sp phase might be disturbed by complicated structure due to ongoing collision subduction in these regions or the data are not yet sufficiently dense a relatively thicker lithosphere of about 120 km was found beneath the sw part of the bohemian massif that was formed during the variscan orogeny we found an lab depth of about 190 km near a single station located in the vrancea area eastern carpathians which is characterized by the occurrence of intermediate deep earthquakes beneath the stations located in the precambrian platform of eastern europe the lab deepens to approximately 200 km even though the converted phase from the lab is not as sharp as found beneath other stations located in central europe or even is missing

magnetotelluric measurements across the sorgenfrei tornquist zone in southern sweden and denmark the results of a magnetotelluric experiment across the sorgenfrei tornquist zone stz in the southwestern part of the baltic shield are presented the stz marks the border between the proterozoic

scattered wave imaging of the lithosphere asthenosphere boundary several recent studies have imaged discontinuities interpreted as the lithosphere asthenosphere boundary using receiver function methods these are exciting results since this boundary is fundamental and yet relatively elusive its location is not well imaged nor are its properties well understood the receiver function method provides high depth resolution and the multitude of studies using this method have increased lateral resolution a better global understanding of the location and nature of the boundary is developing which ultimately will have implications for mantle dynamics and planetary evolution however these new results do not all form a clear and consistent picture and in some cases introduce new complexities and questions some apparent contradictions may represent unmodeled earth structure overall interpretation and synthesis requires careful consideration of the approaches and sensitivities of individual studies

rocks and aeolian features in the mars pathfinder landing site region viking infrared thermal mapper observations albedo rock abundance and thermal inertia derived from viking infrared thermal mapper observations provide information about the surface materials within the mars pathfinder landing ellipse in ares vallis relative to the rest of mars the site has an intermediate albedo 0 19 0 23 intermediate to high bulk thermal inertia 410 540 j m 2 s 0 5 k 1 and high rock abundance 18 25 at local scales the actual surface distribution of rocks might be heterogeneous as on terrestrial catastrophic flood surfaces where rocks tend to be concentrated on fluvially armored surfaces and or bars albedo increases and thermal inertia decreases from the northeast to the southwest in the ellipse owing to interpreted windblown dark sand from acidalia planitia if mars pathfinder lands at the southwest end of the landing ellipse the scene is likely to be the most similar to that of the viking 1 lander while a landing at the northeast end of the ellipse is most likely to provide a view that is different from the viking sites because of low albedo material a landing at the center of the ellipse might put the spacecraft near one of two crater clusters that are interpreted to have dark sandy material in or on their ejecta aeolian activity during the 30 day primary mission might be limited because it is mid northern summer a time in which no historic dust storms have been seen in the region and a season when regional winds should be weak relative to other times during the year

deep resistivity structure of the trans european suture zone in central poland summary the deep resistivity structure was estimated along a 400 km profile in central poland crossing the malopolska massif mm the lysogory unit lu the trans european suture zone tesz and ending at the east european craton eec magnetotelluric transfer functions corresponding to 20 sites were supplemented by magnetovariational responses obtained at the geomagnetic observatories situated at the same tectonic units such a combination made it possible to extend the initial period range which is from fractions of a second to several hours up to months in order to reliably cover crustal and upper mantle depths the geoelectrical structures revealed using 2 d inversions do not contradict the known features of the lithosphere structure determined using seismic and gravity data along the profile the subsurface conductance varying from approximately 10 siemens at the inner part of the eec to about 600 siemens in the tesz is produced by sediments the deep part of which contains conductive highly mineralized water the existence of two crustal conductive faults at the southwest and northeast of the tesz were established mainly by the use of induction arrows it was also revealed that rather high mantle conductivity beneath the mm lu and tesz at depths of about 150 200 km contrasts with the resistive upper mantle of the eec this can be interpreted as the decrease of asthenosphere conductance and or as its submersion beneath the eec generally the results confirm the idea that the tesz forms not only specific seismic boundaries but also causes peculiar conductivity anomalies in the crust and upper mantle

european mantle lithosphere assembled from rigid microplates with inherited seismic anisotropy abstract we have modelled three dimensional seismic anisotropy of the mantle lithosphere from anisotropic parameters of teleseismic body waves we invert jointly shear wave splitting parameters and p residual spheres based on data from dense networks of temporary and permanent stations in four european regions ranging from the variscan belt to the baltic shield changes in orientation of the large scale anisotropy caused by systematic preferred orientation of olivine identify boundaries of domains of mantle lithosphere individual domains are characterized by a consistent large scale orientation of anisotropy approximated by hexagonal or orthorhombic symmetry with generally inclined symmetry axes the domains are separated by mapped tectonic boundaries sutures which cut the entire lithosphere besides the change of anisotropy orientation at domain boundaries we often observe a change of the lithosphere and or crust thicknesses we do not detect any fabric of the mantle lithosphere which could have been produced by a collision of microcontinents in a volume detectable by large scale seismic anisotropy the observations of consistent anisotropy within individual blocks of the mantle lithosphere reflect frozen in olivine preferred orientation most probably formed prior to the assembly of microcontinents that created the modern european landmass therefore our findings support a plate tectonic view of the continental lithosphere as a mosaic of rigid blocks of the mantle lithosphere with complicated but relatively sharp contact zones these contacts are blurred by the easily deformed overlying crust terranes

which forces drive north america understanding the mechanism of plate tectonics is one of the most important problems in the geosciences are the tectonic plates pulled and pushed from the side as the orowan elsasser model suggests or does mantle convection play an active role in driving the plates this question can be addressed by studying the deformation of deep continental roots the application to north america shown here indicates that the deeper mantle moves at a higher velocity than the north american plate and that the mantle plays an important role in driving the plates i suggest that this finding 1 provides a natural explanation for why the motion of north america slowed down dramatically during the past 100 m y and 2 predicts that north american motion will eventually come to a halt

magnetotelluric and seismological determination of the lithosphere asthenosphere transition in central europe abstract we assembled all available magnetotelluric mt sounding curves that cover characteristic provinces of europe the depths of layers with increased electrical conductivities estimated from mt sounding curves by the respective authors were compared with a model of the lithosphere thickness derived from seismic p residuals regions of a relatively thin and thick lithosphere are characterized by positive and negative p residuals respectively we have found a good correspondence with both sets of results in the central part of the pannonian basin the depth of the lithosphere asthenosphere transition is estimated at 60 80 km and 100 120 km at the flanks the results for the bohemian massif show deeper levels of this transition between 100 and 200 km these depths seem to be typical of the lithosphere base beneath the variscan domain of central europe

electrical conductivity and partial melting of mafic rocks under pressure we demonstrate the importance of electric conductivity measurements of partially molten mafic rocks by examining of oman gabbro karelia olivinite ronda and spitzbergen peridotites the electrical conductivities of these rocks were estimated using the impedance spectroscopy at temperatures between 800 c and 1450 c and at pressures between 0 3 and 2 gpa in experiments performed in a piston cylinder apparatus at temperatures below and above melting samples were equilibrated during durations on the order of 200 h our results show that a jump in electrical conductivity can be correlated with the temperature range slightly above the solidus due to the delayed formation of an interconnected melt phase thin sections of quenched samples were used to estimate volume fractions and chemical compositions of the partial melts the increase of the electrical conductivity compares well with the connectivity of melt in partially molten samples above the solidus the electrical conductivity increases by 1 to 2 orders of magnitude in comparison with the conductivity of non melted rock below solidus when a complete melt connectivity is established the charge transport follows the network of the formed melt films at grain boundaries durations of up to 200 h are required in order to reach a steady state electrical resistance in a partially molten rock sample the experimental results were compared with the conductivity data obtained from magnetotelluric mt and electromagnetic em measurements in the northern part of the mid atlantic ridge where a series of axial magma chambers amc are presumably located there is good agreement between the measured electric conductivity of gabbroic samples with a melt fraction of 10 to 13 vol and the conductivity estimated at amc beneath the central part of reykjanes ridge as well as between the conductivity of partially molten peridotites and the source zone beneath the mid atlantic ridge at 60 km

upper mantle heterogeneities beneath central europe summary an inversion of isc travel time data from selected earthquakes in the distance range 30 90 to 53 stations in central europe has been used to model velocity down to 600 km depth the model explains 0 1 0 2s of the residuals as for other array studies leaving 0 5 s unexplained as noise the uppermost 100 km of the mantle and crust contains inhomogeneities that correlate remarkably well with the geology this may be due to deep seated thermal anomalies or in some areas to delays introduced by passage of the rays through sedimentary cover the deeper anomalies are smaller and unrelated to those in the lithosphere which suggests that the asthenosphere is decoupled from the rigid lithosphere the structure at 600 km depth is again quite inhomogeneous and might be due to undulations of the 650 km discontinuity the models show some suggestion of a high velocity slab trending from east to west beneath the alps

seismic anisotropy of the lithosphere around the trans european suture zone tesz based on teleseismic body wave data of the tor experiment abstract a passive teleseismic experiment tor traversing the northern part of the trans european suture zone tesz in germany denmark and sweden recorded data for tomography of the upper mantle with a lateral resolution of few tens of kilometers as well as for a detailed study of seismic anisotropy a joint inversion of teleseismic p residual spheres and shear wave splitting parameters allows us to retrieve the 3d orientation of dipping anisotropic structures in different domains of the sub crustal lithosphere we distinguish three major domains of different large scale fabric divided by first order sutures cutting the whole lithosphere thickness the baltic shield north of the sorgenfrei tornquist zone stz is characterised by lithosphere thickness around 175 km and the anisotropy is modelled by olivine aggregate of hexagonal symmetry with the high velocity ac foliation plane striking nw se and dipping to ne southward of the stz beneath the norwegian danish basin the lithosphere thins abruptly to about 75 km in this domain between the stz and the so called caledonian deformation front cdf the anisotropic structures strike ne sw and the high velocity ac foliation dips to nw to the south of the cdf beneath northern germany we observe a heterogeneous lithosphere with variable thickness and anisotropic structures with high velocity dipping predominantly to sw most of the anisotropy observed at tor stations can be explained by a preferred olivine orientation frozen in the sub crustal lithosphere beneath northern germany a part of the shear wave splitting is probably caused by a present day flow in the asthenosphere

upper mantle temperature determined from combining mineral composition electrical conductivity laboratory studies and magnetotelluric field observations application to the intermontane belt northern canadian cordillera abstract using laboratory derived temperature dependences of the electrical conductivity of mantle minerals coupled with appropriate mixing laws we determine the bulk conductivity of mantle mineral assemblages for the ternary olivine orthopyroxene clinopyroxene ol opx cpx system we calculate physical property bounds hashin shtrikman bounds as a function of the fraction of different phases present these limits correspond to the extreme situations where the most conducting phase is either fully interconnected or fully disconnected the relationships we present between temperature mineral composition and bulk electrical conductivity allow constraining one of them given the other two we apply this approach to an area of the north american cordilleran intermontane belt in the yukon territory northern canada where xenolith evidence indicates bimodal upper mantle mineral assemblages harzburgite and lherzolite this locality coincides spatially with an upper mantle region of low electrical conductivity determined by long period magnetotelluric data given the mantle mineral composition and the maximum and minimum bounds on the electrical conductivity deduced by non linear model appraisal we determine the permitted extremal temperature bounds of the intermontane belt mantle rocks directly below the moho to a depth of some 80 km we show that the mantle in this region is at a minimum temperature of 820 c and a maximum temperature of 1020 c the latter is some 200 c colder than that suggested in a recent interpretation of an observed collocated low velocity zone from a teleseismic survey

the s receiver functions synthetics and data example summary recently the s receiver function method has been successfully developed to identify upper mantle interfaces s receiver functions have the advantage of being free of s wave multiple reflections and can be more suitable than p receiver functions for studying mantle lithosphere however because of specific ray geometry and interference of diverse phases the s receiver function method has some technical difficulties and limitations we use synthetic seismograms to demonstrate the feasibility and limitations of s receiver functions for studying mantle structures full wavefield seismograms were calculated using the reflectivity method and processed to generate synthetic s receiver functions for s sks and scs waves results show that s receiver functions can be obtained from waveforms of s sks and scs waves the synthetic s receiver functions for these incident waves show s to p converted phases at all discontinuities in the crust and upper mantle useful ranges of epicentral distances for calculation of s receiver functions are 55 85 for s 85 for sks and 50 75 for scs waves we apply both the s and p receiver function methods to data recorded at broadband station ykw3 in northwest canada the study shows that there is significant agreement among different receiver function methods and demonstrates the usefulness of s receiver functions for imaging the mantle lithosphere

the lithosphere asthenosphere boundary beneath the western united states summary s receiver functions from 67 broad band seismic stations in the western united states clearly reveal the existence of a mantle discontinuity with velocity reduction downward which we interpret as the lithosphere asthenosphere boundary lab the average depth of the lab is 70 km the boundary is relatively sharp with an overall sharpness of less than 20 km the boundary is more prominent south of the mendocino triple junction where the farallon plate has completely subducted this may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window a double low velocity zone is observed at base of the lithosphere beneath southern sierra nevada implying a second melting zone at a depth of 100 km well correlated with previous studies of lithospheric delamination in the area

lithospheric structure of the aegean obtained from p and s receiver functions combined p and s receiver functions from seismograms of teleseismic events recorded at 65 temporary and permanent stations in the aegean region are used to map the geometry of the subducted african and the overriding aegean plates we image the moho of the subducting african plate at depths ranging from 40 km beneath southern crete and the western peloponnesus to 160 km beneath the volcanic arc and 220 km beneath northern greece however the dip of the moho of the subducting african plate is shallower beneath the peloponnesus than beneath crete and rhodes and flattens out beneath the northern aegean observed p to s conversions at stations located in the forearc indicate a reversed velocity contrast at the moho boundary of the aegean plate whereas this boundary is observed as a normal velocity contrast by the s to p conversions our modeling suggests that the presence of a large amount of serpentinite more than 30 in the forearc mantle wedge which generally occurs in the subduction zones may be the reason for the reverse sign of the p to s conversion coefficient moho depths for the aegean plate show that the southern part of the aegean crustal thickness of 20 22 km has been strongly influenced by extension while the northern aegean sea which at present undergoes the highest crustal deformation shows a relatively thicker crust 25 28 km this may imply a recent initiation of the present kinematics in the aegean western greece crustal thickness of 32 40 km is unaffected by the recent extension but underwent crustal thickening during the hellenides mountains building event the depths of the aegean moho beneath the margin of the peloponnesus and crete 25 28 and 25 33 km respectively show that these areas are also likely to be affected by the aegean extension even though the cyclades crustal thickness of 26 30 km were not significantly involved in this episode the aegean lithosphere asthenosphere boundary lab mapped with s receiver functions is about 150 km deep beneath mainland greece whereas the lab of the subducted african plate dips from 100 km beneath crete and the southern aegean sea to about 225 km under the volcanic arc this implies a thickness of 60 65 km for the subducted african lithosphere suggesting that the aegean lithosphere was not significantly affected by the extensional process associated with the exhumation of metamorphic core complexes in the cyclades

seo3 a new model of olivine electrical conductivity summary standard electrical olivine 3 seo3 is a new model of electrical conductivity for olivine as a function of temperature and oxygen fugacity and is based on a previous study of point defect behaviour in a dunite that study produced quantitative estimates of mobility and concentration for small polarons and magnesium vacancies as a function of temperature and oxygen fucacity unlike earlier so1 and so2 models of olivine conductivity the dunite sample was buffered by pyroxene and avoided alteration at high temperature the low temperature extrapolation produced by seo3 is in good agreement with so2 while the high temperature extrapolation is significantly more conductive which is in general agreement with many field studies that show the earth s upper mantle to be more conductive than predicted by so2 for reasonable temperatures plausible variations in mantle oxygen fugacity correspond to about half an order of magnitude variation in electrical conductivity equivalent to about 100 degrees in temperature

integrated seismic model of the crust and upper mantle of the trans european suture zone between the precambrian craton and phanerozoic terranes in central europe abstract the structure and evolution of the trans european suture zone tesz contact between precambrian europe to the northeast and phanerozoic terranes to the southwest is one of the main tectonic questions in europe the knowledge of the crustal structure lithosphere asthenosphere boundary and mantle transition zone between two seismic discontinuities at depths 410 and 660 km is one of the most important issues to understand the earth s dynamics to create a mantle model of the tesz and surroundings we used different seismic data collected along the 950 km long polonaise 97 profile p4 previous results of 2 d ray tracing and p wave travel time modelling and new results of p wave travel time residuals methods and receiver function sections provide facts about the seismic structure from the surface down to 900 km depth in the tesz a large basin about 125 km wide is filled with sedimentary strata vp 7 0 km s 1 along most of the p4 profile however lower values to the southwest may indicate the termination of baltica high velocity 8 35 km s 1 uppermost mantle lies beneath the avalonia variscan terranes and may be due to rifting and or subduction the seismic lithosphere thickness for the eec is about 200 km while it is only 90 km in the palaeozoic platform pp the mantle transition zone is shallower and about 30 km thicker under the eec which could be due to thermal conditions lower temperature and or the presence of water and feo the result of this paper is a new compiled and integrated seismic velocity model available in digital form down to 900 km depth http www igf fuw edu pl p4 mantle which can be used as a preliminary model of the crust and upper mantle in the tesz area in central europe

the influence of partial melting on the electrical behavior of crustal rocks laboratory examinations model calculations and geological interpretations abstract the complex electrical impedance of a granulite has been determined over the frequency range 10 1 to 105 hz at temperatures between 600 and 1200 c at normal pressure and different oxygen fugacities in the stability field of magnetite the impedance spectroscopic is measurements are compared with the results of melting experiments performed under the same experimental conditions and with the same rock sample as the is measurements a strong increase in the electrical conductivity of about one and a half orders of magnitude is observed during partial melting this increase is explained by the formation of an interconnected network of melt the wetting of the grain surfaces starts slightly above the solidus however complete interconnection of the network is detected at higher temperatures 1070 c and hence higher melt fractions 10 vol this is explained by deviations from the equilibrium fabric of the partially molten sample with a modified brick layer model mblm the electrical conductivity is calculated as a function of the observed melt portion this model is valid if the melt forms a complete interconnected network at the grain boundaries the electrical conductivity at different temperatures was modeled using the results of the melting experiments and the mblm melt 10 s m with ea melt 1 ev solid 0 01 s m with ea solid 1 4 ev the results of the laboratory examinations were applied to different crustal regions where partially molten rocks are assumed to exist the same mblm is used to estimate the melt fraction that is necessary to explain the high conducting zones hczs observed in the crust beneath the andean orogen the pyrenees and the tibetan plateau at least 14 4 and 4 vol of interconnected melt are necessary to produce the observed conductivity respectively the actual amount of melt should be higher due to perturbation problems and melt trapped in pockets

the thickness of tectonic plates a fundamental premise of plate tectonics on earth is that rigid lithospheric plates formed at mid ocean ridges float above a more deformable substratum the asthenosphere 1 the precise nature of the asthenosphere is still debated mechanical models predict a well defined sharp lithosphere asthenosphere boundary lab but evidence for such a boundary from conventional seismic measurements is ambiguous on pages 499 1 and 495 2 of this issue kawakatsu et al 2 and rychert and shearer 3 present analyses of more sophisticated seismic studies that help refine the lab and hence the thickness of the lithosphere and tectonic plates although challenges still remain in picking out this boundary versus other structures within the lithosphere 1 lookup doi 10 1126 science 1169499 2 lookup doi 10 1126 science 1169754

decomposition analysis of the bear magnetotelluric data implications for the upper mantle conductivity in the fennoscandian shield decomposition analysis of the bear magnetotelluric data implications for the upper mantle conductivity in the fennoscandian shield

shear stress at the base of shield lithosphere 1 one of the basic unresolved issues regarding plate tectonics is the degree of coupling between surface plates and convecting mantle below are the plates effectively decoupled from the mantle flow field by a low viscosity asthenosphere or are they strongly coupled to mantle flow while these two views are essentially incompatible they both do a reasonably good job of predicting the motions of the surface plates and cannot therefore be distinguished on this basis the significant distinguishing feature for these models is the magnitude of basal shear stress that is applied to the base of the plate while it is difficult to measure this stress directly it is possible in principle to measure the corresponding deformation of the plate through observations of seismic anisotropy and to infer stress here we focus on the canadian shield for which we expect strong plate mantle interaction we show that seismic anisotropy can be used to constrain the magnitude of the stress level applied to the base of the plate and to document the level of interaction between tectonic plates and the mantle below

evidence for a missing crustal root and a thin lithosphere beneath the central alborz by receiver function studies summary the alborz mountains build the northern part of the alpine himalayan orogen in western asia they are located south of the caspian sea and form an east west range across the north of iran this region is one of the most active tectonic areas as it undergoes extensive crustal deformation and shortening in the present work we used data from 11 permanent stations of the tehran telemetry seismic network to estimate the thickness of the crust and mantle lithosphere beneath the central alborz region by p and s receiver function methods results of both p and s receiver functions revealed a relatively large crustal thickness beneath this region 51 54 km which can be associated with the shortening process related to the orogenic belt no remarkable crustal thickening has been detected below the high topography of central alborz a thick crust 67 5 km is observed locally in the region beneath the damavand volcano which is possibly related to the magmatic addition at the base of the crust beneath the volcanic region the s receiver functions exhibit the existence of a seismic discontinuity in the upper mantle at a depth of 90 km which we interpreted as the base of the lithosphere the missing crustal root and the relatively thin lithosphere beneath the alborz may imply that sublithospheric mantle could be responsible for and support the elevation of the alborz

europe from the bottom up a statistical examination of the central and northern european lithosphere asthenosphere boundary from comparing seismological and electromagnetic observations abstract the lithosphere asthenosphere boundary lab is a fundamental boundary in the plate tectonic paradigm it is the most pervasive on the planet yet comparatively it is one we know little about defined initially on the basis of the mechanical response of the earth to loading its usage has become ubiquitous across the geosciences but the natural differences in its definition due to differences in thermal physical and chemical parameters cause confusion to advance this debate comparisons are made both qualitatively and quantitatively between the delineation of the lab for europe based on seismological and electromagnetic observations we examine statistically using robust methods the labs derived from independent datasets and methods essentially all definitions of the lab as an impedance contrast from receiver functions slabrf a seismic anisotropy change slaba and an increase in conductivity from magnetotellurics elab are consistent with a deeper lab beneath precambrian europe and a shallower lab beneath phanerozoic europe with some exceptionally deep regions in phanerozoic europe such as the alps all three labs increase in depth significantly and rapidly at a location consistent with the surface expression of the trans european suture zone tesz which separates precambrian europe to the north and east from phanerozoic europe in the centre two of the definitions slabrf and elab are consistent for phanerozoic europe with mean values of 90 100 km compared to an average slabrf of 135 km a different two slaba and slabrf are consistent for precambrian europe with mean values of 170 180 km compared to an elab mean of 250 km the deeper elab depths for precambrian europe are more consistent with body wave and surface wave tomography models than are the slaba and slabrf ones suggesting that the lower lithosphere beneath precambrian europe between 170 to 250 km is responding to shearing on the base of the lithosphere from plate driving forces taken together the definitions provide strong constraints on the nature of the lab for example the electrical elab beneath the tesz is anomalously thick to possibly the transition zone whereas slaba and slabrf seismological definitions would place it at far shallower depths given the sensitivity of electrical conductivity to partial melt and water content the thick elab beneath the tesz excludes interpretation of the seismic lab in that location in terms of a thermal structure that would induce partial melting or of hydration

a new absolute arrival time data set for europe summary the main aim of this study is to create a data set of accurate absolute arrival times for stations in europe which do not report to the international seismological centre isc waveforms were obtained from data centres and temporary experiments and a semi automatic picking method was applied to determine absolute arrival times for p and s phases 85 000 arrival times were picked whose distribution of residuals shows generally low standard deviations on the order of 0 5 0 7 s furthermore mean teleseismic station residuals reflect the properties of the underlying crust and uppermost mantle comparison to isc data for matching event stationphase combinations also confirms the good quality of the new absolute arrival time picks most importantly this data set complements the isc data as it fills regional data coverage gaps in europe

velocity conductivity relationships for mantle mineral assemblages in archean cratonic lithosphere based on a review of laboratory data and hashin shtrikman extremal bounds article i nfo can mineral physics and mixing theories explain field observations of seismic velocity and electrical conductivity and is there an advantage to combining seismological and electromagnetic techniques these two questions are at the heart of this paper using phenomenologically derived state equations for individual minerals coupled with multi phase hashin shtrikman extremal bound theory we derive the likely shear and compressional velocities and electrical conductivity at three depths 100 km 150 km and 200 km beneath the central part of the slave craton and beneath the kimberley region of the kaapvaal craton based on known petrologically observed mineral abundances and magnesium numbers combined with estimates of temperatures and pressures we demonstrate that there are measurable differences between the physical properties of the two lithospheres for the upper depths primarily due to the different ambient temperature but that differences in velocity are negligibly small at 200 km we also show that there is an advantage to combining seismic and electromagnetic data given that conductivity is exponentially dependent on temperature whereas the shear and bulk moduli have only a linear dependence in cratonic lithospheric rocks focussing on a known discontinuity between harzburgite dominated and lherzolitic mantle in the slave craton at a

the lithosphere in central europe seismological and petrological aspects abstract the lithosphere thickness in the variscan belt of central europe varies between about 60 and 150 km with typical values of 100 120 km our estimates derived from directionally independent representative p wave residuals are in good agreement with magnetotelluric determinations of a layer with increased conductivity in the upper mantle the large scale anisotropies of the subcrustal lithosphere beneath four seismological stations determined from spatial variations of relative p residuals vary between 6 5 and 15 2 for p velocities the s wave anisotropies determined from sks polarizations vary between 2 2 and 6 7 these values are in reasonable agreement with the anisotropies of peridotites determined in laboratory systematic spatial variations of the directional terms of relative residuals in dependence on azimuths and incidence angles suggest the existence of large dipping anisotropic structures in the subcrustal lithosphere the residual patterns at most stations in the saxothuringicum rhenohercynicum and in the massif central imply northwesterly orientated dips of the anisotropic structures while stations in the moldanubicum the alpine foredeep and most of the alps north of the insubric line suggest southeasterly orientated dips in our interpretation the dipping anisotropic structures may represent paleosubductions which retain olivine preferred orientations originating from an ancient oceanic lithosphere the variscides of central europe may thus represent a collision zone characterized by two systems of paleosubductions divergent relative to the suture between the moldanubicum and the saxothuringicum