the meandering gulf stream as seen by the geosat altimeter surface transport position and velocity variance from 73 to 46 w the surface geostrophic velocity field for the gulf stream region was analyzed for the position structure and surface transport of the gulf stream for 2 5 years of the geosat altimeter exact repeat mission synthetic data using a gaussian velocity profile were generated and fit to the sea surface residual heights to create a synthetic mean sea surface height field and profiles of absolute geostrophic currents most of cross track velocity variance was due to the meandering of the gulf stream rather than to rings or complicated velocity profiles which suggested that the simple model parameters are an efficient description of the gulf stream fluctuations an analysis of the model parameters and the actual geostrophic velocity profiles revealed two different flow regimes for the gulf stream connected by a narrow transition region coincident with the new england seamount chain the upstream region was characterized by relatively straight gulf stream paths long eulerian time scales and eastward propagating meanders the downstream region had more large meanders no consistent propagation direction and shorter eulerian time scales a 25 reduction in surface transport occurred in the transition region with a corresponding reduction in current speed and no change in gulf stream width a significant anomaly in surface transport near 69 w suggested the existence of small scale recirculation gyres there the highest variance empirical orthogonal functions of position and surface transport were significantly correlated with larger surface transports leading more northerly positions by a month

the interrelation of glaciotectonic and glaciodepositional processes within the glacial environment abstract in recent years it has been recognised that ice sediment coupling occurred beneath the quaternary ice sheets that advanced over the soft sediments of lowland areas this paper looks in detail at the effects of this coupling on the sediments which results in glaciotectonic deformation and also discusses the interaction of deformation and deposition within the subglacial environment two types of glaciotectonic deformation are discussed that are produced by the active movement of ice 1 proglacial tectonics at the margin which include compressive fold styles such as listric thrusts and faults and open folding 2 subglacial tectonics formed beneath the glacier which include fold styles resulting from simple shear and represent a soft rock purely dynamic shear zone styles of deformation associated with stagnant ice are also investigated we argue that glaciotectonic deformation is a common phenomenon and an integral part of geological record from continental ice sheets it is suggested that zones of deformation within the sediment are related to similar zones of strain in the ice sheet and complex deformation sequences are produced by the superimposition of these differing styles upon one another as the ice sheet advances and retreats it is also argued that subglacial deposition and deformation are related and that on soft beds underformed till is rare whilst deformed till is very common

strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

strain patterns in models of spreading gliding nappes three experiments have been carried out in order to study the progressive and finite strain in nappes where gravitational spreading and gliding occur together these experimental models were made from analogue materials able to collapse under their own weight on an inclined plane throughout most of the models the stretch trajectories in a vertical plane parallel to the flow show a low amplitude sigmoidal pattern consistent with a previous theoretical model other stretch trajectories exist at the front and back ends of each model the kinematic significance of all these trajectories is discussed in detail highest strain intensities are always found next to the base of the model strain paths calculated at different stages of flow indicate clearly that nowhere in the model is the motion simple there is a complex combination in time and space of simple and pure shear except at the base of the model where the motion approximates a simple shearing near the two parallel lateral sides of the model a lateral boundary effect can be observed from the strong curvature of the transverse markers surface grid and vertical layers strain paths have also been estimated in this complex zone where wrench shearing is superimposed upon vertical shortening and thrust shearing the geological implications of these experimental models are discussed in relation to recent theoretical studies and field work

an investigation of the deforming layer debris rich basal ice continuum illustrated from three alaskan glaciers three small alaskan glaciers with different bed conditions were studied exit glacier had a thin deforming layer and produced subglacial and proglacial glaciotectonic land forms childs glacier also had a thin deforming layer but the upper part was frozen to the ice matanuska glacier had no deforming layer but had subglacial debris rich ice since it has been shown that sediment at the base can account for the majority of ice movement it is suggested that there is a deforming bed debris rich continuum whereby similar processes occur throughout the different subglacial environments these include similar longitudinal deformation patterns compression at the margin extension and simple shear up glacier similar vertical deformation patterns increase in deformation and fabric strength upwards through the sequence leading to the attenuation of stratified ice into dispersed ice and similar and interrelated incorporation processes the major differences were that the processes occurred at a much faster rate within the deforming layer and that probably only the deforming layer will be recorded in the geological record

a thinned lithospheric image of the tanlu fault zone eastern china constructed from wave equation based receiver function migration 1 we apply the newly proposed wave equation based receiver function poststack migration method to the northern china interior structure project broadband data to image the lithospheric structure of the tanlu fault zone area in eastern china our migration result reveals a 60 to 80 km thick present day lithosphere beneath the study region significantly thinned from the paleozoic lithosphere of 180 km the lithosphere asthenosphere boundary lab is coherently imaged along the 300 km east west profile displaying an arc like shape with its apex roughly coincident with the transverse location of the tanlu fault zone on the surface an obvious uplift from 36 km to 32 km of the moho is also clearly detected right below this fault zone the coincidence of the imaged moho uplift and the lab apex with the surface location of the tanlu fault zone provides seismological evidence for the steep geometry and deep penetration of the fault system and indicates that the tanlu fault zone might have acted as a major channel for anthenosphere upwelling during the mesozoic cenozoic continental extension and lithospheric thinning in eastern china frequency analysis and synthetic modeling suggest that both the moho and the lab are sharp and strong the latter in particular is constrained to have a 3 7 drop in s wave velocity over a depth range of 10 km or less such a rapid velocity change at the base of the lithosphere in the study region cannot be solely explained by thermal variation but likely reflects the presence of volatiles or melt in the asthenosphere or is partially attributed to the compositional contrast between the preserved depleted and dehydrated cratonic lithospheric veneer and the uplifted hydrated and fertile asthenospheric materials

eddy energy of the northwest atlantic and gulf of mexico determined from geos 3 altimetry from may 1975 to october 1978 the geos 3 satellite altimeter made numerous repeated observations of sea surface topography in the northwest atlantic and gulf of mexico by comparing members of about 1000 collinear pairs of altimeter profiles we have determined mesoscale sea height variability and eddy kinetic energy in these ocean regions our results agree qualitatively with estimates made from traditional oceanographic surveys but significant quantitative differences exist in certain areas especially in the gulf stream between florida and cape hatteras where the altimetric results show less variability the reason for this appears to be due to differences in spatial sampling historical oceanographic measurements are relatively sparse and must be aggregrated into small geographic boxes to compute variability statistics results therefore represent a combination of temporal and spatial variability in contrast repeated satellite altimeter tracks provide point measurements of changes of sea height and slope this enables determination of temporal variability alone a quantity more representative of eddy energy it is important to make this distinction between space and time because in regions of strong horizontal gradient such as the gulf stream spatial variability can exceed the temporal component in mid ocean where spatial variability is negligible altimetric and oceanographic results are in excellent agreement

the deforming bed debris rich basal ice continuum and its implications for the formation of glacial landforms flutes and sediments melt out till abstract there are two potentially important shear zones at the base of the glacier the deforming layer and or the debris rich basal ice layer these two layers have a significant effect on glacier motion and glacial erosional depositional and sedimentary processes however it is shown that most of the processes operating within these two layers are very similar this includes compression at the ice margin longitudinal extension and simple shear upglacier similar styles of shear zone and associated fabric development and similar incorporation transport and depositional processes in the past many criterion have been developed to identify distinct deposition from the deforming layer and the debris rich basal ice layer however if the processes within these layers are similar then it makes the distinct identification of these two separate layers in the geological record very difficult however it is also shown that the main differences between the two layers include rates of sediment movement processes and preservation potential the later is very important as it appears that the two layers are interconnected and that material is melted out from the debris rich basal ice to form a significant component of the deforming layer in this way the final depositional imprint is from the deforming layer and there is little likelihood of the debris rich basal ice layer being preserved undeformed this is demonstrated from modern glacial environments pleistocene flute studies and studies of pleistocene melt out tills from the u k u s a and germany

north and south china collision along the eastern and southern north china margins abstract lithofacies distribution and petrography and intracontinental deformation in the north china block ncb and the south china block scb show an active continental margin flanked by the eastern ncb and a passive continental margin was present along the west side of the scb both margins initially trending n ne the tanlu fault possibly initiated as part of the subduction zone during paleozoic time along the eastern margin on the ncb the ncb and the scb were divided along the eastern southern north china margin by the qinling dabie tanlu sulu imjingang yanji zone from southwest to northeast these two continental blocks made contact first in the northeast during late early permian time finally in the southeast at late triassic time and the clockwise collision probably lasted to middle jurassic time the eastern suture the yanji imjingang sulu tanlu zone was dominated by contractional deformation the southern suture the qinling dabie zone was a transpression belt dominated by right lateral strike slip

progressive deformation in anisotropic rocks abstract field observations show that the majority of crustal rocks possess a penetrative foliation defined by either compositional layering preferred orientations of crystals or both ductile deformation involving planar anisotropy of viscosity can be characterized by an anisotropy factor q n r s the ratio of the bulk viscosities in pure and simple shear respectively this ratio of the normal and shear viscosities may be determined analytically if the anisotropy is resolvable in a multilayer sequence with individual laminae of isotropic viscosity in that case the resistance to normal compression will be largely controlled by the competent layers whereas the resistance to shear is controlled by the soft layers more specifically the viscosities of the individual laminae add up like resistances in series and in parallel in the expressions for the normal and shear viscosities respectively the reorientation of the bulk stress within an anisotropic multilayer is systematically investigated for a range of anisotropy factors the mode of progressive deformation is controlled only by the anisotropy factor and the orientation of the principal deviatoric stress the rate of deformation can be scaled if the bulk normal viscosity and the magnitude of the principal deviatoric stress are also known the results are illustrated in a series of nomograms showing the spectrum of strain and rotation histories possible in rock volumes deforming with planar anisotropy it appears that with increasing anisotropy factor the deformation spectrum will narrow on simple shear irrespective of the orientation of the background or bulk deviatoric stress axes plane strain and isochoric conditions are assumed throughout the analysis

rayleigh wave tomography of china and adjacent regions 1 this paper presents a tomographic study on the s wave velocity structure of china and adjacent regions group velocity dispersions of fundamental rayleigh waves along more than 4000 paths were determined with frequency time analysis the study region was divided into a 1 1 grid and velocities in between grid nodes were calculated by bilinear interpolation the occam s inversion scheme was adopted to invert for group velocity distributions this method is robust and allows us to use a fine grid in model parameterization and thus helps to restore a more realistic velocity pattern checkerboard tests were carried out and the lateral resolution was estimated to be 4 6 in china and its eastern continental shelves the resulting group velocity maps from 10 to 184 s showed good correlation with known geological and tectonic features the pure path dispersion curves at each node were inverted for shear wave velocity structures the three dimensional velocity model indicates thick lithospheres in the yangtze and tarim platforms and hot upper mantles in baikal and western mongolia coastal area and continental shelves of eastern china and indochina and south china sea regions the tibetan plateau has a very thick crust with a low velocity zone in its middle beneath the crust a north dipping high velocity zone mimicking a subducting plate reaches to 200 km in depth and reaches to the kunlun mountains northward in northern tibet a low velocity zone immediately below the moho extends eastward then turns southward along the eastern edge of the plateau until it connects to the vast low velocity area in indochina and the south china sea

the enigmatic yinshan fold and thrust belt of northern china new views on its intraplate contractional styles the east to east northeast trending yinshan belt lies within north china extending westward at least 1100 km from china s eastern coast to inner mongolia this intraplate jurassic cretaceous belt underwent contractional and normal faulting folding and contemporaneous terrestrial sedimentation and magmatism current views on its contractional deformational style favor relatively limited thick skinned faulting of archean basement and cover units these views are challenged however by recent discoveries in the eastern part of the belt of south directed ductile nappe formation and large displacement 40 45 km thin skinned northward thrust faulting both involving archean and younger rock units collision of the siberian and north china plates upon closure of a jurassic and early cretaceous mongolo okhotsk ocean more than 800 1100 km to the north may have been responsible for yinshan north south contraction some patterns of contraction e g jurassic cretaceous ductile nappe formation appear to have been influenced by a superposed magmatic regime related to westward subduction of a pacific basin plate beneath the north china plate

crustal structure across the yanshan belt at the northern margin of the north china craton abstract we applied an integrated receiver function imaging technique to the teleseismic data from 54 ncisp iii broadband seismic stations to study the fine scale crustal structure beneath the eastern yanshan belt ysb and adjacent areas at the northern margin of the north china craton ncc the robustness and reliability of the resultant crustal image were noticeably improved by achieving imaging consistency between common conversion point stacking and waveform inversion of the receiver functions our imaging result revealed substantial lateral heterogeneities in the crust of the study region as displayed by the undulating intra crustal interfaces highly varying upper middle and lower crust thicknesses and interlayers of low and high velocities the crustal structure of the ysb is characterized by significant structural variations together with a sharp moho a thin to normal crust and a lower crust with lower velocity and normal thickness it differs markedly from the bohai bay basin and the taihang mountains within the interior of the ncc the distinct features and structural complexity of the ysb might be attributed to the complex tectonic history of the region especially the multiple phases of contraction and possible occurrence of crustal delamination which is in large contrast to other subregions of the ncc where tectonic extension likely dominated the crustal deformation since the late mesozoic

a sharp lithosphere asthenosphere boundary imaged beneath eastern north america although a rigid lithosphere about 100 km of crust and uppermost mantle moving over a weaker asthenosphere a few hundred kilometres of more deformable rock is a widely accepted aspect of plate tectonics the properties of the boundary between them are poorly understood when compressional waves p waves from an earthquake encounter an interface at an angle some of the energy is converted to shear waves or s waves these converted waves can be used to construct an image of the interface based on converted waves recorded in eastern north america the lithosphere asthenosphere boundary has been resolved as a sharp gradient that cannot be explained by thermal gradients alone indicating that the boundary is controlled by melt or high water content in the asthenosphere plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere yet the nature of the lithosphere asthenosphere boundary remains poorly understood the gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers for example if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures mantle flow models1 2 indicate that the velocity gradient at its base would occur over tens of kilometres in contrast if the asthenosphere is weak owing to volatile enrichment3 4 5 6 or the presence of partial melt7 the lithosphere asthenosphere boundary could occur over a much smaller depth range here we use converted seismic phases in eastern north america to image a very sharp seismic velocity gradient at the base of the lithosphere a 3 11 per cent drop in shear wave velocity over a depth range of 11 km or less at 90 110 km depth such a strong sharp boundary cannot be reconciled with a purely thermal gradient but could be explained by an asthenosphere that contains a few per cent partial melt7 or that is enriched in volatiles relative to the lithosphere3 4 5 6

observations of eddy fields in the northwest atlantic and northwest pacific by seasat altimeter data sea level variability related to the eddy field activity can be calculated from repeating tracks of altimetric data such as the seasat 3 day repeat data september 13 to october 10 1978 if we consider the eddy field in geostrophic balance the geostrophic relation gives the surface eddy velocities also an estimation of the eddy kinetic energy can be deduced under the assumption of quasi isotropy of the velocity variability this approach is used in the northwest atlantic and northwest pacific through the processing of seasat data high variability of sea surface topography up to 20 cm and high eddy kinetic energy 800 1600 cm sup 2 s sup 2 are detected in the vicinity of the gulf stream and the kuroshio power spectra of the observed sea level fluctuations give wavelengths ranging from 100 to 900 km for time scales between 3 and 25 days maps of sea surface topography variability and eddy kinetic energy are presented and discussed

where is the north china south china block boundary in eastern china the qinling dabieshan belt results from the collision of the north china and south china blocks the eastern extension of the belt the sulu area consists of several stacked units from top to bottom these are 1 a weakly metamorphosed slate sandstone unit of neoproterozoic paleozoic age 2 a gneiss quartzite unit metamorphosed under high pressure p conditions 3 a marble amphibolite unit 4 an ultrahigh p unit that was very deeply subducted and then exhumed and 5 a migmatitic dome resulting from the melting of ultrahigh p rocks all these units underwent the same deformation which was characterized by a top to the northwest extensional ductile shearing during their exhumation granulite facies restites in the migmatite are not significantly different from granulitized eclogites derived from the ultrahigh p rocks the migmatite exhibits the same structural and petrologic evolution as that determined for the other units the resemblance of petrologic and structural features and the lack of ocean basin rock shows that the boundary between the north china block and south china block must be placed north of the sulu area

a wave equation migration method for receiver function imaging 1 theory 1 a wave equation based poststack depth migration method is proposed to image the earth s internal structure using teleseismic receiver functions by utilizing a frequency wave number domain one way phase screen propagator for wave field extrapolation in the migration scheme common conversion point ccp stacked receiver functions are backward propagated to construct a subsurface structural image the phase screen propagator migration method takes into account the effects of diffraction scattering and travel time alternation caused by lateral heterogeneities and therefore it is particularly useful for imaging complex structures and deep discontinuities overlain by strong shallow anomalies synthetic experiments demonstrate the validity of the migration method for a variety of laterally heterogeneous models the migrated images show considerable improvement over the ccp images in recovering the model features influences of several factors on the image quality of the poststack migration are further investigated including interstation spacing noise level of the data velocity model used in migration and earthquake distribution incident direction of source fields on the basis of the sampling theorem and previous statistic results we discuss the relation of spatial resolution and signal to noise ratio of the migrated image with the frequency of the data surface station spacing and number of receiver functions used in stacking we show that both ccp stacking and poststack migration of receiver functions need to be designed in a target oriented way for reliable and efficient imaging our results also suggest that careful consideration of earthquake source distribution is necessary in designing seismic experiments aimed at imaging steeply dipping structures

experiments on folding in spreading gliding nappes abstract a small scale model of spreading gliding nappe was used to study the development of folds which are controlled by the geometry of external boundaries throughout in the model the deformation combines pure shear and simple shear recumbent folds are produced above irregularities of the base the cylindricity of folds is directly dependant on the cylindricity of basal irregularities models with homogeneous rheological properties produce only recumbent folds whereas models of contrasting rheological properties produce recumbent folds and en echelon upright folds in the zones of lateral shear the experiments demonstrate that 1 a large scale recumbent folds in nappes can result from passive amplification processes in unsteady shear flow 2 b upright folds and coaxial refolding result from mechanical instability due to contrasting rheological properties between layers and develop during a unique progressive deformation these laws can be directly applied to a spreading gliding nappe but also on a larger scale to all nappes in which the shear component dominates the deformation

geosat and advanced very high resolution radiometer observations of oceanic planetary waves adjacent to the hawaiian islands geosat observations of the central north pacific ocean show westward propagating mesoscale features with length scales 500 km time scales 105 155 days and observed zonal phase speeds commensurate with those of nearly nondispersive baroclinic rossby waves advanced very high resolution radiometer derived sea surface temperatures observed during april 1988 showed a cusp shaped pattern north of the hawaiian islands congruent with concurrent geosat derived sea level height anomalies it is believed that the cusp shaped pattern is the surface manifestation of a baroclinic rossby wave the observed wavelength of the cusp shaped pattern was 560 km it had an observed meridional extent horizontal amplitude of 200 300 km and an inferred zonal propagation speed of 0 052 m s 1 the westward propagating sea level height structure was observed to amplify as it approached 150 w reaching amplitudes of 0 06 0 08 m in the vicinity of the cusp shaped feature these waves are believed to be locally forced by the topography either as the result of eastward flow over the submerged island ridge or as a result of baroclinic instability during periods of westward flow

determination of finite strain from passive initially planar markers deflected in ductile rocks undergoing simultaneous superposition of heterogeneous strains abstract from an empirical model of strain in ductile heterogeneous shear zones simultaneously undergoing a bulk pure shear we study the influence of the parameters of the deformation on the shape changes of passive initially planar markers and describe a method for determining the finite strain and even sometimes the instantaneous strain from these deflected markers

an improved special sensor microwave imager water vapor correction for geosat altimeter data a wet tropospheric correction for the geosat altimeter data is derived from the special sensor microwave imager ssm i water vapor products by sub objective mapping the correction presents wavenumber spectral characteristics close to those of the original ssm i data in the 200 to 2000 km wave band and is filtered in the short wavelength domain in both respects it behaves significantly better than corrections derived from 4 day averages of the ssm i data interpolated quadratically under the geosat track as an example the along track rms mesoscale variability is lowered by 3 cm for some tracks in the northeast atlantic ocean when this correction is applied rather than the correction of wentz 1990 this suggests that it indeed improves the altimeter data

mantle structure and composition to 800 km depth beneath southern africa and surrounding oceans from broadband body waves abstract average one dimensional p and s wavespeed models from the surface to depths of 800 km were derived for the southern african region using travel times and waveforms from earthquakes recorded at stations of the kaapvaal and south african seismic networks the herglotz wiechert method combined with ray tracing was used to derive a preliminary p wavespeed model followed by refinements using phase weighted stacking and synthetic seismograms to yield the final model travel times combined with ray tracing were used to derive the s wavespeed model which was also refined using phase weighted stacking and synthetic seismograms the presence of a high wavespeed upper mantle lid in the s model overlying a low wavespeed zone lwz around 210 to 345 km depth that is not observed in the p wavespeed model was inferred the 410 km discontinuity shows similar characteristics to that in other continental regions but occurs slightly deeper at 420 km depletion of iron and or enrichment in aluminium relative to other regions are the preferred explanation since the p wavespeeds throughout the transition zone are slightly higher than average the average s wavespeed structure beneath southern africa within and below the transition zone is similar to that of the iasp91 model there is no evidence for discontinuity at 520 km depth the 660 km discontinuity also appears to be slightly deeper than average 668 km although the estimated thickness of the transition zone is 248 km similar to the global average of 241 km the small size of the 660 km discontinuity for p waves compared with many other regions suggests that interpretation of the discontinuity as the transformation of spinel to perovskite and magnesiowustite may require modification alternative explanations include the presence of garnetite rich material or ilmenite forming phase transformations above the 660 km discontinuity and the garnet perovskite transformation as the discontinuity

lithospheric thickness of the chinese continent abstract we invert for the upper mantle temperatures of the chinese continent in the depth range of 70 240 km from a recent s velocity model the depth where temperatures intersect a mantle adiabat with a potential temperature of 1300 c is in close correspondence with the top of the seismic low velocity zone for most regions this correspondence implies that seismic lithosphere estimated from short time scale seismic information may be equivalent to the long time scale geodynamical lithosphere defining the 1300 c adiabat as coinciding with the lithospheric base we estimate the seismic thermal lithosphere thickness the estimated thickness shows obvious dependence on the tectonic settings beneath eastern china which mainly belongs to the circum pacific tectonic domain it has a thickness of 100 km and beneath the qinghai tibet plateau and south to the tarim craton which belong to the tethyan tectonic domain it has a thickness of 160 220 km the lithospheric thicknesses of the three large para platforms cratons range from 170 km for the western yangtze 140 km for tarim and 100 km for sino korean the three cratons may have been reshaped by phanerozoic tectonic activities and are thinner than most cratons in other continents

the eddy field associated with the azores front east of the mid atlantic ridge as observed by the geosat altimeter two years of geosat data november 1986 to december 1988 for the azores madeira region of the north atlantic are analyzed the objective is to study the eddy field associated with the terminal retroflection of the azores current the rms variability of sea level anomaly is 4 to 8 cm with maximum values between 33 degrees and 35 degrees n being associated with the azores current and located on its northern flank higher energy is observed during winter and during the year november 1987 to november 1988 analysis of velocities at crossovers reveals anisotropy with larger meridional velocity variances this is the signature of meanders and or eddies elongated in the north south direction the eddy mean flow interaction is studied through the analysis of the reynolds tensor meridional and zonal accelerations of the mean by the eddy field are consistent with retroflection of the azores current to the south and to the west spectral analyses show that the dominant signals have periods and wavelengths larger than 100 days and 300 km respectively propagation velocities are westward but slow 1 5 km d synoptic mapping of the eddy field shows the wavelike structure of the variability on the northern flank of the current it is likely that the wave generation is not directly due to an instability of the azores current but is attributable to a cause more to the east once formed the wave interacts with the current in winter and forms meanders seasonal variations of the surface circulation are finally analyzed from the 2 year long time series of synoptic maps there is a clear seasonality in the surface circulation which generally appears to be consistent with historical in situ measurements in winter there is a strong meandering and a southward branching while in summer the mean front is narrower and penetrates further to the east

the mesozoic cenozoic east china rift system abstract the mesozoic cenozoic rift system of eastern china extends in a n s direction over a distance of some 4000 km and has a width of 600 1000 km to the east it reaches into the coastal areas of the east and south china sea and extends onto the continental shelf this very large rift system evolved on a heterogenous basement consisting of a number of terranes which were amalgamated during the indosinian orogeny a first rifting phase spanned late jurassic early cretaceous times it was followed by a period of relative tectonic quiescence and thermal relaxation a latest cretaceous phase of compressional tectonics giving rise to partial basin inversion was followed by a renewed rifting cycle spanning eocene early neogene times development of the east china rift system involving vertical as well as lateral crustal movements was probably induced by physico chemical changes within the earth s mantle that can be related to activity along the pacific subduction zones the mesozoic cenozoic east china rifts contain some important hydrocarbon provinces which rely to a large extent on syn rift sediments for source and reservoir rocks

influence of wet tropospheric correction on mesoscale dynamic topography as derived from satellite altimetry we study the influence of the wet tropospheric correction on mesoscale dynamic topography as derived from satellite altimetry for this purpose we use geosat altimeter data in the northeast atlantic and we process separately the tropospheric correction derived from the peridot model following the technique for analysing altimeter height profiles we show that the humidity spatial scales are larger than those of the dynamic topography consequently the wet tropospheric effect is considerably reduced when orbit error corrections are applied after long wavelength filtering the along track rms of the correction is 13 of that of the oceanic signal in addition contrary to the dynamic topographic signal the tropospheric signal is temporally decorrelated between two successive adjacent tracks so that the wet tropospheric contribution to the objectively analysed synoptic maps is only 10 of the signal of mesoscale variability

spatial scales of mesoscale variability in the north atlantic as deduced from geosat data two years of geosat data were used to systematically investigate the mesoscale variability and spatial scales of the north atlantic we first calculated sea surface height variability and eddy kinetic energy these compare well with the eddy kinetic energy obtained with surface drifters then the spatial scales of mesoscale variability were characterized in 10 latitude 10 longitude areas by their wave number spectra and autocorrelation functions comparison with existing in situ data shows good agreement and it is believed that the total error budget is globally below that of the oceanic signal the wave number spectra show significant differences as a function of latitude and longitude and these can be related to different types of forcing instability of a mean current or fluctuating wind scales typically decrease from west to east and south to north simple proportionality with respect to the first internal rossby radii does not apply everywhere

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

assimilation of altimeter eddy fields in a limited area quasi geostrophic model abstract techniques for the synoptic analysis vertical inference dynamical adjustment and forecast of altimetric and deeper in situ data are presented as a first step towards the design of continuous assimilation schemes in limited area oceanic domains a year long time series of streamfunction maps denoted mark 2 drawn in the polymode area of the western north atlantic is used as a benchmark for various tests and simulations an original projection extension scheme using empirical modes of density and or pressure anomalies is used to obtain a first guess of the three dimensional structure of the currents starting from surface topography only in the mark 2 domain this technique works well since the first empirical mode is surface intensified and largely dominant an alternative approach is to incorporate deeper data e g float trajectories below the main thermocline the first guess currents are specified as initial and boundary condition of the harvard 6 level quasi geostrophic open ocean mode

complex strain patterns developed at the frontal and lateral tips to shear zones and thrust zones abstract many of the complex strain patterns seen in shear zones and thrust zones such as variable fabric orientations refolded folds and fabrics together with folds with hinges almost parallel to the main transport direction can be explained in terms of differential movement within the shear zones these strains are developed at the frontal and lateral tips of the zones as they propagate examples are taken from the moine thrust zone of scotland which show variations in strains particularly at the lateral tips the form of differential movement described here may lead to complex strain paths and non plane strain ellipsoids and the spatial variations in finite strain may be used to delineate zones of extensional and compressional flow and differential movement in the shear zones or thrusts

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

recumbent folds key structural elements in orogenic belts abstract this review has two main parts the first of them presents existing ideas and data related to recumbent folds reviewing aspects such as the physical conditions of the development of these folds the strain inside the folded layers the kinematic mechanisms of their formation the role of gravitational forces the tectonic context of their development and the structures associated with them in the second part the above ideas are discussed and possible mechanisms for the development of these folds are presented it is proposed that initial perturbations of the layers are essential to give rise to the asymmetry of recumbent folds these perturbations may be non planarities of the layering or may be linked to the existence of a core or basement of competent rock that hinders the normal propagation of the deformation this could explain why many large recumbent folds have a root zone deformation with an important component of simple shear is a general condition for the formation of recumbent folds in areas with very low grade metamorphism competent layers often play an active role during the deformation and undergo buckling with the development of an overturned fold limb which can be stretched and thinned to finally produce a pair of recumbent folds separated by a thrust in areas with low or medium metamorphism buckling under a simple shear regime is probably the most important mechanism for producing large folds with gentle or moderately dipping axial surfaces subsequent kinematic amplification by coaxial strain components with vertical maximum shortening is important for the formation of recumbent folds these components involve a sub horizontal stretching that can cause a problem of strain compatibility and give rise to a basal thrust in areas deformed under high p and t conditions recumbent folds can develop by flow perturbations and kinematic amplification of folds this is probably a common mechanism in ductile shear zones

folding in the greenland ice sheet the deformation of layering into folds is modeled for a linear viscous medium moving over a decollement folds are generated by flow variations caused by relief on the decollement variations in friction or both the model is applied to folds forming now in the greenland ice sheet near dye 3 for which more complete data are available than for analogous solid earth situations and for which the decollement is at or near the bed the folds wavelength 4 8 km are detected by radio reflection sounding measured surface deformation and deformation rate are used with the radar results to test the theory calculated fold amplitude is only 20 less than that measured which indicates that the theory is substantially correct inversion of the data to calculate basal drag and velocity variations is not helpful for near dye 3 because many different basal boundary conditions can lead to the observed deformations

strain compatibility and shear zones is there a problem in analyzing deformation in rocks it is important to ensure that the solutions obtained satisfy strain compatibility this creates a challenge to understanding patterns of strain associated with shear zones in which measured strain may appear incompatible with the strain in the shear zone walls flattening strains are common in natural shear zones with locally straight and parallel boundaries to satisfy compatibility conditions such strains require volume loss across the shear zone or deviations from plane strain with or without discontinuities between the shear zone and the wall rock in the case of shear zones for which there is no evidence of volume loss or discontinuities along the shear zone walls problems of strain compatibility may be resolved if individual shear zones are linked together in an appropriate fashion shear zones commonly occur in anastomosing arrays and simple configurations of such arrays and the strains associated with them are examined it is shown that local transpression with strain compatibility can be accounted for in this way quite complex local strain patterns can develop in simple arrays

a wave equation migration method for receiver function imaging 2 application to the japan subduction zone 1 the newly developed wave equation poststack depth migration method for receiver function imaging is applied to study the subsurface structures of the japan subduction zone using the fundamental research on earthquakes and earth s interior anomalies freesia broadband data three profiles are chosen in the subsurface imaging two in northeast ne japan to study the subducting pacific plate and one in southwest sw japan to study the philippine sea plate the descending pacific plate in ne japan is well imaged within a depth range of 50 150 km the slab image exhibits a little more steeply dipping angle 32 in the south than in the north 27 although the general characteristics between the two profiles in ne japan are similar the imaged philippine sea plate in eastern sw japan in contrast exhibits a much shallower subduction angle 19 and is only identifiable at the uppermost depths of no more than 60 km synthetic tests indicate that the top 150 km of the migrated images of the pacific plate is well resolved by our seismic data but the resolution of deep part of the slab images becomes poor due to the limited data coverage synthetic tests also suggest that the breakdown of the philippine sea plate at shallow depths reflects the real structural features of the subduction zone rather than caused by insufficient coverage of data comparative studies on both synthetics and real data images show the possibility of retrieval of fine scale structures from high frequency contributions if high frequency noise can be effectively suppressed and a small bin size can be used in future studies the derived slab geometry and image feature also appear to have relatively weak dependence on overlying velocity structure the observed seismicity in the region confirms the geometries inferred from the migrated images for both subducting plates moreover the deep extent of the pacific plate image and the shallow breakdown of the philippine sea plate image are observed to correlate well with the depth extent of the seismicity beneath ne and sw japan such a correlation supports the inference that the specific appearance of slabs and intermediate depth earthquakes are a consequence of temperature dependent dehydration induced metamorphism occurring in the hydrated descending oceanic crust

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

strain in an archean greenstone belt of minnesota abstract we measured strain at more than 60 locations in metasedimentary and metavolcanic rocks of the vermilion district an e w trending archean greenstone belt in minnesota strain ellipsoid orientations and shapes correlate strongly with n s location in the belt but magnitudes do not flattening strains occur near the present vermilion fault which bounds the greenstone belt to the north with constrictional strains to the south the observed strain patterns can be mathematically modeled by deformation paths which produce the flattening strains with west plunging 1 axes by dextral shear of the constrictional strains with east plunging 1 axes using reasonable geologic constraints the shear plane must dip to the north with a subhorizontal shear direction structures throughout the district also indicate dextral shear a geometrical finite element program uses the measured strains to destrain the rocks and find the configuration which most closely satisfies strain compatibility equations the linear e w strain patterns and minor rotations about horizontal axes during the deformation preclude origin of the greenstone belt by infolding and shear off the flanks of a rising granitic diapir by accounting for rotations which result in the deformed curvature of the original surface a true estimate of 50 n s shortening across the belt can be made the data and deformation models favor the origin of the vermilion district rocks at a convergent margin most likely as a n dipping subduction zone complex with shallow slab dip the origin of the constrictional strains remains enigmatic

hanging wall accommodation styles in ramp flat thrust models in this paper we study the dynamic and rheologic control of hanging wall accommodation in ramp flat thrust models in particular we vary the dimensionless ratio of shear strength to gravity stress to model hanging wall accommodation styles in different materials in all models we require that the flat ramp flat footwall provides a surface of low frictional resistance in viscous materials hanging wall accommodation progresses by wedge flow in bingham materials wedge flow is also the preferred mode in cases where the gravity stress exceeds the yield limit of materials such models simulate the flow of salt or snow glaciers above ramp obstructions at high ratios of shear strength to gravity stress the hanging wall blocks translate forward without bending and unbending to the form of the rigid footwall in elastic plastic strain hardening materials ramp flat accommodation progresses by fault bend folding in case there is a near balance between the yield stress and gravity stress in frictional materials hanging wall accomodation progresses by shear or kink band nucleation above fault bends the shear or kink bands which initially nucleate at the lower fault bend change shape and reactivate by normal faulting or tensile failure at the upper fault bend depending on the ratio of shear strength to gravity stress in nature hanging wall accommodation by thrust nucleation above ramps and their subsequent reactivation may be anticipated in frictional sediments at upper crustal levels where temperatures and pressures are low

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

nappe emplacement direction in the central gneiss belt grenville province ontario canada evidence for oblique collision abstract the proterozoic grenville province in canada is widely regarded as a compressional orogen in which thrust transport occurred towards the northwest perpendicular to the orogenic front new structural data from an area near parry sound ontario lead to a re evaluation of this interpretation nappe structures in this area are consistent with northeast rather than northwest directed thrusting and imply that the southwestern part of the province experienced an early sinistral oblique collision which predated the emplacement of the 1 24 ga sudbury dike swarm two nappe piles which contain different rock types and have distinct stacking sequences are separated by a lateral ramp or ductile tear fault that coincides with the interior of the parry sound domain of davidson et al 1982 early nappes and associated structures f 1 are overprinted by nw se striking upright folds f 2 that formed between about 1 16 and 1 12 ga these later folds partially reorient rotated feldspar porphyroclast systems that formed during nappe emplacement and have axes that are consistently parallel to a well developed mineral lineation the orientation of this lin lineation reflects the two stage deformation history of the area rather than the thrusting direction during a single orogenic event

experimental work on the effect of pre existing anisotropy on fabric development in glaciers abstract localization of deformation in ice is known to be important at all scales in deforming glaciers however relatively little is known of the significance of shear localization and the influence of fabric development in anisotropic ice at microscopic scales c axis preferred orientation and the inclination of the primary layering in anisotropic ice masses during both plane strain compression and combined simple shear compression have been examined a series of creep tests in the temperature range of 5 to 1 c over a range of shortening strains varying from 10 to 40 and compressive stresses ranging from 0 to 0 7 mpa have been undertaken significant variations in the strain rate and microstructural development have been observed in the plane strain compression experiments that reflects the varying orientations of the anisotropy and its relationship to easy glide directions in the ice mass in the unconfined combined compression and shear experiments minimum shear stress rates vary between the variously oriented anisotropic ice masses and deviate from the normal power flow law for isotropic ice where annealing occurs such ice masses preserve the pre existing c axis fabric and hence may reflect a contribution from both the recrystallized and inherited ice components

strain distribution in a fold in the west norwegian caledonides abstract strain has been measured from clasts within a deformed conglomerate layer at 17 localities around an asymmetric fold in the rundemanen formation in the bergen arc system west norwegian caledonides strain is very high and a marked gradient in strain ellipsoid shape exists to either side of the fold strain within the conglomerate bed is of the extreme flattening type in the fold especially on the lower fold closure the strain is constrictional mathematical models of perturbations of flow in glacial ice have produced folds of the same geometry as this fold with a strikingly similar pattern of finite strain the fold geometry and strain pattern as well as other field observations suggest that the fold developed passively as the result of a perturbation of flow in a shear zone where the strain was accommodated by simple shear accompanied by extension along y

strain patterns in an ice cap and implications for strain variations in shear zones glacier flow is a form of gravity tectonics in thin sheets characterized by the development of large strains foliation and folds strain is difficult to measure in ice but numerical methods provide a good means of estimating cumulative strains in simple terms flow in an ice cap involves vertical flattening at the center with bottom parallel shear increasing in intensity downwards and outwards in steady state cumulative strains will be plane if flow is two dimensional as in parts of many valley glaciers and some ice caps and of flattening type if lateral extension occurs normal to the flow direction as in many ice sheets in both radial and plane flow strain gradients will be large vertically and low horizontally outwards cumulative strain magnitude can be extremely large near the base and at the margin z is everywhere sub vertical if flow is unsteady more complex strain patterns develop even if changes in the flow field are slight over bedrock ridges flow perturbations can lead locally to rotations greater than those of the quasi simple shear acting near the base in steady state flow this may cause the x direction of the cumulative strain to rotate through the shear plane in the neighborhood of the perturbation in which case strain magnitudes subsequently diminish leading to low or zero strains in plane flow and to constrictional strains in radial flow in the latter case x lies horizontal and perpendicular to the flow direction and we have local constrictional strain in an overall flattening field the zones of perturbed strain tend to be associated with the inverted limbs of recumbent folds shear zones in rocks are expected to show similar patterns of strain perturbation

constraining p wave velocity variations in the upper mantle beneath southeast asia we have produced a p wave model of the upper mantle beneath southeast se asia from reprocessed short period international seismological centre isc p and pp data short period p data of the annual bulletin of chinese earthquakes abce and long period pp p data we used 3d sensitivity kernels to combine the datasets and mantle structure was parameterized with an irregular grid in the best sampled region our data resolve structure on scale lengths less than 150 km the smearing of crustal anomalies to larger depths is reduced by a crustal correction using an a priori 3d model our tomographic inversions reveal high velocity roots beneath the archean ordos plateau the sichuan basin and other continental blocks in se asia beneath the himalayan block we detect high seismic velocities which we associate with subduction of indian lithospheric mantle this structure is visible above the 410 km discontinuity and may not connect to the remnant of the neo tethys oceanic slab in the lower mantle our images suggest that only the southwestern part of the tibetan plateau is underlain by indian lithosphere and thus that the upper mantle beneath northeastern tibet is primarily of asian origin our imaging also reveals a large scale high velocity structure in the transition zone beneath the yangtze craton which could have been produced in multiple subduction episodes the low p wave velocities beneath the hainan island are most prominent in the upper mantle and transition zone they may represent counter flow from the surrounding subduction zones and may not be unrelated to processes beneath eastern tibet 2005 elsevier b v all rights reserved

isotropic points and lines in strain fields abstract in heterogeneous strain fields the strain may be isotropic at some points or along some lines called isotropic points and isotropic lines these isotropic features are known in various strain environments such as folds diapirs nappes or shear zones mechanical instabilities involving flow cells diapirism and folding are able to produce simultaneously isotropic points and lines whose dimensions location and migration are controlled by the flow pattern and history many other isotropic points can be understood in terms of superposition of elementary flows some examples such as neighbouring diapirs synkinematic diapirs shear zone terminations and spreading sliding nappes are presented finally some three dimensional aspects and general properties of isotropic points and lines are discussed

what drove late mesozoic extension of the northern china mongolia tract the northern china mongolia tract exhibited a tectonic transition from contractional to extensional deformation in late mesozoic time late middle to early late jurassic crustal shortening is widely thought to have resulted from collision of an amalgamated north china mongolia block and the siberian plate but widespread late late jurassic early cretaceous extension has not been satisfactorily explained by existing models some prominent features of the extensional tectonics of the northern china mongolia tract are 1 late jurassic voluminous volcanism prior to early cretaceous large magnitude rapid extension 2 overlapping in time of contractional deformation in the yinshan yanshan belt with development of extensionrelated basins in the interior of the northern china mongolia tract and 3 widespread occurrence of alkali granitic plutonism extensional basins and metamorphic core complexes in the early cretaceous a new explanation is advanced in this study for this sequence of events the collision of amalgamated north china mongolia with siberia led to crustal overthickening of the northern china mongolia tract and formation of a high standing plateau subsequent breakoff at depth of the north dipping mongol okhotsk oceanic slab is suggested as the main trigger for late mesozoic lithospheric extension of that tract slab breakoff resulted in mantle lithospheric stretching of the adjacent northern china mongolia tract with subsequent ascent of hot asthenosphere and magmatic underplating at the base of the crust collectively these phenomena triggered gravitational collapse of the previously thickened crust leading to late late jurassic early cretaceous crustal extension and importantly coeval contraction along the southern margin of the plateau in the yinshan yanshan belt the proposed model provides a framework for interpreting the spatial and temporal relationships of distinct processes and reconciling some seemingly contradictory phenomena such as the synchronous extension of northerly terranes during major contraction in the neighboring yanshan yinshan belt d 2003 elsevier b v all rights reserved

using shear wave splitting measurements to investigate the upper mantle anisotropy beneath the north china craton distinct variation from east to west 1 a compilation of shear wave splitting measurements in the north china craton ncc is presented to investigate the upper mantle anisotropy beneath the region by applying the method of silver and chan 1991 the splitting parameters of fast polarization direction and delay time are determined from sks waveforms for 38 broadband stations the results reveal the presence of small to large seismic anisotropy in the mantle beneath the ncc the most striking result is the distinct difference of fast polarization directions between the eastern block eb and the central zone cz of the ncc the fast polarization directions trend se beneath the eb which implies that northwestward mantle flow has played a significant role in the reactivating of the eb during late mesozoic to early cenozoic and the boundary between the cz and the eb was possibly a west limit where the mantle flow was deflected however a collision event occurred in early proterozoic was also a possible cause for the anisotropy of the lithosphere beneath the cz depending on the craton lithosphere beneath the region had not been completely reformed by later tectonic activities

crustal heterogeneity and seismotectonics of the region around beijing china abstract a detailed three dimensional 3 d p wave velocity model of the crust and uppermost mantle under the chinese capital beijing region is determined with a spatial resolution of 25 km in the horizontal direction and 4 17 km in depth we used 48 750 precise p wave arrival times from 2973 events of local crustal earthquakes controlled seismic explosions and quarry blasts these events were recorded by a new digital seismic network consisting of 101 seismic stations equipped with high sensitivity seismometers the data are analyzed by using a 3 d seismic tomography method our tomographic model provides new insights into the geological structure and tectonics of the region such as the lithological variations and large fault zones across the major geological terranes like the north china basin the taihangshan and the yanshan mountainous areas the velocity images of the upper crust reflect well the surface geological and topographic features in the north china basin the depression and uplift areas are imaged as slow and fast velocities respectively the taihangshan and yanshan mountainous regions are generally imaged as broad high velocity zones while the quaternary intermountain basins show up as small low velocity anomalies velocity changes are visible across some of the large fault zones large crustal earthquakes such as the 1976 tangshan earthquake m 7 8 and the 1679 sanhe earthquake m 8 0 generally occurred in high velocity areas in the upper to middle crust in the lower crust to the uppermost mantle under the source zones of the large earthquakes however low velocity and high conductivity anomalies exist which are considered to be associated with fluids the fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes

distinct lateral variation of lithospheric thickness in the northeastern north china craton abstract a detailed knowledge of the thickness of the lithosphere in the northeastern north china craton ncc is important for understanding the significant tectonic reactivation of the craton in the mesozoic and cenozoic time we achieve this goal by applying the newly proposed wave equation based migration technique to the s receiver functions recently collected in the region distinct negative signals are identified below the moho in all the s receiver function migrated images and stacks which we interpret as representing the s to p conversions from the lithosphere asthenosphere boundary lab the imaged lab is as shallow as 60 70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior these observations indicate widespread lithospheric thinning in the study region in comparison with the 180 km lithospheric thicknesses typical of most cratonic regions the revealed topography of the lab generally agrees with the lateral variation in upper mantle seismic anisotropy previously measured through sks splitting analysis in particular a sharp lab step of 40 km is detected at the triple junction of the basin and mountains at almost the same place where an abrupt change from nw se to ne sw in fast polarization direction of shear waves was found these findings suggest a close correlation between the seismic anisotropy and hence deformation of the upper mantle the lithospheric thickness and the surface tectonics of the northeastern ncc while the thinned lithosphere and the nw se fast shear wave polarizations in the east areas probably are related to the dominant nw se tectonic extension in the late mesozoic cenozoic time the thicker lithosphere and the ne sw fast polarization direction in the west mountain ranges may reflect earlier contractional deformations of the region synthetic tests indicate that the lab beneath the northeastern ncc is a well defined zone 10 20 km thick combined with seismic tomography results and geochemical and petrological data this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton

seismological structure of the upper mantle a regional comparison of seismic layering abstract we investigate seismic layering i e discontinuities regions of anomalous velocity gradients and anisotropy and its lateral variability in the upper mantle by comparing seismic models from three tectonic regions old 100 ma pacific plate younger 40 ma philippine sea plate and precambrian western australia these models were constructed by combining two data sets scs reflectivity profiles which provide travel times and impedance contrasts across mantle discontinuities and observations of frequency dependent travel times of three component turning s ss ss sss sss sa and surface r1 g1 waves which constrain the anisotropic velocity structure between discontinuities the models provide a better fit to observed seismograms from these regions than the current generation of global tomographic models the australian model is characterized by high shear velocities throughout the upper 350 km of the mantle with no low velocity zone lvz in the isotropically averaged shear velocities in contrast the oceanic models are characterized by a thin high velocity seismic lid underlain by a distinct lvz with a sharp boundary the g discontinuity separating them the g is significantly deeper beneath the western philippine sea plate than beneath the older pacific 89 and 68 km respectively implying that thermal cooling alone does not control the thickness of the lid we interpret this discontinuity as a compositional boundary marking the fossilized base of the melt separation zone msz active during sea floor spreading no discontinuity is detected at the base of the lvz in the oceanic models the s velocity gradient between 200 and 410 km depth is much steeper in the oceans than beneath australia this high oceanic gradient is probably controlled by a decrease in the homologous temperature over this depth interval the relative depths of the transition zone tz discontinuities are consistent with clapeyron slopes expected for an olivine dominated mineralogy the 660 km discontinuity displays variability in its amplitude that appears to correlate with its depth shallow and bright beneath the pacific deep and dim beneath australia and the philippine sea such behavior is possibly caused by the juxtaposition of the olivine and garnet components of the phase transition radial anisotropy extends through the upper 250 km of the mantle in the australia model and through the upper 160 km of the two oceanic models the magnitude of anisotropy is consistent with that expected for models of horizontally oriented olivine and the localization of anisotropy in the shallowest upper mantle implies that it reflects strain associated with past or present tectonic events

finite strain and deformation within the brianconnais castelvecchio cerisola nappe of the ligurian alps italy abstract the structural history of the castelvecchio cerisola brianconnais nappe ligurian alps italy was dominated by a southwestward overthrust shear that resulted in isoclinal folding and schistosity lineation fabric development within the nappe this deformation was due to the intracontinental shortening related to the collision of the european and adriatic plates during late cretaceous eocene times finite strain produced during the isoclinal folding and the schistosity lineation fabric development was determined from quartzites conglomerates and tuffs using fry s method strain type and intensities are strongly dependent on rock type other factors like the proximity to the major thrusts or the position in relation to the first phase recumbent folds seem to be of secondary importance the quartzites and conglomerates display a constrictional strain or else they approximate to plane strain while the pyroclastic rocks fall into the field of apparent flattening and are characterized by a higher intensity of deformation during nappe emplacement the deformation was not accommodated by simple shear alone the interpretation of strain patterns in the castelvecchio cerisola has led to a model in which longitudinal strain was superposed on layer parallel simple shear schistosity at a low angle to the major thrust faults incremental extension directions deduced from pressure shadows very frequent veins and stylolitic structures parallel to bedding support this hypothesis the data presented here emphasize that subvertical thinning associated with thrust shear and a small extension perpendicular to the emplacement direction occurred in the castelvecchio cerisola nappe

the mean sea surface height and geoid along the geosat subtrack from bermuda to cape cod measurements of near surface velocity and concurrent sea level along an ascending geosat subtrack were used to estimate the mean sea surface height and the earth s gravitational geoid velocity measurements were made on three traverses of a geosat subtrack within 10 days using an acoustic doppler current profiler adcp a small bias in the adcp velocity was removed by considering a mass balance for two pairs of triangles for which expendable bathythermograph measurements were also made because of the large curvature of the gulf stream the gradient wind balance was used to estimate the cross track component of geostrophic velocity from the adcp vectors this component was then integrated to obtain the sea surface height profile the mean sea surface height was estimated as the difference between the instantaneous sea surface height from adcp and the geosat residual sea level with mesoscale errors reduced by low pass filtering the error estimates were divided into a bias tilt and mesoscale residual the bias was ignored because profiles were only determined within a constant of integration the calculated mean sea surface height estimate agreed with an independent estimate of the mean sea surface height from geosat obtained by modeling the gulf stream as a gaussian jet within the expected errors in the estimates the tilt error was 0 10 m and the mesoscale error was 0 044 m to minimize mesoscale errors in the estimate the alongtrack geoid estimate was computed as the difference between the mean sea level from the geosat exact repeat mission and an estimate of the mean sea surface height rather than as the difference between instantaneous profiles of sea level and sea surface height in the critical region near the gulf stream the estimated error reduction using this method was about 0 07 m differences between the geoid estimate and a gravimetric geoid were not within the expected errors the rms mesoscale difference was 0 24 m rms