the computation of body wave synthetic seismograms in laterally homogeneous media synthetic seismograms computed for realistic horizontally stratified media are now routinely used as an aid to seismic interpretation this paper reviews the theoretical background to these methods and presents comparisons of the two popular algorithms reflectivity and wkbj seismograms for a variety of earth models the transformed wave equations are developed from the equations for a spherical gravitating medium in a symmetric form suitable for body wave calculations four methods of solving these equations in general inhomogeneous layers are described the wkbj and langer asymptotic expansions and the wkbj and langer iterative solutions together with the earth flattening transformation and the ray expansion transformed solutions for body waves can then be obtained for realistic layered media four methods of inverting the frequency and wave number transformations are also described the real and complex spectral and slowness methods although realistic seismic models are normally sufficiently complicated that numerical calculations are essential before proceeding with numerical comparisons we have included a review of the canonical signals included in body wave seismograms these analytic results for direct rays partial and total reflections turning rays on forward and reversed branches head waves interface waves airy caustics and fresnel and interface shadows are useful to anticipate and understand numerical problems and results finally a comparison of green s functions for crustal mantle and whole earth models calculated using the wkbj and reflectivity algorithms is included

shear and compressional wave structure of the east pacific rise 9 10 n we use shear and compressional arrivals recorded by 45 ocean bottom seismograph receivers to model regional structure of the 9 10 n region of the east pacific rise modeling indicates that shear conversion occurs at the base of layer 2a producing ps energy that travels through oceanic crust as p and converts to s at the base of layer 2a on upgoing path and ps energy that travels as p through layer 2a and converts to s at the base of layer 2a on downgoing path arrivals the travel times of these arrivals require a shear wave velocity within layer 2a of 0 4 0 8 km s poisson s ratio of 0 46 0 49 waveform inversion was used to model the poisson s ratio structure of layers 2b and 3 for nine instruments with good shear arrivals poisson s ratio within layer 2b was highly variable with values as low as 0 24 and a mean value of 0 263 some of this variability might be due to lateral variability in layer 2a structure the mean poisson s ratio of layer 3 was 0 271 according to the cracking model of shaw 1994 the low poisson s ratios within the upper portion of layer 2b indicate that thick cracks aspect ratio 0 1 extend to depths of 1 5 1 7 km in this region two dimensional travel time modeling indicates that the southwestern portion of our study area is associated with anomalously low seismic velocities with layer 2b and layer 3 velocities reduced by up to 11 from the regional values cracking theory suggests that these low velocities could be caused by porosities of 0 3 5 5 depending on the crack aspect ratio a maximum porosity of 1 5 is predicted from our one poisson s ratio measurement in this area the anomalous velocities are located near the western discordant zone left by the 9 03 n overlapping spreading center we suggest that shearing associated with on axis rotation of the overlap basins is responsible for the low seismic velocities the pattern of anomalies indicates that faulting extends to distances of 10 15 km from the basins

new shrimp u pb zircon ages from the heilongjiang high pressure belt constraints on the mesozoic evolution of ne china the heilongjiang complex is a high pressure accretionary belt composed mainly of serpentinite glaucophane bearing metabasalt epidote glaucophane albite schist greenschist marble two mica schist muscovite albite schist quartz schist and quartzite chert located along the suture zone separating the jiamusi and songliao blocks in ne china shrimp u pb zircon analyses of two samples of granitic gneiss from yilan that are interleaved with the heilongjiang complex record diverse protolith ages that are consistent with derivation from regional permian granitoids 263 7 ma and the adjacent mashan complex 492 6 ma indicating tectonic incorporation of older granitic components into the accretionary complex the age of the basaltic protolith of two samples of glaucophane albite schist from yilan is 245 ma and is slightly younger than the protolith age of two samples of amphibole schist from mudanjiang located about 200 km south of yilan that define weighted mean 206pb 238u ages of 257 4 and 257 5 ma all four samples contain a few older inherited zircon grains and also show the effects of an isotopic disturbance at 200 to 190 ma most likely related to the subsequent high pressure metamorphism the timing of which has been determined in previous studies the basaltic rocks record rifting and ocean development from 260 to 210 ma based on previous studies in which both e morb and oib protolith signatures were identified detrital zircons in two samples of mica schist from mudanjiang show distinct populations at 700 ma 500 to 450 ma 350 to 213 ma peak at 255 ma 290 to 200 ma and 200 ma all except the youngest ages can be matched with rocks from the older components of the jiamusi block and with other areas in the central asia orogenic belt caob supporting the view that the jiamusi block was most likely the eastern segment of the caob prior to rifting the youngest concordant magmatic detrital grain with an age of 207 3 ma defines the maximum age of sedimentation this also limits the timing of high pressure metamorphism in the heilongjiang complex to post late triassic consistent with argon data obtained in previous studies final closure between the jiamusi and songliao blocks took place in the latest triassic early jurassic with the two blocks accreted as a result of pacific ocean subduction this suggests that the heilongjiang complex records the time when north south closure of the caob in china gave way to westward movement related to the pacific ocean subduction which has dominated the tectonics of ne china and far east russia since that time

late variscan tectonic evolution by thinning of earlier thickened crust an 40ar39ar study of the montagne noire southern massif central france abstract 40ar 39ar age determinations on samples selected on deformational criteria form the basis of a proposal of a new detailed tectonometamorphic history of the montagne noire the southern part of the massif central france this range is classically divided into a cover and an axial gneissic and migmatitic core which was metamorphosed under high t medium p conditions the basement is structurally overlain by epimetamorphic paleozoic sediments in the north in the south equivalent rocks form a well known pile of recumbent nappes the variscan history of this area comprises early shortening and thickening of the continental crust the climax of this event took place around 320 ma ago as is shown by previous rb sr data and by 40ar 39ar measurements new developments in structural analyses have led to a better understanding of the late evolution of the area in terms of diapiric uplift of the core south southwestwards verging recumbent nappes have been emplaced and were weakly metamorphosed in their lower parts this movement was accompanied and followed by dextral extensional wrenching of the paleozoic cover along the northern and southern margins of the axial zone attendantly ductile shear zones were formed in order to decipher the complex history three structural levels have been dated with the 40ar 39ar method these levels are 1 the migmatitic and gneissic series in the axial core where micas yield ages of c 315 ma 2 mylonites from the northern and southern shear zones where biotites and muscovites yield ages of c 310 ma 3 muscovites and biotites from the epimetamorphic nappe structures yielding ages of c 297 ma the later ages may represent a younger thermal pulse these data are compared and discussed in the light of previous radiometric studies of the area and precise biostratigraphic constraints a detailed geodynamic evolution of the studied segment is proposed

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

dispersion and attenuation of mantle waves through waveform inversion summary we propose a new approach to the determination of elastic and anelastic parameters of the earth s structure from seismic data instead of measuring such functionals of the medium response as the phase delay or spectral ratios we perturb parameters of the model to satisfy the observed waveforms the advantage of our waveform inversion technique wit is that it uses the properties of the earth as a smoothing filter also because the earth s structure is the common denominator the method allows simultaneous interpretation of different functionals of this structure for example the waveforms of rayleigh and love waves for the same source receiver pair following extensive testing of the method on synthetic and actual data we subject results of our analysis of 37 recordings for various sources and stations to pure path decomposition the period range of analysis extends from 160 to 630s we distinguish four types of regions stable continents areas tectonically active within the last 400myr ocean floors younger than 38 myr and old ocean floors older than 38 myr the results indicate significantly different responses of the oceanic and continental areas at long periods stable continental and tectonic regions have nearly the same dispersion for periods greater than 300 35os and then diverge rapidly with stable continents showing higher velocities the young and old oceans on the other hand become distinct at periods as long as 500 s the old oceans are faster in terms of shear velocity models the data are consistent with a difference of about 4 per cent between the young and old oceans in a depth range from 400 to 670 km while the continental regions are similar and close to the global average at shallower depths our structures are conceptually similar to those inferred from previous pure path analyses and short period surface wave studies while we cannot yet establish statistically significant differences in q for the four regions such differences are obtained if one distinguishes only between continents and oceans the q for the latter is 10 20 per cent lower in the period range of the analysis

the reliability of asymmetric c axis fabrics of quartz to determine sense of vorticity abstract asymmetric c axis fabrics of quartz are commonly used to determine sense of vorticity in ductile shear zones this method seems to work if the fabric pattern resembles a model fabric proposed by lister and hobbs 1980 usually however c axis fabrics are rather vague the reliability of such vague fabrics was tested in a major shear zone with known sense of vorticity only 62 of the c axis fabrics predict the correct sense great care should therefore be taken in applying this method to determine sense of vorticity

an example of multicomponent magnetization in welded tuffs a case study of upper cretaceous welded tuffs of eastern russia abstract four distinct components of natural remanent magnetization were isolated from a single site in welded tuffs in the upper cretaceous kisin group of the sikhote alin mountain range russia in order to contribute toward a basis for an interpretation of multicomponent magnetization rock magnetic experiments were performed on the welded tuffs all four magnetization components essentially reside in magnetite the lowest temperature component up to 300 c component a d 349 3 i 60 9 95 7 3 n 7 is a present day viscous magnetization the third removed component component c d 41 4 i 51 8 95 3 5 n 8 isolated over the temperature range of 450 560 c is a primary remanence the second and fourth demagnetized components component b d 174 7 i 53 1 95 21 2 n 3 and component d d 188 1 i 64 5 95 4 0 n 8 respectively are secondary magnetizations related to a thermal event in sikhote alin between 66 and 51 ma components b and d were acquired through different remagnetization processes component b is ascribed to a thermoviscous remanent magnetization carried by single domain magnetite and component d is a chemical remanent magnetization

location and sinistral displacement of the eastern liaoyuan accretionary belt along the tan lu fault zone ne china abstract the ne to nne striking continental scale tan lu fault zone tlfz in ne china is subdivided into the 900 km long yilan yitong fault yyf in the west and the 1000 km long dunhua mishan fault dmf in the east both faults record sinistral displacement during the earliest cretaceous leading to an obvious offset of the boundary between the liaoyuan accretionary belt lab and the north china craton ncc previous studies have demonstrated that the lab in the south of the eastern central asian orogenic belt caob extends eastwards into an area between the yyf and the dmf the boundaries between the lab and ncc indicate a 35 km sinistral displacement upon the yyf however it remains unclear whether the lab extends to the east of the dmf and the amount of sinistral displacement on the dmf is poorly constrained here we present lithological observations and u pb and hf isotopic data from zircon to constrain the age and position of the lab to the east of the dmf the data indicate an eastward extension of the lab across the dmf suggesting the continuous presence of the lab east of the tlfz in ne china the u pb ages ehf t values and two stage model ages of zircons from the study area constrain the boundary between the lab and ncc to an extension of the ne dipping chifeng kaiyuan fault to the east of the dmf the geological boundaries on both sides of the dmf indicate a 170 km sinistral displacement along the fault thus the tlfz has a total sinistral displacement of 205 km in ne china the eastern branch of the tlfz records significantly more movement due to its proximity to the pacific ocean where subduction of oceanic crust might have caused sinistral faulting along the tlfz

mode of mixture of oceanic fragments and terrigenous trench fill in an accretionary complex example from southern sakhalin abstract four modes of mixture of oceanic materials such as cherts limestones and basalts and terrigenous trench fill sediments are recognized in the cretaceous accretionary complexes in southern sakhalin far eastern pacific margin of the u s s r the first mode is a thin skinned stacking of pelagic bedded cherts with black shale due to duplex formation in association with shallow underplating or off scraping the second mode is a mixture of the uppermost part of the oceanic crust with terrigenous trench fill sediments due to olistostrome formation reconstruction of the original sequence in ascending order suggests that the basement is pillow basalt with overlying pelagic sediments of chert and limestone olistostrome caused by collapse of oceanic crust due to its bending before encountering a trench and finally trenchfill sediments of turbidite this sequence is imbricated by thrusting which appears to have occurred at the time of underplating the third mode is a tectonic mixing of oceanic fragments and terrigenous sediments due to shear which is related to underthrusting of the oceanic plate a reduction in the supply of trench fill sediments causes copious accretion of the upper part of oceanic crust the fourth mode is in situ basalt volcanism at the trench where thick baked margins of terrigenous shale are formed around the injected basalts this mode appears to represent a ridge trench encounter these four different modes of mixing seem to be usual in ancient accretionary complexes in orogenic belts

crustal structure of the colorado plateau arizona application of new long offset seismic data analysis techniques the colorado plateau is a large crustal block in the southwestern united states that has been raised intact nearly 2 km above sea level since cretaceous marine sediments were deposited on its surface controversy exists concerning the thickness of the plateau crust and the source of its buoyancy interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick a thick crust could support the observed topography of the colorado plateau isostatically while a thinner crust would indicate the presence of an underlying low density mantle this paper reports results on long offset seismic data collected during the 1989 segment of the u s geological survey pacific to arizona crustal experiment that extended from the transition zone into the colorado plateau in northwest arizona we apply two new methods to analyze long offset data that employ finite difference travel time calculations 1 a first arrival time inverter to find upper crustal velocity structure and 2 a forward modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals we find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern basin and range province to about 42 km beneath the northern transition zone and southern colorado plateau margin we observe some crustal thinning to 37 km thick and slightly higher lower crustal velocities farther inboard beneath the kaibab uplift on the north rim of the grand canyon the crust thickens to a maximum of 48 km we observe a nonuniform crustal thickness beneath the colorado plateau that varies by 15 and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations crustal compositions as inferred from seismic velocities appear to be the same beneath the colorado plateau as those in the basin and range province to the southwest implying that the plateau crust represents an unextended version of the basin and range some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the proterozoic era

late cretaceous paleomagnetic results from northeast asian continental margin the sikhote alin mountain range eastern russia welded tuffs in the upper cretaceous kisin group have been collected for paleomagnetic study from 24 sites in sikhote alin eastern russia normal paleomagnetic directions from 11 sites are regarded as primary directions whereas reversed paleomagnetic directions from 13 sites are ascribed to secondary origin the normal directions of the kisin group together with the characteristic directions of the sijanov group yield the late cretaceous characteristic direction of sikhote alin d 355 0 i 59 8 with 95 7 2 a significant separation 15 4 11 9 of pole positions is observed between sikhote alin 85 9 n 11 3 e with a95 10 1 and the north china block sikhote alin has been subjected to counterclockwise rotation through 20 5 17 7 with respect to north china block the rotation occurred during 66 51 ma and is ascribed to sinistral motion along the central sikhote alin fault

the magma system of mount st helens non linear high resolution p wave tomography abstract high resolution three dimensional images of p wave velocity anomalies below mt st helens washington were derived using tomographic inversion the model is a 27 5 21 20 km target volume parameterized by blocks 0 5 km per side the area included 39 stations and 5454 local events leading to 35 475 rays used in the inversion to diminish the effects of noisy data the laplacian was constrained to be small within horizontal layers providing smoothing of the model non linear effects were compensated for by iterating three dimensional ray tracing using pseudo bending between inversions and relocating earthquakes relative to the updated three dimensional model the structural differences between the linear and non linear inversions appear to be insignificant although the amplitudes of the anomalies are larger in the non linear models results indicate a low velocity anomaly 7 approximately 1 km in lateral extent from 1 5 to 3 km depths between 3 and 6 km depth the anomaly appears to spread out below 6 km depth the low velocity feature changes to a higher velocity perturbation with lower velocity perturbations flanking around the perimeter of the volcano the higher velocity material which correlates with the higher seismicity at that depth is interpreted as being a plug capping the low velocity magma chamber which begins below 9 km depth

episodicity of mesozoic terrane accretion along the pacific margin of gondwana implications for superplume plate interactions a review of evidence for deformation and terrane accretion on the late triassic early jurassic margins of pangea and the mid cretaceous margins of the palaeo pacific ocean shows that deformation was global and synchronous with probable superplume events late triassic early jurassic deformation appears to be concentrated in the period 202 197 ma and was coeval with eruption of the central atlantic magmatic province onset of pangaea break up a period of extended normal magnetic polarity and a major mass extinction event all possible expressions of a superplume event mid cretaceous deformation occurred in two brief periods the first from approximately 116 to 110 ma in the west palaeo pacific and the second from roughly 105 to 99 ma in the east palaeo pacific with both events possibly represented in northeast siberia this deformation was coeval with eruption of major oceanic plateaus core complex formation and rifting of new zealand from gondwana the cretaceous normal polarity epoch and a major radiation of flowering plants and several animal groups all linked with the mid cretaceous superplume event a simple unifying mechanism is presented suggesting that large continental or oceanic plates when impacted by a superplume tend to break up reorganize associated with gravitational spreading away from a broad thermally generated topographic high and with a resulting short lived pulse of plate marginal deformation and terrane accretion

crustal thinning recorded by the shape of the namurian westphalian leucogranite in the variscan belt of the northwest massif central france in the western part of the variscan belt of the french massif central the gueret massif is an extensional allochthon emplaced to the southeast during namurian westphalian time the internal structure of the coeval leucogranites surrounding the western edge of the gueret massif consists of the plateau d9aigurande brame st sylvestre and st goussaud massifs these plutons are asymmetric diapirs rooted in the marche bussiere madeleine and arrenes faults respectively the plutons indicate diverging extravasation from the gueret massif whatever their shapes the plutons have a northwest trending stretching lineation that is also marked in the granite host rocks by retrograde metamorphic minerals the same extension direction is also inferred from namurian westphalian leucogranites and dikes intruding the gueret massif extension related granitoids present typical structural patterns such as asymmetric shapes extravasation from the collapsing area and persistence of a stretching lineation parallel to the regional extension direction these features may be used to diagnose crustal thinning

the pre cretaceous deep seated tectonics of the abukuma massif and its place in the structural framework of japan abstract the structures and microstructures of the takanuki and hitachi areas in the abukuma massif northeast japan are described in the takanuki area the basic gosaisho series thrusts the pelitic takanuki ones in a hp metamorphic context the nappe structure is afterwards refolded by a migmatitic dome the samegawa dome in a ht metamorphic context microtectonic analysis shows that the nappe was transported from south to north along the stretching lineation geometric features suggest that the samegawa dome was emplaced by diapirism the role of the thrust surface as an instable interface promoting the doming is emphasized the hitachi metamorphic rocks composed of basic schist limestone and sandstone shist thrust the pelitic rocks of the western hitachi gneisses as for the takanuki area the thrusting occurred in ductile synmetamorphic conditions with a north or northeastward displacement owing to lithologic petrologic structural similitudes the nappe of the hitachi metamorphic rocks and that of the gosaisho series are unified into a unique nappe with a northward motion the emplacement occurred between late permian and late cretaceous likely in late jurassic the allochthonous units of the abukuma massif are correlated with the green schist nappe described in southwest japan since they are surrounded by the same zones namely the tanba zone and the kurosegawa kitakami one moreover both in southwest and northeast japan the emplacement of the green schist nappes is due to a shear deformation inducing rotational structures along the stretching lineation indicating the same sense of transport that is eastward in southwest japan and northward in northeast japan owing to the late bending of the japanese islands the late jurassic nappe structure is obliquely overprinted by a ht metamorphism ryoke in southwest japan abukuma in northeast japan and afterwards cut by late faults as the median tectonic line or the tanakura fault giving rise to the present complexity

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

least squares fitting of marine seismic refraction data summary an iterative procedure is presented for fitting waveform data from a marine seismic refraction experiment the wavefunction from the explosive source is known and the crustal structure is refined using the damped least squares procedure the damping parameter serves the dual purpose of stabilizing an under constrained inversion and improving the linearity by suppressing high frequencies the synthetic seismograms and their model differentials are calculated using the wkbj seismogram method both the synthetic seismograms and the linear algebra are sufficiently straightforward that the computations can be performed on an array processor the inversion procedure is then sufficiently rapid that interactive computations are possible the technique is illustrated using the ff2 refraction data from the fanfare cruise of the scripps institution of oceanography these data had been interpreted previously by trial and error using the reflectivity method starting from two different simple models the inversion procedure obtains essentially one unique model its features are very similar to the previous model

a new mechanism of granite emplacement intrusion in active extensional shear zones one of the principal ways in which continental crust grows is by the incorporation from deeper within the earth of large volumes of granitic magma a space problem exists1 as to how these magmas are accommodated in the crust traditionally two main emplacement mechanisms have been emphasized forceful intrusion whereby buoyancy driven magmas physically push the crust aside creating granitic diapirs and balloons and passive emplacement characterized by replacive mechanisms such as cauldron subsidence and stoping although more recent work2 3 has demonstrated that space for granites may be created within bends and offsets of large transcurrent faults the simple view of either forceful or passive still fails to account for the intrusive mechanisms of many granites here we report the discovery of a new igneous intrusion mechanism in the superbly exposed proterozoic continental crust of south greenland we have observed that rapakivi granite was intruded as large scale sheets along ductile extensional shear zones that were active during emplacement in such a process the space problem seems to be simply resolved

the accuracy of models derived by wkbj waveform inversion summary in this paper the accuracy of velocity depth profiles derived by matching wkbj seismograms to observations is quantitatively evaluated seismograms computed with the wkbj method are generally quite reliable but possess predictable systematic inaccuracies in the presence of strong velocity gradients the effects of these inaccuracies on models derived through wkbj waveform inversion are studied using reflectivity seismograms as data the velocity structure used is an oceanic lithosphere model that contains several transition regions separated by relatively homogeneous layers producing partially reflected reverberations in the reflectivity synthetics that are absent from the wkbj seismograms the inversion incorporates the jumping strategy to solve for the smoothest models consistent with the data we find these solutions to be independent of the starting model and to have a stable basic structure that agrees well with the correct model the differences everywhere less than a seismic wavelength depend on the frequency content of the seismograms reverberations in the reflectivity seismograms that are well separated from wkbj arrivals are treated as noise in the inversion

tectonic evolution of the anuy metamorphic rocks sikhote alin russia and their place in the mesozoic geodynamic framework of east asia abstract in sikhote alin the anuy river provides a representative cross section of the pre eocene structure of the area east of the central sikhote alin fault the mesozoic rocks are divided into the following groups from bottom to top anuy metamorphic rocks the tithonian berriasian samarkinsk accretionary complex early cretaceous zhuravlevsk tumnin turbidite and mid cretaceous al rich khungariisk granitoids the anuy metamorphic rocks have a postfolial ne sw trending antiformal structure from bottom to top they consist of 1 low grade schist and sandstone 2 gneiss and micaschist 3 migmatite and 4 ophiolitic nappe east west trending stretching lineation isoclinal folds and top to the east shear criteria are found in the ophiolite and migmatite units these structures are related to ophiolitic thrusting that likely occurred in early cretaceous times 40 ar 39 ar dating of migmatite and metagabbro indicates mid cretaceous ages about 110 ma close to those of the khungariisk granites the main deformation coeval to a high temperature metamorphism is characterized by ne sw trending stretching lineation with divergent senses of shear 40 ar 39 ar dating of biotite and muscovite from several lithologies give late cretaceous to paleogene ages 73 58 ma this tectono thermal event is related to ne sw trending strike slip faulting along the central sikhote alin fault system the unroofing of the anuy metamorphic rocks is likely related to granite emplacement the collision of the anuy microcontinent with asia can be compared to that of the southern japan microcontinent

discriminating geodynamical regimes of tin ore formation using trace element composition of cassiterite the sikhote alin case far eastern russia

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

elemental and sr nd isotopic geochemistry of cretaceous to early paleogene granites and volcanic rocks in the sikhote alin orogenic belt russian far east implications for the regional tectonic evolution abstract the sikhote alin orogenic belt in russian far east is an important late mesozoic to early cenozoic accretionary orogen related to the subduction of the paleo pacific plate this belt was generated by successive accretion of terranes made of accretionary prisms turbidite basins and island arcs to the continental margin of northeastern asia represented by the bureya jiamusi khanka block from jurassic to late cretaceous in order to study the tectonic and crustal evolution of this orogenic belt we carried out zircon u pb dating and whole rock elemental and sr nd isotopic analyses on granites and volcanic rocks from the primorye region of southern sikhote alin zircon dating revealed three episodes of granitoid emplacement permian early cretaceous and late cretaceous to early paleogene felsic volcanic rocks mainly rhyolite dacite and ignimbrite that overlay all tectonostratigraphic terranes were erupted during 80 57 ma postdating the accretionary process in the sikhote alin belt the cretaceous paleogene magmatism represents the most intense tectonothermal event in the sikhote alin belt whole rock major and trace elemental data show arc like affinity for granitoids and volcanic rocks indicating that they were likely generated in a supra subduction setting their initial 87sr 86sr ratios range from 0 7048 to 0 7114 and end t values vary from 1 7 to 3 8 mostly

characteristics structural styles and tectonic implications of mesozoic cenozoic faults in the eastern heilongjiang basins ne china abstract the eastern heilongjiang basins ehbs are the assemblage of a series of meso cenozoic residual basins located in the northeastern corner of china the deformation pattern of the ehbs has significant implications for the history of the pacific plate subduction beneath the eurasia since the late mesozoic in this paper research on the characteristics and structural styles of the meso cenzoic faults in the ehbs has been conducted on the basis of a comprehensive analysis of field geology drilling data and seismic reflection profiles as a result five different stages of the meso cenozoic faults in the ehbs have been recognized these are in accordance with the time and relevant characteristics of fault movements i e the early stage of the early cretaceous normal fault the early stage of the late cretaceous thrust fault the late stage of the late cretaceous thrust fault the cenozoic synsedimentary normal fault and the late stage cenozoic shear fault a regional geological section has been generated across the ehbs by linking four local seismic profiles together a step by step reconstruction has been made to help better understand the mesozoic cenozoic tectonic evolution of the ehbs two phases of extension rifting in the early cretaceous period and the paleogene respectively are demonstrated to be interfered with two phases of regional uplift compression and erosion in the late cretaceous period the complicated development of multiple fault systems within the ehbs has reflected the evolution of a complex tectonic subduction of the pacific plate beneath the eurasia since the cretaceous period

tomo3d 3 d joint refraction and reflection traveltime tomography parallel code for active source seismic data synthetic test we present a new 3 d traveltime tomography code tomo3d for the modelling of active source seismic data that uses the arrival times of both refracted and reflected seismic phases to derive the velocity distribution and the geometry of reflecting boundaries in the subsurface this code is based on its popular 2 d version tomo2d from which it inherited the methods to solve the forward and inverse problems the traveltime calculations are done using a hybrid ray tracing technique combining the graph and bending methods the lsqr algorithm is used to perform the iterative regularized inversion to improve the initial velocity and depth models in order to cope with an increased computational demand due to the incorporation of the third dimension the forward problem solver which takes most of the run time 90 per cent in the test presented here has been parallelized with a combination of multi processing and message passing interface standards this parallelization distributes the ray tracing and traveltime calculations among available computational resources the code s performance is illustrated with a realistic synthetic example including a checkerboard anomaly and two reflectors which simulates the geometry of a subduction zone the code is designed to invert for a single reflector at a time a data driven layer stripping strategy is proposed for cases involving multiple reflectors and it is tested for the successive inversion of the two reflectors layers are bound by consecutive reflectors and an initial velocity model for each inversion step incorporates the results from previous steps this strategy poses simpler inversion problems at each step allowing the recovery of strong velocity discontinuities that would otherwise be smoothened

late permian tectonic evolution at the southeastern margin of the songnen zhangguangcai range massif ne china constraints from geochronology and geochemistry of granitoids abstract this paper reports the la icp ms u pb zircon dating and geochemical data for the huangqigou xiaobeihu and lalagou granitic plutons at the southeastern margin of the songnen zhangguangcai range massif which will not only place important constraints on the rock forming ages source characteristics and tectonic setting of these granitic plutons but will also provide insights into understanding the tectonic evolution of the eastern segment of the central asian orogenic belt zircons from three plutons have distinct oscillatory zoning structures in their cathodoluminescene cl images which combined with the th u ratios of 0 28 1 06 indicate their magmatic origins zircon u pb dating results for these granitic plutons indicate that the granitic magmatism occurred mainly in the late permian 256 252 ma the granites have sio2 of 58 0 74 3 na2o of 2 74 3 76 k2o of 2 75 3 95 na2o k2o of 0 77 1 13 mg 100 mg mg fe2 of 21 8 39 9 and a cnk of 0 88 1 08 with ree patterns characterized by lree hree 7 75 12 82 la yb n 7 88 14 9 and eu 0 56 0 87 the trace element spider diagrams indicate that these granites are enriched in large ion lithosphile elements rb ba th u etc and relatively depleted in high field strength elements ti nb ta with sr and pb ranging from 205 to 350 ppm and from 2 02 to 3 60 ppm respectively the in situ hf isotope analysis reveals that ehf t values range from 2 22 to 11 65 and the two stage hf model ages tdm2 range between 641 and 1274 ma these geochemical characteristics indicate that the late permian granitic plutons at the se margin of the songnen zhangguangcai range massif can be assigned to metaluminous and weakly peraluminous high potassium calc alkaline i type granites with low sr and high yb features and that the magmas to have formed these granites were originated from the partial melting of mafic igneous rocks in the lower crust the source rocks are composed mainly of the juvenile crust with small contributions of ancient continental crust considering the existence of coeval mafic intrusive rocks and sedimentary basins we propose that the late permian magmatism in the study area occurred in a post collisional extensional setting following the collision of the xingkai block and the united jiamusi and songnen zhangguangcai range massifs

amplitude constraints in linear inversions of seismic data linear programming inversions of seismic data may include constraints on the second derivative of the delay time d2 dp2 dx dp x geometric ray theory predicts that the second derivative of the delay time is proportional to the inverse of the complex seismic amplitude squared so that the use of this additional constraint constitutes the incorporation of amplitude information in linear inversions the realizability constraint that velocity be a single valued function of depth must be included explicitly for higher order derivatives of delay time than the second the realizability constraint is nonlinear and cannot be included in a linear scheme given that the data are adequate to allow identification of the major prograde and retrograde branches and the intervening caustics useful bounds on x can be established the function x is calibrated with respect to measured amplitudes by requiring that the t x curve obtained by successively integrating the x curve satisfy the observed travel times this calibration may be cast as a linear least squares problem applications to data from the rose rivera ocean seismic experiment area reveal a welldefined layer 2 layer 3 transition an apparently isotropic and nearly homogeneous layer 3 and a moho transition that is quite gradual this transition appears to be more rapid in the fast pn direction

geochemistry and u pb zircon dating of the toudaoqiao blueschists in the great xing an range northeast china and tectonic implications abstract the toudaoqiao complex is a sequence of high pressure metamorphic rocks located along the suture zone that separates the xing an and erguna blocks in northeast china the rocks were metamorphosed up to epidote blueschist facies at p t conditions of approximately 0 9 1 1 gpa and 320 450 c lithological associations and major and trace element compositions indicate that the blueschists are metabasalts with ocean island basalt oib and normal mid ocean ridge basalt n morb affinities similar to those of the philippines intraoceanic accretionary complex formed by subduction of oceanic crust magmatic zircons extracted from two samples of the epidote blueschist facies metabasalts from the north mountain in the toudaoqiao complex exhibit 206pb 238u ages of 511 5 and 516 17 ma whereas a greenschist from the south mountain has a 206pb 238u age of 511 2 ma these data show that the protoliths of the toudaoqiao complex are of early paleozoic age in addition a granitic dike from the south mountain exhibits a weighted mean 206pb 238u age of 492 1 ma these ages constrain the timing of high pressure metamorphism in the toudaoqiao complex to 490 510 ma the new data support the view that the suture zone between the xing an and erguna blocks is the toudaoqiao xinlin fault rather than the derbugan fault as previously thought this suture which extends from xinlin through toudaoqiao and southern mongolia named the south mongolia toudaoqiao xinlin suture zone may be an important boundary between the central mongolia erguna and xing an blocks

orogeny and relative plate motions example of the japanese islands abstract the paleogeography of the eastern margin of asia near japan for the past 250 ma has been reconstructed on the basis of the relative motions between the oceanic plates and the eurasian plate engebretson et al 1982 and on land geology the relationships between the plate kinematics and orogeny are as follows calc alkaline magmatism is related to the presence of subduction when a plate boundary is a transform fault there is no volcanism very slow convergence rates do not give rise to magmatism migration of rft type triple junctions or ridge subduction produces forearc volcanism near the trench and also a wider volcanic zone uplift of low p t type regional metamorphic belts is related to collision episodes the uplift of the high p t blueschist belts on the contrary is not contemporaneous with them they appear to be related to the start of the subduction of a young plate after the migration of an rft type triple junction because the formation of a voluminous accretionary complex is contemporaneous with this and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high p t type metamorphic belt the subduction of a young plate would thus be an important factor causing these orogenic events highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists presumably along a mechanical plate boundary in the forearc

continental crust under compression a seismic refraction study of south island geophysical transect i south island new zealand 1 the 1996 south island geophysical transect sight active source seismic survey was designed to show the style of lithospheric thickening due to late cenozoic oblique convergence across the australian pacific plate boundary in new zealand as part of this study two seismic refraction lines were shot across central south island and offshore extensions of the continental crust in the tasman sea and pacific ocean we present the data and a 603 km long seismic velocity profile of the crust and uppermost mantle along one of these seismic transects a tomographic inversion of 62 563 travel times from crustal and upper mantle refractions and wide angle reflections resulted in a model with a two layer crust upper crustal velocities were between 5 9 and 6 3 km s and lower crustal velocities were between 6 5 and 7 0 km s continental compression has locally reduced the seismic velocities in the pacific plate crust by 0 2 0 3 km s a possible effect of high strain and fluids in the crust the thickening of the crust from 28 km at the east coast of south island to 37 km beneath the southern alps can account for about 25 of the 80 110 km shortening of pacific plate crust while the rest must be accounted for by rapid erosion of mesozoic sedimentary rocks on the west side of the orogen in our model the lower crust forms a continuous 2 6 km thick layer beneath central south island the asymmetric topography of the southern alps is reflected in the crustal root which has a steeper flank at the west coast this observation is consistent with westward underthrusting of pacific lower lithosphere beneath south island that has been suggested in earlier studies

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

accounting for lateral variations of the upper mantle gradient in pn tomography studies 1 the effect of an upper mantle velocity gradient on regional arrival times has been approximated by a cubic distance term which can be extended to two dimensions for use in tomographic studies to demonstrate this we add a laterally varying upper mantle gradient to the standard pn time term tomography technique and apply to a data set from asia compiled using ground truth event location criteria we observe strong lateral variations in the gradient ranging from 0 001 to 0 003 s 1 with high gradients associated with the tethys convergence zone the gradient patterns may reflect lateral variations in the thermal gradient of the mantle lid variance reduction is 63 with respect to pn tomography without gradients adding gradients allows the use of longer path lengths improving velocity image definition in high gradient regions with sparse station distribution such as tibet

efficient calculation of differential seismograms for lithospheric receiver functions summary a new technique for computing differential seismograms for crustal and upper mantle response to a teleseismic wave receiver functions is developed using the matrix formalism of kennett the work was motivated by the difficulty of modelling teleseismic sv waves and has also proven useful for modelling teleseismic p waves this efficient method for calculating differential seismograms is based on three separate methods for computing synthetic seismograms two of the synthetic seismogram methods save intermediate results then the remaining synthetic seismogram algorithm uses the stored results in an efficient calculation of a new synthetic seismogram for a perturbed velocity model these developments have led to a faster for a 30 layer model a 90 per cent reduction in computation time and more accurate linearized inversion scheme for the determination of velocity models using teleseismic waves

the receiver structure beneath mina nevada abstract receiver functions obtained from broadband teleseismic p waveforms recorded at seismic station mnv near mina nevada are inverted for the vertical shear wave velocity structure southeast of the station the time domain inversion procedure for vertically heterogeneous and laterally homogeneous one dimensional models includes smoothness constraints to minimize model roughness and a pseudo monte carlo approach to investigate the nonuniqueness of the solution models we also report on the potential effects of scattering on a receiver function inversion for a one dimensional velocity structure mnv receiver functions sample the walker belt crust and upper mantle beneath the soda spring valley and pilot mountains to the east and the garfield hills and excelsior mountains to the west of the station the preferred southeast solution models show a smooth positive gradient in the mid and lower crust with the top of the crust mantle boundary at 34 to 36 km depth reaching an upper mantle p velocity of 7 8 to 7 9 km sec 1 between 38 and 40 km these results suggest that the crust beneath mnv may be representative of a transition zone between typical basin and range province crust to the east and a thicker sierran block to the west of the station

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

the sikhote alin orogenic belt russian south east terranes and the formation of continental lithosphere based on geological and isotopic data abstract the sikhote alin orogenic belt russian south east consists of folded terranes made up of jurassic and early cretaceous accretionary prisms turbidite basins and island arc terranes that are overlapped unconformably by undeformed upper cenomanian to cenozoic volcanic deposits the jurassic and early cretaceous accretionary prisms together with the early cretaceous island arc are related to subduction of the paleo pacific plate the turbidite basin which began to form at the beginning of the early cretaceous is related to left lateral movement of the paleo pacific plate along the paleo asian continental margin the collage of terranes that make up the sikhote alin orogenic belt was amalgamated in two stages the first began after jurassic subduction beneath the asian continent was terminated and the second took place in the late albian when the early cretaceous island arc collided with the continental margin intense deformation of the terranes took place along the continental margin in the form of large scale translations from south to north together with oroclinal folding the deformation resulted in rapid thickening of sediments in the upper crust resulting in turn in the formation of granitic metamorphic material in the continental lithosphere in the southwestern part of the sikhote alin orogen granites were intruded during the hauterivian aptian while the entire orogenic belt was affected by intrusions in the late albian early cenomanian synorogenic intraplate volcanic rocks and alkaline ultramafic mafic intrusions also testify to the fact that the orogenic processes in the sikhote alin were related to a transform continental margin and not to subduction geochemical and nd isotopic data indicate the primary continental crust of the sikhote alin was of a hybrid nature consisting of juvenile basic components accreted from an oceanic plate and recycled sedimentary material derived from the erosion of ancient blocks

the complete ordered ray expansion ii multiphase body wave tomography summary in a companion paper clarke 1993 we described a method for the calculation of body wave synthetic seismograms referred to as the complete ordered ray expansion here we present a new method for the inversion of body wave seismograms based on the core algorithm this multiphase waveform tomography allows us to incorporate many body wave arrivals from each seismogram in an inversion vastly increasing the amount of structural information that may be extracted from a set of observed traces we present an example of a synthetic inversion for a low velocity anomaly representing a mantle plume and show that our method allows us to retrieve the major features of the perturbation very successfully our computational scheme does not demand excessive storage but the amount required does grow in proportion to the number of events and stations we thus investigate the effect of partitioning the procedure by initially performing trace by trace inversions and combining the information in a final step the fact that small amplitude phases are less well constrained in the preliminary inversions leads to a somewhat degraded final image nevertheless this technique is useful in situations where a single full inversion is impractical or where data become available over an extended period of time

the late jurassic oblique collisional orogen of sw japan new structural data and synthesis the structural configuration of sw japan mainly reflects a late jurassic early cretaceous orogeny the region is divided into an inner belt and an outer belt on the japan sea and pacific ocean sides respectively by a strike slip fault the median tectonic line mtl both consist of a series of stacked nappes the inner belt is divided into a jurassic olistostrome known as the tanba zone and a hinterland area comprising continental triassic jurassic sediments the tanba zone is sliced into two units a lower one with late jurassic matrix and triassic early jurassic radiolarite olistoliths tectonically overthrust by an upper unit comprising an olistostrome with middle jurassic matrix and blocks which include late paleozoic limestone basic lava and radiolarite the tanba zone is overthrust by a paleozoic nappe complex derived from the hinterland the basal sole of the nappe corresponds to a peculiar unit called the ultra tanba zone in the chugoku area the hinterland is divided into an upper nappe the oga nappe formed by permo carboniferous limestone and permian clastic rocks and a lower one the sangun maizuru nappe formed by paleozoic high pressure hp metamorphics the sangun schists the permian maizuru olistostrome and the dismembered paleozoic yakuno ophiolites in the northern part of sw japan the tanba zone is in faulted contact with the circum hida and the hida zones the former is interpreted as the equivalent of the oga and sangun maizuru nappes of the chugoku domain crushed by post cretaceous tectonics the latter consists of paleozoic high temperature ht metamorphic rocks and late triassic early jurassic granite locally mylonitized and covered by early jurassic sandstone the outer belt is formed by a superficial nappe similar to the tanba zone thrust upon a deep domain characterized by a synmetamorphic ductile deformation the deep domain is divided into a lower unit the oboke unit formed by continental derived arenites and a green schist nappe consisting of oceanic sediments and resedimented ophiolites the green schist nappe overthrusts the oboke unit under synmetamorphic conditions with an eastward displacement the two belts are separated by the ryoke zone which corresponds to the southern part of the tanba zone affected by a cretaceous ht metamorphism and sharply cut by the mtl a geodynamic model is proposed for the jurassic orogeny of sw japan assuming that the evolution of the inner and outer belts are linked in the late triassic early jurassic sw japan is an active plate margin the upper plate or south china block consisted of the hinterland and the tanba belt a forearc basin the lower plate consisted of an oceanic area and the south japan continent the basic mechanism of the orogeny is ascribed to the oblique subduction and collision of the south japan continent

waveform inversion for lateral heterogeneities using multimode surface waves summary we propose a waveform inversion scheme that can be used to invert strong structural heterogeneities the method is based on a ray approximation for surface waves strong structural lateral variations are modelled by vertical discontinuities therefore a geologically heterogeneous region is partitioned into a number of lateral homogeneous subregions synthetic seismograms are calculated by modal summation over incident and transmitted modes with this method the complete waveforms of surface waves including mode cross branch coupling multipathing and scattering are considered in the inversion we test the method with several geophysically realistic structural models an example of waveform inversion for real data a wave path across the iberian peninsula and the oceanic structure off the french coast owing to the gibraltar earthquake ms 5 7 is presented the method allows us to obtain the information represented by different set of structures along the same source receiver minor arc through which the waves have propagated the method is applicable to different models of lateral heterogeneity in its present development it is most appropriate for inverting structures around tectonic scenarios such as continental margins grabens and discontinuities between major plates

late cretaceous palaeomagnetic results from sikhote alin far eastern russia tectonic implications for the eastern margin of the mongolia block summary wepresent palaeomagnetic results from late cretaceous welded tuffs in the kisin group monastirskaya suite primorskaya series collected at 27 sites from the sikhote alin mountain range a high unblocking temperature magnetization component 590 c was isolated after stepwise thermal demagnetization from 25 sites combined with previously reported data reliable characteristic remanence directions from 39 sites are distributed at eight areas ranging from 46 7 n 138 1 e to 43 4 n 134 8 e in the sikhote alin block the bedding tilt test is positive for two area mean directions and inconclusive for the remaining six areas the data set for all 39 sites reveals a positive bedding tilt test at the 99 per cent confidence level and their tilt corrected mean direction is d 335 6 i 54 4 95 8 5 corresponding to a palaeopole at 71 5 n 38 9 e with a95 9 9 this westerly direction is ascertained through the tilt corrected mean direction d 331 1 i 53 5 95 8 5 based on the 25 data selected from four areas kema river terney plastun and moryak rybolov where each data set passes the positive bedding tilt test or reveals an increase in the precision parameter after tilt correction palaeomagnetic declination indicates that the sikhote alin block has rotated counterclockwise by 41 16 with respect to the eurasian continent between late cretaceous times and 53 50 ma compared with palaeomagnetic data from the surrounding regions we find that the rotation recorded in sikhote alin extends westward into the interior of the mongolia block the eastern margin of the asian continent experienced both counterclockwise rotation of the eastern part of the mongolia block and clockwise rotation of the eastern part of the north china block over the cretaceous we interpret these data in terms of a strong net horizontal force towards the ocean side acting on the lithosphere at the eastern margin of the asian continent between the late cretaceous and 53 50 ma intermittently occurring upwellings of mantle and associated horizontal flows may have played an important role in producing the net horizontal force acting on the continental block during late cretaceous times

waveform inversion of seismic refraction data and applications to young pacific crust summary we present a method for constraining the velocity depth structure in the earth using seismic refraction waveform data we test the method with synthetic data from known models and apply it to a set of data collected in 1982 june from the east pacific rise at 13 n from the magma expedition in this iterative process wkbj seismograms are computed for a starting model the difference between these and the observed seismograms is used to update the model subject to physical constraints an important first step in the inverse scheme is the linearization of the wkbj seismogram equation allowing us to compute differential seismograms partial derivatives of the synthetic seismogram with respect to specific model parameters this linearization provides the means for estimating required model perturbations based on the misfit in the seismograms the choice of a suitable numerical strategy for computing an updated model is a crucial second step in formulating a working algorithm because the data contain noise synthetic seismograms can only fit the data to this noise level in this case infinitely many models fit the data to this tolerance and some of these estimates are non physical involving negative layer thicknesses a successful strategy must choose from among these possibilities a well defined physically reasonable new model in a commonly used approach to solving non linear problems the perturbation to the starting model is minimized while improving the fit to the data after several iterations the final model which possesses no special properties still tends to resemble the starting model when used with the magma data this technique essentially does not perturb the model at all a method we find much more satisfactory involves solving for the new model directly while applying physically important constraints as constraints we require the velocity gradient remain below a fixed value and penalize the roughness of the new model we thus solve for the smoothest model fitting the data to the specified misfit

crustal thickening versus lateral expulsion in the indian asian continental collision since the beginning of the continental collision between india and asia there has been about 2500 km of convergence and the northward movement of india has been accommodated by major internal deformation of the asian lithosphere the crustal thickening in and around the tibetan plateau is clearly a direct consequence of this collision but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern asia and southern china numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere we employ a thin viscous sheet model of the lithosphere with a depth averaged nonlinear viscous rheology described by a stress strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless argand number ar the eastern boundary for the collision region is described as a lithostatic boundary the precollision normal stress is determined by static balance and that constant stress is applied throughout the collision the experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1 4 of the indentation rate with only minor variation due to geometry or rheology crustal thickening rates in the region of the plateau are reduced by between 10 and 25 of the corresponding rates determined for similar experiments with a rigid eastern boundary to generalize from the results of these experiments the total north south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3 1 and more probably 4 1

mesozoic strike slip movement of the dunhua mishan fault zone in ne china a response to oceanic plate subduction abstract the ne striking dunhua mishan fault zone dmfz is one of two branches of the continental scale sinistral tan lu fault zone in ne china the field data presented here indicate that the ca 1000 km long dmfz records two phases of sinistral faulting the structures produced by these two phases of faulting include ne sw striking ductile shear belts and brittle faults respectively mylonite hosted microstructures and quartz c axis fabrics suggest deformation temperatures of 450 c 500 c for the ductile shear belts combining new zircon u pb dates for 14 igneous rock samples analyzed during this study with the geology of this region indicates these shear belts formed during the earliest early cretaceous this phase of sinistral displacement represents the initial formation of the dmfz in response to the northward propagation of the tan lu fault zone into ne china a phase of early cretaceous rifting was followed by a second phase of sinistral faulting at 102 96 ma as evidenced by our new u pb ages for associated igneous rocks combining our new data with the results of previous research indicates that the dfmz records a four stage cretaceous evolutionary history where initial sinistral faulting at the beginning of the early cretaceous gave way to rifting during the rest of the early cretaceous this was followed by a second phase of sinistral faulting at the beginning of the late cretaceous and a second phase of local rifting during the rest of the late cretaceous the cretaceous evolution of the dmfz records the synchronous tectonic evolution of the ne china continent bordering the pacific ocean two phases of regional n s compression generated the two phases of sinistral faulting within the dmfz whereas two stage regional extension generated the two phases of rifting the two compressive events were the result of the rapid low angle subduction of the izanagi and pacific plates whereas the two stage extension was caused by the roll back of these respective plates the final closure of the mongol okhotsk ocean at the beginning of the early cretaceous intensified the synchronous compression in ne china causing the northward propagation of the tan lu fault zone

extensional shearing deformation and kilometer scale a type folds in a cordilleran metamorphic core complex raft river mountains northwestern utah cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations in the raft river mountains a mylonitic zone and a late superimposed detachment fault separate precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous paleozoic rocks which suffered a brittle extensive deformation we suggest that the mylonitic ductile deformation is the result of tertiary extensional tectonics strain analysis shows that 1 strain results from large scale progressive shearing deformation in a non coaxial regime 2 e w regional stretching lineations represent the direction of shearing and transport 3 shear criteria indicate an east directed movement 4 strain magnitude is increasing eastward 5 kilometer scale folds parallel to the lineations are interpreted as a type folds formed by eastward shearing of a previously northward dipping layering measurement of striated faults in the upper plate gives an e w direction of extension parallel to the lineations thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics a model is proposed to explain the observed geometry and kinematics of the structures the mesozoic to paleocene cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes important uplift abnormal heating and granite emplacement occurred later during the collapse of the former belt the combined effects of thermal and gravitational instabilities during tertiary extensional tectonics enabled the development of a ductile shear zone localized at the precambrian basement paleozoic cover interface the brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust

the geodynamic evolution of the eastern eurasian margin in mesozoic times abstract the cenozoic features of eurasia such as marginal seas and intracontinental deformation are unraveled using available paleomagnetic and field data the late cretaceous shape of eurasia is then reconstructed from the examples of sw japan and south russian far east the major features of the mesozoic eurasian margin suture zones marked by ophiolites or high pressure metamorphic rocks olistostromes characterized by the lithology of the olistoliths calc alkaline magmatism and strike slip faults are outlined correlations of the ophiolitic belts and the olistostromes along the eastern eurasian margin from borneo to siberia allow the definition of four microcontinents the west philippines south japan and the anuy and okhotsk sea microblocks from south to north the mesozoic structure of the eurasian margin is due to three processes 1 1 oceanic subduction and the accretion of subduction complexes 2 2 the collision of these microcontinents with eurasia between the late jurassic and the late cretaceous and 3 3 the reactivation of the ophiolitic sutures by left lateral wrench faulting

geochemical characteristics and petrogenesis of adakites in the sikhote alin area russian far east abstract adakitic rocks of the sikhote alin area were emplaced during two main periods the cretaceous 132 98 ma and eocene 46 39 ma these rocks primarily occur in the khanka block and less commonly in the sikhote alin orogenic belt the adakitic rocks record the following chemical compositions sio2 57 74 al2o3 15 18 na2o 3 5 6 1 k2o 0 7 3 2 na2o k2o 1 1 3 9 sr y 33 145 and la yb n 11 53 the hree and hfse in these rocks are remarkably depleted the early cretaceous adakites record end t 1 0 to 3 2 and isr 0 7040 0 7090 and the eocene adakitic rocks record nd t 2 0 to 2 2 and isr 0 7042 0 7058 adakitic features suggest different modes of magma generation a comparison of the sr y and la yb ratios and geochemical data on harker diagrams between the two periods of adakitic rocks reveals differences in their petrogenesis the cretaceous adakites may have been generated by the partial melting of meta basic rocks in a subduction zone accompanied by the emplacement of volcanic arc granitoids therefore the subduction of the paleo pacific plate beneath the sikhote alin was probably initiated during this time the eocene rocks which record increasing adakitic features with increasing silica content are most likely the product of andesite that underwent fractionation of mineral assemblage including clinopyoxene orthopyroxene garnet and amphibole these rocks and associated basalts and rhyolite were formed after cretaceous arc magmatism in the sikhote alin area and were most likely generated by rollback of the subducting pacific plate after the eocene abundant adakitic granitoids of early cretaceous and eocene age occur in the kitakami and abukuma mountains of ne japan consequently it is highly probable that a geological correlation existed between sikhote alin and north japan particularly before the opening of the japan sea

main fault systems of the soviet far east the analysis of the distribution of thrusts normal faults and strike slip faults of various ages has allowed us to determine the character of lithospheric block displacements in the soviet far east the early mesozoic late mesozoic and cainozoic kinematics were each essentially different the early mesozoic dzhagdinsk fault system appeared as a result of the collision of the bureinsk khankaisk microcontinent with the siberian continent the largest faults of the system are neither longstanding nor deep but were formed during the latest stage of the structural evolution the multistage formation of the faults of the dzhagdinsk system is conditioned by its position at the margin of the continent the late mesozoic faults are mainly strike slip faults caused by the subduction of the oceanic crust at an acute angle with respect to the strike of the active continental margin the cainozoic faults were formed under compression on the boundary between the siberian platform and the bureinsk massif but under tension in the east of the region

seismic evidence of divergent rifting and subsequent deformation in the southern japan sea and a cenozoic tectonic synthesis of the eastern eurasian margin abstract neogene rift system configuration for the back arc of southwest japan southern rim of the japan sea is argued on the basis of reflection seismic interpretation divergent rifting and subsequent contraction provoked by an arc arc collisional event are manifested by the formation of faulted grabens and their inverted deformation respectively we identified the following four cenozoic tectonic epochs as a decomposition process of the eastern eurasian margin based on reliable paleomagnetic data 1 plate margin rearrangement on a regional left lateral fault through southwest japan and sikhote alin which constituted a continuous geologic province before the early tertiary differential motion 2 early tertiary clockwise rotation 20 of the east tan lu block relative to the north china block 3 oligocene to early miocene divergent rifting and spreading of the japan sea which divided southwest japan from the east tan lu block 4 middle miocene bending and back arc inversion of southwest japan caused by collision with the izu bonin arc according to the estimation of relative motions during these events a paleogeographic reconstruction is presented through cenozoic time

tectonic setting of western pacific marginal basins abstract global kinematics as well as magnetic anomalies of marginal basins with continental geology and paleomagnetic data as additional constraints are used to present a set of reconstructions of the western pacific marginal basins between 56 ma and the present at key periods 56 43 32 20 12 and 3 ma our model accounts for the rapid motion of exotic terranes along the whole of the western pacific convergent zone marginal basins appear to open in a great variety of tectonic settings the two extreme examples being the mariana trough where trench suction may be the predominant driving force and the south china sea where intracontinental deformation appears to be the major mechanism the study of marginal basins is a 3 d problem which must take into account the whole tectonic context subduction related tectonics in cross section and upper plate deformation in map view and not only a 2 d problem the classical trench arc back arc problem

the miocene bending of southwest japan new 39ar 40ar and microtectonic constraints from the nagasaki schists western kyushu an extension of the sanbagawa high pressure belt abstract in the westernmost part of kyushu the nagasaki metamorphics also observed in amakusa consist of oceanic sediments and fragments of ophiolite these rocks underwent a high pressure metamorphism followed by an increase in temperature up to the upper greenschist facies prior to the late cretaceous microtectonic and kinematic analyses indicate a synmetamorphic ductile shear from south to north dating of glaucophane phengite and biotite by 39ar 40ar method suggests that the peak of metamorphism and the related deformation occurred approximately 90 95 ma ago i e cenomanian but a resetting by an oligo miocene tectonic event is also evidenced due to discontinuous exposures the metamorphic rocks of kyushu cannot be continuously traced to the metamorphic belts of honshu and shikoku nonetheless the lithostratigraphy metamorphic environment radiometric data and deformation style strongly support a correlation of the nagasaki metamorphics with the green schist nappe of southwest japan which underwent the late jurassic early cretaceous high pressure sanbagawa metamorphism such a correlation implies that 1 in the amakusa area the nagasaki metamorphics form a window north of a meta olistostrome that belongs to the tanba ryoke zone 2 the trend discrepancy of the stretching lineations between west kyushu and southwest japan is in agreement with a clockwise rotation of the latter with respect to the former assumed to be fixed to eurasia such a bending occurred after paleogene times probably during the opening of the sea of japan

internal deformation of sikhote alin volcanic belt far eastern russia paleocene paleomagnetic results abstract we present paleomagnetic results of paleocene welded tuffs of the 53 50 ma bogopol group from the northern region 46 n 137 e of the sikhote alin volcanic belt characteristic paleomagnetic directions with high unblocking temperature components above 560 c were isolated from all the sites a tilt corrected mean paleomagnetic direction from the northern region is d 345 8 i 49 9 95 14 6 n 9 the reliability of the magnetization is ascertained through the presence of normal and reversed polarities the mean paleomagnetic direction from the northern region of the sikhote alin volcanic belt reflects a counterclockwise rotation of 29 from the paleocene mean paleomagnetic direction expected from its southern region the counterclockwise rotation of 25 is suggested from the paleomagnetic data of the kisin group that underlies the bogopol group these results establish that internal tectonic deformation occurred within the sikhote alin volcanic belt over the past 50 ma the northern region from 44 6 to 46 0 n in the sikhote alin volcanic belt was subjected to counterclockwise rotational motion through 29 17 with respect to the southern region the tectonic rotation of the northern region is ascribable to relative motion between the zhuravlevka terrane and the olginsk taukhinsk terranes that compose the basements of the sikhote alin volcanic belt

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

amplitude ratio tomography for regional phase q 1 we image regional seismic phase attenuation q by extending amplitude ratio techniques which are widely used to determine average q within a sampled region to obtain laterally varying q structure using tomographic techniques the method assumes that source radiation is isotropic and that the amplitude ratios can be modeled by a two dimensional q map and relative site terms application to 2286 1 hz lg amplitudes from 816 events recorded at 35 central and east asia stations yields a misfit of 0 09 log10 units a variance reduction of 73 relative to the uniform q model that best fits the data and laterally varying q from 100 to 1000 with patterns closely following regional geology the image is resolved on average to 2 5 with resolution peaking at 1 5 in the best covered areas which is remarkable given the sparse station geometry employed the high resolution results from the dense distribution of earthquake sources and is aided by the increased confidence in source locations provided by merging catalogs from many different agencies

evolution of the sakhalin fold system abstract the island of sakhalin is subdivided into two late cretaceous isopic zones the miogeosynclinal western zone and the eugeosynclinal eastern zone the boundary between these zones is marked by a band of greenschists and blueschists melange and olistostrome a thick triassic lower cretaceous formation composed of siliceous volcanogenic deposits with lenses of limestones and interlayers of tuff aleurolites clay shales and siliceous argillaceous shales occurs at the base of a miogeosynclinal upper cretaceous section the upper cretaceous deposits of this zone have a cyclic structure and are represented by terrigenous marine near shore and continental formations the paleogene neogene deposits occur on the upper cretaceous ones with a sharp erosion a lithological complex composing the eastern sakhalin eugeosynclinal zone is represented by alternations of deep water sandy aleurolite clay shale formations including olistostromal horizons with siliceous volcanogenic formations the eugeosynclinal regime remained the same up to the late cretaceous the paleogene neogene deposits occur on the cretaceous ones with a sharp structural unconformity the ophiolite blueschist belt of central sakhalin is compared with the kamuikotan belt in hokkaido the belts of granitoid magmatism and calc alkaline volcanism are nearly parallel to the cretaceous and senonian early paleogene ophiolite blueschist belts of sakhalin an eastward migration of these belts to the pacific is observed the formation stage of the mezosoic deep thrusts effecting distribution of the ophiolitic belts has been changed by the formation stage of the cenozoic wrench fault zones

the correction for the travel time effects of seafloor topography in the interpretation of marine seismic data problems associated with the application of topographic corrections to deep ocean marine seismic data are described synthetic travel time data are used to show that corrections for the delay time introduced by the water column are inferior to corrections for the true water path water path corrections place the shots at the ray entry points on the seafloor require no assumptions about the structure under investigation and maintain the correct distribution of data with range the water path correction can be most accurately applied during the stacking operation that transposes the observed data into intercept time slowness space there is then no requirement to make determinations of slowness and for experiments involving seafloor receiver and surface shots the data may be transposed to the seafloor with complete objectivity water path corrected stacks from different locations may then be compared in the sure knowledge that any observed differences are due to real structural changes and not to topographic correction errors or to invalid assumptions of lateral homogeneity

pre eocene synmetamorphic structure in the mindoro romblon palawan area west philippines and implications for the history of southeast asia the structure of the pre eocene rocks considered as the basement of the philippines has been investigated in the mindoro lubang romblon tablas sibuyan and north palawan areas in the former two areas the same pre eocene succession of units is recognized from top to bottom 1 a pre eocene olistostrome 2 an ophiolitic nappe 3 a schistose sequence pelites sandstones basic schists and marbles and 4 a gneissic unit the nature of the contact between the olistostrome and the underlying units is unclear but the ophiolite and the schistose sequence form two thrust sheets of oceanic origin thrust upon the gneissic unit that is considered a part of a continental basement called the west philippines block small scale structures show that the early deformation stage is characterized by a submeridian 0 n40 e lineation formed in greenschist to amphibolite facies conditions during the thrusting kinematic analysis show that the thrusting was from north to south in north palawan metamorphic rocks with similar microtectonic and kinematic characteristics are found they are overlain by a late jurassic olistostrome which is correlated with the olistostromes found in calamian north mindoro carabao and buruanga peninsula north panay the microstructural features and the presence of the olistostrome suggest that the north palawan mindoro tablas romblon sibuyan and carabao islands belong to the same north palawan block of hamilton 1979 which is a continental fragment rifted from asia in cenozoic times it is assumed that the western panay and zamboanga areas which are characterized by mesozoic ophiolites and metamorphic rocks also belong to the north palawan block all these islands experienced to some extent the same mesozoic geohistory the south verging thrusting is interpreted as the result of an oblique collision of the west philippines block with asia the contemporaneous left lateral strike slip faulting and the calc alkaline magmatism widespread along the chinese margin are also included in the geodynamic model

a 100 ma bimodal composite dyke complex in the jiamusi block ne china an indication for lithospheric extension driven by paleo pacific roll back abstract a 125 m wide bimodal composite dyke complex consisting of rhyolite and dolerite dykes was emplaced into cretaceous volcanic strata of the songmuhe formation in the jiamusi block of ne china the dolerite dykes are sub vertical strike north south and intruded into both the country rocks and rhyolite dykes soon after the latter solidified shrimp zircon u pb dating shows that the rhyolite dykes were emplaced at 100 2 ma and the dolerite dykes were also most likely emplaced at 100 2 ma the rhyolite is characterized by enrichment in large ion lithophile elements lile and light rare earth elements lree and depletion in high field strength elements hfse it shows a significant negative eu anomaly and has e nd t values ranging from 0 49 to 1 66 and two groups of initial 87 sr 86 sr ratios at 0 7045 and 0 7061 the rhyolite displays the compositional signature of peraluminous ferroan granitoid indicating it was derived by either differentiation of basalt and or low pressure partial melting of crust the dolerite is also characterized by enrichment in lile and lree and depletion in hfse it has a weak negative eu anomaly and has e nd t 1 22 to 3 26 and 87 sr 86 sr i 0 7057 0 7074 the dolerite originated from partial melting of lithospheric mantle which was affected by subducted oceanic crust and experienced different amounts of crustal contamination such bimodal dyke complexes are an important indicator of crustal extension under the influence of mantle processes thus the dyke complex in the jiamusi block indicates mid cretaceous intra plate extension in ne china related to the subduction of the paleo pacific plate along the eastern eurasian continental margin when compared with mesozoic bimodal magmatism further to the west our new data support a temporal eastward migration of magmatism over a distance 1000 km from 160 ma to 100 ma this was most likely associated with roll back of the paleo pacific plate and consequent upwelling of asthenospheric mantle

timing of deformation and location of the eastern liaoyuan terrane ne china constraints on the final closure time of the paleo asian ocean abstract the liaoyuan terrane lt in ne china formed as a result of accretion during the final closure of the paleo asian ocean between the north china and siberian cratons however the timing of the deformation and the location of the eastern lt remain controversial here we present structural data zircon u pb age dating and hf isotopic data to constrain the kinematics and timing of deformation and the southern boundary of the eastern lt in relation to the ne sw striking yilan yitong fault yyf one of two branches of the continental scale tan lu fault zone in ne china pre jurassic rocks in the eastern lt were widely involved in ductile deformation within wnw ese striking thrust belts the microstructures in these shear zones indicate a top to the ssw sense of shear and deformation temperatures of 450 500 c the northern margin of the north china craton ncc was also affected by the nne ssw contraction as shown by similar ductile deformation in the archean gneisses and wnw ese trending folds in the mesoproterozoic sedimentary cover the ages of crystallization in deformed and undeformed igneous rocks in the lt constrain the deformation to an interval between 241 and 193 ma u pb dating of zircons in syn kinematic migmatitic rocks indicates an age of 234 229 ma for the deformation we consider this middle triassic age to represent the final closure time of the paleo asian ocean along the eastern lt and this timing is consistent with the eastwards scissor style closure model lithological variations zircon age spectra ehf t values and model ages confirm the eastward extension of the lt across the yyf the data constrain the boundary between the lt and ncc to be roughly along the liao river to the west of the yyf and along the northern edge of the exposed archean gneisses to the east the boundary follows the bayan obo chifeng kaiyuan fault which was active as a thrust thrusting of material to the ssw during the middle triassic orogeny the newly determined boundary location indicates a 35 km sinistral displacement along the yyf

paleomagnetic evidence for post early cretaceous tectonic rotation of the sikhote alin superterrane far east russia abstract we present new early cretaceous paleomagnetic results from the zhuravlevka amur terrane of the eurasian continent far east russia out of 34 total sites 14 were collected from komsomolsk on amur area 50 6 n 137 2 e and 20 from vaninsky area 49 1 n 139 2 e thermal demagnetization reveals the presence of two interpretable magnetization components in 19 sites with laboratory unblocking temperatures of 350 c and or 500 580 c the remanent directions of the low temperature component are either parallel or anti parallel to those obtained from the high temperature component results of fold tests show that both components are secondary rock magnetic and reflected light microscopic observations indicate a chemical origin for both of these components as evident from the presence of secondary pyrrhotite and magnetite the komsomolsk on amur area provides an in situ formation mean direction of d 127 5 i 66 7 k 28 2 95 9 3 n 10 sites when combined with the reported paleomagnetic data from early to middle cretaceous accretionary wedge rocks of the kiselevka manoma terrane and the early cretaceous western sakhalin turbidite basin rocks d 94 2 and d 57 1 respectively large magnitude of clockwise rotations of 66 118 is demonstrated for the eastern part of the sikhote alin superterrane with respect to eurasia in addition these three landmasses maintained their e w elongated orientations before the start of rotation implying southward directed subduction of the oceanic plates beneath northern margins these reconstructions of the sikhote alin superterrane provide clues on the tectonic evolution of panthalassa ocean