charging of the neogene penglai 19 3 field bohai bay basin china oil accumulation in a young trap in an active fault zone the penglai 19 3 pl19 3 oil field the largest offshore oil field in china was found in shallow reservoirs 700 1700 m 2297 5577 ft within the most active fault zone in east china the pl19 3 anticline was not finally formed until about 2 0 ma and is cut by densely distributed faults source rock and crude oil samples from the pl19 3 field were analyzed to determine the origin and formation mechanisms of this large oil field three organic rich oil prone source rock intervals exist in the bozhong subbasin each of which has a distinct biomarker assemblage oil samples from different wells have different biomarker associations and three source related oil classes were identified within the pl19 3 field based on biomarker compositions and multivariate analysis of the data the pl19 3 field displays considerable compositional heterogeneity the compositional heterogeneity within the field and comparison between oil samples from the pl19 3 field and those from nearby structures suggest three field filling directions which is consistent with the results of migration pathway modeling the pl19 3 field was charged in the north by oil generated from dongying formation source rocks in the eastern bozhong depression and bodong depression in the southeast by oil generated from shahejie formation source rocks in the miaoxi depression and in the northwest by oil generated from shahejie formation source rocks in the central bozhong depression oil charge from multiple source rock intervals and multiple generative kitchens and focusing of oil originating from a large area of the bozhong depression into the same trap resulted in rapid oil accumulation in the pl19 3 structure and the formation of this large oil field in a very young trap within an active fault zone

reservoir characteristics and their effects on hydrocarbon accumulation in lacustrine turbidites in the jiyang super depression bohai bay basin china abstract a great difference exists between the hydrocarbon charging characteristics of different tertiary lacustrine turbidites in the jiyang super depression of the bohai bay basin east china based on wireline log data core observation and thin section analyses this study presents detailed reservoir property data and their controlling effects from several case studies and discusses the geological factors that govern the hydrocarbon accumulation in turbidite reservoirs the lacustrine fluxoturbidite bodies investigated are typically distributed in an area of 0 5 10 km2 with a thickness of 5 20 m the sandstones of the tertiary turbidites in the jiyang super depression have been strongly altered diagenetically by mechanical compaction cementation and mineral dissolution the effect of compaction caused the porosity to decrease drastically with the burial depths especially during the early diagenesis when the porosity was reduced by over 15 the effect of cementation and mineral dissolution during the late stage diagenesis is dominated by carbonate cementation in sandstones high carbonate cement content is usually associated with low porosity and permeability carbonate dissolution secondary porosity zone and primary calcite dissolution is believed to be related to thermal maturation of organic matter and clay mineral reactions in the surrounding shales and mudstone two stages of carbonate cementation were identified the precipitation from pore water during sedimentation and secondary precipitation in sandstones from the organic acid dissolved carbonate minerals from source rocks petrophysical properties have controlled hydrocarbon accumulation in turbidite sandstones high porosity and permeability sandstones have high oil saturation and are excellent producing reservoirs it is also noticed that interstitial matter content affects the oil bearing property to some degree there are three essential elements for high oil bearing turbidite reservoirs excellent pore types low carbonate cement 15 and average permeability 10 md

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

seismic signature of the collision between the east tibetan escape flow and the sichuan basin abstract gps displacement vectors show that the crust in east tibet is being squeezed in an easterly direction by the northward motion of the indian plate and the sichuan basin is resisting this stream and redirecting it mainly towards indochina the longmen shan containing the steepest rise to the high plateau anywhere in tibet results from the strong interaction between the east tibetan escape flow and the rigid yangtze block sichuan basin but the kinematics and dynamics of this interaction are still the subject of some debates we herein present results from a dense passive source seismic profile from the sichuan basin into eastern tibet in order to study the deep structure of this collision zone using p and s receiver function images we observe a sudden rise of the lithosphere asthenosphere boundary lab from 120 to 150 km beneath the sichuan basin and from 70 to 80 km beneath eastern tibet in contrast the depth of the crust mantle boundary moho increases from 36 to 40 km beneath the sichuan basin and from 55 to 60 km beneath eastern tibet the 410 km discontinuity is depressed below eastern tibet by about 30 km although the 660 remains at nearly the same depth throughout the lms from these observations we conclude that the mode of collision that occurs between tibet and the sichuan basin is very different to that found between india and tibet in southern tibet we observe in essence the subduction of the indian plate which penetrates northwards for several hundred kilometers under central tibet the very thin mantle part of the lithosphere beneath eastern tibet may indicate delamination or removal of the bottom of the lithosphere by hot asthenospheric escape flow this process leads to the exceptionally steep topography at the eastern tibetan margin as a result of gravitational buoyancy this view is supported by the very unusual depression of the 410 km discontinuity beneath eastern tibet which could be caused by the dynamics of the sub vertical downward asthenospheric flow

local magnitude tomography in california lateral variation in crustal attenuation of california is calculated by inverting 25 330 synthetic wood anderson amplitudes from the california integrated seismic network cisn for site source and path effects two dimensional attenuation q or 1 q is derived from the path term which is calculated via an iterative least squares inversion that also solves for perturbations to the site and source terms source terms agree well with initial cisn m l s and site terms agree well with a prior regression analysis q ranges from low attenuation at 0 001 q 1000 to high attenuation at 0 015 q 66 with an average of 0 07 q 143 the average q is consistent with an amplitude decay function log a for california when q is combined with a simple geometrical spreading rate attenuation in california is consistent with the tectonic structure of california with low attenuation in the sierra batholith and high attenuation at the geysers at long valley and in the salton trough possibly due to geothermal effects also path terms are an order of magnitude smaller than site and source terms suggesting that they are not as important in correcting for m l

distinct variations of crustal shear wave velocity structure and radial anisotropy beneath the north china craton and tectonic implications article i nfo this study presents the crustal shear wave velocity structure and radial anisotropy along two linear seismic arrays across the north china craton ncc from ambient noise tomography about a half to one year long ambient noise data from 87 stations were used for obtaining the inter station surface wave empirical green s functions egfs from cross correlation rayleigh and love dispersion curves within the period band 5 30 s were measured from the egfs of the vertical and transverse components respectively these dispersion data were then used to determine the crustal shear wave velocity structure vsv and vsh and radial anisotropy 2 vsh vsv vsh vsv from point wise linear inversion with constraints from receiver function analysis our results reveal substantial structural variations among different parts of the ncc the bohai bay basin in the eastern ncc is underlain by a thin crust 30 km with relatively low velocities particularly vsv and large positive radial anisotropy in the middle to lower crust such a crustal structure is no longer of a cratonic type and may have resulted from the widespread tectonic extension and intensive magmatism in this region since late mesozoic beneath the ordos basin in the western ncc the crust is relatively thicker 40 km and well stratified and presents a large scale low velocity zone in the middle to lower crust and overall weak radial anisotropy except for a localized lower crust anomaly the overall structural features of this region resemble those of typical precambrian shields in agreement with the long term stability of the region the crustal structure under the trans north china orogen tnco central ncc is more complicated and characterized by smaller scale velocity variations strong positive radial anisotropy in the middle crust and rapid change to weak to negative anisotropy in the lower crust these features may reflect complex deformations and crust mantle interactions probably associated with tectonic extension and magmaticunderplating during the mesozoic to cenozoic evolution of the region our structural images in combination with previous seismic geological and geochemical observations suggest that the phanerozoic lithospheric reactivation and destruction processes may have affected the crust especially the middle and lower crust of the eastern ncc and the effect probably extend ed to the tnco but may have minor influence on the crust of the western part of the craton

sedimentary basin inversion caused by igneous underplating northwest european continental shelf a considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the british isles underwent permanent exhumation during the tertiary the most dramatic indicator of this process is the present day absence of as much as 4 km of anticipated postrift thermal subsidence in basins just north and west of scotland any explanation of this observation must take into account the fact that the entire region has very small long wavelength free air gravity anomalies this important constraint implies either that the crust has been thickened or that low density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the north atlantic tertiary epeirogeny is often attributed to compression that is assumed to be related in a general sense to alpine mountain building however to remove 3 km of sedi mentary rock from a basin 100 km wide requires 15 km of shortening minor tertiary compression is observed all over the continental shelf but nowhere is it sufficient to account for the required amount of uplift and erosion in addition exhumation dramatically increases from south to north whereas the observed compression decreases markedly in the same direction at the beginning of the tertiary rifting associated with the initiation of the iceland plume generated substantial volumes of melt inversion of rare earth element concentrations of mgorich igneous rocks suggests that a minimum of 5 km of melt was produced beneath at least part of the continental shelf we infer that much of this melt remains trapped within the lithosphere presumably close to the moho which acted as a density filter such underplating will generate rapid uplift

composition of the crust beneath southeastern china derived from an integrated geophysical data set 1 the nonuniqueness in determining the composition of the crust can be reduced by comparing the data for p wave velocity vp s wave velocity vs vp vs ratio mass density and lame impedances to data determined from laboratory measurements of these physical variables on a variety of crustal rock samples the composition of the crust along the tunxi wenzhou transect southeastern china is presented as a model based on a complete set of geophysical data involving seismic p wave and s wave velocity density gravity heat flow and temperature surveys which allow us to place tighter constraints on possible crustal models the yangtze and the cathaysia blocks have a crustal structure characterized by remarkable differences between the upper middle and lower crust integrated geophysics data are here interpreted to indicate 1 an average composition of granite gneiss for the upper crust with presence of mica quartz schist felsic granulite paragranulite and granite granodiorite beneath the yangtze block and basalt down to the upper part of the middle crust just over the jiangshan shaoxing fault followed laterally by granite gneiss beneath the songyang qintiang and wenping depressions 2 granite granodiorite and biotite gneiss for the upper half of the middle crust with presence of mica quartz schist beneath the cathaysia block 3 gabbro and in less proportion basalt for the lower part of the middle crust and 4 amphibolite for the lower crust with presence of mafic garnet granulite just over the jiangshan shaoxing fault and beneath the wenping depression

mirror symmetry of the crust in the oil gas region of shengli china abstract the concept of mirror image symmetry describes the correlation between the bottom of a sedimentary basin and the top of the underlying mantle the degree of mirror symmetry is analyzed here using thickness data from mesozoic cenozoic sedimentary basins and the moho depth with the purpose of investigating the implications for tectonics and exploratory practices in oil gas bearing basins our attention is focused on different tectonic elements of a multi scale tectonic partition in the oil gas region of shengli southeast of bohai bay china by applying standard least squares analysis methods to both datasets we obtained the average thickness h of the consolidated crust and the mirror symmetry factor a for every tectonic unit thereby allowing us to explore the correlation between the bottom depth of a sedimentary basin and the top of the underlying mantle a detailed analysis of data uncertainties and their potential effects led to small fluctuations of h and a within a narrow band around the mean values of 33 26 km and 1 43 respectively thus the results can be considered valid if they are constrained by these error bounds the issues of crust mantle interaction and the deviation of the crust from isostatic equilibrium are also addressed for their tectonic implications other structural parameters related to the deep rooted crust beneath the tectonic elements that make up the study region such as the ratio r of the thickness of the consolidated crust to the absolute value of the symmetry factor the ratio k of the maximum sediment thickness to the maximum uplift of the mantle and the product kh are also evaluated for their implications in geophysical prospecting the symmetry signature of the crust is compared to the symmetry of other sedimentary basins in china and in the world suggesting symmetry as a feasible prognostic tool in exploration for oil gas reservoirs the key result is that those locations where oil gas reservoirs have been found to date are basins in which the crust mantle interaction is active and the crust is isostatically compensated both of those parameters can be quantitatively constrained conversely oil and gas reserves are poor or nonexistent in regions where the crust mantle interaction is inactive and the crust is far from isostatic equilibrium however an intermediate case between these two extreme scenarios wherein the crust mantle interaction is rather moderate but the crust is in a state close to isostasy cannot be entirely ruled out

an overview of the earth crust under china abstract we assess the results of a number of deep seismic soundings performed in china over the last few decades and study the variations in crustal structure in 18 tectonic units comprised of three platforms and 15 fold systems thickness data on 344 mesozoic cenozoic sedimentary basins as well as data on moho depth are collected in order to discuss the relationship between the thickness of the basins and the average thickness of the consolidated crust in each tectonic unit the degree of mirror image symmetry between mesozoic cenozoic sedimentary basins and the uplifting topmost parts of the mantle is herein analyzed using deep geophysical data on sedimentary basins by applying standard methods of least squares analysis to both datasets we have obtained both the average thickness of the consolidated crust and the mirror image symmetry factor for every platform and tectonic fold system thereby allowing us to explore the correlation between the depths of the bottom of the sedimentary basins and the top of the uplifting mantle the thickness of the consolidated crust in china is found to be between 20 and 63 km following a pattern of gradual thickening from east to west expressed in terms of spatial seismicity and the concentration of seismic energy and according to the sharing out of earthquake hypocenters in the top 80 km of the earth the rheology of the area does not appear to suit the widely accepted jelly sandwich model for the continents which does not seem to be entirely valid in china the findings on the mirror image symmetry factor show that this parameter varies mainly in the range 0 5 to 1 8 compared with each other tectonic unit most of the tectonic systems in china appear to be isostatically compensated tibet is an exception in that the symmetry factor is positive for the gangdise nyainqentanglha 1 0 and himalayan 0 1 fold systems implying that these tectonic zones are far from being in the isostatic equilibrium of the other regions we have also analyzed the zoning characteristics through the logarithmic relationship ln r ln h a using the average thickness of the consolidated crust and the absolute value of the symmetry factor the key finding is that regardless of geographical location all the values of r h a for those sedimentary basins in which oil gas reservoirs have to date been found fall into the narrow range of values between 19 38 and 37 40 there is some appeal in a possible relationship involving the ratio of crustal thickness to symmetry factor more so when the results obtained appear to suggest a prognostic tool for exploratory practice in relation to oil gas reservoirs

carpathian foreland fold and thrust belt and its relation to pannonian and other basins the foreland fold and thrust belt of the carpathians can be traced from austria through the western and eastern carpathians to the south carpathian bend in romania where most of its structural units plunge beneath younger pliocene pleistocene cover folds in the youngest rocks at the south carpathian bend continue westward until all surface expression disappears before reaching the danube river the fold and thrust belt is flanked by elements of the european russian and moesian cratonal areas which are overlain by a variable width and thickness of foredeep deposits that are partly involved within the external fold and thrust belt internal to the fold and thrust belt are older parts of the carpathian orogene formed mainly on continental crust these older structures have significant differences in evolution between the western and eastern carpathians the foreland fold and thrust belt consists predominantly of flysch in its inner parts and molasse in its outer parts these sedimentary rocks are cut into a series of thrust sheets that have moved relatively toward the craton oldest flysch units are late jurassic and the youngest units are miocene some of the flysch units of the inner part of the belt may have been underlain by oceanic crust whereas the younger more external molasse units were underlain by continental crust timing of thrusting is only well constrained locally but suggests that deformation in the western carpathians developed during oligocene to miocene time in the eastern carpathians structural activity continued into the pliocene and deep earthquake activity at vrancea suggests subduction deformation is sti l locally active a volcanic arc of miocene to holocene age lies internal to the fold and thrust belt magmatic activity appears to have migrated externally and eastward in time contemporaneous with thrusting was the development of neogene basins in the intra carpathian region these basins exhibit a two phase subsidence history except for the pannonian and transylvanian basins fast initial subsidence followed by a period of slower linear subsidence the pannonian basin shows only a reasonably fast linear subsidence structural and thermal models indicate the basins were formed by about 100 stretching the convex east loop of the carpathian foreland fold and thrust belt is a result of the collision of continental fragments against a recess in the combined european moesian plate in this region subduction or downbending of the subducted slab results in an extensional stress field suggesting that thrusting and apparent compression along the mountain belt are only thin skinned superficial effects due to the inability to subduct light upper crustal material and detachment of the crust from the underlying lithosphere the dominant forces seem to be those acting on the downgoing slab which produce shortening along the plate boundary and plate geometry that inhibits rigid plate convergence

the north china basin an example of a cenozoic rifted intraplate basin the north china basin was a stable continental region that has undergone several distinct phases of rifting and subsidence during the mesozoic and cenozoic eras the last major phase featuring block faulting rapid subsidence and widespread calc alkaline basaltic volcanism began during early tertiary time this intraplate rifting appears to be the result of an approximately 30 extension and thinning of the lithosphere beneath the northeastern china by the late tertiary period active rifting had slowed and postrift thermal subsidence had begun this regional extension and subsidence resulted in the present day north china basin a large saucer shaped oil producing basin with thinner crust and lower upper mantle p and s wave velocities than the surrounding regions during the quaternary period however the subsidence rate increased in the north china basin the frequent occurrence of destructive earthquakes and observations of relatively high heat flow suggest that the quaternary tectonic activity differs from the late tertiary simple thermal subsidence pattern and may indicate a new phase of rifting

the cenozoic rift system of the north china plain and the deep internal process abstract the cenozoic north china plain rift system generated on the site of a mesozoic rift system is the largest rifted basin in east china it trends nne with a total length of about 1000 km maximum width up to 400 km in its middle part and a width of 50 100 km at its south and north the largest thickness of the cenozoic deposits reaches 12 000 m and the accumulated thickness of the cenozoic basalts about 2000 m this rift system is a region of strong seismicity it has quite considerable differential crust movement at the present day the geophysical data indicate that the thickness of the crust and lithosphere beneath the rift system is approximately 4 8 km and 40 km thinner than that of the surrounding mountains this rift system has rather high heat flow values and residual gravity anomalies so far no low velocity anomaly of pn has been found in this region we infer that the formation of this rift system is caused by upwelling of the asthenosphere which is probably related to the convective roll induced by subduction of the pacific plate underneath the asiatic continent and the disturbance of the upper mantle in this region during the mesozoic

crustal structure and extensional deformation of thinned lithosphere in northern china abstract we herein present an interpretation of a 320 km long wide angle seismic profile between anxin and kuancheng which was obtained in 2002 the profile runs from the north china plain ncp where the lithosphere is just 70 km thick to the yanshan mountain folded belt ymfb where the lithosphere is180 km thick our model shows a crustal thickness that varies from 31 km under the ncp to 36 km under the ymfb the observed thinning of the crust in the ncp is about 14 which compares with an average extension of 24 41 at basin scale and 25 at lithosphere scale this finding suggests that the extensional deformation of the lithosphere in the north china block depends on depth the thin high velocity crust mantle transition zone has most likely originated after a delamination of the bottom of the crust and a concomitant intrusion of materials from the mantle the lower velocity of the lower crust may be attributed to the destruction of the lithosphere which permitted the lateral flow of melting materials above the moho from the ncp to the ymfb the differences found between the crust and the lithospheric mantle help to dispel any of the remaining uncertainty in the extensional factors and they may be attributed to detachment of middle crust and an intrusion of magma that originated in either lithosphere or asthenosphere we infer that the detachment of the middle crust lower crustal flow and magma intrusion probably lead to the underestimation of the crustal scale extensional factor and may represent the crustal response to the thinning of the lithosphere

mechanisms of petroleum accumulation in the bozhong sub basin bohai bay basin china part 1 origin and occurrence of crude oils the origin of the fourteen major oil fields in the bozhong sub basin bohai bay basin was studied based on the results of rock eval pyrolysis on more than 700 samples and biomarker analysis on 61 source rock samples and 87 oil samples the three possible source rock intervals have different biomarker assemblages and were deposited in different environments the third member of the oligocene dongying formation e3d3 32 8 30 3 ma in age is characterized mainly by high c19 c23 tricyclic terpane 0 75 high c24 tetracyclic terpane c26 tricyclic terpane 2 5 low gammacerane c30 hopane 0 15 and low 4 methyl steranes c29 steranes 0 15 ratios and was deposited in sub oxic to anoxic environments with significant terrigenous organic matter input the first e2s1 35 8 32 8 ma and third e2s3 43 0 38 0 ma members of the eocene shahejie formation have low c19 c23 tricyclic terpane and low c24 tetracyclic terpane c26 tricyclic terpane ratios and were deposited in anoxic environments with minor terrestrial organic matter input but have different abundances of 4 methyl steranes and gammacerane the hydrocarbon generating potential and biomarker associations of these three source rock intervals were controlled by tectonic evolution of the sub basin and climate changes three oil families derived from e2s3 e2s1 and e3d respectively and three types of mixed oils have been identified all large oil fields in the bozhong sub basin display considerable heterogeneities in biomarker compositions and originated from more than one source rock interval which suggests that mixing of oils derived from multiple source rock intervals or multiple generative kitchens and or focusing of oils originated from a large area of a generative kitchen is essential for the formation of large oil fields in the bozhong sub basin e2s3 and e2s1 derived oils experienced relatively long distance lateral migration and accumulated in traps away from the generative kitchen e3d3 derived oils had migrated short distances and accumulated in traps closer to the generative kitchen such a petroleum distribution pattern has important implications for future exploration there is considerable exploration potential for dongying derived oils in the bozhong sub basin and traps close to or within the generative kitchens have better chance to contain oils generated from the dongying formation

results of lithospheric studies from long range profiles in siberia several long range seismic profiles obtained during the last ten years in siberia show the complicated lithospheric structure of the siberian platforms the three component observations conducted at distances up to 3000 km made it possible to obtain information on p and s velocities in the crust on p velocity and q factor for the upper mantle and on the seismic boundaries responsible for reflected refracted and converted waves down to a depth of 400 700 km the crustal models are typical of old platforms of eurasia the average thickness of 40 km three layers with p velocities 6 2 6 5 7 0 km s and thicknesses of 10 15 km are distinguished the depth to the m discontinuity varies from 45 50 km beneath the old tunguss depression to 35 40 km beneath the younger vilyui basin the most complicated moho structure is observed in the boundary between the west siberian and the siberian platforms a strong inhomogeneity of p velocity models was revealed for the upper mantle the horizontal inhomogeneities are more larger in the uppermost mantle to depths of 80 100 km where p velocities vary from 8 0 8 2 km s beneath the young west siberian plate to 8 4 8 6 km s beneath some blocks of the siberian craton the fine vertical inhomogeneity was studied with reflections correlated after computer processing of seismograms they outlined several low velocity layers 20 50 km thick the layers were characterized by low q as well intensive waves were recorded from the transition zone between the upper and lower mantle the top of the zone is nearly horizontal in the area its depth is 400 25 km the bottom of the zone lies at about 700 km

mesozoic basins in eastern china and their bearing on the deconstruction of the north china craton abstract mesozoic basins occur widely in the eastern block and the neighboring area of the north china craton including the bohai bay the jiaolai the hefei and the north yellow sea in the north and the jianghan and the subei south yellow sea basins to the south their spatial temporal framework is the consequence of the indosinian and yanshanian tectonic regimes in eastern china and record the events related to mesozoic deconstruction of the north china craton our results demonstrate that the mesozoic tectonic evolution of the eastern north china craton was related to both sub crustal delamination and intra crustal extrusion or escape tectonics thus we propose that the mechanism of uplift of the yanshanian north china plateau and related lithosphere thinning in the eastern north china craton were related to sub crustal delamination at depth however the different distribution patterns of the basins on both sides of the tan lu fault system as well as the co existence of both compressional and extensional basins in the mesozoic indicate that these were controlled by escape tectonics in different tectonic parts of the crust

crustal structure across the three gorges area of the yangtze platform central china from seismic refraction wide angle reflection data abstract we present active source seismic data recorded along a 300 km long profile across the three gorges area of the western yangtze platform central china from west to east the profile crosses the zigui basin huangling dome and jianghan basin the derived crustal p wave velocity structure changes significantly across the tongchenghe fault that lies at the transition from the huangling dome to the jianghan basin west of the tongchenghe fault beneath the zigui basin and the huangling dome we observe a 42 km thick crust of relatively low average velocity 6 3 6 4 km s in contrast east of the tongchenghe fault beneath the jianghan basin the crust is only 30 km thick and has a high average velocity 6 6 6 7 km s a west east variation in crustal composition along the tongchenghe fault is also inferred west of the fault p wave velocities suggest a felsic composition with an intermediate layer at the base of the crust whilst east of the fault felsic intermediate and mafic crustal layers are apparent our results suggest that the crust beneath the jianghan basin has been thinned by rifting accompanied by intrusion of the lower crust by mafic dikes and sills the west to east division of the crust in the three gorges area coincides with first order geophysical contrasts in gravity topography crustal and lithospheric thickness

a comparison of foreland and rift margin sedimentary basins foreland and rift margin basins are compared on the basis of 1 their tectonic setting 2 reasons for subsidence and 3 their large scale geophysical and geological characteristics the thermal and mechanical properties of the underlying lithosphere are shown to be fundamental to the form of tectonic subsidence the lithosphere beneath foreland basins is flexurally downwarped by the loading of the adjacent fold thrust belt whereas tectonic subsidence at rifted margins is caused by mass replacement at depth during lithospheric extension on rifting and subsequent thermal contraction as the lithosphere cools the effects of rheology thermal maturity and lateral changes in properties of the lithosphere are outlined for foreland basins as is the topographic effect of possible phase changes beneath the fold thrust belt the thermal and rheological consequences of lithospheric extension at rift margins makes flexural subsidence relatively less important than in foreland basins flexure may however be partly responsible for uplift landward of the hinge line that is associated with rifting other mechanisms that could cause such uplift include depth dependent extension and thermal expansion due to the lateral diffusion of heat the models describing the evolution of these basins are shown to predict characteristics that are in accord with observations the superposition of foreland and rift margin basins as a result of ocean closure can lead to an overall basin stratigraphy that is complex such phases of basin subsidence must be separated according to the tectonic environment in which they formed in any analysis of the cause and consequences of basin evolution

increase in melt fraction along a south north traverse below the tibetan plateau evidence from seismology abstract a mid crustal low velocity zone and crustal melt fraction contrast the lhasa block in the maxiang yangbajain zone with the tethyan himalayas in the south evidence from wide angle reflections is extended by vertical reflection and differential s to p wave teleseismic delays the temperature and rheology implied allow phase transitions which may change crustal buoyancy and elevation le pichon et al 1997 and may allow lateral mass flow in the uppermost mantle particular path geometries to a tight temporary teleseismic array allow detection of a decrease in velocity northward through the lhasa block a larger relative variation in late teleseismic s with respect to p indicates an increase in poisson s ratio hence of partial melt fraction this may be seen as evidence of a higher position of the mantle asthenosphere level tibet hence appears to be underlain by variation in the structure within the crust across the indus tsangpo suture and in the structure within the mantle further northward of the middle lhasa block these variations are most readily attributable to spatial variation in partial melt fraction and hence temperature which also induces phase transformations mode and amount of deformation and control of buoyancy on elevation may vary accordingly the dynamics of the system is evidenced by seismic anisotropy which we relate to ductile flow

the cenozoic tectonic evolution of the great north china two types of rifting and crustal necking in the great north china and their tectonic implications abstract the great north china underwent widespread continental rifting during cenozoic time that caused extension and necking of the crust in this area and formed four major rift systems in this paper we provide geological and geophysical evidence that rifting and extension in the western part of the great north china differs from that in its eastern part in age and nature the space time relationships show that the origins of the western rift group of the great north china are probably related to the tibet plateau tectonics whereas those of the eastern rift group are related to the subduction events in the western margin of the pacific plate furthermore the quaternary tectonic regime in the eastern part of the great north china differs from the neogene tectonic pattern and shows relations with western rifts we speculate that during the quaternary the processes that caused rifting in the west have spread further east and now affect the eastern part of the great north china

cenozoic faulting of the bohai bay basin and its bearing on the destruction of the eastern north china craton abstract the cenozoic bohai bay basin is located at the center of the eastern block of the north china craton the structural architecture of this basin provides important clues on the deep seated lithosphere thinning of the north china craton the cenozoic regional stress field is characterized by nw oriented extension however the various cenozoic structural patterns of normal faulting and related transverse folding in the bohai bay basin are controlled not only by cenozoic stress field but also by strain field and mesozoic basement fault assemblages in this area regionally the cenozoic tectonic features and the dynamic evolution of the eastern north china craton are dominated by two lithosphere penetrating fault systems including the sinistral tan lu fault system and the dextral lan liao fault system to the west of the lan liao fault system cenozoic extensional tectonics includes nne trending listric normal faults that controlled half grabens however between these two fault systems are wnw trending half grabens which show basement involved faulting in the north and overlapping relations between sedimentary cover and basement in the south to the east of the tan lu fault the north yellow sea basin is a wnw trending fault depression with faulting in the south and overlapping relations in the north these structural features are inherited from the mesozoic tectonic framework of this area whose tectonic characteristics were completely controlled by two opposite strike slipping faults the trans extensional or oblique rifting in the paleogene followed by extensional faulting and subsequent subsidence furthermore the culmination of the decratonization of the north china craton was also related to an eastward jump of cenozoic subduction of the pacific plate and the far field effect of eastward extrusion of cenozoic subduction of the indian plate and was not essentially restricted to the early mesozoic processes therefore the cenozoic especially 25 ma marks the time of cessation of the processes that led to lithosphere thinning and destruction of the eastern block of the north china craton

gravity anomalies flexure of the indian plate and the structure support and evolution of the himalaya and ganga basin bouguer gravity anomalies along four profiles across the western himalaya and ganga basin show large deviations from local isostatic equilibrium a deficit of mass characterizes the ganga basin and an excess underlies the lesser himalaya both can be understood if the indian plate is flexed down by the distributed load of part of the mountains the cross sectional shape of the ganga basin seems to be controlled by the deflection of the indian plate which we compute assuming the indian plate to overlie an inviscid fluid from the shapes of both bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate if the ganga basin is a steady state feature then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin if the rate of convergence of india and the himalaya were constant that rate should equal the distance divided by the corresponding age we find a rate of 10 to 15 mm a for the last 15 to 20 ma which is consistent with a large part of the 50 mm a rate of convergence between india and eurasia being absorbed by the eastward extrusion of parts of tibet profiles of bouguer gravity anomalies show only a small peak or plateau over the southern edge of the lesser himalaya implying that the boundary between the light sediments of the ganga basin and the heavier crustal rocks of the lesser himalaya is not sharp and that there exists some light material beneath the range we infer that some sediment deposited in the ganga basin has been underthrust beneath the lesser himalaya but the quantity is small most of this sediment probably is scraped off the indian plate to make the foothills of the range we find that the load of the high himalaya is too large to be supported solely by elastic stress in the indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate the observed gradient in bouguer gravity anomalies increases from an average of about 1 mgal km over the ganga basin and lesser himalaya to about 2 mgal km over the high himalaya this increase in the gravity gradient implies that the moho dips more steeply 10 15 beneath the high himalaya than beneath the lesser himalaya 2 3 we interpret this steepening of the moho to be due to a weakening of the plate which allows it to bend more sharply beneath the high himalaya than farther south with the inclusion of a weak segment of indian plate beneath the high himalaya calculated anomalies show a somewhat increased gradient beneath the high himalaya but when the weight of the entire himalaya is used as a load on the plate calculated anomalies are more negative than observed therefore an external force system is needed to support much of the weight of the high himalaya as well as to bend the plate sufficiently beneath the high himalaya the magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of india s mantle lithosphere stripped of its crust and underthrust beneath southern tibet

crustal thinning and nature of extension in the northern north sea from deep seismic reflection profiling a regional network of deep seismic reflection profiles recorded in the northern north sea has been used to map mesozoic and cenozoic basin thickness and crustal thickness in the viking graben and adjacent platform areas depth to the reflection moho varies from about 20 km in parts of the viking graben to about 32 km beneath the shetlands and the norwegian margin the shallowing of the reflection moho beneath the viking graben implies crustal stretching factors for mesozoic extension greater than 2 in the center of the north sea basin the axis of crustal thinning is located directly beneath the viking graben the axis of the sedimentary basin steeply dipping basin bounding faults are imaged in the upper crust and several dipping reflectors are observed in the upper mantle but no continuous reflective feature extending from the sedimentary basin to beneath the moho is observed on any of the deep profiles thus these data do not support the existence of lithosphere penetrating low angle detachments zones of simple shear as the cause of extension in the northern north sea some of the mantle reflectors dip to the west and some to the east suggesting that simple shear if it occurs in the upper mantle is not of uniform sense rather these data suggest a complex depth dependent pattern of brittle extensional deformation in the upper crust pervasive ductile extension bulk pure shear in the lower crust which is decoupled from deformation in the mantle and extension accommodated by discrete dipping shear zones in the lithospheric mantle

hydrocarbon habitat of the offshore bohai basin china the bohai basin is a major continental petroliferous basin located in eastern china of which the offshore part of the basin contains more than one third of the total petroleum resources in the basin although the basin has been explored for nearly 40 years the offshore part of the basin is just reaching a moderately mature stage of exploration and its petroleum geological features are gradually becoming known the subduction of the pacific plate to the eurasia plate resulted in the convective upwelling of the mantle in the bohai area and triggered the development of nne and ne striking extensional faults along the mesozoic structures in the late paleocene causing the formation of the rift basin during the oligocene the depocenters and subsidence centers of the greater bohai basin migrated from west to east and from south to north and finally concentrated in the offshore part of the basin since the oligocene especially during the neogene due to the indentation of the india block to the eurasia plate the right lateral strike slip movement of the tan lu fault was relatively intense in the offshore part of the basin these characteristics led to the differences between the offshore and onshore sectors in sedimentation structural features and petroleum geology during the deposition of the dongying formation the offshore sector became the depocenter and subsidence center of the basin resulting in the deposition of the lower dongying formation mudstone source rocks while these mudstones are immature in the onshore part of the basin in the offshore bohai most major discoveries have been made in the drape anticlines on the rises in the upper tertiary reservoirs while the lower tertiary reservoirs within the half grabens only serve as secondary reservoirs as the neogene depocenter of the basin the offshore part accommodated some fine grained shallow lacustrine clastics of the lower minghuazhen formation serving as important regional seals for the neogene reservoirs a set of shallow faults and a central faulted anticlinal belt in the shallow upper tertiary sequences were formed during the neogene in the offshore sector under the influence of right lateral strike slip faulting of the tan lu fault these shallow faults networked with deep extensional faults serving as hydrocarbon conduits the central faulted anticline belt along the tan lu fault system also provides significant traps

modeling the growth and decay of the antarctic peninsula ice sheet a model of the growth and decay of the antarctic peninsula ice sheet during the last glacial interglacial cycle is used to identify the main controls on ice sheet behavior using as input glaciological assumptions derived by w f budd and i n smith 1982 annals of glaciology 3 4249 bedrock topography isostatic compensation and mass balance relationships the model is driven by sea level change over the last 40 000 yr in association with assumed changes in the rate of melting beneath ice shelves an ice sheet dome over 3 5 km thick grows on the offshore shelf and straits west of the antarctic peninsula and reaches a maximum at 18 000 yr b p collapse begins at 14 000 yr b p but becomes rapid and continuous after 10 000 yr b p the present stable ice cover is achieved at 6500 yr b p ice growth and decay are characterized by thresholds which separate periods of steady state from periods of rapid transition the thresholds usually relate to topography tests show that ice sheet behavior is most sensitive to sea level change basal marine melting and accumulation and is less sensitive to isostasy spatial variation in accumulation calving rates and ice flow parameterization tests of the model against field evidence show good agreement in places as well as discrepancies which require further work o

composition of the uralide crust from seismic velocity v p v s heat flow gravity and magnetic data abstract p wave velocity vp s wave velocity vs poisson s ratio heat flow potential field and surface geological data are integrated to constrain a model for the composition of the uralide crust along the urseis transect the model is constructed using published laboratory measurements of vp vs and density for a variety of crustal rock types these laboratory data have been corrected for depth pressure and the uralides temperature depth function the model shows clear differences between the composition of the old continental crustal nucleus of the east european craton and the newly added crust of the accreted arc terranes to the east the crust of the east european craton is more felsic than that of the magnitogorsk and east uralian zones and the latter two have a lowermost crust whose characteristics indicate a high garnet content mafic garnet granulite and or the presence of hornblendite the overall composition of the arc terranes is basaltic physical properties suggest that eclogite is not present in the arc terranes or if present it exists in small amounts that are below the resolution of the data set the lack of eclogite in the lower crust favours an intracrustal differentiation model for the evolution of the bulk composition of the continental crust nevertheless the absence of surface uplift the lack of metamorphism and late orogenic mantle melts and the current crustal thickness indicate that crustal thinning did not affect the bulk composition of the uralide crust

heterogeneous stretching simple shear and basin development abstract the models of basin development which involve either homogeneous stretching of the whole lithosphere or displacement on a lithospheric scale shear zone are but two end members of a range of possible extensional models the homogeneous extension model thins the lower lithosphere beneath the thinned upper crust and superimposes a thermal subsidence basin on the earlier fault bounded basin the shear zone model offsets the zone of lower lithospheric stretching and thermal subsidence it is more likely that the zones of upper and lower lithospheric stretching will be heterogeneous and patchy but will often overlap in plan view this will produce localised uplift and subsequent thermal subsidence within the faulted basin and may explain many of the anomalies between the various stretching estimates made using different structural stratigraphic and geophysical techniques the model which combines heterogeneous lithospheric stretching and associated simple shear may explain 1 variations in dip of the major detachment zones in the large basins 2 variations in types of strain on or beneath the detachment zones 3 regional uplift of part of a basin to erode the earlier fault blocks and 4 the development of volcanism in basins with only low values of upper crustal extension the zone of stretched lower crust and lithospheric mantle may lie beneath the centre of the zone of upper crustal stretching or to one side it may be linked to the upper crustal zone by faults which dip consistently in one direction or by extensional systems which change their dominant dip direction with depth pulling out the mid crust as one or more wedges possible examples are discussed from the basin and range province and northwest european continental shelf

structure of crust and upper mantle beneath the ordos block and the yinshan mountains revealed by receiver function analysis a temporary seismological network of broadband three component stations has been deployed n s to investigate the crust and upper mantle structure across the ordos block and the yinshan mountains p wave receiver functions reveal the moho depth to be about 41 km beneath the central ordos block and down to 45 km beneath the northern ordos block a slight uplifting to 42 43 km beneath the hetao graben increasing to 47 48 km beneath the yinshan mountains and then decreasing to 44 km beneath the northern yinshan mountains along the profile in the ordos block the crustal vp vs ratio about 1 80 south to the hetao graben differs from that about 1 75 beneath the center ordos block the crustal vp vs ratio is significantly lower about 1 65 1 70 beneath the yinshan mountains the p wave receiver function migration imaging suggests relatively flat discontinuities at 410 and 660 km indicating the lack of a strong thermal anomaly beneath this profile at these depths and a low s wave velocity anomaly in the upper mantle beneath the hetao graben we suggest that the low s wave velocity anomaly may be attributable to heat and that the thermal softening advances the evolution of the hetao graben while the lower crustal ductile flows transfer from the hetao graben to the northern ordos block resulting in crustal thickening

geological characteristics and distribution patterns of hydrocarbon deposits in the bohai bay basin east china abstract the bohai bay basin in east china is a meso cenozoic continental fault bounded basin with abundant petroleum resources based on analyses of sedimentation history and tectonic and thermal evolution the vertical and horizontal distribution patterns of oil and gas are discussed in this paper the basin has characteristics of multiple cycles of depression and uplift and each depression has its own sedimentary system and pattern of oil generation the main source rocks are of lower tertiary sediments characterized by multi cyclic sedimentation multiple source reservoir caprock associations were developed abrupt changes of sedimentary facies and the development of numerous faults caused the formation of various types of oil and gas reservoir that constituted multiple oil and gas accumulation zones based on individual structure zones these accumulation zones are mainly distributed around the centre of source rock deposits and are restricted in the areas where source rocks are distributed

structural anatomy and dynamics of evolution of the qikou sag bohai bay basin implications for the destruction of north china craton abstract the qikou sag is a complex cenozoic faulted basin at the center of the bohai bay basin the paleogene sediments in the basin show a maximum thickness of 11 000 m in paleogene representing intense crustal extension the sag has a typical dish like structure in the upper part with the central domain represented by a deep half graben and surrounded by different types of gentle slopes and sub sags linked with the basin margin uplifts the inner structure of the sag is jointly controlled by the basin margin extensional faults intrabasinal extensional tectonics and transfer tectonics the sag is divided into four different extensional tectonic domains by the basement fault along the coastline the haihe xingang fault and the qidong fault these sub sags are in turn controlled by different detachment systems the listric cangdong and hangu faults mark the major boundary faults controlling the extensional structure of the basin the structure and tectonics of the qikou sag provide a direct record of the late stages of rifting and destruction of the north china craton the mesozoic basement structures of the sag determine the tectonic styles in different sub sags the complete inversion of the structures in mesozoic profoundly influenced the development and evolution of the basin during paleogene we identify three main stages in the tectonic evolution of the qikou the oligocene faulting stage the miocene pliocene thermal subsidence stage and the pleistocene quaternary depression stage in the oligocene fault activities and the basin architecture were mainly controlled by the listric boundary fault system of the basin and the secondary fault system in the hanging wall under the detachment and extension in the upper crust since miocene the boundary fault of the basin underwent weak extension the upwelling of the upper mantle and the formation of corresponding shallow depressions contributed to the evolution and tectonic architecture of the qikou sag

isostatic response of the lithosphere with in plane stress application to central australia the relation between gravity and topography in central australia cannot be described by a conventional isostatic response function whether it assumes local or regional compensation or whether the crust behaves as an elastic or viscoelastic medium in particular the response function exhibits considerable anisotropy hence a theory for isostatic response has been developed for a crust in mechanical equilibrium of surface loads buoyancy and in plane forces and subject to erosion sedimentation and stress relaxation the observed north south response function exhibits extreme isostatic overcompensation at some wavelengths and this can be fully explained by the viscoelastic plate model with in plane compression the observed east west response function is consistent with models of regional or local isostatic compensation the response function implies an effective flexural rigidity of 5 10 1022 n m an effective maxwell relaxation time of 25 50 m y an in plane compression of 125 150 mpa and an erosion time constant of about 200 m y these parameters assume that the model evolved over a period of about 700 m y and are wholly consistent with the evolutionary model developed from geological observations

a detailed receiver function image of the sedimentary structure in the bohai bay basin abstract if the site at which receiver functions are constructed is filled by sediments then the waveforms from these receiver functions are dominantly controlled by the sedimentary structures within the first few seconds after the direct p arrival based on this observation waveform data collected at 44 temporary seismic stations have been used to image the sedimentary structure of the bohai bay basin a major continental petroliferous basin in eastern china an adapted hybrid global waveform inversion method was applied to the receiver functions to extract structural information beneath each of the stations the derived s velocity structure provides for the first time a basin scale seismic image of detailed sedimentary stratification the sedimentary cover of the basin is about 2 12 km thick consisting of cenozoic mesozoic and paleozoic strata from top to bottom the structural features presented in the s velocity image coincide quite well with the depression uplift type of tectonic system in the bohai bay basin the reconstructed morphology of the sedimentary layers provides seismological evidence for the two stage evolution of the intracontinental basin that were caused by an intensive tectonic regime transition in late mesozoic immediately following the lithospheric reforming of the eastern china continent

basin modelling how well have we done model developers have been highly successful in building usable useful and reliable one dimensional models and in convincing explorationists to use them development of convenient universal software played an instrumental role in the acceptance of 1 d modelling the remaining weaknesses in technology and training while serious can be resolved through a few years of intensive work in certain specific areas 2 d and 3 d fluid flow modelling in contrast finds itself today largely in the position occupied by 1 d modelling more than a decade ago the concept of fluid flow modelling has not yet been adequately sold to explorationists mainly because existing software is often inadequate to meet the real needs of exploration personnel development of appropriate software will probably have to precede widespread popularization of fluid flow modelling to improve existing software two major areas must be addressed 1 making models more comprehensive by including all relevant geologic phenomena and 2 making software easier to use

some observations on basin evolution and dynamics sedimentary basin evolution is intimately bound in with continental lithosphere dynamics deep seismic reflection profiling has revealed images of the crust and upper mantle that suggest linkages between the shape and structure of sedimentary basins as they evolve with the rheological responses to tectonic stress of upper and lower crust and upper mantle components of the lithosphere this is well illustrated by the european geotraverse lithosphere profile from north cape norway to tunisia the alpine mountain belt has a lithospheric root as well as a crustal root by virtue of its asymmetric disposition this may be the cause of compressive and tensile forces in close association in the upper crust which have created mountain uplift and basin subsidence extension and thrusting all in juxtaposition evidence for an elastic upper layer of crust above a plastic substratum can be used as a basis for modelling basin geometry structure and stratigraphy built upon the flexural nature of the upper crust such models go a long way towards simulating the evolution of basins such as the north sea viking graben in more detail analogue structural models can reveal the geometry of basin fill in relationship to crustal movement and the relationships between structure and sedimentation patterns however sedimentary basin modelling needs to begin with the quantitative simulation of depositional environments in relation to such factors as tectonics sea level variations sediment supply and erosion in order to match sedimentary architecture only then can the range of deposits be placed in space and time within a basin environment as building blocks within a dynamically evolving structural framework to create predictive models for comparison with geological reality