eocene onset and late miocene acceleration of cenozoic intracontinental extension in the north qinling range weihe graben insights from apatite fission track thermochronology abstract intracontinental extension in central china is accommodated by cenozoic grabens flanking the ordos block such as the weihe graben which accommodate 5 6 mm yr of extension to define the onset and evolution of major cenozoic extension we examine the cooling history of exhumed footwall rocks in the north qinling range adjacent to the weihe graben we present results from 47 new apatite fission track aft samples primarily from two vertical transects of 1 5 2 5 km relief located on the northern flank of the north qinling footwall aft ages that get progressively older with increasing distance from the active range bounding normal fault suggest that cenozoic uplift and southward tilting was a response to range parallel extension correlations between aft ages and both elevation and track lengths combined with thermal modeling of representative samples reveal that the north qinling experienced two major stages of cenozoic exhumation 1 relatively slow exhumation in response to a small magnitude of extension that initiated at 50 ma 2 relatively rapid exhumation in response to a large magnitude of accelerated extension that initiated at 10 ma in addition small changes in cooling rate at 35 ma and 25 ma may reflect minor changes in faulting rate we interpret initial extensional at 50 ma to be a far field effect of initial india asia continental collision in contrast accelerated extension after 10 ma along the qinling is likely linked to the upward and outward growth of the tibetan plateau alternatively lower crustal flow beneath the qinling may have progressively accelerated exhumation in late miocene time

lithospheric thermal structure and rheology of the eastern china abstract temperature distributions of the lithosphere beneath the eastern china were estimated by local isostasy equilibrium constrained geothermal calculation in this study maps of the lateral temperature variation at depths of 50 and 100 km as well as moho boundary are presented for the eastern china continent and the 600 c and 1100 c isotherm depth maps are also given meanwhile the thermal thickness of lithosphere is calculated as the depth of 1350 c isotherm the rheological strength of lithosphere is calculated based on a four layer model which consists of the upper lower lowermost crust layers and a wet lithospheric mantle layer the results show that the lithosphere of 160 180 km thick exists under sichuan basin in the western part of yangtze craton but keel is absent beneath the entire north china craton it is notable that the lithospheric thickness is only 130 140 km beneath ordos basin in the western part of north china craton accordingly there is no lithospheric keel beneath ordos basin these geothermal modeling results are in good agreement with those of seismic tomography and other geothermal studies the temperatures in the lower crust of the north china craton estimated are in range of 500 600 c consequently the composition of lower crust of the north china craton should be more mafic than that of previous estimation based on much cooler geotherms the spatial variation of lithospheric rheology in the eastern china is influenced by local geotherms and crustal compositions the creme brulee model which is represent by a strong crust portion but a weak lithospheric mantle portion in vertical strength profile approximates the lithospheric rheological layering for the regions of the western half of north china craton and the northern part of ne china which has thicker crust or moderate to high geotherm on the other hand the jelly sandwich model demonstrates the rheological layering of the western part of the yangtze craton and the eastern half of the north china craton which has low to moderate geotherm and or thinner crust in general thermal age of the tectonic province controls the present day thermal structure and rheology of the lithosphere of the eastern china

deep structures and surface boundaries among proto tethyan micro blocks constraints from seismic tomography and aeromagnetic anomalies in the central china orogen abstract the qinling qilian orogen preserves the records of early paleozoic convergence among the proto tethyan micro blocks in this study we analyze the seismic velocity structure and the aeromagnetic anomalies in the qinling qilian orogen and its adjacent areas showing that the northernmost boundaries of these proto tethyan micro blocks are defined by the guyuan longshoushan fault in the qilian orogen and by the luonan luanchuan fault in the qinling orogen respectively the lithosphere north of the qinling qilian orogen subducted southward under the qinling qilian orogen the boundaries of the micro blocks in the qinling qilian orogen of the proto tethys domain are mostly ew or ne trending combined with early paleozoic geological records our results indicate that the south dipping low velocity anomaly under the alxa block and the south dipping high velocity ones under the north qilian orogen and the north china block might indicate that the alxa block the north qilian oceanic slab and the north china block subducted southward the south dipping high velocity anomaly under the north qilian orogen is interpreted as the remnant of the proto tethys ocean in that area however the north dipping low velocity anomaly under the south qinling and the north qinling orogens the qaidam block and the north dipping high velocity one under the yangtze and the bikou blocks suggest that the south qinling and the north qinling orogens the qaidam the yangtze and the bikou blocks subducted northward the present spatial framework of the qinling qilian orogen is related to early paleozoic convergence among the proto tethyan micro blocks

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

mesozoic large scale lateral extrusion rotation and uplift of the tongbai dabie shan belt in east china the tectonic convergence between the south china block and qinling belt that occurred in mesozoic time was not homogeneous along strike it was mainly concentrated in the shengnongjia and hannan domes which formed a pair of indentors along which the south china block penetrated the qinling belt as a consequence the tongbai dabie shan belt the eastern part of the qinling orogenic belt may have undergone large scale eastward extrusion clockwise rotation and uplift as indicated by its boundary deformation features it was originally located beneath the narrowest part of the qinling belt where it underwent ultrahigh pressure metamorphism in the early triassic

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

multiscale seismic tomography and mantle dynamics abstract in this article we first introduce the methodology of multiscale seismic tomography and the way to interpret the obtained tomographic images and then review the significant recent results of multiscale seismic tomography with emphasis on mantle plumes and subducting slabs global and regional tomography shows that most of the slab materials under western pacific and east asia are stagnant in the mantle transition zone before finally collapsing down to the core mantle boundary as a result of large gravitational instability from phase transitions local and teleseismic tomography studies have imaged clearly the subducting slabs and arc magma chambers in the upper mantle wedge indicating that geodynamic systems associated with arc magmatism and back arc spreading are related to deep processes such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab because most hotspots are located in poorly instrumented continental and oceanic regions 3 d crust and upper mantle structure is determined for only a few hotspots such as iceland yellowstone and eifel which are covered by seismic networks and plume like slow anomalies are revealed under those hotspots global tomography has revealed deep mantle plumes under the major hotspots such as hawaii iceland kerguelen south pacific and africa strong lateral heterogeneities are revealed at the bottom of the mantle which are associated with the deeply subducted slabs and the birth of mantle plumes a thorough understanding of the deep earth structure will only be achieved by a combination of more effective seismic imaging techniques and dense coverage of global seismic networks particularly in the oceans

3 d multi observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle ii general methodology and resolution analysis 1 here we present a 3 d multi observable probabilistic inversion method particularly designed for high resolution regional thermal and compositional mapping of the lithosphere and sub lithospheric upper mantle that circumvents the problems associated with traditional inversion methods the key aspects of the method are as follows a it exploits the increasing amount and quality of geophysical datasets b it combines multiple geophysical observables rayleigh and love dispersion curves body wave tomography magnetotelluric geothermal petrological gravity elevation and geoid with different sensitivities to deep shallow thermal compositional anomalies into a single thermodynamic geophysical framework c it uses a general probabilistic bayesian formulation to appraise the data d no initial model is needed e compositional a priori information relies on robust statistical analyses of a large database of natural mantle samples and f it provides a natural platform to estimate realistic uncertainties in addition the modular nature of the method algorithm allows for incorporating or isolating specific forward operators according to available data the strengths and limitations of the method are thoroughly explored with synthetic models it is shown that the a posteriori probability density function i e solution to the inverse problem satisfactorily captures spatial variations in bulk composition and temperature with high resolution as well as sharp discontinuities in these fields our results indicate that only temperature anomalies of t 150 c and large compositional anomalies of mg 3 or bulk al2o3 1 5 can be expected to be resolved simultaneously when combining high quality geophysical data this resolving power is sufficient to explore some long standing problems regarding the nature and evolution of the lithosphere e g vertical stratification of cratonic mantle compositional versus temperature signatures in seismic velocities etc and offers new opportunities for joint studies of the structure of the upper mantle with unprecedented resolution

tectonic and stratigraphic controls of hydrocarbon systems in the ordos basin a multicycle cratonic basin in central china the ordos basin is the oldest and still an important hydrocarbon province in central china it is a typical cratonic basin developed on the archean granulites and lower proterozoic greenschists of the north china block the development of the ordos basin during the paleozoic mesozoic can be divided into three evolutionary stages cambrian early ordovician cratonic basin with divergent margins middle ordovician middle triassic cratonic basin with convergent margins and late triassic early cretaceous intraplate remnant cratonic basin two hydrocarbon systems are present in the basin the paleozoic gas and mesozoic oil systems in the paleozoic gas system the lower ordovician marine carbonates and pennsylvanian lower permian coal measures serve as source rocks the lower ordovician karst modified dolomites and pennsylvanian bauxitic mudstones form a significant reservoir seal association and the pennsylvanian lower permian deltaic sandstones and upper permian lacustrine mudstones form another effective reservoir seal association in the mesozoic oil system the upper triassic lacustrine mudstones are mature source rocks the upper triassic deltaic sandstones and overlying shallow lacustrine and swamp mudstones form a reservoir seal association and the lower jurassic fluvial sandstones and overlying shallow lacustrine and swamp mudstones form another reservoir seal association in both hydrocarbon systems the stratigraphic variations provide the principal traps the ordos basin is characterized by a stable tectonic setting that controlled the distribution of depositional systems and the development of erosional surfaces and ultimately governed the distribution of oil and gas fields and trap types

s receiver function analysis of the crustal and lithospheric structures beneath eastern tibet abstract s receiver functions from 51 permanent broad band stations are used to investigate the structures of the crust and lithosphere beneath eastern tibet and sichuan basin by stacking the s receiver functions the sp converted phases from the moho and the lithosphere asthenosphere boundary lab are clearly visible the moho depth ranges from 40 to 50 km beneath sichuan basin and from 55 to 60 km on the west side of the longmenshan lms fault the largest depth is 70 km occurring at the northern sichuan yunnan terrane the lab depth beneath sichuan basin ranges from 130 to 170 km while that on the west side of lms ranges from 100 to 120 km from the east to the west across lms there is a sudden rise of the lab followed by a gradual dip western sichuan basin is found to be surrounded by thinner lithosphere extending northeastward along the lms and southward to indochina the thinner lithosphere may indicate that the lower part of eastern tibet lithosphere is heated and delaminated by the underlying hot asthenospheric flow thus reflecting the size and direction of the flow it appears that asthenospheric flow from tibet is resisted by the cold rigid sichuan basin thus branching into northeastward and southeastward parts the complicated topography of the lithosphere likely results from this dynamic process

recent sulfur enrichment in the sediments of little rock lake wisconsin multiple sediment cores collected from little rock lake wisconsin prior to experimental acidification show that total sulfur accumulation rates increased during the past century with most of the increase occurring in the nonacid volatile inorganic and organic fractions the increased sulfur accumulation is a result of diagenetic processes within the sediments rather than changes in seston deposition rates or terrestrial inputs this enrichment has occurred because atmospheric sulfate deposition rates have increased over the past century resulting in increased lake water sulfate concentrations which has in turn increased the diffusional flux to the sediments and increased the extent of diagenetic immobilization individual cores had markedly different sulfur accumulation rates demonstrating the importance of collecting multiple cores to determine lakewide sediment accumulation processes that retain sulfate are of considerable interest in the context of lake acidification because they are important in regulating the alkalinity of many soft water present address u s epa environmental research laboratory 200 sw 35th st corvallis oregon 97333 acknowledgments we thank linda shane for contributing to the pollen analysis peter appleby for the gamma spectroscopy analysis and susan christie for editorial assistance the use of the university of wisconsin s trout lake field station is appreciated carol kelly richard holdren and noel urban reviewed an earlier draft of this paper and made numerous suggestions this work was supported through a cooperative agreement between the u s epa and the university of minnesota cr 8 11540 01 lakes see brezonik et al 1987 the magnitude of net sulfate retention as a fraction of input increases with increasing water residence time baker et al 1986 sulfate retention processes can be the dominant source of acid neutralizing capacity anc in seepage lakes with long residence times in the upper midwest and in florida baker et al 1991 sulfate retention can occur by sedimentation by reduction in anoxic hypolimnia or through reactions within the sediment in particular dissimilatory reduction in surficial sediments at sites with oxic overlying water has been recognized as a major sulfate sink kelly and rudd 1984 rudd et al 1986 baker et al 1985 the relative importance of these processes and the extent to which increasing sulfate loadings have resulted in increased mass retention have

timing of collision of the north and south china blocks controversy and reconciliation the qinling orogen was formed by the joining of the north and south china blocks but the timing of their integration has been debated for more than a decade the controversies obviously stem from different approaches to reconstruction of the integration history two contrasting lines of evidence yield two different ages for collision of the north and south china blocks middle paleozoic and late triassic the shangdan suture within the qinling was regarded in previous studies as the trace along which the north and south china blocks collided our studies however demonstrate that there are two sutures within the qinling the well documented shangdan suture and the newly discovered mianlue suture we show in this paper that the late proterozoic to early mesozoic evolution of the qinling involved interactions between the north china block the north and south qinling orogens and the south china block the middle paleozoic collision along the shangdan suture as constrained by some evidence accreted only the south qinling orogen to the southern part i e the north qinling of the north china block contemporaneous rifting of the south china block and subsequent drifting separated the south qinling from the south china block during the middle to late paleozoic the separation of the south from the north china blocks is supported by other evidence in particular geomagnetic data evidently it was the late triassic collision of the south china block with the south qinling orogen along the mianlue suture that led to final integration of the north and south china blocks

integrated lithospheric modeling combining thermal gravity and local isostasy analysis application to the ne spanish geotransect a two dimensional algorithm to determine the steady state thermal structure of the lithosphere that integrates thermal gravity and local isostasy analyses is presented gravity analyses together with seismic data are used to constrain spatial variations in density and crustal structure while absolute elevation is used to determine the lithospheric mantle thickness the calculation is performed using a finite element technique that links the different physical equations the program optionally calculates the temperature at any material boundary and with given rheological parameters the strength distribution and the total lithospheric strength in selected columns we apply the algorithm to the northeastern spanish geotransect which extends from the pyrenees to the balearic promontory and along which a strong variation in crustal and lithospheric thickness is evident we assess the use of two different inferred density models for the lithospheric mantle the first assumes a linear decrease in density with increasing temperature using the asthenospheric density as a reference the second model assumes a constant density for the whole lithospheric mantle although conceptually the two hypotheses differ substantially the results obtained do not show significant differences lithospheric thicknesses of 120 130 km below the pyrenees 60 65 km in the valencia trough and 65 75 km below the balearic promontory are deduced in all cases the mean lithospheric mantle density has to be 40 60 kg m 3 higher than the asthenospheric density the algorithm is shown to be a powerful tool in lithospheric thermal modeling especially in areas where surface heat flow is poorly constrained because of the temperature density elevation relationship

mesozoic sedimentary evolution of the northwest sichuan basin implication for continued clockwise rotation of the south china block we present new sedimentary data integrated into a regional mesozoic stratigraphic framework to provide a detailed picture of spatio temporal variations in deposition and depocenter migration of the northwest sichuan basin the mesozoic sedimentary evolution is utilized to interpret basin subsidence history and to unravel coeval basin margin tectonics the northwest sichuan basin together with the songpan ganzi terrane behaved as a passive margin south of the qinling paleo tethys from late paleozoic to early middle triassic times and then evolved into a peripheral foreland basin in response to collision of the north and south china blocks since the late middle triassic coeval with strong north south contraction of the songpan ganzi terrane in the late triassic sinistral transpressional deformation of the longmen shan belt led to flexural subsidence of the adjacent western sichuan basin renewed basin margin fold thrust activity triggered recurrence of flexural subsidence of the northwest sichuan basin since the middle jurassic with the depocenter eventually shifting to the northwestern corner of the basin in the early cretaceous sedimentary evolution of the northwest sichuan basin and the basin margin deformation imply that the south china block had been rotating clockwise relative to the north china block throughout the mesozoic with an interim period of early jurassic tectonic quiescence a model is advanced that invokes clockwise rotation of the south china block as a driver for tectonic evolution of both the basin and adjoining structural belts and provides an explanation for several salient features that are otherwise puzzling

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

high resolution body wave tomography models of the upper mantle beneath eastern china and the adjacent areas 1 we present new 3 d tomographic models of vp vs and vp vsratio anomalies in the upper mantle beneath ec and adjacent areas this data was collected and interpreted with the goal of clarifying geodynamic processes that caused spatially variable event histories throughout eastern china ec during mesozoic to cenozoic time the tomographic images were constructed by inverting body wave travel times recorded at 1300 stations within the upgraded china national seismic network and 9 temporary arrays resolution tests for different depths and the featured velocity anomalies verify that the tomographic images capture the velocity heterogeneities in the upper mantle to depths of 700 km the salient features of vp vs and vp vsratio anomalies can be clearly identified these include strong multiscale heterogeneities occupying the upper mantle beneath ec and differences in the spatial scale of anomalies found beneath northern and southern areas of ec these features demonstrate a degree of spatial variability in the geodynamic evolution of ec we propose two mechanisms to explain these patterns first the western front of the subducted slab may have imparted greater horizontal compressional stress in areas where it impinged further eastward into ec these areas would experience stronger convection and an altered stress regime in the upper mantle creating significant thermal anomalies beneath the south china block scb relative to the eastern north china craton ncc second differing thermal states and viscosities for the eastern ncc and the cathaysia block cab resulted in differing responses to regional deformation the archean hinterland of the eastern ncc specifically has a colder thermal state and higher viscosity and therefore exhibits only small scale heterogeneities due to the effect of shear localization the neoproterozoic cab has a relatively warm thermal state with lower viscosity and thus deformed more continuously

the initiation of rifting at constant tectonic force role of diffusion creep we have investigated the effect of diffusion creep on lithospheric extension using a one dimensional numerical model that assumes a constant force is available to drive extension the model is motivated by the fact that continental areas with average heat flow should be too strong to rift using standard estimates of lithospheric strength however such areas do rift and diffusion creep is a mechanism by which lithosphere may deform at a lower stress level than is required for the usually assumed dislocation creep we consider the evolution of strain rate and temperature at the center of an idealized pure shear rift the strain rate will either increase or decrease with time depending on whether lithospheric weakening or strengthening dominates as extension progresses given an initial thermal condition and an assumed lithospheric rheology the applied force must be such that the initial strain rate is greater than some critical value for weakening to dominate the force at which this condition is met we will term the critical force diffusion creep is more efficient at smaller grain sizes and the model results indicate that mantle grain sizes would have to be less than 1 mm for diffusion creep to significantly reduce the critical force we speculate that during the initial stages of continental rifting grain size reduction and diffusion creep deformation mechanisms may have an important effect on lithospheric strength

high resolution lithospheric structure beneath mainland china from ambient noise and earthquake surface wave tomography abstract we present a new high resolution shear velocity model of the lithosphere down to about 160 km beneath china using rayleigh wave tomography we combined ambient noise and earthquake data recorded at 1316 seismic stations the largest number used for the region to date more than 700 000 dispersion curves were measured to generate group and phase velocity maps at periods of 10 140 s the resolution of our model is significantly improved over previous models with about 1 2 in eastern china and 2 3 in western china we also derived models of the study region for crustal thickness and averaged s velocities for upper and mid lower crust and uppermost mantle these models reveal important lithospheric features beneath china and provide a fundamental data set for understanding continental dynamics and evolution different geological units show distinct features in the moho depth lithospheric thickness and shear velocity in particular the north china craton ncc lithosphere shows strong east west structural variations with thin and low velocity lithosphere in eastern ncc and thick and high velocity lithosphere beneath western ncc and the lithosphere of the ordos block seems to have undergone strong erosion the results support the progressive destruction of the ncc lithosphere from east to west at least partly caused by the thermal chemical erosion of the cratonic lithosphere from the asthenosphere another pronounced feature of our model is the strong lateral variations of the mantle lithosphere beneath the tibetan plateau tp the indian lithosphere beneath the tp shows variable northward advancement with nearly flat subduction in western and eastern tp and steep subduction in central tp with evidence for the tearing of indian lithosphere beneath central tp which may be important for the riftings at the surface in himalayas and southern tp the low velocity zone in northern tp shows strong correlation with the region of the mid miocene to quaternary potassic magmatism suggesting that delamination of lithosphere may have played an important role in the rise of the tp

late mesozoic and cenozoic thermotectonic evolution along a transect from the north china craton through the qinling orogen into the yangtze craton central china 1 cretaceous and cenozoic reactivation of the triassic qinling dabie orogen between the north china and yangtze cratons resulted from the combined effects of pacific subduction back arc extension in east china and collisions in west china we report new apatite fission track and apatite and zircon u th he data from east qinling along a 400 km long n s transect from huashan through the qinling orogen to huangling the ages show a general pattern of younging northward three major cooling phases are defined by modeling the multiple thermochronologic data sets the first phase occurred locally in the north and south qinling during the late triassic to early jurassic following heating associated with the triassic yangtze subduction and exhumation of the wudang metamorphic core complex on the cratonal edge a second phase represents regional exhumation between 100 and 60 ma coeval with rifting marked by the late cretaceous eocene k2 e red bed deposition in eastern china and possibly indicating a link with pacific subduction back arc extension in eastern china however it may also have been superimposed by eastward tectonic escape resulting from the lhasa west burma qiangtang indochina collision the third cooling phase was initiated at 45 ma exclusively in the north qinling and in the footwall of the graben bounding normal fault of the weihe graben in the lesser qinling we suggest the third phase was related to reactivation of the qinling fault system as a result of eastward tectonic escape imposed by the india asia collision at 50 ma

lithospheric transition from the variscan iberian massif to the jurassic oceanic crust of the central atlantic abstract a 1000 km long lithospheric transect running from the variscan iberian massif vim to the oceanic domain of the northwest african margin is investigated the main goal of the study is to image the lateral changes in crustal and lithospheric structure from a complete section of an old and stable orogenic belt the variscan iberian massif to the adjacent jurassic passive margin of sw iberia and across the transpressive and seismically active africa eurasia plate boundary the modelling approach incorporates available seismic data and integrates elevation gravity geoid and heat flow data under the assumptions of thermal steady state and local isostasy the results show that the variscan iberian crust has a roughly constant thickness of 30 km in opposition to previous works that propose a prominent thickening beneath the south portuguese zone spz the three layers forming the variscan crust show noticeable thickness variations along the profile the upper crust thins from central iberia about 20 km thick to the ossa morena zone omz and the ne region of the south portuguese zone where locally the thickness of the upper crust is

seismic evidence for significant lateral variations in lithospheric thickness beneath the central and western north china craton abstract we have migrated teleseismic s receiver functions to construct detailed lithospheric structure images that cover the three constituent parts of the north china craton ncc our images show that in contrast to the eastern ncc where significantly thinned lithosphere 60 100 km is widespread the central and western ncc are characterized by the coexistence of both preserved thick and dramatically thinned lithosphere the thick lithosphere 200 km is present beneath the stable ordos plateau and the thinned lithosphere up to 80 km is found in the late cenozoic yinchuan hetao and shaanxi shanxi rift areas with sharp changes occurring over a lateral distance of

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

crustal structure in the junction of qinling orogen yangtze craton and tibetan plateau implications for the formation of the dabashan orocline and the growth of tibetan plateau

lithospheric structure of the ordos block and its boundary areas inferred from rayleigh wave dispersion abstract we have measured rayleigh wave phase velocities of the ordos block and its southern and western boundary areas and inverted these data to describe the upper mantle structure the ordos block was found to be characterized by distinctly high seismic velocities at depths commonly associated with a low velocity asthenosphere to the south abnormally slow velocity anomalies appeared in the weihe rift probably indicating subsolidus grain boundary weakening effect under high temperatures near solidus instead of partial melt in the asthenosphere an intermediate seismic velocity structure was observed beneath the northern qinling orogen nql to the south of the weihe rift a thin high velocity lid which was probably a cratonic relic was found under the haiyuan arcuate tectonic region hatr west of the ordos block a thick lithosphere was present beneath the ordos block 200 km and the nql 140 km while a thinner lithosphere 100 km was found beneath the hatr and the weihe rift the observed structural differences were likely associated with different lithospheric tectonics in the study region whereas the weihe rift and the hatr had been strongly affected by cenozoic lithospheric modification and thinning the ordos block and the nql appeared to have been much less influenced combined with previous studies our results suggested that the far field effect of the indo asia collision may have played an important role in the unique lithospheric process in the study region such as the extension of the weihe rift the preservation of a cratonic relic under the hatr and the localized intense lithospheric modification of these two regions the substantial variations of deep structure and thus contrasting rheological properties between the ordos block and the surrounding notable seismic belts the hatr and weihe rift may be a candidate mechanism for the contrasting seismicity of the region the lateral variations in the lithospheric structure estimated here generally agreed with changes in upper mantle deformation patterns previously derived from sks splitting measurements

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

low apparent stress observed for mw 0 6 microearthquakes recorded during hydraulic fracturing stimulation in the carthage cotton valley gas field understanding the source characteristics of hydraulic fracturing induced microearthquakes is expected to provide better understanding of both the fracturing process and the influence of pre existing structures on the distribution of events however details of the source characteristics of microearthquakes remain largely unknown one controversial issue is whether the mechanism of such events differs from natural tectonic events that is whether significant reductions of effective stress and or volumetric change occur because of the stimulation herein to address this question we estimated the apparent stress of the 61 m w 0 6 microearthquakes observed during the hydraulic fracturing in the carthage cotton valley gas field we first obtained the apparent stress by estimating the radiated seismic energy from the duration of the relative moment rate function and then adjusted the values considering the possibility of seismic energy underestimation the initial value range of 0 001 0 02 mpa is consistent with other past studies of hydraulic fracturing induced microearthquakes although the adjustment increases the original value by a factor of 10 it is still within the lower bound of the range of constant apparent stress commonly observed for natural tectonic events furthermore relatively lower apparent stress was observed for the events striking subparallel to the maximum horizontal compressional stress axis this implies the mechanism of these events could be different from that of events optimally oriented for failure although the uncertainty of their measurements was expected to be large compared with other events because of their small magnitude online material table with information on microearthquakes observed during the hydraulic stimulation in the carthage cotton valley gas field

crust and upper mantle structure of the north china craton and the ne tibetan plateau and its tectonic implications abstract understanding the mesozoic cenozoic tectonic evolution of the north china craton ncc and the ne tibetan plateau tp requires detailed knowledge of the lithospheric structure using dense regional networks and temporary deployments as well as updated reference models we obtain the crust and upper mantle structure to 120 km depth our tomographic results show several major features which have particular implications for the weihe shanxi rift system wsrs deformation of the ne tp and lithospheric evolution of the ncc beneath the wsrs the crust gradually thickens from south to north the lithospheric mantle gradually becomes slower and the mid lower crustal velocities are lower in the weihe rift where rifting of the wsrs initiated we suggest that along strike variations of the lithospheric structures of the wsrs have played an important role in its multistage evolution a low velocity zone lvz in the mid crust beneath the qilian orogen is characterized by relatively higher velocities compared to lvzs in other parts of the tp thus coherent lithospheric deformation may occur due to the high viscosity of the lvz during early plateau growth causing strong anisotropy to develop the western ncc including the ordos block and part of the alashan block shows a high velocity cratonic root extending to the base of our model in contrast the lithosphere of the eastern ncc appears to have been completely modified during the mesozoic through cenozoic and presents a thin lithosphere of relatively low velocities underlain by hot asthenosphere we observed significant upper mantle heterogeneities in the ncc which may reflect its diachronous lithospheric modification

the equivalent elastic thickness te seismicity and the long term rheology of continental lithosphere time to burn out creme brulee insights from large scale geodynamic modeling abstract depending on the conditions and time scale the lithosphere exhibits elastic brittle plastic or viscous ductile properties as suggested by rock mechanics experiments a large part of the long term lithospheric strength is supported in the ductile regime unfortunately these data validated for strain rates 10 6 s 1 small scales few cm and simplified conditions cannot be univocally interpolated to geological time and spatial scales strain rates 10 17 10 13 s 1 100 1000 km spatial scales complex conditions without additional parameterization an adequate parameterization has to be based on real time observations of large scale deformation indeed for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated t e equivalent elastic thickness estimates yet for continents the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates for example in one continental rheology model dubbed jelly sandwich the strength mainly resides in the crust and mantle while in another dubbed creme brulee the mantle is weak and the strength is limited to the upper crust these models have arisen because of conflicting results from distributed earthquake elastic thickness t e and rheology data we address these problems by examining the plausibility of each rheological model from general physical considerations we review the elastic thickness t e estimates and their relationship to the seismogenic layer thickness t s to show that these two quantities have no direct physical relation we also show that some of small t e must be artifacts of inconsistent formulation of the mechanical problem in some free air anomaly admittance models we point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain we then explore by analytical and numerical thermo mechanical modeling the implications of a weak and strong mantle for tectonic structural styles we conclude that rheological models such as creme brulee which invoke a weak lithosphere mantle are generally incompatible with observations the jelly sandwich is in better agreement and we believe provides a useful first order explanation for the long term support of the earth s main surface features

presence of an intralithospheric discontinuity in the central and western north china craton implications for destruction of the craton detailed knowledge of lithospheric structure is essential for understanding the long term evolution and dynamics of continents we present an image of lithospheric structure across the central and western north china craton ncc derived using s and p receiver functions from a dense seismic array a negative velocity discontinuity is identified at 80 100 km depth within the thick lithosphere 160 200 km similar to that observed in many other cratonic regions and roughly at the same depth as the base of the lithosphere in the eastern ncc the intralithospheric discontinuity may indicate an ancient mechanically weak layer within the overall strong cratonic lithosphere and probably also existed beneath the eastern ncc before the mesozoic the presence of such a weak layer could have facilitated simultaneous lithospheric modification at the base and the middle of the lithosphere in the eastern ncc especially under the strong influence of the mesozoic pacific subduction leading to the severe lithospheric thinning and destruction recorded in this region the weak layer probably did not strongly affect the stability and evolution of the central and western ncc and other cratonic regions where effects from plate boundary processes were weak our seismic images integrated with geological data provide new insights into structural heterogeneities in the subcontinental lithospheric mantle and their roles in the dynamic evolution of continents

lithospheric thermal regime under the qinling orogenic belt and the weihe basin a transect across the yangtze and the north china cratons in central china abstract the thermal regime of the lithosphere could be a clue for the processes of geodynamics based on the yuquan yulin seismic reflection transect along the sichuan basin yangtze craton qinling orogenic belt qob weihe basin ordos basin north china craton we modified crustal and mantle lithospheric structures to fit the observed data such as elevation gravity geoid height and heat flow then we studied thermal structure lithospheric thermal thickness and rheological structure of this area the results suggest that the moho temperature along the transect ranges from 580 to 740 c the thickness of the thermal lithosphere varies from 70 to 190 km the rheological strength is characterized by the rheological characteristics of weak crust strong mantle in the sichuan basin the south qob and the ordos basin and strong crust weak mantle in the north qob and weihe basin the sichuan basin has the lowest heat flow the thickest lithosphere and the strongest lithospheric strength while the north qob and weihe basin have the highest heat flow the thinnest lithosphere and the weakest lithosphere the thermal regime of the sichuan and ordos basins is affected by the stability of the craton the thermal regime of the qob and weihe basin respond to the thermal effect of extensional rifting tectonics the thermal regime along the transect indicates that this area is mainly affected by the convergence caused by the continental collision of the indian plate

lateral variation of crustal structure in the ordos block and surrounding regions north china and its tectonic implications abstract crustal thicknesses and poisson s ratios in the ordos block and surrounding regions were estimated by the use of the h k stacking method on teleseismic receiver functions the data came from 353 temporary and permanent seismic stations in 2006 2011 results show that the crustal thickness and poisson s ratio gently vary within the ordos block with an average of 41 3 km and 0 265 respectively consistent with a felsic composition of the crust crustal thicknesses predicted on the basis of airy isostasy are consistent with the estimated thicknesses implying that the topography is approximately compensated the reactivated portion of the north china craton that has undergone mesozoic cenzoic lithospheric thinning is also characterized by the thinning crust while the ordos block maintains normal crustal thickness and average crustal velocity inferred higher densities in the lower crust and the anti correlation between poisson s ratio and crustal thickness in the ordos block may be the result from underplating of mafic magmas in the precambrian around the ordos block the paleoproterozoic khondalite zone in the northern edge has higher poisson s ratio and thickened crust which is consistent with the lower crust being of more mafic composition the weihe shanxi graben in the southeastern edge has mid high poisson s ratio high heat flow and thinning crust which is consistent with the known transtensional tectonic setting in the liupanshan thrust belt along the southwestern edge of the ordos plateau significant variations in crustal thicknesses and poisson s ratios occur on two sides besides a thickened crust a concaved moho implies horizontal shortening of this edge of the ordos block due to its collision with the northeastern tibetan plateau the structural differences between the eastern and western edges of the ordos block reflect that the ordos block is in tectonic stress environment of the western compression and the eastern extension

flat slab subduction trench suction and craton destruction comparison of the north china wyoming and brazilian cratons abstract the mechanisms of growth and destruction of continental lithosphere have been long debated we define and test a unifying plate tectonic driving mechanism that explains the numerous petrological geophysical and geological features that characterize the destruction of cratonic lithospheric roots data from three archean cratons demonstrate that loss of their roots is related to rollback of subducted flat slabs some along the mantle transition zone beneath the cratons during flat slab subduction dehydration reactions add water to the overlying mantle wedge as the subducting slabs roll back they suck in mantle material to infill the void space created by the slab roll back and this fertile mantle becomes hydrated the roll back causes concomitant lithospheric thinning of the overlying craton so the flux of newly hydrated mantle material inevitably rises causing adiabatic melting generating new magmas that gradually destroy the roots of the overlying craton through melt peridotite reactions calculated fluxes of new mantle material beneath cratons that have lost their roots range from 2 7 trillion to 70 million cubic kilometers which is sufficient to generate enough melt to completely replace the affected parts of the destroyed cratons cratonic lithosphere may be destroyed in massive quantities through this mechanism warranting a re evaluation of continental growth rates with time