the oldest known metriorhynchid crocodylian from the middle jurassic of north eastern italy neptunidraco ammoniticus gen et sp nov abstract metriorhynchidae is a clade of marine adapted crocodilians known from several middle jurassic early cretaceous specimens collected predominantly in south america and europe but poorly known in the northern margin of gondwana the portomaggiore crocodile is the most complete specimen of an italian metriorhynchid to date it consists of a partial skeleton that has been provisionally referred to an unnamed species of late jurassic metriorhynchus or geosaurus the specimen is preserved in the reddish nodular limestone of the rosso ammonitico veronese formation bajocian tithonian new data on microfossil associations constrain the age of the metriorhynchid to the late bajocian earliest bathonian on the basis of cranial synapomorphies the portomaggiore crocodile falls as the closest sister taxon of the late jurassic early cretaceous geosaurines and is referred to neptunidraco ammoniticus gen et sp nov it is unique among middle jurassic metriorhynchids in showing an incipient streamlining of the skull shared with late jurassic and cretaceous taxa since neptunidraco is the oldest known member of metriorhynchidae its phylogenetic position supports the hypothesis that the timing of the initial metriorhynchid and geosaurine diversifications should start in the bajocian

calcareous nannofossil zonation of the jurassic cretaceous boundary interval and correlation with the geomagnetic polarity timescale abstract calcareous nannofossil stratigraphy has been investigated in six european land sections and at two deep sea drilling project sites 391c and 534a ranging from the upper kimmeridgian to the lower valanginian most of the sections contain a continuous record of the jurassic cretaceous boundary interval a time of rapid increase in nannofossil abundance and diversity and allow development of a revised nannofossil zonation scheme for this period numerous nannofossil lineages have been studied in detail and the results help to increase potential stratigraphic resolution in this interval the proposed scheme consists of six zones which are units recognizable in all sections regardless of nannofossil preservation and eleven subzones determinable in all but the most poorly preserved material thirty eight additional biohorizons are proposed based on the stratigraphy of the two dsdp sites these can be accurately established only in moderately well preserved material but spotty occurrences of these taxa in other sections may be stratigraphically useful most of the sections studied have been investigated magnetostratigraphically and the results of this study allow correlation between nannofossil zonation and the geomagnetic polarity time scale magnetostratigraphy indicates that many nannofossil events can be quite precisely determined and are not significantly time transgressive finally the magnetostratigraphic definitions of two stage boundaries are refined the kimmeridgian tithonian and berriasian valanginian boundaries are placed within chrons cm22n and cm15n respectively two new genera faviconus and umbria and seven new species are described umbria granulosa rhagodiscus nebulosus cretarhabdus octofenestratus faviconus multicolumnatus nannoconus infans nannoconus compressus and nannoconus wintereri eight subspecies and three taxonomic emendations are also described

surface wave tomography of the western united states from ambient seismic noise rayleigh and love wave phase velocity maps summary we present the results of rayleigh wave and love wave phase velocity tomography in the western united states using ambient seismic noise observed at over 250 broad band stations from the earthscope usarray transportable array and regional networks all available threecomponent time series for the 12 month span between 2005 november 1 and 2006 october 31 have been cross correlated to yield estimated empirical rayleigh and love wave green s functions the love wave signals were observed with higher average signal to noise ratio snr than rayleigh wave signals and hence cannot be fully explained by the scattering of rayleigh waves phase velocity dispersion curves for both rayleigh and love waves between 5 and 40 speriod were measured for each interstation path by applying frequency time analysis the average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station triplets we find that empirical green s functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross correlation without explicit knowledge of the source distribution the average traveltime uncertainty is less than 1 s at periods shorter than 24 s we present rayleigh and love wave phase speed maps at periods of 8 12 16 and 20 s the maps show clear correlations with major geological structures and qualitative agreement with previous results based on rayleigh wave group speeds

the distribution of the mid to lower crustal low velocity zone beneath the northeastern tibetan plateau revealed from ambient noise tomography we collected continuous seismic data recorded between 2007 and 2010 by 208 broadband stations from the chinese provincial digital seismic networks a seismic collaborative experiment of northern tibet and the northeastern tibet seismic experiment cross correlations of vertical component records are computed to extract the rayleigh wave empirical green s functions group and phase velocities are then constructed from the empirical green s functions in 8 to 50 s period at periods 25 s more than 10 lower velocities are imaged beneath the qaidam basin and high velocities are observed beneath the nonbasin regions at periods 30 s up to 10 lower velocities are imaged in the qiangtang and songpan ganze terranes from these group and phase velocity maps a three dimensional 3 d vsv model of the crust is derived the model shows that the qiangtang and songpan ganze terranes have a very thick crust with a prominent low velocity zone lvz in the middle crust the lvz thins out in the vicinity of the eastern kunlun mountains providing a new constraint on the mode of deformation across the tibetan plateau the northwestern qilian orogen where receiver functions reveal a moho deeper than the surrounding areas also features a relatively weak midcrustal lvz which we interpret as an intracrustal response associated with the shortening between the north china craton and the tibetan plateau

cretaceous ridge subduction along the lower yangtze river belt eastern china the lower yangtze river belt is one of the most important metallogenic belts in china the mechanisms responsible for ore genesis and the formation of related cretaceous igneous rocks such as adakite a type granitoid and nb enriched basalt remain controversial mesozoic granitoids in the lower yangtze river belt were mostly formed in the early cretaceous 140 125 ma and three granitoid belts the inner the south and the north have been defined according to petrological and geochemical characteristics previously based mainly on negative end and high initial sr isotope values the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust both of which require crustal thickening mesozoic crustal thickening however is not supported by the development of extensional basins in the region from the late jurassic to cretaceous eastern china was closely associated with subduction of the pacific plate in the south and the izanagi plate in the north the midocean ridge mor between these two plates was drifting toward and likely subducting under the lower yangtze river belt a ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt partial melting of subducting young hot oceanic slabs close to the ridge formed adakitic rocks the negative end values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle and or crustal materials through afc process a slab window opened during ridge subduction as indicated by a type granitoids in the center of the inner belt nb enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths

shear wave properties and poisson s ratios of ultrahigh pressure metamorphic rocks from the dabie sulu orogenic belt china implications for crustal composition 1 shear wave velocities vs anisotropy and shear wave splitting have been measured at pressures up to 600 800 mpa for ultrahigh pressure uhp metamorphic rocks from the dabie sulu orogenic belt china with a focus on three types of eclogites type 1 eclogites are coarse grained unaltered samples showing high densities and high vs values 4 85 0 06 km s at 600 mpa type 2 eclogites are fine grained sheared samples with intermediate vs values 4 53 0 04 km s at 600 mpa and type 3 eclogites are overprinted by amphibolite facies metamorphism and display low vs values 4 33 0 09 km s at 600 mpa the compositional layering and retrograde metamorphism can result in significant anisotropy and shear wave splitting in eclogites suggesting their plausible contribution to seismic anisotropy in the lower crust upper mantle and particularly in subducted slabs integrating our p and s wave velocity results with reliable data from previous studies we estimated the pressure and temperature derivatives of vp vs and poisson s ratios for common rock types in the uhp metamorphic belt the geometric means were used as a mixture rule to invert the lithological and chemical compositions of the layered crust from seismic refraction velocities the inferred crustal composition suggests that the eclogite bearing uhp rocks are tectonic slices of crust that have been thrust along a series of shear zones during the continental collision between the north china and yangtze cratons over a normal uhp free middle lower crust with overall intermediate composition

tectonics and magmatism in turkey and the surrounding area this volume contains 23 papers from a range of international contributors describing recent research into the tectonics and magmatism of turkey and its surroundings this region is sited at the collision zone between eurasia and afro arabia and as such provides an extraordinarily complete and well exposed record of the staged tectonic evolution of this sector of the alpine himalayan orogen the geological history of this area involves separation of continental fragments from the margin of gondwana their migration across the tethyan oceans the subsequent closure of these oceans and finally the development of the neotectonic regime which continues to evolve to the present day such a comprehensive record is relevant to the understanding of collisional zones worldwide the volume is divided into five sections tethyan evolution neotethyan ophiolites post tethyan basin evolution neotectonics and igneous activity the first two sections deal with tethyan oceans whose growth and subsequent closure dominated the geodynamic framework in the mesozoic and cenozoic the subsequent sections deal with more recent geological developments from the balkan peninsula in the west to the transcaucasus in the east that followed consumption of the tethyan oceans there is a broad mix of papers throughout the volume wide ranging review papers on ocean development and extensional tectonics are followed by detailed descriptions of petrology and geochemistry and geographically focused studies on basin evolution specific aspects of extensional and strike slip tectonics and discussions of the relationship of magmatic activity to the tectonic development of the area tectonics and magmatism in turkey and the surrounding area presents up to date results and ideas from a large number of international contributors on a wide range of current research activity in this region it is essential reading for all geoscientists with an interest in both academic and applied aspects of eastern mediterranean geology

the alpine tethys rifted margins reconciling old and new ideas to understand the stratigraphic architecture of magma poor rifted margins this study focuses on the assessment of the stratigraphic architecture of the proximal and distal jurassic alpine tethys rifted margins the aim of the study was to reconcile the major observations performed in the alps through time with what is observed in present day magma poor rifted margins the proximal and distal rifted margins reflect two evolutionary phases of rifting involving different structures and isostatic evolutions leading to different stratigraphic records the proximal rifted margins formed from upper triassic to pliensbachian times and resulted in the formation of classical half graben basins rift evolution changed drastically during pliensbachian to toarcian times when extension started to localize in the future distal rifted margins low angle detachment faults become the new elementary structure controlling final crustal tapering and eventually mantle exhumation to the sea floor new structural elements controlling the stratigraphic architecture of the distal rifted margins include extensional allochthons and breakaway blocks both resulting from the delamination of a keystone block also referred to as hangingwall block during hyper extension processes to define the stratigraphic architecture of the proximal and distal rifted margins three stratigraphic marker horizons are used namely the top of the pre rift sequences the top of the proximal margin syn tectonic sequence and the base of post rift sequences based on the distribution of these stratigraphic marker horizons across the alpine units and a structural re evaluation of the rift structures a synthetic tectono stratigraphic evolution of the alpine tethys domain is proposed that reconciles old and new ideas about how rift systems may evolve in space and time

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

chemostratigraphy of the jurassic system applications limitations and implications for palaeoceanography current chemostratigraphical studies of the jurassic system primarily involve the use of one sedimentary component marine organic carbon one divalent transition metal substituted in carbonate manganese and two isotopic tracers strontium isotope ratios 87 sr 86 sr and carbon isotope ratios 13 c carb and 13 c org in carbonate and in organic matter other parameters such as mg ca and sr ca ratios in calcite oxygen isotope ratios 18 o in carbonate sulphur isotope ratios 34 s in carbonate hosted sulphate nitrogen isotope ratios 15 n org in organic matter osmium isotope ratios 187 os 188 os in black shales and neodymium isotope ratios 143 nd 144 nd in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors stratigraphical patterns in total organic carbon toc can be of either local or regional significance depending on the lateral extent of the former nutrient rich and productive water mass divalent manganese follows a similar pattern being concentrated most probably as a very early diagenetic phase only in oxygen depleted waters that typically underlie zones of elevated organic productivity shifts in mg ca and sr ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes strontium isotope ratios from pristine skeletal calcite provide a global signal 13 c values from carbonates with minimal diagenetic overprint potentially do the same although small spatial differences in palaeo water mass composition may have been locally significant oxygen isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful because they reflect palaeotemperature the evaporation precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase nitrogen isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis neodymium isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems preliminary data on sulphur isotope ratios in carbonates and osmium isotope ratios in organic rich shales both potentially offering global correlation indicate that these tracers may be valuable although the records at present are not sufficiently well established to allow high resolution regional correlation in all cases biostratigraphically well dated reference sections against which the relevant geochemical data have been calibrated are required in the first instance to date studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern boreal and southern tethyan europe carbon isotope stratigraphy has been undertaken primarily but not exclusively on bulk pelagic sediments from the alpine mediterranean or tethyan domain and strontium isotope stratigraphy has been undertaken largely on calcitic skeletal material belemnites and oysters from northern and southern europe in many sections including those containing ammonites multi parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means strontium isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record

mesozoic alpine facies deposition as a result of past latitudinal plate motion the fragmentation of pangaea as a consequence of the opening of the atlantic ocean is documented in the alpine mediterranean region by the onset of widespread pelagic sedimentation1 shallow water sediments were replaced by mainly pelagic limestones in the early jurassic period radiolarian cherts in the middle late jurassic period and again pelagic limestones in the late jurassic cretaceous period1 during initial extension basin subsidence below the carbonate compensation depth ccd is thought to have triggered the transition from early jurassic limestones to middle late jurassic radiolarites1 it has been proposed that the transition from radiolarites to limestones in the late jurassic period was due to an increase in calcareous nannoplankton abundance when the ccd was depressed below the ocean floor1 but in modern oceans sediments below the ccd are not necessarily radiolaritic here we present palaeomagnetic samples from the jurassic cretaceous pelagic succession exposed in the lombardian basin italy on the basis of an analysis of our palaeolatitudinal data in a broader palaeogeographic context we propose an alternative explanation for the above facies tripartition we suggest that the lombardian basin drifted initially towards and subsequently away from a near equatorial upwelling zone of high biosiliceous productivity our tectonic model for the genesis of radiolarites adds an essential horizontal plate motion component to explanations involving only vertical variations of ccd relative to the ocean floor it may explain the deposition of radiolarites throughout the mediterranean and middle eastern region during the jurassic period

shear wave structure in the northeastern tibetan plateau from rayleigh wave tomography 1 the northeastern tibetan plateau has undergone active shortening due to the india asia collision and has been an ideal place for studying the growth of the plateau to reveal deep structure that is responsible for geological observations in ne tibet we have constructed a 3 d shear wave velocity model from rayleigh waves recorded by 36 stations in the northeast tibetan plateau seismic experiment first the two plane wave tomography method was applied to compute the average and 2 d variations of phase velocities in a period range of 20 to 100 s then 1 d and 3 d shear wave velocity models were developed from the obtained rayleigh wave dispersions the entire region is characterized by a weak upper mantle with a relatively thin lithosphere of 80 100 km thick this observation helps to explain the active deformation at the kunlun mountains and the qinling orogen in ne tibet relative low velocity anomalies appear along and to the south of the kunlun fault kf in contrast to high velocity anomalies in the north and northeast one pronounced slow anomaly centered at 120 km depth is imaged right beneath the restraining bend of the kunlun fault which might be a northern extension of the large scale slow anomaly to the south of the fault we interpret it as a local feature due to its large magnitude and coincidence with the restraining bend it is probably related to anomalously high temperature and associated partial melting resulted from localized asthenosphere upwelling after the delamination of a thick lithosphere root the formation of the thick lithosphere root would require coherent shortening in the crust and mantle lithosphere caused by local compression due to the bend of the strike slip fault indicating that the kunlun fault is probably on lithosphere scale and has played an important role in accommodating the eastward extrusion of the tibetan plateau

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

three dimensional s wave velocity model of the bohemian massif from bayesian ambient noise tomography abstract we perform two step surface wave tomography of phase velocity dispersion curves obtained by ambient noise cross correlations in the bohemian massif in the first step the inter station dispersion curves were inverted for each period ranging between 4 and 20 s separately into phase velocity maps using 2d adjoint method in the second step we perform bayesian inversion of the set of the phase velocity maps into an s wave velocity model to sample the posterior probability density function the parallel tempering algorithm is employed providing over 1 million models from the model samples not only mean model but also its uncertainty is determined to appraise the reliable features the model is correlated with known main geologic structures of the bohemian massif the uppermost low velocity anomalies are in agreement with thick sedimentary basins in deeper parts 4 20 km the s wave velocity anomalies correspond in general to main tectonic domains of the bohemian massif the exception is a stable low velocity body in the middle of the high velocity moldanubian domain and high velocity body resembling a promontory of the moldanubian into the tepla barrandian domain the most pronounced high velocity anomaly is located beneath the eger rift that is a part of a tertiary rift system across europe

calcareous nannofossil assemblage turnover in response to the early bajocian middle jurassic palaeoenvironmental changes in the subbetic basin abstract the calcareous nannofossil assemblages from three lower bajocian pelagic successions from the subbetic basin well dated by ammonites have been investigated biostratigraphically they encompass the njt9 to njt10a nannofossil tethyan zones pca factor analysis performed on percentage abundances of selected taxa suggests that the main factors controlling the early bajocian nannofossil assemblages of the subbetic basin were trophic level nutrient availability and nutricline depth a gradual eutrophication is recorded in the calcareous nannofossil assemblages through an increase in the percentage abundance of watznaueria spp especially in the taxa best adapted to unstable and more eutrophic conditions w britannica gr and w manivitiae gr this eutrophication is concomitant with a trend of increasing carbonate 13 c values over the early bajocian and a rise in biogenic silica production and deposits in the study sections and other basins of the western tethys around the middle part of the laeviuscula subzone a decrease in the absolute and relative abundance of the deep dwelling taxa especially crepidolithus crassus suggests a deterioration of the environmental conditions in the lower photic zone this has been interpreted as caused by a nutricline shallowing and or greater turbidity that could be related in turn to surface water stratification two different episodes have been differentiated in relation to nutricline dynamics 1 from the latest aalenian to earliest early bajocian a weak and or deep nutricline with nutrients rather uniformly spread throughout the photic zone and concomitant with mesotrophic conditions and increasing productivity is suggested 2 a shallowing of the nutricline and or increase in surface water turbidity concomitant with more eutrophic conditions is suggested for the latest early bajocian the observed decrease in the deep dwelling taxa supports the hypothesis of a climatically driven fertilization of the surface waters instead of fertilization by upwelling for the early bajocian eutrophication episode

crustal and uppermost mantle structure across the lower yangtze region and its implications for the late mesozoic magmatism and metallogenesis eastern south china abstract we present here high resolution seismic sections of crustal and uppermost mantle structure along two dense linear arrays across the lower yangtze region in the eastern south china block scb our seismic images reveal two low velocity anomalies lvas in the uppermost mantle which we interpreted as relic still cooling mantle magma chambers ponding and underplating beneath the base of the crust in the areas of the dexing porphyry copper deposits and the cretaceous south anhui granites the moho and lab discontinuities imaged also exhibit deformation directly above the two mantle lvas which probably indicate local mantle lithosphere destruction by asthenospheric upwelling in the mesozoic this local asthenospheric upwelling may have triggered melting of the mantle lithosphere and generated the observed mantle magma chamber system through interactions with lower crustal materials low vs bodies revealed in the middle upper crust at depth of 10 km may indicate the corresponding solidified intrusive granitoids these observations provide unambiguous seismological evidence connecting deep magmatism with surface geological features combined with previous petrologic and geochemical studies the findings presented here outline a mantle magma chamber based magmatic system that was underplated beneath the base of the crust and kept supplying heat and materials in forming the voluminous yanshanian igneous rocks and associated metallogenesis in the lower yangtze region

integrated stratigraphy radiolarians and calcareous nannofossils of the middle to upper jurassic alpine radiolarites lombardian basin italy constraints to their genetic interpretation abstract new biostratigraphic data obtained by integrating radiolarians and calcareous nannofossils document a depositional history for the middle to upper jurassic radiolarites of the italian southern alps characterized by strong fluctuations in the sedimentation rates associated to shifts from gravity driven to hemipelagic sedimentary process we studied the radiolarites in four sections located in different settings of the lombardian basin a rifted basin of the northern adriatic plate the sections studied were described and sampled in detail to obtain data on their lithology using facies and microfacies analysis and calcimetry and their nanno and microfossils content especially radiolarians and calpionellids an almost complete record of radiolarians was detected throughout the selcifero lombardo formation so that the radiolarian zonation uazs for the middle to upper jurassic interval partially integrated with calcareous nannofossil and calpionellid biostratigraphic data was successfully applied accordingly the lower member radiolariti correlates with the lower bathonian uaz 5 to the kimmeridgian uaz 11 interval and the upper member rosso ad aptici displays an age spanning from the kimmeridgian to the base of upper thitonian from uazs 10 11 to nannofossil njk zone we interpret the radiolarites as pelagic sediment relatively enriched in radiolarian during a long time span of basin starvation due to the near absence of periplatform exportation if we except bathonian a period of relatively higher sedimentation rates the radiolarites are characterized by very low sedimentation rates implying equally low radiolarian productivity therefore it is not necessary to claim for high tethyan water fertility during middle to late jurassic even if this latter condition likely played a role in the production exportation of the carbonate platforms

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

high temperature geochronology constraints on the tectonic history and architecture of the ultrahigh pressure dabie sulu orogen 1 new u pb zircon and th pb monazite ages are presented from the giant sulu ultrahigh pressure uhp terrane combined with sm nd ages rb sr ages inclusion relationships and geologic relationships they help define the timing of peak recrystallization the timing of subsequent amphibolite facies metamorphism and the architecture of the dabie sulu suture zone between the collided sino korean and yangtze cratons the data indicate a 15 myr record of uhp recrystallization the first clearly documented for a giant uhp terrane this requires that continental subduction in the dabie sulu orogen involved multiple uhp tectonic or recrystallization events a 244 236 ma precursor uhp event seen only in the dabie shan was followed by a second 230 220 ma main uhp event which was itself terminated by a 220 205 ma amphibolite facies overprint older eclogite facies events seen in the qinling segment of this orogenic belt raise the possibility that these rocks have undergone u hp metamorphism three or four times but at present there is no geochronological evidence in the dabie sulu area to support this the subduction of the lower yangtze plate did not proceed in a simple fashion the ages of inherited zircon cores demonstrate that a ribbon continent of yangtze affinity escaped subduction and became wedged against the sino korean plate hanging wall

crustal structure beneath the dabie orogenic belt from ambient noise tomography abstract the qinling dabie sulu orogenic belt in east central china is the largest high and ultrahigh pressure hp and uhp metamorphic zone in the world the dabie mountains are the central segment of this orogenic belt between the north china and yangtze cratons this work studies the nature of the crustal structure beneath the dabie orogenic belt to better understand the orogeny to do that we apply ambient noise tomography to the dabie orogenic belt using ambient noise data from 40 stations of the china national seismic network cnsn between january 2008 and december 2009 we retrieve high signal noise ratio snr rayleigh waves by cross correlating ambient noise data between most of the station pairs and then extract phase velocity dispersion measurements from those cross correlations using a spectral method taking those dispersion measurements we obtain high resolution phase velocity maps at 8 35 second periods by inverting rayleigh wave phase velocity maps we construct a high resolution 3d shear velocity model of the crust in the dabie orogenic belt the resulting 3d model reveals interesting crustal features related to the orogeny high shear wave velocities are imaged beneath the hp uhp metaphoric zones at depths shallower than 9 km suggesting that hp uhp metaphoric rocks are primarily concentrated in the upper crust underlying the high velocity hp uhp metamorphic zones low shear velocities are observed in the middle crust probably representing ductile shear zones and or brittle fracture zones developed during the exhumation of the hp uhp metamorphic rocks strong high velocities are present beneath the northern dabie complex unit in the middle crust possibly related to cooling and crystallization of intrusive igneous rocks in the middle crust resulting from the post collisional lithosphere delamination and subsequent magmatism a north dipping moho is revealed in the eastern dabie with the deepest moho appearing beneath the northern dabie complex unit consistent with the model of triassic northward subduction of the yangtze craton beneath the north china craton

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

surface wave array tomography in se tibet from ambient seismic noise and two station analysis i phase velocity maps summary empirical green s functions egfs between pairs of seismographs can be estimated from the time derivative of the long time cross correlation of ambient seismic noise these egfs reveal velocity dispersion at relatively short periods which can be used to resolve structures in the crust and uppermost mantle better than with traditional surface wave tomography we combine rayleigh wave dispersion estimates from egfs and from traditional two station ts analysis into a new approach to surface wave array tomography with data from dense receiver arrays we illustrate the methodology with continuous broad band recordings from a temporary seismographic network on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china the egfs are robust under temporal changes in regional seismicity and the use of either ambient noise approximated by records without signal from events with magnitude mb 5 or 4 or surface wave coda produces similar results the egfs do not strongly depend on the presence of large earthquakes but they are not reciprocal for stations aligned in the n s direction this directionality reflects the paucity of seismicity to the north of the array using a far field representation of the surface wave green s function and an image transformation technique we infer from the egfs the rayleigh wave phase velocity dispersion in the period band from 10 30 s a classical ts approach is used to determine rayleigh wave phase velocity dispersion between 20 120 s together they constrain phase velocity variations for t 10 120 s which can be used to study the structure from the crust to the upper mantle beneath se tibet short and intermediate period 10 80 s phase velocities are prominently low suggesting that the crust and upper mantle beneath se tibet is characterized by slow shear wave propagation

paleomagnetism and paleogeography of jurassic radiolarian cherts from the northern apennines of italy oriented samples of jurassic radiolarian chert were collected from the tuscan domain continental margin and the ligurid domain oceanic of the northern apennines for paleomagnetic study to determine the paleogeographic origins of these rocks the oceanic rocks are all thermochemically overprinted by a mostly reversed polarity component of magnetization b that was likely acquired during late miocene regional uplift of the northern apennines this component also dominates the lower brittle chert of the tuscan cherts but disappears upsection in the more clay rich and ductile siliceous marlstones in addition the tuscan cherts retain an inferred primary magnetization c isolated at temperatures between 560 and 660 c which passes a fold test and shows a polarity stratigraphy this component indicates a paleolatitude of 11 4 n and a counterclockwise vertical axis rotation of 29 9 with respect to the southern alps of italy of 49 8 with respect to africa and of 91 8 with respect to eurasia our results suggest that the tuscan domain was farther south than other deep water continental margin sections of adria and that transcurrent faulting might have played a significant role in the orogenic evolution of the northern apennines

crustal and uppermost mantle structure in southern africa revealed from ambient noise and teleseismic tomography summary rayleigh wave phase velocity maps in southern africa are obtained at periods from 6 to 40 s using seismic ambient noise tomography applied to data from the southern africa seismic experiment sase deployed between 1997 and 1999 these phase velocity maps are combined with those from 45 to 143 s period which were determined previously using a two planewave method by li burke in the period range of overlap 25 40 s the ambient noise and two plane wave methods yield similar phase velocity maps dispersion curves from 6 to 143 s period were used to estimate the 3 d shear wave structure of the crust and uppermost mantle on an 1 1 grid beneath southern africa to a depth of about 100 km average shear wave velocity in the crust is found to vary from 3 6 km s 1 at 0 10 km depths to 3 86 km s 1 from 20 to 40 km and velocity anomalies in these layers correlate with known tectonic features shear wave velocity in the lower crust is on average low in the kaapvaal and zimbabwe cratons and higher in the surrounding proterozoic terranes such as the limpopo and the namaquanatal belts which suggests that the lower crust underlying the archean cratons is probably less mafic than beneath the proterozoic terranes crustal thickness estimates agree well with a previous receiver function study of nair et al archean crust is relatively thin and light and underlain by a fast uppermost mantle whereas the proterozoic crust is thick and dense with a slower underlying mantle these observations are consistent with the southern african archean cratons having been formed by the accretion of island arcs with the convective removal of the dense lower crust if the foundering process became less vigorous in arc environments during the proterozoic

magnetostratigraphy nannofossil stratigraphy and apparent polar wander for adria africa in the jurassic cretaceous boundary interval article i nfo existing correlations of early cretaceous nannofossil events to polarity zones in italian pelagic limestones are the basis for correlation of polarity chrons to geologic stages and hence calibration of the early cretaceous geomagnetic polarity timescale here we present correlations of nannofossil events to polarity chrons for the late jurassic and jurassic cretaceous boundary interval from the southern alps making the case for revisions of current geologic timescales in this interval the jurassic cretaceous and kimmeridgian tithonian boundaries can be usefully defined at the onsets of polarity chrons cm18r and cm22r respectively the oxfordian kimmeridgian boundary can be assigned to cm25 an apparent polar wander path apwp for the southern alps derived from and time calibrated by bio magnetostratigraphic data is similar to and provides a proxy for the african apwp in the absence of adequate african data of this age the southern alpine apwp is characterized by a cusp hairpin at 30 n 270 e at a time close to the oxfordian kimmeridgian boundary polarity chron cm25 154 ma rapid apwp from high latitude n60 n in the preceding middle jurassic bajocian oxfordian interval coincident with the onset of central atlantic ligurian oceanic rifting and a return to high latitudes during the cretaceous early jurassic poles at high latitudes n60 n are evident for sinemurian to toarcian aalenian time 175 195 ma an apwp for this time interval with this level of temporal calibration cannot be constructed using data from africa itself or from other atlantic bordering continents by rotation to african coordinates

molybdenite re os and albite 40ar 39ar dating of cu au mo and magnetite porphyry systems in the yangtze river valley and metallogenic implications abstract the area of the middle lower yangtze river valley eastern china extending from wuhan hubei province to western zhenjiang jiangsu province hosts an important belt of cu au mo and fe deposits there are two styles of mineralization i e skarn porphyry stratabound cu au mo fe deposits and magnetite porphyry deposits in several nne trending cretaceous fault bound volcanic basins the origin of both deposit systems is much debated we dated 11 molybdenite samples from five skarn porphyry cu au mo deposits and 5 molybdenite samples from the datuanshan stratabound cu au mo deposit by icp ms re os isotope analysis nine samples from the same set were additionally analyzed by ntims on re os results from the two methods are almost identical the re os model ages of 16 molybdenite samples range from 134 7 2 3 to 143 7 1 6 ma 2 the model ages of the five samples from the datuanshan stratabound deposit vary from 138 0 3 2 to 140 8 2 0 ma with a mean of 139 3 2 6 ma their isochron age is 139 1 2 7 ma with an initial os ratio of 0 7 8 1 mswd 0 29 these data indicate that the porphyry skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system albite 40 ar 39 ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 ma i e 10 20 ma later both mineralization styles characterize transitional geodynamic regimes i e the period around 140 ma when the main ns trending compressional regime changed to an ew trending lithospheric extensional regime and the period of 125 115 ma of dramatic ew trending lithospheric extension

reflection seismic imaging of the lujiang zongyang volcanic basin yangtze metallogenic belt an insight into the crustal structure and geodynamics of an ore district abstract five intersecting reflection seismic profiles were shot over the lu zong volcanic basin one of the seven ore districts in the middle and lower yangtze river metallogenic belt in east china the seismic data reveal diversified reflective patterns in the upper crust which are interpreted as horsts and grabens in general the middle crust presents both thrust related structures and boudinage and pinch and well structure on different profiles suggesting a strong structural heterogeneity which developed due to a change from compressional to extensional movement below the central part of a volcanic basin the lower crust shows an obvious reflective anisotropy both at the crust mantle and the brittle ductile transition in the crust this feature may be explained by magma system emplaced along a ne trending fault or weak zone at different depths the tan lu fault zone is present as a northwest dipping fault extending to the middle crust the lower crust beneath the tan lu fault and peripheries is possibly of dabie affinities the lu zong volcanic basin is clearly defined as an asymmetric graben with master faults on both the east and north flanks a contrasting reflection character suggests a division of the basin into northern and southern parts separated by a concealed fault the relatively thin volcanic sequences inferred to be 800 1000 m in thickness cover triassic and paleozoic sequences at depth that provide an exploration target in the northern part for deep porphyry and skarn deposits the dominant deformation reflects nw se oriented compression and extension formed during the yanshanian orogenic episode the contraction deformation is characterized by thrust faults with imbrication of crustal slices whereas the extension deformation is featured by normal faults regional detachments and basins underplating of mantle derived magma and a multi layer magma intrusion model are proposed to explain both seismic features and the ne striking lu zong volcanism and igneous intrusions in the region

sequence stratigraphy and condensed pelagic sediments an example from the rosso ammonitico veronese northeastern italy abstract correlation between eustatic and sedimentary cycles in the jurassic rosso ammonitico veronese suggests that the existence of a genetic link between them should be explored the red nodular limestones of this formation were deposited on a current swept submarine plateau pertaining to the south alpine passive continental margin in this setting of fault bounded blocks shelf derived sediments were trapped in the intervening troughs and only the very slow particle by particle rain of planktonic organisms took place the corresponding rock bodies typically show frequent omission surfaces of variable range current activity seems to be the prime controlling factor which affects the degree of winnowing early cementation and reworking and is decisive in bringing about all lithologic changes this hydrodynamic effect greatly depends on ocean circulation patterns which in turn are controlled by climatic and sea level changes highstands lead to equable climates and hence to a weak regime of currents impinging on sea bottoms the opposite happens during the following lowstands which reactivate ocean circulation through deteriorated climatic conditions it is envisaged that even though most of the geometric and sedimentologic criteria commonly used on shelf slope sediments are lacking depositional sequences i e physically the stratigraphic record of complete eustatic cycles may be recognized also in condensed pelagic successions they consist of thin rock units deposited during periods of sea level highstand when a slowed down current activity allows the preservation of the pelagic input and are bounded by lowstand induced hard grounds when accelerated winnowing prevent deposition these unconformities consequently don t correlate with condensed sections on shelves

a crustal model of the ultrahigh pressure dabie shan orogenic belt china derived from deep seismic refraction profiling we present a new crustal cross section through the east west trending ultrahigh pressure uhp dabie shan orogenic belt east central china based on a 400 km long seismic refraction profile data from our profile reveal that the cratonal blocks north and south of the orogen are composed of 35 km thick crust consisting of three layers upper middle and lower crust with average seismic velocities of 6 0 6 0 2 km s 6 5 6 0 1 km s and 6 8 6 0 1 km s the crust reaches a maximum thickness of 41 5 km beneath the northern margin of the orogen and thus the present day root beneath the orogen is only 6 5 km thick the upper mantle velocity is 8 0 6 0 1 km s modeling of shear wave data indicate that poisson s ratio increases from 0 24 6 0 02 in the upper crust to 0 27 6 0 03 in the lower crust this result is consistent with a dominantly felsic upper crustal composition and a mafic lower crustal composition within the amphibolite or granulite metamorphic facies our seismic model indicates that eclogite which is abundant in surface exposures within the orogen is not a volumetrically significant component in the middle or lower crust much of the triassic structure associated with the formation of the uhp rocks of the dabie shan has been obscured by post triassic igneous activity extension and large offset strike slip faulting nevertheless we can identify a high velocity 6 3 km s zone in the upper 5 km depth crustal core of the orogen which we interpret as a zone of ultrahigh pressure rocks a north dipping suture and an apparent moho offset that marks a likely active strike slip fault

lithospheric structure of hubei province central china from rayleigh wave tomography insight into the spatial contact relationship between the yangtze plate and the eastern qinling dabie orogenic belt

from rifting to drifting tectonic evolution of the south alpine upper crust from the triassic to the early cretaceous abstract the tectonic evolution of the south alpine rifted margin is discussed on the base of four palinspastic upper crustal profiles extension related to the movements between adria and europe began in the norian after variscan orogeny late carboniferous to middle permian orogenic collapse and continental scale wrenching from the late triassic to the early liassic stretching was mostly limited to the lombardian basin during this time extension was mainly controlled by four major listric faults symmetrically centred around the late carboniferous early permian collio grabens smaller faults which also started in the norian were progressively de activated during the late triassic after the middle liassic faulting in the lombardian basin gradually ceased and the site of extension shifted westwards i e towards the future site of crustal separation extension was then controlled by a set of west dipping normal faults oceanic crust was formed not later than 157 ma the overall extension along the profile which had a final length of 290 km was 52 km which corresponds to a stretching factor of 1 22 for the whole length of the preserved margin the strain rate is ca 1 10 16 s the sedimentation history suggests that the extension of the south alpine margin was a continuous process from the norian to the middle jurassic the changes in tectonic pattern are related to progressive hardening of the lithospheric segment undergoing slow extension

3 d velocity model beneath the middle lower yangtze river and its implication to the deep geodynamics abstract a mineralization zone exists in the middle and lower yangtze river mlyr region previous studies have shown that this zone might be caused by the rich ejection of magma in the mesozoic we have applied the teleseismic tomography method to determine a 3 d p wave velocity structure of the mantle down to 500 km depth beneath this region by using 14 740 p wave arrival times collected from 519 teleseismic events recorded by 46 portable and 47 static seismic stations located in this region the relative residual times used for the tomography are calculated by the modified multi channel cross correlation method which increases not only the efficiency but also the data precisions up to 0 01 s the grid space is set as 1 1 horizontally and 50 100 km vertically our tomographic results show that the lithosphere with high velocity anomalies is separated into two parts one locates above the depth of 100 km and the other at the depths between 250 km and 400 km and the asthenosphere with low velocity anomalies locates between these two parts of lithosphere which indicates that the lithosphere delaminated together with the asthenosphere upwelling the detachment of lithosphere might relate to the subduction of the western pacific slab therefore our study provides clear evidence to geoscientists for understanding the deep dynamic process beneath this region and a three stage geodynamic process is developed based on the new evidence

3 d lithospheric structure beneath southern tibet from rayleigh wave tomography with a 2 d seismic array summary a 3 d shear wave velocity model has been established for the lithosphere of southern tibet through rayleigh wave tomography the teleseismic data are acquired from a 2 d seismic array selected out of the second phase of the hi climb project we first inverted for a 2 d phase velocity model using two plane wave tomography approach and then inverted for the 3 d shear wave velocity model with the 2 d phase velocity model and the map of crustal thickness derived from receiver functions the results reveal a pervasive low velocity anomaly in the middle crust of the lhasa block we interpret this low velocity anomaly as the presence of wholesale mid crustal channel flow prominent low velocity anomalies from the lower crust to the mantle lithosphere are observed along the north south trending rifts the tangra yum co rift and the pumqu xianza rift we propose a possible scenario for the formation of these rifts the entire lithosphere is involved in the early stage rifting while the late stage rifting is restricted in the upper crust in the non rift regions a slab like high velocity anomaly is traced beneath both the himalayas and lhasa block combining all the observations we suggest that the central part of southern tibet is currently underlain by the broken indian lower crust and mantle lithosphere

current tufa sedimentation in a changing slope valley the river anamaza iberian range ne spain abstract a three year study of modern carbonate sedimentation was conducted through analysis of sedimentological and hydrochemical parameters measured every six months at 10 sites along a high slope river in northeastern spain river anamaza three stretches of the river were characterised the dominant water inputs from the upstream karstic springs primarily from the jurassic rock aquifer determined the so4 hco3 ca composition of the river water from this area decreasing trends in alkalinity calcium and total dissolved inorganic carbon occurred downstream in both the warm and cool periods as a result of calcite precipitation tufa thickness variations were consistent with such hydrochemical evolution deposition rates increased downstream primarily where the gradient is steeper middle stretch and subsequently decreased at the downstream gently sloped stretch therefore the slope along the river and the distance from the main upstream springs conditioned the spatial distribution of tufa deposits by determining the chemical characteristics of the water the monitored sites represent four primary fluvial subenvironments with distinct sedimentary facies the highest carbonate deposition rates were measured in fast flow conditions stromatolites facies a deposits with calcite coated algae and mosses facies c formed in cascades and small jumps with very fast flowing water exhibited lower deposition rates and typical erosion lower rates corresponded to slow flow conditions in which thin stromatolite crusts and or filamentous and calcareous algae and tufa fragments facies b accumulated in slow flowing water areas near springs upstream stretch calcite was absent linked to the higher pco2 of water microbial films and aquatic flora composed the biofacies facies d in such subenvironment therefore tufa deposition rates in each fluvial subenvironment were controlled by the amount of co2 outgassing linked to local flow conditions the biological substrate type and the chemical attributes of the water along each stretch a six month pattern of deposition rates best marked in the steeper stretch was recorded from thickness measurements spring summer periods 4 30 mm autumn winter periods 1 22 mm parallel variations were recorded using pwp plummer wigley parkhurst and calcite mass balance calculations multivariate statistical analyses hca and pca clearly separated hydrochemical and sedimentological attributes in samples from warm and cool periods exceptions to this pattern were related to changes in the water discharge high discharge events caused the dilution of the chemical composition and erosion of the tufa deposits therefore temperature dependent factors i e physico chemical and biological processes coupled with discharge changes controlled the seasonal variations in deposition rates the stable isotope composition of calcite and river water also reflected the discharge effects however the seasonal variation signature of temperature was preserved in the 18o of sediment

broadband ambient noise surface wave tomography across the united states 1 this study presents surface wave dispersion maps across the contiguous united states determined using seismic ambient noise two years of ambient noise data are used from march 2003 through february 2005 observed at 203 broadband seismic stations in the us southern canada and northern mexico cross correlations are computed between all station pairs to produce empirical green functions at most azimuths across the us coherent rayleigh wave signals exist in the empirical green functions implying that ambient noise in the frequency band of this study 5 100 s period is sufficiently isotropically distributed in azimuth to yield largely unbiased dispersion measurements rayleigh and love wave group and phase velocity curves are measured together with associated uncertainties determined from the temporal variability of the measurements a sufficient number of measurements 2000 is obtained between 8 and 25 s period for love waves and 8 and 70 s period for rayleigh waves to produce tomographic dispersion maps both phase and group velocity maps are presented in these period bands resolution is estimated to be better than 100 km across much of the us from 8 40 s period for rayleigh waves and 8 20 s period for love waves which is unprecedented in a study at this spatial scale at longer and shorter periods resolution degrades as the number of coherent signals diminishes the dispersion maps agree well with each other and with known geological and tectonic features and in addition provide new information about structures in the crust and uppermost mantle beneath much of the us

surface wave array tomography in se tibet from ambient seismic noise and two station analysis ii crustal and upper mantle structure summary we determine the 3 d shear wave speed variations in the crust and upper mantle in the southeastern borderland of the tibetan plateau sw china with data from 25 temporary broad band stations and one permanent station interstation rayleigh wave phase velocity dispersion curves were obtained at periods from 10 to 50 s from empirical green s function egf derived from ambient noise interferometry and from 20 to 150 s from traditional two station ts analysis here we use these measurements to construct phase velocity maps from 10 to 150 s using the average interstation dispersion from the egf and ts methods between 20 and 50 s and estimate from them with the neighbourhood algorithm the 3 d wave speed variations and their uncertainty the crust structure parametrized in three layers can be well resolved with a horizontal resolution about of 100 km or less because of the possible effect of mechanically weak layers on regional deformation of particular interest is the existence and geometry of low shear velocity layers lvls in some regions prominent lvls occur in the middle crust in others they may appear in the lower crust in some cases the lateral transition of shear wave speed coincides with major fault zones the spatial variation in strength and depth of crustal lvls suggests that the 3 d geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity

mantle dynamics and cretaceous magmatism in east central china insight from teleseismic tomograms abstract both the rich mineralization in the lower yangtze block lyb and the post collisional mafic rocks in the dabie orogen dbo are closely related to the cretaceous magmatism in east central china various geodynamic models have been proposed for explaining the mechanism of the cretaceous magmatism but these models are controversial and even contradictory with each other especially on the mechanism of adakites a unified geodynamic model is required for explaining the magmatism in east central china in particular the spatial and temporal correlations of magmatic activity in the dbo and that in the lyb for this purpose we apply teleseismic tomography to study p wave velocity structure down to 800 km depth beneath east central china a modified multiple channel cross correlation method is used to collect 28 805 high quality p wave arrival time data from seismograms of distant earthquakes recorded by permanent seismic stations and our temporary stations in the study region to remove the influence of crustal heterogeneity on the mantle tomography we used the crust1 0 model to correct the teleseismic relative residuals our tomography revealed distinct high velocity high v anomalies beneath the dbo and two flanks of the lyb and low velocity low v anomalies above the high v zones combining our tomographic images with previous geological geochemical and geophysical results we infer that these high v and low v anomalies reflect the detached lithosphere and upwelling asthenospheric materials respectively which are associated with the late mesozoic dynamic process and the cretaceous magmatism we propose a double slab subduction model that a ridge subduction yielded the adakitic rocks in the lyb during 150 135 ma and the subsequent pacific plate subduction played a crucial role in not only the formation of igneous rocks in the lyb but also remelting of the subducted south china block beneath the dbo during 135 101 ma

crustal thickness and poisson s ratios of south china revealed from joint inversion of receiver function and gravity data abstract crustal thickness and poisson s ratio are two important parameters for understanding the crustal structure composition and deformation of the south china block due to the complicated crustal structure and poor signal to noise ratio of teleseismic data at dozens of stations in south china the receiver function h stacking produces large uncertainty and deviation when estimating these two crustal parameters in this paper we improve the technique of the joint inversion of receiver function and gravity data and utilize it to improve the estimates of crustal thickness and poisson s ratio in south china we calculate the receiver functions of 245 permanent seismic stations from the teleseismic data recorded during 2013 2015 and then carry out the joint inversion on the receiver functions and the complete bouguer gravity anomalies data the joint inversion reduces the uncertainty of each single method inversion and enhances the accuracy of estimation our results demonstrate that the crustal thickness ranging 26 1 46 5 km is large in the nw 38 46 5 km and small in the se 26 34 km with a nne trending gradient zone along the line of yichang jishou baise in central south china the crustal poisson s ratio ranging 0 20 0 31 is high 0 28 0 31 in most of the northern south china intermediate 0 26 0 29 around the eastern coastal area very low 0 20 0 24 within the jiangnan orogenic belt and around the southern coastal area and slightly low 0 22 0 26 in other places with the constraints of previous geological and geophysical studies we suggested that the notable low value belt of crustal poisson s ratio between the lines of shitai jiujiang yiyang jishou baise and shaoxing jiangshan pingxiang yongzhou guigang beihai represents the possible suture zone between yangtze and cathaysia sub blocks in south china during the neoproterozoic

the lower changjiang yangzi yangtze river metallogenic belt east central china intrusion and wall rock hosted cu fe au mo zn pb ag deposits abstract the lower valley of changjiang from wuhan of the hubei province in the west to zhenjiang of the jiangsu province in the east contains more than 200 polymetallic cu fe au mo zn pb ag deposits and is one of the most important metallogenic belts in china this metallogenic belt situated at the northern margin of the yangzi craton and bordered by the dabieshan ultrahigh pressure metamorphic belt to the north consists mainly of cambrian triassic marine clastic sedimentary rocks and carbonate and evaporite rocks which overlay a precambrian basement and are intruded by yanshanian 205 to 64 ma granitoid intrusions and subvolcanic complexes repeated tectonism from late proterozoic to triassic resulted in extensively developed networks of faults and folds involving the cambrian triassic sedimentary strata and the precambrian basement the yanshanian granitoid intrusions and subvolcanic complexes in the lower changjiang metallogenic belt are characterized by whole rock 18 o of 8 to 10 initial 87 sr 86 sr of 0 704 to 0 708 and ndt from 10 to 17 and have been interpreted to have originated from mixing between juvenile mantle and old crustal materials also the yanshanian granitoids exhibit eastward younging and increase in alkalinity i e from older calc alkaline in the west to younger subalkaline alkaline in the east which are related to oblique collision between the yangzi and sino korean cratons and tectonic evolution from early compressional to late extensional or rifting regimes most polymetallic deposits in the lower changjiang metallogenic belt are clustered in seven districts where the yanshanian magmatism is particularly extensive from west to east edong jiurui anqing guichi luzhong tongling ningwu and ningzhen mineralization is characterized by the occurrence of three distinct types of orebodies in individual deposits orebodies in yanshanian granitoid intrusions skarn orebodies at the contact zones between the yanshanian intrusions and late paleozoic early mesozoic sedimentary rocks and stratabound massive sulfide orebodies in the late paleozoic early mesozoic sedimentary strata the most important host sedimentary strata are the middle carboniferous huanglong formation lower permian and lower middle triassic carbonate and evaporite rocks the intrusion hosted and skarn orebodies exhibit well developed zonation in alteration assemblages metal contents and isotopic compositions within individual deposits and apparently formed from hydrothermal activities related to the yanshanian magmatism the stratabound massive sulfide orebodies in the late paleozoic early mesozoic sedimentary strata have long been suggested to have formed from sedimentary or volcano sedimentary exhalative processes in shallow marine environments however extensive research over the last 40 years failed to produce unequivocal evidence for syngenetic mineralization on the basis of geological relationships and isotope geochemical characteristics we propose a carbonate hosted replacement deposit model for the genesis of these stratabound massive sulfide orebodies and associated skarn orebodies this model suggests that epigenetic mineralization resulted from interactions between magmatic fluids evolved from the yanshanian intrusions with carbonate and evaporite wall rocks mineralization was an integral but distal part of the larger hydrothermal systems that formed the proximal skarn orebodies at the contact zones and the intrusion hosted orebodies the stratabound massive sulfide deposits of the lower changjiang metallogenic belt share many features with the well studied high temperature carbonate hosted replacement deposits of northern mexico and western united states particularly with respect to association with small shallow granitoid complexes structural and stratigraphic controls on mineralization alteration assemblages geometry of orebodies metal association metal zonation and isotopic systematics

detailed three dimensional shear wave velocity structure of the northwestern united states from rayleigh wave tomography abstract since the mid miocene the northwestern united states has experienced extensive flood basalt volcanism followed by the formation of two time progressive tracks of silicic volcanism the yellowstone snake river plains ysrp and the high lava plains hlp the ysrp track progresses towards the northeast parallel to north american plate motion and has therefore often been attributed to a deep mantle plume source however the hlp track progresses to the northwest over the same time frame in a direction not consistent with any regional plate motion the causes of the mid miocene flood basalts and the tracks of the ysrp and hlp are a matter of ongoing debate we present results of rayleigh wave phase velocity inversions and inversions for 3 d shear wave velocity structure of the northwestern united states using data collected from the high lava plains seismic experiment and the earthscope usarray transportable array ta the large number of stations used in these inversions allows us to show an unprecedented level of detail in the seismic velocity structures of this tectonically complex area our velocity images indicate that low s wave velocities in the uppermost mantle do not well match the track of hlp volcanism while at the surface the newberry caldera appears to anchor the nw end of the hlp hotspot track the seismic results show that it lies in a separate north south trending low velocity band just east of the cascades that is distinct from the main hlp trace the ultra low s wave velocities beneath the ysrp track extend locally to at least 175 km depth and are by far the most prominent seismic anomalies in the region along axis the ysrp hotspot track is characterized by a discrete low velocity channel in the upper mantle that shallows narrows and intensifies to the northeast but then deepens rapidly to the north beneath yellowstone the shallowing of the low velocity anomaly to the northeast is consistent with a stationary heat source beneath a moving plate coupled with basin and range extension and heating

the subduction and continental collision of the north china and yangtze blocks magnetotelluric evidence from the susong anqing section of western anhui china

deciphering processes controlling mid jurassic coccolith turnover the middle jurassic is characterized by major changes within the fossil coccolithophorid community with a transition from lotharingius dominated to watznaueria dominated assemblages concomitant with a significant increase in the pelagic carbonate production the mechanisms that triggered this overturn remain poorly understood here we present a compilation new and previously published data of lotharingius and watznaueria abundances through the early middle jurassic transition alongside this trends in newly acquired and literature derived carbon and oxygen isotope data were used to represent paleoceanographic indicators such as nutrient and temperature changes the nannofossil data show a rapid around 1 5 myr turnover around the aalenian bajocian transition across the aalenian bajocian boundary assemblages dominated by lotharingius spp give way to assemblages dominated by watznaueria spp coinciding with a peak in a particular morphological group of watznaueria species with a cross in the central area the proliferation of this morphogroup occurred during a time of oceanic opening and rearrangement of ocean circulation this led on to the evolution of pioneering coccolithophorid taxa but also to extinctions in several marine groups in the early bajocian the proliferation of two other morphogroups watznaueria without a central area structure and watznaueria with a bar corresponds to the major diversification of watznaueria and the beginning of its meso zoic dominance the watznaueria diversification and dominance are associated with radiation in other marine groups and these biotic changes occurred during a time of putative enhanced oceanic fertility and relatively low temperatures this study suggests that restructuring of fossil coccolithophorid communities may be favored during short turnover intervals related to major paleoceanographic change

sedimentary and biological response to sea level and palaeoceanographic changes of a lower middle jurassic tethyan platform margin southern alps italy abstract the sediment succession formed at the platform to basin transition along the western margin of a jurassic tethyan carbonate platform records a cyclicity in the order of 3 10 my these cycles are interpreted as due to sea level fluctuations associated to palaeoceanographic changes of the surface waters as inferred by the evolutionary dynamics of the calcareous nannofossils overimposed to these variations the skeletal composition of the platform succession records a longer term order of 50 my trend of progressive environmental deterioration related to the break up of pangea our case study deals with the venetian platform margin and the lombardian basin of the southern alps italy a lower to middle jurassic platform basin system of the western tethys four depositional sequences have been recognized in the lower to middle jurassic the local sea level curve derived from sequence analysis records a short lived on the venetian platform the skeletal sediment composition of the carbonate platform changed from sinemurian domerian photozoan to toarcian heterozoan assemblages dominated by suspension feeders this change appears due to increased trophic resources the older event of platform demise correlates to the sea level drop of the latest domerian the final drowning of the venetian platform occurred after the early aalenian drop in the sea level among the basin biota calcareous nannofossils represent the major pelagic carbonate producer since the beginning of the jurassic nevertheless the sinemurian bajocian deep sea carbonates of the lombardian basin consist mainly of allochthonous mud exported off platform with a low nannoplanktonic contribution moreover the platform demise correlates with condensation gaps and mass flows in the basin this relationship suggests that the growth history of the carbonate platform directly controlled the carbonate sedimentation in the adjacent basin the nannofloras recorded in the lower and middle jurassic successions of the lombardian basin consist of two different taxonomic groups the genus schizosphaerella a group of uncertain systematic affinity and several genera of coccolithophorids an inverse correlation between the abundance of schizospheres and coccoliths has been recorded throughout the whole interval investigated moreover during periods of platform and sea level fall schizosphaerella spp dropped in abundance whereas rates of coccolith diversification calculated according to roth s equations reached peak values our evidence shows that schizospheres could be oligotrophic forms while coccolithophorids on the contrary probably diversified during periods of increased nutrient resources the observed sea level falls are in phase with increased trophic resources and holds on up to the transgressive interval the biological response includes a dramatic drop of the platform carbonate productivity and increased corg accumulation on the sea floor the long term evolution of the sedimentary system records increasing eutrophication from liassic to dogger as reflected by the compositional changes of the carbonate producers

origin of mesozoic adakitic intrusive rocks in the ningzhen area of east china partial melting of delaminated lower continental crust to the best of our knowledge modern adakites have not been documented in a nonarc environment we report geochemical and isotopic data for early cretaceous anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction the anjishan adakitic intrusive rocks which are exposed in the ningzhen area of east china have high sr y and la yb ratios coupled with low yb and y as well as relatively high mgo contents and mg numbers mg 0 4 0 6 similar to products from slab melting however low n d t values 6 8 to 9 7 and high 8 7 sr 8 6 sr i 0 7053 0 7066 are inconsistent with an origin by slab melting the tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust high sr y and la yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting implying that the crustal thickness exceeded 40 km in the early cretaceous the present thickness of the crust in the ningzhen area is only 30 km and therefore the crust appears to have been thinned by at least 10 km since the early cretaceous the relatively high mgo contents and mg of the anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks possibly coinciding with lower crustal delamination which would also account for the observed thinning

a tectono genetic model for porphyry skarn stratabound cu au mo fe and magnetite apatite deposits along the middle lower yangtze river valley eastern china abstract the middle lower yangtze river valley metallogenic belt yrb situated along the northern margin of the yangtze craton is characterized by porphyry skarn stratabound cu au mo fe deposits in the areas of uplift and magnetite apatite deposits in cretaceous fault basins following detailed field investigations and a review of published data we recognize two episodes of magmatism and mineralization in the yrb 1 156 137 ma high k calc alkaline granitoids associated with 148 135 ma porphyry skarn stratabound cu au mo fe deposits and 2 135 123 ma shoshonitic series associated with 134 9 122 9 ma magnetite apatite deposits a type granitoids and associated alkaline volcanic have a small age range from 126 5 to 124 8 ma and are temporally spatially and genetically associated with the second episode the geodynamic history of the yrb did not experience the paleozoic to mesozoic lithospheric thickening that took place in the north china craton this process is inferred to be linked to partial melting of the delaminated lower crust at high pressures resulting in the development of c type adakitic rocks the petrochemical and sr nd isotopic data show that both the shoshonitic series and a type granitoids are quite different from adakites with only some of the k calc alkaline granitoids having adakitic signatures previous ore genesis models were established based on an assumed relationship with adakites and a continuous tectono thermal evolution from 150 to 100 ma all data obtained for the middle lower yangtze river region consistently show that the tan lu regional strike slip fault zone initiated at 233 6 to 225 6 ma from the collision between the north china and yangtze cratons and was reactivated at ca 160 ma the tan lu fault was caused by the oblique subduction of the izanagi plate which along the yrb the low angle subducted slab and the overlying crust was disrupted or broken due to the disharmonious movement of the two blocks the high k calc alkaline granitoids magmas were derived from melting of the subducted slab with some input of crustal material these magmas were emplaced at the intersections between ne and ew trending faults and formed porphyry skarn stratabound cu au mo fe deposits between 156 and 137 ma after 135 ma the subducted plate changed its direction of motion to northeast now running parallel to the eurasian continental margin and leading to large scale continental extension the shoshonitic series and subsequent a type granitoids magmatism and the development of magnetite apatite ores in the yrb took place in both fault basins and ne trending rifts between 135 and 124 ma

crustal and uppermost mantle velocity structure and its relationship with the formation of ore districts in the middle lower yangtze river region abstract in this study we conduct ambient noise tomography and teleseismic two plane wave tomography to probe the crustal and uppermost mantle structures in the middle lower yangtze river region the data used include 14 months from july 2012 to august 2013 of continuous vertical component ambient noise data and 219 teleseismic earthquakes recorded at 138 broadband seismic stations from chinese provincial networks and 19 temporary seismic stations deployed by china university of geosciences beijing first we apply ambient noise tomography to the collected ambient noise data to generate rayleigh wave group and phase velocity maps at 5 42 s periods and two plane wave tomography method to earthquake data to generate intermediate to long period phase velocity maps at 20 143 s periods we then combine the short to intermediate period velocity maps from ambient noise and the intermediate to long period phase velocity maps from earthquake data to generate broadband phase velocity maps at periods from 5 to 143 s by inverting these 5 143 s rayleigh wave velocity maps we construct a 3 d shear wave velocity model from the surface to 250 km depth in the middle lower yangtze river region the 3 d model shows that in the upper crust the basin regions including the jianghan hehuai subei hefei and nanyang basins are all featured with low velocities and the mountain regions with high velocities in the uppermost mantle a low velocity zone at 100 200 km depth is observed beneath the middle lower yangtze river metallogenic belt moreover our tomographic results show that the ningwu and ningzhen ore districts are clearly characterized by the strongest low velocity anomaly in the uppermost mantle at 70 200 km depth the depth extent of the low velocity zone becomes shallower and the amplitude of low velocity anomaly becomes larger from the southwest jiurui ore district to northeast ningwu ore districts the change pattern of the low velocity zone in the depth extent and the amplitude is consistent with the fact that peak ages of magmatic events along the middle lower yangtze river metallogenic belt progressively become younger and younger from 148 ma in the southwest to 125 ma in the northeast the observed low velocity zone may represent the cooling hot upper mantle which was partially molten in the past resulting from partial melting of the paleo pacific plate or of an enriched mantle source induced by the westward subduction of the paleo pacific plate the upwelling of the mantle derived magmas may result in the formation of these granitic rocks and coeval ores deposits along the middle lower yangtze river metallogenic belt

ambient noise rayleigh wave tomography in western sichuan and eastern tibet abstract we perform rayleigh wave tomography in western sichuan and eastern tibet by applying the ambient noise method to broadband data recorded at 49 stations from the sichuan digital seismic network cross correlations of vertical component ambient noise data are computed in one hour segments and stacked over 7 months from june to december 2007 then rayleigh wave group dispersion curves from 6 to 48 s were measured using the multiple filter analysis method group velocity maps at periods from 8 to 44 s were inverted the resulting group velocity maps generally show good correlations with tectonic features the sichuan basin is clearly resolved with lower group velocities at the short periods 8 to 20 s due to thick sedimentary layers and a prominent low velocity zone exhibits in the middle crust beneath the eastern margin of the tibetan plateau the longmen shan thrust belt marks not only the rapid lateral velocity variation between the topographically high tibetan plateau and low elevation sichuan basin but also the transition from the thick crust beneath the tibetan plateau to the normal continental crust beneath the sichuan basin

crust and uppermost mantle beneath the north china craton northeastern china and the sea of japan from ambient noise tomography 1 a 3 d shear velocity model of the crust and uppermost mantle to a depth of 100 km is presented beneath the north china craton ncc northeastern china the korean peninsula and the sea of japan ambient noise rayleigh wave tomography is applied to data from more than 300 broadband seismic stations from chinese provincial networks cearray the japanese f net and the iris global seismic network continuous data from 2007 to 2009 are used to produce group and phase velocity maps from 8 s to 45 s periods the model is motivated to constrain the distributed intraplate volcanism crustal extension cratonic rejuvenation and lithospheric thinning that are hypothesized for the study region numerous robust features are observed that impose new constraints on the geometry of these processes but discussion concentrates only on four 1 the north south gravity lineament follows the 40 km contour in crustal thickness and crustal thickness is anticorrelated with water depth beneath the sea of japan consistent with crustal isostasy for a crust with laterally variable composition 2 the lithosphere is thin 70 km beneath the songliao bohai graben but seismically fast 3 even thinner more attenuated lithosphere bounds three sides of the eastern ncc in a horseshoe shape identifying a region of particularly intense tectonothermal modification where lithospheric rejuvenation may have reached nearly to the base of the crust 4 low velocity anomalies reach upward in a y shape in the mantle beneath the eastern and western borders of the sea of japan extending well into continental east asia in the west and are separated by a 60 km thick lithosphere beneath the central sea of japan this anomaly may reflect relatively shallow slab dehydration in the east and in the west may reflect deeper dehydration and convective circulation in the mantle wedge overlying the stagnant slab

from platform to basin to swell orbital control on sedimentary sequences in the oxfordian spain climatic oceanographic and ecological changes that control the formation and deposition of sediment in shallow and deep depositional environments commonly occur with periodicities of a few 10 000 years consequently in order to interpret sedimentary sequences in the geological past high time resolution is required this is best obtained by cyclostratigraphy three sections have been studied in the oxfordian of north eastern spain one represents a shallow siliciclastic carbonate platform with repetitive subaerial exposures one an intraplatform basin with sponge bioherms and one a swell where iron ooids and glauconite formed the platform section displays a well defined stacking pattern of depositional sequences the deeper water sections are well dated by ammonites the correlation between the three sections is a best fit solution integrating biostratigraphy sequence stratigraphy and cyclostratigraphy it is concluded that the small scale depositional sequences formed in tune with the 100 ka orbital eccentricity cycle an additional factor was differential subsidence that ruled basin morphology

late jurassic climate and its impact on carbon cycling abstract the climate of the late jurassic has been characterized by high atmospheric co2 levels and by a monsoonal rainfall pattern in this study we traced the evolution of the global carbon cycle through the late jurassic with the help of carbonate carbon isotope stratigraphy the oxfordian tithonian 13c curve is marked by one major positive carbon isotope excursion with an amplitude 13 c 1 0 middle to late oxfordian and a second minor positive excursion with an amplitude 13 c late kimmeridgian the early kimmeridgian and early tithonian 13c values fluctuate around 13 c 2 0 3 and contrast with the less positive 13c values of early oxfordian and late tithonian age 13 c 1 0 5 a comparison of the late jurassic carbonate carbon isotope curve with the occurrence of organic rich sediments suggests that not only fluctuations in organic carbon burial but also in carbonate carbon burial had an impact on the c isotope record the oxfordian c isotope excursion appears to correspond to a time of overall increased organic carbon burial triggered by increased nutrient transfer from continents to oceans during a time of rising global sea level however episodes of enhanced organic carbon burial during the kimmeridgian and early tithonian are not reflected by prominent spikes in the c isotope record favourable conditions for carbonate platform growth at a time of high global sealevel may have resulted in the stabilisation of the c org c carb burial ratio and hence maintained the 13c record at steady but relatively positive values the middle and late tithonian c isotope values drop below 13 c 1 5 a similar shift to less positive c isotope values was recognized in other c isotope records from the tethys and atlantic oceans and reflects a decrease in the c org c carb burial ratio possibly related to a reorganisation of the global climate system globally widespread marine black shale mature quartzose sandstone assemblages suggest that the relative efficiency of the late jurassic carbon pumps was controlled by weathering erosion and runoff causing widespread marine eutrophication eutrophication favoured the organic carbon pump but it diminished the carbonate platform growth potential the monsoonal rainfall distribution pattern may explain why late jurassic carbonate platforms experienced less severe growth crises than early cretaceous carbonate platforms

the structure of the crust and uppermost mantle beneath south china from ambient noise and earthquake tomography summary two years of continuous recordings of ambient seismic noise observed at 354 stations in south china from 2009 to 2010 are used to estimate rayleigh wave group and phase velocity maps from 6 to 40 s period these results are merged with rayleigh wave phase velocity maps from 25 to 70 s period derived from earthquakes in the same time frame eikonal tomography generates the dispersion maps which by monte carlo inversion are used to estimate a 3 d vsv model of the crust and upper mantle down to a depth of 150 km across all of south china with attendant uncertainties a clear image emerges of the west yangtze block a region of the western yangtze craton characterized by relatively thick crust 40 km overlying a seismic mantle lithosphere that extends to at least 150 km that may have been the nucleus for the formation of the yangtze craton in the archean and may present a present day obstacle to the eastward expansion of tibet the west yangtze block contrasts with the thinner crust 30 km and mantle lithosphere 70 80 km beneath the eastern yangtze craton and south china foldbelt these observed differences are consistent with processes associated with flat slab subduction in the mesozoic that may have eroded the lithosphere of the eastern yangtze craton and the south china foldbelt

crustal structure beneath the middle lower yangtze metallogenic belt in east china constraints from passive source seismic experiment on the mesozoic intra continental mineralization abstract to understand the formation and the tectonic process of the mesozoic middle lower yangtze metallogenic belt ymb the sinoprobe program deployed a quasi linear passive source seismic array across the belt we performed receiver function profiling and measurement of shear wave splitting parameters with the collected data our results show that the moho depth varies significantly along the profile and that a mantle uplift exists right beneath the ymb we also found that the lower crust of the ymb is different from that of its adjacent areas in structure on the receiver function profile it possesses seismic anisotropy with direction roughly parallel to the belt our sks skks shear wave splitting results also show similar belt parallel azimuthal anisotropy right beneath the ymb we interpret the seismic anisotropy in the lower crust of the ymb as the result of mineral crystal alignment caused by melting and belt parallel flow in the mesozoic ore forming process besides we observed a nearly south dipping converter extending from shallow to lower crust beneath the hefei basin which most possibly resulted from the mesozoic crustal extension we interpret the mantle uplift and the crustal extensional structures to be consequences of asthenospheric upwelling during the mesozoic ore forming process our results suggest that the lower crust of the ymb was most likely one part of the multi level metallogenic magma system in the mesozoic magmatism and mineralization processes and the formation of the metallogenic belt to be a result much similar to the mash melting assimilation storage and homogenization cf hildreth and moorbath 1988 richards 2003 process first the asthenospheric upwelling resulted in a crustal extensional environment then the melts from the upwelling asthenosphere intruded into the lower crust of the ymb and assimilation occurred when they mixed with in situ lower crustal materials which led to the formation of adakitic like magma the adakitic like magma rose up along extensional structures and reacted with country rocks to form mineral deposits

crustal and uppermost mantle velocity structure beneath northwestern china from seismic ambient noise tomography summary in this paper we conduct ambient noise seismic tomography of northwestern china and adjacent regions the data include 9 months 2009 january to 2009 september three component continuous data recorded at 146 seismic stations of newly upgraded china provincial digital seismic networks and regional kyrgyzstan and kazakhstan networks empirical rayleigh and love wave green s functions are obtained from interstation cross correlations group velocity dispersion curves for both rayleigh and love waves between 7 and 50 s periods were measured for each interstation path by applying the multiple filter analysis method with phase matched processing the group velocity maps show clear lateral variations which correlate well with major geological structures and tectonic units in the study region shear wave velocity structures are inverted from rayleigh wave and love wave dispersion maps the results show that the tibetan plateau has a very thick crust with a low velocity zone in its mid lower crust along the northern margin of the plateau where a steep topographic gradient is present the low velocity zone does not extend to the tarim basin which may indicate that crustal materials beneath the tarim basin are colder and stronger than beneath the plateau therefore inhibit the extension of mid lower crustal flow and deformation of the tibetan plateau resulting in very sharp topography contrasts in the northeastern margin with a gentle topographic gradient toward the ordos platform the low velocity zone diminishes around the eastern kunlun fault meanwhile our results reveal obvious lateral velocity changes in the crust beneath the tarim basin in the upper crust the manjaer depression in the eastern tarim basin is featured with very low velocities and the bachu uplift in the western tarim basin with high velocities in the mid lower crust the northern tarim basin in general displays lower velocities than the southern part along latitude 40 n with an east west striking which is consistent with the high magnetic anomaly zone and may be related to the central suture belt connecting the south and north of tarim basement blocks together in pre sinian

progress in deep lithospheric exploration of the continental china a review of the sinoprobe abstract the sinoprobe deep exploration in china is a multidisciplinary earth science research program that aims at revealing the composition structure and evolution of the continental lithosphere in china the sinoprobe 2008 2012 has successfully conducted research and field experiments on determining the crustal and mantle structures using new deep seismic and magnetotelluric mt exploration this has allowed the accumulation of new exciting data that have significantly accelerated china s development on deep lithospheric exploration the new data also led to new understandings on the mesozoic and cenozoic geological evolution of the continental china the main results of the sinoprobe so far include 1 a collection of ca 6000 km long seismic reflection profile data 2 a nation wide geochemical baseline 3 a nation wide 4 4 mt array and regional 1 1 mt arrays in the north china and the qinghai tibet plateau 4 three dimensional 3 d exploration of ore districts in the eastern china 5 several continental scientific drilling holes 6 regional in situ stress monitoring networks 7 geodynamic modeling of the lithosphere underneath the continental china and 8 instrumentation development for deep exploration in china etc for the first time the sinoprobe has obtained deep seismic reflection evidence for the moho surface below the thick crust of the central qinghai tibet plateau it also reveals dipping fabrics in the lithospheric mantle beneath the northeastern china the preliminary results from mt array observation of the sinoprobe show an abnormal electric conductivity structure of the lithosphere beneath the ordos basin providing important evidence for the evolution mechanism of the north china craton ncc generally the sinoprobe has conducted successfully during its initial phase 2008 2012 which has settled a foundation for the next phase of the sinoprobe in the future

late mesozoic magmatism of the jiurui mineralization district in the middle lower yangtze river metallogenic belt eastern china precise u pb ages and geodynamic implications abstract the middle lower yangtze river metallogenic belt mlyrmb extending from daye in hubei province in the west to zhenjiang in jiangsu province in the east hosts a number of large polymetallic cu au mo fe zn pb and ag deposits and constitutes one of the most important metallogenic belts in china the cu au mo deposits in the jiurui district are an important component of the mlyrmb in this study we carried out precise and detailed zircon u pb dating for all types of magmatic rocks from the wushan ore deposit in the jiurui district three samples of cu au mo related porphyries from different ore belts at wushan were analyzed and yielded zircon u pb ages of 148 0 1 0 ma 145 4 0 9 ma and 147 3 0 9 ma respectively a series of dykes were emplaced immediately following the cu au mo related porphyries at wushan a dark colored basic dyke which intruded into the granodiorite porphyry at wujia gold deposit near wushan was dated at 144 5 1 2 ma two lamprophyre dykes taken from the north ore belt at wushan underground mining stops were dated at 143 6 0 9 ma and 144 3 0 9 ma respectively a late stage dyke which was also taken from the wushan north ore belt yielded an age of 142 6 1 0 ma and might represent the end of magmatism in the wushan ore deposit these new geochronological data demonstrate that the time range of magmatism in the wushan ore deposit is approximately between 148 ma and 143 ma showing that the magmatic activity at wushan was rapid and intensive the ages of cu au mo related porphyries from other areas in the jiurui district such as the dongleiwan yangjishan and chengmenba ore deposits were also measured and yielded zircon u pb ages of 141 5 1 7 ma 143 4 1 4 ma and 146 6 1 0 ma respectively combined with those previously reported zircon u pb age results from the jiurui district the present age data set demonstrates that extensive magmatism in the jiurui district was coeval and intensive marked by a magmatic activity in the age range of 148 to 138 ma and peaked between 148 ma and 142 ma according to the statistics of all those precise zircon u pb ages the cu au mo related porphyries in the edong and tongling districts in the mlyrmb show similar ages and they have a slightly younger peak age and a longer duration than that of the jiurui district the geographic shape of the mlyrmb in the cretaceous shows an arcuate structure the jiurui district is located at the transitional point of the arcuate structure and the edong and tongling districts are situated on both sides of the arcuate structure considering that the jiurui district has a slightly older peak age and a shorter duration of magmatic activity than that in the edong and tongling districts it seems that the arcuate structure of the mlyrmb played an important role in the formation of these cu au mo related porphyries consequently we suggest that the genesis of the late mesozoic magmatic rocks along the mlyrmb may have been due to a tectonic activity developed from southeast to northwest which probably has a close relationship with the subduction of the paleo pacific plate beneath the eurasian plate in mesozoic times

magnetic stratigraphy of kimmeridgian aptian sections from umbria marche italy new details on the m polarity sequence 1 the magnetic stratigraphy of the maiolica and calcari ad aptici pelagic limestones is documented from four umbro marchean land sections italy the overall biostratigraphic age of the sampled strata encompasses the whole kimmeridgian lower aptian the data were gathered from two new sections arcevia contessa and by resampling more densely than in the past two additional classical sections bosso gorgo a cerbara a detailed record of the polarity chrons m21n m14 and m9 m0 was derived and no difference was observed with respect to the sequence of m polarity chrons inferred from oceanic magnetic anomalies barely measurable remanent magnetization from chron m14 to m10n at contessa concurred with previous evidence from bosso indicating that the maiolica from umbria marche is characterized by a recording gap during chrons m14n to m11 possibly due to diagenetic magnetite dissolution related to the late valanginian carbon isotope event at arcevia and bosso the boundary between maiolica and calcari ad aptici occurs at the top of chron m19n and in the upper part of chron m20n respectively showing that the onset of maiolica sedimentation may be diachronous by 1 5 2 myr along the umbria marche domain arcevia is the most expanded mid upper tithonian land section documented so far here the subchrons m20n 1 and m19n 1 are found to represent 3 2 and 10 1 in time of chrons m20n and m19n respectively evidence is also provided for a reversal excursion occurring within the m20n 1 subchron at bosso an excursion and a reversal excursion were found within chrons m16n and m16 respectively

a transition from carbonate shelf to pelagic basin environments of deposition rifting and depositional systems in the jurassic of northeastern tunisia jurassic deposition in the maghrebian tethys was governed by eustasy and rifting two periods were delineated 1 a carbonate shelf rhaetian early pliensbachian and 2 a platform basin complex early pliensbachian callovian the carbonate shelf evolved in four stages generating three sedimentary sequences j1 to j3 separated by boundary sea level falls drawdown exposure and local erosion sediment facies bear evidence of sea level rises and falls lateral changes in lithofacies indicate shoaling and deepening upward during the sinemurian a major pulse of rifting with an abrupt transition from carbonate shelf to pelagic basin environments of deposition marks the upper boundary of the lower pliensbachian carbonate shelf deposits this rifting episode with brittle fractures broke up the rhaetian early pliensbachian carbonate shelf and has created a network of grabens half grabens horsts and stacked ramps following this episode a relative sea level rise led to pelagic sedimentation in the rift basins with local anoxic environments that also received debris shed from uplifted ramp crests another major episode spanning the whole early pliensbachian bajocian is suggested by early brecciation mass flows slumps olistolites erosion pinch outs and sedimentary prisms a later increase in the rates of drifting marked a progress toward rift cessation during the late jurassic these jurassic carbonates with detrital deposits and black shales as the source rocks in northeastern tunisia may define interesting petroleum plays pinch out flanking ramps onlaps and structurally upraised blocks sealed inside grabens source rock maturation and hydrocarbon migration began early in the cretaceous and reached a maximum during the late tortonian pliocene atlassic orogeny