relative array analysis of upper mantle lateral velocity variations in southern california spatial averaging of short period teleseismic p wave arrivals across scarlet constrains upper mantle lateral velocity variations beneath southern california we compare dt d back azimuth and averaged arrival time estimates determined from the entire network for 154 events to the same parameters derived from small subsets of scarlet the resulting relative array diagrams and plots of net subarray delays computed for 171 overlapping subarrays exhibit large anomalies that vary smoothly as a function of subarray position implying substantial lateral heterogeneity under the network patterns of mislocation vectors for more than 100 subarrays permit identification of an east west striking high velocity antiform beneath the transverse ranges further constraints on both the lateral configuration and depth of the anomaly are obtained through thin lens analysis of the net delay data where all the time perturbations are assumed to originate at a single depth projection of 2798 net delays requires a lens deeper than 100 km a representative lens at 150 km depth features low velocities beneath the salton trough as well as a concentrated high velocity zone between 116 3 118 3 w and 33 7 34 8 n consistent with the relative array diagrams and the pkp delay contours of hadley and kanamori 1977 therefore the bulk of the high velocity material observed by hadley and kanamori may in fact lie at or deeper than 150 km rather than 40 100 km this possibility prompts renewed speculation regarding the relationship of this body to the plate tectonic history of the region

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

propagation of regional seismic phases in the indian shield constraints on crustal and upper mantle velocity models the regional seismic wave field in the central indian shield is studied including propagation characteristics of various prominent phases well recorded in the broadband seismograms of the 21 may 1997 jabalpur deep crustal focal depth 36 km earthquake m w 5 8 the seismograms covering a distance range out to about 1650 km although limited in number seem to well reveal the prominent phases including the p bar and lg as well as the high frequency pn and sn and their coda by detailed numerical simulations the generation and propagation characteristics of the p bar phase from earthquake sources at shallow and deep crustal depths are examined it is found that the p bar phase propagates efficiently at regional distances even for a deep crustal source depth the p bar phase is nevertheless very sensitive to the surface layer velocity structure extremely weak in the absence of a low velocity surface layer and also slightly weak in the presence of a relatively high velocity surface layer analyses of the jabalpur earthquake seismograms and the synthetics reveal that the p bar phase is well generated and propagates efficiently in the deccan traps v p 4 8 4 9 km sec covered regions i e even without a lower velocity surface layer surrounding the earthquake epicenter and the recording stations further the anatomy of the p bar phase is revealed by several computational models its time window composed of several multiple pmp reflections as well as pms conversions the principal boundaries being the moho and the free surface the first observations of high frequency pn and sn phases in the indian shield are recognized from the jabalpur earthquake seismograms by numerical simulations consistent with these observations including the coda a lamellae model with alternating high and low velocity layers for the structure of the indian continental uppermost mantle is inferred a complete synthesis of the regional seismograms is obtained based on the inferred models of the p and s wave velocity structure of the crust and upper mantle in the central indian shield comparison of the synthetics with the jabalpur earthquake seismograms reveals a consistent fit to various observations including the lg phase group velocity window 3 6 3 2 km sec the low pass filtered seismograms at the regional distances are further found to reveal coherent phases consistent with predicted reflection times from discontinuities in the upper mantle a plausible p velocity model inferred from these observations reveals velocity discontinuities at average depths of 230 320 430 and 680 km suggesting significant stratification of the upper mantle b region in the indian shield

syn collisional felsic magmatism and continental crust growth a case study from the north qilian orogenic belt at the northern margin of the tibetan plateau abstract the abundant syn collisional granitoids produced and preserved at the northern tibetan plateau margin provide a prime case for studying the felsic magmatism as well as continental crust growth in response to continental collision here we present the results from a systematic study of the syn collisional granitoids and their mafic magmatic enclaves mmes in the laohushan lhs and machangshan mcs plutons from the north qilian orogenic belt nqob two types of mmes from the lhs pluton exhibit identical crystallization age 430 ma and bulk rock isotopic compositions to their host granitoids indicating their genetic link the phase equilibrium constraints and pressure estimates for amphiboles from the lhs pluton together with the whole rock data suggest that the two types of mmes represent two evolution products of the same hydrous andesitic magmas in combination with the data on nqob syn collisional granitoids elsewhere we suggest that the syn collisional granitoids in the nqob are material evidence of melting of ocean crust and sediment the remarkable compositional similarity between the lhs granitoids and the model bulk continental crust in terms of major elements trace elements and some key element ratios indicates that the syn collisional magmatism in the nqob contributes to net continental crust growth and that the way of continental crust growth in the phanerozoic through syn collisional felsic magmatism production and preservation is a straightforward process without the need of petrologically and physically complex processes

melting of continental crust during subduction initiation a case study from the chaidanuo peraluminous granite in the north qilian suture zone the chaidanuo granite batholith is a similar to 500 km 2 homogeneous high level intrusion in the north qilian oceanic suture zone three types of enclaves have been recognized including 1 supracrustal xenoliths 2 biotite gneiss that represents restite of the upper crustal melting and 3 coeval mantle derived mafic magmatic enclave mme the batholith consists dominantly of peraluminous biotite monzogranite with sio2 69 73 wt and a cnk 1 05 1 28 and shows geochemical affinity with the upper continental crust e g enrichment of large ion lithophile elements liles k rb u th pb depletion of high field strength elements hfses nb ta ti p eu and sr and enriched sr sr 87 sr 86 i 0 731 to 0 744 nd epsilon nd t 6 0 to 7 1 isotopes zircon u pb dating indicates that this batholith formed at 516 505 ma coeval with the mmes 510 ma which represent the early stage of seafloor subduction in the north qilian suture zone the hf isotopic composition of the mme epsilon hf t 6 0 to 2 1 is more depleted than that of the host granite epsilon hf t 14 8 to 4 7 suggesting mixing of mantle and crust derived melts the inherited zircon cores in the host granite yield an age peak at similar to 750 ma with a few detritals of 591 1683 ma zircons from a biotite gneiss enclave yield a protolith age of 744 ma and a metamorphic melting event at 503 ma sr nd isotopic modeling reveals that the batholith was generated by melting of neoproterozoic granitic rocks with the addition of 10 18 mantle derived magma therefore peraluminous granite can be produced by melting of crustal materials heated by mantle derived magmas during the early stage of subduction initiation at the site of a prior passive continental margin c 2014 elsevier ltd all rights reserved

numerical modelling of lg wave propagation across the north sea central graben summary in an effort to study the effect of strong crustal lateral variations on the propagation characteristics of lg waves we perform a numerical modelling of these waves at 1 hz in a graben structure we use a coupled mode method and a simplified 2 d model of the north sea central graben we show that at 1 hz the energy reflected by the structure is negligible and that on the average only 20 per cent of the lg incoming energy is converted to waves propagating into the mantle the energy remaining in the lg wavetrain is subject to a strong redistribution with depth which has an effect at the surface of the model which is very dependent on source type and depth multiplying the surface amplitude by a factor of 0 6 for shallow sources and 0 9 1 4 for mid crustal sources on the average the phase of the wavetrain is shown to be stable enough with period to avoid a destructive interference of the signal crossing the graben perpendicularly or at an angle leads to equivalent results lg wave propagation appears to be surprisingly robust in crustal structures with strong lateral variations the large scale geometric features of the graben prove insufficient to explain the strong attenuation of the lg waves which has been observed in the north sea area

generation of peraluminous granitic magma in a post collisional setting a case study from the eastern qilian orogen ne tibetan plateau abstract early paleozoic peraluminous granites are abundant in the eastern part of the qilian orogen northeastern margin of the tibetan plateau a combined study involving geochronology whole rock geochemical and sr nd hf isotopic compositions for three early paleozoic peraluminous granitic plutons jishishan ledu and shichuan plutons from the eastern qilian orogen was carried out to evaluate the causes of chemical variations and generation mechanisms of peraluminous granitic magmas these granitic plutons have magma crystallization ages of 455 427 ma and are moderately to strongly peraluminous a cnk 1 03 1 18 geochemical and sr nd hf isotopic data indicate that they consist substantially of crust derived melts the jishishan and ledu peraluminous granites were dominantly produced by partial melting of precambrian orthogneisses the shichuan monzogranites with low hree contents e g yb 0 80 1 83 ppm and slightly negative end t 5 3 to 2 3 and positive ehf t 1 6 to 3 4 could be derived from immature crustal materials relatively high average zircon saturation temperatures 750 c for each pluton obvious negative eu anomalies eu eu 0 28 0 80 and low pb ba ratios 0 03 0 16 for the jishishan ledu and shichuan granites are consistent with crustal melting involving biotite breakdown under fluid absent conditions our results suggest that compositional variations of moderately to strongly peraluminous granitic magmas are mainly controlled by source compositions and melting conditions while the processes such as mixing with mantle derived magma fractional crystallization restite unmixing and peritectic assemblage entrainment were insignificant or only play secondary roles in their genesis late ordovician to middle silurian crustal anatexis in the eastern central qilian was probably linked with slab break off which may be an important mechanism in addition to lithospheric delamination for the generation of moderately to strongly peraluminous granites in a post collisional setting

post collisional plutonism with adakite like signatures the eocene saraycik granodiorite eastern pontides turkey the post collisional saraycik granodiorite intruded into a late paleocene to early eocene nappe pile that formed during collision of the pontides in the north and the anatolide tauride platform in the south leading to the formation of the izmir ankara erzincan suture a relatively shallow pluton intrusion depth 5 to 8 km was estimated from al in hornblende geobarometry and contact metamorphic assemblages the emplacement age is tightly constrained to 52 ma by two ar ar plateau and total fusion ages on biotite the main mass of the pluton consists of metaluminous to peraluminous biotite granodiorite and hornblende biotite granodiorite in addition up to 10 m thick dacitic and 25 cm thick aplitic dikes occur granodiorites and dacites show many close compositional similarities to high silica adakites from supra subduction zone settings but tend to be slightly more felsic and to have a higher aluminium saturation index chondrite normalized cn rare earth element patterns are characterized by high ratios of la yb cn concave upward shapes of the hree and a lack of significant eu anomalies in conjunction with relatively high abundances of ba and sr as well as low abundances of y hree and sc these patterns suggest a feldspar poor garnet amphibole rich fractionating mineral assemblage residue all samples have very similar nd sr isotopic characteristics regardless of rock type initial end values range from 0 3 to 1 2 and initial 87sr 86sr ratios from 0 70491 to 0 70529 it is suggested that the magmas formed by partial melting of mafic lower crust at elevated pressures 1 to 2 gpa

geochemistry and zircon u pb ages of adakitic rocks from the dulan area of the north qaidam uhp terrane north tibet constraints on the timing and nature of regional tectonothermal events associated with collisional orogeny abstract geochemistry and u pb ages of leucosomes and tonalites in a high pressure granulite unit of the dulan area have been determined to constrain the tectonothermal evolution related to collision and thickening of lower crust in the north qaidam mountains nqd leucosomes and tonalites show a marked chemical resemblance to adakites 1 high la yb and sr y and low y and hree 2 high al 2 o 3 and low mg values with obvious positive eu anomalies and 3 slightly positive e nd t values zircon u pb analysis of leucosomes and tonalites yielded 206 pb 238 u ages of 428 437 ma and 436 437 ma respectively which constrain the emplacement ages of the adakitic rocks petrological geochronological and geochemical characters indicate that the adakitic rocks may have been derived from partial melting of a thickened mafic lower crust 50 km suggesting a dominating source regime of synchronous high pressure mafic granulites contemporary magmatism in other units of the nqd shows evidence of a widespread tectonothermal event during early silurian 420 450 ma that includes metamorphism magmatism and anatexis related to collision and thickening of lower crust

in situ la mc icp ms and id tims u pb ages of bastnasite ce and zircon from the taipingzhen hydrothermal ree deposit new constraints on the later paleozoic granite related u ree mineralization in the north qinling orogen central china abstract the taipingzhen deposit is a newly discovered ree deposit in the north qinling orogen nqo central china the ree ores are hosted in the altered fracture zones of paleozoic strata and a granitic intrusion the ore minerals mainly include bastnasite ce ce co3 f parisite ce cace2 co3 3f2 and synchysite ce cace co3 2f and this deposit also contains some uncommon minerals such as tornebohmite ce ce2al sio4 2 oh gatelite ce cace3alal2mg si2o7 sio4 3o oh 2 fluocerite la laf3 and f rich cerite ce ce ca 9 mg sio4 3 sio3 oh 4f3 which are identified here for the first time in china to determine the timing of ree mineralization and explore possible genetic relationships between mineralization and granitic magmatism the u pb ages of bastnasite ce from ore and zircon from ore hosting granite are obtained the in situ la mc icp ms u pb age of the bastnasite ce provides a reliable 238u 206pb 207pb 206pb lower intersection point age of 409 16 ma and id tims analyses yield an u pb age of 407 8 3 3 ma the u pb dating of the magmatic zircon reveals that the wall rock was emplaced at 457 1 4 9 ma and the hydrothermal zircon age of 399 4 3 5 ma records the time of hydrothermal alteration thus indicating that the ree mineralization in the taipingzhen deposit occurred at approximately 409 400 ma this age coincides well with the u pb ages of the bastnasite ce and zircon revealing a genetic association between the hydrothermal fluid and the ree mineralization the hf isotopic compositions imply that the partial melting of paleoproterozoic hp and uhp eclogite from the lower crust provided ore materials for the mineralization and the deposit was generated in a retrograde metamorphic setting during the late paleozoic postcollisional extension event in the nqo furthermore combined with the geochronological data published for other u ree metallic deposits in the nqo our study indicates that the late silurian early devonian granite related u ree metallic mineralization throughout the nqo mainly occurred during 421 395 ma

attenuation of multiphase surface waves in the basin and range province part i lg and lg coda summary broad band data recorded by the lawrence livermore laboratory network are used to investigate whether or not lg q and lg coda q have similar values in the basin and range province of north america in order to retrieve spectral estimates of the broad band lg and lg coda signals a hybrid method is used which retains high fidelity at lower frequencies while strongly smoothing the spectral estimates and minimizing the number of spectral points at high frequencies a stochastic approach is used to model the lg signal and ambient noise and to analyse results from two station determinations of lg q in the basin and range the effects of ambient noise frequency dependent station site response and focusing defocusing on lg q inversion are analysed it is shown that after averaging spectral ratios determined from repeated observations over four two station paths a modest site effect will not cause significant errors in the resulting lg q relation using this inversion scheme and spectral ratio estimates averaged over those paths at frequencies between 0 2 and 2 5 hz we obtain the relation qlg f 267 f 56 f0 37 0 06 for the basin and range province further constraints on lg q were obtained from lg coda which due to the natural averaging effect of scattering is insensitive to effects of focusing defocusing to investigate the validity of the single isotropic scattering model for lg coda in the basin and range province we calculated ischrons for scatterers corresponding to later lg coda signals as required by the single scattering model it is found that when significant portions of these isochrons are oceanic lg coda amplitude decay becomes irregular and abnormally slow suggesting a strong scattering reflection effect caused by the rapidly changing crustal structure along the coast applying the single isotropic scattering model to lg coda records with isochrons mostly confined to the continental crust results in q f 275 f 26 f 0 36 0 03 which is nearly identical to our qlg relation this indicates that even in a complicated geological province a joint interpretation of the dynamic properties of lg and lg coda can be successful and fruitful inconsistent lg q and coda q relations found in earlier studies in the basin and range province may be attributed to different settings of parameter trade offs different seismic phases or different dynamic properties being used

cenozoic k rich adakitic volcanic rocks in the hohxil area northern tibet lower crustal melting in an intracontinental setting it is generally accepted that the cenozoic potassic volcanic rocks of northern tibet were derived from a lithospheric mantle source here we report new chronological geochemical and isotopic data for the miocene ca 18 15 ma k rich adakitic volcanic rocks from the hohxil area of the songpan ganzi block in northern tibet we contend that these rocks were generated by partial melting of the mafic lower crust in an intracontinental setting unrelated to subduction of oceanic crust the hohxil rocks exhibit high sr y and la yb ratios high sr and la contents but low yb and y concentrations similar to adakites formed by slab melting associated with subduction however their relatively low e n d values 2 09 to 4 58 high 8 7 sr 8 6 sr 0 7072 0 7075 th u th ba and rb ba ratios and distinctive potassium enrichments k 2 o na 2 o are very different from the composition of typical adakites in addition those k rich adakitic rocks with the highest sio 2 contents 61 wt exhibit the lowest 8 7 sr 8 6 sr ratios and highest e n d values and are the oldest cenozoic volcanic rocks exposed in the songpan ganzi block suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma taking into account the composition of lower crustal mafic xenoliths in tibet as well as the tectonic and geophysical evidence we conclude that the hohxil adakitic magmas were produced by partial melting of amphibole bearing eclogites with a k rich mafic bulk composition in the lower part 55 km of the thickened northern tibetan crust partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward subducted continental lithosphere

petrography and detrital zircon study of late carboniferous sequences in the southwestern north china craton implications for the regional tectonic evolution and bauxite genesis abstract the north china craton ncc has been flanked by the north qilian and north qinling arc accretionary belts to the south and southwest since 400 ma the part of the ncc to the east of the alax terrane e ncc experienced a long sedimentary hiatus and tectonic quiescence between the middle ordovician and the late carboniferous the northern margin of the e ncc was reactivated and uplifted with contemporaneous volcanism during the late carboniferous an event that partly induced the transformation of the e ncc from an erosional platform to a continental sedimentary basin however the factors controlling this transformation are still not fully understood a series of sedimentary rocks overlying ordovician carbonates in the southwestern e ncc contains a lower iron oxide layer and an upper phyllosilicate layer detrital zircons from different parts of the profile from the base to the top of the two layers have similar u pb ages these zircons have a minimum age of ca 300 ma and a prominent peak at ca 450 ma with subordinate peaks at ca 1000 and 2500 ma the near identical minimum age for the two layers suggests they were semi simultaneously deposited in the late carboniferous after the long hiatus in sedimentation detrital zircons with ages of 450 ma have initial hf isotopic compositions that vary from large negative to elevated positive these data together with the trace element compositions of these zircons indicate that these minerals formed in a continental arc environment samples from the upper sedimentary layer contain mica group minerals that are weakly buckled and fractured and have weathered to form clay minerals including chlorite and illite this suggests that the protolith of this sedimentary layer was dominated by mica schist or mica bearing granitoid that most likely located near the adjoined part between the north qilian and north qinling arc accretionary belts detrital zircons with the youngest ages ca 300 ma were considered to derive from the northern marginal region of the e ncc an area characterized by contemporaneous volcanism the co existence of the 450 and 300 ma groups of detrital zircons also suggests that the northern and southern marginal regions of the e ncc underwent almost simultaneous late carboniferous tectonic uplift and denudation a tectonic change that led to the formation of the sedimentary basins within the e ncc late carboniferous phyllosilicate layers within the profile are likely to be the equivalent of economically important diaspore and clay dominated bauxites within the e ncc suggesting that the diaspore in these bauxites was derived from mica group minerals by resolution precipitation processes

syn collisional adakitic granodiorites formed by fractional crystallization insights from their enclosed mafic magmatic enclaves mmes in the qumushan pluton north qilian orogen at the northern margin of the tibetan plateau the qumushan qms syn collisional granodiorite which is located in the eastern section of the north qilian orogen at the northern margin of the greater tibetan plateau has typical adakitic characteristics and also contains abundant mafic magmatic enclaves mmes this recognition offers an unprecedented insight into the petrogenesis of both the adakitic host granodiorite and the enclosed mmes the mmes and their host granodiorites share many characteristics in common including identical crystallization age similar to 430 ma same mineralogy similar mineral chemistry and whole rock isotopic compositions indicating their genetic link the mmes are most consistent with being of cumulate origin formed at earlier stages of the same magmatic system that produced the qms adakitic granodiorite subsequent replenishment of adakitic magmas could have disturbed the cumulate piles as mmes dispersed in the adakitic granodiorite host during emplacement the geochemical data and petrogenetic modeling of trace elements suggest that the qms adakitic host granodiorite is most consistent with fractional crystallization dominated by the mineral assemblage of the mmes the parental magma for the qms granodiorite is best explained as resulting from partial melting of the ocean crust together with recycled terrigenous sediments during continental collision which may have also experienced interaction with mantle peridotite during ascent c 2016 elsevier b v all rights reserved

constraints on timing of peak and retrograde metamorphism in the dabie shan ultrahigh pressure metamorphic belt east central china using u th pb dating of zircon and monazite abstract the dabie shan ultrahigh pressure metamorphic uhpm belt occupies the suture between the yangtze and sino korean blocks in east central china the timing of uhpm in the dabie belt is controversial and most recent data come from dating of zircons monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones and monazite growth has been documented to occur during prograde peak and retrograde metamorphism zircons and monazites from uhp mafic rocks from maowu and a uhp jadeite quartzite from shuanghe were imaged in thin sections and in mineral separates and dated using a high resolution ion microprobe maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite maowu eclogites and garnet pyroxenites contain zircons with mean 206 pb 238 u age of 230 4 ma and monazites from a clinopyroxenite have 208 pb 232 th ages of 209 4 ma multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages i e they do not correspond to the cessation of diffusional pb loss as rocks cooled below the closure temperature during exhumation shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921 22 ma and lower intercept age of 236 32 ma interpreted to represent the ages of the source rocks and of peak metamorphism respectively zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1 5 gpa their pooled age of 238 3 ma agrees well with the lower intercept defined by cores suggesting that pb loss from cores and growth of new rims occurred during uhpm at 235 240 ma monazite records multiple events during retrograde metamorphism maowu clinopyroxenite and shuanghe jadeite quartzite experienced monazite growth at 209 ma interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration only the jadeite quartzite records growth at 223 1 ma estimated exhumation rates for maowu are 7 8 km ma

the genesis of early carboniferous adakitic rocks at the southern margin of the alxa block north china abstract adakite is a highly debated petrologic term that was originally proposed to describe igneous rocks formed by slab melting subsequent studies reported other ways to generate adakitic signatures such as the melting of mafic lower continental crust and fractional crystallization of basaltic magma we studied adakitic rocks from the taohuala mountain at the southern margin of the alxa block north china these rocks are characterized by high sr concentrations 300 450 ppm high sr y 20 75 ppm and la yb n 25 67 ppm ratios and low y

shrimp zircon u pb ages of eclogite and orthogneiss from sulu ultrahigh pressure zone in yangkou area eastern china we report shrimp u pb age of zircons in four samples of eclogite and one sample of orthogneiss from sulu ultrahigh pressure uhp zone in yangkou area eastern china uhp rocks are distributed along the sulu orogenic belt suturing north china block with south china block in yangkou area uhp unit is well exposed for about 200 m along yangkou beach section and consists mainly of blocks or lenses of ultramafic rocks and eclogite together with para and orthogneiss which are highly sheared partly zircon grains examined in this study from eclogite show oscillatory zoning and overgrowth texture in cl images and most of the grains have high th u ratio ranging from 0 8 to 2 1 indicating an igneous origin the weighted mean 206pb 238u ages of zircons from the four samples range from 690 to 734 ma these ages can be correlated to the magmatic stage of the protoliths in rare cases zircon grains possess a narrow rim with very low th u ratio 0 02 epma u th total pb dating of such rim yields younger ages that range from 240 to 405 ma marking the metamorphic stage on the other hand zircons from the orthogneiss show irregular shape and zoning with inclusion rich core and inclusion free rim these grains of zircon yield u pb discordia intercept ages of 226 63 ma and 714 110 ma mswd 0 78 bulk of the areas of the rims rim of the zircons demonstrate younger 206pb 238u ages close to the upper intercept with low th u ratio 0 20 indicating their metamorphic origin in contrast the cores show older 206pb 238u ages close to lower intercept and high th u ratio of 0 14 5 25 indicating their igneous origin the upper intercept age is also commonly noted in zircons from eclogite our results suggest a bimodal igneous activity along this zone during the neoproterozoic probably related to the rifting of the rodinia supercontinent

the generation of sodic granite magmas western palmer land antarctic peninsula subduction related mesozoic to cainozoic granites s l in western palmer land antarctic peninsula have similar chemical compositions to archean tonalite trondhjemite granodiorite ttg suites phanerozoic slab melts adakites and to experimental partial melts of basaltic material in equilibrium with amphibole pyroxene garnet they are predominantly sodic metaluminous and most have al2o3 15 wt and y 36 km between triassic and tertiary times the initial nd t of the magmatism increased and sr t decreased suggesting that new continental crust was produced during this period underplating by mafic magma was an important crustal growth mechanism in the arc the generation of abnormally thick crust and its subse quent fusion is considered to be a consequence of ca 180 ma of subduction and associated magmatism in the region an implication of the model is that dense garnet amphibolite and eclogite residues from partial melting of the lower crust will accumulate in theory the setting was appropriate for such residues to detach from the base of the crust and to sink into the convecting mantle such a process would leave the rest of the crust enriched in large ion lithophile elements lree but depleted in hree y

geochronological and geochemical constraints on the petrogenesis and tectonic significance of paleozoic dolerite dykes in the southern margin of alxa block north china craton abstract dolerite dykes are common in the southern margin of alxa block which is the westernmost part of north china craton the ages and petrogenesis of the dolerite dykes which are important for a better understanding of tectonic magmatic evolution in the region are uncertain this paper reports the results of an integrated geochronological and geochemical study of the dolerite dykes in the jinchuan area previously these dolerite dykes were considered to be coeval with the 831 ma jinchuan ultramafic intrusion by some researchers despite a cross cutting relationship for the dykes our new zircon u pb dating indicates that the dolerite dykes were emplaced at 424 ma 400 ma younger than the jinchuan intrusion the jinchuan dolerite dykes contain 5 9 wt mgo and 47 51 wt sio 2 and are characterized by light ree rare earth elements enrichments relative to heavy ree pronounced negative nb anomaly elevated initial 87 sr 86 sr from 0 7056 to 0 7068 negative end from 2 to 5 and zircon ehf from 2 to 20 the trace element and isotope data are consistent with an asthenosphere derived parental magma that experienced 10 25 crustal contamination prior to final emplacement since the jinchuan dolerite dykes are 20 40 ma younger than the subduction related basaltic volcanic rocks blueschists and eclogites in the nearby north qilian orogenic belt to the south we propose that the jinchuan dolerite dykes are the products of basaltic magmatism induced by delamination of previously subducted oceanic lithosphere and the associated asthenosphere upwelling in a post subduction setting the occurrence of 424 ma post subduction dolerite dykes in the jinchuan area and the 450 ma arc basalts to the south indicate that the southwestern margin of north china craton was an active plate margin in early paleozoic

petrogenesis of the early paleozoic low mg and high mg adakitic rocks in the north qilian orogenic belt nw china implications for transition from crustal thickening to extension thinning abstract the petrogenesis and geodynamic implications of the early paleozoic adakitic rocks in the north qilian orogenic belt remain topics of debate in this study petrology zircon u pb age and lu hf isotopic composition whole rock geochemistry and sr nd isotopes were carried on both low mg and high mg adakitic rocks from the north qilian orogenic belt these results will provide significant constraints on the evolution of the north qilian orogenic belt in general the low mg adakitic granites mainly consist of plagioclase 50 55 alkali feldspar 20 25 quartz 20 25 and biotite 5 in contrast the high mg adakitic granodiorites are generally composed of alkali feldspar 30 35 plagioclase 30 45 quartz 20 25 and amphibole 10 the low mg adakitic granitoids 446 457 ma are characterized by high sio2 69 72 wt low mg 39 42 and low cr and ni contents however the high mg adakitic granitoids 430 431 ma have relatively lower sio2 65 66 wt higher mg 50 59 and higher cr and ni contents the low mg adakitic rocks have high initial 87sr 86sr ratios 0 7068 0 7080 negative end t 2 3 to 3 3 and ehf t values 6 8 to 1 9 with old zircon hf model ages 1 2 1 6 ga however the high mg adakitic rocks show lower initial 87sr 86sr ratios 0 7044 0 7066 higher end t 1 8 to 3 0 and positive ehf t values 2 3 7 7 with younger zircon hf model ages 0 9 1 1 ga these results suggest that the low mg rocks were generated by partial melting of thickened crust whereas the high mg rocks were derived from anatexis of delaminated lower crust which subsequently interacted with mantle magma upon ascent the data obtained in this study provide significant information about the geological and tectonic processes after the closure of the qilian ocean the continent continent collision and thickening probably occurred during 440 467 ma with formation of low mg adakitic rocks and the transition of the tectonic regime from compression to extension probably occurred at 430 ma with formation of high mg adakitic rocks

paleozoic magmatism and porphyry cu mineralization in an evolving tectonic setting in the north qilian orogenic belt nw china abstract the nww striking north qilian orogenic belt records the paleozoic accretion collision processes in nw china and hosts paleozoic cu pb zn mineralization that was temporally and spatially related to the closure of the paleo qilian qinling ocean the wangdian cu deposit is located in the eastern part of the north qilian orogenic belt nw china copper mineralization is spatially associated with an altered early paleozoic porphyritic granodiorite which intruded tonalites and volcaniclastic rocks alteration zones surrounding the mineralization progress outward from a potassic to a feldspar destructive phyllic assemblage mineralization consists mainly of quartz sulfide stockworks and disseminated sulfides with ore minerals chalcopyrite pyrite molybdenite and minor galena and sphalerite gangue minerals include quartz orthoclase biotite sericite and k feldspar zircon la icpms u pb dating of the ore bearing porphyritic granodiorite yielded a mean 206pb 238u age of 444 6 7 8 ma with a group of inherited zircons yielding a mean u pb age of 485 12 ma consistent with the emplacement age 485 3 6 2 ma of the barren precursor tonalite rhenium and osmium analyses of molybdenite grains returned model ages of 442 9 6 8 ma and 443 3 6 2 ma indicating mineralization was coeval with the emplacement of the host porphyritic granodiorite rhenium concentrations in molybdenite 208 9 213 2 ppm suggest a mantle re source the tonalities are medium k calc alkaline they are characterized by enrichment of light rare earth elements lrees and large ion lithophile elements liles depletion of heavy rare earth elements hrees and high field strength elements hfses and minor negative eu anomalies they have ehf t values in the range of 3 6 to 11 1 with two stage hf model ages of 0 67 1 13 ga suggesting that the ca 485 ma barren tonalites were products of arc magmatism incorporating melts from the mantle wedge and the lithosphere in contrast the 40 m y younger ore bearing porphyritic granodiorite is sub alkaline and peraluminous they are enriched in lrees and liles depleted in hfses and show weak negative eu anomalies they display ehf t values of captured or inherited zircons in the range of 8 5 to 10 0 and younger two stage hf model ages of 0 78 ga and 0 86 ga similar to those of ca 485 ma tonalite the ca 445 ma zircons have ehf t values of 2 1 to 9 9 with two stage hf model ages of 0 75 1 27 ga moreover they have relatively high oxygen fugacity than that of the precursor barren tonalite the ca 445 ma magmas at wangdian thus formed in a subduction setting and incorporated melts from the subduction modified lithosphere that had previously been enriched by additions of chalcophile and siderophile element rich materials by the earlier magmatism and metasomatism during the paleo qilian qinling ocean subduction event

contrasting origins of late mesozoic adakitic granitoids from the northwestern jiaodong peninsula east china implications for crustal thickening to delamination two suites of granitoids the late jurassic 158 3 ma linglong suite and the early cretaceous 130 126 ma guojialing suite crop out in the northwestern jiaodong peninsula eastern china the linglong suite is a monzogranite comprising alkali feldspar plagioclase quartz and fe rich biotite the guojialing suite includes at least five plutonic bodies of both granodiorite and monzo granite the rocks are composed of plagioclase alkali feldspar quartz mg rich amphibole and mg rich biotite both the linglong and guojialing suites have adakitic affinity they are enriched in lree with high la yb ratios and show positive eu anomalies the rocks are also enriched in lile and depleted in hfse with high sr y ratios the linglong granite shows very uniform sr nd isotopic compositions with initial 87 sr 86 sr ratios of 0 7119 0 7126 and e nd t values of 21 3 to 21 6 which are similar to those of the local neoarchaean basement the guojialing suite has variable initial 87 sr 86 sr ratios 0 7108 0 7120 and e nd t values 10 8 to 17 2 which are distinct both from those of the neoarchaean basement and from those of the local enriched lithospheric mantle inferred from the coeval mafic dykes in the studied area detailed petrological and geochemical data indicate that the linglong suite was derived by partial melting of neoarchaean metamorphic lower crustal rocks at depth of 50 km with a eclogite residue whereas the guojialing suite was formed by the reaction of delaminated eclogitic crust derived melt with the upwelling asthenospheric mantle the petrogenesis of these two contrasting adakitic granitoids suggests intensive lower crustal delamination during early cretaceous times following a crustal thickening process from the late stage of the early jurassic to early stage of the late jurassic with crustal thickness of 50 km respectively

petrogenesis and tectonic setting of early paleozoic granites and high mg diorites in the northern qilian orogen china abstract the northern qilian orogenic belt n qob was formed by oceanic subduction and collision between the alashan and central qilian belt in the early paleozoic new zircon u pb ages hf isotopic and whole rock elemental data of the intrusive rocks in the central n qob have been conducted to constrain the tectonic evolution of n qob zircon dating results revealed that the ladonggou suyougou granitic plutons and xichagou dioritic pluton were emplaced at 505 3 ma 471 3 m and 425 3 ma respectively the ladonggou monzogranite displays depletion of nb ta ti p and negative ehf t values 6 5 to 0 3 and old hf crustal model ages 1 5 1 9 ga suggesting generation from melting of ancient crust above subducted north qilian oceanic slab the suyougou granite possesses geochemistry of i type granite the relative enrichment of liles depletion of hfses and negative eu anomalies as well as prominent hf isotope heterogeneities and old hf crustal model ages 2 1 2 5 ga suggest a magma source of ancient crust mixed with minor mantle derived melts in a subduction related setting diorites from the xichagou plutonare characterized by high mg values enrichments in cr ni y yb and high sr y ratios and convincing subduction related features suggesting generation from partial melting of subducted sediments mixed with mantle components taking all these geochemistry and geochronology together with the regional geology we interpreted the ladonggou monzogranite suyougou granite and xichagou diorite were related to the long term northward subduction of north qilian oceanic lithosphere during ca 505 and 425 ma

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

late cretaceous gangdese intrusions of adakitic geochemical characteristics se tibet petrogenesis and tectonic implications article i nfo the gangdese batholith emplaced from the cretaceous to eocene in southern tibet has been widely regarded as the major constituent of an andean type convergent margin resulting from northward subduction of the neo tethyan oceanic lithosphere under asia while the gangdese batholith consists predominantly of calc alkaline rocks we identify from the eastern part of the batholith a suite of epidote bearing granodiorites that shows adakitic geochemical characteristics marked with apparently higher la yb and sr y and lower y and hree than other gangdese rocks and common arc magmas shrimp zircon u pb analyses of two of the samples yielded 206 pb 238 u dates of 80 4 1 1 and 82 7 1 6 2 ma which constrain the emplacement ages of the adakitic rocks trace element modeling suggests that these rocks originated from partial melting of a garnet amphibolite source that on the basis of the sr and nd isotope data isr 0 7044 0 7048 end t 3 2 to 0 9 we interpret to be a newly underplated mafic lower crust rather than the subducted neo tethyan oceanic crust this juvenile crust was produced by cretaceous basaltic underplating above the mantle wedge and then thickened by the tectonic contraction owing to flattening of the neo tethyan subduction a process that also led to the adakitic magmatism our interpretation involving a late cretaceous flat slab subduction and related orogenesis in southern tibet is consistent with petrographic data such as the occurrence of magmatic epidote and muscovite rimmed with resorption texture in the granodiorites which indicate deep seated emplacement followed by rapid tectonic exhumation

continuity of the north qilian and north altun orogenic belts of nw china evidence from newly discovered palaeozoic low mg and high mg adakitic rocks in this study the petrology zircon u pb ages lu hf isotopic compositions whole rock geochemistry and sr nd isotopes for newly recognized low mg and high mg adakitic rocks from the north altun orogenic belt were determined the results will provide important insights for understanding the continuities of the north qilian and north altun orogenic belts during early palaeozoic time the low mg adakitic granitoids 445 to 439 ma are characterized by high sio 2 69 70 wt low mg no 43 48 and low cr and ni contents in contrast the high mg adakitic granitoids 425 to 422 ma have relatively lower sio 2 65 67 wt higher mg no 60 62 and higher cr and ni contents the low mg adakitic rocks have high initial 87 sr 86 sr ratios 0 7073 0 7084 negative e nd t 1 9 to 4 0 and e hf t values 6 8 to 2 0 and old zircon hf model ages 1 4 1 7 ga in contrast the high mg adakitic rocks show lower initial 87 sr 86 sr ratios 0 7044 0 7057 higher e nd t 0 7 to 3 1 and positive e hf t values 2 0 to 6 9 with younger zircon hf model ages 0 9 1 2 ga these results suggest that the low mg adakitic rocks were probably generated by the partial melting of thickened crust whereas the high mg adakitic rocks were derived from the anatexis of delaminated lower crust which subsequently interacted with mantle magma upon ascent the data obtained in this study provide significant information about the geological and tectonic processes after the closure of the altun ocean the continent continent collision and thickening probably occurred during 450 440 ma with the formation of low mg adakitic rocks and the transition of the tectonic regime from compression to extension probably occurred at 425 422 ma with the formation of high mg adakitic rocks the geochemical geochronological and petrogenetic similarities between the north altun and north qilian adakitic rocks suggest that these two orogenic belts were subjected to similar tectonomagmatic processes during early palaeozoic times

seismic crustal structure of the north china craton and surrounding area synthesis and analysis we present a new digital model nccrust of the seismic crustal structure of the neoarchean north china craton ncc and its surrounding paleozoic mesozoic orogenic belts 30 45 n 100 130 e all available seismic profiles complemented by receiver function interpretations of crustal thickness are used to constrain a new comprehensive crustal model nccrust the model presented on a 0 25 0 25 grid includes the moho depth and the internal structure thickness and velocity of the crust specified for four layers the sedimentary cover upper middle and lower crust and the pn velocity in the uppermost mantle the crust is thin 30 32 km in the east while the moho depth in the western part of the ncc is 38 44 km the moho depth of the sulu dabie qinling qilian orogenic belt ranges from 31 km to 51 km with a general westward increase in crustal thickness the sedimentary cover is 2 5 km thick in most of the region and typical thicknesses of the upper crust middle crust and lower crust are 16 24 km 6 24 km and 0 6 km respectively we document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as a consequence the depth of the moho there is no systematic regional pattern in the average crustal vp velocity and the pn velocity we examine correlation between the moho depth and topography for seven tectonic provinces in the north china craton and speculate on mechanisms of isostatic compensation

discovery of metamorphic diamonds in central china an indication of a 4000 km long zone of deep subduction resulting from multiple continental collisions the central orogenic belt cob of china is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt here we report discovery of microdiamonds from both eclogites and felsic gneisses in the north qinling zone in the central portion of the cob this discovery demonstrates that the country rocks of continental affinity shared in the ultra high pressure metamorphic uhpm event and provides a bridge connecting the two previously recognized uhpm terranes thereby establishing the existence of a uhpm belt extending more than 4000 km geochronological dating yields early palaeozoic ages in the west and early mesozoic ages in the east recording two separate continental collisions overprinted within the cob occurrence of uhp metamorphism during recurrent continental collision here and in the alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional with significant implications for processes of plate tectonic reorganization and mantle mixing over time

physical description of lg waves in inhomogeneous continental crust summary the view of lg waves as a superposition of seismic surface waves is very useful for explaining the amplitudes of the late parts of seismograms from regional earthquakes we have recently made computations for rayleigh waves interpreting the special attenuation of lg waves in the north sea as being caused by 1 the distribution of energy over longer time intervals combined with 2 the upwards or downwards pumping of wave energy in the diverent crustal structures combined with 3 anelastic absorption no special discussion is necessary of the claim that anelastic absorption in a particular area gives a reduction in wave amplitude whilst a physical description of the geometrical evect is warranted a physical description of the wave propagation phenomena involved is given in the present paper by classifying the mode conversion that is involved in the propagation from one crustal structure into another the physics is described via relations between the phase velocities group velocities and energy dependences on depths of the various surface wave modes that exist in the diverent crustal structures we introduce the concept of energy depth similarity a parameter that is almost as easily computed as the ratios of phase velocities or group velocities but is a far better measure of the transition character the crustal structures in question are representative of 1 the north sea and its surroundings in general and 2 the viking and central grabens in the middle of the north sea wave propagation is fundamentally diverent for diverent wave frequencies it is coherent for long period waves and incoherent for short period waves the propagation may generally be termed well behaved for wave periods longer than several seconds that is energy cross coupling between incoming and outgoing modes follows some regular pattern this regularity is highly dependent on the similarity of the two modes on either side of a discontinuity and also on the similarity of their neighbouring modes a characteristic of the regularity is that there is no mode conversion for very longperiod modes in contrast a pattern cannot be recognized for waves of shorter period and one may use the term ill behaved the lg waves in the continental crust are in the short period range because their wavelengths are short compared to thickness changes as well as to the thicknesses of crustal structures this leads to the view that lg waves are superpositions of many modes changing into even more modes at any discontinuity since rayleigh and love waves are coupled at non perpendicular incidence and the earth has many discontinuities the superposition of modes is constantly changing in relative proportions rayleigh and love modes being intermingled

age and origin of high ba sr appinite granites at the northwestern margin of the tibet plateau implications for early paleozoic tectonic evolution of the western kunlun orogenic belt abstract the buya appinite granite is a typical high ba sr granite emplaced at the northern west kunlun orogenic belt along the northwestern margin of the tibetan plateau the granite is dated at ca 430 ma using the shrimp u pb zircon method it consists of alkaline feldspar granites with coeval appinite enclaves the granite possesses high sio2 69 77 72 69 k2o 4 44 5 10 and total alkalinity k2o na2o 8 80 9 92 sr 655 1100 ppm ba 1036 1433 ppm and lree and low hree and hfse contents and insignificant negative eu anomalies consequently the samples have very high sr y 74 141 and la yb n 37 96 ratios on the other hand they have low mgo or mg cr and ni contents and low radiogenic nd isotopes nd t 8 4 to 10 4 the high ba sr and other geochemical signatures of the granite also appear in the appinite enclaves except that the appinite enclaves have relatively higher abundances in these elements and higher nd t values 5 7 to 6 7 elemental and isotope compositions suggest that the appinites were derived from partial melting of an enriched lithospheric mantle source probably induced by upwelling of the asthenosphere due to the delamination of a subducted slab the granite was likely derived from partial melting of the mafic lower crust with residual garnet associated with involvement of minor lile enriched appinitic magma followed by crystal fractionation of hornblende biotite apatite and allanite in combination with previous investigations on the evolution of the western kunlun we suggest that the buya high ba sr plutons represent the end of an early paleozoic crust thickening event after a terrane accretion on southern tarim craton and the beginning of a post orogenic collapse phase in the paleozoic west kunlun orogenic belt

a large deformation random finite element study failure mechanism and bearing capacity of spudcan in a spatially varying clayey seabed although the random heterogeneity of a clayey seabed has been long recognised it is relatively recently that the influence of spatial variability on the spudcan foundation has been explicitly expl

palaeozoic and mesozoic tectonic evolution and palaeogeography of east asian crustal fragments the korean peninsula in context east and southeast asia comprises a complex assembly of allochthonous continental lithospheric crustal fragments terranes together with volcanic arcs and other terranes of oceanic and accretionary complex origins located at the zone of convergence between the eurasian indoaustralian and pacific plates the former wide separation of asian terranes is indicated by contrasting faunas and floras developed on adjacent terranes due to their prior geographic separation different palaeoclimates and biogeographic isolation the boundaries between asian terranes are marked by major geological discontinuities suture zones that represent former ocean basins that once separated them in some cases the ocean basins have been completely destroyed and terrane boundaries are marked by major fault zones in other cases remnants of the ocean basins and of subduction accretion complexes remain and provide valuable information on the tectonic history of the terranes the oceans that once separated them and timings of amalgamation and accretion the various allochthonous crustal fragments of east asia have been brought into close juxtaposition by geological convergent plate tectonic processes the gondwana derived east asia crustal fragments successively rifted and separated from the margin of eastern gondwana as three elongate continental slivers in the devonian early permian and late triassic late jurassic as these three continental slivers separated from gondwana three successive ocean basins the palaeo tethys meso tethys and cenotethys opened between these and gondwana asian terranes progressively sutured to one another during the palaeozoic to cenozoic south china and indochina probably amalgamated in the early carboniferous but alternative scenarios with collision in the permo triassic have been suggested the tarim terrane accreted to eurasia in the early permian the sibumasu and qiangtang terranes collided and sutured with simao indochina east malaya in the early middle triassic and the west sumatra terrane was transported westwards to a position outboard of sibumasu during this collisional process the permo triassic also saw the progressive collision between south and north china with possible extension of this collision being recognised in the korean peninsula culminating in the late triassic north china did not finally weld to asia until the late jurassic the lhasa and west burma terranes accreted to eurasia in the late jurassic early cretaceous and proto east and southeast asia had formed palaeogeographic reconstructions illustrating the evolution and assembly of asian crustal fragments during the phanerozoic are presented d 2005 international association for gondwana research published by elsevier b v all rights reserved

digital processing of regional network data abstract much more than hypocenter location can be done with digital time histories obtained from regional seismic networks examples are given of how these data can be routinely processed to provide readily accessible intermediate results for subsequent studies among which are velocity inversion using teleseismic p residuals composite focal mechanism studies and spectral analysis the processing must be designed to be as routine and as complete as possible only with these two objectives achieved can the seismic networks be as productive as they should be

constraining the role of the qinling orogen in the assembly and break up of rodinia tectonic implications for neoproterozoic granite occurrences abstract new shrimp and id tims dating results in this study reveal that the widespread early neoproterozoic granites in the qinling orogen were emplaced mainly between 960 and 900 ma major elements mineral compositions and trace elements data also indicate that all these early neoproterozoic granites are characterized by the features of crust derived and typical continent continent syn collisional granites this imply that continent continent collisional event could have occurred in the qinling orogen during the period of 960 to 900 ma which was closely related to the formation of rodinia thus our study argues that the final assembly of rodina occurred at 900 ma on the other hand k feldspar granite dq05 a type granite and granodiorite dq07 from the study area are dated at 711 11 and 717 10 ma respectively and are considered to have generated in an extensional setting this extensional tectonic regime originated from 810 ma when large scale continental bimodal volcanic rocks were formed in south qinling and lasted at least until the time of 710 ma which is consistent with the break up of rodinia this study also suggests that the north china craton did not collide and join together with the south china craton in the neoproterozoic and both of them were probably far away from each other

structural anatomy of the early paleozoic laohushan ophiolite and subduction complex implications for accretionary tectonics of the proto tethyan north qilian orogenic belt northeastern tibet

petrology geochemistry and genesis of high al tonalite and trondhjemites of the cornucopia stock blue mountains northeastern oregon tonalitic and trondhjemitic compositions span a narrow range of trondhjemite sio2 content 65 74 wt and exhibit characteristics of a highal tonalite trondhjemite granitoid ttg suite including light rare earth element lree enrichment low y nb and rb sr and high al2o3 and sr these compositions are consistent with an

age geochemistry and tectonic setting of buqingshan ophiolites north qinghai tibet plateau china abstract the buqingshan ophiolite complex is a sector of the a nyemaqen ophiolite belt in the east kunlun southern marginal suture zone in the northern part of the qinghai tibet plateau the wnw trending ophiolite complex consists of metaperidotite gabbro diabase pillow basalt massive basalt and pelagic sedimentary rocks including radiolarian chert we have determined that early paleozoic and early carboniferous early permian ophiolites are present in this ophiolite complex that were previously thought of permian middle triassic age a zircon u pb age of 467 2 0 9 ma for gabbro and a rb sr isochron age of 481 130 ma for diabase and gabbro have been obtained from the early paleozoic ophiolite slice and middle late ordovician acritarchs have been found in the melange matrix that envelopes the sliver we interpret the first two ages to represent the formation age of the ophiolite granodiorite tonalite plutons with a zircon u pb age of 402 24 ma intrude the slice early carboniferous early permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle most mafic rocks in the early paleozoic ophiolite are of n morb type with small amount of t morb and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination the basalts in the early carboniferous early permian ophiolite have similar characteristics to the mafic rocks and also show dupal anomaly and a mixed source of dmm and em ii the original formation environments of the two stage ophiolites are both mid ocean ridges these lines of evidence suggest that a mature early paleozoic kunlun qilian qinling ocean basin and a mature paleotethyan ocean basin once existed in the study area

late carboniferous early permian high and low sr y granitoids of the xing an block northeastern china implications for the late paleozoic tectonic evolution of the eastern central asian orogenic belt abstract this study presents new zircon u pb geochronological whole rock geochemical and zircon hf isotopic data along with regional geological observations for five late paleozoic granitic plutons within the xing an block of northeastern china these data provide insights into the petrogenesis of these granitoids and the late paleozoic tectonic evolution of the region the geochronological and geochemical data indicate that the intrusions were emplaced during two separate periods with the resulting granites having distinct geochemical features the early late carboniferous 321 310 ma granitic lizishan lzs xiaonangou xng and dabeigou dbg plutons have high sr y values 33 15 126 73 with low concentrations of y and heavy rare earth elements hree indicating adakitic affinities however the xng granodiorites contain higher concentrations of mgo cr and ni and have higher mg values 57 59 than the other plutons the latest carboniferous early permian 301 296 ma granitic weilegesi wlgs and wuyi wy plutons are high k calc alkaline with low sr y ratios 0 18 24 15 and show petrological and geochemical characteristics of i type granites in addition the magmatic zircon crystals within these plutons yield positive ehf t values 4 1 to 12 9 and juvenile two stage model tdm2 ages 1056 499 ma the timing of emplacement and geochemical and isotopic compositions of the lzs and dbg granitoids are indicative of derivation from magmas generated by the partial melting of thickened lower crustal material whereas the xng granitoids were most likely formed from magmas generated by the partial melting of delaminated lower crustal material that subsequently interacted with mantle material during ascent the latest carboniferous early permian low sr y granitoids may have formed from magmas generated by the partial melting of a dominantly juvenile amphibolite facies crustal source under relatively low pressure conditions with these magmas subsequently undergoing variable degrees of fractional crystallization combining these new data with the results of previous research indicates that the terminal collision between the xing an and songliao blocks occurred between the late early and early late carboniferous and the voluminous late carboniferous to early permian granitoids in the xing an block are most likely related to post collisional delamination

lithospheric delamination in post collisional setting evidence from intrusive magmatism from the north qilian orogen to southern margin of the alxa block nw china abstract post collisional granitoids are widespread in the north qilian and southern margin of the alxa block and their petrogenesis can provide important insights into the lithospheric processes in a post collisional setting this paper carries out an integrated study of u pb zircon dating geochemical and sr nd hf isotopic compositions for five early paleozoic intrusive plutons from the north qilian to southern margin of the alxa block the geochronological and geochemical results show that their magmatism can be divided into three periods with distinct geochemical features the early period intrusive rocks 440 ma include the lianhuashan lhs and mengjiadawan mjdw granodiorites both of them display high sr y ratios 52 91 coupled with low y and hree contents implying that they were derived from partial melting of thickened lower crust with garnet in the residue the middle period intrusive rocks 430 ma including the mjdw quartz diorites and yangqiandashan yqds granodiorites are high k calc alkaline with low sr y values the geochemical and isotopic data suggest that they are generated from partial melting of lower crust without garnet in the residue the late period intrusive rocks 414 422 ma represented by the shengrongsi srs and xinkaigou xkg plutons are a type or alkali feldspar granites they are possibly derived from partial melting of felsic crustal material under lower pressure condition our data show decreasing magma crystallization ages from mjdw pluton in the north and lhs pluton in the south to the srs and xkg plutons in the central part of the study area we suggest that such spatial and temporal variations of magmatic suites were caused by lithospheric delamination after the collision between the central qilian and the alxa block a more plausible explanation is that the delamination propagated from the margin part of the thickened lithosphere to inward beneath the north qilian and southern margin of the alxa block

lg and rg waves on the california regional networks from the december 23 1985 nahanni earthquake we investigate lg and rg propagation in california using the central and southern california regional networks approximately 550 stations constitute these two short period networks providing a dense coverage of almost the entire state the waveforms recorded from the december 23 1985 nahanni canada earthquake are used to construct three profiles along the propagation path almost n s and three perpendicular to the propagation path almost e w to look at the nature of propagation of these two types of surface waves groups of records from stations in various geological and tectonic provinces in california are also examined in order to establish regional characteristics of the surface waves we find that the propagation characteristics of lg differ from those of rg across california lg waves are apparently more sensitive to crustal heterogeneities the most striking observations are the similarity of coda for both the lg and the rg waves within geologic provinces and the marked difference in coda between regions these differences are seen in the amplitudes coda duration shape of the energy envelope frequency content and sharpness of the phase initiation in general a decrease in the moho depth near the pacific coast is correlated with a decrease in the surface wave amplitude especially at higher frequencies 0 15 0 2 hz most interesting is the association of the san andreas fault with abrupt changes in the wave train amplitudes the surface waves are amplified in the vicinity of the fault zone and then decrease in amplitude after the zone is crossed in the coast ranges amplitudes are low and waveform coherence is poor the rg phase dominates the record in the sierra nevada and both surface waves are amplified by the thick sedimentary sequence of the great valley

geochronological and geochemical evidence of amphibolite from the hualong group northwest china implication for the early paleozoic accretionary tectonics of the central qilian belt abstract the hualong group located in the central qilian belt northwest china consists mainly of schist amphibolite quartzite and marble ranging from greenschist to amphibolite facies metamorphism on the basis of the medium grade metamorphism the group has been considered to comprise proterozoic basement rocks in this study geochemical sr nd isotopic and zircon u pb geochronological analyses were performed on lentoid amphibolites from the hualong group to characterize their age petrogensis and tectonic setting uranium lead zircon dating of amphibolite revealed a formation age of 456 2 ma and a metamorphic age of 440 1 ma major trace and rare earth element data indicate that the amphibolites are predominantly basaltic andesitic to andesitic rocks with island arc affinities the trace element patterns show enrichment in large ion lithophile elements and depletion in high field strength elements relative to the n morb which confirm their island arc signatures obviously enriched light rees la yb n 2 5 16 9 to chondrite normalized ree patterns further support this interpretation the e nd t values for the amphibolites range from 4 6 to 2 1 indicating subducted sediments as a larger endmember in the source geochemical data for these rocks suggest an island arc setting and the rocks were derived from the depleted mantle that was enriched by melts of subducted sediments in an active continental margin setting at ca 456 ma together with regional evidence it suggests that the hualong group is an accretionary complex that was incorporated into the central qilian belt during the 440 400 ma orogenic event

continuity of the north qilian and north qinling orogenic belts central orogenic system of china evidence from newly discovered paleozoic adakitic rocks abstract adakitic intrusive rocks of 430 450 ma were discovered in the north qilian orogenic belt the western section of the central orogenic system cos in china these adakitic rocks were lower crust melts rather than slab melts as indicated by their crustal ce pb nb u ti eu and nd sm ratios and radiogenically enriched 87sr 86sr i of 0 7053 0 7066 and end t of 0 9 to 1 7 while they are all characterized by low yb 65 and la yb n 13 7 ratios these adakitic rocks are classified into the low mgo ni cr and high mgo ni cr groups the low mgo samples were derived from partial melting of thickened lower crust whereas the high mgo samples were melts from delaminated lower crust which subsequently interacted with mantle peridotite upon ascent adakitic rocks from the adjacent north qinling orogenic belt also originated from thickened lower crust at 430 ma in addition the north qilian and north qinling orogenic belts both consist of lithological assemblages varying from subduction accretionary complexes at south to central arc assemblages which include adakitic rocks then to backarc phases at north such a sequence reflects northward subduction of the qilian and qinling oceans in these two orogenic belts the occurrence of adakitic rocks of common origin and ages together with the similarities in tectonic configurations and lithological assemblages are considered to be the evidence for the continuity between eastern qilian and western qinling forming a 1000 km early paleozoic orogenic belt in such a tectonic configuration the qilian and qinling oceans that subducted from south possibly represent parts of the large proto tethyan ocean this inference is supported by the coexistence of early paleozoic coral and trilobite specimens from asia america and australia in the north qilian orogenic belt post 400 ma volcanic rocks occur in the north qinling orogenic belt but are absent in the north qilian orogenic belt indicating that these two orogenic belts underwent distinct evolution history after the closure of the proto tethyan ocean 420 ma

collision leading to multiple stage large scale extrusion in the qinling orogen insights from the mianlue suture abstract the geologic framework of the phanerozoic qinling dabie orogen was built up through two major suturing events of three blocks from north to south these include the north china craton including the north qinling block the qinling dabie microblock and the south china craton including the bikou block separated by the shangdan and mianlue sutures the mianlue suture zone contains evidence for mesozoic extrusion tectonics in the form of major strike slip border faults surrounding basement blocks a late paleozoic ophiolite and a ca 240 200 ma thrust belt that reformed by 200 150 ma thrusts during a type intracontinental subduction the regional map pattern shows that the blocks are surrounded by complexly deformed devonian to early triassic metasandstones and metapelites forming a regional scale block in matrix melange fabric five distinct tectonic units have been recognized in the belt 1 basement blocks including two types of precambrian basement crystalline and transitional 2 continental margin slices including early paleozoic strata and late paleozoic fluviodeltaic sedimentary rocks proximal and distal fan clastics reflecting the development of a north facing rift margin on the edge of the south china plate 3 out of sequence oceanic crustal slices including strongly deformed postrift deep water sedimentary rocks sheeted dikes basalts and mafic ultramafic cumulates of a late paleozoic ophiolite suite developing independent of the rift margin in a separate basin 4 out of sequence island arc slices 5 accretionary wedge slices all the tectonic units were deformed during three geometrically distinct deformation episodes d 1 d 2 and d 3 during 240 200 ma units 2 4 involved southward thrusting and vertical then southward extrusion of about 20 km of horizontal displacement above the autochthonous basement during the d 1 episode thrust slices 20 km south of the mianlue suture are related to this vertical extrusion due to the same rock assemblages ages and kinematics the d 2 and d 3 episodes folded all the units in a thick skinned style about east west d 2 and west northwest d 3 axes in the mianlue suture zone an early foreland propagating sequence of accretion of late paleozoic rocks deposited above the yangtze craton is not involved in d 1 deformation but is temporally equivalent to the d 2 and d 3 deformation in the mianlue suture two stages of strike slip faulting mainly occurred at the end of d 2 and d 3 respectively during d 2 deformation the bikou block was obliquely indented to the ese into the mianlue suture rather than being thrust over the mianlue suture from the north as a part of the qinling dabie microblock during d 3 deformation however the bikou block was bounded by the south boundary fault of the mianlue suture and the yangpingguan fault on the south these faults are coeval strike slip faults but of opposite senses and accommodated minor southwestward extrusion of the bikou block into songpan ganze orogen the other basement blocks north of the mianlue suture were extruded eastward by about 20 km of lateral displacement based on the offset of the wudang dome during the d 3 episode due to the northeastward indentation of the hannan complex of the south china craton post d 3 emplacement of granite cutting across the strike slip faults such as the mianlue suture provides a minimum age of 200 ma for d 3 deformation therefore based on insights from the evolution of the mianlue suture the d 2 and d 3 episodes in the mianlue suture and its neighbors are not responsible for and associated with the two stage extrusion of the dabie uhp hp terranes from the foping dome to the present erosional surface more than 350 km

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

propagation and attenuation characteristics of the crustal phase lg thelg wave consists of the superposition ofs waves supercritically reflected and thus trapped in the crust this mode of propagation explains the strong amplitude of this phase and the large distance range in which it is observed the numerical simulation leads to successful comparison between observed seismograms in stable continental areas and synthetics computed for simple standard crustal models in regions with strong lateral variations the influence of large scale heterogeneities on thelg amplitude is not yet clearly established in terms of the geometrical characteristics of the crustal structure

formation of wulong mafic dikes via mixing of enriched lithospheric mantle derived and crustal magmas eastern north china craton abstractzircon u pb age and geochemical data are reported for the wulong dike swarms from the liaodong peninsula ne china the studied dikes include diorite lamprophyre and dolerite together with previous dating the diverse rocks have a peak age of ca 120 125 ma they are marked by inversely zoned phenocrysts with mg and cr rich rims and they exhibit mantle like signatures i e high mg cr ni associated with prominent subduction type element enrichments the early diorites have relatively radiogenic sr 0 7143 and less radiogenic nd to 11 0 isotopes coupled with high and variable mgo cr ni contents indicating that they were mainly derived from ancient lower crust with subordinate contributions of enriched lithospheric mantle the transitional lamprophyres are characterized by high rb sr ratios 0 22 0 54 but low initial 87sr 86sr ratios 0 7069 0 7093 moderate negative end t values 14 4 to 12 6 and low promantle elements such as mg cr ni reflecting that juvenile lower crust

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

experimental study of uplift resistance of shallow skirted foundations in clay under transient and sustained concentric loading wind wave and current forces cause significant overturning loads to be transmitted to foundation systems of fixed bottom offshore structures while buoyancy forces in conjunction with environmental loading impart direct tensile loads to foundations of floating facilities shallow skirted foundations are a potentially attractive foundation solution when overturning or tension loading is significant as suctions mobilised within the soil plug provide tensile resistance while passive suctions are maintained undrained reverse end bearing will govern failure providing enhanced uplift resistance compared with vented pull out of the foundation unit with or without the soil plug governed by skirt soil friction currently uncertainty exists over the limiting ratio of skirt depth to foundation diameter to mobilise undrained reverse end bearing and the duration over which passive suctions can be sustained this paper reports results from beam centrifuge tests investigating the response of shallow skirted fou

crustal structure of the northeastern margin of the tibetan plateau from the songpan ganzi terrane to the ordos basin abstract the 1000 km long darlag lanzhou jingbian seismic refraction profile is located in the ne margin of the tibetan plateau this profile crosses the northern songpan ganzi terrane the qinling qilian fold system the haiyuan arcuate tectonic region and the stable ordos basin the p wave and s wave velocity structure and poisson s ratios reveal many significant characteristics in the profile the crustal thickness increases from northeast to southwest the average crustal thickness observed increases from 42 km in the ordos basin to 63 km in the songpan ganzi terrane the crust becomes obviously thicker south of the haiyuan fault and beneath the west qinlin shan the crustal velocities have significant variations along the profile the average p wave velocities for the crystalline crust vary between 6 3 and 6 4 km s beneath the songpan ganzi terrane west qinling shan and haiyuan arcuate tectonic region p wave velocities of 6 3 km s are 0 15 km s lower than the worldwide average of 6 45 km s north of the kunlun fault with exclusion of the haiyuan arcuate tectonic region the average p wave velocity is 6 4 km s and only 0 5 km s lower than the worldwide average a combination of the p wave velocity and poisson s ratio suggests that the crust is dominantly felsic in composition with an intermediate composition at the base a mafic lower crust is absent in the ne margin of the tibetan plateau from the songpan ganzi terrane to the ordos basin there are low velocity zones in the west qinling shan and the haiyuan arcuate tectonic region the low velocity zones have low s wave velocities and high poisson s ratios so it is possible these zones are due to partial melting the crust is divided into two layers the upper and the lower crust with crustal thickening mainly in the lower crust as the ne tibetan plateau is approached the results in the study show that the thickness of the lower crust increases from 22 to 38 km as the crustal thickness increases from 42 km in the ordos basin to 63 km in the songpan ganzi terrane south of the kunlun fault both the conrad discontinuity and moho in the west qinling shan and in the haiyuan arcuate tectonic region are laminated interfaces implying intense tectonic activity the arcuate faults and large earthquakes in the haiyuan arcuate tectonic region are the result of interaction between the tibetan plateau and the sino korean and gobi ala shan platforms

multi stage granitic magmatism during exhumation of subducted continental lithosphere evidence from the wulong pluton south qinling abstract detailed petrology and zircon u pb dating data indicate that the wulong pluton is a zoned granitic intrusive formed from successive increments of magmas an age range of at least 30 ma is recorded from the 225 235 ma quartz diorite on the pluton margin the ca 218 ma granodiorite in the intermediate zone and the ca 207 ma monzogranite at the pluton center all the granitoids display evolved sr nd pb isotopic compositions with 87 sr 86 sr i of 0 7044 0 7062 unradiogenic nd e nd t values of 6 1 to 3 0 nd model ages of 1 1 1 3 ga and moderately radiogenic pb compositions 206 pb 204 pb i 17 500 17 872 207 pb 204 pb i 15 513 15 549 208 pb 204 pb i 37 743 38 001 in combination with variations in zircon hf isotopic compositions with e hf t values in each stage span 12 units and the hf isotopic model ages of 800 1600 ma these features suggest that the granitoids might have been derived from the reworking of an old lower crust mixed with paleozoic and proterozoic materials the rocks also display an adakitic affinity with sr 479 973 ppm high sr y ratios mostly 60 and negligible eu anomalies eu eu 0 78 0 97 but low rb sr ratios low y 4 6 17 ppm hree yb 0 95 1 7 ppm yb lu 6 7 and dy yb 1 9 2 4 ratios suggesting the absence of plagioclase and presence of garnet amphibole in their residue considering a large gap among their crystallization ages we propose that the geochemical evolution from pluton margin to center was controlled mainly by melting conditions and source compositions rather than fractional crystallization mafic enclaves that were hosted in the quartz diorite and granodiorite are mainly syenogabbroic to syenodioritic in composition and are metaluminous and enriched in lree and liles but are depleted in hfse and display an evolved sr nd pb isotopic composition suggesting that they may have been derived from the partial melting of an enriched mantle lithosphere which was metasomatized by adakitic melts and fluids from a subducted continental crust in combination with the results of the triassic ultra high pressure metamorphic rocks in the dabie orogenic belt we apply a model involving the exhumation of subducted continental crust to explain the formation of the wulong pluton at the first stage a dense and refractory mafic lower crust that was trapped at mantle depth by continental subduction witnessed melting under high temperature conditions to produce the quartz diorite magma characterized by low sio 2 60 65 63 98 wt and high tio 2 0 39 0 86 wt the magma subsequently interacted with mantle peridotite leading to high mg 57 67 and the metasomatism of the overriding mantle wedge at the second stage an asthenosphere upwelling that was probably caused by slab break off at ca 220 ma melted the enriched sub continental lithospheric mantle sclm to produce mafic magmas represented by the mafic enclaves that are hosted in the quartz and granodiorite resulting in the partial melting of the shallower subducted crust and generating the granodiorite that is distinguished by high sio 2 69 16 70 82 wt high al 2 o 3 15 33 16 22 wt and a cnk values mostly 1 05 at the third stage the final collapse of the triassic qinling dabie orogenic belt at ca 215 205 ma caused extensive partial melting of the thickened orogenic lower crust to produce the monzogranite which is characterized by high sio 2 67 68 70 29 wt low tio 2 mostly

petrogenesis and tectonic implications of the early paleozoic granites in the western segment of the north qilian orogenic belt china abstract early paleozoic granitic magmatism in the north qilian orogenic belt records a complete wilson cycle and provides critical geological clues for unraveling the regional tectonic history in this study we report the results of zircon u pb ages hf isotopic analysis and systematic whole rock geochemical data for the late ordovician hongliuhe granite and early silurian qingshan monzogranite in the western segment of the north qilian orogenic belt to constrain their emplacement ages petrogenesis and regional evolution history u pb dating reveals that the hongliuhe granite was emplaced around 453 452 ma and the qingshan monzogranite was emplaced about 440 438 ma a geochemical study shows that the two granites belong to the calc alkaline to high k calc alkaline series the hongliuhe granite shows adakitic and peraluminous features while the qingshan monzogranite belongs to metaluminous to weak peraluminous granites zircons in the hongliuhe granite show ehf t values ranging from 15 1 to 11 7 with two stage hf model ages tdm2 of 687 2398 ma whereas zircons in the qingshan monzogranite show ehf t values ranging from 5 7 to 11 0 with two stage hf model ages from 814 to 1057 ma the geochemical characteristics indicate that the hongliuhe granite was a transitional i s type granite and was generated from a thickened lower crust with the addition of minor paleo to mesoproterozoic crustal materials which left a rutile garnet pyroxene plagioclase residue the qingshan monzogranite formed from the partial melting of mafic crust with minor mantle derived materials and the fractionation of ti bearing phases apatite and pyroxene occurred during the magma s evolution which left an amphibole and plagioclase residue we infer that the hongliuhe granite formed during the northward subduction of the north qilian ocean while the qingshan monzogranite was generated during the post collision stage between the qilian and alxa blocks this observation indicates that syn collision stage of the north qilian orogenic belt began before 453 ma and that the extension regime began prior to 444 ma

improved prediction of spudcan penetration resistance by an observation optimized model abstractthe prediction models of penetration resistances of spudcan footings on sand overlying clay have been established by extensive centrifuge tests and large deformation numerical simulations

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

remnants of a neoproterozoic collisional orogenic belt in the core of the phanerozoic qinling orogenic belt china abstract nw se trending paleotectonics are recognized from the wnw ese trending qinling complex in the core of the qinling belt they consist of trending stratum boundary shear zones deformed linear plutons and ophiolite the field relationships and ages of the deformation magmatism and metamorphism suggest that the paleotectonics formed in neoproterozoic the ages of deformed and undeformed plutons approximately constrain the peak deformation during 958 889 ma both the two trending tectonics constitute an orogen scale tectonic replacement by s2 phanerozoic wnw ese tectonics of s1 nnw sse trending neoproterozoic similar to structural replacement in outcrop the strong contractional deformation high pressure metamorphism and granitic magmatism that evolved from syn collisional s type to post collisional i type and to post collisional a type as well as regional extension at 885 700 ma in qinling display a cycle of a collisional orogeny thus the nw se trending paleotectonics could be remnants of a collisional orogenic belt the belt originally could trend nnw sse if reworking and modification of paleozoic and or mesozoic nne ssw contractional deformation are eliminated accordingly assemblage and breakup of some continental blocks occurred in neoproterozoic in qinling which seems to correspond to assemblage and breakup of rodinia supercontinent in the world this study provides new insight into the evolution of the qinling orogenic belt and the reconstruction of proterozoic supercontinent of china and also presents a case study of recognition of an old orogen from a young orogen

discovery of llandovery silurian graptolites and probable devonian corals in the southalpine metamorphic basement of the eastern alps agordo ne italy we report the discovery of aeronian middle llandovery graptolites and corals of probable devonian age in boudins hosted by greenschists within the southalpine metamorphic basement these discoveries provide key constraints to the depositional age range of the protoliths this remarkable occurrence of almost undeformed graptolites and compound corals in boudins within a metamorphic shear zone indicates very marked local strain partitioning

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

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

multi mode analysis of rayleigh type lg part 2 application to southern california and the northwestern sierra nevada the uc diagram technique described in the companion paper part 1 is applied to nine sets of lg phases recorded through the cedar system in southern california and two sets of lg phases recorded along the northwestern margin of the sierra nevada a clear image of the signal is obtained in time frequency wavenumber space and we discuss in particular observations at 2 5 sec period for events 200 to 300 km outside the profiles from the gross features of uc diagrams we conclude that a representation of lg as a single coherent multi mode wave train is oversimplified in the case of southern california but is more appropriate for the sierra block in southern california peaks observed at group velocities smaller than 3 2 km sec are not predicted by realistic crustal models of the area and are probably due to lateral heterogeneities effects such as mode conversion and multipathing on the other hand for group velocities between 3 2 and 3 6 km sec peaks observed in either area can generally be interpreted in terms of overtones excited at the source and propagating through spatially averaged structures although care must be taken to monitor the stability of the algorithm on actual short period records

cenozoic multi phase deformation in the qilian shan and out of sequence development of the northern tibetan plateau abstract uplift of the tibetan plateau and the distribution of deformation across it are the result of india asia collision which bring an opportunity of understanding intracontinental tectonics in the context of continent continent collision the tibetan plateau is bound on the northern margin by the qilian shan thrust belt and the strike slip haiyuan fault these cenozoic fault systems play a critical role in accommodating continental convergence yet the initiation age deformation sequence and mechanisms of deformation are debated in this study integrated geologic mapping field observations and apatite fission track thermochronology were conducted to constrain the initiation ages of the localized thrust faults and the exhumation history of the central and northern qilian shan northern tibet our analyses reveal the central and northern qilian shan underwent rapid cooling during the cretaceous as a result of a far field tectonic event in the eocene oligocene a period of thrust related cooling occurred along the shule nan shan tuolai nan shan and tuolai shan faults reactivation of the proximal thrust faults and initiation of the western segment of the haiyuan fault occurred at ca 16 ma and drove final accelerated miocene cooling and denudation to the surface we argue that the qilian shan thrust belt has persisted as the stationary and internally deformed northern boundary of the himalayan tibetan orogen since the early cenozoic involved overprinting out of sequence development starting by eocene related to initiation of india asia collision and the basins and ranges across the northern tibetan plateau have since experienced multi phase of growth

mineral compositions of syn collisional granitoids and their implications for the formation of juvenile continental crust and adakitic magmatism continental collision zones have been proposed as primary sites of net continental crustal growth therefore studies on syn collisional granitoids with mafic magmatic enclaves mmes are essential for testing this hypothesis the baojishan bjs and qumushan qms syn collisional plutons in the north qilian orogen nqo on the northern margin of the tibetan plateau have abundant mmes in sharp contact with host granitoids sharing similar constituent minerals but with higher modal abundances of mafic minerals in mmes the qms host granitoids have high sr y and la yb ratios showing adakitic compositions different from the bjs granitoids based on bulk rock compositions and zircon u pb age dating recent studies on these two plutons proposed that mmes represent cumulates crystallized early from the same magmatic system as their host granitoids and their parental melts are best understood as andesitic magmas produced by partial melting of the underthrusting upper ocean crust upon collision with some terrigenous sediments under amphibolite facies here we focus on trace element geochemistry of the constituent mineral phases of both mmes and their host granitoids of the qms and bjs plutons we show that different mineral phases preferentially host different trace elements e g most rare earth elements rees and y reside in titanite only found in the qms pluton amphibole apatite epidote and zircon mostly heavy rees and high field strength elements hfses reside in biotite titanite amphibole and zircon based on the mineral chemical data we testify that for these two plutons mmes are of similar cumulate origin crystallized from primitive andesitic melts in the early stage of granitoid magmatism the primitive andesitic melts for these syn collisional granitoids are most likely produced by partial melting of the oceanic crust supporting the hypothesis of continental crustal growth considering the syn collisional granitoids represent juvenile continental crust as evidenced by distinct mineral compositions the two plutons have different parental magma compositions e g higher tio2 content higher sr y and la yb ratios in the qms parental magmas a signature best understood as being inherited from the source the higher tio2 content of the parental magma for the qms pluton leads to the common presence of titanite in the qms pluton absent in the bjs pluton crystallization of which in turn controls the trace element ree y nb ta and others systematics in the residual melts towards an adakitic signature therefore parental magmas with high tio2 content and high sr y and la yb ratios as well as their further fractionation of titanite are important factors in the development of adakitic compositions as represented by the qms host granitoids this model offers a new perspective on the petrogenesis of adakitic rocks the present study further demonstrates that in general mineral chemistry holds essential information for revealing the petrogenesis of granitoid rocks

linking the sulu uhp belt to the korean peninsula evidence from eclogite precambrian basement and paleozoic sedimentary basins abstract considerable debate on whether and how the sulu orogenic belt extends eastward to the korean peninsula has remained over the past decade new results reported here include the following 1 an eclogite and retrograded eclogite bearing complex hongseong complex is discovered in south korea in which the eclogite occurs as lenses in circa 810 820 ma granitic gneiss shrimp zircon dating of the eclogite yields 230 ma for the metamorphic age and 880 ma for the protolith age 2 the basement of the rangnim gyeonggi and yeongnam massifs have affinities to the basement of the north china block ncb however the gyeonggi massif encloses a minor amount of large or small slabs of the hongseong complex that are similar to the rocks of the sulu belt 3 two main paleozoic basins within the rangnim and gyeonggi massifs have a similar paleozoic tectono stratigraphy to the ncb 4 the imjingang and ogcheon belts do not exhibit any metamorphic characteristics of collisional orogenic belts based on these facts we propose a crustal detachment and thrust model and suggest that the collision belt between the yangtze block yb and ncb sino korea craton is preserved along the western margin of the korean peninsula the lower part of the uhp metamorphosed lithosphere of the yb was subducted under the korean peninsula and not uplifted to the surface the lower crust of the yb the hongseong complex was detached from the subducted lithosphere and thrust over the korean peninsula and inserted into the basement rocks of the gyeonggi massif the upper crust of the yb possibly was detached from the lower crust and overthrusted along the honam and chugaryong shear zones the imjingang and ogcheon belts possibly represent the detached upper crust of yb and their present occurrences are controlled by a mesozoic strike slip shear structure all these detached lower and upper crustal slabs were strongly deformed during the late jurassic and early cretaceous tectonic event leading to their present geological distribution and characteristics

early cretaceous adakitic lavas and a type rhyolites in the songliao basin ne china implications for the mechanism of lithospheric extension abstract there is a broad consensus that the extensive late mesozoic igneous rocks in ne china were generated in an extensional setting however the timing and mechanism of the lithospheric extension remain controversial to address this we carried out an integrated study involving la icp ms zircon u pb dating and geochemical analyses major elements trace elements and hf isotopes for the early cretaceous adakitic lavas and a type rhyolites of the songliao basin the adakitic lavas are andesites and dacites the u pb dating of zircons from the adakitic lavas and a type rhyolites yielded ages between 115 and 102 ma geochemically the adakitic lavas are characterized by high sr contents 515 1610 ppm and low y 0 98 17 58 ppm and heavy rare earth element hree contents and they therefore have high sr y 51 112 ratios they also exhibit high mg values 36 57 and high contents of mgo 0 56 3 53 wt cr 15 7 87 3 ppm and ni 6 7 44 7 ppm that are comparable with those of high mg adakitic rocks the a type rhyolites show an affinity with aluminous a type magmatic rocks and they are metaluminous to peraluminous a cnk 0 98 1 35 enriched in alkalis ga zr nb and y depleted in sr and p and exhibit fractionated ree patterns with negative eu anomalies eu eu 0 05 0 77 all the primary zircons from the adakitic lavas and a type rhyolites have positive ehf t values of 3 6 to 12 1 and juvenile two stage model tdm2 ages of 934 392 ma the adakitic lavas probably resulted from the partial melting of a delaminated region of the lower continental crust with the magmas subsequently interacting with mantle materials upon ascent while the a type rhyolites were probably generated by the partial melting of a dehydrated charnockitic middle lower crust the data suggest that the adakitic lavas and the a type rhyolites formed in an extensional environment related to the rollback of the subducting paleo pacific plate the upwelling of asthenospheric mantle and local delamination of the lithosphere which were induced by rollback of the subducting paleo pacific plate extended from the great xing an range southeastward through the songliao basin to eastern heilongjiang and jilin provinces giving rise to the southeastward migration of lithospheric extension and extension related volcanism after ca 140 ma

lg wave propagation and crustal structure differences near denmark and the north sea summary recordings of regional earthquakes near denmark and the north sea have been collected in an attempt to find the lg wave propagation properties in the area propagation paths of lengths between 300 and 1100km are investigated the amplitudes of the lg waves and the other seismic phases in the seismograms are compared the propagation paths are classified according to whether the lg wave propagation compared to p and s wave propagation is good intermediate or poor it is noted that there is a distinct regional pattern in the lg wave attenuation the lg wave propagation is poor across the north sea the north sea is surrounded by areas of good continental lg wave propagation except towards the south east in the netherlands and western germany an area in skagerrak is characterized by good but slow propagation of a surface wave similar to lg since this is exactly the area of a cluster of earthquakes in skagerrak it is suggested that the two phenomena are caused by the same structural inhomogeneities in the area just off the coast of north western denmark the lg wave propagation is as good in the northern part of denmark as it is in the fennoscandian shield the extent to which the propagation is poor across the north sea is dependent on the frequency of the lg waves observed with different seismographs the lg waves are more attenuated relative to the other seismic phases the higher the frequency is between 0 25 and 2 hz as the structural differences between the fennoscandian bedrock area and the norwegian danish basin with 8 10 km of sediments has no effect on the lg wave propagation it is suggested that the structural anomalies causing the poor lg wave propagation across the north sea are deep crustal anomalies possibly significant variations in crustal thickness it is likely that such deep crustal differences are located along the most prominent upper crustal differences in the sedimentary grabens in the middle of the north sea the poor propagation across the north sea is similar to that observed across a sedimentary basin in eastern greenland which also developed mainly before seafloor spreading separated greenland from europe the frequency dependent propagation of the lg waves is explainable in terms of surface wave propagation across structural boundaries as well as in terms of anelastic attenuation

u pb detrital zircon geochronology from the basement of the central qilian terrane implications for tectonic evolution of northeastern tibetan plateau the tuolai group dominates the central qilian terrane and there are different opinions on the age and tectonic attribute of the tuolai group based on large scale geologic mapping and zircon dating the tuolai group is divided into four parts metamorphic supracrustal rocks neoproterozoic acid intrusive rocks early middle ordovician acid intrusive rocks and middle ordovician basic intrusive rocks the metamorphic supracrustal rocks are the redefined tuolai complex group and include gneiss and schist assemblage by faulting contact zircon u pb la mc icp ms dating was conducted on these samples of gneiss and migmatite from the gneiss assemblage quartzite two mica schist and slate from the schist assemblage the five detrital samples possess similar age spectra have detrital zircon u pb main peak ages of 1 7 ga with youngest u pb ages of 1150 ma they are intruded by neoproterozoic acid intrusive rocks therefore the tuolai group belonging to late mesoproterozoic and early neoproterozoic with this caveat in mind we believe that u pb detrital zircon dating together with the geologic constraints obtained from this study and early work in the neighboring regions we suggest that the formation age of the entire crystalline basement rocks of metasedimentary sequence from the central qilian terrane should be constrained between the late mesoproterozoic and the late neoproterozoic but not the previous paleoproterozoic the basement of the central qilian terrane contains the typical grenville ages which indicates the centre qilian terrane have been experienced the grenville orogeny event

undrained bearing capacity factors for conical footings on clay introduction the bearing capacity of circular foundations on undrained clay is of fundamental importance in many geotechnical problems in particular there are a number of designs of offshore foundations where the foundation can be treated approximately as a large circular footing for instance some gravity bases the spudcan foundations of jack up units and the more recently developed caisson foundations in most cases the footing is not placed at the ground surface and it is important to take into account the depth of embedment furthermore the base of a spudcan is generally not flat but approximates a shallow cone for foundations on soft clays the effect of the increase of strength of the soil with depth needs to be taken into account and this is particularly important for large foundations the purpose of this note is to present calculations of bearing capacity factors for shallow circular foundations accounting for embedment cone angle rate of increase of strength with depth and surface roughness of the foundation the results have widespread application particularly in the offshore industry the soil is assumed to be rigid plastic with yield determined by the tresca condition with an undrained strength su the method of characteristics is used for the bearing capacity calculation as described by shield 1955 eason shield 1960 houlsby 1982 and houlsby wroth 1982a for application to undrained axisymmetric problems some previous results have been published for this problem using similar numerical techniques e g houlsby wroth 1982b salencon matar 1982 houlsby wroth 1983 tani craig 1995 martin 2001 but the study presented here involves a much more comprehensive coverage of the parameters where comparisons can be made with the previous solutions the factors differ by up to about 0 5 which gives some indication of the level of accuracy attainable with this numerical technique exceptionally the rough footing results given by tani craig 1995 are higher by up to about 5 but this may be due to a problem with their numerical integration procedures see martin randolph 2001

detrital zircon u pb ages of middle late permian sedimentary rocks from the southwestern margin of the north china craton implications for provenance and tectonic evolution abstract the southwestern margin of the north china craton ncc is located between the alxa terrane to the northwest the north qilian orogen to the west and the north qinling orogen to the south however the paleogeographic and tectonic evolution for the southwestern part of the ncc in the late paleozoic is still poorly constrained in order to constrain the late paleozoic tectonic evolution of the southwestern ncc we carried out detailed field work and detrital zircon u pb geochronological research on middle late permian sedimentary rocks at the southwestern margin of the ncc the u pb age spectra of detrital zircons from six samples are similar showing four populations of 2 6 2 4 ga 2 0 1 7 ga 500 360 ma and 350 250 ma moreover on the basis of the weighted mean age of the youngest detrital zircons 257 4 ma combined with the published results and volcanic interlayers we propose that the shangshihezi formation formed during the middle late permian our results and published data indicate that the detrital zircons with age groups of 2 6 2 4 ga and 2 0 1 7 ga were likely derived from the khondalite belt and yinshan block in the northwestern ncc the junction part between the north qinling and north qilian orogen may provide the 500 360 ma detrital zircons for the study area the 350 250 ma detrital zircons were probably derived from the northwestern part of the ncc the majority of materials from shangshihezi formation within the study area were derived from the northwestern part of the ncc indicating that the northwestern part of the ncc was strongly uplifted possibly resulting from the progressive subduction and closure of the paleo asian ocean a small amount of materials were sourced from southwestern part of the ncc indicating that the north qinling orogen experienced a minor uplift resulting from the northward subduction of the south qinling terrane