an outline of the plate tectonics of china china is principally a part of the eurasian plate but the margins of the indian and philippine sea plates are involved in the himalayas and in the coastal range of taiwan respectively within the eurasian plate the cathaysian paleoplate is separated from the angaraian paleoplate by the junggar hegen suture which contains paleozoic ophiolites and rare blueschists the three microplates of the cathaysian paleoplate consist of precambrian cratons and or phanerozoic accretionary fold belts these coalesced precambrian cratons record at least six stages of intense orogeny before cratonization the paleozoic to cenozoic accretionary fold belts of china can be correlated with similar events now found in west pacific andean and atlantic type active continental margins ophiolites occupying many of these tectonic zones provide evidence for the age and igneous history of oceanic crust formed during the paleozoic to cenozoic the presence of blueschist in some of these chinese sutures reveals evidence of large scale subduction and tectonic exhumation during consolidation of the eurasian plate cenozoic collision of the eurasian and indian plates produced deformation and uplift of the himalayas strongly influencing the tectonics of western china in contrast mesozoic tertiary evolution of eastern china is typical basin range geology similar to that of the western united states which included development of deep sedimentary basins along with calc alkaline plutonic and volcanic activity associated with crustal thinning and high heat flow the complicated tectonic evolution of china is greatly illuminated by the presence of ophiolites and blueschists in proterozoic to tertiary convergent boundaries these petrotectonic assemblages provide evidence of an extremely mobile history of plate movement in china

the closing of tethys and the tectonics of the himalaya recent geological and geophysical data from southern tibet allow refinement of models for the closing of southern neo tethys and formation of the himalaya shelf sediments of the indian passive continental margin which pass northward into deep sea tethyan sediments of the indus tsangpo suture zone were deposited in the late cretaceous an andean type margin with a 2 500 km long trans himalayan kohistan ladakh gangdese granitoid batholith formed parallel to the southern margin of the lhasa block together with extensive andesites rhyolites and ignimbrites lingzizong formation the southern part of the lhasa block was uplifted deformed and eroded between the cenomanian and the eocene in the western himalaya the kohistan island arc became accreted to the northern plate at this time the northern part of the lhasa block was affected by jurassic metamorphism and plutonism associated with the mid jurassic closure of the bangong nujiang suture zone to the north the timing of collision between the two continental plates ca 50 40 ma marking the closing of tethys is shown by 1 the change from marine flysch like to continental molasse like sedimentation in the indus tsangpo suture zone 2 the end of gangdese i type granitoid injection 3 eocene s type anatectic granites and migmatites in the lhasa block and 4 the start of compressional tectonics in the tibetan tethys and indus tsangpo suture zone south facing folds south directed thrusts after the eocene closure of tethys deformation spread southward across the tibetan tethys zone to the high himalaya deep crustal thrusting barrovian metamorphism migmatization and generation of oligocene miocene leucogranites were accompanied by south verging recumbent nappes inverting metamorphic isograds and by south directed intracontinental shear zones associated with the main central thrust continued convergence in the late tertiary resulted in large scale north directed backthrusting along the indus tsangpo suture zone more than 500 km shortening is recorded in the foreland thrust zones of the indian plate south of the suture and 150 km shortening is recorded across the indian shelf zanskar range and the indus suture in ladakh there was also large scale shortening of the karakoram and tibetan microplates north of the suture as much as 1 000 km shortening occurred in tibet the more recent deformation however involved the spreading of this thickened crust and the lateral motion of the tibetan block along major approximately east west trending strike slip fault zones

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

contemporary sand wedge development in seasonally frozen ground and paleoenvironmental implications abstract contemporary sand wedges and sand veins are active in seasonally frozen ground within the extensive discontinuous permafrost zone in northwest territories canada the region has a subarctic continental climate with 291 mm a 1 precipitation 4 1 c mean annual air temperature warm summers july mean 17 0 c and cold winters january mean 26 6 c five years of continuous observations indicate that interannual variation of the ground thermal regime is dominantly controlled by winter air temperature and snow cover conditions at sandy sites thin snow cover and high thermal conductivity promote rapid freezing high rates of ground cooling and low near surface ground temperatures 15 to 25 c resulting in thermal contraction cracking to depths of 1 2 m cracking potentials are high in sandy soils when air temperatures are

evidence for a submerged and displaced continental borderland north banda sea indonesia the banda sea is separated into two oceanic fragments the south and north banda basins by a large intervening region of ridges and basins called the banda ridges the north and south banda basins appear to be trapped pieces of pre tertiary oceanic crust whereas the banda ridges are a submerged and displaced continental borderland structurally very similar to the southern california offshore borderland analysis of dredged rocks from the banda ridges suggests an origin from western irian jaya

reappraisal of the upper weichselian periglacial environment from danish frost wedge casts abstract in ditches dug for the oil and gas pipe line systems in jutland in 1983 many frost wedge casts were recorded sand wedge casts composite wedge casts and ice wedge casts were all common these phenomena indicate the former existence of continuous permafrost probably during the period of advance and presence of the weichselian ice cap the changes of environment which took place during the colder parts of the weichselian may have promoted relatively unstable soil surface and vegetation conditions when compared to present arctic areas where frost wedges form today it is suggested that even though primary sand infilling in wedges was common in denmark the climate need not have been very arid for their formation and further the environmental conditions in europe during the last ice age can not be readily compared with present arctic regions where frost wedges grow today

possible closed system pingo and ice wedge thermokarst complexes at the mid latitudes of utopia planitia mars abstract on earth hydrostatic or closed system pingos csps are perennial ice cored mounds formed by the freeze thaw cycling of water when or as thermokarst lakes lose their water by drainage or evaporation the mounds vary in shape from circular or sub circular to elongate are sub kilometre in their long axes and may reach decametres in height often the mounds show summit depressions radial fractures and single tiers or benches as the csps degrade small scale debris flows or slumps occur end stage degradation typically exhibits debris laden ramparts elevated metres above the surrounding terrain typically the csps originate and develop where deep and continuous permafrost is ice rich i e infused with excess ice as it is in the tuktoyaktuk coastlands of northern canada mid to northern alaska and northeast russia in these areas or regions the terrain often is populated by a thermokarst lakes and alases thermokarst lake basins absent of water b fields of small scale ice wedge polygons either low centred or high centred relative to the polygon margins and c melt water filled or emptied polygon margin troughs or corner pits individual troughs and pits connected by means of the local topographical gradient comprises beaded stream systems or networks here we report describe and discuss a suite of candidate csps at the mid latitudes of utopia planitia up 35 50 n 80 115 e the morphologies scale and height of the martian mounds as well as the surface textures and characteristics landscape features and spatially associated landforms are consistent with the csps observed in ice rich regions on earth the up mounds are nested in or reside adjacent to flat floored possibly thermokarstic depressions small scale polygons some of which exhibit low or high centres relative to their margins and connected polygon margin depressions or pits that mirror beaded stream systems in ice rich terrain on earth the size frequency distribution of craters in our study region precludes a surface younger than 100 ma thus the csp like mounds in up and the pre mound periglacial revisions of the landscape presented by us may be markers of freeze thaw cycling that occurred much earlier in the amazonian epoch than most of the current discussions about periglaciation in up suggest

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

holocene vegetation change at treeline north of yellowknife northwest territories canada abstract two radiocarbon dated cores from small lakes located approximately 25 km north of the mapped boundary between forest tundra and tundra provide records of postglacial vegetation change at the treeline near yellowknife nwt canada basal radiocarbon dates of 6180 and 7470 yr b p were obtained from the cores the fossil pollen evidence suggests that the initial vegetation was betula tundra with a peatland component alnus became an important constituent of the pollen assemblages between 6900 and 5500 yr b p both lakes record sharp increases in picea cf mariana pollen at approximately 5000 yr b p suggesting the establishment of forest tundra by 3500 yr b p picea mariana forest tundra had withdrawn the proportion of organic to inorganic sediment in the cores was at a maximum between 5000 and 3500 yr b p tundra has dominated the region since 3500 yr b p in northwestern canada the maximum northward advance of treeline occurred between 9000 and 5000 yr b p the asynchrony in treeline advance in central and northwestern canada may reflect that glacial ice persisted in the interior nwt longer than previously believed alternatively the asynchronous history of the treeline may be a result of the geometric properties of the long wave westerly disturbance that is manifest in the median summer position of the arctic front and ultimately controls the geographic location of the treeline

response to the discussion by van vliet lanoe et al of the paper features caused by ground ice growth and decay in late pleistocene fluvial deposits paris basin france bertran et al 2018 geomorphology 310 84 101 abstract the response by van vliet lanoe et al to our paper on potential thermokarst features in pleistocene alluvial deposits from the paris basin france presents inconsistencies that we consider here successively these are 1 the map of the maximum extent of pleistocene permafrost in france 2 the genesis of liquefaction and fluidization structures in periglacial environments 3 the origin of thermokarst lakes and recumbent folds 4 the depositional context of sandy units and 5 the age of the studied deposits all structures result from the interplay between 1 the growth and degradation of ice wedges which are responsible for the development of a mound like topography badland thermokarst reliefs on the edge of the alluvial terrace and from the initiation of ponding elsewhere on the terrace 2 the degradation of lithalsa that developed later in the lacustrine deposits the sequence was dated confidently to the late weichselian

evidence for late proterozoic subduction from 700 myr old blueschists in china in a comprehensive review of the world s blueschists ernst1 noted an apparent absence of these high pressure low temperature rocks in the precambrian and ascribed it to a cooling of the earth in the phanerozoic eon as blueschists are thought to be formed in subduction zones the absence of precambrian blueschists has been adduced as evidence that plate tectonics did not operate on the early earth2 although this conclusion is not inevitable in that ancient mobile belts are more likely to have been obliterated than younger ones3 the presence of paired metamorphic belts of kyanite sillimanite and andalusite sillimanite grade in proterozoic gondwanaland supports the existence of a subducted slab but with ancient geothermal gradients too high to form blueschists along the convergent plate boundary4 5 on the other hand data from inclusions in african diamonds support the opposing view that archaean continental geotherms were similar to those of today6 here we present rb sr and k ar ages of 698 718 myr for phengitic mica from high pressure pelitic schists from the aksu region of china which are interleaved with blueschists we interpret these dates as representing the time of the blueschist facies metatmorphism suggesting that subduction was occurring in the late proterozoic

two contrasting accretion v collision orogenies insights from early paleozoic polyphase metamorphism in the altun qilian north qaidam orogenic system nw china

speculations on the mesozoic plate tectonic evolution of eastern china several orogenic belts transecting eastern china are the sites of former convergent plate margins although there have been varying views on the collisional framework of individual continental blocks styles of convergence at these zones and the timing of respective collisions a tectonic study of eastern china mongolia and the southern soviet far east indicates the collision of the south china block with a combined north china northeast china fold zone block in the late triassic early jurassic their collective suturing to eurasia in the late jurassic early cretaceous followed by the sikhote alin japan block in the mid to late cretaceous the evidence is as follows 1 a linear belt of late triassic early cretaceous granites and granodiorites trends east from the qinlingshan through the dabieshan to the huaiyang massif ophiolites flysch subduction zone melange a paired metamorphic belt indicating north dipping subduction and marine strata of carboniferous to late triassic age from the qinlingshan define the suture between the north and south china blocks 2 a sinuous belt of ultramafics blueschists silicic to intermediate magmatism and west and north vergent folds and thrusts trend from the west margin of the ordos basin through central inner mongolia and along the east great khingan range to the amur river coupled with a mid jurassic early creataceous unconformity a suturing of eastern chinese blocks to eurasia along this zone is suggested 3 a fold and thrust belt with ultramafics flysch blueschists and subduction zone melange along the ussuri river in northeast china indicates the suturing of the sikhote alin japan block to eurasia along a west dipping subduction zone in the mid to late cretaceous similarly a tectonic study of southern china and southeast asia has revealed a complex regional mosaic of suture bounded terrains which nucleated about the eastern western and southern margins of the yangtze craton during the late triassic and early jurassic the evidence is as follows 4 a north south trending belt of ophiolites blueschists calc alkaline volcanics and subduction zone melange including granites granodiorites and strongly deformed marine strata all of late triassic age exposed in the longmenshan of sichuan merge with the kekexilishan ophiolite zone into the ailaoshan tengtiaohe ophiolite and blueschist belt in central yunnan along which the songban ganzi complex and the shan thai malaya block join the craton and 5 a southeastern prolongation of the ailaoshan tengtiaohe belt bifurcates into the southeast trending konvoi zone of northern vietnam and the north south trending pak lay luang prabang zone of laos and eastern thailand zones of ophiolite calc alkaline volcanics and strong late triassic deformation they separate the indosinia and shan thai malaya blocks from the craton respectively these findings differ significantly from previous interpretations of a late paleozoic consolidation of south eastern asia as well as disputing the existence of a true pangea

precambrian blueschist terranes of the world blueschists are most common in post paleozoic orogenic belts higher geothermal gradients and a thinner lithosphere in the precambrian are widely thought to have disfavored blueschist formation however several precambrian blueschist belts have been reported from eurasia on the basis of established geologic and geochronologic constraints these belts are grouped as suspect possible and definite precambrian blueschists suspect ones include those in alaska in anglesey u k in the scandinavian caledonides and in yunnan china possible precambrian blueschist belts are those in delhi india in central china in heilongjiang china and in the urals and altai sayan tien shan u s s r definite precambrian blueschist terranes are those in western africa and in aksu china these blueschist belts are described with emphasis on the age constraints the recently investigated aksu blueschist terrane is presented as an example this high p t coherent blueschist greenschist terrane at aksu is unconformably overlain by a thick unmetamorphosed sedimentary sequence ranging in age from late sinian ca 600 800 ma through eocambrian to paleozoic the basal sinian conglomerates contain subrounded to rounded clasts of the underlying schist clearly demonstrating the pre sinian age of blueschist facies metamorphism characteristic features of the above mentioned blueschist belts include 1 1 the blueschist terranes are coherent and primary compositional lithological layers are well preserved melange belts are not common 2 2 protoliths include pelite sandstone bimodal volcanics limestone and small quantities of pillow basalt and pelagic sediment ultramafics are rare 3 3 the protoliths were subjected to multiple deformation and transitional blueschist greenschist facies metamorphism typical precambrian blueschist contains the assemblage crossitic amphibole epidote chlorite albite quartz actinolite rare lawsonite jadeitic pyroxene and aragonite have been described in few belts 4 4 the metamorphic facies series belongs to the high pressure intermediate type of miyashiro 1961 the precambrian blueschist and other pre mesozoic intracontinental blueschist of eurasia formed before rather than during final closure of intervening ocean basins between pairs of continents they do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction underplating and imbrication of accretionary wedges including both oceanic and continent derived elastics the rare preservation of precambrian blueschist is due to greenschist overprinting when the heat flow and geothermal gradient was high rather than reflecting lack of production their existence in china and africa demonstrates that requisite conditions for blueschist facies metamorphism existed at least 700 ma ago

continental growth of northwest china the northwest corner of china is the most interior part of central asia and includes parts of the siberian tarim and kazakhstan plates where the junggar and tarim basins are separated by the east west trending tianshan range a preliminary terrane map divides the area into thirteen separate areas which are classified into four types l continental 2 oceanic crust 3 island arc 4 composite the boundaries for these terranes are faults that have been previously defined either by geologic mapping or holocene activity passive consolidation of the southern margin of paleo asia is marked by accretion and subduction of the paleo tethys oceanic basins and by development of volcanic arcs paleomagnetic data indicate that the major plates and associated smaller terranes did not reach final consolidation until the permian or later the stratigraphic record reveals development of intracontinental basins in the late carboniferous followed by the pan asian thermal event that gave rise to widespread intrusion of a type granites of late paleozoic age the frontal collision of india along the southern border of asia in the paleocene reactivated nearly all of the older major strike slip and thrust faults formed during the late paleozoic consolidation of paleo asia the complex nature of the continental growth of central asia has produced and destroyed many varieties of sutures and reconstruction of the original plate and terrane configurations therefore is enigmatic

metamorphic and tectonic domains of china abstract ten metamorphic domains can be distinguished in china comprising four cratonic three intracratonic and three intercratonic domains each domain contains one or more metamorphic belts each of which in turn contains a characteristic metamorphic facies or facies series that was formed during a distinct metamorphic epoch the metamorphic domains reflect the tectonic domains and tectonic evolution of china ancient continental nucleii in the north china and tarim alxa cratons were probably unified with the yangtze craton during the early proterozoic to form the china platform widespread greenschist facies metamorphism during the middle and late proterozoic accompanied by glaucophane greenschist facies metamorphism represents a rifting and closure event in the china platform a second rifting and closure event in the china platform occurred during the caledonian the china and siberian platforms were closed during the hercynian to form the eurasian continent closure of the ancient tethys ocean occurred in the indosinian epoch and subduction and collision within xizang tibet and taiwan occurred during mesozoic cenozoic time the distribution in time of types of metamorphism in china suggests cyclical changes of metamorphism known as the archaean proterozoic and phanerozoic megacycles each megacycle since the archaean consists of a change from progressive low to intermediate grade metamorphism to lower grade greenschist metamorphism that was superimposed on a general trend in which high grade metamorphism became progressively less important with time the change in metamorphic megacycles shows a general secular decrease in regional heat supply during metamorphism punctuated by episodic high grade progressive metamorphism within orogenic belts

palaeozoic collisional tectonics and magmatism of the chinese tien shan central asia abstract the chinese tien shan range is a palaeozoic orogenic belt which contains two collision zones the older southern collision accreted a north facing passive continental margin on the north side of the tarim block to an active continental margin on the south side of an elongate continental tract the central tien shan collision occurred along the qinbulak qawabulak fault southern tien shan suture the time of the collision is poorly constrained but was probably in in the late devonian early carboniferous we propose this age because of a major disconformity at this time along the north side of the tarim block and because the youshugou ophiolite is imbricated with middle devonian sediments a younger probably late carboniferous early permian collision along the north tien shan fault northern tien shan suture accreted the northern side of the central tien shan to an island arc which lay to its north the north tien shan arc this collision is bracketed by the middle carboniferous termination of arc magmatism and the appearance of late carboniferous or early permian elastics in a foreland basin developed over the extinct arc thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin post collisional but palaeozoic dextral shear occurred along the northern suture zone this was accompanied by the intrusion of basic and acidic magmas in the central tien shan late palaeozoic basic igneous rocks from all three lithospheric blocks represented in the tien shan possess chemical characteristics associated with generation in supra subduction zone environments even though many post date one or both collisions rocks from each block also possess distinctive trace element chemistries which supports the three fold structural division of the orogenic belt it is unclear whether the chemical differences represent different source characteristics or are due to different episodes of magmatism being juxtaposed by later dextral strike slip fault motions because the southern collision zone in the tien shan is the older of the two the tarim block sensu stricto collided not with the eurasian landmass but with a continental block which was itself separated from eurasia by at least one ocean the destruction of this ocean in late carboniferous early permian times represented the final elimination of all oceanic basins from this part of central asia

sand veins and wedges in cold aeolian environments sedimentary structures formed by the progressive primary infilling of thermal contraction cracks with sand are termed primary sand veins and sand wedges in addition to simple vein or wedge shapes irregularities can be caused by sand veins branching from their sides and toes primary sand wedges form widely in sandy polar deserts locally in sandy areas of tundra and probably in seasonally frozen ground there are no unique criteria for distinguishing primary sand veins and wedges from sand veins and wedges of other origins identification of the former depends on the occurrence of distinctive features not always present and on evaluation of their lithofacies and palaeoenvironmental contexts care and caution are advocated in the use of ancient relict primary sand veins and wedges as quantitative palaeoenvironmental indicators because modern active wedge distribution is poorly known and hence inferred thermal climatic threshold values are questionable it is suggested that ancient relict primary sand wedges exceeding 2 m in depth and with well developed vertical lamination probably indicate the former occurrence of continuous permafrost whereas sand veins and narrow sand wedges frost cracks are potentially ambiguous as they may form not only in the active layer above and within continuous permafrost but also in seasonally frozen ground in non permafrost areas

absolute reconstructions of the paleozoic oceans abstract the orientation of three intraplate magmatic belts the mongolian from 130 to 280 m y south siberian from 320 to 400 m y and baltic from 365 to 400 m y belts which can be considered as hot spot tracks are used together with apparent paleomagnetic pole wander paths to reconstruct true motions of continents during paleozoic times the reconstructions obtained show that the old late precambrian pangea continued to exist in the early paleozoic with an inner arrangement of constituent continents strongly different from that of the late paleozoic pangea the continents were constantly located in the eastern hemisphere only the western hemisphere was occupied by the paleo pacific oceans four oceans existed in paleozoic times between the continents in the eastern hemisphere 1 the iapetus which originated in the late precambrian and closed before devonian times 400 m y 2 the asiatic paleo ocean which originated in the late precambrian and closed 450 m y ago 3 the uralian paleo ocean which opened around 500 m y ago and closed in the latest permian 240 230 m y 4 the paleo tethys which opened in the ordovician 480 450 m y and became the mesozoic tethys life duration of the oceans was 200 400 m y the main trend in the earth s evolution during the paleozoic was a break up of the old precambrian supercontinent and construction of the new late paleozoic pangea

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

the impact of latitude and altitude on the extent of permafrost during the last permafrost maximum lpm in north china abstract this paper examines the relationship between the extent of permafrost during the last permafrost maximum lpm largely between 22 and 17 ka bp and the altitudinal and latitudinal temperature gradients that controlled its extent the region chosen namely the qinghai tibet plateau and the transition to the northern adjacent plains in the north china plain is of relatively limited extent in order to avoid regional effects this area extends over 15 degrees of latitude while altitudes vary from 1000 to 1500 m above sealevel asl in the northern plains to more than 4000 m asl in the central to south part of the qinghai tibet plateau the respective lowest southernmost permafrost occurrences at a northern and a southern position are used for reconstruction of the latitudinal and altitudinal gradients as a first approximation we conclude that the elevation compensation during the lpm for a 1 latitude change was between 309 m and 385 m this is considerably higher than the value calculated for existing permafrost in the region that uses thermal data at a depth of zero amplitude change 139 198 m altitudinal compensation for 1 latitude

silurian devonian active margin deep marine systems and palaeogeography alai range southern tien shan central asia analysis of mid palaeozoic successions in the northern part of the alai range kyrgyzstan and bordering uzbekistan southern tien shan central asia has identified a silurian devonian deep marine depositional system of basin slope facies associations here we document the stratigraphy and sedimentology of a region in central asia that through conflict has become inaccessible for geological research the turbidite dominated pul gon formation silurian accumulated in sea floor depressions and within the inferred basin axis the large scale coarse clastic lenses of the dzhidala formation mostly devonian represent the fills of submarine channels canyons or gullies of the palaeoslope other slope apron processes include sediment slides debris flows and olistoliths the partly time equivalent condensed sequences of the mid to late silurian kursala formation and early devonian tamasha formation represent graptolitic mudstone and chert accumulation respectively together with thin dolomitic limestones that accumulated over c 10 15 ma probably on seamounts in the turkestan fergana ocean between the continental margin of kazakhstania and the alai microcontinent the graptolitic shale rich chakush formation early silurian is geographically and petrographically different from the pul gon and dzhidala formations and suggests a different provenance opposing continental margin or seamount talus

morphology and paleoenvironmental significance of quaternary sand veins sand wedges and composite wedges tuktoyaktuk coastlands western arctic canada sand and sand ice fillings of quaternary thermal contraction cracks on summer and hadwen islands western arctic canada comprise sand veins sand wedges and composite wedges sand wedges in diamicton poor ice diamicton rich ice and ice rich sand generally have simple v shapes whereas those in ice poor sand vary from v shaped to irregular forms and may contain inclusions of host sand these morphological differences are explained in terms of the relative tensile strength of the wedge and host materials bundles of sand veins within sand wedges indicate discrete stages of wedge growth criteria previously proposed for identifying relict sand wedges are reevaluated 1 not all wedges are v shaped some are irregular forms with offshoot sand veins 2 a vertical or steeply dipping lamination is not apparent within all wedges some appear to have a massive fill suggesting that the sand source can be texturally and mineralogically very uniform 3 individual sand veins and groups of veins can be just as common within sandy host materials as the better known sand wedges composite sand ice wedges at crumbling point summer island commenced growth as composite wedges continued as sand wedges were modified by thermokarst and in some cases recommenced at a stratigraphically higher level as ice wedges the sand wedge and ice wedge stages reflect environmental change from cold arid and windy proglacial conditions during oxygen isotope stage 2 to warmer and wetter interglacial conditions during oi stage 1

rbsr dating of the kohistan arc batholith in the trans himalaya of north pakistan and tectonic implications the kohistan arc batholith in northern pakistan is situated between the indus suture and the northern suture it belongs to the trans himalayan belt which continues eastwards as the ladakh arc batholith in northwest india and the kangdese batholith in southern tibet in kohistan the island arc consists upward sequence of the chilas stratiform complex of norites noritic gabbros and chromite layered dunites which formed in the sub arc magma chamber plutons of tonalite and diorite one of which has a rbsr isochron age of 102 12 ma the chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites and the yasin group sediments which formed in overlying intra arc basins and which contain albian aptian faunas all these rocks were deformed by major fold structures which are correlated with the formation of the northern suture ne sw trending basic jutal nomal dykes north of gilgit cross cut all the above structures and have a39ar 40ar hornblende age of 75 ma thus the northern suture must have formed in the period 102 12 to 75 ma with the island arc attached to the eurasian plate further northward subduction of the tethyan plate gave rise to an andean type batholith two plutons of which have rbsr isochron ages of 54 4 ma and 40 6 ma the dir utror group of calc alkaline lavas and sediments with eocene fossils are remnants of the volcanic cover of the andean type batholith we suggest that late cretaceous blueschists formed during subduction under the active continental margin and that continental collision and formation of the indus suture was in the eocene the batholith was intruded by layered aplite pegmatite sheets at 34 14 ma and 29 8 ma rbsr ages in post collisional times the87sr 86sr initial ratios of all the dated rocks range between 0 7039 and 0 7052

an analysis of mechanisms of ice wedge casting based on geotechnical centrifuge simulations abstract here we interpret the outcomes of scaled geotechnical centrifuge simulation of ice wedge casting in terms of the likely significance of quaternary ice wedge pseudomorphs observed within different host sediments six experiments were completed in which 1 30th scale models of an ice wedge embedded within frozen host sediments beneath a 25 mm thick unfrozen active layer were thawed from the surface downwards in the geotechnical centrifuge under a 30 times gravity 30 g acceleration host sediment granulometry and or ice contents were varied in each model with host materials comprising medium sands fine sandy silts and silty clays the model ice wedge was 50 mm at the top 150 mm deep and extended across the full width of the 450 mm wide test box centrifuge scaling laws indicate that under an acceleration of 30 g stress distribution was equivalent to a 13 5 m long section of a 4 5 m high and 1 5 m wide full scale prototype ice wedge covered in an active layer of thickness equivalent to 0 75 m thermal regimes measured pore pressures during thaw observed thaw consolidation and measured host sediment geotechnical properties are utilised in the interpretation of casting mechanisms during a single uniform thaw event it is shown that arching of infilling sediment and the formation of a void is likely if negative pore pressures are developed in the host sediment in fine silt and clays high ice contents are more likely than in sands thaw consolidation is greater positive pore pressures encourage complete filling of the ice wedge void and soft sediment deformation is likely to cause deformation of the cast and reduce its width and depth though natural casting mechanisms are likely to be more complex than those simulated here modelling experiments highlight the need for care when inferring original ice wedge geometry from observed shape and size of quaternary ice wedge casts

the extent of permafrost during the last permafrost maximum lpm on the ordos plateau north china abstract this paper presents new field observations that assist in further defining the extent of last permafrost maximum lpm permafrost and frozen ground in north china in that respect this region is specific by its arid environment and continental position bedrock exposures and sand and gravel pits were examined along a south north transect from southern ordos as far as the vicinity of dongsheng district ordos city on the ordos plateau north china attention focussed upon the recognition of sand wedge remains and possible cryoturbations two types of wedge structures were recognized type a wedges extend to depths in excess of 3 0 m and are thought to have formed under conditions of continuous permafrost type b wedges are restricted to the upper 1 5 m and are thought to reflect discontinuous permafrost and or deep seasonal frost osl dating of wedge infills indicates that type a wedges formed between 23 19 ka and younger type b wedges formed until the early holocene it is now possible to delineate a more exact position of the boundary between continuous permafrost and discontinuous permafrost and or deep seasonal frost on the ordos plateau during the lpm this boundaries occur between dongsheng and jinbian at 37 39 n

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

the mineralogy petrology metamorphic ptdt trajectory and exhumation mechanism of blueschists south tianshan northwestern china abstract the south tianshan blueschist belt extends for about 200 km along the south tianshan suture from changawuzhi to the upper kekesu river the high pressure low temperature high p low t metamorphic rocks contain sodic amphibole garnet phengite albite quartz epidote chlorite and stilpnomelane these blueschists occur as small blocks lenses laminae and bedded slabs or sheets in greenschist facies country rocks a few dismembered ophiolitic nappe slices and marble lenses are interlayered in the high p t metamorphic belt metabasite blueschists are mainly e type morb less commonly n type the protoliths of the meta sedimentary rocks are graywackes which contain volcaniclastic material of e type morbs and probably some continental basement materials derived from the tarim plate these protoliths may represent subduction complexes or meta melanges which formed within an accretionary wedge on the southern side of the yili central tianshan plate yctp at the end of the early paleozoic the mineral assemblages include sodic amphibole mainly crossite garnet phengite epidote chlorite albite estimated to have crystallized at 450 c and 9 10 kbar these conditions correspond to the epidote blueschist facies and are the peak metamorphic conditions the prograde metamorphism is represented by lawsonite blueschist facies and the retrograde is greenschist facies the metamorphic ptdt trajectory of the blueschists runs clockwise and reveals that the metamorphism revolved from laumontite facies lawsonite blueschist facies epidote blueschist facies 408 ma to greenschist facies 345 ma during the creation and exhumation of the blueschists in the south tianshan suture we have recognized six stages of deformation in the blueschists the earlier three stages were concomitant with metamorphism the later three postdated the metamorphism on the basis of the deformation characteristics of the blueschists and coherent rocks the regional geology and the metamorphic ptdt path we discuss the exhumation mechanism of the blueschists the post collisional ductile thrust within the suture probably exhumed the blueschists and coherent rocks allowing them to escape retrograde overprinting in the greenschist facies the influence of erosion on the exhumation of blueschists could not be considered directly and quantitatively because of the absence of devonian strata and the few exposures of early carboniferous clastic rocks all without detritus of blueschists

geochronology petrogenesis and tectonic significance of huashugou granitoids in north qilian nw china abstract north qilian orogenic belt nqob is a typical early paleozoic orogenic belt in china however the tectonic setting of nqob remains controversial especially the post subduction processes in this study we report whole rock geochemistry zircon u pb ages and hf nd isotopic compositions of huashugou granitoids in the western segment of the nqob nw china la icp ms u pb dating of zircon from huashugou granitoids indicate magmatic crystallization ages of ca 420 ma with late paleoproterozoic captured zircons of ca 1700 ma the huashugou granitoids show enrichment in light rare earth elements and large ion lithophile elements e g rb th and u depletion of heavy rare earth elements and high field strength elements e g nb ta and ti negative eu anomalies and a cnk

structural evolution of an a type subduction zone lofoten rombak area northern scandinavian caledonides the climactic phase of the caledonian orogeny in scandinavia involved collision of the baltic and greenland cratons in late ordovician to silurian time we infer that the collision resulted in large scale subduction of the leading edge of the baltic craton beneath an andean type greenland continental margin deep erosional levels in the lofoten rombak area norway sweden 69 n permit direct examination and evaluation of the structural evolution of a continental or a type subduction zone the early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km during this process deformational effects in the lower plate were restricted to its upper few hundred meters ultimately en bloc a type subduction should be limted by buoyancy effects as convergence continued the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone the temporal and spatial relationships between these basement involved thrusts are similar to those observed in foreland fold and thrust terrains field and geophysical data bearing on lower plate behavior elsewhere in the caledonldes and in other collisional orogens suggest that the structural features observed in the lofoten rombak area may be generally characteristic of a type subduction in continent continent collisional settings

ubiquity of anomalous fading in k feldspars and the measurement and correction for it in optical dating anomalous fading rates were measured in k feldspars separated from 49 sediment samples mainly from north america the intensity of the optically stimulated luminescence was found to decrease linea

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

distribution and characteristics of mixed transitional mid permian late artinskian ufimian marine faunas in asia and their palaeogeographical implications abstract the permian marine biogeography of the asian region is characterised by the development of three broad realms boreal tethyan and gondwanan in the early early permian asselian to early artinskian there seems to have existed sharp biogeographical boundaries with no transitional zones between the realms presumably due to the effect of gondwanan glaciation and sharp climatic zonation the mid permian late artinskian to ufimian of the asian region is distinguished by the rise of two broad transitional zones in which mixed marine faunas flourished the northern transitional zone north and northeast china sikhote alin central and northeast japan is characterised by an admixture of both warm water cathaysian and cold temperate boreal genera the southern transitional zone arabian peninsula iran afghanistan the karakorum southeast and central pamir salt range central and northwest tibet shan thai terrane timor and western irian jaya on the other hand is distinguished by incorporating faunal elements of both gondwanan and cathaysian origin the mixed faunas of both transitional zones share several antitropically distributed genera e g monodiexodina lytvolasma and spiriferella and are succeeded by palaeotropical tethyan faunas of late permian age it has been postulated that the mixed mid permian fauna of the northern transitional zone was probably formed in an epicontinental sea which was connected to both the asian tethys and the arctic which facilitated the migration of cathaysian and boreal forms into the epicontinental sea and subsequent faunal mixing the formation of the mixed faunas of the southern transitional zone on the other hand is probably related to the breaking off of the cimmerian blocks from northern gondwana and subsequent drifting into a lower latitudinal zone with geographical proximity to both gondwana and cathaysia during the mid permian

lithalsa distribution morphology and landscape associations in the great slave lowland northwest territories canada abstract lithalsas are permafrost mounds formed by ice segregation in mineral rich soil that occur within the zone of discontinuous permafrost nearly 1800 lithalsas were mapped using archival aerial photographs within the great slave lowland northwest territories canada these are up to 8 m high and several hundred meters in length and width one lithalsa examined by electrical resistivity and boreholes rises 4 to 6 m above an adjacent peatland shows clear evidence of ice segregation at depth and ground heave of between 2 5 and 4 0 m and is estimated to have formed within the past 700 years regionally lithalsas are typically located adjacent to ponds and streams with mature forms vegetated by deciduous birch forest or mixed birch and spruce forest with a herb shrub understory and include circular crescentic and linear forms they are abundant within the lowland region which contains widespread glaciolacustrine lacustrine and alluvial fine grained sediments the lithalsas are most common within the first few tens of meters above the present level of great slave lake indicating that many are late holocene and some less than 1000 years in age a comparison with lithalsas in contemporary environments reveals that comparatively warm but extensive discontinuous permafrost fine grained sediments alluvial lacustrine marine or glaciomarine and available groundwater supply provide the climatic and hydro geological parameters for the development of lithalsas in permafrost terrain the identification of lithalsas in this region is important given their sensitivity to climate change and potential hazards to northern infrastructure upon thawing

early paleozoic polyphase metamorphism in northern tibet china abstract the altun qilian kunlun orogenic system in the northern tibetan plateau is considered as the northernmost orogenic collage of the proto tethyan domain early paleozoic polyphase tectonothermal events related to the subduction of the proto tethyan ocean and subsequent collisional orogeny produced two dominant metamorphic belts the north altun nat north qilian nql hp lt metamorphic belt and south altun sat north qaidam nqd uhp metamorphic belt the nat nql hp lt metamorphic belt mainly consists of blueschist eclogite and hp metasediments eclogites record metamorphic conditions of 420 550 c and 21 25 kbar at 510 460 ma the hp lt metamorphic belt is associated with ophiolite subduction accretion complex and arc magmatic rocks suggesting that the nat nql is a typical early paleozoic accretionary orogenic belt in contrast the sat nqd uhp metamorphic belt is characterized by eclogite and garnet peridotite enclosed within continental orthogneiss and paragneiss these rocks record uhp metamorphism at t 700 c and p 27 kbar geochronology suggests that the uhp metamorphism occurred at 500 423 ma and eclogite protolith ages of 850 750 ma suggest that protoliths are related to neoproterozic continental rifting field relationships petrology and geochronology suggest that the sat nqd uhp metamorphic belt resulted from the deep subduction of continental crust moreover in the sat nqd uhp metamorphic belt two types of hp granulites have been recognized the first type of hp granulite was formed by overprinting of eclogites during exhumation the second type of hp granulite has a single metamorphic history and similar age to that of uhp eclogite penecontemporaneous metamorphic ages but different geothermal gradients between hp granulites and related uhp eclogite define a possible paired metamorphic belt which may be considered to be a diagnostic characteristic of collisional orogens high grade metamorphic rocks in the sat nqd commonly record barrovian type metamorphism although it remains unclear whether or not all of these rocks have an earlier u hp metamorphic history the low medium p barrovian metamorphism and associated magmatism are about 20 50 ma later than uhp metamorphism newly recognized eclogites in the northern part of the east kunlun mountains to the south of qaidam basin have a similar field relationship enclosed in continental gneiss and mineral assemblage to the nqd eclogites and a similar or slightly younger metamorphic age 410 430 ma the occurrence of similar continental type eclogites on two sides of the qaidam basin makes it necessary to reconsider the nature of the qaidam block the polyphase metamorphism in northern tibet can be linked to accretion and collisional orogenesis related to the evolution of prototethys through the early paleozoic

multiple metamorphic events recorded in the metamorphic terranes in central inner mongolia northern china implication for the tectonic evolution of the xing an inner mongolia orogenic belt abstract the tectonic evolution of the xing an inner mongolia orogenic belt ximob in the southeastern segment of the central asian orogenic belt is not well understood it is arguable whether the paleo asian ocean was closed in devonian or early mesozoic and whether the tectonic regimes during carboniferous and permian were operated by continuous subduction arc systems or extension after the orogenic process comprehensive studies of metamorphism for four metamorphic terranes including the ondor sum group baoyintu group xilingol complex and early triassic metamorphic sequences in central inner mongolia provide good constraints on the arguable issues about the ximob according to the zircon u pb dating analyses in this study quartz schists of the ondor sum group in the northern and southern orogenic belts nob and sob have a broad detrital zircon age range from late archean to early paleozoic with a similar youngest deposition age of 434 430 ma but have different zircon age populations with a 650 ma peak for the nob but not for the sob biotite gneisses of the xilingol complex have age populations and the youngest deposition age of 436 ma similar to the ondor sum group in the nob amphibolites in the complex have a metamorphic zircon age of 321 3 ma and hornblende 40ar 39ar age of 309 2 ma the early triassic metamorphic sequences metabasites in wulangou and mica schists in shuangjing both have broad zircon age ranges from the late archean to late paleozoic their protoliths are constrained to have formed later than 258 ma and 261 ma respectively and the metamorphism occurred soon afterwards in the early triassic constrained by an actinolite 40ar 39ar age of 241 19 ma in metabasite and a muscovite 40ar 39ar age of 240 2 ma in mica schist combined with the results previously published this study identified four phases of metamorphism in central inner mongolia since paleozoic the silurian 430 426 ma high p t metamorphism of the ondor sum group was related to bidirectional subduction processes the devonian 400 ma medium p t metamorphism of the baoyintu group suggests a crustal thickening orogeny the carboniferous 337 321 ma low p t metamorphism of the xilingol complex is indicative of an extension process of a previous orogen the early triassic 240 ma extensive medium low p t metamorphism along the solonker suture zone was attributed to the closure of limited ocean basins thus an alternative tectonic scenario for the metamorphic terranes in central inner mongolia involves i the early paleozoic trench arc system 500 425 ma ii the devonian collisional orogeny due to closure of the paleo asian ocean 400 360 ma iii the carboniferous and permian extension after the collision 350 250 ma and iv the early triassic within plate orogeny due to the closure of limited ocean basins 240 ma

formation of hp lt rocks and their tectonic implications in the western tianshan orogen nw china geochemical and age constraints the western tianshan high pressure low temperature metamorphic belt in northwestern china formed within a paleozoic accretionary wedge on the south side of the yili central tianshan plate it comprises various lithologies including mafic metavolcanic rocks metavolcaniclastic rocks metagreywacke marble and serpentinite the metavolcanics comprise eclogite omphacitite epidosite and blueschist the trace and rare earth element characteristics of these rocks indicate morb to oib affinities suggesting a seamount like setting for the protoliths of these rocks the end value of the eclogite 8 9 indicates that the protolith was derived from a long term depleted mantle i e an n morb source supporting the trace and rare earth element characteristics of these samples sm nd isochron dating of 343 44 ma omp gln grt wr and an age of 346 3 ma grt gln were determined using an eclogite boudin within blueschist a well defined 40ar 39ar plateau age of 344 1 ma was obtained for crossite from an omphacite phengite bearing blueschist and is concordant with the sm nd isochron age for the eclogite suggesting that neither argon loss nor excess argon affected the crossite a well defined 40ar 39ar plateau age of 331 2 ma was determined for phengite from the same sample the high pressure metamorphic rocks were formed by the b type subduction of a paleozoic south tianshan ocean about 344 ma ago they were however exhumed to higher crustal levels during the collision of the tarim and yili central tianshan plates about 331 ma ago similar to an alpine type tectonic regime

gold deposits of the northern margin of the north china craton multiple late paleozoic mesozoic mineralizing events the northern margin of the north china craton is well endowed with lode gold deposits hosting a resource of approximately 900 tonnes t of gold the 1 500 km long region is characterized by east trending blocks of metamorphosed archean and proterozoic strata that were episodically uplifted during variscan indosinian and yanshanian deformational and magmatic events at least 12 gold deposits from the daqinshan yan liao includes the zhangjiakou yanshan and chifeng gold districts and changbaishan gold provinces contain resources of 20 100 t au each most deposits are hosted in uplifted blocks of precambrian metamorphic rocks although felsic paleozoic and mesozoic plutons are typically proximal and host 30 of the deposits the lodes are characterized by sulfide poor quartz veins in brittle structures with low base metal values and high au ag ratios although phyllic alteration is most common intensive alkali feldspar metasomatism characterizes the wulashan dongping and zhongshangou deposits but is apparently coeval with variscan alkalic magmatism only at wulashan stepwise 40ar 39ar geochronology on 16 samples from gangue and alteration phases combined with unpublished shrimp u pb dates on associated granitoids suggest that gold mineralizing events occurred during variscan indosinian and yanshanian orogenies at circa 350 250 200 180 150 and 129 ma however widespread permo triassic 250 ma and early jurassic 180 ma thermal events caused variable resetting of most of the white mica and k feldspar argon spectra as well as previously reported k ar determinations compiled and new stable isotope and fluid inclusion data show that most 18o values for ore stage veins range from 8 to 14 indicating a fluid in equilibrium with the precambrian metamorphic basement rocks d values from fluid inclusions range widely from 64 to 154 which is indicative of a local meteoric component in some veins and highly variable 34s data 7 to 17 even within individual deposits indicate various local country rock sources for sulfur fluid inclusions from all districts show variable homogenization temperatures between 240 and 400 c and are consistently characterized by low salinity h2o co2 ch4 n2 solutions although the data are largely consistent with that from orogenic gold veins intrusion related veins and epithermal veins are also recognized the multiple episodes of mineralization are coincident with episodic tectonic reactivations and associated magmatism along the northern margin of the north china craton electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00126 001 0239 2

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

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

the himalayas organic carbon burial and climate in the miocene cooling ages of rock in the himalayas imply that rapid exhumation between the main central thrust system and the south tibetan detachment system occurred between 21 and 17 ma the generation of relief and enhanced weathering which followed this event may have resulted in a pronounced increase in the delivery of dissolved strontium carbon phosphorus and other chemical weathering products to the ocean richter et al 1992 the increased supply of nutrients stimulated productivity in oceanic upwelling zones and expansion of the oxygen minimum zone leading to enhanced burial and preservation of organic matter in the monterey formation and other deposits from this interval a downdraw of atmospheric co2 associated with enhanced chemical weathering rates and organic matter burial may have led to global cooling and the expansion of the antarctic ice sheet by 15 ma the above scenario differs from the monterey hypothesis of vincent and berger in that co2 downdraw is primarily via silicate weathering rather than organic carbon burial and that organic carbon burial is driven by increased delivery of nutrients to the ocean rather than by stronger upwelling a carbon mass balance calculation which assumes that river fluxes have been increasing over the last 40 ma predicts that absolute organic carbon burial increased over this interval while at the same time the fraction of carbon buried as organic matter versus carbonate decreased this implies that the organic carbon cycle has acted as a net source of co2 to the atmosphere over the late cenozoic

accretion subduction erosion and tectonic extrusion during late paleozoic to mesozoic orogenesis in ne china abstract the high pressure heilongjiang complex records a long and complicated evolution from the late paleozoic to mesozoic in ne china which is still subject to controversy in this paper we present a new detailed study including structural geochemical and geochronological analyses ar ar dating on metamorphic minerals u pb sims dating on zircons two main periods of oceanward accretionary growth are identified between 305 ma and 195 ma and around 112 101 ma these periods of accretionary growth were associated with the emplacement of voluminous magmatism slab roll back with continental back arc extension and growth of a forearc accretionary prism between these periods of accretionary growth a major phase of regional subduction erosion took place between 195 ma and 142 ma at the eastern edge of the songliao block the main effects of this subduction erosion phase were that regional jurassic early cretaceous extrusion overlapped with mesozoic accretion and erosion in ne china which disturbed the initial orogenic architecture with a review of relevant zircon ages of this accretionary orogen we unravel the complicated geodynamic history of accretion subduction erosion and tectonic extrusion that shaped ne china from the late paleozoic to mesozoic

micro relief development in polygonal patterned ground in the dry valleys of antarctica polygonal patterned ground in polar regions of both earth and mars has received considerable attention in comparison with the size shape and arrangement of the polygons the diverse micro relief and topography termed here simply relief of polygonal patterned ground have been understudied and yet the relief reflects important conditions and processes occurring directly below the ground surface and it can be observed readily in the field and through remote sensing herein we describe the relief characteristic of the simplest and relatively young form of patterned ground in the dry valleys of antarctic we also develop a numerical model to examine the generation of relief due to subsurface material being shouldered aside contraction cracks by incremental sand wedges growth and to down slope creep of loose granular material on the surface we model the longterm subsurface deformation of ice cemented permafrost as a non linear viscous material our modeling is guided and validated using decades of field measurements of surface displacements of the permafrost and relief this work has implications for assessing the activity of surfaces on earth and mars and much larger scale potential manifestations of incremental wedging in icy material namely the distinct paired ridges on europa

metamorphism and deformation of blueschist belts and their tectonic implications north qilian mountains china abstract two blueschist belts in the north qilian mountains occur in middle cambrian and lower ordovician strata and strike n30 35 w for about 500 km along the caledonian fold belt on the south west margin of the sino korean plate the styles of metamorphism and deformation are quite different in the two belts the middle cambrian to ordovician rocks in the high grade belt are mainly blueschists and c type eclogites in which six phases of lower and upper crustal deformation have been recognized the rocks contain glaucophane phengite epidote clinozoisite chlorite garnet stilpnomelane piedmontite albite titanite and quartz the estimated p t conditions of eclogites are 340 10 c 8 1 kbar and of blueschist 380 c 6 7 kbar the ordovician rocks in the low grade belt are characterized by ductile to brittle deformation in the middle to upper crust the low grade blueschists contain glaucophane lawsonite pumpellyite aragonite albite and chlorite the estimated p t conditions are 150 250 c and 4 7 kbar k ar and 39ar 40ar geochronology on glaucophane and phengite from the high grade blueschist belt suggest two stages of metamorphism at 460 440 and 400 380 ma which may represent the times of subduction and orogeny the subduction metamorphism of the northern low grade blueschist belt took place approximately at the end of the ordovician

climatic reconstruction of the weichselian pleniglacial in northwestern and central europe a multiproxy approach is applied to reconstructing accurately the weichselian pleniglacial climate 72 13 ka in northwestern and central europe standardised translations are used to transform proxy data into climate parameter values for six characteristic time windows quantitative reconstructions of the temperature regime are derived from periglacial coleoptera and botanical evidence while aeolian and fluvial evidence provide qualitative information on wind activity and precipitation respectively the early pleniglacial 74 59 ka the cold period between 41 38 ka and the late pleniglacial 27 13 ka are characterised by a strong north to south climatic gradient over northwestern europe during the last glacial maximum discontinuous permafrost was established in northern france whereas the continuous permafrost zone extended from the uk belgium the netherlands germany and poland to the nordic ice sheets prominent wind activity and a relatively low precipitation typify these periods in contrast an indistinct west to east climate gradient was present in the relatively more temperate intervals e g 50 41 ka seasonally frozen ground conditions prevailed in northwestern europe whereas discontinuous permafrost may be suggested for central germany it appears that the climate conditions in northwest and central europe were controlled by three major factors the scandinavian ice sheet the north atlantic surface water circulation and the russian continent 1998 john wiley sons ltd

tectonic evolution of the west junggar region xinjiang china the west junggar region of northwestern china consists of a hercynian age fold belt occupying a late paleozoic zone of convergence between three major eurasian plates siberian tarim and kazakhstan the northern part of the west junggar area includes part of the irtysh zaysan fold belt that extends to the northwest into the ussr where it marks a broad boundary between the siberian and kazakhstan plates the central part of the west junggar area includes the southeastern extension of the chingiz tarbgatay early paleozoic island arc sequence the southern part of the west junggar area is truncated by the deep fundamental junggar alakol fault that has brought the yili microplate into juxtaposiition with the west junggar region in the vicinity of ebin nur lake to the east the junggar area is covered by sediments of the junggar basin paleogeographic reconstructions and geologic evidence indicates that these plates began converging in the middle carboniferous and by late carboniferous alkali granites began invading the imbricated leading edges of the coalescing plates paleomagnetic data reveals that the plates continued to move along strike slip faults bringing the tarim plate and yili microplate into their present positions sometime in the early mesozoic six separate ophiolite occurrences in the west junggar region are strongly tectonized and dismembered the tangbale ophiolite is considered to be late cambrian 508 20 ma and represents the earliest known oceanic crust in western china petrologic and chemical data indicate that the tangbale ophiolite represents a possible back arc or forearc tectonic setting situated close to a developing island arc the darbut ophiolite has been tectonized to a melange and is overlain by late devonian flysch ages of the radiolaria in the cherts associated with the pillow lavas indicate that it formed in the middle devonian other occurrences of ophiolite melange in the northern part of the west junggar region may be as young as late devonian these ophiolites record ocean crust formation in the early to middle paleozoic within the junggarian ocean and their present tectonic setting indicates continental accretion and imbrication shortly after their formation associated blueschist blocks in the ophiolite melange further supports the concept of imbrication brought about by b type subduction and associated obduction postcollisional alkali granites intrude the oceanic sequences of the west junggar region and are considered to represent melted lower crustal material lead isotopic systematics on feldspars from six separate granite plutons indicate ratios that could only be considered oceanic as far as the magmas are concerned the implications of this finding is that the junggar basin basement rocks in the north and west are probably oceanic evidence presented in this study shows a complex history for the development of the west junggar region major movements of consolidating plates in the late paleozoic followed by rejuvenation of these same zones of closure during the himalayan tectonic event of the late tertiary demonstrates that convergence zones and fold belts may respond to vertical and horizontal stress even after many million years of inactivity

ages of alpine tectonometamorphic events in the northwestern himalaya northern pakistan by 39ar 40ar method the main tectonometamorphic alpine events which occurred in the western part of the western himalayan virgation at south of the main suture in northern pakistan are dated by the 39ar 40ar method these events are as follows 1 the blueschist metamorphism 80 5 ma present in tectonic ophiolititic melanges along the suture this event occurred during an early subduction or obduction stage 2 the alpine barrovian metamorphism 50 30 ma developed in the sedimentary cover and orthogneisses of the indian plate during the postcollisional northward underthrusting of this plate below asia and 3 the emplacement of postmetamorphic leucogranites 23 ma resulting from the melting of the indian crust during the climax of the alpine metamorphism

the crustal accretion history and tectonic evolution of the ne china segment of the central asian orogenic belt abstract the basement rocks in parts of ne china constitute a khondalitic sequence of sillimanite and garnet bearing gneisses hornblende plagioclase gneiss and various felsic paragneisses zircon u pb dating of garnet sillimanite gneiss samples from the erguna xing an jiamusi and khanka blocks indicates that high grade metamorphism occurred at 500 ma evidence from detrital zircons in paleozoic sediments from the songliao block also indicates the former presence of a 500 ma component this uniformity of u pb ages across all crustal blocks in ne china establishes a 1300 km long late pan african khondalite belt which we have named the ne china khondalite belt this indicates the blocks of ne china were amalgamated prior to 500 ma contrary to current belief one scenario is that this amalgamated terrane had a tectonic affinity to the siberia craton once forming part of the late pan african 500 ma sayang baikal orogenic belt extensively developed around the southern margin of the siberia craton this belt was the result of collision between currently unidentified terranes with the southeastern angara anabar province at about 500 ma where the rocks were deformed and metamorphosed to granulite facies it appears likely that at sometime after 450 ma the combined ne china blocks rifted away from siberia and moved southward to form what is now ne china the combined block collided with the north china craton along the solonker xar moron changchun suture zone at 230 ma rather than in the end permian as previously thought local rifting at the eastern extremity of the developing central asian orogenic belt caob resulted in the splitting away of the jiamusi khanka bureya blocks however this was only transient and sometime between 210 and 180 ma these were re united with the caob by the onset of pacific plate subduction which has dominated the tectonic evolution of the region since that time

glacial lake mcconnell paleogeography age duration and associated river deltas mackenzie river basin western canada abstract glacial lake mcconnell lasted from 11 8 to 8 3 ka bp while occupying parts of the great bear great slave and athabasca lake basin the retreating laurentide ice front formed the eastern margin whereas low rolling hills formed the north west and south shorelines three major deltas were deposited at the mouths of the laird peace and athabasca rivers the total extent of all phases of the lake was 240 000 km 2 while the largest extent was 210 000 km 2 at 10 5 ka bp downwarping of the basin by glacial ice was the main cause of the lake whereas sediment blockage between jean marie river and fort simpson was secondary initially glacial lake mcconnell occupied the northwestern corner smith arm of the great bear lake basin and discharged through the hare indian river outlet by 11 5 ka bp the enlarged water body flowed out the great bear river but only for a short period of time the mackenzie river formed the third outlet near jean marie river at 11 ka bp and flow in the great bear river ceased until 9 ka bp at 9 9 ka bp glacial lake mcconnell was impacted by a major flood from glacial lake agassiz with a peak discharge of 2 7 10 6 m 3 sec flood water discharged from glacial lake mcconnell peaking at 0 35 0 57 10 6 m 3 sec and receding flow continued for 30 months the massive influx of floodwater into glacial lake mcconnell caused an abrupt increase of discharge which enlarged the outlet channel to between 6 and 13 km wide between fort simpson and jean marie river at 8 3 ka bp isostatic rebound ended the 3500 year old extensive lake by dividing it into the great slave lake and lake athabasca

air temperature snow cover creep of frozen ground and the time of ice wedge cracking western arctic coast the time of ice wedge cracking is examined for several sites with young and old ice wedges along the western arctic coast the correlation between sharp air temperature drops and ice wedge cracking is highest where the snow cover is thin and least where the snow cover is thick the favoured duration and rate of a temperature drop that results in cracking is about 4 days at a rate of about 1 8 c d such temperature drops have a minimal effect in cooling the top of permafrost wherever there is an appreciable snow cover since short duration temperature drops often result in ice wedge cracking the thermal stresses that trigger cracking probably originate more within the frozen active layer than at greater depth in permafrost although most ice wedges tend to crack during periods of decreasing air temperatures about one third of those monitored have cracked during periods of increasing air temperatures long term measurements show that the active layer and top of permafrost move differentially all year in

ice wedges growth thaw transformation and palaeoenvironmental significance frost cracking and ice wedge growth are fundamental processes within the permafrost environment extensive areas of contemporary permafrost terrain are characterised by frost fissure polygons formed by repeated thermal contraction cracking of the ground the incremental growth of ice veins and wedges along the axes of contraction cracks contributes significantly to the volume of ground ice in near surface permafrost in areas beyond the present limit of permafrost the recognition of ice wedge pseudomorphs provides one of the few unambiguous indications of the former existence of permafrost conditions an understanding of the processes of ice wedge growth and thaw transformation is essential if contemporary ice wedges are to be used as analogues for pleistocene frost fissure structures in palaeoenvironmental reconstructions

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

geochronology of the initiation and displacement of the altyn strike slip fault western china abstract 40ar 39ar dating studies have been carried out along the dangjin pass transect across the altyn strike slip fault assf the samples gave ages of 445 2 454 3 ma in the northern belt 164 3 178 4 ma in the mesozoic shear zone and 26 3 36 4 ma in the cenozoic shear zone using the piercing point of the bashikaogong fault and the cangma heihe fault an offset of 350 400 km along the assf has been estimated the 40ar 39ar dating of the syntectonic growth or syntectonic resetting minerals from the samples within the assf belt and offset estimations from different age piercing points suggest that the assf should be initiated in the middle jurassic 178 4 160 ma combined with previously reported ages our studies show that the assf is characterized by multi phase re activation during 85 100 25 40 and 8 10 ma following its initiation in the middle jurassic in the regional tectonic setting of convergence between the indian and eurasian continents

metamorphism and tectonics of the himalaya metamorphism connected with the main growth and deformation stages of the himalaya ranges from mid cretaceous to the quaternary northward subduction gave rise in the early to mid cretaceous to an island are complex with greenschists amphibolites granulites and blueschists in the w kohistan ladakh and an andean type margin with greenschist amphibolite grade metamorphism in the e india and tibet ophiolitic nappes were thrust southwards over the indian continental margin in the palaeogene during the final stages of closure of the southern arm of tethys the indus zangbo suture bifurcates westwards and between the two sutures lies the kohistan ladakh island arc which was converted to an andean type arc in the palaeocene eocene the karakorum range underwent northward subduction at least during the late cretaceous to give rise to a complex and prominent calc alkaline batholith post collisional southward thrusting of crustal slabs over the indian continental margin took place in the early miocene with production of high grade metamorphism inverted isograds and crustal melt granites later southward thrusting in the pliocene to quaternary gave rise to localized low grade recrystallization against the thrusts

preliminary phanerozoic polar wander paths for the north and south china blocks polar wander paths for the north and south china blocks suggest that 1 both were parts of gondwana in the palaeozoic 2 the north china block accreted to siberia in the late permian and 3 the south china block accreted to the north china block in the middle triassic to the early jurassic comparison of the polar wander path for the south china block with that for northern eurasia suggests that relative motion of over 4 000 km has occurred between them

from subduction to collision in the northern tibetan plateau evidence from the early silurian clastic rocks northwestern china abstractthe qilian orogen records early paleozoic collisional suturing of the qaidam block and the central qilian block to the north china craton the composition and u pb age of detrital zircons and the composition of cr spinels from the early silurian lujiaogou and angzanggou formations in the northern part of orogen indicate derivation from evolving oceanic and continental source terranes heavy mineral chemistry indicates the incorporation of suprasubduction zone type ophiolitic detritus in addition to continent derived material integrating these chemical and age data with regional data on the duration of subduction related magmatic activity syn and postcollisional granitic rocks and high pressure metamorphic rocks constrains the transformation from oceanic subduction to continental collision to 450 440 ma the collision resulted in a flood of detritus into the northern part of the orogen from the central qilian block which masked input from the intervening magmatic arc implying rapid exposure o

copper gold and silver enrichment in ore mylonites within massive sulphide orebodies at hongtoushan vhms deposit n e china abstract the hongtoushan archaean cu zn volcanic hosted massive sulphide vhms deposit occurs in the hunbei granite greenstone terrane liaoning province of ne china the ores were metamorphosed 3 0 2 8 ga to upper amphibolite facies at temperatures between 600 and 650 c and underwent three phases of deformation the deformation has transposed the stratiform orebodies into a y shaped configuration ore minerals in the deposit were originally composed of pyrite pyrrhotite chalcopyrite and sphalerite prograde metamorphism obliterated almost all the sedimentary textures in the ores and the present ore textures were formed substantially during peak and post peak metamorphism intensive remobilisations have occurred in the hongtoushan deposit at various scales there are more than 30 oreshoots inside the main massive sulphide orebodies the oreshoots range from 2 to 30 m in length 0 1 to 1 5 m in width and 2 to 30 m in depth extension most run parallel or sub parallel to the strike of the massive sulphide ore layers with only a few being developed in metamorphic rocks in the immediate vicinity of the sulphide orebodies almost all the sulphide and gangue minerals in the oreshoots have been affected by intense shearing pyrite is characterised by cataclastic flow pyrrhotite chalcopyrite and sphalerite exhibit strong plasticity hydrothermal alteration minerals in the oreshoots are dominated by deformation free hornblende actinolite epidote clinozoisite biotite muscovite albite carbonate and quartz cu au and ag in the ore mylonites average 11 wt 1 74 and 235 g t respectively compared to the massive sulphide ores these oreshoots show remarkable enrichment of these elements with enrichment coefficients of 5 3 5 0 and 4 6 respectively we draw the conclusion that these oreshoots are actually ore mylonites overprinted by late stage fluids the presence of annealing textures in pyrrhotite and the absence in pyrite suggest that formation and succeeding annealing of the ore mylonite took place over the temperature range from 550 to 450 c such a temperature range combined with the geology and mineralogy indicates that these ore mylonites and the derivative oreshoots were formed during upper greenschist facies retrograde metamorphism similarity of the ore mylonites to the massive ores in metal varieties and lead isotope ages suggest that cu au and ag of the overprinting fluids on the ore mylonites were derived mainly from the nearby massive sulphide ores however lead isotope compositions of the ore mylonites also suggest that an external source such as the metamorphic host rocks has also contributed to their metal budget

blueschists in major suture zones of china blueschist facies rocks and c type eclogites occur as allochthonous terranes or as small tectonic blocks in almost all the accretionary fold belts and major suture zones within the interior of china and along the circum pacific and tethys himalayan belts at least forty blueschist localities have been described blueschists around the pacific margins and the himalaya alpine belts are mostly mesozoic in age only those in taiwan and new caledonia are cenozoic intracratonal blueschists of china are older than late permian when the collision amalgamation presumably occurred several alleged precambrian blueschist localities ye 1987 are shown in the recently published metamorphic map of china a precambrian blueschist belt in anhui hubei central china extends for about 2300 km however except for the pre sinian blueschist terrane in aksu the age determination for precambrian blueschist metamorphism elsewhere has not been confirmed characteristic features of chinese blueschists include the following 1 blueschist facies metamorphism predates the continental collision amalgamation 2 most blueschists have undergone multistage metamorphism commonly with increasing temperature and or decreasing pressure for later events 3 assemblages include sodic amphibole epidote albite quartz phengite chlorite sphene lawsonite jadeitic pyroxene and aragonite are not common and 4 protoliths are mainly mafic pelagic and clastic rocks blueschist facies metamorphism in china may be divided into two types on the basis of the imposed geothermal gradient hence the progressive change in mineral assemblage the rare lawsonite bearing blueschists produced at lower temperatures and higher pressures are confined to the inner mongolia and yarlung zangbo suture zones where aragonite and possibly jadeitic pyroxene quartz occur and the blueschists are associated with subgreenschist facies rocks most other blueschists of china formed at higher temperatures along the transitional blueschist greenschist facies boundary where epidote sodic and calcic amphiboles are ubiquitous and the blueschists are interlayered with greenschist facies rocks the common occurrence of intracratonal blueschists in major suture zones of china and elsewhere in eurasia indicates pre mesozoic subduction accretion of oceanic crust and flysch sediments these zones of accretion represent growth around the periphery of major precambrian cratons this fact together with the distribution of ophiolite belts and available paleomagnetic data provides evidence for an extremely mobile history of plate movement in eurasia however the paleozoic intracontinental blueschists and ophiolites of china have formed before rather than during final closure between cratons they do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction underplating and imbrication of oceanic materials similar to the accretion history of westernmost north america

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

distribution and activity of ice wedges across the forest tundra transition western arctic canada remote sensing regional ground temperature and ground ice observations and numerical simulation were used to investigate the size distribution and activity of ice wedges in fine grained mineral and organic soils across the forest tundra transition in uplands east of the mackenzie delta in the northernmost dwarf shrub tundra ice wedge polygons cover up to 40 of the ground surface with the wedges commonly exceeding 3 m in width the largest ice wedges are in peatlands where thermal contraction cracking occurs more frequently than in nearby hummocky terrain with fine grained soils there are fewer ice wedges rarely exceeding 2 m in width in uplands to the south and none have been found in mineral soils of the tall shrub tundra although active ice wedges are found there throughout peatlands in the spruce forest zone small relict ice wedges are restricted to peatlands at tundra sites winter temperatures at the top of permafrost are lower in organic than mineral soils because of the shallow permafrost table occurrence of phase change at 0 c and the relatively high thermal conductivity of icy peat due to these factors and the high coefficient of thermal contraction of frozen saturated peat ice wedge cracking and growth is more common in peatlands than in mineral soil however the high latent heat content of saturated organic active layer soils may inhibit freezeback particularly where thick snow accumulates making the permafrost and the ice wedges in spruce forest polygonal peatlands susceptible to degradation following alteration of drainage or climate warming

ancient suture zones within continents ancient suture belts within continents are deformed regions which contain the remnants of former ocean basins they form when two continents or island arcs that earlier were separated by an ocean basin converge and collide during plate tectonic activity these belts provide the only record we have of deep oceanic crust and of ancient sea floor processes for the first 94 percent of the earth9s history that is prior to the oldest preserved crust in the oceans ten criteria for the recognition and interpretation of these ancient belts are discussed a comprehensive program for the study of these belts should have great scientific and economic benefit for the united states and would be relatively cheap compared to other large national scientific efforts

pt os isotopic constraints on the age of hydrothermal overprinting on the jinchuan ni cu pge deposit china platinum group element pge mineralization occurs associated with mafic ultramafic rocks in different environments although the pge enrichment is primarily caused by magmatic processes remobilization of pd and pt by hydrothermal fluids has likely been an important mechanism in increasing the precious metal grade in many cases however the timing of pge enrichment by hydrothermal fluid processes is commonly difficult to constrain the jinchuan ultramafic intrusion in northwest china is ranked the world s third largest magmatic ni cu sulfide deposit besides the main ore body consisting of net textured and disseminated sulfides there is hydrothermal mineralization associated with sheared contact zones of the intrusion which shows elevated cu and pt concentrations the unusually high pt is hosted mainly in sperrylite within altered silicates in this study we applied the pt os geochronometer to a cu pt rich ore body yielding an isochron age of 436 23 ma this age is significantly younger than the main ore formation age of ca 825 ma but similar to that of the continental collision event between the qaidam qilian block and alax block of north china this indicates that the intrusion may have been uplifted during the paleozoic orogenic processes from deeper crust resulting in the generation of the cu pt rich hydrothermal ore body our new data provide the first strong age constraints on the hydrothermal pge enrichment showing that the pt os isotope system is potentially a powerful tool for dating hydrothermal overprinting on ni cu pge sulfide deposits

provenance of eclogitic metasediments in the north qilian hp lt metamorphic terrane western china geodynamic implications for early paleozoic subduction erosion abstract in this contribution we present new petrological geochemical zircon u pb and hf isotopic data for eclogitic metasediments from the north qilian orogenic belt in which early paleozoic ophiolitic sequences and hp lt metamorphic rocks have been previously recognized the studied metasediments contain eclogite facies assemblages reflecting p t conditions of 450 560 c and 1 9 2 4 gpa consistent with those of adjacent eclogites ree compositions of the eclogitic metasediments overlap those of average upper continental crust the shrimp and la mc icpms u pb data of zircons from metasediments demonstrate input from sources with major age components about 500 ma and 1600 1900 ma peak at 1800 ma with a smaller population at 660 800 ma peak at 750 ma and minor 1 9 ga zircon ages the youngest detrital zircons suggest a maximum depositional age of ca 475 ma and combined with p t conditions and previously determined metamorphic age suggest a burial rate of 0 6 0 7 cm yr zircon hf isotopic analyses show that 500 ma zircons have positive ehf t mainly between 8 and 16 by contrast meso to paleoproterozoic detrital zircons show a broad spectrum of ehf t mainly between 5 and 10 with t dm hf of 1800 2500 ma these data suggest that eclogitic metasediments are derived from a mixture of proterozoic continent crust and juvenile early paleozoic oceanic and or island arc crust and their protoliths likely were deposited in a terrigenous dominated continental margin basin rather than a pelagic oceanic crust environment the new results are consistent with subduction erosion of the active continental margin during south dipping subduction as these sediments formed in a fore arc environment close to the qilian block to the south were transported in the subduction zone to 60 70 km depth prior to their exhumation

cessation of ice wedge development during the 20th century in spruce forests of eastern mackenzie delta northwest territories canada ice wedges are presently inactive in white spruce picea glauca forests of eastern mackenzie delta as shown by the absence of vein ice above ice wedges the maintenance of intact breaking cables and the abundance of rootlets propagating across ridge trough sequences at spruce forest sites near surface ground cooling rates and minimum near surface temperatures from the years 2003 2005 were above ice wedge cracking thresholds ground thermal conditions associated with cracking were recorded at a tundra peatland with active ice wedges annual mean permafrost temperatures at the spruce forest sites ranged from 1 8 to 2 9 c whereas at the tundra peatland the permafrost was colder than 6 c although winter air temperatures are similar throughout the study region deeper snow cover thicker active layers and warmer permafrost account for the more gradual seasonal cooling and warmer near surface temperatures recorded at the subarctic forest sites the subtle ridge to trough relief 12 35 cm of permafr

the first 7 years 1978 1985 of ice wedge growth illisarvik experimental drained lake site western arctic coast a large lake measuring 600 m 300 m and with a depth of nearly 5 m was artificially drained on 13 august 1978 observations on the formation width and depth of thermal contraction cracks for the first 7 years show that the crack profiles and ice wedge growth rates differ markedly from those of old ice wedges reported in the literature the first winter s cracks had box like profiles with surface widths to 10 cm and depths to 2 5 m some cracks continued to widen and deepen once opened in early winter and then narrowed or even closed completely in summer mean growth rates for the ice wedges for the first few years have been as much as 3 5 cm year temperature gradients at the time of first cracking have been in the range of 10 15 c m the growth rate of young ice wedges is site specific and temperature dependent varying with factors such as the temperature gradient vegetation and snow cover so an estimate of the age of an ice wedge from its width will usually be impossible a study of crack w

holocene landscape development and climatic change in the low arctic northwest territories canada abstract holocene landscape development in the low arctic tundra zone was studied through fossil pollen analysis and sedimentary studies of a radiocarbon dated core of lake sediment the results indicate that holocene landscape development in this area consists of three main stages the first stage occurred between 8500 and 6300 14c yr b p when the local vegetation shifted from a herb and graminoid tundra to betula shrub tundra and finally to spruce forest tundra following the regional deglaciation soil erosion decreased greatly in response to vegetation development the second stage is between 6300 and 3000 14c yr b p during which spruce forest tundra was developed and catchment soil erosion decreased due to increased vegetative cover the chronological difference of several hundred years in the rise of black spruce population among the treeline sites may have been caused by edaphic conditions during the third stage from 3000 14c yr b p to the present vegetation degraded to the modern betula dominated shrub tundra peatlands expanded to its modern extent in the treeline zone from 1300 14c yr b p it seems treeline advanced northerly ca 50 km beyond its present position between 6300 and 3000 14c yr b p reflecting likely a northward retreat of the polar front following the demise of the ice sheet in the middle holocene the retreat of the treeline at ca 3000 14c yr b p may have been responded to an expansion of the north polar vortex or an intensification of the meridional airflow

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

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

a cold early palaeozoic subduction zone in the north qilian mountains nw china petrological and u pb geochronological constraints the north qilian high pressure hp low temperature lt metamorphic belt is composed mainly of blueschists eclogites and greenschist facies rocks it formed within an early palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved hp lt metamorphic belts in china here we report new lawsonite bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the north qilian mountains five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks peak pressure conditions estimated from lawsonite omphacite phengite garnet assemblages were 2 1 2 4 gpa at temperatures of 420 510 c in or near the stability field of lawsonite eclogite and implying formation under an apparent geothermal gradient of 6 8 c km 1 consistent with metamorphism in a cold subduction zone shrimp u pb dating of zircon from two lawsonite bearing eclogitic metabasites yields ages of 489 7 ma and 477 16 ma respectively cl images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism an age of 502 16 ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph bearing eclogite which is in agreement with the formation age of early ordovician for some ophiolite sequences in the north qilian mountains and may be associated with a period of oceanic crust formation the petrological and chronological data demonstrate the existence of a cold early palaeozoic subduction zone in the north qilian mountains

field observations of syngenetic ice wedge polygons outer mackenzie delta western arctic coast canada 1 low centered ice wedge polygons in the big lake delta plain of the outer mackenzie delta are unusual because their bounding ramparts appear to have a single ridge twenty two ice wedges in the area were examined between 2006 and 2009 to describe their morphology and diagnose their growth processes the ground above ice wedges had a subtle microtopography with ridges of 0 12 m relief and 4 0 m total width bisected by troughs only 0 05 m wide and 0 09 m deep the troughs initially obscured by vegetation growth and organic matter were underlain by ice wedges with average widths that increased downward in the uppermost 1 m of permafrost from 0 03 to 0 95 m shoulders on the ice wedges indicated vertical growth stages temperatures near the top of permafrost were favorable to thermal contraction cracking and ice veins connected to the top of wedge ice were observed in the active layer at five sites these observations indicate the ice wedges are syngenetic and active although without dating control we cannot unequivocally dismiss the possibility that the wedges are epigenetic features that were truncated by a recent thaw unconformity muted relief above the ice wedges which is uncommon above epigenetic ice wedges was largely due to aggradation of the surface secondary ice wedges have not developed within the polygons suggesting that climate variability has not led to polygon network development in this area wedge ice occupied only about 1 5 of the uppermost 1 m of permafrost a much smaller volumetric proportion than in epigenetic settings

early proterozoic granulites in central korea tectonic correlation with chinese cratons abstract a coherent granulite complex has been discovered in the central part of the korean peninsula formerly regarded as the eastern imjingang belt this granulite complex hwacheon granulite complex hgc experienced two cycles of tectonometamorphic events the first event corresponded to crustal thickening followed by peak granulite facies metamorphism and associated partial melting the quasi isobaric cooling path suggested by the occurrences of secondary garnet mantling orthopyroxene and secondary kyanite in mafic and pelitic granulites respectively indicates substantial residence time at midcrustal levels prior to exhumation of the hgc the timing of peak metamorphism was dated from the unzoned overgrowth rims on zircons in a migmatitic granulite at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra