multichronometric evidence for an in situ origin of the ultrahigh pressure metamorphic terrane of dabieshan china abstract u pb zircon data on gneisses indicate a late proterozoic protolith at 700 ma at bixiling and a late archean to early proterozoic for the shuanghe sheet within the ultrahigh pressure metamorphic terrane of dabieshan china u pb zircon and sm nd dates on the gneisses of the shuanghe sheet constrain a metamorphic peak at 230 ma rb sr and 40ar 39ar analyses of phengite from five gneisses in the bixiling complex yield ages at 198 212 ma overlapping the 190 220 ma age on eclogites the host gneisses record a similar metamorphic evolution as the coesite bearing eclogites since both the peak and the retrograde evolution were contemporaneous this argues for an in situ tectonic relationship

a middle silurian early devonian magmatic arc in the qinling mountains of central china a discussion lerch et al 1995 documented an early to middle paleozoic magmatic arc in the qinling orogenic belt of central china by detailed field mapping geochemistry and geochronology in the heihe area of north qinling their u pb analyses of zircons from i type granites developed in the heihe area suggested short lived arc activity from middle silurian to early devonian time this result is very interesting the termination of calc alkaline magmatism in the devonian is plausible and consistent with our studies on geochemistry and geochronology of a mafic pluton developed in lerch et al s 1995 zone 2 we reported a sm nd mineral isochron pyr 2 plag wr age of 402 6 17 4 ma for the lajimiao noritegabbro that intruded into the metabasaltic lava developed in the south of the shangxian area li et al 1989 then demonstrated its island arc origin using trace elements and sr nd isotopes li et al 1994a along the south marigin of zone 2 metamorphic andesite interbedded with metabasaltand intruded by mafic and granitic plutons was found recently in the heihe area their major and trace element compositions show island arc andesite characteristics w sun et al 1995 the 40ar 39ar age of 426 0 1 3 ma li et al unpub age for an amphibole from a metabasalt sample shows that these rocks underwent low amphibolite facies metamorphism in the middle silurian the caledonian metamorphic age was regarded as the time of collision between this volcanic arc and the north china craton li et al 1992 the tectonic system of the north qinling area between 426 and 400 ma might be similar to that of the andes in south america i e an island arc that had already accreted to the continent while the subduction of oceanic crust continued

crustal large scale serpentinized mantle peridotite body in the sulu ultrahigh pressure metamorphic belt eastern china evidence from gravity and magnetic anomalies abstract from analysis of the geological and geophysical data gravity magnetic seismic and petrophysics we propose that geophysical anomalies are produced by a serpentinized mantle peridotite body smpb situated in the middle to lower crust in the sulu belt the smpb was formed by crustal emplacement of mantle peridotites accompanied by ultrahigh pressure uhp metamorphism our finding suggests an emplacement mechanism for the serpentinized mantle wedge smw early in the subduction process this is different from the classic view which holds that the serpentinized forearc mantle is formed by in situ hydration processes blakely et al 2005 the petrophysical properties of the smpb are similar to those of the serpentinized forearc mantle or smw in modern subduction zones worldwide but the formation mechanisms for smpb and smw are different this observation is important for understanding the geodynamic processes that operated in the large uhp metamorphic belt in the dabie sulu area eastern china

comment on paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing by hua ning qiu and j r wijbrans abstract in a recent study qiu and wijbrans 2006 qiu h n and wijbrans j r 2006 paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing geochimica et cosmochimica acta 70 2354 2370 analyzed ar isotopes extracted by crushing garnets from the dabie shan bixiling eclogite and claimed to have constrained a pre triassic 450 ma episode of uhp metamorphism from primary fluid inclusions however in the absence of careful sample characterization and stepped heating analyses the reported ages are more easily explained as experimental artifacts related to ar extracted from either mineral inclusions or the interface sites between mineral inclusions and the garnet matrix dabie shan garnets commonly contain mineral impurities such as k rich omphacite and or k feldspar if dabie shan uhp metamorphism is of the generally accepted triassic age 210 240 ma the apparent age of a phengite sample in equilibrium with the garnet can be explained by the presence of an extraneous 40ar component with mean 40ar 36ar value of 5000 this value is similar to the composition of extraneous 40ar in other eclogite facies terrane

uhp metamorphism and exhumation of the dabie orogen china evidence from shrimp dating of zircon and monazite from a uhp granitic gneiss cobble from the hefei basin a shrimp u pb study of zircons separated from an ultrahigh pressure uhp granitic gneiss cobble sample ds12 from the upper jurassic fenghuangtai formation of the hefei basin north of the dabie orogen has identified three different domains 1 cores some of which show straight boundaries and strong oscillatory zoning with th u ratios of 0 12 0 70 and an imprecise upper intercept age of 777 220 ma mswd 2 4 2 mantles variable in shape and cathodoluminescence cl intensity and containing many uhp mineral inclusions including coesite and omphacite and with th u ratios of 0 02 0 26 and a weighted mean 206pb 238u age of 244 5 ma mswd 4 7 and 3 rims which are more homogeneous and luminescent in cl than the mantles and contain fewer uhp mineral inclusions with th u ratios of 0 01 0 05 and a weighted mean 206pb 238u age of 226 2 ma mswd 0 65 monazites from the same sample do not contain coesite and omphacite inclusions but they do show sector zones and are composed of two contrasting domains the light domains in backscattered electron bse images have a th u ratio of 7 3 18 9 and a weighted mean 206pb 238u age of 221 2 ma mswd 0 94 the dark domains in bse have a th u ratio of 2 8 7 2 and have a weighted mean 206pb 238u age of 218 2 ma mswd 0 96 based on these data and a review of the literature it can be concluded that 1 the protolith of gneiss sample ds12 is a granitoid of neoproterozoic age 2 the ages of 244 and 226 ma for the mantle and rim domains of the zircons are time records of two discrete stages in the uhp evolution of dabieshan 3 the ages of 221 and 218 ma for the light and dark gray domains respectively of the monazites record the times of two stages of retrogressive metamorphism during exhumation of the uhp granitic gneiss to a higher crustal level 4 the exhumation rate of the gneiss is estimated to be 6 km ma and 5 the uhp metamorphic blocks of the dabie orogen provided significant detritus to the hefei basin in the jurassic

fluid rock interactions during uhp metamorphism a review of the dabie sulu orogen east central china comprehensive review on the characteristics of petrology oxygen isotope fluid inclusion and nominally anhydrous minerals nams for many dabie sulu ultrahigh pressure uhp metamorphic rocks including drill hole core samples reveals that fluid has played important and multiple roles during complicated fluid rock interactions attending the subduction and exhumation of supracrustal rocks we have identified several distinct stages of fluid rock interactions as follows 1 the neoproterozoic supercrustal protoliths of uhp rocks experienced variable degrees of hydration through interactions with cold meteoric water with extremely low oxygen isotope compositions during neoproterozoic snow ball earth time 2 a series of dehydration reactions took place during triassic subduction of the yangtze plate beneath the sino korean plate the released fluid entered mainly into volatile bearing high pressure hp and uhp minerals such as phengite zoisite epidote talc lawsonite and magnesite as well as into uhp nams such as garnet omphacite and rutile 3 silicate rich supercritical fluid hydrous melt existed during the uhp metamorphism at mantle depths 100 km which mobilized many normally fluid immobile elements and caused unusual element fractionation 4 the fluid exsolved from the nams during the early exhumation of the dabie sulu terrane was the main source for hp hydrate retrogression and generation of hp veins 5 local amphibolite facies retrogression at crustal depths took place by infiltration of aqueous fluid of various salinities possibly derived from an external source 6 the greenschist facies overprinting and low pressure lp quartz veins were generated by fluid flow along ductile shear zones and brittle faults during late stage uplift of the uhp terrane

triassic polyphase deformation in the feidong zhangbaling massif eastern china and its place in the collision between the north china and south china blocks the feidong zhangbaling massif is located between the dabieshan and sulu areas along the tan lu fault in eastern china five tectonic metamorphic events are distinguished there the earliest deformation d1 corresponds to a southward compression that occurred during subduction of the south china block below the north china block top to the south shearing is coeval with late permian early triassic blueschist facies metamorphism and possibly with the development of south verging recumbent folds in the neoproterozoic paleozoic sedimentary cover of the south china foreland the main ductile deformation d2 is an extensional one characterized by top to the north shearing coeval with the early stage of exhumation of the high pressure rocks a top to the nw ductile shearing and microfolds overturned to the northwest belong to a second deformational phase of exhumation d3 which is distinct from the main event d2 previous 40ar 39ar mica dates ranging between 245 and 212 ma suggest late permian early triassic ages for the d1 to d3 events the d4 event produced ne sw trending folds in the sedimentary cover interpreted as gravity collapse structures a late cretaceous brittle extensional event d5 controls the opening and infill of continental half grabens in the study area the tan lu fault is a cretaceous brittle normal fault the lack of ductile deformation presented along the tan lu fault suggests that it did not play a significant role in the exhumation of high pressure metamorphic rocks

a 1 78 ga large igneous province in the north china craton the xiong er volcanic province and the north china dyke swarm abstract the 1 78 ga xiong er volcanic province xvp and coeval north china giant mafic dyke swarm ncds are the most important magmatic events occurring after the amalgamation of the north china craton ncc the xvp consists of 3 7 km of extrusive volcanics and some feeder dykes sills located along the southern margin of the ncc and extending over an area 0 06 m km2 compositions vary from basalt to rhyolite but are predominantly intermediate in terms of silica content there are also minor sedimentary intercalations and pyroclastic units the sedimentary interlayers indicate an environment changing from continental facies to oceanic facies up section the xvp is characterized by fractional crystallization from an em i type mantle source and both continental arc andean type and rift environments have been proposed the ncds is widespread in the central ncc with an outcrop area 0 1 m km2 and are exposed at variable depths up to 20 km deepest in the north dyke compositions vary from basalt to andesite and dacite but are dominantly mafic and comprise two series of magmatism previous studies revealed that the ncds recorded assimilation and fractional crystallization of an em i type magma source with a minor dm contribution in the younger magmas both syn collisional and intra continental anorogenic environments have been proposed spatial and petrogenic correlations suggest a cogenetic relationship between the ncds and xvp and considered together they define a large igneous province lip of 0 1 m km2 in area and 0 1 m km3 in volume which is also notable for its continuous compositional range from mafic to felsic with no gap at intermediate compositions the petrology is explained by a common magma source that undergoes a silica poor and iron enriched fractionation trend at depth followed by a silica rich and iron poor fractionation trend in shallow level magma conduits dykes and surface lavas a mantle plume is favored as the cause of this 1 78 ga north china lip

late quaternary history of the mackenzie beaufort region arctic canada from modelling of permafrost temperatures 2 the mackenzie delta tuktoyaktuk coastlands in the mackenzie beaufort region maximum permafrost thickness is 750 m in the pleistocene tuktoyaktuk coastlands less than 100 m in the holocene mackenzie delta and 500 m and anomalously warm in the big lake delta plain between the two areas numerical modelling has been used to derive surface temperature histories that fit ground temperatures and permafrost conditions at 12 wells in the unipkat kumak and taglu hydrocarbon fields the models indicate that the present holocene mackenzie delta was built by fluvial processes into a submarine trough the delta front passed a site presently some 20 km from the coast about 4 5 ka and subsequently 58 m of ice bonded permafrost has aggraded in contrast the big lake delta plain was a subaerial platform for much of the wisconsinan it experienced several thousand years of inundation in the holocene probably due to widespread development of thermokarst lakes at sites 8 12 km from the coast the present subaerial conditions were established 0 5 1 5 ka throu

a model of coastal evolution in a transgressed thermokarst topography canadian beaufort sea abstract the coast of the southern canadian beaufort sea consists of bluffs of ice bearing quaternary sediments and of spits and barrier islands enclosing more or less completely lagoons and complex embayments formed by the breaching of thaw lakes the study of the evolution of this coast developed in a thermokarst topography and of the shallow stratigraphy of the inner continental shelf has resulted in the development of a conceptual model of coastal evolution five stages have been defined in this model which form an idealized continuum of coastal evolution according to the model the formation of embayments spits and barrier islands is primarily controlled by the pre existing topography relative sealevel rise and local sediment supply sealevel rise over a topography of thermokarst basins formed in ice bearing sediments leads to the formation of embayments and rapidly eroding headlands from which spits develop geomorphic evidence suggests that barrier islands mainly form by spit breaching or by erosion of tundra islands breached lake basins act as sediment sinks and may restrict the development of coastal accumulation features spits and barrier islands are rapidly retreating landward the mean retreat rates being 2 2 m a 1 and 3 9 m a 1 respectively on an annual time scale spits and barrier islands retreat in response to episodic storm generated overwash events but on a geologic time scale relative sealevel rise in the major forcing mechanism of landward migration while retreating landward these coastal accumulation features may be drowned and possibly partially preserved in breached lake basins during the holocene transgression the shoreface has undergone erosional retreat partly eroding the pre existing topography but preserving the bottom of some breached thaw lakes and lagoons which are now overlain by a transgressive sand sheet on the inner shelf our observations provide the basis of a semi quantitative model of shoreline changes for the barrier coast of the canadian beaufort sea which relates coastal stability to bluff erosion ground ice content sediment texture offshore sediment supply wave energy volume of breached lake basins relative sealevel rise and regional slope

numerical model of the geothermal regime on the beaufort shelf arctic canada since the last interglacial 1 a finite element geothermal model is developed for the outer mackenzie delta beaufort sea shelf to predict permafrost evolution since the last interglacial 130 116 kabp cal the purpose is to reconcile sparse observations of the depth and extent of ice bonded permafrost with sediment properties and the paleoenvironment sea level curves determine as a function of time areas of the shelf that were subaerially exposed promoting permafrost aggradation and areas that were submerged promoting permafrost degradation assuming as a model starting point that a paleoclimate similar to today persisted through the last interglacial permafrost subsequently aggrades in depth and advances seaward from the present shoreline to the shelf slope bathymetric break by the last glacial maximum lgm 26 kabp cal modeled permafrost exhibits reduced growth in depth and seaward progression that correlate with early and middle wisconsin stillstands in sea level following the lgm and rise in sea level offshore permafrost degrades and permafrost base rises 100 m to its present depth of 600 m the offshore limit of modeled ice bonded permafrost lies at the 95 m isobath within 1 km of the bathymetric shelf slope break the model replicates features of offshore permafrost body observed seismically and demonstrates that warm outflow from the mackenzie river depresses the upper surface of offshore permafrost by tens of meters to the 20 m isobath although pleistocene permafrost predated the wisconsinan the model demonstrates that the paleoenvironment of the last 125 000 years is sufficient to develop the depth seaward extent and principal features of the permafrost body

neoproterozoic paleozoic and mesozoic granitoid magmatism in the qinling orogen china constraints on orogenic process abstract the qinling orogen is one of the main orogenic belts in asia and is characterized by multi stage orogenic processes and the development of voluminous magmatic intrusions the results of zircon u pb dating indicate that granitoid magmatism in the qinling orogen mainly occurred in four distinct periods the neoproterozoic 979 711 ma paleozoic 507 400 ma and early 252 185 ma and late 158 100 ma mesozoic the neoproterozoic granitic magmatism in the qinling orogen is represented by strongly deformed s type granites emplaced at 979 911 ma weakly deformed i type granites at 894 815 ma and a type granites at 759 711 ma they can be interpreted as the products of respectively syn collisional post collisional and extensional setting in response to the assembly and breakup of the rodinia supercontinent the paleozoic magmatism can be temporally classified into three stages of 507 470 ma 460 422 ma and 415 400 ma they were genetically related to the subduction of the shangdan ocean and subsequent collision of the southern north china block and the south qinling belt the 507 470 ma magmatism is spatially and temporally related to ultrahigh pressure metamorphism in the studied area the 460 422 ma magmatism with an extensive development in the north qinling belt is characterized by i type granitoids and originated from the lower crust with the involvement of mantle derived magma in a collisional setting the magmatism with the formation age of 415 400 ma only occurred in the middle part of the north qinling belt and is dominated by i type granitoid intrusions and probably formed in the late stage of a collisional setting early mesozoic magmatism in the study area occurred between 252 and 185 ma with the cluster in 225 200 ma it took place predominantly in the western part of the south qinling belt the 250 240 ma i type granitoids are of small volume and show high sr y ratios and may have been formed in a continental arc setting related to subduction of the mianlue ocean between the south qinling belt and the south china block voluminous late stage 225 185 ma magmatism evolved from early i type to later i a type granitoids associated with contemporaneous lamprophyres representative of a transition from syn to post collisional setting in response to the collision between the north china and the south china blocks late mesozoic 158 100 ma granitoids located in the southern margin of the north china block and the eastern part of the north qinling belt are characterized by i type i to a type and a type granitoids that were emplaced in a post orogenic or intraplate setting the first three of the four periods of magmatism were associated with three important orogenic processes and the last one with intracontinental process these suggest that the tectonic evolution of the qinling orogen is very complicated

clinoenstatite exsolution in diopsidic augite of dabieshan garnet peridotite from depth of 300 km clinoenstatite exsolution lamellae have been discovered in clinopyroxene grains of garnet peridotite from the bixiling massif of the ultrahigh pressure metamorphic uhpm terrane in the dabie mountains china using transmission electron microscopy we show here that although the lamellae are low clinoenstatite lclen with p 2 1 c space group symmetry they contain nanometer scale antiphase domains indicating that the originally precipitating phase had c 2 c space group symmetry the lamellae are oriented parallel to 010 and 18 to 001 of the host approximately parallel to 401 both the very cold exhumation path of the dabie uhpm terrane and the orientation of the lamellae preclude the possibility that the originally precipitating phase could have been high temperature clinoenstatite htclen we conclude that the lamellae precipitated as high pressure clinoenstatite hpclen and subsequently inverted to lclen analysis of the incomplete high pressure crystallographic data available for compositions approximating those of the lamellae and host suggests a minimum pressure of precipitation of 9 gpa for dabie and about 12 gpa for similar exsolution previously observed in the alpe arami peridotite switzerland the bixiling complex is a crustal cumulate hence this result extends the minimum depth of subduction of continental rocks in the dabie sulu orogen to 300 km p 9 gpa

timing of collision of the sino korean and yangtse cratons u pb zircon dating of coesite bearing eclogites the suture zone between the sino korean and yangtse cratons in central china is marked by a series of fault bounded belts composed of crustal protoliths in the dabie mountains some of these blocks contain coesite and diamond indicating subduction to minimum depths of 120 km as a result of continental collision the blocks range from the structurally lowest ultrahigh pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest low greenschist facies belt furthermore the faults mark large contrasts in metamorphic grade these features form a structure that is similar to metamorphic core complexes indicating extension as part of the exhumation process timing of the continental collision has previously been inferred as late triassic from regional geologic relations or caledonian from 40 ar 39 ar dating of the adjacent and possibly related qinling metamorphic belt u pb dating of zircon from ultra high pressure eclogites yields a metamorphic age of 209 2 ma this age coincides with later stages of collision and is therefore a minimum estimate of the time of collision

taliks in relict submarine permafrost and methane hydrate deposits pathways for gas escape under present and future conditions we investigate the response of relict arctic submarine permafrost and gas hydrate deposits to warming and make predictions of methane gas flux to the water column using a 2 d multiphase fluid flow model exposure of the arctic shelf during the last glacial cycle formed a thick layer of permafrost protecting hydrate deposits below however talik formation below paleo river channels creates permeable pathways for gas migration from depth an estimate of the maximum gas flux at the present time for conditions at the east siberian arctic seas is 0 2047 kg yr 1 m 2 which produces a methane concentration of 142 nm in the overlying water column consistent with several field observations for conditions at the north american beaufort sea the maximum gas flux at the present time is 0 1885 kg yr 1 m 2 which produces a methane concentration of 78 nm in the overlying water column shallow sediments are charged with residual methane gas after venting events sustained submergence into the future should increase gas venting rate roughly exponentially as sediments continue to warm studying permafrost associated gas hydrate reservoirs will allow us to better understand the arctic s contribution to the global methane budget and global warming

a sea level curve for the canadian beaufort shelf radiocarbon dated peat and peaty clay samples from geotechnical boreholes in the canadian beaufort continental shelf have been used to reconstruct a late quaternary relative sea level rsl curve the samples were carefully selected and evaluated using palynological techniques to ensure that reasonable age error limits could be given to each sample the dated samples were then related to the local geological setting using seismic profiles to determine the environment of deposition the resulting data show a rise of 140 m in rsl since 27 000 years bp a minor lowering of rsl at some time between 20 000 and 10 000 years bp is inferred from acoustic data contributions from basin subsidence sediment loading and consolidation account for 35 m of the total rsl rise the rsl curve is interpreted in the light of recent models of the isostatic and eustatic responses of the earth s crust at the laurentide ice sheet margin ice may have been more extensive during the middle wisconsinan than previously thought a

metamorphic evolution and 40ar 39ar geochronology of the wuguan complex eastern qinling area china implications for the late paleozoic tectonic evolution of the qinling orogen abstract the late paleozoic tectonic evolution of the qinling orogen is poorly constrained the wuguan complex which lies between the north qinling and south qinling tectonic belts represents a late paleozoic metamorphic belt of the qinling orogen in this contribution we report new metamorphic pressure temperature p t constraints and 40ar 39ar ages of garnet amphibolites and garnet bearing metapelites from the wuguan complex textural observations and mineral compositions especially garnets with two stages of growth indicate the wuguan complex experienced two distinct metamorphic events conventional geothermobarometry and phase equilibria modelling indicate that the earlier metamorphism had peak p t conditions of 660 725 c and 8 0 12 0 kbar with a clockwise p t path whereas the later metamorphic overprint records p t conditions of 560 610 c and 7 5 9 0 kbar hornblendes from amphibolites yield 40ar 39ar ages of 306 299 ma whereas muscovites from garnet bearing metapelites yield 40ar 39ar ages of 252 251 ma the two groups of 40ar 39ar ages correspond to the cooling time of the two distinct metamorphic events respectively combining with previously published zircon u pb ages from the wuguan complex and two groups of metamorphic ages from the guishan complex and the xiongdian eclogite belt we infer that the earlier metamorphism probably occurred at the hangingwall of a subduction zone during carboniferous subduction of oceanic crust the later metamorphic overprint might have been related to the initial collision between the sino korean and yangtze cratons during the late permain

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

crustal evolution of the north qinling terrain of the qinling orogen china evidence from detrital zircon u pb ages and hf isotopic composition abstract the north qinling terrain reveals a different geological history from the north china block to the north and the south qinling micro continental block to the south the timing of events and geochemical features of the north qinling terrain are essential for understanding the evolution of the qinling orogenic belt that joins the north and south china blocks this study presents detrital zircon ages and hf isotopic compositions from the guandaokou group a cover sequence of the north china block and from the kuanping group an upper basement sequence of the north qinling terrain the youngest detrital zircon u pb ages constrain the initiation of sedimentation in the guandaokou group to 1 85 ga and the maximum depositional age of the kuanping group to 640 ma zircon u pb ages and hf data testify to a major phase of magmatism in the north qinling terrain at 1000 900 ma the magmatic rocks were largely produced from reworking of early paleoproterozoic 2 5 2 0 ga crust with minor contribution of late paleoproterozoic 1 7 ga juvenile material this age record differs in time from the two important thermal events of 1 85 ga and 2 5 ga widely manifested in the north china block the north qinling terrain can thus be regarded as a separate micro continent with a pre neoproterozoic evolution different from that of the north china block the final assembly of the north qinling terrain to the north china block took place after about 640 ma

detection of subbottom ice bonded permafrost on the canadian beaufort shelf by ground electromagnetic measurements accurate knowledge of the location of subbottom ice bonded permafrost is essential in planning major infrastructure projects such as pipelines in the arctic a combination of refraction and shallow reflection seismic surveys and drilling has been considered the most effective means of permafrost detection in offshore environments field experiments recently carried out on the beaufort shelf by the geological survey of canada indicate that multifrequency horizontal loop electromagnetic hlem measurements are a fast and cost effective alternative during test surveys 60 to 80 soundings were completed in a day analysis of hlem field data shows that a four layer model is appropriate for the local geologic conditions the first layer in the model sea ice is highly resistive 5000 m the second layer consists of seawater and seawater saturated sediments estimated resistivity 1 m because their true resistivities are not significantly different it was not possible to separate the two highly cond

rapid earth expansion an eclectic view abstract paleobiogeography plate evolution and minor amounts of subduction require a rapidly expanding earth during post triassic time not plate tectonics an asteroid impact at the p tr boundary in the congo basin ruptured the lithosphere and together with another impact in the carnian of arizona caused earth s volume to subsequently expand the increase in volume was likely due to inner core and lower mantle transformation growth at the expense of the fluid outer core a modified pacific reconstruction is proposed that closes up the pacific basin in the triassic and allows continents to cover a 55 radius earth some serious weaknesses in the plate tectonics theory are noted and tectonics by high energy impacts is discussed

cryostratigraphy paleogeography and climate change during the early holocene warm interval western arctic coast canada botanical and cryostratigraphic records from northwest canada indicate that the climate of the early holocene was considerably warmer than today tree line was over 100 km farther north and a thaw unconformity dating from 8000 14c years bp formed at the base of an active layer 2 5 times thicker than at present numerous thermokarst lake basins formed in the preceding millennia both the botanical and cryostratigraphic indices described are products of summer conditions previous reconstructions of early holocene climate have not assessed the significance of paleocoastal location on the seasonality and extent of apparent climate warming at present there is a steep gradient in growing season conditions between cooler sites on the beaufort sea coast and warmer inland locations winter conditions are more uniform because both sea and land are snow covered coastal retreat in the region has been rapid due to sea level rising over a gently sloping shelf containing readily erodible sediments the coastlin

glaciotectonic deformation and reinterpretation of the worth point stratigraphic sequence banks island nt canada hill hole pairs comprising an ice pushed hill and associated source depression cluster in a belt along the west coast of banks island nt ongoing coastal erosion at worth point southwest banks island has exposed a section 6 km long and 30 m high through an ice pushed hill that was transported 2 km from a corresponding source depression to the southeast the exposed stratigraphic sequence is polydeformed and comprises folded and faulted rafts of early cretaceous and late tertiary bedrock a prominent organic raft quaternary glacial sediments and buried glacial ice three distinct structural domains can be identified within the stratigraphic sequence that represent proximal to distal deformation in an ice marginal setting complex thrust sequences interfering fold sets brecciated bedrock and widespread shear structures superimposed on this ice marginally deformed sequence record subsequent deformation in a subglacial shear zone analysis of cross cutting relationships within the stratigraphic sequence combined with osl dating indicate that the worth point hill hole pair was deformed during two separate glaciotectonic events firstly ice sheet advance constructed the hill hole pair and glaciotectonized the strata ice marginally producing a proximal to distal deformation sequence a glacioisostatically forced marine transgression resulted in extensive reworking of the strata and the deposition of a glaciomarine diamict a readvance during this initial stage redeformed the strata in a subglacial shear zone overprinting complex deformation structures and depositing a glaciotectonite 20 m thick outwash channels that incise the subglacially deformed strata record a deglacial marine regression whereas aggradation of glaciofluvial sand and gravel infilling the channels record a subsequent marine transgression secondly a later largely non erosive ice margin overrode worth point deforming only the most surficial units in the section and depositing a capping till the investigation of the worth point stratigraphic sequence provides the first detailed description of the internal architecture of a polydeformed hill hole pair and as such provides an insight into the formation and evolution of an enigmatic landform notably the stratigraphic sequence documents ice marginal and subglacial glaciotectonics in permafrost terrain as well as regional glacial and relative sea level histories the reinterpreted stratigraphy fundamentally rejects the long established paleoenvironmental history of worth point that assumed a simple layer cake stratigraphy including the type site for an organically rich preglacial interval worth point fm

regional deformational features of the south china block inferred from middle triassic palaeomagnetic data summary middle triassic red purple sandstones were sampled at four localities in the northeastern part of the south china block scb to detect regional deformational features most of the samples gave a characteristic remanent magnetization with unblocking temperatures up to 680 c the primary nature of this magnetization was ascertained by a positive fold test and a reversal test the tilt corrected mean direction from the yueshan area 30 6 n 116 8 e which is located at a distance of 25 km from the eastern part of the tan lu fault shows a counter clockwise deflection of 67 3 with respect to north however mean directions from the nanzhang 31 5 n 111 7 e tongling 30 9 n 117 9 e and nanjing 32 0 n 118 9 e areas which are located between 75 and 90 km from the northeastern boundary of the scb indicate a northeasterly deflection in declination such as d 38 2 i 25 1 95 6 4 d 32 1 i 25 2 95 9 7 and d 34 9 i 27 8 95 5 4 corresponding to palaeopoles of 50 5 n 220 5 e dp dm 3 7 6 9 55 6 n 231 8 e dp dm 5 6 10 4 and 53 9 n 229 1 e dp dm 3 2 5 9 respectively comparison with the previously reported permo triassic palaeomagnetic results from the scb suggests that very consistent palaeopoles are observed within a stable belt up to 1800 km along the northern margin of the scb covering the changxing nanjing tongling nanzhang and emeishan areas this type of behaviour implies that an area located between 70 and 225 km from northern margin of the scb forms a stable platform where very little relative rotation has been detected however the inner region of this block especially the mesozoic fold thrust belt experienced a clockwise rotation of 11 5 5 0 to 30 3 9 2 with respect to the stable outer region later than the middle triassic areas close to the tan lu fault were subjected to post triassic counter clockwise rotation by 90 with respect to the stable tectonic belt we thus conclude that the northern stable belt of the scb suffered no oroclinal bending during its suturing with the north china block

tectonic evolution of the corsica apennines alps region studied by the method of successive approximations paleotectonic and paleogeographic maps are often very complex users find it difficult to evaluate the extent to which the features shown are based on reliable geologic data and it is not generally possible to estimate uncertainties in shapes and positions in the scientific tradition of designing simple models to represent complex reality this study uses systematically simplified palinspastic models to study the tectonic evolution of the central mediterranean the prime constraint is northwestward motion of the african continent toward europe at 7 5 km m y in the first approximation model only unidirectional plate motion is allowed and the model is unable to generate both the alpine and apennine orogenic belts at right angles to each other the second approximation allows rotations it is surprising how well this very simple model with its rigidly restrictive rules reproduces the known evolution of the central mediterranean this agreement allows some optimism that the second approximation is correctly modeling the primary features of the tectonic evolution of the central mediterranean revising the shapes of the tectonic entities in a third approximation would allow the final pattern to mimic even more closely the present tectonic situation in the mediterranean but here the advantages of rigid simplification would start to diminish the first conclusion of the paper is thus that strongly simplified models are useful in tectonic studies and are justified in light of the uncertainties simplified models are used to explore the recently proposed ancient apennine hypothesis in which apennine deformation begins with a reversal of subduction polarity at about 100 90 ma this view contrasts with the standard young apennine hypothesis in which subduction reversal and the onset of apennine orogensis do not occur until 35 30 ma study of the simplified tectonic models leads to the second conclusion of the paper that the ancient apennine hypothesis honors regional constraints as well as the standard view does the simplified modeling suggests specific tests in alpine corsica for choosing between the young apennine and ancient apennine hypotheses

pattern and kinematic polarity of late mesozoic extension in continental ne asia perspectives from metamorphic core complexes 1 late mesozoic extension in ne asia resulted in the development of a large extensional province metamorphic core complexes mccs are the major features in this province and have 40ar 39ar ages of 130 110 ma for the mylonites and u pb zircon ages of 150 110 ma for the integral granitic intrusions based on this and previous studies this paper summarizes major characteristics of these mccs and recognizes a regional kinematic shear sense most mccs in the transbaikalia region sino mongolia border tract and the northwest central portion of the north china craton ncc show a top to the southeast se shear whereas those in the eastern and southern ncc locally underwent top to the northwest nw shear the three largest basins songliao huabei and ordos in north china are located in the transitional zone between domains of opposing shear sense we interpret the extension in the transbaikalia sino mongolia tract and northwestern part of the ncc to reflect late orogenic collapse of thickened crust following middle late jurassic collision along the okhotsk suture the southeastward extension is probably controlled by crustal scale top to the se tangential shear the transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage ecc i e c in sub horizontal ductile shear zones late in orogenic crustal thickening the combined effect of gravitational loading and thermal uplifting is considered to be the origin of the late or post orogenic collapse the top to the nw extension in the ne of the ncc might reflect antithetic sub extensional zones or mesozoic back arc extension as a far field effect of cretaceous pacific plate subduction

age and composition of granulite and pyroxenite xenoliths in hannuoba basalts reflect paleogene underplating beneath the north china craton abstract in situ u pb and hf isotope systematics of zircons mineral chemistry of silicates and whole rock major element trace element and platinum group element pge data are reported for five mafic xenoliths granulites and pyroxenite in the hannuoba cenozoic basalts north china temperature estimates of 859 886 c suggest that these xenoliths are derived from the lower crust the whole rock compositions of the xenoliths show weakly negative eu anomalies eu 0 67 0 95 high sio 2 al 2 o 3 5 80 13 8 low ir 0 10 0 29 ppb and wide ranges of ni 4 5 235 ppm and la nb 1 70 7 64 most zircons in these samples are structureless and give paleogene 44 5 47 3 ma u pb ages minor populations have early mesozoic 210 220 ma late mesozoic 90 ma and neogene 14 ma ages the fractionation of pyroxene and plagioclase controlled the petrogenesis of the xenoliths which are interpreted as the products of basaltic underplating and fractionation cumulates in paleogene time 45 47 ma with assimilation of some older intermediate crustal components the paleogene underplating corresponds in time to lithosphere scale extension in the north china craton leading to the widespread formation of sedimentary basins most paleogene zircons have positive e hf up to 13 2 with uniform t dm 0 46 0 50 ga whereas most mesozoic zircons have negative e hf this evolution suggests a temporal decrease in the degree of crustal assimilation during the extension of the lithosphere in addition to complex modification in early and late mesozoic times the thermal event related to the eruption of the host magmas ca 14 ma also reheated the lower crust beneath the north china craton

deep subpermafrost thermal regime in the mackenzie delta basin northern canada analysis from petroleum bottom hole temperature data in our studies of the thermal regime of sediments of the young mackenzie delta in the southeastern part of the beaufort mackenzie basin of northern canada we used thermal data from the base of the permafrost layer together with temperature data from petroleum wells by analyzing bottom hole temperature bht data we found that the percentage correction i e the percentage difference between bht and equilibrium temperature is less than 10 with 67 probability for times exceeding 10 hours after circulation ended regardless of circulation time no correlation exists between the percentage correction and depths for the bht data theoretical temperature depth profiles were constructed from the individual heat flow q q q and q q values q is error of estimate of q the interval thermal conductivities and a permafrost base temperature of 0 c estimates of q were based on the maximum bhts from depths 2 7 km the measured and corrected bht values for depths less than 1 5 km lie outside the rang

modeling sub sea permafrost in the east siberian arctic shelf the laptev sea region 1 models of sub sea permafrost evolution vary significantly in employed physical assumptions regarding the paleo geographic scenario geological structure thermal properties initial temperature distribution and geothermal heat flux this work aims to review the underlying assumptions of these models as well as to incorporate recent findings and hence develop an up to date model of the sub sea permafrost dynamics at the laptev sea shelf in particular the sub sea permafrost model developed here incorporates thermokarst and land ocean interaction theory and shows that the sediment salinity and a temperature based parametrization of the unfrozen water content are critical factors influencing sub sea permafrost dynamics from the numerical calculations we suggest development of open taliks may occur beneath submerged thaw lakes within a large area of the shelf

modeling ground thermal conditions and the limit of permafrost within the nearshore zone of the mackenzie delta canada 1 this study examines the interrelated effects of snow and ice on ground thermal conditions beneath regions of shallow water within the nearshore zone of the mackenzie delta field and model based data were used to determine the thermal boundary conditions at the sediment bed surface and to define the contemporary limit of permafrost over two consecutive winters mean sediment bed temperatures deviated up to 9 8 c beneath bottom fast ice that ranged from 10 cm to 100 cm thick with intrasite variability as much as 4 7 c measured and modeled temperatures were found to exponentially relate to the duration of time ice is bottom fast with the sediment bed mean winter ground temperatures at this boundary were predicted within 0 25 c of the observed measurements using numerical thermal modeling as on ice snow depth decreased the limit of equilibrium permafrost shifted toward progressively deeper water because of longer durations of ice contact and greater heat loss from the ground the critical water depth for permafrost under equilibrium conditions was 84 cm calculated from an ice thickness of 93 cm which is equivalent to an ice contact time of 142 days equilibrium permafrost was mapped beneath 393 8 km2 of bottom fast ice an additional 387 9 km2 exhibited seasonal ground freezing in the winter of 2006 2007 areas affected by bottom fast ice represent locations that are actively receiving sediment from distributary channels these results provide the first estimates of contemporary permafrost distribution for shallow water regions of the outer mackenzie delta

fluid inclusion and h o c isotope geochemistry of the yaochong porphyry mo deposit in dabie shan china a case study of porphyry systems in continental collision orogens the yaochong porphyry mo deposit in xinxian county henan province china is located in the hong an terrane that is the western part of the dabie orogen the dabie orogen is part of a 1 500 km long triassic continental collision belt between the north china block and the south china block four types of vein are present paragenetically from early to late they are as follows stage 1 quartz k feldspar pyrite magnetite vein stage 2 quartz k feldspar molybdenite pyrite vein stage 3 quartz polymetallic sulfides k feldspar vein and stage 4 quartz carbonate fluorite vein four compositional types of fluid inclusion pure co2 co2 bearing aqueous and solid bearing are present in quartz from the first three stages only low salinity aqueous fluid inclusions occur in quartz from the last stage all the estimated salinities are 13 1 wt nacl eq and no halite crystals were identified homogenization temperatures for the fluid inclusions from stages 1 to 4 are in the ranges of 262 501 202 380 168 345 and 128 286 c respectively and estimated depths decrease from 6 9 to 8 9 km through 6 2 7 2 to 4 7 km quartz separates from the veins yielded a 18o value of 7 7 11 2 corresponding to 18oh2o values of 1 3 to 6 9 using temperature estimates from fluid inclusion data dh2o values of fluid inclusion vary from 80 to 55 and 13cco2 from 2 3 to 2 7 suggesting that the ore fluids evolved from magmatic to meteoric sources we conclude that the ore forming fluid system at yaochong was initially high temperature high salinity and co2 rich and then progressively evolved to co2 poor lower salinity and lower temperature by mixing with meteoric water which results in ore precipitation

unusual hf contents in metamorphic zircon from coesite bearing eclogites of the dabie mountains east central china implications for the dating of ultrahigh pressure metamorphism metamorphic zircon from coesite bearing eclogites in the dabie mountains encloses high p phases and may have formed at the peak of ultrahigh pressure uhp metamorphism morphologically the metamorphic zircon typically occurs as small multi faceted near spherical grains with homogeneous internal structure and weak backscattered electron bse luminescence geochemically it is characterized by extremely high and relatively constant contents of hafnium hf and very low contents of y u and th reflecting the contraction of the zircon lattice under the uhp conditions high contents of hf may be characteristic of zircon formed during uhp metamorphism which has important consequences for interpretation of geochronological results we propose that the metamorphic zircon extremely enriched in hf may be used to date the peak of uhp metamorphism that produced the coesite bearing eclogites in the dabie mountains and potentially in other uhp terranes

structural geometry of an exhumed uhp terrane in the eastern sulu orogen china implications for continental collisional processes high precision 1 200 1 1000 mapping of yangkou bay eastern sulu orogen defines the structural geometry and history of one of the world s most significant uhp ultrahigh pressure belts at least four stages of folds are recognized in uhp eclogites and associated quartzo feldspathic gneiss uhp eclogite facies rootless f1 and isoclinal f2 folds are preserved locally in coesite eclogite mylonitic to ultramylonitic quartzo feldspathic and coesite eclogite shear zones separate small scale 5 10 m thick nappes of ultramafic mafic uhp rocks from banded quartzo feldspathic gneiss these shear zones are folded and progressively overprinted by amphibolite greenschist facies shear zones the prograde to retrograde d1 d5 deformation sequence is explained by deep subduction of offscraped thrust slices of lower continental or oceanic crust from the down going plate caught between the colliding north and south china cratons in the mesozoic after these slices were structurally isolated along the plate interface they were rolled in the subduction channel during exhumation and structural juxtaposition with quartzo feldspathic gneisses forming several generations of folds sequentially lower grade foliations and lineations and intruded by in situ and exotically derived melts shear zones formed during different deformation generations are wider with lower grades suggesting that deep crustal upper mantle deformation operates more efficiently perhaps with more active crystallographic slip systems than deformation at mid upper crustal levels

luminescence dating of mid to late wisconsinan aeolian sand as a constraint on the last advance of the laurentide ice sheet across the tuktoyaktuk coastlands western arctic canada luminescence dating of pre glacial sand in the tuktoyaktuk coastlands northwest territories discounts an early wisconsinan age for the last laurentide glaciation to cross the arctic coastal plain and supports a late wisconsinan age aeolian dune sand from the kittigazuit formation near cliff point on the southern shore of liverpool bay predates till deposited during the toker point stade potassium rich feldspar from three stratigraphic sections 35 km up ice from the toker point glacial limit provides thermoluminescence age estimates that range from 18 7 2 0 to 9 1 1 0 ka and infrared stimulated luminescence age estimates of 23 8 5 1 to 11 0 2 1 ka quartz from four of the same samples provides optically stimulated luminescence age estimates of 16 5 1 0 to 13 7 0 9 ka collectively these estimates reject the early wisconsinan age for the toker point stade glaciation inferred hitherto from radiocarbon dating a review of pre glacial and post glacial age estimates from the region indicates

plate interior polyphase fault systems and sedimentary basin evolution a case study of the east gobi basin and east gobi fault zone southeastern mongolia abstract structural interpretation of 2 d seismic reflection data and subsurface outcrop correlations reveal six distinct phases of deformation recorded in the paleozoic basement rocks and mesozoic cenozoic basin fill of the east gobi basin egb southeastern mongolia these phases include arc accretion and arc continent collision in late paleozoic time late triassic sinistral shear zone development early jurassic fold and thrust belt style shortening middle jurassic lower cretaceous extension and rift basin development middle cretaceous shortening with basin inversion and regional unconformity development and late cretaceous oligocene thermal subsidence with renewed paleogene left lateral strike slip faulting across the fault zone the five post amalgamation deformation phases are localized along the east gobi fault zone suggesting that preexisting structures and boundary conditions exert fundamental controls on the long term evolution of intracontinental basins such as the egb subsurface geophysical data and outcrop correlations demonstrate that the main subbasins of the egb contain major differences in basement metamorphic and structural fabrics basin fill patterns and distinct mesozoic cenozoic fault generations potential causes related to far field boundary conditions and other driving factors are suggested for each of the major deformation phases

shallow sediment temperature perturbations and sediment thermal conductivities canadian beaufort shelf a regional survey of sediment temperatures and thermal conductivities was conducted at 33 stations across the outer shelf of the canadian beaufort sea techniques developed for deep ocean heat flow investigations were used to study the upper 3 m of sediments temperature depth profiles exhibit curvatures that may be explained by seasonal changes in bottom water temperature some curvatures may arise from other causes it is unlikely that thermal effects of the underlying degradational permafrost can be detected from such shallow temperatures because of the magnitude of and lack of independent knowledge of these transient and local influences thermal conductivities measured on sediment cores and corrected to 1 c range from 0 9 to 2 4 w m 1 k 1 average 1 26 0 2 w m 1 k 1 these values are higher than typical conductivities of deep ocean sediments the wide range of thermal conductivities observed across the outer beaufort shelf may be explained by the presence of a varying fraction of quartz sand

zircon u pb ages and geochemistry of the wenquan mo bearing granitioids in west qinling china constraints on the geodynamic setting for the newly discovered wenquan mo deposit abstract east qinling is the largest porphyry molybdenum province in the world these mo deposits have been well documented in west qinling however few mo deposits have been discovered although granitic rocks are widespread recently the wenquan porphyry mo deposit has been discovered in gansu province which provides an insight into mo mineralization in west qinling in this paper we report pb isotope compositions for k feldspar and sulfides s isotope ratios for sulfides the results obtained from petrochemical study and from in situ la icp ms zircon u pb dating and hf isotopes the granitoids are enriched in lile and lree with ree and trace element patterns similar to continental crust suggesting a crustal origin the mg 40 05 to 56 34 and cr and ni contents are high indicating a source of refractory mafic lower crust the e hf t values of zircon grains from porphyritic monzogranite range from 2 9 to 0 6 and from granitic porphyry vary from 3 3 to 1 9 the zircons have t dm2 of 1014 to 1196 ma for the porphyritic monzogranite and 954 to 1224 ma for the granitic porphyry implying that these granitoids were likely derived from partial melting of a late mesoproterozoic juvenile lower crust the pb isotope compositions of the granitoids are similar to granites in south china showing that the magma was sourced from the middle lower crust in the southern qinling tectonic unit the pb isotopic contrast between the mo bearing granitoids and ores shows that the pb in the ore forming solution was derived from fractionation of a triassic magmatic system 34 s values of sulfides are between 5 02 and 5 66 similar to those associated with magmatic hydrothermal systems la icp ms zircon u pb dating yields crystallization ages of 216 2 1 7 and 217 2 2 0 ma for the granitoids consistent with a previously reported molybdenite re os isochron age of 214 4 7 1 ma this suggests that the mo mineralization is related to the late triassic magmatism in the west qinling orogenic belt in view of these geochemical results and known regional geology we propose that both granitoid emplacement and mo mineralization in the wenquan deposit resulted from the triassic collision between the south qinling and the south china block along the mianlue suture since triassic granitoid plutons commonly occur along the qinling orogenic belt the triassic wenquan mo bearing granitoids highlight the importance of the triassic tectono magmatic belt for mo exploration in order to apply this metallogenic model to the whole qinling orogen further study is needed to compare the wenquan deposit with other deposits

the permafrost regime in the mackenzie delta beaufort sea region n w t and its significance to the reconstruction of the palaeoclimatic history observed depths to the base of the ice bearing permafrost bpf as determined from geophysical well log and survey information in the mackenzie delta and offshore beaufort sea continental shelf show significant variation depths range from over 700m beneath the northeastern portion of richards island and the adjacent offshore region to the north to less than 100m in the modern mackenzie delta and offshore mackenzie bay regions along the tuktoyaktuk peninsula permafrost thicknesses exceed 600m but tend to thin to less than 100m in a southeasterly direction the wide variation in the thickness and distribution of permafrost is due in part to differences in the subsurface deltaic lithologies and to the complexity of the past surface temperature history exhibited for example by fluctuations of the mean annual ground temperature geological cross sections showing variation in the tertiary and upper cretaceous deltaic sequences indicate a good correlation with the permafrost distribution in particular where the sand and gravel units of the iperk and kugmallit sequences predominate permafrost is thick owing to the high thermal conductivities of the ice rich sediments conversely where the low thermal conductivity shales and mudstones of the richards sequence are present permafrost is accordingly thinner numerical models of permafrost aggradation in the mackenzie delta region suggest that early and late wisconsinan surface temperatures were of the order 18 c a mid wisconsinan rise in temperature to approximately 8 c similar to mean surface temperature today is also indicated this proposed palaeoclimate is sufficient to account for the observed maximum permafrost thicknesses in the region

late quaternary history of the mackenzie beaufort region arctic canada from modelling of permafrost temperatures 1 the onshore offshore transition five offshore drill holes northeast of richards island reveal permafrost conditions which are interpreted in terms of the sea level and paleoenviromental history of the canadian beaufort shelf the top of ice bonded permafrost lies 76 88 m below the seabed within 12 km of the shore 11 m below the seabed 20 km offshore and at 60 m some 50 km offshore the base of permafrost is 500 m to over 700 m deep temperature depth gradients are negative or nearly isothermal and permafrost is degrading geothermal modelling of the temperatures and permafrost observations indicates that the sites were subaerially exposed throughout most of the wisconsinan and during the early holocene three sites 2 10 km from the present shoreline experienced inundation at 3 5 4 0 ka these times support dates of marine transgression predicted by the sea level curve but the sites may have been occupied by thermokarst lakes prior to transgression in contrast the site 20 km offshore with shallow ice bonding was inundated about 0 4 ka this area may have persisted as an island until recently shoals observed nearby may be analogues of this process being outliers that are now being eroded below sea level marine transgression occurred about 6 ka some 50 km offshore rbsumb cinq trous de forage en mer au nord est de l ile richards rtvklent la presence de conditions de pergelisol elles sont interpr6t6es en fonction de l histoire du niveau marin et du paltomilieu de la plate forme de beaufort au canada a moins de 12 km vers le large le sommet du perg6lisol cimentk par la glace se trouve entre 76 et 88 m sous le fond marin i 20 km en mer il n est qu i 11 m sous le fond marin et a quelques 50 km en mer il passe b 60 m la

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

ore genesis of the fancha gold deposit xiaoqinling goldfield southern margin of the north china craton constraints from pyrite re os geochronology and he ar in situ s pb isotopes abstract the fancha gold deposit is one of the representative lode type deposit in the xiaoqinling goldfield four stage mineralization process were identified namely i the barren quartz stage ii the pyrite dominated stage iii the quartz polymetallic sulfides stage and iv the quartz carbonate stage correspondingly pyrite crystal can be divided into three generations i e py1 py2 and py3 according to their different textures and paragenesis during the first three mineralization stages in order to better understand the ore genesis an integrated analysis on re os he ar s pb multi isotopes was carried out for each mineralization stage to delineate the timing of the mineralization and the origins of the ore forming materials in detail gold bearing pyrite yields a re os isochron age of 124 3 2 6 ma mswd 1 9 overlapping with the previous hydrothermal mica 40ar 39ar ages suggesting that gold mineralization occurred in the early cretaceous the 3he 4he r ra and 40ar 36ar values of the py2 are 0 68 to 1 17 and 656 55 to 7384 2 and those of the py3 are 0 20 to 0 33 and 647 67 to 8913 55 respectively which reveal a significant contribution of mantle component in the ore forming fluids and the content of crust fluid is increasing in the later evolution in situ 34s values of the sulfides and tellurides in the auriferous quartz veins including the pyrite py1 py2 and py3 chalcopyrite sphalerite galena bismuthinite and tetradymite display a relatively narrow range 4 9 to 3 6 consistent with magmatic sulfur which are quite different from those of the pyrite in surrounding rock 7 8 to 9 3 in situ pb isotopic compositions of the galena and bismuthinite of stage iii are homogeneous with values of 208pb 204pb 207pb 204pb and 206pb 204pb being 37 482 37 508 15 392 15 523 16 995 17 006 and 39 279 39 287 15 588 15 590 18 283 18 288 respectively indicating a mixed pb sources with obvious proportions of mantle combined with previous data we suggest that the ore forming fluids and metals of the fancha gold deposit stemmed directly from the regional mantle derived magmatic hydrothermal system which is coupled with the large scale lithospheric extension and thinning in the eastern north china craton during the early cretaceous

thermal simulation of subsea saline permafrost thermal analyses of the response of offshore permafrost to emergence and submergence have traditionally employed simple closed form solutions where phase change is confined to a discrete freezing temperature these have led to rather rapid rates of return to thermal equilibrium which have proved difficult to explain in the light of recent deep temperature measurements in offshore permafrost profiles this paper reviews the need for an appropriate unfrozen water content relationship for a saline frozen soil and describes some simulations of long term thermal response in offshore permafrost using the author s geothermal simulator simulations of submergence assumed an initial permafrost thickness of 600 m and a mean soil surface temperature of 9 0 c the salinity was assumed constant at 30 the initial temperature profile was linear varying between 9 0 c and a freezing point of 1 8 c at the bottom of ice bonded permafrost the temperature of the ground surface was assumed to have changed to 0 8

the formation of foliated garnet bearing granites in the tongbai dabie orogenic belt partial melting of subducted continental crust during exhumation foliated garnet bearing fgb granites are associated closely with and are usually the major wall rocks of the high pressure hp and ultrahigh pressure uhp metamorphic rocks in the tongbai dabie region the mid segment of the qinling dabie sulu orogenic belt in central china these granites appear either as small plutons or as veins which commonly intrude into or surround the hp and uhp metamorphic eclogites or gneisses the veins of fgb granites usually penetrate into the retrograded eclogites or gneisses along the foliations condensation rims can occasionally be found along the margins of granite veins these granites are rich in si and alkali with high ga al ratios and depleted in ca mg al ti sc v ni co cr and sr which are similar to a type granites in a chondrite normalized diagram the samples are light rare earth elements enriched with different extent of negative eu anomaly moreover rb nb ta sr p and ti show different degrees of negative anomalies whereas ba k la zr and hf show positive anomalies in the primitive mantle normalized diagram negative anomalies of eu and sr indicate strong influence of plagioclase in conventional discrimination diagrams these fgb granites belong to the a type granite with geochemical characteristics affinitive to post collisional granites the end 230 ma values 15 80 to 2 52 and tdm values 1 02 2 07 ga suggest that magma for the fgb granites were derived from a heterogeneous crustal source therefore the fgb granites may provide clues for deciphering the formation of post collisional granites it is proposed that the magma of the fgb granites both in the hp and uhp units was formed in an extensional tectonic setting slightly post dating the hp and uhp metamorphism most likely as a result of decompressional partial melting of uhp retrograded eclogites during exhumation

geoelectric observations of the degradation of nearshore submarine permafrost at barrow alaskan beaufort sea 1 submarine permafrost degradation rates may be determined by a number of interacting processes including rates of sea level rise and coastal erosion sea bottom temperature and salinity regimes geothermal heat flux and heat and mass diffusion within the sediment column observations of ice bearing permafrost in shelf sediments are necessary in order to determine its spatial distribution and to quantify its degradation rate we tested the use of direct current electrical resistivity to ice bearing permafrost in elson lagoon northeast of barrow alaska beaufort sea a sharp increase in electrical resistivity was observed in profiles collected perpendicular to and along the coastline and is interpreted to be the boundary between ice free sediment and underlying ice bearing submarine permafrost the depth to the interpreted ice bearing permafrost increases from 2 m below sea level to over 12 m below sea level with increasing distance from the coastline the dependence of the saline sediment electrical resistivity on temperature and freezing was measured in the laboratory to provide validation for the field measurements electrical resistivity was shown to be effective for detection of shallow ice bearing permafrost in the coastal zone historical coastal retreat rates were combined with the inclination of the top of the ice bearing permafrost to calculate mean vertical permafrost degradation rates of 1 to 4 cm yr 1

tundra lakes and permafrost richards island western arctic coast canada lakes of average size 33 ha occupy a quarter of the surface area of richards island northwest territories most of the lakes have a central pool deeper than the thickness of winter ice and many have prominent shallow littoral terraces the relatively warm lake bottoms cause considerable disturbance to the surrounding continuous permafrost water and lake bottom temperatures the configuration of permafrost and active layer thickness were measured at a tundra lake between 1992 and 1997 the lake is oval 1 6 km long 800 m wide and as deep as 13 m sandy terraces covered by less than 1 m of water extend over 100 m from the shore the terraces are underlain by permafrost which terminates almost vertically at their edge the annual mean temperature measured at lake bottom in the central pool ranged between 1 5 c and 4 8 c depending on depth and between 0 2 c and 5 c on the terraces due to differences in snow cover and proximity to the central pool in consequence the temperature of permafrost at 7 m depth in the terraces also varied from 2 c near shore to 5 c in mid terrace the active layer in the terraces was uniformly 1 4 m deep geothermal modelling of talik configuration indicates that there is no permafrost beneath the central pool of the lake the modelling indicates that under equilibrium conditions about one quarter of the lakes on richards island have taliks that penetrate permafrost and at least 10 15 of the island is underlain by talik short term climatic changes predicted for the region imply a small increase in summer lake water temperature and an extension of the open water season accompanied by thicker snow cover in winter following such changes with longer freeze up and warmer terrace temperatures in winter permafrost may not be sustainable in the lake terraces

lake and channel bottom temperatures in the mackenzie delta northwest territories temperature loggers were placed in 17 lakes and 13 channels throughout the mackenzie delta to determine the annual mean bottom temperature t b and its spatial and temporal variation for june 2009 june 2010 the lakes were classified as perched or connected depending on the duration of their connection to the channel hydrologic system average t b values for nine perched lakes five channels and eight connected lakes distributed throughout the mackenzie delta were 5 5 4 6 and 3 4 c respectively the range of t b among all instrumented water bodies in the delta was 4 0 c over the year bottom temperatures ranged from 20 c in midsummer to 5 c in midwinter with relative stability between freeze up in mid october and breakup at the beginning of june channel perched and connected lake t b and mean annual near surface ground temperatures of 4 c in alluvial sedge wetlands and 2 25 c in forest were used to estimate that about 60 of delta lakes and nearly the entire channel network maintain through taliks

plastic flow of coesite eclogite in a deep continent subduction regime microstructures deformation mechanisms and rheologic implications abstract ultrahigh pressure uhp metamorphic rocks are continental crust that once has been subducted to mantle depth and subsequently exhumed to the surface in collisional orogenic belts despite the long distance transport of the uhp rocks in a deep subduction channel and the fast strain rate required for rapid exhumation little evidence for uhp deformation is found in the exhumed uhp terrains so far identified some uhp rocks have even preserved primary igneous and volcanic structures these observations have led to the postulation that the stress level is low a few mpa in the deep subduction zones deformation microstructures of some eclogite mylonites from a ductile shear complex in the sulu uhp metamorphic belt eastern china provide for the first time convincing evidence that dislocation creep is the predominant deformation mechanism for the major constituent minerals of the uhp rocks in a highly strain localized shear zone at the uhp conditions omphacite phengite and kyanite are the main strain accommodating phases and are dynamically recrystallized through progressive subgrain rotation and grain boundary migration coesite is either fractured with irregular extinction in the moderately deformed eclogite or pseudomorphed by polycrystalline quartz aggregates that have a foam structure garnet does not show optically discernable plastic deformation p t conditions for the deformation are estimated to be p 3 3 gpa and t 624 c using garnet omphacite kyanite phengite coesite quartz thermobarometry suggesting that deeply subducted continental crust remains cold at the upper mantle depths large part of the deeply subducted crust remains relatively undeformed strain is strongly localized into narrow shear zones stress level in the deep subduction channel is extrapolated on the order of tens of mpa much higher than it was previously thought

a coherent lithostratigraphic unit in the coesite eclogite complex of dabie shan china geologic and petrologic evidence abstract as a contribution to the discussion about the in situ vs exotic origin of the ultrahigh pressure metamorphic uhpm rocks and to the real size of the exhumed tectonic units the wuhe pailou unit wpu in eastern dabie shan was studied the wpu typically consists of two micas epidote gneiss marble with minor eclogite and jadeite bearing granofels with rare garnetite and eclogite nodules these lithologies are enclosed within leucocratic granitic gneiss jadeite bearing granofels is the most peculiar lithology of the wpu being the only felsic rock in dabie shan still preserving extensive relics of the uhpm mineral assemblage the uhp peak assemblage is coesite jadeite garnet epidote phengite kyanite with accessory rutile apatite and zircon all eclogites consist of garnet omphacite quartz coesite rutile or high al titanite phengite clinozoisite kyanite two micas epidote gneiss associated with the jadeite bearing granofels typically consists of epidote muscovite biotite plagioclase k feldspar rare garnet and accessory rutile titanite apatite and zircon locally the wpu paragneiss contains garnet with inclusions of phengite kyanite rutile and amphibole na plagioclase symplectite after omphacite a clockwise p t path was inferred for the wpu which consists of an early prograde part up to coesite eclogite facies conditions p 3 3 gpa and t 720 c an early decompression stage and a late part characterised by a near adiabatic decompression down to medium pressure p 1 1 gpa t 670 c and to low pressure amphibolite facies conditions followed by a final cooling to greenschist facies conditions both field and laboratory data consistently suggest that the wpu characterised by the close association of paragneiss marble eclogite and granofels is a supracrustal metasedimentary sequence which underwent subduction uhpm and exhumation as a coherent tectonic unit therefore within collisional orogens large coherent slices of continental crust may be subducted at depths greater than 100 km

triassic granitoids of the qinling orogen central china genetic relationship of enclaves and rapakivi textured rocks abstract granitoids varying in composition from biotite amphibole granodiorite to monzogranite with rapakivi textures have recently been recognized in the qinling orogen central china the granitoids are magnesian calc alkalic and metaluminous a cnk 0 71 0 95 they contain many mafic to intermediate microgranular enclaves the enclaves are quenched mafic melts in granitic host magma as evidenced by fine grained sinuous margins against or transitional contacts with the host granitoids as well as disequilibrium textures and mineral assemblages u pb zircon dating of the granitoids yields 209 2 ma and 212 2 ma and the enclaves 210 2 ma and 211 2 ma respectively the host granitoids and enclaves show obvious silica gap but similar ree and trace element patterns the whole rock 87sr 86sr t end t and zircon ehf t values for the granitoids vary from 0 70516 to 0 70634 2 3 to 4 4 and 0 7 to 7 6 and for the enclaves from 0 70467 to 0 70600 1 1 to 4 0 and 1 2 to 8 8 respectively showing overlapped end t and zircon ehf t values all these suggest that the two magmatic systems represented by the host rock and enclave are probably derived from distinct sources with sufficient interaction or common origin but underwent different degrees of crustal contamination mineralogical and geochemical data point to enclave dissolution in silicic magma as a major process accounting for the origin of the rapakivi texture the possible bimodal magmatism suggested by the coeval granitoids and lamprophyre dykes combined with the structural pattern geochemical features and regional tectonics suggest a post collision setting for the qinling rapakivi textured granitoid plutons

marine transgression shoreline emergence evidence in seabed and terrestrial ground temperatures of changing relative sea levels arctic canada precise temperatures to depths of several hundred meters have been measured in abandoned petroleum exploration wells in coastal and offshore areas of the canadian arctic in these regions changes in relative sea level during the quaternary have left a strong thermal signature on deep ground temperatures surface temperature changes of the order of 10 20 k accompanied shoreline emergence due to uplift in some regions and marine transgression due to eustatic changes in sea levels in others classical analytic techniques are used to demonstrate that these anomalous features are a direct consequence of recent changes in sea level in coastal areas of the queen elizabeth islands modelling of the large near surface temperature depth gradients and the distinct curvature in the measured profiles yield emergence times in general agreement with dates taken from emergence curves the analysis suggests that surface temperatures of these coastal areas were 16 19 k higher than at present from the late wisconsinan until emergence in the holocene suggesting either marine conditions for this period or glacial ice cover with basal temperatures near the pressure melting point in contrast at two offshore wells on the beaufort shelf analyses of the negative temperature depth gradients below the seabed and profile curvature yield times of marine transgression generally consistent with the published sea level curve for the area the analyses suggest that surface temperatures of some present offshore areas were 10 16 k lower than today s sea bottom temperatures from the late wisconsinan until marine transgression in the holocene the characteristic ground temperature profiles measured in such arctic areas provide independent evidence for the relative changes in sea level indicated by proxy data it is especially valuable for these areas where there is little historical record and from which datable material such as shells driftwood and pumice fragments may be scarce or ambiguous these effects may not be observable in temperate areas because of the smaller contrast in sea bottom and terrestrial temperatures

effects of submarine groundwater discharge on the present day extent of relict submarine permafrost and gas hydrate stability on the beaufort sea continental shelf we investigate the role of submarine groundwater discharge on the offshore temperature and salinity field and its effect on the present day extent of submarine permafrost and gas hydrate stability on the north american beaufort shelf with a two dimensional numerical model based on the finite volume method this study finds that submarine groundwater discharge can play a large role in submarine permafrost evolution and gas hydrate stability suggesting that local hydrology may control the evolution of submarine permafrost as strongly as does sea level or paleoclimatic conditions submarine permafrost evolution shows transient behavior over potentially long time scales e g several glacial cycles before a balance of density and pressure driven flows is established with the permeability variations imposed by the overlying permafrost layer the detectable offshore permafrost extent is related to the quasi stationary location of the saltwater freshwater transition larger values of submarine groundwater discharge allow permafrost to extend farther offshore because fresh pore water preserves relict ice therefore differences in the permafrost extent at locations that share similar paleoclimatic history may be explained in part by differences in the local hydrology gas hydrate stability on the north american beaufort shelf may be more widespread than currently thought because low ice saturation highly degraded submarine permafrost likely exists beyond the boundary detectable by common geophysical methods

permafrost heat flow and the geothermal regime at prudhoe bay alaska temperature measurements through permafrost in the oil field at prudhoe bay alaska combined with laboratory measurements of the thermal conductivity of drill cuttings permit an evaluation of in situ thermal properties and an understanding of the general factors that control the geothermal regime a sharp contrast in temperature gradient at approx 600 m represents a contrast in thermal conductivity caused by the downward change from interstitial ice to interstitial water at the base of permafrost under near steady state conditions interpretation of the gradient contrast in terms of a simple mode for the conductivity of an aggregate yields the mean ice content approx 39 and thermal conductivities for the frozen and thawed section 8 1 and 4 7 mcal cm s sup 0 c respectively these results yield a heat flow of approx 1 3 hfu which is similar to other values on the alaskan arctic coast the anomalously deep permafrost is a result of the anomalously high conductivity of the siliceous ice rich sediments curvature in the upper 160 m of the temperature profiles represents a warming of approx 1 8 sup 0 c of the mean surface temperature and a net accumulation of 5 6 kcal cm sup 2 by the solid earth surface during the last 100 years or so rising sea levelmore and thawing of ice rich sea cliffs probably caused the shoreline to advance tens of kilometers in the last 20 000 years inundating a portion of the continental shelf that is presently the target of intensive oil exploration a simple conduction model suggests that this recently inundated region is underlain by near melting ice rich permafrost to depths of 300 500 m its presence is important to seismic interpretations in oil exploration and to engineering considerations in oil production with confirmation of the permafrost configuration by offshore drilling heat conduction models can yield reliable new information on the chronology of arctic shorelines less

temperature changes and thaw of permafrost adjacent to richards island mackenzie delta n w t thawing of ice bonded sediment below the sea floor accompanies sea level rise and shoreline retreat along the beaufort sea coast the rate of thaw is primarily controlled by the warm spring and summer discharge of the mackenzie river which affects sea bottom temperatures to water depths of 10 20 m the intensity of the warming at any one location decreases with depth until continuous sub 0 c temperatures are encountered sea level rise eventually brings the sea floor into the cold sea water zone and thawing stops therefore thawing is also controlled by the residence time in the warm zone sedimentation or scouring can respectively slow or accelerate the increase in water depth and thereby modify the rate of thaw progression this combined with changes in the rate of coastal retreat can produce a subbottom frost table with considerable relief in the extreme ice bonding within a few metres of the sea floor can be explained by accelerated submergence

holocene sea level history of the canadian beaufort shelf new radiocarbon ages pertaining to the holocene sea level history of the canadian beaufort shelf are presented the ages were obtained on samples of freshwater and tidal marsh peat beds from offsho

provenance and tectonic setting of clastic deposits in the devonian xicheng basin qinling orogen central china abstract the qinling orogenic belt is one of world s great collisional orogenic belts which extends from the pamirs west kunlun to the korean peninsula and occupies a key position in understanding of the evolution and assembly of asia this belt separates the north china and south china blocks extensive thick devonian clastic rocks sandwiched between the shangdan and mianlue suture zones are mainly infills of the xicheng basin and provide the principal sedimentary record of tectonism in the qinling orogen devonian infills were subdivided into three subunits the upper devonian dacaotan group the middle upper devonian shujiaba group and the lower upper devonian xihanshui group their provenance and tectonic setting are critical to understanding not only the tectonic evolution of asia but also the global aspects of collisional regions compared with devonian strata in the eastern part of the qinling orogen the xicheng basin was not seriously modified by mesozoic tectonism petrologic studies demonstrate that the dacaotan group consists mainly of feldspathic litharenite with abundant volcanic fragments in contrast the shujiaba and xihanshui groups contain more metamorphic and sedimentary fragments and fewer magmatics than other lithic fragments these relations demonstrate that magmatic plutons were not extensively exposed at the time of deposition of these groups on q f l plots sandstones fall into the field of transitional arcs but on qm p k and lm lv ls diagrams they plot in the fields of continental arc and continental suture rocks the presence of volcanic lithic grains in the sandstones suggests they were derived mainly from a continental arc major element geochemistry of siltstones and mudstones demonstrate that most were derived from a continental arc but some from an oceanic island arc trace element geochemistry favors a continental arc provenance mostly from felsic rocks a high cr content of siltstones and mudstones demonstrates that some mafic or ultramafic rocks were exposed in the source area and provided some detritus rare earth element patterns are very similar to those of sandstones in modern continental arcs devonian paleocurrent indictors demonstrate that this continental arc was located at the northern edge of the basin this deduction demonstrates that the qinling arc in the southern margin of the north china block was the major source of clastic debris in the devonian xicheng basin these relationships suggest that the sediments were most probably deposited in a forearc basin