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

impact of eastern arctic shelf waters on the nansen basin intermediate layers the eurasian shelves supply water to the nansen basin intermediate layers in two ways as buoyancy driven plumes of dense winter water and as permanent inflow of the barents sea branch of atlantic water while the plumes are local and seasonal phenomena the barents sea flow is part of the large scale circulation both interact with atlantic water which enters the arctic ocean through fram strait and moves as a subsurface boundary current eastward along the continental slope during the polarstern cruise ark ix 4 in summer 1993 the fram strait branch was observed as a narrow core within tens of kilometers of the barents sea shelf edge here several patches of cold low salinity water spread across the slope down to about 500 m depth their origin is assumed to be the northern barents sea they mix with the warm saline fram strait branch water fsbw so that the core properties of the latter become modified downstream in the eastern nansen basin the fram strait branch is displaced toward the inner basin by inflow of the barents sea branch of atlantic water bsbw this inflow appears as a broad 200 km wedge extending from 200 to 1300 m depth bsbw is colder and less saline than water of the fram strait branch and it is less dense and less stratified than the ambient water both branches appear to undergo vigorous mixing while spreading eastward so that any eastward continuation of the boundary flow transports about 50 bsbw and 50 fsbw above 600 m and about 80 bsbw and 20 fsbw below that level toward the canadian basin according to available observations the barents and kara seas are the only source areas for shelf waters ventilating the nansen basin below the halocline and these waters constitute a freshwater input rather than a salt input winter shelf water from the laptev sea cannot contribute to layers deeper than the upper halocline

mineralogically mature sandstones in accretionary prisms abstract plate tectonic interpretations of sandstone composition are made in the hope that detrital framework modes of sandstone suites provide information about the tectonic setting of basins of deposition and associated provenances dickinson et al 1983 the underlying assumption that provenance tectonics are directly associated with tectonics of depositional and post depositional sedimentary settings is not however universally true as sandstones from two contemporary accretionary prism complexes nias island indonesia and barbados west indies illustrate mineralogically immature sandstones rich in feldspar and volcanic lithic fragments are expected in such tectonic settings but both nias island and barbadian sandstones are mineralogically very mature dominated by quartz in b th cases this discrepancy between inferred and actual tectonic setting has been explained by postulating that sands from a provenance petrogenetically unrelated to the arc trench system e g continental or recycled orogen sands have been introduced into the accretionary prism by sedimentary or tectonic processes sandstones which are transported sedimentologically or tectonically across the boundaries of sedimentary tectonic provinces and which therefore occur in tectonic settings petrogenetically unrelated to the tectonics of the sediment source area are important components of at least some active accretionary complexes the possibility of trans boundary sedimentary and or tectonic transport must be allowed for in making plate tectonic interpretations based on sandstone petrog aphy especially in reconstructions of ancient orogenic belts

u pb ages of detrital zircons in pacheco pass metagraywackes sierran klamath source of mid cretaceous and late cretaceous franciscan deposition and underplating 1 using laser ablation icp ms techniques u pb ages are reported for 338 detrital zircons separated from four franciscan metagraywacke samples from imbricate thrust sheets cropping out in the pacheco pass area east central california coast ranges structurally higher slabs are clastic quartz albite bearing rocks typified by neoblastic lawsonite jadeitic pyroxene the lowest exposed slab lacks jadeitic pyroxene but contains traces of newly grown pumpellyite and lawsonite in addition to abundant quartz albite studied specimens contain moderate amounts of phengite titanite chlorite stilpnomelane carbonate iron oxides rock fragments and carbonaceous matter these diablo range clastic sediments were deposited in mid cretaceous and late cretaceous time the highest allochthon unit i was deposited after 102 ma unit iv was deposited after 90 102 ma and unit v was deposited after 86 ma metagraywacke depositional accretionary ages within slab iv young upward but ages of the slabs decrease progressively downward in the four sample aggregate most zircons have igneous ages falling in the 135 170 ma range with a smaller population at 85 120 ma the rocks contain rare zircons of middle proterozoic late archean ages a minor source in northern california and nw nevada seems likely for the pacheco pass metagraywackes but the sierran klamath calcalkaline arc provided most of the clastic debris judging by the volcanic nature of lithic clasts in these eastern belt metagraywackes and because massive sierran plutonism occurred at 85 120 ma the abundant 135 170 ma zircons probably were derived chiefly from the eroding comagmatic volcanic arc rather than from the less voluminous jurassic arc plutons

a parameterization of the ice ocean drag coefficient 1 a parameterization of the ice ocean drag coefficient cw was developed through partitioning the oceanic drag force into three components 1 form drag on the floe edge 2 form drag on the ridge keel and 3 skin friction on the ice bottom through these quantities cw was expressed as a function of observable sea ice geometric parameters sensitivity studies were carried out to investigate the influence of varying sea ice conditions on cw the results revealed that cw increases first and then decreases with increasing ice concentration a similar to the observations of the air ice drag coefficient and which is mainly attributed to the nonmonotonic variation of the form drag on the floe edge with ice concentration moreover the form drag on the floe edge is always the dominant component having a proportion of more than 60 in sea ice with a large aspect ratio draft length 1 100 indicating the necessity of including this term in sea ice dynamic models particularly for the marginal ice zone miz the form drag on the ridge keel becomes dominant only when the ridging intensity is extremely high depth spacing 1 20 additionally a large value of cw cannot be caused only by the inclusion of form drag terms but also by large skin friction over rough ice bottoms finally for typical situations in the miz with moderate ridging intensity the parameterization will underestimate cw by approximately 30 for a rough ice bottom and by over 80 for a smooth ice bottom if no form drags are considered

counterclockwise p t t paths from amphibolites franciscan complex california relics from the early stages of subduction zone metamorphism overprinting mineral relationships in amphibolites from the franciscan complex california suggest metamorphic evolution from high temperature amphibolite facies to the blueschist facies with increasing p t ratio a counterclockwise p t t path early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are in turn cut or rimmed by sodic amphiboles some amphibolites are overprinted successively by eclogite and blueschist facies assemblages clinopyroxenes are zoned with increasing jadeite component from core to rim multiple generations of phengites are present in the same sample and are more si rich with each successive generation of growth garnets in eclogite overprinted amphibolites are zoned with an initial mgo increase away from the core to a maximum mgo zone followed by mgo decrease toward the rim garnets in other amphibolites show continually increasing mgo from core to rim several samples representative of the variety of franciscan amphibolites are describ

radiolarian age constraints on mesotethyan ocean evolution and their implications for development of the bangong nujiang suture tibet radiolarian discoveries indicate that deep marine conditions prevailed in central tibet from the early middle jurassic until well into the early cretaceous late hauterivian early aptian 131 121 ma and help to constrain the temporal extent of oceanic conditions along the bangong nujiang suture these new fossils occur in chert or siliceous mudstone blocks associated with the lagkor tso ophiolitic melange basin inversion associated with closure of mesotethys between the qiangtang and lhasa terranes was accompanied by melange formation and predated regionally widespread deposition of overlying shallow marine late aptian albian orbitolinid limestones

compositional trends in arc related deep marine sand and sandstone a reassessment of magmatic arc provenance detrital modes for 524 deep marine sand and sandstone samples recovered on circum pacific caribbean and mediterranean legs of the deep sea drilling project and the ocean drilling program form the basis for an actualistic model for arc related provenance this model refines the dickinson and suczek 1979 and dickinson and others 1983 models and can be used to interpret the provenance tectonic history of ancient arc related sedimentary sequences four provenance groups are defined using qfl qmkp lmlvls and lvflvmilvl ternary plots of site means 1 intraoceanic arc and remnant arc 2 continental arc 3 triple junction and 4 strike slip continental arc intraoceanic and remnant arc sands are poor in quartz mean qfl q 75 they are predominantly composed of plagioclase feldspar and volcanic lithic fragments continental arc sand can be more quartzofeldspathic than the intraoceanic and remnant arc sand mean qfl q values as much as 10 mean qfl f values as much as 65 and mean qmkp qm as much as 20 and has more variable lithic populations with minor metamorphic and sedimentary components the triple junction and strike slip continental groups compositionally overlap both are more quartzofeldspathic than the other groups and show highly variable lithic proportions but the strike slip continental group is more quartzose modal compositions of the triple junction group roughly correlate with the qfl transitional arc field of dickinson and others 1983 whereas the strike slip continental group approximately correlates with their dissected arc field

metamorphic rocks in central tibet lateral variations and implications for crustal structure insights about lateral variations in the age composition and structure of the central tibetan crust are provided by geologic investigations of metamorphic rocks in the qiangtang terrane previous studies have shown that a tectonic melange of triassic age with blueschist and eclogite bearing blocks within a greenschist facies matrix is exposed over an e w distance of 600 km in the central qiangtang terrane new mapping shows that the melange extends over a n s distance of 150 km nearly to the trace of the early mesozoic jinsha suture in the north the melange exposed structurally beneath upper paleozoic to mesozoic strata in the footwalls of early mesozoic normal faults is composed mostly of paleozoic metasedimentary and crystalline rocks these findings support the hypothesis that a large part of the central and northern qiangtang terrane crust is composed of supracrustal rocks the duguer range

provenance of franciscan graywackes in coastal california a systematic comparison of available detrital modes for graywacke sandstones of the franciscan subduction complex and for coeval sandstones of the great valley sequence in the california coast ranges indicates that both were apparently derived from the same general sources the inferred provenance terrane was the ancestral sierran klamath magmatic arc from which mixed volcanic and plutonic detritus readily entered the adjacent great valley forearc basin at intervals along the trend of the arc trench system arc derived detritus also bypassed the forearc region through submarine canyons that fed the franciscan trench longitudinal flow along both the great valley trough and the franciscan trench achieved wide dispersal of the turbidite sediment suites of both franciscan and great valley samples include an array of subquartzose compositions ranging from feldspatholithic to lithofeldspathic mean framework modes of 17 franciscan suites comprising 203 individual samples and of 23 great valley suites comprising 410 individual samples range from 14 to 44 quartz grains 15 to 54 feldspar grains and 7 to 71 total lithic fragments the ratio of quartz to feldspar remains relatively constant as the proportion of lithic fragments changes the compositional variations reflect differences mainly in the admixture of lithic fragments derived principally from volcanic cover with quartz and feldspar derived principally from erosion of underlying plutons despite major overlap in the compositions of the two sets of samples some franciscan sandstones are somewhat more feldspathic and less lithic than any known great valley counterparts and were probably derived from segments of the arc terrane where exposures of plutons were more extensive than within typical great valley sources higher proportions of non volcanic to volcanic lithic fragments in some franciscan sandstones probably reflect complex recycling processes on the trench slope diagenetic effects in many franciscan suites include apparent wholesale replacement of k feldspar by albite present age control is inadequate to test fully for time dependent trends in the compositions of franciscan sandstones analogous to the known stratigraphic variations in the composition of great valley sandstones this question ought to be investigated in future studies

a barotropic inverse tidal model for the arctic ocean 1 we describe high resolution 5 km grid linear dynamics and inverse models of arctic ocean barotropic tides obtained with the osu tidal inversion software otis package the 8 constituent dynamics based model uses the latest ibcao bathymetry and open boundary forcing from the recent tpxo 6 2 global barotropic tidal solution this model performs significantly better than the present benchmark arctic tidal model 14 km grid by z kowalik and a proshutinsky as judged by comparisons with 300 coastal tide gauges the greatest improvements are found in the canadian arctic archipelago nares strait and the baffin bay and labrador sea and can be explained by the higher resolution of the new model in these topographically complex regions the new arctic inverse model assimilates coastal and benthic tide gauges and topex poseidon and ers altimetry for further improvements of the 4 dominant constituents m2 s2 k1 and o1

provenance of late triassic sediments in central lhasa terrane tibet and its implication abstract in southern tibet late triassic sequences are especially important to understanding the assembly of the lhasa terrane prior to indo asian collision we report new data relevant to the provenance of a late triassic clastic sequence from the mailonggang formation in the central lhasa terrane tibet petrographic studies and detrital heavy mineral assemblages indicate a proximal orogenic provenance including volcanic sedimentary and some ultramafic and metamorphic rocks in situ detrital zircon hf and u pb isotope data are consistent with derivation of these rocks from nearby triassic magmatic rocks and basement that comprise part of the newly recognized late permian triassic sumdo cuoqen orogenic belt the new data suggests correlation with the upper triassic langjiexue group which lies on the opposing southern side of indus yarlung ophiolite sediments from both the mailonggang formation and langjiexue group are interpreted to represent formerly contiguous parts of a sequence deposited on the southern flanks of the sumdo cuoqen belt

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

discovery of middle jurassic trench deposits in the bangong nujiang suture zone implications for the timing of lhasa qiangtang initial collision abstract the mesozoic trench deposits in the bangong nujiang suture zone in central tibet can provide critical information for reconstructing the paleotectonic evolution of the bangong nujiang ocean and constraining the timing of the lhasa qiangtang initial collision in this paper the gamulong formation which is well exposed in the dong co area western bangong nujiang suture central tibet was studied the sedimentological analysis shows that the gamulong formation was deposited in a submarine fan environment the youngest detrital zircon age and fossils from limestone pebbles within the gamulong formation combine to determine a middle jurassic age the pebbles in the conglomerate are dominated by limestones and sandstones which is consistent with the sandstone petrology showing limestone and sandstone fragments as the main framework components detrital zircon u pb ages from the sandstones yield age populations of 164 178 ma 200 500 ma 700 1000 ma 1700 2100 ma and 2500 ma the corresponding zircon ehf t values are distributed between 23 5 and 11 1 these zircon features are similar to those from the qiangtang terrane accordingly the provenance results imply that the gamulong formation was recycled from the accretionary complex mugagangri group melange in the bangong nujiang suture zone and the forearc basin deposits lower middle jurassic sewa shaqiaomu jiebuqu formations in the southern qiangtang subterrane the gamulong formation is characterized by sedimentary recycling in the trench likely in response to a tectonic event possibly the continental collision together with the previous studies this work on the occurrence of the gamulong formation indicates that the lhasa qiangtang initial collision could have occurred in middle jurassic time 166 ma

seasonal variation of upwelling in the alaskan beaufort sea impact of sea ice cover 1 data from a mooring array deployed from august 2002 to september 2004 are used to characterize differences in upwelling near the shelf break in the alaskan beaufort sea due to varying sea ice conditions the record is divided into three ice seasons open water partial ice and full ice the basic response is the same in each of the seasons roughly 8 h after the onset of easterly winds the shelf break jet reverses followed approximately 10 h later by upwelling of saltier water which is cold near the shelf break pacific winter water and warm at depth atlantic water the secondary circulation at the outer shelf is to first order consistent with a two dimensional ekman balance of offshore flow in the upper layer and onshore flow at depth there are however important seasonal differences in the upwelling overall the response is strongest in the partial ice season and weakest in the full ice season it is believed that these differences are dictated by the degree to which wind stress is transmitted through the pack ice as the strength of the wind forcing was comparable over the three seasons an eof based upwelling index is constructed using information about the primary flow secondary flow and hydrography the ability to predict upwelling using the wind record alone is explored which demonstrates that 90 of easterly wind events exceeding 9 5 m s 1 drive significant upwelling during certain periods the ice cover on the shelf became landfast which altered the upwelling and circulation patterns near the shelf break

petrofacies and provenance of late mesozoic forearc basin northern and central california data from the great valley group sequence represent the most complete information regarding sandstone petrology of sediment derived from a magmatic arc this information is useful in documenting tectonic and magmatic events within the arc and related terranes and forms the basis for the establishment of petrostratigraphic units for mapping and correlation sandstone and conglomerate compositions are controlled by changes in provenance many of which were basinwide and synchronous clay mineral composition is controlled primarily by burial metamorphism careful attention to sample collection sample preparation and petrographic techniques is essential for uniform results seven petrographic parameters p f lv l m qp q q f and l listed in decreasing importance to petrofacies discrimination define eight petrofacies stony creek platina lodoga grabast boxer cortina los gatos and rumsey listed in approximate order of decreasing age the upper jurassic lower cretaceous petrofacies stony creek platina and lodoga contain higher lithic contents supracrustal sources whereas the upper cretaceous petrofacies especially the rumsey contain higher proportions of plutoniclastic components quartz feldspar and micas the proportion of potassium feldspar increases from near zero in the upper jurassic to nearly 50 of all feldspars in the uppermost cretaceous the lower part of the great valley group upper jurassic and lower cretaceous contains significant quantities of sedimentaclastic and metamorphiclastic material eroded from accreted and deformed terranes tectonic highlands formed by the arc arc collision nevadan orogeny that occurred prior to initiation of the franciscan great valley sierra nevada arc trench system the klamath mountains area provided a major proportion of this detritus ophiolite and serpentinite detritus was deposited locally near the base of the great valley group as a result of deformation along the east side of the growing franciscan subduction complex volcaniclastic detritus was fed into the entire forearc basin as magmatism increased in the sierra nevada area during the cretaceous as the volcanic cover was stripped off plutoniclastic and metamorphiclastic detritus from the underlying batholithic terranes was provided in abundance to the forearc basin crustal components were more continental in the southern sierra nevada and more oceanic in the northern sierra nevada as demonstrated by the higher proportions of metamorphiclastic detritus and by the more felsic nature of volcaniclastic detritus to the south by the middle of the late cretaceous extensive batholithic terranes provided potassium feldspar rich arkosic detritus to the entire forearc basin by the paleogene arc magmatism had migrated eastward sufficiently that deeper levels of the california part of the arc were exposed by erosion tectonic activity decreased in the forearc basin and the basin was filled to sea leve in most parts

modeling winter circulation under landfast ice the interaction of winds with landfast ice 1 idealized models and a simple vertically averaged vorticity equation illustrate the effects of an upwelling favorable wind and a spatially variable landfast ice cover on the circulation beneath landfast ice for the case of no along shore variations in ice upwelling favorable winds seaward of the ice edge result in vortex squashing beneath the landfast ice leading to 1 large decreases in coastal and ice edge sea levels 2 cross shore sea level slopes and weak 05 m s 1 under ice currents flowing upwind 3 strong downwind ice edge jets and 4 offshore transport in the under ice and bottom boundary layers of the landfast ice zone the upwind under ice current accelerates quickly within 2 4 days and then slows as cross shore transport gradually decreases the cross shore sea level slope near the ice edge bottom boundary layer convergence produces ice edge upwelling cross ice edge exchanges occur in the surface and above the bottom boundary layer and reduce the under ice shelf volume by 15 in 10 days under ice along shore pressure gradients established by along and cross shore variations in ice width and or under ice friction alter this basic circulation pattern for a landfast ice zone of finite width and length upwelling favorable winds blowing seaward of and transverse to the ice boundaries induce downwind flow beneath the ice and generate vorticity waves that propagate along shore in the kelvin wave direction our results imply that landfast ice dynamics not included explicitly herein can effectively convert the long wavelength forcing of the wind into shorter scale ocean motions beneath the landfast ice

the yarlung suture melange lopu range southern tibet provenance of sandstone blocks and transition from oceanic subduction to continental collision abstract with the aim of better understanding the history of ocean closure and suturing between india and asia we conducted a geologic investigation of a siliciclastic matrix tectonic melange within the western yarlung suture zone of southern tibet lopu range region 50 km northwest of saga the siliciclastic matrix melange includes abundant blocks of ocean plate stratigraphy and sparse blocks of sandstone metapelite and metabasite blocks in the melange exhibit lower greenschist facies mineral assemblages indicating that they were not deeply subducted we obtained detrital zircon u pb geochronologic and sandstone petrographic data from sandstone blocks in the melange and sandstone beds from tethyan himalayan strata exposed to the south of the suture the sandstones from both units are all similar in u pb detrital zircon age spectra and petrography to the nearby tethyan cretaceous paleocene sangdanlin section which records the earliest appearance at 59 ma of arc affinity strata deposited conformably on indian affinity strata two paleocene sandstones one of which is a schistose block incorporated in the siliciclastic matrix melange yielded indistinguishable maximum depositional ages of 59 ma mesozoic asian affinity sandstone blocks previously documented in the siliciclastic matrix melange 200 500 km along strike to the east are notably absent in the lopu range region we documented a gradational transition in structural style from the block in matrix melange in the northeast to the south vergent tethyan thrust belt in the southwest blocks of tethyan himalayan strata increase in size and the volumetric proportion of matrix decreases from northeast to southwest we conclude that no arc affinity sandstone blocks were incorporated into the subduction complex until india asia collision at 59 ma when the xigaze forearc basin became overfilled and tethyan himalayan strata entered the trench as collision progressed there was a gradual transition in structural style from block in matrix melange formation to imbricate style thrust belt formation

eclogites from central qiangtang northern tibet china and tectonic implications abstract eclogites in central qiangtang northern tibet china are characterized by the garnet omphacite rutile phengite quartz assemblage hornblende plagioclase and sphene occur as retrograde amphibolite facies phases in the eclogites pressure temperature estimates for the primary eclogite assemblage are consistent with minimum pressures of 20 25 kbar at 482 to 625 c the retrograde amphibolite facies assemblage suggests conditions of 10 12 kbar and 460 600 c consequently the eclogites from central qiangtang have undergone unambiguous ultrahigh pressure uhp metamorphism the presence of uhp metamorphic rocks implies a likely deep seated continental subduction along the shuanghu suture within the qiangtang terrain the denudation of these uhp metamorphic rocks correlates well with the main interval that the songpan ganzi turbidites accumulated and could have provided a significant source for these sedimentary rocks

earliest cretaceous accretion of neo tethys oceanic subduction along the yarlung zangbo suture zone sangsang area southern tibet abstract the accretionary complex of the yarlung zangbo suture zone yzsz in southern tibet records the subduction accretion process of neo tethys oceanic lithosphere we report field observations petrographic analysis detrital zircon u pb geochronology and hf isotope data of one sedimentary matrix melange unit of yzsz in sangsang area central southern tibet this melange unit is the northernmost and presumably the oldest one within the yzsz accretionary complex the sandstone matrix and blocks are rich in volcanic detritus linking the main provenance to a juvenile magmatic arc u pb hf analysis of detrital zircons from the sandstones reveals a unimodal age population of 186 121 ma with high positive ehf t values of 6 67 to 14 58 consistent with derivation from gangdese arc on southern margin of lhasa terrane the maximum depositional age of the melange unit is limited to be 149 134 ma by averaging the youngest detrital zircon ages the sandstones of this melange unit probably deposited in the trench basin within a few million years of early accretion driven by the neo tethys oceanic subduction the development of arc trench system verified the initiation of subduction of neo tethys oceanic lithosphere in early jurassic our results and integration of previous studies on subduction accretion process of neo tethys subduction indicate that the accretionary complex in sangsang area documented at least two stages of accretion processes stage 1 in earliest cretaceous and stage 2 in late cretaceous separated by a period of forearc extension forming the yarlung zangbo ophiolites these two stages of accretion correspond to growth and erosion of gangdese arc and crustal shortening in southern eurasian margin

the subduction accretion history of the bangong nujiang ocean constraints from provenance and geochronology of the mesozoic strata near gaize central tibet abstract the mesozoic strata within the bangong nujiang suture zone in central tibet recorded critical information about the subduction accretion processes of the bangong nujiang ocean prior to the lhasa qiangtang collision this paper reports detailed field observations petrographic descriptions sandstone detrital zircon u pb ages and hf isotopic analyses from an accretionary complex preserved as mugagangri group and the unconformably overlying shamuluo formation near gaize the youngest detrital zircon ages together with other age constraints from literature suggest that the mugagangri group was deposited during late triassic early jurassic while the shamuluo formation was deposited during late jurassic early cretaceous based on the differences in lithology age and provenance the mugagangri group is subdivided into the upper middle and lower subunits these units are younging structurally downward southward consistent with models of progressive off scrapping and accretion in a southward facing subduction complex the upper subunit comprising mainly quartz sandstone and siliceous mud shale was deposited in abyssal plain environment close to the qiangtang passive margin during late triassic with sediments derived from the southern qiangtang block the middle and lower subunits comprise mainly lithic quartz sandstone and mud shale containing abundant ultramafic ophiolitic fragments the middle subunit of late triassic early jurassic age records a transition in tectono depositional setting from abyssal plain to trench wedge basin with sudden influx of sediments sourced from the central qiangtang metamorphic belt and northern qiangtang magmatic belt the appearance of ultramafic ophiolitic fragments in the middle subunit reflects the subduction initiation the lower subunit was deposited in a trench wedge basin during early jurassic with influx of jurassic aged zircons originating from the newly active southern qiangtang magmatic arc the lower subunit records the onset of arc magmatism related to the northward subduction of the bangong nujiang ocean the shamuluo formation comprising mainly lithic feldspar sandstone with limestone interlayers was deposited in a post collisional residual sea or pre collisional trench slope basin with sediments derived entirely from the qiangtang block

evolution of the bangong nujiang tethyan ocean insights from the geochronology and geochemistry of mafic rocks within ophiolites abstract the bangong nujiang suture zone bnsz is located in the central tibetan plateau and represents the relict of the bangong nujiang tethyan ocean bnto this paper presents zircon u pb ages and whole rock geochemical and sr nd isotope data for the ophiolitic rocks from the rutog dongco dongqiao amdo and dengqen areas from west to east in the bnsz zircon u pb ages obtained from five gabbros and one leucogabbro from the five areas are 169 2 167 2 187 2 184 2 177 3 and 164 2 ma respectively mafic rocks gabbros and basalts in the bnsz can be divided into morb like and oib type based on geochemical data the morb like rocks are tholeiitic and can be further divided into n and e morb subtypes marked by depleted and flat ree patterns respectively all morb like rocks show supra subduction zone ssz geochemical affinities such as mild enrichment in the large ion lithophile elements and depletion in the high field strength elements coupled with high and positive whole rock e nd t values 8 1 to 5 4 the oib type rocks which formed in the early cretaceous 132 108 ma are exposed in the dongco dongqiao and tarenben areas and they consist mostly of alkali basalts with within plate geochemical characteristics and positive e nd t values 5 9 to 4 5 on the basis of these new data combined with information from the literature we further support that the bnto was an intra oceanic arc backarc basin that existed between the north lhasa subterrane nlt and the south qiangtang subterrane sqt during the early middle jurassic this basin may have been active until the early cretaceous when the oib type basalts erupted

collisional melange development geologic associations of active melange forming processes with exhumed melange facies in the western banda orogen indonesia analysis of block assemblages matrix clay mineral composition and microfauna and offshore seismic reflection profiles reveal that the bobonaro melange in the timor region is sourced from mud rich australian continental margin sequences that are remobilized during accretion and form various facies depending on the structural conditions of emplacement melange facies include broken formation matrix rich mud injections and classic mixed block in clay facies each of these are distinguished by varying degrees of remobilization mixing and dispersion at different structural positions across the orogenic wedge the most important structural control is whether melange was generated beneath or in front of upper plate banda forearc basement banda terrane at the present collisional deformation front in the timor trough seismic reflection profiles show that melange forms mostly by stratal detachment and fluid assisted remobilization above a basal decollement propagating laterally along overpressured jurassic to cretaceous clay rich sequences of the distal australian continental margin the broken clay rich material injects upward through faults to form intrusive bodies at the base of slope cover sediment to form mud ridges at the surface similar patterns of stratal disruption are exposed onshore in the pliocene kolbano fold and thrust wedge of southern timor which is structurally contiguous with the timor trough deformation front melange in the kolbano mountains is mostly broken formation and matrix rich injections of mud from jurassic and cretaceous units deformation mechanisms include intense layer parallel extension associated with emplacement by mud diapirs that rise from near the decollement upward to the surface along fault conduits in the hinterland of the orogenic wedge east timor and northern west timor melange is dominantly of mixed block in clay facies with large blocks derived from roof thrust sheets of intermixed banda terrane and maubisse formation units at the structural base of these thrust sheets is the sonnebait disruption zone sdz which formed the initial suture between the banda terrane and structurally underthrust australian continental margin sequences during the late miocene to early pliocene the thickest accumulations of mixed block in clay melange in the timor region are found at the southern edge outlet of the sdz near the central and viqueque synorogenic basins the extent of block dispersion and mixing in the sdz is indicative of intense shear strains perhaps induced by an oversupply of accretable material when the suture zone was clogged by underthrusting of the australian continental margin

remnants of a late triassic ocean island in the gufeng area northern tibet implications for the opening and early evolution of the bangong nujiang tethyan ocean abstract in this paper we present new major and trace element compositions of basaltic rocks in the gufeng ocean island gfoi area in the western segment of the bangong nujiang suture zone northern tibet our aim was to assess the genesis of these rocks and discuss the implications of this new dataset for the evolution of the bangong nujiang tethyan ocean an ocean island type double layer structure comprising a basaltic basement and an oceanic sedimentary cover sequence found within the gfoi provides direct evidence for the interpretation that the assemblage is a typical ocean island the basalts in the gfoi can be divided into three types named g1 g2 and g3 basalts and these basalts range in composition from morb to oib types which is typical of ocean islands the g1 basalts have morb type affinities possibly indicating the existence of morb oceanic crust under the gfoi the g2 basalts represent the early stage of formation of the gfoi and are produced by the interaction of rising oib type basaltic magma and the existing morb oceanic crust the g3 basalts are typical oib basalts and they are the products of the direct eruption of oib type basaltic magmas the g3 basalts have high la yb n 12 3 14 4 ce yb n 10 8 11 8 la sm n 2 39 2 76 and sm yb n 4 89 5 23 ratios indicating the presence of oceanic lithosphere below the gfoi with a thickness of 50 60 km geochemical analyses of the gfoi cherts show that they contain terrigenous material indicating the gfoi formed close to a continental margin norian conodont fossils within the gfoi limestones indicate the gfoi formed during the late triassic these data combined with geological evidence and a half space model of lithosphere cooling where the thickness of the oceanic lithosphere is determined from the age of the lithosphere indicate that the western segment of the bangong nujiang tethyan ocean opened initially in the late permian expanded rapidly during the early middle triassic and was a mature ocean during the late triassic

the disappearance of a late jurassic remnant sea in the southern qiangtang block shamuluo formation najiangco area implications for the tectonic uplift of central tibet abstract located between the bangong nujiang suture zone and the qiangtang block in central tibet the najiangco area 5 km to the north of nima selinco contains an upper jurassic lower cretaceous sedimentary succession deposited during a period of marine regression the youngest marine sedimentary unit in the najiangco area is the upper jurassic shamuluo formation which consists of sandstone limestone siltstone and shale sedimentary facies analysis shows that tidal flat and subtidal lagoonal facies characterized the northern margin of the basin while delta front and prodelta facies dominated the middle part and carbonate shoal and patch reef facies prevailed along the southern margin provenance analysis including petrographic modal analysis of sandstones and u pb dating of detrital zircons shows that a recycled orogen in the central qiangtang to the north of najiangco area was the source of the sandstones in the shamuluo formation biostratigraphy and u pb zircon dating of a porphyritic granitoid dike 151 2 ma indicate that the shamuluo formation was deposited during the late jurassic oxfordian to kimmeridgian during middle jurassic time the southern qiangtang basin was dominated by shallow marine environments later during the late jurassic oxfordian to kimmeridgian the shallow marine facies retreated to the southern margin of the basin combined with regional paleogeographic data from central tibet two stages of southward retreat of the qiangtang remnant sea and three stages of topographic uplift of the qiangtang block can be recognized during late middle jurassic to early cretaceous time

coupled hydromechanical thermochemical processes in rock fractures the behavior of rock fractures under various coupled processes is reviewed under seven broad categories 1 hydromechanical processes 2 thermohydrologic processes 3 hydrochemical processes 4 thermohydromechanical processes 5 thermohydrochemical processes 6 hydromechanical chemical processes and 7 thermomechanical hydrochemical processes the state of the art and possible directions of further research in these coupled processes are discussed rock fracture behaviors considered include not only dilation closure shear and joint propagation but also changes in flow permeability and chemical sorption and retardation capabilities these properties are of critical concern in practical considerations of the rock mass as a medium to store and isolate chemical and radioactive wastes investigations of coupled processes in two fracture systems and multifracture systems are identified as important areas for future research the need to consider coupled processes in borehole testing procedures involving rock fractures is emphasized

rodingite from the beila ophiolite in the bangong nujiang suture zone northern tibet new insights into the formation ofophiolite related rodingite abstract the beila ophiolite is located in the middle part of the bangong nujiang suture zone on the northern tibetan plateau and rodingites are widespread within serpentinized peridotites in the ophiolite according to the mineral assemblages the beila rodingite can be divided into three types type i rodingites have a mineral assemblage of prehnite chlorite and minor diaspore type ii rodingites comprise diopside prehnite chlorite epidote and minor hydrogrossular and type iii rodingites underwent further rodingitization and contain more hydrogrossular and vesuvianite than the type ii rodingites zircon grains from two rodingite samples yielded shrimpu pb ages of 172 164 ma with positive values of hf t 9 0 to 16 3 and mantle like 18o values 4 31 to 6 82 indicating the beila ophiolite formed during the middle jurassic the rodingitization of the beila ophiolite took place in two stages the early stage involved the production of the type i and ii rodingites during metasomatism by ca rich fluids with high co2 h2o ratios and this stage is characterized by the formation of ca silicates diopside prehnite and hydrogrossular the second stage involved the production of type iii rodingites at the shallower depths in exhumed mantle under the influence of fluids with lower co2 h2o ratios estimates of the migration of elements during rodingitization e g ca alkalis fe cr ni nb ti rare earth elements and large ion lithophile elements combined with sr nd isotopic compositions indicate that the formation of the beila rodingite involved an extensive exchange of materials among the protoliths serpentinized peridotite and subduction related materials the results indicate that analyses of rodingites in ophiolites can contribute to our understanding of material exchange during oceanic plate subduction

paleocene eocene foreland basin evolution in the himalaya of southern tibet and nepal implications for the age of initial india asia collision siliciclastic sedimentary rocks derived from the southern lhasa terrane sitting depositionally upon rocks of the northern indian passive continental margin provide an estimate of the age of initial contact between the continental parts of the indian and asian plates we report sedimentological sedimentary petrological and geochronological data from upper cretaceous paleocene strata in the sangdanlin section located along the southern flank of the indus yarlung suture zone in southern tibet this is probably the most proximal and therefore the oldest record of the india asia collision these strata were deposited by high density turbidity currents or concentrated density flows and suspension settling of pelagic biogenic debris in a deep marine setting an abrupt change from quartz arenitic to feldspatholithic sandstone compositions marks the transition from indian to asian sediment provenance the abrupt compositional change is accompanied by changes in u pb ages of detrital zircons diagnostic of a sediment provenance reversal from indian to asian sources the timing of the transition is bracketed between 60 ma and 58 5 0 6 ma by detrital zircon u pb ages and zircon u pb ages from a tuffaceous bed in the upper part of the section in the context of a palinspastically restored regional paleogeographic framework data from the sangdanlin section combined with previously published data from the northern tethyan himalaya and the frontal nepalese lesser himalaya and subhimalaya suggest that a flexural wave migrated 1300 km southward across what is now the himalayan thrust belt from paleocene time to the present

formation of accretionary prisms influenced by sediment subduction and supplied by sediments from adjacent continents mechanical models are used to investigate the formation of accretionary prisms and related basins fed by sediments supplied from adjacent continents and subjected to sediment subduction results show that prisms forming under the infl uence of a hinterland sediment fl ux exhibit markedly different characteristics compared to classic critical wedges with identical mechanical properties forming by frontal accretion of a preexisting layer the main differences are reduced surface slopes increased spacing between thrusts widespread wedgetop basins and the presence of buried structures these differences are explained by the ability of continuous sedimentation to reduce differential stresses in the wedge leading to stable supercritical geometries thus in regions of active sedimentation wedge geometries cannot be explained solely in terms of relative strengths of the wedge and its decollement as is the case for critical wedges results also show that variations in the relative rates of sediment supply e g linked to climate changes and sediment subduction may lead to pulsed growth and decline of wedges though time as has been evidenced for some natural wedges thus rather than viewing compressive plate margins as accretionary versus erosive the dominant mode may repeatedly switch back and forth through time in response to variations in relative rates of sediment supply and sediment subduction

observations and modeling of dense water cascading from the northwestern laptev sea shelf 1 we examine dense water cascading from the northwestern laptev sea shelf in the arctic ocean this region is believed to be a potential site of dense water formation through ice freezing and brine ejection climate data indicate the presence of saltier denser water on the shelf all year round tongues of dense water spreading from the shelf to the continental slope and reaching a depth of 300 400 m are reported in some historical hydrographic surveys in this study we recreate dense water cascading from the laptev sea shelf by filling a gap between two sequential hydrographic surveys with numerical model output from october 1984 to april 1985 ice formation over the shelf provided salt input almost 5 times greater than the amount that was accumulated in the shelf waters this excessive salt was transported off the shelf mainly by baroclinic eddies and also by mean outflow through the bottom boundary layer shelf origin dense water would have reached at least 1000 m depth but strong stratification prevented it from penetrating deeper than 400 m downslope contribution of mean downslope flow to heat and salt exchange between shelf and slope is on the average 4 times smaller than that caused by eddy fluxes hence although the volume transport associated with cascading is relatively small 0 02 sv heat and salt fluxes are large enough to provide cooling and freshening of atlantic water which propagates along the siberian continental slope

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

perspectives on the roles of melanges in subduction accretionary complexes a review abstract melanges play three principal overlapping roles in the architecture of subduction accretionary complexes sacs during and after sac formation first tectonic melanges serve as zones of concentrated deformation within and below the accreted rocks that are assembled during the subduction accretion process these melanges facilitate preservation of inter melange less deformed accretionary units aus beneath the trench side of the sac the initial deformation zone along the decollement at the top of the down going plate is marked by non melange tectonically dismembered formations or thinner units of scaly rock and breccia that separate accreted rocks above from the subducting rocks below olistostromal rocks may be incorporated into the decollement here in the mid arc to inner arc areas of the sac exotic block bearing melanges develop in zones of tectonic fragmentation and mixing of accreting ocean plate stratigraphy in mud diapirs and along out of sequence faults some of which facilitate uplift of high pressure rocks and serpentinite matrix melanges second after accretion the sedimentary tectonic diapiric and polygenetic melange units serve as single block or sheet architectural aus or subunits within larger accreted aus of the sac diapiric melanges and sedimentary olistostromal melanges formed in and above sac fault blocks respectively may become incorporated into the sac during ongoing deformation and become architectural units as well third melanges serve as post subduction stress guides that focus shear strain during continuing and post accretion deformation of sacs allowing ongoing modification of the sac during progressive deformation of the orogen examples of each type of role reveal the importance of all three processes in the current architecture of outer orogenic belts

analytical models of heat conduction in fractured rocks discrete fracture network models routinely rely on analytical solutions to estimate heat transfer in fractured rocks we develop analytical models for advective and conductive heat transfer in a fracture surrounded by an infinite matrix these models account for longitudinal and transverse diffusion in the matrix a two way coupling between heat transfer in the fracture and matrix and an arbitrary configuration of heat sources this is in contrast to the existing analytical solutions that restrict matrix conduction to the direction perpendicular to the fracture we demonstrate that longitudinal thermal diffusivity in the matrix is a critical parameter that determines the impact of local heat sources on fluid temperature in the fracture by neglecting longitudinal conduction in the matrix the classical models significantly overestimate both fracture temperature and time to equilibrium we also identify the fracture matrix peclet number defined as the ratio of advection timescale in the fracture to diffusion timescale in the matrix as a key parameter that determines the efficiency of geothermal systems our analytical models provide an easy to use tool for parametric sensitivity analysis benchmark studies geothermal site evaluation and parameter identification

structural sequence and geochronology of the qomo ri accretionary complex central qiangtang tibet implications for the late triassic subduction of the paleo tethys ocean abstract the late paleozoic qomo ri accretionary complex qrac in the central qiangtang region of tibet lies at the southern margin of the longmu co shuanghu suture qrac is tectonically linked with the nierong and jitang metamorphic complexes in eastern tibet and the yunling and lancang complexes in western yunnan province sw china this metamorphic complex is mainly composed of late paleozoic passive continental marginal strata of the south qiangtang basin which was underthrust beneath north qiangtang in the late triassic at least three stages of deformation d1 d2 and d3 with two corresponding metamorphic events m1 and m2 were identified within the qrac d1 is characterized by a penetrative foliation s1 bearing a dextral shear and a corresponding m1 mineral assemblage of phengite garnet and quartz this deformation was interpreted to be the result of the northwestward oblique subduction of the paleo tethys ocean and dated at 211 219 ma by phengite 40ar 39ar ages d2 a subsequent compression deformation followed d1 immediately and exhibits flexural folds and pervasive axial crenulation in foliation s2 deformations from d1 to d2 indicate that an oblique oceanic subduction was immediately followed by a collision between the north and south qiangtang blocks along the longmu co shuanghu suture the d3 recorded by minor folds and divisional axial cleavage s3 is related to a giant arc structure with an extension of 50 km

from static to dynamic provenance analysis sedimentary petrology upgraded abstract the classical approach to sandstone petrology established in the golden years of plate tectonics and based on the axiom that detrital modes of sandstone suites primarily reflect the different tectonic settings of provenance terranes has represented a benchmark for decades the composition of sand and sandstone however simply provides us with a distorted image of the lithological structure of source terranes and gives us little clue whether they are allochthonous or autochthonous orogenic or anorogenic young or old what we may able to see reflected in detrital modes is the nature of source terranes continental arc oceanic and the tectonostratigraphic level reached by erosion in space and time the proposed new approach to the petrology of sand and sandstone 1 starts with a simple classification scheme circulated since the 1960s which is purely descriptive objective and free of ill defined ambiguous terms and 2 focuses on the nature and tectonostratigraphic level of source terranes further steps are essential to upgrade provenance analysis acquiring knowledge from modern settings is needed to properly identify and wherever possible correct for physical and chemical processes introducing environmental and diagenetic bias and thus address nature s complexities with adequate conceptual tools equally important is the integration of multiple techniques ideally including bulk sediment multi mineral and single mineral methods bulk sediment petrography remains the fundamental approach that allows us to capture the most precious source of direct provenance information represented by the mineralogy and texture of rock fragments bulk sediment geochemistry applicable also to silt and clay carried in suspension is a superior method to check for hydraulic sorting chemical weathering and fertility of detrital minerals in different sediment sources detrital geochronology thermochronology and isotope geochemistry reveal the diverse time structures of source rocks and have become necessary complementary techniques in modern provenance analysis inferences on geodynamic processes need independent geological information and come last but if tackled properly they can lead us much farther than the standard label obtained by using triangular diagrams uncritically as if they were infallible oracles

the geology of nuclear waste disposal an icsu committee on the geological disposal of high level radioactive wastes has concluded that century long interim storage is essential and that disposal in subduction trenches and ocean sediments deserves more attention

mesozoic subduction accretion history in central tibet constrained from provenance analysis of the mugagangri subduction complex in the bangong nujiang suture zone

tectonic evolution of the early mesozoic blueschist bearing qiangtang metamorphic belt central tibet this is the published version copyright 2003 american geophysical union all rights reserved

sea ice circulation in the laptev sea and ice export to the arctic ocean results from satellite remote sensing and numerical modeling sea ice circulation in the laptev sea and ice exchange with the arctic ocean have been studied based on remote sensing data and numerical modeling ice drift patterns for short and long term periods were retrieved from successive okean radar images and special sensor microwave imager data for the winters 1987 1988 and 1994 1995 seasonal and interannual variabilities of ice drift in the laptev sea and ice exchange with the arctic ocean during the period from 1979 to 1995 were studied with a large scale dynamic thermodynamic sea ice model during an average year sea ice was exported from the laptev sea through its northern and eastern boundaries with maximum and minimum export occurring in february and august respectively the winter ice outflow from the laptev sea varied between 251 000 km2 1984 1985 and 732 000 km2 1988 1989 with the mean value of 483 000 km2 sea ice was exported into the east siberian sea mostly in summers with the mean value of 69 000 km2 out of the 17 investigated summers 12 were characterized by sea ice import from the arctic ocean into the laptev sea through its northern boundary magnitude and direction of ice export from the laptev sea corresponded with the large scale arctic ocean drift patterns during periods of prevailing cyclonic or anticyclonic circulation based on a semiempirical method that has been validated with the large scale model and satellite data ice exchange between the laptev sea and the arctic ocean during the period from 1936 to 1995 has been estimated as 309 000km2 with strong interannual variability and no significant trend apparent

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

dense water formation in the laptev sea flaw lead 1 two primitive equation based models are used to estimate the formation of total volumes either of arctic cold halocline water chw intermediate water imw or deep water dw through freeze related salt rejection in the siberian laptev sea flaw lead system model a assumes that the rejected salt remixes with surface mixed water smw beyond the leads until salinities allow for contribution to the midlayers of either the chw the imw or the dw model b simulates direct salt rejection to the upper layer of the cold halocline and after remixing here further contribution to the midlayers of chw imw or dw averaging both model estimates laptev leads contribute either 0 161 sv of chw 0 075 sv of imw or 0 065 sv of dw which represents as much as 23 16 or 30 of arctic wide lead derived dense water contribution to the appropriate layer respectively northwestern laptev leads produce the greatest amount of dense water these lead sections show very short buoyancy equilibrium timescales 6 to 13 days and local dense water production may potentially be amplified by lateral brine injection into the cold halocline through bottom eddies central southern and southeastern leads generally produce little salt due to low surface water salinities as definite separation mechanisms and proportion distributions of rejected lead brines into chw iwm and dw are still unidentified in nature a combination of lead salt rejection and remixing model a and direct downward expulsion of brine packages model b is assumed to steer laptev lead dense water production

new insight into a subduction related orogen a reappraisal of the geotectonic framework and evolution of the japanese islands abstract the geotectonic framework and the evolutionary history of the japanese islands need revision in accordance with the various geophysical geological evidence gathered by new methodologies in the recent years including seismic tomography vibroseis ground breaking seismic experiments and detrital zircon chronology these investigations have addressed various themes such as 1 seismic profile of the crust and mantle beneath the japanese islands 2 high precision ages of the protoliths of high p t metamorphic rocks and 3 provenance of terrigenous clastics the results have led to a number of important findings including 1 detection of a large mass of slab around the mantle boundary layer suggesting the long term oceanic subduction beneath japan 2 confirmation of the subhorizontal piled nappe structure for the entire crust of japan 3 finding a new high p t metamorphosed accretionary complex unit that represents the youngest blueschist in japan 4 finding of the oldest early cambrian arc batholith and cover sediments and 5 the identification of plural arc batholiths which have already been erased from the surface based on a synthesis of these new data this article presents a re evaluation of the conventional geotectonic subdivision of the subduction related orogen in japan re definition of the elements and their mutual boundaries and re consideration of the geotectonic evolution of the japanese islands in particular the historical change in provenance suggests that proto japan has experienced large scale tectonic erosion in multiple stages and the corresponding large amounts of continental crust materials were subducted for understanding the orogenic growth of japan during the last ca 500 million years the significance of tectonic erosion coupled with continental contraction as well as the oceanward accretionary growth requires further attention

mesozoic cenozoic geological evolution of the himalayan tibetan orogen and working tectonic hypotheses the himalayan tibetan orogen culminated during the cenozoic india asia collision but its geological framework and initial growth were fundamentally the result of multiple previous ocean closure and intercontinental suturing events as such the himalayan tibetan orogen provides an ideal laboratory to investigate geological signatures of the suturing process in general and how the earth9s highest and largest orogenic feature formed in specific this paper synthesizes the triassic through cenozoic geology of the central himalayan tibetan orogen and presents our tectonic interpretations in a time series of schematic lithosphere scale cross sections and paleogeographic maps we suggest that north dipping subducting slabs beneath asian continental terranes associated with closure of the paleo meso and neo tethys oceans experienced phases of southward trench retreat prior to intercontinental suturing these trench retreat events created ophiolites in forearc extensional settings and or a backarc oceanic basins between rifted segments of upper plate continental margin arcs this process may have occurred at least three times along the southern asian margin during northward subduction of neo tethys oceanic lithosphere from 174 to 156 ma 132 to 120 ma and 90 to 70 ma at most other times the tibetan terranes underwent cordilleran style or collisional contractional deformation geological records indicate that most of northern and central tibet the hoh xil and qiangtang terranes respectively were uplifted above sea level by jurassic time and southern tibet the lhasa terrane north of its forearc region has been above sea level since 100 ma stratigraphic evidence indicates that the northern himalayan margin of india collided with an asian affinity subduction complex forearc arc system beginning at 60 ma both the himalaya composed of indian crust and tibet show continuous geological records of orogenesis since 60 ma as no evidence exists in the rock record for a younger suture the simplest interpretation of the geology is that india asia collision initiated at 60 ma plate circuit paleomagnetic and structural reconstructions however suggest that the southern margin of asia was too far north of india to have collided with it at that time seismic tomographic images are also suggestive of a second more southerly neo tethyan oceanic slab in the lower mantle where the northernmost margin of india may have been located at 60 ma the geology of tibet and the india asia suture zone permits an alternative collision scenario in which the continental margin arc along southern asia the gangdese arc was split by extension beginning at 90 ma and along with its forearc to the south the xigaze forearc rifted southward and opened a backarc ocean basin the rifted arc collided with india at 60 ma whereas the hypothetical backarc ocean basin may not have been consumed until 45 ma a compilation of igneous age data from tibet shows that the most recent phase of gangdese arc magmatism in the southern lhasa terrane initiated at 70 ma peaked at 51 ma and terminated at 38 ma cenozoic potassic adakitic magmatism initiated at 45 ma within a 200 km wide elliptical area within the northern qiangtang terrane after which it swept westward and southward with time across central tibet until 26 ma at 26 to 23 ma potassic adakitic magmatism swept southward across the lhasa terrane a narrow 20 km width orogen parallel basin developed at low elevation along the axis of the india asia suture zone the kailas basin and greater himalayan sequence rocks began extruding southward between the south tibetan detachment and main central thrust the kailas basin was then uplifted to 4 km elevation by 20 ma after which parts of the india asia suture zone and gangdese arc experienced 6 km of exhumation between 20 and 16 ma between 16 and 12 ma slip along the south tibetan detachment terminated and east west extension initiated in the northern himalaya and tibet potassic adakitic magmatism in the lhasa terrane shows a northward younging trend in the age of its termination beginning at 20 to 18 ma until volcanism ended at 8 ma we interpret the post 45 ma geological evolution in the context of the subduction dynamics of indian continental lithosphere and its interplay with delamination of asian mantle lithosphere

birth and demise of the bangong nujiang tethyan ocean a review from the gerze area of central tibet abstract reconstructing the evolution of the bangong nujiang tethyan ocean bnto plays an indispensable role in understanding the linkage between gondwana dispersion and tibetan plateau accretion the existence of the bnto is inferred from presence of dismembered ophiolite bodies along the bangong nujiang suture zone which separates lhasa from qiangtang in this paper we present a detailed synthesis of ophiolite metamorphism magmatism lithostratigraphy and tectonism from the western segment gerze area of the lhasa qiangtang collision zone in order to reconstruct the wilson cycle of the bnto our results suggest that the bnto initially opened in the late permian 260 ma as indicated by the earliest record of oceanic crust regionally strong evidence supports a northward subduction of the bnto beneath qiangtang likely initiating in the early jurassic 195 ma as suggested by detrital zircon age spectra from mesozoic arc sourced strata northward oceanic subduction generated three episodes of arc magmatism i e 186 170 ma 169 146 ma and 129 100 ma along the southern qiangtang margin in which the early jurassic magmatism is only sporadically preserved in modern outcrops amdo area meanwhile the recognition of an incomplete arc trench system on the northern lhasa margin supports a southward subduction beneath lhasa as well our new provenance data from lower cretaceous strata on the northern lhasa margin together with available multi disciplinary constraints suggest that final closure of the bnto did not occur until the latest jurassic early cretaceous 150 130 ma therefore the wilson cycle of the bnto is hereby reconstructed providing significant insights into the amalgamation of the tibetan plateau

impact of the arctic ocean atlantic water layer on siberian shelf hydrography this paper examines the role of the arctic ocean atlantic water aw in modifying the laptev sea shelf bottom hydrography on the basis of historical records from 1932 to 2008 field observations carried out in april may 2008 and 2002 2009 cross slope measurements a climatology of bottom hydrography demonstrates warming that extends offshore from the 30 50 m depth contour bottom layer temperature time series constructed from historical records links the laptev sea outer shelf to the aw boundary current transporting warm and saline water from the north atlantic the aw warming of the mid 1990s and the mid 2000s is consistent with outer shelf bottom temperature variability for april may 2008 we observed on shelf near bottom warm and saline water intrusions up to the 20 m isobath these intrusions are typically about 0 2 c warmer and 1 1 5 practical salinity units saltier than ambient water the 2002 2009 cross slope observations are suggestive for the continental slope upward heat flux from the aw to the overlying low halocline water lhw the lateral on shelf wind driven transport of the lhw then results in the bottom layer thermohaline anomalies recorded over the laptev sea shelf we also found that polynya induced vertical mixing may act as a drainage of the bottom layer permitting a relatively small portion of the aw heat to be directly released to the atmosphere finally we see no significant warming up until now over the laptev sea shelf deeper than 10 15 m in the historical record future climate change however may bring more intrusions of atlantic modified waters with potentially warmer temperature onto the shelf which could have a critical impact on the stability of offshore submarine permafrost

sandstone detrital modes in the paleogene liguride complex accretionary wedge of the southern apennines italy abstract the liguride complex is an ophiolite bearing tectonic unit related to westward subduction of the lithosphere of the neo tethys ocean beneath the european margin the complex includes a tectonic melange the calabro lucanian flysch unit containing broken formations of ophiolites and related pelagic and clastic sediments radiolarian chert limestone black shale and quartzarenite olistoliths or tectonic slices of continental crest gneiss granite and amphibolite middle eocene to upper oligocene siliciclastics and a less deformed turbidite unit the saraceno formation that caps the succession detrital modes of upper cretaceous to lower miocene sandstones from the liguride complex accretionary wedge are grouped into four distinct petrofacies that reflect the geodynamic evolution of southern italy an upper cretaceous to middle eocene quartzose petrofacies q90f9l1 is composed of deep water quartzarenitic and subarkosic sandstones that appear to have undergone long distance transport by turbidity currents from the european or african foreland a middle eocene to upper oligocene quartzofeldspathic petrofacies q62f21l17 was produced by mixing of sediment from ophiolitic and pelagic sedimentary sources with volcanic debris an upper oligocene volcanolithic petrofacies q16f24l6054f10l36 is present in the turbidites of the saraceno formation the accretionary wedge sandstones of the liguride complex may record long distance longitudinal transport for hundreds of kilometers from distal sources preexisting forelands and volcanic arc and short distance transverse transport from local sources accreted oceanic terranes and collisional belt terranes in accretionary wedge sandstone assemblages sediments from local and distal sources may be mixed or may record a consistent evolution from one type of transport to the other

salinity trends on the siberian shelves 1 we present an analysis of observed long term 100 year salinity trends on the freshwater dominated siberian continental shelves a multiple regression was performed in the white sea ws the kara sea ks the laptev sea ls and the east siberian sea ess since 1930 the ws has gained freshwater while the ess has lost it consistent with river discharge trends over this period over the past 20 years increases in both river discharge and direct precipitation can explain observed salinity decreases in the ws but not in the ks salinity trends in the ls and ess indicate that ocean circulation plays a dominate role in these areas where in recent years freshwater has been diverted eastward along the coast rather than northward toward the deep ocean

sandstone provenance and tectonic evolution of the xiukang melange from neotethyan subduction to india asia collision yarlung zangbo suture south tibet abstract the xiukang melange of the yarlung zangbo suture zone in south tibet documents low efficiency of accretion along the southern active margin of asia during cretaceous neotethyan subduction followed by final development during the early paleogene stages of the india asia collision here we present integrated petrologic u pb detrital zircon geochronology and hf isotope data on different types of sandstone blocks in the xiukang melange three groups of sandstone blocks with different provenance and depositional setting are distinguished by their petrographic geochronological and isotopic fingerprints blocks of turbiditic quartzarenite originally sourced from the indian continent were deposited in pre cretaceous time on the northernmost edge of the indian passive margin and eventually involved into the melange at the early stage of the india asia collision two distinct groups of volcaniclastic sandstone blocks were derived from the central lhasa block and gangdese magmatic arc one group was deposited in the trench and or on the trench slope of the asian margin during the early late cretaceous and the other group in a syn collisional basin just after the onset of the india asia collision in the early eocene the largely erosional character of the asian active margin in the late cretaceous is indicated by the scarcity of off scraped trench fill deposits and the relatively small subduction complex developed during limited episodes of accretion the xiukang melange was finally structured in the late paleocene eocene when sandstone blocks of both indian and asian origin were progressively incorporated tectonically in the suture zone of the nascent himalayan orogen

mooring based observations of double diffusive staircases over the laptev sea slope abstracta yearlong time series from mooring based high resolution profiles of water temperature and salinity from the laptev sea slope 2003 04 2686 m depth 78 26 n 125 37 e shows six remarkably persistent staircase layers in the depth range of 140 350 m encompassing the upper atlantic water aw and lower halocline despite frequent displacement of isopycnal surfaces by internal waves and eddies and two strong aw warming pulses that passed through the mooring location in february and late august 2004 the layers preserved their properties using laboratory derived flux laws for diffusive convection the authors estimate the time averaged diapycnal heat fluxes across the four shallower layers overlying the aw core to be 8 w m 2 temporal variability of these fluxes is strong with standard deviations of 3 7 w m 2 these fluxes provide a means for effective transfer of aw heat upward over more than a 100 m depth range toward the upper halocline these findings suggest that double diffusion is an imp

the nuclear waste disposal problem an overview from a geological and geochemical perspective abstract in this paper we present a brief review of nuclear waste disposal research from a geological and geochemical perspective examples were chosen to illustrate both laboratory and field aspects of nuclear waste disposal and to show how these could be linked together by geological analogy and observation this strategy of linking geological observations and laboratory experiments and tailoring the disposal concept to simulate the more attractive features of the geology should give greater credence to the overall performance assessment of a nuclear waste disposal system

the evolution of the bangong nujiang neo tethys ocean evidence from zircon u pb and lu hf isotopic analyses of early cretaceous oceanic islands and ophiolites abstract we conducted in situ u pb analyses of zircons from three basalts and one gabbro from the zhonggang oceanic island one basalt from the zhaga oceanic island and one gabbro from the kangqiong ophiolite all located in the middle segments of the bangong nujiang suture zone of tibetan plateau as well as in situ hf isotope analyses of zircons from one gabbro from the zhonggang oceanic island to constrain the tectonic evolution of the bangong nujiang neo tethys ocean all samples contain numerous inherited zircons and all the zircons contain magmatic oscillatory zoning and have th u ratios exceeding 0 4 moreover the average ree content of these zircons is less than 2000 ppm and they display clear negative eu and variable positive ce anomalies indicating a magmatic origin la icp ms u pb dating of the zircons revealed three clear peaks in the age distribution at 248 255 162 168 and 117 120 ma lu hf isotopic analyses of zircons from the gabbro of the zhonggang oceanic island yielded a 269 ma crust mantle separation age taking into account the regional geology previous data and our new analyses we infer that the middle and western segments of the bangong nujiang neo tethys ocean had initially opened in the late permian 254 269 ma and that the ocean opened substantially between the late permian and the early triassic 248 255 ma in addition we infer that the initiation of subduction of the bangong nujiang neo tethys ocean took place at 162 168 ma which is middle jurassic the 117 120 ma age is the time when the oceanic islands and ophiolites were formed indicating that the bangong nujiang neo tethys ocean was to some extent still open at that time

middle permian foraminifers from the zhabuye and xiadong areas in the central lhasa block and their paleobiogeographic implications abstract two new middle permian foraminifer faunas are described from the xiala formation in the zhabuye and xiadong sections in the central lhasa block the presence of some significant taxa in paleobiogeography such as shanita lysites biconcavus hemigordiopsis subglobosa indicates that the lhasa block was in the cimmerian province in paleobiogeography during the middle permian furthermore the nankinella chusenella fusuline assemblage that occurs in the upper part of the xiala formation in the xiadong section resembles contemporaneous fusuline faunas from the xainza and bashor areas in the lhasa block and the northern part of the tengchong block the endemism of the nankinella chusenella assemblage in the lhasa and tengchong blocks indicates that these two blocks were isolated from both the south qiangtang block and the northern gondwanan margin during the middle permian it further suggests that both the bangong nujiang ocean and the neotethys ocean would have opened before the middle permian a new species chusenella tsochenensis sp nov was described

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

discovery of an eclogite belt in the lhasa block tibet a new border for paleo tethys abstract a newly discovered eclogite belt in the eastern part of the lhasa block tibet is about 500 1000 m wide and at least 60 km long in an e w direction the eclogites occur as tectonic slices in garnet bearing mica quartz schist they are generally fresh and form thick massive layers that consist chiefly of garnet grt omphacite omp phengite phe rutile rut quartz qtz p t calculations based on the grt omp phe mineral assemblage yielded peak metamorphic conditions of 2 7 gpa and 730 c close to the phase boundary between coesite and quartz and thus the eclogites can be regarded as part of a very high pressure metamorphic belt petrochemical data suggest that the eclogite protoliths were typical morb basalts derived from depleted mantle shrimp u pb dating of zircons from the eclogite yielded metamorphic ages ranging from 242 15 to 292 13 ma with an average value of 262 5 ma the morb eclogites are interpreted to be remnants of paleo tethyan oceanic lithosphere the eclogites along with permian island arc volcanic rocks to the north are believed to mark a carboniferous permian suture zone dividing the lhasa block into a northern and southern segment this newly identified suture zone suggests that the border of the paleo tethyan ocean jumped southward from north of the bangong nujiang suture to within what is now the lhasa block

wind driven transport pathways for eurasian arctic river discharge distributions of temperature salinity and barium in near surface waters depth 50 m of the laptev sea and adjacent areas of the arctic ocean are presented for the summers of 1993 1995 and 1996 the tracer data indicate that while fluvial discharge was largely confined to the shelf region of the laptev sea in the summer of 1993 surface waters containing a significant fluvial component extended beyond the shelf break and over the slope and basin areas north of the laptev sea in the summers of 1995 and 1996 these distributions of fluvial discharge are consistent with local winds and suggest two principal pathways by which river waters can enter the central arctic basins from the laptev sea when southerly to southeasterly wind conditions prevail river waters are transported northward beyond the shelf break and over the slope and adjacent basin areas these waters can then enter the interior arctic ocean via upper layer flow in the vicinity of the lomonosov ridge under other wind conditions river waters are steered primarily along the inner laptev shelf and into the east siberian sea as part of the predominantly eastward coastal current system these waters then appear to cross the shelf and enter the interior arctic ocean via upper layer flow aligned roughly along the mendeleyev ridge the extent to which either pathway is favored in a given year is largely determined by local wind patterns during the summer months when fluvial discharge is greatest and shelf waters are at the lowest salinity of their annual cycle

tectonostratigraphy and provenance of an accretionary complex within the yarlung zangpo suture zone southern tibet insights into subduction accretion processes in the neo tethys abstract accretionary complexes record critical information about the history of subduction and accretion along the southern margin of asia prior to the india asia collision this paper presents detailed field mapping petrographic and detrital zircon u pb data from an accretionary complex within the yalung zangpo suture zone southern tibet from structurally higher to lower levels the accretionary complex consists of a serpentinite melange the chert matrix tangga melange the mud matrix pomunong melange and the coherent rongmawa formation the tangga melange consists of late triassic early cretaceous abyssal chert and siliceous shale with blocks of chert and mafic to ultra mafic rocks the tangga melange was accreted beneath ophiolitic rocks during the aptian the pomunong melange consists of a late jurassic early cretaceous hemipelagic siliceous shale and chert matrix with blocks from early permian seamounts and from late cretaceous trench fill sandstones it was accreted beneath the tangga melange after the aptian but prior to 71 ma structurally beneath these melanges to the south the uppermost cretaceous rongmawa formation consists of turbiditic sandstone pelagic chert and siliceous shale and records a transition in depositional setting from lower abyssal plain to upper trench detrital zircons from sandstone blocks of the pomunong melange and the coherent rongmawa formation display similar u pb age spectra and are dominated by peaks at 71 231 ma 481 693 ma and 701 1372 ma these age peaks overlap with igneous crystallization ages and detrital zircon ages from sedimentary strata in the lhasa terrane our data indicate that the serpentinite melange tangga melange pomunong melange and rongmawa formation comprise a southward younging accretionary complex that developed during the northward subduction of neo tethyan oceanic lithosphere beneath the south margin of the lhasa terrane and that all exposed ophiolitic and accretionary complex assemblages were accreted to the asian margin prior to the india asia continental collision

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

a study of thermally induced convection near a high level nuclear waste repository in partially saturated fractured tuff we present simple estimates for velocities of buoyancy driven flow of liquid and gas phases near a hypothetical high level nuclear waste repository in partially saturated fractured tuff these estimates indicate that gas phase convection could take place with appreciable velocity of the order of 20 m year while liquid convection is expected to be very slow of the order of 0 1 mm year detailed numerical simulations using an effective continuum approximation to represent fracture effects confirm these estimates and demonstrate a sensitivity of gas convection to the strength of binary diffusion

cross shelf transport of warm and saline water in response to sea ice drift on the laptev sea shelf 1 oceanographic moorings and conductivity temperature depth ctd surveys from september 2009 to september 2010 are used to describe recent changes in the laptev sea hydrography and to highlight wind and ice driven surface ekman transport as the mechanism to translate these changes from the outer to the inner shelf bottom waters in february 2010 moored oceanographic instruments recorded a sudden increase in temperature 0 8 c and salinity 3 near the bottom of the inner laptev sea shelf such warm and saline waters had not been previously observed on the inner shelf in winter they likely originated from the basin and were first observed during a summer 2009 ctd survey in the northwestern shelf break region subsequently spreading east and shoreward across the laptev sea shelf the changes were introduced to the mooring site by the first of a series of bottom intensified flow events with velocities reaching 20 cm s 1 topographically guided along a relic submarine river valley each of the flow events coincided with negative pressure anomalies at the mooring site and offshore directed upwelling favorable winds and ice drift we suggest that the observations to first order resemble a simplified two dimensional two layered ocean where offshore surface ekman transport is compensated for by a barotropic shoreward response flow near the bottom in this paper we use one of the first comprehensive long term laptev sea datasets to highlight ice ocean atmosphere interactions in early and late winter and discuss the role of freshwater stratification and ice mobility on under ice circulation on the laptev sea shelf

sedimentary and tectonic evolution of the southern qiangtang basin implications for the lhasa qiangtang collision timing the mesozoic stratigraphic record of the southern qiangtang basin in central tibet records the evolution and closure of the bangong nujiang ocean to the south the jurassic succession includes toarcian aalenian shallow marine limestones quse formation aalenian bajocian feldspatho litho quartzose to feldspatho quartzo lithic sandstones shallow marine sewa formation and deep sea gaaco formation and bathonian outer platform to shoal limestones buqu formation this succession is truncated by an angular unconformity overlain by upper bathonian to lower callovian fan delta conglomerates and litho quartzose to quartzo lithic sandstones biluoco formation and callovian shoal to outer platform limestones suowa formation sandstone petrography coupled with detrital zircon u pb and hf isotope analysis indicate that the sewa and gaaco formations contain intermediate to felsic volcanic detritus and youngest detrital zircons 183 170 ma with ehf t ranging widely from 13 to 25 pointing to continental arc provenance from igneous rocks with mixed mantle and continental crust contributions an arc trench system thus developed towards the end of the early jurassic with the southern qiangtang basin representing the forearc basin above the angular unconformity the biluoco formation documents a change to dominant sedimentary detritus including old detrital zircons mainly 500 ma ages in the lower part of the unit with age spectra similar to those from paleozoic strata in the central qiangtang area a major tectonic event with intense folding and thrusting thus took place in late bathonian time 166 1 ma when the qiangtang block collided with another microcontinental block possibly the lhasa block

tibetan basement rocks near amdo reveal missing mesozoic tectonism along the bangong suture central tibet the u pb and 4 0 ar 3 9 ar studies of a unique exposure of crystalline basement along the jurassic early cretaceous bangong suture of central tibet reveal previously unrecognized records of mesozoic metamorphism magmatism and exhumation the basement includes cambrian and older orthogneisses that underwent amphibolite facies metamorphism coeval with extensive granitoid emplacement at 185 170 ma the basement cooled to 300 c by 165 ma and was exhumed to upper crustal levels in the hanging wall of a south directed thrust system during early cretaceous time we attribute jurassic metamorphism and magmatism to the development of a continental arc during bangong ocean subduction and early cretaceous exhumation to northward continental underthrusting of the lhasa terrane beneath the qiangtang terrane we speculate that a jurassic arc extended regionally along the length of the bangong suture but in all other places in tibet has been buried either depositionally or structurally beneath supracrustal assemblages

early cretaceous sedimentary evolution of the northern lhasa terrane and the timing of initial lhasa qiangtang collision abstract lower cretaceous strata in the baingoin basin of the northern lhasa terrane record initial collision between the lhasa and qiangtang blocks followed by the early uplift of central tibet north south traverses across the baingoin basin highlight major differences between the duba formation in the north and the quasi coeval duoni formation in the south the duba formation documents upward transition from shallow shelf and deltaic environments to coarse grained siliciclastic fluvial sedimentation abundance of detrital zircons yielding jurassic cretaceous ages with ehf t values mainly between 2 and 10 occurrence of chert cr spinel and pyroxene grains together with southward paleocurrent directions indicate that the duba formation was sourced from the southern qiangtang terrane and bangong nujiang suture zone to the north the duoni formation in the south was deposited in shelfal to fan delta and fluvial environments abundant volcanic clasts detrital zircons yielding cretaceous ages with mainly negative ehf t values and northward paleocurrents indicate an active volcanic source located in the central lhasa terrane to the south with minor input from the northern lhasa terrane only the northern part of the baingoin basin was directly controlled by the lhasa qiangtang collision and may thus be considered a peripheral foreland basin whereas the southern part was mainly influenced by tectonic processes related to the northward subduction of neotethyan lithosphere and may thus be comparable to a retroarc foreland basin but these sedimentary features and the 139 79 ma baingoin plutonic intrusion do not fit well with classical foreland basin models zircon chronostratigraphy constrains the final consumption of bangong nujiang oceanic lithosphere and initial collision between the lhasa and qiangtang microcontinents to have taken place by 122 ma which has major implications for paleotectonic reconstructions of the tibetan plateau

upper triassic turbidites of the northern tethyan himalaya langjiexue group the terminal of a sediment routing system sourced in the gondwanide orogen abstract the upper triassic langjiexue group exposed south of the yarlung zangbo suture zone in south tibet shows sedimentary features different from typical tethyan himalayan successions and its origin is controversial in this article we combine field observations with paleocurrent petrologic geochronological and isotopic data to determine the provenance of langjiexue sandstones these middle to distal deep sea fan turbidites are crosscut by lower cretaceous diabase sills and dikes generated during rifting of india from gondwana indicating that the langjiexue group was originally deposited along or adjacent to the northern passive continental margin of india flute casts at the base of turbidite beds indicate mostly wnw ward paleocurrents pointing to provenance from a source located east of the depositional area common volcanic fragments and plagioclase grains together with a cluster of 400 200 ma aged magmatic zircons with uniform e hf t values from 5 to 10 are incompatible with any nearby sources including the qiantang block the lhasa block or the india subcontinent and indicate instead supply from a long lived magmatic arc terrane considering what is known about late triassic paleogeography a plausible source for langjiexue sediments is represented by the gondwanide orogen generated during subduction of the pan pacific oceanic lithosphere beneath southeastern gondwana this scenario is supported by the age range and hf isotopic signatures of late paleozoic early mesozoic zircons contained in langjiexue turbidites as in coeval turbidites exposed in western myanmar new data are needed to confirm falsify the existence of a thousand km long sediment routing system similar to the modern amazon which sourced in a cordillera type orogen rising along the southeastern margin of gondwana crossed an entire continent to feed turbiditic fans now exposed from western myanmar to the northern tethys himalaya

a field and laboratory study of wave damping by grease ice in a field and laboratory study we discuss the formation growth and wave absorption properties of grease ice our field observations show that grease ice formation occurs under cold windy conditions in both leads and polynyas in leads grease ice forms in the open water then is herded to the down wind edge of the lead in polynyas a langmuir circulation herds the grease ice into long plumes parallel to the wind in the laboratory we grow grease ice in a wave tank and measure its wave absorption properties for single frequency two dimensional waves on a large scale we find that the thickness of the grease ice which increases away from the paddle is determined by a balance between the wave momentum flux and the free surface tilt on a small scale our photographs show that the crystals which make up the grease ice consist of discs measuring about 1 mm in diameter and 1 10 m thick which at low rates of shear sinter together into larger clumps yielding a viscosity increase to measure this non linear viscosity we study the decay of wave amplitude between two critical distances measured inwards from the leading edge the first occurs when the depth of grease ice exceeds k 1 where k is the wave number the second further distance is a line of transition from liquid to solid behavior which we call the dead zone between these two distances the wave amplitude decays with a linear slope which increases as a 0 k 2 where a0 is the wave amplitude in open water concurrent measurements of ice concentration show that it increases from values of 18 22 at the leading edge to a local maximum of 32 44 at the dead zone while the values at the dead zone increase non linearly with a0k finally comparison of the observed to that calculated from a yield stress viscosity model shows if the yield stress coefficient is proportional to the incident wave momentum flux the model predicts the observed

a study of regional temperature and thermohydrologic effects of an underground repository for nuclear wastes in hard rock heat released by the radioactive decay of nuclear wastes in an underground repository causes a long term thermal disturbance in the rock mass the nature of this disturbance for a planar repository 3000 m in diameter at a depth of 500 m below surface is investigated loaded initially with a power density of 10 w m2 of spent fuel assemblies 10 years after discharge from a reactor the maximum increase in temperature of the repository in granite is about 50 c and the epicentral thermal gradient about 70 c km different waste forms and periods before burial have significant effects on the thermal disturbance the effects of temperature changes on the groundwater flow are evaluated with simple models of a vertical fracture connected to a horizontal fracture in the rock mass the buoyancy groundwater flow through the vertical fracture is a function of both the vertical and the horizontal fracture transmissivities as well as the changes in density and viscosity of groundwater caused by the temperature changes finite hydraulic recharge from the surrounding rock mass affects the thermohydrologic disturbance

estimating sea ice coverage draft and velocity in marguerite bay antarctica using a subsurface moored upward looking acoustic doppler current profiler adcp a technique for the analysis of data from a subsurface moored upward looking acoustic doppler current profiler adcp to determine ice coverage draft and velocity is presented and applied to data collected in marguerite bay on the western antarctic peninsula shelf this method provides sea ice information when no dedicated upward looking sonar uls data are available ice detection is accomplished using windowed variances of adcp vertical velocity vertical error velocity and surface horizontal speed adcp signal correlation and backscatter intensity were poor indicators of the presence of ice at this site ice draft is estimated using a combination of adcp backscatter data atmospheric and oceanic pressure data and information about the thermal stratification this estimate requires corrections to the adcp derived range for instrument tilt and sound speed profile uncertainties of 0 20 m during midwinter and 0 40 m when the base of the surface mixed layer is above the adcp for ice draft are estimated based on a a monte carlo simulation b uncertainty in the sound speed correction and c performance of the zero draft estimate during times of known open water ice velocity is taken as the adcp horizontal velocity in the depth bin specified by the range estimate

early cretaceous palaeogeographic evolution of the coqen basin in the lhasa terrane southern tibetan plateau abstract the lower cretaceous sedimentary strata in the lhasa terrane record the palaeogeographic and tectonic history of the tibetan plateau prior to the india asia collision the lower cretaceous strata in the coqen basin of the northern lhasa subterrane include the duoni and langshan formations the duoni formation is composed of conglomerates sandstones siltstones and mudstones in the south deposited in a fluvial environment and quartzose lithic sandstones siltstones and mudstones in the north deposited in a shelf coastal environment the overlying langshan formation is characterized by abundant orbitolina and rudist bearing wackestones and packstones which were deposited on a low energy carbonate ramp the langshan formation in the coqen basin was deposited between 119 115 ma and 98 ma based on its large benthic foraminiferal assemblages age constraints based on the interbedded tuff and conformably overlying langshan formation indicate that the duoni formation was deposited between 123 and 115 ma sandstone detrital modal compositions further indicate that the duoni formation was primarily derived from magmatic arc and recycled orogen sources detrital zircons from the duoni formation yield a primary age population of 160 110 ma peaking at 130 ma with negative e hf t values 18 7 to 2 7 and additional age ranges of 550 500 1200 900 and 1600 1500 ma thereby indicating that the zenong volcanic rocks and palaeozoic sedimentary bedrocks in the northern lhasa subterrane are the most likely sources the palaeogeography of the northern lhasa subterrane can be approximately described in two stages during the aptian 123 115 ma with the eruption of the zenong volcanic rocks the duoni formation accumulated in fluvial and shelf coastal environments and was sourced from the zenong volcanic rocks and basement rocks from the southern region of the northern lhasa subterrane the enhanced magmatic activities in the northern lhasa subterrane may have influenced the duoni formation deposition during the late aptian early cenomanian 115 98 ma a north south transgression occurred causing the majority of the northern lhasa region to be dominated by shallow marine limestones of the langshan formation