interpreting temporal patterns in the late quaternary dust flux from asia to the north pacific abstract a comparative study of the literature on the late quaternary accumulation of aeolian dust in china japan and the pacific ocean shows three different temporal patterns of dust signals the main dust sources in asia the deserts and desert margins contain large amounts of wind suspensible allochthonous material commonly an increase in dust accumulation is taken as an indicator of increased aridity in the interior of asia differences among the signals of asian desert dust accumulation in different depositional areas indicate that this interpretation requires critical evaluation the dust supply of the desert regions was maximal during the pleistocene glacial maxima the widespread assumption that these stages were the most arid periods is not necessarily valid additional factors likely to lead to temporal and regional differences in dust accumulation are 1 eustatic sea level changes associated with changes in the amount of exposed shelf material and changes in the extent of sea ice affecting atmospheric changes 2 the frequency of meteorological situations allowing long range dust transport 3 the strength of the dust transporting winds 4 changes in the wet deposition rate and 5 the presence of a dust trapping vegetation cover temporal variations in dustiness in different depositional areas are the result of specific combinations of these factors simplified assumptions concerning the reasons for dust variability may lead to incomplete or even erroneous palaeoecological conclusions

long term fluvial archives in the fen wei graben central china and their bearing on the tectonic history of the india asia collision system during the quaternary abstract staircases of large scale river terraces are striking features of the landscape in the fen wei graben adjacent to the qinling orogenic belts central china field investigations indicate that all five river terraces are composed of a basal channel gravel and an overlying thick loess palaeosol succession as chinese loess stratigraphy has been well studied it favours age determination of these terraces and their correlation with the marine oxygen isotope stages our research indicates that the ages of the five terraces are approximately 2 6 1 2 0 9 0 65 and 0 15 ma respectively the formation of these river terraces within the fen wei graben has been attributed to nw se crustal extension associated with left lateral displacement between the north and south china blocks in response to the northward movement of india towards asia since cenozoic time thus the stepped terraces in the fen wei graben reflects elements of the india asia collision systems in which terraces define episodes of accelerated northward movement of india towards asia during the quaternary

rise in frequency of surface melting at siple dome through the holocene evidence for increasing marine influence on the climate of west antarctica 1 a new melt layer history from siple dome west antarctica indicates notable late holocene summertime warming visual stratigraphic analyses of the 1004 m ice core identified 62 years with melt layers melting events began around 11 7 ka followed by a period of no melting from 8 8 6 6 ka melt layer frequency increased from 6 6 ka to the present with the 1000 year average melt layer frequency reaching a maximum of 2 at 0 8 ka we use our millennial scale archive of melt events as a unique seasonal paleothermometer to elucidate changes in west antarctic holocene summer climate our calibration suggests the change in melt frequency from 0 to 2 may represent a summer temperature increase of 2 c from the middle to late holocene this temperature change cannot be explained entirely by local change in ice elevation or summer insolation and is in contrast to east antarctic climate records which show peak warmth in the early holocene followed by stable or decreasing temperature we interpret the rise in melt frequency as evidence of an increasing marine influence on the ross sea sector of west antarctica although the surface elevation of siple dome has not changed greatly the continued lateral retreat of the west antarctic ice sheet from its last glacial maximum configuration across the outer continental shelf and the delayed drawdown in ice thickness from the adjacent coastal marie byrd land region in conjunction with periods of increased cyclogenesis perhaps related to variations in enso would allow a moderated maritime climate to more easily reach west antarctica

ross ice shelf oxygen isotopes and west antarctic climate history the ross ice shelf 18o profile at station j 9 covers at least the last 30 000 yr it identifies the depth in the core of ice from i the last glacial interglacial transition 266 to 286 m and ii the 1000 m surface elevation about 140 m various processes contribute to the 18o change observed in the core i climatic warming mainly caused by a decrease in winter sea ice extent around antarctica of about 6 latitude early in the glacial interglacial transition ii decreasing ice sheet thickness later in the glacial interglacial transition and during the holocene and iii decreases in elevation and effective distance from the open ocean as the source of the ice in the core shifts along the flow line toward j 9 average 18o values of the last 3000 yr imply a fairly stable climate yet shorter 102 to 103 yr 18o climatic oscillations up to 6 are seen in both the holocene and the glacial portion of the record

gravel bodies in the kumtagh desert and their geomorphological implications although developed in predominantly arid environments desert geomorphology sometimes bears information about past environmental change china s kumtagh desert has only recently been studied by researchers these investigations revealed the existence of several unique features compared with other chinese deserts including gravel bodies distributed in the northern part of the desert this paper summarizes the results of previous research in the region as well as information obtained from remote sensing and field investigations then discusses the geological background underlying quaternary geomorphology and the types development stages and geomorphological implications of the desert s gravel bodies four main types of gravel bodies and four developmental stages were identified it is suggested that the great elevation difference between the altyn tagh mountains and the kumtagh desert caused by uplift of the qinghai tibetan plateau constitutes the overall background for the development of gravel bodies but subsequent climatic events were the direct causes of their evolution the gravel bodies resulted from alluvial deposits on loose fine sediments mainly of aeolian origin that were subsequently subjected to wind erosion erosion cycles then progressively scattered these gravels to form gravel sheets the gravel bodies have three main geomorphological implications they record anomalous ancient climate events after the dune field formed and have little influence on the development of nearby seif dunes in the northeast demonstrate that the altyn tagh mountains are the main sediment source for the kumtagh desert and suggest that the evolution of the environment and geomorphology of the kumtagh desert have been influenced by neotectonic movements

migration of the siple dome ice divide west antarctica the non linearity of the ice flow law or a local accumulation low over an ice divide can cause isochrones internal layers to be shallower under the divide relative to the flanks forming a divide bump in the internal layer pattern this divide signature is analyzed using ice flow models and inverse techniques to detect and quantify motion of the siple dome ice divide west antarctica the principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide the calculated migration rate is 0 05 0 50 m a 1 toward ice stream d and depends slightly on whether the divide bump is caused by the non linearity of ice flow or by a local accumulation low our calculations also suggest a strong south north accumulation gradient of 5 10 x 10 6 a 1 in a narrow zone north of the divide a consequence of divide migration is that pre holocene ice is thickest about 0 5 km south of the present divide position divide motion indicates that non steady processes possibly associated with activity of the bounding ice streams are affecting the geometry of siple dome the migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths this suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 10 3 10 4 years

morphological record of oxygenic photosynthesis in conical stromatolites conical stromatolites are thought to be robust indicators of the presence of photosynthetic and phototactic microbes in aquatic environments as early as 3 5 billion years ago however phototaxis alone cannot explain the ubiquity of disrupted curled and contorted laminae in the crests of many mesoproterozoic paleoproterozoic and some archean conical stromatolites here we demonstrate that cyanobacterial production of oxygen in the tips of modern conical aggregates creates contorted laminae and submillimeter to millimeter scale enmeshed bubbles similarly sized fossil bubbles and contorted laminae may be present only in the crestal zones of some conical stromatolites 2 7 billion years old or younger this implies not only that cyanobacteria built proterozoic conical stromatolites but also that fossil bubbles may constrain the timing of the evolution of oxygenic photosynthesis

paleogene clockwise tectonic rotation of the xining lanzhou region northeastern tibetan plateau 1 to help understand the deformational history of the northeastern tibetan plateau paleomagnetic samples were collected from 177 sites and two magnetostratigraphic sections at 16 localities distributed among upper jurassic lower cretaceous to pliocene sedimentary and subordinate volcanic rocks within the xining lanzhou region 34 37 n 101 105 e a total of 127 sites at 12 localities yielded primary magnetizations confirmed by fold reversal and conglomerate tests age control on sedimentary rocks is provided by regional synthesis of chronostratigraphic data and our own biostratigraphic and magnetostratigraphic analysis presented in the companion paper by horton et al 2004 analysis of paleomagnetic declination combined with results from previous studies yield a remarkably consistent trend of vertical axis tectonic rotations across the studied region whereas 19 0 7 2 to 37 8 10 6 clockwise rotations are recorded consistently in all paleomagnetic localities in lower cretaceous to eocene rocks all paleomagnetic localities in oligocene to pliocene rocks have recorded minor to insignificant rotations indicating that the xining lanzhou region has undergone a wholesale regional clockwise rotation during late paleogene time consistent with regional chronostratigraphic and thermochronologic results this late paleogene tectonic rotation confirms that deformation reached regions of the northern tibetan plateau shortly after the initial collision of india with asia when compared to rotational paleomagnetic results from adjacent regions several mechanisms can be proposed to explain the clockwise rotation on the basis of consistency with geologic data we prefer a model involving clockwise rotation of the xining lanzhou region through right lateral shear and associated shortening related to northward indentation of the qaidam basin index terms 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic keywords tectonics paleomagnetism tibetan plateau

structural relationships along a greenstone shallow water shelf contact belingwe greenstone belt zimbabwe the belingwe greenstone belt zimbabwe contains a thick tholeiitic basalt and komatiite sequence structurally above shallow water shelf rocks that in turn rest nonconformably over an older gneiss terrain the nature of the contact between the basalt komatiite sequence and underlying shallow water shelf rocks represents one of the most critical relationships upon which general models for the tectonic setting of greenstone belts and the petrotectonic environment of komatiites are based this contact is a 20 to 200 m thick high strain zone characterized by a wide variety of shear zone tectonites derived from both the shelf and overlying lava succession in the ultramafic part of the high strain zone tectonites include ductile ultramafic mylonites ultramafic ultramylonites ultramafic phyllonites and serpentinite schists enclosing less deformed ultramafic serpentinite blocks in the part of the shear zone derived from shallow water sedimentary rocks shear zone tectonites include chert c s mylonite phyllonite and iron rich argillaceous shear bands both parts of the shear zone are cut by brittle fault gouge fabrics we suggest that the tholeiitic basalt and komatiite sequence is allochthonous with respect to the shallow water shelf and refer to it as the mberengwa allochthon contrary to prior suggestions there is no evidence that the tholeiitic basalt and komatiite succession of the belingwe greenstone belt sits conformably over the shallow water shelf rocks documentation of the shear zone at the base of the mberengwa allochthon lends support to the idea that the allochthon represents a fragment of an off axis oceanic plateau structurally emplaced over a shallow water shelf during a convergent plate interaction prior to 2 6 ga

the neogene xiyu formation a diachronous prograding gravel wedge at front of the tianshan climatic and tectonic implications the tarim and junggar basins in central asia are capped by a thick pile of conglomerates called the xiyu formation that are commonly linked to a change in climate and or accelerated uplift near the plio pleistocene boundary in order to better understand their origin and significance we carried out a combined structural and magnetostratigraphic study in the quilitage syncline southern tianshan where the base of the xiyu conglomerates is observed at both sides of the syncline a balanced cross section shows that even at a local scale the base of the xiyu conglomerates cannot be regarded as a single continuous stratigraphic layer on the southern flank of the quilitage syncline we collected 172 samples collected for magnetostratigraphic dating identify 17 polarity chrons that date the new section from 5 2 to 1 7 ma and constrain the base of the xiyu conglomerate here at 1 7 ma this is 4 2 ma younger than the age of the xiyu previously found on the northern limb of the same syncline together with other magnetostratigraphic studies carried out around the tianshan our study unambiguously demonstrates that the onset of deposition of the xiyu conglomerates is diachronous and that the conglomerates are systematically younger toward the basin consequently the xiyu formation should not be considered as a chronostratigraphic marker related to any particular tectonic or climatic event but is instead a prograding gravel wedge that has prograded over the underthrusting forelands a synthesis of chronologic and structural results yields progradation rates over the last 10 ma on the order of 2 0 mm yr and 3 9 mm yr south and north of the tianshan mountains respectively these rates are comparable to the shortening rate across the tianshan range suggesting that underthrusting is the main factor governing the progradation rate of the xiyu formation

δ15n of n2 in air trapped in polar ice a tracer of gas transport in the firn and a possible constraint on ice age gas age differences with respect to gas transport the fim near the surface of an ice sheet can be divided into three zones the uppermost is a convective zone located just below the surface of the ice sheet in which the air is rapidly flushed by convective exchange with the overlying atmosphere below the convective zone there is a diffusive air column in which diffusion is rapid but there is no convection between the bottom of the diffusive air column and the bubble close off region there may be a nondiffusive zone in which diffusion is so slow that negligible gas transport occurs the diffusive air column is characterized by progressive enrichment with depth of 15n in n2 and heavy isotopes of gases in general as predicted using the barometric equation in this paper we present data on the 15n of n2 in recently trapped air samples from 12 ice cores along with numerous downcore samples from byrd vostok and dome c bubble close off depths for these cores calculated from a densification model ranged from 51 to 114 meters below the surface mbs we used these data and the barometric equation to calculate the thickness of the diffusive air column and found that it comprised 46 to 93 of the total firn thickness at our study sites paleo close off depths calculated from the densification model for glacial sections of byrd vostok and dome c are 15 25 m deeper than close off depths today diffusive column heights calculated from 15n varied in a more complex manner the diffusive column height at byrd appears to have decreased from 74 m during the last glacial period to 50 m during the holocene at vostok and dome c the diffusive column height calculated from 15n increase from about 65 m during the last glacial period to about 80 m in the holocene we use records of surface temperature and co2 at byrd and vostok along with their respective chronologies to constrain the ice age gas age difference age throughout the section of the vostok ice core corresponding to the last glacial termination in principle age values calculated from these data can be used to discriminate whether gas in the fim mixes to the bubble close off depth or to a depth equivalent to the diffusive column height in practice however uncertainties in the chronology of byrd and vostok are too great to allow us to distinguish between these two possibilities one can only say that at the time of the last termination age for vostok was between 3 and 10 5 kyr previous estimates fall within this range

early life signatures in sulfur and carbon isotopes from isua barberton wabigoon steep rock and belingwe greenstone belts 3 8 to 2 7 ga

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

late neogene environmental changes in the central himalaya related to tectonic uplift and orbital forcing the linkage between tectonic forces and climate evolution remains a matter of much debate and speculation here we present high resolution oxygen and carbon isotope data from an ancient lake basin in the central himalaya these data together with sedimentologic evidence reveal major changes in drainage systems and depositional settings at 7 2 5 5 and 3 2 ma these environmental changes appear to be driven by regional scale tectonics the oxygen isotope record also reveals alternating wet and dry climates with periodicities of 24 and 100 kyr that were likely controlled by orbital forcing paleo temperatures estimated using a fossil based oxygen isotope paleo thermometer are 21 6 ca t 7 ma which is 19 6 c higher than the present day mean annual temperature in the same area the much warmer environment inferred here is consistent with fossil mammalian and pollen assemblages and sediment clay mineralogy as well as carbon isotope data from fossil tooth enamels and paleosol carbonates the estimated temperature difference would require the study area to have been raised by 2 2 5 km since 7 ma to its current elevation of 4100 4500 m above sea level this result can be interpreted as either indicating the presence of a low altitude intermountain basin in the hanging wall of the already formed main central thrust or a protracted development of a north trending rift basin that has experienced changes in drainage system and depositional environment through time

geochemistry of precambrian carbonates i archean hydrothermal systems abstract carbonate rocks from the superior and slave provinces of canada kaapvaal craton of south africa and the pilbara block of australia considered of hydrothermal origin from field criteria have been characterized mineralogically isotopically sr o c and chemically major and trace elements in agreement with previous studies the bulk chemical composition suggests that the carbonate rocks originated by massive carbonatization silicification and k na metasomatism of intermediate to ultramafic silicate precursors the carbonate component itself defines two partially overlapping populations the ferroan dolomite breunnerite assemblage is confined mostly to shear zones conduits and stockworks while the calcite ferroan dolomite ankerite siderite mineral assemblage represents a wider regional halo this dichotomy may result from a multistory not necessarily coeval plumbing system of the volcanosedimentary piles with the regional halo population being a product of shallow circulation cells while the conduit assemblage probably formed from deeper focussed flows trace element chemistry of carbonates is consistent with their precipitation from relatively low salinity solutions 2 sea water and with derivation of solutes from the contemporaneous volcanosedimentary piles 87 sr 86 sr 0 7020 0 0008 the hydrothermal waters were derived either from metamorphic dewatering at the base of the piles or from a magmatic source with 18o of about 6 7 smow in contrast carbon dioxide in the regional and the conduit assemblages had dissimilar principal sources in the former it was derived from exogenic sources decarbonation of marine carbonates and thermal decomposition and oxidation of organic matter while in the latter the co2 was of mantle origin 13c of 2 to 10 and 4 2 pdb respectively

preferential dust sources a geomorphological classification designed for use in global dust cycle models received 11 april 2011 revised 28 september 2011 accepted 4 october 2011 published 24 december 2011 1 we present a simple theoretical land surface classification that can be used to determine the location and temporal behavior of preferential sources of terrestrial dust emissions the classification also provides information about the likely nature of the sediments their erodibility and the likelihood that they will generate emissions under given conditions the scheme is based on the dual notions of geomorphic type and connectivity between geomorphic units we demonstrate that the scheme can be used to map potential modern day dust sources in the chihuahuan desert the lake eyre basin and the taklamakan through comparison with observed dust emissions we show that the scheme provides a reasonable prediction of areas of emission in the chihuahuan desert and in the lake eyre basin the classification is also applied to point source data from the western sahara to enable comparison of the relative importance of different land surfaces for dust emissions we indicate how the scheme could be used to provide an improved characterization of preferential dust sources in global dust cycle models

late quaternary aeolian sedimentation in central and south eastern tibet abstract the geomorphology and the cover sediments in the central and south eastern part of the tibetan plateau provide information about climatic change luminescence dating of aeolian and colluvial silts on top of different moraine sequences provides the first indications for the timing of these sediments during the late quaternary and holocene although there is a widespread cover of aeolian mantles in tibet only a few papers have focussed on them up to now samples from silt layers on top of pleistocene moraines or solifluction debris from central and southern tibet at elevations above 4000 m asl provide early holocene ages in most sections the aeolian silt on top of the bomi moraine in southeastern tibet situated at an elevation of about 3000 m asl provides an age of 25 ka according to geomorphological investigations the bomi moraine dates from the last ice age which can be confirmed by the luminescence data moreover one sample from a silt layer in between gravel layers xainxa is also pleistocene in age according to its sedimentary and geomorphological situation in the tibetan plateau pleistocene loess mainly occurs below 4000 m asl the holocene material a more sandy silt lying above 4000 m asl however aeolian material capping the moraines provides only minimum ages for the terminal moraines the time interval between glacier retreat and the accumulation of aeolian sediments may be several thousand years the most important fact is the trapping of dust in general lakes and a denser vegetation cover are or were the main traps for the typical loess found in the mountain areas of the tibetan plateau

the morphology of ice streams a b and c west antarctica and their environs airborne radar soundings of the ice sheet surface made in 1984 1985 together with elevations measured by oversnow traverses between 1957 and 1964 have been used to produce a surface elevation map of ice streams a b and c and much of the region around them the flight positions and elevations were tied to many ground control stations whose positions in three dimensions were fixed by satellite tracking ice thicknesses were also measured along the flight lines but have not yet been mapped the surfaces of active ice streams a and b exhibit a longitudinal ridgetrough topography of uncertain origin prominent surface valleys are associated with most but not all of their marginal shear zones there is a deep subglacial trough beneath the grid northeastern side of ice stream a that connects to the subglacial trough beneath reedy glacier the boundary between ice streams bl and b2 the two tributaries of ice stream b persists as a suture zone for 250 km downstream from their merger ice stream bl overlies a relatively deep subglacial trough that continued under the corresponding part of ice stream b until it disappears close to the grounding line between ice streams bl and b2 there is a complex zone containing several regions of undisturbed ice separated by bands of disturbed ice which suggest to us that rafts of ice are being incorporated into the ice streams the previously reported negative mass balance for the drainage system of ice stream b may arise from the expansion of the ice stream into the interior of the west antarctic ice sheet crevasse bands at some places on ridge ab together with high glacial velocities reported elsewhere also suggest this expansion close to the ross ice shelf ice stream b widens into a nearly flat area where the mean surface slope is only 3 5 10 4 and the surface elevations are only a few tens of meters above hydrostatic equilibrium inactive ice stream c differs from active ice streams a and b in surficial and basal characteristics no elongated ridges and troughs are observed instead the ice stream surface exhibits several terraces including some maxima and minima in elevation radar sounding reveals areas where basal echoes are strong and steady indicating subglacial water alternating with areas of weaker echoes with short fading lengths calculations indicate that the strong reflections require a water layer at least several centimeters to several meters thick depending on the electrical conductivity of the water due to reversals in the surface slope the hydrologic potential gradient changes sign several times along ice stream c which implies that subglacial water should collect in some locations

three dimensional coordination number from two dimensional measurements a new method la valeur de n 3 moyenne des coordonnees du neve a trois dimensions peut etre estimee a partir de n 2 nombre moyen des mesures a deux dimensions et a partir d une fonction qui depend uniquement du rapport entre le rayon moyen de voisinage et de celui du grain de l echantillon

binary sources of loess on the chinese loess plateau revealed by u pb ages of zircon the age distribution of detrital zircon has been used to trace sediment sources existing datasets show great similarity of zircon ages between the loess on the chinese loess plateau clp and the sediments from the north tibetan plateau ntp implying that eolian dust is delivered from the ntp to the clp by westerly winds or via the yellow river however ntp dust can also be transported by northwesterly winds from the alxa arid lands aals where materials are received from both the ntp and the gobi altay mountains gams here we report u pb zircon ages for aals sands and ntp and clp loess the results show that the zircons in the aals are mixed from ntp and gams zircons ntp loess is mainly derived from local sources mixing of materials from the ntp and gams defines the zircon ages of the loess on the clp better than the pure ntp source no temporal and spatial heterogeneities of zircon ages have been observed for the loess on the clp which suggests that the well mixed materials in the aals likely have an eolian source

arcuate pamir in the paleogene insights from a review of stratigraphy and sedimentology of the basin fills in the foreland of ne chinese pamir western tarim basin abstract the tectonic evolution of the pamir bears essential information for understanding intracontinental tectonic processes within the context of india asia convergence however it is unclear whether the arcuate shape of the pamir is entirely the result of the india asia collision or may have been in part an inherited pre cenozoic feature in this study we address the evolution of the ne chinese pamir during the paleogene by reviewing the stratigraphy of twelve sections in the western tarim basin and the establishment of sedimentary facies the results indicate three episodes of deposition of alluvial coarse sediments in the foreland regions of ne chinese pamir during the 1 early paleocene 2 late paleocene to early eocene and 3 middle eocene to oligocene respectively these are correspondingly attributed to 1 retroarc deformation in an andean type margin of the southern asia during subduction of the neo tethys ocean 2 the immediate response in the pamir to the india asia collision and 3 continued india asia convergence the alluvial conglomerates with grain size up to 5 10 cm imply a transport distance

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

archaean stromatolites from the steep rock group northwestern ontario canada reinvestigation of the late archaean stromatolites of the steep rock group has shown that a wide variety of forms is present including domed and tabular bioherms and biostromes both columnar and noncolumnar structures are present branching is common in some columnar forms facing directions in the stromatolites are consistent with other field evidence showing that the base of the steep rock group is an unconformity

variations in neoarchean microbialite morphologies clues to controls on microbialite morphologies through time the neoarchean carawine formation hamersley group western australia is a carbonate ramp that preserves diverse microbial structures which are characteristic of specific depositional environments these distinctive structures are distributed in five shallow subtidal and two deeper water facies in the oakover area of the carawine formation the shallow subtidal facies are composed of biohermal and bedded stromatolites centimetre scale ridge shaped microbialites and wavy laminated microbialites the deeper water facies are composed of fenestrate microbialites planar laminated dolostone and dolostone with rolled up microbial laminae microbialites in the carawine formation lie within a continuum of archean to proterozoic microbial facies some shallow water microbial facies in the carawine formation are similar to proterozoic facies such as large bioherms internally composed of a variety of stromatolite morphologies in contrast fenestrate microbialites grew in quiet subtidal environments and are common in archean rocks but have not been documented in similar proterozoic environments the similarity of shallow water facies across the archean proterozoic transition before and after the oxidation of the atmosphere and surface oceans indicates that stromatolite growth in shallow subtidal environments was not strongly affected by the chemical changes associated with oxidation of the oceans or by biological responses to those chemical changes rather stromatolite morphology was controlled mostly by the physical environment and the corresponding biological responses to that environment in contrast the absence of fenestrate microbialites from proterozoic deep subtidal environments suggests that the morphology of deep subtidal microbial structures was influenced by chemical or biological changes that occurred in association with oxidation of the surface oceans

the ages of fractures in the eye dashwa pluton atikokan canada ages of fractures in the eye dashwa granite pluton north of atikokan ontario are estimated from analyses of their filling materials pegmatite aplite hornblendeporphyry epidote chlorite gypsum diabase iron oxides carbonate and clays the analyses include radiometric age determinations of filling materials comparison of crystallization temperatures of filling materials with the expected temperature of the pluton through time and paragenetic relations derived from textures the major fractures primarily epidote and chlorite filled were produced by penetrative deformation during cooling immediately after intrusion of the pluton from 2 6 ga to 2 4 ga they transected the pluton and largely defined the present overall fracture network less severe penetrative deformation occurred about 1 1 ga when a few diabase dikes and small gypsum fractures developed a second source of fracturing appears to have existed through most of geological time and it reopened segments of old fractures within about 400 m of the present erosional surface it is unknown whether the near surface or supergene fractures formed more or less continuously through time or in distinct episodes such as during glacial periods open fractures which are most common in the near surface layer tend to transmit groundwater but fracture sealing due to the accumulation of filling materials seems to occur with time underground excavations such as nuclear fuel waste disposal vaults would be less susceptible to the incursion of groundwater if they were located below the near surface layer

gravel deposits on the margins of the qinghai xizang plateau and their environmental significance abstract the thick deposits of quaternary gravels found along the margins of the qinghai xizang tibet plateau have been studied for decades here we concentrate on two sediment series widely developed along the kunlun tian shan and qilian mountains the xiyu formation a molassic series of lower pleistocene age and the middle and upper pleistocene gobi gravels these two series with their contained fossil assemblages throw light on the question of the mean altitudes and uplift rates of the tibetan plateau during the quaternary and on the related problem of the vigour and penetration of advected summer monsoonal moisture into central asia the sedimentary series on the northern piedmonts of the tibetan plateau considered here challege the view that plateau uplift accelerated in the upper pleistocene they indicate that the plateau had reached mean altitudes in the range 4000 5000 m perhaps by the end of the lower pleistocene and certainly by the middle pleistocene this diminished the incursion of southerly monsoonal moisture and so imposed arid and semi arid conditions across much of a vast region north of the plateau which among other effects precluded widespread mountain glaciation in the lower pleistocene and general ice sheet glaciation of the tibetan plateau as a whole

stratigraphy of the steep rock group northwest ontario a major archaean unconformity and archaean stromatolites the archaean steep rock group of northwest ontario situated in the wabigoon subprovince of the superior province canada comprises five formations wagita formation clastics mosher carbonate jolliffe ore zone dismal ashrock and witch bay formation metavolcanics reinvestigation of the geology of the group has shown that the basal clastics of the wagita formation 0 150 m unconformably overlie the marmion complex a massive tonalite tonalite gneiss terrane 3 ga old overlying the basal elastics is the mosher carbonate 0 500 m containing diverse stromatolite morphologies extensive zones of carbonate breccia occur adjacent to fault zones and mafic dykes stratigraphically above the mosher carbonate is the jolliffe ore zone 100 400 m which is divided into a lower manganiferous paint rock member and an upper goethite member within the jolliffe ore zone thin layers of buckshot ore occur these are horizons of haematitic pisolites and fragments set in a lighter ferruginous matrix of kao

mesozoic cenozoic evolution of the xining minhe and dangchang basins northeastern tibetan plateau magnetostratigraphic and biostratigraphic results 1 accurate stratigraphic ages are crucial to understanding the deformation history of the tibetan plateau prior to and during the indo asian collision efforts to quantify mesozoiccenozoic ages are hindered by limited fossils and a paucity of volcanic horizons and regionally correlative strata magnetostratigraphic and biostratigraphic results from the xining minhe longzhong basin complex and dangchang basin provide an improved chronology of nonmarine basin development over a large region of the northeastern tibetan plateau 34 37 n 101 105 e analyses of 171 magnetostratigraphic levels and 24 palynological assemblages 120 species indicate late jurassic early cretaceous to mid tertiary deposition although magnetic polarity zonation is incomplete independent palynological age control partially restricts possible correlations to the geomagnetic polarity timescale the sediment accumulation record basin provenance structural geology and published thermochronological data support a history of jurassic exhumation late jurassic early cretaceous fault related basin initiation and cretaceous paleogene reduced accumulation these patterns which are compatible with late jurassic early cretaceous extension and cretaceous paleogene postrift thermal subsidence were disrupted at about 40 30 ma when shortening related to the indo asian collision induced localized range uplift vertical axis rotation and amplified subsidence indexterms 1520 geomagnetism and paleomagnetism magnetostratigraphy 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic 9609 information related to geologic time mesozoic keywords tectonics magnetostratigraphy tibetan plateau cenozoic mesozoic sedimentary basins

stable isotope compositions sources and paleoenvironmental significance of holocene calcareous root tubes in the tengger desert northwest china abstract valuable archives of paleoenvironmental changes are scarce in the hinterlands of deserts calcareous root tubes crts a form of pedogenic carbonate are widely distributed in the hinterland of the tengger desert northwest china the investigations of 13c and 18o values within crts from the tengger desert have a great potential to provide new significant archives for paleoenvironmental reconstructions in this area in this study 30 crt samples from the tengger desert were collected all the samples were described using cathodoluminescence cl and their 13c and 18o values were measured the results revealed that limited marine carbonate was incorporated in the crts in the tengger desert and 13c and 18o values of the inner belt of the crts were not affected by recrystallization therefore 13c and 18o values of the crts could accurately record environmental conditions in the tengger desert the carbon source of the crts was a mixture of soil co2 derived from root respiration and atmospheric co2 furthermore 13c values of most samples were lower than 3 0 indicating that c3 vegetation was the main vegetation in the tengger desert during the middle and late holocene moreover 18o values of the crts were ultimately inherited from summer precipitation influenced by the amount effect in summer and strong evaporation the lower 18o values of the crts revealed that relatively higher effective moisture occurred from 7 to 4 cal kyr bp while the higher 18o values indicated that relatively lower effective moisture occurred from 4 to 2 cal kyr bp in the tengger desert the holocene millennial scale moisture changes in the hinterland of the tengger desert suggest an arid early holocene 11 7 7 cal kyr bp a relatively humid mid holocene 7 4 cal kyr bp and a humid to arid late holocene 4 2 cal kyr bp the discrepancy of effective moisture change reconstruction based on the crts in the tengger desert and adjacent lake sediments during the early holocene demonstrated that the reliability of the reconstructed results from lake sediments during this period needs further investigation

magnetostratigraphy of sediments from the yumu shan hexi corridor and its implications regarding the late cenozoic uplift of the ne tibetan plateau abstract the yumu shan located on the northernmost margin of the north qilian shan contributes key information for deciphering the deformation and uplift of the ne tibetan plateau this paper presents magnetostratigraphic results from late cenozoic sediments in the yumu section of the northwestern yumu shan the upper shulehe fm yumen conglomerates fm and jiuquan gravels fm were deposited before 5 23 ma during the periods 3 58 to perhaps 1 77 ma and after 0 8 ma respectively four significant tectonic episodes occurred at 5 89 ma 3 58 ma during 2 88 2 58 ma and 0 9 0 8 ma based on sedimentation magnetic susceptibility and angular unconformities being synchronous with tectonic activity throughout the ne tibetan plateau these tectonic ages do not support the northward growth model for the ne tibetan plateau during the late cenozoic

velocity of ice streams b and c antarctica the essential difference between ice stream and inland ice and the cause of ice streaming are not known in order to study these problems velocities have been measured on and near ice streams b and c by repeated tracking of transit satellites the results confirm the difference in mode of flow between fast on ice stream b and slow on the interstream ice ridges also as expected ice stream c is nearly stagnant which with other evidence of formerly fast flow confirms its recent slowing and current thickening the drainage of ice stream b has a negative balance equivalent to thinning by 0 15 0 05 m yr 1 over the entire catchment the lower portion of the ice stream is thickening at about 2 m yr 1 but the overall behavior is thinning this general thinning is probably partly due to progressive extension of the ice stream into inland ice the transition from inland ice to ice stream seems to occur irregularly both spatially and in time with rafts of inland ice being carried into the ice stream and slowly incorporated

the depth of penetration in antarctic firn at 10 ghz measurements taken by an x band pulse radar system in east antarctica in 1987 were analyzed to determine the depth of penetration of radar energy in polar firn a minimum estimate of the penetration depth at 10 ghz for the cold and dry polar plateau region is approximately 4 7 m spatial variations in the amount of signal penetration were observed and are related to latitude surface elevation and mean annual temperature the amount of signal penetration and its spatial variation are important factors that should be considered when processing datasets from microwave remote sensing systems that operate in the polar regions

isotopic composition of disseminated sulfur in precambrian sedimentary rocks abstract isotopic analyses of trace sulfur in precambrian carbonates and clastic sedimentary rocks support the results of previous studies which have shown that 34s measurements of archean sedimentary sulfur cluster mostly around mantle like values and that a significant overall dispersion of 34 s 32 s ratios commenced at about 2 7 ga ago the outstanding feature of the proterozoic record is the predominance of heavy 34ssulfide values mostly in excess of 0 cdm consideration of these data in combination with the sparse 534s record for proterozoic sulfates suggests that 34ssulfide sulfide may have been smaller than was the case during the phanerozoic a possible scenario that is compatible with the above data is based on a proposition that the terrestrial exogenic sulfur cycle evolved in a tripartite manner in the archean it was coupled to and controlled by the magmatic endogenic cycle the proterozoic for the most part represents a transitional stage into a phanerozoic like exogenic cycle the latter essentially decoupled from its endogenic counterpart and its redox balance maintained and mediated by the biosphere the indication that the mean combined 34s of proterozoic sulfate and sulfide may have significantly exceeded the phanerozoic mean value of 2 cdm is enigmatic and poses problems for balance considerations

plagioclase sub species in chinese loess deposits implications for dust source migration and past climate change plagioclase mineral sub species in the lingtai section in central chinese loess plateau are examined using mineral liberation analyzer techniques showing that loess and paleosol samples exhibit similar patterns in terms of plagioclase feldspar sub species content this suggests that both loess and paleosol units have preserved their primary ca bearing plagioclase compositions of loess source regions weighted average cao in ca bearing plagioclase lies within a narrow range and is equivalent to the average plagioclase composition for upper continental crust this fact supports the hypothesis that chinese loess deposits are the result of a thorough mixing of dust sources the sum of ca bearing plagioclase content exhibits a general increasing trend superimposed by glacial interglacial oscillations in combination with observed plagioclase data in the deserts the variations of ca bearing plagioclase minerals might be used as a proxy for dust source migration and climate changes in the loess source regions furthermore linear relationship between lithogenic magnetic susceptibility ms component input and contents of ca bearing plagioclase in loess units revises a ms proxy for reconstructing paleo monsoon precipitation history the revised ms and plagioclase sub species records help in understanding the mechanism of glaciation across northern tibetan plateau

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

development of the east asian summer monsoon evidence from the sediment record in the south china sea since 8 5 ma 128 samples from ocean drilling program odp site 1143 in the southern south china sea were analyzed for grain size clay minerals biogenic opal content and quartz in order to reconstruct changes in east asian monsoon climate since 8 5 ma an abrupt change of terrigenous mass accumulation rate mar clay mineral assemblage median grain size and biogenic opal mar about 5 2 ma suggests that between 8 5 5 2 ma the source of terrigenous sediment was mainly in the region of surface uplift and basaltic volcanism in southern vietnam a simple model of east asian summer monsoon evolution was based on the clay feldspar ratio kaolinite chlorite ratio and biogenic opal mar the summer monsoon has two periods of maximum strength at 8 5 7 6 ma and 7 1 6 2 ma subsequently there was a relatively stable period at 6 2 3 5 ma continued intensification about 3 5 2 5 ma and gradually weakening after 2 5 ma since i ma the monsoon has intensified with remarkable high frequency and amplitude variability simultaneous increase in sedimentation rates at odp sites 1143 1146 and 1148 as well as in mar of terrigenous materials quartz feldspar and clay minerals at odp site 1143 at 3 5 2 5 ma may be the erosional response to both global climatic deterioration and the strengthening of the east asian summer monsoon after about 3 4 ma c 2006 elsevier b v all rights reserved

high resolution magnetic stratigraphy of fluvio lacustrine succession in the nihewan basin china abstract a detailed paleomagnetic and rock magnetic investigation of nihewan fluvio lacustrine deposits at tai ergou 40 o 12 52 n 114 o 38 22 e hebei province of north china has been carried out the results show that the magnetic mineralogy is dominated by magnetite and high coercivity hematite maghemite occurs mainly in the grayish yellow yellowish brown silt layers and fine grained yellow sand beds either as a detrital component or as the result of low temperature oxidation of magnetite distinct variations in magnetic susceptibility are always associated with changes in lithology i e color and grain size of the sediments reflecting the frequent lowering ascendance of the water table level and change of sedimentation environment based on paleomagnetic results of thermal and af demagnetization experiments we present an unambiguous polarity stratigraphy for the exposed lacustrine fluvial sediments at tai ergou consisting of three normal magnetozones including the brunhes chron the jaramillo and olduvai subchrons and two reversed zones belonging to the matuyama chron this suggests that the nihewan basin carries a long continuous stratigraphic record albeit varying in sedimentation facies and sedimentation rates consequently we concluded that the nihewan lacustrine sediments at tai ergou commenced depositing at the onset of the olduvai subchron and the tai ergou section can be used as a dating tool for biochronological and high resolution magnetostratigraphic purposes

steep rock lake sedimentology and geochemistry of an archean carbonate platform abstract the 2800 ma mosher carbonate formation at steep rock lake in south central canada is one of earth s oldest limestone deposits it is both thick up to 500 m and relatively well preserved and was one of the first precambrian successions to be noted for evidence of early life it continues to provide information on the antiquity of such processes as photosynthesis the carbonate contains a variety of fabrics and facies from apparently fine grained bedded limestones with domal and columnar stromatolites atikokania radial fans and sheet cracks to closely packed metric scale hybrid domes of originally aragonite seafloor crust interlayered with fenestral fabric here we review previous work provide new observations concerning these sediments and their geochemistry and discuss the inception overall development and ultimate demise of the steep rock platform we divide the mosher carbonate into the lower hogarth member and upper elbow point member these units constitute a shallow water carbonate platform succession the hogarth member overlies eroded 3000 ma tonalite and is at least 120 m thick thin horizons of domal pseudocolumnar columnar and fenestral stratiform stromatolites and occasional layers of atikokania radial crystal fans occur within relatively fine grained thin bedded limestone with iron carbonate horizons in the upper part meter scale domes occur locally with sheet cracks and stromatactis the irregular laminae that build the small domal and columnar stromatolites are interpreted to be lithified mats and biofilms presumably cyanobacteria dominated fine grained well bedded facies of the lower hogarth member and its varied association of small stromatolites is interpreted to have accumulated in a relatively protected inner platform environment numerous very thin horizons of iron carbonate in the lower and upper parts of the hogarth member suggest temporally limited influence of iron rich offshore waters vertical trends in fe mn ba sr ce and 13 c probably reflect progressive restriction of circulation with at least one abrupt but short lived change back to more open seawater conditions the upper hogarth member with large banded domes and fenestral crusts and stromatolites is interpreted as back margin facies the overlying elbow point member is at least 70 m thick and appears to be entirely composed of innumerable juxtaposed elongate smooth and thickly layered domes lacking intervening sediment each dome is typically several meters across and composed of centimetric alternations of cuspate fenestral fabric possibly microbial and crystal fan fabric that is probably abiotic seafloor aragonite precipitate adjacent cuspate fenestral and crystal fan fabric show significant differences in concentration of 13 c sr ba fe and mn this giant dome facies is interpreted as a platform margin deposit in which alternating calcite aragonite mineralogies within each dome reflect a laterally fluctuating offshore redox boundary negative ce anomalies positive gd anomalies and reduced positive eu anomalies relative to laterally correlative deep water iron formation occur in the majority of crystal fan samples and in some cuspate fenestral fabric samples ca carbonate precipitation at the margin of an anoxic iron rich sea would have been promoted by oxygenic removal of iron from seawater otherwise fe carbonate would have been favored over ca carbonate precipitation the mosher carbonate formation is interpreted as an early marine oxygen oasis a shallow platform that favored cyanobacterial productivity was sufficiently isolated from open marine circulation for the seawater to become relatively oxygenated and where carbonate sediment aggradation was a positive feedback that sustained these shallow water conditions limestone accumulation terminated as transgressive suboxic and anoxic seawater deposited first manganese oxide rich and then manganese oxide poor iron formation sediments

seasonal variations of glaciochemical isotopic and stratigraphic properties in siple dome antarctica surface snow six snow pit records recovered from siple dome west antarctica during 1994 are used to study seasonal variations in chemical major ion and h 202 isotopic deuterium and physical stratigraphic properties during the 1988 94 period comparison of d measurements and satellite derived brightness temperature for the siple dome area suggests that most seasonal sd maxima occur within 4 weeks of each 1 january several other chemical species h 2o2 non sea salt nss so 4 2 methanesulfonic acid and no3 show coeval peaks with sd together providing an accurate method for identifying summer accumulation sea salt derived species generally peak during winter spring but episodic input is noted throughout some years no reliable seasonal signal is identified in species with continental sources nssca 2 nss mg 2 nh 4 or nsscl visible strata such as large depth hoar layers 5 cm are associated with summer accumulation and its metamorphosis but smaller hoar layers and crusts are more difficult to interpret a multi parameter approach is found to provide the most accurate dating of these snow pit records and is used to determine annual layer thicknesses at each site significant spatial accumulation variability exists on an annual basis but mean accumulation in the sampled 10 km 2 grid for the 1988 94 period is fairly uniform

late quaternary drainage evolution in response to fold growth in the northern chinese tian shan foreland abstract alluvial units are important in understanding the interactions of antecedent drainage evolution with fold growth along the flanks of active orogenic belts this is demonstrated by the anjihai river in the northern chinese tian shan foreland which at present flows northward cutting sequentially through the nananjihai anticline the huoerguos anticline and the anjihai anticline three episodes of alluviation designated as fans fa fb and fc are identified for the anjihai river these three alluvial terrain features comprise a series of terraces where the topographic characteristics geomorphologic structure and up warped longitudinal profiles indicate continuous uplift and lateral propagation of the halaande anticline and the anjihai anticline over the past 50 ky shortly after 3 6 ka when the oldest terrace during the period of the fan fb sedimentation was formed significant rock uplift at the overlapping zone of the anjihai anticline and the halaande anticline led to the eastward deflection of the antecedent anjihai river a series of local terraces with elevation decreasing eastward indicate the gradual eastward migration of the channel of the anjihai river during the period of the fan fc sedimentation finally the anjihai river occupied the previous course of the jingou river when the latter was deflected eastward in response to rock uplift of the anjihai anticline presently flowing across the eastern tip of the anjihai anticline

quaternary geomorphological evolution of the kunlun pass area and uplift of the qinghai xizang tibet plateau abstract there is a set of late cenozoic sediments in the kunlun pass area tibetan plateau china paleomagnetic esr and tl dating suggest that they date from the late pliocene to the early pleistocene analyses of stratigraphy sedimentary characteristic and evolution of the fauna and flora indicate that from the pliocene to the early quaternary about 5 1 1 ma bp there was a relatively warm and humid environment and a paleolake occurred around the kunlun pass the elevation of the kunlun pass area was no more than 1500 m and only one low topographic divide existed between the qaidam basin and the kunlun pass basin the geomorphic pattern in the kunlun pass area was influenced by the kunlun yellow river tectonic movement 1 1 0 6 ma bp the wangkun glaciation 0 7 0 5 ma is the maximum quaternary glaciation in the pass and in other areas of the plateau during the glaciation the area of the glaciers was 3 5 times larger than that of the present glacier in the pass area there was no xidatan valley that time the extreme geomorphic changes in the kunlun pass area reflect an abrupt uplift of the tibet plateau during the early and middle pleistocene this uplift of the plateau has significance on both the plateau itself and the surrounding area

quaternary glacial sediments glaciation style climate and uplift in the karakoram and northwest himalaya review and speculations abstract opinions on the extent and style of the quaternary glaciations of the karakoram and northwest himalaya have ranged from restricted alpine style glaciation to extensive inundation by montane ice caps to overwhelming by sheet ice from the qinghai xizang tibet plateau the sedimentological and geomorphological evidence underpinning these contrasting opinions is used to integrate the sedimentary record with what is known about uplift and incision rates and broad scale changes in the regional climate by reference to the independent record of monsoon versuswesterly strength provided by the lacustrine and loess record of central and east asia increasing relative relief and topographical discordance are considered as influences upon glaciation style glacier extents and sediment budgets since the middle pleistocene ice extent may have progressively declined with increasing constraints on glaciation limits through time but total ice volumes during glaciations may have shown less variation as valley systems enlarged in conditions of quasi balance between uplift and incision

northeastward growth and uplift of the tibetan plateau magnetostratigraphic insights from the guide basin 1 most of the evidence for the uplift of the tibetan plateau comes from its southern and central parts although the northern rim has been less studied it may greatly contribute to the understanding of the mechanism and timing of the uplift recent studies on the northeastern part of the plateau suggest that the uplift can largely be explained by cenozoic thrusting and folding linked to the movement of the altyn tagh and kunlun faults and that diffuse shortening might be a realistic way to explain the thickening of the continental interior nevertheless the age of sediments that record the tectonic processes is poorly known the terrigenous formations in the guide gonghe basin ne tibet show several phases of deformation and include thick deposits of conglomerates interbedded with sandstones and siltstones that are thought to be the result of an increased sediment flux dating of these gravel deposits with magnetostratigraphic and faunal studies has permitted the precise timing of the basin infilling and its tectonic evolution to be established our results indicate that the onset of conglomerate accumulation occurs in the pliocene an observation that is in agreement with those of the linxia basin just east of the plateau if the high sedimentation rates 22 cm kyr in the guide basin are mostly due to tectonism then significant uplift occurred as late as pliocene

geochronological constraints on the tectonic evolution of a late archaean greenstone belt wabigoon subprovince northwest ontario canada abstract u pb isotopic age analyses have been carried out on zircon and titanite from rocks near the contact between the kakagi lake savant lake volcanic belt ksvb within the wabigoon subprovince and the english river subprovince in northwest ontario canada two tonalite gneisses intruded by mafic dykes at tannis lake and caribou lake have minimum zircon ages of 3051 ma and 3075 ma respectively this contrasts with a 2775 1 ma age on zircon from a felsic tuff located within a lower predominantly mafic volcanic sequence in the interior of ksvb these data combined with previous studies indicate that ksvb is presently flanked by older sialic crust in the sioux lookout and kashaweogama lake areas lower mafic volcanic sequences are unconformably overlain by sedimentary units that are in sheared contact with overlying calc alkaline volcanic units titanite and zircon analyses from clasts within a basal conglomerate indicate a maximum age for deposition of 2698 4 ma this late age is confirmed by a 2703 2 ma zircon age for an unconformably underlying porphyry intrusion one clast gives a zircon age of 2903 16 ma suggesting that the sediments were at least partly derived from older material in the english river subprovince data from previous studies show that the structurally overlying calc alkaline volcanic sequences are at least 30 ma older than the sediments the absence of zircon inheritance in all but the youngest ksvb sequences as well as geological and geochemical arguments suggest that the greenstone belt evolved largely in an ensimatic environment in isolation from older sialic material the intervening oceanic crust must therefore have been consumed by subduction which led to arc magmatism and eventual collision between the arc ksvb and continental material english river subprovince the sediments unconformably overlying the lowermost mafic volcanic sequences were deposited in a partly subaerial continental environment since chemical and textural evidence suggests the lower mafic volcanic sequences were deposited in an oceanic environment they must have been obducted onto the edge of a continental block this was perhaps followed by nappe and thrust emplacement on top of the sediments of calc alkaline sequences derived from earlier arc magmatism continued compression resulted in isoclinal folding of the supracrustal sequences into their present vertical positions

aeolian origin of the red earth in southeast china a clay like deposit known as red earth is widely distributed over the terraces and high lands of the yangtze river valley in southeast china its typical pedo stratigraphical features have attracted the interest of pedologists and geologists for many years although its origin is still debated here we report an analyses of the grain size distributions rare earth element bee patterns and upper continental crust ucc normalised major elemental composition of the red earth and compare them with those of the loess and other aeolian deposits in northern china the results show that the red earth in southeast china has two or three end member grain size distributions similar to the sedimentary characteristics and geochemical composition of aeolian deposits found in northern china together with other evidence from field observations these results suggest that the red earth is probably of acolian origin on the basis of these data we suggest that thick aeolian dusts were also deposited in the wet subtropics and that the effect of the winter monsoonal winds upon the dust transport in eastern china was more important than previously believed copyright c 2002 john wiley sons ltd

polar firn densification and grain growth a 50 m firn core from dome c east antarctica was found to consist of coarse firn which comprised 90 to 95 of the core and fine firn coarse firn was characterized by large crystals with a vertical shape orientation near the surface connected to nearest neighbors by relatively large necks in a structure different from closest packing fine firn was of higher density and consisted of smaller more spherical crystals connected by relatively narrow necks in a more nearly closest packed configuration higher surface free energy in fine firn causes crystals and necks to grow more rapidly than in coarse firn however we find that coarse firn densifies more rapidly with time contrary to the predictions of unconfined sintering models load driven densification due to a power law creep mechanism is found to account for the larger coarse firn densification rate however if the exponent in the power law exceeds one then densification rates are predicted to increase with depth due to increasing load contrary to observed behavior we speculate that different mechanisms may control the densification process in fine and coarse firn

secular variation in greenstone sequence development emphasizing superior province canada abstract evidence from the superior province of the canadian shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts 1 quartz arenite and carbonate bearing sequences 2 mafic ultramafic volcanic sequences 3 mafic to felsic volcanic cycles and 4 timiskaming type sequences of fluviatile sediments and calc alkaline to alkaline volcanics recently recognized quartz arenite bearing sequences in the sachigo and wabigoon subprovinces are from base to top quartz arenite stromatolite bearing carbonate oxide facies iron formation and komatiite or tholeiite primary structures in the quartz arenites indicate shallow water deposition and represent mature sedimentation granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates the few dated superior province quartz arenite bearing sequences are 2 85 ga the shallow water origin of the primary structures and biota and the areal extent of the quartz arenite and carbonate sequences invite comparison with platform sediments at passive margins platform sequences unconformably overlie 3 ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities mafic volcanic sequences contain sheet flows iron formations and deep water argillites based on physical volcanology and sedimentology these sequences are compared to modern deep water mafic plain and hawaiian volcanism they generally underlie mafic to felsic volcanic cycles and overlie platform sequences mafic sequences are 2740 2700 ma in age mafic to felsic volcanic cycles consist of deep water tholeiitic flows overlain by shallow water to subaerial intermediate to felsic pyroclastics and flows the facies style of volcanism petrography and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems this type of lithostratigraphic association varies in age from 2775 to 2700 ma in the superior province with minor exceptions timiskaming type sequences consist of fluviatile conglomerate immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry longitudinal and lateral variation within fault bounded basins is extreme and the sequences have generally been affected by only late stages of deformation the combination of rock types facies distribution the fault bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest timiskaming sequences are best compared with modern pull apart basins superior province timiskaming sequences are within the superior province platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts followed by mafic plain sequences arc volcanics and pull apart basins we suggest that platform sequences based upon limited evidence are preceded by an orogeny and that pull apart basins represent a subsequent orogeny the lithostratigraphic units above have distinct metallogenic associations 1 platform sequences based upon compilation of several shields may contain detrital au and u and oxide facies iron formations associated felsic volcanics contain no recognized volcanogenic cu zn deposits in the superior province the following metallogenic associations are noted 2 mafic plain sequences contain komatiite flow related ni deposits cu ni and platinum group deposits are associated with flows and coeval intrusions 3 arc volcanic sequences contain volcanogenic cu zn sulphide deposits and scarce deposits of porphyry type cu mo and 4 timiskaming sequences and related plutonism are related to lode au

a case for late archaean continental emergence from thermal evolution models and hypsometry abstract the secular cooling of the earth s mantle and the growth of the continental crust together imply changes in the isostatic balance between continents and oceans in the oceanic bathymetry and in the area of emerged continental crust the evolution of these variables is of fundamental importance to the geochemical coupling of mantle continental crust atmosphere and ocean to explore this further we developed a model that evaluates the area of emerged continental crust as a function of mantle temperature continental area and hypsometry in this paper we investigate the continental freeboard predicted using different models for the cooling of the earth we show that constancy of the continental freeboard 200 m is possible throughout the history of the planet as long as the potential temperature of the upper mantle was never more than 110 210 c hotter than present such numbers imply either a very limited cooling of the planet or most likely a change in continental freeboard since the archaean during the archaean a greater radiogenic crustal heat production and a greater mantle heat flow would have reduced the strength of the continental lithosphere thus limiting crustal thickening due to mountain building processes and the maximum elevation in the earth s topography rey p f coltice n neoarchean strengthening of the lithosphere and the coupling of the earth s geochemical reservoirs geology 36 635 638 2008 taking this into account we show that the continents were mostly flooded until the end of the archaean and that only 2 3 of the earth s area consisted of emerged continental crust by around 2 5 ga these results are consistent with widespread archaean submarine continental flood basalts and with the appearance and strengthening of the geochemical fingerprint of felsic sources in the sedimentary record from 2 5 ga the progressive emergence of the continents as shown by our models from the late archaean onward had major implications for the earth s environment particularly by contributing to the rise of atmospheric oxygen and to the geochemical coupling between the earth s deep and surface reservoirs

formation and evolution of the badain jaran desert north china as revealed by a drill core from the desert centre and by geological survey abstract the deserts in north china are regarded as direct records of the process of inland asian aridification amongst these deserts those proximal to the chinese loess plateau clp are especially important because of their role as sediment source areas in order to investigate the formation history of the badain jaran desert the largest desert proximal to the clp a 310 m long drill core was obtained from the desert centre which reached the underlying basement rock the lithology of the core consists of an upper layer of aeolian sand with three intercalated layers of lacustrine sediments overlying alluvial fluvial deposits which are developed unconformably over red coloured basement rock a geological survey indicates that the lower red coloured basement rock is of late cretaceous age and is cut by a mesozoic erosion surface widely developed over the badain jaran area on the basis of optically stimulated luminescence and electron spin resonance dating and stratigraphic correlation a preliminary chronological framework was established which was improved by tuning the aeolian sand proportion of the drill core to earth orbital obliquity and precession this final chronology suggests that 1 the basal age of the desert strata is at least 1100 ka corresponding to the formation of the badain jaran desert and 2 that the formation of the erosion surface terminated and the surface had begun to subside by at least 1200 ka the formation of the desert can be summarized as follows the alluvial fluvial fan formed by the heihe river draining the qilian mountains provided sufficient sedimentary material to form the desert under an arid climate while the asian winter monsoon and the westerlies entrained the clastic sediments from the fan area which finally accumulated downwind more distant sediment transport was prevented by the intervening mountainous terrain these coupled processes were driven by tibetan plateau uplift and ongoing global climatic cooling after initial formation of the desert three humid phases occurred in mis 13 15 mis 5 and the holocene during which extensive lakes developed the sequential drying out of the lakes may reflect inter relationships between global climate and the intensity of the asian summer monsoon and the westerlies

tectonic and gravity induced deformation along the active talas fergana fault tien shan kyrgyzstan abstract this paper shows by field palaeoseismological data the holocene activity of the central segment of the intracontinental talas fergana fault tff and the relevance of possible future seismic shaking on slope stability around a large water reservoir the fault striking nw se is marked by a continuous series of scarps deflected streams and water divides and prehistoric earthquakes that offset substrate and holocene deposits fault movements are characterised by right lateral strike slip kinematics with a subordinate component of uplift of the ne block structural geological and geomorphological field data indicate that shallow and deep landslides are aligned along the tff and some of them are active where the tff runs close to the reservoir the fault trace is obscured by a series of landslides affecting rock and soil materials and ranging in size from small slope instabilities to deep seated gravity induced slope deformations dgsds the largest of these which does not show clear evidence of present day activity involves a volume of about 1 km3 and is associated with smaller but active landslides in its lower part with volumes in the order of 2 5 104 m3 to 1 106 m3 based on the spatial and temporal relations between landslides and faults we argue that at least some of these slope failures may have a coseismic character stability analyses by means of limit equilibrium methods lems and stress strain analysis by finite difference numerical modelling fdm were carried out to evaluate different hazard scenarios linked to these slope instabilities the results indicate concern for the different threats posed ranging from the possible disruption of the m 41 highway the main transportation route in central asia to the possible collapse of huge rock masses into the reservoir possibly generating a tsunami

sedimentology and magnetostratigraphy of the tierekesazi cenozoic section in the foreland region of south west tian shan in western china abstract the geology of tian shan provides an excellent example for understanding the intracontinental orogeny in the context of indian eurasian convergence previous studies leave much space in basinfill deposition process to be assessed in the regions west to the talas fergana fault tff we improve the understanding by conducting new investigations on sedimentology and magnetostratigraphy in the tierekesazi section of the foreland region of south west tian shan four lithofacies have been identified i marine lithofacies from the aertashi to bashibulake formations ii lacustrine to fluvial plain lithofacies from the keziluoyi to the middle pakabulake formations iii alluvial sand gravel sheet lithofacies in the upper pakabulake formation and iv conglomerate lithofacies from the atushi to xiyu formations magnetostratigraphic analysis accompanied with biostratigraphic correlation indicates that four lithofacies cover age intervals of ca 65 ma to 34 ma ca 22 1 ma to 12 ma 12 ma to 5 2 ma and 5 2 ma to approximately present with the sediment accumulation rates increasing from ca 2 4 3 3 3 5 compacted decompacted cm ka in the lithofacies i to 12 3 16 17 cm ka in the lithofacies ii to 16 3 19 5 20 6 cm ka in the lithofacies iii and finally to 22 8 22 8 cm ka in the lithofacies iv these results suggest three episodes of sedimentary events combined with previous results these episodes of sedimentary events are attributed to tectonic activities that are widespread along south tian shan we speculate that the oligo miocene boundary event more directly and likely marks the initial underthrusting of the tarim block beneath the south tian shan the mid miocene and mio pliocene boundary events although approximately synchronous between the regions east and west to the tff have different structural expressions in the two regions such difference is proposed to cause the dextral slipping of the tff and more fundamentally likely be driven by the northward indentation of the pamir at this time

nd and sr isotopic characteristics of chinese deserts implications for the provenances of asian dust silicate nd sr isotopes of the fine grained fractions of the 10 major deserts and sandy lands in north china and the loess in chinese loess plateau were systematically investigated wide ranges in nd sr isotopic compositions have been observed the results of the 7 0 b the deserts on the northern margin of tibetan plateau with end 0 ranging from 11 9 to 7 4 and c the deserts on the ordos plateau with the lowest end 0 11 5 the distribution of the threes isotopic regions is controlled by the tectonic setting in north china which implies that the materials of the deserts are derived from the locally eroded rocks from the surrounding mountains and the nd sr isotopic signatures of these deserts could be quit stable over the past million years on the sub tectonic time scales if there is any desert at those times the nd sr isotopic compositions of the loess are mostly close to those of the deserts in isotopic region b suggesting that the main source regions of the last glacial loess in the chinese loess plateau are badain jaran desert tengger desert and qaidam desert also the comparison between the nd sr isotopes of the 5 lm silicate fractions of the deserts and the ancient dust falls in the north pacific and greenland show that the asian end members of these dust falls are derived most from the deserts in the isotopic region b and less from those in the isotopic region c

stratigraphic evidence for the uplift of the tibetan plateau between 1 1 and 0 9 myr ago uplift of the tibetan plateau is manifest not only in widespread denudation but also by an increased deposition rate of sediment near or far from the exhumed regions our results indicate that the mass accumulation rate mar of eolian dust increased between 1 1 and 0 9 myr ago we associate this increase in mar and median grain size with uplift of the tibetan plateau and its adjacent regions during this period this middle pleistocene uplift can also be evidenced by the age of volcanism in the marginal region the existence of thick conglomerate deposits surrounding the uplifted plateau and the increased sedimentation rate of lacustrine deposits in the qaidam basin northeastern tibetan plateau between 1 1 and 0 9 myr ago the correlation between the loess and marine records indicates that after 0 9 myr ago these two records correlate well this good correlation probably suggest that the middle pleistocene upheaval event not only brought the plateau into the cryosphere but also enhanced the coupling of regional scale chinese loess transportation and deposition to the global ice volume variations through its effects on glacial grinding rock denudation and east asian monsoonal circulation

magnetostratigraphy and rock magnetism of the neogene kuitun he section northwest china implications for late cenozoic uplift of the tianshan mountains in order to better constrain the tectonic evolution of central asia under the influence of the india asia collision we carried out a magnetostratigraphic study at the kuitun he section on the northern flank of the tianshan range northwest china a total of 801 samples were collected from a 1559 m thick section which is composed mainly of fluvio lacustrine sandstone and conglomerate stepwise thermal and alternating field demagnetization isolated a linear magnetization component that decays univectorally toward the origin and likely represents a primary magnetization principally carried by magnetite from this component 29 magnetic polarity intervals were identified they correlate between 3 1 and 10 5 ma with the reference magnetic polarity time scale indicating a relatively constant sedimentation rate with an average of 0 21 0 01 mm year we also performed a suite of rock magnetic experiments designed to track time transgressive changes in the sedimentary record from the rock magnetic parameters together with the constant sedimentation rate we conclude that the tianshan mountains were actively uplifting by 10 5 ma

identification of an archean marine oxygen oasis abstract the early earth was essentially anoxic a number of indicators suggest the presence of oxygenic photosynthesis 2700 3000 million years ma ago but direct evidence for molecular oxygen o2 in seawater has remained elusive here we report rare earth element ree analyses of 2800 million year old shallow marine limestones and deep water iron rich sediments at steep rock lake canada these show that the seawater from which extensive shallow water limestones precipitated was oxygenated whereas the adjacent deeper waters where iron rich sediments formed were not we propose that oxygen promoted limestone precipitation by oxidative removal of dissolved ferrous iron species fe ii to insoluble fe iii oxyhydroxide and estimate that at least 10 25 m oxygen concentration in seawater was required to accomplish this at steep rock this agrees with the hypothesis that an ample supply of dissolved fe ii in archean oceans would have hindered limestone formation there is no direct evidence for the oxygen source at steep rock but organic carbon isotope values and diverse stromatolites in the limestones suggest the presence of cyanobacteria our findings support the view that during the archean significant oxygen levels first developed in protected nutrient rich shallow marine habitats they indicate that these environments were spatially restricted transient and promoted limestone precipitation if archean marine limestones in general reflect localized oxygenic removal of dissolved iron at the margins of otherwise anoxic iron rich seas then early oxygen oases are less elusive than has been assumed

magneto and cyclostratigraphy in the red clay sequence new age model and paleoclimatic implication for the eastern chinese loess plateau the chinese loess plateau red clay sequences display a continuous alternation of sedimentary cycles that represent recurrent climatic fluctuations from 2 58 ma to the miocene deciphering such a record can provide us with vital information on global and asian climatic variations lack of fossils and failure of absolute dating methods made magnetostratigraphy a leading method to build age models for the red clay sequences here we test the magnetostratigraphic age model against cyclostratigraphy for this purpose we investigate the climate cyclicity recorded in magnetic susceptibility and sedimentary grain size in a red clay section previously dated 11 myr old with magnetostratigraphy alone magnetostratigraphy dating based on only visual correlation could potentially lead to erroneous age model in this study the correlation is executed through the iteration procedure until it is supported by cyclostratigraphy i e milankovitch cycles are resolved in the best possible manner our new age model provides an age of 5 2 ma for the shilou profile based on the new age model wavelet analysis reveals the well preserved 400 kyr and possible 100 kyr eccentricity cycles on the eastern chinese loess plateau further paleomonsoon evolution during 2 58 5 2 ma is reconstructed and divided into three intervals 2 58 3 6 ma 3 6 4 5 ma and 4 5 5 2 ma the upper part the youngest stage is characterized by a relatively intensified summer monsoon the middle stage reflects an intensification of the winter monsoon and aridification in asia and the earliest stage indicates that summer and winter monsoon cycles may have rapidly altered the use of cyclostratigraphy along with magnetostratigraphy gives us an effective method of dating red clay sequences and our results imply that many presently published age models for the red clay deposits should be perhaps re evaluated

internal fabric of giant domes in the mesoarchean steep rock carbonate platform superior province canada drivers for accumulation of archean seafloor carbonate were mostly very different than those operating today alternating crystal fan fabric and cuspate fenestral fabric forming meter scale giant domes is rare in archean carbonate platforms and limited to this time period their genesis is poorly understood the oldest known examples are in the 2 8ga steep rock group of northwestern ontario canada where 200 to 500m of carbonate was deposited on a semi restricted platform here the elongate domes are composed of cm to dm alternations of calcite crystal fans and microbialite commonly separated by thin dark ferroan dolomite and quartz replaced areas which have herein been referred to as alteration surfaces the cuspate fenestral fabric is formed of draping mat like laminae vertically oriented support structures and cement filled voids the concave up mat like laminae attach to the support structures which similar to the alteration surfaces are primarily composed of ferroan dolomite and quartz one

neogene uplift of the tian shan mountains observed in the magnetic record of the jingou river section northwest china the tian shan mountains constitute central asia s longest and highest mountain range understanding their cenozoic uplift history thus bears on mountain building processes in general and on how deformation has occurred under the influence of the india asia collision in particular in order to help decipher the uplift history of the tian shan we collected 970 samples for magnetostratigraphic analysis along a 4571 m thick section at the jingou river xinjiang province china stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary from this component a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between 1 ma and 23 6 ma with some uncertainty below 21 ma this correlation places precise temporal control on the neogene stratigraphy of the southern junggar basin and provides evidence for two significant stepwise increases in sediment accumulation rate at 16 15 ma and 11 10 ma rock magnetic parameters also undergo important changes at 16 15 ma and 11 10 ma that correlate with changes in sedimentary depositional environments together with previous work we conclude that growth history of the modern tian shan mountains includes two pulses of uplift and erosion at 16 15 ma and 11 10 ma middle to upper tertiary rocks around the tian shan record very young 5 ma counterclockwise paleomagnetic rotations on the order of 15 to 20 which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

ice core analysis at site a greenland preliminary results ice core drilling at site a greenland 70 45 n 35 57 5 w 3145m a s l by the polar ice coring office recovered two cores of about 100 m each which are now in storage awaiting further analysis field studies show that visible strata in the cores record both individual storms and individual years a typical year comprises 15 storm deposits pore close off occurs between 75 and 80 m depth the grain growth rate in isothermal firn and ice is about 7 8 x 1o 3 mm 2 a l the 10m temperature is about 29 5 c and the accumulation rate over the last 200 years is about 290 mm a i ice melt events occur about once every 40 years and major events can be correlated between cores separated by 20 m

five short historical earthquake surface ruptures near the silk road gansu province china five historical earthquakes m 7 have occurred along the hexi corridor at the northeastern margin of the tibetan plateau since a d 180 these are the a d 180 gaotai earthquake m 7 5 the 1609 hongyazi earthquake m 7 25 the 1927 gulang earthquake m 8 0 the 1932 changma earthquake m 7 6 and the 1954 shandan earthquake m 7 3 they are predominantly reverse slip except for the 1932 changma and the 1954 shandan earthquakes the changma earthquake is char acterized by left lateral faulting with a reverse component while the shandan earth quake by right lateral faulting with a normal component fieldinvestigationsindicatethatthelengthofthesurface rupturezoneisonly28km for the a d 180 earthquake 11 km for the 1609 earthquake 23 km for the 1927 earth quake and 5 1 km for the 1954 earthquake much shorter than that predicted using an empirical equation between moment magnitude mw surface rupture length srl coseismic displacement d maximum displacement md average displacement ad and mode value combined displacement statistic mvcds mw 5 91 0 609logsrl dfor strike slip faults mw 5 81 0 653logsrl dfor reverse faults and mw 6 93 0 82logad mvcdsfor all faults this may be common for reverse faulting earthquakes worldwide and suggests that trying to estimate magnitudes of past earthquakes from one type of even the most recent surface rupture data is unreliable and the empirical equations between magnitude coseismic slip and surface rupture length should be applied with caution to seismic hazard assessment on active thrust faults