active deformation to the north of the altyn tagh fault constraints on the northward growth of the northern tibetan plateau abstract sinistral transpressive deformation occurs in the region north of the altyn tagh fault due to the indo eurasian collision in this region several late cenozoic faults have developed and the sanweishan fault is the key feature with clear landforms in this study slip rate measurements combined with several dating methods imply that the sanweishan fault has been active with sinistral strike slip rates of 0 33 0 04 mm a since 14 ka bp and 0 28 0 03 mm a since 20 ka bp which are more than an order of magnitude smaller than that of the atf to the south the horizontal displacements are 1 7 5 5 m in the eastern segment and 25 0 76 0 m in the middle and western segments of the sanweishan fault the increment of horizontal displacement along the main sanweishan fault is 3 0 4 0 m between paleoearthquakes and the magnitude of the latest paleoearthquake on the middle and eastern segments of the fault is up to m 7 2 7 5 the whole length of the sanweishan fault is 360 km or longer which include active faults in the yangguan and danghe reservoirs in the west more components of thrusting occur on the western and eastern segments of the sanweishan fault the growth of the sanweishan fault is similar to that of the atf but the former fault is younger and its slip rate is much smaller the differential slip rates on the sanweishan fault and atf may result in the clockwise rotation and eastward movement of the intervening block which compresses regions to the northeast

aspects of climate variability in the north atlantic sector discussion and relation to the greenland ice sheet project 2 high resolution isotopic signal this paper reviews aspects of climate variability in the north atlantic sector that may influence the seasonal to decadal scale isotopic signal in the greenland ice sheet project 2 gisp2 ice core interpretation of the isotopic signal and its spatial applicability at the seasonal level requires investigation into synoptic scale climatology we discuss possible climatic influences of 1 likely source regions of precipitation reaching the gisp2 site 2 the characteristics of cyclone activity over the north atlantic sector and 3 changes in major atmospheric features such as the mean sea level icelandic low and azores high pressure systems the north atlantic oscillation and the baffin trough next we evaluate correlations between the gisp2 deuterium isotopic signal and coastal temperatures with atmospheric pressure patterns thereby deriving climatic interpretations of the high resolution isotopic record the gisp2 site is influenced by both the icelandic low to the southeast and davis strait baffin bay storms to the southwest and west the north atlantic oscillation influences the gisp2 isotopic signal through the seesaw in winter temperatures between west greenland and northern europe agreement in excursion directions of gisp2 isotopes and east greenland and iceland temperature records is associated with different positions of the baffin trough in winter and also with anomalous 500 mbar net geostrophic flow linkages between isotopic excursion direction and atmospheric variability need to be explored further by comparison with additional seasonal data sets from other greenland ice cores

a method for computing shallow ice core depths sipre style hand augers usually recover core sections whose bottom is somewhat shallower than the drill depth a procedure for calculating the range of possible core depths is presented in the field the drill barrel is lowered down the hole by means of a series of rigid extension rods the apparatus is then rotated to drill through debris left behind by the prior run and into fresh firn or ice care is taken to ensure that the total penetration of debris and firn does not exceed about half the length of the drill barrel otherwise the barrel s capacity for core and chips will be exceeded and the drill likely jammed the entire apparatus is then recovered the workers record the run number the total drill length barrel and extensions the exposed length of extensions above the hole top at the end of drilling and the length of recovered core the cycle is then repeated an example is shown in table i these data are then used to calculate drill depth which is drill length minus the exposed length plus an adjustment if drilling is from the base of a pit distance drilled is then obtained by comparison with the drill depth of the prior run the apparent core loss is the distallce drilled minus the length of recovered core the actual core loss is not readily calculated it is the sum of true core loss ground up core and the collapse of friable strata and the amount of core

the younger dryas termination and north atlantic deep water formation insights from climate model simulations and greenland ice cores results from the gisp2 and grip ice cores show that the termination of the younger dryas yd climate event in greenland was a large and extremely fast climate change a reinitiation of north atlantic deep water formation following a shutdown and its associated winter release of heat to the atmosphere has been suggested as the most likely cause of this climate transition to test this idea two general circulation model experiments using genesis have been completed for yd time 12 000 calendar years ago one with low heat flux in the nordic seas 10 w m deep water shutdown and one with high nordic sea heat flux 300 w m active deep water formation comparison of greenland climate differences between these experiments with the ice core records shows that when deep water is turned on much of the yd termination warming is achieved the increase in precipitation is underestimated because of a model tendency to overestimate summertime precipitation which obscures the dominantly wintertime response to the specified forcing the winter storm track shift toward greenland contributes much of the climate change at the yd termination

propagation of the deformation and growth of the tibetan himalayan orogen a review abstract long standing problems in the geological evolution of the tibetan himalayan orogen include where the india asia convergence was accommodated and how the plateau grew to clarify these problems we review the deformations and their role in the plateau s growth our results show that 1630 km of shortening occurred across the tibetan himalayan orogen since 55 ma with more than 1400 km accommodated by large scale thrust belts these thrust belts display an outward expansion from central tibet and couple with the surficial uplift the development of the tibetan plateau involved three significant steps primitive plateau 90 55 ma proto plateau 55 40 ma and neoteric plateau 40 0 ma several processes have collaborated to produce the proto plateau including the pre existing primitive plateau the india asia collision and subductions of greater india and songpan ganzi beneath the lhasa qiangtang terrane since 40 ma the proto plateau which was dominated by a topographic gradient lower crustal flow and continuous india asia convergence experienced three periods of rapid outward growth 40 23 23 10 and 10 0 ma in general the n s trending rifts were caused by the eastward growth of the plateau dominated by thrusting and crust flow in central tibet while they were the results of intense n s shortening in himalaya

glacio meteorological and isotopic studies along the egig line central greenland the geographical distribution of firn temperature annual accumulation rate as well as deuterium and oxygen 18 content in the firn were determined along an east west transect through central greenland this study is based on isotopic and chemical analyses of shallow firn cores at 18 sites along the egig line and high precision firn temperature measurements in 17 steam drilled boreholes along the eastern part of the transect the firn temperatures at 15 m depth range from 31 6 c at dome grip 3230 m a s l to 11 4 c at caecilia nunatak eastern ice margin at 1600 m a s l and 18 c at t05 near the western ice margin at 1900 ma s l the temperature altitude gradient changes from 0 7 c 100 m 1 in the ice divide region to 1 1 c 100 m 1 in the eastern part of the dry snow zone the temperature latitude gradient in the central part of the egig line is 0 7 c lat 1 the average annual accumulation decreases significantly from the west 47 cm a 1 water equivalent at t05 towards the ice divide 20 25 cm a 1 water equivalent from t99 to t43 accumulation rates are constantly low east of the ice divide 23 17 cm a 1 water equivalent thus dividing central greenland into two climatologically different regions the average 18 o and d values along the whole egig line reflect the well known temperature dependence for greenland very well e g 18 o t m 0 69 c 1 different regression lines for the western and eastern part however should be applied unlike the mean annual temperature the isotopic minimum along the egig line lies east of the ice divide this geographical distribution supports the choice of different water vapour trajectories in central greenland for the west and for the east significant parts of the water precipitated over the western slope are attributed to cyclonic systems entering greenland from the west the deuterium excess shows no significant geographical trend but a uniform seasonal variation at all sites along the egig line suggesting equal contributions from vapour source areas of the water precipitated over central greenland

effects of firn ventilation on isotopic exchange a new model of isotopic diffusion in the upper few meters of firn tracks the isotopic composition of both the ice matrix and the pore space vapor through time in two dimensions stable isotopes in the vapor phase move through the firn by diffusion along concentration gradients and by advection wind driven ventilation carries atmospheric water vapor into the firn where it mixes with existing pore space vapor unlike previous models our model allows disequilibrium between pore space vapor and the surrounding snow grains we also calculate the isotopic effects of ventilation driven sublimation and condensation in the firn model predictions of isotopic diffusion in firn compare favorably with existing diffusion models model results quantify what other investigators have suggested isotopic change in the upper few meters is more rapid than can be explained by the whillans and grootes 1985 model isotopic equilibration with atmospheric vapor is an important component of post depositional isotopic change and ventilation enhances isotopic exchange by creating regions of relatively rapid sublimation and condensation in the firn

variations in melt layer frequency in the gisp2 ice core implications for holocene summer temperatures in central greenland the rare melt features in the gisp2 central greenland deep ice core have decreased in frequency over the most recent 7000 years calibration of this change in melt frequency against modern spatial variation of melt frequency and temperature in central greenland and against modern temporal variability of temperatures in central greenland indicates that mean mid summer temperatures have cooled over the most recent 7000 years probably by slightly more than l c if variability of summer temperatures has not changed comparison to grip isotopic records from central greenland and to the melt record from the agassiz ice cap arctic canada suggests some seasonal and regional coherence for this cooling signal as well as for a cold event about 8000 8500 bp

major features and forcing of high latitude northern hemisphere atmospheric circulation using a 110 000 year long glaciochemical series the greenland ice sheet project 2 glaciochemical series sodium potassium ammonium calcium magnesium sulfate nitrate and chloride provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid low latitudes of the northern hemisphere interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants volcanic and biomass burning events storminess over marine surfaces continental aridity and biogenic source strength plus information related to the controls on both high and low frequency climate events of the last 110 000 years climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long term behavior of atmospheric circulation patterns through changes in ice volume sea level events such as the heinrich events massive discharges of icebergs first identified in the marine record that are found to operate on a 6100 year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics and rapid climate change events massive reorganizations of atmospheric circulation that are demonstrated to operate on 1450 year cycles changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events explanation for the exact timing and global synchroneity of these events is however more complicated preliminary evidence points to possible solar variability climate associations for these events and perhaps others that are embedded in our ice core derived atmospheric circulation records

the north atlantic oscillation signature in deuterium and deuterium excess signals in the greenland ice sheet project 2 ice core 1840 1970 the greenland ice sheet project 2 gisp2 core can enhance our understanding of the relationship between parameters measured in the ice in central greenland and variability in the ocean atmosphere and cryosphere of the north atlantic ocean and adjacent land masses seasonal summer winter to annual responses of d and deuterium excess isotopic signals in the gisp2 core to the seesaw in winter temperatures between west greenland and northern europe from a d 1840 to 1970 are investigated this seesaw represents extreme modes of the north atlantic oscillation which also influences sea surface temperatures ssts atmospheric pressures geostrophic wind strength and sea ice extents beyond the winter season temperature excursions inferred from the d record during seesaw extreme nao mode years move in the same direction as the west greenland side of the seesaw symmetry with the west greenland side of the seesaw suggests a possible mechanism for damping in the ice core record of the lowest decadal temperatures experienced in europe from a d 1500 to 1700 seasonal and annual deuterium excess excursions during seesaw years show negative correlation with d this suggests an isotopic response to a sst land temperature seesaw the isotopic record from gisp2 may therefore give information on both ice sheet and sea surface temperature variability cross plots of d and d show a tendency for data to be grouped according to the prevailing mode of the seesaw but do not provide unambiguous identification of individual seesaw years a combination of ice core and tree ring data sets may allow more confident identification of ga and gb extreme nao mode years prior to 1840

persistent cold climatic episodes around greenland and baffin island links to decadal scale sea surface temperature anomalies two unusually persistent cold episodes around greenland and baffin island in 1971 1972 and 1982 1984 are described in terms of their severity atmospheric circulation variability and changes in sea surface conditions the episodes are significant due to the sizeable station temperature departures many exceeding one standard deviation and their persistence in both episodes unusually persistent cold pools of water developed in the northern atlantic and intensified near davis strait close to the onset of the cold climatic events and then migrated out of the region as the episodes came to a close anomalous northerly surface air flow occurs in most months during the two episodes

snow isotope diffusion rates measured in a laboratory experiment the diffusion of stable water isotopes in snow was measured in two controlled laboratory experiments two batches of snow of different isotopic composition were stacked alternately with varying layer thicknesses the stack was stored in a freezer room at constant temperature for several months and sampled at regular intervals to analyse the diffusion measured isotope profiles were fitted to a theoretical model with diffusion length as the fit parameter in the first experiment we observed a difference in diffusion rates between layers of different thicknesses which is likely caused by layers of snow not being in proper contact with each other in the second experiment we found very good agreement between measurements and model results the measured diffusivity is compared with theory in which we mainly focus on the temperature dependence of the ice vapour fractionation factors this temperature dependence is slightly different for the different isotopes of water which leads to a difference in diffusion rates we illustrate how our set up can be used to measure the ratio between ice vapour fractionation factors of oxygen 18 and deuterium which determine the relation between the difference in diffusion and the firn temperature

late cenozoic transpressional mountain building directly north of the altyn tagh fault in the sanweishan and nanjieshan north tibetan foreland china abstract for many tectonicists the structural development of the northern tibetan plateau stops at the altyn tagh fault atf this study challenges that assumption structural field observations and remote sensing analysis indicate that the sanweishan and nanjieshan basement cored ridges of the archean dunhuang block which interrupt the north tibetan foreland directly north of the atf are bound and cut by an array of strike slip thrust and oblique slip faults that have been active in the quaternary and remain potentially active the sanweishan is a se tilted block that is bound on its nw margin by a steep south dipping thrust fault that has also accommodated sinistral strike slip displacements the nanjieshan consists of parallel but offset basement ridges that record nnw and sse thrust displacements and sinistral strike slip regional folds characterize the extreme eastern nanjieshan and appear to have formed above blind thrust faults which break the surface further west previously published magnetotelluric data suggest that the major faults of the sanweishan and nanjieshan ultimately root to the south within conductive zones that are inferred to merge into the atf therefore although the southern margin of the dunhuang block focuses significant deformation along the atf the adjacent cratonic basement to the north is also affected collectively the atf and structurally linked sanweishan and nanjieshan fault array represent a regional asymmetric half flower structure that is dominated by non strain partitioned sinistral transpression the nw trending dengdengshan thrust fault system near yumen city appears to define the northeastern limit of the sanweishan nanjieshan block which may be regionally viewed as the most northern but early stage expression of tibetan plateau growth into a slowly deforming mechanically stiff archean craton

reconstruction of three centuries of annual accumulation rates based on the record of stable isotopes of water from lomonosovfonna svalbard we use the upper 81 in of the record of stable isotopes of water from a 122 in long ice core from lomonosovfonna central spitsbergen svalbard to construct an ice core chronology and the annual accumulation rates over the icefield the isotope cycles are counted in the ice core record using a model that neglects short wavelength and low amplitude cycles we find approximately the same number of delta 18 o cycles as years between known reference horizons and assume these cycles represent annual cycles testing the validity of this assumption using cycles in deltad shows that both records give similar numbers of cycles using the delta 18 o chronology and decompressing the accumulation records using the nye flow model we calculate the annual accumulation for the ice core site back to ad 1715 we find that the average accumulation rate from 1715 to 1950 was on average 0 30 in w e accumulation rates increased about 25 during the later part of the 20th century to an average of 0 41 in w e for the period 1950 97 the accumulation rates show highly significant 2 1 and 21 year periodicities which gives credibility to our time scale

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

slip rates and seismic moment deficits on major active faults in mainland china 1 the chinese mainland features widespread active faults and intensive seismic activity both can be described in terms of slip rates on these faults previous studies of fault slip rates in mainland china have focused on individual faults or fault segments and large discrepancies exist among results derived from different methods here we derive a self consistent estimate of the slip rates on all major faults in mainland china using the recently updated gps data and an elastic block model the predicted slip rates are high in western and low in eastern china ranging from 25 1 2 0 mm yr in the himalayan thrust system to less than 1 0 mm yr in north china using these slip rates we estimated the rates of moment accumulation on the major fault zones and compared them with the seismic moment released on each fault zone using the chinese historical earthquake catalog that extends for more than 2000 years in many regions the results show nine seismic zones with large moment deficits unreleased moment future refinement of gps measurements and earthquake history will allow better estimates of slip rates on individual faults and better assessment of earthquake hazard on these faults in mainland china

structure of the central altyn tagh fault revealed by magnetotelluric data new insights into the structure of the northern margin of the india asia collision abstract the altyn tagh fault atf is a left lateral strike slip fault that forms the northern margin of the tibetan plateau and plays a significant role in accommodating the convergence between the colliding indian and eurasian plates as a part of the fourth phase of the indepth project magnetotelluric mt data were collected across the central segment of the atf to determine the lithospheric scale structure of the fault system dimensionality analyses demonstrated that the mt data can be interpreted using two dimensional approaches but some localized 3 d effects are seen consequently both 2 d and 3 d inversions were carried out and a joint interpretation was made on the basis of these two types of models inversion models revealed two major conductors beneath the qaidam basin qb and altyn tagh range atr respectively the conductive region beneath the qb was interpreted as a ductile layer in the lower crust to upper mantle that might represent flow beneath the western margin of the qb whereas the large scale south dipping conductor beneath the atr is interpreted as a region with high fluid content formed by metamorphism associated with the oblique underthrusting of the tarim block beneath the northern tibetan plateau these fluids migrate upwards through the fault system and have formed serpentinized zones in the crust combining these interpretations a structural model compatible with diverse geophysical observations is proposed in which we suggest the competing end member rigid block model and continuum model are reconcilable with the continuum model locally dominant for the study region as evidenced by a thickened crust

lithospheric deformation beneath the altyn tagh and west kunlun faults from recent gravity surveys 1 new gravity and topographic elevation data crossing the altyn tagh and west kunlun faults in northern tibet are used to test key features of a recently proposed tectonic model for the northern boundary of the tibetan plateau the observations collected in 1997 and 1998 at 468 stations along three profiles were converted to bouguer gravity anomalies to constrain flexural models for isostatic compensation of the large relief associated with these faults initially we failed to reproduce the main features of the gravity anomalies with any isostatic model that assumes that the main contribution to the observed gravity anomalies is from the flexure of the moho by integrating sedimentary information derived from exploration seismology in the tarim and qaidam basins we obtained satisfactory fits to the data using simple models of isostasy no elastic strength across the altyn tagh is required to fit the gravity data suggesting that the fault breaks the entire lithosphere beneath it and has negligible thrust component currently or in the past this result is consistent with tectonic models that create the thickened crust of northern tibet through thick skinned thrust sheets progressively overriding asia to the northeast as the altyn tagh behaving as a classic transform fault propagates in that direction farther west the gravity observations across the west kunlun are best explained by significant underthrusting of the high topography by an elastic plate with effective thickness between 30 and 40 km in agreement with geologic interpretation and modeling of earlier but very sparse gravity observations

precipitation over greenland retrieved by a dynamic method and its relation to cyclonic activity abstract in order to calculate the vertical motion over some high mountain regions such as greenland an equation without the quasigeostrophic approximation in coordinates has been developed a dynamic method for retrieving precipitation over greenland is based on this equation the retrieved annual mean precipitation distribution for 1987 and 1988 is in very good agreement with the observed annual accumulation pattern over the greenland ice sheet the major weather system producing precipitation over greenland is the frontal cyclone based on the precipitation characteristics greenland can be divided into five subregions precipitation over the north coastal and central interior regions primarily occurs in summer for the three other subregions if the composite monthly mean sea level pressure charts for high and low monthly precipitation amounts are constructed a clear relationship between precipitation and cyclonic activity emerges if a mean cyclone exists in the labrador sea heavy precipita

tibetan plate overriding the asian plate in central and northern tibet at the southern boundary between india and tibet the indian tectonic plate subducts northwards beneath the tibetan plateau seismic imaging shows that at the northern boundary of tibet the asian plate is also subducting southwards beneath the plateau and the tibetan lithosphere is separate

isotopic diffusion in cold snow and firn molecular mixing in cold snow and firn is ordinarily controlled by vapor diffusion diffusion through or along the ice matrix is slow whereas diffusion from the air to within separate grains is comparatively rapid thus mixing occurs in the vapor phase and exchange with the local ice involves all the water molecules a quantification of these concepts is applied to the diffusion of oxygen isotopic depth profiles in southern greenland and at dome c antarctica it successfully describes the smoothing of measured profiles the smoothing rate is strongly dependent on temperature and density of the firn

northeastward growth of the tibet plateau deduced from balanced reconstruction of two depositional areas the qaidam and hexi corridor basins china we address the problem of late cenozoic uplift erosion and growth of northeastern tibet by reconstructing from isopach maps and drill holes the volumes and masses of sediments deposited in the qaidam and hexi corridor basins since 35 myr ago the mass budget is based on simple geometrical assumptions such as regional similarity of the thickness ratios between strata of different ages in the qaidam where our record extends back to the oligocene the budget shows a huge rise of the accumulation rates after the beginning of the pliocene 5 3 ma the early pliocene seems to be the period of maximum deposition with accumulation rates in excess of 1 mm yr 1 2 7 kg m 2 yr 1 of compacted rocks throughout the basin there also seems to be a southeastward shift of the largest depocenters between the upper pliocene 3 4 1 6 ma and the quaternary in the hexi corridor sedimentation is confined to small foreland flexural depressions associated with the frontal thrusts of the qilian shan and occurs at an average rate one order of magnitude smaller than in the qaidam basin the accumulation rate is maximum in the quaternary the sedimentation history appears to support a plateau building mechanism resulting from the combination of two geologicallly common processes crustal scale thrusting and sedimentary basin infilling the time needed to completely fill the qaidam basin and make its catchment a plateau closely resembling that of the highest part of tibet qangtang is of the order of 9 myr the mechanism now at work north of the kunlun which involves rapid infilling of broad flat areas separated by relatively narrow mountain ranges has thus probably been important in producing the high smooth topography that characterizes much of central tibet

tectonic evolution of the qinghai tibet plateau abstract the qinghai tibet plateau composed of several continental slivers within the eastern tethyan domain is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on earth this plateau is generally inferred to be a collage of several continental blocks that rifted from gondwanaland and subsequently accreted to the asian continent however recent recognition of over twenty ophiolite melange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision based on 177 recently finished 1 250 000 scale geological maps and related studies we summarize the main tectonic context of the qinghai tibet plateau and propose a new integrated model to account for the new findings the complex orogen of the immense qinghai tibet plateau consisting of multiple island arc basin systems that developed at different stages while surrounded by the north china yangtze tarim and indian plates is emphasized the entire orogen surrounded by suture zones that mark the locations of oceanic closure is investigated by examining i the first order tectonic units and ophiolitic melanges including arc arc continent collision zones and ii their internally enclosed blocks as the second order tectonic units therefore the qinghai tibet plateau is divided into three major orogenic systems namely from northeast to southwest the early paleozoic qinling qilianshan kunlunshan qin qi kun the late paleozoic triassic qiangtang sanjiang and the late paleozoic to cenozoic gangdese himalaya orogenic systems which are separated by the kangxiwa muzitagh maqin mianxian and the bangong shuanghu changning menglian sutures respectively we propose that the formation and evolution of the qinghai tibet plateau to have been intrinsically related to those of the eastern tethys recorded by the longmu co shuanghu ophiolite melange zone the southern qiangtang paleozoic accretionary arc basin system the bangong nujiang suture zone and their associated composite island arc basin systems the present day bangong shuanghu changning menglian suture system marks the final closure of the tethyan ocean the yarlung zangbo ocean opened as a back arc basin in response to the southward subduction of the tethyan ocean lithosphere in the middle triassic and closed as a result of the india asia collision at the end of cretaceous followed by the northward indention of the indian plate that resulted in significant intra continental deformation and plateau uplift in the cenozoic

controlled experiments on the diffusion rate of stable isotopes of water in artificial firn we studied the diffusion rate of stable isotopes of water ice in a controlled laboratory experiment where isotopically different layers of fabricated firn were stacked this experiment was done in order to study the diffusion rate between the isotopically different layers and to compare the measurements with forward calculated diffusion rates of the fabricated firn stack using three different analytical and numerical methods the layers varied in thickness within the stack and the studied period includes variations in firn temperature we sampled the firn stack on five different dates along the 144 day long experiment we obtained a quantitatively good match between the calculated and measured diffusion rates with a root mean square similarity of 67 80 in the depth averaged diffusion rates despite the good average match we found some qualitative discrepancy between our calculations and our measurements the calculated diffusion rates predict 5 faster isotopic decay of thinner layers and 13 slower decay of thicker layers than our measurements show we speculate that grain size and grain fabric may influence the tortuosity of the firn pack and that the current generalization of tortuosity being a function of firn density as the only free parameter is an oversimplification

transformation of displacement between strike slip and crustal shortening in the northern margin of the tibetan plateau evidence from decadal gps measurements and late quaternary slip rates on faults abstract studies of late quaternary geological slip rates and the gps decadal slip rates along the major strike slip faults in the northern margin of the tibetan plateau show that slip rates remain relatively constant along the middle sections of each fault and decrease rapidly toward the ends our recent studies suggest that the slip rates are low

tomographic evidence for localized lithospheric shear along the altyn tagh fault seismic tomography across the altyn tagh fault at the north edge of the tibetan plateau reveals a low p wave velocity anomaly below the fault down to 140 kilometers this anomaly probably reflects strike slip shear in the lithosphere slip partitioning may also induce a wedge of crust from the tarim basin to plunge into the mantle

svalbard summer melting continentality and sea ice extent from the lomonosovfonna ice core we develop a continentality proxy 1600 1930 based on amplitudes of the annual signal in oxygen isotopes in an ice core we show via modeling that by using 5 and 15 year average amplitudes the effects of diffusion and varying layer thickness can be minimized such that amplitudes then reflect real seasonal changes in 18o under the influence of melt a model of chemical fractionation in ice based on differing elution rates for pairs of ions is developed as a proxy for summer melt 1130 1990 the best pairs are sodium with magnesium and potassium with chloride the continentality and melt proxies are validated against twentieth century instrumental records and longer historical climate proxies in addition to summer temperature the melt proxy also appears to reflect sea ice extent likely as a result of sodium chloride fractionation in the oceanic sea ice margin source area that is dependent on winter temperatures we show that the climate history they depict is consistent with what we see from isotopic paleothermometry continentality was greatest during the little ice age but decreased around 1870 20 30 years before the rise in temperatures indicated by the 18o profile the degree of summer melt was significantly larger during the period 1130 1300 than in the 1990s

glaciological investigations in the crete area central greenland a search for a new deep drilling site the results of the 1984 85 post gisp campaigns in central greenland are presented eight ice cores were obtained some spanning up to 360 years we present i geographical positions and elevations at the drill sites

reconstruction of the undiffused seasonal oxygen isotope signal in central greenland ice cores the climatologically important seasonal signal in ice core oxygen isotope records is altered by diffusion with time here we use the diffusion model of whillans and grootes 1985 to calculate the effects of isotopic diffusion in nine shallow ice cores recovered over a 150 150 km grid centered on summit greenland two different computational methods give similar results for the back diffused 18o seasonal extrema although back diffusing the ice core 18o records improves the correlations between the individual ice core oxygen isotope and greenland coastal temperature records the correlations remain poor particularly for the low accumulation sites stacking the back diffused isotopic records from several cores results in much higher correlations with a strong winter linkage between west coast stations and central greenland these correlations are also significantly greater than the correlations between the stacked measured diffused isotopic profiles and coastal temperatures our results indicate that the use of ice core isotopic records to identify anomalous seasons requires that diffusion effects be accounted for

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

variations of glacial lakes and glaciers in the boshula mountain range southeast tibet from the 1970s to 2009 catastrophic floods originating from glacial lake outbursts have recently become one of the primary natural hazards in the southeastern tibetan plateau here we report observations of glacial lake expansions and glacier recessions in the boshula mountain range southeast tibet derived from multitemporal remote sensing images and digital elevation models during the period from the 1970s to 2009 the area of glacial lakes has expanded from 9 24 0 1 km 2 in the 1970s to 10 96 0 1 km 2 in 2009 specifically the area of moraine dammed lakes has increased by 26 8 from the 1970s to 2009 the glacierized area in the boshula mountain range shrank by 12 7 21 2 km 2 increasing mean summer air temperature was the main cause for the glacier recession and lake expansion from the 1970s to 2001 while the combination of increased summer temperature and decreased summer precipitation led to accelerated glacier recession after 2001 climate warming and ongoing deglaciation play important roles in the expansion of moraine dammed lakes calling for intensified monitoring to properly address the hazard potential in the study area

tibet uplift and erosion abstract the 5 km high tibetan plateau is an outstanding topographic feature on the earth today its horziontal extent elevation and location cause significant effects on modern atmospheric circulation and climate so the history of uplift of the surface of tibet is linked to cenozoic climate changes at local regional and perhaps global scales geological and geophysical studies of the plateau are contributing data on the present and past deformation of the tibetan lithosphere that has formed the plateau primarily during the cenozoic the principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history different parts of tibet probably had different uplift histories but presently available data are not sufficient for distinguishing many contrasts in one scenario cretaceous and early cenozoic north south distributed shortening of tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early miocene thinning of the high density mantle portion of the lithosphere during the miocene may then have allowed the thick tibetan crust to rise close to its present elevation perhaps higher before 8 ma since then slow east west extension of tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late cenozoic erosion of tibet unlike narrow mountain belts has been unable to match the uplift of a broad plateau orographic precipitation and efficient river networks concentrate erosion on the edges while the interior is protected from significant erosion despite its lofty elevation the southern edge of the plateau the himalaya has suffered a minimum of 25 km of denudation since the miocene while central tibet shows little or no sign of major erosion since that time the gangdese arc in southern tibet was rapidly eroded during the mid miocene when 4 km of rock apparently were removed from the surface as shown by mineral cooling ages this pulse of erosion was probably caused by a combination of local thrust system movement and changes in base level and precipitation due to relative elevation changes between the gangdese and the himalaya to the south the modern long wavelength flatness of tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

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

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

temperature history and accumulation timing for the snowpack at gisp2 central greenland previous research has documented a close association between high resolution snow pit profiles of hydrogen and oxygen stable isotope ratios and multi year special sensor microwave imager ssm i 37 ghz brightness temperature data in central greenland comparison of the ssm i data to profiles obtained during the 1989 91 field seasons indicated that d and 18 o data from the near surface snow at the greenland summit are a reliable high resolution temperature proxy to test this new technique further additional stable isotope data were obtained from a 2 m snow pit constructed during late june 1995 near the gisp2 site this new profile supported by pit stratigraphy and chemistry data confirms the utility ofcomparing stable isotope records with ssm i brightness temperatures the sub annual variation of the d record at the gisp2 site was determined using 15 match points from approximately december 1991 through june 1995 and was guided in part by time constrained hoar layers the close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year this is also independently confirmed by analysis of h 2 o 2 data the slope of the multi year t vs 6 correlation was evaluated along with the sub annual variation in the amount rate and timing of accumulation these new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub annual resolution this encourages confident interpretation of the paleoclimatic signal variations in the gisp2 and grip deep cores

post depositional modification of stable water isotopes in winter snowpacks in the canadian rocky mountains to assess the seasonal stability of the do stratigraphy in winter snowpacks in the canadian rocky mountains snow pits were sampled over three accumulation seasons at two field sites these sites opabin and haig glaciers are 160 km apart at similar elevations and represent windward and lee slope environments respectively at both sites snow pits were sampled at one glacier and one forefield location throughout each accumulation season intra seasonal changes in do at each site were examined to determine the extent of post depositional modification of isotope stratigraphies at both glacier sites there was minimal temporal change before the onset of spring melt in all years in addition the similar structure of do profiles from both glacier sites suggests that regional controls govern the isotopic composition of solid phase precipitation across the study area at forefield locations the absence of an insulating layer of ice at the base of the snowpack allowed for vapour transport and post depositional modification of the seasonal do signal this did not result in consistent changes to the mean do deuterium excess and dd do regression line slopes in the lower layers of snow and the observed smoothing of do profiles was less than that simulated by applying a diffusion model to these snowpacks

paleoproterozoic crustal evolution of the tarim craton constrained by zircon u pb and hf isotopes of meta igneous rocks from korla and dunhuang abstract the tarim craton is one of three large cratons in china presently there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis especially an in situ lu hf isotope analysis of zircon in this study precambrian basement rocks from the kuluketage and dunhuang blocks in the northeastern portion of the tarim craton have been analyzed for combined in situ laser ablation icp mc ms zircon u pb and lu hf isotopic analyses as well as whole rock elements to constrain their protoliths forming ages and magma sources two magmatic events from the kuluketage block at 2 4 ga and 1 85 ga are revealed and three stages of magmatic events are detected in the dunhuang block i e 2 0 ga 1 85 ga and 1 75 ga the 1 85 ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings suggesting that the kuluketage and dunhuang blocks are part of the uniform precambrian basement of the tarim craton zircon hf model ages of the 2 4 ga magmatism indicate that the crust of the tarim craton may have been formed as early as the paleoarchean period the 2 0 ga mafic rock from the dunhuang block was formed in an active continental margin setting representing an important crustal growth event of the tarim craton in the mid paleoproterozoic that coincides with the global episode of crust formation during the assembly of the columbia supercontinent the 1 85 ga event in the kuluketage and dunhuang blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the columbia supercontinent while the 1 75 ga magmatism in the dunhuang block resulted from a mixture of the reworked archean crust with juvenile magmas and was most likely related to a post collisional episode

partitioning of localized and diffuse deformation in the tibetan plateau from joint inversions of geologic and geodetic observations abstract the spatial complexity of continental deformation in the greater tibetan plateau region can be defined as the extent to which relative motion of the indian and asian plates is partitioned between localized slip on major faults and distributed deformation potency rates provide a quantitative metric for determining the magnitudes of diffuse and on fault crustal deformation which are proportional to strain rates within crustal micro plates and fault slip rates respectively we simultaneously estimate micro plate rotation rates interseismic elastic strain accumulation fault slip rates on major structures and strain rates within 24 tectonic micro plates inferred from active fault maps in the greater tibetan plateau region using quasi static block models constrained by interseismic surface velocities at 731 gps sites and 9 holocene late quaternary geologic fault slip rates the joint geodetic geologic inversion indicates that geologic slip rates are kinematically consistent with differential micro plate motions estimated left lateral slip rates on the altyn tagh west central kunlun and xianshuihe faults are relatively homogeneous along strike 11 5 10 5 and 11 mm yr respectively while segmentation of the eastern kunlun fault by the intersecting elashan and riyueshan faults results in a decreased slip rate consistent with geologic observations the fraction of total potency rate associated with intrablock strain uncorrected for observational noise ranges from 0 27 in the himalayan range block to 0 91 in the aksai chin block monte carlo simulations are used to quantify the likelihood that internal deformation is statistically distinguishable from the uncertainties in geodetic velocities these simulations indicate that internal block deformation is statistically significant only within the himalayan range front where internal deformation accounts for id 0 11 of block potency rate budget west central plateau id 0 68 ganzi yushu xianshuihe 0 58 burma 0 09 jiali 0 38 and aksai chin 0 68 blocks in the other 18 tectonic micro plates within the plateau region estimated internal block potency is not currently distinguishable from the expected contribution of observational noise to residual velocities of the total potency budget within the tibetan plateau 85 is taken up by slip on major faults with the remaining 15 accommodated by internal processes at sub block scale distinguishable from observational noise the localization of the majority of plate boundary activity is also supported by the spatial distribution of modern and historical crustal earthquakes sixty seven percent of the total moment released by earthquakes in the cmt catalog and 90 of historical moment since 1900 have been released within 25 km of the major faults included in the block model representing only 12 of the characteristic half block length scale of 215 km the localization of deformation inferred from geologic geodetic and seismic observations suggests that forces applied to tectonic micro plates drive fault system activity at the india asia collision zone over decadal to quaternary time scales

seasonal precipitation timing and ice core records this is a commentary on global circulation model experiments of moisture source changes in greenland urging caution in how they are applied because they have important implications for paleoclimate reconstruction from ice cores the work comes from preliminary find is of a ice core gisp2 of the authors the authors conclude that at present anomalies in greenland ice core records should not be interpreted solely in terms of source region variations the combined use of oxygen 18 d and ionic species in the new summit greenland cores should make it possible to answer empirically some of the questions raised by the gcm experiments as to the interpretation of oxygen 18 records in terms of temperature 4 refs 1 fig

electrical resistivity structure at the northern margin of the tibetan plateau and tectonic implications 1 the ene wsw striking altyn tagh fault atf and the wnw ese trending western qilian mountains define the northern margin of the tibetan plateau new magnetotelluric data were collected along three profiles crossing the eastern section of the atf and the southern qilian mountains the basic sounding bandwidth ranged from 0 003 to 7000 s the transverse electric and transverse magnetic mode data sets were inverted into resistivity sections using a conventional two dimensional 2d inversion code and data at periods of 0 3333 5464 s were inverted using a commonly employed three dimensional 3d code the 3d results constrain the interpretation of the 2d models and the final interpretative resistivity models show that lithospheric structures are spatially variable along the atf and across southern qilian in profiles across the atf the main fault is imaged as a vertical resistivity boundary and the high resistivity body in the western profile extends about 15 km deeper than the corresponding body in the eastern profile positive flower structures are apparent in the western profile but are only weakly visible in the eastern profile these observations suggest that the depth of the atf is spatially variable mantle resistivity images indicate a relatively cold and rigid tarim lithosphere which is consistent with a geodynamic model of oblique subduction of the tarim basin below the atf a high resistivity body in the crust of southernmost qilian indicates a huge thick skinned structure low resistivity bodies at the crust mantle boundary south of the main atf are presumed to form a weak layer beneath northern tibet

effect of periodic melting on geochemical and isotopic signals in an ice core from lomonosovfonna svalbard 1 we examine the quality of atmospherically deposited ion and isotope signals in an ice core taken from a periodically melting ice field lomonosovfonna in central spitsbergen svalbard the aim is to determine the degree to which the signals are altered by periodic melting of the ice we use three diagnostics 1 the relation between peak values in the ice chemical and isotopic record and ice facies type 2 the number of apparent annual cycles in these records compared with independently determined number of years represented in the ice core and 3 a statistical comparison of the isotopic record in the ice core and the isotope records from coastal stations from the same region we find that during warm summers as much as 50 of the annual accumulation may melt and percolate into the firn in a median year this decreases to similar to25 as a consequence of percolation the most mobile acids show up to 50 higher concentrations in bubble poor ice facies compared with facies that are less affected by melt most of the other chemical species are less affected than the strong acids and the stable water isotopes show little evidence of mobility annual or biannual cycles are detected in most parameters and the water isotope record has a comparable statistical distribution to isotopic records from coastal stations we conclude that ice cores from sites like lomonosovfonna contain a useful environmental record despite melt events and percolation and that most parameters preserve an annual or in the worst cases a biannual atmospheric signal

structure of the altyn tagh fault and daxue shan from magnetotelluric surveys implications for faulting associated with the rise of the tibetan plateau magnetotelluric measurements in the foreland of the daxue shan have imaged low angle 20 thrust faults that extend to a depth of 3 km the surface projections of the faults at depth coincide with the mapped traces of the chang ma m 7 6 1932 and north hills thrusts minimum cumulative offsets of 7 km on the chang ma thrust and 14 km on the north hills thrust are estimated from the horizontal extent of underthrust sediment assuming that regional thrusting began at 5 6 ma this corresponds to minimum convergence rates of 1 3 mm yr and 2 5 mm yr on the chang ma and north hills thrusts respectively these slip rates correspond to 3 mm yr of sinistral slip on the altyn tagh fault between 96 e and 97 e this is consistent with the 4 2 mm yr of slip estimated by geological studies assuming comparable slip rates and similar fault geometry in the neighboring ranges this requires a minimum of 11 mm yr of shortening parallel to the altyn tagh fault between the dang he nan shan and qilian shan both the style of thrusting and rate of shortening are in agreement with geologic studies that favor a relatively high rate of slip on the altyn tagh fault this in turn implies that lithospheric extrusion contributes significantly in accommodating the ongoing convergence between india and asia farther west the altyn tagh fault is imaged on four magnetotelluric profiles as a vertical resistivity contrast extending to a depth of at least 8 km two strands of the north altyn tagh fault are imaged east of the asperity near subei 39 30 n 95 e

isotopic diffusion in polar firn implications for interpretation of seasonal climate parameters in ice core records with emphasis on central greenland if it were possible to properly extract seasonal information from ice core isotopic records paleoclimate researchers could retrieve a wealth of new information concerning the nature of climate changes and the meaning of trends observed in ice core proxy records it is widely recognized however that the diffusional smoothing of the seasonal record makes a proper extraction very difficult in this paper we examine the extent to which seasonal information specifically the amplitude and shape of the seasonal cycle is irrecoverably destroyed by diffusion in the firn first we show that isotopic diffusion firn is reasonably well understood we do this by showing that a slightly modified version of the whillans and grootes 1985 theory makes a tenable a priori prediction of the decay of seasonal isotopic amplitudes with depth at the gisp2 site though a small adjustment to one parameter significantly improves the prediction further we calculate the amplitude decay at various other ice core sites and show that these predictions compare favorably with published data from south pole and locations in southern and central greenland and the antarctic peninsula we then present numerical experiments wherein synthetic ice core records are created diffused sampled reconstituted and compared to the original these show that alter diffusive mixing in the entire fini column seasonal amplitudes can be reconstructed to within about 20 error in central greenland but that all information about sub annual signals is permanently lost there furthermore most of the error in the amplitude reconstructions is due to the unknowable variations in the sub annual signal finally we explore how these results can be applied to other locations and suggest that dye 3 has a high potential for meaningful seasonal reconstructions while siple dome has no potential at all an optimal ice core site for seasonal reconstructions has a high accumulation rate and a low temperature

late quaternary and present day rates of slip along the altyn tagh fault northern margin of the tibetan plateau 1 both global positioning system gps measurements and studies of late quaternary faulting are consistent with a slip rate of 10 mm yr along the central segment of the altyn tagh fault and a systematic decrease in that rate toward the eastern end of the fault dates of terraces above and below laterally offset terrace risers yield bounds on quaternary slip rates that range from those that agree with gps measurements to values as much as three times faster we argue that offset terrace risers that are protected by topography upstream of them are more closely dated by the age of the upper terrace than by that of the lower terrace in some cases valleys upstream of the fault have been incised into bedrock and few if any terrace risers can be seen within the valleys such streams debouch onto alluviated floodplains or fans that become incised presumably during climate changes to create terrace risers the terrace risers are then displaced so that they lie downslope from bedrock ridges on the upstream side of the fault and thus the risers become protected from further incision in such cases dates of upper terraces should more closely approximate the ages of the risers than those of lower terraces such dates yield slip rates of 10 mm yr in the central segment of the fault and decreasing rates eastward although we cannot with certainty rule out the higher slip rates along the altyn tagh fault our analysis does show that viable interpretations consistent with gps measurements are more likely at least along some segments of the fault not only do these rates support the view that the tibetan plateau deforms internally by slip on a distributed network of faults in the shallow brittle crust and hence behaves as a continuum at depth but the gradual decrease toward the east also shows that the altyn tagh fault does not separate two effectively rigid lithospheric plates correspondingly the relatively low slip rate and the eastward decrease in slip rate suggest that the altyn tagh fault does not transfer a significant portion of the convergence between india and asia into northeastward extrusion of the tibetan plateau thus large scale extrusion of crustal material in india s path into eurasia seems to be limited largely to the confines of the tibetan plateau

atmospheric circulation variability associated with shallow core seasonal isotopic extremes near summit greenland seasonal back diffused 18o time series reconstructed for nine shallow firn cores from the greenland summit for 1959 1987 are evaluated in terms of 1 their spatial and temporal variability and 2 their association to the atmospheric circulation rotated principal component analysis evaluates spatial and temporal patterns of organization in the back diffused 18o ice core time series mean sea level pressure and 500 hpa height fields are obtained for winter and summer seasons that fall in the isotopically warmest and coldest thirds of the seasonal data the winter back diffused 18o data indicate that some degree of longitudinal separation takes place between isotopic records from western and eastern greenland in agreement with coastal station climatic data temporal variability is linked with statistical significance to the variability of the atmospheric circulation over the denmark strait as well as over the norwegian and barents seas the north atlantic oscillation nao and the mean icelandic low in the denmark strait affect isotopic variability over the cores west of the ice divide just as they have been shown to prominently affect west coast air temperature records isotopic variability at some ice cores east of the divide is influenced by pressure variability over the norwegian and barents seas although statistical significance is lower the nao appears insignificant in summer isotopic variability as the circulation in that season is very much localized and dominated by either ridging warm cases or troughs cold cases over eastern greenland and the arctic ocean results suggest that long term winter 18o variability can potentially be linked to climate variation over europe especially from the mediterranean basin northward to about 55 n and as far east as central siberia

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