palaeo moisture evolution in monsoonal central asia during the last 50 000 years abstract the late quaternary climate history of monsoonal central asia was inferred from 75 palaeoclimatic records which provide information about moisture conditions in the last 50 ka or part of this period wet conditions occurred during middle and late marine isotope stage 3 while the last glacial maximum lgm was characterized by dry climate conditions in the region a stepwise climate amelioration is suggested by the climate records following the lgm several climate signals of this period which were reported from high latitude ice core records are preserved in archives from monsoonal central asia as well during the early holocene high effective moisture was inferred from most records from the area dominated by the indian monsoon e g the tibetan plateau suggesting that holocene optimal climate conditions occurred there during this period in contrast areas which are dominated by the south east asian monsoon se monsoon and the westerlies in north western and north central china mongolia do not uniformly show an early holocene climate optimum for this area optimal conditions prevailed during the mid holocene these apparent contradictions can possibly be explained by the regional uplift and descent of air masses in the holocene during the early holocene strengthened insolation possibly caused an enhanced low level convergence over the tibetan plateau which led to the intensification of the summer monsoon the strong air uplift caused intensified precipitation and air divergence in the upper troposphere over the tibetan plateau the areas adjacent to the north therefore experienced an intensified descent of air masses and consequently increased aridity the majority of the palaeoclimatic records suggest reduced effective moisture since the late holocene in the region

quaternary glaciation of the himalayan tibetan orogen glacial geological evidence from throughout the himalayan tibetan orogen is examined to determine the timing and extent of late quaternary glaciation in this region and its relation to similar changes on a global scale the evidence summarised here supports the existence of expanded ice caps and extensive valley glacier systems throughout the region during the late quaternary however it cannot yet be determined whether the timing of the extent of maximum glaciation was synchronous throughout the entire region or whether the response was more varied the lack of organic material needed for radiocarbon dating has hindered past progress in glacial reconstruction however application of optically stimulated luminescence and terrestrial cosmogenic radionuclide methods has recently expanded the number of chronologies throughout the region limits to the precision and accuracy available with these methods and more importantly geological uncertainty imposed by processes of moraine formation and alteration both conspire to limit the time resolution on which correlations can be made to milankovitch timescales several ka in order to put all studies on a common scale well dated sites have been re evaluated and all the published terrestrial cosmogenic nuclide ages for moraine boulders and glacially eroded surfaces in the himalayan tibetan orogen have been recalculated locally detailed studies indicate that there are considerable variations in the extent of glaciation from one region to the next during a glaciation glaciers throughout monsoon influenced tibet the himalaya and the transhimalaya are likely synchronous both with climate change resulting from oscillations in the south asian monsoon and with northern hemisphere cooling cycles in contrast glaciers in pamir in the far western regions of the himalayan tibet orogen advanced asynchronously relative to the other regions that are monsoon influenced regions and appear to be mainly in phase with the northern hemisphere cooling cycles broad patterns of local and regional variability based on equilibrium line altitudes have yet to be fully assessed but have the potential to help define changes in climatic gradients over time copyright 2008 john wiley sons ltd

tree ring evidence of little ice age glacier advances in southern tibet the history of late holocene glacier fluctuations in eastern tibet was studied by determining the ages of trees growing on glacier deposits maximum tree ages yield minimum ages of ad 1760 and 1780 for moraine formation at the maximum extent of the little ice age glacier advances in two glacier forefields subsequent moraines could be dated to the beginning of the nineteenth and the beginning of the twentieth century larch trees from a third glacier forefield in southeastern tibet show evidence of glacier activity from 1580 to 1590 from the end of the eighteenth to the beginning of the nineteenth century and from 1860 to 1880 one glacier at mt gyalaperi recently advanced in both 1951 and 1987 periods of glacier advances can partly be correlated with periods of growth reductions in chronologies of total ring width and maximum latewood density derived from trees growing on slopes above the glacier valleys correlation functions with meteorological data suggest that maximum latewood density of subalpin

latest pleistocene and holocene glacier fluctuations in the himalaya and tibet abstract the timing and extent of latest pleistocene and holocene alpine glacier fluctuations in the himalaya and tibet are poorly defined due to the logistical and political inaccessibility of the region and the general lack of modern studies of the glacial successions nevertheless renewed interest in the region and the aid of newly developing numerical dating techniques have provided new insights into the nature of latest pleistocene and holocene glacier oscillations these studies provide abundant evidence for significant glacial advances throughout the last glacial cycle in most high himalayan and tibetan regions glaciers reached their maximum extent early in the last glacial cycle however true last glacial maximum glacier advances were significantly less extensive notable glacier advances occurred during the lateglacial and the early holocene with minor advances in some regions during the mid holocene there is abundant evidence for multiple glacial advances throughout the latter part of the holocene although these are generally very poorly defined and were less extensive than the early holocene glacier advances the poor chronological control on latest pleistocene and holocene glacial successions makes it difficult to construct correlations across the region and with other glaciated regions in the world which in turn makes it hard to assess the relative importance of the different climatic mechanisms that force glaciation in this region the lateglacial and holocene glacial record however is particularly well preserved in several regions notably in muztag ata and kongur and the khumbu himal these successions have the potential to be examined in detail using newly developing numerical dating methods to derive a high resolution record of glaciation to help in paleoclimatic reconstruction and understanding the dynamics of climate and glaciation in the himalaya and tibet

reconstruction of the late quaternary glaciation of the macha khola valley gorkha himal nepal using relative and absolute 14c 10be dendrochronology dating techniques abstract late quaternary glacier fluctuations in the macha khola valley gorkha himal nepal were reconstructed using relative and absolute dating techniques our results indicate that younger moraine complexes were left by late holocene

subsidence in the geothermal fields of the taupo volcanic zone new zealand from 1996 to 2005 measured by insar abstract a number of the geothermal systems in the taupo volcanic zone tvz new zealand have been utilised on a large scale to provide heat and to generate electricity in some cases causing areas of localised subsidence subsidence monitoring using field based surveys is practically constrained by limits to available resources so we have investigated the use of satellite differential radar interferometry insar for this purpose using ers and envisat radar data spanning 1996 to 2005 we have mapped the deformation at five of the heavily utilised geothermal fields of the tvz subsidence signals were identified at the ohaaki geothermal field from 1999 2004 and at wairakei tauhara from 1996 2005 where our measurements compare well with coeval levelling data across the wider deformation field subsidence was also measured at rotorua from 1996 2000 in favourable conditions the insar measurements provide a relatively dense spatial coverage of the deformation field that extends well beyond the boundaries of the geothermal systems and beyond the scope of the networks of levelling benchmarks in the case of the wairakei tauhara geothermal field using insar it is now possible to improve the spatial resolution near the field margins and to interpret the subsidence signals in the context of the wider more regional deformation our data also provide new insights into possible fault motion at the mokai geothermal field occurring around the time of the commissioning of its first power station we note however that the insar technique is not without limitations high gradient subsidence features are poorly represented although this can be resolved to some extent via a trade off in data processing temporal decorrelation a well known problem for this technique is also an issue for tvz geothermal fields therefore we find that it is possible to provide fortuitous snapshots of the deformation at the tvz geothermal fields but operational monitoring using insar would be difficult as the proportion of suitable interferograms is low

wide band magnetotelluric measurements across the taupo volcanic zone new zealand preliminary results the taupo volcanic zone tvz of new zealand is characterised by intensive geothermal activity and frequent rhyolitic volcanism sixteen wide band 0 01 1 800 s magnetotelluric soundings were measured along a 110 km long profile approximately perpendicular to the strike of the tvz a model obtained from 2d inversion of the soundings shows two near surface regions of high conductance which correspond to low density volcaniclastic sediments up to 3 km thick which infill a sequence of collapse calderas at deeper levels approximately 5 10 km a resistive layer underlies the entire tvz modelling shows other conductive zones occur beneath the tvz with the shallowest lying below the central part at a depth of 10 15 km given the high heat flux and volcanic history of the tvz the high conductivity at depth may indicate the presence of connected melt at greater depth 20 30 km the upper mantle beneath the tvz appears to be anomalously conductive consistent with observed high seismic attenuation

electrical resistivity image of the upper crust within the taupo volcanic zone new zealand the taupo volcanic zone tvz forms a region of back arc extension within the continental crust of new zealand and is notable for its caldera volcanism high rate of magmatic production and high heat flow on the eastern side of the tvz volcanism terminates abruptly along a linear margin we present the interpretation of three multiple source bipole dipole resistivity surveys across this margin the major features of the measured data can be reproduced by using two dimensional models of the resistivity in the upper 10 km of the crust along the margin the high resistivity graywacke basement 300 m increasing to 1300 m is downfaulted to the west across a zone about 6 km wide with a total vertical offset of approximately 2 km the easternmost fault is overlain by an unbroken 0 24 ma ignimbrite sheet suggesting the fault has been inactive for this period further to the west at depths greater than 2 km a conductive zone about 15 km wide and of total conductance 180 280 s has been identified this conductive zone can be modeled by using a range of resistivities 40 3 m and thicknesses 8 2 km models incorporating resistivities less than 15 m require the low resistivity zone to be wedge shaped so that conductance increases to the west the low resistivity is believed to be caused by the presence of clay alteration within volcanoclastic rocks the large volume of volcanic rocks at depth can be linked to a series of caldera forming eruptions that have taken place along the eastern side of the tvz

a 13 000 year climate record from western tibet although the tibetan plateau is important in influencing the atmospheric circulation of the northern hemisphere1 3 there are only a few continuous palaeoclimate records available and these are limited to the plateau s northeastern margin4 6 here we present a 13 000 yr record from sumxi co western tibet constructed from both lake core and shoreline studies which shows that conditions in the early middle holocene were warmer and wetter than at present these results confirm model predictions of an intensified monsoon over the region at 9 000 yr bp owing to an orbitally induced increase in summer insolation7 8 we also find evidence for warm humid pulses at 12 500 and 10 000 yr bp in phase with the steps of the last deglaciation and for a return to cold dry conditions at 11 10 000 yr bp none of which can be explained by orbital variations the existence of the cold episode confirms that the cooling associated with the younger dry as event occurred in continental china6 9 and provides further evidence of the global nature of this event10

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

extent and spatial distribution of pleistocene glaciations in eastern tibet as a contribution to the controversial discussion regarding the nature and extent of the pleistocene glaciation on the tibetan plateau additional observations and results were obtained during several chinese german joint expeditions in the investigation area of the eastern and central tibetan plateau it will be shown that the maximum extent of the last glaciation was limited to the glaciation of isolated mountain groups or smaller plateau glaciations these glaciations are characterized by glacial erosional features such as u shaped valleys in the inner mountain massifs and cirques in the higher summit areas accumulation forms such as terminal and lateral moraines less common than basal moraines are marked by granitic and basaltic erratics using two examples from the eastern margin of the tibetan plateau nianbaoyeze shan and northern chola shan the extent of pleistocene glaciations can be mapped by the distribution of terminal and lateral moraines and erratic granite boulders on the slopes of the surrounding schists two to three main pleistocene glaciations can be separated using weathering criteria paleosols and the overlying stratum on the moraines and associated terrace sequences overlying stratum consist of aeolian sandy silt or solifluction debris the younger moraines are found to be characterized by large erratics were therefore named big boulder moraine and can be found over the investigated area the ice free areas are characterized by pleistocene terraces along the main rivers loess deposits with sand and ice wedges at higher elevations e g the basin of zoige and paleosols in the basins below 3500 m a s l e g aba basin or garze basin the hypothesis of an extensive plateau glaciation could not be verified for this region a rising snowline equilibrium line altitudes elas is revealed in two cross sections during the last glacial maximum lgm at the eastern and northern margin of the tibetan plateau snowlines migrated from about 4000 m in the east minshan to 4300 m in the nianbaoyeze and to about 4700 m in the west bayan har shan a rise was also recorded from about 3800 m in the north central qilian shan up to a range of 4300 4950 m different exposures in the kunlun shan to approximately 5200 m in the tanggula shan in the center of the tibetan plateau the general increase of modern and lgm snowlines or elas towards the plateau corresponds with a decrease of precipitation from east to west and from north to south respectively precipitation was more pronounced at the eastern margin during glacial times than compared to present

pleistocene glaciations of central asia results from 10be surface exposure ages of erratic boulders from the pamir tajikistan and the alay turkestan range kyrgyzstan abstract we have determined the timing of glaciations in the pamir tajikistan and the alay turkestan range kyrgyzstan using 10be surface exposure dating glacial advances in the area have occurred 93 136 60 80 40 55 27 25 22 20 19 17 16 15 15 13 and 11 9 cal ka bp all late pleistocene glaciers in the pamir and the alay turkestan ranges have been valley glaciers except for the most extended glaciers on the pamir plateau which have formed local piedmont glaciations in the eastern pamir these are characterized by ela depressions of 370 380 m thar 0 5 in the turkestan range and alay range ela depressions at the same time were 750 and 600 m respectively late pleistocene glacier advances all over western high asia were contemporaneous with climatic cold phases rather than monsoonal maxima their maximum extent and that of the western hemisphere ice sheets were asynchronous due to increasing aridity in the region over the course of the last glacial late pleistocene climate in central asia seems to have been influenced by the interplay of the westerly circulation and the siberian anticyclone some indirect monsoonal influence in the eastern pamir may be responsible for the existence of some of the lateglacial moraine stages in this area high altitude glaciers seem to have reached their maximum extent earlier mis 5 4 than low altitude glaciers first half of mis 3 possibly due to prolonged glacial aridity imparting with moisture advection onto high altitude sites inducing glacial retreat but prolonged cold during the same time imparting with glacier ablation at lower altitude sites inducing glacial advance

onset of neoglaciation in the southern hemisphere a review of evidence documenting an early neoglacial mid holocene advance of mountain glaciers in south america and new zealand reveals that of 16 glaciers for which radiocarbon age control exists only two have associated dates that may approximate the culmination of an advance advances of the remaining glaciers are controlled only by minimum or maximum limiting ages additional dates for several new zealand glaciers are based on weathering rind measurements having large potential errors twelve of the glaciers now terminate or have terminated in lakes or in tidewater and are subject to instabilities associated with calving termini an extensive debris cover in the ablation zone of nearly half of the glaciers makes their moraine succession potentially suspect as a record of detailed climatic variation the two glaciers with seemingly reliable age control are located in valleys where large rock avalanches could affect glacier mass balance and lead to random advances or where rock avalanche deposits might easily be mistaken for moraines and tills the early neoglacial interval has been defined primarily based on this set of data the ages taken at face value suggest that the early neoglacial advance culminated between ca 5400 and 4900 cal yr ago ca 4600 4400 14c yr bp this chronology however should remain provisional until improved age control is obtained for a large population of glaciers based on closely bracketing radiocarbon dates and or an adequate number of cosmogenic isotope ages data from calving glaciers and those subject to rock avalanche activity should be avoided or assessed critically copyright 2000 john wiley sons ltd

tsunami excitation by submarine slides in shallow water approximation landslide induced tsunamis are receiving increased attention since there is evidence that recent large devastating events have been caused by underwater mass failures normally numerical models are used to simulate tsunami excitation most of which are based on shallow water known also as long wave approximation to the full equations of hydrodynamics analytical studies may handle only simplified problems but help understand the basic features of physical processes this paper is an analytical investigation of long water waves excited by rigid bodies sliding on the sea bottom based on the shallow water approximation which is here derived by properly scaling euler equations for an inviscid incompressible and irrotational ocean in one dimensional 1 d cases where motion depends only on one horizontal coordinate under the further assumptions of small height slide which permits the recourse to linear theory and of flat ocean floor a solution for arbitrary body shape and velocity is deduced by applying the duhamel theorem it is also shown that this theorem can be advantageously used to obtain a general solution in case of a non flat ocean floor when the sea bottom follows a special power law that can be adapted to study reasonable bottom profiles the characteristics of the excited tsunamis are then evaluated by computing solutions in numerous examples with special focus on wave pattern and wave evolution the energy of the wave system is shown to depend on time it grows expectedly in the initial phase of tsunami generation when the moving body transfers energy to the water but it may also diminish later implying that a certain amount of energy may pass back from water waves to the slide

controversy regarding the existence of a large ice sheet on the qinghai xizang tibetan plateau during the quaternary period

u pb dating of zircon in subsurface hydrothermally altered pyroclastic deposits and implications for subsidence in a magmatically active rift taupo volcanic zone new zealand abstract recognising and correlating hydrothermally altered rock units within buried volcanic sequences in the taupo volcanic zone tvz in new zealand is difficult this is because of broad similarities in the lithologies of many major ignimbrite units and the destruction by hydrothermal alteration of distinctive chemical and mineralogical characteristics however magmatic zircons are commonly present are highly resistant to hydrothermal alteration and yield crystallisation ages in intensely altered rocks crystallisation age spectra have been obtained by sims techniques shrimp rg on zircons extracted from cores from altered ignimbrites penetrated by drillholes at the waiotapu te kopia and orakei korako geothermal fields in the central tvz at waiotapu the thick up to 350 m densely welded waiotapu ignimbrite returned a zircon age spectrum with a probability density function pdf peak of 0 79 ma consistent with an eruption age from 40ar 39ar techniques of 0 71 0 06 1 s d ma three older ignimbrite sheets yielded age spectra that were consistent stratigraphically the shallowest of the three yielded sparse zircons that gave a pdf peak of 1 24 ma and it may correlate with the 1 18 0 02 ma ahuroa ignimbrite the middle sheet although 220 m thick yielded an age spectrum identical to that obtained from pumice in the widespread 1 21 0 04 ma ongatiti ignimbrite extending earlier estimates of the likely volume of this large deposit the deepest sheet has a spectrum consistent with an eruption age of 1 45 0 05 ma it has no surficial correlative but its likely coeruptive ash forms part of a concentrated group of primary or secondary tephra in sediments on the ocean floor east of new zealand and in sedimentary basins across the north island these three ignimbrites were previously correlated with either major ignimbrites exposed on the paeroa fault scarp 10 km to the west or the akatarewa ignimbrite that occurs in drillholes at te kopia and orakei korako but these correlations are disproved from our age data drillholes at te kopia tk and orakei korako ok reach but do not penetrate the base of one or more deposits collectively termed the akatarewa ignimbrite samples from 62 tk and 165 m ok below the top contact of this unit yielded closely similar age spectra and pdf maxima of 1 00 tk and 1 02 ma ok implying that the same deposit was penetrated at both fields this ignimbrite is not correlated with a specific surface equivalent several candidate deposits exist but all are areally restricted the akatarewa ignimbrite at tk and ok is however not the same deposit as those earlier correlated with it at waiotapu the age data are consistent with there being two zones of subsidence on both sides of a horst aligned with the three geothermal fields sampled the eastern zone is the reporoa basin which has high heat flow but little surface faulting and the western one is the axis of the taupo rift currently with low heat flow but abundant surface faulting overall subsidence rates at the three geothermal fields studied vary between 2 and

reduced early holocene moisture availability in the bayan har mountains northeastern tibetan plateau inferred from a multi proxy lake record abstract a late glacial and holocene lake sediment record from the northeastern tibetan plateau was investigated using ostracod and geochemical analysis after lake koucha was established about 16 cal ka bp oligohaline conditions gradually converted to mesohaline conditions about 10 cal ka bp clearly suggesting that the water body was a closed basin lake from about 10 cal ka bp the salinity of the lake rose continuously until hyperhaline levels were reached about 7 5 cal ka bp this increase in salinity was accompanied by a falling lake level extreme rainfall and flooding events in a generally dry climate hit the shallow saline lake around 8 5 cal ka bp causing a unique thin gravel containing layer within the continuous fine grained lake sediments the age control of our record does not allow us to correlate this event exactly to the widely recorded 8 2 cal ka bp event however the recovered sediments from lake koucha provide strong evidence that climate changes around 8 5 cal ka bp had significant environmental implications in northeastern tibet after about 7 3 cal ka bp the available moisture significantly increased causing a continuous salinity decline until the present day freshwater conditions of a through flow lake were established about 6 5 cal ka bp in lake koucha this major shift from a shallow saline to a deeper freshwater lake was probably accompanied by a fundamental change from a macrophyte dominated to a phytoplankton dominated lake a slight reduction in available moisture in comparison to the periods before and afterward was recorded between 4 3 and 2 0 cal ka bp in phase with numerous records from monsoonal asia the reconstructed overall change in moisture availability from dry conditions before about 7 3 cal ka bp to wetter conditions afterwards contradicts most palaeoclimate records from the tibetan plateau and its surroundings which show that wetter conditions in the early holocene were replaced by drier and probably cooler conditions in the mid and late holocene however in the semi arid region of lake koucha with a mean annual precipitation of about 500 mm the insolation driven strengthened monsoon in the early holocene may have resulted in a temperature rise and higher evaporation rates which outweighed the increase in precipitation and therefore led to significantly lower moisture availability our record provides further evidence for a regionally non uniform response of the available moisture evolution to the overall monsoonal signal in the holocene climate history of central asia

terrestrial and gps measurements of deformation across the taupo back arc and hikurangi forearc regions in new zealand geodetic measurements of deformation across the taupo back arc and hikurangi forearc regions of new zealand are derived from global positioning system gps measurements made in 1990 and 1991 and triangulation observations made in the 1920s 1950s and 1970s the gps horizontal coordinate differences have precisions of 5 6 mm and the triangulation observations have precisions of 0 7 1 2 arc sec these different kinds of observation allow simultaneous estimation of strain parameters and single epoch coordinates for a total of 184 stations at 68 of which gps observations were made under the assumption that no shift of the scale or orientation of the gps reference frame is embodied in the data from 13 gps stations within the taupo volcanic zone tvz common to the 1990 and 1991 surveys the rotational dilatational and shear components of the deformation rate tensor within the tvz can be estimated for that 1 year interval the principal extension rate from 1990 to 1991 is 0 21 0 09 10 6 yr 68 confidence at an azimuth of 124 13 corresponding to 8 4 mm yr extension over 40 km neither the orthogonal principal extension rate 0 02 0 03 10 6 yr nor the rotation rate 0 06 0 04 rad yr is significant the dilatation rate of 0 23 0 09 10 6 yr is therefore produced by uniaxial extension the data from 75 stations distributed from behind the back arc region and well into the forearc region which were surveyed in at least two of the triangulation or gps epochs between the 1920s and 1991 allow the spatial variation of the shear components of the deformation rate tensor to be estimated the maximum engineering shear rates lie in the range 0 1 0 2 10 6 yr within the tvz and maintain these values with similar orientations in the forearc region to the east the western and southern margins of active extension are reasonably well determined by changes in orientation and magnitude of the shear component these results provide confirmation of the previously less well determined deformation field and in comparison with subduction models indicate that the surface deformation is reflecting variation from north to south of coupling of the subduction plate interface and that both trench suction and gravitational collapse probably contribute to the extension in the forearc region

3 d modeling of the deep electrical conductivity of merapi volcano central java integrating magnetotellurics induction vectors and the effects of steep topography we derive a 3 d model of the electrical conductivity structure of merapi volcano in central java from magnetotelluric mt sounding curves and geomagnetic induction vectors topography has been considered explicitly in the model the overall magnitude of the conductivities is constrained by mt measurements while lateral variations are mainly derived from geomagnetic induction vectors the resulting model is consistent with both the mt transfer functions apparent resistivities and phases and the magnetic transfer functions induction vectors the final model consists of two 3 d structures i e a central conductor and a second conductor lying to the southwest of the summit the high conductivity material is probably hot saline water as suggested by position and lateral extent of the high conductivity material

the hydrothermal vent system of mount ruapehu new zealand a high frequency mt survey of the summit plateau high frequency magnetotelluric mt measurements made on the summit plateau of mount ruapehu some 1 km to the north of the presently active vent beneath crater lake have been used to derive the electrical resistivity structure associated with the volcanic hydrothermal vent system the entire summit plateau area is underlain at shallow depth by low resistivity which is inferred to be the result of hydrothermal alteration caused by rising volcanic gases mixing with local groundwater two areas of localised higher resistivity one between 200 and 500 m depth beneath the central part of the plateau and one at a depth of 1000 m below the northern part of the plateau are interpreted as being the result of hydrothermal alteration at higher temperature forming chlorite dominated alteration products these regions are believed to represent the locations of further heat pipes within the volcanic system both correlate with the locations of eruption centres on ruapehu active within the last 10 ka

radiatively driven convection in ice covered lakes observations scaling and a mixed layer model penetrative convection is discussed where the instability is driven by radiative heating of water below the temperature of maximum density convection of this type occurs in ice covered freshwater lakes in late spring when the snow cover vanishes and solar radiation is absorbed beneath the ice cover the vertical temperature structure bulk mixed layer scaling and mixed layer deepening are examined for a number of temperate and polar lakes a bulk mixed layer scaling for this type of convection is based on energy arguments underlying the classical deardorff convective scaling the depth of the convective layer serves as an appropriate length scale however a modified scale that takes account of the energetics of a distributed radiation source term replaces the surface buoyancy flux velocity scale used by deardorff the scaling compares favorably with large eddy simulations of turbulence kinetic energy tke and with both observations and large eddy simulations of the tke dissipation rate mixed layer deepening is simulated with a model of convection beneath lake ice the model describes the structure of the stably stratified layer just beneath the ice with a stationary solution to the heat transfer equation the structure of the entrainment layer is parameterized with a zero order jump approach the entrainment equation is derived from the mixed layer tke budget and bulk mixed layer scaling entrainment regimes characteristic of convection beneath ice are analyzed it is shown that if the deardorff convective velocity scale is replaced with a scale incorporating the distributed buoyancy flux the entrainment equation describing atmospheric and oceanic convective boundary layers also applies beneath the ice model predictions compare well with data from observations in a number of lakes we propose and compare with observations an extension of the mixed layer model that allows for the inclusion of salinity although the salt concentration is low in most temperate and polar lakes its dynamical effect can be significant close to the temperature of maximum density less

climatic and topographic controls on the style and timing of late quaternary glaciation throughout tibet and the himalaya defined by 10be cosmogenic radionuclide surface exposure dating abstract temporal and spatial changes in glacier cover throughout the late quaternary in tibet and the bordering mountains are poorly defined because of the inaccessibility and vastness of the region and the lack of numerical dating to help reconstruct the timing and extent of glaciation throughout tibet and the bordering mountains we use geomorphic mapping and 10 be cosmogenic radionuclide crn surface dating in study areas in southeastern gonga shan southern karola pass and central western nyainqentanggulha shan and tanggula shan tibet and we compare these with recently determined numerical chronologies in other parts of the plateau and its borderlands each of the study regions receives its precipitation mainly during the south asian summer monsoon when it falls as snow at high altitudes gonga shan receives the most precipitation 2000 mm a 1 while near the margins of monsoon influence the karola pass receives moderate amounts of precipitation 500 600 mm a 1 and in the interior of the plateau little precipitation falls on the western nyainqentanggulha shan 300 mm a 1 and the tanggula shan 400 700 mm a 1 the higher precipitation values for the tanggula shan are due to strong orographic effects in each region at least three sets of moraines and associated landforms are preserved providing evidence for multiple glaciations the 10 be crn surface exposure dating shows that the formation of moraines in gonga shan occurred during the early mid holocene neoglacial and little ice age on the karola pass during the lateglacial early holocene and neoglacial in the nyainqentanggulha shan date during the early part of the last glacial cycle global last glacial maximum and lateglacial and on the tanggula shan during the penultimate glacial cycle and the early part of the last glacial cycle the oldest moraine succession in each of these regions varies from the early holocene gonga shan lateglacial karola pass early last glacial western nyainqentanggulha shan and penultimate glacial cycle tanggula shan we believe that the regional patterns and timing of glaciation reflect temporal and spatial variability in the south asian monsoon and in particular in regional precipitation gradients in zones of greater aridity the extent of glaciation has become increasingly restricted throughout the late quaternary leading to the preservation of old 100 ka glacial landforms in contrast in regions that are very strongly influenced by the monsoon 1600 mm a 1 the preservation potential of pre lateglacial moraine successions is generally extremely poor this is possibly because lateglacial and holocene glacial advances may have been more extensive than early glaciations and hence may have destroyed any landform or sedimentary evidence of earlier glaciations furthermore the intense denudation mainly by fluvial and mass movement processes which characterize these wetter environments results in rapid erosion and re sedimentation of glacial and associated landforms which also contributes to their poor preservation potential

a note on the extent of glaciation throughout the himalaya during the global last glacial maximum abstract quantitative chronologies for the impressive glacial successions that occur throughout the himalaya have until recently been almost totally lacking within the last decade two new techniques have promised to remedy this situation these techniques optically stimulated luminescence and cosmogenic nuclide surface exposure dating enable the age of many glacial features to be determined and have allowed us to study the extent and timing of himalayan glaciation in the late quaternary new data show that the local last glacial maximum lgm occurred during the early part of the last glacial cycle in most areas during marine isotope stage 3 mis 3 mis 3 was a time of increased insolation when the south asian summer monsoon strengthened and penetrated further north into the himalaya the concomitant increased precipitation occurring as snow at high altitudes produced positive glacier mass balances thereby allowing glaciers to advance on the other hand during the global lgm 18 24 ka himalayan glaciation was very restricted in extent generally extending

melt distribution beneath a young continental rift the taupo volcanic zone new zealand 1 taupo volcanic zone tvz is a zone of intense volcanism and rifting associated with the subduction of the pacific plate beneath the continental crust of new zealand s north island an image of the conductivity structure beneath the central part of the tvz has been constructed using 2 d inverse modeling of magnetotelluric data a rapid increase in conductivity at a depth of 10 km beneath the tvz 3 km beneath the base of the seismogenic zone but well above the base of the quartzo feldspathic crust 16 km is interpreted to mark the presence of an interconnected melt fraction 4 within the lower crust beneath the quartzo feldspathic crust the model shows a zone of increased conductivity on the eastern side of the tvz consistent with an increased concentration of melt at deeper levels the pacific plate is resistive compared with the overlying mantle

asymmetric rifting in a continental back arc environment north island new zealand marine seismic reflection data have been used to investigate the structure and evolution of the continental back arc basin forming the volcanic region of central north island new zealand the seismic reflection data defines two parts to the volcanic region in the west an older region showing narrow back tilted blocks and a younger rift basin delineated by the white island and tauranga fault zones to the east the younger rift coincides with the offshore trend of the active taupo volcanic zone tvz listric faulting of basement in the eastern part gives rise to about 80 extension for the basin which is consistent with the thinning of the crust from 25 km along the margins of the basin to 15 km under the basin basement in the region is interpreted to be highly extended greywacke the nature of the faulting and the seismic stratigraphy demonstrates that the extension has been asymmetric in nature being progressively younger towards the plate boundary in the east active faulting offsets the seafloor in the younger eastern part of the basin eight major sedimentary sequences delineated by unconformities are recognised in the basin these sequences based on correlation with major onshore rhyolitic eruptions are considered to be up to about 1 6 ma old a major unconformity within the sequences may coincide with a hiatus in volcanic activity from 0 7 0 4 ma ago a caldera structure is tentatively identified under the western part of the younger rift basin offshore tvz from seismic and other data its age is not well constrained but an age corresponding to the initiation of the younger extension episode 1 6 ma ago is preferred older volcanic sequences underlie the oldest tvz volcanic units to the west of the younger basin and probably correspond to the pliocene miocene volcanics onshore in the coromandel peninsula

seismic velocity structure of the central taupo volcanic zone new zealand from local earthquake tomography abstract the 3 d distribution of p wave velocity vp and the p wave s wave velocity ratio vp vs are derived for the crust in the central taupo volcanic zone tvz new zealand by tomographic inversion of p and s wave arrival time data from local earthquakes resolution in the seismogenic mid crust 4 6 km is good but poorer above and below these depths the 3 d velocity model has several vp anomalies as large as 5 in the mid lower crust 4 10 km and more than 10 in the upper crust 0 4 km the model achieves a 55 reduction in data variance from an initial 1 d model young caldera structures okataina rotorua and reporoa are characterised by low vp anomalies at a depth of about 4 km and these coincide with large negative residual gravity anomalies we attribute these anomalies to large volumes of low vp low density volcaniclastic sediments that have filled these caldera collapse structures although there are no vp anomalies which suggest the presence of molten or semi molten magma beneath the tvz a large high vp anomaly of more than 15 and a high vp vs anomaly are observed coincident with a diorite pluton beneath the ngatamariki geothermal field however vp anomalies cannot be seen beneath the largest geothermal fields waimangu waiotapu and reporoa and consequently if such anomalies exist they must be below the resolution of our data a prominent vp contrast of 5 10 occurs at a depth of about 6 km beneath the boundary between the taupo reporoa depression and the taupo fault belt tfb coincident with the eastern limit of the seismic activity beneath the tfb we interpret this velocity contrast as being caused by the presence of extensive non molten intrusives beneath the taupo reporoa depression we suggest that the high velocity material beneath the taupo reporoa depression is isolated from regional extension in the tvz and from the resulting faulting and seismicity which occurs preferentially within the weaker material of the tfb we are unable to determine whether greywacke which forms the basement beneath the eastern most part of the tvz continues further west or is replaced by a volcanic rock such as andesite which has similar vp and density vp vs anomalies are much smaller than vp anomalies and generally have little spatial relationship to the vp pattern there is a widespread decrease in vp vs from 1 76 to 1 70 1 73 between 4 and 6 km depth over much of the study area and vp vs is high southwest of the okataina caldera where vp is low hypocentres calculated using the 3 d velocity model differ little from those obtained using a 1 d model with station terms however some groups of earthquakes are more tightly clustered following relocation there is a slight decrease in the estimated thickness of the seismogenic zone with 73 of hypocentres between 4 and 7 km depth and a slight increase in the depth of the brittle ductile transition from 6 to 6 5 km

observations on rock glaciers in the himalayas and karakoram mountains of northern pakistan and india abstract rock glaciers are abundant in the lahul and garhwal himalayas of northern india and the karakoram mountains of northern pakistan they exhibit morainic and protalus forms and are restricted altitudinally and climatically to sites above approximately 4000 m asl and regions where annual precipitation is less than 1000 mm in these areas morainic rock glaciers are large usually 1 km long 100 m wide and 15 m thick they are advancing over recent fluvial terraces and modern floodplains the morainic rock glaciers record the advance of ice cored moraines following retreat of glaciers likely since the little ice age these rock glaciers form a major component of the landscape and are important conveyors of debris down valley under the influence of gravity

timing and style of late quaternary glaciation in the eastern hindu kush chitral northern pakistan a review and revision of the glacial chronology based on new optically stimulated luminescence dating abstract glacial landforms and sediments provide evidence for two late quaternary major glaciations in the eastern hindu kush new optically stimulated luminescence osl dating was undertaken to define the timing of these glaciations and associated sediment deposition the drosh glacial was defined by osl dating to have occurred during marine isotopic stage 3 producing an extensive valley glaciation that extended to an altitude of 1300 m above sea level asl in the main valley with an equilibrium line altitude ela depression of 1200 m the pret glacial produced a valley glaciation that extended to an altitude of 1670 m asl in the main valley with an ela depression of 1000 m the new osl dating suggests that this glacial stage probably represents several glacial advances that occurred during the latter part of the last glacial moraines representing two minor glacial advances the shandur and barum glacial stages were also recognized near the contemporary glaciers these probably formed during the middle late holocene and little ice age respectively this study and comparisons with adjacent regions suggest that there was limited glaciation during the global last glacial maximum lgm in the hindu kush deglaciation in the mountain ranges of the himalayas and the consequent meltwater discharge into the adjacent seas was negligible immediately after the global lgm and it was therefore of minor importance as a forcing factor for global climate change during the end of the last glacial cycle however our studies show that glaciation and particularly deglaciation is important in controlling the deposition of thick valley fills and the landscape evolution of the high mountain environments

late quaternary holocene landscape evolution of a monsoon influenced high himalayan valley gori ganga nanda devi ne garhwal the garhwal himalaya provides an excellent natural laboratory in which to examine landscape evolution in a monsoon influenced high mountain environment geomorphic and sedimentological analysis and 10be cosmogenic radionuclide crn surface exposure dating of moraines fans and river and strath terraces in the gori ganga valley of nanda devi ne garhwal indicate that fans and river terraces developed rapidly by debris flow and flood processes during periods of deglaciation these phases of high sediment transfer occurred at 1 2 4 5 and 7 8 ka fan incision rates subsequent to major times of resedimentation after each glacial advance are between 19 and 57 mm year this contrasts with bedrock incision rates based on mid holocene strath terraces of 5 mm year these rates indicate that despite episodes of rapid denudation and resedimentation linked to glacial activity in this region the background rates of denudation are similar to those for other regions of the himalaya averaging several millimeters per year furthermore these data show the importance of climatic controls on landscape evolution and suggest a strong monsoonal control on the dynamics of earth surface processes in this region

a 1400 year high resolution record of atmospheric circulation over the north atlantic and asia major ion series developed from new subannual scale sampling of an ice core from central greenland are calibrated with instrumental series of atmospheric sea level pressure recording major marine icelandic low and terrestrial siberian high atmospheric circulation systems to provide proxy records of atmospheric circulation over the past 1400 years examination of the proxy records reveals major changes in behaviour of these systems c ad 1400 multidecadal and centennial scale periodic components characterization of mean sea level pressure anomaly fields during the little ice age and the mediaeval warm period the potential role of solar forcing coupled ocean atmosphere associations and a perspective within which the characteristics of instrumental era climate can be assessed

rapid slip along the central altyn tagh fault morphochronologic evidence from cherchen he and sulamu tagh consistent with the d 18 o record from the guliya ice cap in the west kunlun the features of interest were all formed by glacial and fluvial processes subsequent to marine isotope stage 5e with the youngest features having formed during the early holocene optimum this near field morphochronological slip rate is averaged over many earthquake cycles and is hence little affected by interseismic strain it is kinematically consistent with other somewhat lower geomorphic slip rate measurements to the east the average rate and lower bounds obtained from alternate interpretational models 18 4 mm yr cannot be reconciled with the most rece geodetic measurements 7 mm yr suggesting that interseismic strain and interactions with adjacent faults may lead to disparate geologic and geodetic rate estimates this late pleistocene holocene morphochronologic rate would imply that at this longitude the altyn tagh fault on the north edge of tibet might absorb almost as much of india s convergence relative to siberia as the himalayan main frontal thrust does on the southern edge of the plateau index terms 1035 geochemistry geochronology 1208 geodesy and gravity crustal movements intraplate 8110 8102 tectonophysics continental contractional orogenic belts 8107 tectonophysics continental neotectonics keywords slip rates cosmogenic dating indo asian collision

downhole geophysical logging downhole geophysical logging is the method of making geophysical measurements in drill holes and interpreting these measurements for evaluation of geological properties of the subsurface the downhole measurements can be made with great spatial resolution with a single sonde containing both source and receivers which are separated by 10 cm to tens of meters measurements can be repeated every few centimeters as the sonde moves upward or downward in the borehole this technique is called well logging it is also possible to make downhole measurements with the sensor in the borehole at depth and the source at the surface when the sensor package is moved to a different depth the source signal is repeated when these measurements are made with seismic waves the process is called vertical seismic profiling vsp in vsp the surface source can be any appropriate system that would generate a seismic pulse dynamite air gun or a sinusoidal wave vibroseis and the downhole sensor can be a hydrophone or borehole geophone vertical seismic profiling is a rapidly expanding technique and some of its applications are described in a series of recent books gal perin 1974 hardage 1983 balch lee 1984 toksoz stewart 1984 another method of downhole measurement is possible when closely spaced drill holes are available with the source placed in one drill hole and the detector receiver in another this technique called cross

holocene variations in the asian monsoon inferred from the geochemistry of lake sediments in central tibet we present a record of monsoon variations for the early and middle holocene that is inferred from the geochemistry of sediment cores from ahung co a lake in central tibet the resolution of this record is better than 50 yr and the age model is derived from radiocarbon ages of terrestrial charcoal which eliminates errors associated with the lake hard water effect we made down core geochemical measurements of carbonate organic carbon c n and 13c of bulk organic matter 13c and 18o of carbonate and dolomite proxy calibration and modern water balance reconstruction show that these are proxies for lake depth and the amount of monsoon precipitation we find that lake level and monsoon precipitation have been decreasing at ahung co since the early holocene 7500 cal yr b p superimposed on this trend are rapid declines in monsoon rainfall at 7000 7500 and 4700 cal yr b p and seven century scale wet dry oscillations the cores do not contain sediment from the last 4000 yr surface sediments from the lake accumulated during the 20th century however from this we argue that lake levels have risen again recently following a late holocene dry period

the reporoa caldera taupo volcanic zone source of the kaingaroa ignimbrites the reporoa caldera occupies the northern end of the reporoa depression previously described as a tectonic fault angle depression earlier confirmation of the topographic basin as a caldera had been hindered by the lack of an associated young pyroclastic flow deposit of large enough volume to have caused caldera collapse new exposures on the eastern margin of the reporoa basin reveal thick lithic lag breccias 30 m interbedded within the 0 24 ma kaingaroa ignimbrites these ignimbrites were previously attributed to the adjacent okataina volcanic centre lag breccia thicknesses and maximum clast sizes decrease rapidly outward from the caldera rim and discrete breccias are absent from ignimbrite sections more than 3 km from the rim the lithic lag breccias together with structural and geophysical evidence confirm reporoa caldera as the source of the c 100 km3 kaingaroa ignimbrites adding another major rhyolitic volcanic centre to the seven previously recognized in the taupo volcanic zone other older calderas may also be present in the reporoa depression

late pleistocene late glacial and holocene glacier advances on the tibetan plateau abstract many end moraine sequences in various mountain areas of the tibetan plateau have already been investigated in detail but a homogeneous chronostratigraphy is still lacking the ice marginal limits are commonly given local names and often the classification does not seem to be secure this is one reason for controversial discussions on the ice extent at the lgm because most organic material in tibet developed at the beginning of the holocene only a few older radiocarbon dates are available up to now late glacial terminal moraines have been found in the western kunlun shan and in the tian shan their radiocarbon ages ca 15 ka bp correspond to a glacier advance south of the qaidam basin which can be correlated with late glacial lacustrine sediments in eastern tibet different ages of peat development in different tongue like basins have been obtained even the bases of peat bogs in basins at nianbaoyeze mountains enclosed by apparent lgm end moraines gave holocene ages three main holocene intervals have been radiocarbon dated from palaeosols and basal peat layers in the area except for a younger glacier re advance at about 3000 bp and those of the little ice age the age of many last glaciation glacier advances on the tibetan plateau are constrained only on the basis of organic matter lying on top of the moraines and therefore might be of late glacial or pleistocene age

cosmogenic radionuclide evidence for the limited extent of last glacial maximum glaciers in the tanggula shan of the central tibetan plateau abstract cosmogenic radionuclide crn exposure ages provide evidence for the limited extent of last glacial maximum glaciers in the tanggula shan central tibetan plateau the most extensive advances occurred during or before marine oxygen isotope stage 6 mis 6 based on previous crn exposure ages the second most extensive advance occurred during or before mis 4 based on previous ages and new ages of 41 400 4300 and 66 800 7100 10 be yr a mis 2 advance of less than 3 km occurred between 31 900 3400 and 16 000 1700 10 be yr

style and timing of glacial and paraglacial sedimentation in a monsoon influenced high himalayan environment the upper bhagirathi valley garhwal himalaya abstract the gangotri glacier at the source of the ganges river has fluctuated greatly throughout the late quaternary in response to climatic oscillations this has resulted in impressive moraines paraglacial debris flow fans and terraces along the upper stretches of the bhagirathi valley cosmogenic radionuclide crn dating of glacial and paraglacial landforms shows that fans terraces and associated moraines formed approximately synchronously as the landscape readjusted to changing environmental conditions this synchronicity suggests that fan and terrace formation is intimately related to glaciation and that fluctuations in glacial and associated environments during times of climatic instability cause rapid sediment transfer and resedimentation of glacial landforms the crn dates show that all existing glacial and paraglacial landforms in the upper bhagirathi valley formed during the late pleistocene and holocene this demonstrates that in this high mountain environment paraglacial and glacial landforms are eroded and resedimented within about 20 000 years furthermore this testifies to the dynamic nature of glacial environments in monsoon influenced high mountain regions

late holocene temperature fluctuations on the tibetan plateau abstract proxy data of palaeoclimate like ice cores tree rings and lake sediments document aspects of climate changes on the tibetan plateau during the last 2000 years the results show that the tibetan plateau experienced climatic episodes such as the warm intervals during ad 800 1100 and 1150 1400 the little ice age between ad 1400 and 1900 and an earlier cold period between the 4th and 6th centuries in addition temperatures varied from region to region across the plateau a warm period from ad 800 to 1100 in the northeastern tibetan plateau was contemporaneous with cooling in the southern tibetan plateau which experienced warming between ad 1150 and 1400 large scale trends in the temperature history from the northeastern tibetan plateau resemble those in eastern china more than the trends from the southern plateau the most notable similarities between the temperature variations of the tibetan plateau and eastern china are cold phases during ad 1100 1150 1500 1550 1650 1700 and 1800 1850

quaternary glacial history of nw garhwal central himalayas a first account of the quaternary glacial history is presented for northwest garhwal central himalaya on the basis of sediments and landforms one glacial stage has been recognised this is called the bhagirathi glacial stage when extensive valley glaciers advanced down the bhagirathi valley to jhala 40 5 km from the snout of gangotri glacier the ela depression during this stage was ca 640 m the bhagirathi glacial stage is constrained by optically stimulated luminescence dates of ca 63 ka and 5 ka bp and this glaciation is considered equivalent to the last glaciation elsewhere in the world the maximum extent of ice occurred ca 63 ka bp this however does not correlate with the last glacial maximum for the northern hemisphere ice sheets 20 18 ka bp a series of sharp crested moraines are present between 1 to 3 km beyond the snouts of most of the present glaciers these moraines formed during the mid holocene 5 ka bp termed here the shivling glacial advance small moraines are inset into these and are dated at about 200 to 300 bp the bhujbas glacial advance ca 300 to 200 bp and considered equivalent to the little ice age in other parts of the world ela depressions of between 40 100 m and 20 60 m occurred during the shivling and bhujbas glacial advances respectively since the bhujbas glacial advance there has been progressive retreat of glaciers initially by downwasting and retreat and then by simple retreat this retreat has accelerated during the last few decades impressive paraglacial fans are associated with deglaciation representing very rapid resedimentation during and soon after ice retreat

sharp decrease in summer monsoon strength 4000 3500 cal yr b p in the central higher himalaya of india based on pollen evidence from alpine peat age constrained pollen data and magnetic susceptibility of an alpine peat profile from the garhwal higher himalaya display a continuous record of climate and monsoon trends for the past 7800 yr about 7800 cal yr b p dominance of evergreen oak quercus semecarpifolia alder alnus and grasses in the pollen record reflect a cold wet climate with moderate monsoon precipitation from 7800 to 5000 cal yr b p vegetation was progressively dominated by conifers indicating ameliorated climate with a stronger monsoon a warm humid climate with highest monsoon intensity from 6000 4500 cal yr b p represents the mid holocene climatic optimum between 4000 and 3500 cal yr b p the abundance of conifers sharply decreased with the greatest increase in evergreen oak this trend suggests progressive cooling with a decrease in the monsoon to its minimum about 3500 cal yr b p two relatively minor cold dry events at ca 3000 and 2000 cal yr b p marked step wise strengthening of the monsoon until ca 1000 cal yr b p after a cold dry episode that culminated ca 800 cal yr b p the monsoon again strengthened and continued until today a sharp decrease in temperature and rainfall at 4000 3500 cal yr b p represents the weakest monsoon event of the holocene record this cold dry event correlates with proxy data from other localities of the indian subcontinent arabian sea and western tibet

holocene late pleistocene climatic ice core records from qinghai tibetan plateau three ice cores to bedrock from the dunde ice cap on the north central qinghai tibetan plateau of china provide a detailed record of holocene and wisconsin wurm late glacial stage lgs climate changes in the subtropics the records reveal that lgs conditions were apparently colder wetter and dustier than holocene conditions the lgs part of the cores is characterized by more negative 18 o ratios increased dust content decreased soluble aerosol concentrations and reduced ice crystal sizes than the holocene part these changes occurred rapidly 10 000 years ago in addition the last 60 years were apparently one of the warmest periods in the entire record equalling levels of the holocene maximum between 6000 and 8000 years ago

resistivity structure of the central taupo volcanic zone new zealand abstract the taupo fault belt tfb is a 15 km wide zone of active normal faulting which is located in the central portion of the taupo volcanic zone tvz new zealand and is aligned sub parallel to the strike of this zone a large scale multiple source bipole dipole resistivity survey has shown that the tfb is characterised by high resistivity ca 250 m to depths of ca 7 km in contrast to the adjacent regions in the east and west of the tvz where resistivities are much lower 1 ma ignimbrites that have undergone slow diagenetic alteration that has produced clay minerals the volcaniclastics in the tfb have high resistivities which suggests that they have not undergone significant alteration and are thus younger than the volcaniclastics in the regions to both east and west these high resistivities are also consistent with the absence of surface geothermal activity and low heat flow the tfb also exhibits high seismicity and active faulting in contrast with its surrounding it was not possible to determine the depth to basement rocks in the tfb because of the lack of resistivity contrast between the basement and the overlying volcanics but gravity data indicate that the young volcanics are no more than 3 km thick

quaternary glaciation of tibet the geological evidence the question of the number and extent of pleistocene glaciations on the qinghai xizang tibet plateau remains contentious there has been considerable research activity in tibet by chinese scientists since the late 1950s and their principal findings including data on glacial and periglacial geomorphology sedimentology tectonics stratigraphy and lake history have been used to reconstruct ice margins and palaeosnowlines three and in places four distinct glaciations have been recognised the most extensive occurring in the later middle pleistocene valley and piedmont glacial systems with some mountain ice caps and a small ice sheet on the upper reaches of the huang he have been recognised but there is no evidence of a single tibetan ice sheet as the plateau and the himalaya karakoram underwent accelerating uplift through the quaternary there occurred progressive desiccation in the interior as the influx of indian ocean moisture was constrained equilibrium line elevations in the last glacial maximum were 4000 m in the south east and northeast plateau margins rising to 5500 m in the northwest interior the heat island provided by the plateau the progressive reduction in precipitation most marked during the glacials and the strong southeast to northwest precipitation gradients produced a glacier distribution pattern dominated by the trans tibetan mountain ranges tectonics and periglacial including aeolian processes have played an equal role with glaciation in generating the plateau s quaternary sedimentary succession

spatial patterns of holocene glacier advance and retreat in central asia glaciers in the southern himalayas advanced in the early holocene despite an increase in incoming summer solar insolation at the top of the atmosphere these glacier advances are in contrast to the smaller alpine glaciers in the western and northern regions of central asia two different glacier mass balance models are used to reconcile this holocene glacier history with climate by quantifying the change in equilibrium line altitudes ela for simulated changes in holocene climate both ela models clearly show that the lowering of elas in the southern himalayas is largely due to a decrease in summer temperatures and that an increase in monsoonal precipitation accounts for less than 30 of the total ela changes the decrease in summer temperatures is a dynamic response to the changes in solar insolation resulting in both a decrease in incoming shortwave radiation at the surface due to an increase in cloudiness and an increase in evaporative cooling in the western and northern zones of central asia both ela models show a rise in elas in response to a general increase in summer temperatures this increase in temperatures in the more northern regions is a direct radiative response to the increase in summer solar insolation

a geophysical study of the taupo volcanic zone new zealand the taupo volcanic zone is a region of intense volcanic activity from which over 16 000 km3 of lavas and pyroclastics have been erupted since the late pliocene analysis of gravity and magnetic anomalies over the zone has revealed the presence of five major centers each associated with an approximately circular negative gravity anomaly and a positive pseudogravity anomaly these centers are interpreted as collapse features resulting from large scale withdrawal of magma from the upper crust four of the depressions occur beneath recognized volcanic centers or calderas the fifth kapenga is inferred to underlie an extinct hidden volcanic center three dimensional models of the thicknesses of low density and magnetized rocks in the taupo volcanic zone have been determined by inversion of gravity and magnetic anomalies beneath three of the centers the thicknesses of low density and magnetized rocks are approximately equal 2 3 km that is the collapse structures are infilled by magnetized volcanics however there is a significant discrepancy between the thicknesses of the gravity 5 km and magnetic models 1 5 km in the region of the okataina volcanic centre it is therefore suggested that a significant volume of low density nonmagnetic rock possibly acidic magma underlies okataina

relative dating of quaternary moraines rongbuk valley mount everest tibet implications for an ice sheet on the tibetan plateau abstract relative dating studies applied to high altitude moraines 5000 5500 m in the rongbuk valley on the northern flank of mt everest reveal strong contrasts in the weathering characteristics of the boulders exposed along moraine crests these differences serve to define three intervals of major pleistocene glaciation that on the basis of the degree of weathering are interpreted to extend back to at least the penultimate glaciation and probably encompass at least one still older glaciation either interpretation indicates that some of these moraines are considerably older than their previously assigned ages the magnitude of equilibrium line lowering during neoglacial and late pleistocene times is calculated to be ca 50 100 and 350 450 m respectively the data described here are incompatible with the recently proposed model kuhle 1987 for large scale ice sheet development on the tibetan plateau the reconstructed equilibrium line lowering in the everest region is only 30 of that cited in the ice sheet model moreover the flow patterns and geometry of the former rongbuk glaciers are in opposition to those proposed by the model based on the data from the everest region it appears that valley glaciation rather than ice sheet growth characterized the southern margin of the tibetan plateau during the middle and late pleistocene glaciations