empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

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

quantitative determination of metamorphic reaction history mass balance relations between groundmass and mineral inclusion assemblages in metamorphic rocks qualitative and quantitative information about metamorphic reaction history and pt paths may be obtained from mineral inclusions in garnet by comparing the mineralogy distribution and compositions of paragenetically related inclusions with minerals in the groundmass assemblage using the algebraic technique of singular value decomposition svd we document mass balance relations between inclusion and groundmass assemblages in metapelitic rocks from two metamorphic terranes that experienced different peak metamorphic conditions and whose transition from inclusion to groundmass assemblage records different pt path segments relative to peak conditions we calculate mass balances relating an inclusion assemblage consisting in part of armored relics of chloritoid to groundmass mineral assemblages in a kyanite staurolite mica schist from the solitude range british columbia and an inclusion assemblage of kyanite staurolite and rutile to groundmass minerals in a sillimanite cordierite gneiss from the skagit gneiss north cascade range washington mass balances for each rock are consistent with reaction histories inferred from petrographic observations in the solitude range schist the results of mass balance calculations are consistent with the growth of staurolite and garnet at the expense of chloritoid during prograde metamorphism and suggest that chlorite although not preserved as an inclusion was involved in initial staurolite growth in the skagit sillimanite gneiss mass balance relations exist between the inclusion suite which formed during high pressure metamorphism and the associated groundmass assemblage which equilibrated at high temperature but much lower pressure mass balance does not exist between the groundmass of the skagit sillimanite gneiss and the groundmass of a nearby kyanite staurolite schist that has been proposed as a possible lower grade equivalent of the sillimanite bearing rocks these results indicate that although compositional modification and selective preservation of minerals must be taken into account mineral inclusion suites may nevertheless preserve enough compositional information to allow reconstruction of complete or nearly complete pre existing assemblages this information may not be retrievable from any other source if no lower grade equivalents of the rocks of interest are exposed

origin and metamorphic history of an early cretaceous polybaric granulite terrain fiordland southwest new zealand regionally extensive two pyroxene granulites in fiordland southwest new zealand are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes the granulite protolith igneous charnockitic rock synkinematically intruded metasediment and other orthogneiss in an early cretaceous subduction related magmatic arc and during cooling experienced deformation induced recrystallization to form granoblastic gneiss the granulites occur side by side with coeval rocks of amphibolite facies mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization 1 an early high temperature comparatively low pressure event accompanying magmatic intrusion andalusite sillimanite facies series recorded locally in the country rock followed by 2 high pressure metamorphism under conditions of 650 700 c at 12 13 kbar garnet granulite locally overprinted earlier formed two pyroxene granulite during the latter event the pressure increase 6 kbar between the two events is attributed to crustal thickening by overthrusting and is equivalent to unloading of a 20 km thick slab over rocks already buried at mid crustal depths both events occurred over a 20 m y interval between the time of magmatic emplacement of the granulite protolith and uplift controlled final cooling of the terrain the phanerozoic granulites in fiordland share some petrologic similarities with precambrian granulite terrains suggesting that at least some aspects of the former may serve as a useful model for development of the latter

desert agricultural systems at eba jawa jordan integrating archaeological and paleoenvironmental records located in the arid basalt desert of northeastern jordan the settlement of jawa is by far the largest and best preserved archaeological site in the region the early bronze age eba settlement phase of jawa 3500 3000 bce is characterized by a highly sophisticated water storage system made of a series of pools dams and canals in addition recent archaeological and geoarchaeological surveys have uncovered agricultural terrace systems in the nearby vicinity in this study four of these runoff terrace systems were investigated by detailed mapping additionally thirteen sediment profiles from inside and outside the terrace systems were recorded and sampled the examined samples were analyzed for bulk chemistry texture phytoliths diatoms and dung spherulites to supply information on the environmental and depositional conditions the terrace systems were dated using optically stimulated luminescence osl ancient terrace agriculture was practiced on slopes small plateaus and valleys close to jawa through the use of surface canals which collected and diverted floodwater from nearby wadis or runoff from adjacent slopes the terraced fields were usually arranged in cascades and comprised a system of risers canals and spillways the terrace fills investigated yield osl ages of around 3300 bce indicating that the terraces were constructed in the early bronze age the terrace fill sequences are composed of mixed unstratified fine sediments of local origin reflecting low energy fluvial deposition regimes the phytolith record is dominated by pooid grasses that include the most common near eastern cereals such as wheat and barley increased phytolith concentrations in terrace fill sediments as compared to samples from non terrace deposits nearby suggest increased plant growth and water availability within the terraces whether the terrace systems were used for growing food crops only or whether they were additionally used for grazing cannot be ascertained overall quantitative phytolith analyses in arid environments are well suited to investigate temporal and spatial distributions of plant microfossil concentrations and their relation to human activity or paleoenvironmental conditions

environmental changes since 8 4 ka reflected in the lacustrine core sediments from nam co central tibetan plateau china the tibetan plateau induces and enhances the asian monsoon that influences the plateau itself east china and even the whole of asia pursuing the changing monsoon history and its impact in this environment sensitive area is a key for understanding the climatic changes here we present the initial results from core sed iments in nam co central tibet to elucidate the holocene environment changes in this area the studied lake nam co is the second largest lake with an elevation of 4718 m water area of 2015 km 2 and catchment area of 10 610 km 2 by using echo seismic profiling and hydro echosounding we investigated the bathymetric dis tribution of the whole lake one 332 cm long piston core was successfully taken in the east part of the lake at 60 m water depth twelve ams 14 c dates from top to bottom of this core established a good time sequence toc tn n alkanes grain size clay minerals fe mn sr ba and caco3 content were analysed with sampling intervals from 1 cm to 10 cm results showed that since 8400 yr bp the environment changes in this region indicate three stages the early stage 8400 6900 yr bp was characterized by slight shifting from warm to cold with a cold dry interval at 8100 7800 yr bp the middle stage was from 6900 to 2900 yr bp during which cli mate changed from warm humid to cold dry however around 2900 yr bp the climate once was slightly cold but strongly dry the late stage from 2900 yr bp to present was characterized by a general tendency toward cold dry the first temperature decline was around 1700 1500 yr bp following which temperature increased once before decreasing again around 600 300 yr bp which might be the result of the little ice age

palaeo water depth estimation for a 600 year record from nam co tibet using an ostracod based transfer function a modern training set of ostracods from the large slightly saline lake nam co on the southeastern tibetan plateau is presented in total 44 samples covering the largest part of the lake and water depth from 2 to 93 m were taken there are eight ostracod species in surface sediments of nam co leucocytherella sinensis leucocythere dorsotuberosa fabaeformiscandona gyirongensis candona xizangensis candona candida ilyocypris cf mongolica eucypris gyirongensis and limnocythere inopinata in order of mean dominance a morphological variant of l dorsotuberosa the forma postilirata is an indicator form of deep water below the thermocline the 36 samples comprise a sufficient number of ostracods for analysis covering water depths between 3 7 and 89 3 m two transfer function models based on weighted average binomial regression wa br and weighted average partial least square regression wa pls using the programs excel and c2 were established to estimate past water depths of nam co transfer functions were applied to ostracod data from two short cores from nam co covering approximately the past 600 years model results show a lake level minimum occurring during the little ice age lia between about 1490 and 1760 ad this minimum corresponds to results from previous stable isotope analysis and indicates a lake level which was about 5 m below the present day level the more exact transfer function model wa pls was set up using c2 and revealed a higher correlation index r o f 0 91 and a lower standard error of 6 7 m however the more selective because of using indicator taxa only wa br model using excel r 0 88 standard error 10 6 m seems to represent palaeo water depths better at least for the studied short cores

holocene vegetation and climate of the alashan plateau nw china reconstructed from pollen data a set of 55 recent pollen spectra from the alashan plateau and the qilian mountains qilianshan northwestern china has been analyzed the established relationships between the pollen spectra and modern vegetation and precipitation patterns of the studied area have been applied to 65 fossil pollen spectra from a 825 cm long sediment record collected in the topographic depression formerly occupied by eastern juyan palaeolake 41 89 n 101 85 e 892 m a s l several qualitative approaches e g indicator plant species artemisia chenopodiaceae and ephedra fragilis type s l ephedra distachya type pollen ratios and quantitative methods of vegetation and climate reconstruction have been tested with the surface pollen and then applied to the eastern juyan pollen record fossil pollen data reveal pronounced environmental changes in the studied area of the alashan plateau between ca 10 700 and 1700 cal yr b p desert taxa as chenopodiaceae nitraria calligonum reaumuria were abundant in the vegetation cover around the study site through the whole record pollen spectra dated to ca 10 700 5400 cal yr b p are characterised by highest values of chenopodiaceae e fragilis type and other desert indicating taxa suggesting rather dry climate most favourable conditions are reconstructed between 5400 and 3900 cal year b p on the basis of relative increase in abundance of artemisia pollen a return to dry conditions occurred at about 3900 cal yr b p the lake finally desiccated after 1700 cal yr b p reconstructed dry climate oscillations between 5900 and 5400 and 3100 cal yr b p correlate well with similar events found in the published records from northern and western china and central mongolia however eastern juyan record does not show the early holocene humidity maximum as suggested by the sedimentary data from monsoon influenced areas of china

pollen and diatom inferred climatic and hydrological changes in sumxi co basin western tibet since 13 000 yr b p although the tibetan plateau greatly influences the atmospheric circulation of the nortbern hemisphere few continuous paleoclimatic records are available from the plateau a 13 000 yr pollen and diatom record from the sumxi longmu co basin in western tibet gives information on major changes both in regional vegetation and in local hydrology after the basin first filled ca 13 000 yr b p a dry spell occurred about 10 500 yr b p within the interval spanned by the european younger dryas chronozone a major environmental change occurred suddenly at 10 000 yr b p with the establishment of wet conditions and was followed by a long term trend toward maximum aridity which lasted approximately 6000 yr short term oscillations are superimposed on this general climatic change with a major reversal event about 8000 yr b p and a second wet pulse leading to a maximum lake volume ca 7500 6000 yr b p maximum aridity occurred 4300 yr b p the major environmental fluctuations recorded at sumxi longmu co appear in phase with climatic changes recognized in north tropical africa suggesting that the 8000 to 7000 yr b p event was caused by an abrupt disequilibrium in the climatic system as was the younger dryas and possibly the 4300 yr b p event

a general cooling trend on the central tibetan plateau throughout the holocene recorded by the lake zigetang pollen spectra abstract a 741 cm long laminated sediment core covering the last 10 800 years was collected from lake zigetang central tibetan plateau 90 9 e 32 0 n 4560 m a s l and analysed palynologically at 69 horizons biome reconstruction suggests a dominance of temperate steppe vegetation mainly artemisia and poaceae on the central tibetan plateau during the first half of the holocene 10 8 4 4 cal ka bp while alpine steppes with desert elements mainly cyperaceae poaceae chenopodiaceae and characteristic high alpine herb families tend to dominate the second half 4 4 0 cal ka bp the artemisia cyperaceae ratio a semi quantitative measure for summer temperature indicates a general cooling trend throughout the holocene dense temperate steppe vegetation and maximum desert plant withdrawal however indicate that a suitable balance of wet and warm conditions for optimum vegetation growth likely occurred during the middle holocene 7 3 4 4 cal ka bp severe early holocene cold events have been reconstructed for 8 7 8 3 and 7 4 cal ka bp

pressure temperature deformation history for a part of the mesoproterozoic fold belt in north singhbhum eastern india abstract the supracrustal rocks in the easternmost part of the proterozoic fold belt of north singhbhum eastern india are folded into a series of large upright folds with variable plunges the regional schistosity is axial planar to the folds the folds were produced by a second phase of deformation d2 and were preceded by d1 deformation which gave rise to isoclinal folds mapped outside the study area and the locally preserved bedding parallel schistosity a shearing deformation during d2 was responsible for the sheath like geometry of a major fold the axial planes were curved by d3 warping the first metamorphic episode m1 of low pressure type produced andalusite porphyroblasts prior to or in the early stage of d1 deformation the main metamorphism m2 responsible for the formation of chloritoid kyanite garnet and staurolite porphyroblasts was late to post d2 in occurrence the staurolite isograd separates two zonal assemblages recorded in the high alumina and the low alumina pelitic schists geothermobarometric calculations indicate the peak metamorphic temperature to be 550 c at 5 5 kb fluid composition in the rocks before and during m2 metamorphism was buffered and fluid influx if any was not extensive enough to overcome the buffering capacity of the rocks from m1 to m2 the p t path is found to have a clockwise trajectory that is consistent with a tectonic model involving initial asthenospheric upwelling and rifting followed by compressional deformation leading to loading and heating

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

softening trigerred by eclogitization the first step toward exhumation during continental subduction direct observation of peak pressure deformation in exhumed subduction channels is difficult because little evidence of this deformation survives later syn exhumation deformation most ultrahigh pressure parageneses are found in continental derived metamorphic rocks making continental subduction the best context to observe peak pressure deformation whereas many studies have enlightened the main driving parameters of exhumation such as buoyancy forces low viscosity in the subduction channel overburden removal by erosion and normal faulting a basic question is seldom considered why is a tectonic unit disconnected from the descending lithosphere and why does it start its way towards the surface this event seminal to exhumation processes must involve some deformation and decoupling of the exhumed slice from the descending slab at peak pressure conditions or close to it our field observations in the bergen arc show that caledonian eclogitization and later amphibolitization of a granulitic terrane was achieved with a consistent component of simple shear compatible with the sense of the caledonian subduction thus the sequence of deformation preserved in the bergen arc documents the decoupling of subducted crustal material from the descending slab at the onset of exhumation this observation suggests that deformation in the subduction channel is largely controlled by kinematic boundary conditions i e underthrusting of the subducting slab in this context of simple shear metamorphic reactions assisted by fracturating fluid infiltration and ductile deformation lower the resistance of rocks and allow the localisation of shear zones and the decoupling of buoyant tectonic units from the subducting slab these tectonic units can then be incorporated into the channel circulation and start their upward travel

synmetamorphic veining origin of andalusite bearing veins in the vedrette di ries contact aureole eastern alps italy discordant andalusite biotite quartz bearing veins occur in the contact aureole of the vedrette di ries pluton italian eastern alps never outside the area of contact metamorphic andalusite development andalusite veins are found only within andalusite bearing hornfelses and vein biotite occurs wherever host rock garnet is partially replaced by biotite veins formed during contact metamorphism synchronously with the crystallization of andalusite and biotite within host rocks their pegmatitic structure and their orientation suggest that vein parageneses crystallized within fluid filled cavities that opened by hydraulic fracturing a mechanism of synmetamorphic veining is proposed to explain rock failure and subsequent mineral deposition within veins during hydrofracturing induced by dehydration reactions in response to heating in the aureole fissures were immediately filled with locally derived fluids the lack of large scale flux together with high fluid pressures required by hydrofracturing suggest fluid in the cavities was a virtually stagnant passive medium and that mass transport toward fractures was driven by intergranular diffusion because temperature and pf values within veins are similar to those in the host rock vein assemblages are interpreted as the stable high t side of reactions taking place within pelitic schists at the time when fractures opened once nucleation of product phases occurred chemical components released by dissolution of reactant minerals were driven to precipitation sites by chemical potential gradients since nucleation was favoured at the strained grains of vein walls andalusite and biotite simultaneously grew in vein and host rock the proposed genetic model contrasts with generally adopted metasomatic mechanisms for the genesis of al2sio5 bearing veins in not requiring large fluid rock ratios or a highly aggressive fluid composition the mechanism of synmetamorphic veining may be particularly useful in the interpretation of vein occurrences in medium and deep crustal rocks which have undergone extensive devolatilization

incomplete blueschist re crytallization in high grade metamorphics from the sesia lanzo unit vasario sparone subunit western alps a case history of metastability abstract the vasario sparone vs subunit is part of the sesia lanzo unit western alps and represents a thin and discontinuous slice of pre alpine deep continental crust it is tectonically interposed between the eclogitic mica schists and the gneiss minuti subunits metastable relics and a sequence of incomplete transformations controlled by a local equilibrium reaction mechanism are preserved in the vs subunit the vs rocks record a pre alpine high grade of metamorphism t 780 50 c p 6 kbar which is comparable to the metamorphism dated around 500 ma in the ivrea zone during the alpine evolution the vs rocks sustained an early blueschist stage t 300 400 c p 8 10 kbar of probable cretaceous age and a later low t low p stage of tertiary age the blueschist transformations are governed by slow reaction kinetics by the heterogeneous distribution of pervasive deformations and by syn metamorphic fluid phase in particular transient kyanite formed in the early stages of the blueschist re crystallization in response to a process controlled by fluid deficient conditions and by internal buffering of a h 2 o at low values

a pollen record of holocene climatic changes from the dunde ice cap qinghai tibetan plateau pollen preserved in ice cores from the dunde ice cap provides a sensitive record of holocene climatic changes and vegetational response in the northern qinghai tibetan plateau at time scales ranging from millennia to centuries and decades pollen analysis of the annually resolvable ice layers for a 30 yr period 1957 1986 suggests that total pollen concentration is correlated positively with summer precipitation and negatively with summer temperature thus it is a sensitive indicator of moisture availability and vegetation density in the steppe and desert regions around dunde high pollen concentrations between 10 000 and 4800 yr b p suggest that the summer monsoon probably extended beyond its present limit to reach dunde and westernmost tibet in response to orbital forcing the summer monsoon retreated time transgressively across the qinghai tibetan plateau during the middle holocene relatively humid periods occurred at 2700 2200 1500 800 and 600 80 yr b p probably as a result of neoglacial cooling prominent pollen changes during the medieval warm period 790 620 yr b p and the little ice age 330 80 yr b p suggest that the vegetation in the qinghai tibetan plateau region is sensitive to abrupt century scale climatic changes such as those anticipated in scenarios of greenhouse warming

eclogite facies shear zones deep crustal reflectors strongly foliated eclogite facies rocks in 30 150 m thick shear zones of caledonian age occur within a grenvillian garnet granulite facies gabbro anorthosite terrain in the bergen arcs of norway the predominant eclogite facies mineral assemblages in the shear zones are omphacite garnet zoisite kyanite in gabbroic anorthosite and omphacite garnet in gabbro eclogite facies rocks in shear zones are generally fine grained alternating omphacite garnet and kyanite clinozoisite rich layers define gneissic layering a strong shape preferred orientation of omphacite kyanite and white mica phengitic muscovite and or paragonite define the foliation the anorthositic eclogites show omphacite b axis maxima approximately normal to the foliation and c axis girdles within the foliation plane p wave velocities v determined at confining pressures to 600 mpa for samples from eciogite facies shear zones range from 8 3 to 8 5 km s l and anisotropy ranges from 1 to 7 the few samples with more pronounced anisotropy tend to be approximately transversely isotropic with minimum velocities for propagation directions normal to foliation and maximum velocities for propagation directions parallel to foliation the fast propagation direction lies within the c axis girdles parallel to foliation and the slow propagation direction is parallel to the b axis concentration normal to foliation in samples for which omphacite crystallographic preferred orientation was determined v for the granulite facies protoliths average about 7 5 km s high calculated reflection coefficients for these shear zones 0 04 0 14 indicate that they are excellent candidates for deep crustal reflectors in portions of crust that experienced high pressure conditions but escaped thermal reactivation

the significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallization the spatial disposition compositional zoning profiles and size distributions of garnet crystals in 11 specimens of pelitic schist from the picuris range of new mexico usa demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks an ordered disposition of garnet centers and a significant correlation between crystal radius and near neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre existing crystals compositional zoning profiles require diffusionally controlled growth the rate of which increases exponentially as temperature increases with time an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process but decline in the later stages because of suppression effects in the vicinity of earlier formed nuclei simulations of porphyroblast crystallization based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics quantitatively replicate textural relations in the rocks the simulations employ only two variable parameters which are evaluated by fitting of crystal size distributions both have physical significance the first is an acceleration factor for nucleation with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock the second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it crystal size distributions for the picuris range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals the same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet phlogopite sphene and pyroxene in rocks from both regional and contact metamorphic occurrences these commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments

oxygen isotopes in lacustrine carbonates of west china revisited implications for post glacial changes in summer monsoon circulation five lakes in the qinghai tibet plateau and north xinjiang have provided oxygen isotope records covering the past 17 11 cal kyr bp the modern lakes are first presented within the major atmospheric circulation and climate patterns of china with the climate related isotope composition of input water equations used to extract hydrological and climatic information from 18o in carbonates are then given after a discussion on potential causes for 18o changes each of the five isotope profiles is interpreted in the light of its specific environmental data the most significant event observed in all records occurred at 12 11 cal kyr bp an abrupt intensification of summer rainfall associated with an extremely rapid 1000 1500 km westward migration of the monsoon fronts the monsoon regime weakened in stages after 8 7 cal kyr bp holocene maximal aridity is recorded at all sites between about 4 5 and 3 5 cal kyr bp long term changes in monsoon strength as inferred from our isotopic interpretation are in good agreement with climate model reconstructions in the tibet qinghai plateau while observed hydrological changes in north xinjiang are not simulated by the models also the non linearity of the holocene monsoon variations cannot be accounted for by orbital forcing

holocene paleohydrology of playa lakes in northern and central spain a reconstruction based on the mineral composition of lacustrine sediments abstract the focus of this study is the reconstruction of holocene limnic conditions of playa lakes in central spain using inorganic components of their sediments it is demonstrated that the mineralogical sediment characters provide valuable paleoenvironmental information if dolomites originate from authigenic precipitation or early diagenetic dolomitization the mg ca ratio of dolomites derived from the position of dolomite s major x ray diffraction line turns out to be an indicator for the reconstruction of paleosalinity in contrast the concentrations of carbonates and sulphatic evaporites and their changing proportion to siliciclastic sediments point to changing depositional environments the comparison of investigations from different playa lakes and drainage basin systems shows that the interpretation of mineralogical sediment features always has to consider the petrographic hydrologic and geomorphic features of the catchment as for the lacustrine sediments investigated there are no data available yet a preliminary chronological framework is derived by comparison with results from neighbouring areas based on this the hypothesis is put forward that during the so called little ice age subhumid environmental conditions occurred over the central part of the iberian peninsula while today climatic conditions are dry subhumid to subarid also for times older than the subatlantic distinct changes of environmental conditions are noticed but various hiatuses in the sediments make correlation difficult

holocene environmental changes in bangong co basin western tibet part 4 discussion and conclusions abstract a 12 4 m core taken in lake bangong provides a continuous holocene climatic record we summarize information on changes in stable isotope and radiocarbon balances in the lake hydrobiology and vegetal cover in the catchment deduced from detailed analytical results given in the three preceding papers the bangong record is then compared with the environmental history of the neighbouring lake sumxi also constructed from multidisciplinary analyses the two records show a major environmental change at 10 9 5 ka b p attributed to a rapid strengthening of the summer monsoonal circulation which led to wet warm conditions this event was followed by a long term trend toward aridity which culminated around 4 3 ka b p in western tibet maximum monsoon rainfall seems to have occurred from 9 5 to 8 7 ka b p and from 7 2 to 6 3 ka b p as two wet pulses separated by a reversal event centered on 8 0 7 7 ka b p our results broadly agree with the records from lake seling in central tibet and lake qinghai at the plateau s northeastern margin and with palaeoclimatic studies in western china which document conditions wetter and warmer than those of today during the early middle holocene the environmental fluctuations recorded in western tibet appear in phase with climatic changes recognized in tropical north africa suggesting that the 8 0 7 7 ka b p and the 4 0 3 0 ka b p dry events were caused by abrupt disequilibrium in the climatic system

stable isotope and ostracode species assemblage evidence for lake level changes of nam co southern tibet during the past 600 years abstract ostracode species assemblages and stable oxygen and carbon isotopes of shells from two 210 pb dated short cores of 78 cm and 47 cm length taken from lake nam co southern tibet provide information about lake level history of the past approximately 600 years at decadal resolution a transfer function is applied to reconstruct paleo water depths from modern ostracodes ostracode assemblages and isotope data from five species allow a subdivision into phases of lake level evolution before 1600 ad relatively low oxygen isotope values and high abundances of deep water 30 m ostracodes i e leucocythere dorsotuberosa leucocythere dorsotuberosa f postilirata suggest a lake level 10 m higher than today between 1600 ad and 1800 ad more positive oxygen isotope values and higher abundances of shallow water ostracodes i e ilyocypris cf mongolica candona xizangensis suggest a lake level approximately 40 m below modern level between 1800 ad and 1920 ad oxygen isotopes are characterized by a trend to more negative values followed by a trend to more positive ones these trends are interrupted by several short term fluctuations but overall oxygen isotope values suggest a drop in lake level a clearer lake level picture is provided by species assemblages that suggest a continuously increasing lake level since 1900 ad precipitation data from meteorological stations nearby show an increase in precipitation in the last 30 years the low lake level between 1600 ad and 1800 ad is contemporary to the little ice age during that cold period meltwater input into the lake decreased this underlines the sensitivity of the level of lake nam co to temperature changes on the tibetan plateau

internal and external fluid sources for eclogite facies veins in the monviso meta ophiolite western alps implications for fluid flow in subduction zones to contribute to our understanding of the mechanisms and pathways of fluid movement through deeply subducted crust we investigate high pressure veins cutting eclogite facies 2 0g pa and 6008c metagabbros of the monviso ophiolite italian western alps the veins consist mainly of omphacite with minor garnet rutile talc and accessory zircon most of the vein minerals have major and trace element compositions that are comparable with the host rock minerals and vein and host rock zircons have similar hf isotopic compositions these observations support the conclusions of previous studies that these veins largely formed from a locally sourced hydrous fluid during prograde or peak metamorphism however the bulk rock cr and ni contents of the veins are significantly higher than those of the surrounding host eclogites we also document distinct cr rich up to weight per cent levels zones in omphacite garnet and rutile in some vein samples vein garnet and talc also have relatively high mgo and ni contents x ray maps of vein garnet and rutile grains reveal complex internal zoning features which are largely defined by micrometre scale variations in cr content some grains have concentric and oscillatory zoning in cr whereas others feature a chaotic fracture like pattern these cr rich zones are associated with high concentrations of ni b as sb nb zr and high ratios of light rare earth elements lree to middle ree mree compared with low cr vein and host rock minerals petrological and mass balance constraints verify that the cr rich zones in the veins were not derived from internally sourced fluids but represent precipitates from an external fluid the external source that is consistent with the distinctive trace element characteristics of the vein components is antigorite serpentinite which forms the structural basement of the high pressure metagabbros we propose at least two separate growth mechanisms for the monviso veins most vein infillings were formed during progressive prograde metamorphism from locally derived fluid influx of the serpentinitederived or other external fluid was transient and episodic and was probably achieved via brittle fractures which preferentially formed along the pre existing vein structures the dehydration of serpentinite at high pressures in subduction zones may provide crucial volatiles and trace elements for arc magmas our results indicate that the movement of these fluids through subducted oceanic crust is likely to be highly channeled and transient so the progressive development of vein systems in mafic rocks may also be crucial for forming channelways for long distance fluid flow at depth in subduction zones

trace element rich brines in eclogitic veins implications for fluid composition and transport during subduction primary and pseudosecondary fluid inclusions occur in oscillatory and sector zoned omphacite in eclogitic veins from the monviso ophiolitic complex in the western alps the inclusions contain aqueous brines and daughter crystals of halite sylvite calcite dolomite albite anhydrite and or gypsum barite baddeleyite rutile sphene fe oxides pyrite and monazite this daughter mineral suite indicates high solubilites of na k ca mg fe zr ti p ba ce la th and s species and provides direct evidence for transport of high fieldstrenght large ion lithophile and light rare earth elements as dissolved species during subduction fluid inclusion heterogeneities preserved within and between adjacent grains in the veins however suggest that the scale of fluid equilibration was small a crack seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins from these observations we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites small scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow and hence limited metasomatism we conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase at monviso despite evidence for high trace element solubilities in saline brines the elements were not removed from the downgoing slab prior to teaching depths of 40 km

the chronology of the last termination a review of radiocarbon dated high resolution terrestrial stratigraphies abstract the last termination is traditionally subdivided into the oldest dryas bolling older dryas allerod younger dryas and preboreal chronozones this paper briefly summarizes the history of the late weichselian chronostratigraphic system and gives an overview of recent developments in radiocarbon dating terrestrial records between 15 000 and 10 000 14 c years bp problems inherent in the traditional chronostratigraphy are discussed in the light of new and more advanced dating possibilities including accelerator mass spectrometry ams measurements on terrestrial macrofossils new results obtained using ams 14 c dated varve sequences are compared with each other and with the marine u th coral record in addition the importance of time synchronous marker horizons as a tool for long distance correlation between different archives is illustrated

role of late jurassic intra oceanic structural inheritance in the alpine tectonic evolution of the monviso meta ophiolite complex western alps

rise and tilt of metamorphic rocks in the lower plate of a detachment fault in the funeral mountains death valley california the funeral mountains in eastern california preserve a record of early cretaceous metamorphism followed by ductile deformation uplift and low angle normal detachment faulting 40ar 39ar age spectra indicate that cooling and uplift of the lower plate began in cretaceous time uplift was accommodated by normal sense movement along a wide northwest dipping shear zone mylonitic fabrics some of which have been dated as late cretaceous deformed older high temperature metamorphic textures analyses of shear bands mica fish and porphyroclasts grain shape fabrics and folds indicate that the upper surfaces moved toward 299 12 top to the northwest relative to lower surfaces uplift continued until the near present the youngest phase being accommodated by top to the northwest movement along the detachment fault which formed subparallel to lower plate mylonitic fabrics fission track apatite data indicate that exposure of the lower plate to the surface occurred sometime after 6 ma reconstruction along the movement vector places the grapevine mountains over the funeral mountains having been displaced at least 40 km isograds and thermobarometry in pelitic schist from the lower plate indicate increasing pressures and temperatures of equilibration toward the northwest the maximum temperature and pressure was determined on a sample from monarch canyon using thermobarometry 700 c at a depth of 32 km at chloride cliff 5 km southeast of monarch canyon 4 samples yielded 575 600 c at depths of 19 27 km at indian pass 17 km southeast of monarch canyon a temperature of 490 c was determined in the southern funeral mountains about 50 km southeast of monarch canyon conodont color alteration indexes indicate temperatures of 325 425 c these data indicate that the lower plate is presently tilted strongly to the southeast from the orientation it maintained at the peak of metamorphism thermochronologic data k ar on muscovite biotite and hornblende 40ar 39ar on hornblende and fission track on apatite titanite and zircon indicate that both tilting and the transition from ductile to brittle styles of quartz deformation are confined to the interval 21 6 ma during the latter part of this interval 11 6 ma rapid uplift and movement along the detachment fault are documented the findings support current theories of detachment fault evolution in which a dipping fault surface undergoes rotation to a subhorizontal orientation while the lower plate undergoes a comparable tilt

asian summer monsoon variability during the late glacial and holocene inferred from the stable carbon isotope record of black carbon in the sediments of muge co southeastern tibetan plateau china a sediment core from an alpine lake muge co from the southeastern margin of the tibetan plateau was analyzed for sediment grain size distribution and the carbon isotopic composition of black carb

the influence of 14c reservoir age on interpretation of paleolimnological records from the tibetan plateau there is a great deal of controversy regarding the fate of glaciers and ice fields on the tibetan plateau in the face of continued anthropogenic global warming paleoclimate reconstructions and spatial analyses aimed at mapping past climate changes are the key to understanding the climatic response of the tibetan plateau to changing conditions specifically the numerous lakes distributed across the tibetan plateau can provide high resolution spatial and temporal climate reconstructions to investigate past changes in the climate system in this paper we review the primary limitation to exploiting these valuable paleoclimate archives errors in radiocarbon based age models we review the techniques that have been used to estimate 14 c reservoir ages on the tibetan plateau and compile the published 14 c reservoir ages to examine their spatial and temporal patterns and to assess the imposed chronological uncertainties using site specific evaluations of bangong co and lake qinghai we demonstrate that 14 c age model uncertainties permit equally probable and contrasting interpretations of existing paleoclimate records we also examine 14 c induced uncertainties in the spatial climatic response on the tibetan plateau to 1 the termination of the last glacial maximum and 2 the holocene thermal maximum we conclude with recommendations for reducing uncertainties in future lake based paleoclimate studies on the tibetan plateau

opposite vergence of nappes and crustal extension in the french italian western alps in the internal pennine zone of the french italian western alps dora maira monviso rocciavre massifs and queyras region the structural trends define an apparent dome geometry that is truncated to the east by the po basin this geometry is classically interpreted as a backthrusting system the kinematic history of the area has been investigated using structural metamorphic and geochronologic data on the basis of which two main deformation episodes have been distinguished that occurred under differing metamorphic conditions 1 the first deformation occurred under greenschist facies metamorphism it caused a regional scale foliation and an ew trending lineation that lie parallel to ductile deformation zones and thrust contacts sense of shear indicators associated with the lineation indicate contradictory top to the east and top to the west senses of shear from these it is deduced that coeval east and west vergent thrust faulting accommodated major displacements between nappes phengite ar ar data yield late eocene early oligocene ages for the greenschist facies metamorphism 2 the second deformation caused the widespread development of high angle shear bands and normal faults on opposite sides of the dora maira massif resulting in the apparent dome geometry of the massif a steep 50 80 network of west dipping normal faults is particularly well developed on the west side of the dora maira massif differences of 2 3 kbar and 100 150 c between juxtaposed monviso rocciavre and queyras nappes implies 8 km of vertical displacement along the monviso upper tectonic contact these normal faults formed in a more brittle regime only a post early oligocene age can be estimated from microstructural studies for these final movements these deformations postdate hp lt metamorphism and resulted in the juxtaposition of nappes of distinctly different origin age grade and p t t history a kinematic model is presented in which opposite vergence of nappes followed by large scale crustal extension occurred in response to uplift during the convergence between the european and south alpine plates this model may account for rapid uplift rapid unroofing and cooling of high pressure low temperature rocks thereby explaining the absence of amphibolite facies metamorphism overprint in this part of the alps that may have led to the preservation of coesite bearing rocks in the southern part of the dora maira massif

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

hydrological and geomorphological basin and catchment characteristics of lake nam co south central tibet abstract for lake nam co south central tibetan plateau geomorphological features and hydrological characteristics have been compiled to provide an overview of the landscape character information is based on primary data collected during field campaigns in the summers of 2005 and 2006 and secondary data as available from satellite images nam co is located at the intersection of the westerlies the winter monsoon the south west asian monsoon and the east asian monsoon it is situated in a graben structure landforms of the drainage basin are highly influenced by tectonics superimposed by fluvial and periglacial processes and locally by glacial and aeolian processes its drainage basin is endorheic thus lake level is primarily controlled by the balance between input through precipitation or inflow streams and evaporation as the output the significance of lake water evaporation is evidenced by the salinity and measurable increase of 2h and 18o of the lake water when compared to the inflow streams and precipitation maximum conductivities total 431 s mm 1 for the inflowing river waters and 1920 s mm 1 for the lake water overall the solute contents are low and characterise nam co as a pristine environment dominant ions are calcium and bicarbonate in river waters sulphate contents are relatively high in nam co and the lake is also characterised by high boron values a significant decrease in lake water salinity during the past two decades is likely related to increased freshwater input

holocene aeolian mantles and inter bedded paleosols on the southern tibetan plateau abstract on the southern tibetan plateau tp a sediment cover of aeolian sand and silt is widely distributed beyond the basins in an elevation belt above 4000 m asl in this geomorphological environment a continuous aeolian sedimentation occurred during the holocene and erosion processes kept subordinate until late holocene a dark humic paleosol which is dated to between 4100 and 2000 years cal bp divides the aeolian sediment cover into two stratigraphic units the lower unit started with the beginning of the aeolian sedimentation since late glacial and early holocene and extended until mid holocene when the paleosol developed the sediments show the bright color of fresh loess like material and no paleosol representing the early to mid holocene climate optimum is detected the upper sediment unit started after the end of paleosol development and lasted until the late holocene with an increased deposition of re transported loess the enhanced aeolian sediment mobilization during late holocene is provoked by drier and cooler climatic conditions reducing the vegetation density in addition human impact on the vegetation by overgrazing of livestock and forest clearing enforced soil erosion processes and thus the supply of fine material the modern soil and vegetation cover on top of this sediment unit are related to the modern climatic conditions in the semi arid region surrounding the lakes nam co and siling co alpine steppe vegetation and phaeozem soils occur in the southern part which is more humid alpine meadow vegetation with cambisols appears all the paleosols are of cambisol type and therefore represent a more humid period from the upper boundary of mid holocene until the beginning of late holocene because of higher availability of effective moisture for the growth of dense vegetation cover and appropriate paleosol formation for this period of paleosol development a general trend from north to south is obvious from 5 5 to 3 8 ka bp in the northeastern tp to between 4 1 and 2 ka bp in the southern tp this coincides with the observed retreat of monsoonal precipitation on the tp during that time in contrast to the arid and semi arid environment of the tp the amount of precipitation is not the primary limiting factor for vegetation density and soil development but temperature and solar radiation have a great influence on local evapotranspiration and effective moisture

linking of regional and local hydrothermal systems in the mid crust by shearing and faulting abstract the concentration of mass during regional metamorphism and metallogenesis requires interactions between regional and local hydrothermal systems that are scale and time dependent during regional low to medium temperature metamorphism in which fluid flow is driven by broad thermal or mechanical gradients fluid flow rates are on average 8 m s and are limited by restricted permeabilities and gentle gradients in hydraulic head in contrast coseismic fluid flow rates in major fault zones may reach 10 2 m s as a consequence there may be a limited capacity for individual faults to substantially control fluid flow in the region because of the large contrast in flow rates and permeabilities between faults and low permeability rocks to produce a major epigenetic ore deposit in overpressured crust fluid flow systems with such diverse flow rates must interact in such a way that metal dissolution and transport from regional fluid flow systems culminate in focused fluid flow with metal deposition this is best achieved through two steps one in which km scale hydrothermal systems channel into 100 m scale ductile shear zones largely by reaction and deformation enhanced permeability in the shear zone and a second one in which rapid fluid pressure changes accompanying brittle faulting or intense shearing within the broader shear zones causes metal deposition and ore genesis

isotope evidence for pleistocene climatic changes in kashmir india stable carbon isotope ratios of organic carbon from palaeosols in loess and oxygen isotope ratios of pedogenic and lacustrine carbonates reveal significant pleistocene climatic changes in the kashmir basin the extreme 13c values of 16 and 25 reported here indicate a shift from arid to sub humid conditions the 18o values of pedogenic carbonates is 2 higher than that of the underlying lacustrine carbonates suggesting that they were deposited in different climatic conditions

mid holocene palaeohydrochemistry and palaeohydrology of yamdrok yumtso southern tibetan plateau reconstructed from δ18o and δ13c of fossil shells of the gastropod radix auricularia palaeohydrochemical and palaeohydrological changes of lakes have seldom been reconstructed from the fossil shells of the gasropod radix auricularia which is a new and potentially high resolution

holocene environmental changes in lake bangong basin western tibet part 1 chronology and stable isotopes of carbonates of a holocene lacustrine core abstract a 12 4 m core collected from the eastern part of lake bangong is analyzed for mineralogy radiometric chronology and stable isotope contents of authigenic inorganic carbonate isotopic compositions of the modern lake waters and of its major tributaries are first presented radiocarbon activity of these waters show that equilibrium with the atmospheric co2 is not reached because of admixture of dead carbon in solution in the system a model is thus developed to correct the radiocarbon ages of fossil material of aquatic origin for this ageing the core provides a continuous holocene record changes in mineralogy of bulk sediments and in 13c and 18o contents of authigenic carbonates reveal extremely large changes in environmental conditions induced by climate changes and or local hydrological factors e g closure opening of the lake system controlled by the altitude of the outflow sill the eastern basin of lake bangong was closed before 9 6 and after 1 2 ka b p in response to aridity on the catchment wet warm conditions attributed to the monsoon influence were established suddenly at 9 6 ka b p and maintained until 6 2 ka b p although minor reversal events occurred between 8 6 and 7 7 ka b p the return towards aridity is non linear dry spells at 6 2 3 9 3 2 and 1 3 ka b p are alternated with short term periods with positive precipitation evaporation balance

hydraulic and acoustic properties as a function of porosity in fontainebleau sandstone laboratory measurements have been made of the permeability k free porosity l compressional velocities vp or ve and compressional attenuations qp or qe in fontainebleau sandstone over a continuous range of porosities from 3 to 28 this large variation was achieved without any composition change fontainebleau sandstone is made of fine quartz grains with regular grain size 250 m permeability was measured with a falling head permeameter velocities and attenuations were obtained either through an ultrasonic experiment for frequencies around 500 khz or through a resonant bar technique experiment for frequencies around 5 khz and in both cases with varying water saturation the results show an excellent correlation between permeability k and total porosity for all our samples for low porosities 3 to 9 permeability in millidarcies is 2 75 10 5 7 33 while for high porosities 9 to 28 permeability k in millidarcies is given by 0 303 3 05 the correlation is also excellent between free porosity and total porosity on the other hand the correlation between acoustic properties and total porosity is not as clear as for hydraulic properties whatever the frequency 500 khz or 5 khz or the water saturation on the average velocity decreases and attenuation roughly increases with increasing total porosity velocity and attenuation values are related to the variation of grain contact structure and two samples with the same porosity and permeability may exhibit different velocities and attenuations the clear correlation between hydraulic properties and porosity is related to constant grain size while the lack of correlation for acoustic properties emphasizes the importance of the microstructure

a comparative study of diffusion profiles in himalayan and dalradian garnets constraints on diffusion data and the relative duration of the metamorphic events cation diffusion rates at 690 30 c have been calculated by inverse modelling of observed manganese mn zonation profiles in 40 garnets from two kyanite bearing metapelite samples from the high himalayan crystalline series zanskar northwest india knowledge of the initial growth profile of mn in garnet is a pre requisite for this technique following previous workers we model mn partitioning into growing garnet in terms of a rayleigh fractionation process and demonstrate that the mngarnet whole rock partition coefficient is 60 100 three dimensional zonation profiles were obtained by successively grinding and polishing 1 cm slabs of each sample at 0 1 0 2 mm intervals and analysing the garnets at each stage thus ensuring that core sections were measured the diffusion model assumes that garnet has a spherical geometry and behaves as a closed system and simulates diffusive modification of the hypothetical mn rayleigh growth profile for each garnet the derived measure of the time integrated diffusion history for each garnet is then combined with radiometric and field relation constraints for the duration of the himalayan metamorphic event to calculate cation diffusion rates the average cation interdiffusion rate calculated for garnets in the two samples examined is 6 3 2 10 23 m2s 1 this interdiffusion rate pertains to a temperature of 690 30 c which is 0 97 t peak the peak temperature conditions experienced by the samples estimated using standard thermobarometric techniques garnet compositions are py2 17alm65 77gro6 16sp1 17 these new diffusion data are consistent with and more precise than existing high temperature 1000 c experimentally determined diffusion data although some uncertainties remain difficult to constrain qualitative comparison between diffusively modified mn growth profiles in garnets from the scottish dalradian and the himalayan garnets suggests that the duration of metamorphism affecting the dalradian garnets was 10 20 times longer than that endured by the himalayan garnets

palynological and stable isotopic study of palaeoenvironmental changes on the northeastern tibetan plateau in the last 30 000 years abstract the tibetan plateau is important in influencing the atmospheric circulation of the northern hemisphere however only a few continuous palaeoclimate records are available here we present a 30 000 year pollen and carbon stable isotope record of two lacustrine sections from the zoige plateau on the northeastern tibetan plateau the pollen spectra show that the territorial palaeovegetational evolution experienced 7 stages in the following order 1 30 0 26 0 kyr b p alpine desert and desert grassland vegetation 2 26 0 18 0 kyr b p grassland and alpine meadow vegetation 3 18 0 14 2 kyr b p alpine desert 4 14 2 10 0 kyr b p grasslands and meadows with islet forests 5 10 0 9 4 kyr b p marsh and meadow vegetation 6 9 4 4 0 kyr b p dark conifer islet and marsh and finally 7 4 0 kyr b p present marsh and subalpine meadows with scattered islets of dark conifer the palaeoclimate was inferred from these variations in palaeovegetation and in particular the variation in the last 12 000 years was confirmed by the 13 c curve of the no 2 pit section major post glacial climatic events are clearly identified in the two study sequences including the late glacial series of bolling allerod old dryas and younger dryas and the holocene optimum the climate records in the two sequences also indicate a long term trend towards aridity in the study area

chemical microstructural changes in eclogite facies shear zones monviso western alps north italy as indicators of strain history and the mechanism and scale of mass transfer abstract variations of eclogite facies mineral chemistry are related to the textural characteristics of shear zones in fe ti metagabbros from the monviso meta ophiolitic complex in the italian western alps three different kinds of microstructural domains are described 1 small lenses of low strain rocks with preserved igneous textures 2 larger high strain domains where deformation caused pervasive mylonitic foliation s1 and a stretching mineral lineation l1 and 3 two types of eclogite facies veins the latter can be subdivided into veins composed of only na pyroxenes filling small tension gashes and dilatation fractures and veins composed of a na pyroxene garnet rutile apatite assemblage occupying large fractures that randomly cross cut the mylonites in low strain domains and mylonites the na pyroxenes are omphacites and the garnets are almandine rich in some mylonite layers containing phengite and blue amphibole crosscutting s1 chloromelanite iron rich omphacites exsolved from omphacite in type 1 veins the pyroxene is either omphacite or concentric and sector zoned omphacite chloromelanite in type 2 veins pyroxenes are omphacite and or concentric and sector zoned omphacite chloromelanite and or zoned kosmochlor chromium rich omphacite omphacites associated garnets are richer in pyrope component than those from low and high strain domains possible origins of both sector and concentric zoning and exsolution are discussed it is emphasized that a miscibility gap between the omphacite and acmite end members partly controlled the omphacite chloromelanite distribution we define a chemical vector coat constrains the strain history in eclogite facies rocks and give indications of the scale and mechanism of mass transfer involved during foliation development and fracturing p t conditions are calculated for each microstructural domain the garnet pyroxene thermometer yields scattered temperature values these values relate to the dependence of temperature estimates on the amount of fe3 in the pyroxene and possibly on non ideal substitution in omphacite acmite solid solution

coronitic reactions and ductile shear zones in eclogitised ophiolite metagabbro western alps north italy abstract the objective of this work is to study the most significant static and syn tectonic transformations which accompanied the eclogite metamorphism 450 50 c 12 15 kbar of an ophiolite metagabbro from the western alps in partly undeformed volumes abundant igneous mineral relics occur and the eclogite metamorphism produced coronitic or incomplete pseudomorphic reactions the disequilibrium or local equilibrium conditions implied by these features suggest that slow diffusion and interface kinetics were the rate limiting factors which prevented a large scale eclogite equilibration the undeformed volumes are bounded by syn eclogite ductile shear zones where the magmatic minerals and textures are obliterated and a new fabric foliation develops the enlargement of equilibrium domains observed in the shear zones indicates that a close approach to eclogite equilibration of these metagabbros was not attained without the catalytic effect of pervasive deformation the bulk rock chemistry also played an important role in determining the kind of eclogite assemblage in particular only sheared gabbros rich in fe can produce bimineralic eclogites

the history and variability of the east asian paleomonsoon climate abstract changes in the east asian paleomonsoon reflect interactions between the global atmosphere ocean land and ice systems and are also an expression of their combined effect within the boundary conditions imposed by the east asian continent and solar radiation the history of the east asian monsoon is an alternation between dominance by the dry cold winter and warm humid summer monsoons high resolution eolian sequences preserved in the chinese loess plateau reveal that the east asian monsoon may have commenced at least 7 2 ma ago they also provide evidence indicating that the pulsed uplift of the tibetan plateau at about 3 4 and 7 2 ma may have played an important role in inducing climate change the palaeoclimatic records of the last glacial cycle show high frequency variability on time scales of 1000 year or even shorter and instability of the east asian paleomonsoon system the high frequency variability could be due to a non linear response to orbital forcing or a result of the coupling processes between different components of the global system cold air activity in northern high latitudes the trans equatorial air streams from the southern hemisphere and possibly enso may have played an important role in east asian monsoon variability the synchroneity of all the palaeoclimatic events along the polar equator polar pep transect is still an open question correlation of limited palaeoclimatic records for the last 30 kyr obtained from east asia and australia suggests that the trans equatorial air streams driven by the monsoon and trade winds may have had an influence on opposite hemispheres

trace element and isotopic zoning in minerals models of compositional fractionation by mineral separation procedures abstract data on compositional zoning of minerals are required to apply thermodynamic models to mineral chemistry and to constrain kinetic models of diffusion and grain growth trace elements and isotopes have not been utilized fully in these applications because in situ methods of analysis e g sims and pixe have not become routine trace element and isotopic compositions of minerals are usually determined from mineral separates however routine mineral separation procedures are not designed to detect and characterize zoning mineral separates are formed by segregating simple monomineralic particles of a mineral from other minerals and compound particles models of the formation of simple and compound particles during rock comminution show how the size and shape of mineral grains and the size of the particles formed affect the constitution of the separates the interior portions of minerals are strongly over represented in mineral separates while the margins of the grains are preferentially included in compound particles the particle size grain size ratio is the dominant factor in the segregation of the rims of minerals from the cores the particle size grain size ratio can be controlled through regulation of rock comminution and sieving procedures separation protocols are suggested to gain information about the nature of chemical and isotopic heterogeneities in minerals in particular chemical and isotopic effects associated with thin zones near grain boundaries can be investigated effectively

effects of porosity and clay content on wave velocities in sandstones the ultrasonic compressional vp and shear vs velocities and first arrival peak amplitude ap were measured as functions of differential pressure to 50 mpa and to a state of saturation on 75 different sandstone samples with porosities ranging from 2 to 30 percent and volume clay content c ranging from 0 to 50 percent respectively both vp and vs were found to correlate linearly with porosity and clay content in shaly sandstones at confining pressure of 40 mpa and pore pressure of 1 0 mpa the best least squares fits to the velocity data are vp km s 5 59 6 93 2 18c and vs km s 3 52 4 91 1 89c deviations from these equations are less than 3 percent and 5 percent for vp and vs respectively the velocities of clean sandstones are significantly higher than those predicted by the above linear fits about 7 percent for vp and 11 percent for vs which indicates that a very small amount of clay 1 or a few percent of volume fraction significantly reduces the elastic moduli of sandstones for shaly

potential forcing mechanisms of holocene lake level changes at nam co tibetan plateau inferred from the stable isotopic composition of shells of the gastropod radix the timing of lake level fluctuations on the tibetan plateau and their relationship with climatic changes is still under debate and the main reason for this is the lack of suitable archives for reconstructing the paleohydrology and paleoclimatology of the lakes here we present the results of analyses of the shell geochemistry of radix sp from an exposed terrace of nam co lake on the south central tibetan plateau optically stimulated luminescence osl dating reveals that deep water lacustrine sediments formed between ca 4 4 and 2 2 ka suggesting a high and stable lake level significantly above the present the results of sr ca 13c and 18o analyses of the fossil shells of radix sp indicate that during the mid to late holocene lake level variations in nam co were mainly controlled by variations in the indian summer monsoon a trend of decreasing evaporation also played an important role comparison with other results suggests a consistent pattern of mid to late holocene lake level changes acro

last glacial maximum and holocene lake levels of owens lake eastern california usa abstract stratigraphic investigations of fluvio deltaic and lacustrine sediments exposed in stream cuts quarry walls and deep trenches east of the sierra nevada in owens valley near lone pine california have enabled the reconstruction of pluvial owens lake level oscillations age control for these sediments is from 22 radiocarbon 14 c dates and the identification and stratigraphic correlation of a tephra which when plotted as a function of age versus altitude define numerous oscillations in the level of pluvial owens lake during the latest pleistocene and early holocene we have constructed a lake level altitude curve for the time interval 27 000 cal yr bp to present that is based on the integration of this new stratigraphic analysis with published surface stratigraphic data and subsurface core data pluvial owens lake regressed from its latest pleistocene highstands from 27 000 to 15 300 cal yr bp as recorded by 15 m of down cutting of the sill from the altitudes of 1160 to 1145 m by 11 600 cal yr bp the lake had dropped 45 m from the 1145 m sill this lowstand was followed by an early holocene transgression that attained a highstand near 1135 m before dropping to 1120 m at 7860 7650 cal yr bp that had not been recognized in earlier studies the lake then lowered another 30 m to shallow and near desiccation levels between 6850 and 4300 cal yr bp fluvial cut and fill relations north of lone pine and well preserved shoreline features at 1108 m indicate a minor lake level rise after 4300 cal yr bp followed by alkaline and shallow conditions during the latest holocene the new latest quaternary lake level record of pluvial owens lake offers insight to the hydrologic balance along the east side of the southern sierra nevada and will assist regional paleoclimatic models for the western basin and range

late quaternary hydrological changes inferred from lake level fluctuations of nam co tibetan plateau china abstract this paper discusses the range of lake level changes using hydro acoustic techniques and sediment core data for lake nam co in central tibet china this second largest tibetan lake is situated at an elevation of 4718 m a s l lake level changes with different high stands documented by beach remnants are widespread on the tibetan plateau and also in lake nam co they indicate significant changes in the hydrological system triggered by the monsoonal circulation using hydro acoustic techniques submerged beach remains at approximately 10 15 m 30 m 45 m 70 m and 95 m below present day lake level were detected indicating a more negative water balance than today the 10 15 m lake level is dated to an age equal to the little ice age all deeper lake levels are assumed to have been formed prior to 10000 cal bp folded older sediments detected in the hydro acoustic profiles indicate a possible larger extent of the glaciers during or prior to the last glacial maximum lgm this is followed by a low stand or even by possible desiccation of the entire lake basin during or after the lgm

variation in amphibole and plagioclase composition with deformation investigation of the mineral chemistry of a shear zone in an amphibolite facies metagabbro has shown that the composition of the amphibole and plagioclase varies with deformation chemical analyses of the rocks indicate that the shear zone approximates an isochemical system the amphibole varies progressively from an initial magnesio hornblende to a ferroan pargasitic hornblende in the shear zone alkalies ti and fe2 fe2 mg increasing while the plagioclase becomes more anorthitic varying from labradorite to bytownite these changes in composition of amphibole and plagioclase from metabasic rocks are normally associated with increasing temperature in prograde regional metamorphic terrains such chemical variations observed in localized shear zones would suggest that shearing stress in regional metamorphism may play an important role in producing the documented discrepancies in amphibole chemistry between different metamorphic belts

age determination of petra s engineered landscape optically stimulated luminescence osl and radiocarbon ages of runoff terrace systems in the eastern highlands of jordan abstract the unfavourable mountainous environment of the petra region in southern jordan was modified by ancient engineers to supply the nabataean roman city of petra with food and water the area was reclaimed by installing extended runoff terrace systems and hydraulic structures the agricultural terrace systems have so far been dated based on surface pottery and the chronology of the systems is under debate in this study optically stimulated luminescence osl and radiocarbon dating techniques were successfully applied to date these terrace systems samples were taken from the fills of agricultural terraces and underneath their walls to determine the chronology of the construction use and abandonment of the agricultural terraces the results suggest that runoff farming in the petra region started around the beginning of the common era and construction use and maintenance lasted at least until 800 ad

deformation and metamorphic history of the western blue ridge eastern blue ridge terrane boundary southern appalachian orogen 1 the western eastern blue ridge boundary involved formation by premetamorphic thrusting of an oceanic fragment accretionary wedge island arc upon laurentian basement and metasedimentary cover detailed outcrop scale mapping structural and petrographic analysis reveal both terranes experienced the same deformation at the same metamorphic grade early f1 and f2 isoclinal folds are overprinted by f3 open to tight folds f3 transposes s2 to an axial planar cleavage s3 the youngest penetrative feature prekinematic to synkinematic relative to s2 barrovian metamorphism attained kyanite grade in the western blue ridge wbr upper amphibolite facies in the eastern blue ridge ebr and anatexis in both terranes postkinematic m2b is characterized by appearance of fibrolite throughout the kyanite zone coarsening to granular sillimanite and identification of a kyanite sillimanite isograd in the wbr d3 strain was accommodated by dynamic recrystallization of quartz neocrystallization folding and basal slip of micas pressure solution of plagioclase folding of kyanite and fracturing of garnet scattered lower greenschist facies m4 static retrograde metamorphism resulted in replacement of kyanite and garnet by sericite and chlorite respectively although the juxtaposition of two lithologically distinct terranes implies a fault relationship polymetamorphism and deformation obliterated any microfabrics mineral assemblages or structures that might have developed during premetamorphic thrusting the boundary cannot be an extension of a continent scale dextral strike slip fault system inferred to have developed along laurentian margin in the devonian

ebsd measured lattice preferred orientations and seismic properties of eclogites we investigated the deformation mechanisms and the seismic properties of 10 eclogite samples from different localities alps norway mali and eastern china through the analysis of their microstructures and lattice preferred orientations lpo these samples are representative of various types and intensity of deformation under eclogitic metamorphic conditions omphacite and garnet lpo were determined from electron backscatter diffraction ebsd technique garnet appears to be almost randomly oriented whereas omphacite develops strong lpo characterized by the 001 axes concentrated sub parallel to the lineation and the 010 poles concentrated sub perpendicular to the foliation in order to analyze the deformation mechanisms that produced such omphacite lpo we compare our observations to lpo simulated by viscoplastic self consistent numerical models a good fit to the measured lpo is obtained for models in which the dominant slip systems are 1 2h110i 11 0 001 110 and 001 100 dominant activation of these slip systems is in agreement with tem studies of naturally deformed omphacite seismic properties of eclogite are calculated by combining the measured lpo and the single crystal elastic constants of omphacite and garnet although eclogite seismic anisotropies are very weak less than 3 for both p and s wave they are generally characterized by a maximum p wave velocity sub parallel to the lineation and by a minimum velocity approximately normal to foliation the mean p and s wave velocities are high respectively 8 6 and 4 9 km s the s wave anisotropy pattern displays complex relationships with the structural frame but the fast polarization plane generally tends to be parallel to the foliation calculated reflection coefficients show that an eclogite crust interface is generally a good reflector rc 0 1 whereas an eclogite body embedded in the upper mantle would be hardly detectable

oxygen and carbon isotopic constraints on the development of eclogites holsnoy norway abstract eclogite formation on the island of holsnoy required the addition of water to anhydrous granulite facies protoliths in order to assess this process oxygen and carbon isotope ratios of whole rock powders and mineral separates from eclogites and granulites have been measured whole rock oxygen isotope ratios range from 7 3 to 6 0 smow in granulites average 6 38 and 7 2 to 6 1 in eclogites average 6 55 field relations permit identification of the granulite protolith of eclogites oxygen isotope measurements show shifts of up to 0 5 between some eclogites compared to their corresponding granulite protoliths indicating open system and locally heterogeneous fluid behavior mineral pair fractionations in the eclogites show disequilibrium are incompatible with slow cooling and diffusive exchange between phases and suggest that open system fluid movement continued after eclogite facies metamorphism carbonate is also present in some of the eclogites as a primary mineral dolomite and as part of a retrograde assemblage calcite textural evidence suggests that carbonate formation occurred during and after eclogite formation however all measured carbonate is out of isotopic equilibrium with eclogite facies minerals due to the influx of retrograde fluids massive calcite marble pods containing amphibolite facies cale silicate minerals have average 18 o of 9 5 0 6 while calcite in retrograded eclogites has 18 o 17 7 2 7 the 13 c 4 0 8 is indistinguishable between these two groups both whole rock and carbonate stable isotope data are interpreted as indicating a continued history of fluid infiltration during and after peak eclogite facies metamorphism the most probable source of fluids are from dewatered sediments tectonically juxtaposed during the caledonian orogeny

lake evolution during the past 30 000 years in china and its implications for environmental change abstract taking china as a whole lake expansion occurred 500 b c 0 a d 650 950 a d and 1250 1650 a d over the past 3000 yr the expansions were mainly due to climatic fluctuations and correspond to frequent flood reports three regional characteristics of lake development have been recognized 1 fluctuations of near coastal lakes and those in other regions as well were out of phase during 500 b c 500 a d which possibly resulted from out of phase fluctuations of climate and sea level on 10 to 100 yr time scale 2 as the lower yellow river channel migrated southward the lakes in the south of the north china plain formed and reached their largest sizes much later than those in the north 3 for the lakes in the middle and lower yangtze river valley the higher a lake basin lies above a neighboring river and the further the lake is from the river s mouth in general the younger the lake is through land reclamation the ancient chinese have changed most of the lakes greatly however human activities were also constrained in certain ways by the natural changes of the lakes reclamation usually was banned and farmland was abandoned to repair reservoirs while lakes were expanding

lake qinghai china closed basin like levels and the oxygen isotope record for ostracoda since the latest pleistocene abstract in 1985 a sino swiss limnogeological expedition to china s largest inland lake recovered seismic profiles and piston cores to help calibrate palaeoclimatic models of monsoon and insolation fluctuations over the northern tibet qinghai plateau since the last glacial maximum lake qinghai a perennial lake with an area of 4278 km2 1985 at 3194 m above sea level receives drainage from 30 000 km2 of catchment near the northwest corner of the plateau the 26 5 m deep terminal lake 1985 with brackish alkaline magnesium sulphate rich waters 14 g 1 1 tds has not overflowed since the last glacial but its levels have responded to climatic and hydrologic changes seismic reflection profiles for the sub bottom sediment structures revealed a complex topography with channels relict terraces tectonic tilting and deformation in deposits buried under a 4 6 m packet of mainly flat bedded sediments from which core samples were recovered the lowermost cored material 14 500 yr b p comprises mainly water lain loess deposited in a smaller flood plain lake of up a few metres depth under cold arid climatic conditions ams radiocarbon dating indicates average sedimentation rates for latest pleistocene and holocene deposits in central basin regions of 0 3 0 5 mm yr 1 the 18o values of fossil ostracod valves provide a record of changes in the isotopic chemistry of the lake waters that can be related to the precipitation evaporation budgets which have controlled the palaeo levels for lake qinghai the precipitation evaporation budget for the lake qinghai region is largely determined by the asian monsoon system the lake qinghai sediment record shows that there was no large influx of glacoer melt waters into the lake basin during the post glacial the regional climate remained arid until at least 12 500 yr b p when an incipient monsoon was possibly initiated river transported silts first reached the basin centre at 11 800 yr b p when the lake depth may have increased to 5 m possibly a monsoonal weakening for several centuries after 10 800 yr b p europe younger dryas caused a further period of aridity at 10 200 yr b p the basin began filling towards modern lake dimensions in response to a permanent strengthening of the asian monsoon substantial lake level rises probably characterised the periods 10 200 9800 9500 8500 8300 7200 yr b p and through perhaps a century at about 6800 yr b p decreasing levels probably characterised the intervening intervals short term lake level fluctuations through the mid holocene climatic optimum were minimal indicating a very stable climatic hydrologic regime an isotopic steady state for the lake waters since 3000 500 yr b p obscures the climatic hydrologic regime dependent 18o gradient but there is little stratigraphic evidence to suggest that the lake level has been significantly below its present level since that time three negative 18o excursions for the lake waters may correspond to known glacier advances on the plateau at 3000 2000 and 1500 yr b p during china s neoglacial

chemical weathering since the little ice age recorded in lake sediments a high resolution proxy of past climate because of the different geochemical behaviour of rubidium and strontium in earth surface processes variations of the rb sr ratios in lake sediments were used as a geochemical proxy of chemical weathering and past climate in a single watershed low magnetic susceptibility low caco3 low sr concentration and hence high rb sr ratio in the lake sediments indicate weak chemical weathering under a cold but wet climate during the little ice age lia in the closed daihai lake watershed the concordant change in both sr and caco3 concentrations with 18o values in the dunde ice core suggests that weak chemical weathering during the wet lia was controlled by air temperature after the lia however precipitation played a dominant role in chemical weathering copyright 2001 john wiley sons ltd

microstructural criteria for reliable thermometry in low temperature eclogites abstract two alpine eclogite bodies sampled in metaophiolitic complexes of the piemonte zone have been studied for geothermometry it is demonstrated that temperature estimates by both exchange thermometry and sizes of antiphase domains in omphacites give rise to erratic results when the defect structures of the minerals are not taken into account microstructural criteria by means of both optical and tem microscopy for reliable thermometry in low temperature eclogites are disscussed the dynamically recrystallized grains are the more suitable sites for the application of exchange thermometry while only low strained omphacites can be used for thermometry based on apd s

annually laminated lake sediments and environmental changes in bashang plateau north china abstract angulinuo lake is the biggest lake on the bashang plateau north china and is 47 6 km2 in area and 2 6 m in depth a core from the inner part of angulinuo lake was sliced and the sediment was observed by scanning electronic microscope sem annual laminations characterized by variable color and grain size were found and interpreted as recording the cyclic deposition of lacustrine clay and aeolian dust the results of chemical analysis of coarse grains conducted by sem edax and grain size analysis of modern aeolian dust in the ice on angulinuo lake support an aeolian origin for the light coarse layers image analysis technique was used to calculate the size and number of coarse grains in each layer the coarse grains were fractionated into four classes 42 m 14 42 m 14 4 2 m and 1 4 4 2 m in general the abundance of the four classes shows similar temporal variation patterns around angulinuo lake the winter monsoon is strong and transports aeolian dust into the lake when the winter monsoon is strong the size and amount of coarse grains are expected to increase we infer that the winter monsoon was weaker during 8430 5440 year bp and was unstable in the later part of this period from 5440 year bp the winter monsoon became stronger and then weaker from 3250 to 2490 year bp during 2490 1170 year bp the winter monsoon was slightly stronger but since 1170 year bp it has become weaker again the changes of the winter monsoon intensity recorded in the annual laminations in angulinuo lake sediments correspond well to environmental changes in north china and to changes in sea level during the same period periods of weaker winter monsoon correspond to times of higher sea levels while the periods of stronger winter monsoon correspond to the neoglaciation stage in china and the periods of lower sea levels

mineralogy and geochemistry of the holocene lacustrine sediments in nam co tibet abstract the carbonates clays and major chemical compositions of lacustrine sediments in nam co lake were examined by x ray diffraction scanning electron microscopy and chemical analysis carbonates include monohydrocalcite mhc first report from china and in a high altitude lake low mg calcite and traces of dolomite mhc in nam co is developed in water 1 8 g l with high mg ca molar ratios 10 03 15 03 high ph 8 04 9 72 and the presence of bacteria algae diatom and ostracoda illite and mg chlorite provide a strong evidence for physical weathering in the holocene most ca and sr in sediments originate from carbonates as the molar ratios of ca and co32 are all less than 1 and the curve of sr is very similar to that of ca however most of the mg fe and rb are from clays the lake water shifted from a fresh water environment to an evaporative alkaline environment by 2 06 cal ka bp there was a depositional event that the depositional rate changed from 0 134 to 1 639 mm a at about 2 cal ka bp

the metamorphic architecture of a transpressional orogen the kaoko belt namibia s 64 61 goscombe b hand m 2001b metamorphic model for the inland branch of the damara orogen a paired metamorphic mountain belt pmmb geological society of australia specialist group in tectonics and structural geology conference ulverstone

wave velocities in sandstones from elastic network simulations an elastic network model is used to calculate compressional and shear wave velocities in a model sandstone the model consists of mineral components and pores distributed at random on a regular cubic lattice springs are used to represent compressional shear and rotational forces between neighboring grains we have used the network model to calculate the dependence of wave velocities on porosity and clay content in sandstones the calculated velocities are a linear function of both porosity and total clay content the coefficients in this bilinear relationship are in quantitative agreement with published experimental correlations for sandstone velocities this result provides theoretical support for the linear velocity porosity clay content relationship commonly assumed in empirical correlations the rotational forces between grains are necessary to obtain quantitative agreement with experimental data wave velocities calculated using the continuum self consistent imbedding approximation agree with the elastic network for quartz water mixtures but not for quartz clay mixtures this disagreement demonstrates the importance of a granular elastic model with rotational forces for calculating velocities in rocks with multiple mineral components

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

pollen inferred climate changes and vertical shifts of alpine vegetation belts on the northern slope of the nyainqentanglha mountains central tibetan plateau since 8 4 kyr bp fossil pollen from nam co and modern pollen from altitudinal vegetation belts around the lake are investigated to reveal alpine vegetation succession in response to climate changes during the holoc

petrological constraints on the eclogite and blueschistfacies metamorphism and p t t paths in the western alps phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high p rocks in the nappes of the western alps very high p eclogite facies including kyanite eclogites and coesite pyrope assemblages eclogite facies paragonite zoisite eclogites high t blueschist facies glaucophane garnet lawsonite assemblages and low t blueschist facies glaucophane lawsonite pumpellyite assemblages the blueschist facies eclogite facies transition is promoted chiefly by increasing t low bulk xmg and relatively low h2o the variety of assemblages and the heterogeneous approach to equilibrium observed in the alpine rocks are not only constrained by the intersection of the reaction surfaces in p t x space but also by the effect and timing of the processes which control kinetics i e pervasive deformation and fluid infiltration the faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures and times all may have induced h2o gradients and contributed to the heterogeneous distribution of deformation through a process of reaction enhanced ductility also mass transfer may have been an effective process in determining the type of high p assemblage in particular rock volumes as regards the p t t paths only the post climax histories are recorded well in the alpine nappes post eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early alpine cretaceous event in contrast more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early alpine event and the subsequent meso alpine palaeogene collision between the european and african plates in the western alps prevalent eclogite facies conditions were attained during the metamorphic climax of the early alpine subduction while blueschist facies recrystallization characterizes the early alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso alpine continental collision

coexisting chromian omphacite and diopside in tremolite schist from the chugoku mountains sw japan the effect of cr on the omphacite diopside immiscibility gap chromian clinopyroxenes with exsolution textures were found in high pressure hp tremolite schist from the osayama serpentinite melange in the chugoku mountains southwestern japan chromian omphacite jadeite jd 23 3 41 4 diopside di 46 7 60 7 kosmochlor ko 4 5 19 6 aegirine ae with up to 6 6 wt cr 2 o 3 occurs as neoblastic crystals in a foliated tremolite rich matrix or as pseudomorphs after relict chromian spinel and contains irregular shaped thin lamellae of chromian diopside jd 3 8 18 2 di 72 5 93 6 ko 1 1 13 9 ae chromian omphacite contains roughly constant jadeite plus kosmochlor components at 38 2 51 6 mol this is equivalent to the jadeite component of ordered p 2 n omphacite systematic analyses of coexisting chromian pyroxenes yield a clear immiscibility gap between omphacite and diopside the compositional gap becomes much narrower with increasing ko component addition of only 10 mol ko component narrows the omphacite diopside gap by an order of magnitude such an effect is similar to but more effective than the introduction of fe 3 on the omphacite diopside immiscibility gap chromian pyroxenes replacing relict chromian spinel are associated with other chromian silicates including phengite chlorite and pumpellyite the wide compositional gap of chromian pyroxenes and the presence of chromian pumpellyite and si rich chromian phengite indicate t p 0 8 gpa this p t estimate is consistent with parageneses of minerals in the host serpentinite the variation of cr content in chromian silicates reflects the extent of cr al substitution and may be related to a chemical heterogeneity of the cr bearing fluid

an ostracod inferred large middle pleistocene freshwater lake in the presently hyper arid qaidam basin nw china abstract exposed sediments of 50 m thickness were investigated in the 120 000 km2 qaidam basin at the northern margin of the tibetan plateau fine grained silty sediments constitute most of the section and were accumulated in a lake intercalated beds of aeolian sand and of fluvial or littoral origin show that the lake experienced periods of desiccation and shallow water levels at the section site the lake sediments provided surprisingly species rich ostracod assemblages with almost all species indicating freshwater to oligohaline conditions the species richness the position of the section in the southern neighbourhood of a topographic rise in the qaidam basin and intercalated layers of fluvial and aeolian sands suggest that a relatively marginal littoral lake setting existed throughout most of the sequence a maximum water depth of several tens of metres may be assumed at least for some parts of the section and a large lake covering about half the size of the qaidam basin may be inferred from our data this inference is in clear contrast to the present day wind shaped topography with salt playas and shallow saline lakes in the lowermost depressions of the qaidam basin dating by optically stimulated luminescence and uranium series measurements did not allow the establishment of a firm chronology for the investigated section however based on infrared stimulated luminescence dates of around 120 140 ka obtained on feldspars and regarded as minimum ages and 230th u ages scattering between 100 and 400 ka a middle pleistocene age between 120 and 400 ka is assumed for the lake formation

late quaternary transition from lacustrine to a fluvio lacustrine environment in the north western nam co tibetan plateau china abstract holocene sediments from a 9 m high outcrop at the gangyasang qu s entry into lake nam co on the south eastern tibetan plateau were studied in a multiproxy approach the lowermost part of the section is of early holocene age and consists of varved sediments deposited in a proglacial lake geomorphological investigations of fossil beach ridges and fossil cliff lines in the littoral zone of nam co indicate a late pleistocene lake level high of 29 m above the present level a warming is visible in ostracode rich silts deposited after 9 6 ka bp an ostracode based transfer function estimates a lake level 28 m higher than today from this time until about 5 2 ka bp a late pleistocene lake level high was accompanied by an increase in lake size causing nam co to overflow into the lower course of gangyasang qu where a bay with slack water conditions formed between 4 2 and 1 3 ka bp the lake level was approximately 10 m higher than today the progradation of the gangyasang qu delta into nam co and the decreasing lake level shifted the study site into a fluvio lacustrine environment the sandy character and relatively low numbers of ostracode valves in the topmost sediments indicate increased fluvial activity possibly due to a temporary increase in flood frequency this facies lasted at least until 1 3 ka bp when the lake level of nam co dropped again and the river incised more than 10 m into the underlying fluvio lacustrine and lacustrine sediments

quaternary paleoenvironmental change on the tibetan plateau and adjacent areas western china and western mongolia abstract this paper summarises the nature of climatic change during the last glacial interglacial cycle on the tibetan plateau and adjacent areas the results are derived from inland glacier fluctuations lake level changes and dust loess records of central asia including tibet these are based on our field investigations from 1988 to 1998 and literature surveys the changing extents of different climatic controlled geomorphic landscapes and vegetation zones help to provide estimates of the magnitude of climatic change features such as ice wedge casts loess and palaeosols ela reconstruction s and palaeobotanical data are used to help reconstruct palaeoprecipitation and palaeotemperatures weathering characteristics the overlying strata and some luminescence dates indicate that there are two main glacial ice advances during the last glacial cycle these correspond to marine oxygen isotope stages ois 2 and 4 in some areas as the eastern or northern margin of the tibetan plateau remnants of older glaciations are preserved higher lake levels in the deserts of central asia and on the tibetan plateau are dated to 40 to 25 ka ois 3 and to late glacial early to mid holocene periods our work supports the view that in many areas of central asia cold phases during the last glacial correspond with the maximum extent of glaciers and the periglacial activity the last glaciation produced large alluvial fans alternating with periods of high lake stands on the tibetan plateau however at the northern margin of the plateau in the qaidam basin and in some particular desert areas in western china high lake levels occurred also during the pleistocene and are related with alluvial fans

oxygen isotopic composition of lacustrine carbonates since 130 ka bp from a tianshuihai lake core tibet an overall increasing δ 18 o trend and its implications abstract the oxygen isotopic composition of lacustrine authigenic carbonates from a core taken from lake tianshuihai in western qinghai tibet plateau shows an overall increasing 18o trend oit from about 130 to 15 ka bp based on the argument by wei and gasse 1999 that the 18o of lacustrine carbonates from lakes in west china is controlled mainly by the 18o of the local precipitation we propose that this oit may have resulted from a similar oit of 18o of precipitation over the drainage area of the tianshuihai lake this oit suggests that from about 130 to 15 ka bp variations of 18o of precipitation on the qinghai tibet plateau were controlled mainly by changes not in temperature but in moisture sources especially in the ratio of precipitation of moisture from marine sources mms to that from continental evaporation mce the changes in the sources of the moisture and in the mms mce ratio may be mainly a function of the uplift of the qinghai tibet plateau

deformation of garnets in a low grade shear zone elongated shapes of garnets in high grade metamorphic rocks have been explained as a result of plastic crystal flow or anisotropic growth in the case of low to medium grade metamorphic rocks a satisfactory explanation has not been proposed yet for elongated garnets we have studied elongated garnets grown under low grade metamorphic conditions in a shear zone with a composite planar linear fabric garnet shapes in three dimensions define oblate ellipsoids drawing on evidence from compositional x ray maps it can be deduced that growth zoning is truncated along the long borders of grains whereas subcircular garnets show non truncated concentric growth zoning this fact shows that selective dissolution along planes parallel to the foliation and the c surfaces can be claimed to be the main mechanism responsible for the deformation of the garnets under examination here more specifically dislocation enhanced dissolution which occurs at low temperatures and low dislocation mobility is arguably the mechanism responsible for partially dissolving the garnet grains selective dissolution is expected to yield plane strain oblate ellipsoids i e with a volume loss as measured in the elongated garnets however the rock as a whole has been deformed by plastic flow in a simple shear regime as shown by the existence of numerous shear criteria and the strain calculations performed 0 1997 elsevier science ltd

reconstructing holocene vegetation and climate history of nam co area tibet using pollen and other palynomorphs abstract a drill core 1 8 m 34 samples was taken out of a swamp near nam co tibet china for palynological analysis c14 dating of the cores sediment at 3 different layers led to an numerical core age of about 8000 bp and the palynomorph content of prepared samples enabled reconstruction of the vegetation and climatic history of nam co area pollen of alpine steppe elements together with cyperaceae dominates the palynoflora reflecting the existing vegetation units around the lake throughout the investigated time span due to availability of water precipitation the different vegetation units extended or had to retreat and hence the amount of their pollen increased or decreased the presence of arboreal plants verified by charcoal particles suggest the existence of woodlands in the lake s surrounding area until possibly human made fires changed the environment over time holocene climate history around nam co shows several phases of a strengthening weakening or even an interruption of monsoonal activity changing conditions between dry and humid evolved from 8017 bp until 4800 bp a mostly humid and relative stable system occurred from 4800 until 700 bp whereas the last 700 years are characterized by drier conditions according to fluctuating monsoonal activity humid or arid periods have been different in duration and intensity the interaction of both parts of the monsoonal system the south asian monsoon and the east asian monsoon influenced the climatic conditions around nam co

deformation processes in eclogitic rocks evidence for the rheological delamination of the oceanic crust in deeper levels of subduction zones the deformation mechanisms and associated growth processes operating in omphacite during foliation development and veining have been studied in a km scale eclogitic ductile shear zone from the monviso meta ophiolitic complex italian western alps dislocation creep due to multiple slip and or climb is the dominant deformation mechanism whereas 100 and 001 twinning fracturing and intergranular fluid phase diffusional creep are subordinate active slip systems include 110 001 12 110 110 100 001 and 12 010 100 fracturing is interpreted in terms of brittle instabilities arising from strain and pore fluid pressure partitioning between adjacent omphacite domains the transition from plastic to brittle behaviour of rocks is cyclic in nature as the internally generated fluids can be released and trapped repetitively during progressive deformation the significance of the eclogitic shear zone is discussed in terms of a mechanically weak zone that developed during burial or at least prior to the peak of temperature at a minimum depth of 40 km in a subduction zone and which subsequently acted as a zone of detachment during a late stage of the alpine orogeny a rheological model for oceanic crust delamination in deeper levels of subduction zones is presented

local origin of high pressure vein material in eclogite facies rocks of the zermatt saas zone switzerland prograde dehydration reactions within metabasalts at the blueschist eclogite facies transition produce kyanite veins usually with quartz sometimes with omphacite or chloritoid of up to several meters length and 20 cm width bulk rock analyses show clear depletion haloes for al2o3 and sio2 near the veins mass balance calculations support the suggestion that the vein material must have been derived from the immediate surroundings these veins are interpreted as local segregation phenom ena and not as fluid dehydration channels of the eo alpine subduction zone calcula tions have been performed for a stagnant vein formation model that considers the rate of the dehydration reactions k responsible for providing the h2o for al and si transport and the rate of diffusion d of these from wall rock to growing vein the model is satisfied for quite a wide range of values for k d and upper limits on the duration of diffusional vein formation of about 1 ma are indicated

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

subduction interface processes recorded by eclogite facies shear zones monviso w alps abstract the monviso ophiolite lago superiore unit constitutes a well preserved almost continuous upper fragment of oceanic lithosphere subducted at c 80 km depth thereby providing a unique opportunity to study mechanical coupling processes and meter scale fluid rock interactions occurring at such depths in present day subduction zones it is made of i a variably thick 50 500 m section of eclogitized basaltic crust associated with minor calcschist lenses overlying a 100 400 m thick metagabbroic body and of ii a c 1 km thick serpentinite sole we herein focus on the three major eclogite facies shear zones found at the top of the unit at the boundary between basalts and gabbros and between gabbros and serpentinites respectively strain localization occurred at lithological interfaces irrespective of material strength while ductile deformation dominates along the shear zones local brittle behavior is demonstrated by the existence of numerous eclogite breccias of fe ti metagabbros and widespread garnet fractures possibly linked with intermediate depth eclogite facies micro seismicity these m to hm sized fragments of fe ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro serpentinite boundary lower shear zone lsz pervasive and focused fluid flow is attested in the lsz by significant alteration of bulk rock compositions weakening of the rocks and widespread crystallization of hydrous parageneses by contrast the intermediate shear zone isz shows evidence for more restricted short range fluid flow the activity of both the isz and lsz ceased during early lawsonite eclogite facies exhumation when deformation localized deeper within the serpentinite sole allowing for the detachment and preservation of this large ophiolitic fragment

melt producing versus melt consuming reactions in pelitic xenoliths and migmatites abstract melt producing reactions have operated both in pelitic xenoliths and in pelitic migmatites but melt consuming reactions related to in situ melt crystallization are largely restricted to migmatites hence xenolith reaction textures may provide clues on the relative roles of both reaction types in migmatites a comparison of xenoliths and migmatites results in a catalogue of textural and chemical features some of which are argued to be diagnostic the results have implications for the identification of restite in igneous rocks and for the relative importance of prograde and retrograde processes in different domains of layered migmatites