late miocene environmental change in nepal and the northern indian subcontinent stable isotopic evidence from paleosols neogene sediments belonging to the siwalik group crop out in the himalayan foothills along the length of southern nepal carbon and oxygen isotopic analyses of siwalik paleosols from four long siwalik sections record major ecological changes over the past 11 m y the carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca 7 0 ma marking the displacement of largely c3 vegetation probably semi deciduous forest by c4 grasslands by the beginning of the pliocene all the flood plains of major rivers in this region were dominated by monsoonal grasslands the floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest miocene a similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of pakistan and in terrigenous organic matter from the bengal fan showing that the floral shift was probably continentwide the latest miocene also witnessed an average change of 4 in the oxygen isotopic composition of soil carbonate as observed previously in pakistan the reasons for this are unclear if not diagenetic a major environmental change is indicated perhaps related to that driving the carbon isotopic shift recently described pollen and leaf fossils from the surai khola section show that evergreen forest was gradually displaced by semi deciduous and dry deciduous forest between 11 and 6 ma the gradual nature of this floral shift which culminated in the rapid expansion of c4 grasses starting 7 0 m y ago is difficult to explain by a decrease in atmospheric pco2 alone cerling et al 1993 but fits well with a gradual onset of monsoonal conditions in the late miocene in the northern indian subcontinent himalayan uplift driving both monsoonal intensification and consumption of co2 through weathering may be the common cause behind major late miocene environmental change globally however the decline of effective moisture associated with monsoon development has probably slowed not increased the rate of consumption of co2 by chemical weathering of himalayan sediments

paleogene island arc collision related conglomerates yarlung tsangpo suture zone tibet abstract coarse clastic rocks in the liuqu conglomerate formed in both terrestrial and subaqueous settings record a paleogene phase in the tectonic evolution of tibet facies changes are commonly abrupt with rapid changes in clast types grain size and stratal patterns sediments were derived from the leading northern edge of the indian margin and a late jurassic cretaceous intraoceanic island arc that lay within tethys the coarse clastic sedimentary rocks of the liuqu conglomerates are extremely proximal but are locally offset relative to their original source terranes they record aspects of the history of collision between these terranes and are interpreted to have been deposited in oblique slip basins that developed along the zone of collision the absence of clasts derived from terranes to the north of the yarlung tsangpo suture suggests that basins associated with deposition of the liuqu conglomerate developed prior to the final collision between india and asia

isotopic identification of the source of methane in subsurface sediments of an area surrounded by waste disposal facilities abstract the major source of methane ch4 in subsurface sediments on the property of a former hazardous waste treatment facility was determined using isotopic analyses measured on ch4 and associated groundwater the site located on an earthen pier built into a shallow wetland lake has had a history of waste disposal practices and is surrounded by landfills and other waste management facilities concentrations of ch4 up to 70 were found in the headspace gases of several piezometers screened at 3 different depths ranging from 8 to 17 m in lacustrine and glacial till deposits possible sources of the ch4 included a nearby landfill organic wastes from previous impoundments and microbial gas derived from natural organic matter in the sediments isotopic analyses included 13c d 14c and 3h on select ch4 samples and d and 18o on groundwater samples methane from the deepest glacial till and intermediate lacustrine deposits had 13c values from 79 to 82 typical of natural drift gas generated by microbial co2 reduction the ch4 from the shallow lacustrine deposits had 13c values from 63 to 76 interpreted as a mixture between ch4 generated by microbial fermentation and the co2 reduction processes within the subsurface sediments the d values of all the ch4 samples were quite negative ranging from 272 to 299 groundwater sampled from the deeper zones also showed quite negative d values that explained the light d observed for the ch4 radiocarbon analyses of the ch4 showed decreasing 14c activity with depth from a high of 58 pmc in the shallow sediments to 2 pmc in the deeper glacial till the isotopic data indicated the majority of ch4 detected in the till deposits of this site was microbial ch4 generated from naturally buried organic matter within the subsurface sediments however the isotopic data of ch4 from the shallow piezometers was more variable and the possibility of some mixing with oxidized landfill ch4 could not be completely ruled out

systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern great basin united states stable carbon and oxygen isotope variations in holocene soil carbonates that formed in the unsaturated zone were examined along several elevation transects in the southern great basin united states a region with a semi arid climate our intent was to study the relationship between the stable isotopic composition of pedogenic carbonates and climate ecological variations differences in parent material and soil depth 13 c of pedogenic carbonate in three different soil profiles from different elevations decreases with soil depth indicating a decrease in the ratio of atmospheric to plantderived co 2 downprofile pedogenic carbonate at the soil air interface approaches a 13 c value in equilibrium with atmospheric co 2 in all three soils observed 13 c profiles for pedogenic carbonate can be described using a one dimensional model for 12 co 2 and 13 co 2 assuming isotopic equilibrium between soil co 2 and soil carbonate the modeled best fit to observed isotopic profiles suggests that the profile differences in part result from differing soil respiration rates at each site 13 c in deep pedogenic carbonate 50 cm varies by about 12 per mil over a 2 440 m elevation range being enriched in 13 c at the lowest elevations the slope of 13 c for these carbonates versus elevation is very similar for soils developed on carbonate and on noncarbonate parent materials depletion by 4 6 to 4 7 per mil per 1 000 m increase in altitude between 300 to 2 740 m above mean sea level for the localities studied this concordance makes it likely that there has been complete isotopic exchange between hco 3 in solution and soil co 2 prior to carbonate precipitation soil co 2 and soil respiration rates increase systematically with elevation the plantderived component of soil co 2 indicates that c 3 plants dominate the biomass at most measured sites in agreement with plant surveys calculated equilibrium fractionation factors between soil co 2 and soil carbonate are very similar to those observed again indicating complete isotopic exchange between carbon species in all the soil co 2 and soil carbonate data suggest that the 13 c variation with elevation observed in the soil carbonates results from differing soil respiration rates at each site as well as from variations in the proportion of c 3 to c 4 and cam plants in each site9s surface biomass 18 o values in pedogenic carbonates are higher at lower elevations due in part to the more positive 18 o values for meteoric waters at lower elevations the average 18 o value of deep 50 cm pedogenic carbonate at all sites however is enriched 2 4 to 3 7 per mil with respect to values we predict should be in equilibrium with the isotopic composition of local meteoric waters this suggests that evaporative isotopic enrichment of soil waters may have occurred at all elevations prior to precipitation of carbonate or that seasonal differences in the isotopic composition of meteoric waters coupled with differential infiltration may be taking place one or both of these processes also may explain the 18 o decrease in soil carbonate with depth observed in two of three soil profiles

palynological evidence for vegetation development and climatic change in the sub himalayan zone neogene central nepal abstract a palynological study on miocene pleistocene sediments exposed at surai khola in central nepal yields new information on vegetation and climate change over the last 11 5 ma our results show that during the late middle miocene to early late miocene 11 5 8 ma lower siwalik the himalayan foothills and the gangetic floodplain of nepal were mainly forested with subtropical to temperate broad leafed b l taxa e g quercus lithocarpus castanopsis alnus and tropical forest taxa respectively grasses were present at the time but were not abundant between the early to late late miocene 8 6 5 ma lower middle siwalik grassland replaced subtropical and temperate b l forest this change may be related to disturbance of the vegetation on the slopes due to uplift perhaps enhanced by intensification of the monsoon between late late miocene to pliocene early pleistocene 6 5 potamogeton and better preservation of all specimens enhanced seasonal flooding in the system during the pliocene 5 5 3 5 ma top middle siwalik may have produced local lacustrine conditions on the overbanks evidenced by an abundance of algae spirogyra and pteridophytes in the pollen assemblages evidence of climatic cooling between 6 5 5 ma ago is indicated by the onset of steppe taxa decrease of tropical forest taxa and disappearance of certain dipterocarpaceae pollen assemblages from the surai khola section suggest complex vegetation changes of which the shift to c 4 grass dominance was only one global climatic cooling the intensification of the monsoon and the late miocene uplift may all have contributed to the phytogeography in the sub himalayan zone

constraining the age of liuqu conglomerate southern tibet implications for evolution of the india asia collision zone abstract controversy over the depositional age and provenance of the liuqu conglomerate along the major structural indus yarlung suture zone in south tibet clouds our understanding of the process of the india asia collision here we report low temperature thermochronometric data apatite fission track apatite and zircon u th he for the liuqu conglomerate in the xigaze area our new data constrain its depositional age to latest oligocene early miocene time indicating that rather than having formed immediately following paleogene india asia collision or collision between india and an intra oceanic arc as previously proposed the conglomerate was probably deposited in an intermontane basin at a slightly later time than the gangrinboche group to the north the liuqu conglomerate should therefore not be used as a key horizon in models constraining the early stages of india asia collision our data together with previous studies suggest that the liuqu conglomerate was sourced from the xigaze forearc basin indus yarlung suture zone as well as the tethyan himalaya furthermore our data indicate that exhumation of the liuqu conglomerate commenced at

oil charge preserves exceptional porosity in deeply buried overpressured sandstones central north sea uk abstract exceptionally high porosity sandstones are reported from oil provinces worldwide yet the mechanism of porosity preservation remains controversial we present strong evidence that the exceptional porosities in the jurassic fulmar formation within the north sea are the result of early oil charging preventing cementation and chemical compaction in sands with relatively low detrital clay contents although overpressure and grain coatings do have some effect the most dramatic evidence that reservoir quality is related to early oil charge is the spatial distribution of the porosity the maximum porosity of the fulmar formation is systematically highest immediately below the top seal and decreases below this despite uniform sedimentology hence porosity is preferentially preserved where oil first accumulated in these unusually homogeneous overpressured reservoir sandstones at the top oil charge prevents or significantly retards chemical compaction and cementation of a reservoir preserving porosity there is independent evidence for early oil charging of reservoirs within the central graben from fluid inclusion studies and k ar age dates of authigenic illite micro quartz grain coatings which have been proposed to preserve porosity during burial are associated with only 5 extra porosity in sands that exceed the worldwide average sandstone porosity by almost 20

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

mass balance reconstruction of a modern vertisol implications for interpreting the geochemistry and burial alteration of paleo vertisols abstract utilizing identical sampling and analytical techniques the morphological and chemical characteristics of a modern vertisol houston black series central texas can be directly compared with an upper mississippian paleo vertisol from the appalachian basin pennington formation east central tennessee mass balance reconstructions suggest retention of primary pedochemical patterns in the paleo vertisol including patterns of soil volume change strain and transport functions translocations of many major and trace elements retention of primary pedochemical patterns suggests that vertisols constitute nearly closed systems during burial diagenesis chemical and mineralogical changes associated with burial diagenesis of the paleo vertisol include oxidation of organic carbon oc illitization of smectites dehydration and recrystallization of fe mn oxyhydroxides and dolomitization of pedogenic calcite significant differences in the chemical behavior of gilgai microhigh and microlow pedons in modern vertisols have implications for interpretation of geochemical data obtained from paleo vertisols overall wetter soil conditions and variable redox potential under gilgai microlows promote greater depths of leaching and mobility of redox sensitive trace elements including co cr cu mn ni and v gilgai microhighs behave as evaporative wicks that draw moisture and soluble phases towards the soil surface resulting in precipitation of metal hydrosylate complexes and sulfates gypsum at the capillary fringe and shallower depths of leaching and fixation of trace elements paleoprecipitation estimates from paleosols based on the depth to the top of the pedogenic carbonate horizon should therefore utilize field petrographic and geochemical data for characterizing maximum depths of leaching loss or gain of exchangeable bases and calcification rather than relying solely upon field data

tibetan garnet records early eocene initiation of thickening in the himalaya tectonic reconstructions of the himalayan orogeny depend on the age at which crustal thickening commenced to investigate this age we analyzed garnet from middle crustal rocks exposed in the north himalayan mabja and kangmar gneiss domes of tibet using lu hf geochronology garnet yielded lu hf ages of 54 52 ma in mabja and 51 49 ma in kangmar samples on the basis of microstructural and major element and rare earth element zoning observations the lu hf ages are interpreted as recording garnet growth during contractional deformation in the middle crust at 54 3 0 6 ma followed by variable recrystallization during subsequent high temperature ductile extension the new lu hf ages are the first to confirm that crustal thickening and contraction in the tibetan himalaya was broadly synchronous with the early eocene collision between greater india and the eurasian plate

tertiary deformation history of southeastern and southwestern tibet during the indo asian collision geologic mapping and geochronological analysis in southwest kailas area and southeast zedong area tibet reveal two major episodes of tertiary crustal shortening along the classic indus tsangpo suture in the yalu river valley the older event occurred between ca 30 and 24 ma during movement along the north dipping gangdese thrust the development of this thrust caused extensive denudation of the gangdese batholith in its hanging wall and underthrusting of the xigaze forearc strata in its footwall examination of timing of major tectonic events in central asia suggests that the initiation of the gangdese thrust was approximately coeval with the late oligocene initiation and development of north south shortening in the eastern kunlun shan of northern tibet the nan shan at the northeastern end of the altyn tagh fault the western kunlun shan at the southwestern end of the altyn tagh fault and finally the tian shan north of the tarim basin such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the eurasian plate and the indian and pacific plates the younger thrusting event along the yalu river valley occurred between 19 and 10 ma along the south dipping great counter thrust system equivalent to the locally named renbu zedong thrust in southeastern tibet the backthrust system in south central tibet and the south kailas thrust in southwest tibet the coeval development of the great counter thrust and the north himalayan granite gneiss dome belt is consistent with their development being related to thermal weakening of the north himalayan and south tibetan crust due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between india and asia or frictional heating along major thrusts such as the main central thrust beneath the himalaya

evidence from paleosols for low to moderate elevation of the india asia suture zone during mid cenozoic time the post collisional uplift history of the india asia suture zone in southern tibet is important for understanding the geodynamic conditions of the india asia collision and how it may have modified regional and global climate here we use stable isotope and major element analyses of paleosols preserved in the liuqu conglomerate to reconstruct india asia suture zone paleoclimate and paleoelevation paleosol carbonate 13c vienna peedee belemnite average values of 9 4 1 1 indicate that liuqu paleosols were well vegetated by c3 plants unlike thinly vegetated and arid modern tibet major element weathering indices show that these soils experienced significant collapse and loss of cations due to weathering of similar intensity to that in the wet low elevations of the neogene himalayan foreland age estimates for the liuqu vary but current evidence points to an early miocene age our evidence requires that the india asia suture zone experienced wet well vegetated conditions during liuqu conglomerate deposition geodynamically this implies that the suture zone was at relatively low elevation and was topographically open to monsoon moisture as late as 40 m y after the start of the india asian collision this challenges the idea that uplift of the india asia suture zone was monotonic and tied directly to crustal thickening

expansion of c4 grasses in the late miocene of northern pakistan evidence from stable isotopes in paleosols stable isotopic clay mineralogic and bulk chemical analyses were conducted on paleosols of the neogene siwalik sections in northern pakistan in order to reconstruct floodplain environments over the past 17 ma the stable carbon isotopic composition of soil carbonate mean 13c pdb 10 2 and associated organic matter mean 13c pdb 24 1 in paleosols representing 17 7 3 ma reveal that floodplain vegetation was dominated by c3 plants at 7 3 ma a shift toward more positive carbon isotopic values began signaling the gradual expansion of c4 grasses onto the floodplain from 6 ma to present carbon isotopic values for paleosol carbonate mean 13c pdb 0 6 and organic matter mean 13c pdb 14 4 are uniformly enriched in 13c indicating the presence of nearly pure c4 grassland the scarcity of kaolinite and abundance of smectite and pedogenic carbonate in most paleosols suggest that rainfall in the region remained 1 0 1 25 m yr or less for the entire 17 ma of record paleosols in the lower portion of the section lack organic a horizons but have reddish b horizons often containing secondary iron oxide nodules leaching depths of soil carbonate in these older paleosols are typically greater than those in the plio pleistocene part of the section where organic a horizons are common and b horizons are markedly more yellow the combined evidence suggests that the mature paleosols in the pre 7 3 ma part of the record are dominantly calcareous alfisols or mollisols that once underlay nearly pure c3 vegetation perhaps trees and shrubs while calcareous mollisols underlying c4 grassland dominate the upper part of the record the carbon and oxygen isotopic trends in the paleosol record in pakistan are also evident in the diet of fossil mammals and in paleosols from nepal thus demonstrating that these paleoenvironmental changes in floodplain vegetation may be continent wide local effects such as the development or intensification of the asian monsoon driven by uplift of the tibetan plateau may have led to the expansion of c4 grasses if however the expansion of c4 grasses proves globally synchronous then a larger scale cause such as a marked decrease in co2 may be the driving mechanism

application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

stable isotopic results from paleosol carbonate in south asia paleoenvironmental reconstructions and selective alteration article i nfo we measured 18 o and 13 c values of paleosol carbonate nodules from a variety of sedimentary units in southern asia in order to reconstruct paleoenvironmental conditions and to evaluate for possible diagenetic alteration paleosol carbonate nodules were collected from lower to upper cretaceous strata exposed in two locations in southern tibet and from lower to middle miocene strata in northern india and nepal additional samples were collected from stratigraphically adjacent marine carbonate units to test for possible resetting of stable isotope values based on the implausibly low 18 o values 13 of most of the marine carbonate samples we interpret the 18 o values of the associated paleosol carbonate nodules as being reset and therefore unreliable for paleoenvironmental or paleoelevation reconstruction importantly the 18 o values of 13 c values of marine carbonate rocks fall within the expected primary

petrochronology of himalayan ultrahigh pressure eclogite the timing and nature of the india asia collision earth s largest ongoing continent continent collisional orogen are unclear ultrahigh pressure metamorphism of indian continental margin rocks is used as a proxy for initial collision because it indicates subduction of india records of this metamorphism are preserved only at kaghan valley pakistan and tso morari ladakh india separated by 500 km and having published ages of peak pressure of 46 2 0 7 ma and 53 51 ma respectively the apparent 6 m y age difference may refl ect multiple subduction events a large promontory along the former indian margin or inadequate constraints on the time of peak pressure recrystallization at tso morari we present 108 coupled in situ u th pb and rare earth element ree analyses of zircons in two tso morari eclogites to obtain age and petrologic information the ages range from ca 53 ma to 37 ma and peak at ca 47 43 ma flat heavy ree slopes and the absence of an eu anomaly are compatible with eclogite facies zircon re crystallization this re crystallization probably occurred at ultrahigh pressure because 64 of the analyses are from zircon included in ultrahigh pressure garnet and omphacite these results are consistent with those from kaghan valley and suggest that a single protracted ultrahigh pressure metamorphic event occurred contemporaneously across much of the orogen following initial contact of the indian and asian continents at ca 51 ma or later

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

provenance of the liuqu conglomerate in southern tibet a paleogene erosional record of the himalayan tibetan orogen abstract the liuqu conglomerate exposed along the yarlung zangbo suture zone in southern tibet has been interpreted as a paleogene molasse deposited during india asia collision or the product of collision between india and an intra oceanic arc detailed petrographic studies and analyses of u pb and hf isotopes of detrital zircon indicate multiple sources for the conglomerate including the sedimentary succession within the xigaze forearc basin to the north of the suture the yarlung zangbo ophiolite within the suture itself and the langjiexue group and tethyan himalayan sedimentary rocks to the south of the suture the occurrence of clasts of both indian and asian origin suggests that the conglomerate was deposited after india asia collision the present data suggest that the onset of himalayan tibetan orogenic erosion occurred as early as the middle eocene when the yarlung zangbo suture zone and xigaze forearc succession were uplifted and became sources for the liuqu conglomerate thus the results are inconsistent with the proposal that the provenance and age of the liuqu conglomerate support the hypothesis that india asia collision did not occur until the oligocene it is further suggested that eocene neotethyan slab breakoff which may result in accelerated exhumation and uplift of the orogen provided a deep level dynamic control on the deposition of the liuqu conglomerate

structural evolution and sequence of thrusting in the tethyan fold thrust belt and indus yalu suture zone southwest tibet regional mapping of a north south traverse from the india nepal china border junction to mount kailas in southwest tibet combined with previously published geochronologic and stratigraphic data is the basis for an incremental restoration of the tethyan fold thrust belt and deformation along the indus yalu suture zone from north to south the major structural features are 1 the indus yalu suture zone composed of five south dipping thrust faults involving rocks interpreted to represent parts of the former indian passive margin and asian active margin 2 the tethyan fold thrust belt composed of a dominantly north dipping system of imbricate thrust faults involving precambrian through upper cretaceous strata and 3 the kiogar jungbwa thrust sheet a linelength cross section reconstruction indicates a minimum of 176 km of north south horizontal shortening partitioned by the tethyan fold thrust belt 112 km and indusyalu suture zone 64 km sequential restoration of the cross section shows that the locus of shortening prior to the late oligocene occurred significantly south possibly 60 km of the indus yalu suture zone within the tethyan fold thrust belt and that a significant amount of unsubducted oceanic lithosphere was present south of the suture in southwest tibet until that time an implication of this result is that postcollision oligocene miocene high k calcalkaline magmatism may be explained by melting due to active subduction of oceanic crust beneath the kailas magmatic complex until the late oligocene a regional profile across the tibetan himalayan orogen from

evaluating the use of weathering indices for determining mean annual precipitation in the ancient stratigraphic record abstract different quantitative methods have been developed to determine paleo rainfall using paleosols of particular interest are methods that use bulk geochemistry to calculate a weathering index and from that mean annual precipitation twenty three paleosols that formed during a time of significant climate change the paleocene eocene thermal maximum petm were analyzed to evaluate the relative merits of two geochemical proxies that have been developed for estimating paleoprecipitation one depends on the chemical index of alteration without potash cia k and the other uses the calmag weathering index which depends on cao and mgo the paleosols are located in the paleogene fort union and willwood formations in the bighorn basin wy morphologic differences among the paleosols indicate that they formed under varied soil moisture conditions a morphology index was developed from pedogenic features that are sensitive to soil drainage and thus to rainfall to assess that variability quantitative mean annual precipitation map values were calculated for each paleosol using the cia k index and the calmag index which was developed specifically for vertisols those results were compared to the morphology index for 23 paleosols the results indicate that increases and decreases in map calculated from both methods correlate relatively well to changes in the soil morphology index however the results from the calmag weathering index show a stronger correlation the results suggest that the calmag proxy provides a more robust map estimate for vertisols in addition to comparing map estimates to the soil morphology index we compared the map values to paleorainfall estimates determined from paleofloras at the same stratigraphic levels as three of the paleosols that comparison confirms that the calmag method is reliable for reconstructing map from ancient vertisols the results also show that in the bighorn basin vertisols with a b horizon

seasonal stable isotope evidence for a strong asian monsoon throughout the past 10 7 m y o of wet season rainfall was signi cantly morenegative 29 5 smow prior to 7 5 ma than after 26 5 smow if this change is attributable to a lessening of the amounteffect in rainfall this agrees with oral and soil geochemical datathat indicate increasing aridity beginning at 7 5 ma keywords tibetan plateau monsoon stable isotopes paleohydrology seasonality introductionthe tibetan plateau is the engine that drives the modern asianmonsoon by generating a high altitude region of low pressure in thesummer as the plateau heats and a region of high pressure in the winteras the plateau cools hastenrath 1991 during the summer warm airrises from the plateau pulling moist air off the ocean across the indiansubcontinent and into the highlands this results in heavy summer rain fall on the subcontinent the opposite occurs in the winter resultingin cold dry air spilling off the plateau and effectively excluding rainfrom the subcontinent thus the presence of a strong wet season dry season alternation implies the presence of a plateau broad and highenough to drive the monsoon the timing of the uplift of the plateau remains a matter of con siderable debate because there are few direct indicators of paleotopog raphy in the geologic record consequently past workers in tibet re lying on indirect indicators of uplift have proposed dates ranging from40 to 3 4 ma on the basis of initiation of potassic volcanism chunget al 1998 turner et al 1993 or extension on the plateau harrisonet al 1995 coleman and hodges 1995 changes in marine sedimen tation rates burbank et al 1993 sediment types rea et al 1998 or biota nigrini and caulet 1992 kroon et al 1991 and changesin stable carbon isotope and palynological patterns on the indian sub continent quade et al 1989 chen 1981 although different areasof the plateau may have risen at different times many workers haveinferred rapid simultaneous uplift of large areas of the plateau at 7 8ma by a process such as lithospheric delamination molnar et al 1993 this inference was based on the following approximately coevalphenomena a major change in plant communities of the indian sub continent quade et al 1989 and shifts in marine upwelling patternsthat are linked to an intense monsoon kroon et al 1991 althoughthere is strong evidence for signi cant climate change at 7 8 ma it isunclear whether this is the onset of the monsoon the oral transitionseems to have been a global rather than local phenomenon cerling etal 1997 and monsoonally driven upwelling may have already beenpresent by 10 12 ma nigrini and caulet 1992 kroon et al 1991 because there is an intimate association between the intense sea sonality of the modern monsoon and a high tibetan plateau and be cause evaporation can be unambiguously recognized in the d

evidence for early 44 ma himalayan crustal thickening tethyan himalaya southeastern tibet abstract comprehensive understanding of himalayan orogenesis is limited in part by a poor knowledge of the eohimalayan episode a phase of tectonic activity predating the better understood neohimalayan crustal thickening associated with coaxial deformation along the main central thrust the cambo ordovician to tertiary metasedimentary sequences outcropping within the tethyan himalaya are the structurally highest units of the himalayan fold and thrust belt they and are inferred to have been the first to have accreted to asia a compilation of data from transects along the length of the main himalayan arc shows that the tethyan sequences have experienced at least five deformation events although the timing of these episodes is poorly constrained emplacement of a series of undeformed granitoid bodies following the second of these events which accounts for the majority of crustal thickening in the tethyan himalayan units is constrained by u pb zircon dating to be older than 44 1 1 2 ma thus significant crustal thickening had occurred along the length of the proto himalayan arc by the mid eocene or within 10 to 20 myr of the initiation of himalayan orogenesis

new weathering index improves paleorainfall estimates from vertisols vertisols are clayey shrink swell soils that are widely recognized in the rock record thus generating the need to better understand the dynamics of elemental concentrations on the development of weathering indexes for climate interpretations we assessed the weathering performance of the four major base forming oxides cao mgo na 2 o k 2 o along a modern vertisol climosequence spanning a strong precipitation gradient and discovered that the concentration of bulk soil cao and mgo yields the strongest correlation to mean annual precipitation map based on this fi nding we introduce the calmag weathering index defi ned as al 2 o 3 al 2 o 3 cao mgo 100 which improves rainfall estimates for vertisols relative to the wellestablished cia k chemical index of alteration minus potassium weathering index rather than documenting the hydrolysis of weatherable minerals common in many other soil orders in vertisols calmag principally tracks the fl ux of calcium and magnesium sourced from calcium carbonate detrital clay and exchangeable ca 2 and mg 2 application to two mesozoic paleosols reveals that in drier climates cia k yields higher map estimates than calmag but that the reverse is true in wetter climates this work improves paleorainfall estimates from vertisols and suggests that a family of weathering indexes is needed for different paleosol types

from a black to a gray box a mass balance interpretation of pedogenesis we utilize chemical elements as tracers in a mass balance analysis that provides functional relationships among soil chemical composition bulk density and volume change in relation to parent material these analytical functions are based on the principle of conservation of mass and include a term quantifying mass flux into out of the soil and between horizons we apply the technique to the oldest member of a chronosequence developed on marine terraces in northern california it is an alfisol that evolved from beach sand to its present state nearly one third of its weight is composed of secondary clay minerals in about 240 ky aside from large increases in organic carbon desilication is the dominant factor in soil evolution 29 50 83 g cm 2 si was leached from the beach sand during pedogenesis the rate of desilication is roughly 2 1 t km 2 yr 1 0 21 g cm 2 ky 1 an order of magnitude slower than that implied by the si denudation rate calculated for the mattole river watershed weathering of primary minerals and synthesis of secondary minerals is relatively well advanced suggesting that the rate of desilication may be declining the local beach is composed of quartz and sodic plagioclase with smaller amounts of chlorite mica and kaolinite the soil has substantially different mineralogy sand is dominated by quartz and clay is dominated by kaolinite halloysite chloritic intergrades and gibbsite bases are also leached though the total mass was much less 57 2 55 g cm 2 of na in the beach sand was lost as plagioclase weathered by focusing on elemental and mineralogical gains and losses we emphasize the essential connection between the pedologic environment and the external hydrochemical environment

does burial diagenesis reset pristine isotopic compositions in paleosol carbonates abstract sedimentological study of early oligocene continental carbonates from the fluvial dagshai formation of the himalayan foreland basin india resulted in the recognition of four different types namely soil palustrine pedogenically modified palustrine and groundwater carbonates stable oxygen and carbon isotopic 18 o and 13 c analyses of fabric selective carbonate microsamples show that although the pristine isotopic compositions are largely altered during deep burial diagenesis complete isotopic homogenization does not occur 18 o and 13 c analyses of 200 calcrete and palustrine carbonates from different stratigraphic horizons and comparison with 18 o of more robust bioapatite fossil vertebrate tooth phase show that dense micrites 70 carbonate invariably preserve the pristine 18 o value mean of 9 8 while altered carbonates show much lower 18 o value 13 8 such inhomogeneity causes large intra sample and intra soil profile variability as high as 5 suggesting that soils behave like a closed system where diagenetic overprinting occurs in local domains a simple fluid rock interaction model suggests active participation of clay minerals to enhance the effect of fluid rock ratio in local domains during diagenesis this places an upper limit of 70 micrite concentration above which the effect of diagenetic alteration is minimal careful sampling of dense micritic part of the soil carbonate nodules therefore does provide pristine isotopic composition and it is inappropriate as proposed recently to reject the paleoclimatic potential of all paleosol carbonates affected by burial diagenesis based on pristine 13 c value of 8 8 0 2 in soil carbonates an atmospheric co 2 concentration between 764 and 306 ppmv is estimated for the early oligocene 31 ma dagshai time these data show excellent agreement between two independent proxy records viz soil carbonate and marine alkenone and support early oligocene survival of the antarctic ice sheet

the paleoaltimetry of tibet an isotopic perspective stable isotopes provide a valuable perspective on the timing of elevation change of the tibetan plateau we begin our paper by looking in depth at isotopic patterns in modern tibet we show that the 18 o value of surface waters decreases systematically up the himalayan front in central nepal by about 2 8 km in agreement with the patterns documented and modeled by previous research on the tibetan plateau itself there is no apparent correlation between elevation and the 18 o value of flowing surface waters both surface waters and soil carbonates display a northward increase in 18 o values of about 1 5 north of the himalayan crest even though elevation increases modestly the isotopic increase with latitude reduces the isotope elevation gradient for water in the northernmost plateau to 1 to 2 km carbonates in both soils and lakes form at higher temperatures than assumed by previous studies on the plateau temperature estimates from clumped isotope 47 analyses of modern soil carbonates significantly exceed mean annual air t and modeled maximum summer soil temperatures by 15 8 2 8 and 9 7 2 5 c respectively similarly elevated temperatures best account for the 18 o values observed in modern soil and lake carbonates we recalculated paleoelevations from previous studies on the plateau using both higher formation temperatures and latitude corrected isotopic values with one notable exception our revised model produces paleoelevation estimates very close to previous estimates the exception is the reconstruction from late eocene age deposits at xoh xil for which we calculate elevations that are higher and much closer to the current elevation than previously reconstructed therefore there is no evidence for northward progression through time of tibetan elevation change instead the available but admittedly very scanty evidence suggests that much of tibet attained its modern elevation by the mid eocene a truly robust test of the various geodynamic models of uplift await expansion and replication of isotopic records all across tibet especially in the center and north and for 15 ma