u pb ages of neoarchean granitoids from the black hills south dakota usa implications for crustal evolution in the archean wyoming province abstract zircons in basement rocks from the eastern wyoming province black hills south dakota usa have been analyzed by ion microprobe shrimp in order to determine precise ages of archean tectonomagmatic events in the northern black hills nbh near nemo phanerozoic and proterozoic meta sedimentary rocks are nonconformably underlain by archean biotite feldspar gneiss bfg and little elk gneissic granite leg both of which intrude older schists the archean granitoid gneisses exhibit a pervasive nw se trending fabric whereas an earlier ne sw trending fabric occurs sporadically only in the bfg which is intruded by the somewhat younger leg zircon crystals obtained from the leg and bfg exhibit double terminations oscillatory zoning and th u ratios of 0 6 0 3 thereby confirming a magmatic origin for both lithologies in situ analysis of the most u pb concordant domains yields equivalent 207 pb 206 pb ages upper intercept u pb concordia of 2559 6 and 2563 6 ma both 2 for the leg and bfg respectively which constrains a late neoarchean age for sequential pulses of magmatism in the nbh unzoned in bse patches of 2560 ma zircon commonly truncate coeval zonation in the same crystals with no change in th u ratio suggesting that deuteric fluid assisted recrystallization accompanied post magmatic cooling a xenocrystic core of magmatic zircon observed in one leg zircon yields a concordant age of 2894 6 ma 2 this xenocryst represents the oldest crustal material reported thus far in the black hills whether this older zircon originated as unmelted residue of 2900 ma crust that potentially underlies the black hills or as detritus derived from 2900 ma crustal sources in the wyoming province cannot be discerned in the southern black hills sbh the peraluminous granite at bear mountain bmg of previously unknown age intrudes biotite plagioclase schist zircon crystals from the bmg are highly metamict and altered but locally preserve small domains suitable for in situ analysis a u pb concordia upper intercept age of 2596 11 ma 2 obtained for zircon confirms both the late neoarchean magmatic age of the bmg and a minimum age for the schist it intrudes taken together these data indicate that the neoarchean basement granitoids were emplaced at 2590 2600 ma sbh and 2560 ma nbh most likely in response to subduction associated with plate convergence final assembly of supercontinent kenorland in contrast thin rims present on some leg bfg zircons exhibit strong u pb discordance high common pb and low th u ratios suggesting growth or modification under hydrothermal conditions as previously suggested for similar zircons from se wyoming the leg bfg zircon rims yield a nominal upper intercept date of 1940 2180 ma which may represent a composite of multiple rifting events known to have affected the nemo area between 2480 and 1960 ma together these observations confirm the existence of a paleoproterozoic rift margin along the easternmost wyoming craton moreover the 2480 1960 ma time frame inferred for rifting in the black hills nemo area corresponds closely to a 2450 2100 ma time frame previously inferred for the fragmentation of supercontinent kenorland

uplift history of the central and northern andes a review the elevation of the andean cordillera is a crucial boundary condition for both climatic and tectonic studies the andes affect climate because they form the only barrier to atmospheric circulation in the southern hemisphere and they intrigue geologists because they have the highest plateau on earth formed at a noncollisional plate margin the altiplano puna yet until recently few quantitative studies of their uplift history existed this study presents both 1 a review of the quantitative paleoelevation estimates that have been made for the central and colombian andes and 2 an examination of the source and magnitude of error for each estimate in the central andes paleobotanical evidence suggests that the altiplano puna had attained no more than a third of its modern elevation of 3700 m by 20 ma and no more than half its modern elevation by 10 7 ma these data imply surface uplift on the order of 2300 3400 m since the late miocene at uplift rates of 0 2 0 3 mm yr paleobotanical and geomorphological data suggest a similar uplift history for the eastern cordillera namely no more than half the modern elevation present by 10 ma no evidence exists for an exponential increase in uplift rate as has been interpreted from fissiontrack data these uplift rates mostly reflect mean surface uplift rather than rock uplift that is uplift of material points because little dissection of the western eastern cordillera has occurred south of lat 19 s and of the altiplano puna thus the central andean plateau appears to be young in the colombian andes paleobotanical data imply rapid uplift of the eastern cordillera between 2 and 5 ma at rates on the order of 0 6 3 mm yr however some of this uplift is likely rock uplift due to erosion driven isostatic rebound rather than mean surface uplift

chronology of internal drainage development and uplift southern puna plateau argentine central andes fundamental to understanding the late cenozoic evolution of the central andes is knowledge of the age that the puna altiplano plateau became a unique tectonomorphic entity within the orogen the two attributes that define the puna altiplano are 1 its average elevation of 3 km and 2 regions of high elevation that correspond spatially to regions with internal drainage the chronology of plateau uplift to elevations similar to modern ones is equivocal owing to an inability to define the absolute elevation of the plateau surface through time however because regions of the puna altiplano have nonmarine evaporites nad because such evaporites must accumulate within regions of internal drainage the chronology of these strata elucidates when the plateau became internally drained we show that for the southern puna between lat 24 and 26 s internal drainage was established between 14 1 and 24 2 ma and probably by 15 ma these results corroborate and refine other workers inferences that the puna altiplano plateau became uplifted and paleogeographically isolated from the adjacent fore arc and foreland during middle to late miocene time

uplift of the altiplano puna plateau a view from the west 1 the western flank of the central andean plateau is a crustal scale monoclinal fold expressed in the geomorphology and in the westward tilt of fore arc basin strata data from three fore arc basins quantify the magnitude and time of displacement of the plateau system relative to the fore arc from 18 30 s to 22 s there is a single monocline strand there other authors documented 2000 m 500 m of early and middle miocene structural relief growth across small scale monoclines and our data reveal 810 m 640 m of 11 5 ma relief growth and 400 m 170 m relief growth since 5 ma across a long wavelength monoclinal fold limb this structural relief growth since 11 ma approximates the topographic relief growth between the fore arc and the altiplano plateau from 22 s to 24 s there are two subparallel long wavelength monoclines structural relief on the east side of the fore arc increased by 2840 m 2510 m during 17 10 ma and by 2320 m 1050 m since 10 ma some part of this monoclinal limb rotation led to topographic uplift of the adjacent puna plateau rotation across the western monocline could have increased the altitude of the fore arc basins by 900 m since 6 ma and may result from intense heating of the eastern margin of the cold and strong atacama lithosphere block although the wavelengths of the monoclines vary along strike and through time the monoclinal style of deformation dominated the uplift history of the eastern fore arc of the central andes throughout the neogene

global vegetation dynamics and latitudinal temperature gradients during the mid to late miocene 15 97 5 33 ma a 617 site palaeobotanical dataset for the mid to late miocene is presented this dataset is internally consistent and provides a comprehensive overview of vegetational change from 15 97 to 5 33 ma the palaeobotanical dataset has been translated into the biome4 classification scheme to enable direct comparison with climate model outputs the vegetation change throughout the langhian seravallian tortonian and messinian is discussed the data shows that the langhian which includes the end of the mid miocene climatic optimum represents a world significantly warmer than today the high northern latitudes were characterised by cool temperate forests the mid latitudes by warm temperate mixed forests the tropics by tropical evergreen broadleaf forests and antarctica by tundra shrub vegetation cool temperate mixed forest existed during the seravallian in the high northern latitudes a reduction in warm temperate mixed forests in the mid latitudes and a loss of tundra on antarctica was noted tortonian vegetation distribution indicates that further cooling had occurred since the seravallian the major changes in vegetation include the first evidence for cold taiga forest in the high northern latitudes and a further reduction of warm temperate mixed forests by the messinian this cooling trend had eliminated warm temperate mixed forests from the western usa and australia and had formed mid latitude deserts despite the cooling trend throughout the mid to late miocene the vegetation distribution of all four reconstructed stages reflect the vegetation of a world warmer than the pre industrial conditions the latitudinal distribution of bioclimatic zones suggests that the latitudinal temperature gradient for the langhian would have been significantly shallower than at present and has gradually but asymmetrically become more modern towards the end of the miocene first the southern hemisphere distribution of bioclimatic zones became more modern probably due to the climatic effects of a fully glaciated antarctica the northern hemisphere bioclimatic zone gradient continued to be shallower than modern throughout the miocene and slowly became more modern by the messinian

palaeoaltimetry from energy conservation principles a knowledge of past changes in the mean elevations of large continental areas is important for understanding both dynamic processes in the earth s mantle 1 and the evolution of the global climate 2 but virtually all methods for determining palaeoelevations are problematic in part because changes in either elevation or climate can give rise to the same observed geological phenomena 4 ideally palaeoelevations would be inferred directly from estimates of palaeopressure and it has recently been shown 5 that vesicles in basaltic lava flows preserve a record of atmospheric pressure at the altitude of emplacement however the elevations thus obtained have large errors 1 4 km and the method can at present be applied only to lavas that have had a simple emplacement history 5 palaeobotanical methods for estimating palaeoelevation have received much attention 6 12 but they rely on empirical temperature elevation relationships the uncertainties in which are difficult to evaluate for past climates here we describe an alternative palaeobotanical approach based on energy conservation in the atmosphere in which fossil leaf assemblages are used to infer enthalpy rather than temperature this approach is relatively insensitive to palaeoclimate with an expected error in palaeoelevation of 700 m

paleobotanical evidence for high altitudes in nevada during the miocene leaf physiognomy provides estimates of environmental parameters including mean annual enthalpy which is a thermodynamic parameter of the atmosphere that varies with altitude analyses of 12 mid miocene floras from western nevada indicate that this part of the basin and range province stood 3 kilometers above sea level at 15 to 16 million years ago which is 1 to 1 5 kilometers higher than its present altitude much if not all of the collapse to present day altitudes seems to have been achieved by 13 million years ago the crust in much of this area has been extended and thinned throughout the past 40 to 50 million years and the isostatic balance of a thinning crust requires subsidence not uplift as suggested by previous paleobotanical work

strontium isotope record of paleohydrology and continental weathering eocene green river formation wyoming sr isotope ratios in carbonate rich lacustrine strata provide highly resolved and geo graphically specific records of past changes in weathering and regional drainage patterns the eocene green river formation is perhaps the best documented pre quaternary la custrine unit in the world therefore these strata are ideally suited for studying the be havior of sr the 87 sr 86 sr ratios measured for primary carbonate in lake expansion contraction cycles of the laney member are directly linked to changing lake facies four distinct cycles are preserved in a 6 4 m interval of the arco washakie basin no 1 core each represented by a vertical succession of transgressive stromatolite facies containing dolomicritic intraclasts laminated micrite facies deposited during lake highstands and lake marginal dolomicrite facies which represent subsequent lowstands initial 87 sr 86 sr ratios range from 0 711 99 to 0 713 31 the least radiogenic isotope compositions are as sociated with laminated micrite that records increased regional runoff from phanerozoic marine carbonate and tertiary volcanic rocks during lacustrine highstands the most radiogenic isotope compositions are associated with dolomicrite recording a higher pro portion of runoff from basin bounding precambrian uplifts and the exposed lake plain during lacustrine lowstand the observed 87 sr 86 sr variations may also be due in part to changes in differential weathering of the surrounding landscape where drier climates are associated with decreased differential weathering and therefore higher 87 sr 86 sr in runoff

tectonic evolution of the central andean plateau and implications for the growth of plateaus current end member models for the geodynamic evolution of orogenic plateaus predict a slow and steady rise during crustal shortening and ablative subduction i e continuous removal of the lower lithosphere or b rapid surface uplift following shortening which is associated with punctuated removal of dense lower lithosphere and or lower crustal flow this review integrates results from recent studies of the modern lithospheric structure geologic evolution and surface uplift history of the central andean plateau to evaluate the geodynamic processes involved in forming it comparison of the timing magnitude and distribution of shortening and surface uplift in combination with other geologic evidence highlights the pulsed nature of plateau growth we discuss specific regions and time periods that show evidence for end member geodynamic processes including middle late miocene surface uplift of the southern eastern cordillera and altiplano associated with shortening and ablative subduction latest

a methodology for targeting palaeo proxy data acquisition a case study for the terrestrial late miocene when planning the acquisition of new palaeo proxy data for reconstructing past climates many factors influence the decision of where the proxies are to be collected one such influence is the likelihood of recording a significant climate change relative to the modern another consideration which is less often considered is the desire to target regions in which there is significant uncertainty in numerical model predictions of past climate change new proxy data in these regions enables a more rigorous test of model simulations and results in better constraints on the various boundary conditions used to force the models in this paper we present a methodology for targeting new palaeo proxy data based on model simulations we use the terrestrial late miocene 11 6 to 5 3 ma as a case study the late miocene climate provides insights into future climate behavior as it is believed to have been considerably warmer than the modern we carry out a suite of late miocene atmosphere only general circulation model simulations which we initially evaluate relative to an example dataset of terrestrial temperature and precipitation in terms of targeting future data acquisition we locate those regions in the model which exhibit the largest change in temperature and or precipitation relative to the pre industrial we also locate those regions which display the largest variability between the model simulations because new data in these regions are most likely to provide a strict test of the reliability of the model results among other regions the amazon basin the chad basin central canada and northern india are identified as potentially useful areas to collect data

the use of a resemblance function in the measurement of climatic parameters from the physiognomy of woody dicotyledons abstract wolfe compiled a data set clamp of the physiognomy of woody dicotyledon leaves with a view to using it to extract climatic information from fossil floras attempts made using these data to identify and describe the relationship between plant physiognomy and climate have all searched for a global relationship applicable throughout data space we investigate the possibility that the relation between climate and physiognomy is so non linear that only local relationships should be sought we use a resemblance function to select sites that are physiognomically similar to each unknown site these similar sites alone yield a climatic estimate for the unknown site data collected from new zealand and australia are then added to wolfe s data set in order to test this new method we show that the use of a resemblance function enables us to produce mean annual temperature estimates for all modern sites that lie within 2 6 c of meteorological data for 90 of the sites this method also allows the objective identification of outliers based on their distance in physiognomic space from their nearest neighbours when a method is used that searches for a global relationship between physiognomy and climate the misfit of estimated mean annual temperature for the combined southern hemisphere and clamp sites is almost twice as great 5 c as that to the clamp sites alone in contrast when the resemblance function is used the rms misfit of estimated mean annual temperature increases by only 0 2 c this result demonstrates the robustness of the new method

a late miocene subtropical dry flora from the northern altiplano bolivia abstract a variety of evidence suggests that the altiplano of the central andes the second highest and largest plateau on earth underwent significant uplift in the late miocene pliocene the most important datum supporting recent uplift is a collection of the 10 66 0 06 ma jakokkota flora from west central bolivia which implies a paleoelevation no more than 1600 1200 m today the site has an elevation of almost 4000 m in order to test the reliability of this estimate the present study analyzes a new collection of the jakokkota flora from a lacustrine unit that is 0 2 0 5 myr younger than the previously analyzed collection from a fluvial unit climate estimates based on leaf morphology for the two collections are statistically indistinguishable the combined flora has a mean annual temperature of 21 5 2 0 c and a mean annual precipitation of 550 180 mm the similarity of the climate estimates for the two floras suggests that there was not a significant climate change between them nor a significant bias in the leaf morphology due to differing taphonomic processes the climate estimate for the combined flora thus presents a representative picture of the late miocene climate of the northern altiplano if one assumes that the climate of the tropics has not changed significantly since the late miocene as is suggested by marine isotopic data then the paleoclimate of the jakokkota flora implies a paleoelevation of 1160 600 m thus the jakokkota flora supports the hypothesis of a young age for the altiplano

controls on p t t deformation path from amphibole zonation during progressive metamorphism of basic rocks estuary of the river vilaine south brittany france edenite tremolite and edenite magnesio hornblende in equilibrium with plagioclase chlorite epidote quartz and vapour involve several types of reactions for which kd can be related to t and p thermodynamic calculation of these equilibria leads to isopleth systems given knowledge of the progressive changes of end member activities in zoned ca mg amphiboles based on microprobe analyses it is possible to construct precise pressure temperature time paths p t t paths which have been followed by metabasites during polyphase metamorphism when applied to basic rocks from the river vilaine area this method allows us to construct a p t t path that can be compared directly to the p t t path constructed from interbedded acid rocks aluminous micaschists in the same structural unit through time both basic and acid rocks underwent the same complex deformation history that can be described conveniently in the l s fabric system of flinn this allows us to construct a p t t deformation path for this structural unit these paths are interpreted in terms of an under overthrusting continental collision belt the hercynian belt and represent an illustration of the time delay caused by stacking of more than two crustal units

multiple skeletal and dental pathologies in a late miocene mesotheriid mammalia notoungulata from the altiplano of bolivia palaeoecological inferences we report a partial adult skeleton of the notoungulate mesotheriid plesiotypotherium achirense showing multiple skeletal and dental pathologies from the late miocene locality of achiri bolivian altiplano the material shows bilateral absence of hypselodont permanent molars further associated with exostosis on various limb bones the main pathology relates with the loss of two first molars on both sides of the jaw perhaps due to long lasting periodontal diseases other craniomandibular afflictions likely related to the latter pathology are an over growth of two first upper molars on both sides and the anomalous development of masticatory muscle insertions m masseter pterygoideus medialis temporalis and temporalis pars profundis a hypothetical pathological sequence of the masticatory apparatus was reconstructed according to distinctive wear of ever growing crowns of two first upper molars and the unequal lower alveoli reabsorption additional pathologies are exostoses observed on several articular surfaces of limb bones from the scapula to distal phalanges they likely decreased the range of movements during locomotion of this individual given its multiple pathologies we might have expected this abnormal individual to be a potential easy prey however the long time survival of this animal suggests a low predatory pressure a hypothesis that is consistent with the virtual absence of carnivorous vertebrates in the achiri fossil assemblage and the composition of coeval mammalian guilds in the bolivian altiplano and or a herd behaviour for plesiotypotherium achirense as seen in many modern grazing herbivore ungulates

onset of convective rainfall during gradual late miocene rise of the central andes a decrease in the ratio of 18o to 16o 18o of sedimentary carbonate from the bolivian altiplano has been interpreted to indicate rapid surface uplift of the late miocene andean plateau ap here we report on paleoclimate simulations of andean surface uplift with an atmospheric general circulation model gcm that tracks oxygen isotopes in vapor the gcm predicts changes in atmospheric circulation and rainfall that influence ap isotopic source and amount effects on eastern ap slopes summer convective precipitation increases by up to 6 millimeters per day 500 for plateau elevations that are greater than about 2000 meters high precipitation rates enhance the isotope amount effect leading to a decrease in precipitation 18o at high elevations and an increase in 18o lapse rate our results indicate that late miocene 18o depletion reflects initiation and intensification of convective rainfall

paleobotanical evidence of eocene and oligocene paleoaltitudes in midlatitude western north america comparisons of the physiognomy of leaves from modern vegetation of known climates with that of eocene and oligocene fossil leaf assemblages from middle latitudes of western north america indicate paleoaltitudes comparable or higher than those at present using canonical correspondence analysis a multivariate statistical approach that includes nonlinear relationships between characters and environmental parameters we relate leaf physiognomy of fossil samples to modern vegetation of known environmental parameters including moist enthalpy of the atmosphere a thermodynamically conserved quantity of the atmosphere that varies with altitude by estimating enthalpy values for both lowland leaf assemblages and coeval interior leaf assemblages altitudes for the interior assemblages can be estimated to 890 m in northeastern washington and southern british columbia the high eocene altitudes indicate subsidence or collapse of an area undergoing crustal extension and may reflect an immediately preceding period of uplift that triggered the extension similarly other areas that have undergone cenozoic crustal extension appear to have been at least as high as they are at present three sites from the rocky mountains also indicate elevations at least as high as at present and therefore suggest subsidence either resulting from cooling of a hot upper mantle or erosion and isostatic compensation of surrounding terrain high altitudes during eocene and oligocene time in western north america appear to have been normal even in areas such as the green river basin and therefore cast doubt on the commonly inferred late cenozoic uplift of that region

abrupt changes in the rate of andean plateau uplift from reversible jump markov chain monte carlo inversion of river profiles abstract knowledge of the rate of surface uplift of the central andean plateau provides important boundary conditions for regional geodynamic models and paleoclimate reconstructions here we present a fully nonlinear inverse method to extract the rock uplift history of the central andean plateau from the cotahuasi river and its tributaries our approach is based on an analytical solution to the stream power model that relates elevation to rock uplift history under the assumptions of constant climatic conditions in space and time and spatially invariant rock uplift we use a bayesian framework that allows us to quantify the full state of knowledge of model parameters i e uncertainties trade offs the erosional efficiency defines the landscape response time and this must be determined using independent data therefore using thermochronometric data from the cotahuasi ocona canyon which record rapid fluvial incision at 13 10 ma we calibrate our results to infer a rock uplift history for the north central andean plateau we infer a pulse of rock uplift between 25 and 10 ma at rates as high as 0 25 km my however the rock uplift rate inferred during this pulse is not as high as is predicted if the central andean plateau grew as a result of loss of unstable lithospheric mantle therefore our results are more consistent with models requiring gradual uplift of the central andean plateau

a new approach to stable isotope based paleoaltimetry implications for paleoaltimetry and paleohypsometry of the high himalaya since the late miocene the change in oxygen isotopic composition of precipitation is modeled using equilibrium fractionation during rayleigh distillation linked to the thermodynamics of atmospheric ascent and water vapor condensation the primary controllers of the vertical variation in isotopic composition with elevation are the low elevation temperature and relative humidity as these control the vertical distribution of condensation an empirical fit of precipitation versus model condensation based on alpine stations is derived this fit is represented in the model as the weighted mean composition of condensation within a 1000 m thick air parcel 1500 500 m above the ground surface and is used for all other regions comparison of model versus observed modern precipitation reveals a close fit particularly of more highly elevated sites comparison of modern waters in the himalayas and southern tibet with model predictions particularly as revealed by comparison of observed and predicted hypsometry provides additional support to the validity of the model finally application of this model to estimates of paleo waters in the himalayas and southern tibet reveals that this region had already achieved its present hypsometry by the late miocene about 10 ma ago fl 2001 published by elsevier science b v

synorogenic evolution of large scale drainage patterns isotope paleohydrology of sequential laramide basins in the past decade we and others have compiled an extensive dataset of o c and sr isotope stratigraphies from sedimentary basins throughout the paleogene north american cordillera in this study we present new results from the piceance creek basin of northwest colorado which record the evolving hydrology of the eocene green river lake system we then place the new data in the context of the broader cordilleran dataset and summarize implications for understanding the synorogenic evolution of large scale drainage patterns the combined data reflect 1 a period of throughgoing foreland rivers heading in the sevier fold and thrust belt and flowing east 2 ponding of freshwater lakes in the foredeep as laramide uplifts blocked drainage 3 hydrologic closure that led to both intensive evaporation in the terminal sink of the piceance creek basin and integration of catchments over length scales 1000 km 4 infilling of basin accommodation by southward migrating magmatism in distal catchments leading to the freshening and demise of intraforeland lakes that also stepped south over time

the effect of drainage reorganization on paleoaltimetry studies an example from the paleogene laramide foreland abstract using multiple isotope systems we examine the complex effects of drainage reorganization in the laramide foreland in the context of stable isotope paleoaltimetry strontium oxygen and carbon isotopic data from lacustrine carbonates formed in the southwestern uinta basin utah between the late cretaceous and late middle eocene reveal a two stage expansion in the drainage basin of lake uinta beginning at 53 ma culminating in the mahogany highstand at 48 6 ma a marked increase in 87 sr 86 sr ratios of samples from the main body of the green river formation is interpreted as the result of water overflowing the greater green river basin in wyoming and entering lake uinta from the east via the piceance creek basin of northwestern colorado this large new source of water caused a rapid expansion of lake uinta and was accompanied by a significant and rapid increase in the o isotope record of carbonate samples by 6 the periodic overspilling of lake gosiute probably became continuous at 49 ma when the lake captured low 18 o water from the challis and absaroka volcanic fields to the north however evaporation in the greater green river and piceance creek basins meant that the waters entering lake uinta were still enriched in 18 o by 46 ma inflows from the greater green river basin ceased resulting in a lowstand of lake uinta and the deposition of bedded evaporites in the saline facies of the green river formation we thus show that basin development and lake hydrology in the laramide foreland were characterized by large scale changes in cordilleran drainage patterns capable of confounding paleoaltimetry studies premised on too few isotopic systems samples or localities in the case of the north american cordillera of the paleogene we further demonstrate the likelihood that 1 topographic evolution of distal source areas strongly influenced the isotopic records of intraforeland basins and 2 a pattern of drainage integration between the hinterland and foreland may correlate in space and time with the southward sweep of hinterland magmatism

predicting paleoelevation of tibet and the himalaya from δ18o vs altitude gradients in meteoric water across the nepal himalaya the 18o value of meteoric water varies with elevation providing a means to reconstruct paleoelevation if the 18o values of paleowater are known in this study we determined the 18o values of water 18omw from small tributaries along the seti river and kali gandaki in the nepal himalaya we found that 18omw values decrease with increasing altitude for both transects 18omw vs altitude along the kali gandaki in west central nepal fit a second order polynomial curve consistent with increasing depletion of 18o with increasing elevation as predicted by a rayleigh type fractionation process this modern 18omw vs altitude relationship can be used to constrain paleoelevation from the 18o values of miocene pliocene carbonate 18oc deposited in the thakkhola graben in the southern tibetan plateau paleoelevations of 3800 480 m to 5900 350 are predicted for the older tetang formation and 4500 430 m to 6300 330 m for the younger thakkhola formation these paleoelevation estimates suggest that by 11 ma the southern tibetan plateau was at a similar elevation to modern

controls on sr isotopic evolution in lacustrine systems eocene green river formation wyoming abstract strontium isotopes from lacustrine carbonates record detailed weathering histories of exposed bedrocks and thus are potentially useful for understanding interactions between tectonics and climate that may be driven by local or regional factors in basin scale hydrologic systems we combine extensive 87 sr 86 sr and 18 o datasets from the green river formation in order to identify environmental factors driving the evolution of sr concentration and isotopic composition in lacustrine systems through the use of sr mass balance modeling two models are developed the first tests the effect of drainage capture that drove a constant volume lake from balance filled to overfilled conditions and is applied to the laney member of the green river formation the second model focuses on lacustrine evolution in response to changing the concentration composition and mass of water influx in a variable size sr reservoir lake this model is applied to the wilkins peak member of the green river formation 87 sr 86 sr ratios are correlated with 18 o values only during the laney member when both are readily explained by a common forcing mechanism drainage capture modeling of sr isotope compositions and concentrations indicates a short residence time for sr 10 3 10 4 years or less in both balance overfilled and underfilled phases of the green river formation lacustrine system this in turn suggests that paleo lacustrine sediments in most lakes can preserve sr isotope records with high resolution 10 2 years timescales rates of sr sequestration in carbonate are shown to have a strong influence on lacustrine sr concentration and high sr concentrations of lacustrine carbonate consistent with high salinity are observed in the underfilled wilkins peak member and the balance filled tipton and lower laney members in the laney member 87 sr 86 sr mass balance modeling results provide additional support for previous interpretations that the introduction of a large drainage system produced an isotopic shift across the lower laclede upper laclede boundary major drainage reorganization is not required to drive high variability in 87 sr 86 sr ratios however modeling shows that variability in 87 sr 86 sr ratios of 0 004 observed in the wilkins peak member can be explained by change in the characteristics of intrabasinal water sources during highstand vs lowstand conditions

a tortonian late miocene 11 61 7 25 ma global vegetation reconstruction for the tortonian age of the miocene epoch 11 6 7 25 ma we present a global palaeobotanical and palaeoecologically based vegetation dataset combined with a best fit late miocene climate vegetation model experiment to create an advanced global data model hybrid biome reconstruction this new palaeoecological database and global vegetation reconstruction can be used both for the purposes of validating future palaeoclimate model simulations as well as a land cover dataset to initialise palaeoclimate modelling experiments our tortonian reconstruction shows significant changes in the distribution of vegetation compared to modern natural vegetation for example in contrast to the modern scenario in the northern hemisphere boreal forests reached 80 n and temperate forests were present above 60 n warm temperate forests covered much of europe coastal north america and south east asia our reconstruction shows a spread of temperate savanna in central usa the middle east and on the tibetan plateau evidence for arid deserts is sparse with the exception of the atacama region south america areas that exhibit arid desert today in the tortonian were instead covered by shrublands grasslands savannas and woodlands the extent of tropical forests in south america was likely reduced but expanded in the indian sub continent and east africa this pattern of global vegetation in the late miocene suggests a warmer and wetter world which is supported by the pattern of climate anomalies predicted by our best fit palaeoclimate vegetation model experiment global mean annual temperature may have been as much as 4 5 c higher than present day with many regions experiencing higher than modern amounts of precipitation over the annual cycle the pattern of temperature and precipitation change reconstructed palaeobotanically and predicted within our climate model experiment infers a global forcing agent on tortonian climate e g such as elevated concentrations of greenhouse gases to explain the observed and modelled climate anomalies this is in contrast to current proxy records of tortonian atmospheric co2 which range from last glacial maximum to mid 20th century levels

plateau style accumulation of deformation southern altiplano 1 employing surface mapping of syntectonic sediments interpretation of industry reflection seismic profiles gravity data and isotopic age dating we reconstruct the tectonic evolution of the southern altiplano 20 22 s between the cordilleras defining its margins the southern altiplano crust was deformed between the late oligocene and the late miocene with two main shortening stages in the oligocene 33 27 ma and middle late miocene 19 8 ma that succeeded eocene onset of shortening at the protoplateau margins shortening rates in the southern altiplano ranged between 1 and 4 7 mm yr with maximum rates in the late miocene summing rates for the southern altiplano and the eastern cordillera we observe an increase from eocene times to the late oligocene to some 8 mm yr followed by fluctuation around this value during the miocene prior to shutoff of deformation at 7 8 ma and transfer of active shortening to the sub andean fold and thrust belt shortening inverted early tertiary graben and half graben systems and was partitioned in three fault systems in the western central and eastern altiplano respectively the east vergent fault systems of the western and central altiplano were synchronously active with the west vergent altiplano west flank fault system from these data and from section balancing we infer a kinematically linked western altiplano thrust belt that accumulated a minimum of 65 km shortening evolution of this belt contrasts with the eastern cordillera which reached peak shortening rates 8 mm yr in between the above two stages hence local shortening rates fluctuated across the plateau superimposed on a general trend of increasing bulk rate with no trend of lateral propagation this observation is repeated at the shorter length and time scales of individual growth structures that show evidence for periods of enhanced local rates at a timescale of 1 3 myr we interpret this irregular pattern of deformation to reflect a plateau style of shortening related to a self organized state of a weak crust in the central south american back arc with a fault network that fluctuated around the critical state of mechanical failure tuning of this state may have occurred by changes in plate kinematics during the paleogene initially reactivating crustal weak zones and by thermal weakening of the crust with active magmatism mainly in the neogene stage

direction and timing of uplift propagation in the peruvian andes deduced from molecular phylogenetics of highland biotaxa abstract physical paleoaltimetric methods are increasingly used to estimate the amount and timing of surface uplift in orogens because the rise of mountains creates new ecosystems and triggers evolutionary changes biological data may also be used to assess the development and timing of regional surface uplift here we apply this idea to the peruvian andes through a molecular phylogeographic and phylochronologic analysis of globodera pallida a potato parasite nematode that requires cool temperatures and thus thrives above 2 0 2 5 km in these tropical highlands the peruvian populations of this species exhibit a clear evolutionary pattern with deeper more ancient lineages occurring in andean southern peru and shallower younger lineages occurring progressively northwards genetically diverging g pallida populations thus progressively colonized highland areas as these were expanding northwards demonstrating that altitude in the peruvian andes was acquired longitudinally from south to north i e in the direction of decreasing orogenic volume this phylogeographic structure is recognized in other independent highland biotaxa and point to the central andean orocline cao as the region where high altitudes first emerged moreover molecular clocks relative to andean taxa including the potato tomato group consistently estimate that altitudes high enough to induce biotic radiation were first acquired in the early miocene after calibration by geological and biological tie points and intervals the phylogeny of g pallida is used as a molecular clock which estimates that the 2 0 2 5 km threshold elevation range was reached in the early miocene in southernmost peru in the middle and late miocene in the abancay segment nw southern peru and from the latest miocene in central and northern peru although uncertainties attached to phylochronologic ages are significantly larger than those derived from geochronological methods these results are fairly consistent with coeval geological phenomena along the peruvian andes they strongly suggest that orogenic volume initially developed in the cao during most of the miocene until a breakthrough in the latest miocene allowed the northward propagation of crustal thickening into central and northern peru possibly by ductile crustal flow from the cao such a combined phylogeographic and phylochronologic approach to regional uplift opens perspectives to estimate the direction s and timing of acquisition of altitude over other cenozoic orogens

tertiary climatic changes at middle latitudes of western north america abstract a new multivariate analysis of leaf physiognomy provides the basis for estimating various climatic parameters for angiosperm leaf assemblages estimates from this climate leaf analysis multivariate program clamp are both accurate valid and precise repeatable application of clamp to fossil leaf assemblages from western north america confirms that eocene mean annual temperature mat was generally high early eocene mat 19 c at paleolat 70 n and 27 c at paleolat 45 n significant fluctuations 5 8 in mat occurred during the eocene each successive warm period was cooler than the preceding warm period mean annual range of temperature mart was generally low during the eocene especially at middle latitudes but then markedly increased by 12 13 c at 33 ma as mat decreased by 7 8 c these changes at 33 ma largely involved a decrease in cold month mean temperature which continued to decline into the early miocene but then increased significantly into the mid miocene thermal optimum warm month mean temperature changed little from the eocene through the early miocene this parameter however markedly decreased at 13 ma which coincides with the growth of the east antarctic ice sheet clamp also indicates that the coastal region of western north america had significantly higher mat than interior regions such as the great basin and rocky mountains these differences in mat appeared by or during the eocene and indicate that much of the interior had high altitudes in the paleogene these inferences have a significant bearing on global circulation models proposed for tertiary climates

geochemistry and petrogenesis of archaean metavolcanic amphibolites from fiskenaesset s w greenland abstract the meta volcanic amphibolites closely associated with the fiskenaesset anorthosite complex can be subdivided on the basis of trace element patterns or mineral chemistry by far the most abundant type has light rare earth element lree depleted ree patterns and displays a wide range in trace element abundances chemically comparable amphibolites can be recognised throughout the ca 2800 m yr west greenland terrain the geochemistry of the basaltic amphibolites is dominantly controlled by fractional crystallisation processes although variable degrees of partial melting may also be important the required crystal extract plagioclase dominated in the proposed fractionation scheme is very similar to the primary mineralogy of cumulates of the fiskenaesset complex and trace element models support a genetic relationship between the anorthosite complex and enclosing host amphibolites the application of trace element discrimination to assign tectonic environment in the archaean is arguable however details of the trace element chemistry especially chondritic la ta ratios are taken to suggest out of a range of likely tectonic environments an ocean floor rather than island arc affinity for the fiskenaesset amphibolites the large ion lithophile lil elements display erratic distribution patterns but are generally enriched relative to the ree this appears not to be related to high grade metamorphism but may be a relict feature of seafloor alteration the association of the cumulate sequence with meta volcanic amphibolites and metasediments probably represents an ocean floor assemblage emplaced into the lower crust during crustal accretion

geochemistry and origin of archean granites from the black hills south dakota the little elk granite 2549 ma and granite at bear mountain bmg 2 5 ga of the black hills formed as a result of a collisional event along the eastern margin of the wyoming province during th

equatorial pacific sea surface temperatures faunal patterns and carbonate burial during the pliocene abstract a high resolution record of radiolarian faunal abundances from the eastern equatorial pacific is compared to records of carbonate and noncarbonate burial to examine the evolution of eastern tropical pacific climate processes during the pliocene these data provide a means to evaluate the sensitivity of the equatorial pacific to the onset of northern hemisphere glaciation around 2 8 2 5 ma to the closure of the isthmus of panama around 4 4 3 2 ma and to orogeny related weathering changes before 4 0 ma radiolarian faunal assemblages and sea surface temperature sst estimates indicate a gradual cooling from early to late pliocene but no significant changes occur near the onset of northern hemisphere glaciation records of carbonate and noncarbonate mass accumulation show a long term decrease from the miocene pliocene boundary to the upper pliocene greater carbonate burial in the early pliocene relative to the late pliocene parallels a gradual cooling from early to late pliocene and may reflect changes related to isthmus closure or widespread orogeny no significant time domain changes are seen in the eastern equatorial pacific that could be related to the onset of northern hemisphere glaciation evolutive spectral analyses of these equatorial pacific climate parameters indicate that variance in sst and seasonality commonly concentrate at frequencies not linearly related to orbital variations furthermore cross spectral comparisons with a high resolution benthic 18 o record indicate that the surface ocean and carbonate flux share little coherent variance with high latitude climate processes during the pliocene given the high degree of chronostratigraphic control in these records these results suggest that milankovitch band surface ocean processes as well as carbonate burial in the equatorial pacific are decoupled from high latitude climate processes during the pliocene

stable isotope paleoaltimetry a critical review stable isotope paleoaltimetry makes use of systematic trends in the distribution and isotopic composition of modern precipitation with climate and topography and of the potential to estimate the isotopic composition of paleoprecipitation from authigenic in situ formed minerals to illustrate the usefulness as well as potential limitations of this method we review 1 processes controlling the isotopic composition of modern precipitation 2 stable isotope data from modern precipitation across regions of high topography and 3 stable isotope data from authigenic minerals that have been used to infer paleotopography from this we conclude that stable isotope studies of authigenic minerals can permit useful inferences on paleotopography with uncertainties that critically depend on a detailed understanding of local to global scale paleoclimate during the time interval of interest

clumped isotope evidence for diachronous surface cooling of the altiplano and pulsed surface uplift of the central andes spatially extensive paleoelevation records of the altiplano plateau are critical to determining the geodynamic mechanisms that formed and support high elevations over a broad area prior stable isotope data reveal a climate history for the northern bolivian altiplano that has been interpreted to show rapid surface uplift of 2 5 1 0 km between 10 and 6 ma this study applies clumped isotope paleothermometry to paleosol carbonates formed at both a low elevation site and temporally overlapping high elevation sites in the southern altiplano eastern cordillera during the middle to late miocene surface paleotemperature decreased by 14 c in the southern altiplano eastern cordillera relative to stable low elevation paleotemperatures implying surface elevation increase of 1 9 0 7 km between 16 and 13 ma and an additional 0 7 0 6 km between 13 and 9 ma both the large magnitude of surface temperature decrease and earlier onset 7 4 myr in the south as compared to the north suggest rapid elevation increase by piecemeal removal of lower lithosphere beneath the plateau and possible northward lower crustal flow

paleoclimatic estimates from tertiary leaf assemblages a primary obstacle to evaluating numerical modeling of tertiary nonmarine climates has been the significance and reliability of proxy data sloan barron 1990 lacking direct measurements of paleoclimatic parameters inferred paleoclimatic values are based on substitute proxy sources such as fossils or isotopes are proxy data significant that is have they been calibrated in modern environments and to what level is the calibration this is the problem of accuracy validity in the statistical sense second are the proxy data reliable can two workers or even the same worker consistently arrive at the same estimates using the same data set this is the problem of precision repeatability can these two problems be resolved quaternary palynologists have used the concept of transfer functions a methodology originally developed for foraminifera imbrie kipp 1971 applied in palynology a response surface for each palynomorph taxon is constructed for the environmental variables under consideration these transformations can then be subjected to a number of different kinds of multivariate analyses including ordination methodologies bartlein et al 1986 webb bryson 1972 these transfer functions can work for quaternary and especially the lateand post glacial of the last 20 to 30 thousand years pollen assemblages because in most regions where the methodology has been applied the flora is of known diversity and the equivalency of a given palynomorph species to one or a few extant species can be reasonably assumed even in the late neogene 1 5 to 10 million years ago however this assumption can no longer be made we know that the palynomorph assemblages contain exotic genera mccartan et al 1990 and the presence of exotic and or extinct species

using fossil leaves as paleoprecipitation indicators an eocene example estimates of past precipitation are of broad interest for many areas of inquiry including reconstructions of past environments and topography climate modeling and ocean circulation studies the shapes and sizes of living leaves are highly sensitive to moisture conditions and assemblages of fossil leaves of flowering plants have great potential as paleoprecipitation indicators most quantitative estimates of paleoprecipitation have been based on a multivariate data set of morphological leaf characters measured from samples of living vegetation tied to climate stations however when tested on extant forests this method has consistently overestimated precipitation we present a simpler approach that uses only the mean leaf area of a vegetation sample as a predictor variable but incorporates a broad range of annual precipitation and geographic coverage into the predictor set the significant relationship that results in addition to having value for paleoclimatic reconstruction refines understanding of the long observed positive relationship between leaf area and precipitation seven precipitation estimates for the eocene of the western united states are revised as lower than previously published but remain far wetter than the same areas today abundant moisture may have been an important factor in maintaining warm frost free conditions in the eocene because of the major role of water vapor in retaining and transporting atmospheric heat

a bimodal archean volcanic series owl creek mountains wyoming wind river canyon in the owl creek mountains of central wyoming contains an approximately 1 5 km long section of isoclinally folded amphibolite grade metavolcanic rocks of dominantly basaltic and dacitic composition few metasedimentary rocks appear to be present the rocks have experienced three separate deformational events including isoclinal folding an amphibolite facies metamorphic event is denoted by garnet biotite temperatures of approximately 570 c the dominant rock in the section is a hornblende amphibolite this lithology has two compositional variants a morb like tholeiitic basalt and interlayered with it a slightly more sio 2 rich mafic rock that spans the compositional range from high silica basalt to basaltic andesite approximately 51 54 sio 2 the lree enriched pattern la 40 x of the basaltic andesites suggests they cannot be derived from the tholeiitic basalts by closed system fractionation in the northern portion of the section the tholeiitic basalts are interlay

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

estimation of rainfall variables from leaf characters in tropical africa abstract regression models are defined for leaf physiognomic and climate variables from 30 modern equatorial african plant communities herbarium specimens provide physiognomic data for published species lists for the 30 sites climate data are from a variety of published sources by limiting the sampled region to equatorial africa where annual amount and seasonality of rainfall are critical determinants of vegetation leaf physiognomy is found to be related linearly to moisture variables principal components analysis demonstrates that the pattern of variation among leaf variables is primarily a function of mean annual and wet months total of all months with 50 mm rainfall multiple regression models are derived for mean annual rainfall wet months rainfall and the number of dry months accuracy of each model is tested by comparing the adjusted predicted values the value for a case when it is not included in calculation of the regression line to the actual climate values at the 30 sites leaf size is the strongest correlate of moisture the mesophyll size class has the most significant positive relationship with annual and wet months rainfall entire margined species increase with wet months rainfall and temperature acuminate tip and length width ratio are also positively related to mean annual and wet months rainfall rounded base emarginate tip and acute tip are inversely related to rainfall variables regression models derived from modern plants and climate can be used to reconstruct seasonal and annual rainfall from cenozoic african leaf fossils

on the origin of the bolivian high and related circulation features of the south american climate abstract the climatological structure in the upper tropospheric summertime circulation over south america is diagnosed using a gcm with and without south american topography a linear model and observational data emphasis is placed on understanding the origin of observed features such as the bolivian high and the accompanying nordeste low to the east results from the linear model indicate that these two features are generated in response to precipitation over the amazon basin central andes and south atlantic convergence zone with african precipitation also playing a crucial role in the formation of the nordeste low the direct mechanical and sensible heating effects of the andes are minimal acting only to induce a weak lee trough in midlatitudes and a shallow monsoonal circulation over the central andes in the gcm the effects of the andes include a strengthening of the bolivian high and northward shift of the nordeste low primarily through changes in the precipitation field the position of

miocene equatorial and southwest pacific paleoceanography from stable isotope evidence abstract stable isotope results from seven miocene deep sea drilling projects in the equatorial and southwest pacific ocean previously correlated using carbon isotope stratigraphy have been examined discussed and interpreted in terms of the development of the miocene pacific ocean the most obvious features of the benthonic foraminiferal stable isotopic records are a major increase in 18o 1 0 during the middle miocene a series of long term oscillations 2 3 my of amplitude 0 5 0 75 and a decrease in 13c values 0 5 1 0 during the latest miocene planktonic foraminiferal 18o records show different trends for high and low latitude regions in the equatorial pacific planktonic 18o values actually decrease during the miocene whereas in the higher southern latitudes planktonic 18o values become more positive in response to cooling surface waters planktonic 13c records show opposite trends to 18o with the high latitude values becoming more negative relative to the tropical regions the development of the miocene pacific ocean in terms of its vertical and horizontal thermal structure and isotopic composition is well illustrated by examining changes in the isotopic difference between planktonic and benthonic foraminifera 18ob p benthonic planktonic is a measure of the thermal structure of the water column 18oph pl high latitude low latitude planktonic values is a measure of the latitudinal temperature gradient 13cb p is an indirect measure of nutrient concentrations in the water column and 13cph pl measures differences in surface water nutrient concentrations between high and low latitude 18ob p increases during the miocene with the greatest increase occurring in the middel miocene at about 14 ma by the latest miocene the isotopic gradient at site 289 in the equatorial pacific approaches the present day isotopic gradient about 4 5 an increase in 18oph pl during the miocene suggests that the latitudinal temperature gradient increased by about 6 c to a value of 12 c in the latest miocene between sites 289 equator and 281 subantarctic 13cb p and 13cph pl values are relatively constant through the early miocene but begin to increase during the middle miocene bottom water 13c values respond similarly at all sites but surface water 13c values exhibit different trends because higher latitude values begin to decrease this decrease perhaps suggests that phosphate concentrations may have increased due to increased upwelling as the circum antarctic circulation system evolved its present day characteristics the isotopic data compiled in this paper suggest that the southwest pacific was responding uniformly to some global or at least pacific wide control during the early miocene in the middle miocene the response became more complex as the low and high latitudes began to show independent trends the changes in the thermal vertical and latitudinal structure probably occurred in respons to the build up of the east antarctic ice sheet intensification of bottom water circulation and an increase in zonal circulation in surface waters in the southern hemisphere the changes in 13c vertical and latitudinal gradients are due to some complex interaction of sea level continental hypsometry climate and biological processes coupled with oceanic circulation changes a strong correlation between estimated sea level changes and 13c values suggests that transgressions and regressions play a critical role in controlling the flux of oxidized organic carbon enriched in12c from the continental shelves and epicontinental seas to the open ocean

paleoaltimetry incorporating atmospheric physics and botanical estimates of paleoclimate we present a method for deducing paleoaltitudes that incorporates basic physical principles of atmospheric science and inferences of paleoclimates from plant leaf physiognomy we exploit the average west to east flow of the atmosphere at mid latitudes and a thermodynamically conserved variable in the atmosphere moist static energy the combined internal latent heat and gravitational potential energy of moist air to develop a method that relies on a parameter that varies with height in the atmosphere in a predictable fashion because the surface distribution of moist static energy is constrained by atmospheric dynamics and thermodynamics the combined internal and latent heat energies also known as the moist enthalpy should only vary with altitude provided we know the distribution of moist static energy thus we avoid having to make assumptions about the mean annual temperature lapse rate which varies spatially and temporally owing to unpredictable variations in atmospheric water vapor to estimate a paleoaltitude therefore we require 1 a priori knowledge of the spatial distribution of moist static energy for the paleoclimate and 2 the ability to estimate paleoenthalpy for two isochronous locations one at sea level the other at some unknown elevation to achieve this we investigated the spatial distribution of moist static energy for the present day climate of north america to estimate the deviations in moist static energy from zonal invariance in parallel we quantified the relation between moist enthalpy and plant leaf physiognomy of modern forests assuming that such deviations from zonal invariance and such relationships between physiognomy and enthalpy apply to ancient climates and fossil leaves these investigations yield an uncertainty estimate of 910 m in the paleoaltitude difference between two isochronous fossil assemblage locations

lapse rates and other variables applied to estimating paleoaltitudes from fossil floras abstract a previous method for determining paleoaltitude from fossil floras has used the difference between sea level and upland paleotemperatures which is then multiplied by the reciprocal of a normal lapse rate the present study examines other variables that can significantly influence such results terrestrial lapse rates have been calculated for 39 areas of the world based upon mean annual temperature mat mean warm month temperature mat and mean cold month ttemperature cmt these show regional and seasonal variability that is explained by factors including continentality elevation of large massifs or high base levels cold air drainage fog mountain barrier effects and the location of the subtropical high pressure zone lapse rates and projected sea level temperatures based on data from the western united states show distinct regional patterns these indicate that within this region the highest lapse rate and temperature values are correlated with 1 continental climates in the subtropical high pressure zone and 2 areas having relief where the lowest elevation land surfaces base levels occur at high altitude the data are applied to evaluate and revise the methodology for estimating paleoaltitudes variables of climatic change sea level fluctuation and paleogeography are also considered limitations in application of the method are emphasized the method is exemplified by application to the latest eocene florissant flora of colorado u s a although the results are conjectural they strongly suggest that the late eocene erosion surface in the southern rocky mountains occurred at a much higher paleoaltitude than previously thought this high elevation may have resulted from earlier laramide crustal thickening

archean geochronological framework of the bighorn mountains wyoming the bighorn mountains of the central wyoming province expose a large tract of archean crust that has been tectonically inactive and at relatively high crustal levels since 2 7 ga seven sensitive high resolution ion microprobe shrimp u pb zircon and titanite age determinations on samples of the main lithologic units provide a geochronological framework for the evolution of this area the oldest precisely dated magmatic event occurred at 2950 5 ma when diorite to granite dykes and sills intruded an older gneiss complex exposed in the central and southern bighorn mountains rocks as old as 3 25 ga may be present in this gneissic basement as indicated by the oldest dates obtained on areas of zircon grains that are interpreted as inherited cores a tonalitic gneiss was intruded into the gneiss complex at 2886 5 ma deformation of the central and southern gneisses preceded the intrusion of the bighorn batholith a tonalitic to granitic intrusion that occupies the northern portion of the uplift this