methane from uv irradiated carbonaceous chondrites under simulated martian conditions 1 a uv photolytic process was studied for the production of methane from carbonaceous chondrites under simulated martian conditions methane evolution rates from carbonaceous chondrites were found to be positively correlated to temperature 80 to 20 c and the concentration of carbon in the chondrites 0 2 to 1 69 wt and decreased over time with murchison samples exposed to martian conditions the amount of evolved methane em per unit of uv energy was 7 9 10 13 mol j 1 for uv irradiation of murchison 1 69 wt samples tested under martian conditions 6 9 mbar and 20 c using a previously described mars uv model moores et al 2007 and the em given above an annual interplanetary dust particle idp accreted mass of 2 4 105 kg carbon per year yields methane abundances between 2 2 to 11 ppbv for model scenarios in which 20 to 100 of the accreted carbon is converted to methane respectively the uv ch4 model for accreted idps can explain a portion of the globally averaged methane abundance on mars but cannot easily explain seasonal temporal diurnal or plume fluctuations of methane several impact processes were modeled to determine if periodic emplacement of organics from carbonaceous bolides could be invoked to explain the occurrence of methane plumes produced by the uv ch4process modeling of surface impacts of high density bolides single airbursts of low density bolides and multiple airbursts of a cascading breakup of a low density rubble pile comet were all unable to reproduce a methane plume of 45 ppbv as reported by mumma et al 2009

mineralogy and geochemistry of diatomite associated with lignite seams in the komnina lignite basin ptolemais northern greece abstract diatomite with variable chemical and mineralogical composition occurs in the komnina lignite basin the diatomite layers which overly lignite seams of upper miocene age are rich in silica or calcium these layers correspond respectively to quartz rich and calcite rich layers the diatomite mainly consists of opal a while other minerals are quartz feldspars calcite smectite illite kaolinite chlorite cristobalite and muscovite the well preserved nature of the diatom species indicates a weak silica diagenesis this is also indicated by the mineralogical composition of diatomite especially the absence of opal ct the organic material present in the lake acted as coating and prevented diatom dissolution the shallow lake where the diatoms were deposited did not allow diagenesis of diatomite diatoms were deposited in the komnina lake as a result of the acidic ph the warm humid conditions and the silica rich environment that occurred due to volcanic activity in the nearby area aridea remains of volcanic ash have been identified in the lignite deposits of the wider ptolemais area the association of diatomite with lignite was brought about by tectonic movements that occurred in the wider area of monastir florina ptolemais kozani elassona sarantaporo in the upper miocene resulting to the development of individual small shallow lakes the paleoenvironment changed to acidic from the previous alkaline ph the vegetation died off the silica produced from the volcanic activity and the calm water conditions allowed the deposition of diatoms

isotopic evidence of the origin of mineralized waters from the central carpathian synclinorium se poland stable isotopes of hydrogen and oxygen were determined in 45 samples of water 27 samples of oil associated waters 17 samples of mineral waters used by spas 1 sample of surface river water from the central carpathian synclinorium covering a stratigraphic range of flysch sediments from upper cretaceous to oligocene moreover oxygen isotope compositions of authigenic calcite vein and cement from core samples of four boreholes were made to evaluate isotopic equilibrium between waters and diagenetic carbonates as a function of temperature the saline and brackish waters tds from 1 g l to 48 9 g l considered here generally belong to four hydrogeochemical classes na cl cl hco3 na hco3 cl na and hco3 na their isotopic composition causes them to fall to the right of global meteoric water line gmwl showing enrichment in 18o and 2h on the other hand relative to standard mean ocean water smow they are depleted in 2h and both depleted and enriched in 18o the observed isotopic composition can be explained by the three component mixing of surface water diagenetically modified sea water kind of connate water and metamorphic water the mixing is accompanied by an exchange of oxygen isotopes between water and carbonate cements causes 18o enrichment of interstitial waters the contribution of isotopic exchange between water and clay minerals in shales was evaluated only theoretically basing of the literature

hydrogen isotopes in eocene river gravels and paleoelevation of the sierra nevada we determine paleoelevation of the sierra nevada california by tracking the effect of topography on precipitation as recorded in hydrogen isotopes of kaolinite exposed in gold bearing river deposits from the eocene yuba river the data compared with the modern isotopic composition of precipitation show that about 40 to 50 million years ago the sierra nevada stood tall 2200 meters a result in conflict with proposed young surface uplift by tectonic and climatic forcing but consistent with the sierra nevada representing the edge of a pre eocene continental plateau

cenozoic migration of topography in the north american cordillera continental topography is the result of complex interactions among mantle convection continental dynamics and climatic and erosional processes therefore topographic evolution of mountain belts and continental interiors reflects directly upon the coupling between mantle and surface processes it has recently been proposed that the modern topography of western north america is partly controlled by the removal of the subducting farallon plate and replacement of lithospheric mantle by hot asthenosphere creating surface uplift of the colorado plateau the southwestern united states and northern mexico while concomitant subsidence characterizes the central united states how the topography of the cenozoic north american cordillera evolved in the past is largely unknown yet currently debated tectonic models each have a predictable topographic response here we examine cenozoic surface uplift patterns of western north america based on a record of 3000 stable isotope proxy data this data set is consistent with eocene north to south surface uplift in the cordillera culminating in the assembly of an eocene oligocene highland 3 4 km in elevation the diachronous record of surface uplift and associated magmatism further supports tectonic models calling for the convective removal of mantle lithosphere or removal of the farallon slab by buckling along an east west axis the eocene oligocene development of rainout patterns similar to present day patterns along the flanks of the cordilleran orogen is therefore unlikely to be the result of late mesozoic crustal thickening and associated development of an andean style altiplano

light stable isotope systematics of large scale hydrologic regimes in california and nevada surface waters shallow groundwaters and precipitation samples were collected along three traverses which begin at the california coast and are oriented in an east west manner roughly parallel to the atmospheric flow path of meteoric water the hydrogen isotope compositions from short term precipitation records varied as much as 40 at one location and were not representative of the average annual isotopic composition of precipitation the hydrogen isotope ratios of surface waters and shallow groundwaters distinguish segments with isotopic variations that correlate with measured vertical fluxes of meteoric water one coastal segment appears to closely approximate an open system in which precipitation dominates over evapotranspiration most segments are described by rather regular variations in d 3 45 per 100 km which represent different degrees of closure of the hydrologic system caused by variable partitioning of precipitation between runoff and evapotranspiration all partially closed systems imply terrestrial recycling of water a simple model estimates this amount to be about 20 across northern and central california three segments representing the great basin show virtually no geographic variation and appear to represent isotopically closed systems

influence of climate change and uplift on colorado plateau paleotemperatures from carbonate clumped isotope thermometry the elevation history of earth s surface is key to understanding the geodynamic processes responsible for the rise of plateaus we investigate the timing of colorado plateau uplift by estimating depositional temperatures of tertiary lake sediments that blanket the plateau interior and adjacent lowlands using carbonate clumped isotope paleothermometry a measure of the temperature dependent enrichment of 13 c 18 o bonds in carbonates comparison of modern and ancient samples deposited near sea level provides an opportunity to quantify the influence of climate and therefore assess the contribution of changes in elevation to the variations of surface temperature on the plateau analysis of modern lake calcite from 350 to 3300 m elevation in the southwestern united states reveals a lake water carbonate temperature lct lapse rate of 4 2 0 6 c km analysis of miocene deposits from 88 to 1900 m elevation in the colorado river drainage suggests that the ancient lct lapse rate was 4 1 0 7 c km and temperatures were 7 7 2 0 c warmer at any one elevation than predicted by the modern trend the inferred cooling is plausible in light of pliocene temperature estimates off the coast of california and the consistency of lapse rates through time supports the interpretation that there has been little or no elevation change for any of the samples since 6 ma together with previous paleorelief estimates from apatite u th he data from the grand canyon our results suggest most or all of the plateau s lithospheric buoyancy was acquired 80 60 ma and do not support explanations that ascribe most plateau uplift to oligocene or younger disposal of either the farallon or north american mantle lithosphere

stable isotopic evidence for neogene surface downdrop in the central basin and range province authigenic calcite and smectite stable isotopic records were determined for five neogene terrestrial stratigraphic sections from the central basin and range province and surrounding regions we recognized a general pattern of increasing authigenic mineral 18 o values with decreasing age since the middle miocene throughout the region we suggest this pattern results from regional elevation decrease and associated changes in atmospheric circulation patterns an anomalous pattern of decreasing 18 o values with decreasing age is observed in the miocene horse spring formation exposed in the lake mead basin which is contemporaneous with local magmatic activity we suggest this pattern results from surficial uplift of local highlands feeding water into the horse spring depositional system the topographic histories suggested here provide insight into the response of the land surface to crustal processes neogene extensional deformation in the central basin and range province resulted in a net regional elevation decrease

a review of paleotemperature lapse rate methods for estimating paleoelevation from fossil floras one widely applied method for estimating paleoelevation from fossil floras uses the relation between vegetation and temperature to estimate mean annual temperature from two isochronous fossil assemblages one at elevation and one at or near sea level and the difference between these temperatures is multiplied by the reciprocal of a temperature lapse rate to estimate paleoelevation three fundamentally different approaches have been developed and these vary both in the method by which paleotemperatures are estimated and in the way by which modern lapse rates can be measured paleotemperature methodologies include the use of plant physiognomy or the use of nearest living relatives lapse rate methodologies use either a global terrestrial lapse rate as the world mean a regional terrestrial lapse rate that represents a large portion of a continent or a local terrestrial lapse rate for areas narrowly confined by latitude and longitude other variables and or corrective factors that should be considered in these applications include the thermal effects of continentality and elevated land surfaces in the continental interior and standardization to compensate for fluctuations or variability in sea level global climate change and paleolatitude these different methodologies can be contrasted by their common application to the late eocene florissant flora of colorado with results ranging from 455 to 4133 meters

formation of dolomite at 40 80 c in the latemar carbonate buildup dolomites italy from clumped isotope thermometry the temperature of formation of replacement dolomite and 18 o h 2 o of dolomitizing fluid in the latemar carbonate buildup dolomites italy were estimated independently from carbonate clumped isotope thermometry dolomite formed at 42 72 9 11 c 2 standard deviations sd from fluid with 18 o h 2 o that averages 0 3 3 3 vienna standard mean ocean water 2 sd the estimated temperature and 18 o h 2 o are similar to those of modern diffuse flow fluids at mid ocean ridges the kind of fluid that has been proposed previously as the dolomitizing fluid in the latemar buildup based on the trace element compositions of dolomite calcite in limestone preserves original 18 o but records clumped isotope temperatures 44 76 9 11 c 2 sd that are higher than those at which the limestone formed temperature recorded by calcite but not 18 o was likely reset during dolomitization clumped isotope thermometry has great potential for application to studies of burial and diagenesis by retrieving independent estimates of temperature and 18 o h 2 o with uncertainties as low as 5 c 2 standard errors se and 0 75 2 se respectively from a single stable isotope analysis of a carbonate mineral

late miocene decoupling of oceanic warmth and atmospheric carbon dioxide forcing measurements from several ocean cores reveal that ocean warmth persisted throughout the late miocene epoch despite co2 levels of only 200 350 p p m v probably driven by a deep thermocline that isolated climate responses from co2 variations geological records of past climate provide the only opportunity to investigate earth s response to large variations in atmospheric carbon dioxide concentrations during the late miocene between 12 million and 5 million years ago atmospheric co2 levels of only 200 to 350 parts per million by volume were accompanied by almost ice free conditions in the northern hemisphere and the continents were warmer than they are at present raising questions about the persistence of co2 climate couplings using measurements from several ocean cores lariviere et al show that ocean warmth persisted throughout the late miocene probably driven by a thick thermocline that isolated climate responses from co2 variations as the depth of the thermocline declined during the late miocene the more familiar relationship between co2 and climate was established deep time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration that is the atmospheric partial pressure of co2 1 although past studies have explored these responses during portions of the cenozoic era the most recent 65 5 million years myr of earth history comparatively little is known about the climate of the late miocene 12 5 myr ago an interval with values of only 200 350 parts per million by volume but nearly ice free conditions in the northern hemisphere2 3 and warmer than modern temperatures on the continents4 here we present quantitative geochemical sea surface temperature estimates from the miocene mid latitude north pacific ocean and show that oceanic warmth persisted throughout the interval of low 12 5 myr ago we also present new stable isotope measurements from the western equatorial pacific that in conjunction with previously published data5 6 7 8 9 10 reveal a long term trend of thermocline shoaling in the equatorial pacific since 13 myr ago we propose that a relatively deep global thermocline reductions in low latitude gradients in sea surface temperature and cloud and water vapour feedbacks may help to explain the warmth of the late miocene additional shoaling of the thermocline after 5 myr ago probably explains the stronger coupling between sea surface temperatures and climate that is characteristic of the more recent pliocene and pleistocene epochs11 12

distribution and provenance of the middle miocene eagle mountain formation and implications for regional kinematic analysis of the basin and range province conglomeratic strata from middle miocene sections in the central resting spring range and nearby eagle mountain california contain a clast assemblage including marble orthoquartzite fusulinid grainstone and coarse 1 m monzogabbro interstratified with tephras yielding laser fusion 40 ar 39 ar ages of 11 6 13 4 and 15 0 ma petrographic and geochronologic evidence ties the clast assemblage to a source area in the southern cottonwood mountains california 100 km west northwest of their present location in the upper 100 m of the resting spring range section conglomerates are derived almost exclusively from the southern cottonwoods source and sandstone modes are as much as 50 angular plagioclase derived from the monzogabbro the lack of dilution of this detritus by other sources and sedimentary features in both sections indicate 1 that deposition occurred on an alluvial fan with a north northeast paleoslope and 2 that transport of the gravels by sedimentary processes was probably 20 km north northeast in a direction normal to the present azimuth to their source therefore we interpret most or all of the net east southeast transport as a result of extensional and strike slip faulting between the cottonwood mountains and the resting spring range since 11 12 ma restoration of these deposits to a position 10 20 km north northeast of the eastern margin of the monzogabbro source east margin of the hunter mountain batholith yields a net tectonic displacement of the cottonwood mountains relative to the resting spring range of 104 km n67 w this result confirms previous reconstructions based on the restoration of isopachs in the cordilleran miogeocline pre cenozoic structural features and other proximal tertiary deposits in the region

active megadetachment beneath the western united states geodetic data interpreted in light of seismic imaging seismicity xenolith studies and the late quaternary geologic history of the northern great basin suggest that a subcontinental scale extensional detachment is localized near the moho to first order seismic yielding in the upper crust at any given latitude in this region occurs via an m7 earthquake every 100 years here we develop the hypothesis that since 1996 the region has undergone a cycle of strain accumulation and release similar to slow slip events observed on subduction megathrusts but yielding occurred on a subhorizontal surface 5 10 times larger in the slip direction and at temperatures 800 c net slip was variable ranging from 5 to 10 mm over most of the region strain energy with moment magnitude equivalent to an m7 earthquake was released along this megadetachment primarily between 2000 0 and 2005 5 slip initiated in late 1998 to mid 1999 in northeastern nevada and is best expressed in late 2003 during a magma injection event at moho depth beneath the sierra nevada accompanied by more rapid eastward relative displacement across the entire region the event ended in the east at 2004 0 and in the remainder of the network at about 2005 5 strain energy thus appears to have been transmitted from the cordilleran interior toward the plate boundary from high gravitational potential to low via yielding on the megadetachment the size and kinematic function of the proposed structure in light of various proxies for lithospheric thickness imply that the subcrustal lithosphere beneath nevada is a strong thin plate even though it resides in a high heat flow tectonic regime a strong lowermost crust and upper mantle is consistent with patterns of postseismic relaxation in the southern great basin deformation microstructures and low water content in dunite xenoliths in young lavas in central nevada and high temperature microstructures in analog surface exposures of deformed lower crust large scale decoupling between crust and upper mantle is consistent with the broad distribution of strain in the upper crust versus the more localized distribution in the subcrustal lithosphere as inferred by such proxies as low p wave velocity and mafic magmatism

the clumped isotope geothermometer in soil and paleosol carbonate we studied both modern soils and buried paleosols in order to understand the relationship of temperature t c 47 estimated from clumped isotope compositions 47 of soil carbonates to actual surface and burial temperatures carbonates from modern soils with differing rainfall seasonality were sampled from arizona nevada tibet pakistan and india t c 47 obtained from these soils shows that soil carbonate forms in the warmest months of the year in the late morning to afternoon and probably in response to intense soil dewatering t c 47 obtained from modern soil carbonate ranges from 10 8 to 39 5 c on average t c 47 exceeds mean annual temperature by 10 15 c due to summertime bias in soil carbonate formation and to summertime ground heating by incident solar radiation secondary controls on t c 47 are soil depth and shading site mean annual air temperature maat across a broad range 0 30 c of site temperatures is highly correlated with t c 47 from soils following the equation maat c 1 20 t c 47 0 21 72 r 2 0 92 where t c 47 0 is the effective air temperature at the site estimated from t c 47 the effective air temperature represents the air temperature required to account for the t c 47 at each site after consideration of variations in t c 47 with soil depth and ground heating the highly correlated relationship in this equation should now permit mean annual temperature in the past to be reconstructed from t c 47 in paleosol carbonate assuming one is studying paleosols that formed in environments generally similar in seasonality and ground cover to our calibration sites t c 47 0 decreases systematically with elevation gain in the himalaya following the equation elevation m 229 t c 47 0 9300 r 2 0 95 assuming that temperature varied similarly with elevation in the past this equation can be used to reconstruct paleoelevation from clumped isotope analysis of ancient soil carbonates we also measured t c 47 from long sequences of deeply buried 5 km paleosol carbonate in the himalayan foreland in order to evaluate potential diagenetic resetting of clumped isotope composition we found that paleosol carbonate faithfully records plausible soil t c 47 down to 2 5 4 km burial depth or 90 125 c deeper than this and above this temperature t c 47 in paleosol carbonate is reset to temperatures 40 c we observe 40 c as the upper limit for t c 47 in modern soils from soil depths 25 cm and therefore that t c 47 40 c obtained from ancient soil carbonate indicates substantially warmer climate regimes compared to the present or non primary temperatures produced by resetting during diagenesis if representative this limits the use of t c 47 to reconstruct ancient surface temperature to modestly buried 3 4 km paleosol carbonates despite diagenetic resetting of 47 values 18 o and 13 c values of the same deeply buried paleosol carbonate appear unaltered we conclude that solid state reordering or recrystallization of clumping of carbon and oxygen isotopes can occur in the absence of open system exchange of paleosol carbonate with significant quantities of water or other phases

late cretaceous extension in the hinterland of the sevier thrust belt northwestern utah and southern idaho cover rocks of the raft river metamorphic core complex located in the sevier belt hinterland preserve a structural and metamorphic history that predates the middle tertiary extension of the region in the eastern raft river mountains cambrian permian rocks form two allochthons that occur in the hanging wall of the mid miocene raft river detachment fault dramatically attenuated metamorphosed cambrian pennsylvanian strata of the lower allochthon yield late cretaceous muscovite sup 40 ar sup 39 ar plateau ages ca 82 90 ma that are interpreted to date cooling during final stages of layer parallel extension devonian permian rocks exposed in the black pine mountains underwent east west extension approximately 160 associated with syntectonic growth of white mica along cleavage sup 40 ar sup 39 ar whole rock spectra of associated slates also indicate a late cretaceous metamorphism together field and sup 40 ar sup 39 ar results suggest that late cretaceous extension occurred in the sevier belt hinterland at the same time as shortening in the eastern foreland and at depth in the hinterland sufficient topography must have been present to drive upper crustal extension in the eastern hinterland

bacterially induced dolomite precipitation in anoxic culture experiments to study the process of microbial mediated dolomite formation growth experiments were carried out with selected bacterial cultures under anoxic environmental conditions simulating those found in lagoa vermelha a hypersaline lagoon in brazil where dolomite precipitation occurs specifically we report the isolation of a particular strain of sulfate reducing bacteria lvform6 from lagoa vermelha sediment which apparently promotes the formation of nonstoichiometric dolomite sulfate reducing bacteria grown in a synthetic liquid medium produced dolomite during 30 days incubation at 30 c the precipitates have morphologies similar to those observed in lagoa vermelha sediment our results demonstrate that sulfate reducing bacteria can influence dolomite precipitation under controlled lowtemperature anoxic conditions and imply that anaerobic microorganisms can play an important role in carbonate sedimentation they may have been particularly significant in earth s earliest history when a more reducing atmosphere existed

hot or not impact of seasonally variable soil carbonate formation on paleotemperature and o isotope records from clumped isotope thermometry quantifying the timing and temperature of soil carbonate formation is important for interpreting isotopic records of earth surface temperature and soil water composition from paleosols pedogenic carbonates typically have been assumed to form at mean annual temperature but recent work suggests warm season bias in carbonate formation could impact the interpretation of d 18 o values and carbonate

rapid localization of pacific north america plate motion in the gulf of california correlation of late miocene volcaniclastic strata across the northern gulf of california shows that the pacific north america plate boundary localized east of the baja california peninsula ca 6 ma dextral offset of the 12 6 ma tuff of san felipe and a pair of overlying ca 6 3 ma pyroclastic flows indicate at least 255 10 km of displacement along an azimuth of 310 isopach and facies trends of the tuff of san felipe support no more than a few tens of kilometers of additional dextral displacement between 12 6 and 6 3 ma these constraints indicate that nearly all of the dextral displacement between the pacific and north american plates prior to 6 3 ma was accommodated outside of the gulf region and by 4 7 ma the plate boundary motion was localized in the gulf of california although continental extension has accounted for a component of plate boundary motion in northwestern mexico since cessation of subduction offshore of southern baja california at 12 5 ma transfer of baja california to the pacific plate was delayed by at least 6 7 m y

a new hypothesis for the amount and distribution of dextral displacement along the fish lake valley northern death valley furnace creek fault zone california nevada 1 the fish lake valley northern death valley furnace creek fault zone a 250 km long predominantly right lateral structure in california and nevada is a key element in tectonic reconstructions of the death valley area eastern california shear zone and walker lane and central basin and range province total displacement on the fault zone is contested however with estimates ranging from 30 to 63 km or more here we present a new synthesis of available constraints preextensional thrust faults folds and igneous rocks indicate that offset reaches a maximum of 50 km neogene rocks constrain its partitioning over time most offset is interpreted as 13 10 ma accruing at 3 5 mm yr in the middle of the fault zone and more slowly toward the tips the offset markers imply 68 14 km of translation between the cottonwood mountains and resting spring nopah range 60 14 km since 15 ma through a combination of strike slip and crustal extension this suggests that a previous interpretation of 104 7 km based on the middle miocene eagle mountain formation is an overestimate by 50 our results also help to mitigate a discrepancy in the 12 0 ma strain budget for the eastern california shear zone displacement has previously been estimated at 100 10 km and 67 6 km for the basin and range and mojave portions of the shear zone respectively our new estimate of 74 17 km for the basin and range is within the uncertainty of the mojave estimate

temporal variations in lake water temperature paleoenvironmental implications of lake carbonate δ18o and temperature records abstract 18 o and clumped isotope measurements of lacustrine carbonates provide important records of past terrestrial climate and paleoelevation however one of the primary challenges interpreting these data is constraining the relationship between mineral formation temperature and seasonal and mean annual climate we examined surface water temperature records from 88 lakes across the globe to develop transfer functions that relate seasonal water surface temperature to mean annual air temperature these transfer functions provide a means of reconciling proxy measures of water temperature with annual climatic conditions mean annual surface water temperature is related to mean annual air temperature maat 0 0318 t water 2 2 195 t water 12 607 r 2 0 96 independent of lake size with consistently higher mean water temperatures than air n hemisphere spring april june mean lake surface temperatures maat 0 0097 t water 2 1 379 t water 8 227 r 2 0 94 are only slightly warmer than maat for lakes in all climate zones while summer water temperatures jja maat 0 0055 t water 2 1 476 t water 18 915 r 2 0 90 may be 10 20 c warmer than maat for cold climate lakes comparison of carbonate stable isotopic data from sites with different timescales of carbonate formation such as found along latitude or elevation gradients can lead to large over or underestimates of past temperature accurate paleoclimatic interpretation of isotopic proxies or use of temperature proxy constraints i e fossil leaves to estimate lake temperature and calculate water 18 o thus requires appropriate application of water temperature air temperature transfer functions particularly when considering ancient cool high elevation lake systems

18o 16o and d h in goethite from a north american oxisol of the early eocene climatic optimum abstract an early eocene oxisol in the ione formation of california formed in a coastal continental weathering environment at a paleolatitude of 38 n the dominant minerals in the oxisol are goethite quartz and kaolinite material balance calculations were applied to new measurements of chemical composition d h and 18 o 16 o ratios of oxisol samples to determine the d 150 3 and 18 o 2 4 0 3 values of the goethite feooh these data in combination with the global meteoric water line mwl yielded an isotopic temperature of 21 4 c the nominal value of 21 c contrasts with the modern mean annual temperature mat of 16 c in that area the warmer temperature is consistent with formation of the goethite during the early eocene climatic optimum the isotopic composition of the goethite and a temperature of 21 c imply ancient water with a d value of 61 4 and a 18 o value of 8 9 0 5 this early eocene 18 o or d value is more negative than values in the range of isotopic scatter observed for modern global precipitation at sites with a mat of 21 c at times of warm global climates the location of a near surface atmospheric isotherm would generally shift relative to its location under modern climatic conditions a simple rayleigh type condensation model indicates that if one follows the isotherm the associated scatter in d and 18 o of precipitation in very warm global climates should shift for a given isotherm to more negative values that may be detectable in proxy records the isotopic results from the goethite of the early eocene oxisol appear to add to evidence in support of this idea

provenance of quartzite clasts in the eocene oligocene sespe formation paleogeographic implications for southern california and the ancestral colorado river quartzite clasts in fluvial sespe formation conglomerates of the santa monica mountains were compared petrographically with potential source rocks in the basin and range and transition zone provinces to determine their provenance modal analysis indicates that sespe orthoquartzite clasts are mainly quartzofeldspathic and derived from source rocks with craton interior provenance such clasts were probably derived from stirling quartzite and wood canyon formation sources in the northern mojave desert the mazatzal peak quartzite of central arizona is the probable source of quartzolithic orthoquartzite clasts containing abundant detrital jasper grains sespe metaquartzite clasts are mainly from sources in the southern mojave desert but some may be from the yavapai terrane of central arizona the dual provenance of orthoquartzite clasts suggests that the sespe paleoriver was a bifurcating system tapping both mojave desert and sonora desert sources after compensating for neogene displacement along the san andreas fault a paleogeographic reconstruction shows that there is a general spatial coincidence between sespe and other cenozoic paleodelta deposits around los angeles that contain exotic clasts and the inferred location of the mouth of the colorado river assuming that it existed during the eocene thus the colorado river may be much older than previously thought as old as late paleocene and perhaps was once part of an ancient fluvial connection with the mojave desert interior

oxygen isotope fractionation during synthesis of camg carbonate and implications for sedimentary dolomite formation hydrous camg carbonate was synthesized at temperatures of 40 degrees 60 degrees and 80 degrees c in the laboratory this material has very similar mineralogical characteristics to natural disordered dolomite from the coorong region in south australia besides the dolomite variable amounts of amorphous carbonate are present in all samples the oxygen isotope compositions of synthesized bulk carbonate samples e g amorphous carbonate plus dolomite plot significantly lower than the northrop and clayton 1966 dolomite water equilibrium fractionated degassing of the samples however revealed relatively low oxygen isotope values for fast reacting using 100 h3po4 amorphous carbonate in contrast slow reacting dolomite has more positive oxygen isotope values and calculated carbonate water oxygen isotope fractionation values are close to strongest known dolomite water oxygen isotope fractionation published earlier on variations of reaction stabilization temperatures during synthesis gave evidence for dolomite formation from hypersaline solutions by a dissolution reprecipitation process it is likely that amorphous carbonate has been a problem in defining the dolomite water fractionation in the past moreover dolomite associated amorphous carbonate contents probably led to incorrect speculations about lower oxygen isotope fractionation in a so called protodolomite water system copyright c 2005 elsevier ltd

oxygen isotope and paleobotanical estimates of temperature and δ18o latitude gradients over north america during the early eocene empirical estimates of climate parameters such as mean annual temperature mat are essential to describe both ancient climate and to ground truth climate model simulations of past climates in terrestrial settings no single proxy record is able to provide detailed temporal yet geographically widespread information and it thus becomes essential that paleoclimatic information obtained from different proxies be comparable as an example of how to compare estimates and to illustrate what can be learned by doing so we measured oxygen isotope ratios of phosphate in the tooth enamel of fossil mammals and the body scales of freshwater fish in order to estimate the mean annual temperature mat in which they lived the samples were collected from paleogene rocks of the bighorn green river and powder river basins of wyoming earliest eocene samples provide mat estimates that are similar to mat estimates for the same times and areas obtained from leaf margin analysis furthermore isotopic data from the paleocene eocene thermal maximum petm which has not yet yielded fossil floras indicate a significantly higher mat of 26 c we also used temperature estimates obtained by both methods to investigate paleoelevation and to reconstruct geographic patterns of mat over north america for the early eocene larger laramide basins have higher temperatures and lower inferred elevations compared to associated volcanic highlands where cooler temperatures are consistent with substantial elevations in turn latitudinal gradients in temperature indicate that warming centered on polar regions played an important role during the eocene hothouse lastly we show that the reconstructed eocene relation between oxygen isotope ratios of precipitation and mat is significantly different from the modern relation between these two variables because the relation has changed over time the modern correlation should not be used to estimate paleo mat from measurements of oxygen isotope ratios in fossil material

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

paleoelevation reconstruction using pedogenic carbonates paleoelevation reconstruction using stable isotopes although a relatively new science is making a significant contribution to our understanding of the recent growth of the world s major orogens in this review we examine the use of both light stable isotopes of oxygen and the new clumped isotope 47 carbonate thermometer in carbonates from soils globally the oxygen isotopic composition 18 o of rainfall decreases on average by about 2 8 km of elevation gain this effect of elevation will in turn be archived in the 18 o value of soil carbonates and paleoelevation can be reconstructed provided 1 temperature of formation can be estimated 2 the effects of evaporation are small 3 the effects of climate change can be accounted for and 4 the isotopic composition of the carbonate is not diagenetically altered we review data from modern soils to evaluate some of these issues and find that evaporation commonly elevates 18 o values of carbonates in deserts an effect that would lead to underestimates of paleoelevation some assessment of paleoaridity using qualitative indicators or carbon isotopes from soil carbonate is therefore useful in evaluating the oxygen isotope based estimates of paleoelevation sampling from deep 50 cm in paleosols helps reduce the uncertainties arising from seasonal temperature fluctuations and from evaporation the new clumped isotope 47 carbonate thermometer expressed as 47 offers an independent and potentially very powerful approach to paleoelevation reconstruction in contrast to the use of 18 o values nothing need be known about the isotopic composition of water from which carbonate grew in order to estimate of temperature of carbonate formation from 47 values using assumed temperature lapse rates with elevation paleoelevations can thereby be reconstructed case studies from the andes and tibet show how these methods can be used alone or in combination to estimate paleoelevation in both cases the potential for diagenetic alteration of primary carbonate values first has to be assessed clear examples of both preservation and alteration of primary isotopic values are available from deposits of varying ages and burial histories 47 values constitute a relatively straightforward test since any temperature in excess of reasonable surface temperatures points to diagenetic alteration for 18 o values preservation of isotopic heterogeneity between different carbonate phases offers a check on diagenesis results of these case studies show that one area of south central tibet attained elevations comparable to today by the late oligocene whereas 2 7 0 4 km of uplift occurred in the bolivian altiplano during the late miocene

lithospheric gravitational potential energy and past orogenesis implications for conditions of initial basin and range and laramide deformation gravitational body forces i e buoyancy forces have come to be seen as critical to the evolution of orogens nevertheless constraining the role of body forces in specific geologic scenarios is made difficult by the substantial number of poorly constrained physical parameters needed to fully relate forces to deformation by separating the calculation of buoyancy forces from the calculation of the resulting deformation models based on relatively simple descriptions of the lithosphere can yield geologically useful constraints among these are the importance of paleoelevation in driving syn and postcontractional extension and in localizing contractional strain although such phenomena have been considered in more complex models of continental deformation the simpler analysis presented here clearly establishes first order limits on lithospheric structures and paleoelevations consistent with buoyancy driven deformation in the early cenozoic great basin of the western united states we show that the low elevations inferred in much of the geologic literature are inconsistent with a body force origin for observed extensional tectonism east of the colorado plateau localization of laramide deformation coincides with pre laramide subsidence of the western interior seaway this subsidence prestressed the lithosphere making the southern rocky mountains the weak link in responding to regional compressional stress

a tibetan model for the early tertiary western united states a comparative study of the geology of the early tertiary western united states and the modern tibetan and turkish iranian plateaux indicates that these three regions situated in the hinterland of major orogenic belts share a common taphrogenic evolutionary path both the tibetan and turkish iranian plateaux have high mean elevations above sea level 5 km and 2 0 2 5 km respectively have experienced fast uplift and widespread unroofing during the last 10 8 ma and are undergoing extension nearly perpendicular to the plate convergence direction successive accretionary and collisional events accompanied by significant shortening and underplating during the formation of these plateaux resulted in the generation of overthickened continental crust 50 km that is currently in the process of extending a two stage magmatism characterized by widespread calc alkaline volcanism followed and or overlapped by bimodal basaltic to rhyolitic volcanism occurs synchronously with crustal extension and thinning the plateaux in general have an internal closed drainage system and are drained by antecedent rivers near their edges the early tertiary western us cordillera displayed similar geological and geomorphic features to those observed in the tibetan and turkish iranian plateaux in the aftermath of a prolonged mesozoic orogeny and underwent an orogenic collapse starting around mid tertiary time a n trending crustal welt with a maximum thickness of 60 km and an elevation around 3 km formed a highland with alpine flora in the hinterland of the cordilleran orogenic belt and separated internal drainage basins and saline lakes on the east from major flood plains of antecedent river systems on the west this highland subsequently became the locus of crustal extension metamorphic core complex formation and wide spread calcalkaline volcanism 9ignimbrite flare up9 this early stage of orogenic collapse was followed by the basin and range extension characterized by block faulting and bimodal volcanism starting around 18 16 ma the basin and range extension and associated subsidence resulted in development of the great basin with a mean elevation of 1 5 km above sea level and a crustal thickness of 30 km similarities in the post orogenic evolutionary paths of these three regions indicate that continental lithosphere reacts to overthickening in a consistent way through large scale crustal remobilization and extensional collapse regardless of the mode and nature of its orogenic build up

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

quantification of large uncertainties in fossil leaf paleoaltimetry 1 estimates of paleoelevation potentially constrain geodynamic models of continental deformation and inform interpretations of landscape and climate evolution one widely used paleobotanical approach reconstructs paleoelevation from the difference in estimated atmospheric enthalpy between a known sea level and a targeted coeval elevated fossil floral site enthalpy is estimated using climate leaf analysis multivariate program clamp on 31 leaf size and shape variables that have been calibrated in living forests errors related to clamp are significantly greater than often reported and there are many sources of large potential error related to this method that are either difficult to quantify or unquantifiable and are thus not documented here we quantify one significant bias toward underestimation of leaf area in the clamp data set 50 that affects all clamp climate estimates including enthalpy crucially errors in paleoelevation when the leaf size bias is included are in the range of 2 km or more at least 2 times the previous estimates and exceeding the plausible paleoelevations of many fossil sites previously published paleoelevations derived from this technique are unlikely to be accurate either in magnitude or in estimated error

collapse of the california current during glacial maxima linked to climate change on land time series of alkenone unsaturation indices gathered along the california margin reveal large 4 to 8 c glacial interglacial changes in sea surface temperature sst over the past 550 000 years interglacial times with ssts equal to or exceeding that of the holocene contain peak abundances in the pollen of redwood the distinctive component of the temperate rainforest of the northwest coast of california in the region now dominated by the california current ssts warmed 10 000 to 15 000 years in advance of deglaciation at each of the past five glacial maxima ssts did not rise in advance of deglaciation south of the modern california current front glacial warming along the california margin therefore is a regional signal of the weakening of the california current during times when large ice sheets reorganized wind systems over the north pacific both the timing and magnitude of the sst estimates suggest that the devils hole nevada calcite record represents regional but not global paleotemperatures and hence does not pose a fundamental challenge to the orbital milankovitch theory of the ice ages

late paleocene high laramide ranges in northeast wyoming oxygen isotope study of ancient river water abstract the distribution and initial timing of the establishment of high surface elevations in the rocky mountains during the early cenozoic remain controversial despite the importance of these data in testing tectonic models for this region we track the timing and distribution of high elevation in the rocky mountains during the late cretaceous early eocene by examining annual and seasonal 18o values of the ancient river water which are calculated from the 18o values of well preserved freshwater bivalve shells in the powder river basin of the eastern laramide province the 18o values of the ancient river water vary between 23 0 and 8 0 smow in both seasonal and annual records in the late paleocene early eocene the large variation suggests that the ancient rivers were fed yearly or seasonally by snowmelt from highlands of 4 5 1 3 km this can be explained by the existence of the bighorn mountains and black hills with a drainage pattern similar to the present in northeast wyoming the 18o values of ancient river water along the front of the sevier thrust belt generally follow a trend from lower values in north 14 2 1 4 in the early paleocene crazy mountains basin to higher values in south 11 1 0 8 in the late paleocene bighorn basin and 7 1 1 6 in the early eocene washakie basin the variations within each basin are relatively small these rivers most likely rise in the sevier thrust belt and may reflect highland elevation of 1 2 km the 18o values in the alberta foreland and williston basin are very low 20 5 in the late cretaceous indicating the rivers were fed by snowmelt from the canadian rocky mountains of 4 3 1 0 km high the attainment of high elevation in the eastern laramide province prior to the western province could be explained by southwestward progression of back thrusts soled into an earlier east directed master detachment which may be formed by the westward rollback of subducted shallow slab

post laramide removal of the farallon slab western united states i propose that post laramide removal of the subhorizontally subducting farallon slab occurred by buckling downward along an approximately east northeast trending axis this process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the united states and propagation of the separated edges toward the central axis of downwelling accompanied by aesthenosphic upwelling behind the trailing edges initial buckling probably began near 50 ma and slab removal was complete by 20 ma this model is based primarily on the space time evolution of the ignimbrite flare up a major mid tertiary igneous event of mantle origin which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central nevada from the north and southeast post laramide uplift extension establishment of the cascadia subduction zone and active magmatism may be consequences of lithosphere scale modifications caused by the laramide removal of the slab and the resulting asthenospheric upwelling

sedimentologic and isotopic constraints on the paleogene paleogeography and paleotopography of the southern sierra nevada california detrital zircon u pb provenance and stable isotopic studies of three paleogene southern sierra nevada california basins place new constraints on the paleoelevation history of the region age spectra from the paleocene witnet formation within the southernmost sierra nevada link these sediments to source terranes that were at or near sea level in the early cenozoic while age spectra from the paleocene goler formation east of the sierra nevada demonstrate isolation of southern sierra nevada basins from the continental interior and tapping of jurassic and triassic arc flank sources during the paleocene west of the sierra nevada strata of the eocene tejon formation are dominated by cretaceous zircons sourced from the sierran batholith goler formation carbonate 18 o suggests paleocene paleoelevations of 1 2 km for the central and southern sierra nevada taken together these data indicate a paleogene southern sierra nevada with modest elevations locally dissected to sea level by rift basins formed by late cretaceous lithospheric collapse these results place new limits on the amount of regional middle to late cenozoic elevation gain that may have resulted from the loss of dense mantle lithosphere from below the central and southern sierra nevada and point to possible north south variations in the topographic evolution of the sierra nevada

paleosol geochemistry of the late paleocene goler formation of southern california abstract paleosol fossilized soil geochemistry can provide a record of paleoclimatic conditions due to the relationships between pedogenic processes and climate in this study paleosols from the late paleocene goler formation of southern california were used to determine the paleoclimatic conditions active during pedogenesis the enrichment and retention of soluble elements ca mg and na and the mobilization of fe and mn within the paleosol profiles suggest a climate with strongly seasonal precipitation similarly variations in the mn content and 13c ratio of pedogenic carbonate nodules also reflect seasonal precipitation regional paleotopography was likely an important control on the paleoclimate of the goler formation due to rain shadow effects although the size and location of these ancient mountains are poorly constrained the identification of climatic effects specifically associated with these variables provides new constraints

stable isotope composition of waters in the great basin united states 1 air mass trajectories 1 isentropic trajectories calculated using the noaa climate monitoring and diagnostics laboratory s isentropic transport model were used to determine air parcel origins and the influence of air mass trajectories on the isotopic composition of precipitation events that occurred between october 1991 and september 1993 at cedar city utah and winnemucca nevada examination of trajectories that trace the position of air parcels backward in time for 10 days indicated five distinct regions of water vapor origin 1 gulf of alaska and north pacific 2 central pacific 3 tropical pacific 4 gulf of mexico and 5 continental land mass deuterium d and oxygen 18 18o analyses were made of precipitation representing 99 of all cedar city events similar analyses were made on precipitation representing 66 of the precipitation falling at winnemucca during the same period the average isotopic composition of precipitation derived from each water vapor source was determined more than half of the precipitation that fell at both sites during the study period originated in the tropical pacific and traveled northeast to the great basin only a small proportion traversed the sierra nevada the isotopic composition of precipitation is determined by air mass origin and its track to the collection station mechanism of droplet formation reequilibration within clouds and evaporation during its passage from cloud to ground the rayleigh distillation model can explain the changes in isotopic composition of precipitation as an air mass is cooled pseudo adiabatically during uplift however the complicated processes that take place in the rapidly convecting environment of cumulonimbus and other clouds that are common in the great basin especially in summer require modification of this model because raindrops that form in the lower portion of those clouds undergo isotopic change as they are elevated to upper levels of the clouds from where they eventually drop to the ground

δ18o and δd of streamwaters across the himalaya and tibetan plateau implications for moisture sources and paleoelevation reconstructions abstract this study presents new 18o and d data from 191 streams across the himalaya and tibetan plateau to better constrain the spatial variability of stable isotopes in modern precipitation over this region moisture penetrating into the southeastern tibetan plateau is predominantly derived from monsoonal airmasses originating from the bay of bengal and transported into the eastern himalayan syntaxis along the brahmaputra river progressive rainout during orographic lifting and cooling results in clear relationships between 18o and d and catchment hypsometric elevation on the plateau margin however monsoonal derived moisture is progressively mixed with central asian airmasses in more western and northern parts of the tibetan plateau as a result predicted isotope elevation relationships that are based on empirical lapse rates or thermodynamic models of the isotopic evolution of an airmass produce large 1 3 km misfits between measured and predicted catchment elevations for much of the tibetan plateau including some areas directly north of the central himalayan crest this suggests that changes in the 18o or d of paleoprecipitation on the central and southwestern tibetan plateau may reflect surface uplift along moisture transport pathways or changes in the penetration of monsoonally derived moisture rather than regional surface uplift histories

deuterium and oxygen isotopes paleoelevations of the sierra nevada and cenozoic climate although geomorphic observations suggest that the sierra nevada has tilted so that the crest has risen 1 2 km since late miocene time deuterium and oxygen 18 isotope concentrations in cenozoic geologic materials decrease eastward across california and nevada similarly to those in modern orographically induced precipitation as if little change in sierra nevadan elevations has occurred since eocene time orographic precipitation however depends on the amount of moisture in the atmosphere which in turn can be much larger in warm air as in eocene or oligocene time and in summer than in the cooler air characteristic of present day dominantly winter precipitation moreover the integrated rainout of vapor and hence presumably in stable isotope concentrations in the remaining vapor depends largely on the difference in heights traversed by air masses not slopes of mountain ranges thus if due simply to orographically induced rainout both eocene and oligocene variations in deuterium isotopes across the sierra nevada and miocene quaternary differences in deuterium and oxygen isotopes between the great valley of california and the basin and range place only weak constraints on the slope or past elevations of the sierra nevada they do not necessarily contradict the inference that the crest of the sierra nevada has risen 1000 m or more since late miocene time

paleoelevation estimates for the northern and central proto basin and range from carbonate clumped isotope thermometry 1 quantitative paleoelevation histories can help explain both why and how widespread cenozoic extension occurred in the basin and range province of western north america we present new estimates of preextensional paleoelevations for the northern and central basin and range using clumped isotope 47 thermometry of lacustrine carbonates collected from each region comparison of carbonate 47 derived mean annual air temperature maat estimates 16 c 20 c for the late cretaceous eocene sheep pass basin of east central nevada with published maat estimates for the eocene coastal northern sierra nevada 20 c 25 c suggests that the early paleogene sheep pass basin had a paleoelevation of 2 km such a modest paleoelevation suggests that either 1 the proto northern basin and range did not attain maximum paleoelevations of 3 4 km until the late eocene early oligocene or 2 the sheep pass basin was a local high relief 1 km setting contained within a 3 km orogenic highland nevadaplano similarity of 47 derived maat estimates 17 c 24 c for carbonates from the central basin and range and the near sea level southern sierra nevada bena basin indicate that middle miocene paleoelevations in the death valley region were 1 5 km these fairly low paleoelevations are incompatible with preextensional crustal thicknesses 52 km and indicate that mean elevation change was minor 500 m and lithospheric mass was not conserved during 100 neogene extension of the central basin and range but was instead likely compensated by synextensional magmatic additions to the crust

rapid uplift of the altiplano revealed through 13c 18o bonds in paleosol carbonates the elevation of earth s surface is among the most difficult environmental variables to reconstruct from the geological record here we describe an approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope compositions of soil waters constrained by measurements of abundances of 13 c 18 o bonds in soil carbonates we use this approach to show that the altiplano plateau in the bolivian andes rose at an average rate of 1 03 0 12 millimeters per year between 10 3 and 6 7 million years ago this rate is consistent with the removal of dense lower crust and or lithospheric mantle as the cause of elevation gain

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

synconvergent surface breaking normal faults of late cretaceous age within the sevier hinterland east central nevada the hinterland of the sevier orogenic belt of western north america is widely interpreted as a cretaceous to paleogene orogenic plateau although evidence for mid crustal extension of late cretaceous age within the sevier hinterland is widespread coeval surface breaking normal fault systems have not been documented new 1 12 000 scale mapping within the type section of the latest cretaceous to eocene sheep pass formation of east central nevada suggests that deposition occurred in response to normal fault movement recording up to 4 km of late cretaceous and paleogene stratigraphic throw intrabasinal normal faulting caused lateral thickness variations within the basal sheep pass formation although upper members are largely unaffected an extensional basin setting best explains the fanning of bedding dips the deposition of megabreccia and the presence of syndepositional normal faults within the sheep pass formation deposition of the basal member of the sheep pass formation is bracketed between ca 81 3 3 7 ma and 66 1 5 4 ma based on the u th he cooling ages of detrital zircons and on a u pb carbonate age derived from the overlying lacustrine limestone member these new data provide the strongest evidence to date for the existence of late cretaceous surface breaking normal faults in the sevier hinterland normal faulting was coeval with mid crustal hinterland extension and with continued contraction within the sevier foreland to the east