timing and episodicity of middle eocene volcanism and onset of conglomerate deposition idaho seventeen 40 ar 39 ar incremental release mineral ages from lava flows and tuffs in the lost river and lemhi ranges idaho 1 demonstrate deposition of thick sequences of syntectonic conglomerate in middle eocene and oligocene time and 2 reveal two temporally and compositionally distinct phases of the challis volcanic group thick sections of coarse conglomerates are preserved in two nnw trending half graben these were previously assigned to the pliocene but are reassigned to the eocene and oligocene based on ages from four tuffs interbedded with conglomerates andesitic and dacitic lava flows and tuffs of the challis volcanic group up to 2 5 km thick were deposited between about 49 and 48 ma whereas rhyolite tuffs only tens of meters thick accumulated between about 46 and 45 5 ma the rhyolite tuffs are intercalated within or underlie the syntectonic conglomerates lithologic stratigraphic and geochronologic evidence correlate these tuffs with the tuff of challis creek which erup

mesozoic tectonics and metamorphism in the pequop mountains and wood hills region northeast nevada implications for the architecture and evolution of the sevier orogen discussion and reply the pequop mountains wood hills east humboldt range region northeast nevada exposes a nearly continuous cross section of precambrian to mesozoic strata representing middle to upper crustal levels of the mesozoic hinterland of the sevier orogen these rocks preserve the transition from unmetamorphosed mesozoic upper crust to partially melted middle crust integration of new structural metamorphic and u pb thermochronologic data from the wood hills and pequop mountains coupled with a regional tectonic reconstruction reveals substantial cretaceous metamorphism contraction and extension in the sevier hinterland in northeast nevada we report two phases of contraction not previously recognized that are accommodated by top to the southeast thrust faults the windermere and independence thrusts contraction was succeeded by two phases of extension along west rooted normal faults the late cretaceous pequop fault and tertiary mary9s river fault system the earliest phase of thrust faulting resulted in as much as 30 km of crustal thickening and an estimated minimum of 69 km of shortening along an inferred fault called the windermere thrust the timing of this thrusting event is bracketed between late jurassic ca 153 ma and late cretaceous 84 ma relaxation of crustal isotherms following and perhaps during thrusting resulted in barrovian style metamorphism of footwall rocks and partial melting of metapelite at deep levels peak metamorphism was attained ca 84 ma and by this time hinterland crustal thickening had reached a maximum during 84 75 ma another minor pulse of shortening and thickening along the independence thrust was followed by partial exhumation of the metamorphic rocks and as much as 10 km of crustal thinning along the pequop fault thus the interval from 84 to 75 ma in northeast nevada marks a fundamental and apparently permanent change from horizontal contraction to extension in the upper to middle crust in the hinterland final exhumation of the metamorphic rocks was accomplished by the tertiary mary9s river fault system our data indicate that much of the metamorphism and some of the contraction in the sevier hinterland in northeast nevada which was previously thought to be largely late jurassic is actually cretaceous in age furthermore the data indicate that widespread metamorphism of the middle crust is a byproduct of tectonic burial and that hinterland and foreland thrust faulting were coeval suggesting that thrust faults in the sevier orogen do not form a simple foreland younging sequence

precise dating of biotite in distal volcanic ash isolating subtle alteration using 40ar 39ar laser incremental heating and electron microprobe techniques precise dating of k rich minerals in volcanic rocks via the 40 ar 39 ar geochronometer has become crucial to resolving many geological problems in some cases ash beds containing biotite phenocrysts but lacking sanidine are the only datable horizons at key stratigraphic intervals necessitating comparison of sanidine and biotite ages within the same chronostratigraphic framework to assess the integrity of biotite and evaluate the accuracy of ages obtained from this often problematic mineral incremental heating 40 ar 39 ar experiments were performed on large millimeter sized euhedral biotite crystals from four key eocene tuffs in the green river and wind river basins and are compared with sanidine 40 ar 39 ar ages unaltered biotite crystals with homogenous k compositions from two tuff beds yielded concordant plateau ages that are indistinguishable from cogenetic sanidine ages in contrast biotite crystals from two other tuffs yielded discordant spectra with relatively young initial steps followed by older down stepping apparent ages plateau ages from the discordant experiments are older than the sanidine ages by 1 to 14 integrated total fusion ages from these experiments exhibit scatter toward both younger and older ages that correlate with the degree of spectral discordance electron microprobe transects reveal that biotite crystals yielding discordant age spectra contain 1 10 m thick k depleted 8 k 2 o alteration zones along internal cleavage planes that are absent in biotite crystals yielding concordant age spectra we propose that these altered zones promote open system behavior phenomena such as loss of k or 40 ar 39 ar k recoil into internal k depleted phases and 39 ar k recoil entirely out of internally corroded biotite crystals are all potential mechanisms that can be related to this alteration due to the presence of these multiple potential pathways that promote the gain or loss of isotopes such altered biotite crystals are unsuitable for high resolution 40 ar 39 ar dating comparisons with sanidine ages or calibration of the geomagnetic polarity timescale

profile of a paleo orogen high topography across the present day basin and range from 40 to 23 ma records of past topography connect earth s deep interior to the surface reflecting the distribution of heat and mass past crustal structure and plate interactions many tectonic reconstructions of the north american cordillera suggest the presence of an altiplano like plateau in the location of the modern basin and range with conflicting timing and mechanisms for the onset of surface lowering extension and orogen collapse here we show through a paleotopographic profile that from the eocene to the oligocene a high broad orogen stretched across nevada with a distinct crest that divided a continuous westward draining slope extending to central california from an internally drained eastern nevada plateau this paleo orogen maintained demonstrably higher than modern elevations reaching 3500 m in the late oligocene despite the long term high gravitational potential energy of the crust supporting this topography surface lowering extension did not occur until the transition to a transform margin changed the external kinematic framework of the system maximum surface lowering was spatially decoupled from brittle upper crustal extension requiring a large component of mid crustal flow

near surface diagnostics of dripping or delaminating lithosphere 1 in various geological regions it has been postulated that the mantle lithosphere has been thinned or completely removed two of the primary removal mechanisms that have been put forward include 1 delamination a wholesale peeling away of a coherent block of the mantle lithosphere and 2 lithospheric dripping viscous rayleigh taylor instability of the mantle lithosphere using computational models we investigate several near surface observables to determine if these may be diagnostic of either often ambiguous removal mechanism surface topography associated with delamination has a broad region of uplift above the lithospheric gap and a localized and mobile zone of subsidence at the delaminating hinge with dripping lithosphere the topographic expression is symmetric and fixed above the downwelling delamination of mantle lithosphere is more efficient than dripping for thermal heating of the crust the onset is more rapid and the elevated temperatures persist longer the resultant crustal p t t paths show modest pressure variations and high temperature increases with large scale delamination or dripping delamination also causes contraction directly above the migrating hinge and distal extension dripping lithosphere induces superimposed contraction and extension above and symmetric about the viscous instability for all the observables if only a portion of the mantle lithosphere is removed by viscous instability delamination inherently removes all of the mantle lithosphere the differences between the two removal mechanisms are even more pronounced with only partial removal of the mantle lithosphere uppermost mantle lithosphere remains well coupled to the crust leading to lower surface temperature variations and broad zones of crustal deformation thickening

capture of high altitude precipitation by a low altitude eocene lake western u s sedimentary facies of the eocene green river formation refl ect a rapid increase in water supply to lake gosiute ca 49 ma marked by a stratigraphic fi ll to spill surface deposits below this surface constitute repetitive lacustrine expansion desiccation cycles whereas those above consist of continuous profundal lacustrine mudstone grading upward into volcaniclastic deltaic sandstone above the fi ll to spill surface calcitic mudstone 18 o decreases from 26 to 20 over an interval representing 100 k y we interpret this shift to have resulted from capture of a foreland river or rivers that drained higher topography north of lake gosiute most likely in north central idaho accurate paleoelevation estimates derived from stable isotopic records in intermontane basins thus may require detailed knowledge of regional drainage systems

subduction adjacent to deep continental roots enhanced negative pressure in the mantle wedge mountain building and continental motion abstract subduction near to a deep continental root is modeled with finite elements the root restricts asthenospheric corner flow which increases mantle wedge suction the suction pulls the continental root toward the subduction zone simultaneously promoting trenchward continental motion and compressive tectonics in the lithosphere between the root and the subduction zone in a 2 d model with a root that extends to 250 km depth and lies 300 km from the mantle wedge corner we find two fold increase in mantle wedge suction and nearly a four fold increase in continental compression to 5 10 12 n m compared to a rootless reference model in 3 d modeling we find that distance to the subduction zone is more important than root width in subduction strike direction or reasonable variations in sub lithospheric viscosity structure with the root enhanced suction effects diminishing rapidly as the wedge corner root distance increases beyond 400 km when finite width roots are wider than the distance to the mantle wedge corner and closer than 400 km compression of the continent between the root and the subduction zone is 50 that found in the infinite width root

the cenozoic climatic and topographic evolution of the western north american cordillera herein we present oxygen isotope records from cretaceous to recent terrestrial sediments in the western north american cordillera the purpose of this analysis is to use oxygen isotope records to understand the coupled surface elevation and climate histories of this region through the cenozoic to do this we constructed 18o maps of surface waters for time intervals that trace the development of topography of western north america these maps are based on 4861 oxygen isotope analyses from both published 4478 and new 383 data we determined the 18o values of surface waters using paleotemperatures derived previously from floral assemblages and the appropriate isotope fractionation factors these data suggest that in the late cretaceous to early eocene the sevier hinterland formed a plateau of unknown height around 50 ma a topographic wave developed in british columbia and eastern washington that swept southward reaching northeastern nevada at 40 to 38 ma and southern nevada 23 ma this southward encroachment of an eocene plateau sweep caused reorganization of drainage patterns such that the intraforeland basins of wyoming and utah drainages extended deep within the sevier hinterland as the wave swept southward the landscape within the sevier hinterland developed into a rugged and high mountain range with the hypsometric mean elevation of 4 km and relief of 1 5 km this eocene highland was bordered on the west by a high sierra nevada ramp and on the east by the intraforeland basins that captured water draining these growing highlands the spatial and temporal evolution of this highland correlates with the timing of volcanism and extension these observations support tectonic models that call for north to south removal of the farallon slab or piecemeal removal of mantle lithosphere the isotopic data show that prior to growth of this highland the north american monsoon nam penetrated much farther north in the paleocene eocene than today the combined effects of global cooling increasing latitudinal temperature gradients and the generation of the orographic barrier created by the growing north to south highland produced a southward migration of the nam front by the oligocene the hydrologic regime that we observe today was in place it has been modified since then as a result of basin and range extension and collapse of the highlands in the mid miocene this collapse allowed the nam to penetrate farther north into the great basin of nevada and utah

elevation or alteration evaluation of isotopic constraints on paleoaltitudes surrounding the eocene green river basin low oxygen isotope values 16 relative to peedee belemnite standard obtained from microbial carbonates of the green river formation have been interpreted as evidence for snowmelt and high elevations surrounding this early eocene lake basin however low values from microbial precipitates could also represent a diagenetic overprint we investigate these alternate hypotheses by measuring the oxygen isotope composition of altered and unaltered freshwater bivalves from the basin we analyzed subannual samples from 3 individuals and bulk samples from 54 individuals subannual samples exhibit clear seasonal fluctuations in oxygen isotope values these fluctuations are large enough to require seasonal shifts in temperature and or the oxygen isotope value of the lake water however the lowest value of any unaltered sample either bulk or high resolution corresponds to lake water 18 o of 12 relative to standard mean ocean water and is not low enough to require the addition of snowmelt to the lake a few bulk samples exhibit very low oxygen isotope values which would seem to suggest snowmelt however these samples also show clear evidence of calcite in x ray diffraction patterns demonstrating the presence of diagenetic precipitates given that 1 diagenetic alteration is a plausible explanation of very low 18 o values 2 alteration has not been examined in microbial carbonates and 3 unaltered aragonitic bivalves provide no independent evidence for low 18 o values for lake waters we conclude that robust isotopic evidence for high elevations surrounding the green river basin during the eocene is currently lacking

physical and chemical signatures of a developing anticyclonic eddy in the leeuwin current eastern indian ocean a multidisciplinary cruise aboard the r v southern surveyor was conducted in may 2006 to sample a developing anticyclonic eddy of the leeuwin current off western australia the eddy formed from a meander of the leeuwin current in mid april 2006 and remained attached to the current until mid august in this study a combination of satellite data altimeter sea surface temperature and chlorophyll a and shipboard measurements acoustic doppler current profiler and conductivity temperature depth were used to characterize the physical and chemical signatures of the eddy the temperature salinity properties of the mixed layer waters within the anticyclonic eddy and on the shelf were both connected to that of the leeuwin current indicating the water mass in the eddy is mainly derived from the leeuwin current and the modified leeuwin current water on the shelf above the salinity maximum near the eddy center there was a regionally significant concentration of nitrate 0 9 mol l 1 and the maximum 2 mol l 1 was at 150 in depth below the photic zone nitrification within the eddy and or local upwelling due to the forming eddy could be responsible for this high concentration of nitrate near the eddy center which potentially makes the eddy a relatively productive feature in the leeuwin current

re evaluation of the ages of 40ar 39ar sanidine standards and supereruptions in the western u s using a noblesse multi collector mass spectrometer abstract the rewards from resolving the tempo of magmatic paleobiologic paleoclimatic and tectonic processes at better than the per mil level of precision have spurred a quest within the geochronology community to improve the precision accuracy and calibration of several chronometers including the 40 ar 39 ar variant of the k ar clock a new generation of multi collector mass spectrometers for 40 ar 39 ar geochronology provides the means to address these issues as well as to evaluate the ages of 40 ar 39 ar standards and key marker horizons deposited by large volume caldera forming supereruptions in the western u s at unprecedented temporal resolution we highlight the utility of a 5 collector noblesse mass spectrometer that has a sufficiently high mass resolving power to distinguish 36 ar h 35 cl from a 36 ar h 35 cl 12 c 3 sum peak at m e 36 maintains a remarkably low and stable background and thus yields 40 ar 39 ar dates of exceptionally high precision we have calibrated a reference gas and developed a simple analytical routine involving one peak hop both of which are used to evaluate detector bias correction factors because the 40 ar 39 ar chronometer is a relative dating technique that relies on calibration against mineral standards of known age precise and accurate determination of the ages of these standards and verification of isotopic composition and homogeneity is essential we present results from both total fusion and incremental heating experiments on single crystals of the widely used alder creek acs and fish canyon fcs sanidine standards several recent independent measurements of acs using multi collector mass spectrometers including those presented herein favor an age of 1 1864 0 0003 0 0012 ma 95 confidence analytical full external uncertainty that is 1 younger than the widely used value obtained by nomade et al 2005 using a single collector mass spectrometer incremental heating experiments on single crystals of fcs most commonly do not yield plateau ages instead many crystals appear to be affected by ar loss or excess ar our findings obtained using ion counters independently confirm those recently obtained using a multiple faraday detectors or a combination of faraday and ion counting detectors thus careful consideration must be given to using fcs in future high precision 40 ar 39 ar calibrations of the geologic timescale notwithstanding 40 ar 39 ar dating of sanidine using the 5 collector noblesse can now provide leverage critical to interpreting the large range of u pb zircon dates commonly found in rhyolitic lavas and tuffs

high times on the tibetan plateau paleoelevation of the thakkhola graben nepal east west extension in the tibetan plateau is generally assumed to have resulted from gravitational collapse following thickening and uplift on the basis of this assumption several studies have dated east west extensional structures to determine when the plateau attained its current high elevation however independent estimates of elevation are needed to determine whether extension occurred before during or after the plateau achieved its current elevation because the isotopic composition of meteoric water decreases with increasing elevation significant change in local elevation throughout the thakkhola graben depositional history should be recorded by change in 18 o values of fluvial and lacustrine carbonates the 18 o values of 16 to 23 of thakkhola graben carbonates reflect meteoric water values similar to modern values and suggest that the southern tibetan plateau attained its current elevation prior to eastwest extension initiation of thakkhola graben extension is constrained between 10 and 11 ma based on magnetostratigraphy of the older tetang formation the 13 c values of soil carbonates suggest an age younger than 8 ma for the base of the thakkhola formation

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

dynamic models for metamorphic core complex formation and scaling the role of unchannelized collapse of thickened continental crust abstract metamorphic core complexes at collisions between cratons and softer terranes such as in the northern north american cordillera share a set of characteristic features including spatial and temporal association of ductile mid crustal deformation with brittle normal faulting spatial coincidence with prior crustal thickening characteristic spatial scaling and limited duration and extent of deformation these properties are reproduced in numerical solutions for gravity driven collapse of a viscous crustal region under conditions where vertical stress is continuous through thickened lithosphere rigid deformable conditions such solutions allow inversion for effective mechanical properties and crustal geometry from direct observations of aspect ratio and exhumation velocity in the northern rockies core complex geometry is consistent with a twofold decrease in viscosity of the thickened cordilleran crustal welt

the 40ar 39ar ages and tectonic setting of the middle eocene northeast nevada volcanic field widespread middle to late eocene calc alkalic volcanism which formed the northeast nevada volcanic field marks the earliest tertiary volcanism in the northern basin and range the central part of this major field in northeast nevada and adjacent utah is herein defined by 23 40ar 39ar ages that range from 42 6 to 39 0 ma rock chemistry from 12 localities stratigraphic position of the volcanic rocks above a regional middle eocene unconformity volcanic setting and lithology the type area is at nanny creek in the northern pequop mountains nevada where rhyolite ash flow tuffs are overlain by a thick section of intercalated andesitic to dacitic flows and flow breccias and rhyolite ash flow tuffs the intermediate composition rocks are locally derived throughout the volcanic field whereas the sources for rhyolite ash flow tuffs are unknown the uniform and widespread occurrence of the andesitic and dacitic flows and flow breccias strongly suggests that the upper crust was perforated by intermediate composition magma across the entire region in the central part of the field the middle eocene volcanic rocks rest with depositional angular discordance on deformed middle paleozoic to triassic strata ostracode bearing limestone probably of early eocene age is locally present below the volcanic rocks in the western and southeastern parts of the field these middle eocene volcanic rocks rest with depositional angular discordance on lower eocene lacustrine strata of the elko and white sage formations respectively this angular discordance documents a middle eocene deformational event previously unrecognized in the region

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

spatial and temporal variation of cenozoic surface elevation in the great basin and sierra nevada the surface uplift of mountain belts caused by tectonism plays an important role in determining the long term climate evolution of the earth however the general lack of information on the paleotopography of mountain belts limits our ability to identify the links and feedbacks between topography tectonics and climate change on geologic time scales here we present a 1 8 o and d record of authigenic minerals for the northern great basin that captures the timing and magnitude of regional surface uplift and subsidence events in the western united states during the cenozoic authigenic calcite smectite and chert 1 8 o values suggest the northern great basin region experienced 2km of surface uplift between the middle eocene and early oligocene followed by 1 to 2km of surface subsidence in the southern great basin and or sierra nevada since the middle miocene these data when combined with previously published work show that the surface uplift history varied in both space and time surface uplift migrated from north to south with high elevations in southern british columbia and northeastern washington in the middle eocene and development of surface uplift in north and central nevada in the oligocene this pattern of north to south surface uplift is similar to the timing of magmatism in the western cordillera a result that supports tectonic models linking magamtism with removal of mantle lithosphere and or a subducting slab

movable bed roughness in the flow of irregular waves and currents over movable beds the bed roughness ks in the flow of irregular waves and currents over a movable bed is studied on the basis of measured current profiles and the model of you 1994 it is found that the movable bed roughness kt is affected by both waves and currents and that the existing formulae derived in purely oscillatory flow generally fail to predict ks in the flow of waves and currents a new formula is proposed to calculate k in the flow of irregular waves and currents over a movable bed the present bed roughness formula together with you s 1994 model gives better prediction of current shear velocity u and especially apparent roughness z than the existing formulae derived in purely oscillatory flow

hinterland drainage closure and lake formation in response to middle eocene farallon slab removal nevada u s a abstract hinterland basins can accumulate high resolution archives of orogenic processes and continental climate but are challenging to reconstruct due to tectonic overprinting and the inherent complexity of their lithofacies assemblages the cordilleran hinterland of northeast nevada has been interpreted to have overlain a flattened farallon slab from the late cretaceous to eocene slab removal and advection of asthenospheric mantle beneath nevada have been invoked to explain a southwestward migrating wave of eocene to oligocene volcanism and proposed as a driver for topographic uplift however the timing of slab removal and possible subsequent delamination of north american lithospheric mantle can only ambiguously be related to the surface record subsequent neogene extension and basin filling has complicated the correlation and interpretation of strata that record these events here we apply single crystal sanidine 40ar 39ar geochronology to 26 ash beds in northeast nevada to reconstruct paleogene geographic and hydrologic evolution we use these ages and legacy geochronology to compare lithofacies and isotope proxy records of meteoric waters to regional tectonics and global climate and assess competing tectonic interpretations for lake basin formation lakes formed locally prior to ca 48 7 ma in northeast nevada coeval with foreland lakes of the green river formation the most expansive phase of lacustrine deposition resulted in onlap onto locally derived fluvial deposits and folded paleozoic bedrock and occurred between ca 43 4 and ca 40 8 ma elko formation strata exhibit a basin wide transition from fluvial lacustrine to fluctuating profundal lithofacies at ca 42 7 ma suggesting a shift towards regional hydrologic closure the stromatolitic upper elko formation is intercalated with ash fall tuffs and several partially welded to unwelded ignimbrites from increasingly proximal volcanism elko formation deposition ended by ca 40 4 ma 40ar 39ar ages for seven ash beds in the dead horse formation at copper basin in northern elko county indicate intermittent ash bed deposition between 45 2 ma and 38 6 ma and an episode of lacustrine deposition between 39 8 ma and 38 6 ma that post dates the main phase of lake elko d values of volcanic glass sampled from dated ash beds reflect changes in the hydrogen isotope compositions of local eocene waters and systematically vary by 80 102 according to their depositional environment the elko formation and overlying volcanic strata are overlain regionally by a pronounced unconformity of 20 m y in the copper basin area deposition continued locally into the oligocene in the hanging wall of a ductile detachment the geochronologic and isotopic framework presented here permits reanalysis of the pinon range carbonate proxy record that was previously interpreted to record both regional uplift and the middle eocene climatic optimum new data suggest instead that isotope values of hydration waters within the elko formation were strongly influenced by evaporation and a change from lacustrine to non lacustrine conditions can account for the 18 o shift that was interpreted to reflect regional uplift moreover the end of elko formation deposition predated the middle eocene climatic optimum we interpret the overall record of drainage ponding and paleovalley inundation progressively more evaporative lacustrine conditions increasingly proximal volcanism and subsequent prolonged unconformity to reflect the surface effects of progressive ne to sw removal of the farallon slab

influence of magmatic hydrothermal activity on brine evolution in closed basins searles lake california evaporites in a 700 m long core km 3 from searles lake california preserve a record of the chemical evolution of inflow waters chemical analyses of fluid inclusions in halite and evaporite minerals show that the major ion composition of inflow waters to searles lake was changed by distant hydrothermal activity associated with magmatism at long valley caldera between 1 27 and 1 0 m y ago below core depths of 291 m the evaporites consist of ca bearing sulfates anhydrite glauberite and halite fluid inclusions in the halite show that parent waters were na cl so42 rich waters above 291 m the evaporites include sodium carbonates pirssonite trona and halite and fluid inclusion brines are na k hco3 co32 cl so42 rich these fluctuations in mineralogy and brine chemistry document an alkalinity spike beginning between 1 27 and 1 0 ma when inflow waters to searles lake crossed the caco3 chemical divide and began to produce alkaline brines that precipitated trona upon evaporation the owens river is a modern chemical analog for inflow water into searles lake beginning between 1 27 ma and 1 0 ma a major contributor of solutes to the owens river is hot creek in long valley caldera which is fed by hydrothermal springs with high alkalinity from magmatically derived co2 the timing of magmatism in owens valley and the appearance of sodium carbonate minerals in core km 3 suggest a causal relationship volcanism and hydrothermal activity provided co2 and elevated alkalinity to searles lake inflow waters 0 5 0 2 m y before the eruptions that formed the bishop tuff and long valley caldera

lithosphere delamination with foundering of lower crust and mantle caused permanent subsidence of new caledonia trough and transient uplift of lord howe rise during eocene and oligocene initiation of tonga kermadec subduction western pacific 1 we use seismic reflection and rock sample data to propose that the first order physiography of new caledonia trough and norfolk ridge formed in eocene and oligocene time and was associated with the onset of subduction and back arc spreading at the australia pacific plate boundary our tectonic model involves an initial cretaceous rift that is strongly modified by cenozoic subduction initiation hence we are able to explain 1 complex sedimentary basins of inferred mesozoic age 2 a prominent unconformity and onlap surface of middle eocene to early miocene age at the base of flat lying sediments beneath the axis of new caledonia trough 3 gently dipping variable thickness and locally deformed late cretaceous strata along the margins of the trough 4 platform morphology and unconformities on either side of the trough that indicate a phase of late eocene to early miocene uplift to near sea level followed by rapid oligocene and miocene subsidence of 1100 1800 m and 5 seismic reflection facies tied to boreholes that suggest absolute tectonic subsidence at the southern end of new caledonia trough by 1800 2200 m since eocene time the cenozoic part of the model involves delamination and subduction initiation followed by rapid foundering and rollback of the slab this created a deep 2 km enclosed oceanic trough 2000 km long and 200 300 km across in eocene and oligocene time as the lower crust detached with simultaneous uplift and local land development along basin flanks disruption of late cretaceous and paleogene strata was minimal during this cenozoic phase and involved only subtle tilting and local reverse faulting or folding basin formation was possible through the action of at least one detachment fault that allowed the lower crust to either be subducted into the mantle or exhumed eastward into norfolk basin we suggest that delamination of the lithosphere with possible mixing of the lower crust back into the mantle is more widespread than previously thought and may be commonly associated with subduction initiation such as cenozoic events in the mediterranean and western pacific

paleogeographic record of eocene farallon slab rollback beneath western north america eocene terrestrial strata in western north america are now dated with sufficient precision to examine the hypothesis that rollback of the farallon flat slab created a trenchward migrating wave of dynamic and thermal topography we use patterns of accumulation and unconformity lake type stratigraphy and physical and isotopic provenance tools to track its migration across the laramide foreland between 53 ma and 47 ma hydrologic ponding and coincident unconformity development began at 53 ma culminating at 51 8 ma in the hydrologic closure of an 900 000 km2 area between the cordilleran and rocky mountain divides four subsequent stream diversions and a southwest expanding regional unconformity record trenchward migration of uplift from 50 to 47 ma volcaniclastic detritus from magmatic centers coincident with the uplifted region progressively filled the green river formation lakes from north to south these findings are consistent with recent numerical and conceptual models of slab rollback that predict initial dynamic subsidence above the slab hinge followed by uplift and volcanism triggered by influx of asthenosphere beneath the overriding plate based on the surface record rollback proceeded systematically at a rate of 6 cm yr across the wyoming craton from 53 to 47 ma resulting in removal of the entire shatsky conjugate oceanic plateau from the base of the north american lithosphere

monazite zircon and garnet growth in migmatitic pelites as a record of metamorphism and partial melting in the east humboldt range nevada monazite and zircon thermometry and geochronology were applied to anatectic pelites of the northern east humboldt range nevada the study area is an exhumed portion of the sevier orogenic root and is characterized by two crustal blocks with different prograde metamorphic histories the winchell lake nappe and the underlying lizzies basin block u th pb secondary ion mass spectrometry shrimp results from zircon and monazite indicate that in the lizzies basin block prograde metamorphism began by 96 5 8 0 ma cooling and melt crystallization was initiated by 80 1 1 4 ma possible reheating and a monazite growth event occurred again at 76 2 2 8 to 68 2 2 0 ma in the upper limb of the overlying winchell lake nappe prograde metamorphism began by 82 8 1 3 ma cooling and melt crystallization recorded by zircon and monazite growth recrystallization at 77 4 12 4 to 58 9 3 6 ma melt may have been present in the winchell lake nappe upper limb for a protracted period or may have formed during several unresolved melting events thin eocene oligocene 40 32 ma high y monazite rims are found in both crustal blocks these probably represent a phase of heating during eocene oligocene magmatism and extensional deformation u th pb geochronology results are consistent with differential burial heating and exhumation of different crustal blocks within the east humboldt range significantly fast exhumation of upper limb rocks of the winchell lake nappe and somewhat slower exhumation of lizzies basin block rock occurred prior to the core complex phase of the exhumation of the ruby mountains east humboldt range metamorphic core complex differences in the timing and tectonic significance of exhumation episodes within the east humboldt range may reflect how the localized presence of partial melt can affect the evolution of an exhumed orogenic terrane

tertiary magmatism and mylonitization in the ruby east humboldt metamorphic core complex northeastern nevada u pb geochronology and sr nd and pb isotope geochemistry u pb analyses of 48 zircon and or monazite fractions from ten intrusive rocks that range in composition from quartz diorite to leucogranite yield crystallization ages between 40 and 29 ma the new data indicate that tertiary intrusive rocks constitute a widespread and significant magmatic component of the ruby east humboldt metamorphic core complex some of the intrusive rocks are gneissic to mylonitic and have been penetratively deformed in a shear zone that can be traced for 100 km along the west flank of the core complex these data when coupled with previously published geochronologic data indicate that an important episode of mylonitization occurred between 29 and 23 ma late oligocene some field relations and geochronologic data however also suggest an earlier history of mylonitization in parts of the core complex this earlier mylonitic history may have occurred in the eocene or even late mesozoic time although more detailed geochronologic data are needed to clearly define the exact time interval sr nd and pb isotopic data from the dated intrusive units indicate that a relatively sharp approximately east west striking crustal boundary separating predominantly archean basement in the north from proterozoic basement in the south transects the complex we speculate that the crustal boundary represents the continuation of the cheyenne belt into this part of northeastern nevada these new data cast doubt on the existence of the proposed 2 0 2 3 ga crustal province in this part of the northern great basin the isotopic data from the plutonic rocks coupled with published data from at least in part coeval volcanic rocks indicate that mantle derived magmas were important in their genesis collectively these data indicate that significant crustal growth via the addition of mantle derived magmas occurred during large magnitude extension in this part of the northern great basin

high elevation of the nevadaplano during the late cretaceous during the late cretaceous central nevada may have been a high elevation plateau the nevadaplano some geodynamic models of the western us require thickened crust and high elevations during the mesozoic to drive the subsequent tectonic events of the cenozoic while other models do not to test the hypothesis of high elevations during the late mesozoic we used carbonate clumped isotope thermometry to determine the temperature contrast between late cretaceous to paleocene carbonates atop the putative plateau in nevada versus carbonates from relatively low paleoelevation central utah site lacustrine carbonates from the nevada site preserve summer temperatures 13 c cooler than summer temperatures from paleosol carbonates from the utah site after correcting for 1 2 c of secular climatic cooling between the times of carbonate deposition at the two sites this 13 c temperature difference implies an elevation difference between the two sites of 2 2 3 1 km including uncertainties from age estimation and climate change broadens this estimate to 2 km our findings support crustal thickness estimates and cenozoic tectonic models that imply thickened crust and high elevation in nevada during the mesozoic

synoptic reconstruction of a major ancient lake system eocene green river formation western united states numerous 40ar 39ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the greater green river piceance creek and uinta basins of wyoming colorado and utah constrain 8 m y of the eocene epoch multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance 40ar loss and 39ar recoil when considered in conjunction with existing radioisotopic ages and lithostratigraphy biostratigraphy and magnetostratigraphy these new age determinations facilitate temporal correlation of linked eocene lake basins in the laramide rocky mountain region at a significantly increased level of precision to compare our results to the geomagnetic polarity time scale and the regional volcanic record the ages of eocene magnetic anomalies c24 through c20 were recalibrated using seven 40ar 39ar ages overall the ages obtained for this study are consistent with the isochroneity of north american land mammal ages throughout the study area and provide precise radioisotopic constraints on several important biostratigraphic boundaries applying these new ages average sediment accumulation rates in the greater green river basin wyoming were approximately three times faster at the center of the basin versus its ramp like northern margin during deposition of the underfilled wilkins peak member in contrast sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled tipton and laney members sediment accumulation patterns thus reflect basin center focused accumulation rates when the basin was underfilled and supply limited accumulation when the basin was balanced filled to overfilled sediment accumulation in the uinta basin at indian canyon utah was relatively constant at 150 mm k y during deposition of over 5 m y of both evaporative and fluctuating profundal facies which likely reflects the basin margin position of the measured section the most rapid sediment accumulation for the entire system 1 m k y occurred between 49 0 and 47 5 ma when volcaniclastic materials from the absaroka and or challis volcanic fields entered the green river formation lakes from the north our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the green river formation basins coupled with previously published paleocurrent observations our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes the overall character of eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding laramide landscape freshwater overfilled lakes were initially dominant 53 5 52 0 ma possibly related to high erosion rates of remnant cretaceous strata on adjacent uplifts expansion of balanced fill lakes first occurred in all green river formation basins at 52 0 51 3 ma and again between 49 6 and 48 5 ma evaporative underfilled lakes occurred in various basins between 51 3 and 45 1 ma coincident with the end of the early eocene climatic optima and subsequent onset of global cooling defined from marine record however evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate evaporative terminal sinks were initially located in the greater green river and piceance creek basins 51 3 48 9 ma then gradually migrated southward to the uinta basin 47 1 45 2 ma this history is likely related to progressive southward construction of the absaroka volcanic province which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient the greater green river and piceance creek basins were eventually filled from north to south with absarokaderived detritus at sedimentation rates 1 2 orders of magnitude greater than the underlying lake deposits

transition from contraction to extension in the northeastern basin and range new evidence from the copper mountains nevada abstract new mapping structural analysis and 40ar 39ar dating reveal an unusually well constrained history of late eocene extension in the copper mountains of the northern basin and range province in this area the northeast trending copper creek normal fault juxtaposes a distinctive sequence of metacarbonate and granitoid rocks against a footwall of upper precambrian to lower cambrian quartzite and phyllite correlation of the hanging wall with footwall rocks to the northwest provides an approximate piercing point that requires 8 12 km displacement in an ese direction this displaced fault slice is itself bounded above by another normal fault the meadow fork fault which brings down a hanging wall of dacitic to rhyolitic tuff that grades conformably upward into conglomerate these relationships record the formation of a fault bounded basin between 41 3 and 37 4 ma the results are consistent with a regional pattern in which volcanism and extension swept southward from british columbia to southern ne

a relict landscape in the centre of fennoscandian glaciation cosmogenic radionuclide evidence of tors preserved through multiple glacial cycles the presence of well developed tors boulder fields and weathering mantles in the parkajoki area of northeastern sweden near the centre of fennoscandian glaciation has been used to suggest that these landscapes were preserved during all glacial cycles since ice sheet initiation in the late cainozoic this implies that all successive large scale glaciations must have had frozen bed conditions across this area to allow for subglacial landscape preservation cosmogenic 10be and 26al data from three tors and a meltwater channel in the parkajoki area were used to test this hypothesis of landscape preservation through multiple glacial cycles apparent exposure ages of tor summit bedrock surfaces ranging between 79 and 37 ka in an area deglaciated at 11 ka are consistent with the interpretation of these features as relict landforms that have survived glaciation with little or no erosion single nuclide minimum exposure age data require that the tors have survived at least two complete glacial cycles this estimate is based on i the approximate duration of periods of ice sheet cover versus ice free conditions as deduced from the dsdp 607 marine benthic foraminifer oxygen isotope record in conjunction with ii a record of fennoscandian ice sheet flow traces and hence ice sheet extent and iii noting that cosmogenic nuclides are accumulated only during ice free periods in addition mean cosmogenic 26al 10be concentration ratios from two of the sites indicate a minimum model total history of 605 ka and a maximum erosion rate of 1 6 m ma 1 thus the numerical ages confirm the overall qualitative interpretation of landscape preservation through multiple glacial cycles

a middle eocene carbon cycle conundrum a 500 000 year long period of warmth in the middle eocene was marked by high atmospheric carbon dioxide concentrations and prolonged dissolution of carbonate in the deep oceans numerical simulations attempting to capture these features identify gaps in our understanding of the causes of this and similar perturbations

continental uplift through crustal hydration isostatic surface uplift of large continental regions lacking deformation remains largely unexplained evidence from the eastern parts of the cordilleran orogen in the western united states suggests that increased buoyancy in the lower crust supports the elevations of the high plains and wyoming craton we suggest that hydration of the lower crust associated with the laramide orogeny produced surface uplift by replacing dense mineral phases such as garnet with less dense phases such as amphibole and mica seismic and petrologic evidence from wyoming and montana is consistent with such changes comparable hydration in the colorado plateau is dated to the early tertiary beyond establishing a newly recognized mechanism for broad continental uplift such hydration suggests that interactions of subduction derived fluids and the lithosphere can be more profound than previously envisioned

relation of flat subduction to magmatism and deformation in the western united states flat subduction of the farallon plate beneath the western united states during the laramide orogeny was caused by the combined effects of oceanic plateau subduction and unusually great suction in the mantle wedge the latter of which was caused by the shallowing slab approaching the north american craton root once in contact with basal north america the slab cooled and hydrated the lithosphere upon removal asthenospheric contact with lithosphere resulted in magma production that was especially intense where the basal lithosphere was fertile in what now is the basin and range province and this heating weakened the lithosphere and made it convectively unstable small scale convection has since affected many areas with slab sinking and unloading the western united states elevated into a broad plateau and the weak portion gravitationally collapsed with development of a transform plate boundary the western part of the weak zone became entrained with the pacifi c plate and deformation there is dominated by shear 2 humphreys mwr204 04 1st pages e surface heat flow p wave velocity d seismic velocity at 100 km background s wave velocity 2 100 175 km c s wave velocity

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

thermal evolution of a portion of the sevier hinterland the northern ruby mountains east humboldt range and wood hills northeastern nevada conventional thermobarometry and gibbs method modeling have been used to obtain new information pertaining to the mesozoic tertiary thermal evolution of the ruby mountains east humboldt range and wood hills metamorphic complexes northeastern nevada kyanite grade assemblages from clover hill in the extreme northeastern part of the east humboldt range were produced at temperatures of 780 810 k and depths in excess of 35 km previously published geochronologic data suggest an early cretaceous or late jurassic age for this event the clover hill area subsequently experienced a major unroofing interval prior to late cretaceous time during which it was brought roughly 20 km closer to the surface the lack of stratigraphic evidence for substantial cretaceous erosion in the region and the topology of the pressure temperature paths for the clover hill samples suggest that the unroofing mechanism may have been tectonic denudation related to the relaxation of large topographic gradients produced by crustal thickening in the hinterland of the sevier orogen metamorphic rocks at clover hill and within the wood hills core complex equilibrated at 820 900 k and 500 640 mpa 18 24 km in cretaceous time prior to 115 ma other parts of the ruby mountains east humboldt range metamorphic complex presently structurally separated from the clover hill sequence were intruded extensively by late cretaceous granitic magmas and attained substantially higher temperatures than the clover hill wood hills block sillimanite grade assemblages from these areas apparently underwent roughly 10 km of unroofing and 130 k of cooling prior to final equilibration at 820 920 k and 360 430 mpa 13 16 km during oligocene extension

chronology of polyphase extension in the windermere hills northeast nevada fission track and 40 ar 39 ar dating and chemical correlation of volcanic strata exposed in the windermere hills and northern pequop mountains northeast nevada indicate a protracted polyphase history of tertiary late eocene late miocene extension along the northern margin of a major cordilleran metamorphic core complex early extension is recorded by a west tilted half graben filled with early oligocene 34 79 0 18 39 18 0 12 ma sedimentary rocks in the eastern windermere hills above the lowangle black mountain detachment fault the early oligocene half graben conformably overlies a widespread suite of late eocene 39 18 0 12 40 38 0 06 ma calc alkaline volcanic rocks reflecting a temporal link between early extension at a high structural level and the end of the ignimbrite flare up these strata are cut by east west striking normal faults which are exposed along and parallel to the northern margin of the metamorphic complex available age data e g between 14 93 0 08 and 34 79 0 18 ma permit the interpretation that the east west striking faults formed at the same time as or after large magnitude unroofing of highgrade rocks we interpret the east west striking faults to accommodate differential uplift of greenschist grade metamorphic rocks in the upper crust above a lateral ramp in a west northwest directed mylonitic shear zone subsequent extension in the windermere hills is defined by deep rapidly filled half grabens of middle miocene 7 42 2 0 to 14 93 0 08 ma age that unconformably overlie older faults and synextensional deposits these are the youngest half grabens in the region and are inferred to be initiated by extensional stresses imparted to the base of the lithosphere by a laterally spreading mantle plume e g the yellowstone hotspot located in southeastern oregon at this time

rapid change in high elevation precipitation patterns of western north america during the middle eocene climatic optimum meco we present eocene terrestrial oxygen 18o carbon 13c and strontium 87sr 86sr isotope lake and paleosol records from two high elevation sites on the western north american plateau these records represent terrestrial stable isotope evidence of the middle eocene climatic optimum meco an enigmatic and rapid global warming event that interrupted protracted eocene global cooling at around 40 0 ma revised stratigraphy based on 15 40ar 39ar ages from air fall and water lain ash in the elko basin nevada places evaporative lake conditions followed by rapid 20 decrease in 13c values already during the pre meco cooling phase requires that in contrast to pre meco climate conditions meco temperature seasonality in western north america was sufficiently strong to re establish regular lake overturn the rate and magnitude of the rapid negative lacustrine 18o shift during meco in the elko lake basin is inconsistent with a simple scenario of regional surface uplift affecting decrease of 18o in precipitation and associated lake freshening we consider the overall negative shift in 18o of non evaporatively 18o enriched lake waters 9 3 1 8 between the 42 to 43 ma elko formation and the ca 38 ma base of the indian well formation to be composed of a ca 3 to 4 permil decrease in 18o of lake carbonate as a consequence of post meco cooling and an additional ca 5 to 6 permil decrease in 18o of riverine lake input as a response to 43 to 38 ma surface uplift when combined with isotope enabled global circulation model results the elko lake data further suggest that once critical elevations were attained a change in upstream moisture transport including strengthening of monsoonal summer rainfall on the south eastern flanks of the cordillera and a larger fraction of air parcel trajectories to the eastern lee of the cordilleran highlands that had passed over the western continental interior were responsible for the rapid middle eocene decrease in 18o of precipitation in the central cordilleran hinterland

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