numerical modelling of lithosphere shortening application to the laramide erogenic province western usa abstract a two dimensional model of lithosphere shortening is presented which quantifies crustal thickening temperature perturbations and flexural isostatic components the model assumes that the upper brittle layer of the crust deforms by thrusting while the lower ductile lithosphere accommodates compressional deformation by a pure shear or squashing mechanism model results suggest that basement uplift foreland basin development and underlying crustal structure are controlled by the amount and configuration of the compressional deformation in the upper and lower lithosphere the perturbation of the temperature field erosion and the flexural rigidity of the lithosphere the model is applicable to regions of thick skinned thrust tectonics and is applied to the laramide orogenic province in the western usa model simulations of structural cross sections across the laramide province show basement uplifts and adjacent foreland basins that are comparable in magnitude with those suggested by geological data the model has also been used to provide insights into the tectonic evolution of the laramide province and an attempt has been made to determine the flexural rigidity of the lithosphere at the time of laramide deformation as well as quantify both the effects of the post shortening re equilibration of the temperature field and thrust uplift erosion the amount of crustal thickening and post shortening cooling of the geotherm predicted by the two dimensional model of lithosphere shortening are used in strength calculations to determine the relative strength or weakness of the lithosphere the results suggest that the lithosphere is relatively strong immediately following shortening due to the cooling of the geotherm following shortening however gradual weakening occurs as the temperature field returns back to its unperturbed state in the presence of an enhanced crustal thickness the results are compatible with the evolution of the laramide province which has experienced a change in tectonic regime to one of extension over the last 20 30 ma

the 2013 wyoming upper mantle earthquakes tomography and tectonic implications on 21 september 2013 two earthquakes m 4 8 and 3 0 focal depths 70 km occurred in the lithospheric mantle beneath central wyoming which provide a rare opportunity to investigate the lithosphere rheology and dynamics we determined a detailed 3 d p wave tomography in the source area of the wyoming earthquakes using a large number of high quality travel time data of local earthquakes and teleseismic events our results show that the 2013 wyoming earthquakes occurred in an isolated high velocity high v anomaly which extends down to a depth of 160 km we propose two possible scenarios about the cause of the 2013 wyoming earthquakes one is that the high v anomaly reflects a remnant of the subducted oceanic slab which fractured under long term negative buoyancy and heating by the infilling asthenosphere the other is removing of a dense mantle lithospheric root the present results shed new light on the rheology and seismotectonics of the continental lithosphere

isotopic constraints on the thermal history of the wind river range wyoming implications for archean metamorphism precise u pb monazite and 40ar 39ar hornblende ages have been obtained from three locations in the high grade archean core of the wind river range wyoming monazites from metapelites in the paradise basin medina mountain and crescent lake have u pb ages of 2718 1 2633 5 and 2657 2 ma respectively hornblendes from amphibolites and granulites from the same locations yield plateau 40ar 39ar isotope ages of 2652 11 2572 9 and 2527 8 ma respectively and are interpreted as cooling ages from the last thermal event the three localities experienced similar peak pressure temperature conditions the timing of high grade metamorphism in the paradise basin is older than the emplacement of large subjacent batholiths at 2 63 2 67 ga calculated cooling rates based on monazite hornblende pairs of 3 4 1 0 c ma for paradise basin 3 8 1 2 c ma for medina mountain and 1 7 0 4 c ma for crescent lake cannot be used to rule out reheating during subsequent pluton emplacement the markedly slowe

effect of regional topography and hydrology on the lacustrine isotopic record of miocene paleoclimate in the rocky mountains drummond c n wilkinson b h lohmann k c and smith g r 1993 effect of regional topography and hydrology on the lacustrine isotopic record of miocene paleoclimate in the rocky mountains palaeogeogr palaeoclimatol palaeoecol 101 67 79 oxygen isotopic compositions of upper miocene 9 2 m y lacustrine limestone from northwestern wyoming range from 21 5 to 35 2 0 pdb and are the lightest yet reported for a non marine carbonate sequence these values require that lake water was greatly depleted in 180 relative to modern meteoric precipitation and indicate that most inflow was sourced as meltwater from glaciers in the adjacent gros ventre range assessment of climatic factors influencing the isotopic composition of global meteoric water indicates either that nearly all of miocene precipitation in the southern rocky mountains was derived from large lake systems in the snake river plain and northern great basin to the west or that gros ventre catchment elevations were up to 2300 m higher than at present because erosion rates in modern alpine regions suggest that subaerial denudation could only account for up to 1200 m of post miocene elevation reduction any additional lowering must reflect the influence of post laramide epeirogeny during basin and range extensional tectonism lacustrine isotopic data therefore provide boundary conditions on the timing and magnitude of changes in late cenozoic paleoclimate topography hydrology and tectonism in the western wyoming portion of the rocky mountains

deep water clastic systems in the upper carboniferous upper mississippian lower pennsylvanian shannon basin western ireland the upper carboniferous shannon basin of western ireland contains a more than 2300 m thick 7540 ft basin fill succession shallowing upward from deep water to deltaic and incised fluvial deposits the deep water basin floor and slope succession is world renowned as an analog for hydrocarbon bearing deep water sandstones on several continental margins such as the basins in east and west africa south america the gulf of mexico and not least offshore northwest europe the shannon basin is frequently visited by both academia and industry for research and training purposes a series of behind outcrop research boreholes reveals the subsurface expression of the deep water rocks and is complemented by seismic scale cliff exposures the succession is interpreted as a first order basin scale linked sedimentary system this system can be analyzed using the principles of source to sink analysis and leaves the visitor with a complete picture of the basin fill enhanced by spectacular sedimentological and stratigraphic detail

neogene tephra correlations in eastern idaho and wyoming implications for yellowstone hotspot related volcanism and tectonic activity the explosive rhyolitic eruptions that define the track of the snake river plain yellowstone volcanism have produced a large volume of tephra found in late miocene and younger basin fill sediments throughout the western united states here we use 40ar 39ar isotopic dating paleomagnetic analysis major and trace element geochemistry and standard optical techniques to establish regional tephra correlations we focus on tephra deposits in three neogene basins in spatially separated areas grand valley in eastern idaho jackson hole in northwestern wyoming and the granite mountains area in central wyoming these basins have experienced relatively continuous deposition from the late miocene to the holocene we found tephra layers that directly tie the stratigraphy between all three basins using these correlations we found that basins experienced discrete pulses of extension separated by long periods of relative quiescence the dates of which are staggered between basins an early pulse of extension occurred in grand valley and jackson hole between 10 3 ma and 16 33 ma with a second pulse initiating between 4 54 ma and 2 09 ma the granite mountains basin experienced a single pulse of extension sometime between 11 14 ma and 6 75 ma these pulses of accelerated extension along with evidence of similar pulses in other basins present a pattern of west to east migration that we suggest is related to the yellowstone hotspot the later pulse of activity in grand valley and jackson hole corresponds to the migration of the north american plate over the tail of the yellowstone hotspot we speculate that the earliest pulse in each basin is related to the more rapid movement of the sublithospheric hotspot head as it spreads out from its earliest known location where the columbia river plateau flood basalt province initiated in southeastern oregon to its outermost edge under central wyoming our results are consistent with this model of a plume head though not unique to it the results of our study also indicate that previous suggestions that the rate of snake river plain explosive volcanism has decreased by a factor of 2 or 3 since emplacement of the middle miocene trapper creek tuffs likely underestimate post trapper creek eruption rates we have discovered a large number of previously unidentified post middle miocene major eruptive events both as ash flow tuffs and as vitric air fall tuffs recalculation to include these newly discovered events results in a rate of major eruptions that is fairly uniform until ca 4 54 ma however there is a substantial gap in major silicic eruptions in the interval between 4 54 ma and 2 09 ma which we call the post heise eruptive gap with the exception of this gap the rate of major eruptions on the snake river plain has been roughly constant since inception of the eastern snake river plain yellowstone volcanic track between 16 ma and 17 ma

fault chronology and uplift history of the southern wind river range wyoming implications for laramide and post laramide deformation in the rocky mountain foreland the stratigraphy provenance and structural involvement of tectogenic sedimentary rocks along the southern and western margins of the wind river range in western wyoming record a complex laramide and later uplift history and identify the timing and position of individual faulting events the chronology of these events is 1 main uplift of the range by displacement on the wind river fault between 90 and 49 to 50 ma 2 the formation of reverse faults in the hanging wall of the wind river fault ca 50 to 51 ma 3 sagging of the toe of the wind river fault into sedimentary fill of the green river basin between 46 and 23 ma 4 uplift of the core of the range along reactivated precambrian shear zones between 30 and 23 ma and 5 collapse of the southern part of the range along normal faults beginning at 13 ma the sedimentary record of uplift and erosion indicates that erosion and basin fill kept pace or caught up with uplift so that by the end of the laramide there was little topographic relief between the crest of the range and the adjacent green river basin the present topography of the range characterized by a high level erosion surface and distinct crestal ridge derives from uplift of the erosion surface by the late laramide reverse faulting post laramide uplift of the crest of the range and neogene exhumation initial uplift of the range as early as 110 ma and prior to displacement on the wind river fault suggests either a pre laramide event of unknown origin or that laramide crustal shortening began nearly 35 m y earlier than commonly assumed

paleotopography in the western u s cordillera issue 2 of volume 312 of the american journal of science brings together several new papers bearing on the history of orogenesis paleotopography and paleoclimate in a region whose geologic record has been more important to the modern development of ideas about these processes than any other in

geomorphic controls on lacustrine isotopic compositions evidence from the laney member green river formation wyoming oxygen isotope values from lacustrine carbonate in the laney member of the green river formation wyoming exhibit a sudden basinwide 6 upsection decrease in 18 o at ca 49 ma 40 ar 39 ar geochronology constrains the duration of the isotopic shift to 200 000 years this change coincides with a sudden change in lake type from balancedfi lled in the lower laclede bed to overfi lled in the upper laclede bed as well as an increase in the proportion of calcitic 80 calcite out of total carbonate by x ray diffraction xrd samples from 32 to 73 the 18 o shift is correlatable through several locations across the greater green river basin and also coincides with a previously observed shift to less radiogenic 87

structural and sedimentological development of footwall growth synclines along an intraforeland uplift east central bighorn mountains wyoming structural sedimentological and provenance data from paleogene synorogenic deposits of the east central flank of the bighorn mountains provide new information about the development of footwall growth synclines the evolution of fault related folds and the erosional unroofing history of intraforeland uplifts three conglomerate units the upper conglomerate member of the fort union formation and the kingsbury and moncrief members of the wasatch formation are incorporated within an asymmetric east verging growth syncline in the footwall of the main range bounding thrust system three stages of footwall deformation are recorded within these conglomerates analysis of mapped progressive unconformities retrodeformed balanced cross sections and conglomerate clast composition data define these stages as part of a continuum of deformation associated with the development of footwall growth synclines development of an anticline syncline pair marked the earliest stage of growth syncline formation stage i rotation of the shared fold limb resulted in amplification of the growth syncline fine grained synorogenic sediment derived from easily eroded mesozoic mudstone bypassed the growth syncline during this stage by the end of lebo shale deposition an average of 12 1 of shortening and 6 46 km of uplift had occurred along the range margin continued growth syncline development was marked by the deposition of the kingsbury conglomerate the kingsbury conglomerate was derived from resistant middle and lower paleozoic carbonate strata in the uplifted source terrane intraformational unconformities recording as much as 55 of bed rotation were developed within the kingsbury conglomerate as fold limb rotation occurred coeval with deposition cross sections indicate that during this early stage of fault related folding an average of 16 9 shortening and 8 12 km of uplift occurred along the eastern flank of the bighorn mountains end of stage i the intermediate stage stage ii of footwall growth syncline development involved partial truncation of the growth syncline by the advancing thrust faults and deposition of the moncrief conglomerate the lower portion of the moncrief conglomerate was rotated basinward in the developing growth syncline the final stage of deformation stage iii was dominated by the thrust faulting of middle and lower paleozoic strata eastward over steeply dipping mesozoic strata and rotated eocene synorogenic conglomerate during this stage of deformation the moncrief conglomerate was deformed as the initially blind thrusts propagated into the near surface conglomerate deposits truncated the entire footwall syncline and overrode the synorogenic conglomerate package cross sections in areas where this final stage of deformation is well developed indicate that an average of 24 1 shortening and 9 7 km of uplift had occurred along the eastern margin of the bighorn mountains the caliber of synorogenic deposition in the powder river basin was linked directly to the lithologic composition of the bighorn mountains approximately half of the 3 6 km thick source stratigraphic section of the eastern bighorn mountains was eroded prior to accumulation of conglomerate the majority of this eroded material was derived from mesozoic mudstone and poorly indurated sandstone that were incapable of generating coarse detritus the first paleogene conglomerates deposited along the east central bighorn mountains therefore do not represent the initiation of laramide uplift but instead represent the exposure of coarse clast forming rock types from the lower half of the hanging wall stratigraphic section i e the mississippian madison limestone and ordovician bighorn dolomite

widespread basement erosion during the late paleocene early eocene in the laramide rocky mountains inferred from 87sr 86sr ratios of freshwater bivalve fossils reconstruction of the timing and pattern of exhumation of basement involved uplifts during the laramide is essential to the understanding of the mechanisms of crustal shortening and thickening in the region we use reconstructions of the 87 sr 86 sr ratios of late cretaceous early cenozoic river water from fossil shells in six basins of the rocky mountains contextualized by modern river water sr chemistry studies to trace the erosion of precambrian basement cores in the laramide ranges the 87 sr 86 sr ratios and sr concentration of modern river water in the rocky mountains are controlled by river bedrock lithology weathering of precambrian silicate rocks in the cores of laramide ranges produces high 87 sr 86 sr ratios in highland rivers weathering of paleozoic and mesozoic carbonates along the basin margins reduces the 87 sr 86 sr ratios and increases sr concentration of rivers as they flow basinward lowland rivers that have headwaters in precambrian basement mostly have 87 sr 86 sr ratios 0 711 whereas rivers confined to or with very long reaches in basins have 87 sr 86 sr ratios between 0 709 and 0 711 river water 18 o values do not change in response to changes in catchment elevation our results from fossil shells show that carbonate weathering dominated the river water sr chemistry in the alberta foreland williston and crazy mountains basins and that proterozoic low grade metamorphic carbonates in the belt purcell supergroup were not exposed in the canadian rocky mountains during late cretaceous early paleocene time silicate weathering influenced the sr chemistry of surface waters in the powder river basin during the late paleocene and the washakie basin during the early eocene suggesting precambrian silicate basement rock was extensively exhumed and eroded the observation that precambrian basement was eroded earlier around the powder river basin than around the washakie basin is consistent with a previous study that suggests laramide ranges in northeast wyoming reached high elevation earlier than the ranges adjacent to the sevier thrust belt widespread basement erosion in late paleocene early eocene time was controlled by tectonic exhumation and may have been intensified by the wet and warm global climate

spatial and temporal evolution of coastal depositional systems and regional depositional process regimes campanian western interior seaway u s a abstract this paper provides a critical review and regional synthesis of late cretaceous shallow marine deposits along part of the western margin of the western interior seaway of north america which contains the most extensively documented outcrop based studies of siliciclastic coastal depositional systems in the world the results of this synthesis are presented in the form of paleogeographic maps covering present day new mexico utah colorado and wyoming usa for five timeslices in the campanian these maps are used to evaluate the spatial and temporal evolution of regional depositional process regimes along a large 1000 m stretch of coastline the evolution of regional depositional process regimes is linked to tectonic and paleoceanographic controls on the western interior seaway which enables the results of this synthesis to be applied to prediction of depositional process regimes in other less intensively studied basins six gross depositional environments have been mapped for each timeslice 1 alluvial to coastal plain sandstones 2 coastal plain coals mudstones and sandstones 3 shoreline sandstones 4 marine mudstones 5 gravity flow siltstones and sandstones and 6 marine marls and chalk shoreline sandstones in each timeslice are interpreted in further detail using documented evidence for the three principal classes of depositional process wave tidal and fluvial and published reconstructions of coastal morphology which is widely considered to reflect depositional process regime based on these interpretations shoreline sandstones are assigned to five categories of depositional process regime 1 regressive wave dominated shorefaces and delta fronts 2 regressive river dominated delta fronts 3 regressive mixed tide and wave influenced delta fronts 4 regressive tide dominated delta fronts and 5 transgressive barrier islands back barrier lagoons and estuaries the accuracy of and uncertainty in classification of depositional process regime are critically evaluated additionally stratal thickness and sediment routing pathways have been interpreted in order to assess the impact of tectonic and paleoceanographic controls on spatial and temporal changes in all of the evaluated shallow marine successions thin tide influenced intervals were deposited in back barrier and lagoon systems associated with net transgressive shorefaces and estuaries however it is notable that all preserved tide dominated and tide influenced regressive deltaic systems are located along the northern margin of the utah bight a tectonically related embayment that had a pronounced expression during the middle campanian the southern margin of the embayment and coastlines to the north are conspicuously more wave dominated which supports their exposure to a larger wave fetch in contrast the northern margin of the embayment was relatively wave protected it is concluded that tidal range was amplified due to resonance of the principal semidiurnal tide in the strongly embayed geometry of the middle campanian utah bight

multidisciplinary approach to constrain kinematics of fault zones at shallow depths a case study from the cameros demanda thrust north spain thrusting at shallow depths often precludes analysis by means of structural indicators effective in other geological contexts e g mylonites sheath folds shear bands in this paper a combination of techniques including structural analysis magnetic methods as anisotropy of magnetic susceptibility and paleomagnetism and paleothermometry is used to define thrusting conditions deformation and transport directions in the cameros demanda thrust north spain three outcrops were analyzed along this intraplate large scale major structure having 150 km of outcropping length 30 km of maximum horizontal displacement and 5 km of vertical throw results obtained by means of the different techniques are compared with data derived from cross sections and stratigraphic analysis mixed layer illite smectite and vitrinite reflectance indicating deep diagenetic conditions and mature stage of hydrocarbon generation suggests shallow depths during deformation thus confirming that the protolith for most of the fault rocks is the footwall of the main thrust kinematic indicators foliation s c structures and slickenside striations indicate altogether a dominant nnw movement of the hanging wall in the western zone and ne in the eastern zone of the thrust thus implying strain partitioning between different branches of the main thrust the study of ams in fault rocks nearly 400 samples of fault gouge breccia and microbreccia indicates that the strike of magnetic foliation is oblique to the transport direction and that the magnetic lineation parallelizes the projection of the transport direction onto the k max k int plane in sites with strong shear deformation paleomagnetism applied to fault rocks indicates the existence of remagnetizations linked to thrusting in spite of the shallow depth for deformation and a strong deformation or scattering of the magnetic remanence vectors in the fault zone the application of the described techniques and consistency of results indicate that the proposed multidisciplinary approach is useful when dealing with thrusts at shallow crustal levels

sedimentary record of seismic events in the eocene green river formation and its implications for regional tectonics on lake evolution bridger basin wyoming abstract outcrops and cores from the top of the lacustrine tipton member and the base of the wilkins peak member 51 5 ma of the eocene green river formation bridger basin in southwestern wyoming yield a wide variety of sedimentary deformation features many of which are laterally extensive for more than 50 km they include various types of folds load structures pinch and swell structures microfaults breccias and sedimentary dikes in most cases deformation is represented by hybrid brittle ductile structures exhibiting lateral variation in deformation style these occur in low energy profundal organic rich carbonate mudstones oil shales trona beds tuffs and profundal to sublittoral silty carbonate deposited in paleolake gosiute the deformation is not specific to the depositional environment because sedimentary units stratigraphically higher with similar facies show no deformation the studied interval lacks any evidence for possible trigger mechanisms intrinsic to the depositional environment such as strong wave action rapid sediment loading evaporite dissolution and collapse or desiccation so endogenic causes are ruled out thus the deformation features are interpreted as seismites and change in deformation style and inferred increase in intensity towards the south suggest that the earthquakes were sourced from the nearby uinta fault system the 22 levels exhibiting seismites recognized in cores indicate earthquakes with minimum magnitudes between 6 and 7 minimum epicentral intensity mcs of 9 and varying recurrence intervals in the seismic history of the uinta fault system with a mean apparent recurrence period of 8 1 k y using average sedimentation rates and dated tuffs in detail however there are two noticeably active periods followed by relative quiescence the stratigraphic position of these deformed intervals also marks the transition between two distinct stages in lake evolution from the balanced filled tipton member to the overlying underfilled wilkins peak member thus these seismites are evidence for regional scale changes in lacustrine sedimentation of eocene lake gosiute in response to syndepositional tectonic activity analysis of synsedimentary deformation features is therefore a promising yet under utilized tool to trace the tectonic evolution of lacustrine deposits of the green river formation and other tectonically active marine and non marine basins

composition structure and evolution of a lacustrine carbonate margin dominated by microbialites case study from the green river formation eocene wyoming usa abstract during mid eocene times the northwestern margin of the gosiute lake wyoming has been characterised by the extensive development of a microbialite dominated carbonate sequence corresponding to the laney member of the green river formation thus providing a potential analogue for petroleum carbonate reservoirs developed in similar marginal lacustrine settings a detailed sedimentological study carried out on the little mesa plateau near labarge sw wyoming allows the reconstruction of 1 lateral and vertical facies variations of the carbonate sequence across the margin 2 development patterns of complex microbial bioconstructions and 3 the evolution of the margin through time the carbonate sequence up to 18 m thick starts with the deposition of sandy limestones which fill channels in the underlying siliciclastic unit of the cathedral bluff formation the increasing microbial contribution in the upper part of the sequence results in the development of large insect microbial build ups which represent up to 80 of the total volume of the carbonate deposits and which aggregated to form reef like geometries up to several tens of metres in diameter including bioherms spur and groove alignments and reef flat structures the initiation and development of the lacustrine carbonate platform was controlled both by the general lake expansion associated to the eocene climatic optimum and by local drainage diversions responsible for a new water inflow in the basin from the northern volcanic provinces short term lake level changes driven by high frequency climate fluctuations which are superimposed to this general transgressive trend may explain the discontinuous and polygenic growth of the individual insect microbial build ups the basin infilling by prograding alluvial volcaniclastic deposits originated from the northwestern volcanic province caused the rapid turn off of the marginal lacustrine carbonate factory during mid eocene time

active alluvial systems in the korla basin tien shan northwest china sedimentation in a complex foreland basin the tien shan is a cenozoic orogenic belt in central asia caused by the post collisional convergence of india with asia that reactivated palaeozoic faults the korla basin is a complex foreland basin within the chinese tien shan elongate sub basins within it have received sediment from both the hangingwall and footwall blocks above and below reverse faults on their northern margins the footwall blocks are themselves uplifted by the next fault to the south the greater part of the sediment supply measured by area comes from the footwall block this sediment supply pattern is analogous to that of extensional half graben

dynamic topography and vertical motion of the u s rocky mountain region prior to and during the laramide orogeny dynamic topography of earth s surface occurs in response to hydrodynamic stresses due to mantle flow beneath a flexible lithosphere here we compare the predicted dynamic topography from an inverse convection model that includes flat slab subduction with the known geologic history of the western united states from late cretaceous through paleocene time to evaluate the validity of the model results downwarping behind the evolving cordilleran volcanic arc took place after its inception in late triassic time culminating in the formation of the western interior seaway in late cretaceous time subsidence behind arcs is a consistent prediction of most models of dynamic topography a more detailed convection model is evaluated here by comparing it to the geologic history of the central rocky mountain region from 100 to 50 ma the match of predicted and observed tectonic subsidence is good up until the time that local deformation by the laramide orogeny begins this is most likely due to local flexural effects of mountain building overwhelming regional dynamic effects maps for successive time intervals show that the model matches the geologic history quite well through time and space in particular the subduction of the previously postulated oceanic plateau the conjugate shatsky rise has a significant impact on surface movements 1 from 95 to 88 ma the locations of most significant regional unconformities match the eastward migration of a zone of high topography 2 from 90 to 85 ma the zone of maximum subsidence coincides with the motion of the leading edge of the conjugate shatsky rise 3 from 78 to 60 ma the site of initiation of laramide deformation migrates coincidently with the position of the center of the conjugate shatsky rise 4 from 70 to 60 ma the time transgressive deposition of thin fluvial conglomerate units in the southern part of the study area is generally coincident with surface uplift caused by the trailing part of the conjugate shatsky rise these results suggest that the inverse model approximates the vertical motion history quite well synchroneity of the position of the center of the passing conjugate shatsky rise the landward limit of the evolving cordilleran volcanic arc and the initiation of laramide deformation suggests that the farallon plate became coupled with the overlying north american plate as the subducted oceanic plateau passed beneath progressively enhanced mechanical coupling between the plates was likely the impetus for laramide shortening this comparison of model results with surface geologic history provides a means to validate but not verify predictions of dynamic mantle flow models

late cretaceous early eocene laramide uplift exhumation and basin subsidence in wyoming crustal responses to flat slab subduction low angle subduction of the farallon oceanic plate during the late cretaceous early eocene is generally considered as the main driver forming the high rocky mountains in wyoming and nearby areas how the deformation was transferred from mantle to upper crust over the great duration of deformation 40 myr is still debated here we reconstruct basin subsidence and compile paleoelevation thermochronology and provenance data to assess the timing magnitude and rates of rock uplift during the laramide deformation we reconstruct rock uplift as the sum of surface uplift and erosion constrained by combining paleoelevation and exhumation with regional stratigraphic thickness and chronostratigraphic information the amount and rate of rock uplift of individual laramide ranges was less than 2 4 4 8 km 0 21 0 32 mm yr during the early maastrichtian paleocene stage 1 and increased to more than 3 km 0 38 0 60 mm yr during the late paleocene early eocene stage 2 our quantitative constraints reveal a two stage development of the laramide deformation in wyoming and an increase of rock uplift during stage 2 associated with enhanced intermontane basin subsidence exhumation and uplift during stage 1 is consistent with eastward migration of cordilleran deformation associated with low angle subduction whereas the change in exhumation during stage 2 seems to follow a southwestward trend which requires an alternative explanation we here suggest that the increase of rock uplift rate during the late paleocene early eocene and the southwestward younging trend of uplift may be a response to the rollback and associated retreating delamination of the farallon oceanic slab

winter snow cover on the sea ice of the arctic ocean at the surface heat budget of the arctic ocean sheba temporal evolution and spatial variability 1 the evolution and spatial distribution of the snow cover on the sea ice of the arctic ocean was observed during the surface heat budget of the arctic ocean sheba project the snow cover built up in october and november reached near maximum depth by mid december then remained relatively unchanged until snowmelt ten layers were deposited the result of a similar number of weather events two basic types of snow were present depth hoar and wind slab the depth hoar 37 of the pack was produced by the extreme temperature gradients imposed on the snow the wind slabs 42 of the snowpack were the result of two storms in which there was simultaneous snow and high winds 10 m s 1 the slabs impacted virtually all bulk snow properties emphasizing the importance of episodic events in snowpack development the mean snow depth n 21 169 was 33 7 cm with a bulk density of 0 34 g cm 3 n 357 r2 of 0 987 giving an average snow water equivalent of 11 6 cm 25 higher than the amount record by precipitation gauge both depth and stratigraphy varied significantly with ice type the greatest depth and the greatest variability in depth occurring on deformed ice ridges and rubble fields across all ice types a persistent structural length in depth variations of 20 m was found this appears to be the result of drift features at the snow surface interacting with small scale ice surface structures a number of simple ways of representing the complex temporal and spatial variations of the snow cover in ice ocean atmosphere models are suggested

fission track thermochronology of the wind river range wyoming evidence for timing and magnitude of laramide exhumation fission track analysis of wind river range basement rocks provides information about the timing magnitude and cooling that occurred in response to laramide uplift eighty samples were collected from precambrian rock along six traverses in the northern central and southern parts of the range over 5 km of elevation apatite fission track ages and track length data suggest that the range did not have a uniform thermal history during late mesozoic and cenozoic time zircon fission track ages are precambrian implying that these rocks have not been at temperatures higher than 200 c 9 10 km burial depth since precambrian time apatite ages from the northeastern and southwestern flanks of the range suggesst that cooling may have initiated in the wind river range by about 85 ma but was definitely occurring by 75 ma data from the green river lakes area and fremont peak in the northern part of the range indicate that cooling was occurring by 62 ma and was most rapid between 60 and 57 ma apparent rates of exhumation vary from 94 m m y to 246 m m y in this part of the range the central part of the range records this same period of rapid cooling but structural and topographic relief are not as great ages from the southern part of the range suggest that uplift and erosion may have been initiated earlier there but continued at much slower rates 34 m m y over a longer period of time than in the north samples from a drill hole in the central wind river range have apatite ages ranging from 37 ma to 86 ma and suggest a significant cooling event at approximately 42 ma involving nearly 2 km of the rock column timing magnitudes and apparent rates of exhumation have varied spatially in the wind river range

middle to late cenozoic cooling and high topography in the central rocky mountains constraints from clumped isotope geochemistry abstract the timing and mechanism of the formation of the high elevations of the central rocky mountains rockies in the western interior of north america remain controversial we examine the middle and late cenozoic paleorelief between the great plains and the central rockies using clumped isotope geothermometry of carbonate cements of fluvial and eolian sandstones our petrographic observations suggest that the studied carbonate cements were formed primarily during eodiagenesis in near surface conditions and retain primary paleoclimate and paleoelevation signals carbonate clumped isotope temperatures decrease 10 c and calculated water o 18 values decrease 4 from 35 ma to 5 ma in both the central rockies and the western great plains following middle and late cenozoic global cooling trend a persistent longitudinal gradient in temperature and o 18 value existed between the central rockies and the great plains during the oligocene and miocene with temperatures 4 8 c higher and calculated water o 18 values 3 6 higher in the great plains these observations suggest the mean elevation of the central rockies was 1 km higher than the western great plains during the middle and late cenozoic similar to the present day regional relief when placed in the context of other paleoaltimetry studies and geological observations our findings support the hypothesis that the high mean topography of the rockies was developed during the late eocene possibly related to isostatic adjustment or dynamic uplift caused by foundering of lower mantle lithosphere or the farallon slab or both

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

structure timing and kinematics of the early eocene south fork slide northwest wyoming usa abstractthe south fork slide sfs is exposed over an area of 800 km2 in northwest wyoming and its evolution is connected in space and time to the adjacent and overlying 48 87 ma heart mountain slide hms the youngest rocks deformed as part of the sfs are the eocene willwood formation sands and mudstones which have a u pb detrital zircon youngest depositional age tuffzirc calculation of 51 80 1 04 1 08 ma as well as a spectrum of older zircons that were eroded from the sevier highlands to the west and the archean beartooth uplift to the north n 379 u pb zircon ages the sfs is overlain by allochthonous paleozoic carbonate and eocene volcanic rocks of the hms the sfs deformation involves shallowly plunging thin skinned folds that trend northeast southwest where the slip surface could have been the top of the jurassic gypsum springs formation detailed mapping reveals the absence of any cleavage or joints and a plethora of minor structures unreported in any thin skinned belt as well as the fac