laramide exhumation of the bighorn mountains wyoming an apatite u th he thermochronology study we report u th he apatite ages from the crystalline core of the bighorn mountains and compare the exhumation history derived from those ages with the exhumation history determined from sedimentary rocks in the adjacent powder river and bighorn basins our u th he apatite ages range from 62 to 369 ma and represent a pre laramide he partial retention zone that was deformed and uplifted at ca 65 5 ma the geometry of the he partial retention zone indicates that the basement in the bighorn mountains is deformed into a doubly plunging anticline the preservation of a pre laramide partial retention zone in the upper few hundred meters of precambrian basement indicates that in general the temperature at the cambrian unconformity did not exceed the apatite he closure temperature this is difficult to reconcile with evidence from adjacent basins for thick sequences of sedimentary rocks 3 4 km prior to 65 ma and normal modern geothermal gradients either the range was never deeply buried 2 3 km its geothermal gradient has been low 20 c since at least the mesozoic or our apatites have higher u th he closure temperatures 80 90 c than those measured for other apatites

mesozoic aseismic ridges on the farallon plate and southward migration of shallow subduction during the laramide orogeny we propose that an aseismic ridge of late jurassic and early cretaceous age on the farallon plate was subducted beneath the unites states cordillera during the laramide orogeny the relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70 40 ma in the existing near trench magmatic arc and laramide uplift and faulting of crystalline basement 1000 1500 km inland from the trench the timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the laramide orogeny and the magmatic lull in the western cordillera because the north northeast trending aseismic ridge was moving northeast relative to the north american plate the point of collision of the aseismic ridge with north america and therefore the locus of shallow subduction migrated southward with time causing the well documented southward migration of the magmatic lull

evidence for synchronous thin skinned and basement deformation in the cordilleran fold thrust belt the tendoy mountains southwestern montana the tendoy mountains contain the easternmost thin skinned thrust sheets in the cordilleran fold thrust belt of southwestern montana and are in the zone of tectonic overlap between the rocky mountain foreland and the cordilleran fold thrust belt the three frontal thrust sheets of the tendoy mountains are from north to south the armstead mckenzie and tendoy sheets near the southeastern terminus of the tendoy thrust sheet is a lateral ramp in which the tendoy thrust climbs along strike from the upper mississippian lombard limestone to lower cretaceous rocks this ramp coincides with the southeastern side of the paleozoic snowcrest trough and projection of the range flanking basement thrust of the blacktail snowcrest uplift suggesting either basement or stratigraphic control on location of the lateral ramp axes of major folds on the southern part of the tendoy thrust sheet are parallel to the direction of thrust transport and to the trend of the snowcrest range they are a result of 1 pre thrust folding above basement faults 2 passive transportation of the folds from a down plunge position 3 minor reactivation of basement faults and 4 emplacement of blind sub tendoy thin skinned thrust faults the tendoy sheet also contains a major out of sequence thrust fault that formed in thick upper mississippian shales and created large overturned foreland verging folds in upper mississippian to triassic rocks the out of sequence fault can be identified where stratigraphic section is omitted and by a stratigraphic separation diagram that shows it cutting down section in the direction of transport the prominent lateral ramp at the southern terminus of the tendoy thrust sheet is a result of fault propagation through strata folded over the edge of the blacktail snowcrest uplift

provenance of synorogenic foreland basin strata in southwestern montana requires revision of existing models for laramide tectonism north american cordillera

retrograding wintertime low frequency disturbances over the north pacific ocean abstract retrograding westward moving features over the middle and high latitudes of the northern hemisphere during winter are examined in observations and data from a gcm simulation using complex principal component analysis the first eigenvector of the low frequency middle troposphere geopotential height over the north pacific has a zonal scale of 120 160 deg longitude and a dipole like meridional structure with maximum amplitude over the gulf of alaska and the bering sea these disturbances propagate westward and go through a life cycle of growth and decay over a period of about three weeks the temporal evolution of the energy and its conversions during the life cycle of the simulated pacific disturbances are composited using the time dependent coefficient of the principal component analysis as an index for their occurrence the composite energy cycle indicates that the kinetic and available potential energy of these disturbances grow and decay together both baroclinic and barotropic conversion

location location location the variable lifespan of the laramide orogeny the laramide orogeny had a spatially variable lifespan which we explain using a geodynamic model that incorporates onset and demise of at slab subduction laramide shortening and attendant uplift began in southeast california usa at ca 90 ma swept to the northeast to arrive in the black hills of south dakota usa at ca 60 ma and concluded in south dakotawithin 10 m y during subsequent slab rollback the areal extent of laramide deformation decreased as the eastern edge of active deformation retreated to the southwest rapidly from ca 55 to 45 ma and more slowly from ca 45 to 40 ma with deformation ultimately ceasing in the southwestern part of the orogen at ca 30 ma geodynamic modeling of this process suggests that changes in the strength of the north america plate and densifcation of the farallon plate played important roles in controlling the areal extent of the laramide orogen and hence the lifespan of the orogenic event at any particular location in western north america

late cretaceous stratigraphy deformation and intrusion in the madison range of southwestern montana dating of orogenic rock units in the central part of the madison range shows that laramide deformation was virtually completed by the end of the cretaceous early campanian k ar dates of about 79 m y were obtained from welded tuffs in the basal part of the livingston formation a volcanic and volcaniclastic assemblage that is conformable with underlying cretaceous clastic rocks and with the overlying sphinx conglomerate no datable materials were obtained from the sphinx but both it and the livingston were deformed by the hilgard fault system a series of thrust faults and associated folds which extend along the western side of the southern two thirds of the range this north trending fault system represents the culmination of laramide shortening within the range k ar and 40 ar 39 ar dating of hornblende from dacitic laccolithic rocks that intruded the hilgard fault system in the central part of the range indicates an approximate date of 68 69 m y b p for emplacement of the igneous suite the dacite postdates movement along faults of the hilgard fault system and postdates the synorogenic sphinx conglomerate

u th pb and rb sr geochronology of middle proterozoic granite and augen gneiss salmon river mountains east central idaho coarsely porphyritic granite and augen gneiss in east central idaho have been dated by the u th pb zircon method when plotted on concordia diagrams the results give nonlinear data arrays that indicate the presence of at least one component of inherited radiogenic lead as well as postcrystallization probably recent episodic lead loss nevertheless a systematic relationship between zircon size and convergence of the 207 pb 206 pb ages for the 100 and 100 150 mesh fractions from several samples are interpreted to provide a valid age of about 1 370 m y for these plutons a single sample which is free of inheritance for all size fractions confirms this conclusion rb sr whole rock samples are highly enriched in radiogenic strontium but do not define an isochron probably because the system did not remain closed following crystallization of the plutons intrusive contacts of the granitic rocks indicate that deformation and regional metamorphism of the host yellowjacket formation occurred prior to 1370 ma rather than prior to 1500 ma as previously suggested by others this refinement of the geochronology 1 removes the necessity that the yellowjacket formation and overlying proterozoic metasedimentary rocks of central and east central idaho be pre belt in age 2 permits the traditional age correlation of the east central idaho metasedimentary sequence yellowjacket through lawson creek formations with the belt supergroup and 3 suggests that a re evaluation is in order for paleogeographic models of central idaho especially the proposed topographic high known as the salmon river arch

observed and gcm simulated westward propagating planetary scale fluctuations with approximately three week periods the structural characteristics and vorticity dynamics of westward traveling patterns wtp in the troposphere are examined using the national centers for environmental prediction national center for atmospheric research ncep ncar reanalyses based on observations for the 1973 95 period as well as the output from a 100 yr integration of a general circulation model gcm with a rhomboidal truncation at 30 wavenumbers and 14 vertical levels an identical set of diagnostic tools including progressive retrogressive variance analysis crossspectra and complex empirical orthogonal functions eofs are applied to the reanalysis and gcm datasets for 300 mb height these diagnoses all indicate that the wtp are most prominent during the cold season in the high latitude zone extending westward from northwestern canada to northeastern siberia with a typical period of 22 days outstanding episodes are identified on the basis of the temporal coefficients of the leading complex eof composite charts of the anomalous 300 mb height sea level pressure and 850 mb temperature fields at various phases of these events are constructed the typical circulation changes accompanying the passage of the wtp are similar to those associated with well known regional weather phenomena such as amplified pressure ridges over alaska cold air outbreaks over western north america and east asia and heavy snowfall over the pacific northwest the occurrence of the wtp over the north pacific is also characterized by notable changes in the spatial distribution and intensity of synoptic scale activity the contributions of relative vorticity advection planetary vorticity advection the b effect and horizontal divergence to the vorticity tendency in various phases of the composite wave at 300 500 and 850 mb are investigated in the mid and upper troposphere the vorticity dynamics of the wtp is similar to that of free external rossby waves with the b effect which leads to westward propagation being the dominant term whereas the eastward advection of relative vorticity is less important due to the weak mean zonal flow in the alaska siberia sector most of the essential characteristics of the observed wtp deduced from the ncep ncar reanalyses are well reproduced by the gcm the realism with which this phenomenon can be simulated in a model environment offers considerable promise for using the gcm as a tool for studying the impact of wtp on intraseasonal atmospheric variability in extended model experiments and for assessing the dependence of the locality and activity level of the wtp on various states of the ambient circulation

the morphology of northern hemisphere blocking abstract the morphology of regional blocking in the northern hemisphere is discussed using the 40 yr ecmwf re analysis era 40 dataset and a measure of blocking based on the reversal at storm track latitudes of meridional contrasts on a potential vorticity pv surface representative of the tropopause the focus is on cyclonic and anticyclonic rossby wave breaking that is inherent to the blocking development and the extent to which this is determined by the climatological jet position and the ambient shears more generally the importance of the climatological planetary scale is discussed the approach is mainly through composite behavior but informed by consideration of many individual events a diversity of behavior is found with longitude in both winter and summer and there is a striking reversal of the sense of the wave breaking between the two seasons that is generally consistent with the difference in the jet locations preferred behaviors are found in various regions and seasons and retrogre

zonal jet structure and the leading mode of variability an observational analysis of the northern hemisphere circulation during winter reveals that the leading mode of variability depends on longitude in particular the first eof of the zonal mean circulation differs over the atlantic and pacific sectors these results provide motivation for a series of model runs where a simple gcm is used to investigate the effects of the zonal jet structure on the leading mode of variability in the northern hemisphere model results indicate that the leading mode of variability depends on the distance between the eddy driven and subtropical jets when the jets are well separated the leading mode of variability describes latitudinal shifting of the eddy driven jet however when the two jets are nearly collocated pulsing of the combined jet dominates the variability this change coincides with a weakening of the positive feedback between the eddies and zonal flow anomalies these results provide a possible explanation for the reduced amplitude of the northern annular mode in the pacific sector relative to that in the atlantic sector during northern hemisphere winter

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

on the zonal structure of the north atlantic oscillation and annular modes abstract the zonal structure and dynamics of the dipolar patterns of intraseasonal variability in the extratropical atmosphere namely the north atlantic oscillation nao and the so called annular modes of variability are investigated in an idealized general circulation model particular attention is focused on the relationships linking the zonal structure of the stationary waves synoptic variability i e the storm tracks and the zonal structure of the patterns of intraseasonal variability large scale topography and diabatic anomalies are introduced to modify and concentrate the synoptic variability establishing a recipe for a localized storm track comparison of the large scale forcing synoptic variability and patterns of intraseasonal variability suggests a nonlinear relationship between the large scale forcing and the variability it is found that localized nao like patterns arise from the confluence of topographic and diabatic forcing and that the patterns are more localized than one would e

reinterpretation of the palynology and age of laramide syntectonic deposits southwestern montana and revision of the beaverhead group new palynological data from syntectonic deposits in southwestern montana have major temporal implications for laramide structural relations in the northern rocky mountains conglomerate and associated sandstones representing the bulk of material eroded from the thrust faulted margin of the blacktail snowcrest foreland massif are middle campanian aquilapollenites senonicus interval zone estimated absolute age 78 81 ma as a major consequence the foreland buttress responsible for the shape of the southwestern montana recess of the cordilleran thrust belt was uplifted and in place by middle campanian time palynomorph assemblages from the campanian deposits include reworked spores of paleozoic age that are evidence of provenance other less precisely dated syntectonic units in the area differ in age and provenance as well as lithology formerly all these deposits were assigned to the beaverhead formation which is raised to group status herein the stratigraphic unit shed from the blacktail snowcrest uplift is raised to formation status as the lima conglomerate the term beaverhead group is restricted to the lima conglomerate and to contemporaneous and younger conglomeratic units that differ in lithology and probably in provenance older sandstone units are removed from the beaverhead group deposits at the original type section of the beaverhead formation are not precisely dated but are no older than campanian and probably no younger than early maestrichtian

reconstructing farallon plate subduction beneath north america back to the late cretaceous using an inverse mantle convection model that assimilates seismic structure and plate motions we reconstruct farallon plate subduction back to 100 million years ago models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy requiring that the farallon slab was flat lying in the late cretaceous consistent with geological reconstructions the simulation predicts that an extensive zone of shallow dipping subduction extended beyond the flat lying slab farther east and north by up to 1000 kilometers the limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten the results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized

sedimentation in a tectonically partitioned nonmarine foreland basin the lower cretaceous kootenai formation southwestern montana the lower cretaceous kootenai formation in southwestern montana was deposited in the nonmarine cordilleran foreland basin in the united states during a period of intensified uplift in the westward adjacent but increasingly impingent sevier fold thrust belt concurrently the foreland basin was partitioned by uplift of intra foreland structural elements and incipient plutonism kootenai fluvial and fluviolacustrine depositional systems developed and evolved in response to changing basin architecture the kootenai thus provides a case study of nonmarine sedimentary responses to tectonic partitioning in a foreland basin the following criteria for recognition of tectonic partitioning of a nonmarine foreland basin are demonstrated by the kootenai 1 complex internalized drainage patterns which locally diverge from the over all regional paleoslope direction and a typical longitudinal pattern 2 intraformational and interformational unconformities both proximal to the fold thrust belt and within the foreland basin 3 evidence for cannibalization and redistribution of foreland basin sediments 4 abrupt deflections of paleodrainage axes in response to initiation of intraforeland uplifts and 5 dramatic base level alterations and consequent changes of fluvial geomorphology as a result of tectonic partitioning and segmentation of the foreland basin kootenai sedimentation can be divided into six phases 1 a tectonic pulse in the sevier fold thrust belt produced a thin but widespread sheet of gravel alluvium referred to as the basal conglomerate member which was deposited by a shallow perhaps ephemeral gravel bed braided stream system drainage was predominantly eastward across the axis of the foreland basin 2 subsequently initial activation of intra foreland structures over the present tobacco root madison gravelly and beartooth archean blocks and the boulder batholith caused reworking of the basal conglomerate in all but the westernmost foreland basin although the topographic manifestations of these uplifts were subtle the foreland basin was effectively partitioned into at least four distinct drainage basins the resulting deposit commonly called the pryor member but here referred to as the first sandstone member was deposited by sand and gravel dominant braided stream systems in the west and east respectively paleodrainage directions in the western foreland basin were southward but over all drainage was northward 3 a period of mud dominant sedimentation followed deposition of the first sandstone probably resulting from a combination of decreased tectonism influx of siliceous volcanic ash and change in source lithology 4 renewed tectonic activity in the fold thrust belt generated a second episode of coarse grained alluvial sedimentation producing the kootenai second sandstone member basin partitioning was accentuated but drainage directions remained essentially unchanged initial activation of the blacktail snowcrest uplift dammed the southern portion of the second sandstone fluvial system base level was elevated and a major anastomosed stream system developed upstream from the structural dam downstream from the axis of the uplift a transport efficient probably entrenched meandering system developed 5 the remainder of kootenai siliciclastic deposition was dominated by muddy fluvial and fluviolacustrine systems the foreland basin configuration remained essentially unchanged as drainage became ponded in response to further elevation of base level as basin partitioning continued an extensive carbonate lacustrine interval punctuated this period of sedimentation 6 gradual southward transgression of the cretaceous sea left a transitional fluviolacustrine to marine imprint on the sedimentary record of the uppermost kootenai and overlying blackleaf formation rejuvenation of orogenesis in the fold thrust belt augmented by cratonic and major intraforeland volcanic and sedimentary lithic sources renewed the supply of coarse grained detritus to the foreland basin during deposition of the blackleaf

flexural subsidence and basement tectonics of the cretaceous western interior basin united states the flexural subsidence history recorded in cenomanian to early campanian 97 to 80 ma strata in the cretaceous u s western interior basin was studied with two dimensional flexural backstripping techniques results indicate that the flexural subsidence resulting from thrust loading was superimposed on epeirogenic subsidence in the foreland basin the flexural component exhibits significant spatial and temporal variations along both the strike and dip relative to the sevier thrust belt the greatest cumulative subsidence occurred in southwestern wyoming and northern utah concurrent subsidence in northwestern montana and southern utah was insignificant temporal trends in subsidence also show a distinct regional pattern from the cenomanian to late turonian 97 to 90 ma subsidence rates were high in utah and much lower in wyoming and montana in contrast during the coniacian and santonian 90 to 85 ma subsidence accelerated rapidly in wyoming increased slightly in montana and decreased in utah we suggest that these spatially and temporally varying subsidence patterns reflect the interplay of several geodynamic factors including 1 temporal and spatial variation in emplacement of the thrust loads 2 segmentation of the basement into adjacent blocks with different rheological properties 3 reactivation of basement fault trends and 4 regional dynamic topographic effects

the impact of the madden julian oscillation on high latitude winter blocking during el nino southern oscillation events abstractwintertime high latitude blocking is associated with persistent changes in temperature and precipitation over much of the northern hemisphere studies have shown that the madden julian oscillation mjo the primary form of intraseasonal tropical variability significantly modulates the frequency of high latitude blocking through large scale rossby waves that alter the global circulation however the characteristics of mjo teleconnections are altered by the el nino southern oscillation enso which modifies the global flow on interannual timescales suggesting that the mjo influence on blocking may depend on enso phase the characteristics of mjo rossby waves and blocking during enso events are examined using composite analysis and a nonlinear baroclinic model the enso phase dependent teleconnection patterns are found to significantly impact pacific and atlantic high latitude blocking during el nino a significant persistent increase in pacific and atlantic blocking follows mjo rmm phase 7 cha

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

wave breaking characteristics of midlatitude blocking in this article northern hemisphere winter midlatitude blocking is analysed through its wave breaking characteristics rossby wave breaking is identified as a key process in blocking occurrence as it provides the mechanism for the meridional reversal pattern typical of blocking two indices are designed to detect the major properties of wave breaking i e the orientation cyclonic anticyclonic direction of breaking or db index and the relative contribution of air masses warm cold relative intensity or ri index the use of the db index differentiates between the anticyclonic cases over europe and asia and the cyclonic events over the oceanic basins one of the three regions displaying cyclonic type was found over the atlantic ocean the other two being over the pacific ocean the first of these is located over the western side of the pacific and is dominated by warm air extrusions whereas the second is placed northward of the exit region of the jet stream where the meridional gradient is much weaker two european blocking types have been detected using the ri index which separates out the cases dominated by warm and cold air masses the latter cases in particular exhibited a well structured dipole with associated strong anomalies in both temperature and precipitation copyright 2011 royal meteorological society

farallon plate dynamics prior to the laramide orogeny numerical models of flat subduction abstract the laramide orogeny 80 50 ma was an anomalous period of mountain building in the western united states that occurred more than 1000 km inboard of the farallon plate subduction margin it is widely believed that this orogeny is coincident with a period of flat subhorizontal subduction however the factors that caused the farallon plate to evolve from a normal steep geometry to flat subduction are not well understood three proposed factors are 1 a westward trenchward increase in north america motion 2 an increased slab suction force owing to the presence of thick colorado plateau lithosphere and 3 subduction of a low density oceanic plateau this study uses 2d upper mantle scale numerical models to investigate these factors the models show that trenchward continental motion is the primary control on subduction geometry with decreasing slab dip as velocity increases however this can only create low angle subduction as the farallon plate was old 100 myr and denser than the mantle a transition to flat subduction requires 1 subduction of a buoyant oceanic plateau that includes an 18 km thick crust that does not undergo metamorphic densification and an underlying depleted harzburgite layer and 2 a slab break off at the landward side of the plateau the break off removes the dense frontal slab and flat subduction develops as the buoyant plateau deflects the slab upward the slab suction force has only a minor effect on slab flattening but the thickness of the colorado plateau lithosphere controls the depth of the flat slab with a continental velocity of 4 cm yr and a 400 km wide oceanic plateau flat subduction develops within 15 ma after plateau subduction the flat slab underthrusts the continent at 200 km depth eventually extending 1500 km inboard of the trench

north atlantic warming and the retreat of greenland s outlet glaciers mass loss from the greenland ice sheet quadrupled over the past two decades contributing a quarter of the observed global sea level rise increased submarine melting is thought to have triggered the retreat of greenland s outlet glaciers which is partly responsible for the ice loss however the chain of events and physical processes remain elusive recent evidence suggests that an anomalous inflow of subtropical waters driven by atmospheric changes multidecadal natural ocean variability and a long term increase in the north atlantic s upper ocean heat content since the 1950s all contributed to a warming of the subpolar north atlantic this led in conjunction with increased runoff to enhanced submarine glacier melting future climate projections raise the potential for continued increases in warming and ice mass loss with implications for sea level and climate

simultaneous atlantic pacific blocking and the northern annular mode a synoptic situation termed high latitude blocking hlb is shown to occur frequently in both the atlantic and pacific sectors and to result in flow anomalies very similar to those associated with the negative phase of the northern annular mode nam in the respective sector there is a weak but significant link between the occurrence of hlb in the two sectors with atlantic hlb tending to lead pacific hlb by 1 3 days this link arises from rare events in which both sectors are almost simultaneously affected by a large scale wave breaking event which distorts the polar trough over northern canada in several cases the tropospheric wave breaking occurs in tandem with a large scale disturbance of the stratospheric polar vortex there is therefore a physical link between the atlantic and pacific sectors but analysis suggests that this does not contribute to determining the pattern of the nam as conventionally defined from monthly mean data however an alternative version of the nam derived directly from daily data does appear to reflect this physical link these conflicting results highlight the sensitivity of the nam to the period over which data are averaged copyright c 2008 royal meteorological society

influence of the vertical and zonal propagation of stratospheric planetary waves on tropospheric blockings 1 case studies are used to elucidate the relationship between stratospheric planetary wave reflection and blocking formation in the troposphere the enhanced upward propagation of a planetary scale wave packet from the eurasian sector involving a euro atlantic blocking leads to a stratospheric sudden warming ssw following the weakening of the stratospheric westerly jet due to polar warming the stratospheric planetary wave packet then propagates downward over the american sector inducing a ridge over the north pacific as well as a trough over eastern canada in the upper troposphere the ridge promotes the formation of a pacific blocking this result explains why pacific blockings tend to form after ssw and why they are associated with suppressed upward propagation of planetary waves

a diagnosis of two blocking events that occurred simultaneously in the midlatitude northern hemisphere using the goddard laboratory for atmospheres goddard earth observing system 1 analyses the horizontal distribution of atmospheric forcing processes involved in the growth maintenance and decay of two northern hemisphere midlatitude blocking anticyclones that occurred simultaneously were examined with the goal of determining whether there was a dynamic connection between the two cases the complete form of the zwack okossi equation which is a generalization of the petterssen sutcliffe development equation was used as the primary diagnostic tool the basic analyses were partitioned into their planetary scale and synoptic scale components noninteraction 500 hpa height tendencies as well as scale interaction 500 hpa height tendencies based on the results of this diagnosis there was no dynamic connection between these blocking anticyclones this result agrees with findings of other studies based on a statistical analysis of simultaneous blocking these results also imply that blocking may be a local phenomenon the diagnostic results from these blocking events were in many respects similar to those of previous studies however some key differences were found for example it was found here that for both mode 2 blocking events temperature advection was an important mechanism in block formation earlier results showed vorticity advection as the primary atmospheric forcing process in block formation for a mode 1 block also the scalepartitioned results show that upstream cyclones contributed to block formation and intensification in these events directly through the synoptic scale component often with the support of the interaction component of the total forcing earlier scale partitioned results showed that the interaction component of the total forcing was most important in the formation of a mode 1 block

new timing constraints for the onset of laramide deformation in southwest montana challenge our understanding of the development of a thick skinned structural style during flat slab subduction

is the north atlantic oscillation a breaking wave abstract given the recent observational evidence that the positive negative phase of the north atlantic oscillation nao is the remnant of anticyclonic cyclonic wave breaking this study uses a multilevel primitive equation model to investigate important dynamical attributes of the above wave breaking behavior for this purpose a hierarchy of different basic states two and three dimensional and initial perturbations are used with the three dimensional climatological flow as the basic state it is found that initial perturbations located equatorward poleward and upstream of the climatological atlantic jet lead to wave breaking similar to that of the positive negative nao phase consistently analysis of observational data indeed shows that the pacific storm track is displaced equatorward poleward prior the onset of the positive negative nao phase this result suggests that the latitudinal position of the pacific storm track plays an important role for determining the phase of the nao sen

late cretaceous subsidence in wyoming quantifying the dynamic component the late cretaceous western interior basin of north america is generally considered to have been a retroarc foreland basin flexural backstripping of the stratigraphic record from 97 2 ma to 73 4 ma along a section perpendicular to the wyoming idaho salient of the sevier belt clearly demonstrates that there were components of subsidence in addition to those driven by the thrust and associated sediment loads the simulation demonstrates that the flexural foredeep is only 180 120 km wide and shows a forebulge located near the subsequent laramide rock springs uplift foredeep strata between the rock springs uplift on the east and the thrust front to the west were mostly involved in the eastward overlapping thrust belt or deeply truncated by inferred rebound after 78 5 ma the difference between the observed decompacted cumulative subsidence and the amount of subsidence that can be explained by simple flexural loading the residual subsidence increases from 800 m in eastern wyoming to 1 8 km near the thrust belt the westward thickening upper cretaceous sediment wedge filled the accommodation space generated by downward tilting to the west of the north american cratonic margin by underplating and mantle flow associated with the shallowly subducted farallon plate

was the laramide orogeny related to subduction of an oceanic plateau numerous models have been presented to explain the late cretaceous early cenozoic laramide orogeny which affected the foreland region of the western cordillera within the us1 the most attractive models invoke low angle subduction2 4 which can develop for various reasons5 6 evidence from south america indicates that one such reason may be the subduction of buoyant ocean floor we here extend the low angle subduction models by attributing the shallowing of the subduction angle during the laramide orogeny to subduction of a large oceanic plateau of anomalously thick and buoyant oceanic crust pacific plate history indicates that a twin to the hess rise may represent the oceanic plateau that triggered laramide events

unmixing detrital geochronology age distributions despite recent advances in quantitative methods of detrital provenance analysis there is currently no widely accepted method of unmixing detrital geochronology data we developed a mixing model that determines mixing proportions for source samples through inverse monte carlo modeling wherein mixed samples are compared to randomly generated combinations of source distributions and a range of best mixing proportions are retained results may then be used to constrain a forward optimization routine to find a single best fit mixture quantitative comparison is based on the kolmogorov smirnov ks test d statistic and kuiper test v statistic for cumulative distribution functions and the cross correlation coefficient for finite mixture distributions probability density plots or kernel density estimates we demonstrate the capacity of this model through a series of tests on synthetic data sets and a published empirical data set from north america mixed in known proportions this proof of concept testing shows the model is capable of accurately unmixing highly complex distributions we apply the model to two published empirical data sets mixed in unknown proportions from colombia and central china neither data set yields perfect model fits which provides a cautionary note of potentially inadequate characterization of source and or mixed samples and highlights the importance of such characterization for accurate interpretation of sediment provenance data set size appears to be a major control on mixture model results small n 100 detrital data sets may lead to misinterpretation of sediment provenance the model is available as a matlab based stand alone executable exe file graphical user interface

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

synoptic behavior of baroclinic eddies during the blocking onset abstract synoptic behavior of individual baroclinic eddies in the course of their interactions with amplifying blocking anticyclones is examined based upon a 30 year record of the tropospheric circulation in the northern hemisphere winter high pass filtered as well as unfiltered fields of geopotential height and potential vorticity were composited relative to the onset of the five different types of blocking patterns before the compositing the entire sequence of the fields was slightly shifted in time and space in such a way that the strongest baroclinic eddy during the onset of each blocking event occupies a prescribed position in the upstream storm track when it reaches its maximum intensity since this shifting considerably reduces the cancellation between the individual high frequency migratory eddies this type of compositing can present a more synoptically oriented view of those interactions than conventional compositing in our composite results one or two pairs of cyclonic and anticyclonic

cenozoic and mesozoic structure of the eastern basin and range province utah from cocorp seismic reflection data cocorp seismic reflection data collected from the eastern basin and range in west central utah provide information on cenozoic extensional tectonics mesozoic thrusting and their interrelationships those data show a series of remarkably continuous low angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15 20 km over that distance none of these events are significantly cut by any high angle normal faults a major detachment beneath the sevier desert can be traced from a surface zone of normal faulting to a depth of 12 15 km with a regional apparent westward dip of 12 tentative correlation of upper and lower plate events suggests 30 60 km of extensional displacement on this detachment whether this structure is a reactivated mesozoic thrust is uncertain west steepening splays off the end of the detachment reach depths of 20 km and may represent a major mesozoic ramp or zones of distributed ductile shearing during extension some events are interpreted to be mesozoic thrusts of which at least one beneath the house range has been reactivated during the cenozoic the snake range decollement dips gently east and has a sense of cenozoic displacement opposite to that of other cenozoic detachments farther east deep events are most numerous beneath the east side of the sevier desert where they occur to depths of 30 km at the top of or perhaps partly within the anomalously low velocity upper mantle

wintertime westward traveling planetary scale perturbations over the euro atlantic region the features of the wintertime westward traveling planetary scale perturbations over the euro atlantic region are examined through the use of space time spectral analysis applied to the 500 hpa geopotential height field the intention is to understand the nature of these phenomena and the performance of climate models data from both ecmwf re analyses and a simulation from the arpege general circulation model are used westward traveling planetary scale transients are found over the region as local perturbations resembling rossby normal modes with a maximum power over the eastern atlantic the westward traveling planetary scale transients north of 40 n have periods larger than 20 days south of this latitude wave periods are shifted to a band around 10 days so that they can be related to subtropical transient waves the atmospheric model used like other models which exhibit reasonable mean climatic properties tend to have less overall intraseasonal variability than observed nevertheless the model is able to capture most of the features of the westward traveling low frequency transients the differences in basic state partially produced by scale interactions would lead to the generation of westward traveling waves in the model distinct from the observed however it is suggested that the improvement of the present model version with regard to previous model versions is due to a better simulation of the time mean state the reasonable simulation of the synoptic scale variability south of 50 n and thus of its barotropic forcing on the basic state may also help to explain the realistic westward traveling transients in the model

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

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

sequence stratigraphy of nonmarine jurassic cretaceous rocks central cordilleran foreland basin system an analysis of the upper jurassic lower cretaceous morrison and cedar mountain formations of utah and colorado has resulted in a general sequence stratigraphic model for nonmarine rocks in this model nonmarine deposition is governed by changes in basin accommodation development and corresponding shifts in depositional base level these fluctuations result in deposition of systematically varying facies and architectural elements that allow nonmarine depositional sequences to be recognized internally nonmarine depositional sequences comprise three systems tracts degradational transitional and aggradational which are analogous to the lowstand transgressive and highstand systems tracts of marine depositional sequences degradational systems tracts overlie sequence bounding unconformities and consist of relatively coarse grained low sinuosity fluvial deposits that are either contained within incised valleys or deposited as widespread thin sheets above shallow erosion surfaces transitional systems tracts represent an increase in basin accommodation development following degradational systems tract deposition they are marked by the transition from laterally continuous low sinuosity fluvial channel sandstones and conglomerates to lenticular and ribbon like meandering and anastomosing channel sandstones aggradational systems tracts are characterized by meandering anastomosing channel sandstones and abundant fine grained overbank and lacustrine deposits the upper jurassic lower cretaceous nonmarine rocks of the study area contain three depositional sequences the first of these the uj 1 sequence consists primarily of an aggradational systems tract overlain by a sequence bounding unconformity however the lower parts of this sequence are transitional with older marine rocks and can be considered the late stages of a marine highstand systems tract the upper morrison sequence uj 2 consists of degradational transitional and aggradational systems tracts above the uj 2 sequence are a sequence bounding unconformity and degradational and transitional systems tracts of the lk 1 sequence represented by the buckhorn conglomerate the buckhorn is overlain by a sequence bounding unconformity and transitional aggradational systems tracts of the lk 2 sequence that is composed of the upper part of the cedar mountain formation the upper jurassic lower cretaceous sequences in utah and colorado can be traced regionally and correlated with nonmarine depositional sequences in central utah and sequences that contain nonmarine transitional and marine rocks in central wyoming these sequences were deposited in the back bulge forebulge and distal foredeep depozones of the late jurassic early cretaceous foreland basin system

the pacific center of action of the northern hemisphere annular mode real or artifact the leading empirical orthogonal function eof of the sea level pressure slp field referred to as the arctic oscillation ao or northern hemisphere annular mode nam consists of a dipole between the polar cap region and the surrounding zonal ring centered along 458n embedded within the outer ring are centers of action over the euro atlantic and pacific sectors in which slp fluctuates in phase that the observed slp fluctuations at these two centers of action are virtually uncorrelated raises the question of whether the pacific center in the annular mode could be an artifact of eof analysis it is argued that sea level pressure fluctuations at the pacific and euro atlantic centers of action of the ao nam would be more strongly correlated were it not for the fact that slp variability over the north pacific is dominated by a pattern in which fluctuations over the north atlantic and north pacific are inversely related evidence of the coexistence of such a pattern which resembles an augmented version of the pacific north american pattern is presented

long distance longitudinal transport of gravel across the cordilleran thrust belt of montana and idaho two newly identified middle eocene paleovalleys 100 km long preserved on top of the southwest montana reentrant of the cordilleran fold and thrust belt indicate long lived longitudinal flow across the thrust belt and resolve a long standing debate about the source of the voluminous quartzite debris in the upper cretaceous to lower tertiary divide harebell and pinyon conglomerates of montana idaho and wyoming geologic mapping stratigraphic provenance and geochronologic studies revealed that eocene volcanic and sedimentary rocks in the paleovalleys are as thick as 2 km onlap preexisting bedrock and interfinger with well rounded conglomerate derived from formations exposed only to the west the middle eocene paleovalleys are the youngest expression of a major paleoriver system that transported sediment toward the foreland during the sevier orogeny an eocene subcrop map shows that the headwaters of the eocene paleovalleys coincided with structural culminations in the thrust belt that supplied sediment to the divide conglomerate of the upper cretaceous to lower tertiary beaverhead group ultimately the lemhi pass and hawley creek paleovalleys provided several thousand cubic kilometers of quartzite debris to the pinyon and harebell conglomerates of northwest wyoming 200 350 km away and formed the northwest half of a giant longitudinal drainage system sevier contraction not the rising idaho batholith first uplifted vast culminations beneath the headwaters of this river system

on the teleconnectivity of the arctic oscillation the term arctic oscillation ao has recently been introduced to describe the leading structure of slp variability over the northern hemisphere a key feature of the ao is its zonally symmetric appearance with a primary center of action over the arctic and opposing anomalies in midlatitudes does the ao s annular appearance result from significant temporal correlations between slp anomalies at distant longitudes the results presented indicate that the temporal coherence between the arctic and midlatitudes is strongest over the atlantic sector with weak correlations between the atlantic and pacific midlatitudes both on intraseasonal and interannual time scales during the past 50 yrs hence the annular character of the ao is more a reflection of the dominance of its arctic center of action than any coordinated behavior of the atlantic and pacific centers of action in the slp field the ao is nearly indistinguishable from the leading structure of variability in the atlantic sector e g the north atlantic oscillation their temporal correlation is 0 95 for monthly data

apatite fission track evidence for laramide and post laramide uplift and anomalous thermal regime at the beartooth overthrust montana wyoming the crystalline rocks of the beartooth overthrust in southwest montana and northwest wyoming were uplifted and thrust over the northwest margin of the bighorn basin during the laramide orogeny the timing and geometry of that event is well documented in the synorogenic paleocene and early eocene sedimentary rocks that were deposited ahead of the rising beartooth basement block and partially over ridden as the block advanced the absence of post laramide sedimentary rocks on the block and in the adjacent bighorn basin has however precluded the reconstruction of the post orogenic tectonothermal history of this region we report here the results of fission track analysis of apatite from the three components of the beartooth over thrust with the aim of reconstructing the laramide and post laramide tectono thermal history of the southeast margin of the beartooth mountains apatite fission track ages and corresponding horizontal confined track length distributions hctlds from precambrian basement rocks constituting the upper plate of the beartooth overthrust indicate that from 7 to 12 km of uplift of the red lodge corner of the beartooth block has occurred since early paleocene time this amount of uplift occurred in two stages with an intervening mid tertiary period of tectonic quiescence the latter was a period of either 1 oligocene and or miocene deposition or 2 tectonic quiescence uplift of 4 to 8 km occurred during the first phase of cooling which lasted from early paleocene time 61 ma to early eocene time 52 ma during the second phase which began in late miocene early pliocene time and continues to the present day about 4 km of uplift occurred our fission track data suggest that the thermal regime in rocks above the beartooth overthrust was relatively stable during tertiary time the maximum geothermal gradient permitted by model thermal histories generated from our observed fission track data during post laramide time is 17 c km this value is the same as that of the present day geothermal gradient measured in the amoco beartooth well and suggests that a low geothermal gradient prevailed throughout tertiary time apatite fission track ages and hctlds from jurassic and cretaceous sedimentary rocks beneath the beartooth overthrust indicate that these rocks have remained 10 20 c cooler than overlying rocks in the shear zone and the lowermost part of the upper plate since 61 ma an observation that is not consistent with the stratigraphic position of these rocks we interpret this temperature to be the result of a persistent thermal regime in which ground water circulating through the sedimentary rocks of the lower plate cooled them and insulated them from conductive heat transfer from hotter overlying rocks this ground water circulation may have been responsible for flushing hydrocarbons out of the rock column explored by the beartooth well shear zone rocks experienced a higher temperature during cenozoic time than did rocks of the upper and lower plates this condition was maintained by flow through the shear zone of ground water heated to higher temperatures at deeper levels along the thrust

a striking example of the atmosphere s leading traveling pattern abstract conventional and complex empirical orthogonal function eof techniques show that for at least four months during the fall and winter of 1979 80 a large amplitude large scale traveling flow anomaly existed in the troposphere and stratosphere this feature evolved in a cyclic manner exhibiting an average period of about 23 days its phase propagation was toward the west while its amplitude was concentrated in a broad envelope centered over canada the phenomenon was equivalent barotropic in the troposphere and increased in amplitude with altitude it represented about 25 of the variance of geopotential heights in the troposphere and stratosphere for the months of november through march the meridional and zonal structure of the zonal wavenumber one component of the traveling anomaly is similar to that of the theoretical 16 day wave however it is distinct from that theoretical mode in that its period is longer and in the troposphere zonal wave two makes a large contribution to its structur

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

palaeoenvironments the archaeological record and cereal pollen detection at clickimin shetland scotland abstract the loch of clickimin shetland is well known for its broch and associated monuments supposedly of the bronze and iron ages although the former date is disputed pollen diatom and sedimentological investigations permit fresh insights into the landscape and economic impacts of those who constructed the monuments reveal that the environs of the site experienced environmental change from neolithic times onward and provide the first holocene radiocarbon dates from the immediate locality the broch building period witnessed a continuation of pre existing pastoral husbandry for which heather burning may have exacerbated the natural spread of blanket peat initially no evidence was adduced for local arable activity in the present investigation and this was thought to be consistent possibly with the low numbers of excavated querns and the absence of cereal macrofossil finds when compared with other shetland broch sites the application of rapid scanning techniques however led to the discovery of a consistent cereal type pollen representation from bronze age times onwards the discrepancies between the palaeoenvironmental evidence and the environmental and palaeoeconomic inferences made by the excavator of the site are explored evidence is presented that both supports and contradicts previous assumptions surrounding an important archaeological site

low frequency variability in a simplified atmospheric global circulation model storm track induced spatial resonance the interaction between two storm tracks is analysed by using idealized experiments with a simplified global circulation model a zonal distance of 180 between the storm tracks drives a retrograde travelling zonal wave number two wave with an amplitude modulation of about 50 days the stream function tendency equation demonstrates that the amplitude modulation is excited in the storm track regions by the high frequency periods between two and eight days eddy forcing spatial resonance of a retrograde propagating zonal wave number two rossby wave and high frequency eddy forcing are proposed as the amplifying mechanisms while friction and self interaction between the low frequency eddies ensure damping sensitivity experiments indicate that the pronounced modulation of 50 days vanishes with decreasing zonal distance between the storm tracks thereby enhancing the intensity and duration of blocking events

arctic oscillation or north atlantic oscillation abstract the definition and interpretation of the arctic oscillation ao are examined and compared with those of the north atlantic oscillation nao it is shown that the nao reflects the correlations between the surface pressure variability at its centers of action whereas this is not the case for the ao the nao pattern can be identified in a physically consistent way in principal component analysis applied to various fields in the euro atlantic region a similar identification is found in the pacific region for the pacific north american pna pattern but no such identification is found here for the ao the ao does reflect the tendency for the zonal winds at 35 and 55 n to anticorrelate in both the atlantic and pacific regions associated with the nao and pna because climatological features in the two ocean basins are at different latitudes the zonally symmetric nature of the ao does not mean that it represents a simple modulation of the circumpolar flow an increase in the ao or nao implies stro

are the north atlantic oscillation and the northern annular mode distinguishable this study addresses the question of whether persistent events of the north atlantic oscillation nao and the northern annular mode nam teleconnection patterns are distinguishable from each other standard daily index time series are used to specify the amplitude of the nao and nam patterns the above question is examined with composites of sea level pressure and 300 and 40 hpa streamfunction along with tests of field significance a null hypothesis is specified that the nao and nam persistent events are indistinguishable this null hypothesis is evaluated by calculating the difference between time averaged nao and nam composites it is found that the null hypothesis cannot be rejected even at the 80 confidence level the wave breaking characteristics during the nam life cycle are also examined both the positive and negative nam phases yield the same wave breaking properties as those for the nao the results suggest that not only are the nao and nam persistent events indistinguishable but that the nao nam events are neither confined to the north atlantic nor are they annular

basal continental mantle lithosphere displaced by flat slab subduction during the subhorizontal flat subduction of an ocean lithosphere plate the overlying continental plate is deformed far inland of the plate boundary in addition arc magmatism which is caused by melting in the asthenospheric wedge during steep subduction wanes or ends when the subduction is subhorizontal the observed upper plate deformation patterns have been explained by an end load exerted at the plate boundary or by stress transmitted into the overlying plate along the top of the flat slab here we present numerical thermal mechanical models of flat slab subduction that show the flattening of the slab results in a compression of the continental plate through end loading the advancing flat slab scrapes off the lowermost 20 50 km of continental mantle lithosphere the displaced continental mantle lithosphere fills the asthenospheric wedge ending arc type melting if the displaced material is buoyant it accumulates in a growing keel that migrates ahead of the slab if it is dense the displaced material sinks with the slab flat slab removal renews the asthenospheric wedge and arc magmatism and leaves a step in the lithosphere asthenosphere boundary and or a keel that consists of displaced continental mantle lithosphere a fossil keel and a fossil step formed during laramide flat subduction are preserved below the western united states continental mantle lithosphere is scraped from the base of the overriding plate by the underlying oceanic slab during flat subduction according to numerical thermal mechanical models

time of initial thrusting in the sevier orogenic belt idaho wyoming and utah reexamination of the distribution of fossils found in the earliest preserved synorogenic conglomerates within the sevier thrust belt suggests that initial thrust movement may be no older than aptian age this interpretation is corroborated by subsidence analyses of sedimentary sequences lying within and east of the idaho wyoming and utah thrust belts a major episode of middle cretaceous aptian cenomanian subsidence is interpreted as recording the initiation of thrust loading deformation in the adjacent sevier orogenic belt an earlier subsidence event took place during middle jurassic time more than 30 m y prior to the cretaceous event and may be the result of tectonic events to the west that are unrelated to thrust deformation in the idaho wyoming and utah thrust belts we find no verifiable evidence to support previous interpretations that sevier belt deformation and uplift began in late jurassic time

the variability of the atlantic storm track and the north atlantic oscillation a link between intraseasonal and interannual variability abstract the winter mean north atlantic oscillation nao index has been mostly positive since the 1980s with a linear upward trend during the period from 1978 to 1990 p1 and a linear downward trend during the period from 1991 to 2009 p2 further calculations show that the atlantic storm track eddy activity is more intense during p2 than during p1 which is statistically significant at the 90 confidence level for a t test this study proposes a hypothesis that the change in the trend of the positive nao index from p1 to p2 may be associated with the marked intensification of the atlantic storm track during p2 a generalized nonlinear nao model is used to explain the observed trend of the positive nao index within p2 it is found that even when the atlantic storm track eddies are less intense a positive phase nao event can form under the eddy forcing if the planetary scale wave has an initial value with a low over high dipole structure during p1 and p2 a blocking flow can occur in the downstream si

a stochastic model for the spatial structure of annular patterns of variability and the north atlantic oscillation abstract meridional dipoles of zonal wind and geopotential height are found extensively in empirical orthogonal function eof analysis and single point correlation maps of observations and models notable examples are the north atlantic oscillation and the so called annular modes or the arctic oscillation minimal stochastic models are developed to explain the origin of such structure in particular highly idealized analytic purely stochastic models of the barotropic zonally averaged zonal wind and of the zonally averaged surface pressure are constructed and it is found that the meridional dipole pattern is a natural consequence of the conservation of zonal momentum and mass by fluid motions extension of the one dimensional zonal wind model to two dimensional flow illustrates the manner in which a local meridional dipole structure may become zonally elongated in eof analysis producing a zonally uniform eof even when the dynamics is not particularly zonally coherent on hemispheric length scales

laramide thrust generated alluvial fan sedimentation sphinx conglomerate southwestern montana the uppermost cretaceous lower tertiary sphinx conglomerate crops out over an area of approximately 20 km2 8 mi2 in the madison range of southwestern montana the sphinx consists of more than 1 000 m 3 300 ft of synorogenic boulder and cobble conglomerate derived from a late cretaceous laramide uplift that was located in the area presently occupied by the madison river valley palynological and radiometric age data indicate that the sphinx was deposited 75 58 ma and that thrusting and folding of the deposit had largely ceased by 56 ma compositions of sphinx clasts and paleocurrent data indicate that the sphinx was produced by uplift and unroofing of mesozoic and paleozoic rocks located on two thrust sheets to the west and southwest the lower s hinx was deposited on the distal portions of an eastward prograding alluvial fan system clast assemblages and lithofacies indicate that deposition of the middle sphinx was controlled by a combination of progradation in response to ongoing thrusting and an influx of resistant clasts derived from middle paleozoic carbonates in the source area deposition of the upper sphinx was probably controlled by source lithology as the influx of very coarse resistant clasts from middle and lower paleozoic carbonates overwhelmed the fan system s ability to organize its load of sediment by normal fan processes a preliminary facies model for thrust generated alluvial fan deposits predicts intraformational deformation cannibalization of proximal synorogenic fan facies and abrupt compositional breaks in response to episodes of thrusting

franciscan complex calera limestones accreted remnants of farallon plate oceanic plateaus the calera limestone part of the franciscan complex of northern california may have formed in a palaeoenvironment similar to hess and shatsky rises of the present north west pacific1 we report here new palaeomagnetic results palaeontological data and recent plate motion models that reinforce this assertion the calera limestone may have formed on farallon plate plateaus north of the pacific farallon spreading centre as a counterpart to hess or shatsky rises in one model2 the plateaus were formed by hotspots close to the farallon pacific ridge axis on accretion to north america plateau dissection in the late cretaceous to eocene 50 70 myr could explain the occurrence of large volumes of pillow basalt and exotic blocks of limestone in the franciscan complex partial subduction of the plateaus could have contributed to laramide 70 40 myr compressional events3

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

four cordilleran paleorivers that connected sevier thrust zones in idaho to depocenters in california washington wyoming and indirectly alaska upper cretaceous sandstones from 17 localities from california to southeastern alaska united states contain unexpectedly large populations of detrital zircons with proterozoic u pb ages with age peaks at 1800 1650 and 1380 ma these peaks are indicative of a sediment source region in the southern part of the proterozoic belt supergoup basin in central idaho which hosts 1800 1650 ma detrital zircons and which was intruded by rift related 1380 ma bimodal plutons and sills belt rocks were strongly uplifted and eroded during late cretaceous sevier shortening and fed four paleoriver systems the lemhi pass hawley creek river system flowed east and sourced the beaverhead harebell pinyon nonmarine megafan in the cordilleran foreland basin the kione river flowed southwest to northern california where it sourced a very large ca 82 80 ma 600 m thick delta and submarine fan complex within the northern great valley forearc basin considerable kione detritus also transited the forearc basin to reach the franciscan trench sourcing a pulse of deposition and subduction accretion in central california and even part of southern california the swakane river flowed northwest out of idaho into washington sourcing the protolith for the high grade swakane gneiss more speculatively a yakutat river may have flowed northwest and deposited yakutat strata in a trench off washington or british columbia before those rocks were translated north to southeastern alaska recognition of a major source area in central idaho for zircons with an uncommon age of 1380 ma helps constrain the ca 85 65 ma paleogeography and paleotectonics of major sectors of the north american convergent margin orogen

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

opposing effects of reflective and nonreflective planetary wave breaking on the nao planetary wave breaking pwb over the subtropical north atlantic is observed over 45 winters december 1958 arch 2003 using ncep ncar reanalysis data pwb is manifested in the rapid large scale and irreversible overturning of potential vorticity pv contours on isentropic surfaces in the subtropical upper troposphere as breaking occurs over the subtropical north atlantic an upper tropospheric pv tripole anomaly forms with nodes over the subtropical midlatitude and subpolar north atlantic the northern two nodes of this tripole are quite similar to the spatial structure of the north atlantic oscillation nao with positive polarity nonlinear reflection is identified in approximately a quarter of all pwb events following breaking two distinct circulation regimes arise one in response to reflective events and the other in response to nonreflective events for reflective events anomalies over the north atlantic rapidly propagate away from the breaking region along a poleward arching wave train over the eurasian continent the quasi stationary wave activity flux indicates that wave activity is exported out of the atlantic basin at the same time the regional poleward eddy momentum flux goes through a sign reversal as does the polarity of the nao for nonreflective events the dipole anomaly over the north atlantic amplifies diagnostics for nonreflective events suggest that wave activity over the azores gets absorbed allowing continued enhancement of both the regional poleward eddy momentum flux and the positive nao

flexural versus dynamic processes of subsidence in the north american cordillera foreland basin 1 dynamic subsidence related to subduction is an important process in retroarc foreland basin systems dynamic subsidence in the north american retroarc foreland has been proposed as dominant in the late cretaceous however questions remain about the nature of the subcrustal load and the basin response to such processes we present new isopach data using 130 data points covering a large portion of the u s and a revised flexural analysis of the sevier foreland higher rigidity associated with the wyoming craton can best model flexural subsidence directly prior to 81 ma we constrain the transition from flexural to nonflexural subsidence to 81 ma and correlate this with large late cretaceous progradation the locus of subsidence and the location of the subducted shatsky oceanic plateau suggest that dynamic loading from viscous flow above the subducting plate is the mechanism driving dynamic subsidence in the retroarc foreland basin of the north american cordillera

the role of oceanic plateau subduction in the laramide orogeny the cause of the laramide phase of mountain building remains uncertain conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant hess or shatsky oceanic plateaux independent verification of these models has remained elusive because the putative ridges or plateaux are no longer at the earth s surface inverse convection models have identified two prominent seismic anomalies on the recovered farallon plate here we combine inverse convection models with reconstructions of plate motions to show that these seismic anomalies coincide palaeogeographically with the restored positions of the shatsky and hess conjugate plateaux as they subducted beneath north america specifically the distribution of laramide crustal shortening events tracked the passage of the shatsky conjugate beneath north america whereas the effects of the hess conjugate subduction were restricted to the northern mexico foreland belt we propose that continued subduction caused the oceanic crust to undergo the basalt eclogite phase transformation during which the shatsky conjugate lost its extra buoyancy and was effectively removed increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward followed by regional scale surface rebound we conclude that laramide uplift resulted from the removal rather than emplacement of the shatsky conjugate

blocking and rossby wave breaking on the dynamical tropopause in the southern hemisphere abstract rossby wave breaking on the dynamical tropopause in the southern hemisphere the 2 pvu surface is investigated using the era 40 dataset the indication of wave breaking is based on reversal in the meridional gradient of potential temperature and persistent large scale wave breaking is taken as a strong indication that blocking may be present blocking in the midlatitudes is found to occur predominantly during wintertime in the pacific and is most vigorous in the east pacific while during summertime the frequency of blocking weakens and its extent becomes confined to the west pacific the interannual variability of blocking is found to be high wave breaking occurs most frequently on the poleward side of the polar jet and has some but not all of the signatures of blocking so it is referred to as high latitude blocking in general cyclonic wave breaking occurs on the poleward side of the polar jet otherwise anticyclonic breaking occurs however at least in wintertime wave breaking in th

a new rossby wave breaking interpretation of the north atlantic oscillation this paper proposes the hypothesis that the low frequency variability of the north atlantic oscillation nao arises as a result of variations in the occurrence of upper level rossby wave breaking events over the north atlantic these events lead to synoptic situations similar to midlatitude blocking that are referred to as high latitude blocking episodes a positive nao is envisaged as being a description of periods in which these episodes are infrequent and can be considered as a basic unblocked situation a negative nao is a description of periods in which episodes occur frequently a similar but weaker relationship exists between wave breaking over the pacific and the west pacific pattern evidence is given to support this hypothesis by using a two dimensional potential vorticity based index to identify wave breaking at various latitudes this is applied to northern hemisphere winter data from the 40 yr ecmwf re analysis era 40 and the events identified are then related to the nao certain dynamical precursors are identified that appear to increase the likelihood of wave breaking these suggest mechanisms by which variability in the tropical pacific and in the stratosphere could affect the nao