strongly negative δ34s values associated with secondary pyrite above and below the j m reef stillwater complex montana abstract previous sulfur isotope studies of the stillwater complex and in particular the j m reef have noted the generally restricted range of 34s values in the range of 0 2 5 e g zientek and ripley 1990 ripley et al 2017 detailed studies of high s zones in both the hanging wall of the reef package which contains the j m reef and the footwall within the reef package but below the mineralized j m reef indicate the presence of significant quantities of secondary pyrite the pyrite rich intervals are characterized by 34s values that strongly differ from those of the j m reef in the range of 0 2 to 17 4 the pyrite rich rocks above and below the j m reef are both serpentinized lherzolites although the j m reef is locally strongly altered to clinozoisite epidote rich assemblages only the sepentinized lherzolites show anomalously low 34s values the highly negative 34s values of the pyrite rich assemblages result from oxidation processes and s isotope fractionation between reduced and oxidized s species one possible process involves the production of a mixed sulfate sulfide fluid with pyrite produced from h2s with low 34s values a second alternative is that infiltration of an oxidized fluid and interaction with sulfide bearing rocks of the complex produced a sulfate rich fluid that then underwent partial reduction to sulfide via reaction with reduced fe bearing minerals strongly negative 34s values would characterize secondary pyrite produced as a result of either process due to the large s isotope fractionation between oxidized and reduced s species at hydrothermal temperatures of 400 c or below the varying degrees of alteration and restricted pyrite rich intervals with highly negative s isotope signatures suggest that fluid infiltration was not pervasive secondary pyrite that was produced as a result of redox driven reactions is preserved only in olivine rich lithologies characterized by a high degree of serpentinization

age of the bird river sill southeastern manitoba canada with implications for the secular variation of layered intrusion hosted stratiform chromite mineralization the archean bird river sill a 20 km long and up to 800 m thick mafic ultramafic layered intrusion in the bird river greenstone belt of southeastern manitoba canada contains significant resources of chromium and nickel copper and locally anomalous concentrations of platinum group elements stratiform chromitite displaying both remarkable lateral continuity and local irregularities due to synmagmatic disruption occurs in up to six main intervals over a thickness of 60 m in the lower ultramafic part of the intrusion ni cu sulfide mineralization is hosted in ultramafic rocks at the base of or just below the sill e g past producing maskwa dumbarton mines in this study we report a u pb age chemical abrasion thermal ionization mass spectrometry for a leucogabbro from the chrome property of 2743 0 0 5 ma n 8 single grains of zircon which is interpreted as the age of crystallization of the sill and defines the timing of the associated cr ni cu platinum group element pge mineralization other neoarchean mafic ultramafic intrusions containing economic concentrations of chromite and or ni cu pges occur in the northwestern part of the superior province in the canadian shield e g mcfaulds lake big trout lake puddy lake shebandowan and share broadly similar geologic relationships the age of intrusion hosted stratiform chromite mineralization worldwide excluding ophiolite hosted chromitites ranges from archean to paleoproterozoic which is comparable to that for komatiite related ni deposits this temporal restriction is consistent with both deposit types requiring the involvement of high mgo cr rich parent magmas produced during large degrees of mantle melting early in earth history

interpretation of porosity and lwd resistivity from the nankai accretionary wedge in light of clay physicochemical properties evidence for erosion and local overpressuring in this study we used porosity to assess the compaction state of the nankai accretionary wedge sediments and any implications for stress and pore pressure however hydrous minerals affect porosity measurements and accounting for them is essential toward defining the interstitial porosity truly representative of the compaction state the water content of sediments was measured in core samples and estimated from logging data using a resistivity model for shale we used the cation exchange capacity to correct the porosity data for the amount of water bound to clay minerals and to correct the porosity estimates for the surface conductivity of hydrous minerals the results indicate that several apparent porosity anomalies are significantly reduced by this correction implying that they are in part artifacts from hydrous minerals the correction also improves the fit of porosity estimated from logging while drilling lwd resistivity data to porosity measured on cores low overall porosities at the toe of the accretionary wedge and in the splay fault area are best explained by erosion and we estimated the quantity of sediments eroded within the splay fault area by comparing porosity effective stress relationships of the sediments to a reference curve additionally a comparison of lwd data with core data resistivity and p wave velocity obtained at site c0001 landward of the mega splay fault area suggested a contribution from the fracture porosity to in situ properties on the formation

magma evolution in the stillwater complex montana ii rare earth element evidence for the formation of the j m reef rare earth element data for 20 cumulus plagioclase feldspars from troctolite anorthosite zone i in the lower banded series and five cumulus and intercumulus plagioclase feldspars from samples that bracket the ultramafic series lower banded series contact of the stillwater complex are consistent with magma mixing as a fundamental process which produced cyclic units in troctolite anorthosite zone i the association of the j m reef with plagioclase rich cumulates in troctolite anorthosite zone i which have a crystallization order different from that observed in the ultramafic series suggests that a geochemically distinct type of magma was added to the chamber this new magma was characterized by a higher al 2 o 3 content ce yb n of 3 7 nd sm n of 1 5 and dy yb n of approximately 1 0 similar to group i gabbronorite dikes in the footwall of the stillwater complex helz 1985 this high al 2 o 3 magma was most likely derived by partial melting of a source of tholeiitic composition in the upper mantle or lower crust the higher al 2 o 3 content of this new magma suggests that it may have been near the sulfur saturation limit for melts of this composition even though the total sulfur concentration may have been low consistent with average sulfur concentrations in the group i gabbronorite dikes rare earth element data for cumulus orthopyroxenes from the ultramafic series presented in the first paper of this two part series lambert and simmons 1987 suggest that magmas parental to cumulates below troctolite anorthosite zone i were derived by multistage dynamic partial melting of a deeper upper mantle source rare earth element and sulfur data for group 2 high mgo gabbronorite dikes also in the footwall of the stillwater complex suggest that these liquid compositions may be similar to magmas parental to the ultramafic series dynamic partial melting of a platinum group element rich source may have permitted enrichment of the platinum group elements in the stillwater magma chamber during the formation of the ultramafic series perturbation of the tholeiitic trend of fractional crystallization observed in the ultramafic series olivine followed by orthopyroxene due to the introduction of an al 2 o 3 rich sulfur saturated magma into platinum group element enriched residual magma remaining in the chamber may have produced the cyclic modal and rare earth element cryptic layering in troctolite anorthosite zone i the identification of feldspars in troctolite anorthosite zone i below the j m reef which have rare earth element affinities with the ultramafic series suggests that progressive mixing of these two liquids was accomplished in a system which preserved the integrity of geochemically different liquid layers double diffusive convective mixing irvine et al 1983 of the two stillwater parental magmas is therefore preferred over mixing via a turbulent plume campbell et al 1983 rare earth element data suggest that complete mixing first occurred with the formation of the sixth olivine bearing member directly below the j m reef in troctolite anorthosite zone i o 5b of todd et al 1982 complete mixing of these two geochemically distinct magmas produced sulfur saturated hybrid magmas parental to the first laterally continuous troctolite in troctolite anorthosite zone i and the platinum group element enriched j m reef

pedogenic carbonate proxies for amount and seasonality of precipitation in paleosols the depth to carbonate nodular bk horizon in soils d in cm is correlated with mean annual precipitation p in mm so that bk horizons are deep in subhumid regions but shallow in semiarid regions previous quantifications of this relationship are unchanged by this new compilation of 807 soils p 137 24 6 45 d 0 013 d 2 where r 2 0 52 and standard error s e 147 mm in most north american postglacial soils the bk horizon is thin and well defined whereas in monsoonal tropical soils of pakistan and kenya the bk horizon is thick and diffuse data from 675 modern soils define the relationship between the thickness of soil with nodules t in cm and mean annual range of precipitation m in mm difference between monthly means of wettest and driest months m 0 79 t 13 71 where r 2 0 58 and s e 22 mm the relationship between carbonate nodule size s in cm and soil age a in ka is quantified by 9 radiocarbon dated soils from las cruces new mexico a 3 92 s 0 34 where r 2 0 57 and s e 1 8 k y these transfer functions are applied to reconstructing paleoclimatic change from paleocene eocene paleosols of the north horn formation and flagstaff limestone of axhandle canyon utah the terminal paleocene spike of warmth recorded by fossil plants in nearby wyoming was coincident with a brief peak in both mean annual precipitation and mean annual range of precipitation in utah

the j m platinum palladium reef of the stillwater complex montana ii origin by double diffusive convective magma mixing and implications for the bushveld complex sequences of cumulates form concurrently by downdip accretion from a column of liquid layers that are primarily separated by diffusive interfaces the low cumulates grow in advance of the upper cumulates in accord with their higher crystallization temperatures an important observation is that mixing of the two parent liquids lowers their crystallization temperatures and consequently should produce unsaturated hybrids that could dissolve earlier cumulates the repetition of cyclic units above the j m reef is explained by having periodic additional injections of dense magma beneath the subsiding column of crystallizing liquid layers neither the j m reef nor the bushveld ore zones should ever have been continuous across the floor of their respective magma chambers and for bushveld the circumstances resulted in zones that are actually minable modified journal abstract

normal polarity magnetosubchrons in 24r and the age of the paleocene eocene boundary a late paleocene to early eocene sequence of flat lying continental strata occurs in an area known as obed mountain west central alberta the upper 110 m consist of interbedded fluvial channel sandstones and overbank mudrocks containing five back swamp coal seams two coreholes 3 5 km apart that extend through the entire coal zone were sampled for magnetostratigraphy and 13c isotope analysis bentonites in the no 1 lowest and no 5 coal seams and a tuff in the no 3 coal seam were sampled for u pb and or rb sr dating of zircon and biotite respectively magnetostratigraphic analysis of 520 samples identified the younger part of chron 25r the whole of chron 25n and the older half of chron 24r we find six normal polarity subzones in this part of chron 24r which we correlate to tiny wiggles 6 to11 in marine magnetic profiles carbon isotope analysis of 14 samples from two cores revealed a negative shift of about 2 peaking near the base of 24r 8r we interpret this as the carbon isotope excursion

the age of the stillwater complex a comparison of u pb zircon and sm nd isochron systematics abstract a zircon u pb age of 2713 3 myr confirms the less precise age of about 2710 myr corrected for new decay constants of jaffey et al 1971 obtained from the same sample of zircon from the stillwater complex chill zone nunes and tilton 1971 this age compares with sm nd mineral and whole rock isochron ages of 2701 8 myr depaolo and wasserburg 1979 or 2706 8 myr if the lugmair et al 1975 normalization procedure is used the agreement of these ages to within about 0 4 indicates that the 147 0 00654 10 9 yr 1 value is less than 0 8 too large relative to the u decay constants determined by jaffey et al 1971 and enables more correct geological syntheses to be made when working out detailed absolute age stratigraphies using precise data from both systems

piggyback basin in the sevier orogenic belt utah implications for development of the thrust wedge timing data on a frontal thrust system indicate that the sevier thrust wedge of central utah advanced rapidly to its ultimate structural front in 30 m y between early albian and campanian time it then persisted by out of sequence thrusting for an additional 25 m y until the early eocene the key to the history of the frontal thrust is a piggyback basin that formed on the hanging wall unconformity bounded sedimentary units within the basin thin both westward and eastward onto structures generated by thrust faulting up to 860 m of upper campanian to lower eocene strata accumulated between these structures which consist of a ramp anticline in the west and a thrust duplex in the east the newly recognized interaction of coeval deposition and thrust deformation permits revised limits on the inception and longevity of the frontal thrust system

weakening of rock salt by water during long term creep the rheological properties of rock salt are of fundamental importance in predicting the long term evolution of salt based radioactive waste repositories and strategic storage caverns and in modelling the formation of salt diapirs and associated oil traps1 2 the short term high stress rheology of rock salt is well known from laboratory experiments however extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures furthermore experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine trace brine is always present in natural salt but sometimes escapes during experiments our tests on dry dilated salt show more or less conventional dislocation creep behaviour but brine bearing samples show marked weakening at low strain rates this is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep because natural rock salt always contains some brine these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long term rheological behaviour of salt in nature

metrology and traceability of u pb isotope dilution geochronology earthtime tracer calibration part i abstract mixed 235u 233u 205pb 202pb tracers for u pb isotope dilution isotope ratio mass spectrometry have been prepared under the auspices of the earthtime initiative the methods and results for the preparation and calibration of the u pb ratio and isotopic abundances are given and the various sources of uncertainty are discussed and quantified the accuracy of the earthtime u pb tracer isotopic composition can be traced back to si units via a series of assay and isotopic composition reference materials combined with the experiments described herein the parameters used in calculating u pb ratios and inferentially u pb dates have correlated uncertainties that result in a total uncertainty contribution to 206pb 238u dates of

world class ni cu pge deposits key factors in their genesis magmatic ni cu sulfide deposits form as the result of segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma and the partitioning of chalcophile elements into these from the silicate melt sulfide saturation of a magma is not enough in itself to produce an ore deposit the appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals and then is concentrated in a restricted locality so that the resulting concentration is of ore grade the deposits of the noril sk region have developed within flat elongate bodies 15 2 0 2 km that intrude argillites evaporites and coal measures adjacent to a major trans crustal fault and immediately below the centre of a 3 5 km thick volcanic basin studies of the overlying basalts have shown that lavas forming a 500 m thick sequence within these have lost 75 of their cu and ni and more than 90 of their pge overlying basalts show a gradual recovery in their chalcophile element concentrations to reach normal values 500 m above the top of the highly depleted zone the ore bearing noril sk type intrusions correlate with those basalts above the depleted zone that contain normal levels of chalcophile elements the high proportion of sulfide 2 10 wt associated with the noril sk type intrusions the high pge content of the ores the extensive metamorphic aureole 100 400 m around the bodies and the heavy sulfur isotopic composition of the ores 8 12 34s are explicable if the ore bearing bodies are exit conduits from high level intrusions along which magma has flowed en route to extrude at surface the first magma to enter these intrusions reacted with much evaporitic sulfur at a low r value and thus gave rise to sulfides with low metal tenors successive flow of magma through the system progressively enriched the sulfides in the conduits losing progressively less of their chalcophile metals and thus accounting for the upward increase in metals in successive lava flows above the highly depleted flows the voisey s bay deposit lies partly within a 30 100 m thick sheet of troctolite interpreted as a feeder for the 1 334 ga voisey s bay intrusion and partly at the base of this intrusion where the feeder adjoins it studies of olivine compositions indicate that an early pulse of magma through the feeder and into the intrusion was ni depleted but that subsequent pulses were much less depleted trace element re os and s and o isotope data and mineralogical studies indicate that the magma pulses interacted with country gneiss probably principally in a deeper level intrusion extracting sio2 na2o k2o and possibly sulfur form the gneiss which accounts for the magma becoming sulfide saturated the jinchuan deposit of north central china occurs within a 6 km long dyke like body of peridotite the compositions of olivine within the dyke the igneous rocks themselves and the ore are all inconsistent with derivation of the body from ultramafic magma as originally supposed and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt magma flow of magma into the intrusion has resulted in olivine and sulfide being retained where the keel was widening out into the intrusion the west australian komatiite related deposits occur in thermal erosional troughs which have developed due to the channelisation of magma flow and the resulting thermal erosion of underlying sediments and basalt by the hot komatiite magma the sediments are sulfide rich and may have contributed substantially to the sulfide of the ores the mineralisation in the duluth complex occurs in troctolitic intrusions along the western margin of the complex as a result of magma interacting with and extracting sulfur from the underlying graphite and sulfide bearing sediments no magma flow channels have been identified so far and the lack of magma flow subsequent to the development of sulfide immiscibility is regarded as the reason why these deposits are not of economic grade when most major ni cu sulfide deposits are compared they prove to have a number of features in common olivine rich magma proximity to a major crustal fault sulfide bearing country rocks chalcophile element depletion in related intrusive or extrusive rocks field and or geochemical evidence of interaction between the magma and the country rocks and the presence of or proximity to a magma conduit the features are thought to explain the three key requirements sulfide immiscibilty adequate mixing between sulfides and magma and localisation of the sulfides discussed and have important implications with respect to exploration

depositional and stratigraphic architecture of the santonian emery sandstone of the mancos shale implications for late cretaceous evolution of the western interior foreland basin of central utah u s a abstract the santonian emery sandstone member of the mancos shale is a continental to shallow marine succession that prograded into the cretaceous western interior seaway of western north america sediment sourced from the uplifted sevier orogen to the west fed a shoreline in the adjacent subsiding north south trending foredeep these sandstones are now superbly exposed along a 120 km long strike parallel continuous outcrop in the cliffs of the wasatch plateau of central utah shoreface parasequences are composed of coarsening upward successions of hummocky cross stratified and intensely bioturbated sandstones indicating deposition at a storm dominated shoreline two distinct depositional complexes record eastward and northeastward shoreline progradation at the northern and southernmost extremities of the plateau respectively in the subsurface emery strata comprise a thick succession of coal bearing coastal plain deposits the emery sandstone records a period of long term paralic progradation to aggradation containing at least 17 parasequences arranged into three 600 kyr regressive transgressive cycles that are recognizable in both complexes each cycle comprises only highstand and transgressive systems tracts and erosional sequence boundaries are absent absence of erosional sequence boundaries is attributed to high rates of subsidence in the foreland basin at this time santonian glacio eustatic sea level fluctuations are proposed to account for the high frequency cyclicity whilst longer term aggradation may be a function of organic controls on shoreface stacking patterns by immobile raised coal mires lying landward of the shorefaces when placed in a temporal and spatial context the emery sandstone provides a missing link in the understanding of long term controls on late cretaceous depositional systems in the central utah segment of the foreland basin and allows inferences to be made regarding foreland basin dynamics in particular thrust tectonics is postulated to have played a major role in governing the architecture of late cretaceous strata in central utah marked along strike thickening of the emery sandstone in particular is attributed to long lived elevated rates of subsidence in northern utah and southern wyoming during the late cretaceous that is adjacent to concentrated zones of thrust nappes a north dipping depositional slope is inferred from this subsidence distribution aligned approximately parallel to the structural grain of the hinterland load northeastward progradational directions of successive turonian santonian and early campanian regressive wedges in this part of the western interior basin are attributed to the presence of this tectonically induced regional slope gradient sediment sourced to these late cretaceous shorefaces is likely to have been sourced through structural recesses within the thrust front resulting in a spatial alignment of several successive regressive wedges and implicates an important long lived kinematic control on sediment supply of late cretaceous regressive wedges only the emery sandstone does not temporally coincide with reported periods of active thrusting suggesting that the regression was a consequence of factors other than uplift related elevated rates of sediment supply

late cretaceous paleocene synorogenic sedimentation and kinematic history of the sevier thrust belt northeast utah and southwest wyoming integration of cross cutting structural relationships overlapping sedimentary units new conglomerate provenance data and radiometric and palynological dates provides a basis for reinterpretation of the distribution and timing of late cretaceous through paleocene thrust faulting in the northeast utah southwest wyoming part of the sevier thrust belt these data indicate a general eastward progression of deformation that was punctuated by local out of sequence and hinterlandward verging events provenance data delimit a sequential restoration of a regional cross section the principal thrust systems in northeast utah and southwest wyoming are the willard ogden crawford absaroka and hogsback thrusts the willard is the westernmost structurally highest and oldest of the thrusts it carries a unique section of thick proterozoic sedimentary rocks as well as paleozoic rocks and was folded during displacement on younger thrust systems the next youngest thrust system is the ogden which comprises several basement rooted imbricate thrusts that together form a large antiformal stack with a structural culmination in the wasatch range growth of the wasatch culmination took place during coniacian through paleocene time contemporaneous with sequential displacement on the frontal crawford absaroka and hogsback thrusts total shortening in this part of the thrust belt was 100 km and structural relief of 25 km developed in the area of the wasatch culmination during coniacian through paleocene time thrusting followed an overall eastward progression that was interrupted by local out of sequence and hinterlandward verging events several episodes of synchronous displacement on two or more thrusts can be demonstrated shortening occurred in three main episodes the first episode 89 84 ma involved 33 km of shortening on the crawford thrust and its footwall imbricates approximately 19 km of structural relief on the basement cover contact developed in the area of the wasatch culmination the second episode 84 62 ma with a break between 75 69 ma involved 30 km of shortening mainly on the absaroka thrust and development of an additional 6 km of structural relief in the culmination the third episode of shortening 56 50 ma took place on the hogsback thrust involved 21 km of horizontal shortening and produced no significant increase in structural relief in the culmination long term rates of shortening ranged between 3 0 mm yr and 6 6 mm yr these three episodes of shortening produced three large accumulations of synorogenic conglomerate totaling 3 km in thickness the henefer formation and echo canyon and weber canyon conglomerates were deposited during crawford thrusting the evanston formation was deposited during and after absaroka thrusting and the lower conglomeratic part of the wasatch formation was deposited during and after hogsback thrusting most of the sediment in these synorogenic units however was derived from repeated uplift of the willard thrust sheet and from the eastern flank of the wasatch culmination in the rear part of the thrust belt only local minor accumulations were derived from the frontal ramp anticlines sediment accumulation and structural deformation were generally out of phase periods of regional shortening and uplift were marked by development of unconformities and sediment bypassing to distal parts of the foreland basin periods of structural inactivity were marked by accumulation of aerially widespread braided river conglomerate on top of the thrust belt one exception to this pattern is the henefer echo canyon weber canyon conglomerate deposit which contains evidence of progressive deformation in close proximity to the tip of the crawford thrust comparison of the sequential restoration with the late cretaceous subsidence history and isopach patterns in the distal foreland basin of western wyoming demonstrates that the principal controls on regional subsidence and sediment supply were the growth and erosion of the wasatch culmination growth of the duplex beneath the culmination may have been a means of maintaining critical taper in the thrust wedge

modeling the influence of sinking anhydrite blocks on salt diapirs targeted for hazardous waste disposal due to the low permeability and high ductility of rock salt many salt diapirs such as those in germany and the netherlands are targeted as long term repositories for disposal of high level radioactive and chemical wastes geophysical and subsurface data show that the gorleben salt diapir which is one of the most extensively investigated diapirs in the world and other salt diapirs of the zechstein formation in germany contain large blocks 80 m thick of high density anhydrite inclusions these blocks which were carried upward by the rising salt are considered to be detached segments of intercalated layers that initially were deposited with the salt results of physical and numerical models presented here show that such detached high density blocks which were entrained and carried upward by the diapir at an earlier stage tend to sink in the late stages of diapiric evolution when the rate of diapiric rise slows down during their descent these high density competent blocks deform by folding and create shear zones at the immediate contact with the less competent salt the descending blocks initiate a secondary internal flow within the salt diapirs they descend and they may deform any repository built within such diapirs which would otherwise be considered as tectonically inactive

watershed reconstruction of a paleocene eocene lake basin using sr isotopes in carbonate rocks provenance studies have used sr isotopes 87 sr 86 sr of silicate source rocks as a link to their eroded basinal equivalents this technique however cannot identify the proportional inputs from different watersheds or define more precisely sedimentation events due to tectonic or climatic change erosion of carbonate rocks dominates the sr input within basin drainage and potentially can be used through 87 sr 86 sr ratios to reconstruct paleohydrology of the entire basin and trace watershed inputs and depositional patterns in continental basins the sr isotopic ratios from waters of the source area allowing for the mixing of shallow groundwater and surface water along the transport path are homogenized in the basinal carbonate sediments mineralogy and diagenesis of carbonate rocks generally do not affect the sr isotopic signal in a near surface system lacking external influence by volcanism eolian dust or deep geothermal waters the 87 sr 86 sr ratios from the source area are directly comparable to those in the receiving continental basin the sr isotopic signal of the paleocene eocene flagstaff formation central utah a carbonate lake deposit in a foreland basin is compared to that of projected source waters draining its thrust front the sevier fold thrust belt freshwater carbonates compose a large portion of the lowermost ferron mountain and uppermost musinia peak members of the formation whereas gypsum and carbonates predominate in the middle cove mountain member previous research had attributed gypsum deposition to the deposition of the middle cove mountain member to either climatic change or unroofing of diapirs of jurassic gypsiferous carbonates to examine more closely the influence of climate versus tectonics on flagstaff sedimentation as well as the efficacy of provenance studies using carbonates we collected rock samples from the three members of the formation on the wasatch plateau of central utah in addition to sampling stream water associated with pennsylvanian permian and jurassic carbonate terrains from the nearby thrust front the 87 sr 86 sr ratios in carbonates and gyprock belonging to the flagstaff formation remained unchanged during deposition the average sr isotope composition of the flagstaff rocks being identical to that of sampled waters draining the projected provenance area there was little change in source rock weathering as the thrust front evolved deposition of gypsum occurred in the basinal lake only during the deposition of the middle cove mountain member despite its constant exposure in the drainage area suggesting a changing balance of tectonic and climatic controls during lake sedimentation the 87 sr 86 sr isotopic studies targeting carbonate rocks and their presumed source waters are a simple but accurate method for reconstructing the paleohydrology of lake basins

numerical analysis of how sedimentation and redistribution of surficial sediments affects salt diapirism abstract two dimensional finite element models are used to study how sedimentation and redistribution of sediments on the upper surface affects the development of subsurface salt diapirs a rising diapir creates a bulge flanked by topographic lows in a generally accumulating sedimentary pile we find that the rate at which this topography is flattened by erosion and redeposition controls the style of diapirism this is because the redistribution of material from topographic highs to flanking lows is equivalent to changing the effective forces acting on the salt redistributing a potential topography modulates diapiric growth rate the main effects of including surficial sediment redistribution in numerical models of diapirism are 1 1 diapirs grow 10 100 times faster 2 2 diapirs may rise above their level of neutral buoyancy and extrude 3 3 diapirs assume finger or stock like shapes rather than mushroom or balloon on string shapes 4 4 layers in the surrounding sediments remain nearly horizontal and only steepen sharply near the diapir in effect the rate of redistribution of surficial overburden strongly controls the mode of diapirism sediment redistribution referred to as erosion for brevity is modeled using a one dimensional diffusion equation we show the results of two different erosion rates infinitely slow no erosion and extremely fast which redistributes surficial sediments but does not remove them from the system we show that the shapes of model diapirs rising beneath surfaces subjected to rapid erosion simulate salt diapirs in the gulf of mexico columnar diapirs indicate rapid deposition on the shelf and plug like diapirs slow sedimentation on the abyssal plane diapirs rising beneath surfaces with negligible erosion have the mushroom shapes interpreted for salt diapirs in central iran

structural evolution of connecting splay duplexes and their implications for critical taper an example based on geometry and kinematics of the canyon range culmination sevier belt central utah abstract critical taper in an evolving orogenic wedge can be maintained by structural thickening in the back of the wedge this is often achieved by folding and uplift of pre existing large internal thrust sheets by the continued growth and or reactivation of duplexes in the hinterland the growth of a blind imbricate complex off a lower thrust provides another mechanism for folding a higher thrust individual splays in a blind imbricate complex may have fault propagation folds and or fault bend folds associated with them if the splays reach the overlying thrust they serve as connecting splays between the lower thrust floor and the upper thrust roof thereby forming a connecting splay duplex csd this type of duplex geometry requires periodic reactivation of the pre existing roof thrust the canyon range thrust in central utah is an internal thrust that is folded into an overturned antiform synform pair and overrides its own synorogenic conglomerates the same conglomerates are infolded into the core of the synform and preserve critical timing information regarding the history of folding and thrust reactivation the folding is caused by the development of a footwall antiformal stack in the core of the canyon range antiformal culmination the geometry and timing relationships suggest that the structure grew as a csd regional timing relationships suggest that the growth of the csd and reactivation of the thrust took place during the later part of the evolution of the orogenic wedge and may have helped the wedge maintain critical taper during thrusting in the external fold and thrust belt

rapid high t decompression recorded by archean granulites in the northern wyoming province insights from petrological modelling this study places new constraints on the pressure temperature p t path and duration of high temperature ht metamorphism recorded by archean granulite facies metasedimentary rocks from the northern wyoming province in the eastern beartooth mountains mt and wy usa these rocks exist as m to km scale xenoliths within a c 2 8 ga calc alkaline granitoid batholith different interpretations of the timing of ht metamorphism relative to batholith intrusion in previous works has led to ambiguity over the mechanism by which these rocks were heated i e batholith intrusion vs a later cryptic event the p t path recorded by these rocks and the duration of this path may be indicative of the heating mechanism but are not currently well constrained here we combine phase equilibria thermobarometry and diffusion modelling of major element zonation in garnet in order to constrain the p t path of ht metamorphism and the durations of different parts of this path it is shown that these rocks record a tight clockwise p t path characterized by near isobaric heating at 6 5 7 kbar to 770 800 c ht decompression to 6 kbar 780 800 c followed by limited decompression while cooling diffusion modelling of major element zonation in garnet suggests that ht decompression was brief likely 1 ma and that cooling rates following this decompression were on the order of 10 100 c ma substantial changes in apparent thermal gradient along this p t path indicate that the rocks record a significant but short lived thermal anomaly that occurred in the wyoming mid crust in the late archean this article is protected by copyright all rights reserved

rheology of rocksalt abstract we review progress in experimental determinations of transient and steady state flow properties and processes of natural rocksalt aggregates focusing primarily on results from avery island louisiana domal salt the steady state flow field established from constant stress and constant strain rate tests at temperatures from 50 to 200 c strain rates from 10 5 to 10 9 s 1 and differential stresses from 20 7 to 2 5 mpa has been separated into two flow regimes each fit by a power law relation at the higher stresses and strain rates this relation is e 1 6 10 4 exp 68 1 rt 10 3 5 3 for which the pre exponential constant is expressed in mpa 5 3 s 1 and the apparent activation energy is in j mol 1 creep rates predicted by equation a do not differ appreciably from those predicted previously the relatively low stress low strain rate data are very well fit by e 8 1 10 5 exp 51 6 rt 10 3 2 4 which is comparable conditions predicts creep rates higher and equivalent viscosities lower than does equation a by two orders of magnitude the change in behavior from a to b is ascribed to a change in rate limiting mechanism from cross slip of screw dislocations to the climb of edge dislocations in this dry material subgrain formation dominates microstructural development during steady state flow of salt and from determinations of average subgrain diameters in crystals from 20 different rocksalt bodies flow stress levels between 0 6 and 1 4 mpa have been estimated these values do not bear directly on arguments concerning the nature of forces initiating salt pillow growth because apparently evidence relating to the early deformational history has been overprinted at that stage fluid assisted grain boundary diffusional processes might dominate dislocation creep leading to a linear stress strain rate mechanical response considering buoyancy forces alone behavior described by equation b rather than a reduces the relief necessary to initiate pillow growth but a 1000 fold amplification is required to produce stress differences near 1 mpa that forces other than buoyancy are important is indicated by the occurrence of these same paleostress levels in bedded salts and in shallow offshore concordant intrusions differential loading generally provides the most plausible initial driving force for the growth of diapiric salt structures

u pb and sm nd ages for the stillwater complex and its associated sills and dikes beartooth mountains montana identification of a parent magma seven analyses from two samples of the lower banded series of the stillwater complex montana yielded a u pb zircon baddeleyite age of 2 705 plus minus 4 ma a more precise age of 2 704 plus minus 1 ma is obtained if the authors regress only five of seven analyses in either case the age is in very good agreement with the previously reported sm nd mineral isochron age of 2 701 plus minus 8 ma for a gabbro from the lower banded series noncumulate mafic sills and dikes that immediately underlie the stillwater complex are separated into five compositional groups u pb zircon ages for a group 1 gabbronorite 2 711 plus minus 1 ma and a group 3 mafic norite 2 703 plus minus 10 ma are comparable to previously reported u pb zircon ages for a group 1 diabase 2 713 plus minus 3 ma and a group 4 high ti norite 2 712 plus minus 3 ma although group 2 high mg gabbronorite and group 6 olivine gabbro have not yet yielded significant amounts of zircon baddeleyite or sphene mineral separates from group 2 yielded a four point pb pb internal isochron age of 2 704 plus minus 25 ma and separates from group 6 yielded a sm nd internal isochron age of 2 706 plus minus 64 ma indicating that these groupsmore are at least contemporaneous with intrusion of the stillwater complex these results indicate that groups 1 and 4 of the associated sills and dikes are perhaps slightly older than the complex and that groups 2 3 and 6 may be coeval with it supporting the idea that these three groups are the best candidates for a stillwater parent magma a conclusion previously presented on the basis of detailed mineralogic and geochemical evidence less

the role of the sediment load in sequence stratigraphy the influence of flexural isostasy and compaction simplified physically based models are utilized in order to examine the effects of sea level sedimentation tectonic subsidence isostatic compensation and compaction on development of unconformities unconformities divide volumes of relatively conformable sediments defining the time and spatial extent of sedimentary sequences the model parameters are varied both individually and in concert in order to isolate their contributions to sequence architecture these models reveal the importance of sedimentation and subsidence rates as well as sea level amplitude and rate in determining the type and extent of sequence boundary formed sedimentation and subsidence rate can vary significantly within and between margins producing different sequence boundaries given an identical eustatic sea level fall it is shown that sea level amplitude plays an important role in determining which systems tracts are present and sequence boundary timing for type 1 sequences and that sea level rate controls the same attributes in type 2 sequences isostatic compensation and compaction which have been described as secondary effects are shown to have considerable influence on sequence architecture these two processes due to isostatic response to sediment loading and sediment self loading initiate feedback by enhancing and partitioning accommodation space these additional effects invalidate application of the equilibrium point as a guide to where and how all accommodation space is created the manner in which flexure distributes accommodation space is a function of lithospheric rigidities higher rigidities partition space laterally producing wide shelves which favor type 1 sequence boundaries whereas lower rigidities partition space vertically forming narrow shelves which favor type 2 sequence boundaries compaction generates a net landward shift of the shelf edge and favors type 2 sequence development both isostatic compensation and compaction introduce time delays in sequence development one consequence of compaction is to rotate sedimentary packages generating growth and leakage of anticlinal structures as fluids are driven from the underlying section this process has important implications for hydrocarbon migration in compaction dominated systems such as the gulf of mexico margin

greenhouse crises of the past 300 million years proxies of past co 2 and climate over the past 300 m y now reveal multiple global climate change events in unprecedented detail evidence for past co 2 spikes comes from expanded and refined data on stomatal index of fossil ginkgo and related leaves new evidence for synchronous climatic change comes from paleosols in montana utah and neighboring states each co 2 spike was coeval with unusually clayey red and decalcified paleosols which can be traced throughout the colorado plateau spikes in atmospheric co 2 also were coeval with increases in paleosol alkali depletion as an indication of high temperature and spikes in paleosol base depletion and depth to calcic horizons as indications of high precipitation in the colorado plateau times of warmer climate were also more humid perhaps due to greater moisture potential of warmer air seasonality of climate did not increase during warm wet spikes the mesozoic greenhouse was not persistently hot with cool spells but warm with hot flashes these data furnish power laws predicting the sensitivity and magnitude of change in mean annual temperature mat and mean annual precipitation map due to rising co 2 in a mid latitude mid continental region the magnitude of the coming anthropogenic greenhouse pales in comparison with past greenhouse spikes at times of global mass extinctions

geochronology of layered mafic intrusions from the pan xi area in the emeishan large igneous province sw china the panzhihua xichang pan xi area hosts mafic ultramafic intrusions which are part of the permian emeishan large igneous province some of these intrusions host giant fe ti v deposits and minor ni cu pge mineralization in the present study zircon u pb ages of 259 3 1 3 and 260 7 0 8 ma have been obtained from the giant fe ti v ore bearing hongge and the unmineralized binggu intrusions respectively by isotope dilution thermal ionization mass spectrometry method in combination with the ages of other ore bearing intrusions this age shows that these mafic ultramafic intrusions were emplaced at ca 260 ma the hongge and binggu intrusions cut the lower part of the rapidly deposited emeishan flood basalt sequence but no further into the upper volcanic sequence in the pan xi area thus emplacement and mineralization of the mafic ultramafic intrusions were almost contemporaneous with the eruption of the emeishan flood basalts during a relatively short time span

modeling the merensky reef bushveld complex republic of south africa the merensky pegmatoid normal reef in the western bushveld complex is commonly characterized as a pyroxene rich pegmatoidal unit with a base that is enriched in chromite and platinum group element bearing sulfides overlying a leuconorite footwall models for its formation have ranged from those that view it as entirely a magmatic cumulate succession to those that have suggested that it is a zone of volatile induced remelting the consequences of the latter interpretation are investigated using the numerical modeling program iridium which links diffusive and advective mass and heat transport with a phase equilibration routine based on the melts program the initial system consists of a simple stratigraphic succession of a partially molten leuconorite overlain by a partially molten pyroxenite both initially at 1 190 c and 2 kbar 2 wt of a volatile fluid composed of 75 mol h2o 20 mol co2 and 5 mol h2s is then added to the lower 20 cm of the pyroxenite the system is then allowed to evolve under conditions of chemical diffusion in the liquid the addition of the volatile components results in a modest increase in the amount of melt in the pyroxenite however chemical diffusion across the leuconorite pyroxenite boundary leads to more extensive melting at and below the boundary with preferential loss of opx from the underlying leuconorite preferential re precipitation of sulfide and chromite and concentration of the pge at this boundary these results mimic actual mineral and compositional profiles across the merensky pegmatoid and illustrate that long term diffusion process can effectively produce mineralogical and compositional layering not present in the original assemblage

quantitative evaluation of color measurements ii analysis of munsell color values from the colton and green river formations eocene central utah abstract a case study of munsell color measurements from the eocene colton and green river formations in central utah shows how easily stereoscopic triaxial scatterplots can display the limited overlap of colors within lacustrine and alluvial facies grays drabbish yellow greens greens and 10 yrs are associated with lacustrine limestones and other known lacustrine facies whereas other yellow reds and reds are predominantly associated with terrestrial deposits including fluvial sandstones lake beds in colton formation show a greater variety of color than those in the green river formation both in terms of a greater range of hues and more extreme lightness and darknesses apparently reflecting the fact that the green river formation represents a single large and relatively uniform lake whereas the many small lakes in the colton formation represent a much greater range of lakes types and more diverse lake and shoreline facies mottles in lacustrine mudrocks and limestones seem to represent biomechanical mixing of adjacent beds whereas mottles in floodplain mudrocks show uniquely vivid colors and apparently reflect in situ oxidation reduction in addition to bioturbation it was also fruitful to subject the three color parameters to individual markov chain analyses

high precision u pb zircon baddeleyite dating of the j m reef platinum group element deposit in the stillwater complex montana usa the stillwater complex montana usa is one of the world s largest layered intrusions and hosts the johns manville j m reef a world class platinum group element pge deposit in this study zircon and baddeleyite were successfully extracted from three mineralized samples of the j m reef the samples which were collected over 15 km of strike length of the deposit provide high precision crystallization ages for the reef and the stillwater complex all samples are coarse grained to pegmatitic and range in composition from olivine bearing gabbronorite to anorthosite and troctolite irregular anhedral morphology and prominent sector zoning characterize interstitial zircon u pb crystallization zircon ages determined by chemical abrasion thermal ionization mass spectrometry ca tims for the three samples are indistinguishable within uncertainty frog pond adit 2709 11 0 56 ma east boulder mine 2709 28 0 32 ma and west fork area 2709 00 0 45 ma a u pb baddeleyite date from the frog pond adit sample 2708 85 0 46 ma is identical within uncertainty with the zircon dates collectively the results illustrate that the j m reef is an intrusion wide time marker that formed at ca 2709 ma during crystallization of the lower banded series of the stillwater complex the recognition that zircon can be successfully extracted from mafic ultramafic rocks associated with magmatic ore deposits provides new opportunities for testing the timing of mineralization and duration of mineralization processes in layered intrusions worldwide

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

the stillwater complex integrating zircon geochronological and geochemical constraints on the age emplacement history and crystallization of a large open system layered intrusion a reply to the comment by rais latypov on wall et al j petrology 59 153 190 2018

a temporal link between the emeishan large igneous province sw china and the end guadalupian mass extinction abstract previous studies have suggested that there were two mass extinction events in the late permian one that occurred at the permo triassic p t boundary 251 ma and a second smaller mass extinction that occurred 5 8 myr earlier at the end of the guadalupian many workers have argued that there is a causal relationship between large scale volcanic activity and mass extinctions the major mass extinction event at the p t boundary coincides with the outpouring of huge quantities of lava that formed the siberian flood basalt province in russia courtillot et al earth planet sci lett 166 1999 177 195 and wignall earth sci rev 53 2001 1 33 suggested that the earlier late permian mass extinction coincided with the eruption of the lavas that formed the emeishan flood basalt efb province in sw china however the age of eruption of the efb lavas is poorly constrained using the sensitive high resolution ion microprobe to analyze zircons we have established the age of the xinjie intrusion believed to be a feeder to the main phase of efb volcanism to be 259 3 ma hence the formation of the efb is coincident with a proposed extinction event at 256 259 ma this result supports a temporal link between the emeishan large igneous province and the end guadalupian mass extinction

baddeleyite zro2 and zircon zrsio4 from anorthositic rocks of the laramie anorthosite complex wyoming petrologic consequences and u pb ages

a three dimensional view of mineralization in the stillwater j m reef drilling by the stillwater mining company in the minneapolis adit area of the stillwater complex has afforded a three dimensional view of platinum group element mineralization in the j m reef stratigraphically mineralization occurs at four levels zones relative to the base of olivine bearing zone i 1 footwall zone below olivine bearing zone i 2 basal zone straddling the basal olivine bearing zone i contact 3 main zone within olivine bearing zone i and 4 upper zone within olivine bearing zone i but above the main zone usually only one zone is present but up to three stacked zones do occur in lateral dimension i e in the plane of the layering all four zones are characterized by elongate or arcuate patterns of mineralization the mineralized areas are irregularly spaced at approximately 60 to 75 m the basal olivine bearing zone i contact shows depressions and highs of similar dimension but the position of mineralization does not correlate with the topography of the contact we propose that olivine bearing zone i formed as a result of injection of a buoyant plume of a secondary melt magma ii which was hotter and less dense than the resident gabbroic melt magma i cooling of magma ii increased its density causing it to fall to the floor as inverse diapirs this sulfide saturated magma ii scavenged platinum group elements from magma i during descent upon reaching the floor the diapirs coalesced and pooled as a hot layer with cooling convection within the layer eventually became laminar and ordered convection cells formed resorption of bronzite and crystallization of olivine in the hot upwelling centers of these cells caused local depressions in the basal olivine bearing zone i contact sulfides accumulated along the cooler downfalling peripheries yielding the crude honeycomb pattern observed periodic turbulence with subsequent calming caused shifting of the convection cells and formation of the discrete stratigraphic levels of mineralization

isotope hydrology of early paleogene lake flagstaff central utah implications for cordilleran evolution lacustrine sediments from paleogene foreland basins of the north american cordillera serve as powerful integrators of information on climatic and tectonic conditions but episodes of hydrologic connection between basins may confound straightforward interpretations we use stable isotope ratios of carbon and oxygen in authigenic carbonates of the flagstaff formation in central utah to investigate the history of the lake system which produced these sediments flagstaff limestone and dolostone exhibit isotopic compositions consistent with having formed from the same fluid at surface conditions indicating high evaporative concentration was unnecessary to drive dolomite precipitation isotopic data are consistent with the flagstaff formation having been produced by a large lake which occupied its basin from the late paleocene into the early eocene lake flagstaff was hydrologically distinct from lake uinta to the north but may have been connected to lake claron in the south paleoaltimetry estimates suggest that the mean elevation of lake flagstaff9s catchment in the sevier highlands did not exceed 1 5 km the growing evidence from stable isotope data that cordilleran elevations were relatively low in the early paleogene is in stark contrast to structural and sedimentological evidence for a high nevadaplano and a topographically rugged sevier thrust belt

the influence of an embedded viscoelastic plastic layer on kinematics and mass transport pattern within accretionary wedges abstract the major aim of this study was to examine the influence of an embedded viscoelastic plastic layer at different viscosity values on accretionary wedges at subduction zones to quantify the effects of the layer viscosity we analysed the wedge geometry accretion mode thrust systems and mass transport pattern therefore we developed a numerical 2d sandbox model utilising the discrete element method starting with a simple pure mohr coulomb sequence we added an embedded viscoelastic plastic layer within the brittle undeformed sediment package this layer followed burger s rheology which simulates the creep behaviour of natural rocks such as evaporites this layer got thrusted and folded during the subduction process the testing of different bulk viscosity values from 1 1013 to 1 1014 pa s revealed a certain range where an active detachment evolved within the viscoelastic plastic layer that decoupled the over and the underlying brittle strata this mid level detachment caused the evolution of a frontally accreted wedge above it and a long underthrusted and subsequently basally accreted sequence beneath it both sequences were characterised by specific mass transport patterns depending on the used viscosity value with decreasing bulk viscosities thrust systems above this weak mid level detachment became increasingly symmetrical and the particle uplift was reduced as would be expected for a salt controlled forearc in nature simultaneously antiformal stacking was favoured over hinterland dipping in the lower brittle layer and overturning of the uplifted material increased hence we validated that the viscosity of an embedded detachment strongly influences the whole wedge mechanics both the respective lower slope and the upper slope duplex shown by e g the mass transport pattern

new isotopic evidence bearing on bonanza au ag epithermal ore forming processes new cu s and pb isotope data provide evidence for a magmatic source of metal loid s and sulfur in epithermal au ag deposits even though their ore forming solutions are composed primarily of heated meteoric ground waters the apparent isotopic discrepancy between ore metals and ore forming solutions and even between the ore and associated gangue minerals indicates two different sources of epithermal ore forming constituents 1 a shallow geothermal system that not only provides the bulk of water for the ore forming solutions but also major chemical constituents leached from host rocks silica aluminum potassium sodium calcium to make gangue minerals and 2 metals and metalloids as te sb etc and sulfur se derived from deeper magma bodies isotopic data are consistent with either vapor phase transport of metal loids and sulfur and their subsequent absorption by shallow geothermal waters or formation of metallic au ag cu phases nanoparticles at depth from magmatic fluids prior to encountering the geothermal system the latter is most consistent with ore textures that indicate physical transport and aggregation of nanoparticles were significant ore forming processes the recognition that epithermal au ag ores form in tectonic settings that produce magmas capable of releasing metal rich fluids necessary to form these deposits can refine exploration strategies that previously often have focused on locating fossil geothermal systems

precise age of the platiniferous merensky reef bushveld complex south africa by the u pb zircon chemical abrasion id tims technique the bushveld complex south africa is the world s largest layered intrusion and host to the majority of the known resources of platinum group elements chromium and vanadium despite intensive study determining the precise crystallization age of the bushveld complex has proven to be difficult in this study zircon and rutile were identified in thin section and separated from a sample of pegmatitic feldspathic orthopyroxenite from the pge rich merensky reef west mine townlands shaft rustenburg section low u 21 105 ppm zircon occurs with interstitial biotite and is locally in direct contact with sulfide rutile occurs mainly as inclusions within or overgrowths on chromite especially in the chromitite stringers that bound the reef and also as acicular grains with biotite a u pb crystallization age of 2054 4 1 3 ma was determined from single crystal chemical abrasion id tims u pb dating of zircon and multigrain fractions of rutile yielded a u pb cooling age of 2055 0 3 9 ma integration of the merensky reef crystallization age with existing u pb age determinations for intrusions that pre and postdate emplacement and crystallization of the layered rocks of the bushveld complex indicate that magmatism occurred at ca 2054 ma and is contemporaneous with related magmatic activity across the northern kaapvaal craton of south africa and botswana

geochemical model of δ18o of pedogenic calcite versus latitude and its application to cretaceous palaeoclimate abstract in the absence of a systematic global data base on the 18 o composition of modern soil calcite predicted isotopic values are calculated as a function of latitude between 60 and 20 using 18 o of summer and winter meteoric water ambient soil temperature equal to either measured values in asia or mean annual temperature and 0 10 20 and 30 evaporation of soil water the results show overall higher isotopic values for summer versus winter precipitation but steeper positive slopes for the winter precipitation curves where soil temperature is equated to mean annual temperature the summer precipitation curves are nearly flat at high latitudes and have a negative slope at low latitudes in all cases the 18 o values increase with increasing percent evaporation of soil water oxygen isotopic data for early aptian and late maastrichtian cretaceous palaeosol calcite as a function of palaeolatitude of the western interior of north america plot in the modern winter precipitation field which along with vertic features may suggest a winter wet palaeoclimate steeper positive slopes for the cretaceous curves compared to the modern curves may be attributable to greater latitudinal gradients in mean annual temperature and soil temperature in the cretaceous compared to today and or to a greater degree of evaporation in cretaceous soils at low latitudes compared to the modern earth distinguishing between these possibilities will require a more complete understanding of the variables that affect oxygen isotopes in pedogenic calcite as well as better cretaceous terrestrial palaeoclimate proxies with which to compare the isotopic data

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

cenozoic paleoclimate on land in north america abstract paleotemperature and paleoprecipitation over the past 40 m yr can be inferred from the degree of chemical weathering and depth of carbonate nodules in paleosols of oregon montana and nebraska paleosol records show that late eocene 35 ma middle miocene 16 ma late miocene 7 ma and early pliocene 4 ma warm climatic episodes were also times of a wet climate in oregon montana and nebraska oregon and nebraska were humid during warm wet times but montana was no wetter than subhumid within the rain shadow of intermontane basins global warm wet paleoclimatic spikes steepened rather than flattened geographic gradients of rocky mountain rain shadows long lived mountain barriers created dusty dry basins with sedimentation rates high enough to preserve milankovitch scale 100 41 kyr global paleoclimatic variation in some sequences of paleosols greenhouse warm wet climates indicated by paleosols were also peaks of diversity for north american plants and animals and coincided with advance

numerical modelling of rise and fall of a dense layer in salt diapirs this thesis uses results of systematic numerical models to argue that externally inactive salt structures which are potential targets for radioactive waste disposal might be internally active due to the presence of dense layers or blocks within a salt layer the three papers that support this thesis use the gorleben salt diapir nw germany which was targeted as a future final repository for high grade radioactive waste as a general guideline the first two papers present systematic studies of the parameters that control the development of a salt diapir and how it entrains a dense anhydrite layer results from these numerical models show that the entrainment of a dense anhydrite layer within a salt diapir depends on four parameters sedimentation rate viscosity of salt perturbation width and the stratigraphic location of the dense layer the combined effect of these four parameters which has a direct impact on the rate of salt supply volume area of the salt that is supplied to the diapir with time shape a diapir and the mode of entrainment salt diapirs down built with sedimentary units of high viscosity can potentially grow with an embedded anhydrite layer and deplete their source layer salt supply ceases however when salt supply decreases dramatically or ceases entirely the entrained anhydrite layer segments start to sink within the diapir in inactive diapirs sinking of the entrained anhydrite layer is inevitable and strongly depends on the rheology of the salt which is in direct contact with the anhydrite layer during the post depositional stage if the effective viscosity of salt falls below the threshold value of around 1018 1019 pa s the mobility of anhydrite blocks might influence any repository within the diapir however the internal deformation of the salt diapir by the descending blocks decreases with increase in effective viscosity of salt the results presented in this thesis suggest that it is highly likely that salt structures where dense and viscous layer blocks are present undergo an internal deformation processes when these dense blocks start sinking within the diapir depending on size and orientation of these blocks deformation pattern is significantly different within the diapir furthermore model results applied to the gorleben diapir show that the rate of descent of the entrained anhydrite blocks differs on different sides of the diapir this suggests that if the anhydrite blocks descent within the gorleben diapir they initiate an asymmetric internal flow within it

petrochronology of zircon and baddeleyite in igneous rocks reconstructing magmatic processes at high temporal resolution zircon zrsio4 and baddeleyite zro2 are common accessory minerals in igneous rocks of felsic to mafic composition both minerals host trace elements substituting for zr among them hf th u y rees and many more the excellent chemical and physical resistivity of zircon makes this mineral a perfect archive of chemical and temporal information to trace geological processes in the past utilizing the outstanding power and temporal resolution of the u pb decay schemes baddeleyite is a chemically and physically much more fragile mineral it preserves similar information only where it is shielded from dissolution and physical fragmentation as an inclusion in other minerals or in a fine grained or non reactive rock matrix it offers the potential for dating the solidification of mafic rocks with high precision through its crystallization in small pockets of zr enriched melt after extensive olivine and pyroxene fractionation zircon and baddelelyite u pb dates are for an overwhelming majority of cases and where we can assume a closed system considered to reflect the time of crystallization the development of the u pb dating tool ca id tims chemical abrasion isotope dilution thermal ionization mass spectrometry since 2005 has led to unprecedented precision of better than 0 1 in 206pb 238u dates bowring et al 2005 increased sensitivity of mass spectrometers and low laboratory blanks due to reduction of acid volumes allow routine u pb age determinations of micrograms of material at sufficiently high radiogenic common lead ratios see schoene and baxter 2017 this volume in situ u pb age analysis using laser ablation or primary ion beam sputtering allows analysis of sub microgram quantities of zircon material from polished internal sections or zircon surfaces with spot diameters ranging from 30 m for laser ablation inductively coupled plasma mass spectrometry la icp ms to 10 m for secondary ion mass spectrometry sims lateral resolutions of 2 5 m for nanosims

polydiapirs multiwavelength gravity structures polydiapirs domes in domes result from the evolution of a multiwavelength rayleigh taylor gravitational instability the present work investigates the initiation and development of polydiapirs by means of two dimensional finite difference models of gravitationally unstable triple layered sequences of newtonian fluids the values of viscosities thicknesses and densities were systematically varied for selected examples in triple layered sequences only two types of density stratification are likely to result in multiwavelength instabilities that will eventually develop into polydiapiric structures the limiting factors and evolution of such structures were studied for both these types of density stratifications finite difference calculations on models with physical parameters appropriate to evaporitic sequences and covered by a viscous clastic overburden showed that a very small downward density decrease in the evaporitic source may cause an internal overturn that will compete with the main overturn between source and overburden this may lead to the development of sequential or simultaneous polydiapirs the mature polydiapirs in the models exhibit spines curtain folds pendant repetitions of the source layers inside the large diapirs and anomalous interdiapiric distances all these features are known in natural salt diapirs suggesting that polydiapirs may be far more common in nature than hitherto recognized

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

mapping the 3d extent of the stillwater complex montana implications for potential platinum group element exploration and development abstract geophysical models characterize the exposed and interpreted buried extent of the stillwater complex critical for understanding the origin of the layered mafic intrusion and its associated high grade platinum group element resources the 3d models constrained by gravity magnetic xenolith seismic borehole and rock property data indicate that the likely maximum extent of the stillwater complex beneath phanerozoic cover is 10 times greater than its outcrop 2240 km2 the thickness values are poorly constrained but vary from 7000 to 12 000 m depending on crustal and mantle density variations and depths to the top of the lower crust and mantle this thickness may include dense metasedimentary units of the basin into which the stillwater complex intruded using the modeled thickness results in a volume estimate of 24 700 km3 albeit poorly constrained new analyses of xenoliths from the cretaceous sliderock and suzie peak intrusions produce ages of 2706 2716 ma corresponding to the age of the stillwater complex and 2813 ma corresponding to the age of archean gneissic basement seismic reflectors in inferred archean crystalline basement possibly including the stillwater complex dip 25 30 north with segments dipping as much as 70 north layered reflectors beneath the phanerozoic sedimentary section and above the inferred archean crystalline basement may represent metasedimentary units perhaps a southern extension of the mesoproterozoic belt basin the potential field models and the seismic reflection data suggest that the stillwater complex was dipping northward prior to deposition of cambrian strata perhaps uplifted in the late archean or proterozoic as previously proposed and that during laramide times faulting and intrusions highly disrupted the complex temperature measurements from boreholes help constrain the depths of feasible mining of the complex

fluid fluxing of cumulates the j m reef and associated rocks of the stillwater complex montana olivine bearing rocks in the lower half of ob i the olivinerich rocks also define pegmatoidal pothole structures 3 rock composition texture and grain size vary considerably in olivinebearing units characterized by segregation between coarser mafic regions and medium grained felsic regions 4 primary plagioclase in olivine rich rocks has an eroded texture 5 modal variations key words layered intrusions partial melting platinum group elements are consistent with progressive incongruent melt reactions and hydrous minerals are most abundant in olivine rich rocks 6 hydrous melt inclusions now crystallized to polyphase hydrous mineral bearing assemblages are present in olivine and chromite

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

mechanisms of petm global change constrained by a new record from central utah catastrophic carbon release and global warming during the paleocene eocene thermal maximum petm profoundly affected continental climate and ecosystems understanding of the details and mechanisms of these effects is limited by the poor geographic coverage of existing continental petm records here we extend the record of this event in north america through identification of the petm carbon isotope excursion within a sequence of paleosols in the north horn formation of central utah some 500 km to the south of previous records from northern wyoming data from the new site suggest that patterns of climatological change were similar across a meridional transect of western north america but that petm climate was relatively more arid in the southern rocky mountains possibly reflecting diversion of precipitation from middle to high latitudes our results are consistent with two proposed mechanisms explaining variation in the amplitude of continental petm carbon isotope records in terms of changes in floral composition or varying environmental wetness and they present opportunities for future tests of proposed north south biotic dispersal during the petm

magnetostratigraphic chronology of cretaceous to eocene thrust belt evolution central utah usa the axhandle basin in central utah is a piggyback basin that formed above the gunnison thrust system the basin contains 1 km of upper cretaceous to eocene strata of the north horn and lower flagstaff limestone formations a significantly more exact chronology based on magnetic polarity stratigraphy has been developed for the basin filling strata the new data provide a detailed temporal framework for the interpretation of the depositional and tectonic evolution of the basin during chrons 33 21 75 49 ma tectonic subsidence curves and stratigraphie relations along the eastern basin margin define two episodes of motion on the gunnison thrust system the majority of tectonic shortening on the system occurred between 83 75 ma and caused widespread non deposition within the axhandle basin a second episode of motion in detail comprising of four separate periods of backthrusting occurred between 70 60 ma the canyon range pavant and nebo thrust sheets were emplaced 30 50 km to the west of the axhandl

the upward transport of inclusions in newtonian and power law salt diapirs abstract this paper studies the ability of salt diapirs to lift large inclusions of dense rocks rafts dense inclusions will be lifted if salt in the diapir rises faster than the inclusions sink the power law rheologies of six different salts and viscosities estimated for newtonian salt are used to calculate the settling velocity of these inclusions as a function of their radii and densiy as well as the temperature of the salt this is done using the known equation describing the velocity of solid spheres settling in unbounded power law fluid two dimensional numerical models are used to study he effect of the ellipticity of inclusions on their sinking velocity as a function of the power law exponentn the calculations of the settling velocities of inclusions neglect several other factors discussed in the paper a the ambient fluid salt is finite bounded b more than one inclusion translates in the salt c further strain rate softening of salt is caused by its diapiric ascent the results suggest that whereas newtonian salt of 1017 1018 pa s and the power law salt of the vacherie dome would have to rise at unreasonably high speeds in order to lift large inclusions most power law salt diapirs would be capable of lifting inclusions of the sizes observed in the iranian domes up to 3 6 km2 if these rise at geologically reasonable velocities and temperatures

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

mechanisms of petm global change constrained by a new record from central utah comment and reply comment bowen and bowen 2008 1 proposed increased aridity at the paleocene eocene boundary carbon cycle perturbation whereas my study retallack 2005 2 found increased rainfall their interpretation of climatic drying during a past co2 spike is also at variance with predictions of increased

u pb and 40ar 39ar geochronology of the saint urbain and lac allard havre saint pierre anorthosites and their associated fe ti oxide ores quebec evidence for emplacement and slow cooling during the collisional ottawan orogeny in the grenville province abstract intrusive suites containing anorthosite mangerite charnockite granite amcg are a characteristic feature of mesoproterozoic magmatic activity in the grenville province of eastern north america in this study the u pb and 40ar 39ar geochronology of two amcg suites saint urbain and havre saint pierre and associated rutile bearing hemo ilmenite deposits within the allochthonous polycyclic belt of the grenville province in quebec are used to constrain their crystallization ages and cooling histories and to assess their tectonic setting both amcg suites separated by 700 km along the st lawrence river have similar crystallization ages ca 1050 1060 ma and durations of magmatism 10 15 million years the 450 km2 saint urbain anorthosite is a composite intrusion with a distinctly younger central leuconorite 1046 2 3 1 ma compared with anorthosite to the north and south 1055 0 2 4 ma and 1053 6 2 6 ma respectively the age of a deformed oxide apatite gabbronorite 1057 4 1 5 ma along the northwest margin of the intrusion is within analytical uncertainty of the anorthosites whereas the oldest intrusion in the saint urbain area is the regionally extensive saint anne du nord orthopyroxene granodiorite 1060 8 2 8 ma the large 11 000 km2 havre saint pierre anorthosite suite comprises a number of intrusive phases including the 2700 km2 lac allard anorthosite which contains the giant lac tio ilmenite deposit and big island ilmenitite dyke u pb zircon ages of three anorthosites from the lac allard intrusion including the host to the lac tio deposit overlap within error 1061 6 3 0 ma 1060 5 1 9 ma 1057 4 8 4 ma and the crosscutting big island dyke yields an age of 1052 9 6 5 ma these results indicate that the havre saint pierre anorthosite should be divided into an older geon 11 part ca 1129 ma and a younger geon 10 part ca 1060 ma average cooling rates of 3 4 c million years were estimated for the two amcg suites by combining crystallization ages u pb zircon and cooling ages u pb rutile 40ar 39ar biotite plagioclase and reflect emplacement of the intrusions into a hot long duration orogen emplacement of these two amcg suites at ca 1050 1060 ma is related to regional extensional tectonics near the end of peak metamorphism associated with the collisional ottawan orogenic phase of the grenville orogeny the results of this study provide strong support for proposals that link amcg magmatism associated with the grenville orogeny to convective thinning or delamination of the lithosphere and subsequent melting of upwelling asthenospheric mantle

geochemistry of the j m howland reef of the stillwater complex minneapolis adit area i sulfide chemistry and sulfide olivine equilibrium platinum mineralization in the stillwater complex occurs within a complex sequence of olivine bearing plagioclase rich cumulates about 400 m above the base of the banded zone the reef consists of a 2 m thick layer of disseminated sulfides extending over the entire strike length of the complex in the area of this study the host rock to the reef sulfides shows a lateral transition over a strike length of 700 m from a heterogeneous olivine plagioclase cumulate mixed rock to a plagioclase olivine cumulate containing a high proportion of postcumulus bronzite with olivine preserved only as rare resorbed cores on bronzite grains platinum group elements within the reef correlate strongly with abundances of sulfides which show convincing textural evidence for having originated as immiscible sulfide liquid strong correlations are also observed between all six platinum group elements ni and cu despite the gradation in host rock mineralogy the sulfide fraction of the reef displays a constant bulk composition over 700 m of strike length containing 9 3 percent ni 6 9 percent cu and approximately 0 7 percent total platinum group elements principally pd these platinum group element concentrations are two to three orders of magnitude higher than those found in typical magmatic sulfide deposits and are attributed to equilibration of sulfide liquid with a very large volume of silicate melt olivines from the reef show a range of compositions from fo 71 to fo 85 and from 2 400 to 4 500 ppm ni a positive correlation is observed between nio and feo in olivines from sulfide bearing samples indicating ni fe exchange between olivine and sulfides on a local scale such that ni contents of olivines were buffered by that of the sulfide phase a value of 9 70 or 1 49 is obtained for the ni fe distribution coefficient between sulfide and olivine in agreement with values obtained at magmatic temperatures by boctor 1982 these observations may be interpreted in the framework of the plume model of campbell et al 1983 a pulse of replenishing magma enters the stillwater chamber as a buoyant plume and subsequently spreads to form a layer of hybrid melt sulfide liquation and olivine crystallization occur within the plume and the hybrid layer in response to mixing and subsequent cooling sulfide liquid attains high platinum group element concentrations during turbulent ascent and spreading of the plume crystallization of olivine from the hybrid layer gives rise to olivine sulfide boulders which settle to the floor of the chamber reaction between the boulders and the evolved liquid at the bottom of the chamber causes olivine to become more iron rich subsequent reequilibration between olivine and sulfide grains gives rise to the observed ni fe distribution

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

deformed anhydrite nodules a new type of finite strain gauge in sedimentary rocks abstract the nodular anhydrite of many sedimentary basins has been deformed by halokinesis ductile faulting and regional folding deformed nodular anhydrite is common and particularly well exposed in the east central sverdrup basin axel heiberg island canadian arctic large oriented specimens containing closely packed nodules were collected on this island and used for a pilot study of the state of paleostrain owing to the asymmetry of many anhydrite nodules robin s method of strain analysis is better suited than most other techniques diameter ratios in about 50 nodule sections must be determined per deformation plane to achieve a reasonable analytical precision given the attitude of bedding and its stratigraphie facing one can split the total strain into two finite increments 1 1 an apparent or real compaction 2 2 the tectonic strain in view of the ubiquity of nodular anhydrite rocks in typical sedimentary piles the procedures described in this paper should be applicable in many regions

platinum group elements in sulphide minerals platinum group minerals and whole rocks of the merensky reef bushveld complex south africa implications for the formation of the reef the concentrations of platinum group elements pge co re au and ag have been determined in the base metal sulphide bms of a section of the merensky reef in addition we performed detailed image analysis of the platinum group minerals pgm the aims of the study were to establish 1 whether the bms are the principal host of these elements 2 whether individual elements preferentially partition into a specific bms 3 whether the concentration of the elements varies with stratigraphy or lithology 4 what is the proportion of pge hosted by pgm 5 whether the pgm and the pge found in bms could account for the complete pge budget of the whole rocks in all lithologies most of the pge 65 up to 85 are hosted by pgm essentially pt fe alloy pt pd sulphide pt pd bismuthotelluride lesser amounts of pge occur in solid solution within the bms in most cases the pgm occur at the contact between the bms and silicates or oxides or are included within the bms pentlandite is the principal bms host of all of the pge except pt and contains up to 600 ppm combined pge it is preferentially enriched in pd rh and co pyrrhotite contains rh os ir and ru but excludes both pt and pd chalcopyrite contains very little of the pge but does concentrate ag and cd platinum and au do not partition into any of the bms instead they occur in the form of pgm and electrum in the chromitite layers the whole rock concentrations of all the pge except pd are enriched by a factor of five relative to s ni cu and au this enrichment could be attributed to bms in these layers being richer in pge than the bms in the silicate layers however the pge content in the bms varies only slightly as a function of the stratigraphy the bms in the chromitites contain twice as much pge as the bms in the silicate rocks but this is not sufficient to explain the strong enrichment of pge in the chromitites in the light of our results we propose that the collection of the pge occurred in two steps in the chromitites some pgm formed before sulphide saturation during chromitite layer formation the remaining pge were collected by an immiscible sulphide liquid that percolated downward until it encountered the chromitite layers in the silicate rocks pge were collected by only the sulphide liquid

experimental modeling of role of gravity and lateral shortening in zagros mountain belt dynamically scaled analogs of the geologic structures of the zagros mountain belt are used to argue that different parts of the zagros mountains of iran record different combinations of the effects of a gravity driven overturn and a southwest northeast lateral shortening superimposed on the zagros overturn partially scaled material models have been used to simulate the zagros geodynamics which involve layer parallel compression of a 6 to 7 km thick phanerozoic carbonate cover containing a pattern of preshortening diapirs the folds in the zagros form rapidly 1 5 mm yr in a 20 to 30 km wide zone reactive some of the preshortening diapirs and generate new synshortening listric diapirs a third set of postshortening diapirs rises from the hormuz decollement behind the fold thrust front model buckle folds superimposed on diapirs or pillows tend to avoid and curve around preshortening diapirs which flatten in the synclines model profiles show that lateral shortening induces residual salt at depth to flow toward and rise through the anticlinal cores as synshortening or postshortening diapirs i suggest that any salt pillows in currently diapir free zones of the zagros fold thrust belt may surface as diapirs through the anticlines in the future in the absence of well data seismic and field observations i have used only three map pattern criteria to recognize the timing of diapirs these are locally obscured by extrusive salt sheets preshortening diapirs are generally small and elongate parallel to fold axes and are restricted to synclines synshortening diapirs are elongate perpendicular to the fold axes and are restricted to anticlines postshortening diapirs can be the same size as synshortening diapirs or larger and can be circular they are more common in the anticlines

origin of stylolites in upper permian zechstein anhydrite gorleben salt dome germany abstract stylolites in evaporites are very uncommon in the gorleben zechstein salt dome stylolite horizons have been found in the so called banderanhydrit z3ha11 at the top of the zechstein hauptanhydrit main anhydrite these pressure solution features seems to be related to the presence of thin magnesite clay interlayers five principal stylolite morphologies can be distinguished in the core sections studied the orientation of the sutured seams is generally bedding parallel with the stylolite axes vertical to the bedding surface this demonstrates that pressure solution features clearly predate the formation of the salt dome and that there is no relation between their formation and halokinesis solution rates of up to about 26 have been calculated by measuring maximum stylolite amplitudes in certain core intervals considering the bedding parallel occurrence of the stylolites and the fact that the stylolite seams are crosscut by anhydrite crystals that we interpret as pseudomorphs after gypsum it must be concluded that the stylolites formed before the dehydration of gypsum to anhydrite was completed the lower parts of the hauptanhydrit were excluded from these processes because they had already been converted to anhydrite by the overburden of overlying zechstein units and by ascending compaction fluids of the underlying zechstein 2 salt we assume that the dehydration of gypsum to anhydrite was completed before the end of zechstein sedimentation and that the formation of stylolites thus must be attributed to the same time interval

the influence of three dimensional grain size distributions on the rheology of polyphase rocks abstract in this study the relation between the deformational behaviour of two phase rocks and the grain size distributions of the constituent phases is investigated samples of hot isostatically pressed 50 50 volume percent mixtures of calcite and anhydrite were experimentally deformed in a triaxial apparatus at a temperature of 500 c a confining pressure of 300 mpa and a strain rate of 2 10 5 s 1 for calcite and anhydrite these are the deformational conditions for dislocation and diffusion creep respectively and for a grain size of 8 m these are the conditions of the equiviscous point it was found that samples from different batches of the synthetic material while having an identical average two dimensional grain size displayed significantly different rheological behaviour the ensuing full analysis revealed that the volumetric three dimensional grain size distributions of the different phases and the different batches varied significantly since the anhydrite is in the transition between the dislocation and the grain size sensitive diffusion creep regime the deformational behaviour of the bulk depends critically on the grain size distribution of the anhydrite phase the usefulness of the concept of an average grain size is discussed

geological and tectonic investigations in the former morsleben salt mine germany as a basis for the safety assessment of a radioactive waste repository abstract the safety assessment of a repository for radioactive waste is based on the results of model simulations and scenario analysis to obtain a well founded geological basis for this extensive geological and structural studies have been carried out for the morsleben repository the morsleben repository for radioactive waste was constructed in zechstein strata the location of the werra formation z1 was located in boreholes and the stassfurt z2 leine z3 and aller z4 formations are exposed in the repository mine the zechstein salt has a total thickness in the western part of the salt body of 580 m in the eastern part it is only 380 m thick the morsleben repository is in the allertal zone salt structure the root zone of a deeply eroded salt body this is evidenced by the relatively small thickness of the zechstein salt as well as by the many blocks of the hauptanhydrit z3ha in the salt which are present mostly in the synclinal hinge zones i e in the deeper parts of the structure as a result of tectonic separation of ductile salt from competent anhydrite the tectonics of the structure are documented by polyphase folds characteristic of the deformation of ductile material the general trend of the fold axes is horizontal nw se parallel to the trend of the allertal zone the western part of the salt structure is characterised by high ne vergent isoclinal folds the z2 to z4 strata involved in the folding extend from the salt table top of the salt body down to the lowest synclinal trough in the eastern part the folds are open and symmetric with considerably smaller amplitude correspondingly z4 strata mostly occur just below the salt table z3 at the repository level and z2 below that the different styles of folding in the east and west are due to the structure of the rocks beneath the zechstein salt the boundary between the two types of folds lies above a major fault in the rocks below the zechstein salt separating a downthrown block in the west from an upthrown block in the east ne vergent isoclinal folds occur above the downthrown block where the major ascent of the salt occurred in contrast the folding in the upthrown eastern block has hardly been affected by the ascending salt hence the wide open symmetrical folds thus the structure of the rocks beneath the zechstein determined the folding style in the overlying salt the present day fold structures chiefly result from late cretaceous compression that affected the north german basin the salt body is therefore primarily a tectonic structure and is not halokinetic

characterizing the u pb systematics of baddeleyite through chemical abrasion application of multi step digestion methods to baddeleyite geochronology u pb baddeleyite geochronology has become a major tool for dating mafic rocks especially dikes associated with large igneous provinces however in many cases post crystallization pb loss and intergrowth of baddeleyite and zircon limit the precision and or accuracy of crystallization ages we present results from multi step digestion experiments designed to understand and reduce these effects experiments were carried out on neoproterozoic baddeleyites with zircon inter and over growths from the gannakouriep dike swarm namibia and on fragments of a large paleoproterozoic baddeleyite from the phalaborwa carbonatite south africa multi step digestion experiments on annealed phalaborwa baddeleyite were designed to test whether the recently developed chemical abrasion technique for u pb zircon geochronology can be applied to baddeleyite the experiments generated complex results individual digestion steps were both normally and reversely discordant suggesting that u and pb were decoupled in the multi step digestions and indicate that the current form of multi step chemical abrasion is not an effective method for reducing the impact of pb loss in baddeleyite a separate set of experiments on the gannakouriep baddeleyite focused on isolating the zircon and baddeleyite components in composite grains conventional single step digestion experiments for this sample resulted in a discordant suite of analyses with significant scatter attributed to inter and over grown zircon and highlight the difficulty of obtaining precise and accurate ages from composite grains to isolate the baddeleyite and zircon in these grains a two step hcl hf chemical abrasion procedure for annealed grains was developed this technique was successful at selectively dissolving the baddeleyite and zircon components secondary zircon inter and over growths of baddeleyite can occur in samples affected by low temperature alteration to granulite facies metamorphism and the new hcl hf chemical abrasion procedure provides a method for resolving both the igneous and metamorphic history of these composite grains

the control of salt supply on entrainment of an anhydrite layer within a salt diapir the influence of four parameters sedimentation rate viscosity of salt stratigraphic location of the anhydrite layer within the salt layer and the perturbation width on salt supply to down built diapirs and its entrainment capacity are studied systematically in numerical models model results show that these four parameters affect salt supply and the evolution history of a salt diapir as such these parameters strongly influence the style and the amount of entrainment of dense inclusions into a diapir in active diapirs i e unburied diapirs salt supply increases with increasing sedimentation rate whereas it decreases with an increase in salt viscosity diapirs initiating from wide perturbation provide more salt supply to feed the diapir presence and initial stratigraphic location of any denser layer e g an anhydrite layer within a salt layer also affects salt supply when lateral forces are negligible salt supply into a diapir depends on these four parameters which directly control the entrainment of any embedded anhydrite layer into the diapir

paleoenvironmental isotope geochemistry and paragenesis of lacustrine and palustrine carbonates flagstaff formation central utah u s a abstract the early paleogene flagstaff formation on the wasatch plateau of central utah preserves a ca 300 meter thick sequence of palustrine and lacustrine limestone and dolostone these rocks formed during a climatically dynamic interval of earth s history and contain important sedimentological and geochemical records of early paleogene greenhouse climate the flagstaff carbonates also document a multiphase history of diagenetic alteration that must be clearly differentiated in order to extract a paleoenvironmental record we have studied the stable carbon and oxygen isotope geochemistry of sample suites from three local stratigraphic sections in conjunction with observations from field studies petrography and x ray diffraction analysis our results provide evidence for a paragenetic sequence including primary carbonate deposition in a shallow freshwater to saline lake system primary or early secondary formation of micritic dolomite episodic exposure and pedogenesis and late stage pressure solution and cementation stable isotope data indicate that the isotopic composition of cements is distinct from but largely determined by that of the host micritic carbonate and suggest that isotopic data from micrites and dolomicrites preserve records of lateral and temporal paleoenvironmental changes in lake flagstaff integration of lithological and isotopic evidence indicates the presence of a persistent lateral paleoenvironmental gradient among our study sites and a secular drying trend represented in all three study sections existing age constraints indicate that the desiccation of lake flagstaff occurred during a multi million year interval of global climate warming that was punctuated by extreme transient 200 kyr warmth during the paleocene eocene thermal maximum petm our analysis suggests that changes in the areal extent and chemistry of lake flagstaff can be attributed largely to climatic changes affecting the lake basin and document a multi million year reduction in precipitation evaporation ratios similar to that previously reported from sites in wyoming to the north although the correlation of the petm within the flagstaff fm remains unclear existing constraints are consistent with this event having corresponded to the maximum lowstand of lake flagstaff perhaps providing evidence for regional climatic drying during this episode of global warmth

climatic and diagenetic signals in the stable isotope geochemistry of dolomitic paleosols spanning the paleocene eocene boundary abstract the precipitation of primary dolomite in soil is rare and controversial and its occurrence indicates highly unusual geochemical and climatic conditions we utilize isotopic and petrographic techniques including clumped isotope thermometry to evaluate diagenetic alteration and preservation of primary climatic signal in dolomitic paleosol nodules from a section spanning the paleocene eocene boundary in central utah isotopic measurements differentiate samples into groups that may be interpreted in terms of alteration however moderate burial depths and extreme warmth during the interval of soil formation make it difficult to reach definitive conclusions from isotope values alone with the addition of petrographic analysis including cathodoluminescence and scanning electron microscopy we are able to identify textural differences between samples suggesting differing amounts of recrystallization we conclude that while some nodules have experienced significant diagenetic alteration the original nodule mineralogy is dolomite and the majority of sampled nodules retain a primary paleoclimate signal early paleogene temperatures recorded at this site are considerably warmer than those suggested for the continental interior from alternate proxies a result that may be partially the result of the clumped isotope temperatures recording warm season soil temperatures rather than mean annual air temperature soil water isotopic composition is suggestive of heightened aridity particularly through the paleocene eocene thermal maximum petm

the shaping of salt diapirs abstract six parameters shape the geometry of passive diapirs associated with stiff overburden rates of salt supply s dissolution d sediment accumulation a erosion er extension e and shortening sh these parameters change in space and time and hence influence the geometry of the structure as it forms a complex six parameter plot representing the evolution history of a salt diapir can be simplified into three separate graphs this study recommends adding to plots of s against a a second of rate of salt supply s this time with extension rate e and a third of rate of sediment accumulation a plotted against extension rate e integrating these three plots on a single diagram results in a complete description of the evolution history of a diapir when it is applied to three dimensional data however if applied to a profile the plot shows only the two dimensional evolution history of the diapir lateral forces extension or compression have a significant role in moulding the geometry of a salt diapir by influencing the space which it occupies by incorporating extension in the moulding plots of salt diapirs this study introduces the rate of sediment accumulation multiplied by extension e a as a significant factor in moulding salt diapirs by using this rate against the rate of salt supply this study redefines the conventionally accepted interpretation of upward narrowing upward widening and columnar diapirs upward narrowing diapirs form when the rate of salt supply is less than the rate of sediment accumulation multiplied by extension upward widening diapirs form when the salt supply is greater than the rate of sediment accumulation multiplied by extension columnar diapirs form when the rate of salt supply is equal to the rate of sediment accumulation multiplied by extension this new relationship explains the absence of columnar and upward widening diapirs in passive margins where thin skinned extension dominates and emphasizes the significance of lateral movement extension and shortening in moulding the geometry of salt diapirs

investigations of the howland reef of the stillwater complex minneapolis adit area stratigraphy structure and mineralization a platinum group mineral deposit occurs within the lower part of the banded zone of the stillwater complex a layered tholeiitic intrusion of archean age the zone of platinum concentration the howland reef is coincident with the reappearance of olivine in the crystallization of the complex host rocks to the mineralization are distinguished by variable proportions of olivine bronzite plagioclase and augite phlogopite hornblende apatite sulfide and chrome spinel are accessory significant lateral and vertical variations in the composition and thickness of the howland reef are evident over 700 m of underground development there are superimposed large and small scale changes in mineral assemblages and rock textures suggesting that processes other than gravity sorting or crystal sedimentation were responsible for reef formation definition drilling in the minneapolis adit and exploratory surface drilling have shown that the howland reef is thickened locally by fault repetitions and elsewhere attenuated by the same system of high angle reverse faults significant concentrations of platinum group elements pge within the howland reef are restricted to sulfide minerals the sulfides show clear evidence of an origin as accumulated immiscible sulfide liquid droplets sulfides occur generally as composite grains or globules consisting mainly of pyrrhotite pentlandite chalcopyrite and platinum group minerals with or without pyrite and assorted low temperature alteration products the dominant platinum group element bearing minerals belong to the braggite vysotskite pt pd ni s solid solution series consistent relationships between sulfide morphologies and the phase assemblages of host silicates provide convincing evidence that the immiscible sulfide droplets formed concurrently with olivine during the earliest stages of reef genesis

multiple s isotope studies of the stillwater complex and country rocks an assessment of the role of crustal s in the origin of pge enrichment found in the j m reef and related rocks abstract the j m reef in the stillwater complex of montana is the highest grade pd deposit in the world averaging approximately 14 ppm pd one suggestion for the origin of the enrichment centers on the sequestration of metals in staging chambers via immiscible sulfide liquids that formed in response to the addition of crustal s because 34s values alone are often insufficient to document in igneous rocks the assimilation of s from archean sedimentary rocks a combined study of both 33s and 34s values and computed 33s values was undertaken 34s and 33s values of cordierite bearing hornfels as well as massive sulfides in the metasedimentary rocks of the contact aureole can be divided into two populations one is characterized by anomalous positive 33s values from 0 to 0 32 and 34s values from 0 5 to 1 6 whereas the other is characterized by anomalous negative 33s values from 0 to 0 22 and 34s values from 1 5 to 5 8 34s and 33s values for rocks from the basal series of the complex range from 0 9 to 3 0 and 0 01 to 0 18 respectively the anomalous s isotope ratios indicate that for some samples at least 50 of the their s was of crustal origin in contrast most of the rocks from the j m reef are characterized by 34s and 33s values between 1 3 and 1 8 and 0 06 to 0 06 respectively these values deviate only slightly if at all from those thought to be representative of mantle s and suggest that i contamination was not an important process for pge enrichment in the complex ii the contaminant was also characterized by near 0 33s values or iii s isotopic exchange in the staging chambers was an important process upgrading of pge values via reaction between sulfide in the chamber and uncontaminated mantle derived magma may also have resulted in the conversion of s isotopic ratios that may have been indicative of crustal s addition to mantle like values rare 34s values up to 7 2 in the j m reef and 8 2 in chromitite from the ultramafic series provide strong evidence for localized assimilation of crustal s which did not experience isotopic homogenization the s isotopic ratios of the j m reef are non unique in terms of a specific genesis as they may be interpreted in either a downward sulfide collection model or an upward fluid transport model

direct dating of ultramafic sills and mafic intrusions associated with ni sulfide mineralization in the thompson nickel belt manitoba canada the paleoproterozoic thompson nickel belt a segment of the northwest margin of the archean superior craton in manitoba is one of the largest nickel producing regions in the world and the second largest ni cu pge platinum group elements mining camp in canada after sudbury the nickel sulfide ores are hosted in or associated with ultramafic sills of komatiitic affinity that were emplaced into sulfur rich metasedimentary rocks of the ospwagan group and subsequently strongly deformed and metamorphosed amphibolite to granulite facies during the trans hudson orogeny a u pb zircon age of 1880 2 1 4 ma for a metaperidotite olivine orthopyroxenite from the upper part of a boudinaged ultramafic sill at the pipe 2 deposit is interpreted as the age of crystallization of the intrusion the first age from an ultramafic sill containing mined nickel sulfide in the thompson nickel belt this age confirms a direct petrogenetic relationship between ni cu pge mineralization and widespread mafic ultramafic magmatic activity molson igneous events at ca 1 88 ga along the superior province craton margin e g thompson nickel belt fox river belt cape smith belt new quebec orogen at the south pit of the thompson mine a u pb zircon age of 1860 7 0 7 ma for a composite mafic intrusion garnet amphibolite to layered metagabbro identifies a new age of mafic magmatic activity in the thompson nickel belt that is unrelated to nickel sulfide mineralization the thermal effects of the trans hudson orogeny are recorded by a range of zircon dates and ca 1760 ma metamorphic titanite from the south pit intrusion and are consistent with interpretations that regional metamorphism occurred as a single progressive event the results of this study open up the possibility of using the high precision u pb geochronology of mineralized ultramafic rocks to precisely constrain the duration of mafic ultramafic magmatism in the thompson nickel belt and assess potential age differences between ultramafic sills across and along strike of this world class nickel metallotect

style and timing of frontal structures thrust belt central utah the cordilleran fold and thrust belt in central utah may be divided into a western belt of precambrian to lower mesozoic strata shortened above ramp style thrust faults and an eastern belt of folded middle to upper mesozoic rocks shortening in the eastern foldbelt occurred above a bedding plane thrust fault system that terminates within a thick section of jurassic shale siltstone and anhydrite this thrust fault system probably failed to reach the synorogenic erosion surface deformation associated with the thrust system takes the form of ramp anticlines in the west and a regionally extensive triangle zone at the eastern extent of the shortening the ramp structures indicate that the thrust system rose from deeper structural levels and became bedding parallel within th jurassic section the middle jurassic section was approximately doubled in thickness by duplexing within the triangle zone shortening across the triangle zone is approximately 50 5 km or 3 mi demonstrable synthrusting deposits within the region are late early to late cretaceous in age late albian to late campanian the synorogenic rocks were folded above the triangle zone and are now overlain unconformably by strata of tertiary age tertiary clastics onlap structures in the foldbelt and were not deformed by the deformation within the triangle zone the age of synorogenic deposits and structural relations of postorogenic strata indicate that deformation was complete by the close of the cretaceous or early paleocene and support a thrust mechanism for much of the folding in the region

pt pd reefs in magnetitites of the stella layered intrusion south africa a world of new exploration opportunities for platinum group elements the 3033 ma stella layered intrusion of south africa consists largely of magnetite gabbros and gabbros that are hosted by greenstones of the kraaipan belt the intrusion contains a 100 m thick platinum group element pge enriched interval that includes a number of laterally continuous pge reefs constituting the oldest mineralization of this type known on earth the richest of the reefs is hosted by magnetitite and contains 10 15 ppm pt pd over 1 m representing by far the highest pge grades known up to this time in magnetitite hosted pt pd reefs the pges are interpreted to have been concentrated by sulfide melt after s saturation had been reached in the advanced stages of magmatic differentiation in response to magnetite crystallization reaction between sulfide melt and oxides led to late magmatic s loss causing a paucity of sulfides in most of the pge mineralized interval as a result the reefs cannot be distinguished macroscopically from their unmineralized host rocks and we suggest that similar mineralization may have been overlooked in the upper parts of other tholeiitic intrusions elsewhere

a lead isotopic study of the stillwater complex montana constraints on crustal contamination and source regions analyses of the pb isotopic compositions of plagioclase from 23 samples covering the stratigraphic thickness of the stillwater complex indicate a narrow range of apparent initial isotopic compositions 206pb 204pb 13 95 207pb 204pb 14 95 15 01 208pb 204pb 33 6 the uniformity of our data is in contrast to but not necessarily contradictory to other recent investigations which give indications that the complex formed by repeated injection of magmas with at least two distinct compositions that were presumably derived from different source regions samples from the basal series of the complex have consistently higher 207pb 204pb ratios suggesting either minor contamination from adjacent country rocks or a slight distinction between parental magmas apparent initial pb isotopic compositions of the complex are very radiogenic compared to late archean model mantle values but are nearly identical to initial pb isotopic compositions found for the the adjacent slightly older 2 73 2 79 ga late archean crustal suite in the beartooth mountains contamination of magmas parental to the stillwater complex by the late archean crustal suite is rejected for two reasons 1 th and u concentrations in stillwater rocks and plagioclase are very low about 0 08 and 0 02 ppm respectively yet th u ratios are uniform at about 4 in contrast to the highly variable 2 26 but often high th u ratios found for the late archean crustal complex 2 it seems improbable that any contamination process would have adjusted the isotopic compositions of the diverse magmas entering the stillwater chamber to near identical values the preferred hypothesis to explain the pb isotopic data for the stillwater complex and the associated late archean crustal suite involves a major late archean crust forming event that resulted in a compositionally complex crust mantle system with relatively homogeneous and unusual pb isotopic compositions the parental magmas of the stillwater complex were generated at different levels within this crust mantle system before isotopic contrasts could develop by radioactive decay within compositionally discrete reservoirs this situation limits the utility of all isotopic tracer systems in discriminating among the various mantle and crustal reservoirs that may have affected the final isotopic character of the stillwater magmas the late archean crustal complex and the stillwater complex melts were ultimately derived from the same distinct mantle without obvious direct interaction with the middle to early archean crust present in the region

late holocene earthquake sequences on the el asnam algeria thrust fault abstract geological analysis of eight trench exposures through the el asnam fault zone reactivated during the october 10 1980 earthquake m 7 3 shows various tectono sedimentary structures associated with large seismic events in the past situated on the southeastern flank of the active sara el maarouf fold the trenches cut across the main 1980 fault traces and also across a secondary normal fault bending moment or extrados fault the erosional action of the cheliff and fodda rivers on the active fault during the late quaternary formed a large gorge and for each coseismic incremental growth of the anticline a thrust fault scarp rises in its path and stops the rivers flow an earthquake induced flood caused by damming of the rivers occurred just after the 1980 event a stratigraphic section 7 in the flood area exhibited 6 flood horizons each of which may represent an earthquake of magnitude greater than 7 close to this area buried fault scarps with cumulative vertical movements and 14 c ages provide an average uplift rate of 0 6 mm y 1 vertical displacements in 1980 were not constant along the fault trend according to the trench locations the estimated uplift rate may be considered as a minimum value for the el asnam fault tectonic and sedimentary data suggest that eight large unevenly distributed earthquakes occurred prior to 1980 and characterize the late holocene seismic activity clusters of large seismic events with m 7 appear around 4000 years b p and also during the last 1000 years which is in agreement with previous evaluations 7 the recurrence interval of large earthquakes is about 720 years but during seismic sequences this recurrence interval decreases and varies between 300 and 500 years

an algorithm for u pb isotope dilution data reduction and uncertainty propagation this is the publisher s version also available electronically from http onlinelibrary wiley com