proterozoic accretionary tectonics at the southern margin of the archean wyoming craton the proterozoic cheyenne belt at the southern margin of the archean wyoming craton consists of strongly deformed lithotectonic blocks bound by major mylonite zones from north to south and structurally lowest to highest these blocks include 1 archean crystalline basement and associated early proterozoic miogeoclinal rocks 2 an amphibolite grade orthogneiss terrane of unknown age containing probable rift related mafic intrusive rocks 3 a 1750 1790 ma marginal basin succession consisting of upper amphibolite grade pelitic and volcano genie schist associated peraluminous granite and minor ultramafic rocks and 4 a 1750 1790 ma intermediate to mafic plutonic metamorphic complex interpreted as the deep roots of an island arc system the earliest deformation d1 produced a synmetamorphic penetrative horizontal transposition foliation and associated recumbent folds tectonic blocks were juxtaposed along major mylonite zones during d2 macroscopic anil microscopic structures associated with d2 indicate northward thrusting along low angle mylonite zones of successively deeper crustal blocks over supracrustal rocks of the wyoming craton at about 1750 ma thrusting occurred at minimum temperatures of 475 c and produced an inverted metamorphic gradient in pelitic rocks north of the belt mylonite zones were subsequently steepened and reactivated under greenschist facies conditions during a period of dextral strike slip d3 between 1750 and 1400 ma cataclasis d4 and brecciation associated with the laramide orogeny d5 locally overprint earlier structures oblique convergence between an island arc and the archean continental nucleus may explain accretion crustal shortening and subsequent strike slip the phanerozoic accretionary events of eastern and western north america provide an analog for this model presence of similar lithologies and shear zones south of the cheyenne belt suggest that the southern margin of the wyoming craton may have been a long lived zone of crustal accretion

evolution of mica fish in mylonitic rocks abstract mineral fish are lozenge shaped porphyroclasts single crystals in a finer grained matrix which occur in ductile shear zones and which are commonly used as shear sense indicators mineral fish of biotite tourmaline k feldspar garnet hypersthene and quartz occur in mylonites but most common are white mica fish these mica fish can be subdivided into six morphological groups that develop by different mechanisms determined by different initial shapes and orientations the principal mechanisms of formation are intracrystalline deformation combined with rigid body rotation concomitant selective grain size reduction occurs by recrystallisation cataclastic separation pressure solution and diffusional mass transfer microboudinage has been proposed for the breakdown of large mica fish into smaller ones but many mica fish undergo shortening rather than extension along their long axes evidence is presented for an alternative process in which the tips of mica fish are isoclinally folded and then break off along the hinge of these microfolds all the presented fish shaped porphyroclasts or mineral fish have a specific shape preferred orientation with their long axis at a small antithetic angle with respect to the foliation they represent a special group of objects with a stable orientation due to flow partitioning into narrow shear bands

evolution of the early proterozoic colorado province constraints from u pb geochronology the colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the archean wyoming craton during the early proterozoic between about 1790 and 1660 ma correspondence in ages between metamorphism deformation and plutonism association of volcanic rocks with comagmatic calc alkalic plutons and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton

palaeotemperatures still exist in the greenland ice sheet the temperature distribution through the greenland ice sheet at the dye 3 borehole is a record of the past climatic changes in the arctic the numerical model of the temperature distribution now presented reproduces the observed temperature distribution within 0 03 k and shows that the basal ice is still cooled 5 k by the cold ice age climate the results suggest a mean ice age temperature of 32 2 c which is 12 k colder than the present temperature and a precipitation rate 50 25 of the present rate calculations of a more detailed temperature history through the present inter glacial period reveal evidence of the ad 1920 50 maximum the little ice age and the atlantic period

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

simple shear deformation and quartz crystallographic fabrics a possible natural example from the torridon area of nw scotland ln this paper the microstructures and crystallographic fabrics of a 30 cm wide deformed quartz vein from the torridon area of nw scotland are described the vein which lies along a joint separating two blocks of proterozoic gneiss was deformed by crystal plastic processes when the rigid joint blocks moved in opposite directions parallel to the vein margins mylonitic foliation within this quartzose ductile shear zone locally displays the classic orientation variation expected for heterogeneous simple shear deformation in thin section the vein is a mylonite composed of dynamically recrystallized elongate quartz grains aligned oblique to the mylonitic foliation fabrics within the central part of the vein have been examined by optical and x ray texture goniometry methods together with orientation distribution function odf analysis the a axis fabric consists of a dominant single point maximum orientated within the xz plane at 9 to the lineation x this point maximum occupies a pole position to the corresponding single girdle c axis fabric for individual positions on the c axis fabric skeleton the orientations of potential crystallographic slip systems slip plane and direction relative to shear zone co ordinates have been calculated by odf analysis the calculation of relative resolved shear stresses schmidfactors for the potential slip systems assuming a simple shear kinematic framework reveals an elegantly simple relationship between individual grain c axis orientation and correspond ing orientations of potential slip systems these results are readily interpreted in terms of bulk simple shear deformation

sedimentary signatures and processes during marine bolide impacts a review abstract studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts this results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters in subaerial impacts the targets are commonly hard rocks frequently of igneous and or metamorphic origin whereas in submarine impacts the targets are mostly unconsolidated or poorly lithified sediments or sedimentary rocks with high volumes of pore water such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area impacts in shallow water marine neritic environments produced craters with low or absent rims and wide and shallow brims as characterize by both the montagnais on the scotian shelf the mjolnir in the barents sea 45 and 40 km in diameter respectively and the chesapeake bay 90 km in diameter lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity as the water in the crater is vaporized during this process resurge gullies can be cut across the rim while mass and debris flows turbidites and other gravity deposits are produced as results of tsunami and crater wall and central high collapse during and after the crater excavation stage such deposits are found both within and outside the crater structure the only difference between gravity deposits triggered by an impact or other rare events such as earthquakes is the admixture of various melt particles and possible enrichments in iridium in the former impacts near the shelf edge may cause partial collapse of the continental margin as shown by the montagnais and chicxulub impacts some of the gravity and debris flows generated by margin collapse may be channelized with final deposits up to several hundred meters thick extending for hundreds of kilometers from the impact site other impact features such as shatter cones tektites spherules and ni spinels shocked quartz isotropication and partial melts are common to both submarine and subaerial impacts theoretical calculations of the destructive forces of mega tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence a dearth of gigantic tsunami evidence for the chicxulub impact outside of the gulf of mexico where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep north atlantic basin should have been 1 m s could be the result of the mitigating effect of the bathymetry of surrounding area causing wave diffraction and interference calculated maximum horizontal orbital velocity near the seafloor at the shelf for the montagnais impact is 22 m s for a 200 m high wave 5 5 m s for a 50 m high wave at 500 km decreasing to 0 5 m s at 1000 km strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record however lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows

boltysh another end cretaceous impact the chixculub impact occurred at the cretaceous tertiary k t boundary and although several other late cretaceous and paleogene impact craters have at times been linked with the k t boundary isotope geochronology has demonstrated that all have significantly different ages the currently accepted age of the 24 km diameter boltysh crater a k ar whole rock age places it in the coniacian at 88 3 ma however comprehensive ar ar dating of a range of melt samples yields a mean age of 65 17 0 64 ma within errors of the k t boundary several of the fresh samples exhibit signs of excess argon but this seems to be concentrated in rapidly crystallized glass rich samples the ar ar age confirms an earlier fission track measurement and thus two dating techniques have yielded an age within errors of the k t boundary for this crater crucially although the ages of boltysh and chixculub are within errors they may not have formed synchronously craters of 24 km diameter occur much more commonly than impacts of chixculub dimensions but their proximity does raise the important question of how many impacts there might have been close to the k t boundary

asymmetrical quartz crystallographic fabrics formed during constrictional deformation numerical simulations predict unique quartz crystallographic fabric patterns for plane strain flattening and constriction multiple studies support the predictions for plane strain and flattening to test predictions for constriction this paper analyzes five examples of quartz crystallographic fabrics from a 1km wide domain of l tectonites in the pigeon point high strain zone klamath mountains california u s a these samples were deformed under greenschist to amphibolite facies conditions quartz c axis fabrics are similar to the predicted double girdle fabrics except that amphibolite facies samples exhibit caxis maxima and are distinctly asymmetrical about the elongation lineations activation of different slip systems combined with small deviations from pure constriction account for the c axis maxima and noncoaxial flow accounts for the fabric asymmetry the simple shear component is randomly oriented in geographic coordinates throughout the domain of l tectonites these data confirm that numerical simulations predict the quartz c axis fabric geometry developed during constriction for some deformation conditions and they confirm the quartz a axis patterns predicted for constriction for the first time these data also demonstrate that the relationship between quartz crystallographic fabrics and strain geometry is not straightforward and they indicate that a axis fabrics may be more useful indicators of strain geometry variations

cometary impacts into ocean their recognition and the threshold constraint for biological extinctions the montagnais impact crater is presently the only site in the ocean where the effect of a meteorite fall on marine organisms has been studied the impact crater is 45 km in diameter and was formed at 50 8 ma by a fall of probably an old cometary nucleus 3 4 km in diameter into shallow 600 m ocean comparison of the impact structure and related deposits with those on land shows several major differences of which the most significant is the absence of an elevated crater rim instead the crater perimeter is bevelled and eroded as a consequence of impact induced bottom currents and turbulent return water flow into the excavating cavity by this process most of the fall out breccia is reworked back into the crater cavity where it accumulates in much larger thickness than in impact craters on land at a microscopic scale the shock metamorphism features are about the same as those for land impacts geochemically impacts of comet nucleii may not leave a recognizable signature at the impact horizon except for a minor increase in iridium thus stratigraphic horizons associated with extinctions and or major changes in biota have to be closely examined for other impact indicators like the presence of tectites glass spherules and quartz grains with shock features occurrence of megatsunami wave deposits extensive erosion on continental margins margin failures and faunal mixing above erosional unconformities are other potential impact indicators there is no single indicator that can provide sufficient proof of an impact event such interpretations have to be based on multiparameter studies of global extent since many of the impact indicators are only of regional extent the lack of extinction of any marine plankton genera or of bottom dwellers at the montagnais impact site allows us to place a lower limit for biological extinctions caused by cometary impacts on those with nucleus 4 km in diameter the calculated frequency for a cometary impact which could result in a 10 extinction of marine genera is about 6 10 7 yr 1 and for the k t boundary type of extinctions about 2 10 8 yr 1 even allowing for a large degree of uncertainty in these estimates it is unlikely that the biological extinction events for the last 250 ma identified by sepkoski 1990 could have been all caused by meteorite impacts

finite strains in transpression zones with no boundary slip abstract a mathematical model describing finite strains has been developed for transpression zones where no slip is allowed in any direction at the zone boundaries this has been adapted to model transpression zones where the boundaries are inclined the model can also be used to describe finite strain patterns where material is incorporated into the deforming zone from one or both sides after the initiation of deformation so that the deforming zone remains at a constant width thus allowing large transpressional strains to be accommodated without the deforming zone closing to zero thickness this model of steady state transpression can be used to explain some of the features of the south mayo trough a late silurian transpression zone in the west of ireland

iridium emissions from kilauea volcano during may 1983 gas and particulate samples were collected at the cooling vents of kilauea volcano in hawaii three vents on the southeast rift zone were sampled using base treated filter packs to absorb acidic gases and teflon filters for particles the samples were analyzed for 40 elements by nondestructive neutron activation analysis as with other volcanoes the chalcophilic and volatile elements were enriched by up to seven orders of magnitude relative to the erupted basalt unlike the case of other volcanoes iridium was observed to be highly enriched 105 fold at the two higher temperature vents and the degree of enrichment appears to be related to both high temperatures and high fluorine content of the gases estimates of the emission rates of ir during eruptions of kilauea based on the measurements reported here and those of the hawaiian volcano observatory yield an emission rate of about 3 g ir per 106 m3 of magma this rate amounts to only about 0 3 of the ir present in the magma that is being released by the volcano this previously unidentified ir source to the atmosphere and ocean may have a significant influence on the geochemical cycle of ir on the earth the uniqueness of kilauea in its emissions of ir is probably related to the deep source of magma which contains significant levels of ir 0 32 ppb and high levels of the halogens f and cl

proterozoic farwell mountain lester mountain suture zone northern colorado subduction flip and progressive assembly of arcs this paper considers the amalgamation of arc and oceanic terranes to be the main mechanism of ca 1 8 1 6 ga continental crustal growth in southwestern laurentia on the basis of geologic and seismic reflection data and teleseismic images we propose the farwell mountain lester mountain suture zone as the northern most paleoproterozoic arc arc suture north dipping farwell mountain seismic reflections project from 18 km depths to the surface and are interpreted to represent conjugate thrusting as the 1 79 1 77 ga green mountain arc was partially underthrust beneath the archean craton we speculate that a north dipping high velocity mantle slab in the teleseismic image is a continuation of this thrust zone south dipping lester mountain reflections project from 22 km depths to the surface and are interpreted to be a thrust zone between the green mountain arc and the 1 76 1 72 ga rawah block surface features of the farwell mountain lester mountain suture zone are 1 marble chert rock with sillimanite pods ultramafic rocks sulfide deposits and pillow basalts which we interpret to be a dismembered accretionary complex 2 an axial planar fabric to north verging isoclinal folds f 2 which we interpret to be part of a north vergent fold and thrust system and 3 a metamorphic break between 500 c rocks to the north and 610 c rocks to the south which we attribute to reactivation of the zone movement on the farwell mountain backthrust is inferred to relate to cheyenne belt suturing at 1 78 1 75 ga we correlate suturing in the farwell mountain lester mountain suture zone to s 1 d 1 1 746 1 74 ga in the soda creek fish creek shear zone we attribute the complexity of this broad suture zone to initial conjugate thrusting plus overprinting and steepening of accretionary structures by subsequent tectonism

air hydrate inclusions in fresh ice core one of the characteristic features of polar glacier ice is the abundance of air bubbles1 3 with increasing depth from the surface of the ice sheet the size of the bubbles generally decreases in response to the change of overburden pressure below a certain depth the bubbles start to shrink faster than expected and finally become invisible although not visible to the naked eye unit mass of the bubble free clear ice includes almost the same volume of air as unit mass of bubbly ice4 5 it is hypothesized that the air exists in the ice structure probably in the form of clathrate hydrates as suggested by miller6 7 but until now the location of the air has been unknown direct observations of air hydrate inclusions were carried out for the first time in fresh ice cores from dye 3 greenland when the ice structure surrounding air hydrate was melted an explosive transition occurred releasing a gas bubble into the melt cavity

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

stratigraphy and depositional setting of the proterozoic snowy pass supergroup southeastern wyoming record of an early proterozoic atlantic type cratonic margin metasedimentary rocks in the medicine bow mountains and sierra madre are divided into four groups the 3 km thick phantom lake metamorphic suite contains strongly deformed metavolcanic and metasedimentary rocks that are crosscut by late archean granites the 2 5 km thick deep lake group unconformably overlies the phantom lake suite and late archean granites and contains fluvial sediments including radioactive quartz pebble conglomerates and glaciomarine deposits both successions are intruded by large sills of tholeiitic gabbro the 4 5 km thick lower libby creek group is inferred to be in thrust fault contact with older units and contains sediments recording transgressions and regressions across a macrotidal delta this succession is intruded by the 2 000 m y old gaps intrusion and comagmatic tholeiitic to weakly alkalic dikes the 3 km thick upper libby creek group is bounded by a thrust fault below and by the cheyenne belt above and contains carbonates and marine slates the early proterozoic deep lake lower libby creek and upper libby creek groups collectively are named the snowy pass supergroup lithologies and stratification sequence in the well preserved medicine bow mountain section suggest transgressive miogeoclinal sedimentation during the early proterozoic paleocurrent data indicate that fluvial then deltaic sedimentation of the deep lake and lower libby creek groups took place on a southwest dipping paleoslope parallel to the inferred south cratonic boundary of the wyoming province this and a few west directed paleocurrents suggest a continental or microcontinental block to the south bounding sedimentation in a northeast elongated basin a rift setting for deposition of these units explains the transgressional character of the sediments the deltaic sedimentation with paleocurrents parallel to the cratonic boundary and the 120 bend in the cheyenne belt between the medicine bow mountains and the sierra madre the upper libby creek group is interpreted to represent open marine conditions following separation of the two continental blocks tholeiitic sills in the deep lake group and tholeiitic to weakly alkalic dikes in the libby creek group are thought to be related to basaltic igneous activity associated with compound early proterozoic rifting between 2 300 and 2 000 m y ago

an early proterozoic volcanic arc succession in southeastern wyoming the green mountain formation of early proterozoic age in the sierra madre range of southeastern wyoming comprises a bimodal mafic and felsic volcanic assemblage the rocks which are chiefly breccias agglomerates flows and volcaniclastic sediments represent both subaerial and submarine eruptions and in part were redeposited in fluvial and nearshore marine environments volcanic rocks are clearly calc alkaline in character and share a large number of geochemical features in common with continental margin arcs or evolved oceanic arc systems the low mg numbers and ni contents of the basalts require 30 40 olivine fractional crystallization and the high contents of the most incompatible elements high la sm n ratios and low zr nb ratios require an undepleted or enriched mantle source geochemical data are consistent with an origin for the felsic volcanics and associated encampment river granodiorite by shallow fractional crystallization of calc alkaline basalt in a tectonic setting similar to modern ar

iridium enrichment in volcanic dust from blue ice fields antarctica and possible relevance to the k t boundary event abstract blue ice fields in antarctica well known as stranding zones of antarctic meteorites contain ice that is several tens to hundreds of thousand years old which records numerous events that have taken place since its formation among the most important structures on the ice fields are dust bands which are mostly of volcanic origin and consist of small volcanic glass shards and lithic fragments petrographic and chemical studies of a number of dust bands during earlier investigations have shown that significant ash layers are deposited at distances of up to several thousand kilometers from the source volcanoes samples of dust layers taken in the lewis cliff beardmore glacier area in antarctica analyzed in the course of this work for major and trace elements were identified as consisting of volcanic ash probably originating from the pleiades melbourne volcanic province northern victoria land some of these dust samples were found to contain iridium in concentrations of up to 7 5 ppb a positive correlation between ir and se as well as enrichments in as sb and other volcanogenic elements has been observed the dust samples investigated here have very small grain size consistent with the distance from the source volcanoes the small grain size of the dust suggests that the ir enrichment is a surface effect and was probably caused by condensation from a vapor phase this is the first time that significant ir enrichments have been found in volcanic dust which demonstrates that ir and possibly also other platinum group metals may be present in some volcanic ash deposits although this has been doubted by the proponents of the pure impact theory for the k t boundary event the results of this study suggest that the ir in the k t boundary clays is not necessarily of cosmic origin but alternatively may have originated from mantle reservoirs that were tapped during extensive volcanic eruptions which may been triggered by impact events

air clathrate crystals from the grip deep ice core greenland a number size and shape distribution study we performed microscopic observations and a statistical study of the number size and shape distribution of clathrates in the grip greenland ice core project deep ice core using 185 samples from a depth range of 1016 3014m spanning a period of 6 to 110 ka sp and encompassing the holocene wisconsin and eemian periods the number concentration of the clathrates varied considerably with climatic changes it was possible to detect the rapid climatic oscillations in the last glacial between 13 and 110 ka bp the dansgaard oeschger cycles in the number concentration profile of clathrates the mean volume of clathrates is less clearly influenced by climatic factors with a tendency towards greater volumes in warmer periods but also a growth of clathrates with depth could be detected this growth rate was calculated to be 3 1 x 10 12 cm 3 a 1 the amount of gases captured in the clathrates is estimated to be significantly smaller than the total amount of air determined by gas concentration measurements this points to diffusion processes of atmospheric gases within the ice matrix

growth stabilization and reactivation of proterozoic lithosphere in the southwestern united states growth of proterozoic continental lithosphere in the southwestern united states involved assembly of tectonostratigraphic terranes during several pulses of convergent tectonism ca 1 74 1 70 and 1 65 1 60 ga prograde metamorphism accompanied orogenic assembly and peak metamorphic conditions outlasted deformation regions now characterized by the highest metamorphic grades underwent slow isobaric cooling and were not uplifted until more than 200 m y after assembly regions of low metamorphic grade were not uplifted substantially after assembly we suggest that 1 relatively thin lithospheric fragments were assembled into isostatically stable normal thickness continental lithosphere 2 assembly did not erase lithospheric scale heterogeneities 3 the present juxtaposition of different crustal levels reflects differential uplift related to 1 4 1 1 ga tectonomagmatic activity and 4 the boundaries between different lithospheric blocks were repeatedly reactivated from precambrian through tertiary time

imaging the mountainless root of the 1 8 ga cheyenne belt suture and clues to its tectonic stability receiver function analysis across the cheyenne belt an archean proterozoic suture images a 100 km wide zone of 50 60 km thick crust this thick crust is thought to be a remnant from the original 1 8 ga suturing event the thick and presumably buoyant crust is remarkable in that it is not associated with a large topographic or gravitational anomaly suggesting isostatic balance and a high density mass at depth to compensate the crustal root these features could account for the lithosphere s long term stability the presence of eclogite below the crustal root is a likely source for the high density mass as it would not produce a seismically unusual moho yet it would provide the required excess mass

mineralogy and phase chemistry of an ir enriched pre k t layer from the lattengebirge bavarian alps and significance for the ktb problem abstract three distinct iridium anomalies are exposed in the cretaceous tertiary boundary ktb section of the lattengebirge bavarian alps comprising a pelagic calcareous sequence from below a continental slope multiple ir anomalies are in contrast to most reported k t sections which are characterized by a single ir peak two of these ir spikes are contained in the 20 cm thick actual ktb termed clayey interval with the main peak of 4 4 ppb precisely at the boundary and the second peak of 2 8 ppb 10 cm above a third peak of 3 6 ppb was detected at 16 cm below the ktb in a thin turbiditic and coal bearing sandstone layer of upper maastrichtian age the distribution of ir anomalies provides evidence of episodic ir delivering events over an extended period of time whereas the worldwide distributed single spiked ktb layers are composed entirely of diagenetically altered materials various phases are discernible in the well preserved ir bearing pre k t layer 1 sandstone clastic minerals in a carbonate matrix 2 coal which is partly burned 3 sulfides chalcopyrite included in coal and 4 silicate melt glasses the glasses represent the first ones reported from a k t section and were formed by combustion metamorphism from sedimentary parent rocks of the coal as indicated by their major element chemistry and ree patterns phase chemistry of the layer components reveals ir concentrations of up to 110 ppb indicating that ir and chalcophile trace elements tre are primarily contained in the sulfides this carrier phase points to ir enrichment by processes unrelated to impact and its stratigraphic position demonstrates availability of ir for incorporation into sediments before the ktb the uptake of ir and chalcophile tre into the coal and subsequent combustion metamorphism occurred on the continent during a time interval of 14 000 9000 yr prior to the ktb as constrained by radiometric age determinations stratigraphy and sedimentation rates in the lattengebirge k t sediments no indications of shock are found the results suggest a volcanic origin for the multiple ir bearing horizons like the worldwide ktb layers they are concomitantly enriched in ir and chalcophile tre and hence have a common source the lattengebirge k t profile is largely continuous and was recently reexamined by micropaleontological methods contribution by d herm and k f weidich the preservation of multiple ir anomalies is attributed to high sedimentation rates of 1 75 cm kyr and the absence of carbonate dissolution and condensation of sediments usually encountered in the worldwide ktb layers

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

the discovery and initial characterization of allan hills 81005 the first lunar meteorite antarctic meteorite alha81005 discovered in the allan hills region of victoria land is a polymict anorthositic breccia which differs from other meteorites in mineralogical and chemical composition but is strikingly similar to lunar highlands soil breccias the petrologic character and several independent lines of evidence identify alha81005 as a meteorite from the moon two small clasts of probable mare basalt occur among the highlands lithologies in thin section 81005 22 this lunar specimen which shows relatively minor shock effects has generated new ideas on the types of planetary samples we may find on the earth and has added fresh excitement to the antarctic meteorite program

tectonic evolution of lower proterozoic rocks uinta mountains utah and colorado rocks of the early proterozoic orogenic system that fringes the archean aged wyoming province are exposed along the uinta fault in the northeastern uinta mountains of utah and colorado exposed here are the red creek quartzite an early proterozoic type miogeoclinal metasedimentary sequence more than 4 km thick and an underlying newly recognized archean gneiss complex more than 2 7 b y old during the hudsonian orogenic period the miogeoclinal sequence was emplaced northward over the archean complex by tectonic translation along a thick mylonite zone in the waning phases of upper amphibolite metamorphism the orogen was disrupted by east trending block faults with several kilometres displacement during initiation of the uinta aulacogen and was buried by more than 7 km of middle proterozoic sediments of the uinta mountain group the middle proterozoic block faults were reactivated with reversed sense of displacement during the laramide uplift of the uinta mountain block

kara and ust kara impact structures ussr and their relevance to the k t boundary event ample evidence exists for at least one major meteorite impact event at the time of the cretaceous tertiary k t boundary and it is therefore important to establish if any recognized terrestrial impact craters are k t in age the kara ussr impact structure consists of two adjacent large impact craters a rare and interesting geologic phenomenon and it has been suggested that this twin impact structure might be related to the k t boundary event however newly determined 40 ar 39 ar and k ar ages presented here suggest that these structures are slightly older than 70 ma and may thus be too old for a 66 ma k t boundary event still these two craters represent a substantial impact event that could have initiated regional if not global degradation of the biosphere their age suggests a possible relation with the campanian maastrichtian boundary

correlation of a 3 200 year old tephra in ice cores from vostok and south pole stations antarctica tephra layers in two antarctic ice cores are correlated on the basis of their chemical compositions and estimated ages we believe the tephra was produced about 3 200 years ago from a major explosive eruption of candlemas island in the south sandwich islands this is the first time that tephra layers have been correlated between two widely separated ice cores and demonstrates that tephra layers may serve as stratigraphic markers for correlating and perhaps in some cases dating antarctic ice cores

heat capacities and entropies of sillimanite fibrolite andalusite kyanite and quartz and the al2sio5 phase diagram low temperature heat capacities for sillimanite fibrolite and both fine grained and coarse grained quartz have been measured superambient heat capacities have been measured for four sillimanite two andalusite one kyanite and two fibrolite samples the following equations are recommended for the temperature dependence of the heat capacities of kyanite andalusite sillimanite and fibrolite

kinematic history of the cheyenne belt medicine bow mountains southeastern wyoming the proterozoic cheyenne belt marks the southern boundary of the archean wyoming province within the medicine bow mountains it consists of strongly deformed lithologically distinct blocks bounded by mylonite zones detailed field and petrographic investigations of the boundary augmented by study of microscopic kinematic indicators allow us to refine existing models for tectonic development of the belt northward thrusting under amphibolite facies conditions along low angle mylonite zones emplaced successively deeper crustal blocks over supracrustal rocks of the wyoming province mylonite zones were subsequently steepened and reactivated under greenschist facies conditions during a period of distributive dextral strike slip we suggest oblique convergence between an island arc and the wyoming craton as a possible mechanism for both deformational events accretionary events in eastern and western north america which involved initial convergence followed by strike slip provide a phanerozoic analog for our model

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

on the age vs depth and optical clarity of deep ice at south pole the first four strings of phototubes for the amanda high energy neutrino observatory are now frozen in place at a depth of 800 to 1000 m in ice at the south pole during the 1995 96 season an addit

the origin of ridge in groove slickenside striae and associated steps in an s c mylonite abstract a three dimensional study of an s c mylonite reveals that the s surfaces anastomose around porphyro clasts and c surfaces are not truly planar but curvi planar with the linear generator parallel to the movement direction it is suggested that it may be common for shear surfaces at all scales to take this form in the material studied surfaces exposed by parting primarily along c surfaces which are a product of ductile deformation and are penetrative have all the characteristics of classic slickensides on the slickensides striae of the ridge in groove type and steps of both congruous and incongruous types are developed the ridge in groove shape of the striae is directly related to the curvi planar shape of the c surfaces incongruous steps are smooth they are the result of parting along s surfaces congruous steps are rough first order congruous steps are formed by fracturing between individual c surfaces whereas second order congruous steps form by breaking off the tips of the dihedra between s and c surfaces thus the smoothness roughness technique can be used to determine the sense of shear in strongly deformed rocks where the angle between s and c surfaces is too small to be recognized in the field this may prove to be a useful shear sense indicator

calibration of laser light scattering measurements of dust concentration for wisconsinan gisp2 ice using instrumental neutron activation analysis of aluminum results and discussion in past work we used laser light scattering lls from bubble free ice to measure how the dust concentration varied along the wisconsinan glacial section of the greenland ice sheet project 2 gisp2 ice core from central greenland in this paper we show how we calibrated this dust profile using instrumental neutron activation analysis inaa of aluminum in the course of the calibration we found that excluding no3 all the major impurities soluble and insoluble present in the ice covaried linearly with respect to the dust concentration we find that the lls signal from ice below a depth of 2600 m follows a different linear trend than the lls signal from shallow ice we attribute this to the presence of numerous microbubbles diameter 0 3 mm that emerged in the deeper ice following core extraction

carbonyl sulfide hydrolysis in antarctic ice cores and an atmospheric history for the last 8000 years 2014 american geophysical union all rights reserved carbonyl sulfide cos was measured in antarctic ice core samples from the byrd siple dome taylor dome and west antarctic ice sheet divide sites covering the last 8000 years of the holocene cos levels decrease downcore in most of these ice cores the magnitude of the downcore trends varies among the different ice cores and is related to the thermal histories of the ice sheet at each site we hypothesize that this is due to the temperature dependent hydrolysis of cos that occurs in situ we use a one dimensional ice flow and heat flux model to infer temperature histories for the ice core samples from different sites and empirically determine the kinetic parameters for cos hydrolysis we estimate e folding lifetimes for cos hydrolysis ranging from 102 years to 106 years over a temperature range of 0 c to 50 c the reaction kinetics are used to estimate and correct for the in situ cos loss allowing us to reconstruct paleoatmospheric cos trends during the mid to late holocene the results suggest a slow long term increase in atmospheric cos that may have started as early as 5000 years ago given that the largest term in the cos budget is uptake by terrestrial plants this could indicate a decline in terrestrial productivity during the late holocene

mineralogic evidence for an impact event at the cretaceous tertiary boundary a thin claystone layer found in nonmarine rocks at the palynological cretaceous tertiary boundary in eastern montana contains an anomalously high value of iridium the nonclay fraction is mostly quartz with minor feldspar and some of these grains display planar features these planar features are related to specific crystallographic directions in the quartz lattice the shocked quartz grains also exhibit asterism and have lowered refractive indices all these mineralogical features are characteristic of shock metamorphism and are compelling evidence that the shocked grains are the product of a high velocity impact between a large extraterrestrial body and the earth the shocked minerals represent silicic target material injected into the stratosphere by the impact of the projectile

shocked quartz in the cretaceous tertiary boundary clays evidence for a global distribution shocked quartz grains displaying planar features were isolated from cretaceous tertiary boundary clays at five sites in europe a core from the north central pacific ocean and a site in new zealand at all of these sites the planar features in the shocked quartz can be indexed to rational crystallographic planes of the quartz lattice the grains display streaking indicative of shock in x ray diffraction photographs and also show reduced refractive indices these characteristic features of shocked quartz at several sites worldwide confirm that an impact event at the cretaceous tertiary boundary distributed ejecta products in an earth girdling dust cloud as postulated by the alvarez impact hypothesis

a new view of an old suture zone evidence for sinistral transpression in the cheyenne belt the type locality of the archean paleoproterozoic suture zone in the southern rocky mountains is marked by a series of subvertical shear zones collectively called the cheyenne belt the cheyenne belt is a key structure for developing models for 1780 1740 ma tectonism along the southern margin of the archean wyoming province which heralded a rapid period of continental amalgamation this paper tests existing structural and plate tectonic models for the cheyenne belt with detailed geologic mapping kinematic analyses quartz crystallographic fabric analyses and deformation mechanism analyses of the northern mylonite zone of the eastern medicine bow mountains mylonites of this zone record a complex deformation history but the main deformation phase was sinistral northwest side up oblique transpression evidence for southeast side up dip slip motion that characterizes many other areas of the belt is confined to ultramylonites immediately adjacent to the terrane boundary hence fabrics related to sinistral transpression were likely overprinted by southeast side up motion sinistral strike slip motion is recorded in at least two other localities in the cheyenne belt because synmetamorphic fabrics on both sides of the suture zone record sinistral strike slip and northwest side up motion this was probably the dominant deformation style in the field area and may have been the dominant deformation style throughout the cheyenne belt based on these data and regional constraints we interpret the cheyenne belt as a subvertical transpressional stretching fault system that simultaneously accommodated sinistral strike slip motion penetrative horizontal shortening and dip slip motion related to differential crustal thickening between the relatively cold wyoming province and younger hotter rocks to the south

a 35 myr record of helium in pelagic limestones from italy implications for interplanetary dust accretion from the early maastrichtian to the middle eocene we have determined the helium concentration and isotopic composition of a suite of early maastrichtian through middle eocene pelagic limestones in the italian apennines the results provide a 35 myr record of the implied flux of extraterrestrial 3he which is a proxy for the accretion rate of interplanetary dust particles idps our measurements show that the 3he flux was fairly constant in the maastrichtian except for possible minor increases factor of two or less from 70 5 ma to 68 ma and at 66 ma which probably reflect transient increases in the accretion rate of asteroidal and or cometary idps we find no evidence for an increase in idp accretion at or immediately before the k t boundary implying that the k t impact was not associated with enhanced solar system dustiness this observation precludes the possibility that the k t impactor was a member of a major comet shower and is more consistent with impact of a lone comet or asteroid our data suggest a 2 to 4 fold increase in idp accretion between 57 and 54 ma followed by a factor of three decrease over an 4 myr period in the early to middle eocene the duration and magnitude of this variability is inconsistent with previous observations attributed to a shower of long period comets and is more likely the result of collisions in the asteroid belt and or kuiper belt in the entire 35 myr record we find no evidence for major enhancements of the idp accretion rate of the type expected from comet showers our results in combination with earlier 3he measurements do not support models that predict recurrent comet showers with periods of 38 myrs if there is a periodicity in the cratering record that is caused by periodic modulation of the oort cloud it is not evident in the apennine sediment data along with the 3he measurements we also obtained 4he concentrations which record temporal changes in the flux or composition of terrigenous matter the most significant change in 4he occurs in the last 4 myrs of the cretaceous over which the concentration of 4he in the detrital component rises by 300 this rise tracks a strong increase in the seawater 87 sr 86 sr ratio suggesting a globally significant change in the composition of continental detritus delivered to the oceans possibly arising from increased continental weathering

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

misalignment of quartz c axis fabrics and lineations due to oblique final strain increments in the ruby mountains core complex nevada abstract to interpret c axis stereonet patterns of deformed quartz the patterns must be viewed in the xz strain plane which is usually assumed to contain the elongation lineation this study describes several c axis patterns from a region of complex amphibolite facies deformation in the northern ruby mountains where the late xz plane is oblique to most of the lineations without the lineation as a guide the proper orientation for interpretation of each pattern is determined by rotating the c axis data around the normal to the foliation until the c axes within the foliation plane lie at the center of the stereonet this new view is inferred to be the xz plane for the final increments of strain the rotation produces an internal symmetry in the patterns in addition the rotated views across the area show a more consistent geographic orientation than the lineations this geographic orientation is aligned with strain in a mylonitic shear zone which represents the latest macroscopic evidence for plastic strain along the edge of the study area the complete reorientation of the quartz textures during the final strain increments implies that they record very little of the finite strain history quartz is the only mineral recrystallized during the late strain thereby providing insight into the geometry of strain increments that cannot be determined from other minerals

comment on snowball earth a thin ice solution with flowing sea glaciers by david pollard and james f kasting 1 pollard and kasting 2005 hereinafter referred to as pk have coupled an energy balance climate model to an ice shelf flow model to investigate the snowball earth episodes of the neoproterozoic 600 800 million years ago when the ocean apparently froze all the way to the equator hoffman and schrag 2002 pk s particular concern was to investigate the possibility that over a wide equatorial band where sublimation exceeded snowfall the bare ice may have been thin enough to permit transmission of sunlight to the water below providing an extensive refugium for the photosynthetic eukaryotes that survived the snowball events this possibility was first proposed by mckay 2000 whose model of radiative transfer and heat conduction predicted tropical ice only a few meters thick for ice albedo up to 0 7 under assumed conditions of sunlight and temperature at the snowball equator however when spectral resolution was incorporated into the radiative transfer model warren et al 2002 the ice albedo had to be reduced below 0 4 to obtain the thin ice solution under otherwise identical conditions it seemed unlikely that such dark sea ice could avoid melting under the equatorial sun but even if it could avoid melting the thin ice would risk being crushed by the inflow of kilometer thick sea glaciers from higher latitudes goodman and pierrehumbert 2003 to further investigate the feasibility of tropical thin ice it was therefore necessary to couple a climate model to models of sea ice thermodynamics and sea glacier flow this is what pk have done surprisingly they conclude that thin ice 3 m thick may have prevailed in a 20 degree latitude band centered on the equator we argue here that this conclusion is too optimistic because pk s thin ice solution apparently required that several controlling variables be set outside their measured ranges the albedo of cold glacier ice the depth of transition from snow to ice and the thermal conductivity of ice we then raise the more general question of how wide is the thin ice domain in the parameter space of model variables 2 choices of model variables that favor thin ice 2 1 albedo of cold glacier ice 2 as sea glaciers flowed equatorward into the tropical region of net sublimation their surface snow and subsurface firn would sublimate away exposing bare glacier ice to the atmosphere and solar radiation this ice would be freshwater meteoric ice which originated from compression of snow so it would contain numerous bubbles giving a high albedo the albedo of cold nonmelting glacier ice exposed by sublimation antarctic blue ice has been measured as 0 55 0 65 in four experiments in the atlantic sector of antarctica bintanja and van den broeke 1995 bintanja et al 1997 liston et al 1999 reijmer et al 2001 0 63 in the transantarctic mountains warren et al 2002 and 0 66 near the coast of east antarctica weller 1968 weller s blue ice albedo is often quoted as 0 69 e g weller 1980 king and turner 1997 but that is an average value which included a few times with patchy snow cover on the ice not all glacier ice is bubbly under several hundred meters of ice thickness the pressure becomes so great that each air bubble dissolves in the ice to form a clathrate crystal and the ice becomes relatively clear price 1995 however when this ice becomes depressurized as its cover sublimates away as in the antarctic blue ice areas where the albedos were measured the bubbles reform lipenkov 2000 so that the albedo of 0 55 0 66 is observed at the surface 3 in contrast to glacier ice which forms by compression of snow sea ice forms by freezing of liquid water in their model pk tried two values of scattering coefficient each of which they applied uniformly to both sea ice and glacier ice with albedo 0 47 a reasonable value for meter thick snowfree sea ice brandt et al 2005 figure 1 the sea glaciers terminated at 9 degrees latitude leaving an equatorial strip of thin ice but with albedo 0 64 i e within the measured range for glacier ice the sea glaciers advanced to the equator with a thickness of 1 km to add realism to the model it would be good to assign different values of scattering coefficient to these two very different types of ice

age vs depth of glacial ice at south pole knowledge of age as a function of depth in glacial ice is important for both glaciology and paleoclimatology for sites near a ridge or dome an ice flow model together with information on accumulation rate provides a first approximation if the accumulation rate is high enough annual layering of isotopes and dust measured in a solid core can provide a precise age vs depth relationship for south pole the flow geometry is not simple and no deep core exists nevertheless by remotely sensing peaks in scattering and absorption of light from pulsed sources buried at depths down to 2200 m we have been able to determine age vs depth for ages up to 65 000 years analysis of radar isochrons by siegert and hodgkins provides a rough extension of the age vs depth model to 165 000 years near bedrock

kinetics of conversion of air bubbles to air hydrate crystals in antarctic ice the depth dependence of bubble concentration at pressures above the transition to the air hydrate phase and the optical scattering length due to bubbles in deep ice at the south pole are modeled with diffusion growth data from the laboratory taking into account the dependence of age and temperature on depth in the ice the model fits the available data on bubbles in cores from vostok and byrd and on scattering length in deep ice at the south pole it explains why bubbles and air hydrate crystals coexist in deep ice over a range of depths as great as 800 meters and predicts that at depths below 1400 meters the amanda neutrino observatory at the south pole will operate unimpaired by light scattering from bubbles

new models for paleoproterozoic orogenesis in the cheyenne belt region evidence from the geology and u pb geochronology of the big creek gneiss southeastern wyoming the disputed age of the deep crust of the colorado province is central to hypotheses for paleoproterozoic crustal growth in the region we studied the high grade big creek gneiss southeastern wyoming as a potential exposure of pre 1780 ma basement rocks new geologic mapping and u pb geochronological data indicate that the big creek gneiss exposes a deeper but coeval level of the green mountain arc relative to the predominantly supracrustal section to the west the big creek gneiss is composed of supracrustal rocks a ca 1780 ma green mountain arc correlative bimodal intrusive suite a ca 1763 ma extensional bimodal intrusive suite and widespread ca 1630 ma pegmatitic leucogranite the mafic member of the younger bimodal suite is documented here for the first time u pb zircon ages from migmatite leucosomes indicate penetrative deformation of the big creek gneiss at ca 1750 ma we find that the postarc intrusive suite is mantle involved implying a second period of crustal growth shortening postdates arc magmatism by 20 m y implying that termination of arc magmatism and accretion were separate events finally criteria previously used to constrain the polarity of subduction for the green mountain arc are not reliable we propose two competing models 1 southward dipping green mountain arc subduction present coordinates with slab breakoff related magmatism following arc accretion or 2 northward dipping subduction with extensional postarc magmatism in both models high temperature deformation coincides with accretion along the cheyenne belt and extensional magmatism is an important component of crustal growth we prefer the northward dipping subduction model because it can be better integrated with regional tectonic events and published isotopic compositions of the igneous rocks

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

evidence for an inverted metamorphic gradient associated with a precambrian suture southern wyoming an inverted metamorphic gradient associated with the northern mylonite zone of the cheyenne belt a deeply eroded precambrian suture in southern wyoming has been documented within metasedimentary rocks of the early proterozoic snowy pass supergroup metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite garnet and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone a minimum temperature increase of approximately 100 c over a km wide zone is required for this transition parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the snowy pass supergroup field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite grade rocks over rocks of the snowy pass supergroup along the northern mylonite zone mineral equilibria and garnet biotite geothermometry on synkinematic mineral assemblages within the snowy pass metasedimentary rocks indicate deformation at minimum temperatures of 480 c and pressures of 350 400 mpa 3 5 4 0 kbar this implies tectonic burial or upper plate thickness of 13 15 km the narrow character of metamorphic zonation and microtextures within the snowy pass supergroup which indicate late synkine matic growth of garnet and staurolite preclude rotation of pre existing isograds by folding as a mechanism for development of the inverted gradient conductive transport of heat from the upper into the lower plate across the originally low angle thrust is insufficient to produce the necessary temperatures in the lower plate shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated up dip advection of metamorphic fluids is a feasible but unproven mechanism for heat transport the possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

gases in ice cores air trapped in glacial ice offers a means of reconstructing variations in the concentrations of atmospheric gases over time scales ranging from anthropogenic last 200 yr to glacial interglacial hundreds of thousands of years in this paper we review the glaciological processes by which air is trapped in the ice and discuss processes that fractionate gases in ice cores relative to the contemporaneous atmosphere we then summarize concentration time records for co2 and ch4 over the last 200 yr finally we summarize concentration time records for co2 and ch4 during the last two glacial interglacial cycles and their relation to records of global climate change

vorticity analysis in a metachert from the sanbagawa belt sw japan abstract common tectonic features of deformed metachert include both sets of extended and shortened veins and quartz c axis preferred orientation patterns in such rocks information is also commonly available on the orientation and magnitude of the finite strain these features can be used to derive two independent estimates of the degree of non coaxiality during deformation in an example from the sanbagawa belt sw japan the two methods give consistent results and show that deformation was intermediate between simple and pure shear such types of progressive deformation distributed over a broad region could cause substantial subvertical thinning and be a significant factor in the exhumation of high p t rocks in the region

does the arc accretion model adequately explain the paleoproterozoic evolution of southern laurentia an expanded interpretation a common model for the paleoproterozoic crustal evolution of southern laurentia involves southerly accretion of successively younger island arcs from 1780 to 1650 ma this model may be oversimplified however for although andesite and basaltic andesite major components of modern island arcs occur they are relatively rare and large volumes of rocks are dominantly bimodal metavolcanic assemblages and related granitoids ophiolites occur but they are also rare and only one accretionary melange has been described inherited zircons and common pb and nd isotopic data indicate involvement of older crust in ca 1750 ma rocks of the mojave province of southeastern california and western arizona and the bimodal assemblages of central colorado these data suggest that the bimodal volcanic rocks were derived from pre existing continental crust likely of ca 1850 ma trans hudson penokean age during extension and partial melting associated with the development of transpressional crustal scale shear zones the occurrence of the 1840 1 ma elves chasm pluton in the grand canyon demonstrates that rocks of trans hudson penokean age are present in southwestern laurentia

relationships between strain and quartz crystallographic fabrics in the roche maurice quartzites of plougastel western brittany abstract an integrated microstructural and petrofabric study of the plastically deformed and partially recrystallized roche maurice quartzites of plougastel western brittany has revealed a clear correlation between the pattern of c axis fabrics displayed by detrital quartz grains and the symmetry of the calculated strain ellipsoid in specimens with flattening k 0 strains c axes lie on a small circle girdle opening angle 28 42 centred about the principal finite shortening direction z for specimens that exhibit approximate plane strain k 1 cross girdle c axis fabrics consisting of a small circle girdle centred about z and connected through the intermediate principal extension direction y were detected within individual specimens c axis fabrics of syntectonically recrystallized new quartz grains within the matrix are similar to those of detrital quartz grains c axes of new grains located within the relatively undeformed sections of the host detrital grains are commonly orientated at angles between 10 and 40 to the host c axis and are in addition statistically orientated at a higher angle to z than their host c axes these relationships are interpreted as indicating that both host grain control and the local strain and or stress field may have influenced the process of recrystallization the relative influence of these factors is however unknown microstructural and petrofabric studies indicate that the roche maurice quartzites have been subjected to essentially coaxial strain histories the role of syntectonic recrystallization in facilitating continued plastic deformation in quartzites subjected to such strain histories is considered

filling the north american proterozoic tectonic gap 1 60 1 59 ga deformation and orogenesis in southern wyoming usa abstract structural and thermochronologic studies in the sierra madre southern wyoming indicate that parts of the 1 78 1 75 ga cheyenne belt an archean proterozoic suture were reactivated at ca 1 6 ga new shear zones notably the brittle battle lake thrust tear fault zone also formed at this time the battle lake fault zone blfz which displaced parts of the cheyenne belt at least 30 km to the north corresponds to the surface expression of a south dipping reflector imaged by regional seismic studies continental dynamics of the rocky mountains experiment 40ar 39ar and u pb data define age patterns in the sierra madre that are delineated by discrete structures north of the 1 78 ga divide peak mylonite zone a foreland structure associated with the medicine bow orogeny 40ar 39ar biotite dates suggest cooling below 300 c by 2 0 ga south of the divide peak mylonite zone but north of the blfz hornblendes yield apparent 40ar 39ar ages of ca 1 80 1 78 ga that may record tectonic burial and cooli

effects of bubbles cracks and volcanic tephra on the spectral albedo of bare ice near the transantarctic mountains implications for sea glaciers on snowball earth 1 spectral albedo was measured along a 6 km transect near the allan hills in east antarctica the transect traversed the sequence from new snow through old snow firn and white ice to blue ice showing a systematic progression of decreasing albedo at all wavelengths as well as decreasing specific surface area ssa and increasing density broadband albedos under clear sky range from 0 80 for snow to 0 57 for blue ice and from 0 87 to 0 65 under cloud both air bubbles and cracks scatter sunlight their contributions to ssa were determined by microcomputed tomography on core samples of the ice although albedo is governed primarily by the ssa and secondarily by the shape of bubbles or snow grains albedo also correlates highly with porosity which as a proxy variable would be easier for ice sheet models to predict than bubble sizes albedo parameterizations are therefore developed as a function of density for three broad wavelength bands commonly used in general circulation models visible near infrared and total solar relevance to snowball earth events derives from the likelihood that sublimation of equatorward flowing sea glaciers during those events progressively exposed the same sequence of surface materials that we measured at allan hills with our short 6 km transect representing a transect across many degrees of latitude on the snowball ocean at the equator of snowball earth climate models predict thick ice or thin ice or open water depending largely on their albedo parameterizations our measured albedos appear to be within the range that favors ice hundreds of meters thick

volcanic ash layers in bare ice areas near the yamato mountains dronning maud land and the allan hills victoria land antarctica

air bubbles and clathrate hydrates in the transition zone of the ngrip deep ice core abstract field studies of air bubble and clathrate inclusions in the ngrip ice core reveal two stages of clathrate formation in the transition zone translucent primary clathrates with rough surfaces and irregular shapes formed from individual bubbles split up and form smaller specimens deeper down in the transition zone 1100 m clathrate metamorphosis sets in giving rise to the formation of transparent secondary clathrates with smooth and regular often poly hedral or faceted shapes rod like crystals of great length 2 mm often show signs of separation into smaller units that fragment into smaller specimens these findings imply that the simplified picture of one bubble forming one clathrate has to be modified

geochemistry of impact glasses from the k t boundary in haiti relation to smectites and a new type of glass we have individually analyzed twelve black and seven yellow glasses from the k t boundary section at beloc haiti plus three smectite mantles around black glasses the main chemical differences between black and yellow glasses are higher s ca mg zn as br sb and au but lower k na and si abundances in the yellow glasses we have found high cao yellow glasses with low s contents 0 08 0 17 wt which may be explained by fusion of carbonate rich sediments microprobe studies of individual glasses show that the black glasses are very homogeneous in their composition at the 10 100 m level while yellow glasses show much more variability and different intra sample elemental correlations one of the black glasses has higher sio2 and k2o abundances but lower concentrations of all other major elements this suggests the existence of a third glass type which we have named the high si k variety hsi k glass the glass shows areas consisting of pure sio2 maybe lechatelierite and some schlieren with lower sio2 content the chemical composition indicates that the hsi k glass may have originated from melting of a sedimentary source rock such as shale or argillite severe changes in major and trace element composition occur during alteration of the glass as documented by analyses of adhering smectites the ree patterns in the glass are similar to average upper crust but the smectites exhibit much lower abundances and flat patterns not unlike the patterns observed in claystones at numerous k t boundary sites the suggestion that the chicxulub structure in yucatan may be the source crater for the k t impact glasses is not fully supported by the currently available data impact glasses and tektites are produced from the surface layers of their targets at chicxulub the target stratigraphy comprised mainly carbonates and evaporites although we cannot exclude chicxulub as a source the present data do not provide any firm chemical evidence for a connection we conclude however that the haiti glasses have formed by impact and have later undergone alteration by low temperature hydrothermal processes to produce clays the boundary claystones at k t boundary sites may very well be alteration products of these impact glasses

testing models for obliquely plunging lineations in transpression a natural example and theoretical discussion theory predicts that stretching lineations in an ideal vertical transpressional zone should be either vertical or horizontal many field descriptions of transpressional zones however indicate a range of lineation orientations between these extremes several theoretical models have been developed to explain such departures from expected lineation orientation and we discuss these in the context of a field example from the archean superior province in the north american craton existing models are insufficient to explain obliquely plunging lineations in this example because 1 obliquely plunging lineations cannot be accounted for by shear zone boundary effects imposed by a no slip condition 2 foliations and lineations vary independently 3 the vorticity normal section is subhorizontal limiting possibilities for inclined simple shear 4 high vorticity is needed for finite strains and lineations to match previously proposed triclinic models but vorticity is relatively low and 5 juxtaposed east and west plunging lineations are unlikely in the previously proposed triclinic models because existing theoretical models are not applicable to our field example we contemplate a new model to explain obliquely plunging lineations within quasi homogeneous transpression

deformation path partitioning within the transpressional white mountain shear zone california and nevada most numerical simulations of transpression zones predict a change in finite stretching direction from subhorizontal to subvertical for simple shear dominated zones we provide a detailed description of a dextral transpression zone the white mountain shear zone wmsz with a range of lineation orientations and compare these natural data to numerical models the wmsz is characterized by steeply dipping foliations with dominant shallowly plunging lineations and coeval subordinate domains of steeply plunging lineations within shallowly lineated domains foliation geometry shear sense indicators and quartz c axis fabrics indicate a large component of simple shear while microstructural and quartz c axis fabric data from steeply lineated domains indicate a large component of pure shear geometric relationships between foliations and lineations and quartz c axis fabrics demonstrate that lineation orientation has remained constant during much of the deformation history comparison of numerical models with the data collected from wmsz shows that the shear zone geometry and the observed strain path partitioning do not match any of these models we propose a conceptual kinematic model for the wmsz involving stable segregated coeval kinematic domains of simple shear dominated fabrics and pure shear dominated fabrics that accommodate the transcurrent and contractional components of deformation respectively

identification of 2 0 to 2 4 ga nd model age crustal material in the cheyenne belt southeastern wyoming implications for proterozoic accretionary tectonics at the southern margin of the wyoming craton a detailed survey was conducted of the nd isotope compositions of precambrian metamorphic rocks both within and adjacent to the cheyenne belt a metamorphosed crustal segment in southeastern wyoming that defines the proterozoic suture zone between the archean wyoming craton and accreted proterozoic island arc s the results of the study indicate that proterozoic miogeoclinal sedimentary rocks exposed north of the cheyenne belt and deposited at the southern margin of the wyoming craton contain only archean crust derived material and no detritus from proterozoic island arcs approaching from the south similarly proterozoic crust exposed south of the cheyenne belt contains no crustal material derived from archean sources however within the cheyenne belt paragneiss of the barber lake block has 2 0 to 2 4 ga nd model ages and is interpreted as metamorphosed intermixed sedimentary detritus from both archean and proterozoic sources the paragneiss is thought to represent remnants of oceanic trench or foredeep sediments deposited in close proximity to both the archean craton and the proterozoic island arcs similar to sedimentary rocks with 2 0 to 2 4 ga nd model ages exposed in the penokean orogen in wisconsin the overall lack of 2 0 to 2 4 ga modal age crustal material in the vicinity of the cheyenne belt may reflect the fact that much of this crustal material was overthrust onto the wyoming craton during accretion and then eroded during postorogenic rebound of the continental margin

high strain zones a unified model abstract where a shear zone is bounded by undeformed wall rocks and there is no volume change the deformation path within the zone on the bulk scale approximates a simple shear where the zone boundaries are stretched and the shear direction between the two boundaries is parallel to one of the principal stretching directions of the boundaries the deformation path within the zone is not a simple shear but still has monoclinic symmetry both simple shear and general monoclinic deformation paths should be regarded as special cases in nature because generally not only are the zone boundaries stretched but also the shear is oblique to the principal stretching directions of the boundaries in this general case the deformation path is triclinic we establish a model which includes this general case also taking into consideration volume change five independent parameters are required to characterize the rate of deformation of the flow four are sufficient to characterize the flow kinematics the model is generally triclinic and orthorhombic and monoclinic deformations are subgroups by varying the values of the characterizing parameters the various types of high strain zones can be represented the kinematics of flow within such zones and the accumulated finite deformation geometry are investigated many field observations such as for example lineations in transcurrent shear zones that vary between vertical and horizontal cleavage transected folds and the absence of sheath folds in some shear zones where folds have been significantly rotated can sometimes be better interpreted in the context of our general model

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known

thick skinned deformation of the archean wyoming province during proterozoic arc continent collision two major mylonite zones are exposed in the southeastern archean wyoming province the cheyenne belt which marks the boundary between the wyoming and colorado provinces and the laramie peak shear zone across which archean rocks have been uplifted differentially at least 10 km between the two mylonite zones is a 60 100 km wide belt of archean migmatitic gneiss which has been intruded by diabase dikes and peridotites and contains enclaves of high grade metasedimentary rocks we interpret this belt of high grade rocks as a tectonically reactivated block of archean crust uplifted during the development of the cheyenne belt collisional zone at ca 1 8 ga this block does not exhibit a decreasing metamorphic gradient away from the mylonite zone but is at uniformly high grade thus rather than a thick skinned thrust ramp model for uplift we invoke vertical uplift along a high angle reverse fault without rotation this style of uplift may be a tectonic response to transpression underplating or tectonic interfingering at depth during collisional orogeny and it may be a more common feature of basement uplifts within foreland fold and thrust belts than has been previously recognized

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

catch a falling star meteorites and old ice a model for the process of meteorite concentration in blue ice regions of the antarctic ice sheet is proposed based on data from near the allan hills and the assumptions that both meteorite influx and glacial flow have been constant the meteorite influx is calculated to be 60 x 10 6 kilogram per square kilometer per year and the age of the exposed ice to be 0 to 600 000 years varying with distance from the allan hills these results are in line with other estimates of influx rate and with measurements of the terrestrial ages of the meteorites providing support for the assumption of steady flow and meteorite influx this may be the oldest sequence of ice in stratigraphic order yet discovered and the results imply that this part of the east antarctic ice sheet has been approximately steady during this time interval

petrogenesis of the 1 43 ga sherman batholith se wyoming usa a reduced rapakivi type anorogenic granite the 1 43 ga sherman batholith southeastern wyoming usa origin that are found throughout the proterozoic anorogenic granite provinces of the southwestern usa adirshows extreme a type petrochemical characteristics compared with other mid proterozoic granite batholiths of north america it ondacks eastern canada southern greenland and the fennoscandian shield anderson 1983 phanerozoic consists of 1 the sherman granite a coarse grained biotite hornblende granite that locally contains fayalite and pyroxenes 2 examples include paleozoic granites of southeastern australia collins et al 1982 and of corsica poitrasson et the lincoln granite a medium grained biotite granite 3 a porphyritic biotite hornblende granite that probably formed by interaction al 1995 and the mesozoic nigerian younger granites jacobsen et al 1958 and the white mountain magma of granitic and mafic magmas and 4 iron enriched mafic dikes and pods the ilmenite series metaluminous sherman granite series foland allen 1991 anorogenic granite refers to plutons that are not associated with compressional exhibits extreme values of feo feo mgo and is rich in structures and that were emplaced long after any known k ree nb and y it crystallized at temperatures exceeding orogenic event anorogenic or a type granites loiselle 900 c and a pressure of 2 5 kbar with water activity of 0 7 wones 1979 are characterized by low h2o and o2 and dlog fo2 of 0 1 to 0 5 the lincoln granite which is fugacities along with high feo feo mgo k2o peraluminous and has less extreme a type geochemical characteristics na2o and k2o contents a type granites typically are rich crystallized at temperatures as low as 750 c and dlog fo2 of in incompatible elements including rare earth elements around 0 5 units above fmq fayalite magnetite quartz the ree zr nb and ta but poor in co sc cr ni ba rocks of the sherman batholith are chemically equivalent to lavas sr and eu minimum magma liquidus temperatures are from the yellowstone hotspot like the yellowstone magmas the 900 1000 c clemens et al 1986 creaser white sherman batholith probably originated by partial melting of under1991 some workers suggest these rocks are derived plated mantle derived mafic rocks from melting of tonalitic or more felsic crust anderson cullers 1978 collins et al 1982 clemens et al 1986 creaser et al 1991 whereas others relate the granites

the chicxulub impact event and its environmental consequences at the cretaceous tertiary boundary abstract an impact mass extinction hypothesis for the cretaceous tertiary k t boundary transition has been confirmed with multiple lines of evidence beginning with the discovery of impact derived ir in k t boundary sediments and culminating in the discovery of the chicxulub impact crater likewise a link between the chicxulub impact crater and k t boundary sediments has been confirmed with multiple lines of evidence including stratigraphic petrological geochemical and isotopic data the environmental effects of the chicxulub impact event were global in their extent largely because of the interaction of ejected impact debris with the atmosphere the environmental consequences of the chicxulub impact event and their association with the k t boundary mass extinction event indicate that impact cratering processes can affect both the geologic and biologic evolution of our planet