empirical and theoretical comparisons of the chicxulub and sudbury impact structures chicxulub and sudbury are 2 of the largest impact structures on earth research at the buried but well preserved chicxulub crater in mexico has identified 6 concentric structural rings in an analysis of the preserved structural elements in the eroded and tectonically deformed sudbury structure in canada we identified ring like structures corresponding in both radius and nature to 5 out of the 6 rings at chicxulub at sudbury the inner topographic peak ring is missing which if it existed has been eroded reconstructions of the transient cavities for each crater produce the same range of possible diameters 80110 km the close correspondence of structural elements between chicxulub and sudbury suggests that these 2 impact structures are approximately the same size both having a main structural basin diameter of 150 km and outer ring diameters of 200 km and 260 km this similarity in size and structure allows us to combine information from the 2 structures to assess the production of shock melt melt produced directly upon decompression from high pressure impact and impact melt shock melt and melt derived from the digestion of entrained clasts and erosion of the crater wall in large impacts our empirical comparisons suggest that sudbury has 70 more impact melt than does chicxulub 31 000 versus 18 000 km3 and 85 more shock melt 27 000 km3 versus 14 500 km3 to examine possible causes for this difference we develop an empirical method for estimating the amount of shock melt at each crater and then model the formation of shock melt in both comet and asteroid impacts we use an analytical model that gives energy scaling of shock melt production in close agreement with more computationally intense numerical models the results demonstrate that the differences in melt volumes can be readily explained if chicxulub was an asteroid impact and sudbury was a comet impact the estimated 70 difference in melt volumes can be explained by crater size differences only if the extremes in the possible range of melt volumes and crater sizes are invoked preheating of the target rocks at sudbury by the penokean orogeny cannot explain the excess melt at sudbury the majority of which resides in the suevite the greater amount of suevite at sudbury compared to chicxulub may be due to the dispersal of shock melt by cometary volatiles at sudbury

preserved flora and organics in impact melt breccias impact cratering can destroy life from local to global scales and result in sudden turnovers of dominant genera and or species here we report that it can also preserve components of the local biology present at the time of impact we have investigated floral matter encapsulated within cenozoic era impact glasses produced by separate bolide impacts into the loessoid sediments of argentina that occurred between 9 2 ma miocene and 6 ka holocene the encapsulation preserved not only macro scale morphological biosignatures such as vascular bundles veins phytoliths and papillae but also structures down to the cellular level in the best preserved samples we also found evidence for organic matter while fossilization typically occurs over an extended time period as minerals slowly replace organic matter and the host rock lithifies under pressure the process documented here is instantaneous preservation of morphological and chemical biosignatures in impact events can provide snapshots of the ecology in environments that do not otherwise promote a diverse fossil record we suggest that this would provide a new strategy for identifying signs of possible early life on ancient mars where similar target conditions once existed

an inverse modeling study in fram strait part i dynamics and circulation abstract in order to reconstruct the circulation in the northern greenland sea between 77 n and 81 n and the exchanges with the arctic ocean through fram strait a variational inverse model is applied to the density field observed in summer 1984 during the mizex 84 experiment an estimate of the three dimensional large scale pressure field is obtained in which the solution is decomposed into a limited number of vertical modes and the mode amplitudes are described by piece wise polynomials on a finite element grid the solution should be consistent with a frictional depth integrated vorticity balance and with the density data the global model parameters are tuned to ensure agreement between the retrieved geostrophic velocity and independent currentmeter data in a companion paper schlichtholz and houssais 1999b the same method but without dynamical constraint is applied to the same hydrographic dataset to perform a detailed water mass analysis and to estimate individual water mass transports a comprehensive picture of the summer geostrophic circulation in fram strait is obtained in which northward recirculations in the east greenland current egc and various recirculations from the west spitsbergen current wsc to the egc are identified it is suggested that the branch of the wsc following the upper western slope of the yermak plateau turns westward beyond 81 n and recirculates southward along the lower slope then merging with a westward recirculating branch south of 79 n at 79 n a southward net transport of 6 5 sv is found in the egc which combined with a northward net transport of only 1 5 sv in the wsc results in a fairly large outflow of 5 sv from the arctic ocean to the greenland sea the inverse solutions show that in summer the local induction of vorticity by the wind stress curl or by meridional advection of planetary vorticity should be small so that in the egc and in the wsc the vorticity balance is mainly achieved between the bottom pressure torque and dissipation of vorticity through bottom friction a substantial barotropic flow associated with along slope potential energy gradients is indeed identified on both sides of the strait

constraints on central uplift structure from the manicouagan impact crater recent drilling operations at the 90 km diameter late triassic manicouagan impact crater of quebec canada have provided new insight into the internal structure of a complex craters central region previous work had indicated that the impact event generated a 55 km diameter sheet of molten rock of relatively consistent originally 400 m thickness floran et al 1978 the drilling data reveals melt sheet thicknesses of up to 1500 m with kilometer scale lateral and substantial vertical variations in the geometry of the crater floor beneath the melt sheet the thickest melt section occurs in a 1500 m deep central trough encircled by a horseshoe shaped uplift of precambrian basement the uplift constitutes a modified central peak structure at least part of which breached the melt sheet mineralogical and compositional segregation differentiation of the thicker melt sheet section coupled with a lack of fractionation in the thinner units shows that the footwall geometry and associated trough structure were in place prior to melt sheet solidification marked lateral changes in sub melt sheet basement relief support the existence of a castellated footwall that was created by high angle impact related offsets of 100s to 1000s of meters this indicates that deformation during the modification stage of the cratering process was primarily facilitated by large displacement fault systems this work suggests that manicouagan is a central peak basin with rings which does not appear to fit with current complex crater classification schemes

introduction summer marginal ice zone experiments during 1983 and 1984 in fram strait and the greenland sea sphere is complicated by numerous land masses which largely confine it during the winter to the sea of okhotsk and to the bering labrador and greenland norwegian seas during the spring and summer the miz retreats from the peripheral seas generally the total extent of the northern miz is less than one half that of the southern numerous studies have demonstrated the sensitivity of weather and climate to the state of the polar ice cover and it is of fundamental importance to understand how annual and interannual variations in ice extent are related to large scale and mesoscale circulations in the atmosphere and ocean successful modeling and parameterization of processes which govern the location and state of the miz are important not only for predicting the impact of sea ice on global climate but also for furthering man s activities in the miz regions e g offshore oil exploration seaborne transport of arctic resources development of the rich fisheries close to the ice margin and naval operations in recent years realization of the scientific and economic significance of the miz has focused attention on the need for coordinated multidisciplinary studies of conditions and processes near ice open ocean boundaries and has led to the development of the marginal ice zone experiment mizex although of substantial interest the remoteness of the vast antarctic miz poses serious logistic difficulties and mizex efforts to date have concentrated on the more accessible northern miz considerable work has been carried out in the

vitric compositions in the onaping formation and their relationship to the sudbury igneous complex sudbury structure the 1 85 ga sudbury structure is the largest well preserved exposed example of a terrestrial impact crater it contains world class ni cu platinum group element pge ores associated with the igneous component a new understanding of the geology and geochemistry of the vitric crater fill sequence the onaping formation provides constraints on the evolution of 1 85 ga igneous rocks in the sudbury structure the onaping formation has a well defined mappable stratigraphy composed of three members seven stratigraphic units and two types of intraformational dikes that are overprinted by an impact generated hydrothermal system stratigraphic and alteration mapping provided a basis for well constrained sampling and allowed the definition of the geochemical characteristics of the less altered rocks from each unit including vitric bombs and blocks and intraformational vitric dikes significantly the identification of a least altered composition of the vitric onaping formation enabled an investigation of the link between the early formed vitric melt phase in the impact crater fill sequence and the sudbury igneous complex and offset dikes the least altered vitric onaping formation represented by the massive xenolith poor cores of vitric bombs blocks and intraformational dikes is andesitic in composition consistent with a crustally derived melt the melt composition 61 6 wt sio 2 4 28 mgo 60 ppm ni is remarkably uniform throughout the onaping formation with a typical enrichment of large ion lithophile and light rare earth elements and pronounced negative nb and ti anomalies incompatible trace element ratios and bulk concentrations of the onaping formation coincide with the main mass norite and offset quartz diorite compositions of the underlying sudbury igneous complex but differ distinctly from the granophyre and quartz gabbro root mean squared differences in composition of the vitric onaping formation with the units of the sudbury igneous complex and quartz diorite offset dikes show that the vitric onaping formation does not directly match any of these igneous units but more closely resembles the composition of the ministic and manchester offset dikes the quartz diorite offset dikes have been proposed as representative of the bulk composition of the original melt of the sudbury igneous complex in detail there are small compositional differences between north and south range offset dikes and between these offset dikes and the concentric manchester offset dike however such differences are lacking in the onaping formation we propose that the least altered vitric composition of the onaping formation represents the best estimate of the bulk composition of an initial quenched melt or shock melt formed early in the evolution of the sudbury structure and is a logical starting liquid to model the evolution of the sudbury main mass and offset dikes the offset dike compositions can be derived from onaping formation vitric shock melt by assimilation of progressively greater amounts of mafic material at deeper levels in the crater this mafic contaminated melt or impact melt developed at the base of the melt sheet and was injected into the offset dikes both prior to and contemporaneously with the earliest stages of sulfide precipitation from the sudbury complex fractionation of the mafic contaminated melt by plagioclase and pyroxene accumulation produced the north range felsic and mafic norite and the south range norites the chalcophile metal content of the onaping formation vitric phase closely reflects the overall metal ratios of the magmatic sulfide ores the vitric onaping formation and unmineralized offset dikes have low liquidus temperatures 1 115 1 125 c and if these units represent the source of metals in the sulfide deposits unusually high mass distribution coefficients for cu and ni and high r factors are necessary to account for the metal endowment

acraman impact ejecta and host shales evidence for low temperature mobilization of iridium and other platinoids geochemical investigations of a widely dispersed late proterozoic impact ejecta horizon and its host shales bunyeroo formation australia have provided strong evidence for low temperature mobilization of the platinum group elements pge including ir pge anomalies are associated with both the ejecta horizon ir up to 2 0 ppb pt up to 270 ppb and the green shales that envelop it several pge anomalies 0 073 0 45 ppb ir 3 11 314 ppb pt also occur in thin green shale beds at other stratigraphic levels within a predominantly red shale sequence average red shale background of 0 019 ppb ir and 0 89 ppb pt all green shale horizons analyzed regardless of their stratigraphic position have relatively high levels of ir and other pge the nonextraterrestrial postdepositional origin of the pge at other stratigraphic levels away from the impact ejecta horizon is indicated by their occurrence in thin permeable green beds in a predominantly red shale sequence their association with enrichments in cu v zn ni and their nonchondritic pge interelement ratios the impact ejecta horizon has a more chondritic pge geochemistry consistent with a meteoritic origin a redox precipitation model similar to that invoked for redbed cu u v deposits is proposed to explain the pge anomalies in the green shales

on the hydrography of the northeast water polynya the water masses and circulation in the area of the northeast water polynya located on the east greenland shelf north of 79 n are described on the basis of an r v polarstern cruise during spring and summer 1993 the baroclinic flow shows northward components close to the east greenland coast and eastward components at the northern limit of the polynya an anticyclonic half circle is formed by this and the southward flowing east greenland current in the south the circle is not closed the upper water column occupied by polar water is affected by this circulation pattern while deeper waters in the trough system of the area seem to spread independently different types of deep waters are found in different troughs all being of atlantic origin though they seem not to be directly connected to return atlantic water it is shown that what is called polar water must be formed at least partly on the greenland shelf and that deepwater formation does not occur in the investigated area

thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from molten fuel coolant interaction experiments 1 thermohydraulic explosions were produced by molten fuel coolant interaction mfci experiments using remelted shoshonitic rocks from vulcano italy the fragmentation history and energy release were recorded the resulting products were recovered and analyzed with the scanning electron microscope fine particles from experiments show shape and surface features that result from melt fragmentation in brittle mode these clasts relate to the thermohydraulic phase of the mfci where most of the mechanical energy is released they are here called active particles the total surface area of such particles is proportional to the energy of the respective explosions other particles from experiments show shape and surface features that result from melt fragmentation in a ductile regime these fragments called passive particles form after the thermohydraulic phase during the expansion phase of the mfci in order to verify thermohydraulic explosions in volcanic eruptions we compared experimental products with samples from phreatomagmatic base surge deposits of vulcano ash particles from the experiments show features similar to those from the deposits suggesting that the experiments reproduced the same fragmentation dynamics to achieve discrimination between active and passive particles we calculated shape parameters from image analysis the mass of active particles in base surge deposits was calculated as the material properties for the natural samples are identical to the experimental ones the energy measurements and calculations of the experiments can be applied for a single phreatomagmatic eruption at vulcano a maximum mechanical energy release of 2 75 1013 j was calculated representing a tnt analogue of 6 5 kt

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

nanosims analysis of organic carbon from the tissint martian meteorite evidence for the past existence of subsurface organic bearing fluids on mars two petrographic settings of carbonaceous components mainly filling open fractures and occasionally enclosed in shock melt veins were found in the recently fallen tissint martian meteorite the presence in shock melt veins and the deuterium enrichments delta d up to 1183 parts per thousand of these components clearly indicate a pristine martian origin the carbonaceous components are kerogen like based on micro raman spectra and multielemental ratios and were probably deposited from fluids in shock induced fractures in the parent rock of tissint after precipitation of the organic matter the rock experienced another severe shock event producing the melt veins that encapsulated a part of the organic matter the c isotopic compositions of the organic matter delta c 13 12 8 to 33 1 parts per thousand are significantly lighter than martian atmospheric co2 and carbonate providing a tantalizing hint for a possible biotic process alternatively the organic matter could be derived from carbonaceous chondrites as insoluble organic matter from the latter has similar chemical and isotopic compositions the presence of organic rich fluids that infiltrated rocks near the surface of mars has significant implications for the study of martian paleoenvironment and perhaps to search for possible ancient biological activities on mars

a case study of atmospheric boundary layer mean structure for flow parallel to the ice edge aircraft observations from cearex this paper describes a case study of the mean structure of the atmospheric boundary layer abl in the marginal ice zone of the barents sea for flow nearly parallel to the ice edge data were collected by the noaa p 3 orion research aircraft on march 24 1989 the geostrophic wind above the boundary layer was oriented approximately 15 off ice with a speed of 12 m s 1 and the atmosphere above the abl showed little cross ice edge variability a lidar on the aircraft revealed over the water the downward slope of a stratocumulus deck toward the ice edge the associated inversion continued to slope downward over the ice creating large thermal wind effects within the boundary layer the inversion slope combined with horizontal boundary layer temperature gradients produced a factor of 2 decrease in surface geostrophic wind from water to ice below the inversion the data revealed an abl that remained well mixed for 100 km into the ice a gust probe system on the aircraft provided a mapping of low level fluxes from a 200 km leg perpendicular to the ice edge

geochemical relationships in the sudbury igneous complex origin of the main mass and offset dikes petrological models relating the different rock types constituting the 1 85 ga sudbury igneous complex are constrained with extensive new geochemical data we show that the main mass felsic norite transition zone quartz gabbro and granophyre have similar ratios of the highly incompatible trace elements e g la sm 4 5 7 la nb 2 8 4 2 th zr 0 04 0 05 and that these variations are consistent with the crystallization and differentiation of magma types largely 80 derived from the upper crust with a smaller contribution from a mantle source although there is presently no conclusive proof that magma was generated in situ as a melt sheet produced by meteorite impact we find no principal reason why this model should be rejected however we propose that a small contribution of mantle derived picritic magma is required to explain the abundant ni cu and platinum group elements pge in the sudbury deposits as well as the compositions of the ultramafic inclusions mgo 12 36 wt fo sub 68 87 olivines with 450 3 700 ppm ni and abundant chrome rich spinel and the magnesian composition of the mafic norite 8 14 wt mgo and the sublayer 6 12 wt mgo we believe that the main mass of the sudbury complex achieved its present composition through incorporation of up to 20 percent mantle derived picritic magma emplaced along crustal fractures produced by the impact event these picritic magmas entered the melt sheet as a dense plume vigorously mixing with it and due to the marked compositional shift the mixed magma formed magmatic sulfides which sank through the magma column depleting the melt in ni cu and pge since both the felsic norite and granophyre have indistinguishable ratios of the incompatible trace elements we see no requirement to derive these units of rock by the crystallization of magmas derived from different sources rather the compositional difference between the felsic norite and granophyre is attributed to the in situ differentiation of the magma we show that the main mass has many compositional traits similar to those of most of the offset dike quartz diorites e g the parkin offset dike la sm 6 3 la nb 4 5 th zr 0 05 and of embayment related leucocratic norites from the whistle mine la sm 6 2 la nb 5 0 th zr 0 02 these rocks have compositions intermediate between the felsic norite and the granophyre and therefore crystallized from the same magma type arguably the unmineralized quartz diorites provide the best possible estimate of the original magma from which the sudbury complex crystallized in detail there are subtle variations in composition within and between offset dikes with the largest difference being between the north and south range offsets the north range offset dikes cut archean granitoids and gneisses and have elevated sr la yb la sm and gd yb and low tio 2 whereas the south range dikes cut early proterozoic sediments mafic volcanics and intrusions and have low sr la yb gd yb la sm and high tio 2 these differences may be caused by the assimilation of different country rocks during emplacement of the dike a strongly mineralized offset dike at the creighton mine has geochemical variations that are different when compared to the main mass and in the case of creighton are more similar to the local mineralized sublayer these data suggest that mineralized and barren quartz diorites have different geochemical compositions and that these traits may be of value in mineral exploration

mechanisms of hydrovolcanic pyroclast formation grain size scanning electron microscopy and experimental studies pyroclasts produced by explosive magma water interactions are of various sizes and shapes data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash grain size and scanning electron microscopy sem reveal information on the formation of hydrovolcanic pyroclasts strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly centimeter sized with increasing water interaction hydrovolcanism increases in explosivity to surtseyan and vulcanian activity these eruptions produce millimeter to micron sized pyroclasts the abundance of fine ash 63 m diameter increases from 5 to over 30 percent as water interaction reaches an explosive maximum this maximum occurs with interactions of virtually equal volumes of melt and water five dominant pyroclast shape types determined by sem result from hydrovolcanic fragmentation 1 blocky and equant 2 vesicular and irregular with smooth surfaces 3 moss like and convoluted 4 spherical or drop like and 5 plate like types 1 and 2 dominate pyroclasts greater than 100 m in diameter types 3 and 4 are typical of fine ash type 5 pyroclasts characterize ash less than 100 m in diameter resulting from hydrovolcanic fragmentation after strong vesiculation fragmentation mechanisms observed in experimental melt water interactions result from vapor film generation expansion and collapse fragments of congealed melt are products of several alternative mechanisms including stress wave cavitation detonation waves and fluid instability mixing all result in rapid heat transfer these mechanisms can explain the five observed pyroclast shapes stress wave fracturing cavitation of the melt results from high pressure and temperature gradients at the melt water interface simultaneous brittle fracture and quenching produces type 1 pyroclasts type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching fluid instabilities promote turbulent mixing of melt and water and produce fine ash this kind of fragmentation occurs during high energy explosions the increased melt surface area due to fine fragmentation promotes high efficiency heat exchange between the melt and water shapes of resulting pyroclasts are determined by maximum surface area type 3 and surface tension effects type 4 type 5 pyroclasts result from nearly simultaneous vesicle burst and melt water fragmentation

petrology of impact melt rocks at the chicxulub multiring basin yucatan mexico compositions and textures of melt rocks from the upper part of the chicxulub structure are typical of melt rocks at other large terrestrial impact structures apart from variably elevated iridium concentrations less than 1 5 to 13 5 0 9 ppb indicating nonuniform dissemination of a meteoritic component bulk rock and phenocryst compositions imply that these melt rocks were derived exclusively from continental crust and platform sediment target lithologies modest differences in bulk chemistry among samples from wells located approximately 40 km apart suggest minor variations in relative contributions of these target lithologies to the melts subtle variations in the compositions of early formed pyroxene and plagioclase also support minor primary differences in chemistry between the melts evidence for pervasive hydrothermal alteration of the porous mesostasis includes albite k feldspar quartz epidote chlorite and other phyllosilicates as well as siderophile element enriched sulfides suggesting the possibility that chicxulub like sudbury may host important ore deposits

drill core lb 08a bosumtwi impact structure ghana petrographic and shock metamorphic studies of material from the central uplift during a recent drilling project sponsored by the international continental scientific drilling progam icdp two boreholes lb 07a and lb 08a were drilled into the crater fill of the bosumtwi impact structure and the underlying basement into the deep crater moat and the outer flank of the central uplift respectively the bosumtwi impact structure in ghana west africa which is 10 5 km in diameter and 1 07 myr old is largely filled by lake bosumtwi here we present the lithostratigraphy of drill core lb 08a recovered between 235 6 and 451 33 m depth below lake level as well as the first mineralogical and petrographic observations of samples from this core this drill core consists of approximately 25 m of polymict clast supported lithic breccia intercalated with suevite which overlies fractured brecciated metasediment that displays a large variation in lithology and grain size the lithologies present in the central uplift are metasediments composed dominantly of fine grained to gritty meta graywacke phyllite and slate as well as suevite and polymict lithic impact breccia the suevites principally present between 235 6 and 240 5 m and between 257 6 and 262 2 m display a fine grained fragmental matrix about 39 to 45 vol and a variety of lithic and mineral clasts that include meta graywacke phyllite slate quartzite carbon rich organic shale and calcite as well as melt particles fractured quartz unshocked quartz unshocked feldspar quartz with planar deformation features pdfs diaplectic quartz glass mica epidote sphene and opaque minerals the crater fill suevite contains calcite clasts but no granite clasts in contrast to suevite from outside the northern crater rim the presence of melt particles in suevite samples from the uppermost 25 meters of the core and in suevite dikelets in the basement is an indicator of shock pressures exceeding 45 gpa quartz grains present in suevite and polymict lithic impact breccia abundantly display 1 to rarely 4 sets of pdfs per grain the shock pressures recorded by the pdfs in quartz grains in the polymict impact breccia range from 10 to 30 gpa we also observed a decrease of the abundance of shocked quartz grains in the brecciated basement with increasing depth meta graywacke samples from the basement are heterogeneously shocked with shock pressures locally ranging up to 2530 gpa suevites from this borehole show a lower proportion of melt particles and diaplectic quartz glass than suevites from outside the northern crater rim fallback impact breccia as well as a lack of ballen quartz which is present in the external breccias similar variations of melt particle abundance and shockmetamorphic grade between impact breccia deposits within the crater and fallout impact breccia outside the crater have been observed at the ries impact structure germany

impact shocked zircons discovery of shock induced textures reflecting increasing degrees of shock metamorphism abstract textural effects specifically characteristic of shock metamorphism in zircons from impact environments have not been reported previously however planar deformation features pdf due to shock metamorphism are well documented in quartz and other mineral grains from these same environments an etching technique was developed that allows sem visualization of pdf and other probable shock induced textural features such as granular polycrystalline texture in zircons from a variety of impact shock environments these textural features in shocked zircons from k t boundary distal ejecta form a series related to increasing degrees of shock that should correlate with proportionate resetting of the u pb isotopic system

upper crustal basement involved folding in the east range of the sudbury basin ontario inferred from paleomagnetic data and spatial analysis of mafic dykes tilting of crystalline basement rocks associated with folding strain at uppermost crustal levels is difficult to recognize if basement rocks are devoid of traceable marker planes here we use the spatial variation in strike of paleoproterozoic mafic dyke segments complemented by compiled paleomagnetic data to identify tilting in archean basement rocks associated with kilometre scale folds of the eastern sudbury basin ontario spatial analysis of the strike of dyke segments is consistent with generation of the ne lobe and a newly identified anticline referred to as the west bay anticline in the layered sudbury igneous complex sic this anticline accounts better for the structural characteristics of the eastern sudbury basin than a previously proposed anticline with west plunging hinge line the west bay anticline is characterized by abrupt plan view thickness variations in the lower sic and curved faults displaying significant strike separations of sic contacts these structural characteristics are co

the sudbury structure controversial or misunderstood the origins of the sudbury structure and associated igneous complex have been controversial most models call for a major impact event followed by impact induced igneous activity although totally igneous models are still being proposed much of the controversy is due in our opinion to a misunderstanding of the size of the original sudbury structure by analogy with other terrestrial impact structures the spatial distribution of shock features and huronian cover rocks at the sudbury structure suggest that the transient cavity was 100 km in diameter which places the original final structural rim diameter in the range of 150 200 km theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in 104 km3 of impact melt more than sufficient to produce a melt body the size of the igneous complex present volume 4 8 103 km3 for the igneous complex to be an impact melt sheet it must have a composition similar to that of the target rocks evidence for this has been presented previously for sr and nd isotopic data which suggest a crustal origin here we also present new evidence from least squares mixing models that the average composition of the igneous complex corresponds to a mix of archean granite greenstone terrain with possibly a small component of huronian cover rocks this is a geologically reasonable mix based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size the igneous complex is differentiated which is not a characteristic of previously studied terrestrial impact melt sheets this can be ascribed however to its great thickness and slower cooling that large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted if this working hypothesis is accepted namely that both the sudbury structure and the igneous complex are impact in origin then previous hybrid impact igneous hypotheses can be discarded and the sudbury structure can be studied specifically for the constraints it provides to large scale cratering and the formation of basin sized multiring impact structures

osmium isotope evidence for a crustal origin of platinum group elements in the sudbury nickel ore ontario canada abstract sulphide ores from the international nickel company s inco creighton mine sudbury were analysed for osmium isotope ratios by icp ms rhenium and osmium abundances were determined by isotope dilution using a mixed spike in solid solution in a nickel sulphide matrix calculated initial 187 os 188 os ratios at 1 85 ga the emplacement age of the sudbury complex cluster around 0 60 the occurrence of less radiogenic compositions is attributed to post emplacement open system behaviour of the re os system the creighton results strongly overlap the initial ratios of published osmium data from two other sudbury mines suggesting a narrow range of isotope ratio in the original ore within the range of estimated osmium isotope compositions in the country rock at 1 85 ga using published and new data therefore the sulphide ores of the sudbury complex can have an entirely crustal source without a mantle derived contribution this conclusion is consistent with published neodymium isotope data for the complex and supports the meteorite impact hypothesis since this is the most effective means of fusing the large quantity of crustal rocks necessary to generate the complex the extraction of platinum group elements pge from this silicate melt by a nickel sulphide liquid was probably analogous to the laboratory fire assay procedure

global event location with full and sparse data sets using three dimensional models of mantle p wave velocity in order to improve on the accuracy of event locations at teleseismic distances it is necessary to adequately correct for lateral variations in structure along the ray paths either through deterministic model based corrections empirical path station corrections or a combination of both approaches in this paper we investigate the ability of current three dimensional models of mantle p wave velocity to accurately locate teleseismic events we test four recently published models two are parameterized in terms of relatively long wavelength spherical harmonic functions up to degree 12 and two are parameterized in terms of blocks of constant velocity which have a dimension of a few hundreds of km these models together with detailed crustal corrections are used to locate a set of 112 global test events consisting of both earthquakes and explosions with p wave travel time data compiled by the international seismological centre isc the results indicate that the supposedly higher resolution block models do not improve the accuracy of teleseismic event locations over the longer wavelength spherical harmonic models for some source locations the block models do not predict the range of observed travel time residuals as well as the longer wavelength models the accuracy of the locations largely varies randomly with geographic position although events in central asia are particularly well located we also tested the effect of reduced data sets on the locations multiple location iterations using 30 p wave travel times indicate that teleseismic events may be located within an area of 1000 km of the true location 66 of the time with only the model based corrections and increasing to 75 if calibration information is available if as few as 8 phases are available then this is possible only 50 of the time further refinement in models and or procedure such as the addition ofp phases azimuth data and consideration of p wave anisotropy may provide further improvement in the teleseismic location of small events

the sudbury igneous complex viscous emulsion differentiation of a superheated impact melt sheet the sudbury igneous complex of ontario canada is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1 85 ga ago the transient cavity and melting zone reached the moho and instantly 2 min relaxed to form a more familiar large shallow crater holding a thick superheated 1700 c melt sheet covered by 2 km of breccia there is little about the resulting bimodal igneous complex that resembles crystallization of well known sheet like bodies of similar composition yet the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions suggesting an intimate common parentage from the surrounding crust this petrogenetic enigma is explained here as a natural unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion which we take as the high temperature equivalent of breccia a wide spectrum of viscously discrete interdispersed parcels of mafic and felsic liquids reflecting the compositional heterogeneity of the target crustal materials formed the emulsion within days to months the emulsion components separated according to their relative densities into a bimodal norite granophyre assemblage that formed the basic structure of the present sudbury igneous complex there is clear evidence of this emulsion in the earliest dikes i e offsets which likely give the earliest state of the nascent melt sheet immediately following emulsion separation the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer these convective motions homogenized and rapidly cooled the magma to the liquidus temperatures whereupon convection ceased the pattern of convection was pinned in place by the embayment topography of the crater floor which in turn played a pivotal role in directing sulfide deposition into the embayments all further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins there is clear evidence of solidification from the floor upward and the roof downward minimal differentiation and compositional modification took place throughout cooling and solidification nevertheless during the solidification stage granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying onaping formation became unstable and entered the melt sheet and the partially melted remnants collected at the interface between the norite and granophyre some interstitial melt from the norite also percolated upward and altogether the blocks and melt produced the distinctive chemical and physical characteristics of the unusual transition zone the sudbury melt sheet is in essence a full scale magmatic experiment the conditions of formation relative to any other large terrestrial magma are precisely known thus the clear lack of any significant modal layering the overall homogeneous nature of each unit and the lack of any significant chemical differentiation through crystal fractionation establish sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers

structural uplift beneath the chicxulub impact structure models of the central structure of large impact craters are poorly constrained partly because of the lack of well preserved terrestrial examples and partly because of the extreme nature of impact events even large impact craters take only a few minutes to form during which time rocks from the deep crust move upward many kilometers interacting with impact melts and breccias before settling to their final position we construct a new model of central uplift beneath the chicxulub crater based upon a well constrained 3 d velocity model obtained by jointly inverting seismic traveltime and gravity data the input tomographic data set has good resolution and many rays cross the central uplift in many directions we use laboratory measurements to convert between velocity and density our velocity model possesses a high velocity zone near the crater center and velocity gradually decreases outside this zone we use regional refraction data to interpret these velocities in terms of a broad 80 km wide zone of structural uplift in which the central rocks originate from the lower crust and the surrounding rocks from the midcrust and upper crust this is in contrast with previous models in which the zone of central uplift is either 40 50 km or 150 km wide our interpretation is consistent with scaling laws yucatan basement lithology other velocity data observations at similar sized terrestrial craters and dynamic modeling of peak ring formation our model of the uplift at chicxulub can be used to help distinguish between competing models of effective target strength in numerical models of crater formation

the nature of the groundmass of surficial suevite from the ries impact structure germany and constraints on its origin surficial suevites from the ries impact structure have been investigated in the field and using optical and analytical scanning electron microscopy the groundmass of these suevites comprises calcite clay minerals impact melt glass crystallites plagioclase garnet and pyroxene francolite and ba phillipsite the latter zeolite is a secondary phase abundant textures have been observed intricate flow textures between the various groundmass phases globules of each phase in the other phases spheroids of pyrrhotite in calcite the budding off of clay globules into silicate glass and or calcite euhedral overgrowths of francolite on apatite clasts and quench textured crystallites in the groundmass groundmass forming calcite displays higher feo mno and sio2 contents than limestone target material the composition of suevite clay minerals is highly variable and not always consistent with montmorillonite three types of glasses are distinguished in the groundmass type 1 glasses are sio2 rich and are clearly derived from sandstones in the sedimentary cover while the protoliths of the other two glass types remains unclear analytical data and micro textures indicate that the calcite silicate glass francolite and clay minerals of the groundmass of the ries suevites represent a series of impact generated melts that were molten at the time of and after deposition on cooling plagioclase pyroxene and garnet crystallized from the groundmass these results are at variance with the current traditional descriptive definition of suevite given that ries is the original type occurrence of suevite some modification to the traditional definition may be in order as the results of this study are most consistent with the groundmass of ries surficial suevites representing a mix of several types of impact generated melts we suggest that a possible origin for these suevites is as some form of impact melt flow s that emanated from different regions of the evolving crater

petrogenesis of an augite bearing melt rock in the chicxulub structure and its relationship to k t impact spherules in haiti geophysical anomalies on the yucatan peninsula define a buried circular structure with an approximate diameter of 180 km refs 1 3 these anomalies along with stratigraphic and petrological data including evidence for shock metamorphism have been used to interpret the structure as an impact crater4 this structure known as chicxulub is particularly interesting because it formed at or near the end of the cretaceous period in the geographical region where an impact is believed to have occurred in large part because of a thick ejecta deposit found on haiti5 glassy tektite like relics in this deposit6 9 are unusually calcic up to 31 wt cao ref 7 providing a further circumstantial link with the chicxulub structure which penetrates a carbonate and evaporite sequence7 8 here we strengthen this link by showing that a simple chemical relationship exists between the glasses and an augite bearing melt rock found within the chicxulub structure and argue that the composition of this melt rock could not easily have been produced by volcanic processes

ries crater and suevite revisited observations and modeling part i observations we report results of an interdisciplinary project devoted to the 26 km diameter ries crater and to the genesis of suevite recent laboratory analyses of crater suevite occurring within the central crater basin and of outer suevite on top of the continuous ejecta blanket as well as data accumulated during the past 50 years are interpreted within the boundary conditions imposed by a comprehensive new effort to model the crater formation and its ejecta deposits by computer code calculations artemieva et al 2013 the properties of suevite are considered on all scales from megascopic to submicroscopic in the context of its geological setting in a new approach we reconstruct the minimum maximum volumes of all allochthonous impact formations 108 116 km3 of suevite 14 22 km3 and the total volume of impact melt 4 9 8 0 km3 produced by the ries impact event prior to erosion these volumes are reasonably compatible with corresponding values obtained by numerical modeling taking all data on modal composition texture chemistry and shock metamorphism of suevite and the results of modeling into account we arrive at a new empirical model implying five main consecutive phases of crater formation and ejecta emplacement numerical modeling indicates that only a very small fraction of suevite can be derived from the primary ejecta plume which is possibly represented by the fine grained basal layer of outer suevite the main mass of suevite was deposited from a secondary plume induced by an explosive reaction fuel coolant interaction of impact melt with water and volatile rich sedimentary rocks within a clast laden temporary melt pool both melt pool and plume appear to be heterogeneous in space and time outer suevite appears to be derived from an early formed melt rich and clast poor plume region rich in strongly shocked components melt clasts and originating from an upper more marginal zone of the melt pool crater suevite is obviously deposited from later formed clast rich and melt poor plumes dominated by unshocked and weakly shocked clasts and derived from a deeper central zone of the melt pool genetically we distinguish between primary suevite which includes dike suevite the lower sublayer of crater suevite and possibly a basal layer of outer suevite and secondary suevite represented by the massive upper sublayer of crater suevite and the main mass of outer suevite

bias in reported seismic arrival times deduced from the isc bulletin summary the accurate timing of seismological data is crucial for most quantitative examinations in seismology we present evidence that traveltime data from many stations contain systematic variations in timing which can be identified by checking the median of station delay times as a function of time this function is expected to be constant but many deviations are found several hundred stations that report arrival times to the isc have been examined the median station delay times of almost 8 per cent of these stations show changes of more than 1 s and thus exceed the structural signal in the data temporal variations of 0 5 1 s are common changes in the distribution of observed earthquakes and other possible explanations of such variations have been tested and fail to explain most of the observations therefore the bulk of the observed changes must be caused by flaws in the timing of the data or by biased picking of arrival times for instance at one station with a strong annual variation of noise level the arrival times are on average picked several tenths of a second later during months with a high noise level because of their systematic nature these errors will not necessarily cancel out by using the large number of traveltimes in the isc bulletin and may therefore introduce a bias in many investigations if the observed timing variations are due to the recording equipment at the stations the errors will be present in the digital waveform data as well tomographic studies could potentially be affected but in particular studies of temporal variations of earth structure based on traveltime data e g inner core rotation need to be looked at with caution as results might be influenced by station effects the exact nature of the bias is study dependent and needs thorough investigation in each individual case

chemical record of the projectile in the graded fall back sedimentary unit from the ries crater germany abstract the so called graded unit comprises 17 2 m of the core from the research borehole nordlingen 1973 and is thought to represent fall back material which has been sorted similar to tephra samples from six levels of these air borne impact debris sediments in the ries crater have been analyzed for ir ni co cr and fe the concentrations of these elements are negatively correlated with average grain size and accordingly the depth of the samples indicating the presence of a surface correlated component and are hence consistent with vapour deposition of meteoritic material on small dust grains the largest ir concentration found is 230 pg g a factor of 28 higher than the indigenous concentration enrichment factors in the uppermost layer of the graded unit are about 28 for ir approximately 2 for fe co ni and 1 5 for cr if the enrichment of all elements determined were of meteoritic origin then the net meteoritic composition is most closely matched by aubrites as suggested previously however it seems more likely that the observed concentration trends of cr fe co and ni are due to mineral sorting therefore the only as yet certain indication of meteoritic material is the presence of ir which is not sufficient to further constrain the nature of the projectile

diverse lithologies and alteration events on the rim of noachian aged endeavour crater meridiani planum mars in situ compositional evidence we report the results of geological studies by the opportunity mars rover on the endeavour crater rim four major units occur in the region oldest to youngest the matijevic shoemaker grasberg and burns formations the matijevic formation consisting of fine grained clastic sediments is the only pre endeavour impact unit and might be part of the noachian etched units of meridiani planum the shoemaker formation is a heterogeneous polymict impact breccia its lowermost member incorporates material eroded from the underlying matijevic formation the shoemaker formation is a close analog to the bunte breccia of the ries crater although the average clast sizes are substantially larger in the latter the grasberg formation is a thin fine grained homogeneous sediment unconformably overlying the shoemaker formation and likely formed as an airfall deposit of unknown areal extent the burns formation sandstone overlies the grasberg but compositions of the two units are distinct there is no evidence that the grasberg formation is a fine grained subfacies of the burns formation the rocks along the endeavour crater rim were affected by at least four episodes of alteration in the noachian and early hesperian i vein formation and alteration of preimpact matijevic formation rocks ii low water rock alteration along the disconformity between the matijevic and shoemaker formations iii alteration of the shoemaker formation along fracture zones and iv differential mobilization of fe and mn and caso 4 vein formation in the grasberg and shoemaker formations episodes ii and iii possibly occurred together but i and iv are distinct from either of these

modeling the formation of the k pg boundary layer abstract in this paper we investigate the formation of the cretaceous paleogene k pg boundary layer through numerical modeling the k pg layer is widely agreed to be composed of meteoritic material and target rock from the chicxulub impact site that has been ejected around the globe and mixed with local material during final deposition the observed composition and thickness of the k pg boundary layer changes with azimuth and distance from the impact site we have run a suite of numerical simulations to investigate whether we can replicate the observational data with a focus on the distal k pg layer and the impact glasses at proximal sites such as beloc haiti previous models of the k pg ejecta have assumed an initial velocity distribution and tracked the ejecta to its final destination here we attempt to model the entire process from impact to the arrival of the ejecta around the globe our models replicate the observed ejecta thickness at proximal sites and the modeled ejecta is composed of sediments and silicate basement rocks in agreement with observational data models that use a 45 impact angle are able to replicate the total ejecta and iridium volume at distal sites and the majority of the ejecta is composed of meteorite and target sediments sub vertical impacts generate too little iridium and oblique impacts of 30 degrees generate too much however in contrast to observations models that involve ballistic transport of ejecta lead to ejecta thickness decreasing with increasing distance and are unable to transport shocked minerals quartz and zircon from the chicxulub basement rocks around the globe we suggest that much of the k pg ejecta is transported non ballistically and that the most plausible mechanism is through re distribution from a hot expanding atmosphere the results are important for future investigations of the environmental effects of the chicxulub impact

the sudbury impact layer in the paleoproterozoic iron ranges of northern michigan usa a layer of breccia that contains fragments of impact ejecta has been found at 10 sites in the paleoproterozoic iron ranges of northern michigan in the lake superior region of the united states radiometric age constraints from events predating and postdating depo si tion of the breccia are ca 1875 ma and 1830 ma the major bolide impact that occurred at 1850 ma at sudbury ontario 500 700 km east of these sites is the likely causative event the michigan sites described here along with previously described sites in minnesota and ontario defi ne an extensive ejecta bearing deposit throughout the paleoproterozoic iron ranges of the lake superior region that we refer to as the sudbury impact layer the layer at the sites in michigan exhibits a range of thicknesses lithologic characters and sedimentary settings the diversity of rock types and internal stratigraphic details of the layer imply that several different processes of transport and deposition are represented but the detailed investigations needed to document them are incomplete many of the sites had been described and interpreted previously as products of common terrestrial processes but the presence of relict shock induced planar deformation features in quartz indicates that the breccia layer is in fact the product of an extra terrestrial impact at most localities this layer also contains relict fragments of altered devitrifi ed glass and or accretionary lapilli one immediate use of the impact layer is as an ultraprecise time line that ties together the well known stratigraphic sequences of the various geographically separated iron ranges the correlation of which has remained controversial for many decades the sudbury impact layer most commonly lies at a horizon that records a signifi cant change in the character of sediments across the region the impact layer marks the end of a major period of banded iron formation deposition that was succeeded by deposition of ficlastic rocks commonly black shales the impact may have produced regional if not global changes in the environment that resulted in this widespread synchronous change in sedimentation style

upper mantle structure beneath central eurasia using a source array of nuclear explosions and waveforms at regional distances we have observed very consistent broadband regional three component p waveforms from a set of 11 explosions that occurred at the former soviet test site in kazakhstan during 1988 and 1989 and have modeled most of the prominent features in these waveforms to determine upper mantle structure beneath central eurasia using a subset of events as a source array we have identified four consistent arrivals during the first 15 s of p waves recorded at the soviet incorporated research institutions for seismology iris stations arti aru d 1500 km and garm gar d 1380 km these arrivals show consistent variations in frequency content relative timing and amplitudes from event to event despite a range in source magnitude mb between 4 9 and 6 1 we have also identified consistent features in p waveforms of these events recorded at obninsk obn d 2880 km we argue that most of the prominent features in these waveforms can be explained by reflections at or refractions near discontinuities or large velocity gradients in the upper mantle we show that a model with discontinuities of approximately 3 0 and 6 5 near 200 km and 400 km respectively produces a better fit to the broadband data at aru and gar than previous models for this region this model also produces a good match to waveforms recorded at obn and noress however this model is not unique for example we show that it is also possible to produce similar waveforms by replacing the 200 km discontinuity with a large notch shaped low velocity zone between approximately 100 and 140 km based on reflectivity synthetics we suggest that additional data especially from distances less than 1000 km should indicate which of these models is more accurate although our models are not unique our observations do constrain the following features 1 there must be reflectors between about 140 and 200 km and near 400 km depth in the upper mantle to explain the large secondary arrivals at both aru and gar 2 based on the relative size of some of the secondary arrivals at aru and gar the discontinuity near 400 km must be large however it cannot be much greater than about 6 5 because there is no evidence in our data of a reflection from 400 km at obn 3 the relatively small amplitude of the first arrivals at both aru and gar suggests that either the gradient in the uppermost mantle is very small or that a shallow low velocity zone is present comparison of our results with other upper mantle studies shows that the central eurasian upper mantle is similar to the upper mantle of the central and eastern united states

emplacement of sharp walled sulfide veins during the formation and reactivation of impact related structures at the broken hammer mine sudbury ontario broken hammer is a hybrid cu ni platinum group element pge footwall deposit located in archean basement rocks below the impact induced sudbury igneous complex sic canada the deposit consists

re os isotope systematics of ni cu sulfide ores sudbury igneous complex ontario evidence for a major crustal component sudbury igneous complex sublayer ores from the levack west falconbridge and strathcona mines were analyzed for their re and os concentrations and os isotopic compositions the re os isotope systematics of three ores from the different mines give isochron ages of1 840 60 ma 1770 60 ma and 1780 110 ma suggesting that the re os system became closed at the time of or soon after the 1850 1 ma crystallization age of the complex the os isotopic compositions of different portions of the complex at the time of crystallization varied considerably with initial187os 186os1850 ranging from 4 64 at levack west to 7 55 at strathcona these heterogeneities require that the os and probably also the other platinum group elements contained in the ores were derived from at least two sources in addition the high initial187os 186os ratios indicate that the os was derived predominantly from ancient crust previous studies have suggested that the complex either crystallized from a mixture of mantle derived basaltic melt and ancient continental crust or was derived exclusively from the fusion of ancient continental crust resulting from a meteorite impact results of modelling suggest that if a contemporaneous mantle derived basaltic melt was involved in the origin of the sic it likely contributed 50 of the os to all three ores the large percentage of ancient crust involved in the production of the ores is most consistent with an interpretation of substantial crustal fusion resulting from meteorite impact

salsa3d a tomographic model of compressional wave slowness in the earth s mantle for improved travel time prediction and travel time prediction uncertainty abstract the task of monitoring the earth for nuclear explosions relies heavily on seismic data to detect locate and characterize suspected nuclear tests motivated by the need to locate suspected explosions as accurately and precisely as possible we developed a tomographic model of the compressional wave slowness in the earth s mantle with primary focus on the accuracy and precision of travel time predictions for p and pn ray paths through the model path dependent travel time prediction uncertainties are obtained by computing the full 3d model covariance matrix and then integrating slowness variance and covariance along ray paths from source to receiver path dependent travel time prediction uncertainties reflect the amount of seismic data that was used in tomography with very low values for paths represented by abundant data in the tomographic data set and very high values for paths through portions of the model that were poorly sampled by the tomography data set the pattern of travel time prediction uncertainty is a direct result of the off diagonal terms of the model covariance matrix and underscores the importance of incorporating the full model covariance matrix in the determination of travel time prediction uncertainty the computed pattern of uncertainty differs significantly from that of 1d distance dependent travel time uncertainties computed using traditional methods which are only appropriate for use with travel times computed through 1d velocity models

the arctic front in the greenland sea during february 1989 hydrographic and biological observations high resolution hydrographic and biological observations in the arctic frontal regime were made in february 1989 along 74 45 n the frontal zone was found to contain four individual frontal interfaces these fronts were a result of the large scale convergence of the currents around the greenland sea gyre bringing together arctic and polar surface waters and two modes of atlantic surface water the distribution of arctic and atlantic copepod species as well as the composition of their developmental stages showed a strong correlation with the distribution of water masses whereby the frontal interfaces acted as biogeographic boundaries whereas the fronts bounding the polar water reached only to a depth of about 100 m the fronts bordering the atlantic water masses could be observed to depths of 600 m at subsurface levels a core of atlantic intermediate water from the jan mayen current was observed the properties of this water type can be explained quantitatively from double diffusive exchange with the overlying water the mixing across the fronts was found to be dominated by fluxes of heat salt and copepods toward the central greenland sea gyre

u pb geochronology of the sublayer environment sudbury igneous complex ontario cu ni sulfide deposits of the sudbury igneous complex are hosted by the inclusion rich sublayer which occurs discontinuously between the footwall and main mass of the sudbury igneous complex by brecciated footwall in the vicinity of the sudbury igneous complex and by offset dikes protruding for tens of kilometers into the country rocks the genesis of mafic ultramafic inclusions in the sublayer and the relationships between the sublayer main mass and offset dikes have long been at the center of debate on sudbury geology this paper reports u pb isotope data on zircon and baddeleyite from samples representing norite matrix melanorite and olivine melanorite pods a metapyroxenite inclusion and diabase from the sublayer at the whistle embayment these samples yield ages ranging from 1848 1 or 1 8 to 1849 1 or 1 1 ma similar to a u pb age of 1849 8 or 2 0 ma obtained for zircons in quartz diorite from the copper cliff offset and indistinguishable from the previously published average age of 1850 or 1 ma for the main mass of the sudbury igneous complex none of the samples yield evidence for inherited zircon components the isotopic data prove that the exotic mafic ultramafic inclusions and pods in the sublayer are not xenoliths scavenged from older layered complexes but require the existence of primitive moderately mg rich magmas that crystallized in the early stages of development of the sudbury igneous complex such mg rich cumulates are unlikely to have crystallized from melted average upper crust

chromium isotope evidence in ejecta deposits for the nature of paleoproterozoic impactors abstract non mass dependent chromium isotopic signatures have been successfully used to determine the presence and identification of extra terrestrial materials in terrestrial impact rocks paleoproterozoic spherule layers from greenland graenseso and russia zaonega as well as some distal ejecta deposits lake superior region from the sudbury impact 1849 0 3 ma event have been analyzed for their cr isotope compositions our results suggest that 1 these distal ejecta deposits are all of impact origin 2 the graenseso and zaonega spherule layers contain a distinct carbonaceous chondrite component and are possibly related to the same impact event which could be vredefort 2023 4 ma or another not yet identified large impact event from that of similar age and 3 the sudbury ejecta record a complex meteoritic signature which is different from the graenseso and zaonega spherule layers and could indicate the impact of a heterogeneous chondritic body

brine formation in relation to climate changes and ice retreat during the last 15 000 years in storfjorden svalbard 76 78 n storfjorden svalbard is an area of intense brine formation the brine is cold dense rich in oxygen and co2 and has reduced ph storfjorden is unique because it contains well preserved agglutinated foraminifera dating back to the beginning of the last deglaciation we have investigated the distribution of calcareous and agglutinated benthic foraminifera benthic oxygen and carbon isotopes calcium carbonate total organic carbon and ice rafted debris in five cores from storfjorden comprising the holocene and the deglaciation the purpose of the study is to reconstruct brine formation in the past under different climate scenarios the data indicate that in storfjorden the ratio of agglutinated to calcareous benthic foraminifera can be taken as a measure of the strength of brine formation the foraminiferal data which are supported by stable isotopes degree of fragmentation and geochemical parameters signify that brine formation intensified during cold periods and weakened during warm periods during the deglaciation increased brine flow coincides with the older dryas the intra allerod cold period and the younger dryas brine formation increased from circa 8200 years b p reaching periodic maxima during the last 4000 years b p in response to the unstable climate maximum brine production correlates with the dark ages cold period circa 1500 1100 years b p and the little ice age circa 600 100 years b p lower production correlates with the roman warm period circa 2500 2000 years b p and the medieval warm period circa 1000 700 years b p

deep drilling into the chesapeake bay impact structure samples from a 1 76 kilometer deep corehole drilled near the center of the late eocene chesapeake bay impact structure virginia usa reveal its geologic hydrologic and biologic history we conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact melt creation and distribution transient cavity collapse and ocean water resurge comparison of post impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long term sedimentation patterns in the mid atlantic region salty connate water of the target remains in the crater fill today where it poses a potential threat to the regional groundwater resource observed depth variations in microbial abundance indicate a complex history of impact related thermal sterilization and habitat modification and subsequent post impact repopulation

carbonaceous megafossils from the precambrian 1800 ma near jixian northern china the upper part of the stratotype section of the changcheng system in the yanshan range 100 km east of beijing peking has yielded small megascopic carbonaceous compressions assignable to tyrasotae

improving teleseismic event locations using a three dimensional earth model abstract a comparison is made between seismic event locations derived from standard spherically symmetric earth models jb prem iasp91 and a recent earth model sp and a published data set of 82 well located earthquakes with a more even global distribution iasp91 and prem are shown to offer similar errors in event location and origin time estimates with respect to the jb model the three dimensional 3d model sp for the earthquakes the improvements are smaller corrections for crustal thickness beneath source and receiver are found to be of similar magnitude to the mantle corrections but use of station corrections together with the three dimensional mantle model provide the best locations

geology of the giant sudbury polymetallic mining camp ontario canada a new 1 100 000 scale compilation bedrock geology map provided with this issue presents a geologic synthesis of canada s most prolific mining camp the world class sudbury structure with total past production and current reserves of over 1 7 billion tonnes of ni cu co pt pd au ag ore lydon 2007 the polymetallic ore is hosted within one of earth s largest preserved impact craters the new map of the ni cu pge sudbury district after ames et al 2005 is included in a pocket at the back of this issue map 1 and is linked to mineral deposit commodity and geochronology data in tables 1 and 2 of this paper view this table table 1 list of mineral deposits and occurrences of the sudbury map area map 1 view this table table 2 u pb isotope data across the map area since 1891 six compilation maps of the geology of sudbury have been published bell 1891 collins 1937 cooke 1946 card 1969 dressler 1984 ames et al 2005 fig 1 some of the ni cu pge mines have operated for over a century whereas new ore deposits discovered as recently as 2004 are already in production or are in the process of being developed i e advanced prospects in the last 15 years sudbury has been the training ground for 60 postgraduate geoscience students in north america fig 1 from 1903 to 1990 only 23 graduate theses were completed on sudbury related topics eight of them in the 1970s after the discovery of shatter cones dietz 1964 fig 1 number of graduate theses of north american geoscientists trained on sudbury geology per decade showing the minor peak in training after the discovery of shatter cones and the recent education boom with over 50 geoscience graduate theses completed since 1990 the dashed lines represent the regional geologic maps of the sudbury area

the critical point and two phase boundary of seawater 200 500 c abstract the two phase boundary of seawater was determined by isothermal decompression of fully condensed seawater in the range of 200 500 c the pressure at which phase separation occurred for each isotherm was determined by a comparison of the refractive index of fluid removed from the top and bottom of the reaction vessel the critical point was determined to be in the range of 403 406 c 285 302 bar and was located by the inflection in the two phase boundary and by the relative volume of fluid and vapor as a function of temperature the two phase boundary of 3 2 nacl solution was found to coincide exactly with that of seawater over the range tested in the present study the boundary for both is described by a single seventh order polynomial equation the two phase boundary defines the maximum temperature of seawater circulating at depth in the oceanic crust thus the boundary puts a limit of about 390 c for seawater circulating near the seafloor at active ocean ridges 2 5 km water depth and about 465 c at the top of a magma chamber occurring at 2 km below the seafloor

resolution identification and stability of broadband acoustic arrivals in fram strait fram strait is the only deep water connection between the arctic and the world oceans an acoustic system for tomography glider navigation and passive listening was installed in the central deep water part of the strait during 2010 2012 with the primary objective of improving the estimates of transport through the strait previous tomographic measurements have relied on the travel times of resolved identified and stable acoustic arrivals the oceanographic conditions and highly variable sea ice in fram strait provide an acoustic environment that differs substantially from those in other tomographic experiments however and results in complex arrival patterns comparisons of the measured arrival patterns with predictions based on hydrographic sections show that it is difficult to resolve and identify individual arrivals in the early part of the arrival patterns in addition the early arrivals are unstable with the arrival structure changing significantly over time later arrivals that are surface reflected bottom reflected tend to be easier to resolve and identify as well as more stable the implication is that inverse methods need to use fluctuations in the overall structure of the early arrivals which tend to sample the ocean similarly in combination with the travel times of the later arrivals

k u band airborne radar altimeter observations of marginal sea ice during the 1984 marginal ice zone experiment pulse limited airborne radar data taken in june and july 1984 with a 13 8 ghz altimeter over the fram strait marginal ice zone are analyzed with the aid of large format aerial photography airborne synthetic aperture radar data and surface observations variations in the radar return pulse waveforms are quantified and correlated with ice properties recorded during the marginal ice zone experiment results indicate that the wide beam altimeter is a flexible instrument capable of identifying the ice edge with a high degree of accuracy calculating the ice concentration and discriminating a number of different ice classes this suggests that microwave radar altimeters have a sensitivity to sea ice which has not yet been fully exploited when fused with ssm i avhrr and ers 1 synthetic aperture radar imagery future ers 1 altimeter data are expected to provide some missing pieces to the sea ice geophysics puzzle

the evolution of the onaping formation at the sudbury impact structure the 1 4 1 6 km thick onaping formation consists of a complex series of breccias and melt bodies lying above the sudbury igneous complex sic at the sudbury impact structure based on the presence of shocked lithic clasts and various glassy phases the onaping has been described as a suevitic breccia with an origin at least in part as fallback material recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases both as clast types and discrete intrusive bodies the nature of the onaping and that of other suevitic breccias overlying impact melt sheets is reviewed the relative thickness internal stratigraphic and lithological character and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the onaping the sudbury structure formed in a foreland basin and water played an essential role in the evolution of the onaping as indicated by a major hydrothermal system generated during its formation taken together observations and interpretations of the onaping suggest a working hypothesis for the origin of the onaping that includes not only impact but also the interaction of sea water with the impact melt resulting in repeated explosive interactions involving proto sic materials and mixing with pre existing lithologies this is complicated by additional brecciation events due to the intrusion of proto sic materials into the evolving and thickening onaping fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two thirds of the onaping

origin of the sudbury complex by meteoritic impact neodymium isotopic evidence samarium neodymium isotopic data on whole rocks and minerals of the sudbury complex in canada gave an igneous crystallization age of 1840 21 x 10 6 years the initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust falling on the crustal trend of neodymium isotopic evolution as defined by shales the rare earth element concentration patterns of sudbury rocks are also similar to upper crustal averages these data suggest that the sudbury complex formed from melts generated in the upper crust and are consistent with a meteoritic impact

atlantic water advection versus sea ice advances in the eastern fram strait during the last 9 ka multiproxy evidence for a two phase holocene a sediment core from the west spitsbergen continental margin was studied to reconstruct climate and paleoceanographic variability during the last 9 ka in the eastern fram strait our multiproxy evidence suggests that the establishment of the modern oceanographic configuration in the eastern fram strait occurred stepwise in response to the postglacial sea level rise and the related onset of modern sea ice production on the shallow siberian shelves the late early and mid holocene interval 9 to 5 ka was generally characterized by relatively unstable conditions high abundance of the subpolar planktic foraminifer species turborotalita quinqueloba implies strong intensity of atlantic water aw inflow with high productivity and or high aw temperatures resulting in a strong heat flux to the arctic a series of short lived cooling events 8 2 6 9 and 6 1 ka occurred superimposed on the warm late early and mid holocene conditions our proxy data imply that simultaneous to the complete postglacial flooding of arctic shallow shelves and the initiation of modern sea ice production strong advance of polar waters initiated modern oceanographic conditions in the eastern fram strait at 5 2 ka the late holocene was marked by the dominance of the polar planktic foraminifer species neogloboquadrina pachyderma a significant expansion of sea ice icebergs and strong stratification of the water column although planktic foraminiferal assemblages as well as sea surface and subsurface temperatures suggest a return of slightly strengthened advection of subsurface atlantic water after 3 ka a relatively stable cold water layer prevailed at the sea surface and the study site was probably located within the seasonally fluctuating marginal ice zone during the neoglacial period

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

physical and chemical evidence of the 1850 ma sudbury impact event in the baraga group michigan an ejecta layer produced by the sudbury impact event ca 1850 ma occurs within the baraga group of northern michigan and provides an excellent record of impact related depositional processes this newly discovered 2 4 m thick horizon accumulated in a peritidal environment during a minor sea level lowstand that punctuated a period of marine transgression common ejecta clasts include shock metamorphosed quartz grains splash form melt spherules and tektites accretionary lapilli and glassy shards suggesting sedimentation near the terminus of the continuous ejecta blanket sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact generated tsunami wave train observed platinum group element anomalies ir rh and ru within the sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite documenting and interpreting the detailed characteristics of the sudbury ejecta horizon in michigan have yielded a fingerprint to identify this chronostratigraphic marker in other paleoproterozoic basins for the first time a foundation exists to assess the consequences of the sudbury impact on precambrian ocean chemistry and early life

isotopic evidence for distinct crustal sources of north and south range ores sudbury igneous complex lead isotope ratios were measured on fe cu ni sulphide ores and associated silicate phases from nine mines on the north and south ranges of the sudbury igneous complex sic these data are compared with the lead isotope signatures of country rocks and shock melted sudbury breccias around the complex both ore and feldspar data from the south range fall along a 1 85 ga reference line the age of the igneous complex and indicate a magmatic origin for lead in the ore data from the north range are also consistent with a magmatic origin but are displaced to lower 206pb 204pb and 207pb 204pb ratios these two suites fall close to the lead isotope composition of country rocks adjacent to the south and north ranges respectively furthermore there is a close correspondence between lead signatures of the sic and sudbury breccia on each side of the complex the evidence suggests an origin for the sic by shock melting of different crustal units on the two sides of the complex this is consistent with the impact of a meteorite on the southward thickening apron of huronian strata which overlay the margin of the superior craton at 1 85 ga

the influence of path corrections and a three dimensional global p wave velocity model on seismic event location in kazakhstan we relocate 81 large nuclear explosions that were detonated at the balapan and degelen mountain subregions of the semipalatinsk test site in kazakhstan during the years 1978 to 1989 the absolute locations of these explosions are available as well as very accurate estimates of their origin times this ground truth information allows us to perform a detailed analysis of location capability we use a sparse network of stations with highly accurate first arrival picks measured using a waveform cross correlation method these high quality data facilitate very accurate location estimates with only a few phases per event we contrast two different approaches 1 a calibration based approach where we achieve improved locations by using path corrections and 2 a model based approach where we achieve improved locations by relocating in a recently published global 3 d p velocity model both methods result in large improvements in accuracy of the obtained absolute locations compared to locations obtained in a 1 d reference earth model ak135 the calibration based approach gives superior results for this test site in particular when arrival times from regional stations are included estimated locations remain well within a 1000 km2 region surrounding the ground truth locations when the path corrections for the balapan and degelen mountain subregions are interchanged but even for the short separation between these two regions we find variations in the path corrections that cause systematic mislocations the model based approach also results in substantially reduced mislocation distances and has the distinct advantage that it is in principle transportable to other source regions around the world

the melt rocks of the boltysh impact crater ukraine ussr the 100 12 m y 25 km diameter boltysh impact crater was formed in precambrian granites and granite gneisses of the ukrainian shield the crater deposits have undergone minimal post impact erosion and it is possible to study a complete vertical section of the underlying 200 m thick melt sheet the melt rocks as sampled in two drill holes can be subdivided into two major textural classes microcrystalline and glassy the microcrystalline melt rocks form an uppermost and two lowermost units with the glassy variety occupying the middle of the melt sheet the microcrystalline units contain 25 zoned plagioclase phenocrysts set in a microcrystalline matrix of intergrown alkali feldspar and quartz pyroxene has been replaced by sheet silicates mineral and lithic clasts make up 5 15 and show varying degrees of shock and resorption the glassy melt rocks are characterized by 10 30 zoned plagioclase and 5 10 orthopyroxene set in a fresh to partially devitrified glassy matrix clast content is 5 chemically the melt rocks are relatively homogeneous and correspond to a mixture of kirovograd granites and gneisses in the ratio of 5 to 1 with ni ir and cr showing slight enrichments over the target rocks there are minor differences in the fe2o3 feo ratio and the alkalis between the microcrystalline and glassy varieties the increase in matrix crystallinity at the upper and lower contacts is contrary to observations at other impact melt sheets where greater matrix crystallinity occurs in the interiors of the melt sheets one possible explanation is that the melt matrix was originally glassy throughout due to its high sio2 content and the microcrystalline matrix is the result of extensive devitrification involving minor alkali exchange with circulating ground waters

deep submarine pyroclastic eruptions theory and predicted landforms and deposits submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows however that significant pyroclastic activity can occur even at depths in excess of 3000 m in order to document and illustrate the full range of submarine eruption styles we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions 1 no gas exsolution 2 gas exsolution but no magma disruption 3 gas exsolution magma disruption and hawaiian style fountaining 4 volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse 5 magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity and 6 volatile content builds up in the top of a dike leading to vulcanian eruptions we also examine the role of bulk interaction steam explosivity and contact surface steam explosivity as processes contributing to volcaniclastic formation in these environments we concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities the most likely circumstance is the quiet effusion of magma with minor gas exsolution and the production of somewhat vesicular pillow lavas or sheet flows depending on effusion rate the amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption the most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity due to the localized concentration of volatiles in magma that has a low rise rate magmatic gas collects by bubble coalescence and ascends in large isolated bubbles which disrupt the magma surface in the vent producing localized blocks bombs and pyroclastic deposits another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions these deposits being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater should be easily distinguishable from strombolian and hawaiian eruptions however we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles in this case a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian style activity the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile induced hawaiian eruption specifically typical pyroclast sizes might be smaller fountain heights may exceed those expected for the purely magmatic hawaiian case cooling of descending pyroclasts would be more efficient leading to different types of proximal deposits and runout distances for density flows would be greater potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer in addition flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles and thus extremely poor in vesicles relative to typical flows associated with hawaiian style eruptions in the primary magmatic gas case we examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid ocean ridges furthermore the latter stages of development of volcanic edifices seamounts formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth but also to 1 the presence of a summit magma reservoir which favors the buildup of magmatic foams enhancing hawaiian style activity and episodic dike emplacement which favors strombolian style eruptions and 2 the common occurrence of alkalic basalts the co2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts these models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles

uppermost impact fallback layer in the bosumtwi crater ghana mineralogy geochemistry and comparison with ivory coast tektites in 2004 an international continental scientific drilling program icdp drilling project at the bosumtwi impact crater ghana 10 5 km in diameter 1 07 myr old was performed to study the sediments that fill the lake as well as the underlying impactites in one lb 05 of 16 cores drilled into the lake sediments the zone between the impact breccias and the post impact sediments was penetrated preserving the final fine grained impact fallback layer this 30 cm thick layer contains in the top 10 cm accretionary lapilli microtektite like glass spherules and shocked quartz grains glass particlesmostly of splash form less than 1 mm size make up the bulk of the grains 70 78 by number in the coarser size fraction 125 m of the top of the fallback layer about one third of all quartz grains in the uppermost part of the layer are shocked with planar deformation features pdfs almost half of these grains are highly shocked with 3 or more sets of pdfs k feldspar grains also occur and some show shock deformation the abundance of shocked quartz grains and the average shock level as indicated by the number of sets of pdfs for both quartz and k feldspar decrease with depth into the layer the well preserved glass spherules and fragments are chemically rather homogeneous within each particle and also show relatively small variations between the various particles on average the composition of the fallback spherules from core lb 5b is very similar to the composition of ivory coast tektites and microtektites with the exception of cao contents which are about 1 5 to 2 times higher in the fallback spherules this is a rare case in which the uppermost fallback layer and the transition to the post impact sediments has been preserved in an impact structure its presence indicates that the impactite sequence at bosumtwi is complete and that bosumtwi is a very well preserved impact crater

the sudbury igneous complex a differentiated impact melt sheet the sudbury structure ontario is the remnant of a 1 85 ga old impact crater which originally had a diameter of 200 to 250 km the sudbury igneous complex occurs within the sudbury structure the sudbury igneous complex is a 2 5 to 3 0 km thick 60 27 km elliptical igneous rock body which consists of four major lithologies from top to bottom traditionally termed granophyre quartz gabbro norite and contact sublayer sulfide and inclusion bearing noritic rock with the exception of the latter all these lithologies are continuous across the structure modal analyses reveal that following the iugs system of nomenclature quartz gabbro samples are in fact quartz monzogabbros a few of the norite samples are quartz gabbros and most norite samples are quartz monzogabbros in view of these observations and in order to clarify the nomenclature an updated terminology is proposed from top to bottom upper unit middle unit lower unit and contact sublayer the bulk composition of the sudbury igneous complex from north range data is granodioritic continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the complex behaved as a single melt system all the sudbury igneous complex lithologies have the same light to heavy rare earth element ree ratio and an overall pattern of increased light ree and depleted heavy ree the occurrence of primary hydrous minerals hornblende and biotite deuteric alteration and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in h2o moreover the granophyric and other far from equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase the sudbury igneous complex differs from traditional layered mafic complexes in the following aspects it has an overall intermediate composition a hydrous nature a crustal isotopic signature normative corundum and an unusually large volume of granophyre the sudbury complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical and therefore mineralogical layering reported here for the first time and similar to those found in impact melt rocks elsewhere are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts the well known ore deposits of the sudbury region are directly related to the genesis of the sudbury igneous complex some ores precipitated from the sudbury melt whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex it is concluded that the sudbury igneous complex is the best exposed and only well documented to date terrestrial impact melt sheet to have differentiated

3 d kinematic restoration of the eastern sudbury igneous complex canada and its importance for cu ni pge sulphide exploration abstract kinematic restorations of deformed igneous terranes have rarely been attempted due to the general absence of 1 planar primary lithological contacts 2 tightly constrained deformation kinematics and 3 geometric constraints on lithologies and structures at depth because of the vast number of exploration drill holes the deformed and layered 1 85 ga sudbury igneous complex sic in ontario canada is reasonably well understood with respect to 1 and 3 we therefore explored the kinematics of folding and faulting in the eastern sic and use the subsurface geometry of faults extracted from a model integrating surface mapping and drill core logs in an attempt to restore the geometry of part of the sic specifically we identified rotation magnitudes and the tilt axis of a kilometer scale plunging fold the west bay anticline which appears to have folded the eastern part of the sic we combined a 3 d model of the faults and lithological contacts of the sic fault kinematics calculated from the inversion of brittle shear faults at surface and limb rotations of the west bay anticline to kinematically restore the eastern part of the sic results of restorations using simple shear and flexural slip algorithms highlight the importance of slip on prominent curved faults that displaced the sic and facilitated its folding flexural slip appears to be the more plausible mechanism of the two as it removed the curvature of and displacement on prominent faults flexural slip restoration helps to estimate primary thickness variations of the sic and the results show that thickened parts of the main mass of the sic correspond spatially with known cu ni pge sulfide deposits this result demonstrates that 3 d restoration can be an important tool for mineral exploration in igneous terranes

geophysical anomalies and quartz microstructures eastern warburton basin north east south australia tectonic or impact shock metamorphic origin abstract the eastern warburton basin northeast south australia features major geophysical anomalies including a magnetic high of near 200 nt centred on a 25 km wide magnetic low 30 000 km large eastern warburton basin the qz pe include multiple intersecting planar to curved sub planar elements with relic lamellae less than 2 m wide with spacing of 4 5 m qz pe are commonly re deformed displaying bent and wavy patterns accompanied with fluid inclusions u stage measurements of a total of 243 planar sets in 157 quartz grains indicate dominance of 10 12 10 13 and subsidiary 11 22 22 41 m 10 11 and x 51 61 planes transmission electron microscopy tem analysis displays relic narrow 1 m wide lamellae and relic non sub grain boundaries where crystal segments maintain optical continuity extensive sericite alteration of feldspar suggests hydrothermal alteration to a depth of 500 m below the unconformity which overlies the qz pe bearing warburton basin terrain the data are discussed in terms of a tectonic metamorphic deformation and b impact shock metamorphism producing planar deformation features qz pdf deformed qz pe are compared to re deformed qz pdfs in the sudbury vredefort manicouagan and charlevoix impact structures a 4 5 km uplift of the big lake granite suite during 298 295 ma is consistent with missing of upper ordovician to devonian strata and possible impact rebound the occurrence of circular seismic tomography anomalies below the east warburton basin the poolowana basin and the woodleigh impact structure signifies a potential diagnostic nature of circular tomographic anomalies

discovery of distal ejecta from the 1850 ma sudbury impact event a 25 70 cm thick laterally correlative layer near the contact between the paleoproterozoic sedimentary gunflint iron formation and overlying rove formation and between the biwabik iron formation and overlying virginia formation western lake superior region contains shocked quartz and feldspar grains found within accretionary lapilli accreted grain clusters and spherule masses demonstrating that the layer contains hypervelocity impact ejecta zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca 1878 ma and 1836 ma the sudbury impact event which occurred 650 875 km to the east at 1850 1 ma is therefore the likely ejecta source making these the oldest ejecta linked to a specific impact shock features particularly planar deformation features are remarkably well preserved in localized zones within the ejecta whereas in other zones mineral replacement primarily carbonate has significantly altered or destroyed ejecta features

suevite breccia of the ries impact crater germany petrography chemistry and shock metamorphism of crystalline rock clasts clasts of deep seated crystalline basement rocks in suevites of the ries crater germany were catalogued lithologically and classified with regard to their degree of shock metamorphism the sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior these clasts can be grouped into seven types of metamorphic and nine types of igneous rocks one hundred forty three clasts representing these lithologies were analyzed for major element bulk composition the fallout suevite contains on average 4 vol of crystalline basement clasts 0 4 vol of sedimentary rocks 16 vol of glass bodies some of them aerodynamically shaped and 79 vol of groundmass on average 52 of all crystalline clasts are from metamorphic sources and 42 are of igneous origin using the shock classification of stoffler 1974 8 of all crystalline clasts appear unshocked 10 gpa and 34 30 and 27 of clasts are shocked to stages i 10 35 gpa ii 35 45 gpa and iii 45 60 gpa respectively the bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations this indicates that the glasses originate by shock fusion of a similarly composed basement the crater suevite contains the same crystalline rock types that occur in the fallout suevites the bore hole nordlingen 1973 yields an average of 62 vol metamorphic and 38 vol igneous rocks the crater suevite differs from fallout suevites by a higher clast glass ratio by preponderance 65 95 of clasts shocked to stage i only and by the absence of aerodynamically shaped glass bodies the source of crystalline clasts and melt particles of suevites is a volume of rocks located deep in the crystalline basement to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 gpa deep burst impact model fallout suevites were ejected propelled by an expanding plume of vaporized rock and withdrew preferentially from this volume melt and highly shocked clasts leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt

floe pair interaction event rates in the marginal ice zone data from an accelerometer package are used to estimate the rates at which interaction events occur between adjacent ice floes in the marginal ice zone observations suggest that the events are principally the result of a modified collision process where brash ice partially filling the gap between two floes acts as an intermediary in the transfer of momentum from one to the other a model is presented from which the rate at which these interactions occur can be predicted comparison of the rates estimated from the data with those predicted by the model shows there to be some agreement although there is considerable room for improvement the predicted event rate depends strongly on the ratio of the root mean square wave height to the amount of open water between pairs of floes the form of this dependence offers an explanation for grouping of events a feature that may be readily observed in many extracts of data

shock wave induced fracturing of calcareous nannofossils from the chesapeake bay impact crater fractured calcareous nannofossils of the genus discoaster from synimpact sediments within the chesapeake bay impact crater demonstrate that other petrographic shock indicators exist for the cratering process in addition to quartz minerals evidence for shock induced taphonomy includes marginal fracturing of rosette shaped discoaster species into pentagonal shapes and pressure and temperature induced dissolution of ray tips and edges of discoasters rotational deformation of individual crystallites may be the mechanism that produces the fracture pattern shock wave fractured calcareous nannofossils were recovered from synimpact matrix material representing tsunami or resurge sedimentation that followed impact samples taken from cohesive clasts within the crater rubble show no evidence of shock induced fracturing the data presented here support growing evidence that microfossils can be used to determine the intensity and timing of wet impact cratering

testing for fullerenes in geologic materials oklo carbonaceous substances karelian shungites sudbury black tuff fullerenes have been reported from diverse geologic environments since their discovery in shungite from karelian russia our investigation is prompted by the presence of onionskin like structures in some carbonaceous substances associated with the fossil nuclear fission reactors of oklo gabon the same series of extractions and the same instrumental techniques laser desorption ionization and high resolution mass spectroscopy electron impact mass spectroscopy were employed to test for fullerenes in samples from three different localities two sites containing putative fullerenes sudbury basin and russian karelia and one new location oklo gabon we confirm the presence of fullerenes c 6 0 and c 7 0 in the black tuff of the onaping formation impact breccia in the sudbury basin but we find no evidence of fullerenes in shungite samples from various locations in russian karelia analysis of carbonaceous substances associated with the natural nuclear fission reactors of oklo yields no definitive signals for fullerenes if fullerenes were produced during sustained nuclear fission at oklo then they are present below the detection limit 100 fmol or they have destabilized since formation contrary to some expectations geologic occurrences of fullerenes are not commonplace

recrystallized impact glasses of the onaping formation and the sudbury igneous complex sudbury structure ontario canada abstract the origin of the sudbury structure and of the associated heterolithic breccias of the onaping formation and the sudbury igneous complex have been controversial while an impact origin of the structure has gained wide acceptance over the last 15 years the origin of the recrystallized onaping formation glasses and of the igneous complex is still being debated recently the interpretation of the breccias of the onaping formation as suevitic fall back impact breccias has been challenged the igneous complex is interpreted either as a differentiated impact melt sheet or as a combination of an upper impact melt represented by the granophyre and a lower impact triggered magmatic body consisting of the norite sublayer formations the onaping formation contains glasses as fluidal and nonfluidal fragments of various shapes and sizes they are recrystallized and our research indicates that they are petrographically heterogeneous and span a wide range of chemical compositions these characteristics are not known from glasses of volcanic deposits this suggests an origin by shock vitrification an interpretation consistent with their association with numerous and varied country rock clasts that exhibit microscopic shock metamorphic features the recrystallized glass fragments represent individual solid state and liquid state vitrified rocks or relatively small melt pods the basal member lies beneath the gray and black members of the onaping formation and where not metamorphic has an igneous matrix igneous textured melt bodies occur in the upper two members and above the basal member a comparison of the chemical compositions of recrystallized glasses and of the matrices of the basal member and the melt bodies with the components and the bulk composition of the igneous complex is inconclusive as to the origin of the igneous complex basal member matrix and melt bodies on average are chemically similar to the granophyre of the sudbury igneous complex suggesting that they are genetically related our chemical results allow interpretation of the entire igneous complex as a differentiated impact melt however they are also consistent with the granophyre alone being the impact melt and the norite and quartz gabbro beneath it representing an impact triggered magmatic body this interpretation is preferred as it is consistent with a number of field observations a re evaluation and extension of structural field studies and of geochemical data as well as a systematic study of the contact relationships of the various igneous phases of the igneous complex are needed to establish a sudbury impact model consistent with all data and observations

the petrogenesis of impact basin melt rocks in lunar meteorite shisr 161 this study explores the petrogenesis of shisr 161 an immature lunar regolith breccia meteorite with low abundances of incompatible elements a feldspathic affinity and a significant magnesian component our approach was to identify all clasts 0 5 mm in size in a thin section characterize their mineral and melt components and reconstruct their bulk major and minor element compositions trace element concentrations in representative clasts of different textural and compositional types indicate that the clast inventory of shisr 161 is dominated by impact melts that include slowly cooled cumulate melt rocks with mafic magnesian mineral assemblages minor exotic components are incompatible element rich melt spherules and glass fragments and a gas associated spheroidal precipitate our hypothesis for the petrologic setting of shisr 161 is that the crystallized melt clasts originate from the upper 1 km of the melt sheet of a 300 to 500 km diameter lunar impact basin in the moon s feldspathic highlands this hypothesis is based on size requirements for cumulate impact melts and the incorporation of magnesian components that we interpret to be mantle derived the glassy melts likely formed during the excavation of the melt sheet assemblage by an impact that produced a 15 km diameter crater the assembly of shisr 161 occurred in a proximal ejecta deposit of this excavation event a later impact into this ejecta deposit then launched shisr 161 from the moon our geochemical modeling of remote sensing data combined with the petrographic and chemical characterization of shisr 161 reveals a preferred provenance on the moon s surface that is close to pre nectarian riemann fabry basin

gravity and magnetic field modeling and structure of the chicxulub crater mexico the chicxulub crater is an 180 km diameter peak ring crater based on drill hole logs and samples potential fields seismic reflection profiles and surface fracture patterns a structural cross section produced based on these constraints has the features expected for a large complex impact crater the bouguer gravity anomaly consists of a broad 90 km radius 30 mgal low with a central 20 km radius 20 mgal high and two 5 mgal concentric lows at 35 and 60 km radius the gravity anomaly is disrupted by large scale basement anomalies and possibly by large scale slumping and backwash erosion effects the magnetic field anomaly over the crater consists of three zones an outer zone from 45 to 90 km radius of low amplitude short wavelength anomalies with an irregular perimeter a middle zone from 20 to 45 km radius of high amplitude short wavelength anomalies slightly elongated nnw sse and an inner 20 km radius single large amplitude anomaly magnetic field modeling indicates that the melt pool averages 90 km in diameter and the melt volume in the crater is estimated at 20 000 km3 the melt pool size constrains the collapsed transient cavity diameter to 90 km gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but 40 km in diameter and underlies or protrudes into the melt pool the preliminary structural cross section indicates that the inferred peak ring is decoupled from the structural uplift the geometry and bouguer gravity signature of the crater indicate that no significant uplift of the moho or relaxation of the crater has occurred

ecology of deep sea benthic foraminifera in the north atlantic during the last glaciation food or temperature control abstract in this study we explore the paleoecology of nineteen species and two species groups of deep sea benthic foraminifera in relation to bottom water temperature and food supply in the central north atlantic during the last glaciation 60 000 28 000 years bp the study is based on core so82 02ggc taken from 1730 m water depth on the western flank of reykjanes ridge the climate of the investigated period was extremely unstable interrupted by thirteen warmings and coolings the so called dansgaard oeschger events at so82 02ggc surface and bottom water temperatures varied with up to 5 c between the warm interstadials and the cold stadials surface productivity as indicated by the flux of planktic foraminifera varied with a factor of up to ten the dansgaard oeschger events had a significant impact on the benthic foraminiferal faunas which experienced up to 70 replacement between the interstadials and stadials multivariate analysis reveals that the investigated species can be separated into two groups group 1 including bulimina costata brizalina subspinescens epistominella decorata and trifarina angulosa was attracted to higher temperatures and high food supply group 2 including pullenia bulloides valvulineria laevigata and melonis barleeanus was attracted to colder water and low food supply three species belonging to the cold group cibicidoides wuellerstorfi oridorsalis umbonatus and eponides tumidulus are almost exclusively confined to sediments rich in ice rafted detritus a few species including nonionella iridea and epistominella vitrea show opportunistic behaviour most species are epifaunal to shallow infaunal and live today in oligotrophic to mesotrophic habitats our investigations indicate that temperature was the primary factor responsible for the relative abundance and main grouping of species however four species show little influence by temperature and several perhaps most were clearly also affected by food supply

improving regional seismic event location in china in an effort to improve our ability to locate seismic events in china using only regional data we have developed empirical propagation path corrections and applied such corrections using traditional location routines thus far we have concentrated on corrections to observed p arrival times for crustal events using travel time observations available from the usgs earthquake data reports the international seismic centre bulletin the preliminary international data center reviewed event bulletin and our own travel time picks from regional data location ground truth for events used in this study ranges from 25 km for well located teleseimic events down to 2 km for nuclear explosions located using satellite imagery we also use eight events for which depth is constrained using several waveform methods we relocate events using the evloc algorithm from a region encompassing much of china latitude 20 55 n longitude 65 115 e we observe that travel time residuals exhibit a distance dependent bias using iaspei91 as our base model to remedy this bias we have developed a new 1 d model for china which removes a significant portion of the distance bias for individual stations having sufficient p wave residual data we produce a map of the regional travel time residuals from all well located teleseismic events residuals are used only if they are smaller than 10 s in absolute value and if the seismic event is located with accuracy better than 25 km from the residual data correction surfaces are constructed using modified bayesian kriging modified bayesian kriging offers us the advantage of providing well behaved interpolants and their errors but requires that we have adequate error estimates associated with the travel time residuals from which they are constructed for our p wave residual error estimate we use the sum of measurement and modeling errors where measurement error is based on signal to noise ratios when available and on the published catalog estimate otherwise our modeling error originates from the variance of travel time residuals for our 1 d china model we calculate propagation path correction surfaces for 74 stations in and around china including six stations from the international monitoring system the statistical significance of each correction surface is evaluated using a cross validation technique we show relocation results for nuclear tests from the balapan and lop nor test sites and for earthquakes located using interferometric synthetic aperture radar these examples show that the use of propagation path correction surfaces in regional relocations eliminates distance bias in the residual curves and significantly improves the accuracy and precision of seismic event locations

dating of a regional hydrothermal system induced by the 1850 ma sudbury impact event new u pb geochronologic data from the sudbury structure directly link extensive hydrothermal activity in crater fill breccias with the 1850 ma impact event and constrain complex impact induced processes to 4 m y semiconformable alteration of breccias in the onaping formation includes silicification albitization chloritization calcitization and complex feldspathization which directly underlies zn cu pb ore deposits geochronologic data demonstrate a middle to lower crustal source for the crater fill deposit that was subsequently affected by the hydrothermal event at 1848 4 3 8 1 8 ma these and other data confirm that the crater fill breccia hydrothermal system sudbury igneous complex and its sublayer ni ore bearing units were emplaced as a result of impact the ages of events associated with the sudbury structure are now more closely constrained than in any other meteorite impact structure

a trishear model for the deformation of the sudbury igneous complex canada abstract the sudbury igneous complex sic canada is an impact induced layered sheet of crystalline rocks deformed into an asymmetrical fold basin the sudbury basin the basin geometry at depth is largely unknown as few attempts were made to quantify displacement and rotation of layer contacts we propose that the dip of layer contacts and foliation surfaces in the southern sic can be approximated by trishear fault propagation folding trishear deformation accounts for 1 angular discordances between upper and basal sic contacts 2 local overturning of the sic 3 progressive steepening of foliation surfaces from nw to se 4 strain gradient in the sudbury basin sedimentary rocks and 5 thickness variations in sic layers moreover the south range shear zone a zone of moderately strong metamorphic fabrics coincides with the surface manifestation of the proposed trishear zone we demonstrate the use of structural data together with forward kinematic modelling to identify the strain distribution within the sic rotation of sic contacts and thickness changes of sic layers backward kinematic modelling provides information on the initial geometry of the sic and is used to restore the shape of the igneous sheet showing that the sic was shortened by approximately 10 km in nw se diameter most of the shortening was accommodated by 40 reduction in the thickness of the upper sic layer

the effect of target lithology on the products of impact melting impact cratering is an important geological process on the terrestrial planets and rocky and icy moons of the outer solar system impact events generate pressures and temperatures that can melt a substantial volume of the target however there remains considerable discussion as to the effect of target lithology on the generation of impact melts early studies showed that for impacts into crystalline targets coherent impact melt rocks or sheets are formed with these rocks often displaying classic igneous structures e g columnar jointing and textures for impact structures containing some amount of sedimentary rocks in the target sequence a wide range of impactgenerated lithologies have been described although it has generally been suggested that impact melt is either lacking or is volumetrically minor this is surprising given theoretical constraints which show that as much melt should be produced during impacts into sedimentary targets the question then arises where has all the melt gone the goal of this synthesis is to explore the effect of target lithology on the products of impact melting a comparative study of the similarly sized haughton mistastin and ries impact structures suggests that the fundamental processes of impact melting are basically the same in sedimentary and crystalline targets regardless of target properties furthermore using advanced microbeam analytical techniques it is apparent that for the structures under consideration here a large proportion of the melt is retained within the crater as crater fill impactites for impacts into sedimentary bearing target rocks thus it is suggested that the basic products are genetically equivalent but they just appear different that is it is the textural chemical and physical properties of the products that vary

an international and multidisciplinary drilling project into a young complex impact structure the 2004 icdp bosumtwi crater drilling project an overview the bosumtwi impact crater in ghana arguably the best preserved complex young impact structure known on earth displays a pronounced rim and is almost completely filled by lake bosumtwi a hydrologically closed basin it is the source crater of the ivory coast tektites the structure was excavated in 2 1 2 2 gyr old metasediments and metavolcanics of the birimian supergroup a drilling project was conceived that would combine two major scientific interests in this crater 1 to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago at a near equatorial location in africa for which very few data are available so far and 2 to obtain a complete record of impactites at the central uplift and in the crater moat for ground truthing and comparison with other structures within the framework of an international and multidisciplinary drilling project led by the international continental scientific drilling program icdp 16 drill cores were obtained from june to october 2004 at six locations within lake bosumtwi which is 8 5 km in diameter the 14 sediment cores are currently being investigated for paleoenvironmental indicators the two impactite cores lb 07a and lb 08a were drilled into the deepest section of the annular moat 540 m and the flank of the central uplift 450 m respectively they are the main subject of this special issue of meteoritics planetary science which represents the first detailed presentations of results from the deep drilling into the bosumtwi impactite sequence drilling progressed in both cases through the impact breccia layer into fractured bedrock lb 07a comprises lithic in the uppermost part and suevitic impact breccias with appreciable amounts of impact melt fragments the lithic clast content is dominated by graywacke besides various metapelites quartzite and a carbonate target component shock deformation in the form of quartz grains with planar microdeformations is abundant first chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and mg but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values core lb 08a comprises suevitic breccia in the uppermost part followed with depth by a thick sequence of graywacke dominated metasediment with suevite and a few granitoid dike intercalations it is assumed that the metasediment package represents bedrock intersected in the