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

vredefort sudbury chicxulub three of a kind vredefort sudbury and chicxulub are the largest known terrestrial impact structures all have been cited as multi ring basins the available data indicate that all have some form of multiple ring attributes most commonly structural features chicxulub however is the only example with morphological ring features there are also commonalities in the structural and lithological features of vredefort and sudbury and it is possible to construct a generalized compilation of the character of 200 300 km diameter impact basins on earth it is not clear however that any of these structures had the original morphological characteristics of large lunar multi ring basins additional data and synthesis are required to fully characterize these structures in order to realize their potential to constrain large scale cratering processes if this is not sufficient incentive for further studies the environmental effects of chicxulub aside the vredefort sudbury and chicxulub impact events are also the reason for

order or chaos origin and mode of emplacement of breccias in floors of large impact structures abstract breccias in the crater floor of large impact structures are pseudotachylites sensu largo authigenic monomict and polymict clastic matrix breccias so called footwall breccias and impact melt breccias pseudotachylite bodies in the center of large impact structures e g vredefort dome south africa appear to have a random distribution and orientation but most dip steeply or vertically large bodies of pseudotachylite in the more distal sectors of the 200 km diameter sudbury structure have been interpreted as ring and terrace collapse features in the vredefort dome networks of randomly distributed pseudotachylite veins accompany large mother lode pseudotachylite dikes in general pseudotachylites in the floors of central parts of impact craters may form through explosive transfer of thermal shock energy in a process that could be termed flash replacement melting whereas pseudotachylites at large distances from the centers of large impact structure are believed to have formed through friction leading to partial or complete melting similar to the formation of tectonic pseudotachylites in smaller structures e g ries and slate islands clastic matrix breccias instead of pseudotachylites occur as the most common breccias in the crater floors they have a chaotic distribution pattern their dips are commonly also steep to vertical melt breccia dikes in the target rocks of the crater floor are associated with melt sheets that fill the lower part of the excavation cavity at vredefort erosion has removed the coherent melt sheet but melt breccia dikes vredefort granophyre in the crater floor are preserved they are characterized by a remarkably homogeneous chemical composition and are believed to represent the initial undifferentiated impact melt near the vredefort collar the granophyre forms more or less concentric dikes in the more central parts of the dome their orientation is more random but in places may be controlled by the archean fabric of the crater basement the offset dikes of the sudbury structure are associated with the sudbury igneous complex that represents in total or in part a differentiated impact melt sheet the dikes in many aspects are similar to vredefort granophyres but their interpretation as undifferentiated bulk melt is problematic on geochemical and structural grounds chemically these dikes are not homogeneous and in places they contain massive sulfide deposits that are unlikely components of an undifferentiated impact melt differentiation and precipitation of massive sulfides are slow processes compared with the presumably high energy and fast emplacement of a supposedly undifferentiated melt pseudotachylites of the central impact crater apparently are mainly produced in the compression phase of the impact process whereas clastic matrix breccias form during the uplift and crater modification phases impact melt breccia dikes contain pseudotachylite bearing inclusions and cut across pseudotachylite bodies such impact melt breccia dikes were probably emplaced during uplift and crater modification

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

vredefort bronzite granophyre chemical evidence for origin as a meteorite impact melt abstract computer assisted compositional modelling studies demonstrate that the unusual chemical composition of the bronzite granophyre in the vredefort structure south africa can be produced by the impact melting and mixing of four pre vredefort target rocks which constitute the lower part of the vredefort stratigraphic section ventersdorp lava witwatersrand shale witwatersrand quartzite and old granite similar models show that a simple igneous assimilation mechanism total assimilation of the same wallrock types using a wide range of possible parent magmas cannot duplicate the granophyre composition unless geologically unreasonable amounts of wallrock 100 of the parent magma are assimilated the unusual granophyre composition reflects the incorporation of significant amounts of basalt 30 80 into the impact melt from which the granophyre was derived for various granophyre compositions the granite component varies inversely with basalt from about 45 5 and shale is relatively uniform at 5 20 a significant and nearly constant amount of quartzite about 20 is an unusual feature of the granophyre which given the generally refractory character of quartzite wallrock is difficult to explain by assimilation the granophyre displays a significant range of chemical variation which is similar to the range observed in impact melt bodies from large and small terrestrial impact structures these variations apparently reflect actual differences in the relative amounts of different target rocks incorporated into discrete volumes of impact melt rather than variations in the composition of individual target rock components these results are consistent with the view that the granophyre is an impact melt and that the vredefort structure formed by the impact of an extraterrestrial object this interpretation also explains several long known geological features 1 the close relationship between granophyre and pseudotachylite both of which were formed simultaneously by shock waves of different intensities 2 the absence of shock effects pseudotachylite and shatter cones in the granophyre and 3 the occurrence of quartzite xenoliths in the granophyre at great distances from the nearest quartzite exposures

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

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

the age and thermal evolution of the vredefort impact structure a single grain upb zircon study abstract single grain u pb shrimp analyses of a heterogeneous zircon population from a small granite body in the archean basement core of the vredefort dome indicate an age of 2017 5 ma for the granite which is identical within errors to the previously reported age determinations for the vredefort impact event petrographic evidence from the granite indicates that it crystallized after the impact shock event but some doubt remains whether it is a true impact melt a decompression melt generated during rapid uplift of the crater basement following the impact event or a melt generated during an earlier 2 05 2 06 ga granulite facies anatectic event that was still at least partially molten at the time of the vredefort impact event rare zircon xenocrysts displaying planar microdeformation features and giving discordant archean ages are overgrown by polycrystalline zircon rims with u th compositions similar to that of the main 2 02 ga zircon population but with discordant 207pb 206pb ages indicating more recent pb loss

the impact pseudotachylitic breccia controversy insights from first isotope analysis of vredefort impact generated melt rocks abstract besides impact melt rock several large terrestrial impact structures notably the sudbury canada and vredefort south africa structures exhibit considerable occurrences of a second type of impact generated melt rock so called pseudotachylitic breccia previously often termed pseudotachylite the term today reserved in structural geology for friction melt in shear or fault zones at the vredefort dome the eroded central uplift of the largest and oldest known terrestrial impact structure pseudotachylitic breccia is well exposed with many massive occurrences of tens of meters width and many hundreds of meters extent genesis of these breccias has been discussed variably in terms of melt formation due to friction melting melting due to decompression after initial shock compression decompression melting upon formation collapse of a central uplift or a combination of these processes in addition it was recently suggested that they could have formed by the infiltration of impact melt into the crater floor coming off a coherent melt sheet and under assimilation of wall rock even seismic shaking has been invoked field evidence for generation of such massive melt bodies by friction on large shear fault zones is missing also no evidence for the generation of massive pseudotachylitic breccias in rocks of low to moderate shock degree by melting upon pressure release after shock compression has been demonstrated the efficacy of seismic shaking to achieve sufficient melting as a foundation for massive pseudotachylitic melt generation as typified by the breccias of the sudbury and vredefort structures has so far remained entirely speculative the available petrographic and chemical evidence has thus been interpreted to favor either decompression melting i e in situ generation of melt upon central uplift collapse or the impact melt infiltration hypothesis importantly all the past clast population and chemical analyses have invariably supported an origin of these breccias from local lithologies only here the first rb sr sm nd and u pb isotopic data for vredefort pseudotachylitic breccias and their host rocks in comparison to data for vredefort granophyre impact melt rock are presented they strongly support that the pseudotachylitic breccias were exclusively formed from local precursor lithologies in agreement with earlier isotopic results for sudbury breccia and chemical results for vredefort pseudotachylitic breccias a contribution from a granophyre like impact melt component to form vredefort pseudotachylitic breccia is not indicated the most likely process for the genesis of voluminous pseudotachylitic breccias in large impact structures remains decompression melting upon formation and collapse of the central uplift during the modification stage of impact cratering

structural characteristics of the sudbury impact structure canada impact induced versus orogenic deformation a review orogenic deformation both preceding and following the impact event at sudbury strongly hinders a straightforward assessment of impact induced geological processes that generated the sudbury impact structure central to understanding these processes is the state of strain of the sudbury igneous complex the solidified impact melt sheet its underlying target rocks overlying impact breccias and post impact sedimentary rocks this review addresses 1 major structural metamorphic and magmatic characteristics of the impact melt sheet and associated dikes 2 attempts that have been made to constrain the primary geometry of the igneous complex 3 modes of impact induced deformation as well as 4 mechanisms of pre and post impact orogenic deformation the latter have important consequences for estimating parameters such as magnitude of structural uplift tilting of pre impact huronian strata and displacement on major discontinuities which collectively have not yet been considered in impact models in this regard a mechanism for the emplacement of offset dikes is suggested that accounts for the geometry of the dikes and magmatic characteristics as well as the occurrence of sulfides in the dikes moreover re interpretation of published paleomagnetic data suggests that orogenic folding of the solidified melt sheet commenced shortly after the impact uncertainties still exist as to whether the sudbury impact structure was a peak ring or a multi ring basin and the deformation mechanisms of rock flow during transient cavity formation and crater modification

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

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

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

the pele offset dykes sudbury impact structure canada the offset dykes are impact melt bearing dykes related to the 1 85 ga sudbury impact structure currently the dykes extend radially outward from or occur concentrically around the sudbury igneous complex which is the remnant of a differentiated impact melt sheet and the source of the dykes the recently identified three pele offset dykes intrude into the archean rocks north of the sudbury igneous complex in this study the pele dykes are characterized for the first time by a combination of fieldwork optical microscopy electron microprobe analyses and bulk geochemical analyses the pele offset dykes stand out from the other offset dykes at sudbury in two significant ways i all other known offset dykes consist of an inclusion rich lithology in the center of the dyke and an inclusion poor lithology along the margins the pele dykes however are only composed of the inclusion poor phase ii the pele dykes particularly the central and eastern dykes have a more evolved chemical composition relative

planar deformation features and impact glass in inclusions from the vredefort granophyre south africa the vredefort granophyre represents impact melt that was injected downward into fractures in the floor of the vredefort impact structure south africa this unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite feldspathic quartzite arkose and granitic material with minor proportions of shale and epidiorite two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features quartz grains in other inclusions display a vermicular texture which is reminiscent of checkerboard feldspar feldspars range from large twinned crystals in some inclusions to fine grained aggregates that apparently are the product of decomposition of larger primary crystals in rare inclusions a mafic mineral probably biotite or amphibole has been transformed to very fine grained aggregates of secondary phases that include small euhedral crystals of fe rich spinel these data indicate that inclusions within the vredefort granophyre were exposed to shock pressures ranging from 5 to 8 30 gpa many of these inclusions contain small rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals including k feldspar quartz augite low ca pyroxene and magnetite the composition of this devitrified glass varies from inclusion to inclusion but is generally consistent with a mixture of quartz and feldspar with minor proportions of mafic minerals in the case of granitoid inclusions melt pockets commonly occur at the boundaries between feldspar and quartz grains in metasedimentary inclusions some of these melt pockets contain remnants of partially melted feldspar grains these melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot 650 to 1610 oc impact melt that crystallized to form the vredefort granophyre

integrated geophysical modelling of a giant complex impact structure anatomy of the vredefort structure south africa abstract only three very large confirmed impact structures are known on earth the chicxulub crater mexico 65 ma ca 180 km wide the sudbury structure canada 1 85 ga 200 km in diameter and the vredefort structure in south africa 2 02 ga while extensive data on large impact structures have been obtained by remote sensing studies of such features on other planetary bodies only this small number of large terrestrial impact structures can provide data crucial to understanding these catastrophic impact processes on earth integrated modelling of gravity and magnetic data constrained by geological as well as refraction and reflection seismic data accomplished the reconstruction of the vredefort impact structure in south africa approximately 250 km wide the original vredefort impact structure covered the whole extent of the archaean witwatersrand basin distinguished by enormous gold resources as it is structurally preserved today in fact it is clear that the preservation of vast volumes of economically important witwatersrand strata is the direct result of the formation of the ring basin around the central uplift the vredefort dome of the impact structure this study is the first attempt to create an integrated and geophysically well constrained model of this very large complex impact structure

opposing decadal changes for the north atlantic meridional overturning circulation hydrographic properties of the north atlantic ocean have changed significantly over the past decades a combination of changes in seawater density calculated from observed properties of sea water and a numerical ocean model reveals that the strength of the meridional overturning circulation has changed in different directions in the subpolar and subtropical basins respectively

the 1946 mount colonel foster rock avalanche and associated displacement wave vancouver island british columbia the 1946 vancouver island earthquake m 7 2 triggered a rock avalanche from the north face of mount colonel foster central vancouver island british columbia approximately 1 5 106 m3 of triassic volcaniclastic rocks detached from between el 1965 m and el 1600 m although just over half of this volume was deposited in the upper part of the track above el 1080 m approximately 0 7 106 m3 descended the lower part of the track and entered the waters of landslide lake at el 890 m the resultant displacement wave ran up a maximum vertical distance of 51 m on the opposite shore and the wave crest was about 29 m high when it spilled over the lip of the lake water displaced during the event destroyed forest in the upper reaches of the elk river valley up to 3 km from landslide lake the wave at landslide lake is comparable to other waves generated by similar magnitude rock avalanches in peru and norway and it is the largest recorded in the canadian cordillera the case history illustrates the conditi

geology mineralization and emplacement of the foy offset dike sudbury impact structure the foy offset dike is the longest of the known radial dikes associated with the 1 85 ga sudbury igneous complex the foy dike extends for at least 30 km from the complex into the north range archean and proterozoic footwall rocks of the sudbury impact structure the dike ranges in width from 400 m where it connects to the sudbury igneous complex to 50 m at its most distal known exposure we divide the dike into a proximal segment and a distal segment based on where it intersects the concentric hess offset dike three main dike lithologies are distinguished all referred to as varieties of quartz diorite inclusion rich pyroxene quartz diorite occurs in the embayment adjacent to the base of the main mass of the sudbury igneous complex this constitutes the sublayer northward the sublayer gives way to amphibole biotite inclusion bearing quartz diorite and amphibole biotite inclusion poor quartz diorite the dike is mineralized and deposits in the proximal segment have been mined mineralization is associated with the inclusion rich dike phase at the nickel offset mine the dike contains inclusion bearing massive ni cu sulfides that also host platinum group minerals the orebodies occurred as strike parallel sheets within the central parts of the dike in association with inclusion bearing quartz diorite the geochemistry of the dike lithologies indicates that both the inclusion poor and inclusion bearing quartz diorite varieties are close in composition to the postulated bulk composition of the sudbury igneous complex this implies that the dike was emplaced prior to any significant fractionation of the main melt body and hence very early in the evolution of the sudbury complex we envisage dike emplacement rapidly following rebound and central uplift formation as part of the impact process early dike emplacement implies that the ore was also introduced early in the evolution of the melt sheet when the inclusion poor quartz diorite phases were still unconsolidated uplift would have caused extension in the target rocks and so dilated the footwall fractures and faults that then became hosts primarily via gravitationally driven intrusion to the primitive polymict impact melt breccias and sulfide ores the chilled margin compositions of the foy inclusion poor quartz diorite are considered good indicators of the original bulk composition of the sudbury igneous complex this idea is supported by comparison with calculated estimations of the bulk composition a shift in geochemical trends a grain size increase and a decrease in inclusion content occurs 22 km along the length of the dike which we believe corresponds to a fault system that downfaults to the north given that the dense ore would have preferentially settled toward the base of the foy dike fracture system postimpact downfaulting of the distal segment of the dike may have displaced any mineralized zones present to deeper levels relative to the proximal segment

grenville age deformation of the sudbury impact structure evidence from magnetic modelling of the sudbury diabase dyke swarm terra nova 24 213 220 2012 abstract the sudbury structure is located 10 km from the grenville front the northern boundary of the grenville metamorphic terrane previous geological studies have suggested that late grenvillian deformation 1000 1070 ma of the sudbury structure was limited to minor brittle deformation olivine diabase dykes which were intruded at 1238 ma before the last pulse of grenville deformation cross much of the sudbury structure any grenville age deformation of the sudbury structure therefore should produce systematic displacements in the geometry of the dykes this study uses information derived from an aeromagnetic survey of the sudbury structure to map and measure fault displacements lateral displacements of dyke segments define a progressive increase in fault rotation from ne to sw across the basin a two dimensional inversion modelling of magnetic anomaly profiles across segments of the dykes outlines a series of fault blocks with common dip the dyke dip detected by the inversion models suggests that displacement on the faults included rotation about an axis perpendicular to the trend of the fault the resulting model of scissor or splay faults is compatible with previously published palaeomagnetic studies of the olivine dykes in the sudbury region

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

differentiation and emplacement of the worthington offset dike of the sudbury impact structure ontario the offset dikes of the 1 85 ga sudbury igneous complex sic constitute a key topic in understanding the chemical evolution of the impact melt its mineralization and the interplay between melt migration and impact induced deformation the origin of the melt rocks in offset dikes as well as mode and timing of their emplacement are still a matter of debate like many other offset dikes the worthington is composed of an early emplaced texturally rather homogeneous quartz diorite qd phase at the dike margin and an inclusion and sulfide rich quartz diorite iqd phase emplaced later and mostly in the centre of the dike the chemical heterogeneity within and between qd and iqd is mainly attributed to variable assimilation of host rocks at the base of the sic prior to emplacement of the melt into the dike petrological data suggest that the parental magma of the worthington dike mainly developed during the pre liquidus temperature interval of the thermal evolution of the impact melt sheet 1200 c based on thermal models of the cooling history of the sic the two stage emplacement of the worthington dike occurred likely thousands to about ten thousand years after impact structural analysis indicates that an alignment of minerals and host rock fragments within the worthington dike was caused by ductile deformation under greenschist facies metamorphic conditions rather than flow during melt emplacement it is concluded that the worthington offset dike resulted from crater floor fracturing possibly driven by late stage isostatic readjustment of crust underlying the impact structure

a 3d model of first order structural elements of the vredefort dome south africa importance for understanding central uplift formation of large impact structures abstract based on detailed structural mapping geophysical information and structural consideration we generated a 3d structural model of a segment of the vredefort dome this model provides insight into the mechanism of mass transfer during central uplift formation of large impact structures the model consists of geometrically and kinematically linked km scale radial and concentric faults that affected the overturned limb of the rim syncline to the vredefort dome the faults accomplished constrictional centripetal rock flow followed by radial spreading of uplifted and gravitationally unstable rocks in the crater centre more specifically concentric faults formed likely as normal faults during transient crater rim collapse steepened and were transformed to reverse faults during central rock uplift and finally were overturned during gravitational collapse of the central uplift radial faults by contrast formed at a later stage of convergent rock flow towards the crater centre and retained largely their orientation during central uplift formation displacement on individual faults is up to 2 km cumulative displacement notably on concentric faults can account for the large displacement magnitudes predicted by numerical models

geochemistry across an exposed section of archaean crust at vredefort south africa with implications for mid crustal discontinuities abstract the central region of the vredefort structure consists of a semi circular multi layered sequence of crystalline rocks which are nearly vertical in attitude and which increase in metamorphic grade towards the core of the structure together with the overlying precambrian strata this sequence provides a cross section through almost the entire crustal section of the kaapvaal craton 36 km the upper part of the vredefort crystalline crust consists of a 3 0 ga sequence of differentiated felsic rocks in amphibolite facies the lower crust consists of a complex and heterogeneous both chemically and isotopically high grade metamorphic terrain of charnockites granulites mafic and felsic and supracrustal rocks the upper crust is separated from the lower crust by the vredefort discontinuity a brittle ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock petrographic chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution this is indicative of different geological environments prior to their present juxtaposition we speculate that the upper and the lower parts of the vredefort crystalline crust were juxtaposed during intracratonic thrusting 2 8 ga ago and that the entire 36 km section of crust including the vredefort discontinuity were rotated and uplifted into their present vertical position during the 2 0 ga vredefort catastrophe

numerical modeling of impact heating and cooling of the vredefort impact structure large meteorite impacts such as the one that created the vredefort structure in south africa 2 ga ago result in significant heating of the target the temperatures achieved in these events have important implications for post impact metamorphism as well as for the development of hydrothermal systems to investigate the post impact thermal evolution and the size of the vredefort structure we have analyzed impact induced shock heating in numerical simulations of terrestrial impacts by projectiles of a range of sizes thought to be appropriate for creating the vredefort structure when compared with the extent of estimated thermal shock metamorphism observed at different locations around vredefort our model results support our earlier estimates that the original crater was 120 160 km in diameter based on comparison of predicted to observed locations of shock features the simulations demonstrate that only limited shock heating of the target occurs outside the final crater and that the cooling time was at least 0 3 myr but no more than 30 myr

re os isotope and geochemical study of the vredefort granophyre clues to the origin of the vredefort structure south africa the origin of granophyric rock dikes the vredefort granophyre at the 2 ga vredefort structure in south africa has been controversial they have been interpreted as either impact melt or igneous intrusions most vredefort granophyre samples have considerably higher os contents than the country rocks and 187re 188os and 187os 188os ratios that scatter about a 2 ga isochron their initial 187os 188os ratios at 2 ga overlap the meteoritic data range indicating that all the granophyre samples contain some meteoritic os the re os isotopic composition of the granophyre is significantly different from that of local precursor rocks and indicates that up to 0 2 of a chondritic component could be contained in the granophyre our results confirm the interpretation that the vredefort granophyre represents an impact melt rock

the dimensions of the chicxulub impact crater and impact melt sheet the chicxulub impact crater which is the principal source of impact debris in cretaceous tertiary boundary sediments is currently buried by a 1 km thick sequence of carbonate sediments because the crater is not exposed for direct scrutiny its size has remained uncertain and in particular estimates ranging from 100 to 300 km have been made on the basis of gravity and magnetic field data in this study the physical properties of the k t boundary impact ejecta and chicxulub impact melt sheet have been used to test the geophysical estimates of the impact crater s size on the basis of the measured thickness of impact ejecta in haiti and north america the calculated volume of the impact melt sheet and the chemical and isotopic composition of the melt sheet the chicxulub impact crater is inferred to be 180 km in diameter and to contain a 3 to 7 km thick melt sheet and breccia sequence within a centrally located 100 km diameter region that corresponds to the limits of the transient cavity

archean age for the granulite facies metamorphism near the center of the vredefort structure south africa granulite facies metamorphic assemblages in rocks exposed near the center of the 2 02 ga vredefort impact structure previously have been interpreted either as early proterozoic genetically related to the 2060 ma bushveld complex or as archean and representative of lower crust that rebounded to upper crustal levels following an impact event zircon and monazite recovered from the granulite facies rocks record high grade metamorphism at 3107 9 ma and a primary age of 3425 ma for detrital zircon a shock deformed but otherwise pristine dolerite dike that intrudes the granulite terrane yields a u pb zircon age of 2560 ma providing a minimum age for the metamorphism these isotopic age data are difficult to reconcile with a regional high grade metamorphic event in the crust beneath vredefort at 2060 ma instead the preimpact high temperature metamorphic history observed in the vredefort lower crustal rocks indicates an enigmatic high temperature event during the stabilization of diamondiferous archean tectosphere

dating the shock wave and thermal imprint of the giant vredefort impact south africa u pb geochronology of single grains of zircon and monazite has been used to date an episode of intense postimpact metamorphism in the core of the deeply eroded vredefort impact structure of south africa results from two basement units exposed in the uplifted central region indicate that the impact and a later pyroxene hornfels metamorphic event were penecontemporaneous at 2020 3 ma discovery of a synimpact to postimpact dike of norite that intruded at 2019 2 ma is the first recognition of mafic igneous activity related to impact the dike is either derived from a sudbury type impact melt layer since eroded or is the product of decompression melting of kaapvaal mantle in response to the ablation of 15 km of crust at the center of the crater the combined heating effects of the shock wave and impact triggered magmas are thought to have created the 300 km 2 thermal imprint of the asteroid collision with kaapvaal craton and account for the nearly coeval timing relationship between core metamorphism and shock revealed by this study

analysis of petrologic hypotheses with pearce element ratios pearce element ratios can test whether the members of a rock suite are comagmatic and can illustrate the causes of chemical diversity in comagmatic suites comagmatic rocks have constant ratios for elements conserved in the system during changes that led to the chemical diversity in basaltic systems the incompatible elements ti k and p are often conserved the slope of the trend on a pearce element ratio diagram is sensitive to the stoichiometry of the crystallizing and segregating phases a judicious choice of ratios as axes for the diagram provides a signature for the phases involved and estimates of their compositions in basaltic rocks diagrams with ti k vs p k can provide a test of the comagmatic hypothesis diagrams with 0 5 mg fe k vs si k have trends that are distinct for each comagmatic suite and different mineral assemblage different suites are distinguished by the intercepts in diagrams whereas mineral assemblages are recognized by the slopes of the trends for example if olivine is the sole crystallizing and segregating phase the trend will have a slope of 1 diagrams with 2ca na k vs al k distinguish plagioclase from augite assemblages and in conjunction with 0 5 mg fe k diagrams unravel the crystallization sequences of suites that have suffered three phase crystallization and segregation analyses from the uwekahuna laccolith kilauea the 1955 and 1967 68 eruptions of kilauea diamond craters volcanic field oregon and experimental data on morb glasses provide illustrations of the interpretations that can be obtained from pearce element ratios

emplacement history of granophyre dikes in the vredefort impact structure south africa inferred from geochemical evidence abstract the central vredefort impact structure is characterised by impact melt rocks known as the vredefort granophyre dikes the mode of emplacement of which is not well known whole rock and petrographic analyses of two dikes were conducted and compared to published geochemical data to elucidate the mode and timing of dike formation the dikes are characterised by compositional and textural heterogeneity between and within individual dikes specifically central dike portions are felsic and rich in wall rock fragments whereas marginal dike phases are more mafic and fragment poor collectively this suggests that melt was derived from compositionally different parental melts and emplaced in at least two pulses in addition the chemical heterogeneity between fragment rich and fragment poor dike zones can be explained by variable assimilation of a mafic component notably ventersdorp basalt at the base of the impact melt sheet from which melt of the granophyre dikes is derived this scenario accounts for the mafic and fragment poor character of melt emplaced first in the dikes and the more felsic and fragment rich nature of melts of the following emplacement pulse i e when the impact melt was less hot and thus less capable of digesting large quantities of mafic wall rock fragments differences in geometrical textural chemical and fragment characteristics between the granophyre dikes and pseudotachylite bodies can be explained by the same process i e impact melt drainage but operating at different times after impact

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