spatial distribution of zircon with shock microtwins in pseudotachylite bearing granite gneisses vredefort impact structure south africa abstract mechanical microtwins in zircon is a characteristic shock feature and a potential tool for estimating shock pressure and or shear stress however so far zircon with microtwins has rarely been described in situ the present study of pseudotachylite bearing granitic gneisses from the vredefort impact structure focuses on the spatial distribution of zircon with microtwins and other deformation features zircon hosted by granitic gneiss does not reveal extensive shock deformation with decreasing distance to pseudotachylite zircon progressively develops sub planar fractures planar deformation bands low angle boundaries rarely thin 112 microtwin lamellae and closest to the vein it is fragmented in contrast zircon hosted by pseudotachylites contains thick and closely spaced microtwin lamellae which form up to three sets within individual grains microtwin lamellae are associated with planar 112 low angle boundaries and with overprinting crystal plastic and brittle deformation we suggest that planar 112 low angle boundaries in zircon are indicative of shock deformation and that microtwins in zircon within pseudotachylite veins are an indicator of the impact origin for the latter as opposed to endogenic seismically induced friction melts locally elevated shock pressure and shear stress induce the formation of pseudotachylites in the target rock as well as shock microtwins in zircon

new observations on shatter cones in the vredefort impact structure south africa and evaluation of current hypotheses for shatter cone formation shatter cones have been described from many meteorite impact structures and are widelyregarded as a diagnostic macroscopic recognition feature for impact however the origin of thismeso to macroscopic striated fracture phenomenon has not yet been satisfactorily resolved and thetiming of shatter cone formation in the cratering process still remains enigmatic here previousresults from studies of shatter cones from the vredefort impact structure and other impact structuresare discussed in the light of new field observations made in the vredefort dome contrary to earlier claims vredefort cone fractures do not show uniform apex orientations at any given outcrop nor do small cones show a pattern consistent with the previously postulated master cone concept simple back rotation of impact rotated strata to a horizontal pre impact position also does not lead to a uniform centripetal upward orientation of the cone apices striation patterns on the cone surfaces are variable ranging from the typically diverging pattern branching off the cone apex to subparallel to parallel patterns on almost flat surfaces striation angles on shatter cones do not increase with distance from the center of the dome as alleged in the literature instead a range of striation angles is measured on individual shatter cones from a specific outcrop new observations on small scale structures in the collar around the vredefort dome confirm the relationship of shatter cones with subparallel sets of curviplanar fractures so called multipli striated joint sets msjs pervasive meter scale tensile fractures cross cut shatter cones and appear to have formed after the closely spaced msj type fractures the results of this study indicate that none of the existing hypotheses for the formation of shatter cones are currently able to adequately explain all characteristics of this fracturing phenomenon therefore we favor a combination of aspects of different hypotheses that includes the interaction of elastic waves as supported by numerical modeling results and which reasonably explains the variety of shatter cone shapes the range of striation geometries and angles and the relationship of closely spaced fracture systems with the striated surfaces in the light of the currently available theoretical basis for the formation of shatter cones the results of this investigation lead to the conclusion that shatter cones are tensile fractures and might have formed during shock unloading after the passage of the shock wave through the target rocks

yarrabubba a large deeply eroded impact structure in the yilgarn craton western australia abstract yarrabubba is a newly discovered impact structure situated within the complex granite greenstone terrain of the yilgarn craton shock metamorphic effects including shatter cones planar deformation features in quartz grains and pseudotachylites were found in deeply eroded archean granites near yarrabubba station southeast of meekatharra western australia aeromagnetic images reveal arcuate demagnetization features at diameters between 11 and 25 km which roughly correspond to the outcropping of the yarrabubba granite and are centered on a magnetic high halo around the barlangi granophyre 119 50 e 27 10 s rapid quench textures in the barlangi granophyre and its inter fingering relationships with pseudotachylites suggest that it is an impact melt that was injected into the yarrabubba granite and spread along fault discontinuities both the potassic yarrabubba granite and the felsic barlangi granophyre are atypical in the northern yilgarn as are the abundant fracturing and frictional melting within the local granitoids these anomalous geological features associated with shock metamorphic effects are indicative of a hypervelocity impact origin the age of the impact is uncertain but geological and geophysical relationships suggest that the yarrabubba structure was formed during the early proterozoic

the manicouagan impact structure an analysis of its original dimensions and form manicouagan quebec 51 23 n 68 42 w is the most intensively studied large complex terrestrial impact structure in a predominantly crystalline target the ground truth data available from manicouagan have considerable potential for interpreting the subsurface characteristics of lunar impact structures of comparable morphology and size two contrasting hypotheses however exist for the preerosional form of manicouagan 1 a multi ring basin with a final diameter of 75 km and a transient cavity diameter of 30 45 km floran and dence 1976 and 2 an endogenically modified peak ring basin with a final diameter of 100 km and a transient cavity diameter of 80 km orphal and schultz 1978 both hypotheses are based largely on topographic data and interpret the prominent 65 km diameter annular moat as a graben like feature formed either 1 by collapse of the outer slope of the transient cavity rim or 2 by uplift of the final crater floor through post impact intrusive activity the present analysis suggests that this annular moat is primarily an erosional feature and that the original form of the impact structure cannot be determined unequivocally on the basis of present topography if all the available topographic geologic and geophysical data are considered however then an internally consistent interpretation of original dimensions is possible the distribution of shock deformation effects in the basement rocks of the present crater floor suggests an original transient cavity diameter of 60 km the 55 km diameter impact melt covered inner plateau provides a minimum estimate for the diameter of the final floor of the crater and the annular moat with its outliers of downdropped ordovician limestone is interpreted as marking the contact between the floor and the innermost slump blocks of the final rim there is no compelling evidence to regard the annular moat as a graben by analogy with fresh lunar and mercurian structures a floor width of 55 km suggests a final rim diameter of 86 95 km this is consistent with the topographic data from outside the annular moat and with the residual peripheral gravity low which suggests a final rim diameter of 85 95 km due to erosion it is difficult to assign manicouagan to a particular morphological class of impact structure but on the basis of the available data it is most likely that the preerosional form was that of a central peak crater or possibly a peak ring basin at the time of its formation manicouagan may have been dimensionally and possibly morphologically similar to the 96 km diameter lunar crater copernicus

studies of the rooiberg group bushveld complex south africa no evidence for an impact origin abstract it has been suggested that the bushveld complex of south africa could be the result of multiple large meteorite or comet impacts according to this hypothesis part of the lower rooiberg group which forms the roof of the complex represents a sheet of impact melt breccia and other impact breccias the present study is an attempt to test the viability of the impact hypothesis for the bushveld complex by interpreting newly acquired field geochemical petrographic and textural data for rooiberg group and associated rocks extensive field work throughout the rooiberg group and particularly at the contact between this unit and the underlying pretoria group metasediments has failed to identify any material that could be interpreted as impact related the rooiberg group is predominantly composed of individual volcanic flows and pyroclastic units representing several geochemically distinct magma types these volcanic units are interbedded with thin laterally extensive sedimentary units a few of which are sedimentary breccias the presence of needles of quartz that may represent paramorphs after tridymite in some rooiberg group units has been used as evidence to support the contention that these rocks represent superheated impact melt however quartz paramorphs after tridymite have been recognized in terrestrial volcanic provinces e g the north shore volcanic group in northeastern minnesota structural data including dips of rooiberg group strata suggest that the lobate shape of the complex which resembles several closely spaced ring features is the result of post rooiberg group deformation microdeformation features in quartz from bushveld related rocks do not satisfy the criteria of shock metamorphic planar deformation features pdfs which would be characteristic of impact induced shock pressures between 10 and 30 gpa these data especially the absence of macroscopic and microscopic evidence of shock deformation in pre bushveld rocks are inconsistent with formation of the bushveld complex by impact

a tem investigation of shock metamorphism in quartz from the vredefort dome south africa the origin of the vredefort structure in south africa is still debated several causes have been discussed namely asteroid impact internal gas explosion or tectonic processes evidence of dynamic rock deformation is pervasive in the form of planar features in quartz grains shatter cones veins of pseudotachylite and occurrence of coesite and stishovite high pressure quartz polymorphs a number of these characteristics is widely believed to support an impact origin however the planar features in quartz which are generally considered as one of the strongest indicators of impact are in the vredefort case considered as anomalous when compared with those from accepted impact structures we have investigated by optical and transmission electron microscopy tem the defect microstructures in quartz grains from different lithologies sampled at various places at the vredefort structure whatever the locality only thin mechanical brazil twin lamellae in the basal plane are observed by tem so far such defects have only been found in quartz from impact sites but always associated with sets of thin glass lamellae in rhombohedral planes 10 1n with n 1 2 3 and 4 at the scale of the optical microscope brazil twins in 0001 are easily detected in vredefort quartz grains because of the numerous tiny fluid inclusions which decorate them similar alignments of tiny fluid inclusions parallel to other planes are also detected optically but at the tem scale no specific shock defects are detected along their traces if these inclusion alignments initially were shock features they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae fine grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen but stishovite was not found even in areas where its occurrence was previously reported finally definite evidence of high temperature annealing was observed in all the samples these observations lead us to the conclusion that our findings regarding microdeformation in quartz are consistent with an impact origin for the vredefort structure most of the original shock defects are now overprinted by an intense post shock annealing episode only the thin mechanical twin lamellae in the basal plane have survived

chicxulub multiring impact basin size and other characteristics derived from gravity analysis the buried chicxulub impact structure in mexico which is linked to the cretaceous tertiary k t boundary layer may be significantly larger than previously suspected reprocessed gravity data over northern yucatan reveal three major rings and parts of a fourth ring spaced similarly to those observed at multiring impact basins on other planets the outer ring probably corresponding to the basin s topographic rim is almost 300 kilometers in diameter indicating that chicxulub may be one of the largest impact structures produced in the inner solar system since the period of early bombardment ended nearly 4 billion years ago

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

relationships between the vredefort structure and the witwatersrand basin within the tectonic framework of the kaapvaal craton as interpreted from regional gravity and aeromagnetic data abstract processed gravity and aeromagnetic data covering the witwatersrand basin and adjacent areas have been used to delineate the boundary of the basin and to investigate other features such as the vredefort structure the bethlehem gravity high and the colesberg magnetic anomaly trend a model is proposed in which the regional aeromagnetic and gravity signatures of these features can be related to the disposition of a semi continuous mid crustal magnetite rich layer which is exposed or has a shallow suboutcrop in three regions in the centre of the vredefort structure along the kaapvaal craton boundary and centrally through the craton northwards from colesberg the bethlehem high is thought to be due to elevation of lower levels of the crust along a nw trending geanticlinal arch the apex of this broad arch is termed the vredefort axis and plunges northwestwards beneath the witwatersrand basin the vredefort structure lies at the intersection of this axis and a major nne trending axis of crustal downwarp which also constitutes the long axis of the witwatersrand basin two contrasting scenarios for the development of these major axes are discussed with either scenario genesis of the vredefort structure should be integrated with the genesis and evolution of the vredefort axis and with the tectonic evolution of the kaapvaal craton

decline of giant impacts on mars by 4 48 billion years ago and an early opportunity for habitability the timing of the wane in heavy meteorite bombardment of the inner planets is debated its timing determines the onset of crustal conditions consistently below the thermal and shock pressure limits for microbiota survival and so bounds the occurrence of conditions that allow planets to be habitable here we determine this timing for mars by examining the metamorphic histories of the oldest known martian minerals 4 476 4 429 gyr old zircon and baddeleyite grains in meteorites derived from the southern highlands we use electron microscopy and atom probe tomography to show that none of these grains were exposed to the life limiting shock pressure of 78 gpa 97 of the grains exhibit weak to no shock metamorphic features and no thermal overprints from shock induced melting by contrast about 80 of the studied grains from bombarded crust on earth and the moon show such features the giant impact proposed to have created mars hemispheric dichotomy must therefore have taken place more than 4 48 gyr ago with no later cataclysmic bombardments considering thermal habitability models we conclude that portions of mars crust reached habitable pressures and temperatures by 4 2 gyr ago the onset of the martian wet period about 0 5 gyr earlier than the earliest known record of life on earth early abiogenesis by 4 2 gyr ago is now tenable for both planets the oldest known minerals from mars have no strong shock features indicating early cessation of giant impacts there according to microanalysis of zircon and baddeleyite grains in meteorites

three dimensional magnetic imaging of the chicxulub crater although few magnetization measurements are available for the structural elements of the chicxulub impact crater the magnetization intensities of the melt sheet upper breccia unit and central uplift are 3 4 orders of magnitude greater than the 3 to 4 km thick carbonate and evaporite stratigraphy covering the yucatan block this allows three dimensional modeling of the crater s structure by inversion using a two layer model two layers are separately inverted by dividing the crater s magnetic field expression into 40 km wavelength components the upper layer average depth 2 km models the distribution of highly magnetized zones in the crater s melt sheet the lower layer average depth 5 km represents relief on the yucatan block s basement surface and effectively maps the crater s 50 km diameter central uplift and possibly the expression of the surrounding collapsed disruption cavity fill the shallower magnetized zones consist of two generally concentric distributions at radii of 20 and 45 km these highly magnetized zones are thought to result from hydrothermal systems localized at the edge of the central uplift and the collapsed disruption cavity having produced magnetic phases during alteration of the melt sheet

shock barometry of the siljan impact structure sweden the siljan impact structure in sweden is the largest confirmed impact structure in western europe despite this the structure has been poorly studied in the past and detailed studies of shock metamorphic features in the target lithologies are missing here we present the results of a detailed systematic search for shock metamorphic features in quartz grains from 73 sampled localities at siljan at 21 localities from an area approximately 20 km in diameter located centrally in the structure the orientations of 2851 planar deformation feature sets in 1179 quartz grains were measured observations of shatter cones outside of the zone with shocked quartz extend the total shocked area to approximately 30 km in diameter the most strongly shocked samples recording pressures of up to 20 gpa occur at the very central part of the structure and locally in these samples higher pressures causing melting conditions in the affected rocks were reached pressures recorded in the studied samples decrease outwards from the center of the structure forming roughly circular envelopes around the proposed shock center based on the distribution pattern of shocked quartz at siljan the original transient cavity can be estimated at approximately 3238 km in diameter after correcting for erosion we conclude that the original rim to rim diameter of the siljan crater was somewhere in the size range 5090 km less

seismic reflection analysis of the manson impact structure iowa our combined interpretation of new high resolution seismic reflection data and reprocessed but previously published industrial vibroseis data indicates that the manson impact structure iowa has an apparent crater diameter of 35 kin an annular trough diameter of around 21 km a shallow floor 0 6 0 7 kin and a central uplift that has a minimum diameter of 7 5 kin the two reflection lines are coincidentally located along an east west radial transect and are consmined by shallow drill information results from the two data sets are correlative both data sets were instnunental to the final interpretation due to the trade offbetween resolution and depth of energy penetration based on the combined interpretation structural uplift of the central peak is estimated to be around 2 8 kin onlapping seismic sequences are present at the eastern edge of the central uplift these seismic packages observed only in the high resolution line are interpreted to represent impact breccia or debris material that was shed from the central peak or dynamically transported from outside of the crater

shatter cones branched rapid fractures formed by shock impact 1 shatter cones are rock discontinuities known only from sites of extraterrestrial impacts and they are assumed to be formed by impact induced shock waves here we characterize the structure of shatter cones by field and microanalyses and explain their formation by dynamic fracture mechanics our analyses reveal that shatter cones always occur as multilevel three dimensional networks 0 01 100 m in size with hierarchal branched fractures a typical individual shatter cone is a curved oblate branch that bifurcates from its parent fracture e g a larger shatter cone and expands to form a spoon like surface the unique shatter cone striations are arranged in v shaped pairs whose enclosed angle is constant for a given sample we propose that shatter cones are the natural consequence of tensile rock fracturing at extreme velocities first the structure of shatter cone networks is strikingly similar to the structure of branched networks of experimental dynamic fractures that propagate at high velocities velocities that approach the rayleigh wave speed vr second fracture front waves generated experimentally by the interaction of a rapidly moving tensile fractures and material inclusions create tracks on the fracture surface that correspond to the v shaped striations of shatter cones third applying the front wave concept to our field measurements vredefort impact south africa shows that the shatter cones propagated at velocities of 0 98 0 90vr with a systematic velocity decrease from the impact center these extreme asymptotic velocities require the intense energy flux of impacts our model explains all of the structural features of shatter cones curved surfaces cone directivity unique striations hierarchic multilevel structure and their exclusive occurrence at impact sites

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

origin of pseudotachylite in the lower witwatersrand supergroup vredefort dome south africa constraints from metamorphic studies small sub millimetre to centimetre thick pseudotachylite veins in metapelitic rocks of the lower witwatersrand supergroup in the collar of the vredefort dome are recrystallized to a biotite cordierite paragenesis indicating the attainment of lower amphibolite facies metamorphic conditions t 500 c following their formation the veins occur along ubiquitous anastomosing fracture networks displaying millimetre to centimetre scale displacements and truncate and incorporate fragments of an older ca 2 05 ga mid amphibolite facies peak metamorphic assemblage the post pseudotachylite metamorphism is attributed to the same event that caused highly variable annealing of ca 2 02 ga shock microdeformation features in rocks across the dome together with the disproportionately large volume of pseudotachylite in the rocks relative to that found in tectonic provinces and the absence of evidence of genetically related large scale shear or fault zones this metamorphic evidence suggests an origin for the pseudotachylite related to the 2 02 ga vredefort bolide impact event that produced the dome possible origins for the pseudotachylite include shock brecciation and melting post shock decompression melting and friction melting associated with the post impact high velocity excavation and modification phases of crater formation the post pseudotachylite metamorphism is attributed to the combined effects of an elevated pre impact crustal geotherm 25 c km related to the waning stages of the 2 05 ga peak metamorphism release of elastic strain energy from the impact shock event and contact effects beneath the impact melt body

metamorphism of the witwatersrand gold fields a review abstract there is widespread evidence for greenschist facies metamorphism in different rock types within all the witwatersrand gold mines the silicate mineral assemblages suggest near uniform peak metamorphic conditions equivalent to the chlorite zone in all the major witwatersrand gold fields assemblages of coexisting pyrophyllite chloritoid have been recorded in most of the gold mines and the critical assemblage of pyrophyllite chloritoid muscovite chlorite quartz tourmaline rutile pyrite is found in most gold fields kaolinite paragonite biotite pyrrhotite and arsenopyrite have more local distribution pelites shales quartzites meta conglomerates metabasalts and some dykes record these metamorphic assemblages albeit in different mineral proportions and pyrophyllite and chloritoid are recorded from many gold producing reefs andalusite kyanite garnet and or cordierite bearing assemblages indicate higher metamorphic grades in three discrete areas along the nw margin of the witwatersrand basin to the ne of the evander gold field and around the collar of the vredefort dome concentric metamorphic facies around the vredefort dome culminate in granulite facies assemblages near the centre it is important to note that there are no definitive published data constraining the temporal relationships between the granulites and amphibolites of the vredefort dome greenschist facies assemblages in the vredefort collar rocks thermal metamorphism related to intrusion of the bushveld complex and the regional greenschist facies metamorphism documented in the gold fields metamorphic conditions of 350 50 c and up to 3 kbars are inferred for the gold fields with temperatures increasing to around 700 c in granulites of the vredefort dome other geothermometric methods including illite crystallinity fluid inclusions carbon elemental ratios and vitrinite reflectance are compatible with the above estimates but are each associated with considerable temperature uncertainties fluids in equilibrium with pyrophyllite muscovite quartz pyrite paragonite are inferred during metamorphism suggesting slightly acid conditions and elevated sulphur activities regional greenschist facies metamorphism documented in the witwatersrand gold mines was accompanied by widespread deformation large fluxes of aqueous fluid and considerable element mobility the low intermediate pressure of this metamorphic terrain and the lack of large scale tectonic thickening favours underplating by mafic magmas as the likely cause of the metamorphic pattern no model for gold genesis in the witwatersrand basin is complete without accommodating post burial processes including metamorphism deformation and alteration any placer gold would be overprinted by these processes and any epigenetic gold could potentially be introduced during these periods of thermal and fluid activity it is also important to note at least two quite unusual features of the witwatersrand metamorphic assemblages the virtual absence of feldspar from the 3 km thick central rand group over 300 km and the narrow range of metamorphic grade inferred for the gold mines over this same distance

dynamic fracture by large extraterrestrial impacts as the origin of shatter cones a large impact by a comet or meteorite releases an enormous amount of energy which evaporates melts and fractures the surrounding rocks distinctive features of such impacts are shatter cones deformed rocks characterized by hierarchical striated features although such features have been used for decades as unequivocal fingerprints of large body impacts the process by which shatter cones form has remained enigmatic here we show that the distinctive shatter cone striations naturally result from nonlinear waves front waves that propagate along a fracture front this explains the observed systematic increase of striation angles with the distance from the impact shatter cone networks typically spanning many scales can be understood as hierarchical bifurcations of the fracture front which is generated by the immense energy flux carried by the initial impact generated shock waves our quantitative predictions based on this theory are supported by field measurements at the kentland and vredefort impact sites these measurements indicate that shatter cones near to the impact site were formed by fractures propagating at nearly the rayleigh wave speed of the host rocks whereas the furthest shatter cones observed about 40 km from the impact site were formed by fronts moving more slowly these results provide insight into impact dynamics as well as dissipative mechanisms in solids subjected to sudden extremely intense fluxes of energy

the sudbury structure ontario canada a tectonically deformed multi ring impact basin the occurrence of shock metamorphic features substantiates an impact origin for the 1 85 ga old sudbury structure but this has not been universally accepted recent improvements in knowledge of large scale impact processes combined with new petrographic geochemical geophysical lithoprobe and structural data allow the sudbury structure to be interpreted as a multi ring impact structure the structure consists of the following lithologies sudbury breccia dike breccias occurring up to 80 km from the sudbury igneous complex sic footwall rocks and footwall breccia brecciated shocked crater floor materials in part thermally metamorphosed by the overlying sic sublayer and offset dikes main mass of the sic and basal member of the onaping formation of geochemically heterogeneous coherent impact melt complex ranging from inclusion rich basal unit through a dominantly inclusion free to a capping inclusion rich impact melt rock grey member of of melt rich impact breccia suevite green member of of thin layer of fall back ejecta black member of of reworked and redeposited breccia material onwatin and chelmsford formations post impact sediments observational and analytical data support an integrated step by step impact model for the genesis of these units analysis of the present spatial distribution of various impact related lithologies and shock metamorphic effects result in an estimated original rim to rim diameter of the final crater of 200 or even 280 km for the sudbury structure prior to tectonic thrusting and deformation during the penokean orogeny

pseudotachylite in impact structures generation by friction melting and shock brecciation a review and discussion abstract fault related pseudotachylites are generated as the result of brittle or brittle ductile deformation related to seismic faulting it is generally accepted among tectonic workers that while cataclasis plays a role in pseudotachylite formation friction melting is involved in addition pseudotachylite and pseudotachylite like breccias have been repeatedly described from the basement exposures of impact and cryptoexplosion structures and as veinlets in shocked meteorites and lunar samples the relevant literature is reviewed with regard to controversial usage of the term pseudotachylite and attention is drawn to several problematic aspects 1 the term pseudotachylite is currently used both as a purely descriptive term and with genetic implications 2 no unambiguous criteria for comparison and distinction of tectonically and impact generated pseudotachylites have been identified 3 it needs to be considered that in impact structures pseudotachylite could be generated by two distinct processes namely by shock brecciation melting and by friction melting pseudotachylite in impact structures could be the result of either of these two processes or of both 4 it is recommended to adhere more stringently when describing breccias from impact or cryptoexplosion structures to nomenclature accepted for the description of fault rocks in tectonic environments this would help to avoid confusion when discriminating between melt rocks pseudotachylite impact melt rock fragmental breccias cataclasite fragmental impact breccia and mylonitic rocks 5 no criteria for the distinction of melt bearing impact breccias impact melt rock and pseudotachylite have been established either in order to solve these problems close interaction between structural geologists and impact workers is warranted it is suggested that the understanding of breccias in impact structures would be improved by paying particular attention to the temporal relationships between different breccia types this would facilitate recognition of possibly different formational processes for impact produced melt and fragmental breccias and their distinction from pre and post shock deformation products

aspects of the dynamic and thermal metamorphic history of the vredefort cryptoexplosion structure implications for its origin abstract the vredefort structure is the oldest and the largest known cryptoexplosion structure on earth an approximately 36 km deep section through the archean sialic crust and the overlying precambrian strata of the kaapvaal craton is exposed in the core of the structure the geology presented in the exposed section includes all the principal metamorphic facies in the crust and records a long and complex thermo tectonic history which dates back to at least 3 5 ga the petrographie and geological observations in the basement rocks indicate that there is a complex interrelationship between the archean geology and the 2 0 ga dynamic and thermal metamorphic overprint some of which are postulated to be indicative of impact processes the dynamic and thermal metamorphic effects do not increase progressively towards the centre of the structure as found at known impact structures in particular dynamic deformation phenomena such as pseudotachylite and planar features in quartz reach maximum intensity in the rocks close to the vredefort discontinuity a brittle ductile shear zone which separates upper crustal amphibolite facies rocks from lower crustal granulites in certain other lithological zones deformation phenomena are noticeably absent or diminished we suggest that changes in the physical and chemical properties of the rocks from margin to centre of the basement may account for the variation in the intensity of the 2 0 ga thermal and dynamic metamorphic effects observed at vredefort in conclusion our overall impression of the vredefort structure is that it is a relic of an ancient meteorite impact crater but that there were thermo tectonic events which occurred both prior to and after the postulated impact event which complicates the interpretation of its origin

a case study of the postdepositional alteration of the witwatersrand basal reef gold placer an investigation of the mineralogy quartz texture fluid inclusions and gold composition of a crossbedded auriferous specimen of the archcan basal reef placer from the welkom gold field witwatersrand south africa has established the degree of postdepositional alteration to which the detrital gold particles were subjected three different postdepositional events can be distinguished 1 regional static recrystallization at a peak metamorphic temperature between 290 degrees and 350 degrees c and a pressure between 2 and 3 kbars 2 local fluid infiltration along fractures and sedimentary interfaces at a temperature between 240 degrees and 300 degrees c and a pressure similar to the above and 3 local k super metasomatism during a later fluid infiltration event along the preexisting zones of weakness intracrystalline diffusion during these events led to homogenization of the gold on a hand specimen scale with respect to ag 8 9 wt and with respect to hg 1 0 wt furthermore during the second event 2 fluid infiltration caused not only homogenization but also partial mobilization of some of the detrital gold with transport over distances of not more than a few millimeters to centimeters these processes were accompanied by microscopic silicification calcitization and possibly hg metasomatism the ph value of this infiltrating fluid was around 4 primary fluid inclusions associated with secondary gold in authigenic quartz consist of cacl 2 rich aqueous solutions whereas secondary fluid inclusions within such authigenic quartz are dilute aqueous solutions having a very low salinity that is attributed to nacl and kcl event 3 it is believed that gold mobilization was coeval with a fracturing and shearing event after deposition of at least parts of the transvaal supergroup on top of the witwatersrand and ventersdorp supergroups the last event 3 the infiltration of k super enriched fluid after the partial mobilization of the gold is tentatively correlated with the bushveld or vredefort events

magnetic fields of lunar multi ring impact basins we survey the magnetic fields of lunar multi ring impact basins using data from the electron reflectometer instrument on the lunar prospector spacecraft as for smaller lunar craters the primary signature is a magnetic low that extends to 1 5 2 basin radii suggesting shock demagnetization of relatively soft crustal magnetization a secondary signature as for large terrestrial basins is the presence of central magnetic anomalies which may be due to thermal remanence in impact melt rocks and or shock remanence in the central uplift the radial extent of the anomalies may argue for the former possibility but the latter or a combination of the two are also possible central anomaly fields are absent for the oldest pre nectarian basins increase to a peak in early nectarian basins and decrease to a low level for imbrian basins if basin associated anomalies provide a good indication of ambient magnetic fields when the basins formed this suggests the existence of a magnetic era possibly due to a lunar core dynamo similar to that implied by paleointensity results from returned lunar samples however the central basin anomalies suggest that the fields peaked in early nectarian times and were low in imbrian times while samples provide evidence for high fields in nectarian and early imbrian times

microscopic planar deformation features in quartz of the vredefort structure anomalous but still suggestive of an impact origin abstract the orientation and distribution of diagnostic shock produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the vredefort structure in south africa these features are widespread occurring in all but four samples the majority of the planar features have a basal 0001 orientation with a few higher pressure so called features and three features occurring near the center of the structure their relative distribution and preservation are however anomalous compared to those at other large terrestrial impact structures and recorded shock pressures calculated on the basis of planar feature orientation do not show a regular decrease radially outward as observed at other known impact structures the apparently anomalous development of planar features is interpreted as a result of post shock recrystallization if the effect of recrystallization of quartz which generally increases inward toward the core of the structure is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure then higher pressures are recorded in the core compared to the collar rocks no evidence of a previously reported additional post recrystallization shock event is found in this study although previous researchers report that pseudotachylite which may or may not be associated with the shock event cuts recrystallized quartzites in is concluded that although anomalous the evidence from planar features in quartz is still consistent with the vredefort structure being an erosional remnant of a large complex impact structure

an early ordovician 40ar 39ar age for the 50 km carswell impact structure canada the formation age of the large 50 km carswell impact structure canada has been a matter of debate ever since its discovery five decades ago with proposed ages ranging from mesoproterozoic to early cretaceous here we present new 40ar 39ar data for aliquots of euhedral adularia separated from vesicles in an impact melt rock from the central uplift of the structure the analyses of the adularia yielded a statistically robust early ordovician crystallization age of 481 5 0 8 ma 2 mean square of weighted deviates 1 06 p 0 30 the most plausible explanation for the formation of vesicle filling adularia is through low temperature mineral precipitation during residual hydrothermal circulation that followed the impact as no other known major intrusive extrusive or thermal events have occurred in the carswell region in the phanerozoic the new age of the carswell impact structure overlaps within uncertainty with the most precise ar ar ages proposed for the l chondrite parent body breakup event but not with the age of the stratigraphic sequence from which the meteorites and micrometeorites from this event were recovered thus either the carswell impact represents a separate unrelated impact event or the dynamic evolution of the l chondrite parent body breakup is more complicated than presently understood and carswell represents one of the earliest and largest known impacts of this event on earth less

constraints on the size of the vredefort impact crater from numerical modeling the vredefort structure in south africa was created by a meteorite impact about two billion years ago since that time the crater has been deeply eroded so to estimate its original size researchers have had to rely heavily upon comparison to other terrestrial impact structures recent estimates of the original crater diameter range from 160 km to as much as 400 km in this study we combined the capabilities of both hydrocode and finite element modeling using the former to predict where the pressure of an impact generated shock wave would have been high enough to form planar deformation features pdfs and shatter cones and the latter to follow the subsequent displacement of these shock isobars during the collapse of the crater we established constraints on the sizes of the projectile and the transient crater and thus on the size of the final crater by comparing the observed locations of pdfs around vredefort to the results of our simulations of impacts by projectiles of various sizes these simulations indicate that a rocky projectile with a diameter of 10 km impacting vertically at a velocity of 20 km s generates shock pressures that are consistent with the distribution of pdfs around vredefort these projectile parameters correspond to a transient crater 80 km in diameter or a final crater 120 160 km in diameter allowing for uncertainties in our modeling procedures we consider final craters 120 to 200 km in diameter to be consistent with the observed locations of pdfs at vredefort the shock pressure contour corresponding to the formation of shatter cones is almost horizontal near the surface making the locations of these features less useful constraints on the crater size however they may provide a constraint on the amount of erosion that has occurred since the impact

frictional melting processes and products in geological materials introduction and discussion this special issue is the outcome of a theme session convened by the above authors entitled frictional melting processes and products in geologic materials held at the 1990 geological society of america annual meeting in dallas texas u s a the goals of the session were threefold to bring together researchers working on various aspects of frictional melting in rock to provide a forum for the exchange of new ideas and to encourage the production of a thematic set of papers dedicated to the topic of the eight papers presented here the first two spray swanson are primarily processand mechanism oriented the next magloughlin maddock o hara deal primarily with chemical aspects of host rock pseudotachylyte relations the last three mainly provide descriptions of naturally occurring koch and masch techmer et al and artificially produced kennedy and spray friction melts natural pseudotachylytes are described from strike slip zones magloughlin swanson thrusts and their listric counterparts koch and masch techmer et al and connecting lateral ramps o hara thus it appears that there exists an association between pseudotachylyte generation and relatively long lived largedisplacement faulting and shearing the prime objective of the meeting was to promote the integration of different approaches

tectonic influences on the morphometry of the sudbury impact structure implications for terrestrial cratering and modeling impact structures developed on active terrestrial planets earth and venus are susceptible to pre impact tectonic influences on their formation this means that we cannot expect them to conform to ideal cratering models which are commonly based on the response of a homogeneous target devoid of pre existing flaws in the case of the 1 85 ga sudbury impact structure of ontario canada considerable influence has been exerted on modification stage processes by late archean to early proterozoic basement faults two trends are dominant 1 the nnw striking onaping fault system which is parallel to the 2 47 ga matachewan dyke swarm and 2 the ene striking murray fault system which acted as a major paleoproterozoic suture zone that contributed to the development of the huronian sedimentary basin between 2 45 2 2 ga sudbury has also been affected by syn to post impact regional deformation and metamorphism the 1 9 1 8 ga penokean orogeny which involved nnw directed reverse faulting uplift and transpression at mainly greenschist facies grade and the 1 16 0 99 ga grenville orogeny which overprinted the se sector of the impact structure to yield a polydeformed upper amphibolite facies terrain the pre syn and post impact tectonics of the region have rendered the sudbury structure a complicated feature careful reconstruction is required before its original morphometry can be established this is likely to be true for many impact structures developed on active terrestrial planets based on extensive field work combined with remote sensing and geophysical data four ring systems have been identified at sudbury the inner three rings broadly correlate with pseudotachylyte friction melt rich fault systems the first ring has a diameter of 90 km and defines what is interpreted to be the remains of the central uplift the second ring delimits the collapsed transient cavity diameter at 130 km and broadly corresponds to the original melt sheet diameter the third ring has a diameter of 180 km the fourth ring defines the suggested apparent crater diameter at 260 km this approximates the final rim diameter given that erosion in the north range is 6 km and the ring faults are steeply dipping impact damage beyond ring 4 may occur but has not yet been identified in the field one or more rings within the central uplift ring 1 may also exist this form and concentric structure indicates that sudbury is a peak ring or more probably a multi ring basin these parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure

gold mineralization within the witwatersrand basin south africa evidence for a modified placer origin and the role of the vredefort impact event the chemical composition of gold within the archaean metasedimentary rocks of the witwatersrand supergroup displays significant heterogeneity at the micro meso and regional scales a detailed electron microbeam analytical and petrological study of the main auriferous horizons in the central rand group throughout the witwatersrand basin indicates that gold has been remobilized late in the paragenetic sequence over distances of less than centimetres contemporaneous chlorite formation was strongly rock buffered gold mobilization occurred under fluid poor conditions at temperatures that did not exceed 350 c widespread circulation of mineralizing fluids within the central rand group is not supported by the gold and chlorite chemical data brittle deformation that affects most of the paragenetic sequence of the central rand group late in its post depositional history is followed by sequences of mineral growth and dissolution that appear throughout the central rand group and have consistent textural relationships with gold the consistent location within the paragenetic sequence the wide regional and stratigraphic extent of the brittle deformation together with mineral chemical and petrological data suggest that the vredefort impact event 2 02 ga was the cause of this late deformation and that post impact fluid poor metamorphism resulted in crystallization of a significant proportion of the gold on and within mineral grains that were deformed during this event

the scaling of complex craters the empirical relation between the transient crater diameter dg and final crater diameter dr of complex craters and basins is estimated using cumulative terrace widths central uplift diameters continuous ejecta radii and transient crater reconstructions determined from lunar and terrestrial impact structures the ratio dg dr is a power law function of dr decreasing uniformly from unity at the diameter of the simple complex crater morphology transition to about 0 5 for large multiring basins like imbrium on the moon the empirical constants in the dg dr relation are interpreted physically to mean that the position of the final rim relative to the transient crater and hence the extent of collapse is controlled or greatly influenced by the properties of the zone of dissociated material produced by the impact shock the continuity of the dg dr relation over the entire spectrum of morphologic types from complex craters to multiring basins implies that the rims of all these structures form in the same tectonic environment despite morphologic differences

radial transpression ridges a new structural feature of complex impact craters the gravity driven collapse of complex impact craters induces the mass transfer of large rock volumes in distal parts of a crater inward movements dominate whereas in the centre of an impact structure outward movements occur if the central uplift collapses the particle trajectory field is centro symmetric which signifies that the conditions for plane strain deformation are not fullfilled converging particle trajectories can be compensated either by a bulk thickening of inward sliding masses folding repetition of rock units along thrust faults plastic flow or by the formation of localised radial transpression ridges rtr at the edges of individual landslides in these transpression ridges material is uplifted to accommodate the converging mass flow different modes of uplift are possible including radial folding lateral overthrusting and the formation of positive flower structures a simple geometric model is used to estimate the amount of transpression thickening and bulk thickening of inward sliding masses on the base of volumetric considerations the existence of rtrs is confirmed by structural investigations at the siljan impact structure sweden as well as at other complex impact craters on earth

geophysical anomalies and quartz deformation of the warburton west structure central australia abstract this paper reports geophysical anomalies and intra crystalline quartz lamellae in drill cores from the warburton west basin overlapping the border of south australia and the northern territory the pre upper carboniferous 450 300 km large warburton basin north eastern south australia is marked by distinct eastern and western magnetic gravity and low velocity seismic tomography anomalies quartz grains from arenite core samples contain intra crystalline lamellae in carbonate quartz veins and in clastic grains similar to those reported earlier from arenites volcanic rocks and granites from the warburton east basin universal stage measurements of quartz lamellae in both sub basins define miller bravais indices of 10 12 and 10 13 in situ quartz lamellae occur only in pre late carboniferous rocks whereas lamellae bearing clastic quartz grains occur in both pre late carboniferous and post late carboniferous rocks the latter likely redeposited from the pre late carboniferous basement quartz lamellae in clastic quartz grains are mostly curved and bent either due to tectonic deformation or to re deformation of impact generated planar features during crustal rebound or and post impact tectonic deformation seismic tomography low velocity anomalies in both warburton west basin and warburton east basin suggest fracturing of the crust to depths of more than 20 km geophysical modelling of the cooper basin which overlies the eastern warburton east basin suggests existence of a body of high density 2 9 3 0 gr cm 3 and high magnetic susceptibility si 0 012 0 037 at a depth of 6 10 km at the centre of the anomalies in the warburton west basin a large magnetic body of si 0 030 is modelled below 10 km with a large positive gravity anomaly offset to the north of the magnetic anomaly in both the warburton east and warburton west the deep crustal fracturing suggested by the low velocity seismic tomography complicates interpretations of the gravity data universal stage measurements of quartz lamellae suggest presence of both planar deformation features of shock metamorphic derivation and deformed planar lamella the latter may be attributed either to re deformation of impact generated lamella impact rebound deformation or and post impact tectonic deformation the magnetic anomalies in the warburton east and west sub basins are interpreted in terms of 1 presence of deep seated central mafic bodies 2 deep crustal fracturing and 3 removal of devonian and carboniferous strata associated with rebound of a central uplift consequent on large asteroid impact further tests of the warburton structures require deep crustal seismic transects

geochemistry and petrology of witwatersrand and dwyka diamictites from south africa search for an extraterrestrial component abstract diamictites are poorly sorted sediments characteristically carrying coarse grained clasts in a fine grained matrix they have generally been considered of glaciogenic or glaciomarine origin recently however it has been suggested that some massive tillite diamictite layers could represent impact breccias an earlier petrographic study of rock and mineral clasts from dwyka group diamictites revealed no evidence for shock metamorphism such as planar deformation features detailed geochemical studies of diamictite samples from the archean witwatersrand supergroup and the dwyka group of the mesozoic karoo supergroup from south africa are reported we studied the contents of the siderophile elements in these breccias as elevated abundances of such elements especially iridium could be indicative for an impact origin by use of coincidence spectrometry and other trace element analysis geochemical tracers of extraterrestrial components were sought however no enrichments of indicator elements for extraterrestrial components compared with ordinary continental crust were found thus neither geochemical nor petrographic evidence supports an impact origin of the diamictites from the dwyka group and the witwatersrand supergroup in south africa

laser argon dating of melt breccias from the siljan impact structure sweden implications for a possible relationship to late devonian extinction events in earlier studies the 6575 km diameter siljan impact structure in sweden has been linked to the late devonian mass extinction event the siljan impact event has previously been dated by kar and ar ar chronology at 342368 ma with the commonly quoted age being 362 7 2 2 ma 2 recalculated using currently accepted decay constants until recently the accepted age for the frasnian famennian boundary and associated extinction event was 364 ma which is within error limits of this earlier siljan age here we report new ar ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from siljan interpreted to be impact melt breccia the analytical results show some scatter which is greater in samples with more extensive alteration these samples generally yield younger ages the two samples with the least alteration yield the most reproducible weighted mean ages one yielded a laser spot age of 377 2 2 5 ma 95 confidence limits and the other yielded both a laser spot age of 376 1 2 8 ma 95 confidence limits and a laser stepped heating plateau age over 70 6 39ar release of 377 5 2 4 ma 2 our conservative estimate for the age of siljan is 377 2 ma 95 confidence limits which is significantly different from both the previously accepted age for the frasnian famennian f f boundary and the previously quoted age of siljan however the age of the f f boundary has recently been revised to 374 5 2 6 ma by the international commission for stratigraphy which is within error the same as our new age however the currently available age data are not proof that there was a connection between the siljan impact event and the f f boundary extinction this new result highlights the dual problems of dating meteorite impacts where fine grained melt rocks are often all that can be isotopically dated and constraining the absolute age of biostratigraphic boundaries which can only be constrained by age extrapolation further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times perhaps resulting in major extinction events in earths biostratigraphic record

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

applications of twin analysis to studying meteorite impact structures abstract this paper describes a technique twin analysis of dolomite and calcite which was used to estimate the level of erosion of an impact crater if the age of impact is known the level of erosion gives the amount of sediments present at that time the estimate of the level of erosion and the presence or absence of tectonic strains in calcite constrains the age of impact this technique might also be useful in identifying deeply eroded impact structures twin analysis gives the greatest shortening direction e 1 tilt corrected e 1 directions converge at or above the present day surface if a shallow gas explosion or meteorite impact explains the structure rather than a deep gas or cryptovolcanic explosion the energy density recorded by twinning distinguishes an impact from a shallow gas explosion for structures 100 m diameter at serpent mound a well established impact structure e 1 directions converge at 1400 390 m 95 confidence interval of the mean above the present day structure and the sample closest to the center of the structure records an energy density of 1 7 10 7 j m 3 these results are consistent with previous studies showing that serpent mound is an impact e g r w carlton c koeberl m t baranoski g a schumacker discovery of microscopic evidence for shock metamorphism at the serpent mound structure south central ohio confirmation of an origin by impact earth and planet sci lett 162 1998 177 185 the level of erosion and other data suggests that serpent mound formed between 290 and 256 ma and was originally 8 5 to 11 km in diameter dolomite twinning is not observed at serpent mound suggesting that this technique may only be useful for studying larger more deeply eroded structures

evolution of the amphibolite granulite facies transition exposed by the vredefort impact structure kaapvaal craton south africa abstract geological mapping is integrated with high precision u pb zircon and monazite geochronology to document in detail the nature and temporal evolution of the amphibolite granulite facies boundary exposed in the vredefort impact structure of the central kaapvaal craton our study has revealed that the preimpact configuration of the amphibolite granulite facies boundary is best preserved in panels between impact related brittle faults and that past concepts of the boundary must be revised a regional lithologic transition exists between amphibolite facies outer granite gneiss 3 084 3 09 ga and a heterogeneous assemblage of granulite facies gneisses ca 3 1 3 3 ga tabular deformed quartz syenite bodies dated at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cy

size and structure of the chicxulub crater revealed by horizontal gravity gradients and cenotes it is now widely believed that a large impact occurred on the earth at the end of the cretaceous period and that the buried chicxulub structure in yucatan mexico is the resulting crater24 knowledge of the size and internal structure of the chicxulub crater is necessary for quantifying the effects of the impact on the cretaceous environment although much information bearing on the crater s structure is available diameter estimates range from 170 to 300 km refs 1a 7 corresponding to an order of magnitude variation in impact energy here we show the diameter of the crater to be 180 km by examining the horizontal gradient of the bouguer gravity anomaly over the structure this size is confirmed by the distribution of karst features in the yucatan region mainly water filled sinkholes known as cenotes the coincidence of cenotes and peripheral gravity gradient maxima suggests that cenote formation is closely related to the presence of slump faults near the crater rim

evaluating the structural character and tectonic history of the witwatersrand basin abstract the witwatersrand basin in south africa has been the focus of gold exploitation for more than 100 years and the subject of hundreds of research investigations principally sedimentological and metallogenic and recently geophysical and geochronological surprisingly the basin wide structural character has not been fully investigated despite the fact that gold metallogenic models have been proposed that want structural architectures as conduits for gold mobilisation and or sites of deposition i e the modified placer and hydrothermal models these models which have polarised viewpoints on the source for gold placer versus metamorphic or hydrothermal source assume an underlying structural character to the basin and structural relationships between goldfields but these assumptions have never been tested through evaluation of the existing structural dataset as corollary the published data for the structural character of the free state welkom klerksdorp west wits line west rand central rand east rand and south rand goldfields are summarised and evaluated to search for basin wide similarities and or correlations that would support tectonic models for the witwatersrand basin and explain its metallogenic character unfortunately the large published historical database 90 years proved to be quite limited with respect to geometric kinematic and or relative chronological detail but must be evaluated against an advanced understanding of earth processes tectonics and the structural geology approach the dataset is largely qualitative rather than quantitative but is useful in establishing broad tectonic trends of the formation and tectonic evolution for the witwatersrand basin it suggests that two deformation events umzawami and ukubambana events mark the end of deposition in the witwatersrand and transvaal basins respectively with development of fold thrust belts at ca 2 7 and 2 2 2 0 ga and these can be correlated with supercontinental cycles the intervening period i e 2 7 2 2 ga was characterised by at least 4 basin forming events that culminated in the deposition of the volcanic sedimentary sequences of the klipriviersberg group and the sedimentary sequences of the platberg chuniespoort and pretoria groups basin formation was facilitated by the reactivation of structures formed during the umzawami event or older as feeder dykes normal listric faults or growth faults the limited structural data for the witwatersrand basin unsurprisingly also leads to the conclusion that gold was macroscopically to mesoscopically remobilised from a primary placer source but remobilisation was localised goldfields scale and not is a basin wide phenomena

friction melt distribution in a multi ring impact basin it is generally accepted that multi ring basins are the consequence of very large impacts but the mechanism by which they form is still a matter of contention1 2 most of what is currently known about multi ring basins is based on remote studies of the moon3 5 and to a lesser extent mars and mercury4 6 but at least two multi ring impact basins have been recognized on earth7 the sudbury8 canada and vredefort9 south africa impact structures providing an opportunity to study their properties directly here we describe the distribution of friction melt pseudotachylyte in the floor of the sudbury impact basin although the veins and dykes of pseudotachylyte decrease in both thickness and frequency of occurrence towards the basin periphery the greatest volumes of friction melt appear to define four rings around the central impact melt sheet field evidence indicates that the rings originated as zones of large displacement which facilitated localized frictional melting of the basin floor during the modification collapse stage of the cratering process by analogy we argue that the rings of other multi ring impact basins are also likely to be the remnants of such large displacement fault zones

evidence for local variations in the depth to the 410 km discontinuity beneath albuquerque new mexico we stack data from the three component broadband seismic stations at pinon flat california pfo ontario canada rson and albuquerque new mexico anmo using the velocity spectrum stacking method by doing so we estimate the transition zone thickness tzt beneath southern california and ontario to be 236 and 263 km respectively beneath anmo we find a bimodal distribution in our estimates of the tzt the transition zone west of anmo appears to be similar in thickness to that beneath pfo 232 km the transition zone east of anmo is 253 km similar to that estimated for rson we conclude that surface tectonics across the rio grande rift are closely related to the transition zone structure beneath it and that the rift which separates the region of active tectonism from the stable high plains of eastern new mexico is also associated with a boundary in transition zone structure most of this 20 km change in tzt appears to be due to topography on the 410 km discontinuity this implies that beneath new mexico lateral variations in temperature do not cross the transition zone and that the 410 km discontinuity reflects surface tectonics more closely than does the 660 km discontinuity

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

mapping chicxulub crater structure with gravity and seismic reflection data the chicxulub crater buried under the yucat n platform of mjxico affords one of the most favourable opportunities on earth for structural studies of a large complex impact crater due to its preservation and accessibility the crater was buried soon after rim erosion in an environment with only minor subsequent tectonic disturbance it is also of interest as the chicxulub impact apparently ended the cretaceous period and the crater s ejecta blanket is known globally at hundreds of localities the buried crater is half overlain by a coastal plain and half by a shallow sea allowing exploration by both marineand land based techniques as well as hybrid combinations the radial symmetry of the crater allows application of data acquired in either terrain to the entire structure using the common data sets of magneticand gravity field anomalies gravity data sets covering the yucat n platform e g 1 2 reveal an 180 km diameter low with internal concentric structure e g 3 4 that is the expression of the chicxulub crater impact lithologies and crater fill the gravity signature is complicated by regional anomalies and early modifications to the crater rim and the northern half of the crater is not well imaged due to the scarcity and imprecision of marine data also an 20 km wide gap in survey coverage due to shallow water occurs between the land data and the nearest seismic profile of camargo and suarez 5 fig 1 which roughly corresponds to the beginning of offshore coverage during september and october 1996 a marine gravity survey was successively conducted on 32 and 10 5 m vessels to survey this gap and offshore seismic lines of the imperial college birps u of texas seismic project offshore coverage was 2 400 line km followed by 1 500 line km inshore with 1 3 and 1 9 mgal unadjusted crossovers respectively data precision was strongly influenced by sea conditions preliminary results from the marine surveys confirm the provisional ties of gravity gradient features to the seismic line the perimeter fault crater fill margin slump faults and collapsed transient cavity margin may all be linked to gradient features unfortunately the clearest crater expression in the gravity field is on the east side where the seismic lines lose resolution at the basin margin to the west the strong peripheral gradient splits with one component corresponding to the perimeter slump fault and a larger feature corresponding to the basin margin this splitting is interpreted as resulting from prompt backwash of crater ejecta into the northwestern part of the crater regional gradients also obscure the crater signature to a greater extent to the west asymmetry of the central high is indicated although a weak feature may extend to the ne the surrounding sharp low continues until it abuts the regional high to the nw additional land surveys support the correlation of the cenote ring to gravity gradient features and suggest that a weak high may correlate to the rim uplift in some places

zircon microstructures record deformation history of shock and tectonically generated pseudotachylites a case study from the vredefort impact structure south africa

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

shock metamorphism in the vredefort collar evidence for internal shock sources shock metamorphic microstructures in the vredefort collar include planar features crystallographically controlled cleavage crystallographically controlled faults and mosaic extinction in addition several recrystallization textures are developed in the quartzites of the collar and quartz c axis distributions for both primary and recrystallized quartz grains are random the degree of recrystallization decreases away from the core collar contact two events of shock deformation have been identified in the collar and using planar feature orientations shock pressures have been estimated using the technique of robertson 1975 the first shock d1 subjected the lowermost witwatersrand rocks to shock pressures of about 150 kbar and the uppermost beds to pressures of about 60 kbar following a period of extensive recrystallization of the quartzites came the second shock event d2 which was weaker than the first and subjected the lowermost strata in the collar to pressures of between 75 and 100 kbar the d2 event has been shown to be separated in time from the d1 event the results are used to show that the shock sources were probably within the earth and that the vredefort ring structure has formed as a result of endogenous processes rather than hypervelocity meteorite impact

the metamorphic history of the vredefort dome at approximately 2 ga as revealed by coesite stishovite bearing pseudotachylites the vredefort dome 2 0 ga represents the central uplift of a very large impact structure this uplift exposed a nearly complete cross section through the continental crust in the region which is 25 30 km thick two metamorphic events took place at about the same time as the impact the first event so called static metamorphism is pre impact and produced lithologies varying from low grade shale to high grade hornfels it resembles contact metamorphism by its lack of schistosity but is more regional as it extends over a large area and is not associated with large intrusions the second event the post shock metamorphism is responsible for the recrystallization of the shock features the investigation of this event has been focused on the degree of alteration of the coesite stishovite bearing pseudotachylite veins that formed during the transit of the shock wave these high pressure silica polymorphs are only present in the upper part of the stratigraphic sequence downward they have been converted to fibrous quartz at the highest grade the fibrous quartz is in turn replaced by triple junctioned mosaic quartz the post shock metamorphism was generated by the heat of the rock before shock plus the heat released by the shock wave the isograds plotted on a map can be translated into depth of burial and therefore provide valuable information regarding the geological setting immediately before impact at the time of impact the rocks were relatively cool and the static metamorphism had ceased with several tens of millions of years separating the two metamorphic events the static metamorphism was probably caused by continental crustal extension in a stress free environment and the lack of deformation is probably due to rapid uplift during the later stages of the impact event

original size of the vredefort structure implications for the geological evolution of the witwatersrand basin abstract historically there have been a range of diameter estimates for the large deeply eroded vredefort impact structure within the witwatersrand basin south africa here we estimate the diameter of the transient cavity at the present level of erosion as 124 140 km based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted transvaal outliers taking erosion into account 6 km and scaling to original final rim diameter an estimate of close to 300 km for the rim diameter is obtained independent estimates of the final rim diameter based on an empirical relation of central uplift diameter to rim diameter spatial distribution of pseudotachylites and concentric large scale structural patterns give a similar estimate of close to 300 km for the original final rim diameter an impact structure of this size is expected to have had an original multi ring form at this size the vredefort impact structure encompasses the bulk of the witwatersrand basin which appears to owe its preservation to the vredefort impact in addition the vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area which in recent interpretations is believed to be a component in the creation of the world class gold deposits of the witwatersrand basin

the nature distribution and genesis of the coesite and stishovite associated with the pseudotachylite of the vredefort dome south africa abstract the vredefort dome represents the uplift centre of a well known 2 00 ga old impact structure of unusually large magnitude shock features like shatter cones planar features and high pressure silica polymorphs are common the present study deals with the description mode of occurrence field distribution and post shock metamorphic alteration of coesite and stishovite which were poorly documented up to now these minerals occur as unusually large crystals in the quartzite of the wall rock in contact with very thin pseudotachylite veins in the pseudotachylite itself which can be interpreted as a friction and or a shock recovery melt fine needles of kyanite are ubiquitous it is proposed that these thin pseudotachylites a type formed during the transit of the shock wave they preceded the development of thick pseudotachylite and microbreccia veins b type which formed during the tensional period which immediately followed from comparison with the model of formation of high pressure polymorphs in porous sandstone it is suggested that higher pressure and stress was concentrated along the a type veins at the arrival of the shock front at this time the quartz was transformed into a high pressure phase which was probably poorly crystalline behind the shock front that is during the rarefaction the pressure was progressively released and the polymorphs crystallized from this initial high pressure phase stishovite crystallized first followed by coesite the high pressure conditions may have lasted an unusually long time due to the magnitude of the vredefort impact this long time probably about one second may account for the large size of the coesite and stishovite crystals most of the high pressure silica polymorphs are corroded to a variable degree by secondary quartz and preserved only in a restricted area of the impact structure this alteration is attributed to post shock metamorphism due to the temperature of the rock before impact plus the heat left after shock recovery some of the evidence put forward by authors against the impact model is discussed according to the facts arising from this investigation it is concluded that an origin by impact remains the simplest and most realistic model to explain the origin of the vredefort dome

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

a record of ancient cataclysm in modern sand shock microstructures in detrital minerals from the vaal river vredefort dome south africa the record of terrestrial meteorite impacts is fragmentary because most impact structures and ejecta are removed by erosion or buried discovery of the missing impact record from hadean to present may be advanced through identifi cation of residual shocked detritus to evaluate which shocked minerals survive erosion and sedimentary transport we investigated modern sands from the vaal river in south africa where it crosses the 2 02 ga vredefort dome the largest terrestrial impact structure known to date shocked minerals were identifi ed in all sediment samples including from the vaal channel and tributaries within the structure in transmitted light detrital quartz preserves discontinuous decorated planar features previously identifi ed as brazil twins which are readily visible as bright continuous features in cathodoluminescence images detrital zircons preserve fi ve orientations of planar fractures pfs which can produce dramatically offset growth zoning and apparent rotation of subgrains other zircons contain fi lled fractures that may represent a new shock microstructure detrital monazite preserves four orientations of pfs and many grains contain oscillatoryzoned shocked zircon inclusions which thus represent shocked inclusions within shocked accessory grains zircon and monazite with granular texture were also identifi ed this study is proof of the concept that shocked minerals can be identifi ed in sediments up to 2 billion years after an impact event and it demonstrates their potential for preserving evidence of ancient impacts the recognition of a new geological repository for impact evi dence provides a means for identifying distal shocked detritus from eroded structures of any age and may be particularly relevant to early earth studies

geoscience impact a synthesis of studies of the sudbury structure geophysical probing results are synthesized into a three dimensional framework necessary for understanding the genesis of the sudbury structure based primarily on seismic reflection results centred on the sudbury igneous complex remnants of crustal melting from a catastrophic meteorite impact are superimposed on the juxtaposition of mid crustal rocks exhumed during the archean against deformed paleoproterozoic sedimentary rocks sedimentation metamorphism deformation and metasomatic overprints are all part of the post impact history of sudbury and tend to dominate the geophysical response of the structure pre impact deformation although certainly preserved in some aspects of sudbury geology is not clearly expressed in the geophysical data nor are any elements of impact induced deformation geophysical views of the sudbury igneous complex are thus somewhat biased in representing mostly the post impact but pre grenvillian history of the region with the exception of igneous events establishing th

aorounga and gweni fada impact structures chad remote sensing petrography and geochemistry of target rocks shuttle radar topography mission data was used to investigate the aorounga and gweni fada impact structures in chad as part of a new remote sensing study we believe the results of various data treatments provide extensive new perspective on the macro structural and topographic divisions for these two impact structures our remote sensing studies indicate revised diameters of aorounga and gweni fada of 16 and 22 km respectively we selected samples from these two structures for their petrographic geochemical and rb sr and sm nd isotopic characteristics in samples from both structures evidence for shock metamorphism was found in the form of single or multiple sets of planar deformation features in quartz which confirms the impact origin for both the aorounga and gweni fada structures the crystallographic orientations of pdfs indicate maximum shock levels of 20 30 gpa for samples from the central parts of both structures the pdf orientations are characteristic for the orientations observed elsewhere in shocked sandstones with the higher angles in the orientation histograms being fairly abundant geochemically the rocks are typical upper crustal sandstones

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

high resolution integrated geophysical investigation at the lancaster gold mine krugersdorp south africa an integrated geophysical approach using seismics and geoelectrical techniques was employed to investigate the architecture of historic narrow reef workings and a proposed open pit mine at lancaster gold mine near krugersdorp south africa the mining activities in the area were mainly carried out within the kimberley reef package in the upper central rand group of the witwatersrand supergroup which hosts several gold bearing conglomerates locally known as reefs the reefs are generally thin 2 m thick and dip between 28 and 32 south the low velocity weathered layer introduces significant static shifts in the reflection seismic data moreover environmental noise from drilling and trucking and the prominent bedrock overburden contact that produces various wave conversions p s conversion caused undesirable noise that contaminates the shot gathers as high amplitude source generated and monochromatic noise the noise was removed from the shot gathers using frequency and velocity filtering techniques the final depth migrated sections are characterised by high resolution images of the subsurface from 10 to 150 m depth which are constrained by the borehole information the reflection seismic data delineate the interfaces between different rock layers and the stopes the refraction and resistivity tomograms on the other hand provide more detailed images of the top 20 50 m of the subsurface and depict the approximate shallow geometry of fluid migration paths mined out areas and bedrock overburden boundaries the integrated results indicate that the study area is characterised by a weathered surface layer with variable low p wave velocity 400 1200 m s and resistivity 150 800 m the deeper layer reveals an increase in resistivity and velocity and it s characterized by discontinuities weak zones cavities or water bearing zones due to the mining activities the combined borehole and geophysical data provide valuable information regarding the physical characteristics of the subsurface and can be helpful for future risk management decisions environmental and engineering studies in the area

evidence of intensified biogenic silica recycling in the black sea after 1970 abstract the black sea has been subject to increased levels of nitrogen and phosphorus loading and a decrease in silicate input after around 1970 changes in phytoplankton community composition from diatoms to non diatom groups have been attributed to the decrease in silicate however a discrepancy between the decreasing silicate input and the increasing silicate pool in the deep sea reported elsewhere implies that another silicate source exists which challenges the current paradigm of widespread silicate limitation in this study we investigate changes in the dissolution state of siliceous protists over the last 140 years and show that siliceous protists became significantly more dissolved after the late 1960s indicating a reduction of the silicate pool preserved in the deep sea sediment we hypothesize that the decline in the dissolution state is caused by increased recycling of biogenic silica in the water column due to an increased annual production driven by nitrogen enrichment

reply to comment on k ar evidence from illitic clays of a late devonian age for the 120 km diameter woodleigh impact structure southern carnarvon basin western australia

large enigmatic crater structures offshore southern california summary digital mosaics of swath and conventional bathymetry data reveal large distinct near circular crater structures in the inner continental borderland offshore southern california two have maximum crater diameters that exceed 30 km and a third has a crater diameter of approximately 12 km all three features exhibit the morphology of large complex craters raised outer rim ring moat and central uplift yet their exact origin remains a mystery preliminary analyses of available seismic gravity and magnetic data over these structures reveal both similarities and distinct differences in geometry structure and geophysical signature to known impact sites all three crater structures however occur within the catalina terrane a highly extended volcanic and metamorphic province floored by catalina schist basement a likely alternative origin may thus involve explosive volcanism caldera collapse and resurgent magmatism and or possibly plutonism and schist remobilization associated with the catalina terrane no single model for crater formation whether impact caldera or pluton fully accounts for all of the present observations regarding the morphology internal structure and known geology of these near circular features timing of crater formation post dates the initial rifting and rotation of the western transverse ranges and appears to predate major right slip along the san clemente and san diego trough fault systems or approximately 18 to 16 ma regardless of their origin these complex craters represent some of the largest structures of their kind in western north america and provide a unique opportunity to better understand the development of unusual crater structures in a submarine environment

new surveys of the chesapeake bay impact structure suggest melt pockets and target structure effect we present high resolution gravity and magnetic field survey results over the 85 km diameter chesapeake bay impact structure whereas a continuous melt sheet is antici pated at a crater this size shallow source magnetic field anomalies of 100 nt instead suggest that impact melt pooled in kilometer scaled pockets surrounding the base of a central peak a central anomaly of 300 nt may represent additional melt or rock that underwent shock induced remagnetization models predict that the total volume of the melt ranges from 0 4 to 10 km 3 a quantity that is several orders of magnitude smaller than expected for an impact structure this size however this volume is within predictions given a transient crater of diameter of 20 40 km for a target covered with water and sedimentary deposits such that melt fragments were widely dispersed at the time of impact gravity data delineate a gently sloping inner basin and a central peak via a contrast between crystalline and sedimentary rock both features are ovoid oriented parallel to larger preimpact basement structures conceptual models suggest how lateral differences in rock strength due to these preimpact structures helped to shape the crater s morphology during transient crater modification

occurrence and orientation of shatter cones in pretoria group quartzites in the collar of the vredefort dome impact origin precluded shatter cones have been found cutting completely indurated fault breccia associated with the down faulting of previously overturned pretoria group quartzites around the vredefort structure the evidence indicates that the shatter cones formed later than the overturning hence they cannot be considered to have been formed by an impact generated shock wave which penetrated the strata just before overturning

transient and disruption cavity dimensions of complex terrestrial impact structures derived from magnetic data 1 accurate transient and disruption cavity dimensions are critical for estimating the energy release associated with impact transient and disruption cavity size can in principle be inferred from morphometric relationships based on crater diameter however locating the crater rim can be difficult for eroded terrestrial craters and existing morphometric relationships are mostly based on observations of extraterrestrial craters where morphologic features at best provide imprecise constraints on the collapsed disruption cavity margin fortunately magnetic survey data collected over terrestrial impact structures demonstrate that collapsed disruption cavity size can be estimated directly from changes in the magnetic anomaly character a lower bound on this parameter can be defined by the outer limit of short wavelength intense magnetic anomalies produced by impact melt and or suevite deposits an upper bound is given by the inner limit of magnetic anomaly trends associated with the pre impact target rock configuration using published values of crater diameters d and values of collapsed disruption cavity diameters dcdc derived from magnetic data for 19 complex terrestrial impact structures we derive the relationship dcdc 0 49d these data and the possibility of geometrical similarity in crater collapse suggest that this relationship is independent of complex crater size over more than a decade of size variation

formation mechanisms of planar deformation features in naturally shocked quartz abstract shock waves induce peculiar defects in quartz shock mosaics high pressure polymorphs coesite and stishovite diaplectic glass and planar deformation features pdfs together these features are the indices of shock metamorphism pdfs appear under the optical microscope as straight and narrow defects parallel to the 10 1n rhombohedral planes with n 3 and 2 as most frequent values although n 1 4 and i e basal plane are also found we report in the first part of this paper a detailed investigation by transmission electron microscopy of the fine structure of pdfs in shocked quartz grains from a variety of sites slate islands la malbaie and manson in north america the vredefort complex in south africa the ries crater in germany and the cretaceous tertiary boundary layer at raton basin colorado we distinguish four pdf fine structures 1 bands of dislocations 2 lamellae of mixtures in various proportions of amorphous silica and small crystallites 3 thin brazil twin lamellae 4 short parallel lamellae forming serrated ladder structures the possible formation mechanisms of these pdfs are discussed in the second part of the paper it is suggested that the dislocation bands are probably not original pdf structures but stem from a later recrystallization stage the brazil twins have been produced by relatively large deviatoric stresses which accompanied the shock wave the other pdf structures which consist of partial amorphization in the 10 1 n planes must result from elastic instabilities of the shear modulus of the quartz structure at high pressure in these planes theoretical estimates of the elastic stiffness coefficients indicate that amorphization should start at approximately 10 gpa the growth of the straight and narrow amorphous lamellae would be driven by the front of the shock wave

impact triggered nanoscale pb clustering and pb loss domains in archean zircon zircon microcrystals are found in many planetary crustal rocks and nanoscale research on grains from well characterized impact environments on earth provide a baseline for reconstructing extreme shock and thermal histories elsewhere however using zircon to date large impact events can be challenging given that shock related isotopic re setting of u pb ratios when measured at micrometre scale is often incomplete and difficult to interpret as the underlying pb migration mechanisms are unclear to better understand shocked zircon u pb systematics we performed atom probe tomography and electron microscopy with the previous sims analyses of two shock metamorphosed mesoarchean zircon grains from deep 15 km beneath the centre of the 2 020 ga giant vredefort impact structure we find evidence of two types of impact related nanoscale pb mobility in one grain clustering has produced 10 nm diameter bodies of radiogenic pb unsupported by u and co located with al with an average 207pb 206pb ratio of 0 50 n 4 the value extant in the grain at the time of impact conversely nearby nanodomains exhibit randomly distributed radiogenic pb u and al and yield 206pb 238u dates consistent with 100 loss of pre impact radiogenic pb atoms during shock metamorphic processes notably domains with multiple pb clusters occur within micrometres of domains that experienced 100 pb loss precluding a uniform radial pattern of thermally driven pb diffusion at the grain scale these cases of broadly coeval clustering and outward pb mobility during geologically instantaneous shock metamorphism point to unusually rapid multi path diffusion processes within sub micrometre volumes which when averaged yield normally discordant u pb dates the isolation of spatially variable styles of pb retention and loss at nanoscale amidst classical grain scale shock microstructures shows promise for recognizing and resolving bombardment histories in planetary crusts using zircon

a marine magnetic study of a carbonate hosted impact structure ile rouleau canada summary marine magnetic and bathymetric data have been acquired in a 6 5 10 km area centred on the ile rouleau impact structure quebec canada the deeply eroded crater is emplaced in weakly magnetic shaley dolomites of the palaeoproterozoic mistassini group subaerial exposure of the structure consists of an island ile mintunikus misaupinanuch in lac mistassini which hosts shatter cones short wavelength and linear nne trending magnetic anomalies that pervade the survey area are inferred to reflect the presence of magnetic glacial landforms on the lake floor the most prominent magnetic feature is a high amplitude anomaly that wraps round the western side of the island the anomaly correlates with an abrupt change in bathymetry but is not consistently circular about the island and its relationship to the impact is unclear to the east a disjointed arcuate anomaly lies parallel to the island 100 m from shore its source may be interpreted to be either 1 a locally thickened layer of magnetic glaciofluvial deposits or 2 post impact chemically induced magnetization along faults the abrupt variations in glaciofluvial thickness needed to account for the anomaly under scenario 1 are however hard to explain geologically and hypothesis 2 is therefore favoured regardless of which model is correct both suggest that the arcuate anomaly coincides with the fault bounded perimeter of a central peak two possible reconstructions for the original crater diameter estimated from the curvature of the magnetic anomaly and using morphometric relationships are 4 4 km and 6 8 km the former is more realistic with regards to the topography and the lack of conspicuous impact related deformation on adjacent ile manitounouc

yallalie a buried structure of possible impact origin in the perth basin western australia an enigmatic buried structure located in mesozoic sedimentary rocks in the perth basin western australia was discovered in 1990 by ampol exploration the basin like yallalie structure centred on 30 26 40 3 s 115 46 16 4 e is circular in plan view and about 12 km in diameter high resolution seismic reflection profiles across the structure show a basin shaped area of chaotic reflec tions that extend to a depth of approximately 2 km below the surface the structure has sharp boundaries with surrounding faulted but otherwise relatively undisturbed rocks in the centre of the structure there is an uplifted area approximately 3 4 km across similar to those described from com plex meteorite impact structures the seismically defined structure coincides with a circular topo graphic depression and image processing of digital elevation data has allowed recognition of concentric and radial structures extending as far as 40 km from the centre of the depression gravity surveys show the structure to be associated with a positive gravity anomaly of about 30 gu aeromagnetic surveys have defined annular anomalies associated with the central uplifted section and possibly margins of the structure a search for siderophile element enrichments by neutron activation analysis in the rocks of the structure which would indicate the presence of a meteorite component proved negative quartz grains in cores that penetrate the structure show the development of prismatic cleavage fractures and irregular slightly curved planes formed by brittle fracture an allochthonous breccia of late cretaceous rocks occurs a few kilometres west of the western margin of the structure quartz grains from a thin veneer of tertiary sediments that drape the structure are essentially unde formed however multiple sets of closely spaced planar deformation features in quartz characteristic of highly shocked rocks have yet to be observed in the rocks of the yallalie structure and the allochthonous breccia the morphology of the yallalie structure determined from topographic and geophysical data suggests strongly that it is of impact origin geological and geochemical evidence is equivocal but is not inconsistent with this interpretation

the importance of being cratered the new role of meteorite impact as a normal geological process this paper is a personal and in many ways incomplete view of the past development of impact geology and of the newly recognized importance of impact events in terrestrial geological history it also identifies some exciting scientific challenges for future investigators to determine the full range of impact effects preserved on the earth to apply the knowledge obtained from impact phenomena to more general geological problems and to continue the merger of the once exotic field of impact geology with mainstream geosciences since the recognition of an impact event at the cretaceous tertiary k t boundary much current activity in impact geology has been promoted by traditionally trained geoscientists who have unexpectedly encountered impact effects in the course of their work their studies have involved 1 the recognition of additional major impact effects in the geological record the chesapeake bay crater the alamo breccia and multiple layers of impact spherules in precambrian rocks and 2 the use of impact structures as laboratories to study general geological processes e g igneous petrogenesis at sudbury canada and archean crustal evolution at vredefort south africa other research areas in which impact studies could contribute to major geoscience problems in the future include 1 comparative studies between low level 7 gpa shock deformation of quartz and the production of quartz cleavage in both impact and tectonic environments and 2 the nature origin and significance of bulk organic carbon kerogen and other carbon species in some impact structures gardnos norway and sudbury canada

benham rise unveiled morphology and structure of an eocene large igneous province in the west philippine basin abstract the benham rise is an oceanic large igneous province at the western margin of the philippine sea it has an ocean island basalt geochemistry a surface area of 0 11 mkm2 and a volume of 0 13 mkm3 a crustal thickness of 15 km is estimated from gravity anomalies volcanism was most active in the early stages of its formation during the lutetian from 47 9 ma to 41 3 ma although volcanic activity extended to 26 ma multibeam bathymetry with almost 100 coverage has revealed the morphology of benham rise for the first time it consists of a main body 310 km by 330 km with the narra loro and molave spurs extending 100 200 km from its eastern side the main body is built on a shield platform from 5200 m to 3800 m sub sea the platform flanks consist of 3 15 km wide terraces with scarps as high as 100 m to 300 m the platform is surmounted by a crest exhibiting caldera morphology at an average depth of 2500 m the crest is named apolaki caldera and may be the world s largest known caldera with a diameter of 150 km features like a breached rim intra caldera benches and a resurgent dome indicate a multi phase volcanic history consisting of both quiet and explosive eruptions the bathymetry and publicly available geological and geophysical data record three main stages in the tectonic history of benham rise shield building caldera formation and post caldera late stage volcanism the platform base is interpreted to have formed over a hotspot beneath the central basin spreading center during the shield building phase between 47 9 ma to 42 5 ma the arrowhead shaped molave spur is interpreted to have formed late in this stage along a ridge that propagated away from the hotspot the caldera formation followed and is interpreted to have occurred before 41 3 41 5 ma when benham rise was still joined with urdaneta plateau the narra spur was formed by late stage hotspot related volcanism as benham rise and urdaneta plateau separated and ended with the eruptions of the vinogradov seamount at 26 ma

combining shock barometry with numerical modeling insights into complex crater formation the example of the siljan impact structure sweden siljan central sweden is the largest known impact structure in europe it was formed at about 380 ma in the late devonian period the structure has been heavily eroded to a level originally located underneath the crater floor and to date important questions about the original size and morphology of siljan remain unanswered here we present the results of a shock barometry study of quartz bearing surface and drill core samples combined with numerical modeling using isale the investigated 13 bedrock granitoid samples show that the recorded shock pressure decreases with increasing depth from 15 to 20 gpa near the present surface to 10 15 gpa at 600 m depth a best fit model that is consistent with observational constraints relating to the present size of the structure the location of the downfaulted sediments and the observed surface and vertical shock barometry profiles is presented the best fit model results in a final crater rim to rim diameter of 65 km according to our simulations the original siljan impact structure would have been a peak ring crater siljan was formed in a mixed target of paleozoic sedimentary rocks overlaying crystalline basement our modeling suggests that at the time of impact the sedimentary sequence was approximately 3 km thick since then there has been around 4 km of erosion of the structure

processed and enhanced gravity and magnetic images over the vredefort structure and their interpretation abstract cryptoexplosion structures are polygonal features which have experienced violent disruption during their formation the vredefort structure is one of the largest known cryptoexplosion structures and the geology is well exposed over its northern half the origin of the violent disruption experienced by the vredefort structure is controversial and many aspects of its history are under intensive study since most of the southern half of the vredefort structure is blanketed by a cover of karoo permian strata geophysical data are essential in any attempt to map the pre karoo geology data sets of the regional gravity and magnetic fields over the vredefort structure have been processed and enhanced with the aid of an image processor the results are presented as a series of images each one enhancing particular attributes of the data sets an interpretation of the geophysical images is presented focusing on the regional geometry and setting of the structure striking features of the geophysical images include a simple concentric geometry and polygonal shape which reflect the distribution of dense and magnetic lithologies within and surrounding the structure the kinematics of the development of a polygonal uplift are discussed inward movement of about seven collar sectors separated by radial faults and uplift of the central sections of each of the sectors may account for the gross polygonal shape pull apart voids formed at kinks in the radial faults might account for the positions of several alkali stocks within the stratified collar of the vredefort structure the enhanced image of the gravity field highlights a se trending gravity low which extends beyond the southeastern quadrant of the vredefort structure this feature forms part of a fundamental nw se trending intracratonic lineament the vredefort axis

cloud creek structure central wyoming usa impact origin confirmed the circular cloud creek structure in central wyoming usa is buried beneath 1200 m of mesozoic sedimentary rocks and has a current diameter of 7 km the morphology morphometry of the structure as defined by borehole seismic and gravity data is similar to that of other buried terrestrial complex impact structures in sedimentary target rocks e g red wing creek in north dakota usa the structure has a fault bordered central peak with minimum diameter of 1 4 km composed predominantly of paleozoic carbonates thickened by thrust faulting and brecciation and is elevated some 520 m above equivalent strata beyond the outer rim of the structure there is a 1 6 km wide annular trough sloping away from the central peak maximum structural relief 300 m and terminated by a detached fault bounded rim anticline the youngest rocks within the structure are late triassic norian clastics and these are overlain unconformably by post impact middle jurassic bathonian sandstones and shales thus the formation of the cloud creek structure is dated chronostratigraphicly as 190 20 ma reported here for the first time are measurements of planar deformation features pdfs in shocked quartz grains in thin sections made from drill cuttings recovered in a borehole drilled at the southern perimeter of the central peak other less definitive microstructures consistent with impact occur in samples collected from boreholes drilled into the central peak and rim anticline the shock metamorphic evidence confirms an impact origin for the cloud creek structure

petrology of impactites from el gygytgyn crater breccias in icdp drill core 1c glassy impact melt rocks and spherules el gygytgyn is a 18 km diameter 3 6 ma old impact crater in ne siberia international continental scientific drilling program el gygytgyn hole 1c was drilled on the frozen crater lake 2 3 km from the crater center to a final depth of 517 m below the lake floor petrographic and geochemical analyses of 26 drill core samples three impact melt rocks from the surface and seven glass spherules from surface deposits outside the crater are used to characterize the impactite inventory at el gygytgyn the bottom 98 m of hole 1c intersected monomict brecciated unshocked rhyolitic ignimbrite with minor intercalations of polymict breccia and mafic inclusions these lithologies are overlain by 89 m of polymict breccia whose components occasionally exhibit scarce low degree shock metamorphic features this unit is succeeded by 10 m of suevite that contains about 1 vol glassy impact melt shards 1 cm in size and a low amount of shock metamorphosed lithic clasts the suevite is capped by a reworked fallout deposit that constitutes a transition over 4 m into lacustrine sedimentation a higher abundance of shock metamorphosed lithic clasts and glass spherules some with ni rich spinel and admixture of an ultramafic component characterize this unit we tentatively interpret this impactite section as allochthonous breccia in the vicinity of el gygytgyn s central ring uplift the geochemical compositions of seven glass spherules from terrace deposits 2 km outside the crater and eight spherules from the reworked fallout deposit in hole 1c show far greater variability than the composition of impact melt shards and impact melt rocks some of these spherules also show strong enrichments in siderophile elements