magnetic properties of polycrystalline diamonds abstract the remanent magnetism and bulk magnetic properties of polycrystalline diamonds have been investigated the diamonds are of two distinct types those occurring together with gem grade diamonds in kimberlite pipes in s africa and carbonados found in placer deposits notably in brazil and the central african republic both types generally possess measurable remanent magnetization the former stronger than the latter and stability tests indicate the presence of primary and secondary components magnetite is the dominant carrier of nrm in the kimberlite diamonds but the very small content of magnetic mineral in the carbonados makes identification of the carrier difficult possible contributors are tetrataenite native iron and cohenite anomalous acquisition of isothermal remanence occurs in some carbonados saturation being unachievable in applied fields in excess of 1 t the ratio of saturation remanence to initial nrm is anomalously low in some of the kimberlite diamonds and the possibility of magnetic contamination is investigated

petalite postdating spodumene in pegmatite as a consequence of the 2 02 ga meteorite impact in the vredefort structure southern africa abstract rare element granitic pegmatites enriched in lithium are economically important hosts for the lithium aluminosilicates spodumene and petalite most spodumene and petalite crystallize from the pegmatite melt depending on pressure and temperature this paper reports the rare occurrence of petalite postdating the subsolidus stage in spodumene pegmatites within the 2 02 ga vredefort impact structure the largest oldest and most deeply eroded meteorite impact structure currently known on earth the studied spodumene bearing pegmatites postdate the deformation fabric in the surrounding amphibolites of the granitoid greenstone terrane impact related veins of fragment rich pseudotachylites cut across the pegmatites spodumene albite and quartz constitute the primary mineral assemblage of the pegmatites muscovite is late magmatic and contains inclusions of spodumene and albite during the subsolidus stage k feldspar replaced spodumene and albite during pervasive metasomatic alteration samples collected closer to the pseudotachylites show reaction textures between spodumene and quartz without the involvement of muscovite raman spectroscopy and microprobe analyses show that the reaction textures consist of quartz and petalite unlike spodumene petalite does not show a replacement by k feldspar the 40ar 39ar muscovite age of 2027 8 ma coincides with the timing of the meteorite impact and documents its effect on the spodumene pegmatites we suggest that the formation of petalite after spodumene was related to heating which was generated in the target rocks by the post shock thermal event thus the unique conditions associated with the meteorite impact can not only result in the formation of rare minerals in the target rocks but can also account for rare scenarios resulting in phases that would not otherwise form in nature

a possible impact origin for the bangui magnetic anomaly central africa abstract bangui is one of the most impressive magnetic anomalies on earth and by far the largest over africa at magsat altitudes it has an amplitude of 28 nt and covers an area of 700 000 km 2 evidence presented suggests that the anomaly might be related to a large impact structure early in the history of the earth north south west east and vertical derivatives of the 5 min topographic data base all reveal a double ring structure with outer ring diameter 810 km and inner ring diameter 490 km the precambrian geology shows several small ancient basins within a large basin with a basement high in the middle such as found in large impact structures the basement complex is early precambrian and hence the impact must be of this age the magnetic anomaly is interpreted as being due to strong remanent magnetism in the ancient crater floor and surrounds the anomaly can be satisfactorily modelled by a 4 5 km thick disc 800 km in diameter beneath the ancient basins to model the amplitude and shape of the anomaly requires an intensity of 10 a m 1 with direction d n 18 w i 25 i e significantly different from the present dipole field the magnetisation is likely to include strong shock remanence srm acquired at the time of impact thermal remanence trm partial thermal remanence ptrm thermochemical remanence tcrm and chemical remanence crm acquired soon after the impact all depend on the size of the impact and all contribute to the high intensity required to explain the anomaly

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

geological modeling of the new champ magnetic anomaly maps using a geographical information system technique 1 reliable global crustal field anomaly maps produced from magnetic measurements of the champ satellite mission now allow for quantitative geological studies of crustal structure and composition we have developed a gis based forward modeling technique to model these anomaly maps on the basis of the geologic and tectonic maps of the world laboratory susceptibility values of the occurring rock types and the seismic thickness of the crust a vertically integrated susceptibility grid is generated in the gis system in addition a remanent magnetization grid is computed for the oceanic crust using a digital isochron map of the ocean floor and rotation models of the paleoplates combining the global vis and remanent magnetization grids the vertical magnetic field anomaly is computed at satellite altitude and compared with the corresponding champ magnetic anomaly map over the oceans induced and remanent magnetization explains well the prominent observed anomalies over the cretaceous quiet zones we also find a good agreement between predicted and observed anomalies over the continents remaining discrepancies between the predicted and observed anomalies can be used to adjust poorly known boundaries and the composition of the buried precambrian provinces until the recomputed anomalies fit the observed anomalies the feasibility of this approach is demonstrated on greenland the west african craton bangui in central africa and the kolyma omolon block in siberia we conclude that quantitative information on the lateral extent the composition and the thickness of the lower crust within a precambrian province can thus be inferred from the new satellite magnetic anomaly maps

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

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

dating ultra deep mine waters with noble gases and 36cl witwatersrand basin south africa abstract concentrations and isotopic ratios of dissolved noble gases 36cl d and 18o in water samples from the ultra deep gold mines 0 718 to 3 3 km below the surface in the witwatersrand basin south africa were investigated to quantify the dynamics of these ultra deep crustal fluids the mining activity has a significant impact on the concentrations of dissolved gases as the associated pressure release causes the degassing of the fissure water the observed under saturation of the atmospheric noble gases in the fissure water samples 70 98 normalized to asw at 20 c and 1013 mbar is reproduced by a model that considers diffusive degassing and solubility equilibration with a gas phase at sampling temperature corrections for degassing result in 4he concentrations as high as 1 55 10 1cm3stp4he g 1 40ar 36ar ranging between 806 and 10331 and 134xe 132xe and 136xe 132xe ratios above 0 46 and 0 44 respectively corrected 134 136 xe 132xe and 134 136 xe 4he ratios are consistent with their production ratios whereas the nucleogenic 4he 40ar and 134 136 xe 40ar ratios generally indicate that these gases are produced in an environment with an average u th k content 2 3 times above that of crustal average in two scenarios one considering only accumulation of in situ produced noble gases the other additionally crustal flux components the model ages for 14 individual water samples range from 13 to 168 ma and from 1 to 23 ma respectively the low 36cl ratios of 4 37 10 15 and comparatively high 36cl concentrations of 8 350 10 15 atoms 36cl l 1 reflect subsurface production in secular equilibrium indicating an age in excess of 1 5 ma or 5 times the half life of 36cl in combination the results suggest residence times of the fluids in fissures in this region up to 3 3 km depth are of the order of 1 100 ma we cannot exclude the possibility of mixing and that small quantities of younger water have been mixed with the very old bulk

magnetic imaging of the vredefort impact crater south africa while most impact craters are characterised by negative magnetic anomalies over their central regions aeromagnetic surveys over the vredefort meteorite impact crater reveal multiple concentric magnetic patterns with no significant anomaly at its centre we performed ground magnetic surveys across a portion of a prominent negative magnetic anomaly that extends in a broad semicircular belt about half way into the basement floor of the crater magnetic anomalies defined by our data are most often negative and occur over a wide range of wavelengths the longest wavelength negative anomaly coincides well with aeromagnetic data we find that this feature is centred over the amphibolite to granulite metamorphic facies transition exposed in the basement floor the transition zone is analogous to the conrad discontinuity observed at depths of about 20 km elsewhere in the kaapvaal craton petrographic studies show a marked increase in the intensity of the impact related thermal and shock metamorphism at this transition which we explain by the focusing and defocusing of shock waves at a rheologic interface during impact we therefore suggest that the magnetic signature at this boundary is caused by a combination of both thermal and shock effects related to the impact event a numerical model of the long wavelength anomaly suggests that it is underlain by a body of coherently magnetised rock whose direction and intensity are similar to those found in pseudotachylites and impact melts that formed during impact on the other hand negative anomalies occurring over smaller 100 to 20 m wavelengths often do not coincide with the surface geology these features cannot be modeled using the same criteria as that for the long wavelength anomaly

experimental constraints on shock induced microstructures in naturally deformed silicates abstract planar deformation features pdfs in various minerals have long been accepted as evidence of impact induced deformation the uniqueness of this association was challenged in the context of the k t boundary extinction debate after mosaicism and microstructures similar to pdfs were reported from the products of explosive volcanism as a result of this debate a significant volume of new experimental and observational data on the development of shock induced microstructures has become available over the last ten years the results reveal that factors such as pre shock temperature pulse duration and crystallographic orientation of target minerals to the shock wave have a primary influence on how these microstructures develop data from diamond anvil cell and high pressure friction experiments reveal that the same solid state amorphization process that produces shock induced pdfs at low temperatures also occurs at much lower strain rates in static experiments the experimental data indicate that the amorphization process is thermally activated and that the character of the resulting pdfs is a function of the applied strain rate shock induced amorphization occurs along those crystallographic planes that are most readily transformed to the high pressure phase during very short pulse durations and produces pdfs that are visible at the optical scale lower strain rate deformation produces tem scale amorphization with orientations that are more homogeneously distributed throughout the target mineral and produces no optically visible pdfs the data confirm the uniqueness of multiple intersecting sets of optically visible pdfs as a diagnostic indicator of hypervelocity impact the data also support the hypothesis that the amorphization process can occur at a wide range of strain rates and that the limiting pressure for the process is controlled by the phase stability of the target mineral under the applied loading conditions not by the hel the data also suggest that the onset pressure the maximum pressure and the pressure range for producing optically visible pdfs decrease with increasing temperature and decreasing strain rate as the pressure range for optically visible pdf formation decreases it is replaced by homogeneous amorphization as observed in the anvil cell experiments thus the uniqueness of multiple intersecting sets of optically visible pdfs to hypervelocity impact is not due to a unique process but rather to a specific set of loading conditions that produce an optically visible microstructure likewise single sets of microdeformations observed in volcanic rocks are produced by the same process but at different loading conditions that preclude the development of multiple intersecting sets of pdfs the data also indicate that shock mosaicism which occurs above the hel represents a plastic response of the target mineral to loading rates that are too large to be accommodated by crystal plastic mechanisms observational data for some naturally deformed samples from the k t boundary the vredefort dome and volcanic rocks along with the experimental observations are used to constrain the range of conditions under which the natural microstructures form and to understand the differences between microstructures produced by impact volcanic and other natural processes finally some possible mechanisms for producing the microstructures observed in volcanic rocks are proposed

lightning leader characteristics in the thunderstorm research international program trip we have used high speed streaking photographic techniques to time resolve the luminous compnents of cloud to ground lightning flashes all recordings were made during our participation in the thunderstorm research international program trip conducted at the kennedy space center florida during the summers of 1977 and 1978 and at the langmuir laboratory near socorro new mexico during the summer of 1979 twenty one dart leaders four dart stepped leaders and three stepped leaders were recorded the majority under daylight conditions the mean two dimensional propagation speed of the dart leaders evaluated over channel lengths less than or equal to 0 8 km above ground is 11 106 m s with a range of 2 9 to 23 106 m s several of the dart leaders reveal a decrease in propagation speed as ground is approached however four of the dart leaders in two separate flashes show an increase in speed near the ground an observation not previously reported in the literature in two multistroke flashes we examine the variation of dart leader propagation speed along the channel and find very similar behavior for different strokes in the same flash the speed variations that we observe may be predominantly caused by geometrical variations of the channel the dart leader propagation speeds reported in this study are compared with the earlier works of schonland mceachron and kitagawa and brook agreement among the studies is good with a common range of observed dart leader propagation speed of 2 to 23 106 m s the major discrepancy among these studies is the observation by schonland of a distribution of dart leader propagation speeds positively skewed toward the lower limit of reported values eleven of the dart leaders are analyzed at upper and lower levels along the visible channel to give 22 dart lengths they range from 7 to 75 m with a mean of 34 m for these 22 determinations we calculate a correlation coefficient of 0 85 between the dart length and the dart leader propagation speed the correlation of greater dart length with higher propagation speed is consistent with the slower decay of channel luminosity due to the greater initial input of energy to the channel by the faster and presumably more energetic dart leader four dart stepped leaders are examined in detail with regard to variation of propagation speed step length stepping interval and luminous intensity during propagation between the cloud base and ground significant differences in the tendencies of these parameters are found within these four leaders for example one dart stepped leader recording shows a decreasing propagation speed and an increasing step interval near ground whereas another shows the opposite behavior in the best event recorded several of the individual steps reveal a photographic film density structure with the lower portion of the step exhibiting a distinct bright tip that fans out into a symmetrically diffuse image in the upper portion of the step our analysis indicates that this spread in the upper portion of the step image is not the result of streaking photography distortion but rather represents the luminous structure of the step we estimate that the step image is recorded in less than 1 s consequently with a measured step length of 20 m the luminous pulse must propagate along the step at a speed of at least 2 107 m s the mean propagation speed for three stepped leaders is found to be 1 1 106 m s all three stepped leaders are very faint and were recorded only in the last 100 200 m above ground two stepped leaders and one dart stepped leader do not propagate completely to ground before initiation of the return stroke apparently these leaders are met by an upward propagating discharge at heights above ground of 20 30 and 20 m respectively other stepped and dart stepped leader cases are indeterminate because an obstacle or the horizon prevent the recording of the leaders near the ground connecting discharges are not observed for any of the dart leader events with a resolution of 10 m at 5 km implying that upward discharges initiated by the approach of dart leader do not occur or are substantially less than a few tens of meters in length dart leaders apparently propagate completely to ground

lightning attachment process involving connection of the downward negative leader to the lateral surface of the upward connecting leader 1 high speed video camera records with a temporal resolution of 20 s and a spatial resolution of 2 4 m per pixel of a downward negative lightning flash that terminated on a 440 m high building are examined the attachment process in this flash exhibited an unexpected behavior in that the downward leader tip connected to the lateral surface of the 400 m upward connecting leader ucl below its tip it appears that the effect of the downward leader on the ucl is significant while the effect of the ucl on the downward leader is negligible except for the final 80 s preceding the beginning of the first return stroke the ratio of speeds of the downward leader and the ucl tends to decrease with time ranging from 1 8 to 0 12 although the lower 80 100 m or so of the ucl were too faint to allow speed measurements

benthic habitat mapping of sorted bedforms using hydroacoustic and ground truthing methods in a coastal area of the german bight north sea abstract the continuously influence of human impacts on the seafloor and benthic habitats demands the knowledge of clearly defined habitats to assess recent conditions and to monitor future changes in this study a benthic habitat dominated by sorted bedforms was mapped in 2010 using biological sedimentological and acoustic data this approach reveals the first interdisciplinary analysis of macrofauna communities in sorted bedforms in the german bight the study area covered 4 km2 and was located ca 3 5 km west of island of sylt sorted bedforms formed as sinuous depressions with an east west orientation inside these depressions coarse sand covers the seafloor while outside predominantly fine to medium sand was found based on the hydroacoustic data two seafloor classes were identified acoustic class 1 was linked to coarse sand type a found inside these sorted bedforms whereas acoustic class 2 was related to mainly fine to medium sands type b the two acoustic classes and sediment types corresponded with the macrofauna communities 1 and 2 the aoinides paucibranchiata goniadella bobretzkii community on coarse sand and the spiophanes bombyx magelona johnstonii community on fine sand a transitional community 3 scoloplos armiger ophelia community with species found in communities 1 and 2 could not be detected by hydroacoustic methods this study showed the limits of the used acoustic methods which were unable to detect insignificant differences in the fauna composition of sandy areas

solar wind interaction with the reiner gamma crustal magnetic anomaly connecting source magnetization to surface weathering abstract remanent magnetization has long been known to exist in the lunar crust yet both the detailed topology and ultimate origin s of these fields remains uncertain some crustal magnetic fields coincide with surface albedo anomalies known as lunar swirls which are thought to be formed by differential surface weathering of the regolith underlying crustal fields due to deflection of incident solar wind protons here we present results from a three dimensional self consistent plasma hybrid model of the solar wind interaction with two different possible source magnetizations for the reiner gamma anomaly we characterize the plasma interaction with these fields and the resulting spatial distribution of charged particle weathering of the surface and compare these results to optical albedo measurements of reiner gamma the model results constrain the proposed source magnetizations for reiner gamma and suggest that vertical crustal magnetic fields are required to produce the observed dark lanes

the bangui magnetic anomaly its geological origin satellite magnetometer data have revealed a long wave length magnetic anomaly in the vicinity of bangui central african republic substantiation of the anomaly has been made through analysis of air and ground magnetic surveys a correlative gravity anomaly has also been observed in ground gravity data the anomaly in an area of highly metamorphosed precambrian rocks does not correlate with any surficial features but is definitely crustal in origin an interpretation consistent with all available geophysical and geological data is that the anomaly could be the result of an early intrusion into the crustal rocks of this region followed by subsidence and deformation

preservation of detrital shocked minerals derived from the 1 85 ga sudbury impact structure in modern alluvium and holocene glacial deposits detrital shocked minerals can provide valuable residual records of eroded impact structures recent studies have reported shocked minerals in modern alluvium in a subtropical climate from the deeply eroded 2 02 ga vredefort dome impact basin in south africa to evaluate the detrital shocked mineral record at a large impact structure in a temperate setting with a holocene glacial erosional history we investigated 4000 detrital zircons and 20 000 quartz grains at the less eroded 1 85 ga sudbury basin in ontario canada for the presence of shocked sand grains modern alluvium from rivers within and outside the basin and holocene glaciofluvial sands eskers and outwash deltas across the basin were investigated for shocked minerals shocked zircon and or quartz were found in all modern rivers and most holocene glacial deposits within but not outside the basin petrography and scanning electron microscopy sem backscattered electron bse cathodoluminescence cl imaging and analysis energy dispersive x ray spectroscopy eds electron backscatter diffraction ebsd were used to document shock microstructures of the total detrital zircons surveyed 3 118 3978 were identified as shocked holocene samples contained higher average percentages of shocked zircon 63 1361 or 4 6 with a high of 29 compared to modern alluvium 55 2617 or 2 1 with a high of 6 ebsd analysis revealed a range of shock microstructures including planar fractures deformation microtwins and crystal plastic deformation at sudbury detrital shocked quartz is rare compared to zircon only 15 grains 0 08 were identified all with decorated planar deformation features pdfs these results demonstrate that a detrital shocked mineral record exists at a large impact basin that is in a youthful stage of erosion despite its age in addition to modern alluvium our results also identify glaciofluvial eskers and deltas as reservoirs for detrital shocked minerals glacial episodes thus enhance the dispersal and preservation of shocked detritus in sedimentary systems despite physical differences the observation that the two largest precambrian impact basins continue to contribute detrital shocked minerals 2 b y after impact suggests that a shocked mineral record of impacts on early earth should reside in precambrian siliciclastic rocks

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

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

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

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

terminal cretaceous environmental events the geologic record of terminal cretaceous environmental events indicates that iridium and other associated elements were not deposited instantaneously but during a time interval spanning some 10 000 to 100 000 years the available geologic evidence favors a mantle rather than meteoritic origin for these elements these results are in accord with the scenario of a series of intense eruptive volcanic events occurring during a relatively short geologic time interval and not with the scenario of a single large asteroid impact event

the carbonado diamond conundrum abstract the reviewer presents the results of a literature search on the enigmatic occurrences of carbonado a form but not the only form of polycrystalline diamond which is mined for industrial diamonds the principal occurrences are in brazil the central african republic car and siberia yakutia in the latter the name yakutite has been coined but though there are differences between these polycrystalline diamonds and the brazil car minerals they would seem to be little more significant than differences within the gondwana occurrences and yakutite should probably be regarded as another name for carbonado most occurrences are in placers and the minerals have probably been recycled more than once by sedimentary processes the ultimate source being unknown polycrystalline diamond has been described from the udachnaya pipe yakutia and though it is strictly framesite defined as an eruptive pipe variety of carbonado it differs little from other carbonados the fact that all occurrences known are in areas with kimberlites and gem diamond developments present suggests that carbonado does stem from terrestrial eruptive processes though not necessarily kimberlite pipes nevertheless this can be only a provisional personal conclusion there is much contradictory evidence in the literature the evidence of subjection to radiogenic agencies in car carbonados seems to be unexplained but it may be a secondary effect and not present in all carbonados this is a conundrum which still calls for much more research

mid crustal granulite facies metamorphism in the central kaapvaal craton the bushveld complex connection abstract the numerous greenstone remnants which occur as inclusions in the archaean gneissic granitoid basement exposed in the core of the vredefort dome display evidence of very high grades of granulite facies metamorphism and an unusually complex three stage metamorphic history 1 in metapelites migmatites and refractory restites were produced in a mid crustal 0 5 gpa m1 anatectic event at temperatures in excess of 900 c in the restites 2 in the migmatites the peak metamorphic assemblages are variably overprinted by a high grade retrograde reaction which involved crystallizing anatectic melts m2 3 both the peak metamorphic assemblages and the retrogression textures are cross cut by pseudotachylitic breccia veins which mark a shock metamorphic event associated with the formation of the vredefort dome and 4 the peak metamorphic assemblages and the m2 retrogression textures are overprinted by new generations of remarkably fine grained post shock high grade phases which crystallised at a pressure some 0 25 gpa lower than that of both the peak metamorphic conditions and the retrogression m3 these data indicate an anticlockwise p t evolution where mid crustal anatexis most probably resulted from the intraplating of bushveld complex related ultramafic magmas at 2 06 ga a period of approximately isobaric cooling and retrogression was followed by the vredefort catastrophe which resulted from the impact of a large meteorite into the terrane at 2 02 ga this shock event halted the natural mid crustal metamorphic progression prior to the terrane cooling below 650 c and resulted in the rapid exhumation of the terrane by some 9 km in the process it provided a window into the deep levels of the central kaapvaal craton not seen elsewhere in the region

lake st martin impact structure manitoba canada magnetic anomalies and magnetizations a ground magnetic survey has further delineated the previously known intense 1000 nt negative anomaly at the centre of the lake st martin impact structure manitoba shocked gneiss in the central uplift has a strong reverse remanent magnetization whose direction is similar to that found after alternating field demagnetization of melt rock specimens from within the impact structure shocked gneiss is interpreted as the source of the central negative anomaly the direction of the remanence is consistent with a permo triassic age for the impact event the magnetization in the shocked gneiss is interpreted as a chemical remanent magnetization carried by hematite formed during an extensive alteration of mafic silicates occurring immediately after the impact

ion microprobe pb pb dating of carbonado polycrystalline diamond abstract we report in situ ion microprobe pb pb dating of quartz rutile and clay mineral inclusions and matrix part of polycrystalline diamonds carbonado derived from a proterozoic conglomerate in diamond bearing deposit of minas gerais in brazil together with u pb ages of zircons also extracted from the deposit thirty four spots on seven quartz grains located in the cavity of the gm01 carbonado yield 207pb 206pb isochron age of 3247 800 ma 2 mswd 0 75 and six spots on three rutile inclusions give the age of 3916 1300 ma 2 mswd 0 092 seven spots on three clay mineral inclusions of the do3 carbonado show common lead compositions while four spots on matrix define 207pb 206pb isochron age of 3811 1800 ma 2 mswd 0 012 seventy six spots on seventy zircon grains derived from the same conglomerate show three major u pb concordant ages of 2151 17 ma 2692 20 ma and 2931 22 ma with two concordant zircon outliers of 1726 58 ma and 3599 12 ma the youngest age is consistent with the formation age of 1750 1700 ma in the proterozoic conglomerate estimated by other workers the 3 6 ga zircon is the oldest record in the south american platform it may be detrital in origin and reflect the remnant of an archean craton the pb pb age of quartz and rutile inclusions in gm01 carbonado and that of do3 carbonado matrix are 3315 720 ma and 3811 1800 ma the matrix age is consistent with pb pb age of 2 6 3 8 ga of central african carbonados in the literature suggesting a close genetic relationship between brazilian and african carbonados

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

benthic habitat mapping concerns using a combined approach acoustic sediment and biological data abstract near shore benthic biotopes in the 2 30 m depth range were analyzed and mapped using a combination of biological sedimentary and acoustic data to interrogate the utility of these methods the surveyed area covered approximately 270 km 2 along 80 km of the southeast coast of portugal the acoustic data were acquired with a single beam ground discrimination system qtc view series v connected to a dual frequency echo sounder 50 and 200 khz sediment grain size and macrofaunal community data were obtained by grab sampling at 88 sites the sedimentary and the biological data were subjected to classification and ordination analysis and the acoustic data obtained with the two frequencies were analyzed individually with the qtc impact v3 4 software and classified into acoustic classes the affinity groups obtained with the three datasets were mapped using a geographic information system the study area showed an inshore offshore pattern with sands ranging from very fine to very coarse revealing a sediment distribution in the near shore shelf comprised by two main areas a very fine fine sand area up to 7 10 m depth and a very coarse coarse sand area offshore in turn the biological data also showed an inshore offshore pattern identifying two main biological assemblages fine very fine sand and coarse very coarse sand communities furthermore the biological pattern clearly reflected the hydrodynamic conditions of the area from highly hydrodynamic shallow depth to the calmer deeper areas accordingly species richness and densities were lower at shallow areas than at deeper depth the acoustic data identified the prevailing biosedimentary gradients along the inshore offshore direction overall the results obtained here showed that the acoustic system could identify two main areas based on sediment grain size which closely related to the two main biological communities groups a and b this study indicates the importance of combining several layers of information in order to increase the spatial resolution of the main biotope distribution validating the acoustic data and detail their sub divisions ground truth sampling furthermore we show that designing ground truth samples on the basis of the acoustic diversity data allows the biological data to speak for itself in general acoustic techniques are used as a means to optimize ground truth sampling this will only be valid if acoustics can capture all the essence of biotope heterogeneity if this is not the case special attention should be given when using broad scale methods acoustic remote sensing devoted to biotope mapping

macrobenthic zonation patterns along a morphodynamical continuum of macrotidal low tide bar rip and ultra dissipative sandy beaches the species composition densities biomass and zonation patterns of the macrobenthos of sandy beaches are greatly influenced by the morphodynamics and morphology of the beaches macrobenthic zonation patterns along a small scale morphodynamic gradient comprising eight belgian beach sites were investigated by taking into account the dimensionless fall velocity and the relative tidal range the beach sites were ordered along the gradient from the ultra dissipative beach type ud to the low tide bar rip beach type ltbr the resulting beach state index varied between 1 8 and 4 2 and the beach profiles were related with the beaches morphodynamic state in total 35 macrobenthic species mainly polychaetes and crustaceans were encountered varying between 19 and 23 species per beach site the species composition was quite similar among beach sites with scolelepis squamata being abundant at all eight sites furthermore the macrobenthic distribution patterns were mainly related to elevation at all beach sites some remarkable difference in metrics largely related to the beach morphodynamics and the consequent hydrodynamics were found at the hydrodynamically benign and consequently macrobenthos rich ud beaches the highest macrobenthic densities and biomass occurred on the upper beach while at the hydrodynamically harsh and thus macrobenthos poor ltbr beaches the maximum densities and biomass occurred lower on the beach species typically occurring on the upper ud beaches such as eurydice pulchra s squamata and bathyporeia sarsi were restricted to the sub optimal middle and lower beach zone at ltbr beaches only bathyporeia pilosa was found on the upper beach of both ud and ltbr beaches the more robust polychaete ophelia rathkei and the interstitial polychaete hesionides arenaria were exclusively found in the hydrodynamically harsh conditions of the middle ltbr beach zone

geochronology of the deep profile through archean basement at vredefort with implications for early crustal evolution the presence of archean granite forming the core of an updomed and overturned sequence of strata at vredefort south africa has been known for over 70 years recent geophysical geochemical and geological evidence has given rise to the proposal that the basement granite core has also been overturned presenting a section of the earth s granitic crust to view and that radial traverses in the vredefort basement penetrate through a 15 km thick section of the archean crust when moving from the contact of collar strata to the center of the dome the upper levels of the vredefort crust are dominated by a relatively homogeneous outer granite gneiss ogg displaying marked gradients in large ion lithophile elements these rocks grade inward into the deeper levels which occur in granulite facies metamorphism the deeper levels of the vredefort crust exposed in the central parts of the dome consist predominantly of felsic inlandsee leucogranofels ilg which occurs interleaved with inclusions of mafic metavolcanic and metasedimentary rocks termed steynskraal metamorphic zone smz a comprehensive isotopic study using four decay schemes 87rb 232th 238u and 235u on this crustal section is now reported the results permit unique and direct tests on the theories of archean crust forming processes in the upper vredefort crust whole rock isochron ages of 3050 m y are obtained from the four decay schemes and almost all evidence of an extended crustal history prior to 3050 m y has been obliterated establishment of strong vertical gradients in rb sr ratio shortly before setting of rb sr whole rock systems has wide implications for the interpretation of sr initial ratios although the vredefort upper levels of the crust has a low 87sr 86sr initial ratio of 0 7019 0 0002 it probably participated in a crustal prehistory for at least 500 m y in the deeper levels of the vredefort basement the geochronology is much more complex rb sr and th pb isochrons of 3500 m y are recorded in the mafic granulite relicts a companion paper welke and nicolaysen this issue provides evidence for an early crust forming event in this sector 3800 m y ago from 3500 m y onward these deeper crustal levels did not undergo addition of new archean crust forming material on a major scale nevertheless a most important younger archean event affected these levels 2800 m y ago cessation of high grade metamorphism is recorded by setting of the rb sr isotopic system in the felsic rocks and zircon u pb ages an episode of strong uranium depletion affected both the felsic and mafic rocks of this sector 2800 m y ago the inlandsee leucogranofels is evidently a rock unit with a long and complex history beginning with the emplacement as a crustal unit at least 3800 m y ago since then it has been subject to reworking and resetting of isotopic systems which terminated 2800 m y ago in the deeper crust there is scant evidence of a 3050 m y event similarly there is no record of a 2800 m y old imprint in the upper vredefort crust thus the last major events that affected the geochemical constitution of the vredefort upper and deeper crustal sectors differed in time and nature nevertheless they brought about isotopic setting in a downward moving sequence while the overlying crust remained cooler rigid and unaffected these events probably reflect significant discontinuities in the cooling of the archean crust

shock experiments on pre heated α and β quartz i optical and density data abstract discs of single crystal quartz unheated and pre heated to 275 c and 540 c i e quartz and 630 c i e quartz were experimentally shocked to pressures ranging from 20 to 40 gpa with the shock front propagating parallel to either 10 1 0 or 0001 refractive indices density and the orientation of planar deformation features pdfs were determined on the recovered quartz samples refractive indices of pre heated quartz are unaffected up to 25 gpa but density starts to decrease slightly up to this pressure above 25 gpa pre heating causes drastic variations refractive indices and birefringence of quartz shocked at ambient temperature decrease continuously until complete isotropization is reached at 35 gpa in quartz shocked at 630 c refractivity drops discontinuously in the interval from 25 to 26 gpa and complete transformation to diaplectic glass is reached at 26 gpa density follows the trends demonstrated by the optical parameters with higher pre shock temperatures yielding lower density at a given shock pressure these results indicate that the threshold pressure for the onset of transformation to diaplectic quartz glass is largely temperature invariant lying at 25 gpa whereas the pressure limit for complete transformation decreases with increasing pre shock temperature from 35 to 26 gpa quartz shocked parallel to 0001 always has a higher density and refractivity than that shocked parallel to 10 1 0 indicating a significant influence of the structural anisotropy this is also evident from the distribution of pdf orientations pressures 25 gpa cause in quartz shocked parallel to 10 1 0 pdfs that are predominantly oriented parallel to 10 1 2 while quartz shocked to the same pressures but parallel to 0001 contains almost exclusively pdfs parallel to 10 1 3 pdf orientations in quartz shocked at ambient temperature parallel to 10 1 0 show the following characteristics i the frequency of 10 1 3 is high at 20 gpa declines with increasing pressure and finally is totally absent at 32 gpa ii 10 1 3 begins to occur at 25 gpa increases and finally is the only remaining orientation at 32 gpa and iii 10 1 2 and 11 2 2 are only present at 20 and 25 gpa in contrast quartz shocked at 630 c parallel to 10 1 0 displays a broad pdf distribution with indistinct maxima and 10 1 3 is totally absent the small structural difference between and quartz which is reflected in the pdf distribution should allow in principle evaluation of the pre shock temperature in naturally impacted metamorphic rocks the results substantiate that formation of shock effects in quartz is dependent on i shock pressure ii pre shock temperature and iii shock wave direction so far the latter two parameters have not been considered in the application of experimental results to impact sites

magsat anomaly data over the kursk region u s s r abstract a new method of reducing magsat orbits was developed to produce a map of the kursk magnetic anomaly at satellite altitude two different techniques to estimate the direction of magnetization were applied to the components of the anomaly to determine the presence of remanent magnetization we propose that there is a significant component of remanent magnetization however we are unable to determine the importance of remanence relative to induced magnetization

vredefort shatter cones revisited shatter cones have been described from a number of circular and polygonal structures worldwide the origin of which has been alternatively ascribed to the impacts of large extraterrestrial projectiles or to catastrophic endogenic processes despite their association with enigmatic catastrophic processes the nature of shatter cones and the physics involved in their formation have not been comprehensively researched results of detailed field and laboratory studies of shatter cones from three areas in the collar of the vredefort dome in south africa are presented vredefort shatter cones are directly related to a widely displayed fracture phenomenon termed multiply striated joint sets msjs msjs are planar to curviplanar fractures occuring at spacings of 1 to several millimeters the joint sets have a fractal character when a new measurement protocol is used in the field involving study of all joint surfaces and all steps and striae exposed on these surfaces new information is gained on the genesis and significance of the msjs and on their relationship to striated conical fractures the internal constitution of a rock specimen with msjs was examined in detail by documenting the precise geometry of many fractures in a suite of parallel thin sections transecting the specimen the steps and striae on shatter cone surfaces have the characteristics of displacement fractures microfaults along which evidence of melting is observed shatter cone and msjs surfaces are often covered with glassy films we evaluate whether these fracture phenomena are linked to the formation of pseudotachylitic friction melt our field and petrographic observations can be interpreted as consistent with the generation of shatter cones msjs relatively late in the formation of the vredefort structure this scenario contrasts sharply with the widely held view that shatter cones are formed during the early compression phase of a shock event that affected horizontal strata

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

response of infaunal macrobenthos to the sediment granulometry in a tropical continental margin southwest coast of india abstract surficial sediment samples collected from the continental margin of the southwest coast of india in july 2004 were examined for the grain size and soft bottom macrobenthic fauna to understand the sediment granulometry and its effect on the faunal distribution samples were collected using smith mcintyre grab from 20 to 200 m depth range consisting of mid shelf outer shelf and slope fine grained sediment located in the mid shelf and supported low faunal abundance polychaetes constituted the bulk of the fauna feeding guild changed with depth and sediment granulometry coexistence of deposit feeders and carnivores in outer shelf and deposit feeders and filter feeders in the slope region indicated the effective utilization of different food resources in general richness and diversity were high in the southern region depth wise the diversity and abundance were relatively high in the 50 75 m depth range correlation and bio env analysis showed that combination of different factors such as sediment texture sediment sorting and depth were found to influence the distribution of macrobenthos hence spatial variations observed in benthic community were presumably linked to the variations in sediment granulometry and the energy level conditions prevailing in the area

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

macrobenthic community structure of soft bottom sediments at the belgian continental shelf within the frame of different research projects a large number of sites at the belgian continental shelf bcs have been sampled for the macrobenthos between 1994 and 2000 these samples cover a diverse range of habitats from the sandy beaches to the open sea from the gullies between the sandbanks to the tops of the sandbanks and from clay to coarse sandy sediments to investigate the large scale spatial distribution of the macrobenthos of the belgian continental shelf the data of all these research projects 728 samples were combined and analysed by means of several multivariate techniques 10 sample groups with similar macrobenthic assemblage structure were distinguished each sample group is found in a particular physico chemical environment and has a specific species composition four sample groups differ drastically both in habitat and species composition and are considered to represent four macrobenthic communities 1 the muddy fine sand abra alba mysella bidentata community is characterized by high densities and diversity 2 the nephtys cirrosa community occurs in well sorted sandy sediments and is characterized by low densities and diversity 3 very low densities and diversity typify the ophelia limacina glycera lapidum community which is found in coarse sandy sediments and 4 the eurydice pulchra scolelepis squamata community is typical for the upper intertidal zone of sandy beaches these macrobenthic communities are not isolated from each other but are linked through six transitional species assemblages the transition between the a alba m bidentata community and the n cirrosa community is characterized by a reduction in the mud content and is dominated by magelona johnstoni the transition between the n cirrosa and the o limacina g lapidum community is distinctive by decreasing densities and coincides with a gradual transition between medium and coarse sandy sediments from the n cirrosa to the e pulchra s squamata community transitional species assemblages related to the transition from the subtidal to the intertidal environment were found each community or transitional species assemblages was found over a specific range along the onshore offshore gradient four types can be discerned 1 almost restricted to the near shore area but possible wider distribution 2 distributed over the full onshore offshore gradient 3 restricted to the near shore area and 4 restricted to the sandy beach environment the diversity pattern on the bcs follows this division with species rich and poor assemblages in the near shore area to only species poor assemblages more offshore the distribution and diversity patterns are linked to the habitat type distinguished by median grain size and mud content

very high resolution side scan sonar mapping of biogenic reefs of the tube worm lanice conchilega reefs of the tube building polychaete lattice conchilega are known to represent hotspots of biodiversity within inter and subtidal soft sediments of the north sea however because of their patchy distribution point sampling does not appropriately map their subtidal spatial distribution this study evaluated the feasibility to detect l conchilega reefs by very high resolution side scan sonar imagery a subtidal very high resolution 410 khz side scan survey combined with grab sampling revealed high densities of l conchilega up to 1979 ind m 2 to coincide with a higher reflectivity patchy and grainy acoustic facies from the side scan sonar imagery individual reefs were estimated to reach a maximum size of 15 m 2 to ground truth the acoustic facies the distribution of intertidal l conchilega reefs was mapped at low tide and side scan sonar imagery was recorded during the following high tide intertidal l conchilega reefs had a patch size of 0 8 m 2 up to 11 6 m 2 elevated 7 5 to 11 5 cm above the surrounding seafloor and covered approximately 10 of the selected area the very high 445 khz resolution side scan sonar imagery revealed a similar acoustic facies as in the subtidal lower resolution 132 khz side scan sonar imagery was less efficient to detect physically less developed l conchilega reefs we conclude that 1 there are no major technical restrictions to map l conchilega reefs using side scan sonar 2 the developmental stage of l conchilega reefs impacts the detectability of the reefs and 3 very high resolution side scan sonar imagery is considered a necessity when mapping small scale structures such as l conchilega reefs c 2008 elsevier inc all rights reserved

the influence of sedimentary heterogeneity on within maerl bed differences in infaunal crustacean community a study was undertaken of the crustacean community associated with maerl habitats in mannin bay ireland based on a visual classification of sedimentary fractions eight sedimentary facies could be distinguished these ranged from live maerl banks and seagrass covered live maerl banks in the shallow low energy parts of the inner bay to maerl debris facies mixed with varying proportions of sand mud and shell gravel in the more exposed high energy parts of the bay amphipoda dominated the crustacean fauna accounting for more than 95 of total numerical abundance overall the crustacean fauna occurring in the studied maerl habitats is similar to the fauna occurring in subtidal gravel habitats with little evidence of elective species only the mud maerl debris facies and the seagrass covered live maerl banks appear to harbour a relatively different crustacean fauna from the remainder of the sedimentary facies these differences were mainly changes in the numerical abundance of dominant species rather than species displacements

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

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

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

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

minimal crustal magnetizations from satellite data abstract the minimum amplitude crustal magnetization needed to explain magsat vector anomaly data is found using harmonic splines as basis functions shure et al 1982 phys earth planet inter 28 215 229 and choosing a subset of the original basis parker and shure 1982 geophys res lett 9 812 815 to represent magnetization we suggest the optimum model of this type is obtained by associating representers only with north and vertically downwards data points given the larger uncertainties associated with east component magsat data following parker et al 1987 rev geophys 25 17 40 we find a model minimizing the r m s volume integral of magnetization over the magnetized lithosphere the numerical effort involved in producing a regional or continental scale model with given horizontal spatial detail with our method is less than that for an equivalent source dipole model when the magnetization is not assumed induced approximately equal to that for an induced magnetization equivalent dipole model and considerably less than that for a spherical harmonic apparent susceptibility model the method is demonstrated by producing a solution for two regions the contiguous usa for comparison with previously published models and africa and its continental margins magnetization models fit the data slightly worse than those of the downward continued anomaly field at the earth s surface comparison of our component of magnetization for the usa in the direction of the core field with the induced magnetization equivalent dipole model of mayhew 1984 earth planet sci lett 71 290 296 is very good as expected for magnetization either induced by or frozen remanently in an approximately dipole field either parallel or antiparallel to the current rotation axis the east component of magnetization is much smaller than the north and vertically downwards components we make no assumptions about the relative contributions of remanent and induced magnetization and the resulting models have a substantial remanent component however our models suffer from the severe non uniqueness common to all magnetization models whereby magnetization in a uniform susceptibility and thickness shell concentric with the earth s centre owing to any internal magnetic field that is then switched off generates no external magnetic field

preliminary correlations of magsat anomalies with tectonic features of africa an overview of the magsat scalar anomaly map for africa has suggested a correlation of magsat anomalies with major crustal blocks of uplift or depression and different degrees of regional metamorphism the strongest magsat anomalies in africa are closely correlated spatially with major tectonic features although a magnetic anomaly caused by a rectangular crustal block would be offset from the block s center by the effects of magnetic inclination an anomaly caused by real crustal blocks of varying uplift depression and degree of regional metamorphism would be located nearer to the locus of greatest vertical movement and highest grade of metamorphism thus the bangui anomaly may be caused by a central old precambrian shield flanked to the north and south by two relatively young sedimentary basins

carbonado physical and chemical properties a critical evaluation of proposed origins and a revised genetic model abstract carbonado diamond is the most controversial of all diamond types and is found only in brazil and the central african republic bangui neither an affinity to earth s mantle nor an origin in the crust can be unequivocally established carbonado diamond is at least 3 8 ga old an age about 0 5 ga older than the oldest diamonds yet reported in kimberlites and lamproites on earth derived from neo to mid proterozoic meta conglomerates the primary magmatic host rock has not been identified discovered in 1841 the material is polycrystalline robust and coke like and is best described as a strongly bonded micro diamond ceramic it is characteristically porous which precludes an origin at high pressures and high temperatures in earth s deep interior yet it is also typically patinated with a glass like surface that resembles melting with exotic inclusions of highly reduced metals carbides and nitrides the origin of carbonado diamond is made even more challenging but the challenge is important because a new diamondiferous host rock may be involved and the development of a new physical process for generating diamond is possibly assured the combination of micro crystals and random crystal orientation leads to extreme mechanical toughness and a predicable super hardness the physical and chemical properties of carbonado are described with a view to the development of a mimetic strategy to synthesize carbonado and to duplicate its extreme toughness and super hardness textural variations are described with an emphasis on melt like surface features not previously discussed in the literature but having a very clear bearing on the history and genesis of carbonado selected physical properties are presented and the proposed origins diverse in character and imaginatively novel are critically reviewed from our present knowledge of the dynamic earth all indications are that carbonado is unlikely to be of terrestrial origin a revised model for the origin of carbonado is developed based on 1 new observations which show that white dwarf stars can have diamond cores 2 c rich exoplanets may have diamond bearing mantles and 3 new shock wave experiments on methane suggest that diamond may be present in large gaseous planets such as saturn uranus and neptune where diamond in the core is solid and in jupiter where liquid diamond may be present it is proposed that carbonado diamond was delivered to earth from extra solar sources during the episode of late heavy bombardment 4 1 3 8 ga if the currently held view that r process heavy elements were added as a late veneer to earth through the collision of white dwarfs in kilonova explosions then why not carbonado diamond during this epic event

late cretaceous and paroxysmal cretaceous tertiary extinctions the various geological signatures at cretaceous tertiary time including iridium and other associated elements microspherules and shock deformation features are compatible with the suggestion that the transition is marked by a period of intense volcanism the volatile emissions from this volcanism would lead to acid rain reduction in the alkalinity and ph of the surface ocean global atmospheric temperature changes and ozone layer depletion these environmental effects coupled with those related to the major sea level regression of the late cretaceous provide the framework for an explanation of the selective nature of the observed extinction record

chemical heterogeneity in carbonado an enigmatic polycrystalline diamond abstract carbonado a polycrystalline variety of diamond is characterized by unusual carbon isotope compositions with bulk 13c values clustered tightly between 23 and 30 relative to pdb these values are significantly lighter than harzburgitic diamond with a range in 13c from 1 to 10 and fall near the lower extreme for eclogitic diamond ranging from 3 to 34 in combination with textural and inclusion data these isotopic compositions have led scientists to question whether carbonado originated in the mantle or in the crust previous studies of carbonado have revealed a bimodal grain size distribution that correlates with cathodoluminescence cl emissions we believe that these textures result from a two step growth process and we have obtained additional chemical evidence that supports the identification of two distinct crystal populations ion microprobe analyses of a central african carbonado reveal a bimodal distribution of 13c values of 24 and 26 with an instrumental precision of 0 29 secondary ion mass spectrometry analyses also demonstrate that this 13c distribution coincides with variations in nitrogen abundance and both of these chemical zonations correlate with cl emission signatures a one dimensional analysis of self diffusion of carbon in diamond suggests that isotopic homogenization occurs extremely slowly even under upper mantle conditions whereas the microscale distribution of carbon isotopes in carbonado does not constrain the temperature pressure or time of carbonado formation it does provide a geochemical signature that recorded the dynamics of the growth process

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

coesite in suevites from the chesapeake bay impact structure the occurrence of coesite in suevites from the chesapeake bay impact structure is confirmed within a variety of textural domains in situ by raman spectroscopy for the first time and in mechanically separated grains by x ray diffraction microtextures of coesite identified in situ investigated under transmitted light and by scanning electron microscope reveal coesite as micrometer sized grains 1 3 m within amorphous silica of impact melt clasts and as submicrometer sized grains and polycrystalline aggregates within shocked quartz grains coesite bearing quartz grains are present both idiomorphically with original grain margins intact and as highly strained grains that underwent shock produced plastic deformation coesite commonly occurs in plastically deformed quartz grains within domains that appear brown toasted in transmitted light and rarely within quartz of spheroidal texture the coesite likely developed by a mechanism of solid state transformation from precursor quartz raman spectroscopy also showed a series of unidentified peaks associated with shocked quartz grains that likely represent unidentified silica phases possibly including a moganite like phase that has not previously been associated with coesite

characteristics of unconnected upward leaders initiated from tall structures observed in guangzhou 1 forty five unconnected upward leaders uuls occurred in 19 downward negative flashes are analyzed each observed uul is initiated by a downward stepped leader before a new strike point is struck for each uul several parameters are determined when possible mainly by using high speed images inception height inception time prior to return stroke rs horizontal distance from the flash s strike point two dimensional 2d distance between the nearest downward leader branch tip and the uul s inception point at its inception time 2d length and 2d average propagation velocity their values range from 40 to 503 m number of samples 45 0 1 to 1 32 ms 38 20 m to 1 3 km 38 99 to 578 m 21 0 48 to 399 m 45 and 5 79 to 33 8 104 m s 1 22 respectively 86 19 22 of the velocities are smaller than 1 7 105 m s 1 no uul with an inception time prior to rs greater than 0 5 ms is initiated from a structure lower than 300 m those uuls with inception heights lower than 300 m seldom exhibit lengths longer than 50 m and only can be initiated by flashes within approximately 600 m while those higher than 400 m can even reach several hundred meters and be initiated by flashes over 1 km away the maximum distances for the downward leaders to attract the uuls with inception heights from 100 to 200 m 200 to 300 m and over 400 m are approximately 350 m 450 m and 600 m respectively

ultramafic rocks in the centre of the vredefort structure south africa geochemical affinity to bushveld rocks abstract the geochemistry and mineral chemistry of ultramafic rocks in the centre of the vredefort structure have been reinterpreted a detailed examination of previously published major element trace and ree analyses as well as mineral analyses strongly suggests a magmatic origin in contrast to previously postulated tectonic emplacement of mantle material a strong geochemical affinity to ultramafic rocks from the bushveld complex is evident the magmatic nature of the ultramafic rocks in the centre of the vredefort structure has implications for the discussion of the metamorphic history of the vredefprt structure which shows evidence of unusually high and as yet not satisfactorily explained geothermal gradients during peak metamorphism

early archaean spherule beds in the barberton mountain land south africa no evidence for impact origin abstract detailed petrographical and geochemical studies are reported for several well documented occurrences of spherule rich layers in the barberton mountain land south africa which were previously inferred by lowe and co workers to be the result of early archaean meteorite impacts in contrast we made the following observations the textures of most spherules are not necessarily the result of impact but could instead be the product of radial or intersertal growth of crystals during secondary mineral formation there is no difference in the content of siderophile elements between spherule layers and layers devoid of spherules the siderophile element abundances are high where sulphide minerals e g pyrite gersdorffite and chalcopyrite and or chromite are present independent of the presence or absence of spherules high contents of e g ir up to 2700 ppb ni 0 96 wt and cr 1 6 wt were found in various samples abundances of these elements in chondritic meteorites are approximately 600 ppb 1 4 wt and 0 35 wt respectively resulting in respective meteoritic components in these samples of 450 70 and 460 we do not accept these high concentrations as primary meteoritic signatures impact melt rocks have typically 1 of a meteoritic component furthermore spherules of any kind are very rare in known impact deposits including the k t boundary if present at all they are not usually associated with any significant ir or pge anomaly the high abundances of the siderophile elements in some barberton samples and their enrichment in secondary minerals indicate that these elements have been remobilized and reconcentrated the pge interelement ratios have changed during remobilization as well thus the pge abundance patterns and ratios are not primary and cannot be used as an argument in favour of an impact origin either nickel rich cr spinels which are found in some of the barberton spherule samples have low fe 3 fe total ratios these ratios are incompatible with an extraterrestrial or impact origin because such impact derived spinels including those found at the k t boundary are highly oxidized and also have different chemical compositions there is also no indication of any evidence of shock metamorphism associated with the barberton spherule layers which is the commonly accepted definitive criterion for recognition of an impact origin this is unusual because such evidence is preserved even in heavily altered samples from deeply eroded archaean impact structures we conclude that while these spherule layers are clearly unusual and deserve further attention there is no convincing evidence for an origin by impact we suggest that they were formed by volcanic processes followed by extensive hydrothermal alteration

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

rock magnetic investigation of possible sources of the bangui magnetic anomaly the bangui magnetic anomaly bma is the largest lithospheric magnetic field anomaly on earth at low latitudes previous studies investigated its geological source using constraints from satellite and ground magnetic field measurements as well as from surface magnetic susceptibility measurements on rocks from the panafrican mobile belt zone pmbz here we combine magnetic field data modelling and rock magnetic property measurements susceptibility and natural remanent magnetization nrm on many samples from this pmbz and the surrounding formations it reveals that nrm is a significant component of the total magnetization mt of the bma source which reaches 4 3 a m with maximum thicknesses of 38 and 54 km beneath the western and eastern parts of the bma only the isolated and relatively thin banded iron formations and some migmatites show such mt values thus we suggest that the thick bma source may be composed either by overlapped slices of such metamorphic rocks or by an iron rich mafic source or by a combination of these two geological structures

how human activities affect the fine sediment distribution in the dutch coastal zone seabed abstract the fine sediment distribution in the seabed is an important indicator for the ecological functioning of shallow coastal seas in this paper we investigate the processes and conditions that determine the fine sediment distribution in the dutch coastal zone surficial seabed while also assessing the response of the system to human interventions an extensive sediment dataset collected in the dutch coastal zone from 2006 to 2014 is presented these data are used to map the distribution of fines in the seabed of the dcz at unique spatiotemporal scales for the entire dutch coastal zone the distribution of fines generally agrees well with previous studies the recent extension of the port of rotterdam the maasvlakte 2 reclamation was found to locally change the distribution of fines in the sand mining pit and directly south of the reclamation fines percentages in the seabed increased by more than 10 we developed a conceptual framework to analyse the distribution of fines and how it is affected by human interventions three components are distinguished within this framework 1 sources of fines 2 transport pathways and 3 accumulation potential these components are determined both qualitatively and quantitatively based on high resolution bathymetric and hydrodynamic model data the distinction between the three components makes it possible to unravel the contributions of different human interventions to the changes in the fines distribution in the case of maasvlakte 2 the local increase of fines percentage in the seabed could thus be attributed to a temporary additional source of fines and enhanced accumulation potential the high spatiotemporal resolution of the new sediment dataset proved crucial to enable development and testing of the framework to evaluate the impact of large engineering works on the spatial distribution of fines

spherical prism magnetic effects by gauss legendre quadrature integration summary regional spherical coordinate observations of the earth s crustal magnetic field components are becoming increasingly available from shipborne airborne and satellite surveys in assessing the geological significance of these data theoretical anomalous magnetic fields from geologic models in spherical coordinates need to be evaluated this study explicitly develops the elegant gauss legendre quadrature formulation for numerically modelling the complete magnetic effects i e potential vector and tensor gradient fields of the spherical prism we also use these results to demonstrate the magnetic effects for the crustal prism and to investigate the crustal magnetic effects at satellite altitudes for a large region of the middle east centred on iran

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

characterization of magnetite particles in shocked quartz by means of electron and magnetic force microscopy vredefort south africa submicroscopic opaque particles from highly shocked granite gneisses close to the core of the vrede fort impact structure have been investigated by means of micro analytical techniques with high spatial resolution such as electron diaraction orientation contrast imagery and magnetic force microscopy the opaque particles have been identified as nano to micro sized magnetite that occur in several distinct modes in one sample magnetite occurs along relict planar deformation features pdfs in quartz generally accepted as typical shock lamellae the magnetite particles along shock lamellae in quartz grains virtually all show uniform crystallographic orientations in most instances the groups of magnetite within diaerent quartz grains are systematically misori entated such that they share a subparallelh101i direction the magnetite groups of all measured quartz grains thus appear to have a crystallographic preferred orientation in space in a second sample orientations of magnetite particles have been measured in microfractures non di agnostic of shock of quartz albite and in the alteration halos e g biotite grains breaking down to chlorite the crystallographic orientations of magnetite particles are diverse with only a minor portion having a preferred orientation scanning electron microscopy shows that magnetite along the relict pdfs is invariably asso ciated with other microcrystalline phases such as quartz k feldspar and biotite petrographic observations sug gest that these microcrystalline phases crystallized from locally formed micro melts that intruded zones of weakness such as microfractures and pdfs shortly after the shock event the extremely narrow widths of the pdfs suggest that heat may have dissipated rapidly re sulting in melts crystallizing relatively close to where they were generated magnetic force microscopy confirms the presence of magnetic particles along pdfs the smallest particles 5 lm with high aspect ratios 15 1 usually exhibit intense uniform magnetic signals characteristic of single domain magnetite consistent oasets between attractive and repulsive magnetic signals of individual single domain particles suggest consistent directions of magnetization for a large proportion of particles