an algebraic reconstruction technique for weakly anisotropic velocity laboratory measurements and field experiments indicate that many sedimentary rocks display velocity anisotropy robertson and corrigan 1983 white et al 1983 thomsen 1986 winterstein 1986 gaiser et al 1987 accounting for anisotropy is proving important in several areas including the processing and interpretation of shear wave data lynn and thomsen 1986 bush and crampin 1987 and the integration of vertical seismic profile vsp data and surface seismic data chiu and stewart 1987a

subsurface cavity imaging by crosshole borehole radar measurements travel time tomography uses direct waves of crosshole radar data a great deal of information concerning the media is contained in later arrivals such as reflections and diffractions more reliable results can be obtained by incorporating both direct waves and later arrivals in this paper the combination of travel time tomography and migration is introduced a smoothed velocity model obtained by travel time tomography is used as the basis of migration then a reverse time migration technique is applied to direct wave suppressed crosshole data the result of tomography indicates that there is an abnormal area and the result of migration gives a more accurate position of the abnormal area which is a cavity the real example shows that the cooperation of tomography and migration is an effective technique for subsurface cavity imaging the synthesis of all information including parallel crosshole measurement and forward simulation assists in the interpretation of the results of tomography and migration

the resolving power of cross borehole tomography the data collected in a cross borehole tomography problem is inherently incomplete since the boreholes do not completely surround the media under study the ability of the experiment to resolve structure is therefore limited by poor experimental geometry we use the method of backus and gilbert to investigate the ability of this technique to resolve variations in acoustic velocity and attenuation between the two boreholes both integrals that appear in the backus gilbert formulation can be performed analytically thus expediting the calculations the resolving kernels have significant horizontal width demonstrating that the horizontal position of structures can be only poorly resolved

coupled modeling of hydrogeochemical and electrical resistivity data for exploring the impact of recharge on subsurface contamination 1 the application of geophysical methods in particular electrical resistivity measurements may be useful for monitoring subsurface contamination however interpreting geophysical data without additional data and without considering the associated hydrogeochemical processes is challenging since the geophysical response is sensitive to not only heterogeneity in rock properties but also to the saturation and chemical composition of pore fluids we present an inverse modeling framework that incorporates the simulation of hydrogeochemical processes and time lapse electrical resistivity data and apply it to various borehole and cross borehole data sets collected in 2008 near the s 3 ponds at the u s department of energy s oak ridge integrated field research challenge site where efforts are underway to better understand freshwater recharge and associated contaminant dilution our goal is to show that the coupled hydrogeochemical geophysical modeling framework can be used to 1 develop a model that honors all the available data sets 2 help understand the response of the geophysical data to subsurface properties and processes at the site and 3 allow for the estimation of petrophysical parameters needed for interpreting the geophysical data we present a series of cases involving different data sets and increasingly complex models and find that the approach provides useful information about soil properties recharge related transport processes and the geophysical response spatial heterogeneity of the petrophysical model can be described sufficiently with two layers and its parameters can be estimated concurrently with the hydrogeochemical parameters for successful application of the approach the parameters of interest must be sensitive to the available data and the experimental conditions must be carefully modeled

time lapse seismic tomography of an underground mining zone abstract in this study we determined p wave tomographic images of the yongshaba deposit an underground mining zone in guizhou province china by inverting arrival time data of micro seismic events and blasts recorded by a passive seismic array consisting of 28 sensors during january to april 2014 using an event location technique and a travel time tomography method that can handle complex seismic discontinuities the damping parameter used in the damped least squares method is determined with the help of trade off curve to reveal internal p wave velocity changes of the study area under severe mining activities a time lapse data partition scheme is used to assess the human influence on the underground structure we introduce a parameter to measure the relative velocity changes in different periods the resolution of the tomographic images and the robustness of the obtained features are examined by conducting a series of checkerboard resolution tests and a restoring resolution test our tomographic results obtained from the entire data set reveal a prominent low velocity low v zone and many obvious high velocity high v zones these features match well with the geological setting and the excavation plan carried out in the mine according to in site surveys the low v zone may reflect empty volumes stress releases and rock breaking and cracking caused by mining activities whereas the high v zones are probably the consequences of local stress concentrations caused by regional stress redistribution the time lapse tomographic images may reveal the process of stress concentrations caused by rock breaking due to the continuing excavations in the entire observation period indicating that the mining activities influenced the rock property and caused complex changes of the underground structure

new zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam ebsd cl eds and isotopic u pb and u th he analysis of the vredefort domethis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh integrated electron nanobeam ebsd cl eds and isotopic measurements u pb u th he of zircon from the collar and centre of the 80 km wide central uplift of the 2020 3 ma vredefort impact structure reveal new shock features in a microstructural progression related to impact basin formation and degree of u pb age resetting 1 planar fractures in 1k0 and 1k2 orientation during initial shock wave compression 2 curviplanar fractures in 1k1 orientation now annealed which host glassy inclusions of partial melt of the host rock 3 microtwin lamellae in an orientation of 65 about 110 attributed to shock wave rarefaction 4 nucleation of impact age crystallites possibly on microtwins during post shock heating by impact melt and 5 crystal plastic deformation linked to crater modification of the core of the central uplift planar fracturing and microtwinning 20 gpa in cold shock zircon in granitoid at a radial distance of 25 km failed to reset zircon age single grain id tims data

experiments and computations of solitary wave forces on a coastal bridge deck part ii deck with girders abstract solitary wave induced forces on a two dimensional model of a coastal bridge are investigated by conducting laboratory experiments and performing cfd computations experimental parameters included four water depths five wave amplitudes four submergence depths and three elevations above the still water level swl for a total of 118 cases submergence depths and elevations are chosen such that the bridge model may be fully submerged partially inundated or fully elevated above the swl euler s equations are solved by use of the cfd program openfoam to compute the wave forces it is found that the forces calculated by openfoam are in close agreement with the laboratory measurements in most cases the effect of formation of entrapped air pockets on the wave forces is studied by including air pressure relief openings on the deck of the model this paper is a companion to part i under the same title the forces on the deck with girders are compared with those on the flat plate given in part i to examine the role of girders on the forces the solitary wave forces are further studied computationally by changing the number of girders in different submergence depths

seismic tomography in boreholes summary synthetic seismic tomographic data sets for cross hole and hole to surface arrays are computed and inverted inversion imaging is performed through an iterative projection solution to a matrix equation the orientation as well as the position of an anomaly relative to the source and recorder arrays affects both the rate of convergence of the solution and the resolution obtained data density and distribution determine the physical validity of the image obtained at convergence borehole tomography is apparently a viable method for imaging inter hole structure

anisotropic velocity tomography a case study in a near surface rock mass cross borehole data were acquired in the surface crown pillar of a massive sulfide ore mine the data consist of five two dimensional 2 d cross borehole panels each with approximately 900 source receiver pairs the panels were located within the crown pillar at either side of and within a major subvertical fault zone that intersects the orebody an initial analysis of the data indicates that the bedrock containing the orebody is seismically anisotropic a rigorous analysis of the traveltimes using anisotropic velocity tomography confirms the initial assessment that anisotropy exists within the crown pillar rock mass anisotropic velocity tomography is the generalization of tomographic methods to anisotropic media as in any geophysical problem the data are insufficient to completely resolve the distributions of the rock properties at all scale lengths we use external constraints on the roughness of the final solution to ensure an algebraically well posed problem plots of the data residuals the t

travel time and amplitude analysis in seismic tomography in cross hole tomography straight lines connecting the source and receiver are often used to approximate the true ray paths thus linearizing the inverse problem a recent paper by bregman et al 1989a has shown that this approximation may be very poor in the regions of greatest interest where large velocity gradients cause significant ray curvature the interpretation method presented by bregman et al 1989a iteratively ray traces and updates the velocity model with model perturbations obtained by a damped least squares inversion of the travel time residuals this paper presents a modification of the travel time tomography method so that it may be applied to the inversion of first motion amplitudes for inverse quality factor q 1 after determining the velocity structure using travel time tomography the amplitudes are inverted using a similar scheme no additional ray tracing is required for the amplitude inversions the effect of the velocity structure previously determined through travel time tomography is taken into account so that amplitude variations due to the focusing and defocusing effects of the velocity structure are not interpreted as being due to attenuation structure both the travel time and amplitude tomography methods are applied here to field data from a cross hole experiment in crystalline rock wong et al 1983 the frequency range of the seismograms is 1 6 6 khz allowing resolution of velocity structure on a scale of several meters the resulting velocity image shows good agreement with other geological and geophysical data reversing the model by placing the model boreholes on opposite sides relative to the original reconstruction yields almost identical results indicating that the interpolation scheme used in the forward modeling has not biased the final model synthetic maslov seismograms calculated for the derived velocity model agree well with the waveform data further confirming the validity of the model in addition the amplitude tomography results place absorptive regions in the same location as the most pronounced low velocity features which correlate with fractures in the boreholes highly transmissive regions also correlate well with high velocity regions

inversion of tracer test data using tomographic constraints 1 we have developed a methodology for inverting tracer test data using zonation information obtained from two dimensional radar tomograms to improve the typically overly smooth hydraulic conductivity fields obtained from conventional inversion of tracer test data the method simultaneously yields two dimensional estimates of hydraulic conductivity as well as petrophysical relationships that relate hydraulic conductivity to radar velocity these relationships can be assumed to be stationary throughout the area of investigation or to vary as a function of zonation using a synthetic three dimensional hydraulic conductivity field we apply the developed inversion methodology and explore the impact of the strength and stationarity of the petrophysical relationship as well as the impact of errors that are often associated with radar data acquisition such as unknown borehole deviation we find that adding radar tomographic data to tracer test data improves hydrogeological site characterization even in the presence of minor radar data errors the results are contingent on the assumption that a relationship between radar velocity and hydraulic conductivity exists therefore the applicability of the proposed method may be limited to field sites where this condition is partially or fully satisfied

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

the transformation of amphibolite facies gneiss to charnockite in southern karnataka and northern tamil nadu india amphibolite facies metamorphic grade gives way southward to the granulite grade in southern karnataka as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes petrologic investigations in the transitional zone south of mysore have established the following points 1 the transition is prograde amphibole bearing gneisses intimately associated with charnockite at kabbal and several similar localities are not retrogressive after charnockite as proved by patchy obliteration of their foliation by transgressive very coarse grained charnockite high fluorine content of biotite and amphibole in gneisses and high large ion lithophile element contents in gneisses and charnockites these features are in contrast to very low fluorine in retrogressive amphiboles and biotites very low large ion lithophile element contents and zonal bleaching of charnockite in clearly retrogressive areas as at bhavani sagar tamil nadu 2 metamorphic temperatures in the transitional areas were 700 800 c pressures were 5 7 kbar and h2o pressures were 0 1 0 3 times total pressures based on thermodynamic calculations using mineral analyses dense co2 rich fluid inclusions in the kabbal rocks confirm the low h2o pressures at the first appearance of orthopyroxene farther to the south in the nilgiri hills and adjacent granulite massif areas peak metamorphic temperatures were 800 900 c pressures were 7 9 kbar and water pressures were very low so that primary biotites and amphiboles those with high f contents are rare 3 the incipient granulite grade metamorphism of the transitional areas was introduced by a wave of anatexis and k metasomatism this process was arrested by drying out under heavy co2 influx charnockites so formed are hybrids of anatectic granite and metabasite of metabasite and immediately adjacent gneiss or are virtually isochemical with pre existing gneiss despite gross recrystallization to granulite mineralogy these features show that partial melting and metasomatism are attendant rather than causative in charnockite development copious co2 from a deep crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis granulite metamorphism of both neosome and paleosome followed the process is very similar to that deduced for the madras granulites by weaver 1980 the massif charnockites for the most part extremely depleted in lithophile minor elements show many evidences of having gone through the same process

target oriented interferometric tomography for gpr data an interferometric form of fermat s principle and traveltime tomography is used to invert ground penetrating radar gpr data for the subsurface velocity distribution the input data consist of gpr traveltimes of reflections from two buried interfaces a reference and b target where the data are excited and recorded by gpr antennas at the surface fermat s interferometric principle is then used to redatum the surface transmitters and receivers to interface a so the associated reflection traveltimes correspond to localized transit times between interfaces a and b the overburden velocity model above interface a is not required the result after tomographic inversion is a high resolution estimate of the velocity between interfaces a and b that does not depend on the velocity model above interface a a motivation for introducing interferometric traveltime tomography is that typical layer stripping approaches will see the slowness error increase with depth as the layers are inverted this suggests that

the geology and gravity field in the central core of the vredefort structure the centre of the vredefort structure in south africa comprises an uplifted granitic basement with an associated 300 g u gravity high ringed by upturned sedimentary strata detailed geological mapping of the basement core has delineated a three fold concentric zoning of outer granite gneisses ogg steynskraal formation sf and inlandsee leucogranofels ilg an increase in metamorphic grade occurs toward the centre of the structure from greenschist relics on the basement perimeter to upper amphibolite and granulite facies in the sf and granulite facies in the ilg a marked lithological change occurs within the sf in which perimeter greenschists and inner amphibolites give way to paragneisses in the ilg paragneiss and mafic granulite relics predominate these divisions are consistent with the standing on edge model for the basement 1 three regional metamorphic episodes and two thermal events have been recognised m1 and m3 both attained granulite facies at low medium load pressures in the ilg m1 granulite facies assemblages extended into the ogg but the m3 facies was restricted to the ilg m2 was a k metasomatism event limited to the ogg an extensive thermal episode m4a is provisionally correlated with the collar metamorphism pyroxene hornfels facies t 800 c was attained in the centre as opposed to conditions of p 4 5 kbar and t 650 c for alkali granite stocks in the sedimentary collar 2 this event was thus pre or syn uplift but not post uplift a high level high temperature p 3 kbar and t 800 900 c event m4b occurred at a late stage in the overall sequence a suite of amphibole orthopyroxene rocks of basaltic komatiite type cao al2o3 1 are chemically related to younger high mg tholeiites these tholeiites are correlated with similar rocks in the sedimentary collar and were intruded syn to post m4a in several episodes of decreasing basicity shock metamorphic features granitic breccias and pseudotachylite are linked to the late stage history of the structure and a case is presented for a dynamic metamorphic front advancing outward and upward from the centre in response to unroofing of the basement the central gravity high3 is delineated by 1239 new gravity stations at 1 km and 0 5 km intervals a low frequency oval shaped anomaly with a long axis trending nw se has eccentrically positioned on it a plateau extending southwards as a spur a dyke is responsible for the spur but the source for the plateau is from a recent borehole thought to be largely due to sheared altered harzburgite modelling of the low frequency gravity field component was based on the mapped divisions of the ogg sf and ilg and a density contrast of 200 kg m3 was determined between the ogg and the sf and ilg lithologies a purely crustal solution for the excess mass distribution in the basement is conclusively demonstrated

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

high resolution aquifer characterization using seismic cross hole tomography an evaluation experiment in a gravel delta we performed field experiments at a well studied aquifer test site to evaluate the geophysical method of seismic cross hole tomography for high resolution aquifer characterization we address the challenge of finding meaningful relationships between seismic velocity and hydraulic conductivity at the field scale for hydraulic conductivity estimation the resolution of seismic cross hole data is limited by inversion ambiguities and diffraction effects which are not accounted for using conventional ray based inversion algorithms for velocity reconstruction we investigate the impact of diffraction effects on seismic velocity inversion and implications for hydraulic modelling our ray based seismic tomograms correlate well with sedimentary cores and hydraulic conductivity logs allowing us to interpret a zone model with distinct geophysical hydraulic zones we use this zone model to constrain seismic inversions using two approaches for forward modelling ray tracing infinite frequency approximation and wavefield modelling at a realistic frequency which accounts for diffraction we show that using the common ray tracing approach seismic velocities in low velocity zones may be significantly overestimated based on the available data we observe a negative linear relationship between logarithmic hydraulic conductivity and seismic velocity this relationship is used to parameterize hydraulic models based on seismic velocity models computer simulations of advective contaminant transport demonstrate that their results are extremely sensitive to an accurate determination of seismic low velocity zones we conclude that seismic tomography is indeed very useful for high resolution aquifer characterization however an inversion approach that accounts for diffraction effects a priori knowledge of the subsurface structure and additional well data should be applied for quantitative interpretations

window constrained inversion of gravity gradient tensor data using dike and contact models we have developed a constrained inversion technique for interpretation of gravity gradient tensor data for dike and contact models striking in the y direction the measured g xz and g zz com

moho depth and crustal composition in southern africa abstract we present new results on structure thickness and composition of the crust in southern africa based on 6300 seismic receiver functions at 85 stations application of hk stacking to the entire sase dataset and use of multi frequency bands improve resolution substantially we observe a highly heterogeneous crustal structure with short wavelength variations in thickness h vp vs ratio composition and moho sharpness which defines 20 blocks that do not everywhere coincide with surface tectonic features in the zimbabwe craton the tokwe block has h 35 38 km and vp vs 1 74 1 79 whereas the thicker crust in the tati block h 47 51 km may be related to deformation of the archean crust along the cratonic margin two distinct crustal blocks with similar crustal thickness 42 46 km but significantly different vp vs ratios are recognized in the limpopo belt extreme values of 1 90 1 94 at the dyke swarms in eastern limpopo and 1 84 at the olifants river dyke swarm and easternmost bushveld intrusion complex bic indicate voluminous magmatic intrusions in the whole crust we find no evidence for magmatic intrusions in the central inferred part of bic where the crust is thick 45 50 km and vp vs is low 1 68 1 70 this thick crustal root may have deflected rising magmas to form the two bic lobes most of central kaapvaal has thin 35 40 km crust and vp vs 1 74 these characteristics are similar to the tokwe block in zimbabwe craton and may indicate delamination of pre existing lower crust which is further supported by a very sharp moho transition the exposed cross section in the vredefort impact crater is non representative of cratonic crust due to shallow moho 34 km and high vp vs 1 80 attributed to shock metamorphism high vp vs 1 76 is typical of the witwatersrand basin and anomalously low vp vs 1 66 1 67 marks the kaapvaal kheis namaqua transition highly heterogeneous crust both in thickness and vp vs ratio is typical of the namaqua natal and cape fold belts

a parametric study of borehole influence in resolving seismic velocity in well to well tomograms the data collected in a well to well tomography experiment is inherently incomplete even when augmented by vsp data the nature of the experiment suggests a geometric limitation to the resolution of any central structure a parametric model has now been developed which examines the effect on reconstructed velocity due to the borehole size transducer standoff altered annulus and the degree of velocity attenuation of the altered annulus using a range of host velocities and hole separations it was found that there are significant velocity variations caused by perturbations in borehole size these errors are particularly significant for boreholes with large diameters and small transducer offsets for an altered annulus errors in both the dimensions and the degree of velocity alteration gave significant velocity variation in reconstructed velocity particularly with large boreholes in all cases it is observed that the variation and alteration in reconstructed velocities increased as hole separation is decreased

broad band trajectory mechanics author s vasco dw nihei k abstract 2019 published by oxford university press on behalf of the royal astronomical society we present a trajectory based solution to the elastodynamic equation of motion that is valid across a wide range of seismic frequencies that is the derivation of the solution does not invoke a high frequency assumption or require that the medium have smoothly varying properties the approach adopted from techniques used in quantum dynamics produces a set of coupled ordinary differential equations for the trajectory the slowness vector and the elastic wave amplitude along the ray path the trajectories may be determined by a direct solution of the governing equations or derived as the by product of a numerical wavefield simulation synthetic tests with interfaces and layers containing increasingly narrow transition zones indicate that the conventional high frequency trajectories associated with the eikonal equation bend too sharply into high velocity regions as the wavelength exceeds the transition zone width tests in a velocity model based upon mapped structural surfaces from the geysers geothermal field in california indicate that discrepancies between high frequency and broad band trajectories can exceed several hundred metres at wavelengths of 1 hz an application to a crosswell tomographic imaging experiment demonstrates that the technique provides a basis for the seismic monitoring of fluid flow along narrow features such as fracture zones

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

crosswell seismic imaging in a contaminated basalt aquifer multiple seismic crosswell surveys have been acquired and analyzed in a fractured basalt aquifer at idaho national engineering and environmental laboratory most of these surveys used a high frequency 1000 10 000 hz piezoelectric seismic source to obtain p wave velocity tomograms the p wave velocities range from less than 3200 m s to more than 5000 m s additionally a new type of borehole seismic source was deployed as part of the subsurface characterization program at this contaminated groundwater site this source known as an orbital vibrator allows simultaneous acquisition of p and s waves at frequencies of 100 to 400 hz and acquisition over larger distances the velocity tomograms show a relationship to contaminant transport in the groundwater zones of high contaminant concentration are coincident with zones of low velocity and high attenuation and are interpreted to be fracture zones at the boundaries between basalt flows the orbital vibrator data show high vp vs values from 1 8 to 2 8 in

crosshole seismic tomography and borehole logging for engineering site characterization in sikeston mo usa abstract this paper presents a geophysical investigation performed in the median separating the east and west bound lanes of u s highway 60 approximately 15 km west from the town of sikeston mo usa two boreholes drilled at depth of 45 m approximately located 6 m from the wahite ditch bridge in u s highway 60 were used to carry out the geophysical tests the objective of the survey was to obtain the density distribution of soil materials and high resolution compressional and shear wave velocities of the shallow subsurface for computation of elastic engineering properties of the unconsolidated material interposed between the two investigated boreholes the studied site is located in the new madrid seismic zone nmsz one of the major seismic source zones in the eastern united states although no major seismic events have occurred in the new madrid area since the catastrophic earthquakes of 1811 1812 more intensive studies in this area are required to better understand the local soil effects and the liquefaction potential of unconsolidated earth materials on strong ground motion joint interpretation of borehole logs and velocity images obtained by p and s wave traveltime inversions outlined shallow anomalies which were interpreted as caused by variation in relative porosity and compactness of saturated unconsolidated soil materials in the interval between 17 and 19 m of depth a velocity anomaly with distinct characteristics is recognized analysis of traveltimes of p wave energy propagated from common source receiver depth positions at that depth interval delineated a low velocity zone lower than estimated p wave velocity in water in which propagated s wave energy was negligible analysis of porosity logs did not outline any porosity anomaly for that region the anomaly is interpreted as caused by the presence of a gas bearing zone extending for 1 5 m 0 76 m from the top and the bottom of the depth interval through which propagation of shear energy was almost prevented and compressional energy traveled a very low speed relative lower values of shear modulus were computed for the gas bearing zone the same depth interval also exhibited relative lower values of bulk modulus relative higher values of compressibility and relative high porosity however the low velocity anomaly is not caused by relative increased porosity but rather to replacement of water by gas in the pore volumes of this zone the presence of a shallow thin gas bearing zone is interpreted as hazardous in the occurrence of strong motion in the new madrid seismic zone shallow water table as detected by seismic and log measurements in the area increases the risk of liquefaction of the sampled soil under strong ground motions

impact tectonics in the core of the vredefort dome south africa implications for central uplift formation in very large impact structures the 80 km wide vredefort dome presents a unique opportunity to investigate the deep levels of the central uplift of a very large impact structure exposure of progressively older strata in the collar of the dome and of progressively higher grade metamorphic rocks toward its center is consistent with differential uplift however the deepest levels exposed correspond to pre impact mid crust rather than lower crust as has been suggested previously pre impact archean gneissic fabrics in the core of the dome are differentially rotated with the angle of rotation increasing sharply at a distance of 1619 km from the center the present asymmetric dips of the collar strata with layering dipping outward at moderate angles in the southeastern sector but being overturned and dipping inward in the northwestern sector and the eccentric distribution of the pre impact metamorphic isograds around the core of the dome can be reconciled with symmetric rotation of an initially obliquely nw dipping target sequence during central uplift formation the rocks in the core of the dome lack distinctive megablocks or large slip magnitude faults such as have been described in other central uplifts we suggest that the large scale coherent response of these rocks to the central uplift formation could have been accommodated by small scale shear and or rotation along pervasive pseudotachylitic breccia vein fractures

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

full waveform processing and interpretation of kilohertz cross well seismic data high frequency cross well seismic data from the midale oil field of southeastern saskatchewan are analyzed for direct and reflected energy the goal of the analysis is to produce interpretable sections to assist in enhanced oil recovery activities co2 injection in this field direct arrivals are used for velocity information while reflected arrivals are processed into a reflection image raw field data show a complex assortment of wave types that includes direct compressional and shear waves and reflected shear waves a traveltime inversion technique layer stripping via ray tracing is used to obtain p and s wave interval velocities from the respective direct arrivals the velocities from the cross well inversion and the sonic log are in reasonable agreement the subsurface coverage of the cross well geometry is investigated it covers zones extending from the source well to the receiver well and includes regions above and below the source receiver depths upgoing and downgoing primary reflections

nature of the archean midcrust in the core of the vredefort dome central kaapvaal craton south africa extreme uplift associated with the formation of the 2 02 ga vredefort dome has exposed a substantial cross section through the crystalline early archean basement complex rocks of the kaapvaal craton the rocks comprise polydeformed high grade tonalite trondhjemite granodiorite ttg gneisses migmatites and late tectonic intrusive granitoids that straddle the upper amphibolite to granulite facies transition field petrographic and geochemical data indicate that compositional heterogeneity occurs on a local scale and reflects the migmatitic character of the rocks rather than crustal scale layering as has been previously proposed no evidence has been found to support exposure of either a melt depleted refractory lower crust or an upper crustal batholithic granite layer however the immense volume of granitic leucosome in the rocks suggests that the exposed section represents an intermediate level between these two zones granitic leucosomes in the upper amphibolite facies migmatites appear to be intrusive into the predominantly trondhjemitic host rocks rather than of in situ derivation leucosome compositions in the granulite facies migmatites are more variable ranging from granitic and charnockitic to enderbitic probably reflecting at least some local derivation leucosomes and small granitoid bodies show local scale geochemical variation that can be explained in terms of variable amounts of melt segregation and migration and fractionation of minerals such as k feldspar within the melts

acoustic velocities in heated block of granite subjected to uniaxial stress acoustic velocities have been measured within a 38 cm cubic block of granite isolated from the surrounding rock the block was subjected to uniaxial stresses in the range 0 24 mpa and to heating in the range 14 sup 0 to 98 sup 0 c an increase in temperature from 14 sup 0 to 72 sup 0 c in the unstressed block resulted in an increase in velocities in directions parallel and perpendicular to the applied stress a further increase in temperature to 98 sup 0 c in the unstressed block resulted in a decrease in velocities application of uniaxial stress with the block at a temperature of 98 sup 0 c resulted first in uniform increases in velocity to an applied stress of mpa and then sharp decreases from 10 to 14 mpa followed by uniform increases in velocity to a maximum stress of 24 mpa a reduction in temperature to 16 sup 0 c resulted in a sharp reduction in velocity 25 percent less than originally measured at ambient temperature it is concluded that at temperatures rising to 72 sup 0 c anisotropic thermal expansion of the crystals comprising the granite causes the closure of microcracks present in the unheated rock between 72 sup 0 and 98 supmore 0 c particularly when stress was applied irreversible mechanical changes occurred in the block less

applications of algebraic reconstruction techniques to crosshole seismic data tomographic imaging techniques were applied to two crosshole data sets to determine the velocity structures and the reliability and resolution of the algorithms on real data the experiments were carried out at the retsoff salt mine in new york and at the underground radioactive waste study site at the stripa mine facility in sweden the traveltimes at restoff were high quality and were obtained over raypaths of up to 500 m in length the structure was quite complicated with velocity contrasts up to 50 percent the stripa site was in granitic rock with velocity contrasts of only a few percent the dimensions of the experiment were small with maximum ray lengths of just over 10 m the data at this site were collected with very high accuracy source and receiver locations were measured to better than 1 0 mm and traveltimes were read to 0 001 ms a number of algorithms similar to the algebraic reconstruction techniques art used in medical imaging have been applied to the data some modifications of the algorithms such as the application of weighting schemes damping parameters and curved raypaths were performed the resulting velocity fields were compared to the known fields and with each other to determine an optimal method the algorithms were found to be a rapid reliable means of reconstructing the slowness field of real data low velocity zones were recovered with accuracy in location and value it was also found that great care was necessary in application of the techniques to ensure that proper damping parameters are used and the proper number of iterations taken otherwise poor reconstructions will result

eigenvector analysis of gravity gradient tensor to locate geologic bodies we have developed a new method to locate geologic bodies using the gravity gradient tensor the eigenvectors of the symmetric gravity gradient tensor can be used to estimate the position of the source body as well as its strike direction for a given measurement point the eigenvector corresponding to the maximum eigenvalue points approximately toward the center of mass of the causative body for a collection of measurement points a robust least squares procedure is used to estimate the source point as the point that has the smallest sum of square distances to the lines defined by the eigenvectors and the measurement positions it s assumed that the maximum of the first vertical derivative of the vertical component of gravity vector gzz is approximately located above the center of mass observation points enclosed in a square window centered at the maximum of gzz are used to estimate the source location by increasing the size of the window the number of eigenvectors used in the robust least squares and

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

crosshole seismology and seismic imaging in crystalline rocks a borehole to borehole audiofrequency seismic system using piezoelectric transducers as active elements has been developed and used to investigate crystalline rocks in canada high quality seismograms have been recorded for transmitter receiver distances of up to 230 meters it has been found that both travel times and signal amplitudes provide diagnostic information about rock quality two dimensional images of seismic velocity and transparency distributions between two boreholes derived from observed arrival times and amplitudes by simple back projection tomography reflect lithology and geological structure the results indicate the potential usefulness of crosshole seismic imaging as a tool for rock mass characterization

geophysical aspects of meteorite impact craters in eroded shield environment with special emphasis on electric resistivity abstract the traditional geophysical methods of magnetics and gravity as applied to meteorite impact crater studies are shortly reviewed their optimal resolution power with respect to crater size and pre existing structures is discussed the morphology of some typical impact craters is illustrated to show the character of the post impact deformation a new geophysical method in impact crater studies the in situ measurement of electrical resistivity is introduced and illustrated with case studies from simple and complex craters it is concluded that the resistivity method represents a useful tool for mapping the extent of authigenic brecciation in crater structures and their surroundings the method constitutes in combination with detailed elevation data gravity and magnetic measurements a powerful tool for constraining the classification of impact craters in crystalline shield environments the resistivity measurements have revealed a complex brecciation pattern in large central uplift structures consisting of a central less brecciated core surrounded by a ring of more intense brecciation typical impact brecciated crystalline rocks have resistivities between 2000 and 8000 ohm m the crater structures treated in this paper are bjorko dellen siljan and uppland all located in sweden

borehole radar applied to the characterization of hydraulically conductive fracture zones in crystalline rock1 this paper discusses the borehole radar system ramac developed within the framework of the international stripa project which can be used in three different measuring modes single hole reflection cross hole reflection and cross hole tomography the reflection modes basically provide geometrical data on features located at some distance from the borehole in addition the strength of the reflections indicate the contrast in electrical properties single hole reflection data are cylindrically symmetrical with respect to the borehole which means that a unique fracture orientation cannot be obtained a method has been devised where absolute orientation of fracture zones is obtained by combining single hole reflection data from adjacent holes similar methods for the analysis of cross hole reflection data have also been developed and found to be efficient the radar operates in the frequency range 20 60 mhz which gives a resolution of 1 3 m in crystalline rock the investigation range obtained in the stripa granite is approximately 100 m in the single hole mode and 200 300 m in the cross hole model variations in the arrival time and amplitude of the direct wave between transmitter and receiver have been used for cross hole tomographic imaging to yield maps of radar velocity and attenuation the cross hole measurement configuration coupled withmore tomographic inversion has less resolution than the reflection methods but provides better quantitative estimates of the values of measured properties the analysis of the radar data has provided a consistent description of the fracture zones at the stripa cross hole site in agreement with both geological and geophysical observations less

vredefort a model for the anatomy of an astrobleme abstract surface mapping deep borehole intersections and geophysical data indicate the existence of a conspicuous set of spatially and genetically related structural forms centered around the town of vredefort in south africa the structure consists of a central granitic core surrounded by concentrically arranged folds and faults that affect strata of sedimentary and volcanic origin it is demonstrated that the structure conforms to that of the root zone of a multi ring impact basin vredefort related characteristics typical of structures of this kind are highlighted and brought into their evolutionary perspective impact occurred at the end of the period of igneous activity that marked the emplacement of the bushveld igneous complex approximately 2000 ma ago most traces of a stage of modification of the original transient crater were removed by erosion and the remnant structural forms reflect mainly the effects of centripetally outward directed horizontally orientated stress the morphology of the structure was eventually modified by later tectonic movement and isostatic adjustment

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

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

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

impact induced melting of archean granulites in the vredefort dome south africa i anatexis of metapelitic granulites mid crustal archean pelitic granulites in the vredefort dome experienced a static low p granulite facies overprint associated with the formation of the dome by meteorite impact at 2 02 ga heating and exhumation were virtually instantaneous with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 c and locally even exceeded 1350 c this led to comprehensive melting of the precursor archean granulite assemblages grt bt qtz pl ksp crd opx sil followed by peritectic crystallization of aluminous alkali feldspar crd spl crn sil parageneses and the segregation of small evolved biotite leucogranite bodies however at a distance of c 6 km from the centre pre impact rock features are largely preserved although partial replacement of garnet by symplectitic coronas of crd opx spl pl and biotite by orthopyroxene indicate that peak temperatures approached 775 50 c thin interstitial moats of k feldspar are closely associated with the orthopyroxene coronas they are interpreted as the remnants of low proportion partial melts generated by biotite breakdown both the textures and mineral compositional data support reduced equilibration volumes in these rocks which reflect rapid isobaric cooling following shock heating and exhumation the high temperatures and strong lateral thermal gradient are consistent with the modelled impact induced isotherm pattern for a 200 300 km diameter impact crater

clast size analysis of impact generated pseudotachylite from vredefort dome south africa clast sizes in pseudotachylite from vredefort dome south africa which is generally regarded as being of impact origin were studied clast size analyses were conducted on the outcrop and thin section scales and the clast size distribution is fractal on each scale on the outcrop scale the clast size distribution of the impact generated pseudotachylite has a small d value d 1 29 0 02 on the thin section scale the d values d 1 68 0 22 and 1 85 0 13 may be slightly larger than those of fault generated pseudotachylites and natural fault gouge and a probable lower fractal limit 50 m is on the same order as that for fault generated pseudotachylites on the thin section scale proportion of clasts for the impact generated pseudotachylites is larger than that for fault generated pseudotachylites and on the outcrop scale the impact generated pseudotachylite has a remarkably high proportion of clasts

discovery of mafic impact melt in the center of the vredefort dome archetype for continental residua of early earth cratering melting by impact heating is thought to have been a significant process in the modification of early planetary crusts however crustally derived melt bodies in ancient terrestrial crust are frequently presumed to be absent due to erosion here we demonstrate that in the central basement uplift of the 2 020 ga vredefort impact basin south africa components of mafic impact melt have survived amid archean gneiss as decimeter scale dikes and lenses of variably foliated gabbronorite zircon microstructural trace element and isotopic analyses u pb lu hf of the gabbronorite reveal a dominant population of 2 02 ga unshocked igneous zircon with apparent ti in zircon temperatures of 800 900 c similar to those from the mafic sublayer of the sudbury impact melt sheet highly negative subchondritic e hf values of 1 4 1 1 to 7 9 1 4 are consistent with a depleted mantle model age of ca 3 ga and gabbronorite derivation from the once superjacent witwatersrand basin lithologies the recrystallized igneous mineral textures and archean felsic gneiss inclusions in the gabbronorite are attributable to the effects of emplacement and crater modification following 20 km elevation of the central uplift long mistaken as preimpact basement the setting and characteristics of the vredefort gabbronorite may provide new benchmarks in the search for remnants of large cratering events and melt residua on earth s cratons

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

arrested charnockite formation in southern india and sri lanka arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high grade terrains of southern india and sri lanka two major kinds of orthopyroxene producing reactions are recognized breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic granitic gray gneiss produced the regional orthopyroxene isograd manifest in charnockitic mottling and veining of mixed facies exposures as at kabbal karnataka and in the kurunegala district of the sri lanka central highlands chemical and modal analyses of carefully chosen immediately adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of cao mgo and feo and gains of sio2 and na2o rb and y are depleted in the charnockite another kind of charnockitization is found in paragneisses throughout the southern high grade area and involves the reaction of biotite and quartz garnet to produce orthopyroxene and k feldspar although charnockite formation along shears and other deformation zones at such localities as ponmudi kerala is highly reminiscent of kabbal close pair analyses are not as suggestive of open system behavior this type of charnockite formation is found in granulite facies areas where no prograde amphibole bearing gneisses exist and connotes a higher grade reaction than that of the orthopyroxene isograd metamorphic conditions of both kabbaltype and ponmudi type localities were 700 800 c and 5 6 kbar lower p h2o in the ponmudi type metamorphism was probably the definitive factor

a new interpretive procedure for whole rock u pb systems applied to the vredefort crustal profile granulite grade precambrian gneisses have usually undergone at least one period of strong u depletion whole rock u pb isotope studies can determine the time s of the severe depletion and this work attempts to place such studies on a more rigorous footing two stage u pb systems can be described in terms of one major episodic differentiation into rocks with varying u pb ratios while three stage systems can be described by two such distinct episodes most of the precambrian granulites that have been isotopically analyzed have histories too complex to be described as two stage systems however it is demonstrated here that u pb data on whole rock suites can yield the complete u pb chemical history of a three stage system in terms of u pb ratios for a suite of granulites present day 207pb 204pb and 206pb 204pb ratios and element concentration data allow these ratios to be calculated at a number of specific past times and plotted as an array the degree of scatter in each of these past arrays is graphed as a function of time the point of least scatter denotes the age of the end of stage 2 in the history of the system the array slope and the dating of the end of stage 2 also permit the beginning of stage 2 to be calculated all other parameters in the system u and pb concentrations pb isotopic ratios can now be determined for each individual rock throughout its history the new interpretive method also distinguishes sensitively among various kinds of uranium fractionation which may have operated during the differentiation episodes it is applied here to uranium depleted granulites in the deeper part of the vredefort crustal profile the times of the two fractionating episodes are calculated at 3860 and 2760 m y respectively the vredefort 3070 m y event when geochemical systems in the upper half of the crystalline basement became permanently closed evidently had little significance for the lower half of the crystalline basement some fundamental features of archaean basement evolution whose detailed significance is still poorly understood were elucidated for the first time by this u pb isotope study on whole rock samples rocks which suffered little uranium depletion at the end of stage 1 suffered least at the end of stage 2 rocks which were most affected at the end of stage 1 suffered the strongest depletion at the end of stage 2 evidently repeated high grade metamorphism purges certain granulites of their u content but leaves others relatively unscathed

inversion strategy in crosshole radar tomography using information of data subsets detecting discrete anomalies such as cavities or tunnels is an important application of crosshole radar tomography however crosshole tomographic inversion results are frequently ambiguous showing smearing effects and inversion artifacts these ambiguities lead to uncertainties in interpretation hence the size and position of anomalies can only be interpreted with limited accuracy and reliability we present an inversion strategy for investigating discrete anomalies with crosshole radar tomography in addition to the full traveltime data set we use subsets of specified ray angle intervals for tomographic inversion by analyzing inversion results from different ray angle intervals a more accurate interpretation of anomalies is possible the second step of our strategy is to develop a good inhomogeneous starting model from joint interpretation of the inversion results from different subsets the third step is to invert the full data set using this new starting model and to evaluate the inversion results by analyzing the distributions of mean square traveltime residuals with respect to the ray angles we use a synthetic model with two discrete anomalies located roughly at the same depth to demonstrate and evaluate our approach this inversion strategy is also applied to a field data set collected to investigate karst cavities in limestone from the inversion results of both examples we show that horizontal smearing of anomalies can be reduced by eliminating near horizontal rays a good starting model can be obtained based on the joint interpretation of the inversion results of the different subsets it leads to a high resolution final image of the full data set

a review of the geochronological constraints on the evolution of the kaapvaal craton south africa abstract all the published u pb geochronological data from zircon titanite sphene and monazite and pb pb evaporation data applicable to the palaeoarchaean to neoarchaean geology of the kaapvaal craton are employed to reconstruct the history and crustal architecture of the ca 3600 2500 ma cratonic basement in south africa only data interpreted as representative of the age of crystallisation or the 207 pb 206 pb ages from zircon xenocrysts have been considered in constructing graphs of cumulative probability versus age of rock formation in this manner it has been possible to identify the main magmatic events that have contributed to the assembly of the craton for convenience of data handling the kaapvaal craton has been subdivided into eastern central northern and western geographic domains these domains are not intended as discrete geological terranes although each one has its own geological and chronological character although consanguinity of events is also evident the oldest rocks so far recognised are located in the swaziland barberton areas where ages 3600 ma have been recorded the early stages of shield development are best exposed in the barberton mountain land where it is now apparent that continent formation took place by magmatic accretion and tectonic amalgamation of small protocontinental blocks at barberton several diachronous blocks formed between 3600 and 3200 ma have been identified each of which represents a cycle of arc related magmatism and sedimentation this phase of crustal development was followed by a period of mesoarchaean cratonic magmatism particularly prevalent between approximately 3100 3000 ma and marked by the development of a major crescent shaped juvenile arc that was accreted onto the northern and western margins of the evolving kaapvaal shield cratonisation was accomplished by the emplacement of major granitoid batholiths which thickened and stabilised the continental crust during the early stages of this cycle subsequent evolution of the craton between 3000 and 2700 ma was associated with continent arc collision during which time the witwatersrand basin and its correlatives were deposited as foreland sequences followed by episodic extension and rifting when the gaborone kanye and ventersdorp sequences were developed

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

surface to tunnel seismic tomography studies at yucca mountain nevada 1 a surface to tunnel seismic survey was conducted to estimate fracture intensity and distribution in the proposed nuclear waste repository area at yucca mountain nevada a 5 km long source line and a 3 km long receiver line were located on top of yucca mountain ridge and inside the exploratory study facility esf tunnel respectively numerical modeling showed that the first arrival p waves are mainly propagated in the repository horizon at depth therefore two dimensional travel time inversions were performed after the actual source locations on the ridge were projected onto the layer of interest at depth static corrections were applied to absorb the scatter in the new source locations and to correct for variations in source and receiver coupling and travel time shifts zero offset amplitude variations of the first arrivals across the receiver line inside the tunnel showed strong correlation with the fracture density determined by measurements along the tunnel wall wave guidance by subhorizontal fractures is suggested as a possible source for this phenomenon tomographic inversion of the travel time data revealed a low velocity zone in the south central area of the repository horizon conversion of the velocity tomography results to fracture density tomograms showed good correlation with fracture intensity mapped along tunnel walls the fractured zone extends over a large area in the southern region of the potential repository in particular southwest of the intensely fractured zone encountered in the esf tunnel the rock in the northern section of the repository appears more competent compared to the highly fractured areas in the south

terrestrial impact melt rocks and glasses abstract the effects of meteorite and comet impact on earth are rock brecciation the formation of shock metamorphic features rock melting and the formation of impact structures i e simple craters complex craters and multi ring basins large events such as the 65 ma chicxulub impact are believed to have had catastrophic environmental effects that profoundly influenced the development of life on earth in this review an attempt is made to summarize some of the voluminous literature on impact melting one important aspect of planetary impact provide some comments on this process and to make suggestions for future research the products of impact melting are glasses impact melt rocks and pseudotachylites our treatise deals mainly with the geological setting petrography and major element chemistry of melt rocks and glasses impact glasses in several petrographic aspects are similar to volcanic glasses but they are associated with shock metamorphosed mineral and rock fragments and in places with siderophile element anomalies suggestive of meteoritic contamination they are found in allogenic breccia deposits within fall back suevite and outside fall out suevite impact craters and as spherules in distal ejecta large events such as the k t boundary chicxulub impact are responsible for the formation of worldwide ejecta horizons which are associated with siderophile element anomalies and shock metamorphosed mineral and rock debris impact glasses have a bulk chemical composition that is homogeneous but exemptions to this rule are common on a microscopic scale however impact glasses are commonly strikingly heterogeneous tektites are glasses ejected from craters over large distances they are characterized by very low water and volatile contents and element abundances and ratios that are evidence that tektites formed by melting of upper crustal sedimentary rocks four tektite strewn fields are known three of which can be tied to specific impact craters impact melt rocks form sheets lenses and dike like bodies within or beneath allogenic fallback breccia deposits in the impact crater and possibly on crater terraces and flanks dikes of impact melt rocks also intrude the rocks of the crater floor they commonly contain shock metamorphosed target rock and mineral fragments in various stages of assimilation and are glassy or fine to coarse grained chemically they are strikingly homogeneous but as with impact glasses exemptions to this rule do exist large and thick melt bodies such as the sudbury igneous complex sic are differentiated or may represent a combination of impact melt rocks sensu strictu and impact triggered deep crustal melts a concerted multidisciplinary approach to future research on impact melting and on other aspects of meteorite and comet impact is advocated impact models are models only and uncritical reliance on their validity will not lead to a better understanding of impact processes especially of melting excavation and deposition of allogenic breccias and the spatial position of breccias in relation to sheets and lenses of melt rocks within the crater impact triggered pressure release melting of target rocks beneath the excavation cavity may be responsible for the existence of melt rocks beneath the impact melt rocks sensu strictu this controversial idea needs to be tested by a re evaluation of existing data and models be they based on field or laboratory research only a relatively small number of terrestrial impact structures has been investigated in sufficient detail as it relates to geological and geophysical mapping in this review we summarize observations made on impact melt rocks and impact glasses in a number of north american brent haughton manicouagan new quebec sudbury wanapitei all in canada asian popigai russia zhamanshin kazakhstan two south african structures morokweng and vredefort the henbury crater field of australia and one european crater ries germany our tables listing major element chemical compositions of impact glasses and melt rocks however include also data from structures not dealt with in further detail

shocked monazite chronometry integrating microstructural and in situ isotopic age data for determining precise impact ages monazite is a robust geochronometer and occurs in a wide range of rock types monazite also records shock deformation from meteorite impact but the effects of impact related microstructures on the u th pb systematics remain poorly constrained we have therefore analyzed shock deformed monazite grains from the central uplift of the vredefort impact structure south africa and impact melt from the araguainha impact structure brazil using electron backscatter diffraction electron microprobe elemental mapping and secondary ion mass spectrometry sims crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal plastic deformation features including shock twins planar deformation bands and neoblasts shock twins were documented in up to four different orientations within individual monazite grains occurring as compound and or type one twins in 001 100 left 10 bar 1 right 110 left 212 right and type two irrational twin planes with rational shear directions in 0 bar 1 bar 1 and bar 1 bar 1 0 sims u th pb analyses of the plastically deformed parent domains reveal discordant age arrays where discordance scales with increasing plastic strain the correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event neoblasts in granular monazite domains are strain free having grown during the impact events via consumption of strained parent grains neoblastic monazite from the inlandsee leucogranofels at vredefort records a 207pb 206pb age of 2010 15 ma 2 n 9 consistent with previous impact age estimates of 2020 ma neoblastic monazite from araguainha impact melt yield a concordia age of 259 5 ma 2 n 7 which is consistent with previous impact age estimates of 255 3 ma our results demonstrate that targeting discrete microstructural domains in shocked monazite as identified through orientation mapping for in situ u th pb analysis can date impact related deformation monazite is therefore one of the few high temperature geochronometers that can be used for accurate and precise dating of meteorite impacts

archean crustal structure of the kaapvaal craton south africa evidence from the vredefort dome abstract crystalline archean basement rocks in the core of the vredefort dome present a profile through a substantial part of the middle and lower crust of the kaapvaal craton previously this profile has been subdivided into two terranes with allegedly distinct lithologies and tectonometamorphic histories that were juxtaposed along a crustal scale late archean brittle ductile thrust zone lithological and structural mapping across the dome indicates however that the basement lithologies share a common polyphase tectonic history culminating in high grade metamorphism and melting at 3 1 ga no evidence was found of the postulated tectonic terrane boundary but the alleged boundary does coincide with a 1 2 km wide transition zone between upper amphibolite facies migmatitic gneisses and more restitic granulite facies gneisses the implications of these results for archean regional tectonic models for the kaapvaal craton are discussed

crustal heterogeneity in south australia earthquake evidence summary lateral velocity variations in the south australian crust have been mapped using the relatively new technique of seismic tomography the input data comprised p1 travel times from 1340 local earthquakes recorded on the south australian seismic network during the period 1978 1983 to reduce errors in the earthquake locations focal depths and origin times we used an averaging technique of dividing the area under consideration into a series of overlapping cylindrical cells all earthquakes and seismic stations were assigned to the centre of the nearest cell travel times between all centres were computed by averaging over all relevant cell pairs after first scaling all travel times by a factor equal to the distance between cell centres divided by the actual ray path length only ray paths appropriate to the p1 phase were considered starting with a first guess model based on the number of ray intersections with each cell the travel times were computed along all ray paths and compared with the observed averaged values the errors and residuals were back projected for each ray over all the chords along the ray and the average cell slowness adjusted in an iterative sense until the computed and observed travel times agreed within a specified error tolerance the areas with poor raypath coverage lack of earthquakes and or stations could not be imaged the area covered lies mostly within the adelaide geosyncline where there is believed to be a thick cover of proterozoic sediments the velocity image is of the upper 20 km of the crust so no direct correlation between the surface geology and the crustal velocity is to be expected results indicate a significant variation in crustal velocity from 5 9 to 6 5 km s 1 the velocities correlate remarkably with time terms computed in a separate study of moho refracted arrivals pn low time terms correspond to high crustal velocity and high time terms reflect anomalously low crustal velocity the residual component of the time terms can be explained in terms of variations in depth to the moho throughout the geosyncline of the order of 10 km the prominent features of the tomographic velocity map are a n s trending velocity low over the spencer gulf graben and a ridge of high velocity beneath the flinders ranges the velocity low can be associated with a region of low gravity aligned parallel to the torrens hinge zone which marks the western boundary of the geosyncline the northern end of the high velocity ridge coincides with a region of positive bouguer gravity teleseismic residuals from both earthquakes and nuclear explosions show a similar trend to the pn time terms and indicate lateral variability in crustal thickness and velocity a component of the residual must be attributed to mantle velocity inhomogeneity which extends to depths of 300 km

seismic characterization of a highly stressed rock mass using tomographic imaging and induced seismicity a seismic tomography study was carried out at falconbridge s lockerby mine sudbury canada in april 1989 the objective was to determine the integrity of a rock mass at 1 km depth and to investigate the correlation between induced seismicity and p wave velocity structure the tomographic image showed two distinct and significant p wave velocity anomalies a low velocity zone at the source region of a mn 2 3 mining induced tremor recorded in 1985 and a high velocity zone at the source region of a mn 2 6 mining induced tremor recorded in 1988 analysis of mining induced microseismicity showed that the high velocity zone was the focus for seismic activity prior to and after the 1988 tremor in contrast to the low velocity zone which remained aseismic for the same period the seismic anomalies were diamond drilled following the tomographic imaging to verify the physical characteristics of the zones the seismically active high velocity zone showed severe core disking indicative of a highly stressed strong rock while the low velocity zone was characterized by extensive fracturing and failed rock the results showed that the 1985 tremor and rock burst sequence resulted in a major failure of the rock mass in the source region of the event and stress was redistributed and concentrated in the source region of the 1988 tremor the 1988 event and rock burst sequence did not result in failure of the source region which therefore remains a seismically active high velocity zone with a potential for significant seismic energy release and further rock bursting the study has highlighted a correlation between p wave velocity structure and induced seismicity which can be used to characterize highly stressed rock masses in underground mines

ultramafic rocks in the centre of the vredefort structure south africa possible exposure of the upper mantle abstract the rocks in the central region of the vredefort structure are interpreted as lower crust that has undergone uplift of 36 km during the vredefort cryptoexplosion event 2 0 ga ago the outcrops in this central region are mainly high grade leucogranulites interbanded with mafic granulites and supracrustals rocks from a borehole located near the centre of the structure indicate that the core region is underlain by ultramafic rocks which probably represent the upper mantle beneath this region of the kaapvaal craton the main lithologies sampled by the borehole are serpentinized hornblende bearing harzburgite with minor bands of pyroxenite and glimmerite the major and trace element chemistry of these ultramafic rocks is similar to that of fertile oceanic lithosphere mineral chemistry and petrographic data suggest that the vredefort ultramafics are metamorphic rocks which have undergone a complex tectonic and metamorphic history sm nd and rb sr isotopic data together with the field relationships provide some evidence that these rocks are archaean but also that they have been chemically and isotopically altered similar hydrated and chemically altered ultramafic rocks occur in the jamestown ophiolite complex in the barberton greenstone belt accordingly we speculate that the crust mantle transition zone in the kaapvaal craton consists of a zone of hydrated archaean oceanic lithosphere