integrated interpretation of 3d seismic data to enhance the detection of the gold bearing reef mponeng gold mine witwatersrand basin south africa we present an integrated approach to the seismic interpretation of one of the world s deepest gold ore body carbon leader reef using three dimensional seismic data ultrasonic velocity measurements at elevated stresses and modified instantaneous attribute analysis seismic wave velocities of the drill core samples quartzite shale and conglomeratic reef from the mine are sensitive to uniaxial stress changes i e they slowly increase with increasing pressure until they reach maximum value at 25 mpa for all the samples seismic velocities are constant above 25 mpa indicating a possible closure of microcracks at stress corresponding to 1 0 km 1 5 km a reflection coefficient of 0 02 computed between hanging wall and footwall quartzites of the carbon leader reef ore body suggests that it may be difficult to obtain a strong seismic reflection at their interface our modified seismic attribute algorithm on the other hand shows that the detection of the lateral continuity of the carbon leader reef reflector can significantly be improved by sharpening the seismic traces three dimensional seismic data reveal that faults with throws greater than 25 m that offset the carbon leader reef can clearly be seen faults with throws less than 25 m but greater than 2 m throw were identified through horizon based attribute analysis while most dykes and sills with thickness less than 25 m were invisible the detection of the lateral continuity of the carbon leader reef reflector and its depth position is greatly improved by integrating the modified instantaneous attributes with controls from borehole observations the three dimensional visualization and effective interpretation of the carbon leader reef horizon shows a host of structurally complex ore body blocks that may impact future shaft positioning and reduce its associated risks

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

integrating palaeocaves into palaeolandscapes an analysis of cave levels and karstification history across the gauteng malmani dolomite south africa abstract the drimolen palaeocave system in the fossil hominid sites of south africa unesco world heritage site is well known for numerous remains of early hominins such as paranthropus robustus and early homo these hominin fossils along with bone tools and notably diverse accumulation of non hominin primates and fauna have all been excavated from the main quarry area of the site where extensive lime mining took place here we report the first radiometric age of 1 712 0 269 ma for hominin bearing deposits associated with the dnh7 paranthropus robustus cranium in the main quarry area of the site which is consistent with recent biochronological estimates this age is similar to recent estimates for swartkrans member 1 hanging remnant somewhere between 2 3 and 1 8 ma which also contains paranthropus and early homo simultaneously we integrate the newly radiometrically dated main quarry deposits with a new fossil deposit the drimolen makondo discovered in 2013 that is situated some 50 m up the hill to the west from the main quarry it has experienced only limited disturbance from mining but much more extensive erosion preliminary excavations and analysis have revealed that the makondo infill is older than the main quarry dating to 2 706 0 428 ma its greater age is confirmed by biochronology the makondo thus overlap with the suggested end of deposition of australopithecus bearing sterkfontein deposits although it is yet to yield any hominin remains these new dates for the two drimolen palaeocave system deposits indicates that contrary to prior age estimates the drimolen site as a whole records the critical hominin and faunal turnover in south african palaeocommunities that occurred around 2 3 1 7 ma finally as the drimolen makondo represents a rare example of a pre 2 ma fossil bearing deposit in the gauteng exposures of the malmani dolomite we also integrate our results into the greater south african record of palaeodeposit formation most of which occur between 2 0 and 1 0 ma an analysis of the age of palaeocave infillings across the malmani dolomite suggests that as is classically the case with karst the height within the dolomite is broadly correlated to their age although with some notable exceptions that are likely related to localised geological features our analysis also indicates that most caves have undergone some form of secondary karstification related to a younger phase of cave formation contrasting with models that suggest the cavities all formed at the same time and that infill is related to erosion and the opening up of cave passages as such the reason that few pre 2 ma deposits have been identified in the gauteng exposures of the malmani dolomite is probably because these older caves have been eroded away identifying such early caves is critical in understanding whether earlier hominins may have once existed in south africa or if erosion of older deposits or an absence of speleogenesis at this time has made such early periods absent from the geological record

nature of and controls on volcanism in the ca 2 32 2 06ga pretoria group transvaal supergroup kaapvaal craton south africa abstract the time of deposition of the pretoria group between 2 32 and 2 06 ga on south africa s kaapvaal craton was characterized by the first major increase in atmospheric oxygen it was accompanied by the extrusion of significant thicknesses of volcanic deposits namely the bushy bend lavas of the timeball hill formation the hekpoort formation and the machadodorp volcanic member of the silverton formation marking the three major volcanic events within the transvaal supergroup which are thought to be precursors to the succeeding bushveld complex magmatism the bushy bend lava member of the timeball hill formation is characterized by subaqueous basaltic andesitic fissure eruptions with fumarolic activity and probably minor subaerial explosive eruptions in the hinterland of the transvaal supergroup basin the subaerial volcanism of the hekpoort formation appears to have been dominated by fissure eruptions with a preponderance of lava flows and locally important pyroclastic material intermittent hiatuses in volcanism were marked by local lacustrine shale deposition finally the machadodorp lava member can be interpreted as the deposits of several seamounts aligned along a fissure probably within an extensional environment the geodynamic control inferred for the evolution of the pretoria group basin encompasses two cycles of prerift uplift subsequent mechanical rifting and long lived thermal subsidence the limited extent of the bushy bend lavas in the south of the pretoria group depository attests to the likelihood that volcanism accompanying the first rifting event was short lived in contrast the second rifting cycle postulated for the pretoria group basin has a strong association with widespread and large scale volcanism of the hekpoort ongeluk flood basalts which may have been plume related the scale of the second cycle volcanism and its importance in influencing the upper part of the pretoria group basin fill is underlined by the machadodorp volcanism probably related to hot spot volcanism as the ongeluk hekpoort plume waned

application of 3d seismic techniques to evaluate ore resources in the west wits line goldfield and portions of the west rand goldfield south africa abstractas expensive as 3d seismic reflection surveys are their high cost is justified by improved imaging of certain ore horizons in some of the witwatersrand basin gold mines the merged historical 3d seismic reflection data acquired for kloof and south deep mines forms an integral part of their ventersdorp contact reef mine planning and development programme the recent advances in 3d seismic technology have motivated the reprocessing and reinterpretation of the old data sets using the latest algorithms therefore significantly increasing the signal to noise ratio of the data in particular the prestack time migration technique has provided better stratigraphic and structural imaging in complex faulted areas such as the witwatersrand basin relative to older poststack migration methods interpretation tools such as seismic attributes have been used to identify a number of subtle geologic structures that have direct impact on ore resource evaluation other improvements include more accurate mapping o

potential vorticity diagnosis of a tropopause polar cyclone long lived coherent vortices located near the tropopause are often found over polar regions although these vortices are a commonly observed feature of the arctic and can have lifetimes longer than one month little is known about the mechanisms that control their evolution this paper examines mechanisms of intensity change for a cyclonic tropopause polar vortex tpv using an ertel potential vorticity epv diagnostic framework results from a climatology of intensifying cyclonic tpvs suggest that the essential dynamics are local to the vortex rather than a consequence of larger scale processes this fact motivates a case study using a numerical model to investigate the role of diabatic mechanisms in the growth and decay of a particular cyclonic vortex a component wise breakdown of epv reveals that cloud top radiational cooling is the primary diabatic mechanism that intensifies the tpv during the growth phase increasing amounts of moisture become entrained into the vortex core at later times near hudson bay allowing the destruction of potential vorticity near the tropopause due to latent heat release to become comparable to the radiational tendency to create potential vorticity

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

anomalous radar backscatter from titan s surface abstract since cassini arrived at saturn in 2004 its moon titan has been thoroughly mapped by the radar instrument at 2 cm wavelength in both active and passive modes some regions on titan including xanadu and various bright hummocky bright terrains contain surfaces that are among the most radar bright encountered in the solar system this high brightness has been generally attributed to volume scattering processes in the inhomogeneous low loss medium expected for a cold icy satellite surface we can test this assumption now that the emissivity has been obtained from the concurrent radiometric measurements for nearly all the surface with unprecedented accuracy janssen et al and the cassini radar team 2009 icarus 200 222 239 kirchhoff s law of thermal radiation relates the radar and radiometric properties in a way that has never been fully exploited in this paper we examine here how this law may be applied in this case to better understand the nature of titan s radar bright regions we develop a quantitative model that when compared to the observational data allows us to conclude that either the reflective characteristics of the putative volume scattering subsurface must be highly constrained or more likely organized structure on or in the surface is present that enhances the backscatter

hydrothermal alteration in the witwatersrand goldfields abstract alteration is recorded throughout much of the 3 km thick upper witwatersrand succession in south africa and extends for 300 km around the basin and 50 km into the basin the evidence for this alteration comes from petrography mineralogy geochemistry rock fabrics and stratigraphy the alteration conceived as a giant halo enveloping each major goldfield requires a major hydrothermal event focused on the auriferous upper wits central rand group the scale and nature of the alteration and its relationship to gold distribution on various scales strongly support a hydrothermal model for gold deposition such a model can account for the 50 000 t of gold mined from the witwatersrand goldfields the model works within the context of the major advances in understanding of the geological framework of the witwatersrand basin including the recognition of the importance of post burial processes it is highly significant that the age of witwatersrand sedimentation is much older than 2700 my and thus the host rocks for gold were deposited at least 100 my before the major global archean greenstone gold event the shale units from 40 major mines covering the seven goldfields are examined to establish the nature and distribution of the alteration and to make the connection between the fluid infiltration that caused the alteration and the introduction of gold into the witwatersrand basin in the hydrothermal model described here an auriferous low salinity fluid is produced at about 500 c during the metamorphism of mafic material that was below and adjacent to the witwatersrand basin large shear zones in the footwall of goldfields are the inferred fluid pathways into the upper wits succession and then unconformities smaller shear zones veins and the reef packages guide local fluid flow in this model gold precipitation occurs in response to the reaction of the fluid with carbon and iron bearing rocks that are abundant in the reef packages immediately overlying unconformity surfaces gold precipitation occurred at about 350 c and 10 km depth with the implication of a high geothermal gradient approaching 35 c per km the alteration of the shales but lack of abundant gold in them is predicted from the paucity of appropriate rocks or minerals in the shale units to precipitate gold but fluid infiltration still changed their composition fundamentally the source of the witwatersrand gold its present distribution and the alteration are explained within this hydrothermal model with similarities to the models invoked for other major gold provinces it is worth noting that quartz pebble conglomerate is a poor term for this gold type and has almost certainly limited exploration opportunities globally

is the ventersdorp rift system of southern africa related to a continental collision between the kaapvaal and zimbabwe cratons at 2 64 ga ago abstract rocks of the ventersdorp supergroup were deposited in a system of northeast trending grabens on the kaapvaal craton approximately 2 64 ga ago contemporary with a continental collision between the kaapvaal and zimbabwe cratons we suggest that it was this collision that initiated the ventersdorp rifting individual grabens strike at high angles toward the continental collision zone now exposed in the limpopo province where late orogenic left lateral strike slip faulting and anatectic granites are recognized we relate the ventersdorp rift province to extension in the kaapvaal craton associated with the collision and see some analogy with such rifts as the shansi and baikal systems associated with the current india asia continental collision

application of seismic to mineral deposit exploration and evaluation mineral deposits occur in different geological envi ronments than oil and gas deposits and are usually at very shallow depths from surface outcrop to hundreds of meters because of internal heterogeneities within most deposit types extensive use of borehole sampling is required to satisfy resource and reserve reporting requirements while borehole sampling can be designed to satisfy the geostatistical variability of a deposit it is usually insufficient to adequately sample the structural continuity of deposits to a level required for accurate mine planning and production scheduling for this reason 2d and 3d seismic surveys are increasingly used in conjunction with borehole drilling and other geophysical methods such as aeromagnetic surveys to map the geological structure of known deposits this paper will examine some challenges and results from anglo american s use of seismic surveying in african coal and platinum deposits

radar investigation of mars mercury and titan radar astronomy is the study of the surfaces and near surfaces of solar system objects using active transmission of modulated radio waves and the detection of the reflected energy the scientific goals of such experiments are surprisingly broad and include the study of surface slopes fault lines craters mountain ranges and other morphological structures electrical reflectivities contain information about surface densities and to some extent the chemical composition of the surface layers radar probes the subsurface layers to depths of the order of 10 wavelengths providing geological mapping and determinations of the object s spin state radar also allows one to study an object s atmosphere and ionic layers as well as those of the interplanetary medium precise measurements of the time delay to surface elements provide topographic maps and powerful information on planetary motions and tests of gravitational theories such as general relativity in this paper we limit our discussion to surface and near surface probing of mercury mars and titan and review the work of the past decade which includes fundamentally new techniques for earth based imaging the most primitive experiments involve just the measurement of the total echo power from the object the most sophisticated experiments would produce spatially resolved maps of the reflected power in all four stokes parameters historically the first experiments produced echoes from the moon during the period shortly after world war ii see e g evans 1962 but the subject did not really develop until the early 1960s when the radio equipment was sufficiently sensitive to detect echoes from venus and obtain the first doppler strip maps of that planet the first successful planetary radar systems were the continuous wave cw radar at the goldstone facility of the caltech s jet propulsion laboratory and the pulse radar at the mit lincoln laboratory all of the terrestrial planets were successfully studied during the following decade yielding the spin states of venus and mercury a precise value of the astronomical unit and a host of totally new discoveries concerning the surfaces of the terrestrial planets and the moon this work opened up at least a similar number of new questions although the early work was done at resolution scales on the order of the planetary radii very rapid increases in system sensitivities improved the resolution to the order of 100 km but always with map ambiguities recently unambiguous resolution of 100 m over nearly the entire surface of venus has been achieved from the magellan spacecraft using a side looking synthetic aperture radar reviews of the work up to the magellan era can be found in evans 1962 muhleman et al 1965 evans hagfors 1968 see chapters written by g pettengill t hagfors and j evans and ostro 1993 the radar study of venus from the magellan spacecraft was a tour de force and is well described in special issues of science volume 252 april 12 1991 and in the journal of geophysical research volume 97 august 25 and october 25 1992 venus will not be considered in this paper even though important polarization work on that planet continues at arecibo goldstone and the very large array vla in this paper we review the most recent work in earth based radar astronomy using new techniques of earth rotation super synthesis at the vla in new mexico operated by the national radio astronomy observatory and the recently developed long code techniques at the arecibo observatory in puerto rico operated by cornell university note it was recently brought to our attention that the vla software doubles the flux density of their primary calibrators consequently it is necessary to half the radar power and reflectivity numerical values in all of our published radar results from the vla goldstone radar the symbiotic relationship in these new developments for recent advances in our understanding of mercury and mars is remarkable vla imaging provides for the first time unambiguous images of an entire hemisphere of a planet and the long code technique makes it possible to map mars and mercury using the traditional range gated doppler strip mapping procedure which was apparently developed theoretically at the lincoln laboratory by paul green based on a citation in evans 1962 richard goldstein was the first to obtain range gated planetary maps of venus as reported in carpenter goldstein 1963 such a system was developed earlier for the moon as reported by pettengill 1960 and pettengill henry 1962 we first discuss the synthesis mapping technique

titan s surface from reconciled cassini microwave reflectivity and emissivity observations abstract the surface of titan saturn s largest moon appears to consist of solid hydrocarbons or co2 with considerable small scale structure according to a new electromagnetic scattering model that uses both cassini radar reflectivity and emissivity measurements our model resolves a well known ambiguity in active and passive observations of surfaces namely that the dielectric constant retrieved from radiometric data is usually less than that obtained from radar backscatter analysis we present here parameters of titan derived from radar and radiometric responses using a single set of surface physical descriptors the observed dielectric constant ranges from 1 75 2 5 consistent with the solids above but also with unconsolidated water or water ammonia ice inferred rms slopes of the surface range from 10 16 at cassini s 2 2 cm with radar albedos of about 30 emissivities of the surface are quite high on the order of 90 we find that the majority of the backscattered radar energy results from volume rather than surface scattering processes thus large scale variations in radar brightness are mainly diagnostic of composition and structure within titan s near subsurface the high level of volume scatter about 15 of incident energy and consequent high radar albedo suggest significant wavelength scale structure and multiple scatter within the medium this is most consistent with a surface composed either of ubiquitous mm size bubbles within the ice analogous to a vesicular cryolava or structures similar to ice pipes and lenses found in terrestrial ice sheets the cryovolcanic hypothesis suggests planet wide resurfacing by active volcanism to explain the large scale uniformity of observed backscatter if the lens pipe mechanism pertains it can imply percolation of precipitating liquid or surface melt through the titan firn

microstructural evolution and trace element mobility in witwatersrand pyrite microstructural analysis of pyrite from a single sample of witwatersrand conglomerate indicates a complex deformation history involving components of both plastic and brittle deformation internal deformation associated with dislocation creep is heterogeneously developed within grains shows no systematic relationship to bulk rock strain or the location of grain boundaries and is interpreted to represent an episode of pyrite deformation that predates the incorporation of detrital pyrite grains into the central rand conglomerates in contrast brittle deformation manifest by grain fragmentation that transects dislocation related microstructures is spatially related to grain contacts and is interpreted to represent post depositional deformation of the central rand conglomerates analysis of the low angle boundaries associated with the early dislocation creep phase of deformation indicates the operation of 100 slip systems however some orientation boundaries have geometrical characteristics that are not consistent with simple 100 deformation these boundaries may represent the combination of multiple slip systems or the operation of the previously unrecognized 120 slip system these boundaries are associated with order of magnitude enrichments in as ni and co that indicate a deformation control on the remobilization of trace elements within pyrite and a potential slip system control on the effectiveness of fast diffusion pathways the results confirm the importance of grain scale elemental remobilization within pyrite prior to their incorporation into the witwatersrand gold bearing conglomerates since the relationship between gold and pyrite is intimately related to the trace element geochemistry of pyrite the results have implications for the application of minor element geochemistry to ore deposit formation suggest a reason for heterogeneous conductivity and localized gold precipitation in natural pyrite and provide a framework for improving mineral processing

the ohio valley wave merger cyclogenesis event of 25 26 january 1978 part ii diagnosis using quasigeostrophic potential vorticity inversion abstract the dynamical interactions between precursor disturbances during the wave merger cyclogenesis event of 25 26 january 1978 over eastern north america are diagnosed using quasigeostrophic potential vorticity qgpv inversion this case is characterized by two prominent preexisting upper level disturbances that induce rapid surface cyclogenesis as they come into close proximity static qgpv inversion is used to attribute a particular geopotential height field to the qgpv associated with each precursor disturbance the full flow is partitioned into the following components the northern upper precursor the southern upper precursor and the background flow prognostic qgpv inversion is used to quantify the instantaneous geopotential height tendencies attributable to each of these flow components the static inversion results for the upper precursors exhibit the structure of baroclinic vortices with maximum amplitude near the tropopause during the 48 h period spanning the period of study of this even

steady activity of microfractures on geological faults loaded by mining stress abstract acoustic emissions ae down to m w 4 were recorded at a site 1 km beneath the surface in the cooke 4 mine south africa several planar ae clusters with lateral extent of 10 100 m were identified most of them were located several tens of meters away from the mining front and exhibited steady activity during the analysis period of about two months some of the clusters coincided with mapped faults the planar cluster aes were sharply aggregated within a thickness of several decimeters likely delineating the fracture interface of the fault and its higher order morphology such as branches bends and stepovers the composite focal mechanism evaluated for each cluster was consistent with slip events on the fracture interface these results imply that numerous shear microfractures occur steadily on a natural fault surface subjected to a mining related stress increase the planar clusters consist of very small aes 99 7 were smaller than m w 2 exhibiting high b values much exceeding unity this contrasts with the more usual b values of the stope cluster aes which were aggregated within 20 m of the mining front and exhibited a more scattered distribution the size distribution of microfractures on a fracture interface may directly reflect fine scale irregularities of the interface on the other hand many other mapped faults near the planar ae clusters were not accompanied by ae activities despite the fact that these quiet faults were subjected to a similar stress history the presence or absence of ae activities on a fault may reflect different states of the fault including stress and strength

composite structure of tropopause polar cyclones tropopause polar vortices are coherent circulation features based on the tropopause in polar regions they are a common feature of the arctic with typical radii less than 1500 km and lifetimes that may exceed 1 month the arctic is a particularly favorable region for these features due to isolation from the horizontal wind shear associated with the midlatitude jet stream which may destroy the vortical circulation intensification of cyclonic tropopause polar vortices is examined here using an ertel potential vorticity framework to test the hypothesis that there is an average tendency for diabatic effects to intensify the vortices due to enhanced upper tropospheric radiative cooling within the vortices data for the analysis are derived from numerical simulations of a large sample of observed cyclonic vortices over the canadian arctic results show that there is on average a net tendency to create potential vorticity in the vortex and hence intensify cyclones and that the tendency is radiatively driven while the effects of latent heating are considerable they are smaller in magnitude and all other diabatic processes have a negligible effect on vortex intensity

large scale antecedent conditions associated with the 12 14 march 1993 cyclone superstorm 93 over eastern north america abstract the results of a multiscale analysis of the 12 14 march 1993 superstonn ss93 over eastern north america are presented a time sequence of overlapping 10 day time mean 5oo hpa geopotential height and anomaly composites shows that the northern hemisphere nh flow pattern from 18 february to 15 march 1993 is characterized by 1 three persistent troughs situated over eastern asia and the northwestern pacific over eastern north america and over northwestern africa and southwestern europe eastward to central russia and 2 a massive blocking anticyclone located over the central and eastern atlantic beginning 8 9 march 1993 the planetary scale flow amplifies substantially the explosive ss93 cyclogenesis and the transport of cold air to very low latitudes occurs a few days later as the nh available potential energy content after peaking on 9 march 1993 decreases by about 6 7 a dynamical tropopause analysis is used to track coherent transient potential vorticity pv anomalies and show their q

mesoarchaean palaeoproterozoic crustal scale tectonics of the central witwatersrand basin interpretation from 2d seismic data and 3d geological modelling abstract the witwatersrand basin represents one of the largest exposures of meso neoarchaean rock on earth and hosts the vredefort dome at its geographic centre the southern half of the basin is covered by thin palaeozoic to early mesozoic marginal sequences of the karoo supergroup however visualisation and analysis of this unexposed portion of the basin can be achieved in a 3d geomodelling environment through the integration of geophysical 2d reflection seismics and traditional geological data in this study the 3d structural architecture around the vredefort dome in the witwatersrand basin is evaluated to establish strato tectonic relationships first order scale structures and a new model for the architecture of basement rocks from the geological modelling several strato structural features have been identified in the seismic sections including a well developed listric fault system in the southwest a prominent fold system observed in the transvaal supergroup and a large listric fault in the east that offsets the aforementioned fold system integration of these geophysical and geological data in 3d space has provided 3d constraints on the volumes for the witwatersrand supergroup ventersdorp supergroup and transvaal supergroup to the south southeast and east of the vredefort dome

interactions of north pacific tropical midlatitude and polar disturbances resulting in linked extreme weather events over north america in october 2007 abstractthis study uses observations and model reanalyses to examine the multiscale processes associated with four high impact extreme weather events ewes over north america during late october 2007 the ewes consisted of wind driven wildfires in california prolonged anomalous cold conditions in mexico linked to two cold surges heavy rainfall in the eastern united states and severe flood producing heavy rainfall in southern mexico the ewes involved a pronounced large scale flow reconfiguration across the north pacific and north america in conjunction with the formation of a high amplitude rossby wave train the flow reconfiguration involved perturbations to the north pacific jet stream linked to polar midlatitude and tropical disturbances including three tropopause level polar disturbances originating over northeastern asia transient extratropical cyclones a diabatic rossby vortex and western north pacific tropical cyclone kajiki eulerian and lagrangian diagnostics indicate that ridge amplifi

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

relationship between detrital zircon age spectra and the tectonic evolution of the late archaean witwatersrand basin south africa abstract shrimp u pb detrital zircon age spectra for 14 samples from the west rand and central rand groups of the witwatersrand basin indicate that felsic source rocks mostly formed at 3090 3060 and 3000 2870 ma with a smaller proportion formed prior to 3100 ma provenance evaluation indicates that source rocks have largely vanished through erosion such that greenstone terrains extending from the amalia kraaipan belt in the southwest to the murchison belt in the northeast are models of but not the actual source areas analysis of detrital zircon age modes reveals that the west rand and central rand groups record different basins the west rand group is characterised by age subpopulations with a much smaller range and less complex distribution than those in the central rand group furthermore detrital zircon age spectra for the west rand group generally decrease in complexity up section whereas they become increasingly more complex in the central rand group it is concluded that the source area for the west rand group is consistent with a passive margin setting with the decrease in complexity up section of zircon age spectra being the consequence of increasing maturity of the hinterland conversely the source area of the central rand group was far more dynamic with an increasing age range of source rocks being exposed through time pulsed tectonic rejuvenation of the source area is implied contemporaneous emplacement uplift and erosion of granitoids supports a magmatic fold thrust belt as the source and a complementary retroarc basin setting detrital xenotime grains record the ages of high u granitoids emplaced in the fold thrust belt during deposition of the central rand group and are much closer to depositional ages than are detrital zircon grains change from a passive margin to a retroarc basin across the west rand central rand boundary is emphasised by change from simple to increasingly complex detrital zircon age spectra this study establishes that detrital zircon geochronology can overcome the problem of overly quartzose compositions in archaean sandstones due to severe chemical weathering and authigenic alteration thereby enabling distinction between passive margin and retroarc basin stages through systematic changes in the age complexity of the source area

bedding parallel shear thrusting and quartz vein formation in witwatersrand quartzites abstract evidence is presented which clearly demonstrates the presence of thrust faults in the gold bearing sedimentary rocks of the witwatersrand supergroup a package of these rocks subjected to shear strain allowed overlying beds on the northern margin of the witwatersrand basin to move outwards in a n nw direction movement took place on ductile shear zones which were parallel to the bedding and often reveal excellent examples of ramp and duplex structures it has also been established that syntectonic foliation planes were utilized as planes of shear in certain circumstances quantitative estimates of strain for the pile which are of necessity minimum values give 1 fluids from which vein quartz formed played a critical role in the shear plane movement and may have relieved a portion of the overburden load during deformation

a potential vorticity based study of the role of diabatic heating and friction in a numerically simulated baroclinic cyclone abstract a particularly intense case of western atlantic baroclinic cyclogenesis was investigated in this study specifically the roles of latent heat of condensation and surface friction were examined from the potential vorticity or pv thinking perspective the methodology used for this study involves three key components 1 a full physics mesoscale model which provides a continuous and dynamically consistent dataset and provides full user control over physical processes 2 a partitioned pv integration which temporally integrates the accumulation of pv due to various physical processes in the model s eulerian framework and 3 the piecewise inversion method of davis and emanuel which calculates the balanced wind and mass field associated with particular pv anomalies potential vorticity features obtained through the partitioned integration technique were inverted to yield their direct contributions to the total circulation in addition sensitivity studies were carried out to determine the overal

the elusive feeders of the karoo large igneous province and their structural controls abstract dolerite sills in the central parts of the karoo large igneous province on the kalahari craton in south africa record the emplacement of magic magmas over 350 000 km2 but the feeders to the voluminous magmatism have yet to be identified extensive exploration and mining data across a 7400 km2 area and to a depth of 2 5 km in the northern karoo basin provide unprecedented access to the subsurface geometry of dolerite sills and their relationship to underlying basement strata the data show saucer shaped sills in the karoo supergroup with distinctly funnel shaped geometries along their base that are seated within basement rocks the funnel shaped dolerite structures are interpreted to represent the feeders to the overlying regional scale sill saucer complexes four distinct saucer geometries can be identified 1 cone shaped saucers that extend outwards from a single funnel shaped point type a 2 elongated saucers that dip towards multiple aligned funnel shaped points type b 3 the more commonly described elongated saucers with flat inner sills type c and 4 elongated saucers with a combination of type b and c geometries type d type a and c saucer geometries and point like contacts are consistent with analogue models of saucers for respective pipe like and linear feeders however the multiple pipe like feeders associated with type b and combination thereof with sheet like feeders type d represent saucer feeder relationships thus far not modelled notably the lower dolerite funnels are commonly rooted in older faults and or dykes within the basement rocks this spatial correlation emphasizes the significance of pre existing basement structures in the kalahari craton for the long range transfer of the magmas through the crust the distributed occurrence of numerous magma feeders across much of the craton is more consistent with thermal incubation of the sub lithospheric mantle beneath gondwana than the impact of a mantle plume

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

tectonic and geochronological constraints on late archaean and palaeoproterozoic stratigraphic correlation within and between the kaapvaal and pilbara cratons recent radiometric dating of carbonates and banded iron formations on the kaapvaal craton southern africa has suggested that late archaean and palaeoproterozoic carbonate sedimentation was diachronous across the craton we present new shrimp u pb zircon ages of 2583 5 ma and 2588 7 ma for two samples of a tuff bed 320 km apart in the upper oak tree formation transvaal supergroup these ages in conjunction with published data support previously established sequence stratigraphic correlations that show the temporal equivalence of late archaean carbonates in the region a review of the sequence stratigraphy and geochronology of the mount bruce megasequence set on the pilbara craton western australia and the newly named highveld megasequence set on the kaapvaal craton indicates a remarkably similar geohistory both cratons are characterized by a divergent megacycle hamersley range and vaal megasequences followed by a convergent megacycle chichester range and drakensberg megasequences our data show that the megasequence boundary is marked by drowning of both cratons at approximately the same time c 2590 ma which permits more meaningful geotectonic correlations between them

volcanic rocks of the witwatersrand triad south africa i description classification and geochemical stratigraphy abstract the witwatersrand triad contains thick volcanic sequences confined largely to the dominion group at the base and the ventersdorp supergroup at the top these volcanic sequences are of late archaean to early proterozoic age and are amongst the oldest supracrustal volcanic sequences erupted onto the archaean kaapvaal craton the volcanic rocks have suffered low grade greenschist facies metamorphism but primary textures and in some samples primary mineralogies are well preserved major and trace element analyses of a large number of samples of volcanic rocks from boreholes in the klerksdorp area western transvaal indicate that the chemical compositions of the lavas are largely unmodified by secondary processes and can be used for classification stratigraphic studies and petrogenetic interpretation the volcanics of the dominion group are a bimodal tholeiitic basalt rhyolite association with considerable compositional variation from basalt to tholeiitic andesite within the basaltic suite the ventersdorp supergroup is a weakly developed bimodal tholeiitic basalt dacite suite basic rocks in the klipriviersberg group at the base and in the overlying platberg group are largely basaltic whereas those of the allanridge formation at the top are largely tholeiitic andesites within the klipriviersberg group the more primitive samples occur at the top of the sequence phenocryst rich porphyries of the makwassie formation in the platberg group are largely of dacitic composition and have textural features suggestive of an ash flow origin a distinct geochemical stratigraphy is present within the volcanic sequences and application of discriminant function analysis and an empirical discrimination grid based on ratios of immobile elements p ti and zr allows for unknown samples to be placed within their correct stratigraphic unit

neoarchaean tectonic history of the witwatersrand basin and ventersdorp supergroup new constraints from high resolution 3d seismic reflection data abstract first order scale structures in the west wits line and west rand goldfields of the witwatersrand basin south africa were mapped using the high resolution 3d reflection seismic method structural models constrain the magnitude of displacement of thrusts and faults the gross structural architecture and neoarchaean tectonic evolution of the west rand and bank fault zones which offset the gold bearing reefs of the basin the merging of several 3d seismic surveys made clear the gross strato structural architecture of the goldfields a macroscopic fold thrust belt is crosscut by a macroscopic extensional fault array these are dissected eroded and overlain by the transvaal supergroup above an angular unconformity the seismic sections confirm that the west rand group ca 2985 2902 ma is unconformably overlain by the central rand group ca 2902 2849 ma with tilting of the west rand group syn to post erosion at ca 2 9 ga the seismic sections also confirm that an unconformable relationship exists between the central rand group and the auriferous ventersdorp contact reef vcr with an easterly verging fold thrust belt being initiated concomitant to deposition of the vcr at approximately 2 72 ga fold thrust formation included development of the 1 newly identified first order scale libanon anticline 2 tandeka and jabulani thrusts which displace the west rand group and 3 parasite folds the fold thrust belt is crosscut by a macroscopic extensional fault array or rift like system of faults which incepted towards the end of extrusion of the ventersdorp lavas and certainly during deposition of the platberg group 2709 2643 ma when a mantle plume may have heated the lithosphere the west rand and bank fault zones formed at this time and include 1 the west rand and bank faults which are scissors faults 2 second and third order scale normal faults in the immediate footwall and hanging wall of the faults 3 drag synclines and 4 rollover anticlines

structural analysis of the collar of the vredefort dome south africa significance for impact related deformation and central uplift formation landsat tm aerial photograph image analysis and field mapping of witwatersrand supergroup meta sedimentary strata in the collar of the vredefort dome reveals a highly heterogeneous internal structure involving folds faults fractures and melt breccias that are interpreted as the product of shock deformation and central uplift formation during the 2 02 ga vredefort impact event broadly radially oriented symmetric and asymmetric folds with wavelengths ranging from tens of meters to kilometers and conjugate radial to oblique faults with strike slip displacements of typically tens to hundreds of meters accommodated tangential shortening of the collar of the dome that decreased from 17 at a radius from the dome center of 21 km to 5 at a radius of 29 km ubiquitous shear fractures containing pseudotachylitic breccia particularly in the metapelitic units display local slip senses consistent with either tangential shortening or tangential extension however it is uncertain whether they formed at the same time as the larger faults or earlier during the shock pulse in addition to shatter cones quartzite units show two fracture types a cm spaced rhomboidal to orthogonal type that may be the product of shock induced deformation and later joints accomplishing tangential and radial extension the occurrence of pseudotachylitic breccia within some of these later joints and the presence of radial and tangential dikes of impact melt rock confirm the impact timing of these features and are suggestive of late stage collapse of the central uplift

a review of reflection seismic investigations in three major metallogenic regions the kevitsa ni cu pge district finland witwatersrand goldfields south africa and the bathurst mining camp canada abstract effective exploration for mineral deposits depends on a sound understanding of the processes and geological structures that contributed to their formation the reflection seismic method has proven to be a powerful tool that provides a high resolution image of the subsurface and information about structural and lithological relationships that control mineral deposits the method has also become an attractive geophysical tool for deep exploration and mine planning in this paper we review the use of reflection seismic methods to obtain a better understanding of the architecture and ore forming processes of three diverse mineral regions the kevitsa ni cu pge district in finland the goldfields of the witwatersrand basin south africa and the bathurst mining camp canada seismic data both 2d and 3d from the kevitsa deposit clearly image the 3d geometry of the ore bearing intrusion and provide information about its relationship to the host rock units and nearby intrusions within a larger tectonic framework 3d seismic data from the witwatersrand basin not only provide clear images of major structures including a distinct reflection that acts as a marker horizon for the gold bearing reef but also provide information that may be useful in resolving a long standing controversy regarding the origin of the gold in the basin for example it might be possible to show that dykes formed impermeable barriers thereby falsifying the epigenetic hydrothermal models 2d and 3d seismic data from the brunswick no 6 area in the bathurst mining camp suggest that the brunswick horizon which contains the bulk of the massive sulfide and associated iron deposits occurs within a reflective package that extends down to at least 6 7 km depth

using tropopause maps to diagnose midlatitude weather systems the use of potential vorticity pv allows the efficient description of the dynamics of nearly balanced atmospheric flow phenomena but the distribution of pv must be simply represented for ease in interpretation representations of pv on isentropic or isobaric surfaces can be cumbersome as analyses of several surfaces spanning the troposphere must be constructed to fully apprehend the complete pv distribution following a brief review of the relationship between pv and nearly balanced flows it is demonstrated that the tropospheric pv has a simple distribution and as a consequence an analysis of potential temperature along the dynamic tropopause here defined as a surface of constant pv allows for a simple representation of the upper tropospheric and lower stratospheric pv the construction and interpretation of these tropopause maps which may be termed isertelic analyses of potential temperature are described in addition techniques to construct dynamical representations of the lower tropospheric pv and near surface potential temperature which complement these isertelic analyses are also suggested case studies are presented to illustrate the utility of these techniques in diagnosing phenomena such as cyclogenesis tropopause folds the formation of an upper trough and the effects of latent heat release on the upper and lower troposphere

pre transvaal wrench tectonics along the northern margin of the witwatersrand basin south africa abstract the pre transvaal tectonic history of the northern margin of the witwatersrand basin is re evaluated in terms of wrench fault tectonics three major east northeast trending left lateral wrench fault systems are recognized the sugarbush rietfontein and kromdraai systems they are characterized by pull apart basins e g bezuidenhout valley and kromdraai grabens narrow slivers of displaced stratigraphy e g langermanskop rietfontein block and marginal convex upward thrusts changes in fault orientation result in areas of compression passing laterally into areas of extension over short distances splays terminate individual fault systems regions of wrench fault overlap are characterized by fold domains e g west rand syncline east rand basin with folds characteristically trending nw se the left lateral strike slip system appears to be of craton wide scale and probably extends at least as far as the barberton greenstone belt the wrench system appears to have come into existence during witwatersrand times but experienced its major movement during middle ventersdorp times the left lateral strike slip system is believed to be related to rifting along the ventersdorp trough and to right lateral faulting along the southwestern margin of the witwatersrand basin these tectonics occurred prior to a post transvaal period of folding and thrusting related to the emplacement of the vredefort dome the latter is responsible for the penetrative regional cleavage and bedding sub parallel faults throughout the area concerned the structural model proposed here has implications for the existence of possible outliers of mineralized upper witwatersrand stratigraphy

cassini radar observations of enceladus tethys dione rhea iapetus hyperion and phoebe cassini 2 2 cm radar and radiometric observations of seven of saturn s icy satellites yield properties that apparently are dominated by subsurface volume scattering and are similar to those of the icy galilean satellites average radar albedos decrease in the order enceladus tethys hyperion rhea dione iapetus and phoebe this sequence most likely corresponds to increasing contamination of near surface water ice which is intrinsically very transparent at radio wavelengths plausible candidates for contaminants include ammonia silicates metallic oxides and polar organics ranging from nitriles like hcn to complex tholins there is correlation of our targets radar and optical albedos probably due to variations in the concentration of optically dark contaminants in near surface water ice and the resulting variable attenuation of the high order multiple scattering responsible for high radar albedos our highest radar albedos for enceladus and tethys probably require that at least the uppermost one to several decimeters of the surface be extremely clean water ice regolith that is structurally complex i e mature enough for there to be high order multiple scattering within it at the other extreme phoebe has an asteroidal radar reflectivity that may be due to a combination of single and volume scattering iapetus 2 2 cm radar albedo is dramatically higher on the optically bright trailing side than the optically dark leading side whereas 13 cm results reported by black et al black g j campbell d b carter l m ostro s j 2004 science 304 553 show hardly any hemispheric asymmetry and give a mean radar reflectivity several times lower than the reflectivity measured at 2 2 cm these iapetus results are understandable if ammonia is much less abundant on both sides within the upper one to several decimeters than at greater depths and if the leading side s optically dark contaminant is present to depths of at least one to several decimeters as argued by lanzerotti et al lanzerotti l j brown w l marcantonio k j johnson r e 1984 nature 312 139 140 a combination of ion erosion and micrometeoroid gardening may have depleted ammonia from the surfaces of saturn s icy satellites given the hypersensitivity of water ice s absorption length to ammonia concentration an increase in ammonia with depth could allow efficient 2 2 cm scattering from within the top one to several decimeters while attenuating 13 cm echoes which would require a six fold thicker scattering layer if so we would expect each of the icy satellites average radar albedos to be higher at 2 2 cm than at 13 cm as is the case so far with rhea black g campbell d 2004 bull am astron soc 36 1123 as well as iapetus

archean foreland basin tectonics in the witwatersrand south africa the witwatersrand basin of south africa is the best known of archean sedimentary basins and contains some of the largest gold reserves in the world sediments in the basin include a lower flysch type sequence and an upper molassic facies both of which contain abundant silicic volcanic detritus the strata are thicker and more proximal on the northwestern side of the basin which is at least locally bound by thrust faults these features indicate that the witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an andean type arc remarkably similar phanerozoic basins may be found in the southern andes we suggest that the continental collision between the kaapvaal and zimbabwe cratons at about 2 7 ga caused further subsidence and deposition in the witwatersrand basin regional uplift during this later phase of development placed the basin on the cratonward edge of a collision related plateau now represented by the limpopo province similarities are seen between this phase of witwatersrand basin evolution and that of active basins north of the tibetan plateau e g the tarim and tsaidam basins the geologic evidence does not appear so compatible with earlier suggestions that the witwatersrand strata were deposited in a rift or half graben

the indigenous origin of witwatersrand carbon in the witwatersrand approximately 40 of the gold is intimately associated with so called carbon in carbon seam reefs which occur in over a dozen paleoplacers many of them concentrated at two stratigraphic levels in the 7000 m thick succession of archean siliciclastic sedimentary rocks this is reduced carbon present as kerogen admixed in various proportions with derivative now solid bitumen s oil generation and migration were active geological processes during early earth history numerous possible source rocks for oil generation including the carbon seams themselves occur within the witwatersrand basin in the witwatersrand ore oil bearing fluid inclusions are also present derived like the bitumen by thermal maturation of the kerogen the presence of kerogen and bitumen in the witwatersrand sedimentary rocks together with a wealth of observations on the spatial distribution of the carbon seams confirm that the carbon originated in situ from living organisms in microbial mat cover as opposed to flowing in from elsewhere as liquid hydrocarbons as some researchers have suggested paleochannels which truncated auriferous carbon seams early in the depositional history are of widespread occurrence and micro synsedimentary faults offset carbon seams the carbon seams are thus indigenous biogenic markers that grew contemporaneously with placer development the various features highlighting the nature and spatial distribution of witwatersrand carbon seams provide a classic case where field evidence trumps laboratory data in the reconstruction of geological processes

pseudotachylite associated with a bedding parallel fault zone between the witwatersrand and ventersdorp supergroups south africa a proterozoic bedding parallel fault zone is described from the witwatersrand basin in south africa the fault zone is dominated by pseudotachylite the youngest tectonite present but also contains quartz veins and cataclasites which post date the spatially associated mylonites both tectonic eliminations and duplications are caused by the fault zone which is dominated by a northerly to westerly overthrusting the fault rocks probably did not form during a single event but as a result of a minimum of two periods of activation of the fault zone separated in time by at least the period required to deposit in excess of 4 km of sediments belonging to the lower part of the transvaal sequence

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

radar bright channels on titan abstract during cassini s t44 flyby of titan may 28 2008 the cassini sar synthetic aperture radar revealed sinuous channels in the southwest of xanadu these channels feature very large radar cross sections up to 5 db whereas the angle of incidence was relatively high 20 this backscatter is larger than allowed by the coherent backscatter model considered to explain the unusual reflective and polarization properties of the icy satellites and only a few radar scattering mechanisms can be responsible for such high radar returns the presence of rounded icy pebbles with size larger than the radar wavelength 2 18 cm is proposed to explain the large radar cross sections measured in these units the radar bright channels are thus interpreted as riverbeds where debris likely shaped and transported by fluvial activity have been deposited similar debris were observed in the landing site of the huygens probe this work may point the way to an explanation for the enhanced brightness of other fluvial regions of titan

integration of underground mapping petrology and high resolution microseismicity analysis to characterise weak geotechnical zones in deep south african gold mines abstract a highly stressed shaft pillar is prone to large seismic events falls of ground and rockbursts which may cause injuries and loss of production especially in weak geotechnical zones it is thus important to identify weak geotechnical zones in order to mitigate risks in this study we present integrated studies underground mapping petrology rock mechanics and high resolution microseismicity analysis to understand the different geotechnical zones in the shaft pillar of cooke 4 mine in south africa the footwall of the remnant shaft pillar comprises the upper elsburg reef of the mondeor formation while the ventersdorp contact reef vcr of the venterspost formation and soft weak lavas of the westonaria formation form the hangingwall results from underground mapping and microscopic analysis show that the shaft pillar is composed of quartzites pebbly quartzites argillaceous quartzites and conglomerates underground mapping further shows that the shaft pillar is characterized by several discontinuities which vary from minor to macro scale fractures laboratory uniaxial compressive strength ucs tests indicate that quartzite has the strongest strength followed by pebbly quartzite argillaceous quartzite and lastly conglomerate analysis of high resolution acoustic emissions aes clusters indicates that the majority of aes are associated with the mining stope faces the clusters show the formation of ortlepp shears ahead of the stope which is caused by the excavation induced stress field microseismic data further reveal that the fracture turning point occurs in the soft strata weak hangingwall lavas the integration of these datasets has allowed us to develop the fracture model for different geotechnical zones which concurs with previous models developed for the similar underground environment i e weak soft lava hangingwall and quartzite conglomerate footwall this has major implications for future mining support production and safety

the vredefort structure a perspective with regard to new tectonic data from adjoining terranes abstract the deformational history of the area surrounding the vredefort structure in south africa is long and complex involving several periods of contraction and extension a general chronology of events covering a time span of 1000 ma is established the vredefort deformation is identified as a separate structural event which is distinct from the earlier deformations it is essential therefore that deformational structures related to the pre vredefort event should be identified and subtracted from the total spectrum of tectonism so that only those structures which can be related with certainty to the formation of the vredefort structure are defined this approach could influence interpretations of the origin of the vredefort structure which attribute most structural and geological associations to the deposition of sediments onto a granitoid basement followed by a vredefort deformational event

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

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

precambrian continental freeboard and geological evolution a time perspective abstract continental crustal growth rates crustal volumes thicknesses and continental freeboard are considered to be intimately related however none are easy to quantify least of all in the precambrian additionally crustal volumes thicknesses and isostasy are related to changing mantle heat and the concomitant variation in thickness of ocean crust another set of variables would be the interaction of plate tectonics and the onset of plate tectonics a contentious issue in itself and mantle plumes superplumes further complication is provided by the ranges of freeboard elevations and eustatic sea level changes overlapping in scale and sharing common genetic causes the constant freeboard model suggests that the net results of the interaction of this complex set of variables has been that continental elevation relative to sea level has remained essentially similar on a global average basis over much of geological time assuming an essentially constant ocean volume since c 3 8 ga application of this model to the precambrian geological record suggests that sigmoidal crustal growth rate models are probably the most reliable a conundrum of the constant freeboard model would be that freeboard would tend to become lowered towards mean sea level for any specific craton over time this freeboard equilibrium would be interrupted for a particular craton by active geodynamic processes resulting from the interplay of plate tectonics and mantle thermal instabilities in this paper we test the validity of this thesis of alternating approximate freeboard equilibrium and active geodynamics by examining the volcano sedimentary record of a number of cratons kaapvaal singhbhum and pilbara for the singhbhum craton and bearing in mind its poor chronological data base freeboard equilibrium appears to have prevailed until c 2 1 ga but for kaapvaal and pilbara until c 1 8 ga the geodynamic processes largely outweighed the tendency towards equilibrium except for c 200 my from c 2 6 to 2 4 ga when global sea level appears to have remained high and transgression was the norm we thus tentatively add the plate tectonic cycle and mantle thermal processes to the already complex association of concepts controlling continental freeboard over time

cassini radar sequence planning and instrument performance the cassini radar is a multimode instrument used to map the surface of titan the atmosphere of saturn the saturn ring system and to explore the properties of the icy satellites four different active mode bandwidths and a passive radiometer mode provide a wide range of flexibility in taking measurements the scatterometer mode is used for real aperture imaging of titan high altitude around 20 000 km synthetic aperture imaging of titan and iapetus and long range up to 700 000 km detection of disk integrated albedos for satellites in the saturn system two sar modes are used for high and medium resolution 300 1000 m imaging of titan s surface during close flybys a high bandwidth altimeter mode is used for topographic profiling in selected areas with a range resolution of about 35 m the passive radiometer mode is used to map emission from titan from saturn s atmosphere from the rings and from the icy satellites repeated scans with differing polarizations using both active and passive data provide data that can usefully constrain models of surface composition and structure the radar and radiometer receivers show very good stability and calibration observations have provided an absolute calibration good to about 1 3 db relative uncertainties within a pass and between passes can be even smaller data are currently being processed and delivered to the planetary data system at quarterly intervals one year after being acquired

3d edge detection seismic attributes used to map potential conduits for water and methane in deep gold mines in the witwatersrand basin south africa abstractinrushes of ground water and the ignition of flammable gases pose risks to workers in deep south african gold mines large volumes of water may be stored in solution cavities in dolomitic rocks that overlie the black reef blr formation while there are several possible sources for methane namely coal seams kerogen found in some gold ore bodies or methane introduced by igneous intrusions potential conduits that may transport water and methane to underground workings were mapped using 3d reflection seismic data edge detection attributes successfully identified many faults some with displacements as small as 10 m faults that displace the ventersdorp contact reef vcr and the blr horizons were of special interest as known occurrences of fissure water and methane in underground workings show a good correlation with faults that were imaged on the vcr and blr horizons because there are uncertainties in determining the linkage of faults with aquifers and methane sources it is considered prud

an early proterozoic three stage rift system kaapvaal craton south africa abstract a three stage model that constitutes a fully evolved rift system is proposed for a number of superimposed early proterozoic basins on the kaapvaal craton based on interpreted depositional systems and syndepositional tectonics this model consists of three superimposed basins 1 1 a pre graben stage that influenced the dominion group witwatersrand supergroup and klipriviersberg group of the ventersdorp supergroup 2 2 graben development that superimposed the platberg group bothaville and allanridge formations of the ventersdorp supergroup 3 3 post graben thermal subsidence that superimposed the black reef quartzite formation and the chuniespoort ghaap group of the transvaal supergroup oblique collision of the kaapvaal craton and zimbabwe craton is interpreted to have generated stresses that initiated the three stage system at different time intervals compressional deviatoric stress appears to have replaced tensional stress and kept the system in operation tectonically the system is analogous to the baikal rift system and the pannonian basin system while the general configuration of the superimposed basins is comparable to the north sea basin or the north china basin

the ohio valley wave merger cyclogenesis event of 25 26 january 1978 part i multiscale case study abstract the long standing observational view of cyclogenesis involves an interaction between tropopause and surface based finite amplitude disturbances there are however instances where more than one upper level disturbance contributes to low level development a subset of these events involves wave or trough merger which has been defined as the amalgamation of two or more distinct 500 hpa vorticity maxima an example of this phenomenon involving a case of very intense continental cyclogenesis 25 26 january 1978 over the eastern united states is selected to elucidate the origin and evolution of precursor disturbances from the planetary scale to the mesoscale the analysis of this event reveals that the two well defined tropopause based disturbances that contribute to cyclogenesis have distinctly different origins and are brought together by confluent planetary scale flow one of these disturbances originates over the western north pacific ocean 10 days prior to cyclogenesis and tracks eastward

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

chronological correlations between the pilbara and kaapvaal cratons abstract the early geological development of the pilbara and kaapvaal cratons has many features in common attempts have been made to correlate geologically similar features of the two cratons and it has been postulated that they originated as contiguous components of a single continent vaalbara during this time the early geological histories of the pilbara and kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed these comparisons indicate significant differences in the chronologies of magmatic events within the granite greenstone crusts of the pilbara and kaapvaal cratons in addition igneous correlatives emplaced during ca 2985 and 2782 ma magmatic events on the kaapvaal craton have not been identified on the pilbara craton and a well defined 2760 ma magmatic event manifest as widespread emplacement of granitic rocks into the pilbara granite greenstone basement and eruption of flood basalts of the lower part of the fortescue group is absent from the kaapvaal craton furthermore similarities in first and second order transgression regression cycles within the sedimentary supracrustal sequences may be attributable to global sea level fluctuations and thus may be irrevelant to the question of former contiguity however similarities in some aspects of the geological development of the pilbara and kaapvaal cratons imply that there were periods extending for between 60 and 200 ma of the archaean era during which the style of crust formation intensity of volcanism and subaerial erosion and magnitude of sea level fluctuations may have varied on a global scale such similarities include the overall duration of formation of the granite greenstone crusts from ca 3650 to 3100 ma the onset of craton wide erosion in the interval ca 3125 to 3000 ma the major episodes of flood basaltic volcanism between 2760 and 2680 ma the predominance of chemical carbonate and banded iron formation sedimentation between ca 2630 and 2440 ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 ma

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

diastathermal extensional metamorphism at very low grades and possible high grade analogues abstract a genetic link between some areas of very low grade of metamorphism and an extensional setting is argued a model is presented in which the metamorphism develops in response to enhanced thermal gradients generated by extensional processes and the pressure temperature time paths are anticipated as being anticlockwise in form examples of granulite facies metamorphism attributed by sandiford and powell 1 to extensional settings may represent the lower crustal analogue of the very low grade examples examined here extensional processes may thus provide the mechanisms by which metamorphism in a variety of crustal levels can develop the traditional association of regional metamorphism with convergent processes is no longer tenable and the termdiastathermal metamorphism proposed by robinson 2 is appropriate to demonstrate the fundamental difference between these two types of regional metamorphism

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

a new variable threshold persistent anomaly index northern hemisphere anomalies in the era interim reanalysis abstractpersistent weather regimes characterized by anomalous temperature or precipitation are often associated with persistent anomalies pas in the tropospheric geopotential height field to ide

textural and geochemical discrimination between xenotime of different origin in the archaean witwatersrand basin south africa abstract xenotime ypo4 of detrital diagenetic and hydrothermal origin within siliciclastic rocks of the archaean witwatersrand basin south africa has been identified on the basis of petrography and in situ ion microprobe shrimp age data the chemical composition of xenotime determined by in situ electron microprobe analysis can be correlated with its origin this allows the origin of any xenotime grain to be assessed by a non destructive microanalytical method prior to ion microprobe geochronology the main chemical discriminators are mree hree abundance normalised hree slope and eu anomaly and in some cases u and th contents igneous detrital xenotime 2800 ma is distinguished from diagenetic 2780 ma and hydrothermal

an overview of the geology of the transvaal sequence and bushveld complex south africa the late archaean early proterozoic transvaal sequence is preserved within the transvaal kanye and griqualand west basins with the 2050 ma bushveld complex intrusive into the upper portion of the succession within the transvaal basin both transvaal and bushveld rocks are extensively mineralized the former containing large deposits of iron manganese asbestos andalusite gold fluorine lead zinc and tin ores and the latter some of the world s major occurrences of pge chromium and vanadium ores transvaal sedimentation began with thin predominantly clastic sedimentary rocks black reef vryburg formations which grade up into a thick package of carbonate rocks and bif chuniespoort ghaap taupone groups these lithologies reflect a carbonate bif platform sequence which covered much of the kaapvaal craton in reaction to thermal subsidence above ventersdorp aged rift related fault systems an erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the pretoria postmasburg segwagwa groups within the three basins under largely closed basin conditions an uppermost predominantly volcanic succession rooiberg group loskop formation is restricted to the transvaal basin a common continental rift setting is thought to have controlled pretoria group sedimentation rooiberg volcanism and the intrusion of the mafic rocks of the rustenburg layered suite of the bushveld complex the dipping sheets of the rustenburg magmas cut across the upper pretoria group stratigraphy and lifted up the rooiberg lithologies to form the roof to the complex subsequent granitic rocks of the lebowa and rashoop suites of the bushveld complex intruded both upper rustenburg rocks and the rooiberg felsites

the gyroscopic earth and its role in supercontinent and metallogenic cycles abstract compilation and analysis of the paleomagnetically constrained plate tectonic reconstructions for the last 2 5 ga of earth s history reveal that continental and oceanic hemispheres are principal and stable first order divisions on the earth with cyclical breakup and reassembly of the continental crust into supercontinents although the breakup of the supercontinent rodinia at 0 75 ga marked important reorganization of the plate tectonic pattern the movements of large continental masses both before and after the breakup of rodinia show remarkable synchronism albeit in significantly rearranged combinations the patterns with disassembled continents coincided with periods when substantially more than 50 of the continental masses occurred either in the northern during palaeoproterozoic and cenozoic times or southern during neoproterozoic to middle palaeozoic times hemispheres whereas the reassembled supercontinents were always symmetrically centred near the equator it is proposed that such regularity might be governed by a convection driven move of the continental fragments towards a pole after the breakup of the supercontinent followed by gyroscopic rebalancing or shift of all earth s solid shells e g entire mantle lithosphere towards the equator relative to the more stable oriented magnetic currents in the liquid core this process is somewhat similar to the true polar wander concept but it takes into account the spinning forces of the earth mantle convection is considered as an important force constantly driving the plates in the oceanic hemisphere and keeping the continental hemisphere intact the periods of gyroscopic rebalancing correspond to the reassembly of the supercontinents at 2 7 2 5 ga kenorland 2 0 0 75 ga columbia and its modification into rodinia and 0 32 0 18 ga pangaea the main reassembly mechanism in addition to rifting spreading and collision is large scale strike slip translation of not only relatively small lithotectonic terranes but also of major cratons these cycles govern changes from the dominantly extension to collision and plume related mineral deposit types in the internal orogens in the continental hemisphere whereas subduction related to collision related mineral deposit types remain persistent through the metallogenic cycles at the oceanic continental hemisphere transition zone just migrating oceanward in time

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

patterns of sedimentation in the precambrian abstract the principle of uniformitarianism may be applied to precambrian basin evolution and to the sedimentary record as a whole the major difference in the precambrian eon lay in variability of rates and intensities of processes controlling weathering erosion transport deposition lithification and diagenesis this paper examines precambrian sedimentation patterns within the larger framework of earth evolution pre rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension all reworked by marine current systems and localised turbidites from c 4 to 3 2 ga greenstone belts provided the predominant settings for the thin passive margin carbonates bif stromatolitic evaporites pelites and quartzites and lesser synorogenic turbidites conglomerates and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings common palaeoenvironments were high gradient alluvial fans low sinuosity braided rivers and relatively shallow marine settings subject to wave and tidal action and turbidity currents although continental crustal growth continued largely through greenstone belts until c 2 7 ga the witwatersrand basin c 3 0 2 7 ga kaapvaal craton south africa reflects initial stabilisation of the oldest craton with an epeiric sea accumulating largely fluvial detritus subject to tidal inland and storm wave craton marginal reworking within a retroarc foreland basin setting neoarchaean palaeoproterozoic sedimentation is discussed within a framework of two global superevents at c 2 7 ga and 2 2 1 8 ga each encompassing major changes in earth s evolution related to the supercontinent cycle mantle superplumes peaks in crustal growth rates and significant biochemical changes within the atmosphere hydrosphere system concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed and deposition of iron formation peaked globally at c 2 5 ga the first global glaciation at c 2 4 2 2 ga provides little support for the snowball earth theory but does suggest a negative feedback loop model whereby intraglacial co 2 related warming and synglacial decreases in weathering alternated up to three times models for palaeo atmospheric and oceanic evolution are mutually exclusive but by c 1 8 ga the existence of large landmasses and free oxygen in earth s atmosphere enabled erg development and red bed sedimentation globally the full spectrum of phanerozoic modern sedimentary environments was thus present on earth a third postulated superevent at c 0 8 0 6 ga essentially recreated conditions experienced at c 2 2 1 8 ga with additionally at least three global refrigeration events

the ventersdorp contact reef model in the kloof gold mine as derived from 3d seismics geological mapping and exploration borehole datasets abstract a model of the ventersdorp contact reef vcr orebody at kloof gold mine was derived by integrating 3d reflection seismic data with information derived from underground mine mapping and exploration drilling the study incorporated the depth converted prestack time migrated pstm seismic cube the mine geomodel faults and dikes mapped in excavations mine development infrastructure and intersections of the vcr by surface and underground exploration drilling the 3d seismic data provide an accurate geometric model of the vcr orebody and its offsets the underground mapping datasets help to define minor faults and dikes that are below seismic resolution limits the integration of the these datasets allowed 1 for better mapping of fault architectures and distributions within the lease area 2 definition of the likely zones of difficult ground conditions around seismically imaged dikes and faults and 3 better predictions of the number and spacing of faults that offset the vcr within minable blocks the model is useful in mitigating both economic and safety risks of deep mining

a chronostratigraphic framework for the north central kaapvaal craton the bushveld complex and the vredefort structure abstract sufficient geochronological data are available pertaining to the north central part of the kaapvaal craton to construct a chronostratigraphic framework for the rock units involved in the vredefort event from the basement rocks at 3170 ma to the waterberg soutpansberg groups deposited between 2025 and 1425 ma the resulting framework forms a convenient chronological guide to the geological evolution of this area and the timing of events such as that at vredefort the basement on which the supracrustal rocks were deposited stabilised from 3170 to 3060 ma but metamorphism and metasomatism of these rocks occurred up to 2800 m y ago in different parts of the craton the first unit of the wits triad the volcaniclastic dominion group was deposited at 3060 ma on the stabilising basement this was followed by the dominantly clastic witwatersrand supergroup and the overlying ventersdorp supergroup the dominantly volcanic ventersdorp rocks are 2700 m y old and form the last unit of the wits triad a considerable time gap of 200 300 ma separates the triad from the lowermost units of the overlying transvaal sequence the age of these volcanic and clastic protobasinal rocks is poorly constrained between 2500 and 2300 ma they may in fact represent eroded remnants of a more extensive depositional cycle the mainly clastic sedimentary part of the transvaal sequence pretoria group has been dated at 2224 ma the upper part of the transvaal sequence rooiberg group is only constrained by the age of the intrusive bushveld complex the bushveld basic layered rocks are well dated at 2060 ma as are the subsequent granites at 2050 ma the vredefort event which post dates the bushveld complex is reasonably well constrained by the age of the bronzite granophyre at 2000 ma deposition of the waterberg and soutpansberg groups which post date the bushveld complex is poorly constrained between the age of the bushveld granites and younger alkaline intrusives at 1425 ma but these sediments and volcanics were probably formed between 2025 and 1752 ma the waterberg group may therefore contain debris from the vredefort structure

mafic dikes displacing witwatersrand gold reefs evidence against metamorphic hydrothermal ore formation the process responsible for the gold mineralization in the witwatersrand basin south africa remains controversial the debate is between a detrital placer enrichment in laterally extensive conglomerate layers and hydrothermal introduction by metamorphic fluids long after sedimentation while textural and geochronological data may be ambiguous a first order geological observation severely delimits the mechanism and timing of gold emplacement the individual gold reefs are cut and displaced by numerous mafic dikes which based on geochemical data presented here are feeders to flood basalts immediately overlying the witwatersrand the displacements preclude fracture controlled fluid focusing over tens of kilometers along a single conglomerate horizon during or after regional metamorphism instead basin scale gold enrichment occurred during sedimentation or early diagenesis well before local redistribution by metamorphic fluids

structural controls on witwatersrand gold mineralisation abstract structural controls on the distribution of gold in the witwatersrand basin can be seen at scales ranging from that of thin sections up to regional seismic data sets at the thin section scale gold occurs largely in fractures with hydrocarbons these fractures are associated with pulses of thrusting deformation that occurred in the latter stages of the basin s development the distribution of thrust fracture networks and therefore gold is controlled by the mechanical attributes of the host stratigraphy thrust displacements are generally very low with structures exploiting depositional contacts and mesoscale sedimentary bedforms many of the small faults are isolated from each other with bimodal vergence between larger scale thrusts of the same age early formed faults and fractures become folded and faulted by subsequent propagation of larger scale thrusts these characteristics are consistent with formation of the mineralised thrust fracture networks in the frontal low displacement parts of a thrust system in the carletonville goldfield the prospectivity of the ventersdorp contact reef vcr has been evaluated by using underground observations to pinpoint the structural habitat of gold and therefore characterise the size of prospective zones around structures intersecting the vcr mapped within 3d seismic data in the welkom goldfield the controls on gold distribution at the tshepong mine have been characterised from 3d seismic data drilling and underground observations and used to develop an ore body model that aids reserve estimation and production planning

transition from diagenesis to metamorphism in extensional and collision settings areas in which there is a transition from diagenesis to metamorphism occur in a variety of tectonic settings some of which are of an extensional character rather than the collision setting normally associated with regional metamorphism in collision settings the diagenetic to metamorphic transition is typically followed by a continuum into the highest grades of metamorphism the baric type is usually of an intermediate pressure variety and metamorphism occurs at a late stage in the tectonic cycle in extensional settings such as marginal basins a restricted facies series is developed that ranges from diagenetic levels up to a low greenschist facies maximum the metamorphism is normally of a low pressure type and develops early in the tectonic cycle these characteristics suggest that it is possible to subdivide broadly the field of low grade regional metamorphism into compressional orogenic and extensional settings

flexural partitioning of the late archaean witwatersrand foreland system south africa abstract the witwatersrand basin is part of a retroarc foreland system formed during the late archaean in response to crustal thickening along the northern margin of the kaapvaal craton this foreland system may be partitioned into foredeep forebulge and back bulge flexural provinces the preserved sedimentary fill occupies the region of the flexural foredeep and displays a typical transition from underfilled deep and shallow marine to overfilled fluvial and alluvial fan phases loading in the witwatersrand thrust belt took place in relation to the oblique convergence of two compressional stress fields as a result the foredeep is restricted along the strike to a distance of the witwatersrand foreland system formed on a newly cratonized continental lithosphere which gave the basin a small flexural wavelength i e 490 km from the center of orogenic loading to the axis of the back bulge relative to most phanerozoic counterparts the forebulge unconformity may be associated with the lack of dynamic loading which is probably a common feature for all precambrian retro foreland systems due to the lower rates of subduction associated with the early stage of plate tectonic processes

hydrothermal gold mineralization in the witwatersrand basin the origin of the gold mineralization in the witwaterstrand basin of south africa the largest known gold province has been controversial for decades with arguments favouring detrital1 2 placer modified placer3 4 and hydrothermal5 6 origins here we present the results of an extensive geological study of witwatersrand rocks which show that the gold and associated uranium mineralization is hydrothermal in origin and postdates a regional high temperature alteration event alteration processes identified on a small scale can be mapped out regionally as roughly strata bound zones of acid metasomatism extending far into the basin the fluid flow responsible for this alteration was concentrated in small scale structures localized along lithological boundaries we find that the gold precipitated as a consequence of interactions of the fluid with shale derived hydrocarbons present within the basin

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

composite synoptic scale environments conducive to north american polar subtropical jet superposition events abstracta polar subtropical jet superposition represents a dynamical and thermodynamic environment conducive to the production of high impact weather prior work indicates that the synoptic scale e

deformation of the continental crust the legacy of mike coward this special publication in memory and celebration of the work of professor mike coward is about the deformation of the continental lithosphere the collected papers discuss geometry structural principles processes and problems in a wide range of tectonic settings and thereby reflect the breadth of coward s interests they encompass the evolution of precambrian basement gneiss terrains the geometry and evolution of thrust systems basement involvement and structural inheritance in basins syn orogenic extension salt tectonics the implication of structural evolution on hydrocarbon prospectivity and structural controls on mineralization examples are drawn from the lewisian and moine thrust belt of nw scotland the italian apennines nw himalayas the cyclades oman zagros mountains colombian cordillera carpathians north sea offshore brazil regional studies of the irumide belt central africa taurus mountains turkey greater south america and from the witwatersrand basin of south africa and the antler orogeny of sw usa

the extratropical tropopause inversion layer global observations with gps data and a radiative forcing mechanism global characteristics of the extratropical tropopause inversion layer identified in radiosonde observations by birner are studied using high vertical resolution temperature profiles from gps radio occultation measurements the gps data are organized according to the height of the thermal tropopause in each profile and a temperature inversion layer above the tropopause with an average magnitude of 3 5 k is found to be a ubiquitous climatological feature the gps data show that the inversion layer is present for all seasons in both hemispheres spanning the subtropics to the pole and there is not strong longitudinal structure dependence of the inversion layer on upper troposphere vorticity is studied while anticyclones exhibit a substantially stronger inversion than cyclones as expected from balanced dynamics the inversion is evident for all circulation types radiative transfer calculations indicate that strong gradients in both ozone and water vapor near the tropopause contribute to the inversion significant absorption of both longwave and shortwave radiation by ozone occurs warming the region above the tropopause water vapor near and immediately above the tropopause contributes to cooling effectively enhancing the inversion

geochronology of a late archaean flood basalt province in the pilbara craton australia constraints on basin evolution volcanic and sedimentary accumulation and continental drift rates abstract eleven high precision 2 5 million years shrimp zircon u pb ages have been obtained from felsic rocks within a single stratigraphic section of late archaean volcanic and sedimentary rocks in the east pilbara of western australia the stratigraphic succession nullagine and mount jope supersequences in sequence stratigraphic terminology fortescue group in lithostratigraphic terminology is interpreted to be the rock record of three major geotectonic cycles that formed in an extensional rift related environment between about 2772 and 2715 ma the geochronology is constrained by a detailed stratigraphic framework based on unconformities and supported by a preliminary magnetostratigraphy field mapping geochemical and petrographic studies have shown that previously unrecognised thin felsic tuff bands are interbedded in subaerial flood basalt piles and mafic tuffs while flood basalts and proximal felsic volcanic rocks comprise by volume most of the volcanogenic components of the succession felsic volcanism is now known to have been active periodically through each geotectonic cycle the succession covers a time period of about 57 million years the lower 1400 m of a thick 1700 m clastic sedimentary succession from the oldest geotectonic cycle was deposited at a rate of about 100 m per million years over a mean time period of 14 million years in contrast a younger 150 m thick cogenetic tuff basalt unit accumulated in less than 3 million years and others probably accumulated at similar rates comparable to those of phanerozoic flood basalts unconformities in the succession are shown to be of variable duration and one unconformity marking the boundary between the first and second geotectonic cycles may represent a time gap of more than 10 million years the unconformity based stratigraphic framework the new geochronology and palaeomagnetic studies j geophys res 108 2003 b12 2551 emp 2 1 to 2 21 have been combined to determine a possible late archaean continental drift rate for one part of the succession implying a period of motion as fast as or up to five times faster than any known from the phanerozoic

titan s diverse landscapes as evidenced by cassini radar s third and fourth looks at titan abstract cassini s third and fourth radar flybys t7 and t8 covered diverse terrains in the high southern and equatorial latitudes respectively the t7 synthetic aperture radar sar swath is somewhat more straightforward to understand in terms of a progressive poleward descent from a high dissected and partly hilly terrain down to a low flat plain with embayments and deposits suggestive of the past or even current presence of hydrocarbon liquids the t8 swath is dominated by dunes likely made of organic solids but also contain somewhat enigmatic probably tectonic features that may be partly buried or degraded by erosion or relaxation in a thin crust the dark areas in t7 show no dune morphology unlike the dark areas in t8 but are radiometrically warm like the dunes the huygens landing site lies on the edge of the t8 swath correlation of the radar and huygens disr images allows accurate determination of its coordinates and indicates that to the north of the landing site sit two large longitudinal dunes indeed had the huygens probe trajectory been just 10 km north of where it actually was images of large sand dunes would have been returned in place of the fluvially dissected terrain actually seen illustrating the strong diversity of titan s landscapes even at local scales

the presidents day cyclone of 18 19 february 1979 influence of upstream trough amplification and associated tropopause folding on rapid cyclogenesis abstract a diagnostic analysis of an amplifying polar jet trough system and associated tropopause fold which preceded the 19 february 1979 presidents day cyclone is presented the analysis is based on conventional radiosonde data infrared and visible satellite imagery 6 7 m water vapor measurements from the temperature humidity infrared radiometer thir and ozone measurements from the total ozone mapping spectrometer toms the case study indicates that dynamically forced meso scale vertical circulations played an important role in the extrusion of stratospheric air along the axis of a polar jet and the subsequent development of the storm system specific findings include 1 tropopause folding accompanying an amplifying polar jet trough system occurred along the axis of the intensifying polar jet in response to subsidence forced by geostrophic deformation patterns associated with the jet streak 2 the folding process extruded dry stratosphere air marked by high values of potential vorticity dow

determining titan surface topography from cassini sar data a technique referred to as sartopo has been developed for obtaining surface height estimates with 10 km horizontal resolution and 75 m vertical resolution of the surface of titan along each cassini synthetic aperture radar sar swath we describe the technique and present maps of the co located data sets a global map and regional maps of xanadu and the northern hemisphere hydrocarbon lakes district are included in the results a strength of the technique is that it provides topographic information co located with sar imagery having a topographic context vastly improves the interpretability of the sar imagery and is essential for understanding titan sartopo is capable of estimating surface heights for most of the sar imaged surface of titan currently nearly 30 of the surface is within 100 km of a sartopo height profile other competing techniques provide orders of magnitude less coverage we validate the sartopo technique through comparison with known geomorphological features such as mountain ranges and craters and by comparison with co located nadir altimetry including a 3000 km strip that had been observed by sar a month earlier in this area the sartopo and nadir altimetry data sets are co located tightly within 5 10 km for one 500 km section have similar resolution and as expected agree closely in surface height furthermore the region contains prominent high spatial resolution topography so it provides an excellent test of the resolution and precision of both techniques

observed cyclone anticyclone tropopause vortex asymmetries relatively little is known about coherent vortices near the extratropical tropopause even with regard to basic facts about their frequency of occurrence longevity and structure this study addresses these issues through an objective census of observed tropopause vortices the authors test a hypothesis regarding vortex merger asymmetry where cyclone pairs are repelled and anticyclone pairs are attracted by divergent flow due to frontogenesis emphasis is placed on arctic vortices where jet stream influences are weaker in order to facilitate comparisons with earlier idealized numerical simulations results show that arctic cyclones are more numerous persistent and stronger than arctic anticyclones an average of 15 cyclonic vortices and 11 anticyclonic vortices are observed per month with maximum frequency of occurrence for cyclones anticyclones during winter summer there are are about 47 more cyclones than anticyclones that survive at least 4 days and for longer lifetimes 1 day survival probabilities are nearly constant at 65 for cyclones and 55 for anticyclones mean tropopause potential temperature amplitude is 13 k for cyclones and 11 k for anticyclones with cyclones exhibiting a greater tail toward larger values an analysis of close proximity vortex pairs reveals divergence between cyclones and convergence between anticyclones this result agrees qualitatively with previous idealized numerical simulations although it is unclear to what extent the divergent circulations regulate vortex asymmetries