laser step heating 40ar39ar age spectra from early archean 3 5 ga barberton greenstone belt sediments a technique for detecting cryptic tectono thermal events abstract samples from sediments of the fig tree group located in the central part of the circa 3 2 to 3 5 ga barberton greenstone belt bgb have been analyzed by the 40 ar 39 ar laser step heating technique this technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include reworked chemical sediments barite clastic sediments sandstone shale and one stromatolite in most cases the apparent age plateaux can be identified by corresponding ca k and cl k ratio plots as belonging to separate mineral phases most of the samples exhibit simple ar diffusion loss during the low temperature part of the experiments while occasionally showing excess ar or possible 39ar recoil effects the various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 ma one barite sample gave t0 original blocking age 2673 3 ma 1 which is close to a calculated model ar diffusion loss age for the same sample of 2688 3 ma 1 this age is interpreted as representing final granitoid activity adjacent to the bgb and or craton scale tectonism associated with the limpopo orogeny two samples gave t0 ages of 2350 2400 ma which may reflect increased thermal gradients associated with the formation of the thick transvaal sedimentary basin that may once have covered the bgb the dominant apparent age plateaux together with modelled diffusive ar loss ages of 2025 2090 ma are thought to represent regional thermal anomalies related to large scale tectono thermal activity in the kaapvaal craton of which the bushveld complex which covers a surface area of 60 000 km2 and formation of the vredefort structure are obvious manifestations the strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere i e 40 ar 36 ar ratios

long period lunar tides at the south pole vertical gravity data from 6 3 years of observations at the geographic south pole have been analyzed to determine the gravimetric factor and phase angle of the long period tides annual and semiannual tides are obscured by the combination of signals arising from instrumental drift and the large annual component of barometric pressure changes at pole gravimetric factors for the three major long period tides are m 1 1589 0 0017 mm 1 1406 0 0085 and mtm 1 1692 0 0068 the only statistically significant phase shift is for the fortnightly component 0 0151 0 0032 days a phase lead our data analysis for the gravitational factor for m at the south pole indicates that the calculation of the influence of the long period ocean tides on the earth tides at the south pole should be reconsidered

thermochronometry and palaeomagnetism of the archaean nelshoogte pluton south africa summary 207pb 206pb single grain zircon 40ar 39ar single grain hornblende and biotite and 40ar 39ar bulk sample muscovite and biotite ages from the nelshoogte trondhjemite pluton located in eastern transvaal south africa show that this granitoid had a protracted thermal history spanning 3213 4 ma to about 3000 ma whole rock 40ar 39ar ages from cross cutting dolerite dykes indicate that these were intruded at about 1900 ma there is no evidence of this or other later events significantly avecting the argon systematics of the minerals from the pluton dated by the 40ar 39ar method the pluton has a well defined palaeomagnetic pole which is dated at 3179 18 2s ma by 40ar 39ar dating of hornblende this pole 18 n 310 e a 95 9 yields a palaeolatitude of 0 significantly diverent from other archaean poles from the kaapvaal craton the palaeolatitude diverence implies that there was significant apparent polar wander during the archaean a second overprinting magnetization seen in the pluton is also seen in the lower proterozoic dolerite dykes and is consistent with other lower proterozoic 2150 1950 ma poles for southern africa

sm nd age of kambalda and kanowna greenstones and heterogeneity in the archaean mantle a precise sm nd age of 2 790 30 myr and an initial ratio corresponding to nd 3 4 0 6 has been determined for the kambalda sequence from the eastern yilgarn block of western australia the high initial 143nd 144nd implies a long lived light rare earth element depleted high sm nd source for the kambalda volcanics and therefore provides definitive evidence for ancient heterogeneities in the archaean mantle

ttgs in the making natural evidence from inyoni shear zone barberton south africa despite the consensus that ttgs the main constituents of the archaean continental crust worldwide originated by partial melting of garnet bearing amphibolites natural evidence is scarce a large variety of archaean amphibole rich rocks including migmatitic amphibolites and hornblende rich cumulates was exhumed as a tectonic melange in the inyoni shear zone isz near barberton south africa likely at the time of emplacement of the 3 2 ga nelshoogte badplaas plutons this unique collection provides the opportunity to investigate partial melting of garnet amphibolites as well as the differentiation processes occurring in the ttg magmas en route to the surface the isz migmatitic amphibolites are characterized by quartz plagioclase leucosomes in equilibrium with garnet amphibole titanite epidote garnet compositions are characterized by high almandine and grossular contents actually the leucosomes and neosomes likely lost part of their melt component and mesosomes may have been also modified towards restite rich compositions due to melt segregation restite bearing rocks are very iron rich according to their high contents in garnet besides the mg rich compositions of the hornblende cumulates and their ree distribution patterns resemble those of the nearby tt tonalite trondhjemite plutons thermobarometric estimates from the isz migmatitic rocks cluster in the range 720 800 c and 1 1 1 2 gpa for the melting reaction and the derived geothermal gradient is ca 17 22 c km these conditions are consistent with either water present melting or more likely water absent epidote dehydration melting such a geothermal gradient is therefore assigned to the genesis of large volumes of medium pressure ttg magmas that will likely form at greater depth unmelted garnet bearing amphibolites correspond to slightly lower conditions the magmatic cumulates crystallized at ca 0 6 gpa the cumulate rocks evidence that the ttg parental magmas en route to the surface experienced fractional crystallization of a large amount of amphibole before crystallizing as tonalite trondhjemite plutons at slightly shallower depths ca 0 5 gpa

observation and analysis of the tidal gravity variations at asuka station located on the antarctic ice sheet tidal gravity observations were made at asuka station 71 5 s 24 1 e antarctica the lacoste romberg model g 805 gravity meter was altered to integrate an electrostatic feedback amplifier and was installed in a snow pit records for tidal analysis were obtained from june 26 to november 13 1987 at a sampling interval of 30 min from the estimated tidal amplitudes corrections for the oceanic effects were made using schwiderski s cotidal maps the ocean tide corrected factors which define the response of the deformable earth to lunar solar tidal forces were found to be 1 169 0 008 o1 1 127 0 017 p1 1 135 0 006 k1 1 261 0 033 m2 and 1 302 0 039 s2 the diurnal factors agree with those obtained from statistically derived formulas by melchior and de becker however the semidiurnal factors m2 and s2 are 10 larger than the theoretical values predicted by wahr this is much larger than the typical 1 2 discrepancy between the statistical model and theory similar tidal gravity observations made at syowa station 69 0 s 39 6 yielded consistent results with those monitored at asuka station the observed m2 factors have a tendency to increase from the equator toward the southern high latitudes the increasing effect of inaccurate ocean load tidal corrections may be due to schwiderski s cotidal chart tidal analysis resulted in a gravity admittance of 0 24 0 04 gal mbar for the air pressure change at asuka station the smaller magnitude compared with the typical value of 0 35 gal mbar for a normal crust may be explained by the loading deformation of the ice sheet under the air mass covering an extent of 30 km over the site

structural setting and geochemistry of archaean i type granites in the bardoc coolgardie area of the norseman wiluna belt western australia abstract regional structures in the bardoc coolgardie area of the archaean norseman wiluna greenstone belt in western australia developed during early large scale thrusting followed by a transpressional regime characterized by enewsw shortening and large scale sinistral strike slip faults structural relationships between granites and greenstone successions indicate emplacement of granitic rocks throughout this deformation history emplacement mechanisms include solid or liquid state diapirism forceful dilation of country rocks and passive stoping i type granites are common and include early and late tectonic intrusions the widespread occurrence of i type granodiorite and monzogranite probably derived by partial melting of lower continental crust or subducted oceanic crust suggests a continental margin setting for the belt alkalic quartz diorite occurs as rounded enclaves in granodiorite and is enriched in ba rb sr ce la p and f which suggests a mantle derived volatile component these quartz diorite melts may have formed by partial melting of metasomatized mantle or alternatively mantle degassing may have lowered solidus temperatures in the source region and thus induced melting emplacement of hot volatilerich quartz diorite melts into the lower crust may have lowered solidus temperatures in the source rocks for granodiorite and monzogranite recent models for archaean gold mineralization suggest that auriferous fluids contained co 2 derived from lower crustal or mantle sources in the bardoc coolgardie area co 2 may have been flushed out during partial melting of the lower continental crust however the origin of the gold in the mineralizing fluids requires some other explanation because gold mineralization is generally a late stage event and took place over a considerably shorter time interval than the formation and emplacement of i type granites

gully and tunnel erosion in the hilly loess plateau region china article i nfo article history although gully development is frequently observed to be associated with piping and tunnel erosion in many parts of the world there is a lack of detailed studies on their spatial interactions such studies not only con tribute to developing geomorphic theories on the evolution of both hillslopes and drainage networks but also are useful to design effective control measures of erosion the present study aims to examine the interactions between gully development and tunnel erosion in the hilly loess plateau region of northern china firstly gullies tunnel inlets and mass movements in the 9 1 km 2 wangjiagou watershed were mapped through ae rial photo interpretation and reconnaissance surveys secondly detailed field studies were carried out to in vestigate the interactions between gully development and tunnel formation processes in the selected gullies thirdly the physiographic variables of gullies derived from gis and field measurements were analyzed to identify differences between tunneled and untunneled gullies a total of 704 channels 967 tunnel inlets and 547 mass movements were identified in the study watershed on the basis of their location and morphology all the channels were classified into four types headwater gullies hillside gullies valleyside gullies and ephemeral river channels tunnels are associated with 79 of headwater gullies 48 of hillside gullies 3 of valleyside gullies and none of ephemeral river channels mass movements are dominated by falls in headwater gullies falls and slides in hillside gullies and soil creeps in ephemeral stream channels statistical tests indicate that there are significant differences in phys iographic variables between tunneled and untunneled gullies tunnel formation in gullies is intricately affect ed by topographic conditions land uses knickpoint distribution soil materials and mass movements published by elsevier b v

landform oriented flow routing algorithm for the dual structure loess terrain based on digital elevation models the loess landform in the loess plateau of china is with typical dual structure namely the upper smooth positive terrain and the lower cliffy negative terrain p n terrain for short obvious differences in their morphological feature geomorphological mechanism and hydrological process could be found in the both areas based on the differences a flow routing algorithm that separately addresses the dual structure terrain would be necessary to encompass this spatial variation in their hydrological behaviour this paper proposes a mixed flow routing algorithm to address aforementioned problems first the loess landform surface is divided into p n terrains based on digital elevation models then specific catchment area is calculated with the new algorithm to simulate the water flows in both positive and negative terrain areas the mixed algorithm consists of the multiple flow routing algorithm multiple flow direction for positive areas and the d8 algorithm for negative areas respectively the approach is validated in two typical geomorphologic areas with low hills and dense gullies in the northern shaanxi loess plateau four indices are used to examine the results which show that the new algorithm is more suitable for loess terrain in simulating the spatial distribution of water accumulation as well as in modeling the flow characteristics of the true surface by considering the morphological structures of the terrain copyright 2013 john wiley sons ltd

granitoid series in terms of magnetic susceptibility a case study from the barberton region south africa abstract magnetic susceptibilities were measured on a representative collection of archaean granitoids of the barberton region using a portable kt5 magnetic susceptibility meter the studied granitoids comprise 1 syn tectonic tonalite trondhjemite granodiorite ttg granitoids 132 samples 2 late tectonic calc alkaline granitoids 402 samples and 3 post tectonic low ca and high ca granitoids 12 samples most of the early stage syn tectonic granitoids 3450 ma have low magnetic susceptibilities less than 3 10 3 si units and correspond to ilmenite series granitoids the late stage kaap valley tonalite pluton 3230 ma contains sporadically distributed higher magnetic susceptibility values greater than 3 10 3 si units which are less than one third in magnetic susceptibility of typical magnetite series ttg of the japanese island arc and thus strictly belong to an intermediate series the barberton ttg suite is essentially derived from reduced amphibolitic lower crust that reflects the anoxic nature of the earth surface during the archaean eon the more oxidized nature of the kaap valley tonalite may be generated in an oxidized lower crust by fluids squeezed out of the subducting plate late tectonic granodiorite adamellite batholithic complexes 3105 ma belong mostly to the magnetite series and seem to suggest that relatively oxidized continental crust reflecting oxic atmosphere and subduction mechanism operating had evolved it by this time post tectonic granitic plutons formed largely between circa 2900 ma and 2700 ma can be subdivided into low ca ilmenite series and high ca magnetite series

long period lunar earth tides at the geographic south pole and recent models of ocean tides for many years the ocean tide models of schwiderski 1980 were the standards used by the earth tide community to interpret deviations of observed earth tides from predictions on the basis of earth models constructed by seismologists recently the topex poseidon altimeter mission provided new and improved information on pelagic ocean tides which led several research groups to generate new models of the major oceanic tides this in turn renewed our own interest in the observations of long period lunar tides at the geographic south pole that were reported many years ago with an attempt to interpret the deviation from predictions using the schwiderski models we used four different models of the fortnightly mf and monthly mm ocean tides to calculate their attraction and loading effects at the south pole and compared the results with the observed gravity tides in our earlier interpretation we did not realize that for long period ocean tides the so called greenwich phase does not refer to the phase of these tides at the latitude of greenwich but to the phase at the equator this resulted in a mistake in the relative phases of earth tide and ocean effect at the south pole for mf we now find that all models predict the phase lead of the observed versus theoretical tides within the formal uncertainties however the amplitude is still underpredicted by 1 5 2 per cent this could be due to several reasons instrument calibration errors in the body and or ocean tide models relaxation of the earth s elastic properties and the huge ice sheet of antarctica these possibilities are discussed for the mm tide the observed amplitude is well predicted within error however the uncertainties in the measurements are rather large

modelling gully erosion for a small catchment on the chinese loess plateau the rolling hills region of the chinese loess plateau is one of the areas with the highest erosion rates on earth a striking feature of this area is the occurrence of many large permanent gullies a 3 5 km2 catchment was selected to study the processes of erosion and to adapt the storm based limburg soil erosion model lisem to the conditions prevailing on the loess plateau part of this work consisted of mapping and measuring the largest gully headcuts the amount of loose soil material beneath the headcuts was also estimated observations suggest that gully headcuts are relatively stable i e do not migrate rapidly but that gullies can nevertheless produce significant amounts of sediment during overland flow events erosion of headcuts occurs mainly by soil falls in between storms the loose soil material produced by these soil falls accumulates on the gully bottom as the lisem simulates storm erosion the development of gullies over time can be ignored and only the amount of material produced by them during runoff events needs to be studied a digital elevation model dem was used to estimate the position of existing gully heads by applying an adapted form of the montgomery and dietrich science 255 1992 826 index using the assumption that headcuts are vertical it is possible to calculate headcut height from the slope angle map a simple stability model which assumes soil falls on gully headcuts to be a function of soil moisture content and headcut height was applied this daily based model can then be used to simulate the accumulation of loose soil material below the headcut the results show that while the dem is not accurate enough to allow the detection of individual headcuts this method can be used to produce a reasonable estimate of the amount of loose soil material available a map showing the amount of loose soil material accumulated can then serve as input for a storm based erosion model such as lisem

modelling the runoff sediment yield relationship using a proportional function in hilly areas of the loess plateau north china based on data observed at the 12 small watersheds in hilly areas of the loess plateau north china the relationship between event runoff volume and sediment yield is examined the results reveal that the runoff sediment yield relationship at the inter event timescale is mainly determined by the runoff sediment concentration relationship at the intra event timescale in the study area the sediment concentration tends to be stable when the flow discharge exceeds a certain critical value many factors that are important for determining the characteristics of low magnitude events such as flow discharge particle size of fluvial sediment and accumulation of loose material on land surfaces prior to a rainstorm appear to have little importance for high magnitude events consequently mean sediment concentration tends to be stable for large flood events suggesting a strong similarity between the two flow sediment relationships at inter and intra event temporal scales furthermore a proportional function is proposed to predict event sediment yield and the correspondence between the predicted and observed sediment yields is examined the performance of the model is good for high magnitude events especially extreme events the applicability of the proposed model at the annual timescale is also discussed

timing of deposition and deformation of the moodies group barberton greenstone belt south africa very high resolution of archaean surface processes abstract thin and largely discontinuous volcanic tuff beds in the alluvial fluvial and marginal marine siliciclastic strata of the middle archaean moodies group barberton greenstone belt bgb contain rare to abundant zircon dating of magmatic zircon grains from five tuff beds and one cross cutting dike yield ages which span the entire spectrum of bgb history from ca 3550 ma to 3215 ma and show variable degrees of post depositional alteration until ca 2500 ma the youngest pristine zircon grains in all tuff samples analyzed by ca id tims chemical abrasion ion dissolution thermal ion mass spectrometry yield upper intercept and concordia ages between 3227 6 0 3 and 3216 0 3 ma and those grains analyzed by la sf icp ms laser ablation sector field inductively coupled plasma mass spectrometry and shrimp sensitive high resolution ion microprobe yield ages between 3228 9 and 3214 9 ma most of these ages are either identical within error or significantly older than a ca id tims age of 3223 4 0 3 ma obtained from zircon grains of a tuff horizon of the schoongezicht formation at the top of the underlying fig tree group we interpret this age to mark the time of volcanic eruption whereas the significantly older and overlapping ages of the stratigraphically higher moodies tuff zircon grains point either to a high degree of magmatic inheritance and or a detrital contribution from surrounding sedimentary rocks during or after ash tuff deposition the minimum age for deposition of moodies group sediments and deformation is provided by a la icp ms zircon u pb age of 3219 9 ma obtained from an undeformed felsic dike which crosscuts folded moodies strata this age overlaps with a precise ca id tims u pb zircon age of 3216 0 0 3 ma obtained from a tuff horizon of the upper moodies group overall the new age data collectively imply that the up to 3 7 km thick sandstone dominated moodies group was deposited and deformed within

origin of granitic magma by crustal remobilisation rb sr and pb pb geochronology and isotope geochemistry of the late archaean qorqut granite complex of southern west greenland abstract rb sr and pb pb whole rock isochrons on the qorqut granite complex yield ages of 2530 30 myr initial 87 sr 86 sr 0 7081 0 0008 and 2580 80 myr respectively a model relating initial sr and pb isotopic compositions of the qorqut granites to the sr and pb isotopic compositions of the amitsoq gneisses ca 3700 myr and nuk gneisses ca 2900 myr at 2550 myr ago as well as sr and pb contents of the gneiss units suggests that between 40 and 50 of the qorqut granite magma was generated by partial melting of amitsoq gneisses and the remainder by partial melting of nuk gneisses

volcanic rocks of the witwatersrand triad south africa ii petrogenesis of mafic and felsic rocks of the dominion group abstract a bimodal suite of volcanic rocks builds the bulk of the dominion group which with an age of 2 72 ga is the oldest cover sequence overlying the granite greenstone archaean basement of the kaapvaal craton in the western transvaal south africa the basic lavas are relatively rich in sio 2 50 58 and aphyric and exhibit a large compositional range this variation is typically tholeiitic in that it is characterized by strong enrichment of ti fe and v in differentiated lavas the different amounts of relative enrichment of these elements allow the basic lavas to be subdivided into two groups one group is characterized by evolution to low ti v ratios in evolved lavas and the other by high ti v ratios for nearly all other elements the composition variation of the two groups is extremely similar and overlaps although the high ti v group contains the most evolved compositions and most of the primitive lavas mg numbers 60 belong to the low ti v group although recognition of the two groups is based on geochemistry they are interlayered in a consistent pattern within the lava pile theoretical modelling indicates that the variation in both lava groups can be explained by low pressure fractional crystallization of phases in a typical tholeiitic sequence primitive basalts of the low ti v group show a large range in major element compositions and a more restricted range of incompatible trace elements and ree abundances in particular fe 2 o 3 total ranges from 9 to 13 5 and correlates positively with mgo cr and ni and negatively with al 2 o 5 and sio 2 there are no correlations of mg number with incompatible trace elements such as zr and ce these lavas also have lree enriched ree abundance patterns and negative nd these features cannot be explained by crustal contamination but are consistent with derivation by uniform amounts of melting of a mantle source which was heterogeneous with regard to major elements particularly fe al and probably ca and na and uniformly enriched in incompatible trace elements at least 0 3 ga before melting the geochemical style of this enrichment is akin to that associated with modern subduction this enrichment is associated with the development of a thick archaean lithosphere the existence of which is corroborated by diamond age and diamond inclusion thermobarometric studies the geochemistry of the dominion basic lavas differs significantly from those of the 3 4 ga barberton komatiitic and tholeiitic suites this suggests that the changing pattern of crustal evolution from metastable greenstone tectonic styles to stable proterozoic styles is reflected in the changing character of the mantle sources tapped by basic magmas the porphyritic dominion rhyodacites or rhyolites exhibit limited compositional variation and are genetically unrelated to the basic lavas nd isotopes and la th and th co ratios suggest that they are partial melts of mafic crustal materials geochemical and petrological features of the dominion suite are entirely consistent with eruption in a continental environment characterized by rifting or lithospheric thinning this is consistent with some models for the tectonic development of the witwatersrand basin

archean crustal evolution in china the taishan complex and evidence for juvenile crustal addition from long term depleted mantle abstract the main framework for crustal evolution of the late archean taishan complex in the shandong province of china is constructed using data from field investigation petrographic examination geochemical analysis and isotopic age determination the history of the complex began with emplacement of basic and tonalitic magmas about 2 7 2 75 ga ago which were metamorphosed designated as metamorphism i to amphibolites and grey gneisses shortly after their emplacement these rocks constitute a metamorphic basement termed the wangfushan gneisses the basement gneisses were later intruded by a plutonic association including 1 alkali and light rare earth element lree enriched dioritic rocks and their differentiates 2 6 ga ago 2 granites and trondhjemites collectively termed the hushan granites also 2 6 ga ago and 3 the aolaishan type granites sensu stricto 2 4 2 5 ga ago the plutonic association in conjunction with the older basement gneisses was later subjected to a second period of deformation and metamorphism metamorphism ii 2 ga ago as inferred from published k ar biotite dates the chronological order of all the above events has been determined by a multichronometric approach and the age results in ma with 2 uncertainties are summarized as follows the relatively low i sr initial 87 sr 86 sr ratio and high positive nd t values for the wangfushan gneisses and the diorite group provides strong evidence for continental growth by addition of juvenile material ultimately separated from long term depleted mantle sources by contrast the sr and nd isotopic data for the aolaishan type granites coherently suggest that they are likely to be derived by anatexis of the basement gneisses at shallow depths crustal evolution of the taishan complex offers an additional example supporting the concept of a continental accretion differentiation superevent that during such a worldwide tectonothermal episode particularly in the archean important differentiation of the upper mantle has contributed to the growth of the continental crust by addition of juvenile material

petrogenetic modelling of in situ fractional crystallization in the zoned loch doon pluton scotland the late caledonian loch doon granitic intrusion ranges in composition from hypersthene diorite at the margin through quartz diorite granodiorite and granite to cordierite microgranite at its core petrogenetic modelling of trace element variations and least squares analysis of major elements indicate that two distinct magmas are involved each magma controlled by crystallization of plag opx cpx bio late stage rocks related to the second magma include the cordierite microgranites and aplites which are interpreted as the final residue which crystallized rapidly after a build up and loss of volatiles analyses of whole rocks and minerals for ree s and other elements of moderate high ionic potential indicate that these elements are strongly controlled by minor phase crystallization apatite zircon sphene and allanite are dominant at intermediate compositions but other accessory minerals such as monazite and xenotime may also become important at acid compositions it is probable that within each magma the mechanisms of crystal settling and filter pressing operated the former being initially dominant and the latter becoming more important with increasing degree of fractional crystallization

observations of load tides at the south pole abstract the use of tidal observations to study the nearly diurnal free wobble mode of the earth is possible if oceanic effects in the tidal record are accurately removed we have analyzed vertical gravity data from the south pole to determine the amplitude and phase of the small daily and semidaily tides observed at the pole since these signals at the pole are most probably caused by oceanic tides our observations provide an excellent target for the oceanic models a comparison with the best models of the oceans now available indicates the need for improvement in measuring and modeling the southern oceans

an archean geomagnetic reversal in the kaap valley pluton south africa the kaap valley pluton in south africa is a tonalite intrusion associated with the archean barberton greenstone belt antipodal paleomagnetic directions determined from the central and marginal parts of the pluton record a geomagnetic reversal that occurred as the pluton cooled the age of the reversal is constrained by an 40 ar 39 ar plateau age from hornblende at 3214 4 million years making it the oldest known reversal the data presented here suggest that earth has had a reversing perhaps dipolar magnetic field since at least 3 2 billion years ago

oxygen and carbon isotope geochemistry of the 3 2 ga kaap valley tonalite barberton greenstone belt south africa abstract the 3 2 ga kaap valley tonalite kvt is a homogeneous hornblende biotite tonalite which is surrounded by the barberton greenstone belt the original assemblage has been altered by varying amounts to chlorite epidote carbonate minerals with the highest proportion of hydrous phases and carbonate occurring along major shear zones within the kvt and the sheared contact with the greenstone material oxygen isotope ratios of silicate minerals are not in equilibrium and indicate limited oxygen exchange between some of the minerals and an external fluid the cation proportions in the chlorite indicate that its temperature of formation was between 150 and 300 c mean 252 c chlorite formed at 250 c would have been in equilibrium with water having a 18o value of about 2 this is consistent with meteoric or sea water which became enriched in 18o through exchange with country rock or the kvt at low w r ratios carbon and oxygen isotope ratios show that the carbonates belong to two generations early calcite was probably precipitated along with the chlorite alteration a later generation was precipitated at much lower temperatures 50 c these stable isotope data are not consistent with the kvt being the source of the fluids responsible for gold mineralisation in the area estimates of the 18o value of the original kvt magma range suggest values of at least 7 6 which is 1 higher than that expected from an entirely mantle derived granite it is possible that the kvt contains a substantial sedimentary crystal component because sediments at 3 2 ga would be expected to be chemically immature having lower 18o values than present day sedimentary rocks

weathering and sediment generation in the archean an integrated study of the evolution of siliciclastic sedimentary rocks of the 3 2ga moodies group barberton greenstone belt south africa abstract alluvial and braided fluvial deposits of the 3 2 ga moodies group in the barberton greenstone belt south africa are analyzed petrographically and geochemically to identify their source rocks provenance and assess the degree and type s of weathering required to produce the observed moodies sediment compositions conglomerate sandstone and shale data give somewhat differing pictures of provenance and weathering due to derivation from different components within the source terrane and size and compositional fractionation during transport the results suggest that shale geochemistry provides the most accurate estimate provenance although both rare earth element and trace element th sc zr cr and ti data must be used in combination to give the best results the source area for moodies group sediments was dominated by tonalite felsic volcanic rock komatiite basalt and granite based on mineralogical and major element divergence from estimated source area composition the moodies group sediments are remnants of an aggressive weathering environment labile materials such as komatiite basalt and coarse plagioclase grains decomposed almost entirely to clays and solutes and the chemical index of alteration for moodies shale is well above the global average an aggressive weathering environment in the archean may have been achieved by increased rainfall higher temperatures and or higher atmospheric p c o 2 more likely a combination of these conditions worked to offset the inhibitory weathering effects of a plant free environment

field geochemical and u pb isotopic constraints from hypabyssal felsic intrusions within the barberton greenstone belt south africa implications for tectonics and the timing of gold mineralization abstract field mapping geochemical and u pb zircon isotopic data are reported for two undeformed feldspar quartz porphyries from within the central region of the barberton greenstone belt 3450 ma and from a porphyry at the deepest level of one of the largest gold mines in the barberton area fairview mine one porphyry the mudpools porphyry has an age of circa 3230 ma and provides a minimum age on unusual bif ferruginous shale hosted mudpool structures field mapping constraints and similar geochemical signatures between the mudpools porphyry and a second porphyry the ironstone pod porphyry suggest that 3230 ma can also be considered the minimum age of the equally unusual ironstone pods the two porphyries cross cut fig tree sediments which are regionally deformed d 2 and intrude up a thrust fault s suggesting pre 3230 ma ages for d 2 deformation and thrusting geochemical data trace elements ree indicate that these two porphyries are high level co magmatic equivalents of the nearby 3227 ma kaap valley tonalite which in part intrudes the lower sequences of the greenstone bell since the edge of the kaap valley pluton is also deformed by the d 2 deformational event either d 2 is diachronous or d 2 in the center of the belt is not equal to d 2 at the edge of the greenstone belt zircon from the fairview mine porphyry indicates a maximum age of 3126 21 ma 2 a gold mineralized shear which cross cuts the fairview porphyryl is responsible for hydrothermal alteration of the porphyry u pb analysis of hydrothermal rutile associated with gold from the porphyry gives an age of 3084 18 ma 2 and may represent the age of gold mineralization associated with this late shearing it appears that 3230 ma is a important age in early barberton greenstone belt history signifying intrusion of the kaap valley tonalite adjacent to the greenstone belt together with the emplacement of high level co magmatic equivalents porphyries of the kaap valley tonalite along regional thrust faults shear zones within the greenstone belt gold bearing hydrothermal fluids have utilized similar shear zones 150 ma after emplacement of both the porphyries in the central region of the greenstone belt and the kaap valley tonalite

digital terrain modelling a review of hydrological geomorphological and biological applications the topography of a catchment has a major impact on the hydrological geomorphological and biological processes active in the landscape the spatial distribution of topographic attributes can often be used as an indirect measure of the spatial variability of these processes and allows them to be mapped using relatively simple techniques many geographic information systems are being developed that store topographic information as the primary data for analysing water resource and biological problems furthermore topography can be used to develop more physically realistic structures for hydrologic and water quality models that directly account for the impact of topography on the hydrology digital elevation models are the primary data used in the analysis of catchment topography we describe elevation data sources digital elevation model structures and the analysis of digital elevation data for hydrological geomorphological and biological applications some hydrologic models that make use of digital representations of topography are also considered

silver isotopes as a source and transport tracer for gold a reconnaissance study at the sheba and new consort gold mines in the barberton greenstone belt kaapvaal craton south africa

a continuing discussion about the correlation of tidal gravity anomalies and heat flow densities on the basis of several objections rydelek et al phys earth planet inter 68 215 219 1991 hereafter noted rzh denied the existence of a correlation between residuals of tidal gravity r and heat flow h suggested by other workers to re examine the whole matter on safe grounds we previously revised and reanalysed all the original tidal data from 300 stations taking care of the rzh objections by eliminating what they considered as the important sources of errors in the data essentially calibration errors of the instruments in amplitude and phase using this new data base we meet here one by one the other criticisms not related to the original tidal measurements and show that most of them are not valid in particular those concerning oceanic load and attraction corrections the findings are as follows 1 no correlation between diurnal tide and semi diurnal tide residuals is normal 2 distance from the sea is not a valid criterion for elimination of several excellent stations 3 unexpected residues at the south pole are not due to imperfections of the cotidal schwiderski maps but to ice shelf tides not considered by rzh 4 the correlation of the tidal gravity residues with the age of tectonic provinces is proposed as an alternative to the correlation with heat flow the data used in 1991 having been corrected are recalculated for comparison with 1986 1991 statistics the correlation is confirmed with the same degree of probability also new tidal gravity data obtained since 1990 with nullified instruments as recommended by rzh in africa 31 stations and latin america 34 stations are included in new statistics which also confirm the correlation the global result obtained from 174 revised and new stations on all continents gives a correlation coefficient k 0 691 with a linear regression r 0 0201 h 57 0 063 0 0013 where r is in microgals and h is in mw m 2 as there is at present no practical or theoretical means to take into account the role of fluids lubrication and stress corrosion owing to deep seated saline waters or fluids in crustal processes such as tidal deformations such small tidal gravity anomalies cannot be correctly calculated the use of a correlation technique therefore remains the only way to treat unexplained residues even if this does not constitute an unequivocal proof other arguments have to be found to deny the existence of this correlation

paraguay araguaia belt conductivity anomaly a fundamental tectonic boundary in south american platform imaged by electromagnetic induction surveys 1 we discovered and mapped a 1200 km long zone of enhanced electrical conductivity through a large array of magnetometers over a narrow corridor parallel to the paraguay and araguaia belts which surround the amazon craton the anomaly has been further studied using magnetotelluric measurements made along a 300 km profile crossing the paraguay belt modeling shows the anomaly at this location as a very strong 100 km wide upper crust to mid crust conductor located under a region where surficial rocks show an increase in deformation and metamorphism within the belt from the high conductivity of the anomalous structure its observed geometry and surface rock exposures the most likely interpretation of its source is graphitized biogenic material in metasediments now deeply underthrust in a neoproterozoic or early cambrian suture zone this result strongly supports the hypothesis that the collision between the amazon plate and the western parana block has probably closed an ocean in the paraguay belt region contrary to former propositions of an ensialic evolution for this belt although the precise age of this ocean closure have been a matter of debate our finding significantly constrains the tectonic setting of west gondwana amalgamation involving the amazon plate and its surrounding blocks

comparison of the performance of flow routing algorithms used in gis based hydrologic analysis flow direction and specific catchment area were calculated for different flow routing algorithms using tapes g and taudem a fuzzy classification was used along with eight topo climatic attributes to delineate six landscape classes from a 10 m usgs dem a series of maps and tabular outputs were produced to compare flow routing predictions in different parts of the study area in the santa monica mountains of southern california the matched pair t test was used to compare the performance of pairs of specific catchment area grids across six user defined fuzzy landscape classes the results show that 1 the source cells predicted with the d demon and fd8 algorithms were confined to hilltops 2 two single flow routing algorithms rho8 d8 produced poor results and 3 the choice of flow routing algorithm has potentially important consequences for the calculation of upslope contributing areas sediment transport capacity topographic wetness and several other topographic indices copyright 2007 john wiley sons ltd

modelling water flow and sediment processes in a small gully system on the loess plateau in china abstract a single gully system was selected in a small agricultural watershed on the loess plateau of china with the objective of measuring and then simulating water and sediment transport and defining alternative land uses to reduce discharge and soil loss the gully had a total length of about 40 m and was about 30 m wide the watershed feeding the gully occupied about 1950 m 2 including the gully itself the gully bottom had a slope of about 32 40 whereas the gully walls were from 40 to over 60 soil water content and water and sediment discharge were measured automatically the physically based hydrological and soil erosion model lisem was used to calculate water and sediment discharge calibration has been carried out for one event by adjusting the saturated conductivity values validation was performed for two additional runoff events calibration results show reasonably comparable hydrographs between measured and calculated discharge lisem could be calibrated on the hydrograph quite satisfactorily using the saturated conductivity as a calibration factor also validation of lisem for two other runoff events showed reasonably good results the calibration on total soil loss also shows good results for the validation runs these results are poor probably due to limitations of the measurement equipment and the incapability of the model to simulate small events scenario analyses showed that forest as an alternative land use for the gully bottom will result in significantly lower water and sediment this strongly supports the implementation of the reforestation policy recently suggested and activated by the central government of the p r china

geology geochemistry and isotopic characteristics of the archaean kaap valley pluton barberton mountain land south africa abstract formed approximately 3500 ma ago the kaap valley tonalite pluton is one of the oldest and the largest of a suite of tonalite trondhjemite gneiss plutons in the immediate environs of the barberton greenstone belt although similar in many respects to the other members of this suite the kaap valley pluton is distinctively more mafic in composition containing hornblende instead of biotite as its dominant mafic mineral and is enriched in rare earth elements these characteristics are not compatible with an origin simply involving a greater degree of partial melting than that required to form more felsic trondhjemites from the same precursor the parental magma to the kaap valley tonalite is considered rather to originally have been a trondhjemitic magma enriched in minor phases such as zircon and apatite and possibly also dissolved co2 vapour which has been contaminated by a mafic component the form and composition of hornblendes in the tonalite are inconsistent with a xenocrystic origin by incomplete digestion of surrounding mafic metavolcanic country rocks and consequently the contamination is inferred to have taken place prior to magma emplacement possible mechanisms include magma mixing and complete assimilation at depth of mafic rocks a subdued form of crystal fractionation occurred during solidification of this magma resulting in the segregation of discrete hornblende and hornblende biotite tonalite phases the tonalite was subsequently diapirically emplaced into its present position perhaps about 3200 ma ago a process which resulted in deformation of the surrounding greenstone belt successions diapirism also perturbed the original probably sub horizontal compositional zoning the present geometry of which can best be explained in terms of an orthogonal pattern of intersecting structural lineaments

late syndepositional deformation and detachment tectonics in the barberton greenstone belt south africa the thick bedded competent quartzose sandstones conglomerates and siltstones of the moodies group 3 225 ma influence structural style in the 3 1 3 5 ga barberton greenstone belt moodies group rocks are folded into a train of tight subparallel doubly plunging overturned synclines separated by narrow strike parallel fault zones structural analysis along a transect across the central part of the greenstone belt suggests that much of the volcanic sedimentary greenstone sequence was buckle folded above a detachment within altered ultramafic rocks located at a depth of 3 6 km early development of brittle structures concentrated shortening and displacement near the fold hinges and in the anticlinal fault zones penetrative strain during folding was low the sense of displacement along faults the original fold vergence and the provenance of moodies group sandstone and conglomerate suggest that tectonic transport was directed toward the interior of the belt from its northwestern and southeastern margins orogenic contraction due to folding alone was at least 56 and subsequent ductile shortening strain accounted for an additional contraction of approximately 10 buoyant rise of the kaap valley tonalite to the north at approximately 3 22 ga and lateral intrusions of sheetlike granitic plutons along both sides of the preserved greenstone belt at 3 1 and 3 0 ga respectively imposed significant ductile strain on the greenstone belt margins and obscured or displaced the orogenic hinterlands of the facing fold and thrust belts the syn and post moodies tectonic history of the central barberton greenstone belt involves the formation and deformation of several shortlived alluvial to shallow marine sedimentary basins on unstable protocontinental crust the probable extensional origin of at least some of these basins and their subsequent shortening buckle folding and displacement above a subhorizontal detachment emphasizes the dominance of horizontal tectonics in the late stage deformation of this archean greenstone belt

testing ocean tide models in the nordic seas with tidal gravity observations summary tidal gravity observations at ny alesund on spitzbergen made with a lacoste romberg gravimeter with electrostatic feedback have been used to assess the accuracy of ocean tide models in the nordic seas eight ocean tide models have been used to compute the ocean tide loading at ny alesund the station is very close to the coast and therefore experiences a significant direct gravitational attraction of the nearby tidal water mass for the bay adjacent to the station the model values have been replaced by the tide gauge values and a special effort has been made to fit the models correctly to the coastline the ocean tide models for m2 and o1 have also been compared with observations made over 30 years ago at longyearbyen spitzbergen with askania gravimeters the comparisons of the tidal gravity observations with computed ocean tide loading show that for the harmonic m2 the phase lag of the ocean tide model of schwiderski and the local tide model of gjevik et al are wrong by 15 and 5 in the norwegian basin for the harmonic s2 the models fes94 1 got00 2 and schwiderski give a poor fit to the observed tidal loading at ny alesund because of phase lags that are too large in the norwegian basin by 10 fes94 1 and got00 2 also have s2 amplitudes that are 5 cm too large in the norwegian basin for n2 fes94 1 fes95 2 got00 2 and schwiderski have phase lags that are 10 too large in the norwegian basin and fes94 1 and 95 2 also have n2 amplitudes that are 2 4 cm too large in that region overall for these three harmonics the global model fes99 gives the best fit to the tidal gravity observations at ny alesund and this agreement is even closer than that obtained using regional arctic ocean tide models

a continental hydrological assessment of a new grid based digital elevation model of australia a rectangular grid digital elevation model dem for australia with a horizontal resolution of 1 40 degree of latitude and longitude approximately 2 5 km has recently been calculated from continent wide coverages of point elevation and stream line data using an elevation gridding technique which incorporates a drainage enforcement algorithm an automated drainage basin analysis of this dem is in close agreement with existing basin analyses both for the better surveyed coastal areas with well defined relief and for large areas of low relief that make up much of the interior lake eyre basin and the murray darling basin the analysis of the arid western plateau region differs from the currently accepted analysis but is in broad agreement with a previous analysis of palaeodrainage systems based on topographic and geological information the drainage analyses is used to calculate cumulative distributions of spatially distributed hydrologically significant terrain parameters for the murray darling and derwent river basins the adequacy of the resolution of the dem for continental hydrological analyses is assessed by comparing drainage basin analyses and cumulative distributions of terrain parameters at resolutions of 1 40th and 1 20th degree of latitude and longitude

modeling the evolution of loess covered landforms in the loess plateau of china using a dem of underground bedrock surface the evolution of loess covered landforms is largely controlled by the pre quaternary underlying bedrock terrain which is one of the most important factors in understanding the formation mechanism of the landforms this study used multiple data sources to detect 1729 outcropping points of underlying terrain in order to construct a digital elevation model dem of the paleotopography of an area of the loess plateau subject to severe soil erosion four terrain characteristics including terrain texture slope gradient the hypsometric curve and slope aspect were used to quantify topographic differences and reveal the loess deposition process during the quaternary a loess thickness map was then created to show the spatial distribution of loess deposits in the test area finally the geomorphological inheritance characteristics of the loess covered landforms were evaluated in different landform divisions the results showed the significant inheritance of modern topography from the underlying topography with a similar general relief trends the average thickness of loess deposits was computed to be 104 6 m with the thickest part located in the xifeng loess tableland area in addition the slope aspects of the north and northwest seem to have favored quaternary loess deposition which supported the hypothesis of an eolian origin for loess in china the modem surface has lower topographic relief compared to the underlying terrain due to loess deposition crown copyright c 2013 published by elsevier b v all rights reserved

upb zircon ages bearing on the nature of early archaean greenstone belt evolution barberton mountainland southern africa abstract u pb zircon systematics for granitic and gneissic clasts from the moodies conglomerate of the barberton greenstone belt south africa exhibit discordancy patterns that suggest variable and multi episodic lead loss and indicate crystallisation ages for the parent granitoids between 3 3 and 3 47 ga metamict zircons in some of these clasts indicate complete resetting of the older material or new growth at about 2 8 2 9 ga ago that is ascribed to a regional thermal event as also shown by published rb sr data for a nearby granitoid pluton since the moodies clasts consist largely of potassic granites our ages suggest the presence of highly evolved continental crust in the source region of the greenstone belt sediments a feature that contradicts the commonly accepted model for the primitive nature of the granitoid crust in early archaean granite greenstone terrains the maximum age of 3 3 ga for the moodies group also shows that greenstone belt evolution in the barberton region took place over a period of at least 150 ma from about 3450 to 3300 ma ago the kaap valley and stentor plutons intrude the lower part of the greenstone succession and their diapiric emplacement caused large scale deformation of earlier structures in the entire greenstone belt sequence at about 3 23 3 25 ga ago detrital zircons in the sediments of the fig tree group date the mean age of their source terrain at c 3520 ma which is also the maximum age of deposition for these strata this age is indistinguishable from the oldest age reported from the barberton greenstones and suggests the possibility that pre greenstone granitoid crust may exist in the eastern kaapvaal craton

sm nd age and isotopic systematics of the bimodal suite ancient gneiss complex swaziland studies of the development and stabilization of the archaean crust often focus on the relative temporal relationships between the metamorphosed basaltic to ultramafic volcanic units greenstone belts and the sialic gneiss terrains that make up the oldest sections of the terrestrial crust at the heart of this interest are the questions of the processes responsible for crust formation in the archaean and whether or not the various units of an archaean crustal section represent new additions to the crust from the mantle or are products of the reprocessing of even older crustal materials one area where this controversy has been particularly pronounced is the archaean crustal section of south west africa1 6 the oldest rocks in the kaapvaal craton consist of the onverwacht group of mafic to ultramafic metavolcanics of the barberton greenstone belt and a grey gneiss complex termed the ancient gneiss complex agc of swaziland we report here the results of a whole rock sm nd isotopic study of the agc and the implications these data may have for crustal evolution in the kaapvaal craton

formation of earth s early archaean continental crust abstract subduction of oceanic crust at an unusually low angle has been proposed as a model for the growth of continental crust older than about 2 5 ga at modern zones of low angle or flat subduction magmatic additions to new crust come from partial melting of both the subducting oceanic crust slab and the thin wedge of mantle above the slab evidence for both a slab and wedge source is preserved in most late archaean 3 0 2 5 ga terrains but we find little evidence that a mantle wedge contributed to crustal growth prior to 3 1 ga this lack of evidence in part reflects a dearth of exposed crust aged between 3 0 and 3 3 ga but also suggests that subduction enriched mantle source regions did not develop before 3 3 ga and possibly not before 3 1 ga in contrast to most modern terrains and some late archaean terrains early archaean 3 3 ga continental crust evolved through direct melting of thick mafic crust we invoke a process of subduction that does not include the development of a mantle wedge and call this process archaean flat subduction to distinguish it from modern low angle subduction

ocean load tides at the south pole a validation of recent ocean tide models small diurnal and semidiurnal gravity tides are seen at the south pole because of the loading by and attraction of the ocean tides these data provide a check on the quality of ocean tide models especially in the southernmost ocean which has historically been the most lacking in tidal data ocean tide models developed in the 1980 s did not predict the gravity tides at this location very well recently developed models based on the topex poseidon altimetric data and improved hydrodynamical modeling agree much better with the observations provided that the tides beneath the ice shelves are included the level of agreement at this remote location suggests that loads from very local tides aside the new generation of ocean tide models can predict the loading tides to very high accuracy

deep learning based approach for landform classification from integrated data sources of digital elevation model and imagery abstract landform classification is one of the most important aspects in geomorphological research dividing the earth s surface into diverse geomorphological types thus an accurate classification of landforms is a key procedure in describing the topographic characteristics of a given area and understanding their inner geomorphological formation processes however landform types are not always independent of one another due to the complexity and dynamics of interior and external forces furthermore transitional landforms with gradually changing surface morphologies are widely distributed on the earth s surface with this situation classifying these complex and transitional landforms with traditional landform classification methods is hard in this study a deep learning dl algorithm was introduced aiming at automatically classifying complex and transitional landforms this algorithm was trained to learn and extract landform features from integrated data sources these integrated data sources contain different combinations of imagery digital elevation models dems and terrain derivatives the loess plateau in china which contains complex and transitional loess landforms was selected as the study area for data training in addition two sample areas in the loess plateau with complex and transitional loess hill and ridge landforms were used to validate the classified landform types by using the proposed dl method meanwhile a comparative analysis between the proposed dl and random forest rf methods was also conducted to investigate their capabilities in landform classification the proposed dl approach can achieve the highest landform classification accuracy of 87 in the transitional area with data combination of dems and images in addition the proposed dl method can achieve a higher accuracy of landform classification with better defined landform boundaries compared to the rf method the classified loess landforms indicate the different landform development stages in this area finally the proposed dl method can be extended to other landform areas for classifying their complex and transitional landforms

early archean foredeep sedimentation related to crustal shortening a reinterpretation of the barberton sequence southern africa abstract the upper dominantly terrigenous sedimentary successions of the early archean barberton sequence kaapvaal province record an evolution similar to that of phanerozoic foredeeps and may represent the oldest known foredeep sequence rather than a passive continental margin sequence as previously proposed pre foredeep ocean crust the 3 5 ga old onverwacht group consists of bimodal volcanics with intercalated shallow water sediments gravity modeling and boundary shear zones indicate that deformed onverwacht crust was obducted northward onto sialic continental crust immediately before during or after burial by the synorogenic terrigenous clastic fill foredeep basin deepening due to crustal loading from the south is recorded by basinal iron formation chert and mudstone of the basal fig tree group which accumulated paraconformably on the onverwacht overlying sediments of the fig tree group consist of 2 km of submarine fan greywackes and mudstones shoaling of the basin is indicated by the transition to the overlying braided alluvial and shallow marine sediments of the ca 3 3 ga old moodies group 3 km thick changing ree patterns and relative abundances of ni cr sr th and ur in the sediments of the fig tree and moodies groups indicate a significant ultramafic mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution a similar trend is reflected in sandstone petrography in which the ratio of extrusive to intrusive rock fragments decreases upwards in the fig tree group whereas the moodies group sediments are enriched in quartz and potassium feldspar a persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of phanerozoic recycled orogens the distribution of facies in the fig tree and moodies groups and paleocurrent petrographic and geochemical data indicate progressive unroofing of a southerly source terrane consisting of the onverwacht volcanics and intercalated sediments and the 3 5 3 2 ga old ancient gneiss complex agc in addition older components of the fig tree group were recycled into overlying stratigraphie units the agc consists of mantle derived 3 5 ga old bimodal metavolcanics and metasediments a tonalite gneiss batholith metabasite dikes and meta anorthosites its structures record progressive northwestward uplift beginning around 3 4 ga ago intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large scale northwestward simple shear followed by imbricate faulting and pseudotachylite formation by 3 3 ga ago the fig tree and moodies groups had also shortened with similar kinematics to those of the agc by partly syndepositional folding and volume loss during the formation of transecting cleavage further folding and slide formation followed during subsolidus diapiric rise of the initially tabular kaap valley tonalite by 3 2 ga and emplacement of trondhjemite plutons from 3 3 to 3 0 ga ago

reassessment of archean crustal development in the barberton mountain land south africa based on u pb dating this paper presents u pb ages for 23 samples from the barberton mountain land an archean granite greenstone terrain in the kaapvaal craton of south africa a reexamination of the geology of this area in the light of previous u pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short lived compressional event within the time span 3230 3215 ma which was synchronous with deposition of the moodies group and the upper part of the fig tree group sediments this event may have been responsible for thrusting of much older sequences in the onverwacht group including slivers of 3 54 ga old gneiss circa 3 47 ga old komatiitic and tholeiitic sequences and circa 3 45 ga old felsic sequences key constraints on the timing of deformation are previously determined 3227 3225 ma ages for detrital zircons in thrusted units of the fig tree group a 3229 4 3 ma age for a deformed porphyry which is pre or synthrusting a previously determined 3227 3 ma age for an undeformed porphyry in an inferred thrust fault 3227 1 ma ages for both the kaap valley pluton and an ignimbrite in the stolzburg syncline both of which predate regional upright folding and a 3216 2 ma age for the dalmein pluton which appears to cut regional upright ne sw trending folds in the kromberg syncline part of the final phase of deformation associated with regional compression areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the barberton region at about 3105 ma the linear configuration of the syenogranitic plutons which are also aligned with the gabbroic usushwana complex as well as the geometry of the granitoid sheets suggests that the circa 3105 ma magmatism was caused by incipient rifting around a nw se trending crustal fracture this episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt some of the above interpretations are in conflict with previous models for early development of the barberton greenstone belt a 3352 6 ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m y younger than the probable age of volcanic rocks which host the dikes this conflicts with the suggestion that much of the onverwacht group comprises an ophiolitic section obducted in an intraoceanic setting at circa 3 45 ga that partially melted to form trondhjemitic magmas which intruded along active thrust faults present data suggest that despite its antiquity and the diversity of ages and lithologies found within it the barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

comparison of single and multiple flow direction algorithms for computing topographic parameters in topmodel single flow direction sfd and multiple flow direction mfd algorithms were used to compute the spatial and statistical distributions of the topographic index used in the watershed model topmodel an sfd algorithm assumes that subsurface flow occurs only in the steepest downslope direction from any given point an mfd algorithm assumes that subsurface flow occurs in all downslope directions from any given point the topographic index in topmodel is in a tan 3 where in is the napierian logarithm a is the upslope area per unit contour length and tan 3 is the slope gradient the in a tan 3 distributions were computed from digital elevation model dem data for locations with diverse topography in arizona colorado louisiana nebraska north carolina oregon pennsylvania tennessee vermont and virginia the means of the in a tan 3 distributions were higher when the mfd algorithm was used for computation compared to when the sfd algorithm was used the variances and skews of the distributions were lower for the mfd algorithm compared to the sfd algorithm the differences between the mfd and sfd algorithms in the mean variance and skew of the in a tan 3 distribution were almost identical for the various dems and were not affected by dem resolution or watershed size topmodel model efficiency and simulated flow paths were affected only slightly when the in a tan 3 distribution was computed with the sfd algorithm instead of the mfd algorithm any difference in the model efficiency and simulated flow paths between the sfd and mfd algorithms essentially disappeared when the model was calibrated by adjusting subsurface hydraulic parameters

computer assisted terrain sketch mapping that considers the geomorphological features in a loess landform abstract in geography a terrain sketch map is necessary to understand the features and internal structures of a landscape due to its ability to depict key information in a geographical scene using as few lines as possible previous computer drawn sketch maps have focused on the artistic effect rather than on depicting terrain features and landform structures and thus they differ considerably from hand drawn sketch maps by geographers or geologists this study develops a dem based method for terrain sketch mapping that considers the typical feature descriptions of a loess landform and the visual hierarchy expression in line with the law of visual perspective the method was tested with experiments on two landforms of chinese loess plateau based on digital elevation models dem with a horizontal resolution of 5 m in the developed method first typical terrain features including the visual outline shoulder line gully and flow lines are extracted from the dem second the map is divided into three visual levels in accordance with the data extension and viewing point then terrain feature lines are assigned to different visual levels finally the visual outlines in the distant view are generalized following the law of visual perspective results are assessed through a questionnaire with specialists experts and students non experts the sketch map was able to characterise loess landforms and is somewhat similar to traditional hand drawn maps the generalisation method realises the near and distant view characteristics of a sketch map which are detailed and simplified respectively the results of the questionnaire also showed that our method presents terrain morphology and geographical scene more accurately and reliably than a hand drawn sketch map

drainage networks from grid digital elevation models current algorithms that deduce the drainage network from a digital elevation model dem represented by a regular array of surface elevations share a fault unless the terrain is rugged the derived water channels tend to flow in parallel lines along preferred directions engendered by the sampling grid orientation we present a simple solution to the problem a second difficulty is the presence of noise that creates artificial pits we briefly describe a method which deals with pits in what we believe to be a more efficient manner for virtual memory environments than previous efforts our system has treated dems of nearly 9 000 000 pixels we show how depth first search of the resulting drainage network permits segmentation of the dem into basins by various criteria analysis of stream sediment anomaly dilution profiles improved hydrological models and other applications

minimizing the grid resolution dependence of flow routing algorithms for geomorphic applications the results of flow routing methods currently used in the geomorphic literature depend on grid resolution this poses a problem for landscape evolution models which must be independent of grid resolution to the greatest extent possible here i illustrate a refinement of currently used flow routing algorithms that yields unit contributing areas i e contributing areas per unit width of flow with minimal grid resolution effects i illustrate the application of this method in idealized topography in high resolution digital elevation models dems of real world topography and by integration into a landscape evolution model for ridge and valley topography the landscape evolution model produces grid resolution independent results in a more straightforward way than previous models for this type of landscape

digital elevation model networks demon a model of flow over hillslopes for computation of contributing and dispersal areas current algorithms for computing contributing areas from a rectangular grid digital elevation model dem use the flow routing model of o callaghan and mark 1984 which has two major restrictions 1 flow which originates over a two dimensional pixel is treated as a point source nondimensional and is projected downslope by a line one dimensional moore and grayson 1991 and 2 the flow direction in each pixel is restricted to eight possibilities we show that large errors in the computed contributing areas result for any terrain topography divergent convergent or planar we present a new model called digital elevation model networks demon which avoids the above problems by representing flow in two dimensions and directed by aspect demon allows computation of both contributing and dispersal areas demon offers the main advantage of contour based models e g moore et al 1988 the representation of varying flow width over nonplanar topography while having the convenience of using rectangular grid dems

climatic and geochemical implications of archean pedogenic gypsum in the moodies group 3 2 ga barberton greenstone belt south africa abstract lithic sandstones of braided fluvial to supratidal facies in the paleoarchean moodies group 3 22 ga barberton greenstone belt south africa include several regionally traceable units with common to abundant in places rock forming nodular concretions of megaquartz pseudomorphs after gypsum barite and calcite concretionary accumulations are stratiform and commonly associated with aqueously reworked fine grained tuffaceous sediment of originally rhyodacitic composition and can grow to fist sized agglomerates in crusts tens of m in lateral extent weathering of tuffaceous material and feldspar delivered alkali cations such as ca ba and k while carbonates were likely supplied by silicate weathering of mafic to ultramafic volcanic rocks during exposure to a co2 rich atmosphere sulfate ions were partly delivered by oxidative pyrite dissolution which may have included microbial and abiotic disproportionation of volcanic s or so2 concretionary growth apparently took place under pedogenic to early diagenetic conditions within unconsolidated granular sediment in the vadose zone dominated by seasonal fluctuations of the groundwater level under evaporitic conditions the concretions likely represent the oldest terrestrial evaporites known to date and form part of the oldest known compound paleosols their formation and composition constrain the local occurrence of sulfate in the archean atmo and hydrosphere their interaction with the emerging biosphere archean weathering regime local climate and vadose zone hydrodynamics

hotspots basalts and the evolution of the mantle the trace element concentration patterns of continental and ocean island basalts and of mid ocean ridge basalts are complementary the relative sizes of the source regions for these fundamentally different basalt types can be estimated from the trace element enrichment depletion patterns their combined volume occupies most of the mantle above the 670 kilometer discontinuity the source regions separated as a result of early mantle differentiation and crystal fractionation from the resulting melt the mid ocean ridge basalts source evolved from an eclogite cumulate that lost its late stage enriched fluids at various times to the shallower mantle and continental crust the mid ocean ridge basalts source is rich in garnet and clinopyroxene whereas the continental and ocean island basalt source is a garnet peridotite that has experienced secondary enrichment these relationships are consistent with the evolution of a terrestrial magma ocean

multi stage alteration rheological switches and high grade gold mineralization at sheba mine barberton greenstone belt south africa abstract structural petrographic and geochemical data from sheba gold mine in the barberton greenstone belt of south africa indicate that gold mineralization formed at the end and as the result of a multi stage structural fluid flow and alteration history this multi stage evolution is a prerequisite for the formation of high grade 40 g t au ore bodies the sheba mine is situated in well bedded metaturbidites of the mesoarchaean fig tree group and structurally infolded ultramafic lavas in the first order refolded ulundi syncline in the northern parts of the belt original komatiitic lava flows are pervasively altered to talc carbonate greyschist this regionally widespread alteration relates to an early phase of deformation and associated fluid flow the reaction softening associated with this alteration accentuates rheological anisotropies within the heterogeneous wall rock sequence this results in the localization of strain along the contact between talc carbonate greyschist and overlying massive greywacke and chert units during later stage refolding and associated flexural slip pervasive quartz veining silicification k enrichment and the progressive replacement of the earlier talc carbonate assemblages by a high variance quartz fuchsite green schist alteration testify to the localized fluid flow along this contact the newly formed finely intergrown green schist assemblage leads in turn to a reaction hardening and embrittlement of the former greyschist chert contact high grade gold mineralization is hosted by brittle quartz carbonate vein networks and breccias developed along brittle ductile structures that cross cut the competent green schist at high angles the siting of gold and predominance of free gold suggest that gold precipitation is related to extreme fluid pressure cycling and phase separation of the mineralizing fluid during the formation of the brittle fracture networks the thrust kinematics of the late stage controlling structure studied here indicate that the mineralized structure accommodated regional nw se subhorizontal shortening when further tightening of the ulundi syncline passed the lock up stage could no longer be accommodated by flexural slip the results highlight that the formation of favourable conditions for high grade mineralization such as those at sheba mine may be the result of the interaction of temporally distinct fluid flow and alteration events and lithological heterogeneities during the progressive structural evolution of a region

ocean tides in the weddell sea new observations on the filchner ronne and larsen c ice shelves and model validation 1 ocean tides under the large weddell sea ice shelves are among the least well observed on earth here we present new spatially extensive observations of the vertical tidal motion of the filchner ronne and larsen c ice shelves using global positioning system gps data spanning a few weeks to years we pay particular attention to the major tidal constituents m2 s2 o1 k1 as well as important grace aliasing periods k2 and s1 we compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves the root sum square errors are 7 8 cm cats2008a and tpxo7 2 and 11 12 cm got4 7 and fes2004 with the energetic m2 rmse 4 8 cm and s2 4 5 cm generally dominating these statistics the fes2004 k1 is particularly inaccurate near the larsen c ice shelf with errors approaching 20 cm meaning that grace release 4 estimates of mass change in the northern antarctic peninsula will be biased we find tidal energy at 3 4 5 6 and weakly at 7 cycles per day at all of our sites the largest amplitudes within these bands are at m4 mo3 and sp3 and approach 30 mm although significant spatial variations exist we show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones comparing model estimates with our m4 ms4 and mn4 values shows that models do not accurately represent these terms

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation

validation of ocean tide models around antarctica using onshore gps and gravity data 1 ocean tide models around antarctica are presently only sparsely tested against independent data ocean tide modeling errors along with subsequent ocean tide loading otl displacement modeling errors alias into altimetry and time variable gravity e g gravity recovery and climate experiment grace time series for example to validate various ocean tide models around antarctica gps data from 15 sites have been used to derive three dimensional displacement estimates at eight diurnal and semidiurnal tidal frequencies using hundreds of days of gps data harmonic parameters were estimated on a daily basis then combined these were then compared with otl displacement estimates derived from global and regional ocean tide models in east antarctica where the tides are well defined submillimeter differences are demonstrated in each coordinate component with the lunar n2 and q1 constituents in closest agreement as found in other studies k1 and especially k2 agree less well the spatial variation in the misfits for these two constituents indicates a site dependency with the k2 errors also suggesting an interaction with satellite dependent effects in west antarctica where sites are nearer the largest ice shelves agreement with the older models csr3 and tpxo 2 and nao 99b is poor for all constituents modeled tidal gravity variations were also compared with gravity measurements at the south pole overall the gps and gravity data agree best with newer tide models namely tpxo 6 cada00 10 fes99 and cats02 01 however validation data are lacking at the southern extents of the large ice shelves and hence some uncertainty still exists in all ocean tide models in these regions