diversity in earth s early felsic crust paleoarchean peraluminous granites of the barberton greenstone belt earth9s oldest preserved granitoid crust dates back to the paleoarchean and consists predominantly of sodic tonalite trondhjemite granodiorite ttg granitoids that arose through the partial melting of hydrated metabasalts in contrast granites sensu stricto typically appear relatively late in the plutonic record of the old cratons however the existence of hadean zircons with mineral inclusion suites that are consistent with crystallization from peraluminous granitic magmas indicates that granitic rocks formed part of the earliest felsic crust although we have direct evidence this earliest felsic crust is not preserved here we present evidence of an unusual variety of markedly low cao k 2 o rich rutile bearing peraluminous granite and rhyolite that was produced concurrently with ttg magmas during three magmatic cycles in the barberton greenstone belt bgb southern africa this material is not preserved as in situ rock units but occurs as clasts within a younger conglomerate within these rocks plagioclase feldspar is a rare inclusion in zircon relative to alkali feldspar and has low anorthite contents an 2 o and sr content as well as the peraluminous character of the magmas is a consequence of phengite melting in a metagraywacke source at pressures in excess of those of plagioclase stability this process contributed to each episode of continental crustal growth through the paleoarchean to mesoarchean in the bgb despite leaving no plutonic record at the typical mid crustal level of exposure that the ttg plutons around the belt represent

the dynamics of motion of the south american plate observations of the intraplate stress field as compiled and evaluated in the context of the world stress map project can be compared with stress modelling results to increase our understanding of the dynamics of motion of the lithospheric plates in this paper we investigate the driving and resistive plate tectonic forces acting on the south american plate in order to establish a detailed basis for future stress modelling our basic assumption is that the plate is in mechanical equilibrium that is the net torque of the forces acting on the plate vanishes the complexity of the forces exerted along the margins of the south american plate requires the dynamical analysis to be performed step wise incorporating the forces which are expected to be of major importance a first order model is defined the effect of additional plate boundary forces is subsequently examined in terms of superpositions onto the first order model with the first order model the relation between three resistive forces and shear stress on the base of the lithosphere is derived the ridge push force driving the motion of the south american plate can be quantified independently and is included in the calculations as a known quantity solutions are obtained using four different determinations of the euler vector of absolute motion of the south american plate which is taken to define the orientation of the basal shear stress the results demonstrate that the concept of a basal shear stress actively driving plate motion cannot be discarded on the basis of a dynamical analysis further with the assumption that basal drag is entirely concentrated below the continental pan of the lithosphere estimates of the maximum amount of continental basal shear resistance are derived evaluation of an alternative model for the south american plate excluding the northernmost pan of the andes demonstrates the significance of the resistive force exerted on the south american plate due to convergence with the caribbean plate it is to be expected that comparison of theoretical distributions of intraplate stress computed on the basis of the present dynamical analysis with observations of the actual state of stress will further constrain the range of feasible force models

geometry and distribution of regional joint sets in a non homogeneous stress field case study in the ebro basin spain abstract three regional joint sets striking n s e w and wnw ese affect the tertiary rocks of the central ebro basin from analysis of their chronological relationships and spatial distribution it is concluded that they correspond to two different tectonic events the n s set oldest and the e w set younger are present in the southern and central sectors while the wnw ese joint set predominates in the northern one the n s joints propagated in response to joint normal and fluid loads under an intraplate stress field with shmax oriented near n s related to forces caused by the convergence of africa iberia and europe and rifting at the valencia trough during the sedimentary infilling of the basin these joints are only present in the southern part of the area the e w joint set in the southern central sector records the same fracturing event as the wnw ese set does in the northern one its orientation was modified by the presence of the older n s set in the south which perturbed the regional stress field the younger wnw ese and e w joint sets are interpreted as unloading joints these propagated as a consequence of flexural uplift and exhumation related to isostatic rebound at the pyrenees and the ebro foreland basin a numerical approach is used to explain the inhomogeneous distribution of the n s joint set in terms of their absence being controlled by the depth of the water table at the time of their formation

ion microprobe ages of zircons from early archaean granite pebbles and greywacke barberton greenstone belt southern africa abstract we report single grain and grain domain u pb zircon ages for three granitoid clasts from the moodies conglomerate and for detrital zircons from a fig tree greywacke from clastic sediments in the early archaean barberton greenstone belt of southern africa the crystallization ages obtained for the moodies clasts vary between 3570 6 ma and 3518 11 ma while the fig tree zircons show a surprisingly uniform source terrain age of 3453 9 ma that may date the stratigraphically underlying onverwacht group felsic metavolcanics the discordancy patterns are ascribed to a combination of variable pb loss at unspecified times in the past and new zircon growth and or recrystallization the shrimp data record details of isotopic variation even within single zircons that result in precise definition of upper concordia intercept ages our results suggest an age range between 3460 and 3570 ma for the source region of the barberton belt clastic metasediments presumably the ancient gneiss complex of swaziland and felsic volcanics of the overwacht group this source included geochemically evolved granitoids that predate the oldest greenstones by some 100 ma

seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

a generalized genetic framework for the development of sinkholes and karst in florida u s a karst topography in florida is developed on the tertiary limestones of the floridan aquifer post depositional diagenesis and solution have made these limestones highly permeable t ca 50 000 m2 d zones of megaporosity have formed at unconformities and dissolution has enlarged joints and fractures erosion of the overlying clastic miocene hawthorn group strata on one flank of a structural arch has exposed the limestone the elevated edge of the hawthorn cover forms the cody scarp ubiquitous solution pipes have previously formed at joint intersections and are now filled downwashing of the fill deeper into solution cavities in the limestone and subsidence of the overlying unconsolidated sediments causes surface collapse a subsidence doline or sinkhole this process may penetrate up to 60 m of the semi consolidated hawthorn cover as occurred when the winter park sinkhole developed dense clusters of solution pipes may have formed cenotes which are now found on the exposed limestone terrain

karst breakdown mechanisms from observations in the gypsum caves of the western ukraine implications for subsidence hazard assessment the term karst breakdown is employed in this paper to denote the totality of processes and phenomena of gravitational and or hydrodynamic destruction of the ceiling of a karst cavity and of the overlying sediments it refers not only to the existence of a surface subsidence collapse feature but first of all to the internal hidden in the subsurface structures that precede development of a surface form this study reports and discusses the results of direct mapping and examination of breakdown structures in the gypsum karst of the western ukraine at the level of their origin i e in caves the accessibility of numerous laterally extensive maze cave systems in the region provided an excellent opportunity for such an approach which made it possible to examine the relationship between breakdown structures and particular morphogenetic or geological features in caves and to reveal stages of breakdown development it is found that breakdown is initiated mainly at specific speleogenetically or geologically weakened localities which classify into a few distinct types the majority of breakdowns which are potent to propagate through the overburden relate with the outlet cupolas domepits that represent places where water had discharged out of a cave to the upper aquifer during the period of transverse artesian speleogenesis distribution of breakdown structures does not correlate particularly well with the size of the master passages several distinct mechanisms of breakdown development are revealed and most of them proceed in several stages they are guided by speleogenetic geological and hydrogeological factors the study confirms that a speleogenetic approach is indispensable to the understanding of breakdown pre requisites and mechanisms as well as for eventual subsidence hazard assessment direct observations in caves aimed both at speleogenetic investigation and breakdown characterization on regional or site specific levels should be employed wherever possible

rates and mechanisms of mesoarchean magmatic arc construction eastern kaapvaal craton swaziland the mechanisms and time scales of magmatism and deformation during orogenesis are important for developing models of lithospheric growth we summarize the first detailed study of a well preserved mesoarchean crustal section from the oldest portion of the kaapvaal craton southern africa which records the complex interactions between deformation and magmatism during craton assembly ca 3 3 3 2 ga we use chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims u pb zircon geochronology and apatite thermochronology in conjunction with geological mapping to show that the tonalitic to granitic usutu magmatic suite intruded into the ca 3 66 3 45 ga ancient gneiss complex over a period of 16 ma ca 3236 3220 ma as discrete pulses of magma with variable intrusive styles usutu rocks retain magmatic fabrics that preserve a history of nw se regional shortening consistent with synchronous deformation recorded in the adjacent barberton greenstone belt u pb zircon dates of ca 3 28 3 23 ga and apatite cooling dates from the nhlangano gneiss se of the ancient gneiss complex reveal that they represent an older largely undocumented period of crustal growth and modification synchronicity in magmatism and similarity in kinematics of deformation north and south of a previously suggested continental suture in the barberton greenstone belt lead us to propose doubly vergent subduction zones that were active from at least ca 3 28 to 3 22 ga geochemistry of the usutu suite reveals differences in magma sources from north to south which when combined with nd isotopic signatures are consistent with their production in a subduction zone the combination of 1 ma precision on crystallization dates with field observations and geochemistry allows us to track the evolution of this magmatic system with temporal resolution unprecedented for archean rocks

determining accurate temperature time paths from u pb thermochronology an example from the kaapvaal craton southern africa thermochronology has revolutionized our understanding of the establishment and evolution of lithospheric thermal structure however many potential benefits provided by the application of diffusion theory to thermochronology have yet to be fully exploited this study uses apatite tc 450 550 c and titanite tc 550 650 c u pb id tims thermochronology at the single to sub grain scale to separate the variable effects of volume diffusion of pb from metamorphic over growth above and below the tc of a mineral data are presented from two ca 3227 ma tonalite samples from north and south of the barberton greenstone belt bgb southern africa two distinct populations of apatite from a sample north of the bgb record fast cooling followed by metamorphic growth 10 myr later both apatite and titanite dates from south of the bgb show a strong correlation with the grain size and record 100 myr of post emplacement cooling complex core rim zoning observed in cathodoluminescence images of apatite is interpreted to reflect metamorphic overgrowth above the tc the age and topology of grain size versus date curves from titanite and apatite are used in combination with a finite difference numerical model to show that slow non linear cooling and not thermal resetting is responsible for the observed distribution the thermal histories from either side of the bgb are very different and provide unique insight into the bgb s tectonic evolution a 70 myr period of apparent stability after ca 3 2 ga terrane assembly was followed by fast exhumation south of the bgb that led to lower crustal melting and intrusion of granitic batholiths ca 3 14 3 10 ga

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

zircon geochronology and hf isotopes of the dwalile supracrustal suite ancient gneiss complex swaziland insights into the diversity of palaeoarchaean source rocks depositional and metamorphic ages abstract we report the results of combined u pb and lu hf isotope analyses of detrital and metamorphic zircon grains from a variety of metavolcano sedimentary rocks of the dwalile supracrustal suite dss from the ca 3 66 to 3 20 ga ancient gneiss complex agc in swaziland the results indicate that the dss is made up by palaeo and mesoarchaean rocks which have been affected by a polymetamorphic overprint and became tectonically assembled during the neoarchaean the oldest dwalile rocks were formed during a phase of felsic to intermediate volcanism at ca 3 46 ga and are derived from the same magma source as the tsawela gneisses based on the combined u pb hf isotope data two groups of metasedimentary rocks can be distinguished an older dwalile group i that is similar in age to the hooggenoeg and kromberg formations 3 46 3 40 ga but with a different source area a younger dwalile group ii is coeval with the fig tree and possibly moodies groups the u pb hf isotope data and petrological observations demonstrate that the dss was affected by three stages of metamorphic overprint an event at 3 23 ga pervasive amphibolite facies metamorphism at 3 15 ga and low p granulite facies metamorphism at 2 99 ga metamorphic zircon in the amphibolite facies rocks resulted from new growth in aqueous fluids at 3 15 ga whereas in the granulite facies rocks new zircon was formed in the presence of melt the 3 23 ga event coincides with the last significant metamorphic overprint documented in the barberton greenstone belt bgb whereas as two younger events are restricted to the agc the data support the model suggesting amalgamation between the agc and bgb terranes at ca 3 23 ga in addition they require crustal thickening in the agc terrane at ca 3 15 ga causing barrovian type metamorphism in the dss and granulite facies metamorphism in the luboya kubuta terrane low p granulite facies metamorphism at ca 2 99 ga is most likely related to crustal extension during the onset of pongola basin rifting causing mantle upwelling mafic magmatism and heat transfer from the mantle into the thinned crust

accretionary history of the archean barberton greenstone belt 3 55 3 22 ga southern africa the 3 55 3 22 ga barberton greenstone belt south africa and swaziland and surrounding coeval plutons can be divided into four tectono stratigraphic blocks that become younger toward the northwest each block formed through early mafic to ultramafic volcanism onverwacht group probably in oceanic extensional island or plateau settings volcanism was followed by magmatic quiescence and deposition of fine grained sediments possibly in an intraplate setting late evolution involved underplating of the mafic crust by tonalitic intrusions along a subduction related magmatic arc yielding a thickened buoyant protocontinental block the growth of larger continental domains occurred both through magmatic accretion as new protocontinental blocks developed along the margins of older blocks and when previously separate blocks were amalgamated through tectonic accretion evolution of the barberton belt may reflect an early archean plate tectonic cycle that characterized a world with few or no large stabilized blocks of sialic crust

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

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

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

a high continental weathering flux into paleoarchean seawater revealed by strontium isotope analysis of 3 26 ga barite controls on archean seawater chemistry remain controversial many studies have suggested that it was largely controlled by oceanic hydrothermal fluid circulation recent work however from clastic sequences hf o isotope data from detrital zircons and models for the rb sr evolution of the continental crust suggest that intense continental weathering and low temperature surface alteration were more important than previously thought during the early archean this is consistent with biogeochemical studies that suggest the archean had a diverse microbial ecology which would in part need to be sustained by nutrients e g phosphorus that were derived from continental weathering to further quantify continental weathering during the early archean we analyzed 3 26 ga barite from the fig tree group south africa for strontium oxygen and sulfur isotope compositions we propose that the seawater component of the barite is characterized by 87sr 86sr ratios 0 701 which is significantly more radiogenic than contemporaneous mantle 0 7007 0 7008 the radiogenic nature of seawater at this time suggests that the continental weathering flux at 3 26 ga had a large impact on ocean chemistry 400 million years earlier than previously suggested

hafnium isotope record of the ancient gneiss complex swaziland southern africa evidence for archaean crust mantle formation and crust reworking between 3 66 and 2 73 ga abstract combined u pb and lu hf isotope analyses of zircons from 16 tonalite trondjemite granodiorite ttg gneiss and granite samples from swaziland reveal that the oldest rocks of the ancient gneiss complex in southern africa formed by reworking of early archaean or perhaps late hadean crust at 3 66 ga and that new crust was extracted from a depleted mantle source during palaeoarchaean events between 3 54 and 3 32 ga this interpretation is supported by ehf t of 1 6 2 0 obtained from 3 66 ga ttg gneisses corresponding to hafnium model ages between 3 77 0 18 ga for a presumed hadean early archaean chondritic mantle and 4 08 0 18 ga for a presumed hadean depleted mantle reservoir with the first model age being the most likely in the light of recent data from worldwide sources furthermore it is reflected by superchondritic ehf t up to 2 2 2 0 for ttgs formed at 3 54 3 45 and 3 32 ga the new datasets additionally show that the palaeoarchaean crust formed between 3 54 and 3 32 ga was intensely reworked afterwards without significant addition of depleted mantle derived material during orogenic and intracratonic melting processes at 3 23 3 1 and 2 7 ga this is well reflected by an array of decreasing ehf t from 2 2 to 7 2 between 3 3 and 2 7 ga which can be forced by 176 lu 177 hf of 0 0113 which is similar to that of present day average continental crust and might result from lower crust zircon fractionation during archaean crust reworking supplementary material results of in situ u pb and lu hf isotope zircon analyses and concordia diagrams are available at www geolsoc org uk sup18465

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

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

transcurrent shearing granite sheeting and the incremental construction of the tabular 3 1 ga mpuluzi batholith barberton granite greenstone terrane south africa structural petrographic and geochronological studies show that the tabular 3 1 ga mpuluzi batholith in the barberton granite gneiss terrane in south africa was emplaced via a combination of external and internal processes external structural controls are indicated by systematic variations in intrusive relationships and strain along the margins of the mpuluzi batholith and are consistent with an emplacement of the granite in a dilational jog within a ne ene trending system of dextral transcurrent synmagmatic shear zones internally the mpuluzi batholith is essentially made up of granite sheets the structurally higher parts of the granite are made up of shallowly dipping sheets that are underlain by an anastomosing network of steeply dipping variably deformed dykes and sheets these granite sheets at lower structural levels intruded either into the actively deforming shear zones or into extensional sectors between and along the bounding shear zones multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3 13 ma the spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting the transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets

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

the early earth geochemical and isotope record of serpentinizing environments from archean stichtite mg6cr2 oh 16 co3 4h2o abstract a record of the conditions and duration of archean serpentinization is preserved as chemical isotopic and textural properties of the magnesium chromium hydroxycarbonate mineral stichtite this provides a window into the conditions associated with a potentially habitable environment on early earth and mars stichtite is associated with archean chromite rich serpentinite rocks of south africa western australia and southern india stichtite compositions range within the cr fe al hydrotalcite group with trends unique to each discrete serpentinite host body elemental and textural analysis of stichtite associated chromite indicates that stichtite forms in fore arc layered intrusive boninite setting rocks through interaction of chromite and methane rich serpentinizing fluids the degree to which chromite is replaced by stichtite is inferred to correlate with the length of time that the host rocks spent within the stichtite window carbon stable isotope values of archean stichtite suggest carbon sourcing from marine kerogen with minor marine dissolved inorganic carbonate in most samples as opposed to the strongly kerogen dominated phanerozoic stichtite the carbon and hydrogen stable isotope profile of archean stichtite ranges from the methane fields of active serpentinizing zones to geothermal with trends suggestive of methane oxidation rather than the substrate depletion noted in phanerozoic stichtite thus isotope values of stichtite appear to record oceanic oxygenation levels the association of the stichtite 2h polytype nee barbertonite with aragonite antigorite suggests this is a higher pressure temperature polytype of stichtite reaction textures isotopic values and qualitative mineral thermobarometric indicators indicate that stichtite forms during serpentinization in a methane h2 rich environment within fluid conduits ranging from near the surface to depths where pressure is up to 2 8 gpa and temperature is up to 300 c

zircon u pb hf isotope systematics of transvaal supergroup constraints for the geodynamic evolution of the kaapvaal craton and its hinterland between 2 65 and 2 06 ga abstract the transvaal basin in south africa hosts a 15 km thick pile of sedimentary successions deposited over a period of 600 ma during the neoarchean to paleoproterozoic presently little is known about the source of these sediments as well as about the tectono magmatic evolution in the hinterland of the transvaal basin preventing detailed geotectonic correlations of the kaapvaal craton kc with other cratons worldwide to solve this problem we present the first systematic study of combined u pb and lu hf isotope data of 2000 detrital zircons from fourteen formations of the transvaal supergroup these reveal that clastic sedimentary rocks were supplied from sources on and off the present day kc detrital zircons in conglomerates of the wolkberg and black reef formations maximum deposition ages at 2769 8 and 2618 11 ma respectively were mainly supplied from surrounding kc either from pietersburg block basement pbb and or from eroded sedimentary successions of the witwatersrand pongola and or ventersdorp supergroups in contrast clastic sedimentary rocks of the rooihoogte duitschland and timeball hill formations maximum deposition ages at 2353 18 ma 2342 18 and 2290 8 ma respectively were predominately supplied from a juvenile neoarchean terrane junat formed at 2570 2500 ma ehf2500 ma 2 to 9 and intensely reworked at 2400 ma and to a minor amount from a composite archean terrane cat emplaced by granitoids between 3540 and 2680 ma and affected by crust reworking at 2570 2430 ma ehf2 5ga 3 to 12 in a neoarchean to paleoproterozoic continental arc terrane npcat subsequent periodic reworking of junat at 2250 2220 ma and 2120 ma is recorded by detrital zircons in sandstones of the overlying boshoek dwaalheuwel daspoort magaliesberg and post magaliesberg formations having maximum deposition ages at 2243 7 2242 7 2240 7 2080 7 and 2068 7 ma respectively the archean zircons age 2650 ma in all these formations were mainly supplied from pbb the new data sets also suggest that the kc was connected to cat npcat and junat at

geodetic networks along the caribbean south american plate boundary abstract three major fault zones form the caribbean south american plate boundary these are the bocono moron and pilar fault zones the first geodetic networks were explored and installed in 1972 1973 first measurements were realized in 1973 and remeasurements were made at several intervals up to date in total 18 sites were studied and explored for local geodetic networks of these only five are completely operational the others are still waiting for monumentation and operation nevertheless for these latter the main part of the installation the exploration work and the feasibility studies in geology and topography have already been realized in the operational networks the field and point vectors show movements and deformations max values yacambu 88 mm 2 years mucubaji 32 mm 5 years the deformational parts of displacements show principally compression vectors from both sides towards the plate boundary

plate motions in the north andean region repeated geodetic measurements with the global positioning system gps provide direct measurements of displacements due to plate motions and active crustal deformation in central america and northern south america an area of complex interaction of the nazca cocos caribbean and south american plates the displacement rates for the period 1988 1991 obtained from the results of the first three central and south america casa gps campaigns are in general agreement with the predictions of the nuvel 1 plate motion model but there are differences in detail between the observations and the model the nazca north andes convergence rate vector measured by gps is different from the nuvel 1 vector at 95 confidence the difference implies that the north andes are moving northward relative to south america the measured convergence between the caribbean plate and the north andes suggests that the southern margin of the caribbean plate is located in the south caribbean deformed belt the april 1991 costa rica earthquake and the cocos caribbean convergence rate determined by gps suggest the possibility of significant ongoing deformation between central america and the stable interior of the caribbean plate our gps results are consistent with deformation of the overriding plates at the convergent margins of central and south america and confirm that active convergence is occurring around much of the southern margin of the caribbean plate from colombia west to costa rica costa rica and panama are not part of the stable caribbean plate instead the south caribbean deformed belt and the north panama fold belt probably represent the southern margin of the caribbean plate

basin formation along the bocono moron el pilar fault system venezuela within the boundary zone between the caribbean south america plates which includes the bocono moron el pilar right lateral fault system that has been active since the late tertiary there are several sedimentary basins in the venezuelan andes and the caribbean mountains these were formed in zones of crustal depression where faults diverge and as pull apart basins in zones of en echelon stepovers or along releasing bends in the faults in addition there are pull apart basins along the avila and la victoria faultzones in the caribbean mountains which are associated with the plate boundary the basins are filled with alluvial lacustrine fluvial and marine sediments mainly of quaternary age on the basis of model studies dimensions of the basins and age of their sedimentary fill it is estimated that right lateral offset necessary to form them along the bocono moron el pilar fault system varies from 2 to 125 km along the avila and la victoria fault zones this right lateral offset is estimated to range from 2 to 30 km on this basis the maximum right lateral offset along the southern caribbean plate boundary that can be documented along the bocono moron el pilar fault system has not exceeded 100 125 km if there has been larger offset it has to be documented by other means or it has taken place along other fault zones within the system

gneiss greenstone relationships in the ancient gneiss complex of southwestern swaziland southern africa and implications for early crustal evolution the ancient gneiss complex agc of swaziland is made up of a variety of archaean gneisses derived from granitoid intrusions of ttg composition emplaced between 3 6 and 3 2 ga ago and infolded amphibolite grade greenstone remnants locally referred to as dwalile supracrustal suite dss the greenstones were considered to postdate the oldest phases of the agc known as ngwane gneiss and this is now confirmed by conventional u pb zircon geochronology and shrimp single zircon ages on detrital grains from dwalile clastic metasediments suggesting that these rocks were derived from a granitoid source some 3 42 to 3 57 ga in age trondhjemitic ngwane gneiss structurally underlying the greenstones has a zircon age of 3521 23 ma and contains a 3683 10 ma old xenocryst this together with nd tdm model ages between 3 7 and 3 9 ga and a pronounced negative euanomaly for the metasediments strongly argues for the presence of very old crustal components in the agc nd isotopic systematics of dwalile metavolcanics suggest these rocks to be contaminated with older crustal material of ngwane type and resulting nd model ages 3 60 3 99 ga are likely to be too old the major and trace element chemistry of the dwalile metavolcanics is remarkably similar to that of onverwacht group rocks in the nearby barberton greenstone belt bgb and this together with a similar age for these two suites points to a genetic link and a similar mechanism of formation our data as well as field relationships do not favour an intra oceanic origin for the dss as has been inferred for the onverwacht group nor do they support a juvenile origin for the exposed ngane gneisses of the agc the ngwane precursors were apparently derived from older crustal protoliths whose subsequent modification particularly through remelting and deformation gave rise to the various compositional varieties now seen in the ngwane suite

gliding and overthrust nappe tectonics in the barberton greenstone belt abstract regional recumbent folds inverted stratigraphy nappes and olistostromes are described from the southern part of the 3 3 3 5 ga barberton greenstone belt overthrusting of thin rigid silicified slabs with minimum dimensions of 25 km2 and up to 500 m in thickness occurred over minimum distances of 86 km more ductile and coherent units were overfolded up to at least 2 km during their emplacement the glide planes on which these nappes travelled were zones of high fluid pressures related to hydrothermal fluid circulation patterns driven by heat sources from igneous intrusions the upwelling areas of the geothermal convection cells were sites of mud pools and hydrothermal vents which may mark the trailing edges pull aparts of the overthrust units progressive silica and carbonate precipitation due to decreasing temperatures within the zones of fluid migration distant from the areas of high heat flow probably acted as built in braking systems below the travelling slabs active sedimentation and metasomatism during this tectonism indicates a protracted history for the evolution of the greenstone belt the recognition of nappe and overthrust tectonics in the barberton belt processes which may have been commonplace in archaean terrains necessitates a re evaluation of the stratigraphy of this belt

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

u pb hf isotope systematics of detrital zircons in high grade paragneisses of the ancient gneiss complex swaziland evidence for two periods of juvenile crust formation paleo and mesoarchaean sediment deposition and 3 23 ga terrane accretion abstract we present the first results of combined u pb and lu hf isotope analyses of detrital zircon grains in high grade paragneisses luboya kubuta granulites from the ca 3 66 3 22 ga ancient gneiss complex agc in swaziland uranium pb ages demonstrate that sedimentation along the se margin of the proto kaapvaal craton took place during two time intervals during the paleoarchaean ca 3 53 and 3 43 ga and mesoarchaean ca 3 22 and 3 16 ga the hf isotope data show that the hinterland to the paleoarchaean metasediments experienced juvenile crust formation from a depleted mantle source at ca 3 53 ga hf 3 53ga values between 0 7 and 4 1 and by reworking of more ancient crust t dm 3 8 3 6 ga in contrast detrital zircons in mesoarchaean metasediments record juvenile crust formation at 3 32 ga hf 3 32ga values between 1 5 and 6 0 t dm 3 45 3 32 ga followed by internal reworking of this crust until 3 22 ga a small number of detrital zircons indicate limited input from older paleoarchaean agc crust compared to sediments of the barberton greenstone belt i e fig tree and moodies group the mesoarchaean metasediments of swaziland are more juvenile in character and show greater complexity in their detrital zircon age spectra pointing towards a different provenance our new u pb hf isotope data together with field and petrological information from the study area suggest that the paleoarchaean metasediments were largely derived from the agc whereas the mesoarchaean metasediments were derived from a ca 3 45 3 32 ga juvenile island arc terrane located se of the agc which became accreted onto the kaapvaal craton at ca 3 23 ga i e the onset of burial and high grade metamorphism in the area

u pb and hf isotope data of detrital zircons from the barberton greenstone belt constraints on provenance and archaean crustal evolution combined u pb and hf isotope analyses of detrital zircons from the fig tree and moodies groups of the barberton greenstone belt south africa yield similar hf isotope compositions and age populations thus pointing to a similar provenance zircon populations of fig tree group greywacke and moodies group quartzarenite are both dominated by age clusters at 3 53 3 47 and 3 28 ga and a minor cluster at 3 36 ga the moodies quartzarenite sample additionally contains a younger age population at 3 23 3 19 ga hafnium isotope data indicate that the source area of both sediments was affected by new crust formation from depleted mantle sources at 3 53 3 47 and perhaps at 3 36 ga hf t between 1 7 and 4 5 accompanied by partial reworking of an eoarchaean crustal component as old as 3 75 3 95 ga in contrast crustal reworking was the predominant process between 3 28 and 3 22 ga hf t between 6 0 and 0 9 probably related to subduction and collision of terranes along the inyoka fault system the zircon u pb and hf isotope datasets favour a southern provenance for the fig tree and moodies sediments comprising granitoids in the vicinity of the southern barberton greenstone belt and in swaziland this finding is in contrast to the sedimentary record of the moodies group which mostly suggests a northern and along strike provenance this discrepancy may be due to reworking of sediments during extensive syn and postorogenic strike slip faulting and high uplift or subsidence between 3 26 and 3 19 ga supplementary material results of in situ u pb and lu hf isotope zircon analyses are available at www geolsoc org uk sup18561

geodetic control of neotectonics in venezuela abstract henneberg h g 1983 geodetic control of neotectonics in venezuela in p vyskocil a m wassef and r green editors recent crustal movements 1982 tectonophysics 97 1 15 net installations for measurements of movement started in 1973 along the bocono fault which is the main fault in the venezuelan andes investigations of horizontal movements are made on three locations along this fault mucubaji at 3700 m height mitisus at 1700 m and yacambu at 700 m at mucubaji there are two nets with 8 stations each and at mitisus we have three nets represented by 29 stations each station has a concrete pillar with a forced centering device on top and has a 1 4 m3 concrete plaform these nets were measured several times giving resulting difference vectors of a range of several cm one of these nets is also a control net of a hydroelectrical power dam installed near the fault trace at yacambu we have a 26 km extended tunnel net which is crossing the fault this net was measured twice with difference vectors up to 6 cm in the maracaibo basin area the vertical component of subsidence due to oil extraction reaches 5 m as a result of continuous movements since 1925 gravimetric research is also carried out over all the mentioned locations in cooperation with the universities of hannover and stuttgart and the german geodetic research institute a programme was started to determine the geoid and deflections of the vertical in the high andes mountains especially around the mucubaji fault site parallel to and in combination with these geodetic investigations seismological studies are carried out by a geophysical group from merida university venezuela

structure of the merida andes venezuela relations with the south america caribbean geodynamic interaction for over 50 years several models based on diverse geologic concepts and variable quality of data have been proposed to explain the major structure and history of the merida andes ma in western venezuela lately this chain growth and associated flexural basins deepening have been related to incipient type a subductions of either polarity accounting for the across chain asymmetry however these recent models have not well integrated the present tectonically active setting driven by neighboring major plate interactions at present this chain exhibits ongoing strain partitioning where cumulative right lateral slip along chain axis is as much as half of or about the same as the transverse shortening since late miocene thus implying that the nne directed maracaibo block extrusion with respect to the south america sa plate is not a secondary feature consequently this paper discusses some limitations exhibited by the se directed continental subduction models maracaibo crust underthrusting the merida andes in the light of available geological and geophysical data besides it is herein proposed that the merida andes structuration is related to a nw directed gently dipping incipient type a subduction where chain growth and evolution are similar to those of a sedimentary accretionary wedge i e barbados but at crustal scale and with ongoing strain partitioning this continental subduction is the se portion of a major orogenic float that also comprises the perija range and the santa marta block d 2002 elsevier science 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

paleoarchean record of the detrital pyrite bearing jacobina au u deposits bahia brazil abstract u pb and lu hf isotope analyses were performed on detrital zircon grains from eight samples distributed along a 1000 m thick section of the au u pyrite deposits of jacobina to identify the primary source areas and define the maximum age for the basin sedimentation these deposits contain detrital pyrite similar to that observed in the witwatersrand gold province although the sequence was deformed by paleoproterozoic orogeny all of the detrital zircon grains n 785 yielded paleoarchean ages between 3 2 and 3 5 ga with primary peaks of approximately 3 3 3 4 ga most of the grains have nearly chondritic compositions with hf t values between 5 6 and 0 1 and hf model ages between 3 5 and 3 8 ga the presence of detrital au u pyrite in the sio 2 rich metasedimentary rocks provide unequivocal evidence that the basin formed in an o 2 poor atmosphere the u pb data set and lithogeochemistry suggest that its infill material was derived from an exclusively paleoarchean source specifically sediments derived from the ttg greenstone assembly of the gaviao block 3 2 3 5 ga these source rocks were strongly hydrothermally altered and weathered before deposition the geochemical composition of the sedimentary rocks and the isotopic signature of hf indicate that the sediments were derived from a juvenile source with minor crustal contamination and a short duration of crustal residence between 150 and 350 myr

detrital zircon geochronology of sandstones of the 3 6 3 2 ga barberton greenstone belt no evidence for older continental crust

juvenile crust formation in the northeastern kaapvaal craton at 2 97 ga implications for archean terrane accretion and the source of the pietersburg gold abstract combined u pb and lu hf isotope data of detrital and magmatic zircon grains provide evidence that the murchison greenstone belt ne kaapvaal craton south africa hosts an important archean suture zone which primarily formed by a collision 2 97 ga ago between a back arc system to the north and an evolved terrane the proto kaapvaal craton to the south this model is supported by the observations that igneous rocks of the northern terrane comprising mafic rocks and tonalites of the rooiwater complex and felsic volcanics of the rubbervale formation were both emplaced at about 2 97 ga and show highly superchondritic hft of 4 4 to 5 1 in contrast u pb hf isotope data of detrital zircon grains from quartzitic schists of the murchison unit and the la france formation both part of the southern terrane provide evidence for a southern provenance which was affected by granitoid intrusions accompanied by crust re working at 3 53 3 42 ga hft 1 8 to 4 8 3 30 3 20 ga hft 1 8 to 6 3 and 3 13 3 05 ga hft 1 3 to 5 6 pointing to a connection with the barberton greenstone belt and swaziland a spatial separation of the two terranes prior to 2 97 ga is furthermore supported by the facts that the youngest detrital zircon grains of the southern terrane having ages between 2 99 and 2 97 ga show subchondritic hft between 6 5 and 1 5 in contrast to the highly superchondritic hft obtained from the contemporaneous magmatic rocks of the northern terrane the juvenile character of the rooiwater and rubbervale magmatic rocks along with the occurrence of vms deposits makes them a likely candidate for the source of the gold pyrite bearing conglomerates exposed in the adjacent pietersburg greenstone belt

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

using the magmatic record to constrain the growth of continental crust the eoarchean zircon hf record of greenland abstract southern west greenland contains some of the best studied and best preserved magmatic eoarchean rocks on earth and these provide an excellent vantage point from which to view long standing questions regarding the growth of the earliest continental crust in order to address the questions surrounding early crustal growth and complementary mantle depletion we present laser ablation split stream lass analyses of the u pb and hf isotope compositions of zircon from eleven samples of the least altered meta igneous rocks from the itsaq amitsoq gneisses of the isukasia and nuuk regions of southern west greenland this analytical technique allows a less ambiguous approach to determining the age and hf isotope composition of complicated zircon results corroborate previous findings that eoarchean zircon from the itsaq gneiss 3 85 ga to 3 63 ga were derived from a broadly chondritic source in contrast to the sm nd whole rock isotope record for southern west greenland the zircon lu hf isotope record provides no evidence for early mantle depletion nor does it suggest the presence of crust older than 3 85 ga in greenland utilizing lass u pb and hf data from the greenland zircons studied here we demonstrate the importance of focusing on the magmatic rather than detrital zircon record to more confidently understand early crustal growth and mantle depletion we compare the greenland hf isotope data with other eoarchean magmatic complexes such as the acasta gneiss complex nuvvuagittuq greenstone belt and the gneissic complexes of southern africa and all lack zircons with suprachondritic hf isotope compositions in total these data suggest only a very modest volume of crust was produced during or survived from the hadean and earliest eoarchean there remains no record of planet scale early earth mantle depletion in the hf isotope record prior to 3 8 ga

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

subsidence hazard avoidance based on geomorphological mapping in the ebro river valley mantled evaporite karst terrain ne spain in the valley of the ebro river to the southeast of the city of zaragoza ne spain the dissolution of evaporite sediments gypsum halite and na sulphates which underlie alluvial deposits gives rise to numerous sinkholes these sinkholes are a potential hazard to human safety particularly where they develop in a catastrophic way even slow developing sinkholes are problematic as they damage urban and agricultural infrastructure necessitating costly repairs and vigilant maintenance to assist in developing avoidance strategies for these hazards the factors controlling sinkhole occurrence have been assessed using geomorphological maps produced from aerial photographs for 1956 and 1981 important controls on sinkhole development are found to include underlying geological structure manifest in preferred orientations of sinkholes on the azimuths n130 150e and n30 40e and the presence of glauberite in the groundwater flow path which apparently promotes accelerated gypsum dissolution perhaps surprisingly alluvium thickness does not appear to significantly correlate with the density of sinkholes on the floodplain in this area the maps for 1956 and 1981 reveal that both human activity and natural processes can serve to obscure the true density of sinkhole development for instance a large number of sinkholes which were conspicuous in 1956 have since been back filled by farmers in the most fluvially active zone of the ebro valley the meander belt the relatively low density of sinkholes compared with adjoining zones suggests that subsidence is being masked by morpho sedimentary dynamic processes aggradation and erosion careful geomorphological mapping for different time periods yields a much more accurate impression of the frequency of sinkhole development than would be gained from surveying currently visible sinkholes in the area of interest

the bastar craton central india a window to archean paleoproterozoic crustal evolution abstract the bastar craton in central india surrounded by cratonic blocks and paleoproterozoic to neoproterozoic orogenic belts is a window to investigate the archean paleoproterozoic crustal evolution and tectonic processes here we propose a new tectonic classification of the craton into the western bastar craton wbc eastern bastar craton ebc and the intervening central bastar orogen cbo we present petrologic geochemical and zircon u pb ree and lu hf data from a suite of rocks from the cbo and along the eastern margin of the wbc including 1 volcanic successions comprising meta andesite and fine grained amphibolite representing arc related volcanics along a convergent margin 2 ferruginous sandstone in association with rhyolite representing a volcano sedimentary succession deposited in an active trench and 3 metamorphosed mafic ultramafic suite including gabbro pyroxenite and dunite invaded by trondhjemite representing the section of sub arc mantle and arc root adjacent to a long lasting subduction system petrologic studies indicate that the mafic ultramafic suite crystallized from an island arc tholeiitic parental magma in a suprasubduction zone environment the chondrite normalized and primitive mantle normalized diagrams of the mafic and ultramafic rocks suggest derivation from morb magma the mixed characters from n morb to e morb of the studied samples are consistent with subduction modification of a morb related magma involving partial melting of the metasomatized mantle wedge our zircon u pb age data suggest that the cratonic nuclei was constructed as early as paleoarchean we present evidence for active subduction and arc magmatism through mesoarchean to neoarchean and early paleoproterozoic with the trench remaining open until at least 2 3 ga two major crust building events are recognized in the bastar craton during mesoarchean recycled paleoarchean subduction related as well as juvenile depleted mantle components and neoarchean accretion of juvenile oceanic crust arc magmatism including granite batholiths and related porphyry mineralization the final cratonization occurred during latest paleoproterozoic followed by collisional assembly of the craton and its incorporation within the peninsular indian mosaic during mesoproterozoic in the global supercontinent context the craton preserves the history of ur the earliest supercontinent followed by the paleo mesoproterozoic columbia as well as minor thermal imprints of the neoproterozoic rodinia and associated grenvillian orogeny

trench investigation along the merida section of the bocono fault central venezuelan andes venezuela abstract the bocono fault is a major ne sw trending dextral fault that extends for about 500 km along the backbone of the venezuelan andes several large historical earthquakes in this region have been attributed to the bocono fault and some of these have been recently associated with specific parts through paleoseismologic investigations a new trench study has been performed 60 km to the northeast of merida in the central venezuelan andes where the fault forms a releasing bend comprising two conspicuous late holocene fault strands that are about 1 km apart the southern and northern strands carry about 70 and 30 respectively of the 7 10 mm yr net slip rate measured in this sector which is based on a 40 vs 85 100 m right lateral offset of the late pleistocene los zerpa moraines a trench excavated on the northern strand of the fault near morros de los hoyos slightly northeast of apartaderos across a twin shutter ridge and related sag pond exposed two main fault zones cutting late pleistocene alluvial and holocene peat deposits each zone forms a shutter ridge with peat deposits ponded against the uplifted block the paleoearthquake reconstruction derived from this trench allows us to propose the occurrence of at least 6 8 earthquakes in the past 9000 yr yielding a maximum average recurrence interval of about 1100 1500 yr based on the northern strands average slip rate 2 6 mm yr such an earthquake sequence should have accommodated about 23 m of slip since 9 ka suggesting that the maximum slip per event ranges between 3 and 4 m no direct evidence for the large 1812 earthquake has been found in the trench although this earthquake may have ruptured this section of the fault further paleoseismic studies will investigate the possibility that this event occurred on the bocono fault but ruptured mainly its southern strand in this region

depositional and tectonic setting of the archean moodies group barberton greenstone belt south africa the 3 22 3 10 ga old moodies group uppermost unit of the swaziland supergroup in the barberton greenstone belt bgb is the oldest exposed well preserved quartz rich sedimentary sequence on earth it is preserved in structurally separate blocks in a heavily deformed fold and thrust belt north of the inyoka fault moodies strata reach up to 3700 m in thickness detailed mapping correlation of measured sections and systematic analysis of paleocurrents show that the lower moodies group north of the inyoka fault forms a deepening and fining upward sequence from a basal alluvial conglomerate through braided fluvial tidal and deltaic sandstones to offshore sandy shelf deposits the basal conglomerate and overlying fluvial facies were derived from the north and include abundant detritus eroded from underlying fig tree group dacitic volcanic rocks shoreline parallel transport and extensive reworking dominate overlying deltaic tidal and marine facies the lithologies and arrangement of moodies group facies sandstone petrology the unconformable relationship between moodies strata and older deformed rocks presence of at least one syndepositional normal fault and presence of basaltic flow rocks and airfall fall tuffs interbedded with the terrestrial strata collectively suggest that the lower moodies group was deposited in one or more intramontane basins in an extensional setting thinner moodies sections south of the inyoka fault generally less than 1000 m thick may be correlative with the basal moodies group north of the inyoka fault and were probably deposited in separate basins a northerly derived southward thinning fan delta conglomerate in the upper part of the moodies group in the central bgb overlies lower strata with an angular unconformity this and associated upper moodies conglomerates mark the beginning of basin shortening by south to southeast directed thrust faulting along the northern margin of the bgb and suggest that the upper moodies group was deposited in a foreland basin timing orientation and style of shortening suggest that this deformation eventually incorporated most of the bgb into a major fold and thrust belt

evidence from physical geodesy for geodynamics in the caribbean area abstract geodynamic studies in the caribbean consider a great deal of geophysical and geological information geodetic data have only been applied but little physical geodesy provides different parameters of the gravity field such as geoid heights deflections of the vertical and gravity anomalies as the gravity field is a function of the mass distribution within the earth these parameters give information about mass inhomogeneities in particular also about those correlated with geodynamics this paper presents some results concerning geodynamic features in the caribbean obtained from the inversion of the gravity field into a theoretical mass distribution of the earth the effect of irregular mass distributions in the lithosphere is separated from the global gravity field and interpreted with respect to plate tectonics a clear correlation between the geometry of the caribbean plate and the lithospheric part of the gravity field can be found

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

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

u pb and hf isotopes of detrital zircons from the pongola supergroup constraints on deposition ages provenance and archean evolution of the kaapvaal craton abstract the c 3 0 2 9 ga pongola supergroup located on the south eastern margin of the kaapvaal craton preserves some of the most pristine archean successions but the timing of volcanic and sedimentary processes and sediment sources remain relatively poorly known combined zircon u pb ages and hf isotope data of sandstones from the nsuze and mozaan groups are presented from key locations of the northern central and southern domains of the 270 km long belt these data indicate important lateral and temporal variations and therefore changes in source areas the basal pongola sedimentary rocks mantonga formation in the northern domain were derived from the c 3 4 ga basement and also most likely from reworked moodies group sedimentary rocks of the barberton greenstone belt zircon detritus in nsuze and lower mozaan groups sandstones shows similar age hf isotope spectra and a remarkable overlap with data from magmatic and meta sedimentary rocks of the southeastern kaapvaal craton the data point to a proximal source for nsuze sandstones in the northern domain and a different provenance for the southern domain sandstones a possible signature of the extensive 3 10 3 07 ga mpuluzi granite is seen only in the nsuze sandstones and basal mozaan of the central domain the data indicate for the first time that cr rich sandstones of the ndikwe thrust segment of the southern domain were deposited at

erosion of archean continents the sm nd and lu hf isotopic record of barberton sedimentary rocks abstract knowing the composition nature and amount of crust at the surface of the early earth is crucial to understanding the early geodynamics of our planet yet our knowledge of the hadean archean crust is far from complete limited by the poor preservation of archean terranes and the fact that less attention has been paid to the sedimentary record that tracks erosion of these ancient remnants to address this problem and get a more comprehensive view of what an archean continent may have looked like we investigated the trace element and sm nd lu hf isotopic records of archean metasedimentary rocks from south africa we focused our study on sandstone and mudstone from drill core in the fig tree group 3 23 3 26 ga of the barberton granite greenstone belt but also analyzed the 3 4 ga buck reef cherts and still older 3 5 3 6 ga meta igneous rocks from the ancient gneiss complex swaziland based on principal component analysis of major and trace element data the fig tree metasedimentary rocks can be classified into three groups crustal detritus rich sediments si rich sediments and ca fe rich sediments the detritus rich sediments have preserved the sm nd and lu hf isotopic signatures of their continental sources and hence can be used to constrain the composition of crust eroded in the barberton area in the paleoarchean period based on sm nd ratios we estimate that this crust was more mafic than today with an average sio 2 content of 60 5 2 wt this composition is further supported by isotopic mixing calculations suggesting that the sedimentary source area contained equal proportions of mafic ultramafic and felsic rocks this implies that the archean crust exposed to weathering was more mafic than today but does not exclude a more felsic composition at depth neodymium and hf crustal residence ages show that the eroded crust was on average 300 400 ma older than the deposition age of the sediments which highlights the importance of intracrustal reworking of older crust at 3 2 ga in the barberton area the si rich sediments have slightly positive e nd t 3 23ga but extremely radiogenic e hf t 3 23ga up to 11 based on analyses of 3 4 ga buck reef cherts we suggest that the radiogenic hf isotopic signature of the si rich sediments can be accounted for by the old chert clasts or detrital silicified rock fragments present in the rocks the latter have extremely high lu hf ratios such that their e hf values would increase dramatically by about 100 epsilon units every 100 ma in the ca fe rich sediments one sample contains carbonate that preserves the typical rare earth element features of seawater precipitates the initial nd isotopic composition of this sample e nd t 3 23ga 1 7 is within the range of previous estimates for archean anoxic seawater

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

sm nd isotopic mapping of lithospheric growth and stabilization in the eastern kaapvaal craton whole rock sm nd isotope systematics of 79 archean granitoids from the eastern kaapvaal craton southern africa are used to delineate lithospheric boundaries and to constrain the timescale of crustal growth assembly and geochemical differentiation c 3 66 2 70 ga offsets in end values for 3 2 3 3 ga granitoids across the barberton greenstone belt bgb are consistent with existing models for c 3 23 ga accretion of newly formed lithosphere north of the bgb onto pre existing c 3 66 ga lithosphere south of the bgb along a doubly verging subduction margin the nd isotopic signature of c 3 3 3 2 ga magmatic rocks show that significant crustal growth occurred during subduction accretion after c 3 2 ga however the nd signature of intrusive rocks c 3 1 and 2 7 ga is dominated by intracrustal recycling rather than by new additions from the mantle signalling cratonic stability

natural and human induced sinkholes in gypsum terrain and associated environmental problems in ne spain the central ebro basin comprises thick evaporite materials whose high solubility produces typically karstic landforms the sinkhole morphology developed in the overlying alluvium has been studied using gravimetry and ground penetrating radar gpr on stream terraces as well as analyzing the evolution of sinkhole morphologies observed in aerial photographs taken in 1928 1957 and 1985 the sinkhole morphologies give some idea of possible subsurface processes as well as an indication of the final mechanisms involve in sinkhole development on stream terraces and cover pediments the most commonly encountered dolines are bowl shaped in their morphology with both diffuse and scarped edges in contrast dolines developed in the gypsiferous silt infilled valleys have a funnel and well shaped morphology the diffuse edged bowl shaped dolines are developed through the progressive subsidence of the alluvial cover due to washing down of alluvial particles through small voids and cracks into deeper subsurface caves resulting in a decrease alluvial density future compaction of the alluvial cover will produce surface subsidences this type of dolines are associated with negative gravity anomalies in contrast the scarped edge dolines are formed by the sudden collapse of a cavity roof the cavities and cracks formed in the gypsum karst may migrate to the surface through the alluvial deposits by piping and they may subsequently collapse in this instance the cavities can be detected by both gravity and gpr anomalies where the voids are not deeper than 4 5 m from the surface these processes forming sinkholes can be enhanced by man induced changes in the groundwater hydrologic regime by both inflows due to irrigation ditch losses or pipe leakages and by outflows from pumping activities

contemporary plate motion and crustal deformation the measurement of relative plate velocities during the past few years is a signal accomplishment in earth science leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins regrettably deformation premonitory to an earthquake has yet to be detected with confidence delineation of the spatial and temporal buildup of strain between earthquakes however has put limits on models of the earthquake cycle a diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike slip faults in contrast the geometry of thrusts and normal faults revealed by earthquake deformation has been found to differ markedly from expectations geodetic observations have proved vital to monitor the ascent of magma through the earth s crust and to predict volcanic eruptions at the earth s surface episodic vertical and horizontal deformation in southern california remains a subject of dispute if anything it is less certain than it once seemed

pseudo oolites in rocks of the ulundi formation lower part of the archaean fig tree group south africa abstract the ulundi formation of the lowermost fig tree group in the type area in the barberton mountain land consists of a sequence of quartz sericite schists banded cherts and tuffaceous rocks up to 40 m thick it overlies talc carbonate schists assigned to the upper part of the onverwacht group in the cherts which in parts represent the sulfide facies of a banded iron formation silicified spheroids have been observed which appear to be diagenetically altered locally zeolitized accretionary lapilli portions of the cherts formed through silicification of fine grained felsic tuffs and contain abundant stylolite seams along which 4 of the rock thickness has been removed by intrastratal solution during deformation a supposedly organogenic spheroidal structure ramsaysphera has been previously described from the upper part of the ulundi formation the spheroids with diameters of up to 8 mm occur in a distinct bed of up to at least 1 6 m thickness and can be traced in outcrop over an area which originally covered 17 30 km the bed was deposited in fairly still water of several 100 m depth the spheroids accumulated with only little finer grained detrital material the interstices between them being filled by cherty silica and minor amounts of dolomite up to 65 of the spheroids consist of tectonically elongated originally round fragments of a micro felsitic groundmass with remnants of phenocrysts the majority of the remaining spheroids consists of concentrically banded bodies of siliceous material with minor amounts of chlorite and sericite as well as small dolomite crystals these spheroids represent highly altered granules of volcanic glass deposited from volcanic ash clouds through a body of water the prototype of ramsay sphera also belongs to this group of spheroids and there is little microscopic or macroscopic evidence for its explanation as an organic structure

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