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

relationships between the vredefort structure and the witwatersrand basin within the tectonic framework of the kaapvaal craton as interpreted from regional gravity and aeromagnetic data abstract processed gravity and aeromagnetic data covering the witwatersrand basin and adjacent areas have been used to delineate the boundary of the basin and to investigate other features such as the vredefort structure the bethlehem gravity high and the colesberg magnetic anomaly trend a model is proposed in which the regional aeromagnetic and gravity signatures of these features can be related to the disposition of a semi continuous mid crustal magnetite rich layer which is exposed or has a shallow suboutcrop in three regions in the centre of the vredefort structure along the kaapvaal craton boundary and centrally through the craton northwards from colesberg the bethlehem high is thought to be due to elevation of lower levels of the crust along a nw trending geanticlinal arch the apex of this broad arch is termed the vredefort axis and plunges northwestwards beneath the witwatersrand basin the vredefort structure lies at the intersection of this axis and a major nne trending axis of crustal downwarp which also constitutes the long axis of the witwatersrand basin two contrasting scenarios for the development of these major axes are discussed with either scenario genesis of the vredefort structure should be integrated with the genesis and evolution of the vredefort axis and with the tectonic evolution of the kaapvaal craton

magnetic mineralogy of the lower continental crust rocks that were once present in the deep continental crust can be found in surface exposure as xenoliths as parts of high grade metamorphic and plutonic terrains and as obducted lower crustal cross sections a review of magnetic and mineralogic studies of samples representing all such occurrences shows that magnetite is the only magnetic phase present in significant amounts in these rocks thermodynamic considerations indicate that tipoor magnetite is the expected magnetic oxide under equilibrium conditions that exist in the lower crust regardless of the primary magmatic composition of the spinel phase furthermore it seems unlikely that serpentinites with metal alloys contribute significantly to the magnetization of the lower crust or mantle thus magnetite in the deep crust is the most likely candidate for the source of the long wavelength magnetic anomalies observed in regional aeromagnetic and satellite magnetometer surveys if so the entire crust should be magnetic in most continental areas including shields in areas of high heat flow the bottom of the magnetic crust may lie considerably above the moho

the distribution of radiogenic heat production as a function of depth in the sierra nevada batholith california geochemical analyses and geobarometric determinations have been combined to create a depth vs radiogenic heat production database for the sierra nevada batholith california this database shows that mean heat production values first increase then decrease with increasing depth heat production is 2 aw m 3 within the 3 km thick volcanic pile at the top of the batholith below which it increases to an average value of 3 5 aw m 3 at 5 5 km depth then decreases to 0 5 1 aw m 3 at 15 km depth and remains at these values through the entire crust below 15 km below the crust from depths of 40 125 km the batholith s root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of 0 14 aw m 3 this is higher than the rates from most published xenolith studies but reasonable given the presence of crustal components in the arc root assemblages the pattern of radiogenic heat production interpreted from the depth vs heat production database is not consistent with the downward decreasing exponential distribution predicted from modeling of surface heat flow data the interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the sierra nevada could be generated within the 35 km thick crust this requires a very low heat flux from the mantle which is consistent with a model of cessation of sierran magmatism during laramide flat slab subduction followed by conductive cooling of the upper mantle for 70 m y the heat production variation with depth is principally due to large variations in uranium and thorium concentration potassium is less variable in concentration within the sierran crust and produces relatively little of the heat in high heat production rocks because silica content is relatively constant through the upper 30 km of the sierran batholith while u th and k concentrations are highly variable radiogenic heat production does not vary directly with silica content d 2005 elsevier b v all rights reserved

stable isotope study of the archaean rocks of the vredefort impact structure central kaapvaal craton south africa the vredefort dome in the kaapvaal craton was formed as a result of the impact of a large meteorite at 2 02 ga the central core of archaean granitic basement rocks is surrounded by a collar of uplifted and overturned strata of the witwatersrand supergroup exposing a substantial depth section of the archaean crust orthogneisses of the core show little variation in whole rock 18o value with the majority being between 8 and 10 with a mean of 9 2 n 35 quartz and feldspar have per mil differences that are consistent with o isotope equilibrium at high temperatures suggesting minimal interaction with fluids during subsequent cooling these data refute previous suggestions that the outer granite gneiss ogg and inlandsee leucogranofels ilg of the core represent middle and lower crust respectively granulite facies greenstone remnants from the ilg have 18o values that are on average 1 5 higher than the ilg host rocks and are unlikely therefore to represent the residuum from the partial melting event that formed the host rock witwatersrand supergroup sedimentary rocks of the collar which were metamorphosed at greenschist to amphibolite facies conditions generally have lower 18o values than the core rocks with a mean value for metapelites of 7 7 n 45 overall through an 20 km thick section of crust there is a general increase in whole rock 18o value with increasing depth this is the reverse of what is normal in the crust largely because the collar rocks have 18o values that are unusually low in comparison with metamorphosed sedimentary rocks worldwide the collar rocks have d values ranging from 35 to 115 average 62 n 29 which are consistent with interaction with water of meteoric origin having a d of about 25 to 45 we suggest that fluid movement through the collar rocks was enhanced by impact induced secondary permeability in the dome structure

heat flow in the witwatersrand basin and environs and its significance for the south african shield geotherm and lithosphere thickness the late archaean early proterozoic witwatersrand basin is situated in the center of the kaapvaal craton south africa two new heat flow determinations and 71 new heat flow estimates permit excellent definition of the heat flow pattern nonvertical heat conduction explains much of the observed local heat flow variability regionally the heat flow in the basin is uniform and the overall mean 51 6 mw m 2 is higher than the mean heat flow for archaean cratons published data indicate that heat generated in the basin s strata 7 5 mw m 2 is largely responsible for the excess heat flow the subsediment heat flux is 43 5 mw m 2 seven new heat flow and heat production determinations in archaean granite domes northwest of the basin average 33 2 mw m 2 and 2 3 0 8 w m 3 respectively contrasted with the heat flux across the floor of the basin the lower heat flow in the domes probably reflects a smaller radiogenic contribution from the crystalline crust below the domes published heat production data from upturned granitic basement exposed in the vredefort dome in the center of the basin indicate that the radioactive contribution from the crust below the basin is 26 5 m w m 2 subtracting this value and the contribution from the strata 7 5 mw m 2 from the basin s surface heat flow 51 mw m 2 indicates that the heat flux through the moho is approximately 17 mw m 2 calculated lithospheric temperatures agree closely with pressure temperature regimes inferred from studies of mantle inclusions in kimberlites in the kaapvaal craton both data sets indicate that the cratonic lithosphere is considerably cooler and thicker than that below an equilibrium ocean implications of the cold thick cratonic lithosphere for understanding the distribution of southern african diamondiferous kimberlites and the density and buoyancy of the lithosphere are discussed

aspects of the dynamic and thermal metamorphic history of the vredefort cryptoexplosion structure implications for its origin abstract the vredefort structure is the oldest and the largest known cryptoexplosion structure on earth an approximately 36 km deep section through the archean sialic crust and the overlying precambrian strata of the kaapvaal craton is exposed in the core of the structure the geology presented in the exposed section includes all the principal metamorphic facies in the crust and records a long and complex thermo tectonic history which dates back to at least 3 5 ga the petrographie and geological observations in the basement rocks indicate that there is a complex interrelationship between the archean geology and the 2 0 ga dynamic and thermal metamorphic overprint some of which are postulated to be indicative of impact processes the dynamic and thermal metamorphic effects do not increase progressively towards the centre of the structure as found at known impact structures in particular dynamic deformation phenomena such as pseudotachylite and planar features in quartz reach maximum intensity in the rocks close to the vredefort discontinuity a brittle ductile shear zone which separates upper crustal amphibolite facies rocks from lower crustal granulites in certain other lithological zones deformation phenomena are noticeably absent or diminished we suggest that changes in the physical and chemical properties of the rocks from margin to centre of the basement may account for the variation in the intensity of the 2 0 ga thermal and dynamic metamorphic effects observed at vredefort in conclusion our overall impression of the vredefort structure is that it is a relic of an ancient meteorite impact crater but that there were thermo tectonic events which occurred both prior to and after the postulated impact event which complicates the interpretation of its origin

investigations and interpretations of the vertical distribution of u th and k south africa and canada investigations of the distribution of u th and k in deep profiles exposed by precambrian doming have been presented for vredefort south africa and have recently been completed for a similar structure near sudbury ontario canada an evaluation of the vredefort data shows evidence of a major crustal repetition of the vertical radioelement distribution with depth a simple explanation is that the south african basement is geochemically layered with repetition occurring from layer to layer this implies a sawtooth or serrated distribution with depth preliminary results from the sudbury investigation show a rise in heat production with distance from the sudbury structure corresponding to decreasing depth in the basement rocks the repetition of the radioelement distribution observed at vredefort does not appear in the sudbury data although th and k make a jump at nearly the same depth the u data show no corresponding increase u loss is suspected in the project area affecting the overall distribution of heat production in the sudbury basement given its complex history and geographic location the material in the sample area may not be representative of the superior province

the depth dependence of heat production in the continental lithosphere derived from seismic velocities an experimentally established relationship enables the conversion of compressional seismic velocity to heat production a the conversion performed for 47 characteristic seismic sections from various tectonic units in central and eastern europe yields a general decrease of a with depth z the pattern of decrease depends on geological age young tectonic units with high heat flow exhibit a more rapid decrease the depth dependence of a does not follow the simple exponential law a z a o exp z d the logarithmic decrement d has the dimension length in general d decreases with depth from 20 30 km at 10 km depth to less than 10 km at the moho in the uppermost 10 km of the crust a certain redistribution of the heat producing radioelements u and th can occur by deep groundwater migration through microcrack networks below the moho heat production attains values which represent only insignificant contributions to heat flow through or to temperature field changes in the lithosphere

seismic refraction studies in the basement of the vredefort structure abstract the 30 mgal positive gravity anomaly in the centre of the basement of the vredefort structure has been a feature of discussion for a number of years to supply constraints on the depth and nature of this body refraction seismic measurements were made on a n s line across the basement anomaly the results support a three fold subdivision of the uppermost structure on the seismic profile granitic gneisses in the north dioritic gneisses in the central sector over the gravity anomaly and carboniferous karoo strata in the southern part of the basement throughout the central section of the refraction profile rocks with p wave velocity of 6 40 6 43 km s dominate the profile whilst to the north and south of this central section basement velocities are all less than 6 0 km s computer travel time simulations of the basic model were carried out using 2 d ray tracing

radioelement concentrations in the deep profile through precambrian basement of the vredefort structure the vredefort basement granite in south africa forms the core of an undomed and overturned sequence of precambrian strata slawson 1976 proposed that the granite core has also been overturned exposing to view a thick section through the archean crust incrasing in depth toward the center of the dome profiles across the vredefort basement provide an opportunity for studying the distribution of u th and k with depth in the upper and middle crust detailed radioelement concentration profiles across the granitic basement showed that 1 the central part of the basement core is depleted in th and u relative to the perimeter and the concentration of th and u falls off regularly from the basement margin inward 2 the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function 3 the felsic rocks of the central basement are characterized by high k u and th u ratios and the ratios k u k th and th u increase from the margin inward and 4 mafic granulites which occur in the central regions of the dome have normal k u k th and th u ratios high grade metamorphic environment resulted in the depletion of bothmore u and th relative to k in the greater proportion of the felsic rocks from the central regions of the vredefort basement depletion of u was far greater than that of th the normal th u k u and k th ratios in mafic rocks in close proximity to the felsic rocks suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the vredefort basement less

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

the time bound character of uranium mineralising processes with special reference to the proterozoic of gondwana abstract it is more useful to consider the time bound character of uranium mineralising processes than the time bound character of uranium deposits this is because present uranium deposits are the result of a series of mineralising steps which occurred over hundreds of millions of years we have identified five periods which are characterised by moderately distinct mineralising processes 1 1 before 2800 ma uranium was fractionated into the upper crust during late archaean early proterozoic cratonisation by magmatic and metamorphic processes the granitic terranes were an important source of uranium for subsequent mineralising events 2 2 2800 2200 ma although the atmosphere was probably not reducing at this time the level of oxygen was sufficiently low for uraninite to be transported and concentrated in sedimentary basins cratonisation was a prerequisite for the formation and preservation of long lived basins rapid burial and reducing conditions during diagenesis were important for preserving the uranitite the uranium was mobilised during diagenesis and metamorphism 3 3 2200 1200 ma after 2000 ma when the oxygen level in the atmosphere increased markedly the movement of uranium in solution as uranyl ions became more important the first marine shelf sequences containing chemical and organic sediments to be deposited on the cratons formed excellent sites for the syn sedimentary diagenetic or metamorphic concentration of uranium in some cases this mineralisation was economic in other cases it may have been just a step in a succession of mineralising events continental clastic sediments which overlie these shelf sediments were excellent hosts for large hydrothermal circulation systems which could remobilise the uranium from either granites or sediments the lack of organic and argillaceous material in these continental sequences resulted in the precipitation of uranium near the basal unconformity where there were reducing sediments 4 4 1200 400 ma with the start of modern plate tectonics there was greater potential for the thermally activated remobilisation of uranium the period 1200 900 ma marked the final mineralisation in some important deposits the pan african and brazilian events 700 400 ma mobilised uranium of uncertain origin and concentrated it in previously deposited marginal marine sediments 5 5 400 0 ma the incorporation of organic material into continental clastic sediments allowed uranyl ions to be precipitated before they reached the sea to form the important sandstone type of deposits any time bound mineralising process may be either the final or an intermediate step in forming an economic uranium deposit which of these it is depends on the subsequent history of the area on whether the uranium is preserved dispersed or reconcentrated that many major uranium deposits occur on late archaean early proterozoic cratons emphasises the importance of the long term preservation of sedimentary sequences even though the uranium may be remobilised several times within the sediments

is there a relationship between seismic velocity and heat production for crustal rocks abstract several investigators proposed an empirical relationship between heat production and seismic velocity for rocks typical of the continental crust these relationships could allow for indirect resolution of the distribution of heat producing elements with depth new heat production data for rock samples from the pikwitonei granulite domain and the sachigo granite greenstone subprovince of the superior province do not follow these relationships indicating that heat production cannot be reliably predicted from seismic velocity these two parameters are not likely to be related as velocity depends upon the major mineral constituents of rocks whereas heat production results from radiogenic components concentrated in both major and trace minerals in rocks

the vredefort structure an introduction and a guide to recent literature abstract the 50 km wide vredefort structure appears to present the eroded roots of an area affected by rapid uplift and violent rock deformation widely regarded as shock deformation on an immense scale the origin of the structure is disputed external impact and internal agencies have been suggested this note introduces the vredefort structure and its geological environment emphasizing information published in the decade prior to the 1987 international workshop on cryptoexplosions and catastrophes in the geological record with a special focus on the vredefort structure this typical cryptoexplosion structure has a central and polygonal basement uplift containing violently deformed and brecciated rocks with the northern half surrounded by steeply dipping and heavily faulted collar strata and then by a rim synclinorium of less deformed strata the gains made in research on all three sectors are noted traverses through the basement from collar to centre effectively penetrate through a 15 km thick section of crust the archaean history of the crustal section has been documented the thermal and fluid inclusion evolution near the time of uplift has been studied and a deep seated co 2 rich gas explosion has been proposed as an alternative to the impact hypothesis for the shock deformation meticulous study of the effects of thermal metamorphism on the shock deformations in the collar strata led to a bold proposal more than one shock event in the evolution of vredefort analysis of the character of the faulting of the collar supports an earlier proposal regarding inward movement of blocks of collar strata renewed research on shatter cone orientations in the collar strata and two detailed studies of the structural history within the rim synclinorium are noted through the intensive mapping geophysical exploration and drilling which have been completed within the witwatersrand and transvaal basins close spatial and temporal connections between vredefort and the bushveld complex have been documented in both cases basin centres were intruded by horizontal mafic eruptives and basin subsidence took place after these eruptives had consolidated and cooled the vredefort structure has long been discussed as a possible deeply eroded remnant of a major cratering event or events the connections of this structure with the bushveld complex carry important implications for such discussion

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

experimental and numerical investigation of local scour around a submerged vertical circular cylinder in steady currents abstract local scour around a submerged vertical circular cylinder in steady currents was studied both experimentally and numerically the physical experiments were conducted for two different cylinder diameters with a range of cylinder height to diameter ratios transient scour depth at the stagnation point upstream edge of the cylinder was measured using the so called conductivity scour probes three dimensional 3d seabed topography around each model cylinder was measured using a laser profiler the effect of the height to diameter ratio on the scour depth was investigated the experimental results show that the scour depth at the stagnation point is independent on cylinder height to diameter ratio when the later is smaller than 2 the increase rate of equilibrium scour depth with cylinder height increases with an increase in shields parameter a three dimensional finite element numerical model is developed for simulating local scour around submerged vertical cylinders steady flow around the cylinder is simulated by solving the reynolds averaged navier stokes rans equations with a k turbulence closure both suspended load and bed load sediment transport rates were included in the model efforts are made to reduce the computational time associated with three dimensional morphological modelling including the use of wall function to avoid resolving the near wall boundary layers large morphological time step and parallel computing techniques scour around a submerged vertical circular cylinder founded on the seabed is simulated and the numerical results are validated against the test data the scour mechanisms around the submerged cylinder are investigated using the numerical model

central pit and dome craters exposing the interiors of ganymede and callisto central pit craters on the large icy satellites ganymede and callisto are an unusual crater class perhaps related to the unusual properties of water ice the detailed morphology and dimensions of pit crater features differ from those of smaller normal complex craters on these two satellites pit crater depths appear to be constant regardless of diameter rim collapse is also restricted relative to that in lunar craters or smaller ganymede craters the floors of pits in craters larger than 60 km are occupied by smooth bright domes pit to crater and dome to crater diameter ratios increase linearly with crater diameter and are indistinguishable on ganymede and callisto and with terrain type or surface longitude the floors and central structures of young bright rayed pit craters such as osiris are covered by a thin uniformly bright impact melt or frost deposit thus domes and pits form rapidly on the time scale of the impact itself rather than by long term postimpact intrusion or extrusion the bright domes in pit craters are most simply explained as the uplift and exposure of relatively ice rich material from depths of 3 5 to 5 km during impact this process is almost directly analogous to the uplift and exposure of anamolous deep seated material observed in terrestrial and lunar craters the unusual pit morphology on icy satellites may be the result of impact into crust that is mechanically much weaker at shallow depth than on rocky bodies such as the moon because crater morphology is strongly dependent on ice rock composition the similarity of pit and dome dimensions on ganymede and callisto indicates that the structure and rheology of the crusts of these bodies down to depths of 10 km are very similar and have been for several b y pit crater morphology indicates that the crusts of both satellites are probably ice rich and differentiated

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

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

heat flow and heat production in the namaqua mobile belt south africa results of heat flow and heat production surveys at 13 localities in the namaqua mobile belt southern africa are presented twelve stations traverse the 1200 1100 m y old gneissic and metamorphic bushmanland and gordonia subprovinces one station is situated in the richtersveld subprovince a 2000 1730 m y old volcano sedimentary domain mildly affected by the 1200 1100 m y old orogeny surface heat flow in the mobile belt ranges from 39 to 81 mw m 2 and averages at 61 11 mw m 2 surface heat production ranges from 0 8 w m 3 in mafic rocks to extreme values of 6 9 12 3 w m 3 in the springbok district western namaqualand high heat production in the springbok district is associated with a thin tabular granitic intrusive and is not representative of the basement the mean heat production of the remaining stations in the mobile belt is 2 3 1 1 w m 3 linear regression analysis of the heat flow heat production data yields a poor fit correlation coefficient 0 62 and imprecise estimates for the intercept 34 6 mw m 2 and slope 11 3 km the eastern margin of the mobile belt has relatively low heat flow and heat production geothermal geological and gravity data suggest a relative depletion of crustal radiogenic heat production in the region however the average namaqua heat flow is higher than worldwide averages for proterozoic terrains there is no geological or geophysical evidence for recent mantle heating below the namaqua mobile belt suggesting that an elevated crustal radioactive component is largely responsible for the elevated heat flow in this respect the mobile belt is similar to several mid late proterozoic heat flow provinces that are characterized by high average surface heat flow and probably have elevated crustal radiogenic heat input these observations and seismic data suggest that the namaqua crust consists of an upper layer that contributes 40 50 on average to the heat flow and a depleted lower layer the inferred heat flow at midcrustal levels is 50 60 of the average surface heat flow the mobile belt s average surface heat flow is 40 85 higher than that from the basement of the adjacent archaean kaapvaal craton and its inferred midcrustal heat flow is similar to the lowest surface values in the craton heat production data from the kaapvaal craton indicate that its upper crust is not depleted of heat sources while enhanced radiogenic heat input from the namaqua upper crust may account for part of the higher than craton heat flow it cannot account for the full excess consequently a larger equilibrium mantle heat flow related to a reduced lithospheric thickness below namaqualand seems to be required by the new data

omori like decay of postseismic velocities following continental earthquakes various mechanisms have been proposed to explain the transient enhanced surface deformation rates following earthquakes unfortunately these different mechanisms can produce very similar surface deformation patterns leading to difficulty in distinguishing between them here we return to the observations themselves and compile near field postseismic velocity measurements following moderate to large continental earthquakes we find that these velocities have a remarkably consistent pattern with velocity inversely proportional to time since the earthquake this suggests that postseismic velocities show an omori like decay and that postseismic displacements increase logarithmically over time these observations are inconsistent with simple linear maxwell or burgers body viscoelastic relaxation mechanisms but are consistent with rate and state frictional afterslip models and power law shear zone models the results imply that near field postseismic surface deformation measurements are primarily the result of fault zone processes and therefore that the inference of lower crustal viscosities from near field postseismic deformation requires care

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

heat flow structure and evolution of the lithosphere of mongolia abstract the geothermal characteristics of mongolia are presented based on heat flow measurements taken in boreholes and at the bottom of lake khubsugul and on heat flow values inferred from 3he 4he isotope ratios in spontaneous spring gases three types of regional heat flow distribution are recognized a narrow linear anomaly in the khubsugul segment of the baikal rift zone an area of regularly varying heat flow controlled by the age of the last tectonomagmatic activity in central mongolia and a zone of anomalously low heat flow in the south mongolian hercynian belt the radiogenic contribution to the terrestrial heat flow is determined a contribution which proved to be different in the southern and northern megablocks of mongolia variations in the thermal regime of the northern central and southern megablocks of mongolia are accounted for by their tectonic evolution

a heat production model of the crust and upper mantle abstract experimental evidence on the relationship between seismic velocity and radiogenic heat production has been used to estimate the vertical distribution of heat sources in the continental lithosphere the derived differentiation rate of the radioelements with depth has been investigated to depths 100 km in various world wide tectonic provinces the results show that more pronounced upward differentiation steeper heat production distribution corresponds to regions of higher heat flow and usually higher crustal temperatures thus crustal distributions of radiogenic heat may be related to tectonic age and crustal evolution the vertical distribution of heat production can be traced even in the subcrustal lithosphere and confirms the upward concentration of a substantial part of the radioelements into the crust at the expense of the upper mantle reflecting the early stages of crustal separation while in the uppermost crust the distribution of the heat sources seems to be controlled by uranium and thorium in the upper mantle it is potassium which controls it data obtained may be important in the geothermal modelling deep temperature calculation and in determinations of moho heat flow

redistribution of u and th in shallow plutonic environments radioactive heat production in the idaho batholith and in the alamosa river stock in the san juan mountains correlates with the degree of hydrothermal alteration of plutons by meteoric groundwater fission track mapping shows that as much as half of the u in whole rock samples of the most intensively altered zone of the alamosa river stock is associated with secondary minerals the hydrothermally altered shallow plutons in the idaho batholith contain significant amounts of u in secondary minerals whereas unaltered deeply emplaced plutons contain virtually all of their u in primary accessory minerals the relative distributions of u and th measured by gamma ray spectrometry the associations of u observed by fission track mapping and a depth dependence in thermally induced hydraulic fracturing suggest a depth dependent geochemical process that could produce an upward increase in the abundance of u and th in batholiths the process adds u and th to chemically receptive secondary minerals during alteration of plutons by convecting groundwater

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

heat production and thermal conductivity of rocks from the pikwitonei sachigo continental cross section central manitoba implications for the thermal structure of archean crust the pikwitonei and sachigo subprovinces of central manitoba provide a cross sectional view of the superior province crust in cross section the upper to mid level crust is composed of synformal greenstone belts surrounded by tonalitic gneisses both of which are intruded by granitoid plutons this crustal structure persists downward into the granulite facies where keels of the greenstone belts can be found to constrain thermal models of the crust we measured heat production and thermal conductivity in 60 rocks from this terrain using standard gamma ray spectrometry and divided bar techniques large vertical and lateral heterogeneities in heat production in the upper crust are evident heat production is high in granites and metasedimentary rocks intermediate in tonalite gneisses and low in the portions of greenstone belts dominated by mafic meta igneous rocks in the deeper granulite facies rocks heat production decreases by a factor of two in the tonalitic gneisses and remains low in the high grad

processed and enhanced gravity and magnetic images over the vredefort structure and their interpretation abstract cryptoexplosion structures are polygonal features which have experienced violent disruption during their formation the vredefort structure is one of the largest known cryptoexplosion structures and the geology is well exposed over its northern half the origin of the violent disruption experienced by the vredefort structure is controversial and many aspects of its history are under intensive study since most of the southern half of the vredefort structure is blanketed by a cover of karoo permian strata geophysical data are essential in any attempt to map the pre karoo geology data sets of the regional gravity and magnetic fields over the vredefort structure have been processed and enhanced with the aid of an image processor the results are presented as a series of images each one enhancing particular attributes of the data sets an interpretation of the geophysical images is presented focusing on the regional geometry and setting of the structure striking features of the geophysical images include a simple concentric geometry and polygonal shape which reflect the distribution of dense and magnetic lithologies within and surrounding the structure the kinematics of the development of a polygonal uplift are discussed inward movement of about seven collar sectors separated by radial faults and uplift of the central sections of each of the sectors may account for the gross polygonal shape pull apart voids formed at kinks in the radial faults might account for the positions of several alkali stocks within the stratified collar of the vredefort structure the enhanced image of the gravity field highlights a se trending gravity low which extends beyond the southeastern quadrant of the vredefort structure this feature forms part of a fundamental nw se trending intracratonic lineament the vredefort axis

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

heat flow pattern at the chicxulub impact crater northern yucatan mexico abstract along an east west profile crossing the chicxulub impact structure in northern yucatan mexico curie depths were obtained from statistical spectral analysis of a grid of aeromagnetic data 256 km wide and 600 km long these depths were corrected for flight height and depth to the sea floor to determine the geothermal gradient assuming a temperature of 580 c for the curie temperature heat flow was then calculated from the geothermal gradients using a value of 2 67 w m k for the mean crustal thermal conductivity the results show a conspicuous heat flow high above on the impact basin in this location the heat flow is 80 mw m 2 approximately available offshore estimates of the depth to the crustal magnetic source bases on the northern yucatan platform and onshore heat flow determination on 8 shallow bore holes and in a 1511 m deep one support the existence of this major high heat flow anomaly associated with the impact crater this high heat flow might be related to the impact through 1 an uplift of the crystalline basement rocks in the center of the crater and 2 impact induced radioactive element concentration into the crust below the impact structure higher thermal conductivities at the lower crust might also play a key role available seismological and thermal property data are compatible with these mechanisms