archaean oil migration in the witwatersrand basin of south africa abstract the late archaean witwatersrand supergroup of south africa hosts the largest known gold uranium pyrite ore deposits oil preserved in fluid inclusions in quartz grains in siliciclastic sedimentary rocks of that supergroup implies that hydrocarbon generation and migration occurred during the archaean and may have been involved in mineralization processes through reference to phanerozoic analogues oil bearing fluid inclusions entrapped in healed microfractures in detrital quartz grains and in early syntaxial quartz overgrowths imply that the onset of oil migration coincided with early to intermediate stages of burial while intra granular porosity was still preserved multiple generations of oil migration are indicated by i oil inclusions within early diagenetic cements at different levels in the stratigraphic succession ii more than one type of oil in entrapment sites iii oil entrapment in multiple stages of the quartz paragenetic sequence oil generation and migration are considered to have occurred throughout and for some considerable time after development of the witwatersrand basin consistent with progressive burial and kerogen maturation in more than one tectonic regime oil bearing fluid inclusions within detrital sandstone fragments suggest that oil migration also occurred in a sedimentary succession on the kaapvaal craton prior to 2 9 ga oil in the witwatersrand supergroup was most likely derived from multiple source areas with the principal source probably being shales within the lower witwatersrand supergroup the hydrocarbon migration history of the basin has important implications for understanding the textural relationship between gold bituminized oil and uraninite in the giant gold uranium pyrite ore deposits

rheological differences between archaean and younger crust can determine rates of quaternary vertical motions revealed by fluvial geomorphology surface uplift is widely observed in subaerial regions of continental crust that are distant from plate boundaries often revealed by river terrace staircases with altitude ranges of several hundred metres which typically span the quaternary this characteristic geomorphology has been explained by forcing of flow in the weak lower crustal layer by surface processes non steady state erosion and cyclical loading by ice sheets and sea level fluctuations this study tests this interpretation by showing that analogous river terrace staircases are absent in archaean crust which lacks the weak lower crustal layer found elsewhere

terrestrial heat flow in botswana and namibia we report terrestrial heat flow measurements from 25 new sites in botswana and namibia twelve sites from the archean kaapvaal zimbabwe craton and the limpopo belt have a mean of 47 4 mw m 2 standard error of the mean but show a trend of increasing heat flow from less than 40 mw m 2 within the cratonic nucleus to about 60 mw m 2 at its edge thirteen sites in the proterozoic and pan african mobile belts that surround the craton have a mean of 66 3 mw m 2 increasing from about 60 mw m 2 at the cratonic margin to more than 70 mw m 2 several hundred kilometers away radiogenic heat production in near surface basement rocks was measured for four sites on the craton and seven sites in the mobile belts the limited heat flow heat production data from the craton are insufficient to establish a correlation heat flow and heat production data from the mobile belts when combined with published data from six sites in zambia exhibit a linear trend with an intercept of 50 3 mw m 2 and a slope of 5 5 1 1 km some but not all of the observed difference in surface heat flow between the mobile belts and the cratonic nucleus may be due to differences in crustal heat production that part of the regional variation in surface heat flow which originates at greater depths implies a fundamental difference in thermal structure of the lithosphere between the craton and the mobile belts in southern africa

kimberlite u th he dating links surface erosion with lithospheric heating thinning and metasomatism in the southern african plateau documenting the surface response to processes at depth is a central challenge in continental tectonics kimberlites bear an unusual record of both surface and deep processes through their downrafted crustal clasts pipe erosion levels and mantle xenolith suites here we combine new apatite u th he ahe data from four south africa kimberlites ranging in emplacement age from ca 143 ma to ca 74 ma with a wealth of other geologic information from the pipes to resolve the timing patterns and causes of erosion across 200 km of the southern african plateau comparison of the ahe dates with the kimberlite eruption ages constrains a strong regional unroofing pulse ca 117 90 ma which was most intense from ca 100 to 90 ma this unroofing phase is synchronous with warming thinning and metasomatism of the mantle lithosphere recorded by xenocrysts and peridotite xenoliths within these same kimberlites these thermochemical changes could be related to still deeper dynamic mantle processes our ahe data appear to directly document the erosional response to surface uplift induced by these various events the ahe results also detect as much as 1 5 km of cenozoic unroofing that is spatially centered on a proposed mid tertiary paleotributary to the orange river suggesting drainage patterns as the cause our results not only tightly refine the surface history of the interior of the southern african plateau but show that kimberlite ahe dating can tie cratonic erosion phases with contemporaneous thermochemical changes in the mantle and thereby point to causative links

thermal hydrous and mechanical states of the mantle transition zone beneath southern africa abstract observations of p to s conversions from the seismic discontinuities near 410 and 660 km depth reveal the shoaling of the 410 km discontinuity and the deepening of the 660 km discontinuity beneath the archaean cratons in southern africa consequently the mantle transition zone is 20 km thicker than beneath post archaean regions and the global average the discontinuity structure combined with tomographically imaged seismic velocities suggests a relatively cold t 100 k and water enriched oh 20 35 saturation transition zone beneath the cratons the thicker than normal transition zone correlates with surface geology and does not follow the track of african plate motion as numerical simulations of small scale convection imply the observations suggest that cold thermal downwelling is not the primary cause of the anomalously thick transition zone beneath the cratons although it may exist in a confined area near the boundary between the kheis thrust belt and the namaqua natal belt if thermal downwelling exists its temperature anomaly at the 660 km discontinuity must be below the levels detectable by the method used in this study t 60 k the spatial correlation between surface geology high velocities beneath the cratons and the transition zone structure indicates that the cratonic keels extend to the base of the upper mantle substantially deeper than most estimates our results demonstrate the need to closely consider the effect of water content on seismic velocities beneath deep continental roots

anthropogenic and geologic influences on subsidence in the vicinity of new orleans louisiana new measurements of ongoing subsidence of land proximal to the city of new orleans louisiana and including areas around the communities of norco and lutcher upriver along the mississippi are reported the rates of vertical motion are derived from interferometric synthetic aperture radar insar applied to uninhabited aerial vehicle synthetic aperture radar uavsar data acquired on 16 june 2009 and 2 july 2012 the subsidence trends are similar to those reported for 2002 2004 in parts of new orleans where observations overlap in particular in michoud the 9th ward and chalmette but are measured at much higher spatial resolution 6 m the spatial associations of cumulative surface movements suggest that the most likely drivers of subsidence are groundwater withdrawal and surficial drainage dewatering activities high subsidence rates are observed localized around some major industrial facilities and can affect nearby flood control infrastructure substantial subsidence is observed to occur rapidly from shallow compaction in highly localized areas which is why it could be missed in subsidence surveys relying on point measurements at limited locations

frozen and active seismic anisotropy beneath southern africa 1 p receiver functions from 23 stations of the sase experiment in southern africa are inverted simultaneously with sks waveforms for azimuthal anisotropy in the upper mantle our analysis resolves the long standing issue of depth dependence and origins of anisotropy beneath southern africa in the uppermost mantle we observe anisotropy with a nearly e w fast direction parallel to the trend of the limpopo belt this anisotropy may be frozen since the archean at a depth of 160 km the fast direction of anisotropy changes to 40 and becomes close to the recent plate motion direction this transition is nearly coincident in depth with activation of dominant glide systems in olivine and with a pronounced change in other properties of the upper mantle another large change in the fast direction of anisotropy corresponds to the previously found low s velocity layer atop the 410 km discontinuity

the thermal regime of south african continental margins abstract we present thermal data from 100 oil exploration boreholes on the continental margins of south africa near surface temperature profiles are constrained with temperatures acquired during reservoir tests corrected bottom hole temperatures and seafloor temperatures the thermal conductivity profiles are estimated from geophysical well logs using a neural network method available data allow us to determine 41 new heat flow values computed from depth ranges greater than 1000 m located on the western and southern continental margins of south africa we believe that the variations in surface heat flow are not controlled by the contribution of the sediments or the basement but rather by the mantle heat flow an upper bound of 30 mw m 2 for the mantle heat flow on the interior of the continent can be inferred from the lowest available estimates located near the coastline on the southern continental margin on both continental margins heat flow values reach a maximum of about 60 mw m 2 100 km away from the shore and consistently decrease further away towards values typically encountered in old oceanic domains 50 mw m 2 this indicates that 1 the mantle heat flow increases over a short distance at the edge of the continent which is consistent with studies on the effect of continents on mantle convection and 2 it may increase to a value greater than heat flow on the adjacent ocean as a possible consequence of the effect of the transition between thin oceanic lithosphere and thick and cold continental lithosphere on mantle convection

geoid anomalies and the structure of continental and oceanic lithospheres geoid anomalies in the wavelength range corresponding to spherical harmonies 6 to 30 are used to bring new constraints on the deep structure of the continental lithosphere on the nature of upper mantle seismic anomalies and on the thermal evolution of the oceanic lithosphere for the shallow lithospheric density anomalies considered here the geoid anomaly is shown to follow approximately the moment law derived from isostasy by a procedure of least squares inversion the geoid over old cratons is found to be equal to that over relatively young 50 myr oceanic lithosphere the oceanic geoid as a function of age follows the expected linear trend with flattening at old ages these results are independent of the degree at which the spherical harmonics are truncated if the tectosphere consisted only of a thick thermal boundary layer marked geoid lows with an amplitude larger than 20 m should be present over cratons in disagreement with the present observation the predicted geoid agrees with the observations if the negative density anomalies associated with a depleted peridotitic layer are present to the bottom of the lithosphere this analysis of the geoid confirms the petrological inferences that the deep roots observed seismically in the top upper mantle form a chemically differentiate reservoir the density of these cold roots is similar to that of mantle material elsewhere because the increase in density resulting from their lower temperature is largely balanced by the decrease in density caused by their depletion analysis in wavelength range corresponding to harmonics 6 to 30 in the oceans confirms that the geoid flattens at old ages and thus supports the view that an important amount of heat is transferred at the base of the lithosphere by small scale convection

late cenozoic uplift of stable continents in a reference frame fixed to south america a reference frame fixed to south america may approximate to an absolute reference frame for the past 100 myr in this frame otherwise stable continents that have moved onto the former positions of oceanic ridges have experienced rapid plateau uplift probably caused by phase changes in the continental mantle and occasional volcanism with time lags of up to 70 myr

the heat flow from the continents the terrestrial heat flow is defined as the quantity of heat escaping per unit time from the earth s interior across each unit area of the earth s solid surface the quantity varies from place to place over the surface of the earth as well as with time throughout earth history the total heat being lost from the earth at a given time is the integral of the heat flow taken over the entire surface of earth the heat flow from the continents is therefore but a part of the larger picture of global heat loss indeed the lesser part when compared to the heat loss through the floor of the ocean basins pollack 1980 sclater et al 1980 davies 1980 this is so not only because of the lesser continental area but also because of the generally smaller heat flow found on the continents than in the ocean basins thus a full discussion of the thermal regime of the earth and the mechanisms by which it exhausts heat from the interior must center on the heat loss through the oceans a topic which lies beyond the scope of this review nonetheless because of the geological antiquity of at least some of their constituent terranes the continents provide the only direct evidence about the thermal tectonic and petrologic processes of the earth at times before the birth of any of the modem ocean basins some two hundred million years ago because inferences about earlier times are drawn wholly from materials now preserved in continents one must be cautious not to overgeneralize about global tectonothermal processes from observations confined to continents

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

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

quaternary sedimentary record in the northern okinawa trough indicates the tectonic control on depositional environment change abstract during the quaternary global sea level was characterized by large fluctuations in amplitude and increased frequency however the eastern marginal seas of china especially the northern east china sea and yellow sea did not experience large scale transgression until the middle pleistocene the zhe min zhejiang fujian uplift was an important barrier in the east china sea preventing transgression of sea water into the northern marginal seas of china but its quaternary evolution is still poorly constrained here we present a thick sand layer record the top age is 416 ka of the integrated ocean drilling program iodp site u1428 from the northern okinawa trough to reconstruct the history of the zhe min uplift since the middle pleistocene provenance analysis indicates that sediments from both sand and imbedded fine grained calcareous rich clay layers at the study site were mainly derived from the yellow river which for the first time indicates that the yellow river sediments could influence the northern okinawa trough since at least the middle pleistocene significant change of lithology at iodp site u1428 at 416 ka was attributed to the large scale subsidence of the zhe min uplift which is probably due to the rapid uplifting of tibet during 0 52 0 38 ma before 416 ka obstacle of the emersed zhe min uplift induced extremely severe reworking and promoted sand ridge formation in the shelf edge of southeastern yellow sea and supplied sand sediments to the northern okinawa trough during transgression and high sea level stages after the subsidence of the zhe min uplift at 416 ka westward of tidal sand ridge and yellow river mouth with the process of shelf transgressions favored fine grained sediments input to the northern okinawa trough even during low sea level stages the flat terrain of the east asian marginal sea shelf and low rainfall amount resulted in low discharge of yellow river and supplied fine grained particles to the northern okinawa trough

a comparison of heat flow from the gondwana continents with velocities of gondwana plates geodynamic implications abstract the velocities of the lithospheric plates generally differ from each other and the cause of the super mobility of the indian subcontinent is an unresolved problem as the lithospheric plates and the sub lithospheric mantle form the two key elements of plate tectonic movement the thermal characteristics of the three constituent plates of gondwanaland i e the indian african and australian landmasses have been compared in this study it has been clearly shown that the mean surface heat flow values q s in those gondwana terrains of india which remained unaffected by tectonic and igneous activity during the last 150 m y i e after the break up of gondwanaland are either equal to or mostly lower by 5 22 mw m 2 than those in similar terrains of australia and even the immobile african continent and that q s values for the terrains of these landmasses which were affected by such activity during the last 150 m y are uniformly equal it is inferred that the indian lithosphere is neither hotter nor thinner than that beneath the above mentioned continents and that the sub lithospheric upper mantle beneath the indian peninsula is not hotter or less viscous than that beneath the african and australian shields these results and inferences evidently imply that the thermal regime and the other related parameters of the continental lithosphere do not contribute to the mobility of the plates to which it is attached that the rapid movement of the indian landmass is not coupled with the thermal characteristics of its lithosphere and that the drag force which the present studies have shown to be insignificant would not have been lower on the base of the indian lithosphere than beneath the african and australian landmasses accordingly the earlier reported postulate that the high velocity of the indian landmass is due to its thin and hot continental lithosphere and less viscous upper mantle is not valid the paleo reconstruction indicates the existence of the reunion and kerguelen plumes one on each side of the indian landmass it is likely that these plumes created in the sub lithospheric mantle favourable conditions which worked together with forces that resulted from slab pull and ridge push to give a high velocity to the indian landmass

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

seismic constraints on dynamics of the mantle of the kaapvaal craton abstract shear wave splitting in the upper mantle of the kaapvaal craton in southern africa is measured by applying the sks technique to the records of a portable array of digital broad band seismographs the data are interpreted in terms of lattice preferred orientation in olivine fast wave polarization directions at all stations of the array are close to the absolute plate motion apm direction of southern africa since the end of the jurassic this alignment is similar to that previously reported for the north american craton and suggests that the large scale component of mantle anisotropy on both sides of the atlantic can be related to resistive drag at the base of the plates a likely depth range of the corresponding deformation is 150 400 km analysis of the mantle converted phases suggests that the high velocity mantle root of the kaapvaal craton which translates coherently with the plate resides in the same depth range the bottom of the root is found at a depth close to 380 km we conclude that the root is deformed by the recent plate motion but the deformations are not strong enough to be seen in the available tomographic models

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

a back projection method for imaging large scale lateral variations of lg coda q with application to continental africa summary a new method is developed which uses back projection tomography to regionalize large scale lateral variations of coda q for lg waves which have traversed long continental paths successful use of this method requires precise and stable single trace measurements of lg coda q xie nuttli 1988 the method converges rapidly and requires minimal computer storage it also allows quantitative estimation of resolution and error in imaging lateral variations of coda q the spatial resolution of this method is limited by the uneven spatial coverage of the data base by our limited knowledge of lg coda generation and by the trade off between the stability and the spatial resolution of the coda q inversion the method is applied to a large set of digital lg coda data from africa where large scale lateral variations of lg coda q are found to correlate well with major tectonic features most of africa is stable and like other stable regions has relatively high coda q values the lowest values of coda q are associated with the african rift system other regions of low q values include the atlas mountains and cape fold belt regions of mesozoic and younger deformation the lateral variation of frequency dependence of lg coda q correlates in most regions with that of q at 1 hz our analysis of the spatial resolution of this method indicates that the resolving power of lg coda q imaging is comparable to that of velocity tomography using long period surface waves using empirical approaches we estimate that uncertainties in lg coda q and its power law frequency dependence parameter are less than about 60 and about 0 2 respectively for most of africa

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

the significance of rb sr ages of biotite and phlogopite for the thermal history of the central and southern marginal zones of the limpopo belt of southern africa and the adjacent portions of the kaapvaal craton abstract generally rb sr biotite and phlogopite ages range from 2050 ma to 1950 ma between the murchison thabazimbi and triangle tuli sabi shear zones of southern africa in the rocks comprising the central and southern marginal zones of the limpopo belt and adjacent portions of the kaapvaal craton in some shear zones however similar ages ocurr as young as 800 ma in contrast within the kaapvaal craton south of the murchison thabazimbi shear zone rb sr biotite ages range between 3150 ma and 2100 ma and show no regular distribution in all cases the ages are interpreted to indicate the time elapsed since the biotite last cooled below its closure temperature approximately 320 c the regular pattern of 2050 ma to 1950 ma rb sr biotite ages north of the murchison thabazimbi shear zone suggests that the present erosional surface approximates a paleo isothermal surface this surface was exposed by uplift and erosion which was completed by 1900 ma to 1800 ma ago when the sedimentary and volcanic rocks of the soutpansberg group were deposited the regional uplift was accommodated by motion on the murchison thabazimbi and triangle tuli sabi shear zones and probably was related to tectonism associated with the emplacement of the anorogenic bushveld and phalaborwa complexes other anorogenic granitic magmatism accompanied uplift the depth represented by the paleo isothermal surface and the magnitude of the uplift that exhumed it cannot be constrained precisely it is suggested however that the majority of the 10 to 15 km by which crust in this area is thinner than that of the adjacent cratons is the manifestation of this uplift

electrical lithosphere beneath the kaapvaal craton southern africa 1 a regional scale magnetotelluric mt experiment across the southern african kaapvaal craton and surrounding terranes called the southern african magnetotelluric experiment samtex has revealed complex structure in the lithospheric mantle large variations in maximum resistivity at depths to 200 250 km relate directly to age and tectonic provenance of surface structures within the central portions of the kaapvaal craton are regions of resistive lithosphere about 230 km thick in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography but thinner than inferred from seismic body wave tomography the mt data are unable to discriminate between a completely dry or slightly damp a few hundred parts per million of water structure within the transitional region at the base of the lithosphere however the structure of the uppermost 150 km of lithosphere is consistent with enhanced but still low conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for kaapvaal xenoliths the electrical lithosphere around the kimberley and premier diamond mines is thinner than the maximum craton thickness found between kimberley and johannesburg pretoria the mantle beneath the bushveld complex is highly conducting at depths around 60 km possible explanations for these high conductivities include graphite or sulphide and or iron metals associated with the bushveld magmatic event we suggest that one of these conductive phases most likely melt related sulphides could electrically connect iron rich garnets in a garnet rich eclogitic composition associated with a relict subduction slab

heat flow and crustal thermal structure in the late archaean closepet granite batholith south india the late archaean closepet granite batholith in south india is exposed at different crustal levels grading from greenschist facies in the north through amphibolite and granulite facies in the south along a 400 km long segment in the dharwar craton two areas pavagada and magadi located in the main mass of the batholith best represent the granitoid of the greenschist and amphibolite facies crustal levels respectively heat flow estimates of 38 mw m 2 from pavagada and 25 mw m 2 from magadi have been obtained through measurements in deep 430 and 445 m and carefully sited boreholes measurements made in four boreholes of opportunity in pavagada area yield a mean heat flow of 39 4 s d mw m 2 which is in good agreement with the estimate from deep borehole the study therefore demonstrates a clear cut heat flow variation concomitant with the crustal levels exposed in the two areas the mean heat production estimates for the greenschist facies and amphibolite facies layers constituting the main mass of the batholith are 2 9 and 1 8 w m 3 respectively the enhanced heat flow in the pavagada area is consistent with the occurrence of a radioelement enriched 2 km thick greenschist facies layer granitoid overlying the granitoid of the amphibolite facies layer which is twice as thick as represented in the magadi area the crustal heat production models indicate similar mantle heat flow estimates in the range 12 14 mw m 2 consistent with the other parts of the greenstone granite gneiss terrain of the dharwar craton

recent deformations of the deep continental root beneath southern africa seismic anisotropy manifested by the splitting of shear waves into two orthogonal polarizations provides evidence for deformation in the earth s upper mantle where such deformation has been inferred beneath precambrian cratons it has been interpreted either as related to recent plate motion1 2 or inherited from the precambrian3 4 here we present data from a portable seismograph array in the kaapvaal craton of south africa which indicate that the inferred direction of mantle flow underneath the array is close to the direction of absolute plate motion for southern africa since the end of the jurassic period such an alignment has also been reported for the north american craton2 suggesting that the flow in both regions is related mainly to shearing of the sublithospheric mantle by the plate above the old continental roots that are likely to exist in this depth range5 6 must therefore be deformed by the plate motion but the deformations are not strong enough to be seen in the presently available seismic tomography data

upper mantle low velocity zone structure beneath the kaapvaal craton from s wave receiver functions summary the southern african plateau is marked by anomalously high elevations reaching 1 2 km above sea level and there is much debate as to whether this topography is compensated by a lower mantle source or by elevated temperatures in the upper mantle in this study we use s wave receiver functions srfs to estimate the lithospheric thickness and sublithospheric mantle velocity structure beneath the kaapvaal craton which forms the core of the plateau to fit the srf data a low velocity zone lvz is required below a 160 km thick lithospheric lid but the lvz is no thicker than 90 km although the lid thickness obtained is thinner than that reported in previous srf studies neither the lid thickness nor the shear velocity decrease 4 5 associated with the lvz is anomalous compared to other cratonic environments therefore we conclude that elevated temperatures in the sublithospheric upper mantle contribute little support to the high elevations in this region of southern africa

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

numerical modeling of impact heating and cooling of the vredefort impact structure large meteorite impacts such as the one that created the vredefort structure in south africa 2 ga ago result in significant heating of the target the temperatures achieved in these events have important implications for post impact metamorphism as well as for the development of hydrothermal systems to investigate the post impact thermal evolution and the size of the vredefort structure we have analyzed impact induced shock heating in numerical simulations of terrestrial impacts by projectiles of a range of sizes thought to be appropriate for creating the vredefort structure when compared with the extent of estimated thermal shock metamorphism observed at different locations around vredefort our model results support our earlier estimates that the original crater was 120 160 km in diameter based on comparison of predicted to observed locations of shock features the simulations demonstrate that only limited shock heating of the target occurs outside the final crater and that the cooling time was at least 0 3 myr but no more than 30 myr

radioelements and heat production of an exposed archaean crustal cross section dharwar craton south india the dharwar craton dc in south india exposes a tilted f30 km thick crustal section equivalent to upper middle and lower archaean continental crust the dc is composed of poly magmatic metamorphic terrains such as the western dharwar craton wdc and eastern dharwar craton edc whose contact is marked by the closepet granite batholith cg in situ gamma ray spectrometric analysis has been carried out to measure k u and th abundance at 1023 sites covering all major rock formations of the dc the data show that the greenschist and amphibolite facies tonalite trondhjemite granodioritic ttg gneisses of the wdc are poorer in radioelements compared to granodioritic gneisses of the edc similarly the wdc granites of the amphibolite facies region have lower abundance indicating their derivation from a depleted lower crust which had suffered an earlier crustal differentiation the elemental data on the gneisses in the wdc and edc do not show a difference between the greenschist and amphibolite facies but there is a marked depletion in the granulite facies the depletion reaches a maximum in the high p granulites in the southernmost part of the dc the elongate cg is an i type calc alkaline metaluminous granite which exposes a f12 km thick batholith where k is remarkably uniform but u and th show several fold increase with decreasing crustal levels heat production was calculated from the radioelemental data for estimating the radiogenic heat contribution of crust to heat flow we arrive at a present day crustal configuration comprising four northerly dipping metamorphic facies layers greenschist amphibolite metasomatized granulite and depleted granulite gross heat production of each layer is computed from heat production of constituent rocks and their abundance the crustal contribution is found to decrease from greenschist to granulite facies regions from 23 to 18 mw m 2 in the wdc 27 to 7 mw m 2 in the edc surface heat flow and the crustal contribution models indicate that mantle heat flow of the wdc is lower 7 10 mw m 2 compared to the edc 17 24 mw m 2 in the greenschist and amphibolite facies regions high mantle heat flow of 29 mw m 2 beneath the depleted granulite facies region of the edc appears anomalous in the dharwar craton

analysis of temperatures in sedimentary basins the michigan basin we develop an analytical and numerical methodology for the analysis of large bottom hole temperature bht data sets from sedimentary basins and test the methodology using temperature stratigraphic and lithologic data from 411 boreholes in the michigan basin least squares estimates of temperature gradients in the formations and lithologies present are calculated as solutions to a large system of linear equations at each borehole the temperature difference between the bottom and top of the hole is represented as a sum of temperature increments through the various formations or lithologies penetrated by the borehole quadratic programming techniques enable bounds to be placed on the gradient solutions in order to suppress or exclude improbable gradient estimates numerical experiments with synthetic data reveal that the estimates of temperature gradients for a given formation or lithology are sensitive to the degree of representation of that unit well represented units have more stable gradient estima

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

u th he thermochronometry constraints on unroofing of the eastern kaapvaal craton and significance for uplift of the southern african plateau the timing and causes of the 1 0 km elevation gain of the southern african plateau since paleozoic time are widely debated we report the first apatite and titanite u th he thermochronometry data for southern africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau the study area encompasses 1500 m of relief within archean basement of the barberton greenstone belt region of the eastern kaapvaal craton titanite dates are neoproterozoic apatite dates are cretaceous with most results clustering at ca 100 ma thermal history simulations confirm mesozoic heating followed by accelerated cooling in mid to late cretaceous time the lower temperature sensitivity of the apatite u th he method relative to previous thermochronometry in southern africa allows tighter constraints on the cenozoic thermal history than past work the data limit cenozoic temperatures east of the escarpment to 35 c and appear best explained by temperatures within a few degrees of the modern surface temperature these results restrict cenozoic unroofing to less than 850 m and permit negligible erosion since the cretaceous if substantial uplift of the southern african plateau occurred in the cenozoic as advocated by some workers then it was not responsible for the majority of post paleozoic unroofing across the eastern escarpment significant mesozoic unroofing is coincident with large igneous province activity kimberlite magmatism and continental rifting within and along the margins of southern africa compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events

mining induced stress transfer and its relation to a text m _w 1 9 seismic event in an ultra deep south african gold mine on 27 december 2007 a text m w 1 9 seismic event occurred within a dyke in the deep level mponeng gold mine south africa from the seismological network of the mine and the one from the japanese german underground acoustic emission research in south africa jaguars group the hypocentral depth 3 509 m focal mechanism and aftershock location were estimated since no mining activity took place in the days before the event dynamic triggering due to blasting can be ruled out as the cause to investigate the hypothesis that stress transfer due to excavation of the gold reef induced the event we set up a small scale 450 times 300 times 310 text m 3 high resolution three dimensional 3d geomechanical numerical model the model consisted of the four different rock units present in the mine quartzite footwall hard lava hanging wall conglomerate gold reef and diorite dykes the numerical solution was computed using a finite element method with a discretised mesh of approximately 10 6 elements the initial stress state of the model is in agreement with in situ data from a neighbouring mine and the step wise excavation was simulated by mass removal from the gold reef the resulting 3d stress tensor and its changes due to mining were analysed based on the coulomb failure stress changes on the fault plane of the event the results show that the seismic event was induced regardless of how the coulomb failure stress changes were calculated and of the uncertainties in the fault plane solution we also used the model to assess the seismic hazard due to the excavation towards the dyke the resulting curve of stress changes shows a significant increase in the last sim 50 text m in front of the dyke indicating that small changes in the mining progress towards the dyke have a substantial impact on the stress transfer

velocity conductivity relations for cratonic lithosphere and their application example of southern africa 1 seismic velocity is a function of bulk vibrational properties of the media whereas electrical resistivity is most often a function of transport properties of an interconnected minor phase in the absence of a minor conducting phase then the two should be inter relatable primarily due to their sensitivity to temperature variation we develop expressions between shear wave velocity and resistivity for varying temperature composition and water content based on knowledge from two kimberlite fields jagersfontein kaapvaal craton and gibeon rehoboth terrane we test the expressions through comparison between a new high resolution regional seismic model derived from surface wave inversion of earthquake data from africa and the surrounding regions and a new electrical image from magnetotelluric mt data recorded in samtex southern african magnetotelluric experiment the data defined robust linear regression between the two is found to be statistically identical to the laboratory defined expression for 40 wt ppm water in olivine cluster analysis defines five clusters that are all geographically distinct and tectonically relate to i fast cold and variably wet kaapvaal craton ii fast and wet central botswana iii slow warm and wet rehoboth terrane iv moderately fast cold and very dry southernmost angola craton and v slow warm and somewhat dry damara belt from the linear regression expression and the mt image we obtain predicted seismic velocity at 100 km and compare it with that from seismic observations the differences between the two demonstrate that the linear relationship between vs and resistivity is appropriate for over 80 of southern africa finally using the regressions for varying water content we infer water content in olivine across southern africa

effects of random horizontal variations in radiogenic heat source distribution on its relationship with heat flow lateral conduction resulting from variations in the radiogenic heat generation is known to modify the heat flow heat generation relation and to bias the evaluation of thickness of radiogenic heat sources in this paper a statistical approach is used to account for a random horizontal variation in radiogenic heat sources whereas their vertical variation is simulated by simple models relationships between spectral densities and autocorrelation functions of heat generation and heat flow are obtained when the horizontal scale length of heat generation distribution is not much larger than the vertical one lateral heat conduction modifies drastically the statistical properties of heat flow distribution and the apparent thickness obtained from the heat flow heat generation plot is highly reduced the procedure is applied to a set of actual heat flow and heat generation data recorded over new hampshire u s a heat production data and especially heat flow data are affected by different types of noise and errors which can be analyzed through their autocorrelation functions the depth scale of heat generation deduced empirically does not seem to have much relation with any geophysical dimension

velocity conductivity relationships for mantle mineral assemblages in archean cratonic lithosphere based on a review of laboratory data and hashin shtrikman extremal bounds article i nfo can mineral physics and mixing theories explain field observations of seismic velocity and electrical conductivity and is there an advantage to combining seismological and electromagnetic techniques these two questions are at the heart of this paper using phenomenologically derived state equations for individual minerals coupled with multi phase hashin shtrikman extremal bound theory we derive the likely shear and compressional velocities and electrical conductivity at three depths 100 km 150 km and 200 km beneath the central part of the slave craton and beneath the kimberley region of the kaapvaal craton based on known petrologically observed mineral abundances and magnesium numbers combined with estimates of temperatures and pressures we demonstrate that there are measurable differences between the physical properties of the two lithospheres for the upper depths primarily due to the different ambient temperature but that differences in velocity are negligibly small at 200 km we also show that there is an advantage to combining seismic and electromagnetic data given that conductivity is exponentially dependent on temperature whereas the shear and bulk moduli have only a linear dependence in cratonic lithospheric rocks focussing on a known discontinuity between harzburgite dominated and lherzolitic mantle in the slave craton at a