zircon shrimp dating confirms a palaeoarchaean supracrustal terrain in the southeastern kaapvaal craton southern africa abstract we report shrimp zircon ages for felsic volcanic rocks of the early archaean nondweni greenstone belt ngb located in northern kwazulu natal province south africa the ngb is part of a chain of greenstone belt remnants that occurs south of the well known barberton greenstone belt bgb two samples of felsic volcanic rocks of the basal toggekry formation yielded zircon ages of 3 53 ga whereas significantly younger ages of 3 41 ga were obtained for two samples of felsic rocks from near the top of the witkop formation the latter date now constrains the age of stromatolite like structures in cherts associated with the felsic rocks which probably represent some of the oldest preserved evidence of life on earth the zircon ages also indicate that the ngb correlates well chronologically with the bgb and is one of the oldest volcano sedimentary successions in the southeastern kaapvaal craton geochemical data for the felsic ngb rocks suggest formation by partial melting or crystal fractionation of a mafic source that was contaminated by older crust possibly in a back arc environment initial e nd values for ngb rocks range from 1 2 to 0 3 and suggest involvement of pre greenstone continental crust that was possibly extensive in the southeastern part of the kaapvaal craton during palaeoarchaean times

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

the crustal role of the agulhas plateau southwest indian ocean evidence from seismic profiling summary its key geographical position near the reconstructed centre of the gondwana break up between antarctica south america and africa has brought attention to the agulhas plateau an oceanic plateau in the southwest indian ocean with regard to its crustal nature and origin the majority of previous studies have suggested a dominantly continental origin as part of the project setarap sedimentation and tectonics of the agulhas ridge and agulhas plateau we conducted an extensive seismic survey over the plateau with the aim of solving the questions about its crustal structure origin and role in a plate tectonic reconstruction context in addition to 1550 km of high resolution seismic reflection profiles we recorded deep crustal large offset and wide angle reflection refraction data from an ocean bottom hydrophone obh profile across the southern plateau the reflection data show clear indications of numerous volcanic extrusion centres with a random distribution we are able to date this phase of voluminous volcanism to late cretaceous time a period when numerous other large igneous provinces formed traveltime inversion of the deep crustal obh records reveals an up to 25 km thick crust with velocities between 7 0 and 7 6 km s 1 for the lower 50 70 per cent of its crustal column we do not find indications for continental affinity but rather a predominantly oceanic origin of the agulhas plateau similar to that inferred for the northern kerguelen and ontong java plateaus in late cretaceous time its main crustal growth was controlled by the proximity of spreading centres and by passage over the bouvet hotspot at 80 100 ma

seismic crustal structure of the limpopo mobile belt zimbabwe abstract a 145 km n s seismic traverse was deployed to determine the crustal structure of the limpopo mobile belt in southern zimbabwe and the nature of its northern boundary with the zimbabwean craton rockbursts from south african gold mines to the south and regional seismicity from the kariba south zambia belt to the north were used as seismic sources p wave relative teleseismic residuals were also measured to assess whether any velocity contrast between the craton and the mobile belt extended into the upper mantle interpretation of reduced travel times from the local buchwa iron ore mine blasts which were broadside to the traverse revealed an upper crustal interface in the limpopo mobile belt at a depth of 5 8 0 6 km dividing material with a velocity of about 5 8 km s from that of about 6 4 km s on the craton arrivals from the same source showed a 4 4 0 5 km thick 5 5 km s layer overlying crust of about velocity 6 5 km s p wave arrivals from the regional seismicity were used to construct a crustal cross section absolute crustal thickness was tentatively estimated from the identification of a moho reflection on the mine blast recordings to the south of rutenga the crust thins from around 34 km to 29 km in association with a positive gravity anomaly centred over the late karoo nuanetsi igneous province and karoo tuli syncline north of rutenga to the boundary with the zimbabwean craton the crust is about 34 km thick the craton boundary was found to be a steeply southerly dipping zone associated with high velocity material which could either be deep seated greenstones or mafic material associated with the margin in the region studied this zone divides cratonic crust which was found to be about 40 km thick from that typical of the mobile belt and implies a step in the moho of around 6 km analysis of relative teleseismic residuals showed that the velocity contrasts are not confined to the crust but extend into the uppermost upper mantle with the cratonic lithosphere being about 4 faster than that of the limpopo mobile belt the resolution of the technique is such that it is difficult to ascertain whether these differences are features of precambrian evolution or are due to reactivation of the upper mantle during karoo igneous and tectonic activity

crustal structure of the kaapvaal craton and its significance for early crustal evolution high quality seismic data obtained from a dense broadband array near kimberley south africa exhibit crustal reverberations of remarkable clarity that provide well resolved constraints on the structure of the lowermost crust and moho receiver function analysis of moho conversions and crustal multiples beneath the kimberley array shows that the crust is 35 km thick with an average poisson s ratio of 0 25 the density contrast across the moho is f15 indicating a crustal density about 2 86 gm cc just above the moho appropriate for felsic to intermediate rock compositions analysis of waveform broadening of the crustal reverberation phases suggests that the moho transition can be no more than 0 5 km thick and the total variation in crustal thickness over the 2400 km 2 footprint of the array no more than 1 km waveform and travel time analysis of a large earthquake triggered by deep gold mining operations the welkom mine event some 200 km away from the array yield an average crustal thickness of 35 km along the propagation path between the kimberley array and the event p and s wave velocities for the lowermost crust are modeled to be 6 75 and 3 90 km s respectively with uppermost mantle velocities of 8 2 and 4 79 km s respectively seismograms from the welkom event exhibit theoretically predicted but rarely observed crustal reverberation phases that involve reflection or conversion at the moho correlation between observed and synthetic waveforms and phase amplitudes of the moho reverberations suggests that the crust along the propagation path between source and receiver is highly uniform in both thickness and average seismic velocity and that the moho transition zone is everywhere less than about 2 km thick while the extremely flat moho sharp transition zone and low crustal densities beneath the region of study may date from the time of crustal formation a more geologically plausible interpretation involves extensive crustal melting and ductile flow during the major craton wide ventersdorp tectonomagmatic event near the end of archean time d 2003 elsevier b v all rights reserved

the agulhas plateau structure and evolution of a large igneous province summary large igneous provinces lip are of great interest due to their role in crustal generation magmatic processes and environmental impact the agulhas plateau in the southwest indian ocean off south africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity with new geophysical data from seismic refraction and reflection profiling we are able to present improved evidence for its crustal structure and composition the velocity depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km where three major units can be identified in the crust in our seismic reflection records evidence for volcanic flows on the agulhas plateau can be observed the middle crust is thickened by magmatic intrusions the up to 10 km thick lower crustal body is characterized by high seismic velocities of 7 0 7 6 km s 1 the velocity depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic lips such as the ontong java plateau or the northern kerguelen plateau the total volume of the agulhas plateau was estimated to be 4 10 6 km 3 of which about 10 per cent consists of extruded igneous material we use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material the agulhas plateau was formed as part of a larger lip consisting of the agulhas plateau itself northeast georgia rise and maud rise the formation time of this lip can be estimated between 100 and 94 5 ma

deep structure of the western south african passive margin results of a combined approach of seismic gravity and isostatic investigations abstract the passive margin of the south atlantic shows typical features of a rifted volcanic continental margin encompassing seaward dipping reflectors continental flood basalts and high velocity density lower crust at the continent ocean transition probably emplaced during initial seafloor spreading in the early cretaceous the springbok profile offshore western south africa is a combined transect of reflection and refraction seismic data this paper addresses the analysis of the seismic velocity structure in combination with gravity modelling and isostatic modelling to unravel the crustal structure of the passive continental margin from different perspectives the velocity modelling revealed a segmentation of the margin into three distinct parts of continental transitional and oceanic crust as observed at many volcanic margins the lower crust is characterised by a zone of high velocities with up to 7 4 km s the conjunction with gravity modelling affirms the existence of this body and at the same time substantiated its high densities found to be 3100 kg m 3 both approaches identified the body to have a thickness of about 10 km yet the gravity modelling predicted the transition between the high density body towards less dense material farther west than initially anticipated from velocity modelling and confirmed this density gradient to be a prerequisite to reproduce the observed gravity signal finally isostatic modelling was applied to predict average crustal densities if the margin was isostatically balanced the results imply isostatic equilibrium over large parts of the profile smaller deviations are supposed to be compensated regionally the calculated load distribution along the profile implies that all pressures are hydrostatic beneath a depth of 45 km

pn arrivals and lateral variations of moho geometry beneath the kaapvaal craton abstract pn arrivals from mining induced earthquakes on the edge of the witwatersrand basin show that the p wavespeeds in the uppermost mantle are almost constant throughout most of the kaapvaal craton the presence of only small wavespeed variations allows the use of a simple method of estimating crustal thicknesses below the stations of the kaapvaal broad band network using pn times that has been compared with results from receiver functions one thousand three hundred thirty seven pn arrivals were used to derive crustal thicknesses at 46 stations on the kaapvaal craton the average crustal thicknesses for 19 centrally located stations on each of the northern and southern regions of the craton that yielded well constrained thicknesses were 50 52 0 88 km and 38 07 0 85 km respectively in contrast the corresponding average thicknesses determined from receiver functions were 43 58 0 57 km and 37 58 0 70 km respectively the systematically lower values for receiver functions in the northern part of the kaapvaal craton that was affected by the bushveld magmatism at 2 05 ga suggest that the receiver functions do not enable the petrological crust mantle boundary to be reliably resolved due to variations in composition and metamorphic grade in a mafic lower crust the pn times also suggest pervasive azimuthal anisotropy with maximum wavespeeds of about 8 40 km s at azimuths of about 15 and 217 in the northern and southern regions of the craton respectively and minimum wavespeeds of about 8 25 km s

fine structure of the lowermost crust beneath the kaapvaal craton and its implications for crustal formation and evolution high quality data from a dense seismic array covering an area of approximately 60 40 km2 are used to obtain tight quantitative estimates of the fine scale velocity and density structure of the lowermost crust and the crust mantle boundary moho beneath the kaapvaal craton in the vicinity of kimberley south africa results based on a modified receiver function waveform analysis of moho conversions and crustal reverberation phases show that the crust beneath the array is thin 35 4 km with an average poisson s ratio of 0 254 the minimum s wave velocity contrast across the moho is 17 3 while the contrast in density is 15 4 the density contrast across the moho is particularly diagnostic for an assumed uppermost mantle density beneath kimberley of 3 3 gm cc as determined from mantle xenoliths the density of the lowermost crust is 2 86 gm cc indicating rocks of felsic to intermediate composition analysis of waveform broadening of the crustal reverberation phases relative to that of the direct p wave shows the thickness of the moho transition zone to be less than 0 5 km and the maximum variation in crustal thickness over the region of the array to be less than 1 km the flat and almost perfectly sharp moho together with the absence of a mafic lower crust suggests large scale crustal reworking in the period between crustal formation and the time of cratonic stabilization

the kapuskasing uplift a geological and geophysical synthesis over the past decade the kapuskasing uplift has been the subject of intense geological and geophysical investigation as lithoprobe s window on the deep crustal structure of the archean superior province enigmatic since its recognition as a positive gravity anomaly in 1950 the structure has been variably interpreted as a suture rift transcurrent shear zone or intracratonic thrust diverse studies including geochronology geothermobarometry and various geophysical probes provide a comprehensive three dimensional image of archean 2 75 2 50 ga crustal evolution and proterozoic 2 5 1 1 ga cooling and uplift the data favour an interpretation of the structure as an intracratonic uplift related to hudsonian collision eastward across the southern kapuskasing uplift erosion levels increase from 10 km in the michipicoten greenstone belt through the wawa gneiss domain 10 20 km into granulites 20 30 km of the kapuskasing structural zone juxtaposed against the low grade swayze greenstone belt al

agulhas plateau sw indian ocean new evidence for excessive volcanism a new set of seismic reflection and refraction lines has been interpreted regarding the basement and crustal structure of the southern agulhas plateau a large number of extrusion centres were identified lava flows dip away from those extrusion centres and form subparallel stratified sequences we interpret those extrusion centres as the result of excessive volcanism in course of the separation of the southern agulhas plateau from the maud rise since the sedimentary layers appear to be little affected by the volcanism that episode obviously ceased before onset of sedimentation in late cretaceous times we have not found evidence for continental fragments within overthickened predominantly oceanic crust we therefore propose that the agulhas plateau belongs to the world wide suite of large igneous provinces lip of predominantly oceanic origin

the lithoprobe abitibi grenville transect two billion years of crust formation and recycling in the precambrian shield of canada we summarize the results of lithoprobe studies in the neoarchean southeastern superior province and the mesoproterozoic grenville province in the southeastern precambrian shield of canada through two composite cross sections based on seismic reflection data which define dramatically different styles of crust formation and tectonic accretion in the neoarchean and mesoproterozoic in the neoarchean the structures at the surface are steep with discontinuous and flatter structures at depth much of the crust appears to be juvenile and the predominant process of crustal growth is inferred to have been subduction accretion of primitive crust in a prograding arc system in the mesoproterozoic surface structures are shallow and the seismic character of the crust is continuous over the entire cross section archean parautochthonous rocks and reworked archean crust comprise a very significant proportion of the preserved crust in the mesoproterozoic and provided the backstop to the grenvillian orogeny result

crustal structure of the siberian craton and the west siberian basin an appraisal of existing seismic data abstract we present a digital model sibcrust of the crustal structure of the siberian craton sc and the west siberian basin wsb based on all seismic profiles published since 1960 and sampled with a nominal interval of 50 km data quality is assessed and quantitatively assigned to each profile based on acquisition and interpretation method and completeness of crustal model the database represents major improvement in coverage and resolution and includes depth to moho thickness and average p wave velocity of five crustal layers sediments and upper middle lower and lowermost crust and pn velocity maps and cross sections demonstrate strong crustal heterogeneity which correlates weakly with tectono thermal age and strongly with tectonic setting sedimentary thickness varies from 0 3 km in stable craton to 10 20 km in extended regions typical moho depths are 44 48 km in archean crust and up to 54 km around the anabar shield 40 42 km in proterozoic orogens 35 38 km in extended cratonic crust and 38 42 km in the west siberian basin average crustal vp velocity is similar for the sc and the wsb and shows a bimodal distribution with peaks at ca 5 4 km s in deep sedimentary basins and 6 2 6 6 km s in parts of the wsb and sc exceptionally high basement vp velocities 6 8 7 0 km s at the northern border between the sc and the wsb indicate the presence of magmatic intrusions and are proposed to mark the source zone of the siberian lip the cratonic crust generally consists of three layers and high velocity lowermost crust vp 7 4 km s is observed only locally pn velocities are generally 8 2 km s in the sc and wsb and abnormally high 8 6 8 9 km s around kimberlite fields we discuss the origin of crustal heterogeneity and link it to regional crustal evolution

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

evidence for pre orogenic early devonian rifting in the variscan belt stratigraphy and structure of the palaeozoic cover of the mauges unit upper allochthon armorican massif france the palaeozoic sediments from the mauges unit armorican massif france are the best preserved pre orogenic sequences belonging to the upper allochthon two coherent sequences are identified the southern unit ch teaupanne unit represents the cover of the proterozoic basement and consists of ordovician sediments unconformably overlain by emsian carbonates followed by emsian to earliest eifelian immature sandstones the northern unit tombeau leclerc unit consists of an hirnantian to emsian condensed sequence in reverse position that has been thrust over the southern unit the devonian unconformity is interpreted as evidence for an early devonian extension recorded by normal faults affecting both the early devonian limestones and the underlying ordovician series this crustal extension recorded here for the first time is possibly related to the opening of a back arc basin saint georges sur loire associated with the subduction of an ocean located further south galicia brittany massif central ocean

the significance of u pb zircon dates in lower crustal xenoliths from the southwestern margin of the kaapvaal craton southern africa the combination of high precision id tims u pb geochronology with cathodoluminescence cl imaging and spatially resolved microsampling of zircon provides a high fidelity record of the temporal evolution of samples of the middle and lower crust a suite of xenoliths exhumed in cretaceous kimberlites at the southwestern margin of the kaapvaal craton including eclogite granulite amphibolite and greenschist facies plutonic rocks are the subject of these detailed zircon studies cl imaging demonstrates the complexity of chemical and growth structure in single zircons and highlights the importance of sampling on an intragrain scale to extract end member ages for zircon growth events for these diverse metamorphic rocks linking the zircon chronology to geologic events and processes requires an understanding of the kinetics and thermodynamics of zircon stability during metamorphism using an empirical approach we attempt to relate zircon crystallization dissolution and regrowth to metamorphic conditions through a detailed analysis of zircon chemical zonation patterns mineral inclusions and petrography these lines of evidence indicate the crystallization of zircon under p t conditions ranging from eclogite through greenschist facies we discuss four examples of this approach 1 at the lowest grade of metamorphism examined primary igneous zircon 1900 ma in a syenite xenolith from the klipfontein 08 kimberlite underwent two episodes 1015 and 964 ma of probable fluid mediated resorption and reprecipitation in the greenschist facies 2 an upper amphibolite facies xenolith from the markt kimberlite contains two distinct populations of blocky and fir tree sector zoned zircon which are suggested to date prograde hornblende breakdown reactions 1107 ma and melt infiltration under retrograde metamorphic conditions 1092 ma respectively 3 at granulite facies metamorphic zircon crystallized at 1114 ma during prograde reactions involving hornblende breakdown in a granulite xenolith also from the markt kimberlite later 960 ma zircon resorption followed by minor secondary crystallization occurred in both the amphibolite and granulite xenoliths during retrograde greenschist facies reactions possibly enhanced by fluids 4 in a much older episode of eclogite facies metamorphism recorded in a xenolith from the lovedale kimberlite zircon crystallized with the peak metamorphic mineral assemblage or possibly early on the cooling path in the late archean 2630 ma each of these examples provides unique information regarding the evolution of the kaapvaal craton while the links to metamorphic reactions and p t conditions clarify the interpretation of zircon dates obtained from these samples of the middle to lower crust

plate tectonics on the terrestrial planets abstract plate tectonics is largely controlled by the buoyancy distribution in oceanic lithosphere which correlates well with the lithospheric age buoyancy also depends on compositional layering resulting from pressure release partial melting under mid ocean ridges and this process is sensitive to pressure and temperature conditions which vary strongly between the terrestrial planets and also during the secular cooling histories of the planets in our modelling experiments we have applied a range of values for the gravitational acceleration representing different terrestrial planets potential temperatures representing different times in the history of the planets and surface temperatures in order to investigate under which conditions plate tectonics is a viable mechanism for the cooling of the terrestrial planets in our models we include the effects of mantle temperature on the composition and density of melt products and the thickness of the lithosphere our results show that the onset time of negative buoyancy for oceanic lithosphere is reasonable less than a few hundred million years for potential temperatures below 1500 c for the earth and 1450 c for venus in the reduced gravity field of mars a much thicker stratification is produced and our model indicates that plate tectonics could only operate on reasonable time scales at a potential mantle temperature below about 1300 1400 c

shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains 1 the nature of the lower crust across the southern african shield has been investigated by jointly inverting receiver functions and rayleigh wave group velocities for 89 broadband seismic stations located in botswana south africa and zimbabwe for large parts of both archaean and proterozoic terrains the velocity models obtained from the inversions show shear wave velocities 4 0 km s below 20 30 km depth indicating a predominantly mafic lower crust however for much of the kimberley terrain and adjacent parts of the kheis province and witwatersrand terrain in south africa as well as for the western part of the tokwe terrain in zimbabwe shear wave velocities of 3 9 km s are found below 20 30 km depth indicating an intermediate to felsic lower crust the areas of intermediate to felsic lower crust in south africa coincide with regions where ventersdorp rocks have been preserved suggesting that the more evolved composition of the lower crust may have resulted from crustal reworking and extension during the ventersdorp tectonomagmatic event at c 2 7 ga citation kgaswane e m a a nyblade j julia p h g m dirks r j durrheim and m e pasyanos 2009 shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains j geophys res 114 b12304 doi 10 1029 2008jb006217

seismic reflection and refraction studies of the deep structure of the agulhas bank summary a 12 s two way time seismic reflection profile 46 km in length and straddling the cape seal arch was surveyed on the agulhas bank during 1985 the contact between the marine sediments and the pre mesozoic basement produces a strong reflection at 2 s the folded cape and kaaimans sediments give rise to occasional strong reflections from 2 6 s strongly reflecting segments occur between 9 and 10 s and with a time to depth conversion made using refraction velocities this zone of occasional strong reflections is identified as the moho the section from 6 9 s does not give rise to significant coherent reflections and is considered to represent the archaean crust an analysis of the faults active during gondwana break up revealed by reflection seismology show the agulhas fracture zone to be a divergent wrench fault system

evolution of the petrological and seismic moho implications for the continental crust mantle boundary the chemical and petrological composition of mafic rocks from the lower continental crust are discussed by comparing mafic granulites and meta gabbroic rocks from the ivrea zone and the northern hessian depression nhd xenolith suite both regions contain contrasting types of meta mafic lithologies i former basaltic rocks with trace element patterns ranging from morb iike to subduction related or intra plate tholeutes and u ca and al enriched plagiodase dominated gabbroic rocks showing positive eu anomalies generated by complex deep crustal magmatic processes such as fractionation accumulation of plagiodase and pyroxene and crustal contamination the absence of typical garnet omphadte parageneses in these rocks indicates that the eclogite stability field was not reached during palaeozoic orogenic processes a compilation of experimentally determined p wave velocities and densities for mafic granulites gabbroic rocks eclogites and peridotites is used to evaluate key physical properties of lower crustal mafic rocks during crystal thickening caused by continent continent collision in a step by step scenario it is demonstrated that the position of the seismic moho defined as a first order velocity discontinuity and the petrological moho defined as the boundary between non peridotitic crustal rocks and olivine dominated rocks is not identical for the case that mafic rocks are transformed into edogites at the base of orogenically thickened crust p wave velocities of eclogites largely overlap with those of peridotites although their densities are significantly higher than common upper mantle rocks as a consequence refraction seismic field studies may not detect edogites as crustal rocks this means that the seismic moho detected by refraction seismic field studies appears at the upper boundary between edogites and overlying crustal units since edogites generally have higher densities than peridotites they might be recycled into the deeper lithosphere thereby transferring excess eu into the upper mantle this process could be a due for understanding the negative euanomaly in the upper continental crust which is apparently not balanced quantitatively by the abundance of common mafic crustal rocks

an 1800 km cross section of the lithosphere through the northwestern north american plate lessons from 4 0 billion years of earth s history the lithoprobe slave northern cordillera lithospheric evolution snorcle study across northwestern north america in combination with related crustal studies has been synthesized into an 1800 km long cross section of the lithosphere that is constrained by high resolution geophysical data seismic reflection refraction electromagnetic potential fields and detailed bedrock geology the cross section offers one of the longest continuous profiles of the continental lithosphere anywhere in the world that is constrained by combined geophysical measurements and electromagnetic properties and exposed bedrock geological relationships the primary conclusion of the study is that during all major orogenic episodes recorded from archean to present in that part of earth s lithosphere the crust and perhaps much of the mantle was reorganized and redistributed rather than being differentiated from the mantle at the time of orogenesis the observed subsurface geometries of relict subduction zones accretion

successive midcrustal high grade metamorphic events provide insight into mid archean mountain building along the se margin of the proto kaapvaal craton in this study we investigate the evolution of continental crust at the paleoarchean to mesoarchean boundary by documenting the deposition deep burial and partial melting of metasedimentary gneisses along the se margin of the proto kaapvaal craton successive high grade metamorphic events recorded by the gneisses between ca 3 23 and 3 07 ga not only coincided with the timing of subduction accretion in the adjacent barberton greenstone belt at ca 3 23 3 22 ga but also with discrete pulses of potassic granitic magmatism during differentiation and consolidation of the newly assembled lithosphere mineral equilibria modeling demonstrates that the granulites evolved along pressure temperature p t paths similar to those documented for metamorphism in modern collisional orogens high temperature deformation at ca 3 11 3 07 ga during uplift of the terrane was coaxial with the main ne sw trending structural grain of the barberton greenstone belt and reflects the regional nw se shortening and ne sw orogen parallel extension exhibited by the younger potassic granites we reconcile these features with mesoarchean terrane assembly in barberton via two nw dipping subduction zones the trace of the first is represented by the main terrane boundary within the barberton greenstone belt the second resulted in accretion burial and high temperature metamorphism along the se margin of the kaapvaal craton with the granulites providing valuable insight into the mid to lower crustal response to what appears to have been a protracted accretionary orogenic event

long lasting epeirogenic uplift from mantle plumes and the origin of the southern african plateau 1 we investigate under what conditions the present day long wavelength anomalous elevation 500 m of the southern african plateau can be attributed to mesozoic plume events the anomalous long wavelength topography of the southern african plateau cannot be easily explained by recent heating of the upper mantle as opposed to other areas of the african superswell and geomorphological studies provide few hard constraints on the timing of the uplift a mesozoic origin for the plateau uplift is attractive in that southern africa experienced several large magmatic events in the mesozoic we formulate numerical and scaling models to investigate if plume material ponded beneath cratonic lithosphere in the mesozoic could produce 500 m of present day elevation we find that starting plume heads are ineffective at producing much long lasting uplift because the material spreads into a thin layer tens of km thick the ponded plume material also suppresses secondary convection by having a stable boundary at its base over time heat continues to flow out of the top of the lithosphere and a net imbalance of heat flow in the lithosphere develops resulting in subsidence even after the plume material has cooled to the temperature of the mantle adiabat secondary convection supplies less heat to the lithosphere than is lost upward by conduction the cratonic lithosphere returns to its original thermal structure on a timescale of less than 200 m y even two mantle starting plume heads one at the time of karoo volcanism ca 183 ma and the other at the time of kimberlite eruption ca 80 90 ma in our models cannot produce much 200 m of the present day anomalous elevation however significant uplift can be generated by plume tails provided they linger beneath the lithosphere for 25 30 m y and if the uplift effects of mesozoic plume heads and tails are considered together then it is possible to account for 500 m of present day elevation consequently a mesozoic plume model for plateau uplift in southern africa appears to be a viable model if plume tails delivered heat to the southern african lithosphere over a period of 25 m y or more

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

a seismic refraction investigation of the namaqualand metamorphic complex south africa the namaqualand metamorphic complex forms part of the precambrian shield of southern africa and hosts several major base metal deposits a long line seismic refraction experiment traversing the complex was conducted in order to complement geological and geophysical studies of the structure and evolution of this region seismograms were obtained at approximately 10 km intervals along the 290 km profile seismic ray tracing techniques were used to model the travel time and amplitude data it was found that the namaqualand lower crust is comprised of a substantial proportion of intermediate velocity 6 2 to 6 9 km s rocks these intermediate velocity rocks approach the surface near the midpoint of the profile moho was found to be at a depth of 42 km no correlation was found between the seismic and electrical resistivity models of the crust when the seismic velocity structure of the middle proterozoic namaqualand metamorphic complex is compared with that of adjacent geological provinces it is found to differ significantly from the archaean kaapvaal province but resemble the late proterozoic damara province these differences might be attributed to variations in the principal mechanisms of crustal growth between archaean and proterozoic times

archaean basin margin geology and crustal evolution an east pilbara traverse a palinspastic reconstruction of a 100 km long traverse through archaean rocks of the east pilbara western australia includes new observations of the deformation preceding the now visible greenstone belt pattern the restoration is time calibrated with all available u pb datings between incompletely preserved basin sequences two superposed palaeoarchaean volcano sedimentary basins the coongan and salgash basins are separated by an eastwards time transgressive interface tentatively interpreted as an onlap surface for over 140 ma the basin margin architecture was structurally controlled by superposed extensional growth fault arrays d 1 with associated dyke swarms in a curved pattern spatially not related to that of the actual distribution of granite domes and greenstone belts the basins are interpreted to have formed by collapse after arching above hotspots due to phase transitions by mini subduction of slabs of cooled water saturated basalt towards the base of an originally c 45 km mafic crust at c 3 31 ga the extension was replaced by plate driven regional nw se compression d 2 inferred from nw over se shear and ramp and flat thrusts partly reversing offsets of the d 1 extension the recognition of widespread d 2 pre doming compression is important because it triggered the c 3 18 ga start of formation of the dome and keel pattern d 3 visible today which culminated at c 2 9 ga supplementary material eight figures numbered supplementary material fig a h and a gps list of observation sites are available at https doi org 10 6084 m9 figshare c 3808243

nature of the archean midcrust in the core of the vredefort dome central kaapvaal craton south africa extreme uplift associated with the formation of the 2 02 ga vredefort dome has exposed a substantial cross section through the crystalline early archean basement complex rocks of the kaapvaal craton the rocks comprise polydeformed high grade tonalite trondhjemite granodiorite ttg gneisses migmatites and late tectonic intrusive granitoids that straddle the upper amphibolite to granulite facies transition field petrographic and geochemical data indicate that compositional heterogeneity occurs on a local scale and reflects the migmatitic character of the rocks rather than crustal scale layering as has been previously proposed no evidence has been found to support exposure of either a melt depleted refractory lower crust or an upper crustal batholithic granite layer however the immense volume of granitic leucosome in the rocks suggests that the exposed section represents an intermediate level between these two zones granitic leucosomes in the upper amphibolite facies migmatites appear to be intrusive into the predominantly trondhjemitic host rocks rather than of in situ derivation leucosome compositions in the granulite facies migmatites are more variable ranging from granitic and charnockitic to enderbitic probably reflecting at least some local derivation leucosomes and small granitoid bodies show local scale geochemical variation that can be explained in terms of variable amounts of melt segregation and migration and fractionation of minerals such as k feldspar within the melts

moho reflectivity patterns a comparison of canadian lithoprobe transects abstract the complex nature of the continental moho may be strongly influenced by the tectonic and magmatic history of the lithosphere observations of moho variability from near vertical incidence seismic reflection surveys suggest that it may be possible to correlate moho reflectivity and lithospheric evolution as an initial step in attempting to determine such correlations we qualitatively compare seismic reflection data from canadian lithoprobe transects that sample regions of exceptionally diverse age and tectonic history the seven selected datasets were collected using consistent acquisition parameters facilitating effective and meaningful comparisons among transects the moho reflections exhibit considerable variability however in virtually all recent transects a distinct moho is observed even in regions where lower crust is considered to have stabilized in the early proterozoic or archean this suggests not only that relationships between moho reflectivity and tectonics can be extended into the archean but that tectonic age is poorly correlated with moho reflectivity the qualitative comparisons do not clearly link a specific tectonic process with a distinctive reflection pattern however well defined moho reflections are associated with lower crustal deformational processes including compression transpression and extension this indicates that ductile shear and the rheologies of the lower crust moho transition and upper mantle play important roles in influencing reflective character indistinct moho reflectivity may be associated with regions that have experienced little ductile deformation or with processes that have disrupted reflective laminations however the majority of incoherent moho reflections appear related to signal degradation associated with data acquisition the study demonstrates that the qualitative characteristics of moho reflectivity are difficult to correlate with tectonic history and thus use as an interpretive tool isolating quantitative reflection characteristics of the moho from distortions imposed by the overlying crust is a challenging problem but appears necessary to further understanding of the crust mantle transition

mesozoic exhumation of the southern cape south africa quantified using apatite fission track thermochronology southern africa s topography is distinctive an inland plateau of low relief and high average elevation is separated from a coastal plane of high relief and low average elevation by a steeply dipping escarpment the origin and evolution of this topography is poorly understood because unlike high plateaus elsewhere its development cannot be easily linked to present day compressional plate boundary processes understanding the development of this regional landscape since the break up of gondwana is a first order step towards resolving regional epeirogenesis we present data that quantifies the timing and extent of exhumation across the southern cape escarpment and coastal plane using apatite fission track analysis afta of 25 outcrop samples and 31 samples from three deep boreholes kw1 67 sa1 66 cr1 68 outcrop fission track aft ages are cretaceous and are significantly younger than the stratigraphic ages of their host rocks indicating that the samples have experienced elevated paleotemperatures mean track lengths vary from 11 86 to 14 23 m the lack of cenozoic apatite ages suggests that major cooling was over by the end cretaceous the results for three boreholes situated seaward south of the escarpment indicate an episode of increased denudation in the mid late cretaceous 100 80 ma an earlier episode of increased denudation 140 120 ma is identified from a borehole north of the escarpment thermal modelling indicates a history involving 2 5 3 5 km of denudation in the mid late cretaceous 100 80 ma at a rate of 175 to 125 m ma the aft data suggest that less than 1 km of overburden has been eroded regionally since the late cretaceous 80 ma at a rate of 10 to 15 m ma but do not discount the possibility of minor in relative amplitude episodes of uplift and river incision through the cenozoic the reasons for rapid denudation in these early and mid cretaceous episodes are less clear but may be related to epeirogenic uplift associated with an increase in mantle buoyancy as reflected in two punctuated episodes of alkaline intrusions e g kimberlites across southern africa and contemporaneous formation of two large mafic igneous provinces 130 and 90 ma flanking its continental margins because cenozoic denudation rates are relatively minimal epeirogenic uplift of southern africa and its distinct topography cannot be primarily related to cenozoic mantle processes consistent with the lack of any significant igneous activity across this region during that time

freeboard revisited continental growth crustal thickness change and earth s thermal efficiency abstract the rate of growth of earth s continental crust is commonly estimated from the age distribution of present continental crust and from the geochemistry of ancient mantle derived lavas in this paper i assess the rate of growth from relationships between the volume and thickness of continental crust the thickness of the continental crust is governed by its total volume and by the relative buoyancy of the continental and oceanic lithospheres the buoyancy of the oceanic lithosphere is governed by the amount of melting at mid oceanic ridges and the mean age of the ocean floor which are functions of the mantle temperature and its relationship to global heat loss thus the relationship between continental crustal thickness continental crustal volume and mantle temperature may be specified provided the functional dependence of global heat loss on mantle temperature is known using this relationship i investigate the implications of crustal growth models for changes in continental crustal thickness over time if archean continental crustal volumes had been the same as or similar to modern ones continental crustal thicknesses would have been significantly greater than today this would be reflected in subsequently deeply eroded archean terrains and is inconsistent with the geological record sigmoidal growth models in which significant crustal growth occurred during the proterozoic also require several kilometers of erosion for strong dependence of heat loss on mantle temperature but permit nearly constant continental crustal thicknesses through time if the dependence is weak with more precise information on the erosion of archean terrains it will be possible to constrain both crustal growth models and the nature of the functional dependence of heat loss on mantle temperature

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

strong rocks sustain ancient postorogenic topography in southern africa the cape mountains of southern africa exhibit an alpine like topography in conjunction with some of the lowest denudation rates in the world this presents an exception to the often cited coupling of topography and denudation rates and suggests that steep slopes alone are not sufficient to incite the high denudation rates with which they are commonly associated within the cape mountains slope angles are often in excess of 30 degrees and relief frequently exceeds 1 km yet be 10 based catchment averaged denudation rates vary between 2 32 0 29 m m y and 7 95 0 90 m m y we attribute the maintenance of rugged topography and suppression of denudation rates primarily to the presence of physically robust and chemically inert quartzites that constitute the backbone of the mountains be 10 based bedrock denudation rates on the interfluves of the mountains vary between 1 98 0 23 m m y and 4 61 0 53 m m y the close agreement between the rates of catchment averaged and interfluve denudation indicates topography in steady state these low denudation rates in conjunction with the suggestion of geomorphic stability are in agreement with the low denudation rates 20 m m y estimated for southern africa during the late cenozoic by means of cosmogenic nuclide thermochronology and offshore sedimentation analyses accumulatively these data suggest that the coastal hinterland of the subcontinent may have experienced relative tectonic stability throughout the cenozoic

did bushmanland extensionally unroof namaqualand abstract the namaqua natal orogen evolved between about 2000 and 1000 ma as part of an orogenic system that assembled the rodinian supercontinent and wraps the archaean kaapvaal craton to the south and west the orogen includes the western ultra high temperature sapphirine hercynite granulite facies namaqualand terrane structurally overlain to the north by the low grade high level igneous richtersveld terrane 1900 1700 ma and to the east by the amphibolite facies granite gneiss pb zn rich supracrustal bushmanland terrane which is overthrust from the north along the groothoek thrust by the high pressure hom terrane west of the dextral tantalite valley pofadder shear zone the gently dipping contacts among these terranes have been considered to be thrusts with the exception of the groothoek thrust the contacts are low angle top down to the east or northeast folded extensional detachments with displacements of up to at least 100 km the namaqualand part of the namaqua natal orogen comprises two distinct event sequences a kibaran phase of crustal shortening and thickening and voluminous granitic sheets at about 1200 ma then a namaquan phase of mafic underplating ultra high temperature metamorphism granitic sheets dextral transtension constrictional fabrics and crustal thinning from about 1060 to 1030 ma a dominant constrictional ls fabric was developed in regionally sub horizontal layered pink granite gneisses and metasediments deformed by ene trending folds extensional detachments developed late in the namaquan phase when the ultra high temperature rocks were exhumed in the bushmanland terrane aggeneys metasediments have mainly constrictional fabrics and are deformed by giant upward facing downward closing sheath folds developed during transtension in which lie 1700 ma pb zn sedex deposits like those of broken hill and flin flon the namaqualand terrane may be the largest and oldest extensional core complex on earth the proterozoic era was characterized by transtensional ultra high temperature horizontally layered lineated pink gneiss terrains with corrugating fold hinges roughly parallel with the finite stretching direction

connectivity between the western and eastern limbs of the bushveld complex abstract the mafic layered rocks of the bushveld complex are 6 8 km thick and crop out over an area of 65 000 km 2 previous interpretations of the bouguer gravity anomalies suggested that the intrusion consisted of two totally separate bodies however the mafic sequences in these arcuate western and eastern limbs are remarkably similar with at least six petrologically distinctive layers and sequences being recognisable in both limbs such similarity of sequences in two totally discrete bodies 200 300 km apart is petrologically implausible and it is suggested that they formed within a single lopolithic intrusion all previous bouguer gravity models failed to consider the isostatic response of the crust to emplacement of this huge mass of mafic magma isostatic adjustment as a result of this intrusion would have caused the base of the crust to be depressed by as much as 6 km with this revised whole crustal model it becomes possible to construct a gravity model consistent with observed data which includes a 6 km thick sequence of mafic rocks connecting the western and eastern limbs of the bushveld complex the exact depth at which the mafic rocks of the bushveld complex lie in the centre of the structure cannot be constrained by the gravity data such a first order model is an approximation because there have been subsequent deformation and structural readjustments in the crust some of them probably related to the emplacement of the bushveld complex specifically the observed geometry of the rocks around the crocodile river dennilton marble hall and malope domes suggests that major upwarping of the crust occurred on a variety of scales triggered by emplacement of the bushveld complex

crustal structure of the southern margin of the african continent results from geophysical experiments 1 a number of geophysical onshore and offshore experiments were carried out along a profile across the southern margin of the african plate in the framework of the inkaba yeafrica project refraction seismic experiments show that moho depth decreases rapidly from over 40 km inland to around 30 km at the present coast before gently thinning out toward the agulhas falkland fracture zone which marks the transition zone between the continental and oceanic crust in the region of the abruptly decreasing moho depth in the vicinity of the boundary between the namaqua natal mobile belt and the cape fold belt lower crustal p wave velocities up to 7 4 km s are observed this is interpreted as metabasic lithologies of precambrian age in the namaqua natal mobile belt or mafic intrusions added to the base of the crust by younger magmatism the velocity model for the upper crust has excellent resolution and is consistent with the known geological record a joint interpretation of the velocity model with an electrical conductivity model obtained from magnetotelluric studies makes it possible to correlate a high velocity anomaly north of the center of the beattie magnetic anomaly with a highly resistive body

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

continental crust beneath the agulhas plateau southwest indian ocean the agulhas plateau lies 500 km off the cape of good hope in the southwestern indian ocean acoustic basement beneath the northern one third of this large aseismic structural high has rugged morphology but basement in the south is anomalously smooth excepting a 30 to 90 km wide zone with irregular relief that trends south southwest through the center of the plateau seismic refraction profiles across the southern plateau indicate that the zone of irregular acoustic basement overlies thickened oceanic crust and that continental crust locally thinned and intruded by basalts underlies several regions of smooth acoustic basement recovery of quartzo feldspathic gneisses in dredge hauls confirms the presence of continental crust the smoothness of acoustic basement probably results from erosion perhaps initially subaerial of topographic highs with redeposition and cementation of debris in ponds to form high velocity beds basalt flows and sills also may contribute locally to form smooth basement the rugged basement of the northern plateau appears to be of oceanic origin a plate reconstruction to the time of initial opening of the south atlantic places the continental part of the southern plateau adjacent to the southern edge of the falkland plateau and both abut the western mozambique ridge both the agulhas and falkland plateaus were displaced westward during initial rifting in the early cretaceous formation of an rrr triple junction at the northern edge of the agulhas continental fragment during middle cretaceous time may explain the origin of the rugged thickened oceanic crust beneath the northern plateau as well as the apparent extension of the continental crust and intrusion of basaltic magmas beneath the southern plateau

short term episodicity of archaean plate tectonics by combining geochemical data and geodynamical models evidence is provided to address the existence and style of archaean plate tectonics a topic of vigorous debate for decades using careful analyses of lithostratigraphic archaean assemblages and numerical model results we illustrate that a short term episodic style of subduction was a viable style of tectonics in the early earth modeling results show how due to the low strength of slabs in a hotter earth frequent slab break off events prevented a modern style long lived subduction system and resulted in frequent cessation and re initiation of the subduction process on a typical time scale of a few million years results fit with geochemical observations that suggest frequent alternation of arc style and non arc style volcanism on a similarly short time scale such tectonics could provide the link between early pre plate tectonic style of tectonics or stagnant lid convection and modern style plate tectonics in which short term episodes of proto subduction evolved over time into a longer term more successful style of plate tectonics as mantle temperature decayed

3 5 billion years of reshaped moho southern africa abstract according to some previous studies archean continental crust is on global average apparently thinner than proterozoic crust subsequently the validity of this statement has been questioned to provide an additional perspective on this issue we present analyses of moho signatures derived from recent seismic data along swaths 2000 km in length across southern africa and its flanking ocean the imaged crust has a near continuous age range between ca 0 1 and 3 7 billion years and the seismic data allow direct comparison of moho depths between adjacent archean proterozoic and phanerozoic crust we find no simple secular change in depth to moho over this time period in contrast there is significant variation in depth to moho beneath both archean and proterozoic crust archean crust of southern africa displays as much crustal diversity in thickness as the adjacent proterozoic crust the moho beneath all crustal provinces that we have analysed has been severely altered by tectono metamorphic and igneous processes in many cases more than once and cannot provide unequivocal data for geodynamic models dealing with secular changes in continental crust formation these results and conclusions are similar to those documented along ca 2000 km swaths across the canadian shield recorded by lithoprobe tying the age and character of the precambrian crust of southern africa to their depth diversities is clearly related to manifold processes of tectono thermal surgery subsequent to their origin the details of which are still to be resolved as they are in most precambrian terranes reconstructing pristine moho of the early earth therefore remains a formidable challenge in south africa better knowledge of fossilised archean crustal sections turned on edge such as at the vredefort impact crater for the continental crust and from the barberton greenstone belt for oceanic crust is needed to characterize potential pristine archean moho transitions

crustal structure beneath southern africa and its implications for the formation and evolution of the kaapvaal and zimbabwe cratons the formation of archean crust appears to in volve processes unique to early earth history initial results from receiver function analysis of crustal structure beneath 81 broadband stations deployed across southern africa re vealsignicantdierences inthenatureof thecrustandthe crust mantle boundary between archean and post archean geologic terranes with the notable exception of the colli sional limpopo belt where the crust is thick and the moho complex the crust beneath undisturbed archean craton is typically thin 35 40 km unlayered and characterized byastrongvelocitycontrastacrossarelativelysharpmoho thiscrustalstructurecontrastsmarkedlywiththatbeneath post archeanterranesandbeneatharcheanregionsaected bylarge scaleproterozoicevents thebushveldcomplexand the okwa magondi belts where the crust tends to be rel atively thick 45 50 km and the moho is complex

initiation development and benefits of lithoprobe shaping the direction of earth science research in canada and beyondthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent lithoprobe contribution 1480 lithoprobe is canada s national collaborative multidisciplinary earth science research project established to develop a comprehensive understanding of the evolution of northern north america it is regarded internationally as one of the most successful national geoscience projects ever undertaken part of lithoprobe s success derives from its history and organizational structure a one year phase i program 1984 1985 was highly successful phases ii to v carried the project forward until 2005 when funding terminated lithoprobe was organized as a decentralized research network of multidisciplinary scientific studies and collaborating scientists about 1500 scientific publications were generated lithoprobe s legacy also includes economic and social benefits new and improved understanding of earth history provides petroleum and mining companies with an enhanced knowledge base lithoprobe demonstrated the applicability of high resolution seismic reflection studies to mineral exploration development of a portable seismic refraction recorder and a long period magnetotelluric system led to technology transfer to canadian companies very high resolution seismic reflection studies applied first by lithoprobe are proving effective for exploration for uranium deposits in saskatchewan in the cratonic areas of canada lithoprobe seismic and magnetotelluric studies have provided significant new information relevant to exploration for diamonds on the west coast of canada lithoprobe studies provided data and a framework for better understanding the mega thrust earthquake hazard in the region lithoprobe established a highly effective public outreach strategy that involved the print and electronic media as well as material for educational purposes unequivocally lithoprobe has enhanced the already strong reputation of the earth sciences in canada

azimuthal anisotropy beneath southern africa from very broad band surface wave dispersion measurements summary seismic anisotropy within the lithosphere of cratons preserves an important record of their ancient assembly in southern africa anisotropy across the archean kaapvaal craton and limpopo belt has been detected previously by observations of sks wave splitting because sks splitting measurements lack vertical resolution however the depth distribution of anisotropy has remained uncertain end member interpretations invoked the dominance of either anisotropy in the lithosphere due to the fabric formed by deformation in archean or palaeoproterozoic orogenies or that in the asthenosphere due to the fabric formed by the recent plate motion each with significant geodynamic implications to determine the distribution of anisotropy with depth we measured phase velocities of seismic surface waves between stations of the southern african seismic experiment we applied two complementary measurement approaches very broad band cross correlation and multimode waveform inversion robust rayleigh and love wave dispersion curves were derived for four different subregions of the archean southern africa in a period range from 5 s to 250 400 s rayleigh and 5 s to 100 250 s love depending on the region rayleigh wave anisotropy was determined in each region at periods from 5 s to 150 200 s sampling from the upper crust down to the asthenosphere the jackknife method was used to estimate uncertainties and the f test to verify the statistical significance of anisotropy we detected strong anisotropy with a n s fast propagation azimuth in the upper crust of the limpopo belt we attribute it to aligned cracks formed by the regional e w extensional stress associated with the southward propagation of the east african rift our results show that it is possible to estimate regional stress from short period surface wave anisotropy measured in this study using broad band array recordings of teleseismic surface waves rayleigh wave anisotropy at 70 120 s periods shows that the fabric within the deep mantle lithosphere of the limpopo belt and northern kaapvaal craton is aligned parallel to the archean palaeoproterozoic sutures at block boundaries this confirms that the fabric within the lithosphere created by pervasive ancient deformation is preserved to this day suture parallel fabric is absent however in the deep lithosphere of the western kaapvaal craton suggesting that it was not reworked in the collision with the craton s core either due to its mechanical strength or because the deformation mechanism was different from those that operated in the north anisotropy at periods greater than 120 130 s shows fast directions parallel to the plate motion and indicates shear wave anisotropy in the asthenosphere the depth distribution of anisotropy revealed by surface wave measurements comprises elements of both end member models proposed previously anisotropy in the asthenosphere shows fast propagation directions parallel to the plate motion anisotropy in the limpopo and northern kaapvaal lithosphere shows fast directions parallel to the archean palaeoproterozoic sutures the distribution of sks splitting orientations across southern africa reflects anisotropic fabric both within the lithosphere dominating the splitting beneath the limpopo belt and northern kaapvaal craton and within the asthenosphere dominating beneath the western kaapvaal craton

sediment supply to the orange sedimentary system over the last 150 my an evaluation from sedimentation denudation balance the south african plateau is bordered by passive margin basins preserving the terrigeneous sediment produced during onshore erosion as such these basins potentially provide a record of the variation in onshore elevation and relief over time here we bring new constraints on the uplift and erosion of the southern african plateau over the last 150 ma from the perspective of the stratigraphic architecture of these basins we review published data to quantify the terrigeneous supply eroded in the drainage area and preserved in the basins the novel aspect of our approach is the integration of the evolution of the whole domain in sedimentation i e not only the platform as well as the onshore eroding region along the south african and namibian atlantic margins we determined the long term signal of sedimentary supply at the 10 ma scale from 3d mass balance calculations comparing sedimentary volumes deduced from offshore isopach maps with a particular attention to associated uncertainties on one hand and denudation volumes deduced from thermochronology data on the other we show that in this case the volumes of denudation and of sediments preserved in the basin are consistent both in total and incremental amount the latter is therefore a reliable estimation of the denudation in the associated continental domain subject to certain assumptions such as the drainage area over time the long term signal of sedimentary supply of the south african and namibian atlantic margins shows that the major part of the denudation occurred during the post rift lower cretaceous and the upper cretaceous and subsequently decreased this is not the expected behavior for the simple relaxation of topography acquired during rifting that would show a progressive decrease of the sediment supply over time this suggests a significant rejuvenation of relief during the upper cretaceous 50 ma after the rifting event if erosion is directly linked to elevation or relief variation this implies 2 generations the first before break up i e before 130 ma presumably during rifting and the second during the upper cretaceous

a revised position for the rotated falkland islands microplate the early stages of transform margin formation are associated with crustal fragmentation and block rotation the restricted size of the resultant microcontinental blocks precludes palaeogeographical reconstructions and reliable estimations of the amount of rotation they can undergo an example considered here is the falkland plateau this is located adjacent to the agulhas falkland fracture zone and its westernmost province is the falkland islands microcontinent the position of the plateau and the islands prior to gondwana break up remains contentious this study integrates seismic reflection and gravity data to propose a revised position of the falkland islands microcontinent constrained by 1 the presence of a mega decollement controlling the gondwanide orogen described north of the falkland islands and underneath south africa and the outeniqua basin and 2 the similar architecture of fault networks mapped north of the islands and in the northernmost outeniqua basin this revised position requires a re evaluation of the timing and rate of rotation of the falkland islands microcontinent and affects the expected crustal architecture adjacent to the islands our model yields rotation rates between 5 5 and 8 ma 1 and two potential times for rotation and predicts more unstretched crust beneath the basin east of the falkland islands than previous models

shear zones of the limpopo belt and adjacent granitoid greenstone terranes implications for late archaean collision tectonics in southern africa abstract granulite metamorphism in the 2 65 ga limpopo belt of southern africa is related to crustal thickening due to late archaean collision tectonics the central zone of the belt is a major thrust sheet of distinctive lithological structural and isotopic character separated from adjacent terranes by crustal scale shear zones and the northern and southern marginal zones are up thrust deep level granulite facies terranes of the adjacent cratons geometrically the marginal zones are mirror images and the limpopo belt is essentially symmetrical in n s section conventionally regarded as a n s collisional orogeny the gross structural and lithological differences between the central zone and the marginal zones kinematic data from shear zones and geophysical data showing the three dimensional geometry of the central zone are here used to suggest that the direction of emplacement of the central zone was also the collision direction this was from northeast to southwest with the upthrust outwardly directed marginal zones developing in mechanical response to this collision

south african seismicity april 1997 to april 1999 and regional variations in the crust and uppermost mantle of the kaapvaal craton abstract events induced by deep gold mining activity on the edge of the witwatersrand basin dominate the seismicity of south africa the deployment of 54 broad band seismic stations at 84 separate locations across southern africa between april 1997 and april 1999 kaapvaal network enabled the seismicity of south africa to be better defined over a 2 year period seismic events located by the south african national network and by localized seismic networks deployed in mines or across gold mining areas were used to evaluate earthquake location procedures and to show that the kaapvaal network locates mining induced tremors with an average error of 1 56 0 10 km compared with 9 50 0 36 km for the south african network travel times of seismic events from the mines recorded at the kaapvaal network indicate regional variations in the thickness of the crust but no clearly resolved variations in seismic wavespeeds in the uppermost mantle greater average crustal thicknesses 48 50 km compared with 41 43 km are observed in the northern parts of the kaapvaal craton that were affected by the bushveld magmatism at 2 05 ga estimates of average crustal thickness for the southern part of the kaapvaal craton from receiver functions 38 km agree well with those from refracted arrivals from mining induced earthquakes if the crustal thicknesses below the sources are assumed to be 40 43 km in contrast the average crustal thickness inferred from refracted arrivals for the northern part of the kaapvaal craton is larger by about 7 km 51 km than that inferred from receiver functions 44 km suggesting a thick mafic lower crust of variable seismic properties due to variations in composition and metamorphic grade pn wavespeeds are high 8 3 8 4 km s indicating the presence of highly depleted magnesium rich peridotite throughout the uppermost mantle of the craton seismic pg and sg phases indicate that the upper crust around the witwatersrand basin is comparatively uniform in composition when averaged over several kilometres

a model of plate kinematics in gondwana breakup summary an accurate model of relative plate motions in gondwana breakup is based on visual fitting of seafloor isochrons and fracture zones fz from the riiser larsen sea and mozambique basin used predictively the model precisely locates kinematic markers in the west somali basin which allows the conclusion that the spreading centres in the west somali and mozambique basins and the riiser larsen sea formed parts of the boundary between the same two plates the locations of fz and less well defined isochrons from neighbouring regions are also consistent with their formation on other lengths of this same boundary and with its relocation from the west somali basin and northern natal valley to the west enderby basin and lazarev sea during chron m10n small independently moving plates thus played no role in the breakup of this core part of gondwana in an inversion procedure the data from these areas yield more precise finite rotations that describe the history of the two plates separation breakup is most simply interpreted to have occurred in coincidence with karoo volcanism and a reconstruction based on the rotations shows the lebombo and mateke sabi monoclines and the mozambique and astrid ridges as two sets of conjugate volcanic margins madagascar s pre drift position can be used as a constraint to reassess the positions of india and sri lanka in the supercontinent

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

remote sensing detection of heavy oil through spectral enhancement techniques in the western slope zone of songliao basin china landsat 7 enhanced thematic mapper plus etm imagery was used to identify anomalous areas caused by hydrocarbon leakage in fula erji in the western slope of songliao basin china three image enhancement techniques including principal component analysis pca band ratioing and false color composition fcc were used based on the spectral characteristics of hydrocarbon induced minerals and the eigenvectors of pca the best four principal components pcs 1345 pc3 the third component of the pca on bands 1 3 4 and 5 1357 pc3 3457 pc4 and 123457 pc4 were selected for fcc combined with band ratios of 3 1 4 3 and 7 5 according to the tonal characteristics of the surface near wells and their visual expression four fcc images were acquired for hydrocarbon induced area delineation the result indicates that anomalies are expressed as a faint tone blob and are mostly located in the holocene sediments an analysis of two cross sections traversing the identified anomalies suggests that they are closely related to the anomalous area interpreted from the four fcc images field data of magnetic susceptibility and delta carbonate are consistent with the distribution of the tonal anomalies and suggest a positive spatial association between the identified anomalous area and the hydrocarbon leakage environment

how the crust meets the mantle lithoprobe perspectives on the mohorovicic discontinuity and crust mantle transition application of regional geophysical and geological methods throughout two decades of canada s lithoprobe project provides new opportunities to analyze the mohorovicic discontinuity moho and crust mantle transition the transect format employed during lithoprobe in which 10 specified regions of canada were targeted for approximately a decade each between 1984 and 2003 permitted teams of scientists to focus on geological geophysical and tectonic issues for each transect as a primary objective was to enhance knowledge of the structure of the crust and lithosphere an obvious target in each transect was the nature and origin of the moho and crust mantle transition accordingly the combined results provide new perspectives on the moho and the relationship of the moho to the crust mantle transition perhaps the most important result is that the continental geophysical moho is a deceptively simple feature it has a variety of signatures at different scales that preclude a single universally applicable interpretation in methods that provide large scale information such as regional seismic studies it is a relatively abrupt refraction velocity contrast that often displays a dramatic downward decrease in seismic reflectivity however its origin in a geological or tectonic sense is perhaps best determined by careful analyses of structural details near the geophysical moho which are complex and varied in some areas within canada it appears that the geophysical moho may be old and perhaps remains from the time the crust formed in other areas it appears to be a relatively young feature that was superimposed onto older crustal fabrics

denudational history along a transect across the drakensberg escarpment of southern africa derived from apatite fission track thermochronology 1 the denudational history of a 500 km long transect across the drakensberg escarpment on the high elevation passive margin of se africa is quantified on the basis of thermal history modeling of apatite fission track data for 15 deep borehole samples supplemented by an additional 10 outcrop samples a minimum of 4 5 km of denudation since formation of the margin 130 myr ago is estimated for the coastal zone with a marked early cretaceous episode of accelerated denudation broadly coincident with continental breakup samples from the swartberg borehole sw 1 67 located 30 km seaward of the present position of the drakensberg escarpment indicate a total depth of denudation of 3 1 1 2 km since 91 myr with a phase of accelerated denudation of 2 1 0 9 km at a mean rate of 95 43 m myr between 91 and 69 myr samples from the ladybrand borehole la 1 68 west of the lesotho highlands indicate 1 7 0 5 km of denudation since 78 myr with a phase of accelerated denudation at 82 43 m myr from 78 to 64 myr average denudation rates declined to about 10 m myr during much of the tertiary although the apatite fission track data do not provide any direct constraints on the denudational history of the lesotho highlands interpolation between the two boreholes constrained by geological evidence and extrapolated in situ produced cosmogenic 36cl derived denudation rate estimates suggests a pattern of denudation compatible with numerical modeling studies of escarpment evolution involving rapid river incision seaward of a preexisting inland drainage divide these patterns of denudation are incompatible with constant retreat of the drakensberg escarpment from an initial position near the present coast we suggest that the drakensberg escarpment formed by rapid post breakup river incision seaward of a preexisting drainage divide located just east of the present escarpment location and became pinned at this divide with subsequent retreat rates of only 100 200 m myr

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

multi genetic origin of the continental moho insights from lithoprobe this paper reviews four hypotheses for the origin of the continental crust mantle boundary and discusses seismic parameters with which these hypotheses might be tested the relict moho hypothesis posits that the oceanic moho is preserved during continental assembly the magmatic underplating hypothesis posits formation of a new moho by episodic emplacement of sill like intrusive bodies the metamorphic or metasomatic front hypothesis posits that the moho is overprinted by a phase transformation and the regional decollement hypothesis posits that the moho behaves as a structural detachment these hypotheses are not mutually exclusive and examples from the canadian lithoprobe program suggest that all four may be applicable in different regions of north america comparison of seismic images from a fossil subduction zone with modern subduction at cascadia suggests that serpentinization of the forearc mantle a previously unrecognized mechanism for overprinting and erasing the reflection moho may have occurred in the proterozoic

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

fine structure of the continental reflection moho comparisons of deep seismic reflection profiles from a variety of tectonic regions that span a large fraction of the earth s history lead to a threefold categorization of reflection patterns near the crust mantle transition type i patterns have no distinct reflections including no reflection moho at all type ii patterns have one or more subhorizontal reflections and type iii patterns have reflections that project from the crust into the mantle subdivisions within each of these are based upon the geometric relationships between the reflection moho and reflections immediately above and in some cases below hence type i examples may have essentially no reflectivity near the crust mantle transition or they may be characterized by crustal reflectivity that fades downward type ii examples may have a single prominent reflection or they may have several subhorizontal reflections type ii patterns may also have crustal reflections that are either listric downward or that appear to be truncated at or near the base of reflectivity type iii examples may have a single prominent reflection that projects below the moho as delineated by seismic refraction data or they may have a series of subparallel dipping reflections more than one type of reflection moho may be visible along a single profile sometimes changing within distances of only a few tens of kilometers this diversity of both geometry and character leads to the conclusion that the variable appearance of the reflection moho is a consequence of variations in the thermal and structural evolution of the crust mantle transition at many scales from a few tens to many hundreds of kilometers

the crustal structure of western europe a thorough understanding of the tectonic processes that formed the con tinental lithosphere under changing geometric and physical conditions through time requires detailed knowledge of the structure of the crust one of the best locations to study this fundamental problem is the northern and western part of europe because it is made up of a number of tectonic provinces ranging in succession from the oldest precambrian areas of fennoscandia to the currently active regions of the mediterranean sea the geology of europe a region extending from the northern tip of scandinavia north cape southward to north africa encompasses the pre 2500 ma archean craton in the northernmost part of the baltic or fennoscandian shield the proterozoic paleozoic and cenozoic pro vinces of northern and central europe that have been added on to this nucleus and the active transition zone between the eurasian and african plates in the mediterranean area cf figure 1 from a tectonic geologic point of view europe can be divided into four main units that cover the whole time span of the history of the continental crust

tectonic model for the evolution of the limpopo belt abstract constraints on which any model for the tectonic evolution of the limpopo belt must be based include the following data crustal thickening to at least 65 km between 2700 and 2650 ma ago was responsible for the formation of the granulite terrane exposed in the limpopo belt today this crustal thickening probably resulted from the thrusting of the kaapvaal craton over the zimbabwe craton along the south dipping triangle tuli sabi shear zone other northwardverging thrusts related to this event occur in the kaapvaal craton south of the limpopo belt in the central and southern marginal zones and the kaapvaal craton this shortening is directed to the southwest while in the northern marginal zone it was directed to the north northwest peak metamorphism was superimposed on the thickened crust and then nearly isothermal decompression of several kbars occurred throughout the belt during this decompression rocks moved upward and spread outward onto the adjacent cratons from the zone of thickened crust along several inward directed shear zones creating a regional pop up high grade rocks were thrust over low grade rocks on the cratons producing along the terrane boundary retrograde metamorphism in the limpopo belt and prograde metamorphism on the cratons

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

seismic studies in the limpopo belt abstract the structure of the archaean limpopo belt of southern africa has been investigated by means of a deep seismic reflection profile and the recording of the traveltimes of mining induced tremors at regional distances interpretation of the traveltimes suggests that a seismic low velocity zone exists in the upper mantle of the northern kaapvaal craton and limpopo belt beginning at a depth of about 42 km the seismic reflection profile provides evidence for a northward dip of the contact between the kaapvaal craton and the southern marginal zone of the limpopo belt no mid crustal decollement zone is detected the tectonic model of van reenen et al 1990 is in accord with these observations the lower crust is essentially seismically transparent and no reflection moho is observed

geology and tectonostratigraphy of the onverwacht suite barberton greenstone belt south africa abstract a synthesis of new and existing data reveals that at least seven major shear zones and an unconformity separate the rocks of the southern barberton greenstone belt into seven units with different geological histories current views that these units constitute a continuous stratigraphic sequence and their subdivision into successive formations cannot be maintained we argue here that the status of the traditional formations of the onverwacht group of the barberton greenstone belt be changed to complexes and collectively these complexes be referred to as the onverwacht suite the total age range and tectonostratigraphic thickness of the suite is about 120 million years and 15 km respectively the precise age range of the rocks present in each complex is largely unknown the original spatial relationships between the complexes can in most cases only be inferred all of the complexes include significant volumes of intrusions that were emplaced into the volcanic and volcaniclastic host rocks the ages of the intrusions in the different complexes vary in some complexes the intrusions and volcanic rocks have similar geochemistry and are probably co magmatic others provide stitching ages between complexes four of the complexes display internal relationships that resemble ophiolites basaltic pillowed and massive lavas are the most common rock types sheeted dykes are generally absent whilst multiple mafic sills are common and dominate one of the complexes the volcanology and geochemistry of the complexes suggest that they formed predominantly in oceanic spreading and intra oceanic arc environments at water depths of 1 7 4 km the complexes were tectonically stacked during the final stages of their development and during two subsequent orogenic events one complex the hooggenoeg complex was uplifted and deeply eroded between about 3460 and 3470 ma with a minimum uplift rate ca 0 5 mm yr similar to that during emplacement of phanerozoic ophiolites unconformably overlying sub aerial coarse clastic sedimentary rocks part of the newly distinguished noisy complex overlap in age with felsic intermediate igneous activity dated between 3445 and 3460 ma that in turn is genetically related to tonalite trondjemite plutons exposed farther to the south the emergence of the deep water hooggenoeg complex above sea level the start of its erosion and fluvial sedimentation on top of it at ca 3458 ma represents the onset of its obduction onto older complexes some of which are now preserved as enclaves in the granitoid terrains to the south other oceanic slabs the kromberg and the mendon complexes were subsequently emplaced onto the noisy complex during latitudinal displacements ca 12 cm yr that were not notably faster than today exhumation with up to 18 km of unroofing along the southern margin of the greenstone belt took place during a second orogenic episode of subduction and accretion between 3140 and 3230 ma this episode was associated with folding thrusting and molasse type sedimentation within the greenstone belt coupled to extension and ductile faulting at a lower crustal level during this period younger complexes were assembled and the older complexes re cycled and eroded at rates and with geotherms comparable to those in modern orogenic belts in the barberton region between 3 2 and 3 5 ga tectonic processes appear to have operated at rates similar to those within modern evolving plate boundaries

an integration of magnetic gravity and seismic data in evaluating the algoa basin in the eastern cape province of south africa for stratigraphic and structural geodynamics the algoa basin is a hydrocarbon potential basin which lies off the southern continental margin of south africa along the coastal region of port elizabeth area with both onshore and offshore components despite the continued consensus on the basin s hydrocarbon potential both its structural and stratigraphic geodynamics are still not well documented in this study gravity data constrained by borehole data was modelled to reveal onshore subsurface geology whereas the seismic sections were used for offshore in addition magnetic data was used to delineate magnetic zones and lineaments the onshore magnetic results show the localised occurrence of large scale geologic bodies with high and positive anomalies forming bounding rims coincidental with the edges of the basin the average depths to the top of the shallowest and deepest magnetic sources were estimated from the power spectrum to be approximately 0 4 0 6 km and 9 5 km respectively the onshore bouguer gravity map is characterised by a gravity low zone over the basin s sediment where it attains its lowest value over the depocentre 2 d gravity modelling results along five profiles show that the basin comprises of two to three v shaped sub basins of shallow crustal depth with sediment thickness of up to 4 6 km the models also revealed discontinuity in sediment units confirming the diachronous nature of the uitenhage group analyses of offshore seven seismic sections show that the stratigraphic sequence consists of diachronous geological units which are characterised by displacements forming a series of minor and major faults complementary to onshore gravity models seismic sections also shows relatively shallow vertical extension to the paleozoic crustal basement between 1 3 and 5 3 km

the thabazimbi murchison lineament of the kaapvaal craton south africa 2700 ma of episodic deformation the thabazimbi murchison lineament is a prominent tectonic feature extending for approximately 500 km in a nne ssw direction across the kaapvaal craton the lineament has a tectonic history of more than 2 5 ga because it appears to form and or influence the boundary between the archaean granite greenstone terrain of the kaapvaal craton and the southern margin of the high grade terrain of the limpopo mobile belt the northern margin of the archaean proterozoic transvaal basin the southern margin of the mesoproterozoic waterberg basin and has probably also influenced the emplacement of the mesoproterozoic bushveld complex mesozoic karoo basalts and sediments formed the springbok flats these lavas erupted into half graben structures formed by the normal reactivation of the previously strike slip zebedelia fault part of the thabazimbi murchison relay system the lineament is cut by a mesozoic dyke swarm associated with the break up of gondwana

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

southern african crustal evolution and composition constraints from receiver function studies 1 stacking of approximately 1500 radial receiver functions recorded at about 80 broadband seismic stations deployed in southern africa reveals systematic spatial variations in the ratio of crustal p and s wave velocities crustal thickness h and the amplitude of the converted moho phases r the eastern zimbabwe and the southern kaapvaal cratons are characterized by small h 38 km small 1 73 and large r 0 15 values suggesting that the relatively undisturbed archean crust beneath southern africa is separated from the mantle by a sharp moho and is felsic in composition the limpopo belt which was created by a collisional event at 2 7 ga displays large h 43 km but similar and r values relative to the cratonic areas the bushveld mafic intrusion complex and its surrounding areas show large 1 78 large h 43 km and small r 0 11 values reflecting the intrusion of mafic material into the original crust as a result of the bushveld event at 2 05 ga excluding the bushveld the spatially consistent and age independent low accentuate the difference between felsic crustal composition and more mafic island arcs that are thought to be the likely source of continental material within such an island arc model our data combined with xenolith data excluding mantle delamination in cratonic environments suggest that the modification to a felsic composition e g by the partial melting of basalt and removal of residue by delamination is restricted to have occurred during the collision between the arcs and the continent

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

crustal structure of precambrian terranes in the southern african subcontinent with implications for secular variation in crustal genesis summary new estimates of crustal thickness poisson s ratio and crustal shear wave velocity have been obtained for 39 stations in angola botswana the democratic republic of congo malawi mozambique namibia rwanda tanzania and zambia by modellingp wave receiver functions using the h stacking method and jointly inverting the receiver functions with rayleigh wave phase and group velocities these estimates combined with similar results from previous studies have been examined for secular trends in precambrian crustal structure within the southern african subcontinent in both archean and proterozoic terranes we find similar moho depths 38 39 3k msd standard deviation crustal poisson s ratio 0 26 0 01 sd mean crustal shear wave velocity 3 7 0 1 km s 1 sd and amounts of heterogeneity in the thickness of the mafic lower crust as defined by shear wave velocities 4 0 km s 1 in addition the amount of variability in these crustal parameters is similar within each individual age grouping as between age groupings thus the results provide little evidence for secular variation in precambrian crustal structure including between meso and neoarchean crust this finding suggests that 1 continental crustal has been generated by similar processes since the mesoarchean or 2 plate tectonic processes have reworked and modified the crust through time erasing variations in structure resulting from crustal genesis

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

reevaluation of plate motion models based on hotspot tracks in the atlantic and indian oceans abstract plate motion models based on hotspot tracks in the atlantic and indian oceans predict minimal movement less than a few millimeters per year between these hotspots and their counterparts in the pacific ocean for the past 100 m yr whereas plate circuit exercises indicate relative motions of 20 mm yr hotspot based models also suggest that the rajmahal traps india were located 1000 km away from the kerguelen hotspot at 115 ma and the deccan traps india were located a similar distance from the reunion hotspot at 65 ma this is at odds with conclusions derived from paleomagnetism plate circuits and geochemical parameters that suggest a genetic link between flood basalt provinces in india and hotspots in the indian ocean these divergent views may be explained by plume action 1000 km from its center or errors in the hotspot motion models the latter hypothesis is scrutinized in this article by examination of the radiometric ages for hotspot tracks in the atlantic and indian oceans the