brittle tectonic evolution along the western margin of south africa more than 500 myr of continued reactivation abstract the brittle structural history of western south africa has been investigated by remote sensing and field studies to build a conceptual scheme for its 500 ma long evolution paleostress tensors were computed from a significant fault slip dataset and a relative geochronological succession of brittle deformation events was established this was aided by separating in time faulting events through the usage of cretaceous weathering horizons silicified fluvial deposits paleosols and 77 54 ma olivine melilitite plugs as time markers the oldest features recognized formed during four compressional episodes assigned to the neoproterozoic pan african evolution this history is expressed by sub vertical conjugate fracture sets and fits well the inferences derived from remote sensing the greatest compressive direction rotated from nw se to nne ssw and finally to almost e w a subsequent ene wsw oriented extensional episode is associated with the local effects of the opening of the atlantic ocean and was followed by a second ca e w extensional episode linked to the well acknowledged mid cretaceous 115 90 ma event of margin uplift a late santonian 85 83 ma nw se compressive paleostress deformed the late cretaceous sequences and was in turn followed firstly by a renewed episode of ne sw extension and later by ca nne ssw late maastrichtian 69 65 ma shortening the latter is broadly coeval with the emplacement of the gamoep magmatic suite a phase of wnw ese cenozoic extension is assigned to the extensional phase recorded in the okawango delta interpreted as reflecting propagation of the east african rift system into southern africa no stress tensor was computed for the present day wegener anomaly stress field oriented nw se however in situ stress measurements were used to perform slip tendency analysis which indicates that under the currently existing stress conditions wnw ese and nnw sse striking faults are critically stressed and are the most likely reactivated in agreement with the present seismicity

plate tectonic development of the south west indian ocean a revised reconstruction of east antarctica and africa a revised sequence of reconstructions for east antarctica relative to africa is generated from published seafloor spreading and fracture zone anomalies these date from the cretaceous tertiary boundary to the late jurassic and lead to a revised predrift reconstruction for east and west gondwanaland euler pole 1 67 s 35 99 e rotation angle 53 43 this reconstruction derived wholly from marine data places east antarctica adjacent to the lebombo mountains and south of and subparallel to the sabi acid volcanic suite supporting the postulate that these rocks represent mesozoic plate boundaries this refit implies that part of the coastal plain of mozambique is underlain by oceanic or very highly extended continental crust the reconstruction is supported by the alignment of 1 the cape fold belt with its antarctic equivalents and 2 the western limit of pan african reactivation of the mozambique mobile belt with the western limit of the sverdrupfjella metamorphic suite antarctica initially moved along a transform fault lying immediately east of and subparallel to the lebombo mountains the eastern face of the tugela cone the southeastern face of the falkland plateau in its reconstructed position and the eastern flank of the agulhas plateau in its reconstructed position at m10 time a triple junction formed near the tip of the falkland plateau implying that the oldest lineated magnetic anomalies in the eastern weddell sea associated with this triple junction cannot predate m10 at m2 time the mozambique ridge rifted from the astrid ridge in a ridge jump episode such an event may also explain the presence of magnetic anomalies in the southern mozambique basin gunnerus ridge conrad rise and del cano rise are all tectonically associated with the madagascar ridge

on the development of electron microprobe zircon fission track geochronology abstract the fission track method has been applied for decades to quantify rates and timing of processes in the shallow crust the most widely used approach the external detector method involves counting fission tracks a decay product resulting from the spontaneous fission of 238 u and a paired set of induced tracks parent proxy from the thermal neutron irradiation of 235 u we propose an alternate method of dating zircons that utilizes an electron microprobe to directly measure uranium concentration u and image the number of spontaneous fission tracks or etch figures that intersect the surface of the crystal using an electron backscatter detector the electron microprobe fission track ep ft method is used to date six zircon samples four of which are widely used as standards fish canyon tuff peach springs tuff buluk member of the bakata formation tuff tardee rhyolite mt dromedary complex hypabyssal granite and browns park formation tuff all samples yield ages that overlap within two standard deviations of published reference ages determined using other radiometric techniques i e k ar 40 ar 39 ar and u pb and also by the traditional fission track external detector method to correct for poorly constrained parameters such as the spontaneous fission decay constant weight to volume percent conversions etching efficiency and selection bias we calculate a preliminary z proportionality factor of 4469 661 1 the ep ft technique avoids the hazard and expense of thermal neutron irradiation allows simultaneous chemical compositions to be determined removes the step of counting an external detector manually and will likely allow much higher track densities to be counted than would be normally possible with optical microscopy the technique is ideal for dating moderate to high u zircons 100 ppm u

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

influence of crustal heterogeneity on normal fault dimensions and evolution southern south africa extensional system abstract the tectonic configuration of southern south africa is dominated by a continental scale mesozoic extensional system superimposed upon a significant and well constrained palaeozoic lithospheric scale heterogeneity through integrating onshore structural analysis with offshore subsurface studies it is possible to evaluate the applicability of established normal fault growth models in a heterogeneous crustal setting at a basin scale for the first time the 480 km long mesozoic extensional system comprises a number of fault arrays that vary in length from 78 to 230 km coupled with displacements of up to 16 km the fault arrays are amongst the longest and largest displacement of high angle normal faults dips of 45 60 documented in continental lithosphere although comprised of co linear segments rather than the en echelon segment configuration typical of many other extensional systems this atypical geometry is considered to be a consequence of the high degree of structural inheritance between the underlying paleozoic cape fold belt and the subsequent extensional system furthermore it is proposed that the overall displacement length dimensions of the extensional faults were inherited from the underlying compressional faults the establishment of a seismic stratigraphic framework for the pletmos and gamtoos offshore basins reveals that the faults established their long lengths 160 and 90 km respectively within 6 myr of rift initiation prior to accruing their substantial displacement 16 km furthermore there is no evidence for the development of intra basin faults the southern south africa extensional system presents an end member case of structural inheritance where extensional structures are parallel to and reactivate underlying compressional structures in this study structural inheritance is considered to have a significant effect on mechanisms of fault growth the pre existing structures not only result in the rapid establishment of fault lengths of up to 160 km but additionally to strain being localised onto the pre existing fabric to such an extent that no intra basin faults evolve and strain is accommodated entirely on the bounding faults

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

southern african topography and erosion history plumes or plate tectonics the physiography of southern africa comprises a narrow coastal plain separated from an inland plateau by a horseshoe shaped escarpment the interior of the inland plateau is a sedimentary basin the drainage network of southern africa is characterized by three river divides broadly parallel to the coastline these features contrast strongly with the broad dome and radial drainage patterns predicted by models which ascribe the physiography of southern africa to uplift over a deep mantle plume the drainage divides are interpreted as axes of epeirogenic uplift the ages of these axes which young from the margin to the interior correlate closely with major reorganizations of spreading regimes in the oceanic ridges surrounding southern africa suggesting an origin from stresses related to plate motion successive epeirogenic uplifts of southern africa on the axes forming the major river divides initiated cyclic episodes of denudation which are coeval with erosion surfaces recognized elsewhere across africa

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

an attempt to constrain the age duration and eruptive history of the karoo flood basalt naude s nek section south africa we have carried out paleomagnetic sampling of a similar to 750 m sequence of the karoo large igneous province naude s nek pass south africa k ar dating cassignol gillot has been performed on four samples from the 650 m upper unit mean age 179 2 1 8 ma and a sample from the lower unit 184 8 2 6 ma a succession of two phases of volcanism is suggested the lower 25 flows 115 m thick have recorded a reversed polarity the next 23 flows 135 m thick are transitional and contribute a detailed record of the van zijl 1962 jurassic reversal the upper 38 flows 500 m thick have normal polarity directional groups dgs of lava flows with quasi identical remanence directions indicate eruption durations too short to have recorded geomagnetic secular variation and hence are interpreted as single eruptive events altogether 19 dgs and 10 sheet lobes yield a sequence of 29 distinct directions this could correspond to a total eruptive activity shorter than 3000 years less than one per mil of the total duration over which the section was emplaced we obtain a new paleomagnetic pole for south africa at similar to 180 ma lambda 75 2 degrees n phi 276 4 degrees e a 95 5 8 degrees n 19 which is consistent with earlier reports

an early cretaceous extinct spreading center in the northern natal valley abstract we have identified an extinct e w spreading center in the northern natal valley on the basis of magnetic anomalies which was active from chron m11 133 ma to 125 3 ma just before chron m2 124 ma in the early cretaceous seafloor spreading in the northern natal valley accounts for approximately 170 km of north south motion between the mozambique ridge and africa this extension resolves the predicted overlap of the continental central and southern mozambique ridge and antarctica in the chron m2 to m11 reconstructions from mesozoic finite rotation parameters for africa and antarctica in addition the magnetic data reveal that the mozambique ridge was an independent microplate from at least 133 to 125 ma the northern natal valley extinct spreading center connects to the spreading center separating the mozambique basin and the riiser larsen sea to the east it follows that the northern mozambique ridge was either formed after the emplacement of the surrounding oceanic crust or it is the product of a very robust spreading center to the west the extinct spreading center connects to the spreading center separating the southern natal valley and georgia basin via a transform fault prior to chron m11 there is still a problem with the overlap of mozambique ridge if it is assumed to be fixed with respect to either the african or antarctic plates some of the overlap can be accounted for by jurassic deformation of the mozambique ridge mozambique basin and dronning maud land it appears though that the mozambique ridge was an independent microplate from the breakup of gondwana 160 ma until it became part of the african plate 125 ma

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

joint seismic geodynamic mineral physical modelling of african geodynamics a reconciliation of deep mantle convection with surface geophysical constraints recent progress in seismic tomography provides the first complete 3 d images of the combined thermal and chemical anomalies that characterise the unique deep mantle structure below the african continent we present a tomography based model of mantle convection that provides an excellent match to fundamental surface geodynamic constraints on internal density heterogeneity that includes both compositional and thermal contributions where the latter are constrained by mineral physics the application of this thermochemical convection model to the problem of african mantle dynamics yields a reconciliation of both surface gravity and topography anomalies to deep seated mantle flow under the african plate over a wider range of wavelengths than has been possible to date on the basis of these results we predict flow in the african asthenosphere characterised by a clear pattern of focussed upwellings below the major centres of late cenozoic volcanism including the kenya domes hoggar massif cameroon volcanicline cape verde and canary islands the flow predictions also reveal a deep seated large scale active hot upwelling below the western margin of africa under the cape verde islands that extends down to the core mantle boundary the scale and dynamical intensity of this west african superplume is comparable to the south african superplume that has long been assumed to dominate the large scale flow dynamics in the deep mantle under africa we evaluate the plausibility of the predicted asthenospheric flow patterns through a comparison with seismic azimuthal anisotropy derived from independent analyses of african shear wave splitting data

an insight into the breakup of gondwana identifying events through low temperature thermochronology from the basement rocks of madagascar 1 fission track analysis was applied to the precambrian suites of madagascar in order to identify the lower temperature cooling histories and their relationships to the phanerozoic events that affected the island apatite ages range from 431 to 68 ma and zircon ages range from 452 to 238 ma thermochronologically the island can be divided into a southern central and northern region each with a subdivision on an east west basis the southern region is sharply separated from the central region by strongly contrasting apparent apatite ages over the northwest southeast striking ranotsara shear zone rsz the change in apparent ages over the rsz is indicative of later reactivation along younger brittle faults the central region has the oldest ages of the island and has a diffuse contact to the third region northward along the entire western margin of the precambrian basement initial paleozoic exhumation was followed by heating burial by sediments during jurassic and cretaceous times a decrease in ages along the eastern margin from 119 to 68 ma coincides with the predicted positions of the marion hot spot after effects of erosion are considered on the other hand these ages may represent progressive opening of the margin in a southward direction together with associated denudation of the rift shoulder the eastern part of the central region has remained very stable since at least devonian times undergoing only long term very slow exhumation at rates of 1 5 m myr

rapid magma emplacement in the karoo large igneous province abstract understanding the dynamics of continental large igneous provinces lips relies on precise dating of basaltic rocks lip research has traditionally focused on dating lavas often neglecting the volumetrically important sill intrusions in underlying sedimentary basins here we present u pb zircon and baddeleyite ages for fourteen new samples of karoo lip sills and dykes spaced by as much as 1100 km across the half million square kilometer karoo basin the samples yield remarkably coherent ages ranging from 183 0 0 5 to 182 3 0 6 myr probability modeling indicates that basin scale emplacement took place within an interval of about 0 47 myrs less than 0 90 myrs with 95 confidence and could even have represented a single magma emplacement event combining the new ages with the estimated volume of sills in the karoo basin gives an emplacement rate of 0 78 km3 yr which is higher than previous estimates upper crustal magma storage may account for these high rates the results challenge the view that melt emplacement in a sedimentary basin is a prolonged process support a scenario of pulsating catastrophic events within a narrow time frame and strengthens the hypothesis linking lips and sill emplacement to global environmental crises

apatite fission track analysis apatite fission track aft analysis has been widely used during the past 30 years to constrain the low temperature thermal histories of many igneous metamorphic and sedimentary rocks in a wide range of geological settings applicable geological settings include orogenic belts rifted margins faults sedimentary basins cratons and mineral deposits the types of geologic problems that can be addressed include the timing and rates of tectonic events sedimentary basin evolution the timing of hydrocarbon generation and ore mineralization the absolute age of volcanic deposits the effects of major climatic changes on the near surface geothermal gradient and long term landscape evolution early work by naeser 1967 and wagner 1968 1969 first established the basic procedures that enabled the fission track dating method to be applied routinely to these geologic problems fleischer et al 1975 summarized the early studies of the broader discipline of nuclear track detection in different solid state materials more recent comprehensive overviews of fission track applications have been provided by naeser and mcculloh 1989 wagner and van den haute 1992 gallagher et al 1998 van den haute and de corte 1998 dumitru 2000 and gleadow et al 2002 etched natural fission tracks in several apatite grains are shown in figure 1 successful aft analysis is limited by the following 1 the availability of apatite from which useful aft data can be obtained often due to a lack of apatite of sufficient grain size and quality within available rock types or alternatively the lack of available rock samples due to minimal outcrop exposure or other reasons 2 economic considerations in terms of the time and money required to obtain sufficient aft data 3 the inherent limitations of aft data to resolve geological thermal history information often related to limited numbers of accumulated spontaneous fission tracks due to

applicability of thin or thick skinned structural models in a region of multiple inversion episodes southern south africa abstract the deformation of the cape fold belt has been attributed to repeated structural reactivation of a mega detachment from the late proterozoic to the mesozoic 650 65 ma through the integration of onshore cross sections with observations from the offshore mesozoic extensional system this study evaluates the applicability of the mega detachment model regional scale cross sections through the permian triassic cape fold belt reveal that it comprises two main structural domains a northern domain dominated by northward verging and asymmetric folds and a southern domain comprising a series of approximately 8 km wavelength box folds the genesis of these box folds is attributed to motion on underlying high angle 45 reverse faults this variation between north and south in the fold belt is reflected by a similar variation in extensional geometry of the mesozoic normal faults as revealed by subsurface data the normal faults demonstrate a progressive increase in dip from 24 in the north to 60 in the south features commonly attributed to thin and thick skinned tectonic models are observed in both domains therefore it is not appropriate to describe the observed deformation as one of the two end members in addition the structures are inferred to have undergone at least two stages of inversion irrespective of dip this is not predicted by either end member model

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

statistical modelling of annealing kinetics of fission tracks in zircon reassessment of laboratory experiments abstract we remodel the annealing kinetics of fission tracks fts in zircon to fit data from 43 laboratory experiments in three earlier publications using spontaneous ft lengths in zircon from the nisatai dacite heating temperature and time are in the ranges of 350 912 c and 10 3 104 h respectively the data sets illustrate a transition in annealing process at a certain temperature zone in order to express the annealing behavior involving the possible transitional process with improved goodness of fit two types of models are constructed the hybrid linear model that consists of the fanning linear model at high temperature and the parallel linear model at low temperature connected with a transitional temperature zone and the parallel curvilinear model that approximates smoothly the transition in the annealing process the validity of the extrapolation of the model to geological time scale is confirmed by comparison with some deep borehole core data the length estimates by the hybrid linear model agree well with the borehole data whilst the estimates by the parallel curvilinear model show a significant disagreement with highly annealed borehole samples although this model provides good agreement with the experimental data for the hybrid linear model estimates of the partial annealing zone corresponding to isothermal heating for 106 and 107 years are 281 352 c and 262 330 c where the top and bottom of the partial annealing zone are defined by the mean length reduction ratio of 0 4 and 0 8 respectively

geochemical distribution of trace elements in kaffioyra and longyearbyen coals spitsbergen norway abstract paleogene bituminous coals of low rank r aver 0 63 and 0 69 respectively occurring in two spitsbergen norway regions longyearbyen and kaffioyra were analyzed to find occurrence and distribution of the following trace elements be cd co cr cu li mn mo ni pb v zn inductively coupled plasma atomic emission spectrometry icp aes was used for ash derived from combusted coals their extracts and macromolecular fractions trace element concentrations were investigated to find occurrence variability and a degree of trace element affinity to organic and inorganic coal fraction in the coals from both regions maceral and elemental composition of these coals was also determined kaffioyra and longyearbyen coals show significant differences in the composition of their organic and mineral matter reflected both in petrographic properties and elemental compositions the kaffioyra coals are characterized by higher contents of vitrinite group macerals lower contents of inetrinite group macerals and lower concentrations of carbon hydrogen and sulphur the kaffioyra coals show higher content of clay minerals and carbonates than the longyearbyen coals trace element contents in ash of coals from both spitsbergen regions are below the average contents of these elements in the world s bituminous coals coals of both regions differ in trace element concentration and their occurrence variability the main carrier of trace elements in coals of both regions is ash of biogenetic and sorptive origin while in the range of high concentrations of trace elements terrigenic input also plays an important role

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

equating the basal unconformity with the palaeoplain a model for passive margins abstract the palaeoplain is the terrestrial continental surface little changed from the landsurface that existed before continental breakup the basal unconformity occurs offshore separating older continental rocks from younger post rift sediments this paper considers the palaeoplain and the basal unconformity to be the same surface it presents a hypothesis for continental passive margins that integrates the geomorphology near the continental boundary with the offshore geomorphology by recognising the identity of the palaeoplain with the basal unconformity the hypothesis links the terrestrial story to the offshore story many details that were separated in earlier models come together in a single model continental rifting starts with a rift valley stage at this stage the pre breakup landsurface the palaeoplain at the edge of the continent is downwarped on the seaward side post rift sediments accumulate on the submerged palaeoplain which thus becomes the basal unconformity this model explains the parallel structures of continental divide present coast line and continental edge it also explains the creation of the basal unconformity in a short time and the comparative volumes of onshore erosion and offshore sedimentation onshore the model explains the modifications of drainage and the great escarpments that are characteristic of passive margins it also explains oldlands observed near the coast

the 29 september 1969 ceres south africa earthquake full waveform moment tensor inversion for point source and kinematic source parameters the ceres earthquake of 29 september 1969 is the largest known earthquake in southern africa digitized analog recordings from worldwide standardized seismographic network stations powell and fries 1964 are used to retrieve the point source moment tensor and the most likely centroid depth of the event using full waveform modeling a scalar seismic moment of 2 2 2 4 1018 n m corresponding to a moment magnitude of 6 2 6 3 is found the analysis confirms the pure strike slip mechanism previously determined from onset polarities by green and bloch 1971 overall good agreement with the fault orientation previously estimated from local aftershock recordings is found the centroid depth can be constrained to be less than 15 km in a second analysis step we use a higher order moment tensor based inversion scheme for simple extended rupture models to constrain the lateral fault dimensions we find rupture propagated unilaterally for 4 7 s from east southwest to west northwest for about 17 km average rupture velocity of about 3 1 km s online material waveform and spectral fits

eroding dynamic topography 1 geological observations of mantle flow driven dynamic topography are numerous especially in the stratigraphy of sedimentary basins on the contrary when it leads to subaerial exposure of rocks dynamic topography must be substantially eroded to leave a noticeable trace in the geological record here we demonstrate that despite its low amplitude and long wavelength and thus very low slopes dynamic topography is efficiently eroded by fluvial erosion providing that drainage is strongly perturbed by the mantle flow driven surface uplift using simple scaling arguments as well as a very efficient surface processes model we show that dynamic topography erodes in direct proportion to its wavelength we demonstrate that the recent deep erosion experienced in the colorado plateau and in central patagonia is likely to be related to the passage of a wave of dynamic topography generated by mantle upwelling

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

post gondwana geomorphic evolution of southwestern africa implications for the controls on landscape development from observations and numerical experiments the relationship between morphology and surficial geology is used to quantify the denudation that has occurred across southwestern africa since the fragmentation of gondwana during the early mesozoic two main points emerge significant denudation of the order of kilometers is widespread except in the kalahari region of the continental interior the denudation is systematically distributed so that the continental exterior catchment draining directly to the cape basin is denuded to a greater depth than the interior catchment inland of the great escarpment the analysis also implies that the majority of the denudation occurred before the beginning of the cenozoic for both the exterior and interior catchments existing models of landscape development are reviewed and implications of the denudation chronology are incorporated into a revised conceptual model this revision implies that the primary effect of rifting on the subsequent landscape evolution is that it generates two distinct drainage regimes a marginal upwarp or rift flank uplift separates rejuvenated rivers that drain into the subsiding rift from rivers in the continental interior that are deflected but not rejuvenated the two catchments evolve independently unless they are integrated by breaching of the marginal upwarp if this occurs the exterior baselevel is communicated to the interior catchment that is denuded accordingly denudation rates generally decrease as the margin evolves and this decrease is reinforced by the exposure of substrate that is resistant to denudation and or a change to a more arid climate the observations do not reveal a particular style of smaller scale landscape evolution such as escarpment retreat that is responsible for the differential denudation across the region it is proposed that numerical model experiments which reflect the observational insights at the large scale may identify the smaller scale controls on escarpment development if the model and natural systems are analogous four numerical experiments are presented in which the roles of antecedent topography resistant substrate climate change and lowering the baselevel of the interior catchment are investigated for an initially high elevation margin bordered by an escarpment the model results suggest several styles of landscape evolution that are compatible with the observations escarpments may retreat in a regular manner but they also degrade and are destroyed only to reform at the drainage divide between exterior and interior catchments

structure and tectonics of the agulhas falkland fracture zone abstract the 1200 km long agulhas falkland transform developed during the early cretaceous break up of west gondwanaland on the african plate the continent bordering the agulhas fracture zone extends between the agulhas bank and the south tugela reentrant it is divided into four distinct parts mallory trough segment i diaz ridge segment ii east london segment iii and durban segment iv from southwest to northeast each segment differs from the ohters in its physiography and in the nature of the continent ocean crustal boundary in segment i a wedge like eastward narrowing deep water basin separates the steep continental slope from a marginal fracture ridge of probable volcanic origin along the transform fault trace along segment ii the south african continental slope is deeply embayed and a great thickness of sedimentary strata has been ponded behind a buried ridge along the northern side of the fracture zone trace above a probable fragment of oceanic crust representing a northerly extension of the jurassic falkland plateau basin in segment iii the continental margin is steep especially along the middle slope it lies directly along the transform trace but conspicuously rugged along the continental slope due to the effects of submarine canyoning and sediment slumping in segment iv the continent ocean boundary also lies directly along the transform fault trace and the margin shows evidence of seaward tilting above landward dipping faults along the middle or lower slope the segmentation is inherited from jurassic structural fabrics formed prior to early cretaceous transform motion when tectonic rotation of microplates caused rifting and partial oceanization along the future agulhas falkland megafault trajectory

mesozoic magnetic anomalies in the southern natal valley the recognition of mesozoic magnetic anomalies m0 m12 within the submarine natal valley south west indian ocean provides a further constraint on the relative motion between south america falkland plateau and africa during the early opening phase of west gondwanaland fig 1 colinear with m12 magnetic anomaly g ref 1 is delineated north of the south tugela ridge2 an acoustic basement high which is suspected to have been emplaced within continental crust immediately north of the continental oceanic crust boundary cob during the initial rifting phase starting from a revised west gondwana reconstruction3 and using palaeo poles of rotation1 3 the falkland plateau can be rotated into the natal valley superposing the corresponding georgia basin and natal valley magnetic anomalies this achieves an optimum continuity of inter continental structural lineaments3 by overlapping of an anomaly intermediate to m10 and m11 m10n4 this suggests that the natal valley cob is located between anomalies m10 and m11 implying that in this zone of south west gondwana seafloor spreading commenced at 125 myr

tectonic subsidence history and thermal evolution of the orange basin the orange basin offshore southwest africa appears to represent a classical example of continental rifting and break up associated with large scale transient volcanism the presence of lower crustal bodies of high seismic velocities indicates that large volumes of igneous crust formed as a consequence of lithospheric extension we present results of a combined approach using subsidence analysis and basin history inversion models our results show that a classical uniform stretching model does not account for the observed tectonic subsidence moreover we find that the thermal and subsidence implications of underplating need to be considered another departure from the uniform stretching model is renewed sub crustal thinning and linked to that uplift in the cenozoic that is necessary to reproduce the observed phases of erosion and the present day depth of the basin the dimension of these events has been examined and quantified in terms of tectonic uplift and sub crustal thinning based on these forward models we predict the heat flow evolution not only for the available real wells but also for virtual wells over the entire study area finally the hydrocarbon potential and the temperature evolution is presented and shown in combination with inferred maturation of the sediments for depth intervals which comprise potential source rocks 2009 elsevier ltd all rights reserved

late mesozoic and tertiary sediment supply to the eastern cape basin se atlantic and palaeo drainage systems in southwestern africa isopach asymmetry and sediment component changes in dsdp cores from the se atlantic orange basin support the hypothesis of major drainage system changes in sw africa during late cretaceous cenozoic time this involved alternations in the use of the 28 s modern orange river and 31 s modern olifants river exit points across the western escarpment by rivers carrying run off from the upper orange vaal catchment areas as well as radical re organizations of internal drainage geometry it is postulated that during late cretaceous times the 28 s exit was used with the middle orange river following a course in the interior well to the south up to 150 km of its modern channel sediment discharge rates from this river were relatively high at least 10 106 m3 yr 1 and resulted in rapid advancement of the continental margin sediment prism west of the mouth by large scale slumping the palaeogene orange vaal river exit was via the 31 s escarpment crossing and during the later part of this period the cape canyon was cut across the continental shelf and slope a significant reduction in sediment discharge to 2 0 106 m3 yr 1 suggests that the lower tertiary climate for sw africa was drier than that of late cretaceous times however aridity did not commence until late miocene times when the orange vaal discharge had switched back to the 28 s exit modern sediment discharge rates 6 5 106 m3 yr 1 are relatively high and reflect soil erosion caused by agricultural activity the two major alterations in exit point of the orange vaal late cretaceous early tertiary and late oligocene early miocene are related to periods of low sea level which promoted river capture adjacent to the western escarpment an additional factor in the first course change may have been the disruption of the middle orange channel by late cretaceous igneous intrusions less important internal reorganizations of the drainage system are postulated in late miocene pleistocene times economic implications for offshore diamond distribution are briefly mentioned

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

linking source and sink evaluating the balance between onshore erosion and offshore sediment accumulation since gondwana break up south africa abstract the geomorphic origin and evolution of the tectonically unique interior highland of southern africa the kalahari plateau and its flanking low lying coastal planes remain largely unresolved because of a lack of regional quantitative analyses of its uplift and erosion history here we focus on the southern cape south africa and link onshore denudation based on new apatite fission track thermochronology results to offshore sediment accumulation using abundant well data and a seismic reflection profile we attempt to relate source and sink in order to resolve some first order issues concerning timing of the exhumation and development of the topographic features of southern africa the volume of sediment accumulated off south africa s south coast is calculated using 173 wells and a seismic reflection profile a total uncompacted sediment volume of 268 500 km3 accumulated off south africa s south coasts since 136 ma in the outeniqua and southern outeniqua basins accumulation volumes and rates were highest in the early cretaceous 48 800 104 km3 at 8150 km3 ma from 136 to 130 ma and 57 500 104 km3 at 5750 km3 ma from 130 to 120 ma and mid late cretaceous 83 700 104 km3 at 3200 km3 ma from 93 to 67 ma volumes and accumulation rates were lowest for the early mid cretaceous 47 400 104 km3 at 1750 km3 ma from 120 to 93 ma and the cenozoic 31 200 104 km3 at 450 km3 ma from 67 to 0 ma although our analysis shows that the accumulated volume of offshore sediments does not match the calculated volume of onshore erosion as quantified through apatite fission track thermochronology e g tinker j h de wit m j brown r 2008 mesozoic exhumation of the 439 southern cape south africa quantified using apatite fission track thermochronology tectonophysics doi 10 1016 j tecto 2007 10 009 the timing of increased sediment accumulation closely matches the timing of increased onshore denudation this suggests that the greatest volumes of material were transported from source to sink during two distinct cretaceous episodes and that the processes driving onshore denudation decreased by an order of magnitude during the cenozoic

passive margin evolution and its controls on natural gas leakage in the southern orange basin blocks 3 4 offshore south africa using a 2d seismic dataset that covers part of the southern orange basin offshore south africa we reconstructed the geological evolution of the basin this evolutionary model was then used to investigate the occurrence of natural gas within the sedimentary column and the distribution of gas leakage features in relation to the observed sedimentary and tectonic structures developed in the post rift succession since the early cretaceous the cretaceous succession has been subdivided into five seismic units the highest sedimentation rates occur within the barremian aptian unit c1 and the turonian coniacian unit c3 two cenozoic units t1 and t2 have been distinguished these show a sudden decrease in sedimentation rate for the whole of the cenozoic three phases of gravitational tectonics with two late cretaceous phases of mass movement in the northern study area and cenozoic slumping in the southern study area have been related to sedimentation rates sea level changes paleoenvironmental evolution and regional tectonics the occurrence of natural gas leakage follows a coast parallel distribution within the study area in the near shore part at water depths shallower than 400 m massive gas chimneys penetrate through the sediment layers and reach the near surface within an intermediate narrow band between 300 and 500 m water depth the gas migrates more diffusely through sub vertical faulted cretaceous sediments while in the outer part of the basin through the cretaceous and cenozoic gravitational wedges only very few signs of gas accumulation and migration can be seen along the faults a conceptual model has been established with the aptian source rock generating gas in the outer part of the basin this source rock underlies the cenozoic wedge in the south and the thick cretaceous wedge in the north and is a postulated source for the natural gas within the sedimentary column this thermogenically generated gas does not migrate directly through the gravitational faults and the above lying sediments but moves buoyancy driven up dip along stratigraphic layers to escape through the sediments to the sea floor in the inner shelf area

segmentation and volcano tectonic characteristics along the sw african continental margin south atlantic as derived from multichannel seismic and potential field data abstract regional seismic reflection and potential field data document the south atlantic s break up history between 39 s and 19 s from the early cretaceous onwards previous maps of distribution of volcanics along the margin showed volcanics along the whole african margin based on extrapolation of data based on previously unpublished marine geophysical data we found the southernmost 460 km long margin segment to be lacking huge volumes of break up related volcanic effusives northwards break up was accompanied by the emplacement of huge volumes of volcanic material prominently featured in seismic sections as huge wedge shaped seaward dipping reflectors sdrs detailed mapping of offsets left and right stepping and variations in structural character of the volcanics reveal the segmentation along and the break up history of the margin several superimposed sdr sequences suggesting episodicity of volcanic emplacement divided by periods of erosion and sedimentation are distinct along southerly lines losing prominence northwards a main outcome of our study is that this passive margin is not continuously of the volcanic type and that the change from a non volcanic to a volcanic margin occurs abruptly we define four distinct first order segments along the 2400 km section of the southwestern african margin covered by our seismic data from south to north these first order segments are magma poor segment i segment ii with enormous sdrs volumes decreasing sdrs volumes in segment iii segment iv again with enormous volcanic output likely influenced by walvis ridge volcanism most important is that there is no systematic increase in the volumes of the effusives towards the tristan da cunha hot spot rather there is an alternating pattern in the sdrs volumes and widths the boundary between the volcanic and magma poor margin segments in the southernmost study area is sharp 10s of km which we propose is reflected in magnetic anomaly data as well we suggest that this variability along the margin is mainly due to a change in stretching rifting character from oblique during the early stages of breakup to conventional seafloor spreading from chron m4 130 ma onwards

natural long term annealing of the zircon fission track system around a granitic pluton we measured 24 ages and 14 length distributions of fission tracks in zircons separated from basement sandstones around a granitic pluton to investigate natural long term annealing behaviors of the zircon fission track zft system samples were collected from three cretaceous coherent units of the shimanto accretionary complex shikoku japan along an eastward traverse at distances of between 1 6 and 10 6 km from the intrusion contact of 15 ma takatsukiyama granite exposing about 5 km in diameter sampling was carried out around the expected position of the transition zone of track retention i e the fossil partial annealing zone fpaz based on previous regional zft age distributions approaching the contact original zft ages of 100 ma show a continuous reduction to 15 ma at 3 km distance their single grain age distributions accordingly change from widespread multimodal patterns reflecting provenance ages to unimodal ones characterizing the secondary heating by the granite intrusion mean and distributions of zft lengths also show notable variations within the fpaz as a result of mixing both preexistent tracks that were shortened by the miocene heating and unannealed ones having original long lengths that have been formed and accumulated after the cooling of the pluton these facts along with observed track segmentation and annealing anisotropy suggest that the shortening patterns of tracks in nature a heating duration of 105 106 years estimated from thermal modeling are indistinguishable from those previously demonstrated at laboratory timescales this basically validates the conventional approach of extrapolating laboratory based kinetic parameters to geologic timescales the observed fpaz and thermal modeling results in conjunction with laboratory heating data previously reported and recent constraints from an ultradeep borehole favor a relatively narrow temperature range for the partial annealing zone i e 230 to 330 c for a heating duration of 106 years

seafloor spreading in the weddell sea from magnetic and gravity data abstract a re compilation of magnetic data in the weddell sea is presented and compared with the gravity field recently derived from retracked satellite altimetry the previously informally named anomaly t an east west trending linear positive magnetic and gravity anomaly lying at about 69 s forms the southern boundary of the well known weddell sea gravity herringbone north of anomaly t three major e w linear magnetic lows are shown and identified with anomalies c12r c21 29 r and c33r on the basis of these and following work by recent investigators isochrons c13 c18 c20 c21 c30 c33 and c34 are identified and extended into the western weddell sea similarly a linear magnetic low lying along the spine of the herringbone is shown and provisionally dated at 93 96 ma anomaly t is tentatively dated to be m5n in agreement with recent tectonic models although current tectonic models are generally in good agreement to the north of t to the south interpretations differ some plate tectonic models have only proposed essentially north south spreading in the region whilst others have suggested that a period of predominantly east west motion relative to present antarctic geographic coordinates occurred during the mid mesozoic spreading between east and west gondwana we identify an area immediately to the south of t which appears to be the southerly extent of n s spreading in the herringbone following recent work the extreme southerly extent of the n s directed spreading of the herringbone is provisionally dated m9r m10 in the oldest weddell sea immediately to the north and east of the antarctic shelf we see subtle features in both the magnetic and gravity data that are consistent with predominantly n s spreading in the weddell sea during the earliest opening of east and west gondwana in between however in a small region extending approximately from about 50 km south of t to about 70 s and from approximately 40 to 53 w the magnetic and gravity data appear to suggest well correlated linear marine magnetic anomalies possible isochrons perpendicular to t bounded and offset by less well defined steps and linear lows in the gravity possible fracture zones these magnetic and gravity data southwest of t suggest that the crust here may record an e w spreading episode between the two plate system of east and west gondwana prior to the initiation of the three plate spreading system of south america africa and antarctica the e w spreading record to the east of about 35 w would then appear to have been cut off at about m10 time during the establishment of n s three plate spreading along the south american antarctic ridge and then subducted under the scotia ridge

cretaceous reconstructions of east antarctica africa and madagascar abstract some cretaceous reconstructions of east antarctica africa and madagascar overlap the mozambique ridge onto the margin of dronning maud land and others place madagascar west of the gunnerus ridge into the riiser larsen sea our m10ny 131 9 ma m0 120 4 ma 96 ma and 34y 83 ma marine free air gravity and topography reconstructions illustrate a solution to the overlap problem and also demonstrate that madagascar separated from antarctica east of the gunnerus ridge our reconstructions chronicle the growth of the conjugate mozambique and riiser larsen basins created at the southwest indian spreading ridge well defined spreading corridors identified by conjugate fracture zones and mesozoic magnetic anomaly identifications extend from one basin into the other we find that the eastern scarp of the mozambique ridge aligns with the astrid ridge and there is no room for madagascar west of the gunnerus ridge the overlap of the mozambique ridge is solved by taking into account the motion of an independent mozambique ridge microplate that existed from chron m11 to circa m2

sedimentation and palaeoenvironments of late cretaceous crater lake deposits in bushmanland south africa a large diameter borehole core from an epiclastic kimberlite remnant on the farm stompoor in the prieska district cape province contains a continuous 76 m section of fossiliferous sediments interpreted as having accumulated within a crater lake during the late cretaceous three distinct facies associations reflect depositional processes that prevailed in offshore areas of the original lake facies association a matrix supported pebble conglomerates comprising a chaotic assemblage of pyroclastic basement and country rocks set in a fine grained matrix flat non erosional basal surfaces with frozen rip up clasts the protrusion of matrix supported clasts above the upper surfaces and a direct relationship between maximum clast size and bed thickness suggest deposition from debris flows that originated subaerially on pyroclastic talus cones surrounding the crater facies association b alternating thin beds of matrix supported granule conglomerate structureless fine grained sandstone and parallel laminated mudrock small fining upward sequences within these beds are comparable to turbidite bouma tade tde numerous partings display petrified fish and frog skeletons as well as bivalve gastropod and ostracode shells leaf impressions insect wings and a possible bird bone these beds were deposited by thin debris flows and turbidity underflows interspersed with periods of pelagic sedimentation facies association c microlaminated mudstone beds containing scattered dropstone lapilli the lamination is imparted by alternating ca rich ca poor layers which may reflect climatic seasonality they are interpreted as the result of seasonally influenced suspension settling through a thermally stratified water column short term periodicities in conglomerate bed thicknesses are interpreted as the result of successive block caving of a slump scar giving rise to several debris flows from the same source area seismic shock from nearby volcanism may have simultaneously triggered slumps on both subaerial and subaqueous slopes dropstone lapilli in type c beds and the preponderance of load casting in type b beds support this interpretation an estimate of the time span involved in accumulating 76 m of crater lake sediments based on the possible seasonal imprint of type c beds gives a figure of some 220 000 yr

the role of fault reactivation and growth in the uplift of western fennoscandia new structural data published aft data and topographic data suggest that the latest cretaceous cenozoic uplift of western norway was associated with normal reactivation of the more trondelag fault complex reactivation focused along the base of today9s topographic rise with maximum displacements in the order of 2 3 km structural and aft data indicate that reactivation occurred along a displacement gradient with least normal displacement in the ne and increasing displacements towards the sw reactivation commenced in connection with the triassic to earliest cretaceous phases of rifting on the norwegian margin and probably continued through most of the tertiary the late cretaceous to cenozoic topographic development of the scandes mountains followed structurally induced templates such as those described from other areas of active normal faulting a fault model for the cenozoic uplift of the norwegian mainland provides a framework for provenance erosion and transport of sediments eventually deposited in the offshore post rift basins in the latest cretaceous palaeogene and neogene the asymmetric topographic profile of fennoscandia is reflected in the aft data and suggests that lithospheric flexure places a first order control on the shape of the scandes mountains and the central fennoscandian craton

structural evolution of the orange basin gravity driven system offshore namibia the orange basin records the development of the late jurassic to present day volcanic rifted passive margin of namibia regional extension is recorded by a late jurassic to lower cretaceous syn rift megasequence which is separated from a cretaceous to present day post rift megasequence by the late hauterivian ca 130 ma break up unconformity the late cretaceous post rift evolution of the basin is characterized by episodic gravitational collapse of the margin gravitational collapse is recorded as a series of shale detached gravity slide systems consisting of an up dip extensional domain that is linked to a down dip zone of contraction domain along a thin basal detachment of turonian age the extensional domain is characterized by basinward dipping listric faults that sole into the basal detachment the contractional domain consists of landward dipping listric faults and strongly asymmetric basinward verging thrust related folds growth stratal patterns suggest that the gravitational collapse of the margin was short lived spanning from the coniacian ca 90 ma to the santonian ca 83 ma structural restorations of the main gravity driven system show a lack of balance between up dip extension 24 km and down dip shortening 16 km gravity sliding in the namibian margin is interpreted to have occurred as a series of episodic short lived gravity sliding between the cenomanian ca 100 ma and the campanian ca 80 ma gravity sliding and spreading are interpreted to be the result of episodic cratonic uplift combined with differential thermal subsidence sliding may have also been favoured by the presence of an efficient detachment layer in turonian source rocks

source rock characteristics and compositional kinetic models of cretaceous organic rich black shales offshore southwestern africa abstract the source rock potential of cretaceous organic rich whole rock samples from deep sea drilling project dsdp wells offshore southwestern africa was investigated using bulk and quantitative pyrolysis techniques the sample material was taken from organic rich intervals of aptian albian and turonian aged core samples from dsdp site 364 offshore angola dsdp well 530a north of the walvis ridge offshore namibia and dsdp well 361 offshore south africa the analytical program included toc rock eval pyrolysis gc bulk kinetics and micro scale sealed vessel pyrolysis mssv experiments the results were used to determine differences in the source rock petroleum type organofacies petroleum composition gas oil ratio gor and pressure volume temperature pvt behavior of hydrocarbons generated from these black shales for petroleum system modeling purposes the investigated aptian and albian organic rich shales proved to contain excellent quality marine kerogens the highest source rock potential was identified in sapropelic shales in dsdp well 364 containing very homogeneous type ii and organic sulfur rich type iis kerogen they generate p n a low wax oils and low gor sulfur rich oils whereas type iii kerogen rich silty sandstones of dsdp well 361 show a potential for gas condensate generation bulk kinetic experiments on these samples indicate that the organic sulfur contents influence kerogen transformation rates type iis kerogen being the least stable south of the walvis ridge the turonian contains predominantly a type iii kerogen north of the walvis ridge the turonian black shales contain type ii kerogen and have the potential to generate p n a low and high wax oils the latter with a high gor at high maturity our results provide the first compositional kinetic description of cretaceous organic rich black shales and demonstrate the excellent source rock potential especially of the aptian aged source rock that has been recognized in a number of the south atlantic offshore basins

long term stratigraphic evolution of atlantic type passive margins a numerical approach of interactions between surface processes flexural isostasy and 3d thermal subsidence the thermal and flexural evolution of passive margins is impacted by the un loading effects of erosion sedimentation processes which in turn affect their relief and sediment accumulation these complex couplings are recorded by the stratigraphic trend of the associated sedimentary basins which is controlled by the balance between sediment accumulation subsidence and eustasy our objective is to constrain the relative contribution of the factors controlling the mechanical response of the lithosphere and the efficiency of the surface processes on parameters such as the denudation accumulation and uplift subsidence history and long termstratigraphic trends the novel aspect of our approach is to integrate the evolution of both domains in erosion and in sedimentation using state of the art modeling of the flexure of the lithosphere including the surface processes erosion sedimentation and the thermal evolution as well as concepts in sequence stratigraphy we investigated numerically the post rift evolution of passive margins testing the influence of the lithosphere s initial geometries thermal states and stretching profiles as well as the efficiency of the surface processes in all simulations the initial flexural rift shoulder is eroded away within 10 to 20 myr in the following stages the sedimentary supply and the evolution of the margin are mostly controlled by the flexural response to the thermal relaxation of the isotherms by cooling and the un loading mass transfer at the surface both the sediment accumulation controlled by relief relaxation and the subsidence rates decrease exponentially with time the evolution of their relative values forms a regressive transgressive sequence we show that variations in the efficiency of the surface processes may impact the uplift subsidence histories and the long term stratigraphic trend within the same range of magnitude as lithospheric parameters such as pre rift crust thickness or depth dependency of stretching the initial crustal thickness is then determinant in the denudation accumulation history whereas the margin width will impact the uplift subsidence history the second most depth of necking and effective elastic thickness are critical mostly during the initial phase of the margin history syn and immediate post rift whereas bulk sediment density and lithosphere thickness become critical in the late post rift

south atlantic fits and intraplate boundaries in africa and south america abstract the misfit problems raised by the pre drift reconstructions of the south and equatorial atlantic compel us to resort to intraplate deformations it is shown that acceptable deformations of the african plate in line with the benue trough and the transafrican rifts and shear zones provide a far from satisfactory solution to the geometrical misfit problems in the south atlantic these problems can instead be alleviated by considering a degree of deformation of the south american plate along an intraplate boundary in line with the parana province the rio grande rise and the walvis ridge in spite of the scarcity of field observations in this area of south america we show that such a deformation cannot be ruled out but it will have to be associated with deformations in the equatorial atlantic and in other areas of the african and south american plates

upper rhine graben role of preexisting structures during rift evolution 1 the evolution of the cenozoic upper rhine graben was controlled by a repeatedly changing stress field and the reactivation of a complex set of crustal discontinuities that had come into evidence during permo carboniferous times a comparison of the spatial and temporal thickness distribution of synrift deposits with preexisting fault patterns permits to infer a sequence of distinct basin subsidence phases that can be related to changes in the ambient stress field reactivation of a system of late palaeozoic fault systems outlining troughs and highs controlled the nucleation of initially separated middle and late eocene basins the depocenters of which coincided with a preexisting wsw ene trend during oligocene crustal extension the individual basins coalesced resulting in the development of the ssw nne striking upper rhine graben during the late oligocene chattian change in stress field the upper rhine graben was probably reactivated as a dextral strike slip system with the central graben segment forming a releasing bend during the early miocene aquitanian a major reorientation of the regional stress field is held responsible for the main subsidence phase of the northern parts of the upper rhine graben this is reflected by a counterclockwise rotation and northeastward shift of the depocenter axis and later by the middle miocene uplift and erosion of the southern parts of the upper rhine graben during the plio quaternary the upper rhine graben was reactivated as a sinistral strike slip system with the central graben segment forming a restraining bend

passive kimberlite intrusion into actively dilating dyke fracture arrays evidence from fibrous calcite veins and extensional fracture cleavage abstract calcite veins are invariably associated with en echelon kimberlite dyke fracture arrays a detailed microstructural study of veining indicates four vein types type i stretched or ataxial veins are defined by high aspect ratio calcite fibers that are crystallographically continuous with calcite of the kimberlite matrix wall rock by elongated phenocrystic phlogopite with sharp crystal terminations centered on contacts between adjacent calcite fibers and by phenocrystic phlogopite that grows or extends across these veins type i vein mineralogy indicates syn dilational crystallization of vein minerals in local tensional areas within the kimberlite vein types ii stretched to syntaxial elongate blocky and iii antitaxial indicate late crystallization vein mineral growth during subsequent or repeated dilation calcite fibers in type i to type iii veins are orthogonal to the contacts of their host dykes regardless of the orientation of vein margins type iv calcite veins with blocky or mosaic polycrystalline textures are attributed to minor post intrusion extension which was potentially accompanied by repeated kimberlite intrusion within a given dyke array syn crystallization syn intrusion type i veins and an ubiquitous dyke parallel fracture cleavage in a zone up to 4 m on either side of dyke contacts suggest that en echelon kimberlite dyke fracture arrays occupied the approximate center of zones of active dilation within the brittle carapace of the upper crust type ii and iii veins indicate that extension or dilation continued independently of an occupying kimberlite fluid phase after initial intrusion arrested mobile hydrofracturing under low differential stress within the upper brittle or seismic carapace of the continental crust followed by repeated dilation of the dyke fracture system is proposed as a mechanism for producing the features observed in this study the conditions constrained in this study indicate passive dyke intrusion into dilating fracture arrays during crustal extension

topographic growth around the orange river valley southern africa a cenozoic record of crustal deformation and climatic change we reconstruct the history of topographic growth in southern africa on both sides of the orange river valley from an integrated analysis of erosion surfaces crustal deformation and climate change first we propose an inventory of erosion surfaces observed in the study area and classify them according to their most likely formative process i e chemical weathering or mechanical erosion among the various land units observed we define a new class of landform the pedivalley which corresponds to a wide valley with a flat erosional floor in the orange river valley we mapped three low relief erosion surfaces each bevelling a variety of lithologies the oldest and most elevated is 1 a stripped etchplain evolving laterally into 2 a stepped pediplain bearing residual inselbergs 3 a younger pediplain later formed in response to a more recent event of crustal deformation these are all cenozoic landforms the etchplain is associated with a late palaeocene to middle eocene weathering event and the two pediplains are older than the middle miocene alluvial terraces of the orange river landscape evolution was first driven by slow uplift 10 m ma followed by a second interval of uplift involving a cumulative magnitude of at least 200 m this event shaped the transition between the two pediplains and modified the drainage pattern a final phase of uplift magnitude 60 m occurred after the middle miocene and drove the incision of the lower terraces of the orange river climate exerted a major control over the denudation process and involved very humid conditions responsible for lateritic weathering followed by more arid conditions which promoted the formation of pedivalleys collectively these produce pediplains

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

structural features of the southwest african continental margin according to results of lithosphere scale 3d gravity and thermal modelling abstract to understand the structure of the southwest african continental margin a lithosphere scale 3d structural model has been developed covering the marginal cretaceous cenozoic orange luderitz walvis and namibe basins the easternmost walvis ridge offshore onshore the model includes two late proterozoic owambo etosha and nama basins this 3d model integrates published thickness maps sediment isopach maps shallow seismic and well data as well as published deep seismic information and has been additionally constrained by 3d gravity and thermal modelling using 3d gravity modelling the first order configuration of the crystalline crust has been resolved with respect to the location of the continent ocean boundary the distribution of a high density lower crustal layer indicates a continuous body extending below the cretaceous cenozoic depocentres and aligned parallel to the coast line in addition high density zones within the continental crystalline crust had to be included in the model to fit observed and calculated gravity the obtained moho topography correlates with the major tectonic units of this continental margin the results of the 3d thermal modelling indicate that there is a clear relationship between the location of thickened sediments and areas with increased temperatures within the upper 10 km of the 3d model this indicates that the low thermal conductivity of the sediments increases heat storage within the areas covered by thick sediments within the deeper crust the main feature of the temperature distribution is the transition across the continental margin from the relatively cold oceanic part to the warm continental one this regional pattern is controlled by the thickness of the crystalline continental crust which is characterized by an increased radiogenic heat production at a depth of 80 90 km the temperature becomes higher beneath the oceanic domain than beneath the continent reflecting the configuration of the lower thermal boundary which is represented by an isothermal lithosphere asthenosphere boundary

post break up tectonic inversion across the southwestern cape of south africa new insights from apatite and zircon fission track thermochronometry the south west african margin is regarded as an example of a passive continental margin formed by continental rifting following a phase of lithospheric extension and thinning recent attention focused on this margin has included theoretical modelling studies of rift processes plate kinematic studies of the opening geometry and timing and empirical studies focused on documenting the crustal structure and offshore sedimentary record here we examine the onshore geomorphic and tectonic response to rifting and breakup with a specific focus on the sw cape of south africa we present 75 new apatite and 8 new zircon fission track analyses from outcrop samples and onshore borehole profiles along the western margin of south africa the data are used to derive robust thermal histories that record two discrete phases of accelerated erosional cooling during the early cretaceous 150 130 ma and late cretaceous 100 80 ma respectively both periods of enhanced erosion are regional in extent involved km scale erosion and extend well inland of the current escarpment zone albeit with spatially variable intensity and style the late cretaceous episode is also expressed more locally by tectonic reactivation and inversion of major faults causing km scale differential displacement and erosion the new aft data do not exclude the possibility of modest surface uplift occurring during the cenozoic but they restrict the depth of regional cenozoic erosion on the western margin to less than c 1 km the inferred pattern and chronology of erosion onshore is consistent with the key features and sediment accumulation patterns within the offshore orange and bredasdorp basins it is suggested that the late cretaceous event was triggered by a combination of regional dynamic uplift augmented along the western margin and in the sw cape by local tectonic forces arising from dextral displacement of the falkland plateau along the falkland agulhas fracture zone

late cretaceous reactivation of major crustal shear zones in northern namibia constraints from apatite fission track analysis abstract namibia s passive continental margin records a long history of tectonic activity since the proterozoic the orogenic belt produced during the collision of the congo and kalahari cratons in the early proterozoic led to a zone of crustal weakness which became the preferred location for tectonism during the phanerozoic the pan african damara mobile belt forms this intraplate boundary in namibia and its tectonostratigraphic zones are defined by ductile shear zones where the most prominent is described as the omaruru lineament waterberg thrust oml wt the prominance of the continental margin escarpment is diminished in the area of the central and northern zone of the damara belt where the shear zones are located this area has been targeted with a set of 66 outcrop samples over a 550 km long 60 km broad coast parallel transect from the top of the escarpment in the south across the damara sector to the kamanjab inlier in the north apatite fission track age and length data from all samples reveal a regionally consistent cooling event thermal histories derived by forward modelling bracket this phase of accelerated cooling in the late cretaceous maximum palaeotemperatures immediately prior to the onset of cooling range from ca 120 to ca 60 c with the maximum occurring directly south of the omaruru lineament because different palaeotemperatures indicate different burial depth at a given time the amount of denudation can be estimated and used to constrain vertical displacements of the continental crust we interpret this cooling pattern as the geomorphic response to reactivation of basement structures caused by a change in spreading geometry in the south atlantic and south west indian oceans

palaeogeographic consequences of conservational models in the south atlantic ocean conservational models like simple shear pure shear or polyphase models that exclude exchanges between the lower continental crust and upper mantle are usually proposed to explain the lithospheric stretching and consequent crustal thinning of passive continental margins these models need large amounts of horizontal movement and have therefore important implications for plate kinematic reconstructions and intraplate deformation in this paper we propose to show these implications in the central segment of the south atlantic ocean in the angola brazilian system these models imply about 240 km of horizontal movement this movement can be com pensated by two end member mechanisms 1 an intraplate deformation located in africa and 2 an intraplate deformation located in south america we detail for each solution the strong geological and geodynamical implications and discuss the consequences for the genesis of passive continental margins

quantifying passive margin denudation and landscape development using a combined fission track thermochronology and cosmogenic isotope analysis approach abstract a new approach to empirically constraining numerical models of denudation and landscape development across passive continental margins is presented involving a combination of apatite fission track thermochronology and cosmogenic isotope analysis this integrated strategy is applied to the namibian sector of the south west african margin where the conventional landscape evolution model of inland retreat of an escarpment initiated at the coast at break up implies a mean rate of retreat of 1 km ma 1 the thermochronological and cosmogenic isotope data are incompatible with a simple escarpment retreat model since they show that denudation rates oceanward of the present escarpment position have been low 1 since the end of the eocene and the estimated rate of escarpment retreat has been only 10 m ma 1 this low rate of retreat is however consistent with numerical landscape evolution models where the escarpment is pinned at an inland drainage divide

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

early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate abstract along the southeast african continental margin a deep sedimentary basin and marginal ridge bordering the agulhas fracture zone formed coevally with south american counterparts underlying the falkland plateau extension in both the southern outeniqua and the falkland basin is related to large angle 100 rotation of the lafonia falkland islands block in middle late jurassic times during attempted propagation of the proto indian ocean across the active circum pacific margin of gondwanaland in the early cretaceous the jurassic microplate boundaries were partly reactivated and partly transected by a right lateral shear along the 1200 km long falkland agulhas transform fault preceded by substantial vorticity of the surrounding crustal blocks this boundary segment separating the south american and african plates was probably generated by complex geodynamic interactions between oceanic plate subduction continental to back arc rift propagation and the impingement of deep mantle plumes on the lithosphere

the african plate a history of oceanic crust accretion and subduction since the jurassic we present a model for the jurassic to present evolution of plate boundaries and oceanic crust of the african plate based on updated interpretation of magnetic gravity and other geological and geophysical data sets location of continent ocean boundaries and age and geometry of old oceanic crust jurassic and cretaceous are updated in the light of new data and models of passive margin formation a new set of oceanic palaeo age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the african plate palaeo stresses and may have influenced the evolution of intracontinental sedimentary basins as a case study we compute a simple model of palaeo stress for the late cretaceous time in order to assess how ridge push slab pull and horizontal mantle drag might have influenced the continental african plate we note that the changes in length of various plate boundaries especially trenches do not correlate well with absolute plate motion but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity

offshore mud volcanoes and onland faulting in southwestern africa neotectonic implications and constraints on the regional stress field abstract recently discovered mud volcanoes in the orange basin offshore southwestern africa denote the existence of neotectonic faults in the submerged continental shelf interpretation of seismic lines perpendicular to the trend of the alignment of the mud volcanoes shows flower structures diagnostic of strike slip faulting along a n nnw direction analysis at the regional scale of onland neotectonic features in southwestern africa shows that recent faulting occurred both in central namibia and namaqualand south africa and that it created both n nnw and nw trending lineaments it is proposed that the newly discovered offshore neotectonic activity and the onland structures described in this paper represent the structural expression of the same stress field these structures form a set of conjugate transtensive faults which constrain the regional horizontal greatest compressive stress in a nw nnw direction such stress orientation also supported by in situ stress measurements defines the so called wegener stress anomaly the predominant present day stress field of southwest africa the wegener anomaly is incompatible with the stress orientation required by plate scale tectonic constraints mainly in the form of recently published gps motion values for the african plate

annealing kinetics of fission tracks in zircon an experimental study a series of laboratory annealing experiments on zircon fission tracks has been carried out under heating conditions of 350 750 c for 10 1 103 hr i e 4 5 min 40 days variation in the confined lengths of spontaneous fission tracks was determined using zircon grains from nisatai dacite the fading contours of normalized mean track length r on the arrhenius diagram showed as sets of straight lines we performed a series of model fittings called the parallel and fanning models in order to describe the decrease in r with increasing temperature or heating time the lowest temperature limit of the zircon partial annealing zone zpaz was defined as r 0 95 and the highest as r 0 4 which approximately corresponded to the total fading of surface tracks extrapolation of the results of the laboratory experiments to the geological timescale gives for a heating duration of 106 yr estimated values of the zpaz of 210 320 c 60 c 2 with the parallel model 190 350 c 50 c 2 with the fanning model critical temperature t0 and 170 390 c 50 c 2 with the fanning model t0 the temperatures of the zpaz decrease by 20 c for an annealing duration that is an order of magnitude longer because the estimated closure temperature of zircon fission track analysis approximately corresponds to the middle of the zpaz these results support the previously estimated closure temperature of 240 c by varying the etching time it was revealed that significant removal of radiation damage occurs at r 0 93

role of continental break up mantle plume development and fault reactivation in the evolution of the gamtoos basin south africa abstract in response to the break up of gondwana the gamtoos basin rifted in the oxfordian kimmeridgian with reactivation of cape fold belt thrusts as extensional structures rifting was rapid with an initial divergent syn rift sequence passing into a parallel passive infill sequence as rifting waned dextral movement on the agulhas falkland fracture zone during the valanginian resulted in reactivation of oxfordian kimmeridgian structures with extensional compressional and strike slip senses of movement post rift thermal subsidence phase of basin evolution began during latest valanginian times with passive infill and onlap of the topography created during the valanginian subsequently the basin began to be uplifted due to the westward migration of the falkland plateau and natal valley spreading centre past the basin resulting in a non depositional hiatus and local channelling before erosion during the cenomanian following renewed subsidence the basin experienced uplift during the palaeocene and eocene the cause of which remains problematic but is contemporaneous with igneous intrusion in southern africa

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

the influence of preexisting structure on the evolution of the cenozoic malawi rift east african rift system this paper analyzes the importance of preexisting structure for the evolution of the cenozoic malawi rift which constitutes the southernmost part of the western branch of the east african rift system kinematic analyses demonstrate that the regional extension direction rotated clockwise from ene to se during rifting cenozoic rift faults of dip oblique and strike slip character rejuvenated crustal structures whenever those were properly oriented with respect to the extension directions depending on these boundary conditions the rift attempts to find the easiest pathway in order to develop in a mechanically modest way proterozoic basement structures represent the basic anisotropy which already influenced pre cenozoic rifts and also the cenozoic malawi rift amongst these basement structures pan african retrograde to cataclastic strike slip shear zones have the strongest influence on later rift structures rift related transfer faults appear as differently striking passive structures and their orientation seems to be controlled by a variety of steep basement structures retrograde shear zones foliation planes and fracture arrays

post rift reactivation of the onshore margin of southeast brazil evidence from apatite u th he and fission track data abstract on the continental margin of southeast brazil there is good geological evidence for post rift tectonic reactivation in the form of onshore tertiary basins up to 800 m deep and cretaceous to paleocene alkaline intrusive bodies at the surface with the aim of constraining the mesozoic and cenozoic exhumation history of this region we present new apatite u th he ages on basement samples which one of us previously used for apatite fission track aft analysis the new he ages range from 115 5 2 2 ma to 47 0 6 4 ma whereas the aft ages range from 330 3 22 ma to 64 5 3 ma inverse thermal history modelling of all the data provides more detail on the post rift cooling history than do the aft data alone furthermore models incorporating radiation damage effects on he diffusion produce thermal histories that are more consistent with independent geological evidence than models without these effects the results imply that only a few samples record an early cooling phase around the time of opening of the south atlantic ocean the new u th he data also imply a late cretaceous phase of exhumation between 90 and 60 ma subsequently the region underwent slow cooling i e erosion rate two or three samples seem to indicate a period of rapid cooling during the neogene although this is at the limit of resolution of the methods the post rift cooling phases imply exhumation from depths between 2 and 5 km this activity we attribute to compression in the south american plate between the andean subduction zone and the mid atlantic ridge in southeast brazil compression acted on a crust that was thermally or mechanically susceptible to reactivation during the late cretaceous deformation in the study area was of regional extent whereas during the tertiary it concentrated along pre existing shear zones of the precambrian brasiliano orogeny

scour protection of partially embedded pipelines using sloping curtains abstracta geotextile mattress with sloping curtain gmsc was introduced to protect underwater pipelines from scour in steady currents gmsc is a newly developed countermeasure against river and es

escarpment evolution on high elevation rifted margins insights derived from a surface processes model that combines diffusion advection and reaction experiments with a surface processes model of large scale 1 1000 km long term 1 100 m y erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent the model describes changes in topographic form as a result of simultaneous short and long range mass transport representing hillslope diffusive processes and fluvial transport advection respectively fluvial entrainment is modeled as a first order kinetic reaction which reflects the credibility of the substrate and therefore the fluvial system is not necessarily carrying at capacity one dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are 1 antecedent topography drainage 2 the timescale or equivalently a length scale in the fluvial entrainment reaction 3 the flexural response of the lithosphere to denudation and 4 the relative efficiencies of the short and long range transport processes when rainfall and substrate lithology are uniform a significant amount of discharge draining over the escarpment top causes it to degrade only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions an additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment this corresponds to a substrate that is hard to detach by fluvial erosion and therefore to fluvial erosion that is not transport limited continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide it is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology low flexural rigidities promote steep and slowly retreating escarpments for given rainfall and substrate conditions the morphology of a retreating model escarpment is determined by the ratio of the short range diffusive and long range advective transport efficiencies a low ratio which is interpreted to correspond to a relatively arid climate and weathering limited conditions promotes steep sharp topped escarpments with straight main slopes and escarpment retreat occurs over a wide range of height scales a high ratio interpreted to correspond to a more humid temperate climate produces a convex upper slope and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock however resistant caprocks are not an essential requirement for model scarps to retreat we conclude that the inferred controls and model behavior are both consistent with the present day morphology of rifted continental margins and with modern conceptual models of landscape evolution

late cenozoic evolution of the outer continental shelf at the head of the cape canyon south africa abstract cenozoic sedimentary successions have a restricted distribution both onshore and offshore along the passive western margin of southern africa and much controversy and uncertainty remains on the geological evolution of this margin siliciclastic authigenic and biogenic sediments varying in age from cretaceous to holocene recovered from a number of vibrocores collected between 200 and 500 m water depth at the head of the cape canyon provide the basis for an integrated bio sr isotopic and lithostratigraphic study the study area is underlain by unconsolidated middle cretaceous interbedded woody quartzose sand and kaolinitic mud a large regional erosional hiatus separates the upper oligocene to lower miocene carbonate sediments from these cretaceous sediments the upper oligocene to lower miocene 18 1 27 2 ma carbonate succession records third order sea level fluctuations superimposed on an overall marine transgression the interpreted late oligocene depositional environments imply increased bottom water activity over time and the peak in phosphogenesis at 23 6 26 1 ma indicates the onset of warm water upwelling calcareous deposition ended at around 18 ma when downwarping of the continental shelf associated with uplift of the continental interior resulted in the seaward tilt of the carbonate succession to greater water depths increased terrigenous input is recorded as inner shelf prograding quartzose delta front deposits following uplift of the interior uplift combined with a long term marine regression resulted in the widespread erosion of middle to upper miocene successions generally recovered only as reworked glauconite carbonate fluorapatite cfa mixed mineral pellets above the erosional hiatus glauconite rich sediments indicate overall low accumulation rates and multiple reworking events of organic rich clayey sediments during the middle to late miocene early pliocene onset of cold water upwelling at least intermittently by 15 ma established hydrographic conditions similar to the benguela upwelling system with a coast parallel mudbelt present on the middle shelf a middle pliocene marine regression reworked 15 8 9 1 ma inner shelf quartzose delta front sediments across the outer shelf late pliocene early pleistocene uplift of the continental interior resulted in the fault controlled incision of the head of the cape canyon along the seaward flank of the buoyant agulhas columbine basement block uplift increased quaternary terrigenous sedimentation on the shelf where the facies distribution was clearly controlled by canyon head bathymetry high frequency glacio eustatic sea level fluctuations resulted in late quaternary erosion and slumping across the outer shelf and the continued downcutting of the cape canyon lowstand shelly gravels are common including the last glacial maximum shell lag which grades into a faecal pellet rich holocene mud drape over much of the study area the late cenozoic evolution of the western margin recorded in the complex condensed sediment record of the study area is characterised by low sediment accumulation rates and multiple reworking events the integrated stratigraphic interpretation emphasises the importance of eustasy tectonic activity fluvial sediment input palaeoceanographic and climatic changes

complex rifted continental margins explained by dynamical models of depth dependent lithospheric extension observations from a number of rifted margins e g south atlantic salt basin mid norwegian margin exmouth plateau reveal wide regions of extremely attenuated crust and depositional environments that indicate depth dependent lithospheric extension although the one dimensional thermal kinematic consequences of depth dependent extension are understood no comprehensive process based explanation for the complex style of these margins exists here we present self consistent numerical models of passive margin formation that explain the depth dependent extension the width of the margin its characteristic tripartite nature and why such margins are prone to deposition of evaporites under appropriate climatic conditions some features that are important to reproducing the observed characteristics include decoupling between upper and lower parts of the lithosphere during stretching contrasting wide and narrow extensional styles above and below the decoupling level and progressive focusing of crustal extension toward the rift axis

indian and african plate motions driven by the push force of the reunion plume head mantle plumes are thought to play an important part in the earth s tectonics yet it has been difficult to isolate the effect that plumes have on plate motions here we analyse the plate motions involved in two apparently disparate events the unusually rapid motion of india between 67 and 52 million years ago and a contemporaneous transitory slowing of africa s motion and show that the events are coupled with the common element being the position of the indian and african plates relative to the location of the reunion plume head the synchroneity of these events suggests that they were both driven by the force of the reunion plume head the recognition of this plume force has substantial tectonic implications the speed up and slowdown of india the possible cessation of convergence between africa and eurasia in the palaeocene epoch and the enigmatic bends of the fracture zones on the southwest indian ridge can all be attributed to the reunion plume an analysis of the plate motions involved in two apparently disparate tectonic events the unusually rapid motion of india between 67 million and 52 million years ago and a transitory slowing of africa s motion at around the same time has uncovered a possible common cause for the two the reunion volcanic hotspot beneath the indian ocean has been active for about 67 million years and the close coupling of the motion of india and africa strongly suggests that it is the waxing and waning of the reunion plume head that is driving these motions

continental rifting parallel to ancient collisional belts an effect of the mechanical anisotropy of the lithospheric mantle analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process rift propagation is not random but tends to follow the trend of the orogenic fabric of the plates systematically reactivating ancient lithospheric structures continental rifts often display a clear component of strike slip deformation in particular in the early rifting stage moreover although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts these observations suggest that the deformation of the lithosphere especially during rifting is controlled by its preexisting structure on the other hand 1 the plasticity anisotropy of olivine single crystal and aggregates 2 the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs and 3 seismic anisotropy data which require a tectonic fabric in the upper mantle coherent over large areas suggest that preservation within the lithospheric mantle of a lattice preferred orientation lpo of olivine crystals may induce a large scale mechanical anisotropy of the lithospheric mantle we use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process we assess the deformation of an anisotropic continental lithosphere in response to an axi symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head model results show that a lpo induced mechanical anisotropy of the lithospheric mantle may result in directional softening leading to heterogeneous deformation during continental rifting this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique 30 55 to the preexisting mantle fabric this directional softening associated with olivine lpo frozen in the lithospheric mantle may also guide the propagation of the initial instability that will follow the preexisting structural trend the preexisting mantle fabric also controls the deformation regime imposing a strong strike slip shear component a lpoinduced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting structural inheritance the plume rift paradox and the onset of transtension within continental rifts fl 2001 elsevier science b v all rights reserved

pre rift tectonic scenario of the eo cretaceous gondwana break up along se brazil sw africa insights from tholeiitic mafic dyke swarms