continental oceanic crustal transition off southwest africa more than 1 426 mi 2 300 km of 48 trace 12 fold seismic reflection profiles were used to examine the nature of the continent ocean boundary off southwest africa south of the orange river faulted blocks which we interpret as rifted continental crust can be traced seaward to the 3 km isobath the transition to oceanic basement occurs in a zone 19 to 25 mi 30 to 40 km wide beneath the continental rise although structural details are obscured by a thick seaward dipping wedge of pre aii pre late early cretaceous overburden oceanic crust at the presumed contact is not older than magnetic anomaly m9 126 to 121 m y b p north of the orange river a pronounced hinge in continental crust correlates with magnetic anomaly g of rabinowitz 1976 seaward of the hinge a complex fault block terrane is evident oceanic basement can not be traced with any certainty landward of anomaly m4 intracontinental stretching and associated volcanism appear to have been important in the early history of the cape basin rifting and local dike intrusion may explain the presence of some lineated magnetic anomalies previously attributed to sea floor spreading related extrusives form at least part of the pre aii wedge just seaward of the hinge zone our interpretation of the seismic data suggests that the initiation of normal spreading in the cape basin postdates by 4 to 9 m y the valanginian age derived from prevailing plate tectonic reconstructions of the south atlantic

linking the basement geology along the africa south america coasts in the south atlantic the work was financially supported by the czech science foundation 15 05988s we thank martin whitehouse and lev ilyinsky for their support while using the nordsim ion probe in stockholm the nordsim facility is jointly funded by denmark finland norway sweden and iceland this is nordsim publication 446 we appreciate the discussions with stano ulrich and pedro oyhantcabal during various stages of this project the paper benefited from the reviews of cees passchier and an anonymous reviewer cees passchier directed us to the work of abraham ortelius mentioned in the introduction and kindly provided the translation from latin

brazilian and african passive margins of the central segment of the south atlantic ocean kinematic constraints the thinning of passive continental margins is usually explained by models using pure stretching or simple shear these models imply hypothetical extensional structures and large horizontal movements between the two conjugate margins more than 250 km for the brazilian and angolan margins refraction reflection data together with the most recent and tightest pre opening fit using continental and oceanic geological and geophysical constraints show that the substratum of the sag basin is divided into an autochthonous part upper continental crust and an allochthonous part exhumed material the thinning process which evolves in an elevated position of the system until at least the break up seems to be depth dependent and to mainly concern the lower middle crust which we postulate is exhumed this exhumation does not explain the entire thinning of the system horizontal motions cannot alone explain the formation of the huge angolan brazilian basin similar observations are made on the whole central segment of the south atlantic ocean part of the lower continental crust is still missing and it seems most improbable that the continental crust maintains its integrity throughout the thinning process

the northeast brazilian rift system the northeast brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the south atlantic these basins represent the locus of intersection of the southern and equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases similar data are not found in its counterpart in africa especially for the neocomian these early cretaceous rift basins of northeast brazil illustrate key three dimensional geometries of intracontinental rift systems mainly controlled by the basement structural framework during the main rift phase syn rift ii neocomian early barremian extensional deformation was distributed over three main rift axes 1 the gabon sergipe alagoas gsa trend 2 the reconcavo tucano jatoba rtj trend and 3 the cariri potiguar cp trend during this phase extensional deformation jumped west from the easternmost basins gsa trend to a series of ne trending intracratonic basins rtj and cp trends characterized by a set of asymmetric half grabens separated by basement highs transfer faults and or accommodation zones these basins are typically a few tens of kilometers wide and trend ne sw roughly perpendicular to the main extension direction during the neocomian preexisting upper crustal weakness zones like the dominantly ne sw trending shear zones of the brazilian pan african orogeny controlled the development of intracrustal listric normal faults internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework the megashear zones of pernambuco brazil and ngaundere africa seem to have behaved like a huge accommodation zone accommodating extensional deformation along the rtj gsa trends with simultaneous extension along the cp trend during the late barremian syn rift phase iii a significant change in rifting kinematics occurred when the cp trend was aborted and major rifting initiated at the equatorial branch during the aptian while the equatorial branch and benue trough africa experienced the main rift phase the rtj trend was aborted and the gsa trend developed a transitional phase between the rift and drift stage the gsa trend and the offshore potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode

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

intracontinental movements in western gondwanaland a palaeomagnetic test abstract early cretaceous intracontinental movements within africa and or south america to account for misfit problem in the pre drift reconstruction of western gondwana have been tested using palaeomagnetic poles from both continents each continent has been considered as comprising separate subplates according to the boundaries proposed by pindell and dewey 1982 and curie 1983 i e northern africa naf southern africa saf northern south america nsa and southern south america ssa visual and statistical distribution of late permian to jurassic poles from the african subplates indicate two distinct groups before rotating saf relative to naf while after the rotation they become indistinguishable the distribution and sparsity of data from south america limit the study in this continent as the data are statistically indistinguishable before and after rotation of ssa with respect to nsa when the test is extended to the reconstruction of western gondwana the tightest grouping of the poles occurs when an intracontinental movement is assumed within africa this suggests that the misfit between the two continents is probably due to intracontinental deformation within africa and not in south america if confirmed this may have profound implications in the kinematics of the processes related to the opening of the south and central atlantic oceans and also the origin of the west african rift system

tectonic evolution of the colorado basin offshore argentina inferred from seismo stratigraphy and depositional rates analysis abstract based on a dense 2d seismic reflection dataset and information from 8 exploration wells we reinterpreted the stratigraphic evolution of the colorado basin the basin is located on the continental shelf and slope within 50 to 2250 m of bathymetry the total sediment fill can be up to 16 000 m seismic to well log correlations provide a chrono stratigraphic framework for the interpreted seismic sequences we show that the colorado basin records the development of a permian pre rift period a triassic jurassic to early cretaceous rift phase and a lower cretaceous to tertiary drift phase this passive margin represents the evolution of lithospheric extension from active rifting to the thermal subsidence drift stage several cretaceous to cenozoic slumping episodes were identified and related to progradation of the sequences and sediment build up in the slope as well as to the development of seaward dipping extensional faults assuming an accumulation time span of 25 ma for the syn rift succession which corresponds to the duration of extensional phases preceding the final break up of the south atlantic sedimentation rates are in the order of 118 m ma by late cretaceous the gondwana break up had stopped in almost all areas and post rift thermal subsidence dominated the basin development during the early phase of subsidence high sedimentation rates of up to 122 m ma are recorded an important decrease of sediment supply 30 m ma occurred until the end of the sag phase during the deposition of the colorado formation and sedimentation took place in elongated depocenters with oceanward progradation a second phase of increased sediment supply takes place during the oligocene and miocene resulting in the development of large basinward prograding sedimentary wedges this event seems to be associated to the interaction between continental dynamics subsidence and increased continental erosion during climate shift from greenhouse to icehouse conditions

observations and modeling of alongshore variability in dune erosion at egmond aan zee the netherlands abstract dunes can erode within a few hours when exposed to high storm surge levels and large waves if the dunes are the primary defense such as in the netherlands this could result in flooding of the hinterland when dunes breach models are often used to analyze how dunes will respond to extreme conditions but they can also be applied to study milder events and with that gain more insight in the processes of dune erosion there is however growing demand for validating existing models under field conditions here we calibrate the xbeach dune erosion model with pre and post storm topography measurements of the dune erosion event in january 2012 at egmond aan zee the netherlands furthermore hydrodynamical data of the intertidal zone along a cross shore transect in the same area collected 3 months prior to the storm event are used to hydrodynamically calibrate and validate xbeach during the january 2012 storm the dune erosion was variable alongshore from the erosion of embryo dunes the forming of a dune scarp to considerable slumping of the entire dune face this caused the erosion volume v to range from 5 to 40 m3 m the calibrated model reproduces the alongshore variability in v with reasonable accuracy additional simulations show that the alongshore variation in v is due to variability on the initial dune topography primarily steepness and that variability in beach and nearshore bathymetry was of secondary importance

evaluation of xbeach performance for the erosion of a laboratory sand dune abstract a new set of laboratory data is used to investigate the bathymetry change of a steep sand dune exposed to waves and high water levels and subsequently compared to the results of numerical simulations using xbeach bichromatic wave boundary conditions are used to simulate a combined short wave and long wave field for two water level elevations corresponding to collision with the dune face and overwashing of the dune crest in the collision regime case episodic slumping due to the undercutting of the dune results in sudden erosional events followed by long periods of wave driven reshaping at the dune toe in the overwash regime case morphological changes are faster and sediment transport rates are higher the xbeach model was used to simulate wave driven erosion of the dune at the two water levels observed in the laboratory the model was not able to precisely recreate the cross shore spatial variability of significant wave height observed in the experiments however near bed wave orbital and mean current velocities were in good agreement with observations following rapid initial adjustment the model results were in agreement with measured dune morphology at successive times xbeach was sensitive to several parameters that control the rate of erosion including the critical avalanching slope under water the threshold water depth and the sediment transport formulation and performed well after careful selection of the best combination of these parameters overall the model predictions were in better agreement with laboratory observations for dune erosion in the overwash regime case than the collision regime case

weathering processes and the composition of inorganic material transported through the orinoco river system venezuela and colombia abstract the composition of river borne material in the orinoco river system is related primarily to erosion regime which in turn is related to tectonic setting especially notable is the contrast between material derived from tectonically active mountain belts and that from stable cratonic regions for a particular morpho tectonic region the compositional suites of suspended sediment bed material overback deposits and dissolved phases are fairly uniform are are typically distinct from whose of other regions for each region a consistent set of chemical weathering reactions can be formulated to explain the composition of dissolved and solid loads in developing these formulations erosion on slopes and storage of solids in soils and alluvial sediments are important considerations compositionally verymature sediment is derived from areas of thick soils where erosion is transport limited and from areas where sediments are stored for extended periods of time in alluvial deposits compositionally immature sediments are derived from tectonically active mountain belts where erosion is weathering limited weathering limited erosion also is important in the elevated parts of the guayana shield within areas of sleep topography compared to the mountain belts sediments derived from elevated parts of the shield are more mature a greater degree of chemical weathering seems to be needed to erode the rock types typical of the shield the major element chemistry and mineral composition of sediment delivered by the orinoco river to the ocean are controlled by rivers that have their headwaters in mountain belts and cross the llanos a region of alluvial plains within the foreland basin the composition of sediments in rivers that drain the shield seems to be established primarily at the site of soil formation whereas for rivers that drain the mountain belts additional weathering occurs during s episodes of storage on alluvial plains as sediments are transported across the llanos to the main stem of the orinoco after mixing into the main stem there seems to be little subsequent alteration of sediment

the thermo tectonic evolution of the southern congo craton margin as determined from apatite and muscovite thermochronology abstract the southern irumide belt sib of zambia consists of predominantly mesoproterozoic terranes that record a pervasive tectono metamorphic overprint from collision between the congo and kalahari cratons in the final stages of gondwana amalgamation this study applies multi method thermochronology to samples throughout southern zambia to constrain the post collisional phanerozoic thermo tectonic evolution of the region u pb apatite and 40ar 39ar muscovite data are used to constrain the cooling history of the region following congo kalahari collision and reveal ages of c 550 450 ma variations in the recorded cooling ages are interpreted to relate to localised post tectonic magmatism and the proximity of analysed samples to the congo kalahari suture apatite fission track data are used to constrain the low temperature thermo tectonic evolution of the region and identify mean central ages of c 320 300 210 200 and 120 110 ma thermal modelling of these samples identifies a number of thermal events occurring in the region throughout the phanerozoic carboniferous to permian triassic heating is suggested to relate to the development of karoo rift basins found throughout central africa and constrain the timing of sedimentation in the basin permian to jurassic cooling is identified in a number of samples reflecting exhumation as a result of the mauritanian variscan and gondwanide orogenies subsequent cooling of the majority of samples occurs from the cretaceous and persists until present reflecting exhumation in response to larger scale rifting associated with the break up of gondwana each model reveals a later phase of enhanced cooling beginning at c 30 ma that if not an artefact of modelling corresponds to the development of the east african rift system the obtained thermochronological data elucidate the previously unconstrained thermal evolution of the sib and provides a refined regional framework for constraining the tectonic history of central africa throughout the phanerozoic

beach and dune erosion during storm surges this paper presents the set up and results of an extensive research programme concerning the erosion of coastal dunes during storm surges a large number of two dimensional and three dimensional mobile bed model tests has been carried out to investigate the process of dune erosion the state of art after a series of small scale tests is summarized attention is focussed on large scale tests carried out in the delta flume with random waves up to 2 m significant height sediment concentration and orbital velocity measurements are discussed the large scale tests have confirmed the validity of a modelling technique based on the dimensionless fall velocity parameter h tw the model results are being applied to check the safety of existing coastal dunes as a water retaining structure that has to protect the major part of the netherlands from inundation during storm surges

birth of a volcanic margin off argentina south atlantic 1 based on a dense grid of multichannel reflection seismic lines we study the evolution of a volcanic rifted margin segment off argentina the segment under study is located between 44 s and 41 s we describe in detail the facies of the extrusive basaltic complexes across the margin these include single and multiple inner seaward dipping reflectors sdrs with varying architecture along the margin outer highs and outer sdr wedges a detailed interpretation of a transitional zone between the inner and outer sdr wedges is presented multiple inner sdrs are concentrated at the southern part of the margin segment while in the north only one steeply dipping wedge is present the dip of the inner sdr wedges changes along the margin and we propose subsidence by loading as main cause the transitional zone between the inner and outer sdr wedges becomes narrower toward the north by correlating with magnetic data we conclude that the emplacement of the 30 100 km wide multiple inner sdrs occurred episodically we propose an injection center which migrated in a seaward direction with proceeding extension as the origin for the multiple inner sdrs a scissor like opening of the margin segment resulted in different amounts of stretching along strike of the margin segment and is likely the cause for the varying distribution of the extrusive basaltic complexes the varying extension rates within the margin segment contribute to decreasing volumes of melts generated in a northward direction within this rift segment

rifting consequences of three plate separation submarine sedimentary basins parallel to the trends of passive continental margins are well explained by tensional processes between two lithospheric plates this leaves open the enigma of many basins which strike across margins we use examples from the north atlantic to show that such basins may be formed during the initial rifting stage due to enhanced lithospheric thinning caused by the separation of three plates we suggest that volcanics can be emplaced in these basins and in the extreme case where mantle temperature and consequently the degree of partial melting are particularly high that basaltic flows can be thick enough to fill and hide the basins

neocomian to early aptian syn rift evolution of the normal to oblique rifted north gabon margin interior and n komi basins abstract the north gabon coastal rift basins consist of a set of 130 150 long segment asymmetrically tilted half grabens interior basin and 000 020 short segment en echelon half grabens n komi basin separated by 040 060 major transverse faults tectono sedimentary analysis of field and subsurface data reveals the control exerted by extensional tectonism over continental sedimentation during berriasian to early barremian times uniform uniaxial 040 060 extension was responsible for the stretching of the brittle upper crust over a 100 km wide domain during late barremian early aptian times the main locus of extension stepped westward resulting in severe end rift uplift and erosion of the failed interior and n komi rift basins early cretaceous coastal rifts in north gabon display a wide range of styles from oblique rifting n komi basin normal rifting interior basin to transform rifting the pre existing precambrian tectonic fabric exerts a strong control over the mode and over the 100 300 km scale segmentation of the rifting

modelling storm impacts on beaches dunes and barrier islands a new nearshore numerical model approach to assess the natural coastal response during time varying storm and hurricane conditions including dune erosion overwash and breaching is validated with a series of analytical laboratory and field test cases innovations include a non stationary wave driver with directional spreading to account for wave group generated surf and swash motions and an avalanching mechanism providing a smooth and robust solution for slumping of sand during dune erosion the model performs well in different situations including dune erosion overwash and breaching with specific emphasis on swash dynamics avalanching and 2dh effects these situations are all modelled using a standard set of parameter settings the results show the importance of infragravity waves in extending the reach of the resolved processes to the dune front the simple approach to account for slumping of the dune face by avalanching makes the model easily applicable in two dimensions and applying the same settings good results are obtained both for dune erosion and breaching

continental rifting and the implications for plate tectonic reconstructions previous plate tectonic reconstructions have tried to recreate the pre rifting pangea configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting that the continental boundaries are isochrons and that the continents rift without distortion this paper proposes a model for continental breakup with distortion where ocean basins are formed by propagating rifts this model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents as rifts propagate through a continent the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading the propagating rift model implies that large amounts of continental extension up to 150 km occur and that the continental boundaries are not isochrons this extension due to rifting results in apparent overlap when the continents are returned to their original configuration because of the gradational nature of this extension the best representation of the pre rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation the overlap is equivalent to the amount of extension that has taken place on both continental edges reconstructions based on this model are presented for the gulf of california the gulf of aden the norwegian greenland sea and the south atlantic ocean they are able to resolve the discrepancies that occur when rigid plate tectonics are applied this model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons

improving predictions of swash dynamics in xbeach the role of groupiness and incident band runup abstract in predicting storm impacts on sandy coasts possibly with structures accurate runup and overtopping simulation is an important aspect recent investigations stockdon et al 2014 palmsten and splinter 2016 show that despite accurate predictions of the morphodynamics of dissipative sandy beaches the xbeach model roelvink et al 2009 does not correctly simulate the individual contributions of set up and infragravity and incident band swash to the wave run up in this paper we describe an improved numerical scheme and a different way of simulating the propagation of directionally spread short wave groups in xbeach to better predict the groupiness of the short waves and the resulting infragravity waves the new approach is tested against field measurements from the delilah campaign at duck nc and against video derived runup measurements at praia de faro a relatively steep sandy beach compared to the empirical fit by vousdoukas et al 2012 the xbeach model performs much better for more extreme wave conditions which are severely underestimated by existing empirical formulations for relatively steep beaches incident band swash cannot be neglected and a wave resolving simulation mode is required therefore in this paper we also test the non hydrostatic wave resolving model within xbeach for runup and overtopping against three datasets results for a high quality flume test show non hydrostatic xbeach predicts the run up height with good accuracy maximum deviation 15 a case with a very shallow foreshore typical for the belgian coast at wenduine was compared against detailed measurements overall the model shows correct behavior for this case finally the model is tested against a large number 551 of physical model tests of overtopping from the clash database for relatively high overtopping discharges the non hydrostatic xbeach performs quite well with increasing accuracy for increasing overtopping rates however for relatively low overtopping rates of less than 10 20 l m s the model systematically underestimates measured overtopping rates

sedimentology and palaeontology of the upper karoo group in the mid zambezi basin zimbabwe new localities and their implications for interbasinal correlation

modeling dune response to an east coast low coastal dunes can act as a method of soft coastal protection against inundation and direct impact of waves during storms if they are substantially large enough in volume to withstand erosion without breaching however the time evolution of sand dunes under direct wave impact is not well understood and many available models require site specific calibration and have had limited verification at field scales here we test three models of varying complexity in their ability to predict both dry beach erosion volumes and dune to a retreat for an east coast low storm event that occurred on the gold coast australia the process based model xbeach which models the entire profile was able to reproduce both dune toe retreat and dry beach volume however was sensitive to calibration parameters the two parametric models that only modeled erosion above the initial dune toe position were capable of accurately predicting dune toe retreat however under estimated dry beach erosion volumes with no calibration the parametric model proposed by palmsten and holman 2012 produced the smallest errors of dune toe retreat with mean error in final dune position of 6 6 m or 18 of the total measured dune retreat with minimal calibration estimated absolute error in average dune toe retreat was less than 13 of observed retreat for all three models

spatial and temporal evaluation of storm induced erosion modelling based on a two dimensional field case including an artificial unvegetated research dune abstract the objective of this study was to evaluate the temporal and spatial performance of the xbeach model predicting storm surge induced erosion and breaching on the basis of a two dimensional field case within the scope of a physical experiment a large scale artificial unvegetated research dune was built at the german baltic sea coastline and surveyed before during and after a surge event to generate high resolution digital elevation models of the temporal and spatial failure process the measured data was used to set up and calibrate two dimensional xbeach models the numerical results showed that the default settings of the latest xbeach x release led to a very good agreement bss 0 82 between the simulated and measured post storm bed levels in the area of the research dune while a higher agreement was observed on the overwash and inundation dominant eastern side than on the collision dominant western side evaluating the model accuracy during the storm surge using 44 digital elevation models of the seaside slope showed a spatially accurate prediction of the two observed breaches 5 m but also the occurrence of additional breaches with respect to timing and breach width evolution however the results highly deviated from the observation and were found to be very sensitive to changes in grid resolution and to changes of default parameters exchanging the nearshore bathymetry with measurements one and two months prior to the physical experiment where a 49 and 51 lower subaerial beach volume was observed resulted in a significantly overestimated dune erosion 31 and 33 and hence in a lower accuracy of the default model bss 0 69 overall the xbeach model is capable of accurately predicting the evolution of storm induced erosion during a sufficiently long and overwash inundation dominant surge event when current high resolution input data is available

two dimensional time dependent hurricane overwash and erosion modeling at santa rosa island abstract a 2dh numerical model which is capable of computing nearshore circulation and morphodynamics including dune erosion breaching and overwash is used to simulate overwash caused by hurricane ivan 2004 on a barrier island the model is forced using parametric wave and surge time series based on field data and large scale numerical model results the model predicted beach face and dune erosion reasonably well as well as the development of washover fans furthermore the model demonstrated considerable quantitative skill upwards of 66 of variance explained maximum bias 0 21 m in hindcasting the post storm shape and elevation of the subaerial barrier island when a sheet flow sediment transport limiter was applied the prediction skill ranged between 0 66 and 0 77 in a series of sensitivity tests in which several hydraulic forcing parameters were varied the sensitivity studies showed that the variations in the incident wave height and wave period affected the entire simulated island morphology while variations in the surge level gradient between the ocean and back barrier bay affected the amount of deposition on the back barrier and in the back barrier bay the model sensitivity to the sheet flow sediment transport limiter which served as a proxy for unknown factors controlling the resistance to erosion was significantly greater than the sensitivity to the hydraulic forcing parameters if no limiter was applied the simulated morphological response of the barrier island was an order of magnitude greater than the measured morphological response

foredunes and blowouts initiation geomorphology and dynamics this paper reviews the initiation dynamics geomorphology and evolution of incipient foredunes established foredunes and blowouts incipient foredunes may be initiated in a variety of ways leading to the formation of ramps terraces or ridges depending on progradation rates vegetation type and cover sediment transport rates and scale of erosional processes vegetation cover morphology growth rates and types combine with aerodynamic processes to determine dune and swale morphology established foredunes are classified into five morpho ecological types and the flow behaviour over two types is detailed a new model of foredune evolutionary paths at various possible time scales is presented blowout initiation dynamics and sediment transport and evolution is examined throughout the review the gaps in present understanding of dune processes and a range of ideas for new research possibilities are provided

modeling cross shore sandbar behavior on the timescale of weeks we compare predictions of a coupled wave averaged cross shore waves currents bathymetric evolution model to observations of onshore and offshore nearshore sandbar migration the observations span a 10 and 44 day period with onshore offshore bar migration at duck north carolina and at hasaki kashima coast japan respectively a 3 5 month period of onshore bar migration at duck and a 22 day period of offshore bar migration at egmond netherlands with best fit parameter values the modeled temporal evolution of the cross shore bed profiles agrees well with the observations model skill defined as 1 minus the ratio of prediction to no change error variances ranges from 0 50 at egmond to 0 88 for the prolonged onshore bar migration at duck localized in time and space reductions in model skill coincide with alongshore variations in the observed morphology consistent with earlier observations simulated offshore bar migration takes place during storms when large waves break on the bar and is due to the feedback between waves undertow suspended sediment transport and the sandbar simulated onshore bar migration is predicted for energetic weakly to nonbreaking conditions and is due to the feedback between near bed wave skewness bedload transport and the sandbar with negligible to small effects of bound infragravity waves and near bed streaming under small waves and conditions when breaking and nonbreaking conditions alternate with the tide the sandbar is predicted to remain stationary the intersite differences in the optimum parameter values are at least partly induced by insensitivity to parameter variations parameter interdependence and errors in the offshore wave forcing

do fracture zones define continental margin segmentation evidence from the french guiana margin abstract plate reconstructions suggest that the french guiana margin in the west equatorial atlantic is a highly segmented margin with both rift and transform style features we describe here the results of modelling coincident multi channel and wide angle seismic gravity and magnetic data acquired along two transects of this margin the resulting models not only highlight the degree of structural segmentation but also demonstrate the effect of trans tension on margin evolution as a whole the margin is characterised by 35 37 km thick pre rift continental crust which is separated from unusually thin oceanic crust 3 4 km by thinned continental and or transitional regions to the north the margin exhibits a 320 km wide zone of thinned continental crust adjacent to a narrow ocean continent transition and is interpreted as a transform margin where the wide zone of thinned crust is a result of profile orientation being highly oblique to the direction of rifting approximately 240 km to the south the margin is characterised by a 70 km wide zone of thinned continental crust which is wider than typical for transform and narrower than typical for rifted margins this crustal structure is interpreted to reflect a leaky transform formed by trans tensional extension these observations suggest that fracture zone influenced geometry of equatorial atlantic rifting did not produce a well defined margin crustal structure but instead resulted in margin segments which display characteristics of both rift and transform tectonic processes the associated abundance of fracture zones has likely also affected the post rift evolution of the margin and provided topographic basement highs which acted as sediment dams to the northwards flux of sediment from the amazon

evaluating the potential of sem eds analysis for fingerprinting suspended sediment derived from two contrasting topsoils sediment source fingerprinting can provide information on the contribution of various sediment sources to the suspended load of a stream this study was conducted to evaluate the potential of sem eds analysis scanning electron microscopy energy dispersive spectrometry to fingerprint two types of topsoil in a research basin a black chernozemic farmland soil and a grey podzolic forest soil simulated suspended sediment samples were prepared from each of the two soils and sem eds analysis of these samples provided information about the size distribution mineralogy and morphology of the particles the two simulated suspended sediment samples were similar in particle size distribution mineralogy and total chemistry so that these characteristics did not provide a distinctive fingerprint for the two topsoils an important difference between the two simulated suspended sediment samples was the morphology of the clay class particles consisting of water stable clay aggregates clay class particles derived from the grey forest soil were irregular whereas those derived from the black farmland soil were rounded this difference in particle morphology can be explained by the difference in land use and may provide a distinct fingerprint for each topsoil

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

large scale dune erosion tests to study the influence of wave periods article i nfo large scale physical model tests were performed to quantify the effects of the wave period on dune erosion attention was focussed on 2d cross shore effects in a situation with sandy dunes and extreme water levels and wave conditions besides profile measurements detailed measurements in time and space of water pressure flow velocities and sediment concentrations were performed in the near near shore area it was concluded that a longer wave period leads to a larger dune erosion volume and to a larger landward retreat of the dune face tests with double peaked wave spectra showed that the influence of the spectral shape on dune erosion was best represented by the tm 1 0 spectral mean wave period better than the peak wave period tp the effect of the wave period on dune erosion was implemented in a dune erosion prediction method that estimates erosion volumes during normative storm conditions for the dutch coast more details of the measurements and additional analyses of physical processes are described in an accompanying paper by van thiel de vries et al van thiel de vries j s m van gent m r a reniers a j h m and walstra d j r submitted for publication analysis of dune erosion processes in large scale flume experiments in this volume of coastal engineering

the predictability of cross shore bed evolution of sandy beaches at the time scale of storms and seasons using process based profile models deterministic and probabilistic profile models have been compared with hydrodynamic and morphodynamic data of laboratory and field experiments on the time scale of storms and seasons the large scale laboratory experiment is a pure 2d case and offers and ideal test case for cross shore profile models as disturbing alongshore non uniformities are absent the field experiments are performed at the egmond site the netherlands during the eu coast3d project and represent storm time scale oct nov 1998 as well as seasonal time scale conditions may 1998 sep 1999 the objective of the paper is to present information of coastal processes on these time scales and to assess the predictive capabilities coastal process based profile models with respect to hydrodynamics and morphodynamics at sandy beaches on the time scales of storms and seasons profile models can quite accurately errors smaller than 10 represent the cross shore significant wave height distribution in the surf zone if the wave breaking model is properly calibrated the wave breaking coefficient should be a function of local wave steepness and bottom slope for most accurate results profile models can reasonably represent the cross shore and longshore currents undertow in a pure 2d case and in 3d field conditions profile models including cross shore mixing effects and breaker delay effects do not produce better predictions of the longshore and cross shore current velocities profile models using default settings can quite reasonably simulate the behaviour of the outer and inner bars on the storm time scale the behaviour of the beach cannot be modelled with sufficient accuracy on the storm time scale profile models can reasonably simulate the post storm onshore bar migration provided that the near bed orbital velocities and wave asymmetry related sand transport are represented in a sufficiently accurate way using non linear wave theories profile models cannot simulate the beach recovery processes on the post storm time scale because these essentially 3d processes are not sufficiently known to be included in the models profile models using default settings cannot simulate the behaviour of the outer and inner bars and the beach on the seasonal time scale the behaviour of the outer bar on the seasonal time scale can only be represented properly after tuning using measured bed profiles the simulation of the inner bar and beach morphology on the seasonal time scale could not be improved by tuning

fault rocks of the kaltungo lineament ne nigeria and their relationship with benue trough tectonics in the upper part of the benue trough of nigeria several major ne sw lineaments are essential components in the structure of both the pan african basement and the cretaceous sediments in order to estimate their influence on the geodynamics of the trough detailed structural analyses have been carried out on the fault rocks of one of them the kaltungo lineament it cross cuts a basement inlier emerging from lower to middle cretaceous sediments this inlier of granitic rocks exhibits a wide fault zone trending n50e which displays an anastomosing network of both ductile and brittle shear zones the former are expressed by mylonitic bands generated by a pan african dextral wrenching on a regional scale the latter are represented by fault rocks of the cataclasite series related to a sinistral wrenching which occurred during the mesozoic the fault zone extends into the cover where it is seen as a thin crush zone characterized by conjugate strike slip and reverse faults from a computer dynamic analysis of microfaults an earlier compression n s shortening axis was found only within the basement whereas a second one n140e shortening axis appears in both sediments and basement the first palaeostress field corresponds to cataclasite genesis which preceded or coincided with the formation of pull apart basins along the sinistral kaltungo wrench fault which initiated the upper benue trough during the lower cretaceous sedimentary basins and uplifted zones kaltungo inlier were formed on releasing and restraining oversteps of such a fault the above statement may be extended to other similar faults at the scale of the entire trough the second palaeostress field corresponds to the late cretaceous compression responsible for the large scale folding in the sediments and the formation of fault breccia along the sinistral kaltungo lineament

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

crustal structure beneath the malawi and luangwa rift zones and adjacent areas from ambient noise tomography abstract crustal shear wave velocity structure beneath the malawi and luangwa rift zones mrz and lrz respectively and adjacent regions in southern africa is imaged using fundamental mode rayleigh waves recorded by 31 safari seismic arrays for african rift initiation stations dispersion measurements estimated from empirical green s functions are used to construct 2 d phase velocity maps for periods between 5 and 28 s the resulting rayleigh wave phase velocities demonstrate significant lateral variations and are in general agreement with known geological features and tectonic units within the study area subsequently we invert rayleigh wave phase velocity dispersion curves to construct a 3 d shear wave velocity model beneath the mrz and lrz low velocity anomalies are found in the upper most crust probably reflecting the sedimentary cover the mid crust of the mrz is characterized by an 3 7 low velocity anomaly which cannot be adequately explained by higher than normal temperatures alone instead other factors such as magmatic intrusion partial melting and fluid filled deep crustal faults might also play a role thinning of the crust of a few kilometers beneath the rifts is revealed by the inversion a compilation of crustal thicknesses and velocities beneath the world s major continental rifts suggests that both the mrz and lrz are in the category of rifts beneath which the crust has not been sufficiently thinned to produce widespread syn rifting volcanisms

continental break up mechanism lessons from intermediate and fast extension settings continental break up mechanisms vary systematically between slow and fast extension systems slow extension break up has been established from studies of the central atlantic european and adria margins this study focuses on the intermediate and fast cases from gabon and east india and draws from the interpretation of reflection seismic gravimetric and magnetic data interpretation indicates continental break up via continental mantle unroofing in all systems with modifications produced by magmatism in faster extension systems break up of the intermediate extension gabon system involves partial upper continental crustal decoupling from continental mantle whereas in the fast east coast india system decoupled and lower crustal regimes underwent upwarping in soggy zones in the footwalls of major normal faults usually upper crustal break up is affected by pre existing anisotropies which form systems of constraining rails for extending continental crust this modifies the local stress regimes they regain a regional character as the function of constraining rails vanishes during progressive unroofing of the upper mantle different regions attain different amounts of upper crustal stretching prior to the break up the break up location is then controlled by the upper crustal energy balance principle of wound linkage by which the minimum physical work is performed for linking upper crustal wounds leading to successful upper crustal break up

benue trough and benue chain it is suggested that the benue trough may have been initiated by the extension of the equatorial oceanic fracture zones onto the continent during the early stages of the separation of africa and south america a transcurrent movement along deep seated faults of the basement is responsible for the structure of the trough after a period of extension characterized by the sedimentary infilling of the trough and igneous activity the trough is turned into a chain through a short phase of compression a discrete pattern of en echelon folds and the strain pattern deduced from the analysis of large scale and microfracturing correspond to a n s to n160 e direction of compression coupled to a sinistral transcurrent movement along a ne sw direction the difference in age of the folding phase in the northern and southern parts of the chain indicates a diachronous nature for this tectonic event in relation to the wrench regime the tertiary evolution of the trough marked by a tensional regime resulting in an e w extension is to be related to the cameroon volcanic line

modelling storm induced beach dune evolution sefton coast liverpool bay uk storm induced dune evolution on a sandy coastal system is investigated using a nested modelling approach applied to the sefton coast liverpool bay uk real time offshore water levels and waves were used as model boundary forcings a delft3d coarse grid setup is used to simulate time and space varying sea surface elevations on which offshore waves are transformed by applying the swan model to establish the wave boundary for the high resolution morphological model xbeach statistical comparisons between model predicted and measured post storm profiles at a number of locations along the coast suggest that xbeach successfully captures storm induced beach change along the sefton coast predicted bed evolution of the beach dune system shows alternate erosion and sedimentation areas in the nearshore strong bed level changes are found at the northern part of the sefton coast when north westerly nw extreme waves and winds coincide with spring high tide morphological changes in the southern part are significantly lower than that in the north as a result of nw wave dissipation on the shoals located to the north of the crosby channel which creates low wave actions in that area in addition erosion of the dune foot is observed at some locations along the beach temporal simulation of beach dune evolution as a result of variable forcing conditions during storms provides useful insight into the morphodynamic processes of beach dune systems during storms using sefton as an example which is very useful for developing coastal management strategies over the existing conceptual tools

crustal structure across the colorado basin offshore argentina summary the geology of the wide shelves surrounding the south atlantic is closely linked to the kinematics and history of the opening of the ocean however several wide sedimentary basins which developed along the margins show peculiarities that are not yet understood in the context of the evolution of the south atlantic the colorado basin a wide sedimentary basin on the broad shelf of argentina extends in ew direction the basin s evolution oblique or orthogonal to the continent ocean boundary indicates that it is not a product of simple progressive extension and crustal thinning in addition a basement high paralleling the continental margin and separating the colorado basin from the deep sea basin is a common interpretation these findings are hardly in accordance with the idea that the colorado basin is an extensional basin that developed in conjunction with the early e w opening phase of the south atlantic in the late jurassic early cretaceous the composition type and structure of the basement key points for the evaluation of the basins evolution are widely speculative in this context multichannel seismic reflection data from the argentine shelf and a 665 kmlong onshore offshore refraction profile running across the colorado basin onto the coast are discussed in combination with gravity data the stratigraphy for the sedimentary successions was adopted from the literature and the reflection seismic marker horizons formed besides the interval velocities the input for the starting model for refraction seismic traveltime modelling the modelling strategy was an iterative procedure between refraction seismic traveltime and gravity modelling the preparation of the density models was coarsely orientated on published velocity density relations the modelling results are in favour of a continuation of the main onshore geological features beneath the sedimentary infill of the colorado basin we interpret the basement along the line from west to east as offshore continuation of the ventana hills the

deep structure of the west african continental margin congo zaire angola between 5 s and 8 s from reflection refraction seismics and gravity data summary the deep structure of the west african continental margin between 5 s and 8 s was investigated using vertical reflection and wide angle reflection refraction techniques during the zaiango project a joint programme conducted in 2000 april by ifremer and totalfinaelf to penetrate below the salt layer a non conventional low frequency seismic source was used in the single bubble mode together with ocean bottom instruments hydrophones and seismometers and a 4 5 km long streamer that recorded multichannel seismic reflection mcs the data show that the continental crust thins abruptly over a lateral distance of less than 50 km from 30 km thick below the continental platform based on gravity data to less than 4 km thick below the lower congo basin that formed prior to the aptian salt deposition this subsalt sedimentary basin 180 km wide 4 km thick with velocities varying from 4 7 km s 1 to 5 8 km s 1 at the bottom is located between the foot of the continental slope and the oceanic domain it is underlain by crust of an intermediary or transitional type between continental crust and what can be recognized as oceanic crust in the transitional zone a crustal upper layer is present below the pre salt sedimentary basin 3 to 7 km thick with velocities increasing from 5 8 km s 1 at the top to 6 8 km s 1 at the bottom of the layer this layer appears to thin regularly from 6 7 km thick below the depocentre of the pre salt sedimentary basin to 3 4 km thick below the western termination of the basin below this upper crustal layer an anomalous velocity layer 7 2 to 7 8 km s 1 is documented below the eastern side of the basin where the crustal thinning is at a maximum the origin of this layer is unknown several arguments like rifting duration between 15 ma and 30 ma or the absence of seaward dipping reflectors precludes the hypothesis of underplated mantle material but other hypotheses such as serpentinized material or high grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks are plausible near the ocean termination of the basin the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles 7 11 and 14 having a dense ocean bottom seismometer hydrophone obs obh spacing on these profiles an anomalous velocity layer is present in the westernmost part of the transitional zone below the basement ridge and in the oceanic domain this layer absent on profile 3 may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process

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

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

numerical simulation of a low lying barrier island s morphological response to hurricane katrina abstract tropical cyclones that enter or form in the gulf of mexico generate storm surge and large waves that impact low lying coastlines along the gulf coast the chandeleur islands located 161 km east of new orleans louisiana have endured numerous hurricanes that have passed nearby hurricane katrina landfall near waveland ms 29 aug 2005 caused dramatic changes to the island elevation and shape in this paper the predictability of hurricane induced barrier island erosion and accretion is evaluated using a coupled hydrodynamic and morphodynamic model known as xbeach pre and post storm island topography was surveyed with an airborne lidar system numerical simulations utilized realistic surge and wave conditions determined from larger scale hydrodynamic models simulations included model sensitivity tests with varying grid size and temporal resolutions model predicted bathymetry topography and post storm survey data both showed similar patterns of island erosion such as increased dissection by channels however the model under predicted the magnitude of erosion potential causes for under prediction include 1 errors in the initial conditions the initial bathymetry topography was measured three years prior to katrina 2 errors in the forcing conditions a result of our omission of storms prior to katrina and or errors in katrina storm conditions and or 3 physical processes that were omitted from the model e g inclusion of sediment variations and bio physical processes

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

the agulhas magsat anomaly implications for continental break up of gondwana abstract a new processing procedure is implemented on magsat data over the southern african region and surrounding oceans the oceanic region of the ensuing map is interpreted in this study the anomaly map is image processed and a composite image of magsat with seasat data is also presented the agulhas high amplitude satellite magnetic anomaly situated off the southeastern coast of africa in the southwestern indian ocean 31 e 35 s is clearly defined in addition a north south oceanic anomaly bordering the west coast of southern africa including in part the walvis ridge anomaly is now recognised the walvis ridge and agulhas oceanic anomalies are interpreted to represent thickened remanently magnetised during cretaceous quiet times ocean crust similarities between the walvis ridge and the agulhas magsat anomalies suggest a common genesis the agulhas anomaly is interpreted to represent the remnant scar of the process that led to fragmentation of gondwana it is hypothesised that fragmentation of gondwana was initiated by an upwelling of hotter than normal asthenosphere centred directly below the agulhas anomaly

paleomagnetism of the lesotho basalt southern africa paleomagnetic study of four sections from middle jurassic karoo lava located in lesotho and south africa yielded a new paleomagnetic pole for africa with coordinates 71 6 n 273 5 e a95 3 7 k 33 n 47 paleosecular variation estimated as the angular standard deviation of vgps distribution is 14 1 2 4 1 8 which is considerably lower than expected for this time interval and latitudinal band preliminary paleointensity results indicate a low field intensity during the acquisition of the magnetization this new result tends to support a possible correlation between secular variation and dipole intensity when the lower to middle jurassic paleomagnetic data set for africa is considered a difference of 13 9 8 7 of arc is found to exist between the mean northwest and south african poles furthermore while the northwest african pole is in very good agreement with the lower to middle jurassic north american pole whichever set of rotation parameters is used to close the north atlantic ocean the south african pole falls some 12 15 of arc apart as intraplate deformation models developed to achieve tighter fit of west gondwana prior to the opening of the south atlantic fail to account fully for the difference between northwest and south african poles the pre cretaceous apparent polar wander path for africa has to be considered with caution

kinematic keys of the santos namibe basins understanding the genesis of the very peculiar 600 km wide santos basin sa o paulo plateau system and its narrow conjugate namibe margin is a kinematic and structural problem several hypotheses have been proposed in order to explain the genesis of this system that imply the same amount of horizontal movement we investigate the consequences of the horizontal movement in the santos basin based in plate kinematic reconstructions the kinematic history of this system that we present here based on the interpretation of seismic profiles and kinematic constraints has the following consequences 1 there is no evidence of a ridge jump sensu stricto but rather a southwards propagation in the central segment of the south atlantic that starts in the northern part between the ne brazilian and gabonese margins 2 the namibe margin evolved as a transform passive margin 3 the opening direction of the santos basin sa o paulo plateau system is oblique to the general opening motions of the south american and african plates and 4 this opening is younger 6 ma than those of the other basins of the central segment of the south atlantic

demerara plateau the structure and evolution of a transform passive margin summary the amazon cone experiment acquired two transects profile sd a across the demerara plateau part of the french guiana northeast brazil continental margin to better understand rift and transform style margin evolution profile a images an intermediate type margin formed as a result of trans tensional extension in this paper we describe the modelling of wide angle and multichannel seismic and gravity data from profile d to reveal whole crustal structure and features exhibiting transform characteristics combining these results with other studies in the region and comparing our results with young rift analogues we develop a model of along margin segmentation that explains the evolution of the west equatorial atlantic interpretation of the velocity depth model for profile d shows a 35 37 km thick continental crust which thins to 10 11 km over a distance of 320 km this thinning is accommodated in two regions the narrowest region associated with the ocean continent transition is interpreted to have formed by dextral shearing of the margin along major transform zones that accommodated the initial break up geometry of the central atlantic given the orientation of the margin relative to local fracture zone traces it is likely that the second region of thinning located 162 km landward of the ocean continent transition is a result of rifting suborthogonal to the profile orientation there is no evidence of rotated faulted blocks half graben structures or riftrelated magmatism manifest as either seaward dipping reflectors or as a high velocity region within the lower crust the demerara plateau is therefore interpreted as a margin segment comprising thinned continental crust bound to the north and south by transform type zones in which trans tensional extension is accommodated in contrast to profile a modelling suggests that the eastern margin exhibits a relatively broad region of crustal thinning associated with extension consistent with a rift type setting offshore unusually thin oceanic crust of 3 3 5 7 km thickness is identified which is consistent with similar observations further south in the absence of identifiable magnetic anomalies best estimates of the initial half spreading rate of 20 mm yr 1 suggest that the thin crust throughout the region is unlikely to be a result of ultra slow spreading but instead it is most likely due to a reduced magma supply at numerous long lived transform faults reflected by those presently offsetting the mid atlantic ridge in this equatorial setting

the guinea continental margin an example of a structurally complex transform margin abstract the guinean continental margin corresponds to a wide hinge zone which developed between the jurassic central atlantic and the cretaceous equatorial atlantic the guinea marginal plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the early cretaceous on the basis of seismic profiles recorded in two selected areas located along the southern guinea margin a seismic stratigraphy is established at depth the plateau is underlain by a thick mesozoic and cenozoic sedimentary wedge made of two main ensembles a lower ensemble or sequence 1 consists of early cretaceous clastics an upper ensemble includes a series of four distinct sequences attributed to albian to tertiary deposits eastward the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of paleocene age beneath the southern margin of the guinea plateau the sedimentary sequences appear deformed by faults and folds resulting from at least two main tectonic episodes a first event only affected sequence 1 which is cut by a set of extensional faults a second tectonic phase including folding reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2 during this tectonic episode former normal faults have been reactivated as reverse faults resulting in a structural inversion a third tectonic phase was responsible for structures transverse to the general e w trend of the southern guinea margin and consisting of n s trending normal faults and associated volcanoes the spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation a first stage is characterized by a divergent rifting leading to n s trending synsedimentary normal faults during the cretaceous the tectonic activity increased as a general transform motion occurred between the guinea plateau and its american conjugate margin the demerara plateau local tensional stresses were responsible for a splay fault system active near the tip of the guinea plateau in late albian times a slight adjustment of the africa plate with respect to the south american one resulted in local squeezing of the whole sedimentary wedge after this short compressive event final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity

a revised fit of south america and south central africa abstract a new set of parameters for the total plate tectonic reconstruction of south america and south central africa is presented euler pole 46 75 n 32 65 w rotation angle 56 40 this fit is constrained by at least three pre drift tectonic features crossing from one continent to the other 1 the geophysically defined eastern and western boundaries of the submarine jurassic outeniqua basin south africa and the falkland plateau basin 2 the late precambrian transcurrent fault and mylonite belts of pernambuco brazil and foumban west africa and 3 the triassic northern tectonic front of the cape fold belt and the major morphological feature on the falkland plateau with which it is closely lined up isotopic ages of falkland plateau gneisses correspond to cape pluton and cape fold belt ages suggesting their palaeoposition was within the realm of the cape fold belt in addition the bathymetrically and geophysically defined northeastern apex of the falkland plateau fits into the re entrant angle defined on the south african margin by the steep southeast facing sheared agulhas margin and the southern face of the tugela cone simultaneously known precambrian outcrops in northeastern brazil and in the gulf of benin area of west africa are juxtaposed rather than overlapped reconstructions producing a closer fit of these cratonic areas are considered untenable

lithostratigraphic geophysical and morpho tectonic studies of the south cameroon shelf abstract a geological and structural sketch of the southern cameroon shelf was drawn on the basis of a high resolution seismic survey dredgings and kullenberg core sampling of the outcropping rocks at sea bed palynological and mineralogical data x ray diffractometry supported by occasional foraminiferal evidence revealed four main lithostratigraphic sets resting on a precambrian basement upper cretaceous albian to senonian paleocene lower eocene miocene pliocene and pleistocene three successive deformational types characterise the structure of this margin 1 n s trending fractures parallel to the onshore precambrian structural directions formed during the rifting period and crosscut by n60 e trending cross faults induced by the transform movement of the south atlantic 2 compressive structures governed by local stresses during the eocene transcurrent reactivation along the n60 e transform faults excluding a salt controlled deformation 3 a set of approximately n s faults affecting the unfolded sequence of the miocene pliocene or the quaternary and indicative of distensive conditions the geodynamic evolution is placed in the general framework of the central and south atlantic rifting and passive margin history interestingly the age of the compressive deformation varies from late albian to mid eocene depending on the regional plate motion events which are generated along the narrow fracture zones extending as far as or even into the continental domain

mass transport deposits and fluid venting in a transform margin setting the eastern demerara plateau french guiana abstract the eastern demerara plateau offshore french guiana was surveyed in 2003 during the guyaplac cruise multibeam bathymetry and acoustic imagery 6 channel seismic reflection and 3 5 khz echo sounding the data show the post transform cenozoic that the series located on the outer part of the plateau below c 2000 m contain at least twelve stacked mass transport deposits mtds that have recorded a history of large scale slope failure as well as two main normal fault sets that provide possible pathways for upward fluid migration through the series reaching at high as the uppermost mtds seabed data show that the area above the failures is characterized by circular to elongate slope parallel depressions interpreted as fluid seeps pockmarks some of them have been modified by along slope currents we suggest that the development of the mtds to results from the combinaiton of the presence of fluid overpressure at depth the geometry of the margin s deep structure in particular the existence of a free borderlateral border on the outermost plateau our results also emphasise the role of stratigraphic decollements within the cenozoic series

reply to comment by d aslanian and m moulin on a new scheme for the opening of the south atlantic ocean and the dissection of an aptian salt basin summary in their comment aslanian moulin argue that our model of south atlantic opening is incompatible with several kinematic and geological constraints as recently evaluated in moulin et al they also claim that we have not appropriately referenced their work we strongly disagree and in addressing their points will argue that our model is compatible with the most important constraints that have been assembled by the many scientists who have worked on the south atlantic and that instead the interpretation by moulin et al suffers from serious flaws

structure of the lithosphere beneath the barotse basin western zambia from magnetotelluric data author posting american geophysical union 2019 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in tectonics 38 2 2019 666 686 doi 10 1029 2018tc005246