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

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

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

thermal history of the rio muni west africa ne brazil margins during continental breakup abstract we document the thermal record of breakup of the conjugate rio muni west africa and ne brazil margins using apatite fission track analysis vitrinite reflectance data and stratigraphic observations from both margins these results permit determination of the timing of four cooling episodes and the temperature of samples at the onset of each episode all samples are interpreted to have experienced higher temperatures in the geological past due to i elevated basal heatflow palaeogeothermal gradient in rio muni 1 well decaying from 58 c km during the mid cretaceous to 21 5 c km in the late cenozoic and ii progressive exhumation from formerly greater burial depth a well constrained history of changing palaeogeothermal gradient allows for much more precise quantification of the thickness of eroded section exhumation than if a constant heatflow is assumed cooling episodes identified from the palaeotemperature data at 110 95 ma both margins and 85 70 ma rio muni only coincide with major unconformities signifying respectively the cessation of rifting breakup and compressional shortening that affected the african continent following the establishment of post rift sedimentation drift the interval between these separate unconformities is occupied by allochthonous rafts of shallow water carbonates recording gravitational collapse of a marginal platform the rift shoulder uplift that triggered this collapse was enhanced by local transpression associated with the obliquely divergent ascension fracture zone and thermal doming due to the coeval st helena and ascension plumes the data also reveal a c 45 35 ma cooling episode attributed to deep sea erosion at the onset of eo oligocene ice growth and a c 15 10 ma episode interpreted as the record of miocene exhumation of the west african continental margin related to continent wide plume development integration of thermal history methods with traditional seismic and stratigraphy based observations yields a dynamic picture of kilometre scale fluctuations in base level through the breakup and early drift phases of development of these margins major unconformities at ocean margins are likely to represent composite surfaces recording not only eustasy but also regional plate margin generated deformation local intra basinal reorganization and the amplifying effect of negative feedbacks between these processes

geochemical and biological marker assessment of depositional environments using brazilian offshore oils abstract a combined geochemical and molecular characterization of a wide selection of oils from the major brazilian offshore basins has been undertaken the elemental sulphur nickel and vanadium and bulk oapi and 13c properties of each sample been considered together with its molecular composition determined using liquid and gas chromatography and quantitative biological marker investigations using gas chromatography mass spectrometry for alkanes the results reveal significant differences in the chemical features of the various oils which enable them to be divided into five groups the distinction of the groups appears to reflect differences in the depositional environment of the source rocks of the oils each group is correlated tentatively with source rocks laid down in a specific depositional regime namely lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate or marine deltaic the diagnostic features that allow this classification are the relative abundance and carbon number distributions of n alkanes pristane phytane ratios sulphur nickel and vanadium contents carbon isotope data the absolute concentrations of hopanes and steranes and their abundance relative to 4 methylsteranes and also the occurrence and abundance of several specific biological markers including 18 h oleanane gammacerane carotane tricyclic terpanes higher acyclic isoprenoids 28 30 bisnorhopane and 25 28 30 trisnorhopane this investigation shows the value of a combined geochemical and molecular approach in the assessment of the palaeoenvironment of deposition of the source rocks which gave rise to the oils

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

rapid climate shifts in the southern african interior throughout the mid to late holocene a detailed climate proxy record based on 18o 13o and grey index of a well dated stalagmite from cold air cave in the makapansgat valley of north eastern south africa suggests that regional precipitation temperatures and vegetation oscillated markedly and rapidly over the last 6500 years on centennial and multi decadal scales the mid holocene prior to 5200 years ago was humid and warm a fundamental transition occurred 3200 years ago leading to drier and cooler conditions that culminated at 1750 ad comparisons with ice core records suggest synchronous changes implicating rapid global teleconnections

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

deep pass an aeromagnetic data filter to enhance deep features in marginal basins abstractan aeromagnetic data filter was tested as a tool in enhancing the magnetic responses of deep sources associated with marginal basins our approach consists in amplifying the responses of magnetic sources which extend through the continental slope and continental rise this amplification compensates for the magnetic response attenuation over the continental slope as a result of 1 the seaward increase of the distance between the source and the magnetometer and 2 the seaward decrease in the volume of sources within the continental crust to amplify the source response we multiply the total field anomaly at a given location by the exponential of the bathymetric depth at the same point tests on synthetic and real data showed that the filter was able to recover the locations of linear magnetic responses not clearly seen in the total field anomaly map when compared with the analytic signal amplitude and the total horizontal derivative filters the proposed filter proved to be complementary in enha

radial gravitational gliding on passive margins abstract gravitational gliding of uppermost sediments down a passive margin is possible if there is a basal layer of evaporite or other soft material to allow detachment in examples from the gulf of mexico and the brazilian margin gliding has produced three main structural domains an uppermost domain of downdip extension an intermediate domain of rigid gliding and a lowermost domain of downdip contraction domain boundaries are established by changes in slope in this paper we examine three kinds of gravitational gliding depending on the paths followed by material particles in ideal parallel gliding particle paths are parallel straight lines trending downslope this should occur where the margin is perfectly straight in ideal radial gliding particle paths are radii of a circle and the margin is shaped like a circular cone natural margins will not have ideal shapes but divergent gliding will tend to occur off coastal salients convergent gliding off coastal re entrants a simple kinematic model based on ductile behaviour illustrates some essential features of radial gliding changes in radius during divergent gliding produce strike parallel extension during convergent gliding they produce strike parallel contraction vertical strains also differ divergent gliding produces an uppermost domain of strong vertical thinning balanced by extensions in all horizontal directions similarly convergent gliding produces a lowermost domain of strong vertical thickening balanced by contractions in all horizontal directions these deformed states cannot be restored by simple techniques based on section balancing we have done three experiments using analogue materials sand to model the brittle behaviour of sediments silicone putty to model the ductile behaviour of basal layers of evaporite the experiments were properly scaled to account for gravitational forces experiment i reproduced convergent gliding above a basement with a conical upper surface strike parallel contraction was responsible for radial folds on the lower slopes the lowermost domain had a complex fold pattern indicating contraction in all horizontal directions experiment 2 reproduced divergent gliding above a basement with a pyramidal upper surface strike parallel extension produced radial rifts on the upper slopes the uppermost domain had a complex polygonal pattern of normal faults indicating extension in all horizontal directions experiment 3 also reproduced divergent gliding but due to non uniform sediment thickness we have examined the many seismic profiles both downdip and alongstrike available for the brazilian margin alongstrike profiles show structures which we attribute to strike parallel strains around the santos re entrant syn sedimentary growthfolds indicate strike parallel contraction increasing downslope this argues for a mechanism of convergent gliding in contrast around the campos salient normal growth faults and associated salt rollers indicate strike parallel extension decreasing downslope this argues for a mechanism of divergent gliding the pattern of oil fields in the campos area including the giant marlim field seems to be closely controlled by divergent gravitational gliding and associated processes radial gliding should occur on other passive margins as well especially where the coastline is sinuous structures around the western gulf of of mexico seem to indicate convergent gliding at the scale of the entire gulf structures at a smaller scale around the mississippi delta seem to indicate divergent gliding

high resolution stratigraphy and sedimentary evolution of coarse grained canyon filling turbidites from the upper cretaceous transgressive megasequence campos basin offshore brazil abstract coniacian to lower maastrichtian coarse grained turbidites fill intra slope fault controlled canyons in the campos basin off shore brazil they form part of an upper albian to lower paleocene transgressive succession characterized by onlapping deepening upward sedimentation throughout the eastern brazilian margin in the carapeba and pargo crp pg oil fields the turbidites consist mainly 95 of graded beds without any other sedimentary structures individual beds are up to 12 m thick and are composed of small pebble 2 cm to granule rich sandstones and medium to very coarse sandstones the finer grained portions of these graded beds show better sorting and higher porosity and these characteristics can be recognized in density logs use of 92 density logs calibrated w th 13 cores permitted the mapping of 198 coarse grained turbidites in the crp pg turbidite system the turbidites form eight thinning and fining upward facies successions some bounded by regional unconformities or local erosion surfaces the successions are 27 140 m thick and contain 7 58 turbidites their durations are estimated to be between 0 4 0 9 m y they form 1 12 km wide tabular or linguoid sandstone bodies in which the younger or more distal turbidites become finer grained thinner bedded and more discontinuous upsection and downcanyon the successions are stacked in an overall retrogradatioual pattern for at least 20 km recording the backfilling of the crp pg canyon turbidites in the crp pg area were probably deposited during falls of relative sea level that punctuated the overall transgressive setting of the late cretaceous and early tertiary there is a mismatch between the number of successions eight and the number of time equivalent third order fluctuations of relative sea level four and part of a fifth haq et al 1988 suggesting that eustatic sea level fluctuation was not the dominant control on most of the successions cross sections and isopach maps of the successions do not suggest internal channeling and a channel fill origin for the fining and thinning upward successions seems unlikely we therefore suggest that episodic tectonic reactivation in the uplifted precambrian source area and faulted basin margin led to relatively abr pt increases in the volume of sediment supplied to the head of the canyon across a narrow shelf steady denudation and decreasing supply of sediment led to formation of the fining and thinning upward successions

distribution of deep sea black shales of cretaceous age in the eastern equatorial atlantic from seismic profiling abstract the presence of thick 200 m carbonaceous shales of cretaceous age in the deep waters of the eastern equatorial atlantic was first revealed by drilling at dsdp sites 367 and 368 near the cape verde islands more recent seismic investigations show that black shales are widely distributed in this region in the gambia basin they are associated with a group of seismic reflectors lying at depths of 400 600 m below the sea floor these can be traced southwards from the drill sites into the sierra leone basin and eastwards beneath the african continental rise to depths at which ambient temperatures fall within the oil generation window in the gambia basin the reflectors continue westwards from the cape verde archipelago before terminating on turonian oceanic basement 92 ma south of the guinea fracture zone near 10 n extension of the same seismic sequence on to younger oceanic crust indicates that deep sea carbonaceous deposits formed close to the present equator as late as the early campanian 80 ma long after a series of oceanic anoxic events in the aptian albian oae1 and over the cenomanian turonian boundary oae2 led to widespread accumulation of black shales this late period of deposition may correspond to the more locally developed oae3 high surface productivity in equatorial upwelling zones during the late cretaceous probably played an important role in delivering large amounts of organic carbon to the sea floor when meridional movement of deep water between the south and north atlantic was restricted by closely spaced east west fracture zones in some areas seismic data show that the black shales level out rather than drape pre existing topography implying significant downslope transport of organic rich material from upwelling cells near the african margin

north african paleodrainage discharges to the central mediterranean during the last 18 000 years a multiproxy characterization using elemental geochemistry clay mineralogy grain size end member modeling and planktonic foraminiferal 18o we characterize the provenance of central mediterranean sediments over the past 18 ka the provenance is dust dominated before and after the african humid period ahp by contrast during the ahp 11 5 ka largely concurrent with organic rich sapropel s1 formation it is predominantly riverine from north african sources such fluvial supply is suggested to come from paleodrainage networks that were reactivated by intensified monsoon precipitation during the ahp the supply is characterized by high mg al and smectite contents and has been accompanied by considerable freshwater influx as indicated by the enhanced grain size and lighter foraminiferal 18o the clay mineral assemblages in our core and in nearby cores correspond with a provenance from the libyan tunisian margin mainly via the paleo river irharhar the inferred fluvial discharge is strongest during the late ahp 8 5 5 ka coinciding with reported enhanced fluvial dynamics and wettest conditions over western libya and tunisia algeria this period is not only synchronous with the largest extension of open water bodies in north africa and lowest saharan dust inputs but also consistent with precipitation records of the west african monsoon moreover our records show a remarkable correspondence with that of a paleodrainage system towards the atlantic west african margin inferring a common headwater region in the central saharan mountains and a similar climate mechanism taken together we suggest a dominant control of north african humid surfaces on the paleodrainage delivery modulated by groundwater level in response to the insolation driven west african monsoon precipitation

wide and narrow margins of the brazilian south atlantic two contrasting end members of passive continental margins here referred to as wide and narrow margins are present in the south atlantic although some margin segments are intermediate between these two styles narrow margins are characterized by a large bounding fault 4 km throw near the shelf edge a sharp transition zone 10 20 km wide from normal thickness to substantially thinned continental crust or oceanic crust where the total width of extended continental crust is usually less than 100 km wide continental margins are characterized by a broad continental shelf and a wide zone 50 600 km of thinned crust with an even distribution of faults wide margins usually have a more gentle continental shelf and slope narrow narrow wide wide and wide narrow conjugate margin pairs exist in the south atlantic and are suggested to be controlled mainly by the opening kinematics and the thermal structure of the lithosphere during rifting basement structure and strain rate appear to have exerted little control on the style of margin extension although basement architecture appears to control the trend of the initial rifts the positions of fracture zones which separate narrow and wide segments and the rate of lateral rift propagation

major controlling factors on hydrocarbon generation and leakage in south atlantic conjugate margins a comparative study of colorado orange campos and lower congo basins abstract we present a supra regional comparative study of the major internal and external factors controlling source rock sr maturation and hydrocarbon hc generation and leakage in two pairs of conjugate margins across the south atlantic the brazil campos basin angola lower congo basin margins located in the central segment and the argentina colorado basin south africa orange basin in the southern segment our approach is based on the analysis and integration of borehole data 1d numerical modeling 2d seismic reflection data and published reports coupling of modeling results sedimentation rate calculation and seal bypass system analysis reveal that 1 oil window is reached by syn rift srs in the southern segment during the early to late cretaceous when thermal subsidence is still active while in the central segment they reach it in late cretaceous neogene during a salt remobilization phase and 2 early hc generation from post rift srs in the southern segment and from all srs in the central segment appears to be controlled mainly by episodes of increased sedimentation rates the latter seems to be associated with the andes uplift history for the western south atlantic basins campos and colorado and to a possibly climate driven response for the eastern south atlantic basins orange and lower congo additionally we observe that the effect of volcanism on sr maturation in the southern segment is very local the comparison of cretaceous mass transport deposit mtd episodes with hc peak of generation and paleo leakage indicators in the southern segment revealed the possible causal effect that hc generation and leakage have over mtd development interestingly paleogene leakage indicators which were identified in the argentina south africa conjugate margins occur contemporaneously to low sedimentation rate periods nonetheless present day leakage indicators which were also identified in both pairs of conjugate margins might be related to seal failure events linked to eustatic sea level drops

episodic burial and exhumation in ne brazil after opening of the south atlantic it is a common assumption that elevated passive continental margins have remained high since rifting and breakup here we show that the atlantic margin of ne brazil has undergone a more complex history our synthesis of geological data landscape analysis and paleothermal and paleoburial data reveals a four stage history 1 after early cretaceous breakup the margin under went burial beneath a thick sedimentary cover 2 uplift episodes in the campanian and eocene led to almost complete removal of these deposits 3 the resulting large scale low relief erosion surface peneplain was deeply weathered and finally reburied at the oligocene miocene transition and 4 miocene uplift and erosion produced a new lower level peneplain by incision of the uplifted and re exposed paleogene peneplain previous studies have identified aspects of this interpretation but we have defined the absolute timing and magnitude of discrete events of burial and exhumation that followed early cretaceous rifting and eocene oligocene peneplanation we suggest that a late sedimentary cover protected paleogene weathering profiles until the present day the uplift phases in brazil are synchronous with uplift phases in africa and the andes the andean phases coincided with rapid convergence on the western margin of south america and the campanian uplift coincided with a decline in spreading rate at the mid atlantic ridge consequently we suggest that both vertical movements and lateral changes in the motion of the plates have a common cause which is lateral resistance to plate motion

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

tectonic control of the triassic santa maria supersequence of the parana basin southernmost brazil and its correlation to the waterberg basin namibia abstract the middle and late triassic santa maria basin exposed in southernmost brazil and waterberg basin in namibia are herein interpreted as part of en echelon small basins in southern west gondwana the main structures are the waterberg omaruru fault which can be connected to a nw strike anastomosed fault zone in brazil based on field structural and stratigraphic analysis two populations of nw strike fractures named a type and b type are recognized a type fractures az 280 290 occur in the sanga do cabral supersequence underlying units of santa maria basin as extension of the waterberg omaruru fault during the early triassic b type fractures az 295 345 are observed in all studied stratigraphic units from the triassic sanga do cabral supersequence to the early cretaceous botucatu serra geral formations based on the structural analysis we propose that nne ssw extension reactivated structures of the damara belt namibia with a propagation towards rio grande do sul state forming an anastomosing normal fault system and related rift basin by early middle triassic time the a type fractures were preferentially active by this phase and the b type ones are interpreted as secondary link segments within the anastomosing system during the gondwana break up and south atlantic opening rift to proto oceanic phase aptian the b type fractures were reactivated as normal faults by n50 e s50 w extension

estimates of yet to find impact crater population on earth abstract one hundred and seventy six craters on earth are currently accepted as resulting from bolide impact this list is growing at a rate of approximately two craters per year because of new discoveries and revised interpretations here we take the subset of craters most relevant to sedimentary basin exploration and utilization phanerozoic kilometre scale craters and tackle the question of how many remain to be found in phanerozoic strata on earth a surprising range in predictions of crater numbers is obtained from published crater population functions reaching two orders of magnitude variation for craters in the range 1 5 km diameter this range in predictions is largely due to variations in treatment of impactor breakup in the atmosphere also required is quantification of the total area and timespan of phanerozoic sediment currently preserved on earth an estimate of 8 87 10 15 km 2 a is derived here together with a relatively conservative crater population function this yields a prediction of 845 craters subtracting the 131 known phanerozoic craters larger than 1 km leaves a predicted population yet to find of 714 of which 228 are larger than 2 5 km uncertainties subsets of this population and spatial distribution are also considered

influence of precambrian shear zones on the formation of oceanic fracture zones along the continental margin of brazil abstract structural inheritance control of oceanic fracture zone formation by a large basement shear zone during continental breakup is still a matter of debate in this research we investigated the reactivation of precambrian shear zones and their role in the formation of the ascension and fernando poo fracture zones during the evolution of the south atlantic ocean we used aeromagnetic and multichannel 2d seismic reflection data associated with previous geological and geophysical data to analyze how the precambrian shear zones may have influenced the formation of oceanic fracture zones our results suggest that the fracture zones are extensions of the shear zones in the transition between continental and oceanic crust the sao miguel do aleixo and pernambuco shear zones were reactivated during the pangea breakup and acted as zones of weakness controlling the location of the ascension and fernando poo fracture zones during the process of oceanic crust spreading our model suggests that the formation of these fracture zones influenced by the shear zones occurred in the early stages of ocean opening it indicates that the fracture zones that formed in the late opening stages were related to the thermal subsidence stage as suggested in previous studies

shelf fed turbidite system model and its application to the oligocene deposits of the campos basin brazil despite the large number of models involving the genesis and sedimentary facies of deep water sandstones none of these models adequately explain the origin and evolution of the extremely clean widespread over 6000 km2 predominantly massive thick over 150 m blanket like sandstones deposited in the deep water environment of the campos basin during the oligocene consequently to explain this sandstone i propose a shelf fed turbidite system model which is strongly based on the campos basin data set the basic framework necessary for the development of a shelf fed turbidite system includes 1 deposition of a large volume of clastics during the buildup of a shelf sand rich unit which later constitutes the main source of sediment for the system 2 localized tectonic pulses that modify the outer shelf declivity and trigger mass flows and 3 a relative fall of sea level which causes the subaqueous exposure of the shelf sediments to reworking in a shallow high energy marine environment these three basic elements are equally important for shelf fed turbidite system development but relative sea level position controls the development of the progradational aggradational and retrogradational depositional phases within the system submarine canyons commonly are scoured during al three phases on the outer shelf and lower slope environments the shelf fed turbidite system model may apply to other sedimentary basins principally to those of the atlantic continental margins that have a thick evaporite sublayer halokinesis can provide the necessary room for the shelf sedimentary unit buildup the tectonic pulses that trigger the flows and even localized relative sea level oscillations that can accelerate or abort any one of the depositional phases of the system

late cretaceous anoxic events in the brazilian continental margin abstract a combined geochemical and micropalaeontological study of cenomanian to maastrichtian pelitic sediments from the brazilian continental margin has been undertaken the organic carbon contents rock eval pyrolysis results and biological marker distributions together with the taxonomic selection and distribution patterns of microfossil assemblages indicate that intermittent anoxic events occurred in the brazilian marginal basins during the cenomanian santonian with deposition taking place in progressively deeper waters from neritic to bathyal such events are characterised by the following 1 i a predominance among the benthonic foraminifera of low diversity small sized specimens mainly composed of gavelinellids buliminids and nodosariids indicating dysaerobic bottom conditions 2 ii levels with virtually no benthonic foraminifera and a well developed planktonic biota foraminifera radiolarians and diatoms indicating anaerobic bottom conditions with oxygenated surface waters 3 iii the occurrence of organic rich calcareous mudstone sediments 4 iv features in the biological marker distributions such as high concentrations of 28 30 bisnorhopane and 25 28 30 trisnorhopane up to 130 ppm a predominance of phytane over pristane and medium to high concentrations of nickel and vanadyl porphyrins up to ca 5700 ppm the widely recognized cenomanian turonian and santonian oceanic anoxic events schlanger and jenkyns 1976 ryan and cita 1977 and jenkyns 1980 are therefore manifest in the cassipore ceara sergipe alagoas and campos basins within the brazilian continental margin as is the presence of such events in the coniacian in contrast the campanian maastrichtian appears to be a time interval when deposition under oxygenated conditions produced sediments with low organic carbon contents and poor hydrocarbon source potential containing highly diversified and abundant calcareous and agglutinated benthonic foraminifera

structural comparison of archetypal atlantic rifted margins a review of observations and concepts abstract in this study we compare three pairs of conjugate rifted margins that are often referred to as archetypes of rift systems despite numerous differences a remarkable first order structural similarity appears between the magma poor iberia newfoundland the magma rich mid norway east greenland and the sediment rich angola brazil rifted margins typical is the seaward arrangement of distinct domains proximal necking distal and outer that each present specific comparable structural characteristics our study also suggests that magma rich systems may go through a stage of hyper extension indicating that extreme crustal thinning does not preclude a magmatic breakup in this contribution we clarify the definition of a number of terms introduced recently in rifted margin studies we review the major features constitutive of these key referenced systems discuss their similarities and differences and examine how the related deformation modes develop in the life cycle of a rift that goes to seafloor spreading we conclude that the distinct domains observed in the margin architecture represent distinct stages in the evolution of rifted margin independently of their later evolution into magma poor or magma rich environments

submarine salt flows in the central red sea the central red sea an oceanic basin floored by miocene evaporites reaching kilome ters in thickness in places is at an early stage of development where seafl oor spreading has geologically only recently replaced continental rifting surveys using a high resolution multibeam echo sounder around thetis deep a new spreading center have revealed a remarkable series of structures resembling viscous gravity fl ows which are here interpreted as originating from fl owage of the evaporites laterally unloaded by axial rifting and other processes developing the relief of the deep the fl ow margins are marked by stream wise lineaments and some apparently rotated markers their fronts in the fl oor of the deep are rounded in plan view and profi le their surfaces contain small closely spaced features resembling extensional faults in one area below declining gradients the surface contains along slope ridges and valleys typical of compression folds ogives flowparallel lineaments and extensional faults lie respectively parallel and orthogonal to the direction of maximum seabed gradient movement is apparently heterogeneous at least in part because of varied blocking by relief in underlying basement observed protruding between fl ows flowage is currently transporting materials into the fl oor of the deep where they have the potential to become incorporated into the young oceanic crust by repeated eruption of axial lavas over them in the light of these new data we reexamine the possibility and implications of fl owage in the south atlantic marginal evaporites in particular whether fl owage contaminated early oceanic crust in such areas

k ar ages and the opening of the south atlantic ocean basaltic rock from the brazilian margin abstract new k ar ages for 13 samples of basalt gabbro wehrlite and trachyandesite drilled from the brazilian continental shelf and coastline yield information about the timing of the opening of the south atlantic and the nature of the crust seaward from the brazil margin the oldest basalt is 138 1 3 5 m y old and from offshore at 24 5 s it represents serra geral flood basalt on attenuated crust in the santos basin basalts from the coastline at 19 5 s are mixed with terrigenous graben sediments and their ages confirm that rifting was underway before 130 m y offshore sites in the campos basin 22 23 s 41 w have basalt ranging from 124 to 112 m y in age and mostly continental flood basalt in composition one 112 m y basalt however is morb like and could therefore represent oceanic lithosphere mixed with continental crust about 50 km from the shoreline at 22 5 s other samples reveal compositionally varied intraplate passive margin magmatism occurring 75 43 m y ago they correlate with profuse contemporary alkalic magmatism on the southeastern brazil coast and probably represent reactivation of zones of weakness i e fracture zone lineaments

controls on the thermomechanical evolution of hyperextended lithosphere at magma poor rifted margins the example of espirito santo and the kwanza basins high quality long offset seismic data from many distal rifted margins show evidence for hyper extended 10 km thick crust direct observation of such domains is challenging as they lie at great water depth buried beneath thick sedimentary sequences and formed by rock assemblages that are hydrated and geophysically indistinguishable only a few drill holes have penetrated basement at ultradistal rifted margins these observations together with outcrops of preserved analogs exposed in collisional orogens suggest that the complex interaction of detachment faults rooted in a subhorizontal shear zone in the hyperextended crust or in the serpentinized mantle controls the formation of the ocean continent transition while depth dependent thinning controls the early phases of rifting conforming to classical rift models we still have a superficial understanding of how normal faults and subhorizontal shear zones form and evolve during rifting and lithospheric breakup here we develop a rheological parameterization to simulate the formation of and slip on large offset normal faults rooted in growing brittle to ductile shear zones the evolution of these structures leads to the creation of a hyperextended crust and eventually exhumed serpentinized mantle we also propose a simplified formulation to simulate magmatic underplating and seafloor spreading the resulting numerical models provide a self consistent picture for the evolution of magma poor rifted margins from initiation of rifting to seafloor spreading the model results are compared with first order observations of the kwanza and espirito santo conjugate margins in the south atlantic as well as of magma poor margins globally