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

evidence for transform margin evolution from the ivory coast ghana continental margin the relative motions between lithospheric plates can be reduced to three major types convergence divergence and transform motion convergence leads to an active margin and eventual collision divergence results in the construction of a rifted passive margin transform motion generates a transform or sheared margin although passive margins have been extensively studied1 2 and many models have been proposed for their origin and subsequent evolution3 5 little is known about transform margins with the exception of a few studies of their morphologies6 7 shallow structures or crustal sections8 9 here we present the main conclusions derived from a recent study of the northern gulf of guinea margins particularly off the eastern ivory coast and ghana where the continental margin is one of the best preserved examples of an extinct transform margin10 12 our observations support a four stage model for transform margin evolution technically active transform contacts first between normal continental crusts and then between thinned margins induce characteristic structures such as pull apart grabens and shear folds the next stage in which thermal exchange between oceanic and continental lithos pheres controls a complex subsidence is followed by the transition to a true intra oceanic fracture zone

continental break up along strike slip fault zones observations from the equatorial atlantic the study focuses on equatorial atlantic margins and draws from seismic well gravimetric and magnetic data combined with thermo mechanical numerical modelling our data and numerical modelling indicates that early drift along strike slip originated margins is frequently characterized by up to 10 20 spreading vector adjustments in combination with the warm thinned crust of the continental margin these adjustments control localized transpression our observations indicate that early drift margin slopes are too steep to hold sedimentary cover which results in their inability to develop a moderately steep slope undergoing cycles of gravitational instability resulting in cyclic gravity gliding these slopes either never develop such conditions or gain them at later development stages our modelling suggests that the continental margin undergoing strike slip controlled break up experiences warming due to thinning along pull apart basin systems pull apart basins eventually develop sea floor spreading ridges margins bounded by strike slip faults located among pull apart basins with these ridges first undergo cooling however spreading ridges leaving the break up trace along its strike eventually pass by these cooling margins warming them again before the final cooling proceeds as a result the structural highs surrounded by several source rock kitchens witness a sequential expulsion onset in different kitchens along the trajectory of spreading ridges

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

thermal consequences of lithosphere extension over continental margins the initial stretching phase summary we compute the thermal evolution of a lithosphere submitted to stretching during a finite duration of time in order to discuss the initial stretching phase of future continental margins the numerical method developed can handle 2 d laterally variable stretching as well as sedimentation it is shown that lateral conduction is more important than vertical conduction over most continental margins during their formation by stretching a simple way to evaluate the relative importance of lateral and vertical conduction effects at the axis of the zone of rifting just prior to oceanization is proposed a simple way to evaluate the amplitude of the thermal uplift on the edges of the zone of rifting at the end of the stretching phase is also presented for small width zones of rifting 70 100 km lateral cooling becomes so large as to prevent large scale melting and presumably prevent the transition to oceanization the effect of highsedimentation rates 100 500 m myr 1 is to increase the surface temperature of the lithosphere and consequently significantly decrease the surface heat flow

long term evolution of the west african transform margin estimates of denudation from benin using apatite thermochronology the benin continental margin was formed during the breakup of gondwana through oblique rifting along transform faults the evolution of topography following breakup directly affects the evolution of sedimentary basins which has major implications for hydrocarbon exploration in the region quantitative constraints on erosion across benin are limited to the cenozoic based on analysis of dissected lateritic palaeolandscapes to resolve the mesozoic erosion history we have obtained apatite fission track and single grain u th sm he data from 18 samples collected across a 600 km long transect through benin we invert these data including available geological and geomorphological constraints to obtain time temperature paths which are used to estimate magnitudes of denudation over the last 200 myr our study suggests that denudation was focused over a c 300 km long seaward sloping limb of the marginal upwarp and at the southern margin of the interior iullemmeden basin from 140 to 100 ma with lower magnitudes of denudation characterizing the continental interior and post cretaceous evolution of the margin models are consistent with modest burial c 1 km of the iullemmeden basin between 120 and 85 ma and of the continental margin between 85 and 45 ma by the eocene the first order relief of benin had developed with regional erosion rates supplementary information full details of the analytical data and modelling results including the various constraints and further data on denudation and burial magnitudes and rates are available at https doi org 10 6084 m9 figshare c 4220804

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

differential opening of the central and south atlantic oceans and the opening of the west african rift system abstract plate tectonic studies of the development of the central and south atlantic oceans using seasat and geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time such models enable an initial assessment of the differential opening between these two oceanic basins to be determined the equatorial atlantic is an integral part of this oceanic rifting process allowing stresses arising from the differential opening to be dissipated into both the caribbean and africa along its northern and southern boundaries respectively the tectonic model for the west african rift system based on geological and geophysical studies shows a series of strike slip fault zones diverging into africa from the gulf of guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones the development of the west african rift system was contemporaneous with the early opening of the south atlantic continued to develop well after the final breakup of south america from africa and did not cease until the late cretaceous when there was a major phase of basin inversion and deformation santonian 80 ma deformation across the benue trough nigeria is broadly contemporaneous with dextral shear reactivation of the central african fracture system which in turn resulted in renewed extension in the sudan basins during the late cretaceous and early tertiary this paper illustrates the close linkage in both time and space between the history of the african rift basins and the opening of the atlantic both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the central and south atlantic in order to dissipate stresses generated by this opening the mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics

development history of the southern terminus of the central atlantic guyana suriname case study the study focuses on the offshore guyana suriname french guiana region it draws from seismic well gravimetric and magnetic data they indicate that the continental break up along the western margin of the demerara plateau took place during the callovian oxfordian associated with the central atlantic opening and accommodated by normal faults the continental break up in the se offshore guyana accommodated by strike slip faults was coeval the continental break up along the ne and eastern margins of the demerara plateau took place during the late aptian albian associated with the opening of the equatorial atlantic and accommodated by dextral strike slip and normal faults respectively different spreading vectors of the central and equatorial atlantic required development of the accommodation block during the late aptian albian paleocene in their contact region and in the region between the central atlantic and its southernmost portion represented by the offshore guyana block which were separated from each other by the opening equatorial atlantic its role was to accommodate for about 20 mismatch between the central and equatorial atlantic spreading vectors which has decreased from the late aptian albian to paleocene down to 0 differential movements between the central and equatorial atlantic oceans were also accommodated by strike slip faults of the guyana continental margin some active until the paleocene

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

sediment loading on the western platform of the new zealand continent implications for the strength of a continental margin abstract mechanical properties of the new zealand lithosphere are determined by forward modelling the gravity and isostatic response of a plio pleistocene sediment loaded shelf margin the giant foreset formation on the western platform of new zealand is a 2 km thick 250 km long and 100 km wide sediment package both the downward flexed pliocene miocene boundary beneath the package and a 35 50 mgal peak to trough gravity anomaly over the sediment package are best matched with a flexural rigidity in the region of 5 6 10 22 1 5 10 23 n m effective elastic thickness t e 20 28 km the giant foresets are a particularly important load to model because unlike most sediment loaded passive margins there is a time gap of about 70 80 ma between rifting and subsequent deposition of the sedimentary load furthermore the width of the giant foreset load is sufficiently small to afford a reasonable resolution of t e

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

the effect of rift shoulder erosion on stratal patterns at passive margins implications for sequence stratigraphy abstract numerical modelling indicates that the erosion of uplifted rift flanks at passive margins has a profound effect on offshore stratigraphic patterns flexural uplift due to isostatic rebound in response to erosion extends far into the basin and causes uplift of the shelf as a result the contemporaneously deposited sedimentary wedge displays a characteristic offlap pattern when the rift shoulder is largely eroded onlap promoting mechanisms related to cooling of the lithosphere enable sediments to onlap onto the basin margin the initial offlapping and subsequent onlapping strata form one complete second order depositional sequence comprising a shelf margin transgressive and highstand systems tract the modelling inferences are in broad agreement with stratal patterns and basin geometries observed on the u s east coast the southeastern brazilian and southeastern australian passive margins and the transantarctic mountains ross sea shelf system the initial offlaps caused by erosion of rift shoulders have important implications for the deriviation of eustatic signals from coastal onlap patterns

spatial patterns of infiltration vary with disturbance in a shrub encroached woodland abstract woody plant encroachment is known to have substantial effects on a range of ecosystem processes research worldwide indicates that the area around shrubs and trees has higher levels of infiltration than the interspaces little is known however about the hydrological consequences of shrub removal on infiltration and how this might be influenced by grazing we examined the spatial patterns of infiltration across three treatments relating to shrub removal and grazing i undisturbed ungrazed unploughed ii grazed but unploughed and iii grazed and ploughed in general we found that disturbance was associated with a greater cover of bare soil but lower infiltrability our laboratory based measure of infiltration at the undisturbed site bare soil was patchy and localized with an autocorrelation range or connectivity of 1 4 m the autocorrelation range of infiltrability at this site a0 3 m was larger than would be predicted from the size of the shrub canopy and this was attributed to the presence of a well developed understorey layer and biological soil crust community at both grazed sites infiltration was confined to the immediate canopy area of the remaining shrubs a0 1 2 m in the unploughed grazed site additionally there was increasing connectivity of bare soil with disturbance up to 6 8 m at the ploughed grazed site with increasing disturbance resource rich shrub patches are likely to become more developed further reinforcing their growth and persistence at the expense of the bare interspaces our results indicate the importance of shrubs for maintaining landscape connectivity and the long term unsustainable practice of removal by ploughing which is likely to promote shrub dominance rather than suppression

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

role of a ductile decollement in the development of pull apart basins experimental results and natural examples abstract traditional models describe pull apart basins as graben or half graben basins with normal or normal oblique slip master faults analogous to death valley california yet many pull aparts are characterized by asymmetric basins with strike slip master faults indicating that not all pull apart basins conform to the simple death valley models we present analogue modelling results that show developmental sequences and structural styles of pull aparts are dramatically different when overburden rides over a ductile horizon and that thickness of the ductile horizon exerts control on basin development in our models synthetic and antithetic strike slip faults control basin geometries while localized normal faulting and local oblique slip on strike slip faults accommodate basin subsidence faults evolve from initial strike slip to normal oblique and normal dip slip to form a system of either isolated sub basins in the case of thick ductile layers or coalescing sub basins in the case of thin ductile layers these results demonstrate distinct differences between non ductile and ductile decollement pull apart structures basin boundaries dominated by normal faults suggest a decollement within or at the base of a non ductile layer similar to death valley california basin bounding faults dominated by strike slip and oblique slip faults indicate basin formation over a ductile layer similar to the gulf of elat aqaba or gulf of paria venezuela and trinidad

shallow structures and evolution of the ivory coast and ghana transform margin abstract seismic profiles across the transform continental margin off the ivory coast and ghana western africa illustrate the structural style resulting from the early cretaceous phase of shear stress which leads to the final separation between the african and brazilian continental margins in this area most of the characteristic tectonic features observed along this portion of margin asymmetric grabens on the ghanean platform folds of the deep ivory coast basin the ivory coast ghana marginal ridge are believed to result from progressive transform contacts between the african and brazilian continents as their margins were created during early cretaceous time a major tectonic unconformity inferred to be of upper albian lower cenomanian age may be a direct consequence of the final separation of the continental margins the later evolution of the transform margin is chiefly explained by thermal subsidence

crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin implications for thermal exchanges across the palaeotransform boundary summary crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin is determined by 2 d modelling of wide angle seismic data and gravity data the resulting models together with geological data recently collected by drilling on the margin are compared with available thermomechanical models of transform margins in particular we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere these effects include i ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it from the continent towards the ocean and ii thermal uplift of the transform margin in the region of the cote d ivoire ghana transform margin the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent ocean boundary is approached the interpreted models lack a transitional domain between continental and oceanic crusts and abnormally thin oceanic crust is found along the margin these results exclude large scale ductile flow of lower continental crust during the margin formation the geological data are moreover not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere moderate heating of the cote d ivoire ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models two main factors may have contributed to reduce thermal exchanges across the continent ocean boundary the first factor is that the oceanic edge may have been colder than assumed by thermal models an abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the cote d ivoire ghana transform margin the oceanic crustal structure is similar to that found along oceanic fracture zones where oceanic crust is also emplaced near a ridge transform intersection in both cases abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of i a cold edge effect of the adjacent continental lithosphere ii accretion at the end of a ridge segment away from the main magma supply point along the spreading axis and iii an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere the second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults which reduces the lateral thermal conduction in the lithosphere since water is much less conductive than rock during the margin formation the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre we propose that this thermal buffer hypothesis may be generalized to most transform margins implying a lower amount of continental heating than assumed by the available thermomechanical models

conjugate divergent margins an introduction the main objective of this book is to provide a global overview of divergent margins based on geological and geophysical interpretation of sedimentary basins along the south central and north atlantic conjugate margins from plate tectonics and crustal scales to a more detailed description of stratigraphical and structural elements that are responsible for petroleum plays these themes are complemented by geodynamic concepts based on physical and numerical models and by comparisons with present day embryonic margins which are succinctly discussed in some papers

morphotectonic evolution of rifted continental margins inferences from a coupled tectonic surface processes model and fission track thermochronology we use a numerical model to study the topographic evolution and erosional history of rifted continental margins the model combines a kinematic description of lithospheric extension tectonic model with a surface processes model that includes short range hillslope and long range fluvial transport the tectonic model predicts the evolution of the lithospheric temperature and strength elastic thickness distribution as well as the tectonic uplift through time this is input into the surface processes model which calculates the degradation of topography and associated isostatic rebound modeled denudation histories across the margin are used to predict apatite fission track age and length patterns modeling results indicate that depending on the adopted parameters an uplifted rift flank should degrade by erosion within 50 100 m y without significant retreat of the topographic elevation maximum the development of an escarpment system at rifted continental margins is in itself not an indication of tectonic rift flank uplift but results from the existence of a high elevation interior plateau erosional base level lowering as a consequence of rifting and regional isostatic response to erosion of the margin however apatite fission track thermochronology reveals that the areas seaward of the escarpment at a number of rifted margins have been exhumed from several kilometers depth such amounts of denudation cannot be accommodated with isostatic rebound alone and require additional tectonic uplift of the rift flanks modeling of apatite fission track patterns suggests that fission track thermochronology dates the onset of rapid erosion which coincides with the initiation of strong relief i e initiation of rifting fission track ages which are younger than the age of rifting thus cannot be unambiguously interpreted as excluding prerift uplift the timing of margin uplift can be established only by careful track length analysis and integration with regional stratigraphic data the model is applied to the saudi arabian red sea margin and the southeastern australian highlands where it is constrained by present day topography and apatite fission track data as well as seismic and gravity data for the saudi arabian red sea margin synrift regional plateau uplift with a magnitude of approximately 1 km is inferred possibly as a result of asthenospheric upwelling flexurally induced tectonic uplift of the rift flank with a magnitude of 2 km is superimposed on this regional uplift the relatively high elevation of the southeastern australian highlands their steep front and the relatively high amounts of erosion suggest that apart from mesozoic synrift flexural uplift tertiary rejuvenation of topography has occurred possibly as a result of renewed lithospheric thinning and underplating the low elevation of the australian interior would inhibit the evolution of a major escarpment in the absence of such renewed uplift

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

fluid flow during early compartmentalisation of rafts a north sea analogue for divergent continental margins high quality 3d seismic data tied to eighteen 18 boreholes are used to investigate the styles of faulting and associated fluid flow features in triassic early jurassic rafts of the broad fourteens basin southern north sea the study area is presented as an analogue for continental margins experiencing early stage gravitational gliding i e prior to complete separation and downslope translation of individual rafts in such a setting and for present day stress conditions fault slip data indicate that chasms and faults separating rafts in the broad fourteens basin comprise structures subject to dip slip and strike slip reactivation chasms and faults sub parallel to these latter chasms comprise the most significant bypass areas for fluid sourced from pre salt strata faults sub parallel to the main chasms show limited propagation into early cretaceous and cenozoic strata draping the rafts a character further stressed by the depth of occurrence of fluid pipes and dim spots this is an important observation and leads us to postulate that faults formed during early stage rafting control fluid flow in regions where gravitational gliding is limited such as west and equatorial africa southeast brazil and parts of the gulf of mexico

basement heat flow in the western canada sedimentary basin abstract the effective thermal conductivity of the entire sedimentary succession is calculated at 487 locations in the western canada sedimentary basin based on net rock analyses and thermal conductivity values corrected for temperature and porosity effects an easterly trend of increasing values is caused by shallower burial depth and by a reduced proportion of low conductivity shales geothermal gradients calculated previously and corrected for pleistocene glaciations are used in estimating the basement heat flow after subtracting the heat generated by radioactive decay in the sedimentary strata the distribution of basement heat flow with values between 30 and 80 mw m 2 is linked with the complex structure of the basement which is comprised of archean blocks accreted terranes orogenic belts and magmatic arcs heat flow heat generation relations in alberta show that the reduced heat flow and the characteristic length scale for the uppercrust thermal layer are of the order of 35 mw m 2 and 6 km respectively

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

the ivory coast ghana transform margin a marginal ridge structure deduced from seismic data abstract the ivory coast ghana icg marginal ridge is a prominent feature of the icg transform margin and includes a fossil ridge partially buried by a thick undeformed sedimentary cover the fossil icg ridge is 130 km long and 25 km wide and towers over the adjacent rifted basin deep ivorian basin dib and the oceanic crust by 1 3 km and more than 4 km respectively it formed in three successive stages 1 1 during the rifting of the dib both vertical and horizontal motions between the dib and the south american plate varied along the plate boundary this relative motion occurred in an accommodation zone that tilted the northern slope of the icg ridge along en echelon mainly strike slip faults 2 2 after the rifting of the dib the relative motion remained constant along the transform plate boundary at this time strike slip deformation was localized into a narrow and highly deformed belt that truncated the accommodation zone 3 3 finally the transform motion occurred between the dib and an occanic plate thermal exchanges between the two adjacent plates induced thermal uplift of the icg ridge that amplified previous tectonic tilting

cote d ivoire ghana margin seismic imaging of passive rifted crust adjacent to a transform continental margin i i summary during may 1990 and january february 1991 an extensive geophysical data set was collected over the cote d ivoire ghana continental margin located along the equatorial coast of west africa the ghana margin is a transform continental margin running subparallel to the romanche fracture zone and its associated marginal ridge the cote d ivoire ghana ridge from this data set an explosive refraction line running 150 km ene wsw between 3 55 n 3 21 w and 4 23 n 2 4 w has been modelled together with wide angle airgun profiles and seismic reflection and gravity data this study is centred on the cote d ivoire basin located just to the north of the cote d ivoire ghana ridge where bathymetric data suggest that a component of normal rifting occurred rafher than the transform motion observed along the majority of the equatorial west african margin traveltime and amplitude modelling of the ocean bottom seismometer data shows that the continental moho beneath the margin rises in an oceanward direction from 24 km below sea level to 17 km in the centre of the line where the crust thins most rapidly there exists a region of anomalously high velocity at the base of the crust reaching some 8km in thickness this higher velocity region is thought to represent an area of localized underplating related to rifting modelling of marine gravity data collected coincident meth the seismic line has been used to test the bestfitting seismic model this modelling has shown that the observed free air anomaly is dominated by the effects of crustal thickness and that a region of higher density is required at the base of the crust to fit the observed data this higher density region is consistent in size and location with the high velocities required to fit the seismic data

late aptian to maastrichtian foraminiferal biogeography and palaeoceanography of the sergipe basin brazil abstract the oldest marine deposits of the sergipe basin are late aptian in age the foraminifera recovered from these strata reveal a characteristic tethyan affinity such as the assemblages of favusellids other rugose and or non rugose hedbergellids ticinellids globigerinelloidids vaginulinids nodosariids polymorphinids gavelinellids eggerellids textulariids lituolids ammosphaeroidinids haplophragmiieds trochamminids and cyclamminids the microfaunal evidence suggests that this area had at least surface water exchange with low latitude central north atlantic western tethyan biogeographic provinces even possibly at intermediate epi to mesopelagic water depths contribution of microfaunal elements from high latitude northern boreal realm and or southern austral regions is also apparent although of lesser significance s similarity index of simpson about 10 20 the occurrence of deep water planktonic morphotypes biticinella ticinella rotalipora dicarinella marginotruncana throughout the mid cretaceous succession confirms a well developed open connection with low latitude western tethyan regions at intermediate water depths since the mid to late albian low diversity gabonita assemblages occur in the uppermost cenomanian middle turonian onshore deposits of the basin they have a more conspicuous occurrence in the lowermost turonian strata similar assemblages have been reported from the cenomanian turonian of gabon and upper cenomanian lower turonian outcrops of morocco and israel west african endemic species of gabonita have not been found in the coniacian maastrichtian of sergipe however on the whole the high diversity foraminiferal microfauna bears striking affinities s about 80 95 with those reported from the gulf coast caribbean and guinean provinces attesting to similar palaeoceanographic conditions at low latitudes on both sides of the atlantic during that time interval the biogeographic distribution of late cretaceous slope assemblages of calcareous and agglutinated benthonic foraminifera was pandemic

denudation of the cote d ivoire ghana transform continental margin from apatite fission tracks apatite fission track analysis of samples from the shoulder marginal ridge of the cote d ivoire ghana transform contin ental margin reveal a cooling of the margin between 85 and 65 ma for the central and eastern parts of the ridge all samples were heated in situ during sedimentary burial with a temperature 120 c except for two samples located in the eastern part which were heated between 105 and 120 c for the first time age depth diagram along a transform margin shows a shape involving erosion starting at the bottom of the continental slope then stepping backwards towards the edge of the slope this retrogressive erosion can result from the deepening of the lithospheric plate sliding along the transform margin from thick continental crust to thin continental crust and finally to oceanic crust this process could be at the origin of the shoulder uplift by flexural response to the important crustal discharge 2 km

the dynamics of rifting deformation of the lithosphere by active and passive driving forces summary the thermal and mechanical behaviour of the lithosphere during rifting is investigated using a two layer model for its rheology and different driving forces for the rift process the uppermost crustal layer is assumed to deform by brittle failure while the lower lithosphere including the lower crust is assumed to behave as a newtonian viscous fluid results of numerical modelling of the deformation flow and heat transport in the viscous layer are presented in which viscosity is a strong function of depth and temperature deformation of the upper brittle layer is complex and difficult to quantify thus deformation of this upper layer was approximated by a simple kinematic model in which the layer is thinned by an amount proportional to the amount of extension it experiences two fundamentally different driving forces were investigated they correspond to active rifting in which the litho sphere is thinned by an upwelling limb of a convection cell or mantle plume and to passive rifting or stretching of the lithosphere the results show that for zones of stretching which are wider than the thickness of the viscous layer thinning at all levels in the lithosphere can be described by a single parameter as in earlier stretching models this is not always true when the rift zone is narrow or near the edges of the rift zone initial subsidence due to isostatic and thermal changes during stretching slows as stretching progresses giving an exponential character to the rift stage subsidence active rifting is associated with thermal uplift in this case lithospheric deformation is confined to a narrow zone near the base of the lithosphere and thinning of the crust does not occur until the base of the lithosphere approaches that level the rate of lithospheric thinning and width of the rift zone depend on the convection velocities in the underlying asthenosphere and on the viscosity depth gradient near the lithosphere asthenosphere boundary also contributing to the elevation changes during rifting are dynamic stresses which result from the flow these dynamic affects may significantly modify the uplift or subsidence of the rift zone another important process which results from viscous flow is the generation of secondary convection in the upwelling asthenosphere ths will further thin the lithosphere and can cause depth dependent stretching in which the lower lithosphere is thinned more than the upper lithosphere both of these properties of the flow dynamic stresses and secondary convection critically depend on absolute values of viscosity and so are not well constrained at present the model results presented here exhibit the degree of complexity which results from viscous flow in the lower lithosphere and which is matched by the wide variety of observations on rifting

flexural unloading and uplift along the cote d ivoire ghana transform margin equatorial atlantic recent ocean drilling program sampling of the cote d ivoire ghana margin of west africa provides for the first time the opportunity to study the development of a marginal ridge that formed along a sheared passive margin adjacent to the continent ocean transition after the end of intracontinental wrenching we model its evolution using a two dimensional flexural backstripping technique the model is constrained by existing seismic refraction and reflection data on the crustal structure and stratigraphy and paleobathymetric evidence from the cores following rifting at 120 ma aptian intracontinental wrenching continued until 105 ma mid albian when south america and africa separated along this transform while the possible presence of thicker crust under the marginal ridge compared to the deep ivorian basin may explain some of the ridge s topography the entire uplift can be readily modeled as a flexural response to unloading along a shallow dipping 25 fault at the time of continental separation forward subsidence modeling of the marginal ridge suggests that an effective elastic thickness of 2 5 km at 105 ma is most appropriate to match the observed structure conduction of heat from the oceanic plate across the continent ocean transition drove temporary uplift of at least 1200 m peaking just before 89 ma when sedimentary data show deposition within the photic zone 0 50 m this study shows that significant transient thermal uplift can be found in sheared continental passive margin settings even where magmatism is insignificant

transform margins development controls and petroleum systems an introduction this paper provides an overview of the existing knowledge of transform margins including their dynamic development kinematic development structural architecture and thermal regime together with the factors controlling these this systematic knowledge is used for describing predictive models of various petroleum system concept elements such as source rock seal rock and reservoir rock distribution expulsion timing trapping style and timing and migration patterns the paper then introduces individual contributions to this volume and their focus supplementary material location table and map of specific transform examples structural elements of the romanche transform margin and glossary of terms used in this article is available at https doi org 10 6084 m9 figshare c 3276407 gold open access this article is published under the terms of the cc by 3 0 license this volume shows dynamic linkage between new transform margin research and increasing transform margin exploration it offers a critical set of predictive tools via an understanding of the mechanisms involved in the development of play concept elements at transform margins and their controlling factors our introduction to this volume reviews the current knowledge of the transform margins focusing on their dynamic development kinematic development structural architecture thermal regimes and petroleum systems the last part of this paper introduces the contributions to this volume

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

coupling of dipolarization front flow bursts to substorm expansion phase phenomena within the magnetosphere and ionosphere 1 recent themis spacecraft observations have shown longitudinally narrow earthward moving plasma sheet flow channels with large abrupt magnetic field dipolarizations that have been referred to as dipolarization fronts we undertake a detailed study of the subset of these events that had good quality auroral imaging we find that many occurred during the substorm expansion phase after onset determined by auroral intensification and were related to auroral streamers both the fronts relation to streamers and their plasma sheet changes are similar to what has been previously shown for other plasma sheet flow channels and to other flow channels seen during the expansion phases of the events considered here surprisingly the dipolarization fronts examined made major contributions to substorm onset ground and space magnetic signatures these include substorm near earth dipolarization which indicates that the current wedge may develop via a series of well defined narrow post onset wedge like structures these also include the auroral zone ground magnetic field which showed only modest responses to the onsets but abrupt large responses to post onset dipolarization front related streamers and midlatitude positive bays which started near the time of streamer formation our observations suggest that traditional magnetic signatures of substorm onset such as near earth dipolarization auroral zone negative bays and midlatitude positive bays may misidentify the expansion phase onset determined by auroral intensification by up to 10s of minutes for some events it should be interesting in the future to determine the generality of the above relations between dipolarization front signatures and onset signatures