variations in magmatic processes along the east greenland volcanic margin summary seismic velocities and the associated thicknesses of rifted and igneous crust provide key constraints on the rifting history the differentiation between non volcanic and volcanic rifted margins the driving force of magmatism at volcanic margins that is active or passive upwelling and the temperature anomaly in the lithosphere this paper presents two new wide angle seismic transects of the east greenland margin and combines the velocity models with a compilation of 30 wide angle seismic velocity models from several publications along the entire east greenland margin compiled maps show the depth to basement depth to moho crustal thickness and thickness of high velocity lower crust hvlc with velocities above 7 0 km s 1 first we present two new wide angle seismic transects which contribute to the compilation at the northeast greenland margin and over the oceanic crust between shannon island and the greenland fracture zone velocity models produced by ray tracing result in total traveltime rms misfits of 100 120 milliseconds and 2 values of 3 7 and 2 3 for the northern and southern profiles with respect to the data quality and structural complexity 2 d gravity modelling is used to verify the structural and lithologic constraints the northernmost profile awi 20030200 reveals a magma starved break up and a rapidly thinning oceanic crust until magnetic anomaly c21 47 1 ma the southern seismic transect awi 20030300 exhibits a positive velocity anomaly associated with the shannon high and a basin of up to 15 km depth beneath flood basalts between shannon island and the continent ocean boundary break up is associated with minor crustal thickening and a rapidly decreasing thickness of oceanic crust out to anomaly c21 the continental region is proposed to be only sparsely penetrated by volcanism and not underplated by magmatic material at all compared to the vast amount of magmatism further south break up is proposed to have occurred at the seaward boundaries of the continent ocean transition zones at between 50 and 54 ma propagating from north to south based on a joint analysis incorporating transects from the kejser franz joseph fjord and godthab gulf secondly the variation of the hvlc along the east greenland margin from 60 to 77 n and from transects of its conjugate margin shows inverted emplacement of prominent landward and seaward hvlc thickness portions from north to south in a distribution chart the differences in the hvlc distribution are attributed to one or more of the following three models in the first model it is inferred that a transfer zone detachment acts as a barrier to northward magma flow in the second model underplating results in thicker and highly intruded lower crust with several small scale feeder dykes that locally increase the lower crustal velocities in the third model a second magmatic event associated with the separation of the jan mayen microcontinent is considered lithospheric scale inhomogeneities might be responsible for the heterogeneous melt generation the inversion of the hvlc distribution in continental and oceanic domains and differences in its velocities

the role of magmatic loads and rift jumps in generating seaward dipping reflectors on volcanic rifted margins abstract the largest volcanic constructs on earth are the seismically imaged seaward dipping reflector sdr units found offshore of many rifted continental margins including most that border the atlantic ocean whether their formation requires large magnitude i e 10 s of km of normal fault slip or results from the deflection of the lithosphere by the weight of volcanic flows is controversial though there is evidence for faulting associated with some sdrs this paper considers the range of structures that can be produced by magmatic and volcanic loading alone to do this an idealized mechanical model for the construction of rift related volcanic flow structures is developed dikes open as plates move away from the center of a model rift and volcanic flows fill the depression produced by the load caused by dike solidification the thin elastic plate flexure approximation allows a closed form description of the shape of both the contacts between flows and between the flows and underlying dikes the model depends on two independent parameters the flexure parameter and the maximum isostatically supported extrusive layer thickness w 0 for reasonable values of these parameters the model reproduces the observed down dip thickening of flows and the range of reflector dip angles a numerical scheme using the analytic results allows simulation of the effect of temporal changes in the locus of magmatic spreading as well as changes in the amount of volcanic infill either jumps in the location of the center of diking or periods with no volcanism result in separate units or packages of model sdrs in which the flow dike contact dips landward consistent with observations previously attributed only to listric normal fault offset when jumps in the spreading center are small i e less than they result in thicker narrower volcanic units on one side of a rift compared to those on the other side this is similar to the asymmetric distributions of volcanic packages seen across some conjugate margins

lack of enhanced preservation of organic matter in sediments under the oxygen minimum on the oman margin the impingement of oxygen minima on continental margins is widely thought to promote the accumulation of sedimentary facies enriched in well preserved organic matter it is shown here however that such a relationship does not clearly apply to the productive oman margin in the arabian sea which hosts one of the most severe oxygen minima in the oceans measurements made on the 0 1 cm depth interval from fourteen box cores collected from the outer shelf upper continental slope area off oman show that 1 1 deposited organic matter is overwhelmingly of marine origin 2 2 there is no significant correlation between the abundance of sedimentary organic carbon corg and the bottom water o2 concentration 3 3 there is no relation between the sedimentary corg n ratio and bottom water o2 4 4 there is no correlation between the hydrogen index hi of the organic matter and bottom water oxygen there are however significant correlations between the corg n ratio and the i corg cr al and zr al ratios as well as between the corg n ratio and the hydrogen index these relationships are interpreted as being the result of winnowing and attendant reworking which produce a progressive increase in the corg n ratio and a decrease in the hi and the i corg ratio as organic matter becomes progressively degraded commensurate rises in the cr al and zr al ratios result from the increased proportion of heavy minerals associated with winnowed lag deposits overall these data suggest that the bottom water oxygen concentration has little effect in governing either the distribution or the degree of preservation of organic matter on this margin thus the generally high but spatially variable corg content of the sediments on the oman margin may not reflect the occurrence of an oxygen minimum but instead be the result of a high settling flux of organic matter supported by monsoon driven upwelling and post depositional redistribution of the organic material by hydrodynamic influences

lack of evidence for enhanced preservation of sedimentary organic matter in the oxygen minimum of the gulf of california a test of the hypothesis that hydrogen rich organic matter accumulates preferentially in sediments under oxygen minima has been carried out by examining the composition of laminated and homogeneous diatomaceous sediments of the guaymas basin in the gulf of california the laminated sediments occur where an intense oxygen minimum intersects the slope around the margins of the basin above and below the depth of the oxygen minimum the sediments are homogenized by a burrowing infauna the two sediment types have very similar bulk chemical and mineral compositions and the organic matter is overwhelmingly marine the total organic carbon contents and rock eval parameters hydrogen and oxygen indices of the two sediment types are not significantly different whereas a road organic carbon maximum in the homogeneous sediments is centered at roughly 1200 m depth on the slope of the basin this maximum lies deeper than the core of the oxygen minimum thus both the concentration and the nature of the organic matter in the sediments of this highly productive marginal marine basin are not related to the concentration of dissolved oxygen in the bottom waters

lack of organic matter accumulation on the upwelling influenced somalia margin in a glacial interglacial transition abstract monsoon induced upwelling has been operating off somalia for at least the last 160 000 years making the nw indian ocean one of the most productive oceanic areas in the world we have studied the sediments recovered during the indusom cruise of the r v marion dufresne from three different environments off this margin a core from the highly productive surface waters core md85 674 a core where the oxygen minimum intersects the slope core md85 664 and a reference core from the deep basin with normal bottom oxygen levels and moderate primary production core md85 668 though the first two environments are a priori favourable to organic matter om accumulation no marine om enrichment can be observed in the cores to understand this anomaly emphasis is placed upon the transition from isotopic stage 6 to stage 5 because the end of the glacial stage 6 is characterised by intensified upwelling and primary productivity in this region whereas stage 5 experienced a marked decrease in productivity various palaeoenvironmental markers such as om v ni cu zn ba and p have been used to identify periods of high productivity and or bottom water oxygen depletion the results suggest that 1 the lack of marine om in two settings favourable to organic carbon storage may be due to the nature of primary productivity coccolithophorid production would not be prone to om accumulation in contrast to the contribution of om by diatoms or particularly naked phytoplankton dinoflagellates or other peridinians 2 ba is not an accurate palaeoproductivity marker in this region although the oceanic environment would have been a priori suitable for the use of ba as a proxy indicator 3 the period of intensified upwelling in stage 6 as well as the stage 6 5 transition left no geochemical imprint on the sediments this means that some upwelling systems may not be accompanied by marked enrichments in marine organic matter within the underlying sediments in other words the somalia upwelling does not participate to the so called biological pump and has no positive effect upon long term carbon storage

magma chambers at oceanic ridges how large seismic models of magma chambers at fast spreading oceanic ridges predict small perched magma chambers above a wide low velocity zone seismic attenuation in a low velocity zone allows for 10 melt in hot gabbros in order to reconcile these data with a structural model of magma chambers that is supported by compelling field data in the oman ophiolite we propose that the low velocity zone belongs to the magma chamber in such a dynamic magma chamber gabbros develop strong shape fabrics during magmatic flow in a concentrated suspension this process results in a drastic reduction of the melt fraction that is compatible with magmatic flow as confirmed by preliminary experiments we propose here that dynamic magma chambers below ridges may contain as little as 10 melt

a mechanism for construction of volcanic rifted margins during continental breakup research into the breakup of continents and formation of ocean basins has advanced in recent years with deep marine seismic data providing much of the impetus for new concepts attention has been focused on nonvolcanic magma poor margins meanwhile a satisfactory explanation for volcanic magmatic margins with their distinctive seaward dipping reflectors sdrs has been missing sdrs are composed of subaerial lava flows of tholeiitic basalt erupted during the transition from continental rifting to seafloor spreading when the crust is both stretched and constructed from asthenosphere derived magma here we present remarkable seismic images down to 40 km offshore east greenland showing that a newly described type of extensional fault plays an integral part in the ascent of magma and the building of transitional crust two such faults define an axial horst block supported by rising magma where the plates separate lava collects in seaward growing half graben on either side of the horst while new gabbroic crust is accreted along the flanks of the magma chamber in dilational shear zones beneath where the faults detach ultimately rising asthenosphere reaches the base of the magma chamber causing the horst to split thereby initiating seafloor spreading the model we describe shows how extension and magmatism interact at the boundaries of rifting plates

southeast baffin volcanic margin and the north american greenland plate separation plate breakup over plumes is characterized by the development of margins showing extensive magma production both underplated at moho level and extruded as thick piles of seaward dipping lava formations the disko svartenhuk area in west greenland is one of the few places in the world with exposed seaward dipping basalts forming a prism whose thickness increases seaward we present a quantitative tectonic study of this margin which we tentatively restored in its geodynamic position during the different stages of plate separation between greenland and north america our structural data are constrained with recently published 39ar 40ar and new and coherent 40k 40ar geochronology in dikes of different orientations the first order structure is that of a tectonically driven seaward crustal flexure linked to definitive plate breakup between greenland and baffin island during the eocene coeval with the formation of the upper part of the exposed seaward dipping volcanic prism this flexing is associated with a significant crustal stretching associated with arrays of continentward dipping normal faults this across strike structure is correlated to a fundamental along strike segmentation with the three segments adopting a zigzag strike there is a clear increase of extensional strain at the extremities of the segments eocene extension trended n060 on average in the northern and southern segments but was nw trending in the central ne trending nugssuaq segment we discuss the interpretation of such an extension perpendicular to the different margin segments from a regional point of view the n060 extension is distinct in orientation from the approximately n s trend of plate separation between north america and greenland in the baffin bay during the eocene however the extension recorded in the margin is nearly perpendicular to the obliquely spreading eocene accretion axis in the baffin bay this latter point suggests a mantle or ridge control of the development of the margin over regional far field lithospheric stress models

a review of the ne atlantic conjugate margins based on seismic refraction data the ne atlantic region evolved through several rift episodes leading to break up in the eocene that was associated with voluminous magmatism along the conjugate margins of east greenland and nw europe existing seismic refraction data provide good constraints on the overall tectonic development of the margins despite data gaps at the ne greenland shear margin and the southern jan mayen microcontinent the maximum thickness of the initial oceanic crust is 40 km at the greenland iceland faroe ridge but decreases with increasing distance to the iceland plume high velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the east greenland ridge and the lofoten margin with the exception of the vestbakken volcanic province at the sw barents sea margin south of the narrow lofoten margin the european side is characterized by wide margins the opposite trend is seen in greenland with a wide margin in the ne and narrow margins elsewhere the thin crust beneath the basins is generally underlain by rocks with velocities of 7 km s 1 interpreted as serpentinized mantle in the porcupine and southern rockall basins while off norway alternative interpretations such as eclogite bodies and underplating are also discussed

an improved tectonic model for the eocene opening of the norwegian greenland sea use of modern magnetic data abstract compilation of new and vintage aeromagnetric data from the norwegian and greenland seas of the ne atlantic provides evidence for a different interpretation of several tectonic elements the previously interpreted oceanic fracture zones gleipne surt bivrost jenegga and vesteralen do not exist these were artefacts of poor navigation and wide line spacing of the vintage dataset this reinterpretation impacts our understanding of the early spreading history of the north atlantic as the opening of the norwegian greenland sea between the jan mayen and senja greenland fracture zones occurred along a stable axis without offsets of the oceanic spreading anomalies or jumps in spreading axis these results contradict the hypothesis that a spatial relationship exists between transfer zones and fracture zones on the lofoten margin and on the ne greenland margin to which they have been projected simplified palaeogeographic reconstruction of the aeromagnetic map to anomaly 22 reveals that a c 50 km wide magnetic anomaly cuts across spreading anomalies 24a 24b and 23 from the voring marginal high on the norwegian margin to traill o on the east greenland coast the anomaly is interpreted to represent an igneous complex referred to as the traill voring igneous complex tvic the complex crosscuts anomaly 22 on the greenland margin suggesting that the igneous activity was active until c 50 ma and can be linked up with the nne trending initial magmatic lineament iml extending between traill o and kangerlussuaq the iml has been suggested to relate to a failed attempt of direct linkage between the reykjanes and mohns ridges the magnetic response of the tvic along the voring margin has previously been interpreted as representing anomaly 24a and 24b such an interpretation required the erroneous introduction of an abandoned spreading ridge

the netherlands indian ocean expedition 1992 1993 first results and an introduction the netherlands indian ocean programme was carried out with the rv tyro in the nw indian ocean the arabian sea and off kenya and the seychelles during 1992 1993 with the objective to study the effect on both spatial and temporal scales of the monsoon on the climate system in the northern indian ocean the programme included five research foci studied in different projects project a was directed towards the effects of the seasonally reversing monsoonal system on the coastal ecosystems in kenya especially to gain a clear understanding of the physical chemical and biological processes occurring in the coastal zone this includes the development of the marine flora and fauna in response to the changing conditions in the coastal zone the second project b aimed at a study of the dynamics of the pelagic system including a quantification of the carbon flux and biogeographical studies with the emphasis on distribution and taxonomy of plankton part of this project was designed as pilot project for the jgofs northwest indian ocean study carried out in 1994 95 covering most of the jgofs core measurements these studies were planned in close cooperation with the geological project c tracing a seasonal upwelling system aimed at determining the spatial and temporal variability of biological chemical and geological parameters that define an upwelling system and recognizing these in the sedimentary record as proxies for upwelling in the waters off somalia and yemen project d concentrated on two main issues the first was the late quaternary productivity and the dynamics of the oxygen minimum zone in the northeast arabian sea the second was the depositional architecture and sediment facies of the middle and lower indus fan the biology of oceanic reef systems of the seychelles formed the last project e within the netherlands indian ocean programme and was mainly directed towards biogeographical taxonomical and ecological studies of the reefs the studies comprised within these projects were and are being carried out at a range of different institutes and preliminary results were discussed at an arabian sea workshop held at nioz texel the netherlands from 13 16 february 1995 the papers in this special form the first group of results from the progamme and as a consequence the articles have a strongly variable character the first paper by kromkamp et al describes temporal and spatial variability of primary production along the kenyan coast and their relationship to the development of the cyanobacterium trichodesmium

evolution of seaward dipping reflectors at the onset of oceanic crust formation at volcanic passive margins insights from the south atlantic seaward dipping reflectors sdrs have long been recognized as a ubiquitous feature of volcanic passive margins yet their evolution is much debated and even the subject of the nature of the underlying crust is contentious this uncertainty significantly restricts our understanding of continental breakup and ocean basin forming processes using high fidelity reflection data from offshore argentina we observe that the crust containing the sdrs has similarities to oceanic crust albeit with a larger proportion of extrusive volcanics variably interbedded with sediments densities derived from gravity modeling are compatible with the presence of magmatic crust beneath the outer sdrs when these sdr packages are restored to synemplacement geometry we observe that they thicken into the basin axis with a nonfaulted diffuse termination which we associate with dikes intruding into initially horizontal volcanics our model for sdr formation invokes progressive rotation of these horizontal volcanics by subsidence driven by isostasy in the center of the evolving sdr depocenter as continental lithosphere is replaced by more dense oceanic lithosphere the entire system records the migration of 10 km thick new magmatic crust away from a rapidly subsiding but subaerial incipient spreading center at rates typical of slow oceanic spreading processes our model for new magmatic crust can explain sdr formation on magma rich margins globally but the estimated crustal thickness requires elevated mantle temperatures for their formation

volcanism on the rockall continental margin continental break up in the northern north atlantic was accompanied by massive volcanism this produced sequences of basalt flows erupted close to sea level which are imaged as seaward dipping reflectors on seismic reflection profiles they comprise the upper 4 5 km of transitional crust in both the edoras bank and hatton bank areas of the rockall continental margin detailed surveys show that the reflectors are convex upward on dip lines but are sub horizontal on strike lines they are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis along the rockall margin two sequences of seaward dipping reflectors occur separated by an 9outer high9 near the foot of the continental margin the landward set of basalt flows are emplaced above stretched and intruded continental crust while the seaward set span the region from highly distended continental crust to fully oceanic crust the outer high is capped by basalt flows but exhibits little coherent internal reflectivity we discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of iceland

comparison of the rift and post rift architecture of conjugated salt and salt free basins offshore brazil and angola namibia south atlantic abstract this study presents a regional comparison between selected 2d seismic transects from large conjugated salt and salt free basins offshore southern brazil campos basin santos basin pelotas basin and southwest africa kwanza basin northern and southern namibe basin walvis basin tectonic stratigraphic interpretation of the main rift and post rift units free air gravity data and flexural isostatic backstripping were used for a comprehensive basin to basin documentation of key mechanisms controlling the present day differences in conjugated and neighbouring south atlantic basins a significant variation in the tectonic sedimentary architecture along strike at each margin and between the conjugated basins across the south atlantic reflects major differences in 1 the structural configuration of each margin segment at transitional phase between rifting and breakup as emphasized in the highly asymmetric settings of the large santos salt basin and the conjugated salt free southern namibe basin 2 the post breakup subsidence and uplift history of the respective margin segment which caused major differences for example between the campos and espirito santo basins and the conjugated northern namibe and kwanza basins 3 variations in the quantity and distribution of post breakup margin sediments which led to major differences in the subsidence history and the related present day basin architecture for example in the initially rather symmetric siliciclastic pelotas and walvis basins and 4 the deposition of aptian evaporites in the large rift and sag basin provinces north of the rio grande rise and walvis ridge highly contrasting the siliciclastic basins along the margin segments south of the ridges the resulting present day architecture of the basins can be generally classified as i moderately symmetric salt free and magma rich in the northern part of the southern segment i highly asymmetric salt bearing and magma poor vs salt free and magma rich in the southern part of the central segment and iii moderately symmetric to asymmetric salt rich and magma poor in the central part of the central segment of the south atlantic

eocene continental breakup in baffin bay abstract we question the timing of continental breakup and early oceanization in baffin bay north east atlantic north of the ungava fault zone the breakup was syn magmatic and led to the development of conjugate volcanic passive margins vpms we investigated the innermost part of the w greenland vpm where a remarkable inner sdr is fully exposed in the svartenhuk area our new radiometric ages and paleomagnetic data from syn tectonic basaltic lavas indicate that continental stretching and thinning spanned the c26r to at least the c24r period giving an eocene lower boundary age for continental breakup in baffin bay these results contradict the proposed flooring of baffin bay by a paleocene oceanic crust older than c24n and also question the accretion of oceanic crust before c22 we confront our results to the dynamics of the northward oceanic rift propagation across the ungava transform fault system and we suggest that plate breakup in baffin bay occurred 8 m y later than in n labrador sea as a result of the thermal and mechanical barrier effect induced by the ungava transform zone

the ocean continent transition in the mid norwegian margin insight from seismic data and an onshore caledonian field analogue understanding the structure of the ocean continent transition oct in passive margins is greatly enhanced by comparison with onshore analogues the north atlantic margins and the fossil system in the scandinavian caledonides show variations along strike between magma rich and magma poor margins but are different in terms of exposure and degree of maturity they both display the early stages of the wilson cycle seismic reflection data from the mid norwegian margin combined with results from ocean drilling program leg 104 drill core 642e allow for improved subbasalt imaging of the oct below the seaward dipping reflector sdr sequences vertical and inclined reflections are interpreted as dike feeder systems high amplitude reflections with abrupt termination and saucer shaped geometries are interpreted as sill intrusions implying the presence of sediments in the transition zone beneath the volcanic sequences the transitional crust located below the sdr of the mid norwegian margin has a well exposed analogue in the seve nappe complex snc at sarek sweden hornfelsed sediments are truncated by mafic dike swarms with densities of 70 80 or more the magmatic domain extends for at least 800 km along the caledonides and probably reached the size of a large igneous province it developed at ca 600 ma on the margin of the iapetus ocean and was probably linked to the magma poor hyperextended segment in the southern scandinavian caledonides these parts of the snc represent an onshore analogue to the deeper level of the mid norwegian margin permitting direct observation and sampling and providing an improved understanding particularly of the deeper levels of present day magma rich margins

three dimensional seismic imaging of a protoridge axis in the main ethiopian rift models of continental breakup remain uncertain because of a lack of knowledge of strain accommodation immediately before breakup our new three dimensional seismic velocity model from the main ethiopian rift clearly images mid crustal intrusions in this active transitional rift setting supporting breakup models based on dike intrusion and magma supply the most striking features of our velocity model are anomalously fast elongate bodies velocity v p 6 5 6 8 km s extending along the rift axis interpreted as cooled mafic intrusions these 20 km wide and 50 km long bodies are separated and laterally offset from one another in a right stepping en echelon pattern approximately mimicking surface segmentation of quaternary volcanic centers our crustal velocity model combined with results from geologic studies indicates that below a depth of 7 km extension is controlled by magmatic intrusion in a ductile middle to lower crust whereas normal faulting and dike intrusion in a narrow zone in the center of the rift valley control extension in the brittle upper crust this zone is inferred to be the protoridge axis for future seafloor spreading

insights into the collection and emplacement of granitic magma based on 3d seismic images of normal fault related salt structures 3d seismic data from salt structures in the southern north sea show how viscous fluids tend to migrate towards and flow upwards along the footwalls of active normal faults where zones of low stress are inherent this process provides one potential solution for the question of how enough space is created to accommodate large granitic bodies in the crust during episodes of regional extension granitic melt may collect in the lower crust below normal detachments in domed areas from which magma may be sourced subsequently in the upper crust emplacement can occur in fault bounded laccoliths at rates commensurate with other geological processes

laterally confined volcanic successions lcvs recording rift jumps during the formation of magma rich margins abstract seaward dipping reflectors sdrs are a characteristic feature of magma rich margins and represent the generation of large volumes of flood basalts at the point of continental breakup a number of recent studies provide new insights into the emplacement and tilting of sdrs and conclude that the majority of sdrs are contained within new magmatic crust that has a close affinity to oceanic crust however the process by which these initial magmatic systems evolve into a fully established spreading centre remains poorly understood several characteristic features of magma rich margins may be explained by the occurrence of rift jumps during sdr emplacement yet the cause and prevalence of such rift jumps remain unknown here we constrain the 3d geometry of the continent ocean transition in the orange basin offshore south africa this allows us to test if where and why such rift jumps occur our results demonstrate an order of along strike segmentation previously unobserved in these settings we demonstrate that the sdr belt is disrupted by the occurrence of a volcanic stratigraphic package defined as the laterally confined volcanic succession lcvs not previously identified on a rifted margin we interpret this as a magmatic spreading centre that was abandoned by a subsequent rift jump identification of lcvss is important for two reasons first we argue that the lcvs formed via the same process as sdrs and hence provides a unique example of sdr geometry prior to their separation onto conjugate plates second as we can map out the 3d geometry of the lcvs and sdrs we propose that rift jumps during magma rich margin formation may be fundamental to the establishment of a laterally continuous incipient spreading centre

surficial sediments of the arabian sea abstract coarse grained low carbonate sediments with high abundances of quartz and feldspar in the northerly areas of the arabian sea close to land reflect significant terrigenous sediment in fluxes primarily from the indus river and to some extent from other sources carbonate content of sediments increases gradually from the northernmost areas to about 18 21 n and reflects the waning influence of terrigenous sediment supply southward in areas farther from significant terrigenous influences the carbonate distribution also clearly reflects bathymetric control thus whereas the caco3 content is high on ridges it becomes less than 10 at about 4800 m ccd in the arabian basin a low carbonate band extends from the areas opposite the indus river to the deep areas 2000 3000 m southward and is confined to the easternmost indus fan valley relatively high bottomwater turbidities in this valley suggest that the low carbonate band may result in part from dilutions due to the influxes of terrigenous sediments transported and deposited by deep turbid water flows although high amounts of organic matter occur in the sediments of westerly northerly and easterly areas close to land it is along the indian continental margin at 200 1500 m water depths that the highest organic carbon values 5 10 are observed however high nutrient abundances in the surface waters and high contents of opal and siliceous fossil remains in the sediments off the arabian peninsula gulf of aden clearly reflect higher biogenic productivities in this region than along the indian margin the high organic matter content in the sediments of the indian margin is primarily due to its preservation which results from the impingement of low oxygenated waters on the sea floor at 200 1500 m water depths and from the high sedimentation rates calculations suggest that the degree of preservation of organic matter along the indian margin is about 15 80 times higher than that in the region off the arabian peninsula

origin composition and relative timing of seaward dipping reflectors on the pelotas rifted margin abstract the mechanism by which seaward dipping reflectors sdrs are formed is a topic of debate two contrasting models exist for their formation the volcanic faulting model and the volcanic loading model each of these models has important implications for the processes which control the structure and formation of magma rich rifted continental margins we have examined high quality deep seismic reflection data across the pelotas basin offshore brazil these data reveal a remarkable set of sdrs for which we have investigated the likely nature of their formation the total package of sdrs has an across strike width of 200 km and a variable vertical thickness of 10 17 km previously interpreted as volcanic flows detailed observations however show changes in seismic character and geometry within the sdr package which suggest a complex and varied evolution we have used gravity anomaly inversion and seismic observations together to investigate the likely composition of the sdrs by determining the proportion of basaltic material to sedimentary volcaniclastic material basalt fraction within the sdrs this has been achieved by minimising the difference between the depth of the gravity moho and seismic moho in order to quantify the lateral variation in basalt fraction taken to be proportional to the bulk density of the package the density of the sdr package together with seismic interpretation is then used to infer the composition depositional environment source and time of formation relative to breakup our analysis suggests that the overall sdr basalt fraction and bulk density decrease oceanwards possibly due a change in the type of volcanic deposits from predominantly subaerial to volcaniclastics possibly deposited subaqueously the sdrs can be split into three sub packages the two inner sdr packages are interpreted to consist of lava flows sourced from syn tectonic subaerial eruptions associated with the onshore parana large igneous province flowing eastwards into an extensional basin the outer sdr package shows reflector geometries that progressively offlap oceanwards interpreted as extrusives sourced from an eastwards migrating newly formed ocean ridge our analysis suggests that both the volcanic faulting and volcanic loading models for sdr formation are applicable to the pelotas rifted margin recording distinct syn rift and syn breakup magmatic events we show that both sdr formation models can be recognised in a naturally occurring example and can coexist on the same margin

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

crustal accretion of thick mafic crust in iceland implications for volcanic rifted margins rifting near hotspots results in mantle melting to create thick mafic igneous crust at volcanic rifted margins vrms this mafic crust is transitional between rifted continental crust with mafic intrusions landward and oceanic crust into which it grades seaward seismic velocities crustal drilling and exhumed margins show that the upper crust in these areas is composed of basaltic lava erupted in subaerial to submarine conditions intruded by downward increasing proportions of dikes and sparse gabbroic intrusions the lower crust of these regions is not exposed but is inferred from seismic velocities vp 6 5 km sec and petrological constraints to be gabbroic to ultramafic in composition limited access to crustal sections generated along vrms have raised questions regarding the composition and structure of this transitional crust and how it evolves during the early stages of rifting and subsequent seafloor spreading active processes in iceland provide a glimpse of subaerial spreading with the creati

magmatic ocean continent transitions abstract continental extension may or may not be coeval with significant mantle melting leading to the formation of distinct types of passive margins respectively magma poor or volcanic with distinct crustal structures especially in inter cratonic mobile areas magmatic breakup and development of volcanic passive margins vpms may postdate the early development of a non magmatic continental rift system the time span between the amagmatic and the magmatic systems may be relatively short or conversely long tens of millions years such evolution is often associated with a significant apparent asymmetry in the wideness of conjugate vpms in this paper we attempt to re interpret the structure of three paired vpms which developed close to but separately from a previous amagmatic aborted rift system due to opposite dips in major crustal detachment faults accommodating extension from sedimentary to volcanic stages those composite margins tend to individualize a fault dissected continental block here designed as l block along one of the conjugate margins in addition to the amount and distribution of amagmatic extension the time span between amagmatic and magmatic extension exerts a major structural and rheological control on the final structure of the ocean continent transitions the finite margin geometry may be erroneously interpreted as resulting from a continuous process from hyperextension to the final magmatic breakup however the early syn sedimentary extension appears in many cases to be low rate or episodic consequently the vpm final breakup of the lithosphere may shift away from the original amagmatic stretched area to the rifted margin

seismic velocity structure of seaward dipping reflectors on the south american continental margin abstract seaward dipping reflectors sdrs are a key feature within the continent to ocean transition zone of volcanic passive margins here we conduct an automated pre stack depth migration imaging analysis of commercial seismic data from the volcanic margins of south america the method used an isotropic ray based approach of iterative velocity model building based on the travel time inversion of residual pre stack depth migration move out we find two distinct seismic velocity patterns within the sdrs while both types show a general increase in velocity with depth consistent with expected compaction and alteration metamorphic trends those sdrs that lie within faulted half grabens also have high velocity zones at their down dip ends the velocity anomalies are generally concordant with the reflectivity and so we attribute them to the presence of dolerite sills that were injected into the lava pile the sills therefore result from late stage melt delivery along the large landward dipping faults that bound them in contrast the more outboard sdrs show no velocity anomalies are more uniform spatially and have unfaulted basal contacts our observations imply that the sdrs document a major change in rift architecture with magmatism linked with early extension and faulting of the upper brittle crust transitioning into more organised dike fed eruptions similar to seafloor spreading

a critical evaluation of depositional parameters controlling the variability of organic carbon in arabian sea sediments abstract organic carbon distribution in the surficial sediments of the arabian sea to some extent mirrors the surface productivity being both high in the peripheral portions and low in the central region suggesting that the productivity is a primary source of organic matter however a critical examination reveals that the organic enrichment on the slope of the indian margin is two to four higher max 16 71 than on the slope of the arabian peninsula max 7 54 while the productivity in the former region is three to four times lower than the latter this observation suggests that the influence of productivity on organic enrichment is of secondary importance high concentrations 4 of organic carbon exactly coincide with the oxygen minimum zone 150 1500 m water depth evidently reflecting the crucial role played by the anoxic bottom waters in the preservation of organic carbon incidently the organic rich band along the western indian slope is wide long and highly concentrated as compared to that of the slope of the arabian peninsula and no organic enrichment is found on other continental slopes of the arabian sea although an equally intense oxygen minima impinge on the floor of the slope such a variability of organic carbon on the slopes is attributed to various geological parameters such as texture of sediments rates of fine terrigenous sediment deposition shelf width slope gradient bottom currents and adsorption capacity of individual clay minerals all these observations suggest that the productivity is not an ultimate control for the organic enrichment of bottom sediments but it is the bottom water anoxia in conjunction with various depositional parameters which determine the degree of preservation

mesozoic rifting evolution of sw gondwana a poly phased subduction related extensional history responsible for basin formation along the argentinean atlantic margin abstract the opening of the south atlantic in the early cretaceous was the final stage of the complex rifting history of sw gondwana in this contribution we reassess the chronology of mesozoic basin formation in southern south america and africa and integrate it in the long term breakup history of sw gondwana triassic rifting is characterized by intracontinental rifting in africa karoo i phase and retro arc extension on the sw margin of gondwana in the early jurassic the impingement of the karoo plume triggered rifting in eastern africa producing the karoo ii basins and the colorado and salado basins on the argentinean shelf east africa rifting ultimately lead the breakup of eastern from western gondwana in the middle jurassic in patagonia the austral malvinas and other related basins formed in association with the synextensional emplacement of the chon aike magmatic province in the patagonian retro arc in the late jurassic the rocas verdes back arc basin opened in southern patagonia while oblique rifting in the core of the late paleozoic gondwanides orogen produced the outeniqua and rawson valdes basins the south atlantic rift initiated in the early cretaceous associated with present day e w extension rifting occurred diachronically from south to north initiating in the previously thinned rawson valdes outeniqua segment a precursor oblique rift system and a larger degree of extension in this segment could explain the lack of seaward dipping reflectors sdr south of the colorado cape fracture zones rifting and sdr emplacement occurred progressively to the north along different rift segments producing strongly asymmetric conjugate margins