the crustal structure of beira high central mozambique combined investigation of wide angle seismic and potential field data abstract the timing and geometry of the initial gondwana break up between africa and east antarctica is still poorly known due to missing information about the continent ocean boundaries along the rifted margins in this context the beira high off central mozambique forms a critical geological feature of uncertain crustal fabric based on new wide angle seismic and potential field data across beira high a p wave velocity model supported by amplitude and gravity modelling provides constraints on the crustal composition of this area in the mozambique basin mainly normal oceanic crust of 5 5 7 km thickness with velocities of 6 5 7 0 km s in the lower crust is present a sharp transition towards beira high marks the continent ocean boundary here the crust thickens to 23 km at maximum a small velocity depth gradient and a constant increase in velocity with basal velocities of maximum 7 0 km s are in good agreement with typical velocities of continental crust and continental fragments the density model indicates the existence of felsic material in greater depths and supports a fabric of stretched but highly intruded continental crust below beira high a gradual decrease in crustal thickness characterizes the transition towards the mozambican shelf area here in the zambezi delta depression 12 km of sediments cover the underlying 7 km thick crust the presence of a high velocity lower crustal body with velocities of 7 1 7 4 km s indicates underplated magmatic material in this part of the profile however the velocity structure in the shelf area allows no definite interpretation because of the experimental setup thus the crustal nature below the zambezi delta and consequently the landward position of the continent ocean boundary remains unknown the difference in stretching below the margins of beira high suggests the presence of different thinning directions and a rift jump during the early rifting stage

mesozoic and cenozoic reconstructions of the south atlantic abstract the movement of antarctica with respect to south america has a number of implications for paleocirculation as well as for the reconstructions of gondwanaland recent papers on the southwest indian ridge have published new or revised poles of opening for africa and antarctica which can be combined with the poles of opening between south america and africa to give resultant motions between south america and antarctica the first indication of a complete closure between south america and the antarctic peninsula is at anomaly 28 time 64 ma as the two continents are now configured between anomaly 28 time 64 ma and anomaly m0 time 119 ma the amount of closure does not change greatly and the small computed overlap can be explained by minor uncertainties in the rotation poles used for the reconstructions or some slight extension between east and west antarctica by 135 ma some rotation or translation of the antarctic peninsula with respect to east antarctica must be postulated in addition to any presumed extension between east and west antarctica in order to avoid an overlap of south america with the antarctic peninsula having determined what we feel to be a viable reconstruction of western gondwanaland and holding south america fixed we rotated africa and antarctica with respect to south america for eight different times during the past africa moved away from south america in a more or less consistent manner throughout the time period closure to present while antarctica moved away from africa in a consistent manner only between 160 ma and 64 ma at 64 ma its motion changed abruptly it slowed its north south motion with respect to africa and began slow east west extension with respect to south america this change supports the hypothesis that a major reorganization of the triple junction between africa antarctica and south america occurred between 60 and 65 ma the triple junction changed from ridge ridge ridge to ridge fault fault at the time of the major westward jump of the mid atlantic ridge just south of the falkland agulhas fracture zone the mesozoic opening of the somali basin moved madagascar from its presumed original position with africa in gondwanaland the closure of sri lanka with india produces a unique fit for india and sri lanka with respect to africa madagascar and antarctica this fit juxtaposes geological localities in southeast india against similar localities in enderhy land east antarctica the late jurassic opening in the somali basin is tied to opening of the same age in the mozambique basin since this late jurassic movement represents the initial break up of gondwanaland it is assumed that similar movement must have occurred in what is now the western weddell sea and may also explain the opening evidenced by the rocas verdes region of southern south america

seafloor alteration of basaltic glass textures geochemistry and endolithic microorganisms 1 the incipient low temperature alteration of glassy margins of recent seafloor lavas from the mohns ridge is characterized by scanning electron microscopy and bulk chemical analyses of major and trace elements these investigations are designed to evaluate the endolithic microbial biomass to identify relations between alteration textures and microorganisms and to identify the chemical processes that take place during alteration the basaltic glass along intersecting fractures and around vesicles is typically altered to concentrically zoned yellow brown amorphous gel palagonite in most fractures microorganisms are observed at the outer surfaces of palagonite rims within porous zones of the rims and frequently also at the glass palagonite interfaces the cells act as nucleation sites for precipitation and become encrusted and embedded in palagonite with time zones of porous palagonite containing numerous hollow fossilized cells alternate with zones of compact palagonite lacking distinct cell structures which together indicate that the microbial growth is discontinuous the palagonite has an average organic carbon content of 0 9 wt 13corg 22 which derive from both living and fossilized biomass the microbial growth and biomineralization are major controls on the porosity and texture of the palagonite and thus likely on the chemical exchange between glass and seawater pit marks in the glass in fractures both with and without microbes indicate that microbial as well as abiotic processes mediate pitting elemental and isotope data show that the transformation of glass to gel palagonite at seafloor conditions with high water rock ratios results in near complete loss of si and alkali elements to seawater formation of fe and al oxyhydroxides complete exchange of alkaline earth elements like sr with seawater retention of v cu y pb th u and trivalent rees accompanied by a strong gain of these and other elements from seawater higher fe ti ratios in the gel palagonite compared to the parental glass suggest that the palagonite represents a mixture of fe ti oxyhydroxides derived from both the glass and seawater

deep structure of the northern kerguelen plateau and hotspot related activity summary seismic refraction profiles were carried out in 1983 and 1987 throughout the kerguelen isles southern indian ocean terres australes antarctiques franqaises taaf and thereafter at sea on the kerguelen heard plateau during the md66 keobs cruise in 1991 these profiles substantiate the existence of oceanic type crust beneath the kerguelen heard plateau stretching from 46 s to 55 s including the archipelago seismic velocities within both structures are in the range of those encountered in standard oceanic crust however the kerguelen isles and the kerguelen heard plateau differ strikingly in their velocity depth structure unlike the kerguelen isles the thickening of the crust below the kerguelen heard plateau is caused by a 17 km thick layer 3 velocities of 7 4 km s 1 or so within the transition to mantle zone below the kerguelen isles are ascribed to the lower crust intruded and or underplated by upper mantle material the crust mantle boundary below the kerguelen heard plateau is abrupt and devoid of any underplated material the difference in structure between the northern edge of the kerguelen plateau including the archipelago and the kerguelen heard plateau may be related to variability of the time dependent hotspot activity the kerguelen heard plateau was emplaced during the cretaceous time 110 ma when the volcanic output rate of the kerguelen plateau and the ninetyeast ridge was high as well as high potential temperature the northernmost kerguelen plateau and the archipelago were emplaced during tertiary time 40 45 ma as the volcanic output rate reduced furthermore intraplate volcanism continued in the kerguelen archipelago for at least 40 ma the isostatic compensation of the kerguelen isles and the kerguelen heard plateau is achieved by low density mantle material as shown by refraction and geoid studies the velocity depth structure below the kerguelen isles is similar to that found below intraplate oceanic islands such as hawaii despite the differences in age the crust below iceland 0ma and the kerguelen plateau 100 120 ma are strikingly akin the similarity between the kerguelen heard plateau and iceland then strongly supports a similar origin for both structures the kerguelen heard plateau being a fossil equivalent of present day iceland crustal thickening beneath the kerguelen heard plateau which may result from an iceland type setting i e an active spreading centre over a hotspot is mostly produced by thickening of layer 3 layer 2 representing 25 per cent only of the thickness of the igneous crust the kerguelen isles despite the initial volcanism near the active southeast indian ridge behave as a midplate volcanic island and are definitely not representative of the whole kerguelen plateau structure

zinc isotope composition of the earth and its behaviour during planetary accretion abstract the terrestrial planets are depleted in volatile elements with respect to chondritic meteorites their possible building blocks however the timing extent and origin of volatile depletion is debated zinc is a moderately volatile element mve whose stable isotopic composition can distinguish when and where this depletion took place here we report data for 40 ultramafic rocks comprising pristine upper mantle peridotites from the balmuccia orogenic lherzolite massif and archean komatiites that together define the zn isotope composition of the earth s primitive mantle peridotites and komatiites are shown to have indistinguishable zn isotopic compositions of 66zn 0 16 0 06 2sd with 66zn the per mille deviation of 66zn 64zn from the jmc lyon standard implying a constant zn isotope composition for the silicate earth since 3 5 ga after accounting for zn sequestration during core formation the earth falls on the volatile depleted end of a carbonaceous chondrite array in 66zn zn mg space implying earth avoided modification of its mve budgets during late accretion e g during a giant impact in contrast to the moon the moon deviates from the chondritic array in a manner consistent with evaporative loss of zn where its 66zn co varies with mn na implying post nebular volatile loss is more pronounced on smaller bodies should the giant impact deliver the earth s volatile complement of pb and ag it cannot account for the budget of lithophile mves e g zn rb mn whose abundances reflect those of earth s nebular building blocks the earth initially accreted from material that experienced chemical and mass dependent isotopic fractionation akin to carbonaceous chondrites though volatile depletion was more pronounced on earth

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

the formation of large neoblasts in shocked zircon and their utility in dating impacts uranium lead u pb geochronology of individual shocked zircon grains has unique potential for dating bolide impact events neoblasts in granular textured zircon have been recognized as the shock related feature most effective at recording the impact age here we report the discovery of large neoblasts 5 100 mu m in dimension in shocked zircon at the sudbury impact structure canada the first report of in situ coarsely granular zircon from a terrestrial impact site other than the vredefort structure south africa the neoblast bearing sample was taken from a heterogeneous lithic clast rich igneous unit associated with the roof rocks of the impact melt sheet making this the first time a crater has been dated using neoblastic zircon from the upper part of its stratigraphy previous in situ discoveries of coarsely granular zircon at vredefort were all in impact generated mafic melt emplaced beneath the impact melt sheet electron backscatter diffraction analysis of the impact aged neoblasts indicates that the high pressure conditions inferred in the formation of many small neoblasts were not necessarily involved in the formation of these large ones their large size internal zonation and occurrence in a slowly cooling environment collectively suggest that large neoblasts at sudbury formed by relatively protracted post impact growth in shocked zircon incorporated into impact related melt based on insight from large neoblast growth in terrestrial settings we suggest that the ca 4 33 ga neoblasts recently reported in lunar zircon may imply a major basin forming event on the moon at that time new knowledge of the cratering environments in which large neoblasts form also raises the prospect of possibly linking ex situ granular zircon in lunar breccias with specific impact structures and thus better calibrating the lunar cratering record with radiometric ages

etude de l equilibre isostatique dans le sud ouest de l ocean indien a l aide des resultats de refraction sismique abstract the aseismic ridges of the southwestern indian ocean are in isostatic equilibrium with respect to the adjacent deep basins the structural units can be split into two different groups according to their mean crustal densities we demonstrate that the crustal thickness within the two groups is a linear function of the water depth this relationship expresses the airy isostatic equilibrium however the southern domain of the madagascar ridge is balanced neither with the northern domain of the ridge nor with any other unit of the area of study the northern domain has undergone only slight subsidence since the middle eocene while the southern domain would have rapidly subsided since the early miocene

vitric compositions in the onaping formation and their relationship to the sudbury igneous complex sudbury structure the 1 85 ga sudbury structure is the largest well preserved exposed example of a terrestrial impact crater it contains world class ni cu platinum group element pge ores associated with the igneous component a new understanding of the geology and geochemistry of the vitric crater fill sequence the onaping formation provides constraints on the evolution of 1 85 ga igneous rocks in the sudbury structure the onaping formation has a well defined mappable stratigraphy composed of three members seven stratigraphic units and two types of intraformational dikes that are overprinted by an impact generated hydrothermal system stratigraphic and alteration mapping provided a basis for well constrained sampling and allowed the definition of the geochemical characteristics of the less altered rocks from each unit including vitric bombs and blocks and intraformational vitric dikes significantly the identification of a least altered composition of the vitric onaping formation enabled an investigation of the link between the early formed vitric melt phase in the impact crater fill sequence and the sudbury igneous complex and offset dikes the least altered vitric onaping formation represented by the massive xenolith poor cores of vitric bombs blocks and intraformational dikes is andesitic in composition consistent with a crustally derived melt the melt composition 61 6 wt sio 2 4 28 mgo 60 ppm ni is remarkably uniform throughout the onaping formation with a typical enrichment of large ion lithophile and light rare earth elements and pronounced negative nb and ti anomalies incompatible trace element ratios and bulk concentrations of the onaping formation coincide with the main mass norite and offset quartz diorite compositions of the underlying sudbury igneous complex but differ distinctly from the granophyre and quartz gabbro root mean squared differences in composition of the vitric onaping formation with the units of the sudbury igneous complex and quartz diorite offset dikes show that the vitric onaping formation does not directly match any of these igneous units but more closely resembles the composition of the ministic and manchester offset dikes the quartz diorite offset dikes have been proposed as representative of the bulk composition of the original melt of the sudbury igneous complex in detail there are small compositional differences between north and south range offset dikes and between these offset dikes and the concentric manchester offset dike however such differences are lacking in the onaping formation we propose that the least altered vitric composition of the onaping formation represents the best estimate of the bulk composition of an initial quenched melt or shock melt formed early in the evolution of the sudbury structure and is a logical starting liquid to model the evolution of the sudbury main mass and offset dikes the offset dike compositions can be derived from onaping formation vitric shock melt by assimilation of progressively greater amounts of mafic material at deeper levels in the crater this mafic contaminated melt or impact melt developed at the base of the melt sheet and was injected into the offset dikes both prior to and contemporaneously with the earliest stages of sulfide precipitation from the sudbury complex fractionation of the mafic contaminated melt by plagioclase and pyroxene accumulation produced the north range felsic and mafic norite and the south range norites the chalcophile metal content of the onaping formation vitric phase closely reflects the overall metal ratios of the magmatic sulfide ores the vitric onaping formation and unmineralized offset dikes have low liquidus temperatures 1 115 1 125 c and if these units represent the source of metals in the sulfide deposits unusually high mass distribution coefficients for cu and ni and high r factors are necessary to account for the metal endowment

observations of beach cusp evolution at melbourne beach florida usa abstract morphological observations terrestrial laser scanning and sediment analysis are used to study beach cusp morphodynamics at melbourne beach florida usa a moderately sloped beach characterized by fine sand the study couples rapid high resolution surveying with surficial sediment sampling over multiple tidal cycles surveys were run 500 m alongshore and sediment samples were collected intertidally over a period of 5 days beach cusp location within larger scale beach morphology is shown to directly influence cusp growth as either gross erosional or gross accretional sediment characteristics within the beach cusp morphology are reported coincident with cusp evolution variations in particle size distribution kurtosis are exhibited as the cusps evolve however no significant correlation is seen between grain size and position between horn and embayment during the end of the study a storm resulted in beach cusp destruction and increased sediment sorting

seismic stratigraphic framework and depositional history for cretaceous and cenozoic contourite depositional systems of the mozambique channel sw indian ocean this study describes previously unrecognized contourite depositional systems cdss in the mozambique channel which constrain palaeoceanographic models for this area the stratigraphic stacking patterns record nine seismic units su1 to su9 separated by eight major discontinuities a to h oldest to youngest key seismic markers in cds evolutionary history occur during aptian albian 122 ma late cenomanian 94 ma early 38 2 36 2 ma and late 25 23 ma oligocene and early middle miocene 17 15 ma epochs these record onset 122 to 94 ma growth 94 to 25 23 ma maintenance 25 23 to 17 15 ma and burial 17 15 ma to the actual time stages for cdss cdss first develop during the onset stage which coincides with the opening and deepening of the african southern ocean gateway at 122 and 100 ma respectively the growth stage beginning in the late cenomanian 94 ma correlates with the opening and deepening of the equatorial atlantic gateway during the growth stage two major shifts in sedimentary stacking pattern occur which coincide with palaeoceanographic changes during the early 38 2 36 2 ma and late 25 23 ma oligocene these in turn coincide with the onset and local enhancement of antarctic water masses cds growth continued until the early middle miocene during the maintenance stage 17 15 ma most cds growth ceased at the end of the maintenance stage circulation of the north atlantic water mass into the southern hemisphere led to a deepening of antarctic water masses in the area

contributions of cretaceous quiet zone natural remanent magnetization to magsat anomalies in the southwest indian ocean the magsat magnetic anomalies over the southwest indian ocean are modeled using a combination of induced plus viscous remanent magnetization im vrm and natural remanent magnetization nrm two broad roughly parallel sw to ne trending triple peaked positive anomalies dominate the region one lying south of africa and the other north of antarctica although these anomaly peaks generally correspond with the agulhas plateau maud rise mozambique plateau astrid ridge and madagascar ridge conrad rise conjugate pairs the im vrm contribution from structural characteristics i e crustal thickness accounts for only about 20 of the anomaly amplitudes a spatially variable but observationally constrained nrm contribution in cretaceous quiet zone kqz crust is required to account for the location shape and amplitude contrast of these anomalies many crustal features in the southwest indian ocean near antarctica have little geophysical data to constrain their structure but do have tectonic conjugates near africa for which much more geophysical data are generally available using geophysical and geological constraints from one member to model the magnetization structure of its conjugate reproduces the observed magsat reduced to pole anomalies over both structures very well this suggests that no significant alteration in their magnetization structure has occurred since the features split models of these conjugate structures show that im vrm reproduces the magsat anomalies associated with non kqz crust but that both im vrm and a dominant nrm component are required to explain the anomalies associated with kqz crust

deep structure of the southern kerguelen plateau southern indian ocean from ocean bottom seismometer wide angle seismic data wide angle seismic data collected during the kerguelen ocean bottom seismometer experiment provide the first images of the deep structure of the southern kerguelen plateau and support a new interpretation of the origin of the plateau velocity models based on travel time inversions and reflectivity synthetic seismograms show a 22 km thick crust composed of 1 6 km of sedimentary cover 5 3 km of upper crust 11 0 km of lower crust and a 4 to 6 km thick reflective zone immediately above moho velocities in the upper crust from 3 8 4 5 km s at top to 6 0 6 5 km s at bottom are consistent with the basaltic nature of this layer the top of which was sampled during the ocean drilling program velocities in the lower crust increase continuously from 6 60 km s at the top to 6 90 km s at 19 5 km depth the reflective zone at the base of the crust identified by wide angle reflections is observed only along the nnw sse direction it consists of alternating high and low velocity layers with an average velocity of 6 70 km s in the nnw sse direction and 6 90 km s in the perpendicular direction strong azimuthal anisotropy is also observed in the upper mantle with velocities of 8 60 and 8 00 km s in the nnw sse and e w directions respectively the absence of high velocities at the base of the crust that characterizes many large volume mafic provinces the reflective lower crust and anisotropy in upper mantle suggest that the southern kerguelen plateau represents a stretched continental fragment overlain by basaltic flows isolated from the antarctic margin during the early opening of the indian ocean

variations in thickness of layer 3 dominate oceanic crustal structure abstract seismic refraction studies have determined that while much of the crust underlying the world s oceans is about 7 km thick it also occurs in lesser and considerably greater thicknesses here we show that vastly different thicknesses of oceanic crust formed at ridges by seafloor spreading are generally characterized by the two gradient velocity structure recognized as typical of 6 7 km thick oceanic crust adopting a standard we study ninety published seismic velocity structures of oceanic crust ranging in thickness from 2 to 37 km several structures formed in midplate settings ocean islands and seamounts are also considered for comparison regardless of origin ridge or midplate we find that the percentage of the whole crust formed by layer 2 systematically decreases with increasing total crustal thickness while the average velocity of layer 2 varies widely for all crust the average velocity of layer 3 increases systematically with increases in the average whole crustal thickness and velocity crust formed in midplate settings differs in that the thickness of layer 2 velocity crust is generally two to three times that of crust formed at ridges residual depths were calculated for oceanic crust by removing the effects of sediment loading and subsidence due to lithospheric cooling most of the data fall close to a model whereby different crustal thicknesses result from different extents of partial melting of oceanic upper mantle the crustal sections were restored to zero age assuming the average depth of emplacement for 7 km thick oceanic crust is 2 5 km below sea level from this restoration it appears that the depth to the top of layer 3 at the time it was formed is 4 25 1 25 km below sea level regardless of whole crustal thickness or spreading rate these results suggest that the mechanism of construction of oceanic crust is in many ways remarkably uniform despite very large changes in the total thickness of the crust produced very basic questions remain it is for instance not at all clear why layer 2 and layer 3 exist in the proportions they do while neutral buoyancy or melt trapping at the base of a brittle lid may account for local properties of the layer 2 3 boundary they do not seem adequate to account for the global phenomenon

anomalous heat flow near the crozet plateau and mantle convection abstract a heat flow study has been performed near the crozet plateau southern indian ocean where the geoid and bathymetry are anomalously high with respect to the nearby deep ocean basins data collected on 67 ma old oceanic crust agree with simple plate model predictions at the stations furthest removed from the plateau heat flow values increase significantly towards the centre of the feature suggesting a thermal origin for this uplift the ratio of geoid to bathymetry confirms that simple crustal thickening with airy local compensation cannot explain the observations the data are compatible with a model of dynamic convective support of the plateau by an underlying thermal plume in the earth s mantle

high spatial resolution geochemistry and textural characteristics of microtektite glass spherules in proximal cretaceous paleogene sections insights into glass alteration patterns and precursor melt lithologies abstract using laser ablation inductively coupled plasma mass spectrometry la icp ms we have conducted spatially resolved trace element analysis on fresh unaltered microtektite glasses linked to the cretaceous paleogene k pg boundary chicxulub crater and on their surrounding alteration phases this unique approach offers the opportunity to study in situ and at high spatial resolution both the mixing of different target lithologies and the variation of the major and trace element budget during the alteration process in addition two dimensional element distribution maps reveal important geochemical information beyond the capabilities of single spot laser drilling glasses from two localities in opposite quadrants from the source crater were studied at the beloc locality haiti the glass population is dominated by the presence of yellow high ca glass and black andesitic glass formed by admixture of carbonate dolomite anhydrite platform lithologies with crystalline basement these glasses alter according to the well established hydration palagonitization model postulated for mafic volcanic glasses rees become progressively leached from the glass to below the detection limit for the applied spot size while immobile zr hf nb and ta passively accumulate in the process exhibiting both inter element ratios and absolute concentrations similar to those for the original glass in contrast the arroyo el mimbral locality ne mexico is characterized by abundant green glass fragments high in si al and alkalis and low in mg ca fe low si black glass is less abundant though similar in composition to the black glass variety at beloc the alteration pattern of high si al green glass at the mimbral locality is more complex including numerous competing reaction processes ion exchange hydration dissolution and secondary mineral precipitation generally controlled by the ph and composition of the surrounding fluid all green high si al glasses are hydrated and variably enriched in sr ba and cs indicating preferred adsorption from seawater during hydration despite the onset of ion exchange reactions which only seem to have affected the alkalis the trace element composition of the green high si al glass is still largely representative of the original melt composition refining the geochemical signature of altered melt lithologies may advance our current understanding of glass stability in the natural environment and provide insight into the origin and emplacement of ejecta material during crater formation

tectonic evolution of the southwest indian ocean since the mid cretaceous plate motions and stability of the pole of antarctica africa for at least 80 myr summary the southwest indian ridge is the boundary where seafloor is being created between the african and antarctic plates east of the newly recognized du toit fracture zone at 27 e it trends north easterly and is cut by a series of six deep near north south clefts marking transform faults this series starts with the dual prince edward fracture zone near 35 e and includes discovery i1 also dual at 42 e indomed at 46 e and gallieni at 52 e terminating with the spectacular atlantis i1 and melville fracture zones at 57 30 e and 60 30 e respectively the trend of these fracture zones is compatible with an instantaneous pole of relative motion for antarctica africa at 8 4 n 42 4 w between 35 e and 55 e the development of the southwest indian ridge has separated the elevated madagascar ridge from the discontinuous crozet plateau that was contiguous to it prior to eocene time in this entire sector the ocean floor overall is elevated with respect to ocean floor of equivalent age elsewhere between these features anomalies 0 22 0 53 ma have been identified north and 13 26 35 60 ma south of the ridge axis these anomalies and 0 34 0 80 ma located just to the west in the mozambique basin lie at the same distance from the ridge axis as a similar sequence east of the bouvet triple junction the similarity in distance from the ridge axis of these anomaly sequences is evidence that africa and east antarctica have been separated by a single plate boundary from 80ma to the present the pole representing the motion of these two plates at the present has not moved in totality far from the position it occupied in the late cretaceous 80 ma furthermore calculation of intermediate finite rotation poles for four intervals within this span reveals only minor 15 excursions from the present pole the spatial reconstruction of the southwest indian ridge at 80ma anomaly 34 places the ridge midway between africa and eastern antarctica and lying at right angles to the north trending mozambique ridge the separation of antarctica from africa following initial opening in the jurassic may have been as straightforward as it appears to have been from 80 ma to the present

shallow impact isotopic insights into crustal contributions to the sudbury impact melt sheet abstract the largest known terrestrial impact melt sheet occurs within the 1850 ma sudbury structure ontario in order to evaluate the relative contributions of different target lithologies to the melt sheet we have investigated the pb isotope compositions of feldspar separates from early formed quartz diorite magmas within offset dykes from around the impact structure the samples define a linear array on plots of age corrected 206 pb 204 pb versus 207 pb 204 pb samples from offset dykes hosted by the huronian supergroup south range have a range of 206 pb 204 pb 1850 from 15 424 to 17 255 and 207 pb 204 pb 1850 from 15 390 to 15 801 whilst those hosted by archean gneisses of the superior province north range cluster around 206 pb 204 pb 1850 14 8 and 206 pb 204 pb 1850 15 1 these values can be approximated by binary mixing between the two major groups of target lithologies a mix of 60 70 of superior province gneisses with 30 40 of huronian metasedimentary material closely matches the pb isotope compositions of north range offset dyke samples whereas in the south range the required huronian component is up to ca 80 these mixing proportions are consistent with sr nd and os isotope and trace element constraints a third minor component either locally exposed paleoproterozoic mafic rocks or the lower crust is also required however the isotopic trace element and ni cu platinum group element characteristics of the melt sheet can be accommodated without the involvement of an average lower crustal or meteoritic component a major contribution of huronian supracrustal material which had a pre impact thickness of up to 12 km is required to explain the chemical characteristics of the impact melts which also have a strong upper crustal affinity e g eu sm 0 22 rb sr 0 2 0 35 as such a shallower level of melting is apparent than that predicted by many previous impact models for the sudbury event this can be accommodated by considering approach trajectories for the impactor oblique to the earth s surface in addition the isotopic and trace element variability identified indicates that the melt sheet was heterogeneous at an early stage and may not have been completely homogenised during crater formation our findings have significant implications for the nature of the sudbury impact event the evolution of the melt sheet and the crustal sources of metals contained in sudbury s world class ni cu pge sulphide ores

earth structure and instrumental seismicity of madagascar implications on the seismotectonics abstract the aim of this study is to improve the knowledge of the seismotectonics of madagascar we first investigate the structure of the earth beneath madagascar through the joint inversion of receiver functions and rayleigh wave group velocities then we use the obtained velocity models to relocate local earthquakes in order to analyse the distribution of seismicity finally we use structural models and earthquake coordinates to compute focal mechanisms our retrieved earth structure models confirm a thin lithosphere beneath madagascar when compared to the nearby east african rift the high plateau in the central region coincides with the thinnest lithosphere over the slowest asthenosphere our results are in good agreement with the gravity anomalies and likely confirm a localised asthenospheric upwelling beneath the central part of madagascar the surface expression of the asthenospheric upwelling consists in a horst graben structure the moderate seismicity is localised along pre existing structures reflecting an e w extension that is mostly accommodated in the lower crust

the kerguelen isles southern indian ocean new results on deep structure from refraction profiles abstract additional seismic refraction profiles shot across the kerguelen archipelago terres australes et antarctiques francaises known as taaf located in the southern indian ocean substantiate the oceanic type crust of the archipelago and the kerguelen plateau as well the crust mantle boundary is better modelled by a transition than by a discrete interface seismic velocities of 6 60 6 80 km s are in the range of those within oceanic layer 3 velocities of 7 2 7 5 km s observed at long ranges may be interpreted according to at least two distinct hypotheses 1 1 notwithstanding their rather low values they represent upper mantle material and might be assigned to peridotites serpentinised by hydrothermal circulations as reported for various oceanic and continental features 2 2 these velocities are assigned to the lower crust intruded by melt produced by adiabatic decompression of upper mantle material during rifting underplating a small increase in mantle temperature caused by the presence of heat sources i e the southeast indian ridge and the kerguelen mantle plume nearby may have produced huge quantities of melt facilited by the slow movement of the antarctic plate then building up the aseismic kerguelen plateau

rubidium isotopic composition of the earth meteorites and the moon evidence for the origin of volatile loss during planetary accretion abstract understanding the origin of volatile element variations in the inner solar system has long been a goal of cosmochemistry but many early studies searching for the fingerprint of volatile loss using stable isotope systems failed to find any resolvable variations an improved method for the chemical purification of rb for high precision isotope ratio measurements by multi collector inductively coupled plasma mass spectrometry this method has been used to measure the rb isotopic composition for a suite of planetary materials including carbonaceous ordinary and enstatite chondrites as well as achondrites eucrite angrite terrestrial igneous rocks basalt andesite granite and apollo lunar samples mare basalts alkali suite volatile depleted bodies e g hed parent body thermally metamorphosed meteorites are enriched in the heavy isotope of rb by up to several per mil compared to chondrites suggesting volatile loss by evaporation at the surface of planetesimals in addition the moon is isotopically distinct from the moon in rb the variations in rb isotope compositions in the volatile poor samples are attributed to volatile loss from planetesimals during accretion this suggests that either the rb and other volatile elements were lost during or following the giant impact or by evaporation earlier during the accretion history of theia

absolute and relative plate motions and hypotheses on the origin of five aseismic ridges in the indian ocean abstract paleopositions of the african indian and antarctic plates are used as an independant set of data to test various hypotheses derived from geophysical data on the origin emplacement mechanism and evolution of five aseismic structures of the western and southern indian ocean crozet and kerguelen plateaus marion dufresne lena and ob seamount chain madagascar and mascarene ridges except for a continental fragment of very limited extent bearing the seychelles islands these structures together with the madagascar and mascarene ridges are probably due to anomalous volcanic episodes at or near active plate boundaries rather than to intraplate emplacement their creation processes are therefore mainly related to the relative motions between the antarctic african and indian plates the volcanic episodes are synchronous with major changes in the relative motions between these plates and thus substantiate the role played by frequent irregularities in the kinematic pattern of the western and southern indian ocean such as ridge jumps asymmetric spreading and rapid variations in spreading velocity or direction finally we think that thermal anomalies located not far from active spreading boundaries may have played a role in the physical processes creating the aseismic ridges

isotopic heterogeneity in the sudbury impact melt sheet abstract a unique terrestrial large impact melt sheet is preserved in the 1850 ma sudbury structure ontario we have undertaken a pb isotope investigation of the southern limb of the melt sheet termed the south range main mass the model initial pb isotope ratios 207 pb 204 pb m vary stratigraphically through the predominantly quartz monzogabbroic lower unit varying from 15 40 to 15 45 at the base to ca 15 35 at the top of the sequence lateral variations of similar range occur in basal lower unit samples over scales of less than 5 km the range of these variations is similar to those of locally exposed upper crustal target rocks and it is evident that the melt sheet has efficiently preserved inherited variability during the violent phases of crater formation superheated impact melts are expected to be well mixed mechanically therefore significant post impact melting of target rocks fallback material and entrained clasts is required to explain such heterogeneity the sudbury structure hosts world class ni cu pge sulphide ore deposits systematic variation in 207 pb 204 pb m occurs throughout sulphide ores within the creighton embayment from massive 15 42 15 45 to interstitial ca 15 40 15 41 and disseminated ca 15 39 sulphide linking the pb isotope composition of these ores to the immediately overlying lower unit stratigraphy a protracted sulphide segregation history is apparent massive sulphides segregated early prior to or during initial silicate crystallisation although the total time involved in sulphide accumulation spanned much of the crystallisation of the lower unit it is also shown that lateral variations in ni depletion throughout the main mass correlate with pb isotopes those segments with the strongest chalcophile element depletion signatures reflecting the accumulation of significant basal sulphides have high initial pb isotope values consistent with early sulphide segregation the characterisation of pb isotopic heterogeneity has therefore provided insights into the evolution and scales of mixing of the melt sheet with the identified chemical variability between melt cells having a significant influence on ore forming processes

reconstruction of the east africa and antarctica continental margins the early jurassic separation of antarctica from africa plays an important role in our understanding of the dispersal of gondwana and pangea previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent ocean boundary cob and determining the amount and distribution of extended continental crust this study focuses on the evolution of the african margin adjacent to the mozambique basin and the conjugate antarctic margin near the riiser larsen sea satellite derived gravity data have been used to trace the orientations and landward limits of fracture zones a 3 d gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust crustal thicknesses together with fracture zone terminations reveal cobs that are significantly closer to the african and antarctic coasts than previously recognized correlation of fracture zone azimuths and identified cobs suggests antarctica began drifting away from africa at approximately 171 ma in a roughly sse direction an areal balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins both margins reveal a trend of increasing extension from east to west our results suggest africa underwent extension of 60 120 km while antarctic crust was stretched by 105 180 km various models tested to determine the direction of extension during rifting suggest that antarctica moved away from africa in a wnw ese direction during the period between 184 and 171 ma prior to the onset of seafloor spreading

crustal structure of the madeira tore rise eastern north atlantic results of a dobs wide angle and normal incidence seismic experiment in the josephine seamount region summary during november december 1988 an extensive geophysical data set was collected over the josephine seamount which is located at the northeasterly end of the madeira tore rise in the eastern north atlantic the josephine seamount lies at the intersection of the madeira tore rise and azores gibraltar fracture zone the latter representing the africa eurasia plate boundary in this part of the eastern north atlantic from this data set a 275 km long explosive refraction line has been modelled together with wide angle airgun profiles seismic reflection and gravity data the velocity depth model shows that the crust either side of the seamount is typically oceanic in character however beneath the seamount there exists a region of anomalously high velocity and crustal thickening to a depth of about 17 18 km gravity modelling also suggests that the josephine seamount is compensated by a crustal root and that the josephine seamount madeira tore rise system is in local isostatic equilibrium calculations of the flexural rigidity and effective elastic thickness of the lithosphere in this region suggest that the madeira tore rise formed contemporaneously with the lithosphere on which it lies this age of crustal loading is consistent with the proposal that the madeira tore rise is an aseismic ridge which formed at or near the mid atlantic ridge

oceanic mafic granulite xenoliths from the kerguelen archipelago oceanic plateaus are large accumulations of volcanic and plutonic rocks on the ocean floor whose detailed nature and origin are only poorly understood their immense volumes frequent occurrence since the cretaceous period1 and conspicuous presence in all major ocean basins point to their significance in understanding the earth s thermal and chemical evolution we report here the discovery of mafic granulite xenoliths from kerguelen island at the northern end of the kerguelen plateau one of the two largest oceanic plateaus granulites are normally found only in continental settings where the crust is thick enough to experience the high temperatures and pressures required for granulite facies minerals to form we speculate that the kerguelen xenoliths represent basaltic magmas underplated during the evolution of the plateau and more generally that such mafic granulites are an important constituent of the lower parts of oceanic plateaus which are inaccessible by drilling

the structure and origin of the azores biscay rise north east atlantic ocean summary the azores biscay rise is a roughly linear north east south west trending feature rising 1500 3000 m above its surroundings which extends from about 45 n 15 30 w towards the azores its south western termination is near 40 30 n 21 30 w about halfway along its length the rise intersects the wnw trending king s trough in 1978 a set of bathymetric magnetic gravity gloria and seismic reflection and refraction data were obtained in the vicinity of the rise together with earlier data these observations suggest that 1 there has been no substantial post emplacement tectonic activity with the possible exception of the construction of some volcanic seamounts at the south western end of the rise and 2 the rise is underlain by a low velocity low density lower crust and is in isostatic equilibrium the rise can be convincingly shown to be the eastern half of a pair of ridges formed by abnormal crustal generation at the mid atlantic ridge crest between the times of anomalies 33 and 24 76 56ma ago the western counterpart of the rise includes gauss and milne seamounts in the newfoundland basin magnetic anomaly 3 1 passes uninterruptedly across the rise and therefore hypotheses that the northern part of the rise was the site of a cenozoic transform fault or subduction zone are not supported by our data it is speculated that king s trough was linked to the north spanish trough by an early cenozoic east west transform fault across the northern iberia abyssal plain this plate boundary became inactive about the middle of the oligocene epoch

the sudbury igneous complex viscous emulsion differentiation of a superheated impact melt sheet the sudbury igneous complex of ontario canada is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1 85 ga ago the transient cavity and melting zone reached the moho and instantly 2 min relaxed to form a more familiar large shallow crater holding a thick superheated 1700 c melt sheet covered by 2 km of breccia there is little about the resulting bimodal igneous complex that resembles crystallization of well known sheet like bodies of similar composition yet the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions suggesting an intimate common parentage from the surrounding crust this petrogenetic enigma is explained here as a natural unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion which we take as the high temperature equivalent of breccia a wide spectrum of viscously discrete interdispersed parcels of mafic and felsic liquids reflecting the compositional heterogeneity of the target crustal materials formed the emulsion within days to months the emulsion components separated according to their relative densities into a bimodal norite granophyre assemblage that formed the basic structure of the present sudbury igneous complex there is clear evidence of this emulsion in the earliest dikes i e offsets which likely give the earliest state of the nascent melt sheet immediately following emulsion separation the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer these convective motions homogenized and rapidly cooled the magma to the liquidus temperatures whereupon convection ceased the pattern of convection was pinned in place by the embayment topography of the crater floor which in turn played a pivotal role in directing sulfide deposition into the embayments all further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins there is clear evidence of solidification from the floor upward and the roof downward minimal differentiation and compositional modification took place throughout cooling and solidification nevertheless during the solidification stage granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying onaping formation became unstable and entered the melt sheet and the partially melted remnants collected at the interface between the norite and granophyre some interstitial melt from the norite also percolated upward and altogether the blocks and melt produced the distinctive chemical and physical characteristics of the unusual transition zone the sudbury melt sheet is in essence a full scale magmatic experiment the conditions of formation relative to any other large terrestrial magma are precisely known thus the clear lack of any significant modal layering the overall homogeneous nature of each unit and the lack of any significant chemical differentiation through crystal fractionation establish sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers

ries crater and suevite revisited observations and modeling part i observations we report results of an interdisciplinary project devoted to the 26 km diameter ries crater and to the genesis of suevite recent laboratory analyses of crater suevite occurring within the central crater basin and of outer suevite on top of the continuous ejecta blanket as well as data accumulated during the past 50 years are interpreted within the boundary conditions imposed by a comprehensive new effort to model the crater formation and its ejecta deposits by computer code calculations artemieva et al 2013 the properties of suevite are considered on all scales from megascopic to submicroscopic in the context of its geological setting in a new approach we reconstruct the minimum maximum volumes of all allochthonous impact formations 108 116 km3 of suevite 14 22 km3 and the total volume of impact melt 4 9 8 0 km3 produced by the ries impact event prior to erosion these volumes are reasonably compatible with corresponding values obtained by numerical modeling taking all data on modal composition texture chemistry and shock metamorphism of suevite and the results of modeling into account we arrive at a new empirical model implying five main consecutive phases of crater formation and ejecta emplacement numerical modeling indicates that only a very small fraction of suevite can be derived from the primary ejecta plume which is possibly represented by the fine grained basal layer of outer suevite the main mass of suevite was deposited from a secondary plume induced by an explosive reaction fuel coolant interaction of impact melt with water and volatile rich sedimentary rocks within a clast laden temporary melt pool both melt pool and plume appear to be heterogeneous in space and time outer suevite appears to be derived from an early formed melt rich and clast poor plume region rich in strongly shocked components melt clasts and originating from an upper more marginal zone of the melt pool crater suevite is obviously deposited from later formed clast rich and melt poor plumes dominated by unshocked and weakly shocked clasts and derived from a deeper central zone of the melt pool genetically we distinguish between primary suevite which includes dike suevite the lower sublayer of crater suevite and possibly a basal layer of outer suevite and secondary suevite represented by the massive upper sublayer of crater suevite and the main mass of outer suevite

video observations of beach cusp morphodynamics beach cusps are a common feature of steep reflective and intermediate beaches although many observations of beach cusp spacing exist there are few observations quantifying the incipient formation evolution and eventual destruction of these features beach cusp morphodynamics were analyzed using a 3 year dataset of video images collected at tairua beach new zealand twenty four beach cusp episodes were selected to monitor the cycle of beach face changes from planar to the appearance of the cusp patterns and back to planar again observations show that beach cusp disappearance can be ascribed not only to the erosive influence of storms but also to the persistence of accretionary conditions leading to the infilling of beach cusp bays and the development of an alongshore continuous berm we also report observations of changes in beach cusp spacing over time which can be attributed to the merging of adjacent cusps within the cusp field with the overall cusp spacing re adjusting to accommodate the disappearance of a horn although the self organization theory provides a better fit to the data and theory we were unable to conclusively refute any of the mechanisms causing beach cusp formation since both existing theories subharmonic standing edge wave and self organization can predict the trend in the observed beach cusp spacing these observations show that initial beach cusp formation primarily occurs around 70 of the episodes analyzed under mildly accretionary conditions and when accretion persists the pattern disappearance is likely to occur as a result of bay infilling to form a featureless berm

palaeomagnetism of cretaceous pacific seamounts summary the palaeomagnetism of cretaceous pacific seamounts as deduced from their magnetic anomalies is reviewed results from 35 seamounts are reported here for the first time of these 35 results 15 are of high enough quality to be useful this has resulted in a considerable increase in the available information concerning the palaeomagnetism of the pacific plate using only the most reliable results we calculate a palaeomagnetic pole for the upper cretaceous at 58 north 350 east with a 95 per cent cone of confidence of only 5 this pole is considerably more accurate than the one previously reported and is farther to the west the previously reported difference between the mean pole position from seamounts close to japan and the mean pole position from seamounts close to hawaii is not real although one group of seamounts close to japan gives an anomalous pole position several mechanisms have been suggested whereby errors might be introduced in the determinations of pole positions from seamounts no error is introduced in the pole position because of the restricted extent of surveys and the consequent inaccuracy in the determination of the regional field the demagnetization effect which can produce differences between the observed direction of magnetization and the direction of the magnetizing field in an elongated body is unimportant for seamounts the new pole position is sufficiently accurate for use in many types of calculations in which it is desired to locate the pacific plate with respect to the earth s spin axis

evolution of the southwest indian ridge from the late cretaceous anomaly 34 to the middle eocene anomaly 20 abstract the determination of the motion of antarctica relative to africa is particularly important when considering the breakup of gondwana two models have been proposed that describe the pattern of seafloor spreading between africa and antarctica during the late cretaceous starting at chron 34 84 ma through to the middle eocene chron 20 46 ma in the first model the motion of antarctica relative to africa can be simply described by a rotation about a single pole of rotation in the second model which we favor the relative motion of antarctica and africa is more complex and a major change in spreading direction between chron 32 74 ma and chron 24 56 ma times is required in this paper we present ten plate tectonic reconstructions of the southwest indian ridge that were produced using a new compilation of magnetic bathymetric and satellite altimetry data in combination with interactive computer graphics these reconstructions illustrate that spreading directions started to change at chron 32 74 ma between chrons 31 and 28 69 64 ma spreading was very slow

the sudbury impact layer in the paleoproterozoic iron ranges of northern michigan usa a layer of breccia that contains fragments of impact ejecta has been found at 10 sites in the paleoproterozoic iron ranges of northern michigan in the lake superior region of the united states radiometric age constraints from events predating and postdating depo si tion of the breccia are ca 1875 ma and 1830 ma the major bolide impact that occurred at 1850 ma at sudbury ontario 500 700 km east of these sites is the likely causative event the michigan sites described here along with previously described sites in minnesota and ontario defi ne an extensive ejecta bearing deposit throughout the paleoproterozoic iron ranges of the lake superior region that we refer to as the sudbury impact layer the layer at the sites in michigan exhibits a range of thicknesses lithologic characters and sedimentary settings the diversity of rock types and internal stratigraphic details of the layer imply that several different processes of transport and deposition are represented but the detailed investigations needed to document them are incomplete many of the sites had been described and interpreted previously as products of common terrestrial processes but the presence of relict shock induced planar deformation features in quartz indicates that the breccia layer is in fact the product of an extra terrestrial impact at most localities this layer also contains relict fragments of altered devitrifi ed glass and or accretionary lapilli one immediate use of the impact layer is as an ultraprecise time line that ties together the well known stratigraphic sequences of the various geographically separated iron ranges the correlation of which has remained controversial for many decades the sudbury impact layer most commonly lies at a horizon that records a signifi cant change in the character of sediments across the region the impact layer marks the end of a major period of banded iron formation deposition that was succeeded by deposition of ficlastic rocks commonly black shales the impact may have produced regional if not global changes in the environment that resulted in this widespread synchronous change in sedimentation style

origin of the earth s moon constraints from alkali volatile trace elements although the moon is depleted in volatile elements compared to the earth these depletions are not in accord with simple volatility for example the csrb ratios of the earth and moon inferred from basalts are approximately one seventh and one half of the ci ratio respectively volatility considerations alone predict that the lunar csrb ratio should be equal to or lower than the terrestrial ratio if the moon was derived entirely from earth mantle material thus hypotheses such as rotational fission which invoke derivation of lunar material entirely from the earth s mantle may be excluded the collisional ejection hypothesis of lunar origin requires at least 18 of lunar material to be derived from a projectile with dehydrated ci composition to match the lunar csrb ratio and 25 50 to match both the lunar csrb ratio and absolute concentrations of cs and rb it remains to be demonstrated that this relatively large contribution of projectile material is consistent with other elemental abundances and element ratios in the moon

crustal architecture and isostatic compensation of the comorin ridge central indian ocean implications for the breakup of east gondwana abstract the comorin ridge located off the southwest margin of sri lanka is an important geological feature with significant relevance to understand the breakup of eastern gondwanaland earlier geophysical studies over this ridge provided several contrasting views due to which its origin and nature of emplacement could not be established very well in this study an integrated interpretation of bathymetry gravity geoid and ship borne magnetic data is carried out to bring out a well constrained evolution model for the ridge results of 3 d gravity inversion of crustal bouguer anomalies and joint gravity magnetic modeling indicate that the southern part of the ridge is underlain by 7 12 km thick crust intruded with volcanic rocks and the presence of 13 18 km of anomalously thicker crust below the northern part of the ridge further crustal models spatial variations in geoid topography ratio gtr and the effective elastic thickness te value obtained in this study suggest a two stage emplacement model for the evolution of the ridge the observed lower te values 7 8 km in the southern part 0 5 n 3 5 n and its proximity to the c34ny spreading anomaly suggest its emplacement on the oceanic lithosphere due to marion plume activity along a fracture zone during 88 84 ma on the other hand relatively higher te values 12 5 15 km in the northern part 3 5 n 6 5 n with absence seafloor spreading magnetic anomalies suggests that it could be a marginal basement high formed at the continent ocean boundary cob during the second phase of rifting between india and sri lanka 94 ma

madagascar basalts tracking oceanic and continental sources extensive upper cretaceous volcanism in southern madagascar was fed in part by mantle sources resembling those expressed today in the indian ocean at marion and prince edward islands and on the central southwest indian ridge in addition very low end t to 17 4 high 87sr 86sr t to 0 72126 tholeiites in southwestern madagascar were variably but highly contaminated by ancient continental material broadly like that affecting the bushe and poladpur formations of the later deccan traps in india alkalic dikes in southwestern madagascar have a rough analogue in the mahabaleshwar formation of the deccan in that they document the influence of a low 206pb 204pb negative end relatively low 87sr 86sr reservoir a very similar reservoir is manifested at present in mid ocean ridge basalts on the central southwest indian ridge near 40 e the original location of this end member appears likely to have been in the madagascan lithospheric mantle a portion of which may have been removed in the middle cretaceous by the action of the marion hotspot or the rifting of indo madagascar an origin within the hotspot itself also may be possible however recent products of the hotspot appear to lack completely the necessary low 206pb 204pb low end signatures

zinc isotopic evidence for the origin of the moon lunar magmatic rocks are shown to be enriched in the heavy isotopes of zinc and to have lower zinc concentrations than terrestrial or martian igneous rocks these variations represent the large scale evaporation of zinc most probably in the aftermath of the moon forming giant impact event the heavily favoured theory for the origin of the earth moon system is a giant impact between the proto earth and a mars sized body such a cataclysmic event would have left its mark on the isotopic composition of the moon because light isotopes evaporate more readily than heavier ones zinc in particular is a powerful indicator of the volatile histories of planets it undergoes strong isotopic fractionation in planetary rocks but is hardly fractionated following volcanic activity on earth this study compares high precision zinc isotopic data for lunar basalts martian meteorites and terrestrial igneous rocks and finds that lunar magmatic rocks are enriched in the heavy isotopes of zinc and have lower zinc concentrations than terrestrial or martian samples the authors conclude that these variations are the result of large scale evaporation of zinc in the aftermath of the moon forming giant impact event volatile elements have a fundamental role in the evolution of planets but how budgets of volatiles were set in planets and the nature and extent of volatile depletion of planetary bodies during the earliest stages of solar system formation remain poorly understood1 2 the moon is considered to be volatile depleted and so it has been predicted that volatile loss should have fractionated stable isotopes of moderately volatile elements3 one such element zinc exhibits strong isotopic fractionation during volatilization in planetary rocks4 5 but is hardly fractionated during terrestrial igneous processes6 making it a powerful tracer of the volatile histories of planets here we present high precision zinc isotopic and abundance data which show that lunar magmatic rocks are enriched in the heavy isotopes of zinc and have lower zinc concentrations than terrestrial or martian igneous rocks conversely earth and mars have broadly chondritic zinc isotopic compositions we show that these variations represent large scale evaporation of zinc most probably in the aftermath of the moon forming event rather than small scale evaporation processes during volcanism our results therefore represent evidence for volatile depletion of the moon through evaporation and are consistent with a giant impact origin for the earth and moon

gravity study of the pitcairn easter hotline shipboard free air gravity and bathymetric anomalies with an extension of 400 km were identified across the pitcairn easter hotline in the south pacific the anomalies are associated with one of the positive geoid undulations observed in the area from satellite data several smaller topographic features volcano tectonic ridges oriented n65 e are superimposed on the topographic high admittance computations and direct modeling show that the swell topography is compensated by a low density zone within the lithosphere 4 to 8 km below the crust the volcano tectonic ridges are locally compensated in a classical airy sense the swell and the associated ridges were probably created by the action of a thermal anomaly resulting from the interaction of the easter island hotspot and of the easter microplate accretion centers

crustal structure and isostatic compensation near the kane fracture zone from topography and gravity measurements i spectral analysis approach summary six gravity and bathymetry profiles perpendicular to the kane fracture zone each more than 300 km long were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages a spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths for remaining wavelengths the admittance the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate about 6 km in thickness after subtracting the gravitational attraction of the bathymetry and its compensation substantial isostatic anomalies remain we interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography except at very long wavelengths the apparent crustal thickness varies by as much as a factor of 2 but there is no evidence indicating systematic thinning of the crust beneath the fracture zone our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface this indicates that there are two different modes of crustal formation with different gravity and topographic signatures effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies and intrusive volcanism or underplating producing gravity anomalies but little topographic relief

development of a shoreline rhythm outer banks of north carolina abstract a shoreline rhythm was studied at duck north carolina during june of 1980 and 1981 the results of both studies show that the shoreline rhythm developed when small waves approaching the shoreline at an oblique angle rebuilt the beach following an erosional event storm waves on the other hand eroded the rhythm when the shore was being rebuilt small waves and associated longshore currents reshaped the nearshore topography from a shoal to a longshore bar one end of the bar became attached to the beach and in turn caused an increase in the horizontal distance between the tip of the horn and the adjacent bay of the rhythm twice amplitude as more sediments were transported from the nearshore and deposited on the horn twice amplitude continued to increase however as the beach prograded at the horn the beach located at adjacent bays did not rebuild nor was it eroded because rip currents were noted in bay regions of the rhythm it is suggested that these currents prevented nearshore sediments from being transported and deposited on bay beaches thus the increase of the rhythm amplitude was caused more by deposition occurring at the horn than of erosion taking place in the adjacent bays

key role of continental margin sediments in the oceanic mass balance of zn and zn isotopes zinc is an essential micronutrient and its concentration and isotopic composition in marine sediments represent promising tracers of the ocean carbon cycle however gaps remain in our understanding of the modern marine cycle of zn including an explanation of the heavy zn isotopic composition of seawater relative to the known inputs and the identity of a required missing sink for light zn isotopes here we present zn isotope data for organic rich and trace metal rich continental margin sediments from the east pacific margins that together provide the first observational evidence for the previously hypothesized burial of light zn in such settings in turn this light zn output flux provides a means to enrich the seawater dissolved pool in heavy isotopes the size and isotopic composition of the margin sink are controlled by the uptake of zn into organic matter in the photic zone and the fixation of this pool probably in the form of zn sulfides in sediments an estimate of its significance to the overall zn oceanic mass balance both in terms of flux and isotopic composition indicates that such settings can fulfill the requirements of the missing zn sink taken together these observations have important implications for the interpretation of zn isotope data for marine sediments in the geologic record

southern ocean hotspot tracks and the cenozoic absolute motion of the african antarctic and south american plates abstract a detailed analysis based on an antarctica africa finite reconstruction at chron c29 64 ma an assumption of no relative wander between the marion prince edward and tristan hotspots and on recently revised bathymetric maps of the southern ocean region shows that the fixed hotspot reference frame is tenable for absolute plate motions bouvet hotspot and probably trinidade as well also shows little or no cenozoic relative motion contrary to previous models bouvet hotspot is unrelated to the meteor rise cape rise seamount chain instead the bathymetric data when compared with the predicted hotspot tracks indicate another hotspot exists near the southernmost south atlantic spreading ridge segment new geochemical evidence from the latter region supports this hypothesis in showing the effects of plume enrichment from a source that is compositionally distinct from bouvet the peculiar zig zag shape of the cape rise meteor rise lineament is the result of this hotspot crossing the active transform segment of the falkland agulhas fracture zone in late mesozoic times followed by an early cenozoic ridge jump to the pre weakened trace on the then south american plate from the averaged cenozoic absolute motions of the african antarctic and south american plates it is evident that antarctica has been most nearly stationary in an absolute motion sense

contemporaneous massive subaerial volcanism and late cretaceous oceanic anoxic event 2 abstract oceanic anoxic events oaes are geological time intervals characterized by extremely high burial rates of organic carbon that led to deposition of organic rich black shales in the global ocean it has been proposed that oceanic anoxic events are ultimately caused by massive volcanism associated with formation of large igneous provinces lips because of chronological similarities but no general consensus has developed yet to investigate the possibility of lip volcanism instigating oaes we measured stable isotopic compositions of bulk organic carbon 13 c org and lead pb isotopic compositions in the silicate sediment fraction across the bonarelli black shale italy a type stratigraphic section for the end cenomanian oae oae 2 94 ma ultra high resolution 13 c org records determined every 1 5 mm capture a 3 sharp negative shift at the base of the bonarelli at the same stratigraphic level pb isotopic compositions in the silicate sediment fraction exhibit significant shifts toward characteristic values of volcanic rocks from contemporaneous lips caribbean and madagascar flood basalts these data suggest a rapid substantial increase in the relative supply of silicate minerals from the two lips massive subaerial volcanism associated with lip formation provides a simple explanation for these two isotopic geochemical signals via release of a huge amount of carbon dioxide 10 5 gt co 2 and particulate materials into the atmosphere which resulted in a rapid negative shift of 13 c in sea water and changes in pb isotopic compositions in the silicate sediment fraction respectively we interpret that massive volcanism triggered significant climatic changes inducing biotic crises and oceanic anoxia

hf isotope evidence for effective impact melt homogenisation at the sudbury impact crater ontario canada abstract we report on the first zircon hafnium oxygen isotope and trace element study of a transect through one of the largest terrestrial impact melt sheets the differentiated melt sheet at the 1 85 ga originally ca 200 km in diameter sudbury impact crater ontario canada yields a tight range of uniform zircon hf isotope compositions e hf 1850 of ca 9 to 12 this is consistent with its well established crustal origin and indicates differentiation from a single melt that was initially efficiently homogenised we propose that the heterogeneity in other isotopic systems such as pb in early emplaced impact melt at sudbury is associated with volatility related depletion during the impact cratering process this depletion leaves the isotopic systems of more volatile elements more susceptible to contamination during post impact assimilation of country rock whereas the systems of more refractory elements preserve initial homogeneities zircon oxygen isotope compositions in the melt sheet are also restricted in range relative to those in the impacted target rocks however they display a marked offset approximately one third up the melt sheet stratigraphy that is interpreted to be a result of post impact assimilation of 18 o enirched rocks into the base of the cooling impact melt given that impact cratering was a more dominant process in the early history of the inner solar system than it is today and the possibility that impact melt sheets were sources of ex situ hadean zircon grains these findings may have significance for the interpretation of the early zircon hf record we speculate that apparent e hf time arrays observed in the oldest terrestrial and lunar zircon datasets may be related to impact melting homogenising previously more diverse crust we also show that spatially restricted partial melting of rocks buried beneath the superheated impact melt at sudbury provided a zircon crystallising environment distinct to the impact melt sheet itself

the crustal structure of the madagascar ridge summary the madagascar ridge is an elongated aseismic plateau in the southwest indian ocean which separates two ocean basins of mid to late cretaceous age from interpretations of a reversed refraction line on the ridge and a continuous gravity profile across it we conclude that it formed during the cretaceous as thickened oceanic crust at a mid ocean ridge which possibly overlay a mantle hot spot the seismic data consist of record sections from five recording sonobuoys forty one shots were fired along a 210 km long nne ssw line across the central part of the ridge simple horizontal layer analysis of first arrivals reveals a lower crustal refractor with vp 7 kms 1 at a depth of 8 8 5 km and the moho at an apparent depth of 18 km but fails to characterize the crustal structure as either oceanic or continental we obtain a definitively oceanic structure by using synthetic seismogram techniques together with tau p analysis and second arrival correlations the model which best fits the data consists of 2 2 km of water and 1 km of sediments overlying a velocity gradient from 4 to 7 kms 1 beneath this at a depth of 7 5 km is a 9 5 km thick layer with vp 6 9 7 1 kms 1 a 7 5 kms 1 layer not observed in the first arrival analysis is found at the base of the crust the moho is a transition zone 1 km thick and occurs at 25 km depth the gravity data consist of a continuous ne sw profile across the ridge and on to the flanking basins two dimensional gravity modelling allows a structure consistent with the synthetic seismogram results and supports the existence of a high density basal layer in the crust we use in addition a time series analysis approach to estimate the depth of compensation assuming an airy model and lithospheric thickness assuming an elastic plate model convolution of bathymetry along the profile with a range of model filters gives minimum rms gravity residuals for a crustal compensation depth of 23 5 km and a plate thickness of less than 2 5 km

geoid height versus topography for oceanic plateaus and swells oceanic plateaus and swells are a major component of the seafloor topography yet they remain among the most poorly understood features this is especially true of the oceanic plateaus which show large variations in crustal thickness to determine the depth and mode of compensation for 53 of the largest plateaus and swells we analyzed the relationship between geoid height and topography in polygonal areas containing each feature both geoid height and topography were first band pass filtered 400 km i 4000 km to isolate the signal associated with local compensation from flexural and deep mantle signals the ratio of geoid height to topography was then determined by fitting a straight line to the data except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with airy and thermal compensation models eighteen features have high geoid topography ratios that cannot be explained by the airy compensation model of crustal thickening these features thermal swells are partially supported by thermal buoyancy forces in the lower half of the lithosphere the ratios are highest for active hot spot swells and decay with the thermal age of the swell to values consistent with airy compensation of the enduring volcanic edifice the remaining features plateaus have lower geoid topography ratios in agreement with the airy compensation model those plateaus with average height greater than 4 km are thought to be continental fragments the shorter plateaus tend to be volcanic features modified continental plateaus presumably small fragments of extended and intruded continental margin crust cluster around heights of n3 km overlapping the range associated with oceanic plateaus since the origin of many plateaus is poorly understood this global geoid topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells

effects of canary hotspot volcanism on structure of oceanic crust off morocco analysis of over 6400 km of multichannel seismics mcs and 50 sonobuoy reflection and refraction experiments reduced both in the domain of x t and tau p shows that a region within the jurassic quiet zone off morocco underwent dramatic changes as a result of the passage of the lithosphere over the canary hotspot commencing approximately 60 ma a seismic unit ucf interpreted as volcanic in origin is observed within the sediments in a region characterized by a broad bathymetric swell it shows diffractions from its upper surface and an internally chaotic seismic facies and pinches out between sedimentary units of continuous subparallel facies a velocity inversion is noted between the ucf 4 7km s and the underlying sediment 3 1 km s the ucf is time transgressive it lies near the cretaceous s tertiary boundary in the northern portion of the study area and is younger to the south kinematic studies of the movement of the canary hotspot relative to africa show that the hotspot first appeared off nw africa about 60 ma and was located beneath oceanic crust in the region where the ucf is observed depth to basement measurements in areas not effected by the hotspot show a consistent linear trend of increased depth with age in areas effected by the hotspot the thermal rejuvenation is evident as basement depths shoal with increased proximity to the present hotspot the reheating of the crust resets the thermal age of the lithosphere with many of the properties of crust of a younger age subsidence curves of the reheated crust off morocco show good correlation to subsidence curves of other reheated crust on a global basis a zone characterized by high crustal velocities 7 1 7 4 km s and greater crustal thicknesses by 1 2 km is observed in an area that corresponds to the bathymetric swell the region of the ucf and the reelevated basement the high velocities and increased crustal thickness are interpreted to be the result of underplating and assimilation of existing oceanic crust caused by the canary thermal anomaly the presence of high crustal velocities coupled with a thickened crustal section has been noted on various passive margins of the world they have generally been attributed to the thermal processes associated with continental rifting off morocco we believe that similar thermally induced phenomena have occurred but that here the heat anomaly was the midplate volcanism associated with the canary hotspot

the evolution of the onaping formation at the sudbury impact structure the 1 4 1 6 km thick onaping formation consists of a complex series of breccias and melt bodies lying above the sudbury igneous complex sic at the sudbury impact structure based on the presence of shocked lithic clasts and various glassy phases the onaping has been described as a suevitic breccia with an origin at least in part as fallback material recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases both as clast types and discrete intrusive bodies the nature of the onaping and that of other suevitic breccias overlying impact melt sheets is reviewed the relative thickness internal stratigraphic and lithological character and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the onaping the sudbury structure formed in a foreland basin and water played an essential role in the evolution of the onaping as indicated by a major hydrothermal system generated during its formation taken together observations and interpretations of the onaping suggest a working hypothesis for the origin of the onaping that includes not only impact but also the interaction of sea water with the impact melt resulting in repeated explosive interactions involving proto sic materials and mixing with pre existing lithologies this is complicated by additional brecciation events due to the intrusion of proto sic materials into the evolving and thickening onaping fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two thirds of the onaping

zn isotope fractionation during uptake into marine phytoplankton implications for oceanic zinc isotopes abstract the extreme scarcity of zinc zn in the euphotic zone coupled to deep enrichments is consistent with biological uptake at the surface and regeneration at depth in the context of a nutrient type depth profile so clearly shaped by uptake into phytoplankton the growing dataset for zn isotopes presents a challenge these data either show very minor isotope effects associated with extreme depletion or enrichment of the light isotopes in the upper ocean in contrast culturing of eukaryotes in the laboratory suggests that light zn isotopes are preferentially taken up into diatoms and coccoliths implying that zn depletion at the surface should be associated with extremely heavy residual dissolved signals here we present the first zn isotope measurements for cultured marine cyanobacteria and compare these data to those for eukaryotic diatoms grown under identical conditions of the four cyanobacteria cultured belonging to the genera synechococcus and prochlorococcus three preferentially take up light zn into the cell with a variability that is not fundamentally different between pro and eukaryotic phytoplankton we also observe only very subtle differences between zn p and fe p uptake ratios for these three cyanobacteria groups relative to diatoms grown under the same conditions a fourth strain exhibits preferential uptake of heavy zn isotopes and very high zn p ratios overall we speculate that the observed variability among cyanobacteria may be related to the molecular structure of their photosynthetic light harvesting apparatus adapted to significantly different light niches these new and published culture data support the hypothesis that cellular 66zn in culture might largely be controlled by the organic ligands that bind zn in the medium given that the zn binding ligands in the ocean have thermodynamic stability constants that are orders of magnitude smaller than the edta used in culture media the surprisingly subtle zn isotope variability in some parts of the surface ocean may be reconciled with culture data by the lesser near zero preference of these weaker complexes for heavy zn isotopes this article is part of a special issue entitled cycles of trace elements and isotopes in the ocean geotraces and beyond edited by tim m conway tristan horner yves plancherel and aridane g gonzalez

la ride asismique de kerguelen heard anomalie du geoide et compensation isostatique abstract gravity surveys implemented throughout the kerguelen archipelago and the kerguelen gaussberg ridge southern indian ocean showed that both features are isostatically compensated regarding the deep ocean basins nearby the crustal thickness derived from refraction lines is only 16 km below the kerguelen archipelago precluding an airy type compensation with a 23 km deep crustal root early investigations demonstrated that the kerguelen ridge and its symmetrical features with respect to the east indian ridge broken ridge have originated near a spreading centre by an anomalous volcanism related to a ridge jump on hot lithosphere this also precludes the isostatic compensation by the deflection of a rigid plate geoid anomaly studies and deep seismic soundings show that the compensation may be achieved by low density material within the upper mantle the compensation level is 50 km deep extraction of basalt from the mantle may be involved in the origin of the depleted mantle and of the unusual thickness of layer 2 below kerguelen i e 9 5 km instead of roughly 2 km within a normal oceanic crust

evaporative loss of moderately volatile metals from the superheated 1849 ma sudbury impact melt sheet inferred from stable zn isotopes the retention of moderately volatile elements on the growing earth remains a major uncertainty in models of terrestrial accretion large impactors were the main carriers of accreted material but their mutual energetic collisions and impacts onto the earth also caused chemical fractionation for which limited experimental data exist the objective of this work was to study several moderately volatile elements in the third largest impact basin preserved on earth at sudbury ontario we conducted a new chemostratigraphic transect n 41 of zn isotope ratios and concentrations by analysing melt sheet and basin fill samples the data were compared to common pb cs cd and sb concentration systematics within the crystallised melt sheet there are strong trends in the extent of moderately volatile element deficits zn isotope composition 66 64znjmc l from 0 18 to 0 47 and initial pb isotope composition the combined evidence suggests that these trends reflect footwall contamination of a melt sheet that had experienced evaporative zn loss of up to 75 80 accounting for plausible isotopic signatures of target rocks the maximum mass dependent zn isotope fractionation e was 0 29 0 04 1 s d which translates to modest fractionation factors 0 99986 to 0 99975 this is comparable to melt fallout glass and fused sands from nuclear detonation sites we attribute the observed zn loss and isotope fractionation to the formation of the impact melt the rapid formation of a solid lid of breccias upon seawater ingress may have prevented stronger evaporative loss and isotope fractionation within the crater fill there is an up stratigraphy increase in zn isotope variability 66 64znjmc l from 0 29 to 1 05 combined with evidence for biogenic reduced c this suggests sedimentation of authigenic particulates within an enclosed crater sea in the melt sheet the zn pb and rb cs pairs experienced different extents of maximum evaporative loss pb up to 98 4 vs zn 78 and cs 90 vs rb 30 the relative loss pattern could reflect evaporation from superheated silicate melt at 1 450 c and 1 atm loss from super liquidus melts formed by bolide impacts could have been a significant process shaping the earth s volatile and moderately volatile inventory

a seismic refraction survey in the kerguelen isles southern indian ocean summary two 65 km long on land refraction lines with shot detonated at sea were implemented in the courbet peninsula eastern region of the kerguelen archipelago terres australes et antarctiques francaises southern indian ocean the first profile pi oriented ne sw runs from cap ratmanoff toward mont ross and the second one p2 from pointe suzanne se of the courbet peninsula through the val studer data from both profiles were supplemented by using the bouguer gravity anomaly map not previously studied the data combined standard travel time interpretation wide angle reflected wave study synthetic seismograms and s wave analysis the velocity depth behaviour shows that the mean crustal thickness ranges from 14 to kerguelen resembles those observed beneath aseismic ridges i e the crozet rise and the madagascar ridge average velocity of 5 5 km s on both lines is in the range of those determined for oceanic layer 2 which is there 8 9 km thick average velocity of 6 6 km s is in the range of velocities within oceanic layer 3 the transition to mantle velocity 7 95 km s is best modelled by a positive velocity gradient within the crust this feature is similar to that observed on structures generated near spreading centres these new data com bined with geological and geochemical investigations in the archipelago support an oceanic origin for the kerguelen heard ridge and also broken ridge both structures were joined 40myr ago as shown by the magnetic anomaly pattern refraction studies invalidate the assumption that isostatic compensation is achieved by a 23 km deep crustal root but substantiate the i 17km only varying with the locality the structure of the crust beneath

the sudbury igneous complex a differentiated impact melt sheet the sudbury structure ontario is the remnant of a 1 85 ga old impact crater which originally had a diameter of 200 to 250 km the sudbury igneous complex occurs within the sudbury structure the sudbury igneous complex is a 2 5 to 3 0 km thick 60 27 km elliptical igneous rock body which consists of four major lithologies from top to bottom traditionally termed granophyre quartz gabbro norite and contact sublayer sulfide and inclusion bearing noritic rock with the exception of the latter all these lithologies are continuous across the structure modal analyses reveal that following the iugs system of nomenclature quartz gabbro samples are in fact quartz monzogabbros a few of the norite samples are quartz gabbros and most norite samples are quartz monzogabbros in view of these observations and in order to clarify the nomenclature an updated terminology is proposed from top to bottom upper unit middle unit lower unit and contact sublayer the bulk composition of the sudbury igneous complex from north range data is granodioritic continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the complex behaved as a single melt system all the sudbury igneous complex lithologies have the same light to heavy rare earth element ree ratio and an overall pattern of increased light ree and depleted heavy ree the occurrence of primary hydrous minerals hornblende and biotite deuteric alteration and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in h2o moreover the granophyric and other far from equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase the sudbury igneous complex differs from traditional layered mafic complexes in the following aspects it has an overall intermediate composition a hydrous nature a crustal isotopic signature normative corundum and an unusually large volume of granophyre the sudbury complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical and therefore mineralogical layering reported here for the first time and similar to those found in impact melt rocks elsewhere are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts the well known ore deposits of the sudbury region are directly related to the genesis of the sudbury igneous complex some ores precipitated from the sudbury melt whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex it is concluded that the sudbury igneous complex is the best exposed and only well documented to date terrestrial impact melt sheet to have differentiated

investigation of a self organization model for beach cusp formation and development a recent numerical investigation of self organization werner and fink 1993 suggests that the feedback process between currents and sediment response can result in self organized patterns and can be used to predict beach cusp formation and spacing a similar model based on self organization is tested here in order to understand the processes occurring during beach cusp formation and development to evaluate the sensitivity toward the parameters used and to examine how the model might relate to field observations results obtained confirm the validity of the self organization approach and its capacity to predict beach cusp spacing with values in fair agreement with the available field measurements with most of the input parameters primarily affecting the rate of the process rather than the final spacing however changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior cusp spacing tends to change with time and cusp regularity shows large long term variations cusps are found to be accretionary in the swash zone and in agreement with most observations mean flows are horn divergent over developed topography simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed results indicate that preexisting large amplitude cusps are destroyed if their spacing is substantially different from that expected under self organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach these findings support the hypothesis that self organization is a robust mechanism for beach cusp formation

heterogeneity of s isotope compositions recorded in the sudbury igneous complex canada significance to formation of ni cu sulfide ores and the host rocks sulfur isotope ratios of sulfide ores and their host rocks and the overlying main mass sequence of norites quartz gabbros and granophyres from the sudbury igneous complex have been analyzed to establish the extent of isotopic heterogeneity of the complex and the impact melt sheet from which it was produced noritic rocks located above the sublayer in the lower portion of the main mass of the sudbury igneous complex are characterized by uniform sulfur isotope ratios with 34s values of 2 59 0 48 rocks from the granophyric portion of the main mass are characterized by elevated and variable 34s values from 4 4 to 14 1 the more 34s enriched s in the granophyre was produced by secondary sulfide precipitation related to hydrothermal alteration systems developed in the upper altered granophyric part of the main mass as it cooled footwall and contact style mineralization from the sublayer show variable 34s values from 0 2 to 4 3 south range mineralization is characterized by low ratios from 0 2 to 2 5 north and northeast range mineralization exhibits locally elevated but variable 34s values for example mineralization from whistle mine varies from 3 1 to 4 3 whereas mineralization from victor varies between 2 4 and 2 9 there is typically only a small difference in s isotope ratio of contact and proximal footwall mineralization country rocks in the area of the sudbury igneous complex are also characterized by a wide range in 34s values and the sulfur isotope homogeneity of the noritic rocks can be explained by mixing of sulfur from several potential sources however the variability present between deposits at the lower contact of the complex requires that country rocks of locally variable sulfur isotope composition interacted with magma at the base of the cooling melt sheet there is no evidence that deposit size correlates with 34s value variations between deposits may involve local s isotope exchange reactions between the country rock and the melt sheet as the melt digested the footwall this study illustrates the application of s isotope data in constraining the scale of convective mixing and contamination in the largest known terrestrial melt sheet

isotopic fractionation of zinc in tektites abstract tektites are terrestrial natural glasses produced during a hypervelocity impact of an extraterrestrial projectile onto the earth s surface the similarity between the chemical and isotopic compositions of tektites and terrestrial upper continental crust implies that the tektites formed by fusion of such target rock tektites are among the driest rocks on earth although volatilization at high temperature may have caused this extreme dryness the exact mechanism of the water loss and the behavior of other volatile species during tektite formation are still debated volatilization can fractionate isotopes therefore comparing the isotope composition of volatile elements in tektites with that of their source rocks may help to understand the physical conditions during tektite formation for this study we have measured the zn isotopic composition of 20 tektites from four different strewn fields almost all samples are enriched in heavy isotopes of zn compared to the upper continental crust on average the different groups of tektites are isotopically distinct listed from the isotopically lightest to the heaviest muong nong type indochinites 66 64 zn 0 61 0 30 north american bediasites 66 64 zn 1 61 0 49 ivory coast tektites 66 64 zn 1 66 0 18 the australasian tektites others than the muong nong type indochinites 66 64 zn 1 84 0 42 and central european moldavites 66 64 zn 2 04 0 19 these results are contrasted with a narrow range of 66 64 zn 0 0 7 for a diverse spectrum of upper continental crust materials the elemental abundance of zn is negatively correlated with 66 64 zn which may reflect that isotopic fractionation occurred by evaporation during the heating event upon tektite formation simple rayleigh distillation predicts isotopic fractionations much larger than what is actually observed therefore such a model cannot account for the observed zn isotope fractionation in tektites we have developed a more realistic model of evaporation of zn from a molten sphere during its hypervelocity trajectory the molten surface of the tektite will be entrained by viscous coupling with air that will then induce a velocity field inside the molten sphere this velocity field induces significant radial chemical mixing within the tektite that accelerates the evaporation process our model albeit parameter dependent shows that both the isotopic composition and the chemical abundances measured in tektites can be produced by evaporation in a diffusion limited regime

the hatton bank continental margin ii deep structure from two ship expanding spread seismic profiles summary the continent ocean transition adjacent to hatton bank was studied using a dense grid of single ship and two ship multichannel seismic profiles the interpretation of the explosive expanding spread profiles esps which were shot as part of this survey are discussed here in detail extensive seaward dipping reflectors are developed in the upper crust across the entire margin these seaward dipping reflectors continue northwards on the faeroes and voring margins where they have been shown to be caused by basaltic lavas as well as on the conjugate margin of east greenland the dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension the esps show clear seismic arrivals out to ranges of 100 km wide angle moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals the determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms the results from the esps show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7 3 7 4 km s this lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the moho the lower crustal lens of high velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas now seen as dipping reflectors on the margin

analyses of multichannel seismic reflection gravity and magnetic data along a regional profile across the central western continental margin of india abstract analyses of multichannel seismic reflection gravity magnetic and bathymetry data along a regional profile across the central western continental margin of india have revealed the depositional pattern of sediments crustal structure and tectonics the four most distinct and varied crustal regions of the margin are palaeo shelf edges shelf margin basin prathap and laccadive ridges and the arabian basin the shelf margin basin is carpeted by 4 5 km maximum thick aggraded and prograded paleocene to holocene sediments six major seismic sequences of the sediments of the margin are identified and their ages are assigned on correlation with drill well results development of the sequence boundaries is attributed to the events of rifting of western india eustatic sea level changes indian and eurasian plate collision and himalayan orogeny tilted fault blocks half grabens located almost equi distance from the igneous construct of the prathap ridge in the shelf margin basin suggest a failed rift associated with stretched continental crust of the basin 2 d model studies of gravity and magnetic anomalies constrained by the seismic results have revealed 6 to 27 km thick crust across the margin the laccadive ridge crust limited by two volcanic intrusives and a steep scarp at its western end is 16 km thick it gradually thins towards offshore and juxtaposed with early tertiary normal oceanic crust 6 km thick of the arabian basin the crustal thickness and velocity and density structure of the ridge are comparable to that of the laxmi ridge a continental sliver the inferences and abrupt change in magnetic and gravity anomaly signatures across the western end of the laccadive ridge mark the zone of transition from continental to oceanic crust

origin and compensation of chagos laccadive ridge indian ocean from admittance analysis of gravity and bathymetry data abstract the chagos laccadive ridge clr is a prominent aseismic volcanic ridge in the northern indian ocean the ridge together with the southern mascarene plateau smp to which it is genetically related is considered as a volcanic trace of the reunion hotspot we have examined the isostatic compensation of the clr through transfer function analysis of gravity and bathymetry data along seven profiles the analysis suggests that the clr is compensated locally with an airy crustal thickness t c of 20 km the rather low elastic plate thickness t e of about 4 km implies that the volcanism of the ridge took place very near a spreading centre the proximity of the chagos fracture zone indicates that the emplacement was probably near a spreading centre transform junction

the hatton bank continental margin iii structure from wide angle obs and multichannel seismic refraction profiles summary in two recent papers normal incidence and expanding spread profiles esps shot across the hatton bank passive continental margin showed seaward dipping reflectors in the upper crust and elevated velocities in the lower crust the reflectors were interpreted as subaerial basalt flows and the high velocities as underplated or intruded igneous material in the lower crust a recent gravity model shows high densities in the lower crust supporting the velocity model in this study we present the forward modelling of two wide angle seismic refraction datasets shot perpendicular to strike and through the esp midpoints the final velocity model across the margin is consistent with all the seismic datasets but differs in some important respects from the esp based model a high velocity volcanic pile is outlined mid way up the continental slope and the inferred velocities in the lower cnst are slightly lower the differences between the two models can be explained by the three dimensional nature of the margin the esps were modelled assuming that the sub sediment structure was invariant along strike the final velocity model is used to obtain an estimate of the total thickness of melt accreted across the margin which is calculated to thicken from zero at hatton bank to about 12 km mid way down the continental slope at the bottom of the continental slope the adjacent igneous section forming the oceanic crust is about 18 km thick and thins rapidly northwestwards to 11 km as the oceanic crust thins its seismic velocity decreases this suggests that it is less mafic and that there is a post rift decrease in the percentage of mantle melting this decrease may be explained by a decrease in asthenospheric temperature and or by a reduction in volatile content

observation of a mesoscale eddy dipole on the northern madagascar ridge consequences for the circulation and hydrography in the vicinity of a seamount abstract based on satellite and in situ data the dynamic characteristics and vertical structure of a surface intensified mesoscale eddy dipole recently expelled from the south east madagascar current semc is described for the first time the dipole was surveyed 250 nautical miles south of madagascar between 14 and 23 november 2016 during west east and south north transects carried out over the northern madagascar ridge the dipole consisted of two counter rotating vortices of similar size 100 km and intensity 0 7 f and an intense southwestward jet 150 cm s 1 in the frontal region between the two eddies the cyclonic eddy was lying on the western side of the anticyclonic eddy with azimuthal velocities reaching 100 cm s 1 at the surface and decreasing slowly with depth 40 cm s 1 at 600 m this dipole was defined as a highly non linear ro 0 7 isolated eddy type structure c 11 cm s 1 and u c 0 7 capable of trapping and advecting water masses over large distances the enhanced concentration of chlorophyll a found in the cyclone relative to the anticyclone could be tracked back to the spin up phase of the two eddies and attributed to eddy pumping the eddy cores were located above the pycnocline 1026 4 kg m 3 within the upper 600 m and consisted of varieties of subtropical underwater stuw found within the semc the stuw found in the anticyclone was more saline and oxygenated than in the cyclone highlighting mixing with the inshore shelf waters from the southeastern coastal upwelling cell off madagascar observations suggest that the dipole interacted strongly with the chaotic bathymetry of the region characterized by a group of five seamounts lying between 240 m and 1200 m the bathymetry blocked its westward advection trapping it in the vicinity of one shallow seamount for more than 4 weeks so enhancing the role of the eddy induced velocities in stirring the surrounding water masses squeezed between the southern madagascan shelf and the northern flank of the anticyclone two filament like dynamic features with very different water mass properties could be observed on the south north transect i one filament highly concentrated in chlorophyll a demonstrating the capacity of the eddy to export shelf water offshore ii intrusions of southern type stuw generally found south of the south indian counter current sicc recirculating on the external flanks of the anticyclone although the observed circulation and hydrography were largely constrained by the presence of the mesoscale eddy dipole unmistakable fine scale dynamics were also observed in the vicinity of the mad ridge seamount superimposed onto the mesoscale eddy flow

foreshore topography tides and beach cusps delaware the development and maintenance of beach cusps were studied along a shore segment of the delaware coast from june 5 through june 28 1979 on three occasions a berm and a series of beach cusps developed together on the foreshore as part of rebuilding processes following an erosional event after the beach was rebuilt and cusps were established the spacing between horns and the elevation of horns remain fixed although the processes which govern the spacing between cusps are still in question the elevation of cusps was controlled by the elevation of swash runup when wave dimensions were fairly constant the elevation of the runup and thus of the cusps was determined by the water level of the spring high tide as processes fluctuated back and forth from depositional to erosional events the horns as compared to the bays responded to a greater extent for example when sediments were being deposited on the cusps the rate of deposition on the horn crest was greater than in the bays when waves eroded the seaward portion of a berm with cusps however most of the sediments lost from the cusps came from the horns the results frequently left beach scarps at former positions of horns and no scarps at bays after an erosional event had left beach scarps at former positions of horns the remnant topography of beach cusps controlled the action of the swash so as to reconstruct horns at their former positions as the beach was being rebuilt the swash was diverted sideways by the beach scarps and the energy of the swash was sufficiently decreased to cause deposition in front of the scarps in turn horns were rebuilt and a new set of cusps was constructed at approximately the same position as of the old set of cusps

thermal evolution and interaction between impact melt sheet and footwall a genetic model for the contact sublayer of the sudbury igneous complex canada 1 the sudbury igneous complex sic and associated ni cu pge mineralization has been interpreted in terms of a large meteorite impact event in this study the thermal relationship between the large cooling melt sheet and the surrounding country rock is examined in terms of its role in an evolving thermal gradient rather than as a passive receptacle for the melt sheet above thermal modeling of this environment is undertaken using physical and thermal constraints appropriate to the sic and assuming heat dissipation from the 2 5 km thick superheated melt sheet 1800 c by either diffusion with zero convection or by rapid convection within the melt sheet with zero convection basal cooling produces a solid base which lowers conductivity such that the immediate footwall rocks reach 1000 c producing partial melting that extends 200 m into the footwall in a rapidly convecting melt sheet the initial footwall chill is remelted and high temperatures maintained within the sheet close to the contact this results in higher temperatures being attained in the immediate footwall 1100 1200 c inducing complete melting of proximal footwall and partial melting to depths of 500 m below the melt sheet proximal footwall consists of paleoproterozoic huronian basalts granitoids and sediments exposed in the south range overlying archaean gneisses and granitoids total and partial melting of this material early in the cooling history of the melt sheet and the subsequent gravitational accommodation of these melts according to density would produce a basalt dominated basal liquid corresponding to the so called contact sublayer the thermal aureole predicted by our models is consistent with that preserved around the north range of the sic assuming 800 m of thermally induced erosion at the contact

a comparison of the isostatic response of bathymetric features in the north pacific ocean and philippine sea summary this paper concerns the calculation and analysis of admittance functions from large and uniform data sets of gravity and topography in four regions of the northern and western pacific ocean the purpose is to separate and describe possible differences in isostatic compensation between several type regions of oceanic crust a mid ocean ridge juan de fuca a mid plate seamount chain hawaiian ridge fracture zone topography on old crust north of hawaii and a marginal basin philippine sea results suggest that there are significant differences in the degree to which long wavelength topography has been compensated which can be distinguished between regions these differences are set in the perspective of three simple compensation mechanisms two of these consider local airy models in which raised topography is compensated at depth either by crustal roots or low density mantle a third considers the effects of an elastic plate of variable thickness supporting crustal variations conclusions are that a a thick plate possibly in excess of 30 km supports the hawaiian ridge b a much thinner plate of 5 to 15 km existed when the fracture zone topography was formed c the juan de fuca ridge is compensated either regionally by a plate 5 to 10 km thick or locally by sub crustal low densities at depths of 15 to 20 km and d the philippine sea shows no evidence for regional support ridges are compensated locally by differences in crustal thickness whereas the basins are underlain by density variations at depths comparable to those of the much younger juan de fuca ridge the major difference between admittance functions for the philippine sea and comparably aged regions of the north pacific ocean adds further new evidence of possible evolutionary differences between it and normal ocean basins