evolution of the passive continental margins of india a geophysical appraisal abstract the passive continental margins of india have evolved as india broke and drifted away from east antarctica madagascar and seychelles at various geological times in this study we have attempted to collate and re examine gravity and topographic bathymetry data over india and the adjoining oceans to understand the structure and tectonic evolution of these margins including processes such as crustal lithosphere extension subsidence due to sedimentation magmatic underplating and so on the eastern continental margin of india ecmi seems to have evolved in a complex rift and shear tectonic settings in its northern and southern segments respectively and bears similarities with its conjugate in east antarctica crustal extension rates are uniform along the stretch of the ecmi in spite of the presence or absence of crustal underplated material variability in lithospheric strength and tectonic style of evolution ranging from rifting to shearing the krishna godavari basin is underlain by a strong 30 km elastic lithosphere while the cauvery basin is underlain by a thin elastic lithosphere 3 km the coupling between the ocean and continent lithosphere along the rifted segment of the ecmi is across a stretched continental crust while it is direct beneath the cauvery basin the western continental margin of india wcmi seems to have developed in an oblique rift setting with a strike slip component unlike the ecmi the wcmi is in striking contrast with its conjugate in the eastern margin of madagascar in respect of sedimentation processes and alignment of magnetic lineations and fracture zones the break up between eastern india and east antarctica seems to have been accommodated along a proterozoic mobile belt while that between western india and madagascar is along a combination of both mobile belt and cratonic blocks

gravity anomalies and segmentation at the east coast usa continental margin summary the free air gravity edge effect anomaly at rifted continental margins has generally been attributed to the transition between thick continental and thin oceanic crust while crustal thinning is a major contributor recent studies suggest that sediment loading and magmatism may significantly modify the edge effect anomaly and cause it at some margins to be highly segmented along their strike in this paper we use a combined 3 d flexural backstripping and gravity anomaly modelling technique to determine the role that sediment loading has played in controlling the segmentation of atlantic type continental margins we focus on the east coast usa since a substantial amount of high quality seismic reflection and refraction gravity and magnetic data already exists for this margin by calculating the gravity anomaly associated with rifting and sediment loading and iteratively comparing it to the observed free air anomaly we have determined the best fit elastic thickness te structure of the margin we show that 0 10 5 a integrated strength of the lithosphere these results imply that weak regions abut strong ones at the east coast usa margin te generally decreases with increase in the amounts of crustal thinning and the flexed basement curvature k suggesting it is controlled at least in part by the mechanical structure of the pre rift lithosphere and yielding due to flexural loading however there is considerable scatter suggesting other factors such as along strike changes in crustal composition irrespective we show that an isostatic anomaly that takes into account the best fit te distribution is significantly reduced in spectral power compared with one which is computed assuming only airy compensation i e te 0 km this is not to imply that rifting and sediment loading completely accounts for the anomalies along strike and across strike the east coast usa margin to the contrary significant isostatic anomaly highs and lows persist especially in inner and middle shelf regions one of the most prominent is an arcuate 670 km long high with flanking lows offshore carolina that we attribute to magmatism during the initial stages of continental break up

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

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

a recipe for microcontinent formation accreted slivers of continental margins are common in the geologic record but the processes that lead to their formation are poorly understood we observe an association of plume related microcontinent isolation and subsequent long term asymmetries in oceanic crustal accretion based on four recent examples the seychelles in the indian ocean jan mayen in the norwegian greenland sea and the east tasman plateau and the gilbert seamount complex in the tasman sea these microcontinents formed by rerifting of a young continental margin 25 m y old in the vicinity of a mantle plume stem followed by asymmetric seafloor spreading two dimensional numerical stochastic basin modeling suggests that a yield strength minimum along the landward edge of a rifted margin thermally enhanced by heating from a mantle plume may cause a spreading ridge to jump onto this zone of weakness this action isolates a passive margin segment the association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

gravity anomalies flexure and crustal structure at the mozambique rifted margin abstract the free air gravity edge effect anomaly associated with passive continental margins is one of the most distinctive features of the marine gravity field early studies attributed the edge effect to the juxtaposition of thick continental with thin oceanic crust while thinning of the crust due to rifting at the time of continental break up is a major contributor it is now recognised that geological processes such as sedimentation and magmatic underplating also contribute the different amounts of sediment and underplate at rifted margins help in fact to explain the diversity of the edge effect although they obscure the mechanics styles and geometry of rifting in this paper combined 2 dimensional flexural backstripping and gravity modelling techniques are used to quantify the contribution of rifting sediment loading and magmatic underplating to the edge effect anomaly at the mozambique margin this margin formed as the result of a two stage break up the first of which resulted in the separation of africa and madagascar in the early jurassic by comparing the observed edge effect to calculations based on simple elastic plate models we have been able to constrain the elastic thickness te and hence flexural rigidity of the lithosphere that underlies the sediments which accumulated at the mozambique margin following break up the model that best explains the shape i e amplitude and wavelength of the edge effect high is one in which the earliest jurassic early cretaceous sediments were deposited on relatively weak lithosphere te 17 5 km while the later miocene recent sediments accumulated on relatively rigid lithosphere te 40 km these results suggest that sedimentation at the mozambique margin has involved lithosphere which responds to loads in a similar manner as would oceanic lithosphere therefore the thinned transitional crust that underlies much of the coastal plain of the mozambique margin is probably of oceanic rather than continental origin

rifting assisted by shear heating and formation of the lomonosov ridge abstract we suggest that thermo mechanical feedback between shear heating and temperature dependent viscosity in continental zones of oblique extension has the potential to significantly weaken the lithosphere and facilitate subsequent rapid breakup we perform thermo mechanical experiments of strike slip and coeval but longer lasting extension using the finite element method the style of rifting in our model depends mainly on the efficiency of shear heating which in turn depends on the magnitude of lithospheric stresses and strain rates we develop and apply a numerical model to explain the formation of the lomonosov ridge microcontinent our proposed model provides a physically consistent mechanism for the detachment of the lomonosov ridge from the barents kara sea margin at 68 56 ma the narrow rift mode predicted by the numerical model is supported by the crustal structure of conjugate passive margins of the eurasia basin

structure of the kerguelen plateau province from seasat altimetry and seismic reflection data the kerguelen plateau in the southern indian ocean stands 2 4 km above the adjacent sea floor and is 2 500 km long fig 1 1 2 seasat provided a unique data set for deriving the free air gravity field of the region fig 2 3 5 here we report the results of a new analysis of the plateau province s structure employing both seasat and newly acquired multichannel seismic mcs data6 7 for ground truth the northern sector is characterized by volcanism and a sedimentary basin the southern sector by a broad anticlinal arch major faulting and a sedimentary basin and the eastern sector by an abyssal basin labuan and bounding ridge william s the three sectors argue for a more complex tectonic evolution of the feature than has been previously proposed

structure of the jan mayen microcontinent and implications for its evolution an extensive seismic survey using ocean bottom seismographs obs was performed in the area across the jan mayen basin north atlantic from the jan mayen ridge to the iceland plateau the jan mayen ridge and surrounding area are considered to be a fragment of a continent which was separated from greenland just prior to magnetic anomaly 6 this study presents the crustal structure of the jan mayen microcontinent and the ocean continent transition to the west of the jan mayen ridge the crustal structures from the centre of the jan mayen ridge to the jan mayen basin are characterized by a deep sedimentary basin a thin basaltic layer within the sedimentary section and extreme thinning of the continental crust towards the iceland plateau the obs data indicate that a continental upper crust v p 5 8 6 1 km s 1 and lower crust v p 6 7 6 8 km s 1 underlie the deep sedimentary basin the thickness of the continental lower crust varies significantly from 12 km beneath the jan mayen ridge to almost zero thickness beneath the northwestern part of the jan mayen basin an ocean continent transition zone is found at the western edge of the jan mayen basin within the 10 km wide transition zone crustal velocities increase towards the iceland plateau and approach the velocities of the oceanic crust obtained at the iceland plateau that is 3 8 5 1 km s 1 oceanic layer 2a 5 9 6 5 km s 1 oceanic layer 2b and 6 8 7 3 km s 1 oceanic layer 3 the crustal model indicates very thin oceanic crust 5 km immediately oceanwards of the ocean continent transition zone beneath the iceland plateau the oceanic crust is thicker 9 km than the typical thickness of normal oceanic crust this might imply that the oceanic crust at the iceland plateau has been generated by asthenospheric material slightly hotter than normal from the crustal structure obtained by the present study it is proposed that the western part of the jan mayen ridge may be referred to as a non volcanic continental margin generated by a long duration of rifting even if the asthenospheric material upwelling along the margin were hotter than normal only small amounts of magmatic intrusions and extrusions would have been generated because of significant conductive cooling under the long duration of rifting

mid ocean ridge jumps associated with hotspot magmatism abstract hotspot ridge interaction produces a wide range of phenomena including excess crustal thickness geochemical anomalies off axis volcanic ridges and ridge relocations or jumps ridges are recorded to have jumped toward many hotspots including iceland discovery galapagos kerguelen and tristan de cuhna the causes of ridge jumps likely involve a number of interacting processes related to hotspots one such process is reheating of the lithosphere as magma penetrates it to feed near axis volcanism we study this effect by using the hybrid finite element code flac to simulate two dimensional 2 d cross section viscous mantle flow elasto plastic deformation of the lithosphere and heat transport in a ridge setting near an off axis hotspot heating due to magma transport through the lithosphere is implemented within a hotspot region of fixed width to determine the conditions necessary to initiate a ridge jump we vary four parameters hotspot magmatic heating rate spreading rate seafloor age at the location of the hotspot and ridge migration rate our results indicate that the hotspot magmatic heating rate required to initiate a ridge jump increases non linearly with increasing spreading rate and seafloor age models predict that magmatic heating itself is most likely to cause jumps at slow spreading rates such as at the mid atlantic ridge on iceland in contrast despite the higher magma flux at the galapagos hotspot magmatic heating alone is probably insufficient to induce a ridge jump at the present day due to the intermediate ridge spreading rate of the galapagos spreading center the time required to achieve a ridge jump for fixed or migrating ridges is found to be on the order of 105 106 years simulations that incorporate ridge migration predict that after a ridge jump occurs the hotspot and ridge migrate together for time periods that increase with magma flux model results also suggest a mechanism for ridge reorganizations not related to hotspots such as ridge jumps in back arc settings and ridge segment propagation along the mid atlantic ridge

the breakup of east gondwana assimilating constraints from cretaceous ocean basins around india into a best fit tectonic model published models for the cretaceous sea oor spreading history of east gondwana result in unlikely tectonic scenarios for at least one of the plate boundaries involved and or violate particular constraints from at least one of the associated ocean basins we link east gondwana spreading corridors by integrating magnetic and gravity anomaly data from the enderby basin off east antarctica within a regional plate kinematic framework to identify a conjugate series of east west trending magnetic anomalies m4 to m0 126 7 120 4 ma the mid ocean ridge that separated greater india from australia antarctica propagated from north to south starting at 136ma northwest of australia and reached the southern tip of india at 126 ma sea oor spreading in the enderby basin was abandoned at 115 ma when a ridge jump transferred the elan bank and south kerguelen plateau to the antarctic plate our revised plate kinematic model helps resolve the problem of successive two way strike slip motion between madagascar and india seen in many previously published reconstructions and also suggests that sea oor spreading between them progressed from south to north from 94 to 84 ma this timing is essential for tectonic ow lines to match the curved fracture zones of the wharton and enderby basins as greater india gradually began to unzip from madagascar from 100 ma in our model the 85 east ridge and kerguelen fracture zone formed as conjugate anks of a leaky transform fault following the 100ma spreading reorganization our model also identi es the afanasy nikitin seamounts as products of the conrad rise hotspot

shear margins continent ocean transform and fracture zone boundaries as exploration extends into deeper water it has become more important to understand the nature of the earth s crust beneath offshore sediments that is heat flow and related source rock potential usually require that sediments be deposited over continental crust hence delineating the boundary between oceanic and continental crust becomes important the material in this article was originally presented at the workshop the crust and its structure at seg s recent annual meeting in calgary these results part of a larger dissertation research project also have been presented to several oil and gas exploration companies in houston

lithospheric strength variations as a control on new plate boundaries examples from the northern red sea region abstract the complex plate boundary between arabia and africa at the northern end of the red sea includes the gulf of suez rift and the gulf of aqaba dead sea transform geologic evidence indicates that during the earliest phase of rifting the red sea propagated nnw towards the mediterranean sea creating the gulf of suez subsequently the majority of the relative movement between the plates shifted eastward to the dead sea transform we propose that an increase in the strength of the lithosphere across the mediterranean continental margin acted as a barrier to the propagation of the rift a new plate boundary the dead sea transform formed along a zone of minimum strength we present an analysis of lithospheric strength variations across the mediterranean continental margin the main factors controlling these variations are the geotherm crustal thickness and composition and sediment thickness the analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone this strength profile also favors the creation of thin continental slivers such as the levant west of the dead sea transform and the continental promontory containing socotra island at the mouth of the gulf of aden calculations of strength variations based on changes of crustal thickness geotherm and sediment thickness can be extended to other geologic settings as well they can explain the location of rerifting events at intracratonic basins of backarc basins and of major continental strike slip zones

quantification and restoration of extensional deformation along the western iberia and newfoundland rifted margins 1 many recent papers describe the structure of the iberia and newfoundland rifted margins however none of them propose kinematic restorations of the complete rift system to quantify the amount of extension necessary to exhume mantle and initiate seafloor spreading in our study we use two pairs of cross sections considered as conjugate lines one across the galicia bank flemish cap and the other across the southern iberia abyssal plain flemish pass both transects have been imaged by reflection and refraction seismic methods and have been drilled during ocean drilling program legs 103 149 173 and 210 drilling penetrated parts of the rift stratigraphy and the underlying basement the cross sections can therefore be considered as the best documented conjugate transects across present day hyperextended magma poor rifted margins the aim of this paper is threefold 1 provide a detailed description of the crustal architecture of the two conjugate sections 2 define the extensional structures and their ages and 3 quantify the amount of strain and strain rate accommodated along these lines this paper proposes a quantitative description of extension along the iberia newfoundland rift system and discusses the limitations and problems in quantifying extensional deformation along hyperextended rifted margins

repeat ridge jumps associated with plume ridge interaction melt transport and ridge migration 1 repeated shifts or jumps of mid ocean ridge segments toward nearby hot spots can produce large long term changes to the geometry and location of the tectonic plate boundaries ridge jumps associated with hot spot ridge interaction are likely caused by several processes including shear on the base of the plate due to expanding plume material as well as reheating of lithosphere as magma passes through it to feed off axis volcanism to study how these processes influence ridge jumps we use numerical models to simulate 2 d in cross section viscous flow of the mantle viscoplastic deformation of the lithosphere and melt migration upward from the asthenospheric melting zone laterally along the base of the lithosphere and vertically through the lithosphere the locations and rates that magma penetrates and heats the lithosphere are controlled by the time varying accumulation of melt beneath the plate and the depth averaged lithospheric porosity we examine the effect of four key parameters magmatic heating rate of the lithosphere plate spreading rate age of the seafloor overlying the plume and the plume ridge migration rate results indicate that the minimum value of the magmatic heating rate needed to initiate a ridge jump increases with plate age and spreading rate the time required to complete a ridge jump decreases with larger values of magmatic heating rate younger plate age and faster spreading rate for cases with migrating ridges models predict a range of behaviors including repeating ridge jumps much like those exhibited on earth repeating ridge jumps occur at moderate magmatic heating rates and are the result of changes in the hot spot magma flux in response to magma migration along the base of an evolving lithosphere the tendency of slow spreading to promote ridge jumps could help explain the observed clustering of hot spots near the mid atlantic ridge model results also suggest that magmatic heating may significantly thin the lithosphere as has been suggested at hawaii and other hot spots

mid cenozoic post breakup deformation in the passive margins bordering the norwegian greenland sea abstract since the initial breakup of the norwegian greenland sea in early eocene time chron 24b 53 7 ma the conjugate passive margins have been subject to compression extension magmatism and broad uplifts and subsidence mid to late cenozoic compressional doming basin inversion and reverse faulting occurred predominantly offshore within the rheologically weak deep cretaceous depocenters on the norwegian margin the domes seem to have formed primarily in middle eocene to early oligocene and in early miocene while doming in the faroes rockall region developed in late paleocene to early eocene oligocene and mid to late miocene we suggest that the main driving force was plume enhanced ridge push a testable working hypothesis is that discrete phases of compression were associated with periods of high plume flux in addition the oligocene chron 13 35 ma change in plate motion appears to have generated local shear adjustments along pre existing basement weaknesses forming compressional structures in the overlying sedimentary section this change in plate motion was first manifest by significant extension between the sw barents sea and the conjugate nne greenland margin and led to spreading along the knipovitch ridge mid cenozoic extension and magmatism also preceded the development of the kolbeinsey ridge the associated separation of the jan mayen microcontinent from east greenland must have been strongly influenced by the position of the iceland plume center in addition to the change in extension direction with respect to initial breakup of the norwegian greenland sea in early eocene time the mid cenozoic extension and magmatism is a post breakup phenomena widely spaced areas of neogene uplift occurred along all bordering margins these enigmatic uplifted areas acted as nucleation sites for growth of ice caps and initiated large glacial deposits that built out on the adjacent shelves

the norway basin revisited from continental breakup to spreading ridge extinction abstract we re evaluate the structure and spreading evolution of the southeastern norway basin nb based on a new high resolution aeromagnetic survey nb 07 the survey covers a complete oceanic spreading segment from the continental oceanic transition of the more margin off norway to the aborted aegir ridge the new survey documents a transform margin an orthogonally rifted segment and an oblique sheared volcanic margin formed during the onset of breakup observed from the east jan mayen fracture zone to the faroe platform the detailed fabric of the nb revealed by the new data indicates that two distinct tectonic phases have reshaped the basin before the cessation of seafloor spreading and abortion of the aegir ridge in the late oligocene after continental breakup phase i from c24 to c21r 52 to 49 ma marks the earliest phase of spreading probably initiated in the central and outer part of the more basin during this period competing oceanic segments led to the formation of overlapping systems and pseudo fault development we observe a significant change in the nb s oceanic spreading system in the late early eocene and based on observations from surrounding areas we suggest that this is a record of a major tectonic event in the norwegian greenland sea around c21r 49 47 9 ma during phase ii from c21r to c10 48 to 28 ma of nb s development spreading rates decreased spreading direction changed and the number of faulting with large displacement increased leading to the formation of unexpected n s oriented oceanic fracture zones the fan shaped development of the spreading system initiated around c21r 49 47 9 ma instead of c18 c17 40 38 ma or c24 53 3 52 3 ma as previously proposed these new observations were used to re evaluate the tectonic evolution of the norwegian greenland sea and discuss some implications on the syn and post breakup development of the surrounding continental margins and the evolution of the jan mayen microcontinent

ridge plume interaction in the north atlantic and its influence on continental breakup and seafloor spreading development of the rifted continental margins and subsequent seafloor spreading in the north atlantic was dominated by interaction between the iceland mantle plume and the continental and oceanic rifts there is evidence that at breakup time a thin sheet of particularly hot asthenospheric mantle propagated beneath the lithosphere across a 2500 km diameter region this event caused transient uplift massive volcanism and intrusive magmatism and a rapid transition from continental stretching to seafloor spreading subsequently the initial plume instability developed to an axisymmetric shape with the 100 km diameter central core of the iceland plume generating 30 40 km thick crust along the greenland iceland faeroes ridge the surrounding 2000 km diameter region received the lateral outflow from the plume causing regional elevation and the generation of thicker and shallower than normal oceanic crust we document both long term 10 20 ma and short term 3 5 ma fluctuations in the temperature and or flow rate of the mantle plume by their prominent effects on the oceanic crust formed south of iceland lateral ridge jumps in the locus of rifting are frequent above the regions of hottest asthenospheric mantle occurring in both the early history of seafloor spreading when the mantle was particularly hot and throughout the generation of the greenland iceland faeroes ridge the northern north atlantic is a classic example of continental breakup above a thermal anomaly in the mantle created by a mantle plume the most obvious consequence of this was the generation of huge volumes of basaltic magmas by partial melting of the mantle at the time of continental breakup white mckenzie 1989 most of this magma bled up to the crust where the majority perhaps 60 80 was frozen in the lower crust while the rest was extruded as lava flows and pyroclastic deposits often after undergoing fractionation in crustal magma chambers if the extrusion rates are sufficiently high lava flows may form extensive sub horizontal flood basalts extending across the continental hinterland on rifted continental margins the extrusive lavas are typically expressed as seaward dipping reflector sdr sequences that are readily identifiable on seismic reflection profiles all these features have been found along more than 2 500 km of the rifted continental margins in the northern north atlantic on both the european and the greenland sides furthermore the mantle plume that gave rise to the thermal anomaly in the mantle is still going strong lying beneath iceland at the present day the northern north atlantic is therefore an excellent natural laboratory in which to investigate the interplay between rifting and magmatism because the history of rifting is well recorded from the time of continental breakup in the early tertiary to the seafloor spreading at the present day magnetic anomaly field reversals during the tertiary were frequent and south of iceland there are few large fracture zones in the oceanic crust so the seafloor spreading magnetic anomalies contain a clear record of the rifting history throughout the ocean basin there have also been several well constrained wide angle seismic surveys of the continental margins on both sides of the atlantic e g barton white 1997b smallwood et al 1999 korenaga et al 2000 often with coincident drilling penetrating the volcanic rocks so both the structure of the rifted margins and the timing of the igneous activity are well known in this paper we examine the interaction between the mantle plume and rifting first of the continental lithosphere to form the present continental margins and subsequently of the oceanic lithosphere to form the mid ocean ridge and the land mass of iceland smallwood white rift plume interaction 3

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

the kerguelen plateau records from a long living composite microcontinent abstract the kerguelen plateau lies in the southern indian ocean and is classically considered as a large igneous magmatic province lip emplaced above an oceanic crust the interpretation of the gravimetric bathymetric anomalies shows that the plateau is structured by nnw and nw trending en echelon fractures along its north eastern border and by wnw trending faults along its western side which separate very differentiated zones i e elan bank and enderby basin a geochemical contamination by continental material has been evidenced for all the studied magmatic rocks of the southern and central part of the plateau only the basalts of the northern zone recording the signature of ridge emitted magmas the facies evolution of the overlying sediments also shows that the deposition began in a subaerial environment for the southern part of the plateau and that the subsidence notably increased after 60 ma all these evidences suggest that the southern part of the kerguelen plateau is underlain by stretched continental crust as it is the case for elan bank and that the central kerguelen plateau is most probably also underlain by attenuated continental crust in reconstructions the south and central kerguelen plateaus resulted from the interaction between these remnants of continental crust with the kerguelen plume while the latter interacted with the south east indian ridge to form the north kerguelen plateau we thus propose a cinematic scenario characterised by the formation of a composite microcontinent composed of fragments of stretched continental crust separated by ridge propagators which were emplaced successively or contemporaneously between india and antarctica during the rifting process being triggered by the kerguelen plume emplacement

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

the role of break up localization in microcontinent separation along a strike slip margin the east india elan bank case study the elan bank microcontinent was separated from east india during the early cretaceous break up the crustal architecture and rifting geometry of east india and the elan bank margins document that the early break up between india and antarctica was initiated in the eastern portions of the cauvery and krishna godavari rift zones and in the southern portion of elan bank however the westwards break up propagation along the krishna godavari rift zone continued even after the break up in the overstepping portion of the cauvery rift zone eventually the western propagating end of the krishna godavari rift zone became hard linked with the failed western portion of the cauvery rift zone by the dextral coromandel transfer fault zone consequently the break up location between india and antarctica shifted from its initial to its final location along the northern portion of the elan bank formed by the western krishna godavari rift zone the competition between the two rift zones to capture continental break up and asymmetric ridge propagation resulted in a ridge jump and the elan bank microcontinent release

three dimensional subsidence analysis and gravity modelling of the continental margin offshore namibia summary seismic re ection pro les and gravity anomaly data have been used to determine the structure and evolution of the namibian continental margin in comparison to other margins the gravity anomaly at the namibian margin shows a number of distinctive features it lacks an oishore gravity low and despite the presence of up to 9 km of sediments the gravity high is displaced landwards of the maximum sediment thickness in an attempt to explain these features the early cretaceous recent stratigraphic record is analysed using a combined 3 d backstripping and gravity modelling technique that enables constraints to be placed on the long term mechanical properties of the lithosphere a neutral depth of necking and a relatively high exural rigidity or equivalent elastic thickness te o f 25 km can explain part of the anomaly at least in the south of the margin however large residual anomalies exist that can be reduced by the presence of pre rift low density sediments within the zone of maximum stretching south of 22 0 s high density volcanic material at the eastern pinch out of the lower rift sequence and a lateral sediment density variation across the shelf slope and rise in addition the presence of a magmatic body at the base of the crust provided that the exural strength of the margin is high signi cantly improves the t between observed and calculated gravity anomalies recovered stretching factors enable predictions of the geometry of the moho to be made palaeobathymetry is estimated along the entire margin through time by comparing the subsidence determined from backstripping to that predicted by simple models of rifting these estimates are within the errors of observed palaeowater depths where well control exists the requirement of high te rather than te that increases with time suggests that stretched continental crust extends oishore for some distance and the continent ocean boundary occurs in relatively deep water near magnetic anomaly m4 reconstruction of the gravity eld of the south atlantic at 100 ma illustrates that in terms of their gravity signature the south atlantic margins are asymmetric

the s reflector west of galicia the seismic signature of a detachment fault the so called s reflector is a group of bright continuous reflections underlying the landward tilted fault blocks of the west galicia rifted margin s has been interpreted as the brittle ductile transition the lop of an intrusion a detachment fault and the crust mantle boundary to constrain the internal structure of the reflector we have carried out detailed analyses of these reflections we compare the waveforms of the seafloor reflection and its first multiple both to determine the amplitude of the seafloor reflection and to show hat the seafloor is in effect a spike in the reflectivity series so that the seafloor reflection can be used as the far field wavelet including both source and receiver ghosts we compare he waveform of the seafloor and 5 and show that within the resolution of our data s is a reflection from a step increase in acoustic impedance this result is confirmed through complex trace analysis and in particular the determination of the apparent polarity of s and the examination of the instantaneous frequency function s is consistently positive polarity and shows no significant frequency anomaly simple modelling shows that s is very unlikely to be a reflection from a thin layer we thus conclude that s is probably a single steplike interface from the varying frequency content of the data we determine a value for the effective q between s and the seafloor and use this to assess the amplitude loss due to attenuation and scattering we use a comparison between the seafloor and the s reflection to constrain the amplitude of s and estimate a reflection coefficient for s of at least 0 2 in places decreasing landwards by analogy with structures developed in the highly extended regions of the western united states we consider that the most likely interpretation of s is as a sharp west dipping detachment fault separating a granitic upper plate from a higher velocity lower plate locally probably serpentinized mantle

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

seismic structure gravity anomalies and flexure of the amazon continental margin ne brazil 1 seismic and gravity data have been used to determine the structure of the sediments crust and upper mantle that underlie the amazon continental margin offshore ne brazil seismic reflection profile data reveal a major unconformity at 7 s two way travel time twtt which we interpret as marking the onset of the transcontinental amazon river and the formation of the amazon deep sea fan system in the late miocene seismic refraction data show mean sediment velocities that decrease by 1 5 km s 1 in a seaward direction we attribute this decrease to facies changes associated with sediment progradation and the development of topset foreset and bottomset beds seismic refraction data show that the sediments are underlain by oceanic crust that has a similar velocity structure compared to elsewhere in the atlantic ocean but is unusually thin 4 2 km we attribute the thin crust to either slow seafloor spreading or a limited magma supply during the initial rifting of south america and africa in the early cretaceous the seismic data have been used to construct a new sediment thickness grid that together with gravity anomaly data suggests the amazon fan loaded lithosphere with an unusually high flexural strength while a high strength lithosphere explains the overall depth of the seismic moho there are discrepancies of up to 2 km beneath the upper fan where the modeled flexed moho is shallower than the seismic moho and beneath the middle fan where it is deeper gravity and seismic modeling suggest these discrepancies are caused by lateral changes in subcrustal density such that the mantle underlying the upper fan is denser than it is beneath the middle fan we attribute these lateral density differences to proximity to the ceara rise which is believed to have formed during the late cretaceous in a mid ocean ridge setting fan loading of a relatively strong dense and hence cold lithosphere predicts stress orientations that are consistent with borehole breakout data and the location and height of the gurupe arch onshore despite its proximity to leaky transform faults the margin that underlies the amazon fan appears to be of nonvolcanic origin the main differences with other nonvolcanic margins such as west iberia and newfoundland are a greater sediment accumulation a narrower zone of transitional crust and a lack of any evidence for extreme extension and mantle serpentinization

large strain vacuum assisted consolidation with non darcian radial flow incorporating varying permeability and compressibility abstracta numerical solution has been developed for large strain consolidation incorporating non darcian nonlinear radial flow with varying compressibility and permeability coefficients the solution can accommodate both conventional fill surcharge as well as vacuum preloading the smear effect caused by mandrel driven vertical drains is also captured in the analysis the proposed model is verified by comparing it with fem simulation existing laboratory data other existing theoretical solutions and its advantage of capturing the multiple factors influencing radial drainage and consolidation is demonstrated the effects of non darcian flow are found to be significant for obtaining an accurate solution unlike numerous past solutions that are based on linear darcy flow the salient finding of this study is that the conventional small strain theory can overestimate the rate of consolidation with radial drainage especially for highly compressible soils such as estuarine clays under substantial preloadin

structure and development of a microcontinent elan bank in the southern indian ocean microcontinents appear to commonly form on young continental margins close to hot spots but difficulties in understanding their geology and evolution have inhibited assessment of their global distribution and significance thick volcanic accumulations in areas affected by hot spot magmatism only complicate the issue elan bank a large western salient of the kerguelen plateau is a microcontinent that originally lay between india and antarctica in gondwana recent regional plate tectonic reconstructions suggest that during gondwana breakup elan bank and india initially separated from antarctica and elan bank became isolated in the southern ocean via a ridge jump to the north between elan bank and india in albian time 108 ma voluminous magmatism attributed to the kerguelen hot spot overprinted and radically altered the original microcontinent and its surroundings recent odp investigations deep seismic reflection data and a wide angle seismic line on elan bank allow us to gain the first insight into the feature s integrated crustal structure and geological evolution and the adjacent continent ocean transition zone our analysis shows that elan bank s crust is at least 16 km thick the upper igneous crust consists of a 2 3 km thick layer with seismic velocities ranging from 4 4 to 5 9 km s that can be interpreted as the result of accumulation of lava flows originating from the kerguelen hot spot seismic velocities at the base of the crust are as low as 6 6 km s which is consistent with a fragment of thinned continental crust 14 km thick a high velocity body located at depths of 5 to 10 km could be interpreted as plutonic rocks emplaced during the major regional magmatic episode on the basis of deep seismic reflection data we interpret extensional structures beneath the volcanic flows in albian time when the area was affected by the kerguelen hot spot volcaniclastic material and lava flows accumulated in faulted grabens and basins both on the bank and within the continent ocean transition zone to the south creating the appearance of flat unstructured basement the seismic structure and inferred composition of elan bank revealed by this study contribute to our understanding of microcontinent formation as well as provide a template for identifying microcontinents in accreted terranes and mountain belts

structure of the nagavali shear zone eastern ghats mobile belt india correlation in the east gondwana reconstruction abstract interpretation of satellite data in combination with regional field traverses delineating the major structural features such as the nagavali and vamsadhara shear zones and associated fold patterns provides a synoptic picture of the regional tectonic framework of the central part of the eastern ghats mobile belt the complex geology of the study area can broadly be grouped into three distinct deformational events d1 fabrics represented by near flat lying gneissic foliations paralleling the lithological banding are best preserved in low strain domains and are related to middle to late archaean thrusting 3000 2600 ma the second deformational event d2 is characterized by the development of shear zones and associated mylonitic fabrics and magmatism probably during 1450 850 ma the pan african thermal 500 550 ma overprint is restricted to shear zones in the form of reworking regionally the central part of the eastern ghats mobile belt can be divided into five distinct structural domains based on structural geometry of folds foliations and lineations a three dimensional block diagram of the nagavali and vamsadhara shear zones involving fold thrust tectonics associated with westward thrusting is presented here a correlation of pan african shear zones in adjacent continents wrapping around the archaean dharwar craton in the reconstruction of rodinia and east gondwana supercontinent suggests an east west convergence

boron isotope geochemistry of australian salt lakes boron isotope geochemistry has been investigated in brines groundwaters and sediments from the modern australian salt lakes of victoria south australia and western australia by applying negative thermal ionization mass spectrometry techniques the geochemical history of the brines has been reconstructed by using 11b bcl and nacl ratios the victorian volcanic crater lakes of southeastern australia have water salinities of up to 60 g l molar nacl ratios 0 87 similar to the marine ratio molar bcl ratios of 2 9 10 4 to 4 9 10 4 and 11b values of 54 to 59 relative to nbs 951 the depletion of total b and the high positive 11b values relative to seawater bclratio 7 9 10 4 11b 39 are attributed to a marine cyclic salt origin together with adsorption processes in closed systems with low water sediment wr ratios in contrast salt lakes from south australia and western australia which are large shallow playas associated with halite gypsum and detrital clay minerals have interstitial and surface brines characterized by salinities of 80 to 280 g l molar nacl ratios of 0 85 to 1 molar bcl ratios of 4 10 6 to 4 10 4 and 11 values of 25 to 48 the 11 values of these brines are different from those of groundwaters from the great artesian basin 11 15 9 to 2 2 with high molar bcl ratios of 1 10 3 to 3 8 10 2 country rocks 11b 8 7 to 6 8 and modern detrital sediments present in these salt lakes 11b 3 2 to 12 3 the 11b values of these salt lakes overlap with those of surface and brackish waters 11b 28 to 35 and with the b isotopic composition of seawater 11b 39 both low molar nacl ratios 39 of some brines indicate interaction of the brines with detrital sediments within the salt lake systems 11 values 39 suggest mixing of brines of marine origin from which b was partly removed by adsorption with waters of terrestrial origin with low 11 values nacl ratios are used as indicators of the origin of the salts as well as of halite dissolution precipitation the 11 values and bcl ratios are sensitive to a marine or non marine origin adsorption of boron onto clays and the effective water sediment ratio at low wr ratios the preferential removal of 10b from the solution affects the bulk solution whereas at high wr ratios the 11 value of a solution is not affected by adsorption although the 11 value of borate minerals may be a discriminant of marine or non marine origin boron isotopes are less distinctive in evaporative environments where boron is not an abundant component and where water sediment interaction occurs

seafloor spreading magnetic anomalies south off sri lanka abstract results obtained from compilation and reinterpretation of about 21 200 line km of bathymetry magnetic and satellite gravity data between 10 s to 10 n latitudes and 75 to 90 e longitudes south off sri lanka are presented here magnetic data and the synthetic seafloor spreading model reveal the presence of mesozoic anomaly sequence m11 through m0 south of sri lanka the oldest magnetic anomaly m11 134 ma occurs between 110 and 140 km away from the sri lankan coast the seafloor created during the early cretaceous is estimated to have evolved with variable half spreading rates ranging from 5 5 to 1 53 cm yr the trends of the fracture zones inferred from the offsets in the magnetic anomalies have been constrained using the satellite gravity mosaic the cretaceous magnetic quiet zone cmqz 121 84 ma crust between the isochrons m0 and a34 has an unequal width and widens from about 170 km in the west 80 e longitude to about 500 km towards east 85 e longitude plate reconstruction models for 160 fit 134 m11 121 m0 and 84 ma a34 are generated under the constraints of newly identified magnetic anomaly isochrons and fracture zones the mismatch in the a34 reconstruction is attributed to the emplacement of the ob lena and marion dufresne seamounts at the spreading ridge axis and or the frequent ridge jumps in the middle cretaceous during the major plate reorganization

provenance of proterozoic garnet biotite gneiss recovered from elan bank kerguelen plateau southern indian ocean at elan bank of the kerguelen plateau in the southeast indian ocean leg 183 of the ocean drilling program recovered clasts of garnet biotite gneiss in a fluvial conglomerate intercalated with basalt flows u pb and pb pb dates of zircons and monazites in these clasts and an overlying sandstone range from 534 to 2547 ma which is much older than the surrounding indian ocean seafloor these dates show that old continental crust resides in the shallow crust of the oceanic kerguelen plateau and that the breakup of gondwana dispersed continental fragments into the nascent indian ocean lithosphere