crustal structure of the northern nova scotia rifted continental margin eastern canada 1 the nova scotia continental margin off eastern canada marks a transition from a volcanic to a nonvolcanic style of rifting the northern nonvolcanic segment of the margin was studied by a 490 km long refraction seismic line with dense air gun shots coincident with previous deep reflection profiles a p wave velocity model was developed from forward and inverse modeling of the wide angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles the continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5 7 6 9 km s crustal thinning down to 3 km occurs in a 180 km wide zone and the sediment cover in this area is up to 15 km thick farther seaward a 150 km wide transition zone is observed with a 5 km thick lower layer 7 2 7 6 km s interpreted as partially serpentinized mantle at the landward end this layer is overlain by highly altered continental crust 5 4 km s extending up to the seaward limit of the jurassic salt province farther seaward the upper layer is interpreted as exhumed and highly serpentinized mantle 5 1 km s separated from the lower layer by subhorizontal reflectivity which probably represents a serpentinization front oceanic crustal thickness is 4 km with layer 2 velocities of 4 6 5 0 km s layer 3 velocities of 6 4 6 55 km s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile this reduced magma production might be related to the proximity of the newfoundland transform margin

getting over continent ocean boundaries the idea of a simple linear boundary between continental and oceanic crust at extended continental margins is widely recognized to be an oversimplification despite this such boundaries continue to be mapped because of their perceived utility in palinspastic and plate kinematic reconstructions to examine whether this perception is justified we review the data and models on which basis continent ocean boundaries are interpreted and map a set of such interpretations worldwide from more than 150 publications the maps show that the location of the continent ocean boundary is rarely consistently estimated within the 10 100 km observational uncertainty that might be expected of the geophysical data used for doing so that this is the case regardless of whether the transition zone behind the boundary is classified as magma rich or magma poor and that the geographical separation of estimates exceeds the width of single study continent ocean transition zones the average of global maximum separations across sets of three or more estimates is large 167 km and mostly a consequence of interpretations published over the last decade we interpret this to indicate an extra component of uncertainty that is related to authors understanding of the range of features that are interpretable at extended continental margins we go on to discuss the implications of this uncertainty for palinspastic and plate kinematic modelling using examples from the literature and from the south atlantic ocean we conclude that a precise continent ocean boundary concept with locational uncertainty defined from the ensembles is of limited value for palinspastic reconstructions because the reconstruction process tends to bunch the ensemble within a region that is i of similar width to the observational uncertainties associated with continent ocean boundary estimates ii narrower than the regions of uncertainty about rotated features implied by the propagation of uncertainties from plate rotation parameters and iii coincident within all the above uncertainties with the more easily mapped continental shelf gravity anomaly secondly we conclude that estimated continent ocean boundaries are of limited use in developing or testing plate kinematic reconstructions because i reconstructions built using them as markers do not within uncertainty limits defined from the ensembles differ greatly from those using more easily determined bathymetric or gravity anomaly contours and ii because it is impossible to segment and date them with useful precision to use as markers of the edges of rigid oceanic lithosphere outside of the constraints of a pre existing plate kinematic model

mineralogical and isotopic record of biotic and abiotic diagenesis of the callovian oxfordian clayey formation of bure france abstract the callovian oxfordian cox clayey unit is being studied in the eastern part of the paris basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal the present study combines new mineralogical and isotopic data to describe the sedimentary history of the cox unit petrologic study provided evidence of the following diagenetic mineral sequence 1 framboidal pyrite and micritic calcite 2 iron rich euhedral carbonates ankerite sideroplesite and glauconite 3 limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks 4 chalcedony 5 quartz calcite pyrite in bioturbations shows a wide range of 34s 38 to 34 5 providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions the most negative values 38 to 22 measured in the lower part of the cox unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply the most positive pyrite 34s values 14 up to 34 5 in the upper part of the cox unit indicate pyrite precipitation in a closed system celestite 34s values reflect the last evolutionary stage of the system when bacterial activity ended however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution the 87sr 86sr ratio of celestite 0 706872 0 707040 is consistent with deposition from jurassic marine derived waters carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates siderite only present in the maximum clay zone has chemical composition and 18o consistent with a marine environment its 13c is however lower than those of marine carbonates suggesting a contribution of 13c depleted carbon from degradation of organic matter 18o values of diagenetic chalcedony range between 27 and 31 suggesting precipitation from marine derived pore waters late calcite crosscutting a vein filled with chalcedony and celestite and late euhedral quartz in a limestone from the top of the formation have lower 18o values 19 suggesting that they precipitated from meteoric fluids isotopically close to present day pore waters of the formation finally the study illustrates the transition from very active biotic diagenesis to abiotic diagenesis this transition appears to be driven by compaction of the sediment which inhibited movement of bacterial cells by reduction of porosity and pore sizes rather than a lack of inorganic carbon or sulphates

crustal anatomy of a transform continental margin abstract this paper describes new deep seismic reflection results across the major transform margin southwest of the grand banks off eastern canada an interpretation of these results is presented supplemented with previous seismic refraction gravity and magnetic measurements the results show a fairly sharp transition between oceanic and continental regions at the transform margin a narrow zone about 40 km wide separates the two regions and is interpreted to be a zone in which shearing has destroyed the original fabric of the continental crust faults in this region extend throughout the crust and there is some evidence that a major strike slip fault may extend some tens of kilometres into the upper mantle magmatism may have further modified the crust in this shear zone which is bounded on the oceanic side by a small seamount the continental crust is thinned by a factor of 3 or more in this zone which may be due to erosion or to flow in the lower crust during shearing the oceanic crust does not appear to thin significantly as the transform margin is approached crustal scale dipping reflectors over a confined region below the shelf are interpreted to represent the contact between two appalachian terranes the meguma and avalon terranes this supports earlier interpretations of a large magnetic lineament in the region as marking this boundary these crustal seismic data crossing a large transform margin are the first of their kind and will help us to constrain evolutionary models of transform margins the picture they provide is different in many respects from that across rifted margins and similar in others in some cases it might be difficult to distinguish between these two classes of atlantic type margins on the basis of crustal structure

subduction initiation at passive margins the scotian basin eastern canada as a potential example thermal evolution modeling based on the geometry of the scotian basin eastern canada and analysis of the shear resistance associated with lithospheric faulting are used to investigate the initiation of subduction at passive margins previous studies have demonstrated the difficulty of subduction initiation at mature passive margins however unique features along a passive margin may facilitate subduction initiation subduction may initiate along a part of the margin 1 by extensional or strike slip deformation rather than shortening 2 by deformation of the weaker continental lithosphere rather than oceanic lithosphere 3 by heating and weakening of oceanic lithosphere due to lateral heat transfer from adjacent continental lithosphere and 4 by heating and weakening of the lithosphere due to the passage of a mantle plume beneath the margin the values of shear resistance under these conditions are still higher than those calculated for ridge push indicating that subduction initiation at passive margins is difficult even with these weakening mechanisms however pulses of compression associated with plate reorganization may be enough to trigger subduction at such passive margins

sustained increase in ice discharge from the amundsen sea embayment west antarctica from 1973 to 2013 we combine measurements of ice velocity from landsat feature tracking and satellite radar interferometry and ice thickness from existing compilations to document 41 years of mass flux from the amundsen sea embayment ase of west antarctica the total ice discharge has increased by 77 since 1973 half of the increase occurred between 2003 and 2009 grounding line ice speeds of pine island glacier stabilized between 2009 and 2013 following a decade of rapid acceleration but that acceleration reached far inland and occurred at a rate faster than predicted by advective processes flow speeds across thwaites glacier increased rapidly after 2006 following a decade of near stability leading to a 33 increase in flux between 2006 and 2013 haynes smith pope and kohler glaciers all accelerated during the entire study period the sustained increase in ice discharge is a possible indicator of the development of a marine ice sheet instability in this part of antarctica key points sustained ase mass flux increase 77 since 1973 thwaites accelerated by 33 during the last 6 years speed changes are pervasive and rapid major implications for ice flow modeling 2014 american geophysical union all rights reserved

comportement d une bordure de massifs anciens et corteges de mineraux argileux l exemple de la bordure occidentale du bassin anglo parisien au callovo oxfordien abstract detailed sedimentological studies of callovian to oxfordian outcrops along the western border of the anglo paris basin maine normandy s w england carried out on a fine biostratigraphical background showed that this sedimentary succession was controlled by two main factors on the margin of the armorican and cornubian massifs 1 a swift levelling of the border area followed by terrigenous sedimentation from uppermost bathonian to lower oxfordian 2 the initiation of a carbonate platform with a transition from terrigenous to carbonate sedimentation at the lower middle oxfordian boundary 3 the rapid build up of the carbonate platform middle oxfordian 4 the burial of corals and the interruption of the platform development favoured by a sudden change from carbonate to siliciclastic sedimentation occurring in early upper oxfordian the sedimentation record was controlled by two main epirogenic structures the eperon du perche with a positive trend between southern normandy and northern maine provinces and the sillon marneux peri armoricain a narrow area with subsident trend beneath the english channel floor and between the armorican and ardenne blocks after the study of clay mineral distribution from the base of the callovian up to the upper oxfordian along this border area of ancient massifs armorican supplies are distinguished from external ones clay mineral supplies from armorican lands strongly reflected epirogenic changes physiographic variations and regional current modifications affecting the future eastern border of the north atlantic these clay minerals are illite kaolinite more scarcely chlorite and mixed layers primary materials such as illite and chlorite occurred as well as smectites and kaolinite there is not any appreciable evidence of diagenesis with depth of burial the clay minerals being dominantly of detrital origin their diversity was conditioned in part by the evolution of continental pedogenesis the origin of smectites is more difficult to account for no sign of volcanic material can be ascertained within the successions under study on the western border so the most likely origin of these smectites was a terrigenous supply in middle oxfordian times three clay mineral assemblages were deposited on the carbonate platform dominant smectites accompanied by kaolinite and illite were deposited near the eperon du perche a strong increase of smectites without kaolinite occurred in south dorset pays d auge and belinois southern maine on the remainder of the western border the composite clay mineral assemblages contained kaolinite illite and smectites in subequal concentrations similar associations also characterize the three other sedimentary episodes but in south dorset and pays d auge the smectites appeared in upper lower oxfordian only during a remobilization of the armorican basement clay minerals proceeded from weak soil erosion their supplies mainly depended on the physiographic evolution of the armorican massif the eperon du perche favored transport of pedogenetic smectites these supplies were also controlled by regional marine current along the coast differential settling processes occur frequently during off shore migration of sediments on the other borders of the anglo paris basin detailed studies of mineralogical assemblages are still too scarce kaolinite was absent from berry to burgundy until the lower oxfordian but was present in the boulogne and le havre regions until middle oxfordian it disappeared from the lower oxfordian in south dorset one of the main external supplies may have come from the northeast or the north smectites were always present in the middle and upper jurassic clay mineral assemblages of the anglo paris basin except on the pays d auge and south dorset they were also wide distributed in upper jurassic deposits on the future margins of the north atlantic western supplies of smectites were carried eastwards through the depression called sillon marneux peri armoricain and deposited in the anglo paris basin

slope induced errors in radar altimetry over continental ice sheets ranges obtained by radar altimetry from a satellite to a sloping surface on the earth are not measurements of the surface elevation at the subsatellite point instead the reflecting point is displaced upslope from the subsatellite point causing a slope induced error between the true range to the subsatellite point and the indicated range for the seasat 1 altimeter this range error is nearly 80 m for a 0 8 degree slope ice sheet surface slopes are frequently large enough to cause errors of 10 m or more in addition undulations in the surface about a mean slope cause pronounced variations in these errors altimeter measurements of ranges to modeled irregular surfaces are simulated and two correction schemes are used to reconstruct the modeled surfaces from the simulated data the results illustrate the fundamental limitations inherent in single beam radar altimetry for mapping irregular surfaces in the simpler two dimensional case for which cross track slopes are neglected a relocation scheme which constructs a surface consistent with the altimeter ranges removes 85 of the rms error the mean error along profiles of about 75 km or longer is usually reduced more than the rms error an alternate slope correction scheme which uses the local slope to calculate the expected error is less effective in the two dimensional case however over a simulated three dimensional surface where groundtracks are widely spaced and cross track slopes are significant the slope correction method must be used in at least the cross track direction the effectiveness of the three dimensional correction depends on the relative size of the errors caused by the local slope on surface undulations in comparison to the regional slope

crustal structure of the french guiana margin west equatorial atlantic summary geophysical data from the amazon cone experiment are used to determine the structure and evolution of the french guiana and northeast brazil continental margin and to better understand the origin and development of along margin segmentation a 427 km long combined multichannel reflection and wide angle refraction seismic profile acquired across the southern french guiana margin is interpreted where plate reconstructions suggest a rift type setting the resulting model shows a crustal structure in which 35 37 km thick pre rift continental crust is thinned by a factor of 6 4 over a distance of 70 km associated with continental breakup and the initiation and establishment of seafloor spreading the ocean continent boundary is a transition zone up to 45 km in width in which the two layered oceanic type crustal structure develops although relatively thin at 3 5 5 0 km such thin oceanic crust appears characteristic of the margin as a whole there is no evidence of rift related magmatism either as seaward dipping sequences in the reflection data or as a high velocity region in the lower crust in the p wave velocity model and as a such the margin is identified as non volcanic in type however there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins consequently the thin oceanic crust the rapidity of continental crustal thinning and the absence of characteristic riftrelated structures leads to the conclusion that the southern french guiana margin has instead developed in an oblique rift setting in which transform motion also played a significant role in the evolution of the resulting crustal structure and along margin segmentation in structural style

spatial and temporal evolution of pine island glacier thinning 1995 2006 we use ers 2 and envisat satellite radar altimetry to examine spatial and temporal changes in the rate of thinning of the pine island glacier west antarctica during the period 1995 to 2006 we show that the pattern of thinning has both accelerated and spread inland to encompass tributaries flowing into the central trunk of the glacier within the 5 400 km 2 central trunk the average rate of volume loss quadrupled from 2 6 0 3 km 3 yr 1 in 1995 to 10 1 0 3 km 3 yr 1 in 2006 the region of lightly grounded ice at the glacier terminus is extending upstream and the changes inland are consistent with the effects of a prolonged disturbance to the ice flow such as the effects of ocean driven melting if the acceleration continues at its present rate the main trunk of pig will be afloat within some 100 years six times sooner than anticipated citation wingham d j d w wallis and a shepherd 2009 spatial and temporal evolution of pine island glacier thinning 1995 2006 geophys res lett 36 l17501 doi 10 1029 2009gl039126

trends in antarctic ice sheet elevation and mass fluctuations in antarctic ice sheet elevation and mass occur over a variety of timescales owing to changes in snowfall and ice flow here we disentangle these signals by combining 25 years of satellite radar altimeter observations and a regional climate model from these measurements patterns of change that are strongly associated with glaciological events emerge while the majority of the ice sheet has remained stable 24 of west antarctica is now in a state of dynamical imbalance thinning of the pine island and thwaites glacier basins reaches 122 m in places and their rates of ice loss are now five times greater than at the start of our survey by partitioning elevation changes into areas of snow and ice variability we estimate that east and west antarctica have contributed 1 1 0 4 and 5 7 0 8 mm to global sea level between 1992 and 2017

rb sr and k ar dating of clay diagenesis in jurassic sandstone oil reservoir north sea rb sr and k ar isotopic determinations were combined with scanning electron microscopy sem observations and x ray diffractometry xrd controls to date the diagenetic formation of clay minerals from oil bearing sandstones of the middle jurassic brent sandstone in the north sea the separated clay fractions often contain detrital components especially k feldspars which are reduced to sizes smaller than 0 4 m either by intense in situ alteration or by destructive sample preparation direct rb sr and k ar isotopic dating of diagenetic illite therefore was not possible isotopic data obtained on mixtures of detrital and newly formed components showed however that illite formed about 40 to 45 ma this value also was obtained directly on clay fractions separated by a gentle method of disaggregation illitization is related to emplacement of hydrocarbons and associated waters into the formation aqueous and hydrocarbon fluids simultaneously trapped in quartz overgrowth inclusions indicate late silicification contemporaneous with this fluid migration minimum trapping temperatures of at least 110 c for these fluid inclusions are too high for the present burial depth hot fluids are assumed to have originated in deep source rocks beneath the brent reservoirs and migrated upward during the middle eocene

crustal structure across the southwest newfoundland transform margin a seismic refraction survey providing deep crustal structure information of the continent ocean boundary across the south west newfoundland transform margin was carried out using large air gun sources and ocean bottom seismometer receivers continental crust 30 km thick beneath the southern grand banks p wave velocity 6 2 6 5 km s thins oceanward to a 25 km wide transition zone in the transition zone paleozoic basement of the grand banks 5 5 5 7 km s is replaced by a basement of oceanic volcanics and synrift sediments 4 5 5 5 km s seaward of the transition zone the crust is oceanic in character with a velocity gradient from 4 7 to 6 5 km s and a thickness of 7 8 km oceanic layer 3 is absent no significant thickness of intermediate velocity 7 km s material is present at the continent ocean transition indicating that no under plating of continental crust has taken place the continent ocean transition across the transform margin is much narrower than across rifted margins supporting the

dual frequency altimeter signal from envisat on the amery ice shelf in antarctica radar altimeter measurements are sensitive to dielectric and penetration properties of the sensed medium snow such that the spacecraft s altitude can be biased since 2002 relatively low frequency radar measurements over the amery ice shelf east antarctica have been acquired using the envisat dual frequency altimeter at s 3 2 ghz and ku 13 6 ghz bands which penetrate a few meters into the firn the altimeter signal is however modified in summer by the presence of snowfilled crevasses indeed the specularity of the snow surfaces in summer makes the altimetric signal sensitive mostly to nadir echoes that increases the ratio between the crevasse signal and the surrounding ice shelf signal at nadir crevasses are distinguished by differences in backscattering behavior compared with the surrounding ice shelf signal crevasses are characterized by a strong backscatter coefficient at ku band and anomalies in the s band altitude estimation these two characteristics make snowfilled crevasses detectable by the dual frequency altimeter of envisat we first retrieve the geometric properties of the crevasses using a hyperbolic shape function created by strong crevasse backscatter in the ku waveform measurements from this retrieved crevasse signal and further waveform analysis we assess the properties of the snow surface and its sub surface the crevasse due to its small size compared to the altimeter footprint is found to be an excellent target to study snow properties of the ice shelf the anomalies in the s band altitude measurements over crevasses can then be explained by the presence of a double echo in the s band waveforms this echo is attributed to a reflection at the base of the snowbridge where we see evidence of sub surface echos in the individual altimeter waveforms based on this observation a methodology is developed to estimate the thickness of the snowbridge we calculate the penetration depths in the summer snow surface of the amery at ku band that is found to be around 6 m

evolution of porewater composition through time in limestone aquifers salinity and d h of fluid inclusion water in authigenic minerals jurassic of the eastern paris basin france past water circulations can significantly reduce the porosity and permeability of marine limestones this is particularly the case in the middle bathonian bajocian to upper oxfordian jurassic limestones from the eastern border of the paris basin the knowledge of the timing the temperature and composition of paleowaters is essential to model the hydrological evolution in this area where the callovian oxfordian claystones are studied for the storage of nuclear wastes in this way fluid inclusions hosted in low temperature 60 c authigenic calcite quartz and celestite crystals were analyzed by raman spectroscopy and mass spectrometry to determine the chlorinity and d h ratios chlorinity measurements mmol cl per liter of water in fluid inclusions trapped in authigenic crystals during the late jurassic early cretaceous period revealed unexpected high values up to 3800 mmol l 1 indicating that brines were involved in some of the diagenetic crystallization processes by contrast fluid inclusions in calcite cements of cenozoic age within the oxfordian limestones have low cl concentration less than 150 mmol l 1 thus showing that a dilution event caused by water infiltrations during the cretaceous uplift of this part of the basin has flushed out the original saline porewater by coupling d of fluid inclusion with 18o of calcite crystals we estimate that calcite precipitation occurred at temperatures between 25 and 53 c the hydrogen isotope composition of calcite forming water is different between the middle jurassic d ranging from 20 to 35 8 v smow and the overlying oxfordian limestone d from 59 5 to 44 8 v smow present day groundwaters are also of distinct composition on both sides of the oxfordian claystones indicating that limestone aquifers underwent independent hydrologic evolutions since the early diagenetic jurassic cementation

illite smectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the texas gulf coast paleocene eocene abstract mudrock samples from a well penetrating eocene and paleocene strata 1235 4455 m of the texas gulf coast have been studied to assess geochemical redistribution in mudrocks during burial diagenesis and the mechanism by which detrital smectitic illite smectite i s is transformed into diagenetic illitic i s idealized smectite and illite end member compositions for interstratified i s are estimated to be k0 00x 10 56mg0 39fe0 57al1 13si3 90o10 oh 2 and k0 53x 10 18mg0 17fe0 16al2 28 si3 40o10 oh 2 respectively there is no obvious difference in the amount of k addition to i s in random and ordered interstratified i s aluminum substitution into both tetrahedral and octahedral sites suggests that the smectite to illite reaction is a complete dissolution precipitation reaction rather than a solid state k 1 al 3 for si 4 substitution reaction depth relat d mineral trends and mass balance calculations suggest that illitic i s may form from kaolinite and possibly illite or mica as well as from smectitic i s although most whole rock element abundances are invariant over the sampled depth interval k2o content increases from ca 2 0 weight percent at 1500 m of burial to ca 3 8 weight percent at 4000 m some of this increase is probably caused by import of k into the shale via fluids derived from interbedded sandstones implying that gulf coast mudrocks behave as open chemical systems during burial diagenesis if this is the case a minimum of 103 pore volumes of fluid must have passed through the most k enriched shales to have introduced the added potassium provided that the k content of present day formation water is representative of that of ancient formation water

increased ice losses from antarctica detected by cryosat 2 we use 3 years of cryosat 2 radar altimeter data to develop the first comprehensive assessment of antarctic ice sheet elevation change this new data set provides near continuous 96 coverage of the entire continent extending to within 215 km of the south pole and leading to a fivefold increase in the sampling of coastal regions where the vast majority of all ice losses occur between 2010 and 2013 west antarctica east antarctica and the antarctic peninsula changed in mass by 134 27 3 36 and 23 18 gt yr 1 respectively in west antarctica signals of imbalance are present in areas that were poorly surveyed by past missions contributing additional losses that bring altimeter observations closer to estimates based on other geodetic techniques however the average rate of ice thinning in west antarctica has also continued to rise and mass losses from this sector are now 31 greater than over the period 2005 2010

a high resolution record of greenland mass balance we map recent greenland ice sheet elevation change at high spatial 5 km and temporal monthly resolution using cryosat 2 altimetry after correcting for the impact of changing snowpack properties associated with unprecedented surface melting in 2012 we nd good agreement 3 cm yr bias with airborne measurements with the aid of regional climate and rn modeling we compute high spatial and temporal resolution records of greenland mass evolution which correlate r 0 96 with monthly satellite gravimetry and reveal glacier dynamic imbalance during 2011 2014 greenland mass loss averaged 269 51 gt yr atmospherically driven losses were widespread with surface melt variability driving large uctuations in the annual mass de cit terminus regions of ve dynamically thinning glaciers which constitute less than 1 of greenland s area contributed more than 12 of the net ice loss this high resolution record demonstrates that mass de cits extending over small spatial and temporal scales have made a relatively large contribution to recent ice sheet imbalance

crustal structure beneath the southern grand banks seismic refraction results and their implications a seismic refraction profile was shot on the southern grand banks using large air gun sources and an array of ocean bottom seismograph receivers a sediment column 1 2 km thick directly overlies paleozoic basement with velocity structure similar to that of the meguma zone of nova scotia the main crustal layer is 27 km thick with seismic velocity of 6 3 km s increasing to about 6 5 km s in the lowest few kilometres complexity is apparent in the crust mantle transition around 32 km depth comparison with deep multichannel reflection data suggests that the increased velocity in the lower part of the crust may be associated with a reflective zone and shows the mohorovicic discontinuity to be delineated by a well defined reflection the absence of a major lower crustal layer of intermediate velocity 7 km s is consistent with observations elsewhere in the region

elevation change of the antarctic ice sheet 1995 2000 from ers 2 satellite radar altimetry we analyzed antarctic ice sheet elevation change dh dt from 1995 to 2000 using 123 million elevation change measurements from european remote sensing 2 ice mode satellite radar altimeter data covering an area of about 7 2 million km sup 2 almost all drainage basins in east antarctica had average dh dt values within spl plusmn 3 0 cm year whereas drainage basins in west antarctica had substantial spatial variability with average dh dt values ranging between 11 to 12 cm year the east antarctic ice sheet had a five year trend of 1 spl plusmn 0 6 cm year where 13 out of the 14 basins had either a positive trend or a trend that was not significantly different than zero the west antarctic ice sheet had a five year trend of 3 6 spl plusmn 1 0 cm year due largely to strong negative trends of around 10 cm year for basins in marie byrd land along the pacific sector of the antarctic coast the continent as a whole had a five year dh dt trend of 0 4 spl plusmn 0 4 cm year finally time series constructed for the pine island thwaites de vicq and land glaciers in west antarctic showed five year dh dt trends from 26 to 135 cm year that were significantly more negative than the average dh dt trends in their respective basins the strongly negative dh dt values for these coastal glacier outlets are consistent with recently reported results indicating increased basal melting at these glaciers grounding lines caused by ocean thermal forcing

25 years of elevation changes of the greenland ice sheet from ers envisat and cryosat 2 radar altimetry abstract the shape of the large ice sheets responds rapidly to climate change making the elevation changes of these ice covered regions an essential climate variable consistent long time series of these elevation changes are of great scientific value here we present a newly developed data product of 25 years of elevation changes of the greenland ice sheet derived from satellite radar altimetry the data product is made publicly available within the greenland ice sheets project as part of the esa climate change initiative programme analyzing repeated elevation measurements from radar altimetry is widely used for monitoring changes of ice covered regions the greenland ice sheet has been mapped by conventional radar altimetry since the launch of ers 1 in 1991 which was followed by ers 2 envisat and currently cryosat 2 the recently launched sentinel 3a will provide a continuation of the radar altimetry time series since 2010 cryosat 2 has for the first time measured the changes in the coastal regions of the ice sheet with radar altimetry with its novel sar interferometric sarin mode which provides improved measurement over regions with steep slopes here we apply a mission specific combination of cross over along track and plane fit elevation change algorithms to radar data from the ers 1 ers 2 envisat and cryosat 2 radar missions resulting in 25 years of nearly continuous elevation change estimates 1992 2016 of the greenland ice sheet this analysis has been made possible through the recent reprocessing in the reaper project of data from the ers 1 and ers 2 radar missions making them consistent with envisat data the 25 years of elevation changes are evaluated as 5 year running means shifted almost continuously by one year a clear acceleration in thinning is evident in the 5 year maps of elevation following 2003 while only small elevation changes observed in the maps from the 1990s

structure of the transkei basin and natal valley southwest indian ocean from seismic reflection and potential field data abstract marine geophysical data from the southern natal valley and northern transkei basin offshore southeast africa were used to study the structure of the crust and sedimentary cover in the area the data includes seismic reflection gravity and magnetics and provides information on the acoustic basement geometry where available features of the sedimentary cover and the basin s development previously mapped mesozoic magnetic anomalies over a part of the basin are now recognized over wider areas of the basin the ability to extend the correlation to the southeast within the natal valley further confirms an oceanic origin for this region and provides an opportunity to amplify the existing plate boundary reconstructions the stratigraphic structure of the southern natal valley and the northern transkei basin reflects processes of the ocean crust formation and subsequent evolution the highly variable relief of the acoustic basement may relate to the crust formation in the immediate vicinity of the continental transform margin renewed submarine seismicity and neotectonic activity in the area is probably related to the diffuse boundary between the nubia and somalia plates 2 5 d crustal models show that a 1 7 3 2 km thick sediment sequence overlies a 6 3 1 2 km thick normal oceanic crust in the deep southern natal valley and transkei basin the oceanic crust in the study area is heterogeneous made up of blocks of laterally varying remanent magnetization 0 5 3 5 a m and density 2850 2900 kg m3 strong modifications of accretionary processes near ridge fracture zone intersections may be a reason of such heterogeneity

mapping of the topography of continental ice by inversion of satellite altimeter data satellite altimeter data over ice sheets provide the best tool for mapping their topography and its possible climatic variations however these data are affected by measurement errors orbit errors and slope errors we develop here a three step inversion technique which accommodates the a priori information on the expected topography and correctly handles and propagates the data errors it estimates first a large scale reference surface then maps the residuals related to undulations and finally iteratively corrects the slope error the method is tested on overlapping small fragments of the antarctic ice sheet using a sub set of seasat data finally a topographic map of terre adelie is produced over areas of small slopes the a posteriori error should be of the order of 0 4 m using ers i data it is therefore expected that climatic variations in the ice sheet topography since the introduction of seasat will be observable

four decades of antarctic ice sheet mass balance from 1979 2017 we use updated drainage inventory ice thickness and ice velocity data to calculate the grounding line ice discharge of 176 basins draining the antarctic ice sheet from 1979 to 2017 we compare the results with a surface mass balance model to deduce the ice sheet mass balance the total mass loss increased from 40 9 gt y in 1979 1990 to 50 14 gt y in 1989 2000 166 18 gt y in 1999 2009 and 252 26 gt y in 2009 2017 in 2009 2017 the mass loss was dominated by the amundsen bellingshausen sea sectors in west antarctica 159 8 gt y wilkes land in east antarctica 51 13 gt y and west and northeast peninsula 42 5 gt y the contribution to sea level rise from antarctica averaged 3 6 0 5 mm per decade with a cumulative 14 0 2 0 mm since 1979 including 6 9 0 6 mm from west antarctica 4 4 0 9 mm from east antarctica and 2 5 0 4 mm from the peninsula i e east antarctica is a major participant in the mass loss during the entire period the mass loss concentrated in areas closest to warm salty subsurface circumpolar deep water cdw that is consistent with enhanced polar westerlies pushing cdw toward antarctica to melt its floating ice shelves destabilize the glaciers and raise sea level

reconstruction of low temperature 100 c burial in sedimentary basins a comparison of geothermometer in the intracontinental paris basin abstract the thermal evolution of sediments in the shallowest part of sedimentary basins is challenging to investigate as most of the geothermometers are at their application limits this is typically the case of the callovian oxfordian claystones at the eastern border of the paris basin a recent 2000 m deep well penetrated the entire jurassic and triassic sedimentary series and permitted coring of sediments never reached by previous drilling in this part of the basin thanks to the use of several independent geothermometers apatite fission track thermochronology fluid inclusion microthermometry clay mineralogy and sedimentary organic matter evolution the burial history of the mesozoic series has been reconstructed modeled and calibrated using kinetic algorithms relevant to the various paleothermometers used in addition to these conventional methods an innovative approach was developed using the temperature dependence of the magnetic mineral assemblage within clay rich sediments mageval thermal modeling indicates that the callovian oxfordian and the lower triassic experienced maximum burial temperatures of 50 5 c and 90 10 c respectively i e about 25 c higher than present day this temperature offset implies the erosion of several hundred meters of sediments which depending on the thermal flux and conductivity considered most likely constituted upper cretaceous chalk deposits falling sea level resulted in the exposure and subsequent dismantling of the chalk cover at the beginning of cenozoic times the eastern border of the basin was then subjected to slow erosion and discrete tectonic deformation

age and origin of albian glauconites and associated clay minerals inferred from a detailed geochemical analysis abstract glauconitic grains are abundant in sandstones and siltstones from albian and cenomanian transition between lower and upper cretaceous formations of southeastern france morphological and crystal chemical data show that the green grains can be considered as glauconite s s with minute amounts of pyrite dolomite calcite and other clay minerals as grain coatings or crack fillings glauconite grains formed early during sedimentation generally from detrital chlorite biotite or clay particles and were found scattered together with other detrital mineral phases muscovite quartz in the silt in which porosity was considerably reduced afterwards by four stages of carbonate calcite dolomite cementation geochemical data especially the fe u and ce contents of the glauconitic grains strongly suggest increasing oxidizing conditions with time in the sedimentation environment the detailed geochemical analyses also allow the pristine glauconite grains to be distinguished from the other clay minerals occurring in the sequence this enabled k ar and rb sr dating of the glauconitic material which yields identical results of 97 9 0 4 and 97 9 3 5 ma respectively these dates point to a synsedimentary origin for the glauconites during the late albian about 97 98 ma the k ar data also indicate that the albian cenomanian boundary should be placed at 96 0 1 9 ma the similarity between the k ar and rb sr ages precludes any further significant pervasive recrystallization of these minerals during burial of the sequence either by heat flux or fluid flow the k ar study of the clay matrix from the same siltstones suggests synsedimentary precipitation of additional clay minerals probably together with the glauconite grains such as mixed layer illite smectite identified in the clay size fraction of the sedimentary succession

the crustal structure across the transform continental margin off ghana eastern equatorial atlantic forward modeling of a suite of wide angle seismic lines across the transform continental margin at the eastern end of the romanche fracture zone off ghana has shown the transition from continental to oceanic crust to be confined to a narrow 6 to 11 km wide zone located at the foot of the steep continental slope the structure of the adjacent oceanic and continental crusts has also been resolved these results are confirmed and enhanced by gravity and magnetic models the crust of the ocean continent transition zone is characterized by high velocities 5 8 7 3 km s 1 a high density 3 10 mg m 3 and high magnetizations 1 1 1 25 a m 1 this rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin the oceanic crust within 70 km of the margin is abnormally thin compared to normal atlantic oceanic crust and shows an average thickness of just 4 4 km we suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded on at least three sides by cold continental lithosphere the preferred seismic model also shows a new fracture zone 30 km southeast of the ocean continent transition which is characterized by low velocities in the upper crust a lens shaped layer with velocities of 7 2 7 4 km s 1 at the base of the crust and a crustal thickness of just 3 4 km the continental crust appears largely unaffected by the proximity of the adjacent oceanic crust there is no evidence for underplating of the continental crust adjacent to the ocean continent transition zone

thermal uplift and erosion across the continent ocean transform boundary of the southern exmouth plateau thermal evolution of the continental lithosphere at a continent ocean transform margin is examined using a two dimensional heat conduction model all heating is assumed to result from the emplacement of new oceanic ridge against the continent the assumed initial continental temperature gradient is probably best applied to regions having experienced finite duration periods of rifting lasting tens of millions of years a deep seismic reflection profile across the southern paleo transform margin of the exmouth plateau northwest australia tests the predictions of the model using this reflection profile it is estimated that up to 3 5 km of sediments have been eroded from the continental rim diminishing to almost no erosion at 60 km from the continent ocean transform boundary this trend and these values for erosion can be matched approximately using the model under the condition of local isostasy a finite difference scheme is employed where the surface elevation is the result of the competing 1 thermal uplift up 2 surficial erosion down and 3 local isostatic rebound up in response to the erosion the model predicts that most of the erosion ceases by 40 ma after ridge emplacement and that of the order of 1000 km3 eroded sediments are shed for every 10 km of transform length

strontium isotopic variations in jurassic and cretaceous seawater a high resolution seawater strontium isotope curve has been generated through the analysis of well dated and well preserved belemnites and oysters from the middle and upper jurassic and the lower cretaceous of great britain analysis of fe and mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered the strontium isotope curve remains relatively flat through the aalenian and early bajocian rapidly descends through the late bajocian and bathonian and reaches a minimum in the callovian and oxfordian it then begins a rapid increase in the kimmeridgian and portlandian that continues through much of the early cretaceous the curve levels off in the barremian suddenly dips downwards in the aptian and recovers gradually through the albian the strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of great britain using the techniques of strontium isotope stratigraphy model results indicate that much of the variation in seawater 87sr86sr between 120 and 40 ma can be explained by changing the intensity of mid ocean ridge hydrothermal fluxes proportionally to estimated mid ocean ridge crustal generation rates it is also possible that the variations during the rest of the mesozoic and the permian are primarily reflections of changing hydrothermal inputs the model results have several important implications first they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs second they suggest that from at least the aptian through the eocene variations in continental weathering were minimal this heightens the importance of the rapid rise in seawater 87sr86sr beginning 40 ma as a significant transition to an extended period of increasing fluvial 87sr fluxes continuing to the present finally the results suggest that several documented short term excursions towards lower 87sr86sr in the latest triassic pliensbachian toarcian callovian oxfordian aptian albian and cenomanian turonian are interpretable as pulses of seafloor hydrothermal activity if so the strontium isotope record offers a means of constraining the timing duration and magnitude of known or proposed hydrothermal events in the geological record

dynamic thinning of antarctic glaciers from along track repeat radar altimetry since 2002 the envisat radar altimeter has measured the elevation of the antarctic ice sheet with a repeat cycle of 35 days this long and regular time series is processed using an along track algorithm to depict in detail the spatial and temporal pattern of elevation change for the whole ice sheet we use this dataset to examine the spatial and temporal pattern of pine island glacier pig thinning and compare it to the neighbouring glaciers we also examine additional areas especially in east antarctica whose mass balance is poorly known one advantage of the finer along track spacing of measurements is that it reveals places of dynamic thinning in regions of rapid ice flow we observe the acceleration of thinning on pig over the entire basin the volume loss increased from 7 km3 a 1 during 2002 06 to 48 km3 a 1 during 2006 10 we also observe accelerated thinning on the lower tens of kilometres of thwaites glacier with a mean thinning of 0 18 m a 1 over its entire basin during our observation period we confirm the dynamic thinning of totten glacier but we do not detect significantly accelerated thinning on any glacier elsewhere than on the coast of the amundsen sea

envisat radar altimeter measurements over continental surfaces and ice caps using the ice 2 retracking algorithm abstract this study presents the first results obtained with the envisat satellite radar altimeter ra 2 over lands and ice caps as radar altimetry was firstly devoted to ocean study this paper starts with a detailed description of the radar altimetry principle over continental surfaces a general formulation of the radar altimeter echoes according to different surface types is given and the retracking procedure ice 2 specially implemented on the envisat satellite ground segment to provide output parameters over continents is clearly presented the envisat ra 2 datasets processed with the ice 2 procedure are then mapped given an overall description of the output parameters over various surfaces the behaviour of these envisat altimetric parameters are analysed and related to main land surface characteristics such as surface volume echo specularity roughness apart from few unexpected signals which will need to be further investigated the new envisat dataset confirms the potential of inland altimetry and provides a new opportunity to monitor continental surfaces the ku band measurements confirm previous results recently obtained with the altimeters on board ers or topex poseidon and offer a temporal continuity the s band altimeter measurements are in accordance with theoretical expected ones and confirm previous topex poseidon observations in c band the s band and the use of the dual frequency thus bring new capabilities for altimetric applications with important deliveries for temporal evolution survey for instance s band will be used over continents to better detect wetlands located under vegetated areas or to better quantify wave penetration in terrestrial snowpack to accurately retrieve snow depth evolution over ice sheets the dual frequency s and ku band waveforms parameters will provide a more accurate height estimation as it will be possible to quantify the error on height measurements induced by snowpack structure change over time this will then provide a more confident survey of the ice sheets surface topography evolution the combination and complementarities with the other altimetry missions such as ers topex poseidon and jason look already promising and offer a 10 year span dataset for continental altimetry applications

admittance signatures of rifted and transform margins examples from eastern canada examination of the gravity and bathymetry data across the rifted scotian margin and the sheared grand banks margin off the canadian east coast using a spectral analysis or admittance approach yielded different results the admittance for the scotian margin best fitted a regional compensation model in contrast a local compensation mechanism reproduced the transfer function for the grand banks margin the results for the longer wavelengths were verified using satellite altimetry data the effect of a hidden or subsurface load was investigated for both margins and this was also found to differ the differences in the compensation mechanism between rifted and sheared margins was attributed to their formation mechanisms

thermal evolution of a sheared continental margin insights from the ballenas transform in baja california mexico abstract the ballenas transform margin in central baja california offers an unparalleled opportunity to study the thermal behaviour of a sheared continental margin during various stages of its evolution apatite fission track and u th he results from two transects perpendicular to the coast reveal a pronounced latest pliocene to pleistocene 1 8 ma heating event related to the neogene opening of the gulf of california proximity to a regional pre rift unconformity indicates that samples remained at near surface levels since paleogene unroofing despite having experienced reheating to maximum paleotemperatures within or above the fission track partial annealing zone in general maximum paleotemperatures during overprinting decrease from 100 120 c near the coast to below 60 c ca 5 8 km further inland suggesting lateral heat flow from a source within the gulf of california heat sources related to the structural development of the ballenas transform fault located approximately 1 5 4 5 km offshore from the two sample transects most likely controlled the observed reheating overprinting patterns do not support conductive reheating due to reburial magmatism or frictional shear instead a pronounced thermal spike in between much less overprinted neighbouring samples strongly favours convective heating by hydrothermal fluids as the dominant overprinting process hydrothermal activity may be caused by either deep fluid circulation along newly formed shear zones of the transform fault or more likely magmatic leaking along the transform fault latest pliocene to pleistocene 1 8 ma activity on the ballenas transform fault is closely linked to extension in the lower and upper delfin basins and provides a minimum age for the structural reorganisation and the relocation of extension in the northern gulf of california this study shows that hydrothermal activity can cause significant thermal events in a transform margin before the passage of the spreading centre paleotemperatures from the ballenas transform are similar to those of other transform margins after passage of a spreading centre which suggests that hydrothermal fluids may have an important thermal buffering effect moderating the maximum temperatures of the ridge segments near their intersection with a continental transform margin

on the evolution of rifted continental margins comparison of models and observations for the nova scotian margin summary a model for the formation and evolution of rifted continental margins based on lithospheric extension during rifting and its thermal and mechanical consequences is proposed model predictions are then compared with geological and geophysical observations from a transect of the 185 ma old rifted margin off nova scotia through the scotian basin three kinematic models of the rifting process are discussed these are 1 the uniform extension model in which the amount of extension is uniform with depth but varies with position across the margin 2 the uniform extension and melt segregation model which has similar properties but also provides an explanation for the properties of the extended continental crust and its transition to oceanic crust by postulating that basaltic melt segregates from the asthenosphere and migrates to the crust and 3 the depth dependent extension model in which the first order consequences of rapidly changing rheological properties with depth in the lithosphere are included by decoupling the lithosphere into two zones with depth each of which undergoes differing amounts of extension these rift models predict the form of crustal and lithospheric thinning subsidence and temperature change once the amount of extension has been determined this is estimated from seismic measurements of present crustal thickness on the assumption that the crust had a uniform thickness equal to that currently measured in the adjacent continental region before rifting occurred rifting is also considered to occur instantaneously a time stepping thermo mechanical model is used to predict the cooling of rifted margin additional thermal contraction subsidence and its amplification by water and sediment loading thermal aspects are calculated using a finite difference model of time dependent conductive heat transport whereas regional isostatic response to loading is calculated by a finite element model the models are coupled because the temperature distribution is used to define a rheological lithosphere an elastic region with thickness that varies in time and space as the model evolves for flexural calculations in the mechanical model secondary coupling occurs through perturbations to the temperature field by sedimentary thermal blanketing and advection of heat during isostatic adjustment the model predictions of 1 sedimentary basin stratigraphy 2 moho position 3 free air gravity anomaly 4 age depth relations for deep exploratory wells and 5 subsidence and temperature histories agree well with observations from the scotian basin additional effects due to sedimentary and crustal radiogenic heat production lateral heat transport and the possible existence of a near surface brittle listric faulted region created during rifting are also considered it is concluded that a model in which rifting occurred by depth dependent extension and which includes a finite thickness for the rheological lithosphere radiogenic heat production in the sediments and crust and a brittle listric faulted crustal layer affords an accurate description of the first order processes that occur during rifting and evolution of this margin

extensional tectonics in the jeanne d arc basin offshore newfoundland implications for the timing of break up between grand banks and iberia

tectono sedimentary evolution and petroleum systems of the mundau subbasin a new deep water exploration frontier in equatorial brazil the brazilian equatorial margin bem evolved in response to transform motion between brazil and africa in 2012 petrobras drilled the pecem well in the mundau subbasin ceara basin of the bem to record the first deep water oil discovery in the region this work investigates the deep water evolution of the mundau subbasin focusing on its structural and sedimentary evolution and characterizes the petroleum systems in this new exploration frontier for such purposes poststack seismic reflection borehole and geochemical data were used three tectono stratigraphic sequences representing synrift mundau formation transitional paracuru formation and drift strata ubarana formation were divided into seven seismic units different tectonic domains were interpreted proximal distal and romanche fracture zone typical structures of transform margins such as marginal ridges and marginal plateaus were not identified in the mundau subbasin instead the subbasin was predominantly deformed by transtensional movements the mundau and paracuru formations are mature within the oil window whereas the ubarana formation is immature main reservoir intervals consist of approximately 1 m 3 28 ft thick intercalations of sandstone between shales siltstones and marls the seal rocks comprise shales in the ubarana formation whereas the hydrocarbon trap is related to an unconformity and a normal fault this work concludes that the paracuru formation is the main source and reservoir in the deep water mundau subbasin effectively comprising a paracuru paracuru petroleum system the results have significant implications for petroleum exploration in the bem by proposing a developed transitional petroleum system in the distal parts of northeastern brazil

refined analysis of radar altimetry data applied to the region of the subglacial lake vostok antarctica satellite altimetry is a powerful tool to map the ice sheet elevation as well as a number of other parameters linked to geometrical and geophysical properties of ice sheets irrespective of new instrumental developments like the laser altimeter on icesat ice cloud and land elevation satellite the well established radar altimeter ra missions ers 1 2 european remote sensing satellites and envisat environment satellite are unique in their temporal coverage over more than a decade and in their temporal and spatial sampling therefore the full exploitation of these ra data by improved methods is imperative here we develop improved techniques to correct for topographically induced errors by a refined consideration of the relevant topographic conditions furthermore we improve the gridding procedure by adapting it to local conditions and thus preserving smaller scale features we apply our methods to the region of the subglacial lake vostok antarctica and derive digital elevation models dems for this region with the aim of improving resolving smaller scales the effect of our improvements is demonstrated in detail by comparing our dems and previously published dems to the icesat laser measurements which are taken as a reference here due to our improvements the standard deviation of the difference between ra based dems of the lake vostok region and icesat data decreases from more than 1 1 m to 0 5 m this remaining error is chiefly the error inherent in the ra observations our ra icesat comparisons supported by fourier analyses also reveal the presence and importance of small scale features that can be detected by laser but not by the ra measurements

crustal structure and transform margin development south of svalbard based on ocean bottom seismometer data abstract the barents sea is located in the northwestern corner of the eurasian continent where the crustal terrain was assembled in the caledonian orogeny during late ordovician and silurian times the western barents sea margin developed primarily as a transform margin during the early tertiary in the northwestern part south of svalbard multichannel reflection seismic lines have poor resolution below the permian sequence and the early post orogenic development is not well known here in 1998 an ocean bottom seismometer obs survey was collected southwest to southeast of the svalbard archipelago one profile was shot across the continental transform margin south of svalbard which is presented here p wave modeling of the obs profile indicates a caledonian suture in the continental basement south of svalbard also proposed previously based on a deep seismic reflection line coincident with the obs profile the suture zone is associated with a small crustal root and westward dipping mantle reflectivity and it marks a boundary between two different crystalline basement terrains the western terrain has low 6 2 6 45 km s 1 p wave velocities while the eastern has higher 6 3 6 9 km s 1 velocities gravity modeling agrees with this as an increased density is needed in the eastern block the s wave data predict a quartz rich lithology compatible with felsic gneiss to granite within and west of the suture zone and an intermediate lithological composition to the east a geological model assuming westward dipping caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust as well as the observed structuring due to the transform margin setting the tectonic thinning of the continental block during opening of the norwegian greenland sea is restricted to the outer 35 km of the continental block and the continent ocean boundary cob can be located to within 5 km in our data distinct from the outer high commonly observed on transform margins the upper part of the continental crust at the margin is dominated by two large rotated down faulted blocks with throws of 2 3 km on each fault apparently formed during the transform margin development analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile and that the downfaulting dominates the whole margin segment from spitsbergen to bjornoya south of bjornoya the faulting leaves the continental margin to terminate as a graben 75 km south of the island adjacent to the continental margin there is no clear oceanic layer 2 seismic signature however the top basement velocity of 6 55 km s 1 is significantly lower than the high 7 km s 1 velocity reported earlier from expanding spread profiles esps and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7 8 km thick sedimentary overburden

ice sheet elevation change in west antarctica from ka band satellite radar altimetry satellite altimetry has been used to track changes in ice sheet elevation using a series of ku band radars in orbit since the late 1970s here we produce an assessment of higher frequency ka band satellite radar altimetry for the same purpose using saral altika measurements recorded over west antarctica altika elevations are 3 8 0 5 and 2 5 0 1 m higher than those determined from airborne laser altimetry and cryosat 2 respectively likely due to the instruments coarser footprint in the sloping coastal margins however altika rates of elevation change computed between 2013 and 2019 are within 0 6 2 4 and 0 1 0 1 cm year of airborne laser and cryosat 2 respectively indicating that trends in radar penetration are negligible the fast flowing trunks of the pine island and thwaites glaciers thinned by 117 10 and 100 20 cm year respectively amounting to a 9 reduction and a 43 increase relative to the 2000s

the yermak plateau formed at a triple junction the yermak plateau is an arcuate marginal plateau located seaward of northwestern spitsbergen the plateau is divided into two zones based on bathymetric trends and magnetic characteristics that have roughly coincident boundaries north of latitude 82 n where the bathymetric contours trend ne the magnetic amplitudes are large whereas the southern portion strikes nnw and exhibits a subdued magnetic signature beneath the northern section of the plateau crustal refraction velocities of 5 0 km s 1 underlain by 7 2 km s 1 are recorded while farther south velocities of 4 3 6 0 and 8 0 km s 1 are measured the 6 0 km s 1 velocity is similar to ones reported on the continental shelf south of spitsbergen dredged material gravity heat flow and seismic reflection and refraction data indicate the northern region of the plateau is oceanic and formed in conjunction with the morris jesup rise in contrast the southern section is thinned continental crust formed by the initial rifting process at a triple junction north of greenland plate reconstructions for anomalies 24 and 13 are consistent with this interpretation little space is required for the yermak plateau at anomaly 24 because it is composed of oceanic and thinned continental crust the development of the yermak plateau and the morris jesup rise paired aseismic ridges took place when the north american eurasian and greenland plates were moving independently

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

greenland 2012 melt event effects on cryosat 2 radar altimetry cryosat 2 data are used to study elevation changes over an area in the interior part of the greenland ice sheet during the extreme melt event in july 2012 the penetration of the radar signal into dry snow depends heavily on the snow stratigraphy and the rapid formation of refrozen ice layers can bias the surface elevations obtained from radar altimetry we investigate the change in cryosat 2 waveforms and elevation estimates over the melt event and interpret the findings by comparing in situ surface and snow pit observations from the north greenland eemian ice drilling project camp the investigation shows a major transition of scattering properties around the area and an apparent elevation increase of 56 26 cm is observed in reprocessed cryosat 2 data we suggest that this jump in elevation can be explained by the formation of a refrozen melt layer that raised the reflective surface introducing a positive elevation bias

a deep seismic transect from hovgard ridge to northwestern svalbard across the continental ocean transition a sheared margin study new seismic refraction data were collected across the western svalbard continental marginoff kongsfjorden ny lesund during the cruise leg ark15 2 of rv polarstern the use ofon and offshore seismic receivers and a dense airgun shot pattern provide a detailed viewof the velocity structure of svalbardos continental interior the continent ocean transition and oceanic crust related to the northern knipovich ridge and the molloy ridge the caledonian central and western terranes of svalbard are not distinguishable on the basisof seismic velocity structure below a 7 to 8 km thick paleozoic sedimentary cover the crystallinecrust reveals a three layer structure with seismic velocities ranging between 6 1 and 6 9 km s the geological suture between the terranes is imperceptible the middle and upper crust belowthe tertiary forlandsundet graben shows striking low velocities this can be related to theearly paleozoic convergent transpressive movements between svalbard and northern greenland followed by an extensional relaxing phase we argue that a brittle fractured rock formation ispresent below the graben which also buries a sedimentary paleozoic core the continent ocean transition can be classified as a sheared margin formed at the continentalpart of the spitsbergen fracture zone moderate crustal thinning is achieved only to the westof the low velocity zone below the forlandsundet graben this leads to the assumption thattranstensional rift movements since oligocene were decoupled from the central terraneof svalbard the moho dips with an angle of 45 eastwards at the continent ocean transitionthat exhibits higher seismic velocities of 7 2 km s on the continental side these can beinterpreted as minor mantle derived intrusions probably induced by convection due to thejuxtaposition of cool continental and hot oceanic lithosphere the oceanic crust generated at the knipovich ridge and the molloy ridge is thin 2 to 4 km compared to the global mean and is characterised by the absence of oceanic layer 3 theseobservations can be ascribed to conductive cooling of the ascending mantle due to theextremely low divergence rate and the neighbouring cool continental crust the underlyingmantle is slightly serpentinized below the knipovich ridge segment reflected by low seismicvelocities of 7 7 km s a thicker sequence of syn and post rift sediments and sedimentaryrocks are observed on the molloy ridge oceanic segment which results from greater subsidencerelative to the knipovich ridge segment

antarctic elevation change from 1992 to 1996 satellite radar altimeter measurements show that the average elevation of the antarctic ice sheet interior fell by 0 9 0 5 centimeters per year from 1992 to 1996 if the variability of snowfall observed in antarctic ice cores is allowed for the mass imbalance of the interior this century is only 0 06 0 08 of the mean mass accumulation rate

episodic mineralization of hydrothermal illite in the soultz sous forets granite upper rhine graben france episodic and localized illite mineralization is documented in the hydrothermally altered soultz sous forets granite upper rhine graben france separated grain size fractions of altered granite and argillite vein samples contain mixtures of 2m1 and 1m trans vacant illite varieties the platy pseudohexagonal 2m1 illite phases dominate the vein fillings whereas the 1m illite occurs largely as a fibrous pore filling variety which is particularly abundant in the granite matrix multiple phases of fluid injections into the granite body have resulted in different illite assemblages each sample containing a mixture of polytype generations formed during different crystal growth events on the basis of mineralogical and k ar isotopic constraints the ages of these vein mineralizing events are determined by plotting the k ar values of the various grain size fractions against polytype abundance and the fitted volume weighted crystallite thickness distributions the results suggest a permian age for the formation of the studied argillite veins characterized by successive injections of hydrothermal fluids secondary episodes of illite crystallization occurred during jurassic and cretaceous or even younger times in both the veins and the granite matrix there are indications that the polytype structure and composition of illite were strongly influenced by variations in fluid chemistry and the degree of fluid rock interaction as the granite was progressively sealed during post variscan episodic hydrothermal activity

strontium distribution and origins in a natural clayey formation callovian oxfordian paris basin france a new sequential extraction procedure strontium is a good monitor of geochemical processes in natural clayey formations in the callovian oxfordian formation of bure in france strontium is sorbed on clay minerals and carried by carbonates detrital minerals and accessory celestite in order to determine the strontium distribution among these different phases four step sequential extractions 1 cobalt hexamine trichloride 2 acetic acid 3 edta and 4 tri acid were performed on samples from different levels of the clayey formation the leachates were also analyzed for strontium isotopes in order to determine the strontium origins this sequential procedure is well suited to determining strontium distribution in claystones although it is less efficient in clay rich limestones and in celestite rich samples the carbonates 38 47 of the total strontium show 87sr 86sr ratios 0 7070 0 7071 that have recorded the isotopic composition of the callovian oxfordian seawater diagenetic carbonates dolomite ankerite and siderite have almost not incorporated any strontium which has been trapped by celestite during the late diagenesis the major part of the celestite shows 87sr 86sr ratios 0 7069 0 7070 quite close to the primary carbonates however a second generation of celestite 0 7074 shows a slight 87sr enrichment and is isotopically in equilibrium with the exchangeable strontium 27 48 of the total strontium with a mean 87sr 86sr value of 0 70745 and the present day porewater 0 7074 this very low 87sr enrichment could be explained by the partial destabilisation of detrital minerals feldspars micas clays which exhibit 87sr 86sr ratios consistent with their pristine hercynian origin 0 7229 0 7350 diffusion of strontium from the subjacent dogger aquifers 0 7076 0 7082 could also be invoked to explain the slight 87sr enrichment

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

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

development of an antarctic digital elevation model by integrating cartographic and remotely sensed data a geographic information system based approach we present a high resolution digital elevation model dem of the antarctic it was created in a geographic information system gis environment by integrating the best available topographic data from a variety of sources extensive gis based error detection and correction operations ensured that our dem is free of gross errors the carefully designed interpolation algorithms for different types of source data and incorporation of surface morphologic information preserved and enhanced the fine surface structures present in the source data the effective control of adverse edge effects and the use of the hermite blending weight function in data merging minimized the discontinuities between different types of data leading to a seamless and topographically consistent dem throughout the antarctic this new dem provides exceptional topographical details and represents a substantial improvement in horizontal resolution and vertical accuracy over the earlier continental scale renditions particularly in mountainous and coastal regions it has a horizontal resolution of 200 m over the rugged mountains 400 m in the coastal regions and approximately 5 km in the interior the vertical accuracy of the dem is estimated at about 100 130 m over the rugged mountainous area better than 2 m for the ice shelves better than 15 m for the interior ice sheet and about 35 m for the steeper ice sheet perimeter the antarctic dem can be obtained from the authors

inland thinning of pine island glacier west antarctica the pine island glacier pig transports 69 cubic kilometers of ice each year from 10 of the west antarctic ice sheet wais it is possible that a retreat of the pig may accelerate ice discharge from the wais interior satellite altimetry and interferometry show that the grounded pig thinned by up to 1 6 meters per year between 1992 and 1999 affecting 150 kilometers of the inland glacier the thinning cannot be explained by short term variability in accumulation and must result from glacier dynamics

diagenetic evolution of clay minerals in oil bearing neogene sandstones and associated shales mahakam delta basin kalimantan indonesia the clay fraction of the mahakam delta basin eastern kalimantan indonesia consists of mixed layer illite smectite kaolinite dickite detrital illite and chlorite kaolinite formed early as vermicular particle aggregates and thick dickite booklets formed late in pore spaces the dickite particles probably grew in deeper intervals of the sequence in contact with brines acidified by decarboxylation of organic matter during dissolution of k feldspar and upward migration of potassium in the handil field on the nearshore anticline evolution of the mixed layer illite smectite is characterized by a decrease in expandability with depth in the tunu field located on the next offshore anticline variable amounts of expandable layers suggest a different evolution the conversion process of smectite to illite layers in the mixed layers depends on the lithology of the host rocks transformation in the shales and dissolution precipitation in the sandstones sample location also played a role in the conversion process because the types of illite clays in the two locations differ when they are compared at similar degrees of evolution present day temperatures and paleotemperatures vitrinite reflectance and illitization of the mixed layer illite smectite all suggest that organic matter and clay particles evolved differently in the handil field but similarly in the tunu field integration of the analytical results obtained here and comparison with a two dimensional numerical model suggest that hydrocarbon generation took place in the deeper synclinal zones of the sequence and that oil migrated upward with brines probably inducing most of the illitization in the upper sequence both models emphasize the role of faults in channelizing fluid flow during the final stages of evolution of the basin

chlorine transfer out of a very low permeability clay sequence paris basin france 35cl and 37cl evidence abstract the marine callovo oxfordian clay formation is found at a depth around 410m in the eastern part of the paris basin france it is a very low permeability formation investigated by the french agency for nuclear waste management andra to study the feasibility of a radioactive waste disposal examining hydrogeological and geochemical characteristics of the clay sequence may test confinement properties of this formation this study uses chlorine isotopes to investigate long term transport processes which may carry chemical elements out of the clay layer to the surrounding rocks detailed chlorine concentration and 37cl depth profiles are examined using pore waters and aquifer waters sampled in the clay formation and its surrounding aquifers the dogger at the bottom and the oxfordian kimmeridgian tithonian unit at the top they are discussed in terms of chlorine budget and hydrogeological processes clay pore waters and aquifer waters show strong chlorine concentration depletion a steep cl concentration gradient from the dogger at 500m in depth 2500 mg l to the oxfordian kimmeridgian tithonian aquifer near the surface 10 mg l is associated to a v shaped profile of the 37cl values modelling cl transport shows that a hydrodynamic dispersion process explains cl concentration and 37cl profiles in oxfordian limestone this process implies a mean upward flux of chloride in the 2 6 10 8 8 2 10 8 mole m2 yr range from the clay formation towards upper limestones where a westward advective flow disperses the chloride the modelling and knowledge of underground water transfer suggest a maximum effective cl hydrodynamic vertical dispersion coefficient vertical cl transport coefficient of 7 6 10 10 m2 s chlorine transfer through the callovo oxfordian clay since deposition 160my ago can be mainly described by the interplay of an early dilution and a later hydrodynamic dispersion event which has apparently erased most of the isotopic effects of diagenetic events such as early diffusion ion filtration etc

chemical and isotopic characterization of hydrocarbon gas traces in porewater of very low permeability rocks the example of the callovo oxfordian argillites of the eastern part of the paris basin abstract a methodology has been developed to determine chemical and carbon isotopic compositions of trace amounts of hydrocarbon gas compounds methane ethane propane iso and normal butane present as dissolved compounds in the porewater of the low permeability callovo oxfordian argillites in eastern paris basin france results indicate that the studied hydrocarbons contain significant amounts of ethane butane and propane in addition to methane carbon isotopic compositions reflect primarily thermogenic origin thermal cracking of organic matter and lack of any significant biodegradation because temperature did not exceed 50 c in the studied argillites investigated hydrocarbons must have originated in hotter deeper organic bearing formations possibly stephanian coals data supports the predominance of high maturity thermogenic gas in the upper part of the callovo oxfordian and low maturity thermogenic gas mixed with minor bacterially produced methane in the lower part of the formation a mixing between three end member gases models quite well the data one thermogenic gas with a low maturity 42 methane with a 13 c of 53 a gas with higher maturity 55 methane with a 13 c of 47 and a bacterial gas 99 45 methane with a 13 c of 80 this study illustrates that migration of hydrocarbon gases can take place in rocks with very low permeability and porosity such as compacted mudrocks given enough time it further suggests that the studied fluid migration and transfer in aquitards would help characterization and understanding of fluid movements in sedimentary basins as a complement to studies focused on water aquifers and hydrocarbon reservoirs chemical and isotopic composition of dissolved hydrocarbons in porewater can be used as natural tracers of fluid circulation in sedimentary basins in addition to more conventional tracers

implications of changing scattering properties on greenland ice sheet volume change from cryosat 2 altimetry abstract long term observations of surface elevation change of the greenland ice sheet gris is of utmost importance when assessing the state of the ice sheet satellite radar altimetry offers a long time series of data over the gris starting with ers 1 in 1991 esa s cryosat 2 mission launched in 2010 provides an invaluable radar altimetry dataset for monitoring the current changes of the ice sheets due to its dense spatial and temporal coverage of these areas here we investigate the effects of including different parameters which describe the shape of the return radar waveform waveform parameters in the elevation change algorithm to correct for temporal changes in the ratio between surface and volume scatter in cryosat 2 observations we present elevation and volume changes for the greenland ice sheet in the period from 2010 until 2014 the waveform parameters considered here are the backscatter coefficient and the leading edge width which are both available in the esa cryosat 2 level 2i data product investigations into relocation of radar reflection points are also included inter comparison of the cryosat 2 derived elevation changes with those derived from operation icebridge laser data suggests waveform parameters to be applicable for correcting for changes in volume scattering the best results in the synthetic aperture radar interferometric mode area of the gris are found when applying only the backscatter correction whereas the best result in the low resolution mode area is obtained by only applying a leading edge width correction using this approach to correct for the scattering properties a volume loss of 292 38 km 3 yr 1 is found for the gris for the time span november 2010 until november 2014 the inclusion of waveform parameter corrections and improved relocation for the gris helps to reconcile the satellite derived elevation changes with those observed by operation icebridge however the bias of temporal changes in the scattering horizons of cryosat 2 is not entirely removed and suggests that future improvements could be made by including climate data and or additional waveform parameters to make additional corrections in the cryosat 2 radar altimetry

water circulation control on carbonate δ18o records in a low permeability clay formation and surrounding limestones the upper dogger oxfordian sequence from the eastern paris basin france abstract upper dogger to oxfordian formations in the eastern part of the paris basin france are currently being investigated by the french nuclear waste management agency andra testing the feasibility of long term deep nuclear waste disposal in the callovo oxfordian claystones characterising the hydrogeological behaviour of the callovo oxfordian claystones is therefore essential in evaluating its potential as a geological barrier in order to evaluate and quantify water rock interactions experienced over geological time by these formations bulk carbonate 13 c and 18 o were measured and calculations of water rock ratios were used to explain carbonate 18 o changes meteoric porewater and a maximum temperature reached of about 40 c were considered the jurassic marine carbonate 13 c was preserved in the callovo oxfordian claystones and in the overlying limestones 0 28 to 3 39 pdb while the 18 o values are lower by 0 5 6 25 to 1 32 pdb calculations show that upper dogger and oxfordian limestone 18 o data i have random like distribution through theoretical 18 o w r curves and ii suggest that water rock ratios 0 08 0 4 needed to explain 18 o changes are higher by a factor of about 2 20 compared to the present day water rock ratio these features indicate advection in both aquifers according to the history of the paris basin this hydrogeological behaviour could have been effective since jurassic cretaceous transition times inversely the carbonate 18 o content trends observed for the callovo oxfordian data show that changes were controlled by post depositional fluid rock interaction with water rock ratio 0 02 0 15 similar to the present day porewater rock ratio the 130 m thick callovo oxfordian claystone appears remarkably homogenous regarding its hydrogeological properties this study suggests an initial marine porewater replacement by meteoric water only after porosity was reduced to its present value thus demonstrating that the callovo oxfordian clay has mainly been isolated from advective meteoric water circulation only the upper 20 m of the callovo oxfordian claystone formation underwent heterogeneous water rock exchange w r from 0 01 to 0 3 probably as a result of its mineralogical heterogeneity and proximity to the advective oxfordian limestone aquifer this study of carbonate 18 o confirms the hydrogeological barrier properties of the callovo oxfordian clay and suggests that it has been a natural hydrological barrier since the earliest times of its diagenesis

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

discrete clay diagenesis in a very low permeable sequence constrained by an isotopic k ar and rb sr study multimethod analyses of several size fractions of clays were used to reconstruct the diagenetic history of the shallow buried claystones within the paris basin a systematic decrease occurred in k ar dates relative to the decrease in size of the clay fractions signifying higher amounts of newly formed clay material in the finer fractions we suggest that the authigenic clay minerals occurring in the fine fractions had an al montmorillonite composition by assuming that the isotopic k ar and rb sr dates obtained on a bentonite layer in the sedimentary sequence stand for pure authigenic clay minerals one may interpret all k ar dates as mixtures of one authigenic and two detrital end members the results imply that a period of low sea level favoured diagenetic smectite type clay formation about 10 15 million years after deposition of the sediments signatures of limited scale chemical and isotopic homogenisation mean that the rock volumes affected by the diagenetic modifications had to be quite limited the study of clay minerals extracted from some stylolites further suggests that any overpressure related to the origin of stylolites had no effect on clay authigenesis

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

carbon cycle and sea water palaeotemperature evolution at the middle late jurassic transition eastern paris basin france abstract a very high resolution carbon and oxygen stable isotope analysis bulk carbonate of a biostratigraphically well constrained callovian oxfordian series is provided here for the first time the homogeneity of the clayey series and the weak diagenetic alteration allow the isotopic signal variations to be considered as primary in origin a prominent and brief negative excursion in the 13c curve 2 occurring at the start of the middle callovian jason zone obductum subzone and correlated regionally suggests a possible methane release the increasing 13c values thereafter up to the early oxfordian concomitant with a warming episode highlight the burial of carbon in organic rich layers which in return may have triggered a decrease in atmospheric pco2 at higher frequencies observed fluctuations of the 13c and 18o values are orbitally driven 405 kyr and 100 kyr eccentricity cycles and may correspond to the salinity and temperature variability recorded in sea water the 18o isotopic measurements from well preserved diagenetically screened belemnites and bivalves along the series compared to available data from tethyan domains agree with the scenario of a global cooling at the middle late jurassic transition the well dated 18o isotopic curve suggests that the onset of this cooling event occurred at the end of the coronatum zone middle callovian

demerara plateau the structure and evolution of a transform passive margin summary the amazon cone experiment acquired two transects profile sd a across the demerara plateau part of the french guiana northeast brazil continental margin to better understand rift and transform style margin evolution profile a images an intermediate type margin formed as a result of trans tensional extension in this paper we describe the modelling of wide angle and multichannel seismic and gravity data from profile d to reveal whole crustal structure and features exhibiting transform characteristics combining these results with other studies in the region and comparing our results with young rift analogues we develop a model of along margin segmentation that explains the evolution of the west equatorial atlantic interpretation of the velocity depth model for profile d shows a 35 37 km thick continental crust which thins to 10 11 km over a distance of 320 km this thinning is accommodated in two regions the narrowest region associated with the ocean continent transition is interpreted to have formed by dextral shearing of the margin along major transform zones that accommodated the initial break up geometry of the central atlantic given the orientation of the margin relative to local fracture zone traces it is likely that the second region of thinning located 162 km landward of the ocean continent transition is a result of rifting suborthogonal to the profile orientation there is no evidence of rotated faulted blocks half graben structures or riftrelated magmatism manifest as either seaward dipping reflectors or as a high velocity region within the lower crust the demerara plateau is therefore interpreted as a margin segment comprising thinned continental crust bound to the north and south by transform type zones in which trans tensional extension is accommodated in contrast to profile a modelling suggests that the eastern margin exhibits a relatively broad region of crustal thinning associated with extension consistent with a rift type setting offshore unusually thin oceanic crust of 3 3 5 7 km thickness is identified which is consistent with similar observations further south in the absence of identifiable magnetic anomalies best estimates of the initial half spreading rate of 20 mm yr 1 suggest that the thin crust throughout the region is unlikely to be a result of ultra slow spreading but instead it is most likely due to a reduced magma supply at numerous long lived transform faults reflected by those presently offsetting the mid atlantic ridge in this equatorial setting

inland thinning of the amundsen sea sector west antarctica 1 together with the pine island glacier pig the thwaites tg and smith sg glaciers are the principal drainage systems of the amundsen sea as sector of western antarctica here we use satellite radar altimetry and interferometry to show that a rapid thinning of ice has occurred within the fastest flowing sections of all as outlet glaciers the pattern of thinning extends to distances greater than 150 km inland between 1991 and 2001 the tg and sg thinned by more than 25 and 45 m at their grounding lines and a total of 154 16 km 3 of ice or 0 43 mm of eustatic sea level rise was lost from the as sector glaciers to the ocean we show that the thickness changes may have caused the pig tg and sg to retreat inland by over 8 4 and 7 km respectively in line with independent estimates of grounding line migration

a refraction seismic transect from the faroe islands to the hatton rockall basin 1 the crustal structure of the faroe rockall plateau was studied by a 790 km long refraction seismic transect consisting of two intersecting lines the air gun shot spacing was 200 m and the signals were recorded by 77 ocean bottom seismometers a p wave velocity model was developed from forward and inverse modeling of the wide angle seismic data and incorporation of coincident multichannel reflection seismic data continental crust with velocities ranging from 5 6 to 6 8 km s can be traced from the faroe islands across the banks to the sw of the faroes and into the hatton rockall basin the thickness of the subvolcanic crust is up to 25 km on the banks but is as little as 8 km in the channels between the banks the thinning in the channels may be related to nw trending shear zones extending from major lineaments in ne rockall trough basalt layers are found along the entire transect with a total thickness of up to 4 km two layers with velocities of 4 9 5 2 and 5 3 5 6 km s are thought to represent paleogene flood basalts that can be correlated from the faroe islands to george bligh bank close to george bligh bank an 80 km wide and up to 9 km thick body with velocities of 6 5 km s is interpreted as intrusion a 5 km thick high velocity lower crustal layer 7 3 km s extends from the area of the intrusion into the northern hatton rockall basin at the northern flank of lousy bank the transition zone to oceanic crust was encountered

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

seismic investigation of an active ocean continent transform margin the interaction between the swan islands fault zone and the ultraslow spreading mid cayman spreading centre

margin segmentation of baffin bay davis strait eastern canada based on seismic reflection and potential field data abstract the continental margin of baffin island was interpreted from a compilation of seismic reflection and potential field data based on these data it was divided into volcanic and non volcanic segments in the northernmost area coast parallel grabens and continental basement highs are superseded beneath the slope by irregular basement topography in combination with the velocities from refraction data and the lack of a magnetic slope anomaly we propose a non volcanic margin in the central region the seismic data show a limited region with a seaward dipping reflector sequence that may be intercalated with a sedimentary section both north and south of cape dyer extensive volcanic rocks are interpreted at and below seabed consistent with a volcanic margin in the southern region the depth to basement drops rapidly offshore a thick prograding sedimentary wedge produces a high amplitude long wavelength free air gravity anomaly that was previously confused with the continent ocean boundary although some volcanic rocks are mapped in the region this area is adjacent to the non volcanic labrador sea margin and is interpreted to be of similar structure segmentation of the margin by major faults can explain the abrupt transitions from volcanic to non volcanic margin segments the continent ocean boundary for the region is determined based on the potential field and seismic data along the greenland plate in davis strait the continent ocean boundary coincides with the ungava fault zone the continent ocean boundary is used in conjunction with published poles of rotation to evaluate plate reconstructions at chrons 33n and 27n the earlier reconstruction produces an extensive overlap of boundaries in the north the later reconstruction shows a gap in the southern region thus additional information is required to refine the reconstructions

uneven onset and pace of ice dynamical imbalance in the amundsen sea embayment west antarctica we combine measurements acquired by five satellite altimeter missions to obtain an uninterrupted record of ice sheet elevation change over the amundsen sea embayment west antarctica since 1992 using these data we examine the onset of surface lowering arising through ice dynamical imbalance and the pace at which it has propagated inland by tracking elevation changes along glacier flow lines surface lowering has spread slowest 6 km yr along the pope smith and kohler psk glaciers due to their small extent pine island glacier pig is characterized by a continuous inland spreading of surface lowering notably fast at rates of 13 to 15 km yr along tributaries draining the southeastern lobe possibly due to basal conditions or tributary geometry surface lowering on thwaites glacier thg has been episodic and has spread inland fastest 10 to 12 km yr along its central flow lines the current episodes of surface lowering started approximately 10 years before the first measurements on psk around 1990 on pig and around 2000 on thg ice dynamical imbalance across the sector has therefore been uneven during the satellite record

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

fold dating a new ar ar illite dating application to constrain the age of deformation in shallow crustal rocks we propose a deformation dating method that combines xrd quantification and ar chronology of submicroscopic illite to determine the absolute ages of folds that contain clay bearing layers two folds in the frontal segment of the mexican fold thrust belt mftb which was deformed from late cretaceous to eocene are used to illustrate the method and its future potential variations in mineral composition illite polytype crystallite size cs and ar total gas ages were analyzed in the limbs and hinge of two mesoscopic folds this analysis examines potential effects of strain variation on illitization and the ar isotopic system along folded layers versus possible regional thermal overprints the ar total gas ages for 9 samples in fold 1 vary between 48 4 and 43 9 ma the of 2m1 detrital illite vs ar total gas ages tightly constrains the age of folding at 43 5 0 3 ma nine ages from three samples in fold 2 range from 76 2 to 62 7 ma which results in a folding age of 63 9 2 2 ma both ages are in excellent agreement with more broadly constrained stratigraphic timing the method offers a novel approach to radiometric dating of clay bearing folds formed at very low grade metamorphic conditions and has the potential to constrain dates and rates of regional and local deformation along and across foreland orogenic belts

effects of lithospheric flow on the formation and evolution of a transform margin a simple model is developed for the processes occurring at a transform continental margin and its predictions are compared with observation the model emphasises the importance of ductile flow in the lithosphere near the transform at initiation within continental lithosphere the upper part of the crust is assumed to be brittle while the lower crust and mantle behave as a newtonian fluid shearing along the transform induces a flow parallel to the direction of motion with the approach and passage of the abutting spreading centre flow beneath the continental side of the transform slows and stops this results in thinning of the lower crust and lithosphere as material is dragged away by the moving oceanic plate the thinning produces an initial thermal uplift combined with isostatic subsidence followed by thermal relaxation and subsidence at and near the margin the model is developed for uniform viscosity and for viscosity that decays exponentially with depth the results are found to be generally consistent with the observed structure at transform margins in particular at the southwest newfoundland transform margin it is suggested that the newfoundland ridge and analogous features may be formed partly from continental crust emplaced by this mechanism

geochronology and mass transfer in gulf coast mudrocks south central texas u s a rbsr smnd and ree systematics whole rock shale and 0 5 m sized clay separates from a well spanning 3500 m of burial in south central texas have been analyzed for there ree sm nd and rb sr contents clay rb sr and sm nd hcl leachate residue pairs yield ages which decrease with depth implying that these samples have not experienced a punctuated diagenetic event rb sr ages from the most deeply buried clay samples are younger than their depositional ages but sm nd leachate residue ages are always equal to or greater than depositional age leachate sr appears to be in isotopic equilibrium with present day sandstone formation waters implying that pore fluids readily pass between the two lithologies during and after diagenesis the ree are redistributed and fractionated on a local mineral grain scale during burial diagenesis the hcl soluble ree reservoirs decrease from 20 to 8 in whole rock and from 35 to 7 in 0 5 m clay and become progressively enriched in the middle ree relative to the light and heavy ree conversely the hcl insoluble silicate reservoirs become progressively enriched in the light and heavy ree relative to the middle ree this fractionation which is more pronounced in the clay than in the whole rock appears to be a result of authigenic illite and phosphate formation during burial diagenesis although the ree are redistributed on a local scale there is little evidence that the ree are gained or lost by the mudrocks during diagenetic redistribution combined nd and sr isotopic data show that the whole rock shale system may be modelled as a three component system composed of young volcanic derived and old craton derived detritus and a seawater derived component these three components are partially homogenized during diagenesis and as a result the sr nd provenance information originally held in the silicate fraction of whole rock shale becomes obscured

changes in west antarctic ice stream dynamics observed with alos palsar data the advanced land observation system alos phased array synthetic aperture radar palsar is an l band frequency 1 27 ghz radar capable of continental scale interferometric observations of ice sheet motion here we show that palsar data yield excellent measurements of ice motion compared to c band 5 6 ghz radar data because of greater temporal coherence over snow and firn we compare palsar velocities from year 2006 in pine island bay west antarctica with those spanning years 1974 to 2007 between 1996 and 2007 pine island glacier sped up 42 and ungrounded over most of its ice plain smith glacier accelerated 83 and ungrounded as well their largest speed up are recorded in 2007 thwaites glacier is not accelerating but widening with time and its eastern ice shelf doubled its speed total ice discharge from these glaciers increased 30 in 12 yr and the net mass loss increased 170 from 39 15 gt yr to 105 27 gt yr longer term velocity changes suggest only a moderate loss in the 1970s as the glaciers unground into the deeper smoother beds inland the mass loss from this region will grow considerably larger in years to come copyright 2008 by the american geophysical union

deep seismic transects across the sheared western barents sea svalbard continental margin abstract we have used recently acquired deep seismic reflection and refraction profiles together with a large amount of conventional mcs data to study the 1000 km long sheared western barents sea svalbard margin with the objective of determining its crustal structure and tectonic history the margin developed from a megashear zone which linked the norwegian greenland sea and the eurasia basin during the eocene opening the margin is composed of two large shear segments and a central rifted margin segment associated with volcanism the continent ocean boundary is confined within a narrow zone 10 20 km at the sheared margin segments but is more obscure and partly masked by volcanics at the rifted margin the crustal structure and tectonic history differ significantly between the three margin segments in the north across the hornsund fault zone the crustal thickness changes abruptly from more than 30 km thick continental crusy on the svalbard platform to 6 8 km thick oceanic crust in the greenland sea this margin segment experienced oblique continent continent and partly continent ocean shear with both transtensional and transpressional component during eocene time while post eocene extension caused crustal stretching west of svalbard the volcanic province at the central rifted margin segment probably comprises an outer part of thick oceanic crust and an inner part of thinned continental crust covered by a mixture of volcanics and sediments this formed at a releasing bend in the margin in response to eocene oligocene extension and two phases of volcanism in the south the crustal thinning occurs over a wider area landward of the senja fracture zone here the margin bounds a basinal province in which as much as 18 20 km of sediments cover a highly attenuated crystalline crust the senja fracture zone evolved due to eocene transtensional continent continent shear followed by continent ocean shear and has been passive since earliest oligocone time we conclude that the first order crustal structure along the margin and its tectonic development is mainly the result of three controlling parameters 1 1 the pre breakup structure 2 2 the geometry of the plate boundary at opening and 3 3 the direction of relative plate motion the interplay between the direction of plate motion and the geometry of the plate boundary gave rise to striking differences in the structural development of the different margin segments

transform tectonics and thermal rejuvenation on the cote d ivoire ghana margin west africa formation of a pronounced basement ridge along many transform continental margins has been attributed to a variety of processes during continental break up including transpressional crustal thickening and thermal rejuvenation and igneous underplating during passage of a spreading ridge odp drill holes on the cote d ivoire ghana margin now provide the first opportunity to quantify the vertical motions along this type of margin apatite fission track dating of detrital sands suggests that large amounts of erosion occurred on the flanks of an intra continental wrench zone that predated margin formation rapid cooling of 120 c at 120 115 ma corresponds to erosion of 3 5 5 km along the conjugate brazilian margin reflecting c 1 km of tectonically driven uplift subaerial erosion and isostatic uplift due to unloading following rift initiation at 120 ma aptian an oceanic spreading axis passed adjacent to this part of the margin at 90 ma cenomanian maximum uplift during the ridge transform intersection was 390 m considerably less than the 2000 m predicted by heat conduction models in local isostatic equilibrium the modern ridge is partially the product of thicker crust 22 km underlying the ridge than the adjacent deep ivorian basin 19 km and partially related to flexural unloading of the transform ridge between the end of intra continental wrenching and ridge transform intersection flexural coupling between the continental and oceanic plates since ridge transform intersection has caused a progressive depression of the offshore margin estimated at about 650 m in the study area

crustal structure of the newfoundland rifted continental margin from constrained 3 d gravity inversion the rifting history of the atlantic continental margin of newfoundland is very complex and so far has been investigated at the crustal scale primarily with the use of 2 d seismic surveys while informative the results generated from these surveys cannot easily be interpreted in a regional sense due to their sparse sampling of the margin a 3 d gravity inversion of the free air data over the newfoundland margin allows us to generate a 3 d density anomaly model that can be compared with the seismic results and used to gain insight into regions lacking seismic coverage results of the gravity inversion show good correspondence with moho depths from seismic results a shallowing of the moho to 12 km depth is resolved on the shelf at the northern edge of the grand banks in a region poorly sampled by other methods comparisons between sediment thickness and crustal thickness show deviations from local isostatic compensation in locations which correlate with faults and rifting trends such insights must act as constraints for future palaeoreconstructions of north atlantic rifting