bottom simulating reflections and geothermal gradients across the western svalbard margin seismic reflection data reveal prominent bottom simulating reflections bsrs within the relatively young 0 78 ma sediments along the west svalbard continental margin the potential hydrate occurrence zone covers an area of c 1600 km2 the hydrate accumulation zone is bound by structural tectonic features knipovich ridge molloy transform fault vestnesa ridge and the presence of glacigenic debris lobes inhibiting hydrate formation upslope the thickness of the gas zone underneath the bsr varies laterally and reaches a maximum of c 150 ms using the bsr as an in situ temperature proxy geothermal gradients increase gradually from 70 to 115 c km 1 towards the molloy transform fault anomalies only occur in the immediate vicinity of normal faults where the bsr shoals indicating near vertical heat fluid flow within the fault zones amplitude analyses suggest that sub horizontal fluid migration also takes place along the stratigraphy as the faults are related to the northwards propagation of the knipovich ridge long term disturbance of hydrate stability appears related to incipient rifting processes

brazilian and african passive margins of the central segment of the south atlantic ocean kinematic constraints the thinning of passive continental margins is usually explained by models using pure stretching or simple shear these models imply hypothetical extensional structures and large horizontal movements between the two conjugate margins more than 250 km for the brazilian and angolan margins refraction reflection data together with the most recent and tightest pre opening fit using continental and oceanic geological and geophysical constraints show that the substratum of the sag basin is divided into an autochthonous part upper continental crust and an allochthonous part exhumed material the thinning process which evolves in an elevated position of the system until at least the break up seems to be depth dependent and to mainly concern the lower middle crust which we postulate is exhumed this exhumation does not explain the entire thinning of the system horizontal motions cannot alone explain the formation of the huge angolan brazilian basin similar observations are made on the whole central segment of the south atlantic ocean part of the lower continental crust is still missing and it seems most improbable that the continental crust maintains its integrity throughout the thinning process

opening of the fram strait gateway a review of plate tectonic constraints abstract we have revised the regional crustal structure oceanic age distribution and conjugate margin segmentation in and around the lena trough the oceanic part of the fram strait between the norwegian greenland sea and the eurasia basin arctic ocean the lena trough started to open after eurasia greenland relative plate motions changed from right lateral shear to oblique divergence at chron 13 times 33 3 ma earliest oligocene a new bouguer gravity map supported by existing seismic data and aeromagnetic profiles has been applied to interpret the continent ocean transition and the influence of eocene shear structures on the timing of breakup and initial seafloor spreading assuming that the onset of deep water exchange depended on the formation of a narrow oceanic corridor the gateway formed during early miocene times 20 15 ma however if the initial lena trough was blocked by terrigenous sediments or was insufficiently subsided to allow for deep water circulation the gateway probably formed with the first well developed magnetic seafloor spreading anomaly around chron 5 times 9 8 ma late miocene paleoceanographic changes at odp site 909 northern hovgard ridge are consistent with both hypotheses of gateway formation we cannot rule out that a minor gateway formed across stretched continental crust prior to the onset of seafloor spreading in the lena trough the gravity seismic and magnetic observations question the prevailing hypotheses on the yermak plateau and the morris jesup rise as eocene oceanic plateaus and the hovgard ridge as a microcontinent

gas hydrate distribution and hydrocarbon maturation north of the knipovich ridge western svalbard margin a bottom simulating reflector bsr occurs west of svalbard in water depths exceeding 600 m indicating that gas hydrate occurrence in marine sediments is more widespread in this region than anywhere else on the eastern north atlantic margin regional bsr mapping shows the presence of hydrate and free gas in several areas with the largest area located north of the knipovich ridge a slow spreading ridge segment of the mid atlantic ridge system here heat flow is high up to 330 mw m 2 increasing towards the ridge axis the coinciding maxima in across margin bsr width and heat flow suggest that the knipovich ridge influenced methane generation in this area this is supported by recent finds of thermogenic methane at cold seeps north of the ridge termination to evaluate the source rock potential on the western svalbard margin we applied 1d petroleum system modeling at three sites the modeling shows that temperature and burial conditions near the ridge were sufficient to produce hydrocarbons the bulk petroleum mass produced since the eocene is at least 5 kt and could be as high as 0 2 mt most likely source rocks are miocene organic rich sediments and a potential eocene source rock that may exist in the area if early rifting created sufficiently deep depocenters thermogenic methane production could thus explain the more widespread presence of gas hydrates north of the knipovich ridge the presence of microbial methane on the upper continental slope and shelf indicates that the origin of methane on the svalbard margin varies spatially

gas hydrate reservoir and active methane venting province in sediments on 20 ma young oceanic crust in the fram strait offshore nw svalbard seafloor pockmarks are common indicators for vertical fluid flow and frequently associated with methane discharge through the gas hydrate stability zone ghsz the present day flux through these degassing systems is presumably at a low level on most rifted continental margins a pockmark field on the nw svalbard passive margin is located on young ocean crust 20 ma and shows evidence of ongoing episodic degassing new geophysical data from the vestnesa ridge 79 n a mounded and elongated sediment drift in the eastern fram strait reveal a gas hydrate free gas and venting system that is exceptionally more dynamic than documented elsewhere along the northeastern north atlantic margin the prominent bottom simulating reflection bsr about 200 mbsf separates anomalously high p wave velocities in the ghsz from a remarkable underlying low velocity zone indicating the presence of gas hydrate and gas in the pore space inversion of p wave velocity data using the differential effective medium theory yields a two dimensional concentration model of methane hydrate and free gas the model predicts saturations of up to 11 in the hydrate reservoir which due to the seafloor topography forms a large anticlinal permeability barrier below in the low velocity zone i e 1350 1500 m s up to 3 of free gas is predicted across the apex of the vestnesa ridge and in the immediate vicinity of extensional faults a conservative estimate indicates that 225 kg m2 of pure methane is stored in hydrate and gas in the upper 230 m of the sedimentary column an elongated pockmark field consisting of 100 individual pockmarks up to 600 m wide systematically aligns the apex of the vestnesa ridge active vigorous degassing from the topography controlled pressure valve system was evident from a 750 m high and 150 m wide gas flare observed in the water column during a cruise with r v jan mayen in october 2008 the gas flare documents dynamic degassing through the corresponding chimney which penetrates the entire ghsz and into the underlying free gas zone cruises in 2006 and 2007 did not detect active gas venting above the pockmark field accordingly vigorous degassing may operate in an episodic mode where hydrothermal circulation systems through young ocean crust may play a significant role

sedimentary markers in the provencal basin western mediterranean a window into deep geodynamic processes deep earth dynamics impact so strongly on surface geological processes that we can use sediment palaeo markers as a window into the deeper earth derived from climatic and tectonic erosive actions on the continents and related to eustasy subsidence and isostasy the sediment in a deep basin is the main recorder of these processes nevertheless defining and quantifying the relative roles of parameters that interact to give the final sedimentary architecture is not a simple task using a 3d grid of seismic and wide angle data boreholes and numerical stratigraphic modelling we propose here a quantification of post rift vertical movements in the provencal basin west mediterranean involving three domains of subsidence seaward tilting on the platform and the slope and purely vertical subsidence in the deep basin these domains fit the deeper crustal domains highlighted by previous geophysical data post break up sedimentary markers may therefore be used to identify the initial hinge lines of the rifting phase and the subsidence laws

magmatic metasomatism and formation of the merensky reef bushveld complex the rare earth element ree contents of pyroxenes and other minerals from the merensky reef and stratigraphically adjacent rocks of the atok section bushveld complex have been determined with the ion microprobe merensky reef clinopyroxene and orthopyroxene contain much higher and more variable concentrations of the ree than their cumulus counterparts in rocks several meters below the reef chondrite normalized merensky clinopyroxene ce contents vary from 10 to 90 for ce and from 4 to 17 for yb they also possess deep negative eu anomalies the eu anomalics being deeper for crystals having high ree contents and relatively shallow for pyroxenes with low ree contents similar compositional characteristics are displayed by cl rich apatite which is an accessory phase in the rocks interstitial pyroxene in cumulates above and below the reef also tends to have elevated ree contents and in general is not in equilibrium with coexisting cumulus minerals the melt from which the cumulus minerals crystallized falls within the compositional range of continental basalts that from which merensky and postcumulus pyroxenes crystallized is inferred to be much more highly enriched in ree than any normal tholeiitic or alkalic basalt despite their highly evolved nature in terms of the ree the merensky reef pyroxenes are not evolved in terms of major elements the decoupling of incompatible trace and major elements is best explained by a metasomatic process it is speculated that metasomatism involved upward percolation of hydrated silicate melt through and its reaction with the crystalline cumulate pile the fact that the rocks enriched in the platinum group elements are also those that show evidence for metasomatism suggests that these elements were also metasomatically redistributed

high resolution p cable 3d seismic imaging of gas chimney structures in gas hydrated sediments of an arctic sediment drift the newly developed p cable 3d seismic system allows for high resolution seismic imaging to characterize upper geosphere geological features focusing on geofluid expressions gas chimneys shallow gas and gas hydrate reservoirs seismic imaging of a geofluid system of an arctic sediment drift at the vestnesa ridge offshore western svalbard provides significantly improved details of internal chimney structures from the seafloor to not vert similar500 m bsf below seafloor the chimneys connect to pockmarks at the seafloor and indicate focused fluid flow through gas hydrated sediments the pockmarks are not buried and align at the ridge crest pointing to recent topography controlled fluid discharge chimneys are fuelled by sources beneath the base of gas hydrate stability zone ghsz that is evident at not vert similar160 170 m bsf as indicated by a bottom simulating reflector bsr conduit centres that are not vertically straight but shift laterally by up to 200 m as well as discontinuous internal chimney reflections indicate heterogeneous hydraulic fracturing of the sediments episodically active pressure driven focused fluid flow could explain the hydro fracturing processes that control the plumbing system and lead to extensive pockmark formation at crest of the vestnesa ridge high amplitude anomalies in the upper 50 m of the chimney structures suggest formations of near surface gas hydrates and or authigenic carbonate precipitation acoustic anomalies expressed as high amplitudes and amplitude blanking are irregularly distributed throughout the deeper parts of the chimneys and provide evidence for the variability of hydrate and or carbonate formation in space and time

the yermak plateau in the arctic ocean in the light of reflection seismic data implication for its tectonic and sedimentary evolution summary the separation of northeast greenland and svalbard is the result of large scale strike slip movements during cenozoic times geological evidence for these movements can be found onshore both on north greenland and svalbard however the role of the submarine yermak plateau yp in this process is unclear the compilation of available multichannel reflection and wide angle seismic data give new insights into the sedimentary and crustal structure and evolution of the plateau the flat surface of the present day plateau is a quite young feature up to 2 km of cenozoic sediments cover a rough basement which show similarities to the rough topography and strike of geological structures of spitsbergen island in some basins more than 4 km of sedimentary rocks could be mapped the most pronounced structure is the sverdrup bank which appears to be part of a larger crustal block p wave velocities of about 4 5 km s 1 derived from sonobuoy data indicate that its uppermost part is most probably composed of sedimentary or volcanic rocks we have made a correlation of previously defined seismic units across the yp to outline the history of sediment deposition in the area the existing graben structures on the plateau might have provided early shallow pathways for water exchange between the arctic and the atlantic oceans a chaotic sedimentary apron east of the sverdrup bank and bright reflections near the mosby seamount interpreted as magmatic sills suggest tectonic and magmatic events during the miocene

crust and uppermost mantle structure of greenland and the northwest atlantic from rayleigh wave group velocity tomography the greenland landmass preserves 4 billion years of tectonic history but much of the continent is inaccessible to geological study due to the extensive inland ice cap we map out for the first time the 3 d crustal structure of greenland and the nw atlantic ocean using rayleigh wave anisotropic group velocity tomography in the period range 10 80 s from regional earthquakes and the ongoing glatis glisn seismograph networks 1 d inversion gives a pseudo 3 d model of shear wave velocity structure to depths of 100 km with a horizontal resolution of 200 km crustal thickness across mainland greenland ranges from 25 km to over 50 km and the velocity structure shows considerable heterogeneity the large sedimentary basins on the continental shelf are clearly visible as low velocities in the upper 5 15 km within the upper continental basement velocities are systematically lower in northern greenland than in the south and exhibit a broadly nw se trend the thinning of the crust at the continental margins is also clearly imaged upper mantle velocities show a clear distinction between typical fast cratonic lithosphere vs 4 6 km s 1 beneath greenland and its ne margin and anomalously slow oceanic mantle vs 4 3 4 4 km s 1 beneath the nw atlantic we do not observe any sign of pervasive lithospheric modification across greenland in the regions associated with the presumed iceland hotspot track though the average crustal velocity in this region is higher than that of areas to the north and south crustal anisotropy beneath greenland is strong and complex likely reflecting numerous episodes of tectonic deformation beneath the north atlantic and baffin bay the dominant anisotropy directions are perpendicular to the active and extinct spreading centres anisotropy in the subcontinental lithosphere is weaker than that of the crust but still significant consistent with cratonic lithosphere worldwide

continent ocean transition at the western barents sea svalbard continental margin the change in crustal type at the western barents sea svalbard margin takes place over a narrow zone related to primary rift and shear structures reflecting the stepwise opening of the greenland sea regionally the margin is composed of two large shear zones and a central rifted margin segment local transtension and transpression at the plate boundary caused the early cenozoic tectonism in svalbard and the western barents sea and might explain the prominent marginal gravity and velocity anomalies

the margin between senja and spitsbergen fracture zones implications from plate tectonics abstract analysis of multichannel seismic data from the continental margin off svalbard between the senja and spitsbergen fracture zones suggests that the transition between continental and oceanic crust is located at or close to the hornsund fault zone in the late paleocene early eoeene 57 m y the region between svalbard and northeast greenland was subjected to regional shear movements associated with a transform system between the young lofoten greenland basin and the arctic ocean approximately 50 m y ago the spreading axis migrated to the northeast creating a deep basin north of the greenland senja fracture zone forming the passive margin between bear island and 76 5 n north of 76 5 n the regional transform was maintained at the time of the main reorganization of relative plate motion 36 m y the northern margin evolved a continental fragment was possibly cut off from the svalbard margin forming a small microcontinent the microcontinent appears as the submarine ridge which has been associated with the hovgaard fracture zone it is suggested that the sediments west of the hornsund fault zone are not older than eocene in the south and mid oligocene in the north the position of the spreading axis has greatly influenced the margin sedimentation

along strike changes in fault array and rift basin geometry of the carboniferous billefjorden trough svalbard norway abstract the billefjorden fault zone bfz of svalbard norway is a long lived major tectonic lineament with significant influence on the structural style of the northwestern barents shelf the fault zone can be traced for around 2 300 km in an n s direction where it is made up of three major fault strands all dipping 50 70 to the east field and seismic data delineate gradual shifts in displacement between master faults northern odelfjellet balliolbreen and southern dronbreen faults and suggest that two c 20 km long 2 3 km wide relay zones exist along strike the western fault strand consistently has a pre carboniferous reverse character placing basement on top of devonian sedimentary units with a throw estimated at 10 km subsequent carboniferous extensional reactivation of segments of the bfz resulted in formation of the billefjorden trough with a rift basin showing a fairly consistent polarity for its length this suggests a profound control by the older devonian reverse faults on the extensional structural style the billefjorden trough extends for a minimum of 110 km along strike and is 20 30 km wide at the most a gentle north plunge of the basin axis in the south is mirrored by an opposite plunge in the north offering a depth thickness of around 2000 m in the central realm diminishing to 500 m in the south geometries of the trough suggest sedimentary response to fault growth with weak lithologies such as evaporites promoting fault monocline formation and associated lenticular basin geometries on the contrary wedged shaped fill geometries toward major faults is consistent with significant fault movement and growth sequences subsequent mild tertiary reactivation of fault segments of the bfz is suggested by local reverse faults and deep seated anticline syncline pairs

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

crustal structure in the svalbard region from seismic measurements abstract seismic refraction measurements along three profiles with lengths ranging from 160 km to 270 km were made in 1976 and 1978 as part of a multinational effort to study the earth s crust and upper mantle in the svalbard region the crustal tliickness is about 26 km along the west coast of spitsbergen and faulting and a strong eastwardly dip of the moho increase the crustal thickness to 40 km beneath the tertiary fold belt on west spitsbergen further to the east pn arrivals indicate a transitional nature of the moho in the area beneath the central spitsbergen basin evidence suggests the presence of major n s trending faults dividing the spitsbergen crust into blocks a down faulted sedimentary basin containing about 3 km of sediments and presumably of cenozoic age is observed in the forlandsundet area the maximum thickness of the cenozoic and mesozoic rock sequences on central spitsbergen is estimated to be about 5 km

synthetic seismogram and surface wave constraints on crustal models of spitsbergen abstract seismic refraction data collected on spitsbergen in 1978 are used to obtain a crustal model assuming plane horizontal layering the observed travel times and wave forms are compared with those of synthetic seismograms computed for various published crustal models the more detailed models adequately explain some but not all of the features of the synthetics these models are adjusted utilizing travel times and wave form amplitudes until a satisfactory fit is achieved the best fitting model consists of a 4 layer crust having thicknesses of 4 1 10 0 7 4 and 5 8 km and compressional velocities of 4 65 6 21 6 30 and 6 65 km sec with increasing depth the uppermost mantle has a velocity of 7 90 km sec a comparison of observed and synthetic p n waveforms supports the existence of a thin low velocity zone beginning at a depth of about 5 km beneath the moho boundary an inversion of seismic surface wave group velocity data yields a shear wave model which is compatible with the compressional wave model

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

effect of thermal contrasts on gravity modeling at passive margins results from the western barents sea the western barents sea passive margin is a key locality to demonstrate the effect of the thermal structure of the lithosphere on forward gravity modeling this margin developed by shear motion between the eurasian and greenland plates during the early tertiary and it is a significant border zone between young hot oceanic lithosphere and cooler continental lithosphere we construct two dimensional gravity models of 125 km thick lithosphere based on expansion of mantle rocks determined from thermal modeling the approach has a substantial impact over traditional shallow gravity models here demonstrated on a previously published model on the basis of a 140 mgal free air anomaly the old model proposes an anomalous high density oceanic crust emplaced in a leaky transform adjacent to the continent during early margin development however the lithospheric models predict a homogeneous oceanic crust while preserving regional isostasy at base lithosphere from continent to ocean two further tests agree with this conclusion a map of bouguer corrected ers 1 satellite data reveals no residual anomalies originating from the oceanic crust at the margin admittance analysis shows a strong oceanic lithosphere and the high coherence between bathymetry and free air gravity discounts a significant subsurface load the high gravity anomalies at the margin are thus an edge effect enhanced by sedimentation onto the strong oceanic lithosphere and shaped by the effect of the lithospheric thermal field other results of this work include a new continent ocean boundary map and two crustal transects across the margin

controls on the formation and stability of gas hydrate related bottom simulating reflectors bsrs a case study from the west svalbard continental slope 1 the growth and stability of the free gas zone fgz beneath gas hydrate related bottom simulating seismic reflectors bsrs is investigated using analytical and numerical analyses to understand the factors controlling the formation and depletion of free gas for a model based on the continental slope west of svalbard a continental margin of north atlantic type we find that the fgz is inherently unstable under a wide range of conditions because upward flow of under saturated liquid depletes free gas faster than it is produced by hydrate recycling in these scenarios the 150 m thick fgz that presently exists there would deplete within 105 106 years we suggest the fgz is in a stable state however that is formed by a diffusion dominated mechanism that produces low concentrations of gas in a fgz of steady state thickness gas forms across a thick zone because the upward fluid flux is relatively low and because the gas water solubility decreases to a minimum several hundred meters below the seabed this newly understood solubility curvature effect is complementary to hydrate recycling but becomes the most important factor controlling the presence and properties of the bsr in environments where the rate of upward fluid flow and the rate of hydrate recycling are both relatively low i e rifted continental margins if the present day fgz is in steady state we estimate that the upward fluid flux in the west svalbard site must be less than 0 15 mm a 1

the origin of chromitites and related pge mineralization in the bushveld complex new mineralogical and petrological constraints this article reports a study of chromitites from the lg 1 to the ug 2 3 from the bushveld complex chromite from massive chromitite follows two compositional trends on the basis of cation ratios trend a decreasing mg mg fe with increasing cr cr al trend b decreasing mg mg fe with decreasing cr cr al the chromitites are divided into five stages on the basis of which trend they follow and the data of eales et al chemical geology 88 261 278 1990 on the behaviour of the mg fe ratio of the pyroxene and whole rock sr isotope composition of the environment in which they occur following eales et al chemical geology 88 261 278 1990 the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aal2o3 and in the case of stage 5 the appearance of cumulus plagioclase buffering the aal2o3 the similarity of pge profiles across the mg 3 and mg 4 chromitites that are separated laterally by up to 300 km and the variation in v in the ug 2 argue that the chromitites have largely developed in situ modelling using the programme melts shows that increase in pressure mixing of primitive and fractionated magma felsic contamination of primitive magma or addition of h2o do not promote crystallization of spinel before orthopyroxene in general they hinder it and that the cr2o3 content of the magma was of the order of 0 25 wt less than 20 of the chromite in the magma is removed before orthopyroxene joins chromite which implies a 13 km thickness of magma for the critical zone it is suggested that the large excess of magma has escaped up marginal structures such as the platreef the pge profile of chromitites depends on whether sulphide accumulated or not along with chromite modelling shows that contamination of critical zone magma with a felsic melt will induce sulphide immiscibility although not chromite precipitation the lg 1 to lg 4 chromitites developed without sulphide whilst those from the lg 5 upwards had associated liquid sulphide much of the sulphide originally in the lg 5 and above has been destroyed as a result of reaction with the chromite

east greenland ridge in the north atlantic ocean an integrated geophysical study of a continental sliver in a boundary transform fault setting 1 the combined greenland senja fracture zones gsfz represent a first order plate tectonic feature in the north atlantic ocean the gsfz defines an abrupt change in the character of magnetic anomalies with well defined seafloor spreading anomalies in the greenland and norwegian basins to the south but ambiguous and weak magnetic anomalies in the boreas basin to the north substantial uncertainty exists concerning the plate tectonic evolution of the latter area including the role of the east greenland ridge which is situated along the greenland fracture zone in 2002 a combined ocean bottom seismometer and multichannel seismic mcs survey acquired two intersecting wide angle reflection and coincident mcs profiles across and along the east greenland ridge we present the results of integrated reflection seismic interpretation first arrival tomography 2d kinematic raytracing full wave amplitude modeling and gravity modeling of the intersecting profiles the results show that 1 the greenland basin is characterized by a normal oceanic crustal velocity structure 2 the velocity structure of the east greenland ridge is of overall continental type and 3 a major faulted basin province above highly extended continental crust exists to the ne of the ridge the results further suggest that a zone of extremely thin and faulted continental crust above partially serpentinized mantle peridotite defines the nw edge of the east greenland ridge and the transition to the ne greenland margin

crustal structure of the eratosthenes seamount cyprus and s turkey from an amphibian wide angle seismic profile abstract in 2010 project cococo incipient continent continent collision recorded a 650 km long amphibian n s wide angle seismic profile extending from the anatolian plateau across southern turkey and cyprus to just south of the eratosthenes seamount esm the aim of the project is to reveal the impact of the transition from subduction to continent continent collision of africa with anatolia arrival picking finite differences ray tracing and inversion of the offshore and on offshore data produced a tomographic model across southern cyprus the accretionary wedge and the esm the main features of this model are 1 crustal p velocities predominantly lower than 6 5 km s beneath the esm 2 crustal thickness between 28 and 37 km 3 an upper crustal reflection at 5 km depth beneath the esm 4 the likely presence of oceanic crust south of the esm and a transform margin north of it and 5 a 12 km thick ophiolite sequence on cyprus land shots on turkey also recorded on cyprus gravity data and geological and previous seismic investigations allow to derive a layered velocity model beneath anatolia and the northern part of cyprus the main features of this model are 1 moho depths of 38 45 km beneath the anatolian plateau 2 an upper and lower crust with large lateral changes in velocity and thickness 3 a north dipping subducting plate below cyprus with a steepening of the dip angle of the plate at about 45 km depth thus the wide angle seismic and gravity data provide detailed insights into the 2 d geometry and velocity structures associated with the cyprus arc collision zone finally integrated analysis of the geophysics and geology allows a comprehensive interpretation of the crustal structure related to the collision process

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

modeling the land surface boundary in climate models as a composite of independent vegetation stands an efficient strategy for modeling the land surface boundary in general circulation models gcms is presented which accounts for the effects of vegetation on surface energy fluxes and allows for an arbitrary number of vegetation types to coexist in a grid square the gcm grid square is depicted as a mosaic of vegetation tiles with each tile consisting of a single vegetation type the energy balance equation for each tile follows closely that of a single vegetation version of the simple biosphere sib model of sellers et al 1986 but is simplified enough to be written in penman monteith form each tile in the square is coupled independently to the gcm atmosphere and tiles affect each other only through the atmosphere this coupling strategy differs conceptually from that of models such as sib that assume a homogeneous mixture of vegetation types within a gcm grid square a quantitative comparison of the two strategies is presented

development of the continent ocean transform boundary of the southern exmouth plateau a two stage model is proposed to explain the principal tectonic and magmatic features observed in multichannel seismic reflection and refraction data across the southern transform margin of the exmouth plateau northwestern australia 1 the rifting stage in which detachment surfaces developed under conditions of extension at a high angle to the future transform and were later sheared by right lateral strike slip faulting final transform rupture was attended by large fault block rotation and mafic intrusions in conditions of pure shear 2 in the drifting stage as the oceanic ridge abutted the continent the continental rim was underplated at this location resulting in a 10 km thick 7 3 km s 3 g cm3 layer resulting in a permanent isostatic uplift of the crust and tilting of synrift sedimentary deposits this wedge extends laterally forming a thickened oceanic layer 3 transient heating of the continental lithosphere induced thermal uplift and erosion of up to 3 5 km of sedimentary units over a 50 km distance from the continent ocean contact

moho and basement depth in the ne atlantic ocean based on seismic refraction data and receiver functions seismic refraction data and results from receiver functions were used to compile the depth to the basement and moho in the ne atlantic ocean for interpolation between the unevenly spaced data points the kriging technique was used free air gravity data were used as constraints in the kriging process for the basement that way structures with little or no seismic coverage are still presented on the basement map in particular the basins off east greenland the rift basins off nw europe are mapped as a continuous zone with basement depths of between 5 and 15 km maximum basement depths off ne greenland are 8 km but these are probably underestimated plate reconstructions for chron c24 c 54 ma suggest that the poorly known ammassalik basin off se greenland may correlate with the northern termination of the hatton basin at the conjugate margin the most prominent feature on the moho map is the greenland iceland faroe ridge with moho depths 28 km crustal thickness is compiled from the moho and basement depths the oceanic crust displays an increased thickness close to the volcanic margins affected by the iceland plume

the western svalbard margin 74 80 n abstract a regional study of the continental margin between the senja and molloy spitsbergen fracture zones reveals that the transition from continental to oceanic crust occurs in a narrow zone beneath the outer shelf and uppermost slope the postulated continent ocean boundary appears to be fault related consisting of sheared and rifted segments the marginal structures are compatible with a plate tectonic model in which the southern greenland sea opened along a northeasterly propagating plate boundary in the eocene whereas the northern greenland sea started opening in the early oligocene the main structure at the margin is the hornsund fault zone which probably reflects an old zone of weakness rejuvenated in the tertiary first by shear and later by extensional movements in the early tertiary local transpressional and transtensional components along the plate boundary are associated with the spitsbergen orogeny emplacement of belts of high density oceanic crust and tectonism in the western barents sea a complex volcanic rifted margin characterized by the bjornoya marginal high links the predominantly sheared margin segments on either side the main ridge like segment of the hovgaard fracture zone was originally part of the spitsbergen margin in a regional sense the hornsund fault zone demarcates the eastern boundary of the tertiary sedimentary wedge which reaches a total thickness of more than 7 km there appears to have been a considerable increase in deposition of sediments the last 5 6 my depocentres located seaward of the east west fjord systems and submarine depressions indicate a relationship between late cenozoic glaciations and high sedimentation rates

prediction of sediment thickness in the norwegian greenland sea from gravity inversion 1 we explore the feasibility of inverting freely available altimetry derived gravity grids for sediment thickness on oceanic crust a four layer earth model is adopted assuming that the gravity signal from the base of the sedimentary layer can be fully isolated from the observed free air gravity the following additional data are required 1 shipboard bathymetry for reliable bouguer correction 2 oceanic crustal ages to correct for gravity anomalies related to the thermal history of lithospheric breakup and seafloor spreading and 3 some wide angle seismic observations of sediment thickness a fixed relationship between the base of the sedimentary layer and the moho is also required because of limited gravity resolution at large depths the final inversion setup searches for three control parameter values density of uppermost sediments density of oceanic crystalline crust and thermal diffusivity of mantle rock that minimize misfit between the earth model and observations applied to the norwegian greenland sea the inversion successfully predicts locations of 20 25 km wide depocenters and crystalline crustal highs yet 1 km thickness errors are common even in well constrained areas the largest errors are associated with underplated crust active mid ocean ridges and the continent ocean boundary indicating locally incorrect assumptions of mantle thermal structure and crystalline crustal thickness the thicknesses of 4 km thick submarine fans are underpredicted probably because of too low sedimentary densities in the earth model if these limitations are acceptable or do not apply because of simple crustal structure gravity inversion may be used for mapping regional sedimentary patterns e g in the arctic ocean

the merensky reef at winnaarshoek eastern bushveld complex a primary magmatic hypothesis based on a wide reef facies the merensky reef on the winnaarshoek property in the central sector of the eastern limb of the bushveld complex belongs to what may be described as a wide reef facies in which platinum group element pge mineralization is largely contained within a relatively thick succession of medium grained nonpegmatoidal feldspathic orthopyroxenite bounded top and bottom by thin chromite stringers the wide reef merensky reef facies at winnaarshoek presents the opportunity to study subtle processes which could potentially be obscured or obliterated in the thinner reef facies found in many areas of the western limb especially in places where the reef zone contains a component of pegmatoidal orthopyroxenite at winnaarshoek a pegmatoidal feldspathic orthopyroxenite is developed below the lower chromite stringer and a second pegmatoidal orthopyroxenite is intermittently developed several centimeters above the upper chromite stringer in contrast to most occurrences in the western limb however both these pegmatoidal orthopyroxenites are only sporadically mineralized if at all in addition rather than being at the base of a feldspathic orthopyroxenite unit as is the case throughout most of the western limb the merensky reef at winnaarshoek occurs toward the top of a sequence of feldspathic orthopyroxenite despite the gross relative uniformity of the merensky reef on a property wide scale at winnaarshoek we demonstrate using underground channel samples that individual intersections spaced a matter of meters apart commonly display remarkable variations in grade distribution our study is based primarily on whole rock and mineral chemical profiles from drill cores through three separate and distinctive subfacies of essentially normal i e nonpothole reef one of the drill core intersections represents typical reef in which pge mineralization forms an upper reef extending from the upper chromitite stringer approximately 1 m into the underlying feldspathic pyroxenite and a much thinner lower reef centered on the lower chromitite stringer the other two drill core intersections represent extremes in terms of the vertical distribution of the pge one having mineralization confined to the immediate vicinity of the two chromitite stringers while the other is richly mineralized throughout the reef zone peaks in pge concentrations in all three intersections are coincident with the most primitive compositions of orthopyroxene plagioclase and chromite the dominant rock forming minerals whereas the cumulate layers below major pge peaks are the most evolved as indicated by mineral compositions and elevated whole rock incompatible element concentrations furthermore the lowest grade merensky reef intersection has the most chemically evolved footwall whereas the richest intersection has a relatively primitive footwall we consider the origin of the merensky reef in the context of its position as the culmination of a sequence of several pge mineralized chromite bearing layers in the upper critical zone and suggest that the regular upward variations in the chemistry of the chromitite from one layer to the next and the overall cyclicity are indicative of complex magma replenishment and mixing processes in the chamber we apply a model of lateral streaming of magma with chromite crystallization and consequent sulfide and platinum group mineral pgm precipitation resulting from magma mixing specifically the merensky reef cannot be properly interpreted outside the context of the sequence of successive events that evolved throughout the development of the upper critical zone genetic models also need to account for lateral facies variations in the reef

a three dimensional geophysical model of the crust in the barents sea region model construction and basement characterization summary barents50 a new 3 d geophysical model of the crust in the barents sea region has been developed by the university of oslo norsar and the u s geological survey the target region comprises northern norway and finland parts of the kola peninsula and the east european lowlands novaya zemlya the kara sea and franz josef land terminate the region to the east while the norwegian greenland sea marks the western boundary in total 680 1 d seismic velocity profiles were compiled mostly by sampling 2 d seismic velocity transects from seismic refraction profiles seismic reflection data in the western barents sea were further used for density modelling and subsequent density to velocity conversion velocities from these profiles were binned into two sedimentary and three crystalline crustal layers the first step of the compilation comprised the layer wise interpolation of the velocities and thicknesses within the different geological provinces of the study region linear relationships between the thickness of the sedimentary rocks and the thickness of the remaining crystalline crust are observed we therefore used the separately compiled area wide sediment thickness data to adjust the total crystalline crustal thickness according to the total sedimentary thickness where no constraints from 1 d velocity profiles existed the barents50 model is based on an equidistant hexagonal grid with a node spacing of 50 km the p wave velocity model was used for gravity modelling to obtain 3 d density structure a better fit to the observed gravity was achieved using a grid search algorithm which focussed on the density contrast of the sediment basement interface an improvement compared to older geophysical models is the high resolution of 50 km velocity transects through the 3 d model illustrate geological features of the european arctic the possible petrology of the crystalline basement in western and eastern barents sea is discussed on the basis of the observed seismic velocity structure the barents50 model is available at http www norsar no seismology barents3d

petrologic evolution of partially molten cumulate the atok section of the bushveld complex the postcumulus evolution of a portion of the bushveld complex that includes the merensky reef is inferred from the study of a continuous 56 m drill core the core penetrated the basal orthopyroxenites of the merensky and bastard units the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the merensky pyroxenite detailed profiles of major minor and rare earth element ree contents of clinopyroxene and orthopyroxene were determined by electron and ion probe good correlations exist between textural and lithological variations and the ree contents of the pyroxenes specifically enrichments in pyroxene ree abundances are observed in the basal pyroxenites of the merensky and bastard units relative to the underlying and overlying rocks in the pyroxenites the nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm 90 chondrite and nd yb ratio is in the range 8 to 25 these extreme enrichments in ree are not accompanied by large variations in major element contents computations of the compaction parameters relevant to the conditions of crystallization of the bushveld complex combined with a consideration of cooling history confirm the importance of compaction as a post cumulus process this analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile a model for the development of the atok section is developed the merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late stage hydrous and incompatible element enriched melt which was thus trapped in the underlying merensky pyroxenite the flow was driven by compaction of the 350 meter thick section of predominantly norite beneath the anorthosite the introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution recrystallization and ree enrichment of the rock forming minerals and in the formation of the main lithologic features of the merensky pyroxenite further upward percolation of the interstitial melt through the merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite this melt accumulated in and metasomatized the bastard pyroxenite in the same manner as the merensky pyroxenite having again been trapped by the overlying bastard anorthosite

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

crustal structure of northwestern svalbard and the adjacent yermak plateau evidence for oligocene detachment tectonics and non volcanic breakup summary in 1999 new seismic refraction data were collected off northwestern svalbard and the adjacent yermak plateau a 260 km long profile provides detailed velocity information for the northeastern edge of the eurasian continent and the adjacent yermak plateau north of forlandsundet graben the depth of the moho varies between 23 and 28 km and remains at this depth to the northern edge of the profile at 81 n the crustal lithology off western svalbard can be related to the basement province west of the raudfjorden fault zone off the northern shoreline of svalbard the structure of the tertiary danskoya basin is mapped below this a late silurian early devonian basin with seismic velocities between 5 1 and 5 8 km s 1 and a thickness of up to 8 km is present a palaeozoic sequence of up to 6 km thickness is expected below the tertiary cover north of the danskoya basin an earlier suggestion that oligocene rift processes affected the southern yermak plateau is confirmed a detachment structure is situated below the palaeozoic basin below danskoya basin which is probably a consequence of simple shear tectonics the middle crust exhibits low seismic velocities above the detachment fault the lowermost crust beneath is slightly contaminated by mantle derived melts which is deduced by the slightly elevated velocities of the lowermost crust these melts can be attributed to decompressive melting caused by modest uplift of the moho during stretching the velocity depth model provides no evidence for large magmatic activity which implies a non volcanic rifted margin history this leads to the assumption that the proposed yermak hotspot during the breakup of svalbard from northern greenland did not exist

the moho boundary above upper mantle peridotites or lower crustal eclogites a global review and new interpretations for passive margins abstract we have performed a global study of 2d crustal scale wide angle profiles across passive margins with regard to local elevations in the moho which could possibly be interpreted as indicative of lower crustal eclogites a total of 16 candidates have been found mainly in the north atlantic and around australia these cases make up c 6 only of the total profile length studied confirming the interpretation of the moho generally representing the top of the mantle the interpreted candidates for lower crustal eclogites indicate that there may be a link between eclogite bodies and continental suture zones in the barents sea off mid norway in the newfoundland appalachians and in the yilgarn craton western australia it is also possible that there is a genetic link between the formation of caledonian eclogites and the jan mayen fracture zone which is the only major fracture zone in the north atlantic several of the inferred lower crustal eclogite bodies are located close to lines of major changes in strain such as coastlines and shelf edges indicating that lower crustal eclogite bodies may be important in guiding the evolution of basin architecture interpreting the moho beneath the bedout high on the northwest shelf of australia as the top of a body of lower crustal eclogites may imply that the northern termination of the lambert shelf represents a paleo suture zone and that the western termination of the broome platform acted as a major transfer zone the significant increase in crustal thickness implied by the eclogite model has important implications for estimates of stretching history subsidence and hydrocarbon maturation modelling

evolution of deep water rifted margins testing depth dependent extensional models 1 a general understanding of rifted margins which form by thinning of the continental lithosphere exists nevertheless the exact form of thinning is unclear this debate has been stimulated by acquisition of dense seismic wide angle and deep reflection surveys from atlantic ocean margins a central issue concerns the way in which thinning changes with depth we have tackled this issue by developing a generalized inverse model this model attempts to fit subsidence and crustal thinning observations by varying strain rate as a function of time and space depth dependent thinning is permitted but we do not prescribe its existence or form here the algorithm is applied to six margins including two of the most contentious conjugate margins newfoundland iberia and brazil angola calculated strain rate histories predict thinning estimates which broadly match estimates inferred from normal faulting the eastern indian and beaufort sea margins formed by largely uniform lithospheric thinning in contrast the newfoundland iberian conjugate margins formed by a pattern of strongly depth dependent strain rate to account for the paucity of syn rift decompression melting of the underlying asthenosphere the lithospheric mantle close to oceanic continent transition must thin more slowly than the overlying crust this form of depth dependency is not common for example the brazil angolan conjugate margin could have formed by uniform lithospheric thinning provided thick layers of salt were deposited in a preexisting 400 m deep topographic depression depth dependent thinning is not required to account for rapid subsidence of presalt strata

rifting in the northern norwegian greenland sea thermal tests of asymmetric spreading the analysis of heat flow seismic and topographic data collected in the northern norwegian greenland sea reveals an asymmetric evolution of the eurasian and north american plates these data are compared to predictions from three kinematic models of extension which produce asymmetry about the knipovich ridge 1 uniform asymmetric pure shear 2 lithospheric simple shear and 3 rift ridge jumping the data are consistent with a range of deformation scenarios from one ridge jump occurring at about 25 m y after the initiation of spreading to continuous asymmetric extension the simple shear model can match the data only when a detachment fault dips more steeply than 45 under svalbard tectonic and heat flow evidence suggests that asymmetry may have evolved from a combination of all three models when the mohns ridge propagating to the east encountered the preexisting northward trending spitsbergen shear zone the direction of ridge propagation shifted to the north being influenced by the abrupt change in the regional stress field at the shear zone ridge axis intersection as a result the nascent knipovich ridge entered into and propagated along the shear zone consequently formerly active shear faults became the new detachment surfaces along which new crust is minted and asymmetrically extended the high level of deviatoric stress about the mohns and knipovich ridge intersection may cause a gradual eastward migration of the knipovich ridge resulting in multiple zones of magma intrusion off axial bands of high heat flow and volcanism located along the barents sea and svalbard margin along the yermak plateau and on continental svalbard just southeast of the yermak plateau may be evidence for this migration propagation from the nansen ridge may have entered the same shear zone from the north explaining the creation of the small molloy ridge and off axis volcanism on the yermak plateau

crustal structure of northern spitsbergen along the deep seismic transect between the molloy deep and nordaustlandet a deep seismic sounding experiment was performed during the expedition arktis xv 2of the rv polarstern and the polish ship eltanin in 1999 in the continent ocean transitionzone of northwestern svalbard along the 430 km long profile awi 99200 the profile runsfrom the molloy deep in the vicinity of an active spreading axis in the northern atlantic tonordaustlandet seismic energy airgun andtntshots was recorded by seismic land onshore stations obss and hydrophone systems with airgun shots recorded up to 200 km onshore and50 km offshore the data recorded along the entire profile provide an excellent database fora detailed seismic modelling of the crustal p wave velocity field along the profile track aminimal depth of about 6 km to the moho discontinuity was found east of the molloy deep here the upper mantle exhibits a p wave velocity of about 7 9 km s 1 and the crustal thicknessdoes not exceed 4 km the continent ocean transition zone to the east is characterized by acomplex seismic velocity structure the moho interface reaches a maximum depth of 28 kmbeneath the continental part of the profile with a p wave velocity in the upper mantle of8 15 km s 1 the continental crust consists of three layers with p wave velocities of about5 5 5 9 6 0 and 6 2 6 6 km s 1 respectively in addition we have found two reflectors in themantle lithosphere at depths of 14 42 and 40 50 km dipping ne the evolution of the regionappears to be within a shear rift tectonic setting the continent ocean transition zone is mostlydominated by extension so the last stage of the development of this margin can be classifiedas rifting the uplifted moho boundary close to the molloy deep can be interpreted as thesouthwestern end of the molloy ridge

accurate measurements of residual topography from the oceanic realm in the oceans our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped these anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle here we analyze a global database of seismic reflection and wide angle profiles from heavily sedimented oceanic crust which abuts continental lithosphere at 449 locations we calculated water loaded subsidence compared it with a reference age depth relationship and determined residual depth we then combined these spot measurements of residual depth with observations from mid oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth our results suggest that the amplitude of residual depth varies by up to 1 km with wavelengths of order 103 km we compare our residual depths with free air gravity and seismic tomographic anomalies our results show that residual depths correlate with long wavelength gravity anomalies in contrast correlations between residual depths and vertically averaged shear velocity anomalies within the upper and or the lower mantle are weaker the largest discrepancies occur at short 1000 km wavelengths these combined observations suggest that residual depth anomalies could be generate by density variations within a thin 102 km low viscosity layer beneath the lithosphere our global compilation should play a significant role in helping to refine predictive geodynamical models

in situ strontium and sulfur isotope investigation of the ni cu pge sulfide ore bearing kevitsa intrusion northern finland the 2 06 ga kevitsa mafic ultramafic intrusion in northern finland hosts a large disseminated ni cu pge deposit the deposit occurs in the ultramafic olivine pyroxene cumulates and shows a range in ni tenors varying from 4 7 wt regular ore to 10 wt ni pge ore there are also a metal poor sulfide mineralization false ore and contact mineralization that are uneconomic ni tenor 0 7045 in comparison to the estimated depleted mantle sr isotope ratio of 0 702 at 2 06 ga the sulfur 34s values are generally higher than 2 which together with the sr isotope data imply involvement of crustal material in the genesis of the kevitsa intrusion and its ores the 87sr 86sr i values obtained from the ore bearing domain of the intrusion show stratigraphic variation and exceed 0 7050 with the maximum value reaching up to 0 7109 in contrast in rocks around the ore domain the initial sr isotope compositions remain more or less constant 0 7047 0 7060 throughout the intrusive stratigraphy the isotope data suggest that the ore bearing domain of the intrusion represents a dynamic site with multiple injections of variably contaminated magma whereas the surrounding part of the intrusion experienced a less vigorous emplacement history no correlation is observed between the strontium and sulfur isotope compositions this is explained by bulk assimilation of the silicate magma in a deeper staging magma chamber and variable assimilation of sulfur during magma transport into the kevitsa magma chamber the low level of metals in false ore and the ni depleted nature of its olivine suggest that some sulfides may have precipitated and deposited in the feeder conduit during the initial stage of magma emplacement cannibalization of early formed sulfides by later magma injections may have been important in the formation of the economic ore deposit

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

chemistry morphology and origin of magmatic reaction chromite stringers associated with anorthosite in the upper critical zone at winnaarshoek eastern limb of the bushveld complex

re os isotope and pd ru variations in chromitites from the critical zone bushveld complex south africa abstract rhenium osmium isotope measurements of selected chromitites from the critical zone of the bushveld igneous complex reveal that a significant component of radiogenic os was present in the magma when it formed 2 06 ga ago measured 187 os 188 os ratios increase with stratigraphic height and individual layers have similar values across the complex initial 187 os 188 os ratios for chromitites from the western bushveld decrease from 0 140 at the lg to 0 129 at the mg2 chromitite increasing to 0 153 at the ug2 chromitite the inflection at mg2 mg3 is the boundary between the lower and upper critical zone consistent with previous mineralogical and geochemical studies indicating a shift in magma composition at this point chromite separates have os values of 12 35 at 2 06 ga and are too radiogenic for a purely mantle derivation assimilation of 5 mafic granulitic lower crust in a tholeiitic parent magma can explain the radiogenic values pd ru ratios for the intergranular and chromite separate components of each layer differ by orders of magnitude but relative shifts from layer to layer are similar combined with the re os data the results suggest that pge were concentrated into chromitites by magmatic or early deuteric processes that were specific to each layer post layering upward mobilization of pge by hydrothermal processes probably did not play a significant role in pge enrichment

evolution and regional stratigraphy of the northeastern canadian polar margin seismic refraction data from the northeastern canadian polar margin were compared to sonic logs and other refraction data in the area to develop a regional stratigraphy for the continental shelf north of the sverdrup rim a schematic geological cross section derived from these data showed 12 km of sedimentary rocks north of the rim based on seismic correlations the section near the edge of the shelf was predicted to consist of 4 km of upper cretaceous to tertiary postrift strata with an initial velocity of 2 1 km s 1 and a high gradient of 0 5 s overlying about 8 km of lower mesozoic to palaeozoic synrift strata with an initial velocity of 4 4 km s 1 and a lower gradient of 0 1 0 15 s all resting on a crystalline basement with a velocity of about 6 2 km s 1 mantle was measured at a depth of 25 km on the outer shelf the geological interpretation of the margin was examined in terms of circum arctic geology and was shown to be consistent with it when the arctic ocean was reconstructed to its predrift position during mississippian to early cretaceous the close spatial relationship of the canadian polar margin basin to the brooks mackenzie basin of alaska which has a mississippian to hauterivian sequence supported the geological interpretation of the margin described here

constraints on gas hydrate distribution and morphology in vestnesa ridge western svalbard margin using multicomponent ocean bottom seismic data source at https doi org 10 1029 2018jb016574 2019 american geophysical union all rights reserved

a new occurrence of merensky reef on the flanks of the zaaikloof dome northeastern bushveld complex relationship between diapirism and magma replenishment exploration for platinum group elements has resulted in the discovery of an occurrence of merensky reef in the eastern limb of the bushveld complex south africa the new discovery was made on the dwarsrand property in the poorly known boundary area between the intensively explored western and central sectors of the eastern limb this boundary area is largely composed of domes and basins that may in part be aligned along a regional lineament the wonderkop fault transvaal supergroup metasediments in the core parts of the domes form prominent hills whereas the bushveld complex in the intervening basins is restricted to areas of poor outcrop the formation of domes is attributed to diapirism triggered by gravitational loading and heating of the transvaal supergroup in response to intrusion of the bushveld complex merensky reef was located beneath cover rocks in an area where the critical and main zones are uplifted and attenuated against the southern flank of the zaaikloof dome drill holes although collared in upper zone intersected the critical and main zones at relatively shallow depths as a result of the uplift the critical and main zones including the merensky reef are steeply dipping the base of the upper zone forms an intrusive unconformity and the layering in this zone has a much shallower dip economically significant grades were obtained on the dwarsrand property but exploration was terminated because of poor strike continuity the new findings suggest that uplift and attenuation of the bushveld complex on the flanks of domes in the northeastern part of the complex are both more extensive and more severe than previously realized a complex history of intrusion and uplift is implied some domes even penetrate into the overlying bushveld granite the occurrence of markedly angular unconformities may be contrasted with the layercake sequences typically associated with the bushveld complex diapirism in response to crustal loading and heating by ultramafic sequences occurred in the early stages of magma emplacement episodic intrusion and uplift are consistent with multiple replenishment models in which each of the many dozens of cycles that comprise the critical and main zones corresponds to a new magma pulse with the oldest layers being dragged to higher structural levels in response to the uplift periodic reactivation of doming is indicated by the lowermost layers within the critical and main zones abutting against the southern flank of the zaaikloof dome at progressively higher levels the transgressive nature of the upper zone and bushveld granite relative to the critical and main zones may indicate that the upper zone formed by new intrusions of magma rather than the generally accepted model of differentiation within the chamber the discovery of hidden merensky reef at relatively shallow depths beneath transgressive upper zone and granite could provide the impetus for further exploration possibilities within the bushveld complex particularly on the flanks of regionally extensive domal structures

resistivity image beneath an area of active methane seeps in the west svalbard continental slope the arctic continental margin contains large amounts of methane in the form of methane hydrates the west svalbard continental slope is an area where active methane seeps have been reported near the landward limit of the hydrate stability zone the presence of bottom simulating reflectors bsrs on seismic reflection data in water depths greater than 600 m suggests the presence of free gas beneath gas hydrates in the area resistivity obtained from marine controlled source electromagnetic csem data provides a useful complement to seismic methods for detecting shallow hydrate and gas as they are more resistive than surrounding water saturated sediments we acquired two csem lines in the west svalbard continental slope extending from the edge of the continental shelf 250 m water depth to water depths of around 800 m high resistivities 5 12 m observed above the bsr support the presence of gas hydrate in water depths greater than 600 m high resistivities 3 4 m at 390 600 m water depth also suggest possible hydrate occurrence within the gas hydrate stability zone ghsz of the continental slope in addition high resistivities 4 8 m landward of the ghsz are coincident with high amplitude reflectors and low velocities reported in seismic data that indicate the likely presence of free gas pore space saturation estimates using a connectivity equation suggest 20 50 per cent hydrate within the lower slope sediments and less than 12 per cent within the upper slope sediments a free gas zone beneath the ghsz 10 20 per cent gas saturation is connected to the high free gas saturated 10 45 per cent area at the edge of the continental shelf where most of the seeps are observed this evidence supports the presence of lateral free gas migration beneath the ghsz towards the continental shelf

the petrogenesis of merensky reef potholes at the western platinum mine bushveld complex sr isotopic evidence for synmagmatic deformation potholes represent areas where the normally planar pge rich merensky reef of the upper critical zone of the bushveld complex transgresses its footwall such geometric relationships being unusual in layered intrusions the recognition of vertical dykes of merensky pyroxenite in the footwall suggests downward collapse of crystal mush into pull apart sites resulting from tensional deformation due to the loading effects of major new magma additions in contrast crosscutting anorthosite veins display physical and isotopic evidence of upward emplacement the merensky reef and its footwall have distinct initial sr isotope ratios r 0 0 7066 and 0 7066 respectively which may be used to constrain these processes related to pothole formation merensky reef in potholes r 0 0 7069 0 7078 shows no isotopic evidence of assimilation of or reaction with footwall material discrete discordant replacement bodies of anorthosite extend from the footwall lithologies to cross cut the merensky reef and its hanging wall the initial sr isotope ratio in these replaced rocks is totally reset to footwall values r 0 0 7066 and immediately adjacent stratiform lithologies are slightly modified towards footwall values in contrast neptunian pyroxenitic merensky dykes cross cutting the footwall lithologies with a large surface area to volume ratio and low sr content do not display footwall like sr isotope initial ratios r 0 0 7077 and thus show no evidence for assimilation of or reaction with footwall material furthermore pegmatoidal replacement pyroxenite replacement pegmatoid at the base of the merensky reef within potholes has a high initial ratio r 0 0 7071 and so models of pervasive metasomatism by footwall material are not applicable this isotopic evidence indicates that there was no active interaction of footwall material with the overlying magma during or after the formation of merensky reef potholes a basic tenet of existing pothole formation hypotheses involving footwall mass transfer in contrast the isotopic data are entirely consistent with an extensional model for pothole formation with the more radiogenic merensky magma migrating laterally to fill extensional zones in the footwall layers

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

trace elements in the merensky reef and adjacent norites bushveld complex south africa trace elements were analysed in rocks and minerals from three sections across the merensky reef in the rustenburg platinum mine in the bushveld complex of south africa whole rocks and separated minerals were analysed by inductively coupled plasma mass spectrometer icp ms and in situ analyses were carried out by ion microprobe and by laser source icp ms merensky reef pyroxenites contain extremely high concentrations of a wide range of trace elements these include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements for major elements and compatible trace elements the measured concentrations in cumulus phases and the bulk rock compositions are similar for highly incompatible elements however concentrations in bulk rocks are far higher than those measured in the cumulus phases in situ analyses of plagioclase have far lower concentrations of th zr and rare earth elements than icp ms analyses of bulk separates of plagioclase a difference that is attributed to the presence of trace element rich accessory phases in the bulk mineral separates we used these data to calculate the trace element composition of the magmas parental to the merensky unit and adjacent norites we argue that there is no reason to assume that the amount of trapped liquid in the merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites we propose that the bushveld complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks the magma that gave rise to the merensky unit was the more contaminated and unusually rich in incompatible trace elements and when it entered the main bushveld chamber it precipitated the unusual phases that characterize the merensky reef the hybrid magma segregated sulphides or platinum group element rich phases during the course of the contamination in the lower chamber these phases accumulated following irruption into the main bushveld chamber to form the merensky ore deposits

contourites in the fram strait abstract multichannel seismic reflection profiles from the fram strait show many features that can be attributed to bottom current influence on the sediments by comparison with well known contourite sequences elsewhere a greater than 2 km thick contourite drift with a smooth internal reflection pattern is observed northwest of svalbard the currents here have probably been controlled by local topography we interpret sequences with abundant diffractions in the broader basins to be current related probably deposited during periods of intensified circulation the continental slopes on the svalbard side have fairly continuous sequences whereas the younger sequences on the greenland lower slope onlaps the lowermost one this can be attributed to differences in current regime sediment supply or margin subsidence the contourites of the lower slope in the northeastern part of the strait about 1000 m water depth can be explained by long term influence of the west spitsbergen current but current influenced patterns down to 3000 m water depth are not compatible with the weak deep water circulation observed at present the oldest contourites west of svalbard are estimated to be of late miocene to pliocene age between 10 and 3 ma based on correlation to the dsdp site 344 and inferred basement ages near the present plate boundary hiatuses may be present beneath the contourites the onset of currents may be related to opening of the gateway between greenland and svalbard to climatic cooling and to palaeogeographic changes outside the fram strait contourite deposition has continued through the late neogene and quaternary but vertical changes in the pattern suggest that current intensities have decreased through time

gas hydrates free gas distribution and fault pattern on the west svalbard continental margin summary a 3 d tomographic analysis of seismic velocity and attenuation fields is presented with the purpose to analyse the intimate relation of gas hydrates and free gas distribution with the fault pattern the 3 d four component seismic data have been acquired offshore western svalbard the analysis of the subbottom topography of the base of the stability field of gas hydrates indicated by the bottom simulating reflector and the thickness of the underlying free gas bearing zone suggest a fault induced compartmentalization of the gas reservoir the proposed fluid and gas circulation scheme assumes deep sourced warm fluids moving upwards mainly along high permeability faults and fractures below the gas hydrate stability zone faults change from being a pathway of fluid flow to being barriers to fluid flow in the hydrate stability zone consequently below the hydrate stability zone free gas is transferred from the permeable faults to non faulted but fault bounded sediments where it accumulates below the base of the hydrate stability zone producing a significantly thicker free gas layer the tomographic approach is therefore proposed as an effective procedure to provide detailed information on 3 d p and s wave velocity and attenuation distribution in marine sediments and specifically to examine the details of the distribution of the gas hydrates and of the free gas for which the dependence on the vp vs and qp is now fairly well known

continental scale water and energy flux analysis and validation for the north american land data assimilation system project phase 2 nldas 2 1 intercomparison and application of model products 1 results are presented from the second phase of the multiinstitution north american land data assimilation system nldas 2 research partnership in nldas the noah variable infiltration capacity sacramento soil moisture accounting and mosaic land surface models lsms are executed over the conterminous u s conus in realtime and retrospective modes these runs support the drought analysis monitoring and forecasting activities of the national integrated drought information system as well as efforts to monitor large scale floods nldas 2 builds upon the framework of the first phase of nldas nldas 1 by increasing the accuracy and consistency of the surface forcing data upgrading the land surface model code and parameters and extending the study from a 3 year 1997 1999 to a 30 year 1979 2008 time window as the first of two parts this paper details the configuration of nldas 2 describes the upgrades to the forcing parameters and code of the four lsms and explores overall model to model comparisons of land surface water and energy flux and state variables over the conus focusing on model output rather than on observations this study seeks to highlight the similarities and differences between models and to assess changes in output from that seen in nldas 1 the second part of the two part article focuses on the validation of model simulated streamflow and evaporation against observations the results depict a higher level of agreement among the four models over much of the conus than was found in the first phase of nldas this is due in part to recent improvements in the parameters code and forcing of the nldas 2 lsms that were initiated following nldas 1 however large inter model differences still exist in the northeast lake superior and western mountainous regions of the conus which are associated with cold season processes in addition variations in the representation of sub surface hydrology in the four lsms lead to large differences in modeled evaporation and subsurface runoff these issues are important targets for future research by the land surface modeling community finally improvement from nldas 1 to nldas 2 is summarized by comparing the streamflow measured from u s geological survey stream gauges with that simulated by four nldas models over 961 small basins

potential serpentinization degassing and gas hydrate formation at a young 20 ma sedimented ocean crust of the arctic ocean ridge system 1 global assessment of methane must consider the role of mid ocean ridges fluids from serpentinized mantle and gabbro material are noteworthy on ocean ridges although they are not very well understood only a few sedimented ocean ridges exist worldwide and they may document past and ongoing serpentinization driven migration of gas rich fluids this study is based on two multichannel reflection seismic profiles acquired across a sedimented segment of the ultraslow spreading knipovich ridge offshore nw svalbard seismic data allow suggesting a potential link between inferred areas of serpentinization transfer of carbon from the deep seated host rocks through the sediments above by diapirism and methane capture within the gas hydrate stability zone at the eastern flank of the knipovich ridge the origin of sediment remobilization features can be related to intrusions and the degassing process from mantle serpentinization these disturbances in sediments overlying the oceanic crust can be observed in seismic data and are interpreted as diapirs in shallower sediments at the predicted base of the gas hydrate stability zone the seismic data show a bright spot with all the characteristics of a gas hydrate related bottom simulating reflector bsr such as enhanced reflection amplitude phase reversal relative to the seabed reflection and crosscutting of sedimentary strata the bsr occurs at about 200 ms two way time within a sequence of marine sediments two dimensional concentration models of methane hydrate using the differential effective medium theory predict saturations of up to 26 of methane hydrate in the pore space of sediments in the gas hydrate reservoir

active gas venting through hydrate bearing sediments on the vestnesa ridge offshore w svalbard abstract gas hydrate systems offshore western svalbard are extensive and include the whole vestnesa ridge an elongated sediment drift north of the molloy transform and just east of the molloy ridge one of the shortest segments of the slow spreading north atlantic ridge system the crest of the vestnesa ridge at water depth between 1200 1300 m is pierced with fluid flow features seafloor pockmarks vary in size up to 700 m in diameter pockmarks are generally larger at the eastern start of the vestnesa ridge than at its western termination a recent cruise with r v jan mayen discovered methane flares in the water column above the pockmark field at the onset of the vestnesa ridge over a period of two days at least 3 pockmarks were continuously active and methane flares in the water column reached a height of approximately 800 m it is still unclear what has triggered the increase in gas expulsion from seafloor sediments high resolution 3d seismic data acquired in 2007 and 2010 show significant differences of the subseafloor expression of these fluid leakage systems at the western end of the vestnesa ridge sub seafloor fluid flow features resemble well described chimney structures however the seismic expression of the active fluid flow features is much broader much more chaotic dome shaped and is not very similar to a typical chimney structure the vestnesa ridge gas hydrate and free gas system occurs within few kilometers of a mid oceanic ridge and transform fault which makes this gas hydrate system unique on earth the close proximity to the spreading center and its hydrothermal circulation system affects the dynamics of the gas hydrate and free gas system the high heat flow together with the high tectonic activity of this region a thick sedimentary cover a shallow maturation window and an accelerated rate of biogenic and thermogenic gas production cause substantial disturbance to the free gas system leading to high variability in gas supply gas migration and gas hydrate build up and dissociation

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

crustal structure of a super slow spreading centre a seismic refraction study of mohns ridge 72 n summary a series of eight high resolution seismic refraction profiles from the ultra slow spreading 16 mm yr 1 full spreading rate mohns ridge in the norwegian greenland sea has been treated with modern inversion methods the profiles were shot parallel to the ridge at an off axis distance of 0 135 km corresponding to crustal ages of 0 22 ma the resulting models are constrained by synthetic seismograms and gravity modelling the crustal thickness in all profiles is well below the global average for typical oceanic crust and shows a high variability with a mean thickness of 4 0 0 5 km this is mainly due to a very thin and variable lower crustal layer layer 3 generally the crust is thicker beneath basement highs and thinner beneath basins implying local isostatic compensation the top of the basement layer 2a consists of a zone with low p wave velocities 2 5 3 0 km s 1 the mean thickness of this layer decreases with distance from the ridge beneath it lies a layer with slightly higher velocities layer 2b its thickness shows less variability along a given profile and an overall increase with age the combined average thickness of the upper two layers remains nearly constant indicating that the boundary between layer 2a and 2b may represent an alteration front upper mantle velocities are generally slow around 7 5 km s 1 for the profile directly within the rift valley a model without a third layer incorporating a constant gradient up to upper mantle velocities and a model with a moho depth inferred from neighbouring profiles and upper mantle velocity as slow as 7 2 km s 1 fit the seismic and gravity data equally well the crustal structure is not mature below the ridge these observations support previous models suggesting the presence of low densities and velocities at about 2 km below the rift axis poisson s ratios determined from converted s wave modelling are incompatible with a layer 3 consisting of purely serpentinized peridotite however a volume fraction of 10 40 per cent serpentinite cannot be ruled out

trace element and platinum group element distributions and the genesis of the merensky reef western bushveld complex south africa the merensky reef of the bushveld complex is one of the world s largest resources of platinum group elements pge however mechanisms for its formation remain poorly understood and many contradictory theories have been proposed we present precise compositional data major elements trace elements and platinum group elements pge for 370 samples from four borehole core sections of the merensky reef in one area of the western bushveld complex trace element patterns incompatible elements and rare earth elements exhibit systematic variations including small scale cyclic changes indicative of the presence of cumulus crystals and intercumulus liquid derived from different magmas ratios of highly incompatible elements for the different sections are intermediate to those of the proposed parental magmas critical zone and main zone types that gave rise to the bushveld complex mingling but not complete mixing of different magmas is suggested to have occurred during the formation of the merensky reef the trace element patterns are indicative of transient associations between distinct magma layers the porosity of the cumulates is shown to affect significantly the distribution of sulphides and pge a genetic link is made between the thickness of the merensky pyroxenite the total pge and sulphide content petrological and textural features and the trace element signatures in the sections studied the rare earth elements reveal the important role of plagioclase in the formation of the merensky pyroxenite and the distribution of sulphide

sr and pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the bushveld complex south africa microdrilling and progressive leaching evidence for sub liquidus contamination within a crystal mush progressive leaching of plagioclase for sr iso topes and microdrilling for sr and pb isotopes from grains of plagioclase and orthopyroxene from the critical zone and the lower zone indicates that these minerals are not in isotopic equilibrium leaching suggests critical zone plagioclase either lost rb or had a more radiogenic sri rim relative to the core whereas plagioclase from an upper zone sample is isotopically homogeneous for sri microdrilling analyses of plagioclase from the lower and critical zones consistently have a higher initial 87 sr 86 sr sri and a less radiogenic modeled 238 u 204 pb composition l2 than coexisting ortho pyroxene the range of calculated sri for plagioclase and orthopyroxene is 0 70506 0 70662 34 and 0 70290 0 70654 36 respectively the average difference in sri between mineral pairs was 0 00095 the range of calculated l2 for plagioclase and orthopyroxene is 9 42 10 30 average 9 7 and 9 83 15 75 average 10 1 respectively the range of measured 208 pb 206 pb for plagioclase and orthopyroxene is 34 757 36 439 33 and 36 669 41 845 85 respectively one orthopyroxenite without evidence for more than one population of crystal size distribution nonetheless had sri 0 70654 36 with calculated l2 of 10 32 for larger grains as compared with sri 0 70290 32 and calculated l2 of 9 97 for smaller grain size fractions isotopic results from this study demonstrate that whole rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the bushveld complex however systematic changes have the potential to elucidate the timing of contamination with regard to other processes crystal aging compaction driven recrystallization and mineral exsolution occurring within a slowly cooled crystal liquid vapor mush system

comparing potential recharge estimates from three land surface models across the western us groundwater is a major source of water in the western us however there are limited recharge estimates available in this region due to the complexity of recharge processes and the challenge of direct observations land surface models lsms could be a valuable tool for estimating current recharge and projecting changes due to future climate change in this study simulations of three lsms noah mosaic and vic obtained from the north american land data assimilation system nldas 2 are used to estimate potential recharge in the western us modeled recharge was compared with published recharge estimates for several aquifers in the region annual recharge to precipitation ratios across the study basins varied from 0 01 15 for mosaic 3 2 42 for noah and 6 7 31 8 for vic simulations mosaic consistently underestimates recharge across all basins noah captures recharge reasonably well in wetter basins but overestimates it in drier basins vic slightly overestimates recharge in drier basins and slightly underestimates it for wetter basins while the average annual recharge values vary among the models the models were consistent in identifying high and low recharge areas in the region models agree in seasonality of recharge occurring dominantly during the spring across the region overall our results highlight that lsms have the potential to capture the spatial and temporal patterns as well as seasonality of recharge at large scales therefore lsms specifically vic and noah can be used as a tool for estimating future recharge rates in data limited regions

the seismic structure across an active oceanic continental transform fault zone a seismic structural section across the southern queen charlotte transform fault zone which separates the oceanic pacific plate from the continental america plate off western canada has been determined from an offshore onshore refraction experiment two explosion profiles one parallel and one perpendicular to the fault strike were recorded on three ocean bottom seismographs obs and seven land based seismographs lbs assuming lateral homogeneity along the parallel profile recorded over oceanic crust at one obs one dimensional amplitude modeling produced a velocity model with the characteristics of standard ocean crust the partially reversed perpendicular profile recorded at two obs s and one good quality lbs extended across the fault zone which in bathymetric cross section shows two linear fault escarpments separated by a flat terrace an initial velocity structure provided by time term analysis of the complete data set was modified by ray tracing until the travel time data for the three stations were satisfied by one seismic structural section this model shows three distinctive crustal blocks the oceanic terrace and continental blocks separated by two major crustally pervasive faults the outer and inner queen charlotte faults the rock units composing the terrace block have lower velocities at equivalent depths than those of the blocks on either side this may have resulted from deformation associated with oblique convergence and or the effects of a jump in the position of the active queen charlotte fault from its outer to inner positions about 1 0 5 ma ago depth to the base of the crustal section increases from 12 to 18 km below sea level across the terrace transition zone representing an eastward dip of about 20 the seismic model after conversion to a density model agrees well with the gravity anomaly along the profile the interpreted structural section is consistent with recent tectonic models that require a component of convergence along the transform fault zone

thermal implications for the evolution of the spitsbergen transform fault abstract heat flow taken between svalbard and greenland reveal three thermal provinces 1 1 the molloy ridge within the spitsbergen transform 2 2 the yermak plateau 3 3 the northeastern margin of svalbard nordaustlandet the molloy ridge is a short spreading segment and the average heat flow is much above the sclater et al 1971 cooling curve but agrees with values from the norwegian greenland sea an additional zone of intrusion identified by heat flow lies to the northwest of the molloy ridge it straddles both the visible fracture zone and part of the yermak plateau a thermal boundary lies between the warm western segment of the yermak plateau and the shelf off nordaustlandet if the thermal subsidence of the western yermak plateau can be traced to the latest heating episode then it is likely that the crust is similar to oceanic in composition and not older than 13 m y approximately 20 m y younger than the northeastern segment of the plateau plate rotation shows that there was no room for the western segment of the plateau prior to anomaly 7 we postulate that the original transform is associated with the hornsund fault zone in response to deviatoric stress across the oblique ridge transform system the nansen ridge propagated southwestward aborting the old transform trace and shifted to its present position it is suggested that this propagation and migration of the ridge transform system across a zone of extensional deviatoric stress allowed the massive intrusion of basalt forming the western yermak plateau the propagation phenomenon coincides with large scale tertiary volcanic activity on svalbard readjustment and migration of the oblique transform is still taking place as the transform ridge system is liberated from continental constraints the migration rate will diminish as orthogonality is approached

strontium isotope disequilibrium of plagioclase in the upper critical zone of the bushveld complex evidence for mixing of crystal slurries we report in situ sr isotope data for plagioclase of the bushveld complex we found disequilibrium sr isotopic compositions on several scales 1 between cores and rims of plagioclase grains in the merensky pyroxenite the bastard anorthosite and the ug1 unit and its noritic footwall 2 between cores of different plagioclase grains within thin sections of anorthosite and pyroxenite of the merensky unit the footwall anorthosite of the merensky reef and the footwall norite of the ug1 chromitite the data are consistent with a model of co accumulation of cumulus plagioclase grains that had crystallized from different magmas followed by late stage overgrowth of the cumulus grains in a residual liquid derived from a different level of the compacting cumulate pile we propose that the rocks formed through slumping of semi consolidated crystal slurries at the top of the critical zone during subsidence of the center of the intrusion slumping led to sorting of crystals based on density differences resulting in a layered interval of pyroxenites norites and anorthosites

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the

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

discussion of the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber by maier et al 2013 miner deposita 48 1 56 we believe the hypothesis presented by maier et al miner deposita 48 1 56 2012 for the formation of the various ore bodies in the bushveld complex to be overly simplistic and we raise concerns that some of our work used in support of this hypothesis has been misrepresented the formation of both diverse metalliferous layers platinum group element pge reefs and ti magnetite layers and some discordant pipe ore deposits has been ascribed by maier et al to the single unifying process of hydrodynamic sorting the problem faced by authors of universal hypotheses for the bushveld complex is the sheer size and complexity of the intrusion we disagree with many aspects of the overall maier et al model and have also identified several minor errors on maps and photographs although some of these do not have a material effect on the model the nature and origin of the layering is however too complex a topic to deal with in the context of this commentary and we restrict ourselves to noting that our preferred hypothesis namely the incremental buildup of layering from numerous episodes of replenishment by different magma lineages is consistent with field relationships our hypothesis for the origin of the ultramafic hosted pge rich reefs i e lateral mixing is applicable to economically mineralized reefs mitchell and scoon econ geol 102 971 1009 2007 and poorly mineralized layers such as the pseudoreef harzburgite scoon and de klerk canad mineral 25 51 77 1987 and the chromitite layers below the ug2 scoon and teigler econ geol 89 1094 1121 1994

lead isotopic disequilibrium between sulfide and plagioclase in the bushveld complex and the chemical evolution of large layered intrusions abstract the pb isotopic compositions of coexisting plagioclase and sulfide from the bushveld complex were determined by laser ablation multi collector icpms la mc icpms the samples are of the upper critical zone in the northeast corner of the complex and were collected from drill core and underground mine exposures all the rocks are fresh and exhibit no evidence for alteration weathering or disruption of the pb isotope systematics subsequent to the initial cooling of the intrusion furthermore individual plagioclase and sulfide crystals do not contain enough u to warrant correction for radiogenic in growth for these reasons the measured pb isotope ratios approximate the initial ones for plagioclase 207pb 206pb ranges from 0 98 to 1 02 and 208pb 206pb from 2 26 to 2 35 low 207pb 206pb and 208pb 206pb ratios characterize grain boundaries and partially annealed microcracks some of which contain minute fragments of sulfide and other phases and this accounts for most if not all the heterogeneity exhibited by individual samples real compositional differences exist however in plagioclase from different lithologic layers for example plagioclase 207pb 206pb values vary from 1 004 in norite beneath the merensky pyroxenite to 1 009 in the mineralized pyroxenite and 0 997 in overlying norite in most samples in which sulfide and plagioclase coexist the sulfide 207pb 206pb ratio is lower and 208pb 206pb ratio higher than the corresponding ones in plagioclase for example in a mineralized merensky reef sample average sulfide 207pb 206pb and 208pb 206pb ratios are 0 993 and 2 313 respectively while those in plagioclase are 1 000 and 2 292 in one sample the sulfide is extremely heterogeneous with 207pb 206pb and 208pb 206pb ratios as low as 0 84 and 2 12 in this particular sample the compositions must represent an isolated occurrence of addition of a young pb component the array of sulfide and plagioclase compositions requires multiple sources of pb at the time of crystallization or soon thereafter the disequilibrium between plagioclase and sulfide implies that some of the pb originated from the isotopically distinct country rocks and was introduced at temperatures at which the composition of sulfide but not plagioclase could be modified thus bushveld sulfide and to some extent plagioclase do not reliably record the initial pb isotopic composition s of the parent magma s

crustal structure of north atlantic fracture zones seismic studies have established that large offset transforms along the slow spreading mid atlantic ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust seismically fracture zone crust in the north atlantic is extremely heterogeneous in both thickness and internal structure it is frequently quite thin 1 2 km thick and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3 a more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the mid atlantic ridge the geological nature of the seismically anomalous crust found within atlantic fracture zones and how this crust forms are still controversial one interpretation consistent with available seismic observations is that the crust within north atlantic fracture zones consists of a thin intensely fractured and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust the existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large scale detachment faults that form preferentially in the cold brittle lithosphere near the ends of segments

switching of a paleo ice stream in northwest svalbard ice streams are the fast flowing zones of ice sheets that can discharge a large flux of ice the glaciated western svalbard margin consists of several cross shelf troughs which are the former ice stream drainage pathways during the pliocene pleistocene glaciations from an integrated analysis of high resolution multibeam swath bathymetric data and several high resolution two dimensional reflection seismic profiles across the western and northwestern svalbard margin we infer the ice stream flow directions and the deposition centres of glacial debris that the ice streams deposited on the outer margin our results show that the northwestern margin of svalbard experienced a switching of a major ice stream based on correlation with the regional seismic stratigraphy as well as the results from odp 911 on yermak plateau and odp 986 farther south on the western margin of spitsbergen off van mijenfjord we find that first a northwestward flowing ice stream developed during initial northern hemispheric cooling starting 2 8 2 6 ma a switch in ice stream flow direction to the present day kongsfjorden cross shelf trough took place during a glaciation at 1 5 ma or probably later during an intensive major glaciation phase known as the mid pleistocene revolution starting at 1 0 ma the seismic and bathymetric data suggest that the switch was abrupt rather than gradual and we attribute it to the reaching of a tipping point when growth of the svalbard ice sheet had reached a critical thickness and the ice sheet could overcome a topographic barrier highlights reflection seismic data reveal two glacial fans at northwest svalbard margin the fans are result of ice stream activities during pliocene pleistocene glaciations based on seismic and bathymetric data we find the flow directions of the ice streams we find a switch in ice stream flow direction the switch resulted as the ice sheet became thick and overcame a topographic barrier