mesoscale physical and bio optical structure of the antarctic polar front near 170 w during austral spring as part of the u s joint global ocean flux study southern ocean program high resolution surveys of the antarctic polar front near 170 w were conducted during october november 1997 with a towed undulating system equipped with conductivity temperature depth and bio optical sensors transects along 170 w and two successive mapping surveys revealed zonal bands with sharp meridional gradients in east west velocity the polar front pf was characterized by a sea surface temperature drop from 1 6 to 1 6 c between 60 35 and 61 10 s with eastward velocities of 0 4 0 5 m s 1 in the core of the pf jet deep mixed layers 200 m were found within and north of the pf but mixed layers shoaled to 100 125 m south of the pf to the edge of loose ice at 62 3 s highest mixed layer chlorophyll concentrations 0 35 mg m 3 in late october along 170 w were to the south of the pf and associated with cold fresh water a large meander of the pf was observed with an alongfront wavelength of 175 km a cross front peak to peak amplitude of 100 km and an eastward phase propagation of 0 05 0 08 m s 1 all of which are consistent with its formation via hydrodynamic instability of the pf jet highest phytoplankton biomass was located just poleward of the center of the pf jet a high chlorophyll up to 1 1 mg m 3 50 by 50 km region was found downstream of the cyclonic bend associated with the meander a survey 7 5 days later revealed growth of this high biomass region so that chlorophyll was in excess of 0 8 mg m 3 over an 80 km cross front by at least 80 km alongfront region high biomass was observed to grow in place with respect to the meander rather than being displaced far downstream as would be expected from advection this pattern is consistent with meander driven upwelling of nutrients and or trace metals which in turn stimulates phytoplankton growth detailed cross sections of the pf reveal narrow 10 20 km wide bands or filaments of phytoplankton biomass that have temperature salinity properties distinct from surrounding water and are coherent for at least 120 km alongfront

age relations among oslo rift magmatic rocks implications for tectonic and magmatic modelling abstract this paper presents a compilation of 67 published and unpublished radiometric age determinations by the rb sr method on magmatic rocks in the continental oslo rift in southeast norway the data include most major rock types and structural units in the subaerial part of the rift the implications of these data with respect to the tectonomagmatic evolution of the rift and processes in the mantle and crust are discussed the oslo rift was active for about 60 million years the earliest recorded magmatic activity was the intrusion of basaltic and syenitic sills in the pre rift lower paleozoic sediments at about 300 ma extrusion of plateau lavas of basaltic and intermediate composition rhomb porphyries started about 295 ma and lasted to about 275 ma with largest magma volumes and highest extrusion rate from 295 to 285 ma in the vestfold graben segment gs one flow per 250 000 years and 0 30 km3 per 103 years the main faulting activity and graben formation occurred during this period the plateau volcanism was succeeded by a stage of bimodal central volcanoes most of which later underwent caldera collapses ring dykes and central intrusions in these calderas give ages in the range 268 266 ma in the vestfold and 266 243 ma in the akershus gs large composite intrusive complexes were emplaced in the periods 27 268 and 273 241 ma in the vestfold and akershus gs respectively a rate of propagation of the rift from ssw towards the nne has been estimated to 1 2 cm y 1 on the basis of 87 sr 86 sr initial ratios and type of magmatic products we estimate magmas to have been emplaced into the crust only during the first 20 25 ma of the magmatic period some of the magmatism appears to be initiated by movement along precambrian fault zones which were reactivated during the oslo rifting event

southern acc front to the northeast of south georgia pathways characteristics and fluxes 1 in december 2000 we conducted high resolution hydrographic and towed undulator transects across the south georgia shelf and into the deep waters of the georgia basin the southern antarctic circumpolar current front saccf was observed flowing northwestward close to the base of the slope at 53 7 s with a second manifestation around 53 4 s having flow in the reverse direction both crossings had a significant barotropic component aligned with the flow the region of the saccf was characterized by large scale undulations of properties on horizontal scales of 5 10 km similar to the local internal rossby radius we observe a distinct surface temperature gradient associated with the saccf in the vicinity of south georgia and demonstrate the usefulness of this by tracing the course of the front around the island with advanced very high resolution radiometer data a distinct northward deflection of the front to the north of south georgia is probably due to topographic steering by the north georgia rise the signature of the saccf is pronounced in the circumpolar deep water with strong isopycnal interleaving indicative of cross frontal mixing the salinity of the weddell sea deep water is marginally higher between the two manifestations of the saccf reflecting the different pathways from its source regions in the weddell sea remarkably strong vertical gradients of potential temperature and salinity exist in the bottom 100 m of the water column at depths of around 3500 m because of the vertical juxtaposition of waters that have taken different routes around the northeast georgia rise

mesoscale biogeochemical responses to iron fertilization in the upper layers of the southern ocean iron experiment areas 1 during the southern ocean iron experiment sofex january february 2002 two iron fertilization experiments were conducted at the south 66 45 s 171 8 w and north 56 23 s 172 w patches the south patch was replete with all macronutrients whereas the north patch was nearly depleted of silicate using a towed water sampling measurement system high resolution three dimensional observations of temperature salinity co2 partial pressure pco2 and the concentrations of total co2 tco2 nitrate phosphate and silicate in the upper 100 m were obtained 10 and 28 days after initial iron additions in the south and north patches respectively tco2 and nutrient drawdowns observed in the south patch were small due to wind events that limited biological response in the early postfertilization period at the north patch drawdown signals were stronger resulting from the longer time since fertilization the north patch was dispersed along a frontal zone between two distinct waters with the denser subducted under the lighter subduction limited co2 uptake from the air but provided alternative mechanisms for vertical carbon export based on the biogeochemical drawdowns and published estimates of the rate of the entrainment of nonfertilized waters into the patch the mean net community productivity over 28 days was estimated to be 94 12 mmol carbon m 2 day 1 to which the air to sea flux contributed only 2 despite low silicate concentrations and potential limitation of growth of silicifying plankton in the north patch our estimates of the enhanced carbon uptake are similar to estimates from higher silicate environments citation hales b and t takahashi 2012 mesoscale biogeochemical responses to iron fertilization in the upper layers of the southern ocean iron experiment areas j geophys res 117 c01018 doi 10 1029 2011jc006956

did hot high heat producing granites determine the location of the oslo rift abstract the late carboniferous early permian oslo rift formed in apparently cold stable lithosphere of the fennoscandian shield in a tensional stress regime widely documented in northwest europe at that time the rift formed obliquely to older crustal structures that display only limited permian reactivation and although numerical modelling suggests that the present day lithospheric structure would serve to focus tensional stresses in the oslo region the assumption that no lithospheric evolution has occurred since the palaeozoic is by no means obvious here i show that up to 5 km thick regional scale late to post sveconorwegian granites in the vicinity of the oslo rift with heat production rates averaging ca 5 w m3 nearly three times higher than the surrounding sveconorwegian gneisses would have increased the temperature in the lower crust and lithospheric mantle by up to 100 c resulting in significant thermal weakening of the lithosphere in this area given a tensional stress regime weakening by these high heat producing element granites would have made the oslo area a favoured site for passive rifting and may have been a first order parameter locating rifting to this part of the fennoscandian shield the thermo rheological effects of such granites must be considered along with other factors in future models of initial rift mechanisms in the oslo rift and probably in other rifts elsewhere

heat flow and thermal structure of the lithosphere across the baltic shield and northern tornquist zone abstract terrestrial heat flow data from archean and proterozoic provinces of the baltic shield its southwestern margin and the tectonic transition to the younger tectonic units of central europe northern segment of the european geotraverse are compiled and examined for local and regional variations a 2 d numerical lithospheric temperature and heat flux model is presented covering 1800 km of the deep seismic fennolora profile across the baltic shield and 500 km of eugeno s northern eugemi seismic lines across the northern tornquist zone and the danish and north german basins thermal lithosphere defined as the outer layer with relatively low temperatures t c in which heat transfer is dominantly by conduction is found to vary markedly in thickness from 85 110 km beneath basins and tornquist zone to about 150 km in central shield areas and 200 250 km in areas of low heat flow along the northern shield profile these results are in very good agreement with seismological information from surface wave dispersion analysis and the refraction wide angle seismic interpretation of fennolora data the combined thermal and seismologic evidence indicates that the uppermost mantle region stratified in seismic velocity is restricted to the upper relatively cold thermal layer which is interpreted to constitute shield lithosphere local and to some extent regional variations in surface heat flow are found to be closely related to variations in near surface radiogenic heat production for the southern baltic shield a heat flow heat production relationship of q 0 32 8 7 6 h 0 is derived regional increase in heat flow from northern and central shield provinces to the shield margin and the tornquist zone is interpreted to be related partly to differences in upper crustal heat generation partly and in southernmost areas mainly to an increased heat flux from the upper mantle upper mantle heat flux is estimated at 20 25 mw m 2 along the northern and central shield transect increasing to 30 40 mw m 2 beneath the shield margin basins and tornquist zone lithospheric thickness variations in this region of northern europe are found to be closely associated with variations in heat flux from the uppermost mantle and perhaps to a large extent controlled by geodynamically determined lateral differences in upper mantle heat flux earthquake focal depth data and information on crustal strength show baltic shield seismicity to be limited generally to the upper 20 25 km brittle part of the crust at temperatures less than 300 400 c maximum source depth of magnetic anomalies at temperatures near 600 c is in the deepest part of the crust near the crust mantle boundary in the southern region about 25 km depth along the tornquist zone and about 35 40 km in southern shield areas and in the uppermost mantle 60 70 km depth in the northern shield areas some petrologic implications are discussed the present deep crust is supposed to be at lower temperature pressure conditions than at early stages of formation however also the present deepest parts of the shield crust 35 40 km are within the stability field of eclogites the general absence of significant gravity anomalies in areas of marked crustal thickness variations indicate high density eclogites to be present in significant amounts in the deep shield crust

hornblende 40ar 39ar geochronology across terrane boundaries in the sveconorwegian province of s norway abstract hornblende incremental heating 40 ar 39 ar data were obtained from augen gneiss and amphibolite of the sveconorwegian province of s norway in the rogaland vest agder and telemark terranes four pyroxene rich samples located close 10 km to the anorthosite charnockite rogaland igneous complex define an age group at 916 12 14 ma and six samples distributed in the two terranes yield another group at 871 8 10 ma the first age group is close to the reported zircon u pb intrusion age of the igneous complex 931 2 ma and the regional titanite u pb age 918 2 ma whereas the second group overlaps reported regional mineral rb sr ages 895 853 ma as well as biotite k ar ages 878 853 ma in the first group the comparatively dry parageneses of low p thermal metamorphism m2 associated with the intrusion of the igneous complex are well developed and hornblende 40 ar 39 ar ages probably record a drop in temperature shortly after this phase in other hornblende biotite rich samples with presumably a higher fluid content the hornblende ages are probably a response to hornblende fluid interaction during a late sveconorwegian metamorphic or hydrothermal event a ca 220 m y diachronism in hornblende 40 ar 39 ar ages is documented between s telemark ca 870 ma and bamble ca 1090 ma differential uplift between these terranes was mostly accommodated by shearing along the kristiansand porsgrunn shear zone the final stage of extension along this zone occurred after intrusion of the herefoss post kinematic granite at 926 8 ma on the contrary the southern part of the rogaland vest agder and telemark terranes share a common cooling evolution as mineral ages are similar on both sides of the mandal ustaoset line the tectonic zone between them the succession within 20 m y of a voluminous pulse of post tectonic magmatism at 0 93 ga a phase of high t low p metamorphism at 0 93 0 92 ga and fast cooling at a regional scale ca 0 92 ga suggests that the southern parts of rogaland vest agder and telemark were affected by an event of post thickening extension collapse at that time this event is not recorded in bamble

the siberian lithosphere traverse mantle terranes and the assembly of the siberian craton abstract the kimberlite fields scattered across the ne part of the siberian craton have been used to map the subcontinental lithospheric mantle sclm as it existed during devonian to late jurassic time along a 1000 km traverse ne sw across the archean magan and anabar provinces and into the proterozoic olenek province 4100 garnets and 260 chromites from 65 kimberlites have been analysed by electron probe major elements and proton microprobe trace elements these data and radiometric ages on the kimberlites have been used to estimate the position of the local paleo geotherm and the thickness of the lithosphere and to map the detailed distribution of specific rock types and mantle processes in space and time a low geotherm corresponding approximately to the 35 mw m2 conductive model of pollack and chapman tectonophysics 38 279 296 1977 characterised the devonian lithosphere beneath the magan and anabar crustal provinces the devonian geotherm beneath the northern part of the area was higher rising to near a 40 mw m2 conductive model areas intruded by mesozoic kimberlites are generally characterised by this higher but still cratonic geotherm lithosphere thickness at the time of kimberlite intrusion varied from ca 190 to ca 240 km beneath the archean magan and anabar provinces but was less 150 180 km beneath the proterozoic olenek province already in devonian time thinner devonian lithosphere 140 km in parts of this area may be related to riphean rifting near the northern end of the traverse differences in geotherm lithosphere thickness and composition between the devonian toluopka area and the nearby mesozoic kimberlite fields suggest thinning of the lithosphere by ca 50 60 km related to devonian rifting and triassic magmatism a major conclusion of this study is that the crustal terrane boundaries defined by geological mapping and geophysical data extended from outcrops in the anabar shield represent major lithospheric sutures which continue through the upper mantle and juxtapose lithospheric domains that differ significantly in composition and rock type distribution between 100 and 250 km depth the presence of significant proportions of harzburgitic and depleted lherzolitic garnets beneath the magan and anabar provinces is concordant with their archean surface geology the lack of harzburgitic garnets and the chemistry of the lherzolitic garnets beneath most of the other fields are consistent with the proterozoic surface rocks mantle sections for different terranes within the archean portion of the craton show pronounced differences in bulk composition rock type distribution metasomatic overprint and lithospheric thickness these observations suggest that individual crustal terranes of both archean and proterozoic age had developed their own lithospheric roots and that these differences were preserved during the proterozoic assembly of the craton data from kimberlite fields near the main archean proterozoic suture the billyakh shear zone suggest that reworking and mixing of archean and proterozoic mantle was limited to a zone less than 100 km wide

a conceptual model of an arctic sea 1 we propose a conceptual model for an arctic sea that is driven by river runoff atmospheric fluxes sea ice melt growth and winds the model domain is divided into two areas the interior and boundary regions that are coupled through ekman and eddy fluxes of buoyancy the model is applied to hudson and james bays hjb a large inland basin in northeastern canada for the period 1979 2007 several yearlong records from instruments moored within hjb show that the model results are consistent with the real system the model notably reproduces the seasonal migration of the halocline the baroclinic boundary current spatial variability of freshwater content and the fall maximum in freshwater export the simulations clarify the important differences in the freshwater balance of the western and eastern sides of hjb the significant role played by the boundary current in the freshwater budget of the system and its sensitivity to the wind forcing are also highlighted by the simulations and new data analyses we conclude that the model proposed is useful for the interpretation of observed data from arctic seas and model outputs from more complex coupled climate models

meso cenozoic morphotectonic evolution of southern norway neogene domal uplift inferred from apatite fission track thermochronology apatite fission track aft thermochronology of precambrian and paleozoic basement samples from southern norway reveals a post paleozoic exhumation history related to offshore mesozoic and cenozoic extensional basin development the data indicate two major phases of rapid exhumation a first mesozoic phase started in the triassic 220 ma in the east and south of the study area and migrated to the west where jurassic 160 ma ages of exhumation predominate a second event is indicated by thermal history modeling of aft ages and track length distributions it is inferred to be neogene in age initiated at about 30 ma and it produced a domal pattern of aft isochrons which follow present day topographic elevation youngest aft ages 100 ma are encountered at sealevel in the inner fjords near the areas of highest topography ages increase radially outward to the mountain peaks and the coastlines forward modeling of age elevation patterns suggests that mesozoic geothermal gradients were 10 15 c km higher than the present value of 20 c km during the triassic and jurassic a total of 1 3 3 5 km of overburden was removed from the study area assuming a 30 c km geothermal gradient for that period we attribute this to rift margin erosion as a result of erosional base level lowering and flank uplift as evidenced by thick continental clastic sequences deposited in triassic jurassic half grabens in the north sea basins we propose that 1 5 2 5 km of neogene exhumation were a result of late stage domal uplift this is supported by basinward dipping pre neogene strata in the basins surrounding southern norway and the infill of a 1 to 2 km thick neogene sediment wedge containing various internal unconformities domal uplift probably started in the late oligocene may have been amplified in the pliocene and was overprinted by plio pleistocene glacial erosion maximum neogene tectonic uplift is estimated at approximately 1 1 5 km radially decreasing outward to a value 500 m near the shoreline neogene domal uplift is coincident with oligocene and pliocene plate reorganizations in the north atlantic similar neogene domes are found around the norwegian greenland sea i e svalbard and the barents sea northern norway east greenland suggesting a regional tectonic cause the onset of neogene uplift postdates major volcanism and continental breakup by 25 m y and predates plio pleistocene glaciations its origin is possibly a combination of induced mantle convection resulting in thermal erosion of the lithosphere and the operation of intraplate stresses

the oslo rift a review the tectonomagmatic history of the oslo rift may be subdivided into 5 main periods stage 1 300 ma development of a shallow depression stage 2 300 295 ma roughly simultaneous onset of widespread basalt volcanism b1 and vertical movement along nnw sse to n s trending faults stage 3 295 275 ma main rifting period accompanied by volcanism dominated by rhomb porphyry lavas latites stage 4 275 240 ma change of magmatic style from basaltic shield volcanism to central volcanoes of mixed compositions many of which went through caldera collapses intrusion of composite batholiths of intermediate to granitic compositions stage 5 240 ma dike intrusions may have continued into triassic time both the crust and total lithosphere under the rift are thinned relative to the precambrian basement on both sides a 12 km thick high density and high velocity body occupies the lower crust along the rift axis and extends under the eastern and western rift shoulder petrological and geochemical data imply that this massive layer represents dense cumulates and gabbroic rocks formed from mantle derived magmas in deep crustal magma chambers two mantle sources one nearly undepleted age corrected nd of about 1 and sr of 10 to 15 the other mildly depleted nd of about 4 sr of 10 contributed melts to the oslo rift magmatism the two sources are interpreted as different parts of a heterogeneous upper mantle which before the rifting event belonged to the subcontinental lithosphere on their way to the surface the mantle derived magmas were retained in deep crustal magma chambers where they underwent extensive fractional crystallization some also suffered moderate contamination anatectic melts and melts formed by mixing between mantle derived and anatectic melts gave rise to syenitic and granitic intrusions

permo carboniferous magmatism and rifting in europe introduction an extensive rift system developed within the northern foreland of the variscan orogenic belt during late carboniferous early permian times post dating the devonian early carboniferous accretion of various neoproterozoic gondwana derived terranes on to the southern margin of laurussia laurentia baltica fig 1 rifting was associated with widespread magmatism and with a fundamental change at the westphalian stephanian boundary in the regional stress field affecting western and central europe ziegler 1990 ziegler cloetingh 2003 the change in regional stress patterns was coincident with the termination of orogenic activity in the variscan fold belt followed by major dextral translation between north africa and europe rifting propagated across a collage of basement terranes with different ages and thermal histories whilst most of the carboniferous permian rift basins of nw europe developed on relatively thin lithosphere the highly magmatic oslo graben in southern norway initiated within the thick stable and presumably strong cold lithosphere of the fennoscandian craton the rift basins in the north sea in contrast developed in younger caledonian age lithosphere which was both thinner and warmer than the lithosphere of the craton to the east

upper ocean water mass characteristics of the california current summer 1993 abstract two large high resolution upper ocean surveys of the california current region were conducted in the summer of 1993 temperature and salinity were measured from a seasoar vehicle and velocity was measured by a ship borne acoustic doppler current profiler both surveys extended from the continental margin to 128 w and consisted of zonal sections 28 km apart the first survey 7 28 june extended from 39 5 n to 36 25 n and the second 16 august to 1 september extended from 39 0 n to 37 0 n the current field was fairly simple in june showing an equatorward surface jet with only one gentle meander a nearly continuous poleward california undercurrent near the continental margin and two anticyclonic subsurface eddies offshore of the jet by august meanders in the surface jet had strengthened and eddies dominated the flow field large scale trends in the water mass characteristics spiciness along three isopycnal surfaces 26 4 26 0 and 25 6 kg m 3 show that temperature and salinity tend to increase towards the equator and towards shore consistent with poleward advection along the continental margin local minima in spiciness tend to coincide with the equatorward surface jet local maxima in spiciness are associated with subsurface anticyclonic eddies which have a core depth of about 150 m and seem to originate in the california undercurrent over the continental slope at local latitudes we identified several different anticyclonic eddies each with different water mass characteristics all of the eddies we observed had diameters larger than the baroclinic radius of deformation water mass characteristics and migration rates are consistent with formation over the continental margin within the preceding 6 8 months

silicon isotope and silicic acid uptake in surface waters of marguerite bay west antarctic peninsula the silicon isotope composition 30si of dissolved silicon dsi and biogenic silica bsi provides information about the silicon cycle and its role in oceanic carbon uptake in the modern ocean and in the past however there are still questions outstanding regarding the impact of processes such as oceanic mixing export and dissolution on the isotopic signature of seawater and the impacts on sedimentary bsi this study reports the 30si of dsi from surface waters at the rothera time series rats site ryder bay in a coastal region of the west antarctic peninsula wap the samples were collected at the end of austral spring through the end of austral summer beginning of autumn over two field seasons 2004 5 and 2005 6 broadly for both field seasons dsi diminished and 30si of dsi increased through the summer but this was accomplished during only a few short periods of net nutrient drawdown during these periods the 30si of dsi was negatively correlated with dsi concentrations the si isotope fractionation factor determined for the net nutrient drawdown periods uptake was in the range of 2 26 to 1 80 when calculated using an open system model and 1 93 to 1 33 when using a closed system model these estimates of are somewhat higher than previous studies that relied on snapshots in time rather than following changes in 30si and dsi over time which therefore were more likely to include the effects of mixing of dissolved silicon up into the mixed layer results highlight also that even at the same station and within a single growing season the apparent fractionation factor may exhibit significant temporal variability because of changes in the extent of biological removal of dsi nutrient source siliceous species and mixing events paleoceanographic studies using silicon isotopes need careful consideration in the light of our new results

integrated geophysical modelling of a lateral transition zone in the lithospheric mantle under norway and sweden summary the scandinavian mountain chain the scandes exhibits characteristics that are unusual for an old palaeozoic mountain belt these include renewed neogene uplift in a passive margin setting and the lack of a pronounced crustal root we investigate the influence of present day thermal compositional and geometric structures in the crust and mantle on the topography of the southern scandes and the resulting implications for the tectonic history of fennoscandia a self consistent 3 d subsurface model of southern norway and sweden is constructed from recent geophysical data sets that constrain the crustal architecture lithosphere geometry density distributions and thermal properties recent seismological studies show evidence for a relatively abrupt transition from seismically slower lithospheric mantle underneath southern norway to seismically faster lithospheric mantle underneath southern sweden we find that a transition from thin subcontinental lithospheric mantle sclm underneath southern norway to a significantly thicker sclm beneath southern sweden can explain this velocitytrend adifferenceincompositionbetweenthetwosclmdomains owingtodifferent depletion refertilization is required to satisfy the gravity field and isostatically compensated topography but contributes only slightly to the observed velocity contrast such a lateral transition zone constituting a major change in the lithospheric structure in southwestern fennoscandia can be related to several major proterozoic and phanerozoic tectonic events such as the sveconorwegian orogeny the permian oslo rift and perhaps even the opening of the atlantic

an array study of lithospheric structure across the protogine zone varmland south central sweden signs of a paleocontinental collision abstract a small seismological array was installed on both sides of the protogine zone pz in varmland south central sweden to study the structure of the mantle lithosphere and lateral variations of its anisotropy no distinct isotropic velocity anomalies were detected by tomography in the upper mantle around the pz the observed velocity variations depending on the direction of propagation can be explained by anisotropy within the subcrustal lithosphere on both sides of the suture the best solution of a joint analysis of anisotropic inferences from teleseismic p residual spheres and an inversion of shear wave splitting parameters resulted in 3d self consistent anisotropic models of blocks of the subcrustal lithosphere the anisotropic structures within the lithosphere are approximated by hexagonal models k p 5 with low velocity symmetry axes the high velocity planes dip to the e in a region westward of the pz and to the nw eastward of the pz the pz can be interpreted as a steep and narrow suture cutting the whole lithosphere and separating the two anisotropic blocks of different origin

a two channel laterally averaged estuarine circulation model laecim a two dimensional two channel laterally averaged estuarine circulation model laecim with a sigma coordinate in the vertical is developed for tides tidal currents and salinity in a two channel estuary with any arbitrary angle the model is forced by oceanic tides composed of 16 major constituents and freshwater discharge the two channels are governed by the same dynamic equations a control volume at the junction between the two channels is designed to conserve mass and volume transport and to guarantee the continuity of momentum in the finite difference schemes based on the physical principle in continuum medium of fluid an explicit leapfrog scheme in time and centered differencing in space are employed together with a time filter to remove the computational mode an euler backward scheme is periodically used to correct retard the phase frequency which is overestimated due to the explicit leapfrog scheme used the model is applied to the entire san francisco bay with its two channels the main north south channel and the golden gate channel which is perpendicular to the former i e with a right angle to the former the simulated time series of tides tidal currents and salinities compare favorably with the observed tidal gauge data and moored current and salinity time series during the summer 1980 the simulated along channel salinity section also compares well to the conductivity temperature depth sectional observations during the same period the model simulated eulerian residual current is consistent with two layer flow theory

basin migration caused by slow lithospheric extension sedimentary basin migration caused by low lithospheric extension rates is investigated using a two dimensional dynamic numerical model of the lithosphere we find that continental breakup will eventually occur when larger extension velocities are used the duration of rifting prior to continental breakup is dependent on the extension velocity stretching the lithosphere with lower velocities does not lead to breakup instead the locus of maximum extension migrates deformation localizes outside the first formed basin that is in turn uplifted this basin then becomes a cold spot in the area in this case syn rift cooling predominates the lithosphere regains its strength during stretching instead of becoming weaker and the lithosphere necking zone becomes stronger than surrounding regions the transition velocity is for the cases studied about 8 mm yr while the locus of maximum thinning migrates after about 50 60 myr a comparison with observations of the mid norwegian galicia and ancient south alpine margins shows a close resemblance of important features fl 2002 elsevier science b v all rights reserved

rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock mechanics based yield criteria for the inelastic deformations at the top and base at mid plate depths stresses are likely to be supported elastically because bending strains and elastic stresses are low the collisional tectonic regime however is far more complex because very large permanent strains are sustained at mid plate depths and this requires us to include the broad transition between brittle and ductile flow moreover important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid plate depths two specific contributions of laboratory rock rheology research are considered in this paper first the high temperature steady state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere the thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm tc above which at geologic strain rates the high temperature ductile strength falls below some arbitrary strength isobar e g 100 mpa for olivine tc is about 700 800 c but for other crustal silicates tc may be as low as 400 600 c suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the moho as suggested by a number of workers on independent grounds put another way the moho is a rheological discontinuity a second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant strain rate tests or as an accelerating creep rate stage at constant differential stress a number of physical mechanisms have been identified that may be responsible for ductile faulting including the onset of dynamic recrystallization phase changes hydrothermal alteration and hydrolytic weakening microscopic evidence for these processes as well as larger scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere

asymmetric lithosphere as the cause of rifting and magmatism in the permo carboniferous oslo graben compared to other permo carboniferous rift basins of nw europe the oslo graben has two distinct characteristics first it initiated inside cold and stable precambrian lithosphere whereas most permo carboniferous basins developed in weaker phanerozoic lithosphere and second it is characterized by large volumes of magmatic rocks despite relatively little extension seismic reflection surveys show that the crust thickens from southern norway to southern sweden the most significant moho deepening occurring from the oslo region eastwards deep seismic studies also suggest that the base of the lithosphere deepens markedly eastwards from the oslo region such a long wavelength lithospberic geometry cannot be explained by the permian or post permian evolution of the area hence the oslo graben appears to have evolved at the transition between two lithospheric domains with contrasting thickness numerical thermo mechanical modelling is applied to test if this transitional position can influence the dynamics of rifting different models with varying lithosphere thickness contrast are considered model results show that a crust and lithosphere thickness contrast comparable to the oslo region can explain rifting and focusing of magmatism in a narrow zone with minor thinning of the crust models also account for the major characteristics of the oslo graben in terms of location and strain distributions in the crust the geological society of london 2004

iron and silicic acid concentrations regulate si uptake north and south of the polar frontal zone in the pacific sector of the southern ocean abstract we investigated the relative roles of fe and silicic acid availabilities in regulating si uptake rates across the polar frontal zone in the pacific sector of the southern ocean 59 68 s 170 w during the us jgofs antarctic environment southern ocean process study aesops meridional gradients in silicic acid concentration si oh 4 of about 0 25 0 56 m km 1 were observed in this area during austral spring and summer 1997 1998 with si oh 4 ranging from 32 si oh 4 and 15 no 3 uptake rates biogenic silica concentrations and si oh 4 no3 uptake ratios fe addition had little or no effect on si uptake rates in enrichments conducted in the low si waters north of the si gradient however fe addition increased si uptake rates 3 5 times over controls in enrichments conducted in the high si waters south of the gradient in both spring and summer fe addition decreased si oh 4 no3 uptake ratios by 2 5 times largely due to stimulation of no3 uptake rates zn addition had no effect on si oh 4 and no3 uptake rates short term 24 h si additions had varying effects on si uptake rates depending on season and location in spring additions of 40 m si to water from bottle enrichments conducted north of the si gradient in situ si oh 4 15 m did not increase si uptake rates initially but did increase uptake rates after 8 days in the summer enrichment north of the si gradient in situ si oh 4 5 m 50 m si additions doubled in situ si uptake rates in the initial water collected for the enrichment and increased si uptake rates as much as 16 fold during the experiment south of the si gradient where in situ si oh 4 was 40 m in both spring and summer si addition had no effect on in situ si uptake rates in the initial enrichment water nor on any si uptake rates measured during the experiment our results indicate that both fe and si availabilities regulate si uptake rates and silica production in the southern ocean along 170 w fe limitation appears to restrict si uptake rates south of the si gradient and plays a role in preventing si depletion south of the acc where ambient si oh 4 never fell below 40 m during 1997 1998 our experiments in the seasonal ice zone at 62 suggest that si uptake in this area switched from being fe limited in the spring when in situ si oh 4 was 40 m to si limited in the summer when si oh 4 was

high resolution biogeochemical investigation of the ross sea antarctica during the aesops u s jgofs program 1 the results of high resolution biogeochemical measurements in the upper 200 m of the ross sea antarctica obtained during the aesops u s jgofs program using the lamont pumping seasoar lps are presented they consist of three west east transects from 170 e to 180 longitude along the aesops study line at 76 5 s and three short north south transects in the ross sea polynya during the initial and maturing stages of phytoplankton blooms in the austral spring and early summer of 1997 the lps carried an in situ instrument array for measurement of temperature salinity fluorescence photosynthetically active radiation par and dissolved oxygen in addition a highpressure pump mounted aboard the lps fish delivered a continuous seawater sample stream to the shipboard laboratory for high speed analysis of its nutrient nitrate plus nitrate phosphate and silicate and total co2 concentrations and co2 partial pressure pco2 vertical resolution of this sampling equaled or exceeded that of hydrostation style conductivity temperature depth ctd casts horizontal resolution nominally equal to a vertical cast every 3 5 km exceeded station resolution by a factor of 10 while not perfectly synoptic the 20 hour duration of these transects is far shorter than the time typically taken to complete the line with conventional sampling methods these surveys clearly identified three distinct deep water masses below about 100 m high salinity shelf water hssw in the western end of the transects modified circumpolar deep water mcdw in the middle of the transects and low salinity shelf water lssw to the east the regions to the west were characterized by high biological productivity with high n p and c p uptake ratios but little si uptake indicating that the production was dominated by phaeocystis to the east biological productivity was lower than in the west and low n p and c p uptake ratios and high si uptake indicated the dominance of diatoms the difference in uptake ratios appears to be entirely due to anomalously high p uptake by diatoms n c uptake ratios are similar in the two regions and very near canonical redfield stoichiometry the area in the center of the transects was characterized by high stratification and low diatom dominated productivity the reason for this low productivity is unclear but is speculated to be due to the short time which this water has been exposed to an ice free surface the presence of strong variability at horizontal length scales of order 10 km is evident in nearly all fields especially in upper 100 m although many of the features are resolved only by two or three lps tracks each separated by only a few kilometers physical bio optical and chemical variability is observed to extend vertically from sea surface to depths as deep as 140 m far below the 1 light level and the mixed layer depth this may be attributed to downwelling of waters associated with eddies and or meandering a simple statistical measure presented to quantify the errors associated with the undersampling of such a highly variable field shows that biogeochemically important parameters like pco2 and oxygen and chlorophyll concentrations are poorly resolved by a commonly used 50 km hydro station spacing longitudinal averages of these parameters however are predicted fairly well at coarser resolutions index terms 4207 oceanography general arctic and antarctic oceanography 4805

elevated mixing at a front 1 the mesoscale submesoscale and microscale structure of a front in the california current was observed using a towed vehicle outfitted with microconductivity sensors thirteen 60 km cross front sections from 0 to 350 m in depth were covered in 3 5 days objectively mapped data are fit via the omega equation to obtain vertical velocity a composite cross front section shows elevated mixing on the dense side within 10 20 km of the front water downwells and gradients are elevated there i e rossby number ro horizontal strain spice gradients and microscale thermal dissipation thermal eddy diffusivity kt reaches 10 3 m2 s 1 and increases 3 10 from the anticyclonic to the cyclonic side with a depth mean of 10 4 m2 s 1 the spatial structure of kt ro and are similar on the dense side suggesting an energy cascade from the mesoscale via the submesoscale to the microscale however it is unclear whether frontogenesis internal wave blocking by elevated vorticity or internal wave trapping by large produces the elevated mixing the mean turbulent heat flux opposes the mean restratifying mesoscale heat flux of 10 w m 2 and may allow the front to persist turbulent nitrate fluxes are 0 1 0 3 mmol m 2 s 1 chlorophyll fluorescence and beam transmission reveal a 6 km wide 100 km long alongfront streamer which is a deep biomass maximum time scales for mixing and nutrient fluxes are 0 3 3 days which are similar to phytoplankton growth rates and the time scale for frontal evolution

observations of seismic reflectors in the lower lithosphere beneath the skagerrak the lithosphere the rigid outer shell of the earth includes both the crust and the upper mantle in the past decade the complex continental crust and underlying shallower mantle lithosphere have been extensively studied using reflection seismology but the lower lithosphere has not been successfully imaged here we report the observation of continuous and virtually horizontal reflectors in the lower continental lithosphere down to depths of 100 110 km below the skagerrak scandinavia these are apparently not structurally associated with the crust whereas previously reported mantle reflectors such as those observed by the birps group around the british isles1 2 are often located near large faults in the overlying crust like the crust the mantle lithosphere shows a seismically transparent upper and a reflective lower part thus the processes that form seismic reflectors seem to have an affinity for ductile rheological environments in the mantle as well as in the crust our observations are consistent with a model consisting of a mechanically strong upper lithosphere underlain by a thermal boundary layer which separates it from the convecting asthenosphere

a stretching model for the oslo rift abstract data on the tectonic and magmatic activity crustal thickness seismic velocity and gravity have been integrated to constrain a stretching model for the late carboniferous early permian oslo rift the crustal structure and numerical modelling of temperature and melt generation during rifting suggest that the oslo rift in skagerrak developed in response to crustal stretching with a factor between 1 4 and 1 6 however upper crustal extension by faulting accounts for a stretching factor of only 1 1 1 2 thermal expansion of the lithosphere due to an increased asthenospheric potential temperature the intrusion of melted mantle material into the lower crust and probably a lowering of the erosional base by a fall in sea level all contributed to the erosion of 2 4 km of lower palaeozoic pre rift sediments over large areas the lateral extent of the thermal anomaly reached far beyond the oslo and skagerrak grabens

rifting in heterogeneous lithosphere inferences from numerical modeling of the northern north sea and the oslo graben oslograbenevolvedinthecoldandstableprecambrian lithosphere of fennoscandia whereas the northern northsearifttookbirthinfreshlyreworkedcaledonian lithosphere huge volumes of magmatic rocks characterize the relatively narrow oslo graben in contrast little magmatism is documented for the wide northern north sea rift differences in timing between both rifts are inferred but still debated we present numerical thermomechanical models along a lithospheric e w section that involves both the oslo graben and the northern north sea area because the modeled section crosses the boundary between caledonian and proterozoic provinces thermal and compositional heterogeneities are considered as is suggested by various geophysical data sets we also consider lithospheric thickness heterogeneities in the precambrian lithosphere modeling results suggest that the northern north sea was on top of weak lithosphere very sensitive to far field stresses consequently we suggest that rifting in the northern north sea began as early as regional extension was effective i e late carboniferous early permian and does not postdate the oslo graben as it is commonly assumed rifting in the strong precambrian lithosphere is unexpected modeling results suggest thatapre existinglithosphericthicknesscontrastwithin the fennoscandian lithosphere favored rifting in the oslo graben index terms 8109 tectonophysics continental tectonics extensional 0905 8159 tectonophysics evolution of the earth rheology crust and lithosphere 8020 structural geology mechanics 9335 information related to geographic region europe 9614 information related to geologic time paleozoic keywords numerical modeling rifting rheology lithosphere oslo graben north sea citation pascal c and s a p l cloetingh rifting in heterogeneous lithosphere inferences from numerical modeling

finite duration extension and decompression melting abstract decompression melting of the upwelling mantle during extension of continental lithosphere provides a viable mechanism for the magmatism observed in many rifted sedimentary basins and passive volcanic margins a model has been developed in which in addition to the adiabatic temperature decrease syntectonic conductive cooling melt transport and emplacement at crustal levels are accounted for the heat equation including latent heat is solved numerically and the incremental melt fraction is traced to simulate dynamic melting the emplaced magma is considered an integral part of the continental crust we find that relative to an instantaneous rifting model with only adiabatic cooling there is a more than 10 decrease in the melt thickness even when the duration of rifting is as short as 1 m y whether extension takes place with constant horizontal velocity or strain rate has important implications for the amount of melt produced consequently pressure release melting does not only depend on the amount of stretching and duration of the rift episode but also on how extension develops through time

tomographic estimates of sub moho seismic velocities in fennoscandia and structural implications abstract pn and sn velocity estimates are derived for the sub moho structure beneath fennoscandia using a tomographic conjugate gradient scheme observational data stem from local earthquake recordings of crustal phases pg sg lg and the sub crustal phases pn and sn by the fennoscandian seismograph network unknowns are pn and sn velocities and event hypocenter parameters crustal thicknesses are assumed as known and are not estimated whereas velocities are allowed to vary the most prominent p and s velocity anomalies are found in the northwest inland lofoten and southwest more of norway and for shear waves also in central finland there is a good correlation between the velocity anomalies in norway and observed negative bouguer anomalies indicating the presence of low density sub moho material in these areas geologically the pn and sn velocity anomalies appear to be associated with geodynamic processes tied to the opening of the norwegian greenland sea

styles of lithospheric extension controlled by underplated mafic bodies abstract the role of underplated mafic bodies upmb in the localization of deformation is examined using a two dimensional thermo mechanical finite element model rheological heterogeneity brought about by the upmb is linked with the following two main physical effects the upmb material is assumed to have 1 an anomalous high temperature and 2 a mafic crustal rock composition that is intrinsically weaker than the mantle the thermal effect will disappear rather quickly but the rock composition can have an effect at any stage of extension we show that the upmb has a strong influence on the style of lithospheric extension which depends on the thermal condition of the uppermost mantle at the time when the upmb is emplaced since the strength contrast between the weakened and non weakened regions is the most important factor the upmb works more efficiently on the localization of deformation for a colder uppermost mantle dependence of the localization of deformation on the temperature and thickness of the upmb i e more significant localization for the upmb with higher temperature and greater thickness also depends on the thermal condition of the uppermost mantle however the width of the upmb has a strong influence on the localization for any thermal condition of the uppermost mantle a greater amount of thinning is distributed into a narrower weakened region such a model behaviour implies that various styles of lithospheric extension including inward or outward migration of deformation and asymmetric extension can be simply obtained by considering the emplacement of the upmb which also plays an important role in controlling the onset of continental break up

location and dynamics of the antarctic polar front from satellite sea surface temperature data the location of the antarctic polar front pf was mapped over a 7 year period 1987 1993 within images of satellite derived sea surface temperature the mean path of the pf is strongly steered by the topographic features of the southern ocean the topography places vorticity constraints on the dynamics of the pf that strongly affect spatial and temporal variability over the deep ocean basins the surface expression of the pf is weakened and the pf meanders over a wide latitudinal range near large topographic features width and temperature change across the front increase and large scale meandering is inhibited elevated mesoscale variability is seen within and downstream of these areas and may be the result of baroclinic instabilities initiated where the pf encounters large topographic features the strong correlations between topography and pf dynamics can be understood in the context of the planetary potential vorticity ppv or f h field mean ppv at the pf varies by more than a factor of 2 along its circumpolar path however at the mesoscale the pf remains within a relatively narrow range of ppv values around the local mean away from large topographic features the pf returns to a preferred ppv value of 25 10 9 m 1 s 1 despite large latitudinal shifts the mean paths of the surface and subsurface expressions of the pf are closely coupled over much of the southern ocean

effect of lithosphere thickness heterogeneities in controlling rift localization numerical modeling of the oslo graben 1 we present 2 d thermo mechanical and numerical modeling at lithospheric scale to assess the impact of pre existing lithosphere thickness contrast on rift localization the results of the modeling suggest that thickness heterogeneity was responsible for focussing rifting and passive asthenospheric upwelling in the permo carboniferous oslo graben at the western margin of the fennoscandian shield these findings are consistent with inferences from recent studies showing that both the crust mantle boundary and the base of the lithosphere are and were deeper at the eastern margin of the oslo graben resulting in contrasting crust and lithosphere thickness

syn rift thermal structure and post rift evolution of the oslo rift southeast norway new constraints from fission track thermochronology abstract the permo carboniferous 240 305 ma oslo graben in southeast norway is a classical magmatic continental rift we present fission track ft data on apatites zircons and sphenes from the rift and surrounding areas in order to clarify its syn and post rift thermal evolution zircons and sphenes within the rift record ft ages of between 270 and 180 ma i e syn post rifting in contrast the precambrian basement surrounding the graben yields zircon ft sphene ft ages of 500 650 ma indicating that heating associated with rifting was focused inside the rift as a result of advective heat flow syn rift to early post rift temperatures reached 240 c at the present erosion level in the graben syn rift advective heating 260 270 ma was probably a result of large scale batholith intrusion post rift 180 220 ma heating seems to have occurred by hydrothermal circulation of high temperature 100 300 c fluids the ft data organic geochemical indicators and fluid inclusions suggest that hydrothermal circulation followed a highly complex pattern controlled by the tectonic and volcanic structure of the rift apatite ft ages and confined track length distributions reveal the post rift exhumation history of the area apatite ft ages are continuous within and outside the graben and decrease from triassic 200 240 ma in the southeast to jurassic 160 ma in the northwest indicating that post rift exhumation took place on a much larger scale than permian rifting modelling of apatite ft ages and track lengths suggests cooling denudation events in the triassic jurassic and neogene accounting for a total post permian erosion of 3 4 km triassic jurassic denudation coincides with the migration of rifting from the oslo skagerrak area to the north sea danish basin as revealed by offshore seismic data and caused destruction of the hydrothermal regime neogene uplift and erosion of the area is also supported by seismic evidence the inferred timing and amount of regional post rift erosion suggests that post rift subsidence and sedimentation within the oslo graben was minimal

extension and magmatism in the oslo rift southeast norway no sign of a mantle plume the late carboniferous permian 300 240 ma oslo graben in southeast norway is a classical continental magmatic rift about 8 12 km of mafic magma was produced during a period of 40 60 my the mantle derived rock volume is 3 105 km3 with an average melt production rate of 0 005 km3y 1 initial subsidence and the onset of magmatism preceded graben formation by 5 10 my upper crust extension inferred from dykes and faults is 10 whereas the total crust appears to have been thinned by up to 30 the mantle upwelled with a velocity 2 mmy 1 and suffered conductive heat loss adiabatic melting models therefore do not apply a melting model that takes into account syn rift conductive cooling depth variable stretching and mantle composition suggests that the lithospheric solidus must have been reduced relative to anhydrous peridotite and that mantle thinning should be greater than crustal stretching the lithospheric affinity of primitive rocks the initial subsidence the absence of a hot spot track and the low magma production rate indicate that oslo graben magmatism was not associated with a mantle plume we conclude that the oslo graben was more wet than hot and that extension and magmatism were caused by passive necking of the lithosphere

observations of the electrical asthenosphere beneath scandinavia the recent recognition that long period i e of the order of hours electromagnetic induction studies could play a major role in the detection of the asthenosphere has led to much interest amongst the geophysical and geological communities of the geomagnetic response functions derived for differing tectonic environments experiments carried out on the ocean bottom have met with considerable success in delineating the electrical asthenosphere i e a local maximum in electrical conductivity minimum in electrical resistivity in the upper mantle in this paper observations of the time varying magnetic field recorded in three regions of scandinavia northern sweden kiruna kir northern finland northeastern norway kevo kev and southern finland sauvamaki sau are analysed in order to obtain estimates of the inductive response function c for each region the estimated response functions are compared with one from the centre of the east european platform eep and it is shown that the induced eddy currents at periods of the order of 103 104 s in the three regions flow much closer to the surface than under the platform centre specifically at a period of 3000 s these currents are flowing at depths of the order of kev 120 km kir 180 km sau 210 km eep 280 km implying that the transition to a conducting zone of 0 2 s m occurs at around these depths one dimensional inversion of ckev and ckir shows that there must exist a good conducting zone of 0 1 1 0 s m under each of the two regions of 40 km minimum thickness at depths of kev 105 115 km kir 160 185 km this is to be contrasted with eep where the d profile displays a monotonically decreasing resistivity with depth reaching 0 1 s m at 300 km finally a possible temperature range for the asthenosphere consistent with the deduced conducvitity is discussed it is shown that at present there is insufficient knowledge of the conditions water content melt fraction etc likely to prevail in the asthenosphere to narrow down the probable range of 900 1500 c

thermomechanical structure of european continental lithosphere constraints from rheological profiles and eet estimates summary the eet equivalent elastic thickness of the lithosphere is a measure of the integrated lithospheric strength it is directly related to the mechanical thickness and rheology of the crustal and mantle lithosphere we present a comparison of eet estimates and strength profiles based on the extrapolation of rock mechanics data for different parts of the european and eurasian continental lithosphere we discuss the temporal and spatial variations in the mechanical thickness and strength inferred from data for precambrian segments of europe s lithosphere variscan europe and the alpine collision belt this analysis demonstrates important spatial and temporal variations in lithospheric rigidity for orogenic belts and sedimentary basins in eastern and western europe and asian parts of eurasia the eet estimates based on synthetic rheological profiles constrained by newly available geophysical data are consistent with the estimates of eet derived from flexural studies of sedimentary basins forelands and orogenic belts these rheological profiles suggest weakening of the major parts of the european and eurasian continental lithosphere by decoupling of the crustal and upper mantle parts a comparison with the seismicity depth distribution for some selected sites suggests that the intra plate seismicity is essentially restricted to the upper crustal parts of europe s lithosphere providing additional support to the notion of the decoupled lithosphere the presence of intra plate stress fields can explain a significant part of the observed variations in eet estimates within individual thermotectonic age groups a comparison of wavelengths of crustal and lithospheric folding with observations shows these wavelengths to be consistent with estimates of eet inferred from the rheological response of basins and orogens at more moderate levels of intra plate stress

role of weak zone orientation in continental lithosphere extension 1 lithosphere extension and continental breakup axes are often sub parallel to orogenic belts and suture zones in an attempt to understand the relation between weak zone orientation and extension direction a 3 d numerical model is developed with which the following aspects are studied the relation between weak zone axis and axis of maximum crustal deformation the amount and distribution of crustal thinning and subsequent implications for rift a symmetry the results suggest that upon oblique extension rifts develop within the weak zone that individually cross the inherited structure while as a group they follow the weak trend this results in an alternating rift asymmetry the pattern of crustal thinning topography and thermal structure are not symmetric around the rift axes of the major rift zones and change along axis over such a rift this is a possible explanation for alternating asymmetric rift structures such as observed in the east african rift system

crustal extension in the oslo graben se norway a method incorporating magmatism and erosion abstract three methods for determining crustal extension were examined for use in the late paleozoic oslo graben of southeast norway 1 1 analysis of faults and dikes 2 2 volume budget isostasy 3 3 analysis of crustal thickness distribution assuming constant crustal volume during extension these methods were found not to be suitable mainly due to rift age and magmatism instead a new integrated model which combines the effect of moho contours volumes of mantle injecta and erosion was used volumes of intracrustal mantle injecta are found from a model combining data on upper crustal felsic magmatic rocks with petrological and gravity inversion models for the vestfold graben segment in the south this implies twice the stretching estimate as compared to stretching derived from moho contours alone and two to three times the estimate from an inadequate set of fault and dike information the batholiths mark zones of high crustal thinning and magmatism was most likely caused by a combination of a thermal anomaly and lithospheric stretching

dissolved fe in the upper waters of the pacific sector of the southern ocean the concentration of dissolved fe was determined in upper ocean waters along 170 degreesw between 53 and 72 degreess during four cruises between october 1997 and march 1998 as part of the us jgofs southern ocean antarctic polar frontal zone cruises fe concentrations were extremely variable in both space and time as a result of advection of meso scale eddys through the region observed values ranged from a maximum of 0 34 nm at 64 degreess in november next to the retreating ice edge to 0 075 nm in march at 71 degreess in the northern ross sea gyre in general the highest average mixed layer fe concentrations and the largest seasonal changes were observed in the two frontal zones at similar to 60 degreess and similar to 64 degreess formation of deep mixed layers during winter and the entrainment of sub surface waters enriched in fe is the primary source of this element to surface waters in this region of the southern ocean additionally upwelling of circumpolar deep water in the circumpolar current along its northern boundary at the polar front and its southern boundary at the southern antarctic circumpolar current front is an important mechanism supplying fe to surface waters in these areas throughout the summer comparison of integrated fe concentrations and th derived poc export and n drawdown all suggest that the maximum changes in these properties are comparable to those that would be predicted by fe availability fe concentrations determined by shipboard flow injection analysis measures et al mar chem 50 1995 3 were significantly higher than those found in replicate samples determined by shored based flameless atomic absorption spectrophotometry after preconcentration by chelation solvent extraction johnson et al mar chem 57 1997 137 the concentration differences which appear to be restricted to the upper 200 300 m do not appear to arise from different sampling or handling processes but instead are believed to result from the different sensitivity of the methods to organic fractions of fe in seawater c 2001 elsevier science ltd all rights reserved

u pb systematics in volcanic and plutonic rocks of the krokskogen area resolving a 40 million years long evolution in the oslo rift abstract the development of the oslo rift started in the latest carboniferous in the northern foreland of the variscan orogen the initial intensive and mainly mafic magmatism at around 300 ma was followed by a period of some 40 m y of mafic and felsic volcanic and plutonic activity the present study applied u pb in zircon on rocks of the krokskogen area northwest of oslo to resolve in greater detail the sequence of events in this evolution a major challenge however has been the absence or paucity of zircon and their low quality for dating in some of the important units especially the alkalic latitic rhomb porphyries the earliest rhomb porphyry flow rp1 yields an age of 299 7 0 4 ma it was thus coeval with the earliest basaltic and alkalic eruptions in the southern part of the oslo graben flows rp6 and rp7 yield overlapping ages of 285 5 0 8 and 284 9 0 5 ma and rp11 is 280 2 0 5 ma the baerum oppkuven and heggelia caldera volcanoes and the slottet kjelsasite pluton developed at 276 7 to 275 9 ma while the oyangen caldera is somewhat younger at 273 272 ma a kjelsasite stock has an age of 265 4 0 6 ma predating the 262 3 0 5 ma blindern rhomb porphyry like dyke which is among the youngest magmatic expressions in the rift the new data stress the importance of the magmatic burst at around 300 ma possibly plume related along the entire oslo rift and sharpens our understanding of the timing of the subsequent protracted sequence of magmatic events reflecting continuous heating and stress in the lithosphere likely broadly controlled by processes accompanying the closing of pangea

location of the antarctic polar front from amsr e satellite sea surface temperature measurements abstract the location of the southern ocean polar front pf is mapped from the first 3 yr of remotely sensed advanced microwave scanning radiometer for the earth observing system amsr e sea surface temperature sst measurements in agreement with previous studies the mean path of the antarctic pf and its standard deviation are strongly influenced by bottom topography however the mean pf path diverges slightly from previous studies in several regions where there is high mesoscale variability although the sst and sst gradient at the pf show spatially coherent seasonal variations with the highest temperature and the lowest temperature gradient during summer the seasonal variations in the location of the pf are not spatially coherent the temporal mean sst at the pf corresponds well to the mean pf path the temperature is high in the atlantic and indian ocean sections and is low in the pacific ocean section where the pf has a more southerly position the relationship of the wind field with the antar

insights into the magmatic architecture of the oslo graben by petrophysically constrained analysis of the gravity and magnetic field 1 a new model of the crustal structure of the oslo graben with a special emphasis on the location of magmatic intrusions is presented based on the interpretation of gravity and magnetic data constrained by petrophysical data seismics and surface geology three dimensional modeling shows that the gravity high above the oslo rift is caused by a combination of crustal thinning and the presence of a high density body in the upper crust related to precambrian gneiss complexes at the western flank of the rift the short wavelength gravity anomalies are correlated with the outcropping low density magmatic rocks of permian age that extend to the depths of 1 5 4 km in addition simultaneous modeling of aeromagnetic data sets collected at two flight altitudes 50 and 3400 m using susceptibility and remanence within a range of reasonable magnetic properties suggests an underlying intrusion with a depth extent up to 15 km magnetization and density of the intrusion are increasing with depth leading to a small density contrast but a large contrast in magnetization with the surrounding rocks therefore only modeling of the magnetic field can resolve the nature of the deep intrusions the three dimensional model provides a new estimate of the volume of magmatic material in the oslo graben system 63 000 km3 this is only two thirds of the values proposed earlier 95 000 km3 as we disregard high volume magmatic underplating correlation with structural data surface geology and deep lithospheric features shows that the distribution of extrusive rocks in the oslo rift can be related to the timing and onset of rifting

intraplate deformation and 3d rheological structure of the rhine rift system and adjacent areas of the northern alpine foreland the lithosphere of the northern alpine foreland has undergone a polyphase evolution during which interacting stress induced intraplate deformation and upper mantle thermal perturbations controlled folding of the thermally weakened lithosphere in this paper we address relationships among deeper lithospheric processes neotectonics and surface processes in the northern alpine foreland with special emphasis on tectonically induced topography we focus on lithosphere memory and neotectonics paying special attention to the thermo mechanical structure of the rhine graben system and adjacent areas of the northern alpine foreland lithosphere we discuss implications for mechanisms of large scale intraplate deformation and links with surface processes and topography evolution

the si cycle in the pacific sector of the southern ocean seasonal diatom production in the surface layer and export to the deep sea abstract dissolved silicic acid si oh 4 and biogenic silica bsio 2 concentrations were measured on four different cruises in the southern ocean along 170 w from late october 1997 to mid march 1998 these data were used to construct a vertically integrated si budget in the surface layer from 58 to 68 s throughout that period there was a distinct southward moving meridional si oh 4 gradient with concentrations increasing to the south in october the gradient was centered on the antarctic polar front apf at 61 s surface layer si oh 4 increased from 10 to 40 mmol m 3 between 58 and 62 s while bsio 2 concentrations were 3 and 2 throughout the study area as the surface mixed layer stratified and shoaled in december diatoms bloomed in response to the more favorable light conditions consuming si oh 4 in the process by late december bsio 2 concentrations were exceptionally high for the open ocean with maximum integrated values of 700 mmol si m 2 around 62 s while vertically integrated si oh 4 in the upper 50 m had dropped from 2500 to 870 mmol m 2 an average decrease of 30 mmol si m 3 the diatom bloom persisted into january with maximum integrated bsio 2 of 600 mmol m 2 between 64 and 65 s while si oh 4 was depleted to 3 in surface waters as far south as 64 5 s the center of the si oh 4 gradient had moved southward to 65 s by late january coincident with maximal diatom biomass at that time the bloom ended by march with the disappearance of most of the diatom biomass from the surface layer deeper mixed layers and increased si oh 4 by late january diatom production had removed 70 of the of si oh 4 initially present in the upper 50 m everywhere north of 65 s and had displaced the si oh 4 gradient 400 km southward while the apf remained stationary between november and late january 80 days an average of 2200 mmol si m 2 had been removed from the upper 50 m between 61 and 65 s based on changes in vertically integrated si oh 4 and bsio 2 concentrations between november and march an average of 2 mol si m 2 was exported from the upper 50 m between 61 and 65 s with 59 of that export occurring between late december and late january the observed seasonal export is very high in comparison with other oceans and the high export coincides spatially with a well documented zone of high opal flux to 1000 m and high opal content in modern sediments north of 61 s bsio 2 export was limited by the low initial standing stock of si oh 4 in surface waters and south of 65 s export was likely limited by the shorter growing season thus the combined effect of a large initial supply of si oh 4 in surface waters and conditions that permit efficient use of that si by diatoms appears to be the primary cause of opal rich sediment accumulation in the pacific sector of the southern ocean

a seasonal progression of si limitation in the pacific sector of the southern ocean we conducted kinetic experiments using the radioisotope 32si to assess si limitation in the pacific sector of the southern ocean between 53 and 71 s from early december 1997 through mid march 1998 a diatom bloom developed near the edge of the pack ice in late november or early december and then propagated southward depleting silicic acid in surface waters from 45 to 5 m and occasionally to 1 m in an area that extended from 61 to 65 s we performed kinetic experiments of two kinds 26 multiple concentration experiments in which the specific rate of silicic acid uptake v was measured as a function of the extracellular silicic acid concentration si oh 4 and 61 two concentration experiments in which v was measured only at the ambient concentration si oh 4 a and one concentration 30 70 m above ambient the rate of si uptake was almost always limited by si oh 4 a in the si depleted waters within the bloom and to its north in those waters v increased with si oh 4 in accordance with the michaelis menten saturation function and si oh 4 a often limited v to 40 of its potential maximum vmax si limiting conditions followed the bloom southward during spring and summer and extended 500 km south of the antarctic polar front apf by late january seventeen of the 87 kinetic experiments showed no si limitation all but three of those non limiting sites were in si rich waters south of the apf however there was occasional relatively weak limitation of si uptake even south of the apf at si oh 4 a 40 m this unexpected result was caused by very low affinity for si uptake high ks in the michaelis menten function seventy percent of all measured ks values 49 of 70 were between 0 7 and 10 m a range only about twice that reported from elsewhere but the remaining 21 ks values ranged from 10 7 to 61 m eleven of these were 20 m which is extraordinarily high in comparison with most values from other oceans there was no clear tendency for ks to change with latitude or with si oh 4 a but there was an apparent seasonal progression in which both the highest ks values and the variability in ks increased with time the initial slopes of the kinetic curves vmax ks a measure of the ability to exploit low concentrations for the southern ocean diatom assemblages were very low compared to other regions of the ocean in the southern ocean data 52 of 73 initial slopes 71 are 2 mm h 1 while only one of 37 initial slopes 3 from other systems is that low the lowest slopes were observed south of the apf which may be a consequence of the high silicic acid concentrations that prevail in that region the ability to compete for silicic acid at low concentrations appears not to impart a significant competitive advantage in the si rich system south of the apf

a three dimensional model of density driven circulation in the irish sea abstract a semi implicit version of the princeton ocean model ecomsi was used to simulate the cyclonic gyre that is found in the western irish sea during the spring and the summer mesoscale seasonal density driven circulations such as this are an important component of the long term flow in shelf seas and they pose a challenge to coastal ocean models extensive comparisons are made here with observational data to assess model performance the model successfully reproduced the development of the temperature field and the associated density driven currents throughout seasonal simulations the results demonstrate conclusively that the gyre is density driven and reinforce the dynamical importance of strong nearbed horizontal density gradients maximum baroclinic currents of 0 14 m s 1 were obtained and results showed that the regional kinetic energy due to the residual flow was 20 25 of that due to tidal flow during periods in which density gradients were strongest the model revealed important int

paleozoic paleo cover of the southern part of the fennoscandian shield fission track constraints abstract granitic intrusions older than 1200 ma from an area west of lake vanern south sweden show fission track ages of 680 50 ma 2 late precambrian for sphene and 220 10 ma late triassic for apatite the samples were taken at the present post glacial erosion surface which is close to the sub cambrian peneplain the c 680 ma sphene fission track ages are thought to be related to the uplift of the area which led to the establishment of this erosion surface at c 600 ma ago the much younger apatite fission track ages and the confined spontaneous track length distributions along with regional stratigraphic constraints suggest that the basement was subsequently buried under 3 4 km of paleozoic supracrustals this induced in the basement a rise in temperature which obliterated the older apatite fission tracks but not those in sphene which has a higher closure temperature at the beginning of the mesozoic the area was uplifted again the cover rocks were removed and the basement rocks cooled slowly below the temperature range of fission track annealing in apatite

seismic structure and composition of the crust beneath the southern scandes norway abstract new results on p and s wave seismic velocity structure in southern norway indicate that the crust has an average poisson s ratio of 0 25 is predominantly of felsic intermediate composition and lacks a significant mafic lower crust a crustal scale refraction seismic study magnus rex mantle investigations of norwegian uplift structure refraction experiment acquired data along three 300 to 400 km long active source seismic profiles across the southwest scandinavian domain in southern norway the youngest section of the fennoscandian shield moho depths in the domain are 36 40 km thinning towards the continental shelf and oslo graben the high vp lower crust beneath the southwest scandinavian domain vp 7 km s is around 4 km thick crustal structure in the adjacent svecofennian domain differs significantly moho depths reach 50 km and an up to 24 km thick high vp lower crust is present strong p and s wave arrivals are recorded from magnus rex from which poisson s ratio for the crust is calculated the unusually strong s wave arrivals allow rare insight into crustal poisson s ratio structure that is not commonly available from active source data the thinness of the mafic lower crust in southern norway is in significant contrast to other parts of the fennoscandian shield and delamination or convective removal of the lower crust is proposed tectonic events that may have triggered detachment of the lower crust include extensional collapse after the caledonian orogeny and permian extension

the moho depth distribution in fennoscandia and the regional tectonic evolution from archean to permian times abstract from a variety of seismic refraction and reflection profiling results a new detailed moho depth map for the fennoscandian part of the baltic shield has been compiled a complementary crystalline crustal thickness map was compiled for denmark the skagerrak sea offshore western norway and adjacent areas where sedimentary strata reach maximum thicknesses of 10 km and even more these maps are discussed in relation to tectonic evolution and crustal ages of fennoscandia seismic velocities bouguer gravity and seismicity areas deviating from the proposed age thickness relationships are the kola peninsula archean and the oslo rift permian the average crustal thickness in the former area is around 44 km while the latter area is associated with a moho elevation of 3 5 km maximum crustal thicknesses in excess of 50 km have been found in southwestern and central finland and correlate with a crust of proterozoic age the caledonides of western norway do not exhibit as elsewhere a crustal root sub moho p n and s n seismic velocities are relatively small in that area and bouguer gravity values are strongly negative intracrustal p velocity variations do not seem to reflect geological age or specific tectonic features although the seismicity is moderate in fennoscandia most of the earthquake activity and in particular the largest earthquakes are confined to areas where the crustal thinning is most pronounced

geophysical characteristics of the tornquist fan area northwest trans european suture zone indication of late carboniferous to early permian dextral transtension

linking changes in tectonic style with magmatism in northern europe during the late carboniferous to latest permian abstract early carboniferous to permian magmatism associated with rifting within the northern foreland of the variscan orogen was widespread across europe during the long period of magmatic activity the regional tectonic setting changed across the region from early carboniferous extension and basin formation to a rifting wrenching style of deformation in the late carboniferous stephanian to early permian wrenching and faulting were accompanied by widespread voluminous and episodic magmatic extrusion intrusion and underplating this was followed by thermal relaxation and the development of the northern and southern permian basins in later permian times thermal relaxation was punctuated by a permo triassic phase of extension and graben formation 40ar 39ar ar step heating dating for mineral separates and whole rock samples of magmatic rocks from southern scandinavia oslo graben and south sweden and rugen north germany provides further radiometric evidence for three of the proposed periods of magmatic activity in the region latest carboniferous to earliest permian ages c 300 310 ma were obtained for volcanic rocks in the oslo graben and dolerite sills and dykes in south sweden and north germany this phase can be time correlated with magmatic activity that occurred throughout europe during large scale dextral wrenching that followed the variscan orogeny a second phase of alkaline intrusions is confined to the oslo graben and related to caldera collapse around c 275 ma the third permo triassic phase c 250 ma is considered to be related to a new tectonic cycle involving extension that triggered minor melting of enriched fertile mantle

fission track thermochronology applied to phanerozoic thermotectonic events in central and southern sweden abstract central and southern sweden is characterized by a precambrian basement and a few lower palaeozoic sedimentary remnants with a probable initial thickness of less than 500 m upper palaeozoic deposits have not been documented in the swedish part of the fennoscandian shield regional deposition of upper palaeozoic sediments on the platform appears possible though since vast amounts of material must have been eroded from the caledonian mountain range during this period fission track thermochronology has been used in this study in order to validate the former existence of a post caledonian sedimentary load trace the variation in thickness and extent of the deposits and get an indication of the cooling history in the area samples from an east west trending profile from the caledonian border zone in the west to the aland archipelago in the east together with a few samples from surrounding areas have been analysed the obtained pooled apparent apatite fission track ages along the profile vary from 250 ma in the west to 500 ma in the east while the mean track lengths decrease from 14 m in the west to 11 m in the east in southern sweden ages between c 170 and 320 ma and a mean track length of 13 4 m were obtained the dating results show that all samples have experienced raised temperatures since the pre cambrian peneplanation temperatures leading to total annealing of fission tracks during the palaeozoic were reached in the western part of central sweden and in southern sweden while partial annealing is traced further east to the aland archipelago the obtained fission track results are interpreted by the former existence of a devonian foreland basin covering most of central and southern sweden an estimation of the upper palaeozoic sedimentary thickness yields 2 5 km of deposits in the ludvika area and at least 1 km in the aland archipelago inverse modelling of the fission track data resulted in several alternative modelled thermal histories one of these suggests that central sweden was covered by palaeozoic and mesozoic sediments until tertiary time in that case the surface relief of western central sweden has been chiselled out during the tertiary another possible thermal history includes denudation due to surface level conditions during the mesozoic in the west in that case reheating to 40 80 c during the late mesozoic cenozoic is required according to the present apatite annealing models none of the proposed modelled thermal histories can be rejected so far

vertical budgets for organic carbon and biogenic silica in the pacific sector of the southern ocean 1996 1998 abstract during the 1996 1998 antarctic environment and southern ocean process study aesops a component of us jgofs we obtained seasonal or longer term data sets on the rates of production vertical transport remineralization and burial of particulate organic carbon poc and biogenic silica bsio2 in the southern ocean at 170 w between 55 s and 68 s the aesops data records enable us to construct vertical c and si budgets for the water column and upper sediments with all estimates derived from direct measurement of the relevant fluxes we constructed annual c and si budgets for each of four ecologically distinct zonal bands within the system for both poc and bsio2 the greatest annual delivery to the sea floor 200 and 1400 mmol m 2 yr 1 respectively and burial 6 and 160 mmol m 2 yr 1 respectively were observed in the southern antarctic circumpolar current acc between 61 5 s and 65 5 s that pattern is consistent with our observation that a diatom bloom propagated southward through the southern acc during the spring and summer of 1997 1998 following the receding ice edge and that this bloom was the main source of both poc and bsio2 in the system on an annual basis in the other zones the annual fluxes of poc and bsio2 to the sea floor ranged from 19 to 67 of those in the zone traversed by the summer diatom bloom the higher benthic fluxes of both poc and bsio2 in the southern acc imply that blooms similar to the one we observed in 1997 1998 occur commonly in the southern acc and that their high productivity signature is transmitted to the sea floor the data show preferential preservation of bsio2 over poc throughout the water column and upper seabed in the four zonal bands we consider bsio2 and poc are produced in mole ratios of 0 1 0 4 exported from the upper 100 m in ratios of 0 2 0 6 arrive at 1000 m in ratios of 1 5 4 5 reach the sea floor in ratios of 2 2 7 6 and are buried in ratios of 11 6 28 despite the preferential preservation of bsio2 accumulation of opal rich sediments beneath the acc does not result from unusually efficient preservation of siliceous material the estimated bsio2 preservation efficiency burial production ranges from 1 2 to 5 5 indistinguishable from the global average of 3 instead opal rich sediment accumulation in this region reflects very high annual rates of bsio2 production in surface waters along with very low accumulation rates of other sedimentary components e g caco3 and detrital material the observed high ratios of bsio2 production to poc production in surface waters are consistent with the known tendency for the si c ratio of diatoms to increase when fe is low if greater fe availability during the last glacial maximum permitted diatoms in the southern ocean to grow with lower more normal si c ratios export of diatom produced poc could have occurred at 2 3 times its present rate a corresponding increase in opal export or opal sediment accumulation is unlikely because silicic acid is almost totally depleted north of 65 s under present conditions thus even large increases in poc production and export during glacial periods would not be reflected in the opal accumulation record

the influence of structural inheritance and multiphase extension on rift development the northern north sea the northern north sea rift evolved through multiple rift phases within a highly heterogeneous crystalline basement the geometry and evolution of syn rift depocentres during this multiphase evolution and the mechanisms and extent to which they were influenced by pre existing structural heterogeneities remain elusive particularly at the regional scale using an extensive database of borehole constrained 2d seismic reflection data we examine how the physiography of the northern north sea rift evolved throughout late permian early triassic rp1 and late jurassic early cretaceous rp2 rift phases and assess the influence of basement structures related to the caledonian orogeny and subsequent devonian extension during rp1 the location of major depocentres the stord and east shetland basins was controlled by favorably oriented devonian shear zones rp2 shows a diminished influence from structural heterogeneities activity localises along the viking sogn graben system and the east shetland basin with negligible activity in the stord basin and horda platform the utsira high and the devonian lomre shear zone form the eastern barrier to rift activity during rp2 towards the end of rp2 rift activity migrated northwards as extension related to opening of the proto north atlantic becomes the dominant regional stress as rift activity in the northern north sea decreases through documenting the evolving syn rift depocentres of the northern north sea rift we show how structural heterogeneities and prior rift phases influence regional rift physiography and kinematics controlling the segmentation of depocentres as well as the locations styles and magnitude of fault activity and reactivation during subsequent events

crustal structure of the fennoscandian shield a traveltime interpretation of the long range fennolora seismic refraction profile abstract the 1979 fennoscandian long range project fennolora was aimed at the determination of the detailed structure in the earth s mantle down to a depth of about 400 km observation distances reached almost 2000 km within scandinavia between shotpoints off the north cape and the southern coast of sweden to achieve an unbiased regarding the upper mantle structure a careful crustal survey was carried out along the entire profile at the same time beneath the fennoscandian shield i e the central section of the profile the crust is characterized by quite a smooth increase in p wave velocity down to the moho which lies at a depth of about 50 km in the southern half of the shield and at about 45 km further north the mean crustal velocity is 6 6 6 7 km s at the base of the crust the velocity increases gradually from about 7 km s to 8 0 8 4 km s in a 5 10 km thick crust mantle transition zone both in the south and in the north the relatively homogeneous crust of the baltic shield borders on areas with a more differentiated velocity structure first order discontinuities at shallower depth characterize the crust mantle boundary near the southeastern tip of sweden 38 km and under the caledonides in the north 43 km

skagerrak evolution derived from tectonic subsidence abstract stratigraphic data gathered from 32 points in the continental skagerrak basin have been analyzed by basin modelling methods non tectonic effects of sediment loading were removed by backstripping giving the observed tectonic subsidence this was compared to synthetic subsidence curves constructed on the basis of a combined lithospheric extension and metamorphism model where gabbroic magma which was emplaced during the permian is transformed to granulits eclogite facies the overall picture is that prior to 230 ma subsidence was roughly similar north and south of the fennoscandian border zone fbz after this time the northern area sagged due to the still preserved lithospheric thermal anomaly and continued metamorphism whereas the southern area was also affected by extension a physical explanation for this contrast in evolution is given by considering the relative lithospheric strength history of the areas the northern area becoming progressively stronger with time relative to the southern area as crustal thicknesses are reduced total post 570 ma extension is approximately 1 4 40 north of the fbz and 1 5 50 south of it north of the fbz the average thickness of the metamorphic layer is between 3 and 7 km whereas south of the zone a thickness of 1 2 km is estimated this difference may reflect variations in the degree of permian magmatism the temperature history of the sediments and the degree of organic maturation has been calculated by the tti method the youngest sediments to have experienced temperatures high enough for significant oil generation are estimated to be of bathonian age the present model heat flow of about 57 mw m 2 is in agreement with observations lateral displacements along the fbz are of the order of 10 20 km and have consequently only played a minor role in the development of the skagerrak basin a thermal lithosphere thickness of about 125 km is inferred

did the variscides collapse or were they torn apart a quantitative evaluation of the driving forces for postconvergent extension in central europe extension following immediately after the end of convergence caused rapid destruction of the european variscan orogen and resulted in the formation of numerous sedimentary basins and widespread magmatism possible driving forces for this phase of extension i e gravitational collapse of the thickened crust or reorientation of the far field stress regime are assessed quantitatively using a two dimensional thermomechanical finite element modeling approach the initial model setup approximates the geometry and physical properties of a lithosphere scale half profile through the variscan orogen and its foreland various numerical simulations and parameter studies suggest that the gravitational instability of the variscan orogen alone was not sufficient to explain the observed amount and timing of crustal extension tensile plate boundary stresses of the order of 10 mpa superposed on the gravitational forces are required to overcome the finite strength of the crust and restore a uniform crustal thickness in the variscan realm modeling results are constrained by comparison with existing data sets from the central european variscides for example crustal structure seismic reflectivity as well as temporal and spatial distribution of magmatic activity

a separating coastal upwelling jet at cape blanco oregon and its connection to the california current system the coastal upwelling region near cape blanco oregon 433n o the west coast of the united states was studied using a towed ctd on seasoar a shipboard acoustic doppler current pro ler adcp satellite sea surface temperature maps and surface drifters during august 1995 the equatorward upwelling jet was inshore of the shelfbreak north of cape blanco meandered gently o shore around the cape and then veered sharply o shore just to the south of the cape analysis of vertical sections of density velocity and spicinessa con rmed the separation of an upwelling jet and front the jet was followed from over the shelfbreak north of the cape to 100 km o shore downstream of the cape the separating jet originates from the coastal upwelling jet leaving the shelf but is augmented by an o shore branching of the poleward undercurrent by combining information from adcp streamfunction maps with analysis of hydrographic data including distributions of the tracer like quantity spicinessa a conceptual model of the three dimensional circulation near the cape emerges a mid shelf upwelling jet encounters a coastal promontory then turns o shore stretching and deepening as it crosses the continental margin the jet then turns back shoreward cyclonically where the deepened equatorward ow encounters the top of the poleward undercurrent owing along the continental slope this causes a portion of the undercurrent to turn o shore to join and strengthen the equatorward transport in the separated jet throughout the separation process the coastal upwelling front and jet are continuous robust features separation of a coastal upwelling jet is an important mechanism for cross shelf transport from the coast to the deep ocean cape blanco appears to be the northernmost point where an equatorward jet regularly separates from the coast to become an oceanic jet and thus serves as the northern boundary of the region where an intense meandering current characteristic of the california current system

mesoscale frontal dynamics shaping the environment of primary production in the antarctic circumpolar current abstract the frontal regions of the antarctic circumpolar current acc differ from other parts of the acc due to higher phytoplankton concentrations and primary production rates we hypothesise that the enhancement of primary production results from the mesoscale frontal dynamics in particular the cross front circulation related to baroclinic instability the hypothesis is corroborated by data collected in austral summer 1995 1996 at the antarctic polar front between 1 w and 12 e during quasi synoptic surveys with a measuring system combining towed and vessel mounted instruments further confirmation is obtained from moored current meters and an earlier section worked across the front in austral winter 1992 the quasi synoptic surveys cover a meander structure of the front and a cold cyclonic eddy located to its south the highest chlorophyll concentrations correlating with enhanced primary production rates are found in a band of mesoscale patches aligned with the front and in a tongue extending southward from the front along the leading edge of a meander ridge the increased chlorophyll concentrations at the meander edge are explained by confluence of surface water which carries with it the phytoplankton grown in favourable light conditions while mesoscale surface confluence structures the synoptic chlorophyll distribution pattern the mean enhancement of primary production at the front can be attributed to the influence of cross front circulation on stratification ageostrophic cross front circulation related to mesoscale upwelling and downwelling was identified at a site of eddy front interaction consistent with the principle of potential vorticity conservation upwelling was found to occur on the anticyclonic equatorward side of the jet and downwelling on the cyclonic poleward side in the frontogenetic situation the associated cross front circulation is characterised by poleward motion of light water at the surface and a reversed flow of dense water at greater depth thus it contributes to stratification and thereby to a more favourable photic environment for the phytoplankton growing in the shallower mixed layer while the cross front circulation varies on horizontal scales of 100 km and time scales of days to weeks it is constrained to sites of available potential energy i e fronts marked by sloping isopycnals with mixed layer shallowing by cross front circulation we suggest a mechanism to explain the enhancement of primary production by mesoscale frontal dynamics in regions and during periods of light limitation this complements the understanding of the importance of frontal dynamics for primary production hitherto focussed on mesoscale upwelling of nutrients in the oligotrophic regime

is there evidence for magmatic underplating beneath the oslo rift we challenge some of the long standing beliefs related to the permian oslo rift structure often referred to as a case example type locality for continental rifting the crustal structure of the oslo rift was long presumed to be thinned proterozoic crust overlying a permian high density layer interpreted as magmatic underplating new data support an alternative view of the crustal structure in the oslo rift region the bouguer gravity high in the region shows a strong asymmetry a steep westward facing gradient to the west of the rift and a much gentler eastern gradient we present a 3d density model based on petrophysical and seismic information which accounts for the bouguer gravity high using an eastward extension of old precambrian structures without invoking a prominent magmatic underplated structure reactivation of old pre rift structures appears to be an important feature affecting the evolution and location of the permo carboniferous oslo rift

on the physical structure of a front in the bellingshausen sea abstract the structure of a front observed at 67 s in austral spring in the bellingshausen sea is described the front extended to full depth and has been identified by read et al 1995 as the southernmost major front of the antarctic circumpolar current where circumpolar deep water cdw outcrops in this paper the structure of the front is examined on scales down to 10 km using a closely spaced ctd section and down to 3 km from underway surveys with seasoar and an accustic doppler current profiler adcp which resolved the density and velocity structure respectively in the top 400 m a seasonal halocline was present throughout the survey area on both sides of the front at the base of a mixed layer 50 70 m deep the signature of the previous winter s mixed layer depth could be seen in the weak stratification between 70 and 150 m the subsurface temperature minimum within the winter mixed layer was significantly colder south of the front 1 7 c than north of it 1 1 c the front defined by eastward velocities greater than 10 cm s 1 was 70 80 km wide with flow at speeds of up to 50 cm s 1 in the surface layer the zone within which water mass changes occurred was narrower about 40 km contained within the zone of eastward flow and extended down to 1000 m the strong velocities in the frontal jet caused differential advection of surface properties from upstream west of the survey area resulting in sharp cross frontal gradients of salinity and chlorophyll a chlorophyll was largest over 5 mg m 3 in a band barely 10 km wide that extended along front for well over 100 km and was tightly constrained along the southern flank of the front where isopycnals outcropped from the winter mixed layer into the surface layer across the seasonal halocline the large values of chlorophyll could only be maintained by advection into the survey area from an unknown source region upstream boyd et al 1995 patches of high chlorophyll were found only south of the front and it is hypothesised that productivity south of the front is a consequence either of some property of the upwelling cdw or of water that had been under ice identified by the 1 7 c subsurface temperature minimum temperature anomalies on the 27 4 kg m 3 isopycnal indicate that patches of water south of the front had broken off from the southern flank of the front so it is probable that the bloom was spreading southwards from the front within the survey area rather than northwards from the ice edge

numerical paleoceanographic study of the early jurassic transcontinental laurasian seaway the forces governing marine circulation of a meridional transcontinental seaway is explored with the princeton ocean model the jurassic laurasian seaway which connected the low latitude tethys ocean with the arctic sea is modeled quantitatively the global ocean is found to have a profound influence on seaway dynamics a north south density difference and hence sea level difference of the global ocean was probably the main factor in forcing the seaway flow when the tethys waters were the denser water the net seaway flow was southward and conversely it was northward for denser arctic waters marine bioprovincial boundaries and sediment data indicate that the seaway probably was dominated by boreal faunal groups and reduced salinities several times in the jurassic the model results suggest that this can be explained by southward flowing seaway currents which may have been related to an oceanic thermohaline circulation where no northern high latitude deep convection occurred

frontal surveys with a towed profiling conductivity temperature depth measurement package seasoar in april 1986 rv oceanus participated in the frontal air sea interaction experiment fasinex1 in the region 27 29 n 68 71 w the objective was to survey a pronounced oceanic front2 4 and its effect on the oceanic and atmospheric boundary layers although moorings have yet to be recovered and much of the data analysis is as yet only preliminary it is possible to report here the typical density structure of a front as derived from seasoar5 6 fig 1a observations which were analysed within a few hours of collection previously unreported detail is revealed including subducted pycnostads areas of weak vertical density gradient with anomalous temperature salinity signature splitting of the frontal jet and three dimensional structure not apparent in the surface temperature field

cenozoic postrift domal uplift of north atlantic margins an asthenospheric diapirism model postrift domal uplifts are a distinct feature of the northern north atlantic rifted margins uplifted regions southern norway finnmark svalbard northern united kingdom occur with a typical spacing of 900 km the amount of domal uplift is substantial 1 2 km thermochronological geomorphological and seismic stratigraphic data suggest a coincident neogene onset of uplift postdating north atlantic breakup and predating onset of glaciation the norwegian domes are characterized by strongly negative gravity anomalies and reduced mantle p wave and s wave velocities previous models for uplift around the north atlantic e g glacial erosion intraplate stresses secondary convection fail to explain the above characteristics we present an asthenospheric diapir model based on a rayleigh taylor instability wherein a hot icelandic asthenosphere layer meets cold cratonic lithosphere the generated asthenospheric diapirs and associated partial melting offer a viable explanation for updoming application of this model to the south norwegian dome succesfully explains most of the currently available data

silicon dynamics within an intense open ocean diatom bloom in the pacific sector of the southern ocean abstract an intense diatom bloom developed within a strong meridional silicic acid gradient across the antarctic polar front at 61 s 170 w following stratification of the water column in late october early november 1997 the region of high diatom biomass and the silicic acid gradient propogated southward across the seasonal ice zone through time with the maximum diatom biomass tracking the center of the silicic acid gradient high diatom biomass and high rates of silica production persisted within the silicic acid gradient until the end of january 1998 ca 70 d driving the gradient over 500 km to the south of its original position at the polar front the bloom consumed 30 to 40 m si oh 4 in the euphotic zone between about 60 and 66 s leaving near surface concentrations our observations indicate that high diatom biomass and high silica production rates previously observed in the marginal seas around antarctica also occur in the deep ocean near the polar front the bloom we observed propagated across the latitudinal band overlying the sedimentary opal belt which encircles most of antarctica implying a role for such blooms in the formation of those sediments comparison of our surface silica production rates with new estimates of opal accumulation rates in the abyssal sediments of the southern ocean which have been corrected for sediment focusing indicate a burial efficiency of 4 6 for biogenic silica that efficiency is considerably lower than previous estimates for the southern ocean

flexural rigidity of fennoscandia inferred from the postglacial uplift the earth s response to glacial loading unloading offers exceptional promise for the study of the physical properties of the lithosphere in particular tilting of paleoshorelines is very sensitive to the lithosphere rigidity to determine the flexural rigidity the isostatic response to deglaciation in fennoscandia is modeled using an earth model with a layered mantle viscosity overlain by an elastic lithosphere the flexural rigidity and asthenosphere viscosity is allowed to vary to get a match between theoretical and observed present rate of uplift and tilting of paleoshorelines five different ice thickness models are used for a relatively thin ice 2500 m in central areas the resulting flexural rigidity is more or less uniform over fennoscandia with a value of 1023 n m elastic thickness te 20 km this is regarded as minimum for the flexural rigidity of central fennoscandia the pattern of the present rate of uplift and the tilts of the paleoshorelines of the area also sets an upper bound of the flexural rigidity 2 5 1025 n m te 110 km in more central areas of fennoscandia the flexural rigidity at the western coast of norway does not seem to exceed 1023 n m te 50 km the most likely glacier model gives a flexural rigidity of 1023 n m te 20 km at the norwegian coast increasing to above 1024 n m te 50 km in central parts of fennoscandia

a three dimensional numerical simulation of hudson bay summer ocean circulation topographic gyres separations and coastal jets abstract the summer ocean circulation in hudson bay is studied numerically using the blumberg mellor model with a 27 5 km 27 5 km horizontal grid and a realistic bottom topography in the control run 1 monthly climatological forcing fields of wind stress oceanic inflow outflow and salt and heat fluxes are used in addition results are presented for a number ot sensitivity experiments 2 no topography otherwise conditions are identical to the control run 3 no wind forcing 4 no oceanic inflow outflow 5 no heat and salt fluxes 6 no temperature and salinity variations and 7 without the nonlinear terms while the overall simulated circulation in hudson bay is cyclonic the strong steering of the flow by the bathymetry is particularly noticeable mesoscale topographic gyres are simulated and the separation of the coastal current due to topographic bumps occurs in several locations the simulated circulation also has well developed vorticity features and narrow density driven coastal jets al

the subpolar front of the japan east sea part ii inverse method for determining the frontal vertical circulation abstract an inverse method for inferring vertical velocities from high resolution hydrographic velocity surveys is formulated and applied to observations collected at the subpolar front of the japan east sea jes taken during several cold air outbreaks the method is distinct from vertical velocity inferences based on the omega equation in that the driving mechanism for the ageostrophic flow is inferred rather than assumed and hence is particularly appropriate for application to wind or buoyancy forced upper ocean currents where friction mixing inertial superinertial motions or higher order effects can contribute along with shear strain of the geostrophic flow to force vertical motions the inferred vertical circulation at the subpolar front of the jes has amplitudes o 100 m day 1 compared to the 20 m day 1 vertical velocities predicted by the omega equation time dependent near inertial motions driven by the winds and modified by the vertical vorticity of the frontal jet appear to be the primary

on upper mantle seismic heterogeneities beneath fennoscandia abstract three dimensional seismic mapping of the upper mantle beneath fennoscandia baltic shield using an ach type of inversion technique in combination with p wave travel time residual observations from the local seismograph network gave the following results the central parts of the baltic shield are characterized by relatively high seismic velocities down to approximately 300 km those parts of the shield most affected by the caledonide orogeny exhibit relatively low velocities particularly in the uppermost 100 km depth interval the lower part of the upper mantle 300 600 km does not exhibit pronounced seismic velocity anomalies and in this respect is in contrast to results from similar studies in regions subjected to neotectonic processes like parts of central and southeastern europe the seismic anomaly pattern in the presumed thickened lithosphere is in quantitative agreement with similar ones derived from surface wave dispersion analysis and inversion of electrical measurements the general orientation of these anomalies coincides with that of the glacial uplift

a north sea southern norway seismic crustal profile summary in this paper results are presented from a seismic refraction experi ment canobe carried out in southern norway ten explosions fired at sea were recorded on land by shifting 13 recording instruments along a pro file with an average station spacing of 5 km the main line runs in a north easterly direction from the south coast at lista along the western margin of the oslo graben into the norsar array with a total length of 5 15 km a separate leg runs across the graben just north of oslo for the first time allow ing a direct comparison of seismic records in the graben with those in the adjacent precambrian shield area the laterally varying crustal structure along the profile is examined by modelling of travel times and amplitudes of p wave arrivals the moho which is the major discontinuity in the lowermost crust appears to sink beneath the coast from 27 km on the seaward side to 34 km onshore the two dimensional modelling procedure adopted proves invaluable in explaining the characteristic amplitude pattern observed in this area beyond the coastal area our picture of the crust is that of a relatively homogeneous one as expected for a shield area there are no indications of significant discontinuities in the crust along the first 300 km of the profile although from the large pg amplitudes within this distance range we infer a strong velocity gradient in the lower crust two structural models are pro posed for the oslo graben where the moho appears to be elevated to between 25 and 29 km

scales of spatial and temporal variability in the southern ocean spatial and temporal variability in geosat altimeter data is used to consider whether the antarctic circumpolar current acc responds on the large scale to global changes in wind forcing or whether its variability is primarily mesoscale the geosat data indicate a spatial decorrelation scale of 85 km and a temporal e folding scale of 34 days using these scales to define autocovariance functions the sea surface height variability is objectively mapped the resulting maps indicate substantial evidence of mesoscale eddy activity over 17 day time intervals meanders of the polar front and subantarctic front appear to elongate break off as rings and propagate statistical analysis of acc variability from altimeter data is conducted using empirical orthogonal functions eofs the first mode eof describes 16 of the variance in total sea surface height across the acc reducing the domain into basin scales does not significantly increase the variance represented by the first eof suggesting that the scales of motion are relatively short and may be determined by local instability mechanisms rather than larger basin scale processes likewise neither complex nor extended eofs indicate statistically significant traveling wave modes

surface circulation and kinetic energy distributions in the southern hemisphere oceans from fgge drifting buoys abstract trajectories of approximately 300 satellite tracked drifting buoys deployed throughout the southern hemisphere oceans during the fiat garp global experiment fgge have been analyzed to infer the mean surface circulation and kinetic energy distributions of the surface flow the resulting picture of mean surface circulation is consistent with expectations based on compilations of historical observations of currents and the density field the distribution of kinetic energy of the mean flow is highly zonal in character with high values in the persistent zonal flows of the antarctic circumpolar current and the south equatorial currents and low values in the broad intervening zone using statistics based on 5 squares the western boundary currents do not emerge as dominant features of the mean flow on the other hand these currents do appear to be a major source of eddy kinetic energy most eddy kinetic energy appears to be due to fluctuations with periods less than one month both mean and eddy kin

3d thermo mechanical modelling of a stretched continental lithosphere containing localized low viscosity anomalies the soft point theory of plate break up abstract we present numerical models of extensional visco elasto plastic 3d continental lithosphere containing weaker areas within its mantle section we aim at understanding the 3d crustal structure of volcanic passive margins that is characterized by both across strike and along strike finite strain gradients with maxima around central igneous complex or their feeding magma reservoirs it is suggested that localized hot melting zones within the lithosphere act as mechanical soft points and result in the local focusing of extension to test this hypothesis 3d thermo mechanical models of extensional continental lithosphere containing thermally induced soft points are implemented results show that crustal extension initiates and is focused over soft points in the mantle reproducing the tectonic segmentation and zig zag pattern of vpms volcanic passive margins

a biogeochemical study in the bellingshausen sea overview of the sterna 1992 expedition abstract a general overview and background of a two ship study to examine biogeochemical fluxes in the marginal ice zone of the bellingshausen sea 65 70 s 80 87 w se pacific sector of the southern ocean is presented the major feature studied was an intense band of chlorophyll that was found geographically close to a receding ice edge however the bloom appeared not to be a result of a shallow mixed layer caused by ice melt stabilisation but rather associated with an oceanic front that coincided with the ice edge details of the hydrographic conditions existing along the 85 w meridian from the ice edge to open water to the north and detailed surveys of surface hydrographic conditions are presented

phytoplankton pigment distribution in relation to silicic acid iron and the physical structure across the antarctic polar front 170 w during austral summer in order to study the factors controlling the phytoplankton distribution across the antarctic polar frontal region pfr surface pigment samples were collected during austral summer january february 1998 near 170 w both the polar front pf and the southern antarctic circumpolar current front saccf were regions of enhanced accumulation of phytoplankton pigments the mesoscale survey across the pf revealed two distinct phytoplankton assemblages on either side of the front the phytoplankton community was dominated by diatoms south of the pf and by nanoflagellates primarily by prymnesiophytes to the north surprisingly chlorophyll a concentrations did not correlate with mixed layer depths however an increase of the dominance of diatoms over prymnesiophytes was observed with decreasing mixed layer depths despite this relationship we conclude that the average light availability in the mixed layer was not an important factor influencing the shift in phytoplankton composition across the pf although no correlation was found between the surface distribution of the major phytoplankton taxa and dissolved iron or silicic acid concentrations the location of the strongest vertical gradient in silicic acid and iron concentration coincides with the maximum abundance of diatoms we conclude that the difference in taxonomic composition is a result of increased silicic acid and iron flux to the upper mixed layer as a result of the increased vertical gradient of these key nutrients south of the front

the lithosphere asthenosphere system in fennoscandia abstract we present a summary of the available information on rayleigh wave dispersion data for the fennoscandian region the observations have been combined to produce regional dispersion relations which have then been subjected to the hedgehog inversion procedure the results are presented on a map outlining the thickness of the lid and the shear velocities in both the lid and the asthenosphere channel lid thickness up to around 135 km is found in the bothnia north central finland area with if any weak shear velocity contrast to the underlying layer the surrounding areas are characterized by lid thickness up to around 75 km a stronger low velocity zone to lid contrast may be found in the caledonian and baltic sea area 0 25 0 45 km s taking into account moho depth data and the aforementioned results a map of the lithosphere asthenosphere system was derived

structure of the scandes lithosphere from surface to depth abstract in this review we give an overview of geophysical data and models available for the scandinavian mountains and adjacent areas as they are of relevance to the debate about the existence and cause of neogene uplift emphasis is given to potential field and petrophysical data of which the earth science communities of norway and sweden have a long tradition of acquisition and interpretation this is reflected in the wealth and dense coverage of data available the topography of the scandes mountain chain correlates to a large degree with a bouguer gravity low which suggests isostatic compensation but comparison with magnetic and petrophysical data shows that the gravity low is partly influenced by the trans scandinavian igneous belt signals due to the effects of surface geology can be further eliminated by considering the near surface density distribution as provided by petrophysical sampling to illuminate the entire lithospheric structure active and passive seismic and magnetotelluric data can be used recent and ongoing experiments are aimed at improving the existing models of the lithosphere integration of these different geophysical data sets allows the structure of the lithosphere of the scandes and its surroundings to be addressed seismic models show no pronounced crustal root below the scandes which could provide isostatic compensation but instead an increase of crustal thickness towards the central fennoscandian shield from the integrated model isostatic compensation is inferred to be largely controlled by the density distribution of the crust and that a high density lower crust to the east of the scandes is necessary to achieve isostatic equilibrium on a regional scale we demonstrate the typical crustal structure with profiles crossing from the scandes into the shield and a combined onshore offshore basement map

the global frequency wavenumber spectrum of oceanic variability estimated from topex poseidon altimetric measurements two years of altimetric data from the topex poseidon spacecraft have been used to produce preliminary estimates of the space and time spectra of global variability for both sea surface height and slope the results are expressed in terms of both degree variances from spherical harmonic expansions and in along track wavenumbers simple analytic approximations both in terms of piecewise power laws and pade fractions are provided for comparison with independent measurements and for easy use of the results a number of uses of such spectra exist including the possibility of combining the altimetric data with other observations predictions of spatial coherences and the estimation of the accuracy of apparent secular trends in sea level

tomographical mapping of the lithosphere and asthenosphere beneath southern scandinavia and adjacent areas abstract in this paper we report on a seismic tomographical survey of the lithosphere asthenosphere beneath southern scandinavia and adjacent areas the ach inversion technique has been used and the observational data in the form of p wave travel time residuals have been obtained from seismogram records from networks in denmark southern norway and southern sweden the volume under investigation subdivided into four levels layers of 7 7 knots blocks each for estimation of velocity perturbations covers the area 54 62 n and 7 20 e and depth range 0 600 km major findings are as follows level 1 0 100 km pronounced velocity lows beneath southern jutland zealand the variscan foredeep and the sveco norwegian province of southeastern norway 1500 ma old the svecofennides of southeastern sweden exhibit relatively high velocities while the sveco norwegian province of southwestern sweden 1700 ma old is characterized by moderately high velocities the transition between velocity highs and lows coincides with the tornquist teisseyre line ttl or fennoscundian border zone level 2 100 300 km velocity anomalies are less pronounced at this level although velocity lows are found beneath the northern jutland zealand area and slightly northwestwards of the oslo rift the only velocity high is found beneath the southeasternmost part of sweden the ttl does not appear to have a clear counterpart in the velocity anomaly pattern in this depth range although a clearly unambiguous answer here would require far more extensive observational data a research goal of the future level 3 300 500 km and level 4 500 600 km velocity anomalies are weak which is not unexpected for the upper mantle beneath shield areas