factors that control the angle of shear bands in geodynamic numerical models of brittle deformation abstract numerical models of brittle deformation on geological timescales typically use a pressure dependent mohr coulomb or drucker prager plastic flow law to simulate plastic failure despite its widespread usage in geodynamic models of lithospheric deformation however certain aspects of such plasticity models remain poorly understood one of the most prominent questions in this respect is what are the factors that control the angle of the resulting shear bands recent theoretical work suggest that both roscoe 45 coulomb angles 45 2 where is the angle of internal friction and arthur angles 45 4 where is the dilation angle as well as all intermediate angles are possible published numerical models however show a large range of shear band angles with some codes favoring arthur angles whereas others yield coulomb angles in order to understand what causes the differences between the various numerical models here i perform systematic numerical simulations of shear localization around an inclusion of given length scale both numerical element type geometrical and rheological viscoplastic versus viscoelastoplastic effects are studied results indicate that the main factor controlling shear band angle is the non dimensional ratio between the length scale of the heterogeneity d and the size of the numerical mesh x coulomb angles are observed only in cases where the inclusion is resolved well d x 5 10 and in which it is located sufficiently far from the boundary of the box in most other cases either arthur or roscoe orientations are observed if heterogeneities are one element in size coulomb angles are thus unlikely to develop irrespective of the employed numerical resolution whereas differences in element types and rheology do have consequences for the maximum obtainable strain rates inside the shear bands they only have a minor effect on shear band angles shear bands initiated from random noise or from interactions of shear bands with model boundaries or other shear bands result in stress heterogeneities with dimensionless length scales d x 1 2 such shear bands are thus expected to form roscoe or arthur orientations consistent with the findings in previous numerical models

late mesozoic magmatism in svalbard a review abstract late mesozoic mafic igneous rocks are widespread across the arctic region and are collectively referred to as the high arctic large igneous province halip in svalbard the halip is represented by the diabasodden suite an extensive system of predominantly basic intrusive doleritic rocks associated lava flows are exposed on the far east of the archipelago two main igneous centres have been proposed 1 central spitsbergen and 2 the eastern svalbard dolerite belt spanning hinlopenstretet and the islands of edgeoya and barentsoya offshore seismic and magnetic data suggest a link between the svalbard and franz joseph land igneous exposures intrusions occur predominantly as sills up to 100 m thick and laterally continuous for up to 30 km dykes are present locally and range in thickness from

making coulomb angle oriented shear bands in numerical tectonic models abstract localization of shear strain can be induced by strain weakening plasticity in continuum models and is often used to represent faults of various scales the orientation of the shear bands is thus required to be consistent with those observed from natural faults and with simple theories of brittle failure although the coulomb angle at which a shear band satisfies the coulomb failure criterion is widely used as an assumed fault orientation the currently available numerical techniques do not always produce shear bands oriented at this angle we demonstrate that under an associated plastic flow rule for which dilation and friction angles are equal the coulomb angle becomes a unique initial shear band orientation regardless of the numerical methods employed and model resolution the known problem of overly expanding shear bands in case of a constant dilation angle is preventable if dilation angle is reduced as shear strain along the shear band increases this treatment corresponds to natural processes reducing roughness of a fault plane

ductile fractures and magma migration from source mechanisms proposed to explain effi cient melt transport away from hot ductile source regions are problematic brittle elastic fracturing is a well known mechanism that allows fast magma migration as dikes through cold crust ductile fractures have been propo sed as an alternative for ductile environments where brittle elastic diking is inhibited ductile fracturing results from rock creep and growth of microscale voids that become interconnected leading to rock failure in this paper we present observations and numerical models supporting the hypothesis that ductile fracture controls early steps in magma migration we postulate that once developed ductile fracture dikes may reach a critical length where magma stresses at dike tips overcome fracture toughness and lead to brittle elastic diking which subsequently controls magma migration

pinch and swell structure and shear zones in viscoplastic layers abstract two dimensional finite element simulations are presented for the extension of stiff viscoplastic layers embedded in a weaker viscous matrix layers and matrix exhibit power law flow laws and the layers exhibit additionally a von mises yield stress the power law flow law applies to rock deformation in the diffusion and dislocation creep regime and the von mises plasticity to the low temperature plasticity regime e g dislocation glide simulations show that pinch and swell structure forms for small viscosity ratio i e 10 20 and typical power law stress exponents i e 1 5 the pinches in layers with initial random geometrical perturbation form consecutively i e not simultaneous in multilayers pinches on both the single layer and the multilayer scale develop furthermore shear zones develop due to the oblique linkage of pinches across the multilayer these shear zones have a stable position cause a normal drag geometry and exhibit significant displacement the numerical results and the importance of low temperature plasticity are supported by field observations microstructural observations and ebsd orientation maps for pinch and swell structure in calcite veins the presented models can explain strain localization by shear zone formation during bulk pure shear extension of viscoplastic multilayers without any material softening or feed back mechanism e g shear heating

effect of quartz inclusions on plastic flow in marble to investigate the effect of variations in volume fraction of a second phase on the strength and microstructure of rocks we produced synthetic marbles with a grainsize of 5 25 m containing quartz of similar size with volume fractions of 0 5 1 and 20 4 vol and between 3 to 9 porosity these rocks were deformed in triaxial compression at a strain rate of 3 x 10 5 s 1 confining pressure of 200 mpa and 600 c marbles containing 20 4 vol quartz were almost 5 times stronger than those containing no quartz the work hardening rate at given strain increased with quartz content marbles containing 20 4 vol quartz work hardened up to 10 strain whereas in single phase samples work hardening was negligible beyond 3 strain the dislocation density in the calcite increased with increasing quartz content the data are consistent with models for the strength of ceramic matrix composites or with the hypothesis that strength is increased by decreasing porosity

segmented lateral dyke growth in a rifting event at barðarbunga volcanic system iceland crust at many divergent plate boundaries forms primarily by the injection of vertical sheet like dykes some tens of kilometres long1 previous models of rifting events indicate either lateral dyke growth away from a feeding source with propagation rates decreasing as the dyke lengthens2 3 4 or magma flowing vertically into dykes from an underlying source5 6 with the role of topography on the evolution of lateral dykes not clear here we show how a recent segmented dyke intrusion in the bar arbunga volcanic system grew laterally for more than 45 kilometres at a variable rate with topography influencing the direction of propagation barriers at the ends of each segment were overcome by the build up of pressure in the dyke end then a new segment formed and dyke lengthening temporarily peaked the dyke evolution which occurred primarily over 14 days was revealed by propagating seismicity ground deformation mapped by global positioning system gps interferometric analysis of satellite radar images insar and graben formation the strike of the dyke segments varies from an initially radial direction away from the bar arbunga caldera towards alignment with that expected from regional stress at the distal end a model minimizing the combined strain and gravitational potential energy explains the propagation path dyke opening and seismicity focused at the most distal segment at any given time and were simultaneous with magma source deflation and slow collapse at the bar arbunga caldera accompanied by a series of magnitude m 5 earthquakes dyke growth was slowed down by an effusive fissure eruption near the end of the dyke lateral dyke growth with segment barrier breaking by pressure build up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries

the relationship between rifting and magmatism in the northeastern arabian sea the rifting of the seychelles microcontinent from india involved two phases of extensional activity the initial separation of the laxmi ridge from india was accompanied by extensive magmatism but the later separation of the seychelles from the laxmi ridge was only weakly magmatic the causal mechanisms linking continental flood basalts lithospheric extension and mantle plumes as well as the relative timing of extension and volcanism are controversial1 2 3 4 the eruption of the deccan flood basalts was approximately contemporaneous with the separation of the seychelles microcontinent from india however between these continental blocks lies the enigmatic laxmi ridge and the sequence of extensional events that formed these various tectonic elements is poorly understood here we present wide angle seismic data along a profile across laxmi ridge that permit delineation of offshore igneous bodies associated with the deccan magmatism these bodies are similar to those associated with flood basalt volcanism and rifting in the atlantic region and elsewhere1 from the geometry of these bodies we infer that there were two periods of extension the first phase which involved extension between laxmi ridge and the indian subcontinent was accompanied by significant deccan related magmatism full development of a continental margin was achieved during the second phase of weakly magmatic extension between laxmi ridge and the seychelles we suggest that between these rifting events the region passed beyond the reach of lateral flow from the source region of the deccan flood basalts

upwelling in the alaskan beaufort sea atmospheric forcing and local versus non local response abstract the spin up and relaxation of an autumn upwelling event on the beaufort slope is investigated using a combination of oceanic and atmospheric data and numerical models the event occurred in november 2002 and was driven by an aleutian low storm the wind field was strongly influenced by the pack ice distribution resulting in enhanced winds over the open water of the chukchi sea flow distortion due to the brooks mountain range was also evident mooring observations east of barrow canyon show that the beaufort shelfbreak jet reversed to the west under strong easterly winds followed by upwelling of atlantic water onto the shelf after the winds subsided a deep eastward jet of atlantic water developed centered at 250 m depth an idealized numerical model reproduces these results and suggests that the oceanic response to the local winds is modulated by a propagating signal from the western edge of the storm the disparity in wave speeds between the sea surface height signal traveling at the fast barotropic shelf wave speed versus the interior density signal traveling at the slow baroclinic wave speed leads to the deep eastward jet the broad scale response to the storm over the chukchi sea is investigated using a regional numerical model the strong gradient in windspeed at the ice edge results in convergence of the offshore ekman transport leading to the establishment of an anti cyclonic gyre in the northern chukchi sea accordingly the chukchi shelfbreak jet accelerates to the east into the wind during the storm and no upwelling occurs west of barrow canyon hence the storm response is fundamentally different on the beaufort slope upwelling versus the chukchi slope no upwelling the regional numerical model results are supported by additional mooring data in the chukchi sea

an analytical benchmark with combined pressure and shear loading for elastoplastic numerical models we discuss the benchmark strategy to check the accuracy of elastoplastic numerical solutions based on a fully two dimensional analytical solution associated rate independent non hardening plasticity with von mises or tresca criteria is assumed throughout algorithms for integration of the rate equations strategies for stress updating over a time step and tangent operators are discussed the accuracy of a simple incremental algorithm as a function of the time step is discussed

mafic intrusions east of svalbard imaged by active source seismic tomography a seismic refraction and reflection tomography experiment was performed across the igneous province east of svalbard which is a part of the cretaceous high arctic large igneous province seismic travel times from 12 ocean bottom seismometers hydrophones deployed along a 170 km line are inverted to produce smooth 2d images of the crustal p wave velocity and geometry of the acoustic basement and moho the inversion of travel times was complemented by forward elastic wave propagation modeling integration with onshore geology as well as multichannel seismic magnetic and gravity data have provide additional constraints used in the geological interpretation the seismic p wave velocity increases rapidly with depth starting with 3 km s at the sea floor and reaching 5 5 km s at the bottom of the upper sedimentary layer the thickness of this layer increases eastward from 2 km to 3 5 km on average the p wave velocity in the crystalline crust increases with depth from 5 5 km s to 6 8 km s the crustal thickness is typical for continental shelf regions 30 34 km finger shaped high velocity anomalies one reaching 12 and two of 4 6 velocity perturbation are obtained these velocity anomalies are concomitant with lower cretaceous basaltic lava flows and sills in the shallow sediments and elongated gravity and magnetic highs traced towards the northern barents sea passive continental margin we interpret the obtained velocity anomalies as signatures of dikes emplaced in the basement during breakup and subsequent spreading in the arctic amerasia basin

structure and evolution of the northern barents kara sea continental margin from integrated analysis of potential fields bathymetry and sparse seismic data summary the northern barents kara sea continental margin is a poorly investigated area because of a permanent ice cover hampering seismic exploration the available geological and geophysical data show that the magma poor margin developed in response to early cenozoic breakup and subsequent opening of the arctic eurasia basin in this study a series of crustalscale geotransects illustrating the architecture of the continental margin are constructed using sparse seismic reflection profiles and a gravity inversion method incorporating a thermal model of rifting the continental side of the northern barents sea margin is underlain by palaeozoic early mesozoic deep sedimentary basins separated from the oceanic side by the marginal uplift a bathymetry analysis complements low resolution seismic data to predict the sedimentary depocenters beyond the shelf break these depocenters are associated with troughs perpendicular to the shelf edge the depocenter in front of the st anna trough may contain a sedimentary section more than 4 km thick the gravity correction for the effect of sedimentary cover was added to the inversion this correction used an exponential density depth function the inversion supports a narrow and steep continent ocean transition cot ca 100 km the conjugate lomonosov ridge margin is modelled using the same technique palaeoreconstructions were made to predict the break up setting the northern barents sea lomonosov ridge rift system can be described as an initially narrow symmetric rift a transitional zone of extreme thinning is assumed between the oldest spreading magnetic anomaly and the stretched continental crust the free air gravity anomaly in the western part ofthemargincanbepredicted bytheupwelling divergentflowmodel implying theexhumation of the lower crust and the continental upper mantle within the cot it is suggested that an episodeofshearorobliqueextensionbeforebreakupisrequiredtoexplaintheobservednarrow symmetric conjugate margins in the eurasia basin

mechanics of kink bands during torsion deformation of muscovite aggregate abstract the growth geometry and bulk mechanical behavior of grain scale kink bands were investigated during layer parallel non coaxial deformation torsion of synthetic polycrystalline aggregates of muscovite at constant displacement rate 3 10 4 s 1 and various confining pressures cp 50 300 mpa temperatures t 300 800 c and finite shear strains 0 2 5 0 the kink instabilities were observed during elastic to plastic transition yield at finite shear strain of ca 0 2 the shear stress required to form the kinks is a function of pressure and temperature and range from ca 20 to 90 mpa within the experimental conditions the kink bands formed in non coaxial torsion tests are asymmetric involve strain partitioning show complex microstructures with increased deformation and have a limited lifetime as the structure is destroyed at ca 5 these results suggest a domain of pressure and temperature conditions where kink band structures should be common results obtained from torsion experiments are compared to those of layer parallel axial compression conducted with the same displacement rate at 600 c and 300 mpa for 5 10 and 17 bulk shortening combining microstructures viewed on surfaces parallel and perpendicular to the torsion plane a 3d geometrical model is constructed to discuss the kink mechanism the microstructural evolution and the mechanical behavior

the brittle plastic transition in experimentally deformed quartz aggregates deformation experiments have been conducted to provide constraints on the processes responsible for the brittle plastic transition in quartz aggregates a correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and or pressure first there is a transition from cataclastic faulting to semibrittle faulting microstructural observations indicate that this transition occurs due to the activation of dislocations in addition faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide second there is a transition from semibrittle faulting to semibrittle flow this transition corresponds to a change from localized to distributed deformation microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime we conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries third there is a transition from semibrittle flow to dislocation creep microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature in addition microstructural observations indicate that a transition from dominantly mode i axial to mode ii shear microcracking occurs with an increase in confining pressure from 0 4 to 0 8 gpa regardless of temperature the differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions however a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the earth the results of this study provide further evidence to indicate that the brittle plastic transition in the continental crust occurs over a relatively wide range of conditions

aspects of the marine nitrogen cycle of the chukchi sea shelf and canada basin abstract as a highly productive seasonally ice covered sea with an expansive shallow continental shelf the chukchi sea fuels high rates of sedimentary denitrification this contributes to its fixed nitrogen n deficit relative to phosphorus p which is among the largest in the global ocean making the chukchi sea severely n limited during the phytoplankton growth season here we examine aspects of the n cycle on the chukchi sea shelf and the downstream canada basin using nutrients dissolved oxygen o 2 and the stable isotopes of nitrate no 3 in the northward flow path across the chukchi shelf bottom waters experienced strong o 2 drawdown from which we calculated a nitrification rate of 1 3 mmol m 2 d 1 this nitrification was likely primarily in sediments and directly fueled sedimentary denitrification historically measured at similar rates we observed significant accumulations of ammonium nh 4 in bottom waters of the chukchi shelf up to 5 m which were inversely correlated with 15 n no 3 indicating a sediment source of 15 n enriched nh 4 this is consistent with a process of coupled partial nitrification denitrification cpnd which imparts significant 15 n enrichment and 18 o depletion to pacific origin no 3 this cpnd mechanism is consistent with a significant decrease in 18 o no 3 relative to bering sea source waters indicating that at least 58 of no 3 populating the pacific halocline was regenerated during its transit across the north bering and chukchi shelves rather than arriving preformed from the bering sea slope this pacific origin no 3 propagates into the canada basin and towards the north atlantic being significantly 15 n enriched and 18 o depleted relative to the underlying atlantic waters

the nutrient salinity and stable oxygen isotope composition of bering and chukchi seas waters in and near the bering strait seawater nutrient salinity and oxygen 18 data collected from 1990 to 1993 in the bering and chukchi seas were used to identify potential sources of nutrients and water masses that result in formation of the arctic ocean upper halocline and its associated nutrient maximum water matching the delta sup 18 o values of the arctic ocean upper halocline and containing sufficient or a nearly sufficient nutrient and salinity concentration was collected in subsurface waters in the summer in portions of the bering sea particularly the gulf of anadyr however nutrient concentrations significantly declined in this north flowing water over the shallow continental shelf before it reached the bering strait as a consequence of biological utilization and dilution with nutrient poor and oxygen 18 depleted fresh water therefore it does not appear likely that the flow of unaltered water through the bering strait in the summer plays a critical role in the formation of the arctic ocean upper halocline the role of other mechanisms for contributing pacific derived waters to the arctic ocean nutrient maximum is considered copyright 1997 american geophysical union

impact of host rock heterogeneity on failure around pressurized conduits implications for finger shaped magmatic intrusions abstract rock failure accommodating the emplacement of magmatic intrusions is controlled by the local stress regime within the host rock most of the mechanical models addressing rock failure conditions e g for a given tectonic regime or magma overpressure simplify the stress calculation by assuming the homogeneity of the host rock properties in this study we highlight the importance of local heterogeneities in controlling the localization of the deformation and the failure mechanism around a magmatic intrusion we numerically model the elastoplastic deformation of a heterogeneous host rock intruded by an overpressurized magmatic body of cylindrical or finger shaped geometry the plastic component of the deformation is considered with a mixed mode mechanism allowing dilatant and shear displacements to act simultaneously our simulations are performed for small strain amplitudes that reflect the pre failure conditions of the host we assess the subsequent failure mechanism of the system based on the development of the localized strain patterns the heterogeneity in the model is introduced by a stochastic perturbation of the host rock cohesion with characteristic wavelength and amplitude we show that a relatively small perturbation of 10 of the cohesion field can efficiently localize the plastic deformation and control the subsequent emplacement mechanism we further investigate a more realistic geological scenario in which the intrusion resides within sedimentary layers of contrasting strength resulting in both heterogeneity and anisotropy of mechanical properties in the host rock our model reproduces similar deformation patterns as observed around finger shaped magma intrusions within the vaca muerta shales in the neuquen basin argentina we conclude that heterogeneities within the host rock may locally seed dilatant shear faults around magmatic conduits and finger shaped intrusions and result in the development of a process zone of length scale in order with the intrusion radius length scale

a barotropic inverse tidal model for the arctic ocean 1 we describe high resolution 5 km grid linear dynamics and inverse models of arctic ocean barotropic tides obtained with the osu tidal inversion software otis package the 8 constituent dynamics based model uses the latest ibcao bathymetry and open boundary forcing from the recent tpxo 6 2 global barotropic tidal solution this model performs significantly better than the present benchmark arctic tidal model 14 km grid by z kowalik and a proshutinsky as judged by comparisons with 300 coastal tide gauges the greatest improvements are found in the canadian arctic archipelago nares strait and the baffin bay and labrador sea and can be explained by the higher resolution of the new model in these topographically complex regions the new arctic inverse model assimilates coastal and benthic tide gauges and topex poseidon and ers altimetry for further improvements of the 4 dominant constituents m2 s2 k1 and o1

rift magmatism on the eurasia basin margin u pb baddeleyite ages of alkaline dyke swarms in north greenland the opening of the arctic ocean involved multiple stages of continental rifting and intrusion of extensive dyke swarms to trace tectonomagmatic processes of the high arctic we present the first u pb ages for alkaline dyke swarms of north greenland concordia ages of 80 8 0 6 and 82 1 1 5 ma indicate that north south and east west dykes are coeval the north south dykes reflect initial east west rifting that led to break up along the gakkel ridge and formation of the eurasia basin the east west dykes reflect local variations in the stress field associated with reactivated palaeozoic faults supplementary materials u pb data are available at http www geolsoc org uk sup18857

hydrographic conditions during the 2002 sbi process experiments abstract a review of the hydrographic data from the 2002 western arctic shelf basin interactions sbi process cruises permits the following conclusions 1 temperature salinity relationships were similar to canonical descriptions but at five stations in the outer shelf slope region warm high salinity atlantic layer water appeared to have risen displaced the lower halocline and mixed with shelf upper halocline water 2 primary production in the sbi study region was strongly influenced by the advection of dissolved inorganic nitrogen din entering via bering strait this import of din ammonium nitrate nitrite is modified by local processes but without the bering strait inflow biological productivity in the sbi region would be much lower 3 in comparison to the inflowing atlantic waters din urea phosphate and din urea silicate ratios in the pacific waters that dominated the upper 150 m of the water column were low they were also low relative to redfield uptake ratios for phytoplankton 4 microbial processes continue to destroy din in significant quantities as the pacific waters transit the sbi region 5 nitrate and ammonium were the principal contributors to din nitrite concentrations were always

robust characteristics method for modelling multiphase visco elasto plastic thermo mechanical problems abstract we have extended our previous 2d method gerya t v yuen d a 2003 characteristics based marker in cell method with conservative finite differences schemes for modeling geological flows with strongly variable transport properties phys earth planet interiors 140 295 320 which is a combination of conservative finite differences with marker in cell techniques to include the effects of visco elasto plastic rheology self gravitation and a self consistently derived evolving curvilinear planetary surface this code is called i2elvis and can solve a new class of computationally challenging problems in geodynamics such as shear localization with large strains crustal intrusion emplacement of magmas bending of realistic visco elasto plastic plates and core formation by vigorous shell tectonics activities related to a global rayleigh taylor instability of a metal layer formed around silicate rich lower density primordial core during planetary accretion we discuss in detail the computational strategy required the rheological constraints to be satisfied at each time step and spatial location we show analytical benchmarks and examples drawn from comparing between numerical and analogue experiments in structural geology subducting slab bending with a visco elasto plastic rheology and equilibrium spherical configurations from self gravitation we have also tested possibilities of future applications by addressing 3d geometries based on multigrid method and including inertial effects in the momentum equation with tracers in order to simulate meteoritic impact events and eventually earthquake instabilities

benchmarking numerical models of brittle thrust wedges we report quantitative results from three brittle thrust wedge experiments comparing numerical results directly with each other and with corresponding analogue results we first test whether the participating codes reproduce predictions from analytical critical taper theory eleven codes pass the stable wedge test showing negligible internal deformation and maintaining the initial surface slope upon horizontal translation over a frictional interface eight codes participated in the unstable wedge test that examines the evolution of a wedge by thrust formation from a subcritical state to the critical taper geometry the critical taper is recovered but the models show two deformation modes characterised by either mainly forward dipping thrusts or a series of thrust pop ups we speculate that the two modes are caused by differences in effective basal boundary friction related to different algorithms for modelling boundary friction the third experiment examines stacking of forward thrusts that are translated upward along a backward thrust the results of the seven codes that run this experiment show variability in deformation style number of thrusts thrust dip angles and surface slope overall our experiments show that numerical models run with different numerical techniques can successfully simulate laboratory brittle thrust wedge models at the cm scale in more detail however we find that it is challenging to reproduce sandbox type setups numerically because of frictional boundary conditions and velocity discontinuities we recommend that future numerical analogue comparisons use simple boundary conditions and that the numerical earth science community defines a plasticity test to resolve the variability in model shear zones

three dimensional tomographic inversion of combined reflection and refraction seismic traveltime data summary a tomographic inversion method is presented for the determination of 3 d velocity and interface structure from a wide range of body wave seismic traveltime data types it is applicable to refraction wide angle reflection normal incidence and multichannel seismic data and is best suited to a combination of these that provides good independent constraints on seismic velocities and interface depths the inversion process seeks a layer interface minimum structure model that is able to explain the given data satisfactorily by inverting to minimize data misfit and model roughness norms simultaneously this regularized inversion and the use of smooth functions to describe velocities and depths allows the highly non linear tomographic problem to be approximated as a series of linear steps the inversion process begins by optimizing the fit to the data of a highly smoothed initial model in each subsequent step structure is allowed to develop in the model with successively greater detail evolving until a satisfactory fit to the data is obtained parameter uncertainties for the final model are then estimated using an a posteriori covariance matrix analysis smooth layer interface models are parametrized using regular grids of velocity and depth nodes from which spline interpolated interface surfaces and velocity fields are defined forward modelling is achieved using ray perturbation theory and a two point ray tracing method that is optimized for a large number of closely spaced shot or receiver points the method may be used to generate 1 and 2 d models from for example vertical seismic profile data or 2 d surveys in which the 3 d geometry of a survey is correctly accounted for the ability of the method to resolve typical target structures is tested in a synthetic salt dome inversion from a set of noisy traveltime data the model converges quickly to a well resolved final model from different starting models the application of this method to real data is demonstrated with a combined 3 d inversion of refraction and reflection data which provide p wave velocity constraints on the methane hydrate stability zone in the cascadia margin offshore vancouver island

seasonal and spatial patterns of sedimentary denitrification rates in the chukchi sea abstract recent constructions of the global nitrogen budget estimate that at least half of the ocean s fixed nitrogen is lost by sedimentary denitrification the majority of which occurs in continental shelves the arctic contains approximately 20 of the world s continental shelf suggesting it is a substantial contributor to the global sedimentary denitrification rate during two cruises in the summer and spring of 2002 and 2004 respectively denitrification rates were calculated from the downward diffusive flux of nitrate in the shelf and slope sediments of the chukchi sea in the western arctic additionally in the spring of 2004 denitrification rates were determined by whole core incubations in which the flux of nitrogen gas out of the sediments was measured measurements were made along three transects crossing the shelf and slope 50 3000 m each transect having different overlying water characteristics denitrification rates generally decreased with increasing water depth rates varied from about 1 6 mmol n m 2 d 1 for the shallow water sediments to undetectable in deep water sediments rates showed little variation between the two seasons however rates were found to correspond with differences in annual overlying primary productivities and particulate organic carbon export fluxes an extrapolation to the whole arctic yielded an average arctic sedimentary denitrification rate of 13 tg n yr 1 taken in the context of the global nitrogen budget it is about 4 13 of the total sink of fixed nitrogen in the ocean

long range inversions for ocean acoustic tomography ocean acoustic tomography was proposed by munk and wunsch 1979 as a method for making measurements of ocean variability over large areas after the successful demonstration of the feasibility of the idea in the 1981 three dimensional mesoscale experiment ocean tomography group 1982 the tomography group has proposed a new experiment to be carried out in 1986 in the eastern pacific ocean on ranges as long as the subtropical gyre scale in this paper the gyre scale experiment is simulated in the model ocean using holland s eddy resolving general circulation quasi geostrophic model the paper addresses the following issues 1 measurement of the heat content vertical profile horizontally averaged along the tomographic section 2 adequacy of the linearized inverse over very long ranges and the need for its improvement 3 possible improvements in the specification of the field statistics to obtain more accurate estimates and to measure properties like average pycnocline trends 4 relationship of possible range dependent information from the inversion to the assigned noise level the results of the modeling simulation can be summarized as follows 1 the linearized stochastic inversion needs to be improved for gyre scale ranges providing estimates of the average heat content that have warm or cold biases iteration is used and shown to provide good estimates of the average heat content 2 a smaller number of iterations is necessary if the initial estimate is improved this can be done by including a spatial mean in the horizontal covariance function for regions of the ocean where the energy level in the mean and in the long length scales may be even more important than the mesoscale energy peak 3 general trends like average pycnocline slopes can be estimated very well by including an inhomogeneous covariance in the inversion 4 the estimates of the mean heat content values and of the average slopes are rather insensitive to the specified noise level 5 the type and number of eigenrays used is not a critical factor in the inversion estimates as long as the eigenrays provide a good sampling of the interior water mass increasing the number of eigenrays does not significantly improve the estimate of the average values 6 range dependent information is primarily a function of the assigned noise for too high a noise the basic quantities that can be measured are the mean value and the average trend

observational and modeling evidence of seasonal trends in sediment derived material inputs to the chukchi sea author posting american geophysical union 2020 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research oceans 125 5 2020 e2019jc016007 doi 10 1029 2019jc016007

rapid magma emplacement in the karoo large igneous province abstract understanding the dynamics of continental large igneous provinces lips relies on precise dating of basaltic rocks lip research has traditionally focused on dating lavas often neglecting the volumetrically important sill intrusions in underlying sedimentary basins here we present u pb zircon and baddeleyite ages for fourteen new samples of karoo lip sills and dykes spaced by as much as 1100 km across the half million square kilometer karoo basin the samples yield remarkably coherent ages ranging from 183 0 0 5 to 182 3 0 6 myr probability modeling indicates that basin scale emplacement took place within an interval of about 0 47 myrs less than 0 90 myrs with 95 confidence and could even have represented a single magma emplacement event combining the new ages with the estimated volume of sills in the karoo basin gives an emplacement rate of 0 78 km3 yr which is higher than previous estimates upper crustal magma storage may account for these high rates the results challenge the view that melt emplacement in a sedimentary basin is a prolonged process support a scenario of pulsating catastrophic events within a narrow time frame and strengthens the hypothesis linking lips and sill emplacement to global environmental crises

seismic characteristics and evolution of post rift igneous complexes and hydrothermal vents in the lingshui sag qiongdongnan basin northwestern south china sea abstract the study of morphology distribution and characteristics of igneous complexes has great significance to the understanding of magma plumbing processes geodynamics and tectonic evolution of continental margins previous studies concentrated partly on the magma rich rifted basins where the lateral magma transport mainly affects the igneous complexes connection and distribution however due to seismic wave shielding effects of the large shallow magmatic bodies the underlying igneous complexes and their corresponding magma plumbing systems in the magma poor rifted margins are still in debate in this study 2d 3d seismic data and well data are utilized to describe the morphology architecture and spatial temporal distribution of igneous complexes in the lingshui sag of the qiongdongnan basin northwestern south china sea margin the identified igneous complexes include 98 intrusive sills and feeder dykes beneath some of the isolated sills twenty six cone shaped mounds that overlie intruded sills through internal disturbed conduits were also described drilled well samples and seismic expressions suggest that these mounds are hydrothermal vents a uniform bottom mounds horizon of these vents suggests that they probably formed at the same time constrained by biostratigraphic data and sedimentation rate of underlying and overlying sedimentary layers the magma emplacement was dated to the middle miocene ca 14 6 ma most of the hydrothermal vents are distributed along the f2 fault zone and have direct linkage with the underlying sills while the large sill complexes that are connected with limited vents are mainly present above the hyperextended continental crust where the crust thins to 6 10 km the sills intruded into different layers from the lower oligocene to the lower miocene and the emplaced depth of sills is 1 2 6 3 km whether or not they feed any vents above unlike most of the large volume and laterally linked sills found in the magma rich rifted margins the scattered distribution of sills at different levels indicates that dykes probably play an important role in magma transport which might coexist with numerous polygonal or small faults and interference reflections this work highlights the critical role of basin structures in controlling the distribution of post rift igneous complexes in magma poor margins including thinned continental crust sedimentary thickness and faults

thermomechanics of mid ocean ridge segmentation the mechanics responsible for the initiation of the orthogonal pattern characterizing mid ocean ridges and transform faults are studied using numerical models the driving forces are thermal stresses arising from the cooling of young oceanic crust and extensional kinematic boundary conditions thermal stress can exert ridge parallel tension comparable in magnitude to spreading induced tension when selectively released by ridges and ridge parallel structure two modes of ridge segment growth have been identified in plan view an overlapping mode where ridge segments overlap and bend toward each other and a connecting mode where two ridge segments are connected by a transform like fault as the ratio of thermal stress to spreading induced stress increases the patterns of localized plastic strain change from the overlapping to connecting mode the orthogonal pattern marks the transition from one mode to the other besides the amount of stress from each driving force the rate of stress accumulation is crucial in determining the emergent pattern this rate dependence is characterized by the spreading rate normalized by a reference cooling rate pe when pe is paired with the ratio of thermal stress to the reference spreading induced stress they unambiguously define stability fields of the two modes the obliquely connecting the orthogonally connecting and the overlapping mode are similar to ridge transform fault intersections observed in ultra slow slow to intermediate and fast spreading centers respectively the patterns are also sensitive to the strain weakening rate fracture zones were created in part as a response to thermal stress

the early cretaceous barents sea sill complex distribution 40ar 39ar geochronology and implications for carbon gas formation abstract mafic igneous rocks of cretaceous age 80 130 ma scattered around the arctic ocean are commonly referred to as the high arctic large igneous province halip we have mapped out the distribution of halip igneous rocks in the barents sea region over the past decade based on integrated seismic gravity magnetic interpretation field work review of publications and analyses of new and vintage borehole and field samples the mapping reveals abundant igneous rocks in the northern and eastern barents sea covering an area of 900 000 km2 with a conservative volume estimate of 100 000 to 200 000 km3 of intrusions the igneous province is dominated by sheet intrusions injected into triassic and permian sedimentary rocks hydrothermal vent complexes are rare and only two potential vent complexes have been identified on seismic data in the eastern barents sea we have further done extensive radiometric dating of the igneous samples in the barents sea region new 40ar 39ar dating of thirteen samples from svalbard reveal ages of crystallization and alteration the large age span 60 140 ma for the raw ages is likely due to partial or complete overprint of the k ar system in plagioclase and the age of the magma emplacement is better represented by u pb tims ages only one of our 40ar 39ar analyses of plagioclase yielded a statistically valid age that is in line with the recently published u pb tims ages of 122 125 ma the new data clearly document that relying on published data from the k ar system can lead to erroneous conclusions on the age of crystallization in this province without a careful use of additional 40ar 39ar degassing data i e k ca we propose that the magmatism on svalbard and franz josef land represents a distinct magmatic event near the barremian aptian boundary 125 ma in the barents sea this early cretaceous barents sea magmatism resulted in the formation of the bssc barents sea sill complex bssc age rocks are also present in arctic canada sverdrup basin and on bennett island new siberia islands the massive injection of hot magma into potentially organic rich sediments in the eastern and northern barents basin caused rapid organic matter maturation and formation of thermogenic gas and oil in contact aureoles we estimate that up to 20 000 gt of carbon were potentially mobilized corresponding to 175 trillion barrels of oil equivalent the production rates and fate of the carbon gases are uncertain however we speculate that rapid release of aureole greenhouse gases methane may have triggered the oceanic anoxic event 1a oae1a and the associated negative 13c excursion in the early aptian some of the methane may also be trapped in the vast hydrocarbon gas accumulations found in the east barents basin

thermal and mechanical aspects of magma emplacement in giant dike swarms we consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts for driving pressure gradients inferred for giant dike swarms thicknesses of 10 m would enable dikes to transport magma laterally over the distances observed in the field up to thousands of kilometers without suffering thermal lock up using time dependent numerical solutions for the thermal evolution of a dike channel under laminar and turbulent flow conditions in the presence of phase transitions we investigate the possibility that the observed dike thicknesses of the order of 100 m result from thermal erosion of the country rocks during dike emplacement this implies that the observed range of dike widths in giant dike swarms may reflect variations in the source volume and not the excess magma pressure it is found that the total volume of intruded magma required to produce an order of magnitude increase in dike width via wall rock melting broadly agrees with the estimated volumes of individual flows in continental flood basalts the presence of chilled margins and apparently low crustal contamination characteristics of some giant dikes may be consistent with turbulent magma flow and extensive melt back during dike emplacement in this case measurements of the anisotropy of magnetic susceptibility most likely indicate magma flow directions during the final stages of dike intrusion shear stresses generated at the dike wall when the dike starts to freeze strongly decrease with increasing dike width which implies that thicker dikes may have less tendency to produce consistent fabric alignment our results suggest that if the dike was propagating downslope off a plume related topographic swell the mechanism responsible for flow termination could possibly have been related to underpressurization and collapse implosion of the shallow magma plumbing system feeding the intrusion radial dikes that erupted at the periphery of the topographic uplift might have increased rather than decreased extensional stresses in the crust within the topographic uplift upon their solidification

finite thickness of shear bands in frictional viscoplasticity and implications for lithosphere dynamics permanent deformations in the lithosphere can occur in the brittle as well as in the ductile domain for this reason the inclusion of viscous creep and frictional plastic deformation is essential for geodynamic models however most currently available models of frictional plasticity are rate independent and therefore do not incorporate an internal length scale which is an indispensible element for imposing a finite width of localized shear zones therefore in computations of localization either analytical or numerical a width of the shear zone is obtained which tends to zero in numerical computations this manifests itself in a severe mesh sensitivity moreover convergence of the global iterative procedure to solve the non linear processes is adversely affected which negatively affects the reliability and the quality of predictions the viscosity which is inherent in deformation processes in the lithosphere can in principle remedy this mesh sensitivity however elasto visco plastic models which are commonly used in geodynamics assume a series arrangement of rheological elements maxwell type approach which does not introduce an internal length scale here we confirm that a different rheological arrangement which put a damper in parallel to the plastic slider kelvin type approach introduces an internal length scale as a result pressure and strain and strain rate profiles across the shear bands converge to finite values upon decreasing the grid spacing we demonstrate that this holds for non associated plasticity with constant frictional properties and with material softening with respect to cohesion finally the introduction of kelvin type viscoplasticity also significantly improves the global convergence of non linear solvers

comments about the age of the canada basin abstract the canada basin appears to have formed over a 60 ma interval during the cretaceous period the opening process can be divided into two parts continental breakup widespread in hauterivian through aptian time about 113 131 ma ago along what was about to become the polar margin of north america and seafloor formation active during the extended cretaceous interval of normal geomagnetic polarity to marine magnetic anomaly 33 time about 118 79 ma ago presently available age controls indicate that all parts of the north american polar margin could have formed at about the same time two styles of margin formation are evident from brock to axel heiberg island there appears to be an extended 70 ma stratigraphic hiatus and northeastward from ellef ringnes island extensive tholeiitic igneous activity associated with continental breakup from banks island to northeastern alaska the breakup interval is abbreviated 20 30 ma and few igneous rocks are observed a progressive difference may be indicated in the amount and or rate of crustal stretching during margin formation with relatively less slower stretching having occurred along the segment southwest of brock island and relatively more faster stretching having taken place along the segment to the northeast

circulation and water mass transformation in a model of the chukchi sea 1 the circulation and water mass transformation in a regional ocean ice model of the chukchi sea are discussed the model has horizontal resolution of o 4 km is forced by fluxes derived from daily ncep reanalysis fields and has seasonally varying transport temperature and salinity imposed at bering strait many of the observed characteristics of the mean circulation and seasonal cycle in the chukchi sea are reproduced the discussion focuses on the branching of the inflow transport into pathways following herald canyon central channel and the alaskan coast the pattern of ice melt and the water mass transformation and formation of winter water and hypersaline water the ice melt pattern and timing is strongly influenced by advection through bering strait high frequency forcing results in a larger region of ice melt particularly over the shoals and in the northern chukchi sea compared to monthly mean forcing in the model the seasonal cycle of salinity in the southern and central chukchi sea is dominated by advection through bering strait while local atmospheric forcing and brine rejection are more important north of herald and hanna shoals and in barrow canyon however since the residence time in the chukchi sea is generally less than 1 year interannual variability in the bering strait salinity will be reflected in the salinity across the chukchi sea and at barrow canyon

upwelling on the continental slope of the alaskan beaufort sea storms ice and oceanographic response 1 the characteristics of pacific born storms that cause upwelling along the beaufort sea continental slope the oceanographic response and the modulation of the response due to sea ice are investigated in fall 2002 a mooring array located near 152w measured 11 significant upwelling events that brought warm and salty atlantic water to shallow depths when comparing the storms that caused these events to other aleutian lows that did not induce upwelling interesting trends emerged upwelling occurred most frequently when storms were located in a region near the eastern end of the aleutian island arc and alaskan peninsula not only were these storms deep but they generally had northward tending trajectories while the steering flow aloft aided this northward progression the occurrence of lee cyclogenesis due to the orography of alaska seems to play a role as well in expanding the meridional influence of the storms in late fall and early winter both the intensity and frequency of the upwelling diminished significantly at the array site it is argued that the reduction in amplitude was due to the onset of heavy pack ice while the decreased frequency was due to two different upper level atmospheric blocking patterns inhibiting the far field influence of the storms

cutting of the european continental lithosphere plasticity theory applied to the present alpine collision presently active european alpine collision has a history of more than 100 myr convergence we investigate the collisional contact problem of an apparently rigid african promontory currently the italy adria block iab steadily penetrating at 1 cm yr over at least the past 45 myr into the deformable european plate we assume that neither continental subduction nor delamination occurs presently in the 300 km wide contact zone enabling use of the indenter indentee approach a key parameter is the aspect ratio h a defined by the width of the deformable foreland over the width of the indenter itself this parameter controls the activation of two fundamentally different modes of deformation 1 classical indentation mode for h a 4 4 described by previous authors and having effect only in a portion of the deformable plate in the vicinity of the indenter and 2 cutting mode for h a 4 4 giving plane strain longitudinal splitting of the entire deformable plate and plastic separation of the two halves we use a matrix operator method to solve for slip lines in the mixed boundary value problem obliquity velocity friction stress of the european alpine case implications are as follows 1 plane stress vertical lithospheric thickening is the first indentation mode of continental collision and 2 for the present ratio of h a 3 deformation is in the cutting mode and slow plane strain intraplate extension rifting is predicted this mode explains the intraplate stress field obtained from borehole and earthquake data considering that the paleoindenter probably has been larger than the iab it is inferred that h a has been below 4 4 since at least the end of the eocene and that consequently the intraplate graben system lower rhine rhine bresse limagne is due to the cutting of the european plate by alpine collision

circulation on the north central chukchi sea shelf abstract mooring and shipboard data collected between 1992 and 1995 delineate the circulation over the north central chukchi shelf previous studies indicated that pacific waters crossed the chukchi shelf through herald valley in the west and barrow canyon in the east we find a third branch through the central channel onto the outer shelf the central channel transport varies seasonally in phase with bering strait transport and is 0 2 sv on average although some of this might include water entrained from the outflow through herald valley a portion of the central channel outflow moves eastward and converges with the alaskan coastal current at the head of barrow canyon the remainder appears to continue northeastward over the central outer shelf toward the shelfbreak joined by outflow from herald valley the mean flow opposes the prevailing winds and is primarily forced by the sea level slope between the pacific and arctic oceans current variations are mainly wind forced but baroclinic forcing associated with upstream dense water formation in coastal polynyas might occasionally be important winter water mass modification depends crucially on the fall and winter winds which control seasonal ice development an extensive fall ice cover delays cooling limits new ice formation and results in little salinization in such years bering shelf waters cross the chukchi shelf with little modification in contrast extensive open water in fall leads to early and rapid cooling and if accompanied by vigorous ice production within coastal polynyas results in the production of high salinity 33 shelf waters such interannual variability likely affects slope processes and the transport of pacific waters into the arctic ocean interior

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

the spatial distribution of solar radiation under a melting arctic sea ice cover 1 the sea ice cover of the chukchi and beaufort seas is currently undergoing a fundamental shift from multiyear ice to first year ice field observations of sea ice physical and optical properties were collected in this region during june july 2010 revealing unexpectedly complex spatial distributions of solar radiation under the melt season ice cover based on our optical measurements of first year ice we found the under ice light field in the upper ocean to be spatially heterogeneous and dependent on wavelength ice thickness and the areal and geometric distribution of melt ponded and bare ice surfaces much of the observed complexity in radiation fields arose because the transmission of light through ponded ice was generally an order of magnitude greater than through bare unponded ice furthermore while many sites exhibited a consistent exponential decay in light transmission through both ponded and bare ice surfaces light transmission under bare ice was also observed to increase with depth reaching maximum values 5 10 m below the bottom of the ice a simple geometric model shows these transmission peaks are a result of scattering in the ice and the interspersion of bare and ponded sea ice surfaces these new observations of complex radiation fields beneath melt season first year sea ice have significant implications for biological production biogeochemical processes and the heat balance of sea ice and under ice ocean waters and should be carefully considered when modeling these sea ice related phenomena

submarine volcanic activity ocean acoustic waves and internal ocean tides 1 submarine volcanic event often generates acoustic waves t waves traveling over long distances through the low velocity channel sofar of the ocean by a method of coherent stacking of t waves from a submarine volcanic activity in northern mariana we found a significant semidiurnal variation of t wave travel times the amplitude of variation is an order of larger than those reported in the previous ocean sound transmission experiments ray theoretical consideration for the numerically simulated ocean tides indicates that such large t phase travel time variation is a consequence of large up and down movement of seawater around the axis of the sofar channel due to the m2 internal tide effectively converted from external tidal forcing t phases a ubiquitous feature of the ocean acoustic noise field can be used to infer internal tidal motion and the associated ocean mixing

the benefits of using a consistent tangent operator for viscoelastoplastic computations in geodynamics strain localization is ubiquitous in geodynamics and occurs at all scales within the lithosphere how the lithosphere accommodates deformation controls for example the structure of orogenic belts and the architecture of rifted margins understanding and predicting strain localization is therefore of major importance in geodynamics while the deeper parts of the lithosphere effectively deform in a viscous manner shallower levels are characterized by an elastoplastic rheological behavior herein we propose a fast and accurate way of solving problems that involve elastoplastic deformations based on the consistent linearization of the time discretized elastoplastic relation and the finite difference method the models currently account for the pressure insensitive von mises and the pressure dependent drucker prager yield criteria consistent linearization allows for resolving strain localization at kilometer scale while providing optimal that is quadratic convergence of the force residual we have validated our approach by a qualitative and quantitative comparison with results obtained using an independent code based on the finite element method we also provide a consistent linearization for a viscoelastoplastic framework and we demonstrate its ability to deliver exact partitioning between the viscous the elastic and the plastic strain components the results of the study are fully reproducible and the codes are available as a subset of m2di matlab routines

photoelastic and numerical investigation of stress distributions around fault models under biaxial compressive loading conditions abstract this paper presents the results of a photoelastic and numerical study of stress distributions contours and trajectories around fault models of various geometries submitted to a biaxial compressive load it aims to describe typical biaxial stress behaviours and emphasize the existing differences with the well known uniaxial compressive load case stress trajectories are sometimes shown by joint sets acting as markers of a paleostress field and they can be interpreted by particular shallow tension compression situations at depth fractures can be reactivated or can dilate under conditions of triaxial compression and behaviour is essentially controlled by a high stress ratio high 3 1 ratio in spite of the potential importance of such stress states on fracture permeability and fluid flow and although they are frequently found at depth in a reservoir context such stress conditions are poorly investigated particularly in terms of stress perturbations the presented analogue experiments consisted of compression tests done on polymethylmethacrylate pmma plates this material has mechanical properties comparable to those of brittle rocks in the upper crust and presents birefringence the samples contained open defects acting as faults and the stress trajectories around these faults were investigated using a photoelastic device comparable numerical experiments were realised with a finite element code franc 2d using frictionless fault models first the effect of an increasing biaxial compressive load ratio 3 1 on stress trajectories around an isolated open defect was explored it was shown that the stress trajectories were drastically modified when 3 1 increased from 0 2 to 0 4 this result being consistent with previous studies in particular when 3 1 was superior or equal to 0 4 external isotropic points around where trajectories diverged called repulsive isotropic points appeared near the tips but away from the defects they tended to move away from the defects towards the main load direction when 3 1 increased the described isotropic points were points of stress trajectory divergence i e points where stress decreased implying that zones around them were strongly unfavourable to shear reactivation second stress trajectories around fault models of various geometries oblique isolated defect dilational jog compressive jog and complex patterns were studied the applied biaxial compressive load ratio being 0 7 these biaxial stress trajectories were compared with similar uniaxial stress trajectories in order to explore the existing differences between the two regimes it enabled new stress trajectory geometries to be described and interpreted in particular numerous external repulsive isotropic points were observed and defect tips were shown to be zones of high convergence of stress trajectories furthermore in contrast to the uniaxial compressive load case stress trajectories and stress contours were geometrically similar within dilational and compressive jogs under biaxial compressive load for both jog types the centre of the overlapping zone and the areas along the wall of a jog defining fault and facing the overlapping tip of the other fault proved to be zones of low mean stress implying that fluids may migrate towards them from the tips of the faults in response to mean stress gradients furthermore the centre of the jogs exhibiting a high differential stress was a favourable area in terms of fracture reactivation which may facilitate fluids transfer and storage it was also observed that for both the uniaxial and biaxial compressive loads isotropic zones were localised at the acute angle between branching defects

mantle convection modeling with viscoelastic brittle lithosphere numerical methodology and plate tectonic modeling the earth s tectonic plates are strong viscoelastic shells which make up the outermost part of a thermally convecting predominantly viscous layer brittle failure of the lithosphere occurs when stresses are high in order to build a realistic simulation of the planet s evolution the complete viscoelastic brittle convection system needs to be considered a particle in cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain dependent yield stress this is applied to a plate deformation problem numerical accuracy is demonstrated relative to analytic benchmarks and the characteristics of the method are discussed

circulation of winter water on the chukchi shelf in early summer abstract using a variety of data sources we investigate the properties and pathways of pacific origin winter water as it spreads across the eastern chukchi shelf in early summer the focus is on the time period june july 2011 during which an extensive shipboard hydrographic velocity survey was undertaken as part of the impacts of climate on ecosystem and chemistry of the arctic pacific environment icescape program a revised circulation scheme is constructed revealing that the transport pathways on the chukchi shelf are more complex than previously thought notably the well known branch progressing northward from the central channel bifurcates as it reaches hanna shoal flowing around both sides of the shoal and dividing into smaller filaments that continue towards barrow canyon mass is conserved in the circulation scheme with approximately 1 sverdrup flowing poleward across the chukchi shelf within these pathways then exiting barrow canyon the salinity of the winter water varied on the shelf in 2011 with saltier water found in the upstream portion of what is defined as the central pathway using sea ice concentration data and atmospheric reanalysis fields we argue that salinization of the winter water in the central pathway occurred via brine rejection as the parcels progressed north and passed through the cape lisburne polynya this demonstrates that winter water pervading the interior shelf can be transformed by convective overturning north of bering strait presumably stirring up nutrients from the sediments and thereby influencing primary productivity in the region of hanna shoal

coupled nitrification denitrification in sediment of the eastern bering sea shelf leads to 15n enrichment of fixed n in shelf waters 1 we studied the nitrogen biogeochemistry of the ice covered eastern bering sea shelf using the isotope ratios 15n 14n and 18o 16o of no3 and other n species the 15n 14n of late winter no3 on the shelf decreases inshore and is inversely correlated with bottom water nh4 consistent with an input of low 15n 14n no3 from partial nitrification of nh4 remineralized from the sediments an inshore 15n 14n increase in total dissolved n tdn suggests that 1 the sediment derived nh4 is elevated in 15n due to the same partial nitrification that yields the low 15n 14n no3 and 2 15n deplete no3 from partial nitrification within the sediments is denitrified to n2 the proportion of newly nitrified no3 on the shelf evidenced by an inshore decrease in no3 18o 16o is correlated with the n deficit further implicating nitrification coupled to denitrification however a simple n isotope budget indicates a comparable rate of denitrification supported by diffusion of no3 into the sediments the isotopic impact of benthic n loss is further demonstrated by a correlation between the 15n 14n of shelf surface sediment and the n deficit of the overlying water column both of which increase inshore and northward as well as by arctic no3 isotope data indicating that the fixed n transported through bering strait has a 15n 14n higher than is found in the open bering sea the significant net isotope effect of benthic n loss on the bering shelf 6 8 is at odds with previous assumptions regarding the global ocean s n isotope budget

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

late spring nitrate distributions beneath the ice covered northeastern chukchi shelf measurements of late springtime nutrient concentrations in arctic waters are relatively rare due to the extensive sea ice cover that makes sampling difficult during the subice cruise in may june 2014 an extensive survey of hydrography and pre bloom concentrations of inorganic macronutrients oxygen particulate organic carbon and nitrogen and chlorophyll a was conducted in the northeastern chukchi sea cold 1 5 c winter water was prevalent throughout the study area and the water column was weakly stratified nitrate no3 concentration averaged 12 6 1 92 m in surface waters and 14 0 1 91 m near the bottom and was significantly correlated with salinity the highest no3 concentrations were associated with winter water within the central channel flow path no3 concentrations were much reduced near the northern shelfbreak within the upper halocline waters of the canada basin and along the eastern side of the shelf near the alaskan coast net community production ncp estimated as the difference in depth integrated no3 content between spring this study and summer historical varied from 28 38 g c m 2 a 1 this is much lower than previous ncp estimates that used no3 concentrations from the southeastern bering sea as a baseline these results demonstrate the importance of using profiles of no3 measured as close to the beginning of the spring bloom as possible when estimating local ncp they also show that once the snow melts in spring increased light transmission through the sea ice to the waters below the ice could fuel large phytoplankton blooms over a much wider area than previously known

role of pressure and temperature in inclusion induced shear localization an analogue experimental approach abstract using analogue model experiments this study investigates the effects of lithostatic pressure and temperature in controlling the pattern of shear localization around rigid inclusions compression experiments were conducted on polymethylmethaacrylate pmma by varying confining pressure p 30 70 mpa homologous temperature t o 0 67 0 80 corresponding to room temperature 80 c and axial strain yield 15 in a triaxial setup mechanical data showed temperature has a greater influence on the bulk yield strength and steady state flow of the analogue material than confining pressure increasing confining pressures multiply the shear bands in number and their overall pattern becomes progressively more complex leading to composite band structures on the other hand under ambient pressure increasing temperature results in a transition from incipient high strain zones to shear bands with sharp boundaries further increase in temperature switches the mode of shear localization sharp to diffuse type we finally show the inclusion induced shear localization as a two stage process and provide a micro mechanical explanation for the p t dependent shear band patterns attributed to three mechanically distinct domains of the inclusion matrix interface compression extensional and shear localization

phytoplankton blooms beneath the sea ice in the chukchi sea in the arctic ocean phytoplankton blooms on continental shelves are often limited by light availability and are therefore thought to be restricted to waters free of sea ice during july 2011 in the chukchi sea a large phytoplankton bloom was observed beneath fully consolidated pack ice and extended from the ice edge to 4100 km into the pack the bloom was composed primarily of diatoms with biomass reaching 1291 mg chlorophyll a m 2 and rates of carbon fixation as high as 3 7 g c m 2 d 1 although the sea ice where the bloom was observed was near 100 concentration and 0 8 1 2 m thick 30 40 of its surface was covered by melt ponds that transmitted 4 fold more light than adjacent areas of bare ice providing sufficient light for phytoplankton to bloom phytoplankton growth rates associated with the under ice bloom averaged 0 9 d 1

seismic interpretation of sill complexes in sedimentary basins implications for the sub sill imaging problem application of 3d seismic reflection data to igneous systems in sedimentary basins has led to a revolution in the understanding of mafic sill complexes however there is considerable uncertainty on how geometries and architecture of sill complexes within the subsurface actually relates to geometries in seismic reflection data to provide constraints on how sill complexes in seismic data should be interpreted we present synthetic seismograms generated from a seismic scale 22 0 25 km outcrop in east greenland constrained by abundant field data this study highlights how overlying igneous rocks adversely affect imaging of underlying intrusions and rocks by decreasing seismic amplitude frequency and making steeply dipping features near impossible to image furthermore seismic modelling in this study shows that because of the high impedance contrast between siliciclastic host rock and dolerites very thin 1 5 m intrusions should in principle be imaged in reflection seismic data at 3 km depth however comparison with actual seismic data with well data shows significant amounts of unimaged sill intrusions and this is likely due to limited seismic resolution overburden complexity inadequate velocity models and interference between reflections from closely spaced sills and sill splays significant improvements to sill imaging and interpretation could be made by better predicting occurrence and geometry of sill intrusions and including these in velocity models supplementary material a scaled version of the input panel fig 5d used for seismic modelling a1 and very high resolution versions of subfigures in figure 11 a2 3 are available at https doi org 10 6084 m9 figshare 5607160

flow mixing object matrix coherence mantle growth and the development of porphyroclast tails abstract the kinematic stability fields of common porphyroclast types have been defined using a computer program that simulates low reynolds number newtonian shearing flow in the vicinity of a rigid spherical object the principal variables governing the geometry of porphyroclast tails at a given finite shear strain are 1 porphyroclast matrix coherence and 2 mantle recrystallization history both of which affect the extent to which mantle material becomes mixed with the matrix as deformation progresses

asymmetric lithospheric extension the role of frictional plastic strain softening inferred from numerical experiments plane strain thermomechanical finite element model experiments of lithospheric extension are used to investigate the effects of strain softening in the frictional plastic regime on the asymmetry of extension strain softening is considered in cases where the crust is either strongly or weakly coupled to the mantle and as the extension velocity varies from 0 3 to 30 cm yr in the absence of strain softening extension is symmetric ss mode when strain softening takes the form of a reduction in the internal angle of friction with increasing strain lithospheric extension may be asymmetric at a lithospheric scale aa mode or exhibit crustal asymmetry concomitant with mantle symmetry as mode the different styles depend on the relative control of the system by the frictional plastic and ductile layers which promote asymmetry and symmetry respectively high extension velocities and weak ductile crust mantle coupling tend to suppress the fundamental asymmetry induced by frictional strain softening this is because they respectively increase the effective strength of the lower lithosphere and decrease the control by frictional plasticity

multipath variability due to the gulf stream a phase coded signal with 64 ms resolution was transmitted at 10 min intervals for a 19 day period over two 300 km ranges the acoustic source was moored at 2000 m depth northwest of bermuda one receiver was moored at 2000 m depth to the northeast of the source and the other receiver was bottom mounted at 1000 m depth near bermuda the large 0 6 s travel time change at the bermuda receiver is probably due in large part to motion of the source mooring in the presence of currents the multipath arrival pattern at the moored receiver undergoes significant modification due to the presence of a southern meander of the gulf stream which intersects this transmission path

a comparison between late summer 2012 and 2013 water masses macronutrients and phytoplankton standing crops in the northern bering and chukchi seas abstract survey data from the northern bering and chukchi sea continental shelves in august september 2012 and 2013 reveal inter annual differences in the spatial structure of water masses along with statistically significant differences in thermohaline properties chemical properties and phytoplankton communities we provide a set of water mass definitions applicable to the northern bering and chukchi continental shelves and we find that the near bottom bering chukchi summer water bcsw was more saline in 2012 and alaskan coastal water acw was warmer in 2013 both of these water masses carried higher nutrient concentrations in 2012 supporting a larger chlorophyll a biomass that was comprised primarily of small

plume induced crustal convection 3d thermomechanical model and implications for the origin of novae and coronae on venus novae and coronae are large volcanotectonic features on venus with a contentious and possibly non unique origin and enigmatic relationship their formation is most commonly explained by flexure and fracturing of the strong rigid venus lithosphere atop mantle upwelling and or downwelling here we present new 3d high resolution thermomechanical model of mantle plume diapir impingement into warm and thin lithosphere with venus like surface temperature numerical results suggests that nova like and corona like structures can result from magma assisted convection of weak ductile crust induced by decompression melting of the hot rising mantle plume during the initial stage nova forms by stellate fracturing of a topographic rise forming atop the growing crustal convection cell few million years later nova can convert to coronae by inward dipping concentric fracturing of the nova rise margins and subsequent outward thrusting of partially molten crustal rocks over the surface an outer annulus of concentric normal faults forms in the outer rise region of the downbending brittle upper crust whereas an inner annulus of concentric thrust faults forms in front of the outward thrusting crustal wedge a trench like depression forms between these two annuli resembling retreating subduction the rudimentary concentric upper crustal slab warms up rapidly and recycles into the convection cell the convection cell remains active for up to 15 million years fueled by heat and magma from the plume predicted surface topography and fracturing patterns agree with some small to moderate size novae and coronae on venus

nucleation growth and structural development of mylonitic shear zones in granitic rock abstract well exposed shear zones in granitic rock of the sierra nevada are preserved in various degrees of development facilitating interpretation of the mechanisms of shear zone nucleation and growth field relations suggest that these shear zones formed by shear localization within pre existing aplite dikes that served as tabular zones of inherent weakness crossing dikes provide a measure of the offset at several points along the shear zones apparently the shear zones grew in width as they accumulated offset and propagated longitudinally by end to end linkage of approximately coplanar aplite dike segments propagation caused discontinuous dikes or dike segments to be joined giving the shear zones a forked or eye shaped trace pattern some shear zones have developed opening fractures at their tips analogous to the splay fractures that form near the tips of small faults these shear zones provide an excellent opportunity to study the processes involved in natural shear zone formation in granitic rocks where shear is localized along pre existing zones of inherent weakness they may serve as useful analogs for the growth and development of larger scale faults that involve ductile shearing

pathways of pacific water across the chukchi sea a numerical model study 1 pathways of pacific water flowing from the north pacific ocean through bering strait and across the chukchi sea are investigated using a two dimensional barotropic model in the no wind case the flow is driven only by a prescribed steady northward flow of 0 8 sv through bering strait the resulting steady state circulation consists of a broad northeasterly flow basically following the topography with a few areas of intensified currents about half of the inflow travels northwest through hope valley while the other half turns somewhat toward the northeast along the alaskan coast the flow through hope valley is intensified as it passes through herald canyon but much of this flow escapes the canyon to move eastward joining the flow in the broad valley between herald and hanna shoals another area of slightly intensified currents there is a confluence of nearly all of the flow along the alaskan coast west of pt barrow to create a very strong and narrow coastal jet that follows the shelf topography eastward onto the beaufort shelf thus in this no wind case nearly all of the pacific water entering the chukchi sea eventually ends up flowing eastward along the narrow beaufort shelf with no discernable flow across the shelf edge toward the interior canada basin travel times for water parcels to move from bering strait to pt barrow vary tremendously according to the path taken e g less than 6 months along the alaskan coast but about 30 months along the westernmost path through herald canyon this flow field is relatively insensitive to idealized wind forcing when the winds are from the south west or north in which cases the shelf transports tend to be intensified however strong northeasterly to easterly winds are able to completely reverse the flows along the beaufort shelf and the alaskan coast and force most of the throughflow in a more northerly direction across the chukchi sea shelf edge potentially supplying the surface waters of the interior canada basin with pacific water the entire shelf circulation reacts promptly to changing wind conditions with a response time of 2 3 days the intense coastal jet between icy cape and pt barrow implies that dense water formed here from winter coastal polynyas may be quickly swept away along the coast in contrast there is a relatively quiet nearshore region to the west between cape lisburne and icy cape where dense water may accumulate much longer and continue to become denser before it is carried across the shelf

source and formation of the upper halocline of the arctic ocean 1 the upper halocline of the arctic ocean has a distinct chemical signature with high nutrient concentrations as well as low oxygen and ph values this signature is formed in the chukchi and east siberian seas by a combination of mineralization of organic matter and release of decay products to the sea ice brine enriched bottom water salinity and total alkalinity data show that the fraction of sea ice brine in the nutrient enriched upper halocline water in the central arctic ocean is up to 4 in the east siberian sea the bottom waters with exceptional high nutrient concentration and low ph have typically between 5 and 10 of sea ice brine as computed from salinity and oxygen 18 values on the continental slope over bottom depths of 150 200 m the brine contribution was 6 at the nutrient maximum depth 50 100 m at the same location as well as over the deeper basin the silicate maximum was found over a wider salinity range than traditionally found in the canada basin in agreement with earlier observations east of the chukchi plateau a detailed evaluation of the chemical and the temperature salinity properties suggests at least two different areas for the formation of the nutrient rich halocline within the east siberian sea this has not been observed before 2004 and it could be a sign of a changing marine climate in the east siberian sea caused by more open water in the summer season followed by more sea ice formation and brine production in the fall winter

evidence from magnetic fabric for the flow pattern of magma in the mackenzie giant radiating dyke swarm continental flood basalts and their associated swarms of mafic dykes are thought to result from melting in mantle plumes1 3 but the processes by which the resulting magma makes its way from mantle to crust remain obscure here we use magnetic anisotropy measurements to reconstruct the pattern of magma flow in the proterozoic mackenzie dyke swarm our results indicate that magma was injected vertically within 500 km of the focal point of the dyke swarm and then travelled horizontally at all further distances out to at least 2 100 km the sharp transition from vertical to horizontal flow at between 500 and 600 km may correspond to the outer boundary of melt generation in the mantle plume believed to be responsible for the mackenzie igneous events

diapirism and topography summary a new numerical technique using markers and a deformable lagrangian mesh is used to study the deformation of the surface above a rising diapir this method allows modelling of a free surface boundary in a self consistent way the method makes it possible to investigate many different problems with non regular geometry the codes were verified through comparison with analytical solutions for the initial stages and with other numerical codes for mature stages our simulations led to the following conclusions 1 the growth rate of the diapir is more strongly influenced by the viscosity contrast than the wavelengths this is due to the strong growth rate difference during the initial stage 2 the maximum elevation above the diapir linearly increases with increasing wavelength and is approximately the same for different viscosity contrasts 3 the elevation will be considerably higher for a low viscosity diapir at a given depth than for an isoviscous diapir comparisons for different thicknesses of the buoyant layer showed that the highest topography is produced when the two layers have equal thickness we showed the dependence of the topographic behaviour on the parameter r the ratio of the density difference between two layers to the density of the upper layer it was found that topography behaves linearly up to r 0 15 it indicates that a posteriori estimation of topography for traditional calculations using the free slip boundary condition works well within this limit

de compaction of porous viscoelastoplastic media solitary porosity waves buoyancy driven flow in deformable porous media is important for understanding sedimentary compaction as well as magmatic and metamorphic differentiation processes here mathematical analysis of the viscoplastic compaction equations is used to develop an understanding of the porosity wave instability and its sensitivity to the choice of rheological model the conditions of propagation size speed and shape of the porosity waves depend strongly on the properties of the solid rock frame whereas most of the previous studies on porosity waves were focused on viscous or viscoelastic mode here we consider the ability of a solid matrix to undergo simultaneous plastic rate independent and viscous rate dependent deformation in parallel plastic yielding is identified as a cause of compaction decompaction asymmetry in porous media this is known to lead to a strong focusing of porous flow speed and amplitude of a porosity wave are given as functions of material parameters and a volume of a source region formulation is applicable to fluid flow in sedimentary rocks where viscous deformation is due to pressure solution as well as in deep crustal or upper mantle rocks deforming in a semibrittle regime

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

dike emplacement on venus and on earth many long linear troughs are visible in synthetic aperture radar images of the surface of venus their geometry closely resembles that of dike swarms on earth such troughs will be formed if dikes fail to reach the surface or if the magma in them is partially drained dikes that form closed ellipses in plan view can be produced by intrusions near a depression and intersecting dikes can result if the stress field changes with time because of the absence of erosion and sedimentation all these effects are better displayed on venus than they are on earth the magnitude of the regional stress on venus can be estimated from the dike patterns to be about 3 mpa or similar to that of earth dikes consisting of en echelon segments whose total length is 1500 km or more occur on both earth and venus on earth their widths are generally greater than 30 m and are sometimes as great as 250 m and they are likely to be similar on venus the melt velocity during their emplacement is predicted to be a few meters per second and the calculated rate of heat loss is sufficiently slow that they can remain molten as they propagate for distances of more than 1000 km the same calculations apply to sills though less is known about their geometry the results therefore suggest that magma intrusion in thin sheets extending over 1000 km or more could produce regional crustal thickening and uplift on both earth and venus

a giant circumferential dyke swarm associated with the high arctic large igneous province halip abstract in this study we identify a giant circumferential mafic dyke swarm associated with the 135 75 ma high arctic large igneous province halip previously a halip giant radiating mafic dyke swarm with portions scattered across the canadian high arctic islands northern greenland svalbard and franz josef land was recognized in a pre drift plate tectonic reconstruction of the arctic region the radiating swarm has been interpreted to focus above a mantle plume responsible for halip magmatism the newly recognized halip giant circumferential swarm has a centre that is near the focus of the halip radiating system and hence is likely related to the halip plume elements of the circumferential swarm are located in each of the four regions where the radiating system is found the circumferential swarm has a quasi circular or slightly elliptical geometry an outer diameter of 1600 km and an arc of 220 it is one of the largest giant circumferential dyke swarms recognized on earth and could be linked to the outer edge of the flattening plume head it is also the first such swarm to have been identified by means of a plate tectonic reconstruction although giant circumferential dyke swarms appear to be relatively rare on earth possible analogues are common on venus and are also found on mars on venus giant circular or elliptical tectono magmatic features termed coronae are characterized by an annulus of graben or fissures and prominent topography some coronae include a radiating graben fissure system both radiating and circumferential graben may be underlain by dykes if so coronae could be analogues for terrestrial giant circumferential dyke swarms such as observed in the case of the halip

the role of elasticity in simulating long term tectonic extension author posting oxford university press 2016 this article is posted here by permission of oxford university press for personal use not for redistribution the definitive version was published in geophysical journal international 205 2016 728 743 doi 10 1093 gji ggw044

a genetic link between transform and hyper extended margins abstract the similarity between the geometry of the west african and south american coastlines is among one of the strongest natural observations supporting the plate tectonic paradigm however using classical plate tectonic approaches to model these conjugate transform margins results in a high degree of variability in palaeogeographic reconstructions using state of the art 3d coupled thermo mechanical numerical models we simulate for the first time crustal deformation at the onset of oceanisation along large offset oblique margins our models show that obliquity causes oceanic rift propagation to stall resulting in an apparent polyphased tectonic evolution and in some circumstances leads to the formation of hyper extended margins as a result conjugate margins located at the edge of future fracture zones are highly asymmetric from rifting to spreading with their lengths differing by a factor of 5 to 10 before the final phase of break up occurs accounting for this discrepancy should ameliorate future palaeogeographic reconstructions

a transpressional origin for the west spitsbergen fold and thrust belt insight from analog modeling 1 the west spitsbergen fold and thrust belt formed along the transform plate boundary between greenland and the western barents sea during paleocene eocene breakup in the northern north atlantic approximately 20 40 km margin perpendicular shortening accumulated in the belt has been attributed to transpression and strain partitioning in a restraining bend but also to head on collision we have applied scaled analog tectonic modeling to test the former hypothesis a pack of brittle quartz sand with a shallow thin layer of silicon putty was deformed by transpression at a 15 convergence angle by movement of a basal plate the kinematics was quantified by means of digital particle image velocimetry the result was a doubly vergent transpressional wedge consisting of a steeply tapered retrowedge and a strongly internally deformed steeply tapered prowedge separated by a central strike slip zone and an adjacent low taper thin skinned fold and thrust belt the doubly vergent wedge evolved through several kinematic phases three main stages were identified namely 1 initial distributed deformation 2 development of an oblique doubly vergent wedge with progressive evolution of local strain partitioning along the marginal shear zones and finally 3 a stage of full strain partitioning between a central strike slip zone and reverse displacement along marginal shear zones with folding and thrusting in a thin skinned belt on the proside the analog model convincingly reproduced the geometry and the kinematic evolution of the west spitsbergen fold and thrust belt supporting the hypothesis of its formation by strain partitioning in transpression with a small angle of convergence and significant lateral displacement

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

tidal effects on long range sound transmission time series of acoustic travel times over long distances have significant tidal components we discuss three experiments first for a 900 km westward transmission from a moored source southwest of bermuda the m2 constituent apart from mooring motion of 8 ms can be accounted for by a variable propagation with and against the 2 cm s barotropic tidal current the situation is different for the k1 and s2 constituents which have a significant solar component particularly for rays with shallow turning points suggesting an effect from a minute daily variation in the temperature of the upper 100 m second for a 500 km transmission from the bahamas northward and parallel to the continental margin the measured m2 constituent is not in phase with the computed effect of the barotropic tidal current unlike the westward transmission mentioned above here the mode conversion from barotropic to baroclinic tides plays a crucial role the principal cause of the travel time oscillations in quadrature with the tidal current is a vertical oscillation by several meters and attendant temperature and sound speed fluctuations associated with the baroclinic tide just seaward of the continental margin in the near zone of mode conversion third for 300 km transmissions from a moored source northwest of bermuda to both a moored receiver and a landbased receiver the tidal components in the travel time are dominated by tidally induced mooring motions the tidal amplitude varies in time as the tidal mooring motion is modulated by the variable current because of the nonlinear drag and the current can be so estimated an important part possibly a predominant part of the nontidal travel time variation of 0 7 s is evidently associated with the mean mooring motion the interpretation of the tidal fluctuations in travel time is varied and complex

interpretation of depth to the magnetic basement in the northern barents sea south of svalbard abstract aeromagnetic data covering some 135 000 km2 with 34 000 km of profiles in the northern barents sea south of svalbard have been interpreted the survey area is situated between 74 30 and 78 n and 15 and 32 e the main result of the study is a contour map of the depth to the top of the magnetic basement indicating the thickness of the sedimentary rocks in the area three different graphical techniques and the autocorrelation method have been applied to determine the depths to magnetic sources from the profile data the different methods yield errors which generally vary between 5 and 15 systematic errors will add to the spread shown by the different methods therefore the inaccuracy of individual point estimates can sometimes be as large as 30 the interpreted depths to the magnetic basement exceed 10 km in the sorkapp basin in parts of storfjorden and towards the olga basin the basement is located at depths of less than 2 to 3 km on the bjornoya sorkapp high and the hopen high and at depths of about 6 km on the gardar banken high depths in the area of the central platform vary smoothly and gradually from 6 to 9 km it is suggested that in some areas pre permian sediments may constitute more than half of the total sedimentary column the location of the continent ocean transition zone has been interpreted from the aeromagnetic data strong gradients in the contour map of depth to magnetic basement coincide with the well known bjornoya sorkapp fault zone the strongest gradients seen elsewhere in the depth to basement map may therefore reveal major basement faults and or flexures which define the margins of structural highs and troughs in the tertiary the continental crust in the western part of the study area was deformed along nnw sse trending fault zones these faults are sub parallel to linear positive magnetic anomalies observed in the same area which reflect trends within the basement and sub parallel to major tectonic lines including basement faults mapped by geologists further to the north on spitsbergen it is therefore inferred that precambrian basement fault or fracture zones have been reactivated in phanerozoic times east of a major tectonic line running n s to nw se through the central part of the area the basement map and the aeromagnetic map show variable trends with ne sw trends dominating comparison with deep seismic reflection data suggests that the interpreted discontinuity may represent a major caledonian compressional structure perhaps the iapetus suture which separates two different crustal terranes

preliminary interpretation of aeromagnetic data from spitsbergen svalbard archipelago 76 79 n implications for structure of the basement abstract new arctic aeromagnetic data from spitsbergen svalbard archipelago have been interpreted the depths to the top of the magnetic source rocks have been interpreted using profile data and the interpretation shows that the magnetic basement is deeper than 3 km below sea level over large areas a picture of basement horsts and grabens emerges from this map a deep n s trending trough more than 8 km deep exists beneath isfjorden it has a graben structure and is bordered to the east by a southward extension of the billefjorden fault zone model calculations show that steep east dipping structures in the pre devonian hecla hoek basement rocks can explain anomalies above the billefjorden fault zone the billefjorden fault zone is interpreted to extend from the shelf north of spitsbergen to south of isfjorden and may extend southwards into the barents sea a new aeromagnetic map covering spitsbergen and the adjacent northern barents sea has been compiled which shows that structures on spitsbergen continue far into the barents sea some of the anomalies that originate within the crystalline basement occur near the boundaries to younger sedimentary basins and suggest that basement tectonics have influenced basin formation