faulting pattern along slow spreading ridge segments a consequence of along axis variation in lithospheric rheology abstract we present here results from an analogue model designed to test the relative influence of along axis variations of the crustal thickness and of the thermal structure of the lithosphere on the geometry and on the faulting pattern of the axial rift of a slow spreading ridge rheologically calculated layered models are employed using quartz sand and silicone putty as analogues of the brittle and ductile components of the lithosphere respectively two parameters have been analysed in detail 1 the thickness of the brittle layer and 2 the viscosity of the ductile layer this study shows that the thickening of the brittle layer and the increase of the viscosity of the ductile layer bring about a widening of the axial valley associated with an increase of the number of faults a decrease of the depth of the axial valley and of the vertical throw of the faults is observed with an increase of the viscosity of the ductile layer and a decrease of the thickness of the brittle layer these results are consistent with observations along mid atlantic ridge segments for segment ends the fault pattern obtained in the models is similar to that described on both sides of the axis for segments bordered by a zero offset discontinuity and on outside corners for segments bordered by a lateral offset discontinuity our results suggest that the viscosity of the ductile layer plays a more important role in the fault pattern than the thickness of the brittle layer the influence of segment length offset length and temporal variation in thermal input could explain the more or less important along axis variation in the deformation pattern observed along segments of slow spreading ridges

oceanic transverse ridges a flexural response to fracture zone normal extension one of the most puzzling characteristics of sea floor morphology is the occurrence of anomalously shallow fracture zone parallel oceanic transverse ridges a model is proposed for the formation of transverse ridges near lat 21 and 24 n on the mid atlantic ridge in which the differential responses of large offset and small offset fracture zones to recent changes in spreading direction result in the generation of normal faults that coincide with the off axis traces of fracture zones numerical models of the flexural response of the lithosphere to normal faulting suggest that modest amounts of extension 5 km along low angle faults 45 are responsible for the transverse ridges

major pathways of atlantic water in the northern north atlantic and nordic seas toward arctic 1 the major pathways of near surface atlantic water in the northern north atlantic and nordic seas are identified as current speeds above 30 cm s using 1014 lagrangian drifters combined with previously published hydrography the inflow over the scotland greenland ridge and establishment of the two branch norwegian atlantic current nwac are described in light of the circulation in the northern north atlantic the western branch of the nwac appears as a jet in the polar front topographically guided from the iceland faroe front through the nordic seas toward fram strait the eastern branch starts as a shelf edge current above the irish scottish continental shelf and after passing through the faroe shetland channel it continues northward along the norwegian shelf edge toward the arctic with a branch bifurcating into the barents sea the nwac appears to maintain its two branch structure throughout the nordic seas with the atlantic water confined to a 200 600 km wide wedge

propagation of the great salinity anomaly of the 1990s around the northern north atlantic 1 time series of t and s extending through 2001 are used to describe propagation of the great salinity anomaly of the 1990s gsa 90s comparison of the distance time relations for the gsa 70s 80s and 90s reveals a substantial intensification of the large scale circulation in the northern north atlantic especially in the subarctic gyre between newfoundland and the faroes the advection rate of the gsa 70s 80s and 90s between newfoundland and the faroe shetland channel is conservatively estimated to have been 3 5 10 and 10 cm s respectively the circulation intensification apparently occurred within a decade between the gsa 70s and 80s during the next decade the advection rate increased from 10 to 13 cm s between newfoundland and iceland basin the gsa 90s was advected towards the faroe shetland channel by the northern iceland basin s branch of the north atlantic current whereas the contribution of the southern branch via the rockall trough was minimal

strength of fracture zones from their bathymetric and gravitational evolution fracture zone evolution is investigated using dynamic models that allow the fault zones to freely slip this is an improvement over past formulations where bathymetric offsets were imposed kinematically the models use a viscoelastoplastic rheology that incorporates the influence of fault friction on fracture zone slip history using viscoelastic plates we assess the role of small scale convection on removal of the lowermost thermal lithosphere beneath fracture zones through a comparison of synthetic gravity to free air gravity across fracture zones we find that the amplitude of the gravity jump across fracture zones is best fit by models with weak faults that have depth averaged yield strengths 10 mpa fracture zones with such low strengths can convert to subduction zones with 100 km of convergence many fracture zones do not fit plate subsidence models with locked or slipping faults but are better fit by systems that are tectonically deformed by modest amounts of extension

are oceanic fracture zones locked and strong or weak new evidence for volcanic activity and weakness traditionally oceanic fracture zones and other lithospheric and crustal faults have been viewed as zones of weakness that are vulnerable to seismic and volcanic activity this notion has recently been questioned on the basis of evidence showing that initial depth offsets scarp height across some pacific fracture zones are preserved even after 100 m y of differential subsidence this evidence suggests that some parts of oceanic fracture zones are locked shortly after they drift away from ridge transform intersections because the fracture zones seem to be strong welds differential cooling stresses cause flexure in the lithosphere on both sides of the fracture in this paper we briefly review the seemingly contradictory evidence for the thermomechanical characteristics of oceanic fracture zones locked and strong vs slipped and weak and present new evidence indicating that parts of some fracture zones are volcanically active volcanism is an indication of relative weakness because strain and failure are required to open volcanic conduits this and other evidence for the inherent weakness of fracture zones can be reconciled with evidence for strength if different segments of fracture zones exhibit contrasting thermomechanical behavior more data are needed to determine whether this is so and whether there are systematic patterns of distribution for weak and strong parts of individual fracture zones

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

atlantic origin of observed and modelled freshwater anomalies in the nordic seas between 1965 and 1990 the waters of the nordic seas and the subpolar basins of the north atlantic ocean freshened substantially1 the arctic ocean also became less saline over this time as a consequence of increasing runoff1 2 3 4 but it is not clear whether flow from the arctic ocean was the main source of the nordic seas salinity anomaly as a region of deep water formation the nordic seas are central to the atlantic meridional overturning circulation but this process is inhibited if the surface salinity is too low2 here we use the instrumental record of nordic seas hydrography along with a global ocean sea ice model hindcast simulation to identify the sources and magnitude of freshwater that has accumulated in the nordic seas since 1950 we find that the freshwater anomalies within the nordic seas can mostly be explained by less salt entering the southern part of the basin with the relatively saline atlantic inflow with seemingly little contribution from the arctic ocean we conclude that hydrographic changes in the nordic seas are primarily related to changes in the atlantic ocean we infer that if the atlantic inflow and nordic seas both freshen similarly this would render the atlantic meridional overturning circulation relatively insensitive to nordic seas freshwater content

fluctuation in porosity and fluid pressure during hydrothermal events textural evidence in the emuford district australia abstract the upper paleozoic granites of the emu suite north queensland show varying degrees of alteration the petrographic and microstructural survey of albite rich zones reveals the spatial overprint of hydrothermal events which are more correlated with changes in physical conditions permeability fluid pressure and flow regime rather than with chemical variations of the percolating fluids the sequence of alteration events chronologically comprises 1 in situ substitution of primary microline to chessboard albite 2 dissolution of magmatic quartz resulting in the formation of interconnected vugs 3 formation of concentric cracks around some vugs 4 infilling of vugs and 5 development of a dense system of closely spaced sheeted veinlets by a crack seal process the first two stages indicate an increase of the global permeability of the albite zones chemically induced porosity whereas the vug and vein filling stages indicate the reverse resulting in an increase of fluid pressure from nearly hydrostatic to nearly lithostatic values mechanically induced porosity this contrast is discussed from micro and mesostructural arguments and from results of a fluid inclusion investigation it is explained by the progressive focusing of the circulating fluids via grain boundaries then vugs and through tapping by late stage microfracturing

oceanic broad multifault transform plate boundaries oceanic transform plate boundaries consist of a single narrow a few kilometers wide strike slip seismic zone offsetting two mid ocean ridge segments however we define here a new class of oceanic transform boundaries with broad complex multifault zones of deformation similar to some continental strike slip systems examples are the 750 km long 120 km wide andrew bain transform on the southwest indian ridge and the romanche transform where the mid atlantic ridge is offset by a lens shaped 900 km long 100 km wide sliver of deformed lithosphere bound by two major transform valleys one of the valleys is seismically highly active and constitutes the present day principal transform boundary however strike slip seismic events also occur in the second valley and elsewhere in the deformed zone some of these events may be triggered by earthquakes from the principal boundary numerical modeling predicts the development of wide multiple transform boundaries when the age offset is above a threshold value of 30 m y i e in extra long 500 km slow slip transforms multiple boundaries develop so that strike slip ruptures avoid very thick and strong lithosphere

some dynamical constraints on upstream pathways of the denmark strait overflow abstractthe east greenland current egc had long been considered the main pathway for the denmark strait overflow dso recent observations however indicate that the north icelandic jet nij which flows westward along the north coast of iceland is a major separate pathway for the dso in this study a two layer numerical model and complementary integral constraints are used to examine various pathways that lead to the dso and to explore plausible mechanisms for the nij s existence in these simulations a westward and nij like current emerges as a robust feature and a main pathway for the denmark strait overflow its existence can be explained through circulation integrals around advantageous contours one such constraint spells out the consequences of overflow water as a source of low potential vorticity a stronger constraint can be added when the outflow occurs through two outlets it takes the form of a circulation integral around the iceland faroe ridge in either case the direction of overall

the sensitivity of the greenland scotland ridge overflow to forcing changes processes that influence the volume and heat transport across the greenland scotland ridge system are investigated in a numerical model with horizontal resolution the focus is on the sensitivity of cross ridge transports and the reaction of the subpolar north atlantic ocean circulation to changes in wind stress and buoyancy forcing on seasonal to interannual timescales a general relation between changes in wind stress or cross ridge density contrasts and the overturning transport of greenland iceland norwegian seas source water is established from a series of idealized experiments the relation is used subsequently to interpret changes in an experiment over the years 1992 97 with realistic forcing on seasonal and interannual timescales there is a clear correlation between heat flux and wind stress curl variability the realistic model suggests a steady decrease in the strength of the cyclonic subpolar gyre of the north atlantic with a corresponding decrease in heat transport during the 1990s

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

recent large increases in freshwater fluxes from greenland into the north atlantic 1 freshwater fw fluxes from river runoff and precipitation minus evaporation for the pan arctic seas are relatively well documented and prescribed in ocean gcms fluxes from greenland on the other hand are generally ignored altogether despite their potential impacts on ocean circulation and marine biology here we present a reconstruction of the spatially distributed fw flux from greenland for 1958 2010 we find a modest increase into the arctic ocean during this period fluxes into the irminger basin however have increased by fifty percent 6 3 0 5 km3 yr 2 in less than twenty years this greatly exceeds previous estimates for the ice sheet as a whole the rate of increase since 1992 is 16 9 1 8 km3 yr 2 the cumulative fw anomaly since 1995 is 3200 358 km3 which is about a third of the magnitude of the great salinity anomaly gsa of the 1970s if this trend continues into the future the anomaly will exceed that of the gsa by about 2025

progress in microboudin method for palaeo stress analysis of metamorphic tectonites application of mathematically refined expression abstract the microboudin method for palaeo stress analysis is mathematically refined by incorporating the recently proposed shear lag model which describes the relationship between stress distribution along a fibre embedded within an elastic matrix and the far field differential stress applied to the matrix the refined probability density function of fractured fibres g with respect to the aspect ratio of the fibre r and the stress parameter is given by g r 1 exp m 1 m r m e f e q m 1 1 e q e f 1 cosh ar m where eq and ef are the elastic constants of the matrix and fibre m is the weibull modulus of the fibre material and a is a constant the stress parameter is defined as 0 s where 0 is the far field differential stress applied to the matrix and s is the modal fracture strength of the fibre material at r 1 this method is applied to tourmaline and piemontite boudins in quartzose rocks from six areas and it is revealed that the probability density function adequately models the proportion of boudinaged tourmaline and piemontite grains the value of is determined for each sample allowing 0 to be determined by this method when s is known

development of microboudinage an estimate of changing differential stress with increasing strain microboudin structure has so far been used in structural analysis as a reliable measure of plastic strain and recently it has been proposed to use it as a measure of relative differential stress this paper demonstrates that stress and strain can be simultaneously deduced if the stress and strain analyses are carried out in combination the analysed samples are quartz schists from four localities of different high p low t type metamorphic belts in japan the samples include microboudinaged competent piedmontite grains within a ductile quartz matrix the analyses strongly suggest that the estimated differential stresses tend to increase with increasing strain and that at the final stage of the microboudinage they reach a maximum

observation of decadal change in the atlantic meridional overturning circulation using 10 years of continuous transport data the meridional overturning circulation moc represents the main mechanism for the oceanic northward heat transport in the atlantic and fluctuations of this circulation are believed to have major impacts on northern hemisphere climate while numerical ocean and climate models and paleo records show large variability in this circulation the use of direct observations of the moc for detecting climate timescale changes has proven difficult so far this report presents the first observational record of moc measurements that is continuous and sufficiently long to exhibit decadal scale changes here a decrease by 20 over the observational period jan 2000 june 2009 and large interannual changes in the flow and its vertical structure data are from a mooring array at 16 n meridional overturning variability experiment move the observed change agrees with the amplitude of multi decadal natural fluctuations seen in numerical ocean and climate models knowledge of the existence and phasing of such internal cycles provides multi decadal climate predictability recently some numerical model simulations have produced results that show a weakening of the moc since the 1990 s and observational confirmation of this now is a high priority

fates and travel times of denmark strait overflow water in the irminger basin the denmark strait overflow dso supplies about one third of the north atlantic deep water and is critical to global thermohaline circulation knowledge of the pathways of dso through the irminger basin and its transformation there is still incomplete however the authors deploy over 10 000 lagrangian particles at the denmark strait in a high resolution ocean model to study these issues first the particle trajectories show that the mean position and potential density of dense waters cascading over the denmark strait sill evolve consistently with hydrographic observations these sill particles transit the irminger basin to the spill jet section 65 25 n in 5 7 days and to the angmagssalik section 63 5 n in 2 3 weeks second the dense water pathways on the continental shelf are consistent with observations and particles released on the shelf in the strait constitute a significant fraction of the dense water particles recorded at the angmagssalik section within 60 days 25 some particles circulate on the shelf for several weeks before they spill off the shelf break and join the overflow from the sill third there are two places where the water density following particle trajectories decreases rapidly due to intense mixing to the southwest of the sill and southwest of the kangerdlugssuaq trough on the continental slope after transformation in these places the overflow particles exhibit a wide range of densities

thermal contraction and the state of stress in the oceanic lithosphere nonuniform contraction of the oceanic lithosphere as it cools and thickens following its formation at the axis of a spreading center results in a complex three dimensional state of deviatoric stress which can be separated into two parts a thermal bending stress due to changes in the vertical temperature distribution and a thermal contraction stress due to lateral variations in the vertically averaged temperature we examine thermal contraction stresses due to temperature changes in a thin semi infinite rectangular plate bounded by the spreading center axis two stress free boundaries representing transforms or fracture zones define the plate width or ridge segment length l with the bottom of the elastic plate defined by a prescribed temperature the plate thickens as the square root of age as expected for a thermal boundary layer due to vertical conductive cooling above this prescribed temperature elastic stresses are assumed to relax quickly stresses are obtained by properly accounting for the rate of accumulation of the vertically averaged stress as initially stress free material is added to bottom of the cooling thickening plate the state of thermal contraction stress calculated from this model is characterized by large tensile stresses at the boundaries of the plate and relatively low stresses in the plate interior at the ridge axis ridge parallel tensile stresses are about 300 mpa the same as if the plate were not allowed to contract in this direction along the transform boundary the maximum transform parallel tensile stress occurs at a distance l 2 from the ridge transform intersection where its magnitude is comparable to the stress at the ridge axis the tangential stresses at the plate boundary decrease rapidly with distance from the boundary at a distance of l 4 from the ridge axis the ridge axis parallel stresses are one tenth of their ridge axis magnitude the stress magnitudes are independent of both spreading rate and ridge segment length a large transform parallel tensile stress may control the length of transform offsets thermal bending moments are influenced by the large thermal contraction stresses near the ridge axis however a short distance from the ridge these moments attain their free horizontal contraction values which previous studies have shown to cause observable bending of the plate and a geoid anomaly at fracture zones flexure due to thermal bending moments will concentrate bending stresses at a distance from the fracture zone determined by the flexural length of the plate thus providing a natural length scale controlling the spacing of transforms

deciphering the pathways for the deep limb of the meridional overturning circulation for the past 50 years it has been assumed that the principal pathway for the deep limb of the atlantic meridional overturning circulation amoc is the deep western boundary current dwbc however recent observations of lagrangian floats have shown that the dwbc is not necessarily a unique dominant or continuous pathway for these deep waters a significant portion of the deep water export from the subpolar to the subtropical gyres follows a pathway through the interior of the newfoundland and subtropical basins which is constrained by the western boundary and the western flank of the mid atlantic ridge the hypothesis that deep eddy driven recirculation gyres are a mechanism for partitioning the deep limb of the amoc into the dwbc and this interior pathway is investigated here eulerian and lagrangian analyses of the output of ocean general circulation models at eddy resolving eddy permitting and non eddy permitting resolutions are used to test this hypothesis eddy driven recirculation gyres simulated in the eddy resolving and eddy permitting models and similar to recirculations inferred from hydrographic data are shown to shape the export pathways of deep water from the subpolar to the subtropical gyres

short wavelength geoid bathymetry and the convective pattern beneath the pacific ocean summary the geoid and bathymetry data in the pacific ocean are used to shed new light on the structure of upper mantle convection at short wavelengths 200 km the geoid pattern is characterized by parallel lineations lineations however appear less clearly at larger wavelengths 400 or 600 km where the signal due to convective thermal anomalies might be expected the most prominent features among the short wavelength lineations are associated with the well known chains of seamounts and islands other lineations observed in the geoid coincide with computed tracks of other volcanic centres although these tracks are not continuous this suggests that long lived plumes are at the origin of the apparently short chains of seamounts apart from the lineations corresponding to the tracks of classical hotspots there are numerous similar lineations visible on the filtered geoid maps which have the same directions as hotspot tracks these preferential directions and the gravity to bathymetry ratios seem to indicate that the lineations are due to magmas implaced just above or within the lithosphere convective plumes fixed with respect to each other and much more numerous than the classical hotspots could be the source of these magmas a geometrical test of this hypothesis is performed by comparing the lineation pattern with the computed tracks for a set of tentative new hotspots in this way numerous plumes long lived and fixed with respect to each other are found to provide an explanation of the observed small wavelength geoid features

geometrical statistical modelling of systems of fracture zones along oceanic ridges summary a mechanical statistical model is presented that aims to help to understand the history and geometry of the process of formation of fracture zones along oceanic ridges it uses ideas of statistical fracture theory used in engineering namely the weibull fracture model the approximate parallelism of the fracture zones along ridges makes it possible to use a onedimensional point process model with points along the ridge axes which represent the transform faults the ratios of the lengths of the corresponding fracture zones to the ocean width are used to obtain a rough estimate of the weibull modulus which is an important material parameter in fracture theory the theory is refined by introducing a hard core point process model the corresponding positive minimum distance between subsequent fracture zones results from stress relaxation in the vicinity of a given fracture zone

evidence for diffuse extension of the pacific plate from pukapuka ridges and cross grain gravity lineations satellite altimeter measurements of marine gravity reveal 100 to 200 km wavelength lineations over a wide area of the pacific plate oriented roughly in the direction of absolute plate motion at least three mechanisms have been proposed for their origin small scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere diffuse n s extension of the lithosphere resulting in lineated zones of extension boudins and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism recently several chains of linear volcanic ridges have been found to be associated with the gravity lineations following ridgelike gravity signatures apparent in high resolution geosat gravity measurements we surveyed a series of volcanic ridges that extend northwest from the east pacific rise flank for 2600 km onto 40 ma seafloor our survey data as well as radiometric dates on samples we collected from the ridges provide tight constraints on their origin 1 individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion 2 the ridges formed along a band 50 to 70 km wide in the trough of one of the more prominent gravity lineations 3 radiometric dates of the largest ridges show no hotspot age progression moreover the directions predicted for minihotspot traces older than 24 ma do not match observed directions of either the gravity lineations or the ridges based on this last observation we reject the minihotspot model the occurrence of the ridges in the trough of the gravity lineation is incompatible with the small scale convection model which would predict increased volcanism above the convective upwelling we favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere however the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model perhaps the fault scarps are buried under more than 30 m of sediments and or covered by the elongated ridges finally we note that if ridge push force is much smaller than trench pull force then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion

active faulting in the dead sea rift abstract manifestations of late quaternary and holocene faulting were studied in a 500 km long segment of the dead sea transform rift most prominent are left slip faults whose characteristic physiographic features are recognizable along most of the studied segment where these faults bend or are stepped to the left rhomb shaped grabens or pull aparts are produced forming depressions in the reverse situation compressional features such as pressure ridges domes and folds form positive topographic features such structures are combined on a variety of scales ranging from a few hundred meters long to tens of kilometers normal faults sub parallel to the left slip faults produce a trough like valley along much of the dead sea transform but are most prominent along the margins of the large rhomb grabens e g the dead sea trough they apparently record a small component of transverse extension generally their motion is slow young slip did not occur along some segments during the last few 10 4 y elsewhere throws of 10 20 m at least occurred in this period the dead sea transform is seismically active the instrumental and historic records indicate a seismic slip rate of 0 15 0 35 cm y during the last 1000 1500 y while estimates of the average pliocene pleistocene rate are 0 7 1 0 cm y either much creep takes place or the slip rate varies over periods of a few 10 3 y

on the origin and propagation of denmark strait overflow water anomalies in the irminger basin denmark strait overflow water dsow supplies the densest contribution to north atlantic deep water and is monitored at several locations in the subpolar north atlantic hydrographic temperature and salinity and velocity time series from three multiple mooring arrays at the denmark strait sill at 180 km downstream south of dohrn bank and at a further 320 km downstream on the east greenland continental slope near tasiilaq formerly angmagssalik were analyzed to quantify the variability and track anomalies in dsow in the period 2007 2012 no long term trends were detected in the time series while variability on time scales from interannual to weekly was present at all moorings the hydrographic time series from different moorings within each mooring array showed coherent signals while the velocity fluctuations were only weakly correlated lagged correlations of anomalies between the arrays revealed a propagation from the sill of denmark strait to the angmagssalik array in potential temperature with an average propagation time of 13 days while the correlations in salinity were low entrainment of warm and saline atlantic water and fresher water from the east greenland current via the east greenland spill jet can explain the whole range of hydrographic changes in the dsow measured downstream of the sill changes in the entrained water masses and in the mixing ratio can thus strongly influence the salinity variability of dsow fresh anomalies found in downstream measurements of dsow within the deep western boundary current can therefore not be attributed to arctic climate variability in a straightforward way

formation and propagation of great salinity anomalies north atlantic arctic ocean and sea ice variability for the period 1948 2001 is studied using a global ocean general circulation model coupled to a dynamic thermodynamic sea ice model forced by daily ncep ncar reanalysis data kalnay et al 1996 variability of arctic sea ice properties is analysed in particular the formation and propagation of sea ice thickness anomalies that are communicated via fram strait into the north atlantic these export events led to the great salinity anomalies gsa of the 1970s 1980s and 1990s in the labrador sea ls all gsas were found to be remotely excited in the arctic rather than by local atmospheric forcing over the ls sea ice and fresh water exports through the canadian archipelago caa are found to be only of minor importance except for the 1990s gsa part of the anomalies are tracked to the newfoundland basin where they enter the north atlantic current the experiments indicate only a minor impact of a single gsa event on the strength of the north atlantic thermohaline circulation thc

a stress transfer model for the development of extension fracture boudinage abstract the term boudinage is used to describe a wide variety of extensional structures in deformed rocks this paper is mainly concerned with boudinage resulting from through layer extension fractures followed by separation of the layer segments thus forming boudins with more or less rectangular cross sections in principle this process is similar to the break up of fibres in fibre reinforced composite materials extended parallel to the fibre direction both processes are controlled by the transfer of stress from the matrix to the fibre or layer and a mathematical model for fibre matrix stress transfer the fibre loading model is well established we have used this as a basis for developing a stress transfer model for boudinage the only difference in the basic mathematical formulation results from geometric differences between the two systems the geometric expressions in the fibre loading model have therefore been rederived for the layer matrix case stress transfer theory predicts that the tensile stress in a layer segment rises from a minimum at the end of a segment to a maximum at the centre this behaviour which is clearly shown by finite element models of boudinage structure suggests that extension fracture boudinage develops by successive mid point fracturing according to stress transfer theory the process will continue until a layer is reduced to segments boudins all of which are shorter than some critical length for which the tensile fracture strength of the layer is equal to the tensile stress at the mid point in practice successive fracturing will be influenced by two other factors 1 in nature the controlling material properties tensile fracture strength elastic moduli will not be single valued but will have a distribution reflecting local variations in lithology and microstructure and 2 major pre deformation flaws may be present in a layer which will control the starting length of layer segments these factors are incorporated with the stress transfer theory into a statistical monte carlo model for extension fracture boudinage which results in a prediction of boudin aspect ratios the predicted distribution compares very closely with the observed distribution of 91 quartzite boudins within lower carboniferous slates at tintagel cornwall the stress transfer model implies that boudin defining fractures occur sequentially so that inter boudin gap lengths will be unequal strain estimates based on boudinage structure will vary according to which part and how much of a layer is sampled a much improved strain estimate is possible based on sequentially closing the inter boudin gaps the stress transfer theory also leads to the possibility of estimating palaeostress from boudinage structure and is the only model available which predicts an aspect ratio distribution of boudins formed by extension fracture other than our own we know of no published data on boudin aspect ratio distributions hence further elaboration of the model is not possible until more field data is available we hope that our work will encourage the systematic measurement of boudinage as well as the development of alternative models

causes of changes in the denmark strait overflow abstract the warming nordic seas potentially tend to decrease the overflow across the greenland iceland scotland ridge gisr system recent observations by macrander et al document a significant drop in the intensity of outflowing denmark strait overflow water of more than 20 over 3 yr and a simultaneous increase in the temperature of the bottom layers of more than 0 4 c a simulation of the exchange across the gisr with a regional ocean circulation model is used here to identify possible mechanisms that control changes in the denmark strait overflow and its relations to changed forcing condition on seasonal and longer time scales the authors establish links of the overflow anomalies to a decreasing capacity of the dense water reservoir caused by a change of circulation pattern north of the sill on annual and shorter time scales the wind stress curl around iceland determines the barotropic circulation around the island and thus the barotropic flow through denmark strait for the overlapping time sc

significant role of the north icelandic jet in the formation of denmark strait overflow water the largest dense water plume feeding the lower limb of the atlantic meridional overturning circulation from the nordic seas comes from denmark strait overflow water measurements of hydrography and water velocity north of iceland and ocean model simulations indicate that a significant part of this water is supplied by the north icelandic jet

evidence for small scale mantle convection from seasat altimeter data evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from seasat altimeter data subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast moving pacific and indo australian plates in particular for the east central pacific ocean the seasat derived data sets reveal lineations with 1 wavelengths in the range 150 500 km perhaps increasing with plate age and 2 peak to trough amplitudes of 5 20 mgal for gravity anomalies the lineated pattern over the pacific plate which first becomes discernable over seafloor 5 to 10 m y old west of the east pacific rise is clearly oblique to the trends of the prominent pacific farallon fracture zones onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 pa s 1019 p which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies recent numerical studies of convection in fluids with a temperature and pressuredependent viscosity support an early onset time for convective instability and low viscosity zones in the upper mantle beneath young lithosphere such low viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

mechanisms for the formation of ridge axis topography atslow spreading ridges a lithospheric plate flexural model the seafloor topography of a slow spreading ridge shows a number of well documented regularities at the ridge segment scale as the result of the complex interplay between ridge axis magmatic and tectonic processes this paper describes the results of a detailed analysis of the seafloor topography of the mid atlantic ridge near the atlantis transform where marine gravity data provide independent although non unique constraints on subseafloor density structure using a combined topography and gravity data set we identified the specific contributions of subseafloor density structure to the seafloor topography we show that the observed along axis deepening 0 3 0 8 km from the midpoint of a ridge segment towards the non transform offsets in the study area can be explained by the vertical deflection of a zero age plate in response to along axis crustal thickness variations however this effect can only account for 50 60 per cent of the observed 1 5 1 7 km deepening towards the atlantis transform suggesting the presence of significant stresses in the lithosphere near a transform results of plate flexural calculations also predict a more elevated rift flank at the inside corner of the ridge transform intersection than at the conjugate outside corner such an asymmetry in rift flank topography is calculated to be greatest near a transform fault with a significant volume of deep transform valley and when adjacent plates across the transform fault are mechanically decoupled or only weakly coupled together these results illustrate the complex interplay between various tectonic processes at a slow spreading ridge

composition and variability of the denmark strait overflow water in a high resolution numerical model hindcast simulation the upstream sources and pathways of the denmark strait overflow water and their variability have been investigated using a high resolution model hindcast this global simulation covers the period from 1948 to 2009 and uses a fine model mesh 1 20 to resolve mesoscale features and the complex current structure north of iceland explicitly the three sources of the denmark strait overflow the shelfbreak east greenland current egc the separated egc and the north icelandic jet have been analyzed using eulerian and lagrangian diagnostics the shelfbreak egc contributes the largest fraction in terms of volume and freshwater transport to the denmark strait overflow and is the main driver of the overflow variability the north icelandic jet contributes the densest water to the denmark strait overflow and shows only small temporal transport variations during summer the net volume and freshwater transports to the south are reduced on interannual time scales these transports are highly correlated with the large scale wind stress curl around iceland and to some extent influenced by the north atlantic oscillation with enhanced southward transports during positive phases the lagrangian trajectories support the existence of a hypothesized overturning loop along the shelfbreak north of iceland where water carried by the north icelandic irminger current is transformed and feeds the north icelandic jet monitoring these two currents and the region north of the iceland shelfbreak could provide the potential to track long term changes in the denmark strait overflow and thus also the amoc

great salinity anomalies in the north atlantic abstract we revisited the great salinity anomaly of the 1970s gsa 70s dickson et al 1988 and documented the newly identified great salinity anomaly of the 1980s hence termed gsa 80s both propagated around the north atlantic in a similar fashion the advective mechanism initially proposed to explain the observed sequence of low salinity low temperature events during the gsa 70s apparently holds also for the gsa 80s the latter was successively observed in the west greenland current 1982 the labrador current 1983 the flemish pass 1984 near the charlie gibbs fracture zone 1984 1985 in the rockall channel 1985 south of iceland 1985 1988 in the north sea 1986 1987 norwegian sea 1987 1988 barents sea 1988 1989 and iceland sea 1989 1990 the advection speed of the gsa 80s seems to be greater than the one of the gsa 70s the 1980s anomaly reached the barents sea 6 to 7 years after peaking in the west greenland current while the 1970s anomaly traveled the same route in 8 to 10 years these anomalies however seem to be of different origin the gsa 70s was apparently boosted remotely by a freshwater sea ice pulse from the arctic via fram strait consequently the gsa 70s was accompanied by a large sea ice extent anomaly in the greenland and iceland seas which propagated into the labrador sea in contrast the gsa 80s was likely formed locally in the labrador sea baffin bay mainly because of the extremely severe winters of the early 1980s but supplemented with a possible contribution of the arctic freshwater outflow via the canadian archipelago facilitated by strong northerly winds which would have enhanced stability and ice formation this anomaly was also associated with a positive sea ice extent anomaly in the labrador sea baffin bay which however had no upstream precursor in the greenland sea thus the gsas are not necessarily caused solely by an increased export of freshwater and sea ice from the arctic via fram strait these results are corroborated by the early 1990s data when a new fresh cold anomaly was formed in the labrador sea and accompanied by a large positive sea ice extent anomaly the harsh winters of the early 1990s were however confined to the labrador sea baffin bay area while the atmospheric and oceanic conditions in the greenland iceland and irminger seas were normal the labrador sea baffin bay area appears therefore to play a key role in formation of gsas as well as in propagation of the gsas formed upstream a likely contribution of the enhanced canadian archipelago freshwater outflow to the gsa formation also seems to be significant two major modes of the gsa origin are thus identified remote generated by an enhanced arctic ocean freshwater export via either fram strait or the canadian archipelago and local resulting from severe winters in the labrador sea baffin bay both modes should be taken into account to model decadal variability of the coupled ocean ice atmosphere system in the north atlantic arctic basin

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

global observations of large oceanic eddies 1 ten years of sea surface height ssh fields constructed from the merged topex poseidon t p and ers 1 2 altimeter datasets are analyzed to investigate mesoscale variability in the global ocean the higher resolution of the merged dataset reveals that more than 50 of the variability over much of the world ocean is accounted for by eddies with amplitudes of 5 25 cm and diameters of 100 200 km these eddies propagate nearly due west at approximately the phase speed of nondispersive baroclinic rossby waves with preferences for slight poleward and equatorward deflection of cyclonic and anticyclonic eddies respectively the vast majority of the eddies are found to be nonlinear

freshwater fluxes in the east greenland current a decade of observations 1 over a decade of mooring measurements in the western fram strait at 78 50 n shows that the annual mean liquid freshwater flux fwf in the east greenland current is relatively constant at 1274 453 km3 yr 1 40 4 14 4 msv despite the fact that the annual mean total volume transport of the egc has more than doubled since 2001 this is shown to be due to an increase of the transport in the deeper ocean and the fact that the largest fw content is present on the east greenland shelf and not in the core of the egc in order to capture the fwf on the shelf modeling results of naosim are included showing that a mean contribution of fwf on the shelf of at least 807 357 km3 yr 1 25 6 11 3 msv should be added to the fwf obtained for the egc when compared to the extra input of freshwater required to account for the 1960 1990 freshening of the northern north atlantic the observed variations in the 1998 2008 egc liquid freshwater fluxes are small

a new path for the denmark strait overflow water from the iceland sea to denmark strait 1 denmark strait overflow water dsow is one of the main components of the thermohaline circulation there has been no consensus on where it is formed and by which way it is brought to the denmark strait it is shown here that it is brought to the sill by a hitherto unknown current that is traced from the sill back into the iceland sea north of iceland the transport of this current is sufficient to account for a major part of the transport of dsow as it has been measured at the sill if some entrainment of ambient water is assumed this supports theories suggesting that the iceland sea is the main source for the dsow and this has consequences for the way in which climate change affects the thermohaline circulation

circulation and transports in the newfoundland basin western subpolar north atlantic the southwestern part of the subpolar north atlantic east of the grand banks of newfoundland and flemish cap is a crucial area for the atlantic meridional overturning circulation here the exchange between subpolar and subtropical gyre takes place southward flowing cold and fresh water is replaced by northward flowing warm and salty water within the north atlantic current nac as part of a long term experiment the circulation east of flemish cap has been studied by seven repeat hydrographic sections along 47 degrees n 2003 2011 a 2 year time series of current velocities at the continental slope 2009 2011 19 years of sea surface height and 47 years of output from an eddy resolving ocean circulation model the structure of the flow field in the measurements and the model shows a deep reaching nac with adjacent recirculation and two distinct cores of southward flow in the deep western boundary current dwbc one core above the continental slope with maximum velocities at mid depth and the second farther east with bottom intensified velocities the western core of the dwbc is rather stable while the offshore core shows high temporal variability that in the model is correlated with the nac strength about 30 sv of deep water flow southward below a density of sigma 27 68 kg m 3 in the dwbc the nac transports about 110 sv northward approximately 15 sv originating from the dwbc and 75 sv recirculating locally east of the nac leaving 20 sv to be supplied by the nac from the south

a strain reversal method for estimating extension from fragmented rigid inclusions abstract in the standard method of strain analysis using fragmented rigid inclusions such as stretched belemnites extension is estimated by comparing the initial length of the inclusion with its final stretched length this approach fails to recognise that not all of the total extension is recorded by separation of the fragments even when cross fractures are present at the onset of deformation this paper presents an alternative approach in which the extensional strain is reversed the inter fragment gaps being incrementally closed until the inclusion is restored to its original state this allows an estimate of the unrecorded extension accommodated by inhomogeneous matrix flow near the inclusion as well as that recorded by fragment separation model experiments in which the fragmented inclusion is represented by wooden blocks set in a ductile matrix allow the strain reversal method to be compared with the standard method the mean percentage relative error is much smaller in the former 10 2 than in the latter 37 2 the main disadvantage of the strain reversal method is that it requires the collection of more data fragment lengths and gap lengths in correct sequence which must be processed by computer a short basic program is provided although there is no one to one correspondence between strain estimates derived from the two methods an approximate numerical transformation from one to the other may be possible one such transformation based on a study of natural boudinage has been applied to the experimental data and reduces the mean percentage relative error in the standard method estimates to 11 5

microboudin structure of piedmontite in quartz schists a proposal for a new indicator of relative palaeodifferential stress abstract this paper proposes a new method to infer the relative differential stress for the rock body bulk differential stress from microboundinage the method is applied to four piedmontite bearing quartz schists collected from the high pressure low temperature metamorphic belts in japan nuporomaporo yamagami asemi and matsunosako in the samples brittle piedmontite grains are broken and subsequently pulled apart within the flowing quartz matrix the method consists of 1 statistical measurement of the fraction of microboudinaged grains 2 theoretical prediction of this fraction as a function of bulk differential stress and 3 inference of bulk differential stress by comparing the measured with the theoretical fraction the theoretical prediction has two aspects one is the model connecting bulk differential stress acting on a whole rock quartz schist with the tensile stress generated in the competent material piedmontite and the other is the model for the fracture strength of the competent material we use a fibre loading theory for the stress and a weakest link theory for the fracture strength the theoretical analysis is found to explain successfully the measured data by a chi squared test and we can obtain relative estimates of bulk differential stress the b value they are 1 0 0 7 0 4 and 0 2 for nuporomaporo yamagami matsunosako and asemi respectively

erosion and flexural uplift along transform faults summary in all geodynamic settings intracontinental continent ocean intraoceanic the main morphological feature of transform plate boundaries is a narrow and elongated valley bounded on at least one side by an uplifted shoulder several mechanisms have been proposed to explain this uplift differential thermal subsidence lateral heat transfer extension perpendicular to the transform but none can be used in all settings we propose that the erosion of a lithospheric plate along the transform boundary may be an alternative mechanism that can be active in all geodynamic settings erosional unloading produces flexural uplift along the transform border which does not exceed 35 40 per cent of the difference in height between the lithospheric plate and the transform valley our model perfectly fits the morphology of several examples in the continental lithosphere the uplifted shoulder of the dead sea transform as well as marginal ridges in continental transform margins along an oceanic transform fault the erosion of the transform border only partially explains the uplift of the highest transverse ridges but may amplify other mechanisms

successive mid point fracturing during microboudinage an estimate of the stress strain relation during a natural deformation abstract this paper describes a piedmontite microboudinage during plastic flow of surrounding quartz in a quartz schist collected from the high pressure low temperature sambagawa metamorphic belt in southwest japan nine microboudins originated from a single grain by successive mid point fracturing and subsequent pull apart such features strongly indicate a fibre loading mechanism for the microboudinage the strain history inferred by successive use of the strain reversal method and stress data quantitatively calculated by a formula of the fibre loading theory provide us with an estimate of the stress strain relation during the microboudinage it apparently proceeded with increasing bulk stress during the last stage of metamorphism