structural analysis of a transform fault rift zone junction in north iceland abstract on the north coast of iceland the rift zone in north iceland is shifted about 120 km to the west where it meets with and joins the mid ocean kolbeinsey ridge this shift occurs along the tjornes fracture zone an 80 km wide zone of high seismicity which is an oblique non perpendicular transform fault there are two main seismic lineaments within the tjornes fracture zone one of which continues on land as a 25 km long wnw trending strike slip fault this fault referred to as the husavik fault meets with and joins north trending normal faults of the theistareykir fissure swarm in the axial rift zone the most clear cut of these junctions occurs in a basaltic pahoehoe lava flow of holocene age where the husavik fault joins a large normal fault called gudfinnugja at this junction the husavik fault strikes n55 w whereas gudfinnugja strikes n5 e so that they meet at an angle of 60 the direction of the spreading vector in north iceland is about n73 w which is neither parallel with the strike of the husavik fault nor perpendicular to the strike of the gudfinnugja fault during rifting episodes there is thus a slight opening on the husavik fault as well as a considerable dextral strike slip movement along the gudfinnugja fault consequently in the holocene lava flow there are tension fractures collapse structures and pressure ridges along the husavik fault and pressure ridges and dextral pull apart structures subparallel with the gudfinnugja fault the 60 angle between the husavik strike slip fault and the gudfinnugja normal fault is the same as the angle between the tjornes fracture zone transform fault and the adjacent axial rift zones of north iceland and the kolbeinsey ridge the junction between the faults of husavik and gudfinnugja may thus be viewed as a smaller scale analogy to the junction between this transform fault and the nearby ridge segments using the results of photoelastic and finite element studies a model is provided for the tectonic development of these junctions the model is based on an analogy between two offset cuts mode i fractures loaded in tension and segments of the axial rift zones or parts thereof in the case of the husavik fault the results indicate that the tjornes fracture zone in general and the husavik fault in particular developed along zones of maximum shear stress furthermore the model suggests that as the ridge segments propagate towards a zero underlapping configuration the angle between them and the associated major strike slip faults gradually increases this conclusion is supported by the trends of the main seismic lineaments of the tjornes fracture zone

effective tension shear relationships in extensional fissure swarms axial rift zone of northeastern iceland abstract the geometry of fracture systems in selected areas of the active krafla fissure swarm mid atlantic ridge northeastern iceland is analysed based on geodetic analysis of the present day topography at the top of holocene basaltic lava flows which fill the axial rift zone the deformation of this initially horizontal surface can be reconstructed extensional deformation is localised at all scales and block tilting though present remains minor using simple models of the surface expression of normal faults the geometrical characteristics of the topographic features related to active deformation during tectonic volcanic events are quantitatively analysed at crustal depths of about 1 km normal faults are present and have an average 70 dip comparison with the dip data of older normal faults observed in the uplifted and eroded shoulders of the rift zone at palaeodepths of 1 2 km indicates that this dip determination is valid comparisons between the local case study and structural analyses of active fissure swarms on a larger scale suggest that normal faulting plays a major role in the middle section of the thin newly formed brittle crust of the rift zone in the axial oceanic rift zone of ne iceland the extensional deformation in the upper crust is dominated by horizontal tension and shear of normal sense their relative importance depending on depth absolute tension dominates in the uppermost several hundred metres of the crust resulting in the development of fissure swarms effective tension plays an important role at a deeper level 2 5 km because of the presence of magmatic fluid pressure from magma chambers which feed dyke injections at crustal depths of about 1 km normal shear prevails along fault planes which dip 60 75 this importance of normal shear at moderate depth between upper and lower crustal levels where tension prevails is pointed out within the extensional context of rifting these variations of tectonic behaviour with depth are controlled by both the lithostatic pressure and the effective tension induced by the presence of magmatic fluid pressure

marine geophysical investigation of the chain fracture zone in the equatorial atlantic from the pi lab experiment the chain fracture zone is a 300 km long transform fault that offsets the mid atlantic ridge we analyzed new multibeam bathymetry backscatter gravity and magnetic data with 100 multibeam bathymetric data over the active transform valley and adjacent spreading segments as part of the passive imaging of the lithosphere asthenosphere boundary pi lab experiment analyses of these data sets allow us to determine the history and mode of crustal formation and the tectonic evolution of the transform system and adjacent ridges over the past 20 myr we model the total field magnetic anomaly to determine the age of the crust along the northern ridge segment to better establish the timing of the variations in the seafloor fabric and the tectonic magmatic history of the region within the active transform fault zone we observe four distinct positive flower structures with several en echelon fault scarps visible in the backscatter data we find up to 10 mgal residual mantle bouguer anomaly in the region of the largest positive flower structure within the transform zone suggesting crustal thickening relative to the crustal thinning typically observed in fracture zones in the atlantic the extensional compressional features observed in the chain transform are less pronounced than those observed further north in the vema st paul and romanche and may be due to local ridge segment adjustments

plate dynamics near divergent boundaries geophysical implications of postrifting crustal deformation in ne iceland the bulk of a tectonic plate is thought to move continously at a rate consistent with the geologic average on the other hand movements are highly episodic at plate boundaries we study the plate dynamics that relate to these two different modes by modelling the displacements observed using the global positioning system in northeast iceland 1987 1990 these observations were made about 10 years after an episodic divergent movement between the north american and eurasian plates 1975 1981 the horizontal displacement field fits well a two dimensional model of postrupture stress relaxation assuming a thin elastic layer overlying a layer of newtonian viscosity this analysis indicates values of about 10 m2 s for the stress diffusivity and 0 3 2 1018 pa s for the newtonian viscosity of the lower layer however no significant correlation exists between the observed and modeled vertical displacements probably because of the relative inaccuracy of the vertical component observations assuming that contemporary plate motion is the sum of many displacements that have diffused from boundaries where episodic displacements occur periodically we simulate the spatial transition from episodic to continuous plate movements the plate boundary zone where movements are episodic or quasi episodic is of the order of 100 km wide depending on the stress diffusivity which may be fairly uniform throughout the world and the frequency of episodic movements

inversion of field data in fault tectonics to obtain the regional stress iii a new rapid direct inversion method by analytical means summary a new method for determining the reduced stress tensor with four degrees of freedom the orientations of the three principal stress axes as well as the ratio of principal stress differences using fault slip data or focal mechanisms of earthquakes is presented from a computational point of view the inversion of fault slip data is made in a direct way by purely analytical means as a result the determination process is extremely fast and adaptable on small microcomputers from a physical point of view the method aims at simultaneously i minimizing the angles between theoretical shear stress and actual slip vector and ii having relative magnitudes of shear stress large enough to induce slip despite rock cohesion and friction examples of application to actual fault slip data sets with good or poor variety of fault slip orientations are shown the double significance of the basic criterion adopted results in a more realistic solution of the inverse problem than the single minimization of the shear stria angle

off rift and rift zone palaeostresses in northwest iceland abstract the vestfirdir peninsula of northwest iceland consists mainly of gently dipping basaltic lava flows 8 14 ma old detailed studies of its principal tectonic elements basaltic dykes normal faults and minor faults and observations of the trends of the major fjords and valleys make it possible to reconstruct the evolution of the associated stress field the results show that a kink shaped curved rift zone existed in this area until about 5 ma ago in the northern part of the peninsula the rift zone trended nne but ene in the southern part it is proposed that the tectonic elements of the vestfirdir peninsula were generated in two stress fields one operating inside the rift zone the other operating outside it the rift zone stress field was responsible for the formation of most normal faults dykes and the main extension direction inferred from the minor faults conversely the off rift stress field was responsible for the formation of several holocene normal faults in the southern part of the peninsula the direction of the secondary extension inferred from the minor faults as well as for most major fjords and valleys in the area boundary element studies suggest that the off rift stress field generated on the concave side of a curved rift zone is different from that generated on its convex side absolute tensile stresses that encourage the formation of valleys and fjords concentrate very near to or at the convex margin but at a considerable distance from the concave margin applying these model results to iceland as a whole it is concluded that the valley formation in northwest iceland occurred at a large distance form the associated rift zone whereas in east iceland the valleys formed near to the margin of the rift zone furthermore the model predicts that zones of relative tension may propagate radially from the convex side of the rift zone and act as magma channels for volcanoes the location and orientation of major fjords and valleys in east and northwest iceland as well as the existence of several major off rift holocene volcanoes support the model predictions

hemispheric asymmetry of deep water transport modes in the western atlantic subtropical studies of the atlantic meridional cold water flow show a hemispheric contrast in the dominant southward transport mode below 2000 m in the north atlantic lower deep water ldw 1 8 2 4 c dominates with small transport of middle deep water mdw 2 4 3 2 c while in the south atlantic the opposite is observed we use numerous observations in the western basins of the tropics to show that the transition occurs rapidly near the equator in the western atlantic a meridional section in the central brazil basin suggests zonal flows are responsible for the transition ldw transport from the guiana basin north of the equator flows eastward in the northern brazil basin and is inferred to continue on through the romanche fracture zone into the eastern atlantic an opposing flow of mdw from the eastern tropical atlantic flows toward the western boundary where it bifurcates to supply mdw to the deep western boundary current dwbc of the brazil basin as well as to feed the northward flow of mdw in the guiana basin offshore of the dwbc the magnitude of each of these oppositely directed flows is roughly 7 sv we furthermore speculate that they are connected predominantly by upwelling from ldw to mdw within the low latitude eastern basin the overall deep water transport system below 2000 m in the western basins of the mid and low latitude atlantic is thus found to comprise the following three distinct components 1 a strong dwbc transport of ldw with associated recirculation dominates the guiana basin north of the equator 2 in the northern brazil basin just south of the equator a narrow eastward flow absorbs the ldw and carries it eastward while a somewhat broader westward flow imports mdw into the western basin 3 this mdw flow then bifurcates with the southward branch causing the mdw dominance in the brazil basin where the mdw dominated dwbc and associated recirculations are the third component of the deepwater transport system

eastward flow through the mid atlantic ridge at 11 n and its influence on the abyss of the eastern basin abstract the dilute antarctic bottom water of the north atlantic eastern trough is supplied from the western trough through fractures in the mid atlantic ridge in particular the influence on eastern trough property distributions of flow through the romanche and vema fracture zones near the equator and 11 n respectively has been noted previously here new observations are reported that document the abyssal circulation of the northeastern atlantic basins gambia abyssal plain south canary basin and north canary basin in particular the dominance of vema influence the absence of romanche influence and the existence of a system of deep western boundary currents and estimated transport deep isopycnals slope steeply across the vema s eastern end near 39 w corresponding to a geostrophic transport through the vema of 2 1 to 2 3 106 m3s 1 colder than 2 0 c this is half or more of the estimated bottom water that flows north across the equator into the subtropical western north atlantic this transp

effect of local and regional stress fields on sheet emplacement in west iceland abstract at the tertiary reykjadalur central volcano in west iceland there is a remarkable change in strike dip thickness and percentage of sheets at a distance of about 9 km from the centre of the volcano at distances of less than 9 km the sheets strike in several main directions their average arithmetic mean dip is 45 and the average thickness 1 m locally the sheets make up nearly 90 of the rock but in 1 1 5 km long profiles they are 5 8 7 9 one third of the sheets are porphyritic containing as much as 50 of plagioclase phenocrysts within and in the vicinity of the central volcano there is thus a clear swarm of inclined cone sheets at distances greater than 9 km the sheets suddenly dip much more steeply average 81 become thicker average 3 15 m strike parallel to the nearby volcanic zones and have all the characteristics of regional dyke swarms in 2 5 10 5 km long profiles these dykes are only 0 83 1 84 of the rock one fourth of the dykes are porphyritic but the percentage of plagioclase phenocrysts is normally less than 10 we interpret this abrupt change from a local swarm of inclined sheets to swarms of regional dykes as reflecting changes in the stress field we propose that a local stress field controlled the strike dip and partly thickness of the sheets to a distance of 9 km from the centre of the volcano this local stress field which is also thought to have given rise to the collapse caldera associated with the volcano is attributed to a crustal magma chamber at a shallow depth below the volcano this crustal magma chamber is regarded as the source of most of the inclined sheets the regional stress fields associated with the volcanic zones is thought to have controlled dyke emplacement at greater distances from the volcano we suggest that many of the regional dykes were injected from a deep seated magma reservoir located at boundary between the crust and upper mantle

a meridional 14c and 39ar section in northeast atlantic deep water 14c 39ar and complementary hydrographic and nutrient data are presented for deep water below 2500 m depth from stations along a meridional section 8 s to 45 n through the romanche trench and along the deep northeast atlantic basins f s meteor cruise 56 leg 5 the large scale 14c distribution along the section is resolved at the 14c data precision of 2 bottom water 14c decreases by 6 from the equator to 45 n and farther up there is a weak 14c minimum 123 over much of the section the 14c data are interpreted as giving a turnover time of about 30 years for the waters below the depth of the 14c minimum 4250 m it is found that water of 1 50 0 05 c potential temperature enters the east atlantic from the west through the romanche trench sill depth about 4000 m and a preliminary value for the inflow rate of 3 6 106 m3 s is deduced this rate greatly exceeds estimated deep inflow rates through the vema fracture zone or across the northern boundary of the east atlantic 39ar data that cover an entire deep ocean circulation system are presented for the first time the observed 14c and 39ar distributions are mutually consistent transit times from the source regions to the equator for water from northern and southern deepwater sources are estimated to be about 170 and 105 years respectively and the 39ar concentration of young antarctic bottom water is deduced as 60 7 modern the 39ar 14c correlation in the ocean appears to be affected by mixing of waters of different age and by more efficient raising of 39ar in the deepwater formation processes

hydrography of the romanche and chain fracture zones the romanche and chain fracture zones rfz and cfz are known as pathways for the lower north atlantic deep water lnadw and the antarctic bottom water aabw from the western trough to the eastern trough of the equatorial atlantic ocean across the mid atlantic ridge we report hydrographic observations describing the evolution of the lnadw and aabw properties during their transit through the equatorial fracture zones to the west of the rfz and cfz sills the lnadw and aabw cores are separated by a deep thermocline pycnocline which marks the transition between the two water masses the deep thermocline is eroded eastward and vanishes in the eastern basin this zonal asymmetry is the signature of a zonal baroclinic pressure gradient associated in each fracture zone with eastward flows of lnadw and aabw below 3800 m depth during the eastward transit through the rfz the near bottom aabw becomes less dense saltier 0 07 and warmer 0 7 c richer in oxygen 0 3 ml l 1 but less rich in silicate 30 mol l 1 simultaneously the lnadw core becomes fresher and colder at a given density and the oxygen maximum characterizing the lnadw core weakens by 0 2 ml l 1 we argue that the property changes within the aabw and lnadw are due to intense vertical mixing in addition to vertical mixing with the aabw it seems likely that isopycnal mixing of the lnadw with the eastern basin deep water contributes to the property evolution of the lnadw exiting the rfz in the cfz evidence is also given of mixing occuring within the aabw and the lnadw a sudden change in the aabw properties observed across the cfz main sill suggests the blocking of the eastward progression of the densest aabw by the cfz main sill however this dense water can reach the eastern basin by flowing northeastward through valleys the sills of which are deeper than thoses of the cfz finally a simple hydraulic control model applied to the rfz and cfz flows predicts a transport of 1 4 0 1 106 m3 s 1 of water colder than 1 9 c across the rfz cfz main sill

evolution of deepwater mixing and weathering inputs in the central atlantic ocean over the past 33 myr the isotopic composition of nd in present day deep waters of the central and northeastern atlantic ocean is thought to fingerprint mixing of north atlantic deep water with antarctic bottom water the central atlantic romanche and vema fracture zones are considered the most important pathways for deep water exchange between the western and eastern atlantic basins today we present new nd isotope records of the deepwater evolution in the fracture zones obtained from ferromanganese crusts which are inconsistent with simple water mass mixing alone prior to 3 ma and require additional inputs from other sources the new pb isotope time series from the fracture zones are inexplicable by simple mixing of north atlantic deep water and antarctic bottom water for the entire past 33 myr the distinct and relatively invariable nd and pb isotope records of deep waters in the fracture zones appear instead to have been controlled to a large extent by contributions from saharan dust and the orinoco amazon rivers thus the previously observed similarity of nd and pb isotope time series from the western and eastern north atlantic basins is better explainable by direct supply of labrador seawater to the eastern basin via a northern pathway rather than by advection of north atlantic deep water via the romanche and vema fracture zones

implications of fracturing mechanisms and fluid pressure on earthquakes and fault slip data in the east iceland rift zone comparison of the eroded off rift zone left inactive by plate motion with the inner active seismic rift zone allows us to constrain the fracturing mechanisms in eastern off rift zone we measured 423 fault slips including normal and strike slip faults inversion of fault slip data reveals the parallelism of the minimum stress 3 computed for the normal and for the strike slip faulting and consistency with the direction of plate divergence north of the vatnajokull in the active rift zone we analysed 14 250 earthquakes recorded by the icelandic meteorological office imo between 2004 and 2009 especially northeast of the askja volcano two main average focal depths were determined at 5 km her ubrei table mountain and 15 km upptyppingar hyaloclastite ridge the double couple focal mechanisms determined by imo revealed that more than half of the mechanisms are strike slip faulting type both in active and off rift zone reveals an unusual importance of strike slip regime in such an extensional tectonic context this can be explained by stress permutations 1 2 similarities in terms of stress orientations and type of faulting are observed both in the old and present day rift zones we assume that the seismic events may be generated by rapid deep magma intrusion also associated with shallower hydrothermal activities we thus propose that the presence of fluids shear failure double couple focal mechanism and stress permutations in both active and off rift zones are closely linked assuming a drucker prager failure criterion we evaluated analytically that a state near of lithostatic pore pressure is a necessary condition for shear failure at shallow 5 km and deep 15 km depth in a simple context of crustal extension that allows for stress permutations however processes favouring stress permutations cannot be further constrained from our observations since the dynamics of fluid materiel heterogeneity and post glacial rebound can also play a significant role

extremely long kelvin helmholtz billow trains in the romanche fracture zone in the atlantic ocean the densest water mass antarctic bottom water aabw can only cross the mid atlantic ridge from its southwestern to northeastern basins in limited because deep conduits at the southwestern entrance of one of these the equatorial romanche fracture zone aabw crosses a sill at 4550 m depth in a 7 km narrow channel before plunging into the deep at the sill slope the rapidly flowing aabw causes shear induced turbulent mixing with the overlying water masses we present an excerpt of 1 hz sampled half yearlong moored observations from 99 high resolution temperature sensors that demonstrate and quantify the turbulence details on top of quasi steady shear flow an internal tide modulates the mixing together they constitute a means for an extremely long train of 250 consecutive kelvin helmholtz billows in a day that vary between 5 and 100 m in vertical scale

transport of bottom water in the romanche fracture zone and the chain fracture zone abstract two moored arrays deployed in the romanche fracture zone and chain fracture zone in the equatorial atlantic ocean provide two year long time series of current and temperature in the lower north atlantic deep water and the antarctic bottom water total time averaged transport of antarctic bottom water potential temperature 1 9 c across the mid atlantic ridge amounts to 1 22 106 m3 s 1 eastward with a standard deviation of 0 25 106 m3 s 1 a time averaged transport of 0 36 0 23 106 m3 s 1 eastward is found for the lower north atlantic deep water in the 1 9 2 1 c temperature range but this may represent only a fraction of the total flow of this water mass across the ridge contributions of the romanche fracture zone and chain fracture zone to the antarctic bottom water transport are similar while the chain fracture zone has the greater share of lower north atlantic deep water transport semiannual and annual periods are detected in the transport time series and together expl

turbulent mixing in a deep fracture zone on the mid atlantic ridge abstractmidocean ridge fracture zones channel bottom waters in the eastern brazil basin in regions of intensified deep mixing the mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones whether affected by the local or the nonlocal canyon topography are still subject to debate to discriminate those mechanisms and to discern the canyon mean flow two moorings sampled a deep canyon over and away from a sill contraction a 2 layer exchange flow accelerated at the sill transports 0 04 0 10 sv 1 sv 106 m3 s 1 up canyon in the deep layer at the sill the dissipation rate of turbulent kinetic energy e increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy cross sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides during spring to neap tides e varies from o 10 9 to o 10 10 w kg 1 below 3500 m around the 2 layer inte

reconstruction of east west deep water exchange in the low latitude atlantic ocean over the past 25 000 years radiogenic neodymium isotopes have been used as a water mass mixing proxy to investigate past changes in ocean circulation here we present a new depth transect of deglacial neodymium isotope records measured on uncleaned planktic foraminifera from five cores spanning from 3300 to 4900 m on the mauritanian margin in the tropical eastern atlantic as well as an additional record from 4000 m on the ceara rise in the equatorial western atlantic despite being located under the saharan dust plume the eastern atlantic records differ from the composition of detrital inputs through time and exhibit similar values to the western atlantic foraminiferal nd across the deglaciation therefore we interpret the foraminiferal values as recording deep water nd isotope changes all six cores shift to less radiogenic values across the deglaciation indicating that they were bathed by a lower proportion of north atlantic deep water during the last glacial maximum lgm relative to the holocene the eastern atlantic records also show that a neodymium isotope gradient was present during the lgm and during the deglaciation with more radiogenic values observed at the deepest sites a homogeneous water mass observed below 3750 m in the deepest eastern atlantic during the lgm is attributed to the mixing of deep water by rough topography as it passes from the western atlantic through the fracture zones in the mid atlantic ridge this implies that during the lgm the low latitude deep eastern atlantic was ventilated from the western atlantic via advection through fracture zones in the same manner as occurs in the modern ocean comparison with carbon isotopes indicates there was more respired carbon in the deep eastern than deep western atlantic during the lgm as is also seen in the modern atlantic ocean

evolution of the deep western boundary current of antarctic bottom water in the brazil basin abstract a synthesis of woce and other hydrographic data shows that the deep western boundary current of antarctic bottom water has a double core structure and that it is differentially modified during its northward transit through the brazil basin at 25 s it descends about 200 m while continuing equatorward as a paired double core current over the relatively broad sloping topography of the western region the downslope core is faster 4 6 cm s 1 versus 3 cm s 1 and is coincident with the high density core equatorward of 19 s the cores begin to diverge and by 15 s the velocity cores have separated and the high speed pattern has inverted between 13 and 11 s the colder slower flowing branch around 3 cm s 1 deepens by 350 m with a noticeable decrease in the vertical stratification of overlying water characteristics subsequently it continues to the equator toward the romanche chain fracture zones the warmer faster flowing branch around 8 cm s 1 proceeds to the equator along the continent

structure of the deep spillway in the western part of the romanche fracture zone structure of the deep spillway was studied on the basis of conductivity temperature depth lowered acoustic doppler current profiler and mooring measurements in 2011 2014 in the romanche fracture zone 22 27 22 32 w the spillway exists quasi permanently in the regime of hydraulic control overflow and consists of two flows in the layer 1 20 c deeper than 4 150 m the first flow enters the fracture from the south through a gap in the southern wall the second flow is directed to the east along the narrow valley of the fracture the sill depths of these passages are 4 570 and 4 430 m respectively the depths over the surrounding ridges are shallower than 4 100 m both flows descend to the local depression of the fracture with 5 000 m depth bottom potential temperatures over both sills were 0 51 c and the measured velocities were 20 40 cm s when the flows descend down the slope they accelerate in supercritical regime maximal measured velocities were up to 65 cm s analysis of the 189 day time series of velocities on the mooring deployed in the flow descending from the gap showed a very high level of fluctuation energy 10 times greater than the energy of the barotropic tide at this point the only energy source of these fluctuations is the available potential energy of the deep thermocline in the west atlantic transforming to the kinetic energy of the deep spillway the total transport of the bottom water below 1 2 c based on the lowered acoustic doppler current profiler measurements is estimated at 0 29 0 35 sv which agrees with the predicted values

structural evolution of sulphide tectonites and their host rocks stratmat mine new brunswick the sulphide orebodies at the stratmat mine in new brunswick are treated as tectonites because their primary characteristics have been so modified by deformation recrystallization and vein injection that most of their original features have been obscured pb zn cu sulphide orebodies at the stratmat mine consist of sulphide and sulphide silicate tectonites gneisses schists phyllites and slates produced by the mixing of two sulphide precursors and silicate host rocks by polyphase deformation much of which relates to progressive non coaxial deformation quartz vein injection during and after this period of deformation and the intermixing of nonsulphide lithotypes led to dilution of the initial ore composition both the deformation of the orebodies and fluid migration manifested by vein injection reflect processes that were operative in a major shear zone no indisputable primary characteristics of the orebodies are preserved although a number of tectonic and or tectonically modified features m

on climate and abyssal circulation in the atlantic ocean during late pliocene marine isotope stage m2 3 3 million years ago abstract marine isotope stage mis m2 3 3 ma is an isolated cold stage punctuating the benthic oxygen isotope 18o stratigraphy of the warm piacenzian interval of the late pliocene epoch the prominent 0 65 18o increase that defines mis m2 has prompted debate over the extent to which it signals an early prelude to the rhythmic extensive glaciations of the northern hemisphere that characterise the quaternary and raised questions about the forcing mechanisms responsible recent work suggests that co2 storage in the deep atlantic ocean played an important role in these events but detailed reconstructions of deep ocean chemical stratification are needed to test this idea and competing hypotheses here we present new records of the nd isotope composition of fish debris and 13c and b ca ratios of benthic foraminifera from the northwest and southeast atlantic ocean our novel geochemical data show that in contrast to major quaternary glaciations such as mis 2 21 ka and mis 100 2 52 ma the deep north atlantic ocean was weakly chemically stratified during mis m2 we show that southern component water incursion into the atlantic ocean was limited to the deep south atlantic basin during mis m2 and peaked well before 10 15 kyr the atmospheric co2 minimum our findings imply that the deep atlantic ocean was not the principle sink of co2 sequestered from the atmosphere during mis m2 implicating a different co2 storage deep water reservoir mechanism presumably southern component water incursion into the pacific ocean weak chemical stratification in the deep atlantic ocean during mis m2 relative to mis 100 and 2 suggests comparatively active atlantic meridional overturning circulation that suggestion is consistent with the warmth of the high latitude north atlantic during mis m2 surface water temperatures cooled during m2 but only close to holocene values our findings may help to explain the paucity of evidence for extensive early glaciation of the northern hemisphere during m2 but leave open the possibility of ice sheet advance on antarctica

nutrient mineralization rates and ratios in the eastern south atlantic the physical and biogeochemical components of nutrients and inorganic carbon distributions along woce line a14 are objectively separated by means of a constrained least squares regression analysis of the mixing of eastern south atlantic water masses contrary to previous approaches essentially devoted to the intricate south atlantic circulation this work is focused on the effects of circulation on nutrients and carbon biogeochemistry with special emphasis on the stoichiometry and the rate of mineralization processes combination of nutrient and apparent cfc age anomalies derived from the mixing analysis indicate faster mineralization rates in the equatorial 12 10 2 mol p kg 1 yr 1 and subequatorial 5 3 10 2 mol p kg 1 yr 1 than in the subtropical 4 3 10 2 mol p kg 1 yr 1 regime at the south atlantic central water sacw depth range lower rates are obtained in the antarctic intermediate water aaiw domain 3 0 10 2 mol p kg 1 yr 1 significant variation with depth of o2 c n p anomalies indicates preferential mineralization of proteins in thermocline waters as compared with the reference redfield composition

structure of regional dykes and local cone sheets in the midhyrna lysuskard area snaefellsnes peninsula nw iceland this paper provides the first detailed structural description of 48 vertical dykes 384 inclined sheets and two large intrusions and the geometry strike dip direction and dip of 1116 fractures in the central area of the snaefellsnes peninsula nw iceland our data show a more complex setting than that depicted by the wnw ese en echelon trend of the volcanic structures at the surface in the miocene basement lavas dykes dominantly strike n50 100 e whereas other directions are also present with a higher dispersion two main swarms of centrally dipping sheets have also been recognized focussing towards two areas sheet dips range from 2 to 75 with the higher frequency between 10 and 45 in section view there is no systematic variation of sheet dip with distance from the focus area gabbro and granophyre laccoliths are present in the studied area but cross cutting relations indicate that most of the inclined sheets are younger comparison with regional tectonics suggests that the n50 80 e striking dykes are coherent with emplacement under the stress field of the pre 6 ma snaefellsnes rift dominated by a nnw sse directed least principal stress 3 the n80 100 e dykes and the late quaternary wnw trending sub aerial volcanic features are instead consistent with the development of a more recent e w right lateral shear zone affecting the snaefellsnes peninsula coherent sets of fractures have also been found within the inclined sheet swarms the stress tensor rotated in response to an excess magma pressure linked to two underlying magma chambers of lobate shape located at an estimated depth of about 400 and 500 m below sea level this local magmatic stress also produced the centrally inclined fracture swarms that have been found in this area

the atlantic deep western boundary current water masses and transports near the equator the distributions and transports of deepwater masses at the western boundary in the tropical atlantic off brazil have been studied on three surveys along 35 degrees w and 5 degrees s and one at 10 degrees s transports are obtained from direct measurements of the velocity fields pegasus profiling system and lowered acoustic doppler current profiler and from geostrophic computations using chlorofluoromethane cfm and hydrographic distributions four water masses could be identified forming the north atlantic deep water nadw system two of these have a high cfm content the shallow upper nadw sunadw and the overflow lower nadw olnadw these exhibit the highest velocity signals at 35 degrees w where distinct flow cores seem to exist most of the southeastward flow of the sunadw centered around 1600 m occurs 320 km offshore between 3 degrees 09 s and 1 degrees 50 s 9 7 3 3 sv farther north in that section a highly variable reversing flow is found in a second velocity maximum the transport of olnadw centered around 3800 m of 4 6 2 6 sv is guided by the parnaiba ridge at 1 degrees 45 s 35 degrees w the water masses located between the two cfm maxima the labrador sea water lsw and the lnadw old water mass lnadw old did not show any persistent flow features however a rather constant transport of 11 1 2 6 sv was observed for these two layers the total southeastward flow of the nadw at 35 degrees w showed a transport of 26 8 7 0 sv if one neglects the reversing sunadw north of 1 degrees 50 s at 5 degrees s the flow of all deepwater masses shows vertically aligned cores the main southward transport occurred near the coast 19 5 5 3 sv the boundary current is limited offshore by a flow reversal present in all three surveys but located at different longitudes at 10 degrees s a southward transport of 4 7 sv was observed in november 1992 however the section extended only to 32 degrees 30 w so that probably a significant part of the flow has been missed an important result is the large transport variability between single cruises as well as variability of the spatial distribution of the flow at 35 degrees w which could lead to large uncertainties in the interpretation of single cruise observations despite these uncertainties we suggest a circulation pattern of the various deepwater masses near the equator by combining our mean transport estimates with other observations

on the conditions of sheet injections and eruptions in stratovolcanoes nearly all eruptions in stratovolcanoes composite volcanoes central volcanoes are supplied with magma through fractures consequently a primary physical condition for an eruption to occur in a stratovolcano is that a magma driven fracture is able to propagate to the surface magma filled fractures frozen or fluid are referred to as sheet intrusions more specifically they are named dykes when subvertical and inclined or cone sheets when inclined field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths for this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions whether a sheet intrusion becomes arrested or alternatively propagates to the surface depends primarily on the stress field in the stratovolcano a stratovolcano normally consists of layers of contrasting mechanical properties such as soft low young s modulus pyroclastic units and stiff high young s modulus lava flows we present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest the models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised we propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress field homogenisation field studies show that the sheet intrusions in individual stratovolcanoes have various dips some are vertical dykes others inclined sheets and still others horizontal sills analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption this effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high we conclude that the shallower the dip of a sheet intrusion the less will be its volumetric magma transport to the surface of a stratovolcano

hydrofracturing stress measurements in the iceland research drilling project drill hole at reydarfjordur iceland two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the iceland research drilling project deep hole at reydarfjordur east iceland as indicated by the continuously extracted drill core the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes the density of the basalt was used to estimate the vertical stress v in megapascals at depth d in meters v 0 027d in the 33 to 270 m depth range the horizontal principal stresses hmax and hmin as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress hmin 2 1 0 016d hmax 3 3 0 016d in the 300 to 550 m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m conditions favoring normal faulting v hmax hmin were established in the 540 to 580 m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of hmax and hmin this would be in accord with the predominant fault type in east iceland the relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near surface high compressive stresses previously determined by hast 1973 in east iceland the large increase in horizontal stress in the range of 300 to 500 m depth is not understood a correlation appears to exist between the zone of almost unintruded thick lava flows 200 500 m and that of high stresses an intuitive model suggesting locked in stresses in this zone is proposed the hydrofracturing determined overall mean direction of hmax is n50 e while a more selective sampling of results yields n40 e the measured stress directions appear to be reasonably compatible with the local structure such as dike direction nne and lava dip wsw significantly hmax direction is subparallel to the n20 e trend of the nearest fissure swarms in the axial rift zone this almost coincidence of directions like the measured stress magnitudes is also in agreement with the model of spreading lithospheric plates being pulled apart

crustal spreading due to dikes and faults in southwest iceland abstract a 30 km long profile extending from 3 million year old pleistocene rocks to the holocene rocks of the rift zone in southwest iceland is dissected by 443 dikes and 156 normal faults nearly all the dikes are basaltic with an average strike of n26 e which is subparallel to the trend of fractures in the nearby rift zone their average dip is 69 and average thickness 1 37 m both values much lower than averages for regional tertiary dike swarms in iceland many dikes are seen to change into sills the great majority of the faults are pure dip slip with an average throw of 10 2 m their average strike n37 e is different from the average strike of the dikes in this area and the fractures in the nearby rift zone their average dip is 75 2 which is somewhat steeper than in the tertiary areas but the dips are less in the interbedded sedimentary layers than in the lava flows growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10 000 years in subsections crustal dilation due to dikes is 0 4 3 7 that due to faults is 0 6 5 3 and that due to both dikes and faults is 1 04 6 44 in these subsections average fault spacing is one fault per 170 563 m spacing decreases toward the active rift zone it is concluded that in typical fissure swarms of the rift zone the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults in the depth range of a couple of hundred metres to perhaps 1 km the non vertical faults contribute most to the dilation at deeper crustal levels however dilation due to dikes becomes dominant

strain accumulation 1986 1992 across the reykjanes peninsula plate boundary iceland determined from gps measurements gps geodetic observations 1986 1992 spanning the oblique plate boundary on the reykjanes peninsula sw iceland show that left lateral shear strain is accumulating in the area the principal strain rates are 1 0 255 0 055 strain yr extension at n121 6 e and 2 0 190 0 053 strain yr contraction at n31 6 e the principal axis of extension is perpendicular to the strike of fractures within volcanic systems on the peninsula the direction of maximum left lateral shear strain is n76 6 e the same as the trend of the seismic zone on the peninsula tensor shear strain in this direction is 0 222 0 054 strain yr continuous aseismic slip of the north american eurasian plates below a 5 11 km locking depth on the peninsula can explain the observed depth of earthquakes the width of the deformation zone determined from recent fissuring and faulting and the gps measurements of strain

mixing in the romanche fracture zone the romanche fracture zone is a major gap in the mid atlantic ridge at the equator which is deep enough to allow significant eastward flows of antarctic bottom water from the brazil basin to the sierra leone and guinea abyssal plains while flowing through the romanche fracture zone bottom water properties are strongly modified due to intense vertical mixing the diapycnal mixing coefficient in the bottom water of the romanche fracture zone is estimated by using the finestructure of ctd profiles the microstructure of high resolution profiler data and by constructing a heat budget from current meter data the finestructure of density profiles is described using the thorpe scaleslt it is shown from microstructure data taken in the bottom water that the ozmidov scale lo is related to lt by the linear relationship lo 5 0 95lt similar to other studies which allows an estimate of the diapycnal mixing coefficient using the osborn relation the thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream eastward of the main sill of the romanche fracture zone in this region a mean diapycnal mixing coefficient of about 1000 3 1024 m2 s21 is found for the bottom water estimates of cross isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the romanche fracture zone and the nearby chain fracture zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the romanche fracture zone although the two fracture zones occupy only 0 4 of the area covered by the sierra leone and guinea abyssal plains the diffusive heat fluxes across the 1 4 8c isotherm in the romanche and chain fracture zones are half that found over the abyssal plains across the 1 88c isotherm emphasizing the role of these passages for bottom water property modifications

hydraulie control and flow separation in a multi layered fluid with applications to the vema channel abstract observations from a recent field experiment in the vema channel are briefly described these show a remarkable change in the configuration of isopycnal surfaces within the channel and the development of thick nearly homogeneous regions near the bottom which are capped by sharp vertical gradients contrary to previous speculation that these bottom boundary layer result from enhanced vertical mixing a dynamical mechanism is explored this involves the hydraulic adjustment of an inertial semi geostrophic flow to the channel geometry first an active two layer flow in a rectangular geometry is studied to show that internal flow separation can occur when the flow is accelerated sufficiently by a narrowing channel almost always this separation accompanies hydraulic control the slowest upstream moving kelvin wave is stopped and upstream and downstream states are not symmetric with respect to the channel width an active three layer flow with a variable bottom profile is then presented as a more

formation of dykes feeder dykes and the intrusion of dykes from magma chambers the paper discussed the formation of dykes and applies the results to iceland it is postulated that dykes follow the pathway of least work and of least tensile strength and thereby intrude the subvertical joints in lava flows it is suggested that dykes form in magma pulses where each dyke is split in two by the next magma pulse and so on in iceland the estimated time between successive magma pulses is of the order of several hundred days statistical considerations indicate that in iceland the probability of seeing dykes end upwards is only 1 2 which agrees with observations it is concluded that many and probably the majority of dykes are non feeders this together with the low probability of finding the connection between feeder and lava flow explains the scarcity of observed feeder dykes it is concluded that overpressure in shallow magma chambers needed to drive magma through crustal fractures in iceland is usually smaller than the tensile strength of the host rock several mpa which thereby is the critical factor in dyke intrusion

formation and growth of normal faults at the divergent plate boundary in iceland the divergent plate boundary in iceland is characterized by 40 80 km long and 5 10 km wide swarms of tension fractures 102 m long and normal faults 103 m long the upper part of the crust is mainly composed of lava flows with abundant columnar joints that are mostly perpendicular to the lava contacts the lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1 for every 150 m depth in the crust at the surface of the rift zone the joints are vertical and parallel to the vertical principal stress because of tilting of the lava pile the columnar joints become oblique to this stress hence becoming potential shear fractures and form echelon sets at greater depths in the crust theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and or large scale tension fractures at crustal depths of 0 5 1 5 km the width depth must be the smallest controlling dimension of many faults nevertheless there is a positive linear relation r 0 91 between maximum throw and length of the holocene faults if the faults grow as self similar structures the throw length relationship can be explained by a similar relation between fault length and width

the flow of antarctic bottom water into the brazil basin the total transport of antarctic bottom water across the rio grande rise including the western boundary the vema channel and the hunter channel is estimated from hydrographic measurements across these pathways the contribution of the vema channel is greatest at 3 9 106 m3 s 1 which is very close to earlier estimates the western boundary current contribution is 2 0 106 m3 s 1 and that of the hunter channel 0 7 106 m3 s 1 the lower values outside the vema channel are offset by the important source of mass they form to the lower density classes of bottom water about 40 of the flow is concentrated in the highest density class representing the source of weddell sea deep water to the brazil basin the flow structure is characterized by horizontal and vertical recirculation

extension direction re orientation in the oceanic rift of iceland and comparison with continental rifts abstract in this study we report the results of a structural analysis carried out on fault systems that form the active rift segments of two areas of iceland namely 1 the northern part of the eastern rift segment northern volcanic zone nvz and 2 the western rift segment that include the western volcanic zone thingvellir area and the reykjanes peninsula in the n s trending nvz mainly krafla volcanic area the obtained extension directions trend around ese at thingvellir the extension directions trend ca nw se and are perpendicular to the main ne sw trending faults in the ene wsw trending tectono magmatic segments of the reykjanes peninsula our data show a marked difference with the gps data and spreading directions the extension directions trend roughly orthogonal to the strike of the boundary faults in all the investigated sectors extension shows an overall southward increase in divergence from the regional directions indicated by gps vectors this behavior has been compared with the continental main ethiopian rift of east africa where structural studies show a re orientation of the extension direction between the rift margins and the internal sectors resulting in a divergence between local and regional extension directions aimed at investigating the causes of these differences we performed a new series of analogue models by varying different parameters such as rift obliquity and width our findings suggest that rift width exerts a primary control on the boundary axis variation of the extension direction in particular significant differences in fault orientation and local direction of extension between the margins and the rift axis are observed in wider rift valleys e g continental rift of ethiopia conversely fault orientation and local extension direction are much more homogeneous in narrower rift valleys e g rifts of iceland where a variations in the direction of extension from the margins to the rift axis are not observed

mechanical interaction between volcanic systems in libya abstract the spatial distributions of monogenetic volcanoes primarily volcanic craters within the four principal volcanic provinces of libya are examined and presented on a volcano density map six main volcanic clusters have been identified referred to as volcanic systems remarkably the al haruj ahvp and nuqay nvp volcanic provinces have double peak volcano density distributions while the gharyan gvp and as sawda svp volcanic provinces have single peak volcano density distributions we interpret each volcano density peak as corresponding to a separate volcanic system so that there is a total of six systems in these four provinces there was an overlap in volcanic activity in these provinces with at least three simultaneously active we propose that each of the 6 volcanic systems was is supplied with magma from a large sill like reservoir similar in lateral dimensions to the systems clusters themselves numerical results show zones of high tensile and shear stresses between the reservoirs that coincide roughly with the main swarms of extension dykes and volcanic fissures and shear faults fractures in the areas the most recent volcanic eruptions in libya fall within the modelled high stress concentration zones primarily eruptions in the volcano waw an namus and the holocene al mashaqaq lava flow there are no known eruptions in libya in historical time but some or all the volcanic systems may have had one or more arrested historical dyke injections in particular part of the recurrent seismic events in the hun graben in the northwest libya may be related to dyke propagation and arrest if some of the inferred magma reservoirs are still fluid as is likely they pose earthquake and volcanic hazards to parts of libya particularly to the city of gharyan and zallah town as well as to many oil field operations

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

the distribution of neodymium isotopes and concentrations in the eastern tropical north atlantic abstract dissolved neodymium nd and its radiogenic isotope composition 143nd 144nd expressed as end belong to the key parameters of the international geotraces program which aims to investigate the processes controlling the distribution of trace elements and their isotopes in the global ocean we present nd isotope and concentration nd data from eleven full depth water column profiles from the eastern sub tropical atlantic ocean between 2 n and 29 n and from the romanche fracture zone sampled during meteor cruise m81 1 geotraces cruise ga11 in february march 2010 the end signatures range from 12 9 in upper north atlantic deep water unadw at the equator to 8 1 in the mixed layer near the canary islands nd concentrations range from 11 9 pmol kg observed within the equatorial undercurrent to 40 2 pmol kg in antarctic bottom water aabw in the romanche fracture zone large variations in surface water nd isotope compositions 12 7 end 8 1 and concentrations 15 7 pmol kg nd 27 7 pmol kg are caused by partial dissolution of saharan dust particles between 2 n and 13 n and volcanic island weathering canaries between 25 n and 29 n in contrast to dust inputs which predominantly affect the nd concentrations and isotope compositions of surface waters and underlying south atlantic central water sacw contributions originating from the canary islands affect the nd isotope composition of the entire surrounding water column without significantly altering nd concentrations thus confirming the concept of boundary exchange in its strictest sense the results confirm that the composition of lower north atlantic deep water lnadw end 12 1 in the abyssal plains of the eastern north atlantic is exclusively set by the mixing ratio of lnadw and antarctic bottom water aabw upper north atlantic deep water unadw in contrast is characterized by end signatures between 12 7 and 12 0 between 2 n and 10 n whereas further north it is clearly affected by admixture of mediterranean water mw and radiogenic inputs from the canary islands and likely also the cape verde islands this article is part of a special issue entitled cycles of trace elements and isotopes in the ocean geotraces and beyond edited by tim m conway tristan horner yves plancherel and aridane g gonzalez

bottom water throughflows at the rio de janeiro and rio grande fracture zones bottom water throughflows at the rio de janeiro fracture zone 22 s and rio grande fracture zone 26 s of the mid atlantic ridge are identified from hydrographic anomalies observed along 9 w in the angola basin the throughflow water is supplied by a meridional band of cold and fresh water lying against the western flank of the ridge

dikes minor faults and mineral veins associated with a transform fault in north iceland abstract the tjornes fracture zone is a transform fault connecting the axial rift zone in north iceland with that of the kolbeinsey ridge off the north coast of iceland on the north coast of the flatcyjarskagi peninsula there is a 3 5 km wide zone of intense deformation which we interpret as the main on land part of this transform fault this transform zone strikes n64 w and is characterized by unusually steeply dipping lavas 25 44 wnw extensive secondary mineralization numerous slickensided surfaces normal faults 1 20 m throw dikes that strike parallel to the zone and occasional 3 20 m wide subzones of crushed rocks striking parallel to the transform fault in a 17 km long continuous coastal profile south of the transform zone there is a 30 clockwise change from south to north in the lava strike the dike strike also changes clockwise north along this profile but only by 4 this indicates that the clockwise change in the lava strike is original and not due to subsequent tectonic rotation when the axial rift zones of north iceland and the kolbeinsey ridge are modelled as single offset mode i cracks finiteelement results show that the tjornes fracture zone developed between these mode i cracks in a region of maximum shear stress the underlap between these axial rift zones has apparently decreased during the past several million years the model predicts that as this underlap decreased the angle between the husavik flatey fault and that of the associated axial rift zones increased and the activity of the husavik flatey fault decreased both these predictions are borne out by the available data

mineralization of biogenic materials in the water masses of the south atlantic ocean i assessment and results of an optimum multiparameter analysis abstract this is the first of two manuscripts dealing with the circulation mixing ventilation and organic matter mineralization of the south atlantic ocean sao the present work quantifies the complex mixing of water masses in the sao using a constrained least squares regression optimum multiparameter omp analysis the omp based on temperature salinity silicate and the conservative parameter no was applied on two world ocean circulation woce lines a17 and a14 in the western and eastern sao respectively the constrained omp sensitivity to sources of error in the end member characteristics measured parameters equation weights and oxygen to nitrogen mineralization ratio rn was carefully assessed using perturbation tests perturbation of rn was the only test that changed significantly the mixing proportions although by less than 5 the constrained omp method allowed defining the realm and identifying the core of flow of each water mass to study its circulation the evolution of its chemical composition and eventually to separate the contribution of physical and biogeochemical processes relevant specific outputs of this first manuscript are 1 north of the south equatorial current the silicate maximum is primarily composed of antarctic intermediate water aaiw rather than circumpolar deep water cdw 2 the two degree discontinuity tdd experiences a dramatic meridional change of water masses composition being dominated by north atlantic deep water nadw north of the vema channel and by cdw southwards 3 the 50 proportion horizon of weddell sea deep water wdsw with a of 0 3 c at the entry of the sao defines the upper limit of the wsdw realm more properly than the classical isopycnals of 46 04 or 46 06 4 where the proportion of wsdw is around 75

intense mixing of antarctic bottom water in the equatorial atlantic ocean the spreading of antarctic bottom water the densest global scale water mass is highly constrained by ocean floor topography in the atlantic ocean the mid atlantic ridge confines this water mass mainly to the western basins the bottom waters in the eastern basins being renewed by flows through gaps in the ridge1 one such gap is the romanche fracture zone a large offset of the ridge which straddles the equator it has been observed2 that sills within this fracture zone block the passage of waters colder than 0 9 c warmer less dense waters passing over the sills appear to cascade downslope where they are modified by mixing here we present direct measurements which quantify these processes the flow is vertically sheared and exhibits remarkably intense turbulence comparable to that seen at the ocean surface in the presence of winds of l0ms 1 this turbulence mixes the densest waters passing through the fracture zone with the warmer overlying waters so that the coldest waters exiting this region have been warmed by 0 6 c during transit topographic obstructions and turbulent mixing together thus determine the properties of the flows renewing the deepest waters of the atlantic ocean s eastern basins

effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies what controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood i propose that for volcanoes at divergent plate boundaries these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them this stress concentration is mainly a function of the applied tensile stress associated with spreading and the aspect height width ratios of the chambers high spreading rates and or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies it is found that for chambers at the same depth in a volcanic zone the one of the highest aspect ratio will normally intrude magma most frequently also if the chambers are of equal aspect ratios the one at the greatest depth will intrude magma most frequently because the extrusion frequency of a volcano is a fraction of its intrusion frequency the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano these results are applied to the divergent plate boundary in iceland where many of the volcanoes appear to be fed by double chambers that is shallow chambers fed by deep seated chambers it is found that except when the aspect ratio of the deep seated chamber is much less than that of the shallow chamber the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep seated chamber it is concluded that whereas the short term i e 103 yrs extrusion frequencies of volcanoes at the plate boundary in iceland may be similar to the dike intrusion frequencies of the source chambers the long term i e 104 yrs extrusion frequencies may be about ten times lower than the intrusion frequencies

renewal rates of east atlantic deep water estimated by inversion of 14c data the renewal of the deep water of the east atlantic and its large scale internal circulation are studied on the basis of the distributions of potential temperature silicate co2 and 14c an isopycnal multibox model including advection mixing and sources and sinks is set up and described tracer data are input for the model and balance equations for the various properties for the boxes of the model serve as constraints for the determination of water fluxes mixing coefficients and source parameters extremal values for various model parameters that are consistent with the tracer data satisfy the balance equations within the estimated tolerances are calculated by linear programming techniques 14c data are seen to be valuable in determining absolute flow rates model results confirm the importance of the romanche facture zone for the renewal of east atlantic deep water eastward flows through the romanche fracture zone were found to be between 2 6 and 5 1 sv flows through the vema fracture zone amount to at most 20 of the romanche fracture zone inflow contributions of antarctic bottom water at the southern end of the east atlantic walvis ridge and of iceland scotland overflow water at the northern end are very small 5 of equatorial inflow diapycnal mixing coefficients are between 1 and 10 cm2 s and values for the dissolution rates of silicate and carbon are in the expected range

overlapping rift zone segments and the evolution of the south iceland seismic zone the south iceland seismic zone is a 20 60 km wide north south and up to 70 km long zone of north and north northeast trending holocene arrays of en echelon tension fractures these fracture arrays are related to dextral strike slip faults buried by holocene lava flows in this zone major destructive earthquake sequences occur at intervals of 45 112 years the largest events reaching magnitude 7 ms we propose that this seismic zone is located between overlapping rift zone segments spreading centers where the eastern segment has been propagating to the south during the past 3 ma we made a finite element study of this configuration with the segments modeled as mode i cracks loaded in tension the results suggest that the south iceland seismic zone in general and the north and north northeast trending dextral faults in particular develop in response to the shear stresses generated between the rift zone segments

tracing lower north atlantic deep water across the equator water whose ultimate source is the overflow at the denmark strait can be traced using maps of salinity dissolved oxygen and silica concentration on deep density surfaces south along the american continent to at least 5 s several property extrema whose resolution is at the limit of accuracy of the measurements occur on these density surfaces which nevertheless may indicate areas of stronger mixing along the crest of the mid atlantic ridge transport estimates for that part of the southward flow within 600 km of the western boundary near 10 n suggest that 10 3 106 m3 s 1 of lower north atlantic deep water moves south there the total southward deep water transport near 10 n is estimated to be closed to 25 106 m3 s 1