nontransform offsets of the pacific cocos plate boundary and their traces on the rise flank a multi beam bathymetric reconnaissance of the fast spreading crest of the east pacific rise between 7 n and 2 6 n found the axial ridge to be interrupted by four 4 13 km non transform offsets they have probably persisted since initiation by a small change in spreading direction 3 5 ma at each offset the volcanic rift zones veer 15 toward each other and overlap for 5 25 km around deep basins intervening lateral displacements of the rift zones are too short 2 5 km to break the linear axial volcanoes but cause local bends in their plan shapes some examples have a morphology that mimics the larger intervolcano offsets interpretation of a thorough bathymetric and magnetic survey around the 9 km 5 5 n offset indicates that the mainly one sided spreading from the overlapped sections of rift zone occurs by a combination of highly asymmetric dike accretion and episodic small scale jumps of the eruptive zone the northern rift zone is elongating propagating at the expense of the southern one the resulting along axis migration of this and other examples at speeds comparable to spreading rates causes the rise flank scars of the nontransform offsets equivalent to the fracture zones of transform faults to be oblique to the spreading direction these traces are recognizable within the highly lineated fault block terrain of the rise flanks because of the distinctive morphology of the 5 25 km wide bands that accreted at overlapped sections of the rift zones here lineation is less marked and oblique to the spreading direction and volcanic relief is more important than relief caused by faulting the west flank trace of the 5 5 n offset is occupied by a line of volcanoes and shallow volcanic ridges several other volcanic chains previously identified as hot spot traces seem to have grown in similar tectonic settings

characteristics of turbulent boundary layers over a rough bed under saw tooth waves and its application to sediment transport article i nfo a large number of studies have been done dealing with sinusoidal wave boundary layers in the past however ocean waves often have a strong asymmetric shape especially in shallow water and net of sediment movement occurs it is envisaged that bottom shear stress and sediment transport behaviors influenced by the effect of asymmetry are different from those in sinusoidal waves characteristics of the turbulent boundary layer under breaking waves saw tooth are investigated and described through both laboratory and numerical experiments a new calculation method for bottom shear stress based on velocity and acceleration terms theoretical phase difference and the acceleration coefficient ac expressing the wave skew ness effect for saw tooth waves is proposed the acceleration coefficient was determined empirically from both experimental and baseline k model results the new calculation has shown better agreement with the experimental data along a wave cycle for all saw tooth wave cases compared by other existing methods it was further applied into sediment transport rate calculation induced by skew waves sediment transport rate was formulated by using the existing sheet flow sediment transport rate data under skew waves by watanabe and sato watanabe a and sato s 2004 a sheet flow transport rate formula for asymmetric forward leaning waves and currents proc of 29th icce asce pp 1703 1714 moreover the characteristics of the net sediment transport were also examined and a good agreement between the proposed method and experimental data has been found

observations of wave induced pore pressure gradients and bed level response on a surf zone sandbar horizontal and vertical pressure gradients may be important physical mechanisms contributing to onshore sediment transport beneath steep near breaking waves in the surf zone a barred beach was constructed in a large scale laboratory wave flume with a fixed profile containing a mobile sediment layer on the crest of the sandbar horizontal and vertical pore pressure gradients were obtained by finite differences of measurements from an array of pressure transducers buried within the upper several centimeters of the bed co located observations of erosion depth were made during asymmetric wave trials with wave heights between 0 10 m and 0 98 m consistently resulting in onshore sheet flow sediment transport the pore pressure gradient vector within the bed exhibited temporal rotations during each wave cycle directed predominantly upwards under the trough and then rapidly rotating onshore and downwards as the wave front passed the magnitude of the pore pressure gradient during each phase of rotation was correlated with local wave steepness and relative depth momentary bed failures as deep as 20 grain diameters were coincident with sharp increases in the onshore directed pore pressure gradients but occurred at horizontal pressure gradients less than theoretical critical values for initiation of the motion for compact beds an expression combining the effects of both horizontal and vertical pore pressure gradients with bed shear stress and soil stability is used to determine that failure of the bed is initiated at non negligible values of both forces

tomographic image of the magma chamber at 12 50 n on the east pacific rise the east pacific rise at 12 50 n fig 1 is a fast spreading ridge with intense hydrothermal activity1 and ophiolite studies2and thermal modelling3 indicate that this is a likely setting for a magma chamber a recent seismic reflection experiment4 imaged the top of the magma chamber at this site at a depth of 1 4 km below the sea floor and found that it is continuous for tens of kilometres along the rise axis here we examine a large set of accurate p wave travel times from a detailed seismic refraction experiment at the same site5 the patterns observed in the travel times demonstrate that a zone of low seismic velocities exists beneath the rise axis throughout the region studied the best fitting two dimensional structure obtained from linear inversion of the travel times includes an axial low velocity zone magma chamber only 6 km wide in which velocities are depressed by more than 0 5 km s 1

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

the empirical age depth relation and depth anomalies in the pacific ocean basin this paper presents a new age depth relation and a comprehensive depth anomaly map for the pacific ocean age depth and sediment thickness were averaged for each 1 1 area bathymetry was corrected for isostatic effects due to sediment and water load and for the variation of sound velocity in water seafloor existing at extreme distance from hot spot tracks is found to be anomalously deep hot spots were treated as thermal anomalies and data within 800 km of them were eliminated from further analysis an age depth equation was determined by fitting a least squares line to the remaining data the resulting depth versus age distribution confirms the linear dependence of depth on age up to 80 ma all older seafloor is within 800 km of hot spots or hot spot tracks and is shallower than predicted this flattening of the age depth distribution requires that a separate function be fit to data older than 80 ma as a result data representing the deepest seafloor older than 80 ma were selected for curve fitting and the age depth distribution was approximated by an exponential function it is impossible to distinguish between the infinite half space and cooling plate models of the lithopshere using the age depth data from the pacific ocean a depth anomaly map prepared by comparing observed depth with the depth predicted by the new age depth formula shows several newly recognized features including swells over the louisville ridge in the south pacific and in the gulf of alaska seamounts

magnetic and tectonic studies of the dueling propagating spreading centers at 20 40 s on the east pacific rise evidence for crustal rotations we present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20 40 s on the east pacific rise a majority of the data used were collected during a cruise aboard the r v moana wave during which continuous seamarc ii coverage was obtained these data were combined with additional data to produce an anomaly map which extends to anomaly 2 aged crust a three dimensional inversion in the presence of bathymetry was carried out for the area the resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity the results indicate consistent asymmetric spreading faster to the east discontinuous high magnetizations in the discordant zone associated with the discontinuity and southward migration of the feature at a rate of 90 100 mm yr between jaramillo and brunhes time 0 95 to 0 73 ma with slowing during the brunhes to less than 10 mm yr there is also an overlapping jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the nazca to the pacific plate in addition areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector results from this second model support the presence of highly rotated pre brunhes nazca crust within brunhes pacific crust which has been deformed by bookshelf faulting this indicates at least two episodes of crustal transfer from the nazca plate to the pacific plate the discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the nazca plate between the discontinuity and the easter microplate to the south the detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges

sediment transport associated with morphological beach changes forced by irregular asymmetric skewed waves 1 the waveshape effects on sediment transport are investigated for cross shore beach profile changes this study is based on experiments performed in the laboratoire des ecoulements geophysiques et industriels wave flume for irregular waves the interest of such experiments resides in presenting complex combinations of wave skewness and asymmetry in bed load ripple and sheet flow regimes net sediment transport rates on typical beach morphodynamics are analyzed in regard to wave skewness and asymmetry undertow and ripple occurrence onshore bar migration is mainly associated with onshore directed sediment transport whereas terrace profile and offshore bar formation correspond to offshore sediment transport as for natural beaches energetic moderate wave climates mostly induce offshore onshore sediment fluxes for a given significant wave height an increase decrease in the wave climate peak period is associated with an increase decrease in wave skewness and leads mostly to offshore onshore sediment fluxes the experiments are fully characterized by unsteady behavior consequently several conditions exhibit phase lag effects where the sediment is mobilized by the wave crest and transported by the following trough which produces a net offshore transport even for a weak undertow the presence of ripples clearly contributes to enhance this behavior an original concept due to its application to skewed asymmetric irregular waves presents the important interaction between wave nonlinearities driving the sediment fluxes the net sediment transport rate under strongly skewed waves is either offshore directed due to phase lag effects or onshore directed when the wave asymmetry is large enough both these mechanisms probably largely contribute to bar formation and migration

formula for predicting bedload transport rate in oscillatory sheet flows abstract at high bed shear stress sheet flows often occur in coastal waters in which high concentration bedload sediments are transported in a thin layer near the bed this paper firstly constructs a theoretical model partial differential equations pdes for the intense transport of non cohesive bedload sediments by unidirectional currents and then seeks a special solution to the pdes to determine the thickness of the bedload particle water mixture which could serve as the reference height that is often invoked in numerical computation and simulation of suspended sediment transport in turbulent flows moreover a modified formula is presented to determine the reference concentration using a uch approach the present study derives a 1d formula for predicting bedload transport rate in sheet flows driven by asymmetric waves with the help of a novel formula for evaluating wave friction factor the new bedload formula can generically take into account slope angle positive and negative wash load concentration in the driving water flow and other factors that affect bedload transport rate it compares well with measured data in a large scale wave flume dohmen janssen c m hanes d m 2002 sheet flow dynamics under monochromatic non breaking waves journal of geophysical research 107 c10 1301 1321 a large scale oscillatory water tunnel hassan w n ribberink j s 2005 transport processes of uniform and mixed sands in oscillatory sheet flow coastal engineering 52 745 770 and in a swash zone of natural beach masselink g hughes m g 1998 field investigation of sediment transport in the swash zone continental shelf research 18 1179 1199

east pacific rise 18 19 s asymmetric spreading and ridge reorientation by ultrafast migration of axial discontinuities a detailed bathymetric side scan and magnetic survey of the east pacific rise out to a seafloor age of 1 ma has been carried out between 18 and 19 s it reveals that some left stepping axial discontinuities have been migrating southward at rates an order of magnitude faster than the spreading rates 1000 mm a or higher these rapid migration events have left on the nazca plate discordant features striking nearly parallel to the ridge axis a discontinuity with an offset of several kilometers has migrated in two stages at around 0 45 and 0 3 ma and has left two large discordant zones consisting of a series of unfaulted hummocky basins bounded to the east by short ridges oriented about n s oblique to the ambient 013 fabric the morphology and reflectivity characteristics of these discordant zones are akin to the overlap basins and abandoned ridge tips which make up the migration trails of large slowly migrating overlapping spreading centers between 18 35 and 19 03 s the ridge axis is flanked a few kilometers to the east by a prominent sedimented ridge previously recognized as a recently abandoned ridge axis the present ridge segment steadily deepens and narrows southward which suggests the abandoned ridge has been rafted onto the nazca plate during the ultrafast southward propagation of the ridge segment rather than by one discrete ridge jump by transferring pacific lithosphere to the nazca plate these migration events account for most of the asymmetric accretion observed faster to the east this process is consistent with the features common to asymmetric spreading namely the sudden onset or demise of asymmetric spreading and the ridge segment to ridge segment variability because the discordant zones left by these rapid migration events are near parallel to the ambient seafloor fabric they are unlikely to be detected by conventional bathymetry or magnetic surveys and so called ridge jumps may actually often represent ultrafast propagation of a ridge segment variations in fault azimuth with age show there has not been any significant change in spreading direction over the past 0 8 m y instead the counterclockwise trend of the east pacific rise relative to the brunhes matuyama reversal 0 78 ma mostly reflects that ultrafast propagation of ridge segments has transferred a larger amount of the pacific lithosphere to the nazca plate at 18 s than at 19 s in keeping with the regional features of the magnetic anomalies we propose that an 8 to 10 km left stepping discontinuity which was located between 17 and 17 30 s at 0 78 ma has been recently redistributed into the present staircase of small left stepping discontinuities between 16 and 19 s this smoothing of the ridge geometry probably occurred through repeated small lateral steps of the ridge segments inside of the discontinuities during ultra fast propagation episodes and may be the consequence of a significant replenishment of the magma reservoir between 17 and 17 30 s during the past 1 m y

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

seabed shear stress and bedload transport due to asymmetric and skewed waves abstract a simple conceptual formulation to compute seabed shear stress due to asymmetric and skewed waves is presented this formulation generalizes the sinusoidal wave case and uses a variable friction factor to describe the physics of the boundary layer and to parameterize the effects of wave shape predictions of bed shear stresses agree with numerical computations using a standard boundary layer model with a k e turbulence closure the bed shear stress formulation is combined with a meyer peter and muller type formula to predict sheet flow bedload transport under asymmetric and skewed waves for a horizontal or sloping bed the predictions agree with oscillatory water tunnel measurements from the literature

bed shear stress under skewed and asymmetric oscillatory flows abstract we present a new formulation to predict the bed shear stress under skewed asymmetric oscillatory flows with or without a co linear mean current extending the work of nielsen 1992 the nonlinearity of the oscillatory flow is incorporated through the use of two parameters the index of skewness or nonlinearity and the waveform parameter the new formulation is tested against the bed shear stress estimated from the log fit and momentum integral methods using oscillatory data from oscillating water tunnel experiments the new formulation and the momentum integral method agree well but differ from those with the log fit method possibly because both methodologies lead to different results for the phase lead between the bed shear stress and the free stream velocity the new bed shear stress formulation is incorporated in a quasi steady bedload formula and accurately reproduces net transport rates under non linear nonbreaking waves with and without an opposing current

reinterpretation of seismic reflection data over the east pacific rise multichannel seismic reflection data over the axial region of the east pacific rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples the data are from the lamont doherty geological observatory s line 17 shot across the rise at 9 sup 0 n the migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof the polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge the asymmetry is probably real and not an artifact of data collection modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally a reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis the shape of the roof reflection is convex upward with an approximate slope of 10 sup 0 and a width of 4 km if extrapolated symmetrically to the other side of the ridge axis the magma chamber roof event is consistent with the funnel shaped chamber proposed by pallister and hopson 1981 for the samail ophiolite if the chambermore roof steepens rapidly beyond the extent of the reflection it would be consistent with the mush filled model of sleep 1975 1978 and dewey and kidd 1977 less

physical removal of contaminative inorganic material from buried human bone abstract introduction of inorganic contaminants to bone during burial poses a serious problem for the interpretaion of the levels of these elements in terms of ancient diet we have found that the problem of non uniform contamination may be reduced or even eliminated for some elements by removal of 1 3 mm of the surface of the bone by physical abrasion whereas surface contamination on unabraded bone is readily visible in human femurs by scanning electron microscopy sem there is no evidence for contamination according to sem after abrasion this process of surface removal significantly reduces the levels of zn cd k al fe and mn but has no effect on na ca mg sr and ba for these woodland and mississippian samples from west central illinois the latter group of elements apparently survived burial without significant contamination the former group of elements was contaminated by the soil environment but at least for zn cd k and possibly fe surface removal has returned the elemental proportions in the remainder of the bone to close to biogenic levels analysis of variance between subgroups after cleaning shows differences in zn and cu between sexes in ca mg and sr over time and in na and ba based on age at death

magmatic processes at superfast spreading mid ocean ridges glass compositional variations along the east pacific rise 13 23 s major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading 150 mm yr east pacific rise epr 13 23 s and near ridge seamounts comprise 212 chemical groups we interpret these groups to represent the average composition of individual lava flows or groups of closely related flows groups slightly enriched in k2o t morb are distributed variably along the axis in contrast to the galapagos spreading center where t morb are extremely rare this result is consistent with the interpretation that t morb magmas arise from low melting temperature k rich heterogeneities in the subaxial epr mantle the galapagos spreading center which is migrating to the west in an absolute reference frame is underlain by mantle previously processed and depleted in the t morb component during melting events giving rise to earlier epr magmas excluding t morb there are nearly monotonic twofold increases in k ti and k p of axial lavas from 23 s to 13 s from 22 s to 17 s these gradients correlate with isotopic ratios but north of 17 s there is a reversal of isotopic gradients indicating recent decoupling of the isotopic and minor element ratios in the subaxial mantle a strong southward increase in degree of differentiation for approximately 200 km north of the large offset at 20 7 s correlates with a gradient in bathymetry consistent with previous interpretations that this offset is propagating to the south samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive evolved fractionation signatures of rift propagation suggesting that propagating rift tips have been abandoned preferentially to failing rift tips glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset and possibly also south of 22 s the latter region may be affected by deformation accompanying northward growth of the easter microplate near ridge seamounts on the pacific plate between 18 19 s comprise two distinct populations those aligned approximately parallel to the spreading direction are extremely variable in major element composition but consistently enriched in sr relative to nearby axial lavas smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and sr relative to axial lavas the degree of differentiation of axial lavas between 18 19 s can be related to the structural development of the rift axis and or vigor of hydrothermal activity of individual segments glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area primary magmatic segmentation mainly reflects mantle source variations the boundaries of which correlate with the largest physical offsets in the rise axis between the easter microplate and garrett transform zone a secondary magmatic segmentation defined by the along axis continuity of similar parental magma compositions or liquid lines of descent occurs with a length scale varying from 11 to 185 km with an average of 69 57 1 km the boundaries of these segments mainly occur at overlapping spreading centers all first second and third order physical offsets correspond to secondary magmatic segment boundaries but some secondary magmatic segment boundaries also occur at small fourth order ridge axis discontinuities the secondary magmatic segments define the length scale of mantle melting variations mainly variations in extent of melting but not the scale of melt extraction processes that feed the axis this scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth order physical discontinuities along axis there is a good positive correlation of average secondary magmatic segment length with spreading rate for four well sampled areas varying from 20 to 150 mm yr secondary magmatic segments also become more variable in axial length with increasing spreading rate the average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature defined by extent of magmatic differentiation which bears little systematic relation to physical or other kinds of magmatic segmentation at 13 23 s the length scale of this variation is 217 60 1 km approximately corresponding to the wavelength of rolls in the gravity field observed off axis taken together the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere magma supply to the axis and crustal magmatic temperature reflect independent regionally decoupled processes

plate bending stresses at axial highs and implications for faulting behavior abstract we develop flexural models of axial high topography which include the effects of plastic yielding and use the results to examine the contribution of plate bending stresses and strains to fault patterns at the east pacific rise epr the lithosphere responds to buoyancy near the ridge axis created by the presence of elevated temperatures and melt within the crust parameterizing these quantities we find topography at wide axial highs can be fit assuming the crust cools to lithosphere temperatures to 3 4 km depth within 2 km from the axis and the presence of 20 30 melt in the lower crust a trade off in buoyancy implies the greater the amount of crustal cooling the less melt is required significant crustal cooling leads to lithospheric strengthening but plastic bending effects allow the plate to deflect over the distance needed to fit the observed axial relief the plate develops at the axis with non zero curvature and unbends as it moves away from the axis placing a section of the upper surface or seafloor into extension seafloor stresses are extensional beginning a few kilometers from the axis and continuing up to distances of 35 km from the axis for models that fit wide axial high topography a compilation of previous faulting studies at the epr reveals that distance from the axis over which fault slip remains active appears to depend on the width of the axial high for narrow highs this region is confined to at most 15 km from the axis but at wider axial highs it reaches distances of 20 45 km comparable to regions of extension predicted by the flexural models the model strain can be as large as 1 9 amounting to half the observed strain assuming a fault slip angle of 45 but most strain assuming a slip angle of 60 these results strongly suggest that bending stresses contribute significantly to normal faulting on the flanks of fast spreading ridges and may have a strong influence on faulting patterns there

effects of wave shape on sheet flow sediment transport 1 a two phase model is implemented to study the effects of wave shape on the transport of coarse grained sediment in the sheet flow regime the model is based on balance equations for the average mass momentum and fluctuation energy for both the fluid and sediment phases model simulations indicate that the responses of the sheet flow such as the velocity profiles the instantaneous bed shear stress the sediment flux and the total amount of the mobilized sediment cannot be fully parameterized by quasi steady free stream velocity and may be correlated with the magnitude of local horizontal pressure gradient or free stream acceleration a net sediment flux in the direction of wave advance is obtained for both skewed and saw tooth wave shapes typical of shoaled and breaking waves the model further suggests that at critical values of the horizontal pressure gradient there is a failure event within the bed that mobilizes more sediment into the mobile sheet and enhances the sediment flux preliminary attempts to parameterize the total bed shear stress and the total sediment flux appear promising

discrete particle model for sheet flow sediment transport in the nearshore fully three dimensional discrete particle computer simulations of high concentration sheet flow transport in oscillatory flows quantify the effect of fluid acceleration on bed load transport in highly unsteady flows typical of nearshore marine environments a simple impulse momentum approach explains simulation results and forms the basis for adding an acceleration related term to widely used energetics sediment transport formulae transport predicted by the acceleration term becomes increasingly significant as wave shape approaches the sawtooth profile characteristic of surf zone bores simulations integrate f ma and a corresponding set of equations for the torques for each sphere normal and tangential forces between contacting particles are linear functions of the distance between sphere centers and the relative tangential displacement at the contact point respectively particle interactions are both inelastic and frictional pressure gradient forces generated by the passage of surface gravity waves drive fluid and particle motion in a stack of thin horizontal fluid layers that exchange momentum and exert fluid drag added mass and buoyancy forces on particles transport properties of the simulated granular fluid assemblage are robust to large variations in material properties of the particles simulated transport rates agree with available experimental data for unsteady transport of coarse sands the mode of bed load motion dispersion of bed load particles and particle segregation by size and density are qualitatively consistent with available particle scale observations of bed load transport of natural particles

boundary layer hydrodynamics and bedload sediment transport in oscillating water tunnels oscillating water tunnels are experimental facilities commonly used in coastal engineering research they are intended to reproduce near bed hydrodynamic and sediment transport phenomena at a realistic scale in an oscillating water tunnel a piston generates an oscillatory motion that propagates almost instantaneously to the whole tunnel consequently flow is uniform along the tunnel unlike the propagating wave motion in the sea or in a wave flume this results in subtle differences between the boundary layer hydrodynamics of an oscillating water tunnel and of a propagating wave which may have a significant effect in the resulting sediment transport in this paper we present a zeroth order analytical model of the turbulent boundary layer hydrodynamics in an oscillating water tunnel by using a time varying eddy viscosity and by accounting for the constraints arising from the tunnel s geometry the model predicts the oscillating water tunnel hydrodynamics and yields analytical expressions to compute bed shear stresses for asymmetric and skewed waves both in the absence or presence of an imposed current these expressions are applied to successfully quantify bedload sediment transport in oscillating water tunnel experiments

breaking new ground estimates of crack depth along the axial zone of the east pacific rise 9 12 54 n using simple fracture mechanics models the depths of fissures that were observed along the axial zone of the east pacific rise epr from 9 12 to 54 n with the deep towed argo z vehicle are estimated the widest cracks 4 8 m along this fast spreading portion of the epr are the deepest and are spatially correlated with the broadest youngest and most hydrothermally active portions of the ridge crest if the widest deepest cracks are not primarily eruptive they should be most abundant in areas of older lava flows having increased in width with time this is not observed some of the widest cracks are located where the 1991 eruption of the ridge crest occurred at 9 45 52 n and may be deep enough to reach the sheeted dikes of layer 2b these cracks may have tapped melt during the eruption and facilitated the flux of vapor rich hydrothermal fluids through overlying lava flows the narrowest presumably shallowest cracks correspond to the narrowest oldest and least hydrothermally active portions of the ridge crest we interpret the wide deep cracks as primarily eruptive in origin and suggest that they may be associated with the inflation of an axial magma chamber whereas the narrow shallow cracks are interpreted as primarily tectonic and are thought to be associated more with far field plate stresses

mechanics of the scouring and sinking of submerged structures in a mobile bed a physical model study abstract wave induced flow velocity and turbulence may cause scouring in the seabed around coastal structures depending on the wave climate and tidal water depth a 3 d physical modelling study was conducted to investigate the possible causes of the sinking of two submerged coastal structures on the santa maria del mar smm beach spain the experimental investigation was conducted by employing a froude similarity law with a geometric scale of 1 20 and the submerged modular structure was subjected to different wave climates and tidal water levels the combinations of the significant wave height and peak period were chosen from in situ real wave conditions that were monitored during the sinking of the prototype structures linking and unlinking conditions for the modules in the structure were investigated in this study the results show that the modules sank to approximately 48 of their height at the end of the tests with storm waves and semi linking conditions of low water depth most of the experimental results were compatible with the prototype monitoring results tests with proper linking among the modules and tests with an appropriate gravel foundation resulted in a sustainable solution because they presented much less or almost no scouring and sinking

volcanic eruptions on mid ocean ridges new evidence from the superfast spreading east pacific rise 17 19 s uniform sediment cover were recovered from lava that buries older faulted terrain the boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector consistent with magmas from two separate reservoirs being erupted in the same event chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments a comparison of our results to global data on submarine mid ocean ridge eruptions suggests consistent dependencies of erupted volume activated fissure lengths and chemical heterogeneity with spreading rate consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths index terms 3035 marine geology and geophysics midocean ridge processes 8414 volcanology eruption mechanisms 8439 volcanology physics and chemistry of magma bodies 3655 mineralogy and petrology major element composition keywords lava flow chemical heterogeneity erupted volume lava morphology side scan sonar

the axial summit graben and cross sectional shape of the east pacific rise as indicators of axial magma chambers and recent volcanic eruptions abstract the axis of the east pacific rise epr undulates up and down hundreds of meters over distances of 30 200 km along strike the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers oscs we have suggested that systematic variations in depth and cross sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment 1 a comparison of recently collected multichannel seismic mcs data 2 with our sea beam and seamarc ii data have allowed us to test and advance this hypothesis along the epr from 9 to 13 n there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment the cross sectional shape of the rise the presence or absence of an axial summit graben and the presence or absence of a shallow axial magma chamber as interpreted from mcs data where the axial magma chamber is present the cross sectional shape of the ridge is broad and an axial summit graben is recognized along the axis in contrast where the cross sectional shape of the rise is narrow and triangular an axial magma chamber is not detected and an axial summit graben is absent these ridge axis characteristics tend to occur along deeper portions of a given ridge segment often near ridge axis discontinuities we suggest that these systematic variations in ridge axis morphology cross sectional shape and structure presence or absence of an axial graben reflect spatial and temporal variations in the magmatic budget of the ridge axis where the magmatic budget is waxing shallow level magma reservoirs in the crust and underlying upper mantle swell creating a broad axial bulge with a summit graben where the magmatic budget is diminished the crustal magma chamber is small this proposed correlation of shape structure and magmatic parameters fails along only two short portions of the ridge in these areas there is evidence for an axial magma chamber and the rise has a broad cross sectional shape but there is no summit graben bottom photographs and submersible results however show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting suggesting that the summit graben has been recently filled in by lava flows and the development of a summit graben or a linear caldera by volcano tectonic collapse has not yet occurred

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

surf zone cross shore boundary layer velocity asymmetry and skewness an experimental study on a mobile bed 1 an experimental study on a physical model of a beach in a two dimensional wave flume was designed to investigate velocity nonlinearities in the wave boundary layer the cross shore velocity was measured in the surf zone along a vertical profile every 3mm from free stream elevation down to the still bed level the skewness and the asymmetry of the phase averaged velocity were computed at each elevation observations indicate that the free stream asymmetry transforms into bottom velocity skewness a linear experimental relation between free stream asymmetry to skewness ratio and bottom skewness to free stream skewness ratio is established a theoretical linear relationship is discussed which predicts the phase lead of the bottom velocity this phase lead is also determined by fourier analyzing the velocity time series the first two fourier components yield the same phase lead at the bed that is found to be about 30 and nearly constant over all the experiments made

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

magma distribution across ridge axis discontinuities on the east pacific rise from multichannel seismic images detailed studies of the morphology of the east pacific rise axis have shown that its linearity is disrupted by many small but distinct non transform offsets including overlapping spreading centres oscs and deviations from axial linearity devals which display variable geochemical signals1 9 using multichannel seismic reflection profiling we have mapped the distribution of a bright shallow reflector that detrick et al 10 have associated with an axial magma chamber we have found that it is neither continuous across the 9 03 osc6 nor separated into two parallel bodies2 and that its lateral offset does not conform to that of the topographic offset associated with the 9 17 deval these observations provide important insight into a causative relationship between morphological and petrological segmentation in this region of the east pacific rise and we speculate that the discontinuities may be the morphological response to fluctuations in the spatial pattern of magma delivery

role of pressure gradients in sheet flow of coarse sediments under sawtooth waves 1 discrete particle model dpm simulations have been performed to directly examine the role of horizontal pressure gradients acting on the wave bottom boundary layer wbbl during the process of sheet flow transport in the surf zone the dpm is a two phase wbbl model that directly couples a one dimensional eddy viscosity fluid flow to a three dimensional particle model newton s third law is enforced at every simulation time step through fluid particle interaction forces of buoyancy drag and added mass simulations are able to reproduce bedload transport rates from a laboratory data set for coarse sediment distributions consequently simulations of a monochromatic sawtooth wave were performed with three coarse grain size distributions to investigate the relative importance of bed shear stresses horizontal pressure forces and fluid particle drag forces on bedload transport processes the simulation results demonstrate that the magnitude of the horizontal pressure force acting directly on sediment embedded in the wbbl is small compared to the magnitude of the particle drag force or particle stress however the phasing of the onshore peak in the horizontal pressure force with respect to the particle drag force is important to sediment mobilization and enhances the observed onshore bias in bed load flux removing the horizontal pressure force on the sediment particles resulted on average in a 30 reduction in net bed load flux in the surf zone free stream fluid accelerations are typically equated to the horizontal pressure gradient thus the simulations may explain why the fluid acceleration can be a successful parameter for aiding in sediment transport predictions in the surf zone

variations in subsidence rates along intermediate and fast spreading mid ocean ridges summary the nature of subsidence near the ridge crest of the intermediate and fast spreading mid ocean ridges of the indian and pacific oceans is investigated using surface ship bathymetry and magnetics profiles the ridge can be divided into discrete sections apparently bounded by distinct structural features such as major fracture zones in which bathymetry plotted against crustal age forms a well defined envelope with a width roughly the amplitude of the local bathymetry the averaged bathymetry in all of the regions studied follows closely a square root of age subsidence curve which in most regions has a subsidence coefficient c1 in the range of 340 390 m myr 1 2 the best fitting subsidence curve however never reproduces the amplitude of the axial topographic high the most notable region displaying unusual behaviour is the east pacific rise between 9 s and 22 s in this region the western flank of the ridge is subsiding at 200 225 m myr 1 2 while the eastern flank is subsiding at normal rates of 350 400 m myr 1 2 other anomalous areas include the region between the easter island hot spot and the chile rise triple junction in which the ridge crest is shallow and which is subsiding at rates of about 290 m myr 1 2 and the region east of the australia antarctic discordance in which the northern flank is subsiding at 440 m myr 1 2 this area may also be subsiding asymmetrically although there is not much data from the southern flank the asymmetric subsidence in the 9 s 22 s region of the east pacific rise begins immediately at the ridge crest and the low subsidence rates on the west flank continue to at least 12 myr old crust oligocene aged crust on the western flank is subsiding at more normal rates but is 500 m shallow with respect both to the world wide average and to the conjugate crust on the eastern flank the simplest model to explain these observations is that the western flank is underlain by a hotter mantle perhaps as the result of upwelling resulting from the large scale return circulation from the trenches depending on the depth of compensation the observed asymmetry could result from a lateral temperature gradient of 0 05 0 10 c km 1 and a total lateral temperature variation of under 100 c

breaker turbulence and sediment suspension in the surf zone field measurements of fluid velocities and suspended sediment concentrations were used to investigate the relative role of coherent vortices related to wave breaking in suspended sediment dynamics the measurements were obtained from a barred intermediate type beach and the instrument rig was exposed to a range of wave types during the experiment including shoaling waves and both spilling and plunging breakers the time scale of the observed breaker vortices was 5 hz and thus in the frequency range of wave orbital motion coherent breaker vortices were separated from wave orbital motions and incoherent higher frequency turbulence using a threshold magnitude for the vertical velocity component plunging breakers displayed the largest root mean square rms and maximum vertical velocities the latter reaching approximately 1 m s 1 these measurements were made at an elevation of 0 15 m above the bed given the observed scale of the breaker vortices and the phase match between suspended sediment concentrations and fluid velocities the turbulence injected into the water column by the plunging breakers played a significant role in suspending bed sediment on the other hand the role of coherent vortex structures in suspending sediment under spilling breakers and shoaling waves is more limited rms vertical velocities under the spilling breakers and shoaling waves in this experiment were respectively a factor 2 and 4 smaller than for plunging breakers different phase relationships were observed between the horizontal fluid velocity and vortex induced sediment suspension for each of the wave types which may contribute to an explanation of net offshore and onshore sediment transport observed during episodes of beach erosion and recovery respectively

full scale cfd simulation of tsunamis part 2 boundary layers and bed shear stresses abstract this paper presents results from numerical simulations of the propagation and run up of full scale tsunamis using a reynolds averaged navier stokes model with emphasis on the resulting boundary layers and bed shear stresses spatial distributions of the shields and rouse parameters during run up and draw down show that for the tsunamis considered with a sediment grain size corresponding to medium sand considerable sediment transport shields parameter o 10 o 100 can be expected during run up and that the sediment transport can be expected to be dominated by suspended load the results likewise show that the expected sediment transport during draw down will similarly be considerable and dominated by suspension the tsunami induced boundary layers are monitored in time and the observed boundary layer thickness ranges from spanning only a small fraction of the water depth to spanning the entire depth the velocity profiles beneath the tsunamis are shown to have good correspondence with a logarithmic profile within the boundary layer similarly the bed shear stresses beneath the tsunamis are investigated and a new and simple engineering model is developed for predicting the temporal variation of the bed shear stress based only on a free stream velocity signal the new engineering model is shown give better predictions for the bed shear stress than a standard manning approach and is likewise shown in the appendix to produce reasonable predictions for shorter wind scale waves it is finally also shown how the temporal evolution of the boundary layer thickness can be predicted based on the free stream velocity signal alone the results presented here are part 2 of a larger study where part 1 presents the model validation and detailed descriptions of the run up process

variation in cross sectional area of the axial ridge along the east pacific rise evidence for the magmatic budget of a fast spreading center along the fast and ultrafast spreading east pacific rise the cross sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation using an automated method we measure the ridge area volume per kilometer along axis where there is complete bathymetric coverage along 3500 km of the northern and southern east pacific rise our two study areas encompass numerous transform faults large overlapping spreading centers small overlapping spreading centers and smaller discontinuities first second third and fourth order discontinuities the cross sectional area variation mimics the undulation of the ridge crest depth local area maxima occur toward the middle of segments and the axial area decreases by 40 or more at first and second order discontinuities third order discontinuities are generally marked by smaller disruptions in the ridge area and fourth order offsets do not systematically correspond with features of the cross sectional area profile the correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross sectional area represents a longer term average of the ridge s magmatic state than axial depth a correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low density crust and mantle also a correlation between larger area and higher mgo content of axial basalts suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone the cross sectional area of the axial ridge appears to correlate with the width of the axis centered low velocity zone in the crust these observations as well as the absence of large relict axial ridges off axis indicate that the axial ridge originates from buoyancy due to thermal expansion and the presence of melt in the crust and mantle within about 10 km of the rise axis portions of the northern east pacific rise underlain by a magma chamber reflector generally occur where the ridge cross sectional area is greatest this supports the connection between processes which inflate the axial ridge and those which heat the crust and upper mantle and produce melt thus while the axial high on fast spreading centers resembles a constructional volcano in cross section it is more like a long narrow balloon whose cross sectional area is a sensitive indicator of magma supply using this relationship we predict with 75 confidence that at least 45 of the unsurveyed northern east pacific rise 18 n to 13 n and 9 n to 5 n and at least 60 of the southern east pacific rise 4 s to 23 s is underlain by a magma chamber reflector

reply to a comment on bathymetry gradients of lineated abyssal hills inferring seafloor spreading vectors and a new model for hills formed at ultra fast spreading rates by k a kriner et al earth planet sci lett 242 2006 98 110 by d r bohnenstiehl

effects of horizontal pressure gradients on bed destabilization under waves we report on new experiments designed to investigate bed destabilization processes in a two dimensional wave flume physical model of a beach the mobile bed consists of non cohesive granular material of low density the wave conditions are provided by repeating a cycle of waves made of two bichromatic groups of different period the horizontal and vertical velocities are acoustically profiled vertically from free stream elevation down to the still bed level in the surf zone additional measurements of the fluid pressure at positions closely aligned horizontally and vertically in and slightly above the sediment bed are undertaken mobile bed interfaces still bed and top interface are detected via acoustic and optical methods both methods are cross compared and give similar results flow turbulence over the bed is analysed the reynolds turbulent shear stress is found negligible compared to the orbital flow induced momentum diffusion the shear stress and the horizontal pressure gradient are computed at near bed elevation and used in the bed incipient plug flow model of sleath cont shelf res vol 19 13 1999 pp 1643 1664 both the model and the measurements confirm that destabilization occurs when the non dimensional pressure gradient or sleath number exceeds the threshold value of 0 3 which is simultaneous with strong flow acceleration the near bottom fluid shear stress detected during these flow accelerations at steep wave fronts is found experimentally to be negative which is retrieved with an unsteady plug flow model this is suggesting that the fluid above the bed resists the sediment layer motion at these particular phases

near axis seamounts on the flanks of the east pacific rise 8 n to 17 n with complete seabeam and seamarcii bathymetry and nearly complete seamarcii side scan coverage of the northern east pacific rise and its flanks from 8 to 17 n we characterize the entire population of seamounts on young seafloor 0 to 2 ma formed along this fast spreading center there are 179 seamounts in this area taller than 200 m most of which belong to one of 21 seamount chains these chains are oriented between the relative and absolute plate motion directions of the pacific and cocos plates and most of the larger chains are more voluminous near their centers than near their ends several of the seamount chains contain bends in orientation which are not coincident with each other the two largest seamount chains at 9 55 n lamont seamounts and 15 40 n are aligned with portions of the east pacific rise crest which are among the shallowest and broadest of this area overall the maximum abundance and size of seamounts increase with shallower and broader ridge axes no near axis seamounts form on the summit of the axial ridge nonetheless within 15 km 0 3 ma of the axis the abundance of seamounts achieves its overall average observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near axis seamounts are formed in a narrow zone on crust aged 0 1 0 3 ma 5 15 km from the axis with significant growth in a zone several times wider side scan sonar reflectivity and seamount magnetization analyses are largely consistent with the predictions of this model two exceptions to this model however are seamount chains at 15 40 n and 8 25 n which show evidence for activity over a broader region this may be due to the presence of fracture zones within about 30 km of these chains a compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge the variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results although these influences cannot be distinguished in this analysis

a two and three dimensional analysis of gravity anomalies associated with the east pacific rise at 9 n and 13 n we have used a unique data set collected during the 1985 east pacific rise multichannel seismic experiment to reevaluate the constraints that gravity data can place on the crustal structure of the east pacific rise epr the close spacing of track lines within the two main survey areas 8 45 n 9 55 n and 12 20 13 30 n allowed us to perform three dimensional analyses of high quality gravity data 1 mgal uncertainity obtained with the bell aerospace bgm 3 gravity meter our gravity modeling was enhanced by the availability of high resolution sea beam bathymetric data and by independent structural constraints provided by seismic reflection and refraction data to model the crustal and upper mantle density structure at the ridge axis we first calculated the gravity anomalies due to the density contrasts at the water crust and crust mantle boundaries and the changes in density caused by the cooling of the lithosphere with age and subtracted these predictable signals from the observed free air anomaly the residual anomalies were then used to place constraints on the magnitude and distribution of anomalous mass at the epr our results show that over 90 of the power in the observed free air anomaly can be modeled by these predictable components of the gravity signal the amplitude and wavelength of the small residual anomaly can be modeled by a broad region 20 km wide centered on the rise axis of slightly lower than normal crustal and or upper mantle densities 0 03 mg m 3 this region of anomalous mass and density variations in the underlying mantle provide the principle isostatic support for the axial topographic high the anomalies are consistent with but do not require the presence of a low density crustal magma chamber at the rise axis if a largely molten magma chamber exists then the gravity data require that it be a narrow volumetrieally small body there are first order differences in the distribution of anomalous mass between the two survey areas with the 9 n region generally being characterized by lower densities in the axial region along axis variations in anomalous mass within each of the survey areas correlate with first order changes in the depth and cross sectional shape of the epr axial high we attribute these differences in axial morphology and the amplitude of the associated gravity anomalies to along strike changes in the size of the crustal magma chamber the width of the surrounding zone of cooling crustal rocks and densities in the uppermost mantle beneath the rise axis

measurements of sheet flow transport in acceleration skewed oscillatory flow and comparison with practical formulations near bed oscillatory flows with acceleration skewness are characteristic of steep and breaking waves in shallow water in order to isolate the effects of acceleration skewness on sheet flow sand transport new experiments are carried out in the aberdeen oscillatory flow tunnel the experiments have produced a dataset of net transport rates for full scale oscillatory flows with varying degrees of acceleration skewness and three sand sizes the new data confirm previous research that net transport in acceleration skewed flow is non zero is always in the direction of the largest acceleration and increases with increasing acceleration skewness large transport rates for the fine sand conditions suggest that phase lag effects play an important role in augmenting positive net transport a comparison of the new experimental data with a number of practical sand transport formulations that incorporate acceleration skewness shows that none of the formulations performs well in predicting the measured net transport rates for both the fine and the coarser sands the new experimental data can be used to further develop practical sand transport formulations to better account for acceleration skewness

observations of turbulence within a natural surf zone abstract here the reynolds stresses u w and w where u and w are the cross shore alongshore and vertical turbulence velocities respectively and the angle brackets represent time averaging are used to diagnose turbulence dynamics beneath natural breaking surf zone waves the data were collected at truc vert beach france during a 12 day period in 1 3 m water depth with strong cross shore and alongshore currents under high energy wave conditions offshore significant wave heights ranged between 2 and 8 m the u w term is predominantly negative increases with the ratio of wave height hs to water depth h degree of wave breaking and decreases in magnitude toward the bed this supports the view that the cross shore shear stress is due to breaking induced vortices that transport high speed cross shore flow downward and disintegrate close to the bed the occasional positive sign of u w within the lower 15 20 of the water column indicates that sometimes surface generated turbule

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

free stream velocity descriptions under waves with skewness and asymmetry abstract in shallow water when waves shoal their orbital velocities exhibit skewness velocity skewness and asymmetry acceleration skewness in response to corresponding changes in the free surface near the bed changes in the skewness and asymmetry of the time varying velocities have important consequences for the residual velocities spun up by the bottom friction and for sediment transport in the wave boundary layer where the largest sediment transport often occurs there is a need to represent the free stream velocity the orbital velocity at the edge of the wave boundary layer in a systematic way such that it can be mimicked in wave tunnel experiments and wave boundary layer models which usually do not represent the free surface since there is no readily accessible dynamical theory for shoaling waves that includes both skewness and asymmetry it is necessary to synthesize the shoaling effect in a free stream velocity description that is general enough to fit observed velocity time series this paper reviews previous free stream velocity descriptions including the description of abreu et al 2010 and introduces a new description based on generalising a sawtooth wave shape the paper quantifies all the relevant measures of skewness and asymmetry for each description it then demonstrates that abreu et al s 2010 description represents the most general one and improves upon their fitting techniques to observed free stream velocities to further simplify it when this description is used in conjunction with a characterisation of the velocity for irregular waves such as that of elfrink et al 2006 it is possible to relate velocity skewness and asymmetry directly to the non dimensional wave and beach parameters wave height surface wavelength and surf similarity parameter as a proxy for beach slope matlab scripts are provided for the improved description of abreu et al 2010 and elfrink et al s 2006 characterisation

volcanic rifting at martian grabens 1 a large fraction of surface extension on mars occurred at segmented grabens having width length ratios akin to oceanic rifts on earth association with volcanic landforms such as pit craters clearly suggests interconnection between tectonic and magmatic processes a martian rift evolution model is proposed on the basis of new geomorphological and structural interpretations of imagery high resolution digital elevation models dems scaled experimental modeling and three dimensional boundary element modeling of magmatic and tectonic processes and a comparison with terrestrial rifts the dems were obtained from mars observer laser altimeter viking orbiter stereo images or a combination of both comparison of terrestrial rifts included afar iceland and the east pacific rise the ambient extensional stress field induced by regional body forces is combined at depth with decompression melting and mantle plume thermal anomaly resulting in emplacement of elongated magma reservoirs along the grabens injection of dikes above the reservoirs and flood basalt eruption result in a reservoir underpressurization of up to hundreds of mpa and induces surface collapse each collapse event is associated with an eruption of volcanic volumes akin to those of individual flow eruptions in large terrestrial igneous provinces the geometry and mechanisms of graben formation and surface collapse are described and used to infer reservoir depth and width we conclude from this study that giant dike swarms akin to typical giant terrestrial dike swarms are unlikely to underlie the volcanic martian grabens on the basis one graben one dike rather every volcanic graben segment appears to own a local dike swarm perhaps analogous to the dike swarms in the icelandic fissure zones index terms 5475 planetology solid surface planets tectonics 8149 5480 planetology solid surface planets volcanism 8450 6225 planetology solar system objects mars 8121 tectonophysics dynamics convection currents and mantle plumes 8010 structural geology fractures and faults keywords rifting graben pit crater magma chamber dyke

a new isostatic model for the east pacific rise crest we have developed a new isostatic model for the east pacific rise epr that explains the origin of both the axial topographic high and the rise crest gravity anomaly our model involves a thin elastic plate with a constant thickness crust that is broken at the ridge crest we assume that a buoyant force beneath the plate bends the free edge of the plate upward at the rise axis forming the topographic high and the rise crest gravity anomaly we have used this model and two conventional isostatic models to examine the isostasy of three segments of the epr at 9 l4 n 6 11 s and 16 21 s rather than modeling individual profiles we have stacked summed the gravity and bathymetry data in each area and computed a single 200 km long mean mirrored profile for each different ridge segment these stacked profiles are a remarkable improvement over even the best individual profiles and clearly show both the axial topographic high and its associated free air gravity anomaly in all three areas the best fitting model parameters are similar and give average compensation depths of about 6 7 km below the seafloor near the base of the oceanic crust and an upper limit on the flexural rigidity of the plate that increases from about 1018 n m at the ridge axis to about 3 1019 n m 5 km from the axis and 1020 n m 25 km or more from the rise crest these results indicate that the epr crest is compensated at much shallower depths than suggested by previous investigators our results are consistent with the presence of a small low density magma chamber in the lower crust although at least part of the compensating root for the axial high must extend into the upper mantle we interpret the low upper mantle densities below the rise crest as a zone of partial melt which accumulates at the base of the oceanic crust between episodic rifting events when the overlying crustal magma chamber is replenished

opposing trends in crustal thickness and spreading rate along the back arc eastern lau spreading center implications for controls on ridge morphology faulting and hydrothermal activity abstract along back arc ridges located near the arc volcanic front crustal thickness is observed to vary systematically with arc proximity independent of spreading rate this effect is especially well expressed by the 400 km long eastern lau spreading center elsc where approaching the arc volcanic front from 100 to 40 km crustal thickness nearly doubles 5 5 9 km as spreading rates decrease by more than half 97 39 mm yr the crustal thickness variations at the elsc appear to result primarily from changes in mantle wedge composition caused by subduction we investigated the effects of these large and opposed crustal thickness and spreading rate variations on ridge morphology faulting and hydrothermal activity as part of the first phase of ridge2000 integrated studies in the lau back arc basin we used deep towed side scan sonar instruments dsl120a and imi30 to continuously map the near axis region within broader coverage ship multibeam bathymetry and acoustic imagery swaths an array of optical sensors maprs concurrently surveyed the near bottom water for hydrothermal plume anomalies hydrocasts made at identified plume sites confirmed their hydrothermal origin the data show that as spreading rates increase and crustal thicknesses decrease along the elsc 1 morphology transitions from shallow peaked volcanic highs to a deeper flat axis 2 faults become larger and more widely spaced 3 hydrothermal activity as measured by plume incidence increases and appears to reach levels higher than the global mid ocean ridge trend with spreading rate the observations indicate that crustal thickness magma supply has a greater control on ridge morphology and faulting than spreading rate even at fast rates where the thermal lithosphere should be thin however heat input from mantle advection proportional to spreading rate perhaps in combination with increased fault permeability appears to have a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge although subduction effects give rise to the opposed trends in crustal thickness and spreading rate at the elsc the observed effects may have generic implications for controls of seafloor spreading characteristics at mid ocean ridges

geological chemical and biological evidence for recent volcanism at 17 5 s east pacific rise the superfast spreading portion of the east pacific rise at 17 5 s is a magmatically robust section of mid ocean ridge plumes characterized by high volatile metal ratios which has been hypothesized to be indicative of recent magmatism were found between 17 22 s and 17 35 s in november december 1993 dives with the french submersible nautile in december 1993 observed young sheet flows and widespread venting centered on the segment s shoalest point at 17 26 s eight submersible dives made one year later with the japanese submersible shinkai 6500 in september november 1994 found glassy unsedimented lavas a range of diffuse and high temperature vents and several types of biologic communities on the ridge crest beneath the volatile rich 1993 plumes the diffuse vents range from small areas in the youngest lavas characterized by very low temperature flow 10 c and a relatively low macrofaunal diversity to larger areas where the apparent older surface was covered with a dense and diverse biotope including anemones tubeworms mussels and serpulids several inactive vents marked by dead mussel clam beds are present in the southernmost portion of the site in the older lavas submersible mapping indicates that the youngest lava l0 unit erupted for at least 4 km along the strike of the ridge at the southern site and for 5 10 km at the northern site the flow is up to 400 m in width at the southern site and may be as much as 1 2 km in width at the northern site comparison of the observed 1993 fe s h2s heat and 3he heat plume values with time series measurements at sites on the northern east pacific rise and the juan de fuca ridge where the onset of the magmatic event is known implies that the last magmatic event at 17 5 s occurred within several years prior to 1993 continued rapid cooling of the underlying heat source is implied from the significant diminishment in the light scattering intensity and rise height of the overlying plumes recorded during 1994 submersible dives the type and diversity of the vent biota found on the l0 vents also suggests a young system the spatial and temporal scales of the accretion event at the 17 5 s epr site are similar in scale to eruptions on the intermediate rate spreading north cleft segment juan de fuca ridge but the 17 5 s event has more subtle gradients in the effusion rate of the eruptions and thermal and chemical character of the hydrothermal system

onshore sandbar migration in the surf zone new insights into the wave induced sediment transport mechanisms we present a novel process based morphodynamic model which includes transport processes due to both velocity and acceleration skewness and a new formulation for intrawave motions that successfully simulates observations of onshore sandbar migration results confirm findings of previous studies in which each process was considered separately and in which sediment transport was computed from the observed water motion however our results indicate that accounting for the joint action of both velocity and acceleration skewnesses causes major improvement of the modeled onshore bar migration and is essential to accurately model the evolution of the entire cross shore bottom profile when compared with observations we also demonstrate that the morphodynamics in the shoaling zone are dominated by velocity skewness bed shear stresses while sediment transport induced by acceleration skewness pressure gradients controls the morphodynamics in the inner surf zone

sediment transport in oscillatory boundary layers in cases of rippled beds and sheet flow an experimental study was focused on the process of sediment transport in unsteady flow conditions due to wave action wave induced oscillatory flow conditions near the seabed were simulated at full scale 1 1 in a new large oscillating water tunnel two sets of experiments series a and b were carried out during series a bed forms and wave cycle averaged suspended sediment concentrations were measured under sinusoidal waves series b focused on measurement of the wave cycle averaged sediment transport rates under regular and random asymmetric oscillatory flows upper shoreface conditions and was aimed at the verification of quasi steady formulas for the description of cross shore sediment transport bedform dimensions appeared to decrease considerably under the influence of wave asymmetry and wave randomness only the measured ripple dimensions under asymmetric waves series b showed good agreement with nielsen s 1979 relations for most of the series b experiments the bed was plane sheet flow the net sediment transport was directed onshore and the measured transport rates showed a strong correlation with the velocity moment an empirical quasi steady transport model is proposed which is based on the new tunnel data and other existing data sets the limitations of the quasi steady model approach became clear in the rippled bed regime and through the presence of a consistent influence of the wave period in plane bed conditions in rippled bed conditions the suspended concentration profiles followed a negative exponential distribution for most of the experiments the measured concentration decay length showed a linear relation with the ripple height in plane bed sheet flow conditions the measured suspended concentration profiles followed a negative power function the power or concentration decay parameter was constant 2 1 for a wide range of velocity conditions it is suggested that the mobile bed sheet flow layer has a strong damping effect on the mixing of suspended sediments turbulence damping

unified view of sediment transport by currents and waves i initiation of motion bed roughness and bed load transport attention is given to the properties of sediment beds over the full range of conditions silts to gravel in particular the effect of fine silt on the bed composition and on initiation of motion critical conditions is discussed high quality bed load transport data sets are identified and analyzed showing that the bed load transport in the sand range is related to velocity to power 2 5 the bed load transport is not much affected by particle size the prediction of bed roughness is addressed and the prediction of bed load transport in steady river flow is extended to coastal flow applying an intrawave approach simplified bed load transport formulas are presented which can be used to obtain a quick estimate of bed load transport in river and coastal flows it is shown that the sediment transport of fine silts to coarse sand can be described in a unified model framework using fairly simple expressions the proposed model is fully predictive in the sense that only the basic hydrodynamic parameters dep

hydrothermal methane and manganese variation in the plume over the superfast spreading southern east pacific rise abstract onboard analyses of dissolved ch4 and mn were conducted on the water column along the southern east pacific rise between 13 50 and 18 40 s during the 1993 ridge flux cruise many hydrothermal plumes were identified by chemical anomalies which overlie more than half of the surveyed ridge crest the ch 4 mn ratio in the plumes shows wide variability ranges from about 0 05 to 3 9 due to wide range of ch 4 concentration the ch 4 rich plumes were detected in the region of 16 00 18 40 s where vigorous magmatic budget was confirmed by previous studies moreover the ch 4 mn ratio of the plumes is highly correlated with other volatile metal parameters such as the s fe ratio in plume particulate these results strongly suggest that volatile input caused by magmatic perturbation to hydrothermal systems is responsible for the ch 4 mn variability in the southern epr plumes based on analogy of the case at the epr 9 50 n site the detection of several volatile rich plumes and the extensive hydrothermal flux of volatile species are attributed to frequent magmatic events in the superfast spreading southern epr

a 220 km2 recently erupted lava field on the east pacific rise near lat 8 s seamarc ii records show a 220 km2 area of unusually high reflectivity on the east pacific rise epr near lat 8 s which we interpret to be a recently erupted lava field this is the only segment of the epr exhibiting high reflectivity over such a large area in our continuous seamarc ii coverage from lat 8 n to 18 n and 3 s to 23 s the lavas appear to issue from the axial neovolcanic zone between lat 8 15 and 8 17 s the lava field extends up to 18km off axis and inundates scarps up to 100 m high on the basis of measurements of scarp heights just outside areas where they are buried we estimate the average thickness of the field to be 70 20 m thus the volume of the lava field is 15 4 km3 in comparison the average annual volcanic budget for the earth is approximately 4 5 km3 and the largest historic basaltic eruption from the icelandic laki fissure in 1783 was approximately 12 3 km3 when did the eruption occur comparative studies of acoustic reflectivity and photographed bottom characteristics elsewhere along the epr suggest that the lava field has little or no sediment cover and is very young this is also one of the only areas on the epr where earthquakes occur that do not appear to be caused by transform faulting if the earthquakes are related to the lava field then major eruptions may have occurred in 1964 1965 and 1969 a large helium plume near lat 13 s may be caused in part by eruptions of the lat 8 s lava field this is consistent with estimates of currents at 2 3 km depth which are such that a plume generated at lat 8 s would eventually appear off axis at lat 13 s our observations suggest that this spreading segment which is bounded by small ridge axis discontinuities at lat 8 05 s and 8 47 s may have been the most volcanically active segment along the epr during the past 25 yr

off axis volcanism at the east pacific rise detected by uranium series dating of basalts recent detailed surveys of the east pacific rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast spreading ocean ridge crests1 7 in parallel with geological and geochemical investigations it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium series disequilibria methods8 11 here we use 238u 230th and235u 231pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9 31 n on the east pacific rise most of the ages are younger than would be expected based on off axis distance and spreading rate we infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0 5 2 km outside the axial summit caldera with some volcanism occurring as far as 4 km off axis melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas

distribution of isolated volcanoes on the flanks of the east pacific rise 15 3 s 20 s volcanic constructions not associated with seamount or volcano chains are abundant on the flanks of the east pacific rise epr but are rare along the axial high the distribution of isolated volcanoes based on multibeam bathymetric maps is approximately symmetric about the epr axis this symmetry contrasts with the asymmetries in the distribution of volcano chains more abundant on the west flank the seafloor subsidence rates slower on the west flank and the distribution of plate motion parallel gravity lineaments more prominent on the west flank most of the isolated volcanoes complete their growth within 14 km of the axis on crust younger than 0 2 ma while seamount chain volcanoes continue to be active on older crust volcanic edifices within 6 km of the ridge axis are primarily found adjacent to axial discontinuities suggesting a more sporadic magma supply and stronger lithosphere able to support volcanic constructions near axial discontinuities the volume of isolated near axis volcanoes correlates with ridge axis cross sectional area suggesting a link between the magma budget of the ridge and the eruption of near axis volcanoes within the study area off axis volcanic edifices cover at least 6 of the seafloor and contribute more than 0 2 to the volume of the crust the inferred width of the zone where isolated volcanoes initially form increases with spreading rate for the mid atlantic ridge 4 km northern epr 20 km and southern epr 28 km so that isolated volcanoes form primarily on lithosphere younger than 0 2 ma 4 6 km brittle thickness independent of spreading rate this suggests some form of lithospheric control on the eruption of isolated off axis volcanoes due to brittle thickness increased normal stresses across cracks impeding dike injection or thermal stresses within the newly forming lithosphere

flow tunnel measurements of velocities and sand flux in oscillatory sheet flow for well sorted and graded sands oscillatory flow tunnel measurements of velocities and fluxes for sands in oscillatory sheet flow conditions are presented the experiments involved a range of well sorted and graded sands in two asymmetric flows velocities were measured using an ultrasonic velocity profiler uvp capable of measuring deep within the sheet flow layer velocity profiles are found to be similar for the same flow but different sand beds and display expected features of oscillatory boundary layer flow the near bed velocity leads the main flow velocity by approximately 21 and a small offshore directed current is generated near the bed measures of the boundary layer thickness are in good agreement with those predicted using an equation formulated for the boundary layer thickness over fixed beds velocity data have been combined with concentration data to produce time dependent sand flux profiles covering the sheet flow and suspension regions there are fundamental differences in the transport processes of sands of different size and grading caused by unsteady effects which dominate in the case of fine sand and are largely absent in the case of coarse sand 1 time averaged flux is onshore directed positive in the case of coarse sand and is confined to a region immediately above the bed in contrast time averaged flux in the case of fine sand extends high above the bed is offshore directed negative in the sheet flow layer and becomes onshore directed in the suspension layer 2 net transport in the case of coarse sand is directed onshore as the percentage of fine sand in the bed increases offshore transport becomes increasingly dominant as the percentage of fine sand increases 3 net transport in the suspension layer is onshore while net transport in the sheet flow layer may be onshore or offshore depending on sand size and grading

variations in axial magma lens properties along the east pacific rise 9 30 n 10 00 n from swath 3 d seismic imaging and 1 d waveform inversion we use three dimensional multistreamer seismic reflection data to investigate variations in axial magma lens aml physical properties along the east pacific rise between 9 30 n and 10 00 n using partial offset stacks of p and s converted waves reflecting off the top of the aml we image four 2 4 km long melt rich sections spaced 5 10 km from each other one dimensional waveform inversion indicates that the aml in a melt rich section is best modeled with a low vp 2 95 3 23 km s and vs 0 3 1 5 km s indicating 70 melt fraction in contrast the aml in a melt poor section requires higher vp 4 52 4 82 km s and vs 2 0 3 0 km s which indicates 40 melt fraction the thicknesses of the aml are constrained to be 8 32 m and 8 120 m at the melt rich and poor sites respectively based on the aml melt mush segmentation imaged in the area around the 2005 2006 eruption we infer that the main source of this eruption was a 5 km long section of the aml between 9 48 n and 51 n the eruption drained most of the melt in this section of the aml leaving behind a large fraction of connected crystals we estimate that during the 2005 2006 eruption a total magma volume of 9 83 106 m3 was extracted from the aml with a maximum of 71 106 m3 left unerupted in the crust as dikes from this we conclude that an eruption of similar dimensions to the 2005 2006 one would be needed with a frequency of years to decades in order to sustain the long term average seafloor spreading rate at this location

geophysical study of the east pacific rise 15 n 17 n an unusually robust segment bathymetric side scan sonar magnetic and gravity data from the east pacific rise epr between 15 and 17 n are used to establish the spreading history and examine melt delivery to an unusually robust spreading segment the axial ridge between the orozco transform fault 15 30 n and the 16 20 n overlapping spreading center osc has an average elevation of 2300 m which is 300 m shallower than typical epr depths and its cross sectional area is double the average value for the northern epr the total opening rate is 86 km myr but the inflated segment appears to have spread faster to the east by more than 20 since 0 78 ma the orientation of magnetic isochrons and lineaments in the side scan sonar indicates a 3 counterclockwise rotation of the spreading direction since 1 8 ma c2 and reflects a change in the pacific cocos plate motion the side scan lineaments also show that the percentage of inward facing faults 83 and the spacing between faults 1 5 km are consistent with the spreading rate dependence shown by carbotte and macdonald 1994 however the mean fault length 4 8 km is 1 5 km shorter than expected for the spreading rate and suggests that extensive off axis volcanism has draped the faults gravity analysis shows that the inflated segment has a 12 mgal bull s eye shaped low in residual mantle bouguer anomaly we offer several possible end member models for the anomaly including a prism of 10 partial melt in the mantle and lower crust or a crustal thickness anomaly of 2 25 km kinematic modeling that is based on structure and magnetic data suggests that two large magmatic pulses occurred at approximately 0 8 ma and 0 3 ma and have reshaped the plate boundary geometry and inflated the segment

shear stress and sediment transport calculations for swash zone modelling it is shown that the observed difference in sediment transporting efficiency by the swash uprush compared with the downrush could be mainly due to greater bed shear stress for a given velocity in the more abruptly accelerated uprush the bed shear stress generated by an arbitrary free stream velocity time series is modelled in terms of usual wave boundary layer models plus a phase lead phi tau of the bed shear stress compared with the free stream velocity at the peak frequency with this approach the total transport amounts in uprush and downrush can be modelled satisfactorily with the same sediment transport formula without the need for different uprush and downrush coefficients while the adaptation of sediment transport formulae from steady flow can thus lead to the right total amounts of sediment moved by this method the timing of the instantaneous sediment transport rates are probably not accurately modelled due to the highly unsteady nature of the swash and the presence of pre suspended sediment in the uprush nevertheless the proposed method is a useful intermediate step before we have a complete understanding of sediment transport under very rapid accelerations and of the relative contribution of pre suspended sediment to the onshore sediment transport in swash zones c 2002 published by elsevier science b v

data analysis of bed concentration of suspended sediment an empirical relation for the bed concentration of suspended sediment cb defined at a few grain diameters above the bed is developed its derivation is based on the experimental data sets of guy et al in 1966 the values of the near bed concentration were obtained by first isolating the suspended load transport from the observed rates of total load transport by use of a bed load formula the suspended load was then equated to einstein s formulation which allowed the determination of cb the influence of large concentrations of suspended sediment and of the reference level for cb on the calculated values of bed concentration has been analyzed the proposed relation compares satisfactorily to values of near bed concentration obtained from independent data sets the procedure used to derive cb provides a simple method for the calculation of the total transport rate of noncohesive sediment

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b

mantle wedge control on back arc crustal accretion at mid ocean ridges plate separation leads to upward advection and pressure release partial melting of fertile mantle material the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away in back arc basins the subducting slab is an important control on the pattern of mantle advection and melt extraction as well as on compositional and fluid gradients modelling studies predict significant mantle wedge effects on back arc spreading processes here we show that various spreading centres in the lau back arc basin exhibit enhanced diminished or normal magma supply which correlates with distance from the arc volcanic front but not with spreading rate to explain this correlation we propose that depleted upper mantle material generated by melt extraction in the mantle wedge is overturned and re introduced beneath the back arc basin by subduction induced corner flow the spreading centres experience enhanced melt delivery near the volcanic front diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away our model explains fundamental differences in crustal accretion variables between back arc and mid ocean settings