a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

mineralogical characterization of low grade nickel laterites from the north oman mountains using mineral liberation analyses scanning electron microscopy based automated quantitative mineralogy abstract afterwards to the late cretaceous obduction of the tethyan semail ophiolite suite intense chemical weathering involved the oman mountains region this weathering produced complex mineral and chemical assemblages that led to the formation of a ni lateritic paleosol in the present study five laterite sections were sampled and studied by using a quantitative mineralogical and petrographic approach mineral liberation analyses mla were carried out for the first time for several selected samples the mla results supported by the electron probe micro analyzer epma and energy dispersive x ray spectroscopy edx analyses revealed that the main ore minerals of the analyzed laterite are chlorite and goethite as the main ni bearing hosts magnetite and chromite are also present as by product minerals the ni bearing chlorite in the saqah 3 and al russayl profiles have coarser grains compared to those in the other saqah localities while the goethite grains are coarse in most of the analyzed samples the presence of approximately 60 to 80 of both the ni bearing minerals chlorite and goethite as liberated phases 95 free is a potential indicator for their future exploration chromite and magnetite grains are present in a wide range of grain sizes seventy percent of the chromite grains in saqah 4 and al russayl profiles and 70 of the magnetite in saqah 2 occur as liberated phases 95 compared to their presence in other profiles where only half of them occur as liberated phases the grain size liberation class relationships showed that ni bearing mineral phases chlorite and goethite are present as liberated phases high grade materials followed by chromite and magnetite which are interlocked with other phases to lower grade materials a better understanding of the mineralogical and geochemical quantitative characterization such as grain size modal analyses mineral association and percentage of interlock between minerals in the deposit can avoid the metallurgical problems thus possibly reducing the operational costs and improving the recovery of metals the above obtained mla parameters can build a good feasibility study during the exploration stage of these low grade ni deposits in the northern oman mountains as well as later during their beneficiation and processing methods if other economic and mining factors are deemed favorable

dynamics of the mid ocean ridge plate boundary recent observations and theory the global mid ocean ridge system is one of the most active plate boundaries on the earth and understanding the dynamic processes at this plate boundary is one of the most important problems in geodynamics in this paper i present recent results of several aspects of mid ocean ridge studies concerning the dynamics of oceanic lithosphere at these diverging plate boundaries i show that the observed rift valley to no rift valley transition globally due to the increase of spreading rate or locally due to the crustal thickness variations and or thermal anomalies can be explained by the strong temperature dependence of the power law rheology of the oceanic lithosphere and most importantly by the difference in the rheological behavior of the oceanic crust from the underlying mantle the effect of this weaker lower crust on ridge dynamics is mainly influenced by spreading rate and crustal thickness variations the accumulated strain pattern from a recently developed lens model based on recent seismic observations was proposed as an appealing mechanism for the observed gabbro layering sequence in the oman ophiolite it is now known that the mid ocean ridges at all spreading rates are offset into individual spreading segments by both transform and nontransform discontinuities the tectonics of ridge segmentation are also spreading rate dependent the slow spreading mid atlantic ridge is characterized by distinct bulls eye shaped gravity lows suggesting large along axis variations in melt production and crustal thickness whereas the fast spreading east pacific rise is associated with much smaller along axis variations these spreading rate dependent changes have been attributed to a fundamental differences in ridge segmentation mechanisms and mantle upwelling at mid ocean ridges the mantle upwelling may be intrinsically plume like 3 d beneath a slow spreading ridge but more sheet like 2 d beneath a fast spreading ridge

extreme waves in shallow and intermediate water depths this paper concerns the description of large transient waves in shallow and intermediate water depths it builds upon recent advances in the description of deep water waves and provides the first quantitative comparisons with a new kinematic model which has hitherto only been validated in deep water conditions a new series of experimental observations is presented in which the wave components within a random or irregular sea state are focused to produce a large transient wave group comparisons with both linear and second order solutions suggest that non linear wave wave interactions produce a steeper wave envelope in which the central wave crest is higher and narrower while the adjacent wave troughs are broader and less deep spectral analysis of the measured water surface elevation suggest that in addition to the development of significant long wave energy there is also a transfer of energy to the shorter high frequency harmonics this energy redistribution has a significant effect upon the underlying kinematics in particular the near bed velocities are shown to be in good agreement with the second order solution which includes the long wave or frequency difference terms first identified by longuet higgins and stewart furthermore the near surface kinematics are shown to be highly non linear indeed a linear wave theory based upon a correct representation of the freely propagating linear harmonics is generally found to under estimate the near surface horizontal velocity however if the linear calculations are based upon a measured wave spectrum the kinematic predictions suffer from high frequency contamination and require empirical correction in contrast the non linear numerical model presented by baldock and swan provides a good description of the measured kinematics over the entire water depth

density changes in upwelling mantle abstract gravity modelling of the litho asthenosphere system in regions involved in extensional tectonics requires an accurate estimation of the vertical density profile in the upwelling asthenosphere to this end the calculation of the variation of several parameters is needed i e the degree of thermal anomaly in the asthenosphere the amount of stretching the thickness of the lithosphere before stretching and the variation of melt fraction generated by adiabatic decompression therefore knowledge of melt density and of the residual mantle rock composition is necessary in order to have the information to calculate the density of the uprising mantle these calculations allow to partially solve the problem related to ambiguity of the gravity interpretation and to provide information about the structural features of the upwelling mantle

the structure of three dimensional convection beneath oceanic spreading centres summary buoyant flow beneath oceanic spreading centres can be driven by density gradients due to thermal expansion and compositional gradients arising from the extraction of melt numerical experiments are used to examine the pattern of mantle flow and melting produced by these two sources of buoyancy compositional buoyancy leads to three dimensional flow beneath the spreading axis with along axis variations in upwelling and melt production thermal buoyancy drives axis parallel circulation and convective rolls aligned perpendicular to the spreading axis that originate at a distance off axis controlled by the spreading rate and mantle viscosity hie distance from the axis to the onset of observed sea floor gravity lineations may show a similar dependence on spreading rate compositional density stratification suppresses the formation of thermally driven rolls near the axis and when it is large causes the thermal rolls to become unstable leading to time dependent behaviour the amount of crust produced is less sensitive to spreading rate than in passive flow models however it has a maximum at intermediate spreading rates due to the presence of thermally driven upwelling in the melting region along axis variations in upwelling and melt production suggest that buoyant flow may have an important effect on the segmentation of spreading centres particularly at slow spreading rates

some theoretical predictions on the relationships among spreading rate mantle temperature and crustal thickness a series of numerical experiments on mantle flow and melting predict a positive relation between mantle temperature and crustal thickness the models also demonstrate that crust formed at slow spreading rates is more sensitive to variations in mantle temperature than crust formed at fast rates so that the range of calculated thicknesses is much greater for crust formed at slower rates an instantaneous mantle temperature increase results in a transient pulse of melt production that is also more pronounced at slower spreading rates the predicted behavior is caused by the interplay between mantle flow driven by plate separation and that driven by thermal compositional and melt related buoyancy a temperature increase results in a decrease in mantle viscosity and an increase in the depth at which melting begins a lower viscosity leads to stronger buoyancy driven flow that carries more mantle to shallow depths below the ridge thermal buoyancy effects which may result in cooling and mixing of depleted and undepleted material under the ridge appear to be of greater importance at slower spreading rates the steady state results are broadly consistent with global compilations of oceanic crustal thickness that show larger variations in crustal thickness at slower spreading rates than at faster rates thicknesses estimated from seismic refraction data from crust formed within a single segment of the mid atlantic ridge but at different spreading rates 1 0 to 1 9 cm yr are consistent with but do not prove the model results the transient pulse of melt production associated with a rapid increase in mantle temperature might occur when a ridge becomes proximal to a hot spot

morphology and tectonics of the kane transform from sea beam bathymetry data high resolution sea beam data have been used to construct an accurate regional scale bathymetric map of the kane transform a large offset slowly slipping 30 mm yr central north atlantic transform the kane transform trends about 100 nearly orthogonal to the regional trend of the adjacent mid atlantic ridge mar rift valley the transform valley walls are composed of an alternating series of ridges and troughs which parallel the regional trend of the mar the ridges narrow and deepen toward the transform valley over distances of 10 20 km and the intervening troughs widen into distinct closed contour depressions that appear to be relict nodal basins the morphologic elements found on either side of the transform valley exhibit along strike continuity and represent undisrupted displacement markers the swath of terrain sandwiched between these markers is only several kilometers wide and is made up of elongate basins and ridges that integrate to create a distinctive transform parallel grain this interval the transform tectonized zone ttz marks the time averaged location of strike slip tectonism as parcels of crust move from each intersection along the transform interface the ttz is widest 6 km near the eastern ridge transform intersection rti and narrows to a strand less than 3 km wide along the central and western portions of the transform along the eastern portion of the transform valley a narrow 3 km median ridge several hundred meters high can be traced for more than 40 km along the valley floor and a similar but shorter feature is also present near the western rti at both rtis the neovolcanic zone from the adjacent rift valley can be traced as a series of constructional edifices across the entire intersection area to the opposing transform valley wall at the eastern rti the neovolcanic zone wraps around the eastern edge of the nodal basin while at the western rti it appears to bisect a paired nodal basin located in the intersection area a line drawn connecting the ends of each neovolcanic zone lies within the narrow ttz and has a strike of 098 using the bathymetric map of the kane transform as a record of the time integrated tectonic evolution of this large slowly slipping ridge transform ridge plate boundary we have constructed a simple model for the evolution of rtis our model emphasizes the interplay between temporal variations in crustal production rates i e variations in the volume of basaltic melt production along oceanic spreading centers and extensional tectonism this model can explain the existence of nodal basins at rtis and relict nodal basins along transforms and fracture zones as well as the occurrence of regularly spaced ridge and trough topography on the ridge flanks

field measurements of rogue water waves abstractthis paper concerns the collation quality control and analysis of single point field measurements from fixed sensors mounted on offshore platforms in total the quality controlled database contains 122 million individual waves of which 3649 are rogue waves geographically the majority of the field measurements were recorded in the north sea with supplementary data from the gulf of mexico the south china sea and the north west shelf of australia the significant wave height ranged from 0 12 to 15 4 m the peak period ranged from 1 to 24 7 s the maximum crest height was 18 5 m and the maximum recorded wave height was 25 5 m this paper will describe the offshore installations instrumentation and the strict quality control procedure employed to ensure a reliable dataset an examination of sea state parameters environmental conditions and local characteristics is performed to gain an insight into the behavior of rogue waves evidence is provided to demonstrate that rogue waves are not go

an experimental study on wave forces on a vertical cylinder due to spilling breaking and near breaking wave groups abstract this paper highlights the importance of spilling breaking waves in the design of offshore structures although wave loading on offshore structures due to non breaking waves has been extensively studied wave loading due to breaking waves is uncertain and not very well understood plunging breaking waves in deep water are very unusual whereas spilling breaking waves are very common in extreme seas nevertheless no significant research efforts have been made to study the effects of spilling breaking waves the present study addresses this comparing the wave loading from highly nonlinear non breaking waves and spilling breaking waves focused wave groups have been used to generate the non breaking and spilling breaking wave conditions in a wave tank the experiments have shown that spilling breaking waves generate significantly larger forces on the cylindrical model than highly non linear non breaking waves of equivalent size this is shown to be due to the direct excitation at high frequency as well as the resonant and impact excitation of higher natural frequency modes it is concluded that spilling breaking waves can generate very significant forces and should therefore be considered in the design of offshore structures

three dimensional gravity and bathymetry analysis of the mid atlantic ridge between 20 n and 24 n flow geometry and temporal evolution of the segmentation bathymetry and gravity data covering 10 m y on both flanks of the mid atlantic ridge south of the kane fracture zone reveal a complex segmentation pattern well correlated with short scale temporal variations in crustal production segment and paleosegment centers are associated with crustal thickening while their extremities appear as elongated bands of thin crust subsidence rates vary between centers and extremities of a segment with higher than world average values at segmert midpoints and closer to average values at the edges this is consistent with segment centers being the site of focused mantle upwellings with associated thin lithosphere and with thick crust a pronounced circular minimum in the long wavelength of the mantle bouguer anomaly associated to a maximum in the long wavelength of the bathymetry marks the site of a mantle upwelling associated with a particularly robust segment located north of a small fracture zone 21 20 n an offset is often observed between extreme values of topography and crustal thickness resulting in a lack of isostatic equilibrium for off axis short wavelength features an obliquity between maxima of crustal thickening associated with active segments and the topographic axial domain suggests that the shift between topography and crustal structure may arise partly from a decoupling between crust and topography formation processes

a characteristics based method for solving the transport equation and its application to the process of mantle differentiation and continental root growth summary purely advective transport of composition is of major importance in the geosciences and efficient and accurate solution methods are needed a characteristics based method is used to solve the transport equation we employ a new hybrid interpolation scheme which allows for the tuning of stability and accuracy through a threshold parameter eth stability is established by bilinear interpolations and bicubic splines are used to maintain accuracy with this scheme numerical instabilities can be suppressed by allowing numerical diffusion to work in time and locally in space the scheme can be applied efficiently for preliminary modelling purposes this can be followed by detailed high resolution experiments first the principal effects of this hybrid interpolation method are illustrated and some tests are presented for numerical solutions of the transport equation second we illustrate that this approach works successfully for a previously developed continental evolution model for the convecting upper mantle in this model the transport equation contains a source term which describes the melt production in pressure released partial melting in this model a characteristic phenomenon of small scale melting diapirs is observed de smet et al 1998 de smet et al 1999 high resolution experiments with grid cells down to 700 m horizontally and 515 m vertically result in highly detailed observations of the diapiric melting phenomenon

solid tidal friction above a liquid water reservoir as the origin of the south pole hotspot on enceladus abstract earth jupiter s moon io and saturn s tiny moon enceladus are the only solid objects in the solar system to be sufficiently geologically active for their internal heat to be detected by remote sensing interestingly the endogenic activity on enceladus is only located on a specific region at the south pole from which jets of water vapor and ice particles have been observed spencer j r and 9 colleagues 2006 science 311 1401 1405 porco c c and 24 colleagues 2006 science 311 1393 1401 the current polar location of the thermal anomaly can possibly be explained by diapir induced reorientation of the satellite nimmo f pappalardo r t 2006 nature 441 614 616 but the thermal anomaly triggering and the heat power required to sustain it over geological timescales remain problematic using a three dimensional viscoelastic numerical model simulating the response of enceladus to tidal forcing we explore the effect of a low viscosity anomaly in the ice shell localized to the south polar region on the tidal dissipation patterns we demonstrate that only interior models with a liquid water layer at depth can explain the observed magnitude of dissipation rate and its particular location at the south pole moreover we show that tidally induced heat must be generated over a relatively broad region in the southern hemisphere and it is then transferred toward the south pole where it is episodically released during relatively short resurfacing events as large tidal dissipation and internal melting cannot be induced in the south polar region in the absence of a pre existing liquid decoupling layer we propose that liquid water must have been present in the interior for a very long period of time and possibly since the satellite formation owing to the orbital equilibrium requirement meyer j wisdom j 2007 icarus 188 535 539 sustaining some liquid water at depth is impossible if heat is continuously emitted at a rate of 4 8 gw at the south pole based on that requirement we propose that the current thermal emission is not in equilibrium with the heat production and that the thermal emission rate is abnormally high at present time alternatively continuous dissipation at a rate of 1 2 gw in the ice shell at the south pole should be sufficient to induce internal melting and it could sustain a layer of liquid water at depth over geologic timescales

shear attenuation beneath the juan de fuca plate implications for mantle flow and dehydration abstract at mid ocean ridge spreading centers the separation of lithospheric plates induces upwelling and pressure release melting in the upper mantle many questions remain about the associated flow pattern melt distribution and the interaction of spreading centers with hotspots based on the propagation of rayleigh waves across arrays of ocean bottom seismometers we present shear wave attenuation and velocity models in the vicinity of the juan de fuca plate with the best resolution to date of any spreading center there is strong attenuation centered at depths of 70 80 km just below the expected dry solidus the top of the attenuating region is somewhat deeper than predicted for the thermal effects of passive mantle upwelling beneath the spreading center the shear velocity structure shows the lowest velocities west of the spreading center particularly near axial seamount and high velocities east of the axis extending to a greater depth than predicted by the passive flow model together these observations support a model in which buoyant upwelling and melt extraction west of the spreading center first depletes and dehydrates the mantle above the dry solidus and then the associated downwelling carries depleted melt free residual mantle downward beneath the juan de fuca plate the shear velocities and attenuation can be well matched with a model in which the dominant changes in the physical properties occur just before the onset of melting

constraints on a buoyant model for the formation of the axial topographic high on the east pacific rise bathymetry and gravity data from four geophysical surveys along the 15 19 s superfast spreading section of the east pacific rise are used to study formation of the axial topographic high after removing a best fitting thermal model the averaged residual axial topography is 265 m high and 15 km wide the residual gravity shows no evidence for shallow isostatic compensation of this topography suggesting deep rooted compensation or dynamic support previous models postulate that the axial high is compensated by low densities in a zone of partial melting within the upper mantle we model the physical properties of the lithosphere and asthenosphere required to match the residual gravity and bathymetry using a nonlinear inversion assuming the high is uplifted by buoyancy forces from low density mantle deep rooted buoyancy forces cannot create a narrow axial high unless they occur in a low viscosity conduit surrounded by high viscosity mantle this high viscosity region could be created by depletion and extraction of all water during the melting process we find that the viscosity of the asthenosphere beneath the melt conduit must also be lower than the viscosity of the residual mantle surrounding the conduit our most reasonable model has a viscosity of 1020 pa s in the residual mantle surrounding a melt conduit which extends 50 km below the seafloor and a viscosity of 1016 6 pa s in the asthenosphere below the melt conduit the viscosity of the melt conduit must be about 1015 pa s or about 105 times lower than the surrounding residual mantle with a density contrast equivalent to the retention of a small fraction of melt 5 because higher melt concentrations are probably required to create even small changes in viscosity and the viscosity of the undepleted mantle is probably higher than 1016 6 pa s we conclude that it is unlikely that the axial topographic high is formed by buoyancy forces in the crust and mantle

scaling wave impact pressures on vertical walls it is widely recognized that the use of froude similarity for scaling up wave impact pressures recorded during physical model tests may lead to over estimation of impact maxima based on reviewing historical work dating back to the 30s and further developments in the 60s and 80s a general method is presented that is suitable for scaling up impact pressures and rise times measured during small scale physical model tests the method accounts for the effect of air leakage and is applicable to most wave impact loads the model is applied to scale wave impact pressures on vertical walls and similar structures and consistent correction factors for the froude scaling law are derived

crustal structure of an extinct rift axis in the labrador sea preliminary results from a seismic refraction survey abstract a preliminary crustal structure for the extinct rift axis in the labrador sea is presented based on a one dimensional analysis of ocean bottom seismometer obs refraction profiles within the extinct rift low p wave velocities are observed for crustal 3 6 6 3 km s and upper mantle 7 3 8 1 km s refractors the velocity depth structures for obss sampling crust within the extinct rift suggest a marked thinning and variability in crustal thickness from 4 7 km to 2 6 km in comparison to typical thicknesses of 6 4 km observed in the adjacent flanks the crustal structures at the extinct rift resemble those observed at the active and inactive transforms of oceanic fracture zones following arguments for crustal thinning at fracture zones we speculate that the first order thinning of the lower crust is due to a reduction in the supply of partial melt to the accretionary axis at the extinct rift the reduced supply is attributed to changes in the thermal field and spreading rate as rifting ceased this is consistent with thermal models which predict that crustal thickness decreases appreciably for spreading rates less than 20 mm yr when plate spreading predominates over buoyancy in driving mantle upwelling

statistics of breaking waves observed as whitecaps in the open sea abstract conventional observations of waves carried out with a buoy in open sea conditions were supplemented with simultaneous visual observations of whitecaps to identify breaking events in the buoy records a statistical wave by wave analysis of these records indicates that such seemingly obvious parameters as wave steepness or wave asymmetry cannot be used to separate breakers from nonbreakers and the breaking occurs at wave steepness values much less than the theoretically expected steepness of a limiting wave the observed fraction of breaking waves varied from about 0 10 to about 0 16 depending on wind speed two thirds of the breaking waves were breaking in one third of the wave groups for which a hrms threshold definition was used

rapid transition from continental breakup to igneous oceanic crust in the south china sea continental breakup represents the successful process of rifting and thinning of the continental lithosphere leading to plate rupture and initiation of oceanic crust formation magmatism during breakup seems to follow a path of either excessive transient magmatism magma rich margins or of igneous starvation magma poor margins the latter type is characterized by extreme continental lithospheric extension and mantle exhumation prior to igneous oceanic crust formation discovery of magma poor margins has raised fundamental questions about the onset of ocean floor type magmatism and has guided interpretation of seismic data across many rifted margins including the highly extended northern south china sea margin here we report international ocean discovery program drilling data from the northern south china sea margin testing the magma poor margin model outside the north atlantic contrary to expectations results show initiation of mid ocean ridge basalt type magmatism during breakup with a narrow and rapid transition into igneous oceanic crust coring and seismic data suggest that fast lithospheric extension without mantle exhumation generated a margin structure between the two endmembers asthenospheric upwelling yielding mid ocean ridge basalt type magmatism from normal temperature mantle during final breakup is interpreted to reflect rapid rifting within thin pre rift lithosphere

buoyancy effects on mantle flow under mid ocean ridges results of a series of two dimensional numerical experiments of mantle flow melting and melt migration under a spreading center are reported the model predicts the distribution of melt in the subridge mantle the width over which most melt is delivered to the crust and the thickness of crust the sources of buoyancy considered are thermal expansion compositional variation caused by melt extraction and the phase change of solid to melt we infer that the steady state average viscosity of the mantle below a ridge cannot be much less than about 1019 pa s for a lower average viscosity thermal convection causes rapid cooling of a large region under a ridge raising the viscosity results imply that transient increases in mantle temperature should lead to larger increases in the oceanic crustal thickness for slow spreading ridges than for fast spreading ridges we assume that the viscosity is proportional to exp 0 where is melt fraction we parameterize the permeability in terms of the reference velocity for percolation of the melt vr where the relative velocity of melt to solid is vr analytic approximations are used to extrapolate the model results to large values of permeability and small values of 0 if vr is less than 1 m yr then more melt would be retained in the subridge mantle than is estimated from analysis of topography and gravity data at fast spreading centers for vr greater than about 100 m yr so little melt would be retained in the mantle that it is difficult to explain the gravity data and the low shear wave velocity structure close to the east pacific rise estimated from seismic surface waves buoyancy effects can lead to a region of mantle upwelling and melting that is as narrow as the observed zone of oceanic crustal accretion for most melt to be added to the crust within a few kilometers of a fast spreading center requires that 0 be less than 0 015 if vr equals 1 m yr and less than 0 003 if vr is 100 m yr

buoyant decompression melting a possible mechanism for intraplate volcanism 1 volcanic activity away from plate boundaries occurs in a variety of settings linear age progressive volcanic chains have been explained as the manifestation of fixed hot spots possibly generated by plumes originating deep in the mantle however important examples of oceanic intraplate volcanism cannot be explained in this way including short lived chains and chains violating the predicted age distance behavior furthermore a significant fraction of volcanism does not occur in linear chains and observations suggest control by lithosphere structure in many cases in this study melting in mantle upwellings that result from the buoyancy associated with the melting itself is considered as a mechanism of intraplate volcanism numerical models of this buoyant decompression melting in a layer that is initially at its melting temperature over some depth range are used to determine the duration and amount of melting in upwellings which organize from an initially small random perturbation for a range of model parameters a predicted inverse correlation between the amount of melt produced and the duration of melting may be diagnostic of the buoyant melting process buoyant melting could occur in a number of geologic settings since the oceanic upper mantle may have previously melted beneath a spreading center and subsequently cooled spontaneous buoyant decompression melting may be possible only in regions where a large scale mantle upwelling can counteract conductive cooling keeping the mantle at its solidus temperature over some depth range where the mantle is at or sufficiently near its solidus buoyant decompression may spontaneously generate convective storms of melting this may explain the abundance of volcanism associated with the south pacific superswell where mantle is slightly cooler than its melting temperature buoyant melting may not occur spontaneously but may be triggered by some initial upwelling this may provide a physical explanation for volcanism that is controlled by lithosphere structure

crustal magnetization and accretion at the southwest indian ridge near the atlantis ii fracture zone 0 25 ma 1 we analyze geophysical data that extend from 0 to 25 myr old seafloor on both flanks of the southwest indian ridge swir lineated marine magnetic anomalies are consistent and identifiable within the study area even over seafloor lacking a basaltic upper crust the full spreading rate of 14 km myr has remained nearly constant since at least 20 ma but crustal accretion has been highly asymmetric with half rates of 8 5 and 5 5 km myr on the antarctic and african flanks respectively this asymmetry may be unique to a 400 km wide corridor between large offset fracture zones of the swir in contrast to the mid atlantic ridge crustal magnetization amplitudes correlate directly with seafloor topography along the present day rift valleys this pattern appears to be primarily a function of along axis variations in crustal thickness rather than magnetic mineralogy off axis magnetization amplitudes at paleo segment ends are more positive than at paleo segment midpoints suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle alteration of the magnetic source layer at paleo segment midpoints reduces magnetization amplitudes by 70 80 within 20 myr of accretion magnetic and ocean drilling program odp hole 735b data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization as the crust ages the lower crust becomes increasingly important in preserving marine magnetic stripes

three dimensional mantle convection beneath a segmented spreading center implications for along axis variations in crustal thickness and gravity segmentation and along axis variations within individual segments indicate the inherently three dimensional nature of mantle up welling and melting beneath oceanic spreading centers numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center the experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher viscosity mantle half space a periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt the relative magnitudes of the buoyant and plate driven components of mantle flow are controlled by the spreading rate and mantle viscosity with buoyant flow more important at lower spreading rates and viscosities buoyant flow beneath the spreading axis amplifies along axis variations in upwelling near a ridge transform intersection and distributes the variations along the entire spreading axis buoyant flow may thus be responsible for the more three dimensional character of slow spreading centers away from the spreading axis thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along axis wavelength controlled by the prescribed thickness of the asthenosphere however the position and stability of rolls are influenced by the segmentation geometry in cases where the spreading center geometry does not allow a stable configuration of rolls the flow is time dependent along axis variations in upwelling cause variations in melt production which imply large variations in crustal thickness that dominate the surface gravity signal the crustal thickness distributions implied by these numerical experiments produce bulls eye shaped negative mantle bouguer anomalies centered over spreading segments as observed at several spreading centers the amplitude of the anomaly increases with decreasing spreading rate

dynamical consequences of compositional and thermal density stratification beneath spreading centers extraction of partial melt to form oceanic crust reduces the density of upwelling mantle beneath spreading centers this compositional buoyancy can significantly affect both the rate of upwelling and the pattern of mantle flow the relative importance of buoyant upwelling and upwelling due to plate spreading depends on the mantle viscosity for a low enough mantle viscosity buoyant upwelling can significantly enhance the thickness of the crust relative to that which would be produced by plate spreading alone for a mantle viscosity of 1022 pa s upwelling due to plate spreading is dominant and crustal thickness is predicted to be a strong function of spreading rate however for a mantle viscosity of 1019 pa s buoyancy dominates mantle upwelling and the calculated crustal thickness like the oceanic crust is nearly independent of spreading rate thermal and compositional density variations result in opposing buoyancy forces that can cause time dependent upwelling for a mantle viscosity of 1020 pa s and a low 1 cm yr spreading rate this behavior takes the form of undamped crustal thickness oscillations with a single well defined period for even slightly lower mantle viscosity the frequency increases and crustal thickness oscillations become more complex with two or more frequencies present while numerical experiments have thus far been carried out only for viscosities significantly greater than that likely to characterize the mantle beneath spreading centers we speculate that these crustal thickness oscillations may explain important aspects of seafloor structure

the role of mantle depletion and melt retention buoyancy in spreading center segmentation abstract numerical experiments are used to examine the structure of mantle flow beneath the axis of a spreading center buoyancy results from the depletion of residual mantle in iron relative to magnesium as melt is extracted mantle depletion buoyancy and from the presence of low density melt melt retention buoyancy 3 d buoyant mantle flow arises spontaneously from an initially 2 d solution for low mantle viscosity and low spreading rate at high viscosity and high spreading rate initially 2 d solutions remain 2 d this may explain the fundamentally different structure of fast and slow spreading centers in a uniform viscosity halfspace the along axis wavelength of 3 d buoyant upwelling scales with the maximum depth of melting the only length scale in this system for reasonable maximum depths of melting 60 90 km along axis wavelengths of 200 300 km are preferred longer than the 50 100 km segmentation length of slow spreading centers in a viscosity layered halfspace the thickness of the asthenosphere introduces another length scale a wavelength of segmentation comparable to the asthenosphere thickness also develops in our numerical experiments this suggests the possibility that a wavelength corresponding to the spacing between gravity lows may be controlled by the asthenosphere thickness while the spacing of major fracture zones corresponds to the longer wavelength 3 times the maximum depth of melting intrinsic to the melting region the along axis structure of 3 d flow varies from narrow focused upwellings at low spreading rates and mantle viscosities to broad regions of upwelling at high spreading rates and mantle viscosity to allow an along axis crustal thickness variation no larger than that which is observed 3 4 km the highly focused upwelling and crustal production predicted at slow spreading rates requires appreciable along axis transport of melt

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

an alternative dynamic model of the axial topographic high at fast spreading ridges recent models that attempt to explain the formation of an axial topographic high at fast spreading ridges invoke the presence of a narrow low density low viscosity melt conduit to buoyantly support the high mantle up welling beneath mid ocean ridges must be highly focused to create the narrow melt zone required by these buoyancy models we show that an alternative model in which extensional stresses in the upper crust are relieved by diking predicts uplift at the ridge and can match the shape and magnitude of the axial high in this model the extensional stresses deepen toward the ridge axis creating dynamic moments within the crust that are balanced by the axial topography along axis variations in the height and width of the axial high may be caused by changes in the depth and width of the axial magma chamber or flexural rigidity of the lithosphere this model is similar to dynamic models of median valley formation at slow spreading ridges in that the axial topography is uncompensated and supported by extensional stresses in the lithosphere and asthenosphere in addition to being able to explain the axial topography observed at fast spreading ridges this model is consistent with either broad passive or narrow dynamic upwelling at mid ocean ridges the change from median valley to axial high is caused by differences in strength of the axial lithosphere and rate of thickening of the plate

investigating solid mantle upwelling beneath mid ocean ridges using u series disequilibria ii a local study at 33 s mid atlantic ridge abstract uranium series disequilibria have been used to study the physical melting regime along a section of the southern mid atlantic ridge where the largest mantle bouguer anomaly yet observed on ridges exists zero age excesses for 230th 238u and 231pa 235u are age constrained by the presence of measured 226ra 230th disequilibria by this criterion 7 out of 15 samples are younger than 8000 years zero age 230th 238u range between 1 06 and 1 25 while zero age 231pa 235u range between 1 77 and 3 57 concentrations of th range from 100 to 866 ppb and strongly correlate with k ti the u series data differentiate the segment with the large gravity anomaly from the surrounding segments specifically this segment shows combined 231pa and 230th systematics unlike any observed in morb thus far having exceptionally low zero age 231pa excesses the u series data can be modeled using upwelling rates similar to the half spreading rate of this area of ridge the disequilibria systematics for the anomalous segment 4 possibly suggest a solid upwelling rate that is a factor of two higher than the adjacent segments the similarity between melting at segment 4 and at ocean islands is noted

distributed extension at the indian ocean triple junction the indian ocean triple junction is a ridge ridge ridge type joining two medium spreading ridges with one slow spreading ridge gloria long range side scan sonar images show that while the axial valleys of the two medium spreading ridges are almost colinear apart from a small 5 km offset the valley of the slow spreading third axis does not meet the other two in a simple fashion the axis of this slow spreading southwest indian ridge swir beyond the rift valley walls of the other two ridges steadily deepens by over 1000 m away from the triple junction to reach 5000 m at 35 km the gloria images show large normal faults around the deep swir rift valley which increase in heave away from the triple junction and crosscut the abyssal hills formed at the faster spreading central indian ridge indicating that extension across the swir is distributed over a zone 10 km or more wide this zone also shows no evidence for the formation of new oceanic crust suggesting that the extension may be amagmatic near the triple junction the high relief of the swir rift flanks containing tilted seafloor of the other two ridges may be an isostatic response of the lithosphere to the deep valley produced by this rifting these observations which suggest the progressive development of a propagating swir rift by the extension of preexisting seafloor may have general implications for the dynamics of oceanic spreading centers in particular the deepening and widening of the valley away from the triple junction is attributed to the competing effects of tectonic thinning and lithospheric cooling thermal models of mid ocean ridges spreading at 30 mm yr e g lin and parmentier 1989 predict that the depth to the 700 c isotherm increases with distance from the spreading axis by a proportion similar to the observed doubling in the total width of the deformation with distance from 10 to 35 km away from the triple junction this rate of widening of the deformation zone may therefore support lithospheric necking models for oceanic rift valleys if the width and depth of the deforming lithosphere are related in a simple manner

seafloor magnetotelluric sounding above axial seamount axial seamount is a large active ridge axis volcano located on the central segment of the juan de fuca ridge in the northeast pacific ocean magnetotelluric mt data have been collected at three sites approximately 4 km apart around the eastern rim of the volcano during a 65 day deployment mt responses in the bandwidth of 10 105 s are almost isotropic with a weakly defined principal direction of strike parallel to the main topographic trends of axial seamount and are relatively flat over the whole bandwidth apparent resistivities are of the order of 7 20 m and phases are as low as 30 at the short periods diagonal terms of the mt tensor are an order of magnitude smaller than the off diagonal terms suggesting that three dimensional effects on the data are minimal two dimensional inversions suggest that seafloor bathymetry and the distant coastlines have a surprisingly small effect on the mt response and one dimensional inversions fit the mt data to within the errors with no serial correlation in the residuals a low crustal resistivity is the most robust part of the model probably due to seawater in fractures and possibly due to a magma chamber an electrical asthenosphere although less well constrained exists over a depth range 30 60 km and the resistivity of this region is compatible with about 8 fraction of melt

teleseismic imaging of subaxial flow at mid ocean ridges traveltime effects of anisotropic mineral texture in the mantle summary deformation of peridotite caused by mantle flow beneath an oceanic spreading centre can result in the development of seismic anisotropy traveltime anomalies and shearwave splitting will develop as seismic energy propagates through such an anisotropic region thus providing a signature of the deformation field at depth in this study we investigate the nature of deformation associated with mantle upwelling for two models of flow in the upper 100 km of the mantle the finite strain fields of the passive upwelling model versus the buoyancy enhanced upwelling model are quite different this suggests that mineral aggregates deform differently in the two models thus developing seismic signatures that are distinguishable numerical estimates of the corresponding mineral textures are made using polycrystal theory for olivine with four operative slip systems the activation of a slip system is determined for each grain on the basis of the local critical resolved shear stress the computed grain deformation reflects a balance between stress equilibrium for the aggregate as a whole and strain continuity between neighbouring grains within the aggregate this approach enables a direct link to be made between the model flow fields and the resulting texture development given these mineral orientation distributions elastic parameters are calculated and wavefronts are propagated through the anisotropic structure traveltimes for teleseismic body waves are computed using ray theory and amplitudes are estimated for an across axis profile extending 100 km from the ridge axis relative p wave residuals of up to 1 s are predicted for the buoyant model with on axis arrivals being earliest since near vertical velocities are fastest beneath the axis on axis p wave arrivals for the passive model are half a second earlier than arrivals 60 km off axis and relative delays continue to increase slowly as distance from the ridge increases s wave splitting of almost a second is predicted for the buoyant model whereas less than a half second of splitting is determined for the passive model

convection beneath young oceanic lithosphere implications for thermal structure and gravity small scale convection under the oceanic lithosphere which begins in the first 5 m y of cooling can produce a gravity signal with the amplitude and wavelength observed for large areas of the central pacific and southern indian oceans using seasat altimeter data the trend of the observed anomalies is parallel to the direction of plate motion as might be expected if they were produced by small scale convection models predict that the wavelengths of gravity anomalies increase more rapidly with age than is observed the persistence of short relatively uniform wavelength anomalies 200 km to crustal ages of 50 ma may indicate that they were produced when the lithosphere was very young and thin and were frozen in as cooling thickened the elastic lithosphere small scale convection which begins under very young lithosphere does not violate other geophysical data such as the rate of seafloor subsidence and variations of geoid height with age after convection has begun the subsidence due to thermal contraction within the lithosphere varies linearly with age in the absence of mantle heat sources although the rate of change of these quantities is affected by convection much of the variation of the geoid height across fracture zones can be fit by a model which includes small scale convection

seismic anisotropy in the mantle beneath an oceanic spreading centre beneath an active mid ocean ridge the mantle upwells in response to the divergence of the newly formed plates leading to high temperatures and pressure release melting below the ridge axis the width of the upwelling region and the amount of melting depend on mantle rheology1 5 but all models predict a maximum decrease in seismic velocity at the ridge axis it has also been suggested however that the alignment of anisotropic minerals by shear in the upwelling mantle will increase seismic velocity for rays travelling subvertically through the upwelling zone6 7 here we report the observation of a consistent pattern of anomalously early p wave arrival times at an array of ocean bottom seismographs deployed across the axis of the southern mid atlantic ridge p waves from distant earthquakes arrive earlier at stations near the axis than at those further away our results are consistent with a model of anisotropy in which the degree of mineral alignment is greatest directly beneath the ridge axis and significant anisotropy extends tens of kilometres from the axis

ringing of a vertical cylinder in waves this paper analyses the results of two series of experiments concerned with the response of a single vertical cylinder in the inertia regime in steep non breaking waves we recorded first the loading on a cylinder when it was held stationary and secondly its response in the same waves when it was pivoted just above the floor of the wave flume and supported at the top by springs in the horizontal plane spring stiffnesses were set to achieve natural frequencies measured in still water in the range between 3 and 11 times the dominant wave frequency the experiments were repeated with cylinders of three different diameters peak loading on stationary cylinders was found to exceed the predictions of a morison model based on kinematics computed from a numerical model of the measured waves though improvements were achieved through the inclusion of slender body terms measured ringing responses are generally in good agreement with those computed on a quasi static basis from the measured loading history but in some conditions particularly at low frequency ratios there is clearly some feedback from the motion to the excitation peak accelerations in the steepest waves are found to be limited approximately to those that would occur if the maximum loading were applied as a step change particular attention is given to a rapid cycle of loading that occurs after the crest has passed the cylinder s axis and to images of the flow around the cylinder at the water surface

gravity anomalies isostasy and mantle flow at the east pacific rise crest bathymetry and gravity data obtained during a detailed hydrosweep survey of the southern east pacific rise from 7 s to 9 s are used to investigate isostasy at the axis of a fast spreading mid ocean ridge in particular we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160 km long 7 12 s 8 38 s ridge segment the crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km the summit is broad and flat and an axial summit caldera is present for the entire length of the ridge segment however the width and cross sectional shape of the crestal high vary systematically along the axis it is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment the ridge crest is marked by a free air gravity anomaly high about 15 20 km wide with an amplitude of 10 15 mgal relative to the ridge flanks mantle bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment the axial mantle bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross sectional area of the crestal bathymetric high the effects of cooling and subsidence away from the axis were removed from the bathymetry and free air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest the residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis the lithosphere was treated as a broken plate either with a constant flexural rigidity or an effective elastic thickness te which grows at a rate proportional to the square root of distance from the axis the best fitting values of te for the constant rigidity case are in the range of 0 3 0 6 km for the growing plate model te increases at a rate of 0 2 0 3 km the gravity constrains the mass deficiency to extend to a depth of 20 30 km for both lithospheric models we interpret this low density material as a region of partial melt feeding magma to the ridge axis the best fitting density anomalies imply that a 4 9 melt fraction is present beneath the crestal high upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis the mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment it thus appears that differences between the along axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three dimensional focused upwelling at slow spreading ridges to two dimensional sheetlike upwelling at fast spreading ridges rather the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along axis magma distribution

crustal thickening in an extensional regime application to the mid norwegian voring margin abstract the structure of the mid norwegian volcanic voring margin at the onset of the maastrichtian paleocene extension phase reflects the cumulative effect of earlier consecutive rifting events lateral structural differences present on the margin at that time are a consequence of migration of the location of maximum extension in time between norway and greenland the most important imprints moho depth thermal structure of these events on the lithosphere are incorporated in a numerical simulation of the final extension phase we focus on a possible mechanism of formation of the voring marginal high and address the relationship between spatial and temporal evolution of crustal thinning and thickening uplift of the surface and strength of the lithosphere it is found that the voring basin formed the strongest part of the margin which explains why the maastrichtian paleocene rift axis was not located here but instead jumped westward with respect to the earlier rift axes locations the modeling study predicts that local crustal thickening during extension can be expected when large lateral thermal variations are present in the lithosphere at the onset of extension negative buoyancy induced by lateral temperature differences increases downwelling adjacent to the rifting zone convergence of material at the particular part of the margin is mainly taken up by the lower crust the model shows that during the final phase of extension the crust in the voring marginal high area was thickened and the surface uplifted it is likely that this dynamic process and the effects of magmatic intrusions both acted in concert to form the marginal high

continental geotherm and the evolution of rifted margins models of melting accompanying mantle upwelling predict far more melt than is observed at nonvolcanic margins the discrepancy may be explained if the paradigm of a uniform asthenosphere is incorrect work on the velocity structure of the continents has shown that the convecting sublithospheric mantle may have a potential temperature as low as 1200 c 100 c cooler than that beneath the oceans the continental geotherm derived from studies on xenoliths brought up by kimberlites is also compatible with a cool sublithospheric mantle except where perturbed by mantle plumes upwelling of such cool mantle during rifting leads to little melt production even for rapid extension rates explaining the formation of amagmatic margins away from mantle plumes however the transition to seafloor spreading and the development of normal thickness oceanic crust requires the invasion of the amagmatic rift by hotter oceanic asthenosphere and or plume material this influx may cause a transient thermal uplift recorded as a breakup unconformity conversely at volcanic margins the onset of seafloor spreading is accompanied by the escape of the hot plume puddle along the mid ocean ridge system away from the volcanic margin leading to a pulse of rapid subsidence

propagating buoyant mantle upwelling on the reykjanes ridge crustal features of the reykjanes ridge have been attributed to mantle plume flow radiating outward from the iceland hotspot this model requires very rapid mantle upwelling and a rheological boundary at the solidus to deflect plume material laterally and prevent extreme melting above the plume stem here we propose an alternative explanation in which shallow buoyant mantle upwelling instabilities propagate along axis to form the crustal features of the ridge and flanks as only the locus of buoyant upwelling propagates this mechanism removes the need for rapid mantle plume flow based on new geophysical mapping we show that a persistent sub axial low viscosity channel supporting buoyant mantle upwelling can explain the current oblique geometry of the ridge as a reestablishment of its original configuration following an abrupt change in opening direction this mechanism further explains the replacement of ridge orthogonal crustal segmentation with v shaped crustal ridges and troughs our findings indicate that crustal features of the reykjanes ridge and flanks are formed by shallow buoyant mantle instabilities fundamentally like at other slow spreading ridges and need not reflect deep mantle plume flow

spreading rate spreading obliquity and melt supply at the ultraslow spreading southwest indian ridge we use bathymetry gravimetry and basalt composition to examine the relationship between spreading rate spreading obliquity and the melt supply at the ultraslow spreading southwest indian ridge swir we find that at regional scales more than 200 km melt supply reflects variations in mantle melting that are primarily controlled by large scale heterogeneities in mantle temperature and or composition focusing on adjacent swir regions with contrasted obliquity we find that the effect of obliquity on melt production is significant about 1 5 km less melt produced for a decrease of 7 mm a to 4 mm a in effective spreading rates esr but not enough to produce near amagmatic spreading in the most oblique regions of the ridge unless associated with an anomalously cold and or depleted mantle source our observations lead us to support models in which mantle upwelling beneath slow and ultraslow ridges is somewhat focused and accelerated thereby reducing the effect of spreading rate and obliquity on upper mantle cooling and melt supply to explain why very oblique swir regions nonetheless have large outcrops of mantle derived ultramafic rocks and in many cases no evidence for axial volcanism cannat et al 2006 dick et al 2003 we develop a model which combines melt migration along axis to more volcanically robust areas melt trapping in the lithospheric mantle and melt transport in dikes that may only form where enough melt has gathered to build sufficient overpressure these dikes would open perpendicularly to the direction of the least compressive stress and favor the formation of orthogonal ridge sections the resulting segmentation pattern with prominent orthogonal volcanic centers and long intervening avolcanic or nearly avolcanic ridge sections is not specific to oblique ridge regions it is also observed along the swir and the arctic gakkel ridge in orthogonal regions underlain by cold and or depleted mantle

ridge jumps and propagations in the south atlantic ocean the normal pattern of mid ocean ridge divergent plate boundaries comprises straight spreading segments with narrow zones of crustal accretion bounded by transform faults where these segments are offset normally the configuration of spreading segments and transform faults is stable for long periods but in places on fast and medium rate spreading ridges the transform offsets have propagated along the ridge axis as some spreading segments have grown at the expense of adjacent ones1 7 although such behaviour has hitherto not been well documented on slow spreading ridges we have mapped multiple ridge jumps and propagating rifts in two areas of the slow spreading 18 mm yr 1 half rate south atlantic ridge crest we attribute this anomalous behaviour in the south atlantic to the presence of hotspots near our study areas causing elevated asthenospheric mantle temperatures and lateral mantle flow with consequent excess volcanism and disruption of the normally well focused melt delivery processes at the ridge crest

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

on the dynamics of mid ocean ridges most extant flow models for ocean ridges suffer either from unrealistic geometry vertical conduit models or a stress singularity at the ridge axis corner flow models i present a model which avoids these difficulties by incorporating a weak plate boundary zone pbz between diverging surface plates an analytical solution for the flow of a constant viscosity asthenosphere and a coexisting melt phase is obtained in elliptic coordinates and used to predict the dynamically supported topography axial valley on the pbz the force required to pull the plates apart and the rate of melt extraction at the ridge stress in the asthenosphere increases linearly with spreading rate but decreases as the plates become thinner and the dynamics of ridge crests is controlled by the balance between these opposing effects the depth of the axial valley is controlled by the plate thinning effect and is therefore a decreasing function of spreading rate the depth of the axial valley on the mid atlantic ridge can be explained if the viscosity of the asthenosphere is 6 1019 1021 pa s and the lower surface of the plate is the 600 c isotherm the plate separation force is controlled by the spreading rate effect and therefore increases with spreading rate about five times as much force is required to maintain a fast spreading ridge 9 cm yr 1 half rate as a slow spreading ridge 1 cm yr 1 melt extraction can produce 6 km of basaltic crust if the volume fraction of melt present in the asthenosphere total degree of melting ranges from about 2 5 4 0 for a slow spreading ridge to 4 0 6 5 for a fast spreading ridge the dynamics of melt extraction does not depend significantly on the finite width of the pbz

laboratory study of plunging wave impacts on vertical cylinders the characteristics of pressures associated with plunging wave impacts on a vertical cylinder are presented despite a high variability in the peak pressures spatial distributions of the impact pressure time histories both in the vertical direction and around the seaward front of the cylindrical surface are found to vary systematically for a range of cylinder locations in the wave plunging region through correlations between the incident wave kinematics and the impact pressures the mechanics of the impact process have been identified and elucidated these results serve as a useful reference for future numerical and theoretical modelling of extreme wave loads on surface piercing vertical cylinders

three dimensional mantle flow beneath mid ocean ridges we have studied the development of multiscalar mantle flow beneath mid ocean ridges using a three dimensional numerical technique modeling includes various modes of heating and the shear flow induced by plate motion away from a spreading center with and without transform offsets the computational box consists of two layers differing in their viscosities it is shown than when flow is generated by heat sources located for the most part in the lower layer convective cells have horizontal dimensions proportional to the entire thickness of the box it is only when flow is driven by buoyancy forces in the top layer or by transient cooling along the top interface that transverse rolls are found to be confined within the lop low viscosity layer these rolls remain generally located far from the ridge plane except in the case of a viscosity contrast of 3 orders of magnitude between the bottom and top layers we conclude that segmentation beneath mid ocean ridges requires both strong viscosity contrasts and the lack of deep convective flow marine geophysical observations suggest that mantle upwelling beneath mid ocean ridges is two dimensional sheetlike beneath intermediate fast and hotspot influenced spreading centers and three dimensional plumelike beneath slow spreading centers and ridges overlying cold regions of the mantle such as the australian antarctic discordance we propose that these observations can be explained by variations in the viscosity structure and temperature of the mantle along the mid ocean ridge system models including a transform offset show that the active portion of the transform is a region of mantle downwelling the toroidal flow induced by the offset shifts the plumes away from the ridge crests the effects of the toroidal field are strongly magnified when the return flow cannot penetrate in the lower layer due to a large viscosity contrast stresses generated by the flow partly explains the development of inside corner highs and the evolution of transform faults

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

small scale convection and divergent plate boundaries this paper explores the suggestion that observations of large igneous crustal thickness at rifted volcanic margins may in part be explained by small scale convection in the upper mantle this may increase the delivery of magma to the overlying lithosphere without the need for anomalously high mantle temperatures this concept is quantitatively assessed by numerical modeling of the flow caused by divergent plate motions in a viscous temperature and pressure dependent nonlinear fluid significant time dependent small scale convection is generated at the lower model viscosities and or higher temperatures and or sharper rift geometries these models can provide the required thick igneous crust observed at volcanic margins however they also give excessive variations in the thickness of igneous crust within the ocean basin and are therefore unacceptable an additional factor we have explored is the recent suggestion that the oceanic mantle may become dehydrated when melt is generated and removed this increases its viscosity by 2 to 3 orders of magnitude in the melting zone above 80 km the high viscosity lid stabilizes the flow and provides a more uniform oceanic crustal thickness while at the same time allowing for vigorous small scale convection during the early history of the rift and the delivery of thick igneous crust against the margin this factor coupled to the models for flow described above allow the main features of volcanic margins to be simulated besides the thick igneous crust predicted at the margins these models suggest that time dependent small scale convection below the margin may persist for tens of million years following the onset of seafloor spreading and that there may be coupling between this flow at the margin and that at the ridge axis the periodicity of this time dependent convection is primarily dictated by the model viscosities the models suggest that episodicities of tens of million years are reasonable these episodicities may be reflected in the record of vertical motions at rifted margins

three dimensional passive mantle flow beneath mid ocean ridges an analytical approach summary we discuss theoretical and computational method on plate driven mantle flow beneath midocean ridges we consider a steady state flow induced by motion of overlying rigid plates in an incompressible viscous mantle beneath a generic ridge transform ridge plate boundary no assumption of orthogonal and symmetric spreading at ridge axis is made analytical solutions for viscosity flow in a half space and in a layered viscosity mantle beneath an infinitesimal thickness lithosphere and beneath plates that thicken with increasing age are presented numerical calculations were carried out using a standard fast fourier transform algorithm the difficulty of using standard fourier methods to predict accurately the mantle flow field in the proximity of the plate boundaries is overcome by applying the gegenbauer reconstruction post processing technique to the fourier pseudo spectral solutions finally we present some examples of flow computations we consider for both models two different ridge transformridge geometries consisting of 100 and 1000 km offsets of two ridge segments spreading at 15 mm yr half rate we found a significant difference in the flow structure between the two flow models close to ridge axis and ridge transform intersections the proposed model and methods are useful for fast mantle flow calculations to investigate melting processes beneath spreading centres and to predict the relationship between mantle temperature crustal thickness and geochemistry of the oceanic crust

effects of planetary thermal structure on the ascent and cooling of magma on venus magellan radar images of the surface of venus show a spatially broad distribution of volcanic features models of magmatic ascent processes to planetary surfaces indicate that the thermal structure of the interior significantly influences the rate of magmatic cooling and thus the amount of magma that can be transported to the surface before solidification in order to understand which aspects of planetary thermal structure have the greatest influence on the cooling of buoyantly ascending magma we have constructed magma cooling profiles for a plutonic ascent mechanism and evaluated the profiles for variations in the surface and mantle temperature surface temperature gradient and thermal gradient curvature results show that for a wide variety of thermal conditions smaller and slower magma bodies are capable of reaching the surface on venus compared to earth primarily due to the higher surface temperature of venus little to no effect on the cooling and transport of magma are found to result from elevated mantle temperatures elevation dependent surface temperature variations or details of the thermal gradient curvature the enhanced tendency of magma to reach the surface on venus may provide at least a partial explanation for the extensive spatial distribution of observed volcanism on the surface

spreading rate dependence of three dimensional structure in oceanic spreading centres segmentation by transform faults and other types of along axis discontinuity1 is a well described but poorly explained characteristic of oceanic spreading centres here we use numerical experiments to explore the dynamics of mantle flow and melting beneath a mid ocean ridge buoyant upwelling driven by compositional density variations resulting from the extraction of the melt that forms the ocean crust exhibits a spreading rate dependent transition between two dimensional and three dimensional upwelling structures for low spreading rates and mantle viscosities an initial two dimensional structure transforms into a three dimensional one at high spreading rates an initially two dimensional structure remains two dimensional these results suggest that the origin of spreading centre segmentation may be different at fast and slow spreading rates

breaking characteristics and geometric properties of spilling breakers over slopes abstract a two phase flow cfd model based on the reynolds averaged navier stokes rans equations coupled with the level set method lsm and k turbulence model is used to simulate spilling breakers over a sloping bed in order to validate the present numerical model the simulated results are compared with the experimental data measured by ting and kirby 1996 the simulated horizontal velocities and free surface elevations are in good agreement with the experimental measurements moreover the present model is able to model the prominent features associated with the breaking process such as the motion of air pockets in the water formation of a forward moving jet the splash up phenomenon and the mixing of air and water in the breaking region the numerical model has been utilized to study the influences of three important environmental parameters water depth offshore wave steepness and beach slope on the characteristics and geometric properties of spilling breakers over slopes a total of 39 numerical experiment cases are performed to investigate the characteristics of breaking waves such as breaking location incipient breaker height and water depth at breaking incipient breaker indices and geometric properties with different offshore wave steepnesses at different water depths over a wide range of beach slopes the geometric properties associated with breaking waves in shallow water are described using the wave steepness and asymmetry factors introduced by kjeldsen and myrhaug 1978 the computed results appear to give reasonable predictions and consistency with previous studies

terrestrial spreading centers under venus conditions evaluation of a crustal spreading model for western aphrodite terra abstract assessment of terrestrial spreading centers under venus environmental conditions indicates that the major difference between earth and venus would be the influence of the enhanced surface temperature on upper mantle temperatures on venus and the resulting increase in crustal thickness and elevation of isostatically compensated topography crust on venus produced at average spreading centers would be about 15 km thick in contrast to the average crustal thickness on earth of about 5 km we have used this model and the pioneer venus altimetry and gravity data to test the hypothesis of head and crumpler 1987 6 that western aphrodite terra represents the location of crustal spreading on venus the topographic data are processed such that all points within equally spaced 100 km wide bins parallel to the proposed axis are stacked together and the average value of topography in each bin is calculated providing average profiles out to a distance of 2000 km on either side of the axis the topography is shown to be very symmetric about the axis with data points less than 1000 km from the axis providing a fit to thermal boundary layer topography comparable to that seen in spreading centers on earth at 1000 km on either side of the axis steep topographic slopes exist this change in topography could be explained by variations of about 15 km in crustal thickness with the crust being thicker in the central domain by a ridge jump or by mechanisms unrelated to crustal spreading for processing line of sight los acceleration data we first show that static and dynamic calculations give the same results for track numbers 427 449 this allows us to determine a synthetic orbit track parallel to the cross strike dis continuities csd s and to compare it with observed los accelerations gravity data for this domain were then processed in a manner similar to that for topography models give a reasonable fit to the data for half spreading rates between 0 3 and 0 5 cm yr we conclude that a spreading center model for ovda regio could account for the observed topography and los gravity anomalies if it had the following characteristics a central domain with a 30 km thick crust that has been spreading at about 0 5 cm yr for the last 200 myr over a mantle whose temperature is about 1600 c at about 200 myr ago a change in upper mantle temperature of about 100 c is required prior to this time the decreased upper mantle temperature would have produced a thinner crust averaging about 15 km at the spreading center this thinner crust would now lie on the flanks of ovda regio at distances greater than 1000 km from the spreading center and would be in excess of 200 myr old on the basis of our analysis we conclude that the hypothesis that ovda regio in western aphrodite terra is the site of crustal spreading on venus is consistent with the topography and gravity data

magmato tectonic cyclicity at the ultra slow spreading southwest indian ridge evidence from variations of axial volcanic ridge morphology and abyssal hills pattern on axis deep tow side scan sonar data are used together with off axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra slow spreading southwest indian ridge differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments we interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development including periods of volcanic construction and periods of tectonic dismemberment using off axis bathymetric data we identify numerous abyssal hills with a homogeneous size for each segment these abyssal hills all display an asymmetric shape with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away we suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment i e a magmato tectonic cycle we observe that large abyssal hills are in ridge sections of thicker crust whereas smaller abyssal hills are in ridge sections of thinner crust this suggests that the magma supply controls the size of abyssal hills the abyssal hills in ridge sections of thinner crust are regularly spaced indicating that the magmato tectonic cycle is a pseudoperiodic process that lasts 0 4 m y about 4 to 6 times shorter than in ridge sections of thicker crust we suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long lived segments by contrast when magma supply strongly decreases and becomes highly discontinuous regular abyssal hills patterns are no longer observed