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

a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

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

ultramafic exposures and the gravity signature of the lithosphere near the fifteen twenty fracture zone mid atlantic ridge 14 16 5 n abstract we analyze geological and geophysical data near the fifteen twenty fracture zone to constrain the processes of magmatic accretion and tectonic extension in the area characterized by extensive outcrops of ultramafic rocks at the seafloor close to the fracture zone 60 km the crust is thicker associated with circular or elongated gravity lows bull s eyes axis parallel faults and abyssal hills and no peridotite outcrops consistent with a more homogeneous and magmatic crust simple melting models predict magmatic crustal thicknesses that differ substantially from the gravity estimates and the outcrop of peridotites seems inconsistent with the high degrees of partial melting in the area inferred from basalt and peridotite geochemistry we explain these discrepancies primarily a by the entrapment of melt in the lithosphere at deep levels particularly near the transform b by along axis melt migration away from the transform and c by variations in the degree and depth of serpentinization of a heterogeneous crust composed of gabbro and peridotite the along axis variations in melt supply lithospheric thickness and alteration of the lithosphere can result in a change of rheological structure and therefore in fault pattern as observed along the ridge axis identified detachment faults in the area provide a mechanism for the uplift of ultramafic rocks to the seafloor these detachments are not systematically associated with or formed at the inside corner of discontinuities as described elsewhere the origin and evolution of these structures is yet unconstrained but we suggest that they can be initiated at mechanical heterogeneities in the deep lithosphere and that once formed they localize strain due to the presence of weak serpentinites along the fault planes

long lived mid ocean ridge segmentation constraints on models long lived large scale segmentation of the mid ocean ridge as defined by variations in subsidence rate of the ocean crust zero age depth and geoid rate has been documented along three mid ocean ridge systems the south atlantic the pacific antarctic and the southeast indian many of the observations in these three areas are inconsistent with standard thermal models for the generation of oceanic crust and lithosphere three models for subsidence rate and geoid rate and two models for zero age depth are examined sensitivity tests are done to determine how much the thermal and physical constants in these models must vary in order to explain the observations and whether or not such values are plausible only the zero age depth variations can be adequately explained using existing models an internally consistent qualitative model is presented which can explain the data it involves regional and local temperature changes in the mantle both along and across strike of the mid ocean ridge

the possible reflection of mantle discontinuities in pacific geoid and bathymetry geoid anomalies over the pacific plate show lineated undulations approximately oriented in the direction of absolute plate motion conventional spectral analyses have revealed a broad range of dominant wavelengths but as filtering is direction dependent results are difficult to interpret here we present a new approach designed to quantify the correlation between the geoid undulations and pacific plate motion we calculate the spatial correlation between geoid lineations and arbitrarily oriented axisymmetric sinusoidal undulations of given wavelength the pole i e the orientation of the sinusoid which maximizes the correlation is determined the distance between this pole and the pacific hotspot pole is calculated and represents a measure of how well a geoid lineation of a given wavelength aligns with pacific plate motion this distance varies with undulation wavelength and has discrete minima for several wavebands these minima moreover occur at wavelengths whose values are remarkably close to the depths of seismically inferred mantle discontinuities a similar analysis of available bathymetry corroborates these findings if these correlations are significant they suggest that geoid and bathymetry undulations are influenced by mantle structure and thus are likely to reflect mantle dynamics

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

pleistocene to recent alkalic volcanism in the region of sanganguey volcano nayarit mexico forty five cinder cones and associated lava flows have erupted within the last 300 000 years along five parallel lines through the calc alkaline volcano sanganguey in the northwestern segment of the mexican volcanic belt lavas erupted from these cinder cones include ne and hynormative alkali basalts hawaiites mugearites and benmoreites it is unusual that this suite has erupted in a calc aikaline volcanic belt where volcanoes in the vicinity have been erupting calc alkaline andesites dacites and rhyodacites

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

regional viscosity variations small scale convection and slope of the depth age1 2 curve cooling plate and cooling half space models are commonly employed to describe the depth age1 2 curve of seafloor these conductive models are not able to reproduce the large range of subsidence rates that are observed with reasonable changes in the physical parameters we examine the influence of small scale convection in the mantle on subsidence rates with a one dimensional parameterized convection model the parameterization is based on a recent experimental study of thermal convection in a fluid with temperature dependent viscosity in our model convection in the asthenosphere begins at the ridge axis enhancing heat transfer out of the asthenosphere and increasing the subsidence rate above that for simple conductive cooling variations in deformation mechanism temperature and water content of the mantle cause changes in viscosity that affect the vigor of convection and may be responsible for large variations in subsidence rates average subsidence rates can range from 250 500 m ma1 2 when small scale convection is considered

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

relationship between axial morphology crustal thickness and mantle temperature along the juan de fuca and gorda ridges we analyze gravity data over the juan de fuca and northern gorda ridges to understand the lithospheric structure of two ridges with contrasting axial morphologies spreading at the same intermediate rate 28 mm yr half rate the cleft segment at the south end of the juan de fuca ridge has an axial high morphology while the northern segment of the gorda ridge has a rift valley residual mantle bouguer anomalies rmba on the northern gorda ridge are elevated relative to the cleft segment by 10 20 mgal indicating thinner crust and or a colder mantle the minimum value 50 mgal of the rmba along the juan de fuca ridge is over axial seamount and gradually increases south toward the blanco transform the observed rmba are interpreted to result from along axis variations in crustal thickness and mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs we estimate that mantle temperatures are elevated by 30 40 c beneath axial seamount resulting in an excess crustal thickness of 1 7 km the cleft segment is associated with crust that is estimated to be only 300 700 m thicker and mantle temperatures are only 10 15 c higher than beneath the northern gorda ridge however even these small differences in crustal thickness and mantle temperature appear to be sufficient to produce a major change in lithospheric strength and axial morphology these results are consistent with the predicted sensitivity of recent thermo mechanical models for rift valley formation to small changes in crustal thickness and mantle temperature at intermediate spreading rates we attribute the systematic differences in axial morphology crustal thickness mantle temperature and lithospheric strength along the juan de fuca gorda ridge system to the presence of the cobb thermal anomaly at axial seamount the hotter mantle beneath the juan de fuca ridge results in greater amounts of decompression melting and the formation of a thicker crust and a thinner weaker lithosphere than along the gorda ridge

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

present and past regional ridge segmentation evidence in geoid data global geoid data from the geosat altimeter satellite and topography data have been analysed in the pacific and atlantic oceans to study present along axis and past off axis regional mid ocean ridge segmentation along the mid atlantic ridge east pacific rise and pacific antarctic ridge axial geoid and depth variations are positively correlated at length scale 800 1000 km respective peak to peak amplitudes are in the range of 0 4 0 8 m and 200 400 m axial variations are correlated with the position of large offset transform faults which coincide with geoid and topography lows triple junctions are also located in geoid and topography lows the observed admittance between geoid and depth is estimated to 2 0 5 m km a value indicative of a dynamic convective origin for the along axis regional variability off axis analysis shows a regional pattern of geoid topography anomalies elongated in the direction of spreading this pattern has also a length scale of 800 1000 km and possibly represents past regional segmentation large offset fracture zones coincide with off axis geoid topography lows

mantle upwelling and melting beneath slow spreading centers effects of variable rheology and melt productivity abstract we examine the effects of non uniform rheology and melting models on the 3d structure of mantle upwelling beneath spreading centers our numerical models identify a range of conditions for which 3d upwelling can be considered as a possible mechanism for magmatic segmentation of slow spreading ridges in a highly viscous shallow region due to dehydration of the solid by melt extraction flow results essentially from the plate spreading this contrasts with a deeper buoyant low viscosity region where melt is present but upwelling solid is not yet dehydrated and where both solid and melt flows acquire their 3d nature the thickness of this buoyant region depending mainly on the temperature interval between the wet and dry solidi results in a specific characteristic length scale for the upwelling smaller segmentation wavelengths occur for smaller values of this thickness in contrast to isoviscous models where minimum segmentation wavelengths were large 150 km minimum wavelengths obtained in models with non uniform rheology and melt production 40 70 km are comparable to smallest observed segment lengths a single wavelength may be preferred for a given set of parameters but such a preferred wavelength is difficult to achieve because a range of initially prescribed wavelengths can persist for long times the persistence of 3d solutions for tens of millions of years of model time indicate that segment lengths observed at slow mid ocean ridges may be inherited from the initial stages of spreading

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

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

mantle flow and melting beneath oceanic ridge ridge ridge triple junctions abstract plate boundary geometry likely has an important influence on crustal production at mid ocean ridges many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting this study investigates how triple junction tj geometry may influence mantle dynamics an earlier study georgen j e lin j 2002 three dimensional passive flow and temperature structure beneath oceanic ridge ridge ridge triple junctions earth planet sci lett 204 115 132 suggested that the effects of a ridge ridge ridge configuration are most pronounced under the branch with the slowest spreading rate thus we create a three dimensional finite element variable viscosity model that focuses on the slowest diverging ridge of a triple junction with geometry similar to the rodrigues tj this spreading axis may be considered to be analogous to the southwest indian ridge within 100 km of the tj temperatures at depths within the partial melting zone and crustal thickness are predicted to increase by 40 c and 1 km respectively we also investigate the effects of differential motion of the tj with respect to the underlying mantle by imposing bottom model boundary conditions replicating a absolute plate motion and b a three dimensional solution for plate driven and density driven asthenospheric flow in the african region neither of these basal boundary conditions significantly affects the model solutions suggesting that the system is dominated by the divergence of the surface places finally we explore how varying spreading rate magnitudes affects tj geodynamics when ridge divergence rates are all relatively slow i e with plate kinematics similar to the azores tj significant along axis increases in mantle temperature and crustal thickness are calculated at depths within the partial melting zone temperatures are predicted to increase by 150 c similar to the excess temperatures associated with mantle plumes likewise crustal thickness is calculated to increase by approximately 6 km over the 200 km of ridge closest to the tj these results could imply that some component of the excess volcanism observed in geologic settings such as the terceira rift may be attributed to the effects of tj geometry although the important influence of features like nearby hotspots e g the azores hotspot cannot be evaluated without additional numerical modeling

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

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

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

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

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

the spreading rate dependence of three dimensional mid ocean ridge gravity structure we analyze over 1300 km of high resolution along axis gravity profiles at ridges with half spreading rates ranging from 1 2 to 5 5 cm yr the results show consistently higher along axis gradients of mantle bouguer anomaly at the slow spreading mid atlantic ridge mar 0 3 1 2 mgal km than at the intermediate to fast spreading cocos nazca ridge and east pacific rise epr 0 1 0 2 mgal km the regional peak to trough amplitude of mantle bouguer anomaly is also greater along the mar 30 60 mgal than the cocos nazca ridge and the epr 10 20 mgal with increasing spreading rate the regional peak to trough amplitude of axial seafloor depth decreases from 1000 1700 m to 200 700 m 3 d numerical experiments suggest that mantle contributions to the gravity can be significant only near large offset transforms at the more commonly observed non transform offsets gravity anomalies will reflect crustal thickness variations the along axis gravity data thus indicate that the amplitude of along axis crustal thickness variation decreases with increasing spreading rate we propose that this spreading rate dependent crustal accretion style may originate in the mantle finite amplitude mantle upwelling is intrinsically plume like 3 d beneath a slow spreading ridge but more sheet like 2 d beneath a fast spreading ridge such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate small amplitude 3 d upwellings may occur at a fast spreading ridge but their effects on crustal thickness variations will be significantly reduced by along axis melt flows along a persistent low viscosity crustal magma chamber in contrast the large crustal thickness variations due to 3 d mantle upwellings will be maintained at a slow spreading ridge because less along axis melt flows can occur in the colder and more rigid crust there

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

anomalously thin crust in oceanic fracture zones new seismic constraints from the kane fracture zone three detailed and carefully positioned seismic refraction experiments have recently been carried out along the kane fracture zone near 24 n 44 w in order to place better geophysical constraints on the extent and origin of the anomalously thin crust reported from parts of this and other large atlantic fracture zones these three experiments when combined with earlier studies result in reversed structural control along about a 300 km long segment of the kane fracture zone extending from the ridge transform intersection out onto crust about 25 m y old an analysis of refraction lines obtained along the fracture zone trough by using both travel time modeling and delay time function techniques reveals large variations in crustal thickness and or velocity however along most of the eastern fracture zone trough upper mantle type velocities are found at depths of only 2 3 km below the seafloor less than half the typical depth to moho in the ocean basins this anomalous fracture zone crust is generally characterized by unusually low compressional wave velocities relatively high velocity gradients throughout most of the thinned crustal section about 1 2 s 1 and a distinct crust mantle boundary the very thin crust appears to be confined to the deepest parts of the kane fracture zone trough 10 km wide although a more gradual crustal thinning may extend up to several tens of kilometers from the fracture zone a specially designed delay time experiment at the intersection of the kane fracture zone and the mid atlantic ridge also revealed the presence of a thinned crustal section of a relatively uniform thickness about 3 km except beneath the nodal basin where the crust may be less than 1 km thick the anomalous crust in the intersection area appears to extend at least part way down the median valley we interpret this anomalous fracture zone crust as a primary accretionary feature that forms as a result of a restricted magma supply near ridge transform intersections although tectonic processes may also play a part at least locally in thinning the crust our results support a ribbon model of the gross seismic structure of the ocean basins in which bands of relatively homogeneous so called normal crust 50 100 km wide extend away from spreading centers each formed at a discrete ridge crest segment and each separated by the thin crust underlying the fracture zone trough

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

late amagmatic extension along the central and eastern segments of the west philippine basin fossil spreading axis the spreading processes within the west philippine basin wpb remain partly unknown this study presents an analysis of the tectono magmatic processes that happened along its spreading axis during the conclusion of the last spreading phase at 33 30 ma we demonstrate that the late episode of n s opening from an e w trending spreading system has been followed by a late tectonic event occurring in the central and eastern parts of the basin this event was responsible for transtensional strain accommodated along the nw se faults cutting through the former e w rift valley in the central part of the basin in its eastern part the same event occurred at a larger extent and led to the creation of a new nw se axis obliquely cutting the older e w spreading segments and their associated spreading fabrics at this location several tens of kilometers of slightly oblique amagmatic extension occurred following a v60 direction we propose that this late event is associated with the onset of e w opening of the parece vela basin located along the eastern border of the wpb at 30 ma extensive stresses within this basin were probably transmitted to the hot and easily deformable rift zone of the wpb the newly created nw se axis most likely propagated from east to west being responsible for scissors opening within the wpb ne sw extension ceased when well organized spreading started at 26 ma in parece vela basin accommodating entirely the global extensive stress pattern 2002 elsevier science b v all rights reserved

upper mantle structure of the tonga lau fiji region from rayleigh wave tomography we investigate the upper mantle seismic structure in the tonga lau fiji region by jointly fitting the phase velocities of rayleigh waves from ambient noise and two plane wave tomography the results suggest a wide low velocity zone beneath the lau basin with a minimum sv velocity of about 3 7 0 1 km s indicating upwelling hot asthenosphere with extensive partial melting the variations of velocity anomalies along the central and eastern lau spreading centers suggest varying mantle porosity filled with melt in the north where the spreading centers are distant from the tonga slab the inferred melting commences at about 70 km depth and forms an inclined zone in the mantle dipping to the west away from the arc this pattern suggests a passive decompression melting process supplied by the australian plate mantle from the west in the south as the supply from the australian mantle is impeded by the lau ridge lithosphere flux melting controlled by water from the nearby slab dominates in the back arc this source change results in the rapid transition in geochemistry and axial morphology along the spreading centers the remnant lau ridge and the fiji plateau are characterized by 60 80 km thick lithosphere underlain by a low velocity asthenosphere our results suggest the removal of the lithosphere of the northeastern fiji plateau lau ridge beneath the active taveuni volcano azimuthal anisotropy shows that the mantle flow direction rotates from trench perpendicular beneath fiji to spreading perpendicular beneath the lau basin which provides evidence for the southward flow of the mantle wedge and the samoan plume this article is protected by copyright all rights reserved

the influence of magma plumbing complexity on monogenetic eruptions jeju island korea terra nova 23 70 75 2011 abstract continuous pyroclastic successions from monogenetic volcanoes in the jeju island volcanic field korea were targeted for detailed geochemical investigations of juvenile ejecta to show that multiple distinct magma pulses may be erupted during a single monogenetic eruption deeply derived and possibly multiply sourced evolving magma pulses may erupt sequentially from the same vent and form generally uninterrupted depositional sequences with continuous chemical trends alternatively pulses can rise independently resulting in breccia horizons and truncation surfaces in depositional sequences and in stepped and mixed chemical trends we infer that uninterrupted eruptions result from clear plumbing systems in which a single large dyke system feeds an eruption and where differently sourced magmas are erupted independently congested plumbing systems consisting of dyke complexes give rise to vent shifts asymmetrical eruptive centres and composite depositional sequences integration of chemical variation and eruptive dynamics provides a powerful means of understanding monogenetic volcanoes

evidence from en echelon cross grain ridges for tensional cracks in the pacific plate sea floor topography in the pacific is mainly aligned with original spreading directions1 but is overprinted by alignments created by mid plate processes spreading produces abyssal hills and fracture zones and mid plate volcanism generates seamounts isolated or in chains a different category of topography the cross grain discovered in geoid height data collected by the seasat radar altimeter2 comprises linear troughs and swells spaced 200 km apart oblique to fracture zones and abyssal hills but parallel to the hawaiian chain three models have been proposed for the cross grain small scale convection organized into longitudinal rolls by the shear of the pacific plate2 compressive buckling3 and lithospheric boudinage resulting from plate wide tensile stresses4 5 none of the previously available data ruled out any of these models here we report multi beam bathymetric data revealing long narrow en echelon ridges along the cross grain interpreted as evidence of tension cracks in the pacific plate

post glacial rebound and asthenosphere viscosity in iceland during the weichselian glaciation iceland was covered with an ice cap which caused downward flexure of the earth s surface the post glacial rebound in iceland was very rapid being completed in about 1 000 years the length of this time interval constrains the maximum value of asthenosphere viscosity in iceland to be 1 times 10 sup 19 pa s or less further clarification of the ice retreat and uplift history may reveal lower viscosity current changes in the mass balance of icelandic glaciers must lead to measurable elevation changes considering this low viscosity expected current elevation changes around the vatnajoekull ice cap are of the order of 1 cm per year due to mass balance change in this century

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

crenulated seafloor evidence for spreading rate dependent structure of mantle upwelling and melting beneath a mid oceanic spreading center abstract the availability of dense geosat gm satellite altimetry data south of 30 s 1 provides the first opportunity to compare at 25 km resolution seafloor generated at ridges spreading with a wide range of spreading rates the axis of the slow spreading southwest indian ridge as well as the seafloor that it has generated has a distinctly crenulated appearance this crenulated seafloor shows regularly spaced gravity lineations in the spreading direction crenulations that record stationary centers of mantle upwelling over the life of the southwest indian ridge a similar pattern of crenulation is seen along sections of the slow spreading northern mid atlantic ridge but along the faster intermediate spreading southern mid atlantic ridge while regularly spaced highs are apparent along sections of the spreading axis stationary crenulation does not persist off the axis this pattern implies that the faster spreading southern mid atlantic ridge has variable centers of time dependent upwelling that migrate along the ridge crenulations are not visible near the axis of the fast spreading pacific antarctic ridge crenulation amplitudes decrease strongly with increasing spreading rate the absence of crenulation at high spreading rate could be due to a 3d 2d transition in flow structure or the effect of mechanisms to redistribute crust along axis while crenulations are not ubiquitous even at slowly spreading ridges where they exist they provide a time record to observe the characteristics of mid ocean ridge upwelling melting and melt migration not only beneath active spreading centers but through a much longer spreading history