transitions in axial morphology along the southeast indian ridge shipboard bathymetric and magnetic profiles across the southeast indian ridge seir were analyzed in order to examine the nature of along axis variations in axial morphology at this intermediate spreading rate ridge three types of axial morphology are observed along the seir an axial high a shallow 200 700 m deep axial valley and a deep 1000 m deep axial valley an axial high is found to the east of the australian antarctic discordance aad east of 128 e and between 82 e and 104 e a shallow rift valley is found from 104 e to 114 e and from 82 e westward past the amerstdam st paul hotspot asp to about 30 s 75 e deep rift valleys are found from 114 e to 128 e in the vicinity of the aad and from the indian ocean triple junction iotj at 25 s 70 e to about 30 s 75 e the transition near 30 s occurs in an area of constant zero age depth and does not appear to result from an increase in mantle temperature it could be the result of the rapid increase in spreading rate along that portion of the seir the most likely cause of the other transitions in axial morphology is variations in mantle temperature the transitions between the different types of axial morphology are well defined and occur over a limited distance transitions in axial morphology are accompanied by significant changes in ridge flank topographic roughness the transitions from axial valleys to axial highs are also accompanied by changes in the amplitude of the seafloor magnetic anomalies our observations suggest that there are distinct modes rather than a continuum of axial morphology on the seir and that there appears to be a threshold mechanism for a rapid change between different states of axial morphology the asp has only a limited influence on the seir the ridge axis is marked by an axial valley for the entire distance from the iotj up to and past the asp the ridge axis becomes shallower as the asp is approached from the northwest but only by about 300 m over a distance of 800 km in addition the ridge continues to become shallower away from amsterdam island toward the transition to an axial high at 82 e 350 km to the east of the asp the kerguelen hotspot appears to exert a major influence on the morphology of the seir by feeding asthenospheric material to the ridge axis a long narrow finger like gravity high extends ene away from the kerguelen plateau for a distance of 500 km shipboard data show that the gravity high results from a large volcanic ridge the ridge appears analogous to the rodriguez ridge extending from the reunion hotspot toward the central indian ridge a series of lower and broader lineated gravity highs extend from the volcanic ridge toward the seir in the ridge segment between the 81 e and 85 e transforms which is the westernmost segment with an axial high the only region of significant off ridge seismicity on the antarctic flank of the seir is a diffuse band of epicenters extending from kerguelen to the seir within the segment between the 81 e and 85 e fracture zones the along axis gradient in depth from 86 e to the aad and the transitions in axial morphology at 104 e and 114 e most likely reflect along axis variations in mantle temperature and melt production rate due to distance from the kerguelen hotspot and the influence of the aad

upper crustal seismic velocity structure and microearthquake depths at the endeavour segment juan de fuca ridge 1 we present the results of a study to invert microearthquake and explosive shot data from the endeavour segment of the intermediate spreading juan de fuca ridge the average isotropic p wave velocity structure derived from the shot data in the uppermost 1 5 km of the oceanic crust is characterized by an increase with age of 8 from the axis to at least 0 5 ma that is attributed to the sealing of layer 2a porosity by hydrothermal processes superimposed on this variation are axis parallel 2 km wide alternating bands of high and low velocity with a peak to peak variation of 5 12 high and low velocities away from the axis correspond to bathymetric trenches and ridges respectively and are likely due to variations in layer 2a thickness p wave azimuthal anisotropy is present in the data that is best fit with a model of 9 anisotropy at 750 m depth decreasing to 1 at 3 km depth and is likely due to the preferential alignment of vertical cracks and fissures in the along axis direction anisotropy and velocity heterogeneity are coupled anisotropy alone may explain the form but not the magnitude of the axis parallel bands there are strong trade offs between the hypocentral depths of microearthquakes and the p and s wave velocity structures changing the mean hypocentral depth by up to 0 5 km leads to only modest increases in the travel time rms but the resulting velocity models appear more feasible when the earthquakes are forced deeper than when they are forced shallower

controls on extrusion at mid ocean ridges a magma lens can erupt to form extrusives only if it is under greater pressure than the static pressure in a column of magma reaching from the lens to the surface the excess pressure results partly from overburden pressure caused by the presence of high and low density rocks dikes and extrusives respectively above the lens the thicker the pile of low density extrusives the lower the average overburden density thus extrusion above a lens should be self regulating in that thickening the extrusive layer reduces the driving pressure for subsequent eruptions flexural stresses may affect extrusion by altering the pressure on a magma chamber for ridges lacking an axial valley we predict that deeper magma lenses should correlate with thicker extrusive layers consistent with recent observations

axial summit trough of the east pacific rice 9 10 n geological characteristics and evolution of the axial zone on fast spreading mid ocean ridge the nature and morphological characteristics of axial summit troughs on fast 90 130 mm yr 1 full spreading rate and superfast spreading 130 mm yr 1 mid ocean ridge crests reflect the time integrated effects of long term magmatic cycles short term volcanic episodicity and the tensional stress regime imposed on young ocean crust two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid ocean ridge crests 1 narrow axial troughs 300 2000 m wide and 30 100 m deep on the east pacific rise crest are classified as axial summit graben the dimensions of axial summit graben as well as the morphological and structural character of their walls and floors suggest a primary tectonic origin an axial summit graben may contain a nested axial summit collapse trough implying that processes responsible for these endemic features may be linked near bottom side looking sonar and observational data collected using the towed vehicle argo i and submersible alvin have been used to characterize the axial summit trough of the fast spreading east pacific rise between 9 and 10 n a four stage model is presented for the evolution of this axial summit collapse trough as well as for other well studied portions of the east pacific rise crest from 21 n to 20 s we propose that the transition from a narrow surface collapse dominated axial trough to a broader fault bounded graben is controlled by the relative importance of diking volcanism hydrothermal cooling and tectonism along a ridge segment over time periods 104 years

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

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

variations in axial morphology along the galapagos spreading center and the influence of the galapagos hotspot the galapagos spreading center gsc is marked by systematic changes in axial morphology between the inca fracture zone fz at 85 5 w and the 95 5 w propagator we analyze these changes using new swath bathymetry and magnetic data acquired aboard the b o hesperides during the galapagos 96 experiment within 350 km of the galapagos hotspot the ridge axis is associated with an east pacific rise epr like axial high at increasing distance from the hotspot the axial high broadens and deepens forming a distinctive transitional axial morphology tam the axis in this transitional region is typically a broad zone 20 km wide consisting of very rough volcanic and fault generated topography west of 95 w this tam evolves into a 20 40 km wide 400 1500 m deep axial valley typical of the slow spreading mid atlantic ridge mar there is not an abrupt change from axial high to rift valley along the gsc but a distinct tam occurs over a distance of 200 300 km along axis and is accompanied by a gravity estimated crustal thickening of 1 2 km the boundary between an axial high and this tam is quite abrupt and occurs along a segment that is less than 9 km long these changes in axial morphology are primarily caused by variations in magma supply along the gsc due to the entrainment and dispersal of plume mantle from the galapagos hotspot however the changes in morphology are not symmetric about the galapagos fz at 91 w the axial high topography extends farther east of the 91 w fz than to the west and the rift valley which develops west of 94 w is not found at comparable distances along the gsc east of the hotspot axial depth variations are also asymmetric across the 91 w fz this asymmetry in both morphology and axial depth variation is attributed to a full spreading rate increase along the gsc from 46 mm yr at 97 w to 64 mm yr at 85 w off axis depth changes are symmetric about the 91 w fz and suggest that 15 40 of on axis depth variation is dynamically supported

constraining crustal emplacement processes from the variation in seismic layer 2a thickness at the east pacific rise abstract a stochastic model for the emplacement of dikes and lava flows at a fast spreading ridge can generate an upper oceanic crustal structure similar to that observed in seismic data from the east pacific rise epr in ocean drill holes and in ophiolites in this model the location of successive dike intrusion events relative to the ridge axis is determined by a gaussian probability function and the cumulative flow lengths of the erupted lavas are chosen to build a gaussian shaped lava pile we interpret wide angle seismic reflections from the steep velocity gradient at the base of seismic layer 2a to be the extrusive sheeted dike contact seismic data from the northern and southern epr place constraints on the on axis extrusive layer thickness 230 50 m the distance over which the thickening of the extrusive layer occurs width of the accretion zone 1 3 km and its off axis thickness 300 800 m ophiolites and ocean drill holes dsdp hole 504b provide additional estimates of the thickness of the extrusive layer and constrain the thickness of the transition region from extrusives to sheeted dikes 100 200 m a simple stochastic emplacement model where the lavas are described by one mean flow length fits the thickening of the extrusive layer off axis inferred from the deepening of seismic layer 2a but the predicted transition from sheeted dikes to extrusives is too thick in order to match the dimensions and flat topped shape of the seismic layer 2a boundary as well as the thickness of the extrusive sheeted dike transition a bimodal distribution of lava flows is used short flows confined within the axial summit caldera asc build up approximately half the extrusive volume occasional voluminous flows spill out of the asc or erupt outside of the asc and pond at a considerable distance off axis to build up the remainder of the extrusive section the upper part of the final extrusive section will be dominated by the off axis flows while the lower portions will be primarily composed of short flows erupted within the asc magnetic transition widths predicted from the overlap of lavas 2 km in this model are similar to those measured in deep tow studies assuming a smoothing function which acts over one seismic wavelength the upper crustal velocity structure predicted by the bimodal lava emplacement model is consistent with the shallow seismic velocity structure measured on the epr the ages of seafloor lavas in this model are younger than the tectonic spreading model ages by 30 70 kyr in agreement with anomalously young lava ages obtained from radioisotope dating of seafloor basalts near the epr

seismic structure and anisotropy of the juan de fuca ridge at 45 n a seismic refraction experiment was conducted with air guns and ocean bottom seismometers on the juan de fuca ridge at 45 n at the northern cleft segment and at the overlapping rift zone between the cleft and vance segments these data determine the average velocity structure of the upper crust and map the thickness variability of the shallow low velocity layer which we interpret as the extrusive volcanic layer the experiment is unique because a large number of travel times were measured along ray paths oriented at all azimuths within a small 20 km by 35 km area these travel times provide evidence for compressional velocity anisotropy in the upper several hundred meters of oceanic crust presumed to be caused by ridge parallel fracturing compressional velocities are 3 35 km s in the ridge strike direction and 2 25 km s across strike travel time residuals are simultaneously inverted for anisotropy as well as lateral thickness variations in the low velocity layer extrusive layer thickness ranges from approximately 200 m to 550 m with an average of 350 m the zone of the thinnest low velocity layer is within the northern cleft segment axial valley in a region of significant hydrothermal activity layer thickness variability is greatest near the cleft vance overlapping rift zone where changes of 300 m occur over as little as several kilometers laterally these low velocity layer thickness changes may correspond to fault block rotations in an episodic spreading system where the low side of each fault block accumulates more extrusive volcanics

bulk rock geochemistry and plagioclase zoning in lavas exposed along the northern flank of the western blanco depression northeast pacific insight into open system magma chamber processes abstract the present study is based on a set of lavas and crosscutting dikes collected by dives along detailed vertical transects on the northern flank of the western part of the blanco transform fault northeast pacific the studied area consists of a small basin the western blanco depression wbd extending from the southern end of the juan de fuca ridge to a pseudofault trace 60 km eastward the northern scarp of the wbd comprises a volcanic unit overlying a sheeted dike complex major and trace element data coupled with sr nd isotope ratios reveal a two component mantle source composed by an isotopically depleted matrix variably veined by more enriched material one chemical group ns2 indistinguishable from the other northern scarp samples on the basis of trace element data has an unusually depleted isotopic composition typical of a nearly pure mantle end member some cogenetic samples of the northern scarp have been used to constrain the differentiation modalities anorthite and mgo content profiles in plagioclase xenocrysts and phenocrysts reveal i the existence of h2o bearing evolved melts in the mushy zones and ii the occurrence of mixing process between these melts and anhydrous mafic liquids the hydration is supported by other petrographic features such as high magmatic fo2 values calculated from fe ti oxide pairs and the presence of pyroxene inclusions in plagioclase phenocrysts mixing consistent with the existence of ni rich ferrobasalts is interpreted to be the consequence of the reservoir refilling by mafic liquids mg 70 these petrological and geochemical evidences are combined with the evolution of mg with depth to suggest a periodic open system magma chamber evolution beneath the southern end of the juan de fuca ridge

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

crustal structure of endeavour ridge segment juan de fuca ridge from a detailed seismic refraction survey a detailed seismic refraction survey was carried out over the endeavour segment of the juan de fuca ridge a medium rate spreading center which lies off western north america to investigate the creation and evolution of oceanic crust a bathymetric high and the presence of hydrothermal vents suggested that the study area was the most recent locus of spreading travel time and amplitude data from 15 in line air gun ocean bottom seismometer profiles were forward modeled using asymptotic ray theory to obtain two dimensional velocity models consisting of four primary layers which correlate well with classic models of oceanic crust significant lateral variations in thicknesses and velocities on the scale of a few to 10 km are superimposed on this basic velocity structure but they appear to be random rather than distributed symmetrically about the ridge we attribute them to fracturing which causes porosity changes hydrothermal circulation which fills voids and fractures with alteration products and variations in magmatic and or deformational processes at the spreading center layer 2a is found to have low 2 6 2 8 km s velocities to average 0 4 km in thickness with variations up to 0 2 km and to be bounded at its base by a sharp velocity increase to 4 8 km s along the axial ridge velocities 0 4 0 6 km s higher than average are interpreted for layers 2b and 2c but these values are confined to a 2 km wide zone centered below the ridge velocities along ridge parallel lines offset 10 km are normal indicating that maturation to off ridge structure has occurred within at most 0 3 ma layer 3 velocities decrease by 0 1 0 2 km s for arrivals traveling along and under the axial ridge perhaps caused by higher temperatures however we find no anomalously low velocities beneath the ridge indicating that no large crustal magma chamber exists on the basis of this study we conclude that magmatic accretion is a fully three dimensional process within ridge segments such as endeavour ridge

variations in upper crustal structure due to variable mantle temperature along the southeast indian ridge 1 there is a systematic variation in axial morphology and axial depth along the southeast indian ridge seir with distance away from the australian antarctic discordance an area of cold uppermost mantle since spreading rate 72 76 mm yr and mantle geochemistry appear constant along this portion of the seir the observed variations in axial morphology and axial depth are attributed to a gradient in mantle temperature in this study we report results from a multichannel seismic investigation of on axis crustal structure along this portion of the seir three distinct forms of ridge crest morphology are found within our study area axial highs rifted axial highs and shallow axial valleys axial highs have a shallow 1500 m below seafloor bsf magma lens and a thin 300 m layer 2a along the ridge crest rifted axial highs have a deeper 2100 m bsf magma lens and thicker 450 m layer 2a on axis beneath shallow axial valleys no magma lens is imaged and layer 2a is thick 450 m there are step like transitions in magma lens depth and layer 2a thickness with changes in morphology along the seir the transitions between the different modes of axial morphology and shallow structure are abrupt suggesting a threshold type mechanism variations in crustal structure along the seir appear to be steady state persisting for at least 1 m y portions of segments in which a magma lens is found are characterized by lower relief abyssal hills on the ridge flank shallower ridge flank depths and at the location of along axis mantle bouguer anomaly mba lows the long wavelength variation in ridge morphology along the seir from axial high segments to the west to axial valley segments to the east is linked to the regional gradient in mantle temperature superimposed on the long wavelength trend are segment to segment variations that are related to the absolute motion of the seir to the northeast which influence mantle melt production and delivery to the ridge

a multichannel seismic investigation of upper crustal structure at 9 n on the east pacific rise implications for crustal accretion reprocessed multichannel seismic profiles from the 9 n segment of the east pacific rise reveal prominent shallow subbasement events these events are identified as wide angle reflections from the base of seismic layer 2a based upon modeling of expanding spread profile data and velocity functions the layer 2a reflections typically increase from 0 15 s after the seafloor reflection at the rise axis to 0 3 0 45 s within 1 2 km of the axis corresponding to an increase in layer thickness of 200 600 m no further systematic increase in layer thickness is observed although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis however the intermittent character of the imaged layer 2a reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer the base of layer 2a is interpreted as corresponding to the transition between the extrusive section pillow basalts and sheet flows and a sheeted dike complex the rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone lateral variability of layer 2a can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2a are approximately half that of layer 2b for an along axis profile apparent along axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2a caused by wandering of the profile relative to axis within the resolution of the data the time delay of the magma chamber reflection relative to the base of layer 2a is constant

hydrothermal circulation at the cleft vance overlapping spreading center results of a magnetometric resistivity survey we report on a magnetometric resistivity sounding carried out in the overlapping spreading center between the cleft and vance segments of the juan de fuca ridge the data collected reveal a strong three dimensionality in the crustal electrical resistivity structure on wavelengths of a few kilometers areas of reduced crustal electrical resistivities with values approaching that of seawater are seen beneath the neovolcanic zones of both active spreading centers we interpret these reduced resistivities as evidence of active hydrothermal circulation within the uppermost 1 km of hot young oceanic crust

fine scale seismic structure of young upper crust at 17 20 s on the fast spreading east pacific rise 1 the detailed upper crustal structure of the east pacific rise epr at 17 20 s is examined by applying a genetic algorithm based waveform inversion to five multichannel seismic lines one on axis and two to either side along 42 and 85 kyr old crust on axis a double stepped velocity pattern is recorded beneath 70 100 m of low velocity extrusives 2 1 2 4 km s 1 we define the upper velocity contrast as the base of seismic layer 2a due to its severity and continuity along and across axis the more subdued and intermittent lower velocity step is not observed off axis material between the two high gradient intervals is proposed to represent the pillow dike transition bounded above by a sharp increase in dike fraction with depth and below by an abrupt change in rheology and or deformation extrusive velocities increase quite rapidly in this area with velocities 3 km s 1 common in crust 85 kyr old this plus a rapid 300 400 m thickening of layer 2a observed within 1 4 km of the rise axis indicates that this segment is undergoing focused melt delivery 500 m wide dike intrusion zone and elevated hydrothermal activity these findings demonstrate the ability of single ship multichannel data to record detailed information on the reflectivity and velocity of the upper crust and the ability of the genetic algorithm to efficiently construct accurate seismic models based on this information

fine scale velocity structure of upper oceanic crust from full waveform inversion of downward continued seismic reflection data at the lucky strike volcano mid atlantic ridge 1 we present a fine scale 2d velocity structure beneath the lucky strike volcano on the mid atlantic ridge mar using an elastic full waveform inversion fwi method the fwi is a data driven procedure that allows simultaneous exploitation of both reflections and refractions energy in multi channel seismic data to create a single self consistent high resolution velocity image of the upper crust that can be used for geologic interpretation the long wavelength background p wave velocity model required by the local optimization approach was created using a combination of downward continuation and 3d first arrival travel time tomography the elastic waveform inversion was applied to carefully windowed downward continued data where wide angle reflections and refractions arrive in front of the water wave and are thus isolated from the high amplitude seafloor scattering energy that is particularly acute in areas of rough igneous seafloor waveform inversion reduces the misfit of the initial model by 76 after 19 iterations and strongly reduced the size of the residuals relative to the signal size the final model shows fine scale structure beneath the northern part of the lucky strike volcano on a resolution of tens of meters evidence for successive lava sequences testifies to the constructional origin of the upper section of layer 2a normal faults are revealed within the shallow crust and are strongly correlated with seafloor observations

seismic images of active magma systems beneath the east pacific rise between 17 05 and 17 35 s seismic reflection data from the east pacific rise between 17 059 and 17 359s image a magma lens that varies regularly in depth and width as ridge morphology changes confirming the notion that axial morphology can be used to infer ridge magmatic state however at 17 269s where the ridge is locally shallow and broad the magma lens is markedly shallower and wider than predicted from regional trends in this area submersible dives reveal recent volcanic eruptions these observations indicate that it is where the width and depth of the magma chamber differ from regional trends indicating an enhanced magmatic budget that is diagnostic of current magmatism

geology of the northern cleft segment juan de fuca ridge recent lava flows sea floor spreading and the formation of megaplumes geologic mapping and lava sampling were carried out after the discovery of large bursts of hydrothermal fluids megaplumes over the southern juan de fuca ridge in 1986 and 1987 our investigations of the northernmost section of the cleft segment have discovered 1 semicontinuous low temperature venting and one major high temperature vent site along 17 km of the neovolcanic zone and 2 very glassy lightly sedimented sheet flows and pillow mounds superimposed on older terrain over about 24 km along the northern most part the pillow mounds are documented to have erupted between 1981 and 1987 the occurrence of the megaplumes during this same time period strengthens the hypothesis that megaplumes are caused by sea floor extension events although the basalts from the entire length of the neovolcanic zone of the cleft segment appear to have been derived from the same mantle source a systematic northward increase in mg number along the segment within the neovolcanic zone indicates less shallow level differentiation to the north possibly related to the development of new magma chambers during the recent phase of sea floor spreading that has occurred there

seismic velocities of the uppermost igneous crust versus age summary seismic refraction velocity data from the acoustic basement called layer 2a have been compiled for different mid ocean ridges the data from post 1970 studies show a strong correlation between velocity and basement age importantly velocities double in less than 10 myr for all ridges but for older crust up to 160 myr in age velocities do not increase significantly

volcanic growth faults and the origin of pacific abyssal hills the topographic features known as abyssal hills characterize 30 of the ocean floor and yet their origin has been the subject of vigorous debate for over 40 years submersible based investigations show that pacific abyssal hills are created on the flanks of the east pacific rise as horsts and grabens which lengthen with time hills are bounded on one side by ridge facing scarps produced by normal faulting and on the other by more gentle slopes produced by volcanic growth faulting

nd and pb isotopic variations through the upper oceanic crust in dsdp odp hole 504b costa rica rift abstract the dsdp odp hole 504b drilled in the 5 9 ma southern flank of the costa rica rift represents the deepest section through modern ocean floor basaltic basement the hole penetrates a 570 m thick volcanic zone a 210 m thick transition zone of volcanic rocks and dykes and 1056 m of dykes a representative selection of these basalt types has been investigated with respect to nd and pb isotopes the nd of the basalts varies from 7 62 to 11 16 this range in the nd isotope composition represents about 67 of the total range reported for pacific morb the pb isotope composition also shows significant variation with 206 pb 204 pb varying from 17 90 to 18 82 the isotopic data show that a small volume of enriched mantle existed in the source the large ranges in isotopic composition in a single drill hole demonstrate the importance of small scale mantle heterogeneities in the petrogenesis of morb fractional melting and extraction of small magma batches by channelled flow and small short lived crustal magma reservoirs with limited potential for mixing of the mantle derived magmas are favored by these isotopic data

lava flows from a mid 1980s submarine eruption on the cleft segment juan de fuca ridge a series of lava flows with a total volume of 0 05 km3 were erupted in the mid 1980s along 17 km of the northern cleft segment of the juan de fuca ridge observations from camera tows and submersible dives show that the new flows are all similar in appearance and consist entirely of pillow lava with a mixture of smooth and striated surface textures suggesting a relatively uniform eruption rate approaching 1 m3 s at point source vents the flows vary in size from small patches to large steep sided ridges and were probably erupted from a dike intruded along the ridge axis because they are aligned along a linear fissure graben system observations at north cleft show that the physical appearance of new flows changes more rapidly than previously realized and that earlier qualitative dating of young lavas based on sediment cover and glassy surface texture were probably overestimates by an order of magnitude sediment accumulation on the lavas is quite variable and locally surprisingly substantial mainly due to hydrothermal deposits that formed while the lava flows were cooling biological vent communities photographed on the new flows in 1989 show that vent animals can colonize new vent sites rapidly but that warm water was still venting only in a few places nonvent animals are much slower to colonize the new flows and rates of colonization observed at north cleft may be useful for making improved age estimates of young 10 years lava flows elsewhere the north cleft eruption represents about 2 of the estimated average annual volcanic output along the global mid ocean ridge implying that many other submarine eruptions are occurring undetected

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

volcanic and structural morphology of the south flank of axial volcano juan de fuca ridge results from a sea marc i side scan sonar survey a 1500 km2 sea marc i side scan sonar survey south of axial volcano investigated the geological structure and constitution of the south axial rift zone sarz and the northern 30 km of the vance spreading segment relative age assignments based on structural relationships indicate that recent volcanism on the southern flank of axial volcano has been restricted to the sarz the surficial volcanic morphology of the sarz changes downrift from linear volcanic ridges north to small 1 km diameter cratered cones south perhaps indicating a variation in eruptive vent geometry from fissures to point sources downrift recent lavas on the northern and central sarz erupted along preexisting faults in the underlying crust by acting as pathways for sarz magmas these faults may have controlled the orientation of sarz volcanic ridges and perhaps the orientation of the entire sarz edifice the vance spreading segment terminates at 45 40 n and is not linked with a transform fault the axis of spreading between 45 40 n and 46 00 n is offset west of the vance segment and is constrained to lie between 129 54 w and 130 06 w a discrete structurally defined spreading axis however is not evident over the southern flank of axial volcano the divergent plate boundary may now underlie the sarz massif although south helium basin an embayment in the southeast flank of axial volcano has undergone and may continue to accommodate crustal extension a 48 km2 lava field discovered east of the sarz is composed of lavas that are inferred to have erupted from the sarz these lavas can be traced eastward from the sarz into the axial valley of the northern vance segment and partially fill the valley with lavas up to 60 m thick the eruption of these lavas which occupy an estimated volume of 1 8 km3 may have contributed to the formation of axial volcano s summit caldera

fine scale seismic structure of the shallow volcanic crust on the east pacific rise at 9 50 n upper crust in a 3 3 km field area centered on the east pacific rise at 9 50 0 n we detonated 18 explosive shots 18 sources in a circular pattern 1 5 km radius on the rise axis and recorded seismic arrivals with eight ocean bottom seismometers eight receivers we observed 30 40 hz compressional body waves from all shots 144 p waves and 1 3 hz stoneley interface waves on a subset of source receiver pairs 58 interface waves using a station correction inversion we find that roughly half of the variance in the p wave first arrival times results from lateral variations in the thickness of the surface low velocity layer slvl a layer of extremely porous lava and basalt breccia with an average p wave velocity of 2 2 km s 1 the slvl thickness increases from 20 m along the axial summit trough ast to 120 m at near axis lava depocenters which are not symmetric about the rise axis depocenters are located 0 5 km to the west and 1 5 km to the east of the rise axis tomographic inversion of the stoneley wave first arrivals reveals that shear velocities in the slvl covary with the layer thickness exhibiting a similar asymmetric pattern with shear velocities increasing from 320 m s 1 near the ast to 520 m s 1 at the near axis depocenters our analysis demonstrates that the seismic characteristics of the extrusive layer near the rise axis are related primarily to volcanic features and processes the thickness and velocity of the slvl are low on the axis and within channel networks that deliver lava flows away from the axis and then increase rapidly at the distal ends of the channels where the lavas are deposited we find that azimuthal anisotropy exerts only a weak influence on our p wave first arrival times which we model as weak 4 seismic azimuthal anisotropy in the upper dikes with a fast axis oriented n23 32 w we find no evidence for seismic azimuthal anisotropy in the extrusive layer index terms 3025 marine geology and geophysics marine seismics 0935 3035 marine geology and geophysics midocean ridge processes 7220 seismology oceanic crust

dike induced faulting in rift zones of iceland and afar geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the earth9s surface elastic modeling of the vertical ground surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth a mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike therefore slip extending into or beyond the dike cavity must occur either 1 on faults that intersect the dike near its top above the zone of dike induced compression or 2 on faults that slip ahead of the dike as it propagates laterally data from iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike

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

no spreading across the southern juan de fuca ridge axial cleft during 1994 1996 direct path acoustic measurements between seafloor transponders observed no significant extension 10 14 mm yr from august 1994 to september 1996 at the southern juan de fuca ridge 44 40 n and 130 20 w the acoustic path for the measurement is a 691 m baseline straddling the axial cleft which bounds the pacific and juan de fuca plates given an expected full spreading rate of 56 mm yr these data suggest that extension across this plate boundary occurs episodically within the narrow 1 km region of the axial valley floor and that active deformation is occurring between the axial cleft and the plate interior a cleft parallel 714 m baseline located 300 m to the west of the cleft on the pacific plate monitored system performance and as expected observed no motion 5 7 mm yr between the 1994 and 1996 surveys

a global analysis of mid ocean ridge axial topography summary current views of mid ocean ridges are strongly influenced by extensive mapping of the mid atlantic ridge and east pacific rise the global picture of the mid ocean ridge system particularly in the sparsely surveyed southern oceans is still based primarily on underway bathymetry profiles collected over the past 40 years this study presents a quantitative analysis of global mid ocean ridge morphology based on 156 of these underway bathymetric profiles thereby allowing commonly recognized features such as axial valleys and axial ridges to be compared on a global basis an empirical orthogonal function eof analysis is used to separate deterministic and stochastic components of axial morphology and to quantify the dependence of each on parameters such as spreading rate and axial depth it is found that approximately 50 per cent of the variance in axial morphology may be described as a linear combination of five independent symmetric and anti symmetric modes the remainder is considered stochastic maximum axial valley relief decreases with spreading rate for rates less than 80 mm yr 1 while axial ridge relief remains relatively constant for all rates greater than 50 mm yr 1 the stochastic component of the axial morphology referred to as bathymetric roughness also decreases with spreading rate for rates less than 80 mm yr 1 and remains relatively constant at higher rates although both axial valley relief and bathymetric roughness near the ridge axis show a similar spreading rate dependence they are weakly correlated at slow spreading centres the distinct differences in morphologic variability of fast and slow spreading ridges may result from the episodicity of magmatic heat input which controls the lithospheric rheology at slow spreading ridges these observations support the notion of a critical threshold separating two dynamically distinct modes of lithospheric accretion on mid ocean ridges

influence of magma supply and spreading rate on crustal magma bodies and emplacement of the extrusive layer insights from the east pacific rise at lat 16 n seismic reflection data from the east pacific rise at lat 16 n which is spreading at the high end of intermediate rates suggest that the depths at which axial magma chambers reside do not vary smoothly as a function of spreading rate rather magma chamber depths form two distinct populations each associated with a distinct axial morphology and with an abrupt transition occurring within the intermediate spreading rate range our data 1 show that the melt lens at high intermediate spreading ridges lies at a shallow level similar to lens depths at faster spreading ridges and 2 provide further support for the spreading rate invariance of ridges with axial highs noted in other ridge properties the axial morphology of the two ridge segments within the study area differs markedly and a large contrast in magma supply is inferred the ridge segment with greater magma supply is associated with a broader and more continuous melt lens a wider region over which the extrusive crust accumulates and a thicker extrusive layer off axis where supply to the ridge segment appears to be centered however on axis the extrusive layer is thinnest where magma supply is robust and a shallower melt lens is observed consistent with a model in which magma pressure controls the thickness of the extrusive layer accumulated above the magma lens

seismic constraints on shallow crustal emplacement processes at the fast spreading east pacific rise we present the results of nine on bottom seismic refraction experiments carried out over young east pacific rise crust the experiments are unusual in that both the source and receiver are located within a few meters of the seafloor allowing high resolution determinations of shallow crustal structure three experiments were located within the axial summit caldera asc over zero age crust the seismic structure at these three locations is fundamentally the same with a thin 60 m surficial low velocity 2 5 km s layer a 100 to 150 m thick transition zone with velocities increasing by 2 5 km s and a layer with velocities of 5 km s at a depth beneath the seafloor of 130 190 m the surficial low velocity layer and transition zone are defined as seismic layer 2a and the 5 km s layer is defined as the top of layer 2b both the surficial low velocity layer and the transition zone double in thickness within 1 km of the rise axis we model layer 2a as the extrusive sequence and transition zone and the 2a 2b boundary as the top of the sheeted dikes the primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from 150 m to 300 m within 1 km of the asc the thickening of the extrusive layer is interpreted to be due to lava that either overflows the asc walls is emplaced through eruptions outside of the asc or travels laterally from the asc through conduits the most probable cause for the thickening of the transition zone is sill emplacement outside of the asc either from magma that does not reach the surface in an off axis eruption or magma that is transported laterally during the drainage process creating the asc we suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2a

the east pacific rise in cross section a seismic model in 1982 we undertook a seismic refraction experiment known as the magma expedition to examine the detailed structure of the east pacific rise near 12 50 n this segment of the rise where the full spreading rate is about 110 mm yr is near the projected trace of the o gorman fracture zone and is the site of an overlapping spreading center as well as black smoker hydrothermal activity in this paper we describe the analysis of a subset of the travel time data collected during the magma expedition namely the data from profiles which were oriented normal to the rise axis these profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross sectional structure of the rise we have modeled these data using a two dimensional ray tracing program we have found that the seismic velocities in the young oceanic crust are rather high with velocity gradients of 4 0 5 5 s 1 in the uppermost crust the highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0 1 ma this decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads the decrease in velocity does not appear to penetrate deeper than about 0 5 km and may reverse itself as hydrothermal alteration fills the pores and cracks the fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits a result consistent with the high temperatures and discrete nature of the vents our best model includes a magma chamber some 4 km wide and extending from the moho to within about 1 1 km of the seafloor this magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the east pacific rise these discrepancies probably arise because the magma chamber under the east pacific rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis

crustal thickness along the western galapagos spreading center and the compensation of the galapagos hotspot swell wide angle refraction and multichannel reflection seismic data show that oceanic crust along the galapagos spreading center gsc between 97 w and 91 25pw thickens by 2 3 km as the galapagos plume is approached from the west this crustal thickening can account for v52 of the 700 m amplitude of the galapagos swell after correcting for changes in crustal thickness the residual mantle bouguer gravity anomaly associated with the galapagos swell shows a minimum of 325 mgal near 92 15pw the area where the gsc is intersected by the wolf darwin volcanic lineament wdl the remaining depth and gravity anomalies indicate an eastward reduction of mantle density estimated to be most prominent above a compensation depth of 50 100 km melting calculations assuming adiabatic passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of v30 c the associated thermal expansion and increase in melt depletion reduce mantle densities but to a degree that is insufficient to explain the geophysical observations the largest density anomalies appear at the intersection of the gsc and the wdl our results therefore require the existence of compositionally buoyant mantle beneath the gsc near the galapagos plume possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the galapagos islands that is later transported to the gsc our estimate for the buoyancy flux of the galapagos plume 700 kg s 31 is lower than previous estimates while the total crustal production rate of the galapagos plume 5 5 m 3 s 31 is comparable to that of the icelandic and hawaiian plumes

high sensitivity of ocean ridge thermal structure to changes in magma supply the galapagos spreading center abstract we explore the physical mechanisms for the observed apparent sensitivity of ridge axis topography and crustal magma systems to small changes in magma supply at intermediate spreading rates numerical experiments were carried out to simulate crustal temperature structure of the galapagos spreading center which spreads at intermediate spreading rates with its various sections appear to have been influenced to different degrees by the nearby galapagos hotspot model results show a strong threshold effect as the crustal thickness decreases from 7 4 km at the 92 w area with an axial high westward to 6 0 km at the 94 w area with a transitional topography the depth to the top of a magma lens is calculated to increase from 1 7 to 2 5 km in contrast at the 97 w area where crustal thickness is only 5 6 km and a rift valley is present the model results predict no steady state magma lens in the crust these model calculations provide a simple physical explanation for the recent observations along the galapagos spreading center where abrupt changes in both magma lens and axial morphology occur within a short distance but crustal thickness changes only modestly results of this investigation illustrate the critical importance of hotspots in influencing mid ocean ridge crustal thickness and the associated changes in thermal structure especially for ridges that spread at the sensitive range of intermediate spreading rates

correlated geophysical geochemical and volcanological manifestations of plume ridge interaction along the galapagos spreading center 1 as the galapagos hot spot is approached from the west along the galapagos spreading center there are systematic increases in crustal thickness and in the k ti nb zr 3he 4he h2o and na2o content of lavas recovered from the spreading axis these increases correlate with progressive transitions from rift valley to axial high morphology along with decreases in average swell depth residual mantle bouguer gravity anomaly magma chamber depth average lava mg ca al ratio and the frequency of point fed versus fissure fed volcanism magma chamber depth and axial morphology display a threshold effect in which small changes in magma supply result in large changes in these variables these correlated variations in geophysical geochemical and volcanological manifestations of plume ridge interaction along the western galapagos spreading center reflect the combined effects of changes in mantle temperature and source composition on melt generation processes and the consequences of these variations on magma supply axial thermal structure basalt chemistry and styles of volcanism

suppression of sea floor scattered energy using a dip moveout approach application to the mid ocean ridge environment multichannel seismic mcs images are often contaminated with in and out of plane scattering from the sea floor this problem is especially acute in the mid ocean ridge environment where sea floor roughness is pronounced energy shed from the unsedimented basaltic sea floor can obscure primary reflections such as moho and scattering off of elongated sea floor features like abyssal hills and fault scarps can produce linear events in the seismic data that could be misinterpreted as subsurface reflections moreover stacking at normal subsurface velocities may enhance these water borne events whose stacking velocity depends on azimuth and generally increases with time making them indistinguishable from subsurface arrivals to suppress scattered energy in deep water settings we propose a processing scheme that invokes the application of dip moveout dmo to deliberately increase the differential moveout between sea floor scattered and subsurface events thereby facilitating the removal of unwanted energy in the stacked section after application of dmo all sea floor scatterers stack at the water velocity while subsurface reflections like moho still stack at their original velocity the application of dmo in this manner is contrary to the intended use that reduces the differential moveout between dipping events and allows a single stacking velocity to be used unlike previous approaches to suppress scattered energy dip filtering is applied in the common midpoint cmp domain after dmo moveover our dmo based approach suppresses out of plane scattering and therefore is not limited to removal of in plane scattering as is the case with shot and receiver dip filtering techniques the success of our dmo based suppression scheme is limited to deep water a few kilometers of water depth for conventional offsets where the traveltime moveout of energy scattered from the sea floor has a hyperbolic moveout with a stacking velocity that depends on the cosine of the scatterer steering angle in a manner analogous to how the moveout of a dipping reflector depends on the dip angle the application of dmo based suppression to synthetics and mcs data collected along the southern east pacific rise demonstrates the effectiveness of our approach cleaner images of primary reflectors such as moho are produced even though present shot coverage along the east pacific rise is unduly sparse resulting in a limited effective spatial bandwidth

bathymetric roughness of the southeast indian ridge implications for crustal accretion at intermediate spreading rate mid ocean ridges the nature of the transition from axial highs to axial valleys at mid ocean ridges and the physical processes involved in the transition are important for understanding how axial morphology changes with spreading rate mantle temperature and lithospheric strength at mid ocean ridges in order to provide observational constraints on the nature of the changes in axial morphology we examined the regional and segment scale variations in axial and flank morphology at the intermediate spreading southeast indian ridge seir using newly collected geophysical data an empirical orthogonal function analysis was used to separate regional and local components of the topography field and to estimate bathymetric roughness three distinct types of axial morphology were identified from the regional component of ridge topography in our area axial highs shallow axial valleys and mid atlantic ridge type deep axial valleys axial depth increases by 2100 m from 88 e and 118 e while off axis depth only increases by 500 m in addition except for one segment with a deep axial valley there is little change in off axis depth within segments in contrast to the large intrasegment variations in axial depth these observations indicate that the overall and intrasegment variations in crustal thickness are much smaller than would be predicted from the variations in axial depth and that the major portion of the variations in ridge axis depth are dynamically supported there are step like increases in bathymetric roughness as axial morphology changes from an axial high to a shallow axial valley and from a shallow axial valley to a deep axial valley the step changes in roughness imply that the change from one mode of axial morphology to another is accompanied by an abrupt change in the strength of the lithosphere the abrupt changes in lithospheric strength may be due to the existence of a threshold mantle temperature or crustal thickness about which the lithospheric strength is very sensitive to small fluctuations systematic intrasegment variations in roughness are also observed roughness shows v shaped patterns within segments with axial highs but no clear pattern within segments with axial valleys the different patterns in roughness at axial highs and axial valleys on the seir may result from the presence or absence of a magma chamber the presence of a magma chamber at a ridge segment with an axial high implies weaker axial lithosphere and hence lower roughness near the center of segments relative to the segment ends

petrological variability of recent magmatism at axial seamount summit juan de fuca ridge 1 a combined study of mapping observational age constraint and geochemical data at the summit of axial seamount juan de fuca ridge has revealed its recent petrological history multiple basalt types erupted at the summit in a time sequence at least three different magma batches have been present beneath the axial summit caldera during the last millennium each with a range in differentiation the first prior to 1100 ce was compositionally diverse dominantly aphyric t morb the second from 1220 to 1300 ce was dominantly plagioclase phyric more mafic n morb erupted mostly in the central portion of the caldera since 1400 ce lavas have been more differentiated and nearly aphyric t morb mostly erupted in the caldera s rift zones parental magmas vary subtly due to small coupled differences in the degree of melting and sources but all share a uniform differentiation trend indicating pooling at similar depths thus melts percolate through melt rich lenses that remain partially isolated in space and or time centennial magmatic timescales at axial seamount are similar to those for fast spreading ridge segments the fluctuation between aphyric and plagioclase phyric lava likely reflects different pathways or velocities of melt migration

evidence for volcanic eruption on the southern juan de fuca ridge between 1981 and 1987 the formation of new ocean crust at mid ocean ridges is known to be a discontinuous process in both space and time but little is known about the frequency and duration of eruptions along an active ridge segment here we present evidence from sea beam surveys and underwater photography for the eruption of lavas along a segment of the juan de fuca ridge between 1981 and 1987 although previous studies have inferred volcanic activity on ridges in areas where recent seismicity or young lava flows have been observed1 4 none has yet had direct evidence to date such a recent submarine eruption the temporal coincidence between this eruptive episode and the megaplumes huge sudden emissions of hot mineral laden water observed over this part of the ridge5 6 in 1986 and 1987 supports previous suggestions5 10 that megaplumes are caused by sea floor spreading events

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

contribution of volcanism and tectonism to axial and flank morphology of the southern east pacific rise 17 10 17 40 s from a study of layer 2a geometry multichannel seismic images from the east pacific rise 17 10 17 40 s are used to study the geometry of seismic layer 2a in the near axis region wave equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2a event processed stacks are compared with seafloor morphology and tectonic fabric imaged in side scan sonar data assuming layer 2a corresponds with the extrusive crust these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge crest and ridge flank morphology we find variations in axial thickening of layer 2a which imply twofold variation in the width of the extrusive layer accumulation zone as well as systematic changes in the pattern of accumulation of this layer on the ridge flanks the layer 2a horizon mimics abyssal hill relief consistent with a horst and graben origin for this topography short wavelength variations in the thickness of layer 2a are superimposed on this relief which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone the integrated off axis thickness of layer 2a is correlated with ridge cross sectional area and indicates persistent 100 000 years spatial gradients in extrusive layer thickness systematic changes in the cross axis shape of the ridge observed over distances of 5 10 km cannot be attributed to the extrusive layer and it appears that axial structure beneath the volcanics governs the cross axis morphology of the ridge

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

dependence of ridge axis morphology on magma supply and spreading rate why do some mid ocean ridge axes have as their topographic expression a median valley 1 2 km deep and 15 30 km wide whereas others have an axial high 100 200 m high and 1 2 km wide in general median valleys exist at the axes of ridges spreading at half rates less than 20 25 mm yr 1 whereas axial highs are found at faster spreading ridges1 2 a recent crustal genesis model3 incorporating hydrothermal cooling and crustal accretion by means of a magma lens agrees well with observations of the spreading rate dependence of axial morphology1 2 the depth of the axial magma chamber3 4 and the thickness of the ridge axis lithosphere5 6 but there are several notable exceptions to the correlation between spreading rate and axial morphology such as the reykjanes ridge which spreads at l0mm yr 1 but has an axial high here we show that by extending the model of ref 3 to include variations in crustal thickness we can explain the observed dependence of axial morphology on crustal thickness and spreading rate our results suggest that the ultimate control on axial morphology is the thermal structure at the ridge axis which is a function of both spreading rate and magma supply

small scale spatial and temporal variations in mid ocean ridge crest magmatic processes data from a suite of closely spaced lava flows recovered within the axial summit caldera and on the crestal plateau of the east pacific rise around lat 9 31 n indicate that eruptions on this fast spreading part of the mid ocean ridge occur throughout the crestal region and are not restricted to the axis these eruptions contribute to a complex distribution of basalts of various ages and a significant thickening of seismic layer 2a away from the axis in our study area small scale 600 m diversity and nonsystematic distribution of lava types may reflect rapid changes in magma chemistry that occur during crystallization and replenishment in small magma lenses coupled with the effects of frequent low volume eruptions both within and outside of the axial summit caldera

relationship between spreading rate and the seismic structure of mid ocean ridges different parts of the global mid ocean ridge system create oceanic crust at rates that differ by more than a factor of ten the rates of supply of heat to these different systems must also vary substantially and it is this heat supply in the form of hot mantle material rising beneath the ridge axis that determines the nature of the magmatic tectonic and hydrothermal processes that control the production of new oceanic crust here we present evidence suggesting that a systematic relationship exists between the depth to zones of elevated temperature and partial melt beneath mid ocean ridges and their spreading rate recent seismic measurements provide robust determinations at six different locations of the depth to the top of the axial low seismic velocity zone which suggest that as full spreading rate decreases from 150 to 20 mm yr 1 the depth below the sea floor to the top of the lvz increases from 1 to 4 km although this relationship might arise in a number of ways our preferred explanation is that the major faults that extend to greater depths on slower spreading ridges1 allow a vigorous circulation of cooling sea water to reach greater depths blocking the upward migration of large melt bodies

structure of the southern juan de fuca ridge from seismic reflection records twenty four channel seismic reflection records were obtained from the axial region of the southern juan de fuca ridge two profiles are normal to the strike of the spreading center and intersect the ridge at latitude 44 40 n and 45 05 n a third profile extends south along the ridge axis from latitude 45 20 n and crosses the blanco fracture zone processing of the axial portions of the cross strike lines resolved a weak reflection centered beneath the axis the reflector is at a depth similar to seismically detected magma chambers on the east pacific rise and a lau basin spreading center we suggest that the reflector represents the top of an axial magma chamber in the migrated sections the top of the probable magma chamber is relatively flat and 1 2 km wide and the subbottom depth of the chamber is greater where the depth to the ridge axis is greater

in situ evidence for the nature of the seismic layer 2 3 boundary in oceanic crust the igneous oceanic crust is typically thought of as comprising two layers an upper crust seismic layer 2 characterized by a rapid increase in seismic velocity with depth and a thicker lower crust seismic layer 3 which is distinguished from layer 2 by both a higher p wave velocity 6 69 0 26 km s l and a much smaller vertical velocity gradient 1 8 km continuous hole drilled into the oceanic crust11 13 and find that at this location the seismic layer 2 3 boundary lies within the sheeted dyke complex where it is associated with gradual downhole changes in crustal porosity and alteration not a litho logical transition from sheeted dykes to gabbro

constructing the crust along the galapagos spreading center 91 3 95 5 w correlation of seismic layer 2a with axial magma lens and topographic characteristics the seafloor and narrow 0 5 1 5 km cross axis width we also image a thin seismic layer 2a 0 24 0 42 km that thickens away from the ridge axis by as much as 150 west of 92 7 w the magma lens is deeper 2 5 4 5 km and wider 0 7 2 4 km and layer 2a is thicker 0 36 0 66 km and thickens off axis by 40 the positive correlation between layer 2a thickness and magma lens depth supports the interpretation of layer 2a as the extrusive volcanic layer with thickness controlled by the pressure on the magma lens and its ability to push magma to the surface our findings also suggest that narrower magma lenses focus diking close the ridge axis such that lava flowing away from the ridge axis will blanket older flows and thicken the extrusive crust off axis flow of lava away from the ridge axis is probably promoted by the slope of the axial bathymetric high which is largest east of 92 5 w west of 94 wt he transitional axial morphology lacks a prominent bathymetric high and layer 2a no longer thickens off axis we detect no magma lens west of 94 7 w where a small axial valley appears the above changes can be linked to the westward decrease in the magma and heat flux associated with the fading influence of the galapagos hot spot on the galapagos spreading center index terms 3035 marine geology and geophysics midocean ridge processes 7220 seismology oceanic crust 0935 exploration geophysics seismic methods 3025

the southeast indian ridge between 88 e and 118 e gravity anomalies and crustal accretion at intermediate spreading rates although slow spreading ridges characterized by a deep axial valley and fast spreading ridges characterized by an axial bathymetric high have been extensively studied the transition between these two modes of axial morphology is not well understood we conducted a geophysical survey of the intermediate spreading rate southeast indian ridge between 88 e and 118 e a 2300 km long section of the ridge located between the amsterdam hot spot and the australian antarctic discordance where satellite gravity data suggest that the southeast indian ridge seir undergoes a change from an axial high in the west to an axial valley in the east a basic change in axial morphology is found near 103 30 e in the shipboard data the axis to the west is marked by an axial high while a valley is found to the east although a well developed axial high characteristic of the east pacific rise epr is occasionally present the more common observation is a rifted high that is lower and pervasively faulted sometimes with significant 50 m throw faults within a kilometer of the axis a shallow axial valley 1200 m deep valley across a transform at 114 e the changes in axial morphology along the seir are accompanied by a 500 m increase in near axis ridge flank depth from 2800 m near 88 e to 3300 m near 114 e and by a 50 mgal increase in the regional level of mantle bouguer gravity anomalies over the same distance the regional changes in depth and mantle bouguer anomaly mba gravity can be both explained by a 1 7 2 4 km change in crustal thickness or by a mantle temperature change of 50 c 90 c in reality melt supply crustal thickness and mantle temperature are linked so that changes in both may occur simultaneously and these estimates serve as upper bounds the along axis mba gradient is not uniform pronounced steps in the regional level of the mba gravity occur at 103 30 e 104 e and at 114 e 116 e and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other but rather represent distinctly different forms of axial structure and tectonics with a sharp transition between them the change from an axial high to an axial valley requires a threshold effect in which the strength of the lithosphere changes quickly the presence or absence of a quasi steady state magma chamber may provide such a mechanism the different forms of axial morphology are also associated with different intrasegment mba gravity patterns segments with an axial high have an mba low located at a depth minimum near the center of the segment at epr like segments the mba low is about 10 mgal with along axis gradients of 0 15 0 25 mgal km similar to those observed at the epr rifted highs have a shallower low and lower gradients suggesting an attenuated composite magma chamber and a reduced and perhaps episodic melt supply segments with a shallow axial valley have very flat along axis mba profiles with little correspondence between axial depth and axial mba gravity

fine scale magnetic anomaly field over the southern juan de fuca ridge axial magnetization low and implications for crustal structure sea surface deep tow and submersible magnetic surveys show that the spreading axis of the cleft segment of the southern juan de fuca ridge sjdf is overlain by a distinctive magnetic anomaly low in contrast to the more typical magnetic anomaly high found over the adjacent vance segment and most mid ocean ridge spreading centers forward iterative thickness modeling based on the near bottom magnetic data shows that this axial magnetic low can be produced by a thin magnetic source layer beneath the cleft axial valley thick crust is required to produce the magnetic anomaly high over the vance axial valley this magnetic modeling demonstrates that variations in the base of the magnetic source layer which is assumed to be equivalent to the base of the extrusive lavas can produce a significant proportion of the observed short wavelength magnetic anomaly signal these magnetic layer thickness models also correlate quite well with nearby seismic results which show that seismic layer 2a thins beneath the axial valley of the cleft and is thicker beneath the vance axial valley recent seismic results over other fast and medium spreading rifts also show rapid changes in the thickness of the uppermost crust near the axis of spreading which could produce significant magnetic contrasts the results of this study and the correlation with independent seismic measurements suggest that while magnetization may vary due to geomagnetic behavior or because of petrology or alteration first order change in upper crustal thickness is one of the most important variables to consider in the production of fine scale magnetic anomalies in young ocean crust

high resolution bathymetric surveys using scanning sonars lava flow morphology hydrothermal vents and geologic structure at recent eruption sites on the juan de fuca ridge the coaxial and cleft segments of the juan de fuca ridge have isolated chronic high temperature and focused hydrothermal vent sites both segments also have experienced recent volcanic eruptions which produced extensive ephemeral low temperature and diffuse hydrothermal venting to study the geologic setting of these sites high resolution bathymetric surveys at eight locations on the coaxial and cleft segments were collected between 1993 and 1999 two 675 khz scanning sonar systems were used mesotech on the submersible alvin and imagenex on the remotely operated vehicle jason the bathymetry from these surveys can be gridded at a scale of 2 4 m and contoured at 1 m and thus can resolve many fine scale features on the seafloor that are indistinguishable in multibeam bathymetry collected at the sea surface bathymetric data at this resolution are particularly useful for identifying geologic features related to diking faulting and lava flow emplacement for example the high resolution bathymetric maps show that submarine fissure eruptions that form pillow lavas last long enough to become localized and to produce point source constructs along their length and their extrusion rate is low enough that no significant drainback occurs in contrast lobate sheet flows are formed by short lived high effusion rate eruptions in which no localization of output occurs along the eruptive fissure and inflation is quickly followed by drainback resulting in extensive collapse features however if the process of submarine lava flow inflation occurs at a slower rate and over a longer period of time it can create lava rises up to 25 m high with distinctive structure and morphology the scanning sonar data also show that fissures and grabens have formed or reactivated where dikes approach the surface adjacent to recent eruptive sites the fine scale bathymetry establishes that all the hydrothermal vent sites studied at the coaxial and cleft segments are located along prominent volcanic or tectonic extensional structures which provide the physical pathway for fluids from the subsurface to the seafloor furthermore the fine scale morphology of recent lava flows can be used as a qualitative indication of eruption duration

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

morphology and segmentation of the western galapagos spreading center 90 5 98 w plume ridge interaction at an intermediate spreading ridge author posting american geophysical union 2003 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geochemistry geophysics geosystems 4 2003 8515 doi 10 1029 2003gc000609

plate motion at the ridge transform boundary of the south cleft segment of the juan de fuca ridge from gps acoustic data 1 we measure the present day plate velocity of the juan de fuca ridge 25 km off axis to be 63 6 3 6 mm a at s67 2 e 7 9 degrees 1 relative to the pacific plate pa this velocity was derived from gps acoustic gpsa measurements in 2000 2001 2002 and 2003 that observed the position of a seafloor array 44 43 n 130 03 w 2900 m depth with a repeatability of 4 6 mm three transient events at the juan de fuca ridge and blanco transform account for 10 of this motion in viscoelastic modeling suggesting that the observed gpsa pa velocity is due primarily to steady state plate dynamics subtracting the modeled transient motion gives a velocity of 57 3 3 9 mm a at s72 9 e 12 1 degrees 1 which is consistent at the 95 confidence level with the velocity calculated from the wilson 1993 0 0 78 ma euler pole therefore this site is interpreted to be in a region of continuous full rate plate motion a robust result of this study which holds with and without correcting for transient motions these results provide direct geodetic evidence that spreading occurs predominantly within 25 km of the axis at this intermediate spreading rate ridge previously reported geodetic monitoring across the 1 km wide axial valley from 1994 1999 and 2000 2003 shows no significant extension chadwell et al 1999 hildebrand et al 1999 chadwick and stapp 2002 w w chadwick personal communication 2006 and seismic monitoring shows no activity this suggests the crust between 0 5 and 25 km off axis accommodates 26 mm of aseismic deformation each year through some combination of near axis fault motion and elastic strain accumulation

active hydrothermal vents and sulfide deposits on the southern juan de fuca ridge massive sulfide deposits rich in zinc and silver were recovered from the juan de fuca ridge 500 km west of oregon in september 1981 the samples recovered are composed largely of zinc sulfide with lesser amounts of iron lead and copper sulfide most of the deposits occur at a series of hydrothermal vents within a relatively continuous depression in the center of a smooth 1 km wide valley along the ridge axis the depression appears to be formed by collapse of a lava lake possibly modified by extensional faulting the axial valley floor outside the depression is underlain by fresh glassy ferrobasalt sheet and lobate flows two types of sulfide mineral deposits were dredged from one of the hydrothermal vents 1 angular slabs of dark gray zinc sulfide and 2 subrounded fragments of porous light gray zinc sulfide the samples contain fresh sphalerite zoned wurtzite pyrite and minor amounts of marcasite galena and chalcopyrite cubanite the spreading rate of the juan de fuca ridge and the composition of the sulfide samples are generally similar to the east pacific rise lat 21 n site however the texture and geologic setting of the sulfide deposits are significantly different

seismic structure of the upper crust from 0 75 ma in the equatorial atlantic ocean on the african plate using ultralong offset seismic data

gravity and seismic study of crustal structure along the juan de fuca ridge axis and across pseudofaults on the ridge flanks variations in topography and seismic structure are observed along the juan de fuca jdf ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation here we analyze gravity data coregistered with multichannel seismic data from the jdf ridge and flanks in order to better understand the origin of crustal structure variations in this area the data were collected along the ridge axis and along three ridge perpendicular transects at the endeavor northern symmetric and cleft segments negative mantle bouguer anomalies of 21 to 28 mgal are observed at the axis of the three segments thicker crust at the endeavor and cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies 3 7 mgal additional low densities elevated temperatures within and or below the axial crust are required to explain the remaining axial mba low at all segments gravity models indicate that the region of low densities is wider beneath the cleft segment gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron enriched crust on the young crust side of the pseudofaults a 10 20 km wide zone of thicker crust is found reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment

uniform accretion of oceanic crust south of the garrett transform at 14 15 s on the east pacific rise using migrated common depth point reflection profiles we find the structural differences along the ultrafast spreading 150 mm yr east pacific rise south of the garrett fracture zone are second order suggesting a remarkably uniform process of crustal accretion the rise axis south of the garrett transform is underlain by a narrow 1 0 km melt lens which shows great along strike continuity the depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading east pacific rise at 9 30 n this observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent melt sill width however shows little variation along the east pacific rise suggesting no dependence of magma chamber size on spreading rate the melt reservoir decreases in width toward across the 14 27 s ridge axis discontinuity by a modest 250 300 m and appears to be continuous across this feature given the small aspect ratio 1 0 km by 50 m by tens of kilometers of the axial melt lens the previously recorded jump in mgo content across the 14 27 s offset is likely the result of a mixing boundary which is sustained through an along strike impedance in convection wide angle reflections originating at the base of seismic layer 2a assumed to coincide with the extrusive layer reveal a twofold to threefold increase 200 250 to 500 600 m in thickness within 1 2 km of the rise axis the pattern of extrusive thickening imaged south of the garrett transform is similar to that observed along the slower spreading 110 120 mm yr east pacific rise at 9 n outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile this implies a degree of temporal stability of the along strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone the inferred uniformity of off axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14 27 s discontinuity are caused by long term reductions in magma supply second order differences in the style of extrusive thickening may be related to structural differences within the low velocity zone underlying the rise axis and or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis images of mono on cross axis profiles may be traced to within 1 0 km of the melt sill edge this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber

petrology and geochemistry of basalts from the southern juan de fuca ridge controls on the spatial and temporal evolution of mid ocean ridge basalt three morphologically distinct regions within the neovolcanic zone of the cleft segment of the southern juan de fuca ridge were investigated and sampled in detail using the dsrv alvin additional along strike and off axis samples were recovered by dredge the southernmost region the southern cleft site is characterized by a 3 km wide axial valley floored by ponded sheet flows and bisected by a 30 to 50 m wide cleft farther north at the young sheet flow site the ridge axis is characterized by a distinct 500 m wide inner graben that is largely covered by distinctly younger looking sheet flows the northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the cleft segment well into the zone of overlap with the neighboring vance segment the pillowed lavas at the young pillow mound site represent the most recent episode of volcanism along the cleft segment strong correlations exist between degrees of fractionation relative ages of lavas and latitude lavas are progressively younger looking and more mafic to the north the compositional range of mid ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35 40 low pressure fractional crystallization of relatively primitive but not primary depleted n type melts scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions generated from small scale mantle heterogeneities furthermore the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern juan de fuca ridge the more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along axis flow in a continuous axial magma chamber or conduit rather the data suggest lavas were derived from discrete magma chambers or lenses each in a different stage of evolution the youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds the oldest stage 40 additional crystallization of the most mafic composition is associated with focused hydrothermal activity and tectonic extension whereas the youngest events are characterized by ridge inflation diking and dispersed hydrothermal activity geochemical and tectonomagmatic features observed along the cleft segment are similar to those recently documented along the east pacific rise from 9 30 n 10 n suggesting the scales processes and stages of magmagenesis are similar along medium to fast spreading ridge segments

downward continued multichannel seismic refraction analysis of atlantis massif oceanic core complex 30 n mid atlantic ridge 1 detailed seismic refraction results show striking lateral and vertical variability of velocity structure within the atlantis massif oceanic core complex occ contrasting notably with its conjugate ridge flank multichannel seismic mcs data are downward continued using the synthetic on bottom experiment sobe method providing unprecedented detail in tomographic models of the p wave velocity structure to subseafloor depths of up to 1 5 km velocities can vary up to 3 km s over several hundred meters and unusually high velocities 5 km s are found immediately beneath the seafloor in key regions correlation within situand dredged rock samples video and records from submersible dives and a 1 415 km drill core allow us to infer dominant lithologies a high velocity body ies found to shoal near to the seafloor in multiple locations is interpreted as gabbro and is displaced along isochrons within the occ indicating a propagating magmatic source as the origin for this pluton s the western two thirds of the southern ridge is capped in serpentinite that may extend nearly to the base of our ray coverage the distribution of inferred serpentinite indicates that the gabbroic pluton s was emplaced into a dominantly peridotitic host rock presumably the mantle host rock was later altered via seawater penetration along the detachment zone which controlled development of the occ the asymmetric distribution of seismic velocities and morphology of atlantis massif are consistent with a detachment fault with a component of dip to the southeast the lowest velocities observed atop the eastern central dome and conjugate crust are most likely volcanics here an updated model of the magmatic and extensional faulting processes at atlantis massif is deduced from the seismic results contributing more generally to understanding the processes controlling the formation of heterogeneous lithosphere at slow rate spreading centers

lava flow layer east pacific rise a multichannel seismic reflection profile across the east pacific rise near 9 n reveals an approximately 400 m thick layer at the top of the oceanic crust with seismic velocity of about 2 8 km s at the rise crest this velocity increases to about 3 8 km s 210 km east of the rise crest these low velocities suggest that this layer is probably a porous lava flow layer layer 2a of seismic refraction studies generated by volcanic activity at the spreading center reflections from the base of the layer imply a relatively sharp transition between the lava flows and the underlying layer presumably a sheeted dike complex the underlying layer is a zone about 1 4 km thick with velocity about 5 7 km s whose base may coincide with the top of a low velocity magma chamber from which the crust develops

rift topography linked to magmatism at the intermediate spreading juan de fuca ridge new seismic observations of crustal structure along the juan de fuca ridge indicate that the axial rift topography reflects magma induced deformation rather than alternating phases of magmatism and tectonic extension as previously proposed contrary to predictions of the episodic models crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2 1 2 6 km the shallow rift valley or axial graben associated with each juan de fuca segment is 50 200 m deep and 1 8 km wide and is well correlated with a magma body in the subsurface analysis of graben dimensions height and width shows that the axial graben narrows and graben height diminishes where the magma body disappears rather than deepening and broadening as expected for rift topography due to tectonic extension we propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor in this model an evolving axial topography results from feedbacks between the rheology of the crust above a magma sill and dike intrusion rather than episodic magma delivery from the mantle

upper crustal structure and axial topography at intermediate spreading ridges seismic constraints from the southern juan de fuca ridge 1 we use multichannel seismic reflection data to image the upper crustal structure of 0 620 ka crust along the southern juan de fuca ridge the study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow cleft and wide vance axial summit grabens asg along most of the axis of both segments we image the top of an axial magma chamber amc the amc along cleft deepens from south to north from 2 0 km beneath the ridge cleft observatory and hydrothermal vents near the southern end of the segment to 2 3 km at the northern end near the site of the 1980s eruptive event along the vance segment the amc also deepens from south to north from 2 4 to 2 7 km seismic layer 2a interpreted as the basaltic extrusive layer is 250 300 m thick at the ridge axis along the cleft segment and 300 350 m thick along the axis of the vance segment however off axis layer 2a is similar in both segments 500 600 m indicating 90 and 60 off axis thickening at the cleft and vance segments respectively half of the thickening occurs sharply at the walls of the asg with the remaining thickening occurring within 3 4 km of the asg along the full length of both segments layer 2a is thinner within the asg compared to the ridge flanks previous studies argued that the asg is a cyclic feature formed by alternating periods of magmatism and tectonic extension our observations agree with the evolving nature of the asg however we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply thus the asg rather than being formed by excess volcanism is a rifted flexural axial high the changes in axial morphology affect the distribution of lava flows along the ridge flanks as indicated by the pattern of layer 2a thickness the fluctuations in magma supply may occur at all spreading rates but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges

causes for axial high topography at mid ocean ridges and the role of crustal thermal structure mid ocean ridge topography is modeled as the flexural response to loads using a thin plate approximation and setting thermal structure of the lithosphere to allow but not require a region of rapid cooling near the axis loads on the lithosphere arise from the presence of low density melt densification due to cooling with distance from the ridge axis and thermal contraction stresses we find two end member classes of temperature and melt structure that can produce axial high topography and gravity observed at the east pacific rise epr one class is very similar to previous models requiring a narrow column of melt extending to at least 30 km depth within the mantle and lithosphere which cools and thickens very gradually with distance from the ridge axis the other is a new class predicting lithosphere which cools rapidly within a few kilometers of the axis and then slowly farther from the axis with melt which is contained primarily within the crust the latter solution is consistent with tomography and compliance studies at the epr which predict rapid crustal cooling within a few kilometers of the axis that is attributed to hydrothermal circulation this solution also allows the melt region to be coupled to crustal thermal structure and requires no melt anomaly within the mantle model fits predict 0 30 melt in the lower crust depending on how temperatures are distributed within the lithosphere and the degree to which thermal contraction stresses are assumed to contribute to topography the model generally predicts a wider axial high for lithosphere which is thin over a wider region near the axis this is consistent with previous correlations between large cross sectional area of the high and indicators of higher melt presence or a warmer crustal thermal regime for a slightly slower rate of lithospheric cooling at distances more than 5 km from the axis the model predicts a trough at the base of the axial high such troughs have been previously observed at the base of the high on the western flank of the southern epr where subsidence rates are anomalously low finally thick axial lithosphere reduces the amplitude of the high making it sometimes difficult to distinguish from long wavelength subsidence this morphology is comparable to that of some intermediate spreading ridges where topography is relatively flat suggesting a transition from fast to intermediate style morphology

channelized lava flows at the east pacific rise crest 9 10 n the importance of off axis lava transport in developing the architecture of young oceanic crust 1 submarine lava flows are the building blocks of young oceanic crust lava erupted at the ridge axis is transported across the ridge crest in a manner dictated by the rheology of the lava the characteristics of the eruption and the topography it encounters the resulting lava flows can vary dramatically in form and consequently in their impact on the physical characteristics of the seafloor and the architecture of the upper 50 500 m of the oceanic crust we have mapped and measured numerous submarine channelized lava flows at the east pacific rise epr crest 9 10 n that reflect the high effusion rate and high flow velocity end member of lava eruption and transport at mid ocean ridges channel systems composed of identifiable segments 50 1000 m in length extend up to 3 km from the axial summit trough ast and have widths of 10 50 m and depths of 2 3 m samples collected within the channels are n morb with mg indicating eruption from the ast we produce detailed maps of lava surface morphology across the channel surface from mosaics of digital images that show lineated or flat sheets at the channel center bounded by brecciated lava at the channel margins modeled velocity profiles across the channel surface allow us to determine flux through the channels from 0 4 to 4 7 103 m3 s and modeled shear rates help explain the surface morphology variation we suggest that channelized lava flows are a primary mechanism by which lava accumulates in the off axis region 1 3 km and produces the layer 2a thickening that is observed at fast and superfast spreading ridges in addition the rapid high volume flux eruptions necessary to produce channelized flows may act as an indicator of the local magma budget along the epr we find that high concentrations of channelized lava flows correlate with local across axis ridge morphology indicative of an elevated magma budget additionally in locations where channelized flows are located dominantly to the east or west of the ast the ridge crest is asymmetric and layer 2a appears to thicken over a greater distance from the ast toward the side of the ridge crest where the channels are located

crustal thickness and the average depth and degree of melting in fractional melting models of passive flow beneath mid ocean ridges in simple models of passive flow beneath mid ocean ridges fractional melting of the upwelling mantle produces unexpected relationships amongst crustal thickness degree of partial melting and the average depth of melting if the degree of melting is estimated on the basis of basalt composition then the average degree of melting is one third the maximum degree of melting and the average depth of melting is about two thirds the maximum depth in the simplest models the crustal thickness is about 1 5 times the product of the average fraction of melting and the height of the melting column if the average degree of melting represented in mid ocean ridge basalts were 10 and melt production continued to the base of the crust then only a 40 km high residual melting column would be required to produce a 6 km thick crust in more realistic models melting beginning at 60 km below the base of the crust and continuing to the top of the mantle would produce a 6 km thick crust with average degree of melting of 6 67 or melting beginning at 67 km and ending at 30 km below the base of the crust would produce a 6 km thick crust with an average degree of melting of only about 5 4 models of composition produced by fractional melting must consider the rate of melt production and the degree of melting throughout the melting region at the points where the melt is actually produced

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

determination of slow slip episodes and strain accumulation along the cascadia margin 1 continuous gps stations in the pacific northwest geodetic array network clearly record subduction related strain accumulation and slow slip episodes along the cascadia convergent margin many of the slow slip episodes have been correlated in time and space with seismic evidence for nonvolcanic tremor leading to the previous discovery of episodic tremor and slip ets in this study we use a hyperbolic tangent curve fitting technique for the identification of slow slip times and displacement magnitudes within the gps time series independent of seismic tremor data we then apply this technique to study the patterns of strain accumulation and release associated with ets events and characterize patterns of coupling associated with the locked and transition zones of the plate interface we demonstrate the effectiveness of this automated technique for both identification of slow slip observations and calculation of slow slip displacements recurrence patterns in the distribution of gps observations demonstrate coherence among neighboring stations over time and apparent along strike segmentation of the subduction interface when slow slip events are removed from the time series we can estimate the total site velocities between slow slip events these velocities decay as depth to the subduction interface increases but they diverge from the long term trends expected from the interseismic cycle at 30 60 km above the interface consistent with the location where slow slip displacements occur forward modeling of coupling on the plate interface reveals that in between slow slip events there is a patch of at least 30 coupling from 20 to 35 km depth which is needed to produce the observed back slip displacements intriguingly our best fitting models have a decrease in coupling down to 30 at 20 km depth followed by a peak of greater than 80 coupling at 30 35 km depth suggesting the source zone for ets events acts as a distinct locking zone that releases strain more frequently than the updip seismogenic locked zone although a zone of constant 30 coupling cannot be ruled out with this data set such a scenario indicates that frictional behavior with depth follows a more complex model than a simple temperature controlled transition we propose that coupling initially decreases with depth due to a decrease in strength of the overriding lower crust but then coupling increases again when the subducting plate comes in contact with the stronger overriding mantle

inconsistent correlation of seismic layer 2a and lava layer thickness in oceanic crust the oceanic crust makes up two thirds of the earth s surface and has been studied for many years yet the processes by which it is formed are still not fully understood one problem is inaccessibility only a tiny fraction of it has been subjected to drilling and sampling the tools of geology so much relies on remote sensing techniques such as seismic exploration the tools of geophysics these two approaches have led to some confusion over crustal structure christeson et al set out to reconcile the two by a direct comparison between seismic interpretations and geologic mapping the new data question the idea that seismic layer 2a correlates with lava layer thickness instead the layer 2a reflection seems to correspond to an alteration boundary within the lava layer at mid ocean ridges with fast to intermediate spreading rates the upper section of oceanic crust is composed of lavas overlying a sheeted dyke complex these units are formed by dykes intruding into rocks overlying a magma chamber with lavas erupting at the ocean floor seismic reflection data acquired over young oceanic crust commonly image a reflector known as layer 2a which is typically interpreted as defining the geologic boundary between lavas and dykes1 2 3 an alternative hypothesis is that the reflector is associated with an alteration boundary within the lava unit4 5 6 many studies have used mapped variability in layer 2a thickness to make inferences regarding the geology of the oceanic crust including volcanic construction dyke intrusion and faulting7 8 9 10 however there has been no link between the geologic and seismological structure of oceanic crust except at a few deep drill holes here we show that although the layer 2a reflector is imaged near the top of the sheeted dyke complex at fast spreading crust located adjacent to the hess deep rift it is imaged significantly above the sheeted dykes section at intermediate spreading crust located near the blanco transform fault although the lavas and underlying transition zone thicknesses differ by about a factor of two the shallow seismic structure is remarkably similar at the two locations this implies that seismic layer 2a cannot be used reliably to map the boundary between lavas and dykes in young oceanic crust instead we argue that the seismic layer 2a reflector corresponds to an alteration boundary that can be located either within the lava section or near the top of the sheeted dyke complex of oceanic crust

accretional curvature of lithosphere at magmatic spreading centers and the flexural support of axial highs fluid magma should tend to rise up to the level of local isostatic equilibrium at an axis of plate spreading the magma and underlying partial melt accretes to the side of the separating plates and gets denser as it cools and freezes this load bends down the lithosphere and additional accretion forms a plate with concave upward curvature the topography resulting from a curved plate passively moving off axis can be described analytically for accretion on the vertical side of uniform lithosphere when its flexural response is that of a thin elastic plate model predictions are consistent with the shape of the axial highs at fast spreading centers typically 400m high and 20 km wide as well as related gravity anomalies this model requires at least several kilometers of lithospheric thickening within a few kilometers of the ridge axis seismic data appear consistent with this lithospheric structure the magnitude and lateral distribution of faulting on the flanks of axial highs is crudely consistent with formation during plate unbending as lithosphere moves away from the axis according to this model abandoned fast spreading centers should retain an elastically supported axial high while for previous models abandoned highs should collapse

bending as a mechanism for triggering off axis volcanism on the east pacific rise we propose that bending stresses play a key role in triggering volcanism on the flanks of the east pacific rise by simultaneously opening tensile cracks near the surface and increasing the pore pressure of melt bodies trapped in the lower crust in addition crack tip stress intensities remain high during tensile opening such that bending cracks propagate downward efficiently and thus have the potential to tap overpressured melt bodies trapped in the lower crust we find that bending increases the vulnerability of the lithosphere to magmatic penetration out to distances of 20 km from the rise axis corresponding to the region of abyssal hill formation and isolated seamount generation

crustal thickness variations along the southeast indian ridge 100 116 e from 2 d body wave tomography 1 axial morphology along the southeast indian ridge seir systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100 116 e west of the australian antarctic discordance aad basalt geochemistry has a consistent indian mid ocean ridge basalt morb type isotopic signature so changes in axial topography are attributed to variations in both mantle temperature and melt supply wide angle seismic refraction lines were shot to four ocean bottom hydrophones within seir segments p1 p2 s1 and t where each segment is characterized by a different morphology we constructed 2 d crustal velocity models by jointly inverting hand picked p wave refraction pg and moho reflection pmp traveltime data using a top down minimum structure methodology the results show a 1 5 km eastward decrease in crustal thickness across the study area with segment averages ranging from 6 1 km at p1 to 4 6 km at t melt generation models require a 30 c decrease in mantle temperature toward the aad to account for the crustal thickness trend significant changes in axial morphology accompany small scale variations in crustal thickness consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature melt supply and cooling from hydrothermal circulation layer 3 thins by 3 0 km as layer 2 thickens by 1 4 km between segments p1 and t reflecting the eastward decrease in melt supply and increase in melt lens depth the trade off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material

upper ocean response to hurricane gilbert the evolving upper ocean response excited by the passage of hurricane gilbert september 14 19 1988 was investigated using current and temperature observations acquired from the deployment of 79 airborne expendable current profilers axcps and 51 airborne expendable bathythermographs from the national oceanic and atmospheric administration wp 3d aircraft in the western gulf of mexico the sea surface temperatures ssts mixed layer depths and bulk richardson numbers were objectively analyzed to examine the spatial variability of the upper ocean response to gilbert net decreases of the ssts of 3o 4oc were observed by the profilers as well as by the airborne infrared thermometer airt along the flight tracks and advanced very high resolution radiometer avhrr imagery the axcps indicated a marked cooling from 29oc to about 25 5oc on september 17 1988 which was about 1 2 inertial periods ip following storm passage this pool of cooler water 3 5 o was located further downstream in the hurricane wake by september 19 2 7 ip following the storm as a result of the near inertial currents in the mixed layer while there was a bias of about 0 6oc and 1 7oc between the in situ and avhrr derived ssts respectively both the avhrr images and the objectively analyzed fields indicated a rightward bias in the upper ocean cooling that extended from the storm track to about 4rmax where rmax the radius of maximum winds is equal to 50 km the larger sst offset of 1 7oc was due to the difference between the time of the avhrr image and the time of the aircraft experiment on september 19 the ssts derived from the avhrr images and the airt also indicated large gradients between the cold wake and the warm eddy in the central gulf of mexico the mixed layer deepened by about 30 35 m on the right side of the track during the storm and 1 2 ip later with little evidence of continued deepening afterward the mixed layer current vectors demonstrate that a strong near inertially rotating current was excited by the passage of gilbert with maxima of about 1 1 4 m s 1 the currents observed during and subsequent to 1 2 ip the storm diverged from the storm track whereas the mixed layer current vectors 2 7 ip after storm passage converged toward the track with relative maxima of 0 8 1 m s 1 this alternating pattern of convergence and divergence of the mixed layer current was associated with the upwelling and downwelling cycles of the baroclinic response considerable current shear existed between the mixed layer and the thermocline currents in the cool wake between the storm track and the 4rmax estimates of the bulk richardson numbers ranged between 0 2 and 1 0 during gilbert and at 1 2 ip which suggests that enhanced current shears were responsible for some of the mixed layer deepening

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data

direct measurement of magnetic reversal polarity boundaries in a cross section of oceanic crust magnetic field measurements made by submersible define the cross sectional geometry of a magnetic polarity rever sal boundary and the vertical variation of crustal magnetization in upper oceanic crust measured polarity boundaries show a systematic pattern of shallow dip towards the spreading axis within the upper extrusive lavas and steeper dip in the lower extrusive lavas this geometry is a consequence of the em placement of extrusive lava at a midocean ridge reversal boundary geometry and magnetization estimates are used to calculate the magnetic contribution of the extrusive lava se quence to the overlying marine magnetic anomaly signal from the forward modeling the highly magnetized extrusive lavas contribute the majority 50 75 of the observed sea surface magnetic anomaly consistent with the extrusive crust forming the primary source layer for young marine magnetic anomalies