mineral and gas chemistry of hydrothermal fluids on an ultrafast spreading ridge east pacific rise 17 to 19 s naudur cruise 1993 phase separation processes controlled by volcanic and tectonic activity during the naudur cruise in december 1993 23 dives using the french submersible nautile were conducted on the axis of the east pacific rise between 17 and 19 s where the spreading rate is among the fastest in the ocean 14 to 16 cm yr twenty hydrothermal fluids located at the topographic high of each segment in the axial domain were collected between 2573 and 2669 m depth on three segments centered respectively at 17 25 s 18 15 s and 18 26 s the fluids exhibit a very wide range of temperature chemical and gas compositions on the 17 25 s and 18 26 s segments fluids have quite uniform compositions low chlorinities are gas enriched and are low in dissolved metals relative to fluids from the 18 15 s segment which show high chlorinities are less gas enriched and show high metal concentrations chloride and metal depletion associated with gas enrichment is consistent with phase separation whereas ch 4 end members show large variations between sites the 13 ch 4 data are very similar with 13 c values in a narrow range 22 0 to 23 9 versus pee dee belemnite pdb 13 co 2 measured in fluids within the 18 15 s and 18 26 s segments are respectively 7 9 and 5 8 versus pdb similar to 13 c of co 2 trapped in mid oceanic ridge basalts suggesting a magmatic origin the variability in fluid composition is linked to the variability of the accretion process observed on the three segments the uniform venting of low chlorinity fluids in the 17 25 s and 18 26 s segments is connected with volcanic activity which causes boiling with preferential venting of vapor enriched fluids high salinity fluids are emitted on the 18 15 s segment where the ridge is tectonics dominated and subseafloor circulation controlled by faults phase separated effluents induced by volcanic and tectonic activity are delivered to the deep ocean in this area as previously observed on the juan de fuca ridge or in the north fiji basin ridge

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

extrusive thickness variability at the east pacific rise 9 10 n constraints from seismic techniques we calculate synthetic shot gathers and their corresponding common depth point cdp profiles over plausible east pacific rise epr shallow velocity structures based on the structures obtained from high resolution on bottom seismic refraction experiments we then use these results to analyze the variability in layer 2a thickness at the epr 9 10 n region as measured by cdp wide aperture profile wap on bottom seismic refraction experiments and conventional air gun refraction data the synthetics indicate that the accuracy of correlating the prominent shallow reflector observed in cdp and wide angle data with the layer 2a 2b boundary is strongly dependent on the structure within layer 2a if layer 2a consists of a surficial low velocity layer overlying a steep velocity gradient our gradient model then there is an excellent correspondence between the two way travel times to the shallow reflector and the base of layer 2a however the shallow reflector may originate from a gradient within layer 2a if the upper crust contains more than one high gradient region our step model this implies that independent estimates of layer 2a velocity structure are needed to properly interpret cdp and wide angle data we also determine that the travel time to the layer 2a reflector for identical velocity structure can vary by as much as 50 ms about 125 m for differing experimental geometries this can explain the discrepancy in two way travel time to the layer 2a reflector imaged on zero age cdp and wap lines the depths to a shallow reflector calculated from cdp and wide angle data in the 9 10 n region of the epr generally correlate with estimated layer 2a thicknesses from on bottom refraction profiles and conventional air gun refraction lines which suggests that the upper crustal structure in this area is similar to the gradient model wap and conventional air gun refraction data indicate that there is a 100 200 m decrease in off axis layer 2a thickness at 9 35 n on the epr the present day location of a deviation in axial linearity deval there is no bathymetric expression of the 50 decrease in layer 2a thickness layer 2a can be interpreted to consist of the extrusive section and transition zone with the layer 2a 2b boundary corresponding to the top of the sheeted dikes we suggest that buoyancy forces associated with the axial magma chamber amc are supporting the extrusive layer and sheeted dikes at the neovolcanic zone with distance from the rise axis the amc solidifies the crust cools the buoyancy forces are reduced and the sheeted dike complex subsides concurrently the extrusive layer thickens resulting in significantly less subsidence of the seafloor we speculate that the 50 decrease in dike subsidence and extrusive thickness at the 9 35 n deval is due to a local reduction in magma supply within the axial magma chamber the off axis pattern of layer 2a thickness suggests that the 9 35 n deval has persisted for 175 000 275 000 years

single crystal 40ar 39ar dating of a late quaternary paroxysm on kos greece concordance of terrestrial and marine ages the late quaternary plinian eruption forming the kos plateau tuff on the island of kos greece was one of the largest in the mediterranean a precise date for this eruption would significantly improve the chronology of the deep sea tephra layer w 3 because reliable correlation of w 3 with the kos plateau tuff has been previously established we have dated sanidine from the eruption at 161 3 1 1 ka using the laser fusion single crystal 40ar 39ar technique this age represents a significant refinement over both conventional and highly precise cassignol method k ar ages and the 40ar 39ar age is shown to be in remarkably good agreement with the age of w 3 inferred from recent astronomically calibrated oxygen isotope chronologies

long submarine lava flows observations and results from numerical modeling long 100 km lava flows are relatively common on mars and venus and have been identified on the moon but they are rarely documented on earth however although 75 of the earth s surface is covered by water only a small percentage of the ocean floor has been investigated at a resolution sufficient to unequivocally identify the boundaries of long submarine lava flows even so basaltic lava flows as long as 110 km have been identified on the deep 1500 m seafloor near hawaii and the east pacific rise ambient conditions on the deep ocean floor may favor the development of long lava flows for the following reasons first high pressures 15 mpa keep volatiles dissolved in basaltic lavas preventing viscosity increases associated with exsolution and vesiculation second seawater rapidly quenches the surface of submarine basalt flows so that an insulating glass layer 1 5 cm thick encases submarine flows within seconds after their emplacement this glass rind effectively insulates the molten flow interior from additional heat loss making submarine basalt flows behave as well insulated subaerial tube fed flows thus for identical basalt flows emplaced on the deep seafloor and subaerially a submarine flow could advance farther before stopping results of numerical modeling indicate that thin 1 m submarine basalt flows behave similarly to identical subaerial flows but thicker submarine flows may advance significantly farther than their subaerial counterparts

episodic dike swarms inferred from near bottom magnetic anomaly maps at the southern east pacific rise fissure eruption at 1728 0 s similar lows are observed at three other drained lava lake troughs including one which is at least 1800 years old residing 400 m away from the present day axis we attribute these lows to the presence of shallow dike swarms the degree to which other geologic features may contribute to the lows is constrained using geologic geophysical and geochemical observations and forward modeling compositional analyses of alvin samples at 1728 0 s do not support fe or ti variations as a primary source hypotheses requiring hydrothermal alteration and porosity variations are both inconsistent with geologic observations and near bottom gravity data analysis from similar areas previous mappings between paleointensity variations and the observed magnetic field over distances of several kilometers from the axis suggest that such variations do not create the field low the dominant source of the magnetization low is most likely the presence of a 100 200 m wide region of shallow dikes which are poorly magnetized relative to extrusives or a region heated above magnetic blocking or curie temperatures by intrusions during the most recent eruption though the latter interpretation cannot explain the low at the fossil trough in the first case this extrusive thinning implies a change in eruptive behavior over the last 750 1500 years given the local spreading rate for the latter case thermal models suggest the anomaly had to have been created by a dike swarms totaling at least 45 m width during the most recent eruption s corresponding to 300 years of plate spreading models indicate that the source of the low is centered slightly east of the axial trough this offset suggests that the axis has been progressively migrating westward over the past millennium consistent with other studies covering greater length and timescales westward migration provides an explanation for the preferential emplacement of recent lavas flows west of the axis evident in abe bathymetry and submersible observations index terms 3035 marine geology and geophysics midocean ridge processes 1517 geomagnetism and paleomagnetism magnetic anomaly modeling 8499 volcanology general or miscellaneous 3045 marine geology and geophysics seafloor morphology and bottom photography 3005 marine geology and geophysics geomagnetism 1550

crustal feeding in the oman ophiolite from the top and from the bottom a thermal and mass balance model the stability of the basalt melt lens identified below fast spreading ridges has been tested by a thermal and mass balance model in order to investigate how the generation of the crust by basaltic melt can be affected by spreading rate the mpdel is constrained using field information from the oman ophiolite the main piece of evidence dealing with the nature and structure of the root zone of sheeted dikes which acts as a thermal boundary layer between the melt lens below and the hydrothermally cooled sheeted dike unit above for a mass balance calculation we first assume that the gabbro unit is entirely created by the melt issued from the lens we examine how the heat generated by crystallization of this mass can be evacuated through the boundary layer on top of the melt lens a steady state situation is found for a given spreading rate and ratio of the nusselt number to the field deduced thickness of the boundary layer if as suggested by geophysical data the melt lens exists only for spreading rates larger than 6 cm yr the boundary layer needs to be essentially conductive with increasing spreading rates and increasing heat supply the system thermally adjusts itself until it must work in a new mode geological evidence suggests that this could be achieved by developing an additional feeding of the gabbro unit by a large number of sills being emplaced below the melt lens into the lower gabbros

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

reply to allen et al s comments on on magma fragmentation by conduit shear stress evidence from the kos plateau tuff aegean volcanic arc by palladino simei and kyriakopoulos jvgr 2008 178 807 817

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

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

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

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

geophysical evidence for a transform margin offshore western algeria a witness of a subduction transform edge propagator for the first time a deep seismic data set acquired in the frame of the algerian french spiral program provides new insights regarding the origin of the westernmost algerian margin and basin we performed a tomographic inversion of traveltimes along a 100 km long wide angle seismic profile shot over 40 ocean bottom seismometers offshore mostaganem northwestern algeria the resulting velocity model and multichannel seismic reflection profiles show a thin 3 4 km thick oceanic crust the narrow ocean continent transition less than 10 km wide is bounded by vertical faults and surmounted by a narrow almost continuous basin filled with miocene to quaternary sediments this fault system as well as the faults organized in a negative flower structure on the continent side marks a major strike slip fault system the extremely sharp variation of the moho depth up to 45 3 degrees beneath the continental border underscores the absence of continental extension in this area all these features support the hypothesis that this part of the margin from oran to tenes trending n65 n70 degrees e is a fossil subduction transform edge propagator fault vestige of the propagation of the edge of the gibraltar subduction zone during the westward migration of the alboran domain

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

an examination of along axis variation of magma chamber width and crustal structure on the east pacific rise between 13 30 n and 12 20 n we investigate the along axis variations of magma chamber width and crustal structure along the east pacific rise epr from 13 30 n to 12 20 n through reprocessed common depth point cdp reflection profiles the magma lens is predominantly a continuous feature in the study area with an average width of 500 m as determined from migrated cross axis cdp profiles this value is similar to widths estimated elsewhere along the epr suggesting that the axial magma chamber amc width is not spreading rate dependent once the threshold for a steady state magma chamber is reached the axial morphology of the 13 n area is generally not a good predictor of magma lens width or continuity a fairly continuous melt lens is imaged where the triangular axial topography might suggest waning magma supply in fact between 13 05 n and 13 01 n a shallow melt lens has been imaged which may be indicative of recent or impending eruptive activity this shoaling is similar to that observed near the 17 26 s region of the epr where the rise axis summit is domed and highly inflated generally the thickness of seismic layer 2a beneath the ridge crest is uniform and comparable to that estimated for 9 n 14 s and 17 s on the epr suggesting that the axial extrusive layer is invariant along fast spreading ridges uniformity of layer 2a thickness along axis implies that variations in magma chamber depth are directly attributed to changes in thickness of the sheeted dike complex seismic layer 2b contrary to expectations of decreasing melt sill depth with increasing spreading rate the average thickness of seismic layer 2b is slightly less 165 m at 13 n than at the faster spreading more robust 9 n area finally geochemical petrologic boundaries which may delineate separate melt supply regions occurring at the 13 20 n and 12 46 n devals deviation in axial linearity are observed to coincide with subtle changes in amc and layer 2a reflection characteristics

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

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

crustal structure of northern juan de fuca plate from multichannel reflection data the northern juan de fuca plate forms the basement of a young 8 ma ocean basin off western canada two subparallel multichannel seismic reflection lines totaling 230 km in length and trending obliquely to the spreading direction were recorded across the basin and provide high quality images of crustal structure the processing sequence applied to the data includes a prestack inside trace mute of common midpoint cmp gathers to reduce noise levels on the deep data cmp stack poststack dip filtering f k migration and band pass filtering coherency filtered stacks are helpful in tracing weaker reflectors the stacked sections reveal a horizontally layered sedimentary sequence overlying a rugged and prominent basement reflector which dips slightly landward topography of the basement was formed at or near the spreading ridge there is no evidence for subsequent deformation a strong fairly continuous reflection from the base of the crust oceanic moho at 2 0 2 2 s two way travel time below the basement surface generally mimics the basement topography and shows the characteristic multicyclic waveform seen in other surveys estimates of reflection coefficients for the moho at several locations indicate that in general the crust mantle transition is unlikely to occur as a discrete first order interface although much of the crust appears acoustically transparent a number of discontinuous intracrustal reflectors are observed over 40 km at the eastern end of the northern line some of these are interpreted to arise from the oceanic layer 2 3 and layer 3a 3b boundaries the noncontinuous nature of these reflectors is an indication of the complexity of the processes producing intracrustal reflectivity and the lateral variability of crustal formation however the interpreted crustal thickness varies by only about 10 away from the distal ends of spreading segments indicating that while the process of crustal formation may be locally variable it produces a broadly uniform result

the ramesses experiment ii evidence for accumulated melt beneath a slow spreading ridge from wide angle refraction and multichannel reflection seismic profiles the ramesses study reykjanes axial melt experiment structural synthesis from electromagnetics and seismics targeted an apparently magmatically active axial volcanic ridge avr centred on 57 45 n at the reykjanes ridge with the aim of investigating the processes of crustal accretion at a slow spreading mid ocean ridge as part of this multicomponent experiment airgun and explosive wide angle seismic data were recorded by 10 digital ocean bottom seismometers obss along profiles oriented both across and along axis coincident normal incidence seismic bathymetry and underway gravity and magnetic data were also collected forward modelling of the seismic and gravity data has revealed layer thicknesses velocities and densities similar to those observed elsewhere within the oceanic crust near mid ocean ridges at 57 45 n the reykjanes ridge has a crustal thickness of approximately 7 5 km on axis however the crust is modelled to decrease in thickness slightly off axis i e with age which implies that full crustal thickness is achieved on axis and that it is subsequently thinned most likely by off axis extension modelling also indicates that the avr is underlain by a thin 100 m narrow 4 km melt lens some 2 5 km beneath the seafloor which overlies a broader zone of partial melt approximately 8 km in width thus the results of this study provide the first clear evidence for a crustal magma chamber beneath any slow spreading ridge the size and depth of this magma chamber the melt lens and underlying zone of partial melt are similar to those observed beneath fast and intermediate spreading ridges which implies that the processes of crustal accretion are similar at all spreading rates hence the lack of previous observations of magma chambers beneath slow spreading ridges is probably temporally related to the periods of magmatic activity being considerably shorter and more widely spaced in time than at fast and intermediate spreading ridges

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

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

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

faulting patterns near 19 30 s on the east pacific rise fault formation and growth at a superfast spreading center 1 structural mapping from high resolution sonar and bathymetric data collected near 19 30 s on the southern east pacific rise epr reveals multiple packets of small closely spaced outward dipping faults that form antithetic to larger inward dipping faults the mean spacing and height of inward dipping scarps are 1 6 and 1 9 times greater than those of outward dipping faults and the median spacing and height of inward dipping scarps are 2 0 and 1 7 times greater respectively accordingly inward dipping faults are less abundant 35 45 but accommodate more 50 55 of the roughly 4 0 strain although previous studies have shown that epr fault populations exhibit exponential size frequency distributions one dimensional throw and spacing data from the 19 30 s region are not described well by this model the first major 40 m scarp height fault is observed within 1 4 km of the ridge crest and invariably dips inward further off axis a transition from greater inward dipping to greater outward dipping fault abundance constrains the minimum half width of the active plate boundary zone to be 5 15 km inspection of additional data sets from 16 and 18 30 s indicates a pattern of large inward dipping faults with associated hanging wall deformation similar to that observed in the 19 30 s region this pattern is distinct from the horst and graben morphology observed along the fast spreading northern epr the pervasive hanging wall deformation observed on the flanks of the southern epr suggests master fault geometries that are more sharply curved or kinked at depth relative to their abyssal hill equivalents on the northern epr boundary element modeling indicates the pattern and geometry of faulting observed along the southern epr is consistent with fault nucleation above a shallow sill like magma chamber with internal overpressure since the influence of the sill on fault growth is strongly dependent on the depth of the sill the different patterns of faulting observed along the northern and southern epr may reflect the small difference in axial magma chamber depth between these two regions with a slightly shallower chamber beneath the southern epr

drilling to gabbro in intact ocean crust sampling an intact sequence of oceanic crust through lavas dikes and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid ocean ridges but it has been an elusive goal of scientific ocean drilling for decades recent drilling in the eastern pacific ocean in hole 1256d reached gabbro within seismic layer 2 1157 meters into crust formed at a superfast spreading rate the gabbros are the crystallized melt lenses that formed beneath a mid ocean ridge the depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid ocean ridges melt lenses occur at shallower depths at faster spreading rates the gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by hole 1256d

shipboard geophysical indications of asymmetry and melt production beneath the east pacific rise near the melt experiment near the mantle electromagnetic and tomography melt experiment seamounts form and off axis lava flows occur in a zone that extends farther to the west of the east pacific rise than to the east indicating a broad asymmetric region of melt production more seamounts slower subsidence and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west variations in axial ridge shape axial magma chamber continuity off axis volcanism and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17 degrees15 s than beneath 15 degrees55 s recent volcanism occurs above mantle with the lowest seismic velocities

tomographic image of the axial low velocity zone at 12 50 n on the east pacific rise the 1982 magma seismic refraction experiment yielded a large set of accurate p wave travel times corrected for bathymetry and anisotropy which sample the structure of the east pacific rise epr at 12 50 n the arrivals were recorded using ocean bottom seismographs at three sites on axis and at 7 km and 16 km east of the axis we invert 2320 travel times for the twodimensional crustal seismic velocity structure across the epr axis assuming that the velocity structure is invariant along strike the travel times provide strong evidence for compressional velocity anisotropy in the upper crust corresponding to 10 faster velocities for propagation parallel to the axis than perpendicular to the axis the travel tunes used for the tomography are corrected for the effects of this azimuthal anisotropy our preferred model contains only the structure clearly required by the data structure which is stable under excessive smoothing and achieves a variance reduction of 81 relative to the laterally homogeneous starting model we resolve a substantial zone beneath the rise axis in which the velocity is reduced by 0 4 to 0 7 km s this low velocity zone lvz is about 7 km wide and extends from a depth of about 1 5 2 0 km down to moho at a depth of 5 5 km the lvz is slightly asymmetric extending 1 km further to the east than to the west of the axis in the shallow 1 0 km depth crust a pattern of velocity variations is imaged in which velocities are high at the spreading axis decrease between 3 km and 7 km east of the axis and then increase again between 10 and 15 km east of the axis we investigate the resolution of the inversion using an impulse response method the lvz and off axis upper crustal variations are well resolved in addition the travel time data indicate that an axial high velocity anomaly less than 2 km wide exists in the upper crust but is not resolved by the inversion the small velocity reductions of the lvz are consistent with hot rock containing only a small quantity of melt these results combined with the multichannel seismic reflection lines and expanded spread profiles from the northern epr suggest that the zone of high melt fraction under the spreading center is confined to a narrow thin lens capping a broad zone of hot plutonic rocks the upper crustal velocity reduction within 1 km of the axis reflects near axial thickening of the extrusive layer and the later reduction probably reflects porosity increases due to near axial tectonism the upper crustal velocity increase beyond 15 km off axis is attributed to porosity decreases associated with hydrothermal alteration

variations in layer 2a thickness and the origin of the central anomaly magnetic high the seismically determined off axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high t ypic ally associated with the ridge crest the positive magnenzauon contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales 2 3 km comparable to that of the proposed layer 2a thickening we present magnetic rema nence data from basalts dredged on and near the east pacific rise axis at 12on which are compatible with a rapid magnetization reduction 20 k y to decay to l e together with near bottom magnetic profiles from the ultra fast spreading east pacific rise at 19 5os these data suggest that previous estimates of the time constant of alteration inferred from slow spreading ridges 0 5 m y may be more than an order of magnitude too high seismic studies of the fast spreading east pacific rise epr indicate a rapid thickening of seismic layer 2a within a few kilometers of the ridge axis vera et al 1990 toomey et ai 1990 detrick et al 1993 these seismic data have been interpreted to reflect an approximate doubling of the extrusive layer thickness from 100 200 m to 300 500 m through addition of flows whi ch may extend 1 2 km off axis christeson et al 992 vera and diebold in press such thickening of the extrusive source layer had been argued on the basis of the widths of magnetic reversal transitions e g macdonald et al 1983 however this geometry for a uniformly magnetized source layer should produce a marked magnetic anomaly minimum over the ridge axis and is therefore difficult to reconcile with the pronounced anomaly high typically associated with the ridge crest klitgord 1976 sea surface magnetic profiles from the ultra fast spreading 77 mrn yr half rate epr near 19 5os figure 1 illustrate the short wavelength axial anomaly high within the 120 km wide central anomaly which has been termed the central anomaly magnetic high camh klitgord 1976 the camh typically has a wavelength of 8 10 km in sea surface profiles at in termediate spreading ridges 15 45 mm yr the camh occupies a larger proportion of the central anomaly and at slow spreading ridges it is not readily distinguishable in sea surface profiles from the central anomaly because the wavelengths become similar near bottom magnetic profiles figure 1 typically have higher amplitude and shorter wavelength 2 4 kin axial anomalies as expected from the closer proximity to the source klitg6rd et al 1975 miller 1977 perram et al 1990 and even allow distinction of the camh from the central anomaly at some slow spreading ridges macdonald 1977 the higher temporal resolution of faster spreading ridges therefore provides the best opportunity to evaluate the processes responsible for the camh thickness variations in the extraslye layer at fast spreading fi dges have a profound effect on the axial magnetic anomaly since the presence of an axial magma chamber and the low re rnanent intensity of intrusive rocks e g smith and banerice 1986 pariso et al 1991 minimize deeper source contributions in the near ridge environment based on seismic

crustal structure beneath the gravity lineations in the gravity lineations intraplate melting petrologic and seismic expedition glimpse study area from seismic refraction data 1 the active source seismic component of the gravity lineations intraplate melting petrologic and seismic expedition glimpse experiment investigated the velocity structure and crustal thickness of the pacific plate along a 450 km line paralleling the southern east pacific rise where prominent intraplate ridges lie within lows of the lineated free air gravity pattern compressional wave traveltime data collected by 10 ocean bottom seismometers indicate that the median crustal thickness is 6 2 km and increases to 8 2 km beneath the sojourn ridge the conspicuous absence of crustal thinning beneath both the sojourn and hotu matua ridge systems is strong support against a boudinage style model for the formation of these volcanic complexes our results suggest the ridge loads are partially supported by both plate flexure and underplating with an additional mantle component inferred from the analysis of gravity data rayleigh wave tomography and teleseismic body wave delays small scale convection could produce the observed crustal thickness pattern but fails to explain the geochemical and formation age trends along the ridges we prefer a model where the channelized flow of low density material in the upper mantle possibly undergoing small scale convection is advected toward the ridge crest lows in the gravity field reflect the density contrast within the asthenosphere and decompression melting amid upwelling limbs of the convective cells sustains the development of the associated ridge systems

stratigraphy of the kos plateau tuff product of a major quaternary explosive rhyolitic eruption in the eastern aegean greece the kos plateau tuff kpt erupted during a moderate volume explosive rhyolitic event approximately 161 ka from a source south of kos in the eastern aegean sea six major stratigraphic units have been identified from a at the base to f uppermost unit a is a widespread vitric ash fall layer that is thickest 1 5 m and most extensive southeast of the source unit b is a 1 to 2 m thick low angle cross stratified armoured pumice lapilli and ash layer found on kos unit c resembles unit b but includes a greater abundance of lithic lapilli less fine ash is only diffusely stratified and is on kos and west of the source unit d includes a sequence of three non welded 1 to 20 m thick ignimbrites that extend radially 38 km from the source in areas of low topography unit e is a sequence of two non welded 3 to 8 m thick ignimbrites which occur radially from the vent regardless of topography 64 km from source unit f has a 6 m thick basal low angle cross stratified armoured pumice lapilli and ash part probably deposited radially from source the upper part of unit f is a widespread 1 m thick vitric ash fall layer found to at least 50 km from the source these six units represent a change in eruptive conditions from initial and final phreatomagmatic activity depositing fallout and internally stratified pyroclastic density current deposits to dry explosive during the more intense phases of the eruption which generated ignimbrites

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

zircon crystallization and recycling in the magma chamber of the rhyolitic kos plateau tuff aegean arc in contrast to most large volume silicic magmas in continental arcs which are thought to evolve as open systems with significant assimilation of preexisting crust the kos plateau tuff magma formed dominantly by crystal fractionation of mafic parents deposits from this 60 km3 pyroclastic eruption the largest known in the aegean arc lack xenocrystic zircons secondary ion mass spectrometry sims u pb ages on zircon cores never older than 500 ka and display sr nd whole rock isotopic ratios within the range of european mantle in an area with exposed paleozoic and tertiary continental crust this evidence implies a nearly closed system chemical differentiation consequently the age range provided by zircon sims u th pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus the age distribution from 160 ka age of eruption by sanidine 40ar 39ar dating smith et al 1996 1 to ca 500 ka combined with textural characteristics high crystal content corrosion of most anhydrous phenocrysts but stability of hydrous phases suggest 1 a protracted residence in the crust as a crystal mush and 2 rejuvenation reduced crystallization and even partial resorption of minerals prior to eruption probably induced by new influx of heat and volatiles this extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned aegean microplate acts as a refractory container for magmas in the dying aegean subduction zone continent continent subduction 1 ref 32

a mechanism for decoupling within the oceanic lithosphere revealed in the troodos ophiolite contrasting kinematic histories recorded in the sheeted dykes and underlying plutonic rocks of the troodos ophiolite provide a new perspective on the mechanical evolution of oceanic spreading centres the kinematic framework of the decoupling zone that partitions deformation between the sheeted dykes and plutonics contrasts with low angle detachment models for slow spreading ridges based on continental rift analogues a model for the generation of multiple horizontal decoupling horizons linked by planar normal faults demonstrates new possibilities for the kinematic and rheological significance of seismic reflectors in oceanic lithosphere

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

the role of density in the accumulation of basaltic melts at mid ocean ridges it is commonly assumed that magma ponds at a level of neutral buoyancy in the shallow crust where melt densities are equal to the bulk density of the surrounding crust at the east pacific rise this neutral buoyancy level lies only 100 400 m below the sea floor significantly shallower than the depths 1 2 km of the magma bodies imaged in multichannel reflection data suggesting that other factors must control the collection of melt in these reservoirs the apparent inverse relationship between magma chamber depth and spreading rate at intermediate and fast spreading ridges suggests that the thermal structure of the rise axis not the buoyancy of melt is the primary factor that controls the depth at which melt ponds in crustal magma chambers beneath mid ocean ridges

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

submersible observations at the axis of the ultra fast spreading east pacific rise 17 30 to 21 30 s abstract manned submersible observations conducted with the diving saucer cyana at the axis of the east pacific rise at four sites near 21 30 21 10 18 30 and 17 30 s demonstrate that the ridge evolves between two end members the volcanic end member is characterized by a non rifted or only slightly rifted topographic ridge with widespread but diffuse water venting the tectonic end member is marked by a well defined axial graben whose floor can be the locus of intense sulfide deposition at one site a small stockwork mineralization exposed through normal faulting was observed at the base of the graben wall the dives illustrate the discontinuous nature of volcanic tectonic and hydrothermal activities even in the case of the fastest accretion found in the ocean

comments on on magma fragmentation by conduit shear stress evidence from the kos plateau tuff aegean volcanic arc by palladino simei and kyriakopoulos jvgr 2008 178 807 817

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

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

magmatic and tectonic segmentation of the intermediate spreading costa rica rift a fine balance between magma supply rate faulting and hydrothermal circulation 3 d tomographic modelling of wide angle seismic data recorded at the intermediate spreading costa rica rift has revealed a p wave seismic velocity anomaly low located beneath a small overlapping spreading centre that forms a non transform discontinuity at the ridge axis this low velocity zone displays a maximum velocity anomaly relative to the background ridge axis crustal structure of 0 5 km s 1 has lateral dimensions of 10 5 km and extends to depths 2 5 km below the seabed placing it within layer 2 of the oceanic crust we interpret these observations as representing increased fracturing under enhanced tectonic stress associated with the opening of the overlapping spreading centre that results in higher upper crustal bulk porosity and permeability evidence for ongoing magmatic accretion at the costa rica rift ridge axis takes the form of an axial magma lens beneath the western ridge segment and observations of hydrothermal plume activity and microearthquakes support the presence of an active fluid circulation system we propose that fracture pathways associated with the low velocity zone may provide the system through which hydrothermal fluids circulate these fluids cause rapid cooling of the adjacent ridge axis and any magma accumulations which may be present the costa rica rift exists at a tipping point between episodic phases of magmatic and tectonically enhanced spreading the characteristics inherited from each spreading mode have been preserved in the crustal morphology off axis for the past 7 myr using potential field data we contextualize our seismic observations of the axial ridge structure at the whole segment scale and find that the proposed balance between magmatic and tectonically dominated spreading processes observed off axis may also be apparent along axis and that the current larger scale magma supply system at the costa rica rift may be relatively weak based on all available geophysical observations we suggest a model for the inter relationships between magmatism faulting and fluid circulation at the costa rica rift across a range of scales which may also be influenced by large lithosphere scale structural and or thermal heterogeneity

lateral variations within coarse co ignimbrite lithic breccias of the kos plateau tuff greece coarse co ignimbrite lithic breccia ebx occurs at the base of ignimbrite e the most voluminous and widespread unit of the kos plateau tuff kpt in greece similar but generally less coarse grained basal lithic breccias dbx are also associated with the ignimbrites in the underlying d unit ebx shows considerable lateral variations in texture geometry and contact relationships but is generally less than a few metres thick and comprises lithic clasts that are centimetres to a few metres in diameter in a matrix ranging from fines bearing f2 10 wt to fines poor f2 0 1 wt lithic clasts are predominantly vent derived andesite although clasts derived locally from the underlying sedimentary formations are also present there are no proximal exposures of kpt there is a highly irregular lower erosional contact at the base of ignimbrite e at the closest exposures to the inferred vent 10 14 km from the centre of the inferred source but no ebx was deposited from 14 to 20 km from source ebx is present over a planar erosional contact at 16 km ebx is a 3 m thick coarse fines poor lithic breccia separated from the overlying fines bearing pumiceous ignimbrite by a sharp contact this grades downcurrent into a lithic breccia that comprises a mixture of coarse lithic clasts pumice and ash or into a thinner one clast thick lithic breccia that grades upward into relatively lithic poor pumiceous ignimbrite distally 27 to 36 km from source ebx is a finer one clast thick lithic breccia that overlies a non erosional base a downcurrent change from strongly erosional to depositional basal contacts of ebx dominantly reflects a depletive pyroclastic density current initially the front of the flow was highly energetic and scoured tens of metres into the underlying deposits once deposition of the lithic clasts began local topography influenced the geometry and distribution of ebx and in some cases ebx was deposited only on topographic crests and slopes on the lee side of ridges the kpt ignimbrites also contain discontinuous lithic rich layers within texturally uniform pumiceous ignimbrite these intra ignimbrite lithic breccias are finer grained and thinner than the basal lithic breccias and overlie non erosional basal contacts the proportion of fine ash within the kpt lithic breccias is heterogeneous and is attributed to a combination of fluidisation within the leading part of the flow turbulence induced locally by interaction with topography flushing by steam generated by passage of pyroclastic density currents over and deposition onto wet mud and to self fluidisation accompanying the settling of coarse dense lithic clasts there are problems in interpreting the kpt lithic breccias as conventional co ignimbrite lithic breccias these problems arise in part from the inherent assumption in conventional models that pyroclastic flows are highly concentrated non turbulent systems that deposit en masse the kpt coarse basal lithic breccias are more readily interpreted in terms of aggradation from stratified waning pyroclastic density currents and from variations in lithic clast supply from source

seismic structure of 15 ma oceanic crust formed at an ultrafast spreading east pacific rise evidence for kilometer scale fracturing from dipping reflectors 1 a grid of seismic reflection data from 15 ma cocos plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading 200 mm yr full rate east pacific rise epr the data contain an unexpected quantity of bright reflectors throughout the crust generally 5 km in length significant reflection dip angles 10 30 are seen only on the isochron profiles and several reflectors have a confirmed isochron dip orientation certain upper crustal reflectors ucrs appear to form a single network with isochron dip the most prominent example apparent reflection coefficient 0 2 projects toward a 70 m disruption in the igneous seafloor and waveform analysis demonstrates that the reflector is likely the product of a low velocity zone on the order of tens of meters thick our observations suggest that the isochron dipping ucrs represent small slip kilometer scale faults fractures and none are primarily related to the seismic 2 3 boundary or to the sheeted dike gabbro transition lower crustal reflectors lcrs have no preferred orientation or regular pattern on the 30 km scale of this survey certain lcrs may be similar to the ucrs in origin while others may originate from near axis ductile flow lithologic banding off axis magmatism or some combination thereof overall reflectors in this area are abundant and complex consistent with previous off axis studies but at odds with observations of young crust 5 ma near the epr this dichotomy may be resolved by a delay in reflector formation through some combination of postaccretionary tectonics magmatism and hydrothermal alteration

shallow axial magma chamber at the slow spreading erta ale ridge thermal models predict that spreading velocity at mid ocean rifts should influence the geometry of the underlying magma chamber insar data from the slow spreading ethiopian rift identify a shallow elongated magma chamber a feature usually associated with fast spreading rifts beneath the erta ale segment implying that spreading velocity may not be so important after all

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

applicability of the k 1bar method to whole rock samples of acid lava and pumice case of the upper pleistocene domes and pyroclasts on kos island aegean sea greece abstract kos pumice which may represent an important marker for the upper pleistocene stratigraphy in the aegean area has been dated at 0 24 0 02 ma by conventional k 1bar dating on feldspar concentrates and whole rock samples this figure is at variance with the interpretation of the tephrochronological record according to which this tephra deposit would be 0 145 ma old this raises the question of a possible excess ar presenting itself in lithic fragments phenocrysts and glassy matrix of the pumice the fact that the same apparent age within error limits is obtained in the three cases makes this presence a priori improbable a large part of the common ar present in pumices is associated with the vacuoles and liberated upon grinding ar purification may be made difficult by the presence of residues of the original volcanic gases or other volatiles of uncertain origin

the seismic attenuation structure of a fast spreading mid ocean ridge the two dimensional p wave attenuation structure of the axial crust of the east pacific rise was obtained from an inversion of waveform spectra collected during an active source seismic tomography experiment the structure shows that attenuation near the surface is high everywhere but decreases markedly within 1 to 3 kilometers of the rise axis the near axis variation is attributed to the thickening of the surface basalt layer and possibly to in situ changes in porosity related to hydrothermal circulation high attenuation is also observed beneath the rise axis at depths ranging from about 2 kilometers less than 1 kilometer beneath the axial magma lens to the base of the crust the levels of attenuation in this deeper region require at most only a small fraction of partial melt

four dimensional upper crustal construction at fast spreading mid ocean ridges a perspective from an upper crustal cross section at the hess deep rift abstract the faulted walls of the hess deep rift in the equatorial pacific provide one of the few significant tectonic windows into the oceanic crust created at a fast spreading mid ocean ridge the 5000 m deep rift graben exposes a slightly dismembered section of oceanic crust created at the east pacific rise 1 ma the upper portion of the north wall of the rift features a laterally extensive exposure of upper crustal rock units similar to those found in many ophiolite complexes for tens of kilometers along the rift wall basaltic lavas with sparse dikes grade down into a spectacular sheeted dike complex that is underlain locally by a heterogeneous assemblage of gabbroic rocks the geometry of lava flows dikes and fault zones that developed during magmatic accretion provide constraints on the cross axis temporal evolution of the upper crust in this area outcrop scale digital imagery from submersible vehicles provides a well constrained geological framework for samples collected from these upper crustal rock units and the geochemistry of lava and dike samples provides additional temporal constraints on crustal accretion and the role of along axis magma transport during accretion integrated results of geological and geochemical investigations suggest a four dimensional model for upper crustal construction whereby tectonic and magmatic processes act in concert to construct the upper oceanic crust within a few kilometers few tens of thousands of years of a fast spreading ridge axis in this model a sub axial melt lens where it exists is recharged from below with distinct parental melts and limited along axis mixing preserves geochemical segmentation of the melt lens dikes emanate from distinct portions of the melt lens and transport magmas vertically as well as significant distances laterally along axis erupting lavas in the special case where magmas ascend through dikes to higher levels in the crust and intersect the seafloor differences in the phenocryst contents and compositions of lavas and dikes suggests that density filtering of buoyant magmas among other factors including magma overpressure and orientations of local stresses plays a role in the eruption of magma on the surface during spreading and magmatic construction subaxial subsidence accommodates thickening of the basaltic lava unit sub axial subsidence faulting and block rotation result in an upper crust with inward dipping toward the axis lava flows and outward dipping away from the axis dikes substantial differences in upper crustal geology may result from segment scale variations in which vertical magma delivery dominates in areas of robust magma supply likely over sites of recently recharged melt lenses at segment centers but lateral magma transport dominates in areas of relatively lower magma supply possibly toward segment ends

seismic images of a magma chamber beneath the lau basin back arc spreading centre we present normal incidence seismic reflection profiles from a back arc spreading centre in the southern lau basin which show a bright mid crustal reflector beneath the ridge axis analysis of wide angle seismic data collected simultaneously with the reflection profiles shows that the reflector coincides with the top of a low seismic velocity zone we interpret the reflector as the top surface of a magma chamber the lau basin is one of only two places where a magma body has been clearly imaged beneath a spreading centre the other being the east pacific rise between 9 and 13 n ref 1 the lau basin reflector is imaged beneath an entire 35 km long ridge segment it is also present beneath both ridges and the overlap basin of a small overlapping spreading centre this is the first observation of a widespread magma body beneath an overlapping spreading centre and indicates that the offset here is not related to a break in continuity of the magma chamber

detection and response to volcanic hydrothermal events on the mid ocean ridge the mid ocean ridge is by far the most continuous volcanic system on the planet but since it lies beneath more than 2 km of seawater a thorough study of its thermal and chemical interaction with the ocean remains very difficult until 1990 the end of the period covered by the last iugg report literally all of our understanding of the dynamics of magmatic accretion on the mid ocean ridge was based on inferences from 1 geochemical geophysical and morphologic studies of long sections of the ridge using a variety of instruments mounted on ships hulls or towed on long cables near the seafloor and 2 submersible based mapping and sampling of small sections of the ridge particularly hydrothermally active ones these types of studies have continued to flourish parmentier 1995 humprhis 1995 but the past 4 years have also seen the first near real time observations of accretion events on two pacific spreading centers and the development of technology and techniques that can monitor the occurrence and extent of such events over a selected portion of the ridge system these monitoring capabilities allow interdisciplinary studies of seafloor hydrothermal systems so that the thermal and chemical effects of hydrothermal venting on the oceans can be assessed

seismic structure and indicators of magma budget along the southern east pacific rise in this paper we re examine the relationship between seismically constrained variations in crustal structure along the southern east pacific rise sepr and the segment scale variations in axial depth morphology basalt geochemistry and hydrothermal activity that have often been attributed to along axis differences in the supply of magma to the mid ocean ridge along 800 km of the fast spreading sepr good correlations exist between axial depth ridge cross sectional area mantle bouguer anomaly and the mgo weight percent of basalts recovered from the rise axis these correlations indicate along axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of 100 000 years in contrast we show that the depth and width of the midcrustal magma sill the thickness of seismic layer 2a and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross sectional area we suggest that the emplacement geometry width of the intrusion zone and flow lengths not magma supply controls extrusive layer seismic layer 2a thickness we hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events dike intrusion eruptions faulting which occur on much shorter timescales 10 100 years than the longer term variations in magma supply reflected in along axis changes in the shape and depth of the ridge axis

can high temperature high heat flux hydrothermal vent fields be explained by thermal convection in the lower crust along fast spreading mid ocean ridges we present numerical models to explore possible couplings along the axis of fast spreading ridges between hydrothermal convection in the upper crust and magmatic flow in the lower crust in an end member category of models corresponding to effective viscosities m lower than 1013 pa s in a melt rich lower crustal along axis corridor and permeability k not exceeding 10 16 m2 in the upper crust the hot melt rich gabbroic lower crust convects as a viscous fluid with convection rolls parallel to the ridge axis in these models we show that the magmatic hydrothermal interface settles at realistic depths for fast ridges i e 1 2 km below seafloor convection cells in both horizons are strongly coupled and kilometer wide hydrothermal upflows plumes spaced by 8 10 km arise on top of the magmatic upflows such magmatic hydrothermal convective couplings may explain the distribution of vent fields along the east epr and south east pacific rise sepr the lower crustal plumes deliver melt locally at the top of the magmatic horizon possibly explaining the observed distribution of melt rich regions pockets in the axial melt lenses of epr and sepr crystallization of this melt provides the necessary latent heat to sustain permanent 100 mw vents fields our models also contribute to current discussions on how the lower crust forms at fast ridges they provide a possible mechanism for focused transport of melt rich crystal mushes from moho level to the axial melt lens where they further crystallize feed eruptions and are transported both along and off axis to produce the lower crust

on magma fragmentation by conduit shear stress evidence from the kos plateau tuff aegean volcanic arc abstract large silicic explosive eruptions are the most catastrophic volcanic events yet the intratelluric mechanisms underlying are not fully understood here we report a field and laboratory study of the kos plateau tuff kpt 161 ka aegean volcanic arc which provides an excellent geological example of conduit processes that control magma vesiculation and fragmentation during intermediate to large scale caldera forming eruptions a prominent feature of the kpt is the occurrence of quite unusual platy shaped tube pumice clasts in pyroclastic fall and current deposits from the early eruption phases preceding caldera collapse on macroscopic and sem observations flat clast faces are elongated parallel to tube vesicles while transverse surfaces often occur at 45 to vesicle elongation this peculiar pumice texture provides evidence of high shear stresses related to strong velocity gradients normal to conduit walls which induced vesiculation and fragmentation of the ascending magma either an increasing mass discharge rate without adequate enlargement of a narrow central feeder conduit or a developing fissure like feeder system related to incipient caldera collapse provided suitable conditions for the generation of plate tube pumice within magma volumes under high shear during the pre climactic kpt eruption phases this mechanism implies that the closer to the conduit walls where the stronger are the velocity gradients the larger was the proportion of plate vs conventional lensoid juvenile fragments in the ascending gas pyroclast mixture consequently plate pumice clasts were mainly entrained in the outer portions of the jet and convecting regions of a sustained plinian type eruption column as well as in occasional lateral blast currents generated at the vent as a whole plate pumice clasts in the peripheral portions of the column were transported at lower altitudes and deposited by fallout or partial collapse closer to the vent relative to lensoid ones that dominated in the inner column portions the plate tube pumice proportion decreased abruptly up to disappearance during the emplacement of the main pyroclastic currents and lithic rich breccias related to extensive caldera collapse at the eruption climax as a consequence of an overall widening of the magma feeder system through the opening of multiple conduits and eruptive vents along with fissure erosion concomitant to the disruption of the collapsing block

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

chemical and isotopic variations along the superfast spreading east pacific rise from 6 to 30 s chemical data of 39 fresh basaltic glasses from the east pacific rise epr between 6 and 30 s and pb sr and nd isotopic compositions of 12 basalt glasses are presented major and trace element data indicate a wide compositional range including primitive basalts mg 0 67 and highly evolved feti basalts mg 0 34 molmg mg fe2 the compositional range can be attributed to low pressure fractional crystallization fractionation corrected major element concentrations provide evidence for varying mantle melting conditions calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar at temperatures between 1430 and 1280 c and total degrees of partial melting between 17 and 20 by weight leached and hand picked basalt glasses display large variations in 87sr 86sr 0 70235 0 70270 143nd 144nd 0 51312 0 51323 and 206pb 204pb 18 064 18 665 but are similar to other n type morb from the epr the isotopic ratios of basalts from 13 to 23 s show strong correlations and delineate two systematic trends from 23 to 17 s 87sr 86sr and pb isotope ratios increase and 143nd 144nd decrease in agreement with previous results mahoney et al 1989 a reverse trend is indicated by basalts from 17 to 13 s however k ti and la sm n continuously increase from 23 to 13 s this opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17 s north of 13 5 s garrett fracture zone isotopic data show no systematic variation with ridge location and display an overall weaker covariation the results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions p t f f degrees of melting there is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30 s we interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components in contrast variations in trace elements are attributed to young mantle differentiation processes

precursory activity of the 161 ka kos plateau tuff eruption aegean sea greece the kos plateau tuff kpt eruption of 161 ka was the largest explosive quaternary eruption in the eastern mediterranean we have discovered an uplifted beach deposit of abraded pumice cobbles directly overlain by the kpt the pumice cobbles resemble pumice from the kpt in petrography and composition and differ from plio pleistocene rhyolites on the nearby kefalos peninsula the pumice contains enclaves of basaltic andesite showing chilled lobate margins suggesting co existence of two magmas the deposit provides evidence that the precursory phase of the kpt eruption produced pumice rafts and defines the paleoshoreline for the kpt which elsewhere was deposited on land the beach deposit has been uplifted about 120 m since the kpt eruption whereas the present marine area south of kos has subsided several hundred metres as a result of regional neotectonics the basaltic andesite is more primitive than other mafic rocks known from the kos nisyros volcanic centre and contains phenocrysts of fo89 olivine bytownite enstatite and diopside groundmass amphibole suggests availability of water in the final stages of magma evolution geochemical and mineralogical variation in the mafic products of the kpt eruption indicate that fractionation of basaltic magma in a base of crust magma chamber was followed by mixing with rhyolitic magma during eruption low eruption rates during the precursory activity may have minimised the extent of mixing and preserved the end member magma types

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

geochemistry of dikes and lavas from the north wall of the hess deep rift insights into the four dimensional character of crustal construction at fast spreading mid ocean ridges 1 an investigation of 1 m y old dikes and lavas from the north wall of the hess deep rift 2 15 n 101 30 w collected during alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust the study area encompasses 25 km of an east west flow line representing 370 000 years of crustal accretion at the east pacific rise samples analyzed exhibit depleted incompatible trace element abundances and ratios la sm n 1 indices of fractionation mgo and incompatible element ratios la sm nb ti show no systematic trends along flow line rather over short 4 m and long 25 km distances significant variations are observed in major and trace element concentrations and ratios modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions these findings combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift dominated systems suggest a more realistic model of the magmatic system underlying the east pacific rise relative to the commonly assumed two dimensional model in this model melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir dikes emanating from these distinct reservoirs transport magma along axis resulting in interleaved dikes and host lavas with different evolutionary histories this model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin

geological indexes of hydrothermal venting hydrothermal venting occurs on mid ocean ridge axes with a diverse array of morphological structural and petrological characteristics consequently it is not clear if certain geological environments are more conducive than others to hydrothermal activity this paper uses complementary data sets recently collected on multiple tectonic segment scales along intermediate to superfast spreading ridge sections to systematically examine this issue the spatial density of venting is gauged by plume incidence the fraction of linear ridge crest overlain by hydrothermal plumes five geological indexes are compared with plume incidence ridge axis elevation ridge axis cross sectional area the percentage of ridge axis underlain by an axial magma chamber amc reflector mgo wt in basaltic glass and spreading rate a mean value for plume incidence and each index where available is calculated for each of 14 second to fourth order tectonic segments from the juan de fuca ridge 44 30 to 48 45 n and the northern 9 to 11 50 n and southern 13 50 to 18 40 s east pacific rise plumes are most common on segments with a cross sectional area 3 5 km2 a net elevation 0 35 km amc coverage 60 and mgo 7 wt spreading rate is not deterministic on the segment scale the data suggest two classes of ridge segments segments with relatively low geological index values have a uniformly low plume incidence indicative of a presently feeble magmatic budget those with high index values have a plume incidence ranging from low to complete coverage apparently a consequence of a magma supply rate sufficient to produce frequent dike intrusions and highly variable hydrothermal activity the best individual predictors of hydrothermal activity are cross sectional area and net elevation applying these indexes to the east pacific rise between 18 n and 33 s suggests that the unexplored segments with the highest probability of activity are those at 15 30 16 n 6 9 n 5 6 s 27 28 s and 30 30 33 s

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

structure and ageing of oceanic crust at 14 s on the east pacific rise a wide angle reflection refraction survey was conducted along a flow line on the eastern flank of the east pacific rise at 14 s the data were acquired with a single 32 litre airgun and ocean bottom hydrophones obh and consist of six profiles on 0 5 8 3 myr old seafloor in total 17 obhs provided seismic records of excellent quality to generate 2 d velocity models traveltimes and amplitudes have been calculated using asymptotic ray theory a comparison of the traveltimes and amplitudes of p wave arrivals on all lines revealed important similarities that correlate well with the classical model of oceanic crust the uppermost crust is composed of a 0 4 0 7 km thick low velocity 2 9 4 6 km s 1 surficial layer layer 2a and is bounded at its base by a sharp velocity increase to 5 5 km s 1 at the top of layer 2b overall layer 2 is 1 4 1 8 km thick small seamounts frequently superposed on fast spreading crust are characterized by velocities lower than within the adjacent upper crust velocities 6 0 km s 1 exist throughout the edifices suggesting that the seamounts are dominantly extrusive decreasing amplitudes beyond 8 10 km offset constrain a moderate gradient lower crust layer 3 with velocities of 6 8 7 2 km s 1 crustal thickness is found to be remarkably uniform at 6 11 0 14 km nevertheless the seismic data provide evidence for age dependent variations in both the low velocity surficial layer and the upper mantle we find that velocities in layer 2a increase from 2 9 to 4 3 km s 1 velocities in the middle and lower crust show uniform values while velocities within the uppermost mantle increase gradually from 7 5 km s 1 to 8 0 km s 1 the most viable explanation for increasing velocities in the upper crust is that off axis hydrothermal circulation was accompanied by the precipitation of secondary minerals in open void spaces thereby decreasing the porosity and increasing the velocity low mantle velocities beneath young crust are suggested to be a temperature controlled phenomenon and may indicate the presence of partial melt in the upper mantle

increase of seismic velocities in upper oceanic crust the superfast spreading east pacific rise at 14 14 s one of the most important observations of oceanic crustal evolution is that seismic compressional wave velocities in the upper igneous crust layer 2a increase as the crust ages the most viable explanation for this phenomenon is decreasing porosity due to filling of open void spaces with hydrothermally generated minerals but the idea that seismic velocity could be a function of plate age however depends primarily on airgun sonobuoy profiles collected more than 20 years ago houtz and ewing 1976 therefore in late 1995 new seismic refraction measurements were obtained on the eastern flank of the superfast spreading east pacific rise south of the garrett transform we found that velocities rapidly increase by about 45 50 within only 0 5 1 m y double within 5 m y or less and remain constant thereafter

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

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

attenuation differences in layer 2a in intermediate and slow spreading oceanic crust in situ seismic attenuationq 1logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate and slow spreading oceanic crust q 1logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in ocean drilling program holes 504b and 896a and in hole 395a located in 5 9 7 3 ma crust on the pacific and atlantic plates respectively our results strongly tie crustal properties to seismic measurables and observed geological structures we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity we interpret a distinct decrease in the q 1log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2a 2b boundary the boundary is located at 465 m depth below the sea floor in both hole 395a and 504b although this is likely to be a coincidence of the sediment thickness at these sites layer 2a is estimated to be approximately 150 m thick in hole 504b and 300 m thick in hole 395a cyclic sequences of high porosity pillows and low porosity massive units in the uppermost 100 m of volcanics in hole 395a result in large velocity heterogeneities which cause 5 times more attenuation in this layer than in hole 504b in hole 896a by contrast fewer pillows more massive flows and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from hole 504b we conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration the observed differences inq 1profiles and layer 2a thickness at these sites may be attributed to variations in the volume and duration of volcanic activity at mid ocean spreading centers for these pacific and atlantic ridge segments

seismic imaging of the axial region of the valu fa ridge lau basin the accretionary processes of an intermediate back arc spreading ridge in 1995 a multidisciplinary geophysical experiment targetted the intermediate spreading valu fa ridge full rate 60 mm yr 1 which is centred on 22 20 s 176 40 w in the lau basin as part of this experiment wide angle and normal incidence seismic profiles were collected both along and across axis to determine the crustal structure of the central valu fa ridge cvfr and its overlap with the northern valu fa ridge nvfr controlled source electromagnetic profiles and underway gravity magnetic and bathymetry data were also collected in this paper we describe the results of forward modelling of the along and across axis wide angle and normal incidence seismic data an axial low velocity block and its underlying slightly broader zone of depressed seismic velocities low velocity zone have been identified and these features are interpreted as corresponding to a melt lens and underlying magma chamber the low velocity block is 1 2 km wide and has a first order upper boundary from which large amplitude reflections are observed amplitude analysis of these indicates an interconnected melt fraction the nature of the lower boundary is more poorly constrained as no reflection event corresponding to the base of the low velocity block is observed modelling indicates that velocities similar to those observed at the base of layer 2 within the axial region 5 5 km s 1 are achieved by 250 m below the upper boundary possibly suggesting a gradational lower boundary with high velocity gradient the low velocity zone lvz is interpreted as an 4 km wide magma chamber delineated by a seismic velocity anomaly of 0 2 km s 1 extending down through layer 3 to within 1 5 2 km of the moho the velocity anomaly and dimensions of the lvz are generally smaller than those observed at the east pacific rise epr and mid atlantic ridge mar the observed along axis continuity of the low velocity block is remarkable extending from the southern tip of the cvfr to the overlapping spreading centre osc with the nvfr a low velocity block is modelled beneath the inside flanks i e the slopes that dip into the overlap basin of both ridges at the osc although the existence of a single low velocity block beneath the overlap basin itself cannot be ruled out the identification of a single lvz centred on the overlap region rather than two merged lvzs beneath each segment implies that the material in each low velocity block originates from the same crustal magma source a reflection event from the moho is observed from directly beneath the axis on both across axis profiles which indicates that a distinct crust mantle boundary may be formed within the axial region many of the observations at the valu fa ridge are consistent with those at the epr and the reykjanes ridge mar which implies that regardless of spreading rate crustal accretionary processes at mid ocean ridges with similar magmatic budgets are also broadly similar

recent tectonic magmatic and hydrothermal activity on the east pacific rise between 17 s and 19 s submersible observations the objective of the naudur cruise december 1993 of the submersible nautile was to study the interaction among magmatic tectonic and hydrothermal processes at a very fast spreading mid ocean ridge axis twenty three dives were completed both along and across the axis in four areas located between 17 10 and 18 45 s on the east pacific rise rock sulfides water and biological samples have been collected along each of the segments two main types of segments have been distinguished characterized either by the predominance of present day volcanic activity or by predominant tectonic activity linked to both types of activity 69 hydrothermal sites have been discovered and sampled they comprise four types interpreted as successive evolutionary stages the first are shimmering water sites which occur immediately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna the second in areas dominated by recent volcanic activity have waters venting directly from lava fissures and more focused discharge areas through black smoker chimneys the third stage is represented by more mature hydrothermal vents and deposits along the faults bounding the eastern side of the axial graben in tectonic dominated areas the associated fauna is well developed the fourth stage corresponds to the reactivation of volcanic activity with lava flows young black smokers and diffuse venting associated with the faults bounding the axial graben fluids collected range from 200 to 340 c and show a wide variability in chemical and gas composition within each of the explored areas evidence of recent volcanic activity has been observed

extensive distribution of hydrothermal plumes along the superfast spreading east pacific rise 13 30 18 40 s a general model relating relative hydrothermal activity to the rate of plate creation requires data from the full spectrum of mid ocean ridge spreading rates to obtain data from a superfast spreading environment the japanese u s ridge flux project used continuous hydrographic optical tow yos in 1993 to map the distribution of hydrothermal plumes along the southern east pacific rise from 13 30 to 18 40 s plume incidence the fraction of the spreading axis length overlain by a significant plume was 0 6 including a virtually continuous vent field stretching 150 km from 17 20 to 18 40 s hydrothermal venting was most concentrated along portions of the ridge crest with an inflated cross sectional area and an observable axial magma chamber reflector this pattern agrees with previous results from the northern east pacific rise such consistency implies that hydrothermal circulation on fast spreading ridges is vigorous where the relative volume of partial melt is high and meager where melt volume is low or undetectable hot rock alone is generally insufficient to drive significant hydrothermal circulation using a mean temperature anomaly of 0 014 0 010 c we estimated the hydrothermal heat flux from the study area as 1 5 1 1 107 mw ux where ux is the cross axis flow at the ridge crest no direct measurements of ux are yet available combining the plume distribution found here with prior data from slow intermediate and fast spreading ridges yields a significant linear correlation between plume incidence and spreading rate that extends across the full range of plate motion this correlation which mirrors that for near axis heat flux calculated by recent models implies that magma supply rate is the principal control on the large scale distribution of axial hydrothermal venting

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