drilling deep into young oceanic crust hole 504b costa rica rift hole 504b is by far the deepest hole yet drilled into the oceanic crust in situ and it therefore provides the most complete ground truth now available to test our models of the structure and evolution of the upper oceanic crust cored in the eastern equatorial pacific ocean in 5 9 m y old crust that formed at the costa rica rift hole 504b now extends to a total depth of 1562 3 m below seafloor penetrating 274 5 m of sediments and 1287 8 m of basalts the site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust as a result the conductive heat flow approaches the value of about 200 mw m predicted by plate tectonic theory and the in situ temperature at the total depth of the hole is about 165 c the igneous section was continuously cored but recovery was poor averaging about 20 the recovered core indicates that this section includes about 575 m of extrusive lavas underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes the latter have been sampled in situ only in hole 504b the igneous section is composed predominantly of magnesium rich olivine tholeiites with marked depletions in incompatible trace elements nearly all of the basalts have been altered to some degree but the geochemistry of the freshest basalts is remarkably uniform throughout the hole successive stages of on axis and off axis alteration have produced three depth zones characterized by different assemblages of secondary minerals 1 the upper 310 m of extrusives characterized by oxidative seafloor weathering 2 the lower extrusive section characterized by smectite and pyrite and 3 the combined transition zone and sheeted dikes characterized by greenschist facies minerals a comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology alteration and porosity into three zones that are analogous to layers 2a 2b and 2c described by marine seismologists on the basis of characteristic seismic velocities many of the logs and experiments suggest the presence of a 100 to 200 m thick layer 2a comprising the uppermost rubbly pillow lavas which is the only significantly permeable interval in the entire cored section layer 2b apparently corresponds to the lower section of extrusive lavas in which original porosity is partially sealed as a result of alteration nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900 1000 m below seafloor within the transition between extrusives and sheeted dikes indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2b and 2c and confirming that layer 2c consists of intrusive sheeted dikes a vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends that between the sheeted dikes of seismic layer 2c and the gabbros of seismic layer 3 which has never been sampled in situ may be within reach of the next drilling expedition to hole 504b therefore despite recent drilling problems deep in the hole current plans now include revisiting hole 504b for further drilling and experiments when the ocean drilling program returns to the eastern pacific in 1991

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

a new magma chamber model based on structural studies in the oman ophiolite abstract structural mapping in the mafic ultramafic transition zone and in the overlying crustal section of the oman ophiolite has revealed the importance of magmatic flow in the formation of layering in the gabbros and the mechanical coupling between this flow and the asthenospheric flow of the underlying peridotites the crustal section near the spreading axis and in particular the magma chamber are fed by two independent melts a dominant gabbroic melt episodically injected from depth and a subordinate wehrlitic crystal mixture produced by compaction of the transition zone layering in the gabbros can be partly produced by tectonic transposition of the gabbroic and wehrlitic mixtures during magmatic flow whatever its origin the layering orientation in the newly created crust does not necessarily reflect its original attitude the final orientation of the layering is determined by the attitude of the magmatic flow plane at the time it was frozen a new magma chamber model is proposed which solves some of the physical difficulties of previous models and accounts for the seismic imaging of magma chambers beneath oceanic ridges

deepening of the axial magma chamber on the southern east pacific rise toward the garrett fracture zone a wide aperture profile along the ridge axis from 14 29 s to 13 39 s 120 km to 30 km south of the garrett fracture zone is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector five areas along the 90 km line are examined in detail with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area a genetic algorithm code is used to find a best fit model from a comparison of the data and wkbj synthetic seismograms one hundred starting models are generated using a predefined set of velocity nodes with a fixed window of allowable depth variations between nodes an evolutionary process favors the better fitting models in each generation and a satisfactory misfit is usually obtained within 40 generations within individual areas the models were in good agreement with the depth of a given velocity node generally varying by not more than 20 m the depth discretization interval for the models a consistent deepening trend of the axial magma chamber amc is observed across the five areas as the garrett fracture zone is approached the depth varies from 0 99 km at area 1 which is approximately 100 km south of the garrett to 1 23 km at area 5 which is approximately 40 km south of the garrett the depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age for the areas examined here layer 2a is observed to be relatively constant in thickness along the axis although it is about 40 m thicker over area 5 where the axial magma chamber is deepest this variation is within the scatter of previously detailed layer 2a measurements at 13 n on the east pacific rise where an effectively constant thickness is observed this implies that layer 2a thickening is not a significant factor along this profile and that the amc deepening is real rather than apparent theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the garrett fracture zone which marks a major interruption of hundreds of kilometers of continuous ridge axis the deepening may also be interpreted as a downward limb from a central injection point however there is no indication of a similar downward trend in the other southern direction furthermore there is no accompanying systematic variation in axial depth or axial volume both of which are proposed to be indicators of central injection and along axis flow

tectonics of the east pacific rise near 12 50 n a submersible study abstract using the diving saucer cyana a portion of the east pacific rise between 12 38 and 12 54 n has been studied and mapped four main zones are defined 1 an active volcanic zone located in the central graben the graben displays a variable relief along strike from less than 10 m up to 55 m and has an en echelon pattern in plan view 2 an active tectonic zone comprising the central graben and extending up to about 2 km from either side of the rise axis 3 an inactive tectonic zone starting 2 km away from the axis and formed by discontinuous horst and graben type of structures and 4 off axis volcanoes seamounts located less than 20 km on either side of the rise axis at 12 50 n the active tectonic zone is extremely narrow and the mode of deformation is through fissuring in strong contrast with that observed in slow or intermediate rate ridges the presence or absence of a magma reservoir at shallow depth explains the difference in the width of the active tectonic zone the state of isostatic equilibrium or lack of equilibrium explains the different modes of deformation

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

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

accumulations of melt at the base of young oceanic crust as hot solid rock ascends through the mantle the pressure around it decreases while its temperature decreases approximately adiabatically during the ascent the melting temperature decreases faster than the adiabat temperature and if the initial temperature of the rock is high enough the rock will begin to melt at the depth where the adiabat crosses the melting temperature curve models of upwelling beneath mid ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge axis region suggesting that some of the ascending melt must arrive away from the ridge axis here i report ocean bottom seismograph observations of an uncommon and previously unreported marine seismic phase which provide strong evidence that melt accumulates in sills at the base of young oceanic crust the phase results from the conversion of compressional p waves to shear s waves on interaction with the crust mantle boundary the moho this phase is rare and very difficult to model but when visible it is strikingly prominent and usually appears at unexpectedly great source to receiver ranges the observations imply the presence of bodies of extremely low rigidity presumably melt sills at the moho close to the receiver melt may be fairly mobile at the base of the crust allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle escape of this melt to the sea floor may be responsible for the formation of off axis seamounts

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

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

crosshole seismic tomography many tomographic interpretations of crosshole seismic traveltimes have approximated the raypaths with straight lines connecting the source and receiver this approximation is valid where the velocity does not vary greatly but in many regions of interest velocity variations of 10 20 percent or more are observed causing significant ray curvature other work has taken this nonlinear effect into account but there do not appear to be many cases of demonstrated success in its application to the crosshole seismic problem we present here an iterative inversion scheme based on two dimensional ray tracing and its successful application to field data the interpretation method iteratively ray traces and then updates the velocity model within each iteration the differences between the data and the current model traveltimes obtained by ray tracing are related to the unknown velocity perturbations through a system of linear equations a damped least squares method solves for the velocity perturbations which updat

travel time and amplitude analysis in seismic tomography in cross hole tomography straight lines connecting the source and receiver are often used to approximate the true ray paths thus linearizing the inverse problem a recent paper by bregman et al 1989a has shown that this approximation may be very poor in the regions of greatest interest where large velocity gradients cause significant ray curvature the interpretation method presented by bregman et al 1989a iteratively ray traces and updates the velocity model with model perturbations obtained by a damped least squares inversion of the travel time residuals this paper presents a modification of the travel time tomography method so that it may be applied to the inversion of first motion amplitudes for inverse quality factor q 1 after determining the velocity structure using travel time tomography the amplitudes are inverted using a similar scheme no additional ray tracing is required for the amplitude inversions the effect of the velocity structure previously determined through travel time tomography is taken into account so that amplitude variations due to the focusing and defocusing effects of the velocity structure are not interpreted as being due to attenuation structure both the travel time and amplitude tomography methods are applied here to field data from a cross hole experiment in crystalline rock wong et al 1983 the frequency range of the seismograms is 1 6 6 khz allowing resolution of velocity structure on a scale of several meters the resulting velocity image shows good agreement with other geological and geophysical data reversing the model by placing the model boreholes on opposite sides relative to the original reconstruction yields almost identical results indicating that the interpolation scheme used in the forward modeling has not biased the final model synthetic maslov seismograms calculated for the derived velocity model agree well with the waveform data further confirming the validity of the model in addition the amplitude tomography results place absorptive regions in the same location as the most pronounced low velocity features which correlate with fractures in the boreholes highly transmissive regions also correlate well with high velocity regions

morphology of extrusive basalts and its relationship to seismic velocities in the shallow oceanic crust recent seismic experiments have revealed that a shallow low velocity zone is a common feature within the uppermost few hundred meters of very young less than 100 kyr oceanic crust at fast spreading ridges the shape of this zone suggests that the low velocities are related to the constructional morphology of the extrusive layer 2a portion of the crust with this in mind we use simple analytical techniques to study the effects of characteristic morphologic elements of the extrusive basalts on their elastic properties we find that high compliance of the contacts between pillows and pillow fragments can account for all of the observed velocity anomaly voids between pillows are by comparison to their effects on porosity considerably less important radial cooling cracks and voids within the pillows themselves will affect the velocities in direct proportion to their effect on the individual pillows the relatively rapid increase in velocity with age in these very young extrusives can be explained by welding of pillow contacts and stiffening of the individual pillows by infilling of radial fractures both of which may take place without a large reduction in porosity if voids between pillows are filled initially by sediments the cementation of these materials can also cause large increases in velocity without a large addition of mass because of the importance of such subtle variations in morphology there can be no one to one relationship between seismic velocities and porosities within the extrusive layer of the oceanic crust therefore independent measures of crustal porosity must be found for example using sea floor gravity surveys

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

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

in situ electrical resistivity and bulk porosity of the oceanic crust costa rica rift in situ electrical resistivity was measured in dsdp hole 504b to a depth of 1 013 m into oceanic basement apparent resistivities are about 10 m in about 600 m of pillow lavas sharply increasing to nearly 1 000 m in the underlying dykes bulk porosities calculated from resistivities show a distinct layering apparently corresponding to geophysical layers 2a 2b and 2c

magma chambers at oceanic ridges how large seismic models of magma chambers at fast spreading oceanic ridges predict small perched magma chambers above a wide low velocity zone seismic attenuation in a low velocity zone allows for 10 melt in hot gabbros in order to reconcile these data with a structural model of magma chambers that is supported by compelling field data in the oman ophiolite we propose that the low velocity zone belongs to the magma chamber in such a dynamic magma chamber gabbros develop strong shape fabrics during magmatic flow in a concentrated suspension this process results in a drastic reduction of the melt fraction that is compatible with magmatic flow as confirmed by preliminary experiments we propose here that dynamic magma chambers below ridges may contain as little as 10 melt

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

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

the ramesses experiment iii controlled source electromagnetic sounding of the reykjanes ridge at 57 45 n summary a controlled source electromagnetic sounding survey centred on an axial volcanic ridge avr segment of the reykjanes ridge at 57 45aen was performed as part of the ramesses experiment low frequency 0 35 11 hz electromagnetic signals were transmitted through the crust to an array of horizontal electric field recorders at the seafloor to ranges of 15 km from the source which was a 100 m long horizontal electric dipole towed at heights of 50 80 m from the seafloor coincident seismic and magnetotelluric studies were conducted during the rest of the ramesses experiment data were interpreted using a combination of 1 d forward modelling and inversion and iterative forward modelling in two dimensions on the axis of the avr the resistivity at the seafloor is 1 v m there is a steep resistivity gradient in the upper few hundred metres of the crust with the resistivity reaching approximately 10 v ma t a depth of 500 m in order to explain the low resistivities the upper layer of the crust must be heavily fractured and saturated with sea water the resistivity increases with distance from the axis as the porosity decreases with increasing crustal age the most intriguing feature in the data is the large diverence in amplitude between fields transmitted along and across the avr axis a significant zone of low resistivity material is required at approximately 2 km depth beneath the ridge crest in order to explain this diverence it is coincident with the low velocity zone required by the seismic data and has a total electrical conductance in excellent agreement with the results of the magnetotelluric study the low resistivity zone can be explained by the presence of a body of partially molten basalt in the crust taken together these results provide the first clear evidence for a crustal magma chamber at a slow spreading mid ocean ridge the data constrain the melt fraction within the body to be at least 20 per cent with a melt volume suycient to feed crustal accretion at this segment of the ridge for of the order of 20 000 years since this body would freeze in the order of 1500 years this finding lends support to the hypothesis that at slow spreading rates crustal accretion is a cyclic process accompanying periodic influxes of melt from the mantle to a crustal melt reservoir

tracking stress and hydrothermal activity along the eastern lau spreading center using seismic anisotropy abstract using p wave refraction data from the l scan controlled source seismic experiment the three dimensional anisotropy structure of the upper crust is determined along the eastern lau spreading center elsc the tomographic image of anisotropy magnitude and orientation is constructed from 197 000 travel time measurements of p wave arrivals recorded on 83 ocean bottom seismometers providing the most detailed and extensive view of upper crustal spreading center anisotropy to date p wave speeds vary with the azimuth of the seismic ray path a type of anisotropy produced by stress aligned lithospheric cracks and microcracks across the study area the fast axes of anisotropy are generally oriented parallel to the trend of the spreading center as expected for ridge parallel cracks that form in association with seafloor spreading the seismic study encompassed four individual spreading segments of the elsc separated by three overlapping spreading centers osc the two larger oscs exhibit high anisotropy that penetrates deep into the upper crust surrounding these oscs and in the wake of the largest are regions where the fast axes of anisotropy are misaligned relative to the general trend in these areas the observations agree with numerical models and seafloor morphology that suggest tensile stress concentrations and a high degree of brittle crack formation the tomographic images indicate that the location and dynamics of persistent ridge offsets can be tracked back through time via their anomalous anisotropy along the spreading centers and away from ridge tips the anisotropy is greater where the melt supply is inferred to be greater and smaller where the melt supply is inferred to be smaller this is opposite to the trend expected if simple tectonic stress models govern anisotropy alternatively hydrothermal activity is expected to increase with magma supply and can explain higher anisotropy via hydrofracturing this study provides the first evidence that seismic anisotropy tracks variations in hydrologic activity along the crests of oceanic spreading centers

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

a finite element computation of seismic diffraction about the rise axis magma chamber in a previous study models of the crust and magma chamber at 12 50 n on the east pacific rise were derived with a two dimensional ray tracing analysis of rise normal refraction lines the best model featured a 4 km wide axial magma chamber but the data contained diffraction effects that were not modeled by geometric ray theory this raised questions as to whether this model would reproduce the waveform data and whether diffraction might be obscuring a much larger magma chamber we have tested the model with a finite element calculation of synthetic seismograms this method incorporates full wave effects including diffraction the synthetics accurately reproduce the data and exhibit diffraction phenomena similar to those in the data the previous travel time analysis was largely successful and was only slightly compromised by diffraction diffraction in rise normal refraction experiments is therefore not obscuring a large low velocity magma chamber

new observations of the shallow seismic structure of young oceanic crust the results are presented of three new experiments carried out on the mid atlantic ridge mar near latitude 23 n using a unique method of studying the seismic structure of the uppermost few hundred meters of the oceanic crust the data were collected using a fixed ocean floor hydrophone receiver and a controllable explosive source that was towed within a few tens of meters of the rugged bottom topography these 1 to 2 km long refraction lines produced for the first time direct observations of the compressional wave velocity structure of the uppermost 200 300 m of the young igneous crust one experiment was carried out over the site of hole 648b of the ocean drilling program on a small volcano within the median valley of the mar the seafloor velocity was observed to be 2 1 km s 1 underlain by an approximately linear velocity gradient of 4 s 1 given that we know the crust at this location consists of fresh basalt lavas with laboratory measured velocities in excess of 5 8 km s 1 porosities at the seafloor of as high as 30 50 are inferred the two other experiments were located over 7 m y old crust near deep sea drilling project site 395 west of the mar separated by only 14 km laterally but by over 1400 m in water depth the experiment positioned in the 100 m thick sediment pond and that located 14 km to the south atop a prominent topographic high produced results that within the data resolution were indistinguishable the velocity of the uppermost basaltic basement was 4 1 km s 1 and the velocity gradient was less than 0 5 s 1 this 2 km s 1 difference in velocity between the two sites is assumed to be a consequence of age related modifications to the physical properties of the crust if the primary change is in only the total porosity then a 15 20 reduction is required to explain these observations this seems unlikely because any such pervasive process would have been previously observed in geophysical and sampling data e g off axis volcanism secondary mineral deposition tectonic compression a combination of many processes acting in unison would be necessary to produce such a decrease in porosity if the geometry of the cracks and voids were modified in just the right way then no change in total porosity is required to explain the velocity difference these on bottom refraction profiles are proven to be practical experiments that can provide precise information on a scale appropriate to many of the ocean floor s geologic features they produce results that may be correlated in a meaningful way with results of ocean drilling and downhole measurements

extensional faulting and caldera collapse in the axial region of fast spreading ridges analog modeling the axial high of the east pacific rise epr is bounded by ridge parallel lateral grabens that develop 2 8 km off axis these troughs appear to lengthen away from the ridge crest suggesting that tectonics is active at least 10 km away from the axis along 15 20 of the length of the ridge the axial high is notched by a summit trough 500 to 1800 m wide these large axial troughs represent elongated collapse calderas that form when the epr magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply under continuous stretching we report the results of analog experiments performed in order to constrain the tectonic magmatic evolution of the crestal region of fast spreading ridges and more particularly the possible link that may exist between the development of axial caldera and the creation of lateral grabens along the crestal region we used inflatable elongated balloons filled with water as an analog of the magma reservoir the balloon is capped with a silicone layer representing hot rocks below the brittle ductile transition and is covered by a sand layer representing the brittle crust the sand surface was given a dome shape that approximates the morphology of a fast spreading ridge mobile walls activated by a stepping motor allowed us to conduct the deflation experiments during continuous extension combination of deflation and extension leads to the creation of two lateral depressions and one axial trough the lateral depressions are controlled by normal faults while the central trough is delineated by reverse faults during balloon deflation tectonic extension is accommodated away from the axis due to the presence of the ductile silicone layer whereas the deflation of the axial reservoir is accommodated by the collapse of the overlaying brittle crust this accounts 1 for the development of axial troughs as collapse calderas and 2 for the ubiquitous formation of lateral grabens of the flanking tectonic province in response to the deformation of the crystal mush

magma chamber profiles from the bay of islands ophiolite complex models of the oceanic lithosphere derived from constraints imposed by the geology of ophiolite complexes require a magma chamber to produce the cumulate and other plutonic portions of the oceanic crust and upper mantle evidence from the bay of islands ophiolite complex indicates that layered plutonic rocks have not formed solely due to differential crystal settling to the horizontal floor of a magma chamber but have formed largely by heterogeneous nucleation and in situ crystallization along gently inclined as well as steeply inclined bounding surfaces of a large magma reservoir coherent well defined and systematically oriented large scale arcuate patterns described by mesoscopic igneous layering have been identified in the plutonic sections of two ophiolite massifs in the bay of islands complex these layering patterns seem to outline the shape of a large continuously evolving steady state magma chamber that extended along a ridge axis and ended at a transform fault intersection

tomographic inversion for three dimensional velocity structure at mount st helens using earthquake data tomographic inversion is applied to 17 659 p phase observations at 21 stations from 2023 earthquakes in the vicinity of mount st helens to study the three dimensional velocity structure block size for the inversion is 2 km horizontally and 2 km or more vertically locations of hypocenters are assumed known and are based on a reference one dimensional layered velocity structure a conjugate gradient technique lsqr is used to invert the large sparse system of equations augmented by regularization with a laplacian roughening matrix resolution is estimated by computing the impulse response of the inversion for various critical locations and uncertainties of the estimates are determined by a jackknife approach the results of the inversion show a remarkable correlation with known geological and geophysical features the spirit lake and spud mt plutons are characterized by high velocity regions extending to approximately 9 km depth the st helens seismic zone a band of diffuse seismicity extending nnw from the volcano is evident as a prominent low velocity lineation the change in character of the velocity anomalies south of st helens corresponds well with the near cessation of seismic activity there a low velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations

ultrasonic velocity and attenuation measurements on basalt melts to 1500 c role of composition and structure in the viscoelastic properties viscoelastic properties of basalt melts of a wide range of composition are determined from ultrasonic compressional wave velocity vp and quality factor qp measurements in the frequency range of 3 4 22 mhz and the temperature range of 1150 1500 c the basalt samples are grouped into two types low magnesia group a containing 5 9 wt mgo and high magnesia group b containing 18 23 wt mgo the vp values of the melts determined at 1400 c are in the range 2 53 2 64 km s for group a basalts and 2 79 2 86 km s for group b basalts the relaxed low frequency velocity vr is a principally linear function of the melt density least squares fits of the data give velocity density relations vr km s 0 691 0 712 g cm3 for group a basalts and vr 0 328 0 907 for group b basalts at 1400 c the mean molecular weights m are 65 0 0 3 and 58 2 0 5 g mol for groups a and b basalts respectively suggesting that m is also a principal factor determining the melt velocity the relaxed modulus mr of the basalt melts range from 16 5 to 22 8 gpa and is also a linear function of the melt density the values of longitudinal l and volume v viscosities are calculated from the measured vp and qp and the available shear viscosity s data the viscosities and related activation energies decrease almost linearly as nbo t nonbridging oxygens per tetrahedrally coordinated cation increases

evolution of the ocean crust results from recent seismic experiments we present results from recent analyses of seismic refraction and sea floor microseismicity studies in the pacific and atlantic oceans which lend support to the hypothesis that processes responsible for the construction of ophiolite suites are similar to phenomena extant at midocean ridges seismicity at fast spreading ridges is characterized by very low magnitude 0 1 and shallow 2 3 km microearthquakes and long term oscillations or harmonic tremor a detailed seismic refraction experiment on a fast spreading portion of the east pacific rise supports the hypothesis of the existence of a crustal magma chamber analyses of these data indicate that the chamber is largely unperturbed by the presence of conjugate spreading centres and the inverted deltashaped zone of partial melt is characterized by a half width in excess of 6 km finally a new approach for obtaining upper crustal velocities when applied to data characterized by the presence of shear waves provides several counter examples to the hypothesis that the shallow crust evolves with time

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

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

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

cracked media poisson s ratio and the structure of the upper oceanic crust summary theoretical models of water filled cracks within a solid are in general agreement and indicate that the poisson s ratio of the composite material will typically increase for thin cracks with aspect ratios less than a but will decrease for thick cracks with aspect ratios between and i near spherical pores cause little change in poisson s ratio this is true both for randomly oriented isotropic crack models and at certain directions for aligned crack models which predict seismic anisotropy observed anomalous values of poisson s ratio in oceanic layer 2 can often be explained using these models as an effect of cracks and do not necessarily require the presence of rocks of unusual composition in particular the low poisson s ratios reported by spudich orcutt 1980 and au clowes 1984 can approximately but not always exactly be fitted with a model consisting of thick cracks within higher poisson s ratio rock more typical of the upper oceanic crust however this interpretation is near the limit of existing theories of cracked material so other explanations for the anomalous observations must also be considered

jurassic coastal park a great diversity of palaeoenvironments for the dinosaurs of the villar del arzobispo formation teruel eastern spain abstract the villar del arzobispo formation cropping out in the western penagolosa sub basin late jurassic eastern spain includes abundant dinosaur tracksites and bones which occur in diverse mixed siliciclastic and carbonate facies deposited from shallow marine to coastal and alluvial paleoenvironments the lower part of the unit mainly composed of bioclastic and oolitic limestone was deposited in an inner carbonate platform which underwent episodic subaerial exposure and siliciclastic inputs from the emergent areas and includes scarce dinosaur tracks this environment evolved into a siliciclastic coastal and alluvial plain that was crossed by channels and affected by periodic flooding events producing the deposition of splay lobes upward the siliciclastic coastal and alluvial deposits are interbedded with inter to supratidal limestone beds these tidal and coastal deposits show the highest abundance diversity and best preservation of dinosaur tracks and bones of the unit this setting gradually evolved upward into an inner carbonate platform producing the deposition of shallow marine bioclastic and oolitic limestone which includes very scarce dinosaur tracks the highest abundance diversity and best preservation of theropod sauropod thyreophoran and ornithopod tracks occur at the top of tidal carbonate beds tracks also occur in the siliciclastic coastal and alluvial plain deposits especially in the flood plain deposits preserved mainly as infillings or natural casts additionally very scarce and poorly preserved tracks occur at the top of shallow marine carbonate beds bones may be articulated and or associated in the flood plain deposits whereas they are isolated and dispersed in the splay lobe deposits although this unit has been previously assigned to the tithonian berriasian the analysis of larger benthic foraminifera suggests a kimmeridgian tithonian age for the villar del arzobispo formation this is consistent with the dinosaur assemblages present in the unit which are strongly related to other european late jurassic faunas

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

seismic propagation across the east pacific rise finite difference experiments and implications for seismic tomography we use a full waveform acousto elastic finite difference technique to investigate seismic propagation across the east pacific rise at 9 30 n for a two dimensional velocity model based on that proposed by vera and others 1990 the primary feature of the model is an upper crustal low velocity region corresponding to the axial magma chamber which includes a small magma body located 1 6 km beneath the seafloor at the rise axis the high velocity gradients in this region result in a complex pattern of propagation which includes considerable scattering of energy above and below the magma chamber a qualitative comparison of finite difference seismograms with data collected by receivers located 9 km and 20 km off axis during a tomography experiment at 9 30 n shows generally good agreement for paths that cross the rise axis the first arrival in the finite difference solutions diffracts above the magma chamber this phase has a very low amplitude and at larger offsets falls below the ambient noise levels observed during the tomography experiment in such cases the first arrival with significant energy is a diffraction from below the magma chamber a high amplitude moho turning pmp phase which results from the large velocity change across the moho beneath the rise axis is apparent in both the finite difference solutions and the observations ray theoretical calculations of the paths of the diffracted arrivals are very unstable and for the diffractions above the magma chamber no solution can be found with a single precision algorithm synthetic delay time inversions using an approximate ray tracing algorithm demonstrate the importance of ensuring that picked arrival times are assigned to paths that pass to the correct side of the magma body synthetic inversions of spectral estimates of t show that q 1 models are compromised not only if the ray paths are incorrect but also if t estimates include significant contributions from more than one phase deterministic scattering from the magma chamber may contribute noticeably to spectral estimates of t but the results of the finite difference experiments imply that high levels of attenuation observed for phases passing below the magma chamber are predominantly the result of intrinsic attenuation

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

a new fe mn geothermometer for hydrothermal systems implications for high salinity fluids at 13 n on the east pacific rise abstract field and experimental investigations demonstrate the chemistry of mid ocean ridge hydrothermal vent fluids reflects fluid mineral reaction at higher temperatures than those typically measured at the seafloor to account for this and in turn be able to better constrain sub seafloor hydrothermal processes we have developed an empirical geothermometer based on the dissolved fe mn ratio in high temperature fluids using data from basalt alteration experiments the relationship t c 331 24 112 41 log fe mn has been calibrated between 350 and 450 c the apparent fe mn equilibrium demonstrated by the experimental data is in good agreement with natural vent fluids suggesting broad applicability when used in conjunction with constraints imposed by quartz solubility associated sub seafloor pressures can be estimated for basalt hosted systems as an example this methodology is used to interpret new data from 13 n on the east pacific rise where high temperature fluids both enriched and depleted in chloride 339 646 mmol kg relative to seawater are actively venting within a close proximity accounting for these variable salinities active phase separation is clearly taking place at 13 n yet the fluid fe mn ratios and the silica concentrations suggest equilibration at temperatures less than those coinciding with the two phase region these data show the chloride enriched fluid reflects the highest temperature and pressure 432 c 400 bars of equilibration consistent with circulation near the top of the inferred magma chamber this is in agreement with the elevated co 2 concentration relative to the chloride depleted fluids the noted temperature derived from the fe mn geothermometer is higher than the critical temperature for a fluid of equivalent salinity this carries the important implication that despite being chloride enriched relative to seawater these fluids evolved as the vapor component of even higher salinity brine

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

constraints on the structure of the east pacific rise from seismic refraction data university of washington seismic data taken during rose phase 1 have been used to study the structure of the east pacific rise in the vicinity of 12 n off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2 5 km a zone of low but uniform velocity gradient from 2 5 to about 5 5 km and a transitional zone about 1 km thick to mantle material there is also evidence for substantial anisotropy in mantle velocity gradients with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise under the rise axis the data allow at most a very narrow zone of low velocity less than about 2 km wide in the crust

numerical models of magma chambers in the oman ophiolite long standing studies conducted in the oman ophiolite have led to models of fast oceanic spreading centers which in some aspects contradict the results of geophysical studies carried out along the east pacific rise epr this is true in particular for the shape size and internal magmatic flow of magma chambers thus justifying numerical modeling our modeling founded on the equations of fluid mechanics is based on a two dimensional finite difference method that takes into account high viscosity contrasts at the walls of the magma chamber boundary conditions deduced from the geophysical data of the epr and geological observations from the oman ophiolite the model predicts a magma chamber shape consistent with field studies whether the magma feeding is from the top by the melt lens or from the bottom by basaltic sills it has only a weak influence on the shape of the chamber the viscosity of the magma plays a crucial role for the setting of convection in the chamber according to geological observations which argue against the existence of convective flow patterns the viscosity of the gabbroic mush within the chamber must exceed 1014 1015 pa s depending on the oceanic plate velocity the flow pattern in the magma chamber and the associated foliation pattern rely critically on whether the underlying upwelling mantle is passive foliations oriented inward or active foliations oriented outward the active structural analysis in the oman model is supported by ophiolite data

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

platinum group element distribution in komatiitic and tholeiitic volcanic rocks from munro township ontario platinum pd ir au mgo cr s ni co cu and zn were determined in volcanic rocks from pyke hill fred s flow and theo s flow in munro township ontario to establish the fractionation trends of the platinum group elements in these rocks and to evaluate the relevant processes controlling such fractionation distinctive and relatively consistent fractionation trends are characteristic of pt pd and ir platinum and pd are usually concentrated in the upper units of flows spinifex textured rocks relative to the lower olivine and pyroxene rich cumulates whereas the reverse is true for ir theo s flow does not show these patterns in that pt pd and ir behave in a similar fashion the platinum group element fractionation trends formed during the crystallization of these flows suggest that sulfide immiscibility has an insignificant role it is concluded that in general ir must be coprecipitated with olivine or pyroxene whereas pt and pd are rejected from the structures of these minerals the ir ferromagnesian association probably does not involve a substitution of ir in the mineral structure of olivine or pyroxene

faroe iceland ridge experiment 2 crustal structure of the krafla central volcano the seismic velocity structure of the krafla central volcano is characterized by large variations in compressional velocity a 40 km wide high velocity dome extends from the lower crust 11 14 km depth beneath the volcano narrowing upward a magma chamber sits at its top near 3 km depth it is defined by both 0 2 0 3 s compressional wave delays and shear wave shadowing to be 2 3 km n s 8 10 km e w and 0 7 1 8 km thick the near surface structure uppermost 2 5 km of the krafla caldera is approximately flat lying with only minor lateral heterogeneities the crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities shear waves reflected from a 19 km deep moho are clearly visible for some paths through the crustal volume below the magma chamber even though the more shallow diving s waves are severely attenuated the midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near solidus temperatures the krafla central volcano plays a major role in crustal genesis along the plate boundary the high velocity dome in our view represents crust generated in and around the magma chamber which has subsequently been advected to greater depths

the use of laboratory velocity data for estimating temperature and partial melt fraction in the low velocity zone comparison with heat flow and electrical conductivity studies peridotite is widely considered to be the dominant component of the upper most mantle seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature the strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low velocity zone of the earth the laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite i e homologous temperature dependence this observation allows us to extrapolate laboratory results to higher pressures greater depths with some confidence requiring only a knowledge of the solidus as a function of pressure using the ratios of lithospheric to asthenospheric velocities temperature and melt fraction in the asthenosphere the low velocity zone can be determined from laboratory velocity data examples of thermal structures of the upper mantle beneath oceanic plates are presented we choose three locations of geophysical interest iceland plateau pacific ocean and philippine sea where reliable seismic velocities have been determined in the upper mantle from surface wave studies a large amount of partial melt 5 vol and a higher temperature than the dry peridotite solidus are inferred in 0 5 m y asthenosphere under the iceland plateau and the active marginal basin of the east philippine sea the partial melt zone appears to extend deeper 100 km and to greater ages 20 m y in the pacific ocean region than under the slowly spreading iceland plateau partial melting is not expected in the asthenosphere older than 5 m y under the iceland plateau even though the volume fraction of partial melt is not so large 3 vol beneath the pacific plate the melting zone may extend to a distance of above 2000 km from the ridge many seamounts observed in the pacific ocean may result from this vast region of melting under the plate we suggest a linear relation between the width of melting zone and the plate velocity the laboratory results are also applied to other low velocity zones it is suggested that a velocity drop up to about 6 in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature this implies that partial melt may not exist generally but only in limited areas in the low velocity zone comparison of seismic results with theoretical models may overestimate the temperature and melt fraction if the velocity drop at subsolidus temperature is neglected temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data indicating that the upper mantle under mid ocean ridges may be fairly dry even a water or co2 undersaturated solidus 0 1 wt does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results it is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study unknown pressure frequency and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy

imaging rapidly deforming ocean island volcanoes in the western galapagos archipelago ecuador using local body wave arrival time tomography methods to determine 3 d seismic velocity structure we imaged the plumbing system of sierra negra volcano galapagos this hot spot volcanic chain includes some of the fastest deforming volcanoes in the world making this an ideal location to study shield volcano plumbing systems we inverted p and s wave arrivals recorded on a 15 station temporary array between july 2009 and june 2011 using an a priori 1 d velocity model constrained by offshore refraction studies with local seismicity from nearby volcanoes as well as the ring fault system the model resolution is good between depths of 3 and 15 5 km the propagation of s waves throughout this volume argues against any large high melt accumulations although a shallow melt sill may exist above 5 km we image a broad low velocity region 25 km laterally below sierra negra at depths 8 15 km no large regional velocity increase is found within the limits of good resolution suggesting that crust is thicker than 15 km beneath the western galapagos archipelago our results are consistent with crustal accretion of mafic cumulates from a large volume magma chamber that may span the boundary between preplume and accreted crust the similarity between our results and those of hawaii leave open the possibility that the crust has also been thickened by under plating

the structure of young oceanic crust near a very fast spreading ridge we have modeled the structure of the young oceanic crust near the fastest spreading ridge ever to be profiled using seismic refraction techniques the data were originally collected during the capricorn expedition of the scripps institute of oceanography in 1953 one of the split leg profiles from that expedition c22 was conducted near the east pacific rise at 15 s where the half spreading rate is over 80 mm yr by digitizing the data and modeling the two splits of c22 using synthetic seismograms we have found that the crust is normal in thickness 5 to 7 km and in velocity structure an upper crustal transition zone is present on both legs and would appear to be a regular feature of young oceanic crust it is probably too deep 1 5 to 2 km beneath the seafloor to be correlated with the boundary between pillow basalts and sheeted dikes instead we attribute this transition zone to changes in crustal porosity and or hydrous minerals in the oceanic crust from the appearance of wide angle moho reflections near the rise axis we suggest that the very large crustal magma chamber that might be expected under this fast spreading ridge is not present this presumably results from the cooling of the crust by pervasive hydrothermal circulation

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

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

seismic structure and crustal magmatism at the mid atlantic ridge 35 n we have determined the three dimensional p wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the mid atlantic ridge mar near 35 n the best fitting one dimensional model is 0 8 km s lower in velocity at a given depth than for fast spreading ridges while the isotropic three dimensional velocity structure shows a strong lateral heterogeneity up to 1 4 km s peak to peak variation at 1 km depth in both the along and cross axis directions an anomalous region with velocities as much as 0 8 km s lower than average and 3 5 km in lateral extent is located beneath a near axis seamount we interpret this feature as a region of near solidus temperatures with possibly a small degree of partial melt from the variation of travel time residuals with azimuth we determine a best fitting p wave anisotropy model that has 4 anisotropy from 500 m to 1 km depth 2 anisotropy in the depth range 1 1 5 km and no anisotropy at greater depth this finding is consistent with vertical cracks aligned parallel to the axial valley we find no evidence for the shallow high velocity ridge parallel feature commonly observed at the axis of fast spreading ridges and attributed to high velocity dikes beneath a thin extrusive layer the absence of this anomaly at the mar implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25 000 years an unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes which may account for the generally lower velocities beneath the mar compared with the east pacific rise and a lateral variation in upper crustal lithology

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

waveform inversion of seismic refraction data and applications to young pacific crust summary we present a method for constraining the velocity depth structure in the earth using seismic refraction waveform data we test the method with synthetic data from known models and apply it to a set of data collected in 1982 june from the east pacific rise at 13 n from the magma expedition in this iterative process wkbj seismograms are computed for a starting model the difference between these and the observed seismograms is used to update the model subject to physical constraints an important first step in the inverse scheme is the linearization of the wkbj seismogram equation allowing us to compute differential seismograms partial derivatives of the synthetic seismogram with respect to specific model parameters this linearization provides the means for estimating required model perturbations based on the misfit in the seismograms the choice of a suitable numerical strategy for computing an updated model is a crucial second step in formulating a working algorithm because the data contain noise synthetic seismograms can only fit the data to this noise level in this case infinitely many models fit the data to this tolerance and some of these estimates are non physical involving negative layer thicknesses a successful strategy must choose from among these possibilities a well defined physically reasonable new model in a commonly used approach to solving non linear problems the perturbation to the starting model is minimized while improving the fit to the data after several iterations the final model which possesses no special properties still tends to resemble the starting model when used with the magma data this technique essentially does not perturb the model at all a method we find much more satisfactory involves solving for the new model directly while applying physically important constraints as constraints we require the velocity gradient remain below a fixed value and penalize the roughness of the new model we thus solve for the smoothest model fitting the data to the specified misfit

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

compressional and shear wave structure of the east pacific rise at 11 20 n constraints from three component ocean bottom seismometer data we report the analysis of seismograms from two orthogonal refraction lines in the rose area of the east pacific rise at 11 20 n recorded by a three component ocean bottom seismometer obs converted shear waves are clearly observed on the horizontal seismometers for both lines including paths crossing the rise axis neither a travel time delay nor measurable attenuation is observed for shear or compressional waves which have passed beneath the rise crest at depths down to 4 km on the basis of these observations we conclude that no large crustal magma chamber is present beneath the rise at this latitude we invert the travel times of p and converted s waves to infer the crustal structure in the vicinity of the rise and we use p wave amplitudes to constrain further the range of solutions the p wave velocity distribution does not differ markedly from that of normal young oceanic crust but the shear velocity distribution results in a poisson s ratio 0 32 that may be more typical of highly fractured crust in rise axis regions a thin 80 m layer of sediment is inferred to be present beneath the obs on the basis of the measured particle motions the observation of an sp phase and the reverberatory character of the seismograms a combined consideration of the results of this experiment and other observations of rise crossing crustal phases elsewhere in the rose area leads us to conclude that any magma within the crust beneath the rise axis of this region must be confined to narrow dikes or pipes or to small isolated bodies with a vertical thickness of less than about 1 km

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

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

petrology and sr nd and pb isotope geochemistry of mid ocean ridge basalt glasses from the 11 45 n to 15 00 n segment of the east pacific rise 1 basaltic glasses from the geophysically well studied section of the east pacific rise epr between 11 45 n to 15 00 n range from normal mid ocean ridge basalts morb to transitional morb and their major element variations correlate with isotopic and trace element indices of enrichment to first order basalts enriched in na8 0 incompatible elements 87sr 86sr and 206pb 204pb but low in fe8 0 and 143nd 144nd are more prevalent along the shallow portions of the ridge axis in detail the samples can be divided into two chemical and geographical groups the southern bathymetric dome extending from the 11 45 n overlapping spreading center to 14 10 n and the northern group extending from 14 10 n to the orozco transform the boundary between these two groups is apparent in a change in isotopic composition results indicate that there are three mantle source components that produce the compositional variability observed among samples from the 11 45 n to 15 00 n segment of the epr a depleted mantle component a seamount type enriched mantle component and an indian morb like mantle component south of 14 10 n the geochemical variability is dominated by binary mixing between a depleted mantle component and an enriched component similar to near ridge seamounts north of 14 10 n the low 206pb 204pb high 207pb 204pb indian morb like component exerts a major influence on the geochemical variability of the axial lavas regional averages of major element composition e g na8 0 and fe8 0 show relatively limited variability consistent with the restricted range in depth for this region and plot within the pacific field of the previously defined global trends major element variations among individual samples however parallel the global array and their correlation with indices of mantle enrichment supports the idea that the pacific type local trend results from small scale heterogeneities in the mantle beneath the epr our results also indicate that tectonic segmentation and magmatic boundaries are probably not causally related in this study area and that the sizes of the present magma chambers are not a dominant factor in determining the compositional variability of erupted lavas

crustal structure of the mid atlantic ridge south of the kane fracture zone from seafloor and sea surface gravity data seafloor and sea surface gravity data are inverted together to construct a model for the near axis crustal structure of a slow spreading ridge the seafloor data set offers two main advantages it allows us to recover shorter wavelengths signal and to constrain the value of a potential field at two different levels the model we propose here would not have been derived from sea surface data alone it is based on a dense sea surface gravity coverage and on 121 sea bottom gravity measurements collected in the mid atlantic ridge at kane mark area during the hydrosnake 1988 and gravinaute 1993 cruises the primary goal of the seafloor surveys was to test for the presence of a magma reservoir beneath the axial neovolcanic ridge first a forward two dimensional 2 d model of the crustal structure across the axis is fit to observed gravity anomalies using constraints from geological and structural observations bouguer anomalies computed from sea bottom measurements and downward continuation of sea surface measurements both constrain the forward modeling this forward model is the starting point of a 2 d monte carlo inversion of seafloor and sea surface data in addition to the crustal thickness variations along axis our data document the amplitude variations of the crustal thickness and or its density in the across axis direction the model resulting from our inversion exhibits several features of the crustal structure in the mark area 1 the presence of a low density ap 300 50 kg m 3 body beneath the neovolcanic ridge is suggested and could correspond to a magma chamber or more probably to a highly hydrothermally fissured zone 2 both long and short wavelength gravity signals exhibit a difference between the western and eastern sides of the axial domain the mean value and the amplitude of bouguer anomalies are higher on the western part this difference suggests that axial processes in this area are very asymmetric 3 abyssal hills are not associated with a single gravity signature for instance on the west side of the axis one of the explored hills has no bouguer anomaly and is interpreted as a neovolcanic ridge whereas the others are associated with a shifted bouguer anomaly high and are interpreted as having thinner magmatic crust 4 the last feature of the crustal fabric we document here is the asymmetric emplacement of some deep rocks outcrops in the mark area we find that pink hill a topographic high where serpentinized peridotites are outcropping is much more serpentinized on its east flank toward the axial valley than on its west flank alteration occurring mainly by fluid circulation through faulted zones the asymmetric serpentinization suggests that deep origin rocks have outcropped by means of a main fault zone and are not emplaced by diapirism

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

high temperature experiments on the elastic and anelastic behaviour of magmatic rocks measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks dunite peridotite gabbro and basalts and a synthetic forsterite at temperatures between 600 and 1500 c depending on the sample material and in the frequency range from 0 003 to 30 hz under forced torsional oscillations the maximum shear stress was 0 3 mpa and the strains were of the order 5 10 6 to 5 10 5 measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting the absolute values of the shear modulus below tm are comparable with the results of other authors the onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity at high temperatures t 0 8 tm the absorption factor q 1 follows a power law q 1 exp a rt with 0 15 0 30 and a between 100 and 200 kj mol 1 towards lower temperatures this high temperature background is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature this peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time exp a rt with a 400 600 kj mol 1 above tm a third contribution to q 1 is observed that can be regarded as independent of frequency between 1 and 30 hz this last component which contributes 50 to the total absorption is attributed to the melt phase in the samples an explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines by some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features at very high temperatures and low frequencies a small influence of steady state flow on q 1 is observed and below t 0 6 tm thermal cracking or thermoelastic losses may contribute to q 1 the influence of lithostatic pressure on q 1 is estimated and the extrapolation of the experimental data to the p t conditions of the uppermost mantle are in good agreement with published earth models an attempt is made to establish a general q t relationship from the experimental data

a baseline for upper crustal velocity variations along the east pacific rise at 13 n a wide aperture profile of the east pacific rise at 13 n provides data necessary to make a high resolution seismic velocity profile of the uppermost crust along a 52 km segment of ridge crest automated and objective processing steps including p analysis and waveform inversion allow the construction of models in a consistent way so that comparisons are meaningful a continuous profile is synthesized from 70 independent one dimensional models spaced at 750 m intervals along the ridge the resulting seismic velocity structure of the top 500 m of crust is remarkable in its lack of variability the main features are a thin low velocity layer 2 a at the top with a steep gradient to layer 2b the seafloor velocity is nearly constant at 2 45 km s 3 along the entire ridge the velocity at the top of layer 2b is 5 0 km s 10 the depth to the 4 km s isovelocity contour within layer 2a is 130 20 m from 13 to 13 20 n north of which it increases to 180 m the increase in thickness is coincident with a deviation from axial linearity deval noted by both a slight change in axis depth and orientation and in geochemistry the waveform inversion providing more details plus velocity gradient information shows a layer 2a with about 80 m of constant velocity material underlain by 150 m of high velocity gradient material putting the base of layer 2a at approximately 230 m depth south of 13 20 n and about 50 m thicker north of the deval the overall lack of variability combined with other recent measurements of layer 2a thickness along and near the axis indicates that the thickness of volcanic extrusives is controlled not by levels of volcanic productivity but the dynamics of emplacement the homogeneity along axis also provides a baseline of inherent variability in crustal structure of about 10 against which other observed variations in similar regimes can be compared

modelling the seismic properties of fast spreading ridge crustal low velocity zones insights from oman gabbro textures although considerable progress has been made in the study of fast spreading mid ocean ridge magma chambers over the past fifteen years the fraction of melt present in the chamber remains poorly constrained and controversial we present new constraints obtained by modelling the seismic properties of partially molten gabbros at the ridge axis p wave velocities at low frequencies are calculated in the foliation lineation reference frame using a differential effective medium technique the model takes into account the lattice preferred orientation of the crystalline phase and the average shape of the melt phase the structural parameters are obtained from the oman ophiolite the structural reference frame is given by the general trend of the gabbro foliation and the melt fraction and shape are estimated using the textures of nine upper gabbro samples the estimated melt fraction and shape depend on the assumptions regarding which part of the observed textures represent the melt in the gabbroic mush of the magma chamber however we can put limits on the reasonable values for the melt fraction and shape our results are consistent with a melt fraction of the order of 10 to 20 in the low velocity zone i e the magma chamber which is anisotropically distributed with the melt pockets preferentially aligned parallel to the foliation and approximated by oblate ellipsoids with approximate dimensions of 4 4 1 these results are also consistent with the seismic structure of the east pacific rise at 9o30 0 the anisotropic melt distribution can at least partially explain the vertical velocity gradient described in the lvz 1999 elsevier science b v all rights reserved

crustal structure in the rose area of the east pacific rise one dimensional travel time inversion of sonobuoys and expanded spread profiles we study the variability of the young oceanic crust as a function of age by analyzing 11 digitally recorded sonobuoys and three expanded spread profiles over 0 5 2 5 and 4 5 m y old oceanic crust the data were collected during the rivera ocean seismic experiment at the east pacific rise between 11 and 13 n we analyze the data within a one dimensional travel time scheme using the tau p method and travel time forward modeling we perform the tau p mapping by using a polynomial least squares technique that is fast and can be applied even to poor quality data as in this case we do not see large changes in the velocity structure between 0 5 and 4 5 m y old oceanic crust the overall structure consists of a rapid increase in velocity from about 2 5 6 km s within the uppermost 1 km of crust layer 2a a zone 1 km in thickness with little or no velocity gradient layer 2b a thin transition layer with a relatively rapid increase in velocity from about 6 2 to 7 km s layer 2c and a thick nearly homogeneous basal layer with velocities around 7 2 km s layer 3 we find that the low velocities at the seafloor and large velocity gradients within the shallow crust can be explained by the hypothesis of large scale cracks and their behavior under confining pressure for the variation of velocity v versus depth below seafloor z this hypothesis yields v v02 bz 1 2 a relationship that fits our solutions

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

axial processes along a segment of the east pacific rise 10 12 n axial segments of the east pacific rise are made up of individual volcanoes each volcano has a distinct magma composition and shows a systematic variation in the fractional crystallization with distance along the axis from the central chamber hydrothermal venting lava morphology and tectonics also vary along the axis lavas erupted near transform faults from the tips of propagating rifts in overlapping spreading centres and from near axis seamounts have different and variable compositions

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