analysis of borehole breakouts boreholes drilled into rock which is subjected to stresses that amount to a significant fraction of the strength of the rock may cause the rock to fail adjacent to the borehole surface often this results in the elongation of the cross section of the borehole in the direction of the minimum principal compressive stress orthogonal to the borehole axis such breakouts are valuable indicators of the direction of the minimum compressive stress orthogonal to the axis of the borehole their shapes may provide information about the magnitudes of both the maximum and minimum stresses relative to the strength of the rock borehole breakouts also may be impediments to drilling and to in situ measurement techniques such as hydraulic fracturing observations and analyses of borehole breakouts raise three important questions first how does the shape of the borehole breakout evolve second why are breakout shapes stable despite the very high compressive stress concentrations that they produce third how is the shape of the breakout related to the magnitudes of the stresses in the rock in this paper extensile splitting of rock in unconfined plane strain compression is assumed to be the process of rock failure adjacent to the circumference of the borehole by which a breakout forms to simulate the evolution of a borehole breakout this process is combined with a numerical boundary element analysis of the stresses around a borehole as its cross section evolves from the originally circular shape to that of a stable breakout the tangential stresses around a stable breakout cross section are found to be everywhere less than the unconfined plane strain tensile or compressive strength of the rock the stresses outside the stable breakout are found to be everywhere less than the limiting values of shear strength given by a mohr coulomb criterion in the regions of great stress concentrations at the ends of a breakout cross section which have a pointed shape the state of stress approaches that of equal biaxial compression in plane strain as it does ahead of a mathematical crack or notch the fact that the stresses around a breakout are less than the relevant strength establishes both the stability of the final breakout cross section and the appropriateness of an elastic analysis of the stresses according to this model the cross sectional shapes of stable breakouts are not related uniquely to the state of stress and the strength of the rock for example stable breakouts created instantly in rock already subjected to stress are much larger than stable breakouts created in the same rock with a preexisting borehole by subsequently increasing the stresses to the same values the results of drilling into an actual rock probably lie between these extremes modest changes in borehole cross section as a result of breakout do not alter significantly the minimum tangential tensile stress around a borehole with internal pressure from that given by the kirsch solution for a circular hole subjected to the same stresses therefore hydraulic fracturing interpretations based on the kirsch solution give the correct values for the far field stresses

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

the distribution of geothermal fields on the juan de fuca ridge near bottom water temperatures were mapped along 400 km of the strike of the juan de fuca ridge as part of a combined sea marc seabeam experiment to image the variability of morphology and structure along a spreading center segment the water temperature data collected by a continuously towed thermistor chain in addition to salinity data indicate that there are four geothermal areas spaced at distances of 100 km from each other south of the cobb propagator and one field just to the north of the propagator on the endeavor ridge segment each thermal region is located above a morphological dome on the spreading center these domes are an average of 100 200 m shallower than the rest of the axis the structure of bottom water suggests that the geothermal regions are on average 20 km long and that the heat from these fields raises the temperature in the water column by a minimum of 0 06 sup 0 c up to 300 m above the bottom two simple models are used to estimate the heat flux associated with these features

borehole coupling at sonic frequencies an elastic plane wave incident on a borehole couples through the borehole wall to produce a compressional wave inside the borehole and the effect of this coupling varies with incident angle imaging experiments that include a downhole source and or hydrophones must account for this effect to achieve optimal results in configurations pertinent to vsp surveys and cross well surveys at seismic frequencies a low frequency source can be assumed recently however both cross well and single well imaging experiments have been undertaken at sonic frequencies above 2000 hz where low frequency approximations are no longer valid we have developed new formulas which are valid over all frequencies and incident angles for both shear and compressional incident waves we present explicit formulas to describe this dependence contrary to what might be expected from low frequency approximations we find that at certain angles and for high enough frequencies marked resonances in the coupling terms for both shear an

acoustic multipole logging in transversely isotropic poroelastic formations the properties of the modes generated by multipole sources in a fluid filled borehole embedded in radially layered formations that include transversely isotropic poroelastic layers are investigated these layers are modeled following biot s and homogenization theories under the assumption that the principal axes of symmetry of both the transversely isotropic skeleton and the complex permeability tensor are parallel to the vertical axis of the borehole a general formulation based on the thomson haskell method accounts for any combination of fluid elastic and poroelastic layers either isotropic or transversely isotropic the investigation is achieved through the computation of dispersion and attenuation curves and frequency dependent sensitivity coefficients with respect to the parameters in the presence of a fast or slow radially semi infinite transversely isotropic poroelastic formation with an impermeable borehole wall the shear wave transverse isotropy of the skeleton can be determined using t

sea beam deep tow investigation of an active oceanic propagating rift system galapagos 95 5 w we have tested and corroborated the propagating rift hypothesis with high resolution sea beam and deep tow data collected over the galapagos 95 5 w propagating rift system the propagating rift is continuously breaking through the cocos plate at a velocity of about 50 km m y with an azimuth of about 273 away from the galapagos hotspot this process transfers cocos lithosphere to the nazca plate including the preexisting spreading center which fails as it is gradually preempted by the propagating rift the spreading center azimuth is being changed by about 8 clockwise by this rift propagation the active propagating and failing rift axes overlap by about 15 km and are joined by a broad and anomalously deep zone of nonrigid plate deformation rather than by a transform fault although rift propagation is continuous on a scale of a few kilometers 50 000 years and lithospheric transferral from one plate to the other occurs mostly continuously as the preexisting rift fails it breaks into discrete segments the geometry of seafloor fabric and tectonic elements such as active and failed rifts the v shaped pattern of pseudofaults and the oblique tectonic fabric in the sheared zone of transferred lithosphere are observed as predicted this quantitative verification provides strong support for the propagating rift hypothesis

detailed tectonics near the tip of the galapagos 95 5 w propagator how the lithosphere tears and a spreading axis develops a model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using seamarc ii sea beam deep tow and alvin near the tip of the propagator growing westward at 52 km m y along the east west trending galapagos spreading axis near 95 5 w initial lithospheric rifting appears to be accommodated along reactivated roughly east west abyssal hill faults at the tectonic tip extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures the initial volcanism initial volcanic tip occurs about 6 5 km behind the tectonic tip as basalt erupts through these fissures covering most of the keystone block the neovolcanic axial tip occurs another 4 5 km back at the western termination of a well defined axial pillow ridge along which volcanism is localized spreading along the propagator axis accelerates to the full rate 29 mm yr half rate at the full rate tip interpreted to be 10 km behind the neovolcanic axial tip or 21 km behind the tectonic tip a complex pattern of strain in the tip area results from the interaction of plate spreading transform shear stress crack propagation dynamic depression of the western end of the axial valley and the transform zone and variations in the strength thickness heterogeneity and anisotropy of the lithosphere southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young thin weak relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction relative to the spreading direction due to the influence of shear stress from the transform zone similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older thick strong heterogeneous and anisotropic lithosphere by oblique slip along a set of left stepping reactivated preexisting east west abyssal hill structures although on a time scale of 106 years the overall process of propagation is mostly continuous rifting at the tectonic tip appears discontinuous at a scale of 1500 m or 30 000 years based on the east west spacing of grabens formed as rifting begins the scale of discontinuity of propagation of the neovolcanic axis is 3 5 km or 50 000 100 000 years based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes the extension involved in rifting is 10 20 based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting these values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting

early paleozoic high mg diorite granodiorite in the eastern kunlun orogen western china response to continental collision and slab break off abstract phanerozoic high mg diorites and granodiorites are known to be related to diverse subduction styles at convergent margin settings but have not been linked to collision processes the santonggou complex in the eastern kunlun orogen is composed of a dominant granodiorite intruded by small stocks and dykes of porphyritic granodiorites and diorites which are all characterized by high mgo and ni but low yb and y contents mafic microgranular enclaves mmes occur in the granodiorite and the porphyritic granodiorites la icpms u pb analyses of zircon from the granodiorite porphyritic granodiorite and porphyritic diorite yield similar ages of 432 2 ma 432 2 ma and 427 3 ma respectively plagioclase in the granodiorite exhibits complex zonations with resorbed and patchy cores plagioclase and hornblende phenocrysts in the porphyritic granodiorites and diorites also show textural and compositional zonations the highest e nd t value of 0 6 in mmes suggests asthenosphere derived mafic magma these lines of mineralogical and geochemical evidence together with results from geochemical modeling suggest that the porphyritic diorites were derived from mantle metasomatized slab melts the porphyritic granodiorites and the granodiorite were generated by simultaneous assimilation and fractional crystallization of the magma approximated by the porphyritic diorites plus mixing of asthenosphere derived mafic and evolved granodioritic magmas pressure estimates from the al in hornblende geobarometer suggest that the granodiorite had been uplifted to near the surface before the emplacement of the porphyritic granodiorites partial melting of the slab upwelling of the asthenosphere and rapid orogenic uplift at 430 ma suggest that the cessation of subduction and the initiation of collision as well as slab break off occurred almost simultaneously in the eastern kunlun orogen

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

effects of in situ permeability on the propagation of stoneley tube waves in a borehole we investigated the theoretical relationship between propagation characteristics of stoneley tube waves in a borehole and in situ permeability by using a modified formulation of a borehole model with a formation that behaves as a biot porous medium we found that stoneley wave attenuation and phase velocity dispersion increased with increasing permeability and porosity and decreased with increasing frequency in rocks with low to medium permeabilities less than 100 md variations in formation velocity and attentuation were major contributors to variations in stoneley wave properties at normal logging frequencies however in high permeability rocks greater than 100 md coupling between the borehole and pore fluids associated with in situ permeability was more important than lithological changes in controlling stoneley wave properties pore fluid viscosity had an effect on stoneley wave propagation equal but opposite to permeability and hence must he taken into account we compared our theoretical

the east pacific rise near 21 n 13 n and 20 s inferences for along strike variability of axial processes of the mid ocean ridge abstract data obtained along various segments of the mid ocean ridge mor are used to construct an idealized model for crustal accretion the model seeks to predict the topographic volcanic tectonic and hydrothermal characteristics of any given spreading segment of the mor as a function of distance away from the bounding transform faults this model is based on a series of detailed mapping efforts carried out on segments of the mor having a broad range of spreading rates between 2 and 16 cm yr this paper includes the results of the french searise program carried out during the summer of 1980 aboard the n o jean charcot and two american cruises conducted in 1981 aboard the r v melville

eocene reorganization of the pacific farallon spreading center north of the mendocino fracture zone during a 1984 sea beam and magnetics survey in the north pacific we collected data in the area north of the mendocino fracture zone where a change in the direction of seafloor spreading occurred between magnetic anomalies 24 and 21 and also conducted detailed surveys of the bends in the surveyor and mendocino fracture zones associated with this change in plate motion we have analyzed these data combined with all available magnetics and bathymetric data from previous cruises to determine whether the ridge reorientation and reorganization occurred by way of rift propagation or rift rotation through differential asymmetric spreading the reorganization began just after anomaly 24 to the north of the surveyor fracture zone but did not begin between the mendocino and surveyor fracture zones until the time of anomaly 22 a 2 5 m y delay produced by the old mendocino transform trend constraining the farallon plate lithosphere between the mendocino and surveyor transforms to move in the old spreading direction until the fracturing through of a new mendocino transform fault in the new orientation most of the reorientation of spreading rift segments occurs within 2 3 m y the entire reorganization requires about 7 m y we find direct evidence for seven propagation episodes accounting for the primary reorientation of 75 of the length of the ridge system and nowhere is the data more consistent with rift rotation than with rift propagation as the principal means of reorientation we conclude that the primary mechanism by which the reorganization occurred was rift propagation assuming that rift propagation can account for all of the ridge reorganization we present forward models of magnetic anomaly profiles which succeed in fitting the data well we then present a model for the tectonic evolution of the reorganization which fits the data well using 11 episodes of both northward and southward propagation

borehole seismic source radiation pattern in transversely isotropic media we extend previous discussions on crosswell tomography in anisotropic formations by deriving the radiation patterns of three typical downhole seismic sources impulsive air gun or dynamite wall clamped vertical vibrators and cylindrical bender inside a fluid filled borehole embedded in a transversely isotropic ti formation the method of steepest descents in conjuncture with the low frequency and far field assumptions is applied to the exact displacement integrals of these sources to obtain their radiation patterns asymptotically in spite of complications caused by quasi p and quasi sv wave coupling and wavefront triplication in homogeneous ti media the final results can still be expressed in slowness components determined by a ray direction which is desired when source radiation effects are to be accounted for by ray based tomography techniques tests with the radiation patterns show that while the effect of anisotropy on p waves is moderate its effect on the s wave pattern is significant eve

high resolution studies of the summit of axial volcano sea beam bathymetry sea marc i side scan 30 khz towed camera and submersible data from axial volcano juan de fuca ridge jdfr have been integrated to produce detailed maps of the volcano s summit region in order to elucidate its volcanic and tectonic evolution the proximity of axial volcano the youngest edifice of the cobb eickelberg seamount chain to the jdfr superposes a hotspot derived radial stress pattern onto the linear stress field of a mid ocean ridge spreading center the morphology of the summit of axial volcano reflects the differing eruptive styles produced from this superposition long rift zones begin at the edges of the caldera and extend as much as 75 km to the north and south axial volcano has a distinct summit plateau similar to those found on many other submarine volcanos the process by which summit plateaus are constructed is illustrated by the lava flows originating from the south rift zone which have partially filled the southern half of the caldera and have smoothed the flanks the summit plateau contains penetrative ne sw fracturing that is approximately parallel to the jdfr the unusual rectangular caldera is 3 8 km in size and its trend is about 160 three possible mechanisms are discussed for the caldera s formation 1 canary island type trapdoor formation 2 overlapping spreading center interaction and 3 magma source migration although none of these models is fully compatible with the present data base the overlapping spreading center model is preferred postcaldera volcanism probably began with the formation of the central caldera eruptive complex which was characterized by relatively low volume high viscosity flows the most recent volcanism has apparently taken place along the northern and southern rift zones which have erupted more fluid lavas resulting in areally extensive flows covering up to 60 of the caldera floor extensive low temperature venting and areas of high temperature venting near the northern and southern rift zones and caldera walls suggest a shallow magmatic source beneath the summit of axial volcano

seamount abundances and distributions in the southeast pacific abstract sea beam bathymetry was recorded for 17 277 km of ship track in the southeast pacific and has been analyzed for seamount population characteristics all the ship tracks are over the pacific plate and most fall along 7 lines parallel to the east pacific rise between 7 s and 22 s the lines fall into three age categories one line is over 0 5 2 ma crust three are over 5 10 ma crust and three are over 30 40 ma crust seamount locations were recorded diameters were manually estimated and heights were measured if the swath crossed the seamount center over 382 features were counted along the entire ship track with heights ranging from 50 m to over 2500 m with sampling most consistent at heights between 100 m and 1000 m height to radius ratios vary considerably suggesting that seamount shapes do not scale to a single parameter the observed variety of morphological forms demonstrates that there is a wide range of seamount shapes for small features the distribution of sizes for the population was approximated by exponential dependence and by power law dependence using methods developed by smith and jordan 17 the power law distribution overestimates abundances at the smallest size ranges and both distributions fail to predict abundances of large seamounts determined from wide beam data 17 size distributions were also determined for seamounts in each seafloor age category almost all small seamounts appear to be produced on crust younger than 0 5 2 ma while the number of larger seamounts increases to 5 10 ma for all size ranges observed here the number of seamounts at 5 10 ma and at 30 40 ma is roughly identical these observations suggest that most small seamounts are formed on very young thin lithosphere that permits the passage of small volumes of magma small volumes would cool in older thicker lithosphere before reaching the surface although larger magma bodies might not numerous small sources of melt must exist near the ridge crest to supply the small seamounts probably trapped remnants of the large scale upwelling

bathymetric artifacts in sea beam data how to recognize them and what causes them sea beam multibeam bathymetric data have greatly advanced understanding of the deep seafloor however several types of bathymetric artifacts have been identified in sea beam s contoured output surveys with many overlapping swaths and digital recording on magnetic tape of sea beam s 16 acoustic returns made it possible to evaluate actual system performance the artifacts are not due to the contouring algorithm used rather they result from errors in echo detection and processing these errors are due to internal factors such as side lobe interference bottom tracking gate malfunctions or external interference from other sound sources e g 3 5 khz echo sounders or seismic sound sources although many artifacts are obviously spurious and would be disregarded some particularly the omega effects described in this paper are more subtle and could mislead the unwary observer artifacts observed could be mistaken for volcanic constructs abyssal hill trends hydrothermal mounds slump blocks or channels and could seriously affect volcanic tectonic or sedimentological interpretations misinterpretation of these artifacts may result in positioning errors when seafloor bathymetry is used to navigate the ship considering these possible geological misinterpretations a clear understanding of the sea beam system s capabilities and limitations is deemed essential

occurrence and origin of submarine plunge pools at the base of the us continental slope high resolution bathymetric data from the new jersey and californian continental margins show a marked depression running along parts of the base of the continental slope detailed analysis reveals that the depressions are a series of discrete plunge pools with associated downslope topographic ramparts we have used new bathymetric data to create our own data base of over 150 examples and systematically analyse plunge pool morphology and location previous observations of plunge pools have been sparse plunge pools are up to 1100 m wide and 75 m deep with a mean diameter of 400 m and a mean depth of 21 m plunge pools only occur where there are sharp decreases in slope of more than 4 and are well developed where changes in slope exceed 15 we propose plunge pools can be created by two mechanisms firstly they may be due to reduced bed shear stress downstream of hydraulic jumps in submarine sediment laden density flows that causes the deposition of bedload and the creation of a sediment bar this bar then defines the downslope margin of a pool secondly the impact of high momentum sediment laden density flows can excavate a depression as has been observed for subaerial snow avalanches sediment deposited downslope of these impact pools is very poorly sorted and partly derived from erosion within the pool both mechanisms influence whether turbidity currents are generated from high density sediment laden density flows influence whether depositional flows are channelised and have implications for base of slope facies models

improving gloria images using sea beam data gloria and sea beam data are processed together with a number of benefits the sea beam data allow an improved slant to ground range projection of the imagery that results in less of the distortions such as layover and foreshortening that are produced when the seafloor is assumed to be flat a calculation originally intended to quantify side scan sonar data has also been found to improve the images in an image enhancement sense this calculation involves estimating the seafloor s backscatter strength from gloria side scan sonar data using a simple physical model and the characteristics of the gloria instrument the backscatter strength is the ratio of backscattered to incident sonar intensity on the sea bottom normalized to the surface area covered by the acoustic pulse it is computed by correcting the gloria data for propagation losses beam directivity pattern time varied amplifier gain and other factors the calculation is improved by accounting for refraction in the water column and for some topographic effects using sea beam data a gloria image from a mid ocean ridge crest is shown to have a large 60 db range that is due to the effect of the system response such as the beam pattern on the images the corresponding image of the seafloor backscatter strength has an almost gaussian distribution of picture element digital numbers with a narrow range 20 db showing that most of the system effects have been removed cross sections through the backscatter strength image show flat trends superimposed with finer variations due to the seafloor geology the conversion of gloria data into seafloor backscatter strength serves two purposes it provides quantitative information that can be used to determine the likely causes of particular features in the images and it performs what is effectively a deterministic shading correction

the morphology of propagating spreading centers new and old recent seabeam investigations over the east pacific rise in the northeast pacific have shown the existence of a propagating spreading center at 18 n the propagating and dying spreading centers are joined by a zone of orthogonal lineations which appear to be the traces of a continuously migrating transform fault similar orthogonal lineations occur at 16 n 107 30 w on the western side of the trace of a 5 m y old propagating spreading center the amplitude and the complexity of the topographic signature over the earlier propagating spreading center are higher and greater than over the presently active center suggesting that the relief of the earlier center may owe its origin to differential subsidence of thermally contrasting segments of lithosphere analogous to fracture zone topography of similar origin traces of former propagating spreading centers are thus a combination of two sets of landforms those resulting from propagation and those resulting from the juxtaposition of two blocks of lithosphere of different thickness

analysis and removal of multiply scattered tube waves because of irregularities in or near the borehole vertical seismic profiling vsp or crosswell data can be contaminated with scattered tube waves these can have a large amplitude and can interfere with weaker upcoming reflections destroying their continuity this type of organized noise cannot always be removed with filtering methods currently in use we propose a method based on modeling the scattered tube wave field and then subtracting it from the total data set we assume that the scattering occurs close to the borehole axis and therefore use a 1 d impedance function to characterize borehole irregularities estimation of this impedance function is one of the first steps our method also accounts for multiply scattered tube waves we apply the method to an actual vsp data set and conclude that the continuity of reflected upcoming events improves significantly in a washout zone

digital image processing of seabeam bathymetric data for structural studies of seamounts near the east pacific rise digital marine bathymetric data are rapidly increasing in both volume and variety these data provide new perspectives on the morphology of the seafloor they also present a number of problems with regard to developing display methods that convey the large amount of information contained in these data we have used digital image processing techniques to reduce and display seabeam multibeam data for three young less than 5 ma seamounts on the eastern flank of the east pacific rise navigational errors in location were corrected by identifying offsets between features observed in crossing tracks and then translating the tracks relative to a control track images of the seamounts were then generated using spatial filtering techniques the images reveal more information than contour maps because 1 more depth intervals can be displayed and 2 the continuous tone nature of the images shown in shaded relief or color coded form more readily conveys morphologic information the images suggest a significant degree of structural control on seamount shape more so than can be delineated by examination of contour maps alone in one case a chain of seamounts strikes at an angle perpendicular to the abyssal hill direction although each seamount within the chain is elongate in a direction parallel to the hills in another case a polygonally shaped flat topped seamount overlies a region of the crust where the abyssal hills display two strike directions images of the third seamount demonstrate a complicated structural control on morphology including a control on the distribution of satellite constructs finally the image presentations clearly depict regions where cross tracks could not be registered because of 1 residual navigation errors and 2 the presence of steep slopes where the depth and location estimates can vary depending on the angular relationship between the ship track azimuth and the strike of the slope

numerical computation of individual far field arrivals excited by an acoustic source in a borehole in this paper i model the acoustic logging problem and numerically compute individual arrivals at far field receivers the ability to compute individual arrivals is useful for examining the sensitivities of each arrival to various factors of interest as opposed to examining the full waveform as a whole while the numerical computation of the mode arrivals peterson 1974 and the numerical computation of the first head waves tsang and rader 1979 have been previously reported the numerical computation of the entire set of head wave arrivals is new and is the major contribution of this paper following roever et al 1974 and others the full wave field is represented as a sum of contributions from both poles and branchcuts in the complex wavenumber plane the pole contributions correspond to mode arrivals while the branch cuts are associated with the body waves i e head waves both the pole and branch cut contributions are computed numerically and results are presented for the cases of a slow an

modeling of low frequency stoneley wave propagation in an irregular borehole a fast modeling method is formulated for low frequency stoneley wave propagation in an irregular borehole this fast modeling method provides synthetic waveforms which include the effects of two borehole irregularities diameter changes washout and formation property changes the essential physics of the low frequency stoneley waves are captured with a simple 1 d model a mass balance boundary condition and a propagator matrix are used to express stoneley wave interactions with the borehole irregularities the accuracy of the proposed method was confirmed through comparison with existing finite difference and boundary integral modeling methods that yielded cross correlations greater than 0 98 comparison of synthetic records calculated for an actual borehole with field records showed qualitative agreement in the major reflections because of the washout zones but showed some disagreements in the reflections caused by the fractures since the synthetic records include only information relating to the borehole geometry and the elastic properties of formation the reflection caused by the fracture will appear only in the field record these results suggest the possibility of distinguishing stoneley wave reflections caused by fractures from those caused by borehole irregularities further the fast computational speed of this method over 300 times faster than either boundary integral or finite difference methods makes it quite suitable for field application

overlapping rift zones at the 5 5 s offset of the east pacific rise a seabeam and magnetometer survey of the pacific nazca plate boundary around 5 5 s mapped a nontransform offset whose geology and kinematics seem typical of a whole class of structures formed where fast spreading rises are laterally offset for distances less than the width of a subaxial magma chamber though the spreading center is abruptly displaced by 15 km right laterally there is no trace of strike slip faulting instead the ends of two 100 km long axial volcanic ridges veer 15 toward each other and overlap for more than 20 km in this overlapping region between 5 22 s and 5 32 s accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one sided spreading a continuous but dog legged magma chamber is inferred to underlie both rift zones and an intervening 9 km wide volcano studded basin a strong magnetic signature from the crust which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank in the past 0 5 106 years the southern axial ridge has propagated north at 40 mm yr 55 of the spreading half rate

the importance of a small wall deformation in the three dimensional acoustic logging results summary a variety of seismic techniques use fluid filled boreholes to host detectors and seismic sources the complexity of the wavefield generated depends on the position of the receivers and the source frequency excited type of formation and type of source in the present paper the boundary element method is used to study the wavefield elicited by monopole or dipole sources within a fluid filled cylindrical cavity drilled through an unbounded elastic formation this model is used to assess the effects of the existence of a small localized wall deformation on the response such a deformation may occur through the mechanical action of the drill string rock failure adjacent to a drilled borehole plastic deformation and washing out of the borehole in soft or poorly consolidated rocks in addition the effect of a source receiver tool placed in an off centre and or tilted position inside the fluid filled borehole on the propagation of both axisymmetric and non axisymmetric wave modes are studied

high resolution bathymetry of the eastern slope of the porcupine seabight the topography of the eastern margin of the porcupine seabight was surveyed in june 2000 utilizing swath bathymetry the survey was carried out during rv polarstern cruise ant xvii 4 as part of the geomound project the main objective was to map and investigate the seafloor topography of this region the investigated area contains a variability of morphological features such as deep sea channels and giant mounds the survey was planned and realized on the basis of existing data so as to guarantee the complete coverage of the margin in order to achieve a resolution of the final digital terrain model dtm that meets the project demands data processing was adjusted accordingly the grid spacing of the dtm was set to 50 m and an accuracy better than 1 of the water depth was achieved for 96 of the soundings

the effect of the invaded zone on full wavetrain acoustic logging most theoretical studies of acoustic borehole logging have employed the simple model of a fluid borehole in an infinite solid this work attempts to account for the invaded zone using a more sophisticated model that additionally includes finite concentric shells surrounding the borehole by appropriately choosing the physical parameters of these cylindrical shells one can study the effect of the invaded zone mudcake or a cased hole on acoustic wavetrain components the model accounts for geometric attenuation but it assumes the formation is perfectly elastic a key consideration is the distance that the acoustic log sees into the formation the body of this report is devoted to studying the effect of the invaded zone mudcake or steel casing on the components of the full wavetrain p wave s wave reflected modes and stoneley mode i conclude that a sonic logging tool has a very shallow depth of investigation this depth for p and s waves depends upon the spacing between the source and receive

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

borehole wave propagation in three dimensions in this paper the three dimensional finite difference method is used to simulate borehole wave propagations in an isotropic as well as an anisotropic formation the finite difference results agree excellently with the analytic solutions of a point force source in the transversely isotropic medium the finite difference synthetics are also in very good agreement with the discrete wave number solutions for fluid filled borehole wave propagation the finite difference synthetics are compared with ultrasonic lab measurements in a scaled borehole model the borehole is drilled along the x axis in an orthorhombic phenolite solid both monopole and dipole logs agree well the observations of the shear wave splitting in the dipole logs are confirmed by the finite difference simulations the 3 d finite difference method is applied to the fluid filled borehole wave propagation in the tilted isotropic formation and in the orthorhombic phenolite formation in a borehole drilled along the z axis in a phenolite formati

periodicity in the accretion process on the southeast indian ridge at 27 40 s abstract image processing of a seabeam bathymetric grid on the southeast indian ridge seir at 27 40 s has been used to analyse the morphotectonics of the ridge flanks steep topographic slopes indicating inward facing or outward facing faults and those representing tilted blocks were automatically separated an autocorrelation process allows an estimation of the periodicity of the fault spacing the outward facing faults first appear at 16 18 km from the axial valley are more common on bathymetric highs and correspond to tilted block zones in adjacent deeper areas the tilting of the blocks is 4 the transformation of the rift valley into the smooth undulating relief of the flanks is accomplished by these outward facing faults and tilted blocks inward facing faults appear either as large faults with major throws or as a series of smaller faults with minor throws the minor faults are mainly localized on bathymetric highs and are densely organized the major faults continuous and symmetrically spaced on each side of the axis zone are organized with a periodicity of 8 2 km these faults seem to be old inner walls which have been moved outwards periodically during the spreading process they alternate with positive magnetic lineaments reflecting old neovolcanic zones linked to intense magmatic periods considering a mean half spreading rate of 31 km m y during the past million years this would mean the development of a major fault each 0 26 m y we propose a model for the evolution of the seir at 27 40 s controlled by a periodic succession of magmatic and tectonic episodes 1 1 faulting and volcanism are concentrated in the axial zone above the melt reservoir where the crust is weakened magmatic period outside this zone the external relief is frozen in supported by the finite strength of the plate 2 2 the axial valley is enlarged fills up with lavas and bulges due to thermal effects 3 3 at the end of the magmatic period this bulged zone collapses by thermal subsidence along lines of weakness such as the faults forming the inner walls 4 4 when the melt reservoir is empty tectonic period the processes are no longer concentrated and active faulting extends over tens of kilometres from the axis the lithosphere is thinned a new axial valley is created the flanks are uplifted and outward facing faults and block tilting are induced

constraints of sea beam data on crustal fabrics and seafloor spreading in the south china sea abstract the south china sea is a mid late tertiary marginal basin the magnetic anomaly lineations in the eastern part of the basin trend approximately east west 1 2 suggesting a north south direction of spreading in the spring of 1985 two cruises on the french research vessel jean charcot provided sea beam coverage seismic reflection magnetic and gravity profiles the sea beam data exhibit two major structural trends scarps striking n50 e 15 interpreted as normal faults and scarps striking n140 e 15 interpreted as fracture zones this fabric implies a northwest southeast direction of spreading up to about 100 km north and south of the inferred spreading axis 3 dense sea beam coverage of a roughly 1 square area northwest of the scarborough seamounts chain shows that the emplacement of these seamounts was at least in its initial stage controlled by faulting in two orthogonal directions n50 e and n140 e magnetic and gravimetric maps of the same area also reveal anomalies trending roughly n50 e which are disrupted by transform zones striking n140 e this detailed study indicates that the fracture zones may be closely spaced less than 20 30 km east of macclesfield bank furthermore magnetic anomalies identified as 6 and 6a 20 myr along two north south profiles located at both edges to the north of this detailed study area may be correlated with the n80 e trend characteristic of such anomalies in the eastern part of the south china sea the east west trend of magnetic anomalies 6 and 6a south of scarborough seamount chain recognised by taylor and hayes 1983 1 is incompatible with the trend of the fault scarps observed on sea beam data we infer that progressive right lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east west average trend of some of the magnetic anomalies and of the scarborough seamount chain which represents the location of the relict spreading axis in spite of a n50 e spreading direction the whole central part of the basin east of 115 e may have formed in this way since n130 140 e striking fracture zones are observed on the sea beam swaths this model may correspond to the second of two successive spreading phases with n s and nw se directions of extension respectively as was presented by the authors in a previous paper pautot et al 1986 3 alternatively we may assume that the direction of extension did not change radically during the opening history 3 implying that oblique spreading occurred along n80 e trending ridge segments creating the n80 e trending scarps and magnetic anomalies observed to the north and to the south of the study area such a direction of extension is compatible with mid late tertiary left lateral movements along large n130 150 e strike slip faults such as the red river and wang chao faults in south china and sundaland

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

the evolution of craters and calderas on young seamounts insights from sea marc i and sea beam sonar surveys of a small seamount group near the axis of the east pacific rise at 10 n a northwest trending group of seamounts located just west of the east pacific rise epr axis has been mapped using high resolution sea marc i side scan sonar and sea beam multibeam bathymetry these seamounts have 9 13 km basal diameters and the shapes of the volcanos change from conical domes near the epr axis to truncated cones and larger elliptical truncated cones farther away from the rise axis the detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions craters to large complex calderas and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse the creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera the concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau the steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera the steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain as a seamount evolves the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits the final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau

inference of manganese nodule coverage from sea beam acoustic backscattering data normal incidence reflectivity from a manganese nodule field was measured with a 12 khz sea beam multibeam echo sounding system aboard the r v thomas washington and used to infer nodule coverage a reflectivity map of the area was produced using the intensity of the specular return from each ping the patchiness of the nodule coverage is evidenced by definite highs and lows in the reflectivity pattern ground truth was provided by near bottom acoustic measurements and photographs taken with the deep tow instrument package of the marine physical laboratory agreement between the simple nodule coverage predictions from sea beam acoustic data and the bottom photographs taken throughout the area is 98 percent although the sea beam system is limited in its dynamic range this paper shows that it can be used very effectively to determine both topography and nodule coverage in potential mining areas

modes of noncircular fluid filled boreholes in elastic formations dispersion curves for4 the modes of noncircular fluid filled boreholes in homogeneous elastic formations are calculated with a boundary integral formulation results are displayed for the propagatory modes of several borehole shapes in both fast and slow formations any departure from circularity reduces the tube wave speed agreement with exact analytical results is obtained for elliptical boreholes while a simple approximate relation involving borehole perimeter and area is verified for arbitrary cross sections higher order surface modes such as the flexural and screw modes split into two distinct branches distinguished by their orientation with respect to the principal axes of the borehole cross section modal dispersion characteristics in elliptical boreholes may be approximately by scaled circular borehole results in fast formations the effective scaled radii are related to transverse dimensions of the borehole but in slow formations they are more closely related to local radii of curvature of the borehole for more complex cross sections with a single axis of symmetry dispersion of the flexural mode is well modeled by that in elliptical boreholes of similar dimensions while that of stoneley and screw modes is not in asymmetrical boreholes significant torsional and out of phase motion may occur

stochastic modeling of seafloor morphology inversion of sea beam data for second order statistics at scale lengths less than 100 km or so statistical descriptions of seafloor morphology can be usefully employed to characterize ridge crest processes off ridge tectonics and vulcanism sedimentation and postdepositional transport we seek to develop methods for the estimation of seafloor statistics that take into account the finite precision resolution and sampling obtained by actual echo sounding systems in this initial paper we restrict our attention to the problem of recovering second order statistics from data sets collected by multibeam devices such as sea beam the seafloor is modeled as a stationary zero mean gaussian random field completely specified by its two point covariance function we introduce an anisotropic two point covariance function that has five free parameters describing the amplitude orientation characteristic wave numbers and hausdorff fractal dimension of seafloor topography we formulate the general forward problem relating this model to the statistics of an ideal multibeam echo sounder in particular the along track autocovariance functions of individual beams and the cross covariance functions between beams of arbitrary separation using these second moments as data functionals we then pose the inverse problem of estimating the seafloor parameters from realistic noisy data sets with finite sampling and beamwidth and we solve this inverse problem by an iterative linearized least squares method the inversion method is applied to sea beam transit data from both the pacific and atlantic oceans sea beam system noise stands out as a sharp spike on the along track autocovariance function and can be modeled as a white noise process whose amplitude generally increases with beam angle the five parameters in our second order model can be estimated from the inversion of data sets comprising 100 200 km of track length in general the cross track wave number is the most poorly determined although uncertainties in the assumed sea beam response may bias the values of the fractal dimension using the assumed beamwidth the measured noise values and the seafloor parameters recovered from the inversion we generate sea beam synthetics whose statistical character can be directly compared with raw sea beam data for most of the track segments we have processed thus far the synthetics are similar to the data in the case of one atlantic profile however the comparison clearly indicates the necessity of incorporating higher order statistics the space domain procedures described in this paper can be extended for this purpose

propagation of acoustic waves in a fluid filled borehole surrounded by a concentrically layered transversely isotropic formation the method of real axis integration is applied to compute the acoustic waveform due to a point source in a fluid filled borehole surrounded by a concentrically layered transversely isotropic formation the problem is of interest in acoustic logging because the earth due to its intrinsic horizontally layering structure is essentially a transversely isotropic medium the presence of an invasion zone introduces concentric layering in the vicinity of the borehole due to the axial symmetry of the problem only two characteristic waves are excited in the transversely isotropic medium the effects of the anisotropy and concentric layering on the acoustic waveforms are studied

multipole acoustic waveforms in nonaxisymmetric boreholes and formations using a 2 5 dimensional velocity stress finite difference vs fd technique waveforms are calculated for multipole excitation of azimuthally nonsymmetric boreholes and formations axial uniformity of the borehole cross section and formation is assumed the vs fd waveforms agree with previous transform methods for circular boreholes and the modes found for elliptical boreholes agree with those derived by a boundary integral technique borehole ellipticity substantially reduces monopole compressional headwave amplitudes in fast formations low frequency dipole waveforms in elliptical boreholes show marked sensitivity to source dipole orientation in fast formations but small sensitivity in slow formations waveforms from dipole receivers perpendicular to the source dipole may be comparable in amplitude to those recorded by parallel dipole receivers dipole waveforms from an exaggerated keyhole shaped borehole are quite similar to those from an elliptical borehole of similar aspect ratio in a slow formation finally the behavior of early compressional headwaves arising from a nearby faster bed parallel to the borehole axis is qualitatively similar to that for a point source above a simple planar interface

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

elastic wave propagation in a fluid filled borehole and synthetic acoustic logs the propagation and dispersion characteristics of guided waves in a fluid filled borehole are studied using dispersion curves and modeling full wave acoustic logs by synthetic microseismograms the dispersion characteristics of the pseudo rayleigh reflected and stoneley waves in a borehole with and without a tool in the center are compared effects of different tool properties are calculated the effect of a rigid tool is to make the effective borehole radius smaller as an approximation dispersion characteristics of the guided waves in a borehole with a tool can be calculated as a purely fluid filled borehole with a smaller effective radius theoretical waveforms microseismograms of elastic waves propagating in a borehole are calculated using a discrete wavenumber integration with an appropriate choice of parameters our results look similar to the acoustic waveforms recorded in a limestone and a shale formation several factors affect the shape of an acoustic log microseismogram the effective rad

submarine salt flows in the central red sea the central red sea an oceanic basin floored by miocene evaporites reaching kilome ters in thickness in places is at an early stage of development where seafl oor spreading has geologically only recently replaced continental rifting surveys using a high resolution multibeam echo sounder around thetis deep a new spreading center have revealed a remarkable series of structures resembling viscous gravity fl ows which are here interpreted as originating from fl owage of the evaporites laterally unloaded by axial rifting and other processes developing the relief of the deep the fl ow margins are marked by stream wise lineaments and some apparently rotated markers their fronts in the fl oor of the deep are rounded in plan view and profi le their surfaces contain small closely spaced features resembling extensional faults in one area below declining gradients the surface contains along slope ridges and valleys typical of compression folds ogives flowparallel lineaments and extensional faults lie respectively parallel and orthogonal to the direction of maximum seabed gradient movement is apparently heterogeneous at least in part because of varied blocking by relief in underlying basement observed protruding between fl ows flowage is currently transporting materials into the fl oor of the deep where they have the potential to become incorporated into the young oceanic crust by repeated eruption of axial lavas over them in the light of these new data we reexamine the possibility and implications of fl owage in the south atlantic marginal evaporites in particular whether fl owage contaminated early oceanic crust in such areas

is the ocean floor a fractal the high resolution topographic structure of the ocean bottom plays an important role in acoustic reverberation mathematical models of the seafloor at different scales are presented based on analyses of the spatial continuity survey devices are usually designed for a certain resolution and to date most studies are concerned with one limited range of scale the question arises whether the statistical behavior at one scale can be carried over to another or what conclusions can be drawn it has been proposed that the ocean bottom can be simulated as a fractal an object of hausdorff dimension strictly larger than its topological dimension analyses of scale dependent measures of spatial continuity nested variograms and covariance functions are used to investigate whether this assumption provides a description coherent with seafloor observations the concepts will be demonstrated in a case study using seabeam bathymetric data from the east pacific rise

rise times of attenuated seismic pulses detected in both empty and fluid filled cylindrical boreholes fourier bessel theory is used to derive filters representing the influence of both empty and fluid filled cylindrical boreholes on particle motion induced in rock by a plane p wave incident perpendicular to the borehole axis for wavelengths greater than 10 times the borehole circumference the effect of the borehole on particle motions is shown to be negligible thus the results have little relevance for the long wavelengths commonly encountered in earthquake seismology the results are however relevant to the study of stress wave propagation at ultrasonic frequencies in rock masses for small wavelengths a 3 0 the filter representing particle motion on the wave incident site of an empty borehole reduces to a linear phase filter which increases all amplitudes by a factor of 2 while the filter representing fluid stress at the center of a fluid filled borehole may be reduced to simple mathematical expressions experimental results were obtained for the interaction of a stress wave with either accele

fast rift propagation along the east pacific rise near easter island during a 1983 seabeam cruise we surveyed the easter nazca spreading center which forms the eastern boundary of the easter microplate we have analyzed all available bathymetry and magnetic anomaly data to determine the present fine scale tectonic configuration and recent tectonic evolution of this spreading center the narrowing of the axial brunhes anomaly is best explained by a young propagating rift growing northward at an average rate of 150 mm yr assuming an average spreading rate of 120 mm yr there are indications of a short episode of faster propagation 350 mm yr prior to 0 4 ma these are the fastest rates of rift propagation yet documented the tip of the propagating rift is at about 25 s 112 25 w 180 km south of the previously hypothesized position the recent spreading direction is n85 w a large well defined sheared zone to the west of the western pseudofault bounding the crust formed by the propagating rift suggests that the easter pacific spreading center 360 km to the west is being replaced by the fast propagator