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

the juan de fuca ridge hot spot propagating rift system new tectonic geochemical and magnetic data underway geophysics deep tow camera work and preliminary geochemical and magnetic studies from a 1980 cruise to the juan de fuca ridge allow the following interpretations 1 the central third between 45 5 n and 46 5 n of the ridge is now actively spreading from a zone which is 10 to 20 km west of the axis bisecting the brunhes matuyama magnetic reversal boundary 2 the apparent terminus of surficial igneous activity at the northern tip of the active cobb propagator has been identified by dredging and camera work the tip appears curved or offset slightly to the west does not correspond to the northernmost tip of the central magnetic anomaly elvers et al 1974 and spreading has been initiated beneath sediments farther north and 3 iron and titanium in fresh glasses from the active volcanic zone exhibit a progressive enrichment northward then a pronounced reversal to more normal concentrations in the vicinity of the cobb offset

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

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

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

the evolution of propagating rifts the magnetic survey carried out by mason and raff1 2 off the north west coast of the us has been central to the development of many of the concepts of seafloor spreading and plate tectonics recently hey and wilson3 have again used this survey to illustrate hey s suggestion4 that ridge segments sometimes increase their length by propagation this process leads to a characteristic pattern of magnetic anomalies offset by discontinuities which hey called pseudofaults because unlike fracture zones they fig 1 were never plate boundaries we demonstrate here how the geometry of the pseudofaults magnetic anomalies and the failed rift can be worked out using velocity triangles and the concept of stability this method was originally developed to understand the evolution of triple junctions between three plates5

analogies to oceanic behaviour in the continental breakup of the western woodlark basin the woodlark basin d entrecasteaux island region off northeast papua new guinea fig 1 offers a rare glimpse of the propagation of active rifting and continental breakup1 into orogenically thickened and subsequently extended lithosphere2 10 rifting continental breakup and the subsequent formation of oceanic lithosphere by sea floor spreading often involve rupture controlled by propagation of a rift tip11 20 we recognize several tectonic elements in the evolving western woodlark intracontinental system that are close geometric analogues of wholly oceanic propagators and a kinematic development that can be viewed as an extrapolation of oceanic tectonics the response to deformational stresses in the final stages of breakup in a thick relatively hot and hence weak continental lithosphere appears akin to that of oceanic lithosphere

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

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

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

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

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

spectral interpolation and downward continuation of marine magnetic anomaly data a two or three dimensional treatment of magnetic anomaly data generally requires that the data be interpolated onto a regular grid especially when the analysis involves transforming the data into the fourier domain we present an algorithm for interpolation and downward continuation of magnetic anomaly data that works within a statistical framework we assume that the magnetic anomaly is a realization of a random stationary field whose power spectral density psd we can estimate by using the psd the algorithm produces an array incorporating as much of the information contained in the data as possible while avoiding the introduction of unnecessary complexity the algorithm has the added advantage of estimating the uncertainty of every interpolated value downward continuation is a natural extension of the statistical algorithm we apply our method to the interpolation of magnetic anomalies from the region around the 95 5 w galapagos propagating rift onto a regular grid and also to the downward continuation of these data to a depth of 2200 m we also note that the observed psd of the galapagos magnetic anomalies has a minimum at low wave numbers and discuss how this implies that intermediate wavelength 65 km 1500 km magnetic anomalies are weaker than suggested by one dimensional spectral analysis of single profiles

three dimensional flow beneath a slow spreading ridge axis a dynamic contribution to the deepening of the median valley toward fracture zones the axial valley along a slow spreading mid ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis along a ridge segment the axial valley floor deepens by as much as 2 3 km over distances of several tens of kilometers approaching a ridge transform intersection this deepening may be explained in part by a pressure gradient associated with horizontal flow in the ridge axis conduit vanishing vertical velocity on a conduit end wall at a ridge transform intersection results in reduced vertical flow within about one conduit width of the intersection horizontal flow along the conduit toward the intersection must occur to form lithosphere at a uniform rate along the ridge axis a simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge transform intersections is considered the model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls this model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection for reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection

continental rifting and the implications for plate tectonic reconstructions previous plate tectonic reconstructions have tried to recreate the pre rifting pangea configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting that the continental boundaries are isochrons and that the continents rift without distortion this paper proposes a model for continental breakup with distortion where ocean basins are formed by propagating rifts this model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents as rifts propagate through a continent the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading the propagating rift model implies that large amounts of continental extension up to 150 km occur and that the continental boundaries are not isochrons this extension due to rifting results in apparent overlap when the continents are returned to their original configuration because of the gradational nature of this extension the best representation of the pre rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation the overlap is equivalent to the amount of extension that has taken place on both continental edges reconstructions based on this model are presented for the gulf of california the gulf of aden the norwegian greenland sea and the south atlantic ocean they are able to resolve the discrepancies that occur when rigid plate tectonics are applied this model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

overlapping spreading centres 3 d inversion of the magnetic field at 9 03 n on the east pacific rise summary overlapping spreading centres oscs represent a new type of plate boundary interaction in which en echelon rise segments overlap significantly and are not joined by a transform fault a three dimensional fourier inversion of the magnetic field was performed on an overlapping spreading centre to remove the effects of topography and ridge orientation a magnetic high exists at the tip of one of the two ridge segments forward modelling suggests that the anomalous magnetic field cannot be attributed to the effects of topography alone the inversion reveals the existence of a magnetization high at the tip of the eastern spreading centre maximum magnetization values are consistent with ones obtained in other high amplitude zones in the pacific as well as with the measured magnetization of samples dredged in the same areas we suggest that the magnetization high over the eastern ridge tip of the 9 03 n osc is associated with highly evolved basalts enriched in iron and titanium such enrichment may be caused by enhanced crystal fractionation within an axial magma chamber which is intermittent and occasionally freezes as the eastern spreading axis propagates into older lithosphere

crustal structure of the tuamotu plateau 15 s and implications for its origin we investigate the sedimentary and volcanic structure of the tuamotu plateau with multichannel seismic seismic refraction and gravity data along a ship track crossing the plateau near 15 s the volcanic basement of the central portion of the plateau is capped with a 1 to 2 km thick sediment layer composed of two compositional sequences the uppermost sequence with semblance derived p wave velocities of 1 6 1 9 km s and thicknesses of 0 2 0 9 km is composed of pelagic sediments the underlying sequence with velocities 2 5 3 5 km s and thicknesses of 0 5 1 5 km is composed of limestone and volcaniclastic sediments sonobuoy refraction data show the upper 1 km of the volcanic basement to have velocities 4 5 5 5 km s the gravity data indicate that the platform is compensated by an elastic lithosphere with effective thickness 5 5 km and that the volcanic thickness is 9 10 km thicker than normal oceanic crust with a volume of 2 0 2 6 l06 km3 the inferred eruption rates of 0 1 0 13 km3 yr are comparable to those of the hawaiian and marquesas island chains but substantially less than those of many oceanic plateaus radiometric and paleontological ages for the plateau and geomagnetic dates of the surrounding seafloor indicate that the northwestern portion of the plateau formed 600 km off the axis of the paleo pacific farallon spreading center on lithosphere of age 10 20 ma linear volcanic ridges and scarps bounding deep sediment filled basins however are similar to features of oceanic plateaus which formed at or near accretionary plate boundaries we suggest that these volcanic ridges and the gross plateau like morphology were formed by magma that was channelled along the lithospheric discontinuities left behind by a southward propagating rift segment of the nearby spreading center we attribute the formation of the northwestern portion of the tuamotu plateau to the passage of two hotspots during times 50 30 ma as they migrated beneath the pacific plate but remained west of the tuamotu propagator

seismicity and crustal structure of the fiji platform southwest pacific data from a telemetered seismic network in fiji provide new constraints on the crustal structure and active deformation within and around the fiji platform an unusual fragmented arc complex situated within the pacific indo australian plate boundary seismic refraction data indicate that crustal thickness in fiji is typical of island arcs averaging about 15 20 km with relatively low midcrustal seismic velocities vp 6 0 km s characteristic of felsic arc related rocks the anticipated higher velocity lower crustal layer is either thin or absent mantle velocity beneath the platform is low 7 6 km s in the same range as that observed beneath the neighboring marginal basins and island arcs the record of crustal seismicity based on historical teleseismic and microearthquake catalogs indicates significant activity both within and along the boundaries of the fiji platform earthquakes located along the edges of the platform are associated with major zones of back arc deformation of the north fiji and lau basins the fiji fracture zone to the north the hunter fracture zone to the south and a deformed zone in the eastern portion of the north fiji basin to the west earthquakes within the platform intraplate events occur more sporadically than those along its margins areas of significant intraplate activity include the taveuni northeastem vanua levu area koro sea the baravi basin and central and south eastern viti levu earthquake depths within the platform are concentrated in the upper 13 16 km of lithosphere as in other areas of extensional strike slip tectonics in fiji however the seismicity apparently extends through the entire crustal section the large 1953 intraplate suva earthquake ms 6 75 was caused by strike slip faulting under the same east west extensional stress field that is deforming the north fiji and lau basins microearthquakes within and around the island of viti levu appear to occur as a result of the same stresses these earthquakes occur in discrete nests that are tightly clustered in space and time they are related to late cenozoic faults that cut the mio pliocene rocks of viti levu the characteristics of its crustal structure the anomalous mantle velocity and the active deformation within the fiji platform distinguish it from microcontinental blocks that have been observed elsewhere the seismicity and focal mechanisms within the fiji platform demonstrate that it is not technically isolated from the active deformational processes that pervade the neighboring arcs and back arc basins

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

the geometry of propagating rifts abstract the kinematics of two different processes are investigated both of which have been described as rift propagation courtillot uses this term to describe the change from distributed to localised extension which occurs during the early development of an ocean basin the term localisation is instead used here to describe this process to distinguish it from hey s type of propagation localisation generally leads to rotation of the direction of magnetisation to hey propagation means the extension of a rift into the undeformed plate beyond a transform fault detail surveys of the galapagos rift have shown that the propagating and failing rifts are not connected by a single transform fault but by a zone which is undergoing shear the principal deformation is simple shear and the kinematics of this deformation are investigated in some detail the strike of most of the lineations observed in the area can be produced by such deformation the mode of extension on the propagating rift appears to be localised for some periods but to be distributed for others neither simple kinematic arguments nor stretching of the lithosphere with conservation of crust can account for the observed variations in water depth

evolution of the iceland hotspot mantle geochemical enrichment beneath the iceland faeroes hotspot has been episodic suggesting that blobs rather than a continuous plume may be rising in the region recurring rift propagation southward from central iceland causing progressive melting of the lower crust followed by the upper mantle best explains the geochemical variations found along the south west and south east neovolcanic zones and their degree of thermal maturity

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

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

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

gloria observations of the propagating rift at 95 50w on the cocos nazca spreading center in this paper we describe a gloria long range sidescan sonar survey of the area around 95 5 w on the cocos nazca rise where a propagating rift has been predicted to occur the sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1 ma old crust the new crust is up to 800 m deeper than the surrounding seafloor although the elevation difference appears to decay with time the pseudofaults which bound this new crust have been observed from the rift tip back to crustal isochron 0 65 ma near the rift tip they consist of scarps several hundred meters high with narrow shallow valleys at their feet the sheared zone between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and nnw sse trending lineaments these may have been produced by the rotation of tectonic fabric formed at the dying rift perhaps accompanied by other deformation the youngest tectonic and volcanic lineaments on both spreading segments strike 273 but on the dying rift short spreading segments are offset en echelon so that the regional trend of the rise axis is 265 older lineaments throughout the area strike 265 the propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction and it is suggested that this is occurring in response to a recent change in spreading direction from 355 to 003 variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273 direction which we take to be normal to the new spreading direction

galapagos hot spot spreading center system 1 spatial petrological and geochemical variations 83 w 101 w we report on the petrology and geochemistry of basalts dredged at 40 50 km intervals along the galapagos spreading center between 83 sup 0 w and 101 sup 0 w 40 stations emphasis is on spatial variations of whole rock major elements rare earths trace metals of the first transition series and the nature of phenocryst assemblages and their abundances these results provide new constraints on the nature and scale of mantle source heterogeneities melting conditions thermal field and dynamics of crustal formation of the region we suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements which appears of worldwide extent we reject vogt and deboer s 1976 model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line because this model would not explain the gradients in ree observed about the galapagos platform our preferred model combines the mantle plume binary mixing model of schilling 1973 with the concept of recurring rift propagation proposed by hey 1977a we further propose that pulsating mantle plume flux perhaps inmore the form of a chain of blobs may initiate the development of new rifts and their propagation the present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow a two million year period is suggested for the last 4 m y from wilson and hey s 1979 information rigorous testing of our preferred model is possible less

galapagos hot spot spreading center system 2 87sr 86 sr and large ion lithophile element variations 85 w 101 w thirty eight samples of basalts dredged from the galapagos spreading center gsc between 85 w and 101 5 w longitudes have been analyzed for k rb cs ba and sr contents as well as for 87sr 86sr ratios basalts between 85 w and 89 w segment of the gsc range from 0 7025 to 0 7028 in 87sr 86sr and those between 95 5 w and 101 5 w range from 0 7026 to 0 7027 these basalts are characterized by lree depleted patterns basalts from 89 w to 95 5 w segment having slightly lree depleted to lree enriched patterns la sm e f between 0 8 and 2 4 range from 0 7027 to 0 7031 in 87sr 86sr the longitudinal 87sr 86sr variation shows a maximum at about 92 w where the gsc is nearest to darwin island the alkali and alkaline earth elements as well as several element ratios also show maxima in their longitudinal profiles but these occur at about 91 w the apparent shift of 87sr 86sr maximum with respect to that of the trace elements may be related to differences in the degree of partial melting the highest 87sr 86sr from the gsc is only slightly lower than the highest 87sr 86sr for tholeiitic basalts from the galapagos archipelago several possible hypotheses have been examined to account for these geochemical and sr isotopic variations in the light of the presently available data the most promising explanation seems to be a binary mixing of hot spot or plume type material with the source of normal mid ocean ridge basalts and resulting rift propagation both end members seem to be heterogeneous and therefore it is at present difficult to put severe constraints on this binary mixing nevertheless the flow pattern of mantle material from the hot spot does not appear to be radial but is preferentially channeled along the 91 w and 92 5 w fracture zones toward the rift

the tectonic evolution of the indian ocean triple junction anomaly 6 to present the tectonic evolution of the indian ocean ridge ridge ridge triple junction the junction of the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir is inferred by mapping structures along its traces on the three plates with a long range side scan sonar geological long range inclined asdic gloria the sonar images to the west of the triple junction show two different styles of tectonic evolution some sections of the trace on the african plate are marked by a swir facing scarp containing fine lineaments that are probably normal faults these correspond with sections of the trace on the antarctic plate of the same age that have a sharp intersection of swir and seir abyssal hills that is to be expected from a simple ridge ridge ridge triple junction evolution a herringbone pattern of isochrons in contrast there are other sections of the trace on the african plate that have a blocky structure with no definite termination of cir abyssal hills against those of the swir the corresponding regions of the antarctic plate show large 10 to 30 km long overlapping fault scarps tapering to the northeast and ending in asymmetric valleys to the southwest these major normal faults are each offset by 1 6 km forming an en echelon series the two different styles of tectonic structures indicate two different modes of evolution although why the triple junction has evolved with two different modes is unclear periods of asymmetric spreading in the cir magnetic anomaly sequence collected on the african plate may possibly indicate that the cir and seir have been offset by a fracture zone at the triple junction during one of these two modes echo sounder traverses on the antarctic and african plates commonly show a steady shallowing of the cir and seir seafloor toward the traces a steep scarp facing the swir and a broad marginal deep at the base of this scarp this rift flank shape resembles that of the swir valley s rift shoulders close to the present triple junction mitchell 1991a suggesting that the mechanical evolution has been similar to the present day and the relief due to the rift flank uplift has been preserved by the cooling and strengthening of the lithosphere with age away from the ridge axes if the shape of the rift shoulder uplift along the trace on the antarctic plate is interpreted with a simple flexural model involving an end loaded elastic plate the inferred rigidity 1019 1021 n m or 2 6 km effective elastic thickness is consistent with the suggestion that the isostatic uplift occurred in young weak lithosphere close to the triple junction interestingly the observed 1 km relief of the trace escarpment and the estimated rigidity suggest that the uplift was produced by a load 2 5 1011 n m that is equivalent to half the present cross sectional area of the swir valley 35 km from the triple junction

the relationship between strain rates crustal thickening palaeomagnetism finite strain and fault movements within a deforming zone the deformation of continental lithosphere generally does not occur on a single fault but is distributed over a zone up to a few hundred kilometers wide if the deformation rate is constant throughout the zone and if the velocity is continuous everywhere and does not change with time the deformation gradient tensor can be obtained analytically this tensor directly determines the amount of crustal thickening and is related in a complicated manner to the finite strain tensor the declination of remnant magnetization measured within the zone can also be rotated by the deformation because palaeomagnetic measurements are generally made on rigid blocks which have not been deformed there is no simple relationship between such rotations and the finite strain expressions for all these quantities are obtained in terms of the velocity gradients which are taken to be constant the history of western north america is used to illustrate the relationship between the present deformation by strike slip and thrust motions the rotations of the remnant magnetization vectors and the shape of accreted terranes at the surface distributed deformation is taken up by motion on faults which separate the zone into blocks the displacement rate on the faults their strike and the finite rotations of the blocks are all controlled by the velocity gradients analytic expressions relating these quantities are obtained a deforming zone containing normal faults which crosses central greece takes up both strike slip and extension and illustrates the relationship between the structures within and the motion across the zone a similar but more complicated zone in northeastern iran involves both strike slip and shortening

near axis seamounts on the flanks of the east pacific rise 8 n to 17 n with complete seabeam and seamarcii bathymetry and nearly complete seamarcii side scan coverage of the northern east pacific rise and its flanks from 8 to 17 n we characterize the entire population of seamounts on young seafloor 0 to 2 ma formed along this fast spreading center there are 179 seamounts in this area taller than 200 m most of which belong to one of 21 seamount chains these chains are oriented between the relative and absolute plate motion directions of the pacific and cocos plates and most of the larger chains are more voluminous near their centers than near their ends several of the seamount chains contain bends in orientation which are not coincident with each other the two largest seamount chains at 9 55 n lamont seamounts and 15 40 n are aligned with portions of the east pacific rise crest which are among the shallowest and broadest of this area overall the maximum abundance and size of seamounts increase with shallower and broader ridge axes no near axis seamounts form on the summit of the axial ridge nonetheless within 15 km 0 3 ma of the axis the abundance of seamounts achieves its overall average observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near axis seamounts are formed in a narrow zone on crust aged 0 1 0 3 ma 5 15 km from the axis with significant growth in a zone several times wider side scan sonar reflectivity and seamount magnetization analyses are largely consistent with the predictions of this model two exceptions to this model however are seamount chains at 15 40 n and 8 25 n which show evidence for activity over a broader region this may be due to the presence of fracture zones within about 30 km of these chains a compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge the variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results although these influences cannot be distinguished in this analysis

a survey of seismic activity near wairakei geothermal field new zealand a five week survey showed that seismic activity within 20 km of wairakei geothermal field took place mainly at shallow depths 2 km in or close to the taupo fault belt and occurred in swarms twenty eight earthquakes with magnitudes m between 1 3 and 2 8 were located 43 other earthquakes with m 0 2 were recorded but could not be located the distribution of located earthquakes did not correlate with known areas of surface geothermal activity no located earthquake occurred beneath the main production borefield or an area of ground subsidence adjacent to the borefield a value for b of 0 7 0 2 was obtained for events with 0 8 m 1 25 within 6 5 km of the borefield no earthquakes were sufficiently clearly recorded to demonstrate attenuation of s phases within the borefield area seismic activity in and near the field is similar to that reported in neighbouring parts of the region operation of the field therefore does not appear to have caused any large increase or decrease of seismic activity in the taupo fault belt within or near the field

evolution of ridges by asymmetrical spreading in late cretaceous time the relatively straight crest of the east pacific rise developed a remarkably orderly crenelate pattern contrary to previous statements such a pattern can evolve by orthogonal asymmetrical spreading without ridges jumping or otherwise affecting plate rigidity such evolution has been observed in wax experiments serrate ridges may also evolve if spreading is not orthogonal to transforms

a detailed study of the cobb offset of the juan de fuca ridge evolution of a propagating rift the cobb offset on the northern portion of the juan de fuca ridge has been identified as the tip of a northward propagating rift hey and wilson 1982 map compilations of magnetic and seismic data from four new cruises define the present locus of spreading and volcanism on the two ridge segments abutting the offset and permit detailed modeling of the recent evolution within this transform zone the axis of recent spreading on the southern ridge segment bends from the normal ridge trend n20 e to a n s trend north of 47 15 n the spreading axis on the northern ridge segment generally defines a n20 e trend except at the southern terminus where the spreading center is offset slightly to the east the two spreading centers overlap by about 33 km in the offset vicinity and there is evidence of recent volcanism on both segments present ridge axis morphology exhibits a transitional sequence from a symmetrical axial high along the more normal portions of each ridge segment to a grabenlike depression as the tip is approached the magnetic anomaly patterns observed in the cobb offset vicinity are not consistent with the patterns predicted by models of continuous northward propagation the magnetic anomaly patterns of the brunhes epoch require an event of rapid northward propagation about 0 7 m y b p followed by a more gradual southward propagation in the middle brunhes epoch most recently the spreading center on the southern ridge has extended northward to its present configuration prior to the brunhes epoch modeling of the magnetic anomaly patterns does not indicate a unique solution however net propagation has been northward we present alternative models for the period beginning 1 7 m y b p in the first model the cobb offset has evolved by a series of northward and southward events of propagation with net advance to the north in the second model stable asymmetric spreading from overlapping ridge segments has evolved into a transform fault offset by gradual decay of the northern ridge both models indicate that propagation has been discontinuous and relatively rapid compared to spreading rate and that realignment of ridge segments near the propagator tip has been an important component

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

analysis of propagators along the pacific antarctic ridge evidence for triggering by kinematic changes abstract we analyze the structure and evolution of two propagators along the pacific antarctic ridge par that we surveyed during the pacantarctic cruise of the n o l atalante a large propagator at 63 30 s 167 w shows a n50 e trending segment of the par propagating southwestward while the adjacent n45 e trending segment retreats the propagating and doomed ridges are offset by about 43 km they both curve into the offset to define an overlap zone about 25 km long the inner pseudofault is juxtaposed to a series of e w trending ridges inferred to represent the failed axes their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure the pseudofaults are 35 5 oblique to the propagating ridge which implies a rate of propagation of 44 8 mm yr using a 62 mm yr full spreading rate comparable to that of other propagators with similar morphology the age of the initiation of the propagation from the heirtzler fracture zone is estimated to be 5 6 ma which coincides with the age of a clockwise change in spreading direction a second smaller southwestward propagator is observed northeast of the major one at 63 15 s 165 10 w with a morphology very similar to that of the larger one it is inferred to have started near 1 ma again at the time of a clockwise change in spreading direction along the par these two propagators are likely to have evolved from extensional relay zones which developed within the heirtzler transform fault tf valley following clockwise changes in spreading direction the present day axial discontinuity is less than 40 km in offset and may not be a tf anymore the development of propagators in this area of the par appears to be triggered by kinematic changes rather than by thermal gradients along the ridge other propagators that have left similar signatures on the flanks of the par appear to have developed at similar spreading rates near 50 60 mm yr full rate as a result of kinematic changes

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

multiple closely spaced transform faults in fast slipping fracture zones the 400 km long offset of the east pacific rise spreading axis between the quebrada fracture zone lat 3 5 s and the gofar fracture zone lat 4 6 s has been surveyed using the long range sidescan sonar gloria the offset is accomplished in three distinct fracture zones each of which contains up to four actively slipping transform faults separated by short spreading centers transform spacings range from 5 to 16 km each transform is associated with its own transverse valley but at quebrada fracture zone and to some extent at the other two these valleys merge into one large slightly oblique fracture zone valley the morphological trends of fracture zone valleys are therefore not necessarily reliable for estimating directions of relative plate movement multitransform fracture zones appear to be much commoner on the fast spreading east pacific rise than on slower spreading ridges the tectonic spreading fabric which comprises closely spaced normal faults formed near the spreading axes and striking subparallel to them is modified to a sigmoidal pattern between the closely spaced transforms even midway between transforms these faults remain oblique to the spreading direction the degree of obliquity increases as the transform separation decreases showing progressive rotation of the stress ellipse as transforms are approached

propagating rift explanation for the tectonic evolution of the northeast pacific the pseudomovie abstract we have developed a new computer graphic technique to use in reconstructing plate tectonic evolution and to make movies illustrating this evolution using this technique we have deciphered the evolutionary history of the juan de fuca area in the northeast pacific and show that a combination of seafloor spreading and rift propagation can explain this seemingly complex area as a simple consequence of the interaction of two rigid plates

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

the galapagos axial magnetic anomaly evidence for the emperor event within the brunhes and for a two layer magnetic source the axial brunhes magnetic anomaly over the galapagos spreading center shows consistent short wavelength anomalies that are not consistent with a uniformly magnetized layer we interpret this anomaly to confirm the presence of the emperor reversed event at 0 49 0 05 mybp with a duration about 0 01 m y further this anomaly is consistent with a two layer magnetic source with the lower layer having very broad transition zones offset away from the axis from those of the upper layer by about 10 km

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

variations in axial morphology segmentation and seafloor roughness along the pacific antarctic ridge between 56 s and 66 s the spreading rate at the pacific antarctic ridge par increases rapidly from 54 mm yr near pitman fracture zone fz up to 76 mm yr near udintsev fz resulting in three domains of axial morphology an axial valley south of pitman fz an axial high north of saint exupery fz and in between the transitional domain extends over 650 km it comprises sections of ridge with an axial valley or an axial high and generally displays a very low cross sectional relief it is also characterized by two propagating rifts two domains of different seafloor roughness appear south of udintsev fz east of 157 w these two domains are separated by a 1000 km v shaped boundary west of 157 w the boundary approximately coincides with chron 3a or chron 4 the southward migration of the transitional area during the last 35 myr explains the v shaped boundary 1 increases in spreading rate above a threshold value produced changes in axial morphology and 2 in the transition zone rotations of the spreading direction were accommodated by the plate boundary either by rift propagation or by transitions from fracture zones to non transform discontinuities leaving trails on the seafloor that presently delineate the v seafloor roughness variations are not controlled by exactly the same spreading rate dependence as changes in axial morphology the transition from rough to smooth seems to have occurred everywhere for spreading rates greater than 50 mm yr except in a domain presently centered on saint exupery fz where it occurred for spreading rates 60 to 65 mm yr independent results from melting model calculations of major elements vlastelicet al 2000 indicate that the upper mantle temperature is likely to be cooler between antipodes and la rose fzs the combination of these two results reveals the existence of a 700 km long segmentation of the upper mantle with a cool area centered on saint exupery fz

three dimensional magnetic modeling of a propagating rift galapagos 95 30 w a combined deep tow and seabeam investigation at 95 30 w on the cocos nazca spreading center has revealed the crustal contact between the propagating rift and the dying rift systems the observed 27 km offset between the axes creates an ideal situation for the application of magnetic methods the normally magnetized crust of the propagating rift tip penetrates into older crust which was created when the earth s main field was reversed as the v shaped structure of the rift clearly violates the conventional two dimensional assumptions a full three dimensional analysis is required in addition the distorting influence of over 1000 m bathymetric relief must be removed from the observed field inversions have been performed on the gridded representations of the observed magnetic field and bathymetry working in the fourier domain the result is a gridded rock magnetization distribution the inversion of the surface data covers a large area 6000 km2 and demonstrates close agreement with magnetization amplitudes of rock samples at existing dredge sites in general the propagating rift process appears to be much more orderly than the dying rift process the magnetic polarity transition widths are narrower and the boundaries have fewer undulations than the dying rift which appears to be quite episodic in behavior the average propagation rate is 52 mm yr compared to the average spreading half rate of 29 mm yr the locations of the boundaries suggest that the acceleration to the normal spreading rate on the propagating rift requires about 250 000 years the inversion of the deep tow data near the seafloor provides a high resolution definition of the location of the tip of the propagating rift at 2 38 1 n 95 30 0 w

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