segment scale variations in the crustal structure of 150 300 kyr old fast spreading oceanic crust east pacific rise 8 15 n 10 5 n from wide angle seismic refraction profiles summary we have simultaneously inverted seismic refraction and wide angle moho reflection traveltimes for the 2 d crustal thickness and velocity structure of 150 300 kyr old crust along the east pacific rise epr between the siqueiros and clipperton fracture zones fzs our results show a strong correlation between ridge segmentation and upper and mid crustal seismic velocities with higher velocities near segment centres and lower velocities near segment ends low crustal velocities at the clipperton and siqueiros fzs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain a relict overlap basin left on the pacific plate by the 9 03 n overlapping spreading centre osc as it propagated southward is associated with a large 1 km s 1 negative upper and mid crustal velocity anomaly this anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration fracturing and shearing arising from rotation of the basin as it was formed the discordant zone left by this osc on the cocos plate is characterized by moderately low crustal velocities probably because of crustal fracturing as the osc propagated into older crust higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust and lower intensity tectonic alteration of the crust than near segment ends the mean crustal thickness along the epr between the siqueiros and clipperton fzs is 6 7 6 8 km the thickest crust is found beneath the lamont seamounts 9 km and in a southward pointing v shaped band located just north of the off axis trace of the 9 03 n osc 7 3 7 8 km the thinnest crust 6 km is found proximal to the clipperton and siqueiros fzs the crust associated with the off axis trace of the 9 03 n osc is not anomalously thin suggesting that magma supply beneath the osc is similar to that of the northern and southern segments we see a similar pattern of crustal thickness variation to that determined using multichannel reflection data including a gradual thickening of the crust from north to south along the northern ridge segment and the location of the thickest crust just north of the 9 03 n osc however the magnitude of the along axis crustal thickness variation we observe along the northern ridge segment between 9 50 n and 9 15 n 1 3 1 8 km excluding the lamont seamounts is significantly less than the 2 3 km of variation previously reported weakening the case for the existence of a low density mantle diapir at 9 50 n inferred from gravity data the band of thick crust located just north of the off axis trace of the 9 03 n osc suggests a close genetic link between this feature and the osc thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward propagating 9 03 n osc over the past 0 5 myr and not to along axis melt migration away from a mantle diapir as previously proposed

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

reversed pyroxene phase equilibria in cao mgo sio2 from 925 to 1 175 c at one atmosphere pressure experiments using v2o5 as a high temperature solvent have produced compositional reversals defining the miscibility gap between enstatite and diopside on the join mg2si2o6 camgsi2o6 between 925 and 1 175 c at atmospheric pressure these experiments locate an equilibrium near 1 000 c among diopside protoenstatite and orthoenstatite they verify the stable coexistence of diopside and protoenstatite above 1 000 c and disprove the hypothesis that orthoenstatite has a stability field which is continuous from temperatures below 1 000 c to the solidus the phase relations suggest that the orthorhombic low ca pyroxene on the solidus in this system formerly identified as orthoenstatite is a phase distinct from the orthoenstatite stable with diopside at low subsolidus temperatures data locating the orthoenstatite diopside miscibility gap validate the use at low pressures of symmetric orthopyroxene and asymmetric clinopyroxene solution models in this system

seismicity and active tectonic processes in the ultra slow spreading lena trough arctic ocean summary with its remote location in the ice covered fram strait lena trough is a poorly known segment of the global mid ocean ridge system it is a prominent member of the ultra slow spreading mid ocean ridges but its spreading mechanisms are not well understood we relocalized teleseismically recorded earthquakes from the past five decades to identify tectonic processes in lena trough and the adjacent spitsbergen fracture zone fz during two cruises with rv polarstern in 2008 and 2009 we deployed seismic arrays on ice floes to record the local seismicity of lena trough we could identify and localize microseismic events which we assume to be present in the entire rift valley in contrast our relocalization of teleseismically recorded earthquakes shows an asymmetric epicentre distribution along lena trough with earthquakes occurring predominately along the western valley flanks of lena trough in 2009 february march several high magnitude earthquakes peaking in an mb 6 6 event occurred in an outside corner setting of the spitsbergen fz this is the strongest earthquake which has ever been recorded in fram strait and its location at the outside corner high of the ultra slow spreading ridge is exceptional comparing the seismicity with the magnetic anomalies and high resolution multibeam bathymetry we divide lena trough in a symmetrically spreading northern part and an asymmetrically spreading southern part south of the south lena fz we propose that a complex interaction between the former de geer megashear zone which separated greenland from svalbard starting at late mesozoic early cenozoic times and the developing rift in the southern lena trough resulted an increasing eastward dislocation towards the spitsbergen fz between older spreading axes and the recent active spreading axis which we believe to be located west of the bathymetric rift valley flanks in a wide extensional plain

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

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

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

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

crustal magnetization of the reykjanes ridge and implications for its along axis variability and the formation of axial volcanic ridges we have inverted the sea surface total magnetic field measurements of the reykjanes ridge between 57 n and 63 n for crustal magnetization intensity assuming a uniformly thick magnetic source layer and direction of magnetization corresponding to the geocentric axial dipole field this section of the north mid atlantic ridge 10 mm yr half spreading rate is unique in that it lies within the influence of the iceland mantle plume which causes it to spread obliquely and in the north to resemble morphologically a fast spreading ridge on a regional scale the axial magnetization anomalies of the reykjanes ridge can be divided into three regimes from south to north these regimes correspond to sections of the ridge with different magmatic and tectonic styles which are believed to represent the effects of a pulsing plume beneath iceland the axis south of 59 10 n has the highest magnetization anomaly with the greatest variability and exhibits the characteristics of a slow spreading ridge unaffected by a plume the axis between 59 10 n and 60 50 n has the lowest magnetization anomaly on average and is where we presume the present day plume front is located the absence of large fault scarps and low magnetization are consistent with the idea of a midcrustal magma reservoir where mixing causes the magma to become less highly fractionated and produces less magnetic lavas finally a moderate increase in the magnetization anomaly and degree of faulting occurs north of 60 50 n signaling a gradual recovery in the crustal production and underlying structure after the passage of the plume front on the local scale with a few exceptions the loci of high magnetization anomalies coincide with the axial volcanic ridges notable lateral offsets in axial magnetization and decay in magnitude were found at 57 50 n 58 05 n and 59 12 n comparisons with seismic layer 2a show that variations in the magnetic source layer thickness may account for as much as half of the observed variability in the along axis magnetization there are also indications that the relative age of the extrusives and the degree of faulting and fissuring may be factors affecting the magnetization intensity of volcanic edifices on the seafloor

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

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

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

mode of seafloor spreading in the northern mariana trough abstract a geophysical study was conducted to clarify the mode of seafloor spreading in the northern mariana trough north of 20 n magnetic anomaly data and single channel seismic reflection profiles were taken along ship s tracks typically with a 2 4 mile spacing the transition from back arc rifting to true seafloor spreading occurs at a structural boundary which has ne sw strike at about 22 n south of this boundary clear magnetic lineations occur and axial rift valleys on the spreading ridges are visible on the seismic profiles our identification of the magnetic lineations shows that the accretion of oceanic crust started at about 3 5 ma with a spreading half rate of 2 cm yr the average spreading rate during the brunhes chron is of the order of 1 cm yr the rate increases northward and is accompanied by a decrease in the relative depths of the axial rift valleys a ridge axis discontinuity is found at about 21 n two spreading axes overlap each other about 50 km along strike and are offset about 30 km we interpret this discontinuity as an on going ridge jump from the western axis to the eastern one the southern end of the western axis is cut by a transform fault but the eastern axis is now propagating northwards arc volcanoes comprising the present volcanic front occur on the eastern flank of the spreading ridges suggesting basinward migration of the volcanic arc the spreading center of the mariana trough is located not at the center but on the eastern or the volcanic arc side of the trough in general offset is caused by the arcward ridge jump and the basinward migration of the volcanic arc asymmetric rifting before the beginning of the accretion of oceanic crust would be another reason for the offset a row of rift grabens along the eastern margin of the trough north of 22 n where the extension of island arc crust is concentrated continues to the spreading ridge south of 22 n the arcward offset of spreading ridges is also found in the lau basin suggesting that it is a common feature of the evolution of back arc basins

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

three dimensional magnetic and gravity studies of the rodriguez triple junction in the indian ocean the rodriguez triple junction rtj is a ridge ridge ridge triple junction connecting three mid ocean ridges in the indian ocean the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir we carried out three dimensional analyses of sea beam bathymetry magnetic and gravity anomaly data covering an area of about 200 km 140 km at the triple junction and obtained the magnetization distribution and the bouguer anomaly field in the magnetic inversion result some of the magnetic lineations of the cir and seir are recognized within the swir valley and the bouguer anomaly field shows little variation over the swir valley in spite of its large topographic relief these results indicate the tectonic rifting at the swir by crustal thinning without volcanic activity a 4 km wide graben in the western cir flank was found to have extremely high magnetization and lower bouguer anomaly than the eastern flank of the cir because the cir and seir have similar spreading directions and rates at the triple junction we consider the graben represents a fossil propagating rift of the seir which began to propagate toward the cir at about 1 ma and then became inactive when the swir grew to reach the cir seir spreading axis at about 0 5 ma we suggest the above process repeats several times and composes part of the triple junction evolution on a longer timescale we also suggest that the whole process is closely related to formation of the cir segments at the triple junction since 20 ma

ridge segmentation and the magnetic structure of the southern mid atlantic ridge 26 s and 31 35 s implications for magmatic processes at slow spreading centers along axis profiles of three dimensional magnetic inversions for the mid atlantic ridge mar 31 35 s show low magnetization near the middle of ridge segments and high magnetization at the segment tips for three adjacent spreading segments thus there is an inverse relation between axial magnetization and axial topography the ridge segment at 26 s on the mar has the same inverse relationship between magnetization and topography the common occurrence of this relationship suggests that it reflects a fundamental process of crustal accretion at the mar we analyze the rock magnetic properties from 42 locations within the four ridge segments in the south atlantic to constrain the inherent trade off between source intensity and source thickness in the magnetization model the natural remanent magnetization nrm intensities from the four ridge segments averaged together correlate with the magnetic inversion profiles this finding implies that changes in the magnetization of the extrusives may account for much of the observed magnetic anomaly amplitude variation a direct correlation of feo content and magnetization suggests that magnetic anomaly amplitudes may be an indicator of feti rich basalts at the slow spreading mar even though the iron content of the basalts from high magnetization areas is not as high as observed at pacific spreading centers despite the different magma plumbing systematics of the pacific spreading centers and the mar it appears that the segment scale magma system of the mar also results in segment scale crustal magnetization variations further evidence that the axial magnetic variations result from source intensity variations is that older isochrons have higher intensities near the ridge discontinuities similar to the behavior on axis between 0 and 5 ma the decay in magnetization is 50 independent of location within a spreading segment

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

magnetization of 0 29 ma ocean crust on the mid atlantic ridge 25 30 to 27 10 n a sea surface magnetic survey over the west flank of the mid atlantic ridge from 0 to 29 ma crust encompasses several spreading segments and documents the evolution of crustal magnetization in slowly accreted crust we find that magnetization decays rapidly within the first few million years although the filtering effect of water depth on the sea surface data and the slow spreading rate 13 km m y preclude us from resolving this decay rate a distinctly asymmetric along axis pattern of crustal magnetization is rapidly attenuated off axis suggesting that magnetization dominated by extrusive lavas on axis is reduced off axis to a background value off axis we find a statistically significant correlation between crustal magnetization and apparent crustal thickness with thin crust tending to be more positively magnetized than thicker crust indicative of induced magnetization in thin inside corner ic crust in general we find that off axis segment ends show an induced magnetization component regardless of polarity and that ic segment ends tend to have slightly more induced component compared with outside corner oc segment ends possibly due to serpentinized uppermost mantle at ic ends we find that remanent magnetization is also reduced at segment ends but there is no correlation with inside or outside corner crust even though they have very different crustal thicknesses this indicates that remanent magnetization off axis is independent of crustal thickness bulk composition and the presence or absence of extrusives remanence reduction at segment ends is thought to be primarily due to alteration of lower crust in oc crust and a combination of crustal thinning and alteration in ic crust from all these observations we infer that the remanent magnetization of extrusive crust is strongly attenuated off axis and that magnetization of the lower crust may be the dominant source for off axis magnetic anomalies

central anomaly magnetization high documentation of crustal accretion along the east pacific rise 9 55 9 25 n near bottom magnetic data collected along the crest of the east pacific rise between 9 55 and 9 25 n identify the central anomaly magnetization high camh a geomagnetic anomaly modulated by crustal accretionary processes over timescales of 104 years a significant decrease in camh amplitude is observed along axis from north to south with the steepest gradient between 9 42 and 9 36 n the source of this variation is neither a systematic change in geochemistry nor varying paleointensity at the time of lava eruption instead magnetic moment models show that it can be accounted for by an observed 50 decrease in seismic layer 2a thickness along axis layer 2a is assumed to be the extrusive volcanic layer and we propose that this composes most of the magnetic source layer along the ridge axis the 9 37 n overlapping spreading center osc is located at the southern end of the steep camh gradient and the 9 42 9 36 n ridge segment is interpreted to be a transition zone in crustal accretion processes with robust magmatism north of 9 42 n and relatively low magmatism at present south of 9 36 n the 9 37 n osc is also the only bathymetric discontinuity associated with a shift in the camh peak which deviates 0 7 km to the west of the axial summit trough indicating southward migration of the osc camh boundaries defined from the maximum gradients lie within or overlie the neovolcanic zone nvz boundaries throughout our survey area implying a systematic relationship between recent volcanic activity and camh source maximum flow distances and minimum lava dip angles are inferred on the basis of the lateral distance between the nvz and camh boundaries lava dip angles average 14 toward the ridge axis which agrees well with previous observations and offers a new method for estimating lava dip angles along fast spreading ridges where volcanic sequences are not exposed

do layer 3 rocks make a significant contribution to marine magnetic anomalies in situ magnetization of gabbros at ocean drilling program hole 735b we estimate the average magnetization of a vertical section of oceanic gabbros using paleomagnetic and downhole magnetic logging techniques and evaluate the ability of these crustal rocks to contribute to marine magnetic anomalies results from ocean drilling program hole 735b show that the drilled crustal section has a mean effective remanent magnetization of 2 5 a m olivine gabbros which make up 60 of the section have an average effective magnetization between 1 and 2 a m we used two dimensional forward modeling and the average effective magnetization of olivine gabbros to predict the magnitude of a typical layer 3 anomaly observable at sea surface these results indicate that if polarity boundaries within oceanic layer 3 are near vertical crustal sections similar to 735b would contribute from 25 to 75 of the overlying marine magnetic anomalies at the sea surface the shape of the polarity boundaries in lower crust will be controlled by its thermal evolution and potentially could be constrained by systematic analyses of the amplitude and skewness of marine magnetic anomalies at individual spreading centers

the rise flank trails left by migrating offsets of the equatorial east pacific rise axis sea beam and magnetic surveys of the young 1 ma rise flank around nontransform rise crest offsets at 5 5 n 4 9 n 3 4 n 2 0 n and 2 8 s mapped distinctive trails of obliquely lineated highly magnetic crust that spreads from the characteristically curving overlapping rift zones a deep tow survey around the small 1 5 km 3 9 n offset found only normal rise flank terrain near bottom deep tow observations at the medium size 9 km 5 5 n offset and the large 27 km 2 0 n offset provided more detailed information on the formation of crust in the offset trails and of the structure of tectonically modified crust at interrift rise crest sites patches of interrift crust which forms deep overlap basins at medium size offsets and basins and plateaus of rotated abyssal hills at large offsets are periodically shed from the axial zone together with abandoned portions of overlapped rift zone to become important elements of the rise flank trails the oblique azimuths of the paired fracture zone trails that diverge from the rise crest offsets show that most have migrated along the rise crest at average speeds of 25 125 mm yr net movement of the 27 km 2 8 s offset has been slower with alternating periods of rapid northward and southward migration information on the migration history of the surveyed offsets was supplemented by inferring the migration direction of another eight offsets on the equatorial east pacific rise epr whose trails were less thoroughly mapped most offsets of the southern pacific cocos boundary have migrated south and most on the northern pacific nazca boundary have migrated north the patterns are not readily explained by the inferred distribution of mantle upwellings along the epr axis

tectonics of the failing spreading system associated with the 95 5 w galapagos propagator the failing spreading system associated with the galapagos 95 5 w propagator system comprises a complex set of volcanic and structural provinces data from detailed surveys using gloria seamarc ii sea beam deep tow the submersible alvin magnetics and bottom cameras constrain the plate boundary geometry and evolution of this area although the failing spreading system may involve diffuse spreading the patterns of sedimentation volcanism and faulting suggest localization the failed spreading axis composed of a set of right stepping en echelon basins extends and ages southeastward from offset and overlapping doomed failing axes the failing axis is offset 13 km left laterally from the doomed axis although there may have been a preexisting offset in the doomed axis lack of structural evidence for this and lack of an offset of this magnitude in the brunhes matuyama magnetic reversal argue against this possibility a model whereby the offset is transient does not explain the pattern of failed spreading basins which leads directly away from the currently failing axis detailed structural and volcanic patterns suggest that this offset may be caused by a westward progression of northward jumps in the location of the failing axis like a secondary propagator probably in association with the migrating strain stress field of the migrating transform zone of the propagator system this is modelled as a migrating extensional relay zone merz similar to the relay zones observed at some fast slipping transform faults except that it propagates westward with the overall system bathymetric and magnetic evidence for tectonic elements associated with the merz supports this model although the expected extinct trace of the doomed axis is not obvious in the present incomplete data set unlike other models for the plate boundary geometry the merz model does not require serendipity for temporal and spatial juxtaposition of the primary propagator and the offset within the failing spreading system observation of a similar geometry in the lau basin supports this preferred model present data from the galapagos failing spreading system are inconclusive however and the merz model remains speculative if valid this model implies that in at least some cases paired propagators act together the merz is too short and deep to drive its own propagation according to standard cracking models

near axis crustal structure and thickness of the endeavour segment juan de fuca ridge a model of crustal thickness and lower crustal velocities is obtained for crustal ages of 0 1 1 2 ma on the endeavour segment of the juan de fuca ridge by inverting travel times of crustal paths and non ridge crossing wide angle moho reflections obtained from a three dimensional tomographic experiment the crust is thicker by 0 5 1 km beneath a 200 m high plateau that extends across the segment center this feature is consistent with the influence of the proposed heckle melt anomaly on the spreading center the history of ridge propagation on the cobb overlapping spreading center may also have influenced the formation of the plateau the sharp boundaries of the plateau and crustal thickness anomaly suggest that melt transport is predominantly upward in the crust lower crustal velocities are lower at the ends of the segment likely due to increased hydrothermal alteration in regions influenced by overlapping spreading centers and possibly increased magmatic differentiation

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

overlapping spreading centers implications from crack growth simulation by the displacement discontinuity method overlapping spreading centers osc s are a fundamental aspect of accretionary processes at intermediate and fast spreading centers and typically occur at deep points along the axial depth profile they have a characteristic geometry consisting of two en echelon overlapping curving ridges separated by an elongated depression the length to width ratio of this overlap basin is typically 3 1 we have been successful in reproducing the overlapping spreading center geometry by modelling the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity a boundary element displacement discontinuity method was used to solve this problem our calculated results are compared with seafloor observations in terms of the size and shape of the overlap region for small osc s there is a very good agreement between calculations and observations but for large ones the overlap basin tends to be longer than our predicted results indicate this suggests that the assumptions made in the model i e perfectly elastic isotropic and homogeneous medium are perhaps valid for the brittle lid above the magma chamber that underlies osc s with small offsets 2 km but oversimplified for osc s with large offsets our modelling shows that the initial interaction of closely spaced surface ruptures along spreading centers is to deflect away from one another as they approach the deflection will be the greatest for small misalignments of the fracture systems thus even minor misalignments of the spreading centers may result in the development of osc s where the misalignment is less than the width of the cracking front the fracture systems may meet head on creating a saddle point along the axial depth profile our results support the hypothesis suggested by macdonald et al 1984 in which overlapping spreading centers develop where two magmatic pulses migrate toward each other along the strike of the spreading center following fracture systems and magmatic conduits which are imperfectly aligned

kinetics of conversion of air bubbles to air hydrate crystals in antarctic ice the depth dependence of bubble concentration at pressures above the transition to the air hydrate phase and the optical scattering length due to bubbles in deep ice at the south pole are modeled with diffusion growth data from the laboratory taking into account the dependence of age and temperature on depth in the ice the model fits the available data on bubbles in cores from vostok and byrd and on scattering length in deep ice at the south pole it explains why bubbles and air hydrate crystals coexist in deep ice over a range of depths as great as 800 meters and predicts that at depths below 1400 meters the amanda neutrino observatory at the south pole will operate unimpaired by light scattering from bubbles

a one million year history of the 11 45 n east pacific rise discontinuity bathymetric magnetic and side scan sonar data from a june 1987 seamarc ii cruise aboard the r v moana wave have been combined with additional data in an area 140 km by 154 km centered on the overlapping spreading center pair osc located at 11 45 n on the east pacific rise epr the purpose of this study is to correlate and interpret these data sets in order to determine the recent 1 m y history of this osc the data sets include gridded bathymetry and magnetic field anomalies a side scan sonar mosaic and a fault map the bathymetry and anomaly grids were used as inputs in a three dimensional fourier inversion resulting in a crustal rock magnetization distribution solution the most prominent feature of all five maps is a roughly v shaped anomalous zone centered on the osc the zone is characterized by a positive depth anomaly a negative magnetic field anomaly within the central positive anomaly and a zone of high magnetization in the inversion solution a hummock y texture and sparse curved faults mark the zone in the side scan mosaic and fault map the shape of this zone indicates a short period of northward migration of the discontinuity 0 72 0 6 ma at approximately 75 mm yr followed by slow less than 30 mm yr and then fast 140 200 mm yr southward migration for the past 0 6 m y the anomalous zone is not well defined beyond the edge of the central anomaly which is straight and continuous indicating that if this discontinuity of the epr existed 0 72 ma its offset was negligible the present day axis has an orientation of 349 the edge of the central anomaly has an orientation of 353 and jaramillo age crust to the east of the epr has an orientation of 353 355 this variation in magnetic linealion directions suggests a 4 to 6 counter clockwise change in the direction of spreading in the past 1 m y which seems to have been accommodated by asymmetric spreading variations in mean fault trend with age are consistent with a counter clockwise change in the direction of spreading for the same time period

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

high magnetization zones near spreading center discontinuities abstract three dimensional inversions of the magnetic field reveal the presence of zones of high magnetization over the propagating limb of large spreading center discontinuities the presence of high magnetization zones is confirmed by rock magnetic measurements where available magnetization highs are associated with basalts which tend to have high fe contents and low mg numbers these data suggest that high magnetization zones observed over large ridge axis discontinuities are associated with highly differentiated basalts enriched in iron following christie and sinton 1 such highly evolved basalts may be the result of shallow level crystal fractionation in small magma bodies with a low supply rate these small magma bodies are postulated to correspond to the first stages in the development of a sub axial magmatic system as a result of the propagation of one of the limbs of the offset into older lithosphere because high magnetization zones at large ridge axis discontinuities often correspond to gaps in the along axis extent of a seismically detectable magma chamber these magma bodies may be smaller than a few hundred meters rock magnetic measurements suggest that the enrichment in iron associated with increased differentiation may be accompanied in a few cases by an increase in the concentration of titanomagnetite within the basalts and in the magnetization of the rocks however the exact relationship between high magnetization intensities and iron enrichment is complex and unclear and may be significantly affected by factors such as magnetic mineralogy and crystallization history

fine scale crustal magnetization variations and segmentation of the east pacific rise 9 10 9 50 n sea surface scalar magnetic field measurements collected over the east pacific rise near 9 30 n and within the brunhes chron have been inverted for magnetization of the upper crust a close track spacing 1 3 km and careful corrections for sources of background noise make this data set particularly suitable for investigating fine scale crustal magnetization a well defined axial magnetization high can be traced throughout the survey area but varies considerably along axis marked changes in the amplitude of axial magnetization are often accompanied by offsets in its along axis linearity e g at 9 25 9 37 and 9 46 n on the basis of comparisons with seismic reflection data and geochemical measurements on basalts dredged from this rise area we argue that the along axis variation in the axial magnetic high is the combined result of variations in the thickness of the magnetic source layer and in the magnetization of the layer caused by variations in the iron and titanium abundances in the extrusive upper crust these two explanations are probably coupled in that a thicker source layer arises from a deeper axial magma chamber which is more prone to the production of evolved magmas rich in iron and titanium because crustal magnetization represents a time integrated signal averaged over a substantial volume of rock the observation of offsets in the linearity of the axial magnetization high implies that such features which we suggest mark boundaries between distinct magmatic systems within the axial crust are stable on timescales of at least several tens of thousands of years a distinct pattern of short wavelength 5 km magnetization anomalies parallel to the rise axis to the north of 9 25 n correlates well with short reversed subchrons within the brunhes chron although independent observations are needed to confirm this explanation the correlation offers the potential that the determination of fine scale magnetization anomalies may prove useful for investigating temporal details of crustal evolution at fast spreading ridges