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

seismic structure of the mid atlantic ridge 8 9 s 1 the mid atlantic ridge at 8 9 s is characterized by a transition from axial valley to axial high and recent episodes of ridge jumping and ridge propagation we present constraints on the structure of 0 4 ma crust in this region on the basis of the analysis of wide angle seismic data from a grid of profiles across and parallel to the current and abandoned spreading centers a 350 800 m thick oceanic layer 2a interpreted as high porosity extrusive basalts is underlain by a 2 0 2 5 km layer 2b with velocities which increase with age and decrease in the vicinity of the pseudofaults layer 3 velocities are uniform across the area except for a possible localized anomaly at the ridge axis the crustal thickness varies from 6 7 km near the pseudofaults formed by ridge propagation to 9 10 km at the segment center of the recently 0 3 ma abandoned spreading center seismically determined crustal thickness and density variations and age related lithospheric cooling can plausibly account for all observed variations in gravity across the area and there is no requirement for the thicker crust at the segment center to be underlain by hot mantle the transition from axial valley to axial high occurs at a crustal thickness of 8 km

a slab in depth three dimensional geometry and evolution of the indo australian plate geoscientists use plate tectonics to explain many aspects of both continental evolution and evolution of the planet as a whole the subduction of material at convergent plate boundaries forms a fundamental component to the theory of plate tectonics plates continents subduction zones and spreading centers all exhibit motion and geometric evolution so to try and resolve the past geometries of the planet geologists have utilized plate tectonic reconstructions here we present a three dimensional image of the subducted indo australian plate below southeast asia and show that the geometry of the subducted slab at depth is intimately related to the geometric evolution of se asia over the past 50 ma including the collision of india with the asian continent we show how the once semicontinuous subducting indo australian plate has been segmented during collision between india australian and the subduction margin to the north thus we have found that the geometry of the subducted plate should form a key component to the interpretation of the evolution of earth s surface as complexities and evolution of the subducted plate are manifest in the evolution of the overriding plate

half graben versus large offset low angle normal fault importance of keeping cool during normal faulting 1 we study the effect of temperature in the crust on the style of faulting during the extension of a model lithosphere two distinct fault patterns are seen depending on whether the brittle layer thickness hl is greater or less than a critical value hc for a thin layer hl hc a rift develops with multiple faults bounding an evolving set of grabens numerical results show that the value of hc depends on how the thermal structure and therefore the thickness distribution of the lithosphere evolve during extension in the region where the displacement occurs along the fault the brittle lithosphere thins and we show that this promotes development of multiple faults in models with heat transfer only by rock motion and conduction the advective thinning effect can greatly reduce hc with rift velocities greater than 1 cm yr 1 hc is reduced by a factor of 5 to unreasonably small values however inclusion of simulated hydrothermal cooling increases hc this allows large offsets to develop in model lithospheres with brittle layers of order 10 km thick extension of layers thicker than 10 km tends to produce a rift with multiple graben structures finally we compare the result of a simplified analytic model of extension to data we find that the style of faulting seen in a variety of continental rifts is broadly consistent with our model predictions

variable crustal structure along the juan de fuca ridge influence of on axis hot spots and absolute plate motions multichannel seismic and bathymetric data from the juan de fuca ridge jdfr provide constraints on axial and ridge flank structure for the past 4 8 ma within three spreading corridors crossing cleft northern symmetric and endeavour segments along axis data reveal south to north gradients in seafloor relief and presence and depth of the crustal magma lens which indicate a warmer axial regime to the south both on a regional scale and within individual segments for young crust cross axis lines reveal differences between segments in moho two way traveltimes of 200 300 ms which indicate 0 5 1 km thicker crust at endeavour and cleft compared to northern symmetric moho traveltime anomalies extend beyond the 5 15 km wide axial high and coincide with distinct plateaus 32 and 40 km wide and 200 400 m high found at both segments on older crust moho traveltimes are similar for all three segments 2100 100 ms indicating little difference in average crustal production prior to 0 6 and 0 7 ma the presence of broad axis centered bathymetric plateau with thickened crust at cleft and endeavour segments is attributed to recent initiation of ridge axis centered melt anomalies associated with the cobb hot spot and the heckle melt anomaly increased melt supply at cleft segment upon initiation of axial volcano and southward propagation of endeavour segment during the brunhes point to rapid southward directed along axis channeling of melt anomalies linked to these hot spots preferential southward flow of the cobb and heckle melt anomalies and the regional scale south to north gradients in ridge structure along the jdfr may reflect influence of the northwesterly absolute motion of the ridge axis on subaxial melt distribution

microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment we report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the mid atlantic ridge segment at 29 n and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes the seismic velocity structure is obtained from two dimensional 2 d and three dimensional 3 d tomographic inversions of travel times from shots along an axial refraction line inversion solutions indicate that the velocity structure in the lower crust is heterogeneous with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high the thickness of the lower crust at the segment end is asymmetric across axis with thinner crust beneath the inside corner the indicated variations in crustal thickness are consistent with those inferred from mantle bouguer gravity anomalies the microearthquakes located along axis during the 41 day recording period cluster in three separate along axis regions 1 the southern segment end near 28 55 n 2 the central along axis topographic high at 29 11 n near and north of the broken spur hydrothermal vent field and 3 a region midway between beneath a volcano near 29 02 n the greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset this pattern of off axis microearthquake activity and the cross axis asymmetry in crustal thickness at the segment end support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions anomalous focal mechanisms for microearthquakes beneath the along axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center in contrast may be the result of volcanic and hydrothermal processes such as magma movement or thermal stresses generated near cooling plutons a comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust along axis well resolved focal depths determined with a 3 d velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center these observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state

discovery of a magma chamber and faults beneath a mid atlantic ridge hydrothermal field crust at slow spreading ridges is formed by a combination of magmatic and tectonic processes with magmatic accretion possibly involving short lived crustal magma chambers the reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast spreading centres but it has been difficult to image such magma chambers beneath slow spreading centres owing to rough seafloor topography and associated seafloor scattering in the absence of any images of magma chambers or of subsurface near axis faults it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow spreading ridges here we report the presence of a crustal magma chamber beneath the slow spreading lucky strike segment of the mid atlantic ridge the reflection from the top of the magma chamber centred beneath the lucky strike volcano and hydrothermal field is approximately 3 km beneath the sea floor 3 4 km wide and extends up to 7 km along axis we suggest that this magma chamber provides the heat for the active hydrothermal vent field above it we also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward dipping faults cutting through the volcanic edifice suggesting continuous interactions between tectonic and magmatic processes

role of melt supply in oceanic detachment faulting and formation of megamullions normal faults are ubiquitous on mid ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases numerous long lived detachment faults that form megamullions with large scale corrugations have been identifi ed on magma poor mid ocean ridges but recent studies suggest counterintuitively that they may be associated with elevated magmatism we present numerical models and geological data to show that these detachments occur when 30 50 of total extension is accommodated by magmatic accretion and that there is signifi cant magmatic accretion in the fault footwalls under these low melt conditions magmatism may focus unevenly along the spreading axis to create an irregular brittle plastic transition where detachments root thus explaining the origin of the enigmatic corrugations morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid ocean ridges

deep lithospheric structures along the southern central chile margin from wide angle p wave modelling p crustal and upper mantle structures of the subduction zone in south central chile between 42 degrees s and 46 degrees s are determined from seismic wide angle reflection and refraction data using the seismic ray tracing method to calculate minimum parameter models three profiles along differently aged segments of the subducting nazca plate were analysed in order to study subduction zone structure dependencies related to the age that is thermal state of the incoming plate the age of the oceanic crust at the trench ranges from 3 ma on the southernmost profile immediately north of the chile triple junction to 6 5 ma old about 100 km to the north and to 14 5 ma old another 200 km further north off the island of chiloe remarkable similarities appear in the structures of both the incoming as well as the overriding plate the oceanic nazca plate is around 5 km thick with a slightly increasing thickness northward reflecting temperature changes at the time of crustal generation the trench basin is about 2 km thick except in the south where the chile ridge is close to the deformation front and only a small 800 m thick trench infill could develop in south central chile typically three quarters 1 5 km of the trench sediments subduct below the decollement in the subduction channel to the north and south of the study area only about one quarter to one third of the sediments subducts the rest is accreted above similarities in the overriding plate are the width of the active accretionary prism 35 50 km and a strong lateral crustal velocity gradient zone about 75 80 km landward from the deformation front where landward upper crustal velocities of over 5 0 5 4 km s su 1 su decrease seaward to around 4 5 km s su 1 su within about 10 km which possibly represents a palaeo backstop this zone is also accompanied by strong intraplate seismicity differences in the subduction zone structures exist in the outer rise region where the northern profile exhibits a clear bulge of uplifted oceanic lithosphere prior to subduction whereas the younger structures have a less developed outer rise this plate bending is accompanied by strongly reduced rock velocities on the northern profile due to fracturing and possible hydration of the crust and upper mantle the southern profiles do not exhibit such a strong alteration of the lithosphere although this effect may be counteracted by plate cooling effects which are reflected in increasing rock velocities away from the spreading centre overall there appears little influence of incoming plate age on the subduction zone structure which may explain why the m w 9 5 great chile earthquake from 1960 ruptured through all these differing age segments the rupture area however appears to coincide with a relatively thick subduction channel

small scale structure of the kane oceanic core complex mid atlantic ridge 23 30 n from waveform tomography of multichannel seismic data 1 frequency domain waveform tomography applied to deep sea multichannel seismic data collected across the kane oceanic core complex occ reveals the small scale structure of this section of oceanic lithosphere formed at the mid atlantic ridge 3 3 2 1 myr ago that has been uplifted and exhumed at the seafloor along an oceanic detachment fault geological and geophysical studies indicate that the central eastern area of the kane occ is formed by a large gabbroic intrusion the new high resolution seismic velocity models show that this gabbroic core is 1 000 1 350 m thick and it is underlain by a 300 m thick layer of low velocity material interpreted as serpentinized peridotite the models also reveal the presence of a deeper 200 m thick by 1 700 m long high velocity body interpreted as a small gabbroic sill embedded in serpentinzed peridotite

modes of faulting at mid ocean ridges abyssal hill bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the earth the recognition that these faults form at plate spreading centres came with the plate tectonic revolution recent observations reveal a large range of fault sizes and orientations numerical models of plate separation dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast spreading centres conversely plate stretching with differing amounts of constant rate magmatic dyke intrusion can explain the great variety of fault offset seen at slow spreading ridges very large offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion

continuous exhumation of mantle derived rocks at the southwest indian ridge for 11 million years the global mid ocean ridge system where tectonic plates diverge is traditionally thought of as the largest single volcanic feature on the earth yet wide expanses of smooth sea floor in the easternmost part of the southwest indian ridge in the indian ocean lacks the hummocky morphology that is typical for submarine volcanism at other slow spreading ridges the sea floor can extend by faulting the existing lithosphere along only one side of the ridge axis however the smooth sea floor in the easternmost southwest indian ridge also lacks the corrugated texture created by such faulting instead the sea floor is smooth on both sides of the ridge axis and is thought to be composed of altered mantle derived rocks here we use side scan sonar to image the sea floor and dredge samples to analyse the composition of two sections of the southwest indian ridge between 62 050 e and 64 400 e where the sea floor formed over the past 11 million years we show that the smooth floor is almost entirely composed of seawater altered mantle derived rocks that were brought to the surface by large detachment faults on both sides of the ridge axis faulting accommodates almost 100 of plate divergence and the detachment faults have repeatedly flipped polarity we suggest that this tectonic process could also explain the exhumation of mantle derived rocks at the magma poor margins of rifted continents

variations in upper crustal structure due to variable mantle temperature along the southeast indian ridge 1 there is a systematic variation in axial morphology and axial depth along the southeast indian ridge seir with distance away from the australian antarctic discordance an area of cold uppermost mantle since spreading rate 72 76 mm yr and mantle geochemistry appear constant along this portion of the seir the observed variations in axial morphology and axial depth are attributed to a gradient in mantle temperature in this study we report results from a multichannel seismic investigation of on axis crustal structure along this portion of the seir three distinct forms of ridge crest morphology are found within our study area axial highs rifted axial highs and shallow axial valleys axial highs have a shallow 1500 m below seafloor bsf magma lens and a thin 300 m layer 2a along the ridge crest rifted axial highs have a deeper 2100 m bsf magma lens and thicker 450 m layer 2a on axis beneath shallow axial valleys no magma lens is imaged and layer 2a is thick 450 m there are step like transitions in magma lens depth and layer 2a thickness with changes in morphology along the seir the transitions between the different modes of axial morphology and shallow structure are abrupt suggesting a threshold type mechanism variations in crustal structure along the seir appear to be steady state persisting for at least 1 m y portions of segments in which a magma lens is found are characterized by lower relief abyssal hills on the ridge flank shallower ridge flank depths and at the location of along axis mantle bouguer anomaly mba lows the long wavelength variation in ridge morphology along the seir from axial high segments to the west to axial valley segments to the east is linked to the regional gradient in mantle temperature superimposed on the long wavelength trend are segment to segment variations that are related to the absolute motion of the seir to the northeast which influence mantle melt production and delivery to the ridge

evidence from the asymmetry of fast spreading ridges that the axial topographic high is due to extensional stresses along fast spreading mid ocean ridges such as the east pacific rise there is an axial topographic high 5 20 km wide which stands 200 400 m above the background slope caused by thermally induced seafloor subsidence there are also smaller topographic lows flanking the axial high along most of the east pacific rise from 20 s to 15 n the existence of these lows is predicted by models of the origin of the axial high one model postulates that the axial high is created by buoyant uplift from a narrow zone ofconcentrated partial melt extending tens of kilometres down into the mantle1 2 3 4 another model requires no such buoyant zone suggesting instead that the axial high is generated by dynamic extensional stresses in the lithosphere and shallow asthenosphere5 here we show that the observed asymmetry of the flanking lows can be used to distinguish between these two proposed mechanisms although either model can be adapted to match the asymmetry on individual profiles the along axis variation in degree of symmetry favours the model of dynamic extensional stresses for the origin of the axial high and its flanking lows

microseismicity of the mid atlantic ridge at 7 s 8 15 s and at the logatchev massif oceanic core complex at 14 40 n 14 50 n lithospheric formation at slow spreading rates is heterogeneous with multiple modalities favoring symmetric spreading where magmatism dominates or core complex and inside corner high formation where tectonics dominate we report microseismicity from three deployments of seismic networks at the mid atlantic ridge mar two networks surveyed the mar near 7 degrees s in the vicinity of the ascension transform fault three inside corner high settings were investigated however they remained seismically largely inactive and major seismic activity occurred along the center of the median valley in contrast at the logatchev massif core complex at 14 degrees 45n seismicity was sparse within the center of the median valley but concentrated along the eastern rift mountains just west of the serpentine hosted logatchev hydrothermal vent field to the north and south of the massif however seismic activity occurred along the ridge axis emphasizing the asymmetry of seismicity at the logatchev segment focal mechanisms indicated a large number of reverse faulting events occurring in the vicinity of the vent field at 3 5 km depth which we interpret to reflect volume expansion accompanying serpentinization at shallower depth of 2 4 km some earthquakes in the vicinity of the vent field showed normal faulting behavior suggesting that normal faults facilitates hydrothermal circulation feeding the vent field further a second set of cross cutting faults occurred indicating that the surface location of the field is controlled by local fault systems

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

crustal structure of the peruvian continental margin from wide angle seismic studies keine zusammenfassung in englischer sprache vorhanden no summary in english language

accretion mode of oceanic ridges governed by axial mechanical strength oceanic spreading ridges exhibit structural changes as a function of spreading rate mantle temperature and the balance of tectonic and magmatic accretion the role that these or other processes have in governing the overall shape of oceanic ridges is unclear here we use laboratory experiments to simulate ridge spreading in colloidal aqueous dispersions whose rheology evolves from purely viscous to elastic and brittle when placed in contact with a saline water solution we find that ridge shape becomes increasingly linear with spreading rate until reaching a minimum tortuosity this behaviour is predicted by the axial failure parameter f a dimensionless number describing the balance of brittle and plastic failure of axial lithosphere slow spreading fault dominated and fast spreading fluid intrusion dominated ridges on earth and in the laboratory are separated by the same critical f value suggesting that the axial failure mode governs ridge geometry values of f can also be calculated for different mantle temperatures and applied to other planets or the early earth for higher mantle temperatures during the archaean our results preclude the predicted formation of large tectonic plates at high spreading velocity the shape of mid ocean ridges is influenced by lithospheric mechanical strength according to laboratory simulations of diverging plates the results imply that large tectonic plates probably could not have formed on a younger hotter earth

the 3 d geometry of detachment faulting at mid ocean ridges seismic images of cretaceous slow spreading crust from the eastern central atlantic provide new constraints on the process of seafloor spreading and the importance of detachment faulting the seismic depth sections image detachment faults that appear to have exhumed footwall massifs of similar geometry to massifs of plutonic and mantle rocks mapped at the present mid atlantic ridge the detachments are consistent with the structure and microearthquakes of the mid atlantic ridge at 26 n with the footwall rotation inferred from paleomagnetic data and with numerical modeling of oceanic detachments other seismically imaged detachments have similar dimensions and geometry but are covered by a layer of small fault blocks the detachment types differ in whether or not the fault locks up in the subsurface probably controlled by the fault strength the elastic thickness and whether the exhumed footwall is partly covered by basalts toward the segment middle decreasing mantle serpentinization decreasing elastic thickness and thicker median valley basalts all increase the likelihood that the fault locks up and a new fault propagates upwards from the still active root zone transferring a slice of the hanging wall to the footwall to be rafted with the footwall out of the median valley as a result an oceanic detachment fault exhuming the footwall at a segment end to form an oceanic core complex may disappear laterally beneath rafted blocks detachment faulting may be more widespread at slow spreading ridges than interpreted from seafloor mapping

oceanic detachment faults generate compression in extension in extensional geologic systems such as mid ocean ridges deformation is typically accommodated by slip on normal faults where material is pulled apart under tension and stress is released by rupture during earthquakes and magmatic accretion however at slowly spreading mid ocean ridges where the tectonic plates move apart at rates 244 000 events detected along 25 km of the ridge axis to depths of 10 km below seafloor surprisingly the majority of these were reverse faulting events restricted to depths of 3 7 km below seafloor these reverse events delineate a band of intense compressional seismicity located adjacent to a zone of deeper extensional events this deformation pattern is consistent with flexural models of plate bending during lithospheric accretion our results indicate that the lower portion of the detachment footwall experiences compressive stresses and deforms internally as the fault rolls over to low angles before emerging at the seafloor these compressive stresses trigger reverse faulting even though the detachment itself is an extensional system

evidence of an axial magma chamber beneath the ultraslow spreading southwest indian ridge ultraslow spreading ridges are a novel class of spreading centers symbolized by amagmatic crustal accretion exposing vast amounts of mantle derived peridotites on the seafloor however distinct magmatic centers with high topographies and thick crusts are also observed within the deep axial valleys this suggests that despite the low overall melt supply the magmatic process interacting with the tectonic process should play an important role in crustal accretion however this has been obscured due to the lack of seismic images of magma chambers using a combination of seismic tomography and full waveform inversion of ocean bottom seismometer data from the southwest indian ridge at 50 28 e we report the presence of a large low velocity anomaly lva 4 9 km below the seafloor representing an axial magma chamber amc in the lower crust this suggests that the 9 5 km thick crust here is mainly formed by a magmatic process the lva is overlain by a high velocity layer possibly forming the roof of the amc and defining the base of hydrothermal circulation the steep velocity gradient just below the high velocity layer is explained by the ponding of magma at the top of the amc this could provide the overpressure for lateral dike propagation along the ridge axis leading to a complex interaction between magma emplacement tectonic and hydrothermal processes and creating a diversity of seafloor morphology and extremely heterogeneous crust

contrast in crustal structure across the clipperton transform fault from travel time tomography a three dimensional 3 d seismic refraction study of the clipperton transform fault northern east pacific rise reveals anomalously low compressional velocities from the seafloor to the moho we attribute this low velocity anomaly to intensive brittle deformation caused by transpression across this active strike slip plate boundary the seismic velocity structure south of the clipperton transform appears unaffected by these tectonic forces but to the north seismic velocities are reduced over 10 km outside the zone of sheared seafloor this contrast in seismic velocity structure corresponds well with the differences in mid ocean ridge morphology across the clipperton transform we conclude that the amount of fracturing of the upper crust which largely controls seismic velocity variations is strongly dependent on the shallow temperature structure at the ridge axis intermittent supply of magma to the shallow crust north of the clipperton transform allows seawater to penetrate deeper and the cooler crust is brittle to a greater depth than south of the transform where a steady state magma lens is known to exist the crustal thickness averages 5 7 km only slightly thinner than normal for oceanic crust and variations in moho depth in excess of 0 3 km are not required by our data the absence of large crustal thickness variations and the general similarity in seismic structure imply that a steady state magma lens is not required to form normal east pacific rise type crust perhaps a significant portion of the lower crust is accreted in situ from a patchwork of short lived gabbro sills or from ductile flow from a basal magma chamber as has been postulated in some recent ophiolite studies

magmatic controls on axial relief and faulting at mid ocean ridges abstract previous models do not simultaneously reproduce the observed range of axial relief and fault patterns at plate spreading centers we suggest that this failure is due to the approximation that magmatic dikes open continuously rather than in discrete events during short lived events dikes open not only in the strong axial lithosphere but also some distance into the underlying weaker asthenosphere axial valley relief affects the partitioning of magma between the lithosphere and asthenosphere during diking events the deeper the valley the more magma goes into lithospheric dikes in each event and so the greater the average opening rate of those dikes the long term rate of lithospheric dike opening controls faulting rate and axial depth the feedback between axial valley depth d and lithospheric dike opening rate allows us to analytically relate steady state values of d to lithospheric thickness h l and crustal thickness h c a two dimensional model numerical model with a fixed axial lithospheric structure illustrates the analytic model implications for axial faulting the predictions of this new model are broadly consistent with global and segment scale trends of axial depth and fault patterns with h l and h c

neogene stratigraphy of the submerged u s atlantic margin abstract thirty boreholes and several hundred sea floor samples provide a detailed but incomplete record of neogene strata and depositional environments along the submerged part of the united states atlantic margin three major sedimentary basins the blake plateau basin the baltimore canyon trough and the georges bank basin contain neogene sedimentary prisms as thick as 1200 m comprising bathyal marine to nonmarine strata middle miocene rocks compose the most widely represented and thickest unit 600 m in the baltimore canyon trough calcareous microfossils provide excellent biostratigraphic and paleoenvironmental indicators except in some sparsely fossilferous shallow water intervals and where shallow water diatomaceous assemblages predominate however in outer sublittoral assemblages of the blake plateau basin and baltimore canyon trough the co occurrence of abundant diatoms radiolarians and planktic foraminifera provide an excellent opportunity for intercalibration of biozones preliminary correlations between chronostratigraphic and paleoenvironmental interpretations indicate a close association of depositional sequences with sea level fluctuations but some local exception persist

influence of magma supply and spreading rate on crustal magma bodies and emplacement of the extrusive layer insights from the east pacific rise at lat 16 n seismic reflection data from the east pacific rise at lat 16 n which is spreading at the high end of intermediate rates suggest that the depths at which axial magma chambers reside do not vary smoothly as a function of spreading rate rather magma chamber depths form two distinct populations each associated with a distinct axial morphology and with an abrupt transition occurring within the intermediate spreading rate range our data 1 show that the melt lens at high intermediate spreading ridges lies at a shallow level similar to lens depths at faster spreading ridges and 2 provide further support for the spreading rate invariance of ridges with axial highs noted in other ridge properties the axial morphology of the two ridge segments within the study area differs markedly and a large contrast in magma supply is inferred the ridge segment with greater magma supply is associated with a broader and more continuous melt lens a wider region over which the extrusive crust accumulates and a thicker extrusive layer off axis where supply to the ridge segment appears to be centered however on axis the extrusive layer is thinnest where magma supply is robust and a shallower melt lens is observed consistent with a model in which magma pressure controls the thickness of the extrusive layer accumulated above the magma lens

length and timescales of rift faulting and magma intrusion the afar rifting cycle from 2005 to present although fault and magmatic processes have achieved plate spreading at mid ocean ridges throughout earth s history discrete rifting episodes have rarely been observed this paper synthesizes ongoing seismic structural space based geodetic and petrologic studies from the subaerial red sea rift in ethiopia where a major rifting episode commenced in september 2005 our aims are to determine the length and timescales of magmatism and faulting the partitioning of strain between faulting and magmatism and their implications for the maintenance of along axis segmentation most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the dabbahu manda hararo rift segment strain is accommodated primarily by axial dike intrusions fed from mid segment magma chamber s these findings show that episodic approximate century interval rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation the scale 65 km 8 km and intensity of crustal

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data

crustal thickness along the western galapagos spreading center and the compensation of the galapagos hotspot swell wide angle refraction and multichannel reflection seismic data show that oceanic crust along the galapagos spreading center gsc between 97 w and 91 25pw thickens by 2 3 km as the galapagos plume is approached from the west this crustal thickening can account for v52 of the 700 m amplitude of the galapagos swell after correcting for changes in crustal thickness the residual mantle bouguer gravity anomaly associated with the galapagos swell shows a minimum of 325 mgal near 92 15pw the area where the gsc is intersected by the wolf darwin volcanic lineament wdl the remaining depth and gravity anomalies indicate an eastward reduction of mantle density estimated to be most prominent above a compensation depth of 50 100 km melting calculations assuming adiabatic passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of v30 c the associated thermal expansion and increase in melt depletion reduce mantle densities but to a degree that is insufficient to explain the geophysical observations the largest density anomalies appear at the intersection of the gsc and the wdl our results therefore require the existence of compositionally buoyant mantle beneath the gsc near the galapagos plume possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the galapagos islands that is later transported to the gsc our estimate for the buoyancy flux of the galapagos plume 700 kg s 31 is lower than previous estimates while the total crustal production rate of the galapagos plume 5 5 m 3 s 31 is comparable to that of the icelandic and hawaiian plumes

magmatic and tectonic extension at the chile ridge evidence for mantle controls on ridge segmentation we use data from an extensive multibeam bathymetry survey of the chile ridge to study tectonomagmatic processes at the ridge axis specifically we investigate how abyssal hills evolve from axial faults how variations in magmatic extension influence morphology and faulting along the spreading axis and how these variations correlate with ridge segmentation the bathymetry data are used to estimate the fraction of plate separation accommodated by normal faulting and the remaining fraction of extension m is attributed primarily to magmatic accretion results show that m ranges from 0 85 to 0 96 systematically increasing from first order and second order ridge segment offsets toward segment centers as the depth of ridge axis shoals relative to the flanking highs of the axial valley fault spacing however does not correlate with ridge geometry morphology or m along the chile ridge which suggests the observed increase in tectonic strain toward segment ends is achieved through increased slip on approximately equally spaced faults variations in m along the segments follow variations in petrologic indicators of mantle melt fraction both showing a preferred length scale of 50 20 km that persists even along much longer ridge segments in comparison mean m and axial relief fail to show significant correlations with distance offsetting the segments these two findings suggest a form of magmatic segmentation that is partially decoupled from the geometry of the plate boundary we hypothesize this magmatic segmentation arises from cells of buoyantly upwelling mantle that influence tectonic segmentation from the mantle up

high sensitivity of ocean ridge thermal structure to changes in magma supply the galapagos spreading center abstract we explore the physical mechanisms for the observed apparent sensitivity of ridge axis topography and crustal magma systems to small changes in magma supply at intermediate spreading rates numerical experiments were carried out to simulate crustal temperature structure of the galapagos spreading center which spreads at intermediate spreading rates with its various sections appear to have been influenced to different degrees by the nearby galapagos hotspot model results show a strong threshold effect as the crustal thickness decreases from 7 4 km at the 92 w area with an axial high westward to 6 0 km at the 94 w area with a transitional topography the depth to the top of a magma lens is calculated to increase from 1 7 to 2 5 km in contrast at the 97 w area where crustal thickness is only 5 6 km and a rift valley is present the model results predict no steady state magma lens in the crust these model calculations provide a simple physical explanation for the recent observations along the galapagos spreading center where abrupt changes in both magma lens and axial morphology occur within a short distance but crustal thickness changes only modestly results of this investigation illustrate the critical importance of hotspots in influencing mid ocean ridge crustal thickness and the associated changes in thermal structure especially for ridges that spread at the sensitive range of intermediate spreading rates

glacial cycles drive variations in the production of oceanic crust glacial cycles redistribute water between oceans and continents causing pressure changes in the upper mantle with consequences for the melting of earth s interior using plio pleistocene sea level variations as a forcing function theoretical models of mid ocean ridge dynamics that include melt transport predict temporal variations in crustal thickness of hundreds of meters new bathymetry from the australian antarctic ridge shows statistically significant spectral energy near the milankovitch periods of 23 41 and 100 thousand years which is consistent with model predictions these results suggest that abyssal hills one of the most common bathymetric features on earth record the magmatic response to changes in sea level the models and data support a link between glacial cycles at the surface and mantle melting at depth recorded in the bathymetric fabric of the sea floor

microearthquake seismicity of the mid atlantic ridge at 5 s a view of tectonic extension we report measurements made with an ocean bottom array which was operated for 10 days on the mid atlantic ridge just south of the 5 s transform fault fracture zone a total of 148 locatable earthquakes with magnitudes 0 5 2 8 were recorded seismic activity appears to be concentrated within the western half of the median valley the median valley seismic zone is bounded in along axis direction by the transform fault to the north and the tip of the axial volcanic ridge to the south a few scattered events occurred within the inside corner high on the transform fault and in the western sidewall close to the segment center earthquakes reach a maximum depth of 8 km below the median valley floor and appear to be predominantly in the mantle although a few crustal earthquakes also occurred the presence of earthquakes in the mantle indicates that it is not strongly serpentinized we infer the median valley seismic activity to primarily arise from normal faulting

seismicity and structure of the 85 e volcanic complex at the ultraslow spreading gakkel ridge from local earthquake tomography accretion mechanisms at ultraslow spreading ridges are still only poorly understood due to difficult survey conditions for seismic experiments at these ridges melts gets focused in distinct magmatic centres with thin crust in between at the 85 e 85 n volcanic complex at gakkel ridge arctic ocean where a recent spreading episode has been observed perennial sea ice cover challenges traditional investigations of seismic structure and microseismicity we used an unusual survey set up to gather seismological data during 16 days in july 2007 using seismometer arrays mounted on ice floes despite only 12 stations the drift of the ice floes over the survey area resulted in a multitude of crossing rays in the rift valley the data included 303 microearthquakes of which 248 events could be confidently located we compiled a 1d velocity model by localizing a subset of these earthquakes with a suite of randomly created velocity models in this model the moho is placed at 7 km depth below seafloor inferring a thick basaltic crust using 124 events which were recorded by at least two arrays we inverted for p wave velocity structure in a local earthquake tomography resolution tests indicate reliable results in the central rift valley illuminating the thermal structure underneath the asgard volcanic chain in the aftermath of its most recent spreading episode our results show microearthquake activity down to 16 km beneath seafloor inferring a cold lithosphere most hypocentres cluster at the centre of the rift valley at the site the asgard volcanic chain this may mean that existing thermal models for this class of ridges have to be refined an area of decreased seismic velocities crosses the rift valley at this location and microearthquake activity is located at its eastern fringe where the velocity gradient is highest we therefore speculate that the reduced velocities may be caused by warm intruded material and that the observed seismicity predominatly reflects the relaxation of thermal stresses following recent intrusion

magmatic and tectonic extension at mid ocean ridges 1 controls on fault characteristics we use 2 d numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction m of the total spreading rate i e m 1 for fully magmatic spreading heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust we examine a range of intrusion rates and axial thermal structures by varying m spreading rate and the efficiency of crustal cooling by conduction and hydrothermal circulation fault development proceeds in a sequential manner with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis fault spacing and heave are primarily sensitive to m and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis contrary to what is often cited in the literature but consistent with prior results of mechanical modeling we find that thicker axial lithosphere tends to reduce fault spacing and heave in addition fault spacing and heave are predicted to increase with decreasing rates of off axis lithospheric thickening the combination of low m particularly when m approaches 0 5 as well as a reduced rate of off axis lithospheric thickening produces long lived large offset faults similar to oceanic core complexes such long lived faults produce a highly asymmetric axial thermal structure with thinner lithosphere on the side with the active fault this across axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes

initiation of salt diapirs with frictional overburdens numerical experiments we have studied the initiation of salt diapirs with frictional overburdens under lateral compression and extension the overburden is assumed to be elasto plastic with mohr coulomb yield criterion and non associated flow rule plastic flow localizes into narrow shear bands which may represent faults the geometry and the evolution of these shear zones are simulated for various rheological parameters the dilational and frictional properties of frictional overburdens are found to be important controls on the types of shear zones formed in lateral compression and extension the friction and dilation angles influence the angle of shear and the width dip spacing and development history of conjugate zones of shear plus dilation fault zones that define wedges and grabens

why meter wide dikes at oceanic spreading centers abstract numerical models show that maximum dike width at oceanic spreading centers should scale with axial lithospheric thickness if the pre diking horizontal stress is close to the andersonian normal faulting stress and the stress is fully released in one dike intrusion dikes at slow spreading ridges could be over 5 m wide and maximum dike width should decrease with increasing plate spreading rate however data from ophiolites and tectonic windows into recently active spreading ridges show that mean dike width ranges from 0 5 m to 1 5 m and does not clearly correlate with plate spreading rate dike width is reduced if either the pre diking horizontal stress difference is lower than the faulting stress or the stress is not fully released by a dike partial stress release during a dike intrusion is the more plausible explanation and is also consistent with the fact that dikes intrude in episodes at iceland and afar partial stress release can result from limited magma supply when a crustal magma chamber acts as a closed source during dike intrusions limited magma supply sets the upper limit on the width of dikes and multiple dike intrusions in an episode may be required to fully release the axial lithospheric tectonic stress the observation of dikes that are wider than a few meters such as the recent event in afar indicates that large tectonic stress and large magma supply sometimes exist

seismic velocity constraints on the material properties that control earthquake behavior at the quebrada discovery gofar transform faults east pacific rise 1 mid ocean ridge transform faults rtfs vary strongly along strike in their ability to generate large earthquakes this general observation suggests that local variations in material properties along rtfs exert a primary control on earthquake rupture dynamics we explore these relationships by examining the seismic structure of two rtfs that have distinctly different seismic coupling we determine the seismic velocity structure at the gofar and quebrada faults on the east pacific rise epr using p wave traveltime tomography with data from two active source wide angle refraction lines crossing the faults we image low velocity zones lvzs at both faults where p wave velocities are reduced by as much as 0 5 1 0 km s 10 20 within a several kilometer wide region at the gofar fault the lvz extends through the entire crust into the seismogenic zone we rule out widespread serpentinization as an explanation for the low velocities owing to the lack of a corresponding signal in the locally measured gravity field the reduced velocities can be explained if the plate boundary region is composed of fault material with enhanced fluid filled porosity 1 5 8 local seismic observations indicate that the high porosity region lies within a 10 km long portion of the fault that fails in large swarms of microearthquakes and acts as a barrier to the propagation of large m 6 0 earthquakes tomographic images of fault structure combined with observed earthquake behavior suggest that epr transform segments capable of generating large earthquakes have relatively intact gabbro within the seismogenic zone whereas segments that slip aseismically or via earthquake swarms are composed of highly fractured 2 km wide damage zones that extend throughout the crust

flexure and thickening of the lithosphere at the east pacific rise we have formulated a new class of plate bending equations which simulates the flexure of the lithosphere at mid ocean ridges the stress accumulation effect in a growing moving and thickening lithosphere significantly changes the flexural behavior it is shown that the growing plate flexes more easily than the conventional static plate due to the fact that the bending stress vanishes on the base of the plate we apply this new plate model to the east pacific rise compared with madsen et al 1984 the fits to the gravity and topography observations are improved assuming the thickness varies in proportion to the square root of distance from the ridge crest the best fitting thickening rate for the growing plate model is 0 7 km1 2 or 5 5 km m y 1 2 which further implies that the 600 or 700 c isotherm marks the bottom boundary of the effective elastic thickness of the lithosphere

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

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

coupled mechanical and hydrothermal modeling of crustal accretion at intermediate to fast spreading ridges the genesis of oceanic crust at intermediate to fast spreading ridges occurs by the crystallization of mantle melts accumulated in at least one shallow melt lens situated below the ridge axis seismic reflection data suggest that the depth of this melt lens is inversely correlated with spreading rate and thereby magma supply the heat released in it by crystallization and melt injection is removed by a combination of hydrothermal cooling and diffusion due to the different time scales of hydrothermal cooling and crustal accretion numerical models have so far focused on only one of the two processes here we present the results from a coupled model that solves simultaneously for crustal accretion and hydrothermal cooling our approach resolves both processes within one 2d finite element model that self consistently solves for crustal mantle and hydrothermal flow the formation of new oceanic crust is approximated as a gabbro glacier in which the entire lower crust crystallizes in one shallow melt lens we find that the depth of the melt lens and the shape of hot potentially molten lower crust are highly dependent on the ridge permeability structure the predicted depth of the melt lens is primarily controlled by the permeability at the ridge axis whereas the off axis permeability determines the width of hot lower crust a detailed comparison of the modeling results with observed locations of the melt lens at intermediate to fast spreading ridges shows that only a relatively narrow range of crustal permeabilities is consistent with observations in addition we find significant deviations between models that resolve or parameterize hydrothermal cooling the predicted crustal thermal structures show major differences for models that predict the same melt lens location this illustrates the importance of resolving hydrothermal flow in simulations of crustal accretion

crustal structure of the southwest indian ridge at 66 e seismic constraints the southwest indian ridge represents a slow spreading end member of the global mid ocean ridge system and as such its structure places important constraints on models of melt supply and delivery from the mantle at ridges we present results from a wide angle seismic experiment conducted at the ridge axis at 66 e in a region that has comprehensive swath bathymetric gravity and magnetic data coverage and where the full spreading rate is 12 mm yr 1 based on these data the experiment traversed four spreading segments crustal thickness and velocity structure were determined along three intersecting profiles each 100 km long using shots from a 10 gun 71 l tuned airgun array towed at 15 m depth and fired at 40 s intervals recorded on three ocean bottom hydrophones on each profile obh data show high amplitude arrivals from oceanic layer 2 lower amplitude arrivals from layer 3 and wide angle reflections from the moho forward modelling and inversion of traveltime picks from these data show that the crust consists of a 1 5 2 5 km thick layer 2 with a high velocity gradient and a 0 5 3 0 km thick layer 3 with a low velocity gradient and a crustal thickness of 2 2 5 4 km additional constraints on the models come from 2 d modelling of gravity data along the profiles corrected for 3 d effects of off line bathymetry along axis the thickness of layer 2 varies little but layer 3 is thick at segment centres and very thin at segment boundaries along a flowline profile crustal thickness varies by up to 75 per cent from its minimum value in 3 myr the reduced crustal thickness is consistent with observations from very slow spreading ridge axes elsewhere and may be explained by conductive cooling of the upwelling mantle the large along axis variations in layer 3 thickness indicate that magmatic accretion is focused at segment centres and melt is delivered to segment ends perhaps only by lateral dyke propagation flowline variations in crustal thickness may result from episodicity of melt supply on timescales of 3 myr and by tectonic extension during amagmatic periods velocities at the top of layer 2 are poorly correlated with crustal age based on magnetic anomalies suggesting also that episodicity is decoupled between adjacent segments

relationship between spreading rate and the seismic structure of mid ocean ridges different parts of the global mid ocean ridge system create oceanic crust at rates that differ by more than a factor of ten the rates of supply of heat to these different systems must also vary substantially and it is this heat supply in the form of hot mantle material rising beneath the ridge axis that determines the nature of the magmatic tectonic and hydrothermal processes that control the production of new oceanic crust here we present evidence suggesting that a systematic relationship exists between the depth to zones of elevated temperature and partial melt beneath mid ocean ridges and their spreading rate recent seismic measurements provide robust determinations at six different locations of the depth to the top of the axial low seismic velocity zone which suggest that as full spreading rate decreases from 150 to 20 mm yr 1 the depth below the sea floor to the top of the lvz increases from 1 to 4 km although this relationship might arise in a number of ways our preferred explanation is that the major faults that extend to greater depths on slower spreading ridges1 allow a vigorous circulation of cooling sea water to reach greater depths blocking the upward migration of large melt bodies

magmatic and tectonic extension at mid ocean ridges 2 origin of axial morphology 1 we investigate the origin of mid ocean ridge morphology with numerical models that successfully predict axial topographic highs axial valleys and the transition between the two the models are time dependent simulating alternating tectonic and magmatic periods where far field extension is accommodated by faulting and by magmatism respectively during tectonic phases models predict faults to grow on either side of the ridge axis and axial height to decrease during magmatic phases models simulate magmatic extension by allowing the axial lithosphere to open freely in response to extension results show that fault size and spacing decreases with increasing time fraction spent in the magmatic phase fm magmatic phases also simulate the growth of topography in response to local buoyancy forces the fundamental variable that controls the transition between axial highs and valleys is the rise sink ratio fm ft t m where fm ft is the ratio of the time spent in the magmatic and tectonic periods and t m is the ratio of the characteristic rates for growing topography during magmatic phases 1 m and for reducing topography during tectonic phases 1 t models predict the tallest axial highs when fm ft t m 1 faulted topography without a high or valley when fm ft t m 1 and the deepest median valleys when fm ft m t 1 new scaling laws explain a global negative correlation between axial topography and lithosphere thickness as measured by the depths of axial magma lenses and microearthquakes exceptions to this trend reveal the importance of other behaviors such as a predicted inverse relation between axial topography and spreading rate as evident along the lau spreading center still other factors related to the frequency and spatial pervasiveness of magmatic intrusions and eruptions as evident at the mid atlantic and juan de fuca ridges influence the rise sink ratio fm ft t m and thus axial morphology

crustal structure of the trans atlantic geotraverse tag segment mid atlantic ridge 26 10 n implications for the nature of hydrothermal circulation and detachment faulting at slow spreading ridges new seismic refraction data reveal that hydrothermal circulation at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 10 n is not driven by energy extracted from shallow or mid crustal magmatic intrusions our results show that the tag hydrothermal field is underlain by rocks with high seismic velocities typical of lower crustal gabbros and partially serpentinized peridotites at depth as shallow as 1 km and we find no evidence for low seismic velocities associated with mid crustal magma chambers our tomographic images support the hypothesis of tivey et al 2003 that the tag field is located on the hanging wall of a detachment fault and constrain the complex dome shaped subsurface geometry of the fault system modeling of our seismic velocity profiles indicates that the porosity of the detachment footwall increases after rotation during exhumation which may enhance footwall cooling however heat extracted from the footwall is insufficient for sustaining long term high temperature hydrothermal circulation at tag these constraints indicate that the primary heat source for the tag hydrothermal system must be a deep magma reservoir at or below the base of the crust

mid ocean ridge seismicity reveals extreme types of ocean lithosphere along ultraslow spreading ridges where oceanic tectonic plates drift very slowly apart conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous along such spreading centres long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes hence melt supply lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest spreading ridges however owing to the lack of seismic data the lithospheric structure of ultraslow ridges is poorly constrained here we describe the structure and accretion modes of two end member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow spreading ridge axis we observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere but unusually contain earthquakes down to depths of 35 kilometres this observation implies a cold thick lithosphere with an upper aseismic zone that probably reflects substantial serpentinization we find that regions of magmatic lithosphere thin dramatically under volcanic centres and infer that the resulting topography of the lithosphere asthenosphere boundary could allow along axis melt flow explaining the uneven crustal production at ultraslow spreading ridges the seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought with important implications towards seafloor deformation and fluid circulation

effect of lithospheric geometry on rift valley relief 1 continental and mid oceanic rifts commonly show a rift valley bounded by normal faults and uplifted shoulders in this paper we numerically model the evolution of rift valley relief in a notched brittle lithosphere results show that the rift valley has a maximum valley relief which is controlled by the competition between isostasy and lithospheric geometry if the axial lithosphere is much thinner than the flanking lithosphere the isostatic force will break outward dipping faults on the axis and stop the deepening of the rift valley we call this the axial failure mode on the other hand if the thickness of axial lithosphere is close to that of the flanks flexural bending will break new faults on the flanks and stop the valley deepening we call this the flanking failure mode triggering of either mode gives the maximum valley depth and the selection is controlled by the interplay between isostasy and lithospheric geometry on the basis of these mechanisms we propose a scaling equation that relates the maximum valley depth to the geometry flanking thickness axial thickness and notch width and strength friction angle and cohesion of the lithosphere the model explains along axis variation of valley relief at mid oceanic ridges by changes of the axial lithosphere thickness

kinematics and geometry of active detachment faulting beneath the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge newly acquired seismic refraction and microearthquake data from the trans atlantic geotraverse tag segment of the mid atlantic ridge at 26 n reveal for the first time the geometry and seismic character of an active oceanic detachment fault hypocenters from 19 232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an 15 km long dome shaped fault surface that penetrates to depths 7 km below the seafloor on a steeply dipping 70 interface a tomographic model of compressional wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall which appears to roll over to a shallow dip of 20 5 and become aseismic at a depth of 3 km outboard of the detachment the exhumed lithosphere is deformed by ridge parallel antithetical normal faulting our results suggest that hydrothermal fluids at the tag field exploit the detachment fault to extract heat from a region near the crust mantle interface over long periods of time

seismic observation of an extremely magmatic accretion at the ultraslow spreading southwest indian ridge the oceanic crust is formed by a combination of magmatic and tectonic processes at mid ocean spreading centers under ultraslow spreading environment however observations of thin crust and mantle derived peridotites on the seafloor suggest that a large portion of crust is formed mainly by tectonic processes with little or absence of magmatism using three dimensional seismic tomography at an ultraslow spreading southwest indian ridge segment containing a central volcano at 50 28 e here we report the presence of an extremely magmatic accretion of the oceanic crust our results reveal a low velocity anomaly 0 6 km s in the lower crust beneath the central volcano suggesting the presence of partial melt which is accompanied by an unusually thick crust 9 5 km we also observe a strong along axis variation in crustal thickness from 9 5 to 4 km within 30 50 km distance requiring a highly focused melt delivery from the mantle we conclude that the extremely magmatic accretion is due to localized melt flow toward the central volcano which was enhanced by the significant along axis variation in lithosphere thickness at the ultraslow spreading southwest indian ridge

three dimensional seismic structure of the mid atlantic ridge 35 n evidence for focused melt supply and lower crustal dike injection 1 we gathered seismic refraction and wide angle reflection data from several active source experiments that occurred along the mid atlantic ridge near 35 n and constructed three dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness the tomographic images reveal anomalously thick crust 8 9 km and a low velocity bull s eye from 4 to 10 km depth beneath the center of the ridge segment the velocity anomaly is indicative of high temperatures and a small amount of melt up to 5 and likely represents the current magma plumbing system for melts ascending from the mantle in addition at the segment center seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust our results also indicate that the segment ends are colder receive a reduced magma supply and undergo significantly greater tectonic stretching than the segment center

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

opposing trends in crustal thickness and spreading rate along the back arc eastern lau spreading center implications for controls on ridge morphology faulting and hydrothermal activity abstract along back arc ridges located near the arc volcanic front crustal thickness is observed to vary systematically with arc proximity independent of spreading rate this effect is especially well expressed by the 400 km long eastern lau spreading center elsc where approaching the arc volcanic front from 100 to 40 km crustal thickness nearly doubles 5 5 9 km as spreading rates decrease by more than half 97 39 mm yr the crustal thickness variations at the elsc appear to result primarily from changes in mantle wedge composition caused by subduction we investigated the effects of these large and opposed crustal thickness and spreading rate variations on ridge morphology faulting and hydrothermal activity as part of the first phase of ridge2000 integrated studies in the lau back arc basin we used deep towed side scan sonar instruments dsl120a and imi30 to continuously map the near axis region within broader coverage ship multibeam bathymetry and acoustic imagery swaths an array of optical sensors maprs concurrently surveyed the near bottom water for hydrothermal plume anomalies hydrocasts made at identified plume sites confirmed their hydrothermal origin the data show that as spreading rates increase and crustal thicknesses decrease along the elsc 1 morphology transitions from shallow peaked volcanic highs to a deeper flat axis 2 faults become larger and more widely spaced 3 hydrothermal activity as measured by plume incidence increases and appears to reach levels higher than the global mid ocean ridge trend with spreading rate the observations indicate that crustal thickness magma supply has a greater control on ridge morphology and faulting than spreading rate even at fast rates where the thermal lithosphere should be thin however heat input from mantle advection proportional to spreading rate perhaps in combination with increased fault permeability appears to have a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge although subduction effects give rise to the opposed trends in crustal thickness and spreading rate at the elsc the observed effects may have generic implications for controls of seafloor spreading characteristics at mid ocean ridges

rift topography linked to magmatism at the intermediate spreading juan de fuca ridge new seismic observations of crustal structure along the juan de fuca ridge indicate that the axial rift topography reflects magma induced deformation rather than alternating phases of magmatism and tectonic extension as previously proposed contrary to predictions of the episodic models crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2 1 2 6 km the shallow rift valley or axial graben associated with each juan de fuca segment is 50 200 m deep and 1 8 km wide and is well correlated with a magma body in the subsurface analysis of graben dimensions height and width shows that the axial graben narrows and graben height diminishes where the magma body disappears rather than deepening and broadening as expected for rift topography due to tectonic extension we propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor in this model an evolving axial topography results from feedbacks between the rheology of the crust above a magma sill and dike intrusion rather than episodic magma delivery from the mantle

microearthquake seismicity and focal mechanisms at the rodriguez triple junction in the indian ocean using ocean bottom seismometers hypocenters and focal mechanisms of microearthquakes have been investigated at the rodriguez triple junction in the indian ocean little was known on microearthquake activity in this region we deployed 18 ocean bottom seismographs during the kh93 3 cruise of the r v hakuho maru ocean research institute university of tokyo from july 30 to august 20 1993 we obtained 579 well constrained hypocenters and 13 focal mechanisms microearthquakes were found to be active along all of the three ridges the central indian ridge the southeastern indian ridge and the southwestern indian ridge especially at the triple junction there was an earthquake swarm within narrow area of approximately 15 5 km2 all of the 13 focal mechanisms showed normal or strike slip faultings which means that the extensional stress field characterizes this region

seismic constraints on caldera dynamics from the 2015 axial seamount eruption seismic observations in volcanically active calderas are challenging a new cabled observatory atop axial seamount on the juan de fuca ridge allows unprecedented real time monitoring of a submarine caldera beginning on 24 april 2015 the seismic network captured an eruption that culminated in explosive acoustic signals where lava erupted on the seafloor extensive seismic activity preceding the eruption shows that inflation is accommodated by the reactivation of an outward dipping caldera ring fault with strong tidal triggering indicating a critically stressed system the ring fault accommodated deflation during the eruption and provided a pathway for a dike that propagated south and north beneath the caldera s east wall once north of the caldera the eruption stepped westward and a dike propagated along the extensional north rift

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

a numerical model of hydrothermal cooling and crustal accretion at a fast spreading mid ocean ridge 1 there are two competing models for lower crustal accretion at fast spreading ridges in the gabbro glacier model all the lower crust solidifies in a magma lens at the dike gabbro boundary and then flows to its final level in the sheeted sill model the lower crust is formed by sills emplaced at a variety of depths we present a steady state numerical representation of the sill model that explicitly includes hydrothermal circulation the crust is accreted uniformly at the ridge axis at all depths and hydrothermal circulation occurs in crustal rocks that have cooled below a threshold temperature the results show that when the permeability exceeds a threshold value 4 10 14 m2 for a cracking temperature of 800 c and a uniform permeability hydrothermal circulation can cool the entire crust near the ridge axis producing vertical isotherms in the lower crust at lower permeabilities the properties of seawater lead to bimodal cooling with the lowermost crust cooling more slowly than shallower depths these results are reasonably consistent with studies of the seismic structure of the east pacific rise and of plagioclase crystal size distributions in gabbros from oman we infer that the sheeted sill model cannot be discarded on thermal grounds

crustal structure of the flanks of the east pacific rise implications for overlapping spreading centers tomographic inversion of seismic refraction data from the flanks of the east pacific rise epr 17 15 s shows that the thickness of layer 2 varies by as much as 500 meters off axis a thick layer 2 is found in crust affected by migration paths of overlapping spreading centers osc however no significant variation in crustal thickness is detected throughout the study area the crustal structure differences documented in this paper are primarily related to this paleo tectonic setting rather than the east west asymmetries characteristic of this region of the southern epr

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

sensitivity of seafloor bathymetry to climate driven fluctuations in mid ocean ridge magma supply recent studies have proposed that the bathymetric fabric of the seafloor formed at mid ocean ridges records rapid 23 000 to 100 000 years fluctuations in ridge magma supply caused by sealevel changes that modulate melt production in the underlying mantle using quantitative models of faulting and magma emplacement we demonstrate that in fact seafloor shaping processes act as a low pass filter on variations in magma supply strongly damping fluctuations shorter than about 100 000 years we show that the systematic decrease in dominant seafloor wavelengths with increasing spreading rate is best explained by a model of fault growth and abandonment under a steady magma input this provides a robust framework for deciphering the footprint of mantle melting in the fabric of abyssal hills the most common topographic feature on earth

mapping of seismic layer 2a 2b boundary above the sheeted dike unit at intermediate spreading crust exposed near the blanco transform 1 we present results from a study mapping the seismic layer 2a 2b boundary in young ocean crust located adjacent to the north wall of the blanco transform fault btf we review seafloor features geochemical and petrologic data seismic imaging and seismic structure and conclude that the btf seismic study region is representative of crust formed at the intermediate spreading juan de fuca ridge the mean layer 2a two way travel time beneath the seafloor is 0 37 0 10 s for the 42 12 km seismic survey area several regions are observed where layer 2a is consistently thin or thick over lateral distances of 5 10 km both in a ridge parallel and ridge perpendicular direction layer 2a thicknesses appear more variable in the ridge parallel isochron direction than the ridge perpendicular flow line direction there is no systematic pattern of layer 2a thickness variability with distance from the btf nor is there a correlation between seafloor topography and layer 2a thickness calculated mean layer 2a thickness for the seismic study region is 485 135 m assuming an interval velocity of 2 65 km s the imaged layer 2a 2b boundary projects on average 600 650 m above the top of the sheeted dike complex mapped on the adjacent btf north wall we argue that the geologic context of the layer 2a 2b boundary will vary as competing processes of magmatic construction and fracturing increasing porosity and crustal thickening compaction dike intrusion and hydrothermal sealing decreasing porosity vary as the crust spreads laterally from the ridge axis to ridge flanks

axial crustal structure of the lau back arc basin from velocity modeling of multichannel seismic data located west of the tonga trench the lau back arc basin is a prime environment for studying the interplay between oceanic spreading systems and an active subduction zone within the basin lie two complementary intermediate rate spreading systems the central lau spreading center clsc and the eastern lau spreading center valu fa ridge elsc vfr that are positioned 170 to 40 km away from the active volcanic arc respectively multichannel seismic mcs images of both systems reveal systematic variations in axial crustal structure primarily related to the proximity of the volcanic arc but also related to spreading rate morphology and petrology upper crustal refraction data selected from the along axis seismic lines collected during the 1999 mcs survey were modeled in both the time versus range t x and intercept time versus slowness p domains to provide validation and detail to the seismic reflection observations the results show that as both the proximity to the arc and the spreading rate decrease southward 1 seismic layer 2a thickens by 0 6 km between the clsc and vfr from 0 4 km to 1 0 km 2 the average depth of the axial magma chamber amc increases from 1 5 km at the clsc to 2 8 km at the southern vfr excluding the northern section of the elsc that shows no continuous amc reflector but does show an isolated melt sill 3 the upper crustal basement velocity decreases from 2 1 km s at the clsc to 1 8 km s at the vfr and 4 the velocities of both layer 2a and 2b decrease between the clsc and northern vfr from 3 2 to 2 5 km s and 5 0 to 3 9 km s respectively along with a broad axial high morphology these features of the clsc thinner layer 2a shallower amc and faster crustal velocities correlate best with the fast spreading east pacific rise and intermediate spreading juan de fuca ridge conversely the structural characteristics of the central elsc vfr have no known counterpart in the global mid ocean ridge system we attribute this primarily to the volatile concentration in the magmas coming from the tonga subduction zone

rift valley no rift valley transition at mid ocean ridges a simple mechanical model for the axial topographic and gravity variations with spreading rate is developed the model emphasizes the rapid transition from one mechanism to another with changing spreading rate our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis there exists a potential failure zone of width lf where the stresses within the brittle plate exceeds its yield strength if the yield strength of the brittle plate is nearly constant this width is largely determined by the strain rate and the viscosity of the strong mantle the high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region a critical feature in this model the size of the decoupling region ld is determined by the thickness of the oceanic crust and temperatures within the crust if the decoupling region is small ld lf as shown to be the case for fast spreading ridges then each of the two layers responds separately to the horizontal stretching the brittle plate will be broken only at the axis and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy the changing size of the decoupling region gives rise to the transition from atlantic type axial topography to pacific type as well as the transition from a high amplitude highly variable gravity signature to a low amplitude very uniform gravity signature parameters can be chosen so that such a transition occurs at a half rate of 35 mm yr which is the critical half rate suggested by both topographic and gravity data

accretional curvature of lithosphere at magmatic spreading centers and the flexural support of axial highs fluid magma should tend to rise up to the level of local isostatic equilibrium at an axis of plate spreading the magma and underlying partial melt accretes to the side of the separating plates and gets denser as it cools and freezes this load bends down the lithosphere and additional accretion forms a plate with concave upward curvature the topography resulting from a curved plate passively moving off axis can be described analytically for accretion on the vertical side of uniform lithosphere when its flexural response is that of a thin elastic plate model predictions are consistent with the shape of the axial highs at fast spreading centers typically 400m high and 20 km wide as well as related gravity anomalies this model requires at least several kilometers of lithospheric thickening within a few kilometers of the ridge axis seismic data appear consistent with this lithospheric structure the magnitude and lateral distribution of faulting on the flanks of axial highs is crudely consistent with formation during plate unbending as lithosphere moves away from the axis according to this model abandoned fast spreading centers should retain an elastically supported axial high while for previous models abandoned highs should collapse

abyssal hills formed by stretching oceanic lithosphere tectonic plates are formed and move apart at mid ocean ridges some portion of this plate separation process can occur by stretching of the crust resulting in a complex pattern of extensional faults abyssal hills the most ubiquitous topographic features on earth1 are thought to be a product of this faulting2 3 here we report the results of a self consistent numerical model of lithospheric formation and stretching that includes spontaneous fault creation in this model an axial valley develops where the fault activity is most concentrated the frozen fault generated topography rafted out of the axial valley is visually and statistically similar to observed abyssal hills formed at many slower spreading ridges faults appear to be replaced by new faults because their offset changes the local stress field we accordingly need no temporal variation in magmatism as required by some previous models4 5 6 to control the spacing or offset of faults our model results suggest instead that the irregularity of abyssal hill relief may result from a self organized critical stress state at spreading centres

crustal accretion at the reykjanes ridge 61 62 n we report results of a seismic gravity and magnetic survey of the reykjanes ridge spreading center at 61 62 n about 600 km from the center of the iceland mantle plume anomalously shallow water on the ridge crest enabled us to record seismic refractions on a 2 4 km hydrophone streamer the velocity within layer 2a is 2 4 0 3 km s 1 and its mean thickness is 400 100 m the velocity at the base of layer 2a is 3 3 0 3 km s 1 on the ridge axis increasing with crustal age to 4 0 km s 1 at 1 5 ma and 4 5 km s 1 at 5 ma assuming that seismic layer 2a on the ridge axis is also the extrusive layer i e the magnetic source layer we have successfully modeled the variations in amplitude of the magnetic field the best magnetic model includes enhanced magnetization within layer 2a at the sites of recent volcanic activity as independently recognized in side scan sonar data we also present a full crustal seismic model based on wide angle seismic recordings on digital ocean bottom hydrophones and disposable sonobuoys the seismic model is complemented by gravity modeling which further suggests that the ridge crest is in isostatic equilibrium the zero age crust is 10 0 km thick while crust of age 5 ma is 7 8 km thick these crustal thicknesses are greater than those of normal oceanic crust which we attribute to the presence of anomalously hot asthenospheric mantle beneath the spreading center we suggest that the variation in thickness between 0 ma and 5 ma crust is caused by temporal variation in the plume fed asthenospheric temperature beneath the reykjanes ridge

decrease in oceanic crustal thickness since the breakup of pangaea earth s mantle has cooled by 6 11 c every 100 million years since the archaean 2 5 billion years ago in more recent times the surface heat loss that led to this temperature drop may have been enhanced by plate tectonic processes such as continental breakup the continuous creation of oceanic lithosphere at mid ocean ridges and subduction at deep sea trenches here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time we find that oceanic crust formed in the mid jurassic about 170 million years ago is 1 7 km thicker on average than crust produced along the present day mid ocean ridge system if a higher mantle temperature is the cause of thicker jurassic ocean crust the upper mantle may have cooled by 15 20 c per 100 million years over this time period the difference between this and the long term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss we also find that the increase of ocean crustal thickness with plate age is stronger in the indian and atlantic oceans compared with the pacific ocean this observation supports the idea that upper mantle temperature in the jurassic was higher in the wake of the fragmented supercontinent pangaea due to the effect of continental insulation thicker oceanic crust forms from a hot mantle observations of unusually thick oceanic crust that formed 170 million years ago in the atlantic and indian oceans suggest that the ancient supercontinent pangaea helped insulate and warm the mantle

topographic controls on dike injection in volcanic rift zones abstract dike emplacement in volcanic rift zones is often associated with the injection of blade like dikes which propagate long distances parallel to the rift but frequently remain trapped at depth and erupt only near the tip of the dike over geologic time this style of dike injection implies that a greater percentage of extension is accommodated by magma accretion at depth than near the surface in this study we investigate the evolution of faulting topography and stress state in volcanic rift zones using a kinematic model for dike injection in an extending 2 d elastic viscoplastic layer we show that the intrusion of blade like dikes focuses deformation at the rift axis leading to the formation of an axial rift valley however flexure associated with the development of the rift topography generates compression at the base of the plate if the magnitude of these deviatoric compressive stresses exceeds the deviatoric tensile stress associated with far field extension further dike injection will be inhibited in general this transition from tensile to compressive deviatoric stresses occurs when the rate of accretion in the lower crust is greater than 50 60 of the far field extension rate these results indicate that over geologic timescales the injection of blade like dikes is a self limiting process in which dike generated faulting and topography result in an efficient feedback mechanism that controls the time averaged distribution of magma accretion within the crust

crustal thickness variations along the southeast indian ridge 100 116 e from 2 d body wave tomography 1 axial morphology along the southeast indian ridge seir systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100 116 e west of the australian antarctic discordance aad basalt geochemistry has a consistent indian mid ocean ridge basalt morb type isotopic signature so changes in axial topography are attributed to variations in both mantle temperature and melt supply wide angle seismic refraction lines were shot to four ocean bottom hydrophones within seir segments p1 p2 s1 and t where each segment is characterized by a different morphology we constructed 2 d crustal velocity models by jointly inverting hand picked p wave refraction pg and moho reflection pmp traveltime data using a top down minimum structure methodology the results show a 1 5 km eastward decrease in crustal thickness across the study area with segment averages ranging from 6 1 km at p1 to 4 6 km at t melt generation models require a 30 c decrease in mantle temperature toward the aad to account for the crustal thickness trend significant changes in axial morphology accompany small scale variations in crustal thickness consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature melt supply and cooling from hydrothermal circulation layer 3 thins by 3 0 km as layer 2 thickens by 1 4 km between segments p1 and t reflecting the eastward decrease in melt supply and increase in melt lens depth the trade off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material

seismic structure of the southern gulf of california from los cabos block to the east pacific rise 1 multichannel reflection and coincident wide angle seismic data collected during the 2002 premier experiment sea of cortez addressing the development of oblique rifting pescador experiment provide the most detailed seismic structure to date of the southern gulf of california multichannel seismic mcs data were recorded with a 6 km long streamer 480 channel aboard the r v maurice ewing and wide angle data was recorded by 19 instruments spaced every 12 km along the transect the mcs and wide angle data reveal the seismic structure across the continent ocean transition of the rifted margin typical continental and oceanic crust are separated by a 75 km wide zone of extended continental crust dominated by block faulted basement little lateral variation in crustal thicknesses and seismic velocities is observed in the oceanic crust suggesting a constant rate of magmatic productivity since seafloor spreading began oceanic crustal thickness and mean crustal velocities suggest normal mantle temperature 1300 c and passive mantle upwelling at the early stages of seafloor spreading the crustal thickness width of extended continental crust and predicted temperature conditions all indicate a narrow rift mode of extension on the basis of upper and lower crust stretching factors an excess of lower crust was found in the extended continental crust total extension along transect 5w is estimated to be 35 km following crustal extension new oceanic crust 6 4 km thick was formed at a rate of 48 mm a 1 to accommodate plate separation

crustal structure and magmato tectonic processes in an active rift asal ghoubbet afar east africa 2 insights from the 23 year recording of seismicity since the last rifting event 1 the asal ghoubbet ag rift has sustained a major volcano tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys it is therefore an ideal place to study the transient magmato tectonic processes that operate after a rifting episode we examine the space time evolution of 2500 md 2 8 earthquakes recorded in the rift from 1979 to 2001 we focus on the relationships between this seismic activity and both the three dimensional structure of the rift and its postrifting behavior depicted from geodesy the results highlight the major role of the central magmatic system fieale shark bay on the structure seismic activity and overall behavior of the rift from 1978 to 1986 the rift opens at a fast rate yet mainly aseismically the opening is magmatically driven and accommodated since 1986 when the opening rate abruptly decreased the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system these pulses result from that magmatic zone undergoing alternating stretching and inflating episodes thus while the plate driven induced stresses have been rebuilding in the rift since 1987 the rift opening is still essentially accommodated in the axial magmatic zone the ag rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode that transient opening has essentially occurred aseismically and most tectonic faults remain relaxed and locked

along and across axis variations in crustal thickness and structure at the mid atlantic ridge at 5 s obtained from wide angle seismic tomography implications for ridge segmentation two end member styles of crustal accretion are observed at two adjacent spreading segments at the mid atlantic ridge at 5 s focused accretion to the segment center with rapid crustal thinning toward the transform in the northern segment and crustal thickening toward the transform at an oceanic core complex in the southern segment our results were obtained by tomographic inversion of wide angle seismic reflection and refraction data collected along three intersecting profiles the segment north of the 5 s fracture zone is characterized by a well developed median valley with a pronounced seafloor bulge in the segment center a discrete portion of anomalously low velocities 0 4 to 0 5 km s relative to average off axis structure at depths of 2 5 km beneath this bulge is possibly related to the presence of elevated temperatures and perhaps small portions of partial melt this suggests that this segment is currently in a magmatically active period which is confirmed by the observation of fresh lava flows and ongoing high temperature hydrothermal activity at the seafloor close to the current spreading axis the crust thins rapidly from 8 5 km beneath the segment center to less than 3 km beneath the transform fault which indicates that melt supply here is strongly focused to the segment center the reduction in crustal thickness is almost exclusively accommodated by the thinning of velocity portions indicative of seismic layer 3 the transform fault is characterized by more uniform velocity gradients throughout the entire crustal section and very low upper mantle velocities of 7 2 7 3 km s indicating that serpentinization could be as much as 25 at 3 5 km depth in contrast 4 1 ma old crust of the northern segment shows only minor thinning from the segment center toward the segment end here the transform is characterized by a normal seismic layer 2 3 transition suggesting robust melt supply to the segment end at those times in the adjacent southern segment the crust thickens from 2 5 km beneath the flank of an oceanic core complex to 5 0 km at the segment boundary the observed changes in crustal thickness show a significant temporal and lateral variability in melt supply and suggest a more complex crustal emplacement process than predicted by models of focused melt supply to the segment centers