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

high b values in the leaky segment of the tjornes fracture zone north of iceland are they evidence for shallow magmatic heat sources near the seamount holl in the tjornes fracture zone tfz north of iceland two anomalies of high seismic b values have been found in a depth shallower than 10 km according to the results of similar b value anomalies beneath volcanic edifices such as mt st helens a magma chamber was suspected in the region on first view the existence of a long living magma body between the island of kolbeinsey and holl seamount appears to be confirmed by reflection seismic surveys conducted in the area with shallow water depths of 400 m they show a clear deep reflector however the reflector is best imaged in a migrated record at 1500 m s i e the speed of sound in water and it can be modeled by single scattering from the bathymetry in the area which leads to the conclusion that the apparent reflection images the seafloor rather than a magma chamber the recent volcanism along manareyjar ridge with the last eruption in 1868 69 and the hydrothermalism east of grimsey island would be trivially explainable by a magma chamber in the subsurface however there is no evidence for a magma chamber in this more southern part of the grimsey lineament gl even in a volcanic area such as the gl as the leaky part of the tfz neither the existence of high b values nor an apparent deep reflector prove the existence of a magma chamber instead the seismic imaging of magma chambers in areas of intermediate to slow spreading ridge areas appears to be questionable

morphology and genesis of slow spreading ridges seabed scattering and seismic imaging within the oceanic crust a grid of 32 across axis and five axis parallel multichannel seismic mcs reflection profiles were acquired at an axial volcanic ridge avr segment at 57 45 n 32 35 w on the slow spreading reykjanes ridge mid atlantic ridge to determine the along axis variation and geometry of the axial magmatic system and to investigate the relationship between magma chamber structure the along axis continuity and segmentation of melt supply to the crust the development of faulting and the thickness of oceanic layer 2a seismic reflection profiles acquired at mid ocean ridges are prone to being swamped by high amplitude seabed scattered noise which can either mask or be mistaken for intracrustal reflection events in this paper we present the results of two approaches to this problem which simulate seabed scatter and which can either be used to remove or simply predict events within processed mcs profiles the 37 mcs profiles show clear intracrustal seismic events which are related to the structure of oceanic layer 2 to the axial magmatic system and to the faults which dismember each avr as it ages through its tectono magmatic life cycle and which form the median valley walls the layer 2a event can be mapped around the entirety of the survey area between 0 1 and 0 5 s two way traveltime below the seabed being thickest at avr centres and thinning both off axis and along axis towards avr tips both avr parallel and ridge parallel trends are observed with the pattern of on axis layer 2a thickness variation preserved beneath relict avrs which are rafted off axis largely intact each active avr is underlain by a mid crustal melt lens reflection extending almost along its entire length similar reflection events are observed beneath the offset basins between adjacent avrs these are interpreted as new avrs at the start of their life cycle developing centrally within the median valley the east west spacings of relict avrs and offset basins is 5 7 km corresponding to a life span of the order of 0 5 0 7 myr during which avrs appear to undergo multiple 20 60 kyr tectono magmatic cycles

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

observed and modelled effects of ice lens formation on passive microwave brightness temperatures over snow covered tundra abstract immediately before an april 2007 snow survey and passive microwave radiometer field campaign in the northwest territories canada a rain on snow event deposited a thin 3 mm continuous layer of ice on the surface of the snowpack at eight sites the brightness temperature t b of the undisturbed snow pack was measured with a multi frequency dual polarization 6 9 19 37 and 89 ghz ground based radiometer system the ice lens was then carefully removed and the t b s were measured again the individual v pol channels and the 37 v 19 v difference were largely unaffected by the presence of the ice lens exhibiting a systematic shift of about 3 k in comparison the ice lens had a considerable effect on the h pol t b at all frequencies with a mean difference ice lens present ice lens removed of 9 k 5 3 k at 6 9 ghz 40 k 11 3 k at 19 ghz 33 k 7 6 k at 37 ghz and 19 k 8 0 k at 89 ghz the effect of the ice lens on h pol measurements was also observed with spaceborne data from the advanced microwave scanning radiometer amsr e satellite data simulations of t b were produced for each site using a new two layer formulation of the helsinki university of technology hut snow emission model the ice lens was used as the top layer and the underlying snowpack considered as a homogenous second layer the agreement between observations and simulations was variable with agreement strongest at 19 ghz a comparison with simulations produced using the microwave emission model of layered snowpacks memls suggests hut model uncertainty is related not to the ice lens but to difficulties in simulating emission from deep snow overall the observations and simulations suggest h pol measurements are capable of detecting new ice layers across the tundra snowpack while v pol measurements are more appropriate for snow water equivalent swe retrievals due to their relative insensitivity to ice layers

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

backscattered coherent noise and seismic reflection imaging of the oceanic crust an example from the rift valley of the mid atlantic ridge at 23 n seismic reflection surveys shot over oceanic crust are contaminated by source generated coherent noise that limits the identification and interpretation of subbasement reflections coherent noise arrivals are generated around the seismic line by scattering from the upper surface of the basaltic layer and by backscattering of turning waves from the underside of the same interface both types of arrival may be enhanced by conventional common midpoint cmp stacking some turning arrivals also exhibit negative moveout in a cmp gather due to the increase of crustal velocity with depth in unsedimented areas coherent arrivals scattered from the top of the seafloor can be aligned by dip moveout with or without a subsequent zero offset migration prestack and suppressed but turning arrivals will usually remain turning arrivals generated by linear features in the seafloor such as fault scarps appear on a stack section as nearly linear coherent arrivals the apparent dip of which is related to the features angle to the seismic line after migration these turning wave arrivals can extend close to the topography that produced them and continue to late times leading to the possibility that such arrivals might be incorrectly interpreted as intrusions that crosscut but do not offset the moho careful identification of crustal reflections on a seismic profile along the mark area rift valley indicates that the snake pit hydrothermal field marks the northern limit of identifiable crustal reflections at least some of which appear to be related to magma injection the recent extensive volcanism of the northern mark area may well be a result of faulting caused by the stong northward growth of the southern spreading center permitting melt to rise more easily to shallow crustal levels

heterogeneous seismic velocity structure of the upper lithosphere at kane oceanic core complex mid atlantic ridge the kane oceanic core complex occ is a large corrugated megamullion that was formed by a long lived detachment fault at the axis of the mid atlantic ridge adjacent to kane fracture zone between 2 1 and 3 3 ma we use refracted arrivals recorded along a 6 km long hydrophone streamer during a multichannel seismic survey to constrain the shallow seismic velocity structure of the occ results are presented in high resolution traveltime seismic tomographic models along six lines that cover all of the main morphological features of the megamullion the models show large lateral variability in p wave velocity within the upper 0 5 2 0 km of the lithosphere and these variations correlate to first order with observed variations in lithology documented by in situ basement samples and seafloor morphology lithological interpretation of the velocity models indicates that there is marked lateral variability in distribution of gabbroic intrusions serpentinized peridotites and basalts at scales of a few kilometers to 10 km serpentinized peridotites appear to dominate the central and older parts of the occ high velocity gabbros are consistently but not exclusively present closer to the termination of the kane detachment fault and toward the ends of the occ the structure of the lithosphere exhumed by the kane detachment fault is far from the standard ophiolite based penrose model and it does not show segment centered magmatism that is commonly interpreted at slow spreading ridges if the gabbros exhumed toward the termination of the occ were emplaced deep 10 km beneath the spreading axis they may have constituted a weak zone that focused initiation of the kane detachment fault alternately as the occ footwall was being exhumed the gabbros may have been emplaced because of dynamic changes in melt supply changes in mantle fertility or decompression melting late stage volcanism is clearly associated with a major high angle normal fault that cuts the detachment surface this volcanism may have been stimulated or enhanced by bending stresses in the bending footwall the shape of the large scale corrugated morphology of the occ is nearly invariant in the dip direction across major changes in basement lithology indicating that once established the form of the kane detachment fault was highly resistant to modification

seismic evidence for the stratified lithosphere in the south of the north china craton 1 the north china craton ncc formed in the paleoproterozoic and suffered cratonic destruction in the mesozoic it consists of the western ncc wncc the trans north china orogen tnco and the eastern ncc encc we investigated the upper mantle structures in the southern ncc by using receiver functions analysis polarization analysis was applied to increase the quality of receiver functions the seismic images show significant stratified structures in the upper mantle in the south of the ncc in the south of the wncc the lithosphere is 280 km thick and contains a low velocity zone at 110 220 km depth this low velocity zone is interpreted as an intracratonic partial melting zone in the south of the tnco and encc there is a low velocity zone at 60 200 km depth which results from the partial melting during the mesozoic cratonic destruction its upper boundary is interpreted as new lithosphere asthenosphere boundary at 60 100 km depth below this low velocity zone there are the remains of archean lithosphere ral blocks of ral stagnate in the asthenosphere and reach depths over 300 km a high velocity slab extends from the uppermost mantle of the wncc into the ral with a dip angle of 20 this slab is interpreted as the paleoproterozoic slab which indicates an eastward subduction between the wncc and the encc the wncc is stable the lithosphere in the south of the tnco and encc suffered the mesozoic cratonic destruction while the destruction is relatively weaker than that in the north of the encc

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

an analysis leading to the reduction of scattered noise on deep marine seismic records the rough and uneven basaltic basement in deep marine environments functions as a potential source of scattering the source generated noise not only interferes with the weak and deep primary reflections but often resembles primary reflections thereby confusing the seismic interpretation an analysis of the arrival time of scattered noise indicates that nmo and apparent dip of scattered noise exhibit azimuth dependence for isotropic scattering in line scattering at ranges near 0 degree azimuth and sideswipe from 90 degree azimuth should dominate the seismic section actual marine data displayed in various data gathers illustrate these characteristics of scattered noise and also correct a common misconception about the appearance of scattered noise the analysis suggests the use of cdp stacking and velocity filtering to attenuate the scattered noise the combined effect shows a total reduction of scattered noise by 21 db for the frequency bandwidth from 20 to 32 hz

magnetic studies of magma supply and sea floor metamorphism troodos ophiolite dikes abstract dikes of the eastern troodos ophiolite of cyprus intruded at slow ocean spreading axes with dips ranging up to 15 from vertical and with bimodal strikes now ne sw and n s due to post 88 ma sinistral microplate rotation varied dike orientations may represent local stress fields during dike crack propagation but do not influence the spatial distributions or orientation distributions of dikes magnetic fabrics nor of their palaeomagnetic signals anisotropy of magnetic susceptibility ams integrates mineral orientation distributions from each of 1289 specimens sampled from dikes at 356 sites over 400 km2 in the eastern troodos ophiolite of cyprus in 90 of dikes ams fabrics define a foliation kmax kint parallel to dike walls and a lineation kmax that varies regionally and systematically magma flow alignment of accessory magnetite controls the ams with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea floor metamorphism titanomagnetite has less influence on anisotropy occasionally intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility kmax still defines the magmatic flow axis such blended subfabrics of kinematically compatible mafic silicate and misaligned multidomain magnetite subfabrics are rare areas of steep magma flow kmax plunge 70 and of shallow magma flow alternate in a systematic and gradual spatial pattern foci of steep flow were spaced 4 km parallel to the spreading axes and 6 km perpendicular to the spreading axes ridge parallel separation of steep flow suggest the spacing of magma feeders to the dikes whereas ridge perpendicular spacing of 6 km at a spreading rate of 50 mm a implies the magma sources may have been active for 240 ka the magma feeders feeding dikes may have been 2 km in diameter stable paleomagnetic vectors in some cases verified by reversal tests are retained by magnetite and titanomagnetite in all specimens the stable components were isolated by three cycles of low temperature demagnetization ltd followed by 10 steps of incremental thermal demagnetization td 47 of primary a components 338 2 57 2 n 207 95 3 9 mean tub 397 8 c are overprinted by a b component 341 4 63 5 n 96 95 8 7 mean tub 182 11 c a and b components are ubiquitous and shared equally by the n s and ne sw striking dikes a component unblocking temperatures tub are zoned subparallel to the fossil spreading axis their spatial pattern is consistent with chemical remagnetization at some certain off axis distance determined by sea floor spreading a components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge axis thus their magnetizations are younger than those of the overlying volcanic sequence for which chrms are commonly reported as 274 33 88 ma

comparison of snow mass estimates from a prototype passive microwave snow algorithm a revised algorithm and a snow depth climatology abstract while it is recognized that no single snow algorithm is capable of producing accurate global estimates of snow depth for research purposes it is useful to test an algorithrn s performance in different climatic areas in order to see how it responds to a variety of snow conditions this study is one of the first to develop separate passive microwave snow algorithms for north america and eurasia by including parameters that consider the effects of variations in forest cover and crystal size on microwave brightness temperature a new algorithm gsfc 1996 is compared to a prototype algorithm chang et al 1987 and to a snow depth climatology sdc which for this study is considered to be a standard reference or baseline it is shown that the gsfc 1996 algorithm compares much more favorably to the sdc than does the chang et al 1987 algorithm for example in north america in february there is a 15 difference between the gsfc 1996 algorithm and the sdc but with the chang et al 1987 algorithm the difference is greater than 50 in eurasia also in february there is only a 1 3 difference between the gsfc 1996 algorith rn and the sdc whereas with the chang et al 1987 algorithm the difference is about 20 as expected differences tend to be less when the snow cover extent is greater particularly for eurasia the gsfc 1996 algorithln performs better in north america in each month than does the chang et al 1987 algorithm this is also the case in eurasia except in april and may when the chang et al 1987 algorithm is in closer accord to the sdc than is the gsfc 1996 algorithm

coherent noise in marine seismic data three mechanisms that might produce noise patterns on stacked sections are examined direct and trapped waves that propagate outward from the seismic source cable motion caused by the tugging action of the boat and tail buoy and scattered energy from irregularities in the water bottom and sub bottom different noise patterns can be observed on shot profiles and common midpoint cmp gathers these patterns can be diagnostic of the dominant mechanism in a set of data field data suggest that the dominant noise is from waves scattered within the shallow sub bottom this type of noise is enhanced by cmp stacking moreover it can be as strong as surface wave noise on stacked land seismic data as well moveout filtering is best for suppressing the noise while preserving signal since the scattered noise does not exhibit a linear moveout pattern on cmp sorted gathers moveout filtering must be applied either to traces within shot records and common receiver gathers or to stacked traces our data example demonstrates that although it is more costly moveout filtering of the unstacked data is particularly effective because it conditions the data for the critical data dependent processing steps of predictive deconvolution and velocity analysis modified journal abstract

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

evidence for accumulated melt beneath the slow spreading mid atlantic ridge the analysis of data from a multicomponent geophysical experiment conducted on a segment of the slowspreading 20 mm yr 1 midatlantic ridge shows compelling evidence for a significant crustal magm

composite synvolcanic intrusions associated with precambrian vms related hydrothermal systems large subvolcanic intrusions are recognized within most precambrian vms camps of these 80 are quartz diorite tonalite trondhjemite composite intrusions the vms camps spatially associated with composite intrusions account for 90 of the aggregate sulfide tonnage of all the precambrian intrusion related vms camps these low alumina low k and high na composite intrusions contain early phases of quartz diorite and tonalite followed by more voluminous trondhjemite they have a high proportion of high silica 74 sio2 trondhjemite which is compositionally similar to the vms hosting rhyolites within the volcanic host rock successions the quartz diorite and possibly tonalite phases follow tholeiitic fractionation trends whereas the trondhjemites fall within the composition field for primitive crustal melts these transitional m i type primitive intrusive suites are associated with extensional regimes within oceanic arc environments subvolcanic composite intrusions related to the archean sturgeon lake and noranda and paleoproterozoic snow lake vms camps range in volume from 300 to 1 000 km3 three have a sill morphology with strike lengths between 15 and 22 km and an average thickness between 1 500 and 2 000 m the fourth has a gross stock like shape the vms deposits are principally restricted to the volcanic strata above the strike length of the intrusions as are areally extensive thin exhalite units the composite intrusions contain numerous internal phases which are commonly clustered within certain parts of the composite intrusion these clusters underlie eruptive centers surrounded by areas of hydrothermal alteration and which contain most of the vms deposits early quartz diorite and tonalite phases appear to have intruded in rapid succession evidence includes gradational contacts magma mixing and disequilibrium textures they appear to have been emplaced as sill dike swarms these early phases are present as pendants and xenoliths within later trondhjemite phases the trondhjemite phases contain numerous internal contacts indicating emplacement as composite sills common structural features of the composite intrusions include early xenolith phases abundant small comagmatic dikes fractures and veins and in places columnar jointing internal phases may differ greatly in texture from fine to coarse grained aphyric and granophyric through seriate to porphyritic mineralogical and isotopic evidence indicates that early phases of each composite intrusion are affected by pervasive to fracture controlled high temperature 350 450 c alteration reflecting seawater rock interaction trondhjemite phases contain hydrothermal magmatic alteration assemblages within miarolitic cavities hydrothermal breccias and veins this hydrothermal magmatic alteration may in part be inherited from previously altered wall rocks two of the four intrusions are host to cu mo rich intrusive breccias and porphyry type mineralization which formed as much as 14 ma after the main subvolcanic magmatic activity the recognition of these precambrian subvolcanic composite intrusions is important for greenfields vms exploration as they define the location of thermal corridors within extensional oceanic arc regimes which have the greatest potential for significant vms mineralization the vms mineralization may occur for 2 000 m above the intrusions in some cases vms mineralization has been truncated or enveloped by late trondhjemite phases of the composite intrusions evidence that much of the trondhjemitic magmatism postdates the principal vms activity is a critical factor when developing heat and fluid flow models for these subseafloor magmatic hydrothermal systems

seismic stratigraphy of the oceanic moho based on ophiolite models synthetic seismograms that model vertical incidence seismic reflections from the oceanic moho were calculated from detailed laterally varying velocity depth models of two ophiolite complexes the synthetic seismograms reproduce the rapid lateral variability of waveforms amplitudes and two way travel times of reflections from the oceanic moho that are observed in existing mcs data the large variations observed in the synthetic seismograms were caused solely by horizontal changes in the geologic structure along the crust mantle boundary because focusing or defocusing of seismic energy by the upper crustal units was not modeled the petrologic contrast producing the strongest reflector from the oceanic crust mantle transition is highly variable so it is difficult to identify what the particular moho reflections represent geologically although with broad band data velocity gradients can be distinguished from laminated zones more important however is our finding that the seismic refraction oceanic moho and the seismic reflection oceanic moho usually do not coincide and that either or both may cross major geologic contacts

magmatic processes at slow spreading ridges implications of the ramesses experiment at 57 45 n on the mid atlantic ridge summary this paper is the first in a series of three this issue which present the results of the ramesses study reykjanes axial melt experiment structural synthesis from electromagnetics and seismics ramesses was an integrated geophysical study which was carefully targeted on a magmatically active axial volcanic ridge avr segment of the reykjanes ridge centred on 57 45aen it consisted of three major components wide angle seismic profiles along and across the avr using ocean bottom seismometers together with coincident seismic reflection profiles controlled source electromagnetic sounding csem and magnetotelluric sounding mt supplementary data sets included swath bathymetry gravity and magnetics analyses of the major components of the experiment show clearly that the sub axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids the avr is underlain by a significant crustal magma body at a depth of 2 5 km below the sea surface the magma body is characterized by low seismic velocities constrained by the wide angle seismic data a seismic reflection from its upper surface and a region of anomalously low electrical resistivity constrained by the csem data it includes a thin ribbon like melt lens at the top of the body and a much larger region containing at least 20 per cent melt in a largely crystalline mush zone which flanks and underlies the melt lens ramesses is the first experiment to provide convincing evidence of a significant magma body beneath a slow spreading ridge the result provides strong support for a model of crustal accretion at slow spreading rates in which magma chambers similar to those at intermediate and fast spreading ridges play a key role in crustal accretion but are short lived rather than steady state features the magma body can exist for only a small proportion of a tectono magmatic cycle which controls crustal accretion and has a period of at least 20 000 years these findings have major implications for the temporal patterns of generation and migration of basaltic melt in the mantle and of its delivery into the crust beneath slow spreading mid ocean ridges

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

new seismic images of oceanic crustal structure remarkable new images of the internal structure of oceanic crust have been recorded from a two ship multichannel seismic survey in the western north atlantic results include strong planar dipping reflectors in the lower crust evidence of whole crustal failure generating reflectors from the top of the basement to the moho and banded layering in the lower crust and in some places in the upper mantle anomalous crust in the blake spur fracture zone is documented from both normal incidence and wide angle seismic profiles the internal structure of the crust records the magmatic and tectonic history of the crust as it was generated at and then moved away from the spreading center

axial magnetic anomaly amplitude along the mid atlantic ridge between 20 n and 40 n after reduction to correct for the topography spreading rate latitude and lineation azimuth effects axial magnetic anomalies amplitudes are compared with bathymetric gravity seismological and geochemical data along a 2600 km long section of the mid atlantic ridge axis between 20 n and 40 n significant results are obtained at two different scales a regional one 100 km related to sublithospheric thermal perturbations such as those associated to hotspots and a local one 100 km related to ridge segmentation at a regional scale two long wavelength highs of the axial magnetic anomaly amplitudes are observed between 37 5 n and 40 n and between 27 n and 30 n the first high associated with marked bathymetric gravity geoid seismic velocity and geochemical anomalies is related to the azores hotspot and is interpreted as reflecting the existence of a thick and or fe ti enriched magnetic source layer a second long wavelength high centered at 28 5 n corresponds to a weaker bathymetric and gravity signature it is associated with a large low seismic velocity anomaly and a marked intermediate wavelength positive geoid high which suggest higher sublithospheric temperatures deeper and more extensive partial melting and a thicker and or fe ti enriched magnetic source layer at a local scale all but one of the 25 ridge segments investigated show that axial magnetic anomaly amplitude is higher at segment ends by a factor of 2 compared to segment centers a clear correlation is observed between the range of axial magnetic anomaly amplitude and the range of the mantle bouguer anomaly mba within each segment more complex relationships exist between median amplitude amplitude at segment centers and at segments ends and mba separation of segments in two groups hotter and colder based on mba shows different types of variations among segment with grouped amplitudes at segment centers and scattered amplitudes at segment ends for hotter segments and the opposite for colder segments these observations support shallow fractionation and the presence of serpentinized bodies in the vicinity of discontinuities as the major processes which control the axial magnetic anomaly amplitude variations

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

axial magnetic anomalies over slow spreading ridge segments insights from numerical 3 d thermal and physical modelling the axial magnetic anomaly amplitude along mid atlantic ridge segments is systematically twice as high at segment ends compared with segment centres various processes have been proposed to account for such observations either directly or indirectly related to the thermal structure of the segments 1 shallower curie isotherm at segment centres 2 higher fe ti content at segment ends 3 serpentinized peridotites at segment ends or 4 a combination of these processes in this paper the contribution of each of these processes to the axial magnetic anomaly amplitude is quantitatively evaluated by achieving a 3 d numerical modelling of the magnetization distribution and a magnetic anomaly over a medium sized 50 km long segment the magnetization distribution depends on the thermal structure and thermal evolution of the lithosphere the thermal structure is calculated considering the presence of a permanent hot zone beneath the segment centre the best fitting thermal structure is determined by adjusting the parameters shape size depth etc of this hot zone to fit the modelled geophysical outputs mantle bouguer anomaly maximum earthquake depths and crustal thickness to the observations both the thermoremanent magnetization acquired during the thermal evolution and the induced magnetization which depends on the present thermal structure are modelled the resulting magnetic anomalies are then computed and compared with the observed ones this modelling exercise suggests that in the case of aligned and slightly offset segments a combination of higher fe ti content and the presence of serpentinized peridotites at segment ends will produce the observed higher axial magnetic anomaly amplitudes over the segment ends in the case of greater offsets the presence of serpentinized peridotites at segment ends is sufficient to account for the observations

seismic evidence for a magma chamber beneath the slow spreading mid atlantic ridge seismic reflections from magma chambers have been observed along the fast spreading east pacific rise1 2 and the intermediate spreading valu fa ridge3 4 sub axial reflections also exist beneath the intermediate spreading juan de fuca ridge5 but no magma chambers have been identified beneath the slow spreading mid atlantic ridge suggesting that here magma chambers lie unusually deep or are transient features6 11 seismic reflection profiles acquired in 1989 over the snake pit hydrothermal area in the rift valley of the mid atlantic ridge 25 km south of the kane fracture zone showed no evidence of magmatic activity12 although geochemical analyses of hydrothermal vent fluids suggest the existence of magma at depths as shallow as 1a 2 km13 14 by suppressing in these data high amplitude coherent noise generated at the sea floor i have obtained images in an otherwise non reflective crust of seismic reflections beneath and just south of the snake pit hydrothermal area these reflections define a small 4 km wide dome whose apex is 1 200 m beneath the sea floor as bright reflections from the upper flanks of this dome occur in the depth range suggested by the vent fluid geochemistry i interpret the dome to be the seismic expression of a small magma chamber

structure and seismotectonics of the vema fracture zone atlantic ocean the structure and seismo tectonics of the vema fracture zone and the adjacent regions of its transverse ridge to the south and the mid atlantic ridge spreading centre to the north is discussed based on an analysis of seismic refraction profiles micro earthquake activity and gravity anomalies crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers micro earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge transform intersection immature crust similar to that reported by fowler 1976 from the famous area lies beneath the spreading centre the lower crust thins and normal mantle appears as the fracture zone is approached the thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection this is also the locus of current microearthquake activity at sub bottom depths of 3 4 km which a composite fault plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley within the central region of the fracture zone valley the crust thickens but still retains an anomalous velocity structure seismic layer 3 is absent and crustal velocities are low this is probably the consequence of attenuated crustal accretion of faulting and cracking and of hydrothermal alteration the pattern of residual gravity anomalies after correction for topography for sediment thickness and for mantle temperature variations is consistent with the crustal structure defined by the seismic experiments the gravity measurements indicate that there is a broad thinning of the crust over 20 40 km in the vicinity of the fracture zone on which are superimposed localized regions of low and high densities to the south of the fracture zone the crust thickens and assumes a normal velocity structure including a mature seismic layer 3 the seismic moho is warped upwards under the transverse ridge suggesting a tectonic origin from buckling rather than uplift by serpentinization

mid ocean ridge sulfide deposits evidence for heat extraction from magma chambers or cracking fronts numerous seafloor observations show that the sizes of high temperature hydrothermal sulfide edifices vary dramatically with spreading rate on fast spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m while on slow spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105 106 m3 we argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast spreading ridges where spreading occurs predominantly by magmatism and slow spreading ridges where there is a significant component of tectonic extension along fast spreading ridges a steady state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability in such systems episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis over a time scale of the order of a decade mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions on slow spreading ridges tectonic extension maintains the fluid pathways necessary to support vigorous convection in the waning stages of magmatism hydrothermal circulation driven by a downward migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits as the depth of circulation increases overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively once hydrothermal fluids penetrate the moho serpentinization clogs the fluid pathways and high temperature venting ceases until it is reactivated by a fresh magmatic intrusion

no evidence from multichannel reflection data for a crustal magma chamber in the mark area on the mid atlantic ridge the role of crustal magma chambers in the construction of oceanic crust along the slowly spreading mid atlantic ridge mar is still highly disputed1 some investigators have argued for small short lived crustal magma bodies2 whereas others have postulated the presence of permanent reservoirs centred between ridge offsets which expand and contract in response to a fluctuating magma supply3 thermal modelling4 5 detailed seismic refraction experiments at several different locations along the mar6 9 and micro earthquake10 11 and teleseismic source mechanism studies12 preclude the existence of large steady state magma chambers under the entire mar rift valley however smaller localized crustal magma bodies could be present at least intermittently here we use multichannel seismic reflection profiles to look for evidence of a crustal magma chamber along the mar south of the kane fracture zone in one of the most magmatically robust and hydro thermally active areas presently known along the mar although some subbasement reflectors are imaged we do not see a shallow large amplitude intracrustal reflector that might be associated with a crustal magma body if a crustal magma chamber was associated with the most recent volcanic episode along this part of the mar our results suggest that it was extremely short lived

manganese and iron profiles during early diagenesis in loch etive scotland application of two diagenetic models abstract sediment cores were taken from two locations in loch etive scotland the first location bonawe 145 m depth was a site with high surficial sediment manganese 1 8 extractable mn beneath waters that were usually hypoxic and the second location airds bay 75 m depth was a contrasting site with lower surficial manganese 0 29 extractable mn beneath a usually well mixed water column cores were analysed for porewater manganese iron and ammonium and solid phase extractable non lithogenic precipitated manganese and iron the manganese and iron profiles were analysed using two models released into the public domain by their authors van cappellen wang 1996 slomp et al 1997 both models assume steady state the former is a multicomponent transport and reaction model for carbon oxygen nitrogen sulphur iron and manganese whereas the other is an analytical single component model for the diagenesis of manganese or iron at the low manganese airds bay site good fits to the measured manganese and iron concentration profiles were obtained by application of the slomp model at this site the van cappellen model gave an acceptable fit to the manganese profiles but not as good as that of the slomp model and a poor fit to the iron profiles with a modelled dissolved maximum approximately 5 times higher than measured and strong decrease in solid phase iron with depth which was not observed at the high manganese bonawe station the slomp model again produced a good fit to the iron concentration profiles but with the manganese profiles it was able to produce a good fit to either the dissolved or to the solid phase extractable but not simultaneously to both at this site the van cappellen and wang model again gave an acceptable fit to the manganese profiles but a poor fit to the iron profiles the sub oxic diagenetic processes which gave profiles resembling the measured profiles were then examined using the models to compare their contribution to that of the measured direct oxygen uptake this showed that at the bonawe site manganese reduction amounted to at least 40 of the oxygen uptake van cappellen model and possibly up to 80 slomp model the reductions of iron oxides 0 4 and of sulphate 8 were of less importance and the balance was assumed to be direct oxidation of carbon by oxygen at the airds bay site direct oxidation of carbon by oxygen appeared to be the dominant process with manganese oxide iron oxide and sulphate accounting for only approximately 8 3 and 5 of oxygen uptake the similarities between the results of the two models used give extra weight to the results and the differences indicate where caution is needed and possibly more work

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

single channel seismic reflection data from the east pacific rise axis between latitude 11 50 and 12 54 n two single channel seismic profiles were recorded in 1981 during a cruise of r v jean charcot along the axis of the east pacific rise between lat 11 50 n and 12 54 n the seismic source was a 1 3 1 80 in 3 water gun the ship9s track was kept on the rise axis by using the real time mapping capabilities of the seabeam echo sounder the seismic data were recorded in analog form and no signal processing was performed the seismic sections revealed the existence of segments of 7 to 30 km long coherent arrivals located at about 0 6 s two way traveltime below the sea floor reflection the same time difference observed for the water borne multiple of these arrivals is a strong indication that these segments of reflection are not side echoes but vertical incidence reflections originating from the subbottom the correlation between the seismic data and some geologic observations in the area near lat 12 13 n indicates that the observed reflections are probably associated with the roof of discontinuous magma chambers situated at an average depth of about 1 5 km below the sea floor

a deep seismic reflection profile across the extinct mid labrador sea spreading center in 1990 a deep multichannel seismic reflection line was shot along a flow line across the labrador sea the results from the central portion of this line between magnetic anomalies 25 and across the extinct central ridge are described here spreading rates in this part of the labrador sea are very low from 10 mm yr to less than 3 mm yr so that the line provides a unique opportunity to examine the relationship between very slow spreading and crustal structure a clear division is observed between two types of crust the oldest crust between dirons 21 and 25 which formed at a mean half spreading rate of 10 mm yr exhibits smoothly undulating basement with only minor normal faulting this region shares many of the reflection characteristics of north atlantic crust formed at moderate to low spreading rates in contrast the younger central region between chrons 21 and 13 which formed at a mean half spreading rate of 3 mm yr displays evidence of intense normal faulting of the crust giving a total extension of about 70 inward facing normal faults on both sides of the extinct ridge with large offsets many of which extend to lower crust or moho depths dominate the seismic section the axial region is characterized by a deep fault bounded median valley these results suggest that mechanical extension plays a more important role in seafloor spreading at low spreading rates than previously documented integration of the reflection data with previous refraction measurements and with gravity modeling of the region shows variations in crustal thickness which can be correlated with spreading rates the region formed at a mean spreading rate of 10 mm yr where about 15 extension is observed exhibits slightly thinner than normal crust 4 8 km or less versus a normal thickness of about 7 km at a lower spreading rate of 3 mm yr across the axial region where extension is about 70 an average crustal thickness of 3 km is obtained thus lower spreading rates are associated with regions of thinner crust and greater amounts of extension while many studies suggest that thin crust at slow spreading rates may result from a reduced magma supply this study suggests that extension is at least equally important and may be responsible for most of the variations in crustal thickness the increased cooling of young oceanic lithosphere formed at these very low spreading rates approximately 3 mm yr may have amplified brittle failure in response to plate separation however the timing of extension is still uncertain and some of it may be related to postextinction tectonics and not to the spreading process the role of extension as a control on crustal thickness needs to be considered further in studies of crustal generation and magmatism at slow spreading ridges extension will decrease the importance of magmatism in generating thin oceanic crust and will favor models of magmatic processes which produce thicker crust

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

anomalously thin crust in oceanic fracture zones new seismic constraints from the kane fracture zone three detailed and carefully positioned seismic refraction experiments have recently been carried out along the kane fracture zone near 24 n 44 w in order to place better geophysical constraints on the extent and origin of the anomalously thin crust reported from parts of this and other large atlantic fracture zones these three experiments when combined with earlier studies result in reversed structural control along about a 300 km long segment of the kane fracture zone extending from the ridge transform intersection out onto crust about 25 m y old an analysis of refraction lines obtained along the fracture zone trough by using both travel time modeling and delay time function techniques reveals large variations in crustal thickness and or velocity however along most of the eastern fracture zone trough upper mantle type velocities are found at depths of only 2 3 km below the seafloor less than half the typical depth to moho in the ocean basins this anomalous fracture zone crust is generally characterized by unusually low compressional wave velocities relatively high velocity gradients throughout most of the thinned crustal section about 1 2 s 1 and a distinct crust mantle boundary the very thin crust appears to be confined to the deepest parts of the kane fracture zone trough 10 km wide although a more gradual crustal thinning may extend up to several tens of kilometers from the fracture zone a specially designed delay time experiment at the intersection of the kane fracture zone and the mid atlantic ridge also revealed the presence of a thinned crustal section of a relatively uniform thickness about 3 km except beneath the nodal basin where the crust may be less than 1 km thick the anomalous crust in the intersection area appears to extend at least part way down the median valley we interpret this anomalous fracture zone crust as a primary accretionary feature that forms as a result of a restricted magma supply near ridge transform intersections although tectonic processes may also play a part at least locally in thinning the crust our results support a ribbon model of the gross seismic structure of the ocean basins in which bands of relatively homogeneous so called normal crust 50 100 km wide extend away from spreading centers each formed at a discrete ridge crest segment and each separated by the thin crust underlying the fracture zone trough

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

magma dynamics at mid ocean ridges by noble gas kinetic fractionation assessment of magmatic ascent rates abstract despite its impact in understanding oceanic crust formation and eruptive styles of related volcanism magma dynamics at mid ocean ridges are poorly known here we propose a new method to assess ascent rates of mid ocean ridge basalt morb magmas as well as their pre and sin eruptive dynamics it is based on the idea that a rising magma can reach a variable degree of both co2 supersaturation in melt and kinetic fractionation among noble gases in vesicles in relation to its ascent rate through the crust to quantify the relationship we have used a model of multicomponent bubble growth in morb melts developed by extending the single component model of proussevitch and sahagian a a proussevitch d l sahagian dynamics and energetics of bubble growth in magmas analytical formulation and numerical modeling j geophys res 103 1998 18223 18251 to co2 he ar gas mixtures after proper parameterization we have applied it to published suites of data having the required features glasses from pito seamount and mid atlantic ridges our results highlight that the investigated morb magmas display very different ranges of ascent rates slow rises of popping rock forming magmas that cross the crust 0 01 0 5 m s slightly faster rates of energetic effusions 0 1 1 m s up to rates of 1 10 m s which fall on the edge between lava effusion and hawaiian activity inside a single plumbing system very dissimilar magma dynamics highlight the large differences in compressive stress of the oceanic crust on a small scale constraints on how the systems of ridges work as well as the characteristics of the magmatic source can also be obtained our model shows how measurements of both the dissolved gas concentration in melt and the volatile composition of vesicles in the same sample are crucial in recognizing the kinetic effects and definitively assessing magma dynamics an effort should be made to correctly set the studied samples in the sequence of volcanic submarine deposits where they are collected enhanced knowledge of a number of physical properties of gas bearing mor magmas is also required mainly noble gas diffusivities to describe multicomponent bubble growth at a higher confidence level

asymmetric deep crustal structure across the juan de fuca ridge a multichannel line was shot across the endeavour segment of the juan de fuca ridge careful processing of 25 km of the line centered about the rise axis shows that young crustal structure is asymmetric an intermittent reflector from the top 1 km of crust follows topography on both the pacific and the juan de fuca plates it could be either the pillow basalt diabase dike contact or a metamorphic front within the pillow basalts a reflection exists 2 3 km below the rift valley floor diffractions from its edges indicate that the rift valley9s velocity structure is markedly different from the structure of the flanks the deep crustal structure is highly asymmetric moho exists within 4 km of the rift valley on the pacific plate in contrast to the juan de fuca plate which shows a reflector at intermediate depths dipping away from the rise to reach moho depth 12 km from the rift valley this reflector9s origin is enigmatic we speculate that the thermal history and structure of the two plates differ because of local differences in mantle structure caused by either ridge migration or a melting anomaly

siberia snow depth climatology derived from ssm i data using a combined dynamic and static algorithm abstract one of the major challenges in determining snow depth sd from passive microwave measurements is to take into account the spatiotemporal variations of the snow grain size static algorithms based on a constant snow grain size cannot provide accurate estimates of snow pack thickness particularly over large regions where the snow pack is subjected to big spatial temperature variations a recent dynamic algorithm that accounts for the dependence of the microwave scattering on the snow grain size has been developed to estimate snow depth from the special sensor microwave imager ssm i over the northern great plains ngp in the us in this paper we develop a combined dynamic and static algorithm to estimate snow depth from 13 years of ssm i observations over central siberia this region is characterised by extremely cold surface air temperatures and by the presence of permafrost that significantly affects the ground temperature the dynamic algorithm is implemented to take into account these effects and it yields accurate snow depths early in the winter when thin snowpacks combine with cold air temperatures to generate rapid crystal growth however it is not applicable later in the winter when the grain size growth slows combining the dynamic algorithm to a static algorithm with a temporally constant but spatially varying coefficient we obtain reasonable snow depth estimates throughout the entire snow season validation is carried out by comparing the satellite snow depth monthly averages to monthly climatological data we show that the location of the snow depth maxima and minima is improved when applying the combined algorithm since its dynamic portion explicitly incorporate the thermal gradient through the snowpack the results obtained are presented and evaluated for five different vegetation zones of central siberia comparison with in situ measurements is also shown and discussed

geological indexes of hydrothermal venting hydrothermal venting occurs on mid ocean ridge axes with a diverse array of morphological structural and petrological characteristics consequently it is not clear if certain geological environments are more conducive than others to hydrothermal activity this paper uses complementary data sets recently collected on multiple tectonic segment scales along intermediate to superfast spreading ridge sections to systematically examine this issue the spatial density of venting is gauged by plume incidence the fraction of linear ridge crest overlain by hydrothermal plumes five geological indexes are compared with plume incidence ridge axis elevation ridge axis cross sectional area the percentage of ridge axis underlain by an axial magma chamber amc reflector mgo wt in basaltic glass and spreading rate a mean value for plume incidence and each index where available is calculated for each of 14 second to fourth order tectonic segments from the juan de fuca ridge 44 30 to 48 45 n and the northern 9 to 11 50 n and southern 13 50 to 18 40 s east pacific rise plumes are most common on segments with a cross sectional area 3 5 km2 a net elevation 0 35 km amc coverage 60 and mgo 7 wt spreading rate is not deterministic on the segment scale the data suggest two classes of ridge segments segments with relatively low geological index values have a uniformly low plume incidence indicative of a presently feeble magmatic budget those with high index values have a plume incidence ranging from low to complete coverage apparently a consequence of a magma supply rate sufficient to produce frequent dike intrusions and highly variable hydrothermal activity the best individual predictors of hydrothermal activity are cross sectional area and net elevation applying these indexes to the east pacific rise between 18 n and 33 s suggests that the unexplored segments with the highest probability of activity are those at 15 30 16 n 6 9 n 5 6 s 27 28 s and 30 30 33 s

vapor transport grain growth and depth hoar development in the subarctic snow measurements from the subarctic snowpack are used to explore the relationship between grain growth and vapor flow the fundamental processes of dry snow metamorphism due to extreme temperature gradients the subarctic pack undergoes extensive depth hoar metamorphism by the end of the winter a five layered structure with a pronounced weak layer near the base of the snow evolves grain size increases by a factor of 2 3 while the number of grains per unit mass decreases by a factor of 10 observed growth rates require significant net inter particle vapor fluxes stable isotope ratios show that there are also significant net layer to layer vapor fluxes soil moisture enters the base of the pack and mixes with the bottom 10 cm of snow while isotopically light water vapor fractionates from the basal layer and is deposited up to 50 cm higher in the pack end of winter density profiles for snow on the ground compared with snow on tables indicate the net layer to layer vapor flux averages 6 x 10 7 kg m 2 s 1 though detailed measurements show the net flux is episodic and varies with time and height in the pack with peak net fluxes ten times higher than average a model driven by observed temperature profiles reproduces the layer to layer flux pattern and predicts the observed weak layer at the base of the snow calculated layer to layer vapor fluxes are ten times higher than inter particle fluxes which implies that depth hoar grain growth is limited by factors other than the vapor supply this finding suggests that gain and loss of water molecules due to sublimation from grains takes place at a rate many times higher than the rate at which grains grow and it explains why grains can metamorphose into different forms so readily

crustal structure of the propagating tammar ridge segment on the mid atlantic ridge 21 5 n active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time the mechanisms controlling ridge propagation however are poorly understood at the slow spreading mid atlantic ridge at 21 5 n a seismic refraction and wide angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply thus crust is thickest 8 km at the domed segment center and decreases in thickness toward both segment ends however thicker crust is formed in the direction of ridge propagation suggesting that melt is preferentially transferred toward the propagating ridge tip further while seismic layer 2 remains constant along axis seismic layer 3 shows profound changes in thickness governing variations in total crustal thickness this feature supports mantle upwelling at the segment center thus fluid basaltic melt is redistributed easily laterally while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply the onset of propagation seems to have coincided with the formation of thicker crust suggesting that propagation initiation might be due to changes in the melt supply after a rapid initiation a continuous process of propagation was established the propagation rate seems to be controlled by the amount of magma that reaches the segment ends the strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length

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

crustal velocity structure of the lucky strike segment of the mid atlantic ridge at 37 n from seismic refraction measurements we estimate the seismic structure of the slow spreading lucky strike segment of the mid atlantic ridge located approximately 300 km south of the azores platform using seismic reflection and seismic refraction data acquired in june 2005 as a part of the seismic study for monitoring of the mid atlantic ridge sismomar survey the three dimensional velocity model shows an upper crustal low velocity anomaly running parallel to the ridge axis which is limited by the median valley bounding faults the velocity models also show a low velocity anomaly underlying the axial melt lens reflector located at the segment center below the lucky strike volcano this lower crustal low velocity region can be explained by elevated temperatures and possibly small amounts of melt the lower crustal low velocity anomaly and the axial melt lens reflector constrain the geometry of the magma chamber responsible for the construction of the lucky strike volcano the presence of this magma chamber and thick crust at the segment center are consistent with a focused melt supply to the segment center

seismic imaging of the axial region of the valu fa ridge lau basin the accretionary processes of an intermediate back arc spreading ridge in 1995 a multidisciplinary geophysical experiment targetted the intermediate spreading valu fa ridge full rate 60 mm yr 1 which is centred on 22 20 s 176 40 w in the lau basin as part of this experiment wide angle and normal incidence seismic profiles were collected both along and across axis to determine the crustal structure of the central valu fa ridge cvfr and its overlap with the northern valu fa ridge nvfr controlled source electromagnetic profiles and underway gravity magnetic and bathymetry data were also collected in this paper we describe the results of forward modelling of the along and across axis wide angle and normal incidence seismic data an axial low velocity block and its underlying slightly broader zone of depressed seismic velocities low velocity zone have been identified and these features are interpreted as corresponding to a melt lens and underlying magma chamber the low velocity block is 1 2 km wide and has a first order upper boundary from which large amplitude reflections are observed amplitude analysis of these indicates an interconnected melt fraction the nature of the lower boundary is more poorly constrained as no reflection event corresponding to the base of the low velocity block is observed modelling indicates that velocities similar to those observed at the base of layer 2 within the axial region 5 5 km s 1 are achieved by 250 m below the upper boundary possibly suggesting a gradational lower boundary with high velocity gradient the low velocity zone lvz is interpreted as an 4 km wide magma chamber delineated by a seismic velocity anomaly of 0 2 km s 1 extending down through layer 3 to within 1 5 2 km of the moho the velocity anomaly and dimensions of the lvz are generally smaller than those observed at the east pacific rise epr and mid atlantic ridge mar the observed along axis continuity of the low velocity block is remarkable extending from the southern tip of the cvfr to the overlapping spreading centre osc with the nvfr a low velocity block is modelled beneath the inside flanks i e the slopes that dip into the overlap basin of both ridges at the osc although the existence of a single low velocity block beneath the overlap basin itself cannot be ruled out the identification of a single lvz centred on the overlap region rather than two merged lvzs beneath each segment implies that the material in each low velocity block originates from the same crustal magma source a reflection event from the moho is observed from directly beneath the axis on both across axis profiles which indicates that a distinct crust mantle boundary may be formed within the axial region many of the observations at the valu fa ridge are consistent with those at the epr and the reykjanes ridge mar which implies that regardless of spreading rate crustal accretionary processes at mid ocean ridges with similar magmatic budgets are also broadly similar

three dimensional analysis of gravity anomalies in the mark area mid atlantlc ridge 23 n we have carried out a three dimensional analysis of the gravity field over the mid atlantic ridge at kane mark area a well studied portion of the mid atlantic ridge south of the kane fracture zone between 23 40 n and 22 30 n the mid atlantic ridge in this area is composed of two distinct spreading segments separated by a small nontransform discontinuity at 23 15 n the mantle bouguer anomalies in the mark area are dominated by v shaped bands of anomalous mass and small amplitude gravity lows and highs located both on and off axis which indicate that the anomaly distribution is more complex than the simple bulls eye pattern described along some other parts of the mid atlantic ridge within the kane fracture zone and in the ridge transform intersection area mantle bouguer anomalies do not accurately reflect the extent of crustal thinning that has been documented seismically this is attributed to lower average crustal and mantle densities due to fracturing and alteration within the fracture zone and the inability of gravity data to resolve the abrupt change to thinner fracture zone crust which is confined to a narrow zone along the fracture zone trough and beneath the nodal basin at the ridge transform intersection outside of the kane fracture zone the magnitude of the crustal thickness variations predicted solely from the residual gravity anomalies compare reasonably well with the crustal structure derived from seismic refraction studies the largest residual gravity anomalies implying significant reductions in crustal thickness are observed over the small nontransform offset at 23 15 n the anomalous mass associated with this feature forms a v shape that can be traced well off axis suggesting that these discontinuities are fundamental ridge segmentation boundaries that can persist for millions of years the distinctive v shaped pattern of gravity anomalies in the mark area reflects systematic crustal thickness variations related to the northward migration of this nontransform offset over the past 3 4 m y

evolution of high latitude snow mass derived from the grace gravimetry mission 2002 2004 since march 2002 the grace mission provides monthly global maps of geoid time variations these new data carry information on the continental water storage including snow mass variations with a ground resolution of 600 700 km we have computed monthly snow mass solutions from the inversion of the 22 grace geoids 04 2002 05 2004 the inverse approach developed here allows to separate the soil waters from snow signal these snow mass solutions are further compared to predictions from three global land surface models and snow depths derived from satellite microwave data we find that the grace solutions correlate well with the high latitude zones of strong accumulation of snow regional means computed for four large boreal basins yenisey ob mac kenzie and yukon show a good agreement at seasonal scale between the snow mass solutions and model predictions global rms 30 40 mm of equivalent water height and 10 20 mm regionally

role of atmospheric circulation with respect to the interannual variability in the date of snow cover disappearance over northern latitudes between 1988 and 2003 1 this paper analyzes the main spatial patterns in the dates of snow cover disappearance variability over northern latitudes between 1988 and 2003 the dates of snow cover disappearance were calculated using satellite passive microwave data from the special sensor microwave imager spatial and temporal patterns were obtained using principal components analysis in the s mode we identified eight components each representing a large region characterized by homogeneous interannual variability in the dates of snow cover disappearance we found that atmospheric circulation summarized by means of teleconnection indices had an important impact on the date of snow cover disappearance for most of these regions a role is played by the arctic oscillation in western siberia the spring east atlantic west russian pattern in central siberia and the pacific north

seismic structure across the rift valley of the mid atlantic ridge at 23 20 mark area implications for crustal accretion processes at slow spreading ridges the results from a 53 km long wide angle seismic profile across the rift valley of the mid atlantic ridge south of the kane transform near 23 20 n mark area provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid ocean ridges anomalously low middle and lower crustal p wave velocities beneath the neovolcanic snake pit ridge are consistent with elevated axial temperatures and with the presence of 4 1 partial melt evenly distributed within the lower crust in preferentially oriented elongated thin films if the melt inclusions have larger aspect ratios melt fractions can be up to 17 3 this and other geological observations suggest that the study area is presently in a magmatically active period the igneous crust is anomalously thin beneath both flanks of the median valley 2 3 2 5 km thus the mantle rocks observed along the western rift valley wall at pink hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting the crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous with igneous crustal thickness variations of 2 2 km over horizontal distances of 5 km this heterogeneity indicates that the magma supply in the area has fluctuated during the last 2 m y thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data