petrographic and geochemical variation along the reykjanes ridge 57 n 59 n abstract high density sampling of the neo volcanic zone along the reykjanes ridge has been used to test the relationship between bathymetric features and geochemistry the axial region has short wavelength features which correspond to elongate en echelon volcanic ridges these ridges sit atop intermediate wavelength features termed swells and troughs a broad correspondence is found between the sections containing more fractionated samples and the intermediate wavelength swells conversely the troughs are dominated by more primitive lavas this suggests that swells represent areas of greater melt flux resulting in thicker crust and hence generally more fractionation progressively higher concentrations of incompatible elements are found northwards along the ridge with increases occurring as a series of steps that do not correlate with short or intermediate wavelength bathymetric features the northward increase in ratios such as nb zr suggests that sections of the ridge are fed by similar aggregate melt fractions and that melt fractions become slightly smaller northwards changes or consistency in ratio particularly rb nb cannot be explained by progressively greater northward incorporation of a component from the icelandic plume mantle

extrusive thickness variability at the east pacific rise 9 10 n constraints from seismic techniques we calculate synthetic shot gathers and their corresponding common depth point cdp profiles over plausible east pacific rise epr shallow velocity structures based on the structures obtained from high resolution on bottom seismic refraction experiments we then use these results to analyze the variability in layer 2a thickness at the epr 9 10 n region as measured by cdp wide aperture profile wap on bottom seismic refraction experiments and conventional air gun refraction data the synthetics indicate that the accuracy of correlating the prominent shallow reflector observed in cdp and wide angle data with the layer 2a 2b boundary is strongly dependent on the structure within layer 2a if layer 2a consists of a surficial low velocity layer overlying a steep velocity gradient our gradient model then there is an excellent correspondence between the two way travel times to the shallow reflector and the base of layer 2a however the shallow reflector may originate from a gradient within layer 2a if the upper crust contains more than one high gradient region our step model this implies that independent estimates of layer 2a velocity structure are needed to properly interpret cdp and wide angle data we also determine that the travel time to the layer 2a reflector for identical velocity structure can vary by as much as 50 ms about 125 m for differing experimental geometries this can explain the discrepancy in two way travel time to the layer 2a reflector imaged on zero age cdp and wap lines the depths to a shallow reflector calculated from cdp and wide angle data in the 9 10 n region of the epr generally correlate with estimated layer 2a thicknesses from on bottom refraction profiles and conventional air gun refraction lines which suggests that the upper crustal structure in this area is similar to the gradient model wap and conventional air gun refraction data indicate that there is a 100 200 m decrease in off axis layer 2a thickness at 9 35 n on the epr the present day location of a deviation in axial linearity deval there is no bathymetric expression of the 50 decrease in layer 2a thickness layer 2a can be interpreted to consist of the extrusive section and transition zone with the layer 2a 2b boundary corresponding to the top of the sheeted dikes we suggest that buoyancy forces associated with the axial magma chamber amc are supporting the extrusive layer and sheeted dikes at the neovolcanic zone with distance from the rise axis the amc solidifies the crust cools the buoyancy forces are reduced and the sheeted dike complex subsides concurrently the extrusive layer thickens resulting in significantly less subsidence of the seafloor we speculate that the 50 decrease in dike subsidence and extrusive thickness at the 9 35 n deval is due to a local reduction in magma supply within the axial magma chamber the off axis pattern of layer 2a thickness suggests that the 9 35 n deval has persisted for 175 000 275 000 years

the central anomaly magnetic high implications for ocean crust construction and evolution the central anomaly magnetic high camh is a long lineated short wavelength 10 km wide at the sea surface magnetic anomaly overlying many of the world s midocean spreading centers this anomaly is particularly well developed over the northern juan de fuca ridge including the northern symmetrical and endeavour segments the ubiquitous presence of this zone of high magnetic contrast directly in the zone of active oceanic plate formation is a key to our understanding of near axis crustal plate evolution recently obtained deep tow magnetometer data from segments of the juan de fuca spreading center have caused us to revise the present models of the source of the camh and to review the implications the new model would have on our understanding of the evolution of the magnetic structure of oceanic plates in this model we attribute the near axis magnetization contrast to tectonically controlled low temperature alteration of the upper vertical section of extrusive basement extensive and systematic fissuring of the upper extrusive rocks allows penetration of sea water and rapid alteration of the magnetic minerals in fissured areas approximately 3 km off axis the correlation between fissured crustal blocks and near bottom magnetic lows and unfissured crustal blocks with magnetic highs which is observed at the axis is also seen in a single off axis tow over an outer flanking ridge

temporal and spatial cyclicity of accretion at slow spreading ridges evidence from the reykjanes ridge summary a unifying model of oceanic crustal development at slow spreading rates is presented in which accretion follows a cyclic pattern of magmatic construction and tectonic destruction controlled by along axis variation in melt supply and coupled to along axis variation in spreading rate and across axis asymmetry in spreading this study focuses on the reykjanes ridge mid atlantic ridge south of iceland which is divided along its entire length into numerous axial volcanic ridges avr five adjacent avrs have been analysed located between 57 30 n and 58 30 n and south of any strong iceland hotspot influence the seabed morphology of each avr is investigated using sidescan sonar data to determine relative age and eruptive history along axis gravity profiles for each avr are modelled relative to a seismically derived crustal reference model to reveal the underlying crustal thickness and density structure correlating these models with seabed features crustal structure ridge segment morphology and relative ages a model of cyclic ridge segmentation is developed in which accretion results in adjacent avrs with a range of crustal features which when viewed collectively reveal that second order segments on the reykjanes ridge have an along axis length of 70 km and comprise several adjacent avrs which in turn reflect the pattern of third order segmentation tectono magmatic accretion is shown to operate on the scale of individual avrs as well as on the scale of the second order segment as a whole

axial summit trough of the east pacific rice 9 10 n geological characteristics and evolution of the axial zone on fast spreading mid ocean ridge the nature and morphological characteristics of axial summit troughs on fast 90 130 mm yr 1 full spreading rate and superfast spreading 130 mm yr 1 mid ocean ridge crests reflect the time integrated effects of long term magmatic cycles short term volcanic episodicity and the tensional stress regime imposed on young ocean crust two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid ocean ridge crests 1 narrow axial troughs 300 2000 m wide and 30 100 m deep on the east pacific rise crest are classified as axial summit graben the dimensions of axial summit graben as well as the morphological and structural character of their walls and floors suggest a primary tectonic origin an axial summit graben may contain a nested axial summit collapse trough implying that processes responsible for these endemic features may be linked near bottom side looking sonar and observational data collected using the towed vehicle argo i and submersible alvin have been used to characterize the axial summit trough of the fast spreading east pacific rise between 9 and 10 n a four stage model is presented for the evolution of this axial summit collapse trough as well as for other well studied portions of the east pacific rise crest from 21 n to 20 s we propose that the transition from a narrow surface collapse dominated axial trough to a broader fault bounded graben is controlled by the relative importance of diking volcanism hydrothermal cooling and tectonism along a ridge segment over time periods 104 years

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

do lower crustal rocks record reversals of the earth s magnetic field magnetic petrology of oceanic gabbros from ocean drilling program hole 735b ocean drilling program odp leg 118 at the southwest indian ridge recovered the first vertically oriented continuously cored gabbros from in situ ocean crust magnetic properties and oxide petrography of drillcore samples were analyzed in order to determine the processes which control the remanent magnetization of oceanic layer 3 and the ability of gabbros to record reversals of the earth s magnetic field primary igneous magnetite is common only in evolved fe ti oxide rich gabbros and averages by volume 2 of the rock although substantially higher contents are common secondary magnetite produced by high temperature exsolution and hydrous alteration of olivine and pyroxene is observed in all gabbros studied and is the most important magnetic phase in the majority of recovered gabbros cumulate olivine gabbros because of the high temperatures associated with this alteration it is likely that the primary magnetization of this crustal section was a thermal remanent magnetization or a very high temperature chemical remanent magnetization acquired very near the ridge crest hysteresis loop parameters indicate that all of the studied samples are capable of carrying significant stable remanent magnetization finally previously published drilling results and field studies from odp site 735b indicate that although this crustal section has been tectonically uplifted it formed at a mid ocean ridge spreading center similar to much of the oceanic crust therefore the results of this magnetic study can be extrapolated to include the oceanic gabbro section in general and argue strongly that layer 3 crustal rocks are capable of recording reversals of the earth s magnetic field

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

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

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

circumpolar mapping of antarctic coastal polynyas and landfast sea ice relationship and variability abstractsinking of dense water from antarctic coastal polynyas produces antarctic bottom water aabw which is the densest water in the global overturning circulation and is a key player in climate change as a significant sink for heat and carbon dioxide very recent studies have suggested that landfast sea ice fast ice plays an important role in the formation and variability of the polynyas and possibly aabw however they have been limited to regional and case investigations only this study provides the first coincident circumpolar mapping of antarctic coastal polynyas and fast ice the map reveals that most of the polynyas are formed on the western side of fast ice indicating an important role of fast ice in the polynya formation winds diverging from a boundary comprising both coastline and fast ice are the primary determinant of polynya formation the blocking effect of fast ice on westward sea ice advection by the coastal current would be another key factor these effects on the variability in

a first description of the antarctic peninsula coastal current abstract we present hydrographic and shipboard adcp data collected during the fall april june and winter july august and moored velocity observations collected from 2001 to early 2002 on the west antarctic peninsula wap shelf during the southern ocean global ecosystems dynamics so globec program in fall a geostrophically balanced buoyant current flows southward along the coast this antarctic peninsula coastal current apcc forms during the ice free season and extends from adelaide island to alexander island although its path inside marguerite bay is uncertain during the fall of 2001 the apcc had a volume transport of 0 32 0 13 sv and a freshwater transport relative to a reference salinity of 34 4 of 126 50 km 3 yr 1 from early july to late october the apcc disappears from the coast as the freshwater input from the coast diminishes and sea ice forms on the shelf an examination of the relative sizes of the freshwater sources suggests runoff from land and precipitation over the ocean are the primary sources for the apcc

a multichannel seismic investigation of upper crustal structure at 9 n on the east pacific rise implications for crustal accretion reprocessed multichannel seismic profiles from the 9 n segment of the east pacific rise reveal prominent shallow subbasement events these events are identified as wide angle reflections from the base of seismic layer 2a based upon modeling of expanding spread profile data and velocity functions the layer 2a reflections typically increase from 0 15 s after the seafloor reflection at the rise axis to 0 3 0 45 s within 1 2 km of the axis corresponding to an increase in layer thickness of 200 600 m no further systematic increase in layer thickness is observed although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis however the intermittent character of the imaged layer 2a reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer the base of layer 2a is interpreted as corresponding to the transition between the extrusive section pillow basalts and sheet flows and a sheeted dike complex the rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone lateral variability of layer 2a can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2a are approximately half that of layer 2b for an along axis profile apparent along axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2a caused by wandering of the profile relative to axis within the resolution of the data the time delay of the magma chamber reflection relative to the base of layer 2a is constant

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

ridge segmentation and the magnetic structure of the southwest indian ridge at 50 30 e 55 30 e and 66 20 e implications for magmatic processes at ultraslow spreading centers 1 the aim of this paper is to investigate the relationships between the segmentation and the magnetic structure of the ultraslow spreading southwest indian ridge contrary to faster spreading ridges magnetization usually decreases from high values along the neovolcanic axis to low values in the nontransform discontinuities there is a direct correlation between the deepening of the axial valley and the decrease of the magnetization from the neovolcanic axis toward the deepest parts of the axial discontinuities we suggest that less frequent eruptions as the distance from the segment center and the length of these discontinuities increase result in thinner extrusive lavas and thus control the along axis magnetization variations by thinning the magnetic source layer a unique segment centered at 50 28 e shows a marked low magnetization anomaly at its center similarly to the segments of the slow spreading mid atlantic ridge we suggest that in this segment both the mantle temperature and the magmatic activity are high enough for the lavas not to be highly fractionated a higher rate of melt production to the west of gallieni transform fault may have created some form of reservoir where mixing of melts occurs and where crystalline fractionation is low producing low magnetization lavas to the east magma chambers may be smaller with cooler mantle temperatures resulting in restricted mixing and significant fractionation which may lead to relatively high intensity magnetization lavas finally we propose that serpentinization of peridotites has no significant contribution to the variation of the magnetization along the axial valley off axis in thin crust areas upper mantle rocks may become progressively more altered as distance from the axis increases the strong faulting and alteration of a thin basaltic cap and underlying upper mantle rocks can produce the disappearance of the magnetic reversal pattern and the increase of the magnetization which is observed along the traces of the largest amagmatic discontinuities by contrast in thicker crust areas the upper mantle rocks are shielded from the alteration and the serpentinization process may be delayed resulting as on the mid atlantic ridge in slightly more positive magnetization values along the traces of axial discontinuities regardless of polarity

magmatic processes at superfast spreading mid ocean ridges glass compositional variations along the east pacific rise 13 23 s major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading 150 mm yr east pacific rise epr 13 23 s and near ridge seamounts comprise 212 chemical groups we interpret these groups to represent the average composition of individual lava flows or groups of closely related flows groups slightly enriched in k2o t morb are distributed variably along the axis in contrast to the galapagos spreading center where t morb are extremely rare this result is consistent with the interpretation that t morb magmas arise from low melting temperature k rich heterogeneities in the subaxial epr mantle the galapagos spreading center which is migrating to the west in an absolute reference frame is underlain by mantle previously processed and depleted in the t morb component during melting events giving rise to earlier epr magmas excluding t morb there are nearly monotonic twofold increases in k ti and k p of axial lavas from 23 s to 13 s from 22 s to 17 s these gradients correlate with isotopic ratios but north of 17 s there is a reversal of isotopic gradients indicating recent decoupling of the isotopic and minor element ratios in the subaxial mantle a strong southward increase in degree of differentiation for approximately 200 km north of the large offset at 20 7 s correlates with a gradient in bathymetry consistent with previous interpretations that this offset is propagating to the south samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive evolved fractionation signatures of rift propagation suggesting that propagating rift tips have been abandoned preferentially to failing rift tips glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset and possibly also south of 22 s the latter region may be affected by deformation accompanying northward growth of the easter microplate near ridge seamounts on the pacific plate between 18 19 s comprise two distinct populations those aligned approximately parallel to the spreading direction are extremely variable in major element composition but consistently enriched in sr relative to nearby axial lavas smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and sr relative to axial lavas the degree of differentiation of axial lavas between 18 19 s can be related to the structural development of the rift axis and or vigor of hydrothermal activity of individual segments glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area primary magmatic segmentation mainly reflects mantle source variations the boundaries of which correlate with the largest physical offsets in the rise axis between the easter microplate and garrett transform zone a secondary magmatic segmentation defined by the along axis continuity of similar parental magma compositions or liquid lines of descent occurs with a length scale varying from 11 to 185 km with an average of 69 57 1 km the boundaries of these segments mainly occur at overlapping spreading centers all first second and third order physical offsets correspond to secondary magmatic segment boundaries but some secondary magmatic segment boundaries also occur at small fourth order ridge axis discontinuities the secondary magmatic segments define the length scale of mantle melting variations mainly variations in extent of melting but not the scale of melt extraction processes that feed the axis this scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth order physical discontinuities along axis there is a good positive correlation of average secondary magmatic segment length with spreading rate for four well sampled areas varying from 20 to 150 mm yr secondary magmatic segments also become more variable in axial length with increasing spreading rate the average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature defined by extent of magmatic differentiation which bears little systematic relation to physical or other kinds of magmatic segmentation at 13 23 s the length scale of this variation is 217 60 1 km approximately corresponding to the wavelength of rolls in the gravity field observed off axis taken together the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere magma supply to the axis and crustal magmatic temperature reflect independent regionally decoupled processes

marine magnetic anomalies the origin of the stripes the results of recent observational and theoretical investigations of lineated magnetic anomalies on the ocean floor are summarized in tables graphs and diagrams and analyzed topics addressed include early lineation models inversions of magnetic anomalies to obtain source functions deep tow studies of magnetic anomalies evidence from the long wavelength component of the magnetic field including magsat observations and direct measurements of the magnetic properties of oceanic rocks it is concluded that the source of the lineated anomalies must reside in most of the oceanic crust not just in the pillow lavas of layer 2a

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

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

propagating rift model for the v shaped ridges south of iceland 1 we present new marine geophysical data which constrain the seafloor spreading history of the reykjanes ridge near iceland and the origin of its flanking v shaped topographic and gravity ridges contrary to the geometry assumed in pulsing plume models the v shaped ridges are not symmetric about the reykjanes ridge axis and seafloor spreading has not been symmetric about a stable axis thus existing models must at least be modified to include an additional asymmetry producing mechanism the best understood and documented such mechanism is rift propagation one possibility is that plume pulses drive the propagators however rift propagation also produces v shaped wakes with crustal thickness variations suggesting the possibility that a pulsing iceland plume might not be necessary to explain the reykjanes v shaped ridges scarps and troughs

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

wavelet analysis of deep tow magnetic profiles modeling the magnetic layer thickness over oceanic ridges 1 the interpretation of marine magnetic anomalies usually consists either in determining the magnetization distribution assuming the source geometry and magnetization direction or in determining the magnetic layer thickness assuming the magnetization direction and intensity in this paper we introduce a new technique that allows modeling of the thickness of the magnetic source layer with very few a priori assumptions about the magnetization the magnetic layer is assumed to be made of a series of bodies each having a constant unknown magnetization and an unknown size this technique is based upon the application of the continuous wavelet transform recently introduced for the interpretation of potential field data as a multipole decomposition we present applications to synthetic data to one deep tow magnetic profile recorded across the juan de fuca ridge jdf and to three deep tow magnetic profiles recorded across the central indian ridge cir our results confirm that despite significant source thickness variations 100 1200 m across the cir measured magnetic anomalies mostly reflect past geomagnetic field intensity fluctuations however we show that within the axial region of high magnetization thickness variations have a significant contribution to short wavelength variations of deep tow magnetic signals 100 nt

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

the characteristic variability and connection to the underlying synoptic activity of the amundsen bellingshausen seas low 1 recent studies have noted an asymmetrical climate change across antarctica with significant warming in west antarctica and the antarctica peninsula and primarily insignificant trends in east antarctica due to its proximity variations in the position and intensity of the amundsen bellingshausen seas low absl are a suspected atmospheric mechanism here we investigate the absl to understand its characteristic variability and underlying synoptic scale influences based on three reanalysis data sets the absl is defined as the minimum monthly pressure in the 45 75 s 180 60 w domain using this criterion a significant north south and east west progression is noted in the climatological 1979 2001 average absl which is strongly tied to the location of the maximum cyclone system density and minimum cyclone central pressures more than 550 cyclones a year were identified in the vicinity of the absl during spring significant trends in their central pressures are noted in the ross sea the implied changes in temperature advection by these stronger systems are consistent with the warming in west antarctica the strongest cyclone events i e the ten with the deepest central pressures also demonstrate a connection to the climatological absl albeit weaker moreover these strong cyclone events are significantly linked to the southern annular mode sam particularly in their annual frequency and location steering in the summer this shows that large scale forcing such as from the sam may influence the strongest cyclones in the region and could allow for the prediction of such events

contribution of serpentinized ultramafics to marine magnetic anomalies at slow and intermediate spreading centres insights from the shape of the anomalies summary detailed characteristics of marine magnetic anomalies 33r and 20r suggest that the magnetization of the deeper magnetic layers including the lower crust and possibly the uppermost mantle is horizontally displaced with respect to that of the upper crust we examine the possibility that serpentinization of ultramafics in the lower crust and possibly the uppermost mantle delays the acquisition of magnetization and introduces a shift between the upper and lower crustal magnetization patterns thermal evolution models and the resulting magnetization patterns of the oceanic lithosphere are calculated for a wide range of physical parameters such as the nusselt number and the depth of hydrothermal circulation in the crust and the temperature range of serpentinization the models with moderate hydrothermal cooling of the whole crust and serpentinization temperatures ranging between 200 and 300 dc successfully explain the anomalous skewness and the hook shape of observed sea level magnetic anomalies created at slow and intermediate spreading rates

a comparison of volcanic edifices at the reykjanes ridge and the mid atlantic ridge at 24 30 n a detailed analysis of high resolution side scan sonar data shows that the volcanic edifices built within the axial zones of the slow spreading 20 mm yr reykjanes ridge and the slow spreading 25 mm yr mid atlantic ridge are remarkably similar in fact individual seamounts hummocks rounded mounds and hummocky ridges at the reykjanes ridge have nearly identical morphological counterparts at the mid atlantic ridge mar arguing that commonly eruptive conditions are similar at the two ridges the construction of these volcanic edifices typical of slow spreading ridges occurs at the reykjanes ridge despite its proximity to the iceland hot spot and the influence of the hot spot on other characteristics of the ridge axis such as its overall shape an axial high north of about 59 n its shallow axial depths and its thicker crustal section the volume of magma that goes into constructing volcanic edifices at each of 12 segments of the mar and four regions of the reykjanes ridge is calculated using multibeam bathymetry data our results indicate that edifice volume per unit seafloor area equivalent to a uniform thickness over the area of interest is larger at the mar than at the reykjanes ridge on average taking into account spreading rate we then estimate b the average building rate per unit length along the axis which is also larger at the mar we conclude that the same volcanic edifices build the crust at the reykjanes ridge and mar but that their emplacement is more frequent at the mar to explain our results and reconcile them with those of magde and smith 1995 who documented that seamounts are taller and more abundant at the reykjanes ridge than at the mar we consider the controls on diking and eruption within the axial zones of the two ridges we suggest that the combination of slower spreading rate and the oblique geometry of spreading at the reykjanes ridge leads to fewer eruptions which are more widely dispersed across the axial zone than at the mar if this hypothesis is correct the proportion of seamounts modified by subsequent eruptions will be less this might also explain the taller heights of the seamounts if there is less overlap and infilling between edifices

mid atlantic ridge volcanism from deep towed side scan sonar images 25 29 n abstract we present deep towed side scan sonar mosaics of the inner valley floor of eight spreading segments at the slow spreading mid atlantic ridge between 25 and 29 n an analysis of these images which well resolve features a few tens of meters in size confirms that the multitude of small seamounts with diameters between 0 5 and 3 km identified on the inner valley floor from previously collected multibeam bathymetry data are volcanically constructed moreover these images reveal that these volcanoes have distinct surface morphologies not evident in the coarser resolution multibeam bathymetry maps 83 of the seamounts have a hummocky bulbous morphology the other 17 have a smooth morphology in addition to near circular seamounts small 1 2 km long volcanic ridges are abundant in our study regions and are not in general seen in the bathymetry maps we combine these new morphological data with existing models for the construction of the shallow oceanic crust to obtain a better understanding of the melt delivery system that builds the distinctive seafloor topography at the slow spreading mid atlantic ridge

hydrothermal activity on the reykjanes ridge the steinaholl vent field at 63 06 n we have completed a systematic survey for hydrothermal activity along the reykjanes ridge from 57 45 n to 63 09 n shipboard determinations of total dissolvable mn tdmn and dissolved si ch4 and h2 have revealed evidence for only one site of hydrothermal activity along this section of ridge crest the steinaholl vent field at 63 06 n 1 our measurements confirm this site to be a stable hydrothermal convection cell of at least two years duration the site is situated in just 250 350 m of seawater and unlike deeper sited vent fields e g tag 3650 m 26 n mar is notable for the formation of bubble rich plumes which have been imaged using a high frequency 38 khz echo sounder high dissolved gas concentrations of up to 18 nmol l ch4 and 30 nmol l h2 coincide with these bubble rich plumes the steinaholl plume is also characterised by high total dissolvable mn tdmn anomalies up to 60 nmol l and dissolved si anomalies 2 mol l yielding molar tdmn ch4 and tdmn si ratios of 3 5 and 0 03 respectively directly comparable to the tag hydrothermal plume the absence of any indication of hydrothermal activity along the remainder of the reykjanes ridge is intriguing between 11 n and 40 n on the mid atlantic ridge previous studies have revealed evidence for hydrothermally active sites approximately every 150 km for the 300 km of ridge crest studied in detail here 750 km total only one site has been found thus despite its proximity to the iceland hot spot the incidence of hydrothermal activity along this section of ridge crest appears to be only 20 50 of that measured elsewhere along the slow spreading mid atlantic ridge

on the characteristics of internal tides and coastal upwelling behaviour in marguerite bay west antarctic peninsula abstract internal waves and coastal upwelling have important roles in both physical oceanography and marine ecosystems via processes such as mixing of water masses and transfer of heat and nutrients to biologically active layers in this paper we use quasi weekly hydrographic profiles and moored records of temperature salinity and water velocity to investigate the nature of internal tides and coastal upwelling behaviour in northern marguerite bay at the western antarctic peninsula within ryder bay a near coastal site in northern marguerite bay atmospherically forced oscillations of the water column with periods around 2 7 days are observed associated with wind induced coastal upwelling and downwelling sea ice cover is seen to play a role in the seasonal suppression of these oscillations significant internal tides also are observed at this site a range of processes are seen to be important in controlling internal tide variability including changes in local stratification and sea ice conditions both diurnal and semidiurnal internal tidal species are observed despite the study region being poleward of the critical latitude for diurnal internal tides this suggests that at least the diurnal internal tides are generated close to the study location and we investigate likely sources

magnetization of axial lavas from the southern east pacific rise 14 23 s geochemical controls on magnetic properties although the spatial association of iron rich lavas and high amplitude magnetic anomalies is well documented a causal link between enhanced iron content and high remanent magnetization has been difficult to establish here we report magnetic data from approximately 250 samples with 8 16 feo total iron as feo from the southern east pacific rise epr that provide strong support for the presumed geochemical dependence of remanent intensity the limited age range 0 6 ka of axial lavas from this ultrafast spreading ridge 150 mm yr full rate effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low temperature alteration systematic sampling relative to the chilled margin illustrates that substantial grain size related variations in magnetic properties occur on a centimeter scale both microprobe data and curie temperatures suggest that the average groundmass titanomagnetite composition in the southern epr samples is approximately constant modal modified ulv6spinel content 0 67 over a wide range of lava compositions saturation magnetization and saturation remanence are highly correlated with feo r 0 73 and 0 83 respectively indicating that more iron rich lavas have higher abundances of otherwise similar titanomagnetite we show that there is a good correlation between natural remanent magnetization nrm and feo provided that sufficient specimens are used to determine the average nrm of a sample r 0 63 because the range of iron contents in mid ocean ridge basalts is limited the best fit slope 4 44 a m per feo in an ambient field of 0 030 mt should provide reasonable bounds on the equatorial magnetization of submarine lavas 10 a m at 8 5 feo and 50 a m at 16 feo finally we demonstrate that along axis variations in nrm closely parallel geochemical changes along the southern epr where magnetization values deviate significantly from those predicted from the range of measured feo contents these discrepancies may reflect additional unrecognized geochemical variability

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

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

en echelon axial volcanic ridges at the reykjanes ridge a life cycle of volcanism and tectonics abstract new deep towed sidescan records and multibeam bathymetric data along the reykjanes ridge allow us to assess the varying thermal effect of the icelandic hotspot the increasing predominance of faulting speculated to be derived from mantle upwelling and outflow over simple extensional neotectonics is traced from northeast to southwest numerous en echelon volcanic ridges built along the plate boundary are seen in various stages of growth and decay and a cycle of alternating construction and dismemberment is proposed unfaulted short narrow lensoid ridges occasionally in small groups are interpreted as immature and developing axial volcanic ridges broad convex flanked ridges appear to be mature sites of volcanic construction where the magma supply has been maintained narrow bow form ridges occasionally with rectilinear flanks represent axial volcanic ridges which have ceased construction and are in initial stages of disaggregation isolated polygonal blocks generally seen off axis represent the most advanced stage of dismantling of the surface crustal accretionary features while this cyclicity is speculated to be related to periodic magmatic and tectonic focusing and defocusing along the axis due to the restricted magma supply at this slow spreading ridge it is recognised that this needs modelling although the oblique spreading of the reykjanes ridge allows one to clearly distinguish between structures attributable to plate spreading from those related to small scale accretionary processes we propose that the major processes identified in this study are directly applicable to all slow spreading sections of mid ocean ridges

segmentation and a nontransform ridge offset on the reykjanes ridge near 58 n multibeam bathymetry deep towed side scan sonar gravity and magnetic data over a 50 km square area all suggest the presence of a second order nontransform offset on the obliquely spreading reykjanes ridge north atlantic ocean near 58 n latitude it is the first such offset to be recognized on the reykjanes ridge this region is characterized by a shallow median valley containing en echelon axial volcanic ridges avrs similar to those found on more northerly parts of the reykjanes ridge the side scan sonar shows that all the avrs are constructional in nature the one immediately south of the offset basin backscatters strongly is unmarked by faulting and so appears extremely young other avrs appear older having lower backscatter and off axis being cut by faults and fissures at 57 55 n the progression of avrs is interrupted by a 600 m deep 20 km 10 km basin residual mantle bouguer anomalies corrected for two dimensional lithospheric cooling display a high of at least 8 mgal over the basin two small off axis basins occur roughly along the flow line from this basin and are also characterized by gravity highs the basins are interpreted as regions of crustal thinning and are believed to represent the discontinuous trace of the ridge offset which has thus been in existence for at least 2 m y and has been slowly propagating south inversion of the magnetic field shows a very low magnetization 3 4 a m 1 associated with the offset basin which is interpreted as indicating a reduced magnetic layer thickness due to poor magma supply to the offset area the brunhes matuyama reversal boundary and anomaly 2 traces are offset about 3 km dextrally across the proposed offset trace the avr immediately north of the offset displays high amplitudes of magnetization which steadily increase southward toward its tip adjacent to the offset suggesting the presence of increasingly highly fractionated basalts toward the avr tip polarity transition widths across the brunhes matuyama boundary are comparable with those on other slow spreading ridges but are wider than individual avrs probably because avrs overlap in the crustal accretion zone transition widths are also asymmetrical across the spreading center being narrower 4 5 km to the east than the west 8 km suggesting a certain degree of asymmetry in the accretionary and rifting processes at the ridge axis

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

rapid drawdown of antarctica s wordie ice shelf glaciers in response to enso southern annular mode driven warming in the southern ocean abstract here we investigate the largest acceleration in ice flow across all of antarctica between 2008 insar and 2014 landsat velocity mappings this occurred in glaciers that used to feed into the wordie ice shelf on the west antarctic peninsula which rapidly disintegrated in 1989 between 2008 and 2014 these glaciers experienced at least a threefold increase in surface elevation drawdown relative to the 2002 2008 time period after 20 yrs of relative stability it is unlikely that the ice shelf collapse played a role in the large response instead we find that the rapid acceleration and surface drawdown is linked to enhanced melting at the ice ocean boundary attributable to changes in winds driven by global atmospheric circulation patterns namely the el nino southern oscillation enso and southern annular mode sam linking changes in grounded ice to atmospheric driven ocean warming

the northern mid atlantic and reykjanes ridges spreading center morphology between 55 50 n and 63 00 n seamarc ii and hydrosweep acoustic imagery and bathymetry reveal that the northern mid atlantic ridge mar and reykjanes ridge between 55 50 n and 63 00 n are segmented seafloor spreading centers that exhibit structural morphologies that vary with distance from iceland the northern 100 km of the mar 55 50 n 56 47 n is oriented 005 perpendicular to the spreading azimuth and contains four third order spreading segments separated along strike by right and left stepping axial discontinuities the boundary between mar and reykjanes ridge occurs at 56 47 n where the bight transform fault offsets the neo volcanic axis 15 km right laterally across a single linear fault oriented 092 this is the only transform fault between 55 50 n and 63 00 n and is inferred to have generated the bight fracture zone the reykjanes ridge lies between the bight transform and iceland s reykjanes peninsula and is oriented 30 oblique to the spreading orthogonal direction axial faults and volcanic systems on the reykjanes ridge are arranged in a right stepping en echelon pattern and display strikes intermediate between the spreading orthogonal direction and the strike of the ridge which is compatible with experimental models of oblique extension spreading segments on the northern mar and reykjanes ridge are defined structurally and bathymetrically at the smallest scale the volcanic axis is segmented into individual volcanic systems 4 45 km long which are separated along strike by intervolcano gaps or offsets these features are analogous to fourth order spreading segments documented elsewhere on the mar axial volcanic systems are superimposed on intermediate wavelength 13 65 km axial topographic highs that constitute third or second order spreading segments segment boundaries occur at axial depth maxima which correspond to offsets in the volcanic axis four primary characteristics of the spreading axis axial valley depth axial boundary fault throw relief along the neovolcanic axis and degree of inter segment structural discontinuity decrease toward the north this variation is consistent with higher crustal temperatures near iceland due to either thicker crust or elevated mantle temperatures

formation of large summit troughs along the east pacific rise as collapse calderas an evolutionary model summit troughs wider than 500 m and 30 110 m deep are present along 15 20 of the fast spreading east pacific rise epr they occur only along ridge segments with large cross sectional areas indicative of a time averaged robust magma supply where available seismic data confirm that these troughs are underlain by an axial magma chamber amc 1 2 1 6 km below the seafloor furthermore detailed investigation of the large summit troughs which notch the epr between 17 56 and 18 35 s indicates that the vertical relief of the troughs tends to be maximum where the amc is shallowest both of these observations are inconsistent with the predictions from a model in which large summit troughs form by rifting of the brittle upper crust during phases of amagmatic extension rather we propose that they represent elongated collapsed calderas that form when the melt supply to formerly inflated amcs wanes or nearly ceases because seismic studies constrain the melt lens along the epr to be only 30 80 m thick the proposed existence of collapsed calderas 40 110 m deep implies that the entire magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply in particular we suggest that a voluminous crystal mush zone will stretch in response to steady state seafloor spreading when the magma supply temporarily decreases the resulting caldera will further widen with each subsequent dike intrusion when an abundant melt supply finally resumes the associated tumescence of the neovolcanic zone and profuse lava flows will combine to smooth out the caldera

antarctic surface melting dynamics enhanced perspectives from radar scatterometer data 1 antarctic ice sheet surface melting can regionally influence ice shelf stability mass balance and glacier dynamics in addition to modulating near surface physical and chemical properties over wide areas here we investigate variability in surface melting from 1999 to 2009 using radar backscatter time series from the seawinds scatterometer aboard the quikscat satellite these daily continent wide observations are explored in concert with in situ meteorological records to validate a threshold based melt detection method radar backscatter decreases during melting are significantly correlated with in situ positive degree days as well as meltwater production determined from energy balance modeling at neumayer station east antarctica these results support the use of scatterometer data as a diagnostic indicator of melt intensity i e the relative liquid water production during melting greater spatial and temporal melting detected relative to previous passive microwave based studies is attributed to a higher sensitivity of the scatterometer instrument continental melt intensity variability can be explained in part by the dynamics of the southern annular mode and the southern oscillation index and extreme melting events across the ross ice shelf region may be associated with el nino conditions furthermore we find that the antarctic peninsula accounts for only 20 of antarctic melt extent but greater than 50 of the total antarctic melt intensity over most areas annual melt duration and intensity are proportional however regional and localized distinctions exist where the melt intensity metric provides greater insight into melting dynamics than previously obtainable with other remote sensing techniques

uniform accretion of oceanic crust south of the garrett transform at 14 15 s on the east pacific rise using migrated common depth point reflection profiles we find the structural differences along the ultrafast spreading 150 mm yr east pacific rise south of the garrett fracture zone are second order suggesting a remarkably uniform process of crustal accretion the rise axis south of the garrett transform is underlain by a narrow 1 0 km melt lens which shows great along strike continuity the depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading east pacific rise at 9 30 n this observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent melt sill width however shows little variation along the east pacific rise suggesting no dependence of magma chamber size on spreading rate the melt reservoir decreases in width toward across the 14 27 s ridge axis discontinuity by a modest 250 300 m and appears to be continuous across this feature given the small aspect ratio 1 0 km by 50 m by tens of kilometers of the axial melt lens the previously recorded jump in mgo content across the 14 27 s offset is likely the result of a mixing boundary which is sustained through an along strike impedance in convection wide angle reflections originating at the base of seismic layer 2a assumed to coincide with the extrusive layer reveal a twofold to threefold increase 200 250 to 500 600 m in thickness within 1 2 km of the rise axis the pattern of extrusive thickening imaged south of the garrett transform is similar to that observed along the slower spreading 110 120 mm yr east pacific rise at 9 n outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile this implies a degree of temporal stability of the along strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone the inferred uniformity of off axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14 27 s discontinuity are caused by long term reductions in magma supply second order differences in the style of extrusive thickening may be related to structural differences within the low velocity zone underlying the rise axis and or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis images of mono on cross axis profiles may be traced to within 1 0 km of the melt sill edge this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber

mid ocean ridge eruptive vent morphology and substructure evidence for dike widths eruption rates and evolution of eruptions and axial volcanic ridges high resolution side scan sonar data of mid atlantic ridge mar inner valley floor axial volcanic ridges reveal details of their architectural elements we develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features inhibition of gas exsolution lack of magma disruption and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling leading to more rapid centralization of eruptions along the widest places in the dike dikes are predicted to initially feed eruptions from fissure vents producing lines of hummocky ridges centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts the widest dikes are predicted to produce smooth flows up to several kilometers in length which should pond in adjacent lows edifice sizes predicted on the basis of volume fluxes and flow lengths implied by the widths of dikes observed on the seafloor in iceland and in ophiolite complexes are in quantitative agreement with the dimensions of observed features each ridge is made up of the products of a variety of these individual dike emplacement and extrusive events involving various dike widths cooling times and eruption durations on the basis of typical mar spreading rates about one dike emplacement event would be expected every 40 years since not all dikes reach the surface the actual mean interval between eruptions will exceed this mar dikes should solidify between average dike emplacement events dike emplacement events are more frequent on the east pacific rise epr and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes differences between the morphology of the mar axial volcanic ridge and the sheet flow dominated epr are attributed to on average wider dikes erupting with greater frequency along the epr

building the crust at the mid atlantic ridge the morphology of the sea floor at mid ocean ridge spreading centres provides a key to understanding how ocean crust is constructed images of the axial zone of the slow spreading mid atlantic ridge obtained at a range of spatial scales show that crustal construction is complex and highly variable reflecting an underlying three dimensional variability in magmatic tectonic and hydrothermal processes

propagating buoyant mantle upwelling on the reykjanes ridge crustal features of the reykjanes ridge have been attributed to mantle plume flow radiating outward from the iceland hotspot this model requires very rapid mantle upwelling and a rheological boundary at the solidus to deflect plume material laterally and prevent extreme melting above the plume stem here we propose an alternative explanation in which shallow buoyant mantle upwelling instabilities propagate along axis to form the crustal features of the ridge and flanks as only the locus of buoyant upwelling propagates this mechanism removes the need for rapid mantle plume flow based on new geophysical mapping we show that a persistent sub axial low viscosity channel supporting buoyant mantle upwelling can explain the current oblique geometry of the ridge as a reestablishment of its original configuration following an abrupt change in opening direction this mechanism further explains the replacement of ridge orthogonal crustal segmentation with v shaped crustal ridges and troughs our findings indicate that crustal features of the reykjanes ridge and flanks are formed by shallow buoyant mantle instabilities fundamentally like at other slow spreading ridges and need not reflect deep mantle plume flow

recent ice surface elevation changes of fleming glacier in response to the removal of the wordie ice shelf antarctic peninsula regional climate warming has caused several ice shelves on the antarctic peninsula to retreat and ultimately collapse during recent decades glaciers flowing into these retreating ice shelves have responded with accelerating ice flow and thinning the wordie ice shelf on the west coast of the antarctic peninsula was reported to have undergone a major areal reduction before 1989 since then this ice shelf has continued to retreat and now very little floating ice remains little information is currently available regarding the dynamic response of the glaciers feeding the wordie ice shelf but we describe a chilean international polar year project initiated in 2007 targeted at studying the glacier dynamics in this area and their relationship to local meteorological conditions various data were collected during field campaigns to fleming glacier in the austral summers of 2007 08 and 2008 09 in situ measurements of ice flow velocity first made in 1974 were repeated and these confirm satellite based assessments that velocity on the glacier has increased by 40 50 since 1974 airborne lidar data collected in december 2008 can be compared with similar data collected in 2004 in collaboration with nasa and the chilean navy this comparison indicates continued thinning of the glacier with increasing rates of thinning downstream with a mean of 4 1 0 2 m a 1 at the grounding line of the glacier these comparisons give little indication that the glacier is achieving a new equilibrium

understanding the sam influence on the south pacific enso teleconnection the relationship between the el nino southern oscillation enso and the southern hemisphere annular mode sam is examined with the goal of understanding how various strong sam events modulate the enso teleconnection to the south pacific 45 70 s 150 70 w the focus is on multi month multi event variations during the last 50 years a significant p 0 10 relationship is observed most marked during the austral summer and in the 1970s and 1990s in most cases the significant relationship is brought about by la nina el nino events occurring with positive negative phases of the sam more often than expected by chance the south pacific teleconnection magnitude is found to be strongly dependent on the sam phase only when enso events occur with a weak sam or when a la nina el nino occurs with a positive negative sam phase are significant south pacific teleconnections found this modulation in the south pacific enso teleconnection is directly tied to the interaction of the anomalous enso and sam transient eddy momentum fluxes during la nina sam and el nino sam combinations the anomalous transient momentum fluxes in the pacific act to reinforce the circulation anomalies in the midlatitudes altering the circulation in such a way to maintain the enso teleconnections in la nina sam and el nino sam cases the anomalous transient eddies oppose each other in the midlatitudes overall acting to reduce the magnitude of the high latitude enso teleconnection

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

variations in oceanic crustal structure and implications for the fine scale magnetic anomaly signal marine magnetic anomalies are traditionally modelled using a constant thickness source layer usually defined as the extrusive lavas associated with seismic layer 2a recent seismic studies reveal that the thickness of layer 2a can vary by more than a factor of two an observation which has important implications for the magnetic anomaly source layer we model deep tow magnetometer data from the endeavour segment of the northern juan de fuca ridge in combination with nearby seismic data and find a positive correlation between the thickness of the seismically defined layer 2a and the magnetic anomaly source layer this suggests that short wavelength magnetic anomalies observed within individual magnetic polarity units can arise from variations in the thickness of the extrusives rather than geomagnetic field variations or secondary magnetization

life and death of axial volcanic ridges segmentation and crustal accretion at the reykjanes ridge abstract in this paper we present a description of ridge segmentation and segment evolution at the slow spreading reykjanes ridge based on the combined and integrated interpretation of several geophysical datasets acquired along this section of the mid atlantic ridge at the reykjanes ridge segmentation is manifest at the seabed most clearly as axial volcanic ridges avrs these correspond to third order segments while multiple adjacent avrs collectively represent the larger scale pattern of second order segmentation avrs are known to undergo a life cycle of multiple phases of magmatic accretion tectonic extension and dismemberment our combined interpretation of the results of the previously independently analysed datasets indicates that magma influx from the mantle to the crust associated with these cycles is initially focused towards second order segment centres adjacent avrs within a second order segment are progressively rejuvenated from the segment centre towards the segment tips during each cycle with the redistribution of magma along axis occurring within individual avrs at mid crustal to upper crustal level during the magmatic phase in some cases offset basins between adjacent avrs are characterised by significant crustal melt accumulation and increased crustal thickness indicating that they are currently sites of incipient avr growth the initiation of new avrs within former offset basins and the abandonment of other avrs indicates that third order segments have a finite and limited life span as part of this study we have mapped the geometry and location of all abandoned inactive relict avrs preserved off axis to form the basis of a reconstruction of the pattern of asymmetric spreading using this reconstruction we have developed a new model of avr evolution spanning 2 myr of crustal accretion that reveals nested scales and phases of accretion in which each avr undergoes multiple tectonomagmatic cycles before ending its life once it has migrated sufficiently far across the median valley to become isolated from its melt supply once extinct avrs are rafted further off axis and are preserved in the morphology of the seabed and the thickness of layer 2a

structure and petrology of the reykjanes ridge between 62 55 n and 63 48 n geophysical and petrologic data are presented on the northern extent of the reykjanes ridge as it nears iceland the crest of the reykjanes ridge is a small belt of rough topography superimposed on an elevated plateau the rough topography consists of a series of en echelon ridges immediately to the southwest of the reykjanes peninsula the ridges strike about n35 e and apparently represent a rotational adjustment toward the n40 e trends on iceland to the south the en echelon ridges strike about n25 e magnetic profiles place the peak of the magnetic anomaly either directly below or along the eastern side of the topographic highs eight distinctive en echelon volcanic systems swarms are identified and are structurally and petrologically indistinguishable from those of the western reykjanes ridge petrologically three rock types picrite basalts olivine tholeiites and tholeiites were recovered these form distinctive groups which match those described from the western reykjanes peninsula

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

age dependent variation in the magnetization of seamounts the magnetization of pacific seamounts shows a surprisingly large variation when viewed as a function of time similar to normal upper oceanic crust the basalts that form the external shell of volcanic seamounts have a high average magnetization immediately after formation this high magnetization decays rapidly with time resulting in a factor of 5 reduction in intensity in less than 500 000 years because submarine volcanoes appear to consist of the same basalts that form the upper part of the seafloor low temperature oxidation of the magnetic mineral titanomagnetite is assumed to be responsible for the reduction in magnetization for both types of crust some large oceanic volcanoes are of sufficient volume that the upper capping rocks erupt subaerially and submerge completely due to thermal subsidence only after several million years these capping rocks can be locally abundant i e hawaii are exposed to a different post formation environment and can have a magnetic mineralogy that is different from submarine volcanoes in spite of these differences the magnetization of subaerially erupted rocks of the hawaii emperor chain also may show a similar rapid reduction in intensity with age finally seamounts of cretaceous age show a factor of 2 higher average magnetization when compared to tertiary seamounts similar to the previously observed increased magnetization of cretaceous seafloor basalts recovered from deep sea drilling project ocean drilling project cores this increase in cretaceous magnetization appears to be reflected in the increased amplitude of magnetic anomalies in some but not all of the ocean basins prior and subsequent to the cretaceous normal superchron the processes potentially responsible for the elevated cretaceous magnetization values include a high geomagnetic field intensity large scale variations in whole rock major element geochemistry of the extrusive rocks or a systematic acquisition of a secondary magnetization during the cretaceous normal superchron