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

growth history of a diffusely venting sulfide structure from the juan de fuca ridge a petrological and geochemical study centimeter scale mineralogical and chemical analyses of a diffusely venting 280 c sulfide structure called roane from the mothra hydrothermal field on the juan de fuca ridge reveal a complex growth history these analyses document four well defined zones which from the exterior to the interior of the structure include a barite silica zone fossil worm tube zone a silica sulfide zone silica zone a sulfide silica zone outer sulfide zone and a sulfide gordaite silica zone inner sulfide zone these features are a product of a myriad of processes that include extensive mineral replacement ingress of seawater fracturing and breakout of hot hydrothermal fluids significant conductive cooling and the sustained broad scale outflow of warm fluids from the walls through a porous and permeable matrix roane lacks an open throughgoing chalcopyrite lined central conduit instead it hosts an anastomosing discontinuous network of tortuous channels within the interior of a sponge like matrix of amorphous silica sulfide sulfate and clay minerals megafaunal communities at the summit and sides of this mature diffusely venting chimney provide a constructional framework for new growth isolation of hydrothermal fluid from seawater during progressive fossilization of megafauna and the early formation of barite silica assemblages promote flow of higher temperature fluids within the interior continued isolation of interior higher temperature fluids through increased mineral precipitation and expansion of the structure leads to a zn sulfide pyrite dominated permeable matrix and a network of tortuous channels that form a central porous conduit this conduit is shielded by a weakly silicified metal sulfide inner wall and a strongly silicified outer wall the most outer portions of the walls are highly porous and sites of significant advection and mixing between hydrothermal fluids and seawater the mineralogy and chemistry of roane are strongly affected by ammonia ammonium buffering of the ph during cooling of the hydrothermal fluids the two major results of the buffering are 1 precipitation of zn sulfide at higher temperatures than are typical for bare rock systems leading to the correlation of cu zn cd and se and 2 increased deposition of amorphous silica and clay minerals

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

controls on extrusion at mid ocean ridges a magma lens can erupt to form extrusives only if it is under greater pressure than the static pressure in a column of magma reaching from the lens to the surface the excess pressure results partly from overburden pressure caused by the presence of high and low density rocks dikes and extrusives respectively above the lens the thicker the pile of low density extrusives the lower the average overburden density thus extrusion above a lens should be self regulating in that thickening the extrusive layer reduces the driving pressure for subsequent eruptions flexural stresses may affect extrusion by altering the pressure on a magma chamber for ridges lacking an axial valley we predict that deeper magma lenses should correlate with thicker extrusive layers consistent with recent observations

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

patterns of wind drifted snow on the alaskan arctic slope detected with ers 1 interferometric sar we tested whether spaceborne interferometric synthetic aperture radar insar could be used to reveal patterns of redistribution of wind drifted snow in arctic alaska based on a simple geometric model we found that lateral variations in new snow assuming a density of 0 3 g cm 3 accumulation of 11 cm or redistribution of the existing snow into dunes of half this height could produce decorrelation of c band interferograms comparison of interferograms with field observations for two periods from winter 1993 94 one with wind but little new snow and the second with wind and new snow indicates the interferograms delineated areas where the snow depth had changed due to drifting striking patterns of windward scouring and leeward deposition were revealed the interferograms also showed that during one high wind event conspicuous interferometric bands a few kilometers wide and 30 km long were formed downwind of a mountain ridge we speculate that these bands were caused by large scale alterations in the concentrations of moving snow particles a finding consistent with ground observations of alternating bands of clear air and blizzard for the same area and similar to phenomena observed with the advanced very high resolution radiometer and the geodetic earth observing satellite during blizzards in north dakota and iowa u s a

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

variable morphologic expression of volcanic tectonic and hydrothermal processes at six hydrothermal vent fields in the lau back arc basin ultrahigh resolution bathymetric maps 25 cm grid are used to quantify the physical dimensions of and spatial relationships between tectonic volcanic and hydrothermal features at six hydrothermal vent fields in the lau back arc basin supplemented with near bottom photos and nested within regional dsl 120a side scan sonar data these maps provide insight into the nature of hydrothermal systems along the eastern lau spreading center elsc and valu fa ridge vfr along axis transitions evident in localized volcanic morphology and tectonic characteristics include a change from broad low relief volcanic domes hundreds of meters wide 10 m tall that are dominated by pillow and lobate lava morphologies and are cut by faults and fissures to higher aspect ratio volcanic domes tens of meters wide tens of meters tall dominated by aa type lava morphologies with finger like flows and few tectonic structures these along axis differences in localized seafloor morphology suggest differences in hydrothermal circulation pathways within the shallow crust and correlate with regional transitions in a variety of ridge properties including the large scale morphology of the ridge axis shallow axial valley to axial high seafloor lava compositions and seismic properties of the upper crust differences in morphologic characteristics of individual flows and lava types were also quantified providing an important first step toward the remote characterization of complex terrains associated with hydrothermal vent fields

modelling of early winter snow density using fully polarimetric c band sar data in the indian himalayas abstract the seasonal snow cover contributes significantly to the water resource in the indian himalayas and snow density is one of the vital parameters in the determination of the hydrological potential of snow the application of conventional methods for snow density retrieval using fully polarimetric sar data is constrained by the properties of snow primarily due to the melt and frost cycles as compared to fresh dry snow the surface component of backscatter is significant in case of melt and frost in the conventional decomposition based methods the surface component is not considered for the inversion of permittivity as proposed in this study in this paper we also propose a methodology for the estimation of snow density using bi temporal fully polarimetric c band radarsat 2 synthetic aperture radar sar data we utilize the relation between the differential modified mueller matrix components the attenuation constants and the fresnel transmission coefficients the permittivity of snow is derived using the inversion of the fresnel transmission coefficients which is used to determine the snow density using a state of the art empirical relation the snow density estimates from the proposed method are compared with other methods based on coherency matrix decomposition and evaluated against in situ measurements collected during a field campaign carried out in dhundi in the state of himachal pradesh in india the snow density estimates using the proposed method are observed to correlate with the in situ measurements and were also found to be better than the decomposition based methods

insar assessment of surface deformations in urban coastal terrains associated with groundwater dynamics monitoring ground deformations arising from groundwater dynamics in dense urban coastal terrains is crucial for the sustainable development of infrastructures in these highly populated areas the city of montreal which is located in the saint laurent plain in eastern canada with its fast growing populations is a unique case study for other similar cities in coastal terrains the city undergoes high level house foundation damages with densities reaching up to 89 repairs km2 resulting from time dependent ground deformations that are correlated to groundwater dynamics and evapotranspiration using radarsat 2 c band synthetic aperture radar interferometry we observe 3 to 5 mm ground line of sight displacement variations temporally outphased by few months relative to the 2 m subartesian aquifer hydraulic head variations the deformations are observed over a 60 km2 area located in the central part of the montreal island in canada from 2008 to 2010 we observe displacements of 1 mm year uplift in the areas covered by 15 m thick clay layer these displacements are well correlated to the number of house repairs we also observe 2 mm year subsidence on elevated terrains associated with evapotranspiration the amplitudes of the displacements observed during this two year study are significant when integrated over the average lifetime of urban structures we conclude that the observed ground deformations are related to the seasonal variation of hydraulic head in most of the areas of montreal moreover wetter climate forecasts over upcoming decades for this area will accentuate groundwater level fluctuations thus more ground deformations are foreseen and have to be considered in future infrastructure design standards

a 65 k y time series from sediment hosted glasses reveals rapid transitions in ocean ridge magmas studies of ocean ridge magmatism have been hampered by the difficulty in constructing time series data over more than a few thousand years sediment rapidly covers newly formed ocean crust and older rocks even when recovered from fault scarps cannot be dated accurately ridge eruptions however disperse pyroclastic glass over distances as far as 5 km and these glasses have been shown to persist for thousands of years in on ridge sediment push cores here we present data on such glasses from a piston core that impacted basement in much older 600 ka sediment the age of deposition was determined using established stratigraphic methods to date the host sediment yielding an average sample resolution of a few thousand years and a continuous 65 k y time series the new time series data show systematic temporal variations in magma compositions related to a change to the dynamics of crustal storage which led to greater extents of pre eruptive differentiation shortly thereafter was a small but discernable shift toward more enriched primary melt compositions these events coincide with the onset of enhanced crustal production previously identified using seismic data and interpreted to reflect the capture of a hotspot by the ridge these results show the long term preservation of pyroclastic glasses and suggest that the construction of high resolution volcanic stratigraphy over a million years or more may be possible at ocean ridges using multiple piston cores that impact basement sediment hosted glasses have the potential to transform ocean ridges from the volcanic setting with the worst time series data to that with the best

evolution of seismic layer 2b across the juan de fuca ridge from hydrophone streamer 2 d traveltime tomography how oceanic crust evolves has important implications for understanding both subduction earthquake hazards and energy and mass exchange between the earth s interior and the oceans although considerable work has been done characterizing the evolution of seismic layer 2a there has been little analysis of the processes that affect layer 2b after formation here we present high resolution 2 d tomographic models of seismic layer 2b along 300 km long multichannel seismic transects crossing the endeavour northern symmetric and cleft segments of the juan de fuca ridge these models show that seismic layer 2b evolves rapidly following a different course than layer 2a the upper layer 2b velocities increase on average by 0 8 km s and reach a generally constant velocity of 5 2 0 3 km s within the first 0 5 myr after crustal formation this suggests that the strongest impact on layer 2b evolution may be that of mineral precipitation due to active hydrothermal circulation centered about the ridge crest and driven by the heat from the axial magma chamber variations in upper layer 2b velocity with age at time scales 0 5 ma show correlation about the ridge axis indicating that in the long term crustal accretion processes affect both sides of the ridge axis in a similar way below the 0 5 ma threshold differences in 2b velocity are likely imprinted during crustal formation or early crustal evolution layer 2b velocities at propagator wakes 5 0 0 2 km s where enhanced faulting and cracking are expected and at areas that coincide with extensional or transtensional faulting are on average slightly slower than in normal mature upper layer 2b analysis of the layer 2b velocities from areas where the hydrothermal patterns are known shows that the locations of current and paleohydrothermal discharge and recharge zones are marked by reduced and increased upper layer 2b velocities respectively additionally the distance between present up flow and down flow zones is related to the amount of sediment cover because as sediment cover increases and basement outcrops become covered direct pathways from the igneous basement through the seafloor are cut off forcing convective cells to find alternate paths

measuring snow and glacier ice properties from satellite satellite remote sensing is a convenient tool for studying snow and glacier ice allowing us to conduct research over large and otherwise inaccessible areas this paper reviews various methods for measuring snow and glacier ice properties with satellite remote sensing these methods have been improving with the use of new satellite sensors like the synthetic aperture radar sar during the last decade leading to the development of new and powerful methods such as sar interferometry for glacier velocity digital elevation model generation of ice sheets or snow cover mapping some methods still try to overcome the limitations of present sensors but future satellites will have much increased capability for example the ability to measure the whole optical spectrum or sar sensors with multiple polarization or frequencies among the methods presented are the satellite derived determination of surface albedo snow extent snow volume snow grain size surface temperature glacier facies glacier velocities glacier extent and ice sheet topography in this review emphasis is put on the principles and theory of each satellite remote sensing method an extensive list of references with an emphasis on studies from the 1990s allows the reader to delve into specific topics

dike induced faulting in rift zones of iceland and afar geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the earth9s surface elastic modeling of the vertical ground surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth a mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike therefore slip extending into or beyond the dike cavity must occur either 1 on faults that intersect the dike near its top above the zone of dike induced compression or 2 on faults that slip ahead of the dike as it propagates laterally data from iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike

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

sentinel 1 insar measurements of deformation over discontinuous permafrost terrain northern quebec canada abstract differential synthetic aperture radar interferometry d insar has been applied in permafrost environments to detect surface deformation caused by freeze thaw processes in the active layer and underlying permafrost the effectiveness of sentinel 1 insar in monitoring ground surface deformation over continuous permafrost terrain above the treeline has been proven the heterogeneous landscape and developed vegetation cover increase the difficulty of applying d insar in sub arctic discontinuous permafrost terrain the potential of sentinel 1 insar in such an environment has not been fully explored in this study we explore the capabilities and limitations of applying sentinel 1 time series data for monitoring surface deformation over discontinuous permafrost terrain the interferometric coherence time series from september 2016 to april 2018 were analyzed over typical landscapes in discontinuous permafrost environments and their thaw subsidence curves are revealed by the small baseline subset sbas insar technique the seasonal thaw subsidence in the summer of 2017 was in the range of 15 80 mm in the study area the land cover types with thaw subsidence magnitudes from low to high are exposed land peatland lichen low shrub lichen dominated and wetland low vegetation the difference in displacement pattern between lichen dominated and wetland low vegetation dominated permafrost terrains is especially clear at the end of the thawing stage in september and october the differences in thaw subsidence magnitude and pattern reveal the influence of the soil water content in the active layer and permafrost properties on the thaw subsidence patterns we also compared the sentinel 1 retrieved cumulative displacement with the x band terrasar x and l band alos palsar results the difference of retrieved deformation magnitude using the three sensors is amplified when shrubs are more developed the findings indicate that sentinel 1 time series with a 6 day or 12 day span work well over discontinuous permafrost terrain above the tree line i e tundra tundra wetlands and less developed shrub tundra environments during the thawed season but the results and accuracy are not promising over developed shrub tundra and especially forest tundra environments

quantifying diffuse and discrete venting at the tour eiffel vent site lucky strike hydrothermal field 1 the relative heat carried by diffuse versus discrete venting of hydrothermal fluids at mid ocean ridges is poorly constrained and likely varies among vent sites estimates of the proportion of heat carried by diffuse flow range from 0 to 100 of the total axial heat flux here we present an approach that integrates imagery video and temperature measurements to accurately estimate this partitioning at a single vent site tour eiffel in the lucky strike hydrothermal field along the mid atlantic ridge fluid temperatures photographic mosaics of the vent site and video sequences of fluid flow were acquired during the bathyluck 09 cruise fall 2009 and the momarsat 10 cruise summer 2010 to the lucky strike hydrothermal field by the rov victor6000 aboard the french research vessel the pourquoi pas ifremer france we use two optical methods to calculate the velocities of imaged hydrothermal fluids 1 for diffuse venting diffuse flow velocimetry tracks the displacement of refractive index anomalies through time and 2 for discrete jets particle image velocimetry tracks eddies by cross correlation of pixel intensities between subsequent images to circumvent video blurring associated with rapid velocities at vent orifices exit velocities at discrete vents are calculated from the best fit of the observed velocity field to a model of a steady state turbulent plume where we vary the model vent radius and fluid exit velocity our results yield vertical velocities of diffuse effluent between 0 9 cm s 1 and 11 1 cm s 1 for fluid temperatures between 3 c and 33 5 c above that of ambient seawater and exit velocities of discrete jets between 22 cm s 1 and 119 cm s 1 for fluid temperatures between 200 c and 301 c above ambient seawater using the calculated fluid velocities temperature measurements and photo mosaics of the actively venting areas we calculate a heat flux due to diffuse venting from thin fractures of 3 15 2 22 mw discrete venting of 1 07 0 66 mw and by incorporating previous estimates of diffuse heat flux density from tour eiffel diffuse flux from the main sulfide mound of 15 6 mw we estimate that the total integrated heat flux from the tour eiffel site is 19 82 2 88 mw and that the ratio of diffuse to discrete heat flux is 18 we discuss the implication of these results for the characterization of different vent sites within lucky strike and in the context of a compilation of all available measurements of the ratio of diffuse to discrete heat flux

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

high resolution near bottom vector magnetic anomalies over raven hydrothermal field endeavour segment juan de fuca ridge high resolution near bottom vector magnetic data were collected by remotely operated vehicle jason over the raven hydrothermal vent field 47 57 3 n 129 5 75 w located north of main endeavour vent field on the endeavour segment of the juan de fuca ridge the survey was part of a comprehensive heat flow study of the raven site using innovative thermal blanket technology to map the heat flux and crustal fluid pathways around a solitary hydrothermal vent field raven hydrothermal activity is presently located along the western axial valley wall while additional inactive hydrothermal deposits are found to the nw on the upper rift valley wall magnetic inversion results show discrete areas of reduced magnetization associated with both active and inactive hydrothermal vent deposits that also show high conductive heat flow higher spatial variability in the heat flow patterns compared to the magnetization is consistent with the heat flow reflecting the currently active but ephemeral thermal environment of fluid flow while crustal magnetization is representative of the static time averaged effect of hydrothermal alteration a general nw to se trend in reduced magnetization across the raven area correlates closely with the distribution of hydrothermal deposits and heat flux patterns and suggests that the fluid circulation system at depth is likely controlled by local crustal structure and magma chamber geometry magnetic gradient tensor components computed from vector magnetic data improve the resolution of the magnetic anomaly source and indicate that the hydrothermally altered zone directly beneath the raven site is approximately 15 106 m3 in volume

ridge asymmetry and deep aqueous alteration at the trench observed from rayleigh wave tomography of the juan de fuca plate using rayleigh wave tomography of noise removed ocean bottom seismometer obs data from the cascadia initiative we illuminate the structure of the upper mantle beneath the juan de fuca plate beneath the juan de fuca ridge there is strong asymmetry with a pronounced low velocity zone in the 25 65 km depth range extending to the west from the spreading axis this anomaly has velocities low enough to indicate the presence of melt the asymmetry in velocity structure and the much greater abundance of seamounts on the west flank of the ridge suggest that dynamic buoyant upwelling is important perhaps triggered by thermal or compositional anomalies beneath axial seamount in contrast there is no evidence for asymmetry in the axial zone or lower than expected velocities beneath the gorda ridge on the eastern flank of the juan de fuca ridge the shear velocity in the 25 65 depth range is higher than expected the lithosphere appears to be colder and thicker than predicted by standard plate cooling models perhaps caused by the downwelling counterpart of the upwelling on the west side of the ridge close to the trench there is a sharp decrease in shear velocity we interpret this as aqueous alteration caused by hydrothermal circulation through deep normal faults associated with bending of the plate beneath the astoria and nitinat fans where abyssal plain sediment is thickest the velocity decrease is much smaller which is consistent with a thick sediment cap that prevents hydrothermal alteration of the plate

comment on sensitivity of seafloor bathymetry to climate driven fluctuations in mid ocean ridge magma supply olive et al reports 16 october 2015 p 310 argue that 10 fluctuations in melt supply do not produce appreciable changes in ocean ridge bathymetry on time scales less than 100 000 years and thus cannot reflect sea level forcing spectral analysis of bathymetry in a region they highlight as being fault controlled however shows strong evidence for a signal from sea level variation

mapping small elevation changes over large areas differential radar interferometry a technique that uses synthetic aperture radar sar images to measure very small 1 cm or less surface motions with good resolution 10 m over large swaths 50 km is presented along with experimental results the method could be used for accurate measurements of many geophysical phenomena including swelling and buckling in fault zones residual displacements from seismic events and prevolcanic swelling the method is based on sar interferometry where two images are made of a scene by simultaneously flying two physically separated antennas then the phases of corresponding pixels are differenced and altitude formation is deduced from some simple computation and image rectification it is also possible to use one antenna flown twice over the same scene then if the second flight exactly duplicates the track of the first an interesting possibility occurs there would be no phase changes between the images at all unless there was a physical change in the scene such as ground swelling that would alter the distance from some resolution element to the antenna since the phase changes all occur at the short carrier wavelength the basic limitation on sensitivity is only the phase noise in the system when the two imaging passes are made from flight tracks that are separated which is the case with the seasat images used here it is no longer possible to distinguish surface changes from the parallax caused by topography however with some additional computation a third image made at some other baseline may be used to remove the topography and leave only the surface changes this method was applied using seasat data to an imaging site in imperial valley california where motion effects were observed that were ascribed to the expansion of water absorbing clays phase change images of this area are shown along with associated ground truth about the presence of water problems with the technique are explored along with a discussion of future experimental possibilities on upcoming sar missions like earth observing system eos earth resources satellite ers 1 sir c and the venus imaging radar magellan

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

magmatic and tectonic segmentation of the intermediate spreading costa rica rift a fine balance between magma supply rate faulting and hydrothermal circulation 3 d tomographic modelling of wide angle seismic data recorded at the intermediate spreading costa rica rift has revealed a p wave seismic velocity anomaly low located beneath a small overlapping spreading centre that forms a non transform discontinuity at the ridge axis this low velocity zone displays a maximum velocity anomaly relative to the background ridge axis crustal structure of 0 5 km s 1 has lateral dimensions of 10 5 km and extends to depths 2 5 km below the seabed placing it within layer 2 of the oceanic crust we interpret these observations as representing increased fracturing under enhanced tectonic stress associated with the opening of the overlapping spreading centre that results in higher upper crustal bulk porosity and permeability evidence for ongoing magmatic accretion at the costa rica rift ridge axis takes the form of an axial magma lens beneath the western ridge segment and observations of hydrothermal plume activity and microearthquakes support the presence of an active fluid circulation system we propose that fracture pathways associated with the low velocity zone may provide the system through which hydrothermal fluids circulate these fluids cause rapid cooling of the adjacent ridge axis and any magma accumulations which may be present the costa rica rift exists at a tipping point between episodic phases of magmatic and tectonically enhanced spreading the characteristics inherited from each spreading mode have been preserved in the crustal morphology off axis for the past 7 myr using potential field data we contextualize our seismic observations of the axial ridge structure at the whole segment scale and find that the proposed balance between magmatic and tectonically dominated spreading processes observed off axis may also be apparent along axis and that the current larger scale magma supply system at the costa rica rift may be relatively weak based on all available geophysical observations we suggest a model for the inter relationships between magmatism faulting and fluid circulation at the costa rica rift across a range of scales which may also be influenced by large lithosphere scale structural and or thermal heterogeneity

shallow axial magma chamber at the slow spreading erta ale ridge thermal models predict that spreading velocity at mid ocean rifts should influence the geometry of the underlying magma chamber insar data from the slow spreading ethiopian rift identify a shallow elongated magma chamber a feature usually associated with fast spreading rifts beneath the erta ale segment implying that spreading velocity may not be so important after all

correction to a statistical test of phase closure to detect influences on dinsar deformation estimates besides displacements and decorrelation noise two case studies in high latitude regions displacements of the earth s surface can be estimated using differential interferometric synthetic aperture radar the estimates are derived from the phase difference between two radar acquisitions when at least three such acquisitions are available one can compute the displacement between the first and the third acquisition and compare it with the sum of the two intermediate displacements these two are expected to be equal for a piston like spatially uniform deformation however this is not necessarily the case in measured data such lack of phase closure can be due to decorrelation noise alone it has also been attributed to complex scattering processes such as soil moisture changes or multiple scattering sources however the nature of these nonrandom effects is only poorly understood in cold regions as the role of snow and freeze thaw processes has not been studied to date to distinguish the noise like and the systematic effects an asymptotic wald significance test is proposed it detects situations when the observed closure error cannot solely be explained by noise such situations with p are observed at the ku band during snow metamorphism and melt and following a summer precipitation event in sodankyla finland they can also be prevalent 25 in the x band observations of ice rich permafrost regions in the lena delta russia indicating the presence of processes that can have systematic and deleterious impacts on the estimation of surface movements satellite based monitoring of these displacements is thus possibly subject to complex error sources in high latitude regions

temporal and spatial history of the 1999 2000 endeavour segment seismic series juan de fuca ridge 1 two large clusters of earthquake activity in june of 1999 and january of 2000 have dominated recent seismicity along the endeavour segment of the juan de fuca ridge the impacts of the june 1999 sequence on the hydrologic system which include changes in vent temperature and chemistry within the main endeavour vent field have been well documented previously analysis of seismic and hydroacoustic data indicates that both sequences exhibit a swarm like behavior characterized by the absence of a dominant main shock event the epicentral locations of events within the two swarms overlap spatially with centroid positions near 47 49 and 47 46 n latitude during the june 1999 swarm the initial activity spans the along axis region where a shallow axial magma chamber reflector was later imaged the epicenters then migrate 12 km to the south at a rate of 0 3 m s consistent with lateral dike propagation a distinct subcluster of events also occurred in the vicinity of surveyor volcano on the overlapping portion of the cobb segment given its distance from the main swarm this activity may represent a triggered response to dynamic shaking the january 2000 swarm has a more limited along axis extent relative to the june 1999 swarm with no indication of lateral migration much of this activity is concentrated in a region predicted to have undergone extension due to dike propagation in 1999 although it contains fewer total events and is of shorter duration relative to the june 1999 swarm the january 2000 activity exhibits a higher peak rate of seismicity and greater mean event magnitude as in situ temperature monitoring was not in place during january 2000 and vent fluids were not sampled until june 2000 the impacts of this swarm on the hydrothermal system are unknown the southernmost tip of the endeavour segment also is found to be a region of repeating swarm activity although morphologic evidence indicates the cobb segment has been propagating northward recently this seismic activity suggests that the western limb of the endeavour cobb overlap zone remains active

termination of a 6 year ridge spreading event observed using a seafloor seismic network on the endeavour segment juan de fuca ridge 1 we present automatically determined epicenters and magnitudes for 36 523 earthquakes recorded along the endeavour segment between august 2003 and october 2006 using a local ocean bottom seismometer obs network the catalog is dominated by two swarm sequences in january and february 2005 in the vicinity of the endeavour overlapping spreading center which included earthquakes in west valley the northern portion of the endeavour segment southwest endeavour valley and the endeavour vent fields these swarms are attributed to volcanism including a dike intrusion on the northern endeavour in february 2005 and smaller diking events on the propagating tip of the west valley segment in both swarms the dike on the northern endeavour propagated to the south which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment following the swarms seismic activity on the endeavour segment decreased on average to 15 of pre swarm values and almost ceased at the segment ends we infer that a 6 year non eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate spreading stresses the inferred coupling between the 1999 and 2005 events the observation of extensive precursory activity prior to the 2005 swarms and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation hydraulic diffusion and magma withdrawal and replenishment

repeat pass sar interferometry over forested terrain repeat pass synthetic aperture radar sar interferometry provides the possibility of producing topographic maps and geocoded as well as radiometrically calibrated radar images however the usefulness of such maps and images depends on our understanding of how different types of terrain affect the radar measurements it is essential that the scene coherence between passes is sdcient in this paper we derive a general system model including both radar system and scene scattering properties the model is used to interpret measurements over a forested area where the scene coherence varies between 0 2 and 0 5 the coherence is found to be sensitive to temperature changes around 0 c but surprisingly insensitive to wind speed the interferometric height discontinuity at the forest to openfield boundary shows good agreement with in situ tree height measurements for a dense boreal forest but is observed to decrease for a less dense forest this suggests the possibility of estimating bole volume from the interferometric tree height and a ground dem the decrease of scene coherence over a dense forest with increasing baseline is also used to estimate the effective scattering layer thickness

plate motion at the ridge transform boundary of the south cleft segment of the juan de fuca ridge from gps acoustic data 1 we measure the present day plate velocity of the juan de fuca ridge 25 km off axis to be 63 6 3 6 mm a at s67 2 e 7 9 degrees 1 relative to the pacific plate pa this velocity was derived from gps acoustic gpsa measurements in 2000 2001 2002 and 2003 that observed the position of a seafloor array 44 43 n 130 03 w 2900 m depth with a repeatability of 4 6 mm three transient events at the juan de fuca ridge and blanco transform account for 10 of this motion in viscoelastic modeling suggesting that the observed gpsa pa velocity is due primarily to steady state plate dynamics subtracting the modeled transient motion gives a velocity of 57 3 3 9 mm a at s72 9 e 12 1 degrees 1 which is consistent at the 95 confidence level with the velocity calculated from the wilson 1993 0 0 78 ma euler pole therefore this site is interpreted to be in a region of continuous full rate plate motion a robust result of this study which holds with and without correcting for transient motions these results provide direct geodetic evidence that spreading occurs predominantly within 25 km of the axis at this intermediate spreading rate ridge previously reported geodetic monitoring across the 1 km wide axial valley from 1994 1999 and 2000 2003 shows no significant extension chadwell et al 1999 hildebrand et al 1999 chadwick and stapp 2002 w w chadwick personal communication 2006 and seismic monitoring shows no activity this suggests the crust between 0 5 and 25 km off axis accommodates 26 mm of aseismic deformation each year through some combination of near axis fault motion and elastic strain accumulation

on the derivation of coseismic displacement fields using differential radar interferometry the landers earthquake we present a map of the coseismic displacement field resulting from the landers california june 28 1992 earthquake derived using data acquired from an orbiting high resolution radar system we achieve results more accurate than previous space studies and similar in accuracy to those obtained by conventional field survey techniques data from the ers 1 synthetic aperture radar instrument acquired in april july and august 1992 are used to generate a high resolution wide area map of the displacements the data represent the motion in the direction of the radar line of sight to centimeter level precision of each 30 m resolution element in a 113 km by 90 km image our coseismic displacement contour map gives a lobed pattern consistent with theoretical models of the displacement field from the earthquake fine structure observed as displacement tiling in regions several kilometers from the fault appears to be the result of local surface fracturing comparison of these data with global positioning system and electronic distance measurement survey data yield a correlation of 0 96 thus the radar measurements are a means to extend the point measurements acquired by traditional techniques to an area map format the technique we use is 1 more automatic 2 more precise and 3 better validated than previous similar applications of differential radar interferometry since we require only remotely sensed satellite data with no additional requirements for ancillary information the technique is well suited for global seismic monitoring and analysis

geology and hydrothermal evolution of the mothra hydrothermal field endeavour segment juan de fuca ridge detailed characterization of the mothra hydrothermal field the most southern and spatially extensive field on the endeavour segment of the juan de fuca ridge provides new insights into its geologic and hydrothermal development meter scale bathymetry side scan sonar imagery and direct dive observations show that mothra is composed of six actively venting sulfide clusters spaced 40 200 m apart chimneys within each cluster have similar morphology and venting characteristics and all clusters host a combination of active and extinct sulfide structures black smoker chimneys venting fluids above 300 c are rare while more common lower temperature diffusely venting chimneys support dense colonies of macrofauna and bacterial mat hydrothermal sediment and extinct sulfide debris cover 10 15 m of the seafloor surrounding each vent cluster obscuring the underlying basaltic substrate of light to moderately sedimented pillow lobate sheet and chaotic flows basalt talus and collapse terrain extinct sulfide chimneys and debris between the clusters indicate that hydrothermal flow was once more widespread and that it has shifted spatially over time the most prominent structural features in the axial valley at mothra are regional 020 trending faults and fissures and north south trending collapse basins the location of actively venting clusters within the field is controlled by 1 localization of fluid upflow along the western boundary fault zone and diversion of these fluids by antithetic faults to feed vent clusters near the western valley wall and 2 tapping of residual magmatic heat in the central part of the axial valley which drives flow beneath vent clusters directly adjacent to the collapse basins 70 90 m east of the western valley wall these processes form the basis for a model of axial valley and hydrothermal system development at mothra in which the field is initiated by an eruptive diking episode and sustained through intense microseismicity and non eruptive diking events

gravity and seismic study of crustal structure along the juan de fuca ridge axis and across pseudofaults on the ridge flanks variations in topography and seismic structure are observed along the juan de fuca jdf ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation here we analyze gravity data coregistered with multichannel seismic data from the jdf ridge and flanks in order to better understand the origin of crustal structure variations in this area the data were collected along the ridge axis and along three ridge perpendicular transects at the endeavor northern symmetric and cleft segments negative mantle bouguer anomalies of 21 to 28 mgal are observed at the axis of the three segments thicker crust at the endeavor and cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies 3 7 mgal additional low densities elevated temperatures within and or below the axial crust are required to explain the remaining axial mba low at all segments gravity models indicate that the region of low densities is wider beneath the cleft segment gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron enriched crust on the young crust side of the pseudofaults a 10 20 km wide zone of thicker crust is found reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment

accuracy of three dimensional glacier surface velocities derived from radar interferometry and ice sounding radar measurements we present a method for analyzing the errors involved in measuring three dimensional glacier velocities with interferometric radar we address the surface parallel flow assumption and an augmented approach with a flux divergence fd term the errors in an interferometric ers 1 2 satellite radar dataset with ascending and descending orbit data covering storstrommen glacier northeast greenland are assessed the fd error assessment is carried out on airborne 60 mhz ice sounding radar data from the same area a simple model of an interferometric radar system is developed and analyzed the error sources considered include phase noise atmospheric distortions baseline calibration errors a dry snow layer and the stationary flow assumption used in differential interferometry the additional error sources in the analysis of fd errors are noise bias and unknown variations of the ice thickness and approximations of the ice flow model the example glacier is now building up following a surge the analysis shows that in the case study presented the errors are small enough to justify the use of both the estimated surface parallel flow term of the vertical velocity and the estimated fd term of the vertical velocity

spatio temporal variability of x band radar backscatter and coherence over the lena river delta siberia abstract satellite based monitoring strategies for permafrost remain under development and are not yet operational remote sensing allows indirect observation of permafrost a subsurface phenomenon by mapping surface features or measuring physical parameters that can be used for permafrost modeling we have explored high temporal resolution time series of terrasar x backscatter intensity and interferometric coherence for the period between august 2012 and september 2013 to assess their potential for detecting major seasonal changes to the land surface in a variety of tundra environments within the lena river delta siberia the terrasar x signal is believed to be strongly affected by the vegetation layer and its viability for the retrieval of soil moisture for example is therefore limited in our study individual events such as rain and snow showers that occurred at the time of terrasar x acquisition or a refrozen crust on the snowpack during the spring melt were detected based on backscatter intensity signatures the interferometric coherence showed marked variability the snow cover onset and snow melt periods were identified by significant reduction in coherence principal component analysis provided a good spatial overview of the essential information contained in backscatter and coherence time series and revealed latent relationships between both time series and the surface temperature the results of these investigations suggest that although x band sar has limitations with respect to monitoring seasonal land surface changes in permafrost areas high resolution time series of terrasar x backscatter and coherence can provide new insights into environmental conditions

differentiation of environments of dolomite formation lower cretaceous of central tunisia combined field sedimentological mineralogical isotopic and geochemical study of the lower cretaceous dolomites of central tunisia has demonstrated considerable diversity in origin environments of dolomite formation include deep phreatic karst lacustrine and evaporitic sabkha all four groups of dolomite are composed of non stoichiometric and or disordered crystals which are more or less rich in calcium and in iron petrographic fabrics are of three types replacement recrystallization and cementation these three fabrics are proposed among the various criteria for the different environments of dolomitization average isotope 18 o and 13 c contents for these four dolomite groups range from 10 5 to 0 4 o pdb and 3 9 to 3 7 o respectively the distribution of strontium is related both to the degree of recrystallization and to the palaeosalinity ferrous iron also very common is regarded as an indicator of relatively deep reducing conditions mainly in meteoric groundwaters sodium distribution is related to inclusions within the dolomite its distribution being relatively constant in all four groups it cannot be regarded as a reliable criterion for palaeosalinity of dolomitizing fluids this study confirms that dolomitization may occur under widely different palaeoenvironments either at the surface or during burial with the exception of the sabkha environment dolomitizing fluids seem to have been essentially meteoric

using a ground based sar interferometer and a terrestrial laser scanner to monitor a snow covered slope results from an experimental data collection in tyrol austria in this paper we report on an experimental activity aimed at investigating the potential of two terrestrial remote sensing techniques namely ground based sar gb sar interferometry and terrestrial laser scanning in order to retrieve snow depth sd measurements in mountainous regions terrestrial laser scanning is a more consolidated technique based on the measurement of the optical near infrared reflectivity and it is affected by the surface of the snow layer a temporal data sequence allows us to estimate the absolute sd variation recent use of sar interferometry to evaluate snow mass characteristics is based on relating the measured interferometric phase shift to a change in the snow mass interferometric gb sar measurements and terrestrial laser scanner scans were collected together with pointwise conventional measurements of physical snow parameters during the winters of 2005 2006 and 2006 2007 the experiment was carried out in the wattener lizum a high alpine area at about 2000 m elevation north of the main ridge of the austrian alps in tyrol notwithstanding the difficulty of providing both lengthy data record in dry snow conditions and detailed knowledge of the observed snow characteristics the obtained results confirmed the presence of a clearly measurable interferometric phase variation in relation to the growing height of the snow layer a comparison of the sd maps obtained through the two techniques shows differences partly due to the different nature of the two observations

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

upper crustal structure and axial topography at intermediate spreading ridges seismic constraints from the southern juan de fuca ridge 1 we use multichannel seismic reflection data to image the upper crustal structure of 0 620 ka crust along the southern juan de fuca ridge the study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow cleft and wide vance axial summit grabens asg along most of the axis of both segments we image the top of an axial magma chamber amc the amc along cleft deepens from south to north from 2 0 km beneath the ridge cleft observatory and hydrothermal vents near the southern end of the segment to 2 3 km at the northern end near the site of the 1980s eruptive event along the vance segment the amc also deepens from south to north from 2 4 to 2 7 km seismic layer 2a interpreted as the basaltic extrusive layer is 250 300 m thick at the ridge axis along the cleft segment and 300 350 m thick along the axis of the vance segment however off axis layer 2a is similar in both segments 500 600 m indicating 90 and 60 off axis thickening at the cleft and vance segments respectively half of the thickening occurs sharply at the walls of the asg with the remaining thickening occurring within 3 4 km of the asg along the full length of both segments layer 2a is thinner within the asg compared to the ridge flanks previous studies argued that the asg is a cyclic feature formed by alternating periods of magmatism and tectonic extension our observations agree with the evolving nature of the asg however we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply thus the asg rather than being formed by excess volcanism is a rifted flexural axial high the changes in axial morphology affect the distribution of lava flows along the ridge flanks as indicated by the pattern of layer 2a thickness the fluctuations in magma supply may occur at all spreading rates but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges

sulfide geochronology along the endeavour segment of the juan de fuca ridge 1 forty nine hydrothermal sulfide sulfate rock samples from the endeavour segment of the juan de fuca ridge northeastern pacific ocean were dated by measuring the decay of 226ra half life of 1600 years in hydrothermal barite to provide a history of hydrothermal venting at the site over the past 6000 years this dating method is effective for samples ranging in age from 200 to 20 000 years old and effectively bridges an age gap between shorter and longer lived u series dating techniques for hydrothermal deposits results show that hydrothermal venting at the active high rise sasquatch and main endeavour fields began at least 850 1450 and 2300 years ago respectively barite ages of other inactive deposits on the axial valley floor are between 1200 and 2200 years old indicating past widespread hydrothermal venting outside of the currently active vent fields samples from the half graben on the eastern slope of the axial valley range in age from 1700 to 2925 years and a single sample from outside the axial valley near the westernmost valley fault scarp is 5850 205 years old the spatial relationship between hydrothermal venting and normal faulting suggests a temporal relationship with progressive younging of sulfide deposits from the edges of the axial valley toward the center of the rift these relationships are consistent with the inward migration of normal faulting toward the center of the valley over time and a minimum age of onset of hydrothermal activity in this region of 5850 years

models of ocean ridge lithospheric deformation dependence on crustal thickness spreading rate and segmentation we use three dimensional 3 d temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid ocean ridge lithospheric structure we find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along axis gradients in crustal temperature and a pita pocket shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle in contrast fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis the overthickened crust produced by ridge hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling creating an extremely weak lithosphere in a steady state model we apply a simple two dimensional 2 d cyclic faulting model to across axis sections of temperature and rheological structure slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing in contrast fast spreading and hotspot affected segments are predicted to have much smaller variations in faulting styles except in the immediate vicinity of a major transform fault the presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness spreading rate and the ability of rift bounding normal faults to propagate along axis for fast spreading and hotspot affected segments the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma chamber supported isostatic features this is in contrast to most slow spreading segments where axial rift valleys are predicted to be a dominant morphological feature for a slow spreading segment of great length however the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center

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

magmatic cycles pace tectonic and morphological expression of rifting afar depression ethiopia the existence of narrow axial volcanic zones of mid oceanic ridges testifies of the underlying concentration of both melt distribution and tectonic strain as a result of repeated diking and faulting axial volcanic zones therefore represent a spectacular topographic expression of plate divergence however the submarine location of oceanic ridges makes it difficult to constrain the interplay between tectonic and magmatic processes in time and space in this study we use the dabbahu manda hararo dmh magmatic rift segment afar ethiopia to provide quantitative constraints on the response of tectonic processes to variations in magma supply at divergent plate boundaries the dmh magmatic rift segment is considered an analogue of an oceanic ridge exhibiting a fault pattern extension rate and topographic relief comparable to intermediate to slow spreading ridges here we focus on the northern and central parts of dmh rift where we present quantitative slip rates for the past 40 kyr for major and minor normal fault scarps in the vicinity of a recent september 2005 dike intrusion the data obtained show that the axial valley topography has been created by enhanced slip rates that occurred during periods of limited volcanism suggestive of reduced magmatic activity probably in association with changes in strain distribution in the crust our results indicate that the development of the axial valley topography has been regulated by the lifetimes of the magma reservoirs and their spatial distribution along the segment and thus to the magmatic cycles of replenishment differentiation 100 kyr our findings are also consistent with magma induced deformation in magma rich rift segments the record of two tectonic events of metric vertical amplitude on the fault that accommodated the most part of surface displacement during the 2005 dike intrusion suggests that the latter type of intrusion occurs roughly every 10 kyr in the northern part of the dmh segment

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

insar for estimation of changes in snow water equivalent of dry snow this paper describes the theoretical relation between interferometric phase and changes in snow water equivalent swe and show results from experiments using ers 1 2 tandem data the main scattering contribution from a dry snow cover is from the snow ground interface however the radar wave will be refracted in the snow thus only small changes in the snow properties between two interferometric synthetic aperture radar sar images will change the interferometric phase this phase change is shown to introduce a significantly increase in the digital elevation model dem height error although no effects are observed on the degree of coherence the phase change is also shown to affect the differential interferometric results and may wrongly be interpreted as range displacement the presented theory and results implies that light snowfall and or small changes in snow properties between interferometric sar insar image acquisitions may introduce significant height errors in dem derived from glaciers ice sheets or bare ground even in the case of high degree of coherence thus meteorological observations in addition to degree of coherence must be considered when generating dem in areas covered with snow or where snow fall is likely to have occurred

penetration depths inferred from interferometric volume decorrelation observed over the greenland ice sheet radar interferometry provides a novel way to study the subsurface of glaciers interferometric correlation since the two complex images that comprise the interferogram have slightly different incidence angles at each point on the ground a decorrelating phase noise with statistics related to the scattering medium is present in the interferogram the amount of surface decorrelation is increased by volume scatter the larger the vertical extent of the scatterers contributing to the radar echo the greater the decorrelation will be by modeling this effect the authors can estimate radio wave penetration depths within the upper layers of the glacier or ice sheet observations of the greenland ice sheet using ers data yield penetration depths one way 1 e point for power that range from 12 to 35 m due to the contribution of volume scatter the critical interferometer baseline is decreased and the authors find for the greenland data the baseline must be restricted to be less than 300 m the authors also compare penetration depths measured within the dry snow zone with those found in the percolation zone and coastal areas they find that as expected the rocky coastal areas evidence minimal penetration interestingly the penetration depths that the authors measure in the percolation zone spl sim 23 m indicate a large degree of volume scatter which is contrary to earlier results that found the scattering in the percolation zone dominated by structures in the first few meters this discrepancy may be due to unmodeled scattering behavior or the radar return may indeed include significant contributions from scatterers far beneath the surface