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

seismic evidence for variations in axial magma chamber properties along the southern juan de fuca ridge abstract multichannel seismic data collected along the cleft segment on the southern juan de fuca ridge show that this intermediate spreading center is underlain by a mid crustal reflector interpreted as the top of an axial magma chamber amc the amc reflection is present along most of the segment and deepens gently from 2 0 km near the southern end of the segment beneath the ridge cleft observatory site to 2 3 km at the northern end beneath the site of the mid 1980s submarine eruption we analyzed the one dimensional seismic structure of the amc at two locations with contrasting lava chemistry beneath two different hydrothermal vent fields at the northern site waveform modeling in the time intercept slowness p domain indicates that the amc is 100 m thick and it is characterized by a decrease in p wave velocity from 6 km s to 3 7 km s in contrast the p wave velocity within the shallower 100 m thick amc at the southern site is higher 5 0 km s the decrease in seismic velocity within the amc indicates that it is partially molten and that it is not a cracking front as previously suggested for other intermediate spreading segments the data show a coherent seismic phase interpreted as the p to s wave conversion at the amc pamcs stacking of this event shows that the pamcs is only present along the northern part of the segment our results thus suggest along axis variations in the crystallinity of the amc the amc along cleft varies from a high crystal content

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

numerical modeling of phase separation at main endeavour field juan de fuca ridge 1 before being disrupted by a magmatic event in 1999 the vent temperatures and salinities along the axis of the main endeavour field on the juan de fuca ridge exhibited a quasi steady spatial gradient in which the southern vent fluids were hotter and less saline than the northern vent fluids we present 2 d numerical models of two phase flow in a nacl h2o system to understand these gradients we consider homogenous permeability models with a range of bottom boundary temperature distributions and heterogeneous permeability models by imposing layer 2a extrusives with a constant bottom boundary temperature distribution the aim is to understand the impact of both bottom boundary temperature and layer 2a permeability on hydrothermal fluids and to determine what combination of these controlling factors could cause the observed trend we find that variations in bottom boundary temperature alone cannot explain the span of surface temperatures and salinities measured at the main endeavour field heterogeneous permeability within layer 2a that has higher overall permeability in the northern part of the vent field than the southern part can reproduce the observed north to south temperature gradient but such a permeability distribution cannot reproduce the observed salinity gradient we conclude that both deep seated heterogeneous permeability perhaps localized by a fault zone and a heterogeneous layer 2a are required to produce the trend of temperatures and salinities in vent fluids at the main endeavour field prior to the 1999 event

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

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

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

magmatic architecture within a rift segment articulate axial magma storage at erta ale volcano ethiopia abstract understanding the magmatic systems beneath rift volcanoes provides insights into the deeper processes associated with rift architecture and development at the slow spreading erta ale segment afar ethiopia transition from continental rifting to seafloor spreading is ongoing on land a lava lake has been documented since the twentieth century at the summit of the erta ale volcano and acts as an indicator of the pressure of its magma reservoir however the structure of the plumbing system of the volcano feeding such persistent active lava lake and the mechanisms controlling the architecture of magma storage remain unclear here we combine high resolution satellite optical imagery and radar interferometry insar to infer the shape location and orientation of the conduits feeding the 2017 erta ale eruption we show that the lava lake was rooted in a vertical dike shaped reservoir that had been inflating prior to the eruption the magma was subsequently transferred into a shallower feeder dike we also find a shallow horizontal magma lens elongated along axis inflating beneath the volcano during the later period of the eruption edifice stress modeling suggests the hydraulically connected system of horizontal and vertical thin magmatic bodies able to open and close are arranged spatially according to stresses induced by loading and unloading due to topographic changes our combined approach may provide new constraints on the organization of magma plumbing systems beneath volcanoes in continental and marine settings

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

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

segmentation and eruptive activity along the east pacific rise at 16 n in relation with the nearby mathematician hotspot the 16 n segment of the east pacific rise is the most overinflated and shallowest of this fast spreading ridge in relation with an important magma flux due to the proximity of the mathematician hot spot here we analyze the detailed morphology of the axial dome and of the axial summit trough ast the lava morphology and the geometry of fissures and faults in regard to the attributes of the magma chamber beneath and of the nearby hotspot the data used are 1 m resolution bathymetry combined with seafloor photos and videos at the dome summit the ast is highly segmented by 10 third order and fourth order discontinuities over a distance of 30 km often two contiguous and synchronous asts coexist such a configuration implies a wide 1100 m minimum zone of diking the existence of contiguous asts their mobility their general en echelon arrangement accommodating the bow shape of the axial dome toward the hotspot plus the existence of a second magma lens under the western half of the summit pla teau clearly reflect the influence of the hotspot on the organization of the spreading system the different asts exhibit contrasted widths and depths we suggest that narrow asts reflect an intense volcanic activity that produces eruptions covering the tectonic features and partially filling the asts ast widening and deepening would indicate a decrease in volcanic activity but with continued dike intrusions at the origin of abundant sets of fissures and faults that are not masked by volcanic deposits

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

influence of ridge migration and proximity to hot spots on the morphology of slow and intermediate spreading centers recent observations of ridge bathymetry suggest magmatic segmentation at fast and intermediate spreading centers is linked to the migration of the spreading axis over the mantle at fast and intermediate spreading centers carbotte et al 2004 observed that leading segments those that are offset in the direction of ridge migration are typically shallower interpreted to be magmatically robust relative to trailing segments across first and second order discontinuities the model set forth for this correlation invokes asymmetrical mantle upwelling in response to the absolute motion of the ridge axis and the entrainment of melt from across discontinuities in this investigation differences in ridge axis depth across first and second order discontinuities are examined within the context of absolute plate motions for portions of the slow spreading mid atlantic ridge mar 22 36 n and 25 35 s latitude intermediate spreading galapagos spreading center gsc 83 98 w and intermediate spreading south east indian ridge seir 77 114 e portions of each of these areas display chemical and or physical anomalies resulting from hot spot ridge interaction along non hot spot influenced sections of intermediate spreading ridges leading segments coincide with shallower segments across 72 of the first and second order discontinuities 86 of transform faults and 55 of second order discontinuities depth asymmetries vary with ridge offset length with maximum asymmetries for ridge offset lengths of 50 100 km a weaker correlation is observed between ridge migration direction and ridge morphology at the slow spreading mar where leading segments are shallower across 60 of first and second order discontinuities for hot spot influenced spreading centers hot spot proximity dominates ridge morphology at intermediate spreading centers but it is not a consistent predictor of axial depth asymmetries at slow spreading centers this spreading rate dependent influence of ridge migration and hot spot proximity on axial morphology may reflect a more limited entrainment of melt from across slow spreading discontinuities due to the predominance of three dimensional upwelling and melt focusing to segment centers

waning magmatic activity along the southern explorer ridge revealed through fault restoration of rift topography 1 we combine high resolution bathymetry acquired using the autonomous underwater vehicle abe with digital seafloor imagery collected using the remotely operated vehicle ropos across the axial valley of the southern explorer ridge ser to infer the recent volcanic and tectonic processes the ser is an intermediate spreading ridge located in the northeast pacific it hosts the magic mountain hydrothermal vent we reconstruct the unfaulted seafloor terrain at ser based on calculations of the vertical displacement field and fault parameters the vertical changes between the initial and the restored topographies reflect the integrated effects of volcanism and tectonism on relief forming processes over the last 11 000 14 000 years the restored topography indicates that the axial morphology evolved from a smooth constructional dome 500 m in diameter to a fault bounded graben 500 m wide and 30 70 m deep this evolution has been accompanied by changes in eruptive rate with deposition of voluminous lobate and sheet flows when the ser had a domed morphology and limited extent low effusion rate pillow eruptions during graben development most of the faults shaping the present axial valley postdate the construction of the dome our study supports a model of cyclic volcanism at the ser with periods of effusive eruptions flooding the axial rift centered on the broad plateau at the summit of the ridge followed by a decrease in eruptive activity and a subsequent dominance of tectonic processes with minor low effusion rate eruptions confined to the axial graben the asymmetric shape of the axial graben supports an increasing role of extensional processes with a component of simple shear in the spreading processes

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

plate tectonics magma for 50 000 years intrusions of magma into the crust help accommodate the divergence between tectonic plates a magnetotelluric survey of the crust and mantle beneath afar ethiopia has identified enough magma to accommodate plate separation there for about 50 000 years

climatology synoptic conditions and misanalyses of mississippi river valley drylines abstractthe dryline is an important focal point for convection initiation although drylines most commonly occur on the southern great plains dryline passages and subsequent severe weather outbreaks have been documented in the mississippi river valley this study presents a 15 yr 1999 2013 climatology of these mississippi river valley drylines and associated severe weather additionally synoptic patterns are identified that may result in drylines moving atypically far eastward into the mississippi river valley in total 39 mississippi river valley drylines hereafter referred to as mrv drylines were identified from the north american regional reanalysis narr dataset through the study period mean and anomaly synoptic composites were created for these drylines mrv dryline events typically occur under synoptically active conditions with an amplified upper air pattern a 500 hpa shortwave trough to the west or northwest of the dryline and a strong surface cyclone to the north these boundaries are

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

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

upper crustal seismic structure along the southeast indian ridge evolution from 0 to 550 ka and variation with axial morphology 1 the seismic structure of uppermost crust evolves after crustal formation with precipitation of alteration minerals during ridge flank hydrothermal circulation however key parameters of crustal evolution including depth extent and rates of change in crustal properties and factors contributing to this evolution remain poorly understood here long offset multichannel seismic data are used to study the evolution of seismic layer 2a and uppermost 2b from 0 to 550 ka at three segments of the intermediate spreading rate southeast indian ridge the segments differ in on axis morphology and structure with crustal magma bodies imaged at axial high and rifted high segments p1 and p2 but not at axial valley segment s1 and marked differences in thermal conditions within the upper crust are inferred one dimensional travel time modeling of common midpoint supergathers is used to determine the thickness and velocity of layer 2a and velocity of uppermost 2b at all three segments layer 2a velocities are higher in 550 ka crust than on axis by 7 14 with the largest increases at segments p1 and p2 velocities increase more rapidly by 125 ka at p1 with spatial variations in velocity gradients linked to location of the underlying crustal magma body we attribute these differences in crustal evolution to higher rates of fluid flow and temperatures of reaction at these ridge segments where crustal magma bodies are present layer 2a thickens off axis at segments p1 and p2 but not at s1 both off axis volcanic thickening and downward propagation of a cracking front linked to the vigor of axial hydrothermal activity could contribute to these differences in zero age crust layer 2b velocities are significantly lower at segments p1 and p2 than s1 5 0 5 4 and 5 8 km s respectively whereas similar velocities are measured off axis at all segments 5 7 5 9 km s lower on axis 2b velocities at segments p1 and p2 can be partly attributed to thinner layer 2a with lower overburden pressures leading to higher porosities in shallowest 2b however other factors must also contribute likely candidates include subaxial deformation due to magmatic processes and enhanced cracking with axial hydrothermal activity at these segments with crustal magma bodies

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

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

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

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

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

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

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

anatomy of an active submarine volcano most of the magma erupted at mid ocean ridges is stored in a mid crustal melt lens that lies at the boundary between sheeted dikes and gabbros nevertheless images of the magma pathways linking this melt lens to the overlying eruption site have remained elusive here we have used seismic methods to image the thickest magma reservoir observed beneath any spreading center to date which is principally attributed to the juxtaposition of the juan de fuca ridge with the cobb hotspot northwestern usa our results reveal a complex melt body which is 14 km long 3 km wide and up to 1 km thick beneath the summit caldera the estimated volume of the reservoir is 18 30 km3 more than two orders of magnitude greater than the erupted magma volumes of either the a d 1998 or 2011 eruption our images show a network of sub horizontal to shallow dipping 30 features that we interpret as pathways facilitating melt transport from the magma reservoir to the eruption sites

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

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

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

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

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

upper crustal evolution across the juan de fuca ridge flanks 1 recent p wave velocity compilations of the oceanic crust indicate that the velocity of the uppermost layer 2a doubles or reaches 4 3 km s found in mature crust in 10 ma after crustal formation this velocity change is commonly attributed to precipitation of low temperature alteration minerals within the extrusive rocks associated with ridge flank hydrothermal circulation sediment blanketing acting as a thermal insulator has been proposed to further accelerate layer 2a evolution by enhancing mineral precipitation we carried out 1 d traveltime modeling on common midpoint supergathers from our 2002 juan de fuca ridge multichannel seismic data to determine upper crustal structure at 3 km intervals along 300 km long transects crossing the endeavor northern symmetric and cleft ridge segments our results show a regional correlation between upper crustal velocity and crustal age the measured velocity increase with crustal age is not uniform across the investigated ridge flanks for the ridge flanks blanketed with a sealing sedimentary cover the velocity increase is double that observed on the sparsely and discontinuously sedimented flanks 60 increase versus 28 over the same crustal age range of 5 9 ma extrapolation of velocity age gradients indicates that layer 2a velocity reaches 4 3 km s by 8 ma on the sediment blanketed flanks compared to 16 ma on the flanks with thin and discontinuous sediment cover the computed thickness gradients show that layer 2a does not thin and disappear in the juan de fuca region with increasing crustal age or sediment blanketing but persists as a relatively low seismic velocity layer capping the deeper oceanic crust however layer 2a on the fully sedimented ridge flank sections is on average thinner than on the sparsely and discontinuously sedimented flanks 330 80 versus 430 80 m the change in thickness occurs over a 10 20 km distance coincident with the onset of sediment burial our results also suggest that propagator wakes can have atypical layer 2a thickness and velocity impact of propagator wakes is evident in the chemical signature of the fluids sampled by odp drill holes along the east endeavor transect providing further indication that these crustal discontinuities may be sites of localized fluid flow and alteration

coupling volcanism and tectonics along divergent plate boundaries collapsed rifts from central afar ethiopia magma along divergent plate boundaries is erupted from fissures or vents from central volcanoes with limited impact on rift architecture here i summarize the geological and structural features accompanying the eruption of part of a km thick volcanic sequenc e stratoids along the red sea divergent boundary in central afar in the area of tendaho and dobi grabens more than 4700 km 3 ma per 100 km of rift length of magma have been produced by repeated fissure eruptions from within tendaho graben the graben sides show distinctive structural features as steep topographic gradients coinciding with inward tilted blocks forming dominoes coeval to the emplacement of the km thick volcanic sequence similar features are observed also in the dobi graben this allows proposing an original mechanism where the distinctive structure of the grabens results from the collapse at the surface induced by magma withdrawal during the emplacement of the volcanic sequence this portion of afar shows how rift architecture is shaped by voluminous fissure eruptions forming collapsed rifts these occur in continental domains during the breakup stage central afar and in oceanic domains where rifts narrow east pacific rise collapsed rifts represent an end member type of volcano tectonic activity where the width of the erupting reservoir balances that of the active rift zone along divergent boundaries the width of the reservoir influences the style of surface deformation a progressively higher ratio of the width of the reservoir emptied re to that of the active rift zone ri generates in sequence axial grabens calderas and collapsed rifts

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

a large spectral survey of small lunar craters implications for the composition of the lunar mantle a global spectral survey of 4506 immature craters with diameters 1 km was carried out using near ir data from the kaguya spectral profiler to characterize the composition of the lunar megaregolith on the basis of band minima and radiative transfer mixing models small crater spectra fall into three groups 1 mare basalts with strong absorptions at relatively long wavelengths indicating high ratios of high to low ca pyroxene 2 norites containing about 50 plagioclase and with pyroxene assemblages dominated by low ca pyroxene that occur within the south pole aitken basin spa near apollo 14 and other locations near imbrium basin and three major cryptomaria deposits and 3 noritic anorthosites occurring within the feldspathic highlands terrane containing about 20 wt pyroxene with a pyroxene assemblage containing exclusively very low ca pyroxene very few pure anorthosites are present in this survey and there are no occurrences of pyroxene poor olivine rich assemblages models of the composition of basin ejecta incorporate large amounts of mantle material and the spectral results require that that the sampled mantle is orthopyroxenite basin depth diameter ratios used in the models required to match the measured composition are consistent with prior estimates for the largest basins the composition found in the spa and imbrium regions are consistent with mafic impact melt breccias or basaltic impact melts of basin origin for spa we model this composition and find it requires an extremely low impact angle while this is consistent with prior work on an oblique impact for the spa event a more robust solution invokes the production of norite in impact melt seas

quantitative study of the deformation at southern explorer ridge using high resolution bathymetric data abstract we present preliminary results of a morphological study of the summit of the southern explorer ridge ser the ser is an inflated intermediate rate spreading center located in the northeast pacific off the west coast of canada that hosts a large hydrothermal vent complex known as magic mountain a quantitative assessment of faulting on the axial summit graben floor close to the ridge summit is accomplished through the analysis of high resolution near bottom bathymetric data these data were acquired using a multibeam system mounted on an autonomous underwater vehicle operated a few tens meters above the seafloor structural mapping reveals numerous subvertical fissures and normal faults that nucleated from tension fissures the ratio between the length and the maximum scarp height of normal faults is not constant contrary to what is generally observed on subaerial faults highlighting the probable importance of fault segment linkage and fault growth processes within the relatively thin brittle layer populations of small faults exhibit an exponential size frequency distribution that reflects the importance of linkage in the fault growth history and the relatively large amount of tectonic strain 3 7 to 18 4 accommodated by the normal faults we propose that the 500 to 600 m wide and 60 m deep asymmetric axial summit graben of the ser formed due to magma chamber deflation as well as normal faulting that initiated on the present eastern border of the graben we find a well defined geographic distribution in the types of lava flows which indicates a general decrease of the eruption rate through time we also find that the magic mountain hydrothermal field is located in the vicinity of the large eastern axial summit graben bounding fault whose dimensions suggest it may reach to the brittle ductile transition depth this fault likely has provided an efficient physical pathway for fluids from the subsurface to the seafloor for a significant period of time thus allowing the hydrothermal system to grow and mature