mechanisms and depths of atlantic transform earthquakes we have studied 40 earthquakes on seven major transforms along the mid atlantic ridge the majority show the expected transform parallel strike slip motion on steeply dipping fault planes some anomalous mechanisms suggesting either unusually shallow dip or a component of dip slip motion occur near ridge transform intersections the vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform centroid depths determined by waveform inversion are limited to a shallow zone corresponding approximately to the area above the 400 c isotherm of the thermal structure expected from lithospheric cooling the depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600 c isotherm assuming that the areas above the 400 c or 600 c isotherms fail seismically moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion suggesting that some aseismic slip occurs if the time period sampled is representative in contrast on the vema transform the inferred seismic slip is greater than predicted unless faulting extends to the 650 c isotherm alternatively the vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history in contrast to transform events centroids for oceanic intraplate earthquakes extend to approximately the 750 c isotherm this result is intriguing if depths reflect lithospheric strength as a function of depth transforms have much higher strain rates than intraplate areas so if both had the same rheology the limiting isotherm for transforms should be higher alternatively if a weaker rheology were appropriate the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra plate regions transforms may thus be either weaker and or have higher temperatures than expected in the latter model the required high temperatures perhaps due to intrusion at depth predict measurable heat flow anomalies the second possibility unusual rheological weakness is reminiscent of the unexplained weakness of the san andreas fault where heat flow data exclude anomalously high temperatures

southern kermadec submarine caldera arc volcanoes sw pacific caldera formation by effusive and pyroclastic eruption abstract three newly identified submarine caldera volcanoes brothers healy and rumble ii west of the southern kermadec arc within water depths 80 and low density 0 3 0 5 g cm 3 erupted pumiceous clasts these observations are interpreted as evidence of subaqueously quenched pyroclastic eruption s sheathed by ambient water flashing to steam discharging some 5 km3 of pyroclastic material from healy volcano caldera collapse is inferred to be penecontemporeous with the pyroclastic eruption s although it is possible formation as an explosive crater occurred but we consider the latter unlikely given the volume of erupted material and water depth extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas and syn eruptive caldera collapse is possible within water depths of 500 1000 m

detailed volcanic geology of the marnok area mid atlantic ridge north of kane transform we present the results of a detailed study of the volcanic geology of the median valley floor of the mid atlantic ridge in two adjacent but contrasting spreading segments immediately north of the kane transform fault based on a high resolution deep towed sidescan sonar tobi survey near bottom photographic traverses and geochemical analyses from 28 precisely sited sets of dredge samples this has allowed us to assess the relative importance of small intersegment and large transform boundaries on magmatic processes at slow spreading ridges we find that the petrological effect of the transform so called transform fault effect is minor compared to the manifestation of the second order segmentation the southern narrowgate type segment has a poorly developed neovolcanic zone which tapers towards its bathymetric centre where continuous faulting causes rapid dismemberment flat topped seamounts are preferentially located at the ends of the segment particularly near the non transform offset there is a striking variation in the degree of fractionation along the length of this segment primitive lavas were sampled at the centre while more fractionated basalts showing a greater range of parental compositions form discrete volcanic edifices at the ends in contrast the northern segment has a wider inner valley and syn magmatic faults extend up to 15 km along the crest of its robust axial volcanic ridge the along segment trend of increasing mgo towards the bathymetric crest of the axial volcanic ridge is similar to but less well defined than that for the southern segment there is no variation in the bulk degree of melting along the segments as determined from the major element geochemistry hence melt migration is favoured over focused mantle upwelling as the main cause of the crustal thickness variations which define second order segmentation radigenic isotopes show only very small variations with no systematic pattern emerging within or between the ridge segments the isotopic irregularities are not related to the variations in incompatible elements implying that the latter result from dynamic melting processes rather than from long lived source heterogeneity a model is developed which relates the volcanology tectonic style and the geochemical trends to the episodicity of magma supply and eruption

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

on the cause of the asymmetric distribution of seamounts about the juan de fuca ridge ridge crest migration over a heterogeneous asthenosphere abstract the distribution of non hotspot seamounts in the northeast pacific is highly asymmetric small seamount chains and isolated edifices are numerous on the pacific plate but nearly absent on the juan de fuca plate we propose a hypothesis for the asymmetric generation of seamounts near a ridge axis in which upwelling and early melting of upper mantle heterogeneities occurin advance of a spreading centre which migrates with respect to the asthenospheric frame of reference plate motion solutions indicate that the juan de fuca ridge is migrating to the west at a ridge perpendicular rate of 20 mm a 1 which is large compared to the half spreading rate of 30 mm a 1 migration of the ridge axis will result in the initiation of upwelling in the upper mantle in advance of the spreading centre if slightly enriched lower melting temperature heterogeneities are present in the upper mantle they will intersect their solidus at a greater depth and will begin to melt earlier than the host peridotite seamount volcanism will occur preferentially on the pacific plate because of two factors firstly more asthenosphere and hence more early melt heterogeneities must ascend to supply the pacific plate which is translating more rapidly than the relatively stationary juan de fuca plate secondly the asthenosphere that is required to ascend to supply the thickening of the juan de fuca plate will have been flushed of its significant shallow level heterogeneities by the previous advance and passing of the ridge few should remain to cause volcanism on the juan de fuca plate testable physical and petrologic predictions of this model can be identified for example within a small seamount chain the age difference between individual seamounts and the crust on which they lie will decrease toward the ridge axis variation in lava chemistry along a small seamount chain which is interpreted to represent magmatism from a single large heterogeneity will provide the major discriminant of this hypothesis systematic variations should be observed with earlier formed edifices being constructed of more enriched lavas and later younger ones of more depleted lavas isotope ratios should be unaffected by varying degrees of partial melting but could display increased dilution towards the ridge axis as melting of the host mantle and magma mixing become significant

the largest au deposits in the st ives goldfield yilgarn craton western australia may be located in a major neoarchean volcano sedimentary depo centre the largest neoarchean gold deposits in the world class st ives goldfield western australia occur in an area known as the argo junction region e g junction argo and athena why this region is so well endowed with large deposits compared with other parts of the st ives goldfield is currently unclear because gold deposits at st ives are hosted by a variety of lithologic units and were formed during at least three different deformational events this paper presents an investigation into the stratigraphic architecture and evolution of the argo junction region to assess its implications for gold metallogenesis the results show that the region s stratigraphy may be subdivided into five regionally correlatable packages mafic lavas of the paringa basalt contemporaneously resedimented feldspar rich pyroclastic debris of the early black flag group coarse polymictic volcanic debris of the late black flag group thick piles of mafic lavas and sub volcanic sills of the athena basalt and condenser dolerite and the voluminous quartz rich sedimentary successions of the early merougil group in the argo junction region these units have an interpreted maximum thickness of at least 7 130 m and thus represent an unusually thick accumulation of the neoarchean volcano sedimentary successions it is postulated that major basin forming structures that were active during deposition and emplacement of the voluminous successions later acted as important conduits during mineralisation therefore a correlation exists between the location of the largest gold deposits in the st ives goldfield and the thickest parts of the stratigraphy recognition of this association has important implications for camp scale exploration

a geometric technique for relocating hotspots and refining absolute plate motions an age independent geometric relationship is presented that links hotspots to the seamounts which they produce and so permits the use of undated seamounts to refine the motion of tectonic plates this technique has the potential to rigorously assess hotspot fixity and to locate extinct hotspots the present application of this method points to a recent change in pacific plate motion and suggests a relocation of the louisville hotspot to the hollister ridge south of the eltanin fracture zone

the pattern of circumferential and radial eruptive fissures on the volcanoes of fernandina and isabela islands galapagos maps of the eruptive vents on the active shield volcanoes of fernandina and isabela islands galapagos made from aerial photographs display a distinctive pattern that consists of circumferential eruptive fissures around the summit calderas and radial fissures lower on the flanks on some volcano flanks either circumferential or radial eruptions have been dominant in recent time the location of circumferential vents outside the calderas is independent of caldera related normal faults the eruptive fissures are the surface expression of dike emplacement and the dike orientations are interpreted to be controlled by the state of stress in the volcano very few subaerial volcanoes display a pattern of fissures similar to that of the galapagos volcanoes some seamounts and shield volcanoes on mars morphologically resemble the galapagos volcanoes but more specific evidence is needed to determine if they also share common structure and eruptive style

carboniferous submarine volcanism at herbornseelbach lahn dill area germany a 100 m thick sequence of carboniferous submarine dominantly basaltic volcanic rocks at herbornseelbach lahn dill area is subdivided into six units two pillow volcanoes show a characteristic change from basal megapillows up to 5 m across through normal pillows 0 5 to 0 8 m across to a top facies of small pillows 0 5 m across and or breccias of scoriaceous clasts and discontinuous lava stringers sheet flows 0 5 to 4 m thick can be distinguished from sills by asymmetric cooling structures and textures vesicle concentrations near the top basal flow wrinkles and ponding of sediments on irregular flow tops

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

seamount statistics in the pacific ocean we apply the wide beam sampling technique of jordan et al 1983 to approximately 157 000 km of wide beam profiles to obtain seamount population statistics for eight regions in the eastern and southern pacific ocean population statistics derived from wide beam echograms are compared with seamount counts from sea beam swaths and with counts from bathymetric maps we find that the average number of seamounts with summit heights h h is well approximated by the exponential frequency size distribution h oe h the exponential model for seamount sizes characterized by the single scale parameter 1 is found to be superior to a power law self similar model which has no intrinsic scale in describing the average distribution of pacific seamounts and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance large scale regional variations in seamount populations are documented we observe significant differences in seamount densities across the murray fracture zone in the north pacific and the eltanin fracture zone system in the south pacific the eltanin discontinuity is equally evident on both sides of the pacific antarctic ridge in the south pacific regions symmetrically disposed about the ridge axis have very similar seamount densities despite the large difference between pacific plate and antarctic plate absolute velocities evidently any differences in the shear flows at the base of the pacific and antarctic plates do not affect seamount emplacement systematic variations in o and are observed as a function of lithospheric age with the number of large seamounts increasing more rapidly than small seamounts these observations have been used to develop a simple model for seamount production under the assumptions that 1 an exponential size frequency distribution is maintained 2 production is steady state and 3 most small seamounts are formed on or near the ridge axis the limited data available from this study appear to be consistent with the model but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters however the data from the south pacific indicate that the off axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m

helium isotope disequilibrium and geochronology of glassy submarine basalts the u th he dating method is the oldest method of geochronology that involves radioactivity but historically it has not been particularly successful even when problems of poor helium retention and loss due to radiation damage are circumvented by analysing suitable mineral phases1 4 problems frequently remain with inherited helium5 6 here we show that isotope disequilibrium in the 3he 4he ratio between helium trapped in vesicles and that dissolved in the glass phase of some young seamount basalts can be used to determine u th he ages for the basalts we use the 3he 4he in vesicles extracted by crushing in vacuo to correct the dissolved phase he by fusion of the remaining powder for the inherited component and compute the radiogenic helium concentration the method is applicable to rocks containing phases with different u th he and the results have implications for dating lavas in the age range of 103 to 106 years thus filling a gap between the limits of the 14c and k ar methods and for reconstructing the geochemical history of young volcanic systems

hot spot ridge crest convergence in the northeast pacific evolution of the juan de fuca ridge during the past 7 m y has been reconstructed taking into account both the propagating rift history and migration of the spreading center in the absolute fixed hot spot reference frame northwestward migration of the spreading center at a rate of 30 km m y has resulted in progressive encroachment of the ridge axis on the cobb hot spot and westward jumping of the central third of the ridge axis more recently than 0 5 ma seamounts in the cobb eickelberg chain are predicted to display systematic variations in morphology and petrology and a reduction in the age contrast between the edifice and underlying crust as a result of the ridge axis approach relative seamount volumes also indicate that magmatic output of the hot spot varied during this interval with a reduction in activity between 2 5 and 4 5 ma compared with relatively more robust activity before and after this period spatial relationships determined in this reconstruction allow hypotheses relating hot spot activity and rift propagation to be evaluated in most cases rift propagation has been directed away from the hot spot during the time period considered individual propagators show some reduction in propagation rate as separation between themore propagating rift tip and hot spot increases but cross comparison of multiple propagators does not uniformly display the same relationship no obvious correlation exists between propagation rate and increasing proximity of the hot spot to the ridge axis or increasing hot spot output taken together these observations do not offer compelling support for the concept of hot spot driven rift propagation however short term reversals in propagation direction at the cobb offset coincide with activity of the heckle melting anomaly suggesting that local propagation effects may be related to excess magma supply at the ridge axis less

two forms of volcanism implications for mantle flow and off axis crustal production on the west flank of the southern east pacific rise seamarcii side scan sonar and bathymetric data on the west flank of the east pacific rise 16 18 s reveal a large number of seamounts organized primarily in chains the easternmost ends of the chains near the ridge axis appear to be active as indicated by fresh lava flows in addition areas of unusually high reflectivity representing recent lava flows were found as far as 60 to 80 km from the spreading axis the flows are sometimes dammed by abyssal hill scarps formed near the ridge axis that have maximum relief of the order of 100 m in other cases the scarps appear to be buried by flows the disappearance of scarps can be used to estimate the extent of old lava flows or volcanic debris that are not detected acoustically in the sonar image under this criterion lava flows and volcanic debris surrounding the seamounts cover 40 50 of the survey area of about 18 000 km2 with an estimated average thickness of about 100 m the volume of lava flows and volcanic debris is about 0 8 0 3 of that of the total crust the volume of the seamounts is estimated by isolating the seamounts from the background topography with an anisotropic median filtering technique the volume of seamount edifices is 1 05 0 05 of the volume of the crust thus the total volume of volcanic material extruded off axis is about 1 5 to 2 2 of the volume of the crust the decreasing volcanic activity away from the ridge axis suggests that near axis seamounts may be associated with the upwelling system beneath the ridge perhaps by preferential melting of embedded passive heterogeneities however the decrease in volcanism with increasing distance from the axis also could be caused by a decrease in lithospheric vulnerability to penetration by magma since no apparent east west faults and no simultaneous volcanic activity along the length of a seamount chain are observed north south extension and ridge perpendicular convection are unlikely to be major causes of seamount formation in the study area the existence of closely spaced linear chains and the tendency for fresh flows to be found at near ridge ends of chains suggest that there are discrete sources in the upper mantle active for extended periods over 1 8 m y passive heterogeneities embedded in the upwelling mantle would have to be highly elongated along streamlines to produce persistent nearly stationary melting anomalies a variety of mechanisms may be responsible for the off axis volcanism but the single hypothesis most consistent with all observations is a mini hotspot origin although fresh lavas 80 km off axis are consistent with broad mantle upwelling beneath the ridge the possibility of narrow upwelling cannot be ruled out because the seamounts could be caused by independent buoyant upwelling of mini plumes that can penetrate thin lithosphere more easily near the axis

new global seamount census from altimetry derived gravity data summary recent revisions to the satellite derived vertical gravity gradient vgg data reveal more detail of the ocean bottom and have allowed us to develop a non linear inversion method to detect seamounts in vgg data we approximate vgg anomalies over seamounts as sums of individual partially overlapping elliptical polynomial functions which allows us to form a non linear inverse problem by fitting the polynomial model to the observations model parameters for a potential seamount include geographical location peak vgg amplitude major and minor axes of the elliptical base and the azimuth of the major axis the non linear inversion is very sensitive to the initial values for the location and amplitude hence they are constrained by the centre and amplitude of the uppermost contours obtained with a 1 eotvos contour interval with these initial conditions from contouring we execute a step wise and fully automated inversion and obtain optimal model estimates for potential seamounts these are statistically evaluated for significance using the akaike information criterion and f tests a logarithmic barrier technique is applied to ensure positivity of all seamount amplitudes after automatic and manual inspections of the model parameters we estimate actual heights and basal ellipses of the inspected potential seamounts directly from the predicted bathymetry grid in this study we find globally 24 643 potential seamounts h 0 1 km that are located away from continental margins 8458 potential seamounts are taller than 1 km although our global estimate is significantly lower than predictions from previous studies a first order reconciliation of the size frequency statistics obtained from those studies reveals that the previous counts are systematically overestimated because of the ambiguity of gravity signals due to small seamounts of h 1 km and the overlap with abyssal hills we estimate the global seamount census to lie in the 40 000 55 000 range the seamount data from this study are accessible from http www soest hawaii edu pt smts

offset caldera and crater collapse on juan de fuca ridge flank volcanoes forty three volcanoes located along the flanks of the juan de fuca ridge were selected to study relationships between their morphologies and off axis magmatic processes the volcanoes occur both in chains consisting of up to seven distinct cones and isolated edifices nearly all of the volcanoes are circular truncated cones with steep flanks and large relatively flat summit plateaus in addition most of these volcanoes also have prominent and distinctly offset calderas or craters the most striking characteristic of the volcanoes morphology is that nearly all of their collapse structures are located on the sides of the volcanoes which face the juan de fuca ridge and many are breached with openings toward the ridge a simple model based on these observations accounts for these ridge facing features as plate motion transports a volcano away from its magma source beneath the lithosphere the volcano s magma supply conduits tend to lag behind eventually these conduits are abandoned and ridgeward collapse structures are formed it can be inferred from the model that on average individual volcanoes were active for approximately 50 000 years and that most eruptions took place early in this interval if most of the cone building eruptions occurred during the first thousand years or so associated hydrothermal activity may have temporarily rivaled the present day yearly time averaged hydrothermal output along the entire juan de fuca ridge axis

seamount abundances and distributions in the southeast pacific abstract sea beam bathymetry was recorded for 17 277 km of ship track in the southeast pacific and has been analyzed for seamount population characteristics all the ship tracks are over the pacific plate and most fall along 7 lines parallel to the east pacific rise between 7 s and 22 s the lines fall into three age categories one line is over 0 5 2 ma crust three are over 5 10 ma crust and three are over 30 40 ma crust seamount locations were recorded diameters were manually estimated and heights were measured if the swath crossed the seamount center over 382 features were counted along the entire ship track with heights ranging from 50 m to over 2500 m with sampling most consistent at heights between 100 m and 1000 m height to radius ratios vary considerably suggesting that seamount shapes do not scale to a single parameter the observed variety of morphological forms demonstrates that there is a wide range of seamount shapes for small features the distribution of sizes for the population was approximated by exponential dependence and by power law dependence using methods developed by smith and jordan 17 the power law distribution overestimates abundances at the smallest size ranges and both distributions fail to predict abundances of large seamounts determined from wide beam data 17 size distributions were also determined for seamounts in each seafloor age category almost all small seamounts appear to be produced on crust younger than 0 5 2 ma while the number of larger seamounts increases to 5 10 ma for all size ranges observed here the number of seamounts at 5 10 ma and at 30 40 ma is roughly identical these observations suggest that most small seamounts are formed on very young thin lithosphere that permits the passage of small volumes of magma small volumes would cool in older thicker lithosphere before reaching the surface although larger magma bodies might not numerous small sources of melt must exist near the ridge crest to supply the small seamounts probably trapped remnants of the large scale upwelling

paleomagnetism of northern cocos seamounts constraints on absolute plate motion paleodeclinations of several small seamounts on the northern cocos plate demonstrate that it rotated approximately 30 counter clockwise about a pole close to its northern boundary during the past 1 to 6 m y paleoinclinations agree with models of absolute plate motion in which the northern component of velocity for the cocos plate is about 50 mm yr the paleomagnetic data dredge samples and seamount morphology either independently or in concert are reasonably reliable for distinguishing fault blocks from central volcanoes and for determining the approximate age of a structure relative to that of the surrounding sea floor

heck and heckle seamounts northeast pacific ocean high extrusion rates of primitive and highly depleted mid ocean ridge basalt on off ridge seamounts we analyzed petrographically and chemically basalt from eight dredge hauls from the heck and heckle seamounts northeast pacific ocean major elements were determined for mineral glass and whole rock samples and trace and rare earth elements were determined for glass and whole rock samples the dredge hauls included hyaloclastites and fragments from sheet flows and pillows the clinkery fragments are interpreted to be deformed sheet flow tops characteristic of high effusion rates the hyaloclastites recovered are reworked deposits as indicated by the wide compositional range of the glass shards abundance of clay and calcite matrix and bedding most rocks are aphyric but the analyzed plagioclase and olivine phenocrysts and microcrysts are equilibrium compositions and show minor compositional zonation up to 7 5 an 1 fo calculated quenching temperatures range from 1228 o to 1216oc decrease with decreasing mg and are consistent with phase equilibria the seamount lavas are primitive mg 68 7 59 5 and restricted in compositional range la ce 0 65 0 77 the basalts are equivalent to highly depleted mid ocean ridge basalt la sm 0 29 0 41 and are at the depleted end of the range in composition of basalts from east pacific seamounts they are more depleted than basalts from the adjacent west valley spreading axis the lavas can be produced by partial melting of a depleted source mineralogical and geochemical evidence is consistent with rapid ascent of the heck and heckle lavas through oceanic crust with little residence time in magma chambers lavas of the heck and heckle seamount chains have a limited range of incompatible element ratios whereas the adjacent west valley segment of the juan de fuca ridge is highly heterogeneous in contrast lavas from the east pacific near ridge seamounts exhibit a wider range of incompatible element ratios than do the adjacent east pacific rise basalts on the west valley segment magma supply is less robust associated with lower spreading rates compared to the east pacific rise at 10on in contrast at fast spreading centers robust melting produces a mixed mantle signature in axial lavas while suppressed melting at the seamounts reveals the heterogeneities we suggest that at some spreading ridges more fertile portions of the mantle are preferentially melted such that the outwelled portions of the mantle tapped by the seamounts are more depleted

evolution of macquarie ridge complex seamounts implications for volcanic and tectonic processes at the australia pacific plate boundary south of new zealand abstract the macquarie ridge complex mrc coincides with the australia pacific plate boundary south of new zealand and is characterised by a rugged bathymetry that comprises numerous seamounts along its length tectonic plate reconstructions show that the australia pacific plate boundary evolved from a spreading centre to a transpressional boundary from ca 40 to 6 ma however limited sampling and high resolution mapping of the region have meant that the composition and morphology of the mrc remain relatively unknown here we present results from multibeam mapping photographic investigation and petrologic and geochemical analyses from 10 seamounts along the mrc dredge samples collected from the seamounts are alkaline to sub alkaline basalts containing 45 1 50 8 wt sio2 3 5 12 0 wt mgo and 1 9 4 9 wt total alkalis all samples are enriched mid ocean ridge basalts that have k ti 0 15 which originated as low degree partial melts during the late stages of mid ocean ridge volcanism this oceanic crust has been sheared and accreted along the plate boundary since ca 6 ma by cumulative transpressional relative plate motion such that lavas from distinct magma chambers have been juxtaposed at individual seamounts along the ridge mrc seamounts have formed as discrete elevations as a result of dip slip and strike slip faulting of the ridge axis three guyot type seamounts have flat top summit plateaux that were presumably formed by wave erosion via relative sea level lowering due to uplift of the ridge these seamounts have since subsided due to extension associated with the step over fault geometry of the transform plate boundary along the mcdougall and macquarie ridge segments while the seamounts are of volcanic origin they are manifestations of tectonic and geomorphic processes seamounts distal from the ridge axis however are not subjected to plate boundary deformation and instead have morphologies formed through volcanic processes

volcanic development of small oceanic central volcanoes on the flanks of the east pacific rise inferred from narrow beam echo sounder surveys abstract narrow beam bathymetric surveys of about 30 volcanoes on young crust of the cocos plate were collected in order to help determine the origin and evolution of oceanic central volcanoes the volcanoes are located on ocean crust 1 5 7 5 m y in age and range in size from to 600 km 3 these volcanoes differ greatly in shape development of linear features small scale morphologic characteristics and crater development we conclude that these large differences in morphologic characteristics are due to the complex interaction of many factors in their development such as conduit geometry magma ascent rates eruption rate history tectonic activity crater formation and filling and others elongation directions of individual volcanoes and clear linear bathymetric features near these volcanoes are among the lines of evidence that strongly suggest these volcanoes originate and evolve on and near oceanic fracture zones their origin and morphologic evolution is thus closely tied to the tectonic processes and thermal regimes at fractures we speculate that these volcanoes on fast spreading lithosphere are generated by episodic pulses of melt production beneath ridge crests in the vicinity of fracture zones petrologic evidence shows that these volcanoes cannot be fed from sub axial magma chambers beneath the east pacific rise instead they tap the same mantle source as the ridge crest at mantle depth

single plume model for asynchronous formation of the lamont seamounts and adjacent eeast pacific rise terrains sonar reflectance is used to quantify the relative ages of young volcanic terrains on a seamount chain and a nearby spreading center in the north equatorial pacific the reflectance is a measure both of the burial of lava by pelagic sedimentation and of the microrelief of the lava the extent of sediment cover and the lava type distribution are calibrated with thousands of bottom photographs the relative ages indicate that the locus of volcanic eruptions has migrated toward the east pacific rise axis zero age activity lies just to the west of the east pacific rise south of the clipperton transform and is probably the cause of an anomalously shallow neovolcanic zone the evidence fits a scenario in which eruptive centers appear evolve and become extinct in succession with little or no overlap in time the eruptive centers are fed in sequence through conduits from buoyant tightly focused magma plumes the near identical heights of many of the centers at the time of their latest eruptions suggest that the seamounts tap the same mantle depth range broad summit plateaus with calderas are formed if the plume volume is such that the hydraulic head is reached and magma is trapped within the volcano in ephemeral magma chambers or conduits if the plume volume is small and the hydraulic head is not reached the seamount remains small with only a small crater the plumes ascend asynchronously from a common region of nucleation in the asthenosphere plumes that follow in close succession build a composite seamount with a string of summit calderas plumes that rise less often result in a line of discrete circular volcanoes

new insights into the magmatism in the northern margin of the south china sea spatial features and volume of intraplate seamounts the extensive intraplate seamounts are obvious features in the northern south china sea scs however the distribution volume and origin of these seamounts are not well understood which greatly hinders our understanding of magmatism in the scs based on high resolution bathymetric data and 147 seismic profiles and combining gravity and magnetic data we first identify 45 seamounts in the northern margin of the scs and simulate their shape with elliptical cones results show that the total volume of these 45 seamounts above seafloor is estimated at about 1885 3078 km3 and the total volume of intrusive magma above moho is about 0 15 mkm3 which is close to the estimates for classic large igneous provinces across the world these seamounts are mostly located on the continental slope with thin crust approximately 12 18 km which reduces the overlying pressure and shortens the magmatic conduits the dominant azimuth of elliptical major axis in seamounts is consistent with the synrift and synspreading fault strikes ne nee indicating that these pre existing faults provide magmatic conduits for the subsequent postrift intraplate seamounts based on three existing clues i e 1 the intraplate seamounts high velocity layer and hainan mantle plume are contiguous in 3 d space 2 the high velocity layer is thicker beneath the continental shelf but thinner beneath the slope and 3 the basalts dredged from certain seamounts show oib type geochemical features we propose a magmatic upwelling pattern which contains hainan mantle plume to explain the spatial and morphological characteristics of these intraplate seamounts

palaeomagnetism of cretaceous pacific seamounts summary the palaeomagnetism of cretaceous pacific seamounts as deduced from their magnetic anomalies is reviewed results from 35 seamounts are reported here for the first time of these 35 results 15 are of high enough quality to be useful this has resulted in a considerable increase in the available information concerning the palaeomagnetism of the pacific plate using only the most reliable results we calculate a palaeomagnetic pole for the upper cretaceous at 58 north 350 east with a 95 per cent cone of confidence of only 5 this pole is considerably more accurate than the one previously reported and is farther to the west the previously reported difference between the mean pole position from seamounts close to japan and the mean pole position from seamounts close to hawaii is not real although one group of seamounts close to japan gives an anomalous pole position several mechanisms have been suggested whereby errors might be introduced in the determinations of pole positions from seamounts no error is introduced in the pole position because of the restricted extent of surveys and the consequent inaccuracy in the determination of the regional field the demagnetization effect which can produce differences between the observed direction of magnetization and the direction of the magnetizing field in an elongated body is unimportant for seamounts the new pole position is sufficiently accurate for use in many types of calculations in which it is desired to locate the pacific plate with respect to the earth s spin axis

abundances distribution and sizes of volcanoes in the pacific ocean and implications for the origin of non hotspot volcanoes abstract in this paper i present data on the abundances sizes and crustal age for all volcanoes volcanic islands and seamounts which appear on published bathymetric charts of the pacific ocean these new data shed light on the origin of non hotspot volcanoes and are important in combination with data on the chemical compositions of seamounts and volcanic islands for estimates of the bulk composition of ocean crust these data also provide firm constraints on off ridge oceanic volcanism models results of this study show that the size frequency distribution of pacific volcanoes is poisson like and that the smallest volcanoes are much more abundant than large ones this study shows clearly that the most abundant volcanoes on the earth are the submerged oceanic volcanoes which comprise 5 25 of the oceanic volcanic layer on pacific crust of eocene age and younger the abundance of volcanoes number of volcanoes per unit area increases monotonically with increasing age assuming steady state the production rate of new off ridge volcanoes number of volcanoes per unit area per unit time is inversely proportional to the square root of the lithosphere age 1 on crust older than eocene the number of volcanoes per unit area of crust decreases monotonically with increasing age however the total volume of lava represented by these edifices increases with increasing age size frequency distributions of volcanoes on swaths of successively older crust indicate that these abundance patterns are partly due to the effect of sediment burial of small edifices on old pacific crust as well as the effect of increased lithosphere thickness on seamount size these general patterns are not appreciably changed by omitting from consideration known hotspot volcanoes 2 and volcanoes built at fossil constructional plate margins 3

nonlinear 40ar 39ar age systematics along the gilbert ridge and tokelau seamount trail and the timing of the hawaii emperor bend over the last three decades the first order correlation in morphology and orientation of seamount trails has been called upon to support the concept of a fixed pacific hot spot frame of reference and to explain the hawaii emperor bend heb by a dramatic change in pacific plate motion in this paper however we present 40ar 39ar ages for the gilbert ridge and tokelau seamounts pacific that show similar changes or bends in their orientation but at different geological times up to 20 myr earlier than the heb changes in pacific plate motion alone cannot explain these observations because these asynchronous bends should have been reflected in the morphology of each of these seamount trails together with the lack of linear age progressions and inconsistent apparent local plate velocities of 131 and 87 mm yr we rule out a fixed hot spot origin for the gilbert ridge and tokelau seamount trails instead we invoke secondary or alternate processes to explain the complex age systematics and morphologies in these seamount trails we propose here that the heb type bends in these seamount trails were likely formed by short term jerk like plate extensions in the studied southwestern region of the pacific plate reactivating a preconditioned lithosphere that can be characterized by a complex structure and precursory magmatic impingements the remarkable differences observed in these colinear seamount trails fundamentally question the existence of heb type bends in the formation of pacific volcanic lineaments they also show us that applying geometric and morphologic observations alone is insufficient in constraining past plate motions nevertheless the need and search for alternate volcano tectonic mechanisms offer opportunities to better understand intraplate volcanism in general

the lifecycle of mid ocean ridge seamounts and their prodigious flank collapses abstract volcanic seamounts are spectacular bathymetric features representing undersea mountains comparable in size to volcanic islands they are often sites of active volcanism represent major marine habitats and contain potentially globally significant commodities of trace metals however we know comparatively little about how they form the extent of their volcanic lifecycle and what risks they pose from periodic flank collapse here we study a uniquely extensive 43 myr record of volcaniclastic and calciclastic turbidites accumulated on the madeira abyssal plain in the ne atlantic which originate from large submarine landslides from the adjacent great meteor cruiser seamount complex on the flanks of the mid atlantic ridge these turbidites present the first long term temporal record of the recurrence magnitude and emplacement mechanism of seamount flank collapses from a single seamount complex throughout its lifecycle landslides from the great meteor cruiser seamount complex are comparable size to those from the canary islands with average volumes of 30 40 km3 but can be as large as 130 km3 comparable to many volcanic island landslides these seamount landslides were predominantly multi stage failures similar to those form neighbouring volcanic islands but in contrast occurred preferentially during sea level lowstands we show that volcanic seamounts above mantle plumes have long protracted lifecycles and that substantial flank failures are capable of occurring at all stages of development temporal change in turbidite composition has for the first time enabled reconstruction of the inception around 43 45 myrs ago ascension taking around 20 25 myrs emergence after 30 myrs and later recession last 9 myrs cessation in volcanism last 4 myrs and subsidence of the seamounts from 2 myrs onwards these seamounts above a mantle plume have taken around 26 myrs to emergence above sea level from inception while their volcanic island contemporaries only took 2 to 5 myrs here we use the history of turbidites derived from landslides from the seamounts to show how the seamounts developed and suffered mass wasting during their life cycles

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

petrologic evolution of large failed rifts in the eastern pacific petrology of volcanic and plutonic rocks from the mathematician ridge area and the guadalupe trough

digital image processing of seabeam bathymetric data for structural studies of seamounts near the east pacific rise digital marine bathymetric data are rapidly increasing in both volume and variety these data provide new perspectives on the morphology of the seafloor they also present a number of problems with regard to developing display methods that convey the large amount of information contained in these data we have used digital image processing techniques to reduce and display seabeam multibeam data for three young less than 5 ma seamounts on the eastern flank of the east pacific rise navigational errors in location were corrected by identifying offsets between features observed in crossing tracks and then translating the tracks relative to a control track images of the seamounts were then generated using spatial filtering techniques the images reveal more information than contour maps because 1 more depth intervals can be displayed and 2 the continuous tone nature of the images shown in shaded relief or color coded form more readily conveys morphologic information the images suggest a significant degree of structural control on seamount shape more so than can be delineated by examination of contour maps alone in one case a chain of seamounts strikes at an angle perpendicular to the abyssal hill direction although each seamount within the chain is elongate in a direction parallel to the hills in another case a polygonally shaped flat topped seamount overlies a region of the crust where the abyssal hills display two strike directions images of the third seamount demonstrate a complicated structural control on morphology including a control on the distribution of satellite constructs finally the image presentations clearly depict regions where cross tracks could not be registered because of 1 residual navigation errors and 2 the presence of steep slopes where the depth and location estimates can vary depending on the angular relationship between the ship track azimuth and the strike of the slope

the t reflection and the deep crustal structure of the voring margin offshore mid norway seismic reflection data along volcanic passive margins frequently provide imaging of strong and laterally continuous reflections in the middle and lower crust we have completed a detailed 2d seismic interpretation of the deep crustal structure of the voring margin offshore mid norway where high quality seismic data allow the identification of high amplitude reflections locally referred to as the t reflection using a dense seismic grid we have mapped the geometry of the t reflection in order to compare it with filtered bouguer gravity anomalies and seismic refraction data the t reflection is identified between 7 and 10 s sometimes it consists of one single smooth reflection however it is frequently associated with a set of rough multiple reflections displaying discontinuous segments with varying geometries amplitudes and contact relationships the t reflection seems to be connected to deep sill networks and is locally identified at the continuation of basement high structures or terminates over fractures and faults the spatial correlation between the filtered positive bouguer gravity anomalies and the deep dome shaped reflections indicates that the latter represent a high impedance boundary contrast associated with a high density and velocity body in 50 of the outer voring margin the depth of the mapped t reflection is found to correspond to the depth of the top of the lower crustal body lcb which is characterized by high p wave velocities 7 km s we present a tectonic scenario where a large part of the deep crustal structure is composed of preserved upper continental crustal blocks and middle to lower crustal lenses of inherited high grade metamorphic rocks deep intrusions into the faulted crustal blocks are responsible for the rough character of the t reflection whereas intrusions into the ductile lower crust and detachment faults are likely responsible for its smoother character deep magma intrusions can be responsible for regional metamorphic processes leading to an increasing velocity of the lower crust to more than 7 km s the result is a heterogeneous lcb that likely represents a complex mixture of pre to syn breakup mafic and ultramafic rocks cumulates and sills and old metamorphic rocks such as granulites and eclogites an increasing degree of melting towards the breakup axis is responsible for an increasing proportion of cumulates and sill intrusions in the lower crust

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

comparison of the shapes and sizes of seafloor volcanoes on earth and pancake domes on venus the fact that flat topped pancake domes observed on venus resemble flat topped volcanoes on earth s seafloor suggests that volcanic processes on the seafloor and on venus may operate to construct volcanic forms with similar characteristics to test this the shapes and size distributions of terrestrial submarine volcanoes seamounts from several magmatic and tectonic provinces are quantitatively compared with those of venusian pancake domes seamount heights and diameters are highly correlated suggesting that there may be a universal mechanism for their growth the 2014 point sample mean of the height to diameter ratio is d 0 10 0 04 in contrast heights and diameters of 52 pancake domes exhibit no obvious trend and values of d are as much as three orders of magnitude smaller with a 52 point sample mean of d 0 02 0 03 seamounts with basal diameters db 10 km have flatness values ratio of top to bottom diameter that span the entire range between 0 pointed cone and 1 truncated cone larger sized seamounts tend to have small values of flatness and more pointed tops this is not true for pancake domes which show large flatnesses at all sizes additionally seamounts have distinctive profiles with their flank slopes increasing in gradient toward the summit flank slopes of pancake domes however commonly increase in gradient toward the base with respect to sizes the diameter distributions of the different seamount populations are consistent with a model in which the number of seamounts decreases exponentially with increasing diameter the diameter distribution of 126 pancake domes by contrast is more uniform its mean is db 22 13 km this size is at least an order of magnitude larger than the estimated characteristic diameters of the seamount populations based on the above results i conclude that seamounts have significantly different shapes and size distributions than venusian pancake domes that have been identified and measured to date this suggests that one or more of the variables e g magma composition crystallinity magma volume effusion rate cooling rate pre existing topography governing their formation differs in terms of shapes and size distributions the small db 20 km volcanic edifices first identified from venera data are good morphologic matches to seamounts and should be the focus of future work investigating the formation of comparable volcanic features in these two different planetary environments

shape analysis of pacific seamounts abstract shape statistics have been compiled from 85 profiles of well surveyed pacific seamounts in the height range 140 3800 m a flat topped cone was fit to each seamount s cross sectional profile maintaining the slopes of the sides as closely as possible on each profile a basal width d b a summit width d t and a maximum height h were measured the height to basal radius ratio is h is estimated by the ratio 2hd b and flatness f by the ratio d t d b slope angle arctan e is estimated from e 2h d b d t summit height and basal radius are found to be highly correlated r 0 93 the 85 point sample mean of the height to basal radius ratio is h 0 21 0 08 implying that a seamount s summit height is typically one fifth its basal radius despite the high correlation individual points show some scatter and there may be groupings into different morphological types for example all but one of the seamounts with summit heights above 1000 m have values of h that are larger than the sample mean the 85 point sample mean of flatness is f 0 31 0 18 data points show a large scatter with values of f varying between 0 a pointy cone and 0 69 a flat topped cone a histogram representation of flatness however indicates that certain values of f may be more common than others the histogram shows a bimodal distribution with maxima occurring at values of f in the ranges 0 10 0 20 and 0 35 0 50 moreover there is some evidence that the mean flatness decreases with summit height so that the preferred shape of a large sized seamount may be a pointy cone slope angle has an 85 point sample mean of 18 6 individual values of vary between 5 and 36 in addition to having a lower than average mean flatness seamounts with heights above 2600 m also have a lower than average mean slope angle 15 to determine which variables account for most of the observed variation in the seamount shapes a multivariate principal component analysis was performed on the data using five shape variables summit height basal radius summit radius flatness and slope the analysis indicates that most of the variation is described by two variables flatness and summit height

susceptibility of mid ocean ridge volcanic islands and seamounts to large scale landsliding 1 with a view to assessing the incidence of large scale landsliding a morphologic database was created for volcanic islands and seamounts on young oceanic lithosphere the database included 44 mid ocean ridge seamounts jasper seamount and the islands of ascension bouvet guadalupe and several of the galapagos and azores islands supplemented with published reports from a further five volcanic edifices the data reveal that major landslides are common on edifices taller than 2500 m but are rare in shorter edifices implying a threshold of instability at around 2500 m a number of causes of this threshold are discussed for example many structures taller than 2500 m are or were originally volcanic islands and therefore their flanks probably include extensive weak hyaloclastite built up from lava sea interactions around coasts compaction of hyaloclastites in larger edifices lead to regions of low permeability which may help to explain the more deeply seated slope failures it is intriguing that the threshold also coincides with the edifice height at which volcanic ridges become observable a stage at which dike intrusion is probably common because slope oversteepening or excess pore pressure associated with dike intrusions have been proposed elsewhere as landslide triggers the current indications of landsliding reveal no observable relation to rainfall as landslides occur equally in wet and dry climates and no relation to tectonic setting as there are relatively few major landslides in the seismically active azores group

physical volcanology and structural development of cerro azul volcano isabela island galapagos implications for the development of galapagos type shield volcanoes erupted from circumferential vents near the summit these flows are nearly an order of magnitude smaller in volume than the predominantly aa flows erupted from radial eruptive fissures near the break in slope 0 06 0 1 km3 the differences in volume and flow morphology with altitude are due to slower eruption rates from summit vents than from flank vents which in turn are attributable to the different heights the magmas must ascend from shallow reservoirs these observations support the contention that the steep upper flanks were constructed by the buildup of short lava flows rather than by the structural deformation of originally gently dipping flanks in addition to the higher eruption rates a subdued lower flank geometry is promoted by the deposition of lava deltas onto the shallow galapagos platform on the western northern and eastern flanks of the volcano 40ar 39ar geochronology and volume estimates show that despite their morphologic differences the growth of the western galapagos shields has been nearly synchronous precluding an evolutionary model for their development the wide variations in elevation volume area and the distribution of slope angles among the western volcanoes can be linked instead to different long term eruption rates and to a lesser degree the position of each volcano relative to the edge of the galapagos platform

multi beam sonar surveys of guyots of the gulf of alaska abstract this paper describes in detail the three guyots in the gulf of alaska the form of these features has been charted from total bottom coverage bathymetric surveys using swath mapping sonar data were collected by the u s naval oceanographic office statements and descriptions of the geomorphology of these guyots welker durgin and pratt are used to speculate on the ultimate demise of the juan de fuca hot spot

morphology and distribution of lava flows on mid ocean ridges a review abstract both pillow lavas and sheet flows are important features of oceanic volcanism at mid ocean ridges submersible surveys of the neovolcanic zone and off axis areas of these complex seafloor volcanic terrains conducted during the past two decades have uncovered a wide array of basaltic surface forms with pillow basalts displaying bulbous spherical and elongate tubular patterns and sheet flows exhibiting smooth lobate rippled wrinkled ropy whorly hackly and jumbled configurations the genesis of pillow lavas and sheet flows appears to be coupled to variations of spreading rates within the plate boundary zone while pillow lavas predominate at slow spreading centers i e mid atlantic ridge sheet flows prevail along the active volcanic zones at intermediate i e galapagos rift and juan de fuca ridge fast and superfast i e east pacific rise spreading centers yet both types of flows are present at each setting the distribution of pillow lavas and sheet flows along a ridge segment is related to the topographic gradient of the ridge axis hence the formation of sheet flows tends to be favored by the development of topographic highs between ridge segment boundaries where the crustal lid is thinnest and rifting directly taps the underlying magma chamber fissure eruptions here release copious amounts of surface fed fluid lavas which decrease in volume down the topographic gradient away from the bathymetric high of the ridge segment lava flows become more distal and channelized thereby fostering greater concentrations of tube fed pillow flows toward the segment boundaries the type of lava flow morphologies encountered at any specific site on the seafloor is contingent upon the effusion rate cooling rate and slope as demonstrated by laboratory simulations

the geometric relationship between hot spots and seamounts implications for pacific hot spots abstract hot spots and the seamounts produced by them provide both geometric and temporal evidence for changes in absolute plate motion the main limitation in using hot spot produced seamounts in plate tectonic reconstructions arises from the multiple sources of error and ambiguity that plague radiometric age estimates in particular unless the hot spot has maintained a steady and voluminous flux rate over long periods of time the exact location of a hot spot which represents the zero age origin along the hot spot trail is poorly known here we discuss a unique geometric relationship between a hot spot and the seamounts produced by it that we recently have discovered i e hot spot produced seamounts have seafloor crustal flow lines that intersect at the hot spot location furthermore we obtain images of cumulative volcano amplitudes cva by convolving seamount shapes with their flow lines hot spots correspond to clear local maxima in this image and the amplitudes are proportional to cumulative hot spot flux this technique dubbed hot spotting allows us to determine hot spot locations based only on a set of seamount locations no age information is required we use the hot spotting technique to examine the pacific plate hot spots in general and the bowie and cobb hot spots in the gulf of alaska in particular we find that the hawaii louisville caroline cobb and bowie hot spots have clear representations in the cva images rurutu and or rarotonga are close to a large cva high while the other french polynesian hot spots in general exhibit a much more subdued and blurred expression we also conclude that the cobb hot spot plume may have been entrained by the juan de fuca ridge about 2 ma ago or alternatively is in a waning phase and cannot penetrate juan de fuca plate lithosphere the bowie hot spot appears to have encountered the ridge more recently

morphological and structural studies of the society and austral hotspot regions in the south pacific abstract the southeastern extension of the society teahitia hotspot and austral macdonald hotspot volcanic chains were extensively studied 1986 1989 using multi channel echo sounding seabeam bottom imaging systems and submersible dives sea floor within these hotspot regions consists of recently constructed submarine highs built on crust that shows morphological and structural discontinuities with the surrounding ancient abyssal hill regions 4200 m in depth recent volcanic and hydrothermal activity is present on several volcanoes within the society hotspot and on the macdonald volcano previous studies carried out in the society hotspot region showed several geological provinces 1 1 eight major highs 1000 m in height which extend from above sea level down to a depth of 3750 m these features have been formed by lateral eruptive events along rift zones which are aligned along four main structural directions n150 180 n80 n30 40 and n110 which correspond to the orientations of ancient ridge axes transform faults and riedel type shear zones respectively 2 2 the sea floor which slopes away from the base of each major volcano 3750 4200 m depth has an average slope of 0 5 and is characterized by smooth topography and volcanic cones less than 500 m high 3 3 the surrounding abyssal hill region which has an average slope of 0 1 near the society hotspot consists of faulted terrane with a n170 orientation and ancient elongated structural highs from the structural lineations observed in the society hotspot volcanoes a reconstruction of volcanic features has been made it is proposed that the formation of the seamounts occurs in an organized fashion along pre existing preferential structural orientations that are associated with zones of crustal weakness

cretaceous volcanism and jurassic magnetic anomalies in the nauru basin western pacific ocean the nauru basin contains a well documented pattern of late jurassic magnetic anomalies and a cretaceous volcanic complex that has not disturbed the older magnetic structure the following hypotheses are offered to explain this paradox the cretaceous basalts are normally and uniformly magnetized making them invisible to a surface towed magnetometer tension cracks and fractures associated with thermal uplift and or magma wedging provided pathways for the cretaceous magma through the jurassic basement without disrupting the jurassic magnetic structure the same tension cracks admitted sea water that convectively removed heat from the magnetic layer and thus maintained the jurassic basement below its magnetic blocking temperature for extended periods of time during intrusive thermal episodes finally the maximum uplift associated with cretaceous volcanism in the nauru basin could have been equal to but was probably something less than that associated with nearby seamount and atoll chains

seamount formation and associated caldera complex and hydrothermal mineralization in ancient oceanic crust troodos ophiolite cyprus abstract the inter graben zone between the solea and mitsero grabens in the northern part of the troodos ophiolite cyprus displays igneous tectonic and physiographic features that are characteristic of seamount volcanism at modern oceanic spreading centers in this zone the 200 m high alestos hill volcanic edifice consisting of pillow lavas with preserved lava tubes and cone sheet intrusions is interpreted to have developed as a seamount fed by a late stage off axis shallow level gabbroic pluton that was intrusive into young oceanic crust concentric cone sheet intrusions and associated faults and shear zones in and around the alestos hill seamount collectively form a seafloor caldera complex which probably formed during the extrusion of lavas and subsequent cauldron subsidence over the gabbroic pluton the ene trending elongated massive sulfide ore deposit formation in the alestos memi area represents a focused upflow zone for hydrothermal fluids driven by the same off axis gabbroic intrusion responsible for formation of the alestos hill seamount the ene trending and up to 150 m wide alestos memi fault zone dissects the elongated ore body in a dextral sense and might have provided a high permeability conduit in the upper crust for upwelling hydrothermal flow at the time of magmatism the geometry of this fault zone and its kinematic relations with a n s striking alestos forest normal fault in the west are suggestive of its transfer fault origin reminiscent of mineralized transfer faults found at the mid atlantic ridge the active tag hydrothermal mound with massive sulfide deposits that is located on a small volcanic dome 3 km off axis and along a transfer fault in the trans atlantic geotraverse tag area at 26 n provides a modern analogue for the alestos hill seamount and associated faulting and mineralization in the ancient oceanic crust of the troodos ophiolite these findings indicate that off axis magmatism played a major role in crustal accretion hydrothermal alteration and ore mineralization during the late stage evolution of the troodos ophiolite

geography and history of the louisville hotspot chain in the southwest pacific the louisville ridge is a 4300 km long late cretaceous and cenozoic hotspot chain the south pacific equivalent of the hawaiian emperor chain its northwestern end is being consumed by the tonga kermadec subduction zone but seabeam and magnetic mapping shows that the chain still includes more than 60 major volcanoes of both normal and reversed polarity distributed along a 75 km wide band that can be fitted to small circles about three successive poles of pacific plate hotspot rotation this band obliquely crosses fracture zones of the eltanin system but there is little interaction and no evidence of any genetic connection between the two structures forty of the louisville volcanoes grew above sea level and are preserved as high latitude i e coral free guyots they are spaced less than 100 km apart along most of the chain but there are none within 750 km of the inferred present location of the hotspot beneath a swell at the southeast end of the chain the rate of volcano building by the louisville hotspot declined sharply about 20 m y ago after being remarkably constant at 3 4 103 km3 m y for the previous 50 m y and none of the louisville volcanoes built during the past 10 12 m y the time of most profuse hawaiian volcanism has reached sea level however a seamount from which pleistocene lavas were dredged rises to within 540 m of the sea surface from the crest of the hotspot swell at 50 5 s 139 2 w guyot heights demonstrate that a hotspot swell several hundred meters high has persisted throughout the known life of the chain and sometimes had an isostatic depression on its loaded crest the depths of guyot shelf breaks increase systematically to the northwest from 530 to 2100 m proportional to the square root of volcano age with some scatter and tilting caused by plate boundary tectonism and local isostatic subsidence and uplift

near axis seamounts on the flanks of the east pacific rise 8 n to 17 n with complete seabeam and seamarcii bathymetry and nearly complete seamarcii side scan coverage of the northern east pacific rise and its flanks from 8 to 17 n we characterize the entire population of seamounts on young seafloor 0 to 2 ma formed along this fast spreading center there are 179 seamounts in this area taller than 200 m most of which belong to one of 21 seamount chains these chains are oriented between the relative and absolute plate motion directions of the pacific and cocos plates and most of the larger chains are more voluminous near their centers than near their ends several of the seamount chains contain bends in orientation which are not coincident with each other the two largest seamount chains at 9 55 n lamont seamounts and 15 40 n are aligned with portions of the east pacific rise crest which are among the shallowest and broadest of this area overall the maximum abundance and size of seamounts increase with shallower and broader ridge axes no near axis seamounts form on the summit of the axial ridge nonetheless within 15 km 0 3 ma of the axis the abundance of seamounts achieves its overall average observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near axis seamounts are formed in a narrow zone on crust aged 0 1 0 3 ma 5 15 km from the axis with significant growth in a zone several times wider side scan sonar reflectivity and seamount magnetization analyses are largely consistent with the predictions of this model two exceptions to this model however are seamount chains at 15 40 n and 8 25 n which show evidence for activity over a broader region this may be due to the presence of fracture zones within about 30 km of these chains a compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge the variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results although these influences cannot be distinguished in this analysis

geochemical and structural studies of the lamont seamounts seamounts as indicators of mantle processes abstract a chain of seamounts located just west of the east pacific rise epr axis at 09 55 n herein called the lamont seamounts have been investigated using sea marc i and sea beam sonar surveys alvin submersible dives deep sea camera tows and dredging the most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors the distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts ring fractures could be important in focusing post caldera collapse eruptions around summit craters and building summit plateaus structural and morphological data and the petrology of recovered lavas suggest that calderas in the lamont seamounts probably formed by collapse associated with magma withdrawal into large primary vertical magma conduit systems rather than by repeated inflation deflation cycles associated with steady state shallow level magma chambers calderas in the lamont seamounts have evolved through coalescence of adjoining craters and the plan view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time compared to lavas from the adjacent epr the majority of lamont seamount and cone lavas are more primitive depleted in incompatible elements and compositionally heterogeneous variations in light rare earth elements relative to major elements e g tio 2 na 2 o na 2 o cao and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single n type morb source rather it appears that lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the lamont seamounts are separate from those along the epr in particular seamount magmas appear to reflect near primary melting events in the mantle whereas epr lavas are the products of shallow level crystallization and mixing in crustal magma chambers absence of long lived magma chambers within the lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain furthermore because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the lamont chain as a whole we propose that the scale of mantle chemical heterogeneities is less than 5 km

on the applicability of thermal conduction models to mid plate volcanism comments on a paper by gass et al the proposition that mid plate hot spot magmatism is strongly discouraged by a combination of thick lithosphere and rapid plate motion is examined empirically on a global basis and found to be inadequate although hot spots perhaps tend to avoid cratonic areas the nonrandom worldwide distribution of hot spots appears primarily to reflect the state of the sublithospheric mantle plate thickness or speed is of lesser importance the high level of african mid plate volcanism since 25 m y b p appears to be part of a global pattern controlled by events in the mantle rather than a result of the africa plate coming to rest over sublithospheric thermal sources about 40 m y b p

constraints of sea beam data on crustal fabrics and seafloor spreading in the south china sea abstract the south china sea is a mid late tertiary marginal basin the magnetic anomaly lineations in the eastern part of the basin trend approximately east west 1 2 suggesting a north south direction of spreading in the spring of 1985 two cruises on the french research vessel jean charcot provided sea beam coverage seismic reflection magnetic and gravity profiles the sea beam data exhibit two major structural trends scarps striking n50 e 15 interpreted as normal faults and scarps striking n140 e 15 interpreted as fracture zones this fabric implies a northwest southeast direction of spreading up to about 100 km north and south of the inferred spreading axis 3 dense sea beam coverage of a roughly 1 square area northwest of the scarborough seamounts chain shows that the emplacement of these seamounts was at least in its initial stage controlled by faulting in two orthogonal directions n50 e and n140 e magnetic and gravimetric maps of the same area also reveal anomalies trending roughly n50 e which are disrupted by transform zones striking n140 e this detailed study indicates that the fracture zones may be closely spaced less than 20 30 km east of macclesfield bank furthermore magnetic anomalies identified as 6 and 6a 20 myr along two north south profiles located at both edges to the north of this detailed study area may be correlated with the n80 e trend characteristic of such anomalies in the eastern part of the south china sea the east west trend of magnetic anomalies 6 and 6a south of scarborough seamount chain recognised by taylor and hayes 1983 1 is incompatible with the trend of the fault scarps observed on sea beam data we infer that progressive right lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east west average trend of some of the magnetic anomalies and of the scarborough seamount chain which represents the location of the relict spreading axis in spite of a n50 e spreading direction the whole central part of the basin east of 115 e may have formed in this way since n130 140 e striking fracture zones are observed on the sea beam swaths this model may correspond to the second of two successive spreading phases with n s and nw se directions of extension respectively as was presented by the authors in a previous paper pautot et al 1986 3 alternatively we may assume that the direction of extension did not change radically during the opening history 3 implying that oblique spreading occurred along n80 e trending ridge segments creating the n80 e trending scarps and magnetic anomalies observed to the north and to the south of the study area such a direction of extension is compatible with mid late tertiary left lateral movements along large n130 150 e strike slip faults such as the red river and wang chao faults in south china and sundaland

transition from circular to stellate forms of submarine volcanoes large volcanic islands and guyots have stellate forms that reflect the relief of radiating volcanic rift zones multiple volcanic centers and embayments due to giant flank failures small mid ocean ridge volcanoes in contrast are commonly subcircular in plan view and show only embryonic rift zones in order to characterize the transition between these two end members the morphology of 141 seamounts and guyots was studied using the shape of the depth contour at half the height of each edifice irregularity was characterized by measuring perimeter distance elongation and moment of inertia of the contours assuming an ideal edifice is circular the analysis reveals a general transition over 2 4 km edifice height best transition estimate 3 km while some large edifices 4 5 km high show no major embayments or ridges suggesting considerable variation in the effectiveness of mechanisms that cause flank instability and growth of rift zones the various origins of the transition are discussed and the upper limit of magma chambers many of which lie above the basement of the larger edifices is proposed to affect the morphologic complexity via a number of mechanisms and is an important factor affecting the mode of growth the origins of the truncated cone shape of mid ocean ridge volcanoes are also discussed of the eruption mechanisms that have been proposed to explain their flat summits the most likely mechanisms involve eruption from small ephemeral magma bodies lying within the low density upper oceanic crust the discussion includes speculations on factors affecting the depths of magma chambers beneath oceanic volcanoes supporting table is available via web browser or via anonymous ftp from ftp kosmos agu org directory append username anonymous password guest subdirectories in the ftp site are arranged by paper number information on searching and submitting electronic supplements is found at http www agu org pubs csupp about html

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

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

can we image beneath salt a decade has passed since petroleum geologists first recognized that jurassic salt in the extensional gulf of mexico often appeared to be thrust between layers of younger sediments since then many teams of geoscientists have been faced with the problems of either 1 improving subsalt quality through better data collection or 2 making subsalt interpretations from data of very poor quality these problems aren t new in fact they have faced the seismic industry from the beginning in the gulf of mexico however the cause of the problem salt is obvious in particular high velocity structurally contoured salt bodies act as sonic lenses that disperse or concentrate seismic signals in haphazard ways many advances in seismic imaging technology have been in response to these salt related problems of velocity and structure

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

submarine analogs to venusian pancake domes abstract the morphology and dimensions of the largediameter steep sided flat topped pancake domes on venusmake them unlike any type of terrestrial subaerial volcano comparisons between images of hawaiian seamounts andpancake domes show similarities in shapes and secondaryfeatures the morphometry of pancake domes is closer to thatof pacific seamounts than subaerial lava domes consideringboth morphology and morphometry seamounts seem a betteranalog to the pancake domes the control of volatileexsolution by pressure on venus and the seafloor can causelavas to have similar viscosities and densities although thelatter will be counteracted by high buoyancy underwater however analogous effects of the venusian and seafloorenvironments alone are probably not sufficient to producesimilar volcanoes rather venusian lavas of variouscompositions may behave like basalt on the seafloor ifappropriate rates and modes of extrusion and planetary thermalstructure are a so considered introduction

guyots of the mid emperor chain mapped with multibeam sonar abstract bathymetry of seven of the middle emperor chain guyots from u s naval oceanographic office multibeam narrow beam swath mapping surveys is presented these data are contoured at a 100 fm interval using different theories on formation juxtaposed with the accurate bathymetry of this chain tectonic possibilities are shown for changes in plate direction and conduit activity fluctuations the range for one complete pulse of this activity is shown to be 280 nm 518 km for the emperor chain

submarine central volcanoes on the nazca plate high resolution sonar observations abstract this paper describes observations of 214 submarine central volcanoes made with long range side scan sonar and narrow beam echosounder between peru and the east pacific rise they are all circular in plan with basal diameters ranging from less than 1 to over 10 km profiles of four volcanoes were obtained with the narrow beam echosounder and those of a further nine were inferred from their acoustic shadows all the profiles show steep mostly concave sides a distinct summit region could be identified on about two thirds of the volcanoes in just under half of these the volcano culminates in a simple crater but the rest have broad slightly convex summit plateaus with or without craters almost half of all the volcanoes have one or more craters summit plateaus are relatively more common on the larger volcanoes so that for central volcanoes with a basal diameter greater than 4 km the commonest morphology is a steep sided truncated cone with a slightly domed summit plateau in most cases this form must represent the mode of formation not the result of subsequent wave erosion it is suggested that this form is the result of relatively fluid lavas travelling outward on low slopes from a central vent but terminating in steeply dipping hyaloclastites and volcanic breccias the spatial distribution of volcanoes is very non random in the area studied the east pacific rise out to 6 5 ma has less than one volcano per 10 4 km 2 whereas a field hundreds of kilometers across with a peak density of over 60 per 10 4 km 2 was seen north of mendana fracture zone on 6 5 26 ma old crust

the evolution of craters and calderas on young seamounts insights from sea marc i and sea beam sonar surveys of a small seamount group near the axis of the east pacific rise at 10 n a northwest trending group of seamounts located just west of the east pacific rise epr axis has been mapped using high resolution sea marc i side scan sonar and sea beam multibeam bathymetry these seamounts have 9 13 km basal diameters and the shapes of the volcanos change from conical domes near the epr axis to truncated cones and larger elliptical truncated cones farther away from the rise axis the detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions craters to large complex calderas and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse the creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera the concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau the steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera the steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain as a seamount evolves the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits the final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau

wave planation surfaces in the mid western east sea sea of japan indicators of subsidence history and paleogeographic evolution of back arc basin abstract this study focuses on revealing the origin of deep marine wave planation surfaces and its geomorphic and tectonic implications for the back arc evolution based on multi channel seismic reflection profiles acquired from the mid western east sea sea of japan on seismic reflection profiles the wave planation surfaces are recognized as continuous and high amplitude single reflectors outlining the flattened top of submarine terraces or isolated topographic highs the wave planation surfaces are classified into four types depending on the geomorphic characteristics and origins of substratal landmasses i e continental margin terrace type 1 rifted continental fragment type 2 volcanic edifices type 3 and uplifted sedimentary successions type 4 which have common origin of wave erosion near the sea level but different subsidence history the mean subsidence rate of wave planated substratum is calculated by dividing the maximum depth of a wave planation surface below modified datum sea level d e by the time elapsed since the onset of subsidence t information of local subsidence history enables reconstruction of paleogeographic evolution of mid western east sea during the neogene quaternary time 1 incipient continental rifting and onset of marine incursion ca 23 18 ma 2 progressive continental rifting and onset of wave planation ca 18 11 ma 3 final eruption of post rift volcanoes ca 11 ma 4 uplifting of shelf margin banks and onset of the pliocene holocene volcanic activities ca 11 2 5 ma 5 change in the locus of volcanic eruption and cessation of contractile uplifting in the basin margin ca 2 5 0 ma

morphological characteristics and emplacement mechanism of the seamounts in the central indian ocean basin abstract the morphotectonic features of the central indian ocean basin ciob provide information regarding the development of the basin multibeam mapping of the ciob reveals presence of abundant isolated seamounts and seamount chains sub parallel to each other and major fracture zones along 73 e 79 e and 75 45 e morphological analyses were carried out for 200 seamounts that occur either as isolated edifies or along eight sub parallel chains the identified eight parallel seamount chains that trend almost north south and reflecting the absolute motion of the indian plate probably originated from the ancient propagative fractures inspite of the differences in their height the seamounts of these eight chains are morphologically correlatable in the study area the seamounts are clustered north and south of 12 s latitude interestingly in the area north of 12 s area ii 9 12 s the seamounts are distinctly smaller 400 m height whereas the area south of 12 s area i 12 15 s has a mixed population of seamounts the normalized abundance of the ciob seamount is 976 seamounts 10 6 km 2 but on a finer scale this value varies from 500 to 1600 seamounts 10 6 km 2 which is less than the seamount concentrations of the pacific and atlantic oceans 9000 to 16 000 seamounts 10 6 km 2 three categories of seamounts are present in the ciob e g 1 single peaked 2 multi peaked and 3 composite the study indicate that single peaked seamounts are dominant 89 while multi peaked is less 8 and composite ones are rare 3 in the ciob the progressive northward movement of the indian continent caused collision between india and asia at around 62 ma ago a majority of the near axis originated seamounts in the ciob seemed to have formed as a consequence of the temporally widespread cretaceous 65 ma to late eocene

propagating rift and overlapping spreading center in the north fiji basin abstract the present day geometry of the north fiji marginal basin at the pacific and indo australian plate boundary results from a polyphased tectonic evolution since the beginning of the opening spreading occurred along several ridge axes that are characterized by highly variable trends from 15 s to 21 40 s the present day trends of the axis are mainly n20 w n20 e and n n5 e the structural analysis of multi beam echo sounder and seismic data combined with submersible observations 18 dives in this area allow a precise study of relationships between the n n5 e and n20 e axes near 18 30 s two main oceanic features occur in this area the most striking one consists in a broad northward propagating rift of the n n5 e spreading ridge the northern tip of the propagator is located at 18 10 s on 173 30 e and is characterized by a northern v shaped end and a slight camber the diving observations corroborate the present day magmatic and tectonic activities along the n n5 e spreading axis located between 18 10 s and 20 30 s and its northward propagation to the detriment of the n20 e axis which is less active at its southern end moreover the youngest axis trends inside the n20 e axial domain confirms this northward propagation of the n n5 e structural trends the n n5 e spreading axis shows very close similarities with those of an intermediate to fast spreading ridge either in its morphology or in its spreading rate on the other hand the n20 e axis displays a large tectonically active overlapping arm that is counterposed to the northern end of the n n5 e propagating system the transform zone between the two overlapping arms is characterized by typical oblique and curved structural patterns in the sea floor the whole tectonic system constitutes thus a typical large overlapping spreading center the average propagating rate is calculated at 5 7 cm yr that is compatible with backward migration of the n20 e overlapping arm this study confirms the high instability of the north fiji basin spreading system through time and space due to the regional tectonic stress produced by the plate convergence and the double sphenochasmic opening of the basin

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

formation of submarine flat topped volcanic cones in hawai i high resolution bathymetric mapping has shown that submarine flat topped volcanic cones morphologically similar to ones on the deep sea floor and near mid ocean ridges are common on or near submarine rift zones of kilauea kohala or mauna kea mahukona and haleakala volcanoes four flat topped cones on kohala were explored and sampled with the pisces v submersible in october 1998 samples show that flat topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage similarly shaped flat topped cones on the northwest submarine flank of ni ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage submarine postshield stage eruptions on hilo ridge mahukona hana ridge and offshore ni ihau form pointed cones of alkalic basalt and hawaiite the shield stage flat topped cones have steep 25 sides remarkably flat horizontal tops basal diameters of 1 3 km and heights 300 m the flat tops commonly have either a low mound or a deep crater in the center the rejuvenated stage flat topped cones have the same shape with steep sides and flat horizontal tops but are much larger with basal diameters up to 5 5 km and heights commonly greater than 200 m the flat tops have a central low mound shallow crater or levees that surrounded lava ponds as large as 1 km across most of the rejuvenated stage flat topped cones formed on slopes 10 and formed adjacent semicircular steps down the flank of ni ihau rather than circular structures all the flat topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades these and other submarine volcanic cones of similar size and shape apparently form as continuously overflowing submarine lava ponds a lava pond surrounded by a levee forms above a sea floor vent as lava continues to flow into the pond the lava flow surface rises and overflows the lowest point on the levee forming elongate pillow lava flows that simultaneously build the rim outward and upward but also dam and fill in the low point on the rim the process repeats at the new lowest point forming a circular structure with a flat horizontal top and steep pillowed margins there is a delicate balance between lava heat supply to the pond and cooling and thickening of the floating crust factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents moderate to high confining pressure at moderate to great ocean depth long lived steady eruption years to decades moderate effusion rates probably ca 0 1 km3 year and low but not necessarily flat slopes with higher effusion rates sheet flows flood the slope with lower effusion rates pillow mounds form hawaiian shield stage eruptions begin as fissure eruptions if the eruption is too brief it will not consolidate activity at a point and fissure fed flows will form a pond with irregular levees the pond will solidify between eruptive pulses if the eruption is not steady lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones as occurs during the post shield stage the steady effusion of lava on land constructs lava shields which are probably the subaerial analogs to submarine flat topped cones but formed under different cooling conditions

global distribution of seamounts inferred from gridded geosat ers 1 altimetry using a forward modeling approach based on an axisymmetric gaussian seamount i characterize the global seamount distribution by locating circular maxima in the gridded vertical gravity gradient field derived from altimetry collected by the geosat and ers 1 satellite missions the global seamount distribution is long tailed and resembles a power law distribution for seamounts in the height range 2 7 km smaller seamounts are not well isolated by my technique nor are they well resolved in the gridded data several factors are likely to influence the height of volcanic seamounts such as melt availability magma driving pressure and plate thickness the observed relationship between seamount gravimetric amplitudes and the age of the underlying seafloor implies that there is an upper limit on seamount heights whether a seamount will reach that height depends most likely on supply side factors such as melt availability and magma driving pressure but the limiting height itself seems more likely to be controlled by the strength of the oceanic plate specifically compressional stresses directly beneath the seamount as a consequence of the lithosphere s flexural response to loading may eventually exceed the magma driving pressure and prevent magma from reaching the surface thus limiting the growth of the seamount because oceanic plate strength primarily is controlled by plate age the limit on seamount height is inferred to be a simple function of plate age at the time of seamount emplacement using analytical solutions i present a simple flexural model that predicts the observed global height age relationship

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

the indian ocean nodule field petrotectonic evolution and ferromanganese deposits abstract the indian ocean nodule field ionf is significant from several points of view roughly bordered by 10 s to 16 30 s and 72 e to 80 e and located within the central indian ocean basin ciob the field hosts the world s second largest and second high grade manganese nodule deposit after the equatorial nodule belt in the north pacific ocean moreover the crust underlying this field is characterised by unique morphotectonic signatures owing to its formation between 60 and 49 ma under three variable spreading conditions fast intermediate and slow from the indian ocean ridge system iors the nodule field has been surveyed both extensively more than 0 4 million km2 area and intensively comprising of a large geophysical data set and geological sample inventory during the last two decades several morphotectonic features such as seamounts hills ridge normal lineaments and ridge parallel lineations have disturbed the apparently smooth topographic gradient 1 7000 of the seafloor here variations in the rate of spreading and formation of new oceanic crust along the ridge crest during more than one episode of india eurasia collision are imprinted in the ionf based on the nature of the ridge parallel lineations which are related to the rate of crustal accretion the field is divided in to four sectors a b c and d from north to south sectors a and c were formed at a fast rate of spreading 90 95 mm year half rate and sectors b and d were formed at an intermediate 55 mm year and slow 26 mm year rates respectively the predominance of tensional stress in sectors a and c caused asymmetrical flexuring of the seafloor resulting in widely spaced faults and folds with low amplitude and large wavelength in contrast the seafloor flexuring in sector d are closely spaced long symmetrical and of high amplitude the timing and intensity of the collision of india with eurasia is constrained by the variable intensity of these flexures suggesting probably a soft touch at 58 ma and the hard collision at about 51 ma the nodule field hosts several seamounts both as isolated entities and in linear chains which are arranged parallel to the flow lines along the direction of absolute motion of the indian plate the distribution morphology and growth patterns of a majority of these seamounts are related to spreading rate suggesting their formation at the ridge crest however many of the seamounts show more than one stage of growth with local intraplate volcanism contributing to the enlargement of the larger ancient seamounts varieties of volcanics such as tholeiitic basalts spilites ferrobasalts and pumice occur within the ionf the alteration of some of these volcanics has resulted in palagonitisation of the glass and formation of zeolites subsequently during its journey away from the ridge crest to the abyssal areas the crust underlying the nodule field witnessed intraplate volcanism this is evident from the addition of younger rocks at the base of the ancient seamounts inconsistent growth of volcanoes eruption of ferrobasalt corresponding to the areas of high amplitude magnetic zones and presence of volcanogenic hydrothermal materials vhm of 10 ka age these findings collectively hint that the ionf is geodynamically unstable and may have been volcanically active in the recent past during the last 8 ma growth of authigenic ferromanganese deposits in the form of manganese nodules and crusts has occurred in the nodule field the deposits occur at an average water depth of 5000 m the basinal geomorphology intraplate tectonic deformations and volcanic eruptions considerably influence the formation development morphology mineralogy and composition of these deposits the data show that the large seamounts reverse faults and fracture zones fzs supply rock fragments as seeds for the nodule formation the hydrogenous precipitation from the overlying water column is the primary source of metal accumulation in the nodules the secondary relatively weak intraplate eruptions along the base of ancient seamounts or lineaments subsurface igneous activity and diagenetic remobilisation of metals have also played significant role in the growth and enrichment of the deposit based on a large data set we estimate the contribution of various physico chemical parameters and model the probable conditions of formation of the ferromanganese deposits in the ionf the model also hints at the location where the resource exploitation should be rewarding

surveying the flanks of the mid atlantic ridge the atlantis basin north atlantic ocean 36 n new seismic reflection and okean 9 5 khz sidescan sonar data were used to characterise the seismic stratigraphy and structure of the region south of the azores islands north atlantic ocean the new data provide constraints on the tectono sedimentary evolution of deep ocean basins located on the flanks of slow spreading ridges south of the azores thick sedimentary sequences 1 0 s two way travel time correlated with units in the madeira abyssal plain odp sites 950 952 overlay an irregular basement generated by a combination of volcanic and tectonic processes three main cenozoic depositional events are recognised and signed by distinct seismic megasequences the moderate deformation of the sedimentary units and the structural fabric of the underlying oceanic crust shown on seismic and sidescan data suggest the existence of a complex tectonic setting south of the azores illustrating this complex setting is the change from a basement with prominent abyssal hills west of 30 w to a segmented oceanic crust denoting an extensional transtensional tectonic regime east of the latter meridian

seamount morphology in the bowie and cobb hot spot trails gulf of alaska full coverage multibeam bathymetric mapping of twelve seamounts in the gulf of alaska reveals that they are characterized by flat topped summits rarely with summit craters and by terraced or step bench flanks these summit plateaus contain relict volcanic features e g flow levees late stage cones and collapse craters and as such must have been constructed by volcanic processes such as lava ponding above a central vent rather than by erosion above sea level the terraced flanks are composed of a sequence of stacked lava deltas and cones probably tube fed from a central lava pond a morphology which is suggestive of long lived stable central lava sources and low to moderate eruption rates indicative of significant time spent above a hot spot outlet most of these seamounts have summit plateaus surrounded and cut into by amphitheater headwall scarps and flanks that are scarred by debris chutes but lack visible debris accumulations at their base we interpret the lack of blocky debris fields as evidence that the slope failures are mainly small scale debris flows rather than large scale flank collapses however we cannot rule out the possibility that large flank collapse blocks from early in the histories of these seamounts are now hidden beneath the thick glacio fluvial fan deposits that cover the gulf of alaska seafloor these slope failure features become smoother and longer and increase in size and abundance with increasing age of a seamount suggesting that slope failure processes continue long after volcanic activity ceases

lithospheric age dependence of off ridge volcano production in the north pacific data for numbers of seamounts on north pacific ocean crust of different age indicate that the production rate of new off ridge seamounts volcanoes is proportional to the inverse of the square root of the age of the lithosphere this observation is consistent with several hypotheses which have been offered to explain the origin of off ridge oceanic volcanism and in combination with petrologic and paleomagnetic evidence for small oceanic volcanoes leads to a new self consistent model for off ridge volcanism in this model the production rate of off ridge volcanoes is controlled primarily by the availability of fracture zone conduit systems in the thickening lithosphere in contrast mantle plume or hotspot volcanoes may punch through the oceanic lithosphere decreasing production rate of off ridge volcanoes on old oceanic lithosphere may be associated with decreasing extent of partial melting of chemically and isotopically heterogeneous mantle material

styles of eruptive activity on intraplate volcanoes in the society and austral hot spot regions bathymetry petrology and submersible observations submersible observations carried out in the society and austral hot spot regions south pacific during the teahitia ii cruise 1988 1989 enable us to propose a model relating the style of volcanism to the depth of eruption on large intraplate seamounts 1 the bases of volcanic edifices are characterized by broad flat flows formed during a high rate of discharge of a very fluid magma on the sea floor 2 the edifices flanks deeper than 500 m are mainly made up of bulbous tubular pillows and large drained lava tunnels occurring on slopes steeper than 60 70 these lava flows occur on lateral volcanic vents about 200 300 m high along linear rift zones which are formed on the flank of the major edifices 3 volcanic ejecta due to hydromagmatic eruptions occur at shallow depths 500 m in height are individual conical features built at a high rate of volcanic discharge localized along radial linear eruptive fissures called rift zones the slopes located between the rift zones are covered by talus material are limited and unimportant mass wasting sector collapse features were observed on the edifices the conical shaped features are the primary morphology of the intraplate volcanoes formed during edifice growth which is closely linked to the hydrostatic pressure of the magmatic column the mean values of the slopes 11 17 of intraplate seamounts are the same as those of subaerial intraplate shield volcanoes when the seawater hydrostatic pressure is taken into account

submarine volcanism surrounding tenerife canary islands implications for tectonic controls and oceanic shield forming processes abstract using high resolution bathymetric data 97 seamounts within two large submarine areas n and se of tenerife have been identified and studied in detail main dimensions and associated parameters of these seamounts have been measured allowing a morphometric and morphologic classification on a shape basis in order to be certain of their volcanic origin several restrictive criteria were applied conical shape with maximum height 150 m basal ratio 1 aspect ratio 0 1 to the initial tally obtaining finally 31 regular shapes that can be identified without any reasonable doubt as volcanic seamounts these are very similar to submarine volcanoes described elsewhere on the sea floor most of these volcanic seamounts are aligned along two lines that follow the main volcano tectonic trends of the canary archipelago the nw se line crosses the island while the ne sw line coincides with a seismically active reverse fault that separates the tenerife and gran canaria insular blocks the distribution of the submarine volcanism to the north of tenerife permits a better knowledge of the island from its base on the sea floor and defines the tectonic control of the growth of the shield between 4 and 2 ma therefore the evolution of the submarine edifice of tenerife shows that the widespread parasite or satellite eruption centres are also very important in the overall growth of large shield edifices of volcanic islands